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  <row Id="1" PostTypeId="1" AcceptedAnswerId="25" CreationDate="2013-07-16T20:51:47.093" Score="68" ViewCount="4500" Body="&lt;p&gt;Given current technology, how far could we send someone before they get a lethal dose of hard radiation?  What if they don't need to make a return trip?  I know that last I had heard, Mars was too far to be able to take a return trip without being exposed to too much radiation, but it's possible that technology has improved since then.&lt;/p&gt;&#xA;" OwnerUserId="7" LastEditorUserId="68" LastEditDate="2013-07-24T16:48:29.880" LastActivityDate="2020-02-26T12:41:17.353" Title="How far could we send someone before they get a lethal dose of radiation?" Tags="|radiation|medical|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2" PostTypeId="1" AcceptedAnswerId="162" CreationDate="2013-07-16T20:58:20.037" Score="33" ViewCount="2438" Body="&lt;p&gt;The concept of a space elevator unfortunately still seems like one confined to fiction, at least for now. However, I've heard some sources claim that carbon nanotubes could be the first material strong enough to construct one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this the case, or are they still not strong enough? And if so, is the material the only real technological barrier to constructing such a structure, or are there other major issues that need to be overcome as well?&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorDisplayName="user40" LastEditDate="2013-07-16T21:28:34.097" LastActivityDate="2013-12-16T12:59:23.563" Title="What technological barriers do we need to overcome to build a space elevator?" Tags="|space-elevator|materials|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3" PostTypeId="1" AcceptedAnswerId="2417" CreationDate="2013-07-16T20:59:46.853" Score="30" ViewCount="1966" Body="&lt;p&gt;I know that the Voyager missions were launched years ago, and are quite a long way out there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the problem of communications taking forever to get to the probe and back, what are we currently doing with the probes for science?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-10-14T15:20:06.477" Title="What science are the Voyager missions still doing after so many years away from the Solar System?" Tags="|voyager|science|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4" PostTypeId="1" AcceptedAnswerId="23" CreationDate="2013-07-16T21:01:26.857" Score="17" ViewCount="1417" Body="&lt;p&gt;Is the technology currently in place to launch a manned mission to Mars (and is it thus just a resource constraint that's preventing such a mission from occurring), or are there still other technologies we need to develop for this to be feasible? If so, what are they?&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorUserId="68" LastEditDate="2013-07-24T16:46:24.247" LastActivityDate="2013-11-14T00:02:53.687" Title="What technological barriers, if any, prevent a manned mission to Mars at this time?" Tags="|mars|technology|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5" PostTypeId="1" AcceptedAnswerId="3501" CreationDate="2013-07-16T21:02:16.623" Score="59" ViewCount="6568" Body="&lt;p&gt;The runway at the &lt;a href=&quot;http://en.wikipedia.org/wiki/Shuttle_Landing_Facility&quot; rel=&quot;noreferrer&quot;&gt;Shuttle Landing Facility&lt;/a&gt; in Florida has a unique feature that looks rather intentional: it is surrounded by &quot;moats&quot; or &quot;canals&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/File:Shuttle_Landing_Facility.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rLA8N.jpg&quot; alt=&quot;Shuttle Landing Facility&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They certainly look man-made, so I presume this is intentional. I could only speculate as to why this was made a part of it, I'm curious what the official purpose of this is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My initial instinct would be to say that it is there to provide water for fire suppression in case of an emergency, but I've never seen something quite like it at another runway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there any official documentation explaining what the water surrounding the runway is for?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="17" LastEditorDisplayName="user12" LastEditDate="2013-07-19T00:52:14.290" LastActivityDate="2019-03-11T00:53:59.677" Title="Why is the Shuttle Landing Facility runway surrounded by water?" Tags="|space-shuttle|kennedy-space-center|nasa|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6" PostTypeId="1" AcceptedAnswerId="475" CreationDate="2013-07-16T21:02:28.733" Score="12" ViewCount="620" Body="&lt;p&gt;I've heard that there might be some creative ways to get to Geosynchronous Orbit that could save a lot of fuel. How might I manage to make this happen?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="14" LastEditDate="2013-07-19T10:27:55.140" LastActivityDate="2013-07-19T13:16:35.233" Title="What creative orbital maneuvers might I do to save fuel for a geosynchronous spacecraft?" Tags="|orbital-mechanics|orbit|orbital-maneuver|fuel|geosynchronous|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7" PostTypeId="1" AcceptedAnswerId="37" CreationDate="2013-07-16T21:03:09.730" Score="17" ViewCount="887" Body="&lt;p&gt;I know that on Mars, radiation levels are quite high (turns out lack of an atmosphere does that). On the rovers that are sent to Mars, etc. How is radiation kept from playing havoc with the rover's electronics?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-17T01:35:14.417" LastActivityDate="2013-07-17T01:35:14.417" Title="How does NASA keep radiation from frying rovers?" Tags="|radiation|mars|rovers|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8" PostTypeId="1" AcceptedAnswerId="15" CreationDate="2013-07-16T21:03:46.940" Score="24" ViewCount="2691" Body="&lt;p&gt;What is the optimal orientation to place 4 reaction wheels, such that complete 3 axis control can be maintained at all times?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2023-03-23T17:16:19.507" Title="Optimal placement of 4 reaction wheels?" Tags="|reaction-wheel|attitude|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9" PostTypeId="1" AcceptedAnswerId="138" CreationDate="2013-07-16T21:06:28.487" Score="21" ViewCount="995" Body="&lt;p&gt;The oort cloud has long been hypothesized as being around a light year away from the sun, but no observations have yet been made. Is it completely impractical with current science to make such an observation, or are there techniques we could try to use to observe it (and thus confirm or deny its existence?)&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorDisplayName="user40" LastEditDate="2013-07-16T21:36:38.897" LastActivityDate="2020-02-28T11:32:30.477" Title="How could we observe the Oort cloud, if it exists?" Tags="|oort-cloud|observation|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10" PostTypeId="1" AcceptedAnswerId="101" CreationDate="2013-07-16T21:06:54.537" Score="23" ViewCount="1126" Body="&lt;p&gt;I've heard of something called the South Atlantic Anomaly (SAA), which seems to have a lot of Low Earth Orbit Satellite Operators quite worried. What is it, and how can I protect my spacecraft against it?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2019-08-21T14:49:52.790" Title="What is the South Atlantic Anomaly, and what can I do to protect satellites from it?" Tags="|radiation|orbit|low-earth-orbit|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11" PostTypeId="1" AcceptedAnswerId="171" CreationDate="2013-07-16T21:09:51.883" Score="16" ViewCount="907" Body="&lt;p&gt;According to Scientific American (February 2009), ion engines start off slow, but build up speed as they travel.  At what travel distance would an ion engine (such as an ion thruster, a Hall thruster, or a magnetoplasmadynamic thruster) be faster and/or more efficient than conventional (solid/liquid) fuel engines.&lt;/p&gt;&#xA;" OwnerUserId="30" LastEditorUserId="30" LastEditDate="2013-07-16T21:32:39.550" LastActivityDate="2021-02-07T16:57:22.333" Title="At what travel distances are ion engines faster and more efficient than conventional fuel engines?" Tags="|engine-design|fuel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12" PostTypeId="1" AcceptedAnswerId="536" CreationDate="2013-07-16T21:10:26.597" Score="19" ViewCount="596" Body="&lt;p&gt;This is something I was thinking about right now when this beta was opened ... perfect!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In radio astronomy, there is an technique which is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Very-long-baseline_interferometry&quot;&gt;very long baseline interferometry&lt;/a&gt;. The idea is to combine a number of telescopes to resemble a somewhat larger non-existing telescope. The intended purpose is usually described as gaining a higher angular resolution when observing the sky. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now imagine that you do something similar with a number of dishes somewhere in deep space for the purpose of picking up signals from Earth. Compared to that, current deep space missions such as New Horizons use a fairly large single dish for receiving commands from Earth. Imagine, you instead have a number of small spacecrafts with very small dishes but at high distance to Earth. Imagine that those small space crafts are somehow capable of performing very long baseline interferometry and are supposed to receive commands from Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, could this array of small dishes with the small spacecrafts be used to &quot;substitute&quot; a single bigger dish in this context? Or are an improved angular resolution and picking up signals over long distances somewhat different problems? &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="49" LastEditDate="2013-07-19T06:47:08.430" LastActivityDate="2013-07-21T15:16:08.903" Title="VLB interferometry in deep space - does it make sense for communication?" Tags="|deep-space|vlbi|communication|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13" PostTypeId="1" AcceptedAnswerId="47" CreationDate="2013-07-16T21:10:51.990" Score="18" ViewCount="3699" Body="&lt;p&gt;We knew that Voyager 1 and 2 exploited the gravitational slingshot from Saturn and Jupiter back in 70s, and the trajectories were predicted by Newton's laws of motion, and this law we used a lot in designing a trajectory for an interstellar trip. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions are: What other branches of mathematics are used in astrodynamics? Is it possible to simulate a complete eco-system in a spaceship on computer using numerics? How about probability theory?&lt;/p&gt;&#xA;" OwnerUserId="11" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:58:30.657" LastActivityDate="2013-08-20T21:58:30.657" Title="What branches of mathematics are widely used in astrodynamics?" Tags="|orbital-mechanics|mathematics|astrodynamics|orbit-determination|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14" PostTypeId="1" AcceptedAnswerId="167" CreationDate="2013-07-16T21:11:42.167" Score="22" ViewCount="506" Body="&lt;p&gt;On the &lt;a href=&quot;http://morpheuslander.jsc.nasa.gov/about/&quot;&gt;about page&lt;/a&gt; for &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Morpheus&quot;&gt;Project Morpheus&lt;/a&gt;, the possibility of converting moon dust into oxygen is mentioned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It was manufactured and assembled at JSC and Armadillo Aerospace. Morpheus is large enough to carry 1,100 pounds of cargo to the moon – for example, a humanoid robot, a small rover, or a &lt;strong&gt;&lt;em&gt;small laboratory to convert moon dust into oxygen&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm curious if it's theoretical or if it's something practical that has been successfully done. I'm not sure of the amount of moon dust we brought back to be able to test this sort of thing. Is this something that is a legitimately possible?&lt;/p&gt;&#xA;" OwnerUserId="17" LastEditorDisplayName="user12" LastEditDate="2013-07-18T20:19:17.060" LastActivityDate="2013-09-16T18:51:07.713" Title="Is converting moon dust into oxygen a viable option for sustainability?" Tags="|the-moon|gasses|life-support|" AnswerCount="2" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15" PostTypeId="2" ParentId="8" CreationDate="2013-07-16T21:13:35.283" Score="25" Body="&lt;p&gt;Most often they're configured in a &quot;pyramid&quot;, which is done for both controllability and redundancy reasons (it's tolerant to one wheel failure).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, there are many subtleties to this problem, and all spacecraft with reaction wheels do not necessarily follow the same design. The paper &quot;Sun-Tracking Commands and Reaction Wheel Sizing with Configuration Optimization&quot; by Hari Hablani is available &lt;a href=&quot;http://www.aero.iitb.ac.in/~hbhablani/Papers_for_Homepage/JGCD_1994_Sun_Tracking_Wheel_Configuration.pdf&quot;&gt;here&lt;/a&gt; [PDF], and discusses reaction wheel sizing and configuration considerations.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-07-16T21:13:35.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16" PostTypeId="2" ParentId="5" CreationDate="2013-07-16T21:14:18.990" Score="27" Body="&lt;p&gt;I too have wondered this in the past, and an official explanation has been completely lacking in my research! However, the closest I've found is the suggestion that it's for drainage. The following link of a &lt;a href=&quot;http://www.nasa.gov/multimedia/podcasting/landingsites.html&quot; rel=&quot;noreferrer&quot;&gt;podcast transcription&lt;/a&gt; states the following:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Richard Merritt, a landing support manager with United Space Alliance, says Florida's marshy terrain is the main reason it took nearly a decade to move from one coast to another.&lt;/p&gt;&#xA;&lt;p&gt;Richard Merritt/Landing Support Manager: We were still a research and development type of aircraft/spacecraft. They just weren't comfortable with the target here. If you look from above and looking on the runway, each side has a lot of water. So, if you didn't make the runway here, you'd be talking to the alligators.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;They go on to talk about how it was easier to land on a dry desert (another landing site) because you didn't have to worry about that - so I think that hints that the canal was probably constructed for drainage purposes to keep the runway from being swamped.&lt;/p&gt;&#xA;&lt;p&gt;There are other theories that have been put forward, but none seem plausible to me:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The most popular seems to be water needed if a fire should break out - but digging a water filled trench all the way around the runway is an incredibly archaic way to do this, especially when multiple fire hydrants would have been far cheaper to construct and this facility is only found at this runway! In addition, since the shuttle isn't carrying much fuel at all on landing, the chance of a massive fire requiring this much water is rather low.&lt;/li&gt;&#xA;&lt;li&gt;Again, security is another theory, but NASA doesn't use this approach anywhere else, and again it's incredibly archaic - we have much better, and cheaper ways of securing facilities these days.&lt;/li&gt;&#xA;&lt;li&gt;Material needed to level the runway is another theory, but if this was the only reason then it would have likely all come from one big hole rather than digging a neat trench around the runway.&lt;/li&gt;&#xA;&lt;li&gt;If it was needed to catch debris that might blow onto the runway, then we'd see structures with similar purposes in other locations, which we don't. If this was the case, then a simple reinforced fence would have done the job arguably more effectively for a fraction of the cost.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="19" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-18T23:57:15.517" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17" PostTypeId="1" AcceptedAnswerId="632" CreationDate="2013-07-16T21:19:38.103" Score="28" ViewCount="2796" Body="&lt;p&gt;In the past two decades, NASA has launched at least three missions that use RTG's:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Cassini&lt;/li&gt;&#xA;&lt;li&gt;Mars Science Laboratory&lt;/li&gt;&#xA;&lt;li&gt;New Horizons&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Those launches include plutonium, which is a reason for some to oppose the missions in their form, because the launch is involved with considerable risk.  For each of them, NASA has published a quite extensive environmental impact statement.  This includes an estimate of the number of &quot;latent cancer deaths&quot; in the worst case scenario:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Cassini worst case, probability &quot;less than 1 in 1 million&quot;, nicely&#xA;formulated as &quot;could be a 0.0005 percent increase above the normally&#xA;observed 1 billion cancer fatalities&quot;.  0.0005% * 1 billion = 5000&#xA;deaths. (see &lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/safety/safetyeis/&quot;&gt;Cassini Environmental Impact Statement&lt;/a&gt;, chapter 4, page 4-63)&lt;/li&gt;&#xA;&lt;li&gt;New Horizons: 1:1 million for 100 &quot;latent cancer fatalities&quot;, 1:62000 for 5 fatalities and 10 km² contamination (at \$93–\$520 million/km² cleanup costs, see &lt;a href=&quot;http://pluto.jhuapl.edu/overview/deis/docs/NH_DEIS_Full.pdf&quot;&gt;New Horizons Draft Environmental Impact Statement&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;li&gt;Mars Science Laboratory, similar probability but with 60 deaths (see &lt;a href=&quot;http://science.nasa.gov/media/medialibrary/2010/11/05/MSL-FEIS_Vol1.pdf&quot;&gt;Mars Science Laboratory Enviromental Impact Statement&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What alternatives are available?&lt;/strong&gt;  NASA considered solar&#xA;for Cassini in Chapter 2 of &lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/safety/safetyeis/&quot;&gt;the EIS&lt;/a&gt; and for the Mars Science Laboratory in&#xA;Chapter 2 at &lt;a href=&quot;http://science.nasa.gov/media/medialibrary/2010/11/05/MSL-FEIS_Vol1.pdf&quot;&gt;the EIS&lt;/a&gt;.  The risk for Cassini was much higher than for MSL&#xA;(because it had much more plutonium), but the cost of implementing the&#xA;alternative was also much higher (It might be possible now, but I&#xA;question if it was possible when Cassini was launched).  There was some discussion, e.g. &lt;a href=&quot;http://www.space4peace.org/cassini/solar.htm&quot;&gt;here&lt;/a&gt;.  Both Juno and Juice, missions to Jupiter, use solar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a mission like New Horizons, but also Voyager 1 and 2, solar power&#xA;really is not feasible.  Is there any alternative here?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One esoteric, science-fiction solution that I can think of would be to&#xA;produce plutonium in space.  It's futuristic, but with a nuclear&#xA;reactor on an asteroid, operated by robots, one could produce fuel&#xA;that makes exploration of the outer solar system possible.  This is,&#xA;of course, not currently possible, but would get rid of the launch safety issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another, closer-by alternative would not get rid of plutonium in&#xA;Earth-based sources completely, but to limit it further.  &lt;a href=&quot;http://en.wikipedia.org/wiki/Advanced_Stirling_Radioisotope_Generator&quot;&gt;New-style&#xA;ASRG's&lt;/a&gt; use only a quarter of the fuel than traditional RTG's do,&#xA;and payloads get more efficient, so a future beyond Saturn might do&#xA;with 10–20% of the plutonium.  This was proposed for the now-cancelled&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Titan_Mare_Explorer&quot;&gt;Titan Mare Explorer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any alternatives for exploring the outer solar system that do not carry nuclear launch risks?  For the sake of this question, I define outer solar system as Saturn or beyond, where solar has never been used.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="415" LastEditDate="2014-05-05T14:23:41.733" LastActivityDate="2020-05-29T12:48:59.820" Title="Are there any safe-to-launch alternatives to RTG's for outer solar system exploration?" Tags="|safety|rtg|power|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="18" PostTypeId="1" AcceptedAnswerId="509" CreationDate="2013-07-16T21:20:00.097" Score="9" ViewCount="530" Body="&lt;p&gt;For extremely long journeys, deterioration of spaceship materials becomes a concern beyond mere rust or corrosion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a ship is sufficiently protected against radiation and other colliding objects&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What materials would be problematic in terms of naturally decaying into different atomic elements? &lt;/li&gt;&#xA;&lt;li&gt;Can anything be done to prevent said radioactive decay?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="23" LastEditorUserId="288" LastEditDate="2014-02-04T18:44:26.293" LastActivityDate="2014-02-04T18:44:26.293" Title="What spaceship materials could naturally experience radioactive decay and transmute on a million year interstellar journey?" Tags="|interstellar-travel|long-duration|radioactive-decay|" AnswerCount="1" CommentCount="6" ClosedDate="2013-07-21T21:22:00.847" ContentLicense="CC BY-SA 3.0" />
  <row Id="19" PostTypeId="1" AcceptedAnswerId="143" CreationDate="2013-07-16T21:20:15.513" Score="20" ViewCount="2025" Body="&lt;p&gt;A lot is heard about space junk these days, but how much of a problem is it really to orbiting satellites? Are there many cases of spacecraft being damaged by it, or is it a bit overhyped?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Following on from that, if it is an issue then what measures could be taken to &quot;clean up&quot; the junk we have orbiting the earth? Is the technology available to do an effective job of cleaning it up?&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorUserId="51" LastEditDate="2013-12-27T12:38:07.057" LastActivityDate="2024-03-23T08:57:08.077" Title="How much of a problem is space junk, and how can we clean it up?" Tags="|debris|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20" PostTypeId="2" ParentId="6" CreationDate="2013-07-16T21:21:09.410" Score="6" Body="&lt;p&gt;I guess you are referring to concepts like this ... &lt;a href=&quot;http://www.google.com/patents/US6116545&quot; rel=&quot;noreferrer&quot;&gt;http://www.google.com/patents/US6116545&lt;/a&gt; - lunar flybys. In a way, stuff like this has been done before, although not initially intended by the mission designers: &lt;a href=&quot;http://en.wikipedia.org/wiki/HGS-1&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/HGS-1&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-16T21:21:09.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21" PostTypeId="1" AcceptedAnswerId="80" CreationDate="2013-07-16T21:21:17.257" Score="22" ViewCount="2425" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Dyson_sphere&quot; rel=&quot;noreferrer&quot;&gt;Dyson sphere&lt;/a&gt;, a concept coined by the physicist Freeman Dyson, is a solar megastructure built around our Sun to harvests most of the radiated energy from it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this sounds impossible for now, some of its variants look promising in the near future, for example a Dyson swarm or a Dyson bubble around the Sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we have the technology to do that?&lt;/p&gt;&#xA;" OwnerUserId="11" LastEditorUserId="49" LastEditDate="2015-11-01T00:30:20.713" LastActivityDate="2015-11-01T00:48:42.683" Title="Is it possible to build a Dyson sphere or its variant in our solar system based on our current technology?" Tags="|energy|construction|conceptual|megastructure|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22" PostTypeId="1" AcceptedAnswerId="31" CreationDate="2013-07-16T21:21:44.663" Score="12" ViewCount="1639" Body="&lt;p&gt;Have there ever been any cases of any countries or constituencies laying claim to any space territory? Is there any legal framework in place for such a claim (or indeed, to nullify such claims?)&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorDisplayName="user12" LastEditDate="2013-07-18T20:17:02.850" LastActivityDate="2021-09-30T11:36:24.320" Title="Have any countries laid claim to territory in space?" Tags="|law|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23" PostTypeId="2" ParentId="4" CreationDate="2013-07-16T21:22:44.730" Score="8" Body="&lt;p&gt;There is a fellow by the name of Robert Zubrin who has made a case by which humans could travel to Mars within existing (actually long existing) technology, and within NASA's existing budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If all his facts are correct, then there is no technological or financial barrier to landing humans on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/The_Case_for_Mars&quot; rel=&quot;noreferrer&quot;&gt;The Case For Mars&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastEditorDisplayName="user39" LastEditDate="2013-07-25T05:31:03.050" LastActivityDate="2013-07-25T05:31:03.050" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24" PostTypeId="1" CreationDate="2013-07-16T21:23:25.587" Score="16" ViewCount="837" Body="&lt;p&gt;For outer solar system exploration, virtually the only feasible power subsystem are &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Radioisotope thermoelectric generators&lt;/em&gt; (&lt;em&gt;RTGs&lt;/em&gt;)&lt;/a&gt;.  These include plutonium, which may carry considerable risks (see also &lt;a href=&quot;https://space.stackexchange.com/questions/17/are-there-any-safe-to-launch-alternatives-to-rtgs-for-outer-solar-system-explor&quot;&gt;this question&lt;/a&gt;).  According to &lt;em&gt;Space Mission Analysis and Design&lt;/em&gt; (Larson and Wertz, Third Edition, Eight Printing, 2006), page 335:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We must also consider safety issues, but RTG sources are probably safer than most propellants.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How true is this statement?  In the case of catastrophic launch failure of a spacecraft carrying several kg of plutonium, how does the risk posed by the plutonium compare to the risk posed by propellants?  By risk, I mean the danger posed to human health and to the local and global environment.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: I'm looking for quantitative calculations.  The question I linked above links to environmental impact statements that NASA did for the launch of Cassini, New Horizons, and Mars Science Laboratory.  These contain risk calculations: for certain catastrophic scenarios, they estimate the consequences to human health (latent cancer risks) and cleaning costs.  I'd like to see similar calculations for the consequences of large amounts of propellants, or their partial burning, being released locally or through a larger region.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2022-05-22T04:54:54.087" Title="How does the launch risk posed by plutonium compare to the launch risk posed by propellants?" Tags="|fuel|safety|nuclear|power|rtg|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="25" PostTypeId="2" ParentId="1" CreationDate="2013-07-16T21:24:21.143" Score="59" Body="&lt;p&gt;In order to properly assess this, you need to take a few things into account:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What are the typical exposure levels?&lt;/li&gt;&#xA;&lt;li&gt;How would Solar Storms be protected against?&lt;/li&gt;&#xA;&lt;li&gt;What kind of shielding do you need?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Much of my answer to this was taken with help from The Case for Mars, by Robert Zubrin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, let's start with the long term limits. As a starting point, let's look at the legal limits according to OSHA and NASA. These are defined in the following table, taken from &lt;a href=&quot;http://srag-nt.jsc.nasa.gov/Publications/TM104782/techmemo.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;. These limits provide for an exposure per year of 50 REM, and a career of 100 to 400 REM, measured internally. To simplify this, I'm going to assume that the trip is a return trip, specked at 50 REM per year, and a 45 year old male is making the trip. That gives a 6 year mission, assuming we can keep the radiation exposure below 50 REM per year, and longer given reduced portions, such as while on the surface of a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CzmmU.jpg&quot; alt=&quot;Radiation Exposure Limits&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Case for Mars, they cite an average cosmic ray dosage of between 20 and 50 REM per year, depending on where the sun is at during its cycle. The minimum takes place when solar activity is at it's max, as the sun is blocking the rays (They come from outside the solar system). This is the primary source of day to day radiation exposure. This is below the limits easily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the second part, you need to consider rare high energy events, namely Solar Flares. If you were outside of a spacecraft with no shielding these events could be quite deadly in a short amount of time. However, we do know a few things about these. First of all, there is always advanced notice of them coming, at least &lt;a href=&quot;https://books.google.com.mx/books?id=0UowtSAL5J4C&amp;amp;pg=PA40&amp;amp;lpg=PA40&amp;amp;dq=spe%20time%20to%20reach%20earth&amp;amp;source=bl&amp;amp;ots=sYZRFeLAfD&amp;amp;sig=XVwNs_qQGDWvncZhW2epkIz1U_c&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=OpKVVMydNsXboAT93YCIAg&amp;amp;ved=0CBwQ6AEwAA#v=onepage&amp;amp;q=spe%20time%20to%20reach%20earth&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;18 minutes&lt;/a&gt; at the distance of Earth's orbit. Secondly, the primary source of energy is charged particles, which can be repelled with a modest amount of shielding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The typical plan for such storms is to have a specific shelter for these storms. Hydrogen has been found to be effective in absorbing the radiation quite well. Basically, if one hides out in the pantry, one would be relatively safe. One could do so for the few days required to wait out the solar storm, and the radiation would be reduced to a manageable level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, a 45 year old man could take a 6 year trip in space, or a 45 year old woman could take a 5 year trip in space, while meeting all OSHA requirements for astronaut safety. That could enable a person to make a trip to Jupiter, the Asteroid belt, and everything closer, without any major concern of radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that this isn't a lethal dose of radiation. This would increase a person's lifetime risk for cancer by a few percent. Even longer trips could probably taken, especially explorer type missions where there is a high risk of death anyways to the point that a small increase in cancer rates isn't an issue.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2020-02-26T12:41:17.353" LastActivityDate="2020-02-26T12:41:17.353" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="26" PostTypeId="1" AcceptedAnswerId="147" CreationDate="2013-07-16T21:25:00.927" Score="21" ViewCount="833" Body="&lt;p&gt;Nuclear pulse propulsion seemed to be quite actively researched up until the 1990's, with a few projects that saw some rather promising results (some that could theoretically reach over 4% light speed.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it all seems to have gone a bit quiet since then - is there any ongoing research into this area, or have plans been shelved for now? For a very promising technology (on the surface at least) that could hugely cut journey times, it seems a shame if not much is being invested in it.&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorUserId="25" LastEditDate="2013-07-18T19:45:36.513" LastActivityDate="2013-07-18T19:45:36.513" Title="Is there any ongoing research in nuclear pulse propulsion?" Tags="|launch|propulsion|nuclear-pulse-propulsion|research|nuclear|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="27" PostTypeId="1" AcceptedAnswerId="293" CreationDate="2013-07-16T21:27:28.817" Score="10" ViewCount="274" Body="&lt;p&gt;It is hard enough to exactly aim a rocket when you know all the forces on it, but on a long journey there must be any number of minor gravitational attractions to rocks and other mass in space, and the effects must add up considerably over millions of miles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My assumption is that adjustments must be made based on comparing actual position to estimated position, and correcting accordingly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflightnow.com/news/n1209/04juno/&quot;&gt;From spaceflightnow.com:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first &quot;deep space maneuver&quot; occurred last Thursday, Aug. 30, more than 300 million miles from home, to begin fine-tuning the craft's course to intercept Earth. The Leros-1b main engine was fired for 29 minutes and 39 seconds, changing Juno's velocity by about 770 mph while consuming around 829 pounds of fuel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So how often does this measurement need to be done, and how is it done? Triangulation from known stars?&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="6537" LastEditDate="2018-01-12T09:46:22.037" LastActivityDate="2018-01-12T09:46:22.037" Title="How much adjustment of position is required when making long flights through the solar system?" Tags="|orbital-maneuver|mission-design|navigation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="29" PostTypeId="1" AcceptedAnswerId="119" CreationDate="2013-07-16T21:28:09.947" Score="15" ViewCount="6038" Body="&lt;p&gt;The gas giants in our solar system have abundant quantities of Helium and Hydrogen. Is it possible to harvest these resources there? Does NASA have any plans to build some permanent base in space for the purpose of harvesting these resources? &lt;/p&gt;&#xA;" OwnerUserId="11" LastEditorUserId="6944" LastEditDate="2021-03-28T18:20:57.783" LastActivityDate="2021-03-28T18:20:57.783" Title="How feasible is it to harvest isotopes of Helium or Hydrogen from the gas giants?" Tags="|engineering|mining|hydrogen|science|helium|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="30" PostTypeId="2" ParentId="22" CreationDate="2013-07-16T21:29:11.487" Score="4" Body="&lt;p&gt;According to &lt;em&gt;Who Owns the Moon?: Extraterrestrial Aspects of Land and Mineral Resources Ownership&lt;/em&gt; by Virgiliu Pop, people have been attempting to say they own lunar or Martian real estate since the 1890s.  For example, A. Dean Lindsay of Ocilla, Georgia attempted to say he owned tons of extraterrestrial real estate.  However, none of these purchases of extra terrestrial territory have any legal standing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This information is also available on Wikipedia (which references the book as well) at &lt;a href=&quot;http://en.wikipedia.org/wiki/Extraterrestrial_real_estate&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Extraterrestrial_real_estate&lt;/a&gt; .&lt;/p&gt;&#xA;" OwnerUserId="30" LastActivityDate="2013-07-16T21:29:11.487" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="31" PostTypeId="2" ParentId="22" CreationDate="2013-07-16T21:29:33.587" Score="14" Body="&lt;p&gt;Although this may be surprising, there is a whole discipline called &amp;quot;Space Law&amp;quot;.&#xA;As there is space law, there are space lawyers. You should look into the work and activities of e.g. &lt;a href=&quot;http://www.virgiliupop.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.virgiliupop.com/&lt;/a&gt; (link broken as of 2021)&lt;/p&gt;&#xA;&lt;p&gt;There are countless private claims on territory in space. The maybe strangest one is this: &lt;a href=&quot;https://en.wikipedia.org/wiki/Nation_of_Celestial_Space&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Nation_of_Celestial_Space&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;You might want to have a look at &lt;a href=&quot;https://iislweb.space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://iislweb.space/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;As for the exact legal regulations, have a look at &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Outer_Space_Treaty&lt;/a&gt; The current philosophy is, that space is common ground and, as it is &amp;quot;extraterrestrial&amp;quot;, no terrestrial nation can claim territory in space.&lt;/p&gt;&#xA;&lt;p&gt;EDIT: As stated in the comments, it is worth mentioning that there are analogies with the protection of Antarctica. For a good introduction, have a look into this paper: &lt;a href=&quot;http://si-pddr.si.edu/dspace/handle/10088/16170&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://si-pddr.si.edu/dspace/handle/10088/16170&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="12180" LastEditDate="2021-09-30T11:36:24.320" LastActivityDate="2021-09-30T11:36:24.320" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="32" PostTypeId="5" CreationDate="2013-07-16T21:30:13.847" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-16T21:30:13.847" LastActivityDate="2013-07-16T21:30:13.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="33" PostTypeId="4" CreationDate="2013-07-16T21:30:13.847" Score="0" Body="Questions pertaining to materials used for construction of space-worthy objects. " OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-16T21:58:10.490" LastActivityDate="2013-07-16T21:58:10.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="34" PostTypeId="2" ParentId="6" CreationDate="2013-07-16T21:31:54.313" Score="7" Body="&lt;p&gt;The general physics behind transfer orbits is pretty mundane, you really can't save fuel using a direct approach. The key item is to thrust as close to the apogee as possible, using the highest ISP possible, and get as much thrust in as you can.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there is at least one very creative maneuver which has been done, and &lt;a href=&quot;http://www.google.com/patents/US6116545&quot;&gt;patented&lt;/a&gt;. Essentially, this involves raising the apogee of the spacecraft such that it will orbit the moon, and have the moon raise it's perigee, and fix the inclination. In order to do so, the spacecraft must be launched in either April or October.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was done once to a spacecraft, which was placed in such an orbit where it was impossible to lift it to Geosync orbit, and thus was made quite useless. The satellite was originally known as &lt;a href=&quot;http://en.wikipedia.org/wiki/HGS-1&quot;&gt;AsiaSat 3&lt;/a&gt;, and was placed in a highly inclined orbit (51 degrees). The lifetime was reduced, and the satellite was perfectly stationary, but it was still usable, for a few years.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-16T21:31:54.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="36" PostTypeId="1" AcceptedAnswerId="44" CreationDate="2013-07-16T21:32:18.300" Score="18" ViewCount="1948" Body="&lt;p&gt;I'm looking for a list of objects with a semi-major axis between 0.95 AU and 1.05 AU that are probably larger than 1 km in diameter.  For this, the &lt;a href=&quot;http://ssd.jpl.nasa.gov/sbdb_query.cgi#x&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Small-Body Database Search Engine&lt;/a&gt; seems to be the best option.&lt;/p&gt;&#xA;&lt;p&gt;Here is an example search query:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a &amp;gt;= 0.95 (AU) (Semi-major axis)&lt;/li&gt;&#xA;&lt;li&gt;a &amp;lt;= 1.05 (AU)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;This gives 403 objects.  Estimated diameter is only given for 6.  Largest is 2.48 km.  &lt;a href=&quot;http://en.wikipedia.org/wiki/3753_Cruithne&quot; rel=&quot;nofollow noreferrer&quot;&gt;3753 Cruithne&lt;/a&gt; has a diameter of about 5 km and orbits at a=0.998 AU.  It's even in those search results, but it just doesn't have a diameter listed.&lt;/p&gt;&#xA;&lt;p&gt;As far as I know, diameter estimates are made roughly from magnitude information.  Obviously albedo factors into this, but let's say I'm happy with the uncertainty of using a standard albedo and standard density.  I have absolute magnitude, H, as a parameter for all objects, but the problem is I don't know &lt;em&gt;how far away&lt;/em&gt; it was when the measurement was taken.  It seems obvious to me that you couldn't get orbital parameters without at least having a distance estimate at the time of when you recorded magnitude.  In other words, there absolutely must be enough information taken to estimate the object's diameter, but it's not obvious how to do it.&lt;/p&gt;&#xA;&lt;p&gt;There is &lt;a href=&quot;http://neo.jpl.nasa.gov/nhats/&quot; rel=&quot;nofollow noreferrer&quot;&gt;another database tool&lt;/a&gt; for Near Earth Objects that NASA has online for candidates for an asteroid mission.  However, the objects in this database are already restricted by some criteria that excludes larger objects I'm trying to tabulate.&lt;/p&gt;&#xA;&lt;p&gt;Using the output fields available from the JPL database, how can I calculate an estimate of object diameter?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="11902" LastEditDate="2022-07-08T06:52:47.530" LastActivityDate="2022-07-08T06:52:47.530" Title="How can I derive an mass/size estimate of an asteroid from JPL parameters?" Tags="|asteroid|jpl-horizons|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="37" PostTypeId="2" ParentId="7" CreationDate="2013-07-16T21:34:15.793" Score="11" Body="&lt;p&gt;This is the same as if you were to protect a satellite from radiation in outer space. It is a field of study in &lt;a href=&quot;http://en.wikipedia.org/wiki/Radiation_hardening&quot;&gt;Radiation Hardening&lt;/a&gt;. For instance, your desktop pc may not have ECC ram so if an error occurs windows just crashes. In server hardware however there is ECC, redundancy and test patterns. These are things that are used to mitigate errors in electronics so that the software does not produce errors. Satellites and Rovers take it to the next level.&lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastActivityDate="2013-07-16T21:34:15.793" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="38" PostTypeId="2" ParentId="5" CreationDate="2013-07-16T21:34:17.277" Score="6" Body="&lt;p&gt;Some research I've done on the web has turned up some interesting theories, but no official explanation.  The theories that I think are most likely are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Dirt was needed to level the runway and was taken from the immediate vicinity&lt;/li&gt;&#xA;&lt;li&gt;Drainage (per berry120's answer)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;A couple other ideas include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Catching debris that might blow onto the runway&lt;/li&gt;&#xA;&lt;li&gt;Water supply in case of an emergency&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="36" LastActivityDate="2013-07-16T21:34:17.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="39" PostTypeId="1" AcceptedAnswerId="908" CreationDate="2013-07-16T21:36:19.470" Score="28" ViewCount="3539" Body="&lt;p&gt;The Saturn V rockets were the &quot;tallest, heaviest, and most powerful rocket ever brought to operational status and still holds the record for heaviest payload launched and heaviest payload capacity to Low Earth orbit (LEO)&quot;&lt;sup&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Saturn_V&quot;&gt;[1]&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;em&gt;&lt;a href=&quot;http://www.billbryson.co.uk/books_shorthistory.html&quot;&gt;A Short History of Nearly Everything&lt;/a&gt;&lt;/em&gt;, the author states the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We no longer possess a rocket powerful enough to send humans even as far as the Moon. The last rocket that could, Saturn 5, was retired years ago and has never been replaced.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In recent years, there has been on and off discussion about manned flights to go &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_proposed_missions_to_the_Moon#Manned_missions&quot;&gt;back to the Moon&lt;/a&gt;. Does NASA have plans for a rocket that will replace the deprecated Saturn V? Do modern (post-1972) advancements drastically change the design of the rocket NASA would use to get back to the Moon, or would it closely resemble the Saturn V design?&lt;/p&gt;&#xA;" OwnerUserId="17" LastEditorUserId="17" LastEditDate="2014-12-10T17:23:18.860" LastActivityDate="2015-08-09T16:47:06.263" Title="What will be NASA's successor to the Saturn V rocket?" Tags="|rockets|nasa|saturn-v|crewed-spaceflight|hllv|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="40" PostTypeId="1" AcceptedAnswerId="419" CreationDate="2013-07-16T21:36:33.007" Score="29" ViewCount="14349" Body="&lt;p&gt;If I were to launch a CubeSat, how long would it last before falling out of orbit and burning up in the atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorUserId="14" LastEditDate="2013-07-19T10:22:28.043" LastActivityDate="2018-11-26T07:19:31.653" Title="How long does a CubeSat last?" Tags="|cubesat|orbital-lifetime|orbital-mechanics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="41" PostTypeId="2" ParentId="19" CreationDate="2013-07-16T21:39:07.050" Score="2" Body="&lt;p&gt;Let me begin by touting the Anime &lt;a href=&quot;http://en.wikipedia.org/wiki/Planetes&quot; rel=&quot;nofollow&quot;&gt;Planetes&lt;/a&gt; which has as it's main plot device; space junk. &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_debris&quot; rel=&quot;nofollow&quot;&gt;Space Debris&lt;/a&gt; is a serious problem. It is so big of a deal that when you are planning to launch anything, you have to plan in a method of disposing of it at the end of the useful life. There are even orbits for dead stuff so that they don't interfere with other equipment. The means that is portrayed in the animation is to have people in space searching for an retrieving it. Apart from that, all current strategies are around avoiding the debris in the first place.&lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastActivityDate="2013-07-16T21:39:07.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="42" PostTypeId="1" AcceptedAnswerId="102" CreationDate="2013-07-16T21:39:10.063" Score="14" ViewCount="5826" Body="&lt;p&gt;With robots that can run for years on nuclear batteries, and in some cases even perform basic repairs to themselves, they seem like pretty capable instruments for space exploration. Aside from the obvious study of how the human body copes in extra-terrestrial situations, are there any advantages to sending manned space missions, as opposed to solely robotic ones?&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorDisplayName="user12" LastEditDate="2013-07-18T20:16:48.520" LastActivityDate="2013-07-19T05:35:30.300" Title="What advantages do manned missions have over robot missions?" Tags="|crewed-spaceflight|robotic-missions|" AnswerCount="3" CommentCount="1" ClosedDate="2013-07-22T00:51:03.467" ContentLicense="CC BY-SA 3.0" />
  <row Id="43" PostTypeId="1" CreationDate="2013-07-16T21:39:58.307" Score="9" ViewCount="377" Body="&lt;p&gt;NASA, ESA, Roscosmos, and others, all have somewhat different cultures and attitudes when it comes to building and launching spacecraft.  I don't have a source, but one anecdote I heard from someone with a long time experience in the field, is that whereas the Europeans and Americans carefully and literally documented every screw turned, Russian engineers went by experience and didn't document and test everything nearly as meticulous as the Europeand and Americans did — and that their failure rates were around the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is about the failure rate.  Is there any significant difference in the launch failure rate between the Americans, Europeans, and Russians, respectively?&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2013-07-17T01:58:32.370" Title="Are there any significant differences in launch failure rates between different agencies/countries?" Tags="|launch|failure|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="44" PostTypeId="2" ParentId="36" CreationDate="2013-07-16T21:41:16.400" Score="10" Body="&lt;p&gt;As Wikipedia mentions, &lt;a href=&quot;http://en.wikipedia.org/wiki/Absolute_magnitude&quot; rel=&quot;noreferrer&quot;&gt;Absolute Magnitude&lt;/a&gt; is a standard measurement of how bright an object would be, if measured from $1\ ㍳$ (Earth Mean distance from sun), away. Furthermore, it uses the same logarithmic scale to measure relative brightness, such that a $1.0$ magnitude change relates to a $10^{0.4}=2.512$ scale. Put all of the pieces together, and you should be able to estimate how big an object is, with some assumed albedo.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2013-10-30T08:36:27.303" LastActivityDate="2013-10-30T08:36:27.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="45" PostTypeId="2" ParentId="40" CreationDate="2013-07-16T21:41:37.890" Score="13" Body="&lt;p&gt;That depends very much on the launch you're piggybacking with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you launch a CubeSat, you're entirely at the whim of the launch you're piggybacking on.  CubeSats usually don't have propulsion so if it's a low orbit, it may take only weeks before you burn up in the atmosphere.  At a slightly higher orbit it may be a couple of months.  As most CubeSats are for technology demonstration or as student projects, this is entirely sufficient.  But there is no generic answer that applies to all CubeSats.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2013-07-16T21:41:37.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="46" PostTypeId="1" AcceptedAnswerId="53" CreationDate="2013-07-16T21:43:02.753" Score="14" ViewCount="880" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon_Treaty&quot; rel=&quot;noreferrer&quot;&gt;Moon Treaty&lt;/a&gt; states that no extraterrestrial property can be owned by any organization or person, with the exception of international and governmental organization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, taking the current state of space law, what are the legal prerequisites for starting the exploitation of natural resources on some asteroid by private company? The private company can't legally possess the asteroid, but it could be claimed by the government, which could grant the license for extracting resources from it to private company. Is it possible according to actual space law and its interpretation? &lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastEditorUserId="7062" LastEditDate="2023-07-06T20:01:47.207" LastActivityDate="2023-07-06T20:01:47.207" Title="What are the actual legal prerequisites for extracting natural resources from asteroids by private entities?" Tags="|asteroid|law|mining|natural-resources|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="47" PostTypeId="2" ParentId="13" CreationDate="2013-07-16T21:45:47.483" Score="15" Body="&lt;p&gt;Orbit propagation (calculating the trajectory of an object under the influence of, at a minimum, gravity of a central body) is at the heart of astrodynamics, and high precision propagation means numerical solutions of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ordinary_differential_equation&quot;&gt;&lt;strong&gt;ordinary differential equations&lt;/strong&gt;&lt;/a&gt;. This, in turn, requires relatively deep understanding of &lt;a href=&quot;https://en.wikipedia.org/wiki/Numerical_analysis&quot;&gt;&lt;strong&gt;numerical analysis&lt;/strong&gt;&lt;/a&gt; so you can implement your algorithms on a computer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Probability theory&lt;/strong&gt; can also come in handy, particularly when coupled with the &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbit_determination&quot;&gt;&lt;strong&gt;estimation/determination&lt;/strong&gt;&lt;/a&gt; problem. We have the ability to take very precise measurements of a satellite on-orbit, and these may be used to update the state of the satellite (via &lt;a href=&quot;http://en.wikipedia.org/wiki/Kalman_filter&quot;&gt;&lt;strong&gt;filtering&lt;/strong&gt;&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Least_squares&quot;&gt;&lt;strong&gt;least squares&lt;/strong&gt;&lt;/a&gt;, among other techniques), but inherent uncertainty in the measurements and the dynamics models will lead to uncertainty in the state estimate. Understanding how to characterize and propagate this uncertainty is currently a very hot topic of research. Additionally, most of the processes involved in orbit propagation and determination are nonlinear, which adds complexity to the estimation process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, good ol' &lt;strong&gt;multivariable calculus&lt;/strong&gt; always comes in handy, both for propagation (calculating &lt;a href=&quot;http://en.wikipedia.org/wiki/Jacobian_matrix_and_determinant&quot;&gt;Jacobians&lt;/a&gt;) as well as for understanding the underlying effects of different perturbations, such as the &lt;a href=&quot;http://adsabs.harvard.edu/full/1984SvA....28..594E&quot;&gt;asphericity of Earth&lt;/a&gt;. In fact, &lt;a href=&quot;http://en.wikipedia.org/wiki/Perturbation_%28astronomy%29#General_perturbations&quot;&gt;&lt;strong&gt;General Perturbations&lt;/strong&gt;&lt;/a&gt; (GP) theory, while not used much for high precision propagation any more, still proves useful in the orbit design process, and involves analytically averaging equations of motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I wouldn't classify it as &quot;advanced&quot; per se, a deep understanding of &lt;strong&gt;geometry&lt;/strong&gt; is an absolute must. It's manifested largely in &lt;a href=&quot;http://en.wikipedia.org/wiki/Direction_cosine&quot;&gt;&lt;strong&gt;coordinate system transformations&lt;/strong&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low thrust trajectory design, another hot topic, often boils down to an &lt;a href=&quot;http://en.wikipedia.org/wiki/Mathematical_optimization&quot;&gt;&lt;strong&gt;optimization&lt;/strong&gt;&lt;/a&gt; problem, which is a whole field unto itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know someone will get mad because I didn't include their sub-specialty here, but I think I captured the topics that come up most often. It's a very interesting, diverse field.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-07-16T21:45:47.483" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="48" PostTypeId="1" AcceptedAnswerId="81" CreationDate="2013-07-16T21:46:01.203" Score="15" ViewCount="2729" Body="&lt;p&gt;It seems that some people maintain the existence of a &amp;quot;planet X&amp;quot; is all but science fiction, while others claim that its existence is a possibility, even if that's relatively a remote one. Others take this further and make the argument that &lt;a href=&quot;http://en.wikipedia.org/wiki/Tyche_%28hypothetical_planet%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;its existence is likely&lt;/a&gt;. Is there a compelling case that such a planet could exist, and if so how come it hasn't yet been discovered - is it a simple case of it being too far away, or is there more to it?&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorUserId="37744" LastEditDate="2020-09-17T05:13:04.343" LastActivityDate="2020-09-17T05:13:04.343" Title="Is it possible that a giant 10th planet exists in our solar system?" Tags="|solar-system|planet|" AnswerCount="3" CommentCount="4" ClosedDate="2013-07-17T18:35:12.500" ContentLicense="CC BY-SA 4.0" />
  <row Id="49" PostTypeId="2" ParentId="3" CreationDate="2013-07-16T21:46:33.317" Score="19" Body="&lt;p&gt;From what I understand, the probes are at the end of their lives; their nuclear fuel is almost out and power is limited to run just a few systems. As for the science, they are just reporting on the conditions of deep space. The Voyagers lasted longer than even it's designers thought it would. &lt;a href=&quot;http://voyager.jpl.nasa.gov/news/factsheet.html&quot;&gt;http://voyager.jpl.nasa.gov/news/factsheet.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The probes instruments are:  infrared, ultraviolet, magnetometers, plasma detectors, cosmic-ray and charged-particle sensors. They report on the light, magnetism, and radiation of deep space.&lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastEditorDisplayName="user39" LastEditDate="2013-07-21T14:21:50.810" LastActivityDate="2013-07-21T14:21:50.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="50" PostTypeId="1" AcceptedAnswerId="98" CreationDate="2013-07-16T21:47:07.957" Score="25" ViewCount="4645" Body="&lt;p&gt;Are there any physical, mental, or health-related medical conditions that could permanently exclude someone from going into space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the criteria differ from one country's (government sponsored or private) program to another?&lt;/p&gt;&#xA;" OwnerUserId="44" LastEditorUserId="25" LastEditDate="2013-07-18T19:44:11.947" LastActivityDate="2019-04-18T19:15:52.997" Title="What medical conditions can impede someone from going into space?" Tags="|astronauts|medical|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="51" PostTypeId="1" CreationDate="2013-07-16T21:47:14.490" Score="13" ViewCount="743" Body="&lt;p&gt;Many have said that rocket launches are an outdated and inefficient technology and have predicted that other technologies will take over, yet to date, it's still the only thing we have to launch into orbit.  In the past, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_HARP&quot;&gt;HARP&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Super_High_Altitude_Research_Project&quot;&gt;SHARP&lt;/a&gt; have been projects researching to basically &lt;em&gt;shoot&lt;/em&gt; projectiles into orbit — they might still need some propulsion for orbital adjustments, but energy usage should be much less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is any research still going on in this direction, specifically for launching payloads into orbit?  If not, why not?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="1142" LastEditDate="2014-03-06T14:18:13.217" LastActivityDate="2014-03-06T14:18:13.217" Title="Is there any research going on launching into orbit with &quot;space guns&quot;? Why (not)?" Tags="|launch|technology|space-gun|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="52" PostTypeId="2" ParentId="46" CreationDate="2013-07-16T21:47:35.350" Score="8" Body="&lt;p&gt;I think the current philosophy is somewhat different. It was said that asteroids are so small that they barely match the definition of territory. They are more like an object. Furthermore, if you equip them with any propulsion system and change their trajectory or orbit, respectively, they literally become space-crafts. Based in this train of thought, the argument is, who claims it first, takes it as private property. For reference: &quot;Mining the Sky, Untold Riches from the Asteroids, Comets, and Planets&quot;, John S. Lewis. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-16T21:47:35.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="53" PostTypeId="2" ParentId="46" CreationDate="2013-07-16T21:51:42.197" Score="9" Body="&lt;p&gt;According to current space law, the 1967 Outer Space Treaty: (in Article 6)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;“The activities of non-governmental entities in outer space, including&#xA;the Moon and other celestial bodies, shall require authorization and&#xA;continuing supervision by the appropriate State Party to the Treaty. ”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Which means that any company that wants to operate in space must be approved by its country and supervised by its country, i.e. Planetary Resources must be supervised by the US government (article &lt;a href=&quot;https://talkingpointsmemo.com/idealab/planetary-resources-space-mining-plans-blaze-uncharted-legal-territory&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="30" LastEditorUserId="10273" LastEditDate="2022-06-24T17:01:41.040" LastActivityDate="2022-06-24T17:01:41.040" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="54" PostTypeId="2" ParentId="17" CreationDate="2013-07-16T21:52:48.937" Score="6" Body="&lt;p&gt;Depending on the usage and also what you mean by the &quot;outer solar system&quot;, solar panels are getting to the point where they can be used.  For example, &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Juno&lt;/a&gt;, currently &lt;s&gt;en route to&lt;/s&gt; in orbit around Jupiter, uses solar panels.  As solar panels become more efficient, they may be more useful for the planets which are more remote.  On the other hand, and as you noted, the Mars Science Lab's &lt;a href=&quot;http://en.wikipedia.org/wiki/Curiosity_%28rover%29&quot; rel=&quot;nofollow&quot;&gt;Curiosity rover&lt;/a&gt; uses RTG's.&lt;/p&gt;&#xA;" OwnerUserId="36" LastEditorUserId="36" LastEditDate="2016-07-06T18:41:38.070" LastActivityDate="2016-07-06T18:41:38.070" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="55" PostTypeId="1" AcceptedAnswerId="226" CreationDate="2013-07-16T21:53:14.450" Score="25" ViewCount="2185" Body="&lt;p&gt;Consider the situation, where the private company (let's say, registered on some Caribbean country, but owned by the citizen of United States) does the unauthorized launch of satellite or other spacecraft from territory, for example, Kazakhstan. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite falls on the territory of, say, some EU country (Germany for example) causing significant damage to the property of private company registered there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What could be the most severe consequences, according to current state of space law? According to which law could the owner of the launching company be processed? Are there possible repercussions that the suffering country could issue against other country (those from where the spacecraft was launched, those where the company is registered or those where the owner of the launching company has citizenship)?&lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastEditorUserId="25" LastEditDate="2013-07-18T16:50:54.153" LastActivityDate="2013-08-29T21:54:57.737" Title="What are the possible legal consequences of an unauthorized launch of a satellite?" Tags="|launch|law|liability|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="56" PostTypeId="5" CreationDate="2013-07-16T21:54:07.127" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-16T21:54:07.127" LastActivityDate="2013-07-16T21:54:07.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="57" PostTypeId="4" CreationDate="2013-07-16T21:54:07.127" Score="0" Body="Questions about international law (treaties, implementations, interpretation, jurisdiction etc.) as it applies to space exploration." OwnerDisplayName="user43" LastEditorDisplayName="user12" LastEditDate="2013-07-21T19:23:21.537" LastActivityDate="2013-07-21T19:23:21.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="58" PostTypeId="5" CreationDate="2013-07-16T21:54:51.817" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-16T21:54:51.817" LastActivityDate="2013-07-16T21:54:51.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="59" PostTypeId="4" CreationDate="2013-07-16T21:54:51.817" Score="0" Body="Questions about the liability of the private entities or countries for damages done to other parties during space exploration activities." OwnerDisplayName="user43" LastEditorDisplayName="user43" LastEditDate="2013-07-16T21:57:17.190" LastActivityDate="2013-07-16T21:57:17.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="60" PostTypeId="1" CreationDate="2013-07-16T21:56:05.787" Score="13" ViewCount="2080" Body="&lt;p&gt;In the United States access to space is restricted by the FCC / FAA, which licenses private, commercial, and government entities for potential launches into space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any international oversight on space launches? For instance, if a company were to build  a launch platform out in the middle of the Pacific Ocean, would there be any organization with authority to monitor or restrict that activity? &lt;/p&gt;&#xA;" OwnerUserId="4" LastEditorUserId="58" LastEditDate="2020-12-01T16:44:40.987" LastActivityDate="2020-12-01T16:44:40.987" Title="Are there international regulations for launching objects into space?" Tags="|launch|law|international-organizations|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="61" PostTypeId="2" ParentId="51" CreationDate="2013-07-16T21:56:05.947" Score="1" Body="&lt;p&gt;Little to none, as far as I know, but there still are a few people bringing this topic up every few years. E.g. &lt;a href=&quot;http://www.iafastro.net/iac/archive/browse/IAC-08/D4/1/423/&quot; rel=&quot;nofollow&quot;&gt;http://www.iafastro.net/iac/archive/browse/IAC-08/D4/1/423/&lt;/a&gt; Find the author on twitter: &lt;a href=&quot;https://twitter.com/justinpark01&quot; rel=&quot;nofollow&quot;&gt;https://twitter.com/justinpark01&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-16T21:56:05.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="63" PostTypeId="2" ParentId="48" CreationDate="2013-07-16T22:00:22.110" Score="9" Body="&lt;p&gt;From basic physics if there is one it cannot be massive, or must be incredibly far away, as we would see the perturbations in the orbits of the other planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, if it was massive but very low density, for some reason, so we couldn't see it's effect on the orbits of the other planets, we would expect to see something with a regular orbit from astronomical observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what we know of planetary distribution, massive planets are expected, well, about where Jupiter and Saturn sit, with smaller ones closer to the sun, and also smaller ones further out, with Uranus and Neptune the last of the gas giants.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2013-07-16T22:07:21.940" LastActivityDate="2013-07-16T22:07:21.940" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="64" PostTypeId="2" ParentId="43" CreationDate="2013-07-16T22:00:54.957" Score="2" Body="&lt;p&gt;Need to take a side approach to answer this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Futron did a study for SpaceX on launch failure root causes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read it &lt;a href=&quot;http://www.spacex.com/FutronDesignReliability.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They focussed on root causes, vs country of origin.  I.e. Stage separation issues vs engine failures, etc.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-07-16T22:00:54.957" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="65" PostTypeId="2" ParentId="36" CreationDate="2013-07-16T22:01:04.920" Score="15" Body="&lt;p&gt;The prior answer cleared up the issue I was having.  It is true that with only the absolute magnitude (H) and albedo (p) you can estimate the size.  &lt;a href=&quot;http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html&quot; rel=&quot;noreferrer&quot;&gt;The formula&lt;/a&gt; is the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$D(km) = \frac{1329}{\sqrt{p}}10^{-0.2H}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For objects in my search range, here are some examples:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(object name)   (JPL diameter estimate) (formula result)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Aten  1.1 km     1.14 km&lt;/li&gt;&#xA;&lt;li&gt;Khufu    0.7 km   0.634&lt;/li&gt;&#xA;&lt;li&gt;Amun  2.48 km    2.48 km&lt;/li&gt;&#xA;&lt;li&gt;Cruithne  none   2.84 km&lt;/li&gt;&#xA;&lt;li&gt;Xanthus  1.3 km    1.3 km&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As you can see, this is probably the exact method JPL uses for the diameter calculation.  Also, most objects that do not have a listed diameter also have no listed albedo, which explains why they didn't provide an estimate.  For my calculations I assumed an albedo of 0.2 where I did not have one.  This is likely the reason that Wikipedia gave 5 km for Cruithne while I found about half that - it could be a fairly dark object.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorDisplayName="user12" LastEditDate="2013-10-30T03:48:13.533" LastActivityDate="2013-10-30T03:48:13.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="66" PostTypeId="2" ParentId="48" CreationDate="2013-07-16T22:01:18.153" Score="11" Body="&lt;p&gt;No. A planet of significant mass would cause detectable perturbations in the orbits of the other planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is how Neptune's existence was postulated. Deviations from predictions in Uranus' orbit suggested where an 8th planet  must be located. A focused search in the projected area led to its discovery. &lt;/p&gt;&#xA;" OwnerUserId="48" LastActivityDate="2013-07-16T22:01:18.153" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="67" PostTypeId="1" AcceptedAnswerId="77" CreationDate="2013-07-16T22:01:28.320" Score="47" ViewCount="5770" Body="&lt;p&gt;According to NASA, roughly &quot;1,000 pounds of methane are produced on International Space Station and dumped overboard as waste gas every year&quot;&lt;sup&gt;&lt;a href=&quot;http://morpheuslander.jsc.nasa.gov/about/&quot;&gt;[1]&lt;/a&gt;&lt;/sup&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like an awful waste to just get rid of it. The Morpheus Project page talks about possibly using the methane as fuel, it's curious that they don't do that already. &lt;strong&gt;What is all that methane gas a byproduct of, and why can't it be used for something more productive?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="17" LastEditorUserId="25" LastEditDate="2013-07-16T22:12:52.390" LastActivityDate="2017-10-23T21:37:13.667" Title="Why does the International Space Station produce so much methane?" Tags="|iss|space-station|gasses|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="68" PostTypeId="2" ParentId="51" CreationDate="2013-07-16T22:04:12.127" Score="3" Body="&lt;p&gt;One primary problem with launch guns is that it is not a very friendly environment to soft and squishy things.  Usually very high G forces during launch.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, you need something to circularize the orbit once you get high enough still.  Orbit is not just about speed and height but the right speed in the right direction.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-07-16T22:04:12.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="69" PostTypeId="5" CreationDate="2013-07-16T22:04:41.863" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-16T22:04:41.863" LastActivityDate="2013-07-16T22:04:41.863" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="70" PostTypeId="4" CreationDate="2013-07-16T22:04:41.863" Score="0" Body="Questions relating to the use of technology to further space exploration." OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-17T00:23:54.437" LastActivityDate="2013-07-17T00:23:54.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="71" PostTypeId="1" AcceptedAnswerId="97" CreationDate="2013-07-16T22:07:02.800" Score="13" ViewCount="431" Body="&lt;p&gt;What would be the jurisdiction for contracts signed in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say, an american citizen runs company registered on Cyprus, and operates a space station  launched from the territory of Kazakhstan. He signs a contract with other party, specifying that for any disputes the Nigerian law applies. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the second party feels to be harmed, and the Nigerian law is in that particular case on the side of the unhonest partner (this was the reason, the Nigerian law was chosen). In which court could the second party sue the space station operator? Could it choose the court based on citizenship, company registration, or the country from which the spacecraft was launched?&lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastEditorDisplayName="user12" LastEditDate="2013-07-20T16:34:54.077" LastActivityDate="2013-07-20T20:26:51.493" Title="What is the jurisdiction for contracts signed in space?" Tags="|law|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="72" PostTypeId="1" AcceptedAnswerId="82" CreationDate="2013-07-16T22:07:56.603" Score="19" ViewCount="870" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot;&gt;Interplanetary Transport Network&lt;/a&gt; is a network of mathematically-defined paths through our solar system that would require little to no energy for a spacecraft to follow them, albeit at very low speeds. While these pathways would not be practical for manned spacecraft due to the low speeds they require, they have been proposed for use as transport pathways and trajectories for scientific missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that NASA has indeed used these pathways for a few of their missions, including the Genesis mission and the Solar and Heliospheric Observatory. The Chinese spacecraft Chang'e 2 also used this network on its voyage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While there appear to be a relatively vast number of these pathways scattered throughout our solar system, not all of them are currently known. How many of these pathways have we documented so far? Are there any official (or unofficial) estimates out there for what proportion of these pathways have been discovered to date? &lt;/p&gt;&#xA;" OwnerUserId="47" LastEditorUserId="49" LastEditDate="2014-12-06T17:50:47.243" LastActivityDate="2014-12-06T17:50:47.243" Title="How much of the Interplanetary Transport Network is currently known?" Tags="|solar-system|transport|low-energy-transfer|uncrewed-spaceflight|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="73" PostTypeId="5" CreationDate="2013-07-16T22:08:16.523" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-16T22:08:16.523" LastActivityDate="2013-07-16T22:08:16.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="74" PostTypeId="4" CreationDate="2013-07-16T22:08:16.523" Score="0" Body="Questions about determining which legal system and which court would be in charge of settlement of disputes occurring in connection with space exploration." OwnerDisplayName="user43" LastEditorDisplayName="user43" LastEditDate="2013-07-17T00:26:59.463" LastActivityDate="2013-07-17T00:26:59.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="75" PostTypeId="2" ParentId="24" CreationDate="2013-07-16T22:09:11.493" Score="4" Body="&lt;p&gt;Plutonium is astonishingly poisonous.  But conversely, the amount of Pu used by a mission is fairly small. And it naturally decays.&lt;/p&gt;&#xA;&lt;p&gt;Comparing double digit kilograms of horribly poisonous stuff (say 10 kg), vs hundreds of thousands (if not millions) of kilograms of propellant (even tetrazine, MMDH) makes it seem true.&lt;/p&gt;&#xA;&lt;p&gt;Additionally, knowing how bad the Pu is, it is surprisingly armored. People forget that Apollo 13 dropped the LM's RTG into the deepest part of the Pacific, and the belief is that the containment vessel survived one of the fastest reentries to Earth's atmosphere. (I like how Top Gear America's first driver on their track was &lt;em&gt;'the fastest man they could find'&lt;/em&gt; Buzz Aldrin, but of course the Apollo 13 had him beat by a bit).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="47149" LastEditDate="2022-05-22T04:54:54.087" LastActivityDate="2022-05-22T04:54:54.087" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="76" PostTypeId="2" ParentId="21" CreationDate="2013-07-16T22:10:29.567" Score="11" Body="&lt;p&gt;We definitely could not build a Dyson sphere with current technology. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first problem is a lack of material. To build it at any reasonable distance from the sun, the volume of even an extremely thin shell is greater than all the planets, plus the asteroids, and the Oort cloud. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After that problem, one has to move all the mass to the shell. That is a huge amount of energy.&lt;/p&gt;&#xA;" OwnerUserId="48" LastEditorUserId="38" LastEditDate="2013-07-17T00:26:02.520" LastActivityDate="2013-07-17T00:26:02.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="77" PostTypeId="2" ParentId="67" CreationDate="2013-07-16T22:10:49.207" Score="52" Body="&lt;p&gt;The methane is produced as a by-product of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Sabatier_reaction&quot; rel=&quot;noreferrer&quot;&gt;Sabatier Reaction&lt;/a&gt;, where Carbon Dioxide is removed from the air using a catalyst and hydrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other waste product is water, which can then be split into Hydrogen and Oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So basically, the amount of methane is directly related to the amount of Carbon Dioxide the astronauts breathe out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @KeithS commented:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the second part of the question, &quot;why can't it be used for something more useful,&quot; the answer there is that almost anything you'd do with the methane involves combustion, requiring O2 (more than you'd get back from the Sabatier reaction in the first place). Energy is relatively plentiful on the ISS; it's got two tennis-court-sized solar arrays providing virtually all of the ISS's power needs. What's scarce is oxygen, so they retain as much of it as they can&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2017-10-23T21:37:13.667" LastActivityDate="2017-10-23T21:37:13.667" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="78" PostTypeId="1" AcceptedAnswerId="85" CreationDate="2013-07-16T22:11:57.320" Score="18" ViewCount="1190" Body="&lt;p&gt;My friends and I were trying to figure this out a while ago.&lt;br&gt;&#xA;I was thinking that by transporting smaller pieces into space via a lighter re-entry spacecraft, there would be overall less fuel used because there would be less &quot;feedback&quot; (ie more fuel needs more fuel to lift that fuel, etc).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My friends were thinking it would be the same because the same overall mass of the craft would require the same amount of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be more efficient to transport a spacecraft into space in pieces, or only once it is assembled?&lt;/p&gt;&#xA;" OwnerUserId="30" LastEditorUserId="14" LastEditDate="2013-07-19T10:34:39.667" LastActivityDate="2013-07-19T10:34:39.667" Title="Is it more efficient to assemble spacecraft in orbit, or assemble on the ground and launch?" Tags="|launch|propulsion|spacecraft-development|orbital-assembly|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="79" PostTypeId="2" ParentId="39" CreationDate="2013-07-16T22:13:34.310" Score="12" Body="&lt;p&gt;NASA believes you need a large 70mT or 110mT rocket (SLS v1 and v2) to do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.directlauncher.org/&quot;&gt;Direct 3.0&lt;/a&gt; believes you can build such a rocket cheaper than SLS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pretty much anyone NOT involved in building shuttle components (and thus future SLS components) seems to think that fuel depots are the way to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You launch your translunar vehicle in orbit out of pieces.  You build a fuel depot (It better be reusable more than one mission or you are wasting your time) and then you let any commercial launch vendor compete (and generate a market) for lots of flights as cheap as possible.  And if any of the fuel/oxidizer launches fail, oh well. Launch again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The days of the super large, single launch is probably long past.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="63" LastEditDate="2013-07-22T19:06:37.637" LastActivityDate="2013-07-22T19:06:37.637" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="80" PostTypeId="2" ParentId="21" CreationDate="2013-07-16T22:16:15.310" Score="21" Body="&lt;p&gt;For a solid sphere - Absolutely not!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This calculation from &lt;a href=&quot;http://www.aleph.se/Nada/dysonFAQ.html&quot;&gt;http://www.aleph.se/Nada/dysonFAQ.html&lt;/a&gt; explains why:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;8) &lt;strong&gt;How strong does a rigid Dyson shell need to be?&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Very strong. According to Frank Palmer:&#xA;  Any sphere about a gravitating body can be analysed into two hemispheres joined at a seam. The contribution of a small section To the force on the seam is&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$\mathbf{g}\text{(ravity)}\cdot\mathbf{d}\text{(ensity)}\cdot\mathbf{t}\text{(hickness)}\cdot\mathbf{A}\text{(rea)}\cdot\mathbf{cos}(\text{angle})$.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The integral of $\mathbf{A}\cdot\mathbf{cos}(\text{angle})$ is $\mathbf{\pi}\cdot\mathbf{R^2}$.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So the total force is $\mathbf{g}\cdot\mathbf{d}\cdot\mathbf{t}\cdot\mathbf{\pi}\cdot\mathbf{R^2}$. Which is independent of distance, neatly enough.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The area resisting the force is $2\cdot\mathbf{\pi}\cdot\mathbf{R}\cdot\mathbf{t}$.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Thus, the pressure is $\mathbf{g}\cdot\mathbf{d}\cdot\mathbf{R}/2$; this can be translated into a cylindrical tower of a given height on Earth. If that tower built of that material can stand, then the compression strain is not too great.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;At 1 AU, that comes to $2\cdot(\mathbf{\pi}\cdot\text{AU}/\text{YR})^2$, or -- by my calculations -- in the neighborhood of &lt;strong&gt;80 to 90 THOUSAND kilometers high&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For the thin version held up by solar wind and photon pressure - nope. The numbers are better but still out of our reach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the swarm of independent solar panels - yes. We would just need to build a hell of a lot of them - might need to take apart a small planet for raw materials though...&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="49" LastEditDate="2015-11-01T00:48:42.683" LastActivityDate="2015-11-01T00:48:42.683" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="81" PostTypeId="2" ParentId="48" CreationDate="2013-07-16T22:16:23.013" Score="20" Body="&lt;p&gt;This topic is actually pretty nicely covered in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Planets_beyond_Neptune&quot;&gt;Planets beyond Neptune&lt;/a&gt; Wikipedia page, so I recommend reading it, if for nothing else then for a convenient collection of references. But to quote the most relevant part on &lt;a href=&quot;https://en.wikipedia.org/wiki/Robert_Sutton_Harrington&quot;&gt;Harrington&lt;/a&gt;'s Planet X (beyond Pluto) of that Wiki page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Planet X disproved&lt;/strong&gt;&lt;br/&gt; Harrington died in January 1993, without having found Planet X. Six months before, E. Myles Standish had&#xA;  used data from &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2&quot;&gt;Voyager 2&lt;/a&gt;'s 1989 flyby of Neptune, which had revised&#xA;  the planet's total mass downward by 0.5% — an amount comparable to the&#xA;  mass of Mars — to recalculate its gravitational effect on&#xA;  Uranus. When Neptune's newly determined mass was used in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Developmental_Ephemeris&quot;&gt;Jet&#xA;  Propulsion Laboratory Developmental Ephemeris&lt;/a&gt; (JPL DE), the supposed&#xA;  discrepancies in the Uranian orbit, and with them the need for a&#xA;  Planet X, vanished. There are no discrepancies in the trajectories&#xA;  of any space probes such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_10&quot;&gt;Pioneer 10&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_11&quot;&gt;Pioneer 11&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1&quot;&gt;Voyager 1&lt;/a&gt; and&#xA;  &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2&quot;&gt;Voyager 2&lt;/a&gt; that can be attributed to the gravitational pull of a large&#xA;  undiscovered object in the outer Solar System. Today, most&#xA;  astronomers agree that Planet X, as &lt;a href=&quot;https://en.wikipedia.org/wiki/Percival_Lowell&quot;&gt;Lowell&lt;/a&gt; defined it, does not exist.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-07-16T22:16:23.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="82" PostTypeId="2" ParentId="72" CreationDate="2013-07-16T22:16:30.053" Score="17" Body="&lt;p&gt;The question is somewhat odd. The &quot;Interplanetary Transport Network&quot; may be a misleading term. When probes are sent into deep space, most of them make use of flybys or gravity assist manoeuvres. Virtually every celestial body can therefore be used for increasing the speed of a probe or decreasing it. The &quot;network&quot; refers to series of such manoeuvres. In the end, it is a pure mathematical question of how many interesting sequences one can find between planets, moons and the sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might have a look at this: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nasawatch.com/archives/2010/10/crowdsourcing-e.html&quot; rel=&quot;noreferrer&quot;&gt;http://nasawatch.com/archives/2010/10/crowdsourcing-e.html&lt;/a&gt;  &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sophia.estec.esa.int/thespacegame/&quot; rel=&quot;noreferrer&quot;&gt;http://sophia.estec.esa.int/thespacegame/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/g3iUn.jpg&quot; alt=&quot;The ITN&quot;&gt;&lt;br&gt;&lt;em&gt;&quot;This stylized depiction of the ITN is designed to show its (often convoluted) path through the Solar System. The green ribbon represents one path from among the many that are mathematically possible along the surface of the darker green bounding tube. Locations where the ribbon changes direction abruptly represent trajectory changes at Lagrange points, while constricted areas represent locations where objects linger in temporary orbit around a point before continuing on.&quot; &lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot; rel=&quot;noreferrer&quot;&gt;Source: wiki/ITN&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2014-03-14T17:12:28.373" LastActivityDate="2014-03-14T17:12:28.373" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="83" PostTypeId="2" ParentId="60" CreationDate="2013-07-16T22:19:49.767" Score="14" Body="&lt;p&gt;First of all, let's get a few things straight.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The FCC has a partial agreement for space launches, specifically, they are allowed to regulate the use of frequencies, etc.&lt;/li&gt;&#xA;&lt;li&gt;The FAA is allowed to regulate the spacecraft, launch site, and the timing of the launch, and similar such things. But their power pretty much stops at the atmosphere.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In essence, as virtually every satellite has to talk to someone at some point in time. As a result, the FCC is allowed to regulate satellites and other similar machines pretty well. They have done so, by doing things like ensuring that the spacecraft can de-orbit prior to allowing it an FCC license, for instance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as inter-government organizations, there are quite a few that regulate how things work. The main one is the International Telecommunication Union, which is the equivalent to the FCC from the United Nations. Specifically, they have a &lt;a href=&quot;http://www.itu.int/ITU-R/go/space&quot;&gt;Space Services Department&lt;/a&gt;, which &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Department handles capture, processing and publication of data and&#xA;  carries out examination of frequency assignment notices submitted by&#xA;  administrations for inclusion in the formal coordination procedures or&#xA;  recording in the Master International Frequency Register (MIFR).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-16T22:19:49.767" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="84" PostTypeId="1" AcceptedAnswerId="392" CreationDate="2013-07-16T22:20:41.133" Score="20" ViewCount="1553" Body="&lt;p&gt;Would it be possible to construct a working spacecraft using only materials extracted from the lunar surface? If not, what materials would we have to be ferried from Earth for this purpose?&lt;/p&gt;&#xA;" OwnerUserId="47" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:49:38.727" LastActivityDate="2018-08-25T12:36:36.570" Title="Could a spacecraft be made out of lunar materials?" Tags="|the-moon|materials|natural-resources|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="85" PostTypeId="2" ParentId="78" CreationDate="2013-07-16T22:21:07.987" Score="12" Body="&lt;p&gt;There is clearly a trade off that needs to be made.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your components are only 2 feet cubed, then it is not more efficient to build a ship for humans out of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your components are 10,000 cubic feet each, then it is not more efficient easier, or conversely, you only need one or two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It depends on many things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have cheap launch for a useful size chunk (say Falcon 9 gets reusable and dirt cheap) but a larger launch does not linearly scale (Say Falcon Heavy does not get as cheap) then it might still be cheaper to launch many times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Propellant is cheap compared to launch cost.  Elon Musk has claimed F9 propellant cost is on the order of 3% of total cost.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-07-16T22:21:07.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="86" PostTypeId="2" ParentId="42" CreationDate="2013-07-16T22:23:32.250" Score="8" Body="&lt;p&gt;As well as the political and outreach benefits, we also have:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Machines still do not have our problem solving capability, so if something outwith their programming happens, they may well fail to respond appropriately&lt;/li&gt;&#xA;&lt;li&gt;We can learn skills and techniques to help us prepare for migrating off this planet (yep, long term, but is likely to be needed at some point)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-16T22:23:32.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="87" PostTypeId="1" AcceptedAnswerId="94" CreationDate="2013-07-16T22:31:29.050" Score="18" ViewCount="1500" Body="&lt;p&gt;From Mercury silver suits to orange Space Shuttle ones, color of space suits have changed over years. Is there any reason for it?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="14" LastEditDate="2013-07-17T14:37:12.453" LastActivityDate="2014-01-21T01:03:10.433" Title="Is there any reason why American spacesuits color changed over years?" Tags="|space-shuttle|nasa|spacesuits|project-mercury|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="88" PostTypeId="1" AcceptedAnswerId="5779" CreationDate="2013-07-16T22:31:34.530" Score="38" ViewCount="9219" Body="&lt;p&gt;I would like to find a good estimate of the &lt;em&gt;sum total&lt;/em&gt; amount of payload that humans have put into space, that is, over all years, all space programs, and all types of payload that reached orbit.  Obviously a large fraction of these quickly spent more of their mass as propellant or de-orbited, so it has no physically meaning interpretation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total number of orbital launches each year can be &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:Orbital_launches_by_year.svg&quot;&gt;found on Wikipedia&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mz57U.jpg&quot; alt=&quot;orbital launches&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would follow that if you had the payload for each one of these launches, or a good estimate of average payload, you could find the total mass that has been sent into orbit over human history.  I found one source that gives the total number of orbital launches as &lt;a href=&quot;http://www.planet4589.org/space/lvdb/index.html&quot;&gt;5,210&lt;/a&gt;, which I think is as of 2010.  An &lt;a href=&quot;http://boards.straightdope.com/sdmb/showthread.php?t=643250&quot;&gt;obvious approach&lt;/a&gt; is to take the number of launches, and multiply by some generic payload mass to get an estimate.  So far this is the best I can find online.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is the problem of military launches, in that we categorically don't have information about those payloads.  These seem to be less than 1/3rd of the launches, so it shouldn't introduce any more error than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did find &lt;a href=&quot;http://books.google.com/books?id=2PD2H3X9LG0C&quot;&gt;one source&lt;/a&gt; that estimates that 255 tons were put into orbit in 2007.  That year is probably not representative.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="6537" LastEditDate="2018-01-11T20:19:01.193" LastActivityDate="2023-12-03T06:54:23.927" Title="What is the total mass sent into orbit over all history?" Tags="|spacecraft|low-earth-orbit|statistics|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="89" PostTypeId="1" CreationDate="2013-07-16T22:35:10.403" Score="7" ViewCount="302" Body="&lt;p&gt;One of the biggest challenges in long space flight is the psychological impact it has on the crew.  I've seen a lot of research into preventing the effects, but what about &quot;preventing&quot; the cause without making a huge ship?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any research into using virtual reality to try and give the astronaut the experience of being in a open environment?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was only able to turn up &lt;a href=&quot;http://ec.europa.eu/enterprise/policies/space/files/research/fp8_space_hearing/presentations/rittweger-joern_en.pdf&quot; rel=&quot;nofollow&quot;&gt;one mention&lt;/a&gt; of the idea, but not if it had been studied or if there were any results:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, possibly of interest, is &lt;a href=&quot;http://online.liebertpub.com/doi/abs/10.1089/cpb.2006.9926&quot; rel=&quot;nofollow&quot;&gt;a study that found&lt;/a&gt; that VR could help reduce the symptoms of patients with claustrophobia.&lt;/p&gt;&#xA;" OwnerUserId="57" LastEditorUserId="6537" LastEditDate="2018-01-12T09:49:29.953" LastActivityDate="2018-01-12T09:49:29.953" Title="What research has been done into using virtual reality to counteract the psychological effects of extended space travel?" Tags="|medical|psychology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="90" PostTypeId="2" ParentId="84" CreationDate="2013-07-16T22:35:17.720" Score="11" Body="&lt;p&gt;According to a couple of sources, such as &lt;a href=&quot;http://www.space.com/13247-moon-map-lunar-titanium.html&quot;&gt;space.com&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/File%3aComposition_of_lunar_soil.svg&quot;&gt;Wikipedia,&lt;/a&gt; the Moon has an abundance of silicon, iron, aluminum, and titanium.  The potential for other materials from millions of years of asteroids and comets suggests all the necessary building blocks for the hull and structure of the ship should be there. &lt;a href=&quot;http://en.wikipedia.org/wiki/Helium-3#Abundance&quot;&gt;Helium-3&lt;/a&gt; is also thought to be more abundant on the Moon, making it a potential fuel source. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What the Moon doesn't have is hydrocarbons for making plastics and rubber. You might be able to create airtight seals and electrical circuits without these things. Not being an electrical engineer I don't know. All of the electronics I've seen make heavy use of plastics and rubber for enclosing circuitry and shielding wires. &lt;/p&gt;&#xA;" OwnerUserId="4" LastEditorDisplayName="user12" LastEditDate="2013-07-17T02:55:21.840" LastActivityDate="2013-07-17T02:55:21.840" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="91" PostTypeId="1" AcceptedAnswerId="95" CreationDate="2013-07-16T22:37:24.417" Score="20" ViewCount="2162" Body="&lt;p&gt;During reentry phase into the Earth's atmosphere the heat produced by air friction does not allow any communication with the surface. Why does the heat interfere with electronic frequencies and signals?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="4" LastEditDate="2013-07-16T23:08:33.627" LastActivityDate="2013-07-16T23:08:33.627" Title="Why is it that during reentry phase a capsule cannot communicate with mission control?" Tags="|capsule|reentry|mission-control|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="92" PostTypeId="2" ParentId="78" CreationDate="2013-07-16T22:37:26.917" Score="8" Body="&lt;p&gt;There are a couple of things that have to be considered in this assessment.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What is the total launch cost?&lt;/li&gt;&#xA;&lt;li&gt;How does the cost of the components scale, with changes in the launch?&lt;/li&gt;&#xA;&lt;li&gt;How difficult is the assembly?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Big rockets typically are more efficient, especially if they are aerodynamically shaped. However, there is somewhat of an optimal ratio of diameter to length. The object you are going to get in to space will pretty much have dimensions of a can, scaled to an appropriate size, no matter the rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Multiple launches were required to build the ISS, simply because it couldn't fit in to any rocket available, and it probably wouldn't hold together without it. However, some things can be made cheaper if they are a single entity, like a big mirror for instance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is this:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Cost typically goes down per pound the larger the rocket.&lt;/li&gt;&#xA;&lt;li&gt;The volume and shape of the object vary considerably after you get to that stage.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-16T22:37:26.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="93" PostTypeId="1" CreationDate="2013-07-16T22:37:28.790" Score="15" ViewCount="332" Body="&lt;p&gt;Various sources, for example the &lt;a href=&quot;http://en.wikipedia.org/wiki/Spacelab#Legacy&quot;&gt;Spacelab wikipedia page&lt;/a&gt;, report that several modules used in American sector of the ISS are based on the Spacelab legacy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To what extent has its legacy been used? Was there any ready to use hardware left from the Spacelab project? Or were significant pieces of blueprint design reused? Or was it just the common interfaces, approaches and experience?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="49" LastEditDate="2013-08-05T23:15:33.093" LastActivityDate="2013-08-06T05:39:38.070" Title="To what extent are the US modules of ISS based on the Spacelab design?" Tags="|iss|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="94" PostTypeId="2" ParentId="87" CreationDate="2013-07-16T22:40:07.393" Score="18" Body="&lt;p&gt;You are referring to the materials protecting from insulation and the outer layers of space suits. Look carefully, they have not seen much change since the Apollo era. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Mercury, the suits were still at an rather early stage of development. They were intended as pressure-suits protecting the astronauts from decompression in case there was a leak in the capsule. They were not intended for EVA. In a similar way, the orange suits used in space shuttles at the end of the program were just pressure-suits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suits for EVAs, on the contrary, have been white to grey for more than 40 years now, both in the US and in Russia. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: &lt;a href=&quot;http://en.wikipedia.org/wiki/Advanced_Crew_Escape_Suit&quot;&gt;http://en.wikipedia.org/wiki/Advanced_Crew_Escape_Suit&lt;/a&gt; &quot;The orange color allows rescue units to easily spot the astronauts in the case of an Orbiter bailout over the ocean.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT2: This explains why EVA suits are usually white: &lt;a href=&quot;http://spaceflight.nasa.gov/station/eva/white.html&quot;&gt;http://spaceflight.nasa.gov/station/eva/white.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorDisplayName="user12" LastEditDate="2014-01-21T01:03:10.433" LastActivityDate="2014-01-21T01:03:10.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="95" PostTypeId="2" ParentId="91" CreationDate="2013-07-16T22:40:43.190" Score="22" Body="&lt;p&gt;Heat doesn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The intense heat does, however, strip off electrons from molecules in the upper atmosphere, leaving an electrically charged plasma. This plasma interferes with radio communications to such an extent there is a complete radio blackout until the capsule has decelerated to such an extent plasma is no longer formed - this can take 30 seconds or more.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-16T22:40:43.190" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="96" PostTypeId="1" AcceptedAnswerId="358" CreationDate="2013-07-16T22:42:04.003" Score="25" ViewCount="1475" Body="&lt;p&gt;Enceladus, the sixth-largest moon of Saturn, is probably unique in being the only body in our solar system besides Earth (and possibly Europa) to have significant amounts of liquid water. Most of this water is contained in vast oceans underneath a thick layer of ice on the surface, and is kept warm by the planet's geologic activity. As such, it has been proposed as a potential place that could harbor life. Irregardless of that debate (of whether there is life on Enceladus), are there any Earth-based lifeforms known that could hypothetically survive in Enceladus's oceans?&lt;/p&gt;&#xA;" OwnerUserId="47" LastEditorUserId="14" LastEditDate="2013-07-17T13:55:56.157" LastActivityDate="2022-09-26T23:16:18.047" Title="Could any life from Earth survive in Enceladus' oceans?" Tags="|life|water|forward-contamination|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="97" PostTypeId="2" ParentId="71" CreationDate="2013-07-16T22:46:41.577" Score="5" Body="&lt;p&gt;From the Wikipedia page on &lt;a href=&quot;http://en.wikipedia.org/wiki/Conflict_of_contract_laws&quot;&gt;contract law conflict&lt;/a&gt;, there is an indication of what the answer might be (however as with all things law, ymmv)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The section on Closest and most real connection states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In default, the court has to impute an intention by asking, as just and reasonable persons, which law the parties ought to, or would, have intended to nominate if they had thought about it when they were making the contract. In arriving at its decision, the court uses a list of connecting factors, i.e. facts which have an unambiguous geographical connection, and whichever law scores the most hits on a league table created from the list will be considered the proper law. The current list of factors includes the following:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;the flag of any ship involved&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;the place where the contract is made (which may not be obvious where negotiations were concluded by letter, fax or e-mail)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;the place(s) where performance is to occur;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Have a read of the page for the fuller list.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer might be that it will depend where the spacecraft is registered, but it could also depend on other factors.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-16T22:46:41.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="98" PostTypeId="2" ParentId="50" CreationDate="2013-07-16T22:49:20.510" Score="22" Body="&lt;p&gt;For NASA, candidates must be able to pass the long-duration space flight physical. Height, blood pressure, and vision are three of the physical requirements included in this test. From their &lt;a href=&quot;http://www.nasa.gov/audience/forstudents/postsecondary/features/F_Astronaut_Requirements.html&quot; rel=&quot;noreferrer&quot;&gt;website&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;h2&gt;Astronaut Requirements&lt;/h2&gt;&#xA;&lt;h3&gt;Pilots&lt;/h3&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Distant visual acuity: 20/100 or better uncorrected, correctable to 20/20 each eye.&lt;/li&gt;&#xA;&lt;li&gt;Blood pressure: 140/90 measured in a sitting position.&lt;/li&gt;&#xA;&lt;li&gt;Height between 62 and 75 inches.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;h3&gt;Mission Specialists&lt;/h3&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Distance visual acuity: 20/200 or better uncorrected, correctable to 20/20, each eye.&lt;/li&gt;&#xA;&lt;li&gt;Blood pressure: 140/90 measured in a sitting position.&lt;/li&gt;&#xA;&lt;li&gt;Height between 58.5 and 76 inches.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="17" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-16T22:49:20.510" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="99" PostTypeId="1" AcceptedAnswerId="106" CreationDate="2013-07-16T22:49:50.110" Score="13" ViewCount="1074" Body="&lt;p&gt;By having a gun on board, one can conduct military kinds of experiments. Exploring armoring approaches for spacecraft, warning systems for the personnel, orbital ballistic studies for the multi-body attacker-projectile-target systems. The actual presence of weaponry in space has an immediate impact on the design of certain classes of spacecrafts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Zvezda module of the Russian ISS sector is the direct descendant of Almaz (Salyut) space stations. The Almaz stations had the &lt;a href=&quot;http://en.wikipedia.org/wiki/Nudelman-Rikhter_NR-23&quot; rel=&quot;noreferrer&quot;&gt;Nudelman-Rikhter gun&lt;/a&gt; onboard. Is an NR gun installed on Zvezda?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If yes, have any experiments been conducted with it?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="12102" LastEditDate="2020-01-04T02:43:02.337" LastActivityDate="2020-01-04T05:04:02.457" Title="Is the Nudelman-Rikhter gun installed on Zvezda module?" Tags="|iss|russia|roscosmos|weapon|zvezda|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="101" PostTypeId="2" ParentId="10" CreationDate="2013-07-16T22:54:48.413" Score="22" Body="&lt;p&gt;The South Atlantic Anomaly is  a region of high particle flux above the Atlantic Ocean off the coast of Brazil, caused by a slightly lower magnetic field above this point on the Earth. The problem it causes satellites is that above the anomaly, the magnetic field being weaker allows particles to get closer to the Earth. These particles can damage detectors on satellites so they may need to be shut off. It has on occasion caused laptops on the ISS to crash.&lt;/p&gt;&#xA;&lt;p&gt;From NASA:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970307a.html&quot; rel=&quot;noreferrer&quot;&gt;The South Atlantic Anomaly comes about because the Earth's field is not completely symmetric. If we were to represent it by a compact magnet (which reproduces the main effect, not the local wiggles), that magnet would not be at the center of the Earth but a few hundred miles away, in the direction away from the &amp;quot;Anomaly.&amp;quot;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;And a lovely image from &lt;a href=&quot;http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/misc_saad.html&quot; rel=&quot;noreferrer&quot;&gt;http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/misc_saad.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/XeXgM.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;And just realised 6 years later I hadn't really answered the &amp;quot;how to protect&amp;quot; part of your question. So:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Shutting off sensitive detectors is a practical solution where possible&lt;/li&gt;&#xA;&lt;li&gt;Shielding components or circuits that cannot be shut off is essential if you want to avoid them crashing like the laptops (generally space-hardened circuitry should be fine, it's more standard items that could be a problem - items you'd expect to be protected by Earth's magnetic field in a low orbit)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-08-21T14:49:52.790" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="102" PostTypeId="2" ParentId="42" CreationDate="2013-07-16T22:59:13.540" Score="10" Body="&lt;p&gt;To add on to what Rory Alsop said about machines lacking problem solving capability:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least the rovers we have sent to Mars so far are not fully autonomous - they are not controlled at all time either, but are queued up with a set of plans.  But if something unexpected does happen, it is not easy for a person to take the wheel, so to speak.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the relative positions of Earth and Mars, the time delay to issue a command could be about 12 minutes, and the time it takes to find out what happened the same.  By the time your signal gets there it could already be too late to react.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earlier this year Curiosity sat around for a month because the sun came in between the Earth and Mars.  They could have queued up exploration instructions for a month, but of course they wouldn't have trusted the rover to handle things on its own.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But AI that could manage on its own is probably not far off, especially for a rather inactive planet such as Mars.  &lt;/p&gt;&#xA;" OwnerUserId="57" LastActivityDate="2013-07-16T22:59:13.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="103" PostTypeId="1" AcceptedAnswerId="111" CreationDate="2013-07-16T23:07:11.440" Score="13" ViewCount="24987" Body="&lt;p&gt;&quot;Failure is not an option&quot; is attributed to Gene Kranz, flight director of Gemini, Apollo and Space Shuttle missions. In the movie Apollo 13, Ed Harris says it but IMDB trivia states that it was not said during that mission. Did he really say this?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="25" LastEditDate="2018-05-01T16:06:35.220" LastActivityDate="2018-05-01T16:06:35.220" Title="&quot;Failure is not an option&quot; is really a quote of flight director Gene Kranz?" Tags="|crewed-spaceflight|apollo-program|history|apollo-13|project-gemini|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="104" PostTypeId="1" CreationDate="2013-07-16T23:10:47.617" Score="11" ViewCount="364" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Vega_program#Balloon&quot;&gt;Vega program&lt;/a&gt; included a balloon floating in the Venus atmosphere.  It seems no more balloons have been used on other planets since then. The Wikipedia article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerobot&quot;&gt;Aerobots&lt;/a&gt; mentions JPL feasibility studies for Mars, Venus, Titan, and Jupiter, but does not cite any sources for Jupiter and the information there might be original research in the Wikipedia article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since then, have there been any official feasibility studies into descending balloons into the atmosphere of Jupiter or other outer planets?  By official, I mean by or with funding from a major space agency or related institute, as opposed to semi-hobbyists doing calculations.  What would be the biggest challenges, compared to the balloons that have been used on Venus?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2013-07-17T16:40:59.713" LastActivityDate="2013-07-17T20:14:35.993" Title="Have there been any official feasability studies into descending balloons into outer planetary atmospheres?" Tags="|balloons|jupiter|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="105" PostTypeId="2" ParentId="12" CreationDate="2013-07-16T23:15:36.663" Score="3" Body="&lt;p&gt;It should work - the challenge you would have is positioning the dishes accurately on that baseline, or at least knowing how far off the baseline they are at any moment, in order to calculate the relative phase of an incoming waveform.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, their positions can be measured very accurately, and they tend to stay where they are. In space, they will each have their own orbit and be affected by impact from debris in space, solar wind and other particles etc.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-16T23:15:36.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="106" PostTypeId="2" ParentId="99" CreationDate="2013-07-16T23:20:35.940" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Almaz&quot; rel=&quot;nofollow&quot;&gt;Almaz&lt;/a&gt; was the military version of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Salyut_programme&quot; rel=&quot;nofollow&quot;&gt;Salyut&lt;/a&gt;. At that time while military meant reconnaissance &quot;only&quot;; Salyut 2, 3 and 5 are referred to as Almaz. Some sources say that the Almaz editions were equipped with guns while the civil editions of Salyut were not. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is safe to assume that with the ISS being a rather civil and international cooperation, its Russian segments were based on the civil edition of the Salyut.&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="288" LastEditDate="2013-10-17T05:39:17.300" LastActivityDate="2013-10-17T05:39:17.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="107" PostTypeId="2" ParentId="96" CreationDate="2013-07-16T23:21:19.220" Score="12" Body="&lt;p&gt;Depending on exactly what temperature the water on Enceladus is, the answer is yes. Bacteria are an obvious choice. But there is a higher lifeform which can almost certainly survive:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tardigrade&quot;&gt;Tardigrades&lt;/a&gt; (Water bears) can survive for days at −200 °C - longer at higher temperatures, and can cope with extremely acid or alkaline environments. They can even survive in outer space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The probable water temperature on Enceladus is much higher than this, and while there will be dissolved salts, this should not be a problem to the hardy tardigrade.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-16T23:21:19.220" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="108" PostTypeId="1" AcceptedAnswerId="109" CreationDate="2013-07-16T23:23:05.450" Score="16" ViewCount="2313" Body="&lt;p&gt;Is it possible to visit some of the famous American capsules in any museum?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="26446" LastEditDate="2020-04-18T17:20:56.410" LastActivityDate="2020-04-18T17:20:56.410" Title="Are Mercury/Apollo/Gemini capsules visible in any museum?" Tags="|apollo-program|history|project-mercury|project-gemini|museum-exhibits|" AnswerCount="5" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="109" PostTypeId="2" ParentId="108" CreationDate="2013-07-16T23:27:29.263" Score="23" Body="&lt;p&gt;Yes, visiting some of the famous (American) capsules is possible. Larger accumulations of capsules, excluding mock-ups and never-flown equipment, can be found at the following places in the US. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;National Air and Space Museum (Smithsonian Institution), Washington, DC: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury-Redstone-3 [suborbital], &quot;Freedom 7&quot; (Shepard) &lt;/li&gt;&#xA;&lt;li&gt;Mercury-Atlas-6, &quot;Friendship 7&quot; (Glenn)&lt;/li&gt;&#xA;&lt;li&gt;Gemini 4, (McDivitt, White)&lt;/li&gt;&#xA;&lt;li&gt;Gemini 7, (Borman, Lovell)&lt;/li&gt;&#xA;&lt;li&gt;Apollo 11, &quot;Columbia&quot; (Armstrong, Aldrin, Collins) &lt;/li&gt;&#xA;&lt;li&gt;Skylab 4, (Carr, Gibson, Pogue) &lt;/li&gt;&#xA;&lt;li&gt;[Russian] Soyuz TM-10 (Manakov, Strekalov)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Kansas Cosmosphere and Space Center, Hutchinson, KS: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury-Redstone-4 [suborbital], &quot;Liberty Bell 7&quot; (Grissom)&lt;/li&gt;&#xA;&lt;li&gt;Mercury-Atlas-1, Capsule #4 (unmanned)&lt;/li&gt;&#xA;&lt;li&gt;Apollo 13, &quot;Odyssey&quot; (Lovell, Swigert, Haise)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Museum of Science and Industry, Chicago, IL: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury-Atlas-7, &quot;Aurora 7&quot; (Carpenter)&lt;/li&gt;&#xA;&lt;li&gt;Apollo 8 (Borman, Lovell, Anders)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Astronaut Hall of Fame, Titusville, FL: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury-Atlas-8 &quot;Sigma 7&quot; (Schirra)&lt;/li&gt;&#xA;&lt;li&gt;Apollo 14, &quot;Kitty Hawk&quot; (Shepard, Roosa, Mitchell)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Space Center Houston (NASA Johnson Space Center's Visitor Center), Houston, TX: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury-Atlas-9, &quot;Faith 7&quot; (Cooper)&lt;/li&gt;&#xA;&lt;li&gt;Gemini 5 (Cooper, Conrad)&lt;/li&gt;&#xA;&lt;li&gt;Apollo 17, &quot;America&quot; (Cernan, Evans, Schmitt)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;California Science Center, Los Angeles, CA: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury-Redstone-2 [suborbital] (chimpanzee &quot;Ham&quot;) &lt;/li&gt;&#xA;&lt;li&gt;Gemini 11 (Conrad, Gordon)&lt;/li&gt;&#xA;&lt;li&gt;Apollo-Soyuz Test Project, &quot;ASTP&quot; (Stafford, Brand, Slayton) &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A full list ordered by missions can be found here: &lt;a href=&quot;http://www.live555.com/misc/CapsuleLocations.html&quot;&gt;http://www.live555.com/misc/CapsuleLocations.html&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-17T18:20:02.227" LastActivityDate="2013-07-17T18:20:02.227" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="110" PostTypeId="1" CreationDate="2013-07-16T23:28:04.220" Score="12" ViewCount="625" Body="&lt;p&gt;The &lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/safety/chap2.pdf&quot;&gt;Cassini Environmental Impact Statement, Chapter 2&lt;/a&gt; (page 53–54) describes the status of Concentrated Solar Energy in space:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A 1981 conceptual study of the system impacts of using a concentrated&#xA;  solar array (CSA) on the Galileo spacecraft, launched in 1989 to Jupiter and powered by two RTGs, concluded that such an effort could be performed but would require an &quot;extensive development effort&quot; and that &quot;..the severe environmental constraints and the embryonic state of CSA development indicates that CSA will not displace the RTG on the Galileo mission&quot; (Rockey et al. 1981). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(...)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Concentrators have not been demonstrated in space, and a number of significant technical&#xA;  problems would have to be solved before a concentrator could be considered feasible for space&#xA;  missions, such as Cassini. The problems include how to regulate the concentrator's temperature&#xA;  for acceptable performance as the spacecraft traverses a Sun-to-spacecraft range from 0.63 AU to&#xA;  9.3 AU; how to predict the behavior of the optics over the mission lifetime, because small changes&#xA;  in the concentrator condition (e.g., yellowing, aging, and sagging) can lead to significant power&#xA;  losses; and how to improve the alignment of the concentrator elements due to the dependence of&#xA;  the concentrator's power-generating ability on the Sun's incident angle. In addition, concentrator&#xA;  performance depends on clear, unobscured optics, and estimating the buildup of interstellar (and&#xA;  Saturnian) dust on the optics would be difficult. Moreover, vibration testing of any concentrator&#xA;  array would have to be performed to verify post-launch optical alignments and operating&#xA;  characteristics in zero gravity environments. The size of the concentrator arrays that would be&#xA;  needed for the exploration of the Saturnian system would not easily integrate into the Titan IV&#xA;  (SRMU)/Centaur and would not satisfy the launch mass constraints. Furthermore, it is not clear&#xA;  that concentrator arrays would provide any advantage over planar arrays for this mission (JPL&#xA;  1994a).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;1981 is &lt;em&gt;a long time ago&lt;/em&gt;, and there might have been plenty of &quot;extensive development effort&quot;.  What is the current status of concentrated solar arrays in space?  Have they ever been used, or studied more recently than in the cited study?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: I think this is about &lt;a href=&quot;http://en.wikipedia.org/wiki/Concentrated_photovoltaics&quot;&gt;Concentrated photovoltaics&lt;/a&gt;, not &lt;a href=&quot;http://en.wikipedia.org/wiki/Concentrated_solar_power&quot;&gt;concentrated solar power&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2013-07-16T23:36:59.413" LastActivityDate="2016-06-03T20:50:09.660" Title="What is the status of concentrated solar energy (CSE) in space exploration?" Tags="|energy|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="111" PostTypeId="2" ParentId="103" CreationDate="2013-07-16T23:30:58.057" Score="15" Body="&lt;p&gt;If I remember it correctly, Gene Kranz too confirmed in an interview, that this sentence was made up for the film. But he liked it so much that he used it as the title for his memoirs / autobiography. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-16T23:30:58.057" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="112" PostTypeId="1" AcceptedAnswerId="398" CreationDate="2013-07-16T23:51:50.357" Score="16" ViewCount="447" Body="&lt;p&gt;Although a substantial changes in the Earth's magnetic field (e.g., as associated with a &lt;a href=&quot;http://en.wikipedia.org/wiki/Geomagnetic_reversal&quot;&gt;geomagnetic reversal&lt;/a&gt; where the magnetic field strength may be reduced to 5% of the current strength) are rare events, the impact on space weather as felt by objects in Earth orbit would seem to be great enough that some consideration would have been given to this issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What, if any, planning has been or is being done to address this potential danger to satellite operations? What kind of measurements are performed - in space - for better understanding the situation and corresponding processes? Is there any kind of monitoring - in space - on the current state of Earth's magnetic field, its development and space weather phenomena, which may influence the field?&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastEditorUserId="8607" LastEditDate="2015-03-01T17:25:39.303" LastActivityDate="2015-03-01T17:25:39.303" Title="What, if any, plans have been made for handling events like a geomagnetic reversal?" Tags="|radiation|magnetic-field|space-weather|geomagnetic-reversal|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="113" PostTypeId="5" CreationDate="2013-07-16T23:52:18.623" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-16T23:52:18.623" LastActivityDate="2013-07-16T23:52:18.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="114" PostTypeId="4" CreationDate="2013-07-16T23:52:18.623" Score="0" Body="Questions pertaining to activities of the National Aeronautics and Space Administration, a United States government agency that deals with space exploration and research." OwnerDisplayName="user40" LastEditorDisplayName="user12" LastEditDate="2013-11-01T13:27:13.267" LastActivityDate="2013-11-01T13:27:13.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="115" PostTypeId="5" CreationDate="2013-07-16T23:54:14.820" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-16T23:54:14.820" LastActivityDate="2013-07-16T23:54:14.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="116" PostTypeId="4" CreationDate="2013-07-16T23:54:14.820" Score="0" Body="Questions regarding scientific research, by means of the scientific method, into topics discussed. Investigative work based on empirical and measurable evidence that can be repeated, tried, and tested." OwnerUserId="-1" LastEditorUserId="49" LastEditDate="2013-11-09T20:52:49.667" LastActivityDate="2013-11-09T20:52:49.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="117" PostTypeId="5" CreationDate="2013-07-16T23:54:57.760" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-16T23:54:57.760" LastActivityDate="2013-07-16T23:54:57.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="118" PostTypeId="4" CreationDate="2013-07-16T23:54:57.760" Score="0" Body="Questions pertaining to the history of space exploration. Includes how current things will be viewed historically from the future (e.g. Apollo Moon landing sites 100 years from now)" OwnerDisplayName="user40" LastEditorUserId="12102" LastEditorDisplayName="user40" LastEditDate="2019-07-26T22:31:22.813" LastActivityDate="2019-07-26T22:31:22.813" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="119" PostTypeId="2" ParentId="29" CreationDate="2013-07-16T23:55:31.373" Score="7" Body="&lt;p&gt;There are studies into mining in space, but not for the gas giants.  The outer planets are massive.  Very massive.  This means they have very big gravity wells.  Lifting anything from Jupiter is going to be extremely energy-intensive, getting it back from Jupiter to Earth is too.  So, currently, this is the realm of science-fiction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the second part of your question: not from the outer planets; this is, if anything, way too far off for NASA (or any other space agency) to have concrete plans for a permanent base.  NASA used to have an &lt;a href=&quot;http://www.niac.usra.edu/&quot; rel=&quot;nofollow&quot;&gt;Institute for Advanced Concepts&lt;/a&gt; where people got paid to theorise about things currently impossible, but even there I don't think they studied mining the outer planets.  It's simply not feasible.  However, mining other resources in space, such as asteroids, Luna, or the inner planets, is within the reach of feasibility studies, although NASA does not have concrete plans at this point.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="63" LastEditDate="2013-07-21T19:32:52.407" LastActivityDate="2013-07-21T19:32:52.407" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="120" PostTypeId="2" ParentId="22" CreationDate="2013-07-16T23:57:14.897" Score="2" Body="&lt;p&gt;A chilean called Jenaro Gajardo claimed ownership of the entire moon back in the 50s. The legend says that even President Nixon asked him about the Apollo XI mission but I have not found any reliable source for the latter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Jenaro_Gajardo_Vera&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article about Genaro Gajardo&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://news.google.com/newspapers?nid=1665&amp;amp;dat=19690715&amp;amp;id=Ju4eAAAAIBAJ&amp;amp;sjid=KyQEAAAAIBAJ&amp;amp;pg=1400,1171216&quot; rel=&quot;nofollow&quot;&gt;Old NY Times article about Genaro Gajardo&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="60" LastEditorUserId="17" LastEditDate="2013-07-17T00:26:46.633" LastActivityDate="2013-07-17T00:26:46.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="121" PostTypeId="2" ParentId="110" CreationDate="2013-07-17T00:02:15.270" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://dx.doi.org/10.1109/PVSC.2000.915745&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; is a paper (or abstract, at least) stating that a Hughes 702 bus flew in 2000 with solar array concentrators. I don't believe they're totally uncommon, nowadays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the quotes you posted seem to be referring to deep space missions, which have significantly different power issues. I'm not aware of any interplanetary missions that have used concentrated solar arrays as their main power source.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-07-17T00:02:15.270" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="122" PostTypeId="1" AcceptedAnswerId="126" CreationDate="2013-07-17T00:04:15.760" Score="20" ViewCount="10589" Body="&lt;p&gt;What would happen if an astronaut took his/her glove off in space but had the rest of their body completely sealed off?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that the hand would start to boil or freeze depending on location.&lt;/p&gt;&#xA;" OwnerUserId="66" LastEditorUserId="6537" LastEditDate="2018-01-11T09:58:24.687" LastActivityDate="2019-06-11T13:58:38.870" Title="How would an astronaut's hand react to a hard vacuum?" Tags="|spacesuits|humans|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="123" PostTypeId="1" CreationDate="2013-07-17T00:08:22.487" Score="17" ViewCount="943" Body="&lt;p&gt;I always wondered about this. Is it because the fuel is too costly? Because the equipment will be trashed?&lt;/p&gt;&#xA;" OwnerUserId="60" LastEditorUserId="63" LastEditDate="2013-07-17T20:20:19.987" LastActivityDate="2015-07-03T00:53:09.433" Title="What specifically makes launching objects from Earth into space expensive?" Tags="|launch|cost|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="124" PostTypeId="5" CreationDate="2013-07-17T00:12:48.993" Score="0" Body="&lt;p&gt;The ISS (International Space Station) is a combination of the Soviet/Russian plans for a successor to the Mir space station, and the US Freedom space station model. The merging of the two programs was meant to save money and generate political good will.&lt;/p&gt;&#xA;&lt;p&gt;The ISS now consists of many modules, delivered over a decade in dozens of launches on many different launch vehicles).&lt;/p&gt;&#xA;" OwnerUserId="46507" LastEditorUserId="46507" LastEditDate="2023-03-23T16:55:39.307" LastActivityDate="2023-03-23T16:55:39.307" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="125" PostTypeId="4" CreationDate="2013-07-17T00:12:48.993" Score="0" Body="Questions about the ISS (International Space Station), a habitable man-made low Earth orbit satellite whose first component was launched in 1998 and used to conduct experiments for a wide variety of research fields in a microgravity environment." OwnerUserId="17" LastEditorUserId="46507" LastEditDate="2023-03-23T16:55:45.640" LastActivityDate="2023-03-23T16:55:45.640" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="126" PostTypeId="2" ParentId="122" CreationDate="2013-07-17T00:16:41.327" Score="25" Body="&lt;p&gt;Without a glove, a space suit would basically lose its integrity. It is like not wearing a suit in the first place. Then, the question is what happens next. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest known resemblance of such a scenario happened during a test of an Apollo space suit in a vacuum chamber: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/5cElpqJzLXY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; (In 1965, a tube pressurising a space suit of a test subject, a man inside an evacuated vacuum chamber, had become disconnected from the suit by accident. Essentially without any pressure on his body, he lost consciousness within about 14 seconds, while he was still able to notice fluids on the tip of his tongue starting to boil before passing out. Because the chamber could be re-pressurized to a safe level within a total of about 25 seconds, this accounts for one of the rare cases in which a human being has survived such an incident without obvious damage.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a very nice video explaining what would happen to a human body in empty space: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/pm6df_SExVw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; (It is the best and shortest explanation that I know of. It's worth watching it.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A bit more reading with less chance of being taken off-line: &lt;a href=&quot;https://web.archive.org/web/20140326132000/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html&quot; rel=&quot;noreferrer&quot;&gt;https://web.archive.org/web/20140326132000/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2017-05-29T10:22:53.510" LastActivityDate="2017-05-29T10:22:53.510" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="127" PostTypeId="5" CreationDate="2013-07-17T00:16:49.043" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T00:16:49.043" LastActivityDate="2013-07-17T00:16:49.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="128" PostTypeId="4" CreationDate="2013-07-17T00:16:49.043" Score="0" Body="Questions pertaining to natural satellites of planetary objects." OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-17T22:00:13.830" LastActivityDate="2013-07-17T22:00:13.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="129" PostTypeId="5" CreationDate="2013-07-17T00:21:51.350" Score="0" Body="&lt;p&gt;The first mission to Mars was in 1960 and was conducted by the former Soviet Union (the first successful landing occured in 1971 &lt;a href=&quot;/questions/tagged/mars-3&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;mars-3&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;mars-3&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-mars-3-tooltip-container&quot;&gt;mars-3&lt;/a&gt;), and there are several current missions studying the planet. Several space agencies from around the world are studying its surface features, atmosphere, and its potential for hosting life in either the past or present.&lt;/p&gt;&#xA;&lt;p&gt;Questions that are related to the past, current, or future exploration and study of Mars should use this tag. For some more information:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_of_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Exploration of Mars - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://marstrek.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA - Mars Trek (navigate the surface of Mars)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;For artificial satellites that are stationed in Mars orbit, see &lt;a href=&quot;/questions/tagged/mars-orbiters&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;mars-orbiters&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;mars-orbiters&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-mars-orbiters-tooltip-container&quot;&gt;mars-orbiters&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;For the Martian dust storms, see &lt;a href=&quot;/questions/tagged/dust&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;dust&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;dust&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-dust-tooltip-container&quot;&gt;dust&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="46507" LastEditDate="2023-03-23T08:07:45.957" LastActivityDate="2023-03-23T08:07:45.957" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="130" PostTypeId="4" CreationDate="2013-07-17T00:21:51.350" Score="0" Body="Questions about the science or exploration of the planet Mars. See the tag wiki for related tags." OwnerUserId="109" LastEditorUserId="46507" LastEditDate="2023-03-23T08:08:03.093" LastActivityDate="2023-03-23T08:08:03.093" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="131" PostTypeId="1" AcceptedAnswerId="146" CreationDate="2013-07-17T00:22:00.443" Score="7" ViewCount="2141" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/questions/29/harvesting-isotopes-of-helium-or-hydrogen-from-outerspace&quot;&gt;answer&lt;/a&gt; suggests that outer planets are very heavy and that lifting things on it is very energy expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If someone was to workout on a planet that is considered 'heavy' (whatever that means) would they have super strength once back on earth? (Think DBZ and goku training under heavy gravity.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what are the effects of body building on a heavy planets? &lt;/p&gt;&#xA;" OwnerUserId="66" LastEditorUserId="6537" LastEditDate="2018-01-11T19:57:33.763" LastActivityDate="2018-01-11T19:57:33.763" Title="What are the effects of body building on heavy planets?" Tags="|humans|training|physical-activity|" AnswerCount="6" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="132" PostTypeId="1" AcceptedAnswerId="330" CreationDate="2013-07-17T00:39:50.800" Score="16" ViewCount="4757" Body="&lt;p&gt;One of the most often cited drawbacks to settlement of other planets and satellites is the negative health effects of reduced gravity. Among the most severe of these is loss of bone density, reduced muscle mass, and weakened immune system. It seems that at least the first two of these effects could be directly solved by simply increasing a person's effective weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, would wearing heavy clothing or other equipment be enough to mitigate the negative effects of reduced gravity?&lt;/p&gt;&#xA;" OwnerUserId="47" LastEditorUserId="68" LastEditDate="2013-07-24T16:46:59.390" LastActivityDate="2018-04-27T03:50:00.237" Title="Would wearing heavy clothing be enough to solve the health effects of reduced gravity on other planets?" Tags="|health|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="133" PostTypeId="5" CreationDate="2013-07-17T00:41:52.700" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T00:41:52.700" LastActivityDate="2013-07-17T00:41:52.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="134" PostTypeId="4" CreationDate="2013-07-17T00:41:52.700" Score="0" Body="Questions relating to human physiology, as it affects capabilities in space." OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-17T08:32:18.457" LastActivityDate="2013-07-17T08:32:18.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="135" PostTypeId="1" AcceptedAnswerId="139" CreationDate="2013-07-17T00:47:42.163" Score="12" ViewCount="961" Body="&lt;p&gt;How do people drink from cups (an activity which heavily relies on the presence of gravity) in space?&lt;/p&gt;&#xA;&lt;p&gt;I know they could try to wait for the liquid to 'split up' and try to catch a chunk, but is there some sort of a 'special space cup' they use for their coffee?&lt;/p&gt;&#xA;" OwnerUserId="70" LastEditorUserId="37744" LastEditDate="2020-09-17T04:10:28.597" LastActivityDate="2020-09-17T04:10:28.597" Title="How do people drink from cups in Space?" Tags="|food|astronaut-lifestyle|microgravity|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="136" PostTypeId="5" CreationDate="2013-07-17T00:50:08.303" Score="0" Body="&lt;p&gt;The takeoff or the liftoff phase of the flight of a rocket and the set of activities required for preparation of the launch vehicle leading to it, that might be initiated off the surface of the Earth, its atmosphere (to include air-launch), another man made object, another celestial body or its atmosphere in order to gain distance from it and achieve flight independent of any surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast, start of a rocket or a rocket stage in space and independent of another mass body is not considered a launch, and term &quot;ignition&quot; should be used instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also: &lt;a href=&quot;/questions/tagged/launch-site&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch-site&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch-site&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/launch-vehicle&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch-vehicle&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch-vehicle&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/launchpad&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launchpad&amp;#39;&quot; rel=&quot;tag&quot;&gt;launchpad&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/launch-sequence&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch-sequence&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch-sequence&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/launch-escape-system&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch-escape-system&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch-escape-system&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/launch-trajectories&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch-trajectories&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch-trajectories&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/air-launch&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;air-launch&amp;#39;&quot; rel=&quot;tag&quot;&gt;air-launch&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/launch-window&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch-window&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch-window&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/launch-system&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch-system&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch-system&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/high-altitude-launch&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;high-altitude-launch&amp;#39;&quot; rel=&quot;tag&quot;&gt;high-altitude-launch&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/multi-launch&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;multi-launch&amp;#39;&quot; rel=&quot;tag&quot;&gt;multi-launch&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/launch-assist&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch-assist&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch-assist&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/hllv&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;hllv&amp;#39;&quot; rel=&quot;tag&quot;&gt;hllv&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="58" LastEditDate="2019-09-19T15:58:24.977" LastActivityDate="2019-09-19T15:58:24.977" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="137" PostTypeId="4" CreationDate="2013-07-17T00:50:08.303" Score="0" Body="Questions regarding the takeoff or the liftoff phase of the flight of a rocket and the set of activities required for preparation of the launch vehicle leading to it." OwnerDisplayName="user40" LastEditorUserId="49" LastEditDate="2014-02-20T20:24:05.610" LastActivityDate="2014-02-20T20:24:05.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="138" PostTypeId="2" ParentId="9" CreationDate="2013-07-17T00:52:53.807" Score="12" Body="&lt;p&gt;Well, we can't observe it with the Hubble telescope. It's just too small.&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;http://astroengine.com/2008/08/22/why-cant-we-see-the-oort-cloud/&quot; rel=&quot;noreferrer&quot;&gt;astroengine.com&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Using the equation: &lt;span class=&quot;math-container&quot;&gt;$(d / D) × c = φ$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where &lt;span class=&quot;math-container&quot;&gt;$d$&lt;/span&gt; is the diameter of the Oort Cloud comet (some estimates put this number at an upper limit of &lt;strong&gt;300 km for the diameter of a cometary nucleus&lt;/strong&gt;), &lt;span class=&quot;math-container&quot;&gt;$D$&lt;/span&gt; is the distance from the Oort Cloud to Hubble (&lt;strong&gt;0.3 light years, or &lt;span class=&quot;math-container&quot;&gt;$3×10^{15}$&lt;/span&gt; metres&lt;/strong&gt; – distance at which it is theorized there is the highest density of Oort Cloud objects), &lt;span class=&quot;math-container&quot;&gt;$c$&lt;/span&gt; is a constant (&lt;span class=&quot;math-container&quot;&gt;$c = 206265$&lt;/span&gt;) and &lt;span class=&quot;math-container&quot;&gt;$φ$&lt;/span&gt; is the telescope resolution.&lt;/p&gt;&#xA;&lt;p&gt;So what resolution do we need to image an Oort Cloud object, 300 km in diameter, from 0.3 light years away? If we plug in the numbers we get:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$φ = 2.06×10^{-5} = 0.00002~\text{arc-seconds}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;The resolving power of Hubble is &lt;span class=&quot;math-container&quot;&gt;$0.1~\text{arc-seconds}$&lt;/span&gt;, and is therefore &lt;strong&gt;useless at detecting anything below this angular size&lt;/strong&gt;; Oort Cloud comets (although pretty big at an upper limit of 300 km) simply cannot be observed by the world’s most advanced space-based optical observatory.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;How big would a telescope have to be? Well, from the same article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But how big would an Oort Cloud observing telescope have to be to resolve a cometary nucleus 300 km wide at a distance of 0.3 light years away? Using the simple relationship &lt;span class=&quot;math-container&quot;&gt;$R = 11.6 / w$&lt;/span&gt;, where &lt;span class=&quot;math-container&quot;&gt;$R$&lt;/span&gt; is the resolving power (&lt;span class=&quot;math-container&quot;&gt;$R = 0.00002/2$&lt;/span&gt;; the reason for halving our resolving power is given by Phil), and w is the width of the telescope mirror, we rearrange to get:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$w = 11.6 / 0.00001 = 1.16×10^6~\text{cm} = 11.6~\text{km}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;As you can see, such a telescope would be &lt;em&gt;&lt;strong&gt;huge&lt;/strong&gt;&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditorDisplayName="user29" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2020-02-28T11:32:30.477" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="139" PostTypeId="2" ParentId="135" CreationDate="2013-07-17T00:55:11.737" Score="15" Body="&lt;p&gt;There's a bunch of videos online with a demo of drinking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically for coffee you can watch that here:&#xA;&lt;a href=&quot;http://www.youtube.com/watch?v=pk7LcugO3zg&quot; rel=&quot;noreferrer&quot;&gt;http://www.youtube.com/watch?v=pk7LcugO3zg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;http://www.collectspace.com/ubb/Forum14/HTML/000725.html&quot; rel=&quot;noreferrer&quot;&gt;post&lt;/a&gt;, he uses a special cup that has a specific shape and surface tension. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During STS-126 last November, mission specialist Don Pettit devised a&#xA;  cup that took advantage of its special shape and surface tension such&#xA;  that it was perfect for use in microgravity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This special object is actually just a report cover. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As Pettit describes in the video, the plastic for the cup was taken&#xA;  from the cover of a flight data file, so I stopped by an office supply&#xA;  store and purchased a report cover. In orbit, Kapton polymide tape was&#xA;  used to hold the cup together. Despite my best effort, I was unable to&#xA;  find a local store that stocks Kapton, so another 3M product, Scotch&#xA;  tape was substituted (if I ever come across a few strips of Kapton, I&#xA;  will use it to cover the clear tape).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;oddly enough there was an article written yesterday (July 15th, 2013) on this exact question, pointing out the same information and same video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ibtimes.com/nasa-figures-out-how-drink-coffee-space-zero-gravity-coffee-cup-video-1345635&quot; rel=&quot;noreferrer&quot;&gt;http://www.ibtimes.com/nasa-figures-out-how-drink-coffee-space-zero-gravity-coffee-cup-video-1345635&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="66" LastEditorUserId="66" LastEditDate="2013-08-15T19:11:35.607" LastActivityDate="2013-08-15T19:11:35.607" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="140" PostTypeId="2" ParentId="135" CreationDate="2013-07-17T01:02:14.037" Score="16" Body="&lt;p&gt;Historically cups have not been used.  For spaceflight, liquids have been stored in sealed bag-like containers and then drunk through a tube.  These are somewhat similar to how many children's drinks are packaged and stored. (There was a link to a document showing some pictures of these sorts of containers and some food containers, but the link has gone dead.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As &lt;a href=&quot;https://space.stackexchange.com/a/139/36&quot;&gt;another answer&lt;/a&gt; has said, there have been demonstrations of drinking containers that take advantage of certain physical properties which allow a liquid to stay inside an open container.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also &lt;a href=&quot;http://www.youtube.com/watch?v=4o1lr9LxODc&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; of an astronaut letting water loose inside his cabin and then taking it into his mouth by moving his head into a position which allows that.  This was part of an educational video in which astronauts were showing how liquid behaves in a low gravity environment.&lt;/p&gt;&#xA;" OwnerUserId="36" LastEditorUserId="36" LastEditDate="2019-12-17T15:49:33.897" LastActivityDate="2019-12-17T15:49:33.897" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="141" PostTypeId="2" ParentId="131" CreationDate="2013-07-17T01:03:18.417" Score="4" Body="&lt;p&gt;Thinking the effect when an astronaut comes back to earth from long time exposure in some low gravitational environments, the lost bone density won't be immediately regained. He/She faces greater chances of fracture. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe human will face similar situation when going to a relatively high gravity planet. You will face higher chances of fracture, but gradually your body will adapt. Eventually you will gain higher bone density and better muscle strength (like workout with extra weight).&lt;/p&gt;&#xA;" OwnerUserId="11" LastActivityDate="2013-07-17T01:03:18.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="142" PostTypeId="1" AcceptedAnswerId="161" CreationDate="2013-07-17T01:05:04.500" Score="15" ViewCount="1954" Body="&lt;p&gt;What items are not allowed to be brought aboard space exploration missions? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What can't be brought back from a space mission (without prior authorization)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why were these regulations originally put into place?&lt;/p&gt;&#xA;" OwnerUserId="44" LastEditorDisplayName="user40" LastEditDate="2013-07-17T22:01:23.100" LastActivityDate="2015-07-22T22:16:14.453" Title="What types of items are prohibited on space missions?" Tags="|law|safety|history|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="143" PostTypeId="2" ParentId="19" CreationDate="2013-07-17T01:08:32.593" Score="12" Body="&lt;p&gt;I actually asked myself that question a while ago. And after some search I found some really good answers and some pretty interesting ones. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space junk is a huge problem. We spend billions on space programs, and junk should be the least of our worries. Over the past few years it has troubled some scientists, with space debris crashing into communication satellites, causing huge problems and interference with operations. Also the fact that their trajectory is unreliable can cause problems for us to understand and draft a good route to dodge all that junk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a few ideas that have been dreamed up to clean up space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;DARPA's &lt;a href=&quot;http://www.darpa.mil/our_work/tto/programs/phoenix.aspx&quot; rel=&quot;nofollow&quot;&gt;Phoenix&lt;/a&gt; Project: DARPA plans on using robots that tag along with communication satellites to launch into space. And once in space they detach themselves and hook onto dysfunctional satellites and try to refurbish them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Switzerland's &lt;a href=&quot;http://www.usnews.com/news/articles/2012/02/17/switzerland-announces-plan-to-clean-up-space-junk&quot; rel=&quot;nofollow&quot;&gt;CleanSpaceOne&lt;/a&gt; Program: CleanSpaceOne is a satellite that goes into space and collects all the debris it can and re-enters Earth's atmosphere, where the satellite and its components burn off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;THE MOST INTERESTING ONE! &lt;a href=&quot;http://www.nbcnews.com/science/japan-go-fishing-space-debris-6C10403432?franchiseSlug=sciencemain&quot; rel=&quot;nofollow&quot;&gt;Japan Aerospace Exploration Agency&lt;/a&gt;: They plan on deploying a gigantic net to sweep up space junk back into Earth's atmosphere where it will burn off. They have spent 6 years developing that net.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.sen.com/news/esa-plans-to-clear-up-space-junk.html&quot; rel=&quot;nofollow&quot;&gt;European Space Agency&lt;/a&gt;: They plan to launch ATVs (Automated Transfer Vehicles). These are autonomous vehicles which can use lasers to detect debris and collect it and send it back to earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://theweek.com/article/index/213197/nasas-plan-to-clean-up-space-junk-lasers&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt;: They plan on using lasers, not to destroy the junk but to nudge them away from the shuttle's path.&lt;/p&gt;&#xA;" OwnerUserId="70" LastEditorUserId="6816" LastEditDate="2015-04-29T19:51:30.627" LastActivityDate="2015-04-29T19:51:30.627" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="144" PostTypeId="1" AcceptedAnswerId="197" CreationDate="2013-07-17T01:09:22.740" Score="37" ViewCount="25357" Body="&lt;p&gt;Curiosity, we know, has no little nuclear reactor, it has a RTG. Great! But how long will it last? For how many years will our little 1-ton friend be roaming around another planet?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="15906" LastEditDate="2017-08-30T20:48:20.733" LastActivityDate="2020-07-11T21:54:15.223" Title="About how long will Curiosity last on its nuclear power?" Tags="|mars|rovers|energy|curiosity|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="145" PostTypeId="2" ParentId="144" CreationDate="2013-07-17T01:09:22.740" Score="5" Body="&lt;p&gt;Let's take a closer look at the nuclear cell.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to According to &lt;a href=&quot;http://mars.jpl.nasa.gov/msl/mission/technology/technologiesofbroadbenefit/power/&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt;;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Radioisotope power systems are generators that produce electricity from the natural decay of plutonium-238, which is a non-weapons-grade form of that radioisotope used in power systems for NASA spacecraft. Heat given off by the natural decay of this isotope is converted into electricity, providing constant power during all seasons and through the day and night.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It contains about 10lbs of plutonium-238. Great! Now how long will &lt;em&gt;that&lt;/em&gt; last?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, from the same NASA page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This type of power supply will give the mission an operating lifespan on Mars' surface of a full Martian year (&lt;strong&gt;687 Earth days&lt;/strong&gt;, a little less than &lt;strong&gt;two Earth years&lt;/strong&gt;) over a wide latitude range.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So we have about 2 years of the rover being able to be run on nuclear power. After that, though, it should be able to run for another twelve-ish years on its MMRTG.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-17T04:16:22.287" LastActivityDate="2013-07-17T04:16:22.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="146" PostTypeId="2" ParentId="131" CreationDate="2013-07-17T01:15:23.083" Score="10" Body="&lt;p&gt;No, you wouldn't become a superhuman. Well, at least not for the workout part of this endeavour. Essentially, every mass object would be heavier due to stronger gravitational pull of a larger mass planet than Earth. Say, you use weights to exercise on these gas giants the question you're referring to is inquiring about. And let's, for the sake of argument, pretend you are protected from all the environmental hazards of being on (in?) such planets, and you actually have somewhere to stand (gas giants like Jupiter and Saturn don't really have a solid surface, so you'd be suspended at a point of equal atmospheric density than the average of your own body and whatever you carry on your person, and ice giants like Uranus and Neptune, while they have a solid surface, it is at a depth of a rather uncomfortable atmospheric pressure).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These weights would become heavier by &lt;a href=&quot;https://en.wikipedia.org/wiki/Weight&quot; rel=&quot;nofollow&quot;&gt;weight&lt;/a&gt;, while their &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass&quot; rel=&quot;nofollow&quot;&gt;mass&lt;/a&gt;, of course, stays the same. Something you can do just as well on the good old Earth by simply using heavier weights in the first place. To be more accurate, this gravitational pull wouldn't work on weights only, but on your body as well, so even lifting your arms would involve a lot more effort on its own. You could simulate this by attaching additional weights to your limbs here on Earth, for example. Or doing weight lifting exercises while submerged in water and account for weight difference in displaced water (&lt;a href=&quot;https://en.wikipedia.org/wiki/Buoyancy&quot; rel=&quot;nofollow&quot;&gt;buoyancy&lt;/a&gt;) by using heavier weights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, Jupiter's &lt;a href=&quot;https://en.wikipedia.org/wiki/Surface_gravity&quot; rel=&quot;nofollow&quot;&gt;equatorial surface gravity&lt;/a&gt; is 2.528 &lt;a href=&quot;https://en.wikipedia.org/wiki/G-force&quot; rel=&quot;nofollow&quot;&gt;g&lt;/a&gt;. &lt;code&gt;g&lt;/code&gt; stands for g-force, or &lt;em&gt;a measurement of acceleration felt as weight&lt;/em&gt;. This means that the 10 kg weights you'd take from Earth with you to the Jupiter would weight there roughly 25 kg. You, the man doing the exercises with these weights however, still have the same physical strength, your bone structure can support same amount of stress (if you didn't spend too much time in &lt;a href=&quot;https://en.wikipedia.org/wiki/Weightlessness&quot; rel=&quot;nofollow&quot;&gt;zero-g&lt;/a&gt; travelling there, and your cardiovascular and respiratory systems can take the added strain and you don't simply collapse due to &lt;a href=&quot;https://en.wikipedia.org/wiki/G-LOC&quot; rel=&quot;nofollow&quot;&gt;G-LOC&lt;/a&gt; induced hypoxia, heart or lung failure once at the hypergravity environment),...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, you would probably &lt;em&gt;feel&lt;/em&gt; stronger upon your return (not taking into account your travel back to Earth that would mostly be in zero gravity, negating any such positive effects from before, like muscle atrophy, bone loss, and so on), but that's something you can try e.g. by using a heavy training basketball for a few minutes, and then switching back to a normal one. The &lt;em&gt;normal basketball&lt;/em&gt; will appear to &quot;fly&quot; off you as if it was featherweight. Well, for a few minutes at least, until you &quot;recalibrate your senses&quot;. Either case, you could practice in becoming a superhuman in a lot safer environment than gas giant outer planets.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-23T15:39:41.393" LastActivityDate="2015-07-23T15:39:41.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="147" PostTypeId="2" ParentId="26" CreationDate="2013-07-17T01:15:24.937" Score="9" Body="&lt;p&gt;&lt;a href=&quot;http://www.centauri-dreams.org/?p=440&quot;&gt;This article&lt;/a&gt;, titled &lt;em&gt;Nuclear Pulse Propulsion Re-Examined&lt;/em&gt;, was published in late '05.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also &lt;a href=&quot;http://www.scifiideas.com/sfi/technology/nuclear-pulse-propulsion/&quot;&gt;this article&lt;/a&gt;, which was posted in early '12.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, despite much Googling, I wasn't able to find any real &lt;em&gt;research&lt;/em&gt; on the subject after about the '90s, when &lt;a href=&quot;http://en.wikipedia.org/wiki/Nuclear_pulse_propulsion#Medusa&quot;&gt;Project Medusa&lt;/a&gt; was started by NASA.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-07-17T01:15:24.937" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="148" PostTypeId="2" ParentId="84" CreationDate="2013-07-17T01:32:17.977" Score="5" Body="&lt;p&gt;Constructing a spacecraft will be a ghastly expense for everyone involved, and even more in a non-accommodating surface such as the Moon's. Building a space craft is challenging engineering. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As suggested before, the Moon has an abundance of silicon, iron, aluminum and titanium. But do remember that these are raw materials, which are not processed in any way. And the materials used on our spacecraft are sent to us machined with certain specifications. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make &lt;em&gt;that&lt;/em&gt; possible, we will need a whole support structure of secondary industries such as these to actually process the raw materials into usable materials. Without processing facilities, it's as good as not having resources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aspects such as these make such a task even more daunting and economically taxing. The same goes for electronics - they &lt;em&gt;will&lt;/em&gt; need a lot of tiny but crucial parts for construction. And these can't be produced without the appropriate machinery or labor (again economically, not preferable) On the other hand, if you were talking just about just the &lt;em&gt;materials&lt;/em&gt;, then yes, you will find the above mentioned materials, but you will also be needing polymers of different kinds which I don't believe have been found.&lt;/p&gt;&#xA;" OwnerUserId="70" LastEditorUserId="1142" LastEditDate="2017-01-18T22:38:17.413" LastActivityDate="2017-01-18T22:38:17.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="149" PostTypeId="1" CreationDate="2013-07-17T01:38:53.580" Score="19" ViewCount="907" Body="&lt;p&gt;I am specifically interested in on-ground tests that were conducted after the full shuttle was pieced together (orbiter, tank, engine, boosters) for the first time (before STS-1).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course the number of individual tests may have been quite large, but what were the overall characteristics of the process?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who ultimately decided if the shuttle was space-worthy? &lt;/p&gt;&#xA;" OwnerDisplayName="user40" LastEditorUserId="12102" LastEditorDisplayName="user40" LastEditDate="2017-09-20T04:56:35.317" LastActivityDate="2017-09-20T10:25:46.137" Title="What was the space-worthiness determination procedure for STS-1?" Tags="|space-shuttle|safety|history|nasa|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="150" PostTypeId="2" ParentId="39" CreationDate="2013-07-17T01:45:41.503" Score="8" Body="&lt;p&gt;It is actually true that NASA, at the moment, does not have any way of transporting people into space. And lately, they are finding servicing their spacecrafts so expensive that they are involving private contractors to ferry goods from Earth to the ISS. I believe it is a possibility that NASA will hire private contractors for any manned missions, at least in the near future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has been exploiting SpaceX's services for a while now. SpaceX &lt;em&gt;probably&lt;/em&gt; has the largest operational spaceship fleet in the US as of now. They recently used their &lt;a href=&quot;http://www.spacex.com/dragon.php&quot;&gt;Dragon&lt;/a&gt; for an official NASA resupply mission to the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A to some sources, SpaceX should be capable of &lt;a href=&quot;http://www.wunderground.com/news/spacex-sending-humans-to-space-20130110&quot;&gt;manned&lt;/a&gt; missions by early 2015. Therefore, it could be a possibility that NASA uses SpaceX for their manned missions as well. &lt;/p&gt;&#xA;" OwnerUserId="70" LastEditorDisplayName="user12" LastEditDate="2013-07-17T03:17:36.843" LastActivityDate="2013-07-17T03:17:36.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="153" PostTypeId="2" ParentId="43" CreationDate="2013-07-17T01:58:32.370" Score="4" Body="&lt;p&gt;If you are feeling up to the task you can look at the raw statistics of every space flight &lt;a href=&quot;http://www.spacelaunchreport.com/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. But you are right in saying that different cultures and attitudes do have effects on their Space Programs. Take for example the Space Race during the cold war. There was a literal rush to send the first satellite or man in space. And in that competition people could have found it not as important to document every aspect of the building process. Like the questions suggests, the Russians. So events, history, practices and cultures do in fact affect the space programs. &lt;/p&gt;&#xA;" OwnerUserId="70" LastActivityDate="2013-07-17T01:58:32.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="156" PostTypeId="2" ParentId="123" CreationDate="2013-07-17T02:03:56.920" Score="22" Body="&lt;p&gt;Three big factors:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The energy required is large (&lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_orbital_energy&quot;&gt;http://en.wikipedia.org/wiki/Specific_orbital_energy&lt;/a&gt;).  For a low orbit like the one the ISS is in, it's about a 33 MJ/kg addition from the surface to orbit -- and that neglects atmospheric drag. &lt;/li&gt;&#xA;&lt;li&gt;The rocket equation (&lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot;&gt;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&lt;/a&gt;).  In a nutshell, you must not only burn fuel to provide delta-V to your payload, you must also burn fuel to provide delta-V to the fuel you need to burn to provide delta-V to your payload.  Yeah, this is an exponential relationship.&lt;/li&gt;&#xA;&lt;li&gt;Complexity.  The system you use to rapidly (it must be rapid if you are using rocketry) convert this energy into delta-V is complicated and must be reliable.  With our current technology, reliable, complex systems are expensive to engineer and operate.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Various current technologies try to address these factors to reduce the costs.  For example, most space launches launch to the east to reduce that delta-V/energy requirements, item #1, for a mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Future technologies may seek to address these issues and therefore reduce these costs.  For example, space elevators might eliminate #2 by separating the energy source from the payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reducing complexity or increasing our ability to deal with complexity, item #3, can/would reduce costs as well.  For many missions, using robots instead of humans removes whole systems (life support, re-entry systems) and thus dramatically reduce costs.  This is a double-edged sword though as some new systems (machine intelligence, dextrous manipulators) might need to be added.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, there are other, smaller factors to consider as well:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fuel and the other materials used are, as you suggested expensive as well.  Aerospace systems generally have extreme material requirements.  The metal must be a specific alloy, the cryogenics must be of a high purity, etc.  This adds up.&lt;/li&gt;&#xA;&lt;li&gt;Fabrication of components typically requires precision and small tolerances -- both of which increase cost.&lt;/li&gt;&#xA;&lt;li&gt;Vehicle and systems are typically built in small quantities.  The advantageous economics of Henry Ford's assembly line don't apply.&lt;/li&gt;&#xA;&lt;li&gt;The systems are generally contracted for and operated by state entities.  These entities have the advantage of generating large amounts of funds via taxation, but have the disadvantage of inefficiencies due to the lack of a profit motive.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-17T19:15:43.183" LastActivityDate="2013-07-17T19:15:43.183" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="157" PostTypeId="1" AcceptedAnswerId="158" CreationDate="2013-07-17T02:09:37.570" Score="24" ViewCount="2112" Body="&lt;p&gt;We all know about the fancy 'sky crane' system used on NASA's Curiosity rover. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The rockets slow the descent to 1 ½ mph and power a sideways parry to avoid the faster falling chute. As Sky Crane descends to 60 feet above Mars' surface, the rover inches down from underneath it on three nylon ropes like a spider spinning strands of its web. With Curiosity dangling 20 feet below, Sky Crane continues its downward progress until the rover is resting on the surface. Explosive bolts cut Curiosity's last physical attachments to the outside world, and Sky Crane flies away to death-plunge into the red sands, its incredible job done.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why did they use this seemingly overly complex system? Why wasn't a simple airbag system used?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-17T03:49:28.867" LastActivityDate="2019-10-18T17:54:58.413" Title="What problems led to the use of the Sky Crane system used by Curiosity?" Tags="|mars|rovers|curiosity|skycrane|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="158" PostTypeId="2" ParentId="157" CreationDate="2013-07-17T02:13:17.573" Score="23" Body="&lt;p&gt;&lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2012/30jul_skycrane/&quot; rel=&quot;noreferrer&quot;&gt;This NASA doc&lt;/a&gt; explains fairly clearly why the skycrane system was used:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not rockets, like the Viking missions?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;With a payload this size, the rockets could kick up enough dust to compromise the rover and its instruments,&quot; explains Sell. &quot;And the rockets could excavate craters Curiosity would have to avoid as it drives away. Add to that the risk of a big, heavy vehicle driving down off the lander via an exit ramp to reach the surface.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How about airbags?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Bags big enough to soften its landing would be too heavy or too costly to launch. Besides, you'd have to drop the payload so slowly for the bags to survive the load, you may as well place the rover right on its wheels.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This image really explains the difference in size between Curiosity and previous rovers, which make many methods not feasible:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9zl9t.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/mars/images/pia15280.html&quot; rel=&quot;noreferrer&quot;&gt;Three Generations of Rovers with Crouching Engineers&lt;/a&gt;. Front: Sojourner, 1997; Left: Spirit/Opportunity, 2004; Right: Curiosity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are Earthbound prototypes. Curiosity had not yet been launched when this photo was taken.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="23773" LastEditDate="2019-10-18T17:54:58.413" LastActivityDate="2019-10-18T17:54:58.413" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="160" PostTypeId="2" ParentId="142" CreationDate="2013-07-17T02:38:41.450" Score="7" Body="&lt;p&gt;Most space launches are conducted by sovereign states, so there really isn't any authority to prohibit anything.  There are some private organizations that launch, but these are generally done under the flag of a sovereign state.  In the future, private organizations may choose to operate independently from international waters (ala &lt;a href=&quot;http://en.wikipedia.org/wiki/Sealaunch&quot;&gt;SeaLaunch&lt;/a&gt;), but there is a lack of precedent there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said that, there are some things that receive more scrutiny than others.  Here are some to consider:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Biological materials.  For planetary missions that include a re-entry vehicle, biological materials that might cause planetary cross-contamination.  This falls under the name of &lt;a href=&quot;http://en.wikipedia.org/wiki/Back-contamination&quot;&gt;planetary protection&lt;/a&gt; and covers contamination in both directions.&lt;/li&gt;&#xA;&lt;li&gt;Radioactive materials.  In the event of a launch failure, these materials may be scattered over a wide area and pose a significant health hazard.&lt;/li&gt;&#xA;&lt;li&gt;Weaponry.  While each nation is sovereign, most nations (certainly space-faring ones) have treaties and agreements in place concerning the weaponization of space.  Probably the most significant of these is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Outer_Space_Treaty&quot;&gt;Outer Space Treaty&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Unstable systems.  Things like spent stages and remains from pyros can create space debris -- a long-term hazard for future missions.&lt;/li&gt;&#xA;&lt;li&gt;Heavy stuff.  Chalk this one up to prohibition via physics.  Stuff that has a low utility/unit mass is rarely going to make it on the manifest of present-day launch vehicles.  Think iron for example.  Most structures are better engineered by other materials with a better material properties.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-17T02:50:28.550" LastActivityDate="2013-07-17T02:50:28.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="161" PostTypeId="2" ParentId="142" CreationDate="2013-07-17T02:43:14.837" Score="11" Body="&lt;p&gt;Disclaimer: &lt;strong&gt;IANASL - I Am Not A Space Lawyer&lt;/strong&gt; :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would greatly depend on individual nations carrying out space missions and their &lt;a href=&quot;http://www.oosa.unvienna.org/oosa/en/SpaceLaw/national/state-index.html&quot; rel=&quot;nofollow&quot;&gt;national space laws&lt;/a&gt;, which would be by an order of magnitude too broad to be reasonably covered within this Q&amp;amp;A, but I presume you're limiting your question to &lt;a href=&quot;/questions/tagged/international-law&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;international-law&amp;#39;&quot; rel=&quot;tag&quot;&gt;international-law&lt;/a&gt;, as the tag you chose implies, and what international laws and regulations are there limiting or prohibiting use of, placement and removal of  certain items?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps most historically important, the &lt;a href=&quot;https://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967&quot; rel=&quot;nofollow&quot;&gt;Outer Space Treaty of 1967&lt;/a&gt; &lt;strong&gt;prohibits placement and use of nuclear weapons or any other weapons of mass destruction&lt;/strong&gt; in Earth's orbit and beyond (celestial bodies, outer space,...). Additionally, it &lt;strong&gt;prohibits building military installations, weapons tests, and similar militarisation of space&lt;/strong&gt; in it's &lt;a href=&quot;https://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967#Article_IV&quot; rel=&quot;nofollow&quot;&gt;Article IV&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;States Parties to the Treaty undertake not to place in orbit around&#xA;  the earth any objects carrying nuclear weapons or any other kinds of&#xA;  weapons of mass destruction, install such weapons on celestial bodies,&#xA;  or station such weapons in outer space in any other manner. The moon&#xA;  and other celestial bodies shall be used by all States Parties to the&#xA;  Treaty exclusively for peaceful purposes. The establishment of&#xA;  military bases, installations and fortifications, the testing of any&#xA;  type of weapons and the conduct of military manoeuvres on celestial&#xA;  bodies shall be forbidden.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is later extended to any &lt;em&gt;&quot;potentially harmful interference with activities of other States Parties in the peaceful exploration and use of outer space&quot;&lt;/em&gt; in the &lt;a href=&quot;https://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967#Article_IX&quot; rel=&quot;nofollow&quot;&gt;Article IX&lt;/a&gt; of the treaty, but is non-specific regarding any particular items / equipment that is supposed to cause this &lt;em&gt;harmful interference&lt;/em&gt;. Most of these &quot;space laws&quot; read as generally agreed upon principles regarding space exploration and use, but aren't specifically mentioning prohibited items.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This said, I personally wouldn't know of any other explicitly prohibited items, but the &lt;a href=&quot;http://www.oosa.unvienna.org/oosa/en/SpaceLaw/index.html&quot; rel=&quot;nofollow&quot;&gt;International Space Law&lt;/a&gt; is constantly evolving under the umbrella of &lt;a href=&quot;http://www.oosa.unvienna.org/oosa/en/OOSA/index.html&quot; rel=&quot;nofollow&quot;&gt;United Nations Office for Outer Space Affairs (UNOOSA)&lt;/a&gt; in form of international treaties, resolutions, and principles, so I suggest UNOOSA webpage as your first source of information. Naturally, I would be curious to read findings of any other answerers on this matter just as well, and my answer should by no means be considered as complete. Hope it helps, perhaps as a starting point, and others can shed a bit more light on the subject at hand.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-22T22:16:14.453" LastActivityDate="2015-07-22T22:16:14.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="162" PostTypeId="2" ParentId="2" CreationDate="2013-07-17T02:50:26.973" Score="20" Body="&lt;p&gt;(with considerable help from &lt;a href=&quot;http://io9.com/5984371/why-well-probably-never-build-a-space-elevator&quot;&gt;&quot;Why we'll probably never build a space elevator&quot;&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have laid out a good foundation, for the first, and largest challenge, namely the material for the cable itself. Carbon Nanbotubes are the best substance we know of to build a Space Elevator. In their purest form, they have a tensile strength of over 100 GPa. The exact number varies, but the general consensus is 130 GPa would result in a safe elevator. The next challenge beyond that is to build one that is the required length, and combine them into a workable cable, which isn't an easy task.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second significant obstacle is how to keep the cable stable. It would vibrate with time, which could make managing the cable quite difficult. Add in space debris already in orbit (a ton), and it would be extremely challenging. Keeping the cable steady while climbing would take a considerable amount of time, perhaps up to a month for the trip to orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there are the issues of where to build the space elevator, security, and other related issues. It's an extremely challenging problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some hope, principally in constructing an elevator at the Moon, Mars, or similar locations. In fact, one could be built around the Moon using technology easily available, and there are far fewer societal issues with that than would exist around the Earth. Around Earth would be quite difficult, at least, in the foreseeable future.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="76" LastEditDate="2013-12-16T12:59:23.563" LastActivityDate="2013-12-16T12:59:23.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="163" PostTypeId="2" ParentId="142" CreationDate="2013-07-17T02:51:06.513" Score="8" Body="&lt;p&gt;This would differ from country to country. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the items not allowed to be brought into onto shuttle programs, the items on board the shuttle are very mission specific and rarely are they allowed to carry personal items on board the shuttle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like the Space Shuttle program had a Personal Preference Kit or PPK for each crew member.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The contents of a PPK were limited to 20 separate items, with a total weight of 1.5 pounds and all of these items would be placed in a bag provided by NASA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz has a little looser allowances in this matter, each crew member could take up to 2.2 pounds of material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Separate from the PPK are crew care packages.  These are manifested by the psychological support teams and include personal items considered to be for the well being of the crew members, such as books, CDs, religious supplies, holiday decorations, and favorite condiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, some personal items can be manifested under necessary supplies like, a crew member can have their own clothes included as part of their clothing allowance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And these packages are checked and once cleared off by NASA's staff. Where they check for any breakable objects. Such as pencils(hence people use special pens in space). or weapons of any sort(That weigh less than 1.5 Pounds).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This weight limit that the Operating bodies have, makes it difficult or rather impossible for them to carry any sort of weapons on board. So the only real danger from the things brought on board by crew members are breakable things which are avoided. Especially because of the lack of gravity, things floating around can hurt crew members &lt;/p&gt;&#xA;" OwnerUserId="70" LastActivityDate="2013-07-17T02:51:06.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="164" PostTypeId="1" AcceptedAnswerId="242" CreationDate="2013-07-17T02:52:55.250" Score="19" ViewCount="514" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/197/881&quot;&gt;As we've already determined&lt;/a&gt;, Curiosity should last about 14 years on its nuclear power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens near that timeframe? Will NASA start to manage the rover's use of power by shutting down unused tools? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-02-04T17:39:23.023" Title="How will Curiosity manage its power when its nuclear power starts to run low?" Tags="|curiosity|energy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="165" PostTypeId="5" CreationDate="2013-07-17T02:56:46.580" Score="0" Body="&lt;p&gt;Curiosity is the name of a rover that NASA launched to Mars on November 26, 2011 as part of its Mars Science Laboratory mission. It runs on nuclear power and has an expected lifespan of about fourteen years. Its original mission was planned for two years, but has been extended indefinitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mars.nasa.gov/msl/mission/science/objectives/&quot; rel=&quot;nofollow&quot;&gt;Curiosity's mission objectives&lt;/a&gt; include the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Determine the nature and inventory of organic carbon compounds&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Inventory the chemical building blocks of life (carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Identify features that may represent the effects of biological processes&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Investigate the chemical, isotopic, and mineralogical composition of the martian surface and near-surface geological materials&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Interpret the processes that have formed and modified rocks and soils&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Assess long-timescale (i.e., 4-billion-year) atmospheric evolution processes&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Determine present state, distribution, and cycling of water and carbon dioxide&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Characterize the broad spectrum of surface radiation, including galactic cosmic radiation, solar proton events, and secondary neutrons&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h3&gt;Additional Resources:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mars.nasa.gov/msl/&quot; rel=&quot;nofollow&quot;&gt;Mars Science Laboratory - NASA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mars.nasa.gov/msl/news/pdfs/MSL_Fact_Sheet.pdf&quot; rel=&quot;nofollow&quot;&gt;Mars Science Laboratory/Curiosity Fact Sheet - NASA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_(rover)&quot; rel=&quot;nofollow&quot;&gt;Curiosity - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Science_Laboratory&quot; rel=&quot;nofollow&quot;&gt;Mars Science Laboratory - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="10234" LastEditDate="2015-07-15T20:27:53.160" LastActivityDate="2015-07-15T20:27:53.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="166" PostTypeId="4" CreationDate="2013-07-17T02:56:46.580" Score="0" Body="Questions regarding the Curiosity rover launched by NASA for the purpose of Mars exploration." OwnerDisplayName="user40" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:09:18.257" LastActivityDate="2013-10-26T15:09:18.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="167" PostTypeId="2" ParentId="14" CreationDate="2013-07-17T02:58:46.370" Score="26" Body="&lt;p&gt;Yes.  Lunar regolith contains oxygen -- about 50% by weight.  Specifically, you can heat (perhaps with solar energy) &lt;a href=&quot;https://en.wikipedia.org/wiki/Ilmenite&quot;&gt;Ilmenite&lt;/a&gt; and liberate oxygen.  The reaction requires hydrogen, but the hydrogen is not consumed by the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, the reaction is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FeTiO&lt;sub&gt;3&lt;/sub&gt; + H&lt;sub&gt;2&lt;/sub&gt; → FeTiO&lt;sub&gt;2&lt;/sub&gt; + H&lt;sub&gt;2&lt;/sub&gt;O → H&lt;sub&gt;2&lt;/sub&gt; + ½O&lt;sub&gt;2&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Engineering a reliable system on the moon to do this is, of course, challenging but certainly viable.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="17" LastEditDate="2013-07-19T15:20:43.027" LastActivityDate="2013-07-19T15:20:43.027" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="168" PostTypeId="1" AcceptedAnswerId="183" CreationDate="2013-07-17T03:07:45.580" Score="19" ViewCount="2577" Body="&lt;p&gt;As of July 2013, the New Horizons Long Range Reconnaissance Imager (LORRI) can resolve Pluto and Charon as separate objects. From what I've read, it will still be some time until the spacecraft is close enough to provide higher resolution imagery than the Hubble Space Telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the expected timetable for this to occur?  Also, what sort of work would the New Horizons team be able to do at that time?&lt;/p&gt;&#xA;" OwnerUserId="62" LastEditorUserId="87" LastEditDate="2013-07-17T08:32:50.693" LastActivityDate="2013-12-17T18:44:34.893" Title="At what approximate distance will the New Horizons spacecraft resolve Pluto better than the Hubble Space Telescope?" Tags="|new-horizons|pluto|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="169" PostTypeId="2" ParentId="27" CreationDate="2013-07-17T03:15:36.110" Score="3" Body="&lt;p&gt;Using Voyager as your example, the vast majority of the fuel (hydrazine) was used for attitude control -- pointing the spacecraft at various targets.  This of course includes pointing at the Earth for communication with ground control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft, including Voyager, often use a star tracker (&lt;a href=&quot;http://www.ballaerospace.com/page.jsp?page=104&quot; rel=&quot;nofollow&quot;&gt;like this one from Ball Aerospace&lt;/a&gt;) to determine their attitude.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-17T03:15:36.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="170" PostTypeId="1" AcceptedAnswerId="186" CreationDate="2013-07-17T03:16:35.223" Score="12" ViewCount="753" Body="&lt;p&gt;I have read a lot of literature concerning manned missions to the Moon, Mars, and the Jovian moons.  Something I have not heard much about, despite it's relative distance and &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/e/e5/Ceres_Cutaway.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;interesting composition&lt;/a&gt;, is a manned flight to Ceres.&lt;/p&gt;&#xA;&lt;p&gt;What sort of engineering challenges would be posed by landing on Ceres? The diagram provided by Wikipedia suggests a thin, dusty crust with an inner layer of water-ice.  Could a landing conceivably disrupt the crust in that area ?&lt;/p&gt;&#xA;" OwnerUserId="62" LastEditorUserId="19335" LastEditDate="2020-06-27T15:37:47.943" LastActivityDate="2020-06-27T19:15:37.407" Title="What engineering challenges would be posed by a manned mission to Ceres?" Tags="|crewed-spaceflight|ceres|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="171" PostTypeId="2" ParentId="11" CreationDate="2013-07-17T03:20:10.517" Score="14" Body="&lt;p&gt;At the risk of not answering your question directly, I will suggest to you that you consider two other, better figures of merit for propulsive efficiency than distance and speed: &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific impulse&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust&quot; rel=&quot;nofollow noreferrer&quot;&gt;thrust&lt;/a&gt;.  These two measures are more meaningful because they impact the key hurdle in space travel: energy.  A good illustration of this is available &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Delta-Vs_for_inner_Solar_System.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;on Wikipedia here&lt;/a&gt;.  Notice how the Moon and Mars are about the same energy-distance from LEO.&lt;/p&gt;&#xA;&lt;p&gt;Two points in space may be far apart but require very little energy to move between.  Conversely, two points in space may be relatively close but require a large amount of energy to move between.&lt;/p&gt;&#xA;&lt;p&gt;Chemical rockets will probably always provide higher thrust and ion engines will probably always provide higher specific impulse.  These measures then usually map into transit time and mass efficiency.  If you must have a fast transit time, chemical rockets will usually win up to the practical limit imposed by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket equation&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann transfer orbits&lt;/a&gt;.  From then on, it is all ion drive and slow, spiraling trajectories.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="12180" LastEditDate="2021-02-07T16:57:22.333" LastActivityDate="2021-02-07T16:57:22.333" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="172" PostTypeId="5" CreationDate="2013-07-17T03:20:37.663" Score="0" Body="&lt;p&gt;The only recognized dwarf planet in the Asteroid Belt is named Ceres.&lt;/p&gt;&#xA;" OwnerUserId="36462" LastEditorUserId="36462" LastEditDate="2020-07-07T16:17:34.807" LastActivityDate="2020-07-07T16:17:34.807" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="173" PostTypeId="4" CreationDate="2013-07-17T03:20:37.663" Score="0" Body="Questions about the only recognized dwarf planet in the main asteroid belt." OwnerUserId="36462" LastEditorUserId="36462" LastEditDate="2020-07-07T16:17:46.080" LastActivityDate="2020-07-07T16:17:46.080" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="175" PostTypeId="1" AcceptedAnswerId="181" CreationDate="2013-07-17T03:39:36.023" Score="13" ViewCount="10957" Body="&lt;p&gt;On March 1, 1982, the Venera 13 lander survived for 127 minutes in an environment with a temperature of 457 °C (855 °F) and a pressure of 89 Earth atmospheres (9.0 MPa).  This says nothing of the sulfuric acid in the atmosphere.  If we use that temperature and pressure as an example, are there any materials that would be better candidates for a future landing vehicle on Venus?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if the material would only increase the mission duration by a couple of hours, I am interested in knowing what sort of material advances we have made since the Venera missions.&lt;/p&gt;&#xA;" OwnerUserId="62" LastEditorUserId="25" LastEditDate="2013-09-26T12:26:49.370" LastActivityDate="2020-09-26T08:54:30.103" Title="What material properties would be necessary to shield a lander from the environment of the Venusian surface?" Tags="|materials|venus|engineering|environment|thermal|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="176" PostTypeId="1" CreationDate="2013-07-17T03:42:41.293" Score="35" ViewCount="27282" Body="&lt;p&gt;In terms of practicality, which of the planets proposed for terraforming would require the least resources to make fully habitable? In other words, which would be the most practical given technology available today or in the foreseeable future?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be somewhere like Mars, which is already relatively hospitable but large, or something smaller but more alien, like Ceres?&lt;/p&gt;&#xA;" OwnerUserId="47" LastEditorUserId="25" LastEditDate="2016-09-14T14:48:59.517" LastActivityDate="2016-09-14T14:48:59.517" Title="Which planet or moon would be most feasible to terraform?" Tags="|colonization|terraforming|" AnswerCount="5" CommentCount="3" ClosedDate="2016-09-14T14:52:50.783" ContentLicense="CC BY-SA 3.0" />
  <row Id="177" PostTypeId="1" AcceptedAnswerId="184" CreationDate="2013-07-17T03:48:30.440" Score="9" ViewCount="739" Body="&lt;p&gt;There are lots of engineering challenges associated with long-term space flight.  Many of these are driven by the humans on board the vehicles -- specifically the mitigations required to control health risks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the health risks facing humans on long-term space flights?&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="68" LastEditDate="2013-07-24T16:49:15.600" LastActivityDate="2013-07-24T16:49:15.600" Title="What are the health risks facing humans on long-term space flights?" Tags="|health|long-duration|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="178" PostTypeId="2" ParentId="2" CreationDate="2013-07-17T04:01:35.813" Score="9" Body="&lt;p&gt;The technological barriers to building a space elevator are utterly dependent on the site selected.  By this I mean which planet or moon you are &quot;elevating&quot; from.  This is directly related to the mass of the body and its diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, building a terrestrial space elevator is probably several decades off - at least.  But, the technologies exist today to build a &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_space_elevator&quot; rel=&quot;noreferrer&quot;&gt;lunar space elevator&lt;/a&gt;.  Unlike a terrestrial space elevator, a lunar space elevator has much lower tensile strength requirements that are well within today's technologies limits.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorDisplayName="user12" LastEditDate="2013-07-22T00:49:08.850" LastActivityDate="2013-07-22T00:49:08.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="179" PostTypeId="5" CreationDate="2013-07-17T04:04:22.737" Score="0" Body="&lt;p&gt;The term is sometimes used more generally as a synonym for planetary engineering, although some consider this more general usage an error. The concept of terraforming developed from both science fiction and actual science. The term was coined by Jack Williamson in a science-fiction story (&quot;Collision Orbit&quot;) published during 1942 in Astounding Science Fiction, but the concept may pre-date this work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on experiences with Earth, the environment of a planet can be altered deliberately; however, the feasibility of creating an unconstrained planetary biosphere that mimics Earth on another planet has yet to be verified. Mars is usually considered to be the most likely candidate for terraforming. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. Several potential methods of altering the climate of Mars may fall within humanity's technological capabilities, but at present the economic resources required to do so are far beyond that which any government or society is willing to allocate to the purpose. The long timescales and practicality of terraforming are the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics, and methodology of altering the environment of an extraterrestrial world.&lt;/p&gt;&#xA;" OwnerUserId="6" LastEditorUserId="6" LastEditDate="2013-07-17T08:31:45.673" LastActivityDate="2013-07-17T08:31:45.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="180" PostTypeId="4" CreationDate="2013-07-17T04:04:22.737" Score="0" Body="Questions regarding the artificial creation of Earth-like conditions on another celestial body." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-21T21:01:42.990" LastActivityDate="2013-09-21T21:01:42.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="181" PostTypeId="2" ParentId="175" CreationDate="2013-07-17T04:36:04.783" Score="11" Body="&lt;p&gt;We have plenty of metallic materials that could stand the heat of Venus's atmosphere, including copper, nickel, cobalt, iron, titanium, tungsten, and chromium, to name but a few (&lt;a href=&quot;http://www.lenntech.com/periodic-chart-elements/melting-point.htm&quot;&gt;here's a list of elemental melting points&lt;/a&gt;), as well as a large number of alloys including carbon steel and stainless steel. Even the sulfuric acid isn't a huge problem with some of these metals (tungsten is entirely inert to the stuff), and some alloys perform even better at this. It appears that the standard material for working with sulfuric acid here on Earth is carbon steel, even in extremely high temperature ranges. (From &lt;a href=&quot;http://www.stress.com/pdf/coke/TB_acid_materials_mfg.pdf&quot;&gt;this report&lt;/a&gt;--an analysis of the various metals currently used in sulfuric acid manufacturing furnaces)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the metallic components of a Venus-bound spacecraft really aren't the problem. The limiting factor seems to be the electronics. Even though silicon wont melt at those temperatures, heating up a circuit board to 457°C together with sulfuric acid has repercussions, and will quickly ruin most of the circuitry on the craft. With our current slate of electronics-worthy materials, we are not going to be able to make a circuit board that could survive on Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, we &lt;em&gt;can&lt;/em&gt; insulate the important electronics, and much research has been done on that front since the last Venus mission. In particular, it appears that &lt;a href=&quot;http://www.sbir.gov/sbirsearch/detail/355873&quot;&gt;aerogel materials&lt;/a&gt; are being looked at to insulate the circuitry from the combined force of the heat and the sulfuric acid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I can tell, this technology is probably advanced enough to make a craft that, in a perfect scenario, could function fine on Venus for a fairly long time, maybe even as long as the Mars rovers. But there are always kinks to be worked out, and I doubt that we could reasonably expect anything as long-lasting as the Mars rovers for a while.&lt;/p&gt;&#xA;" OwnerUserId="47" LastEditorUserId="47" LastEditDate="2013-07-17T06:05:37.833" LastActivityDate="2013-07-17T06:05:37.833" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="182" PostTypeId="1" CreationDate="2013-07-17T04:56:45.253" Score="17" ViewCount="1354" Body="&lt;p&gt;Additive manufacturing is currently being proposed as a way to greatly decrease the cost of establishing a permanent (robotic or human) presence on the Moon or Mars by sending only 3D printers to establish a &quot;settlement&quot; (at least basic shelters). This would prevent having to send raw materials and/or expensive/bulky items out of the Earth gravitational field. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the current 3D printing technology good enough to achieve this goal?&#xA;If not, what needs to be overcome? I would think protecting the printer from dust storms would be the most difficult task...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This could be seen as the first step towards one of the holy grail of space exploration, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Self-replicating_spacecraft&quot;&gt;Von Neumann probe&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="71" LastEditorUserId="12102" LastEditDate="2019-06-23T22:56:47.173" LastActivityDate="2019-06-23T22:56:47.173" Title="Is current state-of-the-art additive manufacturing (aka 3D printing) good enough to build parts out of Moon/Mars dust?" Tags="|mars|the-moon|construction|regolith|reduced-gravity-manufacturing|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="183" PostTypeId="2" ParentId="168" CreationDate="2013-07-17T05:05:53.423" Score="21" Body="&lt;p&gt;A bit of good old Google-fu reveals &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/0218-new-horizons-pluto-better-hubble.html&quot; rel=&quot;noreferrer&quot;&gt;this blog post&lt;/a&gt; by &lt;a href=&quot;http://www.planetary.org/about/staff/emily-lakdawalla.html&quot; rel=&quot;noreferrer&quot;&gt;Emily Lakdawalla&lt;/a&gt; for when &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;New Horizon&lt;/a&gt;'s LORRI imagery of Pluto is expected to be of better resolution than that of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt;'s &lt;em&gt;Wide Field Camera 3 (WFC3)&lt;/em&gt; (relevant excerpts only, no copyright infringement intended):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;So New Horizons' vision is is about 25 times less sharp than Hubble's.&#xA;  Therefore, New Horizons will have to be about 25 times closer to Pluto&#xA;  than Earth is before its images will begin to improve on Hubble's.&#xA;  Right now it's &quot;only&quot; 4 times closer to Pluto than Earth is. It still&#xA;  has a long way to go!&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Pluto will appear 3 pixels across to LORRI when New Horizons is about&#xA;  160 million kilometers away. How did I get that number? Pluto is 2400&#xA;  kilometers across, so 3 pixels means each pixel is 800 kilometers.&#xA;  Divide that by 5 microradians -- which is the same as dividing it by 5&#xA;  and then tacking the word &quot;million&quot; on the end -- and you get 160&#xA;  million kilometers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Her calculations regarding the date when that's supposed to happen (1st of January, 2015) are a bit off from dates quoted in Wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Key_mission_dates&quot; rel=&quot;noreferrer&quot;&gt;New Horizons' key mission dates&lt;/a&gt;, though:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Feb 2015    Observations of Pluto begin&#xA;            New Horizons is now close enough to Pluto for the main science mission to begin.&#xA;5 May 2015  Better than Hubble&#xA;            Images exceed best Hubble Space Telescope resolution.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;This discrepancy suggests there were quite some approximations involved in the calculation of the date and uncertainty on which of the two calculations is more correct, or which of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt;'s cameras is meant in the Wiki page adds to confusion, but math behind the calculations of actual distance when this will happen (as per your question) appear correct (although the numbers used mightn't be). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This obviously called for a peer review, and Derek Szymanski was more exact in the comments to the mentioned blog, and uses more precise resolution numbers for &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt;'s &lt;em&gt;Wide Field Camera 3 (WFC3)&lt;/em&gt; (~ 2.68 pixels wide photographs of Pluto), coming up with a distance of &lt;strong&gt;roughly 180 Million kilometers&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do note though these calculations are for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt;'s &lt;em&gt;Wide Field Camera 3 (WFC3)&lt;/em&gt;, and not the &lt;em&gt;ESA Faint Object Camera&lt;/em&gt;, which has approximately 5.3x finer resolution (by my calculations)&lt;sup&gt;*&lt;/sup&gt; and has produced this photograph:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/T2BEe.gif&quot; alt=&quot;ESA Faint Object Camera&amp;#39;s view of Pluto&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;ESA Faint Object Camera's views of Pluto and computed interpolations  (Credit: ESA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/hst_pr96_09a.html&quot; rel=&quot;noreferrer&quot;&gt;NASA's page describing this photograph&lt;/a&gt;, each pixel is over 150 km across. Taking diameter of Pluto into account (2390 km), this translates into resolution of roughly 16 x 16 pixels, and the photographs appears to confirm that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the other part of your question, &lt;em&gt;what sort of work would the New Horizons team be able to do at that time&lt;/em&gt;, this is pretty much when the LORRI images become more usable than HST's own observations, but observations of Pluto by LORRI begin months before that (Feb. 2015). Wiki provides these &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Pluto_flyby&quot; rel=&quot;noreferrer&quot;&gt;details on Pluto flyby mission objectives&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Primary objectives (required)&lt;/strong&gt;&lt;br /&gt;&#xA;  - Characterize the global geology and morphology of Pluto and Charon&lt;br /&gt;&#xA;  - Map chemical compositions of Pluto and Charon surfaces&lt;br /&gt;&#xA;  - Characterize the neutral (non-ionized) atmosphere of Pluto and its escape rate&lt;br /&gt; Loss of any of these objectives will constitute a failure&#xA;  of the mission.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Secondary objectives (expected)&lt;/strong&gt;&lt;br /&gt;&#xA;  - Characterize the time variability of Pluto's surface and atmosphere&lt;br /&gt;&#xA;  - Image select Pluto and Charon areas in stereo&lt;br /&gt;&#xA;  - Map the terminators (day/night border) of Pluto and Charon with high resolution&lt;br /&gt;&#xA;  - Map the chemical compositions of select Pluto and Charon areas with high resolution&lt;br /&gt;&#xA;  - Characterize Pluto's ionosphere, and its interaction with the solar wind&lt;br /&gt;&#xA;  - Search for neutral species such as H2, hydrocarbons, HCN and other nitriles in the atmosphere&lt;br /&gt;&#xA;  - Search for any Charon atmosphere&lt;br /&gt;&#xA;  - Determine bolometric Bond albedos for Pluto and Charon&lt;br /&gt;&#xA;  - Map surface temperatures of Pluto and Charon&lt;br /&gt; It is expected, but not demanded, that most of these objectives will be met.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Tertiary objectives (desired)&lt;/strong&gt;&lt;br /&gt;&#xA;  - Characterize the energetic particle environment at Pluto and Charon&lt;br /&gt;&#xA;  - Refine bulk parameters (radii, masses) and orbits of Pluto and Charon&lt;br /&gt;&#xA;  - Search for additional moons, and any rings&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More can be read &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Pluto_flyby&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;*&lt;/sup&gt; &lt;strong&gt;Correction&lt;/strong&gt;: My calculations on the resolution of WFC3 vs. FOC were fairly accurate, considering they were based on approximate data and visual observations alone, but I've nevertheless found more precise resolution difference (slightly in favor of FOC), which is in the order of 5.55x, with WFC3's resolution of 0.04 arcsecond pixels as mentioned before vs. &lt;a href=&quot;http://www.spacetelescope.org/about/general/instruments/foc/&quot; rel=&quot;noreferrer&quot;&gt;FOC's 0.0072 arcsecond pixels&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-17T18:44:34.893" LastActivityDate="2013-12-17T18:44:34.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="184" PostTypeId="2" ParentId="177" CreationDate="2013-07-17T05:13:01.083" Score="5" Body="&lt;p&gt;There are a number of potential health risks humans face in outer space, though probably the two most important are: microgravity and cosmic radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Microgravity:&lt;/strong&gt; Even if the ultimate destination of a long space flight is somewhere with reasonable gravity (IE: Mars), the humans on board a long-distance space flight would still experience very low gravity for an extended period of time. From experiments conducted on the ISS, and similar orbital stations, we know that microgravity causes humans to experience bone degeneration, muscular atrophy, and suppressed immune response, for starters. Fortunately, we can mitigate the effects of this slightly by having people exercise regularly and wearing heavy clothing, but it still can cause problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Cosmic Radiation:&lt;/strong&gt; Outside of the Earth's magnetic protection, astronauts on a long-distance flight would be exposed to a lot of high-energy cosmic radiation. This can increase the risk of getting cancer, and generally decrease immune function. Fortunately, the levels of this radiation are ordinarily pretty low, so an astronaut could make it from here to about Jupiter with our current technology before they passed NASA's safety standards for radiation. The problem comes when there's an intense burst of radiation all at once from solar flares. A human, exposed unprotected to a solar flare, would get fried pretty fast from it and die from even a small exposure. Fortunately, however, water (it's actually the Hydrogen in the water) is an excellent insulator from cosmic radiation, and most solar flares give at least some advance warning, so astronauts could essentially lock themselves in the food/water storage area for a few days and wait it out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither of these problems are insurmountable, which is why long-distance space flight is even a possibility, but these are certainly two very important issues that must be kept in mind when planning for this type of mission.&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2013-07-17T05:13:01.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="185" PostTypeId="1" AcceptedAnswerId="223" CreationDate="2013-07-17T05:37:55.487" Score="28" ViewCount="5393" Body="&lt;p&gt;According to International Law, to what altitude does the jurisdiction of a state extend? Is there a definite limit mandated either through treaties or court judgements? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After reaching that limit, do we find ourselves in the outer space regulated by space law? Is there a layer of international space, regulated by other treaties?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a very practical question: To what altitude may launched spacecraft or satellite ascend/descend without violating the air territory of a sovereign state?&lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastEditorUserId="288" LastEditDate="2013-07-28T14:32:31.050" LastActivityDate="2021-03-24T19:59:35.003" Title="At what height does it become international territory / outer space, instead of state-controlled air territory?" Tags="|launch|law|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="186" PostTypeId="2" ParentId="170" CreationDate="2013-07-17T05:38:25.343" Score="10" Body="&lt;p&gt;Landing on Ceres would probably be much like landing on the Moon or Mars--it's mostly flat probably with a few craters, so it wouldn't be that novel of an event. And we've sent unmanned missions far further than Ceres, so propulsion obviously isn't an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The single biggest engineering obstacle that would have to be worked out would simply be figuring out how to keep humans alive and healthy over the course of such a long flight. Ceres is roughly four times as far away from Earth as Mars is at its closest (270 million km vs 54 million km), and would take well over a year even with our fastest technology. At that point, simply having the space for enough &lt;em&gt;food&lt;/em&gt; for a few astronauts is a daunting task, especially if you have to worry about a return trip.&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2013-07-17T05:38:25.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="187" PostTypeId="1" AcceptedAnswerId="194" CreationDate="2013-07-17T05:46:40.940" Score="10" ViewCount="1914" Body="&lt;p&gt;Is it possible to prevent muscular atrophy in microgravity (space station, spacecraft etc.) by proper exercises?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Urban dwellers here on Earth can train/develop arm muscles to the equivalent of their ancestors - who used those muscles far more. It could be theoretically possible to substitute &quot;working&quot; with legs under Earth gravitation by intense exercises with specialized fitness equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any successes with preparing such set of exercises? Or, is exercise deemed trivial - do astronauts have enough time to exercise so hard while on space station? &lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastEditorUserId="288" LastEditDate="2013-09-21T13:56:30.270" LastActivityDate="2013-09-23T17:40:49.767" Title="Is it possible to prevent muscular atrophy in microgravity with proper exercises?" Tags="|astronauts|health|microgravity|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="188" PostTypeId="5" CreationDate="2013-07-17T05:47:30.740" Score="0" Body="&lt;p&gt;Astronauts are people who are trained to travel in a spacecraft. Depending on the country, they could also be called cosmonauts‎, spationauts, or taikonauts‎. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first person to travel to space was Yuri Gagarin of the Soviet Union. He completed his first orbit on 12 April 1961. Since then more than 560 people have been trained as astronauts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All questions related to specific astronauts, or astronauts in general, from any nation are appropriate for this tag. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Information:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Astronaut&quot; rel=&quot;nofollow&quot;&gt;Astronaut - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_astronauts_by_name&quot; rel=&quot;nofollow&quot;&gt;List of Astronauts by Country - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.howmanypeopleareinspacerightnow.com/&quot; rel=&quot;nofollow&quot;&gt;How Many People Are In Space Right Now?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-14T16:55:06.583" LastActivityDate="2015-07-14T16:55:06.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="189" PostTypeId="4" CreationDate="2013-07-17T05:47:30.740" Score="0" Body="Questions regarding people present on spacecraft or space stations." OwnerDisplayName="user43" LastEditorDisplayName="user12" LastEditDate="2013-08-08T14:18:34.617" LastActivityDate="2013-08-08T14:18:34.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="190" PostTypeId="5" CreationDate="2013-07-17T05:48:07.037" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T05:48:07.037" LastActivityDate="2013-07-17T05:48:07.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="191" PostTypeId="4" CreationDate="2013-07-17T05:48:07.037" Score="0" Body="Questions about the impact of the absence of weight in space on humans and equipment. " OwnerDisplayName="user43" LastEditorDisplayName="user12" LastEditDate="2013-09-21T21:25:12.990" LastActivityDate="2013-09-21T21:25:12.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="192" PostTypeId="5" CreationDate="2013-07-17T05:48:58.900" Score="0" Body="&lt;p&gt;Questions about impact of radiation on space exploration, such as on humans and equipment.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-22T06:32:46.330" LastActivityDate="2013-07-22T06:32:46.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="193" PostTypeId="4" CreationDate="2013-07-17T05:48:58.900" Score="0" Body="Questions about impact of radiation on space exploration." OwnerDisplayName="user43" LastEditorDisplayName="user12" LastEditDate="2013-07-22T06:32:17.703" LastActivityDate="2013-07-22T06:32:17.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="194" PostTypeId="2" ParentId="187" CreationDate="2013-07-17T05:59:46.577" Score="10" Body="&lt;p&gt;On the International Space Station, crewmembers regularly exercise to help maintain their muscle/bone mass. A number of scientific studies have been done (&lt;a href=&quot;http://jap.physiology.org/content/106/4/1159&quot;&gt;here's one&lt;/a&gt;) on the effectiveness of their exercise regimen, and it appears that even with their rather modest amount of exercise (5 hr/wk), the decrease in muscle mass is slight and more of a muscle relocation than a loss anyway. Exercise regimens have improved since that study was published, and other techniques, such as wearing a heavy shirt, have been proposed to boost the effects of exercise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So long story short, yes, exercise does help prevent bone/muscle loss in microgravity situations. While we haven't perfected the techniques yet, we've probably refined the exercise regimens required so that even if astronauts might lose some muscle, it likely won't result in any long-term damage.&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2013-07-17T05:59:46.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="196" PostTypeId="1" AcceptedAnswerId="234" CreationDate="2013-07-17T06:28:55.950" Score="19" ViewCount="2571" Body="&lt;p&gt;Are there plans for another space station when the ISS is deorbited or de-manned? What are the details of these plans, if any?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus: How would a space station be built without the Space Shuttle?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user12" LastEditDate="2013-09-17T23:05:51.103" LastActivityDate="2016-03-31T17:27:02.507" Title="Is there an ISS replacement planned?" Tags="|iss|space-station|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="197" PostTypeId="2" ParentId="144" CreationDate="2013-07-17T06:30:10.080" Score="30" Body="&lt;p&gt;Because of the nature of radioactive decay, Curiosity is going to get a whole lot more than 2 years out of its power source. Curiosity will be getting enough power to keep active for a bare minimum of 14 years (&lt;a href=&quot;http://en.wikipedia.org/wiki/Curiosity_%28rover%29&quot;&gt;see &lt;em&gt;Specifications&gt; Power Source&lt;/em&gt;&lt;/a&gt;), though the amount of power it has will steadily decrease over time (from 125 Watts initially to 100 Watts after 14 years). I don't think the battery is likely to be the limiting factor in this case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: The two year figure cited in Undo's answer is actually only the time of the rover's first mission, though it will continue to be reassigned new missions until it eventually gives out.&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2013-07-17T06:30:10.080" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="198" PostTypeId="1" AcceptedAnswerId="201" CreationDate="2013-07-17T06:31:59.883" Score="6" ViewCount="153" Body="&lt;p&gt;We are lucky on Earth, the ozone layer protects us from UV-C by absorbing wavelengths below about 280nm.  However, the same can not be said for in space and on places like Luna and Mars.  I would imagine that generating and sustaining an artificial ozone layer would not be feasible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What materials would provide adequate protection from UV-C, while allowing UV-B (needed for vitamin D production) to penetrate into a habitat?&lt;/p&gt;&#xA;" OwnerDisplayName="user92" LastEditorUserId="6537" LastEditDate="2018-01-12T09:51:39.937" LastActivityDate="2018-01-12T09:51:39.937" Title="What would an effective means be of protecting a habitat from UV-C radiation?" Tags="|habitat|humans|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="199" PostTypeId="1" CreationDate="2013-07-17T07:31:53.500" Score="4" ViewCount="1364" Body="&lt;p&gt;Commercial cubesat cameras are a bit expensive:&#xA; &lt;a href=&quot;http://www.cubesatshop.com/index.php?page=shop.product_details&amp;amp;flypage=flypage.tpl&amp;amp;product_id=63&amp;amp;category_id=16&amp;amp;option=com_virtuemart&amp;amp;Itemid=80&quot; rel=&quot;nofollow&quot;&gt;http://www.cubesatshop.com/index.php?page=shop.product_details&amp;amp;flypage=flypage.tpl&amp;amp;product_id=63&amp;amp;category_id=16&amp;amp;option=com_virtuemart&amp;amp;Itemid=80&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what are the experiences with using cheaper commercial off-the-shelf cameras regarding radiation and low temperatures in LEO.&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorUserId="25" LastEditDate="2013-09-24T12:53:27.090" LastActivityDate="2013-09-24T12:53:27.090" Title="What would be the most cost effective camera solution for CubeSat satellite" Tags="|cubesat|camera|remote-sensing|" AnswerCount="1" CommentCount="0" ClosedDate="2013-07-17T18:26:49.563" ContentLicense="CC BY-SA 3.0" />
  <row Id="200" PostTypeId="1" CreationDate="2013-07-17T07:34:32.277" Score="3" ViewCount="500" Body="&lt;p&gt;I think the Radiation beyond earth will be the greatest challenge for space exploration. what are other factors we face in this area?&lt;/p&gt;&#xA;" OwnerUserId="73" LastActivityDate="2013-07-17T11:22:04.723" Title="What are the hurdles we face in deep space exploration?" Tags="|deep-space|" AnswerCount="2" CommentCount="0" ClosedDate="2013-07-17T11:25:38.037" ContentLicense="CC BY-SA 3.0" />
  <row Id="201" PostTypeId="2" ParentId="198" CreationDate="2013-07-17T07:43:59.580" Score="5" Body="&lt;p&gt;Glasses can be manufactured such that they transmits the desired parts of the UV. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the picture below (found &lt;a href=&quot;http://www.shimadzu.com/an/industry/ceramicsmetalsmining/chem0501005.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;), together with a somewhat official definition of &lt;a href=&quot;http://en.wikipedia.org/wiki/Ultraviolet&quot; rel=&quot;noreferrer&quot;&gt;ultraviolet wavelength ranges&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;UVA: 400-315 nm&lt;/li&gt;&#xA;&lt;li&gt;UVB: 315-280 nm&lt;/li&gt;&#xA;&lt;li&gt;UVC: 280-100 nm&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rqqAL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;it is clear that most ordinary glasses are mostly opaque to UVC, and start being partially transparent from UVB up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From that same wiki page: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ordinary glass is partially transparent to UVA but is opaque to&#xA;  shorter wavelengths, whereas silica or quartz glass, depending on&#xA;  quality, can be transparent even to vacuum UV wavelengths.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A mixture of various glass types (possibly with coatings) can thus be manufactured to allow any desired portion of UVA, UVB and UVC through.&lt;/p&gt;&#xA;" OwnerUserId="97" LastActivityDate="2013-07-17T07:43:59.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="202" PostTypeId="2" ParentId="200" CreationDate="2013-07-17T07:50:02.870" Score="1" Body="&lt;p&gt;There are many dangers in space and some hurdles are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;food supplies: linked with the long distances, the time taken to travel implies lots of food for the crew&lt;/li&gt;&#xA;&lt;li&gt;metheorites and other obstacles: they can be a real pain in the neck and cause quite a lot damage&lt;/li&gt;&#xA;&lt;li&gt;distances: everything is far away in space&lt;/li&gt;&#xA;&lt;li&gt;the return voyage: if needed&lt;/li&gt;&#xA;&lt;li&gt;costs: who will pay for all the technology and materials?&lt;/li&gt;&#xA;&lt;li&gt;low gravity: can a human go a very long time with nearly no gravity?&lt;/li&gt;&#xA;&lt;li&gt;time: We don't live long enough to reach anywhere far away&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To name just a few.&lt;/p&gt;&#xA;" OwnerUserId="87" LastEditorUserId="87" LastEditDate="2013-07-17T10:28:10.673" LastActivityDate="2013-07-17T10:28:10.673" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="203" PostTypeId="1" AcceptedAnswerId="211" CreationDate="2013-07-17T07:54:39.433" Score="18" ViewCount="6084" Body="&lt;p&gt;Due to the large amount of energy required, the standard method for leaving the Earth's atmosphere is rocket propulsion. Is there any other methods in development that don't involve rockets? E.g. solar sails, space elevators?&lt;/p&gt;&#xA;" OwnerUserId="65" LastEditorUserId="49" LastEditDate="2014-06-01T21:05:10.343" LastActivityDate="2015-08-02T08:48:04.917" Title="Methods for propulsion other than rockets for leaving Earth's atmosphere?" Tags="|launch|energy|propulsion|rockets|spacecraft-development|" AnswerCount="6" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="204" PostTypeId="1" CreationDate="2013-07-17T08:03:51.640" Score="12" ViewCount="237" Body="&lt;p&gt;The processing power in current exploration rovers like the MER's RAD6000 (20Mhz) and Curiosity's RAD750 (200Mhz) is far below anything you would find in any mobile phone today. As far as I understand this is mainly due to the radiation hardening and possibly temperature ranges. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would happen if you used terrestrial processors and other electronic components for a rover on a lunar mission? Could you use other measures to prevent these effects (e.g. shielding)?&lt;/p&gt;&#xA;" OwnerUserId="99" LastActivityDate="2013-07-17T12:00:50.550" Title="What shielding would be required to use commercial components in lunar exploration?" Tags="|radiation|electronics|" AnswerCount="2" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="205" PostTypeId="1" AcceptedAnswerId="217" CreationDate="2013-07-17T08:07:14.810" Score="13" ViewCount="783" Body="&lt;p&gt;If you're going on a space walk, you need to be shielded from cosmic rays, temperature, and possibly other issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What components of cosmic rays are dangerous? What sort of protection is used to shield from these as well as from the temperature issues?&lt;/p&gt;&#xA;" OwnerUserId="96" LastEditorUserId="25" LastEditDate="2013-09-26T12:31:14.843" LastActivityDate="2013-09-28T02:19:59.670" Title="What level of protection is required for spacewalkers?" Tags="|radiation|spacesuits|shielding|eva|protection|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="206" PostTypeId="2" ParentId="176" CreationDate="2013-07-17T08:21:54.510" Score="7" Body="&lt;p&gt;Here are a list of properties I consider to be needed to make a planet Earth-like:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Solid or liquid surface at an atmospheric pressure of about 1 bar.&lt;/li&gt;&#xA;&lt;li&gt;Atmospheric constituents suitable for plants, animals, fungi, bacteria and so on to survive unaided. This basically means copying Earth's ratios as close as possible.&lt;/li&gt;&#xA;&lt;li&gt;Tectonic activity or technological equivalent, to replenish the atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Gravity similar to Earth's.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The &lt;strong&gt;hardest thing to change&lt;/strong&gt; about a planet in terms of bringing it close to Earth in all properties that matter to Humans, &lt;strong&gt;would be its gravity&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whilst the atmosphere of Mars is closer to Earth's at the present time, it will always be one fifth of the mass of Earth, and will therefore never (unless we add mass to it) be close to Earth's gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, I am of the opinion that &lt;strong&gt;Venus would be a better terraforming target&lt;/strong&gt;. Thinning out its atmosphere, e.g. by dispersing bacteria into the air, which will turn gasses into solids or liquids to then rain down to the surface, or by putting a big Mylar parasol in front of the planet to cool it down and freeze-out some molecules, would be comparatively much easier then smashing hundreds of moons into Mars to make it more massive.&lt;/p&gt;&#xA;" OwnerUserId="101" LastEditorUserId="101" LastEditDate="2015-02-10T14:25:36.010" LastActivityDate="2015-02-10T14:25:36.010" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="208" PostTypeId="1" AcceptedAnswerId="405" CreationDate="2013-07-17T08:28:27.147" Score="9" ViewCount="379" Body="&lt;p&gt;With the Hubble telescope to be decommissioned any time now, and the James Webb Space Telescope at least in 2018 and unlikely to be active while Hubble still remains, will there be a gap when we have no space telescope at all? Or is there (planned, or present) another one to fill in?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2013-07-18T18:34:40.943" LastActivityDate="2016-11-11T07:59:22.703" Title="What is the current state of affairs concerning space telescopes?" Tags="|telescope|astronomy|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="209" PostTypeId="1" AcceptedAnswerId="225" CreationDate="2013-07-17T08:32:07.117" Score="16" ViewCount="2187" Body="&lt;p&gt;I see two parallels to the forthcoming lunar exploration and human colonisation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The influx of Europeans to America in the 17th Century.&lt;/li&gt;&#xA;&lt;li&gt;The division of, and disputes about, ownership of Antarctica.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In the first, nation-states who were at war in Europe just continued warring in the New World. To the victor went the spoils. In the second, a less belligerent kind of diplomacy has left us with disputed, overlapping claims.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will countries (or corporations) go to war over its resources? Will ownership of it simply be handed to a non-profit like IANA by a body such as the United Nations, and this non-profit then divvy up the moon on a need-by-need basis?&lt;/p&gt;&#xA;" OwnerUserId="101" LastEditorUserId="25" LastEditDate="2013-07-18T17:36:51.750" LastActivityDate="2016-08-11T06:18:03.653" Title="Who owns the moon?" Tags="|the-moon|law|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="210" PostTypeId="2" ParentId="51" CreationDate="2013-07-17T08:42:20.133" Score="3" Body="&lt;p&gt;According to wikipedia, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;nofollow&quot;&gt;delta-v neccessary&lt;/a&gt; to reach low earth orbit is 9.4 km/s. This is roughly 30 times the speed of sound. Launched from a cannon, a payload would need enough initial velocity to have that speed at the top of the atmosphere, after encountering several km of athmospheric drag. I did not work out the math of what that means for initial velocity, but it will be tremendous. While there are laboratory guns that fire very fast projectiles  - &lt;a href=&quot;http://www.nytimes.com/1994/03/22/science/fastest-gun-on-earth-goals-go-beyond-planet.html&quot; rel=&quot;nofollow&quot;&gt;up to 16 km/s&lt;/a&gt; - those fire only little specks of dust. Scaling up the technology will be hard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To take a look at the numbers and g-forces, at 1000 g constant acceleration we would need 1 seconds to reach 9.8 km/s, and a barrel of 500 m length (!). The speed was picked to arrive at round numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A projectile from a cannon has it's highest speed right at launch, where the athmosphere is thickest and thus the drag the highest, but would not need to lift the mass of the fuel. I think it's hard, but doable, to calculate which technology may be more energy efficient for a given payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To sum it up, the speeds needed are beyond what is achievable with common cannons and the like, we need different approaches that may be doable but have not been implemented near the scale of what is needed to shoot a payload into space.&lt;/p&gt;&#xA;" OwnerUserId="102" LastActivityDate="2013-07-17T08:42:20.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="211" PostTypeId="2" ParentId="203" CreationDate="2013-07-17T08:46:15.470" Score="12" Body="&lt;p&gt;Currently existing, and for leaving the atmosphere? No. It's always a rocket engine of one kind of another, whether for vertical launch or horizontal, solid fuel or oxygen-hydrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Developed - definitely yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar sail is only viable in space, considering solar wind it catches is a miniscule fraction of physical (air) wind force, meaning it would be torn to shreds in the upper atmosphere and never provide enough thrust to overcome earth gravity. Same for the ion engine which has minimal thrust nowhere near to overcoming earth gravity, but can produce that thrust for years at a time using up minimal amounts of energy and fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is a number of planned methods of launch that wouldn't involve rocket engines. There are already a few groups working on the space elevator plans - we're quite far from it, because the only viable material - nanotubes - despite dirt cheap raw materials, are still prohibitively expensive to manufacture. There was a plan to use antarctic ice to build a launch tube similar to multi-stage cannon, to launch vehicles diagonally using land-bound explosive charges; that was stalled due to environmental concerns. There was a plan for a &quot;partial space elevator&quot; - a rope orbiting the Earth at LEO, with trailing end reachable by vehicles with air-breathing jet engines, docking their payload to it, then the payload lifted to the orbit and launching from the other end of the rope, using space propulsion (solar sail, ion drive) from there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with these all is that while operating them would be significantly cheaper than rocket engines - per-launch cost a fraction of current, their initial cost outweighs the cost of a rocket-powered vehicle many times over. With current economy, they are all on hold or proceeding at glacial pace with only minor, inexpensive side tasks completed, while the bulk of the work awaits funding.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2015-08-02T08:48:04.917" LastActivityDate="2015-08-02T08:48:04.917" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="212" PostTypeId="5" CreationDate="2013-07-17T08:46:27.443" Score="0" Body="&lt;p&gt;Relating to treaties, laws, ownership and disputes during the exploration of space. For non-space exploration related law questions, see our sister site &lt;a href=&quot;https://law.stackexchange.com/&quot;&gt;Law Stack Exchange&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10273" LastEditorUserId="10273" LastEditDate="2020-07-01T10:48:12.560" LastActivityDate="2020-07-01T10:48:12.560" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="213" PostTypeId="4" CreationDate="2013-07-17T08:46:27.443" Score="0" Body="Relating to treaties, laws, ownership and disputes during the exploration of space. For non-space exploration related law questions, see our sister site Law Stack Exchange." OwnerUserId="101" LastEditorUserId="10273" LastEditDate="2020-07-01T10:47:57.723" LastActivityDate="2020-07-01T10:47:57.723" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="214" PostTypeId="2" ParentId="182" CreationDate="2013-07-17T09:05:01.397" Score="10" Body="&lt;p&gt;Definitely not 3D printing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You should look for the mirror side of 3D manufacturing: substractive methods like CNC milling. Take a solid lunar rock, carve it into any arbitrary shape you desire, construct whatever you wish from carved components, add minuscule amounts of organic sealant brought from Earth. Use solar furnace to melt metal, cast it in CNC-carved rock die into rough shape with surplus metal, finish off to precise shape using a multi-axis CNC mill, weld using solar-powered electric welder (vacuum makes for a very pleasant environment for welding, no oxidation problems.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3D printing materials are fickle and hard to obtain. Most likely once rudimentary base is established, and enough power produced to create glass (or artificial lighting, if glass covers are not viable) to grow plants, stuff like corn-based plastic could become a thing. Until then, plastic, which is pretty much essential for 3D printing is a luxury; meanwhile metal and stone which is the daily bread of milling, is a commodity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe with enough hard work, a 3D printer that utilizes metal or glass as its work material could be doable, but I'm afraid the technological problems would make it prohibitively complex and likely fault-prone.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2013-07-17T12:29:56.017" LastActivityDate="2013-07-17T12:29:56.017" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="215" PostTypeId="1" AcceptedAnswerId="218" CreationDate="2013-07-17T09:54:47.837" Score="17" ViewCount="614" Body="&lt;p&gt;Do Lagrange points ever move or are they on the same spot as long as the solar system exists in it's current form with all it's planets in its current order?.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They could move due to orbital objects relative distance to each other. For example the moon is (slowly) moving away from earth, does such a movement (of an orbital object, not 'just' the moon) have an effect on the position and stability of Lagrange points?&lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorUserId="25" LastEditDate="2013-07-18T18:43:16.620" LastActivityDate="2013-11-17T04:53:14.287" Title="Are Lagrange points stable over a long period of time?" Tags="|orbit|lagrangian-points|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="216" PostTypeId="1" AcceptedAnswerId="386" CreationDate="2013-07-17T10:12:53.747" Score="15" ViewCount="1897" Body="&lt;p&gt;This refers to the sterilization of space exploring tools like rovers and probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is important to not send out rovers that contain any bacteria, because another rover, from another mission, could find traces of such bacteria and scientists could conclude there would have been life previously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is it accomplished that space-rovers, space shuttles and other equipment is clean before it is launched?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose a probe has to return something to Earth for analyzing it on the ground. How is ensured it is safe to be handled by humans, not being contaminated by any bacteria?&lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorUserId="47" LastEditDate="2013-07-17T17:01:06.710" LastActivityDate="2013-07-18T15:55:57.093" Title="How is equipment sterilized for space missions?" Tags="|environment|forward-contamination|sterilization|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="217" PostTypeId="2" ParentId="205" CreationDate="2013-07-17T10:22:47.773" Score="8" Body="&lt;p&gt;A &lt;a href=&quot;http://news.google.com/newspapers?nid=1817&amp;amp;dat=19950917&amp;amp;id=7jsdAAAAIBAJ&amp;amp;sjid=MaYEAAAAIBAJ&amp;amp;pg=3480,5230453&quot;&gt;newspaper article&lt;/a&gt; from september 1995 says that even when the astronauts &lt;em&gt;tried&lt;/em&gt; to get cold, by staying in the shadow of the shuttle out in the vacuum of space they were still warm and comfortable.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But to NASA's delight, Endeavour's two spacewalking astronauts kept warm Saturday thanks to new heated gloves, thermal socks, boot liners and toasty long underwear.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In this scenario the spacewalk lasted at least an hour at temperatures as cold as -120 or -130&#xA;degrees Fahrenheit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This kept them warm even though&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the spacewalkers remained motionless on the robot arm in an attempt to get cold&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Though this apparently had no effect as at one point one of the spacewalkers&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;had to turn off the fingertip heaters in his gloves&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This information is probably quite old as this occurred about 18-19 years ago and technology is likely to have advanced. Though i imagine the basics would be the same as it appears relatively simplistic to maintain heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the front of protection from radiation on spacewalks it seems like, other than the suit, there is none!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://resources.yesican-science.ca/trek/radiation/final/assignment_reading6-1.html&quot;&gt;website&lt;/a&gt; talks about the plans for the EVARM badges to be attached to various parts of the space suit to allow measurement of exposure to radiation to see if any further protection is needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, I told a small lie, there is one other form of protection that they get from radiation other than the space suit. Timing. Much like launches, NASA make sure to monitor the sun for solar flares and generally make sure there is no astronauts about when one is going off. Making sure to only send them out into space when radiation is at its lowest provides one of the highest forms of protection at this time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Coming from a hopefully more recent, though perhaps not entirely reliable, source we have this &lt;a href=&quot;http://science.howstuffworks.com/space-suit1.htm&quot;&gt;HowStuffWorks article&lt;/a&gt; on spacesuits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a section on temperature control which talks about how they layer the fabrics of the space suit to ensure as much heat is kept in as possibble. It also bring up the interesting point of heat removal, heat builds up seemingly easily and if not released on occassion then the astronaut risks dehydration and a foggy visor from perspiration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cooling systems range from fans which blow air to water cooled systems to remove heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That same article also gives us a little more insight onto how they are protected from radiation. It shows that the spacesuit is reflective to purposefully reduce the amount of radiation. It concurs with my previous point about spacewalks being planned to avoid solar flares. Being caught in a solar flare in just a space suit would be about as effective as trying to walk on the sun wearing tin foil and leaves.&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastActivityDate="2013-07-17T10:22:47.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="218" PostTypeId="2" ParentId="215" CreationDate="2013-07-17T10:29:13.750" Score="13" Body="&lt;p&gt;The Lagrange points are not some physical objects, just locations where gravity of other bodies coincides to be of specific value/&quot;shape&quot;, and as the strength of gravity field changes, so do the point move; obviously whatever affects the gravity does influence these points, like trajectory of given body that generates them, its mass (influenced by capturing meteorites or losing mass due to massive impacts), other passing bodies etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course such changes are rather infrequent and/or slow (or minuscule) and so the points change rarely and by not much; also, L4 and L5, being stable, will drag whatever fell into them as they shift following the changes, unless the changes are very drastic (say, given celestial body split into multiple pieces by a massive impact.) and other bodies passing by rapidly shift them too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, never to forget, they permanently travel along with the orbit of the body - they are &quot;almost immobile&quot; in relation to the Earth-Sun system, but that system is mobile!&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2013-11-17T04:53:14.287" LastActivityDate="2013-11-17T04:53:14.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="219" PostTypeId="1" AcceptedAnswerId="348" CreationDate="2013-07-17T10:29:48.153" Score="22" ViewCount="940" Body="&lt;p&gt;Life on a space station is very confined. For months on end, astronauts are kept in a structure that's about the size of a football field on the outside and &lt;em&gt;much&lt;/em&gt; smaller on the inside. The nearest viewable objects beyond 100 yards are on Earth's surface, 230 miles below. This leaves a &lt;em&gt;lot&lt;/em&gt; of distance in which the astronauts generally do not have objects to regularly focus their eyes on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this affect an astronaut's ability to focus on things within that range after returning to Earth? For example, might they have more trouble focusing on distant vehicles or signs while driving on the highway?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there anything the astronauts do, or could be doing, to compensate for or prevent such problems?&lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorUserId="49" LastEditDate="2014-03-08T09:24:25.227" LastActivityDate="2014-03-10T19:56:08.150" Title="Does long-term residence on the ISS affect eyesight?" Tags="|iss|health|humans|long-duration|ocular|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="220" PostTypeId="2" ParentId="209" CreationDate="2013-07-17T10:48:25.217" Score="6" Body="&lt;p&gt;Strictly speaking, no one owns the moon, per the &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon_Treaty&quot;&gt;Moon Treaty&lt;/a&gt;, and should be administered by international law (AKA, the United Nations). Of course, the Moon Treaty hasn't been ratified by any of the major players in Manned Spaceflight, so it strictly doesn't apply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Practically speaking, I would assume the moon will follow the squatters rights principal, he who first lands on the Moon will claim some reasonable area for them to do their work, and so on. When enough people have done this, then a better framework will be in place to manage it, much as was done in most periods of colonization.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-17T10:48:25.217" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="221" PostTypeId="1" AcceptedAnswerId="230" CreationDate="2013-07-17T10:49:37.273" Score="24" ViewCount="9614" Body="&lt;p&gt;Most of &lt;a href=&quot;https://en.wikipedia.org/wiki/Category%3aArtificial_satellites_orbiting_the_Moon&quot;&gt;artificial satellites of the Moon&lt;/a&gt; has depleted their batteries, lost communication, been deorbited or impacted the surface after orbital decay. The list doesn't exactly make it clear which ones are still functional and active. Could you provide the current, complete list of active lunar orbiters and what functions they fulfill?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="14" LastEditDate="2013-07-18T18:47:18.917" LastActivityDate="2016-12-02T21:12:53.440" Title="Which artificial satellites in lunar orbit are currently active?" Tags="|artificial-satellite|the-moon|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="222" PostTypeId="2" ParentId="209" CreationDate="2013-07-17T10:50:11.843" Score="5" Body="&lt;p&gt;Have a look at my answer to a similar question: &lt;a href=&quot;https://space.stackexchange.com/questions/22/have-any-countries-laid-claim-to-territory-in-space/31#31&quot;&gt;Have any countries laid claim to territory in space?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is most likely that property claims will eventually cause trouble. Today, the Moon is just too far away for causing any trouble, so it is save to ignore it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Antarctica is a useful example for this philosophy. Although there are tons of territorial claims there and although Argentina for instance even keeps a military presences manned at all time, disputes have never resulted in military action yet. Instead, Antarctica became protected under the United Nation's Antarctica treaty.  The fun part is, that it is just technologically complicated to make use of resources there, so nobody defended claims. On the contrary, the treaty explicitly becomes &quot;open for review&quot; in the 2040s - once technology has seen a significant improvement. Guess, what will happen next ... &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-07-18T17:47:54.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="223" PostTypeId="2" ParentId="185" CreationDate="2013-07-17T10:53:09.380" Score="10" Body="&lt;p&gt;The general rule of thumb on this type of issue is that if it is in orbit, then you don't have to worry about traveling through other countries, and if it is not yet in orbit, you should ask permission from whatever country you are flying through. For this reason, and safety reasons in general, most spacecraft launch from the coast. Thus, a sub-orbital flight is still maintained by the country below it, but an orbital flight would not be.&lt;/p&gt;&#xA;&lt;p&gt;Of course, like ships, Orbiting vehicles are subject to their registered nation, but that's another issue entirely.&lt;/p&gt;&#xA;&lt;p&gt;A few other things of some interest:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Geostationary satellites are theoretically governed by the country around which they orbit, to an extent. However, the countries that have tried to maintain this claim can't enforce it, so it hasn't been a significant issue. Equatorial countries try to make such claims, but they really aren't enforceable, so...&lt;/li&gt;&#xA;&lt;li&gt;Once you go beyond the surface of the Moon, then international law about what frequency you can use no longer applies. However, if you plan on talking back to Earth, you should coordinate. This could be used for a special purpose radar orbiting Mars, for instance.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2021-03-24T19:59:35.003" LastActivityDate="2021-03-24T19:59:35.003" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="224" PostTypeId="2" ParentId="208" CreationDate="2013-07-17T10:56:31.607" Score="5" Body="&lt;p&gt;While Hubble and the James Webb Space Telescope are not the only telescopes in space, let me make a statement about &quot;another one to fill in&quot;. Please note the complexity of space telescopes and the time needed for their development. The ideas and first concepts for Hubble came up at the same time the development of the Space Shuttle started. This was essentially around 1970. It took two decades to build and launch it and even a few more years to bring it to its full capacity. The time required for the development of the James Webb Space Telescope is at similar orders of magnitude. With decades for building such stuff, it is hard to talk about another one just &quot;filling&quot; in. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-17T10:56:31.607" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="225" PostTypeId="2" ParentId="209" CreationDate="2013-07-17T10:56:40.997" Score="11" Body="&lt;p&gt;While obviously the question does not have a single distinct answer There are some existing examples that can lead us to some conclusions&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per the &lt;a href=&quot;http://history.nasa.gov/1967treaty.html&quot; rel=&quot;nofollow&quot;&gt;Outer Space Treaty of 1967&lt;/a&gt; - No one owns the moon; &quot;Outer space, including the moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Antarctic being similar to the moon, in having no aborigines, and being rather difficult to get to and survive at, has an existing treaty and some &lt;a href=&quot;http://en.wikipedia.org/wiki/Territorial_claims_in_Antarctica&quot; rel=&quot;nofollow&quot;&gt;conflicting claims&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly as pointed out the fictional work &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/The_Man_Who_Sold_the_Moon&quot; rel=&quot;nofollow&quot;&gt;The Man Who Sold the Moon&lt;/a&gt;&quot; by Robert A. Heinlein, if you want to own it, all you have to do is buy it. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2015-06-22T10:15:40.423" LastActivityDate="2015-06-22T10:15:40.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="226" PostTypeId="2" ParentId="55" CreationDate="2013-07-17T11:10:12.043" Score="14" Body="&lt;p&gt;First of all, let me give you what the typical requirements are for a launch, so we can compare them to your hypothetical situation. Managing a launch requires keeping an eye on where your rocket will be at any time, and what would happen to it if it exploded. Most launch facilities impose &quot;Range Safety&quot; requirements which must be demonstrated prior to launch, which minimize the damage that would be caused in the event of an incident. This has resulted in several rockets being blown up prematurely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this &lt;a href=&quot;http://www.hq.nasa.gov/pao/History/SP-4204/ch9-6.html&quot;&gt;NASA website&lt;/a&gt;, if a spacecraft deviates more than 3 sigma away from it's desired course, it is deemed a malfunctioning rocket, and subject to self destruction. Of course, if it has obtained orbit, it will not be subject to this requirement. The flight path is determined such that this will not happen over any major city. All of this work is done to reduce the likelihood of an incident.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do not do this level of calculation, then you are subject to liability. If you launched from an island on the Pacific that was unclaimed, and an issue resulted, I'm sure that the lawyers could find a way to sue you, and would. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you do not need to apply for an international permit to launch. Take North Korea, which has had several launches in spite of strong international opposition to their launches. However, if they did actual damage to people as a result, they would be dealt with in an even stronger manner than they had previously been.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-17T11:10:12.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="227" PostTypeId="2" ParentId="204" CreationDate="2013-07-17T11:13:14.777" Score="5" Body="&lt;p&gt;It is a bit a question of philosophy - between a zero-fault system or a fault-tollerant system. A typical terrestrial processor would cause a higher number of faults during computation, such as bit flips etc. leading to unpredictable results. Yes, one can shield them to some degree. But it is much easier to build the computer and its software so that they can handle such glitches. This is essentially what is done in a number of academic satellite projects. Those projects either just can not effort radiation-hardened hardware or are just not allowed to buy it (dual-use issues). Although academic satellites are mostly sent to low Earth orbit only, they have also an increased problem with radiation and solar activity. Nevertheless, a wide range of cheap embedded and even desktop CPUs have been successfully used. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-17T11:13:14.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="228" PostTypeId="1" AcceptedAnswerId="235" CreationDate="2013-07-17T11:14:18.893" Score="12" ViewCount="801" Body="&lt;p&gt;I have heard about people paying lots of money to become Russian cosmonauts, essentially space tourists. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it still possible to 'buy your way' into space? What sort of training would be required for a &quot;space tourist&quot;?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-07-28T01:01:45.137" LastActivityDate="2013-10-18T12:07:27.870" Title="How could I travel to space as an ordinary citizen?" Tags="|tourism|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="229" PostTypeId="2" ParentId="200" CreationDate="2013-07-17T11:22:04.723" Score="0" Body="&lt;p&gt;&lt;em&gt;Funding!&lt;/em&gt; Funding! &lt;strong&gt;Funding!&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From an engineering perspective, there is not any problem which can not be overcome. Ironically, the hugest problem causing the most and longest discussions in every (deep) space project every single time is cash. How are we going to build this or that with this little amount of money? It is the hardest &quot;engineering&quot; constrain of all. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-17T11:22:04.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="230" PostTypeId="2" ParentId="221" CreationDate="2013-07-17T11:23:54.237" Score="21" Body="&lt;p&gt;Orbiting the moon is notoriously difficult for long periods of time. The lunar gravity field varies by as much as 1%. There has been some work to maximize the lifetime, but it hasn't been particularly successful to date. For a complete technical explanation, see &lt;a href=&quot;http://www.cs.odu.edu/~mln/ltrs-pdfs/tp3394.pdf&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;. The bottom line is, there are 4 orbits which one could orbit to stay for an extended period of time, namely 27º, 50º, 76º, and 86º (See also &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_orbit&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&quot; rel=&quot;noreferrer&quot;&gt;Science@NASA&lt;/a&gt;). Given this, there aren't many satellites that are even orbiting the moon, let alone functioning. It is difficult to tell which satellites have crashed into the moon, but it is safe to say that any spacecraft orbiting from more than 20 years ago will have crashed in to the moon already.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only known spacecraft currently orbiting the moon arethe &lt;a href=&quot;http://lunar.gsfc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Lunar Reconnaissance Orbiter (LRO)&lt;/a&gt;, as well as the two &lt;a href=&quot;https://en.wikipedia.org/wiki/THEMIS#ARTEMIS&quot; rel=&quot;noreferrer&quot;&gt;ARTEMIS probes&lt;/a&gt;, which are functioning. Beyond that, it is unlikely that any spacecraft is still orbiting, let alone functioning. There are plans for a few more launches in the next few years, but for now, it is only these 3.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-12-02T21:12:53.440" LastActivityDate="2016-12-02T21:12:53.440" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="231" PostTypeId="1" AcceptedAnswerId="631" CreationDate="2013-07-17T11:27:48.733" Score="21" ViewCount="5634" Body="&lt;p&gt;Obviously the asteroid belt is pretty sparsly populated. But how sparsly exactly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the state of knowledge regarding how many particles are there in a given size bracket and a volume, what would be typical distances between asteroids in a given size bracket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit to add:&lt;br&gt;&#xA;The answers so far talk about asterois sized around a km. I was thinking also about smaller asteroids or even dust, down to the scale of a mm. I understand that we may know little about such small asteroids.&lt;br&gt;&#xA;Anyway, an answer that gives me the mean free paths for several different sizes, all to within an order of magnitude, would be great.&lt;/p&gt;&#xA;" OwnerUserId="102" LastEditorUserId="49" LastEditDate="2013-11-21T01:45:20.887" LastActivityDate="2018-04-03T16:13:10.967" Title="What's the (particle) density of the asteroid belt?" Tags="|asteroid|solar-system|meteorites|space-dust|asteroid-belt|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="232" PostTypeId="2" ParentId="199" CreationDate="2013-07-17T11:31:16.283" Score="1" Body="&lt;p&gt;In all honesty, you might want to ask the right people. I have my doubts that you can find all of them here right now. I suggest you join this mailing list: &lt;a href=&quot;http://www.cubesat.org/index.php/about-us/how-to-join&quot; rel=&quot;nofollow&quot;&gt;http://www.cubesat.org/index.php/about-us/how-to-join&lt;/a&gt; There, you basically find members of most cubesat-projects in existence. From experience, quiet a lot of projects did a lot of interesting stuff, but the level of documentation is usually rather low. So there is basically no way around asking the people directly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: If you receive a good answer, I would be glad if you could post it here! &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-17T11:31:16.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="233" PostTypeId="1" CreationDate="2013-07-17T11:32:43.623" Score="19" ViewCount="2747" Body="&lt;p&gt;The Lagrangian Points are points in space, where the combination of gravitational pull of a set of two bodies and the centripetal force of orbiting one of them add up to zero. The special property of $L_4$ and $L_5$ points - in Earth-Sun system, located on Earth orbit a sixth of its length away from Earth, in leading and trailing direction respectively, is that they are actually &lt;em&gt;stable&lt;/em&gt; - while, similarly to other Lagrangian Points they are local maxima of gravity field, unstable gravity-wise, the Coriolis Force of the set of the bodies creates local minimum there, overcoming the force of gravity locally and making them act similar to gravity wells; anything that gets sufficiently near and moving sufficiently slowly, will remain there indefinitely (or until impacted by a sufficiently fast meteorite). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That makes them very interesting from space exploration point of view, as potential points where e.g. deep space observatory unaffected by requirement of constant rotation around Earth could reside; also they are expected to have collected quite a few meteorites and shed some light on Earth history, and generally provide a very valuable point at stable distance from Earth and not just barely over its surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I'm aware the sum of forces keeping bodies inside $L_4$ and $L_5$ is fairly &lt;em&gt;weak&lt;/em&gt;. I'm interested &lt;em&gt;how&lt;/em&gt; weak it is are. Since they are just empty points in space, and not planets with own gravity, obviously quite a few measurements are not applicable, but I guess we could get something indicative, like what is the escape speed from these points' &quot;force well&quot;, what is the centripetal acceleration at the &quot;steepest part of their slopes&quot; or such.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm quite interested if a space station located there would require active stabilizing to prevent escaping them, or opposite, they would cost significant extra fuel and energy just to escape them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[see comments for discussion - this question used to ask about &quot;gravity well&quot; of these points, but it was shown they don't actually have any, being maxima like others; still, Coriolis Force seems to act like gravity for all practical purposes there, and I'd be quite interested in learning how strong it is there.]&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="168" LastEditDate="2013-08-11T11:08:58.177" LastActivityDate="2015-02-13T16:30:15.460" Title="How deep is the force well of L4 and L5 Lagrangian Points of Earth-Sun set?" Tags="|orbital-mechanics|gravity|lagrangian-points|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="234" PostTypeId="2" ParentId="196" CreationDate="2013-07-17T11:38:35.660" Score="16" Body="&lt;p&gt;For specifically the ISS, there are no replacement plans yet. Besides, China is about to set-up its own (big) space station. In a way, this could become the next habitable place in low Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You do not need a space shuttle for building a space station. MIR was built without the shuttle. Similar to the segments of the Russian part of the ISS, the blocks of MIR were all their own independent space crafts, including individual attitude and orbit control systems. Docking could be done via remote control or even automatically.&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="80" LastEditDate="2013-07-17T15:20:01.783" LastActivityDate="2013-07-17T15:20:01.783" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="235" PostTypeId="2" ParentId="228" CreationDate="2013-07-17T11:51:50.737" Score="8" Body="&lt;p&gt;I think you are looking for Space Adventures Ltd. They handle the &quot;tourists&quot; on Russian Soyuz flights to the ISS: &lt;a href=&quot;http://www.spaceadventures.com/&quot; rel=&quot;nofollow&quot;&gt;http://www.spaceadventures.com/&lt;/a&gt; This is the only business &lt;strong&gt;currently&lt;/strong&gt; capable of actually sending you to space for cash. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-10-18T12:07:27.870" LastActivityDate="2013-10-18T12:07:27.870" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="236" PostTypeId="2" ParentId="204" CreationDate="2013-07-17T12:00:50.550" Score="5" Body="&lt;p&gt;An alternative to hardened processors is the concept of multiple redundant negotiating processors, where each processor negotiates its view of the world with the others, so if one suffers a bit flip it can be corrected by consensus, or even shut down if it is faulty. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space shuttle used a basic version of this with 3 computers needing to agree, or if necessary 2 out of 3 winning. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course hardening and simplification can provide provable robustness, whereas using a complex configuration with 9 high end processors will have to cope with frequent incidents requiring correction. &lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-17T12:00:50.550" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="237" PostTypeId="1" AcceptedAnswerId="266" CreationDate="2013-07-17T12:05:17.103" Score="39" ViewCount="15318" Body="&lt;p&gt;What concerns, besides orbit decay and continued cost of operation (and potentially, a critical failure) are the motivations behind decommissioning the ISS by 2020? I mean, I'm fairly sure it would be cheaper to develop a weak-thrust motor that would move the station to a higher orbit, instead of just dumping it and building a new one, and it's probably quite some time before it begins failing as badly as MIR would while still operating.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorDisplayName="user12" LastEditDate="2013-07-18T18:37:54.537" LastActivityDate="2015-11-03T05:31:33.433" Title="Why decommission the ISS?" Tags="|iss|end-of-life|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="238" PostTypeId="2" ParentId="231" CreationDate="2013-07-17T12:12:27.107" Score="4" Body="&lt;p&gt;Apart from the points were an actual asteroid is, the density is very low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be improbable to reach an asteroid without aiming carefully.&quot; (&lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid_belt&quot; rel=&quot;nofollow&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore the distribution of the asteroids is not uniform, thus the mean density might not be very characteristic.&lt;/p&gt;&#xA;" OwnerUserId="87" LastActivityDate="2013-07-17T12:12:27.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="239" PostTypeId="2" ParentId="237" CreationDate="2013-07-17T12:15:04.357" Score="14" Body="&lt;p&gt;I will use an analogy. As your car gets older it wears out. It gets more and more expensive to maintain as parts fail. At some point it is cheaper to get rid of it and buy a new car. This point may depend on how safe you feel in the car. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An old ramshackle rustbucket may be drivable slowly, despite having holes in the floor, broken suspension, no windscreen etc. , but the ISS is a much higher risk. If parts fail, there is a high likelihood that astronauts will die. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS is old, and the amount of maintenance is getting quite high-the schedule for each astronaut includes essential maintenance tasks. &lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-17T12:15:04.357" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="240" PostTypeId="2" ParentId="231" CreationDate="2013-07-17T12:38:42.343" Score="8" Body="&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/a/26714/17607&quot;&gt;This answer and question on physics&lt;/a&gt; cover a similar topic, I will link to it here and block quote the most relevant parts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most notable part of this answer is most likely going to be:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Asteroids are not distributed uniformly in the asteroid belt, but could be approximated to be evenly spaced in a region from 2.2 AU (1 AU is 93 million miles, or the average distance between Earth and the Sun) to 3.2 AU from the Sun and extending 0.5 AU above and below the ecliptic (the plane of Earth's orbit, which is a convenient reference for the solar system). That yields a volume of roughly 16 cubic AU, or about 13 trillion trillion cubic miles. (Note: space is big!)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So you can see that there is a huge gap in the asteroids, bigger than the distance between the earth and the sun by at least double! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As referenced by the answer on physics there is currently a NASA mission underway to venture to the asteroid belt to view it in more detail, relevant information for this mission can be found at:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://dawn.jpl.nasa.gov/mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://dawn.jpl.nasa.gov/mission/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2013-07-17T12:38:42.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="241" PostTypeId="1" AcceptedAnswerId="243" CreationDate="2013-07-17T12:41:29.910" Score="10" ViewCount="741" Body="&lt;p&gt;Other than accepting space tourists, that is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean, does the ISS perform any commissioned experiments for private-owned entities, manufacture products that require space conditions, produce commercially sold media, or otherwise &quot;earn money&quot;?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-17T13:13:53.493" Title="Does ISS perform any commercial operations?" Tags="|iss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="242" PostTypeId="2" ParentId="164" CreationDate="2013-07-17T13:13:41.730" Score="15" Body="&lt;p&gt;It will survive the way that pretty much all space missions survive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, most space missions are speced such that the power at the end of the mission will meet 100% of the demand of the spacecraft. The mission of Curiosity is 2 years, thus, the spacecraft will have 100% of required power after 2 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond that, its power will decrease. However, Curiosity has two batteries which will allow it to store charge, allowing it to still perform high power tasks, just for less time than it otherwise would be able to. So what will happen is the amount of moving, science, etc will be decreased over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The half life of the Plutonium 238 is 87.7 years. In effect, that means that the amount of power will only be halved every 87 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, the limiting factor will probably be the lithium ion batteries, and not the RTG. But the effect will still be the same, you can't use your instruments as much, or move as far, but you can still do science to some extent, as time continues.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="921" LastEditDate="2014-02-10T13:27:32.283" LastActivityDate="2014-02-10T13:27:32.283" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="243" PostTypeId="2" ParentId="241" CreationDate="2013-07-17T13:13:53.493" Score="8" Body="&lt;p&gt;Yes, commercial experiments happen on the ISS, but no, at least space agencies do not turn an explicit profit in business terms out of them. If you, as a company, want to run an experiment on-board the ISS, it is best to talk to your &quot;local&quot; space agency. Most of the stuff is done in cooperation between agencies and companies (and universities) anyway. Besides, research is intended to advance humankind and therefore your knowledge, so you can turn a profit out of it for sure. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-17T13:13:53.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="244" PostTypeId="2" ParentId="144" CreationDate="2013-07-17T13:15:36.677" Score="12" Body="&lt;p&gt;First of all, most space missions are speced such that the power at the end of the mission will meet 100% of the demand of the spacecraft. The mission of Curiosity is 2 years, thus, the spacecraft will have 100% of required power after 2 years. However, there comes to be a bigger question, does the spacecraft need 100% power to continue on?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond that, it's power will decrease. However, Curiosity has two batteries which will allow it to store charge, allowing it to still perform high power tasks, just for less time than it otherwise would be able to. So what will happen is the amount of moving, science, etc will be decreased over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, as Plutonium 238, the element used, has a half life of 87 years, the effect of the radioactive decay will be minimal. I would be more worried about the lithum ion battery than the RTG.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-17T13:15:36.677" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="245" PostTypeId="1" AcceptedAnswerId="384" CreationDate="2013-07-17T13:44:50.803" Score="8" ViewCount="258" Body="&lt;p&gt;It is well known that Russian manned spacecrafts gave birth to multiple unmanned satellite projects (such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Bion_%28satellite%29&quot;&gt;Bion&lt;/a&gt; spacecrafts, for example).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have Mercury/Gemini/Apollo spacecrafts or their direct descendant projects ever been used as unmanned satellite projects (not counting the test flights for the manned programs). &lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user12" LastEditDate="2013-11-05T04:19:10.530" LastActivityDate="2013-11-05T04:19:10.530" Title="Have Mercury/Gemini/Apollo produced any unmanned descendants?" Tags="|crewed-spaceflight|russia|uncrewed-spaceflight|united-states|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="246" PostTypeId="1" AcceptedAnswerId="279" CreationDate="2013-07-17T13:46:28.550" Score="10" ViewCount="695" Body="&lt;p&gt;There &lt;em&gt;is&lt;/em&gt; stuff floating around in space - everything from asteroids to planets to molecules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was thinking - would it be possible to harvest stray oxygen molecules, compress them, and get breathable oxygen?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="14" LastEditDate="2013-07-17T14:31:08.623" LastActivityDate="2017-02-03T09:23:12.050" Title="Are there enough oxygen molecules in space that they could be compressed into breathable oxygen?" Tags="|oxygen|interstellar-medium|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="247" PostTypeId="1" AcceptedAnswerId="436" CreationDate="2013-07-17T13:49:00.843" Score="52" ViewCount="5559" Body="&lt;p&gt;One might think that spacecraft would be on the cutting edge of technology.  However, when looking over details of spacecraft, it seems their computer systems are often very much behind the times.  For example, the Curiosity rover was launched in 2011, when consumer laptop systems ran in the GHz and had GB's of memory. Curiosity's CPU runs at 132 MHz and the system only has 256 MB of RAM (source: &lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_embedded_computer_systems_on_board_the_Mars_rovers&quot;&gt;http://en.wikipedia.org/wiki/Comparison_of_embedded_computer_systems_on_board_the_Mars_rovers&lt;/a&gt; ).  I realize there may be some lag from obtaining the parts for the spacecraft before it is built and ultimately launched, but this seems extreme. Why don't spacecraft have more up-to-date data systems?&lt;/p&gt;&#xA;" OwnerUserId="36" LastEditorUserId="9092" LastEditDate="2021-01-31T15:55:12.027" LastActivityDate="2021-01-31T15:55:12.027" Title="Why are spacecraft computers obsolete at launch?" Tags="|radiation|spacecraft-development|flight-computer|computer|reliability|" AnswerCount="9" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="249" PostTypeId="5" CreationDate="2013-07-17T13:51:02.680" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T13:51:02.680" LastActivityDate="2013-07-17T13:51:02.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="250" PostTypeId="4" CreationDate="2013-07-17T13:51:02.680" Score="0" Body="Questions about deep space travel and observations that are related to the distance between the object in question and the Earth. For example, questions about communications delay, radial resolution, means of propulsion through the interstellar medium, e.t.c. Deep space does not exclude space near or around other celestial bodies past our Solar system's heliosphere, and should not be confused with the term &quot;interstellar&quot;." OwnerUserId="-1" LastEditorUserId="49" LastEditDate="2013-08-23T18:49:03.487" LastActivityDate="2013-08-23T18:49:03.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="251" PostTypeId="5" CreationDate="2013-07-17T13:51:03.983" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T13:51:03.983" LastActivityDate="2013-07-17T13:51:03.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="252" PostTypeId="4" CreationDate="2013-07-17T13:51:03.983" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T13:51:03.983" LastActivityDate="2013-07-17T13:51:03.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="253" PostTypeId="5" CreationDate="2013-07-17T13:57:24.350" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T13:57:24.350" LastActivityDate="2013-07-17T13:57:24.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="254" PostTypeId="4" CreationDate="2013-07-17T13:57:24.350" Score="0" Body="Forward contamination is the transfer of microbial life from Earth to another celestial body. For the reverse process, see: back contamination. " OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-17T14:24:27.167" LastActivityDate="2013-07-17T14:24:27.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="255" PostTypeId="2" ParentId="247" CreationDate="2013-07-17T13:59:38.843" Score="34" Body="&lt;p&gt;A big part of it is reliability. NASA &lt;em&gt;could&lt;/em&gt; probably put in an Intel Xeon chip made in 2012 that has a crazy high amount of processing power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the &lt;a href=&quot;http://en.wikipedia.org/wiki/RAD750&quot; rel=&quot;noreferrer&quot;&gt;chip that was used&lt;/a&gt;, the RAD750, has years of experiments and usage behind it, such as being used in a variety of spacecraft including:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Deep Impact comet chasing spacecraft, launched in January 2005 - first to use the RAD750 computer.&lt;/li&gt;&#xA;  &lt;li&gt;XSS 11, small experimental satellite, launched April 11, 2005&lt;/li&gt;&#xA;  &lt;li&gt;Mars Reconnaissance Orbiter, launched August 12, 2005.&lt;/li&gt;&#xA;  &lt;li&gt;WorldView-1 satellite, launched Sept 18, 2007 - has two RAD750s.&lt;/li&gt;&#xA;  &lt;li&gt;Fermi Gamma-ray Space Telescope, formerly GLAST, launched June 11, 2008&lt;/li&gt;&#xA;  &lt;li&gt;Kepler space telescope, launched in March 2009.&lt;/li&gt;&#xA;  &lt;li&gt;Lunar Reconnaissance Orbiter, launched on 18 June, 2009&lt;/li&gt;&#xA;  &lt;li&gt;Wide-field Infrared Survey Explorer (WISE) launched 14 December, 2009&lt;/li&gt;&#xA;  &lt;li&gt;Solar Dynamics Observatory, launched Feb 11, 2010&lt;/li&gt;&#xA;  &lt;li&gt;Juno spacecraft, launched August 5, 2011&lt;/li&gt;&#xA;  &lt;li&gt;Curiosity rover, launched November 26, 2011&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Because of use since '05, NASA can be fairly confident that the chip won't fail due to radiation problems, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why? Well, I would say that &lt;a href=&quot;https://space.stackexchange.com/a/256&quot;&gt;John Besin's answer&lt;/a&gt; summed it up pretty well, and I won't try to top it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I wouldn't think this would be the case at all. If anything, NASA would want to use hardware (and software) that has been extensively tested throughout years of use, both in NASA and in industry as a whole. &lt;strong&gt;The last thing NASA wants is to find a bug in a spacecraft's system at an inopportune moment&lt;/strong&gt;, and when you’re talking about devices that need to travel potentially hundreds of thousands of miles through space, &lt;strong&gt;there are many inopportune moments&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2013-07-21T00:25:04.847" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="256" PostTypeId="2" ParentId="247" CreationDate="2013-07-17T13:59:53.553" Score="29" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;One might think that spacecraft would be on the cutting edge of technology.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wouldn't think this would be the case at all. If anything, NASA would want to use hardware (and software) that has been &lt;em&gt;extensively&lt;/em&gt; tested throughout years of use, both in NASA and in industry as a whole. The last thing NASA wants is to find a bug in a spacecraft's system at an inopportune moment, and when you’re talking about devices that need to travel potentially hundreds of thousands of miles through space, there are &lt;em&gt;many&lt;/em&gt; inopportune moments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might also find &lt;a href=&quot;https://softwareengineering.stackexchange.com/q/159637/96927&quot;&gt;this question on Programmers.SE&lt;/a&gt; interesting; it addresses the programming languages, hardware, etc. used to construct the Mars Curiousity rover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I imagine that the lower-spec hardware NASA uses has lower power requirements than cutting-edge, high-powered hardware. For example, if the rover doesn't &lt;em&gt;need&lt;/em&gt; a faster processor to operate, why waste space and weight by powering such a processor when a lower-spec one will suffice?&lt;/p&gt;&#xA;" OwnerUserId="114" LastEditorUserId="-1" LastEditDate="2017-04-12T07:31:59.047" LastActivityDate="2013-07-18T16:59:48.730" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="257" PostTypeId="5" CreationDate="2013-07-17T13:59:59.470" Score="0" Body="&lt;p&gt;The exact definition of a Low Earth Orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (therefore making at least 11.25 orbits per day) and an eccentricity less than 0.25. This is a common type of orbit as it is the easiest to get into and is generally useful for satellites that don't have to spend much time imaging or sending data between places on Earth. There are exceptions, such as Starlink is an internet system in LEO.&lt;/p&gt;&#xA;" OwnerUserId="46507" LastEditorUserId="7062" LastEditDate="2023-03-23T18:49:03.167" LastActivityDate="2023-03-23T18:49:03.167" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="258" PostTypeId="4" CreationDate="2013-07-17T13:59:59.470" Score="0" Body="Questions regarding Low Earth Orbit or LEO, which refers to orbits at an altitude between approximately 160 kilometers / 100 miles and 2,000 kilometers / 1,200 miles." OwnerUserId="14" LastEditorDisplayName="user43968" LastEditDate="2021-09-17T02:32:01.507" LastActivityDate="2021-09-17T02:32:01.507" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="259" PostTypeId="2" ParentId="246" CreationDate="2013-07-17T14:02:25.403" Score="6" Body="&lt;p&gt;&lt;em&gt;Theoretically&lt;/em&gt; yes. But it would take hell of a lot of time.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In all phases, the interstellar medium is extremely dilute by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 10&lt;sup&gt;6&lt;/sup&gt; molecules per cm&lt;sup&gt;3&lt;/sup&gt;. In hot, diffuse regions of the ISM, matter is primarily ionized, and the density may be as low as 10&lt;sup&gt;-4&lt;/sup&gt; ions per cm&lt;sup&gt;3&lt;/sup&gt;. Compare this with a number density of roughly 10&lt;sup&gt;22&lt;/sup&gt; molecules per cm&lt;sup&gt;3&lt;/sup&gt; for liquid water. By mass, 99% of the ISM is gas in any form, and 1% is dust. Of the gas in the ISM, 89% of atoms are hydrogen and 9% are helium, with 2% of atoms being elements heavier than hydrogen or helium, which are called &quot;metals&quot; in astronomical parlance. The hydrogen and helium are a result of primordial nucleosynthesis, while the heavier elements in the ISM are a result of enrichment in the process of stellar evolution. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(from &lt;a href=&quot;http://en.wikipedia.org/wiki/Interstellar_medium&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Interstellar_medium&lt;/a&gt;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also have a look at this source: &lt;a href=&quot;http://hypertextbook.com/facts/2000/DaWeiCai.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://hypertextbook.com/facts/2000/DaWeiCai.shtml&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closer you are to a star, as in e.g. the Sun, (or planet with an atmosphere) the more atoms, dust and molecules you may find. But the difference in orders of magnitude is the most relevant aspect. The amount of atoms per cubic meter here near Earth's surface is just out of proportion to the amount of atoms in space. So the question would be how much volume (with how much efficiency) you might need to harvest before you can use the gas for e.g. your living environment. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="897" LastEditDate="2017-02-03T09:23:12.050" LastActivityDate="2017-02-03T09:23:12.050" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="260" PostTypeId="1" AcceptedAnswerId="263" CreationDate="2013-07-17T14:14:40.410" Score="31" ViewCount="2797" Body="&lt;p&gt;Would it be technologically and economically practical to build and operate a space elevator that only provided transport between lower and higher orbital paths?&lt;/p&gt;&#xA;&lt;p&gt;Such a mechanism would only avoid &lt;em&gt;some&lt;/em&gt; of the issues with a fixed surface-orbit elevator and would introduce other issues such as needing to dock with a platform moving at a velocity different than the local orbital velocity.&lt;/p&gt;&#xA;&lt;h3&gt;Somewhat related&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2/what-technological-barriers-do-we-need-to-overcome-to-build-a-space-elevator&quot;&gt;What technological barriers do we need to overcome to build a space elevator?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/203/methods-for-propulsion-other-than-rockets-for-leaving-earth-atmosphere&quot;&gt;Methods for propulsion other than rockets for leaving earth atmosphere?&lt;/a&gt; (&lt;a href=&quot;https://space.stackexchange.com/questions/203/methods-for-propulsion-other-than-rockets-for-leaving-earth-atmosphere#211&quot;&gt;S.F.'s answer&lt;/a&gt; mentions a &amp;quot;partial space elevator&amp;quot; linking a jet-accessible platform to LEO.)&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-10-01T20:35:46.627" Title="Could a partial space elevator be practical and useful?" Tags="|orbital-mechanics|orbital-maneuver|conceptual|space-elevator|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="261" PostTypeId="5" CreationDate="2013-07-17T14:39:18.433" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T14:39:18.433" LastActivityDate="2013-07-17T14:39:18.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="262" PostTypeId="4" CreationDate="2013-07-17T14:39:18.433" Score="0" Body="Project Mercury was the first human spaceflight program of the United States." OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-17T16:11:11.653" LastActivityDate="2013-07-17T16:11:11.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="263" PostTypeId="2" ParentId="260" CreationDate="2013-07-17T14:40:56.833" Score="17" Body="&lt;p&gt;In short, no. The reason is, the hardest part by far of getting to space is getting to Low Earth Orbit. As the saying goes, once you've done that, you're half way to anywhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be difficult to make the elevator stay put to non-anchored locations. Furthermore, it would mess with the speed required to get to each of these locations, and in the end, wouldn't be very practical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given this, the most practical thing that could be done along these lines would be to build a very large platform on top of a mountain, and raise the object to the top of the platform to launch. That gives very little theoretical benefit, although it does cut down on wind resistance and such. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is, however, a system that could help once you have obtained orbit, using magnetic tethers. I won't bother to explain all of the physics, but I will point you to &lt;a href=&quot;https://web.archive.org/web/20190320031643/http://www.tethers.com/orbittoorbit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Tethers&lt;/a&gt;, which discusses it further, and also provide a graphic from their website which demonstrates it further.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rKAMr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-10-01T20:35:46.627" LastActivityDate="2019-10-01T20:35:46.627" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="264" PostTypeId="1" AcceptedAnswerId="353" CreationDate="2013-07-17T14:42:20.900" Score="16" ViewCount="3318" Body="&lt;p&gt;There was an &lt;a href=&quot;http://www.astronautix.com/craft/stsation.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;independent proposal&lt;/a&gt; that the space shuttle external tank could have been lifted all the way to orbit, and then used as a structural material in space stations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of propellant budget, how much would it have cost to do this?  Presumably, you would impart more Δv to the external tank, which would have reduced the payload you can take to orbit.  How close was the external tank to orbital velocity, how much extra propellant would it have taken to get it the rest of the way, and would this have been mechanically possible with the space shuttle design?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="6537" LastEditDate="2018-01-11T20:43:44.700" LastActivityDate="2020-11-02T22:27:46.963" Title="What would the Δv cost of bringing the space shuttle external tank to orbit be?" Tags="|space-shuttle|low-earth-orbit|space-station|wet-workshop|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="265" PostTypeId="2" ParentId="260" CreationDate="2013-07-17T14:46:02.557" Score="9" Body="&lt;p&gt;Practical and useful - yes. Economically viable or technologically possible? Not sure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely it wouldn't require nearly as much materials so incredibly durable as the &quot;full elevator&quot;. Jet engine powered airplanes are vastly cheaper and easier to construct and operate than rocket-powered vehicles. Once in LEO any vehicle can engage low-thrust, low-energy propulsion like a solar sail, and depart Earth's gravity pit at fraction of the cost of corresponding jet propulsion. Essentially, that would provide the benefits of space elevator at the part of the travel where its benefits matter most - between the atmosphere (where jet engines work) and the LEO (where orbital engines work). Currently that gap is jumped with rocket thrusters that are simply extremely costly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That much for benefits. Now for the problems.&#xA;The problems of delivering several hundreds kilometers of a space rope to the orbit aside (we've been there with classic space elevator) we have actual air drag, and no anchor that would pull it and provide energy, plus each vehicle riding up would pull it down. That thing wouldn't support itself like a classic space elevator, it would require its own propulsion to keep it afloat. (OTOH, fuel for said propulsion could be delivered by the same airplanes, and it could be any of the number of the neat &quot;orbital engines&quot;, no need for the troublesome rockets). There would be the whole problem of docking the payload in the stratosphere at supersonic speeds. I'm not sure about meteorology of the stratosphere, but I think it could be troublesome (not worse than against classic space elevator though). And obviously the usage cost would be considerably higher than of classic elevator which could use efficient electric motors to bring payload from ground far beyond geostationary orbit - while jets are an order of magnitude cheaper than rockets, electric motors leave jets far behind in terms of energy efficiency = cost of operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One more problem: the active propulsion can't fail for prolonged periods of time. In case of a classic elevator, it would sit there completely inert with no problems. In case of the partial, it would fall. If lengthy repairs are needed, it could be propelled to a higher orbit, as high as needed, and repaired there over time, but unexpected faults would simply destroy it.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-17T14:46:02.557" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="266" PostTypeId="2" ParentId="237" CreationDate="2013-07-17T14:51:12.090" Score="34" Body="&lt;p&gt;There are some seals, on some of the modules that have a limited lifespan.  The lifespans are in the 20 year range, but they are very hard to replace. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of them are on the Russian end of the station, (Oldest part of course, in terms of launch, but since also was built even earlier than launch for Mir II usage) and these are the ones exposed to the fuel/oxidizer for station reboost.   Thus they are exposed to Hydrazine and MMH which is horribly corrosive, and even seals safe with those things, have a limited lifespan.  I imagine these seals are buried fairly deep inside the module, and not greatly accessible for replacement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely, the Russians have talked about taking their modules and going home, if the US decides to end the ISS 'early' from their perspective which argues against my point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US side is much harder to support without Zvezda/Zarya for orbital control.  US side does much of the power, and heat radiation, but not much of the orbital maintenance.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-07-21T11:38:01.417" LastActivityDate="2013-07-21T11:38:01.417" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="269" PostTypeId="5" CreationDate="2013-07-17T15:16:05.107" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T15:16:05.107" LastActivityDate="2013-07-17T15:16:05.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="270" PostTypeId="4" CreationDate="2013-07-17T15:16:05.107" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T15:16:05.107" LastActivityDate="2013-07-17T15:16:05.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="271" PostTypeId="2" ParentId="237" CreationDate="2013-07-17T15:16:10.707" Score="20" Body="&lt;p&gt;The orbital decay is not the reason for abandoning ISS at all. The orbit is constantly being corrected by the cargo ships, plus several modules have their own thrusters for orbit correction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS is 15 year old. Mir was in orbit for 15 year when it was deorbited.&#xA;The ageing shows itself in many aspects on all the stations. &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station_maintenance&quot;&gt;ISS&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Salyut_7#Technical_and_crew_problems&quot;&gt;Salyut-7&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Mir#Safety_aspects&quot;&gt;Mir&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Skylab#Solar_activity&quot;&gt;Skylab&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last Mir expeditions tried to find and fix up small holes in the hull with an epoxide based glue. Not much luck. They found one, but could not find the others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Salyut-7 station was left without a crew for several months. A power failure caused a loss of contact, so the ground control could not control its flight. And what's even worse, it had stopped the life support systems. The water froze. The pipelines were seriously damaged. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The failure to account for increased solar activity invalidated previous calculations and brought down Skylab from the parking orbit much earlier than anticipated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, critical events are happening all the time. The plastics degrade, the metals corrode, micrometeorites cause damage from outside. And if left without attention a space station produced with the current level of technologies will quickly become a useless and dangerous piece of space garbage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason for deorbiting Mir was a need to spend resources on the newer ISS project. The Russians have had previous experience of using two stations at the same time. That was the brand new Mir and the old good Salyut-7. (They even flew in Soyuz there and back between the stations). Two stations are just too expensive to support together. So if the Russians or Americans or all together decide to spend money on something newer and better, the station will have to be abandoned.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="1142" LastEditDate="2014-03-20T01:18:59.330" LastActivityDate="2014-03-20T01:18:59.330" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="272" PostTypeId="5" CreationDate="2013-07-17T15:19:27.687" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T15:19:27.687" LastActivityDate="2013-07-17T15:19:27.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="273" PostTypeId="4" CreationDate="2013-07-17T15:19:27.687" Score="0" Body="Wet workshop is the idea of using a spent rocket stage as a makeshift space station." OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-17T16:11:34.280" LastActivityDate="2013-07-17T16:11:34.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="274" PostTypeId="5" CreationDate="2013-07-17T15:25:47.763" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T15:25:47.763" LastActivityDate="2013-07-17T15:25:47.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="275" PostTypeId="4" CreationDate="2013-07-17T15:25:47.763" Score="0" Body="Project Gemini was the second human spaceflight program of the United States. It was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966." OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-17T16:11:08.497" LastActivityDate="2013-07-17T16:11:08.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="277" PostTypeId="5" CreationDate="2013-07-17T15:30:22.817" Score="0" Body="&lt;p&gt;&lt;strong&gt;The Apollo program was&lt;/strong&gt; a mission that was born thanks to the famous John F. Kennedy speech that declared that NASA will put a human on the Moon by the end of the 1960-1970 decade. The program was started in 1961 and ended in 1975.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Questions tagged with this tag&lt;/strong&gt; usually address a question related to an Apollo mission, gadget, vehicle, rocket, crew or other entities related to Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basic needed knowledge of all the &lt;em&gt;important&lt;/em&gt; Apollo missions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 1&lt;/strong&gt; was the mission where three astronauts died from a fire in a pure-oxygen environment&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 7&lt;/strong&gt; demonstrated the capability of Apollo vehicles and spacecraft, was the first manned mission&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 8&lt;/strong&gt; was the first manned mission to the moon, orbited the moon and took pictures of the lunar surface&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 9&lt;/strong&gt; tested the Lunar Module (LM)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 10&lt;/strong&gt; exhibited a lunar orbit&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 11&lt;/strong&gt; was the first Apollo mission to land on the moon, landed in &lt;em&gt;Sea of Tranquility&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 12&lt;/strong&gt; landed on the moon, known for launching during a storm and producing two lightning bolts during flight, landed in &lt;em&gt;Ocean of Storms&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 13&lt;/strong&gt; is notoriously known for the oxygen tank rupture mid-flight, orbited the moon, tied to the quote &lt;em&gt;&quot;Failure is not an option&quot;&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 14&lt;/strong&gt; landed on the moon, landed in &lt;em&gt;Fra Mauro&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 15&lt;/strong&gt; landed on the moon, landed in &lt;em&gt;Hadley Rille&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 16&lt;/strong&gt; landed on the moon, landed in &lt;em&gt;Descartes&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Apollo 17&lt;/strong&gt; landed on the moon, known for successfully recording Lunar Module (LM) ascent, landed in &lt;em&gt;Taurus-Littrow&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;You can learn more about Apollo with&lt;/strong&gt; the following links:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/planetary/lunar/apollo.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Official NASA Archives Apollo Mission List&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia Page&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=ez0bFWKR9-0&quot; rel=&quot;nofollow noreferrer&quot;&gt;40th Anniversary Documentary&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.netflix.com/title/80138278&quot; rel=&quot;nofollow noreferrer&quot;&gt;BBC &quot;Space Race&quot; Show&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.hq.nasa.gov/pao/History/SP-350/cover.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Apollo Expeditions to the Moon&quot; book&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="20596" LastEditDate="2017-08-14T03:10:47.867" LastActivityDate="2017-08-14T03:10:47.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="278" PostTypeId="4" CreationDate="2013-07-17T15:30:22.817" Score="0" Body="NASA's Apollo program was a 17-mission program that focused on landing a human on the lunar surface. The program was the third human spaceflight by NASA, after Mercury and Gemini. " OwnerUserId="14" LastEditorUserId="20596" LastEditDate="2017-08-13T21:01:50.437" LastActivityDate="2017-08-13T21:01:50.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="279" PostTypeId="2" ParentId="246" CreationDate="2013-07-17T15:38:11.050" Score="8" Body="&lt;p&gt;Even at the high atmosphere level, far below 'space' it is barely possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at Skylon, whose core focus is to inject enough oxygen from the very thin air, cool it and compress it very fast down to LOX to then use it as oxidizer, since normally oxygen at high atmosphere is not usable for this.  I.e. Extend ability to run 'air-breathing'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And Skylon is finally getting successes though not an actual flight yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now extend that to actual 'space' and you can see how hard it probably is.  Conversely the use case of breathing is significantly lower volume than collecting oxygen for actual use as oxidizer in a rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just trying to offer some boundaries to the discussion.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-07-17T15:38:11.050" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="280" PostTypeId="1" AcceptedAnswerId="860" CreationDate="2013-07-17T16:05:11.570" Score="15" ViewCount="394" Body="&lt;p&gt;In 1996-1997, I became extremely enthusiastic about the space exploration prospects for aerobot technology. There seem to be at least 10 celestial bodies suitable for aerobot deployments:  Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Enceladus, Europa, &amp;amp; Titan. I even published my &lt;a href=&quot;http://hdl.handle.net/10945/8787&quot;&gt;master's thesis&lt;/a&gt; on the subject.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the time, Kerry Nock was leading a group of scientists at JPL who were studying the subject with institutional funding. They did several experiments and published some of their results:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ALtItude Control Experiments (&lt;a href=&quot;http://hdl.handle.net/2014/30077&quot;&gt;ALICE&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Balloon Experiment at Venus &amp;amp; Venus Flyer Robot (&lt;a href=&quot;http://hdl.handle.net/2014/29140&quot;&gt;BEV &amp;amp; VFR&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But in the nearly two decades since that time, there have been no aerobot missions (that I know of) and I've read very little about even proposed aerobot missions. Of those few I have encountered, they seem to get a little funding for study and then get abandoned in favor of more difficult missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think an airborne aerobot on Titan must be one of the single most promising (in terms of discovering exobiology) and technologically feasible missions imaginable for the near future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why has this idea apparently been abandoned?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there scientific or engineering problems that make it impractical or too expensive? I have been unable to find any such issues myself. Or is it a political thing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If &lt;a href=&quot;http://en.wikipedia.org/wiki/DEPTHX&quot;&gt;DEPTHX&lt;/a&gt; (which strikes me as a kind of aerobot floating in water instead of air) got funded by NASA with the goal of developing technology that can explore the oceans of Jupiter's moon Europa (which first requires getting through miles of ice; no mean task), then why have there been no (or few) NASA or ESA funded projects for aerobots in less challenging environments like Titan or the upper atmosphere of Venus?&lt;/p&gt;&#xA;" OwnerUserId="108" LastEditorUserId="108" LastEditDate="2013-07-17T16:14:52.677" LastActivityDate="2013-07-30T22:16:01.743" Title="Why so few aerobot missions?" Tags="|aerobot|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="281" PostTypeId="1" AcceptedAnswerId="288" CreationDate="2013-07-17T16:33:28.433" Score="38" ViewCount="21166" Body="&lt;p&gt;A structure with a radius of 224m rotating at 2 rotations per minute will generate 1g of force on the inside (&lt;a href=&quot;http://www.artificial-gravity.com/sw/SpinCalc/&quot;&gt;spincalc&lt;/a&gt;). It will generate that force on the feet, but as you travel up the body the amount of force applied reduces. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Rotation&quot;&gt;Wikipedia&lt;/a&gt; (citation needed) a larger radius and a slower rotation should be make the effect more consistent for a standing human. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Playing around with the &lt;a href=&quot;http://www.artificial-gravity.com/sw/SpinCalc/&quot;&gt;spincalc&lt;/a&gt; tells me that with a 1000 meter radius and a rotation of 0.95 rotations per minute is also at 1g, but I have no idea how that will affect the reduction in inertia felt as you travel away from the outer edge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What radius and rotation would be needed to produce 1g consistently from the floor to a height of about 6ft (2m) within a tolerance of a few percentage points (maybe 5%)?&lt;/p&gt;&#xA;" OwnerUserId="4" LastEditorUserId="58" LastEditDate="2020-03-10T15:00:02.400" LastActivityDate="2021-08-19T20:24:28.317" Title="What would the size and rotation of a station need to be to produce 1g gravity from head to toe?" Tags="|artificial-gravity|humans|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="282" PostTypeId="2" ParentId="104" CreationDate="2013-07-17T16:36:50.983" Score="4" Body="&lt;p&gt;I suspect there is nothing that really addresses your question, however, I do see Jupiter mentioned in these two although I don't think it is as carefully studied as are some other heavenly bodies like Venus and Titan:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;[Cutts et al. 1995] J. A. Cutts, K. T. Nock, J. A. Jones, G.&#xA;Rodriguez, and J. Balaram. &lt;a href=&quot;http://dx.doi.org/10.1007/BF00710794&quot; rel=&quot;nofollow&quot;&gt;Planetary exploration by robotic&#xA;aerovehicles&lt;/a&gt;. Journal of Autonomous Robots, 2(4):261-282, 1995.&lt;/li&gt;&#xA;&lt;li&gt;[DiCicco et al. 1995] A. G. DiCicco, K. T. Nock, and G. E. Powell.&#xA;Balloon experiment at Venus (BEV). In JPL Planetary Aerobot&#xA;Technology Development Program Papers presented at the AIAA 11th&#xA;Lighter-Than-Air Technology Conference, Clearwater, May 1995. AIAA,&#xA;American Institute of Aeronautics and Astronautics. (sorry, but I&#xA;couldn't find a digital reference for this one)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Not sure how much funding these papers received though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Disclaimer: I took these references from &lt;a href=&quot;http://hdl.handle.net/10945/8787&quot; rel=&quot;nofollow&quot;&gt;my thesis&lt;/a&gt; and it's been many years since I read them in their entirety. Jupiter is mentioned in the abstract of the first, however.&lt;/p&gt;&#xA;" OwnerUserId="108" LastActivityDate="2013-07-17T16:36:50.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="283" PostTypeId="2" ParentId="196" CreationDate="2013-07-17T16:44:36.433" Score="5" Body="&lt;p&gt;A trivial answer could be: &lt;em&gt;the same kind of station&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians will keep producing modules of Salyut-Mir-Zvezda and TKS-FGB lineage, with futher Soyuz-Progress modifications for transportation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Europeans will keep building their MPLM modules and ATV based spacecrafts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;US will keep producing Dragons, integrated truss structure components and MPLM based modules. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;China will participate with Tiangong modules and manned transportation on Shenzhou spacecrafts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While Russian and Chinese habitable modules have their own propulsion systems, and hence are able to get to the station by themselves, the European and US modules are not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, there will be a need for a powerful space tug able to bring 10-15 ton components to the station. It can be based on TKS (similar to &lt;a href=&quot;http://en.wikipedia.org/wiki/Kvant-1#Launch_and_docking&quot; rel=&quot;noreferrer&quot;&gt;FSM used to bring 11 tons Kvant-1 module&lt;/a&gt; to Mir). But Russia will hardly be able to produce enough space tugs to construct all the station in reasonable time. And 11 tons is probably the maximum for Proton buster + FSM tug combination. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there will probably be another one in the same range, something US could spend its spare money on to speed things up and to have some independence and control in the project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the smaller, 1-5 tons loads (the size of Cupola, Poisk module or Dextre) the existing ATV and &lt;a href=&quot;http://en.wikipedia.org/wiki/Progress_M-MIM2&quot; rel=&quot;noreferrer&quot;&gt;Progress propulsion system&lt;/a&gt; or Dragon thrusters&lt;br&gt;&#xA;should be enough.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-07-17T16:44:36.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="284" PostTypeId="1" AcceptedAnswerId="286" CreationDate="2013-07-17T16:50:46.500" Score="37" ViewCount="10184" Body="&lt;p&gt;We all know that James Webb Space telescope is planned to be launched in 2018. It has been decided that the orbit of JWST &lt;a href=&quot;http://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Orbit&quot;&gt;will be&lt;/a&gt; elliptical around the Lagrange point L2, which has been declared as one of the unstable points (L1, L2, L3).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So then, why &lt;strong&gt;should&lt;/strong&gt; it orbit the point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've got a few points. The telescope's instruments are quite sensitive and should always be maintained away from Sun (Really? Is that so?) and also in a cold environment - both these are satisfied by L2. Wikipedia says this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The combined gravitational forces of the Sun and the Earth can hold a spacecraft at this point, so that in theory it takes no rocket thrust to keep a spacecraft in orbit around L2. In reality, the &lt;strong&gt;stable&lt;/strong&gt; point is comparable to that of a ball balanced upon a saddle shape. Along one direction any perturbation will drive the ball toward the stable point, while in the crossing direction the ball, if disturbed, will fall away from the stable point. Thus some station-keeping is required, but with little energy expended (only 2–4 m/s per year, from the total budget of 150 m/s)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;First, is it really stable at all? If that is &lt;em&gt;so&lt;/em&gt; unstable in reality, why should it be positioned there? I mean, L4 &amp;amp; L5 points are fine. Why can't the telescope be positioned in such a way that it's always facing outward from the Sun? (Earth too, if they don't want the reflection from it to crack itself)&lt;/p&gt;&#xA;" OwnerUserId="116" LastEditorUserId="10132" LastEditDate="2015-07-22T23:57:03.063" LastActivityDate="2022-01-13T19:06:11.393" Title="Why should the James Webb Space telescope stay in the unstable L2?" Tags="|orbital-mechanics|lagrangian-points|telescope|space-telescope|james-webb-telescope|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="285" PostTypeId="2" ParentId="216" CreationDate="2013-07-17T16:57:32.273" Score="5" Body="&lt;p&gt;It appears that most of the true sterilization actually takes place &lt;em&gt;after&lt;/em&gt; launch. The harsh conditions of the trip there and the landing should be enough to kill off a majority of the microbes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars rover projects all receive multiple sterilization treatments here on Earth before launch, typically UV treatment, but &lt;a href=&quot;http://www.astrobio.net/exclusive/4196/microbe-risk-when-rover-wheels-hit-martian-dirt&quot; rel=&quot;noreferrer&quot;&gt;what really kills off the bacteria&lt;/a&gt; is the exposure to cosmic radiation on the trip over and the intense UV radiation on Mars' surface.&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2013-07-17T16:57:32.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="286" PostTypeId="2" ParentId="284" CreationDate="2013-07-17T17:02:29.503" Score="51" Body="&lt;p&gt;There are a couple of reasons.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The distance from the L2 to Earth is only 1.5 million km away. The L4/L5 are 1 AU, or about 150 million km away. That leads to a reduction in link margin of 40 db, or 1/10000. That is quite significant. In order to compensate for that difference, you either need a bigger radio dish, more power, or a loss in data.&lt;/li&gt;&#xA;&lt;li&gt;As you mentioned, the fuel usage is quite low to maintain that position, only on the order of 150 m/s delta v for the entire mission. That isn't a whole lot, and in fact, is less than &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_station-keeping#Station-keeping_in_geostationary_orbit&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Orbital Station-keeping on Wikipedia&quot;&gt;what is required to keep a satellite in geostationary orbit&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The satellite is much closer, reducing the time to command an object. Light only will take 5 seconds to reach James Webb, whereas it will take 9 minutes to reach L4/L5. This limits the ability to do real time commands, which occasionally are useful (Think Gamma Ray Bursts, Super Novas, etc).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Bottom line is, the communication problem is simplified with a closer telescope, and that more than makes up for having to take a bit more fuel.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="716" LastEditDate="2022-01-13T13:50:54.847" LastActivityDate="2022-01-13T13:50:54.847" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="287" PostTypeId="1" AcceptedAnswerId="292" CreationDate="2013-07-17T17:05:32.723" Score="15" ViewCount="10185" Body="&lt;p&gt;Yes, I know, &lt;em&gt;c&lt;/em&gt; is the limit, but let's say we want to build a shelter that will be meterorite-impact-proof, on the Moon, or some asteroid or anywhere without the atmosphere. We can spot anything bigger than, say, 10cm early enough to destroy or deflect it with remote weaponry, but anything smaller than that will get through and the shield must hold. $Mass = 1{dm}^3 * 2g/{cm}^3$ (average asteroid density), $E=0.5mv^2$. We know how to convert energy to shield thickness, but we still need the &lt;em&gt;v&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are reasonable expected speeds of &quot;fast&quot; meteorites?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="168" LastEditDate="2013-08-11T11:23:47.217" LastActivityDate="2021-08-19T19:12:26.547" Title="What maximum speeds can be expected from meteorites?" Tags="|habitat|protection|meteorites|" AnswerCount="6" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="288" PostTypeId="2" ParentId="281" CreationDate="2013-07-17T17:09:01.903" Score="40" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What radius and rotation would be needed to produce 1g consistently from the floor to a height of about 6ft (2m)?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Infinity.  Technically there will always be a vertical gradient of artificial gravity.  Realistically, people will not care.  Even with a radius of 224 m the difference isn't much.  The acceleration for anything attached to the structure will be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Dozb1.png&quot; alt=&quot;a = ω²r&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This makes the problem simple because the rotation rate (omega) is constant, so the difference between your head and feet is r1/r2.  For a person standing in a 224 m radius structure, that's 2/224 = 0.9%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the tidal forces on Earth cause a difference in gravity of 0.00006 % from your head to your toe.  Earth has an exceptionally constant gravitational field.  If you like, you can calculate the radius needed to produce this degree of consistency.  It is about half the radius of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A percent difference in acceleration from head to toe shouldn't bother someone too much.  The main concerns of discomfort in artificial gravity are dynamic Coriolis (false) forces.  These are not static like the effect your mention.  The terms depend on velocity, not position, so someone standing still will not feel them (discounting any moving fluid in their body).  For normal motion, these are much more significant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacefuture.com/archive/artificial_gravity_and_the_architecture_of_orbital_habitats.shtml&quot;&gt;Here&lt;/a&gt; are some images of dropping an object in artificial gravity.  For the 2 rpm case, there is significant noticeable deflection.  But again, due to forces that only occur when something is moving relative to the ground.  So you could have 1% difference in gravity due to radial location, but several centimeter displacement from dropping something.  The latter will be more noticeable.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="17" LastEditDate="2013-07-19T16:30:13.160" LastActivityDate="2013-07-19T16:30:13.160" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="289" PostTypeId="2" ParentId="29" CreationDate="2013-07-17T17:16:04.713" Score="5" Body="&lt;p&gt;I know that it has been theorized about harvesting isotopes of Helium or Hydrogen space, including studies for / by NASA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, NASA did study mining celestial bodies, including other planets, and still does. The study report &quot;Space Resources&quot; (NASA SP-509, 1992) has become a classic. It can be found here &lt;a href=&quot;http://www.nss.org/settlement/spaceresources/library.htm&quot; rel=&quot;nofollow&quot;&gt;http://www.nss.org/settlement/spaceresources/library.htm&lt;/a&gt; Another example for a more up-to-date study is the Robotic Asteroid Prospector (RAP) concept, which is currently under investigation: &lt;a href=&quot;http://www.nasa.gov/directorates/spacetech/niac/2012_phase_I_fellows_cohen.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/directorates/spacetech/niac/2012_phase_I_fellows_cohen.html&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though, it should be noted that those studies primarily apply to moons, asteroids and inner planets, while gas gians as not investigated in detail. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-17T19:29:45.020" LastActivityDate="2013-07-17T19:29:45.020" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="290" PostTypeId="2" ParentId="287" CreationDate="2013-07-17T17:20:27.583" Score="2" Body="&lt;p&gt;The &quot;American Meteor Society&quot; states that meteorites typically enter the Earth's atmosphere at  &lt;a href=&quot;http://www.amsmeteors.org/meteor-showers/meteor-faq/&quot; rel=&quot;nofollow&quot;&gt;11 - 72 km/s&lt;/a&gt;. This is not cited, but after some Googling it is a figure that is repeated often.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-07-17T17:20:27.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="291" PostTypeId="2" ParentId="287" CreationDate="2013-07-17T17:39:14.987" Score="6" Body="&lt;p&gt;According to the &lt;a href=&quot;http://www.amsmeteors.org/meteor-showers/meteor-faq/#3&quot; rel=&quot;noreferrer&quot;&gt;American Meteor Society&lt;/a&gt;, meteorites usually hit the Earth's atmosphere going around 160,000 MPH.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Meteors enter the atmosphere at speeds ranging from &lt;strong&gt;11 km/sec (25,000 mph), to 72 km/sec (160,000 mph!)&lt;/strong&gt;...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The 70~ish top figure is also repeated in this &lt;a href=&quot;http://wiki.answers.com/Q/How_fast_does_a_meteor_go_through_space&quot; rel=&quot;noreferrer&quot;&gt;answers.com answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why such a big range, between 25k and 160k MPH?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The wide range in meteoroid speeds is caused partly by the fact that the &lt;strong&gt;Earth itself is traveling at about 30 km/sec (67,000 mph).&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also, there was recently a two to four meter meteorite that &lt;a href=&quot;http://blogs.scientificamerican.com/observations/2012/12/20/california-meteor-broke-speed-record-for-atmospheric-entry/&quot; rel=&quot;noreferrer&quot;&gt;hit over California&lt;/a&gt; that was calculated to be going around 64k MPH.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;researchers have calculated that the parent object of the Sutter’s Mill meteorite entered the atmosphere at &lt;strong&gt;28.6 kilometers per second (64,000 mph).&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-17T17:49:01.093" LastActivityDate="2013-07-17T17:49:01.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="292" PostTypeId="2" ParentId="287" CreationDate="2013-07-17T18:04:21.527" Score="24" Body="&lt;p&gt;I kept wondering where this 72 km/s maximum is coming from, and I figured it out!  This is the calculation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mDirX.png&quot; alt=&quot;30 km/s + (30√2) km/s = 72 km/s&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why?  Firstly, it's obvious that the Earth is traveling at 30 km/s in its orbit.  But what possible directions can the asteroid hit from?  The most logical approach is to hit it moving the exactly opposite direction.  That means we demand an asteroid in a retrograde orbit.  Normally this won't happen for orbits in the inner solar system around the sun, but this came come from the Ort cloud or somewhere far away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea is that an object &lt;em&gt;very&lt;/em&gt; from the sun is disturbed and begins a highly elliptical orbit.  These can be retrograde.  It would also need to intersect with our orbit at its closest point to the sun (this is why we sum the two velocities).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The kinetic energy of a body in a circular orbit is half of its gravitational potential energy. Since the far point of the object's orbit (formally a comet I think) is nearly infinity, that means its kinetic energy at the close approach will be exactly equal to its potential energy at 1 AU.  That means it's &lt;em&gt;specific&lt;/em&gt; kinetic energy (just 1/2 v^2) will be twice that of Earth.  That means it will be traveling at the square root of 2 times faster than Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously this would be rare, but the principle is that anything moving faster than this when it hits the atmosphere obviously came from somewhere outside our solar system.  This is my brief illustration of the concept.  Earth is green, sun is yellow, and the object is grey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WFrSv.png&quot; alt=&quot;object hit&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="17" LastEditDate="2013-07-19T15:22:09.853" LastActivityDate="2013-07-19T15:22:09.853" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="293" PostTypeId="2" ParentId="27" CreationDate="2013-07-17T18:25:35.353" Score="4" Body="&lt;p&gt;This is pretty mission-specific. Using Mars Science Laboratory as an &lt;a href=&quot;http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&amp;amp;NewsID=1200&quot; rel=&quot;nofollow&quot;&gt;example&lt;/a&gt;, they initially scheduled for six possible midcourse correction maneuvers. According to the link, the second maneuver imparted about 5.5 m/s total delta-V, whereas the first imparted about six times that amount.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;So how often does this measurement need to be done?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The trajectory is measured quite often, with an estimate being provided multiple times per day or even more frequently (depending on the phase of the mission).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;and how is it done?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Range measurements are usually taken between a ground station antenna and an antenna on the spacecraft. Think of it as a glorified &quot;time of flight&quot; calculation, in which the time it takes for the signal to travel between the antennas is measured, which in turn produces an estimate of the distance traveled. Sometimes the first time derivative of range (range rate) can be measured as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These measurements can be used to update the trajectory, and the difference between the nominal trajectory and the actual one is used to calculate the required correction maneuver.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-07-17T18:25:35.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="295" PostTypeId="2" ParentId="247" CreationDate="2013-07-17T19:19:48.287" Score="4" Body="&lt;p&gt;There is an anime called &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket_Girls&quot; rel=&quot;nofollow&quot;&gt;Rocket Girls&lt;/a&gt;&quot; in which the protagonist asked this same question. The answer she got was that they only use Classic Technology; technology that has built up a reputation of success over time. This is true for medicine and general aviation as well. In fact this is true for most branches of engineering, its primarily software engineering that keeps using the &quot;latest&quot; stuff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, &lt;a href=&quot;https://en.wikipedia.org/wiki/CMOS&quot; rel=&quot;nofollow&quot;&gt;CMOS&lt;/a&gt; is more susceptible to Radiation than &lt;a href=&quot;http://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic&quot; rel=&quot;nofollow&quot;&gt;TTL&lt;/a&gt;, so when you are doing &lt;a href=&quot;http://en.wikipedia.org/wiki/Radiation_hardening&quot; rel=&quot;nofollow&quot;&gt;Radiation Hardening&lt;/a&gt; it may be better to have a slow 100 mhz TTL based chip than a fast 3.4 Ghz CMOS based chip.&lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastActivityDate="2013-07-17T19:19:48.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="296" PostTypeId="1" AcceptedAnswerId="303" CreationDate="2013-07-17T19:49:20.287" Score="18" ViewCount="1106" Body="&lt;p&gt;We know that it is possible for humans to stay physically in shape during long term space missions (see for example &lt;a href=&quot;http://en.wikipedia.org/wiki/Valeri_Polyakov&quot; rel=&quot;noreferrer&quot;&gt;Valeri Polyakov&lt;/a&gt; who stayed on Mir for more than 14 months for one trip). However, the challenges of manned space exploration are different and would include even longer durations.&lt;/p&gt;&#xA;&lt;p&gt;Reducing the need of the constant physical training required by the Zero-G environment could help to make manned exploration missions more viable. This could be achieved by artificial gravity.&lt;/p&gt;&#xA;&lt;p&gt;Is artificial gravity feasible today? Would it indeed help to make manned exploration viable?&lt;/p&gt;&#xA;" OwnerUserId="121" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-21T00:51:03.393" Title="Is artificial gravity feasible in manned long-term space exploration?" Tags="|crewed-spaceflight|long-duration|artificial-gravity|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="297" PostTypeId="5" CreationDate="2013-07-17T19:51:09.643" Score="0" Body="&lt;p&gt;This refers to laws passed by nations to regulate space exploration from the Earth.  For more details on the distinction between this tag and &lt;a href=&quot;/questions/tagged/space-law&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged 'space-law'&quot; rel=&quot;tag&quot;&gt;space-law&lt;/a&gt;, please see &lt;a href=&quot;http://meta.space.stackexchange.com/questions/5/tags-space-law-vs-international-law&quot;&gt;this meta post&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-17T20:28:11.323" LastActivityDate="2013-07-17T20:28:11.323" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="298" PostTypeId="4" CreationDate="2013-07-17T19:51:09.643" Score="0" Body="Questions pertaining to international regulations on activities on Earth that affect space exploration." OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-17T20:28:08.073" LastActivityDate="2013-07-17T20:28:08.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="299" PostTypeId="5" CreationDate="2013-07-17T19:56:49.783" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T19:56:49.783" LastActivityDate="2013-07-17T19:56:49.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="300" PostTypeId="4" CreationDate="2013-07-17T19:56:49.783" Score="0" Body="Questions about human medical conditions that affect and are affected by space travel." OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-17T20:28:02.010" LastActivityDate="2013-07-17T20:28:02.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="301" PostTypeId="1" CreationDate="2013-07-17T20:12:20.603" Score="9" ViewCount="4186" Body="&lt;p&gt;In order to maintain a Colony on Mars, we will need to be able to provide consistent, safe, and sustainable power on the planet.  The extremely cold temperature means that the need to maintain temperature in the habitat will require consistent power generation.  What are the viable options for a Martian colony to provide this power?&lt;/p&gt;&#xA;" OwnerUserId="123" LastEditorDisplayName="user12" LastEditDate="2013-07-19T15:21:50.633" LastActivityDate="2014-04-05T20:13:46.883" Title="What power source options provide reliable and sustainable power generation for a Martian Colony" Tags="|mars|colonization|power|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="302" PostTypeId="2" ParentId="104" CreationDate="2013-07-17T20:14:35.993" Score="3" Body="&lt;p&gt;I think project ARCHIMEDES of the German Mars Society is somewhere within the grey zone between major agency and semi-hobbyists project. It is intended to deploy a balloon into the Mars' atmosphere. The project is backed by a number of public research institutes and the German space agency (DLR), while I believe that most of its funding is private. Regarding your question, they went significantly beyond just calculations and did actual high atmosphere tests with real hardware (at least two that I know of) using sounding rockets here on Earth (at Esrange near Kiruna in Sweden). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further reading: &#xA;&lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.IAC-04-Q.P.02&quot; rel=&quot;nofollow&quot;&gt;http://arc.aiaa.org/doi/abs/10.2514/6.IAC-04-Q.P.02&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The German Mars Society's site (in German, English translation is out-dated): &lt;a href=&quot;http://archimedes.marssociety.de/&quot; rel=&quot;nofollow&quot;&gt;http://archimedes.marssociety.de/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-17T20:14:35.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="303" PostTypeId="2" ParentId="296" CreationDate="2013-07-17T20:16:37.820" Score="14" Body="&lt;p&gt;&quot;Artificial&quot; gravity is the name given to techniques to create acceleration that mimics gravitational force.  There are two major ways to do this -- both of which are very feasible:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Rotation -- in this case, the acceleration is created by &lt;a href=&quot;https://en.wikipedia.org/wiki/Centripetal_force&quot;&gt;centripetal force&lt;/a&gt;.  The rotating structure accelerates the crew by forcing them to follow a curved (usually circular) path.  This is usually depicted as a rotating torus, but is probably easier to do with a crew compartment tethered to a counterweight spinning about their combined center of gravity.&lt;/li&gt;&#xA;&lt;li&gt;Continuous acceleration -- in this case, the vehicle is undergoing constant thrust, which accelerates the crew and gives the impression of gravity.  This is probably only practical for very long (interstellar?) missions and in fact is probably a beneficial side effect of the propulsion required.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Both of these methods probably improve the long term environment for the crew, but do add significant complexities to the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-17T20:21:40.580" LastActivityDate="2013-07-17T20:21:40.580" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="304" PostTypeId="1" AcceptedAnswerId="856" CreationDate="2013-07-17T20:24:56.377" Score="10" ViewCount="477" Body="&lt;p&gt;I know that the high atmospheric pressure on these planets make a unique challenge to learn about the planets interiors.  Are there any active projects designed to learn more about the interior of the gas giants?  &lt;/p&gt;&#xA;" OwnerUserId="123" LastEditorUserId="690" LastEditDate="2014-06-23T14:30:54.410" LastActivityDate="2019-05-21T07:14:42.960" Title="Are there any experiments in progress designed to explore the interiors of gas giants like Jupiter or Saturn?" Tags="|gas-giant|instrument|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="305" PostTypeId="2" ParentId="233" CreationDate="2013-07-17T20:32:11.287" Score="4" Body="&lt;p&gt;Probably the best way to think about gravity wells is to look at the delta-V required (proportional to energy required for a given mass) to move from one point to another in the well.  &lt;a href=&quot;https://en.wikipedia.org/wiki/File%3aDelta-Vs_for_inner_Solar_System.svg&quot; rel=&quot;nofollow&quot;&gt;This graphic&lt;/a&gt; is particularly good.  From it, you can calculate the delta-V to &quot;get around the neighborhood.&quot;&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-17T20:32:11.287" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="306" PostTypeId="1" AcceptedAnswerId="321" CreationDate="2013-07-17T20:34:30.047" Score="15" ViewCount="588" Body="&lt;p&gt;In one of the short stories of Stanislaw Lem the main character, Ijon Tichy, accidentally drops off a piece of pork out of his spaceship, and this pork becomes a satellite of the spaceship, causing the eclipse of one of the stars on regular basis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While theoretically it is possible, under which circumstances something like that would be possible? Even if the mass of the spaceship would be very big, the object would have to leave it with very little relative speed, but how little should it be? And how dangerous such 'satellites' would be? They would be annoying, because they would disrupt observations a bit, but is there any other potential danger of such event?&lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastEditorUserId="14" LastEditDate="2013-07-19T10:23:47.047" LastActivityDate="2013-07-19T13:24:15.750" Title="Under what circumstances would an object falling out of spacecraft start orbiting that spacecraft?" Tags="|orbital-mechanics|artificial-satellite|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="307" PostTypeId="1" AcceptedAnswerId="313" CreationDate="2013-07-17T20:40:47.377" Score="9" ViewCount="1973" Body="&lt;p&gt;Are there any international regulations about littering in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What, except from general social norms, would prevent private operator from discarding waste into deep space? And, in case of damage caused by rubbish, what options would have the suffering party to demand compensation?&lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastEditorDisplayName="user40" LastEditDate="2013-07-17T21:04:13.440" LastActivityDate="2018-11-25T10:36:42.400" Title="International regulations about littering in space?" Tags="|law|liability|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="308" PostTypeId="1" AcceptedAnswerId="354" CreationDate="2013-07-17T20:46:19.397" Score="21" ViewCount="5764" Body="&lt;p&gt;This is not nice perspective, but eventually it will happen. An astronaut falls out of spaceship because of damage caused by collision with other object, or because of suit decompression. The fluids from the body would evaporate, and if any bacteria would survive, than only as spores. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it mean the perfect mummification of the body? Or there will be some decay, caused by enzymes from damaged cells, for example?&lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastEditorUserId="3185" LastEditDate="2016-04-20T12:35:47.913" LastActivityDate="2023-11-09T01:45:39.603" Title="Decay of a corpse in space?" Tags="|medical|vacuum|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="309" PostTypeId="2" ParentId="306" CreationDate="2013-07-17T20:51:26.993" Score="10" Body="&lt;p&gt;In the conventional sense of a satellite this is wrong.  For two very good reasons here.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The space ship's GM term is so small that even the slightest movement will put the bacon outside its escape velocity&lt;/li&gt;&#xA;&lt;li&gt;Even if the space ship were HUGE, it becomes a &lt;a href=&quot;http://en.wikipedia.org/wiki/Newton%27s_cannonball&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's cannonball&lt;/a&gt; problem, where it will travel back to exactly its release point&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There is a very &lt;em&gt;different&lt;/em&gt; kind of circular motion that can be observed, however.  For the mathematical specifics you can find sufficient treatment here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/24816/what-exactly-is-the-microgravity-field-in-orbit&quot;&gt;https://physics.stackexchange.com/questions/24816/what-exactly-is-the-microgravity-field-in-orbit&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, if your space ship is orbiting another body, like the Earth, then depending on how you release something, it may float in circles around your vessel.  One way to see this is taking on complementary elliptical orbits.  The spaceship achieves the high point of its orbit around Earth as the bacon achieves the low point of its orbit, and vice versa.  It is a subtle detail of orbital mechanics that they can seem to dance around each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this is not a &quot;true&quot; orbit and the microgravity field of orbit has very unique properties.  To begin with, two objects are not bound to each other - over time they can float apart with ever increasing distance.  Objects can also swap back and fourth in just &lt;em&gt;one&lt;/em&gt; direction as the orbits cross each other.  They have videos of this on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Newton's cannonball problem would also apply.  If you gave a nudge to a wrench from the ISS, it can make its way back to you within 90 minutes - the time taken for one orbit.  The release motion to start it in an circular path around the ISS would require separating it first.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2013-07-17T20:51:26.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="310" PostTypeId="5" CreationDate="2013-07-17T20:55:34.813" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T20:55:34.813" LastActivityDate="2013-07-17T20:55:34.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="311" PostTypeId="4" CreationDate="2013-07-17T20:55:34.813" Score="0" Body="Questions about the physics and problems of manned space flight, i.e. physically moving people from one region of space to another region.&#xD;&#xA;" OwnerDisplayName="user40" LastEditorDisplayName="user12" LastEditDate="2013-08-06T20:22:02.977" LastActivityDate="2013-08-06T20:22:02.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="312" PostTypeId="1" AcceptedAnswerId="337" CreationDate="2013-07-17T21:01:00.493" Score="28" ViewCount="17969" Body="&lt;p&gt;What time zone is used on ISS to define when a day starts? Is it the same as mission control? Do Russian cosmonauts use a different time zone respect to European and American astronauts?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="25" LastEditDate="2013-07-18T19:47:36.850" LastActivityDate="2014-03-15T21:33:47.277" Title="What time zone is used on ISS to define a day?" Tags="|iss|time|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="313" PostTypeId="2" ParentId="307" CreationDate="2013-07-17T21:03:37.597" Score="5" Body="&lt;p&gt;Yes, there are international regulations, which shall be listed in this reply. How private entities fit in is still a disputed question. There is a lack of relevant cases, which could be used as a reference. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;Inter-Agency Space Debris Coordination Committee&quot; (IADC) has released the &quot;Space Debris Mitigation Guidelines&quot; (October 2002). If I remember it correctly, it boils down to a life-time limit in geosynchronous orbit or below of 25 years. Satellites launched in 2008 or later are required to able to be de-orbited or moved into a graveyard-orbit beyond geosynchronous orbit at the end of their life or the maximum of 25 years. The usual reference given for this statement in scientific publications is the &quot;Space debris mitigation guidelines&quot; of the Scientific and Technical Subcommittee of the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS), (A/AC.105/720, 1999 and A/AC.105/890, February 2007).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IADC: &lt;a href=&quot;http://www.iadc-online.org/&quot;&gt;http://www.iadc-online.org/&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some further reading: &lt;a href=&quot;http://users.soe.ucsc.edu/~johnm/Publications/J1-actaastro.2010.08.019.pdf&quot;&gt;An International Environmental Agreement for space debris&#xA;mitigation among asymmetric nations&lt;/a&gt;, 2010. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-17T21:03:37.597" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="314" PostTypeId="2" ParentId="312" CreationDate="2013-07-17T21:08:52.760" Score="6" Body="&lt;p&gt;UTC. It is kind of middle ground between mission controls in Houston and Moscow. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-17T21:08:52.760" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="315" PostTypeId="2" ParentId="312" CreationDate="2013-07-17T21:11:52.163" Score="21" Body="&lt;p&gt;Like many armed  forces, and Stack Exchange, they use UTC (or Zulu time)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Historically, Greenwich Mean Time was accepted as being the definitive time zone in 1884 to aid navigation (which then, as now, relied on accurate time) - and the Prime Meridian was defined as running through Greenwich at 0 degrees Longitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Coordinated Universal Time (UTC) succeeded Greenwich Mean Time in 1963 and is basically GMT with better (sub-second) accuracy.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2013-07-17T21:58:50.420" LastActivityDate="2013-07-17T21:58:50.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="316" PostTypeId="1" AcceptedAnswerId="318" CreationDate="2013-07-17T21:16:46.217" Score="5" ViewCount="1312" Body="&lt;p&gt;Has there ever been an experiment were a plant was brought on board? I would think that if a plant was able to go through normal cycles of photosynthesis, it would be helpful to bring a plant on board to create oxygen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is a foreign idea, why hasn't it been explored yet?&lt;/p&gt;&#xA;" OwnerUserId="66" LastEditorUserId="68" LastEditDate="2013-07-24T16:44:52.960" LastActivityDate="2013-07-24T16:44:52.960" Title="Do spaceships carry plants? If not, why?" Tags="|plants|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="317" PostTypeId="1" CreationDate="2013-07-17T21:17:08.897" Score="17" ViewCount="1873" Body="&lt;p&gt;Scientific spacecraft can generate very large amounts of data.  However, both the storage available onboard the spacecraft and the bandwidth available to transmit the data to ground stations are limited.  Thus, data compression is needed.  What algorithms have been used successfully on spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="36" LastEditorUserId="36" LastEditDate="2013-08-19T14:26:53.613" LastActivityDate="2013-08-19T14:26:53.613" Title="What data compression algorithms have been successfully used in spacecraft?" Tags="|communication|data-storage|data-systems|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="318" PostTypeId="2" ParentId="316" CreationDate="2013-07-17T21:31:19.937" Score="13" Body="&lt;p&gt;It depends on your definition of exploration missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Talking about human space flight into low Earth orbit, yes, there have been plenty of plants and successful experiments involving growing plants in space. Some are still going on on-board the ISS. Astronauts also, intentionally or unintentionally, let stuff grow which is actually part of their food supply (fruits etc.) sometimes. However, plants have never been used in a &quot;self-sustaining&quot; way over multiple generations for e.g. generating oxygen. Although, literature on past experiments with plants in space comes to the conclusion that this is possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If exploration means out of Earth orbit, there are only very few occasions where plants have been carried. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Zond_program#Circumlunar_missions&quot;&gt;Soviet circumlunar Zond missions&lt;/a&gt; carried, besides lots of different animals, also plants. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-17T22:15:25.233" LastActivityDate="2013-07-17T22:15:25.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="319" PostTypeId="5" CreationDate="2013-07-17T21:34:21.550" Score="0" Body="&lt;p&gt;This tag is to be used when question refers to how human usual activities change when in space: sleeping, eating, washing and so on.&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-07-17T21:59:47.373" LastActivityDate="2013-07-17T21:59:47.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="320" PostTypeId="4" CreationDate="2013-07-17T21:34:21.550" Score="0" Body="Question tagged with &quot;space–life&quot; are related to how life is conducted aboard a space ship or an artificial satellite." OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-07-17T21:59:53.047" LastActivityDate="2013-07-17T21:59:53.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="321" PostTypeId="2" ParentId="306" CreationDate="2013-07-17T21:43:05.583" Score="15" Body="&lt;p&gt;For an object &lt;strong&gt;A&lt;/strong&gt; to be gravitationally bound to another body &lt;strong&gt;B&lt;/strong&gt; that is orbiting a larger body &lt;strong&gt;C&lt;/strong&gt; itself, &lt;strong&gt;A&lt;/strong&gt; must be inside body &lt;strong&gt;B&lt;/strong&gt;'s &lt;a href=&quot;http://en.wikipedia.org/wiki/Hill_sphere&quot;&gt;Hill sphere&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the derivation of the radius of the Hill sphere does not take into account things like weirdly-shaped space stations with very complex gravitational fields, but rather it assumes perfectly spherically symmetric bodies &lt;strong&gt;B&lt;/strong&gt;, &lt;strong&gt;C&lt;/strong&gt;, and a massless &lt;strong&gt;A&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the concept doesn't apply as-is, but let's use it anyway by making the assumption that all of the ISS is compressed into a small sphere of homogeneous density, as dense as its materials will allow. Taking this &quot;idealized&quot; ISS as an example, &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station&quot;&gt;the following numbers apply&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mass: roughly 450,000 kg&lt;/li&gt;&#xA;&lt;li&gt;altitude: between 435 km and 330 km.&lt;/li&gt;&#xA;&lt;li&gt;with the mean radius of Earth 6371km, this implies &#xA;&lt;ul&gt;&#xA;&lt;li&gt;a semi-major axis of 6753.5 km&lt;/li&gt;&#xA;&lt;li&gt;an orbital eccentricity of 0.0078&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Then, using 5.972e24 kg for the mass of the Earth, the radius of the ISS' Hill sphere is about 2 metres. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hill sphere is a more complete definition of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Sphere_of_influence_%28astrodynamics%29&quot;&gt;sphere of influence&lt;/a&gt;, which is the region in space where the gravity of body B dominates over body C. For the ISS, the sphere of influence is about 15 cm. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So given these numbers, and knowing that it's &lt;em&gt;true&lt;/em&gt; gravitational field is vastly more complex than just that little idealized sphere, it's pretty much impossible to orbit the ISS. As AlanSE noted, you can put things in &lt;em&gt;apparent&lt;/em&gt; orbits, but these are normally only transient and will cease being close to the ISS after a few dozen of these &quot;orbits&quot;. Another way to understand that is by looking at the &lt;a href=&quot;http://en.wikipedia.org/wiki/Three-body_problem#Circular_restricted_three-body_problem&quot;&gt;three body problem&lt;/a&gt;, particularly at the derivation of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point&quot;&gt;Lagrange points&lt;/a&gt;. The thing to note is that the Hill sphere is the region where there is &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_stability&quot;&gt;orbital stability&lt;/a&gt; (in the phase space of the differential equations that is, not celestial orbits), meaning, body's that start just outside the Hill sphere will show &lt;em&gt;diverging&lt;/em&gt; orbital behaviour, whereas bodies that start just inside the Hill sphere will show &lt;em&gt;stable&lt;/em&gt; or &lt;em&gt;converging&lt;/em&gt; orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things will change though if the ISS were placed in deep interalactic space, far away from any celestial body. In principle, perturbations from all those remote sources will be completely negligible, and the ISS would gravitationally dominate a vast region in space, making orbits about it possible. Don't expect these orbits to be &lt;em&gt;fast&lt;/em&gt; though; I haven't run the numbers, but I expect anything thrown faster than a few mm/s will already be moving beyond ISS escape velocity. Also, don't expect these orbits to be anywhere near Keplerian; as I mentioned, the mass distribution of the ISS is nowhere near regular, and so the orbits about it will also deviate significantly from nice conic sections.     &lt;/p&gt;&#xA;" OwnerUserId="97" LastEditorUserId="97" LastEditDate="2013-07-19T13:24:15.750" LastActivityDate="2013-07-19T13:24:15.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="322" PostTypeId="1" AcceptedAnswerId="324" CreationDate="2013-07-17T21:43:59.220" Score="11" ViewCount="938" Body="&lt;p&gt;After returning from the Apollo 11 mission, astronauts were &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11#Splashdown_and_quarantine&quot; rel=&quot;noreferrer&quot;&gt;placed in quarantine&lt;/a&gt; for 18 days (July 24 - August 10, 1969).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/T1yxy.jpg&quot; alt=&quot;The crew of Apollo 11 in quarantine after returning to Earth, visited by Richard Nixon&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Photo: The crew of Apollo 11 in &lt;a href=&quot;https://en.wikipedia.org/wiki/Quarantine&quot; rel=&quot;noreferrer&quot;&gt;quarantine&lt;/a&gt; after returning to Earth, visited by Richard Nixon (source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Apollo_11_crew_in_quarantine.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was this (useless) practice continued after that mission?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="49" LastEditDate="2013-07-18T02:35:19.150" LastActivityDate="2013-07-18T02:35:19.150" Title="Was quarantine continued after Apollo 11?" Tags="|apollo-program|quarantine|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="323" PostTypeId="1" CreationDate="2013-07-17T21:44:50.427" Score="17" ViewCount="6124" Body="&lt;p&gt;I understand ISS uses &lt;a href=&quot;http://en.wikipedia.org/wiki/Control_moment_gyroscope#International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;control moment gyros&lt;/a&gt; as the primary attitude control system. As the article states, CMGs eventually get &quot;saturated&quot;, a state in which they can no longer absorb momentum. This can only be rectified with an external torque, from the onboard thrusters or the (significant) gravity gradient torque. How often does this need to be done on ISS, typically?&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="6816" LastEditorDisplayName="user29" LastEditDate="2017-04-06T03:31:03.770" LastActivityDate="2017-04-06T03:31:03.770" Title="How often must the ISS desaturate its control moment gyros?" Tags="|iss|attitude|operations|reaction-wheel|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="324" PostTypeId="2" ParentId="322" CreationDate="2013-07-17T21:48:11.280" Score="13" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_11#Splashdown_and_quarantine&quot;&gt;Wikipedia&lt;/a&gt;, it continued for the Apollo 12 &amp;amp; 14 missions as well.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This practice would continue for &lt;strong&gt;two more Apollo missions, Apollo 12 and Apollo 14&lt;/strong&gt;, before the Moon was proven to be barren of life and the quarantine process dropped.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-07-17T21:48:11.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="325" PostTypeId="2" ParentId="317" CreationDate="2013-07-17T21:50:49.323" Score="13" Body="&lt;p&gt;I will focus on meteorological satellites.  They're a prime example of satellites that measure very large quantities of data (and one I have personal experience with).  The bottom line is: they don't really compress the data, but they might degrade/limit it.  A specific property of meteorological satellites is that the users want the data &lt;em&gt;fast&lt;/em&gt;, so that could make a difference compared to other kinds of satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c3/sec3-1.htm&quot;&gt;Advanced Very High Resolution Radiometer (AVHRR)&lt;/a&gt; is a meteorological imager that has flown on meteorological satellites from NOAA and EUMETSAT since 1979.  It images the entire Earth at a resolution of 1 km in five simultaneously operated channels.  This resolution is so high, that the relatively short time during which it has a downlink connection is not enough for the NOAA satellites to downlink all the data.  Therefore, they downlink global data only in a special format known as &lt;em&gt;Global Area Coverage&lt;/em&gt; (GAC): 4 adjacent pixels are averaged, and 2 out of 3 scanlines are ignored.  Arguably, this is a kind of lossy compression.  Users that need the full-resolution data for their own region can download it directly from the satellite (and national meteorological agencies do), or pre-order full-resolution data for specific regions (but not globally).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three properties are limited:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;bandwidth&lt;/li&gt;&#xA;&lt;li&gt;storage&lt;/li&gt;&#xA;&lt;li&gt;computational power &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;With enough computational power, the NOAA satellites could compress all the data and downlink all of it.  Alas, the NOAA KLM series dates from 1999 and they don't have the computational power.  Since then, all the three aforementioned properties have grown, and as far as meteorological satellites are concerned, it appears bandwidth and storage have grown more than computational power.  Hence, I don't think there's any advanced compression going on for the vast amounts of data measured by meteorological satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, there are other satellites that also collect large quantities of data.  I have no expertise on those, but maybe other people can contribute with relevant answers.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2013-07-17T22:07:16.357" LastActivityDate="2013-07-17T22:07:16.357" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="326" PostTypeId="5" CreationDate="2013-07-17T22:48:51.950" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T22:48:51.950" LastActivityDate="2013-07-17T22:48:51.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="327" PostTypeId="4" CreationDate="2013-07-17T22:48:51.950" Score="0" Body="Questions about objects in orbit which are human made, such as communications and GPS satellites, orbital observatories and space stations, and other man-made object in permanent or semi-permanent orbits around the Earth, other celestial objects, aside from the Sun, or even other satellites. Not to be confused with non-orbited man-made objects in space, that are usually referred to as &quot;probes&quot;, or simply &quot;spacecraft&quot;." OwnerDisplayName="user40" LastEditorUserId="168" LastEditDate="2015-02-22T11:12:04.260" LastActivityDate="2015-02-22T11:12:04.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="328" PostTypeId="5" CreationDate="2013-07-17T22:50:57.447" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-17T22:50:57.447" LastActivityDate="2013-07-17T22:50:57.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="329" PostTypeId="4" CreationDate="2013-07-17T22:50:57.447" Score="0" Body="Questions about objects in orbit. Consider tagging with artificial-satellites or natural-satellites." OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-18T18:43:42.447" LastActivityDate="2013-07-18T18:43:42.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="330" PostTypeId="2" ParentId="132" CreationDate="2013-07-17T23:55:50.700" Score="13" Body="&lt;p&gt;To fully counteract the reduced gravity, the extra weight would need to be &lt;strong&gt;distributed evenly across &lt;em&gt;every cell&lt;/em&gt; in your entire body.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of weighted clothing would &lt;strong&gt;certainly help with muscles&lt;/strong&gt; like those in your legs, which serve to support you. &lt;strong&gt;Other areas of your body&lt;/strong&gt;, however, &lt;strong&gt;wouldn't be helped&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take, for example, your &lt;strong&gt;vital organs&lt;/strong&gt;. Your organs are optimized to function at 9.81-ish m/s². What if a problem with the heart is discovered when immensely long amounts of time are spent in a different gravitational field? You would need to apply the extra weight to at least every cell in the heart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another area to note is &lt;strong&gt;digestion&lt;/strong&gt;. Since your digestive system is optimized to function at 1 g, there may be problems when in 0.7 g, etc. You would then probably need to apply the extra weight to &lt;em&gt;every cell of food&lt;/em&gt; that enters the astronaut. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nonetheless, the idea of wearing a weighted vest &lt;em&gt;would&lt;/em&gt; solve, or at least reduce, some of the problems with reduced gravity, such as reduced bone mass in your legs, etc. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;&lt;sup&gt;But really, who wants to wear a weighted vest when you could &lt;em&gt;jump nine feet high&lt;/em&gt;?&lt;/sub&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2016-03-05T16:09:32.017" LastActivityDate="2016-03-05T16:09:32.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="331" PostTypeId="1" AcceptedAnswerId="362" CreationDate="2013-07-18T00:02:16.970" Score="15" ViewCount="746" Body="&lt;p&gt;Of all the destinations within our solar system, the one that intrigues me the most is Europa. Every location has its own merits and beauty, but I'm most optimistic about finding life on Europa. It's all under ice, but perhaps there's enough warm water to be hospitable to life. Putting aside the marketing and political barriers, are there unsolved technical problems that would keep us from completing this successfully?&lt;/p&gt;&#xA;" OwnerUserId="79" LastEditorUserId="70" LastEditDate="2013-07-18T13:43:01.330" LastActivityDate="2016-07-06T09:51:34.157" Title="What are the constraints for a successful robotic mission to Europa?" Tags="|life|robotic-missions|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="332" PostTypeId="5" CreationDate="2013-07-18T00:14:49.957" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-18T00:14:49.957" LastActivityDate="2013-07-18T00:14:49.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="333" PostTypeId="4" CreationDate="2013-07-18T00:14:49.957" Score="0" Body="Questions about the Kennedy Space Center, the NASA launch site in Florida. " OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-18T18:45:39.627" LastActivityDate="2013-07-18T18:45:39.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="334" PostTypeId="5" CreationDate="2013-07-18T00:15:33.790" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-18T00:15:33.790" LastActivityDate="2013-07-18T00:15:33.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="335" PostTypeId="4" CreationDate="2013-07-18T00:15:33.790" Score="0" Body="Questions regarding space exploration safety issues and procedures." OwnerDisplayName="user40" LastEditorDisplayName="user12" LastEditDate="2013-08-13T15:33:42.040" LastActivityDate="2013-08-13T15:33:42.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="336" PostTypeId="2" ParentId="323" CreationDate="2013-07-18T00:16:21.937" Score="9" Body="&lt;p&gt;Virtually all systems that do this use some sort of a continuous process. The typical process is something like this:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A large change in momentum is required. The momentum change is handled by the reaction wheel. Reaction wheels basically work by changing the rotation of the spacecraft. You can think of it like standing in the center a &lt;a href=&quot;http://en.wikipedia.org/wiki/Roundabout_%28play%29&quot; rel=&quot;noreferrer&quot;&gt;roundabout&lt;/a&gt; and turning around in a circle. The roundabout will actually start turning the other direction. Reaction wheels have a maximum speed about which they can rotate, thus limiting their maximum limits of the absorbed momentum.&lt;/li&gt;&#xA;&lt;li&gt;The system dumps the momentum slowly in other forms. For most low earth orbiting spacecraft, they use magnetometers, which essentially re-orient the spacecraft using the magnetic field of the Earth. Other systems might include slight thruster maneuvers, or holding the momentum until the spacecraft needs to stop. Alternatively, small thrusters, often known as Reaction Control System (RCS) can be used. Typically, the reaction wheels try to bleed of momentum as quickly as they can, in a continuous fashion, if using passive means or magnetorquers, or they hold off as long as they possibly can and use an RCS sytem&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The gravity gradient works basically because if you are rotating an object, the heaviest part tends to point towards the Earth. This is a completely passive system, which can work quite well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.dept.aoe.vt.edu/~cdhall/courses/aoe4065/NASADesignSPs/tp3677.pdf&quot; rel=&quot;noreferrer&quot;&gt;information&lt;/a&gt; that I have available indicates that the ISS does not use magnetorquers, but rather will store the momentum until it is so high that it must desaturate, when it accomplishes via the use of small thrusters. This is modeled quite accurately in &lt;a href=&quot;http://forum.kerbalspaceprogram.com/showthread.php/37945-Can-reaction-wheel-kill-a-certain-angular-momentum-of-ship-without-working-forever&quot; rel=&quot;noreferrer&quot;&gt;Kerbal Space Program&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal then of the reaction wheels is to do two things.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Sometimes the momentum is counteracted by the reverse direction after a small period of time. &lt;/li&gt;&#xA;&lt;li&gt;Minimize the number of times the RCS thrusters have to fire.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;How often this happens isn't public knowledge, but I would speculate it is around a couple of times per day. However, as @Chris pointed out, there is a presentation that covers some of this information from &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100024204_2010020499.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;. Specifically, there's a few numbers that can be pulled out:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The average amount of torque stored before a desaturate is around 14000 lb-ft/sec, or 18000 N.&lt;/li&gt;&#xA;&lt;li&gt;&quot;Hundreds of kg/year of propellant&quot; can be saved by reducing RCS use via clever algorithms.&lt;/li&gt;&#xA;&lt;li&gt;Assuming a thruster ISP of 300 (Slightly high), the fuel per wheel desaturate is approximately 60N, or 6 kg.&lt;/li&gt;&#xA;&lt;li&gt;To save &quot;Hundreds of kg/year&quot;, and still using some fuel, let's estimate that the fuel usage for RCS/year is about 300 kg.&lt;/li&gt;&#xA;&lt;li&gt;Put all of that together, and there is roughly 50 desaturate maneuvers per year. I believe this estimate is somewhat low, I would expect 1-2 per week.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-18T13:25:31.807" LastActivityDate="2013-07-18T13:25:31.807" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="337" PostTypeId="2" ParentId="312" CreationDate="2013-07-18T00:25:24.617" Score="11" Body="&lt;p&gt;Virtually all spacecraft use one of two time conventions, or more likely, both of these&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Universal Time Coordination (UTC)&lt;/li&gt;&#xA;&lt;li&gt;GPS Time&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The difference between these two is slight, but changes periodically. The difference is currently 16 seconds, as can be seen on &lt;a href=&quot;http://leapsecond.com/java/gpsclock.htm&quot;&gt;this clock&lt;/a&gt;. The reason for the difference is the failure for GPS time to keep track of leap seconds (This is deliberate, to not have to worry about missing seconds). The reason for this is that it is an international standard, established for things where a common time metric is needed across the world. Satellites move through 24 time zones in a very short period of time, the ISS less than 100 minutes, so it makes sense to have a common anchor point. Typically, navigation systems use GPS time, while other systems use UTC time, and that is exactly what is used on the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typically, the spacecraft time is set to UTC even for non-Earth orbiting systems, where the speed of light delay is significant. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-18T00:25:24.617" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="338" PostTypeId="2" ParentId="331" CreationDate="2013-07-18T00:48:24.927" Score="2" Body="&lt;p&gt;The answer to your question very much depends upon what your mission objectives are. Let's look at some of the possibilities, in order of difficulty.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Crashing an object into Europa:&lt;/strong&gt; I think we can manage this one. We successfully crash-landed the Huygens probe on the surface of Titan in 2005 (incidentally resulting in the &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/thumb/b/bc/Huygens_surface_color.jpg/200px-Huygens_surface_color.jpg&quot; rel=&quot;nofollow&quot;&gt;only photograph&lt;/a&gt; we have from the surface of any celestial body outside the inner solar system.)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Landing a rover/craft on the surface of Europa:&lt;/strong&gt; We have fortunately had lots of practice landing rover-style craft on Mars, so the technology to do this successfully is already mostly developed. In fact, NASA is planning on &lt;a href=&quot;http://news.yahoo.com/jupiters-moon-europa-target-possible-nasa-lander-212201869.html&quot; rel=&quot;nofollow&quot;&gt;doing exactly this&lt;/a&gt; starting in 2020. This could give us valuable information about the viability of life on Europa, though it wouldn't be able to bring us into direct contact with it. These rovers could, for example, be able take very detailed analyses of Europa's chemical composition, paving the way for future underwater probes. Also, over the course of geologic time water from the deep oceans could migrate into the surface ice, which would mean that &lt;em&gt;even surface rovers could detect signatures of ancient life&lt;/em&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Drilling down into the ice crust to the oceans underneath:&lt;/strong&gt; In order to successfully either prove or disprove whether there is indeed &lt;em&gt;currently&lt;/em&gt; life on Europa, we would need to send a probe of some kind under Europa's surface ice into its liquid oceans. Unfortunately, this presents a number of very difficult (and unprecedented) engineering challenges that we would have to overcome first. If Europa even has liquid oceans (which we don't know for sure yet), they would be buried under at somewhere between 5km to 100km of granite-hard ice. At the upper ends of that scale, we don't even fully understand how ice behaves at those pressures, much less how to drill through it properly. Even assuming we could get Earth's finest drilling equipment to Europa, we would have to take enormously complicated precautions to reseal and reinforce the hole as it is dug, both to prevent the tunnel from collapsing on itself and to keep Europa's internal pressure from suddenly erupting through the hole. An underwater probe for Europa is probably out of the question both because of current understanding of the conditions on Europa, and our drilling technology.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="47" LastEditorUserId="47" LastEditDate="2013-07-18T06:37:16.727" LastActivityDate="2013-07-18T06:37:16.727" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="339" PostTypeId="5" CreationDate="2013-07-18T00:57:28.923" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-18T00:57:28.923" LastActivityDate="2013-07-18T00:57:28.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="340" PostTypeId="4" CreationDate="2013-07-18T00:57:28.923" Score="0" Body="Questions regarding information transfer between spacecraft, and between spacecraft and Earth." OwnerDisplayName="user40" LastEditorDisplayName="user12" LastEditDate="2013-08-15T17:37:20.740" LastActivityDate="2013-08-15T17:37:20.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="341" PostTypeId="2" ParentId="323" CreationDate="2013-07-18T01:11:59.873" Score="10" Body="&lt;p&gt;I did some more digging with this, and while I didn't find an explicit answer to my question, I did come across some interesting resources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.45.3919&amp;amp;rep=rep1&amp;amp;type=pdf&quot;&gt;this&lt;/a&gt; paper states that they use reaction control jets to desaturate when the total momentum reaches 13000 ft-lbf-sec. However, there is no information about how long it takes for this amount to accumulate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100024204_2010020499.pdf&quot;&gt;this (fairly recent) presentation&lt;/a&gt; is very informative. It states that a &quot;Torque Equilibrium Attitude&quot; is used so the cumulative torque over one orbit is &quot;approximately zero&quot;. So, as in the first paper, CMG torques will be (nominally) periodic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Towards the end, a great chart of propellant mass vs. time (over a 3 month period) is shown. Excluding thruster tests and propellant transfers, I count four maneuvers, all during rendezvous/docking activities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of this leads me to conclude three things. One, the frequency with which this needs to happen is dependent on the docking schedule, as they impart significant non-periodic torques. Two, the nominal attitude is such that there is very little secular buildup in angular momentum. Three, the desaturation maneuvers with thrusters are not needed very often (e.g. daily), and in fact over a three month period, the only thruster firings occurred during docking activities. So, desaturation maneuvers &lt;strong&gt;not&lt;/strong&gt; due to docking happen, &lt;strong&gt;at most&lt;/strong&gt;, perhaps 3-4 times per year, and likely less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: I should also mention that there are quite a few journal publications from the 90s written about optimal control laws for the ISS. However, they were not open access, and an answer to this question (simulation-based or on-orbit) was not found in the abstract/first page.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-07-18T01:24:21.387" LastActivityDate="2013-07-18T01:24:21.387" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="342" PostTypeId="2" ParentId="264" CreationDate="2013-07-18T01:28:24.403" Score="3" Body="&lt;p&gt;I'm not sure I can give you a specific amount of propllent, but I can give you a back of the envelope answer.  Perhaps someone can add details from Shuttle Program documents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The External Tank (ET) came off shortly after Main Engine Cutoff (MECO).  After that, the Shuttle made one or more OMS burns depending on when the launch was made in the Program's history.  These burns raised the orbit's perigee and circularized the orbit.  The OMS pods had about 300 m/s delta-V available for the Orbiter alone.  Making a rough estimate that half (?) of this (150 m/s) was used for orbital insertion and half was used for the deorbit burn, you would need to provide an additional 150 m/s of delta-V to the ET to get it into the shuttle's low orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that an ET at this altitude would quickly re-enter due to the small but significant atmospheric drag.  You would therefore have to either add additional delta-V to further raise the orbit or plan on reboosting the tank every 90-180 days like the ISS does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Don't forget the rocket equation either.  In addition to providing the additional delta-V to the ET, you have to provide additional delta-V for the fuel you use to provide this additional delta-V and so on, and so on, and....&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-18T01:33:58.317" LastActivityDate="2013-07-18T01:33:58.317" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="343" PostTypeId="5" CreationDate="2013-07-18T01:40:14.840" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-18T01:40:14.840" LastActivityDate="2013-07-18T01:40:14.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="344" PostTypeId="4" CreationDate="2013-07-18T01:40:14.840" Score="0" Body="Questions about a hypothetical transportation system between Earth and space that would not require large rockets to transport payloads." OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-18T23:33:40.637" LastActivityDate="2013-07-18T23:33:40.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="345" PostTypeId="2" ParentId="312" CreationDate="2013-07-18T01:48:49.827" Score="15" Body="&lt;p&gt;While it is true that just about everything uses UTC on the ISS, the crew's schedule moves around.  For example, if there is an upcoming EVA that must be done at a particular time, or a spacecraft arrival/departure scheduled, ground control will adjust the sleep cycle of the crew over several days.  Thus the &quot;day&quot; as defined on the station by when the crew is awake can and does change.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-18T01:48:49.827" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="346" PostTypeId="2" ParentId="89" CreationDate="2013-07-18T02:10:30.963" Score="2" Body="&lt;p&gt;I came across a study, which I believe comes close to the question, is using VR to treat the fear of Flight and claustrophobia. The abstract of the study can be found &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/12381038&quot; rel=&quot;nofollow&quot;&gt;here.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal of the study was to understand the effect of Virtual Reality Graded Exposure Therapy. They had a small sample size of thirty individuals diagnosed with the fear of flying. They were then randomly assigned to one of three groups.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Three Groups: VRGET with no physiological feedback (VRGETno), VRGET with&#xA;  physiological feedback (VRGETpm), or systematic desensitization with&#xA;  imaginal exposure therapy (IET). Eight sessions were conducted once a&#xA;  week. During each session, physiology was measured to give an&#xA;  objective measurement of improvement over the course of exposure&#xA;  therapy.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Observations during the three month followup revealed significant difference in flying behavior between the groups. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Only one participant (10%) who received IET, eight of the ten&#xA;  participants (80%) who received VRGETno, and ten out of the ten&#xA;  participants (100%) who received VRGETpm reported an ability to fly&#xA;  without medication or alcohol at three-month followup.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The results suggest that VRGET can be a possible treatment for the fear of flying.&#xA;Implications of this procedure could, essentially, be used for treatments of other psychological issues. &lt;/p&gt;&#xA;" OwnerUserId="70" LastEditorUserId="70" LastEditDate="2013-07-24T13:50:41.140" LastActivityDate="2013-07-24T13:50:41.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="347" PostTypeId="2" ParentId="149" CreationDate="2013-07-18T02:27:13.080" Score="9" Body="&lt;p&gt;For your question of &quot;Who ultimately decided if the shuttle was space-worthy?&quot; the answer is the signatories of the Certificate of Flight Readiness (CoFR) Endorsement.  You can see the STS-116 CoFR Endoresement here: &lt;a href=&quot;http://www.nasa.gov/pdf/163551main_116-CoFR.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/pdf/163551main_116-CoFR.pdf&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-18T02:27:13.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="348" PostTypeId="2" ParentId="219" CreationDate="2013-07-18T02:53:42.497" Score="14" Body="&lt;p&gt;According to &lt;a href=&quot;http://abcnews.go.com/Technology/space-station-astronauts-eyesight-affected-long-stays-orbit/story?id=15904338#.UedW72RgYfs&quot; rel=&quot;noreferrer&quot;&gt;ABC News&lt;/a&gt;, yes:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But now, as American astronauts spend more and more time in space, they've noticed they're returning to Earth with a surprising malady: &lt;strong&gt;They cannot focus their eyes properly after they come home&lt;/strong&gt;, and for some the problem &lt;strong&gt;seems permanent&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;p&gt;A fifth of the astronauts tested showed a &lt;strong&gt;flattening of the rear of the eyeball&lt;/strong&gt;, affecting their ability to focus their eyes. A third showed &lt;strong&gt;expansion of the space surrounding the optic nerve&lt;/strong&gt; that's normally filled with cerebral spinal fluid.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Also according to &lt;a href=&quot;http://lightyears.blogs.cnn.com/2012/02/10/astronaut-feels-spaces-toll-on-his-body/&quot; rel=&quot;noreferrer&quot;&gt;CNN&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In the past few years, &lt;strong&gt;about half&lt;/strong&gt; of the astronauts aboard the international space station have &lt;strong&gt;developed an increasing pressure inside their heads, an intracranial pressure that reshapes their optic nerve&lt;/strong&gt;, causing a significant shift in the &lt;strong&gt;eyesight of male astronauts&lt;/strong&gt;. Doctors call it &lt;strong&gt;papilledema&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Interestingly, only &lt;em&gt;male&lt;/em&gt; passengers seem to have been affected:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Female space travelers have &lt;strong&gt;not&lt;/strong&gt; been affected.&lt;/p&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;p&gt;Barratt is one of &lt;strong&gt;10 male astronauts&lt;/strong&gt;, all &lt;strong&gt;older than 45&lt;/strong&gt;, who have &lt;strong&gt;not recovered&lt;/strong&gt;. Barratt returned from a &lt;strong&gt;six-month stint&lt;/strong&gt; aboard the station in October 2009 and has &lt;strong&gt;experienced a profound change&lt;/strong&gt; in his sight.&lt;/p&gt;&#xA;&lt;p&gt;He &lt;strong&gt;used to be nearsighted&lt;/strong&gt;. But now, the space veteran says he’s &lt;strong&gt;eagle-eyed at long distance but needs glasses for reading&lt;/strong&gt;. There is no treatment and &lt;strong&gt;no answers as to why female space flyers are not affected.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Also &lt;a href=&quot;http://www.aviationweek.com/Article.aspx?id=/article-xml/asd_06_07_2013_p03-01-586136.xml&quot; rel=&quot;noreferrer&quot;&gt;Aviation Week&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Nineteen ISS astronauts have &lt;strong&gt;developed symptoms of impaired vision&lt;/strong&gt; since the ailment was first recognized in 2005, according to Dr. Christian Otto, a Universities Space Research Association remote medicine specialist who serves as the principal investigator for the NASA-sponsored Prospective Observational Study of Ocular Health.&lt;/p&gt;&#xA;&lt;p&gt;The study, ultimately involving a &lt;strong&gt;dozen closely followed international astronauts&lt;/strong&gt;, will search for a link between the &lt;strong&gt;blurred vision and the long observed shift of fluid from the lower torso to the chests and heads of fliers as they adjust to weightlessness&lt;/strong&gt;. The fluid shift now appears to affect the &lt;strong&gt;eyes&lt;/strong&gt; as well as the &lt;strong&gt;cardiovascular&lt;/strong&gt; and &lt;strong&gt;central nervous&lt;/strong&gt; systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;With this in mind, I think we can say that the reduced &lt;em&gt;gravity&lt;/em&gt; affects astronaut's vision - not the lack of objects for them to focus on.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-18T03:50:33.860" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="350" PostTypeId="2" ParentId="317" CreationDate="2013-07-18T04:07:23.420" Score="15" Body="&lt;p&gt;Image data is typically compressed using standard image compression algorithms. For example, the &lt;a href=&quot;http://www.gwg.nga.mil/ntb/baseline/docs/2500c/index.html&quot;&gt;National Imagery Transmission Format&lt;/a&gt;, MIL-STD-2500C, defines a standard, extensible format for the transmission of imagery data within the military. The standard calls out several types of image compression compliant with the standard (such as JPEG). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Telemetry data is often not compressed at all. One rationale is that it is often several orders of magnitude less data than what is being generated by the payload (be it imagery, communications, or data from another type of instrument or sensor), and thus in the broad scheme of things not worth compressing. Telemetry is also often sent as a stream of data (rather than a file), which limits the types of compression schemes that could be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, radio frequency signals are often intentionally increased in size through techniques like &lt;a href=&quot;http://en.wikipedia.org/wiki/Forward_error_correction&quot;&gt;forward error correction&lt;/a&gt; (FEC) and &lt;a href=&quot;http://en.wikipedia.org/wiki/Convolutional_code&quot;&gt;convolution coding&lt;/a&gt;, which are means of adding redundant data to a signal to make the transmission more robust to transmission errors endemic to space communication links.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, there are standards for the lossless and lossy compression of spacecraft data. The &lt;a href=&quot;http://public.ccsds.org/about/default.aspx&quot;&gt;Consultative Committee for Space Data Systems (CCSDS)&lt;/a&gt; has published a set of standards for Space communication links. There are currently &lt;a href=&quot;http://public.ccsds.org/publications/SLS.aspx&quot;&gt;four released standards&lt;/a&gt; that discuss lossless data compression and image data compression.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="127" LastEditDate="2013-07-18T09:41:16.653" LastActivityDate="2013-07-18T09:41:16.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="351" PostTypeId="2" ParentId="219" CreationDate="2013-07-18T04:13:22.980" Score="6" Body="&lt;p&gt;While astronauts on the ISS &lt;em&gt;have&lt;/em&gt; developed eyesight problems, it is for completely different reasons than what you suggest in your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The distance of objects that these astronauts focus on is not a problem. In terms of optics, the human eye treats everything past about 20 feet away the exact same, so looking out of the cupola at Earth should give the astronauts plenty of healthy long-distance viewing. In fact, these astronauts don't probably experience anything different than the average white collar worker here on Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as mentioned in Undo's answer, the microgravity environment &lt;em&gt;can&lt;/em&gt; have very negative effects on astronauts vision, because of the relocation of fluids to the head. Because gravity is not acting like normal to push internal fluids down towards the feet, body fluids have more of a chance to build up in the cranial space. If the fluid movement is strong enough, it can actually physically push against the back of the eyeball, causing it to permanently warp, and ultimately resulting in distorted vision.&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2013-07-18T04:13:22.980" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="352" PostTypeId="1" AcceptedAnswerId="361" CreationDate="2013-07-18T04:18:46.390" Score="35" ViewCount="25405" Body="&lt;p&gt;Are there currently any projects underway to develop a telescope which will have the ability to directly observe an exoplanet with any clarity?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't mean blurry (yet impressive) images like this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/File:ESO_VLT_image_of_exoplanet_HD_95086_b.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/edrd9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But maybe something where we can see actual features on the surface or atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that this may not be possible with conventional telescopes, but I was wondering what technologies would be needed to see surface or atmospheric features on an exoplanet say between 5-50 light years from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If no current projects are underway, when would be a feasible timeframe to see such a telescope?&lt;/p&gt;&#xA;" OwnerUserId="84" LastActivityDate="2022-06-08T07:18:55.583" Title="When will we have the technology to directly observe an exoplanet with significant clarity?" Tags="|exoplanet|telescope|imaging|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="353" PostTypeId="2" ParentId="264" CreationDate="2013-07-18T04:37:12.773" Score="17" Body="&lt;h1&gt;Shuttle Answer&lt;/h1&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt; I corrected a (huge) mistake that incorrectly penalized the vehicle for lifting the nominal payload to MECO-1, when the question was asking about eliminating a nominal payload in favor of boosting the empty ET into a stable orbit instead.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Using rough numbers, it would cost everything / not work. Putting the ET in orbit would have eliminated the ability of the shuttle to carry any other payload, and an ET used during launch would need significant retrofit to be useful for anything else.&lt;/p&gt;&#xA;&lt;p&gt;First the math, then a few other reasons why this seems like a bad idea.&lt;/p&gt;&#xA;&lt;h2&gt;Math&lt;/h2&gt;&#xA;&lt;h3&gt;Assumptions&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mass of &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_external_tank#Standard_Weight_Tank&quot; rel=&quot;nofollow noreferrer&quot;&gt;ET (standard weight)&lt;/a&gt; = 35,000 kg&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle#Overview&quot; rel=&quot;nofollow noreferrer&quot;&gt;typical payload capacity mass&lt;/a&gt; = 22,700 kg&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://library.thinkquest.org/21149/crafts/shuttleorb.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbiter dry mass&lt;/a&gt; = 82,288 kg (using largest of the numbers because orbiter is not really dry at burnout, so this conservatism helps to account for residuals, crew, other consumable fluids, etc.)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Orbital_Maneuvering_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific impulse of OMS engines&lt;/a&gt; = 316 s&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; required to circularize orbit after MECO = 150 m/s (see other answer for justification)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h3&gt;Calculations&lt;/h3&gt;&#xA;&lt;p&gt;For a nominal ascent trajectory 140,000 kg can be put into the MECO-1 orbit, which is divided between the ET, Orbiter, and Payload.&lt;/p&gt;&#xA;&lt;p&gt;Nominally, the ET is then staged, which reduced the mass prior to the OMS circularization burn to 105,000 kg. Using the rocket equation we can calculate the OMS propellant required for that burn:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$mp_{nominal} = mf_{nominal} \cdot \left(e^{\frac{\Delta v}{g \cdot I_{sp}}} - 1\right)$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$I_{sp}       = 361 s  $&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$ g         = 9.81 \frac{m}{s^2}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\Delta v        = 150 m/s  $&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$    mf_{nominal} = 105,000 kg  $&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$    mp_{nominal} = 4,542.91kg  $&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;If instead we want to raise the orbit of the entire MECO-1 mass, we get a higher propellant weight.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$    mp_{BoostET} = mf_{BoostET} \cdot \left(e^{\frac{\Delta v}{g\cdot I_{sp}}} - 1\right)$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$    mf_{BoostET} = 140,000 kg$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$    mp_{BoostET} = 6,057.22 kg$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;So boosting the ET requires an additional 1,514 kg of OMS propellant. This is less than the 22,700 kg payload allocation, so it appears possible (although the Shuttle would have to be retrofitted to hold additional OMS propellant in the cargo bay and supply it to the OMS engines — surely possible, but not trivial).&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;Note: One obvious option for increasing performance is using the higher specific impulse SSMEs rather than the OMS engines for the circularization burn. This would either require a direct-ascent trajectory (possible, but likely only for low-altitude orbits), or an ability to restart the SSMEs (or at least one of them). Again possible but non-trivial.&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;h2&gt;Challenges&lt;/h2&gt;&#xA;&lt;p&gt;But despite the first-order technical feasibility, there would be significant challenges associated with this approach.&lt;/p&gt;&#xA;&lt;p&gt;The main issue is that upon reaching orbit the tank would need significant rework to be useful. Remember, it wasn't designed to be a habitat, it was designed to hold fuel and oxidizer for the shuttle during ascent. Any accommodations for it to serve a dual purpose would have a cost or weight penalty. More importantly, any on-orbit modifications would be additional missions — probably EVAs by astronauts on follow-up missions.&lt;/p&gt;&#xA;&lt;p&gt;Once empty, the tank is also a (relatively) light-weight object for its size (i.e. it has a low ballistic coefficient). This would cause it to re-enter more quickly than typical payloads, which might be 2/3 the mass but (maybe) 1/10 the cross section. This would require putting the ET in a higher than normal orbit (reducing mass available for other payload) or working on a clock to add adjunct station-keeping propulsion to the ET husk (before de-orbit).&lt;/p&gt;&#xA;&lt;p&gt;So I'll temper my original response and say technically this approach was probably feasible, but likely to be expensive and represent a non-trivial evolution for a program that historically struggled to maintain a high flight rate, demonstrate a good safety record, or operate affordably.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="8693" LastEditDate="2020-11-02T22:27:46.963" LastActivityDate="2020-11-02T22:27:46.963" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="354" PostTypeId="2" ParentId="308" CreationDate="2013-07-18T05:03:45.330" Score="9" Body="&lt;p&gt;It would seem that the ultra-cold vacuum would kill off most of the bacteria, etc in the body, as well as quickly boil off all of the water content. Here's a lovely description of the process from &lt;a href=&quot;http://scienceinfocus.co.uk/qa/would-corpse-decay-space&quot; rel=&quot;noreferrer&quot;&gt;Focus magazine&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In space we can assume that there would be no external organisms such as insects and fungi to break down the body, but we still carry plenty of bacteria with us. Left unchecked, these would rapidly multiply and cause putrefaction of a corpse on board the shuttle or the ISS. Drifting exposed in the vacuum of space itself, however, this process would rapidly slow to a halt.&lt;/p&gt;&#xA;&lt;p&gt;The low pressure would initially boil off most of the water and what was left would freeze, halting any biological processes. Depending on the trajectory of the corpse, there might be some warming from the side facing the Sun, but all this would do is accelerate the rate of water loss, leaving only a dried husk.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="47" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-18T05:03:45.330" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="358" PostTypeId="2" ParentId="96" CreationDate="2013-07-18T05:54:54.453" Score="13" Body="&lt;p&gt;Probably the closest thing to that environment on Earth would be Lake Vostok, which is 4 kilometers below the antarctic surface. Very little, if any, light gets down there, and it's really really cold, but still liquid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scientists &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-23230864&quot;&gt;recently found&lt;/a&gt; bacteria living there, so it stands to reason that bacteria would survive on Enceladus, but the really amazing thing is that the bacteria are ones that go with fish and crustaceans and mollusks, which would indicate that there are actually molluscs, crustaceans and even fish living in Lake Vostok!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the BBC &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-23230864&quot;&gt;article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A large number of bacterial sequences, reports the team, were from&#xA;  &quot;animal commensals, mutualists and pathogens… including those&#xA;  associated with annelids, sea anemones, brachiopods, tardigrades and&#xA;  fish.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also discovered: eukaryotes and archaea.&lt;/p&gt;&#xA;" OwnerUserId="23" LastActivityDate="2013-07-18T05:54:54.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="359" PostTypeId="1" CreationDate="2013-07-18T06:01:55.507" Score="14" ViewCount="1431" Body="&lt;p&gt;How do spacecraft measure onboard (micro)gravity at any given point in time (especially when subject to the gravitational fields of multiple bodies)? I'm guessing that rudimentary accelerometers won't be sufficient.&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="25" LastEditDate="2013-07-20T22:52:26.210" LastActivityDate="2019-07-02T01:46:46.723" Title="How do spacecraft measure onboard gravity?" Tags="|gravity|technology|physics|instrument|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="360" PostTypeId="2" ParentId="359" CreationDate="2013-07-18T06:32:23.520" Score="4" Body="&lt;p&gt;As far as I can tell, we don't actually have reasonably-sized sensors capable of precisely determining microgravity, and that our official gravitational values are instead calculated from other information. With enough information about the masses of and distances to nearby objects, and the acceleration of a given spacecraft, we can pretty accurately calculate the gravitational force on the spacecraft without need for sensors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the International Space Station has &lt;a href=&quot;http://www.nasa.gov/centers/glenn/about/fs11grc.html&quot; rel=&quot;nofollow&quot;&gt;two systems of accelerometers&lt;/a&gt; (SAMS-II and MAMS) used to detect small vibrations on the frame of the ISS, but neither of them are actually sensitive enough to measure the net effective gravitational force on the ISS.&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2013-07-18T06:32:23.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="361" PostTypeId="2" ParentId="352" CreationDate="2013-07-18T07:19:27.263" Score="26" Body="&lt;p&gt;I'm afraid it would be extremely difficult - simply the number of photons reflected off a planet surface and reaching Earth (and the telescope lens, however big) within timeframe for a solid photo is too small to create any meaningful image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planets are not stationary; they orbit their stars, and that means long-exposure photo will show them as a trails. Of course the telescope could be made to follow the orbital movement and we could actually achieve the planet disc image eventually. Unfortunately though, they also pivot around their axis, and that means we are not getting their surface photos, just blurred lines around the disc. Now if we were smart enough, we could take short-timed photos multiple times on the same &quot;hour&quot; of the planet's &quot;day&quot;, and combining them we might get what we want - providing we somehow make out how long given planet's &quot;day&quot; is. But that's only for planets without or with thin atmosphere. If the planet has weather - that's the end, it's not repeatable at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, there - we already do have two techniques for taking decent photos of the exoplanets. First one - send a probe there, make it take photos and return - would take thousands of years to complete. The other - construct a telescope with lens enormous enough to capture enough photons reflected off given planet within a time frame that won't blur the surface beyond recognition - would cost 100s of trillions of dollars. The James Webb Space Telescope (the world's largest space telescope) costs almost 20 billion dollars and will not be able to 'resolve' exoplanets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;This actually could be done within somewhat more reasonable budget. You'd need a high-precision (not necessarily enormous lens size=brightness) telescope, with a sensor capable to register separate photons, not their sum over time - &quot;record a movie&quot; instead of just acquiring a still. The telescope would still need to follow the planet orbit, but registering the observations over long time and using autocorrelation function of the measurements it could determine the rotation period (day length) of given planet - specific features of terrain would reappear at specific places in regular intervals (one day apart) creating a cyclic function in the general noise. Knowing the &quot;day length&quot; and precise time of each photon, you could remap all your measured points onto their right locations of rotating sphere over time, and that way recreate the image of the whole surface - similarly how a modern photo camera uses its movement path recorded by accelerometers to recreate a static image from a long-exposure photo shot from shaky hand. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course this still requires telescopes better than anything we have, but it's well within reach of our contemporary technology and not on excessive budget.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="14447" LastEditDate="2016-05-18T01:22:23.970" LastActivityDate="2016-05-18T01:22:23.970" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="362" PostTypeId="2" ParentId="331" CreationDate="2013-07-18T07:21:27.770" Score="13" Body="&lt;p&gt;I think the major obstacle is the level of &lt;strong&gt;autonomousness&lt;/strong&gt; required for a full-fledged lander mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The communication delay between Earth and Jupiter &lt;a href=&quot;http://www.spaceacademy.net.au/spacelink/commdly.htm&quot; rel=&quot;nofollow&quot;&gt;is between half an hour and an hour&lt;/a&gt; (depending on the time of year), &lt;strong&gt;one way&lt;/strong&gt; (so the real delay is twice as long), but intelligent decisions have to be made real-time during the landing, and especially during the drilling phase (and/or when &lt;em&gt;diving&lt;/em&gt;, if it is to go down itself or send a probe).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Landing&lt;/strong&gt; as Europa's surface changes a lot, the spacecraft will have to explore the surface from a distance and somehow select a suitable landing spot. Of course, this can be done from low Europa orbit with the aid of people, since there is no rush yet. But given the communication delay, the landing itself will have to be carried out almost fully autonomously. As this will likely be a multi-billion dollar flagship mission, and the consequences of most failure types in this phase all mean end-of-mission, you &lt;em&gt;cannot&lt;/em&gt; afford to have anything go wrong here. This implies a &lt;em&gt;very&lt;/em&gt; robust AI landing system, that has withstood even the most challenging of tests. We've practiced a lot on Mars, but still, there is no system yet that the space agencies are willing to bet their billions on.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Drilling&lt;/strong&gt; after a successful landing, it's time to cut through the ice. Ideally, we've selected our landing spot well enough to have landed near a fracture, or at the very least on a thin piece of the ice crust. But of course, good engineers always design for worst case, so, we've just landed on the thickest, densest, coldest part on Europa's surface ice. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drilling through through ice is probably a lot harder than just dropping a small nuclear furnace on it, that just melts its way through. This can be a very sensitive issue, as getting to Jupiter is usually easiest after a few Earth-flybys. There was a huge political debate when the Cassini/Huygens probe did this, because there was a slight chance of the probe disintegrating in the Earth's atmosphere, meaning, it could break its RTG and spread several kilograms of plutonium out into the atmosphere. Plutonium is the most toxic substance known to man, so that even those few kg would do a lot of damage, regardless of &lt;em&gt;where&lt;/em&gt; it was dropped. So bringing a nuclear furnace could prove difficult to pull off &lt;em&gt;politically&lt;/em&gt;, or at the very least, increase flight time significantly because we can do no Earth flybys. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other concerns with the nuclear option as well; because of its high temperature, it might destroy any organics it might encounter prior to detecting them. It's also difficult to turn off, so that when it has finally reached the liquid water ice, it will probably have to cut through completely and let itself sink to the bottom. There will be concerns of contamination -- dropping an open nuclear furnace onto what &lt;em&gt;could&lt;/em&gt; be pristine life will not make you very popular. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drilling mechanism, be it an actual drill, a heat source like the nuclear furnace, or something else entirely, will have to be an intelligent thing as well. We don't know too much about the ice, so it may be partially rocky (I'll skip that here), and partially &quot;slush&quot;. This means, we have to design for the scenario where liquid water will likely fill up the hole above the drilling mechanism, and re-freeze. The only way you can &lt;em&gt;guarantee&lt;/em&gt; to make this work, is to leave the lander on the surface, and drop the intelligent drilling/exploration probe down the ice, with a tether providing communication/power. Note that that entire length of the tether will have to be &lt;em&gt;on the drilling/exploration probe&lt;/em&gt;, because you can't slack a tether down re-frozen ice.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Diving&lt;/strong&gt; once &lt;em&gt;all&lt;/em&gt; the drilling problems have been solved, and the drilling/exploration probe has reached the liquid ocean, it's time to do some exploring. Most likely the only thing this thing will do during the &lt;em&gt;first&lt;/em&gt; Europa mission is collect some samples in its immediate surroundings and do some biochemical experiments on it. Ideally however, what has reached the ocean is a full-fledged submarine, capable of maneuvering itself to spots of its choosing, not being hindered by any tether (so the tip of its tether will be a WiFi transmitter and battery-recharge point :) and fully-packed with all robotic arms, labs-on-a-chip, and other sensors known to man. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Basically it's a three or four-part mission. Part &lt;strong&gt;A&lt;/strong&gt; will be in low Europa orbit to serve as a communication relay station to Earth. Part &lt;strong&gt;B&lt;/strong&gt; will be the lander, and serve as a relay station of communications to part &lt;strong&gt;A&lt;/strong&gt;. Part &lt;strong&gt;C&lt;/strong&gt; will be the ice-penetrating probe, which will communicate with &lt;strong&gt;B&lt;/strong&gt;, that relays it to &lt;strong&gt;A&lt;/strong&gt;. Possibly, there will be a part &lt;strong&gt;D&lt;/strong&gt; as well -- a relay station in a high Jupiter orbit, to minimize communication eclipses between the Earth and Europa, and/or serve as a temporary data storage device. Data from &lt;strong&gt;C&lt;/strong&gt; will most likely be communicated to &lt;strong&gt;A&lt;/strong&gt; in bursts, because &lt;strong&gt;A&lt;/strong&gt; will only come by one per orbital period (unless you place it in Europa-stationary orbit, which is &lt;em&gt;unstable&lt;/em&gt; due to the proximity to Jupiter, and would only further complicate the mission). &lt;strong&gt;A&lt;/strong&gt; will have to store and transfer these large volumes to Earth, and communication rates at such distances are considerably lower than what is possible between &lt;strong&gt;A&lt;/strong&gt;, &lt;strong&gt;B&lt;/strong&gt; and &lt;strong&gt;C&lt;/strong&gt;. So possibly, &lt;strong&gt;C&lt;/strong&gt; will communicate its data over the course of a Europa orbit to &lt;strong&gt;D&lt;/strong&gt;, which stores it and transmits it at the lower data rate to Earth receivers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This particular mission I've sketched here is the most popular one, but in all honesty, also the least likely to happen anytime soon. The last two phases will have to be done &lt;strong&gt;fully autonomously&lt;/strong&gt;, with &lt;strong&gt;space-grade reliability&lt;/strong&gt;. This is all technology we don't have yet. So there are technological challenges, political challenges, monetary challenges, challenges concerning contamination, etc. etc. etc. It's too big of a leap, given the amount of resources we can realistically hope to spend on it (alas, the Appollo days, where a &lt;em&gt;military&lt;/em&gt; budget scale was used, are long gone).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;far&lt;/em&gt; more likely mission is one that puts an orbiter around Europa, which is equipped with a very powerful ice-penetrating radar (or similar device) and do high quality remote sensing. The only real challenge here is that radar (of which ground-versions and airborne-versions already exist), which will ideally be able to detect not only the shapes and sizes of sub-surface objects, but also, which substances those objects are composed of. &lt;/p&gt;&#xA;" OwnerUserId="97" LastEditorUserId="97" LastEditDate="2016-07-06T09:51:34.157" LastActivityDate="2016-07-06T09:51:34.157" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="363" PostTypeId="2" ParentId="359" CreationDate="2013-07-18T07:35:30.090" Score="18" Body="&lt;p&gt;It's fundamentally impossible to measure gravity from an object that is in freefall. This is the first principle of &lt;a href=&quot;http://en.wikipedia.org/wiki/General_relativity&quot;&gt;general relativity&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What accelerometers will give you is &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;accelerations (linear or rotational) induced by thrusters, atmosperic drag, reaction wheels, etc.&lt;/li&gt;&#xA;&lt;li&gt;vibrations from rotating solar panels, crew-induced forces, etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The only thing you &lt;em&gt;can&lt;/em&gt; measure from a body in freefall is &lt;em&gt;inhomogeneities&lt;/em&gt; in the gravity field, or in other words, the &lt;em&gt;gravity gradient&lt;/em&gt;. You can arrange a collection of accelerometers such that it becomes a &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_gradiometry&quot;&gt;gravity gradiometer&lt;/a&gt;. A gradiometer does not measure the gravity itself, but the &lt;em&gt;change&lt;/em&gt; in the gravitational field as the gradiometer moves along it. &lt;/p&gt;&#xA;" OwnerUserId="97" LastEditorUserId="97" LastEditDate="2013-07-18T07:43:23.277" LastActivityDate="2013-07-18T07:43:23.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="364" PostTypeId="1" AcceptedAnswerId="365" CreationDate="2013-07-18T07:54:02.613" Score="12" ViewCount="4954" Body="&lt;p&gt;I read &lt;a href=&quot;http://www.slate.com/articles/technology/future_tense/2011/02/space_stasis.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this essay by Neal Stephenson&lt;/a&gt; recently, which makes the case that, had we not been locked-into chemical rocket technology through years of investments, other approaches might have been more viable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of today, what non-chemical rocket technologies appear to be the most promising for the future of space travel?&lt;/p&gt;&#xA;" OwnerUserId="132" LastEditorUserId="6816" LastEditDate="2016-11-24T01:32:45.200" LastActivityDate="2016-11-24T01:32:45.200" Title="What are the most promising non chemical rocket space launch approaches?" Tags="|propulsion|advanced-propulsion|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="365" PostTypeId="2" ParentId="364" CreationDate="2013-07-18T08:38:22.040" Score="8" Body="&lt;p&gt;There has been a lot of researching regarding Non-Chemical Rocket propulsion Systems.&#xA;The most recent one being &lt;a href=&quot;http://www.space.com/7067-laser-propulsion-wild-idea-finally-shine.html&quot;&gt;beamed thermal propulsion&lt;/a&gt;. It involves propelling a rocket by using microwaves from the ground. &#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Non-rocket_spacelaunch&quot;&gt;Here&lt;/a&gt; you can find more Systems which have been researched on. Some have already been used, such as the Air Launch, where rockets are launched from conventional horizontal-takeoff aircraft, to carry satellites to low earth orbit.&#xA;Also some other examples are:&#xA;Static Structures: Space Elevators, Skyhook&#xA;Dynamic Structures: Space Fountain, Orbital Ring&#xA;And Mechanical Systems such as Pneumatic launchers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Non-rocket_spacelaunch#Comparison&quot;&gt;This&lt;/a&gt; table gives the estimate build cost, the technological readiness for most of these Non-Rocket Space launch Methods.&lt;/p&gt;&#xA;" OwnerUserId="70" LastActivityDate="2013-07-18T08:38:22.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="366" PostTypeId="1" AcceptedAnswerId="367" CreationDate="2013-07-18T08:48:29.227" Score="9" ViewCount="1432" Body="&lt;p&gt;On Earth one can group multiple telescopes to observe a specific region. They can be moved around and focus different areas. Also they give a higher resolution of the area they observe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would this be possible for telescopes that are not located on the ground?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of building larger and larger telescopes, building smaller ones and make them work together.&lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorUserId="25" LastEditDate="2013-07-18T17:30:14.927" LastActivityDate="2013-07-18T18:19:45.907" Title="Can multiple space telescopes be grouped together to explore the space" Tags="|telescope|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="367" PostTypeId="2" ParentId="366" CreationDate="2013-07-18T09:00:47.000" Score="14" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Astronomical_interferometer&quot;&gt;This is already done&lt;/a&gt;, especially with radio telescopes. For example, we've had the &lt;a href=&quot;http://en.wikipedia.org/wiki/Very_Large_Array&quot;&gt;Very Large Array&lt;/a&gt; for a couple of decades, and we now have &lt;a href=&quot;https://en.wikipedia.org/wiki/LOFAR&quot;&gt;LOFAR&lt;/a&gt; and the &lt;a href=&quot;http://en.wikipedia.org/wiki/Very_Large_Telescope&quot;&gt;VLT&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It certainly would be possible to have an array of telescopes spread across the globe, or even an array of satellite telescopes. &lt;a href=&quot;http://www.ligo.caltech.edu/&quot;&gt;LIGO&lt;/a&gt; (not for electromagnetic wave detection though) will eventually have a couple of stations on the other sides of the world, and the long term plan is to have a satellite version of LIGO, &lt;a href=&quot;http://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna&quot;&gt;LISA&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also: &lt;a href=&quot;http://www.nasa.gov/mission_pages/deepimpact/main/&quot;&gt;Deep Impact&lt;/a&gt; and &lt;a href=&quot;http://keckobservatory.org/&quot;&gt;Keck&lt;/a&gt;. Keck is another observatory that uses this technique. Deep Impact was a project where many of the world's telescopes were focussed at a single event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;http://en.wikipedia.org/wiki/HALCA&quot;&gt;HALCA&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/RadioAstron&quot;&gt;Spektr-R&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="96" LastEditorUserId="96" LastEditDate="2013-07-18T18:19:45.907" LastActivityDate="2013-07-18T18:19:45.907" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="368" PostTypeId="1" AcceptedAnswerId="380" CreationDate="2013-07-18T09:58:04.137" Score="16" ViewCount="386" Body="&lt;p&gt;With NASA planning to eventually get people to Mars, and with privately funded attempts such as Mars-One planning to do the same, and relatively soon, there is the issue of having to feed anyone you send for the 6 or so months journey. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Obviously the issue extends to when you reach the planet but this question is focused on in flight food production as this deals more with the realm of long distance, manned space exploration)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What technologies currently exist or what research is being done focusing on sustainable food production for these long, manned, exploration trips?&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="58" LastEditDate="2016-09-15T14:10:53.413" LastActivityDate="2016-09-15T14:10:53.413" Title="Are there any existing technologies, or research, that investigate the sustainable production of food for manned exploration trips?" Tags="|crewed-spaceflight|colonization|health|food|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="369" PostTypeId="1" AcceptedAnswerId="373" CreationDate="2013-07-18T10:40:51.143" Score="11" ViewCount="289" Body="&lt;p&gt;It is well known that micro meteorites, space junk, and other small objects can put a dent (Or rip through) in a spacecraft quite effectively. How can I protect people, sensitive electronics, and other items from impacts from these items?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-18T11:23:38.203" LastActivityDate="2013-07-18T11:51:06.387" Title="How can I shield against micro-meteors?" Tags="|debris|shielding|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="370" PostTypeId="1" AcceptedAnswerId="691" CreationDate="2013-07-18T10:45:48.160" Score="51" ViewCount="8992" Body="&lt;p&gt;Solar sails are a great, inexpensive way to move out from the sun, if you’re not in a hurry.  Ocean going ships are able to tack against the wind, and to move up wind. Could a solar sail gain inbound (towards the sun) momentum by tacking? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="14" LastEditDate="2013-07-19T10:23:19.343" LastActivityDate="2021-05-05T07:10:38.230" Title="Can you tack against the sun using a solar sail?" Tags="|propulsion|orbital-maneuver|orbital-mechanics|solar-sail|" AnswerCount="7" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="371" PostTypeId="5" CreationDate="2013-07-18T10:47:51.890" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-18T10:47:51.890" LastActivityDate="2013-07-18T10:47:51.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="372" PostTypeId="4" CreationDate="2013-07-18T10:47:51.890" Score="0" Body="Travel between planents in a solar system" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2013-07-18T18:42:24.427" LastActivityDate="2013-07-18T18:42:24.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="373" PostTypeId="2" ParentId="369" CreationDate="2013-07-18T11:40:38.770" Score="10" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Whipple_shield&quot;&gt;Whipple shields&lt;/a&gt; are used for this very thing. The basic idea is that when the object strikes the first layer, it breaks apart, and the resulting pieces have a low enough energy to be stopped by the other layer(s). They have the added benefit of being much lighter than, say, a single thick slab of aluminum or titanium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ares.jsc.nasa.gov/ares/hvit/basic.cfm&quot;&gt;This site&lt;/a&gt; has some good photos of ground-tested Whipple shields and their variations.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-07-18T11:51:06.387" LastActivityDate="2013-07-18T11:51:06.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="374" PostTypeId="1" AcceptedAnswerId="375" CreationDate="2013-07-18T12:12:15.313" Score="17" ViewCount="7536" Body="&lt;p&gt;During space race, the Russians were leading the game until the Gemini program (Gemini 6/7 rendezvous was the overtaking moment in my opinion). I do not know of any Soviet claim similar to Kennedy's famous speech. (In fact, there was no proclamation until missions ended successfully.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the Russians state somehow that they could reach Moon as well? What are the political, technical or economical reasons they concentrated on low orbit instead? Could Korolëv's death be considered one of main reasons why the Soviets could not keep the pace?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="25" LastEditDate="2013-07-18T17:53:55.290" LastActivityDate="2017-05-30T16:47:34.110" Title="Why did the Russians never land on the Moon?" Tags="|the-moon|space-race|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="375" PostTypeId="2" ParentId="374" CreationDate="2013-07-18T12:39:48.810" Score="23" Body="&lt;p&gt;Without overtly saying it, the Soviets too were aiming for the moon.  It was only after it was clear they lost that they decided they never were even trying.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also Gemini was post-Kennedy speech and developed to test the techniques needed for Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem they had was a large engine, was not something in house, and was harder to develop than expected.  So for the &lt;a href=&quot;http://en.wikipedia.org/wiki/N1_%28rocket%29&quot;&gt;N-1 launcher&lt;/a&gt; (Their answer to the Saturn-V) instead for 5 F-1 engines (about 1.5 million lbs thrust) they used 30 (!!!) NK-15s with about 370,000 lbs thrust.  (In fact those engines are now being used on Antares! Just renamed to NK-33. Some seem to have actually been built back then for the N-1!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The F-1 development was very hard, but the US succeeded.  And with only 3 stages, and 5 large engines on the first stage, it was possible to launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The N-1 with 30 engines on the first stage, and 5 stages in total (A sure sign you lost the design battle is more stages) had all sorts of issues with interactions between so many engines, and due to political pressures was forced to test in all up configuration, with insufficient ground testing to work out those interactions.  Thus it failed on all 4 launch attempts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is often used as a critique of the SpaceX approach where Falcon 9 is approaching that level of interaction, and Falcon Heavy with 27 main engines even more so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, that ignores the causes of the Soviet failures (insufficient ground testing before launch attempts) and just focuses on the numerical number of engines.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-07-18T12:49:37.590" LastActivityDate="2013-07-18T12:49:37.590" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="376" PostTypeId="2" ParentId="370" CreationDate="2013-07-18T12:55:55.773" Score="5" Body="&lt;p&gt;In sailing, tacking is used to sail as close to the wind as you can (obviously not straight into it, though the hard wing Americas Cup boat might have been able to) while still generating lift in the sail.  For normal boats this means at best a 25-30 degree angle to the wind.  Thus you sail a zig zag pattern and need to tack, else you get way off course. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wind sails do not work by pressure of the wind, pushing them, except in straight downwind situations or in older square sail you see in movies about pirates, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern sails have curvature and loft and work as vertical wings, generating lift from the pressure differential created by airflow at different speeds over the two sides of the wing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar sails do not have that ability, as photons do not act like air molecules in an atmosphere.  Thus they need to sail, straight 'down wind' as it were. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-07-18T22:30:49.090" LastActivityDate="2013-07-18T22:30:49.090" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="377" PostTypeId="2" ParentId="370" CreationDate="2013-07-18T13:34:58.343" Score="28" Body="&lt;p&gt;This is actually somewhat easier than you would think. In the world of Orbital Dynamics, you only have to accelerate or decelerate your orbit to move closer/further away from the object you are orbiting. So, all you have to do is create a net momentum that pushes to slow down your orbital velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, a big part of what makes tacking work is the fact that you are forcing the water to act as a friction medium against the wind, in essence, causing it to slow you down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not an expert in such motion, but I believe a configuration like below would work, assuring the arrow is the direction of orbital motion, and the sail is the T like item. The direction could be off by 90 degrees to make this happen, and it might not work really close to the sun, but it should get you in the right direction at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xENSt.gif&quot; alt=&quot;Travelling towards the sun&quot;&gt;&lt;br&gt;&#xA;&lt;sup&gt;Source: Ben Diedrich via &lt;a href=&quot;http://wiki.solarsails.info/index.php/File:Sailmch2.gif&quot; rel=&quot;noreferrer&quot;&gt;SolarSailWiki&lt;/a&gt;, CC BY-SA 3.0&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, this has already been done, by a Japanese probe called &lt;a href=&quot;http://m.guardiannews.com/world/2010/may/17/space-yacht-ikaros-japan-venus&quot; rel=&quot;noreferrer&quot;&gt;Ikaros&lt;/a&gt;. It sailed using the sun to Venus, from Earth Orbit, and thus demonstrated that this is possible.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="27793" LastEditDate="2018-12-29T03:16:19.360" LastActivityDate="2018-12-29T03:16:19.360" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="378" PostTypeId="2" ParentId="370" CreationDate="2013-07-18T13:40:23.240" Score="40" Body="&lt;p&gt;&lt;strong&gt;You cannot directly propel the solar sail towards the sun.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A solar &quot;sail&quot; is basically a mirror. The analogy of wind and sails on ships is not useful for understanding how solar sails work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each photon from the sun which strikes the sail is reflected. Each photon imparts a small amount of momentum. If the sail is pointed directly at the sun then you get twice the photon's momentum added to the sail. If you angle the sail, then you are sending each photon off in a direction that is not directly back towards the sun; That gives you a net force to one side. So you can control the vector of the total force of the reflecting photons, but the net direction is always more than 90 degrees from the sun. As the mirror approaches edge-on to the sun, the net force vector would approach 90 degrees from the sun and drop to zero magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that the pressure from the sun's photons applies to anything. It doesn't have to be a designed sail. Orbital mechanics currently take &quot;light pressure&quot; into account for accurate determinations of space craft orbits.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;You can alter your orbit using a solar sail.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can use the momentum from the solar sail to alter the orbital eccentricity to move a part of the orbit closer to the sun, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to go directly towards the sun from the earth, you don't need a force pushing you directly toward the sun. You need a force pushing against your normal orbital direction. That decreases your angular momentum about the sun, and you then fall towards the sun due to gravity.&lt;/p&gt;&#xA;" OwnerUserId="48" LastEditorUserId="48" LastEditDate="2013-07-18T13:45:43.853" LastActivityDate="2013-07-18T13:45:43.853" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="379" PostTypeId="2" ParentId="370" CreationDate="2013-07-18T13:49:55.640" Score="8" Body="&lt;p&gt;While you can't do tricks common for normal sails due to lack of water to keep your keel from drifting sideways and normally lets the ship travel upwind, you are still able to extract force diagonal to the sun radius (the angle of incidence equals the angle of reflection; resulting force is perpendicular to the surface), aimed outside the Solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, this wouldn't let you travel inwards, except... - you can apply said force with the lateral component directed against your orbital speed vector. That way, despite the parallel component pushing you outside, against the Sun gravity, your orbital speed and resulting centrifugal force drops; and while the outwards component of the solar sail push is only momentary, your orbital speed loss accumulates and leads to continuous reduction of your orbital radius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words: Solar sail lets you change orbital speed. Sun gravity can make you travel towards the Sun, depending on said speed.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-18T13:49:55.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="380" PostTypeId="2" ParentId="368" CreationDate="2013-07-18T13:53:08.857" Score="10" Body="&lt;p&gt;The largest scale experiment to test out such ideas was the &lt;a href=&quot;http://en.wikipedia.org/wiki/Biosphere_2&quot;&gt;Biosphere 2&lt;/a&gt;. The experiment was probably more akin to setting up a habitat on Mars than growing in a spacecraft, but it is a good starting point for all such technologies. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, the facility was set up so that 8 people could live there for an indefinite period of time, growing their own food, and maintaining the equipment and environment. In practice, it is quite similar to the type of environment that might exist on a Mars colony.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The program was plagued with issues, which I will point you to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Biosphere_2&quot;&gt;Wikipedia&lt;/a&gt; page to learn more about, but it was largely sustainable, and later versions actually worked quite well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, there has been several programs to grow plants on board the International Space Station, and while they aren't a significant part of the diet yet, they &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/veggie.html&quot;&gt;have been eaten&lt;/a&gt;. They are working to improve these programs constantly.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-18T13:53:08.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="381" PostTypeId="1" AcceptedAnswerId="420" CreationDate="2013-07-18T14:11:55.437" Score="8" ViewCount="777" Body="&lt;p&gt;One potential issue that will be faced with long, manned, space exploration will be exposure to fluctuating radiation levels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current method of safety seems to be launching at times when radiation is low, avoiding solar flares, or hiding between the water tanks when it peaks unexpectedly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isn't really a sustainable, nor highly safe method of protecting from radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What technologies or research is underway that will provide a higher level of safety to humans inside the craft on long exploration voyages?&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="68" LastEditDate="2013-07-24T16:42:19.403" LastActivityDate="2016-06-24T14:33:36.823" Title="What advances are being made to shield astronauts from peaks in radiation?" Tags="|radiation|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="382" PostTypeId="2" ParentId="381" CreationDate="2013-07-18T14:48:21.543" Score="1" Body="&lt;p&gt;Bigelow in its inflatable designs uses a water balloon as part of the meteor protection.  But it also adds some shielding.  I have not seen anything that suggests it is sufficient for much protection.  But that seems like an extensible (to a point) approach.  (Obviously mass of water is a pain, but then again, water is simple and low cost short of launch cost).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-07-18T14:48:21.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="383" PostTypeId="2" ParentId="264" CreationDate="2013-07-18T15:01:22.843" Score="5" Body="&lt;p&gt;We have the rocket equation over two segments.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;v_e = 4,440 m/s&lt;/li&gt;&#xA;&lt;li&gt;v2 = 150 m/s&lt;/li&gt;&#xA;&lt;li&gt;v1 = 7,900 m/s - 150 m/s = 7,750 m/s&lt;/li&gt;&#xA;&lt;li&gt;Orbiter mass itself = m_o = 68,585 kg&lt;/li&gt;&#xA;&lt;li&gt;Payload (inside orbiter) = m_p = 24,400 kg&lt;/li&gt;&#xA;&lt;li&gt;external tank = m_t = 35,000 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I will refer to 4 different mass values.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mL - the mass at launch&lt;/li&gt;&#xA;&lt;li&gt;m2 - the mass right before MECO&lt;/li&gt;&#xA;&lt;li&gt;m2' - the mass right after MECO (if there is a seperation)&lt;/li&gt;&#xA;&lt;li&gt;m3 - the mass that makes it to orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The essence of the problem is that we have a reference case where the mass drops by the mass of the external tank at seperation, and then we want to find out how much we have to decrease the payload weight in order to still reach orbit with the external tank, meaning that m2=m2'.  But first, we need to fill in all the values for the reference case.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;m3 = m_o + m_p = 68,585 kg + 24,400 kg = 92,985 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The mass after seperation can be found from the rocket equation.  Add the external tank mass to find the mass right before seperation.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;m2' = (92,985 kg) * exp( (150 m/s) / (4,440 m/s) ) = 96,180 kg&lt;/li&gt;&#xA;&lt;li&gt;m2 = 96,180 kg + 35,000 kg = 131,180 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Apply the rocket equation yet again to find the mass at liftoff.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mL = (131,180 kg) * exp( 7,750 / 4,440 ) = 751,496 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The actual weight on the launchpad is 2 million kg.  However, I just need something to apply consistently right now between the two cases.  This degree of error was actually rather predictable, since I used a fuel velocity that's too high and didn't account for other structural materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;-------- end reference case ---------&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Returning to the premise, we get the ET into orbit by sacrificing payload weight.  That will change the weight of the space shuttle on the launchpad, and therein lies the difficultity.  For this problem, however, we can actually apply a single stage rocket equation to full orbital velocity because in our false model there are no seperations at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll introduce new primed variables.  Consider them to be defined by the following equations.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mL = mL' + m_p = 727 096.026 kg + m_p&lt;/li&gt;&#xA;&lt;li&gt;m3 = m3' + m_t + m_p = 68 585 kg + m_t + m_p&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/l7sXp.png&quot; alt=&quot;equation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;m_p = ( 727,096.026 - ( 68585 + 35000 ) * exp( 7900/4440 ) ) / exp( 7900/4440 ) = &lt;strong&gt;19,120 kg&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The payload mass was lowered by 5280.5 kg by my calculation.  That sounds reasonable - that &lt;strong&gt;we lose 5 tons of payload in order to push 35 tons of tank material the last little bit&lt;/strong&gt; of the path to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, regarding the other answer:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To be slightly more precise, the sum of the orbiter burnout mass and the ET, the OMS specific impulse, and the required ΔV can be plugged into the rocket equation to solve for how much excess payload mass is available. The answer is a negative number.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I think I figured out what happened here.  I think it is this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;m_p = ( (106,780 kg) - (103,585 kg) exp( (150 m/s) / (4,440 m/s) ) ) / ( exp( (150 m/s) / (4,440 m/s) ) - 1) = -10 601.5052 kilograms&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This calculation and number comes from applying the rocket equation to the final stage, after the MECO.  The problem with that calculation is that you're reducing the payload weight, but don't account for the fact that you'll have more fuel at (what was previously) MECO because you reduced the payload.  So basically, this is a one-segment application of the rocket equation and it doesn't get the right answer.  Because of the nature of the beast, you need to consider two segments of application.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-07-18T15:01:22.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="384" PostTypeId="2" ParentId="245" CreationDate="2013-07-18T15:02:18.417" Score="5" Body="&lt;p&gt;First of all, yes, all mentioned systems have been &lt;em&gt;tested&lt;/em&gt; in space in &lt;em&gt;unmanned&lt;/em&gt; flights as you have noted. Some of those flights carried animals (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Atlas_5&quot; rel=&quot;noreferrer&quot;&gt;Mercury-Atlas 5&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Redstone_2&quot; rel=&quot;noreferrer&quot;&gt;Mercury-Redstone 2&lt;/a&gt;) and a few experiments on e.g. life support. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the contrary, nothing has actually been build which could match &lt;a href=&quot;http://en.wikipedia.org/wiki/Bion-M_No.1&quot; rel=&quot;noreferrer&quot;&gt;Bion&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/Zond_program#Circumlunar_missions&quot; rel=&quot;noreferrer&quot;&gt;Zond&lt;/a&gt; spacecrafts. As of today, those Soviet / Russian programs are still fairly unique. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were a number of proposals for &quot;spin-offs&quot; of the mentioned programs. &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Gemini#Proposed_extensions_and_applications&quot; rel=&quot;noreferrer&quot;&gt;Gemini&lt;/a&gt; saw a few ideas towards an advanced transportation craft and, most notable, an intensive program on potential military use. For Apollo, there was the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Applications_Program&quot; rel=&quot;noreferrer&quot;&gt;Apollo Applications Program&lt;/a&gt;. There were tons of concepts, e.g. even one for a manned Venus-Flyby with Apollo technology. In the end, Skylab became the only actual &quot;spin-off&quot; of the Apollo program. Politically, the focus was directed on manned space flight, thus astronauts supervising experiments in space. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-18T15:02:18.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="385" PostTypeId="1" CreationDate="2013-07-18T15:53:33.037" Score="1" ViewCount="91" Body="&lt;p&gt;What technological and financial barriers prevent proposals for partial nonrocket spacelaunch such as magnetic levitation trains from becoming a reality?  These systems, if created, could substantially reduce the delta-v required by rockets to achieve orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would exclude space elevators from this discussion, as those have already been discussed at length elsewhere.&lt;/p&gt;&#xA;" OwnerUserId="120" LastActivityDate="2013-07-18T15:53:33.037" Title="What are the barriers to partial nonrocket spacelaunch?" Tags="|launch|" AnswerCount="0" CommentCount="3" ClosedDate="2013-07-18T17:20:40.870" ContentLicense="CC BY-SA 3.0" />
  <row Id="386" PostTypeId="2" ParentId="216" CreationDate="2013-07-18T15:55:57.093" Score="7" Body="&lt;p&gt;There is actually a whole discipline, which deals with such issues (among others), in one way or another: &lt;a href=&quot;http://en.wikipedia.org/wiki/Astrobiology&quot;&gt;Astrobiology&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After &lt;a href=&quot;http://en.wikipedia.org/wiki/Reports_of_Streptococcus_mitis_on_the_Moon&quot;&gt;reports of Streptococcus mitis on the Moon&lt;/a&gt; 1969/1970, the topic become more and more an issue of discussion. Today, there are sets of regulations on how a spacecraft, depending on its target, needs to be sterilized. This had led to the term &lt;a href=&quot;http://planetaryprotection.nasa.gov/methods&quot;&gt;Planetary Protection&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methods for handling space probes basically include assembly in clean rooms and  microbial reduction through heat and chemicals. In more detail, this means e.g. dry-heating, sterilizing wipes and aseptic integration. With heat being the safest method for sterilization, there are still problems with overheating electronics or scientific experiments. So further methods such as  low-temperature, vapor-phase or hydrogen peroxide-based sterilization processes are under investigation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/COSPAR&quot;&gt;Committee on Space Research&lt;/a&gt; (COSPAR) has introduced a system of 5 categories for different types of exploration missions. For example, Category IV refers to landers on celestial bodies, which have the potential to host life and for which there is a possibility of contamination (e.g. Mars, Europa &amp;amp; Titan). Missions under this category are supposed to receive the best possible measures of sterilization. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the second part of your question, Category V refers to sample-return missions to/from celestial bodies with a potential to support life. They are then referred to as &quot;restricted Earth return missions&quot;. It is recommended that all returned equipment and samples shall be handled in accordance to &lt;a href=&quot;http://en.wikipedia.org/wiki/Biosafety_level_4#Biosafety_level_4&quot;&gt;biosafety level 4&lt;/a&gt;, which, although being a US regulation, is internationally accepted. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-18T15:55:57.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="387" PostTypeId="1" CreationDate="2013-07-18T16:14:21.933" Score="7" ViewCount="697" Body="&lt;p&gt;Considering photons only travel at the speed of light (and cannot travel faster), our main technique of digital communication (at least I believe, but I could be wrong) is off the table.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What techniques are being investigated for digital superluminal communication?  Would these techniques be applicable to space missions where communication lag is significant?&lt;/p&gt;&#xA;" OwnerUserId="30" LastEditorUserId="12102" LastEditDate="2021-07-05T22:19:40.087" LastActivityDate="2021-07-05T22:19:40.087" Title="Techniques for digital superluminal communication" Tags="|communication|physics|radio-communication|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="388" PostTypeId="1" AcceptedAnswerId="393" CreationDate="2013-07-18T16:17:02.123" Score="25" ViewCount="30455" Body="&lt;p&gt;For long-distance manned missions, such as a mission to Mars, we are inevitably going to have to shield astronauts from cosmic radiation, especially in the event of a solar flare or SEP. What materials provide the best protection from the kinds of high-energy cosmic radiation astronauts would be exposed to on these journeys?&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2019-03-16T16:21:54.783" Title="What materials provide the best protection from cosmic radiation?" Tags="|crewed-spaceflight|radiation|materials|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="389" PostTypeId="2" ParentId="387" CreationDate="2013-07-18T16:20:04.130" Score="10" Body="&lt;p&gt;Currently, no techniques &lt;em&gt;exist&lt;/em&gt; to send information faster than the speed of light. However, there are some theories:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Group velocity &gt; c experiments&lt;/li&gt;&#xA;&lt;li&gt;Evanescent wave coupling&lt;/li&gt;&#xA;&lt;li&gt;Tachyons&lt;/li&gt;&#xA;&lt;li&gt;Quantum nonlocality&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://en.wikipedia.org/wiki/Superluminal_communication&quot;&gt;Wikipedia article&lt;/a&gt; on the subject:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to the currently accepted theory, three of those four phenomena &lt;strong&gt;do not produce superluminal communication&lt;/strong&gt;, even though they may give that appearance under some conditions. The third, tachyons, &lt;strong&gt;arguably do not exist as their existence is hypothetical&lt;/strong&gt;; even if their existence were to be proven, attempts to quantize them &lt;strong&gt;appear to indicate that they may not be used&lt;/strong&gt; for superluminal communication, because experiments to produce or absorb tachyons &lt;strong&gt;cannot be fully controlled&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If wormholes are possible, they &lt;em&gt;could&lt;/em&gt; be used to transmit information:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;If wormholes are possible&lt;/strong&gt;, then ordinary subluminal methods of communication &lt;strong&gt;could be sent through them to achieve superluminal transmission speeds&lt;/strong&gt;. Considering the immense energy that current theories suggest would be required to open a wormhole large enough to pass spacecraft through it may be that only &lt;strong&gt;atomic-scale wormholes would be practical&lt;/strong&gt; to build, &lt;strong&gt;limiting their use solely to information transmission&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is some &lt;em&gt;small&lt;/em&gt; possibility that quantum entanglement would achieve the desired effect:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;some argue that superluminal communication &lt;strong&gt;could be achieved via quantum entanglement&lt;/strong&gt; using other methods that &lt;strong&gt;don't rely on cloning a quantum system&lt;/strong&gt;. One suggested method would use an &lt;strong&gt;ensemble of entangled particles to transmit information&lt;/strong&gt;, similar to a type of quantum eraser experiments. As the quantum eraser experiments &lt;strong&gt;rely on a classical, subluminal channel&lt;/strong&gt; for coincidence detection, it is &lt;strong&gt;unclear whether superluminal communication would be possible&lt;/strong&gt; by this method. Physicist John G. Cramer at the University of Washington is &lt;strong&gt;attempting to replicate one of these experiments&lt;/strong&gt; and demonstrate whether or not it can produce superluminal communication.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Therefore, we &lt;strong&gt;currently&lt;/strong&gt; don't have any means of superluminal communications, but there are theories. If we can create a wormhole (very unlikely anytime soon), then it would be &lt;em&gt;possible...&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-18T16:33:27.800" LastActivityDate="2013-07-18T16:33:27.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="390" PostTypeId="2" ParentId="388" CreationDate="2013-07-18T16:22:01.257" Score="8" Body="&lt;p&gt;One possible material that was mentioned in Scientific American is fecal matter.  The hydrocarbons in it can absorb the radiation safely.&lt;br&gt;&#xA;However, most of the general public would reject this possibility because of the grossness factor (just like recycling water by purifying and sanitizing urine).&lt;/p&gt;&#xA;" OwnerUserId="30" LastActivityDate="2013-07-18T16:22:01.257" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="391" PostTypeId="2" ParentId="388" CreationDate="2013-07-18T16:25:59.500" Score="2" Body="&lt;p&gt;There is talk that it might be possible to use magnets as a shield from cosmic radiation:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Astronauts travelling to the International Space Station are protected from much of this radiation by the Earth’s atmosphere as well as by its “magnetosphere”, the magnetized bubble of plasma surrounding the Earth created by its magnetic field. However, people on longer flights will not have this natural shielding and are therefore at greater risk.&lt;/p&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;p&gt;injecting a supersonic plasma into a 1.5 m long vacuum vessel lined with magnetic coils, with a target magnet placed at the far end of the vessel. Using both optical imaging and an electromagnetic probe, Bamford’s team showed that the target magnet deflected the plasma such that the volume of space surrounding the magnet was almost entirely free of plasma particles.&lt;/p&gt;&#xA;&lt;p&gt;-- &lt;a href=&quot;http://physicsworld.com/cws/article/news/2008/nov/06/magnetic-shield-could-protect-spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;physicsworld.com&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;An image showing how Earth's magnetic field takes care of this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YKYNr.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Neat!&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-18T16:41:55.477" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="392" PostTypeId="2" ParentId="84" CreationDate="2013-07-18T16:30:55.420" Score="9" Body="&lt;p&gt;Short answer: Yes, it is certainly possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer: As has been noted before, there are abundances of metals. Although you can not really find large quantities of other important classes of substances such as hydrocarbons, you can actually find a lot of their chemical components (elements) contained in a number of minerals on the Lunar surface. So the question boils down to how to extract and process them. Then, the question should be referred to mineralogy and chemistry ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a wonderful book about this topic, edited by Viorel Badescu and released in 2012: &lt;a href=&quot;http://books.google.co.uk/books/about/Moon.html?id=ho65r66FVEsC&quot; rel=&quot;nofollow&quot;&gt;&quot;Moon: Prospective Energy and Material Resources&quot;&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="49" LastEditDate="2015-05-06T22:55:17.480" LastActivityDate="2015-05-06T22:55:17.480" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="393" PostTypeId="2" ParentId="388" CreationDate="2013-07-18T16:34:18.973" Score="20" Body="&lt;p&gt;There's a pretty good &lt;a href=&quot;https://en.wikipedia.org/wiki/Health_threat_from_cosmic_rays&quot;&gt;Wikipedia page&lt;/a&gt; about it, which lists a couple of options:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Water makes for fairly good radiation shielding (also discussed with land based radiation &lt;a href=&quot;http://whatif.xkcd.com/29/&quot;&gt;here&lt;/a&gt;), but is relatively heavy and is consumed during flight.&lt;/li&gt;&#xA;&lt;li&gt;Liquid hydrogen is also good, and is used as fuel, so it will already be on board.  However, this too is consumed during flight.&lt;/li&gt;&#xA;&lt;li&gt;We could change the materials that spacecraft are made out of.  Since hydrogen rich materials work well to shield the most common types of cosmic radiation, some plastics could work.  However, this would require some reengineering to be practical.&lt;/li&gt;&#xA;&lt;li&gt;Like JKor said, human waste works well, but has &quot;grossness&quot; problems.  However, this is unique in that it increases instead of decreases as flight goes on, so it could supplement liquid hydrogen and water.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One of the largest problems with bringing up extra shielding is that it tends to be heavy, and more weight == more cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia page mentions active magnetic shielding, but that is at this point mostly a theoretical idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shielding is important for unmanned missions as well (though not as much), as radiation can have effects on computer systems by interfering with magnetic storage - see &lt;a href=&quot;http://newswatch.nationalgeographic.com/2010/05/07/voyager_2_speaking_in_code/&quot;&gt;this&lt;/a&gt; National Geographic article and &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2010-151&quot;&gt;this&lt;/a&gt; NASA press release for an instance of such an event happening on Voyager 2.&lt;/p&gt;&#xA;" OwnerUserId="120" LastEditorDisplayName="user12" LastEditDate="2013-07-18T16:46:38.407" LastActivityDate="2013-07-18T16:46:38.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="394" PostTypeId="1" AcceptedAnswerId="397" CreationDate="2013-07-18T16:37:13.897" Score="9" ViewCount="1389" Body="&lt;p&gt;As a member of a pulsar search team that works through the Pulsar Search Collaboratory, I've noticed that many data plots (except the tests) are radio frequency interference (RFI) or noise. As a result, I have gained an appreciation for what kinds of issues antennas can cause.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What techniques can satellite ground-station operators, programmers, and installers use to recognize RFI or noise before we have to manually review the plots? How can this problem be reduced or removed?&lt;/p&gt;&#xA;" OwnerUserId="30" LastEditorUserId="25" LastEditDate="2013-07-18T17:28:57.843" LastActivityDate="2013-07-18T17:28:57.843" Title="Techniques to reduce radio interference or noise" Tags="|ground-station|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="395" PostTypeId="5" CreationDate="2013-07-18T16:54:01.423" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-18T16:54:01.423" LastActivityDate="2013-07-18T16:54:01.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="396" PostTypeId="4" CreationDate="2013-07-18T16:54:01.423" Score="0" Body="The study of satellite orbits and space flight dynamics. " OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-07-18T18:43:22.417" LastActivityDate="2013-07-18T18:43:22.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="397" PostTypeId="2" ParentId="394" CreationDate="2013-07-18T17:02:07.857" Score="6" Body="&lt;p&gt;Removing Radio Frequency Interference (RFI) can be quite a challenging issue, but it can be solved. There's a number of things that should be done to minimize the effect of this noise. A basis for this can be found from the &lt;a href=&quot;http://www.arrl.org/radio-frequency-interference-rfi&quot; rel=&quot;noreferrer&quot;&gt;American Radio Relay League (ARRL)&lt;/a&gt;, but some of it is just from experience.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;All of your sensitive equipment should be in a Faraday cage, or at least a metal box.&lt;/li&gt;&#xA;&lt;li&gt;Try to use a simple radio scanner to find the source of your frequency. Those items should also be in a metal box or Faraday cage.&lt;/li&gt;&#xA;&lt;li&gt;Isolate power lines using high end filtering devices.&lt;/li&gt;&#xA;&lt;li&gt;Use proper grounding and keep wire lengths short.&lt;/li&gt;&#xA;&lt;li&gt;Keep anything in the radio pathway clean, with solid connections.&lt;/li&gt;&#xA;&lt;li&gt;Pay attention to where your antennas are pointed when you see the RFI issues, what time of day, etc. This can be useful information in pinpointing the exact location, which makes the problem easier to solve.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line is that you have to find out where the RFI is coming from, and fix it. In most cases, it is technically illegal to produce RF without a license, so vendors, neighbors, and power companies should be willing to help you remove it if you can track down what device is causing the issues. Good luck!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-18T17:28:01.213" LastActivityDate="2013-07-18T17:28:01.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="398" PostTypeId="2" ParentId="112" CreationDate="2013-07-18T17:10:13.460" Score="11" Body="&lt;p&gt;First of all, it is a bit odd to talk about specific plans. If you think about emergency procedures for other natural phenomena, such as Hurricanes or Earth quakes, you need to keep in mind that a &lt;a href=&quot;http://en.wikipedia.org/wiki/Geomagnetic_reversal&quot;&gt;reversal&lt;/a&gt; is a long-term process which is supposed to take decades to centuries. This is what is called geological time scales. In a way, some scientist say, that the &quot;next&quot; reversal has already started. For proving this statement, scientist usually refer to the current characteristics of Earth's magnetic field. It is somewhat different from a &quot;clean&quot; dipole, which it is supposed to be according to basic physics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the process of Earth's magnetic field's reversal was first simulated by computers, it has been agreed on that the overall field strength (around 50µT at the Earth's surface) will not drop significantly. For further reading, have a look at the ground-breaking work of Gary A. Glatzmaier, Paul H. Roberts et al. &lt;a href=&quot;http://www.nature.com/nature/journal/v377/n6546/abs/377203a0.html&quot;&gt;A three-dimensional self-consistent computer simulation of a geomagnetic field reversal&lt;/a&gt;, Nature, 1995. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Talking about space exploration in this context, there are plenty of missions which relate to this issue. First, you need plenty of data / observations of Earth's magnetic field for understanding it (and for tuning &amp;amp; verifying the computer models). Second, you need monitoring of the field with respect to sudden changes. Sudden changes may only be caused by solar activity, so from a scientific perspective, such missions actually monitor the solar weather. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But talking about plans and although such sudden changes should not trigger a reversal on their own, those effects account for most problems here on Earth with respect to Earth's magnetic field. The most prominent example is the &lt;a href=&quot;http://en.wikipedia.org/wiki/March_1989_geomagnetic_storm&quot;&gt;geomagnetic storm of March 1989&lt;/a&gt;, which had significant influence on the power grid in North America. Although there have been improvements to e.g. power grids, to the best of my knowledge, there is no overall plan of how to handle a massive solar storm (thus a massive change in Earth's magnetic field) in most countries. The space probes at the Earth-Sun-system's Lagrangian points remain the first (and last) line of defence (for early warning) against such events. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notable missions for looking at Earth's magnetic field: &lt;a href=&quot;http://en.wikipedia.org/wiki/Magsat&quot;&gt;Magsat&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/%C3%98rsted_%28satellite%29&quot;&gt;Ørsted&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Swarm_%28ESA_mission%29&quot;&gt;Swarm&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notable missions monitoring solar activity: &lt;a href=&quot;http://en.wikipedia.org/wiki/STEREO&quot;&gt;STEREO&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Cometary_Explorer&quot;&gt;International Sun/Earth Explorer 3 (ISEE-3)&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/WIND_%28spacecraft%29&quot;&gt;WIND&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Advanced_Composition_Explorer&quot;&gt;ACE&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory&quot;&gt;SOHO&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-18T17:32:29.593" LastActivityDate="2013-07-18T17:32:29.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="400" PostTypeId="2" ParentId="247" CreationDate="2013-07-18T17:59:50.143" Score="11" Body="&lt;p&gt;Another big reason is there simply isn't a need to do anything more powerful. There exist many applications on Earth where reliability is more important than speed. For instance, a vending machine contains a simple computer. You don't want that to crash and take your money. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vast majority of the processing used today by computers is in the graphical interface. As there is no satellite that runs a graphical interface, it doesn't really make that much of a difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point of a satellite's computer is to keep the satellite alive, pointed in the right direction, manage power, and collect data for use on the ground. Thus, they don't need to have gigahertz processors, they just need to be a data pipe. They need to do this with a high level of precision. You can't go and push the power button on a spacecraft, you need its systems to work flawlessly at all times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Computers are regularly used on the ISS by the astronauts, but these are done for non-critical systems. It's only when the computer has to do significant processing of the data that the speed matters, and except for some compression, most of that is still done on Earth. Furthermore, most of the image laden systems out there have custom on board chips that assist with processing the images quicker, allowing less of the work to be done on the main processor.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-18T17:59:50.143" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="401" PostTypeId="5" CreationDate="2013-07-18T18:10:16.897" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-18T18:10:16.897" LastActivityDate="2013-07-18T18:10:16.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="402" PostTypeId="4" CreationDate="2013-07-18T18:10:16.897" Score="0" Body="Questions regarding observation of spacecraft or phenomenon in space" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-22T14:53:35.603" LastActivityDate="2015-12-22T14:53:35.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="403" PostTypeId="1" AcceptedAnswerId="412" CreationDate="2013-07-18T18:30:27.567" Score="8" ViewCount="682" Body="&lt;p&gt;The habitable modules of US sector of the ISS didn't have any propulsion systems to rendezvous the ISS by themselves. They were intended to be brought there by the Space Shuttle. Most of the Russian heavy modules of Mir and the ISS have arrived to the stations by themselves. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To bring 11 tons Kvant-1 module to Mir, Russia has used the disposable &lt;a href=&quot;http://en.wikipedia.org/wiki/Kvant-1#Launch_and_docking&quot; rel=&quot;nofollow&quot;&gt;FSM space tug, based on TKS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has NASA announced any realistic space tug projects to be able to bring a habitable module, such as, say, &lt;a href=&quot;http://en.wikipedia.org/wiki/Node_4&quot; rel=&quot;nofollow&quot;&gt;Node 4&lt;/a&gt; or a similar payload to the ISS? &lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="25" LastEditDate="2015-12-17T22:41:23.970" LastActivityDate="2015-12-17T22:43:58.323" Title="Does NASA have a space tug project to bring a habitable module to the ISS?" Tags="|iss|nasa|orbital-assembly|logistics|space-tug|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="404" PostTypeId="2" ParentId="403" CreationDate="2013-07-18T18:36:26.240" Score="2" Body="&lt;p&gt;Googling around, I couldn't find any mention of a space tug program being officially announced by NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_tug&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A &lt;strong&gt;reusable space tug was studied in 1969&lt;/strong&gt; as part of NASA's Space Transportation System, but went &lt;strong&gt;unfunded,&lt;/strong&gt; as did every other component of that system except the Space Shuttle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="25" LastEditDate="2015-12-17T22:43:58.323" LastActivityDate="2015-12-17T22:43:58.323" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="405" PostTypeId="2" ParentId="208" CreationDate="2013-07-18T18:40:13.860" Score="1" Body="&lt;p&gt;The the United States National Reconnaissance Office has &lt;a href=&quot;http://en.wikipedia.org/wiki/2012_National_Reconnaissance_Office_space_telescope_donation_to_NASA&quot; rel=&quot;nofollow&quot;&gt;donated&lt;/a&gt; two identical declassified reconnaissance telescopes to NASA in 2012. They say the telescopes are more powerful than Hubble. &lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-07-18T18:50:07.060" LastActivityDate="2013-07-18T18:50:07.060" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="406" PostTypeId="5" CreationDate="2013-07-18T18:40:55.113" Score="0" Body="&lt;p&gt;While this tag is &quot;the-moon&quot; the proper name for our moon is &quot;Moon&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not The Moon; not Luna; simply Moon.&lt;/p&gt;&#xA;" OwnerUserId="17839" LastEditorUserId="17839" LastEditDate="2017-12-13T01:09:23.417" LastActivityDate="2017-12-13T01:09:23.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="407" PostTypeId="4" CreationDate="2013-07-18T18:40:55.113" Score="0" Body="Questions about Earth's Moon. Also for questions about spacecraft, manned missions, potential missions, etc. related to Earth's Moon.  " OwnerDisplayName="user40" LastEditorUserId="13710" LastEditDate="2017-12-13T13:17:29.573" LastActivityDate="2017-12-13T13:17:29.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="410" PostTypeId="1" AcceptedAnswerId="417" CreationDate="2013-07-18T20:43:42.007" Score="11" ViewCount="811" Body="&lt;p&gt;China is supposed to land a rover on the Moon soon (2013). This will be the first landing on the Moon from any nation in quiet a few decades - if none of the Google-Lunar-X-Price teams is faster, of cause.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing is going to happen as part of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Chang%27e_3&quot;&gt;Chang'e 3 mission&lt;/a&gt;. There are &lt;a href=&quot;http://news.qq.com/a/20120812/000404.htm&quot;&gt;some pictures&lt;/a&gt;, very few bits and pieces of information and only very little papers about the details (design etc.) of this mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know a good source of information? I am really interested in the rover, its design, its deployment, its method of control and its instruments. If there is any good stuff out there in Chinese - and if someone here actually speaks Chinese - could you give an overview of what is going on right now? What is the current status of when it will be launched exactly? &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorDisplayName="user12" LastEditDate="2013-07-18T21:01:34.313" LastActivityDate="2013-08-09T06:42:19.317" Title="The upcoming landing of Chang'e 3 on the Moon - Is there detailed information about it anywhere?" Tags="|the-moon|rovers|lunar-landing|china|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="411" PostTypeId="1" AcceptedAnswerId="888" CreationDate="2013-07-18T20:49:52.160" Score="12" ViewCount="3273" Body="&lt;p&gt;There is an ongoing debate about the pros and cons of space exploration. One of the oft-cited pros is integrating the spin-off technologies developed for space into everyday life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the research put in to the Apollo program produce any technologies that are still used today for non-space exploration uses?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="49" LastEditDate="2014-12-26T13:53:14.810" LastActivityDate="2014-12-29T21:07:04.487" Title="What are the &quot;everyday&quot; technologies that are a byproduct of the Apollo program?" Tags="|apollo-program|technology|science|spinoff|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="412" PostTypeId="2" ParentId="403" CreationDate="2013-07-18T21:03:20.680" Score="7" Body="&lt;p&gt;At NASA, the type of vehicles you are describing go by the name Orbital Transfer Vehicles (OTVs).  While none are currently funded or planned for, there were &lt;a href=&quot;http://www.astronautix.com/craft/otv.htm&quot;&gt;studies&lt;/a&gt; underway as recently as the 80s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Upper stages are sometime considered &quot;space tugs&quot; but I don't think that is a good description as they are only used once and are really just, well, an upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA delivered the ISS modules it was responsible for delivering directly via the STS.  Also, there are no additional modules planned for the ISS as it is at &quot;assembly complete.&quot;&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-18T21:09:50.460" LastActivityDate="2013-07-18T21:09:50.460" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="413" PostTypeId="1" CreationDate="2013-07-18T21:11:16.707" Score="11" ViewCount="171" Body="&lt;p&gt;Tomographies of various kinds are basically the only possibility right now to look deeper into &lt;strong&gt;asteroids&lt;/strong&gt; or planets. On Earth, &quot;signals&quot; generated by earthquakes are for instance used for performing a tomography in a mathematical sense of the Earth and its core structure (&lt;a href=&quot;http://en.wikipedia.org/wiki/Seismic_tomography&quot; rel=&quot;nofollow&quot;&gt;seismic tomography in wikipedia&lt;/a&gt;, more details in a &lt;a href=&quot;http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/7/SeismicTomography_Background.pdf&quot; rel=&quot;nofollow&quot;&gt;document of the Incorporated Research Institutions for Seismology&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, as far as I know, the only mission making use of a similar concept (for an asteroid) is &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Rosetta&lt;/a&gt; with its &lt;a href=&quot;https://en.wikipedia.org/wiki/CONSERT&quot; rel=&quot;nofollow&quot;&gt;&quot;COmet Nucleus Sounding Experiment by Radiowave Transmission&quot;&lt;/a&gt; - please correct me if I am wrong. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been plenty of proposals for seismic networks on the Moon, etc. But are there any currently seriously investigated mission concepts with hopefully some actual funding, which contain at least one experiment/instrument of any sort that could perform a tomography of a celestial body? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;EDIT: What I am kind of looking for are concepts, which result in a 3D representation of the interior of an object as you can find it in medical applications. Due to the fair number of seismometers and quakes here on Earth, you can actually search for anomalies within a plain shell model of the Earth. On the Moon, there were similar attempts during the Apollo era. Have a look at this paper: &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0040195113001236&quot; rel=&quot;nofollow&quot;&gt;A. Khan, A. Pommier, G.A. Neumann, K. Mosegaard, 2013. The lunar moho and the internal structure of the Moon: A geophysical perspective. In: Tectonophysics.&lt;/a&gt; Figure 4 is interesting. Here, the limitation was the number of seismometers and useful seismic events. Besides, all seismometers were located on one hemisphere, thus further limiting the value of their data. The answer of @DeerHunter refers to InSight for Mars, which is based on the same concept and essentially has the same limitations.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-08-12T19:29:48.780" LastActivityDate="2013-08-12T19:29:48.780" Title="Mission concepts for a tomography of an asteroid or other celestial body" Tags="|asteroid|planetology|geophysics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="414" PostTypeId="1" CreationDate="2013-07-18T21:49:17.600" Score="20" ViewCount="791" Body="&lt;p&gt;I was wondering whether there are any sound studies on &lt;strong&gt;mining in microgravity&lt;/strong&gt;. For example, smelting iron (out of ore) in weightlessness has been described and (in theory) solved. Also, there are people working on, for example, excavators for the Lunar surface, which does have a noticeable field of gravity compared to microgravity one would experience near an average asteroid. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am looking for &lt;strong&gt;studies on drilling, digging and rock blasting&lt;/strong&gt; (or &lt;em&gt;processes substituting the previous&lt;/em&gt;) on asteroids, which have a rather weak field of gravity. How can this be done? I am also interested in how the excavation material can be handled, if it was ever described. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: In Undo's answer, there is a link to a website. It includes the following statement: &quot;Some studies adopted &lt;strong&gt;tunnelling to mine an asteroid&lt;/strong&gt;.&quot; But it does not give any references. I would really be interested in such a study. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-26T00:15:39.020" LastActivityDate="2018-08-14T07:15:04.347" Title="Mining in microgravity: are there sound studies?" Tags="|asteroid|microgravity|mining|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="415" PostTypeId="2" ParentId="260" CreationDate="2013-07-18T22:01:27.380" Score="7" Body="&lt;p&gt;This is a very practical and useful idea.  It is often called a skyhook and the &lt;a href=&quot;http://en.wikipedia.org/wiki/Skyhook_%28structure%29&quot;&gt;Wikipedia article&lt;/a&gt; has lots of useful information and links about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably the most useful trait of a skyhook is it potential to store orbital momentum over time via electrical propulsion and transfer it to a &quot;hooked&quot; spacecraft quickly -- reducing the need for propellant and the it's associated mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting to LEO is the hardest part to getting into space and that applies to the fuel you need to go beyond LEO.  So something that might reduce this is very valuable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest challenge here is probably connecting a vehicle on a suborbital trajectory to the end of the skyhook.  The rendezvous wouldn't be hard, but you'd only get one chance I suspect.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-18T22:10:17.573" LastActivityDate="2013-07-18T22:10:17.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="416" PostTypeId="1" AcceptedAnswerId="418" CreationDate="2013-07-18T22:05:07.070" Score="12" ViewCount="206" Body="&lt;p&gt;Changing velocity rapidly in space is a bit of a problem requiring either the expense of propellant, or aiming past a planet for a gravitational slingshot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you could fire out a strong tether and anchor to a large mass such as an asteroid temporarily, would it be an effective and efficient way to change course rapidly? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My analogy is with the Batmobile grappling hook as illustrated by this screenshot hosted on chickslovethecar.com:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EifQF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-22T18:47:03.643" Title="Would tethering to a large mass be an efficient way to change velocity?" Tags="|navigation|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="417" PostTypeId="2" ParentId="410" CreationDate="2013-07-18T22:10:28.907" Score="11" Body="&lt;p&gt;I was able to find quite a bit in the way of the Chang'e 3 rover's specifications and equipment, so I will only summarize its most interesting features.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Power supply:&lt;/strong&gt; The rover is powered by &lt;em&gt;both&lt;/em&gt; nuclear power and solar power, giving it a potential lifespan of approximately 30 years or more, even though its original mission is only for three months. It will be carrying a 45kg RTG, which they expect will provide approximately 140 Watts of power initially.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;First mission:&lt;/strong&gt; The Chang'e 3 rover's first mission is to explore roughly a 3 square kilometer area around its landing site, while testing &quot;lunar surface topography, geological structure, mineral composition, and chemical composition.&quot; The rover will land at Rainbow Bay on the Sinus Iridium at a latitude of 44 degrees North.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Spectroscopy equipment:&lt;/strong&gt; It will carry a single infrared spectrophotometer, and an alpha-particle x-ray spectrometer.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Radar equipment:&lt;/strong&gt; The rover will have radar equipment attached to its underside, which will allow it to measure the underground structure of the lunar soil to a depth of 30 meters, with additional capacities extending down a few hundred more meters into the crust.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;The lander:&lt;/strong&gt; The 100 kg lander also has its own power supply and transmission capabilities, as well as an extreme ultraviolet camera and a near-ultraviolet optical telescope and after releasing the rover will function as a lunar observatory.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Object avoidance:&lt;/strong&gt; I was unable to find what exact algorithms/software they were using, but it seems they have a larger-than-normal number of distance/movement sensors to guide the rover's movements.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Rover wheels:&lt;/strong&gt; The Chang'e 3 rover has a novel design for the patterning on its wheels, which will help it maneuver easier on the lunar surface. Also, the rover's front wheels are capable of being extended and used as legs to navigate over difficult terrain.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Launch date:&lt;/strong&gt; I was unable to find anything more specific than &quot;late 2013&quot;. Since the Chang'e 3 project received an approximately 10 month set back a while ago, China has not as far as I can tell announced a new official launch date.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;English articles:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Chang%27e_3&quot;&gt;http://en.wikipedia.org/wiki/Chang%27e_3&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Chinese articles:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://news.qq.com/a/20120812/000404.htm&quot;&gt;http://news.qq.com/a/20120812/000404.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://news.qq.com/a/20120725/000108.htm&quot;&gt;http://news.qq.com/a/20120725/000108.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.morningpost.com.cn/xwzx/guonei/2012-07-25/338237.shtml&quot;&gt;http://www.morningpost.com.cn/xwzx/guonei/2012-07-25/338237.shtml&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://space.kexue.com/2013/0517/31222.html&quot;&gt;http://space.kexue.com/2013/0517/31222.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://baike.baidu.com/view/2241905.htm&quot;&gt;http://baike.baidu.com/view/2241905.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note: The first four Chinese articles largely duplicate what the English Wikipedia page has to say, while adding details, such as the size/specs of the RTG and the official landing site for the rover. If you're interested in more detail about the rover, look at the baidu link.&lt;/p&gt;&#xA;" OwnerUserId="47" LastEditorUserId="47" LastEditDate="2013-07-19T15:58:56.883" LastActivityDate="2013-07-19T15:58:56.883" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="418" PostTypeId="2" ParentId="416" CreationDate="2013-07-18T22:15:17.737" Score="13" Body="&lt;p&gt;If you can make and break this connection safely and reliably, you would be able to exchange momentum very efficiently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is effectively what a &lt;a href=&quot;http://en.wikipedia.org/wiki/Skyhook_%28structure%29&quot;&gt;skyhook&lt;/a&gt; does with the addition of making the tethered connection in reverse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Challenges:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You probably won't be able to make and break this connection safely and reliably, so you will need a back-up plan!&lt;/li&gt;&#xA;&lt;li&gt;These are probably large distances, so the mass of the tethering system will probably be large.&lt;/li&gt;&#xA;&lt;li&gt;Because the mass will be large, you will probably be exchanging momentum in the wrong direction to make the connection.  So you might make your situation worse if you try and fail to make the connection.&lt;/li&gt;&#xA;&lt;li&gt;Tether dynamics are crazy complicated.  They will probably bite you.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-22T18:47:03.643" LastActivityDate="2013-07-22T18:47:03.643" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="419" PostTypeId="2" ParentId="40" CreationDate="2013-07-18T22:21:06.347" Score="22" Body="&lt;p&gt;gerrit is right, the most important restricting factor for the life-time in orbit is the orbit of the launcher. But I think a few details could be useful, anyway ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although cubesats are fairly similar (well, cubes), there are a few differences. There are 1, 2, 3, 6 and 12U cubesats. Besides, a lot of projects do not care about the weight restriction (one kg per one U) anymore, so there are plenty of over-weight (and some under-weight) cubesats in space. Some cubesats have deployable solar arrays, a lot have (different types of) deployable antennas. All of those factors account for differences in orbital decay. However, fundamentally, the decay still depends on the altitude (perigee and apogee) of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few numbers for circular orbits and 1U cubesats: ~1 year @ 450 km, 25 years @ 640 km (maximum allowed life time in or below geosynchronous orbit for every satellite launched in 2008 or later - &lt;a href=&quot;http://www.iadc-online.org/&quot; rel=&quot;noreferrer&quot;&gt;IADC&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interesting paper by AGI: &lt;a href=&quot;https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1144&amp;amp;context=smallsat&quot; rel=&quot;noreferrer&quot;&gt;An Evaluation of CubeSat Orbital Decay&lt;/a&gt;, 2011. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;em&gt;Feel free edit in a graph, which is not copyright-protected - cant find anyone.&lt;/em&gt;)&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2018-11-26T07:19:31.653" LastActivityDate="2018-11-26T07:19:31.653" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="420" PostTypeId="2" ParentId="381" CreationDate="2013-07-18T22:43:11.707" Score="5" Body="&lt;p&gt;There are two novel approaches that I am aware of:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The first is to have a safe room.  This is basically at the core of&#xA;the ship and is probably surrounded by shielding or water.  The crew&#xA;gets notified of a solar event and has some time to get into the&#xA;room.  The downside is they don't know how long they will have to be&#xA;in the room.  There are some statistics on how long events usually&#xA;last though.  This doesn't particularly help with the constant background radiation.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The second is to magnetize the ship so that it repels the charged&#xA;particles.  &quot;A powerful electromagnet using super conductors would encircle the crew quarters with a donut shaped magnetic field of up to 20 Teslas.&quot;  &lt;a href=&quot;http://www.patrawlings.com/default.cfm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pat Rawlings&lt;/a&gt; made a piece of art to depict this:&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1ozGW.jpg&quot; alt=&quot;Magnetically shielded spacecraft could deflect cosmic rays, thereby preventing cell damage and cancer-causing damage to the occupant’s DNA. A powerful electromagnet using super conductors would encircle the crew quarters with a donut shaped magnetic field of up to 20 Teslas. SAIC engineers have developed Superconducting Magnetic Energy Storage technology to provide protection against short-term electrical power loss. Digital artwork by Pat Rawlings/SAIC, 2006&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like a combination of these two (and other?) approaches would make sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course if you are on a planet or a moon for example, burying yourself in regolith is the preferred approach...&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-18T22:56:53.847" LastActivityDate="2013-07-18T22:56:53.847" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="421" PostTypeId="1" CreationDate="2013-07-18T22:47:38.997" Score="14" ViewCount="192" Body="&lt;p&gt;There are actually plenty of ftp-servers etc. in the U.S. and Europe, which host raw and pre-processed data of various instruments from old space exploration missions. Usually it is for free, although in a few cases one must leave a name and an e-mail-address before being granted access to the archives. Some of them host bits and pieces of old Soviet stuff too, such as data form &lt;a href=&quot;http://en.wikipedia.org/wiki/Venera&quot;&gt;Venera missions to Venus&lt;/a&gt;. But there is only very limited data on the internet as far as I have seen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know of websites, ftp-servers or similar, which host such data? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;em&gt;I must admit, that I do not speak Russian, so I have a hard time googling through the depth of the Russian internet ...&lt;/em&gt;)&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorDisplayName="user12" LastEditDate="2013-07-21T14:42:29.370" LastActivityDate="2013-07-21T14:42:29.370" Title="Are there any (free) online data archives for data from instruments on Soviet / Russian missions?" Tags="|science|scientific-data|raw-data|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="422" PostTypeId="5" CreationDate="2013-07-18T22:52:32.573" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-18T22:52:32.573" LastActivityDate="2013-07-18T22:52:32.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="423" PostTypeId="4" CreationDate="2013-07-18T22:52:32.573" Score="0" Body="Space tourism is space travel for recreational, leisure or business purposes. " OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-18T23:33:48.620" LastActivityDate="2013-07-18T23:33:48.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="424" PostTypeId="2" ParentId="46" CreationDate="2013-07-18T23:01:29.330" Score="2" Body="&lt;p&gt;Private property rights are a &lt;a href=&quot;http://en.wikipedia.org/wiki/Natural_and_legal_rights&quot; rel=&quot;nofollow&quot;&gt;natural right&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Natural rights are rights not contingent upon the laws, customs, or&#xA;  beliefs of any particular culture or government, and therefore&#xA;  universal and inalienable.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you can get to the asteroid and make a claim or remove material, it is yours.  No treaty or law changes this.  Whether or not a court in Bangladesh or Arizona disagrees with you is immaterial.  This is especially true if those states have no way of getting to your asteroid.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-18T23:06:39.433" LastActivityDate="2013-07-18T23:06:39.433" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="425" PostTypeId="2" ParentId="416" CreationDate="2013-07-18T23:06:10.393" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Momentum_exchange_tether&quot; rel=&quot;nofollow&quot;&gt;Momentum exchange tethers&lt;/a&gt; are similar/related mechanisms that have been studied in the past. It has not been flown/tested however, due to the various complexities involved, some of which have been mentioned by @Erik.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-07-18T23:06:10.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="426" PostTypeId="2" ParentId="416" CreationDate="2013-07-19T00:11:33.277" Score="6" Body="&lt;p&gt;Not only can it be done, it has been done. But how, might you ask. Well, it hasn't been done using a rope, but it has been done using gravity, and it's done all the time. That's the basic premise of a Gravity Slingshot, come close enough to a big object to have it help you change your course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, this will only change your velocity with respect to the outside world. Your speed relative to the Asteroid will be more or less the same after. But you will change speed with respect to the outside world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the use of the tether itself, it would have to be very strong, and have a very strong grip. You'd have to get it placed very close to the target, and let go at the precise moment, or else it would spell disaster. But it could be done, in theory, and would save some fuel, if you can make everything line up perfectly. But any object small enough to allow for this to happen would be difficult to get with the required precision to intercept it, most likely losing most of the gain you would get.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-19T00:11:33.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="427" PostTypeId="2" ParentId="414" CreationDate="2013-07-19T00:57:50.417" Score="13" Body="&lt;p&gt;There is one such article &lt;a href=&quot;http://www.permanent.com/asteroid-mining.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, based on &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930007686.pdf&quot; rel=&quot;noreferrer&quot;&gt;a NASA paper&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Most Earth mining &lt;strong&gt;depends upon gravity&lt;/strong&gt; to hold the cutting edge against the ore. (However, for many Earth mining operations this is not enough, and other means are employed, e.g., cables and reels.) Scraping away at the surface of the asteroid requires holding the cutting edge against the outer surface of the asteroid. This would &lt;strong&gt;require either local harpoons or anchors imbedded into the surface of the asteroid&lt;/strong&gt;, or cables or a &lt;strong&gt;net around the asteroid for the cutter to hold onto.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;h3&gt;Mining an asteroid with a canopy&lt;/h3&gt;&#xA;&lt;p&gt;Strip mining would result in a lot of dirt being thrown up. An unconventional space mining method sees this not only as a problem but also as an opportunity. &lt;strong&gt;A canopy around the mining site can be used to collect ore purposely kicking up&lt;/strong&gt;, the canopy shaped and rotating to use the centrifugal force to channel the ore to the perimeter for collection, as this NASA artwork shows.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bYMtf.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;If no canopy were put up, a lot of debris would cloud and cover the mining environment and probably interfere with mining operations. (Mining without a canopy would certainly be unacceptable in Earth orbit. Companies will most probably use a canopy also because the canopy would be quite profitable in terms of the amount of loose ore it would collect.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I wasn't able to find any real &lt;em&gt;studies&lt;/em&gt; on the subject, although the concept of using a canopy sounds quite promising.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-08-14T07:15:04.347" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="429" PostTypeId="1" AcceptedAnswerId="430" CreationDate="2013-07-19T01:13:39.033" Score="16" ViewCount="1237" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.reuters.com/article/2013/03/29/us-russia-usa-space-idUSBRE92R0Z520130329&quot;&gt;this article&lt;/a&gt;, Russia recently tested a new launch trajectory that shaved over 45 hours off their normal flight time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What allowed for this difference? Doesn't it seem like the shortest distance will be the closest thing to a straight line?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-19T02:18:09.997" LastActivityDate="2014-10-16T07:19:08.880" Title="The Russians recently tested a new launch trajectory that goes to the space station in about six hours. What allowed for the difference?" Tags="|launch|iss|russia|trajectory|docking|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="430" PostTypeId="2" ParentId="429" CreationDate="2013-07-19T02:03:37.703" Score="9" Body="&lt;p&gt;The main difference between the two options is that the amount of time is cut down for orbital correction maneuvers. Essentially, a docking maneuver looks something like this:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Launch the vehicle to an almost identical, but not quite identical, orbit to the desired object.&lt;/li&gt;&#xA;&lt;li&gt;Very carefully identify the orbit you are in, and the orbit you want to get to. Do this somewhat far away, to ensure no collision.&lt;/li&gt;&#xA;&lt;li&gt;Maneuver your spacecraft to be just above the space station. This will have you slowly approaching the station.&lt;/li&gt;&#xA;&lt;li&gt;Match the speed when you just barely reach the station.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The main difference is that there was more time spent in the second step, and the original launch was quite a bit further away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give you an idea, the Dragon approach looks something like this.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Launch into an insertion orbit, with the perigee around 200 km, and the apogee around the space station altitude (But slightly lower).&lt;/li&gt;&#xA;&lt;li&gt;Check out systems&lt;/li&gt;&#xA;&lt;li&gt;Raise the orbit to be slightly above the space station. This takes at least 2 burns, spaced an hour apart.&lt;/li&gt;&#xA;&lt;li&gt;Match the speed at the correct time.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The main difference is that the spacecraft is launched deliberately in to an insertion orbit. However, this gives the spacecraft more opportunity to correct for mistakes.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-19T02:03:37.703" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="431" PostTypeId="2" ParentId="429" CreationDate="2013-07-19T02:16:43.697" Score="24" Body="&lt;p&gt;They have shaved off 45 &lt;em&gt;hours&lt;/em&gt;, not &lt;em&gt;minutes&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to be a whole complex of factors:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the Space Shuttle retirement, the ISS orbit could be raised to provide better ballistic possibilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The new on-board computer allows to conduct ballistic calculations faster and  with less dependence on ground control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbit measurements precision has improved over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The communication has improved over time, going through the station and satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a constant pressure from the cosmonauts themselves, because one gets very sick during the two days scheme. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the sequence of events is now denser and the ballistic gaps are narrower, they always have a backup variant to go by 2 days scheme.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Altogether it made the new scheme possible, and they promise to bring it down to four and even two hour scheme.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This answer is based on two interviews given &lt;a href=&quot;http://gagarin.energia.ru/index.php?option=com_content&amp;amp;view=article&amp;amp;id=111:2013-03-25-05-13-17&amp;amp;catid=5:2011-10-25-09-30-46&amp;amp;Itemid=&amp;amp;lang=ru&quot;&gt;by Rafail Murtazin&lt;/a&gt; the deputy head of Energia ballistics department, and &lt;a href=&quot;http://polit.ru/news/2013/05/08/space_soyuz_mks_3_hours/&quot;&gt;by Sergey Krikalev&lt;/a&gt; the head of the Yuri Gagarin Cosmonauts Training Center.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-07-19T02:16:43.697" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="432" PostTypeId="2" ParentId="228" CreationDate="2013-07-19T02:24:32.963" Score="8" Body="&lt;p&gt;Space is commonly defined as starting at the &lt;a href=&quot;http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot;&gt;Kármán line&lt;/a&gt;. I believe you can buy a ticket for an upcoming flight via Virgin Galactic to cross this line on a suborbital flight today. In addition, you can also purchase a much more expensive, orbital trip, from &lt;a href=&quot;http://spaceadventures.com&quot;&gt;Space Adventures&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-19T02:24:32.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="433" PostTypeId="2" ParentId="247" CreationDate="2013-07-19T02:39:06.250" Score="4" Body="&lt;p&gt;A couple of things I might add to the good answers already here:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Selection timeframe.  The decision as which hardware to use for a vehicle is made long before (years?) the vehicle is launched.  Thus, at launch it is probably obsolete.&lt;/li&gt;&#xA;&lt;li&gt;Radiation hardening.  Often, these comparisons focus on one or two specs that are interesting for terrestrial uses: CPU clock speed and RAM for example.  While these are important, fault tolerance in a radiated environment is more important while flying by Jupiter than it is when playing Doom.  This tolerance creates a trade-off that doesn't help the other specs.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-19T02:39:06.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="434" PostTypeId="1" AcceptedAnswerId="508" CreationDate="2013-07-19T03:39:56.703" Score="14" ViewCount="2753" Body="&lt;p&gt;SpaceX is often called a &quot;commercial&quot; company and participates in NASA's &lt;a href=&quot;http://www.nasa.gov/exploration/commercial/crew/index.html#.Uel9tWTOvBg&quot;&gt;Commercial Crew Program&lt;/a&gt;.  In SpaceX's early days, all of SpaceX's revenue came from non-government sources.  But today, SpaceX is very similar to other NASA contractors in that it has several government contracts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How &quot;commercial&quot; is SpaceX as measured by the fraction of historical revenue from non-government contracts to total revenue over the life of the company?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: I would be open to other ideas regarding a better metric for being &quot;commercial.&quot;  Perhaps something like the summation of historical earnings before interest, taxes, depreciation, and amortization (EBITDA) for example.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="25" LastEditDate="2013-07-19T19:21:56.413" LastActivityDate="2014-04-21T15:49:13.897" Title="What portion of SpaceX's revenue comes from private sources?" Tags="|spacex|economics|commercial|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="435" PostTypeId="2" ParentId="403" CreationDate="2013-07-19T04:29:53.070" Score="2" Body="&lt;p&gt;The Orbital Sciences entry in the CRS program is the Cygnus vehicle.  It has two components a cargo module and an orbital module.  While not officially offered as a tug, it functions as a tug to bring the cargo module to the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the Cygnus payload module is not that big, it seems likely that Orbital could provide such functionality, should the serious need arise (I.e. Someone willing to pay for it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whereas the thrusters and systems for orbital maneuvering on a Dragon capsule are integrated in the capsule, as the trunk is mostly empty space. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-09T23:37:03.717" LastActivityDate="2013-08-09T23:37:03.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="436" PostTypeId="2" ParentId="247" CreationDate="2013-07-19T05:32:51.333" Score="60" Body="&lt;p&gt;There are a number of reasons why spacecraft electronics typically lag what is commercially available by several years.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Radiation tolerance&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Electronics are very susceptible to radiation phenomenon that terrestrial electronics are largely protected from by the Earth's atmosphere and magnetic field. Common radiation-based failure mechanisms are Single-Event Event/Upset (SEE/SEU) — most commonly thought about as a flipped bit, latch-up — where a bit gets stuck in a certain state and the part needs to be powered down to be reset, burn-out — where a high energy particle (e.g. proton or neutron) destroys the part, and total dose — where long-term exposure (rather than a freak event) degrades the part. As chips and circuits advance and pack transistors more tightly, the probability of these events increases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several techniques and testing methods exist to demonstrate if electronic assemblies are robust in space radiation environments, but these tests are expensive. So once they've been done for a part, component, or assembly, the trade is often made to live with less performance and save the cost of re-testing and avoid the risk of complete mission failure. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Reliability&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;It is easier to do maintenance on a terrestrial computer, and the costs of failure are often much lower than for spacecraft. Ground systems also don't have the same tight power, size, and mass budgets that space systems do, which limit the amount of redundancy that is feasible. A solution is to continue to use parts that have been shown to have high reliability. Another way to increase reliability is to perform parts screening and to perform lots of electronics testing (e.g. bake-out to find infant mortality, random vibration testing to mimic launch environments, shock testing to mimic pyrotechnic events like fairing jettison, and thermal vacuum testing to mimic space. This testing takes time and is expensive. The time delay alone puts most space systems at least one Moore's law cycle behind the latest consumer parts. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Build time for satellites&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;To say nothing of the avionics, satellites take a long time to build. Even when the computers are done the rest of the vehicle has to be assembled and tested. For large spacecraft this can take years. Meanwhile, the computer isn't getting any younger and an aversion (often justified) to risk means upgrading it would require many of these tests to be re-done. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Power consumption&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Over time Moore's law helps chips to increase in processing power and decrease in power consumption, but generally speaking, when comparing contemporaneous parts more powerful chips consume more power. Spacecraft are almost universally power starved, so there's little incentive to use a more power-hungry chip than is absolutely necessary. Everything in a spacecraft is a trade-off: a Watt of power used for the main flight computer carrying around unused cycles is a Watt that can't be used for RF communications, or providing power to a payload (when that payload isn't communications), etc.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Paperwork&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Paperwork and process can be as dominate as any of the other reasons. The aerospace industry as a historically high barrier to entry. Once reason is the human capital required to build and launch spacecraft, but equally as important is the space heritage of the software and components that go into them. Space environments are more challenging that terrestrial ones in a variety of ways and often require unique solutions (for avionics, rejection of heat without convective cooling is a good example). Launch environments were discussed above. Qualification of components is a real-world hardware-centric task, but there is a paper trail that backs up this analysis and provides confidence to a spacecraft builder's customers and the launch provider that the vehicle will be safe during ascent and that it will operate in space. This is proven through a combination of test and analysis and demonstration, but most of the people who care don't witness or oversee these activities directly, so they rely on excellent paperwork to provide that confidence. Once you've gone through the trouble of getting buy-in on widget X — the effort associated with a obtaining Δ buy-in for widget Y or even X+ is harder to justify if the older part still works. Aerospace suppliers (prime contractors and all the way down the supply chain) are often also required to have rigorous quality processes in place — i.e. more paperwork and process. All of this slows down the pace of innovation and change for in exchange for predictability. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Launch delays&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Then once the spacecraft is ready it needs to be launched, and launches can slip months if not years. &lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="127" LastEditDate="2013-07-24T13:56:12.320" LastActivityDate="2013-07-24T13:56:12.320" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="437" PostTypeId="2" ParentId="42" CreationDate="2013-07-19T05:35:30.300" Score="3" Body="&lt;p&gt;If we could find a way to exploit &lt;a href=&quot;http://en.wikipedia.org/wiki/Action_at_a_distance_%28physics%29&quot; rel=&quot;nofollow&quot;&gt;spooky action at a distance&lt;/a&gt; we could eliminate manned space flight altogether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the other answers suggest, the inability to make decisions in a timely manner limits what a robot can do in space exploration (such as the rovers). The speed of light and great distance limits how fast decisions can be sent to a rover or deep space probe. If we can find a way to communicate with a probe on Europa at the same speed as a machine right next to you then we would have no need to &quot;experience&quot; space from our own eyes any more. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately, robotic missions are far better (from a scientific point of view) than manned missions, so long as we can communicate with the robots in real time.&lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastActivityDate="2013-07-19T05:35:30.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="438" PostTypeId="1" AcceptedAnswerId="557" CreationDate="2013-07-19T06:04:25.783" Score="10" ViewCount="5525" Body="&lt;p&gt;Back in the old days of the cold war the US and Russia were trying to develop a Nuclear powered aircraft. Problems with radiation shielding and &quot;safety&quot; held them back. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say that they got this technology to work and developed a large flying fortress type aircraft capable of flying at 600mph at 100,000 feet. Now if you wanted to launch a space payload off that platform (weighing 1kg) how would you adjust the launch equations to calculate how much cryogenic fuel you will need? Also, taking into consideration that the atmosphere up there is very cold and there is very little air pressure. This would mean that the fuel would stay colder longer and need lighter tanks. Also since the pressure is reduced the nozzle velocity of the propellent would be higher at those altitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Similar to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Pegasus_%28rocket%29&quot;&gt;Pegasus&lt;/a&gt;, in the fact that it is launched from 40,000 feet from a carrier aircraft. Inclination: 51.6, Apogee: 421Km, Perigee: 431 (basically the ISS' orbit), launched eastward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Expanding further: I want to create a model in c# that will calculate and plot the amount of fuel required to launch 1kg into orbit from several altitudes and speeds. Over time I want to add in factors like drag etc, to determine what the performance of the carrier craft should be.&lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastEditorDisplayName="user12" LastEditDate="2013-07-20T20:25:22.430" LastActivityDate="2018-04-25T14:35:41.407" Title="How much fuel would one need to launch a 1kg object from 100,000 feet?" Tags="|launch|fuel|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="439" PostTypeId="1" AcceptedAnswerId="486" CreationDate="2013-07-19T06:58:23.300" Score="17" ViewCount="3311" Body="&lt;p&gt;The pressure in the Space Shuttle's main engines must be very high to get the vehicle off the ground (with the SRB assist, of course). With such high pressures inside the engine, how do you inject fuel into the engine? I understand that the fuel is passed around the engine so that it cools the nozzle, so that the exhaust doesn't melt it, then into the engine. However I always assumed that the pressure of the fuel going into the engine must be higher than the pressure the exhaust gases exert. If so, how does the Shuttle pump such high pressure fuel into the engines? &lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastEditorUserId="49" LastEditDate="2014-03-13T00:43:55.830" LastActivityDate="2015-02-26T14:17:16.120" Title="In a cryogenic fuel rocket, at what pressure is the fuel injected into the engine?" Tags="|space-shuttle|fuel|cryogenics|rockets|ssme|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="440" PostTypeId="2" ParentId="434" CreationDate="2013-07-19T08:03:43.753" Score="8" Body="&lt;p&gt;It is entirely commercial:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Commercial: prepared, done, or acting with sole or chief emphasis on salability, profit, or success: a commercial product;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From SpaceX's perspective it doesn't matter where the contracts come from, they are a commercial, profit-making organisation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't redefine commercial to exclude revenue from a particular slice of the market.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-19T08:03:43.753" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="441" PostTypeId="1" AcceptedAnswerId="442" CreationDate="2013-07-19T08:05:13.947" Score="6" ViewCount="9713" Body="&lt;p&gt;What would be a viable means to simulate and maintain Earth's gravitational acceleration in a future colony built on another planet? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am mainly focusing on terrestrial planets, where a colony could be built on the surface, such as on Mercury, Venus, Luna, Mars, gas and ice giant moons, minor and dwarf planets where the gravitational constant is less than on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or would it instead be a case of a specific physiological adaptation occurring over time to the lower gravitational effects on these planets?&lt;/p&gt;&#xA;" OwnerDisplayName="user92" LastEditorDisplayName="user12" LastEditDate="2013-07-19T15:21:14.537" LastActivityDate="2018-09-11T16:57:10.330" Title="How to simulate Earth's gravity in future colonies on other planets?" Tags="|gravity|colonization|planet|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="442" PostTypeId="2" ParentId="441" CreationDate="2013-07-19T08:23:48.107" Score="2" Body="&lt;p&gt;There are a few potential ways to go about this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1. Increase the mass of the planet&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, if the mass of the planet is equal to that of earth's then you have the same gravitational attraction (provided the spin is the same, I'll get to that).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For planets with more mass than earth, you would need to decrease the mass to achieve this effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's not easy though so an alternative is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what if rather than create a gravity field of the same strength we just make it feel like a gravity field of the same strength?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2. Apply a downwards force&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This technology doesn't really exist, that I know of, but if one could apply a 'repulsing' effect from the ceilings on each person in the room then the added downwards force would feel like a stronger gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely, if you wanted to decrease it you would want to apply an attractive force towards the ceiling. But once again, these technologies don't exist yet, nor do I have any thoughts on how would create them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;3. Magnetic Boots&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To me, this seems like the most viable of the options. Have metallic flooring and boots that are magnetic using electromagnetic induction. This allows you to 'turn on and off' the magnetism. If you are being attracted to the floor by the magnets then you get the advantage of it feeling like a stronger gravity on your legs and holds you to the ground, but your body will still experience less weight and there will still have less weight on those objects that aren't being magnetized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best thing about this technology is that it could also be used in a spacecraft for long exploration flights&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="-1" LastEditDate="2018-09-11T16:57:10.330" LastActivityDate="2018-09-11T16:57:10.330" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="443" PostTypeId="1" AcceptedAnswerId="456" CreationDate="2013-07-19T08:26:30.383" Score="8" ViewCount="733" Body="&lt;p&gt;One of the problems with space flight is the speed required to achieve orbit; 17,500 mph. One of the ways I've always thought to achieve this was to launch at a low speed to a very high altitude then let the rocket fall back to earth, being speed up by gravity until it reaches orbital velocity and begins orbiting. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question: Are there any programmes for this by the major space agencies, or papers written about the practicality of it?&lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastEditorUserId="14" LastEditDate="2013-07-19T10:20:29.097" LastActivityDate="2013-07-19T18:55:02.167" Title="Can a projectile fall into a low Earth orbit?" Tags="|low-earth-orbit|rockets|orbital-mechanics|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="445" PostTypeId="1" AcceptedAnswerId="474" CreationDate="2013-07-19T09:08:13.697" Score="13" ViewCount="1931" Body="&lt;p&gt;What is the actual feasibility of interstellar travel? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, a manned journey to Proxima Centauri, given the distance, what technology would need to be developed/perfected for such a journey?&lt;/p&gt;&#xA;" OwnerDisplayName="user92" LastActivityDate="2017-12-28T18:38:45.043" Title="How would manned interstellar travel become feasible?" Tags="|interstellar-travel|feasibility|" AnswerCount="4" CommentCount="1" FavoriteCount="0" ClosedDate="2017-12-28T17:04:00.160" ContentLicense="CC BY-SA 3.0" />
  <row Id="446" PostTypeId="1" CreationDate="2013-07-19T09:47:47.477" Score="13" ViewCount="533" Body="&lt;p&gt;There are many factors in planning a long space voyage.  The human interaction being one of them.  Group dynamics have been studied in detail for naval and other military applications, the stations on Antarctica have also contributed to the body of knowledge.  As seen with &lt;a href=&quot;http://en.wikipedia.org/wiki/Biosphere_2#The_ethnosphere%3a_psychology_and_conflict&quot;&gt;Biosphere 2&lt;/a&gt; the answers for space (or simulated space) trips are not as simple.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Logistical limitations put more restrictions on the number of members of space crew then common Military or Arctic journeys.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have heard that the number of members of a team, can impact the relationship dynamics, where poor dynamics lead to mission failure.  Presumably civilian vs military and/or gender of crew members would be considerations as well,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any significant research done to find the optimal configuration and number of members of a small (&amp;lt;= 5) team?  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="63" LastEditDate="2013-07-19T16:31:37.057" LastActivityDate="2013-08-02T14:25:53.460" Title="Is there research for the optimal small crew size for a long space voyage?" Tags="|crewed-spaceflight|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="447" PostTypeId="1" CreationDate="2013-07-19T09:48:00.270" Score="2" ViewCount="35" Body="&lt;p&gt;Supposing a planned long-term manned-flight with a spaceship that has to be fairly large to accommodate many people and meet some requirements like decent shielding from radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it better to build a large space ship in total and launch it from Earth, considering places of production, technology available, resources for logistics and costs of fuel, or is it better to construct modules/parts, which are brought to and assembled in space, also considering places of production, technology available (especially for the later assembly), resources for logistics and costs for fuel?&lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorUserId="14" LastEditDate="2013-07-19T10:34:01.023" LastActivityDate="2013-07-19T10:34:01.023" Title="Is it better to build a spaceship in total or in parts for orbital assembly?" Tags="|technology|orbital-assembly|" AnswerCount="0" CommentCount="2" ClosedDate="2013-07-19T11:02:44.043" ContentLicense="CC BY-SA 3.0" />
  <row Id="448" PostTypeId="2" ParentId="93" CreationDate="2013-07-19T09:49:16.607" Score="9" Body="&lt;p&gt;I will try to answer my own question. Gradually editing the answer as I found out any details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that while the Spacelab modules, Harmony,  Tranquility,  ATVs, Cygnus, Columbus, Leonardo, Rafaello, Donatello, all belong to different legal entities and attributed to different space agencies or companies,  in reality, the pressurized compartments for all of those systems have been produced by &lt;a href=&quot;http://www.thalesgroup.com/Portfolio/Space/Orbital_Transport_and_Infrastructure/&quot; rel=&quot;noreferrer&quot;&gt;Thales Alenia Space&lt;/a&gt; as the primary contractor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So since it is all happening inside one company, they all share large common pieces of design and, quite probably, share the hardware leftovers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the Unity, Destiny and Quest modules have been produced by Boeing.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-05T13:18:28.110" LastActivityDate="2013-08-05T13:18:28.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="449" PostTypeId="1" AcceptedAnswerId="455" CreationDate="2013-07-19T10:02:55.050" Score="7" ViewCount="220" Body="&lt;p&gt;Supposing a long-term manned flight is planned would a biological resource-system be considered as an option?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a biological resource-system work or is a technical solution superior?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently technical solutions turn urine into drinkable water again and turn CO2 (carbon dioxide) to O2 (oxygen) [and C (carbon)]. Would the same be feasible with plants or other biological solutions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isn't limited to oxygen and water; food could be grown as well. Would it be considered or dismissed, because testing such a system could be difficult.&lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorUserId="25" LastEditDate="2013-07-19T13:20:37.717" LastActivityDate="2021-11-13T18:31:43.263" Title="Would a biological resource-system be feasible for long-term manned flights?" Tags="|long-duration|life-support|gasses|food|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="450" PostTypeId="1" CreationDate="2013-07-19T10:13:59.663" Score="25" ViewCount="4227" Body="&lt;p&gt;There is an estimate at &lt;a href=&quot;http://genetics.thetech.org/ask/ask113&quot;&gt;Understanding Genetics&lt;/a&gt; that the minimum number of humans required for a colonization process is 160.  It also mentions some animal populations that have rebounded from near extinction with much lower numbers. Strangely there is no mention of haplogroups in the answer there, and it seems to assume a random sampling of humans rather than a picked group.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would seem that if you were trying prevent mutation in all haplotypes you would probably need a larger gene pool. Common sense also implies that it would be easier to get a diverse gene pool from a smaller group by purposely selecting from different haplogroups.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If hand picking with gene profiling, what is the smallest group of humans that can form a viable human colony, where new gene infusion is unlikely?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Clarifications:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;This question is looking for a &lt;strong&gt;lower limit&lt;/strong&gt; number.&lt;/li&gt;&#xA;&lt;li&gt;The number should (at least, theoretically) provide population self-sustainability and long-term survival.&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The answer should take two different approaches: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;An intra-solar-system colony that may have future additions to its numbers&lt;/li&gt;&#xA;&lt;li&gt;An interstellar colony that does not expect future additions to its numbers&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;li&gt;Let's define &quot;long-term&quot; as &lt;strong&gt;at least 200 years&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;li&gt;This question is focusing on genetic/population considerations. Practically, a real colony may require more than the answer here indicates.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-11-07T21:11:30.540" LastActivityDate="2020-06-07T03:20:27.817" Title="What is the smallest number of humans required for colonization?" Tags="|colonization|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="451" PostTypeId="5" CreationDate="2013-07-19T10:21:07.210" Score="0" Body="&lt;p&gt;In the frictionless vacuum of space, rockets move with mathematical precision along a trajectory.&lt;/p&gt;&#xA;&lt;p&gt;Around a single point mass, or even any spherically symmetric body due to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shell_theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;shell theorem&lt;/a&gt;, these orbits are simple conic sections and can thus be modelled with simple geometry. Circles and ellipses form cyclic orbits bound by the gravitational field of the parent body, while parabolas and hyperbolas lead to an eventual escape.&lt;/p&gt;&#xA;&lt;p&gt;Commonly used properties for these orbits are:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;r_P: The periapsis distance, the closest approach to the parent.&lt;/li&gt;&#xA;&lt;li&gt;r_A: The apoapsis distance, the furthest away&lt;/li&gt;&#xA;&lt;li&gt;a: The semi-major axis, half the distance between the periapsis and apoapsis.&lt;/li&gt;&#xA;&lt;li&gt;e: The eccentricity, how stretched the conic section is. 0 for a circle and 1 for a parabola&lt;/li&gt;&#xA;&lt;li&gt;μ: The gravitational parameter, an object's mass times the gravitational constant.&lt;/li&gt;&#xA;&lt;li&gt;v_e: Escape velocity, the necessary speed (in any direction) to escape a parent body from a given distance.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Velocity in these orbits are governed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler's laws&lt;/a&gt;, easily applied through the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vis-viva equation&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;When three or more bodies are involved &lt;a href=&quot;https://en.wikipedia.org/wiki/Three-body_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;(three body problem)&lt;/a&gt;, systems typically behave chaotically and can't be solved analytically. A notable exception is the circularly restricted three body problem (CR3BP), which has solutions such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrange_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lagrange points&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;For spacecraft, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Patched_conic_approximation&quot; rel=&quot;nofollow noreferrer&quot;&gt;patched conics&lt;/a&gt; approximation is often used to model interplanetary flight. Here the trajectory is broken up into multiple conic sections with different parent bodies, the assumption being that the gravity of one object usually dominates.&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="8693" LastEditDate="2023-02-04T22:20:35.330" LastActivityDate="2023-02-04T22:20:35.330" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="452" PostTypeId="4" CreationDate="2013-07-19T10:21:07.210" Score="0" Body="Orbital mechanics (also called astrodynamics) is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft on an orbital or escape trajectory. For the movements of celestial bodies, use [celestial-mechanics], not this." OwnerUserId="14" LastEditorUserId="46507" LastEditDate="2023-03-23T18:50:12.580" LastActivityDate="2023-03-23T18:50:12.580" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="453" PostTypeId="5" CreationDate="2013-07-19T10:30:57.060" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T10:30:57.060" LastActivityDate="2013-07-19T10:30:57.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="454" PostTypeId="4" CreationDate="2013-07-19T10:30:57.060" Score="0" Body="In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft." OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-19T16:44:34.163" LastActivityDate="2013-07-19T16:44:34.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="455" PostTypeId="2" ParentId="449" CreationDate="2013-07-19T10:31:01.447" Score="4" Body="&lt;p&gt;If I understand what you are asking correctly, this is essentially the same idea as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Biosphere_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Biosphere 2&lt;/a&gt;, where your spaceship air and water is managed by plants, to keep it livable. The Biosphere experiment provided a baseline for what would be required to pull that off, and actually tested such a system. The design called for 8 people to stay inside of the 204,000 square meter facility for 2 years, with the only outside environmental change to come from the form of managing the temperature and rainfall, all from within the contained system. A photo of the system can be seen below, sourced from &lt;a href=&quot;http://en.wikipedia.org/wiki/File:Wiki_bio2_sunset_001.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/F39yn.jpg&quot; alt=&quot;The Biosphere&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to pull this off, you need to have a huge facility. Just to put this in perspective, the entire volume of the International Space Station is 837 meters squared. The biosphere is thus about 240 times larger than the ISS. Granted, there are aspects of the Biosphere which would not need to be reproduced to completely maintain systems on a space station (Variable air chambers, etc), but that would only reduce the size maybe by a factor of two. And even at that, the system wasn't able to maintain a truly independent environment, during the first test, they had to bring in oxygen and resorted to using emergency food stores.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, to use this would require an extensive system. A hybrid system could be put in place, where machines do some of the conversion from Carbon Dioxide to Oxygen, and food is grown, with a significant reduction in the required space, and that's the most likely solution that we could see anytime in the near future.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-19T10:31:01.447" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="456" PostTypeId="2" ParentId="443" CreationDate="2013-07-19T10:32:25.210" Score="7" Body="&lt;p&gt;One of the major show stoppers for slow ascent to orbital heights and beyond is that &quot;slow&quot; by necessity means &quot;longer time&quot;, i.e. increases the time needed to achieve this, thus increasing your overall energy output needs (in a nutshell, &lt;a href=&quot;https://en.wikipedia.org/wiki/Power_%28physics%29&quot;&gt;&lt;code&gt;W = PT&lt;/code&gt;&lt;/a&gt;). While we could probably be able to achieve this, it's just not economical. It might be practical to combine a bit of both, and release the rocket off an airplane carrying it till the heights airplanes can operate, and maybe require a rocket stage less, but the total energy required will still be greater than doing it as fast as possible.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-07-19T10:37:44.360" LastActivityDate="2013-07-19T10:37:44.360" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="457" PostTypeId="1" AcceptedAnswerId="460" CreationDate="2013-07-19T10:32:49.643" Score="26" ViewCount="12136" Body="&lt;p&gt;For launching satellites or other spacecraft, there is a significant advantage in being close to the equator: angular momentum helps in gaining initial speed and one can launch into any inclination.  Most spaceports for orbital launches are, indeed, at lower latitudes, with at least one major exception: &lt;a href=&quot;http://en.wikipedia.org/wiki/Plesetsk_Cosmodrome&quot;&gt;Plesetsk Cosmodrome&lt;/a&gt; is at 63°N, and is, according to Wikipedia, &lt;a href=&quot;http://en.wikipedia.org/wiki/Spaceport&quot;&gt;the most active spaceport on Earth for launches to orbit or interplanetary space&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the fall of the Soviet Union, it's the only domestic Russian spaceport, and empty land around is beneficial for the safety of letting different rocket stages fall to Earth.  But Russia has plenty of empty land at lower latitudes (although even Kazakh Baikonur is at 46°N), so why was Plesetsk even built at its present place to begin with?  Are there any advantages of launching from such a high latitude?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="27091" LastEditDate="2020-01-16T13:56:59.453" LastActivityDate="2020-01-16T13:56:59.453" Title="Is there any advantage in launching spacecraft from a high latitude, or why was Plesetsk built so far north?" Tags="|launch|russia|soviet-union|spaceport|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="459" PostTypeId="2" ParentId="457" CreationDate="2013-07-19T10:42:19.307" Score="19" Body="&lt;p&gt;Building just another (domestic) launch side is not exactly easy or cheap. There is a lot of logistics to take care of. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For your question, you need to understand the history of Russian space flight and Plesetsk. It was build for launching ICBMs over the north pole into north America. Keep in mind that Soyuz rockets were derived from the R-7 rocket, which was initially developed to carry nuclear warheads. Both, modern Soyuz and R-7 share the same type of launch pads, assembly etc. Besides, keeping Plesetsk open for e.g. polar launches (high inclination) still makes sense from a pure technical point of view. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-19T10:57:32.317" LastActivityDate="2013-07-19T10:57:32.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="460" PostTypeId="2" ParentId="457" CreationDate="2013-07-19T10:44:57.847" Score="24" Body="&lt;p&gt;There are a few advantages to launching a spacecraft from a high latitude, depending on what you want exactly. The primary advantage to such a system is for very high inclined orbits. Most low earth satellites benefit from a high inclination, and in fact, many are placed in to what is known as a &quot;Sun Synchronous Orbit&quot; with a slight retrograde inclination. If you are launching to a retrograde inclination, you actually have to take out all of the rotation of the location from which you launch, making it more difficult to achieve the orbit from near the equator. However, it would be more difficult to launch an equatorial satellite, for instance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rotation speed at the equator is approximately 40000 km/ 24 hours, or 1670 km/hour The minimal Earth orbital velocity is about 29000 km/hour. Thus, while you do get an advantage in rotational speed from the Earth's rotation, it isn't significant enough where other factors don't play an important part, in fact, there are a good deal more factors to contend with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as @ernestopheles provided in his answer, there are many non-technical reasons to have that as a location, namely that it was a launch facility for nuclear ICBMs, and as many of those are decommissioned, they are readily available, but transporting them elsewhere doesn't make much sense. In addition, the facilities are also available.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2013-10-05T13:51:40.527" LastActivityDate="2013-10-05T13:51:40.527" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="461" PostTypeId="5" CreationDate="2013-07-19T10:48:40.313" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T10:48:40.313" LastActivityDate="2013-07-19T10:48:40.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="462" PostTypeId="4" CreationDate="2013-07-19T10:48:40.313" Score="0" Body="In-orbit assembly refers to the assembly of large structures in orbit around a body, usually but not limited to the Earth. Components are lifted into space in a series of launches, enabling the use of smaller rockets - or - the construction of structures beyond the capacity of existing rockets / launchers. " OwnerUserId="14" LastEditorUserId="12102" LastEditDate="2018-11-16T03:31:29.047" LastActivityDate="2018-11-16T03:31:29.047" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="463" PostTypeId="1" CreationDate="2013-07-19T10:57:30.537" Score="13" ViewCount="569" Body="&lt;p&gt;There are a multitude of resources about growing food in space, &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/10-074.html&quot;&gt;NASA research&lt;/a&gt; indicates we have been growing foods in space for years. &lt;a href=&quot;http://www.nss.org/settlement/ColoniesInSpace/colonies_chap09.html&quot;&gt;Colonies in Space by T. A. Heppenheimer&lt;/a&gt; seems to have some research behind it, but it measures production in acres, and assumes large colonies of thousands of people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a long space journey or a planetary settlement, abundant solar energy or sufficient electrical energy to simulate solar energy would be a basic requirement.  Everything else would need to be self sustaining in the food energy life cycle.  After sufficient air and water recycling have been addressed the next concern is meeting the human dietary requirements.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a small group of humans with limited area for growing, and a hostile environment (space or planet) what would be the optimal garden/farm products for self sustaining indefinitely? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="68" LastEditDate="2013-07-24T16:41:39.820" LastActivityDate="2022-05-31T05:01:36.293" Title="What is the optimal selection for a small self sustaining space farm?" Tags="|long-duration|food|colonization|life-support|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="465" PostTypeId="5" CreationDate="2013-07-19T11:21:43.187" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T11:21:43.187" LastActivityDate="2013-07-19T11:21:43.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="466" PostTypeId="4" CreationDate="2013-07-19T11:21:43.187" Score="0" Body="Questions about long-duration spaceflight." OwnerDisplayName="user40" LastEditorDisplayName="user40" LastEditDate="2013-07-25T19:38:26.117" LastActivityDate="2013-07-25T19:38:26.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="467" PostTypeId="5" CreationDate="2013-07-19T11:53:37.990" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T11:53:37.990" LastActivityDate="2013-07-19T11:53:37.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="468" PostTypeId="4" CreationDate="2013-07-19T11:53:37.990" Score="0" Body="A space station (or orbital station) is a spacecraft capable of supporting a crew, which is designed to remain in space (most commonly in low Earth orbit) for an extended period of time. For questions about specific space stations (e.g. the ISS, MIR, Salyut 1-7, Tiangong or Skylab), please use corresponding tags. " OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-19T16:46:29.250" LastActivityDate="2013-07-19T16:46:29.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="469" PostTypeId="5" CreationDate="2013-07-19T12:00:35.373" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T12:00:35.373" LastActivityDate="2013-07-19T12:00:35.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="470" PostTypeId="4" CreationDate="2013-07-19T12:00:35.373" Score="0" Body="Refers to questions concerning the solar system containing Earth. For other solar systems, use the 'planetary-system' tag." OwnerUserId="14" LastEditorDisplayName="user12" LastEditDate="2013-08-03T04:31:42.330" LastActivityDate="2013-08-03T04:31:42.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="471" PostTypeId="5" CreationDate="2013-07-19T12:12:48.017" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T12:12:48.017" LastActivityDate="2013-07-19T12:12:48.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="472" PostTypeId="4" CreationDate="2013-07-19T12:12:48.017" Score="0" Body="Space mining refers to the general possibility of exploiting raw materials from the Moon, asteroids and other minor planets, including near-Earth objects. For questions targeting specifically the mining of asteroids, please use the tag 'asteroid-mining'." OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-19T16:43:45.487" LastActivityDate="2013-07-19T16:43:45.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="474" PostTypeId="2" ParentId="445" CreationDate="2013-07-19T12:30:25.943" Score="10" Body="&lt;p&gt;Given no time constraint, there are only two pieces missing for a manned journey to Proxima Centauri.  Resources (money) and commitment. Getting there and back in a single human life time would be a totally different question/answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several, drives that can get us there, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;noreferrer&quot;&gt;solar sail&lt;/a&gt; probably being the simplest and easiest to maintain over the long time/distance. All of current propulsion options would take time measured in generations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have had humans constantly in space for decades now, so technologically speaking all the major hurdles of maintaining life in space are overcome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some medical and psychological considerations, most of the medical having to do with adapting to space in a way that make re-adapting to earth difficult. The psychological considerations can likely not be fully addressed without going on the journey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several logistical issues, to balance available resources to allow for a successful mission.  You could capture an asteroid (preferably mostly metal) and hollow it out, put 500 people in it, and farms/gardens to support 5,000, and send it on its way.  but the resources to accomplish this would be significantly greater then a simple trip to the moon, which we have not repeated for several decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Practically you could send a small envoy in a smaller man made craft, a crew of 3 to 10, with frozen sperm/egg/embryo DNA to maintain genetic diversity.&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-07-19T12:30:25.943" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="475" PostTypeId="2" ParentId="6" CreationDate="2013-07-19T13:05:01.600" Score="7" Body="&lt;p&gt;The lunar flyby of AsiaSat 3 is, of course, creative, but it was only necessary (and more efficient than alternatives) because of a launch failure which left it in a much higher inclination orbit than it should have. For a more typical launch, one in which a good amount of inclination is taken out by the upper stage, there is little you can do to save large amounts of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Boeing has been marketing an &lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_702#702SP&quot; rel=&quot;noreferrer&quot;&gt;all-electric-propulsion&lt;/a&gt; satellite bus. While it takes much longer to reach your operational slot, the EP thrusters are much, much more efficient, and allow you to drastically decrease the mass of your bus at launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you're in GEO, there's more you can do to save fuel. Because of the asphericity of Earth and the pull of the Sun and Moon, GEO spacecraft have to perform &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_station-keeping&quot; rel=&quot;noreferrer&quot;&gt;stationkeeping&lt;/a&gt; maneuvers to stay in their assigned &quot;slot&quot;, or longitude band. It's a somewhat technical problem, but there are strategies for performing this stationkeeping such that you can do it in a near-optimal way, and save a significant amount of propellant as a result. A Google Scholar search for &quot;geostationary stationkeeping&quot; will bring up lots of interesting papers for the technically-inclined.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-07-19T13:16:35.233" LastActivityDate="2013-07-19T13:16:35.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="476" PostTypeId="2" ParentId="301" CreationDate="2013-07-19T13:22:35.130" Score="2" Body="&lt;p&gt;There are a number of options, including &lt;a href=&quot;http://news.discovery.com/space/mars-colonies-powered-by-mini-nuclear-reactors-110830.htm&quot; rel=&quot;nofollow&quot;&gt;nuclear power&lt;/a&gt; and even the possibility of &lt;a href=&quot;http://www.universetoday.com/21293/despite-dust-storms-solar-power-is-best-for-mars-colonies/&quot; rel=&quot;nofollow&quot;&gt;solar power&lt;/a&gt; (with highly efficient photovoltaic cells - such as those used on the Mars rovers). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A suggestion discussed in &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/11858273&quot; rel=&quot;nofollow&quot;&gt;&quot;Self-sustaining Mars colonies utilizing the North Polar Cap and the Martian atmosphere.&quot;&lt;/a&gt; (Powell et al. 2001) is that the colony should be near the northern ice cap, utilising any water ice - separating it into hydrogen and oxygen for power generation.  The article suggests that this process could be initiated using nuclear reactors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the linked Abstract:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The colonies would be located on the North Polar Cap of Mars and&#xA;  utilize readily available water ice and the CO2 Martian atmosphere as&#xA;  raw materials to produce all of the propellants, fuel, air, water,&#xA;  plastics, food, and other supplies needed by the colony. The colonists&#xA;  would live in thermally insulated large, comfortable habitats under&#xA;  the ice surface, fully shielded from cosmic rays. The habitats and&#xA;  supplies would be produced by a compact, lightweight (~4 metric tons)&#xA;  nuclear powered robotic unit termed ALPH (Atomic Liberation of&#xA;  Propellant and Habitat), which would land 2 years before the colonists&#xA;  arrived. Using a compact, lightweight 5 MW (th) nuclear reactor/steam&#xA;  turbine (1 MW(e)) power source and small process units (e.g., H2O&#xA;  electrolyzer, H2 and O2 liquefiers, methanator, plastic polymerizer,&#xA;  food producer, etc.) ALPH would stockpile many hundreds of tons of&#xA;  supplies in melt cavities under the ice, plus insulated habitats, to&#xA;  be in place and ready for use when the colonists landed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user92" LastEditorUserId="123" LastEditDate="2013-07-19T15:14:09.767" LastActivityDate="2013-07-19T15:14:09.767" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="477" PostTypeId="5" CreationDate="2013-07-19T13:27:37.963" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T13:27:37.963" LastActivityDate="2013-07-19T13:27:37.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="478" PostTypeId="4" CreationDate="2013-07-19T13:27:37.963" Score="0" Body="The facilities necessary to launch a rocket, including launch pad, integration facilities, and other related infrastructure." OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-19T16:45:25.510" LastActivityDate="2013-07-19T16:45:25.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="479" PostTypeId="5" CreationDate="2013-07-19T13:40:07.280" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T13:40:07.280" LastActivityDate="2013-07-19T13:40:07.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="480" PostTypeId="4" CreationDate="2013-07-19T13:40:07.280" Score="0" Body="Questions related to the growth or use of food on spacecraft or colonies. Also, any questions related to drinkable liquids either made or consumed in space." OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-21T04:08:08.920" LastActivityDate="2013-07-21T04:08:08.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="481" PostTypeId="1" AcceptedAnswerId="507" CreationDate="2013-07-19T13:48:38.687" Score="11" ViewCount="1654" Body="&lt;p&gt;This is follow-up to &lt;a href=&quot;https://space.stackexchange.com/a/303/49&quot;&gt;@Erik's answer&lt;/a&gt; to the question &lt;a href=&quot;https://space.stackexchange.com/q/296/49&quot;&gt;Is artificial gravity feasible in manned long-term space exploration?&lt;/a&gt; I'm curious, whether there's any alternative methods to simulating effects of gravitational acceleration as felt on Earth (9.78 - 9.82 m/s&lt;sup&gt;2&lt;/sup&gt;), if there's any ongoing research, or proposed methods of achieving this? Could, for example, dynamic principles of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gyroscope&quot; rel=&quot;nofollow noreferrer&quot;&gt;gyroscope&lt;/a&gt; (i.e. &lt;a href=&quot;https://en.wikipedia.org/wiki/Gyroscopic_exercise_tool&quot; rel=&quot;nofollow noreferrer&quot;&gt;powerball&lt;/a&gt;) be used to create equally distributed centripetal force on the whole inner spherical body of a space station? Could such rotation on all three axis be sustained with inertia?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't want to limit my question to &lt;a href=&quot;/questions/tagged/artificial-gravity&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;artificial-gravity&amp;#39;&quot; rel=&quot;tag&quot;&gt;artificial-gravity&lt;/a&gt; only, but would also like to know of possible ways to actually create long-term sustainable &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_field&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational fields&lt;/a&gt;, be it directly by producing &lt;a href=&quot;https://en.wikipedia.org/wiki/Graviton&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitons&lt;/a&gt;, or by other ways of producing gravitational effects of a large mass body with small mass bodies, if this is at all feasible (&lt;a href=&quot;https://en.wikipedia.org/wiki/Micro_black_hole&quot; rel=&quot;nofollow noreferrer&quot;&gt;quantum mechanical black holes&lt;/a&gt; maybe?), and how far are we from seeing any of such systems demonstrated either on paper and/or actual model-scale systems?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not interested in infeasible far-out / fictional ideas, but actual scientific research in this area and pragmatics of proposed (if any?) alternative methods.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2022-06-04T21:47:58.370" Title="Alternatives to centripetal force and constant acceleration to simulate Earth-like gravity" Tags="|artificial-gravity|gravity-fields|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="483" PostTypeId="2" ParentId="443" CreationDate="2013-07-19T14:22:40.893" Score="5" Body="&lt;p&gt;There are several issues with this approach, but three in particular come to mind:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Slow ascents by a rocket (I know you might be considering other modes) are counterproductive due to &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_drag&quot; rel=&quot;nofollow&quot;&gt;gravity drag&lt;/a&gt;.  This is why rockets accelerate as fast as their payloads can withstand.  Unfortunately, with humans this isn't very fast.&lt;/li&gt;&#xA;&lt;li&gt;Atmospheric drag.  If you were doing this around a planet with no atmosphere, and you were able to avoid the gravity drag issue, you'd be golden.  Unfortunately, without an atmosphere, you are probably using rocketry and therefore gravity drag will bite you.  Even then, if you could attain slow ascent, why not just keep going...&lt;/li&gt;&#xA;&lt;li&gt;Impact.  Your &quot;dive&quot; would need to follow a path that doesn't intersect the planet's surface -- which means you will probably need to ascend a very long ways or find a way to provide significant velocity perpendicular to the line bewteen you and the planet's center of gravity.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-19T18:55:02.167" LastActivityDate="2013-07-19T18:55:02.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="484" PostTypeId="1" CreationDate="2013-07-19T14:23:33.023" Score="9" ViewCount="139" Body="&lt;p&gt;The process of how a &quot;government space mission&quot; is initiated is kind of an interesting question to me. In the US and Europe, one has to deal with a series of institutions, agencies, politicians, write proposals, submit stuff, get it reviewed, lobby, make feasibility studies, call for instruments, go through certain phases of development, pass defined reviews e.t.c., before a mission reaches a state where it can (or can not) be approved for being fully build towards an actual launch. A lot of good ideas die along the way before any piece of hardware is ever integrated. I have e.g. some understanding of how this works for ESA projects, but none of the stuff coming from JAXA. They actually build and launch really cool hardware, such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;noreferrer&quot;&gt;Hayabusa&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to know how a robotic government (as opposed to fully private) space mission in Japan comes together. Where does it start? Who is setting up the programs? What are the steps? Who is calling for mission concepts (if this is the way they handle it)? Who is developing the concepts? Which government authorities, besides JAXA, are involved? Who needs to approve it and when? How is the funding handled (funding agencies etc)? Where do universities fit in? How does JAXA manage projects? Who is building the stuff (agencies in house or industry via contracts)? &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="49" LastEditDate="2013-08-07T02:34:45.070" LastActivityDate="2013-08-07T02:34:45.070" Title="How are (robotic) government space exploration missions in Japan initiated?" Tags="|japan|jaxa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="485" PostTypeId="2" ParentId="296" CreationDate="2013-07-19T14:37:24.577" Score="4" Body="&lt;p&gt;In manned long-term space exploration, where your objective is interstellar, and your time frame is within a single human lifetime. Lack of simulated gravity is not the problem, rather it is the limiting factor for transit time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming sufficient propulsion (i.e. &lt;a href=&quot;http://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;nofollow&quot;&gt;Bussard ramjet&lt;/a&gt;) the ship would accelerate at 1 gravity until half way to the destination, then at the halfway point, turn around and decelerate for the other half of the journey. Other then a brief period of zero gee at turn over, the content acceleration would provide &quot;artificial gravity&quot;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can only accelerate so fast without squishing your ships occupants. So being limited to a single gravity for acceleration is one of the key limiting factors in getting very far in a human lifetime.&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-07-19T14:37:24.577" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="486" PostTypeId="2" ParentId="439" CreationDate="2013-07-19T15:20:28.817" Score="10" Body="&lt;p&gt;This is complicated, but here is the gist:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thrust is generated by flowing high pressure gas into a lower pressure environment.  This flow is supersonic, so what goes on downstream of the throat (top of the the nozzle) cannot be sensed by what is going on in the combustion chamber (sound is just a pressure wave).  The nozzle is supersonic, the combustion chamber is subsonic.  The nozzle expands the gas to create the high stagnation pressure required -- mostly by increasing the velocity of the gas.  Note that &lt;a href=&quot;http://en.wikipedia.org/wiki/Stagnation_pressure&quot;&gt;stagnation pressure&lt;/a&gt; (the value you are thinking of) is not just static pressure -- it also includes a velocity component.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in summary, the the Shuttle doesn't have to pump fuel and oxidizer into the combustion engine at a pressure higher than that at the nozzle exit because:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;No pressure information is transmitted back from the nozzle exit to the combustion chamber due to the supersonic flow in the nozzle.&lt;/li&gt;&#xA;&lt;li&gt;A large portion of the nozzle's exit pressure (stagnation pressure) is from the velocity of the gases -- velocity created by expanding the supersonic gas flow.&lt;/li&gt;&#xA;&lt;li&gt;Fuel and oxidizer is pressurized however to some degree by turbopumps to insure positive delivery into the combustion chamber.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Hope this helps.  Propulsion is a very complicated field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot;&gt;The main combustion chamber pressure on the SSMEs at full power is approximately 3,008 pounds per square inch.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-19T16:00:09.853" LastActivityDate="2013-07-19T16:00:09.853" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="487" PostTypeId="1" CreationDate="2013-07-19T15:26:54.570" Score="8" ViewCount="168" Body="&lt;p&gt;I imagine the lift-off process itself puts a large amount of stress and strain on the rockets materials. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the amount of debris up in space there is always the risk of rocks to collide with the craft. Depending on their speed and where they hit this could cause untold havoc, especially if it were to penetrate the craft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand we have a question asking about how we could shield rockets in the future using a shielding method, but what i am interested in is how do they currently test the rockets on earth to ensure the parts can withstand a blow from space debris? Are any physical tests done or is it all theoretically calculated into the design of the rocket?&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:52:30.817" LastActivityDate="2013-10-28T19:52:30.817" Title="How are current spacecraft tested for their impact durability?" Tags="|rockets|debris|safety|shielding|impact|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="488" PostTypeId="5" CreationDate="2013-07-19T15:29:38.767" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T15:29:38.767" LastActivityDate="2013-07-19T15:29:38.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="489" PostTypeId="4" CreationDate="2013-07-19T15:29:38.767" Score="0" Body="Questions regarding the collision of one thing with another" OwnerDisplayName="user106" LastEditorDisplayName="user106" LastEditDate="2013-07-19T15:49:56.760" LastActivityDate="2013-07-19T15:49:56.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="490" PostTypeId="2" ParentId="487" CreationDate="2013-07-19T15:30:04.860" Score="1" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/centers/wstf/laboratories/hypervelocity/&quot; rel=&quot;nofollow&quot;&gt;Hypervelocity impact testing&lt;/a&gt; is key and is done in many &lt;a href=&quot;http://ares.jsc.nasa.gov/ares/hvit/index.cfm&quot; rel=&quot;nofollow&quot;&gt;places&lt;/a&gt;.  These facilities are capable of simulating Micrometeoroid and Orbital Debris impact on spacecraft materials and components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the capabilities of these facilities are very limited.  They are therefore primarily used to confirm theoretical models which are then used to model larger systems.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-19T15:40:17.153" LastActivityDate="2013-07-19T15:40:17.153" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="491" PostTypeId="5" CreationDate="2013-07-19T15:55:21.043" Score="0" Body="&lt;p&gt;For the equation used to calculate the exhaust velocity needed to achieve a specific delta-v, see &lt;a href=&quot;/questions/tagged/rocket-equation&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;rocket-equation&amp;#39;&quot; rel=&quot;tag&quot;&gt;rocket-equation&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For amateur rocket development and testing, see &lt;a href=&quot;/questions/tagged/amateur-rocketry&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;amateur-rocketry&amp;#39;&quot; rel=&quot;tag&quot;&gt;amateur-rocketry&lt;/a&gt;,  &lt;a href=&quot;/questions/tagged/model-rocketry&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;model-rocketry&amp;#39;&quot; rel=&quot;tag&quot;&gt;model-rocketry&lt;/a&gt;, and &lt;a href=&quot;/questions/tagged/sugar-rocket&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;sugar-rocket&amp;#39;&quot; rel=&quot;tag&quot;&gt;sugar-rocket&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For sub-orbital rockets used for scientific measurements, see &lt;a href=&quot;/questions/tagged/sounding-rocket&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;sounding-rocket&amp;#39;&quot; rel=&quot;tag&quot;&gt;sounding-rocket&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For rockets which utilize a solid and liquid propellant, see &lt;a href=&quot;/questions/tagged/hybrid-rocket&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;hybrid-rocket&amp;#39;&quot; rel=&quot;tag&quot;&gt;hybrid-rocket&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For questions about rockets exploding, see &lt;a href=&quot;/questions/tagged/rocket-explosion&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;rocket-explosion&amp;#39;&quot; rel=&quot;tag&quot;&gt;rocket-explosion&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For rockets affixed to a sled for acceleration testing, see &lt;a href=&quot;/questions/tagged/rocket-sled&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;rocket-sled&amp;#39;&quot; rel=&quot;tag&quot;&gt;rocket-sled&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="58" LastEditDate="2020-02-25T14:55:45.993" LastActivityDate="2020-02-25T14:55:45.993" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="492" PostTypeId="4" CreationDate="2013-07-19T15:55:21.043" Score="0" Body="Questions regarding the boosters or thrusters used to propel man-made objects. For rockets firing opposite the primaries, see [retrorockets]. See wiki for other related tags." OwnerDisplayName="user12" LastEditorUserId="58" LastEditDate="2020-02-25T14:55:45.993" LastActivityDate="2020-02-25T14:55:45.993" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="493" PostTypeId="5" CreationDate="2013-07-19T15:56:40.047" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T15:56:40.047" LastActivityDate="2013-07-19T15:56:40.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="494" PostTypeId="4" CreationDate="2013-07-19T15:56:40.047" Score="0" Body="Questions regarding the attractive force present between two masses. If about research in artificial gravity (i.e. manmade substitutes for gravity), please use the artificial-gravity tag." OwnerDisplayName="user106" LastEditorDisplayName="user12" LastEditDate="2013-07-21T21:44:35.733" LastActivityDate="2013-07-21T21:44:35.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="495" PostTypeId="5" CreationDate="2013-07-19T15:58:03.723" Score="0" Body="&lt;p&gt;Questions with this tag could be about colonizing a new planet with humans or with other living animal species.&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorDisplayName="user106" LastEditDate="2013-07-19T16:43:27.300" LastActivityDate="2013-07-19T16:43:27.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="496" PostTypeId="4" CreationDate="2013-07-19T15:58:03.723" Score="0" Body="Questions regarding the formation of a new colony on another planet or other body." OwnerDisplayName="user106" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:47:45.720" LastActivityDate="2013-10-28T19:47:45.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="497" PostTypeId="5" CreationDate="2013-07-19T15:59:00.100" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T15:59:00.100" LastActivityDate="2013-07-19T15:59:00.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="498" PostTypeId="4" CreationDate="2013-07-19T15:59:00.100" Score="0" Body="Questions regarding travel on a cosmic scale, from one star to another.&#xD;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-14T14:20:51.630" LastActivityDate="2013-08-14T14:20:51.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="499" PostTypeId="5" CreationDate="2013-07-19T16:00:05.573" Score="0" Body="&lt;p&gt;Questions could be either about the machines that support life on spacecraft or those that support it on planets / moons&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorDisplayName="user106" LastEditDate="2013-07-19T16:45:30.290" LastActivityDate="2013-07-19T16:45:30.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="500" PostTypeId="4" CreationDate="2013-07-19T16:00:05.573" Score="0" Body="Questions about life support systems in use on spacecraft, space stations, extraterrestrial colonies, or simulated extraterrestrial environments, about their function, design, effectiveness, historical references, or any other considerations regarding their use in space exploration." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-06T23:19:13.547" LastActivityDate="2013-08-06T23:19:13.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="501" PostTypeId="5" CreationDate="2013-07-19T16:02:01.283" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T16:02:01.283" LastActivityDate="2013-07-19T16:02:01.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="502" PostTypeId="4" CreationDate="2013-07-19T16:02:01.283" Score="0" Body="Questions regarding the spacesuits that astronauts wear whilst in space" OwnerDisplayName="user106" LastEditorDisplayName="user106" LastEditDate="2013-07-19T16:43:32.737" LastActivityDate="2013-07-19T16:43:32.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="503" PostTypeId="5" CreationDate="2013-07-19T16:12:36.883" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-19T16:12:36.883" LastActivityDate="2013-07-19T16:12:36.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="504" PostTypeId="4" CreationDate="2013-07-19T16:12:36.883" Score="0" Body="Questions related to protecting craft from their environment. Should be focused on the protection aspect and not the managing the effects of said environment." OwnerDisplayName="user106" LastEditorUserId="25" LastEditDate="2013-09-26T12:32:52.383" LastActivityDate="2013-09-26T12:32:52.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="505" PostTypeId="2" ParentId="308" CreationDate="2013-07-19T16:52:14.983" Score="10" Body="&lt;p&gt;Other than decay caused on Earth by microorganisms, other factors for decay in space, where decay is defined as loss of pristine mummification of the original, freeze-dried corpse:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;A body in orbit around the Sun would be impacted by the solar radiation.  A quick search only found research on live tissue.  Presumably, this would cause some disintegration of the mummified cells as well. It is unclear if orbital decay or solar wind damage would be the more significant erosion/decay vehicle.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A body in orbit around a planet would likely lose all cohesiveness to orbital decay prior to any significant loss due to other means.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A body drifting in open space, would be subject to micro meteor strikes as well as attraction to any other drifting bodies.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;There are many factors; while the mummified body would not decay as fast as an Earth-bound body, it would not be eternally preserved.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="47149" LastEditDate="2022-06-07T01:14:09.107" LastActivityDate="2022-06-07T01:14:09.107" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="506" PostTypeId="1" AcceptedAnswerId="510" CreationDate="2013-07-19T17:14:11.857" Score="12" ViewCount="1431" Body="&lt;p&gt;With the change at the 6th launch from the original Falcon 9 design for the first five launches, to the new design, either called Falcon 9 v1.1 or Falcon 9R, it would be useful to understand the changes made.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also not entirely clear (at this time) if the v1.1 config is the F9-R config, or if they are two different models.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, how do the side boosters on the Falcon Heavy vs the core booster on the Falcon Heavy relate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is clear more than the public info is of interest.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorDisplayName="user12" LastEditDate="2013-08-15T00:02:41.147" LastActivityDate="2018-04-08T23:58:45.647" Title="What are the differences between the Falcon 9 (1.0) and the Falcon 9 (1.1 or F9R)?" Tags="|rockets|spacex|falcon-9|falcon-heavy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="507" PostTypeId="2" ParentId="481" CreationDate="2013-07-19T17:58:09.400" Score="8" Body="&lt;p&gt;Wikipedia has a &lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Methods_for_generating_artificial_gravity&quot; rel=&quot;noreferrer&quot;&gt;good page&lt;/a&gt; on the subject of artificial gravity, highlighting many (very interesting) possibilities.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Rotation&quot; rel=&quot;noreferrer&quot;&gt;Rotation&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Linear_acceleration&quot; rel=&quot;noreferrer&quot;&gt;Linear acceleration&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Mass&quot; rel=&quot;noreferrer&quot;&gt;Mass&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Magnetism&quot; rel=&quot;noreferrer&quot;&gt;Magnetism&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Gravity_generator.2Fgravitomagnetism&quot; rel=&quot;noreferrer&quot;&gt;Gravity generator/gravitomagnetism&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Rotation&quot; rel=&quot;noreferrer&quot;&gt;Rotation&lt;/a&gt;&lt;/strong&gt; is exactly what you would expect. The spacecraft is a big rotating cylinder, just like in physics textbooks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Linear_acceleration&quot; rel=&quot;noreferrer&quot;&gt;Linear acceleration&lt;/a&gt;&lt;/strong&gt;, however, is a very interesting idea:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Any spacecraft could, in theory, continuously accelerate in a straight line, forcing objects inside the spacecraft in the opposite direction of the direction of acceleration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note, however, that you need to keep accelerating. As soon as you need to slow down, your gravity goes away. Therefore, this would probably be best for very long-range voyages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of using &lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Mass&quot; rel=&quot;noreferrer&quot;&gt;Mass&lt;/a&gt;&lt;/strong&gt; for gravity basically involves installing an ultra-high density mass in the spacecraft to attract objects toward it, simulating gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Magnetism&quot; rel=&quot;noreferrer&quot;&gt;Magnetism&lt;/a&gt;&lt;/strong&gt; is another interesting one:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A similar effect to gravity has been created through diamagnetism. It requires magnets with extremely powerful magnetic fields. Such devices have been made that were able to levitate at most a small mouse and thus produced a 1 g field to cancel the Earth's; yet it required a magnet and system that weighed thousands of kilograms, was kept superconductive with expensive cryogenics, and required 6 megawatts of power.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DL4hE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Using this technique to levitate a frog.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity#Gravity_generator.2Fgravitomagnetism&quot; rel=&quot;noreferrer&quot;&gt;Gravity generator/gravitomagnetism&lt;/a&gt;&lt;/strong&gt; is highly theoretical, and probably won't work anytime soon:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In science fiction, artificial gravity (or cancellation of gravity) or &quot;paragravity&quot; is sometimes present in spacecraft that are neither rotating nor accelerating. At present, there is no confirmed technique that can simulate gravity other than actual mass or acceleration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-19T18:04:58.707" LastActivityDate="2013-07-19T18:04:58.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="508" PostTypeId="2" ParentId="434" CreationDate="2013-07-19T19:21:09.050" Score="7" Body="&lt;p&gt;Let's take a look at their launch manifest-&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Falcon 1&lt;/strong&gt;- 5 launches&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;DARPA (US Government) failed&lt;/li&gt;&#xA;&lt;li&gt;DARPA (US Government) failed&lt;/li&gt;&#xA;&lt;li&gt;Multiple, primarily NASA/DoD (US Government), but one from Celestis (Private) failed&lt;/li&gt;&#xA;&lt;li&gt;SpaceX (Really just a demo, not a real satellite)&lt;/li&gt;&#xA;&lt;li&gt;ATSB (Malaysian government)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Falcon 9 v 1.0&lt;/strong&gt; - 5 launches&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Test payload only&lt;/li&gt;&#xA;&lt;li&gt;COTS demo 1 (NASA, US Government)&lt;/li&gt;&#xA;&lt;li&gt;COTS demo 2-3 (NASA, US Government)&lt;/li&gt;&#xA;&lt;li&gt;CRS 1 (NASA, US Government)+ ORBCOMM OG2 Prototype (Failed, Commercial)&lt;/li&gt;&#xA;&lt;li&gt;CRS 2 (NASA, US Government)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So, that totals to the following numbers:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;4 successful NASA missions&lt;/li&gt;&#xA;&lt;li&gt;2 demo missions&lt;/li&gt;&#xA;&lt;li&gt;1 foreign government&lt;/li&gt;&#xA;&lt;li&gt;2 failed commercial launces&lt;/li&gt;&#xA;&lt;li&gt;3 failed DARPA/DOD missions&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, almost none of SpaceX's revenue to date has come from commercial companies, it almost all has come from the government. However, if you check the plans for the Falcon 9 v1.1, you will see this (Current (July 19) &lt;a href=&quot;http://www.spacex.com/launch_manifest.php&quot;&gt;public manifest&lt;/a&gt;, until end of 2013, in the listed launch order:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;MDA Corp (Defense Contractor)&lt;/li&gt;&#xA;&lt;li&gt;SES (Commercial)&lt;/li&gt;&#xA;&lt;li&gt;Thaicom (Commercial)&lt;/li&gt;&#xA;&lt;li&gt;COTS 3 (NASA, US Government)&lt;/li&gt;&#xA;&lt;li&gt;Orbcomm (Commercial)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So there is yet hope that it will turn in to a company that doesn't depend on government to function.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-19T19:21:09.050" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="509" PostTypeId="2" ParentId="18" CreationDate="2013-07-19T19:40:22.330" Score="5" Body="&lt;p&gt;As shown by this table from &lt;a href=&quot;http://en.wikipedia.org/wiki/File%3aPeriodic_Table_Radioactivity.svg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; there really are not any natural elements that are appropriate for building a space ship where radioactive decay is significant issue, eight of the most stable radioactive isotope/elements have half-lives of over four million years.  Most of the elements don't have meaningful half-lives in a million year period.&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-07-19T19:40:22.330" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="510" PostTypeId="2" ParentId="506" CreationDate="2013-07-19T20:19:38.070" Score="12" Body="&lt;p&gt;There is a &lt;a href=&quot;http://theclimategap.com/falcon9-evolution/&quot; rel=&quot;noreferrer&quot;&gt;great summary article&lt;/a&gt; that reviews the various versions with changes in each version described and referenced.&lt;/p&gt;&#xA;&lt;p&gt;Changes:&lt;/p&gt;&#xA;&lt;h3&gt;Length of booster core&lt;/h3&gt;&#xA;&lt;p&gt;Here's an image showing the two Falcons side-by-side:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/065XV.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;As you can see, Falcon 9 1.1 is much longer than its predecessor, Falcon 9 Block 1 had a liftoff height of &lt;strong&gt;53 meters&lt;/strong&gt;, versus Falcon 9 1.1's &lt;strong&gt;69.2 meters&lt;/strong&gt;.  The stretch is to accommodate the additional fuel and oxidizer needed for the new more powerful engine.&lt;/p&gt;&#xA;&lt;h2&gt;Engine in use&lt;/h2&gt;&#xA;&lt;p&gt;The engine was changed from the Merlin 1-C to the Merlin 1-D which has higher thrust at 145,000 lbs at sea level. It is also reputed to be more easily manufactured.&lt;/p&gt;&#xA;&lt;h2&gt;Engine layout&lt;/h2&gt;&#xA;&lt;p&gt;The engine layout has changed.  It changed from a 3X3 grid (tic tac toe) layout to the Octo-web layout of eight engines in a cirle, with a ninth in the center.&lt;/p&gt;&#xA;&lt;h2&gt;Landing Legs&lt;/h2&gt;&#xA;&lt;p&gt;With the Jul 2013 update of the SpaceX web site, the Falcon 9 v1.1 has been pictured with landing legs. This is for the Reusable part of the F-9R moniker. Tweets from Elon Musk have suggested that they will also act as aerodynamic surfaces of some kind during descent.&lt;/p&gt;&#xA;&lt;h2&gt;Additional statistics&lt;/h2&gt;&#xA;&lt;p&gt;Here's some more statistics in chart-ish form, from near the bottom of &lt;a href=&quot;http://www.spacelaunchreport.com/falcon9.html&quot; rel=&quot;noreferrer&quot;&gt;this page&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/24gDv.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-04-08T23:58:45.647" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="511" PostTypeId="1" CreationDate="2013-07-19T21:32:54.813" Score="50" ViewCount="5821" Body="&lt;p&gt;Have you seen the &lt;a href=&quot;http://www.imdb.com/title/tt0796366&quot;&gt;first of the two new Star Trek movies&lt;/a&gt;? Kirk (Chris Pine), Sulu (John Cho) and a &lt;a href=&quot;http://en.memory-alpha.org/wiki/Redshirt&quot;&gt;red shirt&lt;/a&gt; perform something really awesome in this film: They jump from space down to a planet, basically only protected by some suit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question(s): &lt;strong&gt;Is a jump from actual space down to Earth possible? If yes, how? What are the actual problems related to it? Has it ever been researched? If yes, what was the outcome?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's &lt;strong&gt;assume two scenarios&lt;/strong&gt; for my question. One jump from the true edge of space at &lt;strong&gt;100 km altitude&lt;/strong&gt; and another jump from &lt;strong&gt;400 km&lt;/strong&gt;, the approximate altitude of the ISS. Both &lt;strong&gt;jumps happen from fixed positions relative to the Earth's surface&lt;/strong&gt; (&lt;em&gt;not&lt;/em&gt; from an orbit, of cause). &lt;strong&gt;Imagine someone doing a base-jump from a gigantic tower.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Intuition tells me that rapid deceleration once deep in the atmosphere would not even be the issue. Trouble should come from heat caused by friction and its 'disposal', although I am not sure about that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Giving some context to this question, first of all, there was &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Excelsior&quot;&gt;Project Excelsior&lt;/a&gt;, in which Joseph Kittinger did similar jumps, among them one from an altitude of 31.33 km, in 1960. Further jumps of this kind happened within projects &lt;a href=&quot;http://en.wikipedia.org/wiki/Red_Bull_Stratos&quot;&gt;Red Bull Stratos&lt;/a&gt;, during which Felix Baumgartner jumped from a maximum altitude of 38.97 km in 2012. Both projects featured jumps from within Earth's atmosphere by definition, to be more precise from the stratosphere. Although, both parachutists experienced a rather long phase of virtual free fall before they 'hit' the 'atmosphere', as they described it.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;A while ago I had to deal with &lt;strong&gt;sounding rockets&lt;/strong&gt;. Straight up to about &lt;strong&gt;100 km&lt;/strong&gt; in powered flight and immediately straight down again in &lt;strong&gt;'free' fall&lt;/strong&gt;. Temperature measurements on the outer shell indicated a maximum of about 250°C +/- 50K on re-entry, although the temperatures had already reached about 70°C at apogee because of the high-speed ride upwards. I dug for an example in terms of speed and deceleration on the way down and made a plot, here it is:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nU2i4.png&quot; alt=&quot;free fall from 87 km and (re-) entry&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;It is only from 87 km, but it should do the trick. The object was a cylinder, about 2.5m in length and 0.3m in diameter, weighing something less than 100kg (weight and dimensions are slightly similar to a human body). Yes, it did tumble. You can see the parachute opening at about 6km. The &lt;strong&gt;peak-deceleration&lt;/strong&gt; on the way down was at about &lt;strong&gt;5.5 G&lt;/strong&gt;, within limits for a human to survive. It includes the one G, which you experience here on Earth's surface. Be careful with the data above 60km - it is GPS data, which sucks at high altitude and high vertical speeds. If someone is interested in, the rockets were &lt;a href=&quot;http://www.astronautix.com/lvs/imporion.htm&quot;&gt;Improved&lt;/a&gt; &lt;a href=&quot;http://www.astronautix.com/lvs/oriocket.htm&quot;&gt;Orions&lt;/a&gt;.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-29T20:52:49.383" LastActivityDate="2017-01-18T22:45:30.760" Title="How possible are 'space jumps'?" Tags="|spacesuits|reentry|medical|" AnswerCount="5" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="512" PostTypeId="1" AcceptedAnswerId="665" CreationDate="2013-07-19T21:48:47.257" Score="32" ViewCount="1892" Body="&lt;p&gt;Voyager 1 and 2 are the most distant man made objects.  How long will we be able to still communicate with them?  What will be the first thing that prevents communication?  RTG exhaustion?  Signal attenuation over the vast distance?  Something else?&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="25" LastEditDate="2013-07-20T10:53:18.053" LastActivityDate="2013-07-23T12:32:45.930" Title="How long will communication with the Voyager probes continue?" Tags="|voyager|rtg|power|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="514" PostTypeId="5" CreationDate="2013-07-19T22:27:44.610" Score="0" Body="&lt;p&gt;Space Exploration Technologies Corporation, or SpaceX, is a private space exploration company. It was the world's first privately held company to successfully launch a cargo mission into space. They were also the first company to launch cargo to the ISS. In addition, they were the first company to recover and reuse the first stage boosters. Finally, they were also the first company to launch crew into orbit.&lt;/p&gt;&#xA;&lt;p&gt;They developed launchers:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon 1 (retired): &lt;a href=&quot;/questions/tagged/falcon-1&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;falcon-1&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;falcon-1&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-falcon-1-tooltip-container&quot;&gt;falcon-1&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 (v1.0, v1.1 retired, Full Thrust active): &lt;a href=&quot;/questions/tagged/falcon-9&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;falcon-9&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;falcon-9&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-falcon-9-tooltip-container&quot;&gt;falcon-9&lt;/a&gt; &lt;a href=&quot;/questions/tagged/falcon9-r&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;falcon9-r&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;falcon9-r&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-falcon9-r-tooltip-container&quot;&gt;falcon9-r&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Falcon Heavy (active): &lt;a href=&quot;/questions/tagged/falcon-heavy&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;falcon-heavy&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;falcon-heavy&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-falcon-heavy-tooltip-container&quot;&gt;falcon-heavy&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Super Heavy, and Starship, formerly BFR (under development): &lt;a href=&quot;/questions/tagged/spacex-super-heavy&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;spacex-super-heavy&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;spacex-super-heavy&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-spacex-super-heavy-tooltip-container&quot;&gt;spacex-super-heavy&lt;/a&gt; &lt;a href=&quot;/questions/tagged/starship&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;starship&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;starship&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-starship-tooltip-container&quot;&gt;starship&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;They developed the engines:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Kestrel (retired): &lt;a href=&quot;/questions/tagged/kestrel&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;kestrel&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;kestrel&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-kestrel-tooltip-container&quot;&gt;kestrel&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Draco (active): &lt;a href=&quot;/questions/tagged/draco&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;draco&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;draco&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-draco-tooltip-container&quot;&gt;draco&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;SuperDraco (active): &lt;a href=&quot;/questions/tagged/super-draco&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;super-draco&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;super-draco&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-super-draco-tooltip-container&quot;&gt;super-draco&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Merlin family (1D is active): &lt;a href=&quot;/questions/tagged/merlin-1d&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;merlin-1d&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;merlin-1d&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-merlin-1d-tooltip-container&quot;&gt;merlin-1d&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Raptor (under development): &lt;a href=&quot;/questions/tagged/raptor&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;raptor&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;raptor&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-raptor-tooltip-container&quot;&gt;raptor&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;They developed space vehicles:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dragon (active): &lt;a href=&quot;/questions/tagged/dragon&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;dragon&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;dragon&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-dragon-tooltip-container&quot;&gt;dragon&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Dragon 2 (active): &lt;a href=&quot;/questions/tagged/dragon-v2&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;dragon-v2&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;dragon-v2&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-dragon-v2-tooltip-container&quot;&gt;dragon-v2&lt;/a&gt; &lt;a href=&quot;/questions/tagged/crew-dragon&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;crew-dragon&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;crew-dragon&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-crew-dragon-tooltip-container&quot;&gt;crew-dragon&lt;/a&gt; &lt;a href=&quot;/questions/tagged/red-dragon&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;red-dragon&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;red-dragon&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-red-dragon-tooltip-container&quot;&gt;red-dragon&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Starship (under development): &lt;a href=&quot;/questions/tagged/spacex-starship&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;spacex-starship&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;spacex-starship&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-spacex-starship-tooltip-container&quot;&gt;spacex-starship&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Famous payloads:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Deep Space Climate Observatory (DSCOVR): &lt;a href=&quot;/questions/tagged/dscovr&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;dscovr&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;dscovr&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-dscovr-tooltip-container&quot;&gt;dscovr&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Elon Musk's Tesla Roadster (carrying Starman): &lt;a href=&quot;/questions/tagged/tesla-roadster&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;tesla-roadster&amp;#39;&quot; aria-label=&quot;show questions tagged &amp;#39;tesla-roadster&amp;#39;&quot; rel=&quot;tag&quot; aria-labelledby=&quot;tag-tesla-roadster-tooltip-container&quot;&gt;tesla-roadster&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Dragon 2 capsule &lt;em&gt;Endeavour&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="46507" LastEditDate="2023-03-23T14:10:24.840" LastActivityDate="2023-03-23T14:10:24.840" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="515" PostTypeId="4" CreationDate="2013-07-19T22:27:44.610" Score="0" Body="Space Exploration Technologies Corporation, or SpaceX, is a private space exploration company. It was the world's first privately held company to successfully launch a cargo mission into space , cargo to the ISS (International Space Station), land and reuse a booster, and a crewed mission into orbit. See the wiki for related tags. " OwnerDisplayName="user12" LastEditorUserId="46507" LastEditDate="2023-03-23T11:18:02.780" LastActivityDate="2023-03-23T11:18:02.780" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="516" PostTypeId="2" ParentId="511" CreationDate="2013-07-19T22:45:43.300" Score="29" Body="&lt;p&gt;From &lt;a href=&quot;https://physics.stackexchange.com/q/41573/2498&quot;&gt;this question&lt;/a&gt; on Physics.SE:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But other than that, there is no reason why a man couldn't be lobbed from behind Jupiter, make a slow-down loop around the Moon, then spiral down to Earth... given some marvelous suit that will withstand the atmospheric entry.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;https://physics.stackexchange.com/q/40829/2498&quot;&gt;this question on Felix Baumgartner&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Note that even if he jumped from &amp;quot;infinity&amp;quot;, he would only reach the escape velocity which is 11,200 m/s for the Earth, just like the slowest meteoroids. I guess that a good enough (and cooled) suit inspired by NASA rockets might be capable of protecting a human against such relative speeds even though for generic surfaces, they would almost certainly start to burn at the surface.&lt;/p&gt;&#xA;&lt;p&gt;However, it wouldn't be pleasant to slow down from such speeds in the atmosphere. ;-) You see that if you uniformly slow down from 10 km/s to 0 km/s while flying through 10 km of the atmosphere, the penetration through the atmosphere takes about 2 seconds. However, getting from 10 km/s to 0 km/s in two seconds means that the deceleration is 5000 m/s/s or 500 g. I guess that not even he could survive that. ;-)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So the interesting bit of information I get from those two is that your trajectory is going to be key. You couldn't drop straight in, so like the space shuttle, you will need to have a long glide path. This will give you lower friction, leading to a lower g-loading and lower temperatures. Obviously you will then need more stored air - as this could take some time, and possibly thicker ablative material on your suit (I haven't got numbers on this, but while the temperatures may be a bit lower, you are still going to have to ablate to protect the contents of the suit)&lt;/p&gt;&#xA;&lt;p&gt;You might need winglets or other control surfaces to manage this glide slope.&lt;/p&gt;&#xA;&lt;p&gt;In fact - you'd be better off with a capsule...&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-19T22:45:43.300" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="517" PostTypeId="2" ParentId="511" CreationDate="2013-07-19T23:50:44.277" Score="17" Body="&lt;p&gt;While Rory's answer is close, let me give a few additional details.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The orbital speed is about 7.8 km/s in low Earth Orbit.&lt;/li&gt;&#xA;&lt;li&gt;If you are orbiting, you will not fall straight down. It just won't happen. In fact, the maximum speed would result from a minimal burn, which would take you through the atmosphere quite slowly.&lt;/li&gt;&#xA;&lt;li&gt;You will start slowing down to an extent at around 50 km high, which is where re-entry really starts. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So, there are 2 scenarios which should be discussed.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The straight down approach- Think the &lt;a href=&quot;http://en.wikipedia.org/wiki/Free_fall#Record_free_fall_parachute_jumps&quot;&gt;Felix Baumgartner freefall record of 39 kilometer (24 mile)&lt;/a&gt;, but about 500 km high.&lt;/li&gt;&#xA;&lt;li&gt;The slow approach- This would be more like the space shuttle.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The straight down approach- Somehow you are on a space station, and you need to abort. You only have a rocket, and no spaceship. So you fire enough to stop your orbital velocity, and fall straight down. This sequence of events is rather unlikely, BTW. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your max speed would likely be around 2000 m/second. Let's say you hit the atmosphere at 10 km, that would give you a time to decelerate of 10 seconds. That's about 20g of acceleration, not enough to kill you, but it wouldn't be a pleasant experience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the second one, you are only slightly falling vertically. Your G force wouldn't be anything more than that of the space shuttle. Presumably, if you could design the suit just right, it would work, but it would likely be extremely heavy, and risky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I believe it could be done in either case, but it would be rather dangerous. The hardest part would be starting the de-orbit maneuver, and building the suit just right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Far more likely is the ability to survive an aborted launch, such as the Challenger. You might be going very fast, or high, but these types of things are more likely to happen inside of the atmosphere, slowing you down considerably.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="109" LastEditDate="2013-07-20T00:06:36.953" LastActivityDate="2013-07-20T00:06:36.953" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="519" PostTypeId="1" AcceptedAnswerId="522" CreationDate="2013-07-20T00:50:18.403" Score="5" ViewCount="1929" Body="&lt;p&gt;I don't use the term artificial gravity because, the gravity from a black hole is real.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have harnessed and are able to control a black hole would you be able to use it as portable gravity device?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't really have the physics and the math to to figure it out.  But it would seem that if you are in a low gravity environment, you could place a black hole under the floor, and have gravity.  Presumably by changing the distance between the floor and the black hole you could adjust to 1 gravity or partial gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would also seem to follow that if you were in a high gravity environment (like a ship under high acceleration) you could place the black hole over the ceiling and counter the force of gravity you are being subjected to.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-07-20T02:29:14.593" LastActivityDate="2015-12-17T22:45:13.453" Title="Could a black hole be used as portable gravity device?" Tags="|gravity|conceptual|black-hole|" AnswerCount="3" CommentCount="1" ClosedDate="2013-07-20T06:30:59.760" ContentLicense="CC BY-SA 3.0" />
  <row Id="520" PostTypeId="2" ParentId="519" CreationDate="2013-07-20T02:12:32.793" Score="2" Body="&lt;p&gt;Cool idea. If it's massive enough to help, it seems like it would be difficult to manage. You'd have difficulty moving it about your vessel, and your propulsion system would have all that mass to accelerate.&lt;/p&gt;&#xA;" OwnerUserId="79" LastActivityDate="2013-07-20T02:12:32.793" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="521" PostTypeId="2" ParentId="519" CreationDate="2013-07-20T02:28:48.420" Score="7" Body="&lt;p&gt;Well, it's a neat idea, but let's take a few things in to account first.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Let's make the assumption that you don't want the gravity to be more than 1% different from your feet to your head. Assuming you are 2m, that gives us a 200m difference between the location of the black hole and you.&lt;/li&gt;&#xA;&lt;li&gt;The amount of mass that would be required to give, let's say, 1g. Using &lt;a href=&quot;http://www.calculatoredge.com/chemical/gravitational.htm&quot;&gt;this calculator&lt;/a&gt;, I figured the amount of mass required would be 5900 000 000 000 000 kg. That is 5.9*10^15, about 1 billionth of the Earth's mass, or about the mass of 50 Mt. Everests.&lt;/li&gt;&#xA;&lt;li&gt;The requirements to thrust become quite complex with all of that extra mass.&lt;/li&gt;&#xA;&lt;li&gt;The managing of a black hole of that size becomes a challenge.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It is an interesting idea, but unlikely to ever be done in the real world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it should be noted that black holes are very efficient converters of mass to energy, and might someday be used to power spacecraft. But that's pretty far out there for right now.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-20T02:28:48.420" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="522" PostTypeId="2" ParentId="519" CreationDate="2013-07-20T02:51:01.903" Score="7" Body="&lt;p&gt;This question is out of scope, not in the simple sense of just this site.  You need to realize that critical details that your question depends on are literally unknown at this point in history.  That's because small black holes (which are easily implied by your question) involve quantum gravity - basically the area of physics where we know for a fact current theories don't work completely, and that there's new stuff to be discovered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, as a matter of scope, it's not something we can do on the scale of society today.  The reason is that small black holes are not naturally occurring.  The only natural way we know of black holes forming are from collapse of stars, and only stars much larger than our sun.  Obviously, that's not very small.  Could &quot;we&quot; make small black holes?  Yes, although the pronoun &quot;we&quot; is used extremely liberally here.  Some advanced civilization could, this is almost certain.  There is a size lower limit, which wasn't even known until people like Hawking.  The black hole radiates faster and faster with lower mass.  That means you need above a certain size to have it stable.  Estimates I've seen range from the mass of Mt. Everest to the mass of the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To address the &quot;portable gravity device&quot; I need to refer you to &lt;a href=&quot;http://en.wikipedia.org/wiki/Gauss%27s_law_for_gravity&quot; rel=&quot;nofollow&quot;&gt;Gauss' law for gravity&lt;/a&gt;.  This relates the mass within a volume to the gravitational flux over the surface.  The gravitational flux is the gravity times the area.  Gravity is a parameter we set due to the requirement that we create Earth gravity (which might be an absurd requirement by the time we're capable of making such a thing, for the record).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that you would use a rigid sphere with the black hole in the inside.  The principle is then that you could stand on the outside of the sphere.  Let's say that you're using the smallest stable black hole.  Because of Gauss's law of gravity, the mass of the black hole needed is proportional to the area.  This constant of mass divided by area is the same as for the Earth.  But since surface area to volume ratio goes up with smaller size, you need higher density.  This is the utility that black holes can provide.  They are subatomic, and effectively points in space, allowing your structure to envelop any given density (provided you meet the stability requirement).  With this setup, a black hole would allow you to create a sort of &quot;micro-planet&quot;.  You would have to have something to hold in the atmosphere still, but this could be tethered to the ground and wouldn't be particularly exotic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you have a sphere with a black hole in the middle tugging.  That means the sphere has to have material strength to resist that tug.  The requirement is almost identical to that of rotating artificial gravity habitats actually.  It's the difference between the strength requirement for a spherical pressure vessel versus a tube, which is a factor of 2.  Practically, this could be up to 1 km or so with fairly normal materials.  It's a similar problem to building skyscrapers.  Your surface has some load, which is your habitat, including people and all their things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this a stable construction?  No.  Your structural sphere exerts no pull on the black hole because of the shell theorem.  In reality, it's not perfectly symmetric, which means that the black hole will always be looking for some weak point in your mass distribution and will use that to eat your habitat.  Naturally you would engineer around this, and it wouldn't even be particularly difficult.  It could be done with gravitational &quot;tugs&quot; in the interior.  Basically, large lead balls that you lower and raise according to how the black hole is moving.  I should mention that this is only one of the several methods that are possible.  Electromagnetic forces would be preferable.  However, I know that some people caution about small black hole's values of angular momentum and charge values, because they might like to expel them quickly.  But we can't say for sure.  These is an open research area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is actually an extra proviso that can be used.  The small black hole will emit radiation.  Hopefully nice, roughly 5000 K radiation.  You could reflect this off the inner walls.  It would affect the growth/decay rate of the black hole, but who cares, its mass will budge very little for millions of years.  That reflection would provide some lift to the surface, and it could even be used to eliminate the structural requirement.  Basically, radiation could keep the surface afloat.  The same principle has been noted for Dyson spheres.  However, there are limited design variables.  The upward pressure you can get from this is limited by the blackbody radiation law.  That surface power rate is then limited by temperature.  Temperature is what limits how small you can make the black hole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, this is all plausible.  Whether it's practical is a different story.  Even if everything goes perfectly you're still limited in the surface area to mass department.  In short, if you tore apart Earth and made many micro-planets from it, you would end up with &lt;em&gt;exactly&lt;/em&gt; the same surface area.  That's not a very compelling business case.  If we're so advanced that we can make black holes, then matter will probably have other uses, such as powering fusion power plants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is another hazard I have yet to mention of small sizes.  Small size correlates with higher temperature, but in the strange world of quantum gravity, it can even make new particles at the temperature that roughly corresponds to the particle's rest mass.  Photons can be made at any temperature - that's why we &lt;em&gt;all&lt;/em&gt; give off thermal radiation.  But at higher temperature, neutrinos, electrons, and others can be made.  This isn't particularly very good news.  If the black hole is too small, it will make an exotic zoo of particles that would make any modern particle physicist envious (it's also dangerous).  It is just one more reason that small black holes present dangers.  But thankfully we won't encounter them anytime soon.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="25" LastEditDate="2015-12-17T22:45:13.453" LastActivityDate="2015-12-17T22:45:13.453" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="523" PostTypeId="1" AcceptedAnswerId="524" CreationDate="2013-07-20T04:24:17.290" Score="12" ViewCount="1106" Body="&lt;p&gt;I know that the farthest man-made object from Earth is Voyager 1, and was wondering if and when it will ever come relatively close to another star system, or some other object located outside the Solar System? If this will happen, when, and will it come close enough where it will begin orbiting the object, for example a star or planet?&lt;/p&gt;&#xA;" OwnerUserId="6" LastEditorUserId="25" LastEditDate="2013-07-20T10:52:37.237" LastActivityDate="2013-07-20T10:52:37.237" Title="What is the first non-solar-system object Voyager 1 will run into?" Tags="|orbital-mechanics|voyager|extra-solar-object|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="524" PostTypeId="2" ParentId="523" CreationDate="2013-07-20T04:28:36.197" Score="8" Body="&lt;p&gt;I don't know what lies along either of the Voyagers' flightpaths (and would be skeptical of someone that claimed they did), but I am pretty sure that they will have escape velocity for any solar system or object they pass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why?  Because escape velocity is the velocity where the object's kinetic energy and potential energy relative to another body is the same as a stationary object an infinite distance from that body.  Since the Voyagers are a very very large (effectively infinite) distance from any body they might orbit and have a non-zero speed, they will have escape velocity for that other body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only way this might change is if there was some significant but non-destructive drag imposed on the spacecraft as it approached another object.  It is more likely that any such drag would tear the spacecraft apart.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-20T04:46:46.560" LastActivityDate="2013-07-20T04:46:46.560" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="525" PostTypeId="2" ParentId="88" CreationDate="2013-07-20T05:51:48.170" Score="6" Body="&lt;p&gt;Here is part of the answer: &lt;a href=&quot;http://space.skyrocket.de/directories/chronology.htm&quot;&gt;a chronologigal list of all orbital launches and launch attempts for each year&lt;/a&gt;.  The other part is probably to cross this with the payload each mission delivered to orbit.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-20T05:51:48.170" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="526" PostTypeId="2" ParentId="484" CreationDate="2013-07-20T06:07:28.400" Score="6" Body="&lt;p&gt;Probably the best description of JAXA's process is the example provided on JAXA's website of the &lt;a href=&quot;http://www.isas.jaxa.jp/e/enterp/missions/mm/ascaproj/index.shtml&quot; rel=&quot;noreferrer&quot;&gt;Development of the ASCA (ASTRO-D) Project&lt;/a&gt;.  This example is in line with JAXA's &lt;a href=&quot;http://www.isas.jaxa.jp/e/enterp/missions/mm/selection.shtml&quot; rel=&quot;noreferrer&quot;&gt;ISAS Mission Selection Procedures&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;ISAS Mission Selection Procedures&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;ISAS has a character of an inter-university research institute, and is&#xA;  run in cooperation with scientists and engineers across the nation.&#xA;  Mission proposals are solicited from the team, and mission selections&#xA;  are made by committees comprising about equal numbers of internal and&#xA;  external members. Approved projects are then implemented with the&#xA;  collaboration of scientists and engineers, again both inside and&#xA;  outside ISAS.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The first stage of the mission selection is endorsement by the&#xA;  Steering Committee of Space Science or the Steering Committee of Space&#xA;  Engineering. Each Committee consists of about 30 members, about half&#xA;  of which are from outside ISAS. The process usually starts with the&#xA;  formation of a Working Group under the Committee.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Comprising scientists and engineers, the Working Group defines&#xA;  objectives, identifies scientific requirements and performs&#xA;  feasibility studies. When planning is complete, the Working Group&#xA;  proposes the mission to the respective committee depending on the&#xA;  nature of the mission. The Committee studies the proposal from various&#xA;  aspects including scientific significance, technical and financial&#xA;  feasibility, and maturity of the group behind the mission. When&#xA;  intensive study is needed, a subcommittee is created for a detailed&#xA;  assessment which sometimes results in setting priorities among&#xA;  multiple proposals.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A Committee's decision to endorse a specific mission proposal is&#xA;  reported promptly to the Director General. In case both Committees&#xA;  endorse new missions starting in the same fiscal year, priority may be&#xA;  recommended by the Director of Project Coordination.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Working Groups do not receive funds for their studies. However, the&#xA;  development of hardware technologies essential for future missions is&#xA;  supported by a &quot;basic development fund&quot; after being refereed by groups&#xA;  appointed by the above Committees.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The second stage of mission selection is the decision at the Advisory&#xA;  Council for Research and Development. The Council, whose 21 members&#xA;  again comprise roughly equal numbers of internal and external members,&#xA;  is the principal decision-making body of the Institute, and it deals&#xA;  also with budgets and appointments.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The selected mission is then proposed to the government both for&#xA;  mission approval by the Space Activities Commission and for budget&#xA;  allocation by the Ministry of Finance. The selection process is&#xA;  completed by a Diet resolution. A proposal for the new start from a&#xA;  given fiscal year (which starts on April 1) must be submitted to the&#xA;  government in June of the previous year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Source &amp;amp; Copyright: &lt;a href=&quot;http://www.isas.jaxa.jp/&quot; rel=&quot;noreferrer&quot;&gt;Courtesy of JAXA&lt;/a&gt; (In accordance with JAXA website's &lt;a href=&quot;http://www.jaxa.jp/policy_e.html&quot; rel=&quot;noreferrer&quot;&gt;Scope and Conditions for Use of the Contents of the Site&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this helps, or is at least a starting point.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="49" LastEditDate="2013-08-07T02:29:10.403" LastActivityDate="2013-08-07T02:29:10.403" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="528" PostTypeId="1" AcceptedAnswerId="543" CreationDate="2013-07-20T10:26:51.813" Score="11" ViewCount="619" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Recommended_Daily_Allowance&quot; rel=&quot;nofollow&quot;&gt;Dietary Reference Intake&lt;/a&gt; (aka RDA) is formulated for humans operating at one Earth gravity. Is there a similar reference or any research supporting specific recommendations for a human in prolonged freefall?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="12102" LastEditDate="2021-09-06T08:14:09.647" LastActivityDate="2021-09-06T08:14:09.647" Title="Is there any research or recommendations for a diet in microgravity?" Tags="|crewed-spaceflight|microgravity|health|food|reduced-gravity-cooking|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="529" PostTypeId="2" ParentId="523" CreationDate="2013-07-20T10:48:48.513" Score="5" Body="&lt;p&gt;There are no major objects that either Voyager spacecraft is planning on reaching anytime soon in the Solar System. The best that I could find after considerable digging around is that Voyager 1 will come 1.6 light years from the star designated AC+79 3888 in about 40,000 years, sourced &lt;a href=&quot;http://www.space.com/17688-voyager-1.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is always the oddball chance that one of them will run in to an Oort cloud object, but we can't even see any of them from Earth, so that's quite difficult to determine. For a rough pass of the two spacecraft, see &lt;a href=&quot;http://www.theplanetstoday.com/voyager_flight_path.html&quot; rel=&quot;noreferrer&quot;&gt;this animation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By far, the best science that they can do is to learn more about the heliosphere, which they were designed to do and have power to do. And as it surrounds the Earth, they will run in to it from any direction.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-20T10:48:48.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="530" PostTypeId="1" AcceptedAnswerId="533" CreationDate="2013-07-20T11:14:22.080" Score="15" ViewCount="5178" Body="&lt;p&gt;Due to gravity and movement an object can be accelerated (positive/negative) and slingshot into another direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has been used various times, but as far as I know with orbital objects like a Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could be the case that black holes have far too much gravity to be used for swing-by or that one would have to get too close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Would it be possible to use black holes for the same purpose?&lt;/strong&gt;&#xA;For example for interstellar flights.&lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorDisplayName="user12" LastEditDate="2013-07-20T20:01:50.820" LastActivityDate="2020-01-13T14:54:21.357" Title="Could black holes be used for swing-by?" Tags="|interstellar-travel|black-hole|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="531" PostTypeId="1" AcceptedAnswerId="537" CreationDate="2013-07-20T11:24:08.417" Score="11" ViewCount="1525" Body="&lt;p&gt;I wrote a program that simulates a solar system. I was able to calculate the locations for every planet on its elliptical route for any given time.&lt;br&gt;&#xA;In a second project, I managed to simulate newtonian gravitational behavior (n-body problem, time-step approach).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I'm wondering how it is possible to:&lt;br&gt;&#xA;  (1) find routes (different possibilities) from a given location/planet to another&lt;br&gt;&#xA;  (2) choose the best route according to duration or fuel consumption&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So where's a good place to start?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be more exact: It's not about writing another simulation, it's about understanding the physics behind it!&lt;/p&gt;&#xA;" OwnerUserId="139" LastEditorDisplayName="user12" LastEditDate="2013-07-20T16:35:37.393" LastActivityDate="2014-02-11T17:23:41.007" Title="Physics and math behind flight through solar system" Tags="|interstellar-travel|physics|trajectory|mathematics|" AnswerCount="3" CommentCount="0" ClosedDate="2014-02-11T17:44:38.573" ContentLicense="CC BY-SA 3.0" />
  <row Id="532" PostTypeId="2" ParentId="531" CreationDate="2013-07-20T11:33:47.027" Score="5" Body="&lt;p&gt;This is exactly how it is done - &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every space mission has used Gravity Assisted Trajectories in order to not only set the route to the destination(s) but also to gain additional velocity - otherwise the amount of propellant required would make these missions unfeasible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The n-body simulation is the main way this is solved - throw some compute power at it and you are good. It does just come down to solving the maths.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/N-body_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/72gF2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other solutions, such as crowd-sourcing - check out &lt;a href=&quot;http://sophia.estec.esa.int/thespacegame/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Space Game&lt;/a&gt; - or analysis of the &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20130011675&quot; rel=&quot;nofollow noreferrer&quot;&gt;vast data store which exists already&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="1142" LastEditDate="2014-02-10T15:51:15.827" LastActivityDate="2014-02-10T15:51:15.827" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="533" PostTypeId="2" ParentId="530" CreationDate="2013-07-20T11:41:13.520" Score="7" Body="&lt;p&gt;This is actually a wonderful question for physics, you asking about rather specific aspects of astrophysics. I'll give it a try anyway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oversimplified, seen 'from a distance', a back hole is just another massive mass point in space. So the answer here is yes, it can be used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is, however, that you need to keep 'a distance'. There are two issues here: As you mentioned, you want to be able to escape. So you must not cross the event horizon of the back hole. Below this 'horizon', all trajectories of objects, even at light speed, will eventually fall back to the black hole. This brings you to the second issue: Relativity. A fly-by near a black hole can theoretically accelerate you to virtually the speed of light, at least temporarily. It depends, whether you actually want this, including all side effects. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-20T12:14:06.473" LastActivityDate="2013-07-20T12:14:06.473" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="534" PostTypeId="5" CreationDate="2013-07-20T11:43:13.500" Score="0" Body="&lt;p&gt;Asteroids are defined as &lt;code&gt;&quot;any of numerous small celestial bodies composed of rock and metal that move around the sun (mainly between the orbits of Mars and Jupiter).&quot;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first asteroid, Ceres, was discovered between the orbits of Mars and Jupiter in 1801. At the time it was considered to be a planet, but as astronomers discovered more similar bodies in the same region of space, the term &quot;asteroid&quot; was created by Sir William Herschel in the mid-nineteenth century to differentiate them from planets (Ceres has since been reclassified again as a Dwarf Planet). The region between Mars and Jupiter, known as The Asteroid Belt, potentially contains millions of asteroids, but most of them are less than 1 km in diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other asteroids exist in co-orbital points around planets, especially Jupiter. These asteroids are referred to as Trojan Asteroids. There are also asteroids with different orbits, some that even cross the paths of planets, including Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In June 2010, JAXA returned the first collected sample of an asteroid from space with its Hayabusa mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Questions related to past, current, and future missions to asteroids, and the related science, should use this tag. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Information:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/51-asteroids-formation-discovery-and-exploration.html&quot; rel=&quot;nofollow&quot;&gt;Space.com - Asteroids: Fun Facts and Information About Asteroids&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid&quot; rel=&quot;nofollow&quot;&gt;Asteroids - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-14T01:35:22.657" LastActivityDate="2015-07-14T01:35:22.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="535" PostTypeId="4" CreationDate="2013-07-20T11:43:13.500" Score="0" Body="Questions about asteroids, bodies in the solar system smaller then planets which orbit the sun." OwnerUserId="109" LastEditorDisplayName="user12" LastEditDate="2013-07-21T21:44:33.267" LastActivityDate="2013-07-21T21:44:33.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="536" PostTypeId="2" ParentId="12" CreationDate="2013-07-20T14:58:47.783" Score="8" Body="&lt;p&gt;The problem with very distant communications is one of power, not &quot;resolution&quot; (which doesn't quite make sense in that context anyway). So you could have a whole array of receivers, but they're all going to be receiving the same (small) amount of power, and since the assumption is that one receiver is not enough to close a link, any number of receivers is not going to help you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, let's say a signle receiver just &lt;em&gt;barely&lt;/em&gt; doesn't close a link. Something like you see a coherent signal, but there is too much noise for it to be usable. In that case, if you have an array of receivers like this, it's possible you could work some communications wizardry and lower the error rate in the signal (if all receivers in your array were receiving at a slightly different phase, for instance), but that goes beyond my understanding of signal processing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Although I use &quot;receivers&quot; in the  answer, the same holds for &quot;transmitters&quot;, since it's the same problem, but in reverse).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-07-20T15:07:00.123" LastActivityDate="2013-07-20T15:07:00.123" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="537" PostTypeId="2" ParentId="531" CreationDate="2013-07-20T15:28:59.023" Score="5" Body="&lt;p&gt;I think you have to start with the basics if you want to really learn the physics.  Start with the most basic transfer between two circular orbits: a &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Hohmann transfer orbit&lt;/a&gt;.  This is the least expensive way to transfer a vehicle between two circular orbits.  It requires a two burns.  After that, you can move on to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Patched_Conic_Approximation&quot; rel=&quot;nofollow&quot;&gt;Patched Conic Approximation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Create your numerical integrator using the n-body time step method you described to simulate your flight path and see how close your calculations get you when you use these techniques.  You will very quickly get a feel for their capabilities and their shortcomings.  It is very cool to see how bad the day can get if you are using a simple Hohmann transfer to get to Mars from Earth...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck -- fun stuff!&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-20T15:35:24.470" LastActivityDate="2013-07-20T15:35:24.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="538" PostTypeId="2" ParentId="284" CreationDate="2013-07-20T15:41:53.573" Score="7" Body="&lt;p&gt;I believe it is as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point#L2&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia sub-article on L2&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Sun–Earth L2 is a good spot for space-based observatories. Because&#xA;  an object around L2 will maintain the same relative position with&#xA;  respect to the Sun and Earth, shielding and calibration are much&#xA;  simpler.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-20T15:41:53.573" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="539" PostTypeId="2" ParentId="71" CreationDate="2013-07-20T15:58:13.510" Score="2" Body="&lt;p&gt;Firstly, a jurisdiction (in the sense of this question) is the geographical (spatial in this case) area over which a state has validity in asserting their authority.  This area is typically limited to the state's sovereign territory or vessels flagged by that sovereign state.  In this case, it doesn't really matter as you aren't really worried about something that was done on the space station (like a murder for example).  The contract was executed in Cyprus (?) under Cypriot jurisdiction for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, anyone can sue anyone in most places -- US included.  You don't have to have a good reason to sue someone.  So, technically, the answer to your question, &quot;which court could the second party sue the space station operator?&quot; is any (or at least most any) court.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps what you really want to know is where would the second party have the best chance of suing with a favorable outcome?  The space station really isn't involved here since we are just talking about breach of contract so you would then have to fall back to where the station operator has the most assets where the contract is recognized as legally binding.  I suspect this would be Cyprus in your example, but perhaps the company doesn't have many assets there -- making a favorable judgement worthless.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-20T16:04:14.907" LastActivityDate="2013-07-20T16:04:14.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="540" PostTypeId="1" AcceptedAnswerId="541" CreationDate="2013-07-20T16:05:38.173" Score="14" ViewCount="1628" Body="&lt;p&gt;The Dawn spacecraft is scheduled to arrive in initial orbit around Ceres in February 2015 with a final planned orbital change in December to reduce its orbit to a 700km altitude.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At present I can't find any speculative information about an extended mission to be conducted afterwards.   Is this just NASA not doing anything that far in advance to avoid a potential letdown if hardware failures end up precluding something planned; or will Dawns fuel supply be too highly depleted to depart from orbit and go somewhere else?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming it is able to depart from Ceres, does it have enough fuel to enter orbit around any additional asteroids, or would it be limited to doing an occasional flyby past a target of opportunity?&lt;/p&gt;&#xA;" OwnerUserId="157" LastEditorUserId="49" LastEditDate="2014-06-30T13:06:41.117" LastActivityDate="2015-08-22T13:56:05.303" Title="Does the Dawn spacecraft have the potential for an extended mission?" Tags="|nasa|asteroid|mission-design|asteroid-belt|dawn-mission|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="541" PostTypeId="2" ParentId="540" CreationDate="2013-07-20T16:12:16.787" Score="4" Body="&lt;p&gt;NASA doesn't &lt;strong&gt;&lt;em&gt;seem&lt;/em&gt;&lt;/strong&gt;&lt;sup&gt;1&lt;/sup&gt; to currently have any plans for an extended mission so far. See this screenshot from &lt;a href=&quot;http://dawn.jpl.nasa.gov/mission/faqs.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;this page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KGcx2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also from &lt;a href=&quot;http://en.wikipedia.org/wiki/Dawn_(spacecraft)#Objectives&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An extended mission following the completion of the Ceres study is also possible, although unlikely, as greater scientific returns may be attained by spending more time at Vesta and Ceres. Although 2 Pallas would have been a feasible extended target for the originally scheduled launch date, launch delays have meant that this may no longer be the case. Fuel was not specifically allocated to break orbit from Ceres, so doing this depends upon the details of the flight to Ceres.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1: This is very important that it only &lt;em&gt;seems&lt;/em&gt; this way. Politics, for example, might get in the way of &lt;s&gt;disclosing&lt;/s&gt; openly displaying such info, and instead may hide the info deep in the jungle of non-Google-indexed pages.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-20T21:43:57.357" LastActivityDate="2013-07-20T21:43:57.357" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="542" PostTypeId="2" ParentId="540" CreationDate="2013-07-20T16:13:15.303" Score="11" Body="&lt;p&gt;Dawn uses an ion drive which is very efficient but does still require xenon as a &lt;a href=&quot;http://en.wikipedia.org/wiki/Working_mass&quot;&gt;reaction mass&lt;/a&gt;.  Dawn started with 425kg of xenon and used about 275kg to get to Vesta and will use about 110kg to get to Ceres.  The remaining 40kg is for stationkeeping and margin.  Dawn does not, nor was it planned to have, enough xenon left after arriving at Ceres to depart Ceres.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-20T16:20:35.470" LastActivityDate="2013-07-20T16:20:35.470" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="543" PostTypeId="2" ParentId="528" CreationDate="2013-07-20T16:57:22.813" Score="12" Body="&lt;p&gt;The &quot;Nutrition and Human Spaceflight&quot; section of NASA's &lt;a href=&quot;http://www.nasa.gov/pdf/71426main_FS-2002-10-079-JSC.pdf&quot;&gt;Space Food factsheet&lt;/a&gt; has some good, basic information with links to specific research and studies.  For example:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nutrients astronauts need in space are the same ones all  people&#xA;  need, but the amounts of some differ. Astronauts need  the same number&#xA;  of calories for energy during spaceflight as they need on the ground.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Most of the vitamins and minerals  they need are the same as on the&#xA;  ground.  The amount of iron in an astronaut’s diet should be less than&#xA;  10 milligrams per day for both men and women. Astronauts  have fewer&#xA;  red blood cells while they are in space. Most of the  iron absorbed&#xA;  from food goes into new red blood cells. If astro­nauts were to eat&#xA;  foods high in iron, the iron would be stored in their bodies and&#xA;  could cause health problems. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Sodium and vitamin D affect bone. The amount of sodium in  the&#xA;  astronauts’ diet is limited because too much can lead to bone  loss as&#xA;  well as other health problems. The body usually makes  vitamin D when&#xA;  the skin is exposed to sunlight, but spacecraft  are shielded to&#xA;  protect the astronauts from harmful radiation.  On Earth and in&#xA;  microgravity, people need vitamin D for  healthy bones. Vitamin D&#xA;  supplements are recommended for  space travelers on the ISS, since the&#xA;  current space foods do not  provide enough of this vitamin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-20T16:57:22.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="544" PostTypeId="1" CreationDate="2013-07-20T17:11:08.980" Score="8" ViewCount="138" Body="&lt;p&gt;On manned-missions which take its participants further and further into space it will be quite difficult to have a control center on Earth as it takes longer an longer to communicate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes the light around 8 minutes to travel from the surface of the sun to Earth. Without any delay this would be 16 minutes for two-way communication and there's surely a point/zone on/in which it will be impractical to communicate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there research on this topic?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;For missions with 'M' crew members that will take its crew 'X' AU&#xA;  (astronomical unit[s]) away from Earth it is reasonable to have&#xA;  mission control solely on Earth. The crew members are not allowed to&#xA;  make mission-critical decisions on their own&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;For missions with 'M' crew members that will take its crew 'Y' AU&#xA;  away from Earth it is reasonable to have a hybrid-solution. After 'X'&#xA;  AU (...). Crew members should decide democratically.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;For missions with 'M' crew members that will take its crew 'Z' AU&#xA;  away form Earth it is reasonable to have mission control solely in the&#xA;  hands of the crew. It's recommended to base this on a hybrid solution.&#xA;  After 'Y' AU (...). When 'M' exceeds 'N' high ranked members voices&#xA;  count twice.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="107" LastActivityDate="2015-04-17T13:21:11.147" Title="Is there research on optimal control/command strategy/hierachy?" Tags="|crewed-spaceflight|mission-control|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="545" PostTypeId="2" ParentId="544" CreationDate="2013-07-20T17:18:55.020" Score="2" Body="&lt;p&gt;There is this work at USC, &lt;a href=&quot;http://news.usc.edu/#!/article/43871/research-to-explore-psychological-effects-of-communication-delays-in-space/&quot; rel=&quot;nofollow&quot;&gt;&quot;Research to explore communication delays in space&quot;&lt;/a&gt;, going on right now under a NASA grant. The investigation is being supervised by Professor Lawrence Palinkas: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The three-year study will involve simulating communication lag between&#xA;  astronauts on the International Space Station and ground control staff&#xA;  to determine how crew members cope with stressful situations and new&#xA;  forms of isolation.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Crew members will be asked to perform a set of specific tasks during&#xA;  the course of four weeks, split between the beginning and end of their&#xA;  six-month stay on the space station. During two of those weeks,&#xA;  they’ll experience a 50-second delay in interaction with mission&#xA;  control.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In addition, the jobs will vary in terms of how novel and critical&#xA;  they are in nature — that is, whether the astronauts received any&#xA;  training for each task and whether each situation is life-threatening&#xA;  or menial. “Cleaning out the toilet is not novel or critical,”&#xA;  Palinkas said, “but responding to a medical emergency could be.”&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Palinkas is hopeful that findings from the project will prove useful&#xA;  in terms of improving training programs for astronauts to handle&#xA;  anxiety and stress, as well as developing technology that helps&#xA;  mitigate the negative effects of communication delays.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="63" LastEditorUserId="6312" LastEditDate="2015-04-17T13:21:11.147" LastActivityDate="2015-04-17T13:21:11.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="546" PostTypeId="1" AcceptedAnswerId="549" CreationDate="2013-07-20T17:32:01.410" Score="16" ViewCount="2805" Body="&lt;p&gt;I've always liked the idea of exploring the moon Titan, most likely because of the atmosphere, pressure, and the fact that it has stable bodies of liquids at the surface. Although it's very cold, these factors may increase the chance of life being there. So, does anyone know if and when we will send another probe (most likely unmanned) to Titan? Are there any missions planned within the near future?&lt;/p&gt;&#xA;" OwnerUserId="6" LastEditorUserId="17" LastEditDate="2013-07-22T12:21:20.033" LastActivityDate="2019-01-16T15:05:29.950" Title="Are there plans for another mission to Titan?" Tags="|titan|probe|future-missions|" AnswerCount="4" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="547" PostTypeId="2" ParentId="546" CreationDate="2013-07-20T17:48:20.327" Score="5" Body="&lt;p&gt;NASA's currently planned and funded (to some extent) future missions are listed here: &lt;a href=&quot;http://www.nasa.gov/missions/future/&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/missions/future/&lt;/a&gt;.  Alas, nothing is currently planned for Titan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess other agencies/organizations might have something.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-20T17:48:20.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="548" PostTypeId="2" ParentId="528" CreationDate="2013-07-20T18:30:31.740" Score="6" Body="&lt;p&gt;There's a great PDF provided by NASA on &lt;a href=&quot;http://www.nasa.gov/pdf/143163main_Space.Food.and.Nutrition.pdf&quot; rel=&quot;noreferrer&quot;&gt;space food and nutrition&lt;/a&gt; from an educational stand point for grades k-8. It's also under public domain and does not have a copyright. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The beginning of the document goes into how Project Mercury, Gemini, Apollo, skylab and others prepared food for that period of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also learn about the types of space foods (rehydratable food, thermostableized food, frozen food, etc) and how microgravity affects how food is eaten and packaged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are interested in learning how to make space food there is an example of a Space tortilla recipe on page 54.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Ingredients % by Mass&#xA;Wheat 61.79 &#xA;Water 26.58 &#xA;Glycerin 4.02 &#xA;Shortening 3.71&#xA;Mono/Diglycerides 1.24&#xA;Salt 0.99&#xA;Baking Powder 0.87&#xA;Dough Conditioner 0.31&#xA;Fumaric Acid 0.19&#xA;Potassium Sorbate 0.15 &#xA;Carboxymethyl Cellulose 0.12&#xA;Calcium Propionate 0.03&#xA;100.00%&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Also according to this they reference USDA Food Guide Pyramid. Which indicates astronauts follow the same guideline.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BV1GM.png&quot; alt=&quot;USDA FOOD GUIDE PYRAMID&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is obviously a good document for children interested in space food and nutrition because it's a lower level for better understanding.&lt;/p&gt;&#xA;" OwnerUserId="66" LastActivityDate="2013-07-20T18:30:31.740" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="549" PostTypeId="2" ParentId="546" CreationDate="2013-07-20T19:37:20.447" Score="11" Body="&lt;p&gt;A colleague worked on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_Mare_Explorer&quot; rel=&quot;noreferrer&quot;&gt;Titan Mare Explorer&lt;/a&gt; (the concept did not proceed beyond proposal stage).  It was an ultra-cool idea that would include to lower a boat to sail the seas of Titan (doesn't that sound catchy?).  Alas, it wasn't selected; instead, NASA selected the &lt;a href=&quot;https://en.wikipedia.org/wiki/InSight&quot; rel=&quot;noreferrer&quot;&gt;InSight&lt;/a&gt; Mars lander — NASA keep changing their priorities as to where they want to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are currently no active plans to return to the Saturn system.  Personally, I believe that the next landing in the outer solar system will not be on Titan, but on another moon, such as Europa or Enceladus.  But it will be a bit down the road; neither upcoming &lt;a href=&quot;http://en.wikipedia.org/wiki/Jupiter_Icy_Moon_Explorer&quot; rel=&quot;noreferrer&quot;&gt;JUICE&lt;/a&gt; (ESA Jupiter mission), nor &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Juno&lt;/a&gt; (NASA Jupiter mission) include any landers.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2016-06-02T15:06:11.200" LastActivityDate="2016-06-02T15:06:11.200" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="550" PostTypeId="2" ParentId="546" CreationDate="2013-07-20T20:16:28.197" Score="3" Body="&lt;p&gt;The boat mentioned by gerrit reminded of another concept. The idea is to put a blimp into Titan's atmosphere and fly around. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.iafastro.net/iac/archive/browse/IAC-09/A3/I/4764/&quot; rel=&quot;nofollow&quot;&gt;Colombatti, G., La Gloria, N., Aboudan, A., Debei, S., 2009. &lt;strong&gt;Airship's Dynamics Modeling and Control Strategy for Titan exploration&lt;/strong&gt;. Proceedings of the 60th International Astronautical Congress - IAC-09.A3.I.12&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of background for your question: There are certainly plenty of types of missions investigated right now. Unfortunately, the chances of such a mission actually receiving the required funding are slim. So the result is a wealth of good ideas, some even developed in very high detail, but virtually non of them fly. My habit is to &quot;ignore&quot; them unless they turn up in high level agency documents and political discussions. Otherwise it is just too painful to see them catching dust over time. From this point of view, there are no missions as of today planed for another visit of Titan. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorDisplayName="user12" LastEditDate="2013-07-20T22:09:07.957" LastActivityDate="2013-07-20T22:09:07.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="551" PostTypeId="2" ParentId="71" CreationDate="2013-07-20T20:26:51.493" Score="-1" Body="&lt;p&gt;The Governing Law clause on the contract would be Nigerian. So no matter which court hears the case the dispute would have to be settled using Nigerian Law.&lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastActivityDate="2013-07-20T20:26:51.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="553" PostTypeId="1" AcceptedAnswerId="564" CreationDate="2013-07-20T21:08:17.177" Score="12" ViewCount="1637" Body="&lt;p&gt;If I have a set of rechargeable batteries, I count myself lucky if I can get them to last for more than 2 years. Yet there are many spacecraft out there, which use their batteries much more regularly than me, and yet they have batteries that still work after years. How are they able to maintain the lifespan of these batteries for such an extended period of time?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2013-07-20T21:59:39.867" LastActivityDate="2013-07-21T00:24:31.970" Title="How do space missions preserve their batteries for so long?" Tags="|power|end-of-life|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="554" PostTypeId="5" CreationDate="2013-07-20T21:08:50.663" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-20T21:08:50.663" LastActivityDate="2013-07-20T21:08:50.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="555" PostTypeId="4" CreationDate="2013-07-20T21:08:50.663" Score="0" Body="Questions related to extending the lifetime, and determining when the useful period of a satellite has ended" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-20T22:37:47.493" LastActivityDate="2013-07-20T22:37:47.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="556" PostTypeId="2" ParentId="553" CreationDate="2013-07-20T21:27:17.733" Score="7" Body="&lt;p&gt;&lt;a href=&quot;http://www.electrochem.org/dl/interface/fal/fal99/IF8-99-Pages25-30.pdf&quot; rel=&quot;noreferrer&quot;&gt;This document&lt;/a&gt; provides a bunch of useful information on the topic. Some applicable quotes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Space missions impose several critical performance requirements on batteries and fuel cells. Batteries required&#xA;  for space applications must be capable&#xA;  of operating in a hard vacuum and&#xA;  withstand severe launch environments&#xA;  (vibration, shock, and acceleration).&#xA;  Space applications also require batteries&#xA;  that can provide maximum electrical&#xA;  energy in minimum weight and&#xA;  volume&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Long cycle life (&gt; 30,000&#xA;  cycles) is the critical driver for orbiting&#xA;  spacecraft, and long active shelf life is&#xA;  the driver for planetary probes (&gt; 7-10&#xA;  years). Radiation resistance and operation at temperatures as low as -80°C is&#xA;  essential for some planetary missions.&#xA;  No single battery system can meet all&#xA;  these complex requirements.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The &lt;strong&gt;Ag-Zn battery was the first&#xA;  choice in the early days of space missions&lt;/strong&gt;. The &lt;strong&gt;Ni-Cd battery&lt;/strong&gt; became the&#xA;  major energy storage device &lt;strong&gt;over the&#xA;  next 20&lt;/strong&gt; years because of its long cycle&#xA;  life. The &lt;strong&gt;Ni-H2&#xA;  battery&lt;/strong&gt; started to play a&#xA;  role in the &lt;strong&gt;80s&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Shiny table!:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IAoV8.png&quot; alt=&quot;table&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also plenty of information about the longevity of power sources. For example, the Space Shuttle didn't use batteries:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The space shuttle&#xA;  requires a power source that can provide 6-12 kW for 2,000 hours. &lt;strong&gt;Batteries&#xA;  cannot satisfy this application&lt;/strong&gt;, as the&#xA;  required battery &lt;strong&gt;weight would be prohibitive&lt;/strong&gt;. The application can and has,&#xA;  however, been met with an &lt;strong&gt;alkaline&#xA;  fuel cell operating on hydrogen and&#xA;  oxygen stored separately in cryogenic&#xA;  tanks&lt;/strong&gt;. International Fuel Cells is providing alkaline fuel cells for the shuttle&#xA;  orbiter applications. This system has&#xA;  been in use for the past &lt;strong&gt;18 years&lt;/strong&gt;. Plans&#xA;  are underway to replace the alkaline&#xA;  fuel cell system with an &lt;strong&gt;advanced&#xA;  PEM-based system&lt;/strong&gt;. The replacement of&#xA;  the hydrazine-powered auxiliary&#xA;  power unit (APU) with an electric APU&#xA;  powered by 100-150 kWh Li-ion batteries is also underway.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I won't just copy and paste the whole thing here, but it's a good read, with lots of rich information on the topic of batteries in space.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-07-20T21:27:17.733" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="557" PostTypeId="2" ParentId="438" CreationDate="2013-07-20T22:04:25.007" Score="10" Body="&lt;p&gt;&lt;em&gt;Since, as asked, the question isn't answerable, I'll discuss the elements which affect how one finds the answer.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact amount of propellant is going to vary depending upon what propellant, what the efficiency of the stage is, what the current speed is while crossing 100,000', the structural mass needed for that fuel, the desired course (Orbital type and/or escape),  and the current weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that at 100K feet, the only real effect of the weather is going to be mild changes in density; this is probably one of the smallest effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The specific fuels will make tremendous differences; which fuel determines how much the casing mass is going to be as well as the fuel mass to accelerate the casing and the payload. H2/O2 is high efficiency, but loses due to increased casing mass. Aluminium-Rubber is significantly lower energy, but also much more dense, and thus has a much lower casing mass. (Casing mass is everything that isn't fuel nor payload.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The altitude alone reduces the density of air and the local gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Local gravity effect at 100,000': approximate earth equitorial diameter 41850758', we get a radius of 20925379'; adding 100000 we get 21025379'. Applying the inverse square law, 1/((21025379/20925379)^2), we get about 0.9905G. This means that the payload will need less fuel, as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atmospheric density at 100,000' the air pressure averages 10millibars; &quot;standard&quot; is 1 bar. This will vary slightly by temperature, but not nearly as much as near-surface.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="415" LastEditDate="2014-10-15T13:15:08.120" LastActivityDate="2014-10-15T13:15:08.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="559" PostTypeId="2" ParentId="530" CreationDate="2013-07-20T22:29:32.297" Score="6" Body="&lt;p&gt;Slingshots work by gravity interaction between a massive body in motion and a second body in motion. Any massive object can be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problems with a black hole are (1) avoiding the capture range, (2) surviving the radiation, (3) surviving the accretion disk contents, (4) the extreme distances involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind: black holes can have tremendous distances with gravitational effects. Distances which can be measured in light years in some cases, and thousands of AU even for smaller ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The strength of the boost is a factor of how close one gets, and the correct angle of insertion relative to the motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The further out, the less the acceleration/deceleration. But also, the further out, the less the radiation from the accretion disc collisions, the less dense the accretion disk, and the less Hawking Radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The immense distances also require immense amounts of time. &lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-07-20T22:29:32.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="560" PostTypeId="2" ParentId="359" CreationDate="2013-07-20T22:52:05.817" Score="8" Body="&lt;p&gt;The best way to measure gravity from a spacecraft is not by using an instrument on a spacecraft, but using a pair of instruments on two spacecraft. The best example of this is &lt;a href=&quot;http://www.nasa.gov/mission_pages/grail/main/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Grail&lt;/a&gt; spacecraft. What they did in essence was to orbit around the Moon such that they had a more or less constant distance between the two. As an area of higher gravity passed, then the spacecraft would actually drop a bit. The two spacecraft were able to very accurately detect the range between the two spacecraft, and if the spacecraft went in a particular direction. Using some complex math, they were able to figure out the gravity map of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, this can be done using an Earth based radar type system, where the frequency of the spacecraft is very carefully monitored for &lt;a href=&quot;https://en.wikipedia.org/wiki/Doppler_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Doppler shift&lt;/a&gt;, searching for small changes in the orbit caused by differing gravitational fields. This is complex, but has been done. The prime example of this was &lt;a href=&quot;http://www2.jpl.nasa.gov/magellan/fact1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Magellan&lt;/a&gt;, but I believe it has been done by other spacecraft, and this Doppler effect &lt;a href=&quot;https://space.stackexchange.com/a/3682/49&quot;&gt;will also be used to determine Jupiter's gravity field&lt;/a&gt; by gravity perturbed orbit of &lt;a href=&quot;http://www.nasa.gov/mission_pages/juno/main/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno spacecraft&lt;/a&gt; as it starts orbiting it in 2016.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-02-15T22:56:36.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="561" PostTypeId="2" ParentId="132" CreationDate="2013-07-20T23:53:49.170" Score="9" Body="&lt;p&gt;Known problems from reduced gravity:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Osteoporosis&lt;/li&gt;&#xA;&lt;li&gt;Motor muscle atrophy&lt;/li&gt;&#xA;&lt;li&gt;Heart muscle atrophy&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The natural process of bone replacement is compromised in 0-G. It's also been shown by experiments to be compromised by lack of long-axis stress on the long bones - stresses like walking induce microfractures which proceed to get &quot;healed&quot; with stronger bone. How much gravity is needed to maintain bone health is not yet known, but the lack of long-bone stress can be compensated by vigorous exercise with elastic bands holding one down. (This has been demonstrated on US space stations.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Motor muscle atrophy can be partially compensated by vigorous exercise with elastic resistance. This is not sufficient to maintain full pre-flight strength for those in microgravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Peripheral cardiovascular effects are apparently temporary (&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/301.html&quot; rel=&quot;noreferrer&quot;&gt;cite&lt;/a&gt;). The heart muscle, like all muscle, will not maintain condition and muscle density when loading is reduced. Lower gravity will require less pressure to push blood up into the brain; this reduced effort will result in lowered heart muscle condition, and eventually, atrophy. (&lt;a href=&quot;http://www.jappl.org/content/91/2/645.full&quot; rel=&quot;noreferrer&quot;&gt;cite&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The use of a weighted vest will increase both required muscle effort for walking, and impact stress from walking. The decreased velocity, however, will still not result in the same effect as normal gravity, at least not without excessive mass.  (Remember, E=MV^2.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The use of a weighted vest will not, however, result in increased direct load on the cardiovascular system, so heart atrophy is likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TLDR: No, a weighted suit will not eliminate the health effects of reduced gravity. It will ameliorate some of them.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-07-20T23:53:49.170" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="562" PostTypeId="2" ParentId="247" CreationDate="2013-07-21T00:14:34.213" Score="3" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Selection Time&lt;/strong&gt;: spacecraft are designed and built years prior to launch. The selected processor at build time, even if top of the line, will have been eclipsed by launch time.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Vibration Tolerance&lt;/strong&gt;: launch of spacecraft requires vibration tolerant computer systems; many newer processors are not yet rated at design time.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Radiation Resistance&lt;/strong&gt;: Smaller circuits are more subject to radiation induced errors than larger circuits. most more advanced processors use smaller circuitry to reduce energy costs, thermal loads, and operation cycle times.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Price&lt;/strong&gt;: Older processors can be bought for far less than current leading edge processors; prices drop noticeably once patents expire.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;lack of need&lt;/strong&gt;: Not all satellites need highly robust processing solutions.&lt;br&gt; &lt;em&gt;The entire Apollo mission was run with a processing power equivalent to a couple of high end linux workstations... this includes the mainframes at JSC and Cape Kenedy. The on-board computer on Apollo was about as powerful as many digital watches. (80kB total memory; that's 37kB words of 2B each for ROM, plus 2K words of RAM.) It ran at 1 MHz, rather fast for its day. I've bought $20 calculators with better specifications than the AGC.&lt;/em&gt; &lt;br&gt;The tasks of most satellites can be reliably run with older processors without mission compromise.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-07-21T00:14:34.213" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="563" PostTypeId="1" AcceptedAnswerId="590" CreationDate="2013-07-21T00:21:08.500" Score="11" ViewCount="396" Body="&lt;p&gt;GNSS satellites (at least the GPS ones) are in MEO, so LEO spacecraft can use GNSS for their positioning.  I've read that it's quite low-cost compared to the alternatives.  How does the precision of GNSS-based positioning compare to other methods?  Is it sufficiently accurate, for example, for earth observation satellites?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorDisplayName="user29" LastEditDate="2013-07-21T17:16:55.063" LastActivityDate="2013-07-21T17:16:55.063" Title="In LEO, what is the precision of GNSS-based position compared to traditional methods?" Tags="|navigation|gnss|orbit-determination|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="564" PostTypeId="2" ParentId="553" CreationDate="2013-07-21T00:24:31.970" Score="4" Body="&lt;p&gt;Generally speaking, there's a couple of things that should be done to prevent this from happening.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Order good batteries. This helps, to an extent, but isn't a sure thing.&lt;/li&gt;&#xA;&lt;li&gt;Try to keep the thermal limits of the batteries within limits. Don't allow the batteries to get too warm, or too cool, and vigorously monitor this.&lt;/li&gt;&#xA;&lt;li&gt;Charge the batteries fully, but don't overcharge them. This is probably one of the big reasons that consumer batteries fail. It is actually common for them to be somewhat overcharged, which is why most batteries are noticeably warmer after they are done charging. They aren't super overcharged, but this does contribute to wear and tear.&lt;/li&gt;&#xA;&lt;li&gt;Minimize the depth of discharge. Essentially, charge your batteries when you have used less than a third of their capacity. You are much more likely to have issues if you completely discharge your batteries.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If you do all of these things, your batteries will last longer. The same goes with personal electronics, by the way. Of course, making sure your battery is charged in an optimal manner can be a very difficult and time consuming matter, which is why more systems don't do it (Every battery is a bit different).&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-21T00:24:31.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="565" PostTypeId="1" AcceptedAnswerId="567" CreationDate="2013-07-21T00:25:07.127" Score="22" ViewCount="1259" Body="&lt;p&gt;GNSS satellites (at least GPS ones) are in Medium Earth Orbit (MEO).  LEO satellites can use GNSS for positioning.  Is this still possible for satellites in elliptical, such as Molniya, orbits?  For geostationary satellites, all GNSS satellites will be in a relatively small solid angle (but still much larger than the earth one), but in principle, it should still be possible to use GNSS.  How far up has GNSS-based positioning been used, and how does the precision degrade with distance to Earth, theoretically and in practice?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="6816" LastEditDate="2015-08-05T19:54:13.957" LastActivityDate="2021-08-05T22:51:49.407" Title="How far up have satellites used a GNSS for positioning, and how does the precision degrade with altitude?" Tags="|navigation|altitude|gps|gnss|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="566" PostTypeId="1" AcceptedAnswerId="879" CreationDate="2013-07-21T00:30:54.853" Score="19" ViewCount="1725" Body="&lt;p&gt;In principle, by placing a GNSS-receiver on all extremities of a spacecraft (or aircraft, for that matter), one can determine the orientation of the satellite.  Google &lt;a href=&quot;https://www.google.com/search?q=gnss+based+attitude+determination&quot;&gt;lists some studies&lt;/a&gt;, but have such systems been used in real spacecraft?  On the one hand, I'd think cubesats might go for such a low-cost solution, on the other hand, the smaller the spacecraft, the larger the error in attitude determination by using GNSS.  The technique is not listed on &lt;a href=&quot;http://en.wikipedia.org/wiki/Attitude_control&quot;&gt;the Wikipedia page on attitude control&lt;/a&gt;.  Has this been used in practice at all?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="25" LastEditDate="2013-07-21T00:34:58.240" LastActivityDate="2016-10-19T07:11:20.253" Title="Do any spacecraft use GNSS for attitude determination?" Tags="|attitude|gnss|gps|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="567" PostTypeId="2" ParentId="565" CreationDate="2013-07-21T00:31:34.990" Score="15" Body="&lt;p&gt;The furthest satellite that I have seen use GPS is the AO-40, an amateur built satellite. In fact, they did a &lt;a href=&quot;http://www.emergentspace.com/assets/1/7/AIAA_GNC_2002_AMSAT_A040.pdf&quot; rel=&quot;noreferrer&quot;&gt;research paper&lt;/a&gt; on the subject. The paper states that it was able to achieve a navigation solution at 60,000 km, which is about 1/6th of the way to the moon, and well beyond the belt of Geosynchronous satellites. I'm sure there are other satellites which have demonstrated this technology even further, but just to receive the signal beyond the GPS constellation is impressive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be stated that this problem is difficult because the GPS antennas are all focused at the Earth, thus, not much radiation extends beyond the GPS constellation. Still, it can be done, if you do it carefully. Furthermore, there are now special receivers for HEO GPS reception, such as can be found &lt;a href=&quot;http://techtransfer.gsfc.nasa.gov/ft_tech_gps_navigator.shtm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is at least one mission that continues to try to break the record further away, being &lt;a href=&quot;https://www.sciencedaily.com/releases/2016/11/161104190848.htm&quot; rel=&quot;noreferrer&quot;&gt;NASA's MMS mission&lt;/a&gt;. They have gone beyond AO-40's record to hit 70,000 km, since I originally wrote this answer.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-11-09T14:31:26.663" LastActivityDate="2016-11-09T14:31:26.663" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="569" PostTypeId="2" ParentId="296" CreationDate="2013-07-21T00:51:03.393" Score="5" Body="&lt;p&gt;Assuming a sufficient budget, a spin-habitat is a highly viable option. To maintain 1G and an acceptable ≤2 RPM rotation, one needs a radius of 223m or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given human needs, a torus of 5m habitat tube at 223m median radius, with a pair of 1mm steel hull shells, is a mass of about 55 cubic meters of steel or about 300 metric tons, just for a fairly thin structural torus. Adding additional support structure should at least double that mass. That's the prohibitive expense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.artificial-gravity.com/sw/SpinCalc/&quot;&gt;SpinCalc Online Calculator&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-07-21T00:51:03.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="572" PostTypeId="5" CreationDate="2013-07-21T03:34:27.663" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-21T03:34:27.663" LastActivityDate="2013-07-21T03:34:27.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="573" PostTypeId="4" CreationDate="2013-07-21T03:34:27.663" Score="0" Body="An extra solar object is any object located outside of the Solar System." OwnerUserId="6" LastEditorUserId="6" LastEditDate="2013-07-21T05:45:46.623" LastActivityDate="2013-07-21T05:45:46.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="574" PostTypeId="1" AcceptedAnswerId="577" CreationDate="2013-07-21T03:53:26.553" Score="13" ViewCount="880" Body="&lt;p&gt;There are currently many programs that are attempting to find intelligent extraterrestrial life. Putting aside the fact that I believe that the chances of this are slim to none, I'm interested in exactly what would happen if such a discovery was made.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Has any country/nation specifically stated what it would do if intelligent life was discovered elsewhere?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-13T00:17:37.097" LastActivityDate="2017-04-06T00:46:17.200" Title="Has any country stated its official position on how it would respond to a discovery of intelligent extra terrestrial life?" Tags="|life|astrobiology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="575" PostTypeId="1" AcceptedAnswerId="8252" CreationDate="2013-07-21T03:59:16.957" Score="14" ViewCount="1799" Body="&lt;p&gt;Have any of the rock samples (or meteors from Earth's moon or from Mars) provided a measurable amount of gold, silver, platinum or any other precious metals?&lt;/p&gt;&#xA;" OwnerUserId="137" LastEditorUserId="14" LastEditDate="2013-07-21T15:15:37.900" LastActivityDate="2015-02-24T12:27:44.027" Title="Which precious metals have been detected via samples taken from Earth's moon or Mars?" Tags="|the-moon|mars|mining|research|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="576" PostTypeId="1" AcceptedAnswerId="580" CreationDate="2013-07-21T04:06:14.910" Score="11" ViewCount="356" Body="&lt;p&gt;Before a rover or other exploration vehicle is launched, it undergoes rigorous cleaning and disinfection, such as a bath in UV rays to kill off bacteria.&lt;/p&gt;&#xA;&lt;p&gt;The official reasoning on this seems to be that the bacteria on the vehicle (deposited by people touching it, spiders on the ceiling, &lt;em&gt;whatever&lt;/em&gt;), may cause a false positive for life on the other planet by &amp;quot;riding&amp;quot; the vehicle to space and leaving the vehicle for the other planet. There, it would colonize. We would later detect it and say that it's a sign that life exists there.&lt;/p&gt;&#xA;&lt;p&gt;However, my understanding of &lt;a href=&quot;https://space.stackexchange.com/questions/216/how-is-equipment-sterilized-for-space-missions/285#285&quot;&gt;this answer&lt;/a&gt; is that cosmic radiation will take care of it on its long journey:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It appears that most of the true sterilization actually takes place after launch. The harsh conditions of the trip there and the landing should be enough to kill off a majority of the microbes.&lt;/p&gt;&#xA;&lt;p&gt;The Mars rover projects all receive multiple sterilization treatments here on Earth before launch, typically UV treatment, but what really kills off the bacteria is the exposure to cosmic radiation on the trip over and the intense UV radiation on Mars' surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;If cosmic radiation will kill it all off anyway, what is the real risk to leaving life on a spacecraft headed for another planet?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-12-18T01:54:40.140" Title="Is the rigorous disinfection process prior to launch necessary?" Tags="|life|sterilization|cosmic-radiation|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="577" PostTypeId="2" ParentId="574" CreationDate="2013-07-21T04:34:21.417" Score="20" Body="&lt;p&gt;Apparently on August 21, 2011, NASA put together a report titled &amp;quot;&lt;a href=&quot;http://arxiv.org/pdf/1104.4462v2.pdf&quot; rel=&quot;noreferrer&quot;&gt;Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis.&lt;/a&gt;&amp;quot; that answers just that. Also found on &lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/com/B005IGI2GA&quot; rel=&quot;noreferrer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Amazon for 1.99&lt;/a&gt;. This doesn't cover how a country would respond but how NASA would handle several scenarios to the idea. I'm guessing since NASA is heavily funded by the United States and this document was published, the United States would obviously have regulations with NASA in mind if this were to occur.&lt;/p&gt;&#xA;&lt;p&gt;In the first contact scenario, NASA specifies that we should train our minds in the case that we become in contact with ETI (Extraterrestrial intelligence) so that &amp;quot;our minds grow accustomed to thinking about, identifying and analyzing specific scenarios and variations of them&amp;quot;. They believe that by training our minds we will have a better capacity to analyze and respond to ETI contact. They break these scenarios into two parts: intentionally or unintentionally harmful, which personally seems very science-fiction.&lt;/p&gt;&#xA;&lt;p&gt;Being that this is a scientific paper, I found it very inspiring that they talked about handling communication. Since our common form of communication is through specific wavelengths it's impossible to know if we could ever be able to communicate with them. If an ETI is transmitting on an uncommon communication method, it's unknown how we could communicate.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The possibility also remains that ETI do not use electromagnetic&#xA;radiation for communication but instead have discovered some other&#xA;method (possibly something more efficient or effective) for exchanging&#xA;information across astronomical distances.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Also, it is noted that communication over vast distances might limit the ability for ETI to engage in warfare. Thus, even if we do discover ETI, the time it takes to communicate could be hundreds of years due to sheer distance.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This difficulty in communicating across such vast distances also might&#xA;limit the ability for ETI to engage in interstellar warfare for the&#xA;simple reason that the communications problem renders such warfare too&#xA;logistically difficult to coordinate.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;which starts the next interesting subject of&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Another implication of these long communication times across the&#xA;galaxy is that ETI might become alerted to our presence without us&#xA;realizing it&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Once ETI become alerted to our presence, it will take at least as many&#xA;years for us to realize that they know we are here. During the&#xA;intervening time, ETI can respond to our presence or prepare for&#xA;contact in ways that we would have no knowledge of or influence on.&#xA;Even if humanity can successfully exchange signals with ETI, there is&#xA;no guarantee that the information will be successfully communicated.&#xA;In order for information to be exchanged, it is also necessary that&#xA;humans and ETI understand the contents of each others’ messages.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The answer to the concern with communication is that we would obviously communicate through mathematical conversation. Which is hinted to be our first few exchanges with ETI. They also talk about increasing the framework with METI to increase the probability of communication any time a transmission is sent from earth.&lt;/p&gt;&#xA;&lt;p&gt;In the same section of communication this paragraph stood out to me:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For example, we are almost guaranteed to lose in a fight between us&#xA;and them, and there is a strong likelihood that such a loss would be&#xA;so severe that we would cease to survive as a civilization. On the&#xA;other hand, if ETI decide to use their superior abilities to help us,&#xA;then they may be able to help solve many of our problems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It's obvious that depending on the situation how we handle a discovery depends on the nature of the ETI. I find it kind of sad that throughout this document that it's a worry that ETI will be harmful to civilization. You would think that if we are searching and they are searching for life outside of their own, we would be peaceful not looking to destroy one another due to fear. In any case, if the ETI are war-minded and more advanced, the odds of survival are small, so we are prepared to lose, but if they come with open-arms, we are ready to work together to &amp;quot;solve many of our problems&amp;quot;. It would be pretty awesome to have ETIs come to us with cures to our diseases.&lt;/p&gt;&#xA;&lt;p&gt;I don't think people are interested in my analysis of the paper but I just wanted to give you a bit of an introduction of what's inside. I haven't read the entire thing yet and what I have covered has only been 8 pages in. NASA obviously has interest in how we should respond. They even go into the topics of &lt;strong&gt;Extraterrestrial ethics: Selfishness and universalism, Possible ETI heterogeneity, benefits, handling mere detection, how we would cooperate, and so much more&lt;/strong&gt;. I am really excited to read the entire article.&lt;/p&gt;&#xA;&lt;p&gt;Great question again, I am glad I took the time to answer this. After I read it and find more interesting information, I am going to post more information on this topic on my personal &lt;a href=&quot;http://johnriselvato.com/how-nasa-world-handle-eti-alien-discovery/&quot; rel=&quot;noreferrer&quot;&gt;website&lt;/a&gt;. Mainly because I don't want to get an award for the longest answer on a Stack Exchange site. :D&lt;/p&gt;&#xA;" OwnerUserId="66" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-04-06T00:46:17.200" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="580" PostTypeId="2" ParentId="576" CreationDate="2013-07-21T08:27:09.483" Score="6" Body="&lt;p&gt;I do know that the &lt;a href=&quot;http://en.wikipedia.org/wiki/Deinococcus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deinococcus&lt;/a&gt;, an earth bacteria, has the ability survive in high energy levels of radiation. Chances are NASA is just trying to limit the possibility of an organism going into space alive. Anything is possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also would just seem careless to send earth life into space. This is especially true knowing that &lt;a href=&quot;http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040230&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermal adaptation in bacteria&lt;/a&gt; is possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://web.mst.edu/~microbio/bio221_2007/D_radiodurans.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deinococcus radiodurans&lt;/a&gt; seems like a perfect bacteria example of a life form that should not be taken into space:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;radiodurans is a gram-positive, red-pigmented bacterium that typically&#xA;  grows in a tetrad structure. It is a nonsporulating and strictly&#xA;  aerobic bacterium, but it has been known to survive in vacuum&#xA;  conditions. D. radiodurans is a chemoorganotroph with an optimal&#xA;  growth temperature of 25º – 35º C; however, it is very resistant to&#xA;  extreme temperatures. The bacterium has been isolated from numerous&#xA;  locations, including elephant dung and the granite of Antarctica’s dry&#xA;  valleys. The natural habitat has not been determined.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="66" LastEditorUserId="27346" LastEditDate="2019-12-18T01:54:40.140" LastActivityDate="2019-12-18T01:54:40.140" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="582" PostTypeId="2" ParentId="576" CreationDate="2013-07-21T11:40:28.020" Score="0" Body="&lt;p&gt;Currently the majority of interplanetary travel is funded by government, government is maintained by politics, politics are governed by politicians, politicians are governed by... (&lt;em&gt;well I am not sure, but in any case that would be out of scope for this site&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously a purely scientific mission searching for life would want to observe good hygiene, and ensuring your vehicle is clean before it leaves ensures that anything it finds is new. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Filling your vehicle with all the known bacteria of the world and seeing how it survives when exposed to cosmic radiation is a test that I don't believe has actually be conducted yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But lastly, even if you transfer dead organisms to Mars, when you find a dead organism on Mars there is no way to know its origin. If you can find dead organisms on mars that did not have terrestrial origins that you have proven extraterrestrial life does or has existed. As we don't have proof of extraterrestrial life, nor a method for categorizing life as clearly terrestrial in origin, these questions remain important so the precautions remain necessary, from a scientific perspective. &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-07-21T11:40:28.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="583" PostTypeId="2" ParentId="421" CreationDate="2013-07-21T12:55:41.453" Score="3" Body="&lt;p&gt;I've been searching around, and found some piecemealed data. Here's what I can find.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Some very limited data is available from the &lt;a href=&quot;http://pds.jpl.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;Planetary Data Archives&lt;/a&gt;, mostly in the form of cooperative missions with NASA&lt;/li&gt;&#xA;&lt;li&gt;I found some data on &lt;a href=&quot;http://www.iki.rssi.ru/eng/da.html&quot; rel=&quot;nofollow&quot;&gt;this website&lt;/a&gt;, from the Space Research Institute&lt;/li&gt;&#xA;&lt;li&gt;Some of it appears to be hosted on a private network similar to the WW2, known as the &lt;a href=&quot;http://www.rssi.ru/&quot; rel=&quot;nofollow&quot;&gt;Russian Space Science Network&lt;/a&gt;. Not being a member, I can't confirm.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, Soviet data is just harder to get at, due to the heightened security during the time of the Cold War, and much of that data just isn't public even today. Hopefully this will change someday&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-21T12:55:41.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="584" PostTypeId="1" AcceptedAnswerId="2281" CreationDate="2013-07-21T12:57:48.800" Score="13" ViewCount="509" Body="&lt;p&gt;Popular &lt;a href=&quot;https://en.wikipedia.org/wiki/Far_side_of_the_Moon#Potential&quot; rel=&quot;nofollow&quot;&gt;potential uses of the far side of the moon&lt;/a&gt; include radio telescopes &amp;amp; mining for Helium-3 (&lt;em&gt;good potential for use as a fuel in fusion reactors&lt;/em&gt;). These two activities are not necessarily mutually complementary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are terrestrial examples of areas that are selected purely for their complementary attributes of reasonable accessibility and otherwise are relatively undesirable locations, such as in the Mojave Desert with &quot;Goldstone Deep Space Communications Complex&quot; (&lt;em&gt;seeks relative quiet for study&lt;/em&gt;) and &quot;Fort Irwin National Training Center&quot; (&lt;em&gt;seeks seclusion for military training&lt;/em&gt;). Experience with these indicates that competition for non-complementary use of such space needs to be proactively managed for mutual use to be practical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any existing organization that would work to manage these types of projects appropriately? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would seem that with proper management, these two examples could be mutually helpful, but without it could be unpleasant at best.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="58" LastEditDate="2020-12-01T16:47:31.643" LastActivityDate="2020-12-01T16:47:31.643" Title="What plans are in place or being considered for management of the far side of the moon?" Tags="|the-moon|international-organizations|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="586" PostTypeId="5" CreationDate="2013-07-21T15:02:05.870" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-21T15:02:05.870" LastActivityDate="2013-07-21T15:02:05.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="587" PostTypeId="4" CreationDate="2013-07-21T15:02:05.870" Score="0" Body="Questions about what country a person is a citizen of, as pertaining to space exploration." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-21T19:06:16.157" LastActivityDate="2013-07-21T19:06:16.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="588" PostTypeId="1" AcceptedAnswerId="595" CreationDate="2013-07-21T15:03:43.253" Score="17" ViewCount="286" Body="&lt;p&gt;The HiRISE camera on the Mars Reconnaissance Orbiter has 3 color sensors, red (440-850nm), blue-green (400-600nm), and near-infrared (800-1000nm). Why these particular channels? Why not standard visible colors?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-02-11T23:02:35.840" LastActivityDate="2018-02-11T23:02:35.840" Title="Why did the HiRISE camera team choose the wavelengths they did?" Tags="|mars|camera|instrument|mars-reconnaissance-orbiter|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="589" PostTypeId="2" ParentId="12" CreationDate="2013-07-21T15:16:08.903" Score="5" Body="&lt;p&gt;Interferometry is used to calculate distance. In VLBI they use multiple telescopes so that they can check the &lt;strong&gt;arrival time&lt;/strong&gt; of the signal (like how GPS works).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the wiki:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth. The distance between the radio telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes. This allows observations of an object that are made simultaneously by many radio telescopes to be combined, &lt;strong&gt;emulating&lt;/strong&gt; a telescope with a size equal to the maximum separation between the telescopes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The key part is emulating; as in it gives the effect of using a large telescope. I think that is where the core of your question is coming from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using small dishes on several deep space satellites may not work because they just may not be able to amplify the signal enough to receive anything. Also, VLBI would require that the time the signal arrives and the position of the receiver are known with great precision. Probes flying out into space at different speeds will have different drifts in their atomic clocks, as well as problems in determining the exact position (within millimetres) of each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're going to launch multiple satellites to improve deep space communication then it may be better/simpler to launch them so that they act as repeaters for the signal. &lt;/p&gt;&#xA;" OwnerDisplayName="user39" LastActivityDate="2013-07-21T15:16:08.903" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="590" PostTypeId="2" ParentId="563" CreationDate="2013-07-21T15:57:47.110" Score="4" Body="&lt;p&gt;This is a somewhat nuanced issue. Yes, GNSS can be used by LEO spacecraft, and yes it's done often and is often more cost-effective than the alternative (mostly because a ground system that does orbit determination is expensive). I'll speak about GPS because that is by far the most common.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are basically two aspects to this: autonomous, on-board estimation using GPS, and &quot;post-processed&quot; estimation. A GPS receiver will give you what it calculates as your state (i.e. position and velocity). This can be used, and is often accurate enough (when, say, your accuracy requirement is just postion within 1km).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, often the accuracy requirement will be much higher (centimeter-level). In this case, using the GPS receiver navigation solution will not be adequate. As an alternative, you can record/downlink the GPS pseudorange measurements (basically raw GPS measurement data), and with advanced estimation techniques (not to mention the more accurate post-processed GPS ephemeris catalog), you can get to your required level of accuracy. Note that this will not be real-time, unlike the on-board solution described above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to how this compares with &quot;traditional methods&quot; (e.g. ground-based ranging or SLR), that also depends on how many observations are taken, etc., which in turn is driven by the accuracy requirement. I think it is safe to say that both GNSS and &quot;traditional methods&quot; can be made to meet the accuracy requirement of the mission.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-07-21T15:57:47.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="591" PostTypeId="5" CreationDate="2013-07-21T17:21:39.780" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-21T17:21:39.780" LastActivityDate="2013-07-21T17:21:39.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="592" PostTypeId="4" CreationDate="2013-07-21T17:21:39.780" Score="0" Body="Satellite orbit determination (OD) is the method of determining the position and velocity of an orbiting object from onboard or ground observations. The OD problem is generally described by the computational process (generally solved by applying statistical estimation techniques) of determining the state of a satellite as a function of time using the observations. Based on https://secwww.jhuapl.edu/techdigest/content/techdigest/pdf/V27-N03/27-03-Vetter.pdf" OwnerDisplayName="user29" LastEditorUserId="6944" LastEditorDisplayName="user29" LastEditDate="2024-02-14T22:07:48.367" LastActivityDate="2024-02-14T22:07:48.367" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="593" PostTypeId="1" AcceptedAnswerId="755" CreationDate="2013-07-21T17:30:56.423" Score="22" ViewCount="5141" Body="&lt;p&gt;I seek detailed information on the computer and/or data handling system of Rosetta. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Who manufactured it? &lt;/li&gt;&#xA;&lt;li&gt;What type(s) of CPU is used? &#xA;&lt;ul&gt;&#xA;&lt;li&gt;What kind of architecture does it belong to and at what frequency does it operate?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;What is the operating system? &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;How much data can the system store?&lt;/strong&gt; &#xA;&lt;ul&gt;&#xA;&lt;li&gt;What types of storage does it include (RAM, ROM, disks, whatever)? &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;What are the I/O rate of the various storage devices?&lt;/strong&gt; &lt;/li&gt;&#xA;&lt;li&gt;Is there any buffering system? (At what rate can the data be send back to Earth?) &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2014-01-31T15:09:05.757" LastActivityDate="2014-11-13T03:36:33.157" Title="On-board computer / data handling system of Rosetta - details?" Tags="|flight-computer|data-storage|rosetta|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="594" PostTypeId="1" AcceptedAnswerId="754" CreationDate="2013-07-21T18:06:58.377" Score="9" ViewCount="391" Body="&lt;p&gt;Redshift occurs whenever an observer moves away from (shifted to the red) or approaches (shifted to the blue; blueshift) an object that emits radiation and the other way around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Traveling towards something that mostly emits radiation in the spectrum of visible light like our sun will shift it into spectra of higher energy. Going from ultra-violet over x-ray to gamma depending on the speed on a space ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would a space ship have to be designed to protect the crew and equipment from increasing radiation as its speed increases?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A cylindrical object with shielding at its top with the crew directly behind it? Or should the crew be as far away as possible from the space ship's top? Would a different shape be more effective for shielding against radiation? &lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorUserId="25" LastEditDate="2013-07-25T17:45:07.097" LastActivityDate="2013-07-25T17:45:07.097" Title="How to design a spacecraft to cope with redshift?" Tags="|crewed-spaceflight|radiation|interstellar-travel|communication|spacecraft-development|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="595" PostTypeId="2" ParentId="588" CreationDate="2013-07-21T18:56:24.290" Score="12" Body="&lt;p&gt;Red because of the color of the planet - we need to be able to see the most red detail, as that's what most of the planet is colored. Blue-green because we don't really need to see blue and green individually. If it's not red, it's enough to know that it's blue or green. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the near-infrared sensor. Why? It turns out that information on mineral groups can be effectively acquired with a near-infrared camera, and has been in use for years. For example, &lt;a href=&quot;http://www.spectroscopynow.com/details/ezine/136e9ff0ecd/Vendor-column-Mining-mineral-analysis-using-near-infrared-technology-Dont-laugh-.html?tzcheck=1&quot;&gt;this article&lt;/a&gt; on this use of near-infrared cameras is quite informative:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mineral analysis by near-infrared (NIR) analysis is not well known, even though spectral information on minerals has existed for years. This is primarily because NIR users typically have focused on the elemental minerals rather than geologic mineral species. Researchers using near-infrared for minerals have applied a remote sensing approach rather than a chemometric-based approach, and unfortunately these two groups seldom talk to one another. By bringing together NIR, software and chemometric modeling, advanced technology applications are being created for diverse industries, including mineral analysis in mining exploration and control ore processing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's a good read for anyone interested in the subject - and shows just how useful this camera is on the HiRISE orbiter.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-21T19:47:45.857" LastActivityDate="2013-07-21T19:47:45.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="596" PostTypeId="1" AcceptedAnswerId="616" CreationDate="2013-07-21T19:05:22.287" Score="9" ViewCount="390" Body="&lt;p&gt;Are there any alternative energy sources to power inter-stellar probes such as Voyager I and II developed or in development by any space agency?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plutonium (238-Pu) has a half-life of about 87,7 years, which is a big limitation to the lifespan of such missions, even if we have a lot of spare energy at the start. Full nuclear reactor is too heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any concept of powering such probes with an energy source, that could work for hundreds of years? Otherwise, the only possibility to reach further is to accelerate probes to higher velocities. &lt;/p&gt;&#xA;" OwnerDisplayName="user43" LastActivityDate="2013-11-01T22:42:42.410" Title="Alternative energy sources for inter-stellar probes?" Tags="|energy|probe|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="597" PostTypeId="5" CreationDate="2013-07-21T19:10:59.357" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-21T19:10:59.357" LastActivityDate="2013-07-21T19:10:59.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="598" PostTypeId="4" CreationDate="2013-07-21T19:10:59.357" Score="0" Body="Questions about equipment used to gather information during space exploration." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-21T19:48:20.080" LastActivityDate="2013-07-21T19:48:20.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="599" PostTypeId="5" CreationDate="2013-07-21T19:12:01.583" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-21T19:12:01.583" LastActivityDate="2013-07-21T19:12:01.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="600" PostTypeId="4" CreationDate="2013-07-21T19:12:01.583" Score="0" Body="Questions about planets in the solar system comprised primarily of gases, such as Jupiter, Saturn and Neptune." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-21T19:23:32.883" LastActivityDate="2013-07-21T19:23:32.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="601" PostTypeId="1" AcceptedAnswerId="615" CreationDate="2013-07-21T19:19:45.367" Score="10" ViewCount="874" Body="&lt;p&gt;The problem how it is possible to receive data from Voyager I at all is dealt in this question: &lt;a href=&quot;https://physics.stackexchange.com/questions/13227/how-earth-communicates-with-voyager-i&quot;&gt;https://physics.stackexchange.com/questions/13227/how-earth-communicates-with-voyager-i&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are still problems if we want to go really far away and still be able to send scientific data back to Earth:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The receivers on Earth must go really big&lt;/li&gt;&#xA;&lt;li&gt;With more and more precise directional antennas, more and more precision is required to position them, otherwise the signal will simply miss the target&lt;/li&gt;&#xA;&lt;li&gt;Transmission bitrate for Voyager I is quite low, too low to be able to send photos or other big volume data&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Are there any alternatives currently developed or conceptualized by space agencies? I can think of 2: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Building intermediate receiver stations, which is also problematic, because it would be very hard to keep them in constant distance to probe and Earth&lt;/li&gt;&#xA;&lt;li&gt;Sending back data on something like flash card, every few years, but we have a limited number of such transmissions, we need much power to launch them in the direction of Earth, and they could be quite hard to intercept.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerDisplayName="user43" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2016-06-13T22:36:07.303" Title="Alternative data transfer channels for inter-stellar probes?" Tags="|probe|communication|interstellar-travel|data-transmission|" AnswerCount="6" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="602" PostTypeId="5" CreationDate="2013-07-21T19:29:28.630" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-21T19:29:28.630" LastActivityDate="2013-07-21T19:29:28.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="603" PostTypeId="4" CreationDate="2013-07-21T19:29:28.630" Score="0" Body="The Voyager program is an American scientific program that launched two unmanned space missions, the probes Voyager 1 and Voyager 2. Although they were designated officially to study just the planetary systems of Jupiter and Saturn, the space probes were able to continue their mission into the outer solar system, and they are expected to push through the heliosheath in deep space." OwnerUserId="6" LastEditorUserId="6" LastEditDate="2013-07-21T19:31:46.360" LastActivityDate="2013-07-21T19:31:46.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="604" PostTypeId="1" AcceptedAnswerId="645" CreationDate="2013-07-21T19:50:36.007" Score="33" ViewCount="3444" Body="&lt;p&gt;The Russians did some early testing with rats &lt;a href=&quot;http://books.google.com/books?id=cg0ZR4rzzQoC&amp;amp;pg=PA244&amp;amp;dq=zero%20gravity%20pregnancy&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=-DfsUdDYGbbe4AOz2oDADA&amp;amp;ved=0CC0Q6AEwAA#v=onepage&amp;amp;q=pregnancy&amp;amp;f=false&quot;&gt;(Packing for Mars: The Curious Science of Life in the Void, By Mary Roach pg 245&lt;/a&gt;) and natural means did not result in pregnancy. Later testing shipped pregnant rats into space, where some health issues developed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA research indicates that female astronauts have given birth after flying in space, some difficulties in achieving pregnancy are reported, but attributed to age, secondary to delaying attempts while pursing careers. (&lt;a href=&quot;http://books.google.com/books?id=QYrH9P0iPb8C&amp;amp;pg=PA387&amp;amp;dq=zero%20gravity%20pregnancy&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=-DfsUdDYGbbe4AOz2oDADA&amp;amp;ved=0CD4Q6AEwAw#v=onepage&amp;amp;q=pregnancy&amp;amp;f=false&quot;&gt;Principles of Clinical Medicine for Space Flight edited by Michael R. Barratt, Sam Lee Pool page 384&lt;/a&gt;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any research on mammal impregnation occurring naturally in space?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;, clarify that this question is about the impregnation --&gt; Egg and Sperm join, Cells divide and attach to the uterus; the 1979 Russian study indicates there were problems at this stage. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorDisplayName="user12" LastEditDate="2013-12-13T00:18:12.383" LastActivityDate="2018-12-20T14:38:17.033" Title="Is it possible to get pregnant through natural means in space?" Tags="|medical|life|astrobiology|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="605" PostTypeId="1" CreationDate="2013-07-21T20:22:17.563" Score="12" ViewCount="460" Body="&lt;p&gt;Observed rotation rates of small asteroids &lt;a href=&quot;https://physics.stackexchange.com/questions/26476/can-a-minimum-ultimate-tensile-strength-uts-for-an-asteroid-be-established-bas&quot;&gt;point to a limit&lt;/a&gt; on the material strength of these bodies.  This limit is not very high&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that's only part of the picture, because an upper limit on rotation rate only tells you about the &lt;em&gt;global mechanics&lt;/em&gt; of the body.  There's also the question of whether we expect collisions to cause high rotation rates which stress the bodies in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the fact that we have direct observations of asteroid material (falling from the sky all the time), what material quality could we expect?  It's simple to look up &lt;a href=&quot;http://www.stanford.edu/~tyzhu/Documents/Some%20Useful%20Numbers.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;values for ordinary Earth rocks&lt;/a&gt;.  Compare the two, there is 3 orders of magnitude difference.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Asteroid rotation rate based limit: 20 kPa&lt;/li&gt;&#xA;&lt;li&gt;Granite, Basalt, Quartzite, etc tensile strength: around 20 MPa&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The most obvious use of asteroids seems like it could be cutting it into beams, and using those for station construction, like what we do with certain kinds of lumber or granite counter-tops on Earth.  If we did that, would these materials be worth a darn?  Do we have a measured range of asteroid material strengths?  Would the rock be too fragmented to use? &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Also note that the target for &lt;a href=&quot;http://www.youtube.com/watch?v=rzOHVQA4dqA&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Asteroid Redirect Initiative&lt;/a&gt; is intended to be soft rock.  This eliminates some dangers of losing its (unstable) orbital location and falling to Earth, although I do not know if it's the reason for the requirement.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2018-02-19T12:01:51.530" Title="What is the range of strength for asteroidal materials?" Tags="|asteroid|mining|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="606" PostTypeId="2" ParentId="605" CreationDate="2013-07-21T20:31:43.327" Score="1" Body="&lt;p&gt;I may be misunderstanding your derivation of asteroidal material strength, but I believe your analysis doesn't measure something that is, say, related to &lt;a href=&quot;http://en.wikipedia.org/wiki/Von_Mises_yield_criterion&quot; rel=&quot;nofollow&quot;&gt;von Mises yield strength&lt;/a&gt;.  Instead, you are measuring the cohesion of the asteroid's &lt;a href=&quot;http://en.wikipedia.org/wiki/Rubble_pile&quot; rel=&quot;nofollow&quot;&gt;rubble&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think we can do a fairly good analysis of various asteroids' composition and then do terrestrial tests to determine material properties of the raw or alloyed material.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-21T20:31:43.327" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="607" PostTypeId="2" ParentId="605" CreationDate="2013-07-21T20:34:14.417" Score="3" Body="&lt;p&gt;Not entirely answer for your question but answer of what would be used as the construction materials in space, instead of raw asteroid rock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that the considerable number of meteorites are iron, while the asteroid rock structural endurance itself is poor, with abundance of solar energy forging them into iron is totally viable, and there, &lt;a href=&quot;http://en.wikipedia.org/wiki/Iron#Mechanical_properties&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; provides abundance of information on mechanical properties of various iron alloys. I mean, why settle for brittle rock when good iron is abundant?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-21T20:34:14.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="608" PostTypeId="5" CreationDate="2013-07-21T20:43:39.460" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-21T20:43:39.460" LastActivityDate="2013-07-21T20:43:39.460" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="609" PostTypeId="4" CreationDate="2013-07-21T20:43:39.460" Score="0" Body="Questions about the extraction of materials from other planets or asteroids." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-07-21T21:44:26.843" LastActivityDate="2013-07-21T21:44:26.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="610" PostTypeId="5" CreationDate="2013-07-21T20:44:37.980" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-21T20:44:37.980" LastActivityDate="2013-07-21T20:44:37.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="611" PostTypeId="4" CreationDate="2013-07-21T20:44:37.980" Score="0" Body="Questions about imagery devices on spacecraft and probes." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-06T21:11:35.117" LastActivityDate="2013-08-06T21:11:35.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="612" PostTypeId="5" CreationDate="2013-07-21T20:45:08.763" Score="0" Body="&lt;p&gt;The rate of data-transfer is limited by both the transmission-power of the probe, and the size of the antennas receiving the signals. An unmanned probe is of no use if it can not send any of its scientific data of images back to Earth. For manned spacecraft, communication is important for the crew to get orders and navigational information.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-06T21:11:19.580" LastActivityDate="2016-01-06T21:11:19.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="613" PostTypeId="4" CreationDate="2013-07-21T20:45:08.763" Score="0" Body="Questions regarding transfer of data from spacecraft and probes" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-06T21:10:57.887" LastActivityDate="2016-01-06T21:10:57.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="614" PostTypeId="1" AcceptedAnswerId="617" CreationDate="2013-07-21T22:22:13.073" Score="13" ViewCount="3360" Body="&lt;p&gt;If a manned mission to mars was made with the hope of colonization, we would probably need to make many of our structures there out of materials found there - they would probably be simply too heavy to launch from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With that in mind, have any materials/minerals been found on Mars that would allow the building of a colony, such as iron, etc.?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2014-02-28T05:15:53.957" LastActivityDate="2018-04-25T21:11:16.857" Title="Does Mars contain materials that could be used to build a space colony?" Tags="|mars|colonization|materials|construction|natural-resources|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="615" PostTypeId="2" ParentId="601" CreationDate="2013-07-21T23:15:52.110" Score="7" Body="&lt;p&gt;First of all, to solve the receiver strength issue, it is merely a matter of using higher frequencies. The reason this works it it allows for a smaller dish to have a high gain, which allows for more efficient data processing. The main reason the Mars Reconnaissance Orbiter is able to process so much more data than its predecessors is that it uses X band, at 8 GHz. Technology is allowing for higher datarates from spacecraft, from algorithms, frequency, and pointing accuracy, it's all looking better. MRO, if magically placed where the Voyager probes are, would have significantly more data capacity than the Voyager probes have. The key has to be to use more directional pointing to get the dish pointed right at Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is worth noting that higher frequencies continues to work, and it is frequently proposed that really long distance missions use either lasers, or even higher frequencies, like X-Rays or Gamma Rays. These of course require increasingly accurate pointing, but could theoretically be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, the pulses could be short in time duration, and achieve a similar affect. This would work best for a beacon of source. We could place one of these beacons to let the satellite know where to point the antenna, and it should be able to find a very precise location. See &lt;a href=&quot;http://seti.harvard.edu/oseti/allsky/&quot; rel=&quot;nofollow&quot;&gt;Optical SETI projects&lt;/a&gt;. Still, it is a challenge, but it is one that is getting more solvable every day.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-01-23T14:09:29.893" LastActivityDate="2015-01-23T14:09:29.893" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="616" PostTypeId="2" ParentId="596" CreationDate="2013-07-21T23:21:59.650" Score="2" Body="&lt;p&gt;Actually, a full nuclear reactor is not too heavy, and in fact, has been accomplished. There have been quite a few, Wikipedia has &lt;a href=&quot;http://en.wikipedia.org/wiki/Nuclear_power_in_space&quot; rel=&quot;nofollow&quot;&gt;an article&lt;/a&gt; on the subject. The United States has launched 1 nuclear reactor into space, the Soviet Union has launched around 40. These would have a lifetime of much longer than RTGs, as the half-life of Uranium 235 is in the millions of years, in other words, it won't go away unless it is being used. One would simply have to be careful enough not to use the entire supply of fuel before reaching the destination, but that shouldn't be too difficult of a problem.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2013-11-01T22:42:42.410" LastActivityDate="2013-11-01T22:42:42.410" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="617" PostTypeId="2" ParentId="614" CreationDate="2013-07-22T04:12:01.317" Score="15" Body="&lt;p&gt;Mars has extensive iron oxide ore. Much of it on the surface. Not readily extracted, but extractable with the correct smelting process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has silicates, as well; this allows for glass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has carbon dioxide; with a solar panel farm, and a cracking solution, that's oxygen for the taking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's just materials available readily at surface. Mars will have other metal resources available via mining, and other non-metal resources as well. What it lacks is free oxygen and surface water; water can be extracted from Mars' polar &quot;ice&quot; (most of which isn't actually water).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars has sufficient resources to be able to expand for a considerable time; it's general composition is thought to be similar to Earth's, though with fewer heavy elements, and having cooled sufficiently to have a solidified core (and thus a very weak magnetosphere - 1/(10^4) that of Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars may or may not have sufficient resources for a permanent self-sustaining colony, but it does have a plentitude of resources that could make a colony viable in an interim colonial mode.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="108" LastEditDate="2013-07-23T23:40:20.960" LastActivityDate="2013-07-23T23:40:20.960" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="618" PostTypeId="1" AcceptedAnswerId="641" CreationDate="2013-07-22T07:51:13.887" Score="24" ViewCount="1669" Body="&lt;p&gt;When a project like Rosetta is being developed, how are flight plans calculated? Do planners simply input the desired target into a program or a flight computer which spits out the optimum flight path incorporating (multiple, if need be) gravity assists also taking other considerations like fuel into account? Do all space agencies have their own in-house software or do they use a common application? I'd appreciate more details on this.&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="25" LastEditDate="2013-07-22T08:43:05.410" LastActivityDate="2013-07-22T20:16:08.127" Title="How are flight plans calculated?" Tags="|orbital-mechanics|flight-computer|orbital-maneuver|trajectory|planning|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="619" PostTypeId="2" ParentId="530" CreationDate="2013-07-22T08:50:24.393" Score="2" Body="&lt;p&gt;Yes, as long as the orbits are long and far enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that relativistic mass IS a gravitational mass. The faster an object moves, the heavier gravity-wise it is, and while at speeds below, say, 0.5 &lt;em&gt;c&lt;/em&gt; this mass gain is insignificant, as you're approaching speed of light about all of acceleration energy is converted into mass - and as result, into gravity force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if you try to approach a black hole through elliptical orbit where the &quot;near pass&quot; speed would exceed speed of light if counted in Newton and Kepler way, obviously that won't work. Instead of speed that would counteract the centripetal force of gravity of the black hole,   you'll start gaining mass, and that will make the black hole's gravitational pull on you so much stronger. &quot;Falling&quot;, instead of speeding up you keep gaining weight, and so the gravity pull grows not only with waning distance but with your kinetic energy growing - until the inevitable demise.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-22T08:50:24.393" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="620" PostTypeId="2" ParentId="596" CreationDate="2013-07-22T09:13:22.753" Score="1" Body="&lt;p&gt;When (if) we get cold fusion under control, the probe could be scooping free hydrogen out of the interstellar space. While there's not much of it, if the probe moves fast enough it will sweep huge volumes of space per each second of flight. That would be fully sustainable, since the hydrogen is pretty much everywhere, but the probe would eventually reach a break-even speed where drag of captured particles would outweigh energy gain from fusion of &quot;burning them up&quot; for accelerating.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-22T09:13:22.753" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="621" PostTypeId="1" AcceptedAnswerId="622" CreationDate="2013-07-22T09:22:55.573" Score="18" ViewCount="3251" Body="&lt;p&gt;We've seen quite a few probes examining the Solar System in the recent years, but beyond Voyager delivering new measurements every couple months now, I haven't heard about any of the probes underway towards &quot;out beyond&quot; the Solar System.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any successors to the Voyagers currently, aiming towards neighbor stars or just &quot;out of the system&quot; in general? If so, could you summarize their equipment and missions?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2019-03-01T07:58:14.747" Title="Are there any successors to Voyagers underway currently?" Tags="|voyager|probe|extra-solar-object|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="622" PostTypeId="2" ParentId="621" CreationDate="2013-07-22T10:01:12.180" Score="23" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;New Horizons&lt;/strong&gt;&lt;/a&gt; has examined Pluto on 14 July 2015 and &lt;a href=&quot;https://en.wikipedia.org/wiki/(486958)_2014_MU69&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ultima Thule&lt;/a&gt; on 1 January 2019.  It may continue to explore a third Kuiper Belt object, but it's not aiming to measure the interstellar medium, let alone nearby stars; it's too slow for that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It didn't go into orbit in the Pluto-Charon system; there's no atmosphere thick enough for aerobraking, and it doesn't have the propulsive capability to slow down. It made a flyby on 14 July 2015.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After Pluto and the &lt;a href=&quot;https://en.wikipedia.org/wiki/(486958)_2014_MU69#Exploration&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ultima Thule flyby&lt;/a&gt;, it is flying on and ultimately, in the 2040s or so, leave the solar system.  It's slower than the Voyager spacecrafts and will no longer be operational when it enters the interstellar medium.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2019-03-01T07:58:14.747" LastActivityDate="2019-03-01T07:58:14.747" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="623" PostTypeId="1" AcceptedAnswerId="7834" CreationDate="2013-07-22T10:35:32.897" Score="19" ViewCount="3978" Body="&lt;p&gt;Space probes headed for the far reaches of our Solar System rely on radioisotope thermoelectric generators for power (&lt;a href=&quot;http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot;&gt;Cassini–Huygens&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1&quot;&gt;Voyager 1&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_2&quot;&gt;Voyager 2&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably this is because solar power isn't feasible at large distances from the Sun. With the current technology available, what is considered the &quot;safe zone&quot; that solar arrays can be used as a reliable source to power a space craft? &lt;/p&gt;&#xA;" OwnerUserId="17" LastEditorUserId="25" LastEditDate="2013-07-22T13:20:00.437" LastActivityDate="2022-09-27T12:10:54.247" Title="How far from the Sun can solar power be used as a reliable energy source?" Tags="|energy|rtg|solar-power|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="624" PostTypeId="1" CreationDate="2013-07-22T10:58:29.260" Score="15" ViewCount="2090" Body="&lt;p&gt;Somewhere I read that there are serious plans to send a cubesat into lunar orbit. A few years ago, the ISS was also offering a launch opportunity for such a mission. I guess that it was actually booked. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is the communication supposed to work with a cubesat over such long distances? Has it been researched or proposed for 1U cubesats? Are there any special requirements on frequencies, antennas, power consumption ... ? What needs to be taken care of if you want to use your hobby ground station for something like this, if it is already working for cubesats in LEO? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt; A few interesting links ... &lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://amsat-uk.org/2013/06/28/uk-radio-ham-plans-lunar-cubesat/&quot; rel=&quot;noreferrer&quot;&gt;http://amsat-uk.org/2013/06/28/uk-radio-ham-plans-lunar-cubesat/&lt;/a&gt; &lt;br&gt;(the original announcement of amsat, which raised my attention) &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.kickstarter.com/projects/aresinstituteinc/lunarsail-the-worlds-first-crowdsourced-solar-sail&quot; rel=&quot;noreferrer&quot;&gt;http://www.kickstarter.com/projects/aresinstituteinc/lunarsail-the-worlds-first-crowdsourced-solar-sail&lt;/a&gt; &lt;br&gt;(a related campaign on kickstarter) &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.lunarcubes.com/&quot; rel=&quot;noreferrer&quot;&gt;http://www.lunarcubes.com/&lt;/a&gt; &lt;br&gt;(website of the first international workshop on 'lunarcubes - the next frontier') &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="14" LastEditorUserId="12102" LastEditDate="2016-04-20T18:14:55.697" LastActivityDate="2022-06-07T00:41:32.150" Title="Cubesats in lunar orbit and beyond - how does communication work?" Tags="|the-moon|deep-space|communication|cubesat|amateur-radio|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="625" PostTypeId="2" ParentId="623" CreationDate="2013-07-22T11:32:19.497" Score="14" Body="&lt;p&gt;There are at least two problems with solar photovoltaic cells (not considering concentrators) in the outer solar system: the low power of the sun, and the low temperature of the cells.&lt;/p&gt;&#xA;&lt;p&gt;For the Cassini mission to Saturn (9–10 AU from the Sun), NASA investigated solar as an alternative.  They calculated the surface area that would be required, and concluded that the mass of the solar arrays required would result in a spacecraft with a mass exceeding anything that could be launched with existing technology, and would severely inhibit manoeuvrability.  They concluded that it would have been possible, but that the scientific cost would be too high:&lt;/p&gt;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/safety/chap2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini Environmental Impact Statement, chapter 2&lt;/a&gt;, page 2-53 onward.  For one alternative configuration,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The addition of this size array, in conjunction with the other&#xA;modifications required to implement solar power, increased the spacecraft dry mass by 1,337 kg&#xA;(2,948 lb). With the mass of the propellants, the Huygens Probe, and the launch adapter, the total&#xA;spacecraft mass would increase to 7,228 kg (15,935 lb), far exceeding the launch capacity of the&#xA;Titan IV (SRMU)/Centaur of 6,234 kg (13,743 lb) for a trajectory to Saturn (JPL 1994a).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;or, for another one,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To further reduce the size of the arrays, the power available to the&#xA;science instruments was reduced by 50 percent. Because of the large moment of inertia created&#xA;2&#xA;by the large solar panels (397 m²[4,269 ft²] and 585 kg [ 1,290 lb]) (JPL 1994a), the time&#xA;required to turn and maneuver the spacecraft during its exploration of the Saturnian system would&#xA;increase by a factor of between 4 and 18 compared with the compact RTG-powered spacecraft.&#xA;The resulting impacts on the mission's science objectives would be serious and include increased&#xA;times for image mosaics, inadequate turn rates for fields and particles instruments, reduced image&#xA;resolution due to inadequate target motion compensation, loss of instrument observation time&#xA;during turns for communicating with Earth, and insufficient turn rates to support radar&#xA;observation of Titan's cloud-enshrouded surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;More recently, two missions to Jupiter (4.9–5.5 AU from the Sun) use solar arrays: NASA's &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno&lt;/a&gt; is currently (2022) orbiting Jupiter.  ESA's &lt;a href=&quot;http://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juice&lt;/a&gt; is to launch in 2023.  Both use solar photovoltaics, and are the furthest-away spacecraft to do so to date.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2022-09-27T12:10:54.247" LastActivityDate="2022-09-27T12:10:54.247" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="626" PostTypeId="1" CreationDate="2013-07-22T11:38:49.787" Score="11" ViewCount="1274" Body="&lt;p&gt;This is (again) something from the back of my head, that I am looking for right now. I think that in 2008, I saw a presentation on &lt;strong&gt;foamed / expanded aluminium&lt;/strong&gt; for structural components (1). The promise was to enable roughly 10 times lighter structures in aluminium as compared to a structure with an equivalent strength made from &lt;em&gt;solid&lt;/em&gt; aluminium. It was at an ESA meeting, so it should come from either somewhere in Europe or Canada. I can not find it and actually I do not know how it is called. &lt;em&gt;Foam&lt;/em&gt; does not seem to be the term. It was about &lt;em&gt;really small bubbles&lt;/em&gt; of some sort, similar to or smaller than in foam plastic. I do not know about outgassing in this context, maybe the &lt;em&gt;bubbles&lt;/em&gt; were evacuated. It would be interesting to find out more. There are some composites / polymers in use in space, for which foam has at least been investigated, but this is sort of not what I am looking for. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(1) EDIT: I need to specify this. I am talking about structural components of space crafts or satellites. Think of structures in terms of skeletons. Launchers are interesting, but not the scope of this question. It also means, that I prefer a source, which talks about the use in space (longer duration, weeks, months, years), space applications (proposals for or documentation of structural component xyz of mission abc) or space qualification of the material (an analysis of its mechanical properties plus [thermal] vacuum tests is probably most interesting in this case).&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-07-22T22:26:24.477" LastActivityDate="2013-07-23T02:48:35.177" Title="Ultra-lightweight material for structural components - aluminium foam?" Tags="|materials|construction|outgassing|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="627" PostTypeId="2" ParentId="624" CreationDate="2013-07-22T12:10:51.197" Score="7" Body="&lt;p&gt;So, let's first compare the distances. Your average LEO satellite is placed in orbit maybe 600 km away. The distance to the Moon is 384000 km. That is 640 times further. Using &lt;span class=&quot;math-container&quot;&gt;$r^2$&lt;/span&gt; losses, and converting to decibels, that is an additional 56 dB loss. Of course, a groundstation needs to be able to communicate in LEO beyond directly overhead, so let's just assume it's at the horizon at that distance. I'm going to estimate that horizon distance at about 3000 km. That reduces the difference in power requirements to just 42 dB difference. So, is that achievable for an amateur station?&lt;/p&gt;&#xA;&lt;p&gt;Most cubesats are build by students or other hobbyists, and use the Amateur Radio frequencies to communicate. There is actually a pretty common feat to test out an amateur radio station that involves pointing an antenna at the Moon, and trying to communicate with other people. For the most part, the data rates are quite low, but a large station can actually talk using Morse code, which can be fairly fast. This is actually quite a bit harder than talking to a satellite orbiting the Moon, for a couple of reasons. First of all, the Moon don't reflect all of the energy back to Earth, and then you have to consider the two way path loss in signals. So, it can in theory be done, so, how do you do it?&lt;/p&gt;&#xA;&lt;p&gt;First of all, you will need to use a lower data rate than the LEO satellite. If you cut the rate down by 16, that's about a 12 dB gain. If you use a preamplifier/amplifier, you can get another 20 dB or so. And then using higher gain antennas will get you the rest of the way, all the way to the required 42 dB extra gain. The lower data rates could be compensated by the fact that you have near constant communication for 12 hours each day. Alternatively, you could use a higher frequency, and a dish antenna, which will allow you to get much higher gain, and a similar feat could be ensured.&lt;/p&gt;&#xA;&lt;p&gt;Bottom line is, this is a doable problem, although it would be a bit tricky to manage. It should be tested first by bouncing a signal off of the Moon and verifying you hear the echo. This should be done using the same modulation you plan on listening with. If you pull this off, you should be good to go.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="47149" LastEditDate="2022-06-07T00:41:32.150" LastActivityDate="2022-06-07T00:41:32.150" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="628" PostTypeId="2" ParentId="626" CreationDate="2013-07-22T12:59:46.190" Score="5" Body="&lt;p&gt;What you are looking for is Aerogel - Www.aerogel.org. There are a few different kinds, some of which are flexible and light, at above 99.9% air, and others that are more rigid, with a lower air content.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solid used can vary too, from graphite to aluminium or silica, depending on the required properties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The silica aerogels are incredibly good at blocking heat transfer, as you can see in this pic from &lt;a href=&quot;http://stardust.jpl.nasa.gov/tech/aerogel.html&quot; rel=&quot;noreferrer&quot;&gt;http://stardust.jpl.nasa.gov/tech/aerogel.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VUWaY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-22T12:59:46.190" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="629" PostTypeId="2" ParentId="626" CreationDate="2013-07-22T13:03:27.633" Score="10" Body="&lt;p&gt;&lt;strong&gt;Usage as blast containers&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023870_2011025092.pdf&quot; rel=&quot;noreferrer&quot;&gt;short article by NASA&lt;/a&gt; investigating the use of aluminium foam. Whilst they don't intend to use it for the majority of the space craft design they do intend to use it to replace their blast containers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA were looking for a replacement for their blast containers, the job of which is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The blast containers are&#xA;  used to safely trap the debris&#xA;  from small explosions that&#xA;  are initiated at liftoff to sever&#xA;  frangible nuts on hold-down&#xA;  studs that secure the spacecraft to a mobile launch platform until liftoff&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The results of their testing with aluminum foam were as below. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the basis of&#xA;  the results of these tests, aluminum foam&#xA;  liners with densities of 16 and 24 percent&#xA;  were selected. Sample foam liners were&#xA;  tested in a simulated launch configuration (see figure). The results of the tests&#xA;  showed that suitably designed and fabricated aluminum foam liners absorb energy more effectively than the solid lead&#xA;  liners do.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Uses as Shielding&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from blast containers we also have their plans for it to be used as &lt;a href=&quot;http://ston.jsc.nasa.gov/collections/trs/_techrep/TM-2009-214793.pdf&quot; rel=&quot;noreferrer&quot;&gt;shielding from micro meteoroids and debris.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This paper compares existing honeycomb structures to that of the foam&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The purpose of this study was to evaluate the effect on shielding performance achieved by replacing &#xA;  metallic honeycomb cores with metallic open-cell foam cores in a double-sandwich panel MMOD &#xA;  shielding configuration representative of those used onboard the ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And the conclusion of the experiment was that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At normal incidence, the foam-modified shield was found to provide a 15% improvement in critical &#xA;  projectile diameter at low velocity (3 km/s) and a 3% increase at high velocity (7 km/s)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The technical details below this point explain mostly why they intend to use it, its strength to weight ratio and ability to contain blasts being the main one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Properties of the foam&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aluminium Foam does seem to be the correct name for what you are thinking of. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aluminium Foam is produced from an aluminium alloy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.goodfellow.com/catalogue/GFCat4J.php?ewd_token=dYepC9mINoxB1BQgLQabdwzFe4xwaZ&amp;amp;n=W7S8wAkxXaN7tKH5Q61NDDNp3c4fyz&quot; rel=&quot;noreferrer&quot;&gt;mechanical properties&lt;/a&gt; of this foam are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;High Surface area to unit volume&lt;/li&gt;&#xA;&lt;li&gt;High strength to weight ratio&lt;/li&gt;&#xA;&lt;li&gt;Completely Isotropic load response&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The foam also takes on a lot of the properties of the alloy it is made from, namely:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Corrosion Resistance&lt;/li&gt;&#xA;&lt;li&gt;Electrical Conductivity&lt;/li&gt;&#xA;&lt;li&gt;Thermal Conductivity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The aluminium foam itself has a bubbly appearance much like &lt;a href=&quot;http://commons.wikimedia.org/wiki/File%3aClosed_cell_aluminium_foam_with_small_cell_size.JPG&quot; rel=&quot;noreferrer&quot;&gt;This Image of the aluminium foam&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Further Reading&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further reading can be found in &lt;a href=&quot;http://www.metalfoam.net/papers/simancik1997b.pdf&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; paper on its potential uses structurally.&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorDisplayName="user12" LastEditDate="2013-07-23T02:48:35.177" LastActivityDate="2013-07-23T02:48:35.177" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="630" PostTypeId="2" ParentId="626" CreationDate="2013-07-22T13:18:29.773" Score="4" Body="&lt;p&gt;There is a commonly used Aluminum Honeycomb design. The idea is, instead of using solid aluminum, sandwich an aluminum honeycomb design between two aluminum plates. This works well for structural purposes, provides a bit of flexibility, and reduces the weight. That might be what you are referring to. There is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Honeycomb_structure&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; that shows some common designs, including &lt;a href=&quot;https://en.wikipedia.org/wiki/File:CompositeSandwich.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SdJYB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-22T13:18:29.773" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="631" PostTypeId="2" ParentId="231" CreationDate="2013-07-22T13:38:27.170" Score="17" Body="&lt;p&gt;I just want to add that a lot of work goes into predictions of abundance, for objects including those we have not detected so far.  There is some similarity to exoplanets - where we know that a method has a detection bias.  If you can quantify the detection bias perfectly, then you can get the total abundance for different sizes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sdss.org/news/releases/20010605.edr.img9.html&quot; rel=&quot;noreferrer&quot;&gt;One source&lt;/a&gt; gives a pretty good idea.  Many references employ the D^-2.3 relationship, but it can occur as D^-1.3 as a cumulative metric.  It's interesting that smaller sizes have been found to follow a different pattern.  I strongly doubt that this pattern would continue to extremely small sizes, below D=0.1 km.  As a mathematical statement, integrating to zero diverges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0fxJn.gif&quot; alt=&quot;abundance with size&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I built some numbers for coefficients of the above relations with simple Excel calculations.  To give you an idea of how the count changes with size, I took two regions of the above pdf.  The region from 6 km to 1000 km contains on the order of 90,000 objects.  But you're also interested in small bodies, so I also integrated the D^-4 relationship from 0.1 km to 6 km.  That gives about 15,000,000,000 objects (15 billion).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you decreased the lower bound from 0.1 km to something smaller, the count would be even more (likely by orders of magnitude).  But we have no idea what relationship should hold there.  Notice that the error bars grow as you go smaller?  We don't have a good idea what the prevalence of small and microscopic bodies should be.  Go small enough and you're at molecular sizes, so you could get a measure of the particle density in the solar system.  But at that point, completely different factors are driving it (like atmospheric escape and solar wind), as opposed to the gravitational collection and breakup of bodies, which is what we think of as asteroids.  We can put some bounds on the numbers - because space probes apparently didn't get smashed by micrometeorites.  Regolith studies could also give some indication.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get particle density, take the number you like for number of objects and divide by some rough metric of the volume that defines the asteroid belt.  The uncertainty from that will probably be less than the count itself so I wouldn't worry too much about accuracy.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-07-22T13:38:27.170" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="632" PostTypeId="2" ParentId="17" CreationDate="2013-07-22T14:11:38.540" Score="21" Body="&lt;p&gt;Fission reactors can work just fine for space probes, and that will probably happen.  Projects are currently underway at US agencies to develop designs for this.  Notably, &lt;a href=&quot;http://www.lanl.gov/newsroom/news-releases/2012/November/11.26-space-travel.php&quot;&gt;Demonstration Using Flattop Fissions (DUFF)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why a fission reactor?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It is not highly radioactive at launch&lt;/li&gt;&#xA;&lt;li&gt;It can be compact&lt;/li&gt;&#xA;&lt;li&gt;It can have high power&lt;/li&gt;&#xA;&lt;li&gt;It's not subject to limited supply of fuel&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The assumption is that you would use enriched Uranium.  Such a reactor would probably use 20% enriched Uranium, because that is the boundary between officially weapons-grade material.  Although this material somewhat politically problematic, there are no health concerns until the reactor is turned on.  You could hold it in your hands completely safely, although they would never let you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next concern people have is &quot;what if it accidentally turns on?&quot;  This is why a space reactor will use control drums.  We have lots of research on nuclear safety for space reactors, because they've been considered for moon missions and all kinds of things, over many decades.  Control rods can be forced in the reactor if a crash happens, but control drums have to rotate, and there's no density difference to cause that.  They are locked in place until pretty far from Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the reactor fell into the ocean it's designed to not go critical.  You would race like hell to retrieve it, because you don't want any suspicious group to pick up a free nuclear reactor, and the IAEA tightly watches flows of nuclear materials around the globe.  If the thing was burnt up and strewn across a large area, that would be troubling, but no cancers will be caused.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A fission reactor in space can produce extremely high powers.  A space probe would use an extremely modest design, with low burnup (fuel efficiency) and passive cooling.  Even so, it will give more power than any RTG or solar array would.  You'll need a lot of shielding between the reactor and the probe itself, and there will be a good deal of physical separation between the parts.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-07-22T14:11:38.540" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="633" PostTypeId="1" AcceptedAnswerId="714" CreationDate="2013-07-22T14:25:58.393" Score="60" ViewCount="6617" Body="&lt;p&gt;Obviously asteroids are abundant in many of the raw materials we use today, but considering the (presumably) large expense involved in mining from asteroids it may not be worthwhile to mine some materials since it is more cost-effective to get them on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are mainly three issues here:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What is the approximate cost to develop an asteroid mining enterprise?&lt;/li&gt;&#xA;&lt;li&gt;What would the approximate on-going cost of operations be?&lt;/li&gt;&#xA;&lt;li&gt;What resources are available on asteroids that would be more cost-effective to mine that way (considering the above expenses) than to obtain using conventional methods?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I don't know if any of these issues have been addressed in a technical paper or discussion to this level of detail, but I would appreciate as much detail as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Another matter of consideration is &lt;strong&gt;when&lt;/strong&gt; we start developing the asteroid enterprise and &lt;strong&gt;how long&lt;/strong&gt; it would take to develop, because these things affect both the cost and benefit. Please take these into account in your answer.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit for clarification&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking primarily for asteroid mining of resources &lt;strong&gt;for use on Earth&lt;/strong&gt;, though an answer addressing asteroid mining of resources for use in space would also be interesting. Obviously the answer to the latter would be very different since it is probably &lt;strong&gt;more&lt;/strong&gt; worthwhile to mine resources in space for use in space.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="49" LastEditDate="2014-02-28T05:23:13.733" LastActivityDate="2023-10-18T01:59:43.583" Title="What resources could be gained from asteroid mining that would be worth the effort?" Tags="|asteroid|mining|natural-resources|space-industry|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="634" PostTypeId="1" AcceptedAnswerId="635" CreationDate="2013-07-22T14:51:09.183" Score="61" ViewCount="16858" Body="&lt;p&gt;Mars is a planet commonly brought up when talking about terraforming. Even if we were to manage to get past all economic barriers, and somehow form a thick atmosphere, would the lack of a magnetic field on Mars stop any attempt to terraform it? Would the atmosphere be destroyed by solar wind?&lt;/p&gt;&#xA;" OwnerUserId="6" LastEditorDisplayName="user12" LastEditDate="2013-08-10T17:42:21.387" LastActivityDate="2017-06-23T02:05:22.517" Title="Would terraforming Mars be possible?" Tags="|mars|magnetic-field|terraforming|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="635" PostTypeId="2" ParentId="634" CreationDate="2013-07-22T15:00:51.970" Score="43" Body="&lt;p&gt;Without a magnetic field any changes we do are seemingly temporary. &lt;a href=&quot;http://adsabs.harvard.edu/abs/1967JGR....72.5275S&quot;&gt;As we make atmosphere it will be torn away by double solar winds.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.popsci.com/environment/article/2010-03/sorry-terraformers-periodic-bursts-solar-radiation-destroy-martian-atmosphere&quot;&gt;Double solar winds&lt;/a&gt; are the worst, occurring about 15% of the time this occurs when a faster solar wave catches a slower and rolls into one bigger wave.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And these happen frequently, very frequently!&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;First Edberg and his colleagues identified 41 doubled solar particle waves and solar powerful particle waves from what are called coronal mass ejections from 2007 to 2008 detected by the Advanced Composition Explorer (ACE) spacecraft that monitors the space weather near Earth. Then they identified 36 of the same events hitting Mars in Mars Express data.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The thin atmosphere left on Mars is due to the small amounts protected at each pole by what remains of the magnetic fields, (which still exist at the poles).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I hear you asking, why does Venus have a thick atmosphere but Mars does not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is much smaller than earth and Venus and it is theorized (not definite) that its liquid outer core cooled and solidified, this had two effects on Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly it meant the large volcanoes became extinct, with no liquid core there was nothing left for them to eject. The volcanoes were responsible for the production of a large amount of the gasses released into the atmosphere, much like they were on Earth millions of years ago. Without these, there was nothing replenishing the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly it meant that the magnetic field slowly died off at all places but the poles, which still retain a small magnetic field. This meant that these double solar waves we mentioned before are now ripping whats left of the atmosphere off of mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sake of Venus, Venus no longer has a magnetic field, despite popular belief, though the solar winds have torn away all of the lighter hydrogen and oxygen layers. It is unknown to me at this time why the solar winds are unable to tear away the remaining gasses though I think this might be due to them being larger and heavier, such as the carbon dioxide and sulfuric acid layers that make up Venus atmosphere today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you could theoretically terra-form Mars by producing atmosphere quicker than its torn away, like the volcanoes tried to do, but that will be expensive and highly resource intensive and not really worth it compared to just sticking a dome there and filling that with air. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was once talk of liquidating the core with a thermo-nuclear explosion but again, were talking very expensive and difficult. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one could get the core moving again, presuming it has stopped and solidified, then the magnetic field should replenish itself, and an atmosphere would stay put when generated, but the risks and costs are insanely high compared to finding a planet that already has a magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lack of magnetic field wouldn't &lt;em&gt;stop&lt;/em&gt; our attempts, but it would &lt;em&gt;reverse&lt;/em&gt; them all over time due to these double solar waves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lack of field also wouldn't stop radiation, so any life, such as trees, would die very quickly when exposed to that un-absorbed level. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, terra-forming Mars is theoretically possible, but practically improbable. You are more likely to find bio domes on atmosphere-less planets until we find a simple way to regenerate the magnetic fields. &lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorDisplayName="user106" LastEditDate="2013-10-14T14:43:49.023" LastActivityDate="2013-10-14T14:43:49.023" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="636" PostTypeId="1" CreationDate="2013-07-22T15:57:36.590" Score="23" ViewCount="2420" Body="&lt;p&gt;It is well known that there are a number of negative health issues (loss of bone density, muscle mass, (eyesight degeneration&lt;sup&gt;1&lt;/sup&gt;, etc.) that can impair astronauts who spend long periods of time in space. That said, are there no positive effects on the human body at all?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Edit 1:&lt;/strong&gt; To address the comments, let's assume that the astronauts are eventually going to return to Earth.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Edit 2:&lt;/strong&gt; ... And hope to do it alive.&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;See &lt;a href=&quot;https://space.stackexchange.com/q/131/12102&quot;&gt;What are the effects of body building on heavy planets?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="12102" LastEditDate="2021-08-12T00:01:47.617" LastActivityDate="2022-06-02T23:47:30.207" Title="Does space flight have any positive effects on the human body?" Tags="|crewed-spaceflight|health|medical|tourism|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="637" PostTypeId="1" AcceptedAnswerId="649" CreationDate="2013-07-22T17:13:27.440" Score="30" ViewCount="5851" Body="&lt;p&gt;If sending a payload to GEO (geosynchronous orbit) or any other high orbit, you could start off launching to LEO (low Earth orbit), and then use a &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot;&gt;Hohmann transfer&lt;/a&gt; to raise the height of the orbit.  I think this is pretty much the standard of how it's done.  Hohmann transfers are the most efficient way to raise orbit, but that doesn't mean that going from ground-&gt;LEO-&gt;GEO is the most efficient compared to other options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do launches ever bypass LEO altogether?  For GEO, couldn't you just launch &lt;em&gt;straight&lt;/em&gt; to GEO?  So at an altitude of 300 km or so, you would be flying almost vertically.  Is this ever done or seriously proposed for real launches?  Would it be more or less efficient?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="25" LastEditDate="2013-07-22T17:37:36.463" LastActivityDate="2024-02-29T21:39:15.493" Title="Do any launches bypass LEO?" Tags="|launch|orbital-mechanics|low-earth-orbit|trajectory|geosynchronous|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="638" PostTypeId="2" ParentId="637" CreationDate="2013-07-22T17:36:30.490" Score="17" Body="&lt;p&gt;The common method for entering GEO orbit is to launch in what is known as a Geosynchronous Transfer Orbit (GTO), which has an apogee at GEO altitude, and a perigee of a few hundred km. Effectively all GEO missions insert their payloads into a GTO (and not into a LEO parking orbit as the OP suggests). There is no benefit of stopping in LEO first, as the only difference is that the GTO orbit has a much higher velocity at rocket burnout (it is this extra velocity which is translated into gravitational potential energy at apogee).&lt;/p&gt;&#xA;&lt;p&gt;The booster upper stage is separated from the payload at low-Earth altitudes and the satellites coasts to apogee where it uses on-board propulsion to circularize the GTO orbit into GEO, typically over the course of several orbits. Take a look at the &lt;a href=&quot;https://www.spacex.com/media/falcon-users-guide-2021-09.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9 user's&lt;/a&gt; guide, page 27, for how a mission to GTO looks. The main reasons to use this concept of operations are to gain the benefits of staging the upper stage rocket mass&lt;/p&gt;&#xA;&lt;p&gt;Launching straight to GEO is practically impossible — assuming that this would be defined as the rocket being responsible for separating a payload in GEO. The only feasible means of doing it would be to add an upper stage to the rocket that would accomplish the same function that the satellite propulsion does for GTO-to-GEO circularization. But since the sequence of events in the same this is a distinction without a difference where you've just renamed the satellite propulsion system as a part of the rocket. GEO orbits are at ~42,000 km, where as high LEO is ~1,000 km. You simply have to coast your way to GTO as it takes hours to get there (using anything remotely like currently available rocket technologies).&lt;/p&gt;&#xA;&lt;p&gt;It is worth mentioning that technically most rockets do enter LEO for some point in time, although they typically don't stay there for very long. I believe most Falcon 9 missions to GTO are in a very low &amp;quot;LEO&amp;quot; orbit for on the order of 20 minutes between first stage burns, for instance.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="881" LastEditDate="2024-02-29T21:39:15.493" LastActivityDate="2024-02-29T21:39:15.493" CommentCount="14" ContentLicense="CC BY-SA 4.0" />
  <row Id="639" PostTypeId="1" AcceptedAnswerId="657" CreationDate="2013-07-22T17:49:32.447" Score="4" ViewCount="669" Body="&lt;p&gt;I am an enthusiast of space exploration. &lt;strong&gt;Is it possible that strength of a human can be increased or decreased by keeping them in microgravity?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.huffingtonpost.com/2010/08/20/astronaut-strength-in-spa_n_689866.html&quot; rel=&quot;nofollow&quot;&gt;this link&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Astronauts can become as weak as 80-year-olds after six months at the International Space Station&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I imagine that gravity plays a main role in transforming strength. Please shed some light on my thoughts.&lt;/p&gt;&#xA;" OwnerUserId="82" LastEditorUserId="158" LastEditDate="2013-07-23T09:34:46.210" LastActivityDate="2013-07-23T18:27:45.280" Title="Does living in space affect humans' strength?" Tags="|gravity|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="641" PostTypeId="2" ParentId="618" CreationDate="2013-07-22T20:16:08.127" Score="11" Body="&lt;p&gt;While I do not have any special insight into the how the Rosetta flight profile was calculated, the general idea is as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, you start with an ephemeris model which contains the position data of all the bodies of interest over the time of your mission.&#xA;These values used to be tabulated for planetary bodies and published in almanacs but are now usually numerical models that may contain the descriptions of thousands of bodies.&#xA;As an example, the Jet Propulsion Laboratory (JPL) maintains an online ephemeris tool called HORIZONS which can be found &lt;a href=&quot;http://ssd.jpl.nasa.gov/?horizons&quot;&gt;here.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you know where everything is, or is going to be, you need to calculate your flight path.&#xA;At this point, your problem is now a combination of orbital mechanics and multi-variable optimization.&#xA;As far as I know, there is no standard program used in the calculation of these mission profiles.  In fact, this is an area of active research.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the orbital mechanics of &lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/missiongravityassistprimer/&quot;&gt;gravity assist&lt;/a&gt; are well understood so the active research focuses on the different optimization methods to find the best mission profile.&#xA;Since you mentioned the Rosetta mission, the European Space Agency (ESA) has a pair of papers (&lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/MAD/ACT-RPR-MAD-2009-EvolutionaryNeurocontrol.pdf&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/MAD/pub/ACT-RPR-MAD-2007-%28JOGO%29SearchSpacePruningAndGlobalOptimisation.pdf&quot;&gt;here&lt;/a&gt;) available on its website on the topic.&#xA;The introduction to the first paper, in particular, explains both the &quot;traditional&quot; approach and outlines some of the improved optimization techniques currently being investigated.&lt;/p&gt;&#xA;" OwnerUserId="167" LastActivityDate="2013-07-22T20:16:08.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="642" PostTypeId="2" ParentId="637" CreationDate="2013-07-22T21:12:49.490" Score="11" Body="&lt;p&gt;The reason one doesn't normally launch to GEO is that of fuel requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A hohman transfer orbit is usually the lowest energy method to achieve a particular orbit other than LEO. Boosting to LEO, then using a hohman transfer orbit saves fuel, and thus launcher mass. And, since orbital maneuvering is in upper stages, typically, reducing the upper stage mass reduces launch costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been non-LEO launches; mostly, they have been non-earth-orbit trajectories.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's worth noting that even the moon shots have generally used an Earth Orbital staging point; this allows for system checks prior to boosting for the moon. Four of the Ranger probes failed in parking orbit prior to lunar boost.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-07-22T21:12:49.490" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="643" PostTypeId="5" CreationDate="2013-07-22T21:54:30.417" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-22T21:54:30.417" LastActivityDate="2013-07-22T21:54:30.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="644" PostTypeId="4" CreationDate="2013-07-22T21:54:30.417" Score="0" Body="The Space Shuttle was a crewed, partially reusable low Earth orbital spacecraft operated by NASA from 1981 to 2011." OwnerUserId="6" LastEditorUserId="6" LastEditDate="2013-07-23T01:34:04.407" LastActivityDate="2013-07-23T01:34:04.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="645" PostTypeId="2" ParentId="604" CreationDate="2013-07-22T23:53:58.090" Score="11" Body="&lt;p&gt;&lt;a href=&quot;http://spacebiosciences.arc.nasa.gov/staff/tore-straume&quot;&gt;Dr. Tore Straume&lt;/a&gt; of NASA's BioSciences Division actually published a paper on this very topic. Titled: &quot;&lt;a href=&quot;http://journalofcosmology.com/Mars124.html&quot;&gt;Radiation Hazards and the Colonization of Mars: Brain, Body, Pregnancy, In-Utero Development, Cardio, Cancer, Degeneration&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The study conducted by them concluded that the Space radiation negatively affects Male fertility. The Cosmic Radiation during travel is supposed to so high that it can significantly decrease the male sperm count and they would also sterilize human fetus. The only solution to that seems to be better shielding from cosmic radiation. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The critical health effect (most radiosensitive) for human colonization of Mars may turn out to be infertility in women resulting from radiation exposure in utero. Although direct human data are not available for this effect, studies in non-human primates have found that oocytes are extremely radiosensitive during gestation, i.e., 50% killed following only 0.07 Sv of chronic tritium beta rays (similar in biological effectiveness to x rays and GCR protons). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also another obstacle or problem seems to be the effect of the lack of gravity on the bone development process. &lt;/p&gt;&#xA;" OwnerUserId="70" LastActivityDate="2013-07-22T23:53:58.090" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="646" PostTypeId="1" CreationDate="2013-07-23T01:03:32.103" Score="113" ViewCount="64525" Body="&lt;p&gt;I used to be familiar with the various choices out there for orbital mechanics simulation software.  Alas, those days are gone.  What are the choices today, preferably sorted by platform?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/637/do-any-launches-bypass-leo/&quot;&gt;This question&lt;/a&gt; and others on this site might benefit from their use in making answers more visual.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Please note&lt;/strong&gt; that list questions such as this are typically discouraged on SE sites. An exception was made in this case due to its obvious utility and relevance.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="63" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2023-06-07T21:43:16.167" Title="What are the choices today for orbital mechanics simulation software?" Tags="|orbital-mechanics|software|astrodynamics|" AnswerCount="13" CommentCount="4" CommunityOwnedDate="2013-08-22T21:02:33.717" ContentLicense="CC BY-SA 3.0" />
  <row Id="647" PostTypeId="2" ParentId="646" CreationDate="2013-07-23T01:05:45.910" Score="29" Body="&lt;p&gt;Here are the options I'm aware of off the top of my head:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.agi.com/products/stk/modules/default.aspx/id/stk-free&quot;&gt;AGI/STK (Systems Toolkit)&lt;/a&gt; — PC, free&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://orsa.sourceforge.net/&quot;&gt;Orbit Reconstruction, Simulation and Analysis (ORSA)&lt;/a&gt; — Linux/Mac/PC, free (last version 2011-02-17)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="63" LastEditorUserId="1332" LastEditDate="2013-12-27T14:17:12.993" LastActivityDate="2013-12-27T14:17:12.993" CommentCount="8" CommunityOwnedDate="2013-08-22T21:02:33.717" ContentLicense="CC BY-SA 3.0" />
  <row Id="648" PostTypeId="2" ParentId="646" CreationDate="2013-07-23T01:16:20.463" Score="58" Body="&lt;p&gt;To add to @Erik's list:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://software.nasa.gov/software/GSC-17177-1&quot; rel=&quot;noreferrer&quot;&gt;GMAT&lt;/a&gt; — Cross platform, Free. NASA open source product.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://ai-solutions.com/freeflyer/&quot; rel=&quot;noreferrer&quot;&gt;FreeFlyer&lt;/a&gt; — PC, commercial. Probably AGI's biggest competitor.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;http://jat.sourceforge.net/&quot; rel=&quot;noreferrer&quot;&gt;Java Astrodynamics Toolkit&lt;/a&gt; — Cross platform, free. Another open source product, more of a software library than a full-fledged simulation environment.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://fiveforfiveapps.com/index.php/orbit-designer&quot; rel=&quot;noreferrer&quot;&gt;Orbit designer&lt;/a&gt; — Android, free. Not even close to the same ballpark of these other packages, but might be a fun way to play around with different orbits. &lt;strong&gt;Edit:&lt;/strong&gt; I actually just downloaded this and I'm absolutely hooked. Highly recommended. (Caveat: I'm a nerd for things like this, and it may in fact be a rather boring app for most people).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="28040" LastEditDate="2018-11-12T18:40:03.330" LastActivityDate="2018-11-12T18:40:03.330" CommentCount="3" CommunityOwnedDate="2013-08-22T21:02:33.717" ContentLicense="CC BY-SA 4.0" />
  <row Id="649" PostTypeId="2" ParentId="637" CreationDate="2013-07-23T01:26:10.747" Score="21" Body="&lt;p&gt;To attempt to answer this question a little more directly, yes, there have been launches which bypass a LEO parking orbit. According to &lt;a href=&quot;http://www.esa.int/esapub/bulletin/bullet103/biesbroek103.pdf&quot;&gt;this&lt;/a&gt; paper, Luna-2 launched almost directly into a lunar insertion orbit (although intervening coast segments may have been &quot;LEO&quot;, &lt;em&gt;per se&lt;/em&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: &lt;a href=&quot;http://www.zarya.info/Diaries/Luna/Luna.php&quot;&gt;Zarya&lt;/a&gt; states that Luna-1, Luna-2, and Luna-3 &lt;em&gt;all&lt;/em&gt; launched into direct lunar transfers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As others have mentioned, though, there are several good reasons to launch into a LEO parking orbit, and that's what the majority of super-LEO missions do.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-07-23T01:35:34.250" LastActivityDate="2013-07-23T01:35:34.250" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="650" PostTypeId="1" AcceptedAnswerId="663" CreationDate="2013-07-23T01:52:53.217" Score="13" ViewCount="324" Body="&lt;p&gt;Say I operate a satellite in LEO, and I think I have instruments on board that will be sensitive to &lt;a href=&quot;http://www.swpc.noaa.gov/SWN/&quot;&gt;space weather&lt;/a&gt; events. If I keep an eye on space weather activity levels, when do I start turning off these sensitive instruments? How often should I expect to take action like this? Finally, how dependent is all of this on my orbit altitude?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sake of being specific, let's say I operate &lt;a href=&quot;http://cloudsat.atmos.colostate.edu/&quot;&gt;CloudSat&lt;/a&gt;, a member of the so-called &quot;&lt;a href=&quot;http://www.nasa.gov/mission_pages/a-train/a-train.html&quot;&gt;A-Train&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-07-23T02:54:49.290" LastActivityDate="2021-09-02T16:28:36.630" Title="If I am a spacecraft operator, at what space weather activity levels do I start getting nervous?" Tags="|radiation|space-weather|operations|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="651" PostTypeId="2" ParentId="637" CreationDate="2013-07-23T02:58:09.210" Score="6" Body="&lt;p&gt;You can't launch straight to Geostationary Orbit (GEO) as you can't get the perigee high enough without a burn at geosynchronous altitude. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Proton_%28rocket_family%29&quot; rel=&quot;nofollow&quot;&gt;Proton&lt;/a&gt; has done that, though with a Low Earth Orbit (LEO) parking orbit along the way. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5&quot; rel=&quot;nofollow&quot;&gt;Ariane 5&lt;/a&gt; normally launches directly into Geostationary Transfer Orbit (GTO) &amp;mdash; 250 km perigee and (close to) synchronous apogee. It has finished all its activity by about 27 minutes after liftoff. &lt;/p&gt;&#xA;" OwnerUserId="176" LastEditorUserId="49" LastEditDate="2014-10-20T13:55:31.720" LastActivityDate="2014-10-20T13:55:31.720" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="652" PostTypeId="2" ParentId="646" CreationDate="2013-07-23T08:40:09.487" Score="27" Body="&lt;p&gt;Apart from these serious software mentioned above there is an interesting game with quite realistic orbital calculations, quite suitable for teaching kids about space:&#xA;&lt;a href=&quot;https://kerbalspaceprogram.com/&quot;&gt;Kerbal space program&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for AGI non-free version is a lot more powerful.&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorUserId="1332" LastEditDate="2013-12-27T14:17:25.417" LastActivityDate="2013-12-27T14:17:25.417" CommentCount="7" CommunityOwnedDate="2013-08-22T21:02:33.717" ContentLicense="CC BY-SA 3.0" />
  <row Id="653" PostTypeId="1" AcceptedAnswerId="659" CreationDate="2013-07-23T09:26:43.720" Score="62" ViewCount="6355" Body="&lt;p&gt;Currently, the ISS is resupplied with fresh clothes from the Earth. I remember reading a report on NTRS stating that the requirements for fresh clothes proved to be much higher than expected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, several related questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Will it be feasible to put a washing machine into the ISS?&lt;/li&gt;&#xA;&lt;li&gt;What kinds of challenges are there to be overcome (vibration isolation, energy budget, water disposal, contamination etc.)? Preferably with breakdown into types of COTS washers most suited for space operations.&lt;/li&gt;&#xA;&lt;li&gt;Will the benefits outweigh the costs for LEO operations (impact on resupply budgets, clothes wear and tear, smell etc.)?&lt;/li&gt;&#xA;&lt;li&gt;What about interplanetary missions (Mars, asteroids)? How could washers contribute to getting job done at lower costs?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="51" LastEditDate="2014-04-20T11:20:17.770" LastActivityDate="2022-06-05T18:59:47.783" Title="Washing clothes in space: are there any technical challenges?" Tags="|iss|life-support|long-duration|astronaut-lifestyle|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="654" PostTypeId="2" ParentId="636" CreationDate="2013-07-23T09:29:33.720" Score="15" Body="&lt;p&gt;There have been no known long-term positive health effects reported.&lt;/p&gt;&#xA;&lt;p&gt;Then again, euphoria and elation are considered negative health effects, and both have been reported, and many other causes are used as recreational stress relief.&lt;/p&gt;&#xA;&lt;p&gt;Almost all astronauts report positive mental attitude changes from their time in orbit. This isn't exactly measurable, tho', and I've not seen any well done studies done on Astronauts who flew versus those who didn't on various psychological effects. †&lt;/p&gt;&#xA;&lt;p&gt;Short term, ISTR reading that blood pressure generally drops. And there is a study reporting that blood pressure and heart rate drop while in microgravity. ①. This does, however, appear to be maladaptive for return, as it also is accompanied by cardiac atrophy.&lt;/p&gt;&#xA;&lt;h1&gt;Into the Realm of Wild Speculation.&lt;/h1&gt;&#xA;&lt;p&gt;This reduction could be of use for certain individuals at grave risk of stroke or heart attack from hypertension; the reduction in blood pressure and heart rate is known to be a direct reduction in stroke and heart attack risks. The problem being that current launch protocols involve a highly stressful period of increased workload for launch, which is counter-indicated by the very condition which would benefit from the reduction. ‡&lt;/p&gt;&#xA;&lt;p&gt;Certain forms of depression might benefit from spaceflight and microgravity. The profound &amp;quot;life changing experience&amp;quot; reported by almost all astronauts may be of benefit, but until it is more affordable, it's unlikely to be tested.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;p&gt;① Microgravity decreases heart rate and arterial pressure in humans.&lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/8964756&quot; rel=&quot;noreferrer&quot;&gt;www.ncbi.nlm.nih.gov/pubmed/8964756&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h1&gt;Digressions&lt;/h1&gt;&#xA;&lt;p&gt;† The problem being that doing the study is laden with confounding factors and a lack of a viable control group. You'd need a control group who were selectable for astronaut training, but told from the outset they are never going to get to go... Which would be an interesting study in and of itself, but would be impractical financially, might be unethical, and certainly still wouldn't be entirely free of confounding factors.&lt;/p&gt;&#xA;&lt;p&gt;‡ If some form of gravitic lift is developed, then it's quite likely that this could be of use; it's equally likely that any such lift would render spaceflight unneeded for the same benefit, as any such system would likely be able to generate local microgravity conditions.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-23T09:29:33.720" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="655" PostTypeId="2" ParentId="24" CreationDate="2013-07-23T10:14:43.780" Score="3" Body="&lt;p&gt;Solar is feasible for at least as far as Jupiter; NASA &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Solar_panels&quot; rel=&quot;nofollow&quot;&gt;is using it&lt;/a&gt; for &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;the Juno Mission&lt;/a&gt;. Solar panels are heavy compared to equivalent-power RTGs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fission reactors of several different kinds are also practical, but due to political concerns (and treaties) are not used by any space agencies to date. They too are heavy by comparison to RTGs, but lighter than Solar, in the typical configurations. They also have shorter lifespans, but much higher power output. (And, in theory, once expended, could have an RTG thermocouple system module which then turns them into an RTG.) The Uranium is not &quot;safe&quot;, but isn't itself toxic other than its radioactive properties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radio-Thermal Generators (RTGs) have the advantage of having no moving parts, and being extremely light. There is almost no chance of failure that doesn't involve mechanical damage. They last for decades (the Pioneer and Voyager probes are all still operating †). The drawback is that the plutonium used is itself highly toxic, as well as radioactive. While there have been failures, the normal launch protocols have resulted in those failures not having significant population-affecting effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Versus propellants, the question is almost irrelevant. The total propellant used will be used pretty much no matter the weight of the on-probe power. This is because of (1) the various stages being general-use designs, (2) the normative design mode being to design a mission for the maximum payload capacity of the launcher for the trajectory desired, and (3) the desire to pack as much science on board as one can. Thus, the power source choice usually affects only what instrumentation is aboard, not how much fuel gets used; the exception being when it results in a mission being either scrapped as unworkable or having to be turned into multiple missions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;† Pioneer 10 was still broadcasting as of 2003; the broadcast antenna is one of the more high-power instruments aboard, and there isn't enough electricity for it to continue to operate the radio. But it's still got SOME power...  over 30 years in flight and still under power. Just not enough to hit the 57W needed to operate the 8W transceiver.  &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2001/ast03may_1/&quot; rel=&quot;nofollow&quot;&gt;http://science1.nasa.gov/science-news/science-at-nasa/2001/ast03may_1/&lt;/a&gt; and &lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?showtopic=2362&quot; rel=&quot;nofollow&quot;&gt;http://www.unmannedspaceflight.com/index.php?showtopic=2362&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="415" LastEditDate="2014-10-15T13:13:59.783" LastActivityDate="2014-10-15T13:13:59.783" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="656" PostTypeId="1" CreationDate="2013-07-23T10:29:07.390" Score="1" ViewCount="2257" Body="&lt;p&gt;Are there night photos from curiosity or early of Mars's skies?&lt;/p&gt;&#xA;&lt;p&gt;Two moon on the sky. Photos from ground. From rovers.&lt;/p&gt;&#xA;&lt;p&gt;Perhaps not exactly like this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cVEUi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cVEUim.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;but more like this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/o1aTv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/o1aTvm.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;but at night and two moons.&lt;/p&gt;&#xA;" OwnerUserId="178" LastEditorUserId="27627" LastEditDate="2020-12-14T15:10:19.253" LastActivityDate="2020-12-14T16:04:53.383" Title="Are there photos taken by rovers, with both moons visible in the martian night sky?" Tags="|mars|rovers|photography|moons|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="657" PostTypeId="2" ParentId="639" CreationDate="2013-07-23T10:29:12.900" Score="7" Body="&lt;p&gt;When you are first in the space station having gone straight from earth you will &lt;em&gt;feel&lt;/em&gt; stronger. All of the items still have mass but they have negligible weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without constant exercise and strain though, your body will begin to break down your muscles and bones. You aren't using them as much as before because of the reduced gravity, so your body 'recycles' the materials used to make muscle and bone into other things it needs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is part of the cause of the pains some astronauts feel after extended periods out of gravity, though some of this is just through word of mouth than through scientific evidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Returning to Earth is a painful ordeal for some who didn't exercise enough as the extra stress on the body due to gravity no longer has the previous muscle and bone mass to support your own weight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the strength isn't increased in microgravity, but without excessive use it will be decreased over time.&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="80" LastEditDate="2013-07-23T18:27:45.280" LastActivityDate="2013-07-23T18:27:45.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="658" PostTypeId="1" AcceptedAnswerId="662" CreationDate="2013-07-23T10:50:44.490" Score="8" ViewCount="2081" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/questions/623/how-far-from-the-sun-can-solar-power-be-used-as-a-reliable-energy-source&quot;&gt;this question&lt;/a&gt; we know that solar cells be problematic the farther you get from the sun.  We know that solar sails provide viable propulsion far from the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be practical to use a solar windmill to generate enough energy to power a space station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that at least two mills turning on opposite directions would be required for stability. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2018-04-20T12:06:40.433" Title="Would a solar windmill in space be practical?" Tags="|energy|space-station|solar-wind|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="659" PostTypeId="2" ParentId="653" CreationDate="2013-07-23T10:59:04.527" Score="50" Body="&lt;h1&gt;Methods of cleaning&lt;/h1&gt;&#xA;&lt;p&gt;Current washing technologies are mostly solvent based. Most normal &amp;quot;soiling&amp;quot; of clothing is a mixture of oils and salts (Both of which are products of sweat), and sloughed skin cells, often bound by those same oils. When one finds clothing irritable from extended wear, it's usually due to the effects of accumulated sweat and skin cells, and the bacteria that eat them.&lt;/p&gt;&#xA;&lt;p&gt;Normal washing uses a mix of water (as a solvent) and a surfactant to bind oil to the water for more complete cleaning. The agitation is mostly to move the water and surfactant mix through the garment and its fibers.&lt;/p&gt;&#xA;&lt;p&gt;Dry Cleaning uses any of a number of chemical solvents to remove the various oils, salts, bacterial and skin cells.&lt;/p&gt;&#xA;&lt;p&gt;It is possible to use abrasive processes to remove skin and oil accumulations; it won't generally remove them from between fibers, but &amp;quot;sand-washed&amp;quot; is a viable option for extending the wearability of a garment short term, at the expense of long term damage to the garment.&lt;/p&gt;&#xA;&lt;p&gt;Air-washing is also possible; the use of high pressure and temperature air can remove large portions of the cells and bacterial colonies. It can remove some of the salts and oils, as well.&lt;/p&gt;&#xA;&lt;p&gt;Evaporative cleaning can also be used; this can be used to remove oils, but won't remove particulates between the fibers.&lt;/p&gt;&#xA;&lt;p&gt;Thermal degradation is used for certain forms of cleaning. That is to say, you heat it until the biologicals break down. It's not generally conducive to clothing, but in combination with other techniques, can be used for enhancement of other methods.&lt;/p&gt;&#xA;&lt;h1&gt;Why not use?&lt;/h1&gt;&#xA;&lt;p&gt;Water? It's present. In theory, astronauts could rinse garments with extant systemry. The thing is, water and other fluids are not easily handled in microgravity (as the wringing a washcloth video shows). It's certainly useable for limited applications. It's best used for removing salts; salt accumulation is primarily an artifact of sweat evaporation in a high heat environment, and is almost a non-issue in space. Plus, water is a limited quantity.&lt;/p&gt;&#xA;&lt;p&gt;Surfactant? Most laundry surfactants are contact irritants. A spill in station would be a major hazard. Further, water used with them requires different purification from normal. Some specific ones could be used, but again, the environmental risks due to breathing in the material make it a risk, as does the increased risks to the water purification.&lt;/p&gt;&#xA;&lt;p&gt;Dry Cleaning? Almost all the chemicals are highly toxic.&lt;/p&gt;&#xA;&lt;p&gt;Air-washing? Noisy, and requires particulate capture aboard station.  Also, it's not highly effective.&lt;/p&gt;&#xA;&lt;p&gt;Evaporative? Slow, doesn't remove particulates (including bacteria and skin cells, as well as crystalline salt). Plus, anything removed now has to be scrubbed from the atmosphere. But at least, given enough time, the bacteria starve.&lt;/p&gt;&#xA;&lt;p&gt;Thermal Degradation is a heating issue - both for the garment and for the habitat. Heat management is a major part of spacecraft design.&lt;/p&gt;&#xA;&lt;h1&gt;If you're going to anyway...&lt;/h1&gt;&#xA;&lt;p&gt;A combined pressurized steam to recapture system, possibly followed by vacuum drying, might be a reasonable system.&lt;/p&gt;&#xA;&lt;p&gt;Use of pressurized steam gets maximum use of both thermal and air washing, plus removes some salts and oils from even inside the fiber bundles of the fabric, for a minimum amount of water. It's also a reasonably small system; a tractor feed unit could be created that washes a shirt at a time with a tractor feed. It also is of limited hazard to crew.&lt;/p&gt;&#xA;&lt;p&gt;Adding vacuum drying on a warm surface would maximize volitalization of remaining liquids.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-13T00:34:21.087" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="660" PostTypeId="1" CreationDate="2013-07-23T11:01:29.817" Score="7" ViewCount="950" Body="&lt;p&gt;Orbital stations going to 3rd generation. China starting its own program. Russians MIR (world) was flooded in the ocean. Humanity has lived in space for half of century. But there is still no moon base. What is the main problem of moon's base?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My vision of moon's base lay in more logically constructive way than earth's orbit base. There may be placed unbordered number of space modules. Move everything what need to be by some orbital catapult. Thereafter, all of this modules may stay on the moon for ages. No needs in normalization of orbit , no needs in fuel. Just sun batteries and portable nuclear generators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And no needs to flooded that station as it was with Russians MIR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the principle difference with trip to orbit or trip to moon? The radius of moon is 1.7 thousand meters and the radius of earth is 6 thousand meters. The range from earth to moon is 384.5 thousand meters. And there is no atmosphere in this 384 thousand meters, just little gravity maneuver and earth's force of gravity self move you to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to suggest that there is less need for fuel for this maneuver, than to get to the earth orbit, just precision equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity rover mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Need less than order resources for setup the same station without chassis on the moon. There may be 10 such stations with full monitoring of moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 3 orbital stations, may be one on the moon instead.&lt;/p&gt;&#xA;" OwnerUserId="178" LastEditorUserId="6816" LastEditDate="2017-01-03T23:25:32.053" LastActivityDate="2017-08-04T14:35:01.923" Title="What is the main problem of Luna's base? Why is there a 3rd generation of orbital bases and no Luna base?" Tags="|the-moon|colonization|" AnswerCount="2" CommentCount="5" ClosedDate="2013-07-23T19:06:27.190" ContentLicense="CC BY-SA 3.0" />
  <row Id="661" PostTypeId="2" ParentId="656" CreationDate="2013-07-23T11:02:58.777" Score="14" Body="&lt;p&gt;Yes. And I found it on &lt;a href=&quot;https://en.wikipedia.org/wiki/Spirit_(rover)#Astronomy&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;. Available on &lt;a href=&quot;https://mars.nasa.gov/mer/gallery/press/spirit/20050909a.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Images&lt;/a&gt; as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eTCXL.png&quot; alt=&quot;Phobos &amp;amp; Deimos as seen by the rover Spirit.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Image feature: Phobos &amp;amp; Deimos as seen by the rover Spirit.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="28148" LastEditDate="2018-11-30T09:21:43.253" LastActivityDate="2018-11-30T09:21:43.253" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="662" PostTypeId="2" ParentId="658" CreationDate="2013-07-23T11:08:03.193" Score="19" Body="&lt;p&gt;Not really. The pressure due to solar wind is approximately 1-6 nPa (See &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_wind#Solar_wind_pressure&quot;&gt;Wikipedia&lt;/a&gt;). The pressure due to just the light from the Sun at Earth is actually higher, at around 9 nPa (&lt;a href=&quot;http://en.wikipedia.org/wiki/Radiation_pressure#In_interplanetary_space&quot;&gt;Source&lt;/a&gt;). The mass makes somewhat of a difference, but let's just assume somehow you can get a mass of a 1 square meter windmill at 1 gram. The effective power would be very small, far less than a Watt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a 35% efficiency with solar panels, which is achievable, the amount of power that 1 square meter of solar cells could generate would be about 450 W. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then there's the issues that the pressure would tend to push your device further away from the Sun with each passing moment, and that would be very difficult to correct for. Not to mention that this would take away from the effectivity even more, as you would essentially be moving with the solar wind, thus not allowing it to power your station. Bottom line, it's an interesting idea, but just won't work.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-24T20:44:33.777" LastActivityDate="2013-07-24T20:44:33.777" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="663" PostTypeId="2" ParentId="650" CreationDate="2013-07-23T11:52:04.947" Score="12" Body="&lt;p&gt;To boil this down to a simple answer is difficult, but here's some of the factors that are required to make this assessment.&lt;/p&gt;&#xA;&lt;p&gt;First of all, the question comes, what is your satellite is specified to. When you build a satellite, you specify a level of radiation tolerance you desire. If you have more than that level of radiation, then you have to start taking special precautions. That level depends very much if you are in LEO, GEO, or worse, an elliptical orbit. The requirement should take care of you for your day to day operations, but probably won't take care of you for the really extreme stuff (Unless you are willing to pay for it).&lt;/p&gt;&#xA;&lt;p&gt;So, what are some typical requirements for different orbits? One example of the typical radiation levels can be found from &lt;a href=&quot;http://space-env.esa.int/index.php/radiation-level-survey.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;, which has several graphs like the following:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rT2m4.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Suffice to say, if you have anything much larger than the normal levels, you will start to worry about it. You can take a look at space weather today from &lt;a href=&quot;http://www.swpc.noaa.gov/today.html#satenv&quot; rel=&quot;nofollow noreferrer&quot;&gt;NOAA&lt;/a&gt;, and see how the radiation events look. Or look at some of the &lt;a href=&quot;http://www.swpc.noaa.gov/ftpmenu/plots/proton.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;archives&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;In reality, this is often somewhat of a trial and error. If you notice something bad starting to happen during one solar storm, then you might look at the parameters and set limits based off of that. Or you might simply end up ultra conservative. Look through historical data, and decide how often it is acceptable to be off the air. For many communications, defense, and other uses, it's just simply not acceptable to be off the air ever, some, like special telescopes, limited outage might be preferred to the added expense of shielding.&lt;/p&gt;&#xA;&lt;p&gt;There are also metrics which show that you aren't having a problem yet, but you might be experiencing higher levels of radiation than is healthy for your system. The best indicator is some idea as to the number of times your Error Detection and Correction algorithms are firing off. If you see a spike in them, it might indicate that you have a radiation problem that you need to deal with.&lt;/p&gt;&#xA;&lt;p&gt;A number I've often found to be useful is looking at the proton flux, and stating if it's over 10 cm&lt;sup&gt;-2&lt;/sup&gt;s&lt;sup&gt;-2&lt;/sup&gt;sr&lt;sup&gt;-1&lt;/sup&gt;, for LEO. I'm not very familiar with eclipse/ GEO, but I do know the satellites experience more radiation overall. Thus, they probably have more protection, and overall are better off.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2021-09-02T16:28:36.630" LastActivityDate="2021-09-02T16:28:36.630" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="664" PostTypeId="2" ParentId="660" CreationDate="2013-07-23T12:10:00.297" Score="9" Body="&lt;p&gt;MIR: Flooded is the wrong term. It was &lt;a href=&quot;http://www.orbiterwiki.org/wiki/Basic_orbit_maneuvers#Deorbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;de-orbited&lt;/a&gt; and fell into the ocean. At the time, it was found to be &lt;a href=&quot;http://en.wikipedia.org/wiki/Mir#Final_days_and_deorbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;damaged beyond repair&lt;/a&gt;. So it was a natural decision for avoiding any future &lt;a href=&quot;http://en.wikipedia.org/wiki/2009_satellite_collision&quot; rel=&quot;nofollow noreferrer&quot;&gt;collisions&lt;/a&gt; for preventing the generation of more &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_debris&quot; rel=&quot;nofollow noreferrer&quot;&gt;space debris&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current advantage of Earth's orbit is that you can easily conduct research in &lt;a href=&quot;http://en.wikipedia.org/wiki/Micro-g_environment&quot; rel=&quot;nofollow noreferrer&quot;&gt;microgravity&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon is certainly an interesting place to go, but much further away. It takes a lot more energy to carry any payload or module to the Moon. Compared to that, you first need to &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;go into an orbit around Earth&lt;/a&gt;. You then &lt;a href=&quot;http://en.wikipedia.org/wiki/Trans-lunar_injection&quot; rel=&quot;nofollow noreferrer&quot;&gt;leave this orbit and Earth&lt;/a&gt; and Earth's field of gravity (almost) and fly towards the Moon. Then you need to &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbit_insertion&quot; rel=&quot;nofollow noreferrer&quot;&gt;brake again to come into a Lunar orbit&lt;/a&gt;. Then you need to &lt;a href=&quot;http://www.orbiterwiki.org/wiki/Basic_orbit_maneuvers#Deorbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;brake another time&lt;/a&gt; to land in powered flight. It is not about precision really, it is about fuel / energy. The whole process takes a lot. Therefore it is a hell of a lot more expensive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Building modules and even space ships from Lunar material has been discussed, but it is still a far way to go, both from an engineering point of view and again the required funding - and it would require launching the machinery to do it (more mass to launch from Earth). The necessary materials however do exist on the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is, what you want to do on the Moon. Right now, most interesting is its exploration. On the contrary, it is the wrong place to conduct microgravity research. Microgravity is just interesting to a lot of people right now. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="19916" LastEditDate="2017-08-04T14:35:01.923" LastActivityDate="2017-08-04T14:35:01.923" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="665" PostTypeId="2" ParentId="512" CreationDate="2013-07-23T12:32:45.930" Score="19" Body="&lt;p&gt;Popular Mechanics had a neat article about it last year. Bottom line, nobody knows. We will lose communication with the probes at some point in the next 10-15 years because the fuel supply will run out. The probes are powered by nuclear reactors and scientists expect them to be depleted sometime in the 2020's. They have been proactively shutting down systems in order to conserve fuel and extend the life of the probes, but there is no hard and fast date they expect to lose communications. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.popularmechanics.com/science/space/deep/how-much-longer-will-we-talk-to-the-voyagers-11479518&quot;&gt;Voyager 1&amp;amp;2&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="169" LastActivityDate="2013-07-23T12:32:45.930" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="666" PostTypeId="2" ParentId="660" CreationDate="2013-07-23T12:35:44.960" Score="20" Body="&lt;h1&gt;Distance&lt;/h1&gt;&#xA;&lt;p&gt;The moon is much further away. Just a hair shy of 385000km, versus a peak of about 450km for the planned LEO stations.&lt;/p&gt;&#xA;&lt;p&gt;Distance within a gravity well is a major issue; the moon is still inside Earth's gravity well, roughly 0.0027 meters per second per second...&lt;/p&gt;&#xA;&lt;p&gt;This means having to hit a velocity high enough to not stop until gravity of the moon captures it. That means more fuel per unit payload. That means bigger launchers and/or more launches.&lt;/p&gt;&#xA;&lt;h1&gt;The ISS - an LEO Station.&lt;/h1&gt;&#xA;&lt;p&gt;The ISS was assembled from over three dozen missions's efforts; over 2 dozen carried significant superstructure units. It can house at best a dozen people. It's not self sufficient. It is one of the most expensive habitation units ever built by mankind, and per capita, probably the single most expensive.&lt;/p&gt;&#xA;&lt;p&gt;Oh, and it's about 450 tons. ①&lt;/p&gt;&#xA;&lt;h1&gt;So what portion of LEO mass can we expect for Delivery to the moon?&lt;/h1&gt;&#xA;&lt;p&gt;About 1/10th. Here's the math...&lt;/p&gt;&#xA;&lt;p&gt;Let's look at the Saturn V. It was capable of delivering 118 tons to LEO, but only 47 tons to translunar orbit insertion (and the payload has to stop itself at the moon, at that).② The Apollo-Saturn missions were essentially 6 stages. The S-1C first stage, the S-II second stage, the S-IVB third stage, the Service Module as 4th stage, and the Lunar module is two (descent and ascent). Each of which is a rocket. Without the SM, figure the SIVB would launch even less to the moon. The actual payload to the moon was the command module and the lunar module; the service module is actually essential rocketry as well as essential support for the CM.&lt;/p&gt;&#xA;&lt;p&gt;So... let's assume 7500kg the CM mass is the 4th stage to the moon, and calculate the mass to the moon of useful lunar payload.&lt;/p&gt;&#xA;&lt;p&gt;Loaded Masses:&#xA;LM Mass: 14,696 kg &lt;code&gt;    &lt;/code&gt; ③&lt;BR&gt;&#xA;SM Mass: 24,523 kg &lt;code&gt;    &lt;/code&gt; ④&lt;BR&gt;&#xA;CM Mass: 5,806 kg &lt;code&gt;      &lt;/code&gt; ⑤&lt;br&gt;&#xA;Remove: -7500 kg (Fuel, engines on SM, estimate)&lt;br&gt;&#xA;Total: 37525 kg to lunar orbit.&lt;/p&gt;&#xA;&lt;p&gt;The Descent stage is about 10,500 kg ⑥, of that roughly 14,700 LM, or about 71% descent stage. So, using the same ratio upon that 37525... we get about 10700kg to surface... from a Saturn V. About 1/10 the payload to the moon as to LEO&lt;/p&gt;&#xA;&lt;p&gt;The best current heavy lift launchers only get 23 tons to LEO; the Falcon Heavy is supposed to get 53 tons to leo. ⑦ Which means a mere 5 tons to the moon per launch.&lt;/p&gt;&#xA;&lt;p&gt;Which means MANY launches to get a useful payload to a lunar base. Even assuming a railgun return launcher, it's  going to be an insanely expensive project to build a moonbase with current launcher technology.&lt;/p&gt;&#xA;&lt;p&gt;Which leads to the other issue. Return Mass. Figure an LM/CM combination unit for every crewing. It's just within reach of the Falcon Heavy... It's 5 tons, and two men, and not happy. And not carrying supplies. That's a second Falcon Heavy.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;Conclusions&lt;/h1&gt;&#xA;&lt;p&gt;Without a mega-rocket like the Saturn V, it's just NOT practical to consider a station. And the political considerations of the Carter Administration basically sealed the doom of manned lunar spaceflight.&lt;/p&gt;&#xA;&lt;p&gt;Note that SpaceX founder Elon Musk has stated categorically his goal is a manned colony on Mars in his lifetime. He's got the money, the brain-power, and the will. And the heaviest working launch vehicle in the approval process. The question becomes, &amp;quot;Can he get a working colonial craft into orbit, equipped, and staffed?&amp;quot; We can expect the Falcon 9 Heavy will not be the final stage in his ultra-heavy lift needs.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;p&gt;&lt;strong&gt;①&lt;/strong&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/International_Space_Station&lt;/a&gt; Wikipedia — International Space Station.&#xA;Didn't need an exact figure, so Wiki's good enough.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/structure/iss_assembly.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/mission_pages/station/structure/iss_assembly.html&lt;/a&gt; NASA ISS Assembly Missions.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;②&lt;/strong&gt;&#xA;&lt;a href=&quot;http://www.astronautix.com/lvs/saturnv.htm#more&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/lvs/saturnv.htm#more&lt;/a&gt; Encyclopedia Astronautica - Saturn V&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;LEO Payload: 118,000 kg (260,000 lb) to a 185 km orbit at 28.00 degrees. Payload: 47,000 kg (103,000 lb) to a translunar trajectory. Success Rate: 100.00%. Launch data is: complete. Development Cost &lt;span class=&quot;math-container&quot;&gt;$: 7,439.600 million. Launch Price $&lt;/span&gt;: 431.000 million in 1967 dollars in 1966 dollars. &lt;br&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;③&lt;/strong&gt;&#xA;&lt;a href=&quot;http://www.astronautix.com/craft/apollolm.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/craft/apollolm.htm&lt;/a&gt; Encyclopedia Astronautica - Lunar Module&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;④&lt;/strong&gt;&#xA;&lt;a href=&quot;http://www.astronautix.com/craft/apollosm.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/craft/apollosm.htm&lt;/a&gt; Encyclopedia Astronautica - Service Module&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;⑤&lt;/strong&gt;&#xA;&lt;a href=&quot;http://www.astronautix.com/craft/apollocm.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/craft/apollocm.htm&lt;/a&gt; Encyclopedia Astronautica - Command Module&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;⑥&lt;/strong&gt;&#xA;&lt;a href=&quot;http://www.braeunig.us/space/specs/lm.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.braeunig.us/space/specs/lm.htm&lt;/a&gt; Lunar Module&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;⑦&lt;/strong&gt;&#xA;&lt;a href=&quot;http://www.spacex.com/falcon_heavy.php&quot; rel=&quot;noreferrer&quot;&gt;http://www.spacex.com/falcon_heavy.php&lt;/a&gt; SpaceX Falcon Heavy page&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-23T12:35:44.960" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="667" PostTypeId="1" AcceptedAnswerId="676" CreationDate="2013-07-23T12:49:55.943" Score="15" ViewCount="1113" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Grasshopper_%28rocket%29&quot;&gt;SpaceX Grasshopper&lt;/a&gt; vehicle (formally a VTVL vehicle) tests have been increasing the height that their tests can climb to before doing the vertical landing.  Throughout all these tests it keeps upright.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in practice, operating as a rocket stage, this vehicle will fall from a substantial height.  Before it gets close to the ground, would it keep vertical as it falls, or would it tumble, or fall oriented horizontally?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="25" LastEditDate="2013-07-23T16:26:27.323" LastActivityDate="2015-01-28T14:12:08.533" Title="Vertical-takeoff, vertical-landing (VTVL) vehicle falling orientation" Tags="|rockets|spacex|reuse|landing|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="668" PostTypeId="1" AcceptedAnswerId="670" CreationDate="2013-07-23T13:09:01.083" Score="39" ViewCount="8692" Body="&lt;p&gt;On Earth, there are plenty of examples of vehicles that roam difficult terrain having tracks instead of wheels, while all (at least those I am aware of) extraterrestrial rovers have wheels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a special reason (like reliability) why wheels are chosen instead of tracks?&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorUserId="1142" LastEditDate="2017-01-26T14:34:20.393" LastActivityDate="2022-06-06T00:56:43.593" Title="Wheels vs. tracks for rovers" Tags="|design|rovers|movement|" AnswerCount="8" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="669" PostTypeId="2" ParentId="668" CreationDate="2013-07-23T14:15:36.723" Score="23" Body="&lt;p&gt;A similar question was asked on the &lt;a href=&quot;https://robotics.stackexchange.com/questions/558/why-do-mars-rovers-designers-prefer-wheels-over-tracks&quot;&gt;Robotics SE&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wheels provide a lot of flexibility, like with the &lt;a href=&quot;https://robotics.stackexchange.com/questions/1235/how-does-rocker-bogie-keep-the-body-almost-flat/1241#1241&quot;&gt;rocker bogie system&lt;/a&gt;. where the rover can climb over obstacles up to twice the diameter of the wheels&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And Tracks are usually heavier than wheels. Making it more expensive for deployment . It's also easier to maneuver with wheels than rely on the skid turning of the tracks. &lt;/p&gt;&#xA;" OwnerUserId="70" LastEditorUserId="-1" LastEditDate="2017-04-13T12:49:18.033" LastActivityDate="2013-07-23T14:15:36.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="670" PostTypeId="2" ParentId="668" CreationDate="2013-07-23T14:36:18.693" Score="35" Body="&lt;p&gt;Tracks are heavy, high-friction, and primarily useful in soft/muddy/slippery terrain where the weight distribution is essential to prevent sinking and slipping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They take much more energy to move than wheels, and while on Earth-bound robots that's not so much of a problem, on a lander this is to be or not to be of the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They weigh quite a bit. I can't imagine sensible tracks that wouldn't be at least several kilograms in the case of rubber, much more in the case of metal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as long as we don't plan missions to places that are &lt;em&gt;swampy&lt;/em&gt;, six-wheeled design is already sufficient for when one wheel loses traction or encounters obstacle higer than its axis. The only scenario when that wouldn't suffice and tracks are superior is when the whole rover starts sinking in the soil. On Earth, where soil can be damp and loose, moved by erosion and rains tracks make sense for heavyweight, multi-ton vehicles. They make sense for much smaller vehicles like snowmobiles for material as loose as snow. But so far, we haven't discovered a planet (other than our own) where wheels would be so inferior to tracks to grant being replaced, despite the problems with tracks.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="84" LastEditDate="2013-07-24T04:00:30.790" LastActivityDate="2013-07-24T04:00:30.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="671" PostTypeId="2" ParentId="667" CreationDate="2013-07-23T14:48:52.260" Score="7" Body="&lt;p&gt;This is not meant to be cheeky, but if I understand your question correctly, then the answer seems to be contained in the name of the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;VTVL: Vertical Takeoff &lt;strong&gt;Vertical Landing&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think that even if the Grasshopper had no active flight controls, a large gyroscope would suffice to keep it vertical throughout flight, but the article makes it clear that it does have active flight controls. The Wikipedia article mentioned that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;...a third flight in December&#xA;2012 of 29 seconds duration, with extended hover under rocket engine&#xA;power, in which it ascended to an altitude of 40 metres (130 ft)&#xA;before &lt;strong&gt;descending under rocket power to come to a successful&#xA;vertical landing&lt;/strong&gt;.&quot;&lt;/li&gt;&#xA;&lt;li&gt;CEO Musk said in November 2012: &quot;Over the next few months, we’ll&#xA;gradually increase the altitude and speed. ... I do think there&#xA;probably will be some craters along the way; we’ll be very lucky if&#xA;there are no craters. &lt;strong&gt;Vertical landing is an extremely important&#xA;breakthrough...&lt;/strong&gt;&quot;&lt;/li&gt;&#xA;&lt;li&gt;During a Phase 1 test, the Grasshopper RLV would be launched and&#xA;ascend to 240 feet AGL and &lt;strong&gt;then throttle down in order to descend&lt;/strong&gt;,&#xA;landing back on the pad approximately 45 seconds after liftoff.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So in short, I think the answer to your question is: &quot;vertical.&quot;&lt;/p&gt;&#xA;" OwnerUserId="108" LastEditorDisplayName="user12" LastEditDate="2013-07-23T16:29:54.057" LastActivityDate="2013-07-23T16:29:54.057" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="672" PostTypeId="2" ParentId="668" CreationDate="2013-07-23T15:21:07.207" Score="21" Body="&lt;p&gt;From a logistical perspective, solid wheels have a significant benefit over tracks: maintenance. Tracks lower the ground pressure of the vehicle to something on the order of human foot; the trade off for that is twofold:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;You still have &amp;quot;wheels&amp;quot; inside the track, usually lots of them, and they each have hubs and bearings and potential places to fail.  Additionally, there are flex points (or one big one for rubber tracks) all of which are just another point of potential failure.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Tracked vehicles turn, by dragging one track, in soft or muddy terrain they very easily fill with &amp;quot;ground&amp;quot;.  Bad things happen when the tracks get full, either the track breaks or it falls off the vehicle; in either case, it takes significant resources to return to functionable, none of which would be available for a lone vehicle.  On side slopes, even moving forward without turning can fill the track, leading to failure.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="109" LastEditorUserId="47149" LastEditDate="2022-06-06T00:56:43.593" LastActivityDate="2022-06-06T00:56:43.593" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="675" PostTypeId="2" ParentId="668" CreationDate="2013-07-23T16:24:30.583" Score="4" Body="&lt;p&gt;Another point is that 6 wheeled robots can still move when one or more individual wheels' motors fail (okay not as elegantly), however, a tracked vehicle is doomed at this point.&lt;/p&gt;&#xA;" OwnerUserId="186" LastActivityDate="2013-07-23T16:24:30.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="676" PostTypeId="2" ParentId="667" CreationDate="2013-07-23T16:26:12.023" Score="9" Body="&lt;p&gt;As the concept video originally released shows, the Grasshopper System will be able to stabilize itself, and land vertically. No doubt there will be some loss of complete control in places, but in the end, the spacecraft must be controlled, and will land vertically. See &lt;a href=&quot;http://youtu.be/sSF81yjVbJE&quot; rel=&quot;nofollow&quot;&gt;the YouTube video&lt;/a&gt; below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/sSF81yjVbJE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This makes sense, for a lot of reasons. The primary reason is that the rocket is more stable in a vertical configuration than a horizontal, allowing for more accurate control. No doubt some re-orientation will occur, but as a whole, the system will work better if operating in a vertical configuration.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-01-28T14:12:08.533" LastActivityDate="2015-01-28T14:12:08.533" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="677" PostTypeId="2" ParentId="668" CreationDate="2013-07-23T17:13:05.407" Score="4" Body="&lt;p&gt;In addition to weight as other people have mentioned, tracked vehicles are extremely maintenance intensive relative to wheeled variants.  When the nearest mechanic is hundreds of thousands to tens of millions of miles away, not breaking or wearing out under use is a critical requirement.&lt;/p&gt;&#xA;" OwnerUserId="157" LastActivityDate="2013-07-23T17:13:05.407" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="678" PostTypeId="2" ParentId="668" CreationDate="2013-07-23T18:33:21.980" Score="6" Body="&lt;p&gt;I should actually add that there is a possible third option. A 4 or 6 legged walker can probably work well on any terrain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A possible disadvantage is that it could be maintenance heavy.&lt;/p&gt;&#xA;" OwnerUserId="193" LastActivityDate="2013-07-23T18:33:21.980" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="679" PostTypeId="1" CreationDate="2013-07-23T18:44:24.303" Score="4" ViewCount="128" Body="&lt;p&gt;Gathering and delivering resources from Earth is ultimately too expensive and unsustainable for space exploration in the long term. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Emphasis has been made on extracting resources while in space, such as establishing a space elevator for launching cargo and people, a lunar or orbital production base for making products like structural elements, fuel, food, and other things you need for long term operations. Modern computers and networking technology makes it possible to do much of the work by remote control. So it's possible to have robots to work preparing for when the humans arrive, and to assist us once we get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which supply chain burdens that need to be addressed in the next 30 years for more accessible private and commercial space travel? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What technologies are being developed to address these hurdles? Which have the most potential?&lt;/p&gt;&#xA;" OwnerUserId="44" LastEditorUserId="44" LastEditDate="2013-07-24T19:48:01.967" LastActivityDate="2013-07-24T19:48:01.967" Title="What technologies are being developed to ease the supply chain burden for space travel?" Tags="|technology|supply|robotics|" AnswerCount="0" CommentCount="6" ClosedDate="2013-07-24T23:09:55.810" ContentLicense="CC BY-SA 3.0" />
  <row Id="680" PostTypeId="1" CreationDate="2013-07-23T19:37:31.867" Score="8" ViewCount="1071" Body="&lt;p&gt;Does anyone know of any missions planned or in progress that are searching in the very vacuum of space for the complex organic molecules that are key indicators of life (such as DNA, RNA, etc.), or perhaps even life itself (as currently defined; ie. one cell or multi-cellular), or even lifelike things such as viruses?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know there are several missions in search of life or evidence of life on celestial bodies (eg. Mars), but I'm talking about just in the &quot;emptiness&quot; of space itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm envisioning some kind of vehicle with a solar-sail for propulsion and perhaps also some kind of sensors built into these vast areas of the sails that also looks for the presence of certain biochemical molecules or cells that may have come into contact with the sails and stuck there as the vessel swept through space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That kind of a discovery would seem to me to represent a potentially major change in humanity's search for extra-terrestrial life.&lt;/p&gt;&#xA;" OwnerUserId="108" LastEditorUserId="25" LastEditDate="2013-07-23T20:59:39.813" LastActivityDate="2013-08-02T06:34:29.103" Title="DNA or even life in vacuum of space" Tags="|life|astrobiology|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="681" PostTypeId="2" ParentId="653" CreationDate="2013-07-23T20:19:12.920" Score="14" Body="&lt;p&gt;Good question, and this is not really an answer to your question I know, but it's too long for a comment. My reaction to reading it was (perhaps strangely) to wonder about even the necessity of clothes in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that on Earth, we wear clothes for a number of reasons (some practical, others more cultural and traditional than any purely objective, rational reason):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;to protect our fragile bodies from weather (in space, aside from&#xA;EVA, we should be able to absolutely control the weather so as to&#xA;make it suitable for whatever needs or preferences we have)&lt;/li&gt;&#xA;&lt;li&gt;to protect our fragile bodies from getting caught in hazardous&#xA;machinery (clothes would probably be required in space for this&#xA;reason just as they are on Earth, but if this is the &lt;em&gt;only&lt;/em&gt; reason&#xA;to wear clothes in space, then it seems to me that we might be able&#xA;to do with a minimum of clothes—maybe spandex shorts for males and&#xA;a scarf of some kind for anyone with long hair—and thus minimize the need for washing them)&lt;/li&gt;&#xA;&lt;li&gt;to protect our modesty (this seems to be strictly cultural as&#xA;several Earthbound cultures such as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Zo%27%C3%A9_people&quot;&gt;Zo'é people&lt;/a&gt; have&#xA;largely done away with clothing; we humans have shown ourselves to&#xA;be one of the most adaptable species on Earth, so it seems to me&#xA;that we ought to be able to adapt to space with regards to clothing&#xA;too)&lt;/li&gt;&#xA;&lt;li&gt;Decorative (see modesty)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If the technical challenges of washing clothes in space are very demanding, then perhaps we should seriously consider creating a space-faring culture that has little or no need for intra-vehicular clothing. Extra-vehicular clothing would obviously still be required, but the high risk of EVA would seem to make for strong justification to minimize them, thus further minimizing the need for washing clothes in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, great question; very thought-provoking.&lt;/p&gt;&#xA;" OwnerUserId="108" LastActivityDate="2013-07-23T20:19:12.920" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="682" PostTypeId="2" ParentId="680" CreationDate="2013-07-23T20:24:29.490" Score="5" Body="&lt;p&gt;I don't know of any projects currently planned to find such materials in the &quot;vacuum&quot; of space - the problem with this is that due to space being inherently huge, you're lucky to pick up any molecules at all, let alone the complex organic ones. You also get the issue that if you find such a molecule, then what would it really tell you about where it may have originated from? If we pick up molecules from solid bodies (not necessarily just planets, but meteors / comets) then we've got a starting point we can use to make assertions about its origin, whereas such a molecule collected in deep space wouldn't really tell us a great deal (it could have separated from pretty much any comet, at any time, anywhere.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this isn't to say that only planets are useful in this regard - comets and other such bodies are all good things to investigate. NASA is working towards harvesting material from comets more easily in the not too distant future with structures such as the &lt;a href=&quot;http://www.nasa.gov/topics/solarsystem/features/comet-harpoon.html&quot; rel=&quot;nofollow&quot;&gt;comet harpoon&lt;/a&gt; - something like this should, statistically speaking, prove far more fruitful in its approach.&lt;/p&gt;&#xA;" OwnerUserId="19" LastEditorUserId="108" LastEditDate="2013-08-01T02:40:04.030" LastActivityDate="2013-08-01T02:40:04.030" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="683" PostTypeId="1" AcceptedAnswerId="708" CreationDate="2013-07-23T21:01:44.210" Score="16" ViewCount="1731" Body="&lt;p&gt;With more and more people going into space in future it is probable that crimes will be committed sooner or later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who will have jurisdiction on prosecuting such crimes?&#xA;And who will be able to define what is the crime (let's assume that theft and violence against other person will be universally regarded as criminal behaviour, but there are lot of cases when some actions are regarded as crime in one society and are perfectly legal in other societies.&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorUserId="63" LastEditDate="2013-07-24T00:38:51.657" LastActivityDate="2013-07-24T11:48:37.507" Title="Jurisdiction over crime in space" Tags="|law|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="684" PostTypeId="1" CreationDate="2013-07-23T21:13:54.970" Score="10" ViewCount="1453" Body="&lt;p&gt;I think that other than in a star, nobody has achieved any fusion process with a net positive energy output.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But reading about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot;&gt;Bussard ramjet&lt;/a&gt; a few minutes ago, it seems that this might be theoretically possible given adequate speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anyone calculated the minimum velocity necessary for a Bussard ramjet to be able to achieve hydrogen fusion with a net positive energy output?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some assumptions would obviously be necessary (average hydrogen density et. al.), but this seems like it would be a fun calculation. Though it would probably require at least days for me to try.&lt;/p&gt;&#xA;" OwnerUserId="108" LastEditorUserId="49" LastEditDate="2013-08-09T01:40:50.283" LastActivityDate="2013-08-22T15:45:05.817" Title="Minimum velocity to achieve fusion with Bussard ramjet" Tags="|propulsion|interstellar-travel|ramjet|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="685" PostTypeId="2" ParentId="683" CreationDate="2013-07-23T21:14:38.603" Score="1" Body="&lt;p&gt;No one will have jurisdiction since there is no sovereign territory in space.  It is only when you return to someone's sovereign territory that you enter someone's jurisdiction.  This gives rise to the occupation known as bounty hunting...  This is no different than how it was when there was unclaimed terrestrial territory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is important to note that your right to self-defense is a natural right and not granted by any sovereign nation.  So take some guns with you just in case.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-23T21:14:38.603" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="686" PostTypeId="2" ParentId="683" CreationDate="2013-07-23T21:29:20.507" Score="0" Body="&lt;p&gt;Because space is not the sovereign territory of any particular country the jurisdiction likely falls to the UN and by extension the International Criminal Court.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some cases, such as murder, countries claim sovereignty over their citizens outside there territory; so a US citizen would be subject to US laws.  Also space exploration requires space ships and countries would likely try to exercise their laws over ships flagged in their country much like seafaring vessels.&lt;/p&gt;&#xA;" OwnerUserId="203" LastActivityDate="2013-07-23T21:29:20.507" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="687" PostTypeId="2" ParentId="683" CreationDate="2013-07-23T22:01:49.100" Score="7" Body="&lt;p&gt;Nobody has Jurisdiction, because there is no governing body.  Historically Captains have full power of command/judicial proceedings in/on their ships.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Jurisdiction&quot;&gt;Wikipedia&lt;/a&gt; &#xA;&lt;em&gt;Jurisdiction (from the Latin ius, iuris meaning &quot;law&quot; and dicere meaning &quot;to speak&quot;) is the practical authority granted to a formally constituted legal body or to a political leader to deal with and make pronouncements on legal matters and, by implication, to administer justice within a defined area of responsibility. The term is also used to denote the geographical area or subject-matter to which such authority applies.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-07-23T22:01:49.100" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="688" PostTypeId="2" ParentId="683" CreationDate="2013-07-23T22:06:25.887" Score="3" Body="&lt;p&gt;In theory, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon_Treaty&quot; rel=&quot;nofollow&quot;&gt;Moon Treaty&lt;/a&gt; turns everything over to international country, however, none of the spacefaring countries ratified the treaty. Thus, it is unlikely that this will be the case. There are two likely possibilities which we can predict now, based off of two real world examples.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most likely, the laws will most likely follow the same laws as &lt;a href=&quot;http://people.howstuffworks.com/cruise-ship-law.htm&quot; rel=&quot;nofollow&quot;&gt;shipping&lt;/a&gt;, and international airplanes, namely, that the host country of the ship is responsible for the laws of the ship, and the people are somewhat subject to their native country. Of course, these are untested, as there hasn't been any private enterprise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second possibility is that the rules of the International Space Station will be followed. &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/International_Space_Station/International_Space_Station_legal_framework&quot; rel=&quot;nofollow&quot;&gt;ESA&lt;/a&gt; has a great write up on this, a few quotes of interest:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Intergovernmental Agreement allows the Space Station Partners&#xA;  States to extend their national jurisdiction in outer space, so the&#xA;  elements they provide (e.g. laboratories) are assimilated to the&#xA;  territories of the Partners States.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The basic rule is that 'each partner shall retain jurisdiction and&#xA;  control over the elements it registers and over personnel in or on the&#xA;  Space Station who are its nationals' (Article 5 of the&#xA;  Intergovernmental Agreement).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can read the complete agreement from the &lt;a href=&quot;http://www.state.gov/documents/organization/107683.pdf&quot; rel=&quot;nofollow&quot;&gt;US State Department&lt;/a&gt;, take a look at Article 5.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-23T22:31:17.027" LastActivityDate="2013-07-23T22:31:17.027" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="690" PostTypeId="1" AcceptedAnswerId="701" CreationDate="2013-07-24T02:37:44.540" Score="20" ViewCount="2870" Body="&lt;p&gt;Can physical activity from within a space craft affect it somehow? For example if somebody on the ISS performs repairs involving a lot of hammering, would it perhaps affect the station's orientation a little bit? The vibrations can't be a good thing at any rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, how about the act of simply moving around inside the craft?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Are any changes substantial enough to require the ISS or other craft to correct them every now and then?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="80" LastEditDate="2013-07-24T05:57:40.407" LastActivityDate="2013-07-25T17:44:40.143" Title="How does astronaut activity from within affect a spacecraft?" Tags="|iss|space-station|attitude|physical-activity|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="691" PostTypeId="2" ParentId="370" CreationDate="2013-07-24T03:15:25.547" Score="43" Body="&lt;p&gt;No, because there's nothing like water for a keel to work against.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In water sailing there are two force vectors&lt;/strong&gt;, the vector from the reaction of the wind against the sail, and the vector from the keel and rudder against the water. These vectors add together to propel the sailboat.  This works for almost any direction on the compass except where the wind is coming from, or about 45 degrees either side of that.  &quot;Beating close to the wind&quot; as @geoffc mentions, is traveling up against that limit.  &quot;Tacking&quot; is zigzagging - going for a while 45 degrees clockwise from the wind, and then for a while 45 degrees counterclockwise.  Working your way upwind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In solar sailing there's just one force vector&lt;/strong&gt;, because there's only one medium, the solar photons, as @Craig mentions.  With one vector the direction limits become more than 90 degrees to either side of where the sunlight is coming from.  &lt;strong&gt;So no zigging, no zagging, using anything like tacking.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now maybe gravity can be used as another vector via orbital mechanics as @Pearson and @SF mention, and maybe one day this will be called &quot;tacking&quot; but the physics are wholly different than a keel through water.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;UPDATE, &lt;a href=&quot;https://www.economist.com/news/science-and-technology/21729424-electric-solar-wind-sail-brings-high-hopes-new-way-propelling-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;September 23rd 2017 issue of The Economist, p 73&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In some ways an &lt;strong&gt;E-sail&lt;/strong&gt; [made of 20km-long tethers with a positive charge, repelling solar wind particles] resembles a solar sail, a rival idea for powering craft cheaply through space. A solar sail provides propulsion because the sunlight it reflects exerts pressure on the sail, pushing it forward. But E-sails have an important advantage over solar sails. Once unfurled, there is no easy way to stop a craft with a solar sail gathering speed. An E-sail-powered craft can be prevented from accelerating simply by switching off its electron gun. This means it can return to Earth under the influence of the sun's gravity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="213" LastEditorUserId="213" LastEditDate="2018-12-30T03:22:28.617" LastActivityDate="2018-12-30T03:22:28.617" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="693" PostTypeId="5" CreationDate="2013-07-24T03:54:24.100" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-24T03:54:24.100" LastActivityDate="2013-07-24T03:54:24.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="694" PostTypeId="4" CreationDate="2013-07-24T03:54:24.100" Score="0" Body="Questions regarding small points in space where there is infinite density, with an event horizon past which the gravitational forces are so great even light cannot escape." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-27T02:01:01.130" LastActivityDate="2013-08-27T02:01:01.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="695" PostTypeId="5" CreationDate="2013-07-24T03:55:49.667" Score="0" Body="&lt;p&gt;Questions about devices that collect information, usually in the electromagnetic spectrum (Infra red, Visible light, Ultra violet, Gamma Ray, X-Ray) about distant objects with the goal of better discerning the objects.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-08T20:19:22.280" LastActivityDate="2013-08-08T20:19:22.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="696" PostTypeId="4" CreationDate="2013-07-24T03:55:49.667" Score="0" Body="Telescopes are devices that collect data (usually electromagnetic spectrum, visible or otherwise) to better resolve distant objects." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-08T20:19:08.310" LastActivityDate="2013-08-08T20:19:08.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="697" PostTypeId="5" CreationDate="2013-07-24T05:53:09.807" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-24T05:53:09.807" LastActivityDate="2013-07-24T05:53:09.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="698" PostTypeId="4" CreationDate="2013-07-24T05:53:09.807" Score="0" Body="Questions regarding the systematic study of the world through observation and experimentation." OwnerUserId="-1" LastEditorDisplayName="user12" LastEditDate="2013-10-26T17:54:25.977" LastActivityDate="2013-10-26T17:54:25.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="699" PostTypeId="5" CreationDate="2013-07-24T05:56:42.837" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-24T05:56:42.837" LastActivityDate="2013-07-24T05:56:42.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="700" PostTypeId="4" CreationDate="2013-07-24T05:56:42.837" Score="0" Body="Questions regarding the path that a spacecraft will take due to forces acting on the spacecraft, such as gravitational or radiation from the sun.&#xD;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-13T15:19:26.697" LastActivityDate="2013-08-13T15:19:26.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="701" PostTypeId="2" ParentId="690" CreationDate="2013-07-24T08:19:44.297" Score="20" Body="&lt;p&gt;&lt;a href=&quot;http://youtu.be/S_p7LiyOUx0?t=26s&quot;&gt;This video&lt;/a&gt; may help to answer your question. Starting at about 00:24, you can see an astronaut running around the &quot;exercise wheel&quot; of Skylab (an early NASA space station program, which followed the Apollo moon landings). Basically after some time, NASA told the astronauts to stop running around like this because it was causing more propellants to be used to maintain Skylab's correct attitude (orientation) in space (at least this is what I've been told...it would be nice to find a reference to back this claim up).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyways, the short answer is yes, an astronaut's physical activity within a spacecraft can absolutely affect its orientation and even its orbit. As to whether it is a large or small effect, this depends on the amount and type of activity and (most importantly) the size of the spacecraft. Even on the ISS, however, astronauts are instructed to avoid or even stop altogether some activities because they can have an adverse affect on the amount of propellants used for attitude control (I have some memory of such instructions during my time working in the ISS Program). Another issue is that an astronaut's activities can affect the microgravity experiments being conducted on the ISS. These are typically conducted over a specified period of time, so an astronaut may be instructed to avoid motion in a certain part of the ISS for a certain period of time, in order not to disturb a particular experiment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not really a simple matter of analyzing starting and ending velocities of an astronaut and assuming that the net change is zero. If you think of it in terms of kinetic energy, an astronaut is all the time transferring small amounts of kinetic energy to his or her spacecraft (the energy being converted from chemical bonds in the food he or she eats into kinetic energy from the work his or her muscles do). Changes in kinetic energy, of course, imply a change in velocity [E = (0.5) m * v^2]. On average, these velocity changes most probably cancel each other out, if they are not applied in a consistent direction. This is why there is no large overall change in a spacecraft's orbit due to these motions. However, the motions can absolutely cause small changes in the orientation of the spacecraft, and these are most often controlled by an active attitude control system (usually small rockets that fire in small bursts to counter unwanted changes in attitude).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will add, however, that these effects (i.e. astronaut motion) are probably not the largest source of attitude disturbance on a spacecraft. I'm not really an attitude control expert, so I can't tell you the relative magnitude of the different disturbances, but one of the largest (for low Early-orbit spacecraft) is actually aerodynamic drag, caused by the thin layer of atmosphere present at such altitudes.&lt;/p&gt;&#xA;" OwnerUserId="219" LastEditorUserId="25" LastEditDate="2013-07-25T17:44:40.143" LastActivityDate="2013-07-25T17:44:40.143" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="702" PostTypeId="2" ParentId="690" CreationDate="2013-07-24T08:36:29.240" Score="8" Body="&lt;p&gt;Internal activity, other than activation of the drives, will have minimal effects.&lt;/p&gt;&#xA;&lt;h1&gt;Attitude&lt;/h1&gt;&#xA;&lt;p&gt;If everyone goes into one round area, and starts running the walls, the walls will (slowly) start to rotate the opposite direction; this may not be exact, due to the station wanting to rotate upon its own center of mass, but it will attempt to rotate as close as it can to opposite.&lt;/p&gt;&#xA;&lt;h1&gt;Thermal Radiation Acceleration.&lt;/h1&gt;&#xA;&lt;p&gt;If action internal generates significant localized heat, the radiation of that heat from the nearby exterior walls will provide a very tiny thrust. For comparison, the asymmetric thermal loads on the Pioneer probes were sufficient to generate 400km/year off-coast position differences;  that's 8e-10 m/s² (0.8 nanometers per second per second).&lt;/p&gt;&#xA;&lt;p&gt;Note that this force is dwarfed in the inner system by other interactions, but is a real effect that crew could cause.&lt;/p&gt;&#xA;&lt;h1&gt;Kinetic linear Acceleration&lt;/h1&gt;&#xA;&lt;p&gt;Any kinetic acceleration from kicking off one side is canceled by impact with the other side, save for the very very tiny amount converted to heat by friction with internal atmosphere. But, to reset for additive, you're knocking about the same amount off in the other direction, for a long-term net of 0 acceleration (excepting asymmetric thermal loading).&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-24T08:36:29.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="703" PostTypeId="2" ParentId="511" CreationDate="2013-07-24T09:14:53.400" Score="11" Body="&lt;p&gt;Science Fiction has shown several interesting possibilities for surviving the reentry, most notably, either a suit that's got a high thermal load, or an ablative shield that one rides down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Science fact has an even more interesting possibility: the shuttlecock mode. Inspired by a badminton shuttlecock, Scaled Composites uses it as the reentry mode for the SS1 and SS2 spacecraft; SS1 rose to a level where atmosphere was no longer useful to affect attitude of the craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A system of extensible vanes could be used to generate a shuttlecock drogue; a high expansion foam or gas into rolled tubing could generate a nice large drogue effect, and keep friction levels from reaching thermal danger to the suited astro-parachutist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issue is one of not entering at a speed sufficient to damage the drogue and/or astro-parachutist.¹  And that's a de-orbit issue. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, the Aerobraking inflatable shield exemplified in A.C. Clark's &lt;em&gt;2010: Odyssey 2&lt;/em&gt; is from an actual proposal to NASA (by Clark, if I remember correctly). &lt;a href=&quot;http://www.space.com/16695-nasa-launches-hypersonic-inflatable-heat-shield.html&quot; rel=&quot;noreferrer&quot;&gt;NASA finally got around to testing the idea in 2012&lt;/a&gt;... IRVE-3 passed initial tests about a year ago - July, 2012.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A combination of an inflatable shield for the high speed portion², and then shuttlecock drogues after slowing enough to not be injured by atmosphere itself, and finally a parachute for final landing could make a jump from LEO or even GTO survivable. Whether or not the rig is of a practical weight as an escape system is as yet dubious, but the technology does exist.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;¹: Noting that speed, in this case, is purely relative to the atmosphere. Orbital velocity is about 7.8 km/sec for low earth orbit; surface speed at the equator is about 0.46 km/sec. So that's a considerable large amount of speed to shed: about 7.3 km/sec. &lt;br&gt;Also note: Kittinger and Baumgartner had a near-zero relative velocity due to use of a lighter than air vehicle. Any speed below about 0.1 km/s is a non-issue - 360 kph isn't too much of a problem, and the drogue can handle well more than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;²: That's the point while still above surface speed, yet below orbital speed.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2013-08-26T05:46:55.027" LastActivityDate="2013-08-26T05:46:55.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="704" PostTypeId="1" AcceptedAnswerId="706" CreationDate="2013-07-24T09:21:03.403" Score="17" ViewCount="981" Body="&lt;p&gt;We had &lt;a href=&quot;https://space.stackexchange.com/questions/690/how-does-astronaut-activity-from-within-affect-a-spacecraft&quot;&gt;this question&lt;/a&gt; which discussed if space craft had to be corrected in their orbits from the movements from the people inside. (No as their net movement cancels each force out)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Following on from that, the &lt;a href=&quot;http://www.slashgear.com/spacex-dragon-detached-from-iss-to-return-home-31231051/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Dragon capsule was detached&lt;/a&gt; last year and moved away from the ISS by a mechanical arm. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the course of the ISS have to be corrected to account for the force exerted on itself by the mechanical arm when pushing the SpaceX Dragon capsule away, and it's now reduced weight due to the loss of the capsule and the old equipment it took with it?&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-10-11T06:56:33.830" Title="Did the ISS have to adjust its course when the SpaceX Dragon capsule was detached?" Tags="|iss|orbital-mechanics|spacex|dragon|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="705" PostTypeId="1" AcceptedAnswerId="707" CreationDate="2013-07-24T11:10:17.757" Score="17" ViewCount="3094" Body="&lt;p&gt;After Sojourner success, repeated by Spirit and Opportunity, it seemed like we had developed a fail-proof system for atmospheric landing for our probes: there's very little that can go wrong with the parachute+airbag landing system. Most of it is passive and self-stabilizing/self-protecting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then we're back to rocket-based braking system, which, at least to me, seems like an accident waiting to happen. One of the engines fails to fire, the stabilizing sensors go awry, thrust strength is wrong, the tether disconnects too early and a thousand of other possible failure points seem rather dangerous to me. So, what makes that solution preferred to parachute landing on planets with atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorDisplayName="user12" LastEditDate="2013-10-29T04:42:59.787" LastActivityDate="2022-02-05T17:55:46.790" Title="What are the disadvantages of parachute+airbag for atmospheric landing, versus rocket-based?" Tags="|mars|rockets|landing|parachute|air-bag|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="706" PostTypeId="2" ParentId="704" CreationDate="2013-07-24T11:22:42.717" Score="12" Body="&lt;p&gt;The ISS orbital speed is 7.71 km/s or 27 756 km/hour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure the best thrust it could give the Dragox-X with the robotic arm is an order of 20km/h and with Dragon-X weighing roughly 8 tons with the payload, and ISS weighing  450 ton, less than 2% of that speed has been transferred to the station. That means its speed changed by 0.2km/hour, which is 2/277000 or 0.0007% of its orbital speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can assure you, 0.0007% speed change doesn't need to be compensated for. That's much less than what it loses daily to atmospheric friction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTOH, if the thrust was applied off-center, it could have made ISS to spin (slowly, albeit it would bring the solar panels out of optimal alignment eventually), and that would need to be compensated for. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2013-07-24T12:17:35.460" LastActivityDate="2013-07-24T12:17:35.460" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="707" PostTypeId="2" ParentId="705" CreationDate="2013-07-24T11:47:00.907" Score="18" Body="&lt;p&gt;It's quite simple. The new Mars Science Laboratory rover is too heavy to survive an airbag landing. The airbags themselves are made from fabric and had to be strengthened for the Mars Exploration Rovers project. There are some nice videos on-line of JPL in the vacuum chamber at Plum Brook shredding up some Sojourner-era airbags when testing them at MER entry speeds and weights, along with the results: &lt;a href=&quot;http://hdl.handle.net/2014/38594&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Exploration Rover Airbag Landing Loads&#xA;Testing and Analysis&lt;/a&gt;‎. This brought the materials used (which included kevlar) to their limits, and NASA/JPL knew they would not be able to use this system with a lander the size of MSL.&lt;/p&gt;&#xA;&lt;p&gt;Also see &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/q/157/101&quot;&gt;What problems led to the use of the Sky Crane system used by Curiosity?&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="101" LastEditorUserId="11855" LastEditDate="2022-02-05T17:55:46.790" LastActivityDate="2022-02-05T17:55:46.790" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="708" PostTypeId="2" ParentId="683" CreationDate="2013-07-24T11:48:37.507" Score="5" Body="&lt;p&gt;The answer depends partly on your definition of &quot;crime&quot; and whether international law is suitable for dealing with criminal issues. Adopting the prevailing viewpoints, however, legal issues related to activities in space are addressed by a set of international treaties, most notably the &lt;a href=&quot;http://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;noreferrer&quot;&gt;Outer Space Treaty of 1967 (OST)&lt;/a&gt;, which has been signed and ratified by all &quot;space-faring&quot; nations. Under this treaty, countries have agreed essentially to be responsible for all activities in space that are conducted by their respective &quot;entities&quot;, including governmental and non-governmental entities and private individuals. So if a citizen of Country X causes harm to a citizen of Country Y in space, then (assuming both countries are signatories of the OST) Country X has an obligation to restore the damages to Country Y. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What this means in terms of specific &quot;crimes&quot; committed by specific people against other people is largely an untested question. As someone else mentioned, there are more specific agreements between countries for specific cases, such as the &lt;a href=&quot;http://www.state.gov/documents/organization/107683.pdf&quot; rel=&quot;noreferrer&quot;&gt;Intergovernmental Agreement of the ISS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even though the OST addresses non-governmental entities, it is largely geared towards governmental actors in space. For example, if the US launches a rocket and it accidentally falls on, e.g. the Palace of Versailles, the US would probably be shelling out a large reparation payment to the French Republic. Countries generally supervise and restrict their own citizens from launching things in space, unless they insure those activities. For example, rocket launches conducted by commercial companies are almost always insured against third-party damages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your question, however, speaks to a big open hole in space law. If private individuals will start to operate and even live in space, Earth-bound governments may no longer wish to remain responsible for their actions. For example, one could imagine that a country might rescind the citizenship of a &quot;rouge&quot; space-traveler, precisely in order to avoid liabilities caused by this &quot;space pirate&quot;. Countries would probably still be expected to provide protection and assistance to their own space-based citizens and assets against such pirates, but prosecuting the pirates criminally (or under an international legal framework) might be near impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To gain deeper insight, note that the OST treaty and much of space law is largely based on long-standing customs and treaties concerning activities in international waters (e.g. &lt;a href=&quot;http://en.wikipedia.org/wiki/Convention_on_the_High_Seas&quot; rel=&quot;noreferrer&quot;&gt;Convention on the High Seas&lt;/a&gt;). If you think about it, international waters are very similar to space in terms of jurisdiction. Every vessel is normally required to operate under a certain country's flag, and that country then assumes responsibilities for certain liabilities. While there are certainly some pirate vessels (even today), usually the actions of those vessels are traced back to a particular country, and international pressure is applied to that country to control the pirates' actions and perhaps in seeking reparation for damages. Many wars have erupted or escalated precisely because pirates from a particular country committed crimes against ships of another country.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia page for &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_law&quot; rel=&quot;noreferrer&quot;&gt;Space Law&lt;/a&gt; is also a good starting place for learning more about these issues.&lt;/p&gt;&#xA;" OwnerUserId="219" LastActivityDate="2013-07-24T11:48:37.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="709" PostTypeId="2" ParentId="690" CreationDate="2013-07-24T11:53:38.277" Score="5" Body="&lt;p&gt;Physical activity can absolutely affect a space craft from within, and measures are taken to mitigate the effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a few exercise machines aboard the International Space Station that the occupants use to stay fit and maintain bone strength: the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1001_prt.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced Resistive Exercise Device (ARED)&lt;/a&gt;, &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/CEVIS.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cycle Ergometer with Vibration Isolation and Stabilization System (CEVIS)&lt;/a&gt;, and the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/976.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Treadmill with Vibration Isolation and Stabilization System (TVIS)&lt;/a&gt;. Those are all really fancy names for a weight lifting machine, an exercise bike, and a treadmill.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These machines are not bolted to the wall because they do not want to add any unwanted vibration forces to the solar arrays. Sunita Williams provides a great explanation in her &lt;a href=&quot;http://www.youtube.com/watch?v=doN4t5NKW-k#t=3m14s&quot; rel=&quot;nofollow noreferrer&quot;&gt;departure video&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You'll probably see that the bike bounces around a little bit. As I move it, it's not steady and held to the wall firmly. The reason for that is the space station is pretty big; you saw that there's also solar arrays on the space station. If we start putting any forces into the space station, it's going to make those solar arrays bounce around a little bit. So to prevent that, the machines bounce around a little bit and move around a little bit. That way, we don't put any forces on to the structure of the space craft out to the solar arrays. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;She goes on to demonstrate the ARED at &lt;a href=&quot;http://www.youtube.com/watch?v=doN4t5NKW-k#t=13m26s&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Departing Space Station Commander Provides Tour of Orbital Laboratory&quot;&gt;13:26&lt;/a&gt; which also &quot;floats&quot; around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/ARED2.jpg&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Advanced Resistive Exercise Device (ARED)&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2rNXa.png&quot; alt=&quot;Advanced Resistive Exercise Device (ARED)&quot;&gt;&lt;/a&gt; &lt;a href=&quot;http://spaceflight.nasa.gov/gallery/images/station/crew-23/hires/iss023e029750.jpg&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Cycle Ergometer with Vibration Isolation and Stabilization System (CEVIS)&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mbDzQ.png&quot; alt=&quot;Cycle Ergometer with Vibration Isolation and Stabilization System (CEVIS)&quot;&gt;&lt;/a&gt; &lt;a href=&quot;http://www.nasa.gov/images/content/173305main_iss014e10591_hires.jpg&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Treadmill with Vibration Isolation and Stabilization System (TVIS)&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TYP4r.png&quot; alt=&quot;Treadmill with Vibration Isolation and Stabilization System (TVIS)&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;sup&gt;All images courtesy of NASA. Click any of them for full resolution&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="17" LastEditorUserId="17" LastEditDate="2013-07-24T15:53:45.843" LastActivityDate="2013-07-24T15:53:45.843" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="710" PostTypeId="2" ParentId="633" CreationDate="2013-07-24T12:49:17.703" Score="6" Body="&lt;p&gt;This is somewhat a matter of opinion, but I believe the answer to your title question is &quot;NONE&quot;. That's why there aren't any major mining companies that are even remotely close to doing this. If there were resources on asteroids that merited the costs of mining them, then such companies would probably start to do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first two questions you raise (the ones labeled #1 and #2) are not really answerable in a general sense. There are so many parameters affecting the costs (either development or operating) that any &quot;general&quot; answer would be pretty much meaningless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, there may be a time in the future when mining asteroids becomes economically reasonable. And despite the current economics, there have been plenty of serious attempts to start asteroid mining companies. The most notable presently is &lt;a href=&quot;http://www.planetaryresources.com/&quot; rel=&quot;noreferrer&quot;&gt;Planetary Resources, Inc.&lt;/a&gt;. Despite the all-star line-up, however, none of the people involved have any mining experience (as far as I'm aware). They are mostly space enthusiasts who believe strongly in the long-term potential of exploiting space commercially. I would argue that they are motivated largely by their ideals, rather than by economics (which isn't a bad thing, and I wish them success!).&lt;/p&gt;&#xA;" OwnerUserId="219" LastActivityDate="2013-07-24T12:49:17.703" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="711" PostTypeId="5" CreationDate="2013-07-24T12:52:02.743" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-24T12:52:02.743" LastActivityDate="2013-07-24T12:52:02.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="712" PostTypeId="4" CreationDate="2013-07-24T12:52:02.743" Score="0" Body="Questions regarding how astronauts live in space, including day to day activities, chores, etc." OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2013-09-12T20:06:21.670" LastActivityDate="2013-09-12T20:06:21.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="713" PostTypeId="2" ParentId="633" CreationDate="2013-07-24T13:35:00.500" Score="3" Body="&lt;p&gt;The answer will depend on how we deplete our Earth resources. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm fairly sure uranium would be extremely welcome since I've seen estimates that we have maybe 100 years worth of its supply, although it's yet to be found in bulk in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While iron is abundant on Earth, smelting its ores is costly both environmentally and financially, so a &quot;space forge&quot; using meteoritic iron ores and processing them into ready products (and delivering ready forms directly to the customer through aerial drop) would be viable. In this case lack of gravity and air could reduce the costs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lunar He&lt;sub&gt;3&lt;/sub&gt; is targetted as a potential energy source.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still more in realm of sci-fi than actual science, but Jupiter's vast hydrogen supply could provide energy both for long-distance space travel and for power for Earth (although projects of &quot;beaming power&quot; (powerful masers) are more viable than just hauling hydrogen).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In more distant future, when sources of hydrocarbons on Earth are depleted, methane sources like Titan may become economically viable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, though, robguinness' answer &quot;None&quot; is correct. We're still quite a bit away from the break-even point of costs vs profits. I'm sure once &quot;cheap&quot; sources of energy (both oil and uranium) get depleted, that will begin changing rapidly.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-24T13:35:00.500" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="714" PostTypeId="2" ParentId="633" CreationDate="2013-07-24T13:46:45.923" Score="42" Body="&lt;p&gt;NASA has actually published &lt;a href=&quot;http://www.nasa.gov/mission_pages/asteroids/news/asteroidmining.html#.Ue_RNaxnBW0&quot; rel=&quot;noreferrer&quot;&gt;a study&lt;/a&gt; on this. This study was the primary motivator for Planetary Resources to start its work on mining an asteroid. And there is &lt;a href=&quot;http://www.mining.com/nasa-takes-asteroid-mining-serious-funds-viability-study-45382/&quot; rel=&quot;noreferrer&quot;&gt;more work&lt;/a&gt; being done by NASA to learn more as well. And there's &lt;a href=&quot;http://www.reuters.com/article/2012/04/24/us-space-asteroid-mining-idUSBRE83N06U20120424&quot; rel=&quot;noreferrer&quot;&gt;the approach&lt;/a&gt; that Planetary Resources has set in place, which seems to be the best overall &lt;a href=&quot;http://www.gizmag.com/planetary-resources-asteroid-mining/22298/&quot; rel=&quot;noreferrer&quot;&gt;approach&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first thing that should be noted is that Asteroids have large amounts of precious metals. Estimates show that even a small asteroid could have a fortune in Platinum, at least at today's prices. Work would need to be done to ensure that the value didn't plummet the economy. But, there are other options as well out there as to how an asteroid could be valuable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 3 primary types of asteroids. C-type, S-type, and M-type. The latter two contain significant amounts of iron, and the C type carbon. Now, metal on the ground is quite cheap. The cost of Iron Ore is about \$115 per ton. It would therefor be difficult to make money on Earth using Iron Ore. However, there is an alternative way to make money. The Falcon 9 has a cost per pound of \$1800+ if loaded to it's maximum capacity. Thus, if you can make something useful in space of your asteroid, it's value has suddenly increased dramatically. The key for the short term profitability in space is the ability to manufacture and otherwise assist satellites. The amount of money that can be made increases dramatically if you can pull it off. Rocket Fuel, which could easily be made from an asteroid, has the same cost of \$1800/pound, if you can get it to the source. It might even be worth more, as fuel would extend the lifetime of the satellites, which are significant investments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what does it take to make a space mining operation profitable? Manufacturing. If you can develop an autonomous set of manufacturing robots, then you might be able to make something work. There is a number &lt;a href=&quot;http://www.space.com/15405-asteroid-mining-feasibility-study.html&quot; rel=&quot;noreferrer&quot;&gt;often quoted&lt;/a&gt; of \$2.6 billion to break even, but that depends on making these advanced robotics work. It could work, but I think only time will tell.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a whole world of microgravity manufacturing possibilities which has barely been touched. It's far easier to make some things in microgravity, but we haven't really explored that space very much yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest we've done to real space mining is the Apollo program. In total, it brought 382 kg of rocks, for a cost of around \$125 billion in today's dollars. Asteroids would be cheaper to go to than the Moon, and using unmanned spacecraft should make it cheaper, but the proven numbers we have gives a cost of about \$9200/ounce (\$325/g). If that cost can be reduced, which should happen by making it unmanned and the primary mission, then gold could be profitable fairly soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, it's a long time before the Earth mining industry will be affected by Space Mining, but it would likely significantly affect the aerospace industry, moving operations to on orbit. And some day, some of those materials will return to Earth.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="33" LastEditDate="2017-04-26T16:34:34.730" LastActivityDate="2017-04-26T16:34:34.730" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="715" PostTypeId="1" AcceptedAnswerId="720" CreationDate="2013-07-24T15:09:32.697" Score="23" ViewCount="15295" Body="&lt;p&gt;Various sci-fi movies treat Mars atmosphere differently, anywhere between &quot;you just need oxygen rebreather&quot; and &quot;Your eyes will pop out and your head will explode&quot;. Meanwhile, I haven't seen any solid scientific data on human surviving requirements in Mars conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely the suit would have to be warm, probably with active heating or at the very least good insulation, like aerogel. Surely one would need to at least provide oxygen, probably the neutral gas too to prevent acidosis from Mars' CO&lt;sub&gt;2&lt;/sub&gt; in its atmosphere. But how would human body react to the pressure? I'm aware humans can survive in pressures considerably lower than normal, with correspondingly increased oxygen content - Apollo cabin pressures were about 25% of Earth's atmospheric pressure. How is the situation on Mars?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2013-07-24T15:51:42.193" LastActivityDate="2018-08-10T01:28:25.870" Title="Does Mars space suit have to be pressurized?" Tags="|mars|spacesuits|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="716" PostTypeId="1" CreationDate="2013-07-24T17:46:27.290" Score="10" ViewCount="319" Body="&lt;p&gt;The &lt;a href=&quot;http://100yss.org/initiatives/overview&quot;&gt;100YSS mission&lt;/a&gt; currently includes a discussion about Alpha Centauri (our closest star) as a potential target destination, but is it really the best candidate for the 100YSS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To narrow the question's scope, I'll add that my own criteria in asking the question make the discovery of extraterrestrial life the highest priority, so I would add:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;stars with larger numbers of Earth-like planets are better&#xA;than stars with fewer (in hopes of maximizing the probability of finding life)&lt;/li&gt;&#xA;&lt;li&gt;closer stars are better than farther stars (to minimize travel time)&lt;/li&gt;&#xA;&lt;li&gt;stars with more stable surroundings (like our star) are better than&#xA;stars with less stable surroundings (like stars in a binary system&#xA;such as HDE 226868/&lt;a href=&quot;http://en.wikipedia.org/wiki/Cygnus_X-1&quot;&gt;Cygnus X-1&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="108" LastEditorUserId="25" LastEditDate="2013-11-01T19:50:34.170" LastActivityDate="2013-11-01T19:50:34.170" Title="How can we determine the optimal destination for a manned mission beyond our solar system?" Tags="|crewed-spaceflight|interstellar-travel|future-missions|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="717" PostTypeId="1" CreationDate="2013-07-24T19:26:36.600" Score="28" ViewCount="938" Body="&lt;p&gt;Have any caves been spotted on the moon, in an area where Astronauts could potentially seek shelter from radiation?&lt;/p&gt;&#xA;" OwnerUserId="232" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:55:42.657" LastActivityDate="2015-10-16T21:12:16.693" Title="Moon Caves spotted?" Tags="|the-moon|colonization|caves|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="718" PostTypeId="2" ParentId="717" CreationDate="2013-07-24T19:33:30.747" Score="6" Body="&lt;p&gt;Yes -- if you count &lt;a href=&quot;http://lunarscience.nasa.gov/articles/lava-tube-lunar-base/&quot;&gt;lava tubes&lt;/a&gt;.  And why wouldn't you?&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-24T19:33:30.747" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="719" PostTypeId="2" ParentId="716" CreationDate="2013-07-24T20:06:17.000" Score="3" Body="&lt;p&gt;Through unmanned missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A set of probes of moderate complexity would be able to gather enough data to help making the decision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can imagine prerequisites of such probes.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;as it gains speed, it discards trailing segments. Possibly another probe just with such segments is sent. At preprogrammed time the segments activate and start acting as radio relays - we don't need the probe back, just its data, so the &quot;way back home&quot; of the data would be at speed of light, from relay to relay.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The probe contains a central module with a good spectrometric telescope to provide automated survey of the planets, and a couple landers similar to simple Mars landers, autonomous vehicles capable of airbraking and landing on potentially inhabitable planets. We don't need inhabited, just inhabitable by humans or at least resources-rich so that the actual 100YSS could establish a base of operations. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The core module picks promising candidate planets using the telescope data, and deploys the landers to these. The probes relay their findings to the core module, the core module sends it back home through the relay system.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;One such probe is sent to each candidate star system.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;As the probes are underway, we use the time to develop actual spaceship, develop needed technologies for easily deployable sustainable base, and so on. We should have it ready once the replies start coming in, then we pick the best candidate planet using the gathered data - if there is any life in given system, a most human-habitable planet in that system. If there is no life in any of them, most human-habitable of them all.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-24T20:06:17.000" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="720" PostTypeId="2" ParentId="715" CreationDate="2013-07-24T20:25:02.833" Score="20" Body="&lt;h2&gt;Yes&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Mars&quot;&gt;atmospheric pressure&lt;/a&gt; on Mars is about 0.6% of Earth's sea level pressure. That's about 0.6 kilopascals (0.087 psi). Whereas even one of the highest points on earth, &lt;a href=&quot;http://en.wikipedia.org/wiki/Mount_Everest#Environment&quot;&gt;Mount Everest&lt;/a&gt; is 33.7 kilopascals (4.89 psi). &lt;strong&gt;That means Mount Everest has about 56 times the atmospheric pressure of the surface of Mars.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Humans wouldn't survive very long in those sorts of conditions. According to this graph from NASA, .087 psi puts us below the &quot;Bubbles in blood&quot; level:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/XoCnS.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note the graph goes from high pressure at the top to low pressure at the bottom)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even Everest is just barely in the safe region for humans. The graph assumes the individual is breathing the atmosphere they're in, which isn't the case in your proposed question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is evidence that a person could &lt;a href=&quot;http://www.scientificamerican.com/article.cfm?id=survival-in-space-unprotected-possible&quot;&gt;survive briefly&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="234" LastActivityDate="2013-07-24T20:25:02.833" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="721" PostTypeId="1" CreationDate="2013-07-24T22:21:48.973" Score="14" ViewCount="385" Body="&lt;p&gt;There are some experiments and projects that are searching for extraterrestrial intelligence.(&lt;a href=&quot;https://en.wikipedia.org/wiki/Search_for_extraterrestrial_intelligence%E2%80%8E&quot;&gt;SETI&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to know what signs exactly are they looking for? Do they just look for monotone sinusoidal radio signals?&lt;/p&gt;&#xA;" OwnerUserId="160" LastEditorUserId="-1" LastEditDate="2016-04-28T15:27:12.310" LastActivityDate="2016-11-25T18:18:51.230" Title="What exactly do they look for when searching for extraterrestrial intelligence?" Tags="|astrobiology|seti|radio-communication|technomarkers|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="723" PostTypeId="2" ParentId="715" CreationDate="2013-07-24T22:53:52.143" Score="14" Body="&lt;p&gt;There is no requirement for the suit itself to be pressurized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a requirement for some pressure on the skin to prevent vacuum bruising. That can be provided by either of two means - fluid pressure from the environment, or mechanical pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current &quot;bleeding edge&quot; designs are a wire-reinforced elastic material, with a pressurized helmet that seals to the shoulders. The skin is actually encountering 0 atmospheric pressure, and the suit is porous to allow sweat to escape, but maintains even mechanical counter-pressure to the 3 or so pounds per square inch needed to avoid vacuum injury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that eye damage from vacuum isn't instant, either - but is fairly rapid. Individuals in vacuum experiments accidentally exposed to vacuum experienced immediate eye irritation due to rapid loss of moisture, but all made full recoveries from momentary exposures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, lung damage is rapid, but not instant, and is mostly due to drying and edema. Several minutes of extremely low pressure on skin resulted in profound edema, but no lasting injury, for the early weather ballon jump tests.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceindustrynews.com/mits-next-mars-space-suit/&quot;&gt;http://spaceindustrynews.com/mits-next-mars-space-suit/&lt;/a&gt;&#xA;&lt;a href=&quot;http://www.edudemic.com/2012/09/mit-researcher-unveils-incredible-new-mars-space-suit/&quot;&gt;http://www.edudemic.com/2012/09/mit-researcher-unveils-incredible-new-mars-space-suit/&lt;/a&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/offices/oce/appel/ask/issues/45/45s_building_future_spacesuit.html&quot;&gt;http://www.nasa.gov/offices/oce/appel/ask/issues/45/45s_building_future_spacesuit.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.universetoday.com/66061/early-nasa-vacuum-chamber-test-gone-wrong/&quot;&gt;http://www.universetoday.com/66061/early-nasa-vacuum-chamber-test-gone-wrong/&lt;/a&gt;&#xA;&lt;a href=&quot;http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html&quot;&gt;http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-07-24T22:53:52.143" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="724" PostTypeId="2" ParentId="721" CreationDate="2013-07-24T23:14:49.337" Score="5" Body="&lt;p&gt;This is a question of philosophy, clearly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically people are looking for every type of signal which supposedly can not occur naturally/inartificially. The general assumption is, that e.g. prime numbers can not be found in any significant role in nature thus every occurrence of them must come from some kind of thinking process-intelligent life. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the contrary, it is hard to identify such criteria. Humans can only judge based on what they know, so one could easily miss any signal which does not comply with &lt;em&gt;human&lt;/em&gt; criteria. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wiki page on SETI, for which you provided a link, goes a bit more into the details. It is also worth the effort to have a look into the work of &lt;a href=&quot;http://en.wikipedia.org/wiki/Carl_Sagan&quot; rel=&quot;nofollow&quot;&gt;Carl Sagan&lt;/a&gt;. You might know his novel &lt;a href=&quot;http://en.wikipedia.org/wiki/Contact_%28novel%29&quot; rel=&quot;nofollow&quot;&gt;Contact&lt;/a&gt;. But he also wrote scientific / philosophic texts about this topic. Most interestingly, he also advocated to 'calibrate' instruments on space probes intended to find life on other planets (like Mars) against Earth as a test subject first. Some of them failed to verify life on Earth. (In this context, people assumed for a long time that life in space had to look like life on Earth and therefore searched for e.g. spectral signatures of hydrocarbons.) It is a similar problem with the search for criteria in SETI. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="289" LastEditDate="2013-07-28T14:32:36.160" LastActivityDate="2013-07-28T14:32:36.160" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="725" PostTypeId="1" AcceptedAnswerId="728" CreationDate="2013-07-24T23:39:31.287" Score="22" ViewCount="1333" Body="&lt;p&gt;&lt;a href=&quot;http://space-track.org&quot;&gt;Space Track&lt;/a&gt; is a website dedicated to passing along information about satellites to consumers, primarily satellite operators. The information comes from the US military. In addition to providing the &lt;a href=&quot;http://en.wikipedia.org/wiki/Two-line_element_set&quot;&gt;Two Line Elements&lt;/a&gt; that inform operators of exactly where their satellites are, they also produce conjunction reports, of predicted near misses to other objects in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am trying to use the information they provide in order to predict with a medium fidelity the probability of impact. Some of the information they provide is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The In Track, Radial, and Cross Track miss distances.&lt;/li&gt;&#xA;&lt;li&gt;A 3x3 correlation matrix with the above values&lt;/li&gt;&#xA;&lt;li&gt;Radar Cross Sectional data for each object.&lt;/li&gt;&#xA;&lt;li&gt;Other miscellaneous data.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'm trying to figure out the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How can I determine a minimum distance apart? It's clear if the two objects are 1 m diameter spheres, if they are less than 1 meter from each other, it is a hit, but is there a good rule of thumb for an arbitrary object, and a known object (Our satellite) without extensive modeling applied?&lt;/li&gt;&#xA;&lt;li&gt;Given that I have a concept on how much of a miss distance is considered a hit, how do I determine that miss distance?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-08T20:22:29.073" LastActivityDate="2022-05-25T02:20:41.967" Title="What is the probability of impact?" Tags="|orbital-mechanics|impact|space-track|conjunction|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="726" PostTypeId="5" CreationDate="2013-07-24T23:43:42.967" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-24T23:43:42.967" LastActivityDate="2013-07-24T23:43:42.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="727" PostTypeId="4" CreationDate="2013-07-24T23:43:42.967" Score="0" Body="The United States outlet for unclassified data from the military for tracking space objects. Includes satellite elements, conjugation reports, and tracking reports of newly launched objects." OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-25T14:21:16.183" LastActivityDate="2013-07-25T14:21:16.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="728" PostTypeId="2" ParentId="725" CreationDate="2013-07-25T00:07:55.023" Score="18" Body="&lt;p&gt;I... like this question. First things first, TLE sets are nearly useless for precise probability of collision estimation, because they do not contain an estimate of the uncertainty in the orbit, which is ultimately what affects your probability of collision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, assuming you get a full message with a high-precision state vector and an associated covariance matrix (full 6x6 state is better, but 3x3 can work) &lt;em&gt;for each satellite&lt;/em&gt;, there are a few things you can do. There are quite a few different methods for estimating this, look up work by Alfano and Patera for a couple examples. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What the problem mostly boils down to is estimating the overlap of the uncertainty at the time of closest approach. Imagine &quot;uncertainty&quot; (in 3 dimensions) as an &lt;a href=&quot;http://en.wikipedia.org/wiki/Ellipsoid&quot; rel=&quot;noreferrer&quot;&gt;ellipsoid&lt;/a&gt; - if you're familiar with concepts of probability, it's your multivariate probability density. Now, the probability of collision can be thought of as the amount of overlap between those two ellipsoids. In your specific situation, you do not have full state and uncertainty information for both objects, but similar concepts apply here (they essentially make some assumptions and combine the uncertainty of the objects).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To try to answer your questions more directly:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;I assume you're asking what minimum distance you (as an operator) should tolerate. This is hard to put a specific value on. Normally, you get a probability of collision estimate and assign a threshold in that space, such as &quot;anything higher than a 1e-4 probability of collision and we will maneuver&quot;. As far as what distance constitutes a &quot;hit&quot;, what's usually done is assuming both objects are spherical. This can be hard if you don't know much about the other object.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;See above for the basic concepts, but implementation obviously requires knowledge of probabilistic concepts, orbit dynamics, and (preferably) state estimation techniques.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Finally, this is very much an active topic of research, and there are often papers describing a &quot;new&quot; way to do this. I should also mention that while what I explained is often done in practice, there are several reasons to believe that it does not, in fact, provide an &lt;em&gt;accurate&lt;/em&gt; probability of collision estimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, open literature on this topic is sparse, but searching for the authors I mentioned above, and in fact just the terms &quot;satellite probability of collision&quot; will get you some very useful hits if you have the right access (AIAA journal papers, for instance).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;However, you can estimate this numerically instead of analytically using Monte Carlo methods. Here's a quick and dirty way to do this:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Sample from your &quot;primary&quot; satellite (the one you got the conjunction report for). To do this, generate a 3x1 normally-distributed random vector with zero mean and unit variance (the &lt;a href=&quot;http://mathworld.wolfram.com/StandardNormalDistribution.html&quot; rel=&quot;noreferrer&quot;&gt;standard normal&lt;/a&gt;. Now, take  your 3x3 covariance for this object, find its &lt;a href=&quot;http://mathworld.wolfram.com/CholeskyDecomposition.html&quot; rel=&quot;noreferrer&quot;&gt;Cholesky decomposition&lt;/a&gt; (you should be able to find a routine to do this in your software package - 'chol' in Matlab), and multiply it by your random vector. You've now sampled your probability space.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Repeat #1 for the conjuncting object. However, once you sample, you need to translate each point according to its position, which is given to you in RIC coordinates. Just add your position vector to the random vector.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Calculate the &lt;a href=&quot;http://en.wikipedia.org/wiki/Euclidean_distance&quot; rel=&quot;noreferrer&quot;&gt;distance&lt;/a&gt; between your two samples. Record said distance.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Repeat steps 1-3 for some very large number of samples (think tens of thousands - generally, the more, the merrier).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Now, you have a list of &quot;miss distances&quot;. There are a couple things you can do with this, but one thing that's particularly helpful is to construct an empirical &lt;a href=&quot;http://en.wikipedia.org/wiki/Cumulative_distribution_function&quot; rel=&quot;noreferrer&quot;&gt;cumulative distribution function&lt;/a&gt;. You can look up how to do this, but you should be able to find a function to do this in most software packages (in Matlab, it's 'cdfplot'). This will give you a plot that looks like this (only with x&gt;0): &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TP9kA.gif&quot; alt=&quot;cdfplot&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, one way you can read this is by choosing a &quot;miss distance&quot; you're comfortable with (say, 20m), and finding the value of $F(x)$ at this value of $x$. That's the probability that you will come within 20m or less of the offending satellite. If it's greater than some threshold you've decided upon with your management, you begin the fun task of planning an avoidance maneuver. If not, you wait and watch. Alternatively, you can specify a probability you're comfortable with, and read off a miss distance associated with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing you'll notice is that you're really focused on the tail of your plot... and you'll see that to get a smooth representation at the tail, you need a whole hell of a lot of samples. So like I said, the more, the merrier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One last thing: if you've followed these steps, the probability number you have is $1\sigma$. For, say, a $3\sigma$ estimate, just multiply your two covariance matrices by your factor (in this case 3).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-09T00:51:14.587" LastActivityDate="2013-10-09T00:51:14.587" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="729" PostTypeId="1" AcceptedAnswerId="3192" CreationDate="2013-07-25T00:25:06.690" Score="15" ViewCount="4815" Body="&lt;p&gt;In late 1997, the European Space Agency started the LEON project to provide higher performance processors for ESA missions. An open instruction set architecture was a first-level requirement (both to avoid availability issues and to allow customization), but suitability for use in space was also a major consideration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ESA chose to use SPARC Version 8 (from 1991/1992 based on the copyright notice in &lt;em&gt;The SPARC Architecture Manual: Version 8&lt;/em&gt;, the base ISA was released in 1987). SPARC had some advantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It was perhaps the only fully open ISA with significant backing.&lt;/li&gt;&#xA;&lt;li&gt;It was a reasonably simple ISA (RISC), friendly to lower-effort implementation. &lt;/li&gt;&#xA;&lt;li&gt;Workstations existed using SPARC v8 compatible processors, so cross compiling could be avoided.&lt;/li&gt;&#xA;&lt;li&gt;Development tools for SPARC were reasonably mature, though presumably more oriented toward server/workstation workloads.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, SPARC also had some disadvantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;By 1997 (with the introduction of the Pentium II) the future of SPARC in workstations would have begun to be under question (making the cross compiling issue somewhat moot).&lt;/li&gt;&#xA;&lt;li&gt;SPARC was not being broadly adopted as an open ISA, so there was not a significant commodity effect. (This was probably not a major consideration for LEON, but influences the availability of software tools. The openness of SPARC also has limited advantage in terms of patents.)&lt;/li&gt;&#xA;&lt;li&gt;As a classic RISC SPARC had somewhat poor code density. (Code size is a significant factor for space-based computers.)&lt;/li&gt;&#xA;&lt;li&gt;SPARC was not being broadly adopted for use in embedded systems. (This would have influenced the availability of development tools suitable for such systems.)&lt;/li&gt;&#xA;&lt;li&gt;SPARC's register windows would have increased minimum core size, slightly increased hardware design complexity, and involved more complex development for tightly constrained real time operation.&lt;/li&gt;&#xA;&lt;li&gt;SPARC included less useful features like tagged arithmetic. (This was probably not significant since a subset of the architecture could be used.)&lt;/li&gt;&#xA;&lt;li&gt;SPARC's instruction format was somewhat less regular than other RISCs. (This is a rather trivial objection; the size/power difference between instruction decoders for an Alpha-like ISA and SPARC would be less than 1%.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;With somewhat guaranteed use by the ESA of whatever architecture was chosen and the special requirements for space-based systems, it seems that a custom ISA might have been practical. A custom, open ISA would have &lt;em&gt;significant&lt;/em&gt; disadvantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The lack of existing development tools would have added cost and linked compiler development to hardware development (increasing schedule risk). (Non-open tools for SPARC development would have represented some risk to LEON, but this could rightly have been considered a minor issue.)&lt;/li&gt;&#xA;&lt;li&gt;The lack of same-ISA workstations would have added complexity and cost to initial development.&lt;/li&gt;&#xA;&lt;li&gt;Even excluding the cost of producing software development tools, developing a new ISA has significant cost and risk. (Getting consensus without design-by-committee effects is challenging and &lt;a href=&quot;http://en.wikipedia.org/wiki/Second-system_effect&quot;&gt;second system dangers&lt;/a&gt; may have been significant.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Any increase in (early) cost and risk may have disproportionately increased the probability of project failure. On the other hand, a custom ISA could have been more fit for the purpose and &lt;em&gt;might&lt;/em&gt; have been able to generate more external adoption in aerospace both of the ISA and of chips developed for the ESA (which would have increased the quality of software, increased the testing of implementations, and reduced hardware costs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how did the ESA come to choose SPARC over a custom ISA (or perhaps negotiating a limited perpetual license for another RISC ISA)?&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastEditorDisplayName="user12" LastEditDate="2013-12-17T15:54:49.987" LastActivityDate="2015-12-17T22:48:29.213" Title="Why did the ESA choose SPARC for LEON?" Tags="|technology|flight-computer|esa|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="731" PostTypeId="1" AcceptedAnswerId="748" CreationDate="2013-07-25T00:50:05.207" Score="8" ViewCount="555" Body="&lt;p&gt;It seems that most mining proposals involve landing on the body (blasting pieces of asteroids up from the surface for measurement has been proposed), would it be practical to exploit the volatile nature of cometary material (and availability of solar energy) to extract cometary material without having to land on the comet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scooping from the ion tail might be more practical (dust might require a more robust scoop) and desirable (comets are more special for their volatile content).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost of developing specialized comet mining technology (not just the scoop but also the navigation) would be spread over a smaller number of uses, so it is probably unlikely to be economically practical. However, this seemed like an interesting question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(One of my concerns with landing on a comet was the perception of greater danger near Earth orbit--when the comet if fully active--than an asteroid.)&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastEditorDisplayName="user56" LastEditDate="2013-07-25T15:02:12.837" LastActivityDate="2023-10-12T18:36:51.817" Title="Could a comet-trailing &quot;scoop&quot; be practical for mining?" Tags="|mining|comet|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="732" PostTypeId="5" CreationDate="2013-07-25T01:05:51.053" Score="0" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Dragon_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dragon capsule&lt;/a&gt; is produced by Space Explorations Technologies of California.  Designed to launch on a Falcon 9 capsule, it was initially used in cargo mode, delivering payloads to the ISS. Recovery is by parachute into the ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second model, that incorporates pusher abort engines (Super Draco) will be used for manned flight.  When not needed for abort usage, the Super Draco engines are designed to be used for a propulsive touchdown on land. Use &lt;a href=&quot;/questions/tagged/dragon-v2&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;dragon-v2&amp;#39;&quot; rel=&quot;tag&quot;&gt;dragon-v2&lt;/a&gt; for questions about Crew Dragon (SpaceX's official name for the crewed capsule)&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="22326" LastEditDate="2020-02-27T19:38:21.383" LastActivityDate="2020-02-27T19:38:21.383" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="733" PostTypeId="4" CreationDate="2013-07-25T01:05:51.053" Score="0" Body="SpaceX cargo capsule for lifting payloads to orbit.  Launched on a Falcon 9 rocket. Use [dragon-v2] for the Crew Dragon version of this project. " OwnerUserId="51" LastEditorUserId="22326" LastEditDate="2020-02-27T19:38:21.383" LastActivityDate="2020-02-27T19:38:21.383" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="734" PostTypeId="5" CreationDate="2013-07-25T01:14:54.043" Score="0" Body="&lt;p&gt;Developed by SpaceX, the initial Falcon 9 version used 9 Merlin-1C engines in a tic-tac-toe layout on the first stage and a Merlin 1C with a vacuum nozzle for the second stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the first five launches it was replaced with the Falcon 9 V1.1 or F-9R (R for reusable). The engine was upgraded to the Merlin-1D which due to higher thrust consumes more fuel and oxidizer, so the first stage tanks were stretched to provide more consumables.  The engine layout was changed to a circle of eight engines, with the ninth in the center, known as the Octaweb layout.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A further improvement informally known as the V1.2 configuration, but officially as the Falcon 9 Full Thrust increased the thrust of the Merlin-1D engines again, using subcooled fuel (RP-1) and oxidizer (LOX). This extra performance allows for recovery of the first stage on most missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first stage got landing legs, which are used to recover the first stage on land with propulsive landing. Three engines fire before reentry to the atmosphere to slow the booster stage and a single (center) engine is used for actual landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For missions with spare performance, the stage can boost back to the launch site and land on land. For those without sufficient performance left after the mission is launched (most GTO missions) the stage can land on the ASDS Barge (Autonomous Spaceport Drone Ship). SpaceX has one deployed in the Atlantic Ocean (Marmac 303 &amp;mdash; &quot;Of Course I Still Love You&quot; or OCISLY) and one in the Pacific Ocean (Marmac 304 &amp;mdash; &quot;Just Read The Instructions&quot; or JRTI).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6816" LastEditDate="2016-12-20T12:42:58.960" LastActivityDate="2016-12-20T12:42:58.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="735" PostTypeId="4" CreationDate="2013-07-25T01:14:54.043" Score="0" Body="Questions regarding the Falcon launch vehicle developed by SpaceX for medium payloads (15,000 lbs) to LEO and GEO (via GTO, second stage cannot inject into GEO direct)" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-06-29T13:34:42.417" LastActivityDate="2016-06-29T13:34:42.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="736" PostTypeId="2" ParentId="731" CreationDate="2013-07-25T02:47:23.860" Score="6" Body="&lt;p&gt;Comets are typically on much higher energy trajectories compared to the asteroids looked at for mining.  I suspect that that alone would make them un-economic.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-25T02:47:23.860" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="737" PostTypeId="2" ParentId="653" CreationDate="2013-07-25T04:58:01.293" Score="22" Body="&lt;p&gt;I decided to search through NTRS &lt;a href=&quot;http://hdl.handle.net/2060/20130012684&quot; rel=&quot;nofollow noreferrer&quot;&gt;and found this&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;#Waterless Clothes-Cleaning Machine&lt;/p&gt;&#xA;&lt;p&gt;This machine can be used wherever water is at a premium, or to minimize washing with water.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Lyndon B. Johnson Space Center, Houston, Texas&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;A waterless clothes-cleaning machine has been developed that removes loose&#xA;particulates and deodorizes dirty laundry with regenerative chemical&#xA;processes to make the clothes more comfortable to wear and have a fresher&#xA;smell. This system was initially developed for use in zero-g, but could be altered for 1-g environments where water or other resources are scarce. Some of&#xA;these processes include, but are not limited to, &lt;strong&gt;airflow&lt;/strong&gt;, &lt;strong&gt;filtration&lt;/strong&gt;, &lt;strong&gt;ozone generation&lt;/strong&gt;, &lt;strong&gt;heat&lt;/strong&gt;, &lt;strong&gt;ultraviolet light&lt;/strong&gt;, and &lt;strong&gt;photocatalytic titanium oxide&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The machine has a chamber large enough to contain and agitate several articles of clothing, as well as a self-sealing door for insertion and removal of&#xA;the clothing. The agitation and removal of particulate and volatiles in the&#xA;clothes is done by airflow and some kind of agitation mechanism, possibly&#xA;by rotating the chamber and/or alternating airflow and/or beater panels for&#xA;the zero-g environment. Agitation in 1-g could be done with tumbling. One of&#xA;the main purposes of the airflow is to remove particulate from the clothing and&#xA;to deposit it into a filter where the particulate can be removed from the filter&#xA;at the end of the cycle. This airflow can also carry ozone into the chamber to&#xA;penetrate into the clothing to kill off bacteria and break down odorizing proteins or other organics. The chamber can also contain an ultraviolet light&#xA;source to expose the agitating clothes to bacteria-killing wavelengths of light.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;This light source could also expose a photocatalytic material such as titanium&#xA;oxide, embedded coated on the interior of the chamber walls or on agitation&#xA;mechanisms, to energies that would produce hydroxyl ions from the chamber humidity to aid in the removal of organic compounds from the cloth&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Heat could be introduced into the clothing chamber either by heating the&#xA;airflow or by heating the clothing chamber directly using electrical heater strips on the chamber walls. The heat would aid in the killing of bacteria, breaking down proteins, and evaporating volatiles from the clothes. The airflow for this system could either be completely recycled back through the system or vented out, depending on the needs of the clothes cleaner’s surrounding environment. Airflow, ozone, UV light, and the heat can be controlled independently so each can be turned on or off without affecting the others to allow for the needs of the specific type of clothing or different types of soiling on the clothes.&lt;/p&gt;&#xA;&lt;p&gt;This work was done by &lt;strong&gt;Glenn Johnson&lt;/strong&gt; and &lt;strong&gt;Shane Ganske&lt;/strong&gt; of &lt;strong&gt;United Space Alliance&lt;/strong&gt; for Johnson Space Center.&lt;/p&gt;&#xA;&lt;hr/&gt;&#xA;&lt;h1&gt;A small bibliography on the subject:&lt;/h1&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Colombo, Gerald V.; Putnam, David F.; Lunsford, Teddie D.; Streech, Neil D.; Wheeler, Richard R., Jr.; Reimers, Harold. Single phase space laundry development. SAE PAPER 932092. 1993.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Laundry Study for a Lunar Outpost. Ewert, Michael; Jeng, Frank. JSC-17326. 2009. (&lt;strong&gt;A design tradeoff study!&lt;/strong&gt;)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://hdl.handle.net/2060/20120011664&quot; rel=&quot;nofollow noreferrer&quot;&gt;Next Generation Life Support Project: Development of Advanced Technologies for Human Exploration Missions&lt;/a&gt;. Daniel J. Barta. JSC-CN-26608. 2012. (Status of Alternative Water Processor to treat, inter alia, laundry wastewater).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr/&gt;&#xA;&lt;h1&gt;Break-even point&lt;/h1&gt;&#xA;&lt;p&gt;Ewert, Michael K.; Broyan, James Lee, Jr. &lt;a href=&quot;http://hdl.handle.net/2060/20130011292&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mission Benefits Analysis of Logistics Reduction Technologies&lt;/a&gt;. JSC-CN-28324. 2013.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;No clothing laundry system was assumed in the baseline model because &lt;strong&gt;a one year mission is likely not long enough for the benefits of laundry to significantly exceed the direct clothing mass&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;(Which doesn't mean I agree with them.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="47149" LastEditDate="2022-06-05T18:59:47.783" LastActivityDate="2022-06-05T18:59:47.783" CommentCount="0" CommunityOwnedDate="2013-07-25T04:58:01.293" ContentLicense="CC BY-SA 4.0" />
  <row Id="738" PostTypeId="2" ParentId="717" CreationDate="2013-07-25T06:51:13.853" Score="26" Body="&lt;p&gt;There are a number of confirmed instances of caves on the Moon formed by old lava tubes (e.g.: &lt;a href=&quot;http://www.universetoday.com/90031/russia-eyes-caves-on-moon-for-setting-up-a-lunar-base/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;http://www.wired.co.uk/news/archive/2011-02/09/lunar-cave-photos&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;http://www.universetoday.com/73659/lro-takes-closer-look-at-moon-caves/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;). These could potentially form a very advantageous site for a lunar colony or base in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.lpi.usra.edu/decadal/leag/AndrewWDagaFINAL.pdf&quot; rel=&quot;nofollow&quot;&gt;Here is a formal study&lt;/a&gt; of the presence and possible usage of lava tubes, both on the Moon and Mars, for exploration and colonization purposes. Building a base in a lava tube has a number of attributes that makes it highly desirable for a lunar base or colony, in particular:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Protection from radiation&lt;/strong&gt; (as you noted)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Minimal structural needs&lt;/strong&gt; Using these tubes as structural support for a lunar colony could significantly reduce the amount of building materials that would need to be brought as payload. This could reduce building costs drastically.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Simplified containment&lt;/strong&gt; A long-term base or colony requires a lot of oxygen to stay sustainable, and lava tubes help to preserve this precious resource. First, it's relatively easy to seal the base off from the outside, just by filling in a few entryways. Second, oxygen can be easily extracted from the lunar soil/rock forming the walls of these structures.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Constant temperatures&lt;/strong&gt; By making the base underground, you get to avoid the temperature extremes that can occur on the surface. On the Moon, an underground base would have a &lt;a href=&quot;http://adsabs.harvard.edu/abs/1992ntlr.work...51Y&quot; rel=&quot;nofollow&quot;&gt;year-round temperature of -20 °C&lt;/a&gt; (−4 °F), reducing the need for heavy temperature maintenance. The temperature could be raised by solar heaters on the surface, and since the base is contained underground, the heat would escape only very slowly.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Exploration &amp;amp; research&lt;/strong&gt; An underground base is probably one of the most lucrative spots for conducting scientific research and exploration on the Moon, since the lunar subterranean environment is still largely unexplored--especially if combined with a telescope on the surface without the interference of atmospheric distortion.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="47" LastEditorUserId="49" LastEditDate="2015-10-16T21:12:16.693" LastActivityDate="2015-10-16T21:12:16.693" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="739" PostTypeId="1" CreationDate="2013-07-25T06:55:37.967" Score="20" ViewCount="1578" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Deep_Space_Network&quot;&gt;NASA&lt;/a&gt; and the &lt;a href=&quot;http://en.wikipedia.org/wiki/ESTRACK&quot;&gt;ESA&lt;/a&gt; have &lt;em&gt;Deep Space Networks&lt;/em&gt; that circle the globe enabling them to track their spacecraft constantly. However, &lt;a href=&quot;http://en.wikipedia.org/wiki/Soviet_Deep_Space_Network&quot;&gt;Russia&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Chinese_Deep_Space_Network&quot;&gt;China&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Indian_Deep_Space_Network&quot;&gt;India&lt;/a&gt;, and possibly also &lt;a href=&quot;http://en.wikipedia.org/wiki/Usuda_Deep_Space_Center&quot;&gt;Japan&lt;/a&gt;, only list tracking stations within their national borders. While China is apparently planning on setting up an additional station in South America, how do these countries currently track their spacecraft when they are not in sight? Do they rely on the NASA or ESA networks?&lt;/p&gt;&#xA;" OwnerUserId="80" LastActivityDate="2014-12-05T10:25:02.553" Title="How do Russia, China, Japan, and India track their spacecraft?" Tags="|ground-station|space-track|deep-space-network|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="740" PostTypeId="1" CreationDate="2013-07-25T07:52:11.123" Score="14" ViewCount="510" Body="&lt;p&gt;It has now been two years since the last Space Shuttle mission ended (July 21, 2011). Since then, the United States has not had its own capability to launch astronauts into space. To my knowledge neither President Obama nor any high-level administration official has discussed the topic publicly in more than three years. Even going to the NASA website for &lt;a href=&quot;http://www.nasa.gov/content/human-missions-list&quot;&gt;Human Space Missions&lt;/a&gt; and clicking on &lt;a href=&quot;http://www.nasa.gov/exploration/home/index.html&quot;&gt;Future Exploration Plans&lt;/a&gt;, one does not easily find a clearly-stated plan with any kind of timeline. NASA released some details of the planned &lt;a href=&quot;http://www.nasa.gov/home/hqnews/2011/sep/HQ_11-301_SLS_Decision.html&quot;&gt;Space Launch System&lt;/a&gt; in September 2011, but at least I haven't seen any major updates or detailed timeline released since then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This leads me to my question: What is the United States' current plan to get back to launching people into space? Furthermore, &lt;em&gt;when&lt;/em&gt; can we expect the United States to regain such capability?&lt;/p&gt;&#xA;" OwnerUserId="219" LastEditorUserId="243" LastEditDate="2013-07-25T16:41:31.613" LastActivityDate="2013-07-29T12:03:31.980" Title="What is the United States' current plan to get back to launching people into space?" Tags="|crewed-spaceflight|future-missions|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="741" PostTypeId="1" AcceptedAnswerId="742" CreationDate="2013-07-25T08:03:23.573" Score="19" ViewCount="8466" Body="&lt;p&gt;...other than budget, obviously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our computers nowadays are good enough that we should be capable of sending a probe to a neighbor star system and back, and get some pretty good analysis reports. We do have batteries that would survive a hundred years before self-discharge renders them useless, and media to keep the collected data for long after that. The current roadblock seems to be speed of the probes — it would take much longer to reach that far than any of our equipment could survive in working order. We need better propulsion to receive the results in any reasonable time frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me ask the question first in a very short and non-constructive way: Why don't we have any better propulsion?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for something less subjective: What roadblocks do the scientists currently struggle against that keep the propulsion of our probes too weak to think practically of missions outside the Solar System? Are there any reasonable predictions or projects of propulsion systems that would considerably improve on what we have? Or is it just budget? Give it a fuel tank big enough and it will fly as fast as we desire? Or are there other considerations like safety in the case of nuclear?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="289" LastEditDate="2013-07-28T00:19:24.367" LastActivityDate="2017-08-26T15:09:46.517" Title="What limits the speed of space probes currently?" Tags="|interstellar-travel|propulsion|" AnswerCount="6" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="742" PostTypeId="2" ParentId="741" CreationDate="2013-07-25T08:52:27.157" Score="15" Body="&lt;p&gt;It's partly the same issues as the launch problem. If you put more fuel in the fuel tanks of the rockets, you then increase the mass. Then to lift that fuel you need to add a bit more fuel to lift that fuel, and so on and so forth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A similar problem exists with the current propulsion system on probes but before I go into that I'm going to (very briefly) explain travel in space so we can understand the problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Travel In Space&lt;/strong&gt; &lt;em&gt;(assuming traveling in a straight line)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Travel in space is not the same as travel on land. Travel on land requires the constant burning of fuel to be able to replace the speed lost to friction, air resistance, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Travel in space doesn't work the same way, it doesn't require a constant burning, it requires you burn enough fuel to propel the mass to that initial speed, then enough to burn in reverse to slow itself down at its destination. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Which isn't half and half, it requires more burned fuel to speed up than to slow down, as the slowing down portion has less mass because we can discount the lost fuel burned to accelerate in the first place.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Back to the problem&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ok so we &lt;em&gt;could&lt;/em&gt; just add more fuel to the probes, but then we run into the same issue as the launch issue, adding extra fuel adds extra mass which requires extra fuel of its own to burn in order to propel the increased mass to the same speeds that we wanted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So really what we want is a method of propulsion other than the existing liquid and solid process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see &lt;a href=&quot;http://education.jsc.nasa.gov/explorers/p4.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, NASA already has some alternative ideas they want to try out for propulsion, I will briefly cover them below in case the link dies. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://education.jsc.nasa.gov/explorers/p4_glossary.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nuclear Thermal Propulsion&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nuclear thermal propulsion – heats a fluid, usually hydrogen, in a&#xA;  high temperature nuclear reactor that creates thrust to move the&#xA;  rocket in space&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA expects this type of propulsion system to be much lighter and a more efficient method of propelling ships in space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, every silver lining has a cloud and this is no exception. What currently stands in our way of using this system is the extreme difficulty to keep hydrogen in it's liquid form. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see &lt;a href=&quot;http://www.nasa.gov/topics/technology/hydrogen/hydrogen_fuel_of_choice.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, hydrogen needs to be kept at 20 Kelvin to stay in a liquid form. This proposes many technical challenges, first to reduce the temperature to such a level, then again when trying to stop the liquid fuel from heating up from the high temperatures of the exhaust!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And don't be mistaken, the technical problems with using liquid hydrogen isn't for lack of trying. in fact the idea to use liquid hydrogen as a fuel has been around since &lt;a href=&quot;http://history.nasa.gov/SP-4404/ch8-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;at least the 1950's!&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;Plasma based propulsion&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA is also investigating a plasma based propulsion system called project VASIMR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea is to use a nuclear reactor (again) and hydrogen (again) to ionize the hydrogen and blast it through a magnetic nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously this is very technically challenging but there is also the issue that the plasma has to be magnetically shielded form the hardware of the ship or it causes electrode erosion in the engines themselves. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Excuse my lack of knowledge of the physics around how this bit actually works.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not to mention you would also need energy to power the nuclear reactors in each design. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Conclusion&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So really, we use chemical based propulsion systems because the alternatives are technologically expensive and difficult. We will struggle to make the chemical propulsion systems propel more because of the multiplying fuel issue (unless we find more efficient fuels). But really the biggest issue isn't so much the propulsion, its the distance!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example the space station currently orbits us at about &lt;a href=&quot;http://education.jsc.nasa.gov/explorers/p4.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;18,000 miles per hour&lt;/a&gt;, orbiting the earth once every 90 minutes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo spacecraft which flew to the moon traveled faster than that, at about &lt;a href=&quot;http://education.jsc.nasa.gov/explorers/p4.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;24,000 miles an hour&lt;/a&gt;. These types of speed are inconceivable for travel on earth, being hundreds of times faster than any jet can go. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So really my argument rests on these points, fuel problem, lack of easy alternatives, cost of fuel, sheer distance.&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="1054" LastEditDate="2017-08-26T15:09:46.517" LastActivityDate="2017-08-26T15:09:46.517" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="743" PostTypeId="2" ParentId="731" CreationDate="2013-07-25T11:01:04.807" Score="1" Body="&lt;p&gt;Logistically it is not rewarding to capture, debris from an comets tail.  Compare it to placing a net in a flood swollen river, sure you can capture lots of stuff, and the only energy is casting and retrieving the net.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the goal of mining is generally to acquire specific resources for specific needs. All mining on earth is focused in specific areas known or believed to harbor larger than average amounts of specific resources.  It is reasonable to expect similar when mining resources in space.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2013-07-25T13:00:22.520" LastActivityDate="2013-07-25T13:00:22.520" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="744" PostTypeId="1" AcceptedAnswerId="750" CreationDate="2013-07-25T11:09:45.753" Score="37" ViewCount="16038" Body="&lt;p&gt;What is the approximate influence of atmospheric drag on the cost of rocket launches? Is it beneficial to have launch sites located at higher altitudes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cape Canaveral is at sea level, but I've noticed that Chinese launch sites are located on quite high altitudes (although they could still be higher if located in Tibet).&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorUserId="49" LastEditDate="2013-09-10T20:45:27.400" LastActivityDate="2021-05-21T21:23:20.803" Title="Effect of atmospheric drag on rocket launches and benefits of high altitude launch sites" Tags="|launch|rockets|atmospheric-drag|atmosphere|high-altitude-launch|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="745" PostTypeId="2" ParentId="739" CreationDate="2013-07-25T11:10:16.443" Score="-4" Body="&lt;p&gt;Probably GPS-like system, swarm of satellites solve this task easily. &#xA;See &lt;a href=&quot;https://en.wikipedia.org/wiki/GLONASS&quot; rel=&quot;nofollow&quot;&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/GLONASS&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/GLONASS&lt;/a&gt; &lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="240" LastActivityDate="2013-07-25T11:10:16.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="746" PostTypeId="2" ParentId="741" CreationDate="2013-07-25T11:50:37.480" Score="4" Body="&lt;p&gt;Simply put: Fuel and drive efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a maximum amount of vector change possible based upon the fuel load aboard, and the efficiency of the drive in converting that fuel into a change of vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To accelerate (which is a synonym for change of vector) for a given time requires a set amount of fuel. All that fuel needs to be aboard, and there's no way to refuel effectively in flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the fuel and the drive efficiency combine to create a maximum total vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And total vector is a synonym for speed.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2013-07-27T05:47:57.473" LastActivityDate="2013-07-27T05:47:57.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="747" PostTypeId="2" ParentId="740" CreationDate="2013-07-25T12:16:04.877" Score="8" Body="&lt;p&gt;The US plan is to buy launches for LEO, and develop its own capsule and launcher for harder missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The options are Soyuz to the ISS, and if any of them work; SpaceX's Dragon, Boeing's CST-100, and SNC's Dream Chaser. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the COTS program, NASA solicited commercial offerings to provide cargo to the space station, trying to wean itself from reliance on Progress/ATV/HTV missions, or at least have their own alternative.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX and Orbital Sciences have developed launchers (Falcon 9 and Antares) and carriers of cargo (Dragon and Cygnus) for that need.  SpaceX has launched 3 times to the station (as of this writing) with 10 more booked.  Orbital is set for its first launches by the end of 2013.  Dragon is unique in that it can return meaningful amounts of cargo, Cygnus will burn up on reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For crew capabilities instead of following through with the COTS-D option, the program became the CCiCAP and SpaceX, Boeing, and Sierra Nevada got funding to develop manned products. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has said it's Dragon design was meant to be manned from the get go, and just needed primarily an abort system, which they are using the CCiCAP program to develop.  (Pusher type using Super Draco thrusters, which are planned for propulsive landing in non-abort cases).  They have a pad abort scheduled for the end of 2013 from a test stand (Why waste a Falcon 9 core on an abort test?) and a full up abort midflight later the following year. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boeing's CST-100 has passed through the various design review phases NASA has expected and making progress.  Same for Dream Chaser.  A test model is in the process of being tested for landing tests (Dragged by a pickup truck along the runway, to test maneuvering and its front skid (instead of a wheel)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA is working on the Orion capsule to be launched by the SLS (or maybe Delta 4 - Heavy) and is meant for longer missions.  None of the commercial alternates are really designed for long missions (Greater than the 2-3 days to the ISS, and day or two to return), whereas Orion is designed for flights to the moon, L1, asteroid missions, or potentially more. There are all sorts of major differences between a 2-3 day mission to get to the ISS then dock, vs dozens to hundreds of days on your own. Things like life support, radiation shielding, power, and just plain endurance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the US manned space flight plan is let commercial companies do service to LEO, where clearly there is a market if three competitors believe there is enough business to compete.  For the 'hard stuff' that is not commercially available, develop it through NASA.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-07-29T12:03:31.980" LastActivityDate="2013-07-29T12:03:31.980" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="748" PostTypeId="2" ParentId="731" CreationDate="2013-07-25T12:22:53.747" Score="6" Body="&lt;p&gt;The advantage of asteroid mining is that there's very little mining involved; almost all of the resources are exposed, free to take. It's &quot;bringing them home&quot; that's the problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is most expensive in space? Air can be reused. Solar power is abundant as long as you don't fly too far away. The resources are there free for picking. What is most expensive is the ΔV.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Blasting, picking, splitting asteroids, that's all baby level difficulty compared to accelerating the acquired material to speeds at which it will reach its destination within this decade or even century. Transport is the most significant cost in any space-based economy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, what would you pick, chasing a comet on elliptic orbit running far beyond orbit of Pluto, average ΔV comparable to that required to reach Saturn, struggling with not only slowing them down enough to get them to Earth but curving their orbit to make it circular instead of elliptic, or just bundle a bunch of mined material, strap it to a solar sail and gradually &lt;a href=&quot;https://space.stackexchange.com/questions/370/can-you-tack-against-the-sun-using-a-solar-sail&quot;&gt;tack against the Sun&lt;/a&gt; until they reach their destination - then drop them on a desert or leave for Earth orbital manufacturing, and return the same &quot;solar barque&quot; using the same solar sail back to the asteroid belt where another batch of mined resources awaits, all that practically for free?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-07-25T12:22:53.747" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="749" PostTypeId="2" ParentId="744" CreationDate="2013-07-25T13:20:31.633" Score="4" Body="&lt;p&gt;The flight path of a rocket is barely focused on 'up'.  It is easy to get high enough to where orbit is possible.  Consider the case of sounding rockets, which even smaller organizations or amateur rocket groups can achieve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hard part is going fast enough to orbit.  I.e. Getting up to speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus a rocket typically heads straight up quickly to get out the thick part of the atmosphere and then turns and primarily accelerates to orbital speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starting from a higher altitude would be minorly helpful (I cannot quantify the number, alas) for the very first few moments of flight, but after that would be of little benefit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As noted in the comments the extra cost of logistics for getting fuel, oxidizer, parts, and payloads to the higher altitudes would mostly likely not be worth it.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-07-25T13:20:31.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="750" PostTypeId="2" ParentId="744" CreationDate="2013-07-25T15:21:34.557" Score="30" Body="&lt;p&gt;I'll give you the numbers.  I'm breaking this up into 3 different terms.  There's atmospheric drag, what I'll call the &quot;hover&quot; term, and the gravitational potential climb.  I will more or less assume a flight directly up.  You're welcome to use whatever term for velocity you want, as none of them will be representative.  I'll take &lt;a href=&quot;http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml&quot;&gt;the Shuttle's speed at halfway to max Q&lt;/a&gt;.  This is 1000 ft/s, or about 300 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would think atmospheric drag would be very difficult.  It's actually not.  In any case, you would probably use the v^2 relationship for drag.  But if you think about where that comes from, it basically assumes that all the air in front of you is accelerated to the speed of your craft (minus any departure from unity in the drag coefficient).  So for a good approximation, just take the mass-thickness (I call mu) for the entire atmosphere, and multiply by the metric for velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I'll use the numbers for Falcon 9, which is a diameter of 3.66 meters and launch mass of 333,400 kg.  Yes, a lot of these numbers change over the course of the flight, but in ways that are fairly obvious if you changed this to do numerical integration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V (drag) = 1/2 \mu C_d A v / M $$&#xA;$$= (0.5)(10 \text{ tonnes} / m^2)(0.5)\pi(3.66/2 m)^2(300 m/s)/(333.4 \text{ tonnes}) $$&#xA;$$= 23.7 m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wow.  That is not much.  Maybe velocity should be higher.  But still, out of 10 km/s total, this is a tiny amount.  Atmospheric drag complicates launches, but not much because of its Delta v value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, the &quot;hover&quot; term.  This represents the gravity drag.  Again, I'm forced to assume a pretty much upward launch.  I'll also compare sea-level to Mt. Everest, at a height of 8,848 m.  Not that you'd set up a launchpad there, but we need this to answer the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V = g h / v&#xA;= (9.8 m/s^2)(8,848 m)/(300 m/s) = 298 m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now this is much more significant.  This isn't all of the gravity drag either.  It's still sucking your delta v budget after you're out of the atmosphere, until you get to full orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's move on to the gravitational potential itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V = \sqrt( g h )&#xA;= \sqrt( (9.8 m/s^2)  (8,848 m) ) = 294.5 m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sum of all these is a ballpark estimate of the benefit you would get from changing your launch location from sea-level to Mt. Everest.  Honestly though, you save a comparable amount just by moving it down to the equator, where the Earth's rotation gives you a bigger boost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, this is 616.7 m/s out of a total budget of 10 km/s.  So it would be less than 10%.  By the rocket equation, this can still make a difference.  But then again, actual costs are complicated.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-09-11T16:26:43.530" LastActivityDate="2013-09-11T16:26:43.530" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="751" PostTypeId="5" CreationDate="2013-07-25T16:04:27.953" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-25T16:04:27.953" LastActivityDate="2013-07-25T16:04:27.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="752" PostTypeId="4" CreationDate="2013-07-25T16:04:27.953" Score="0" Body="Missions expected to happen over the next decade, but that have not launched yet. In particular if such is only a concept at this time." OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-25T16:41:56.727" LastActivityDate="2013-07-25T16:41:56.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="753" PostTypeId="2" ParentId="739" CreationDate="2013-07-25T16:37:45.537" Score="10" Body="&lt;p&gt;Unless there is direct mission cooperation with NASA or ESA, these countries generally do not rely on help from those two organizations. Exceptions to this are when there is a failure or other spacecraft emergency, as with the recent &lt;a href=&quot;https://en.wikipedia.org/wiki/Fobos-Grunt#Contact&quot;&gt;Phobos-Grunt&lt;/a&gt; mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What do they do instead? Well, if we're just talking tracking, there's usually no need to constantly track a spacecraft. A few passes per day will give you enough info to nail down your orbit, and in LEO you will get at least a couple passes per day over your home country. Of course, these countries also have at least some semblance of a GNS system, which can also be used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of data relay, often strategically-placed relay satellites help solve this problem. For instance, Russia has its own &lt;a href=&quot;http://en.wikipedia.org/wiki/Luch_%28satellite%29&quot;&gt;Satellite Data Relay Network&lt;/a&gt;, similar to the US's &lt;a href=&quot;http://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite&quot;&gt;TDRSS&lt;/a&gt;, which can receive transmissions from a satellite in orbit and send the signal to a ground station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One more thing: NASA's Deep Space Network is normally only used for very distant interplanetary missions. Significantly smaller ground station antennae can be used for Earth-orbiting satellites. For non-ESA/US countries, the same principle applies for interplanetary missions... just track it when you can see it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To expand a bit more... yes, &lt;a href=&quot;http://www.nasaspaceflight.com/2012/07/long-march-3c-tracking-data-relay-satellite/&quot;&gt;China&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Indian_National_Satellite_System&quot;&gt;India&lt;/a&gt; appear to have data relay satellite systems, although if you're interested in interplanetary mission communication these would not be used. Instead, I just discovered that &lt;a href=&quot;http://en.wikipedia.org/wiki/Soviet_Deep_Space_Network&quot;&gt;Russia&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Chinese_Deep_Space_Network&quot;&gt;China&lt;/a&gt;, and &lt;a href=&quot;http://en.wikipedia.org/wiki/Indian_Deep_Space_Network&quot;&gt;India&lt;/a&gt; all have their own DSN's (which I now see you've linked to in your OP, oops).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-07-25T17:06:15.703" LastActivityDate="2013-07-25T17:06:15.703" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="754" PostTypeId="2" ParentId="594" CreationDate="2013-07-25T16:57:25.330" Score="2" Body="&lt;p&gt;So, there are two aspects to this problem. The first is one that we deal with today on a regular basis, that of the Doppler shift of a spacecraft orbiting the Earth, or otherwise moving towards/away from the ground-station. This is the same phenomena as the Red Shift, just at a lower magnitude. Why is this important? For a 150 MHz uplink, the Doppler shift is around 3 kHz for a satellite in LEO. That actually is a channel in some modes of communication. The higher the frequency, the larger the jump, such that an FM channel could entirely be passed, if the doppler is not taken in to account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, how do you correct for that? Basically, calculate what the doppler should be, and send at a frequency such that the spacecraft will get the right signal. Additionally, you can build a control circuit such that the detected frequency window can drift slightly, according to the carrier frequency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the second problem, that of radiation becoming higher energy as you approach it. How is this managed? First of all, let's see where it would become an issue. Let's assume a frequency of 10^15, which is in the realm of visible light. The energy becomes ultraviolet at an increase of about 10 fold frequency increase. To do that, you have to be moving at 0.9c, thus, you are hit by the wavelength 10 times more (1/(1-speed)), giving you an increase of 10 in frequency. That is a very significant speed. Furthermore, this will only be coming from the front half of the spacecraft, the back half will see a 50% decrease in frequency, which is harmless (See &lt;a href=&quot;http://www.calctool.org/CALC/phys/default/doppler&quot; rel=&quot;nofollow&quot;&gt;this calculator&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what else is a spacecraft going to need to do that is moving that fast? There must be a way to protect it against small particles, even the most minute particle would cause issues at that speed. That would involve some physical shielding, no doubt, on the forward site. In addition, there is high amounts of radiation anyways in space, which will need protection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, radiation from moving this fast isn't likely to be a problem. It will likely be solved when dealing with other issues the spacecraft will have.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-25T16:57:25.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="755" PostTypeId="2" ParentId="593" CreationDate="2013-07-25T17:52:13.143" Score="18" Body="&lt;h2&gt;Orbiter&lt;/h2&gt;&#xA;&lt;p&gt;The Solid State Mass Memory (SSMM) is 25 Gbit (about 3 GByte) with a 5Mbps data rate.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The telemetry rate shall be switchable between 8 bps and 65536 bps&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The bus throughput shall be minimal 131 kbps.&lt;/p&gt;&#xA;&lt;p&gt;The SSMM shall support an input data rate of up to 5 Mbps useable data (physical data rate excluding IEEE-1355 protocol overhead)&lt;/p&gt;&#xA;&lt;p&gt;The telecommand rate shall be switchable between at least 4 bit rates, i.e. 7.81 (4000/2 ), 250, 1000 and 2000 bps.&lt;/p&gt;&#xA;&lt;p&gt;The usable memory size shall be at least 1MWord RAM and 512 KWords EEPROM for each of 4 processors, and 512KWords PROM (redundant) accessible from each processor.&lt;/p&gt;&#xA;&lt;p&gt;The SSMM shall have a size of at least 25 Gbit at end of life taking into account failures in memory cells.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;ftp://ftp.estec.esa.nl/pub2/sci-spd/roserid/System%20Requirements%20Specification,%20RO-EST-RS-2001%20issue%206.pdf&quot; rel=&quot;noreferrer&quot;&gt;Found Here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h2&gt;Lander&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Central Interface Unit (CIU) provides the communication among the units of the Lander. This is a synchronous, serial data transmission channel with the transmission rate of 32kbit/s.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Harris RTX2010 processor has been selected for the DPU boards&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Note that there are two Data Process Units (DPU) in warm-redundant mode. Also note that this is a 16 bit processor.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.researchgate.net/publication/234265387_Embedded_Computer_System_on_the_Rosetta_Spacecraft&quot; rel=&quot;noreferrer&quot;&gt;Found Here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Data: 235 Mbit during primary mission, 65 Mbit during each subsequent&#xA;60 h period&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Data rate 16 kbit/sec&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;also&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;central computing and data storage capability (2x2 Mbyte, RAM, EEPROM)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.planetaryprobe.eu/IPPW7/proceedings/IPPW7%20Proceedings/Presentations/Session7B/pr395.pdf&quot; rel=&quot;noreferrer&quot;&gt;Found Here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-07-25T20:33:21.577" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="756" PostTypeId="1" AcceptedAnswerId="1685" CreationDate="2013-07-25T18:10:55.247" Score="27" ViewCount="900" Body="&lt;p&gt;There are dozens of speculative proposals to explore &lt;a href=&quot;https://en.wikipedia.org/wiki/Europa_(moon)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Europa&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Enceladus_(moon)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Enceladus&lt;/a&gt;, and possibly other Solar System bodies (including &lt;a href=&quot;https://en.wikipedia.org/wiki/90377_Sedna&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sedna&lt;/a&gt; and other Trans-Neptunians!) that may be endowed with liquid water under a crust of ice (due to tidal heating or radioactive decay). One of them (IIRC) was sending crawling robots to exploit (possibly non-existing) cracks in the ice. Another set of rather fantastic-looking proposals hinges on manned exploration and drilling. There are also &lt;a href=&quot;https://en.wikipedia.org/wiki/Cryobot&quot; rel=&quot;nofollow noreferrer&quot;&gt;ice-melting cryobot&lt;/a&gt; ideas floating aroung.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in cheaper and preferably reliable ways of getting knowledge about the properties of these oceans up to tests for presence of life &lt;em&gt;in situ&lt;/em&gt;, without sample return.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;What is the limit of data obtainable from orbital missions? (please state notional or existing instruments and orbits required, including perhaps &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Recovery_and_Interior_Laboratory&quot; rel=&quot;nofollow noreferrer&quot;&gt;multi-satellite formations&lt;/a&gt;)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How can we get data from inside the oceans? (Can, for instance, penetrators be used - cf. &lt;a href=&quot;https://en.wikipedia.org/wiki/LCROSS&quot; rel=&quot;nofollow noreferrer&quot;&gt;LCROSS&lt;/a&gt;?)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sci.esa.int/juice&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter Icy Moons Explorer&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://users.ictp.it/~chelaf/searching_for_ice.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Searching for ice and ocean biogenic activity&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Related questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/331/what-are-the-constraints-for-a-successful-robotic-mission-to-europa&quot;&gt;What are the constraints for a successful robotic mission to Europa?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-12-27T14:19:54.693" Title="How to explore subsurface oceans (cheaply)?" Tags="|life|water|planet|" AnswerCount="2" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="758" PostTypeId="1" AcceptedAnswerId="761" CreationDate="2013-07-25T19:58:28.520" Score="0" ViewCount="296" Body="&lt;p&gt;I heard about an 'ocean' under Europa's ice surface and that there are higher temperatures.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible that there are amino acids or simple organic molecules like there was on the early Earth?&lt;/p&gt;&#xA;" OwnerUserId="139" LastEditorUserId="108" LastEditDate="2013-07-26T00:27:05.350" LastActivityDate="2013-07-26T00:27:05.350" Title="How possible are simple life forms on Europa?" Tags="|astrobiology|" AnswerCount="1" CommentCount="7" ClosedDate="2013-07-26T00:57:01.133" ContentLicense="CC BY-SA 3.0" />
  <row Id="759" PostTypeId="1" AcceptedAnswerId="760" CreationDate="2013-07-25T20:13:07.220" Score="8" ViewCount="295" Body="&lt;p&gt;Is is possible with today's technology, to take a photo of an extra-solar planet?  This applies not only to the visual wavelength (380-780 nm), but any wavelength.  &lt;/p&gt;&#xA;" OwnerUserId="139" LastEditorUserId="25" LastEditDate="2015-12-17T22:46:54.550" LastActivityDate="2015-12-17T22:46:54.550" Title="Is it possible to take images of extrasolar planets?" Tags="|imaging|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="760" PostTypeId="2" ParentId="759" CreationDate="2013-07-25T20:22:11.240" Score="12" Body="&lt;p&gt;Yes.  Here is the first image of an extrasolar planet taken from 2005: &lt;a href=&quot;http://apod.nasa.gov/apod/ap050510.html&quot;&gt;http://apod.nasa.gov/apod/ap050510.html&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-07-26T05:59:04.400" LastActivityDate="2013-07-26T05:59:04.400" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="761" PostTypeId="2" ParentId="758" CreationDate="2013-07-25T22:39:15.953" Score="8" Body="&lt;p&gt;Since we've never discovered life outside our own planet there's really no basis to judge the likelihood of life on Europa. We just don't know how common life is, because we only have the one sample.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, we do know that &lt;a href=&quot;https://en.wikipedia.org/wiki/Extremophile&quot;&gt;life can be found&lt;/a&gt; in some of the most extreme environments Earth has to offer. This leads us to believe that it's possible for life to survive in such an &lt;a href=&quot;https://en.wikipedia.org/wiki/Europa_%28moon%29#Potential_for_extraterrestrial_life&quot;&gt;environment as Europa&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But &lt;em&gt;how possible&lt;/em&gt;? We just don't have enough information yet to make a reasonable guess at that. Given the information we have, in our solar system &lt;em&gt;alone&lt;/em&gt; there are about 175 natural satellites and planets (and that number is always growing), and only one of which has confirmed life. We might want to narrow it down to which bodies have at least the conditions where an extremophile could survive (and that's only if we're looking for Earth-like life) (&lt;a href=&quot;http://www.wired.com/wiredscience/2009/01/et-life/&quot;&gt;or at least the top five?&lt;/a&gt;) (or &lt;a href=&quot;http://kokogiak.com/solarsystembodieslargerthan200miles.html&quot;&gt;those over 200 miles in diameter&lt;/a&gt;?). However even Earth life has shown that it can &lt;a href=&quot;http://www.newscientist.com/article/dn14690-water-bears-are-first-animal-to-survive-space-vacuum.html&quot;&gt;survive in &lt;em&gt;vacuum&lt;/em&gt;&lt;/a&gt;, which opens up the list again to everything we can find in the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, &lt;strong&gt;.5%&lt;/strong&gt;?&lt;/p&gt;&#xA;" OwnerUserId="234" LastActivityDate="2013-07-25T22:39:15.953" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="762" PostTypeId="1" AcceptedAnswerId="764" CreationDate="2013-07-25T23:49:29.087" Score="12" ViewCount="2041" Body="&lt;p&gt;I think that many 3D modeling software packages like &lt;a href=&quot;http://www.blender.org/&quot; rel=&quot;noreferrer&quot;&gt;Blender&lt;/a&gt; and &lt;a href=&quot;http://www.rhino3d.com/&quot; rel=&quot;noreferrer&quot;&gt;Rhino-3D&lt;/a&gt; assume the presence of gravity acting downwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would a person use to model a structure in microgravity (more specifically, in free fall) like a space station or a space habitat like those proposed by &lt;a href=&quot;http://en.wikipedia.org/wiki/Gerard_K._O%27Neill&quot; rel=&quot;noreferrer&quot;&gt;Gerard K. O'Neill&lt;/a&gt; and the &lt;a href=&quot;http://ssi.org/&quot; rel=&quot;noreferrer&quot;&gt;Space Studies Institute&lt;/a&gt; in a 3D software package?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YqrNz.jpg&quot; alt=&quot;Bernal Sphere Size Comparison&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="108" LastEditorUserId="12102" LastEditDate="2020-10-28T06:52:00.417" LastActivityDate="2020-10-28T06:52:00.417" Title="3D modeling software for zero gravity" Tags="|microgravity|3d-modeling|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="763" PostTypeId="2" ParentId="762" CreationDate="2013-07-25T23:53:33.107" Score="10" Body="&lt;p&gt;Most 3D modelling packages let you define forces - I wasn't aware Blender had gravity turned on by default, but you can turn it off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally though, modelling is unlikely to require it unless you are doing detailed stress analysis, in which case I'm not sure Blender would be up to the job.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-25T23:53:33.107" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="764" PostTypeId="2" ParentId="762" CreationDate="2013-07-26T00:44:39.580" Score="11" Body="&lt;p&gt;I have a fair amount of experience using Blender, Maya, and 3DS Max, three of the most popular 3D programs, and all allow you to set gravity to any value, including zero, for physics simulations. I believe that most of the Autodesk/AutoCAD software allows you the same freedom, though I can't speak from experience there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, a vast majority of 3D modeling doesn't require gravitational calculations to begin with, and these forces only become relevant when you turn physics simulations on. Unless you are specifically interested in computing object collisions, you don't need to worry about what gravity is set to.&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2013-07-26T00:44:39.580" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="765" PostTypeId="1" AcceptedAnswerId="766" CreationDate="2013-07-26T10:51:57.383" Score="20" ViewCount="4676" Body="&lt;p&gt;Given sufficient liquid in the form of water or other suitable medium, would the use of the lunar analogue of &lt;a href=&quot;http://en.wikipedia.org/wiki/Geothermal_energy&quot; rel=&quot;nofollow noreferrer&quot;&gt;geothermal energy&lt;/a&gt; be practical or feasible for powering a lunar colony?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Internal_structure_of_the_Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; (referenced) gives an internal temperature for the Moon's core of 1600–1700 K, which would seem to easily be sufficient for creating steam to power turbines.  Unlike Earth, there is no volcanic activity near the surface so drilling would likely have to be much deeper, how deep I am not sure.&lt;/p&gt;&#xA;&lt;p&gt;Other challenges are &lt;a href=&quot;http://en.wikipedia.org/wiki/Moonquakes#Moonquake&quot; rel=&quot;nofollow noreferrer&quot;&gt;moonquakes&lt;/a&gt; and the lack of water in the ground. Which could lead to higher risk of fracturing a pipe and leakage of heat transferring fluid (a parched Luna would more quickly leach away fluids).&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="7062" LastEditDate="2022-06-08T11:54:08.600" LastActivityDate="2023-01-12T06:54:09.773" Title="Would the lunar analogue of geothermal energy be practical on a lunar base?" Tags="|the-moon|colonization|temperature|energy|geology|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="766" PostTypeId="2" ParentId="765" CreationDate="2013-07-26T11:33:29.333" Score="16" Body="&lt;p&gt;The &amp;quot;much deeper&amp;quot; drilling would have to be roughly 900km to get near to the core. No drilling of this scale has been ever attempted on Earth. Lower gravity will make it easier but not much easier, and you &lt;em&gt;will&lt;/em&gt; have to have pumping stations from time to time - no pipe will withstand 900km long column of water, even in lunar gravity.&lt;/p&gt;&#xA;&lt;p&gt;On top of the difficulties you list, there's filling the pipes with water - to make it worthwhile and not cool down to zero within first 100 meters the pipes should have some reasonable diameter. Say, 1m&lt;sup&gt;2&lt;/sup&gt; cross section &lt;span class=&quot;math-container&quot;&gt;$\times$&lt;/span&gt; 900,000m length, times two (up and down) that's 1,800,000 tons of water just to fill it. How do you plan to bring it there?&lt;/p&gt;&#xA;&lt;p&gt;Sure, you might just try to generate electricity deep down there and send it up by wires. This requires the heating&lt;span class=&quot;math-container&quot;&gt;$\rightarrow$&lt;/span&gt;cooling cycle for the water though and while you have heating available locally, cooling might pose some problems. You can't use cheap evaporation, or convection. You're pretty much stuck with dissipation into native rock, a huge grid of thin pipes conducting the heat away into the rock far enough from the &amp;quot;hot zone&amp;quot; to make it worthwhile. The further you get the more water you need.&lt;/p&gt;&#xA;&lt;p&gt;On the other hand:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;there is no air to disperse solar heat;&lt;/li&gt;&#xA;&lt;li&gt;there are no clouds to obstruct it;&lt;/li&gt;&#xA;&lt;li&gt;the dust flies very short way without wind to carry it, to cover the panels;&lt;/li&gt;&#xA;&lt;li&gt;there is one rotation per month, so automatic tuning systems are idle most of the way;&lt;/li&gt;&#xA;&lt;li&gt;the moonquake risks are minimal on the surface;&lt;/li&gt;&#xA;&lt;li&gt;there is no need for bulk amounts of expensive materials like water. Quartz is abundant on the surface.&lt;/li&gt;&#xA;&lt;li&gt;Sure, you have a half-month-long night (when you might need to use stored energy or pull it by wire from the other side of the Moon... or at least use an orbital reflector to bring it to the panels) but then you get half a month of constant energy.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;In other words, if we didn't have the option of abundant, efficient, cheap solar energy we might feel compelled to seek other options like geothermal on the Moon. Currently though, there is simply no reason to pick a solution several orders of magnitude more expensive when you have something so cheap and efficient as solar power.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="47149" LastEditDate="2022-06-01T23:24:59.667" LastActivityDate="2022-06-01T23:24:59.667" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="767" PostTypeId="2" ParentId="765" CreationDate="2013-07-26T11:41:32.493" Score="7" Body="&lt;p&gt;The first question is, how far do you have to drill? I found &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2012/pdf/1564.pdf&quot;&gt;an article&lt;/a&gt; that provides a reasonable temperature profile. Given that profile, how far would we need to go? It seems that most geothermic energy sources rely on water near the boiling point, or ~400 K. Given that as our benchmark, the model seems to say we'd need to go about 70 km deep to pull it off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, how far have we gone on Earth? &lt;a href=&quot;http://en.wikipedia.org/wiki/Kola_Superdeep_Borehole&quot;&gt;This Wikipedia article&lt;/a&gt; references a 12 km hole dug. That's a far cry from 70 km! And that requires lots of heavy machinery to achieve this feat!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it could be done, and probably will be done some day, but there are far more practical ways of getting power for now.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-26T11:41:32.493" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="768" PostTypeId="1" AcceptedAnswerId="1005" CreationDate="2013-07-26T12:23:24.197" Score="15" ViewCount="2055" Body="&lt;p&gt;ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta spacecraft&lt;/a&gt; is scheduled to orbit &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow noreferrer&quot;&gt;67P/Churyumov–Gerasimenko&lt;/a&gt; after it reaches the comet by mid-2014:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rosetta's main objective is to rendezvous with, and enter orbit&#xA;  around, comet 67P/Churyumov-Gerasimenko and to perform observations of&#xA;  the comet's nucleus and coma. During the period that Rosetta orbits&#xA;  the comet, 67P/Churyumov-Gerasimenko will reach the closest point to&#xA;  the Sun in its orbit, allowing for the consequent increase in activity&#xA;  to be measured. A lander, named Philae, will be deployed and it will&#xA;  attempt to make the first ever controlled landing on a comet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(source: &lt;a href=&quot;http://sci.esa.int/rosetta/47366-fact-sheet/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA's Rosetta Fact Sheet&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5UWRu.jpg&quot; alt=&quot;Rosetta&amp;#39;s 67P/Churyumov–Gerasimenko rendezvous&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Trajectory of the Rosetta Space Probe (source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the comet approaches the Sun, ejected materials will increase its coma and tails in size and particle density due to increase in &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pressure&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiation pressure&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar wind&lt;/a&gt;, and heat exchange on the Sun-facing surface of the comet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I'm curious is, what (if any) adjustments in orbital trajectory are planned by &lt;a href=&quot;http://www.esa.int/ESA&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt; due to this increase in particle density (either it having a deceleration effect on spacecraft's inertia, or to avoid collision with larger comet's debris), and which or the coma and tails (dust tail, ion tail,...) is the spacecraft's orbit planned to intersect along the comet's path around the Sun. In essence, what are ESA's estimates regarding particle density the Rosetta spacecraft will have to travel through while orbiting 67P/Churyumov–Gerasimenko, and the acceptable / planned for margins of error?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T02:44:06.773" LastActivityDate="2014-03-24T10:33:07.840" Title="Will Rosetta have to adjust its orbit around Chury due to the comet's coma and tails?" Tags="|orbital-mechanics|orbit-determination|comet|rosetta|exosphere|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="769" PostTypeId="2" ParentId="744" CreationDate="2013-07-26T12:23:30.783" Score="11" Body="&lt;p&gt;Let's compare two rockets with somewhat similar specifications, but one very large difference.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Falcon 1- Carries about 670 KG to LEO (See the &lt;a href=&quot;http://www.spacex.com/Falcon1UsersGuide.pdf&quot;&gt;User's Guide&lt;/a&gt;) Mass 38555 KG (&lt;a href=&quot;http://en.wikipedia.org/wiki/Falcon_1&quot;&gt;Wikipedia&lt;/a&gt;). Launched from sea level.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Pegasus- Carries about 450 KG to LEO (See &lt;a href=&quot;http://en.wikipedia.org/wiki/Pegasus_%28rocket%29&quot;&gt;Wikipedia&lt;/a&gt;). Mass 18,500 KG. Launched from 40,000 feet.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This makes a lot of assumptions, but let's just assume you could scale linearly the mass of the Pegasus. That would give the rocket a mass of 27000 KG to lift the payload of the Falcon 1. That is a difference of 40% roughly. Why the difference?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Falcon 1 is a LOX/RP liquid propellant rocket whereas Pegasus is solid rocket based. The technologies have vastly different thrust/weight ratios, specific impulse, and mass fractions. Solid rockets tend to have higher mass fractions and T/W because the engines are less complex (i.e. no pressurization systems, or plumbing, or turbo-pump machinery. But solids are typically more expensive, and typically have fixed impulse burns (i.e. they cannot be turned off). This is less of an issue for early stages, but the reason the Pegasus added the optional HAPS (hydrazine) final stage for precision orbit insertion.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Because it is air-launched Pegasus can fly a different trajectory. Instead of a low angle-of-attack ascent followed by a gravity turn, Pegasus has wings. It flies at positive angle of attack and uses lift to aid in ascent.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Engines at altitude can use a more efficient engine design (i.e. rocket nozzle expansion ratios tuned to atmospheric pressure at drop vice sea level.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There is a small velocity gain from the air launch. This isn't significant (~2% of orbital velocity), but is there.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Pegasus doesn't have to worry about changing inclinations, as the Falcon 1 does. But the specified numbers for Falcon 1 don't take in to account a varied inclination.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There is considerably less air density at 40,000 feet, which results in lower integrated drag losses.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You are 10 km higher at altitude. This probably isn't significant.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, raising your altitude of your launch site will give you an increase in performance, both by increasing your engine efficiency, and reducing your drag. These numbers wouldn't be as drastic for a launch site of 10,000 feet, but they still would represent a measurable change in performance.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-28T18:15:46.237" LastActivityDate="2013-07-28T18:15:46.237" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="770" PostTypeId="1" AcceptedAnswerId="773" CreationDate="2013-07-26T15:05:16.487" Score="23" ViewCount="2435" Body="&lt;p&gt;Some proposals for living in space envision finding an asteroid that is a solid chunk of something (i.e. Nickel Iron), hollow it out, slap on an air lock, and you have a home.  Add a drive and you have a space ship.  Of course the reality is a little more complex, but the idea has merit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting aside, spin and thrust forces (way too many variables for a simple question), what is the largest naturally occurring space body that we know of or can reasonably hypothesise the existence of that could be hollowed, filled with air at a human comfortable range, and lived in?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is easy enough to image an asteroid that is essentially the iron core of a planet, so it reasonable to assume that could be an option.  Also a planetoid at or near the size of our moon could conceivably have a core that was cooled sufficiently to tunnel to the center of and create a hollow.  Our Moon being in orbit around Earth, has significant tidal stress, (likely keeping the core warm) so would not be a candidate.  Though an identical body in a solar orbit, or interstellar trajectory, might be a candidate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to Clarify Scope&lt;/strong&gt;&#xA;I was thinking, with a planetoid in a significant gravity well (like the moon or earth), all the mass would be left on the body. With a small body, “mined” materials would likely be sold or ejected. Some materials from the body or outside source would be used for sealing &amp;amp; supporting.  For this question I define hollow as allowing essentially an obstructed view of the interior, as would be the case with a Dyson Sphere.  Cross supports would be practical in a small body of a few hundred meters, (though would violate the definition of hollow as used here).  For a hollow measured in hundreds of kilometers, cross braces would be impractical for the same reason that a space elevator is impractical; there are no reasonable means of spanning with sufficient strength, without resulting in honeycombing or tunneling.  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2013-07-26T16:25:31.387" LastActivityDate="2015-03-05T21:25:05.673" Title="What is the largest naturally occurring body that could be hollowed and safely lived in?" Tags="|interstellar-travel|asteroid|space-station|colonization|" AnswerCount="2" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="771" PostTypeId="1" CreationDate="2013-07-26T15:35:48.510" Score="1" ViewCount="1501" Body="&lt;p&gt;Where is the center of universe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/AXqM4.jpg&quot; alt=&quot;Universe&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is cosmic microwave background radiation, and there is its Doppler effect. Is there any map with geographical center of universe? What is situated there? How far from us this center? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found this:&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/History_of_the_Center_of_the_Universe&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/History_of_the_Center_of_the_Universe&lt;/a&gt;&#xA;, but it's like half-mythology theory and no facts/photos.&lt;/p&gt;&#xA;" OwnerDisplayName="user267" LastEditorDisplayName="user12" LastEditDate="2013-08-05T04:09:15.427" LastActivityDate="2013-08-05T04:09:15.427" Title="Where is the center of space / universe?" Tags="|deep-space|" AnswerCount="1" CommentCount="14" ClosedDate="2013-07-26T16:59:57.993" ContentLicense="CC BY-SA 3.0" />
  <row Id="772" PostTypeId="2" ParentId="771" CreationDate="2013-07-26T15:56:36.317" Score="5" Body="&lt;p&gt;When you believe in the Big Bang theory, which is the standard theory, the universe started from a singularity. With the Big Bang space evolved. The room itself evolved so you can either say there is no center or the center is anywhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason there is no background radiation wave that passed from the center over everything else is because there's no such thing as a center. The space evolved and therefore the center. The background radiation is everywhere in the universe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't take a picture of the universe in total.&lt;/p&gt;&#xA;" OwnerUserId="107" LastEditorUserId="107" LastEditDate="2013-07-26T17:30:10.933" LastActivityDate="2013-07-26T17:30:10.933" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="773" PostTypeId="2" ParentId="770" CreationDate="2013-07-26T16:19:35.560" Score="31" Body="&lt;p&gt;&lt;strong&gt;Caves&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any large body where you can survive on the surface to begin with can be used for caves, artificial or natural.  This observation isn't off-topic because you ask about holding in atmospheric pressure.  Moon caves can hold 1 atmosphere of pressure about 60 meters below the surface with no stresses on the surrounding rock.  I'm using very simple math here.  If you assume a density of 1 g/cm&lt;sup&gt;3&lt;/sup&gt;, then 10 meters head of water is needed to get 1 atmosphere of pressure.  The moon has 1/6th the gravity of Earth.  QED.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is possible on many bodies.  It's possible on Ceres, Vesta, even.  But as we decrease the mass of the body, the cave has to get deeper to maintain 1 atmosphere of pressure.  The gravity in the living area also decreases (obviously, because it's a smaller body).  So there's only a limited set for which this would make sense.  Plus, many bodies have an atmosphere to begin with, so caves are either supplementary or useless there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the most compelling places for caves is Mercury.  This is argued very well by &lt;a href=&quot;http://einstein-schrodinger.com/mercury_colony.html&quot; rel=&quot;noreferrer&quot;&gt;Jim Shifflett&lt;/a&gt;.  I don't mean that in the sense that a Mercury colony would be preferable to Mars or the Moon (although there is apparently disagreement on that count), but I do mean that caves &lt;em&gt;are the only way&lt;/em&gt; to set up bases on Mercury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Center Caves&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's return to the idea of Ceres.  Its central pressure is something like 200 Earth atmospheres.  Gravity increases roughly linearly with radius, so your caves would be roughly 1/400th of the dwarf planet's radius under the surface.  You might also be swimming, but that's a separate topic.  What happens when the object is &lt;em&gt;so small&lt;/em&gt; that the cave would have to wind up in the center?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, to begin with, you would have zero gravity.  According to the shell theorem, gravity would cancel out perfectly due to the rock of the asteroid itself.  There is &lt;a href=&quot;https://physics.stackexchange.com/questions/21298/keeping-air-in-a-giant-gravitationally-bound-space-balloon&quot;&gt;a particular size&lt;/a&gt; at which the center is naturally at 1 Earth atmosphere exactly.  This puts you at an asteroid of a diameter of about 20 km.  Many objects of this size are well cataloged, and we know a decent amount about them.  Gaspra is an example.  It is also not round.  Now, onto that subject...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is room for debate as to whether the requirement of no structural forces is a reasonable one.  This is the topic of &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_differentiation&quot; rel=&quot;noreferrer&quot;&gt;Planetary differentiation&lt;/a&gt;.  Many asteroids are rock, forged in the interior of a larger body, and they only separated because of collisions.  That means they could have been formed of molten materials under lots of pressure, making a fairly cohesive body.  This is actually fairly common.  The limit for differentiation is generally thought to be well above the 20 km diameter point, and likely closer to the 100s of km range.  This we know from studies of moons.  We have relatively less information about free asteroids because moons have historically been a more obvious type of object to study, and their sizes are shifted more towards larger sizes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to sum up this rant, let me establish this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Q: could you empty the center of a 20 km size asteroid and fill it with gas?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A: It would leak out, but if you kept it from leaking out the laws of physics tell us definitively &quot;yes&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Q: could you empty the center of a 20 km size asteroid and expose it to the vacuum of space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A: It might collapse in on itself, but we don't know for sure.  It lies near the boundary of required material strength.  It would also depend on the size of the cavity you carve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also some obvious arguments of scale that follow.  For instance, could you hollow out the center of a 1 km asteroid and expose it to space without collapse?  Sure.  We have pretty good odds of that working, because that's only fighting its self-gravitation.  That fact that much larger objects are undifferentiated means that odd-shaped structures of km-scale can hold against their self-gravity.  But &quot;can&quot; is different from &quot;will&quot;.  This is what you need the civil engineers for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Gravity Balloons&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's get ridiculous.  What if I took a 20 km size object, started hollowing out the center, filling it with a sealed pocket of air... and I just kept going?  How big could I go before it collapses into a big mess?  Well, there is no limit, per se.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have the problem of natural differentiation - that works against you.  It wants to upturn your air bubble, putting low density (air) on the outside and high density (rock) in the middle.  But it's not differentiated to begin with, so that's not going to happen very easily.  So say you've hollowed out &lt;em&gt;nearly&lt;/em&gt; a 20 km diameter air bubble in the center.  Now you've moved almost all of the rock, breaking it up into little pieces.  Well, studies of asteroids have established tumbling angles on the order of 7% to 14% rise-to-run, the same for sand piles on Earth (not surprising).  You could avoid steep angles while rearranging the rocks, although the idea of dynamical failure is still terrifying.  So it's &lt;em&gt;possible&lt;/em&gt; that you could corral material to hold in a very large volume of air - all with no material strength.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is the detail that the naturally supported pressure changes with the size of the air bubble.  But this follows very predictable math, which is trivial to work out.  The only real limit is when the air volume becomes so amazingly massive that the &lt;em&gt;gravity of the air itself&lt;/em&gt; limits the habitable range.  Someone wrote a sci-fi book about this exact very thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.kschroeder.com/my-books/sun-of-suns&quot; rel=&quot;noreferrer&quot;&gt;http://www.kschroeder.com/my-books/sun-of-suns&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now there are two major flaws in the vision that this author painted.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;He thought the envelope had to be made of carbon nano-tubes.  This is ridiculous.  You could make it out of a sheet of rock 10 km thick.  You have stability problems, but what do you care if you can make Carbon nanotubes anyway...&lt;/li&gt;&#xA;&lt;li&gt;Artificial gravity habitats are rotating in free space.  This creates impossible drag forces.  I wrote about a reasonable way to do it &lt;a href=&quot;http://gravitationalballoon.blogspot.com/&quot; rel=&quot;noreferrer&quot;&gt;in a blog&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This is all very fanciful thinking.  There are only 2 objects in the entire inner solar system that could hold breathable air in their center to begin with - Eros and Phobos.  And even that would be thin, Oxygen-rich atmosphere (like Skylab).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might be that producing breathable air in the first place is a greater challenge.  Near Earth asteroids are all far too small for the crazy scheme I described.  But they might also lack sufficient Hydrogen and Nitrogen.  Getting the materials for life would require infrastructure to begin with.  If you need a place to park those gases you produce, it shouldn't be too hard in the cosmic sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll get back to the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;what is the largest naturally occurring space body that we know of or can reasonably hypothesise the existence of that could be hollowed, filled with air at a human comfortable range, and lived in?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The issue is that large bodies have a central pressure that's too high for humans.  If your limit is about 3 Earth atmospheres (a reasonable biological limit), the largest body would be something like &lt;a href=&quot;http://en.wikipedia.org/wiki/132_Aethra&quot; rel=&quot;noreferrer&quot;&gt;132 Aethra&lt;/a&gt;, an unassuming body on the inner edge of the asteroid belt.  If you made the center air bubble large enough, however, that pressure would drop.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-03-05T21:25:05.673" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="774" PostTypeId="2" ParentId="705" CreationDate="2013-07-26T23:42:06.937" Score="15" Body="&lt;p&gt;First, there's a &lt;em&gt;lot&lt;/em&gt; that can go wrong with a parachute + air bag landing system.  Second, the Mars Pathfinder / Mars Exploration Rover systems also required rockets to fire just before reaching the ground.  In that case they were solid rocket motors as compared to the throttled liquid monopropellant engines used by Mars Science Laboratory.  Third, all of the systems use a parachute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the first point, an air bag landing system is vulnerable to ripping open the bags on impact against a natural surface with rocks.  Or even without rocks if you hit hard enough.  The solid rockets fired at terminal descent only provide coarse control over the impact velocity, which is why the air bags were required.  It was an approach to develop a low-cost Mars landing system that did not require more expensive throttled thrusters or a more capable landing radar which would be needed for finer control over the impact velocity.  While lower cost, the air bags could be taken out by a very rough surface, high horizontal velocities due to wind, or a high total velocity of impact due to radar spoofing from, for example, varying terrain such as mesas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When first proposed, the air bag system was not viewed as robust, but rather extremely risky due to the velocity of impact, the number of impacts (let's replace one landing with thirty!), and the associated accelerations.  And it just plain looked crazy.  That risk was accepted in order to lower cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end the system did work, with some changes in MER over MPF to improve the reliability by compensating for some wind effects and significantly toughening the air bags.  The funny thing is how before it flies everyone says it's crazy and will never work.  After it flies and works, and you try to go do something else, now the new thing is crazy and everyone asks why you're not using that really robust air bag system?  Go figure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As noted, an air bag landing system does not scale well when increasing the rover from 170 kg to 900 kg.  The only way to get an air bag system to work at that scale would be to reduce the landing velocity.  So you take the rover and air bags hanging on the tether in MER, and replace the solid rockets at the top of the tether with throttled hydrazine thrusters and replace the altimeter radar with a doppler radar for better control of the impact velocity.  Once you take those steps, you can reduce the landing velocity dramatically.  So much so, that you can eliminate the air bags entirely!  You can even eliminate the lander structure which avoids having to get the rover off of a lander to start the mission.  You need to toughen the wheels and suspension to have them serve as landing gear, but that is a happy price to pay to get rid of all that other stuff.  Voila.  You have the skycrane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to make a landing system as reliable as possible, you reduce the impact velocity as much as possible.  The complexity required to lower the velocity does not inherently make something unreliable.  It just means it will cost you more to make it reliable.  In return, you can make the landing much less sensitive to the atmospheric and surface environments, which you have no control over.  Overall, the MSL landing system is much more reliable.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="6816" LastEditDate="2018-05-15T20:38:40.550" LastActivityDate="2018-05-15T20:38:40.550" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="775" PostTypeId="1" AcceptedAnswerId="783" CreationDate="2013-07-27T10:32:09.367" Score="9" ViewCount="903" Body="&lt;p&gt;We know from &lt;a href=&quot;https://space.stackexchange.com/questions/770/what-is-the-largest-naturally-occurring-body-that-could-be-hollowed-and-safely-l&quot;&gt;this question&lt;/a&gt; that an asteroid 20 km in diameter is feasible as a hollow space habitat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many variables in real life, but assuming you had a spherical solid iron asteroid 20 km in diameter orbiting the Solar system in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kuiper_belt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kuiper belt&lt;/a&gt;. When you start hollowing it out to form your habitat, you use a &lt;a href=&quot;http://en.wikipedia.org/wiki/Coilgun&quot; rel=&quot;nofollow noreferrer&quot;&gt;coilgun&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/Railgun&quot; rel=&quot;nofollow noreferrer&quot;&gt;railgun&lt;/a&gt; to impart thrust. Now you potentially have an interstellar ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming you have decided to leave an outer shell 1 km thick, this gives you 18 km diameter sphere of iron as &lt;a href=&quot;http://en.wikipedia.org/wiki/Working_mass&quot; rel=&quot;nofollow noreferrer&quot;&gt;reaction mass&lt;/a&gt;. Assuming we could eject this mass with the velocity of 0.1 c, and the rate of ejection at 10 k tons per hour, what would be the theoretical maximum velocity such an object could reach, if we eventually want to also decelerate to zero at some distant location? Would the speed reached be sufficient to exit the Solar system and start an interstellar voyage?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Size and mass of this space vehicle's engine is negligible, and we don't care that the projectiles might hit some other object in their way.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-07-27T17:01:17.603" Title="Where could you go in a 20km iron asteroid?" Tags="|interstellar-travel|asteroid|propulsion|orbital-maneuver|rail-gun|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="776" PostTypeId="1" AcceptedAnswerId="777" CreationDate="2013-07-27T10:59:42.857" Score="8" ViewCount="922" Body="&lt;p&gt;Given a large iron body and a &lt;a href=&quot;http://en.wikipedia.org/wiki/Coilgun&quot; rel=&quot;noreferrer&quot;&gt;coilgun&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/Railgun&quot; rel=&quot;noreferrer&quot;&gt;railgun&lt;/a&gt; to impart thrust, You can turn an asteroid into a fuel tank (&lt;em&gt;sort of&lt;/em&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sending large or small bits of iron at high speed towards someone is likely to be considered unfriendly at best.  Obviously using this form of propulsion in or near earth orbit would be &lt;strong&gt;BAD&lt;/strong&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could it safely be used anywhere in the solar system?  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-07-27T22:24:43.723" LastActivityDate="2013-07-27T23:31:04.823" Title="Is it safe to use a coilgun or railgun as an engine inside the solar system?" Tags="|propulsion|safety|fuel|rail-gun|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="777" PostTypeId="2" ParentId="776" CreationDate="2013-07-27T11:09:40.677" Score="5" Body="&lt;p&gt;It could be used anywhere, with the following conditions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;You fire those bits of iron at greater than solar escape velocity and aim the trajectory so that they won't hit anyone on the way out of the solar system&lt;/li&gt;&#xA;&lt;li&gt;You fire them in such a way that they &lt;strong&gt;will&lt;/strong&gt; deliberately hit something (eg. the Moon) in such a way that they are then easily mine-able&lt;/li&gt;&#xA;&lt;li&gt;You fire them on a trajectory that ultimately ends in the sun&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;All of these require calculation of the trajectory carfully to ensure they won't hit anything they aren't supposed to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would expect that of these 3 options, number 2 would be best, as it doesn't waste resources, but to be honest they are all a bit fraught with danger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, why not use the asteroid as a base to launch your space craft from? The mass ratios work much better this way.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-07-27T11:09:40.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="778" PostTypeId="1" AcceptedAnswerId="785" CreationDate="2013-07-27T14:40:49.160" Score="16" ViewCount="1127" Body="&lt;p&gt;Would it be possible, using current technology and renewable energy sources only (e.g. photovoltaics), to magnetically polarize external shields of orbiting satellites in such a way, to effectively prevent or substantially delay orbital decay, or to perform micro adjustments to their orbits (vertical only), by pushing or leaning against Earth's own magnetic field? Which considerations need accounting for, have such mechanics been proposed before, or even already in use? If not, how does Earth's magnetic field affect magnetically neutral and inert artificial satellites and their orbits?&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-08-16T14:22:35.543" Title="Preventing orbital decay and micro adjustments to orbits by &quot;leaning&quot; on Earth's magnetic field" Tags="|orbital-mechanics|magnetic-field|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="779" PostTypeId="1" AcceptedAnswerId="780" CreationDate="2013-07-21T19:35:22.850" Score="15" ViewCount="12281" Body="&lt;p&gt;I just finished reading Clarke's &quot;Prelude to Space&quot;. In chapter XXVII the character Taine gives a talk where he states&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It sounds paradoxical, but it's easier to make the forty-million-mile journey from a lunar base to Mars than it is to cross the quarter of a million miles between Earth and Moon. It takes much longer, of course [...] but it doesn't take more fuel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;My question is: Can this statement still be considered true, looking from the perspective of today's science advances? In &quot;Prelude&quot;, when Clarke talks about &quot;fuel&quot; he means rockets powered by nuclear fission. Since that technology has not come to pass, the statement should possibly be abstracted to &quot;but it doesn't take more &lt;strong&gt;energy&lt;/strong&gt;&quot;. In answering the question, a theoretical lunar base should be taken into account. &lt;/p&gt;&#xA;" OwnerUserId="257" OwnerDisplayName="herzbube" LastEditorUserId="6816" LastEditDate="2016-12-31T00:43:20.840" LastActivityDate="2018-03-05T03:24:08.123" Title="Energy consumption for travelling to the Moon vs. to Mars" Tags="|orbital-mechanics|mars|the-moon|delta-v|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="780" PostTypeId="2" ParentId="779" CreationDate="2013-07-21T20:01:21.743" Score="12" Body="&lt;p&gt;The requirements for this can be found at &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space_.E2.80.94_high_thrust&quot;&gt;Wikipedia&lt;/a&gt;, and here's the general budget. These are measured in terms of Delta-V, which is the only thing that really matters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take this 1 at a time. The total requirement to land on Mars from the Moon is the sum of the lunar escape velocity and the Mars Insertion Orbit. That is 2.8+0.6 km/s, or 3.4 km/s. The fuel required to hit LEO from the moon is 2.74 km/s Thus, it does take more fuel to get to Mars than it does to get to Earth from the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you use a continual thrust approach, the numbers are quite a bit different, and in fact, favor the trip to Mars over the trip to Earth from the Moon. That's probably what Clarke was referring to.&lt;/p&gt;&#xA;" OwnerUserId="25" OwnerDisplayName="PearsonArtPhoto" LastActivityDate="2013-07-21T20:01:21.743" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="781" PostTypeId="2" ParentId="779" CreationDate="2013-07-22T15:16:19.817" Score="1" Body="&lt;p&gt;I assume a big part of this equation has to do with the force and material needed to achieve escape velocity. Earth his a much higher gravity, not to mention air pressure (and an atmosphere) that any space-faring vessel must push through. If I remember correctly, a shuttle has to blast off at something like 5km/sec^2 to reach Lagrange point 5. While the distance is much greater (Moon is about a quarter of a million miles away, while Mars is just shy of .5 AU's) you could theoretically use all the fuel that is needed for leaving Earth for the trip from the Moon to Mars by means of constant acceleration/deacceleration, or even use the Moon for a Slingshot (using a celestial body's gravity well to achieve higher speeds via complicated math, ratios, angles, and one very ballsy pilot). Mathematically, it is probably easier by numbers, but not so much fuel consumption and supplies used; the time for the trip would require a great amount of supplies, not to mention fuel to leave Mars and the return trip home. I know that the really huge red rocket and the boosters are there just to reach spacebound velosity, so it makes you wonder how well they would do in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe we should get warp drives &lt;em&gt;a la&lt;/em&gt; Farscape.&lt;/p&gt;&#xA;" OwnerDisplayName="Jersey" LastActivityDate="2013-07-22T15:16:19.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="782" PostTypeId="2" ParentId="779" CreationDate="2013-07-22T22:45:06.037" Score="7" Body="&lt;p&gt;There are a few other parameters to consider. Launching out of Earth's gravity well is challenging. A 6.2-million-pound (2800-tonne) Saturn 5 rocket could put 260,000 lbs (120 tonnes) into low Earth orbit (4% of initial mass into orbit). Launching from the moon can put more payload into lunar orbit with the same mass of propellant. The lunar module weighed 10,300 lbs (4700 kg) including 5200 lbs (2400 kg) of fuel and could achieve lunar orbit with that small mass of fuel (~50% of initial mass into orbit, albeit with different fuels/efficiencies).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in orbit, because the Moon's mass is lower, its escape velocity is lower than Earth's as well and so is its Mars transfer orbit velocity. In addition, you can factor in the delta-V that you get from the moon's orbit around the Earth, which is 1 km/sec in the direction of a Mars traveler at one time during the month.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So from ground to ground (Earth surface to Moon surface vs. Moon surface to Mars surface) it is no surprise the Moon to Mars would require less propellant, especially considering you could aerobrake to slow the craft down at Mars. It would, however, take much longer. This also presumes you can manufacture a rocket on the moon. If you have to bring the rocket and its fuel from Earth, there's no point, except you might be able to slingshot past the moon after an Earth launch, using its orbital velocity to increase your craft's velocity as it approaches and passes, but because the Moon's mass is low, you don't get much of an effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Clarke's perspective, it was true then and is true now. Propulsion techniques don't change the delta-V, only the time in which the delta-V is applied (assuming no massless propulsion) and the shape of the transfer orbit.&lt;/p&gt;&#xA;" OwnerDisplayName="craig" LastEditorUserId="6816" LastEditDate="2016-12-31T01:27:47.523" LastActivityDate="2016-12-31T01:27:47.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="783" PostTypeId="2" ParentId="775" CreationDate="2013-07-27T17:01:00.773" Score="5" Body="&lt;p&gt;For simplicity's sake, let's assume a perfectly uniform sphere, as you described. The amount of volume to begin with would be about 4188 cubic km. The volume of material evacuating the sphere would be about 3053 cubic km. Thus, about 72% of the asteroid would become fuel. Let's ignore relativistic effects, because my mind isn't quite working that well yet today. First of all, how fast could you get if you don't want to stop? I'll do some handwaving calculus and say that it should go about 0.134 c, if you don't want to stop. If you do want to stop eventually, then you should accelerate to half of that speed, and then coast, using the rest of the acceleration to stop. Thus, you could go about 0.067 * c. So, how long would it take you to get somewhere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alpha Centauri is about 4.367 light years. Assuming a negligible acceleration/deceleration period, that's about a 65 year trip. I'll leave that to you if that's acceptable to make the trip.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-27T17:01:00.773" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="784" PostTypeId="1" AcceptedAnswerId="786" CreationDate="2013-07-27T17:02:43.487" Score="8" ViewCount="102" Body="&lt;p&gt;I'm reading about the 30-or-so USSR &lt;a href=&quot;http://www.space4peace.org/ianus/npsm2.htm&quot;&gt;RORSAT missions&lt;/a&gt; that had unshielded, fast spectrum, highly enriched Uranium nuclear reactors powering a satellite, which was later moved up to a higher &quot;disposal&quot; orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently one of the biggest debris issues is the liquid NaK (former) coolant &lt;a href=&quot;http://www.space.com/6322-nuclear-powered-soviet-satellite-acts.html&quot;&gt;leaking&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But these are just liquid droplets.  I can easily see how the &lt;em&gt;number of drops&lt;/em&gt; would multiply much faster than solid metal breaking up... but how do the impact risks compare?  Will a liquid drop do the same impact damage as a solid chunk of metal of the same mass?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="168" LastEditDate="2013-07-27T17:23:08.273" LastActivityDate="2013-08-11T17:19:16.613" Title="Liquid micrometeorites compared to solids" Tags="|debris|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="785" PostTypeId="2" ParentId="778" CreationDate="2013-07-27T17:04:00.853" Score="15" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RcQjN.png&quot; alt=&quot;Tethers&quot;&gt;&#xA;&lt;hr/&gt;&#xA;&lt;a href=&quot;http://ntrs.nasa.gov&quot; rel=&quot;noreferrer&quot;&gt;NTRS&lt;/a&gt; to the rescue:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Alternating Magnetic Field Forces for Satellite Formation Flying (2012)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://hdl.handle.net/2060/20120015763&quot; rel=&quot;noreferrer&quot;&gt;http://hdl.handle.net/2060/20120015763&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Youngquist, Robert C.; Nurge, Mark A.; Starr, Stanley O.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Selected future space missions, such as large aperture telescopes and multi-component interferometers, will require the precise positioning of a number of isolated satellites, yet many of the suggested approaches for providing satellites positioning forces have serious limitations. In this paper we propose a new approach, &lt;strong&gt;capable of providing both position and orientation forces&lt;/strong&gt;, that resolves or alleviates many of these problems. We show that &lt;strong&gt;by using alternating fields and currents that finely-controlled forces can be induced on the satellites&lt;/strong&gt;, which can be individually selected through frequency allocation. We also show, through analysis and experiment, that near field operation is feasible and can provide sufficient force and the necessary degrees of freedom to accurately position and orient small satellites relative to one another. In particular, the case of a telescope with a large number of free mirrors is developed to provide an example of the concept. We also discuss the far field extension of this concept.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;EDIT (thanks and a tip hat go to Tildal Wave, the author of the original question):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This design solution has gotten a Wiki page all on its own: &lt;a href=&quot;https://en.wikipedia.org/wiki/Electromagnetic_Formation_Flight&quot; rel=&quot;noreferrer&quot;&gt;Electromagnetic Formation Flight&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't make use of Earth's magnetic field, though (one would have to pull multiTesla magnets into orbit for that), so a notice to upvoters: please keep in mind that this answer doesn't seem to match the question fully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think a better answer would contain calculations on the heights and inclinations where using Earth's own field is feasible for a given intensity of the spacecraft's magnet (that is, average altitude gain during a month of operation through magnet manipulation exceeds altitude loss due to atmospheric drag).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Necessary parameters/data for calculations: full Earth's magnetic field model, solar activity index (to account for drag and magnetic field variations), craft's ballistic coefficient, magnet size, orbital inclination. Basically, one has to derive optimal control law for the magnet, and apply it along a fixed period of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's been a recent article (&lt;a href=&quot;http://dx.doi.org/10.2514/1.57479&quot; rel=&quot;noreferrer&quot;&gt;http://dx.doi.org/10.2514/1.57479&lt;/a&gt;) called &quot;Orbit Boosting Maneuvers for Two-Craft Coulomb Formations&quot; in the &lt;em&gt;Journal of Guidance, Control, and Dynamics&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-16T14:22:35.543" LastActivityDate="2013-08-16T14:22:35.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="786" PostTypeId="2" ParentId="784" CreationDate="2013-07-27T19:42:14.870" Score="7" Body="&lt;p&gt;I'm looking through Heinrich Klinkrad's 2006 book (Space Debris: Models and Risk Analysis) where he describes this very hazard (p.26).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total amount of NaK released (tentative!): 208 kg, at 13 kilos per event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbit: inclination 65 degrees, altitudes 900-950 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The text states that possible size of droplets remaining in the initial orbit and not taken off by drag or evaporation is from 10 to 55 mm, with the total mass less than 60kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The density is stated to be similar to water (I presume that calculations were based on drag terms in ballistic equations, although the density of liquid NaK is around 0.855 g/cm^3).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/aEEZP.png&quot; alt=&quot;klinkrad-1&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now on to damage from hypervelocity impact:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;at hypervelocities, all solids behave like liquids (&quot;the resulting impact damage for an aluminum projectile is almost unaffected by its solid or liquid state)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;droplets with round shape will have lower penetration capability than elongated projectiles&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;NaK alloy in the droplets has density like water ($~1.0 g/{cm^3}$), approx. 7.8 times less than steel. Let's assume we're talking about projectiles weighing 30 grams each. The NaK droplet will have diameter of 3.855 cm, the steel bullet - 1.9439 cm.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Ballistic limit of a NaK droplet will be &lt;strong&gt;1.409 times smaller&lt;/strong&gt; than that of a round steel bullet (for the same angle and velocity, the maximum thickness of a wall penetratable by a steel bullet will be 1.409 times more than that penetratable by a NaK droplet): we use ESA parameters $\lambda=1.056$ and $\beta=0.519$. $\lambda$ is for the diameter, and $\beta$ is for projectile density. You can grab Klinkrad's book and re-do the computations for another density of the droplets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$t_{t, lim}=K_1 d^\lambda_p \rho^\beta_p \rho^\kappa_t v^\gamma_p {(\cos \alpha_p)}^\xi$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$K_1$ - calibration factor&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t_{t, lim}$ - ballistic limit (min. required single-wall thickness), cm&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$d_p$ - diameter of the projectile, cm&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\lambda, \beta, \kappa, \gamma, \xi$ - calibration exponents&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\rho_t, \rho_p$ - densities of the target and projectile, $g/{cm^3}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v$ - projectile velocity at impact, $km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\alpha_p$ - impact angle (w.r.t. surface normal), degrees&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An important caveat: do not trust simple back-of-the-envelope computations; do numerical simulations, and above all, experiment!&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-11T17:19:16.613" LastActivityDate="2013-08-11T17:19:16.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="787" PostTypeId="1" AcceptedAnswerId="788" CreationDate="2013-07-27T20:27:13.257" Score="20" ViewCount="2741" Body="&lt;p&gt;According to JPL: &lt;a href=&quot;http://voyager.jpl.nasa.gov/mission/weekly-reports/&quot;&gt;http://voyager.jpl.nasa.gov/mission/weekly-reports/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 1 travels at a velocity relative to the Sun of 19.03 km/s (or 0.000063478 c, or a time difference of ~ 0.0005 seconds per year), and Voyager 2 travels at 15.428 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the &lt;a href=&quot;http://voyager.jpl.nasa.gov/news/factsheet.html&quot;&gt;factsheet&lt;/a&gt;, Voyagers do not have a time synchronization system on board, atomic clock or something like that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they need to synchronize the time with the earth? Or do deep space exploration probes need this in general? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, how do we do it?&lt;/p&gt;&#xA;" OwnerUserId="11" LastEditorUserId="38" LastEditDate="2013-07-28T19:11:17.667" LastActivityDate="2013-10-13T20:36:35.700" Title="Do the Voyager space probes perform time synchronization regularly?" Tags="|voyager|probe|time|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="788" PostTypeId="2" ParentId="787" CreationDate="2013-07-27T23:18:26.723" Score="15" Body="&lt;p&gt;According to &lt;a href=&quot;http://voyager.jpl.nasa.gov/faq.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL's Voyager FAQ&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is no clock chip, as such, in the spacecraft. The &quot;clock&quot; is&#xA;  really a counter, based on one of several electronically generated&#xA;  frequencies. These frequencies, based on a reference, generated by a&#xA;  very stable oscillator, are converted and fed to different locations&#xA;  in the spacecraft as synchronization signals, timers, counters, etc.&#xA;  The &quot;clock&quot; signal is part of the information telemetered to the&#xA;  ground and it is with ground software that we convert to day of year,&#xA;  time of day Greenwich Mean Time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This means that any data it still collects (it wouldn't be much, according to &lt;a href=&quot;https://space.stackexchange.com/a/49/49&quot;&gt;this answer&lt;/a&gt;) is accompanied by values of Voyager's internal counter, and this counter data later converted to time here on the good old Earth (where, I imagine, any such time difference due to Voyager's velocity relative to Sun will be accounted for, whenever necessary). I also can not think of a single instrument onboard Voyager that would require clock synchronisation with some timekeeper on Earth, and they can easily rely on the internal clocks, relative to which Voyager makes observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put it differently, if Voyager needs to take measurements of something every 60 seconds, it will inspect its internal counter and take another measurement after its internal counter says 60 seconds have passed, not when those 60 seconds would have passed for us on Earth (which might take an extremely tiny amount longer).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-07-27T23:18:26.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="789" PostTypeId="1" AcceptedAnswerId="801" CreationDate="2013-07-27T23:26:47.040" Score="12" ViewCount="445" Body="&lt;p&gt;Is there any evidence to support the likelihood of widespread consumer space travel in, say, the next 20 years? I attempted to do some research on commercial space travel, and came across &lt;a href=&quot;http://www.virgingalactic.com/booking/&quot;&gt;Virgin Galactic&lt;/a&gt;'s website, where you can book a flight to space for a cool quarter million USD. However the website seemed to be missing two vital pieces of information: when flights would actually commence, and &lt;em&gt;where&lt;/em&gt; said flight takes you. What are we talking about here, a trip up to the space station? To the moon and back? Just a bit further than the current airplane flights that take you just outside the atmosphere? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A bit more googling found &lt;a href=&quot;http://www.thenational.ae/news/uae-news/christmas-day-lift-off-into-space-for-virgin-galactic-and-abu-dhabi&quot;&gt;an article&lt;/a&gt; which claims Virgin Galactic will commence flights this December. But I wasn't able to unearth any information about exactly where the flights will take passengers, and honestly—there isn't a huge subset of people who can afford $250,000 tickets. So, my questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Is Virgin Galactic the only company reportedly working on consumer space travel, or are there others? &lt;/li&gt;&#xA;&lt;li&gt;Is there any information on where exactly these flights will take you? If the current vehicles are only capable of near-Earth travel, is there any information about plans/research to achieve craft that can fly further (ex. to the moon)? &lt;/li&gt;&#xA;&lt;li&gt;I assume the $250 grand price tag is in part to make up for the costs of creating the craft, and in part due to cost of fuel. Are there plans in progress anywhere for consumer spacecraft that are cheaper to build/fuel so that the price could drop down to something more budget-friendly for the average person? &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Essentially I'm just looking for any hard facts and information that can be provided about current and near-future plans for consumer space craft in the areas of launch timeline, route/distance from earth, and pricing. It would be nice to know if I'm likely to get the opportunity to go to space in my lifetime :)&lt;/p&gt;&#xA;" OwnerUserId="289" LastEditorUserId="25" LastEditDate="2013-07-28T01:01:11.743" LastActivityDate="2013-07-28T15:43:44.963" Title="Near-future consumer space travel" Tags="|crewed-spaceflight|tourism|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="790" PostTypeId="2" ParentId="776" CreationDate="2013-07-27T23:31:04.823" Score="2" Body="&lt;p&gt;You are actually just turning the asteroid into your source of reaction mass.  Your real propulsion source is whatever energy source you are using to power the railgun.  Unless you are using charged particles, your railgun will have a pathetic specific impulse too.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-07-27T23:31:04.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="791" PostTypeId="2" ParentId="721" CreationDate="2013-07-28T00:47:47.030" Score="3" Body="&lt;p&gt;They basically look for unnatural signals, for example a carrier signals (one narrow band which is very energetic against the noise background is a good example).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with this approach is that you can't differentiate between noise and highly compressed data (so the propability to catch usual communication data is very very impropable).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another problem can be other communication channels, for example communication with neutrinos is &lt;em&gt;very&lt;/em&gt; lucrative because it can only absorbed by black holes and neutron stars.&lt;/p&gt;&#xA;" OwnerUserId="291" LastActivityDate="2013-07-28T00:47:47.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="792" PostTypeId="1" CreationDate="2013-07-28T01:18:49.697" Score="11" ViewCount="4911" Body="&lt;p&gt;Is it possible to get an impulse, if you shoot at a solar sails with lasers mounted on the spaceship?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, why doesn't a laser create a force, if it fires/releases a photon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the impulse conservation rule broken with such a construction?&lt;/p&gt;&#xA;" OwnerUserId="291" LastEditorUserId="109" LastEditDate="2017-04-03T18:00:12.040" LastActivityDate="2017-04-03T18:00:12.040" Title="Laser propulsion possible?" Tags="|propulsion|solar-sail|laser|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="793" PostTypeId="2" ParentId="792" CreationDate="2013-07-28T02:04:51.923" Score="6" Body="&lt;p&gt;Yes, it is entirely possible for lasers to give a propulsive power. In fact, there is a fairly commonly discussed idea of pointing a large laser at a solar sail to accelerate it faster than would be possible by just the sail itself. The key is, the lasers cannot be on the spaceship, they have to be away from the spaceship. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yes, given these laws, lasers do in fact exert a small force back on the object they are launched from. It's just that the momentum is so tiny, it's barely noticed with highly sensitive devices and powerful lasers. The most powerful laser in the world is the laser located at &lt;a href=&quot;http://news.nationalgeographic.com/news/2012/11/pictures/121129-lasers-technology-beams-science-nuclear-fusion-energy/&quot;&gt;NIF&lt;/a&gt;. It can produce a pulse at 500 trillion watts, albeit only for an instant. If you concentrated that on a single point, it would exert a force of about 367 N for an absorptive surface. That same force would be applied to the sail, except doubled as it would be reflected (Ideal). It should be noted that while this is actually quite a bit of momentum, it can only happen for a very small fraction of a second, making it more difficult to apply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, you are on a spacecraft, and you want to propel yourself with a laser stationed on the spacecraft itself. What do you do? There are two options you could do. First of all, if you bounce the light off of a reflective surface, then the light will bounce back, in essence, it will provide a slight bit of momentum, because the net effect is that light is being pushed off of the sail and outside of the spacecraft. But, there's a better way. Instead, point the laser as you would a rocket engine, and use it for thrust. Then you would get 100% of the thrust from said laser, and you'll be slowly accelerating in the direction you want to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, one has to ask where the power comes to power this laser. The only conceivable source in the long run is solar power. And to do that, you might as well use solar sailing, it will give you the same return on your space, with less weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, the only way that makes sense to use this is if a laser from a remote location is firing at you. Otherwise, it just wouldn't make any sense.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-28T14:22:38.720" LastActivityDate="2013-07-28T14:22:38.720" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="794" PostTypeId="2" ParentId="792" CreationDate="2013-07-28T02:49:31.687" Score="4" Body="&lt;p&gt;I'm going to borrow an analogy from &lt;a href=&quot;https://en.wikipedia.org/wiki/Baron_M%C3%BCnchhausen&quot; rel=&quot;nofollow noreferrer&quot;&gt;Baron Münchhausen&lt;/a&gt; (see Fig. 1) later on to illustrate the &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's three laws of motion&lt;/a&gt; that are in effect with such a system you're proposing and why it wouldn't be possible to create a propulsive force, if the lasers and the solar sail are a part of the same closed system (or a single body), but let's first go through these laws of motion:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;&lt;strong&gt;First law&lt;/strong&gt;: When viewed in an inertial reference frame, an object either is at rest or moves at a constant velocity, unless acted upon&#xA;  by a force.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Second law&lt;/strong&gt;: The acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body,&#xA;  and inversely proportional to its mass. Thus, &lt;code&gt;F = ma&lt;/code&gt;, where &lt;code&gt;F&lt;/code&gt; is&#xA;  the net force acting on the object, &lt;code&gt;m&lt;/code&gt; is the mass of the object and&#xA;  &lt;code&gt;a&lt;/code&gt; is the acceleration of the object.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Third law&lt;/strong&gt;: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and&#xA;  opposite in direction to that of the first body.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia - Newton's laws of motion&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the first two laws seem promising, but I've promised to illustrate the problems of your proposal with an analogy, so here it is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/SXYcP.jpg&quot; alt=&quot;Baron Münchhausen pulls himself out of a mire by his own hair&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Fig. 1 - Baron Münchhausen pulls himself out of a mire by his own hair. Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/M%C3%BCnchhausen_trilemma&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not trying to be smart, or mock your proposal in any way whatsoever. I just genuinely love &lt;a href=&quot;https://en.wikipedia.org/wiki/Baron_M%C3%BCnchhausen&quot; rel=&quot;nofollow noreferrer&quot;&gt;Baron Münchhausen's&lt;/a&gt; stories since I was a kid, and this one, where Münchhausen &lt;em&gt;allegedly pulled himself and the horse on which he was sitting out of a swamp by his own hair&lt;/em&gt;, is as good of analogy as I can currently think of, and it taught me the very same dynamics of a closed system that I'm describing here when I was a kid and didn't yet even hear of Newton until then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, while lasers will exert &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pressure#Particle_argument:_photons&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiation pressure&lt;/a&gt; upon solar sails (see &lt;a href=&quot;https://space.stackexchange.com/a/793/49&quot;&gt;@PearsonArtPhoto' answer&lt;/a&gt; for more information), they will also exert &lt;em&gt;a force equal in magnitude and opposite in direction&lt;/em&gt; on the vehicle's body, as per Newton's third law of motion. And because the lasers are a part of the same system (not two separate bodies, as quoted in Newton's third law of motion - they're attached to the vehicle and it to the solar sails, possibly by some long tether), this force on the solar sail will be canceled out by the &lt;em&gt;equal and opposite&lt;/em&gt; force on the lasers that are emitting the photons. It would stretch the tethers the sails are connected to with the spaceship, but that's about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a bit of generalisation, as lasers work as &lt;em&gt;concentrators&lt;/em&gt; of photons (&lt;a href=&quot;https://en.wikipedia.org/wiki/Optical_amplification&quot; rel=&quot;nofollow noreferrer&quot;&gt;optical amplification&lt;/a&gt;) and travel many times back and forth within the gain medium, but they travel exactly one length of the gain medium longer in the direction of the output coupler, so our initial presumption still stands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simple point in case (reusing Münchhausen's &quot;idea&quot;, but let's make it less potentially painful and unpleasant) - put your hands around your waist and try lifting yourself up without moving your legs at all. You will find it quite impossible, unless you exert some force on another body that is not a part of yours, and that you can push against.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if the laser was stationed on another body, such propulsion would be quite possible. It could also be used to constantly change direction (spin on its axis), even if it was part of the same body, but it wouldn't work as a long term propulsion in any desired direction. Not by targeting the photon emitting lasers on a sail attached to the same vehicle. If would work without a sail though. Sadly, there's no free lunch here, not even with photons.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: I see in the comments that my and &lt;a href=&quot;https://space.stackexchange.com/a/793/49&quot;&gt;@PearsonArtPhoto's&lt;/a&gt; answers have caused some concerns over their accuracy, due to supposed &quot;free lunch&quot; concept of pushing on solar sails using an onboard generated laser beam. So what follows is a bit of background on the actually proposed mode of propulsion, as found in scientific papers. If anyone can find a proposed solution that does this in a closed-loop system, and propels a spaceship in any direction by focusing a laser beam that is aboard the spaceship on the solar sails attached to the same spaceship, I'll be extremely interested to read the paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, from &lt;a href=&quot;https://en.wikipedia.org/wiki/Beam-powered_propulsion#Direct_impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Beam-powered propulsion - Direct impulse&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The beam has to have a large diameter so that only a small portion of&#xA;  the beam misses the sail due to diffraction and the laser or microwave&#xA;  antenna has to have a good pointing stability so that the craft can&#xA;  tilt its sails fast enough to follow the center of the beam. This gets&#xA;  more important when going from interplanetary travel to interstellar&#xA;  travel, and when going from a fly-by mission, to a landing mission, to&#xA;  a return mission. The laser or the microwave sender would probably be&#xA;  a large phased array of small devices, which get their energy directly&#xA;  from solar radiation. The size of the array obsoletes any lens or&#xA;  mirror.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Right. Not really directly dismissing the idea of carrying the source of the laser beam aboard the spacecraft, but it is suggested the beam source would have to be stationary by implying the greater radii (due to angular resolution) the more distant the spaceship is to the energy beam source. Let's take this a step farther and quote actual scientific papers, not only Wikipedia (which is fine in most cases, but one can't trust conclusions there with any degree of certainty, unless also absolutely certain it doesn't change implied meaning of its quoted sources);&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/4Landis.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced Solar and Laser-pushed Lightsail Concepts, Final Report, May 31, 1999 - NASA Institute for Advanced Concepts (PDF)&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;1.11 Beamed-energy Propulsion&lt;/strong&gt; (pages 4-6)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;An alternative solution to the problem of the mass ratio required for&#xA;  high velocity flight is to use beamed energy. &lt;strong&gt;In beamed-energy&#xA;  propulsion, the energy source is left stationary, and the probe is&#xA;  pushed at a distance.&lt;/strong&gt; Since the propulsion system does not move, the&#xA;  weight of the energy source is not critical, and fuel does not have to&#xA;  be carried.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;An example of the beamed-energy propulsion is the photon-pushed sail.&lt;/strong&gt; Since a photon has momentum, a photon beam can “push” a&#xA;  reflective sail. In practical terms, the force produced by reflecting&#xA;  a light beam is 6.7 newtons per gigawatt of light reflected. This&#xA;  force comes with no expenditure of fuel whatsoever. Thus, it is&#xA;  extremely favorable for high delta-V missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there we have it. Such systems describe a stationary beam source, and a moving spaceship pushing against the momentum force reflected photons exert upon the solar sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have also watched the first laboratory tests performed on feasibility of using this momentum of the photons targeted at an extremely light solar sail, but sadly don't exactly remember where this study was conducted, or who the publisher was (it was a fair bit ago, probably around 10 years?). Still, the test system was assembled in a vacuum chamber with the high-power laser stationary, and a small (roughly 1 square inch in size) solar sail held in place by an extremely sensitive lever that didn't impart movement of the test piece of the solar sail in the direction of the light beam. The forces measured were extremely tiny, but the concept worked and the method proposed was confirmed. Again - by using a stationary laser beam source!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this clarifies things a bit.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-07-28T13:40:39.260" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="795" PostTypeId="1" AcceptedAnswerId="803" CreationDate="2013-07-28T11:29:15.373" Score="11" ViewCount="993" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermoelectric_effect&quot;&gt;Thermoelectric effect&lt;/a&gt; allows for electrical energy to be created, based on a difference of temperature.  In fact the &lt;a href=&quot;http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;Radioisotope thermoelectric generator&lt;/a&gt; is based on this principal and does/has supplied energy to several NASA vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space is cold, and the inside of human occupied vehicle is warm. So if you just made the shell of your ship a big Thermoelectric Generator, and kept the inside at a comfortable temperature, how much electrical energy would you produce?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-07-28T12:34:25.193" LastActivityDate="2013-07-28T19:16:00.587" Title="In space can the difference in temperature between the inside and outside, provide useful energy?" Tags="|energy|thermal|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="796" PostTypeId="2" ParentId="795" CreationDate="2013-07-28T12:33:49.703" Score="3" Body="&lt;p&gt;An RTG is based off of the flow of heat. You have to have a way to make heat flow from one spot to another spot. You can't directly just use one end directly to space, there just isn't anything there. What you end up having to do is use radiators, which will radiate the heat in to space, but these tend to be warmer than the spacecraft itself, as they have waste heat. In addition, efficiency numbers for a thermocouple are always less than 10%, and could be much lower than that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, where would the heat come from, if not from an RTG? It would either have to be waste heat from the electronics, or heat coming from the Sun. Waste heat can typically be prevented in most cases by careful design, and usually isn't a significant problem. From the sun is possible, but solar cells have an efficiency of around 20%+, and sometimes quite a bit higher than that. As solar cells are more efficient, why use thermocouples?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line: in theory, one could use a thermocouple to produce a small amount of electricity from &quot;Waste heat&quot; that is radiated out of a radiator. But the best case, it could provide a small amount of heat that could improve your efficiency slightly.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-28T19:16:00.587" LastActivityDate="2013-07-28T19:16:00.587" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="797" PostTypeId="1" AcceptedAnswerId="862" CreationDate="2013-07-28T13:07:54.117" Score="17" ViewCount="3429" Body="&lt;p&gt;As I understand it; being away from earth subjects you to significant health risks from radiation.  On Earth most of this risk is deflected by &lt;a href=&quot;http://en.wikipedia.org/wiki/Earth%27s_magnetic_field&quot;&gt;Earth's magnetic field&lt;/a&gt;.  I did not find a reference but presumably this magnetic field could be duplicated in a spaceship given sufficient energy. I also understand that some radiation shielding can be created by use of materials &lt;em&gt;(lead being a well known example)&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have a spacecraft comparable to the ISS in mass and volume &lt;em&gt;(different structure of course)&lt;/em&gt; moving around the solar system under a constant 1g thrust &lt;em&gt;(turn over and decelerate halfway)&lt;/em&gt;, what would be the more efficient use of energy; Moving the extra mass or the energy required to generate the magnetic field?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-07-28T13:27:38.210" LastActivityDate="2018-03-15T02:12:48.053" Title="Radiation shielding magnetic or mass, which is more efficient?" Tags="|radiation|health|spacecraft-development|shielding|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="798" PostTypeId="2" ParentId="789" CreationDate="2013-07-28T14:24:52.137" Score="2" Body="&lt;p&gt;From what I can tell, the company is offering a &lt;strong&gt;suborbital flight&lt;/strong&gt;. Everything on their &lt;a href=&quot;http://www.virgingalactic.com/overview/experience/&quot; rel=&quot;nofollow&quot;&gt;experience page&lt;/a&gt;, such as that it seems the flight takes only one day, suggests this.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-07-28T14:24:52.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="799" PostTypeId="2" ParentId="792" CreationDate="2013-07-28T14:40:45.310" Score="3" Body="&lt;p&gt;The others covered some essential points concerning use of remote beam sources as propulsion to solar sails. Let me add something else:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar sail is huge to gather a lot of relatively weak, scattered solar energy and use its (very weak) kinetic component to push against (huge but cheap) mirror surface - Sun shines in all directions and you need enormous surface to catch its energy to provide reasonable propulsion to your vehicle (but the surface can be any cheap, light reflective material).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you use light source of higher intensity than sunlight, concentrated over a small area, you might be better off exploiting the photovoltaic effect of the beam. Concentrate the laser beam over much smaller (but more expensive to build) area of a solar cell. While shining laser at ten square meters of solar batteries you're unlikely to generate direct kinetic push of magnitude comparable to a square kilometer of solar sail, you'll give it a fairly good voltage which then can be used to accelerate minuscule amounts of matter to relativistic speeds providing quite efficient propulsion in form of ion drive, that uses very little of solid fuel, and produces thrust proportional to amount of electricity you pump into it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if you decide to shine a laser at a craft, don't use that laser to push that craft. Use it to power the drive of that craft.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-07-28T14:40:45.310" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="800" PostTypeId="1" AcceptedAnswerId="805" CreationDate="2013-07-28T15:28:54.060" Score="22" ViewCount="1403" Body="&lt;p&gt;ISS will be closed by 2020. Are there any other space stations planned to be put in orbit in decade 2014-2024 that have really chances to be built?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-07-28T15:42:28.760" LastActivityDate="2013-10-03T18:28:53.890" Title="Is any space station planned to be put in orbit in decade 2014-2024?" Tags="|iss|space-station|future-missions|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="801" PostTypeId="2" ParentId="789" CreationDate="2013-07-28T15:43:44.963" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;http://www.virgingalactic.com/overview/experience/&quot; rel=&quot;nofollow&quot;&gt;Virgin Galactic&lt;/a&gt; website has several areas that clearly indicate a flight on SpaceShipTwo is a sub-orbital space tourism flight. Specifically, passengers are taken above the &lt;a href=&quot;http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow&quot;&gt;Von Karman line&lt;/a&gt; (roughly 100 km altitude), which is considered the edge of space, and high enough for any passengers to qualify as astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding the specific questions: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Passengers would experience several minutes of weightlessness above the atmosphere before re-entering and gliding back to the launch site. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I haven't read anything about expansion plans for tourism beyond sub-orbital flight, although VG is working on a &lt;a href=&quot;http://www.virgingalactic.com/launcherone&quot; rel=&quot;nofollow&quot;&gt;launch vehicle&lt;/a&gt; for orbital payloads. Elon Musk of SpaceX has &lt;a href=&quot;http://io9.com/5963349/spacex-founder-unveils-plan-to-send-80000-people-to-mars&quot; rel=&quot;nofollow&quot;&gt;publicly discussed plans to send humans to Mars&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I would assume, as with any business, that price is set by the company's perception of supply and demand. So obviously that would be informed by costs (development, operational, etc.) as well as market demand. Generally prices are high in new markets and come down over time. Air transportation is probably a good analogy: expensive at first, but gradually falling in price over the last several decades. Without having any data, I would guess the cost of fuel is a negligible fraction of the operations cost. Rubber and LOX are cheap.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="127" LastActivityDate="2013-07-28T15:43:44.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="802" PostTypeId="2" ParentId="667" CreationDate="2013-07-28T16:09:43.043" Score="5" Body="&lt;p&gt;The full flight profile of a VTVL Grasshopper-like vehicle hasn't been shared by SpaceX, but it certainly won't &quot;stay vertical&quot; for the entire flight — assuming that vertical is defined as 90° flight path angle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ground launch rockets typically start by flying more or less straight up. Because they are at low angle of attack this keeps lateral aerodynamic forces manageable and it gets the rocket out of the thick lower atmosphere as quickly as possible. But as soon as possible (exactly where depends on all sorts of parameters but above 100,000 ft would be an order of magnitude altitude) they begin what's called a gravity turn. The trajectory begins to turn over and the flight path angle decreases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it is likely that the vehicle will have started to turn over by staging and thus will no longer be vertical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the vehicle falls ballistically before restarting the main engines for landing, there are really only two high-level options: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Actively control attitude: use an attitude control system to control how the vehicle is pointing&lt;/li&gt;&#xA;&lt;li&gt;Let the vehicle attitude float (i.e. not controlled).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Which is best to choose would be a trade buried in the details of the rocket and trajectory design, and which we'll only know the answer to when SpaceX decides to share it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Either way, the vehicle will have to be properly re-oriented for engine ignition before it got too deep into the atmosphere. Depending on the aerodynamic stability of the vehicle this could be done in part passively or with (or augment with) active control. Because it's only a first stage it will only be going several Mach, and thus probably won't need a thermal protection system as heavy, complex, or expensive as what was used for Shuttle or even the Dragon or Apollo re-entry vehicles.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2013-07-28T16:09:43.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="803" PostTypeId="2" ParentId="795" CreationDate="2013-07-28T16:31:27.477" Score="11" Body="&lt;p&gt;This question is intriguing because of the nature of the thermal cycles.  This is the maximum theoretical efficiency of a thermal cycle (Carnot cycle), no ifs, ands, or buts about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ABGVj.png&quot; alt=&quot;Carnot efficiency&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can obtain very low temperatures from space, because space is at a low temperature.  You'll often hear the Cosmic Microwave Background (CMB) cited for this, but that's overly optimistic.  A radiator could theoretically couple to that temperature, but in practice you would have to shield line-of-sight from every star in the sky.  Practically, the averaged temperature of CMB + stars is a better lower bound, and you'll never get anywhere near to that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a blackbody was exposed to Earth's radiation and space in LEO, but was shielded from the sun, it could get to &lt;a href=&quot;https://physics.stackexchange.com/questions/41605/temperature-of-a-black-body-in-leo-on-the-dark-side-of-the-earth&quot;&gt;-30 degrees C&lt;/a&gt;.  Obviously deep space probes don't have to worry about Earth radiation, and you could thinkably shield from that anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We're left with the conclusion that T_C, in the Carnot limit, can be made very low.  That suggests a very real possibility of a thermal cycle between the radiator and &quot;room temperature&quot;.  An engineer will hem and haw at the mention of this, but you &lt;em&gt;could&lt;/em&gt; have a thermal cycle between those two things at 90% efficiency.  I think we need to recognize that the answer to the question, as asked, is &quot;yes&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, onto the qualifiers.  There are a lot of penalties for ever setting up something like this.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Space stations have to spend money on insulation to begin with, so you would increase that cost (dramatically), in order to avoid heat flow bypassing your radiator.&lt;/li&gt;&#xA;&lt;li&gt;What physical mechanism would you make this thermal cycle out of?&lt;/li&gt;&#xA;&lt;li&gt;The system would affect the energy balance of the station.  It would have to be a part of the heat engineering from the ground up.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The only thing this really tells us is that it's not a serious near-term consideration.  There are good reasons that we don't have a experience base for #2, for instance.  There's a lot of work going on with recovery of waste heat, but often over small T differentials, but we don't often consider heat rejection at very low temperatures.  It would only come into play for heat recovery systems in liquid nitrogen plants, for instance.  If memory serves me correctly, I think thermoelectric systems depend on a very high temperature source.  Everything is turned on its head when changing the temperature ranges like this.  But there might be good options out there.  This would be an interesting study area, because we just don't see those super low temperatures on Earth naturally.  So it would rarely make sense to use it for heat rejection.  Keep in mind, however, that good heat engines are not guaranteed.  Thermal plants on Earth can often only produce 30-40% efficiency when the theoretical limit is 70% or so.  That's because higher efficiency systems just won't scale up like we need.  It's hard to say what efficiencies we could get.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we wouldn't do it soon because space radiators are extremely large in size, modern designs require lots of mass, and insulation costs money too.  The physical limit is area, from thermal radiation laws, but these are often made of metal sheets, or other things which are heavy.  This is reasonable when you consider they'll have to hold in a fluid at pressure (and leaking is bad).  The real limit is robustness for the space environment, ability to go up in a rocket, and the ability to unfurl itself into space.  If you could spread out a much lighter sheet, it will cool much faster.  But how could you recoup that coldness?  And how is it going to hold up?  In the distant future, space heat rejection systems have a lot of room to obtain very good performance through large, fragile systems in zero gravity.  Same goes for insulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heat balance with the station isn't completely self-defeating.  This would &lt;em&gt;only&lt;/em&gt; work if used with a station that was virtually perfectly insulated to begin with (long way off), but the &quot;useful work&quot; would be put back into the station.  That creates an interesting situation where you get back a lot of the energy you consume.  Low temperatures have economic value just as high temperatures do.  But you'll have some other heat source (the sun, for instance) anyway, so any proposal for heat cycles coupling from station temperature to space also have to compete with coupling to the higher temperature.  The ultimate design for heat movement between reservoirs would have to be considered in a holistic sense.  But I could see how the OP's idea would make sense in a station powered by PV panels, for instance.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2013-07-28T16:31:27.477" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="804" PostTypeId="2" ParentId="575" CreationDate="2013-07-28T16:56:30.437" Score="6" Body="&lt;p&gt;I'm shamelessly clutching the straw given by Major Stackings (who considers titanium to be a precious metal) and present the results of Mars Odyssey's survey with Gamma Ray Spectrometer (&lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA04257&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://photojournal.jpl.nasa.gov/catalog/PIA04257&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the map of relative abundance of &lt;a href=&quot;https://en.wikipedia.org/wiki/Thorium&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thorium&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xb0gC.jpg&quot; alt=&quot;GRS survey map&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The truth is, &lt;strong&gt;any metal should be considered precious on Mars&lt;/strong&gt;. However, in the chemical sense, precious metals are very strictly defined and neither titanium nor thorium fit the bill.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-02-24T12:27:44.027" LastActivityDate="2015-02-24T12:27:44.027" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="805" PostTypeId="2" ParentId="800" CreationDate="2013-07-28T17:31:42.000" Score="17" Body="&lt;h2&gt;Possible US based future stations&lt;/h2&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bigelowaerospace.com/&quot; rel=&quot;noreferrer&quot;&gt;Bigelow Aerospace&lt;/a&gt;, has launched two pathfinder station models (Genesis I in 2006 and Genesis II in 2007).  I think they are both still in orbit, not active.  They licensed the Transhab technology from NASA and ran with it.  The pathfinder modules were not human habitable.&lt;/p&gt;&#xA;&lt;p&gt;They have primarily been waiting for someone to be able to deliver passengers and cargo, that they can pay to deliver them.  Bigelow tried offering a $50 million prize, and a contract for services in order to spur development a bunch of years ago, but it did not succeed in generating the service they needed.&lt;/p&gt;&#xA;&lt;p&gt;Bigelow plans on building a commercial station for 'rent' to national governments who wish to have a 'space' program, but cannot afford to develop it from scratch.&lt;/p&gt;&#xA;&lt;p&gt;The BA-330 model is meant to have 330 Cubic meters of space, and originally they had planned 2 BA-330's sharing a connecting node with a smaller module, as a station.  Looks like they are suggesting on BA-330 by itself should suffice now.&lt;/p&gt;&#xA;&lt;p&gt;With SpaceX (Falcon9/Dragon), Boeing (Atlas5/CST-100), and Sierra Nevada Corp (Atlas 5/Dream Chaser) all working to deliver passengers to ISS, the side effect should be options for Bigelow.&lt;/p&gt;&#xA;&lt;p&gt;Bigelow had to downsize staff for a while, once core development was done, since they were still several years away from commercial crew options.  The expectation is once SpaceX/Boeing/SNC can deliver crew, they will ramp up fairly quickly.&lt;/p&gt;&#xA;&lt;p&gt;In the interim, &lt;a href=&quot;http://www.nasa.gov/home/hqnews/2013/jan/HQ_M13-011_NASA-Bigelow_Event.html&quot; rel=&quot;noreferrer&quot;&gt;NASA has contracted Bigelow&lt;/a&gt; to attach a module to the ISS as a demonstration. It is not really meant to be a proper module, since it will be maintained with its hatch closed, and only opened periodically for testing. However, it is a great next step. Of course, this is to be brought up as an external trunk payload on a Dragon/Falcon9 launch in 2015.  Which gives you a feel for its size, if it fits in the Dragon trunk.  This seems MUCH smaller than a BA330 and smaller than even the Genesis I and II modules currently in orbit.&lt;/p&gt;&#xA;&lt;h2&gt;Possible Chinese future stations&lt;/h2&gt;&#xA;&lt;p&gt;The Chinese have expressed their plans for a station as part of a three phased approach.  The first two were manned launches, that they have declared operational.  The third phase starts with the Tiangong-2 space laboratory to be launched in 2015, with an experimental core space station module around 2018. The ultimate aim is to build a 60-ton multi-module space station by 2020.&lt;/p&gt;&#xA;&lt;p&gt;They are limited by the size of their launchers, the Long March 5, and the coming Long March 7.  Thus they will take the Russian/Soviet approach that Mir took, of a core module, with additional modules docked to it.  (I distinguish this from ISS, which of course is a series of modules docked together, since the ISS has a number of elements that were not delivered with their own propulsive systems.  May be a bit of a quibble).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.eurasiareview.com/22072013-china-space-program-and-strategy-analysis/&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt;, is a good discussion of the various Chinese space ambitions and strategies.&lt;/p&gt;&#xA;&lt;p&gt;Space.com has a nice infographic on what it might look like:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/i9oOI.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;h2&gt;Possible Russian stations&lt;/h2&gt;&#xA;&lt;p&gt;The Russians have been testing the current ISS modules, to see if they could detach and use as the base modules of a new station called Opsek.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/13xQj.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-10-03T18:28:53.890" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="806" PostTypeId="1" CreationDate="2013-07-28T18:40:37.953" Score="13" ViewCount="362" Body="&lt;p&gt;A large number of symptoms of long duration stays in microgravity seem to be associated with the headward shift of organs and blood.  I've always wondered, couldn't these symptoms also happen in ordinary gravity, by just lying down?  If the cause is lack of a vertical gravity gradient, that seems logical. For instance, do long term bedridden patients get &lt;a href=&quot;http://www.nsbri.org/humanphysspace/introduction/intro-bodychanges.html&quot;&gt;puffy face&lt;/a&gt; like astronauts?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What are the symptoms caused by long stays in microgravity, and how do they compare to the symptoms experienced by bedridden patients?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="1028" LastEditDate="2015-06-09T09:17:47.400" LastActivityDate="2015-07-24T09:13:13.387" Title="How are the symptoms of long-term exposure to microgravity similar to those of bedridden patients?" Tags="|health|astronauts|microgravity|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="807" PostTypeId="1" AcceptedAnswerId="808" CreationDate="2013-07-28T20:34:08.150" Score="10" ViewCount="639" Body="&lt;p&gt;After a long period in space and once back to earth, what do astronauts to get again used to the Earth's gravity? &lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="25" LastEditDate="2015-12-17T22:51:09.783" LastActivityDate="2015-12-17T22:51:23.847" Title="Back to Earth's gravity, what astronauts do?" Tags="|long-duration|gravity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="808" PostTypeId="2" ParentId="807" CreationDate="2013-07-28T22:42:26.860" Score="12" Body="&lt;p&gt;NASA have an intensive rehabilitation program. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Body fluid including blood moves from the legs to the head and upper body&lt;/li&gt;&#xA;&lt;li&gt;The total amount of blood in the body decreases&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And from &lt;a href=&quot;http://www.space.com/21413-hadfield-astronaut-health-return-earth.html&quot; rel=&quot;nofollow&quot;&gt;space.com&lt;/a&gt;, &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Bone density lessens at a rate of 1 percent a month&lt;/li&gt;&#xA;&lt;li&gt;Muscle mass shrinks&lt;/li&gt;&#xA;&lt;li&gt;Eyeball pressure changes, with roughly one-fifth of astronauts reporting vision issues&lt;/li&gt;&#xA;&lt;li&gt;The sense of balance is partially lost&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are all counteracted by exercise, both cardiovascular, and strength exercises. In order to regain the muscle mass and fluids, general exercise seems to work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To increase bone density, calcium supplements are used, along with relatively high impact exercise such as running.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These exercises start off gently and increase in intensity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The physical exercise requirements are fairly well understood (and described in part on their &lt;a href=&quot;http://www.nasa.gov/centers/johnson/slsd/about/divisions/hacd/project/nxpcm.html&quot; rel=&quot;nofollow&quot;&gt;Non-exercise Physiological Countermeasures&lt;/a&gt; project page) but of perhaps more interest are the other areas listed there.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="25" LastEditDate="2015-12-17T22:51:23.847" LastActivityDate="2015-12-17T22:51:23.847" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="809" PostTypeId="1" AcceptedAnswerId="893" CreationDate="2013-07-29T07:53:43.983" Score="9" ViewCount="1236" Body="&lt;p&gt;There's a related question: &lt;a href=&quot;https://space.stackexchange.com/questions/795/in-space-can-the-difference-in-temperature-between-the-inside-and-outside-provi&quot;&gt;In space can the difference in temperature between the inside and outside, provide useful energy?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with that one is that it relies on the concept of &quot;hot inside&quot; and using space as the heat sink - while there are sources (like the Sun) which would pretty much heat up the &quot;outside&quot; more than the &quot;inside&quot;, and generally a range of other problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, there should be a much simpler way of exploiting heat transfer: Let's take a pad of Peltier cells, with the side exposed to the Sun, with highly absorbent coating, and an array of radiators of high emmissivity on the &quot;dark side&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic problem on Earth is the &quot;ambient temperature&quot; - about as much heat as is radiated out of the &quot;cooled side&quot; is returned to it by radiating Earth, and balanced out by convection+conduction of heat from air. In space, there's Cosmic Microwave Background (CMB) which, if I understand correctly, is fairly weak. There is no convection but radiation is fairly efficient. That means considerable amounts of heat could be transferred from the Sun side to the &quot;outer space&quot; side, at least as long as we remain relatively near to the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course such heat transfer could be exploited through any energy source that exploits heat transfer - like a Peltier cell, or even if you're willing to go that far, through a Stirling engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, how would such systems fare against &quot;tried old&quot; photovoltaic cells? Could someone throw some numbers at me, comparing what we could expect from the same surface/mass/price of one versus the other?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2019-10-21T19:50:10.130" Title="Peltier cell as solar cell in space" Tags="|energy|solar-power|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="810" PostTypeId="1" AcceptedAnswerId="822" CreationDate="2013-07-29T08:04:41.233" Score="34" ViewCount="20869" Body="&lt;p&gt;Earth's rotation provides considerable horizontal thrust to all launched satellites, nearly 1700km/hour near the Equator. That means considerable fuel savings and generally lower costs, and so most satellites accelerate in the earth rotation direction, adding to that speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, satellites at GEO will not travel against the Earth surface, and ones beyond GEO will &quot;apparently&quot; travel &quot;backwards&quot; as Earth's rotation overtakes them, but they still travel in the same direction as Earth rotates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any satellites currently that do travel in the opposite direction? Of course, their launch would have been significantly more expensive as they'd have to first brake against Earth rotation and then accelerate from zero in the other direction, but that doesn't mean it can't be done. Has it been done?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2013-08-04T23:03:56.510" LastActivityDate="2015-04-14T18:25:28.930" Title="Do any (LEO) satellites move east-to-west?" Tags="|low-earth-orbit|orbit|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="811" PostTypeId="1" AcceptedAnswerId="1690" CreationDate="2013-07-29T08:13:39.583" Score="19" ViewCount="835" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/792/laser-propulsion-possible&quot;&gt;another question&lt;/a&gt; I've been corrected that batteries don't weigh that much in ion drive powered probes. Well, I thought a can of xenon would have a chance to be lighter than a solid block of plutonium, but truth of the matter, I have no clue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could someone clue me on what are the proportions of mass between fuel(plus container), power source and the engine itself in ion engines used nowadays to propel probes into regions so distant from the Sun that solar panels are not practical to use there?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-08-30T21:39:05.963" Title="What contributes the majority of the mass in an ion drive?" Tags="|propulsion|ion-thruster|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="812" PostTypeId="1" AcceptedAnswerId="813" CreationDate="2013-07-29T10:43:36.220" Score="17" ViewCount="1539" Body="&lt;p&gt;In zero (or micro) gravity there are lots of problems for a birthday cake. Cakes &quot;rise&quot;, and flames &quot;burn up&quot;, without gravity how could any of this happen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will my cake fail to rise?  Will my candles burn?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorDisplayName="user12" LastEditDate="2013-08-07T17:08:45.613" LastActivityDate="2013-08-07T17:08:45.613" Title="Can you have birthday cake in space?" Tags="|food|microgravity|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="813" PostTypeId="2" ParentId="812" CreationDate="2013-07-29T11:31:45.397" Score="25" Body="&lt;p&gt;The cake will &quot;rise&quot; for the same reasons it does in gravity (yeast or other agents releasing gases as a metabolic byproduct, when heated, or as a chemical process and reacting to other ingredients, given sufficient time and providing for other required conditions to activate the rising agent). The only problem is, it will &quot;rise&quot; equally in all directions in a zero gravity environment, so you might want to start shaping your dough well before it starts increasing in volume due to internal pressure upon release of gases, maybe even contain this process completely within a cake shaped container that you bake the dough in (there's no real names for such &quot;kitchen tools&quot; yet for those that will be required in zero gravity, so I'm just using some generic terms - I'm sure Gordon Ramsay would have something to yell about regarding my choice).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the candles will burn without the flame &quot;rising&quot; in a single direction, but again - equally in all directions, so the &quot;flame&quot; will be more or less spherical, with only the candle wick slightly misshaping it towards a cylinder shape (assuming it will distribute wax due to suction - a byproduct of burning and expanding in volume - equally along its length). So the flame would look like a product of a sphere and a cylinder (slightly flattened &quot;top&quot;), but nearly completely flattened where the candle wax is still solid (&quot;bottom&quot;) due to candlestick's surface tension, melting of wax, and suction of air from all directions equally - but the candlestick is slightly in the way and this air pressure slightly increases there as the path narrows). This experiment has been performed on Mir and &lt;a href=&quot;https://www.youtube.com/watch?v=IgzCMKdAYuI&quot; rel=&quot;noreferrer&quot;&gt;the video is available on YouTube&lt;/a&gt;, and the shape the flame forms confirms all the physical processes described, albeit the experiment seems to have been conducted in a smaller to sea-level Earth's atmospheric pressure, or in atmosphere with slightly smaller percentage of oxygen (the colour of the flame suggests the former to be more probable cause of it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Ods1K.jpg&quot; alt=&quot;Candle flame in Zero-G on the Mir&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Candle flame in Zero-G on the Mir&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, you could have a cake in zero-g, and you could eat it, but then you wouldn't have it any more. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-07-30T19:00:37.437" LastActivityDate="2013-07-30T19:00:37.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="814" PostTypeId="1" AcceptedAnswerId="818" CreationDate="2013-07-29T16:33:18.543" Score="15" ViewCount="1225" Body="&lt;p&gt;The two Viking landers had an experiment on board where they attempted to find life on the planet. The initial results said yes, there was life, but later there were questions raised about the results. What is the final verdict on the matter?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-09T14:40:08.627" LastActivityDate="2016-05-03T16:06:38.303" Title="Did the Viking landers detect life on Mars?" Tags="|mars|planetary-science|astrobiology|lander|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="815" PostTypeId="1" AcceptedAnswerId="816" CreationDate="2013-07-29T15:08:19.763" Score="59" ViewCount="8119" Body="&lt;p&gt;In 'From the Earth to the Moon' (1873) by Jules Verne, A huge cannon is used to send a spaceship to the moon.  A lively discussion in &lt;a href=&quot;http://en.wikisource.org/wiki/From_the_Earth_to_the_Moon/Chapter_IX&quot;&gt;chapter IX&lt;/a&gt; leads to using 400,000 pounds of fulminating cotton to launch their ship to the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two questions;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Does 1,600,000 pounds (725,748 kg) of powder equal 400,000 pounds (181,437 kg) of fulminating cotton, for lift?&lt;/li&gt;&#xA;&lt;li&gt;Putting aside all the other issues, would either amount of lift get you to the moon?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="109" OwnerDisplayName="James Jenkins" LastEditorDisplayName="user12" LastEditDate="2013-08-17T20:49:10.050" LastActivityDate="2015-02-12T20:59:19.800" Title="Can gunpowder get you to the moon?" Tags="|launch|propulsion|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="816" PostTypeId="2" ParentId="815" CreationDate="2013-07-29T17:21:24.180" Score="51" Body="&lt;p&gt;The Delta V requirement to launch is about 14 km/s to low lunar orbit, per &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;. That means that you would have to achieve a speed of 14 km/s in order to orbit the moon. Some of that will need to be done from space, but most of it could theoretically be achieved from the ground. So, what do you need to do to make that happen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://hypertextbook.com/facts/2003/GabrielaBis.shtml&quot; rel=&quot;noreferrer&quot;&gt;In World War II&lt;/a&gt;, the Germans developed an artillery shell that could travel at 1.67 km/s. It used 200 kg of powder, and fired a 106 kg shell. Let's just suppose that you could scale that upward infinitely (Not likely, but we'll just assume for a moment). Furthermore, let's assume a mass of 1000 kg for the ship (Likely would be higher). Given all of that, you would need 10 times as much to launch the ship the same speed, and about 72 times as much to launch the ship to lunar orbit. That would scale to about 14400 kg of powder, or about 16 tons of powder, much less than Jules Vern stated you would need. So, why don't we do that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While in theory one could get to the moon like this, pure cannon thrust would not be sufficient to land on the moon, at least in a controlled manner. You would end up landing on the moon at the lunar escape velocity of 2.4 km/s, without a rocket to stop you. Furthermore, the gravity forces exerted on you would at launch would be enormous, artillery shell electronics &lt;a href=&quot;http://www.gforces.net/a-discussion-on-typical-examples.html&quot; rel=&quot;noreferrer&quot;&gt;have to be rated&lt;/a&gt; at 15000 Gs. Good luck getting a person to survive that. And also, the physics doesn't quite scale as I indicated here, but the numbers provide a good first order approximation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch profile of a rocket is near best case for getting astronauts to Space, in terms of the amount of gravity. You really do need to be continually thrusting for some time. However, a rail gun could provide some of the velocity needed to orbit, if you plan carefully to make this happen.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-02-12T20:59:19.800" LastActivityDate="2015-02-12T20:59:19.800" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="817" PostTypeId="2" ParentId="815" CreationDate="2013-07-29T17:36:00.047" Score="11" Body="&lt;p&gt;The absolute theoretical maximum mass that 1,600,000 lbs of powder could launch to the moon is a bit under 35,500 kg. The calculation is not all that difficult (introductory calculus-based physics), but is somewhat long and involved, and would be quite ugly on a site like this without mathjax.&#xA;However, that makes two major assumptions that cannot actually happen:  no air resistance, and all of the powder burns instantly, transferring all its energy to the rocket (none to a visual flame, none to sound, etc.  I'm still working on a quantitative analysis of these effects, but I'm quite certain that accounting for either air resistance or the finite burn speed of the rocket would make it impossible to reach the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The maximum speed a propellant-powered rocket can reach depends on the rocket's mass, the mass of propellant, and the exhaust velocity of the exhaust. As per James Jenkins, the ship was 20,000 lbs; the propellant is, of course, 1,600,000 lbs. Using a typical black powder exhaust velocity of 800 m/s.  Without fighting gravity, that amount of gunpowder could propel the ship to a bit over 3,500 m/s, well short of the Earth's escape velocity of 11,200 m/s.  Turning it around a bit, that mass ratio would require an exhaust velocity of nearly 2,550 m/s.  And for completeness, the given 160M lbs of propellant could launch a rocket of a bit under 1.5 lbs; it would take 24 billion pounds of propellant to launch the full 20,000-lb rocket.&lt;/p&gt;&#xA;" OwnerUserId="303" LastEditorUserId="303" LastEditDate="2013-07-30T02:04:52.033" LastActivityDate="2013-07-30T02:04:52.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="818" PostTypeId="2" ParentId="814" CreationDate="2013-07-29T18:17:39.307" Score="14" Body="&lt;p&gt;Initially it looked like there was life, then that was dashed, but newer research might be indicative of life after all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://csep10.phys.utk.edu/astr161/lect/mars/viking.html&quot; rel=&quot;nofollow&quot;&gt;http://csep10.phys.utk.edu/astr161/lect/mars/viking.html&lt;/a&gt;, back in 1976 we had:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The 4 basic experiments that the Vikings carried out to search for evidence of life were:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Gas Metabolism: look for changes in the atmosphere induced by metabolism in the Martian soil.&lt;/li&gt;&#xA;  &lt;li&gt;Labeled Release: Look for release of radioactive carbon dioxide by metabolism from organic material labeled by radioactive carbon.&lt;/li&gt;&#xA;  &lt;li&gt;Pyrolytic Release: Search for radioactive compounds in soil by heating soil exposed to radioactive carbon dioxide.&lt;/li&gt;&#xA;  &lt;li&gt;Mass Spectrometer: Search directly in Martian soil for organic compounds known to be essential to Earth life.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;Of them, only the Labeled Release experiment gave positive results, and the complete absence of any organic compounds in the Martian soil according to the mass spectrometer experiment suggests that the positive results were not evidence for life, but rather evidence for a complex inorganic chemistry in the Martian soil. Thus, the Viking verdict was that there was no evidence for present or past life on Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But then in 2012, this was published in the &lt;a href=&quot;http://ijass.org/PublishedPaper/year_abstract.asp?idx=132&quot; rel=&quot;nofollow&quot;&gt;International Journal of Aeronautical and Space Sciences&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(summary taken from a Discovery.com article, for those who can't access the ijass)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The new study took a different approach. Researchers distilled the Viking Labeled Release data, provided as hard copies by the original researchers, into sets of numbers and analyzed the results for complexity. Since living systems are more complicated than non-biological processes, the idea was to look at the experiment results from a purely numerical perspective.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;They found close correlations between the Viking experiment results' complexity and those of terrestrial biological data sets. They say the high degree of order is more characteristic of biological, rather than purely physical processes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They conclude that the complexity pattern seen in active experiments strongly suggests biology while the different pattern in the control responses is more likely to be non-biological. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, no final verdict yet&lt;/strong&gt; - as this is relatively new research, not universally accepted, and stemming from one set of data. &lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2016-05-03T16:06:38.303" LastActivityDate="2016-05-03T16:06:38.303" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="819" PostTypeId="1" AcceptedAnswerId="824" CreationDate="2013-07-29T18:27:50.917" Score="18" ViewCount="1837" Body="&lt;p&gt;The question speaks for itself. If the answer is &quot;yes&quot;, what are the problems in creating a large-scale space structure, maybe even dozens of kilometres large? What about moving &amp;amp; repositioning, building, handling, supplying, maintaining and repairing issues of space hyperstructures (for example, a nuclear factory in space)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could check only one aspect: moving and repositioning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretically, larger space structures (like space stations) don't need serious propulsion because they can be built in space (as it happened with the International Space Station: it was built piece by piece) and moving them should be necessary at repositioning - but if they could get an orbit (around a moon, a planet or maybe even a star), they are moved by the force of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; with the word &lt;em&gt;structure&lt;/em&gt;, I mean a man-made facility that has certain functions. I wouldn't like to use the word &lt;em&gt;space station&lt;/em&gt;, because with very large scales, a similar facility would be more than a station.&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="168" LastEditDate="2013-08-09T02:59:18.777" LastActivityDate="2020-11-13T22:35:25.760" Title="Are there any theoretical size limits of man-made space stations/structures?" Tags="|space-station|construction|facility|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="820" PostTypeId="1" AcceptedAnswerId="826" CreationDate="2013-07-29T18:33:10.397" Score="10" ViewCount="4862" Body="&lt;p&gt;Before the space race or modern technology of 20th century, what was the earliest point in history that space exploration was attempted? If available, I'd love to read the documentation.&lt;/p&gt;&#xA;" OwnerUserId="66" LastEditorUserId="12102" LastEditDate="2021-09-13T00:13:40.637" LastActivityDate="2021-09-13T00:13:40.637" Title="What was the first attempt in history for space exploration?" Tags="|launch|rockets|history|spaceflight-firsts|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="821" PostTypeId="2" ParentId="815" CreationDate="2013-07-29T18:35:01.927" Score="5" Body="&lt;p&gt;There is a story told that the first manmade object to achieve escape velocity, was a &lt;a href=&quot;http://nuclearweaponarchive.org/Usa/Tests/Plumbob.html#PascalB&quot; rel=&quot;nofollow&quot;&gt;man hole style cover&lt;/a&gt;, over an exhaust vent, from an underground nuclear bomb test.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, they quote this on the website.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But the assumption that it might have escaped from Earth is&#xA;  implausible (Dr. Brownlee's discretion in making a priority claim is&#xA;  well advised). Leaving aside whether such an extremely hypersonic&#xA;  unaerodynamic object could even survive passage through the lower&#xA;  atmosphere, it appears impossible for it to retain much of its initial&#xA;  velocity while passing through the atmosphere. A ground launched&#xA;  hypersonic projectile has the same problem with maintaining its&#xA;  velocity that an incoming meteor has. According to the American Meteor&#xA;  Society Fireball and Meteor FAQ meteors weighing less than 8 tonnes&#xA;  retain none of their cosmic velocity when passing through the&#xA;  atmosphere, they simply end up as a falling rock. Only objects&#xA;  weighing many times this mass retain a significant fraction of their&#xA;  velocity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From another amusing perspective, there is a great science fiction story, called &lt;a href=&quot;http://en.wikipedia.org/wiki/King_David%27s_Spaceship&quot; rel=&quot;nofollow&quot;&gt;King Davids Spaceship&lt;/a&gt; by Jerry Pournelle that postulates a universe where the interstellar faring societies will not intervene unless your planet can reach orbit, and for political reasons a planet requires space flight as soon as possible, so they build a manned craft, that uses the approach of firing a gun downward, (sort of chemical version of Orion) to reach orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pournelle is fun for writing good science fiction, and it has interesting discussions of the issues involved.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-03-03T13:24:01.370" LastActivityDate="2014-03-03T13:24:01.370" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="822" PostTypeId="2" ParentId="810" CreationDate="2013-07-29T18:41:37.220" Score="21" Body="&lt;p&gt;The correct term for this type of orbit is &lt;strong&gt;retrograde&lt;/strong&gt;. Here's a shiny animated thing showing the orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5nStl.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the difficulty to achieve such an orbit, it is rarely used. Most commonly, artificial satellites using this orbit are commercial earth-observing satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some satellites which use this orbit are Israel's &lt;a href=&quot;http://en.wikipedia.org/wiki/Shavit&quot; rel=&quot;noreferrer&quot;&gt;Shavit&lt;/a&gt; satellites and two US &lt;a href=&quot;http://en.wikipedia.org/wiki/Future_Imagery_Architecture&quot; rel=&quot;noreferrer&quot;&gt;Future Imagery Architecture&lt;/a&gt; satellites.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-07-29T18:41:37.220" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="823" PostTypeId="2" ParentId="810" CreationDate="2013-07-29T19:23:05.227" Score="32" Body="&lt;p&gt;As Undo mentioned, the type of orbit is called a retrograde. A pure retrograde orbit is quite rare, in fact, NORAD currently isn't tracking any (unclassified) objects with a 180 inclination (Pure Retrograde). However, there are a fair number of satellites with a retrograde inclination (Over 90 degrees). Of particular note is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;Sun Synchronous orbits&lt;/a&gt; of about 98 degrees. The largest (Unclassified) inclination of any satellite currently in orbit that I can find any information about appears to be &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/minisat-0.htm&quot; rel=&quot;noreferrer&quot;&gt;Minisat 01&lt;/a&gt;, with an inclination of about 150 degrees. The reasons for this unusual orbit are not explained.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, there's a nice little graph of what inclination space collisions are expected in &lt;a href=&quot;http://www.fp7-accord.eu/files/ACCORD_Register_Growth_Factors.pdf&quot; rel=&quot;noreferrer&quot;&gt;this presentation&lt;/a&gt;, page 5. It should give an idea of how common slight retrograde orbits are.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-04-14T18:25:28.930" LastActivityDate="2015-04-14T18:25:28.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="824" PostTypeId="2" ParentId="819" CreationDate="2013-07-29T19:38:30.290" Score="20" Body="&lt;p&gt;&lt;strong&gt;Electromagnetism&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;To be as practical as possible, the Space Shuttle mission &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-75&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-75&lt;/a&gt; experimented with a space tether.  For the record, the tether did break.  To be fairly exact, it did break for reasons that wouldn't have broken it if the line had been shorter.  So in a certain sense (but only a limited sense), we have already seen a space tether break because of its size.&lt;/p&gt;&#xA;&lt;p&gt;That was because of wire current interacting with trapped air from manufacturing in the line.  The current was caused by movement within the Earth's magnetic field.  That was 20 km long.  We can't call that a limit in any meaningful sense, because NASA could make a better tether now, and the entire point of the mission was to experiment with induced current.  So an insulator would fair better.&lt;/p&gt;&#xA;&lt;p&gt;Nonetheless, induced current is still &lt;em&gt;size specific&lt;/em&gt;.  It's also specific to velocity, magnetic field, and probably a few other things.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Tidal Forces&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The reason there was tension in the wire to begin with was tidal forces.  There are abundant tether proposals for various cislunar systems that take advantage of the gravity gradient.  It was even considered to make structures for the ISS that would use the tidal forces to help maintain orientation.&lt;/p&gt;&#xA;&lt;p&gt;There are some qualifiers.  Tidal forces only extend in one direction, so it may not actually stress your structure depending on geometry and how it's pointed.&lt;/p&gt;&#xA;&lt;p&gt;For a basic mathematical treatment, the tidal field grows linearly with movement from the central point.  If you have a mechanical member, then that is integrated over the length to give a tensile force on the order of (length)&lt;span class=&quot;math-container&quot;&gt;$^2$&lt;/span&gt;, I believe.&lt;/p&gt;&#xA;&lt;p&gt;In a meaningful sense, a space elevator has to fight against tidal forces, and it would require futuristic materials. My poor &lt;span class=&quot;math-container&quot;&gt;$l^2$&lt;/span&gt; approximation would be insufficient on that scale.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Self-gravitation&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Something can collapse under its own weight. Of course, we'll have to assume that it's not relying on internal pressure. Planets rely on their own internal pressure to hold itself stable against self-gravitation. This is fairly unspectacular, and since we're looking for man-made structures, I'll imagine something like the Death Star.&lt;/p&gt;&#xA;&lt;p&gt;If a spherical space station has a constant density, the gravity grows linearly from the center. Because of that, it would have the same (length)&lt;span class=&quot;math-container&quot;&gt;$^2$&lt;/span&gt; scale of structural requirements, but this would be compressive.  You can push materials limits further with tensile loads than compressive loads in general.&lt;/p&gt;&#xA;&lt;p&gt;It's important to note that self-gravitation depends on the average density of the structure. Actually, it's a (density)&lt;span class=&quot;math-container&quot;&gt;$^2$&lt;/span&gt; relationship. The argument for the square is that on Earth your weight is just (mass), because it's gravitation between you-Earth. Self-gravitation is between you-you. Thus, your mass term gets entered twice. This means that a very sparse structure could theoretically span a size greater than that of Earth, while not collapsing under its own self-gravitation.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Others&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;There's another way you could push this even further - use kinetic forces.  You could have a rigid structure that rotates in a big ring, which avoids some self-gravitation compression.  You could push that idea very far, with huge self-gravitating Dyson swarms, or something along those lines.  But maybe that would fail the requirement for not being &amp;quot;rigid&amp;quot;.  There may be other workarounds.  My creativity fails me at this point.&lt;/p&gt;&#xA;&lt;p&gt;At this point, reaching absurdity, there are some odd and even comical limits you can think of.  For instance, if you assume that our energy growth continues to grow at 1% per year, &lt;a href=&quot;http://www.theoildrum.com/node/8155&quot; rel=&quot;nofollow noreferrer&quot;&gt;we will cook the Earth in 1000 years or so&lt;/a&gt;.  It's not a complicated argument.  Just assume we continue to exponentially grow and the conclusion is obvious.  This can be applied on any boundary, including the solar system or the galaxy.&lt;/p&gt;&#xA;&lt;p&gt;Ultimately, yes, the limit would be a becoming a black hole.  It's difficult to see how that would happen sooner than the thermal limit, because it's highly sensitive to the matter density.  Theoretically you could make a black hole without a catastrophic event like a supernova, because large black holes can have a &amp;quot;density&amp;quot; (defined with the event horizon) less than water.  So if you were flying giant blocks of lead around in space in a carefully planned formation over many light years, you could turn into a black hole in a very novel way.  But why?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="33864" LastEditDate="2020-11-13T11:05:29.093" LastActivityDate="2020-11-13T11:05:29.093" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="825" PostTypeId="2" ParentId="67" CreationDate="2013-07-29T20:02:48.477" Score="23" Body="&lt;p&gt;The methane is produced as a by-product of the Sabatier reaction, where carbon dioxide is removed from the air using a catalyst and hydrogen.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;CO2 + 4H2 → CH4 + 2H2O&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The molar mass of Methane is 16.0425  (grams per mole) and the molar mass of carbon dioxide is 44.0095  (grams per mole).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means that the 1000 pounds of methane came from about 2700 pounds of carbon dioxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 2700 pounds of carbon dioxide per year is about 3 people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The water produced by the Sabatier reaction can be split into hydrogen and oxygen providing about half of the 300 pounds per year of hydrogen that this reaction needs.&lt;/p&gt;&#xA;" OwnerUserId="16" LastActivityDate="2013-07-29T20:02:48.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="826" PostTypeId="2" ParentId="820" CreationDate="2013-07-29T20:07:25.277" Score="20" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Wan Hu&lt;/strong&gt; (16th century A.D.):&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to legend, Wan Hu (possibly 萬虎 or 萬戶) was a minor Chinese&#xA;  official — supposedly of the middle Ming dynasty (16th century) — who&#xA;  attempted to become the world's first &quot;astronaut&quot; by being lifted by&#xA;  rockets into outer space. The crater Wan-Hoo on the far side of the&#xA;  Moon is named after him.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7QPWp.png&quot; alt=&quot;Wan Hu&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Quote and picture source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Wan_Hu&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Wan Hu&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Lagâri Hasan Çelebi&lt;/strong&gt; (17th century A.D.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Lagâri Hasan Çelebi was a legendary Ottoman aviator who, according to&#xA;  an account written by traveler Evliya Çelebi, made a successful&#xA;  manned rocket flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ICkMH.jpg&quot; alt=&quot;Lagâri Hasan Çelebi&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Quote and picture source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Lag%C3%A2ri_Hasan_%C3%87elebi&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Lagâri Hasan Çelebi&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;First man-made object in space&lt;/strong&gt; (3 October 1942)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first steps of putting a man-made object into space were taken by&#xA;  German scientists during World War II while testing the V-2 rocket,&#xA;  which became the first man-made object in space on 3 October 1942 with&#xA;  the launching of the A-4.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/8t8eY.jpg&quot; alt=&quot;A U.S. Army cut-away of the V-2.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Quote source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_exploration#History_of_exploration_in_the_20th_century&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Space exploration - History of exploration in the 20th century&lt;/a&gt;, cut-away source: &lt;a href=&quot;https://en.wikipedia.org/wiki/V-2&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on V-2 rocket&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;First flights&lt;/strong&gt; (4 October 1957)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first successful orbital launch was of the Soviet unmanned Sputnik&#xA;  1 (&quot;Satellite 1&quot;) mission on 4 October 1957. The satellite weighed&#xA;  about 83 kg (184 pounds), and is believed to have orbited Earth at a&#xA;  height of about 250 km (160 mi). It had two radio transmitters (20 and&#xA;  40 MHz), which emitted &quot;beeps&quot; that could be heard by radios around&#xA;  the globe.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/jO79h.jpg&quot; alt=&quot;Sputnik 1, the first artificial satellite&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Quote and photograph source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_exploration#History_of_exploration_in_the_20th_century&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Space exploration - History of exploration in the 20th century&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="25" LastEditDate="2015-12-17T22:50:13.147" LastActivityDate="2015-12-17T22:50:13.147" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="827" PostTypeId="2" ParentId="820" CreationDate="2013-07-29T20:20:29.223" Score="5" Body="&lt;p&gt;The first recorded attempt with a credible method and strong historic basis would have to be 20th century - and that would be the rocketry approach of the Russian and American space programs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Historic attempts before the 1950's all used bad methods - methods that cannot work according to our current understanding of physics. So we can just ignore those using balloons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we accept legends using the correct methodology but incorrect math, we get a wide variety of &quot;early attempts.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The legendary Icarus and his wax wings can be excluded, as it's a method reliant upon atmosphere and impossible physiology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wang Tu, aka Wan Hu, is the next oldest legend. He is said to have attempted to fly to the heavens by mixture of kites and rockets. Rockets are a viable method - and kites are a good way to get a weight airborne. High efficiency parasails, anchored to the ground, can be used to easily lift a human to upwards of 1000' AGL in areas of constant 15 knot winds. Wang Tu couldn't have succeeded due to the nature of the available materials, and the traditional date given of 2000BC is implausible, but that the tradition is recorded in October 1909 by Scientific American indicates that it was considered to be at least informative. At the earliest, Wang Tu's attempt would be after gunpowder in China, which puts it somewhere around 1100 AD.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some legends put the use of a large canon or catapult as Western equivalents, but they post-date Wang Tu even in their claimed dates. A sufficiently strong one could achieve orbital launch, but would kill the human due to the accelerations involved, or, like the proposed US Space Canon, be several miles long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, reliable and effective? The US and Russian missile programs leading to Sputnik and Vanguard. At least on the right track? The Legendary Wang Tu.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-07-29T20:20:29.223" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="828" PostTypeId="2" ParentId="815" CreationDate="2013-07-29T20:22:02.000" Score="26" Body="&lt;p&gt;As far as I know, the shock wave in detonating explosives does not go faster than about 2.5 km/s, so a bullet will not be propelled beyond that speed, however many barrels of gunpowder you accumulate. The shock wave can be sped up if the operation occurs in a high pressure environment, though, but reaching enough speed to get to orbit (about 8 km/s) seems hard, let alone going to the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can make a multi-stage system, though: one cannon which shoots another cannon which shoots a third cannon, and so on. Ultimately you end up with a rocket, not a cannon. It is a bit of a question of definition...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(A variant with &lt;a href=&quot;http://nextbigfuture.com/2009/02/nuclear-orion-home-run-shot-all-fallout.html&quot;&gt;an explosive a bit more punchy than gunpowder&lt;/a&gt; is being seriously investigated, but I doubt it will happen any time soon.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The acceleration would also flatten in a very literal way any vertebrate unlucky enough to have been selected for this trip. Space cannons are more than idle speculation, but they are for launching bulk materials, not people.&lt;/p&gt;&#xA;" OwnerUserId="311" LastActivityDate="2013-07-29T20:22:02.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="829" PostTypeId="1" CreationDate="2013-07-29T20:31:33.527" Score="28" ViewCount="12725" Body="&lt;p&gt;I think I might have understood what this &lt;a href=&quot;http://xkcd.com/1244/&quot; rel=&quot;noreferrer&quot;&gt;XKCD comic&lt;/a&gt; is trying to do. The diagram is proposing to use the &lt;a href=&quot;http://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;noreferrer&quot;&gt;Oberth effect&lt;/a&gt; at &lt;em&gt;closest approach to the sun&lt;/em&gt; to escape the solar system!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/R1gLH.png&quot; alt=&quot;xkcd comic&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(image license: CC BY-NC 2.5)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With current technology (something like the New Horizons or Voyager engines), could this kind of assist actually get it outside the solar system?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Additional note:  I thought the bottommost planet in the comic was meant to indicate Jupiter. This is apparently &lt;a href=&quot;http://www.explainxkcd.com/wiki/index.php?title=1244:_Six_Words&quot; rel=&quot;noreferrer&quot;&gt;a minority opinion on other forums&lt;/a&gt; - their logic is that &quot;Kuiper&quot; could refer to a maneuver that ultimately crosses the Kuiper Belt.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="10132" LastEditDate="2015-07-08T23:41:51.643" LastActivityDate="2021-07-26T12:13:38.480" Title="Could the sun be used as a gravity assist outside the solar system (with current tech)?" Tags="|orbital-mechanics|interstellar-travel|gravity-assist|xkcd|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="830" PostTypeId="2" ParentId="756" CreationDate="2013-07-29T20:39:40.933" Score="12" Body="&lt;p&gt;I'll have to dig through the &lt;a href=&quot;http://www.lpi.usra.edu/decadal/2013_2022/decadalReport.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;decadal survey&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_and_Planetary_Institute&quot; rel=&quot;nofollow noreferrer&quot;&gt;LPI&lt;/a&gt; proceedings a few times, so I decided I have to make this answer a community wiki anyway. As soon as there is a decent answer by anybody else, I'll accept it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three basic mission modes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;getting inside the ocean&lt;/li&gt;&#xA;&lt;li&gt;getting onto the surface&lt;/li&gt;&#xA;&lt;li&gt;doing research from low orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The main goal: determining habitability of the subsurface ocean (if it exists).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What can be done from low orbit:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mapping gravity (yeah, that's what was meant by GRAIL-like twin craft formation-flying missions)&lt;/li&gt;&#xA;&lt;li&gt;doing fine-grained laser altimetry&lt;/li&gt;&#xA;&lt;li&gt;mapping volatiles escaping from the body&lt;/li&gt;&#xA;&lt;li&gt;employing GPR (ground-penetrating radar)&lt;/li&gt;&#xA;&lt;li&gt;characterizing magnetic field&lt;/li&gt;&#xA;&lt;li&gt;studying the plasma around the body (&lt;a href=&quot;https://en.wikipedia.org/wiki/Langmuir_probe&quot; rel=&quot;nofollow noreferrer&quot;&gt;Langmuir probe&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;undertaking detailed multispectral &lt;em&gt;dynamic&lt;/em&gt; stereo imaging to map and monitor the surface (bet you've all seen those dunes move in &lt;a href=&quot;http://hirise.lpl.arizona.edu/&quot; rel=&quot;nofollow noreferrer&quot;&gt;HiRISE&lt;/a&gt; pictures, so you know what I'm talking about)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What can't be done from low orbit:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;In situ chemistry analysis&lt;/li&gt;&#xA;&lt;li&gt;...&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Getting under the ocean:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Melting the ice with heat from a nuclear reactor&lt;/li&gt;&#xA;&lt;li&gt;Drilling the ice (the thick ice idea of drilling 100 km is slightly unappealing)&lt;/li&gt;&#xA;&lt;li&gt;Crashing artificial penetrators into the ice (won't work for thick layers presumably present on Europa)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Crashing a captive asteroid into the ice shield&lt;/strong&gt; (not cheap or quick at all, and may not work for thick ice)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Resources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.lpi.usra.edu/opag/&quot; rel=&quot;nofollow noreferrer&quot;&gt;OPAG — LPI's Outer Planets Assessment Group&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Europa Study 2012 Report&lt;/li&gt;&#xA;&lt;li&gt;An article in &lt;em&gt;Astrobiology&lt;/em&gt; on &lt;a href=&quot;http://online.liebertpub.com/doi/pdfplus/10.1089/ast.2013.1003&quot; rel=&quot;nofollow noreferrer&quot;&gt;Europa lander mission definition&lt;/a&gt; (available on-line free of charge)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/inab9.jpg&quot; alt=&quot;Europa lander — preliminary drawing as of mid-2013&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(to be continued)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please feel free to edit the answer if you feel like it.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="1332" LastEditDate="2013-12-27T14:19:54.693" LastActivityDate="2013-12-27T14:19:54.693" CommentCount="1" CommunityOwnedDate="2013-07-29T20:39:40.933" ContentLicense="CC BY-SA 3.0" />
  <row Id="832" PostTypeId="2" ParentId="829" CreationDate="2013-07-29T21:00:06.473" Score="18" Body="&lt;p&gt;No. What a gravity assist does is change the velocity with respect to other objects, but not the one you were approaching. &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/grav/primer.php&quot; rel=&quot;noreferrer&quot;&gt;Nasa&lt;/a&gt; provided a nice diagram to assist with understanding this.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/OH9yl.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, in more ordinary terms, it could be though of as the below diagram shows. The baseball is thrown at the train at 30 miles per hour. From the view point of the train, the ball is first coming at it at 80 miles per hour, then leaving at up to 80 miles per hour. From the train's perspective, the relative velocity is the same. However, from the perspective of the person, the ball is moving much faster after hitting the train.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8Chpu.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, enter the idea of using the sun as a gravity slingshot to leave the solar system. It won't work, because the relative speed of flying by the sun won't be changed by flying close it it. One could fly by the sun, use a rocket to accelerate it's movement and leave the solar system behind (Due to the previously mentioned &lt;a href=&quot;http://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;noreferrer&quot;&gt;Oberth effect&lt;/a&gt;), but that's about the best that could be accomplished by a flyby of the sun. In fact, this was proposed in &lt;a href=&quot;http://www.niac.usra.edu/files/library/meetings/misc/trieste_may02_mtg/McNutt_Ralph.pdf&quot; rel=&quot;noreferrer&quot;&gt;a paper&lt;/a&gt;, which states that if a large amount of acceleration occurs near the Sun quickly, the spacecraft could achieve a speed of 20 AU/year. That would allow for exploring the interstellar medium, but still make it difficult to explore another star.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-30T14:06:43.480" LastActivityDate="2013-07-30T14:06:43.480" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="833" PostTypeId="2" ParentId="511" CreationDate="2013-07-29T21:36:24.520" Score="11" Body="&lt;p&gt;If I'm reading the question correctly, this is a question about how difficult the engineering challenges are. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the data in the question itself(amazingly helpful) the real question is keeping the person you're dropping from being crushed/catching fire. I believe the density of air and the spirit of the question prevents effective parachuting at high altitude. Your space jumper is going to free fall for some time, decelerate as they hit the atmosphere, then presumably open a traditional parachute(at traditional terminal velocity) and land safely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hitting the atmosphere after free fall if you are a sounding rocket or person is nonfatal(by crushing), albeit unpleasant. 5 g is completely survivable, even without countermeasures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that leaves breathing(not too hard, just some oxygen) and heating problems from air compression. The design of heat shields is actually to &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmospheric_reentry#Blunt_body_entry_vehicles&quot;&gt;maximize the drag coefficient and minimize the heat load&lt;/a&gt;, so if you're willing to bring like a &lt;a href=&quot;http://en.wikipedia.org/wiki/Aeroshell&quot;&gt;toboggan&lt;/a&gt; made of ceramic composites to push the air out of the way, certainly. (Could be strapped to your back. Envision a ninja turtle lying on his back with legs and arms pointed straight up) If you want to dive headfirst Captain Kirk style you are going to have to have more than just a viewpane. It might be possible, but it would not be safe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you want to sacrifice dignity, lying on your back with an aeroshell could, in my estimation, be an entirely practical way to fall from geostationary orbit.&lt;/p&gt;&#xA;" OwnerUserId="319" LastActivityDate="2013-07-29T21:36:24.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="834" PostTypeId="1" AcceptedAnswerId="838" CreationDate="2013-07-29T22:43:56.250" Score="18" ViewCount="2352" Body="&lt;p&gt;Aside from the obvious answer of being able to easily avoid the larger bodies in the Solar System.  I'm primarily curious as to the mechanics of this endeavor -- would there be any benefit to attempting to leave the solar system say... perpendicular to the ecliptic?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which direction would get you out of the influence of the Sun fastest, and could you take advantage of that environment on the way to speed things up?&lt;/p&gt;&#xA;" OwnerUserId="62" LastEditorUserId="62" LastEditDate="2013-09-20T00:52:44.823" LastActivityDate="2017-10-24T06:52:24.667" Title="Would leaving the ecliptic plane offer any benefit to interstellar travel?" Tags="|interstellar-travel|trajectory|ecliptic-plane|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="835" PostTypeId="1" AcceptedAnswerId="1329" CreationDate="2013-07-29T22:59:23.860" Score="12" ViewCount="710" Body="&lt;p&gt;I understand if technology isn't quite there yet. If it isn't, is there any research being developed along these lines? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm trying to wrap my head around the composition of these planets, and the thought of &quot;What are things like under the Great Red Spot as compared to other points on/in Jupiter&quot; is a fairly tantalizing one.&lt;/p&gt;&#xA;" OwnerUserId="62" LastEditorUserId="25" LastEditDate="2013-07-29T23:15:21.717" LastActivityDate="2019-05-21T07:20:56.320" Title="Do we have instruments that can measure the depth of storms like the Great Red Spot on Jupiter and the Great Dark Spot on Neptune?" Tags="|instrument|gas-giant|atmosphere|weather|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="836" PostTypeId="5" CreationDate="2013-07-29T23:16:34.487" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-07-29T23:16:34.487" LastActivityDate="2013-07-29T23:16:34.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="837" PostTypeId="4" CreationDate="2013-07-29T23:16:34.487" Score="0" Body="Questions regarding effects of weather on space exploration and how it can be forecasted and managed on Earth and on other bodies with atmospheres." OwnerUserId="25" LastEditorUserId="12102" LastEditDate="2021-02-20T00:19:40.733" LastActivityDate="2021-02-20T00:19:40.733" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="838" PostTypeId="2" ParentId="834" CreationDate="2013-07-29T23:22:34.753" Score="21" Body="&lt;p&gt;The fastest way to leave the Solar System is to pass by as many of the Gas Giants as you can, and use their gravity to slingshot you faster. As there are no planets outside of the ecliptic, it would not be advantageous to avoid the ecliptic plane. This holds true until we start to get really fast space probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, passing by the planets would give you a chance to abort, by slingshoting around them to return to Earth, in the event of a quick catastrophic failure (Entirely possible). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to prove this point, New Horizon's left Earth as the fastest spacecraft to ever leave Earth Orbit, reaching the orbit of the Moon in only 9 hours. Yet it will never catch up to the Voyager probes, because they used the gravity of both Jupiter and Saturn to speed up. I can't put it any better than &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons#Launch&quot;&gt;Wikipedia&lt;/a&gt;, so here goes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;New Horizons is often given the title of Fastest Spacecraft Ever&#xA;  Launched, although the Helios probes are arguably the holders of that&#xA;  title as a result of speed gained while falling toward the Sun. New&#xA;  Horizons, however, achieved the highest launch velocity and thus left&#xA;  Earth faster than any other spacecraft to date. It is also the first&#xA;  spacecraft launched directly into a solar escape trajectory, which&#xA;  requires an approximate velocity of 16.5 km/s (59,000 km/h; 37,000&#xA;  mph), plus losses, all to be provided by the launcher. However, it&#xA;  will not be the fastest spacecraft to leave the Solar System. This&#xA;  record is held by Voyager 1, currently traveling at 17.145 km/s&#xA;  (61,720 km/h; 38,350 mph) relative to the Sun. Voyager 1 attained&#xA;  greater hyperbolic excess velocity from Jupiter and Saturn&#xA;  gravitational slingshots than New Horizons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Furthermore, the amount of objects in the ecliptic plane is vastly overstated. Space is really really big, and we have to plan very carefully to get a spacecraft to another planet purposely. Even a small miss will cause issues. There isn't much there, and there is much to gain by going through the ecliptic plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The day that we have spacecraft capable of very high thrust for extended periods of time, we likely won't need this shortcut, but for now, it's an invaluable tool.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-07-29T23:33:34.433" LastActivityDate="2013-07-29T23:33:34.433" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="839" PostTypeId="2" ParentId="834" CreationDate="2013-07-29T23:29:54.070" Score="1" Body="&lt;p&gt;&lt;em&gt;Assuming you have sufficient energy for a constant 1g thrust&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On an interstellar voyage, the ecliptic plane has little to no meaning. If your direction of travel is generally at the right angle (90°) to the plane, then there is little point in scooting around all the obstacles in the Solar system. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-07-29T23:42:33.750" LastActivityDate="2013-07-29T23:42:33.750" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="840" PostTypeId="1" AcceptedAnswerId="5016" CreationDate="2013-07-29T23:51:26.023" Score="66" ViewCount="171376" Body="&lt;p&gt;If you have the energy for a constant 1G thrust, how long would it take to get to the planets in our solar system?  How long for the 5 nearest solar systems?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming turn over and decelerate at halfway.&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2015-01-12T16:54:50.000" Title="How fast will 1g get you there?" Tags="|propulsion|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="841" PostTypeId="2" ParentId="840" CreationDate="2013-07-30T02:04:55.727" Score="52" Body="&lt;p&gt;Not assuming any time taken for orbital maneuvering, turning halfway 180° to decelerate, assuming closest distance of planets (and Luna) to the Earth, and not accounting for fuel burn (i.e. literal constant 1g acceleration):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;The Moon / Luna&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth (&lt;a href=&quot;https://en.wikipedia.org/wiki/Supermoon&quot;&gt;Supermoon&lt;/a&gt;): 356,577 km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 2h 22m 12s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 3h 20m 24s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Mercury&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth: 77.3 million km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 1d 10h 52m 48s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 2d 1h 19m 12s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Venus&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth: 40 million km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 1d 1h 5m 2s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 1d 11h 28m 48s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Mars&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth: 65 million km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 1d 7h 58m 5s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 1d 21h 13m 1s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Jupiter&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth: 588 million km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 4d 0h 11m 2s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 5d 16h 2m 2s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Saturn&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth: 1.2 billion km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 5d 17h 25m 1s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 8d 2h 20m 24s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Uranus&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth: 2.57 billion km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 8d 9h 6m 0s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 11d 20h 24m 0s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Neptune&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth: 4.3 billion km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 10d 20h 7m 48s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 15d 7h 52m 48s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Pluto&lt;/strong&gt;:&lt;br /&gt;&#xA;Closest to Earth: 4.28 billion km&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, no deceleration): 10d 19h 31m 12s&lt;br /&gt;&#xA;Travel time (at 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, decelerating halfway): 15d 7h 1m 12s&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-07-22T09:16:34.427" LastActivityDate="2014-07-22T09:16:34.427" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="842" PostTypeId="2" ParentId="820" CreationDate="2013-07-30T02:11:48.710" Score="7" Body="&lt;p&gt;If we accept bibical references, it's arguable the Tower of Babel tried to reach space, given that it was an attempt to build a city with a tower &quot;whose top may reach unto heaven; and let us make us a name, lest we be scattered abroad upon the face of the whole earth.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/r8uox.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(from &lt;a href=&quot;http://en.wikipedia.org/wiki/File:Confusion_of_Tongues.png&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Book of Jubilees (not biblical) mentions the tower's height as being 5,433 cubits and 2 palms, or 2,484 m (8,150 ft), about three times the height of Burj Khalifa, or roughly 1.6 miles high. &lt;/p&gt;&#xA;" OwnerUserId="328" LastActivityDate="2013-07-30T02:11:48.710" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="844" PostTypeId="1" AcceptedAnswerId="854" CreationDate="2013-07-30T09:01:30.397" Score="13" ViewCount="2258" Body="&lt;p&gt;Most launches that I've seen have occurred during the day. But I've also seen a couple of missions that have been launched at night. Are there any specific advantages or disadvantages to a day launch versus a night launch? Or is it completely irrelevant?&lt;/p&gt;&#xA;" OwnerUserId="80" LastActivityDate="2019-06-20T02:13:33.110" Title="Day versus night launches" Tags="|launch|time|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="845" PostTypeId="1" AcceptedAnswerId="852" CreationDate="2013-07-30T10:48:12.120" Score="14" ViewCount="1538" Body="&lt;p&gt;I have looked around a bit, Wikipedia has an article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_toilet&quot;&gt;space toilets&lt;/a&gt; and one on &lt;a href=&quot;http://en.wikipedia.org/wiki/Toilet_paper&quot;&gt;toilet paper&lt;/a&gt;, but the two don't seem to be used together.  According to &lt;a href=&quot;http://www.worldwatch.org/node/5142&quot;&gt;one reference&lt;/a&gt;, annual consumption of toilet tissue in North America is 23.0 kg per capita.  Recycling air and fluids, even solids in a farm, are all pretty well accepted concepts, but what about toilet paper?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do you meet this essential need in space? How could you meet it on a long journey or colonization? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="58" LastEditDate="2013-10-17T16:32:36.240" LastActivityDate="2021-03-06T06:02:28.340" Title="Without renewable resources, how do you wipe in space?" Tags="|colonization|astronaut-lifestyle|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="846" PostTypeId="1" AcceptedAnswerId="848" CreationDate="2013-07-30T11:23:50.413" Score="15" ViewCount="796" Body="&lt;p&gt;NASA have already sent several surface rover vehicles to the surface of planets, mostly to Mars. Just think about Curiosity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compared to the fact that these rovers are remotely controlled and have strongly limited energy resources (not so much they could bring with them, and have &quot;only&quot; solar panels to collect some), they can travel quite big distances. I heard that one of these Mars rovers (don't know which one?) could travel up to 20 kilometres on the surface of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, is this enough to make general assumptions about the whole planet? I mean, even on Earth it matters, where those 20 kilometres would be; be it in the Amazonas, in the Himalayas, or in the New York city, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, prior to organizing a landing project like the Curiosity rover, NASA have monitored the surface of Mars with additional devices — but is it accurate enough, coupled with data gathered from surface rovers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I wasn't clear, here's an example: Curiosity measures a temperature inside a crater, but this value may be different inside other craters and valleys.&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="58" LastEditDate="2013-12-17T16:43:08.473" LastActivityDate="2013-12-17T16:43:08.473" Title="Are data we get from surface rovers accurate?" Tags="|mars|rovers|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="847" PostTypeId="2" ParentId="634" CreationDate="2013-07-30T11:35:52.010" Score="1" Body="&lt;p&gt;A global terraforming would cost incredible amount of energy, work and money, but I think it's possible by starting with some giant domes that acts like green houses for the vegetation and then connecting them with additional green house tunnels and then, slowly expanding them. It terraforms the surface, but without the defense of domes, it would be useless, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding enormous amount of oxygen, nitrogen, ozone, carbon-dioxide and other components would help making pressure and composition of air to non-lethal - vegetation would help making this method easier by releasing oxygen they create. Then, the domes may be removed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's just my theory, and might be totally useless, so feel free to vote down. However, I think it's a possible way of terraforming. &lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="19946" LastEditDate="2017-06-23T02:05:22.517" LastActivityDate="2017-06-23T02:05:22.517" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="848" PostTypeId="2" ParentId="846" CreationDate="2013-07-30T11:42:06.123" Score="14" Body="&lt;p&gt;The general philosophy that NASA and others has taken with landing missions is that they are used to validate something strange seen from on orbit, or to answer questions that can't be observed from on orbit, plus safety considerations in their landing spot. For example, here's the reasons that some of the landers landed where they did:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://mars.jpl.nasa.gov/msl/mission/timeline/prelaunch/landingsiteselection/&quot;&gt;Curiosity&lt;/a&gt;&lt;/strong&gt; - Clay was observed from on orbit, plus interesting layers observed.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://planetary.chem.tufts.edu/Phoenix/landing_site_selection.htm&quot;&gt;Phoenix&lt;/a&gt;&lt;/strong&gt; - Selected because of ice observed just below the surface.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://www.mars.asu.edu/christensen/docs/golombek_mersites_jgr.pdf&quot;&gt;Opportunity&lt;/a&gt;&lt;/strong&gt; - Observation of Hemetite.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://www.mars.asu.edu/christensen/docs/golombek_mersites_jgr.pdf&quot;&gt;Spirit&lt;/a&gt;&lt;/strong&gt; -  Signs of erosion, questions relating to volcanic activity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These findings can validate the orbit data, and help us to better understand the orbital data. However, if we send a thousand landers, we can never completely be convinced that there was never life on Mars, for instance. All we can do is say things about the locations we visited. But that can help us better understand Mars as a whole, particularly when combined with on orbit data.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-07-30T11:42:06.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="849" PostTypeId="1" AcceptedAnswerId="853" CreationDate="2013-07-30T12:20:56.323" Score="8" ViewCount="26555" Body="&lt;p&gt;We already know, we are even taught at school that planets of the Solar System beyond Mars (Jupiter, Saturn, Uranus and Neptune) are &quot;gas-giants&quot;. But what does it mean exactly? Also, can any kind of ground vehicles land on its surface?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:08:31.050" LastActivityDate="2013-10-26T15:08:31.050" Title="Do gas planets have surfaces suitable for landing on it?" Tags="|landing|gas-giant|" AnswerCount="2" CommentCount="10" ClosedDate="2013-07-31T15:17:12.337" ContentLicense="CC BY-SA 3.0" />
  <row Id="850" PostTypeId="2" ParentId="849" CreationDate="2013-07-30T12:20:56.323" Score="2" Body="&lt;p&gt;No, they don't have. Cores of these gas giants are actually solid, but above them, there's a thick layer of gas, with thousands of kilometres of thickness. Obviously, its concentration is increasing as you approach the core, but it's not solid. Bodies are sinking into it, like into wateror quicksand. Except if they have something with smaller density than the gas (like hydroplan's &quot;foots&quot; with full of air)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of it, ground vehicles (like rovers) can't be placed on these planets. However, some kind of gliders would be effective in the atmosphere of these planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(This is my first attempt to answer my own question, so I may be wrong. Please fix it if I am.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="306" LastEditDate="2013-07-31T11:00:55.273" LastActivityDate="2013-07-31T11:00:55.273" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="851" PostTypeId="2" ParentId="844" CreationDate="2013-07-30T13:54:40.947" Score="9" Body="&lt;p&gt;Assuming you are talking about a 'classic' launch, one with rockets pushing something into orbit, no - &lt;strong&gt;it's completely irrelevant&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Darkness? Bah. Space programs have enough lights to send darkness cowering away in a corner, just like football games. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately, it's all about ballistics (the physics of getting from point A to point B). If the space station you need to dock with is at the right place at night, so be it. No problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some &lt;em&gt;weather&lt;/em&gt; concerns, though - such as icing (the cold kind) on some spacecraft, high winds, thunderstorms, 6-inch hail, etc. These can occur during the day, though, so &lt;strong&gt;the time of day that a launch occurs is completely irrelevant.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="23891" LastEditDate="2019-06-19T21:11:57.717" LastActivityDate="2019-06-19T21:11:57.717" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="852" PostTypeId="2" ParentId="845" CreationDate="2013-07-30T14:27:43.007" Score="13" Body="&lt;h1&gt;Current Practice&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Right now, toilet paper is used &lt;a href=&quot;http://www.space.com/6444-space-toilet-road-test.html&quot; rel=&quot;noreferrer&quot;&gt;(as well as wet wipes)&lt;/a&gt;. It is disposed of in air-tight bags, according to &lt;a href=&quot;http://blogs.smithsonianmag.com/aroundthemall/2013/03/how-do-astronauts-go-to-the-bathroom-in-space/&quot; rel=&quot;noreferrer&quot;&gt;Smithsonian.com&lt;/a&gt;. From the &lt;a href=&quot;http://www.spaceref.com/iss/ops/sm.life.support.book1.pdf&quot; rel=&quot;noreferrer&quot;&gt;reference&lt;/a&gt; provided by Deer Hunter, &quot;toilet wipes are used to wipe not only the person but the receptacle and the seat.&quot; It also mentions that the air-tight bags are discarded in a trash container.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Future Missions&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;As far as future missions such as long journeys or colonizations, I can't find anything addressing toilet paper use, though it isn't hard to imagine the toilets of future spaceships going completely paperless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The University of Arizona paper &lt;a href=&quot;http://mozart.as.arizona.edu/~apai/Daniel_Apai/ASTR475/ASTR575FinalPaper.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Mars Settler: A Manned Mission to Explore Mars&quot;&lt;/a&gt; (Sheehan, Seo, and Bilinski) discusses the recycling of toilet paper in future manned Mars missions, which implies that they forsee the use of toilet paper.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-01-13T19:53:21.643" LastActivityDate="2014-01-13T19:53:21.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="853" PostTypeId="2" ParentId="849" CreationDate="2013-07-30T15:14:26.867" Score="11" Body="&lt;p&gt;Interesting question,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;to answer the question, gas planets do probably have a solid core, because the pressure is so enormous that the atoms connect into a crystal-like structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know that water for example can have more than 3 material states &lt;a href=&quot;http://en.wikipedia.org/wiki/Ice_X#Phases&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is even possible/hypothesized that you get an metal if you compress a gas highly enough. This can explain the extreme magnetic fields of Jupiter &lt;a href=&quot;http://en.wikipedia.org/wiki/Metallic_hydrogen&quot;&gt;metallic hydrogen&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you can't land on this solid core because the preasure is so enourmous.&lt;/p&gt;&#xA;" OwnerUserId="291" LastEditorUserId="382" LastEditDate="2013-07-31T20:46:16.433" LastActivityDate="2013-07-31T20:46:16.433" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="854" PostTypeId="2" ParentId="844" CreationDate="2013-07-30T15:21:31.663" Score="8" Body="&lt;p&gt;For a launch into orbit, it is mostly irrelevant, as Undo's answer correctly states.  One aspect that has not been mentioned that might favour a night-time launch is to clear airspace.  Perhaps some spaceports are located in areas that have some flight movements during the day, but none during the night; that would favour a night-time launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For sub-orbital launches, it all depends on the payload.  For example, the sounding rocket &lt;a href=&quot;http://en.wikipedia.org/wiki/Maxus_%28rocket%29&quot; rel=&quot;nofollow&quot;&gt;Maxus&lt;/a&gt; reaches up to 700 km; higher than many LEO satellites, but it return to Earth immediately after.  Many payloads are mostly concerned by the ~14 minutes of microgravity it provides, but for payloads that do atmospheric measurements of any kind, time of day can be relevant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, sounding rockets are the only possible platform for in-situ measurements in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Mesosphere&quot; rel=&quot;nofollow&quot;&gt;mesosphere&lt;/a&gt; (50–100 km up), which is too high for balloons, and too low for satellites.  Research into &lt;a href=&quot;http://en.wikipedia.org/wiki/Noctilucent_cloud&quot; rel=&quot;nofollow&quot;&gt;noctilucent clouds&lt;/a&gt; might want to have coincident surface-based observations, which requires a night-time launch.  Other atmospheric phenomena might also experience a marked diurnal cycle.  So, in this case, it certainly matters.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="25" LastEditDate="2015-12-18T16:43:44.713" LastActivityDate="2015-12-18T16:43:44.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="855" PostTypeId="2" ParentId="835" CreationDate="2013-07-30T15:24:09.193" Score="4" Body="&lt;p&gt;A recent analysis of Voyager 2's flybys of Uranus and Neptune found that the weather layers on both planets are at most 1000km thick.  This was done by modeling how density variations in the atmosphere would affect the spacecrafts trajectory by minutely varying the planets gravitational field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.skyandtelescope.com/news/207909201.html&quot; rel=&quot;nofollow&quot;&gt;http://www.skyandtelescope.com/news/207909201.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="157" LastActivityDate="2013-07-30T15:24:09.193" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="856" PostTypeId="2" ParentId="304" CreationDate="2013-07-30T15:31:18.097" Score="8" Body="&lt;p&gt;The Juno probe will be doing some of this for Jupiter's atmosphere.  Due to the extreme densities involved it will be doing so from orbit; not by entering into it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Scientific_objectives&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Scientific objectives Image of Jupiter aurora in UV by the Hubble&#xA;  Space Telescope. Bright streaks and dots are caused by magnetic flux&#xA;  tubes connecting Jupiter to its largest moons (Io: bright streak on&#xA;  the far left; Ganymede: bright dot below center; Europa: dot on the&#xA;  right).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Juno spacecraft's suite of science instruments will:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Determine the ratio of oxygen to hydrogen, effectively measuring the abundance of water in Jupiter, which will help distinguish among prevailing theories linking the gas giant's formation to the Solar System.&lt;/li&gt;&#xA;  &lt;li&gt;Obtain a better estimate of Jupiter's core mass, which will also help distinguish among prevailing theories linking the gas giant's formation to the Solar System.&lt;/li&gt;&#xA;  &lt;li&gt;Precisely map Jupiter's gravity to assess the distribution of mass in Jupiter's interior, including properties of the planet's structure and dynamics.&lt;/li&gt;&#xA;  &lt;li&gt;Precisely map Jupiter's magnetic field to assess the origin and structure of the field and how deep in Jupiter the magnetic field is created. This experiment also will help scientists understand the fundamental physics of dynamo theory.&lt;/li&gt;&#xA;  &lt;li&gt;Map the variation in atmospheric composition, temperature, structure, cloud opacity and dynamics to pressures far greater than 100 bars (10 MPa; 1450 pound/sq inch) at all latitudes.&lt;/li&gt;&#xA;  &lt;li&gt;Characterize and explore the three-dimensional structure of Jupiter's polar magnetosphere and its auroras.[17]&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/835/do-we-have-instruments-that-can-measure-the-depth-of-storms-like-the-great-red-s/855#855&quot;&gt;A recent analysis&lt;/a&gt; of data from the Voyager 2 flyby's of Uranus and Neptune found their weather layers were at most 1000km thick.  I don't know if a followup to repeat the analysis for Jupiter and Saturn is planned or not.&lt;/p&gt;&#xA;" OwnerUserId="157" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-07-30T15:31:18.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="857" PostTypeId="1" AcceptedAnswerId="1099" CreationDate="2013-07-30T16:54:16.717" Score="11" ViewCount="1373" Body="&lt;p&gt;After reading and answering James Jenkins's interesting &lt;a href=&quot;https://space.stackexchange.com/questions/845/without-renewable-resources-how-do-you-wipe-in-space&quot;&gt;question on wiping in space&lt;/a&gt;, I was curious about the usage of solid waste on a manned space vessel. In his question he mentions recycling solids in a farm, and during my research to answer his question I discovered that one paper discussed the possible use of dried human waste as a radiation shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not really concerned with what is done with waste presently, but if there are any current recycling methods for human waste (excluding urine) they may certainly be addressed. Here's what I'm wondering: what is the most optimal method of recycling human solid wastes for sustainibility on a manned space vessel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll set the maximum limit on future development to be the rearrangement of molecules. Once we have technology on that scale, the reuse question is not as big of a concern (given that the energy requirements are low enough). Any future technology short of that, however, can be considered in the answer.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2018-02-02T21:11:41.213" LastActivityDate="2018-02-02T21:11:41.213" Title="What is the most sustainable method of recycling (human) solid waste on a manned space vessel?" Tags="|crewed-spaceflight|life-support|waste|sustainability|" AnswerCount="2" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="858" PostTypeId="2" ParentId="301" CreationDate="2013-07-30T18:24:01.387" Score="11" Body="&lt;p&gt;Many life support and mission design questions have answers depending on scale: how large is the colony?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far viable solutions discussed in Mars mission papers revolve around two major sources of energy:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;solar (photoelectric panels ) and&lt;/li&gt;&#xA;&lt;li&gt;nuclear (meaning conventional fission reactors on Uranium-235, most likely highly enriched)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Until now, we have faced several challenges as far as photovoltaics are concerned:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;dust accumulation&lt;/li&gt;&#xA;&lt;li&gt;the need to rotate the panels to track the Sun and corresponding unreliability of any mechanisms with bearings in Martian environment (even with human maintenance)&lt;/li&gt;&#xA;&lt;li&gt;PV decay with time&lt;/li&gt;&#xA;&lt;li&gt;seasonal variations of PV output&lt;/li&gt;&#xA;&lt;li&gt;the need to store energy for consumption at night and the refreshing idea of having to add extra mass to the deliverable goods and to watch out for any problems (including thermal ones) with the batteries (although flywheels will become more efficient as the colony scales)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Nuclear reactors with Brayton cycle turbines do not obviate the need for energy storage, but they have problems of their own:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;cooling requires large radiators (since they are vertical, no need to worry about the dust)&lt;/li&gt;&#xA;&lt;li&gt;siting (behind berms and in craters) due to the need to shield the colony from neutron flux&lt;/li&gt;&#xA;&lt;li&gt;the choice between refuelling (for large and long-term colonies) and replacement&lt;/li&gt;&#xA;&lt;li&gt;nuclear waste (and reactor) disposal&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, there are two other possible solutions for sufficiently large colonies:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;solar thermal power plants&lt;/li&gt;&#xA;&lt;li&gt;orbital solar PV power plants which send the energy through microwaves (thus, one doesn't need to provide aeroshields and all the other dead mass needed for landing the plant) - a rather scalable solution, I'd gather.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To re-cap:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a colony/habitat will need power &lt;strong&gt;generation&lt;/strong&gt; and &lt;strong&gt;storage&lt;/strong&gt;, no matter what&lt;/li&gt;&#xA;&lt;li&gt;reliability means diversity: for sufficiently large habitats, one would like to have both a reactor and a solar panel field, flywheels and batteries&lt;/li&gt;&#xA;&lt;li&gt;resupplying the colony is a pain due to the need to pinpoint land the cargo, so any energy solutions that don't require landing are really welcome&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In my humble opinion, having a nuclear reactor as the main source of energy, flywheels for energy storage, and panels and batteries for backup would provide a conservative starting point for future designs. However, calculations are needed to refine the designs and choose between competing solutions.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2013-07-30T18:24:01.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="859" PostTypeId="2" ParentId="857" CreationDate="2013-07-30T21:59:39.737" Score="4" Body="&lt;p&gt;Assuming we as race we colonize beyond our planet, with sustainable methods in our biological needs. Large quantities of organic material will be required, regardless if you are farming &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydroponics&quot; rel=&quot;nofollow&quot;&gt;hydroponically&lt;/a&gt; or in soil, organic components are required to grow plants. &lt;a href=&quot;https://en.wikipedia.org/wiki/Soil&quot; rel=&quot;nofollow&quot;&gt;Soil&lt;/a&gt; is a combination of sand (&lt;em&gt;minerals&lt;/em&gt;) and organics. The only known source of organics in the universe are byproducts of earth life. As there is a significant cost in raising these organics to orbit, it makes financial sense to husband those organics we do get to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Compost&quot; rel=&quot;nofollow&quot;&gt;Composting&lt;/a&gt; is a time honored, simple, first step in the recycling of organic products like &lt;a href=&quot;https://en.wikipedia.org/wiki/Night_soil&quot; rel=&quot;nofollow&quot;&gt;Night soils&lt;/a&gt;.  It does not require significant investments in technology, infrastructure or human resources. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current space habits are uniquely suited to support composting of night soils, as separation of solids and liquids is mutually desired.  There many articles available on the subject, as &lt;a href=&quot;http://www.motherearthnews.com/nature-and-environment/composting-human-waste-zmaz73sozraw.aspx#axzz2aZ3g2FOz&quot; rel=&quot;nofollow&quot;&gt;this one&lt;/a&gt; points out, layering the dry night soils with materials that are high in cellulose or carbon is good first step.  Generally composting principals of turning the compost (&lt;em&gt;speeds breakdown&lt;/em&gt;), and encouraging a warm moderately dry (&lt;em&gt;but moist&lt;/em&gt;) environment are all that is required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The composted organic materials are then mixed with water (Hydroponics) or sand (soil), and used to grow plants. As with any farming operation, plants that are grown and eaten raw should be properly washed before consumption. Processing methods appropriate for destroys pathogens are used in other applications. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It can be expected that early space farm produce would be fresh vegetables for human/animal consumption and fiber rich plants that can be made into paper (toilet paper!), cloth, and for feeding &lt;a href=&quot;http://en.wikipedia.org/wiki/Ruminant&quot; rel=&quot;nofollow&quot;&gt;Ruminants&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course everything goes back into the composting cycle.  Note that while composting animal protein (meat) is possible it requires a level of skill beyond this simple method. &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-07-30T21:59:39.737" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="860" PostTypeId="2" ParentId="280" CreationDate="2013-07-30T22:16:01.743" Score="7" Body="&lt;p&gt;Aerobots occupy an intermediate position between landers and orbiters. Turns out they cannot do well some things that these two are capable of.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;No ability to do geological studies. A big objection if you remember the main drive in planetary exploration today - the search for life, present or extant, or at least habitable conditions. You simply cannot have a second look at this piece of what might be clay over there. You can't grab it and put into SAM.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;No predictability of a polar orbiter mission: you cannot plan a revisit cycle to spot changes in successive images (one of the reasons people are extremely enthusiastic about HiRISE). Neither can you set up regular relay links if you've got a rover or two down there.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Yes, there are many things you can do with aerobots, but most of them are about weather and the atmosphere. It would be fantastic to combine wide-area exploration (aerosols, magnetic field, chemistry analysis, vertical atmospheric profiles) by aerobots with landers and orbiters, but...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...&lt;strong&gt;money matters&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, missions are de-scoped all the time in favor of modes with greater potential for results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, planetary destinations are juggled around so that bodies where aerobots would be ideal are moved to the back of the queue. Why? Because &lt;strong&gt;in the present conditions Mars is our best bet to find (possibly extant) life&lt;/strong&gt;. Look at Discovery where Titan Mare Explorer failed to score - and TiME was much more promising in terms of science payload than a venusian aerobot.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2013-07-30T22:16:01.743" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="862" PostTypeId="2" ParentId="797" CreationDate="2013-07-31T02:46:00.157" Score="20" Body="&lt;p&gt;In space (within the Solar system), you will get mostly two types of &quot;radiation&quot; that have health consequences:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Photons of various energy, from long wave radio to gamma rays.&lt;/li&gt;&#xA;&lt;li&gt;High-energy charged particles, mostly electrons and protons ejected from the Sun upper atmosphere (this is known as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_wind&quot;&gt;solar wind&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Main source for these is, of course, the Sun. Photons, being electrically neutral, totally laugh at magnetic fields; a &quot;magnetic barrier&quot; will work only for charged particles. We know what UV can do to human skin despite an atmosphere, so one can imagine that some extra shielding will be needed in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming that you have &lt;a href=&quot;http://en.wikipedia.org/wiki/Superconductivity&quot;&gt;superconductors&lt;/a&gt;, you can maintain a powerful magnetic field for indefinite times, with energy being consumed only when a particle is indeed deflected. The shape and position of this field requires some care, though. For instance, the Earth magnetic field is not very good at protecting the Earth from solar wind; instead, it just moves around the impact point: high energy particles concentrate on polar regions, producing beautiful &lt;a href=&quot;http://en.wikipedia.org/wiki/Aurora_%28astronomy%29&quot;&gt;auroras&lt;/a&gt;. An awful lot of research on the subject of optimal magnetic shielding for spaceships is referenced from &lt;a href=&quot;http://engineering.dartmouth.edu/~d76205x/research/shielding/&quot;&gt;this page&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An aggravating circumstance of radiations in space is that it does not occur with a continuous flow; instead, it comes in bursts of considerable intensity, when &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_flare&quot;&gt;solar flares&lt;/a&gt; occur. A good space shielding will be utter overkill most of the time, but will occasionally become an absolute necessity to avoid the crew being, well, killed over. A mitigating characteristic, though, is that the source position is well known (the Sun tends to be highly visible) and flares can be observed &quot;visually&quot; some hours before the onslaught of high-energy particles, giving time to raise extra shields.&lt;/p&gt;&#xA;&#xA;&lt;hr /&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Outside of the solar system&lt;/strong&gt;, things change quite a bit. The solar wind actually creates a kind of &quot;bubble&quot; around the Sun, called the &lt;a href=&quot;http://en.wikipedia.org/wiki/Heliosphere&quot;&gt;heliosphere&lt;/a&gt;, which acts a bit like a magnetic shield against the rest of the Universe. On the border of the heliosphere is a rather confused situation about which much is theorized but little is known; the Voyager 1 probe is &lt;a href=&quot;http://uk.news.yahoo.com/nasas-voyager-1-approaches-outer-limit-solar-system-223553489.html#Ps87M72&quot;&gt;currently moving through it&lt;/a&gt;. Beyond, there is not much to fear about solar wind, but a lot more about other high energy particles of many types, collectively known as &lt;a href=&quot;http://en.wikipedia.org/wiki/Cosmic_ray&quot;&gt;cosmic rays&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We don't really know where cosmic rays come from, but the sources appear to be multiple. For our present discussion, this means that cosmic rays don't come from a unique predictable direction, and happen at seemingly random times, so shields of any kind must be up at all times. Moreover, not all of these particles are charged, so magnetic shields won't be enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that cosmic rays are also a problem within the solar system, even close to the Earth, but leaving the heliosphere increases the issue dramatically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An extra hazard is beautifully exposed in Arthur C. Clarke's &lt;a href=&quot;http://www.amazon.ca/Songs-Distant-Earth-Arthur-Clarke/dp/0345322401&quot;&gt;&quot;The Songs of Distant Earth&quot;&lt;/a&gt;. If you are outside of the heliosphere, then you are traveling to the stars -- so you must be traveling &lt;em&gt;fast&lt;/em&gt;, because stars are far, far away. This implies that low-energy particles or bigger fragments (e.g. stray atoms or molecules from &lt;a href=&quot;http://en.wikipedia.org/wiki/Nebula&quot;&gt;nebulae&lt;/a&gt;) will have a high &lt;em&gt;relative&lt;/em&gt; speed, and the repeated impacts will be damaging for the ship and its inhabitants. In the book, they add a big layer of ice in front of the ship, and must renew it regularly.&lt;/p&gt;&#xA;&#xA;&lt;hr /&gt;&#xA;&#xA;&lt;p&gt;As for &lt;strong&gt;materials&lt;/strong&gt; for more tangible shields (which will protect from neutral particles as well), a good candidate is not lead, but &lt;em&gt;water&lt;/em&gt;. Water has a very good ratio of absorbing power per weight; also, water has other uses that lead does not offer, such as bathing, watering plants, raising fishes (&lt;a href=&quot;http://en.wikipedia.org/wiki/Tilapia&quot;&gt;tilapias&lt;/a&gt; offer a lot of protein while requiring only limited amounts of swimming space), and, come what may, even drinking, should the onboard stocks of decent beverages become depleted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A popular design is a space ship as a big tumbling cylinder, creating &quot;artificial gravity&quot;. The &quot;ground&quot; (the cylinder surface, from the inside) can be a big pool, and habitats would then be floating, like fish farms. The water maintains the inner ecosystem &lt;em&gt;and&lt;/em&gt; provides excellent radiation shielding at the same time. Astronauts double as sailors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other possible materials include various polymers, gold (used for &lt;a href=&quot;http://info.goldavenue.com/Info_site/in_glos/in_glos_foil.html&quot;&gt;lunar modules on Apollo missions&lt;/a&gt; -- when you go to the Moon, you do it &lt;em&gt;with style&lt;/em&gt;), and even &quot;biological waste&quot; from the crew. This whole radiation issue is still one of the unsolved problems for the trip to Mars, so that's an active research area.&lt;/p&gt;&#xA;" OwnerUserId="311" LastActivityDate="2013-07-31T02:46:00.157" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="864" PostTypeId="1" AcceptedAnswerId="875" CreationDate="2013-07-31T07:16:14.577" Score="21" ViewCount="25373" Body="&lt;p&gt;What would happen, if the space suit of a spacewalking crew was punctured during the extravehicular activity (EVA)? What are the chances of this happening? Has this already happened to anyone in the history of space exploration?&lt;/p&gt;&#xA;" OwnerUserId="73" LastEditorUserId="14" LastEditDate="2013-07-31T16:04:24.180" LastActivityDate="2018-12-09T05:28:25.010" Title="Puncturing space suit during EVA. What would happen?" Tags="|health|spacesuits|life-support|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="866" PostTypeId="1" AcceptedAnswerId="876" CreationDate="2013-07-31T10:19:39.283" Score="7" ViewCount="1588" Body="&lt;p&gt;In my mind, I picture a &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;noreferrer&quot;&gt;gravity assist&lt;/a&gt; sling shot boost like a ball on the end of string being swung until the centrifugal force grows strong and then is released.  Where gravity is the string, the planet is the swinger, and ship is the ball.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your ship is under acceleration when performing a gravity assist, would there be centrifugal force applied?  Or asked another way, what would be the perceived gravitational increase to the occupants/cargo of ship using gravity slingshot while under thrust?&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-07-31T14:01:13.033" Title="Does a gravity assist increase the g force on the ship?" Tags="|propulsion|gravity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="867" PostTypeId="1" AcceptedAnswerId="895" CreationDate="2013-07-31T10:45:09.250" Score="8" ViewCount="1727" Body="&lt;p&gt;I understand the basic concept inasmuch that the gyroscopic effect is used and that the rigid body is given an initial spin around an axis of maximum mass moment of inertia (MMOI). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, can someone clarify this concept, perhaps with some examples?&lt;/p&gt;&#xA;" OwnerUserId="357" LastEditorUserId="357" LastEditDate="2013-07-31T14:49:16.467" LastActivityDate="2013-07-31T22:33:51.617" Title="How is spin stability used in spacecraft?" Tags="|attitude|spacecraft|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="868" PostTypeId="1" AcceptedAnswerId="872" CreationDate="2013-07-31T12:12:56.143" Score="46" ViewCount="9515" Body="&lt;p&gt;There's this interesting story: &lt;a href=&quot;http://www.enterprisemission.com/NukingJupiter.html&quot; rel=&quot;nofollow&quot;&gt;Did NASA Accidentally “Nuke” Jupiter?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long story short:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On September 21, 2003 Galileo is deorbited in order to prevent eventuality of contamination of Europa. It descends into Jupiter atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;October 19, 2003 an unexplained &quot;blotch&quot; appears on surface of Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speculation is that Galileo gradually sunk into thicker layers of Jupiter body, and eventually reached level where pressure squeezed its RTG battery so much the plutonium went supercritical. The plasma bubble takes some time to expand and reach the surface, producing the spot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How likely is this scenario? Is this just a hoax, a baseless speculation, or is this a fact? Can someone confirm or debunk this story?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="4660" LastEditDate="2016-04-02T23:07:44.760" LastActivityDate="2016-04-02T23:07:44.760" Title="Did the Galileo spacecraft cause a &quot;blotch&quot; on Jupiter after it crashed?" Tags="|jupiter|nuclear|myth-busting|galileo|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="869" PostTypeId="1" AcceptedAnswerId="880" CreationDate="2013-07-31T12:17:39.543" Score="16" ViewCount="11873" Body="&lt;p&gt;Would certain types of flying be possible on planets like Venus, Titan (which is a moon, I know :P), or gas-giants? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm thinking on the aspects of these types of aircrafts: airships (that uses only enormous amount of air to float and fly), gliders (that uses only the wind to fly), planes with propellers, helicopters (they use similar ways to fly) and planes with jet engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, most of them wouldn't work on planet with no or very thin atmosphere like the Moon or Mercury, so I specified the question to those planets and moons which have dense and/or thick atmosphere. Also, by aircrafts I mean only those vehicles which are not able to leave the atmosphere - since those that can travel in space, must be able to travel within the atmosphere too. But vice versa, it's not true.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, gliding on Jupiter (or even on Venus) would be strongly possible, since the heavy storm in their atmosphere generate strong winds. But it obviously can't go to space, since there's no wind to throttle it in higher layers of the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorDisplayName="user29" LastEditDate="2013-08-01T16:07:42.880" LastActivityDate="2017-04-27T19:16:26.617" Title="Flying on dense atmosphere planets &amp; moons" Tags="|planet|atmosphere|aerobot|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="870" PostTypeId="2" ParentId="864" CreationDate="2013-07-31T12:39:12.197" Score="11" Body="&lt;p&gt;I depends on what caused the tear / puncture and how fast the depressurisation is due to it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, for example, the puncture was caused by a micrometeorite, the relatively fast depressurisation of the space suit might not be of the greatest concern for the unfortunate astronaut, as the micrometeorite could pass through the astronaut's body at speeds possibly an order of magnitude greater than a bullet exiting a shotgun can travel, causing anything from punctured through projectile wound, to instantly internally liquefying the poor astronaut's body in an unrecognisable mix of all of his/her internals, if the micrometeorite hits the space suit at extreme velocity relative to the movement of the EVA engaging astronaut and is big enough (where total energy released is $E_\text{k} =\tfrac{1}{2} (m_\text{in} - m_\text{out})(v_\text{in}-v_\text{out})^2$ where $m$ is mass and $v$ velocity of the penetrating object).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, assuming the tear / puncture was made by other (non-lethal on their own) causes, such as parts of the space suit unwittingly getting trapped between some equipment, brushing against a sharp object, or similar, then the problem becomes one of trying to temporarily re-pressurise the space suit's internal pressure with whatever life support oxygen and other gases in the breathable mix are left available, hopefully adding to the suit's internal pressure faster than the depressurisation is occurring and moving to the nearest airlock as soon as possible (as @ernestopheles mentions in his comment). If the puncture is within the reach of the astronaut (or one of his/her team), they might also try to apply pressure on the puncture manually, or by applying some adhesive patch in hope it stops or limits the speed of the depressurisation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Frequency of such micrometeorites is tough to judge, and depends on where the EVA is taking place. On planets with at least some atmospheric pressure, chances are they would burn up upon entry to the planet's atmosphere. Around Earth's orbit, there might be bigger risks involved also due to loads of space junk orbiting Earth, that we've put in space during previously aborted or retired missions, lost tools, e.t.c. that might be orbiting the Earth due to inertia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the other part of your question, if this already happened to anyone, I don't have any knowledge of such accident happening to anyone while actually in space, but it did happen while testing space suits and EVA equipment here on Earth, in relatively controlled environment of a vacuum chamber, which was luckily repressurized fast enough not to cause permanent health issues or even death to the test subject involved. See &lt;a href=&quot;https://space.stackexchange.com/a/126/49&quot;&gt;this answer&lt;/a&gt; for more information on that accident and what would happen, if your body is exposed to the vacuum of space.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-03-27T18:58:15.260" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="871" PostTypeId="2" ParentId="868" CreationDate="2013-07-31T13:16:25.970" Score="30" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Galileo_%28spacecraft%29#Electrical_power&quot;&gt;Galileo probe&lt;/a&gt; carried two radionucleid batteries, each one carrying 7.8kg of Plutonium-238. But Plutonium 238, while radioactive, is not the kind of Plutonium used for nuclear bombs. That's another isotope: Plutonium-239.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's a big difference, because only the kinds of Plutonium with odd isotope numbers are fissile. &lt;a href=&quot;http://en.wikipedia.org/wiki/Fissile&quot;&gt;Fissile&lt;/a&gt; means, that they are capable of sustaining a nuclear chain-reaction. Plutonium 238 is not fissile. That means that it is impossible for Plutonium-238 to undergo nuclear explosion.&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2013-07-31T13:16:25.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="872" PostTypeId="2" ParentId="868" CreationDate="2013-07-31T13:17:00.220" Score="48" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;The speculation is that Galileo gradually sunk into thicker layers of Jupiter body, and eventually reached level where pressure squeezed its RTG battery so much &lt;strong&gt;the plutonium went supercritical&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Galileo was powered by RTGs.  That is a heat source powered by the decay of Plutonium-238.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nuclear decay rate does not change.  It isn't affected by pressure, density, or &lt;em&gt;literally&lt;/em&gt; any chemical process, because the decay is a nuclear process.  Since the decay is constant, the heat production rate is constant.  That shouldn't cause anything like a bubble detectable from space, although admittedly, it could cause bubbling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nuclear criticality is a process of a fission chain reaction.  So the question becomes:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Does Pu-238 have a fission cross section sufficient to go critical?&lt;/li&gt;&#xA;&lt;li&gt;Are there any other radioisotopes that could go critical? (other Pu isotopes probably)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The answer to both of these is pretty handily &quot;no&quot;.  The only fissile Pu isotopes are Pu-239 and Pu-241.  While I don't doubt that someone might be able to detect some Pu-239 atoms in an RTG, its fission cross section would have to dominate over the other cross sections (neutron dead-ends) within the material.  Looking up the &lt;a href=&quot;http://ie.lbl.gov/ngdata/sig.htm&quot;&gt;data&lt;/a&gt;, the neutron absorption cross section of Pu-238 looks sufficiently large to shut down any reaction.  This mechanism would preclude criticality regardless of size.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-07-31T21:02:34.753" LastActivityDate="2013-07-31T21:02:34.753" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="873" PostTypeId="2" ParentId="797" CreationDate="2013-07-31T13:23:16.353" Score="2" Body="&lt;p&gt;Another item to consider with solar flares is that since there is the potential ability to &quot;see them coming&quot;, there is a good potential for having a significantly reduced amount of shielding. Specifically, the ship could have a sort of bomb shelter, but in this case a solar flare shelter.  A subsection of the ship large enough at least for the inhabitants could be shielded much more heavily than the rest of the ship, thereby reducing the costs associated with shielding (energy usage and $ in general).&lt;/p&gt;&#xA;" OwnerUserId="361" LastActivityDate="2013-07-31T13:23:16.353" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="874" PostTypeId="1" AcceptedAnswerId="878" CreationDate="2013-07-31T13:32:16.987" Score="8" ViewCount="510" Body="&lt;p&gt;Robert Truax is said to have advocated a &quot;space truck&quot;.  From &lt;a href=&quot;http://www.nytimes.com/2010/09/30/us/30truax.html&quot;&gt;the NYT&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;He did calculations showing that complexity, not size, drove costs. Hence, his proposed “space truck,” two football fields long but relatively simple in design. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I can't easily find anything online that tells what this idea was exactly.  The quote would seem to point toward fewer stages, but that's speculative.  Are there any references that give an idea of what he had in mind?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-08-08T13:03:56.987" Title="What is the basic idea of the Truax &quot;Space Truck&quot;?" Tags="|launch|spacecraft-development|reference-request|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="875" PostTypeId="2" ParentId="864" CreationDate="2013-07-31T14:00:50.327" Score="16" Body="&lt;p&gt;Chances: &lt;a href=&quot;http://en.wikipedia.org/wiki/Murphy%27s_law&quot; rel=&quot;noreferrer&quot;&gt;Murphy's law&lt;/a&gt; - bad things eventually happen. Reasons: Besides (micro-)meteorites, as mentioned by @TildalWave, the risk is with the astronauts themselves. &lt;a href=&quot;http://en.wikipedia.org/wiki/Extra-vehicular_activity&quot; rel=&quot;noreferrer&quot;&gt;EVAs&lt;/a&gt; are for repairing stuff, 'plumbing', handling tools and equipment etc. It is natural to break stuff while you work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of numbers, although there were plenty of EVAs in history, everybody survived them. And besides problems with heating and cooling, water at the wrong place inside the suit or just overpressure, there have never been severe problems yet. So it is hard to put a number on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space suits (just like manned space crafts) are designed to handle leaks or holes of a certain size for a certain amount of time. In a space craft, the idea is to give astronauts time to 'jump' into their pressure suits or another available safe space craft before they run out of air. In a suit, the idea is similar – provide enough time for a safe return to an air lock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are interested in details, have a look at the specifications of space suits for EVAs. I found an example for the &lt;a href=&quot;http://www.colorado.edu/ASEN/asen3036/Orlan.pdf&quot; rel=&quot;noreferrer&quot;&gt;Russian Orlan suit&lt;/a&gt;. The system is referred to as &quot;leak compensation system&quot; (section 4 | page 28).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that there is virtually &lt;em&gt;no absolutely airtight suit&lt;/em&gt;. There are just too many connections and o-ring-seals. All suits therefore leak permanently to some degree. If the amount of leaked air increases above 'normal', for whatever reason, a suit's life support system can increase the amount of air pumped into the suit at the expense of the duration of an EVA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Real life incidents of leaking or punctured suits are rare. As far as I know, there are no documented cases of suits loosing air rapidly in space. But keep in mind, every suit leaks, so there were in fact a few suits leaking more than they should. Partially punctured suits are also a known problem. A famous modern example happened on &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-118#Wednesday_15_August_.28Flight_day_8.29&quot; rel=&quot;noreferrer&quot;&gt;STS 118&lt;/a&gt;, where two of the five layers of material of the glove of an American suit were punctured. Another interesting incident, although initially unnoticed, happened on &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-37#Spacewalks_2&quot; rel=&quot;noreferrer&quot;&gt;STS 37&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="12408" LastEditDate="2016-07-03T16:05:08.063" LastActivityDate="2016-07-03T16:05:08.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="876" PostTypeId="2" ParentId="866" CreationDate="2013-07-31T14:01:13.033" Score="8" Body="&lt;p&gt;No, there would not be any perceptible acceleration of the occupant. This is because the occupant is accelerated as much as the craft. With a ball on a string, the force is communicated through the point of attachment of the string on the ball. With gravity the force is communicated directly to all the mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(If one were using something like a neutron star to provide a gravitational assist, it would be possible to experience gravity different than the center of gravity of the [rigid] ship due to the force of gravity varying by square of the distance. This tidal force is a major plot point in Larry Niven's short story &quot;Neutron Star&quot;. [If the ship was very large and still rigid, a less dense star could provide sufficient tidal force to be noticeable.])&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastActivityDate="2013-07-31T14:01:13.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="877" PostTypeId="2" ParentId="874" CreationDate="2013-07-31T14:25:23.977" Score="11" Body="&lt;p&gt;Truax was advocating &quot;design for minimum cost&quot;. As &lt;a href=&quot;http://www.dunnspace.com/leo_on_the_cheap.htm&quot;&gt;this paper here&lt;/a&gt; (starting on page 140) explains, this philosophy of design would probably include a number of changes to how spacecraft were designed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this sentence is the key:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The fundamental premise that the DFMC concept rests upon is that, by&#xA;  using a clean-sheet design approach, a space launch vehicle can be&#xA;  optimized for minimum cost, instead of being designed like current&#xA;  launch vehicles, which are optimized for maximum performance and&#xA;  minimum weight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From reading through the section titled &quot;Cultural Changes to Get a Space Truck&quot;, I think that the main point of the term &quot;space truck&quot; is to emphasize that it doesn't really matter how pretty or elegant the final design is, it only matters how much it costs to get the payload into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For non-engineers this sounds completely obvious, but it goes completely against the grain for many engineers. &quot;Elegant&quot; design (where the exact criteria for what is elegant varies from discipline to discipline) is considered particularly beautiful and is often striven for at the expense of other concerns. (Yes, I have been guilty of this) Mostly, this is a matter of culture (simple, robust design is rarely stressed, either in training or practice) and egotism (engineers like to brag about the beauty of their designs).&lt;/p&gt;&#xA;" OwnerUserId="113" LastEditorUserId="113" LastEditDate="2013-07-31T15:36:16.880" LastActivityDate="2013-07-31T15:36:16.880" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="878" PostTypeId="2" ParentId="874" CreationDate="2013-07-31T14:35:51.293" Score="8" Body="&lt;p&gt;There's a good reference to more specifics of the design in &lt;a href=&quot;http://books.google.com/books?id=g-4agkc8_IAC&amp;amp;lpg=PA42&amp;amp;ots=skVeuNI8_j&amp;amp;dq=robert%20truax%20%22space%20truck%22&amp;amp;pg=PA41#v=onepage&amp;amp;q=robert%20truax%20%22space%20truck%22&amp;amp;f=false&quot;&gt;&quot;Realizing Tomorrow: The Path to Private Spaceflight&quot;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, it's a huge 2 stage rocket constructed out of 8mm steel that wasn't expected to be as reliable as most rockets, but because of the simplicity in design and higher tolerances while being able to deliver much greater lift capacity would make it cheaper.  Further cost reductions would come from lower transportation costs to the launch site and relatively mass production.&lt;/p&gt;&#xA;" OwnerUserId="361" LastActivityDate="2013-07-31T14:35:51.293" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="879" PostTypeId="2" ParentId="566" CreationDate="2013-07-31T14:37:28.213" Score="11" Body="&lt;p&gt;The ISS does it. Source: &lt;a href=&quot;http://spacestationlive.jsc.nasa.gov/handbooks/adcoHandbook.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://spacestationlive.jsc.nasa.gov/handbooks/adcoHandbook.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Attitude Determination&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;How am I currently oriented?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The ISS also uses GPS to determine how the ISS is&#xA;  oriented, or facing, as it orbits the Earth. &lt;strong&gt;This&#xA;  orientation, or attitude, can be determined by&#xA;  measuring the difference in time that the GPS signals&#xA;  are received by four antennas&lt;/strong&gt;. These antennas&#xA;  receive the same GPS signal at slightly different times,&#xA;  with the signal traveling at a constant speed (the&#xA;  speed of light).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;To calculate attitude, at least four of the nearly 24 GPS&#xA;  satellites in orbit must be in view of these antennas.&#xA;  &lt;strong&gt;The GPS receiver calculates the ISS attitude about&#xA;  once a minute&lt;/strong&gt;, providing information on where the ISS&#xA;  is pointed at that point in time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, in more detail on that:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://sunnyday.mit.edu/16.355/Gomez-GPS-Lessons.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://sunnyday.mit.edu/16.355/Gomez-GPS-Lessons.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ISS GPS antenna array consists of four antennas in a&#xA;  3 meter by 1.5 meter rectangle on the S0 element of the&#xA;  ISS main truss. The array center is slightly to the port&#xA;  side (4 meters) of the vehicle centerline, as shown in&#xA;  Figure 10.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lsPih.png&quot; alt=&quot;Figure 10&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ad7SK.png&quot; alt=&quot;ISS in orbit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;GPS alone can not meet the attitude&#xA;  accuracy requirements. The multipath environment on&#xA;  ISS is such that the unfiltered GPS attitude solutions can&#xA;  not meet the 0.5 degree requirement. The unfiltered GPS&#xA;  attitude solutions are used as an input in the ISS’s on&#xA;  board software attitude filter.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-07-31T14:57:22.213" LastActivityDate="2013-07-31T14:57:22.213" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="880" PostTypeId="2" ParentId="869" CreationDate="2013-07-31T14:38:15.767" Score="20" Body="&lt;p&gt;I was actually just reading a great What If? article on this found &lt;a href=&quot;http://what-if.xkcd.com/30/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. Flight on other planets is possible. I think the included comic strip summarizes it wonderfully:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0zCQm.png&quot; alt=&quot;Some Description&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for each valid body in our solar system (barring Earth of course), I'm going to paraphrase a bit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The Sun&lt;/strong&gt;: Attempting flight on the sun is more or less useless as any vessel close enough to feel its atmosphere would be instantly vaporized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mars&lt;/strong&gt;: The article goes into a lengthy discussion about simulation via &lt;a href=&quot;http://www.x-plane.com/desktop/home/?utm_expid=25103718-8&quot; rel=&quot;noreferrer&quot;&gt;X-Plane&lt;/a&gt;. X-Plane, as it turns out, can be made to closely simulate the conditions found on Mars. Unfortunately, as was also found, flight on Mars is possible but difficult. To achieve flight on Mars, you need to be going &lt;em&gt;fast&lt;/em&gt;. The article states that a speed of mach 1 is required merely to achieve flight. Problem is, once you achieve flight, the inertia makes it nearly impossible to change course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Venus&lt;/strong&gt;: Venus is interesting. The atmosphere on Venus is 60 times denser than Earth's atmosphere. You could easily achieve flight at incredibly low speeds (a Cessna 172 Skyhawk, the aircraft the article is based around, could achieve flight at running speed). Problem is, the air on Venus is hot enough to melt lead. You can always get around this by flying in Venus's upper atmosphere. The upper atmosphere is rather earth-like and would be quite easy to fly a plane in. Only, you'd have to ensure no metal is exposed as sulfuric acid in the upper atmosphere introduces the threat of corrosion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Jupiter&lt;/strong&gt;: Flight on Jupiter is unrealistic. Jupiter's gravity is much too strong. The power required to maintain flight is about 3x that of Earth making flight there highly unrealistic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Saturn&lt;/strong&gt;: Weaker gravity and slightly denser atmosphere than Jupiter means an aircraft might fair better but ultimately would succumb to cold or high winds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Uranus&lt;/strong&gt;: Flight on Uranus could be sustained slightly longer but ultimately the aircraft would still succumb to the conditions found there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Neptune&lt;/strong&gt;: The temperature and turbulence make it impossible to achieve flight on Neptune. It's assumed your aircraft would quickly break apart in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Titan&lt;/strong&gt;: Titan is perhaps the best plan to attempt flight on. To quote the article: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;When it comes to flying, Titan might be better than Earth. Its&#xA;  atmosphere is thick but its gravity is light, giving it a surface&#xA;  pressure only 50% higher than Earth’s with air four times as dense.&#xA;  Its gravity-lower than that of the Moon-means that flying is easy.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Flight on Titan &lt;strong&gt;IS&lt;/strong&gt; easy. A human could theoretically achieve flight with a wingsuit and mere muscle power. The problem is, Titan is cold, 72 Kelvin cold. Flight would require some major heating modifications but, barring the heat factor, Titan is the absolute best place to attempt flight in our solar system. It's even better than Earth. As an interesting note, Titan, thus far, has actually been too cold for even unmanned probes to explore. Again, quoting the article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The batteries would help to keep themselves warm for a little while, but eventually the craft would run out of heat and crash. The Huygens probe, which descended with batteries nearly drained (taking &lt;a href=&quot;http://www.beugungsbild.de/huygens/huygens.html&quot; rel=&quot;noreferrer&quot;&gt;fascinating pictures&lt;/a&gt; as it fell), succumbed to the cold after only a few hours on the surface. It had enough time to send back &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/New_images_from_Titan&quot; rel=&quot;noreferrer&quot;&gt;a single photo&lt;/a&gt; after landing—the only one we have from the surface of a body beyond Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth&lt;/strong&gt;: Earth's conditions are quite optimal for flying. Earth's gravity is 9.78 m/s². As a comparison, Jupiter's gravity is 24.79 m/s² and Titan's gravity is 1.352 m/s². Earth's atmosphere is, at sea level, 1 standard atmosphere or 101.3 kPa or 14.7 psi compared to Mars's average which is about 0.006 standard atmosphere or  600  Pa or 0.087 psi and Venus's average which is about 9.2 mPa or 1,330 psi. Takeoff speed for our Cessna 172 Skyhawk is 64 KIAS (Knots Indicated Air Speed) and the best rate of climb is 73 KIAS. Normal cruise speed in a Cessna 172 Skyhawk is 122 knots (140mph, 226 km/h). As a comparison, flight on mars would require speeds over Mach 1 which translates to 768 mph or 1,236 kph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To summarize&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Sun&lt;/strong&gt;: Instant vaporization.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Mars&lt;/strong&gt;: Atmosphere's too thin to fly below mach 1, above mach 1 you&#xA;essentially can't steer.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Venus's Lower Atmosphere&lt;/strong&gt;: Flight is possible but the air's hot as&#xA;lead. You'd melt.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Venus's Upper Atmosphere&lt;/strong&gt;: Flight is possible but corrosion is a&#xA;factor due to sulfuric acid so no exposed metal.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Jupiter&lt;/strong&gt;: High gravity makes flight extremely unrealistic.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Saturn&lt;/strong&gt;: Flight's possible but your aircraft might ultimately succumb to the&#xA;cold and weather conditions.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Uranus&lt;/strong&gt;: Same as Saturn but you &lt;strong&gt;MIGHT&lt;/strong&gt; last a bit longer.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Neptune&lt;/strong&gt;: Your aircraft would break apart quickly from the extreme&#xA;turbulence.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Titan&lt;/strong&gt;: Flight could be achieved with artificial wings and mere muscle&#xA;power. Unfortunately, Titan's cold. To quote the XKCD article:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;blockquote&gt;&#xA;    &lt;p&gt;If humans put on artificial wings to fly, we might become Titan versions of the Icarus story—our wings could freeze, fall apart, and send us tumbling to our deaths.&lt;/p&gt;&#xA;  &lt;/blockquote&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FL6na.png&quot; alt=&quot;Icarus on Titan&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Earth&lt;/strong&gt;: We know flight on Earth works due to firsthand knowledge. We&#xA;don't have the most optimal conditions in our solar system, but&#xA;the conditions here are still great for all types of manned aircraft.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Anywhere Else&lt;/strong&gt;: No atmosphere, so you would crash ballisticly.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;As a small note&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Titan is the absolute best environment for flight using a conventional aircraft if you don't factor in the cold. I imagine it would be much easier and less costly to attempt flight in Venus's upper atmosphere by protecting all exposed metal from corrosion than it would be to make major modifications to a conventional aircraft so that it and its pilot can withstand the extreme cold found on Titan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Another Small Note&lt;/strong&gt;: Mach 1 is measured relative to earth so 340.29 m / s. The speed of sound on Mars is different. The speed of sound is 226 m/s.&lt;/p&gt;&#xA;" OwnerUserId="315" LastEditorUserId="315" LastEditDate="2013-08-01T15:56:51.327" LastActivityDate="2013-08-01T15:56:51.327" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="881" PostTypeId="1" AcceptedAnswerId="907" CreationDate="2013-07-31T15:24:08.463" Score="40" ViewCount="5407" Body="&lt;p&gt;The ISS is a huge (relatively) satellite which, according to NASA, is longer than a football field. I am curious about what would happen if the station's skin was punctured - perhaps by a careless astronaut, by a small object flying through space, or a monkey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this were to happen, I would imagine that air would start to leak out of the station, eventually depriving the crew of breathable air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the crew do? Are there ways to block off parts of the station? Would you just float up to it and patch it?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="1255" LastEditDate="2013-12-13T16:48:44.300" LastActivityDate="2022-02-08T10:20:17.020" Title="Are there any safety procedures in place on the ISS in case of puncture?" Tags="|iss|life-support|shielding|debris|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="882" PostTypeId="1" AcceptedAnswerId="884" CreationDate="2013-07-31T15:28:02.760" Score="11" ViewCount="1158" Body="&lt;p&gt;Curiosity should last a very long time - up to around 14 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is whether NASA has any plans for a Mars rovers &lt;em&gt;after&lt;/em&gt; Curiosity. If so, are any specifics known?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-07-31T15:42:26.000" Title="Does NASA have any plans for future Mars rovers after Curiosity?" Tags="|mars|rovers|curiosity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="884" PostTypeId="2" ParentId="882" CreationDate="2013-07-31T15:34:29.327" Score="14" Body="&lt;p&gt;Nasa is planning another rover for a 2020 arrival.  To keep costs down the plan is to use a chassis similar to Curiosity but with a different set of instruments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA's Science Definition Team issued a report earlier this month outlining four major goals for the mission:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Explore an astrobiologically relevant ancient environment on&#xA;Mars to decipher its geological processes and history, including the&#xA;assessment of past habitability.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Assess the biosignature preservation potential within the&#xA;selected geological environment and search for potential&#xA;biosignatures.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Demonstrate significant technical progress towards the&#xA;future return of scientifically selected, well-documented samples to&#xA;Earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Provide an opportunity for contributed Human Exploration &amp;amp;&#xA;Operations Mission Directorate (HEOMD) or Space Technology Program&#xA;(STP) participation, compatible with the science payload and within&#xA;the mission’s payload capacity.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The planetary Society did an in depth &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/van-kane/20130727-mars-rover-2020-in-depth.html&quot; rel=&quot;noreferrer&quot;&gt;writeup&lt;/a&gt; of the report.  NASA's &lt;a href=&quot;http://mepag.jpl.nasa.gov/reports/mep_report.html&quot; rel=&quot;noreferrer&quot;&gt;report&lt;/a&gt; is available if you'd prefer to read the original.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Concept Sketch for new rover:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/DeoSl.jpg&quot; alt=&quot;Concept Sketch for new rover&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="157" LastEditorUserId="157" LastEditDate="2013-07-31T15:42:26.000" LastActivityDate="2013-07-31T15:42:26.000" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="885" PostTypeId="2" ParentId="881" CreationDate="2013-07-31T15:42:15.610" Score="19" Body="&lt;p&gt;Quoth &lt;a href=&quot;http://www.quora.com/International-Space-Station/How-does-the-international-space-station-ISS-handle-meteorite-impacts-and-have-there-been-ones-large-enough-to-breach-the-hull#&quot;&gt;Robert Frost&lt;/a&gt;, self-described as &quot;instructor/engineer in the NASA Mission Operations Directorate&quot;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If the object is in LEO and larger than around 10 cm, the ISS can be&#xA;  warned and moved, a few orbits in advance of the potential impact. &#xA;  For smaller objects, they just accept the risk.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The ISS has shields called Whipple bumpers.  They are multi-layered&#xA;  with spaces between the layers.  The intent is that impact with a&#xA;  layer will both slow and hopefully break apart the projectile, so that&#xA;  by the time it gets to the bottom layer it is no longer harmful.  That&#xA;  shielding is heaviest near the front of the vehicle, since that is&#xA;  where impacts of the greatest velocity would likely happen.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Should a projectile puncture the hull into the pressurized cabin, the&#xA;  crew have patch kits that they can use to plug the hole.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The cabin has not been penetrated, but there are several punctures on&#xA;  the vehicle.  The solar arrays, radiators, and some truss have scars&#xA;  from hits.  The crew have occasionally been made aware of a hit by&#xA;  hearing a loud banging noise.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="370" LastEditorUserId="370" LastEditDate="2013-07-31T16:24:08.433" LastActivityDate="2013-07-31T16:24:08.433" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="886" PostTypeId="1" CreationDate="2013-07-31T17:34:37.720" Score="15" ViewCount="1734" Body="&lt;p&gt;How feasible was Project Orion and how feasible would it be if started today? Specifically, what would be the real world impact of nuclear fallout? And, would there be methods to mitigate or eliminate the fallout, so Project Orion could became a realistic method of space travel? &lt;/p&gt;&#xA;" OwnerUserId="374" LastEditorUserId="49" LastEditDate="2013-07-31T18:27:50.570" LastActivityDate="2020-01-01T12:35:21.370" Title="Feasibility of Project Orion" Tags="|nuclear-pulse-propulsion|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="887" PostTypeId="2" ParentId="800" CreationDate="2013-07-31T18:15:28.883" Score="8" Body="&lt;p&gt;There are multiple plans for new space stations. Bigelow Aerospace plans on completing their next-gen commercial space station by 2014. However, the future of space stations lies with the Chinese who plan to build on the success of the Tiangong-1 with the Tiangong-2 and the Tiangon-3 with the expectation of large manned station by 2020. This is coincidentally the same year the the ISS is de-orbiting, so we shouldn't be without a foothold in space for long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the current landscape of space exploration being dominated by governments, the Chinese are more likely to be successful due to the fact that, unlike companies like Bigelow, they don't need to return a profit. That, along with the Chinese trying to prove their dominance in space, means that most likely the next big space station will come from China&lt;/p&gt;&#xA;" OwnerUserId="374" LastActivityDate="2013-07-31T18:15:28.883" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="888" PostTypeId="2" ParentId="411" CreationDate="2013-07-31T18:39:37.667" Score="15" Body="&lt;p&gt;Here are some Apollo specific innovations: microchip, cordless tools, joystick, CAT scans, technology in MRI machines, modern shoe designs, freeze dried food, vacuum sealed packages, dampening material, retro-reflector (detects chemical leaks), water purification, silicon based storage of records, fly-by-wire, ground water cleaning, large fabric roofs used in landmark buildings, anti-tip rafts, insulation blankets, and countless others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And those are just commonly used innovations which had a specific link to the Apollo program. There are much more everyday innovations from the space program overall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spinoff.nasa.gov/spinoff/database&quot;&gt;http://spinoff.nasa.gov/spinoff/database&lt;/a&gt; contains articles about all the spin offs from NASA programs.&lt;/p&gt;&#xA;" OwnerUserId="374" LastEditorUserId="374" LastEditDate="2013-08-08T12:55:41.410" LastActivityDate="2013-08-08T12:55:41.410" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="889" PostTypeId="1" AcceptedAnswerId="891" CreationDate="2013-07-31T19:26:17.670" Score="15" ViewCount="1503" Body="&lt;p&gt;Since we already use gravity assist to launch satellites through the solar system, is there any way we can use gravity assist to speed up a manned trip to a planet such as Titan or Europa, while also avoiding the problem of needing lots and lots of fuel to lift fuel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be a rough time frame for a trip using this method?&lt;/p&gt;&#xA;" OwnerUserId="374" LastEditorUserId="26613" LastEditDate="2019-03-26T12:46:41.500" LastActivityDate="2019-03-26T12:46:41.500" Title="Gravity assist for manned travel" Tags="|orbital-mechanics|crewed-spaceflight|trajectory|gravity-assist|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="890" PostTypeId="2" ParentId="889" CreationDate="2013-07-31T19:52:34.597" Score="15" Body="&lt;p&gt;A gravity assist (or a slingshot) is one of the many compromises in mission design. Instead of going somewhere directly you go somewhere else first and use the momentum of a planetary body to speed up your own movement thus fitting into a Delta-V budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the crux of the problem is that gravity assists take time. For unmanned missions, this is acceptable since avionics and science instruments are shielded, and can be shut off when not needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For manned missions, existence of unforgiving radiation environment due to solar wind and galactic cosmic rays means you have to make the trip as short as possible. Muscular deconditioning and bone loss are also threats that arise in microgravity. What's more, if the trip is shorter, you don't need as much mass dedicated to life support (oxygen, food, clothes, toilet tissue). Thus, in general, gravity assists aren't suited for manned flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are &lt;strong&gt;exceptions&lt;/strong&gt;: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;slingshots around giant planets (and their heavier moons) when you want to study the moons,&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;free return trajectories (in case your propulsion malfunctions) and&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Buzz Aldrin's &lt;strong&gt;cycler orbit&lt;/strong&gt; options (with or without powered gravity assists - these are the orbits that revisit say Mars and Earth without much expenditure of fuel) - a bit of a variation on &quot;free return&quot;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr/&gt;&#xA;&#xA;&lt;p&gt;On a purely personal note (and if you ask me), I'd prefer a brachistochrone constant 1g acceleration/deceleration trajectory to any gimmick to solve the artificial gravity and rad protection problems simultaneously.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-07-31T20:01:15.750" LastActivityDate="2013-07-31T20:01:15.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="891" PostTypeId="2" ParentId="889" CreationDate="2013-07-31T19:52:48.150" Score="15" Body="&lt;p&gt;In general, Gravity Assists do not reduce the amount of time, unless you are going really far out there. For instance, Galileo took 6 years to make it to Jupiter after making 3 flybys, one of Venus, two of Earth. Comparatively, New Horizons made it there in just over a year, Voyager 18 months, and Pioneer about 2 years. However, there is considerable fuel savings from doing the flybys.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you are already going to pass through the orbit of an object, then a flyby could speed you along. Thus, Voyagers are going faster through space than New Horizons will ever be, because of the extra flyby of Saturn the Voyagers received.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting to Mars would not have any candidates for speeding the journey time to it except perhaps the moon, as Mars is the easiest planet to reach from Earth. A flyby of Jupiter would probably make a mission to Saturn faster, if you line it up just right.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-01-21T18:09:47.737" LastActivityDate="2014-01-21T18:09:47.737" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="892" PostTypeId="1" AcceptedAnswerId="1740" CreationDate="2013-07-31T20:03:21.653" Score="12" ViewCount="972" Body="&lt;p&gt;Many of my favorite ideas for space exploration, such as large outer solar system probes, or the Mars Direct human Mars exploration proposal, require nuclear reactors to supply the high-power requirements of the spacecraft. However I have never very much liked the idea of a large amount of refined uranium being put on top of a possible bomb (the rocket fuel). I am specifically concerned the possibility of the rocket exploding, sending radioactive uranium around the globe in a mini-Chernobyl. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming we will not be getting fusion reactors anytime soon, what alternatives are there to nuclear fission power where solar power is not enough for a mission. Also, since there needs to be a high-power source on the surface of a planet (Mars) for Mars Direct, what high-power sources could be used on a planet such as Mars?&lt;/p&gt;&#xA;" OwnerUserId="383" LastEditorUserId="49" LastEditDate="2014-03-07T09:31:54.933" LastActivityDate="2015-06-05T12:15:47.923" Title="Is there an alternative to nuclear reactors for high-power spacecraft?" Tags="|propulsion|technology|power|natural-resources|nuclear|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="893" PostTypeId="2" ParentId="809" CreationDate="2013-07-31T20:04:10.087" Score="4" Body="&lt;p&gt;This post doesn't include any real world construction problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;if we guess that the temperature on the cold side is -30.0 degrees F (243 K) and the temperature at the warm side is 250 F (394 K) (&lt;a href=&quot;http://my.execpc.com/~culp/space/spacecraft.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;),&#xA;the &lt;a href=&quot;http://en.wikipedia.org/wiki/Carnot_cycle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Carnot cycle&lt;/a&gt; predicts an efficiency of 0.383.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Multiplying by the maximal efficiency of a peltier element of 15% (&lt;a href=&quot;http://en.wikipedia.org/wiki/Thermoelectric_cooling&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thermoelectric cooling&lt;/a&gt;), the net efficiency is 0.057, or 5.7%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 1 square meter surface lit with an energy of &lt;code&gt;120 W&lt;/code&gt; (&lt;a href=&quot;http://en.wikipedia.org/wiki/Sunlight&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;), at 5.7%, yields only 6.9 W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is much lower that the energy you get with typical solar panels (in the rage of 20% &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_panel&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="291" LastEditorUserId="1235" LastEditDate="2019-10-21T19:50:10.130" LastActivityDate="2019-10-21T19:50:10.130" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="894" PostTypeId="1" CreationDate="2013-07-31T21:59:36.843" Score="34" ViewCount="8586" Body="&lt;p&gt;By &lt;a href=&quot;https://space.stackexchange.com/questions/864/puncturing-space-suit-during-eva-what-would-happen&quot;&gt;another question&lt;/a&gt; I was reminded how hard it is to build airtight equipment such as vacuum chambers from multiple pieces. &lt;a href=&quot;http://en.wikipedia.org/wiki/Spacecraft_docking_and_berthing_mechanisms&quot; rel=&quot;noreferrer&quot;&gt;Docking or berthing space ships / space station modules&lt;/a&gt; is a rather similar task, only with a reversed pressure gradient. &lt;strong&gt;I am wondering how the &lt;em&gt;seals&lt;/em&gt; work.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When &lt;a href=&quot;http://en.wikipedia.org/wiki/Vacuum_flange&quot; rel=&quot;noreferrer&quot;&gt;screwing two pieces of a vacuum chamber together&lt;/a&gt;, you can pick between a few materials in terms of seals &lt;a href=&quot;http://en.wikipedia.org/wiki/Vacuum#Measurement&quot; rel=&quot;noreferrer&quot;&gt;depending on the target quality of the vacuum&lt;/a&gt;. For an ultra-high vacuum, copper is rather common. But copper rings can only be used once (&lt;em&gt;copper gasket vs. knife-edge flange&lt;/em&gt;), so they have to be exchanged each time you open up some particular flange. For a high / medium vacuum, you can work with &lt;a href=&quot;http://en.wikipedia.org/wiki/Viton&quot; rel=&quot;noreferrer&quot;&gt;Viton&lt;/a&gt; / cheap rubber. Viton and rubber can be used multiple times, but even this has limits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lmS2n.jpg&quot; alt=&quot;Space Shuttle - APAS-95&quot;&gt;&#xA;This is an &lt;a href=&quot;http://en.wikipedia.org/wiki/Androgynous_Peripheral_Docking_System#APAS-95&quot; rel=&quot;noreferrer&quot;&gt;APAS-95&lt;/a&gt; docking mechanism, mounted on a space shuttle. You can clearly see the two brown rings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QkrCP.jpg&quot; alt=&quot;ISS/Kibo - Passive CBM&quot;&gt;&#xA;This is a passive interface of a &lt;a href=&quot;http://en.wikipedia.org/wiki/Common_Berthing_Mechanism&quot; rel=&quot;noreferrer&quot;&gt;Common Berthing Mechanism&lt;/a&gt; as found on Kibo at the ISS. There are basically three rings, made of some brown material. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a few interesting aspects about those rings in space, which make me curious. Apparently, they can be &lt;em&gt;re-used&lt;/em&gt; rather often. Remember, how often for instance modules at the space station MIR were re-arranged and therefore un-docked and docked again. Besides, this stuff stays in space (and vacuum) for very long times while it does not seem to degrade significantly (by out-gassing etc).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Which &lt;em&gt;materials&lt;/em&gt; are used and have been used in this context? What are their &lt;em&gt;limits&lt;/em&gt;? How many docking / un-docking cycles can they handle? How air-tight are those seals in terms of numbers (e.g. loss of air in kg per day)?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-02-19T20:12:11.370" Title="How do manned spacecraft achieve an airtight connection while docking?" Tags="|crewed-spaceflight|spacecraft-development|engineering|docking|vacuum|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="895" PostTypeId="2" ParentId="867" CreationDate="2013-07-31T22:33:51.617" Score="4" Body="&lt;p&gt;You are asking for examples ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stabilizing a space craft or rocket with a spin is a rather easy way of keeping its trajectory straight while it is under powered flight. You do not need to gimbal rocket engines and you can work without thrusters or active control surfaces. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A typical example in terms of rockets are &lt;a href=&quot;http://en.wikipedia.org/wiki/Sounding_rocket&quot; rel=&quot;nofollow&quot;&gt;sounding rockets&lt;/a&gt;. Within the first few seconds of the launch, they are spun up to a few rotations per second (by fixed control surfaces) and therefore keep their pre-arrange trajectory while under powered flight. At motor burnout, you can de-spin them with yo-yos, if you like. There is no better way to show this than with a video: &lt;a href=&quot;http://www.youtube.com/watch?v=5nlVcRtBTLQ&quot; rel=&quot;nofollow&quot;&gt;http://www.youtube.com/watch?v=5nlVcRtBTLQ&lt;/a&gt; (yo-yos are used at 1:50). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some space probes are stabilized in a similar fashion. The following video is a computer animation of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Exploration_Rover&quot; rel=&quot;nofollow&quot;&gt;MER&lt;/a&gt;s' flights to Mars: &lt;a href=&quot;http://www.youtube.com/watch?v=XRCIzZHpFtY&quot; rel=&quot;nofollow&quot;&gt;http://www.youtube.com/watch?v=XRCIzZHpFtY&lt;/a&gt; The several stages of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta_II&quot; rel=&quot;nofollow&quot;&gt;Delta II&lt;/a&gt; itself are stabilized with gimbaling rocket engines and gyros. However, at 1:18, you can see how the final 'stage' for insertion on a trajectory towards Mars is spun up before it is ignited. Once the engine has stopped at 1:37, you can see the yo-yos. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nice aspect about spin stabilization is, that it allows rather simple designs. You can save a lot of moving parts, weight and complexity. If you do not need to change your orientation while engines are running, it is a perfect concept. &lt;/p&gt;&#xA;" OwnerUserId="14" LastActivityDate="2013-07-31T22:33:51.617" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="896" PostTypeId="2" ParentId="868" CreationDate="2013-07-31T22:38:59.270" Score="8" Body="&lt;p&gt;As both @AlanSE and @Philipp said, &lt;code&gt;Pu-238&lt;/code&gt; won't go boom no matter what you do to it.  However, lets pretend it was &lt;code&gt;Pu-239&lt;/code&gt; and see what happens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the battery puts out only about 1/2% of the power of a &lt;code&gt;Pu-238&lt;/code&gt; battery, Galileo doesn't work at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, while it is just barely possible to make a mass supercritical with pressure, you would have to start with a mass that's just a hair below critical and then subject it to some really extreme pressure. While we think of metal as incompressible, it will squash a bit under planetary core type pressures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, if you manage to do this, you end up with a criticality accident, not a mushroom cloud. The one I recall reading about knocked a nearby worker off a catwalk – he lived long enough to run out of the building. If you want a boom, you have to convert the subcritical mass to a very supercritical mass very quickly – which is why all the complex mess with explosives and very precise detonators.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorDisplayName="user12" LastEditDate="2013-08-13T16:45:28.523" LastActivityDate="2013-08-13T16:45:28.523" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="897" PostTypeId="1" AcceptedAnswerId="901" CreationDate="2013-07-31T22:56:53.830" Score="9" ViewCount="10356" Body="&lt;p&gt;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1494996839&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;A Princess of Mars&lt;/em&gt; (1912) by Edgar Rice Burroughs&lt;/a&gt; in &lt;a href=&quot;http://en.wikisource.org/wiki/A_Princess_of_Mars/Chapter_III&quot; rel=&quot;noreferrer&quot;&gt;Chapter III&lt;/a&gt; describes this &quot;superhuman leap&quot; by the hero John Carter:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;My effort was crowned with a success which appalled me no less than it&#xA;  seemed to surprise the Martian warriors, for it carried me fully&#xA;  thirty feet into the air and landed me a hundred feet from my pursuers&#xA;  and on the opposite side of the enclosure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The novel was written at a time when the possibility of breathable air on &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars&quot; rel=&quot;noreferrer&quot;&gt;Mars&lt;/a&gt; still existed; knowledge of the lesser gravity of Mars spawned such assumptions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will Humans from Earth really possess the ability to leap 30 feet high and land 100 feet away on Mars?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="1142" LastEditDate="2017-03-06T01:23:12.760" LastActivityDate="2020-01-31T00:04:53.180" Title="Will colonists on Mars have enhanced physical mobility relative to an Earth-normal environment?" Tags="|mars|colonization|physical-activity|reduced-gravity-sports|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="899" PostTypeId="1" AcceptedAnswerId="900" CreationDate="2013-07-31T23:29:33.233" Score="14" ViewCount="2181" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Molniya_orbit&quot;&gt;Molniya orbits&lt;/a&gt; are popular with Russian communication satellites, because they spend a lot of time at high latitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Europe and Canada are also quite far north.  Do any European, American, or perhaps Canadian satellites use Molniya orbits?  If yes, which ones are those?  If not, why not?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="25" LastEditDate="2013-08-01T00:44:19.043" LastActivityDate="2019-02-14T13:19:45.447" Title="Are Molniya orbits used apart from Russian communication satellites?" Tags="|orbit|communication-satellite|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="900" PostTypeId="2" ParentId="899" CreationDate="2013-08-01T02:18:33.113" Score="6" Body="&lt;p&gt;The vast majority of Molniya satellites are Russian, but a bit of research found a few. Wikipedia has a list of &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_NRO_Launches&quot; rel=&quot;nofollow&quot;&gt;NRO launches&lt;/a&gt; that include a few satellites in Molniya orbit. Being NRO, and classified, it's hard to verify this claim, but it doesn't surprise me. A list of all &lt;a href=&quot;http://www.wmo-sat.info/oscar/satellites&quot; rel=&quot;nofollow&quot;&gt;Earth Observing satellites&lt;/a&gt; popped up &lt;a href=&quot;http://www.wmo-sat.info/oscar/satelliteprogrammes/view/233&quot; rel=&quot;nofollow&quot;&gt;a NASA satellite&lt;/a&gt; as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other countries seems to prefer constellations of LEO satellites for communication at high latitudes, usually in some sort of a polar or otherwise high inclination orbit. There is also a special orbit known as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Tundra_orbit&quot; rel=&quot;nofollow&quot;&gt;Tundra orbit&lt;/a&gt;, where a Geosynchronous, but not stationary, satellite orbits in a particular ground pattern over a particular region. Sirius Satellite Radio is the only current users of this particular orbit.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-01T17:06:52.023" LastActivityDate="2013-08-01T17:06:52.023" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="901" PostTypeId="2" ParentId="897" CreationDate="2013-08-01T02:21:29.227" Score="18" Body="&lt;p&gt;Let's first see what calculations we have available on the Internet, maybe from what could be considered relatively reliable sources:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://phoenix.lpl.arizona.edu/mars102.php&quot;&gt;University of Arizona - Phoenix Mars Mission - Mars 101&lt;/a&gt;&lt;/strong&gt;:&lt;br /&gt;&#xA;  Gravity on Mars is only about 38% of Earth's. So, if you weighed 100&#xA;  pounds on Earth, you would only weigh about 38 pounds on Mars. And if&#xA;  you can jump one meter (3.3 feet) high on Earth, &lt;strong&gt;you would be able to&#xA;  jump 2.64 meters (almost 9 feet) high on Mars&lt;/strong&gt;. The lower gravity on&#xA;  Mars could prove beneficial to future astronauts, as it would permit&#xA;  them to easily walk around the surface wearing large space suits and&#xA;  carrying heavy backpacks.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But let's put this claim to the test. Not because we don't trust their calculations, but because we want to understand the physics involved a bit better than that description provides. We could assume the following;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can jump 1 m high in Earth's gravity (distance between feet and the ground when feet are stretched in the air), your initial velocity (or linear momentum) would have to be 4.43 m/s. This is the constant that wouldn't change no matter at what gravitational conditions you jump, as it is the ability of your body to produce momentary force, in our case in the vertical direction required for a jump, producing momentum force in the opposite direction to gravitational acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saying it differently, your legs can't spring any faster just because the gravitational acceleration working on them is smaller. They can't, because of your muscles and bones only being capable of producing that much force in that much time. Point in case, if this wasn't so, astronauts on the IIS would be able to move their limbs at the speeds that even slow-motion cameras would have hard time recording. And it isn't the atmospheric pressure (drag, or air resistance) slowing you down much either, not in our case at least (and even less so on Mars, with atmospheric density equal to the density found 35 km above the Earth's surface), otherwise those ISS astronauts would be kicking really lively during extravehicular activity (EVA).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, I hope I demonstrated well enough, that we can consider this 4.43 m/s as constant. So let's take this number and gravitational acceleration on Mars (0.38 of Earth's) into our own calculations, and we get &lt;strong&gt;2.633135 m&lt;/strong&gt; for the height of our jump, and &lt;strong&gt;2.3776 seconds&lt;/strong&gt; &quot;in the air&quot;. Accounting for a small margin of error and rounding up of the end results, the calculations on the page of the University of Arizona are indeed correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how far could you jump? Ballistics teach us that the best angle to achieve the longest range is 45°, if we don't have to consider aerodynamics. Since we don't have to consider aerodynamics at all (our bodies are hardly airworthy at such low speeds and the atmosphere on Mars is utterly neglectable in our case anyway), this is really convenient, because we can apply simple trigonometry to our previous results, where the hypotenuse length in a real triangle is those 2.633135 m from before, when we jumped straight up. And we know that hypotenuse &lt;em&gt;c&lt;sup&gt;2&lt;/sup&gt; = sqrt(a&lt;sup&gt;2&lt;/sup&gt; + b&lt;sup&gt;2&lt;/sup&gt;)&lt;/em&gt;, where &lt;code&gt;a&lt;/code&gt; and &lt;code&gt;b&lt;/code&gt; legs are of equal length in a right triangle with the hypotenuse at a 45° angle. A bit of basic arithmetic, and we end up with the calculated length of our &lt;em&gt;&quot;ballistically best&quot;&lt;/em&gt; &lt;strong&gt;hop range of 3.723815 m&lt;/strong&gt; (plus as far as you can stretch your legs and pull yourself back up on landing without falling on your back, what basically athletes do with their long jumps). This is of course a jump from a stationary position, or a hop, and not with a run before the jump, gaining horizontal momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where the quoted author gets those numbers from, is however beyond my knowledge. Maths aren't all that sound anyway; If we apply our hop logic from before (&lt;em&gt;l = 2(sqrt(c&lt;sup&gt;2&lt;/sup&gt;/2))&lt;/em&gt;) to those &lt;em&gt;&quot;30 ft high&quot;&lt;/em&gt;, and don't assume humans living on Mars in time grow wings, we get a distance of 42.43 feet (plus slightly more with stretching legs forward and pushing oneself back in the upright position upon landing). If the author didn't mean &quot;hops&quot; but actual &quot;long jumps&quot; with a run before the jump, then it becomes a problem of jump technique, which I'm afraid I can't calculate for conditions on Mars, but I presume the major problem in gaining much speed than on Earth before the jump on Mars would be one of traction due to lesser gravity, and &lt;strong&gt;I highly doubt jumps much longer than 30 ft in length would be possible for an average person&lt;/strong&gt; (the one we assumed a high jump of &lt;em&gt;feet 1 m off the ground when stretched&lt;/em&gt; in our previous calculations), &lt;strong&gt;and maybe roughly double as much (~ 60 ft) for a top athlete&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, current long jump world record holder Mike Powell (USA) managed 8.95 m (29 ft 4¼ in) in Tokyo, back in 1991. If we multiply this value with &lt;code&gt;1/0.38&lt;/code&gt; (or 2.6316 to approximate conditions on Mars based on 38% of Earth's gravity), we get 23.55 m (or 77 ft 3&lt;sup&gt;17&lt;/sup&gt;⁄&lt;sub&gt;64&lt;/sub&gt; in), which sounds about right, if we take some non-negligible percentage off it due to loss in traction. I.e., we get back to roughly 60 ft (let's sum it up to 20 meters) for a top athlete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then again, that work you're quoting is fiction, and I'm making too much of it again... ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-08-01T02:21:29.227" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="902" PostTypeId="1" AcceptedAnswerId="903" CreationDate="2013-08-01T02:42:33.237" Score="24" ViewCount="10707" Body="&lt;p&gt;When did &quot;Space&quot; become the accepted terminology for the area beyond the Earth's atmosphere? What, if anything, was it called before it became known as &quot;Space&quot;?&lt;/p&gt;&#xA;" OwnerUserId="137" LastEditorUserId="49" LastEditDate="2013-08-01T03:40:04.340" LastActivityDate="2021-04-12T00:32:06.353" Title="When did the term &quot;Space&quot; first come into use?" Tags="|history|terminology|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="903" PostTypeId="2" ParentId="902" CreationDate="2013-08-01T03:30:21.390" Score="12" Body="&lt;p&gt;The English word &lt;em&gt;space&lt;/em&gt; originates from Latin word for expanse - &lt;em&gt;&lt;code&gt;spatium&lt;/code&gt;&lt;/em&gt; (also written &lt;em&gt;spacium&lt;/em&gt; in Medieval Latin), and later French word &lt;em&gt;&lt;code&gt;espace&lt;/code&gt;&lt;/em&gt;, through the use of which English variant &lt;em&gt;space&lt;/em&gt; was formed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The earliest mention of space in relation to the &lt;em&gt;'outer space'&lt;/em&gt; is attributed to the 17th-century English poet &lt;a href=&quot;https://en.wikipedia.org/wiki/John_Milton&quot;&gt;John Milton&lt;/a&gt;, that used it in his epic poem &lt;a href=&quot;https://en.wikipedia.org/wiki/Paradise_Lost&quot;&gt;Paradise Lost&lt;/a&gt; to much the same effect we now use it to describe &lt;em&gt;'outer space' beyond the Earth’s atmosphere too thin for aeronautical purposes&lt;/em&gt; (or other non-arbitrary definitions):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;When I behold this goodly frame, this World,&lt;br /&gt; Of Heaven and Earth consisting, and compute&lt;br /&gt; Their magnitudes—this Earth, a&#xA;  spot, a grain,&lt;br /&gt; An atom, with the Firmament compared&lt;br /&gt; And&#xA;  all her numbered stars, that seem to roll&lt;br /&gt;&#xA;  &lt;strong&gt;Spaces&lt;/strong&gt; incomprehensible (for such&lt;br /&gt; Their distance argues, and their swift return&lt;br /&gt; Diurnal) merely to officiate light&lt;br /&gt;&#xA;  Round this opacous Earth, this punctual spot,&lt;br /&gt; One day and night,&#xA;  in all her vast survey&lt;br /&gt; Useless besides—reasoning, I oft&#xA;  admire,&lt;br /&gt; How Nature, wise and frugal could commit&lt;br /&gt; Such&#xA;  disproportions, with superfluous hand&lt;br /&gt; So many nobler bodies to&#xA;  create,&lt;br /&gt; Greater so manifold, to this one use,&lt;br /&gt; For aught&#xA;  appears, and on their Orbs impose&lt;br /&gt; Such restless revolution day&#xA;  by day&lt;br /&gt; Repeated, while the sedentary Earth,&lt;br /&gt; That better&#xA;  might with far less compass move,&lt;br /&gt; Served by more noble than&#xA;  herself, attains&lt;br /&gt; Her end without least motion, and receives,&lt;br /&gt;&#xA;  As tribute, such a sumless journey brought&lt;br /&gt; Of incorporeal&#xA;  speed, her warmth and light;&lt;br /&gt; Speed, to describe whose swiftness&#xA;  number fails.&lt;/em&gt; — viii. 15-38.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Milton's description of the Universe and its celestial bodies is described in detail in &lt;a href=&quot;http://www.gutenberg.org/files/28434/28434-h/28434-h.htm&quot;&gt;The Project Gutenberg EBook of The Astronomy of Milton's 'Paradise Lost'&lt;/a&gt;, by Thomas Orchard.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-08-01T03:30:21.390" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="904" PostTypeId="1" AcceptedAnswerId="909" CreationDate="2013-08-01T04:16:21.573" Score="10" ViewCount="1334" Body="&lt;p&gt;I was thinking, and those little plastic bags that hold the astronauts' food have to go &lt;em&gt;somewhere&lt;/em&gt;. Where do they go? Are they compacted into a big cube like on WALL-E, and sent out into space, or are they sent back down to earth on the next mission?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-08-01T08:41:09.503" Title="What happens to waste on the ISS?" Tags="|iss|debris|waste|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="905" PostTypeId="1" AcceptedAnswerId="906" CreationDate="2013-08-01T04:49:49.543" Score="26" ViewCount="7543" Body="&lt;p&gt;What is the largest number of human beings in space at the same time, and what vessels were they on when it happened?&lt;/p&gt;&#xA;" OwnerUserId="137" LastEditorUserId="7062" LastEditDate="2021-09-17T00:10:23.723" LastActivityDate="2022-01-03T16:53:48.933" Title="What is the largest number of humans in space at the same time and what vessels were they on when it happened?" Tags="|crewed-spaceflight|history|astronauts|record|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="906" PostTypeId="2" ParentId="905" CreationDate="2013-08-01T05:21:57.767" Score="22" Body="&lt;p&gt;The record appears to be 13. This has happened at least three times, once in 1995 and twice in 2009.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://hypertextbook.com/facts/2006/KieshaHarding.shtml&quot; rel=&quot;noreferrer&quot;&gt;March, 1995&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;There are many other interesting records related to human spaceflight. On March 14, 1995, a record number of thirteen people were in space at one time. Seven of them were Americans that were on the STS 67 Endeavour, three cosmonauts were on the Mir space station, and an astronaut from the United States, along with two cosmonauts, were aboard Soyuz TM21.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/6503-population-space-historic-high-13.html&quot; rel=&quot;noreferrer&quot;&gt;March, 2009&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Soyuz TMA-14: Three people aboard, including space tourist Charles Simonyi and the new Expedition 19 crew for the station which numbers two, a Russian and an American. Launched Thursday and will arrive at the station Saturday morning.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Space Shuttle Discovery: Seven people aboard, returning from the space station after delivering the last pair of U.S. solar wings to the orbiting laboratory, boosting it to full power during their STS-119 mission. The shuttle is due to land Saturday in Florida to end a 13-day spaceflight.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;International Space Station: Currently home to three astronauts, one each from the United States, Russia and Japan. Two will return home April 7 with Simonyi to end their Expedition 18 mission.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/7003-space-station-population-hits-record-high.html&quot; rel=&quot;noreferrer&quot;&gt;July, 2009&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The population boost came from the influx of seven newcomers aboard the space shuttle Endeavour, which docked Friday at 1:47 p.m. EDT (1747 GMT), and opened its hatch into the station a couple hours later.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;The crew of STS-127 is extremely happy to be here,&amp;quot; replied Endeavour's STS-127 commander Mark Polansky. &amp;quot;Thirteen is a pretty big number but it's going to be an outstanding visit for us.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Polansky came with pilot Doug Hurley, and mission specialists Chris Cassidy, Julie Payette, Tom Marshburn, Tim Kopra and Dave Wolf. Waiting onboard the station with Padalka were Roman Romanenko, Michael Barratt, Robert Thirsk, Frank De Winne and Koichi Wakata.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="80" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-01T05:21:57.767" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="907" PostTypeId="2" ParentId="881" CreationDate="2013-08-01T05:33:18.450" Score="33" Body="&lt;p&gt;There are two kinds of depressurization alarm:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ΔP/ΔT (rapid depressurization)&lt;/li&gt;&#xA;&lt;li&gt;ATM PRESS (slow leak, &lt;strong&gt;or&lt;/strong&gt; pressure hitting the thresholds)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Main steps that are taken:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Record incident and contact mission control&lt;/li&gt;&#xA;&lt;li&gt;Take pressure gauge&lt;/li&gt;&#xA;&lt;li&gt;Evacuate into the Soyuz&lt;/li&gt;&#xA;&lt;li&gt;Check if Soyuz descent module is leaking&lt;/li&gt;&#xA;&lt;li&gt;If time remaining (T.res) is less than X minutes, say &amp;quot;Bye-bye&amp;quot; to the station, prepare the Soyuz for flight and undocking. Contact Mission control&lt;/li&gt;&#xA;&lt;li&gt;If T.res is more than X minutes and the Descent module is leaking, return to the ISS, and deactivate the station orderly&lt;/li&gt;&#xA;&lt;li&gt;If T.res is more than X minutes and the Descent module is safe, return to the ISS, close vacuum valves through the laptop and &lt;strong&gt;pinpoint leak&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Isolate the leaking compartment&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Roughly speaking, X is the time to deactivate the station.&lt;/p&gt;&#xA;&lt;p&gt;How to spot the leak:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Use the ISS computer to find the leaking compartment&lt;/li&gt;&#xA;&lt;li&gt;If fails, use pressure gauge in each compartment with closed hatches to determine the culprit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;strong&gt;Any steps to patch leaks are taken only after consulting Mission Control&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;h1&gt;CREW SURVIVAL IS THE HIGHEST PRIORITY&lt;/h1&gt;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceref.com/iss/ops/iss.emergency.ops.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Space Station Emergency Operations&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceref.com/iss/ops/russian.segment.emergency.pdf&quot; rel=&quot;noreferrer&quot;&gt;Integrated Russian Segment Emergency Operations&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceref.com/iss/ops/emergency.procedures.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Space Station Complex Operations Emergency Procedures All Expedition Flights&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Note: if anybody knows where to find an up-to-date SODF please let me know.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-01T15:01:09.617" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="908" PostTypeId="2" ParentId="39" CreationDate="2013-08-01T06:38:49.923" Score="18" Body="&lt;p&gt;The largest US rockets currently in practical development are the Falcon Heavy (aka Falcon 9 Heavy) and the NASA SLS Block 1.&lt;/p&gt;&#xA;&lt;p&gt;NASA is likely to use the Falcon Heavy should SLS funding be cut; the SLS is subject to Congressional appropriations and executive budget decisions, neither of which could kill the Falcon project.&lt;/p&gt;&#xA;&lt;p&gt;According to the Wikipedia entry, the SLS Block I is slated to have a 70,000kg lift to LEO, and an estimated in-service date of 2017. The Block II is supposed to life 129,000kg - higher than the Saturn V's 118,000kg.&lt;/p&gt;&#xA;&lt;p&gt;The Falcon Heavy has a current design lift capacity of 53,000kg. and an estimated in service date of late 2014.&lt;/p&gt;&#xA;&lt;h1&gt;Discussion and Extrapolation&lt;/h1&gt;&#xA;&lt;p&gt;Elon Musk has noted that his goal is to get a colony off-Earth; he's also said the Falcon Heavy is not the endgame for SpaceX. It is likely that, in the event of a budget crisis canceling the SLS, SpaceX will upscale to a larger engine than the Merlin, but use the same basic pattern of design as the Falcon Heavy.&lt;/p&gt;&#xA;&lt;p&gt;It's already noted by SpaceX sources that the goal is reusable stages; the Falcon Heavy is slated to use reusable 1st stage cores once the vertical landing system is perfected.&lt;/p&gt;&#xA;&lt;p&gt;Given the rapid development pace of SpaceX, and the slower pace of SLS development, it's entirely possible that SpaceX will have a comparable lift option before the 2017 first service launch date slated for the SLS.&lt;/p&gt;&#xA;&lt;p&gt;In the interim, NASA may need to make use of the Falcon Heavy; 3 Falcon Heavy launches should cost less than 1 SLS launch, and have another 20,000kg total capacity, some of which will be lost to interconnection equipment, for deep space capability.&lt;/p&gt;&#xA;&lt;h1&gt;Bottom Line&lt;/h1&gt;&#xA;&lt;p&gt;&lt;strong&gt;NASA Plans to use the SLS; Political &amp;amp; Budget considerations may require them to use the Falcon Heavy.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_orbital_launchers_families&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Comparison_of_orbital_launchers_families&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h3&gt;SpaceX Falcon Heavy&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.buildtheenterprise.org/spacex-breaking-the-1000-per-pound-launch-cost-barrier&quot; rel=&quot;noreferrer&quot;&gt;http://www.buildtheenterprise.org/spacex-breaking-the-1000-per-pound-launch-cost-barrier&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.spacex.com/missions&quot; rel=&quot;noreferrer&quot;&gt;http://www.spacex.com/missions&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.spacex.com/falcon-heavy&quot; rel=&quot;noreferrer&quot;&gt;http://www.spacex.com/falcon-heavy&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Falcon_Heavy&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Falcon_Heavy&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h3&gt;SLS&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Space_Launch_System&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/audience/foreducators/rocketry/home/what-is-heavy-lift-launch-vehicle-k4.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/audience/foreducators/rocketry/home/what-is-heavy-lift-launch-vehicle-k4.html&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/exploration/systems/index.html#.Ufn5vWTwKzo&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/exploration/systems/index.html#.Ufn5vWTwKzo&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/exploration/systems/sls/index.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/exploration/systems/sls/index.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h3&gt;Elon Musk's Mars plans&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.forbes.com/sites/alexknapp/2012/11/27/spacex-billionaire-elon-musk-wants-a-martian-colony-of-80000-people/&quot; rel=&quot;noreferrer&quot;&gt;http://www.forbes.com/sites/alexknapp/2012/11/27/spacex-billionaire-elon-musk-wants-a-martian-colony-of-80000-people/&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.space.com/18596-mars-colony-spacex-elon-musk.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.space.com/18596-mars-colony-spacex-elon-musk.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-01T06:38:49.923" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="909" PostTypeId="2" ParentId="904" CreationDate="2013-08-01T08:41:09.503" Score="8" Body="&lt;p&gt;&lt;a href=&quot;http://www.spaceref.com/iss/ops/SSP50481.nonrec.cargo.plan.pdf&quot;&gt;This paper&lt;/a&gt; shows how trash and waste were planned to be handled on the ISS in the year 2000.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ISS Program shall provide accommodations and capabilities for the preflight, flight, on-orbit and return to earth of trash, waste, and non-recoverable&#xA;  materials in order to ensure the safety and health of both flight crew and ground&#xA;  operations personnel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If we look lower down at the Waste section of the paper we can see that waste consists of:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;chemicals, radioactive materials, batteries, sharps,&#xA;  and biologically/biomedically active products and consumables of no further use&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So this definitely covers the waste of which you specifically speak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This waste is handled by storing it onboard safely (away from public living areas) and then returning it to Earth at earliest convenience or by jettisoning it in orbit. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Standard stowage devices shall be made available for on-orbit and return based&#xA;  upon manifest approval of systems and payloads. A pantry of standard&#xA;  trash/waste stowage devices is maintained by JSC-OC/Logistics and&#xA;  Maintenance. The pantry of standard trash/waste stowage devices shall&#xA;  accommodate both planned and off-nominal operations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So as we can see from this, even if they aren't able to send it back to Earth they have the facilities on board to presumably chuck it into space to ensure the health of the crew.&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastActivityDate="2013-08-01T08:41:09.503" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="910" PostTypeId="2" ParentId="881" CreationDate="2013-08-01T09:15:22.920" Score="8" Body="&lt;p&gt;TsSKB Progress has released the unclassified 1979 educational film &lt;a href=&quot;http://www.youtube.com/watch?v=OKuhog3fbMc&quot; rel=&quot;nofollow noreferrer&quot;&gt;Razgermetizatsiya Stantsii&lt;/a&gt; (that is &lt;em&gt;Depressurization of the Station&lt;/em&gt;)&#xA;created for Salyut stations. The films shows mandatory procedures step by step.&lt;/p&gt;&#xA;&lt;p&gt;Of course the ISS is bigger and the crew can be larger than just the commander and the flight engineer, but it can be assumed that while the Soyuz is used as a lifeboat, the crew still follows a similar set of procedures.&lt;/p&gt;&#xA;&lt;p&gt;The cosmonauts have 5 minutes to perform necessary actions and get to Soyuz landing module.&lt;/p&gt;&#xA;&lt;p&gt;In the case of pressure going down in Soyuz after the hatch has been closed, the problem is in Soyuz itself. They need to get back to the station and isolate the station from the leaking spacecraft.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2022-02-08T10:20:17.020" LastActivityDate="2022-02-08T10:20:17.020" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="911" PostTypeId="1" AcceptedAnswerId="915" CreationDate="2013-08-01T10:47:50.923" Score="15" ViewCount="3706" Body="&lt;p&gt;Obviously a balance scale is not going to add value to the task in free-fall. Weight in space is more accurately described as mass, but the need to define it still exists. Regardless of if you're doing scientific experiments, mixing ingredients, or calculating mass to thrust values for delta-v, you need to know the mass or weight of things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how do you figure out the mass of an object, if you can't use a balance scale?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="603" LastEditDate="2014-10-31T17:21:10.280" LastActivityDate="2014-10-31T17:21:10.280" Title="How do you weigh things in a micro-gravity environment?" Tags="|microgravity|mass|measurement|" AnswerCount="6" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="912" PostTypeId="2" ParentId="911" CreationDate="2013-08-01T11:04:38.803" Score="0" Body="&lt;p&gt;You can use rotation to make a balance scale work.  Attach the object being weighed on one end of a rod attached to a pivot and a moveable counterweight to the other.  Only one position on the rod for the counterweight will allow the system to be balanced for any given test mass.&lt;/p&gt;&#xA;" OwnerUserId="157" LastActivityDate="2013-08-01T11:04:38.803" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="913" PostTypeId="2" ParentId="911" CreationDate="2013-08-01T11:08:43.953" Score="8" Body="&lt;p&gt;You can basically use all effects, that depend on the mass of an object. Spring pendulums, some magic with centrifugal forces, angular momentums ..... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For determining the mass of for instance a human body, see &lt;a href=&quot;http://www.lsda.jsc.nasa.gov/scripts/hardware/hardw.aspx?hardware_id=963&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Linear Acceleration Mass Measurement Device&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some really cool website by NASA, which provides some nice insight and an experiment, which demonstrates an option of how it can work: &lt;a href=&quot;http://quest.nasa.gov/space/teachers/microgravity/4inert.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://quest.nasa.gov/space/teachers/microgravity/4inert.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8ZPiE.jpg&quot; alt=&quot;SLAMMD&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="168" LastEditDate="2013-08-07T11:01:11.590" LastActivityDate="2013-08-07T11:01:11.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="914" PostTypeId="2" ParentId="911" CreationDate="2013-08-01T11:29:53.980" Score="0" Body="&lt;p&gt;The following is only an example. As ernestopheles already said, one could use any effect that depends on mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To measure the mass of small solid objects, I'd put a small sliding carriage between 2 springs of known properties. Place the mass in the carriage, deflect the carriage a bit and let it go. Measure the frequency of oscillation.&lt;/p&gt;&#xA;" OwnerUserId="402" LastActivityDate="2013-08-01T11:29:53.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="915" PostTypeId="2" ParentId="911" CreationDate="2013-08-01T11:31:54.327" Score="11" Body="&lt;p&gt;For pretty precise measurement you use linear acceleration of the body with fixed force (say, spring pulled until its force reaches nominal value) and then you measure its speed when launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kinetic energy $ E={{1}\over{2}}mv^2 $ will be equal to potential energy of the &quot;launcher&quot; (which can be easily calibrated by launching an object of known mass and measuring its speed using the same assembly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, given known energy and measurable speed we can calculate the mass:&#xA;$$ m={{2E}\over{v^2}} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, assume the launch assembly is $ 5 kg $.  You launch $5 kg$ weight out of the assembly and measure the distance it travels over the course of 1 second. It goes $ 10 m/s $; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ E= {{1}\over{2}}(5kg+5kg)(10m/s)^2 = 500J $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;that's the force of the assembly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We launch a human, and the result of measurement is $ 4 m/s $. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ m = {{2 \cdot 500J}\over{(4m/s)^2}} = 62.5kg $$&#xA; Substract the known $5 kg$ of assembly, and the human mass is $57.5 kg$.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2013-08-11T18:16:07.807" LastActivityDate="2013-08-11T18:16:07.807" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="916" PostTypeId="1" CreationDate="2013-08-01T11:36:20.067" Score="14" ViewCount="1066" Body="&lt;p&gt;Sun-synchronous orbits are popular around Earth, and the &lt;a href=&quot;http://mars.jpl.nasa.gov/mro/mission/timeline/mtscienceops/scienceopssciorbit/&quot;&gt;Mars Reconnaisance Orbiter&lt;/a&gt; uses one as well (so I think &lt;a href=&quot;http://en.wikipedia.org/w/index.php?title=Sun-synchronous_orbit&amp;amp;qoldid=564797014&quot;&gt;Wikipedias definition as a geocentric orbit is wrong&lt;/a&gt;).  Considering that probably any rotating body has some oblateness, is a sun-synchronous orbit always possible, or are there some fundamental limitations that make sun-synchronous orbits impossible around certain planets, minor planets, moons, or other bodies in the solar system?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(A heliocentric orbit is of course a special case)&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="14" LastEditDate="2013-08-01T12:27:24.770" LastActivityDate="2021-11-17T13:53:24.250" Title="Are sun-synchronous orbits possible around any body?" Tags="|orbital-mechanics|orbit|sun-synchronous|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="917" PostTypeId="2" ParentId="916" CreationDate="2013-08-01T11:56:10.913" Score="7" Body="&lt;p&gt;No. First, the matter of oblateness which introduces the necessary precession. In some cases it will force the sun-synchronous orbit altitude below the body's surface (obviously impossible). In other cases other bodies will disturb the orbital motion too strongly, destabilizing the orbit. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2018-01-28T15:50:30.130" LastActivityDate="2018-01-28T15:50:30.130" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="918" PostTypeId="1" AcceptedAnswerId="921" CreationDate="2013-08-01T14:15:49.137" Score="11" ViewCount="1071" Body="&lt;p&gt;From the &lt;a href=&quot;http://science1.nasa.gov/missions/tiros/&quot;&gt;NASA TIROS page&lt;/a&gt; on TIROS-1:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The craft was spin-stabilized and space-oriented (not Earth-oriented). Therefore, the cameras were only operated while they were pointing at the Earth when that portion of the Earth was in sunlight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What does it mean that the craft was &lt;em&gt;space-oriented (not Earth-oriented)&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://maia.usno.navy.mil/whatiseop.html&quot;&gt;US Navy describes Earth orientation&lt;/a&gt;, but this appears to describe an axis definition and the orientation of planet Earth, whereas the NASA TIROS page describes something related to the actual spacecraft orientation.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="12102" LastEditDate="2020-06-22T23:47:19.703" LastActivityDate="2020-06-22T23:47:19.703" Title="What is the difference between a &quot;space-oriented&quot; and an &quot;Earth-oriented&quot; satellite?" Tags="|terminology|tiros-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="919" PostTypeId="1" AcceptedAnswerId="920" CreationDate="2013-08-01T14:21:50.390" Score="10" ViewCount="402" Body="&lt;p&gt;Upon reading about the history of the &lt;a href=&quot;http://science1.nasa.gov/missions/tiros/&quot; rel=&quot;noreferrer&quot;&gt;TIROS satellite series&lt;/a&gt; and looking at some of the photographs taken by TIROS-1, I wonder what kind of orbit they were in.  Clearly they were polar or at least with a high inclination, but were they sun-synchronous?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where can I find such information?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/czT3E.jpg&quot; alt=&quot;TIROS-1 photograph&quot;&gt;&#xA;&lt;br /&gt;&lt;sup&gt;TIROS-1 photograph, 28 April 1960, 16:00 GMT&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="12102" LastEditDate="2020-06-22T23:47:35.957" LastActivityDate="2020-06-23T00:05:52.100" Title="Where can I find orbital parameters for historical satellites and other spacecraft?" Tags="|orbit|history|tiros-spacecraft|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="920" PostTypeId="2" ParentId="919" CreationDate="2013-08-01T14:32:23.500" Score="9" Body="&lt;p&gt;&lt;a href=&quot;http://www.planet4589.org/space/jsr/jsr.html&quot;&gt;Jonathan McDowell&lt;/a&gt; keeps a great launch record, as well as a satellite catalog. If you cannot find something there, you likely will not find it anywhere else online.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TIROS 1 appears to have been in a 690x754 km orbit at 48.4deg inclination (I think... Hard to read on mobile).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-01T14:32:23.500" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="921" PostTypeId="2" ParentId="918" CreationDate="2013-08-01T14:44:58.757" Score="9" Body="&lt;p&gt;What they mean in this context is that the orientation (attitude) of the satellite is fixed inertially (i.e. it points at a fixed point in space, such as a star, although it doesn't actually have to be pointed at any physical object). This is as opposed to an attitude that is Earth-referenced, i.e. maybe it points 15 degrees &quot;off-nadir&quot;, where nadir essentially means &quot;straight down&quot;.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-01T14:44:58.757" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="922" PostTypeId="1" CreationDate="2013-08-01T15:12:23.187" Score="7" ViewCount="673" Body="&lt;p&gt;I have heard that antimatter has been produced and collected in the Large Hadron Collider and that it reacts violently with matter. Could this be used as a method of propulsion and if so what would be the theoretical thrust and impulse generated?&lt;/p&gt;&#xA;" OwnerUserId="374" LastEditorDisplayName="user12" LastEditDate="2013-09-13T00:13:10.260" LastActivityDate="2015-12-17T22:52:37.113" Title="Could antimatter be used for spacecraft propulsion?" Tags="|propulsion|antimatter|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="923" PostTypeId="1" AcceptedAnswerId="1053" CreationDate="2013-08-01T15:13:02.447" Score="26" ViewCount="4499" Body="&lt;p&gt;Do all launches involve self-destruct mechanisms? How do they usually work? Is it just the boosters that are required to self-destruct? In the case of the space shuttle, how about the external tank?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are the mechanisms only manually triggerable?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="415" LastEditDate="2014-11-02T01:32:47.270" LastActivityDate="2014-11-02T01:32:47.270" Title="Do all launches include self-destruct mechanisms?" Tags="|launch|rockets|failure|range-safety|flight-termination-system|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="924" PostTypeId="1" AcceptedAnswerId="933" CreationDate="2013-08-01T16:02:28.867" Score="16" ViewCount="10482" Body="&lt;p&gt;The Apollo lunar module had thin skins of foil for protection / air containment. What would have happened if a hole was made in this skin by a flailing astronaut, screwdriver, asteroid, etc.? Would the tear have gotten larger and larger before it could be patched?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-02T00:47:40.110" LastActivityDate="2018-07-18T06:09:23.503" Title="What would have happened if the skin of an Apollo lunar module was ruptured?" Tags="|life-support|apollo-program|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="925" PostTypeId="1" AcceptedAnswerId="1737" CreationDate="2013-08-01T16:16:30.753" Score="30" ViewCount="4399" Body="&lt;p&gt;Miniaturisation has many advantages, and in recent times, nanosatellites, picosatellites, and femosatellite concepts have emerged.  The Wikipedia article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Miniaturized_satellite&quot;&gt;miniaturised satellite&lt;/a&gt; has some pointers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the smallest &lt;em&gt;operational&lt;/em&gt; artificial satellite that exists or has existed?  By &lt;em&gt;operational&lt;/em&gt;, I mean any satellite that performs or has performed a particular function, however modest, but at least communication with Earth (either through a relay, or directly to a ground station), thereby excluding orbital debris (that would also classify as artificial satellites).&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2019-07-26T10:21:52.363" Title="What is the smallest operational artificial satellite?" Tags="|artificial-satellite|record|" AnswerCount="6" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="926" PostTypeId="1" AcceptedAnswerId="927" CreationDate="2013-08-01T17:08:13.293" Score="29" ViewCount="940" Body="&lt;p&gt;A &lt;a href=&quot;http://ssi.org/2010-conference-space-manufacturing-14/proceedings/&quot; rel=&quot;noreferrer&quot;&gt;paper&lt;/a&gt; (&lt;a href=&quot;http://ssi.org/2010/SM14-proceedings/Active-Debris-Removal-EDDE-ElectroDynamic-Debris-Eliminator-Pearson-Carroll-%20Levin.pdf&quot; rel=&quot;noreferrer&quot;&gt;Active Debris Removal: EDDE&lt;/a&gt;) contained this diagram of orbital debris.  This is the angle seen by the spacecraft horizontally and altitude vertically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/S2xEp.png&quot; alt=&quot;Space Debris&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the reasons for the lines and clusters of objects on this scale?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at lines around 1000 km and 1500 km, I'm wondering if a part of it is arbitrary numerology.  That is, scientists picking 1000 km because it's a round number.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="25" LastEditDate="2013-08-01T19:41:44.753" LastActivityDate="2013-08-01T19:41:44.753" Title="Reason for space debris clustering in LEO" Tags="|low-earth-orbit|debris|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="927" PostTypeId="2" ParentId="926" CreationDate="2013-08-01T17:28:06.890" Score="26" Body="&lt;p&gt;The two bands near ~750km and ~975km (the more pronounced one) correspond with two large debris events: the &lt;a href=&quot;https://celestrak.org/events/asat.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fengyun 1C&lt;/a&gt; ASAT test occurred near -- but not quite at, although it's hard to tell from this plot -- the higher altitude (Gabbard plot &lt;a href=&quot;https://celestrak.org/events/FY1C-Gabbard.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (PDF warning)), and the &lt;a href=&quot;http://en.wikipedia.org/wiki/2009_satellite_collision&quot; rel=&quot;nofollow noreferrer&quot;&gt;Iridium Cosmos&lt;/a&gt; collision occured at 789 km.&lt;/p&gt;&#xA;&lt;p&gt;As for the higher band, I'm not sure, but some quick Googling produces &lt;a href=&quot;http://adsabs.harvard.edu/full/2005ESASP.587...79J&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; relatively recent paper (from 2005) describing unknown debris objects near 1400km.&lt;/p&gt;&#xA;&lt;p&gt;I'll add that &amp;quot;round numbers are nice&amp;quot; is likely not a reason for this, as orbit design is a relatively complex process, and mission planners rarely (if ever) enjoy the flexibility to just choose a number they like for altitude.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="-1" LastEditorDisplayName="user29" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2013-08-01T18:12:21.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="928" PostTypeId="2" ParentId="925" CreationDate="2013-08-01T17:50:18.767" Score="18" Body="&lt;p&gt;There are only very few projects, which are smaller than cubesats (1U @ 10x10x10cm). One of the key guys behind the original &lt;a href=&quot;http://www.cubesat.org/images/developers/cds_rev12.pdf&quot;&gt;cubesat standard&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Bob_Twiggs&quot;&gt;Bob Twiggs&lt;/a&gt;, is now &lt;a href=&quot;http://www.space.com/8838-tiny-satellites-big-science.html&quot;&gt;promoting &quot;pocket cubes&quot;&lt;/a&gt;. The &lt;a href=&quot;http://pocketqub.org/standard/&quot;&gt;new standard&lt;/a&gt; specifies satellites at a size of about 5x5x5cm, 1/8 of a cubesat in volume. A future launch including 'myPocketQub 442' is currently &lt;a href=&quot;http://amsat-uk.org/2013/07/18/ukube-1-launch/&quot;&gt;scheduled in October 2013&lt;/a&gt;, after some delays. Most projects appear to be related to amateur radio, see for example &lt;a href=&quot;http://www.amsatuk.me.uk/iaru/finished_detail.php?serialnum=188&quot;&gt;this description&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;There are currently no pocket cubes orbiting the Earth and there have not been any yet. This leaves &lt;strong&gt;1U cubesats&lt;/strong&gt; as the smallest known satellites in orbit at the present time.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-08-01T22:26:58.663" LastActivityDate="2013-08-01T22:26:58.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="929" PostTypeId="2" ParentId="925" CreationDate="2013-08-01T19:54:01.600" Score="6" Body="&lt;p&gt;The smallest satellites seem to be the Cubesat design, at 10cmx10cmx10cm. There are a number of these in orbit, perhaps around a dozen, with about the same number failing during the launch phase. For a list of them (And other amateur satellites) see &lt;a href=&quot;http://en.wikipedia.org/wiki/OSCAR&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-01T19:54:01.600" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="930" PostTypeId="1" AcceptedAnswerId="939" CreationDate="2013-08-02T03:46:16.647" Score="20" ViewCount="2276" Body="&lt;p&gt;Space junk, orbital debris, or space waste pose a risk on functional satellites and space laboratories / stations in orbit around Earth. According to Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Currently about 19,000 pieces of debris larger than 5 cm are tracked,&#xA;  with another 300,000 pieces smaller than 1 cm below 2000 km altitude.&#xA;  For comparison, ISS orbits in the 300–400 km range and both the 2009&#xA;  collision and 2007 antisat test events occurred at between 800–900 km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is a nuisance at best, a potential threat to working equipment, extravehicular activities of astronauts working on or repairing satellites, telescopes, space stations, e.t.c., even space stations themselves. They serve absolutely no purpose, so my question is, &lt;strong&gt;are there any safety related, environmental, or political reservations&lt;/strong&gt; to prevent us from simply deorbiting larger pieces of such defunct debris into Earth's atmosphere and let them burn up upon reentry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is assuming, that we do have means of deorbiting larger orbital debris, and is not a question of technical, or economical feasibility. Let's, for the sake of argument, assume we can build and launch into orbit above targeted debris a railgun, that is shooting high-precision soft body projectiles at these larger debris that we're tracking, and knock them off orbit into the atmosphere, where they hopefully burn on re-entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What, besides financing and technical challenges, is stopping us?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I'm editing to re-emphasise the request to describe the non-technical and non-economical nature, or challenges such deorbiting of space junk might come with. We already have a good answer on &lt;a href=&quot;https://space.stackexchange.com/a/143/49&quot;&gt;which methods have been proposed to do so, at what stage of development they are, how much they'd cost, e.t.c.&lt;/a&gt;. I'm explicitly seeking more information on possible safety, environmental, or political reservations, maybe even legal? For example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How hazardous materials are we talking of?&lt;/li&gt;&#xA;&lt;li&gt;Can we expect all of such materials to burn up completely in the atmosphere on re-entry?&lt;/li&gt;&#xA;&lt;li&gt;Are there any legal repercussions we should consider, such as this junk being a property of some foreign body?&lt;/li&gt;&#xA;&lt;li&gt;What political challenges we need to address, has this been discussed by leaders of the Earth in any detail before, e.t.c.?&lt;/li&gt;&#xA;&lt;li&gt;How much threat there is any attempt at deorbiting debris will only end up with them orbiting at even less convenient altitudes concerning safety of future launches?&lt;/li&gt;&#xA;&lt;li&gt;e.t.c.?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-07-24T16:32:50.713" Title="Any possible setbacks in deorbiting larger space junk and let it burn up upon reentry into atmosphere?" Tags="|orbit|debris|safety|feasibility|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="931" PostTypeId="2" ParentId="930" CreationDate="2013-08-02T06:15:14.080" Score="2" Body="&lt;p&gt;The big challenges are financial and technical. Above a certain size big space junk could land on people, but I think that's more or less the only danger. About a hundred tons of space dust burns up in the atmosphere every day(most of it microscopic, so estimates vary widely) so it would be safe to assume the addition of a few more tons of satellites wouldn't impact upper atmosphere chemistry too heavily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently one of the challenges in satellite senescence is that it is much cheaper(financially, in rocket fuel, timewise) to park a dead satellite in a 'graveyard orbit' than it is to pull it down into a decaying orbit.&lt;/p&gt;&#xA;" OwnerUserId="319" LastActivityDate="2013-08-02T06:15:14.080" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="932" PostTypeId="2" ParentId="680" CreationDate="2013-08-02T06:34:29.103" Score="1" Body="&lt;p&gt;You do get complex organics formed in dust clouds and comet tails and so on. Not enzymes, that would really be a big deal(on a level with finding a stainless steel 3/8th inch socket wrench in a comet), but nucleotides and amino acids and some even simple sugars are definitely out there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's one hypothesis that most of the precursor organic molecules needed to get life kickstarted just fell to earth over time, having formed out in space. And it's not implausible, you get all kinds of weird chemistry in interstellar dust. When there are only three carbon atoms, you get all kinds of strange ions and bonds that aren't stable if there are other things around to react with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Life in a way we would recognize it cannot exist in the vacuum of space. It can survive(often for a surprisingly long time, especially inside rocks and things), but it can't be life in a way we recognize. Life is probably possible in space, but if it's there, it doesn't look like anything we know about or could test for. Checking solar sails for cells is like checking trees for elephant poop. It's a way of looking for life, but even if it's there, you're not going to find it that way.&lt;/p&gt;&#xA;" OwnerUserId="319" LastActivityDate="2013-08-02T06:34:29.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="933" PostTypeId="2" ParentId="924" CreationDate="2013-08-02T09:57:34.007" Score="26" Body="&lt;p&gt;A hole in the LEM during lunar operations would likely have been grounds to abort, and return to the CSM. For the flight up, it's known that duct tape was carried in the LEM; Apollo 17 made use of it repairing their rover, and proved it's ability to hold in high quality vacuum. It's thus very likely that they'd have patched a single small hole or pair of small holes with duct tape from the inside, and checked for pressure. It's almost certain such a repair would still have them make the trip in suit, ready to button up immediately.&lt;/p&gt;&#xA;&lt;p&gt;A hole in the CSM might be more of an issue, since large portions of the pressure vessel are inaccessible during flight. If such an inacessible hole happened, it's likely the crew would have gone to suits, on umbilicals, and taken the very next opportunity to aim for Earth. An accessible hole would have been patched with duct tape or better materials.&lt;/p&gt;&#xA;&lt;p&gt;Further note: the Apollo astronauts had a suit repair kit; it would likely have been used in the case of a breach.&lt;/p&gt;&#xA;&lt;h1&gt;Discussion&lt;/h1&gt;&#xA;&lt;p&gt;Keep in mind: the wall thickness was 0.012 inches (≅0.305mm); this is about 3.8x the thickness of a typical soda can (≤0.1mm), and those are not easily holed (it tends to deform rather than puncture). Further, the internal pressure makes the skin rigid, just like a soda can; a can of 7-up is 30psi, and a standard atmosphere about 14.7 - so a soda can is thick enough to hold in 1 atmosphere. The walls of the LEM are three to four times thicker... thus harder to pierce, and less likely to tear if punctured.&lt;/p&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2008/21apr_ducttape/&quot; rel=&quot;noreferrer&quot;&gt;Moondust and Duct Tape (http://science1.nasa.gov/science-news/science-at-nasa/2008/21apr_ducttape/).&lt;/a&gt; Retrieved 2 Aug 2013.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.today.com/id/31923804/ns/today-today_travel/t/events-exhibits-years-after-man-moon/#.Uft6EGTwKzo&quot; rel=&quot;noreferrer&quot;&gt;Events, exhibits 40 years after man on moon (http://www.today.com/id/31923804/ns/today-today_travel/t/events-exhibits-years-after-man-moon/#.Uft6EGTwKzo)&lt;/a&gt; Retrieved 2 Aug 2013.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://h2g2.com/approved_entry/A3770192&quot; rel=&quot;noreferrer&quot;&gt;The Saturn/Apollo Stack - Lunar Module (http://h2g2.com/approved_entry/A3770192)&lt;/a&gt; Retrieved 2 Aug 2013.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.canmaker.com/news/index.php?option=com_content&amp;amp;view=article&amp;amp;id=1504&amp;amp;Itemid=126&quot; rel=&quot;noreferrer&quot;&gt;The Canmaker Frequently Asked Questions (http://www.canmaker.com/news/index.php?option=com_content&amp;amp;view=article&amp;amp;id=1504&amp;amp;Itemid=126)&lt;/a&gt; Retrieved 2 Aug 2013.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://hypertextbook.com/facts/2000/SeemaMeraj.shtml&quot; rel=&quot;noreferrer&quot;&gt;Pressure in a Can of Soda (http://hypertextbook.com/facts/2000/SeemaMeraj.shtml)&lt;/a&gt; Retrieved 2 Aug 2013.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-02T09:57:34.007" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="934" PostTypeId="2" ParentId="930" CreationDate="2013-08-02T10:20:04.413" Score="4" Body="&lt;p&gt;The upper atmosphere is a sensitive environment.  Unlike compounds released in the troposphere, compounds that end up in the stratosphere or mesosphere have a long lifetime, so stuff accumulates there.  This issue has not been adressed very much, but unlike @JeremyKemball's answer, I do not think it is safe to assume that none of it matters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, the true reason that it's not being done routinely are not environmental.  A lot of things &lt;em&gt;are&lt;/em&gt; being deorbited on purpose, and so far the impact on the upper atmosphere has never been a reason to look for different solutions.  So, possible environmental setbacks — yes.  But what's stopping us in practice are not the possible environmental setbacks — rather, the technical and financial challenges are.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2013-08-02T10:20:04.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="935" PostTypeId="1" AcceptedAnswerId="936" CreationDate="2013-08-02T10:31:37.370" Score="11" ViewCount="1222" Body="&lt;p&gt;We know from &lt;a href=&quot;https://space.stackexchange.com/questions/869/flying-on-dense-atmosphere-planets-moons&quot;&gt;Flying on dense atmosphere planets &amp;amp; moons&lt;/a&gt; that many bodies in our solar system have sufficient atmosphere for at least the concept of flying to be considered.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about the beauty of a rainbow, the sun reflecting through drops of water hanging in the air, the magical way they move as you do.  Would moving to another planet or moon in our solar system mean the end of seeing rainbows? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-05-18T15:46:59.663" Title="Can you have a rainbow on any bodies in the solar system besides Earth?" Tags="|planetary-science|atmosphere|solar-system|optical|physics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="936" PostTypeId="2" ParentId="935" CreationDate="2013-08-02T11:39:58.517" Score="13" Body="&lt;p&gt;There is a possibility of rainbows appearing naturally on other celestial bodies that have sufficient water vapor or a large number of water droplets in their atmospheres, or indeed gases or droplets of other molecules that are capable of acting upon the light from the Sun (or other sources) as tiny prisms and refract it to its visible components (different wavelengths). Another possibility is also mineral dust scattered in the atmosphere, or indeed in planetary rings:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/gtAUs.png&quot; alt=&quot;Refraction of transparent minerals&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Scatter plot of refraction of various transparent minerals and their density (source: &lt;a href=&quot;http://physics.info/refraction/&quot; rel=&quot;noreferrer&quot;&gt;physics.info&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem is, we don't really know which celestial bodies in our Solar system might that be. Mars has possibly too thin atmosphere and not enough water vapor (210 ppm) for any such effect to be really noticeable, and we don't know enough about atmospheric conditions of other planets and their moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are other possibilities. For example Titan, one of Saturn's moons, has liquid methane (refraction index of 1.29):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/FwCIH.jpg&quot; alt=&quot;Titan&amp;#39;s Many Layers&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Titan's layers of atmosphere in the UV + visible spectrum, image from the Cassini–Huygens spacecraft (source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Titan&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even stronger case for possible rainbow effect forming on the surface of Titan comes from Cassini and Huygens observations, indicating that Titan might have a pretty complex weather system and precipitation in the form of liquid methane rain: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Conditions in Titan's troposphere are near the triple point of&#xA;  methane, the second most abundant component of its atmosphere. Our&#xA;  understanding of Titan's lower atmosphere has shifted considerably in&#xA;  the past decade. Ground-based observations, Hubble Space Telescope&#xA;  images, and data returned from the Cassini and Huygens spacecraft show&#xA;  that Titan's troposphere hosts a methane-based meteorology in direct&#xA;  analogy to the water-based meteorology of Earth. What once was thought&#xA;  to be a quiescent place, lacking in clouds or localized weather and&#xA;  changing only subtly on long seasonal timescales, is now understood to&#xA;  be a dynamic system with significant weather events regularly&#xA;  occurring against the backdrop of dramatic seasonal changes. Although&#xA;  the observational record of Titan's weather covers only a third of its&#xA;  30-year seasonal cycle, Titan's atmospheric processes appear to be&#xA;  more closely analogous to those of Earth than to those of any other&#xA;  object in our solar system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(Source: &lt;a href=&quot;http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-040809-152548&quot; rel=&quot;noreferrer&quot;&gt;Titan's Methane Weather, Annual Review of Earth and Planetary Sciences&lt;/a&gt;, Lowell Observatory, May 2012)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: In July 2011 a &lt;em&gt;glory&lt;/em&gt;, a rainbow-like feature has been observed for the first time on another planet by &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Venus_glory&quot; rel=&quot;noreferrer&quot;&gt;ESA's Venus Express&lt;/a&gt; probe:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The atmosphere of Venus is thought to contain droplets rich in&#xA;  sulphuric acid. By imaging the clouds with the Sun directly behind the&#xA;  Venus Express spacecraft, scientists hoped to spot a glory in order to&#xA;  determine important characteristics of the cloud droplets.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;They were successful. The glory in the images here was seen at the&#xA;  Venus cloud tops, 70 km above the planet’s surface, on 24 July 2011.&#xA;  It is 1200 km wide as seen from the spacecraft, 6000 km away.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/nm2Wd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;This simulated view shows how a glory would appear on Venus (left) versus Earth (right). (Source: &lt;a href=&quot;http://www.nature.com/news/why-the-venus-rainbow-is-actually-a-glory-1.14869&quot; rel=&quot;noreferrer&quot;&gt;Nature.com&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's beyond me why this took nearly 3 years to make its public debut in April 2014, but here we have it, a rainbow on another celestial body than Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KO57F.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;An optical effect called a glory seen by the Venus Express probe from an altitude of 6,000 kilometres. The false-colour image&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;is composed of images at ultraviolet, visible, and near-infrared wavelengths taken ten seconds apart, so that they do not&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;overlap perfectly. (Caption credit: &lt;a href=&quot;http://www.nature.com/news/why-the-venus-rainbow-is-actually-a-glory-1.14869&quot; rel=&quot;noreferrer&quot;&gt;Nature.com&lt;/a&gt;, Image source: &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Venus_glory&quot; rel=&quot;noreferrer&quot;&gt;ESA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Discovery of this glory isn't merely a pretty sight, but gives Cytherean atmospheric physicists clues about its composition, among other things. For example, &lt;a href=&quot;http://www.newscientist.com/article/dn25203-first-alien-rainbow-image-holds-clues-to-venus-mystery.html&quot; rel=&quot;noreferrer&quot;&gt;New Scientist article&lt;/a&gt;, referencing &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S001910351400061X&quot; rel=&quot;noreferrer&quot;&gt;Icarus, DOI: 10.1016/j.icarus.2014.01.030&lt;/a&gt; (Markiewicz et al.), puts it like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The team found that sulphuric acid droplets alone&#xA;  cannot explain the glory, but droplets coated with elemental sulphur&#xA;  or mixed in with ferric chloride fit the data well.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;&quot;This could be the so-called unknown absorber that people had been trying to identify for years,&quot; Markiewicz says. &quot;We cannot say for&#xA;  sure, but we can say that this is one more piece of the puzzle for the&#xA;  whole thing.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-05-18T15:44:44.247" LastActivityDate="2014-05-18T15:44:44.247" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="937" PostTypeId="1" AcceptedAnswerId="27987" CreationDate="2013-08-02T11:41:27.003" Score="17" ViewCount="1614" Body="&lt;p&gt;Earth observation from space occurs throughout the electromagnetic spectrum.  With my meteorological bias, I until recently believed it wasn't at frequencies higher than UV, but &lt;a href=&quot;http://wwwpem.space.swri.edu/axis.html&quot;&gt;the X-Ray detector AXIS proves me wrong&lt;/a&gt;.  What is the highest frequency instrument that is &lt;em&gt;dedicated&lt;/em&gt; to Earth observation from space?  The &lt;a href=&quot;http://currents.ucsc.edu/04-05/02-21/flashes.asp&quot;&gt;RHESSI spacecraft apparently measured γ-rays from Earth&lt;/a&gt;, but rather as a side-effect to observing γ-rays from the Sun.  AXIS measures up to 300 KeV (12 picometre, 25 exahertz).  Is this the highest frequency for any dedicated Earth observation from space, or are there even higher-frequency sensors?  I'm interested in either historical instrument, current instruments, or instruments confirmed to be launched soon.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="48" LastEditDate="2013-08-02T12:25:31.377" LastActivityDate="2018-06-20T17:29:36.067" Title="What is the highest frequency at which Earth is, or has been, observed from space?" Tags="|earth-observation|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="938" PostTypeId="2" ParentId="446" CreationDate="2013-08-02T14:02:06.097" Score="8" Body="&lt;p&gt;Russia has been conducting a series of experiments since 1967 with the most recent one being &lt;a href=&quot;http://en.wikipedia.org/wiki/MARS-500&quot;&gt;Mars-500&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before that there were &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;1967 a year long experiment of 3 people in 12 square meters hermocapsule, imitating mars spaceship designed by Korolyov. There is a documentary (in Russian) &lt;a href=&quot;http://www.youtube.com/watch?v=tqmi9v4E3P8&quot;&gt;&lt;em&gt;See Mars... without Going Crazy&lt;/em&gt;&lt;/a&gt; available on youtube.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1972 &lt;a href=&quot;http://en.wikipedia.org/wiki/BIOS-3&quot;&gt;&lt;em&gt;BIOS-3&lt;/em&gt;&lt;/a&gt; closed ecosystem experiment with crews of 1-3 people.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.imbp.ru/WebPages/WIN1251/Science/Stend/GEK.html&quot;&gt;Ground Experimental Complex&lt;/a&gt; has been used for the following experiments by the Russian Academy of Sciences Institute of Biomedical Problems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;1976-1977 a 120 day experiment. Group dynamics and interaction with a visiting crew.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1980 25 days. The program studied interaction of a male only crew with a female-only visiting crew.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1983-1984 60 days study of mixed gender group dynamics, and 2 other 90 days experiments.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1987 a 90 days experiment with several emergency simulation periods, a week long each.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1989 a 30 days experiment, reduced levels of oxygen.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1994-1995 &lt;a href=&quot;http://www.esa.int/For_Media/Press_Releases/The_HUBES_experiment_a_ground-based_simulation_of_a_135-day_manned_spaceflight&quot;&gt;Human Behavior in Extended Spaceflight (&lt;em&gt;HUBES-94&lt;/em&gt;)&lt;/a&gt; as part of EuroMir-95 collaboration.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1995-1996 &lt;em&gt;ECOPSY-95&lt;/em&gt; (aka &lt;em&gt;EKOPSI-95&lt;/em&gt;) a 90 days experiment&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1999-2000 &lt;a href=&quot;http://www.sisc.org/Space/IBMP/engl/sfincss_e.html&quot;&gt;&lt;em&gt;SFINCSS-99&lt;/em&gt;&lt;/a&gt; a 240 days experiment. That one resulted in a &lt;a href=&quot;http://www.jamesoberg.com/04142000assualt_rus.html&quot;&gt;crew member defection, fisticuffs and a sexual assault&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;2007-2011 &lt;em&gt;Mars-500&lt;/em&gt; included 14, 105 and 520 days experiments with 6 people crews.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some results of HUBES-94, ECOPSY-95, SFINCSS-99 and MARS-500 experiments &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/?term=SFINCSS&quot;&gt;can be found in PubMed database&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-02T14:25:53.460" LastActivityDate="2013-08-02T14:25:53.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="939" PostTypeId="2" ParentId="930" CreationDate="2013-08-02T14:16:05.570" Score="13" Body="&lt;p&gt;Reentry &lt;em&gt;can&lt;/em&gt; be hazardous, for several reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The debris might not burn up completely. Whether a given object will be consumed in the upper atmosphere depends a lot on its materials, shape, velocity, reentry trajectory, and even direction (to take an extreme example, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Gemini&quot;&gt;Gemini spacecraft&lt;/a&gt; had a heat shield on one side, so that the way the craft was pointed made all the difference between a safe landing and astronaut BBQ). A non-burned debris can make some damage on land. It seems that a debris from a failed Thor rocket launch on November 30th, 1960, killed a cow in Cuba (referenced from &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Thor_and_Delta_launches_%281960%E2%80%9369%29&quot;&gt;here&lt;/a&gt;, but the link appears to be dead); granted, this was not a debris &quot;from orbit&quot; but this still highlights the issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a more recent example: in late 2011, a &lt;a href=&quot;http://www.dailymail.co.uk/news/article-2077708/Metallic-spaceball-Metal-sphere-fell-Earth-sparks-alien-speculation.html&quot;&gt;metallic, obviously man-made object&lt;/a&gt; fell in a field in Namibia, most probably from orbit. It seems to have been an hydrazine tank from an old satellite, which survived re-entry.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;em&gt;If&lt;/em&gt; the debris burns up, then what the debris was made of becomes spread out in upper atmosphere. This can be problematic, especially if the debris included a &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;RTG&lt;/a&gt;. People, generally speaking, are not very keen on the idea of spreading out radioactive material over their head; indeed, this was what really prompted the ban on atmospheric nuclear tests.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A country who develops a way to drop debris on Earth is a country who can drop &quot;debris&quot; on &quot;Earth&quot;, where &quot;debris&quot; can be &quot;heavy pieces of hard materials with a lot of kinetic energy&quot; and &quot;Earth&quot; can be more specific, like &quot;the Kremlin&quot;. That's not necessarily the kind of technology that we want to be seen deployed and employed. Diplomatic consequences are probable.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A more important point, however, might be that &lt;strong&gt;large debris are not a big issue&lt;/strong&gt; because their position and trajectory is known and predicted. The really scary debris are the one in the 1 cm size range, because at orbital speed they have enough kinetic energy to destroy a satellite or spacecraft, and yet they are small enough to evade detection. And there are a lot of them; a lot more than large debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If financial and technical aspects are &quot;not a problem&quot; then instead of dropping the debris, we should push them outside of Earth orbit, and then drop them into the Sun. In the long term, that would be much safer.&lt;/p&gt;&#xA;" OwnerUserId="311" LastActivityDate="2013-08-02T14:16:05.570" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="940" PostTypeId="1" AcceptedAnswerId="1198" CreationDate="2013-08-02T15:25:28.217" Score="22" ViewCount="1412" Body="&lt;p&gt;CubeSats are small, but so are some propulsion systems (cold gas thrusters, for instance, really aren't too complex and can be low volume/mass). Has any CubeSat actually &lt;strong&gt;flown&lt;/strong&gt; with its own propulsion system? If so, what were the results?&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2015-08-04T06:29:31.923" Title="Has any CubeSat flown with an active propulsion system?" Tags="|propulsion|technology|cubesat|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="941" PostTypeId="1" AcceptedAnswerId="945" CreationDate="2013-08-02T17:01:43.380" Score="20" ViewCount="555" Body="&lt;p&gt;What density of satellites in LEO is permissible before a single collision has a reasonable chance of triggering the catastrophic destruction of everything in orbit?  How many orders of magnitude are we below this level presently--is this a practical concern that limits our ability to utilize space in the long run?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The reason a single collision could result in near-complete destruction is that when satellites collide, or are badly damaged, they release many pieces of debris that may badly damage other satellites and cause further collisions.  If the probability of collision with debris (before deorbiting) times the number of pieces of debris produced by collision is greater than one you end up in a supercritical regime as with viral pandemics and nuclear meltdowns/explosions.)&lt;/p&gt;&#xA;" OwnerUserId="407" LastEditorUserId="407" LastEditDate="2013-08-03T22:34:00.467" LastActivityDate="2013-08-03T22:34:00.467" Title="Criticality in space junk" Tags="|low-earth-orbit|debris|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="942" PostTypeId="1" AcceptedAnswerId="944" CreationDate="2013-08-02T17:10:11.667" Score="28" ViewCount="1062" Body="&lt;p&gt;My understanding is that probes within the Solar system primarily rely on Earth-based tracking networks and ephemeris data to navigate. Once they get really close to their destinations, they can optionally also use visual tracking to position themselves. But how would spacecraft, e.g., those on extrasolar voyages, navigate without Earth being in range to spoonfeed them positional information? &quot;Old school&quot; star navigation?&lt;/p&gt;&#xA;" OwnerUserId="80" LastActivityDate="2021-07-20T18:21:06.263" Title="How can spacecraft navigate without contact from Earth?" Tags="|navigation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="944" PostTypeId="2" ParentId="942" CreationDate="2013-08-02T17:41:29.770" Score="21" Body="&lt;p&gt;There are two problems here:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Where am I (and what is my velocity vector in some inertial frame of interest)? (&lt;strong&gt;Navigation&lt;/strong&gt;) (&lt;em&gt;that's what I suppose you are asking&lt;/em&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Am I flying head over heels or vice versa (and how fast am I rotating)? (&lt;strong&gt;Attitude determination&lt;/strong&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For &lt;strong&gt;navigation&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The prevailing mode of thinking considers X-ray &lt;a href=&quot;https://en.wikipedia.org/wiki/Pulsar&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;pulsars&lt;/strong&gt;&lt;/a&gt; to be the principal avenue of advance. This is called XNAV (x-ray pulsar-based navigation and time determination).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The underlying principle here is the stability of millisecond pulsars emitting in the X-ray spectrum. A combination of pulse counts from known MSPs is transformed into pseudo-ranges and solved in a filter like any other TDOA (multilateration) problem. Please note that it is more difficult and time-consuming than navigation using GPS, where pseudo-random sequences transmitted by satellites make it easy to disambiguate the position (pulsars emit the same pulses all over).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NICER: A battery of X-ray detectors planned for installation on the ISS&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/D4RMd.jpg&quot; alt=&quot;An artist&amp;#39;s rendition of NICER&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Credit: NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;2012 - Pulsar Navigation and X-ray Communication Demonstrations with the &lt;a href=&quot;https://en.wikipedia.org/wiki/NICER&quot; rel=&quot;noreferrer&quot;&gt;NICER&lt;/a&gt; Payload on the ISS &lt;a href=&quot;http://hdl.handle.net/2060/20120016975&quot; rel=&quot;noreferrer&quot;&gt;http://hdl.handle.net/2060/20120016975&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;1981 - Navigation using X-ray pulsars NTRS Doc.ID 19810018591 Chester, T. J., Butman, S. A.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Approximately one dozen X-ray pulsars are presently known which emit strong stable pulses with periods of 0.7 to approximately 1000 s. By comparing the arrival times of these pulses at a spacecraft and at the Earth (via an Earth orbiting satellite), a three dimensional position of the spacecraft can be determined. One day of data from a small onboard X-ray detector yields a three dimensional position accurate to approximately 150 km. This accuracy is independent of spacecraft distance from the Earth. Present techniques for determining the two spacecraft coordinates other than range measure angles and thus degrade with increasing spacecraft range. Thus navigation using X-ray pulsars will always be superior to present techniques in measuring these two coordinates for sufficiently distant spacecraft. At present, the break even point occurs near the orbit of Jupiter. The Crab pulsar can also be used to obtain one transverse coordinate with an accuracy of approximately 20 km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For &lt;strong&gt;attitude determination&lt;/strong&gt;, star trackers are and will be enough in the foreseeable future.&lt;/p&gt;&#xA;&#xA;&lt;hr/&gt;&#xA;&#xA;&lt;p&gt;Related multimedia:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.youtube.com/watch?v=LXwugoQ6GL0&quot; rel=&quot;noreferrer&quot;&gt;GSFC video on SEXTANT/NICER&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-03T07:40:30.923" LastActivityDate="2013-08-03T07:40:30.923" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="945" PostTypeId="2" ParentId="941" CreationDate="2013-08-02T17:47:55.900" Score="19" Body="&lt;p&gt;What you're describing was popularized some years ago by Donald Kessler at NASA. It has since been termed &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Kessler_syndrome&quot;&gt;Kessler Syndrome&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, a collision could feasibly set off a cascading chain reaction of collisions. Note that the time scales for this aren't necessarily on the order of minutes or even hours... collisions may just start happening at an ever increasing rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to what that density actually is, there is no hard number. Depending on which study you read, we may even be at this critical point now. &lt;a href=&quot;http://webpages.charter.net/dkessler/files/Kessler%20Syndrome-AAS%20Paper.pdf&quot;&gt;Here's&lt;/a&gt; a recent one by the man himself (PDF Warning).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-02T17:55:12.663" LastActivityDate="2013-08-02T17:55:12.663" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="946" PostTypeId="1" CreationDate="2013-08-02T18:57:45.543" Score="-6" ViewCount="253" Body="&lt;p&gt;Studies have shown that binary star systems doom any surrounding planets.&#xA;&lt;a href=&quot;http://www.universetoday.com/71934/tight-binaries-are-death-stars-for-planets/&quot; rel=&quot;nofollow&quot;&gt;http://www.universetoday.com/71934/tight-binaries-are-death-stars-for-planets/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean that we will never be able to inhabit a Tatooine like planet or are there exceptions to this rule in which a habitable planet can exist with two suns. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the difference on flight through that solar system be. For example if you were launching Saturn V from that planet how would the affects on it from the binary starts be different than from a single star.&lt;/p&gt;&#xA;" OwnerUserId="374" LastEditorUserId="14" LastEditDate="2013-08-04T17:20:15.430" LastActivityDate="2013-08-04T17:20:15.430" Title="Dreams of Tatooine doomed" Tags="|orbital-mechanics|colonization|" AnswerCount="1" CommentCount="2" ClosedDate="2013-08-03T01:04:00.227" ContentLicense="CC BY-SA 3.0" />
  <row Id="947" PostTypeId="2" ParentId="946" CreationDate="2013-08-02T19:10:20.600" Score="4" Body="&lt;p&gt;The Universe Today article is out of date. Since it's 2010 publication, NASA has in fact found planetary candidates in binary systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following links are more current infographics:&lt;br&gt;&#xA;&lt;a href=&quot;http://phys.org/news/2013-03-capture-picture-tatooine-planet-orbiting.html&quot; rel=&quot;nofollow&quot;&gt;http://phys.org/news/2013-03-capture-picture-tatooine-planet-orbiting.html&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.space.com/17348-tatooine-alien-planets-two-suns-kepler-74-infographic.html&quot; rel=&quot;nofollow&quot;&gt;http://www.space.com/17348-tatooine-alien-planets-two-suns-kepler-74-infographic.html&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://arstechnica.com/science/2013/01/binary-star-systems-make-for-unstable-planets/&quot; rel=&quot;nofollow&quot;&gt;http://arstechnica.com/science/2013/01/binary-star-systems-make-for-unstable-planets/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is worth noting for habitability purposes that binary systems with planets can be grouped into 3 broad categories: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Close Binary Pair - the stars are close enough for planets to orbit them jointly.&lt;/li&gt;&#xA;&lt;li&gt;Near Pairs - the pair are close enough to disrupt planets orbiting one or the other, and far enough apart to preclude habitable zone orbits.&lt;/li&gt;&#xA;&lt;li&gt;Far Pairs - The pair is far enough away (≥1000 AU) to not interfere with habitable zone worlds orbiting a single star within the pair.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also - far binary companions will be pretty much just a very bright star, much as Sol is from Pluto's aphelion.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-02T19:10:20.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="948" PostTypeId="1" CreationDate="2013-08-02T19:40:15.040" Score="28" ViewCount="2274" Body="&lt;p&gt;What happens to defunct lunar and Mars rovers? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zeoId.jpg&quot; alt=&quot;snow covered rover&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What technological barriers prevent humanity from creating a perpetual rover with sun batteries or nuclear portable reactors for use on Mars, the Moon, etc., with daily visual transmissions and scientific data transmissions forever?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If radiation was the main issue, does humanity now have the technology to overcome this? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, &lt;a href=&quot;http://www.dvice.com/archives/2010/07/unmanned-solar-1.php&quot; rel=&quot;noreferrer&quot;&gt;this device named Zephyr&lt;/a&gt; can, in theory, fly forever in the Earth's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;                                                                &lt;/p&gt;&#xA;" OwnerUserId="408" LastEditorUserId="1108" LastEditDate="2013-11-27T01:54:08.733" LastActivityDate="2022-05-20T19:00:21.557" Title="What happens to defunct lunar and Mars rovers? What technological barriers prevent humanity from creating a perpetual rover?" Tags="|mars|the-moon|rovers|technology|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="949" PostTypeId="2" ParentId="948" CreationDate="2013-08-02T20:22:45.177" Score="30" Body="&lt;h2&gt;Temperature shifts&lt;/h2&gt;&#xA;&lt;p&gt;Earth's temperature swings from extreme to extreme is quite moderate.  That is, coldest is generally around -40C and warmest is around 40-45C.&lt;/p&gt;&#xA;&lt;p&gt;The Moon has two weeks in blazing hot sunlight, much hotter (depending on your reflectance) but well over 100C, and two weeks of much much colder.  Not quite absolute zero (-273C), but pretty cold to around -173C.&lt;/p&gt;&#xA;&lt;p&gt;That is a 273 degree Celsius difference.  Versus, worst case of 80C on Earth.  Things expand and contract a lot more over 273 vs 80.  So designing circuitry that can handle the extremes is tough.  Most commercial circuit boards will crack due to thermal expansion alone.  Welds/solder are very tough to maintain across these temperature variations.&lt;/p&gt;&#xA;&lt;p&gt;Most rovers and other probes that have an RTG, use it for heat, as much as for power as, to keep them warm overnight.&lt;/p&gt;&#xA;&lt;p&gt;Mars is not as bad since it actually has an atmosphere to moderate it a bit.  The temperature swings are not as bad, but still pretty bad.  From a high of 20C to -153C (173 degree vs 273 for the moon).&lt;/p&gt;&#xA;&lt;p&gt;Clearly it can be done as the current rovers have shown, but it is tricky and hard.  It requires hibernating and using battery power to keep the heat going for some of it.&lt;/p&gt;&#xA;&lt;h2&gt;Radiation&lt;/h2&gt;&#xA;&lt;p&gt;The moon, lacking an atmosphere does not have much to protect from solar radiation.  Mars has a bit better having a thin atmosphere.&lt;/p&gt;&#xA;&lt;h2&gt;Distance&lt;/h2&gt;&#xA;&lt;p&gt;Stuff breaks.  Nothing lasts forever. Grease needs to be replaced eventually.  With no way to fix it, more redundancy is required.  However, because launches are so expensive, you are limited by mass.&lt;/p&gt;&#xA;&lt;h2&gt;Dust&lt;/h2&gt;&#xA;&lt;p&gt;Both the moon and Mars have surprisingly fine dirt that seems to get into and onto everything.  The Apollo astronauts had trouble getting it off the space suits after the moon walks.  Mars also has a lot of dust, as the various rovers have shown.&lt;/p&gt;&#xA;&lt;p&gt;This stuff destroys moving parts as it manages to get in there somehow.&lt;/p&gt;&#xA;&lt;p&gt;If you use Solar arrays for power on Mars, it also covers the arrays reducing power, as the wind deposits it.  The Opportunity/Spirit missions found that the wind both deposited, and occasionally cleaned off the dust.&lt;/p&gt;&#xA;&lt;p&gt;Now you need a solar array cleaning mechanism to maintain power, and yet another mechanical component to break, far away from anyone to fix it.&lt;/p&gt;&#xA;&lt;h2&gt;Battery lifespan&lt;/h2&gt;&#xA;&lt;p&gt;Rechargeable batteries have finite lifespans before they stop holding charge, or before it degrades to a level too low to survive.&lt;/p&gt;&#xA;&lt;h2&gt;Bearings&lt;/h2&gt;&#xA;&lt;p&gt;Moving parts can be affected by the dust, but the bearings and some kind of grease will eventually wear out.&lt;/p&gt;&#xA;&lt;h2&gt;Funding&lt;/h2&gt;&#xA;&lt;p&gt;While not a technological limit per se, when funding runs out to run the program, even if the vehicle/orbiter/probe is still working, it may have its life cut short by program management.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-06-10T17:08:52.483" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="950" PostTypeId="2" ParentId="930" CreationDate="2013-08-02T23:43:54.133" Score="6" Body="&lt;p&gt;First off, the things we put into space, even the things we use to put people and other things into space, are not really friendly when you get right up close. Keep in mind that in order for the Space Shuttles to be decommissioned and turned into aerospace museum exhibits, they had to go through an extensive decontamination process to remove all the volatile, toxic chemicals used as propellants, fuels and even lubricants on the orbiter, replacing them with substances that only have to do the job well in Earth's atmosphere and ambient temperatures, instead of at +-200*C in a vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One example, a common fuel used for lift, deorbit and station-keeping/orbit-modification rockets, is &lt;a href=&quot;http://en.wikipedia.org/wiki/Hydrazine&quot;&gt;hydrazine&lt;/a&gt;, N&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;4&lt;/sub&gt;. Ideally, one molecule of hydrazine and one molecule of diatomic oxygen combine to produce two molecules of water vapor, one molecule of diatomic nitrogen, and about &lt;strong&gt;592 kJ/mol&lt;/strong&gt;. For the size of the molecules involved, that is a lot of heat released, which is why it's used in rocket fuel; pound for pound, it's better than the well-known explosive C4 (C4's primary active ingredient, RDX, gives you a little over twice as much energy per mole at about 1120 kJ/mol, but weighs almost 4 times as much at 222g/mol to the hydrazine and oxygen's combined 64g/mol). With a rocket being little more than a long, controlled explosion, the rocket's fuel tank is not something I'd like to see entering the oxygen-rich atmosphere at hypersonic speeds; that's a one-way ticket to a short, uncontrolled explosion, and a powerful one (especially with the &lt;a href=&quot;http://en.wikipedia.org/wiki/LOX&quot;&gt;LOX&lt;/a&gt; tank entering right beside it bringing lots of concentrated oxygen goodness to the party).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrazine is also acutely toxic; the NFPA, which rates chemical hazards for use in industrial settings, gives it &lt;strong&gt;the maximum rating of 4&lt;/strong&gt; as a health hazard, placing it in the same category as substances we have developed specifically to kill each other, like hydrogen cyanide, sarin, and VX nerve gas. The substance will also spontaneously ignite at ambient Earth temperatures and the presence of oxygen (another reason it's used as fuel; no ignition source needed), and it will go ahead and spontaneously &lt;em&gt;detonate&lt;/em&gt; given a good physical shock (like, say, the impact with the ground after re-entry if it makes it that far). With this overall NFPA 704 rating of 4-4-3, it's among the nastiest substances our chemical industry produces in bulk (there are nastier, but they're typically not sold and shipped by the tanker-load).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, the ideal reaction between hydrazine and oxygen isn't the only possibility; given various mixtures of the two, you'll end up with ammonia gas, various oxides of nitrogen, and other not-nice things (though almost anything's better than hydrazine itself). And that is if LOX is used as the oxidizer; there are things known to modern science that oxidize &lt;em&gt;better&lt;/em&gt; than liquid oxygen, like the halo-trihalides (chlorine trifluoride, chlorine tribromide and bromine trifluoride), perchlorates, peroxides etc, all of which could be in the satellite's oxidizer tank and would combine with hydrazine in new and exciting ways. A common one is nitric acid (in its white fuming form, often &quot;inhibited&quot; by adding hydrofluoric acid which creates a protective layer of metal fluoride, to prevent the nitric acid eating through the tank), because it doesn't require cryogenic handling; the ideal reaction would be one part fuel to two parts oxidizer producing nitrogen gas, water and oxygen, but nitric acid doesn't like to act &quot;ideally&quot;, and you're &lt;em&gt;much more likely to get a (un)healthy dose of toxic nitrogen dioxide&lt;/em&gt; instead of the clean nitrogen and oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is all just from one example, with &quot;interesting&quot; consequences from a chemistry perspective. More glaring is the simple physics; a spacecraft in geostationary orbit is travelling at about 11,068km/h, or about 3,074 m/s. It's an interesting contradiction in projectile physics and orbital motion, that &quot;you speed up to slow down&quot;. The opposite holds; by slowing this spacecraft's angular velocity down to de-orbit, you end up going faster relative to the earth's surface. By the time you begin re-entry, you're actually going faster than 8,000m/s, or faster than Mach 25; 10 times faster than a .223 Remington round fired from an M-16. And the satellite weighs a hell of a lot more than a .223 slug. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just as a rough estimate, let's say the spacecraft weighed a modest 5 tons (~22000 kg), which is the current average for a communications satellite. At escape (or reentry) velocity, the craft would have a kinetic energy of .5 * 22000 * (8000)^2 = 7.04E11 J. In perspective, that's a little more energy than you'd get by detonating 10 &lt;a href=&quot;http://en.wikipedia.org/wiki/GBU-43/B_Massive_Ordnance_Air_Blast_bomb&quot;&gt;MOAB&lt;/a&gt;s simultaneously. Adding back in the hydrazine goodness, let's say 400kg of the craft's weight was hydrazine fuel. At 32g/mol, and 592 kJ/mol, the potential chemical energy of the hydrazine fuel is 7.4E9 J, one one-hundredth the energy of the spacecraft itself, but still about the energy inherent in the average lightning bolt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All figures from Wikipedia.&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="288" LastEditDate="2013-09-03T18:50:26.797" LastActivityDate="2013-09-03T18:50:26.797" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="951" PostTypeId="1" AcceptedAnswerId="954" CreationDate="2013-08-03T00:07:27.910" Score="9" ViewCount="304" Body="&lt;p&gt;There is no doubt that a solar sail can move you around the Solar system, and even interstellar, if you're not in a hurry. But it will not get you off the planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming your ship/satellite is in orbit, what is the lowest orbit a solar sail could lift from? Can you use a rocket to get to LEO, and use a sail to get higher? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-08-03T12:04:01.023" LastActivityDate="2013-08-03T12:13:30.057" Title="What is the closest to Earth you can expect lift from a solar sail?" Tags="|low-earth-orbit|solar-sail|atmospheric-drag|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="952" PostTypeId="1" AcceptedAnswerId="953" CreationDate="2013-08-03T01:56:27.313" Score="13" ViewCount="811" Body="&lt;p&gt;It's been said before that a solar sail could be used for solar and extrasolar missions - if you aren't in a hurry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as you can't spell solar sail without the &lt;em&gt;solar&lt;/em&gt; part, I'm wondering if you could expect to get reasonable propulsion through the universe once you are lightyears away from the sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that eventually the propulsion coming from the sun would die down, and all that would be left is noise coming from other sources, all trying to push you one way or another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't a solar sail's value diminish greatly once you are in the middle of nowhere?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-11-01T22:13:35.710" Title="Once lightyears out of the solar system, could you expect to get feasible propulsion with a solar sail?" Tags="|propulsion|solar-sail|extra-solar-object|interstellar-travel|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="953" PostTypeId="2" ParentId="952" CreationDate="2013-08-03T11:47:04.940" Score="10" Body="&lt;p&gt;You could expect to get a reasonable propulsion only if you plan ahead. Specifically, if you were to shine a very high powered laser on said solar sail from somewhere, you might be able to get propulsion even further out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without a laser, you really can't do much once you get far away. As &lt;a href=&quot;http://www.earthmatrix.com/sciencetoday/solar_constants_%20planets.html&quot;&gt;this table&lt;/a&gt; shows, here's a few solar constants at the orbits of particular planets (W/m&lt;sup&gt;2&lt;/sup&gt;):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Planet       Mean      Perihelion       Aphelion&#xA;―――――――――――――――――――――――――――――――――――――――――――――――――&#xA;Mercury      9116.4       14447.5         6271.1&#xA;Venus        2611.0        2646.4         2575.7&#xA;Earth        1366.1        1412.5         1321.7&#xA;Mars          588.6         715.9          491.7&#xA;Jupiter        50.5          55.7           45.9&#xA;Saturn        15.04         16.76          13.53&#xA;Uranus         3.72          4.11           3.37&#xA;Neptune       1.510         1.515          1.507&#xA;Pluto         0.878         1.571          0.560 &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;At Neptune, you are at less than a tenth of a percent of what you'd get at Earth. At that point, it just doesn't make any sense to keep trying, unless you can increase it artificially somehow. One could even argue that it's not worth it at a point far before reaching Neptune as well.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-03T16:21:02.787" LastActivityDate="2013-08-03T16:21:02.787" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="954" PostTypeId="2" ParentId="951" CreationDate="2013-08-03T12:13:30.057" Score="6" Body="&lt;p&gt;There is &lt;a href=&quot;http://www.centerforspace.com/downloads/files/pubs/AIAA%202008-6442%20A%20Critical%20Assessment%20of%20Satellite%20Drag%20.pdf&quot; rel=&quot;noreferrer&quot;&gt;a paper&lt;/a&gt; on what orbital drag looks like as a function of orbit. In order to determine if the solar sail would work to raise the orbit, let's make a few assumptions.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Approximately 1/4th of the orbit, the sail will be providing useful energy. That is probably a high assumption, but I'll put it out there. &lt;/li&gt;&#xA;&lt;li&gt;Drag is a constant effect.&lt;/li&gt;&#xA;&lt;li&gt;The orbit is roughly circular.&lt;/li&gt;&#xA;&lt;li&gt;Attitude is manageable, ie, no attitude problems resulting from being low in the atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;The only drag on the spacecraft is due to the Solar Sail.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Okay, so, a solar sail produces around 4.67e-6 N/m^2 (From &lt;a href=&quot;http://science.howstuffworks.com/solar-sail1.htm&quot; rel=&quot;noreferrer&quot;&gt;How Stuff Works&lt;/a&gt;). Given the 25% efficiency in an Earth orbit, that means that the drag caused by the solar sail must be less than 1.16 uN in order to get any benefit at all. Given the table previously mentioned, that requires that the sail be deployed at at least 900 km, or else it will cause more drag than it can overcome through the sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There might be some alleviation of this depending on the orientation of the sail with respect to the Earth, but it should give you an idea at least. It also will depend somewhat on the solar flux levels, which change depending on the Solar Cycle. But this should give you at least a first order point to do further analysis.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-03T12:13:30.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="955" PostTypeId="1" AcceptedAnswerId="1763" CreationDate="2013-08-03T17:47:04.103" Score="25" ViewCount="4660" Body="&lt;p&gt;Reading about the Sea Dragon idea, I came upon &lt;a href=&quot;http://engineerinprogress.blogspot.com/2010/10/out-there-ideas-of-yesteryear.html&quot; rel=&quot;noreferrer&quot;&gt;this argument against it&lt;/a&gt;, which sounds compelling, but relies on more detailed knowledge than I have:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Those are to be pressure-fed to avoid &quot;complexity&quot;. Complexity? How about combustion instability from runaway pressure waves the size of houses? It took seven years to stop combustion issues from killing the F1 on test stands, and if it hadn't been started in the late fifties as a research project, it would have delayed the entire moon program.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;AFAIK, the Sea Dragon would have a pressurized bag of Helium at around 60 psi which acts as the pusher of the cryogenic liquids into the engine.  It seems completely counter-intuitive that this would have more mechanical problems than a turbine driven system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this?  What causes an astoundingly simple pressure driven engine to suffer from major pressure instabilities propagating from the engine?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="9092" LastEditDate="2021-03-15T10:14:05.580" LastActivityDate="2021-03-15T10:14:05.580" Title="Why do pressure fed engines have combustion instabilities?" Tags="|launch|engine-design|engineering|sea-dragon|pressure-fed-engine|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="956" PostTypeId="1" AcceptedAnswerId="957" CreationDate="2013-08-03T21:51:41.640" Score="9" ViewCount="2450" Body="&lt;p&gt;Movie &lt;a href=&quot;http://www.imdb.com/title/tt0086197/&quot; rel=&quot;nofollow&quot;&gt;The Right Stuff (1983)&lt;/a&gt; claims at the end that &lt;a href=&quot;http://en.wikipedia.org/wiki/Gordon_Cooper&quot; rel=&quot;nofollow&quot;&gt;Gordon Cooper&lt;/a&gt; was the last man to fly alone in space (if I remember it correctly and dubbing is right). Beside that, Wikipedia also states that he was in 1963 the &lt;em&gt;last American&lt;/em&gt; to do a full solo orbital mission. Is he still holding this as world record or some other not American astronaut did the same more recently?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Disclaimer: really I did not like that movie. :-) &lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="25" LastEditDate="2015-07-09T02:15:00.200" LastActivityDate="2016-08-01T17:21:35.103" Title="Was Gordon Cooper the last man to fly a whole mission in space alone?" Tags="|crewed-spaceflight|astronauts|record|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="957" PostTypeId="2" ParentId="956" CreationDate="2013-08-04T01:27:53.903" Score="17" Body="&lt;p&gt;The last person to fly solo in space was &lt;a href=&quot;https://en.wikipedia.org/wiki/Brian_Binnie&quot; rel=&quot;nofollow&quot;&gt;Brian Binnie&lt;/a&gt; of the Space Ship One flight on October 4, 2004. That was a sub-orbital flight, just barely touching space. The Space Shuttle required 2 people to launch minimum, thus, it was never launched Solo. Neither were Apollo and Gemini, so it has been since the Mercury program since an American could launch solo in to space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of some note is the fact that, while launched together, the Command Module pilot was in space by himself for an extended period of time. Ronald E Evans was alone on the Command Module for 80 hours on Apollo 17. The Russians, however, orbited Earth for slightly longer solo. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Chinese have flown one solo mission, and the pilot was &lt;a href=&quot;http://en.wikipedia.org/wiki/Yang_Liwei&quot; rel=&quot;nofollow&quot;&gt;Yang Liwei&lt;/a&gt;. The date was October 15, 2003. He was the last astronaut to orbit the Earth solo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last solo &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_Soviet_manned_space_missions&quot; rel=&quot;nofollow&quot;&gt;Russian Cosmonaut&lt;/a&gt; was &lt;a href=&quot;http://en.wikipedia.org/wiki/Georgi_Beregovoi&quot; rel=&quot;nofollow&quot;&gt;Georgy Beregovoy&lt;/a&gt;, launched 30 October 1968. Since then, the Russian program has used the Soyuz capsule, which typically requires two pilots. Soyuz 4/5, however, transferred crew in space. &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_4&quot; rel=&quot;nofollow&quot;&gt;Soyuz 4&lt;/a&gt;, launched 17 January 1969, was launched with just Vladimir Shatalov, while &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_5&quot; rel=&quot;nofollow&quot;&gt;Soyuz 5&lt;/a&gt; landed with just Boris Volynov, having two astronauts from Soyuz 5 who switched in space. This was the last time a Russian spacecraft had only a single occupant in space, per &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Soviet_manned_space_missions&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the movie was filmed in 1983. That leaves &lt;a href=&quot;https://en.wikipedia.org/wiki/Georgy_Beregovoy&quot; rel=&quot;nofollow&quot;&gt;Georgy Beregovoy&lt;/a&gt; as the last person to fly an entire mission solo in space prior to the movie being released.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-08-01T17:21:35.103" LastActivityDate="2016-08-01T17:21:35.103" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="958" PostTypeId="1" AcceptedAnswerId="960" CreationDate="2013-08-04T03:58:03.580" Score="40" ViewCount="81545" Body="&lt;p&gt;I recently studied that Voyager 1 has traveled a distance of 125.01 AU as of August 3rd, 2013. I wonder, how would it be able to communicate with the Earth? How much time does it take for the signal from ground station to reach Voyager 1?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-08-05T04:09:57.570" LastActivityDate="2022-05-30T10:26:00.913" Title="How does Voyager 1 send signals to Earth?" Tags="|voyager|communication|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="960" PostTypeId="2" ParentId="958" CreationDate="2013-08-04T04:50:10.780" Score="42" Body="&lt;p&gt;&lt;strong&gt;Communication system&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The radio communication system of Voyager 1 was designed to be used up&#xA;to and beyond the limits of the Solar System during the extremely long&#xA;flight of this space probe. The communication system includes a 3.7&#xA;meter diameter parabolic dish high-gain antenna to send&#xA;and receive radio waves via the three Deep Space Network stations on&#xA;the Earth. These modulated waves are placed in the S-band (about 13 cm&#xA;in wavelength) and X-band (about 3.6 cm in wavelength) which provided&#xA;a bit rate as high as 115.2 kilobits per second when Voyager 1 was at&#xA;the distance of Jupiter from the Earth, and many fewer kilobits per&#xA;second at larger distances.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1#Communication_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Voyager 1 - Communication system&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Voyager 1 communication is received on Earth by the Deep Space Network (DSN):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NASA's Deep Space Network (DSN) has been in partnership with Voyager 1&#xA;and Voyager 2 since 1977, providing daily communications support to&#xA;the two very distant spacecraft. The excellent partnership continues&#xA;as the Voyager twin spacecraft explore the regions of our universe&#xA;near the area where the solar wind meets the interstellar winds –&#xA;areas never before explored by human-made objects.&lt;/p&gt;&#xA;&lt;p&gt;Because of the enormous distances and the resultant weak signals from&#xA;the spacecraft, the large antennas and the very sensitive receivers of&#xA;the DSN are required to provide the necessary communications&#xA;capabilities. The DSN is the world's largest and most sensitive&#xA;spacecraft communications network. It consists of three deep space&#xA;communications complexes located approximately 120 degrees of&#xA;longitude apart around the world: at Goldstone, California; near&#xA;Madrid, Spain; and near Canberra, Australia. This placement permits&#xA;continuous communication with a spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://voyager.jpl.nasa.gov/news/profiles_dsn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Voyager - Deep Space Network&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Communications delay&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;p&gt;Radio communications travel at the speed of light, or &lt;span class=&quot;math-container&quot;&gt;$299\ 792\ 458\ \frac{\text{m}}{\text{s}}$&lt;/span&gt;. And one astronomical unit (AU) is &lt;span class=&quot;math-container&quot;&gt;$149\ 597\ 871\ \text{km}$&lt;/span&gt;. The quoted &lt;span class=&quot;math-container&quot;&gt;$125.01\ \text{AU}$&lt;/span&gt; is Voyager 1's distance it travelled as it navigated through our Solar system, and not the distance from the Earth on that date, so we'll first have to find an accurate distance from the Earth, and do our calculations on the time it takes for its signals to reach the Earth after we find that information.&lt;/p&gt;&#xA;&lt;p&gt;JPL has a convenient web page with &lt;a href=&quot;http://voyager.jpl.nasa.gov/where/&quot; rel=&quot;nofollow noreferrer&quot;&gt;exact current Voyager's distance from the Earth&lt;/a&gt;. It also provides the calculation for how long it takes the light from the Sun to reach it, but that's not exactly what we need (we want the time to reach the Earth with the speed of light). We could make a shortcut in our calculations, since the Sun is on average &lt;span class=&quot;math-container&quot;&gt;$1\ \text{AU}$&lt;/span&gt; away from the Earth, but that would result in some margin of error due to the Earth not necessarily along the direct path between Voyager 1 and the Sun (it could be closer, or farther, depending on the time of the year and the Earth's position in the orbit around the Sun).&lt;/p&gt;&#xA;&lt;p&gt;So let's do our own calculations; Currently, Voyager 1 is &lt;span class=&quot;math-container&quot;&gt;$124\ 56269764\ \text{AU}$&lt;/span&gt;, or &lt;span class=&quot;math-container&quot;&gt;$18\ 634\ 314\ 372.96\ \text{km}$&lt;/span&gt; away from the Earth. Entering these numbers and the speed of light in our calculator (distance divided by speed of light, after we do unit conversion from kilometers to meters), we get the &lt;span class=&quot;math-container&quot;&gt;$62157.382\ \text{seconds}$&lt;/span&gt; or &lt;strong&gt;17 hours, 15 minutes, 57 seconds and 382 milliseconds, and increasing every day&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="47149" LastEditDate="2022-05-30T10:26:00.913" LastActivityDate="2022-05-30T10:26:00.913" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="962" PostTypeId="1" AcceptedAnswerId="964" CreationDate="2013-08-04T09:24:31.980" Score="6" ViewCount="1120" Body="&lt;p&gt;&lt;em&gt;Gravitational slingshot&lt;/em&gt; is proven to be one of the best ways to accelerate an satellite. But how does it actually help accelerating a satellite? During this process, will satellites use chemical thrusters to move out of the gravitational pull of the planet, in the vicinity of which a gravity slingshot is performed?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-04T13:40:16.103" LastActivityDate="2013-08-04T13:40:16.103" Title="What is a gravitational slingshot?" Tags="|orbital-maneuver|gravity-assist|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="964" PostTypeId="2" ParentId="962" CreationDate="2013-08-04T10:26:07.367" Score="2" Body="&lt;p&gt;The hyperbola of the spacecraft past the planet trades a tiny bit of the planet's vector (from its orbital velocity) for a large boost to the spacecraft's vector; this is because the amount of energy traded is the same on both sides, but the planet's mass is millions or more times larger. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simplest case is an approach along the line of planetary motion. If you are approaching from ahead, as you pass, you swing around (preferably in direction of rotation) the planet, and come out on a course 180° from where you started, with an increase of 2x the speed of the planet. (The planet DOES slow down, but so little that it's impossible to notice.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slowing can be performed as well; approach from behind, and come out 180° to the rear, and for maximum slowing, do so in a counter-rotational path. This slows you by twice the vector of the planet, accelerating the planet ever so minutely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pass direction needing to be in rotational direction for acceleration isn't entirely true; but is a tiny consideration. If you move in the direction of rotation, you transfer less energy to the planet in the form of rotation, as you move past less of its mass; conversely, if you go counter-rotational,you maximize interaction with the mass of the planet. Note that the effects of the planet's mass are more profound than the spacecraft's, but it all ties in, and the effect of mass passed by is signficant enough that NASA calculates for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further: a ship can maximize gains by thrusting on the exit leg, from periapsis on, especially if thrusting so as to increase the radius (and decrease the drag from planetary gravity.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-04T10:26:07.367" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="965" PostTypeId="1" AcceptedAnswerId="966" CreationDate="2013-08-04T12:06:02.603" Score="6" ViewCount="294" Body="&lt;p&gt;Is it possible to observe a satellite orbiting a distant exoplanet? If its possible, it would seem to indicate that we have found intelligent life on that planet, since you need advanced technology to send a satellite to orbit. &lt;/p&gt;&#xA;" OwnerUserId="416" LastEditorUserId="25" LastEditDate="2013-08-04T12:14:16.307" LastActivityDate="2013-08-04T18:52:49.307" Title="Is it possible to observe satellites orbiting exoplanets?" Tags="|observation|exoplanet|astrobiology|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="966" PostTypeId="2" ParentId="965" CreationDate="2013-08-04T12:13:53.023" Score="5" Body="&lt;p&gt;How could you possibly observe a satellite on an exoplanet? There are 3 ways that I could see it being done. I'm going to assume you are talking about satellite similar to what we have now, there are objects conceived of in Science Fiction that could be more easily seen.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Look for the satellite when it passes in front of the sun. (Occultation)&lt;/li&gt;&#xA;&lt;li&gt;Direct observation&lt;/li&gt;&#xA;&lt;li&gt;Looking for its communication patterns.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Occultation&lt;/strong&gt;- This would basically look the same as a space rock, so it's not very likely to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Direct Observation&lt;/strong&gt;- This would require a telescope that is enormous. It's more likely we would see ground objects than satellites, as they are much larger typically. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Communication&lt;/strong&gt;- This one is possible, but typically satellites are highly focused on their home planet. Signals from the ground tend to be more powerful, and less directional. It could be that we would just get lucky, but aside from that, I don't think it would happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, there's easier ways to search for extra terrestrial life. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-04T18:52:49.307" LastActivityDate="2013-08-04T18:52:49.307" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="967" PostTypeId="1" AcceptedAnswerId="979" CreationDate="2013-08-04T12:25:41.163" Score="17" ViewCount="698" Body="&lt;p&gt;Currently, Voyager 1 is floating through the emptiness of space. Does it do anything to indicate its presence, like a light source onboard or broadcasting a signal that makes it more observable to others than just any inert object flying through space?&lt;/p&gt;&#xA;" OwnerUserId="416" LastEditorUserId="103" LastEditDate="2013-08-04T22:11:26.393" LastActivityDate="2013-08-07T08:48:46.710" Title="Does Voyager 1 do anything to be more observable?" Tags="|voyager|deep-space|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="969" PostTypeId="1" AcceptedAnswerId="1058" CreationDate="2013-08-04T13:07:25.620" Score="22" ViewCount="9837" Body="&lt;p&gt;On Earth is possible to define the position of something using &lt;a href=&quot;http://en.wikipedia.org/wiki/World_Geodetic_System&quot;&gt;WGS&lt;/a&gt;, and the same system is used for navigation. What is used in space instead? Are different systems used for Earth-orbiting objects (the ISS for example) as opposed to objects beyond LEO (Voyager 1/2) due to practical reasons?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user29" LastEditDate="2013-08-04T16:06:01.490" LastActivityDate="2021-04-26T19:42:53.000" Title="What coordinate system is used in space for navigation?" Tags="|iss|voyager|navigation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="970" PostTypeId="1" AcceptedAnswerId="972" CreationDate="2013-08-04T17:02:35.737" Score="10" ViewCount="471" Body="&lt;p&gt;Could a fixed wing &lt;a href=&quot;http://en.wikipedia.org/wiki/Entomopter&quot;&gt;entomopter&lt;/a&gt; with a wingspan of 2 meters actually fly in the Martian atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, could it reach a flight speed of over 250 mph in the thin atmosphere of Mars?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-04T17:09:25.153" LastActivityDate="2013-08-04T19:35:20.550" Title="Will the Mars Entomopter actually work?" Tags="|mars|aerobot|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="971" PostTypeId="2" ParentId="834" CreationDate="2013-08-04T19:19:09.247" Score="17" Body="&lt;p&gt;There are two major reasons you may be going outside the ecliptical plane:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You want to get to a particular star.&lt;/li&gt;&#xA;&lt;li&gt;You want to put the Sun on a direct line between your craft and another point in the sky (usually a star system or an interstellar probe to that star system) to exploit Sun's gravitational lens (that is, your destination starts around 550-740 AU from the Sun) to achieve &lt;strong&gt;awesome&lt;/strong&gt; extra amplification of received/transmitted radio signal (57 dB at 1.42GHz, for instance).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In any case, the trajectory to get you out of Dodge fast and right will have to use perihelion propulsive maneuver near the Sun (as close as your heat rejection will tolerate) after gravity assists from Saturn and Jupiter (the &lt;a href=&quot;https://en.wikipedia.org/wiki/Krafft_Ehricke&quot;&gt;Krafft Arnold von Ehricke&lt;/a&gt; trajectory). You would choose the point of perihelion to rotate your velocity vector in the direction required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens Claudio Maccone. Springer, 2009.&lt;/li&gt;&#xA;&lt;li&gt;Krafft Arnold von Ehricke, &quot;Saturn-Jupiter Rebound. A method of high-speed spacecraft ejection from the Solar System&quot;. Journal of the British Interplanetary Society, Vol.25, 1972. Pp.561-571.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastActivityDate="2013-08-04T19:19:09.247" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="972" PostTypeId="2" ParentId="970" CreationDate="2013-08-04T19:24:17.337" Score="8" Body="&lt;p&gt;From that linked Wikipedia page:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Several NASA Research Centers have noted its unique ability to fly on the planet Mars. Fixed wing aerial Mars rovers would have to fly at over 250 mph just to stay aloft in the rarefied Mars atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This is for an entomopter with a 15 to 18cm wingspan.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.niac.usra.edu/files/studies/abstracts/448Colozza.pdf&quot; rel=&quot;noreferrer&quot;&gt;Anthony Colozza's paper&lt;/a&gt; at the Ohio Aerospace Institute, has this:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A Mars aircraft with a 1 meter wingspan, would operate at a similar Reynolds number as terrestrial insects. Using the enormous lift producing mechanism of the entomopter may be an effective way to design vehicles capable of flying in the Martian atmosphere. Another advantage is that the reduced Mars gravitational force enables substantially lighter structures.&lt;/p&gt;&#xA;&lt;p&gt;Georgia Tech Research Institute has confirmed that this concept, on a preliminary level, appears feasible for a Mars application and may, in fact, be easier to accomplish due to vehicle scaling. It would greatly enhance mission capability allowing the vehicle to take off, fly slowly or hover, and land.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So to answer your question, yes, it should be able to fly, but no, it will not fly at 250mph, but considerably slower, which will be a benefit as it can spend more time over target, observing.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-04T19:35:20.550" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="973" PostTypeId="1" AcceptedAnswerId="2469" CreationDate="2013-08-04T23:14:41.577" Score="10" ViewCount="2110" Body="&lt;p&gt;Could someone provide a graph of speed vs mass of a typical spacecraft - say, Soyuz or Dragon, as it rises towards LEO? If not graph, then a single fixed point of mass at speed of 2 km/s will do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rationale: &lt;a href=&quot;https://en.wikipedia.org/wiki/Vactrain&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vactrain&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q5ISn.jpg&quot; alt=&quot;VacTrain&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A segment of this train might be used to propel vehicles to about 2 km/s. A branch in the track to separate &quot;communal transport&quot; from a NASA launch facility, a section with a fast-acting airlock, the vehicle exits the vacuum tube at a friendly angle upwards, engages rocket engines, crosses the atmosphere and enters the orbit. We gain 2 km/s at initial stage very cheaply, but we are limited by the dimensions and tolerances (like curve angles) of the communal part used primarily for plain human transport. This limits the amount of fuel we can fit in the launch vehicle, and as result puts a total cap on how much we can get to the orbit (if anything at all).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, would this mode of launch be viable at all?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="49" LastEditDate="2013-08-04T23:30:46.543" LastActivityDate="2013-10-19T17:51:56.047" Title="Speed-mass graph of starting rocket?" Tags="|launch|propulsion|low-earth-orbit|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="974" PostTypeId="5" CreationDate="2013-08-05T00:59:16.657" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-05T00:59:16.657" LastActivityDate="2013-08-05T00:59:16.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="975" PostTypeId="4" CreationDate="2013-08-05T00:59:16.657" Score="0" Body="Questions pertaining to objects in Low-Earth orbit." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-05T15:24:13.300" LastActivityDate="2013-08-05T15:24:13.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="976" PostTypeId="1" AcceptedAnswerId="20469" CreationDate="2013-08-05T01:13:47.973" Score="39" ViewCount="1824" Body="&lt;p&gt;I was wondering if it was possible to download the software that runs any of the computers on existing or decommissioned space probes, landers, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, could one download the software that runs any of the components on Curiosity?  If someone wanted to build their own similar robot, could they use the existing code developed by JPL?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it open source?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about other space probes?&lt;/p&gt;&#xA;" OwnerUserId="84" LastEditorUserId="9092" LastEditDate="2021-03-23T00:34:44.987" LastActivityDate="2021-03-23T00:34:44.987" Title="Is the software running on any existing or decommissioned space probes available for download?" Tags="|spacecraft|software|open-source|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="977" PostTypeId="2" ParentId="976" CreationDate="2013-08-05T07:03:39.967" Score="35" Body="&lt;p&gt;The NASA software for the Apollo Guidance Computers is released. Further, a low-level hardware emulator running the software is available; the source for the AGC is in several released manuals. Likewise, the Apollo Landing Computer has been emulated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current software generally isn't released due to the risks to the hardware; NASA eventually has to release all of it to the National Archives, per Federal Law, but the Archives is not obligated to release the code if the Archivist feels it would endanger current missions. (This author used to work for the Archives, as an archival aid - what does and doesn't get released is a complex balance of public right to know, individual right to privacy, and prevention of undue risk to ongoing programs.)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.galaxiki.org/web/main/_blog/all/build-your-own-nasa-apollo-landing-computer-no-kidding.shtml&quot;&gt;Build Your Own NASA Apollo Landing Computer (http://www.galaxiki.org/web/main/_blog/all/build-your-own-nasa-apollo-landing-computer-no-kidding.shtml)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ibiblio.org/apollo/&quot;&gt;Virtual AGC page (http://www.ibiblio.org/apollo/)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-05T07:03:39.967" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="979" PostTypeId="2" ParentId="967" CreationDate="2013-08-05T08:57:03.733" Score="17" Body="&lt;p&gt;Any object in space has several &lt;strong&gt;sensor signatures&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Reflected optical&lt;/strong&gt; - light reflected from surfaces. If you illuminate Voyager, its glint will be detectable.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Emitted optical&lt;/strong&gt; - light emitted by bulbs aboard. For energy conservation reasons there are none.&lt;/li&gt;&#xA;&lt;li&gt;If there is an insanely sensitive optical sensor exactly on the line between a star and Voyager, &lt;strong&gt;occultation detection&lt;/strong&gt; may be possible.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Infrared&lt;/strong&gt; - due to the work of RTGs and equipment onboard Voyager dumps heat into the environment by radiation. At mission start, the heat loss from the MHW-RTGs was estimated at 200 W.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Radar&lt;/strong&gt; - Voyager was not optimized for stealth, with lots of sharp corners and no radar-absorbents in use, so its radar cross-section must be quite high (NASA should know the numbers, actually).&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Radiotransmissions&lt;/strong&gt; - Voyager is still communicating with the DSN back here on Earth. Depending on range and aspect, one can detect the mainlobe, side- and backlobes from this particular source. Other equipment on board may also have detectable side emissions - if you get sufficiently close, you'll spot them. &lt;strong&gt;Note&lt;/strong&gt;: X-band (8.4GHz) transmission comes through two TWT amplifiers of 12.5 and 21.6 W; the antenna dish is 3.65 m in diameter.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Radioactivity&lt;/strong&gt; from the RTGs - only when you come VERY close for inspection (Pu-238 emits alpha-particles, which get caught by the berillium shell and transformed into free neutrons which decay into protons).&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Kinetic signature&lt;/strong&gt; - you'll know it when your craft is hit by a Terran probe.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now let's suppose you've succeeded in detecting something in the vast expanses of space. My bets are on optics and passive radiowave detection, although I don't know who you are and what detectors you can choose from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does one tell an artificial object from a natural one using currently available technologies?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Rotation&lt;/strong&gt;. Voyagers are three-axis stabilized, which is sufficiently unusual for rocks to warrant investigation.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Active radio transmissions&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Infrared signature&lt;/strong&gt;. Temperature above the background of deep space for such a small object in the middle of nowhere cannot come from tidal heating or other natural sources.&lt;/li&gt;&#xA;&lt;li&gt;...and, last, when you are close enough, you will see a complex metallic/composite object. No chance for something like that to emerge naturally.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Raymond L. Heacock. The Voyager Spacecraft. Proceedings of the Institution of Mechanical Engineers. V.194, no.28. 1980.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr/&gt;&#xA;&#xA;&lt;p&gt;Thanks to &lt;strong&gt;TildalWave&lt;/strong&gt;'s comment, we have an interesting reminder of human imperfection:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rosetta probe was misidentified for a near-Earth asteroid &quot;about 20 meters in diameter&quot; and designated 2007 VN84. Its trajectory was calculated to pass Earth closer than 6.000 km, which caused a bit of concern that it will impact Earth and even an alert was issued, until its trajectory was later recognized to match that of the Rosetta spacecraft. Just a short anecdote, I thought it's worth mentioning here. Here's a &lt;a href=&quot;http://web.archive.org/web/20100226073812/http://www.newscientist.com/article/dn12914&quot; rel=&quot;noreferrer&quot;&gt;relevant article&lt;/a&gt; from the New Scientist (archived).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-07T08:48:46.710" LastActivityDate="2013-08-07T08:48:46.710" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="980" PostTypeId="1" AcceptedAnswerId="1756" CreationDate="2013-08-05T09:06:09.880" Score="9" ViewCount="502" Body="&lt;p&gt;What kind of instruments are currently used, when ground control is monitoring a docking of new ship (or) segment to the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they monitor it at all?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-08-06T01:48:16.623" LastActivityDate="2013-09-04T13:01:33.103" Title="How does ground control monitor the docking of ships to the ISS?" Tags="|iss|docking|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="982" PostTypeId="1" AcceptedAnswerId="39822" CreationDate="2013-08-05T18:35:36.540" Score="36" ViewCount="4649" Body="&lt;p&gt;I've been reading a number of online &quot;pop-sci&quot; articles on the subject of &quot;warp drive&quot; - derived from work done initially by Alcubierre.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the most recent articles say that there are researchers at NASA who have found theoretical basis for not only &quot;warping&quot; space in the manner described by Alcubierre, but that estimates on the quantities of &quot;exotic&quot; matter required to make it work are far more reasonable than initially thought (~500kg instead of ~1 Jupiter mass), and that there is bench-top experimentation under way to validate it all. Furthermore, that it would easily permit an interstellar trip of say, 20 light-years to be completed in only a couple of months. Some statements have been made that we may be only a few decades away from a real spacecraft capable of such a voyage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: &lt;strong&gt;What has really been accomplished, proven or disproven to-date?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="49" LastEditDate="2013-12-31T18:34:52.980" LastActivityDate="2023-02-28T23:25:08.347" Title="Is warp drive a legitimate avenue of scientific investigation?" Tags="|propulsion|interstellar-travel|advanced-propulsion|alcubierre-drive|ftl|" AnswerCount="4" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="983" PostTypeId="2" ParentId="982" CreationDate="2013-08-05T19:01:23.757" Score="15" Body="&lt;p&gt;This is a repository community-wiki post with references to current work on Alcubierre drive at NASA (Harold White) and in other places.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Harold White. Warp Field Mechanics 101 (2011). &lt;a href=&quot;http://hdl.handle.net/2060/20110015936&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://hdl.handle.net/2060/20110015936&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Harold White. Warp Field Mechanics 102 Energy Optimization (2013). &lt;a href=&quot;http://hdl.handle.net/2060/20130011213&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://hdl.handle.net/2060/20130011213&lt;/a&gt; (See also YouTube videos: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/n_VQPqKUwhQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; and &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/gy_UFmGh83E?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;arXiv preprints on keywords &lt;strong&gt;(Alcubierre and warp)&lt;/strong&gt;: &lt;a href=&quot;http://arxiv.org/find/all/1/all:+AND+Alcubierre+warp/0/1/0/all/0/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://arxiv.org/find/all/1/all:+AND+Alcubierre+warp/0/1/0/all/0/1&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Metamaterial-based model of the Alcubierre warp drive. Igor I.Smolyaninov — Physical Review B, 2011. &lt;a href=&quot;http://arxiv.org/pdf/1009.5663&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://arxiv.org/pdf/1009.5663&lt;/a&gt; (&lt;strong&gt;peer-reviewed&lt;/strong&gt;)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Alcubierre Warp Drive: On the Matter of Matter. Brendan McMonigal, Geraint F. Lewis, Philip O'Byrne. 2012. &lt;a href=&quot;http://arxiv.org/pdf/1202.5708v1&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://arxiv.org/pdf/1202.5708v1&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.techrxiv.org/articles/preprint/Positive_Energy_Density_for_the_Alcubierre_Warp_Field_Equations_Using_an_RF-Driven_Dielectric_Resonant_Cavity/19224552/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Glenn, Chance (2022): Positive Energy Density for the Alcubierre Warp Field Equations Using an RF-Driven Dielectric Resonant Cavity&lt;/a&gt;. TechRxiv. Preprint. &lt;a href=&quot;https://doi.org/10.36227/techrxiv.19224552.v1&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://doi.org/10.36227/techrxiv.19224552.v1&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;see also &lt;a href=&quot;https://thedebrief.org/how-darpa-star-trek-and-ufos-inspired-this-engineer-to-unravel-the-secrets-to-warp-drive-propulsion/&quot; rel=&quot;nofollow noreferrer&quot;&gt;How DARPA, Star Trek, and UFOs Inspired This Engineer to Unravel the Secrets to Warp Drive Propulsion&lt;/a&gt; and other overly-credulous articles from early this year&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The original work:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;M.Alcubierre. The warp drive: hyper-fast travel within general relativity. Classical and Quantum Gravity, V.11, no.5, 1994. Pp.L73–77. &lt;a href=&quot;http://dx.doi.org/10.1088/0264-9381/11/5/001&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://dx.doi.org/10.1088/0264-9381/11/5/001&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Limitations:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Fundamental limitations on &amp;quot;warp drive&amp;quot; spacetimes. F.S.N. Lobo and M. Visser, Classical and Quantum Gravity 21, 5871 (2004). &lt;a href=&quot;http://arxiv.org/pdf/gr-qc/0406083&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://arxiv.org/pdf/gr-qc/0406083&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Stefano Finazzi, Stefano Liberati, and Carlos Barceló, &amp;quot;Semiclassical instability of dynamical warp drives, Physical Review. Link: &lt;a href=&quot;http://journals.aps.org/prd/abstract/10.1103/PhysRevD.79.124017&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://journals.aps.org/prd/abstract/10.1103/PhysRevD.79.124017&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="11262" LastEditDate="2023-02-28T23:25:08.347" LastActivityDate="2023-02-28T23:25:08.347" CommentCount="0" CommunityOwnedDate="2013-08-05T19:01:23.757" ContentLicense="CC BY-SA 4.0" />
  <row Id="984" PostTypeId="2" ParentId="982" CreationDate="2013-08-05T21:49:40.587" Score="21" Body="&lt;h3&gt;Original Answer&lt;/h3&gt;&#xA;&lt;p&gt;Given Alcubierre's math, and White's calculations, it's a viable avenue of research to pursue.&lt;/p&gt;&#xA;&lt;p&gt;Whether or not it is practical as FTL, and given the expected maximum apparent speed of about 10 times the speed of light (White), and that the math says it should be able to be done, an attempt to implement a prototype series should be of immense benefit. Even if the drive fails to work, the drive testing will provide interesting measurements.&lt;/p&gt;&#xA;&lt;p&gt;Therefore, investigations of Alcubierre's Warp Drive equations in a practical (rather than theoretical) mode are of use scientifically.&lt;/p&gt;&#xA;&lt;p&gt;Also note: &lt;em&gt;Any measured effect, even if not what is predicted, will lead to improvements in the theory.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;Further, NASA is allowing White to pursue a practical experiment.&lt;/p&gt;&#xA;&lt;p&gt;Note that Warp Drive, even if it doesn't allow for FTL (or as Alcubierre's theory notes, &lt;em&gt;apparent&lt;/em&gt; FTL by STL movement of the craft within STL movement of the space the craft is moving through), a STL version will still massively reduce travel times. A 0.01C drive puts Mars a mere few hours away. At furthest distance, Mars is about 18 light minutes away; 1800 minutes travel time is 30 hours - not even 2 days.&lt;/p&gt;&#xA;&lt;p&gt;Note that, for logistical and health purposes, a roughly 60-90 day mission (round trip) is about the limit.  White has implied that he expects 10 C to be practical - this puts Alpha Centauri, home of the nearest detected extrasolar planet, at 156 days travel time. So, for practical purposes, a 10 C drive isn't an interstellar one. At 40 C apparent velocity, the Alpha Centauri system becomes reasonably practical. This means that, unless apparent speeds higher than White's worked 10 C examples are practical, the Warp Drive is going to still be an in-system drive.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;2015 Update&lt;/h3&gt;&#xA;&lt;p&gt;It is worth noting that, as of May 2015, NASA thinks the EM-Drive may in fact be generating an Alcubierre warp, and is in fact generating thrust in a vacuum. Dr. White is working on the team testing this drive.&lt;/p&gt;&#xA;&lt;p&gt;NASA has yet to release any data that this author can find on Dr. White's approved tests of a pure Warp Drive, but if the EM-Drive is generating a warp field, this may be a result of two lines of unrelated research converging.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;References&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015936_2011016932.pdf&quot; rel=&quot;noreferrer&quot;&gt;White, Harold &amp;quot;Sonny&amp;quot;, &lt;em&gt;Warp Field Mechanics 101&lt;/em&gt; (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015936_2011016932.pdf)&lt;/a&gt; Retrieved 5 August 2013.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://techland.time.com/2012/09/19/nasa-actually-working-on-faster-than-light-warp-drive/&quot; rel=&quot;noreferrer&quot;&gt;Peckham, Matt, &lt;em&gt;NASA Actually Working on Warp Drive&lt;/em&gt; (http://techland.time.com/2012/09/19/nasa-actually-working-on-faster-than-light-warp-drive/)&lt;/a&gt; Retrieved 5 August 2013.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2015/04/evaluating-nasas-futuristic-em-drive/&quot; rel=&quot;noreferrer&quot;&gt;José Rodal, Ph.D, Jeremiah Mullikin and Noel Munson, &lt;em&gt;Evaluating NASA's Futuristic EM-Drive&lt;/em&gt;, (http://www.nasaspaceflight.com/2015/04/evaluating-nasas-futuristic-em-drive/)&lt;/a&gt;, Retrieved 3 May 2015.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-05-04T06:25:44.167" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="985" PostTypeId="2" ParentId="93" CreationDate="2013-08-05T22:53:30.120" Score="4" Body="&lt;p&gt;NASA started the process for the ISS as a shuttle delivered components system.&lt;/p&gt;&#xA;&lt;p&gt;Spacelab was designed for maximal size for the payload bay, for any given length within the bay; in other words, you're not loading larger diameter modules in a shuttle cargo bay. The lab habitation modules were 4.12m diameter, 2.7m long.&lt;/p&gt;&#xA;&lt;p&gt;Note that larger modules could and were built to fit in the space shuttle payload bay's 4.6x18m capacity, but Spacelab was intentionally spaced away from the walls.&lt;/p&gt;&#xA;&lt;p&gt;Modules of up to 4.477m diameter have been built and launched on the shuttle; all the modules I've been able to find show the same 4.89m (16') bay width limitation in mind, and the shuttle payload design limit of 4.6m (15').&lt;/p&gt;&#xA;&lt;p&gt;Note that the Delta IV has been proposed for future launches; present non-shuttle launches of modules to the ISS have all been on Soyuz-U and Proton-K rockets. The Delta IV has a 5m diameter.&lt;/p&gt;&#xA;&lt;p&gt;The Falcon 9 has carried components to orbit in the trunk of the Dragon Cargo Capsules; such components have been smaller, to match the roughly 3.5m internal limit of the 3.7m diameter trunk.&lt;/p&gt;&#xA;&lt;p&gt;Both the Falcon and Delta IV can carry wider payloads; the Falcon 9 fairing is capable of a shuttle-loadable payload inside it's 5m diameter, 13m long fairing.&#xA;The Delta IV likewise has a 5m payload fairing. Either could, in theory, carry a wider fairing; current components in process, however, do not require such capacity.&lt;/p&gt;&#xA;&lt;p&gt;Further, the largest limit at present is primarily the lack of lift capacity, more than the volume lifted. The Falcon could carry most of the extant components, for example, but would be near its capacity limit. The Delta IV would be overloaded by many of the current modules. The Heavy Variants of either could carry larger modules, but given the weight and volume ratios, those larger modules are likely to be longer, and fit in the standard extant fairings, and to be in the 4.4m to 4.7m diameter range.&lt;/p&gt;&#xA;&lt;h1&gt;TL/DR:&lt;/h1&gt;&#xA;&lt;p&gt;Due to the common carriage requirements between Spacelab and ISS, components had to fit in the shuttle bay. Given current launcher needs, launchers only need to carry components that were designed to fit the shuttle's bay.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-06T05:39:38.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="986" PostTypeId="1" AcceptedAnswerId="987" CreationDate="2013-08-06T00:41:43.627" Score="11" ViewCount="1247" Body="&lt;p&gt;I'm curious as to whether or not any country has any real plans for a manned mission to Mars. Not a conceptual thing, but real, hard plans. If so, are there any specifics? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2015-03-17T20:31:00.153" Title="Does any country have real plans for a manned mission to Mars?" Tags="|mars|crewed-spaceflight|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="987" PostTypeId="2" ParentId="986" CreationDate="2013-08-06T01:26:37.827" Score="1" Body="&lt;p&gt;I do not know of a country, but &lt;a href=&quot;http://www.news.com.au/technology/sci-tech/wanted-humans-to-for-mars-colony-mission/story-fn5fsgyc-1226552015338&quot; rel=&quot;nofollow&quot;&gt;Mars One are looking for volunteers&lt;/a&gt; to send someone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sorry to extend your question to corporations rather than &quot;countries&quot; (assuming government-led).&lt;/p&gt;&#xA;" OwnerUserId="444" LastActivityDate="2013-08-06T01:26:37.827" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="988" PostTypeId="5" CreationDate="2013-08-06T01:42:15.470" Score="0" Body="&lt;p&gt;Generically docking is the process and equipment used to join two spacecraft together. In the context of the ISS, docking is performed with an active spacecraft docking to a passive spacecraft.  This is in contrast to berthing where a passive spacecraft is moved by the CanadArm2 to a passive port.&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="51" LastEditDate="2014-01-13T05:18:14.993" LastActivityDate="2014-01-13T05:18:14.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="989" PostTypeId="4" CreationDate="2013-08-06T01:42:15.470" Score="0" Body="Questions about the process and equipment used to join two spacecraft together. &#xD;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-15T09:32:31.520" LastActivityDate="2013-08-15T09:32:31.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="990" PostTypeId="2" ParentId="986" CreationDate="2013-08-06T03:43:57.927" Score="0" Body="&lt;p&gt;Something not discussed much is we currently do not have the technology to depart earth, land on Mars, and then again depart Mars.  There is way too much fuel required for such a round trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right now with our chemical rockets it is strictly a one way trip.  The definition of a suicide mission.  So people may talk and speculate, but I doubt anyone is going to seriously spend the kind of money needed in the hopes there might be qualified people who would actually accept the mission (ethical issues aside).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For those who would like to read more about the challenges this type of mission must solve before we can even leave the atmosphere I suggest you read &lt;a href=&quot;http://www.marsdrive.com/Libraries/Downloads/Mars_Exploration_Entry_Descent_and_Landing_Challenges.sflb.ashx&quot; rel=&quot;nofollow&quot;&gt;Robert Braun's paper&lt;/a&gt; on the subject.  Or at least read &lt;a href=&quot;http://www.wired.co.uk/news/archive/2013-05/31/getting-to-mars&quot; rel=&quot;nofollow&quot;&gt;something lighter by wired&lt;/a&gt; which covers a wide range of challenges many of which we still haven't technically found proven solutions for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:  Undo specifically stated is there any real plan which means: &quot;We have most of the technology to do this, and are anticipating doing this around 2018&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since people don't seem to read the links...The honest answer is no, we don't have the technology (meaning &quot;application of scientific knowledge for practical purposes&quot;) to do this.  We &lt;em&gt;think&lt;/em&gt; we understand the &lt;em&gt;some&lt;/em&gt; of issues well enough to investigate building the hardware.  Since no human has left earth orbit, survived in space and returned to earth, no one can even claim we understand all the technical hurdles. Even long term experiments in isolated biospheres in the Earth's gravity, atmosphere and sunlight have failed miserably.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact it wasn't until 2005 that we realized it was impossible to send a chemical rocket there and back.  Yet ask the general public in 2004 and they would tell you we have the technology to do it--build a big rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is an illustration of why it seems our progress to voyage into space has stalled.  Not because it would just cost too much, but we do lack technology knowledge in many areas for such a trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example: NASA's popular solution barring a technology breakthrough is to use heavy lift rockets (which we don't have yet) to build the ship in space and haul all the mass out there first then launch from there.  Do we know how to build a ship in space?  Not really, but ISS has taught us a lot about what we don't know any we can start looking at tools to develop the technology for exo-construction.  But we (humans in general) are way too early in the technological process to claim we can go to Mars and back by year XXXX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT2:  As a recent follow-up on Feb. 2015, Gerard ’t Hooft, a Dutch Nobel laureate and ambassador for Mars One project, said he did not believe the mission could take off by 2024 as planned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;“It will take quite a bit longer and be quite a bit more expensive. When they first asked me to be involved I told them ‘you have to put a zero after everything’,” he said, implying that a launch date &lt;strong&gt;100 years from now&lt;/strong&gt; with a budget of tens of billions of dollars would be an achievable goal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.theguardian.com/science/2015/feb/23/mars-one-plan-colonise-red-planet-unrealistic-leading-supporter&quot; rel=&quot;nofollow&quot;&gt;http://www.theguardian.com/science/2015/feb/23/mars-one-plan-colonise-red-planet-unrealistic-leading-supporter&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="446" LastEditorUserId="446" LastEditDate="2015-03-16T21:52:16.797" LastActivityDate="2015-03-16T21:52:16.797" CommentCount="31" ContentLicense="CC BY-SA 3.0" />
  <row Id="991" PostTypeId="2" ParentId="986" CreationDate="2013-08-06T03:51:07.990" Score="0" Body="&lt;p&gt;Perhaps you could say the Dutch, but Mars One is a Dutch corporate venture, not a government program.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2013-08-06T03:51:07.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="992" PostTypeId="1" CreationDate="2013-08-06T04:02:29.770" Score="2" ViewCount="70" Body="&lt;p&gt;Using off-the-shelf technology, would it be possible to create a community-built array-style radio telescope?  Much like the &lt;a href=&quot;http://en.wikipedia.org/wiki/Very_Large_Array&quot; rel=&quot;nofollow&quot;&gt;Very Large Array&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/Square_Kilometre_Array&quot; rel=&quot;nofollow&quot;&gt;Square Kilometer Array&lt;/a&gt; but using home grown antennae and the Internet to connect them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If enough people got involved in such a project, could the resolution of such a telescope rival that of the above mentioned telescopes?  The area of such a telescope could potentially be measured on a continental scale.  Has anyone attempted such a project?  Is it even feasible?&lt;/p&gt;&#xA;" OwnerUserId="84" LastActivityDate="2013-08-06T04:02:29.770" Title="Would it be possible to create a community-built array-style radio telescope?" Tags="|telescope|radio-telescope|" AnswerCount="0" CommentCount="3" ClosedDate="2013-08-06T14:37:47.690" ContentLicense="CC BY-SA 3.0" />
  <row Id="993" PostTypeId="1" CreationDate="2013-08-06T04:06:55.143" Score="8" ViewCount="2660" Body="&lt;p&gt;There is one new project, like Google Maps, which would be an online video transmission of the Earth's surface from a satellite:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.urthecast.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.urthecast.com/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Another good one is a web cam situated on the International Space Station (ISS), and looks like it works:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/multimedia/nasatv/iss_ustream.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/multimedia/nasatv/iss_ustream.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;What is a major problem in situating a camera on Mars and another on the Moon?&lt;/p&gt;&#xA;" OwnerDisplayName="user267" LastEditorUserId="12180" LastEditDate="2022-05-14T08:14:11.810" LastActivityDate="2022-05-14T08:14:11.810" Title="Why are there no video transmissions from the Moon and Mars?" Tags="|mars|the-moon|space-station|deep-space|camera|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="995" PostTypeId="5" CreationDate="2013-08-06T06:51:57.153" Score="0" Body="&lt;p&gt;The planet Jupiter is a gas giant, 5th planet from the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could also refer to the Jupiter booster used during the Apollo project to test technologies.&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="51" LastEditDate="2014-01-07T21:55:34.077" LastActivityDate="2014-01-07T21:55:34.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="996" PostTypeId="4" CreationDate="2013-08-06T06:51:57.153" Score="0" Body="Questions about the exploration of the planet Jupiter." OwnerUserId="84" LastEditorUserId="84" LastEditDate="2013-08-06T13:58:16.483" LastActivityDate="2013-08-06T13:58:16.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="997" PostTypeId="2" ParentId="993" CreationDate="2013-08-06T07:32:25.540" Score="3" Body="&lt;p&gt;The view from Mars is going to be entirely boring. To get an idea of how boring, try videoing Mars from Earth's surface. What happens is the little red dot moves through the sky. Quite often you can't see it, because it is cloudy, or daytime, or the dot is behind the Earth. The same would happen from the other end, only the dot would be bluish, and the clouds would be dust rather than water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the moon - well, it could look quite nice, but isn't going to be as useful from a scientific viewpoint as satellite footage, which is much closer, and cheaper to get to.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-08-06T07:32:25.540" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="998" PostTypeId="1" AcceptedAnswerId="1004" CreationDate="2013-08-06T08:00:53.540" Score="20" ViewCount="21522" Body="&lt;p&gt;Comparing ESA's Kourou (5° off) with NASA's Cape Canaveral (28°) and JAXA's Tanegashima (30°) launch sites, how advantageous is proximity to the equator? Considering the fact that the ESA and all of Arianespace's clients are happy to transport equipment and payloads all the way to French Guiana, why don't NASA and JAXA relocate to a more suitable station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the US, which has a number of overseas territories, there are a number of options. For a start the &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_Virgin_Islands&quot;&gt;US Virgin Islands&lt;/a&gt; are 10° closer to the equator and not too far away from Florida either. Then there are possessions like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Palmyra_Atoll&quot;&gt;Palmyra Atoll&lt;/a&gt; which is on par with Kourou in terms of latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why doesn't NASA use a site closer to the equator as a launch site? Is it for security reasons? Or is Kourou preferred purely for historical reasons as well as safety? Setting aside safety and political issues, how beneficial is Kourou to the ESA purely in terms of launch metrics?&lt;/p&gt;&#xA;" OwnerUserId="80" LastActivityDate="2018-02-05T02:17:36.303" Title="How much of an advantage do equator-proximal launch sites provide?" Tags="|launch|nasa|jaxa|esa|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="999" PostTypeId="1" CreationDate="2013-08-06T09:13:27.527" Score="20" ViewCount="3417" Body="&lt;p&gt;In case there will be zero space launches starting at some point in time. How long will it take for the space debris mass in LEO to be halved?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-06T11:02:54.093" LastActivityDate="2014-02-03T11:16:22.423" Title="Space debris half life" Tags="|low-earth-orbit|debris|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1000" PostTypeId="2" ParentId="112" CreationDate="2013-08-06T09:58:51.527" Score="6" Body="&lt;p&gt;I'm going to attempt to answer your later questions : &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What kind of measurements are performed - in space - for better understanding the situation and corresponding processes? Is there any kind of monitoring - in space - on the current state of Earth's magnetic field, its development and space weather phenomena, which may influence the field?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, there's monitoring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ernestopheles already mentioned two magnetospheric missions (Swarm hasn't launched yet).  There's also ESA's &lt;a href=&quot;http://sci.esa.int/cluster&quot; rel=&quot;nofollow&quot;&gt;Cluster-II&lt;/a&gt; mission, the JAXA/NASA &lt;a href=&quot;http://www.jaxa.jp/projects/sat/geotail/index_e.html&quot; rel=&quot;nofollow&quot;&gt;Geotail&lt;/a&gt;, and NASA's &lt;a href=&quot;http://twins.swri.edu/index.jsp&quot; rel=&quot;nofollow&quot;&gt;TWINS&lt;/a&gt;.  There's also the &lt;a href=&quot;http://vanallenprobes.jhuapl.edu/&quot; rel=&quot;nofollow&quot;&gt;Van Allen Probes&lt;/a&gt; (formerly RBSP) to study the radiation belt, and magnetometers in the various GOES satellites.  NASA's also working on &lt;a href=&quot;http://mms.gsfc.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;MMS&lt;/a&gt; that might launch next year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the type of measurements -- well, there's the magnetometer data, but there's also insitu measurements of the solar wind (flux, composition, velocity, etc.), and remote sensing (spectroscopy for the most part)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the planning for operations impact ... I don't know if any satellites are prepared for long-term bombardment by space weather.  My understanding is that for more recent spacecraft, even geostationary and LEO ones have some measures that they can take in case of an Earth-directed &lt;a href=&quot;http://en.wikipedia.org/wiki/Coronal_mass_ejection&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;coronal mass ejection&lt;/em&gt;&lt;/a&gt; (CME).  As the CMEs have mass, they also have a charge and magnetic field associated with them, and so may cancel out the Earth's magnetic field.  There are a number of ground-based magnetometer sites that record this information, as well as the daily fluctuations of the magnetic field in general.  I believe that &lt;a href=&quot;http://supermag.jhuapl.edu/&quot; rel=&quot;nofollow&quot;&gt;SuperMAG&lt;/a&gt; is an aggregator, and not a direct collection project, but they'd show where those sites are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some cases, to protect against CMES, operators tilt the  panels so it's a smaller target ... the problem is, this also means they're getting less power, so not all spacecraft can do this long-term.  If the panels are fixed to the spacecraft, you have to tilt the whole thing to do it, and this can lead to thermal issues.&lt;/p&gt;&#xA;" OwnerUserId="46" LastEditorUserId="14" LastEditDate="2013-08-17T18:36:34.527" LastActivityDate="2013-08-17T18:36:34.527" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1001" PostTypeId="1" AcceptedAnswerId="1002" CreationDate="2013-08-06T10:09:04.870" Score="5" ViewCount="358" Body="&lt;p&gt;The ISS with its 73m x 108m &lt;a href=&quot;http://en.wikipedia.org/wiki/Integrated_Truss_Structure&quot; rel=&quot;nofollow&quot;&gt;Integrated truss structure&lt;/a&gt;, apparently, is the largest (by dimensions) artificial object in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know about the &lt;a href=&quot;http://journals.ru/attach/653/65203/974979.jpg&quot; rel=&quot;nofollow&quot;&gt;Krab experiment&lt;/a&gt; which produced two self assembly rings of 20m diameter, attached to a Progress spacecraft. That gives 40m x 20m. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both Space Shuttle orbiter and Mir station were a bit shorter than 40m. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither of thin object such as &lt;a href=&quot;http://en.wikipedia.org/wiki/IKAROS&quot; rel=&quot;nofollow&quot;&gt;IKAROS&lt;/a&gt; solar sail (20m) or &lt;a href=&quot;http://en.wikipedia.org/wiki/Znamya_%28space_mirror%29&quot; rel=&quot;nofollow&quot;&gt;Znamya reflector&lt;/a&gt; (20m) come even close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Inflatable_Antenna_Experiment&quot; rel=&quot;nofollow&quot;&gt;Inflatable Antenna Experiment's&lt;/a&gt; struts were shorter (28m) too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the question is: &lt;strong&gt;have there been any (other than ISS) artificial objects longer than 40 meters in orbit?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-09T22:30:45.600" LastActivityDate="2013-08-10T03:48:22.933" Title="Artificial objects longer than 40 meters in space" Tags="|dimensions|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1002" PostTypeId="2" ParentId="1001" CreationDate="2013-08-06T10:26:19.200" Score="7" Body="&lt;p&gt;That one is easy: &lt;a href=&quot;http://www-istp.gsfc.nasa.gov/Education/wtether.html&quot; rel=&quot;noreferrer&quot;&gt;http://www-istp.gsfc.nasa.gov/Education/wtether.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been a number of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_tether_missions&quot; rel=&quot;noreferrer&quot;&gt;space tether missions&lt;/a&gt;, the longest (verified!) was the TSS-1R on February 25, 1996, when the Columbia in the STS-75 flight unrolled the tether to 19.7 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Video: &lt;a href=&quot;http://www.youtube.com/watch?v=pq4brZBqXvU&quot; rel=&quot;noreferrer&quot;&gt;http://www.youtube.com/watch?v=pq4brZBqXvU&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/missions/sts-75/mission-sts-75.html&quot; rel=&quot;noreferrer&quot;&gt;http://science.ksc.nasa.gov/shuttle/missions/sts-75/mission-sts-75.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/missions/sts-75/sts-75-day-04-highlights.html&quot; rel=&quot;noreferrer&quot;&gt;http://science.ksc.nasa.gov/shuttle/missions/sts-75/sts-75-day-04-highlights.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-06T11:00:37.830" LastActivityDate="2013-08-06T11:00:37.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1003" PostTypeId="2" ParentId="993" CreationDate="2013-08-06T10:55:02.083" Score="11" Body="&lt;p&gt;Pure and simple, cost&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mars&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting a webcam on or in orbit of Mars would involve sending the data back to earth, and sending data to and from Mars isn't cheap. There's only so much bandwidth, the Mars Reconnaissance Orbiter in the ideal conditions puts out 6Mbits, and Curiosities direct to earth data rate is ~15-32kbps, or at up to 8Mbits to the orbiters ( but only with an 8 minute window each day ), with the signal taking between 4 and 22 seconds to arrive once sent. Other orbiters are in a comparable situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So any basic webcam would be competing with scientific data that's collected far faster than it can be sent. To get it to work we'd need more hardware in Mars orbit to increase bandwidth enough that communicating with Mars and its surface would have a larger window of opportunity, data rate, and capacity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said that I don't think a webcam would be very interesting, a handful of photographs showing night day and various weather would sum it up pretty much. Hardly justifies spending the hundreds of millions of $ to put one there. Anything interesting would likely be scientific in nature, so a realtime video from orbit is more likely, and probably of so high a resolution it isn't realtime. Perhaps Mars weather systems?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The Moon&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again cost, and interest. The moon isn't a terribly active place as far as a normal webcam would be concerned, but you could put a webcam up there and point it at earth or the surrounding landscape. Data rate isn't so much of an issue as it is with Mars as Earth is comparatively close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem then is funding it, and making it last for an appreciable amount of time. The temperature ranges on the Moon are quite large, and there's no atmosphere to protect against the various amounts of radiation coming from space&lt;/p&gt;&#xA;" OwnerUserId="419" LastActivityDate="2013-08-06T10:55:02.083" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1004" PostTypeId="2" ParentId="998" CreationDate="2013-08-06T11:58:17.190" Score="13" Body="&lt;p&gt;The question should be broken in several parts.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Why was Cape Canaveral chosen?&lt;/li&gt;&#xA;&lt;li&gt;Why does NASA not relocate somewhere else?&lt;/li&gt;&#xA;&lt;li&gt;What is the gain in terms of payload mass when launching from Kourou?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;I will not answer the third part since for each launcher and inclination the answer will be slightly different.&lt;/p&gt;&#xA;&lt;p&gt;From &lt;strong&gt;Moonport&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Cape Canaveral, better known as &amp;quot;the Cape,&amp;quot; had been earmarked as a missile testing range in 1947. An elbow of land jutting out into the Atlantic midway between Jacksonville and Miami, the Cape covers about 60 square kilometers. Early Spanish sailors, marking it down as the only major feature of the long Florida coast line, named it for its abundance of cane reeds. Its choice as a missile range was dictated by several factors: the planners could set up &lt;strong&gt;a line of tracking stations&lt;/strong&gt; stretching southeasterly over the Atlantic to provide the longest range necessary for missile testing; the Banana River Naval Air Station could serve as a &lt;strong&gt;support base&lt;/strong&gt;; and the launch area was &lt;strong&gt;accessible to water transportation&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The selection was made by a Joint Chiefs of Staff committee. When the armed services went into rocketry in 1945, the Army stationed its launch team of German V-2 experts at White Sands, New Mexico - near the scene of Robert Hutchings Goddard's pioneering work in the 1930s. &lt;strong&gt;The southwestern desert proved too small for rockets&lt;/strong&gt;, On 29 May 1947, a modified V-2 went the wrong way and landed in a cemetery south of Juarez, Mexico - one of the factors that decided the Joint Chiefs to move rocket experiments to the east coast of Florida.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Factors not mentioned above:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Access by road and railroad from elsewhere in the CONUS&lt;/li&gt;&#xA;&lt;li&gt;Availability of land to set up oxygen (later - hydrogen) production&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Historical curiosities:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&amp;quot;Historical precedent&amp;quot; - Florida was &amp;quot;chosen&amp;quot; by Jules Verne in 1865&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Other factors that came into play later:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Launch ranges need a lot of uninhabited or sparsely inhabited land/sea in the direction of useful launch azimouths, especially where spent stages fall.&lt;/li&gt;&#xA;&lt;li&gt;The likely list of inclinations to launch into.&lt;/li&gt;&#xA;&lt;li&gt;Conversion of the Atlantic range into an operational ICBM base (and you simply don't build ICBM bases on tiny coral atolls in the middle of nowhere).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;So, did NASA consider changing sites? &lt;strong&gt;YES&lt;/strong&gt;, there was one particularly stubborn person out there.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ctPx0.jpg&quot; alt=&quot;Wernher von Braun&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Moonport:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A Saturn Launch Site&lt;/p&gt;&#xA;&lt;p&gt;With better than 20 years' experience, the von Braun team preached and practiced that rocket and launch pad must be mated on the drawing board, if they were to be compatible at the launching. The new rocket went hand in hand with its launching facility.&lt;/p&gt;&#xA;&lt;p&gt;The short-lived plan to transport the Saturn by air was prompted by ABMA's interest in launching a rocket into equatorial orbit from a site near the Equator; &lt;strong&gt;Christmas Island&lt;/strong&gt; in the Central Pacific was a likely choice. Equatorial launch sites offered certain advantages over facilities within the continental United States. A launching due east from a site on the Equator could take advantage of the earth's maximum rotational velocity (460 meters per second) to achieve orbital speed.&lt;/p&gt;&#xA;&lt;p&gt;The more frequent overhead passage of the orbiting vehicle above an equatorial base would facilitate tracking and communications. Most important, an equatorial launch site would avoid the costly dogleg technique, a prerequisite for placing rockets into equatorial orbit from sites such as Cape Canaveral, Florida (28 degrees north latitude). The necessary correction in the space vehicle's trajectory could be very expensive - engineers estimated that doglegging a Saturn vehicle into a low-altitude equatorial orbit from Cape Canaveral used enough extra propellant to reduce the payload by as much as 80%.&lt;/p&gt;&#xA;&lt;p&gt;In higher orbits, the penalty was less severe but still involved at least a 20% loss of payload. There were also significant disadvantages to an equatorial launch base: &lt;strong&gt;higher construction costs&lt;/strong&gt; (about 100% greater), &lt;strong&gt;logistics problems&lt;/strong&gt;, and the &lt;strong&gt;hazards of setting up an American base on foreign soil&lt;/strong&gt;. Moreover in 1959 there was a question as to &lt;strong&gt;how many U.S. space missions would require equatorial orbits&lt;/strong&gt;. The only definite plans for equatorial orbits were in connection with communications and meteorological satellites operating at 35,000 kilometers. (6)&lt;/p&gt;&#xA;&lt;p&gt;While there was disagreement over the merits of an equatorial base for future Saturn operations, the Atlantic Missile Range was the clear choice for the developmental launchings. At the range's launch site, Cape Canaveral, the Air Force Missile Test Center provided administrative and logistical support. The range's ten tracking stations, stretching into the South Atlantic, gave good coverage of test flights. Moreover, ABMA's launch team, the Missile Firing Laboratory (MFL), had launched missiles from Cape Canaveral since 1953. Cost and time considerations agreed. As an MFL study noted, the Atlantic Missile Range met &amp;quot;the established [launch] criteria in the most efficient, timely manner at a minimum cost. (7)&lt;/p&gt;&#xA;&lt;p&gt;(6) NASA Special Committee on Space Technology, Recommendations Regarding a National Civil Space Program (Stever Committee Report), Washington, 28 Oct. 1958; ABMA, Juno V Development, pp. 19-20, 65; Army Ordnance Missile Command (hereafter cited as AOMC), Saturn Systems Study, by H. H. Koelle, F. L. Williams, and W. C. Huber, report DSP-TM-1-59 (Redstone Arsenal, AL, 13 Mar. 1959), pp. 16-19, 61- 63. 183-89; House Committee on Science and Astronautics, Equatorial Launch Sites - Mobile Sea Launch Capability, report 710, 87th Cong., 1st sess., 12 July 1961, pp. 1-5 (see hearings of same committee and topic, 15-16 May 1961, for fuller discussion): Mrazek interview. The debate over the merits of an equatorial launch site or a mobile sea launch capability continued for several years with congressional hearings in the spring of 1961. Vice Adm. John T. Hayward was a leading advocate of shipboard launches.&lt;/p&gt;&#xA;&lt;p&gt;(7) Missile Firing Laboratory, &amp;quot;Project Saturn, Facilities for Launch Site,&amp;quot; n.d.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Why does NASA not relocate someplace else now? The answer is simple - first, and foremost, COST. Building the whole supporting infrastructure from the ground up at a new place is very very costly and &lt;strong&gt;will not bring any money&lt;/strong&gt; since &lt;strong&gt;NASA does not do commercial launches&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Moonport: A History of Apollo Launch Facilities and Operations.&lt;/strong&gt;&#xA;By Charles D. Benson and William Barnaby Faherty. NASA SP-4204. &lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-13T13:50:05.170" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1005" PostTypeId="2" ParentId="768" CreationDate="2013-08-06T13:15:48.253" Score="7" Body="&lt;p&gt;&lt;strong&gt;&lt;sub&gt;Since my question didn't yet receive an answer, I did some digging around of my own. This should be considered as a temporary answer, before more precise ones will be actually possible.&lt;/sub&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The answer is, that &lt;strong&gt;for the time being, we don't know yet&lt;/strong&gt;, because we don't yet know what orbit around &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow noreferrer&quot;&gt;67P/Churyumov–Gerasimenko&lt;/a&gt; will be first attempted, or is indeed the most desirable one. And we likely won't know until the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta spacecraft&lt;/a&gt; is awaken from its hibernation mode it's currently in (entered on 8 June 2011) to conserve energy, and it makes its own observations of the 67P/Churyumov–Gerasimenko.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Approach itself will be based on ground observations alone, due to onboard camera capabilities, but orbital maneuvers after approach will be determined based on Rosetta's observations, too, presumably during last stages of the approach in &lt;strong&gt;May 2014&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The deep sleep is made necessary by the craft's enormous distance from&#xA;  the Sun and the weakness of the sunlight falling on its solar panels,&#xA;  which cannot produce enough electricity to power the probe fully. (Source: &lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/Rosetta_comet_probe_enters_hibernation_in_deep_space&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is scheduled to be awoken from this deep sleep, or hibernation, on January 2014, when it will begin its preliminary observations and a controlled slow approach to the comet's core, where it will enter a slow speed (relative to comet's core) orbit around it, to about &lt;strong&gt;25 metres per second&lt;/strong&gt;. Calculations to Rosetta's approach and orbiting trajectory will be made synchronous to its own observations:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As Rosetta drifts towards the heart of the comet, &lt;strong&gt;the mission team&#xA;  will try to avoid any comet dust and achieve good comet illumination&#xA;  conditions&lt;/strong&gt;. The first camera images will dramatically improve&#xA;  calculations of the comet’s position and orbit, as well as its size,&#xA;  shape and rotation. (Source: &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Comet_rendezvous&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/RJKV4.jpg&quot; alt=&quot;ROSETTA ORBITING COMET 67P/CHURYUMOV-GERASIMENKO&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Artist’s impression of Rosetta approaching the nucleus of Comet 67P/Churyumov-Gerasimenko. Source and hi-res image at: &lt;a href=&quot;http://spaceinimages.esa.int/Images/2002/11/Rosetta_orbiting_Comet_67P_Churyumov-Gerasimenko&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rosetta spacecraft's approach and orbit will be operated from the ground, but the spacecraft does have ability to react to observed conditions on its own, since the communication with the ground control can take up to 50 minutes in a single direction:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rosetta will be operated from the ground. It is impossible to have the&#xA;  manoeuvres for the whole mission programmed before the launch because&#xA;  &lt;strong&gt;adjustments will have to be made at each stage of the journey&lt;/strong&gt;. At the&#xA;  appropriate time, commands will be sent from Earth to adjust the&#xA;  spacecraft’s trajectory. But instructions from the ground can take up&#xA;  to 50 minutes to reach the spacecraft, so to overcome the delay,&#xA;  Rosetta must have the ‘intelligence’ to look after itself. This is&#xA;  done by its on-board computers, whose tasks include data management&#xA;  and attitude and orbit control. If any problems arise during the&#xA;  cruise, back-up systems will ensure that the spacecraft can remain&#xA;  operational during critical mission phases. For example, the&#xA;  spacecraft will automatically position itself with the solar panels&#xA;  facing the Sun, to avoid it becoming powerless. (Source: &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Short of the last two quotes above, I wasn't able to find any additional source of information on what orbit around 67P/Churyumov–Gerasimenko would be considered most desirable, or estimated trajectory corrections for the spacecraft due areas of increased particle density it might be flying through, especially as the comet approaches the Sun and slingshots past it (November 2014 – December 2015), and the coma and tails increase in size and density.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-09T05:34:21.960" LastActivityDate="2013-08-09T05:34:21.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1006" PostTypeId="2" ParentId="993" CreationDate="2013-08-06T13:23:07.933" Score="14" Body="&lt;p&gt;&lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsn/&quot;&gt;DSN&lt;/a&gt; time is competitive, and not cheap.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, besides the costs that Tom mentioned in getting the camera there (and an appropriate relay satellite), you have to consider the costs of getting the data back to the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the space station, it's only ~370km (230 miles) up ... relatively close, and they can use much smaller dishes than what's required to get decent telemetry from spacecraft at Mars or even the moon (359 Mm to 406 Mm).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take as an example, the &lt;a href=&quot;http://stereo.gsfc.nasa.gov/&quot;&gt;STEREO&lt;/a&gt; mission -- &lt;a href=&quot;http://stereo-ssc.nascom.nasa.gov/plans/dsn_schedule.shtml&quot;&gt;each spacecraft gets one or two DSN passes per day&lt;/a&gt;, but they're also &lt;a href=&quot;http://www.swpc.noaa.gov/stereo/STEREO_SWB_Support.html&quot;&gt;tracked by smaller dishes in the U.S., Asia, and Europe&lt;/a&gt; to try to get near-real-time space weather information. (it was as small as 5.5m dishes early in the mission vs. the 34m and 70m ones from the DSN; they're now using a mix of dishes; I know &lt;a href=&quot;http://www.amsat.org/amsat/articles/g3ruh/127.html&quot;&gt;Bochum is 20m&lt;/a&gt; )&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right now, the spacecraft get 480 kbps from the DSN (to decrease to 160kbps when they're near 2AU (300 Gm) from earth).  The beacon mode data, as it needs to be downlinked on the smaller dishes ... is closer to 600bps. (sorry, couldn't find a comprehensive telemetry rate table)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;a href=&quot;http://stereo-ssc.nascom.nasa.gov/plans.shtml&quot;&gt;current plans&lt;/a&gt;, under 2013 week 37, they expect 'Significant telemetry shortfalls'.  That's the competitive nature -- it might be that some other spacecraft with a higher priority is in the same region of the sky.  (eg, we get less time when one of the spacecraft is in the same direction from earth as mars, or when there's a major launch)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We get donated time for STEREO beacon because the data's of value to many groups ... I'm not sure that you'd be able to get the same for something that's just for public outreach.  (Crowd funding might work, but if the images are really boring, maybe not)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Disclaimer : I work for the Solar Data Analysis Center, which oversees the STEREO beacon data processing.&lt;/p&gt;&#xA;" OwnerUserId="46" LastActivityDate="2013-08-06T13:23:07.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1007" PostTypeId="2" ParentId="999" CreationDate="2013-08-06T13:56:41.203" Score="19" Body="&lt;p&gt;NASA has a decent FAQ section on their website about orbital debris. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How many orbital debris are currently in Earth orbit?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;More than 21,000 orbital debris larger than 10 cm are known to exist. The estimated population of particles between 1 and 10 cm in diameter is approximately 500,000. The number of particles smaller than 1 cm exceeds 100 million. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How long will orbital debris remain in Earth orbit?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The higher the altitude, the longer the orbital debris will typically remain in Earth orbit. Debris left in orbits below 600 km normally fall back to Earth within several years. At altitudes of 800 km, the time for orbital decay is often measured in decades. Above 1,000 km, orbital debris will normally continue circling the Earth for a century or more. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Since most of the accounted for orbital debris is located in the 750-800 KM range, it would take several decades for orbital decay to significantly reduce the amount of man made space debris. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/faqs.html&quot;&gt;Orbital Debris FAQ&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="169" LastActivityDate="2013-08-06T13:56:41.203" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1009" PostTypeId="1" AcceptedAnswerId="1010" CreationDate="2013-08-06T17:03:49.120" Score="21" ViewCount="4254" Body="&lt;p&gt;ISS is at an altitude of about 400 km (approximately). But there will be some of space debris at that altitude (since some of the space debris orbits decay over time). So the debris has a probability of coming in contact (colliding) with the ISS. So, how does the ISS avoid those collisions?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="80" LastEditDate="2013-08-06T17:13:25.647" LastActivityDate="2015-05-02T22:22:49.653" Title="How is the International Space Station protected against orbital debris?" Tags="|iss|debris|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1010" PostTypeId="2" ParentId="1009" CreationDate="2013-08-06T17:57:35.957" Score="16" Body="&lt;p&gt;I will assume you are asking about collision risk management. As usual, it comes in several guises:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Avoidance&lt;/li&gt;&#xA;&lt;li&gt;Protection&lt;/li&gt;&#xA;&lt;li&gt;Mitigation&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;See this reference: &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/news/orbital_debris.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nasa.gov/mission_pages/station/news/orbital_debris.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key assets in Collision Avoidance are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the &lt;a href=&quot;https://en.wikipedia.org/wiki/Haleakala_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;telescopes&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Air_Force_Space_Surveillance_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;radars&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Based_Space_Surveillance&quot; rel=&quot;nofollow noreferrer&quot;&gt;satellites&lt;/a&gt; in the U.S. Department of Defense &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Surveillance_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Surveillance Network&lt;/a&gt; that help in detecting, classifying and estimating orbital parameters of space debris&lt;/li&gt;&#xA;&lt;li&gt;computers in DoD’s &lt;a href=&quot;https://en.wikipedia.org/wiki/JSpOC&quot; rel=&quot;nofollow noreferrer&quot;&gt;Joint Space Operations Center&lt;/a&gt; cranking through the large volumes of data obtained by the surveillance network and identifying dangerous stuff up there&lt;/li&gt;&#xA;&lt;li&gt;computers, qualified personnel and procedures at NASA &lt;a href=&quot;https://en.wikipedia.org/wiki/MCC-H&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mission Control Center-Houston&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;counterparts of the above in Russia (&lt;a href=&quot;https://en.wikipedia.org/wiki/TsUP&quot; rel=&quot;nofollow noreferrer&quot;&gt;MCC-M&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Centre_for_Outer_Space_Monitoring&quot; rel=&quot;nofollow noreferrer&quot;&gt;MSIC&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;When &lt;a href=&quot;https://en.wikipedia.org/wiki/Debris_avoidance_maneuver&quot; rel=&quot;nofollow noreferrer&quot;&gt;Debris Avoidance Maneuvers&lt;/a&gt; cannot be performed due to late detection of the threat, risk avoidance procedures force the ISS crew to go for the boats (err, Soyuz vehicles).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Protection is indeed &lt;a href=&quot;https://en.wikipedia.org/wiki/Whipple_shield&quot; rel=&quot;nofollow noreferrer&quot;&gt;Whipple shields&lt;/a&gt;, as &lt;a href=&quot;https://space.stackexchange.com/users/49/tildalwave&quot;&gt;TildalWave&lt;/a&gt; graciously noted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mitigation comes into play after the collision. As discussed elsewhere (&lt;a href=&quot;https://space.stackexchange.com/questions/881/are-there-any-safety-procedures-in-place-on-the-iss-in-case-of-puncture&quot;&gt;Are there any safety procedures in place on the ISS in case of puncture?&lt;/a&gt;), if there is time to isolate leaking compartments, the crew may do so. However, repairing the station is considered to be the job of followup expeditions.&lt;/p&gt;&#xA;&#xA;&lt;hr/&gt;&#xA;&#xA;&lt;p&gt;Broadly speaking, there are three other possible solutions to the problem that have not been implemented on the ISS:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Prevention of debris generation (by responsible design)&lt;/li&gt;&#xA;&lt;li&gt;Debris collection&lt;/li&gt;&#xA;&lt;li&gt;Active defense (with kinetic interceptors or &lt;a href=&quot;https://dx.doi.org/10.1016/j.actaastro.2015.03.004&quot; rel=&quot;nofollow noreferrer&quot;&gt;laser ablation&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;A hat tip to &lt;a href=&quot;https://space.stackexchange.com/users/2843/osgx&quot;&gt;osgx&lt;/a&gt; for references on ideas on installing a debris detection telescope and a laser on the ISS:&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;sub&gt;Toshikazu Ebisuzaki, Mark N. Quinn, Satoshi Wada et al. Demonstration designs for the remediation of space debris from the International Space Station. Acta Astronautica, July 2015. DOI &lt;a href=&quot;http://dx.doi.org/10.1016/j.actaastro.2015.03.004&quot; rel=&quot;nofollow noreferrer&quot;&gt;10.1016/j.actaastro.2015.03.004&lt;/a&gt;&lt;/sub&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;sub&gt;Wolfgang O. Schall. Laser requirements for the removal of space debris from orbit. Proceedings of SPIE 3574 (1998) pp. 426–436. DOI &lt;a href=&quot;https://dx.doi.org/10.2514/2.3785&quot; rel=&quot;nofollow noreferrer&quot;&gt;10.2514/2.3785&lt;/a&gt; - the first paper proposing an ISS-based laser.&lt;/sub&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-05-02T22:22:49.653" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1011" PostTypeId="1" AcceptedAnswerId="1012" CreationDate="2013-08-06T18:50:51.130" Score="13" ViewCount="365" Body="&lt;p&gt;It seems that most rover-related attention recently has been on Mars, with Curiosity recently getting there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it seems there are also benefits to exploring places a little closer to home, such as the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have there been any unmanned surface missions to the moon in the past decade? Have there been &lt;em&gt;any&lt;/em&gt;?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2019-07-13T04:29:25.870" Title="Have there been any recent lunar rovers?" Tags="|the-moon|rovers|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1012" PostTypeId="2" ParentId="1011" CreationDate="2013-08-06T19:04:14.083" Score="8" Body="&lt;p&gt;Not in the last decade, no. The Russian &lt;a href=&quot;http://solarsystem.nasa.gov/missions/profile.cfm?Print=0&amp;amp;Sort=Nation&amp;amp;MCode=Luna_24&amp;amp;Nation=USSR&amp;amp;Display=ReadMore&amp;amp;FIELDNAMES=&quot; rel=&quot;noreferrer&quot;&gt;Luna 24&lt;/a&gt; was the last unmanned lunar rover, and it was deployed in 1976.&lt;/p&gt;&#xA;&lt;p&gt;However, there are several &lt;em&gt;planned&lt;/em&gt; lunar rovers, such as the Chinese &lt;a href=&quot;http://www.americaspace.com/?p=35272&quot; rel=&quot;noreferrer&quot;&gt;Chang'e 3&lt;/a&gt; rover. It is scheduled for launch in &lt;strong&gt;December 2013&lt;/strong&gt; - fairly soon!&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dX9gR.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;After landing, a solar-powered rover will roll off the lander and commence its mission, which is expected to last for at least three months, although presumably that will include a lot of down-time while the two-week-long lunar nights prevail.&lt;/p&gt;&#xA;&lt;p&gt;The Chang’e 3 lander itself will continue to operate as a stationary science platform. It will be powered by a radioisotope thermoelectric generator and hence will be largely unaffected by the presence or absence of direct sunlight. The lander will operate a number of science instruments, including an optical telescope and a “soil probe” to conduct analyses of lunar regolith.&lt;/p&gt;&#xA;&lt;p&gt;The Chang’e 3 rover will have a mass of 120 kilograms, including a 20 kg science payload.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-06T19:04:14.083" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1013" PostTypeId="5" CreationDate="2013-08-06T20:10:40.453" Score="0" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Rover_(space_exploration)&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A rover (or sometimes planetary rover) is a space exploration vehicle designed to move across the surface of a planet or other celestial body. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Rovers are usually unmanned, with the only exception being the Apollo Lunar Roving Vehicle which was used by six of the Apollo astronauts on three missions (Apollo 15, 16, 17). The first successful unmanned rover mission was sent to the Moon in 1970 by the former Soviet Union, and there are several current examples of rovers on the Moon and Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This tag is appropriate for any questions related to the past, present or future rover missions to any celestial body. This can include the history, science, technology, and management of the vehicles.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-14T15:46:58.757" LastActivityDate="2015-07-14T15:46:58.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1014" PostTypeId="4" CreationDate="2013-08-06T20:10:40.453" Score="0" Body="Questions about vehicles sent to extraterrestrial bodies, generally for use in data collection." OwnerDisplayName="user12" LastEditorUserId="10234" LastEditDate="2015-07-14T15:46:53.943" LastActivityDate="2015-07-14T15:46:53.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1015" PostTypeId="5" CreationDate="2013-08-06T20:15:35.727" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-06T20:15:35.727" LastActivityDate="2013-08-06T20:15:35.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1016" PostTypeId="4" CreationDate="2013-08-06T20:15:35.727" Score="0" Body="Questions about debris in space, often in Earth orbit. Usually referring to nonoperational man-made satellites or parts of satellites." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-06T20:58:53.250" LastActivityDate="2013-08-06T20:58:53.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1017" PostTypeId="5" CreationDate="2013-08-06T20:28:48.310" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-06T20:28:48.310" LastActivityDate="2013-08-06T20:28:48.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1018" PostTypeId="4" CreationDate="2013-08-06T20:28:48.310" Score="0" Body="Questions relating to the exploration of Titan, a moon of Saturn." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-06T21:11:12.973" LastActivityDate="2013-08-06T21:11:12.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1019" PostTypeId="5" CreationDate="2013-08-06T21:56:37.693" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-06T21:56:37.693" LastActivityDate="2013-08-06T21:56:37.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1020" PostTypeId="4" CreationDate="2013-08-06T21:56:37.693" Score="0" Body="Relating to creating, testing, and refining technology for spacecraft." OwnerDisplayName="user12" LastEditorUserId="4660" LastEditDate="2015-04-23T16:22:44.973" LastActivityDate="2015-04-23T16:22:44.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1021" PostTypeId="2" ParentId="247" CreationDate="2013-08-06T23:56:18.227" Score="3" Body="&lt;p&gt;Same thing happens in aviation as you've indentified for space technology. Major factors would be reliability, &quot;hardness&quot; and development time frames, but there are other considerations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any life-critical system has to be trustworthy, and when you can't get at it to fix it if it breaks (like robotic space probes), reliability becomes paramount also. The longer a thing has been around and accumulated experience, the more it can be trusted. Also, the more complex a system is, the more difficult it can be to verify that all &quot;working parts&quot; are working as they should. The newest technologies are always pushing boundaries of one form or another, challenging the limits of what can be done. That can put a thing on the edge of catastrophe - not a good place to be when lives are on the line. Newer computing technology is always more sophisticated (more complex) than what it replaces, making verification/validation more difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Airplanes and rockets operate in harsh environments; the vehicles themselves create harsh or perhaps extreme environments for some of their own components. It's difficult to build electronic components and systems which can operate in such conditions - temperature, shock/vibration, EMI, radiation, etc. without some challenge to reliabilty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes a long time (years) for a new aircraft or space system to make it from the initial design to &quot;first launch&quot;, and design of subsystems (including those employing computers) has to be frozen at some point in the process. Computer technology moves much faster, so the designs get frozen with what is trustworthy (perhaps already becoming obsolete), and computer technology marches further along before the aircraft or rocket takes flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It really might not be a wise thing to try to do it any other way. When your life is in the balance, much better to have an old, crude but dependable system than something new and snazzy, but not fully proven.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2013-08-08T22:34:36.803" LastActivityDate="2013-08-08T22:34:36.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1022" PostTypeId="5" CreationDate="2013-08-07T00:17:36.150" Score="0" Body="&lt;p&gt;Spacecraft often have the need of changing their velocity, for example when performing an orbital transfer. To do so, propulsion is needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the spacecraft's velocity is changed, momentum has to be conserved. This problem is usually solved by accelerating mass in the opposite direction. That is the case for:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Chemical rockets, using energy from a chemical reaction to expand the exhaust products through a nozzle.&lt;/li&gt;&#xA;&lt;li&gt;Thermal rockets, using some externally provided power (beamed or nuclear) to accelerate a propellant.&lt;/li&gt;&#xA;&lt;li&gt;Ion engines, using an electric or magnetic field to accelerate charged particles.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Momentum can also be transferred without sacrificing mass by photons, either by reflecting sunlight or laser light with a sail, or by emitting photons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The preferred form of propulsion is chosen from the requirements for acceleration and the magnitude of the acceleration change (known as delta-V or $\Delta V$), and by how much propellant the spacecraft can carry or how much energy is available.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Existing technologies:&lt;/h3&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Chemical rockets, high acceleration, low propellant efficiency. Widely used.&lt;/li&gt;&#xA;&lt;li&gt;Nuclear rockets, modest acceleration, good propellant efficiency. Prototypes built by the US and the Soviet Union.&lt;/li&gt;&#xA;&lt;li&gt;Ion engines, low acceleration, exceptional propellant efficiency. Used by some small probes for interplanetary missions, and by a number of satellites for maintaining orbit (stationkeeping).&lt;/li&gt;&#xA;&lt;li&gt;Solar sails, minuscule acceleration, does not require propellant. Some prototypes tested.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2016-12-20T12:43:06.450" LastActivityDate="2016-12-20T12:43:06.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1023" PostTypeId="4" CreationDate="2013-08-07T00:17:36.150" Score="0" Body="Systems for propelling a spacecraft from one place to another, such as rockets." OwnerDisplayName="user12" LastEditorUserId="6816" LastEditDate="2016-12-20T13:38:32.337" LastActivityDate="2016-12-20T13:38:32.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1024" PostTypeId="5" CreationDate="2013-08-07T02:27:17.330" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-07T02:27:17.330" LastActivityDate="2013-08-07T02:27:17.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1025" PostTypeId="4" CreationDate="2013-08-07T02:27:17.330" Score="0" Body="Questions about telescopes used to detect electromagnetic radiation; specifically radio waves." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-07T12:47:44.593" LastActivityDate="2013-08-07T12:47:44.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1026" PostTypeId="1" CreationDate="2013-08-07T04:34:32.567" Score="1" ViewCount="513" Body="&lt;p&gt;I am wondering what type of telescope I could use to see the surface of the moon. I am fascinated by seeing marks of human feet.&lt;/p&gt;&#xA;" OwnerUserId="463" LastEditorUserId="168" LastEditDate="2013-08-07T07:15:17.980" LastActivityDate="2013-08-07T07:22:32.277" Title="What type of telescope should I use in order to view the surface of the moon?" Tags="|the-moon|telescope|" AnswerCount="1" CommentCount="1" ClosedDate="2013-08-07T15:37:18.110" ContentLicense="CC BY-SA 3.0" />
  <row Id="1027" PostTypeId="2" ParentId="1026" CreationDate="2013-08-07T05:30:41.697" Score="3" Body="&lt;p&gt;Depends if you want to do astrophotography or just visual imaging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Visual imaging then a Newtonian &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astro imaging then a refractor  or bigger scopes for getting those craters really big Cassegrain telescopes  the choice is yours...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is just a sample what a refractor and a webcam can do: -&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/v2yHn.jpg&quot; alt=&quot;Moon Mosaic&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="466" LastEditorUserId="168" LastEditDate="2013-08-07T07:22:32.277" LastActivityDate="2013-08-07T07:22:32.277" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1028" PostTypeId="1" AcceptedAnswerId="1045" CreationDate="2013-08-07T07:59:46.123" Score="10" ViewCount="2021" Body="&lt;p&gt;Yes, I know it's a hard question considering the tight interaction between military and &quot;civilian&quot; space programs and the Great Wall, but it is really interesting to know the scale of the &quot;enterprise&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that speculative opinions aren't welcome. Please state references used and the year for which the data are valid.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-07T08:15:13.367" LastActivityDate="2017-10-26T19:13:42.680" Title="How many people work in the Chinese space program?" Tags="|china|space-industry|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1029" PostTypeId="1" AcceptedAnswerId="1611" CreationDate="2013-08-07T08:36:02.133" Score="7" ViewCount="175" Body="&lt;p&gt;This is a question at the nexus of image interpretation and methodology of planetary science.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How can &lt;a href=&quot;http://hirise.lpl.arizona.edu&quot;&gt;HiRISE images&lt;/a&gt; be used in confirming or refuting hypotheses about the past and present geology of Mars in the areas which our rovers haven't explored?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Have there been any meta-studies on the diversity of sources used in scientific articles (e.g. HiRISE alone, HiRISE + THEMIS, HiRISE with digital terrain models etc.) on Martian geology? How many scientists down here rely on their interpretation of imagery?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How many of the ideas proposed in the articles were subsequently disproven by in-place exploration by the rovers?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="25" LastEditDate="2018-02-11T23:02:23.713" LastActivityDate="2018-02-11T23:02:23.713" Title="How reliable is HiRISE imagery as a source of knowledge about Mars?" Tags="|mars|instrument|mars-reconnaissance-orbiter|planetology|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1030" PostTypeId="1" AcceptedAnswerId="1033" CreationDate="2013-08-07T10:35:45.630" Score="16" ViewCount="941" Body="&lt;p&gt;Because the moon always has the same &lt;a href=&quot;http://en.wikipedia.org/wiki/Tidal_locking&quot;&gt;orientation to earth&lt;/a&gt;, if we want to have communications with manned or robotic explorers on the far side some type of relay would be needed for communication.  Would a communication satellites used in lunar orbit be possible or would the tidal effects of earth disrupt the orbit to make it impossible/impractical?&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2020-04-09T01:32:37.677" Title="Can communication satellites be used in lunar orbit?" Tags="|orbital-mechanics|the-moon|communication-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1031" PostTypeId="2" ParentId="911" CreationDate="2013-08-07T10:42:42.503" Score="8" Body="&lt;p&gt;A quick search through the space agencies' sites yielded this photo of the IM-01M mass measuring device in the Russian orbital segment:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Anatoli_Ivanishin&quot; rel=&quot;noreferrer&quot;&gt;Anatoly Ivanischin&lt;/a&gt; undertakes periodic weighing of his body on the &quot;Mass measuring&quot; device. Credits: &lt;a href=&quot;http://www.roscosmos.ru/main.php?lang=en&quot; rel=&quot;noreferrer&quot;&gt;Roscosmos&lt;/a&gt;. The likely author of the photo is &lt;a href=&quot;https://en.wikipedia.org/wiki/Anton_Shkaplerov&quot; rel=&quot;noreferrer&quot;&gt;Anton Shkaplerov&lt;/a&gt;.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/uhpis.jpg&quot; alt=&quot;Ivanisin&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Peggy_Whitson&quot; rel=&quot;noreferrer&quot;&gt;Peggy Whitson&lt;/a&gt;: We weigh ourselves with a Russian-built device called the body mass measuring system. It calculates inertia as it moves a mass back and forth on a calibrated spring. It's very simple system, but seems to work very well. After we calibrate the spring, we climb on the device and release the spring. Our mass is calculated from the inertial forces on the spring.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://spaceflight.nasa.gov/feedback/expert/answer/isscrew/expedition5/index.html&quot; rel=&quot;noreferrer&quot;&gt;http://spaceflight.nasa.gov/feedback/expert/answer/isscrew/expedition5/index.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-08T10:46:01.997" LastActivityDate="2013-08-08T10:46:01.997" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1032" PostTypeId="1" AcceptedAnswerId="1038" CreationDate="2013-08-07T11:22:12.260" Score="-2" ViewCount="386" Body="&lt;p&gt;Will astronauts in space stations such as the International Space Station which orbits the Earth feel their weight?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-08-07T17:08:23.767" LastActivityDate="2013-08-07T17:08:23.767" Title="Do astronauts feel their weight in orbit?" Tags="|microgravity|" AnswerCount="3" CommentCount="4" FavoriteCount="0" ClosedDate="2013-08-08T04:57:58.137" ContentLicense="CC BY-SA 3.0" />
  <row Id="1033" PostTypeId="2" ParentId="1030" CreationDate="2013-08-07T11:24:39.903" Score="18" Body="&lt;p&gt;It's not really about tidal effects of the Earth but about irregularities of &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects&quot; rel=&quot;noreferrer&quot;&gt;lunar gravity&lt;/a&gt; (&lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_concentration_%28astronomy%29&quot; rel=&quot;noreferrer&quot;&gt;Mascons&lt;/a&gt;) necessitating large fuel expenditures for maintaining an orbit around the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/k1j8I.jpg&quot; alt=&quot;GRAIL gravity anomaly map&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taken from: &lt;a href=&quot;http://boingboing.net/2013/06/23/lunar-gravity-maps.html&quot; rel=&quot;noreferrer&quot;&gt;http://boingboing.net/2013/06/23/lunar-gravity-maps.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solution (first proposed in 1966 by R. Farquhar) is to place a comms relay into a &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot; rel=&quot;noreferrer&quot;&gt;halo orbit&lt;/a&gt; around the Earth-Moon L2 &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrange_point&quot; rel=&quot;noreferrer&quot;&gt;Lagrange point&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1973 Farquhar and Kamel found a &quot;natural&quot; halo orbit providing almost constant coverage (in-plane oscillation should be greater than 32379 kilometers):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0dGzD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Farquhar, R. W. The Control and Use of Libration-Point Satellites. Ph.D. Dissertation, Dept. of Aeronautics and Astronautics, Stanford University, Stanford, CA, 1968.&lt;/li&gt;&#xA;&lt;li&gt;J.Masdemont. Libration Point Orbits and Applications. 2003. P.47.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastActivityDate="2013-08-07T11:24:39.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1034" PostTypeId="1" AcceptedAnswerId="1040" CreationDate="2013-08-07T11:42:20.263" Score="17" ViewCount="24266" Body="&lt;p&gt;The Sun emits electromagnetic radiation which may consist of UV, IR, X-rays and also the charge particles that are emitted from the sun which are harmful to humans. But during intense solar flares it emits more radiation and even gamma rays. How are astronauts in the ISS protected from the harmful radiation?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-08-07T11:58:06.317" LastActivityDate="2020-12-11T17:55:25.847" Title="How are the astronauts in the ISS protected from solar flares?" Tags="|iss|space-weather|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1035" PostTypeId="1" AcceptedAnswerId="1057" CreationDate="2013-08-07T12:50:21.903" Score="9" ViewCount="1185" Body="&lt;p&gt;How can I guarantee global coverage with the fewest number of satellites? Assume that link margin isn't a problem.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-08T01:36:20.703" Title="What is the fewest number of satellites required to have constant global coverage?" Tags="|orbit-selection|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="1036" PostTypeId="2" ParentId="634" CreationDate="2013-08-07T13:19:20.680" Score="21" Body="&lt;p&gt;Using liberally &quot;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/145160811X&quot;&gt;The Case For Mars&lt;/a&gt;&quot; second edition. Here's a few facts that help out in the discussion.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;There is substantial Carbon Dioxide at the poles, which if all of it melted, would thicken the atmosphere considerably. &lt;/li&gt;&#xA;&lt;li&gt;The theory is current that a 4 degree centigrade rise in temperature in the South Pole (Sustained) would trigger a run away greenhouse effect, more drastic than is predicted by Earth global warming scenarios.&lt;/li&gt;&#xA;&lt;li&gt;We don't really know how much gas Mars loses every day, but there is &lt;a href=&quot;http://abcnews.go.com/Technology/curiosity-reveals-mars-losing-atmosphere/story?id=19701439&quot;&gt;definitely some lost&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The absence of a magnetic field is a significant contributor to the loss of atmosphere on Mars.&lt;/li&gt;&#xA;&lt;li&gt;The atmospheric levels &lt;a href=&quot;http://hypertextbook.com/facts/2000/LaurenMikulski.shtml&quot;&gt;change rapidly&lt;/a&gt; as a result of gas being switched between poles during season changes. That level is 30%. I believe that accounts for the 30% @RhysW quoted in his answer, but it comes back on a regular basis.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line, some level of terraforming on Mars is entirely possible. We would likely have to replenish the atmosphere periodically, perhaps once every thousand or tens of thousands of years. &lt;a href=&quot;http://www.nasa.gov/mission_pages/maven/main/index.html&quot;&gt;MAVEN&lt;/a&gt; will answer many of the unanswered questions (Specifically, what is the rate of loss of atmosphere on Mars)&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-07T16:53:03.157" LastActivityDate="2013-08-07T16:53:03.157" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1037" PostTypeId="2" ParentId="1032" CreationDate="2013-08-07T13:51:06.127" Score="1" Body="&lt;p&gt;No.  They're in freefall.  They don't weigh anything as a result.&lt;/p&gt;&#xA;" OwnerUserId="157" LastActivityDate="2013-08-07T13:51:06.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1038" PostTypeId="2" ParentId="1032" CreationDate="2013-08-07T14:09:44.933" Score="3" Body="&lt;p&gt;No, because they are basically in free fall, as they are constantly accelerating - just as if you had jumped off of an airplane - you would &lt;em&gt;feel&lt;/em&gt; weightless, as gravity can move you exactly as it wants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, how does it work that the space station doesn't 'fall into' earth? Well, it basically falls 'around' Earth - which is how any orbit works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6d81P.png&quot; alt=&quot;orbit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Image credit &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Centripetal_force_diagram.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this image, the big blue dot is the ISS, while the little dot is the Earth (don't ask me why.) As you can see, the ISS has velocity tangential to it's orbit - it wants to fly off into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, think of the Earth's gravity as a string - it's basically a force that really wants to keep the space station a certain distance away, for all intents and purposes here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now imagine you have, down on Earth, one of those rubber bouncy balls with a hole drilled through it and a string through that hole. If you start the ball moving tangentially to the circle the string would make, the ball would travel along that circle. If there wasn't any friction, the ball would just keep doing this... just like the ISS.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-07T14:39:50.137" LastActivityDate="2013-08-07T14:39:50.137" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1039" PostTypeId="2" ParentId="1032" CreationDate="2013-08-07T14:26:35.263" Score="1" Body="&lt;p&gt;Yes! The astronauts do in fact feel their mass (not technically their weight) when they try to move themselves or other objects. You can watch videos of astronauts unpacking the SpaceX or Progress vehicles, and when they move packages it is quite obvious that both the packages &lt;em&gt;and&lt;/em&gt; the astronaut move about as a reaction.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2013-08-07T14:26:35.263" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1040" PostTypeId="2" ParentId="1034" CreationDate="2013-08-07T14:38:41.647" Score="20" Body="&lt;p&gt;So far, Solar flares haven't proven as dangerous as one would possibly think. They're nothing to easily dismiss, mind you, and the ISS crew does have procedures in place to &lt;em&gt;&quot;weather the storm&quot;&lt;/em&gt;, but nothing too spectacular has yet happened due to them. The ISS crew did receive a Solar weather warning several times and were advised to enter the more protected areas of the ISS, such as the US built &lt;a href=&quot;https://en.wikipedia.org/wiki/Destiny_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Destiny laboratory&lt;/a&gt;, or the Russian built service module &lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Zvezda&lt;/a&gt;, but they were in all instances well within the safety margins. On one occasion (in &lt;a href=&quot;http://www.space.com/3247-astronauts-sleep-safety-solar-flare.html&quot; rel=&quot;noreferrer&quot;&gt;December 2006&lt;/a&gt;) even sleeping throughout the event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most interesting thing about Solar &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot; rel=&quot;noreferrer&quot;&gt;coronal mass ejections&lt;/a&gt; (the cause of &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_flare&quot; rel=&quot;noreferrer&quot;&gt;Solar flares&lt;/a&gt;) is that these are highly magnetised particles, and they actually help push away other charged particles (such as high-energy protons in &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_ray&quot; rel=&quot;noreferrer&quot;&gt;Cosmic rays&lt;/a&gt;) with the effect known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Forbush_decrease&quot; rel=&quot;noreferrer&quot;&gt;Forbush decrease&lt;/a&gt;. Such particles are lot bigger concern to astronauts stationed on ISS:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Cosmic rays are super-charged subatomic particles coming mainly from&#xA;  outside our solar system. Sources include exploding stars, black holes&#xA;  and other characters that dwarf the sun in violence. Unlike solar&#xA;  protons, which are relatively easy to stop with materials such as&#xA;  aluminum or plastic, cosmic rays cannot be completely stopped by any&#xA;  known shielding technology.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Even inside their ships, astronauts are exposed to a slow drizzle of&#xA;  cosmic rays coming right through the hull. The particles penetrate&#xA;  flesh, damaging tissue at the microscopic level. One possible&#xA;  side-effect is broken DNA, which can, over the course of time, cause&#xA;  cancer, cataracts and other maladies.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/wvyCs.gif&quot; alt=&quot;September 2005 Forbush decrease&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;(Excerpts and September 2005 Forbush decrease graph source: &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2005/07oct_afraid/&quot; rel=&quot;noreferrer&quot;&gt;NASA Science News&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of high-energy subatomic particles in cosmic rays that would penetrate ISS hull with ease (compared to protons of coronal mass ejection, that can't pass through ISS external shield / hull that easily) actually decreases during Solar flares on their way towards the Earth, forming a massive magnetic field on top of Earth's own.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another point is, that while Solar flares can reach head spinning speeds of several millions of kilometers per hour, the average distance from the Sun is still in the vicinity of 150 million kilometers, giving ISS crew ample warning time to react, with average of at least a day's worth of advance warning, so they can prepare and move to better protected areas of ISS to &lt;em&gt;&quot;weather the storm&quot;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On top of advance warnings due to observations of the Sun's activity by several satellites, and also ground based observations, ISS is still well within the Earth's magnetic field, which helps reduce the effects of prolonged exposure to Cosmic radiation. Solar storms can however cause other problems, such as burning sensitive electronic equipment with highly charged EM pulse, so the ISS would try and limit possible damage by other means too, and if deemed necessary, rotate photovoltaic arrays (solar panels wings) perpendicular to the impending Solar flare, rotate the whole station, or any other &lt;a href=&quot;https://en.wikipedia.org/wiki/Risk_management&quot; rel=&quot;noreferrer&quot;&gt;risk management&lt;/a&gt; procedure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ead30.jpg&quot; alt=&quot;ISS risk management example&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image: Example of risk management: A NASA model showing areas at high risk &lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;from impact for the International Space Station. Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Risk_management&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="10234" LastEditDate="2017-06-29T19:20:54.250" LastActivityDate="2017-06-29T19:20:54.250" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1042" PostTypeId="5" CreationDate="2013-08-07T16:14:15.173" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-07T16:14:15.173" LastActivityDate="2013-08-07T16:14:15.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1043" PostTypeId="4" CreationDate="2013-08-07T16:14:15.173" Score="0" Body="Questions regarding cosmic radiation sent out by objects in space such as the sun." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-07T16:49:33.750" LastActivityDate="2013-08-07T16:49:33.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1044" PostTypeId="1" AcceptedAnswerId="1049" CreationDate="2013-08-07T16:59:54.827" Score="13" ViewCount="184" Body="&lt;p&gt;I recently &lt;a href=&quot;http://www.spacedaily.com/m/reports/The_Suns_Magnetic_Field_is_about_to_Flip_999.html&quot;&gt;read&lt;/a&gt; that the sun's polarity is going to change. Will this effect cause damage to satellites and/or affect its communication with ground stations?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-08-16T16:24:35.660" LastActivityDate="2013-08-16T16:24:35.660" Title="Can change in sun's polarity affect satellites?" Tags="|magnetic-field|space-weather|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1045" PostTypeId="2" ParentId="1028" CreationDate="2013-08-07T17:17:02.403" Score="14" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Chinese_space_program&quot; rel=&quot;noreferrer&quot;&gt;Chinese Space Program&lt;/a&gt; is largely handled by two entities, &lt;a href=&quot;http://en.wikipedia.org/wiki/China_National_Space_Administration&quot; rel=&quot;noreferrer&quot;&gt;China National Space Administration (CNSA)&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/China_Aerospace_Science_and_Technology_Corporation&quot; rel=&quot;noreferrer&quot;&gt;China Aerospace Science and Technology Corporation (CASC)&lt;/a&gt;. They are both state agencies with the former handling the planning and the latter, the implementation.&lt;/p&gt;&#xA;&lt;p&gt;A Wired article dated July 1, 2013, and titled, &lt;a href=&quot;http://www.wired.co.uk/news/archive/2013-07/01/china-space-programme&quot; rel=&quot;noreferrer&quot;&gt;A dragon in space: China's space programme can no longer be ignored&lt;/a&gt; contributes a couple of figures:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;What you find in China is that there are many different actors involved in the space programme,&amp;quot; says Bergquist. The Chinese National Space Agency (CNSA) is little more than a clearing house that co-ordinates funding and international co-operation, to the extent that such co-operation exists. According to Berquist, it has &lt;strong&gt;only between 30 to 40 staff&lt;/strong&gt;, making it much more like the UK Space Agency and much less like Nasa.&lt;/p&gt;&#xA;&lt;p&gt;The real work is done by an array of contractors, most of which fall under the umbrella of the state-owned China Aerospace Science and Technology Corporation (CASC). &lt;strong&gt;Employing over 100,000 people, CASC&lt;/strong&gt; develops &lt;strong&gt;military hardware&lt;/strong&gt; like ballistic missiles and UAVs, as well as an array of space technology.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;NTI, a watchdog of some sort, &lt;a href=&quot;http://www.nti.org/facilities/64/&quot; rel=&quot;noreferrer&quot;&gt;notes&lt;/a&gt; (2012):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;120,000 employees, including more than 30 scholars of the Chinese Academy of Sciences (CAS) and the Chinese Academy of Engineering (CAE)&lt;a href=&quot;http://en.wikipedia.org/wiki/China_National_Space_Administration&quot; rel=&quot;noreferrer&quot;&gt;2&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The 120,000 number for CASC is also listed on this &lt;a href=&quot;http://www1.uwe.ac.uk/students/careersandemployability/gradlink/china/meettheemployers/casc.aspx&quot; rel=&quot;noreferrer&quot;&gt;University of the West of England profile page&lt;/a&gt; for the corporation.&lt;/p&gt;&#xA;&lt;p&gt;Kosmos Directive, an &amp;quot;international society that was inaugurated in 2011 in Moscow - Russia, dedicated to the development of studies in International Space Law and Space History of Humanity&amp;quot; &lt;a href=&quot;http://kosmosdirectives.com/news120507.html&quot; rel=&quot;noreferrer&quot;&gt;estimates CASC's employee numbers&lt;/a&gt; to be 110,000.&lt;/p&gt;&#xA;&lt;p&gt;So, a 100,000-120,000 employees appears to be the general consensus.&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-07T17:17:02.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1046" PostTypeId="1" AcceptedAnswerId="1048" CreationDate="2013-08-07T19:06:04.047" Score="33" ViewCount="3292" Body="&lt;p&gt;Curiosity &lt;a href=&quot;http://news.cnet.com/8301-17938_105-57597122-1/mars-curiosity-rover-sings-happy-birthday-to-itself/&quot;&gt;celebrated&lt;/a&gt; it's first birthday by playing, on Mars, Happy Birthday. How was this accomplished, and why would this capability even be included on Mars?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-07T19:20:36.500" Title="Why would NASA include a speaker on Curiosity?" Tags="|mars|curiosity|sound|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1047" PostTypeId="2" ParentId="1046" CreationDate="2013-08-07T19:19:37.400" Score="32" Body="&lt;p&gt;Curiosity does not have speakers. As the OP's linked article states:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This was accomplished using Curiosity's Sample Analysis at Mars (SAM) instrument. &lt;strong&gt;It &amp;quot;sang&amp;quot; the song by vibrating at different frequencies.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;More information on how this was accomplished is available in this &lt;a href=&quot;http://www.latimes.com/news/science/sciencenow/la-sci-sn-curiosity-anniversary-nasa-sam-happy-birthday-20130806,0,1786759.story&quot; rel=&quot;noreferrer&quot;&gt;LA Times article&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Sample Analysis at Mars instrument, or SAM, isn't a musical instrument. It doesn't have keys or strings. It's part of the lab in the rover's belly that analyzes rock samples and helped discover a habitable environment on Mars -- a place where microbes could have hypothetically lived in the past.&lt;/p&gt;&#xA;&lt;p&gt;But SAM does make noise. In order to shake powdery rock samples so that they settle down, the instrument vibrates at various frequencies, Florence Tan, SAM’s lead electrical engineer, said in a video.&lt;/p&gt;&#xA;&lt;p&gt;These frequencies can be used as musical notes. &lt;strong&gt;And by making SAM shake faster and slower, the team was able to tap out a little birthday tune for the rover on Monday.&lt;/strong&gt; (They went with Aug. 5 because Curiosity landed around 10:30 p.m. Pacific time on that date.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="80" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-07T19:19:37.400" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1048" PostTypeId="2" ParentId="1046" CreationDate="2013-08-07T19:20:36.500" Score="17" Body="&lt;p&gt;An &lt;a href=&quot;http://www.washingtonpost.com/blogs/the-switch/wp/2013/08/06/lonely-curiosity-rover-sings-happy-birthday-to-itself-on-mars/?tid=pm_business_pop&quot;&gt;article in the Washington post explains:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the rover’s sample analysis unit vibrates at different frequencies to&#xA;  move soil samples. Normally, those vibrations sound remarkably like&#xA;  the noises robots make in Disney’s Wall-E, but when you string them&#xA;  all together, something similar to “Happy Birthday” results.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="123" LastActivityDate="2013-08-07T19:20:36.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1049" PostTypeId="2" ParentId="1044" CreationDate="2013-08-07T19:24:59.033" Score="6" Body="&lt;p&gt;Most satellites are very, very far from the sun.   The closest spacecraft to the sun thus far has been Helios 2 at 0.29 AU (~43 million km).  Right now, I think the closest operating would be MESSENGER (perihelion at 46 million km).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although the sun's magnetic field is strong, this is still really, really far away.  Those fluctutations aren't enough to be detected by most spacecraft.  (the CMEs that get produced, yes, but not the flip itself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The detection of the &lt;a href=&quot;http://magnetograph.msfc.nasa.gov/magmore.shtml&quot;&gt;sun's magnetic field is done through remote sensing&lt;/a&gt; -- Phil Scherrer (one of the two quoted in the recent NASA press release) is the PI for &lt;a href=&quot;http://hmi.stanford.edu/&quot;&gt;HMI&lt;/a&gt; ... which is on &lt;a href=&quot;http://sdo.gsfc.nasa.gov/&quot;&gt;SDO&lt;/a&gt; ... which has an inclined geosyncronous orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's not to say that future spacecraft won't be affected ... there are two  upcoming missions to get closer to the sun -- ESA's &lt;a href=&quot;http://sci.esa.int/solar-orbiter/&quot;&gt;Solar Orbiter&lt;/a&gt; will attempt 45 Rsun (31 million km) and NASA's &lt;a href=&quot;http://solarprobe.jhuapl.edu/&quot;&gt;Solar Probe Plus&lt;/a&gt; will attempt 8.5 Rsun (6 million km) ... but they'll both be designed for going in that far.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(disclaimer : I work for the Solar Data Analysis Center, and help to maintain the caching system that's used to distribute the science-quality HMI data ... but I'm not a solar physicist)&lt;/p&gt;&#xA;" OwnerUserId="46" LastActivityDate="2013-08-07T19:24:59.033" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1050" PostTypeId="1" AcceptedAnswerId="1054" CreationDate="2013-08-07T19:27:17.410" Score="15" ViewCount="919" Body="&lt;p&gt;I was reading about the &lt;a href=&quot;http://en.wikipedia.org/wiki/Minotaur_V&quot; rel=&quot;noreferrer&quot;&gt;Minotaur V&lt;/a&gt; that Orbital Sciences is readying for launch from Wallops Island, VA before 2014.  (I really do not trust launch dates). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is a Minotaur IV+ with a fifth stage.  Ok, it is an all solids launcher, based off the Peacekeeper ICBM motors, so I get the extra staging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle is hard to quantify in terms of stages, sort of 1.5 stages  (SRB's, ET, and shuttle, but engines are on the shuttle, ET is just a tank.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Early Atlas that dropped two engines and tankage, but kept a sustainer engine firing is another half stage approach. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 is easy with 2 stages (maybe 3 with Dragon).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saturn V was mostly four stages (counting the SM/CM combo sort of as a stage).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there ever been a real launcher (that launched, not paper designs) with more stages?  (Seems like the Soviet N1 should count as real, since they tested a physical rocket, even if it never succeeded).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-07T23:33:00.507" LastActivityDate="2013-08-07T23:33:00.507" Title="Highest stage counts in actual launchers?" Tags="|rockets|stages|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1051" PostTypeId="5" CreationDate="2013-08-07T19:33:06.250" Score="0" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;rocket equation&lt;/a&gt; suggests that carrying all the empty weight to orbit is a waste.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus a stage allows you to optimize the lower stage for the lower altitude flight regime.  Then have a different stage design for higher altitude operations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This tag is for discussion of stage design, usage, and ideas.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-07T19:44:22.787" LastActivityDate="2013-08-07T19:44:22.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1052" PostTypeId="4" CreationDate="2013-08-07T19:33:06.250" Score="0" Body="Rather than carry empty weight all the way to orbit, rockets usually use a stage system.  Stages can take many forms and have over the years." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-07T19:44:33.193" LastActivityDate="2013-08-07T19:44:33.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1053" PostTypeId="2" ParentId="923" CreationDate="2013-08-07T20:46:41.070" Score="9" Body="&lt;p&gt;Let's take a survey of some of the powers that have launched something in to orbit, taken from &lt;a href=&quot;http://en.wikipedia.org/wiki/Timeline_of_first_orbital_launches_by_country&quot;&gt;Wikipedia&lt;/a&gt;. This list is in no particular order.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;United States- Highly likely, even &lt;a href=&quot;http://en.wikipedia.org/wiki/Range_safety&quot;&gt;manned missions&lt;/a&gt; have self destruct.&lt;/li&gt;&#xA;&lt;li&gt;North Korea- &lt;a href=&quot;http://en.rian.ru/world/20120413/172795198.html&quot;&gt;It seems they do&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Russia- Nope, &lt;a href=&quot;http://www.spaceflightnow.com/news/n1307/01proton/&quot;&gt;they aren't required&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;France- &lt;a href=&quot;http://en.wikipedia.org/wiki/Ariane_5&quot;&gt;Ariane 5&lt;/a&gt;, yes, most likely all.&lt;/li&gt;&#xA;&lt;li&gt;Japan- &lt;a href=&quot;http://www.space-travel.com/reports/Japan_rocket_blasts_off_with_space_yacht_and_Venus_probe_999.html&quot;&gt;Yes&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;China- &lt;a href=&quot;http://www.sinodefenceforum.com/strategic-defense/chinas-space-program-news-views-145-1143.html&quot;&gt;Yes, it appears&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it seems like everyone but Russia has self-destruct capabilities in all of their rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few other facts. Some of the abort systems are automatic, but most require ground intervention. Essentially, they key is to get the fuel out of the rocket, preferable burnt. A small explosive device is usually sufficient, rockets are very light, and most rocket fuels are explosive in nature, a small charge in the right place will be sufficient to explode the tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the Shuttle had 3 systems total, one for each solid rocket booster, and one for the external tank. The two solid rocket booster self destructs were used after the Challenger disaster, to prevent them from doing any damage to the ground. From the &lt;a href=&quot;http://history.nasa.gov/rogersrep/v1ch9.htm&quot;&gt;Challenger Report&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Range Safety&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Television coverage of the Challenger accident vividly showed the&#xA;  Solid Rocket Boosters emerging from the ball of fire and smoke. The&#xA;  erratic and uncontrolled powered flight of such large components could&#xA;  have posed a potential danger to populated areas. The responsible&#xA;  official accordingly destroyed the Solid Rocket Boosters.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;To understand how the booster rockets were destroyed, one must&#xA;  understand the purpose of a range safety system, its functions, and&#xA;  the special considerations that apply to Shuttle launches.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Eastern Space and Missile Center operates a range safety system&#xA;  for all Department of Defense and NASA launch activities in the Cape&#xA;  Canaveral area. The primary responsibility of the range safety system,&#xA;  run by the U.S. Air Force, is to protect people and property from&#xA;  abnormal vehicle flights during first stage ascent.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;To fulfill its range safety responsibilities, the Eastern Space and&#xA;  Missile Center staff supervises on-site launch preparations and tracks&#xA;  rockets and vehicles until they are far enough away from populated&#xA;  areas to remove any danger. When such a danger arises during the&#xA;  ascent stage of a launch, the vehicle may have to be destroyed to&#xA;  minimize harm to persons and property on the ground. Every major&#xA;  vehicle flown from the Cape Canaveral area has carried an explosive&#xA;  destruct system that could be armed and fired by the range safety&#xA;  officer.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Range safety procedures in launch activities from Kennedy are governed&#xA;  by Department of Defense and NASA documents. The primary regulatory&#xA;  publication is DOD Document 3200.11, Use, Management, and Operation of&#xA;  DOD Major Ranges and Test Facilities.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-07T20:53:27.333" LastActivityDate="2013-08-07T20:53:27.333" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1054" PostTypeId="2" ParentId="1050" CreationDate="2013-08-07T21:27:46.750" Score="11" Body="&lt;p&gt;If, counting a stage as a unit of a craft that fires, providing thrust, and all thrust cuts off before a different unit in the craft starts to thrust...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V - Apollo - LEM stack is 6 stages. The Saturn stack of 3 stages, plus the 2 stages of the LEM, and the single stage CSM pairing.&#xA;In sequence: Saturn stage I, Stage II, and Stage III, then the CSM, then the LEM Descent, then the LEM ascent, then back to the CSM. The LEM is part of the whole package, and is an important part of the mission to the moon. In those early Apollo launches, the LEM was the real payload, not the CM; the CM was there to keep the LEM weight down... so that astronauts would not be stranded on the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Later Apollo missions to LEO used the Saturn 1B, itself 2 stages, plus the CSM.)&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-07T21:27:46.750" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1055" PostTypeId="5" CreationDate="2013-08-07T22:03:56.113" Score="0" Body="&lt;p&gt;Falcon Heavy is an evolution of the &lt;a href=&quot;https://space.stackexchange.com/questions/tagged/falcon-9&quot;&gt;Falcon 9&lt;/a&gt; design.  Using the basic Falcon 9 first stage as a building block, the center core, and two side boosters are used to increase payload to LEO to a reported 63,800&amp;nbsp;kilograms (140,700&amp;nbsp;lbs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each core is a LOX/RP1 booster using 9 Merlin 1-D engines in the Octaweb configuration.  (Eight engines in a circle, with the ninth in the center.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initially anticipated to be launched in 2013, the Falcon Heavy eventually had its maiden flight on February&amp;nbsp;6, 2018, carrying Elon Musk's &lt;a href=&quot;https://space.stackexchange.com/questions/tagged/tesla-roadster&quot;&gt;Tesla Roadster&lt;/a&gt; to a heliocentric orbit beyond Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.flickr.com/photos/spacex/40126461411/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ssYNM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="10335" LastEditDate="2018-02-07T23:39:34.953" LastActivityDate="2018-02-07T23:39:34.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1056" PostTypeId="4" CreationDate="2013-08-07T22:03:56.113" Score="0" Body="SpaceX's heavy lift launcher, based on the Falcon 9, using two strap-on boosters with a center core." OwnerUserId="51" LastEditorUserId="10335" LastEditDate="2018-02-07T23:38:57.103" LastActivityDate="2018-02-07T23:38:57.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1057" PostTypeId="2" ParentId="1035" CreationDate="2013-08-08T00:22:59.027" Score="6" Body="&lt;p&gt;As others mentioned in the comments, a 4-satellite constellation is the minimum that will give you global coverage. &lt;a href=&quot;http://www.google.com/patents/US4809935&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; is Draim's patent (it also includes a 3-satellite constellation for hemispherical coverage), and a coverage map from &lt;a href=&quot;http://personal.ee.surrey.ac.uk/Personal/L.Wood/software/SaVi/Draim/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; is shown below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding GEO satellites, while a 3-satellite constellation in GEO &lt;em&gt;will&lt;/em&gt; give you full coverage for all &lt;em&gt;longitudes&lt;/em&gt;, the same is not true for all &lt;em&gt;latitudes&lt;/em&gt;. Coverage as you approach the poles gets spotty, particularly when you include considerations like minimum elevation angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1eGZc.png&quot; alt=&quot;Coverage map for Draim tetrahedral constellation&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="25" LastEditDate="2013-08-08T01:36:20.703" LastActivityDate="2013-08-08T01:36:20.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1058" PostTypeId="2" ParentId="969" CreationDate="2013-08-08T00:44:50.790" Score="7" Body="&lt;p&gt;While coordinate systems are used for all sorts of things in space flight, from your position in the solar system down to individual instruments, I'll assume for the purposes of this question that you're talking specifically about &lt;strong&gt;coordinate systems used for determining satellite position relative to something&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Also, I'm going to assume you're really interested in &lt;strong&gt;reference frames&lt;/strong&gt;, instead of coordinate systems. For the reference frames I'll talk about, &lt;a href=&quot;http://en.wikipedia.org/wiki/Cartesian_coordinate_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cartesian&lt;/a&gt; coordinates are typically used (&lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;nofollow noreferrer&quot;&gt;Keplerian elements&lt;/a&gt; are, of course, another popular alternative).&lt;/p&gt;&#xA;&lt;h2&gt;Earth-orbiting satellites&lt;/h2&gt;&#xA;&lt;p&gt;There are two main types of reference frames: &lt;em&gt;inertial&lt;/em&gt; and &lt;em&gt;rotating&lt;/em&gt;. Specifically, when they're Earth-centered (origin at the center of the Earth, as they nearly always are, except when dealing with measurements from the Earth's surface), the two main classes are &lt;em&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Earth-centered_inertial&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth-centered inertial&lt;/a&gt;&lt;/em&gt; (ECI), and &lt;strong&gt;Earth-centered, Earth-fixed&lt;/strong&gt; (ECEF). ECI frames have axes defined by some inertial reference, such as pointing one of the axes at a specific star or constellation. The Earth rotates in these axes (i.e. the ECI axes do not correspond to any fixed location on Earth's surface). Alternatively, ECEF frames rotate with the Earth.&lt;/p&gt;&#xA;&lt;p&gt;There are many variations on these two frames, and most of them have to do with how they account for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Nutation#Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;precession and nutation&lt;/a&gt; of the Earth.&lt;/p&gt;&#xA;&lt;h2&gt;Other&lt;/h2&gt;&#xA;&lt;p&gt;This is a much more broad category, but generally they tend to be barycentric frames. You could define a reference frame with the origin at the &lt;a href=&quot;http://en.wikipedia.org/wiki/Barycentric_coordinates_(astronomy)&quot; rel=&quot;nofollow noreferrer&quot;&gt;barycenter&lt;/a&gt;, or center of mass, of several bodies (usually two, e.g. Earth-Sun or Earth-Moon).&lt;/p&gt;&#xA;&lt;p&gt;The International Celestial Reference Frame (&lt;a href=&quot;https://www.iers.org/IERS/EN/DataProducts/ICRF/ICRF/icrf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ICRF&lt;/a&gt;) is valid throughout the solar system, and has its origin at the barycenter of the solar system.&lt;/p&gt;&#xA;&lt;p&gt;Of course, if you have a mission to, say, Mars, you would define a Mars-centric reference frame to work in.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="21050" LastEditDate="2021-04-26T19:42:53.000" LastActivityDate="2021-04-26T19:42:53.000" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="1059" PostTypeId="1" AcceptedAnswerId="20181" CreationDate="2013-08-08T05:38:06.343" Score="16" ViewCount="16436" Body="&lt;p&gt;What is the highest number of satellites that have been launched on a single rocket? What were the number of stages used in that launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(It may be from the past or a future mission)&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="25" LastEditDate="2013-08-08T14:55:53.353" LastActivityDate="2021-01-24T18:34:17.050" Title="Highest number of satellites launched on a single rocket" Tags="|launch|rockets|record|" AnswerCount="11" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1063" PostTypeId="1" AcceptedAnswerId="1068" CreationDate="2013-08-08T07:36:57.687" Score="9" ViewCount="1575" Body="&lt;p&gt;Is the &lt;a href=&quot;http://en.wikipedia.org/wiki/PSLV&quot;&gt;Polar Satellite Launch Vehicle&lt;/a&gt; capable of taking a 1,350 kg orbiter from the Earth to Mars as &lt;a href=&quot;http://www.timesofindia.com/india/Isro-kicks-off-Mars-mission-campaign-with-PSLV-assembly/articleshow/21636884.cms&quot;&gt;this article&lt;/a&gt; states?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="288" LastEditDate="2013-11-12T15:48:45.673" LastActivityDate="2016-05-17T00:04:59.080" Title="Can Indian PSLV take ISRO to Mars?" Tags="|mars|isro|pslv|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1064" PostTypeId="2" ParentId="413" CreationDate="2013-08-08T08:37:24.857" Score="8" Body="&lt;p&gt;The &lt;a href=&quot;http://insight.jpl.nasa.gov/science.cfm&quot; rel=&quot;noreferrer&quot;&gt;InSight&lt;/a&gt; mission fits your criteria. Quoting JPL:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;SEIS: To capture Mars' pulse, or its internal activity, InSight will carry a &lt;strong&gt;seismometer&lt;/strong&gt; called &lt;code&gt;SEIS&lt;/code&gt; to the surface of the Red Planet. &lt;code&gt;SEIS&lt;/code&gt; will take precise measurements of quakes and other internal activity on Mars to better understand the planet's history and structure.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;HP3: To take Mars' temperature, a key indicator of planetary evolution, InSight will deploy a heat flow probe on the surface of Mars. The instrument, known as &lt;code&gt;HP3&lt;/code&gt;, will hammer &lt;strong&gt;five meters&lt;/strong&gt; into the Martian subsurface, deeper than all previous arms, scoops, drills and probes, to learn how much heat is coming from Mars' interior and reveal the planet's thermal history.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1539p.png&quot; alt=&quot;InSight&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Why do I consider it &lt;strong&gt;tomography&lt;/strong&gt; if there's only one sensor?&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It is possible to spot impact events with the help of &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;Mars Reconnaissance Orbiter&lt;/a&gt;...&lt;/li&gt;&#xA;&lt;li&gt;... and thus correlate seismic records with quantitative estimates of the force of impact and its precise areographic coordinates.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;In words of Bruce Banerdt at the July 2013 meeting of MEPAG:&#xA;(&lt;a href=&quot;http://mepag.jpl.nasa.gov/meeting/jul-13/files/Banerdt%202013-0732%20MEPAG.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://mepag.jpl.nasa.gov/meeting/jul-13/files/Banerdt%202013-0732%20MEPAG.pdf&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/NYBVQ.png&quot; alt=&quot;InSight Capabilities&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-10T18:31:21.703" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1066" PostTypeId="2" ParentId="1059" CreationDate="2013-08-08T10:58:29.093" Score="22" Body="&lt;p&gt;(This answer is up to date as of the end of 2013, the answer changes periodically, however, so...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depends on your definition a bit. I'm going to say that satellites must be able to communicate with the ground independently. Given that definition, The highest attempted (That I know about) was the &lt;a href=&quot;http://www.cnn.com/2006/TECH/space/08/30/cloudsat/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;DNEPR launch&lt;/a&gt; of 2006, with 18 satellites on board. The highest successful appears to be an &lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/dnepr-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;DNEPER launch on 17.04.2007&lt;/a&gt;, with 12 satellites on board. The highest planned is a 29 satellite launch of a &lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/dnepr-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;DNEPR&lt;/a&gt; planned sometime in 2013. The most I can find of a single design that was successful appears to be the latest ORBCOMM &lt;a href=&quot;http://www.spacex.com/news/2015/12/11/orbcomm-2-mission-update&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9 launches&lt;/a&gt;, which carried 11 satellites of the same design. Globalstar  attempted to launch 12 on the same mission on 9/9/1998 on a &lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/zenit-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zenit 2&lt;/a&gt;, but the &lt;a href=&quot;http://www.spacelaunchreport.com/zenit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;mission failed&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For non-communicating satellites, the most is no doubt &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_West_Ford&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project West Ford&lt;/a&gt;, which launched 480,000,000 needles in to space to serve as an artificial ionosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the record, here's some of the statistics about the number of stages for each vehicle (Thanks Paul!):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/dnepr-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;DNEPR&lt;/a&gt;- 3 stages&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_lau_fam/pegasus.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pegasus XL&lt;/a&gt;- Airplane plus 4 stages&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/kosmos-3m.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kosmos-3M&lt;/a&gt; - 2 stages&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/titan-3c.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Titan-3C&lt;/a&gt; 3 stages&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/zenit-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zenit 2&lt;/a&gt;- 2 stages&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;EDIT- The record changed a few times in 2013, and now the record for the most satellites in a single launch is 32, set by &lt;a href=&quot;http://spaceflightnow.com/news/n1311/21dnepr/#.Uo41dSfW6HM&quot; rel=&quot;nofollow noreferrer&quot;&gt;the DNEPR launch on Nov 21, 2013&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-02-15T18:37:15.187" LastActivityDate="2017-02-15T18:37:15.187" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1067" PostTypeId="1" CreationDate="2013-08-08T12:19:54.040" Score="5" ViewCount="493" Body="&lt;p&gt;Staying in space is different from staying on Earth. When astronauts return to Earth from orbit, they usually suffer from muscle loss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any genetic (abnormal) changes in their cells or in their offspring?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-08-08T16:47:52.167" LastActivityDate="2014-04-10T17:03:00.750" Title="Do astronauts experience any genetic changes?" Tags="|health|astronauts|" AnswerCount="1" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1068" PostTypeId="2" ParentId="1063" CreationDate="2013-08-08T12:40:48.743" Score="6" Body="&lt;p&gt;Yes, but to do so they're having to make significant compromises to the mission.  It was &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/07290600-MAVEN-MOM.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;originally&lt;/a&gt; intended to fly on India's next generation, significantly higher performing, Geosynchronous Satellite Launch Vehicle (&lt;a href=&quot;http://en.wikipedia.org/wiki/Geosynchronous_Satellite_Launch_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;GSLV&lt;/a&gt;) but delays and problems (3 of 7 flights have failed catastrophically, only 2 were fully successful) in that program forced them to use the older Polar Satellite Launch Vehicle (&lt;a href=&quot;http://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;PSLV&lt;/a&gt;) instead. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Mangalyaan&quot; rel=&quot;nofollow noreferrer&quot;&gt;MOM/Mangalyaan&lt;/a&gt; probe has a higher fuel to payload mix than the NASA/ESA probes mentioned above, and will be entering in a highly inclined orbit to save on the amount of fuel needed to enter orbit.  My supposition is that a large fraction of the fuel in the orbiter originally intended to put it in a low Mars orbit is instead being used to compensate for the less powerful launch and send it into interplanetary transfer orbit instead.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/jx9sc.jpg&quot; alt=&quot;MOM/Mangalyaan mission profile&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;MOM / Mangalyaan mission profile compiled by and featured in &lt;a href=&quot;http://www.dailymail.co.uk/indiahome/indianews/article-2308766/Mars-Ahoy-Indian-scientists-gung-ho-putting-orbiter-Red-Planet.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mail Online India article&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The highly inclined 80,000 / 580 km orbit means it will only spend a minority of its time close to the planet, and severely limiting it's sensors effectiveness except for a small fraction of each orbit. However, in some ways this is less severe a problem than it may first seem. Some of the instruments are cameras: And for imaging purposes the limiting factor is not how fast pictures can be taken by the orbiter; but the low bandwidth available to return data to Earth. As a result for imaging, the limitation will primarily be in what they can take pictures of at any given time; not the total number they can send home.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparing it to other Mars orbiters:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;NASA's &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbite&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Reconnaissance Orbiter&lt;/a&gt; is 2180 kg fueled / 1031 kg empty. It was launched on an Atlas 401 which can deliver 9800 kg to a Low Earth Orbit (LEO). The standard PSLV can only deliver 3250 kg to LEO. The &lt;a href=&quot;http://articles.timesofindia.indiatimes.com/2013-08-06/india/41130623_1_mars-orbiter-mission-lyman-alpha-photometer-mars-colour-camera&quot; rel=&quot;nofollow noreferrer&quot;&gt;TOI article&lt;/a&gt; says the launch will be done with the PSLV-XL which can deliver 12.5% more mass to a Sun-Synchronous Orbit (SSO); assuming a similar increase to LEO it would scale to 3650 kg. That's only 37% of the rocket power; but for an orbiter that's 62% as massive.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The 1030 kg &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Global_Surveyor&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Global Surveyor&lt;/a&gt; was launched on a Delta 7925 rocket. The Delta family can put between 2700 and 6100 kg in LEO; but Wikipedia doesn't have a per variant breakdown in performance. If launched on a lower power model, performance would be similar to what the PSLV offers; if it was a high performance model, the ~2:1 gap vs the MRO Atlas V-401 remains.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;ESA's &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Express&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Express&lt;/a&gt; was 1123 kg fueled and launched on a &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz-FG&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz-FG/Fregat&lt;/a&gt; combo that could put 7800 kg into LEO. This is an even larger performance gap than with the MRO.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="157" LastEditorUserId="157" LastEditDate="2016-05-17T00:04:59.080" LastActivityDate="2016-05-17T00:04:59.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1069" PostTypeId="5" CreationDate="2013-08-08T12:46:59.267" Score="0" Body="&lt;p&gt;RTG is a Radio-isotope Thermo-electric Generator.  It uses the heat generated by the decay of a radioactive material, typically Plutonium (Pu238) to generate power.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For missions beyond the asteroid belt, solar panels stop being as effective, due to reduced concentration of solar power.  Thus RTGs are commonly chosen for such missions.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-08T13:56:35.903" LastActivityDate="2013-08-08T13:56:35.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1070" PostTypeId="4" CreationDate="2013-08-08T12:46:59.267" Score="0" Body="RTG is a Radio-isotope Thermo-electric Generator.  It uses the heat generated by the decay of a radioactive material to generate power.  Often the heat as well, to hold off the cold temperatures in space or other planets." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-08T13:56:32.997" LastActivityDate="2013-08-08T13:56:32.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1071" PostTypeId="2" ParentId="874" CreationDate="2013-08-08T13:03:56.987" Score="6" Body="&lt;p&gt;Source: &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/surfaceorbit.php&quot; rel=&quot;noreferrer&quot;&gt;http://www.projectrho.com/public_html/rocket/surfaceorbit.php&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h1&gt;Sea Dragon&lt;/h1&gt;&#xA;&lt;p&gt;Payload mass delivered to LEO &lt;strong&gt;550 metric tons&lt;/strong&gt;&#xA;Cost per payload kilogram &lt;strong&gt;&lt;span class=&quot;math-container&quot;&gt;$59/kg to $&lt;/span&gt;600/kg&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Details &lt;a href=&quot;http://neverworld.net/truax/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;http://www.astronautix.com/lvs/searagon.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;http://en.wikipedia.org/wiki/Sea_Dragon_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Sea Dragon was designed by Robert Truax in &lt;strong&gt;1962&lt;/strong&gt; to be a low-cost heavy lift launch vehicle. To reduce costs for launch pads and gantries, the vehicle was to be &lt;strong&gt;launched from the ocean&lt;/strong&gt;. It would be towed out to the watery launch site, and the ballast tank in the first stage exhaust nozzle would be flooded. This would drag the tail down and the nose up, orienting the rocket into launch position. The rocket would then float with the second stage cargo hatch conveniently just above the waterline, ready to be loaded.&lt;/p&gt;&#xA;&lt;p&gt;At 150 m long and 23 m in diameter, Sea Dragon would have been the largest rocket ever built. To lower the cost of the rocket itself, it was designed to be build of inexpensive materials, specifically 8 mm steel sheeting.&lt;/p&gt;&#xA;&lt;p&gt;The project was shut down by NASA in the mid-1960's due to budget cuts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dge67.jpg&quot; alt=&quot;Sea Dragon schematics&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-08T13:03:56.987" CommentCount="0" CommunityOwnedDate="2013-08-08T13:03:56.987" ContentLicense="CC BY-SA 3.0" />
  <row Id="1072" PostTypeId="5" CreationDate="2013-08-08T13:53:55.703" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-08T13:53:55.703" LastActivityDate="2013-08-08T13:53:55.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1073" PostTypeId="4" CreationDate="2013-08-08T13:53:55.703" Score="0" Body="Questions regarding extremes or records, such as the smallest, largest, first, last, heaviest, etc. of a thing or event." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-08T14:18:30.993" LastActivityDate="2013-08-08T14:18:30.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1074" PostTypeId="1" AcceptedAnswerId="1077" CreationDate="2013-08-08T13:59:08.397" Score="17" ViewCount="2425" Body="&lt;p&gt;If a manned mission to another far-off planet were to take place, the crew probably would need something more sustainable than tubes for a food resource - plants would probably be the best option. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, almost all plants have a gene specifying that they grow against gravity. This would lead me to believe that they wouldn't do very well in micro-gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would normal, non-genetically-modified, plants be able to survive and yield fruit in space? What special considerations or procedures would need to be taken into account?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="113" LastEditDate="2013-08-08T15:00:09.473" LastActivityDate="2024-02-14T19:18:00.367" Title="Can plants grow in microgravity?" Tags="|life|microgravity|plants|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1075" PostTypeId="5" CreationDate="2013-08-08T14:00:49.720" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-08T14:00:49.720" LastActivityDate="2013-08-08T14:00:49.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1076" PostTypeId="4" CreationDate="2013-08-08T14:00:49.720" Score="0" Body="Questions regarding living organisms of kingdom Plantae, such as tomatoes, squash, and apple trees." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-08T17:14:49.823" LastActivityDate="2013-08-08T17:14:49.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1077" PostTypeId="2" ParentId="1074" CreationDate="2013-08-08T14:18:38.290" Score="17" Body="&lt;p&gt;So far the studies that have been performed in orbit have shown that plants grow perfectly normal (shoots up, roots down--so to speak) in microgravity. They also produce healthy offspring which can grow new plants in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Experiments have been performed on &lt;em&gt;Arabidopsis&lt;/em&gt; and other &lt;em&gt;Brassicaceae&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aboard the ISS a special “plant growth chamber” called Advanced Astroculture (or ADVASC) is used for &lt;em&gt;Arabidopsis&lt;/em&gt;. Within this enclosed chamber the environmental and growth conditions of the plants can be controlled.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ADVANCED ASTROCULTURE™ controls temperature, humidity, light, atmospheric conditions and delivery of nutrients to plants. It requires no power during Shuttle ascent and descent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(FS-2001-11-187-MSFC)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The seeds are planted in a tray containing a special material composed of crushed clay (Arcillite) for rooting. The plant receives its nutrition via basins in ADVASC that contain water and other nutrients.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another unit with plant growth chambers called the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/ABRS.html&quot; rel=&quot;noreferrer&quot;&gt;Advanced Biological Research System (ABRS)&lt;/a&gt; has also been used on the ISS for &lt;em&gt;Arabidopsis&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/a2UQS.jpg&quot; alt=&quot;ABRS&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vegetables have been grown in space as well. Mizuna, a type of Japanese lettuce, has been grown in the Lada Validating Vegetable Production Unit. Experiments have been performed in the Lada unit since 2002.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dpa2s.jpg&quot; alt=&quot;Mizuna on the ISS&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This year (2013) the ISS will begin to grow its first crop of lettuce that will actually be eaten on board! Red-leaved romaine lettuce will be grown under LED lighting starting in December (2013) with the goal for it to eventually be eaten by the astronauts. The plants are expected to grow to edible size in 28 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/tages.html&quot; rel=&quot;noreferrer&quot;&gt;Getting to the Root of Plant Growth Aboard the Space Station - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/centers/marshall/news/background/facts/advasc2.html&quot; rel=&quot;noreferrer&quot;&gt;Advanced Astroculture Fact Sheet (FS-2001-11-187-MSFC) - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://proceedings.aip.org/resource/2/apcpcs/458/1/376_1?isAuthorized=no&quot; rel=&quot;noreferrer&quot;&gt;Reproduction on orbit by plants in the Brassicaceae family - Mary E. Musgrave, Anxiu Kuang, Ying Xiao, and Sharon W. Matthews, &lt;em&gt;American Institute of Physics&lt;/em&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/10-074.html&quot; rel=&quot;noreferrer&quot;&gt;Growing Plants and Vegetables in a Space Garden - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.hortweek.com/Edibles/article/1212016/Extra-terrestrial-lettuce-feed-space-station-crew/&quot; rel=&quot;noreferrer&quot;&gt;Extra-terrestrial lettuce to feed space crew - HorticultureWeek&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-10-08T13:47:47.143" LastActivityDate="2014-10-08T13:47:47.143" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1078" PostTypeId="1" AcceptedAnswerId="1079" CreationDate="2013-08-08T14:20:46.200" Score="25" ViewCount="1938" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Galileo_%28spacecraft%29&quot;&gt;Galileo&lt;/a&gt; took six years from Earth to Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Cassini-Huygens&quot;&gt;Cassini-Huygens&lt;/a&gt; took three years to reach Jupiter and six years to reach Saturn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons#Key_mission_dates&quot;&gt;New Horizons&lt;/a&gt; took one year to reach Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29&quot;&gt;Juno&lt;/a&gt; was launched in 2011 and is planned to arrive at Jupiter in 2016, after five years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2022, the ESA mission &lt;a href=&quot;http://en.wikipedia.org/wiki/Jupiter_Icy_Moon_Explorer&quot;&gt;JUICE&lt;/a&gt; will depart for Jupiter.  Arrival is not expected before 2030, &lt;em&gt;eight years&lt;/em&gt; after launch.  Or, more precisely: &lt;a href=&quot;http://sci.esa.int/juice/50074-scenario-operations/&quot;&gt;ESA JUICE Operations&lt;/a&gt;: 7.6 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is JUICE so slow?  Are agencies using a slow technology to save costs and energy, are the planets poorly aligned&lt;sup&gt;heh heh&lt;/sup&gt;, or there some other boundary conditions?  &lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="51" LastEditDate="2016-07-08T13:35:52.213" LastActivityDate="2023-04-15T12:22:03.657" Title="Why will JUICE take 8 years to reach Jupiter, longer than any earlier spacecraft?" Tags="|orbital-mechanics|trajectory|interplanetary|jupiter|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1079" PostTypeId="2" ParentId="1078" CreationDate="2013-08-08T14:33:59.670" Score="18" Body="&lt;p&gt;From the &lt;a href=&quot;http://sci2.esa.int/cosmic-vision/JUICE_Yellow_Book_Issue1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Yellow Book&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;06/2022 - Launch by Ariane-5 ECA + EVEE-type Cruise&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;EVEE means Earth Venus Earth Earth - a long series of gravity assist maneuvers to fit more payload into the Ariane 5 ECA launcher capabilities.&lt;/p&gt;&#xA;&lt;p&gt;Why an Earth slingshot comes first:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The fact that the first arc includes an Earth gravity assist for both launch opportunities removes the dependency of the relative declination of Venus, and therefore the baseline and backup launches have the same escape velocity, resulting in similar launch mass performance.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;ECA performance:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The ECA version is able to carry a satellite weighing 4100 kg towards the following earth escape orbit:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;infinite velocity V∞ = 3475 m/s&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;declination δ = - 3.8°&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;JUICE:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The mass injected into the Earth escape trajectory would be 4800 kg (without launch adapter), with a hyperbolic escape velocity of 3.15 km/s.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Ariane 5 ECA user's manual: &lt;a href=&quot;http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-08T14:55:29.270" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1081" PostTypeId="1" CreationDate="2013-08-08T15:00:04.793" Score="7" ViewCount="1459" Body="&lt;p&gt;How can the optimal number of satellites be determined for a plane of satellites? Specifically, I would like to see how to do this in general, and a specific example worked out, such as the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Altitude - 780 km&lt;/li&gt;&#xA;&lt;li&gt;Horizon - 5 degrees&lt;/li&gt;&#xA;&lt;li&gt;Overlap - Let's say that there should be 200km overlap (or something in that range) for each satellite track, to ensure a reasonable continuity.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="527" LastEditDate="2014-05-27T11:20:53.223" LastActivityDate="2016-01-31T14:52:29.843" Title="How can the optimal number of satellites in an orbital plane be determined?" Tags="|artificial-satellite|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1082" PostTypeId="2" ParentId="1074" CreationDate="2013-08-08T16:05:06.410" Score="6" Body="&lt;p&gt;In addition to the gravity, an important influence for plant growth is &lt;em&gt;light&lt;/em&gt; (cf. &lt;a href=&quot;http://en.wikipedia.org/wiki/Photoperiodism&quot; rel=&quot;nofollow noreferrer&quot;&gt;Photoperiodism&lt;/a&gt;). So, in microgravity plants still &quot;know&quot; which direction to grow - leafs towards the light (for &lt;a href=&quot;http://en.wikipedia.org/wiki/Photosynthesis&quot; rel=&quot;nofollow noreferrer&quot;&gt;Photosynthesis&lt;/a&gt;), roots away from it, towards water/nutrients.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As others pointed out, space experiments confirm that plants do grow in microgravity.&lt;/p&gt;&#xA;" OwnerUserId="380" LastEditorUserId="380" LastEditDate="2018-02-07T15:30:39.637" LastActivityDate="2018-02-07T15:30:39.637" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1083" PostTypeId="2" ParentId="1081" CreationDate="2013-08-08T16:20:40.377" Score="7" Body="&lt;p&gt;The question is badly underspecified, so let's try a more precise, but probably simplistic scenario.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You want to place a sequence of satellites in a circular orbit. All satellites follow the same orbit. The orbit is in the Earth's equatorial plane. You want to know, how many satellites you will need in total, if you want that an observer, located on a specific point on the Equator, can always &lt;em&gt;&quot;see&quot;&lt;/em&gt; at least one satellite, where &lt;em&gt;&quot;seeing&quot;&lt;/em&gt; means &lt;em&gt;&quot;having a direct line of sight which is at least 5 degrees above the horizon&quot;&lt;/em&gt;. The Earth is assumed to be a perfect ellipsoid, and we don't take into account refraction of light and radio waves in the upper atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case, let $R$ be the Earth's radius (6378 km), and $h$ the altitude from its surface (780 km in your example). Let $a$ be the minimum angle of the line of sight above the horizon. Seen from a polar point of view, with the Earth centre as the origin, we can define the angle $b$ such that the piece of orbit that the observer can &quot;see&quot; has angle $2b$. In that case, we get the following equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$(R+h) \tan a \sin b = (R+h) \cos b - R$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and we want to solve that for $b$. The left size of the equation grows with $b$ and the right side decreases with $b$ ($b$ is in the $[0;\pi/2]$ range) so there will be one intersection point. It is rather tedious to make an actual expression, but numeric resolution is easy with some dichotomy. For the parameters above, this yields $b = 22.42^\circ$ (approximately), for an orbit chunk of size about 5602 km. If you want to always &lt;em&gt;&quot;see&quot;&lt;/em&gt; at least one satellite, then they will have to be spaced by no much than that amount. With an orbit radius of $R+h = 7158 \, \mathrm{km}$, the orbit length is about 44975 km, and a bit more than 8 satellites are needed (so, in practice, 9 satellites).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note that the conditions are restrictive.&lt;/strong&gt; If the observer is not &quot;right under&quot; the orbit (geometrically, on a point which is not at the intersection of the orbital plane and the Earth surface) then the orbit chunk that the observer sees is smaller. If the orbit is not in the equatorial plane, then we can no longer ignore the Earth rotation and the satellite speed. For &lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System&quot; rel=&quot;nofollow&quot;&gt;GPS&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_%28satellite_navigation%29&quot; rel=&quot;nofollow&quot;&gt;similar systems&lt;/a&gt;, a complex set of orbits is used (instead of a string of satellites following a single orbit), to get enough coverage with &lt;em&gt;at least three&lt;/em&gt; satellites(*) visible from any point at any time, and preferably more, if only to be able to cope with satellite failures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(*) &lt;em&gt;Four&lt;/em&gt; satellites are needed to get a 3D position, but with three satellites you can compute a &lt;em&gt;line&lt;/em&gt; of possible positions, and you can infer your position as the intersection of the line and the surface on which you are. This works well for boats, which tend to be at sea level.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="311" LastEditorUserId="13262" LastEditDate="2016-01-31T14:52:29.843" LastActivityDate="2016-01-31T14:52:29.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1084" PostTypeId="5" CreationDate="2013-08-08T16:27:38.527" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-08T16:27:38.527" LastActivityDate="2013-08-08T16:27:38.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1085" PostTypeId="4" CreationDate="2013-08-08T16:27:38.527" Score="0" Body="Questions about satellites designed to aid in communications, such as phone, TV and Internet satellites." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-08T17:14:08.757" LastActivityDate="2013-08-08T17:14:08.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1086" PostTypeId="5" CreationDate="2013-08-08T16:28:42.703" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-08T16:28:42.703" LastActivityDate="2013-08-08T16:28:42.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1087" PostTypeId="4" CreationDate="2013-08-08T16:28:42.703" Score="0" Body="Questions regarding the mechanics or attributes of objects falling 'around' other objects, thereby orbiting them." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-08T17:15:17.683" LastActivityDate="2013-08-08T17:15:17.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1088" PostTypeId="1" AcceptedAnswerId="1092" CreationDate="2013-08-08T16:35:14.043" Score="9" ViewCount="482" Body="&lt;p&gt;It seems obvious that water has been on Mars at some point, be it long ago or recent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question, then, is this: Why are we &lt;em&gt;still&lt;/em&gt; looking for evidence of water on Mars? Shouldn't we be focusing on evidence of &lt;em&gt;life&lt;/em&gt; instead?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-08T16:46:55.303" LastActivityDate="2013-08-08T17:51:57.397" Title="Why are Martian rovers still looking for water?" Tags="|mars|life|water|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1089" PostTypeId="5" CreationDate="2013-08-08T16:38:07.057" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-08T16:38:07.057" LastActivityDate="2013-08-08T16:38:07.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1090" PostTypeId="4" CreationDate="2013-08-08T16:38:07.057" Score="0" Body="Questions regarding living organisms, searches for life, or human life in space." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-08T17:14:03.380" LastActivityDate="2013-08-08T17:14:03.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1091" PostTypeId="1" CreationDate="2013-08-08T17:06:47.520" Score="12" ViewCount="1360" Body="&lt;p&gt;I'm trying to find out information about the mechanics of data transmission from planetary probes: techniques, devices, methods, software.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, &lt;strong&gt;How reliable data transmission is undertaken in interplanetary missions?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a &lt;a href=&quot;https://physics.stackexchange.com/questions/13227/how-earth-communicates-with-voyager-i&quot;&gt;related question&lt;/a&gt; on physics.SE, but an elaborate answer with citations and links is absent from the thread.&lt;/p&gt;&#xA;&#xA;&lt;hr/&gt;&#xA;&#xA;&lt;p&gt;I have little or no knowledge about &lt;em&gt;radio comms&lt;/em&gt;, and introductory knowledge of &lt;em&gt;information theory&lt;/em&gt; disciplines.&lt;/p&gt;&#xA;" OwnerDisplayName="user496" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2013-08-10T03:35:48.740" Title="How is data transferred from interplanetary missions?" Tags="|communication|data-transmission|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1092" PostTypeId="2" ParentId="1088" CreationDate="2013-08-08T17:45:46.310" Score="9" Body="&lt;p&gt;Yes, and we are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The focus now is on the four ingredients for life, raw materials, energy, water, and favorable conditions.  Water is always there, but is not the primary focus.  We now seem to be able to find places on Mars where water once flowed using orbital data.  It is the job of the surface missions to first confirm that (Curiosity has) and then look for the other stuff.  Previous missions have been said to fall under a &quot;Follow the water&quot; strategy.  We have now graduated to &quot;Follow the carbon&quot;, in particular reduced carbon.  That is the main raw material to look for, especially since it is not found easily on Mars -- on the surface and in the atmosphere, all the carbon, and iron as well, is oxidized.  Curiosity has already found iron that is reduced, not oxidized, by drilling down a few cm.  So it seems to be on the right track.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the search is for past life, not current life.  Our ability to find sites on Mars where current life is accessible is limited or non-existent, and even the possibility of extant life is really a long shot.  We are far more likely to meet with success if we look for evidence of past life, since we have good evidence that Mars was much like Earth a few billion years ago.  Not so much now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity is armed with the instrumentation it needs to explore those four ingredients, at least at the scale of a drill sample.  Now it just needs to drive to the primary science target in the foothills of Mount Sharp.  Stay tuned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finding such evidence of life will be highly suggestive, but not conclusive.  Unless we get really lucky and a rover finds some gross, obvious morphological evidence of life, another long shot, we will need to bring selected samples to Earth for the much more detailed examinations required to rule out abiotic mechanisms for that evidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars 2020 mission is planned to start us on that path, both with more detailed, fine scale searches for evidence of past life in situ, and the collection of selected samples for possible transport to Earth.  The &lt;a href=&quot;http://mars.nasa.gov/files/m2020/SDT_July2013.zip&quot;&gt;report of the Mars 2020 Science Definition Team&lt;/a&gt; outlines the overall strategy for the search for life on Mars in great detail.  It is highly recommended reading for those who would like an introduction to this topic.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-08-08T17:51:57.397" LastActivityDate="2013-08-08T17:51:57.397" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1093" PostTypeId="2" ParentId="1078" CreationDate="2013-08-08T18:01:08.213" Score="13" Body="&lt;p&gt;Deer Hunter's covered JUICE's flight itself, so I'm limiting my answer to addressing why New Horizons and Cassini were so much faster.  There is a de facto upper bound in how long it can take for a probe to reach its destination because the scientists who do all the work to design and build a mission also want to be around for the payoff when it arrives.  From initial design to arrival is already typically 10-15 years which is a large fraction of a career; if you add the years of lobbying and advocacy needed to get a mission approved it can easily use up half a scientists career just to get a probe in place to begin studying its target.&lt;/p&gt;&#xA;&lt;p&gt;Cassini had to travel ~2x as far from the Earth as the Jupiter missions, New Horizons has ~8x as far to go.  Total transit times from Earth to destination are almost 7 years for Cassini and 9.5 for New Horizons.  The length of time New Horizons is taking has been described as a challenge for building and maintaining the team; and is why it's only doing a flyby not entering into orbit:  The later would have required either a far larger probe to decelerate by rocket thrust on arrival, or a much slower transit to keep the total mass down.  The first option wasn't realistic since it already was using the most powerful Atlas variant ever flown; and no other readily available rocket with significantly more power was available.&lt;/p&gt;&#xA;" OwnerUserId="157" LastEditorUserId="157" LastEditDate="2023-04-14T12:22:26.090" LastActivityDate="2023-04-14T12:22:26.090" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1094" PostTypeId="2" ParentId="1091" CreationDate="2013-08-08T18:02:00.763" Score="8" Body="&lt;p&gt;Start with the &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/index.php&quot;&gt;Basics of Space Flight&lt;/a&gt;, in particular the chapters on telecom, navigation, and the Deep Space Network.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you are specifically asking about reliable data transmission, I'm guessing that you thinking about all the retransmission protocols we use in Earth communications.  The direct-to-Earth communications to date have generally not used automatic retransmission protocols, due to the long light-time delays and the value of bits on the link.  Usually humans are in the loop to decide whether or not to request a retransmission of something that didn't quite make it.  Sometimes they do, and sometimes the information has lost value due to the passage of time, and there are better uses for the scarce downlink bits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reliability in deep space communications comes from the use of advanced error-correcting codes, high power, big antennas, and very careful link calculations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We do use automatic retransmission protocols on short relay links in space, as well as automatic data rate adjustments, in particular between Mars rovers and landers and Mars orbiters.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-08-08T18:02:00.763" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1095" PostTypeId="1" CreationDate="2013-08-08T18:02:46.057" Score="2" ViewCount="79" Body="&lt;p&gt;When is Fēngyún-3C planned to launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://database.eohandbook.com/database/missionsummary.aspx?missionID=368&quot; rel=&quot;nofollow&quot;&gt;CEOS/ESA EO Handbook&lt;/a&gt; states July 2013, but this was last updated October 2012, and I don't think it's actually been launched.  &lt;a href=&quot;http://www.wmo-sat.info/oscar/satellites/view/115&quot; rel=&quot;nofollow&quot;&gt;WMO-sat.info&lt;/a&gt; says &gt;= 2013, but is not specific either.  Googling Fēngyún-3C or FY-3C does not lead me to more up-to-date information.  I don't know any Chinese, so perhaps there is more current information accessible to those who do through the &lt;a href=&quot;http://www.nsmc.cma.gov.cn/NewSite/NSMC/Home/Index.html&quot; rel=&quot;nofollow&quot;&gt;CMA NSMC website&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2013-08-08T18:02:46.057" Title="When is Fēngyún-3C planned to launch?" Tags="|launch|" AnswerCount="0" CommentCount="10" ClosedDate="2013-08-09T05:21:01.480" ContentLicense="CC BY-SA 3.0" />
  <row Id="1096" PostTypeId="1" AcceptedAnswerId="1118" CreationDate="2013-08-08T18:19:44.033" Score="5" ViewCount="3204" Body="&lt;p&gt;The Indian Space Research Organization (ISRO) is the space program of the Republic of India. How many people are employed by ISRO?&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2013-08-12T19:39:41.863" Title="How many people does ISRO employ?" Tags="|space-industry|isro|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1097" PostTypeId="2" ParentId="1096" CreationDate="2013-08-08T18:19:44.033" Score="2" Body="&lt;p&gt;It would seem according to &lt;a href=&quot;http://www.isro.org/pdf/rit10.pdf&quot; rel=&quot;nofollow&quot;&gt;this table&lt;/a&gt; on the ISRO site that they had 15,483 employees as of 2010.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-08-12T19:20:59.783" LastActivityDate="2013-08-12T19:20:59.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1098" PostTypeId="1" CreationDate="2013-08-08T18:22:05.123" Score="22" ViewCount="7987" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wm88S.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The geostationary ring clearly seen in the above picture looks alarmingly crowded. How full is it? How much space does a satellite in that orbit require to operate safely? Surely, (other) geosynchronous orbits will also need to be taken into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do countries have to reserve or announce positions prior to launch? &lt;/p&gt;&#xA;" OwnerUserId="80" LastActivityDate="2013-08-13T02:01:52.767" Title="How full is the geostationary belt?" Tags="|orbit|geosynchronous|artificial-satellite|geostationary|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1099" PostTypeId="2" ParentId="857" CreationDate="2013-08-08T18:48:09.460" Score="5" Body="&lt;p&gt;While answering according to &lt;em&gt;actual&lt;/em&gt; optimal sustainability may not be strictly possible at the present, there is a method being currently researched by NASA that may provide one of the best solutions to managing human waste on a manned space vessel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(As of 2004) Dr. Bruce E. Rittman of Northwestern University lead the NASA-funded team to experiment with the use of human waste as fuel for electricity in a process involving the use of &lt;em&gt;Geobacter&lt;/em&gt; microbes for the generation of the electric energy. According to the NASA article, the best solution may be a combination of recycling the water from the waste, using it as fertilizer, and using it for electricity using this method that is currently being studied.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has not released any updates on the progress of this research since 2007, though it has been alluded to in Earth-oriented work on microbial fuel cells as recent as &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0013468616301141&quot; rel=&quot;nofollow&quot;&gt;2016&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2004/18may_wastenot/&quot; rel=&quot;nofollow&quot;&gt;Waste Not - NASA Science&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-05-10T20:58:28.730" LastActivityDate="2016-05-10T20:58:28.730" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1100" PostTypeId="1" CreationDate="2013-08-08T19:22:34.050" Score="5" ViewCount="97" Body="&lt;p&gt;In addition to the benefits of smaller and more frequent launches (and perhaps other benefits), could fuel depots also provide a modest benefit in reducing the mass of fuel tanks for vehicles leaving LEO?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This assumes that stresses on fuel tanks during the trip to LEO would be reduced by not being loaded with fuel, that stresses from LEO outward are lower, and that support for any inflation or other reconstruction in LEO would not add mass that must be carried out of LEO. (Any extra mass that is left in LEO would still be an issue.)&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastActivityDate="2013-08-09T00:10:06.690" Title="Would fuel depots allow lighter fuel tanks for post LEO use?" Tags="|launch|fuel-depots|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1101" PostTypeId="2" ParentId="1098" CreationDate="2013-08-08T19:30:16.970" Score="5" Body="&lt;p&gt;The International Telecommunication Union allocates orbital slots, generally for Geosync. So yes, countries do have to reserve slots.  Specifically, the &lt;a href=&quot;http://en.wikipedia.org/wiki/ITU-R&quot; rel=&quot;noreferrer&quot;&gt;Radiocommunication Sector (ITU-R)&lt;/a&gt; division of the ITU is responsible for orbital slot allocations.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A broadcast or communications satellite needs to be properly spaced to avoid frequency interference/overlap.  But a weather satellite, or earth observation satellite might not interfere and could coexist. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a ton of great detail on the Wikipedia page for &lt;a href=&quot;http://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;noreferrer&quot;&gt;Geosync orbit&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-08-08T19:59:46.243" LastActivityDate="2013-08-08T19:59:46.243" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1102" PostTypeId="1" AcceptedAnswerId="1106" CreationDate="2013-08-08T19:44:15.147" Score="25" ViewCount="4578" Body="&lt;p&gt;A documentary on Apollo program (I can't remember the name at the moment) stated that &lt;em&gt;Aldrin&lt;/em&gt; and not &lt;em&gt;Armstrong&lt;/em&gt; was the first astronaut designated to walk on the moon. But after LEM landed, mission control realized that it was not possible for Aldrin to open the door and exit since the way was blocked by Armstrong. So they decided that since Armstrong was the mission commander, it would not be a problem if he would be the &lt;em&gt;first&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;I know that the LEM was phone booth sized and there was not even a place to sleep on the floor. Moreover, Apollo 11-13 were considered &lt;em&gt;attempts&lt;/em&gt; to land &amp;quot;before this decade is out&amp;quot;, so probably no one expected success on the first landing mission (in fact, the landing was somewhat a mix of luck and ability).&lt;/p&gt;&#xA;&lt;p&gt;But...it seems incredible to me that after all the simulations done, this aspect of the mission was not considered at all. Is the documentary right?&lt;/p&gt;&#xA;&lt;p&gt;Disclaimer: I respect Aldrin, as well as Armstrong, for their contribution to space exploration, I just want to fact-check that statement.&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="7062" LastEditorDisplayName="user55" LastEditDate="2022-06-08T12:11:24.973" LastActivityDate="2022-06-08T12:11:24.973" Title="Is it true that Armstrong was not designated as being the first to walk on the moon?" Tags="|the-moon|apollo-program|history|lunar-landing|spaceflight-firsts|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="1103" PostTypeId="5" CreationDate="2013-08-08T20:25:33.857" Score="0" Body="&lt;p&gt;Various kinds of facilities are used in the development, manufacturing, launching and operations of space craft. Examples include:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Research and development facilities (e.g. &lt;a href=&quot;https://www.nasa.gov/centers/ames/about/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ames Research Center&lt;/a&gt;, &lt;a href=&quot;https://www.esa.int/About_Us/ESTEC&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA ESTEC&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Manufacturing facilities (e.g. &lt;a href=&quot;https://www.nasa.gov/centers/marshall/michoud/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Michoud Assembly Facility&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_South_Texas_launch_site&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX South Texas site&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Test facilities (e.g. &lt;a href=&quot;https://www.nasa.gov/centers/stennis/home/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stennis Space Center&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_South_Texas_launch_site&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX South Texas site&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Launch facilities (e.g. &lt;a href=&quot;https://www.nasa.gov/centers/kennedy/home/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kennedy Space Center&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikonur_Cosmodrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Baikonur Cosmodrome&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Mission control (e.g. &lt;a href=&quot;https://www.nasa.gov/centers/johnson/home/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Johnson Space Center&lt;/a&gt;, &lt;a href=&quot;http://www.esa.int/About_Us/ESOC&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESOC&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="20987" LastEditDate="2020-09-24T19:59:59.163" LastActivityDate="2020-09-24T19:59:59.163" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1104" PostTypeId="4" CreationDate="2013-08-08T20:25:33.857" Score="0" Body="Questions related to facilities used in space exploration, such as factories, test sites, launchpads and mission control centers." OwnerUserId="-1" LastEditorUserId="20987" LastEditDate="2020-09-24T19:59:59.163" LastActivityDate="2020-09-24T19:59:59.163" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1105" PostTypeId="1" AcceptedAnswerId="1706" CreationDate="2013-08-08T20:29:22.403" Score="15" ViewCount="474" Body="&lt;p&gt;The International Space Station has a Ham Radio station, and I've heard that the station frequently makes contact with people on the ground. I've seen the &lt;a href=&quot;http://www.nasa.gov/content/international-space-station-timelines-july-2013-1/#.UgP_gaw844o&quot;&gt;crew timelines&lt;/a&gt;, but they only publish what happened in the past. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="168" LastEditDate="2013-08-23T07:09:10.930" LastActivityDate="2013-09-10T20:39:06.353" Title="Is there a pre-published time that astronauts on the ISS will be available on Ham Radio?" Tags="|iss|amateur-radio|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1106" PostTypeId="2" ParentId="1102" CreationDate="2013-08-08T20:32:19.970" Score="29" Body="&lt;p&gt;&lt;strong&gt;Apollo 11 (AS-506) was considered to be the first landing mission. And yes, up to early 1969 nobody thought the plan out thoroughly enough.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Note on lunar module crew positions:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Neil A. Armstrong - commander&lt;/li&gt;&#xA;&lt;li&gt;Edwin E. Aldrin, Jr. - lunar module pilot&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Above them, in the 183x186 orbit:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Michael Collins - command module pilot&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Now let's see: &lt;strong&gt;Chariots for Apollo&lt;/strong&gt; :&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Landing, surface work, and ascent were going to be difficult, complex, and demanding tasks. George Mueller, the manned space flight chief in Washington, had therefore urged in mid-1968 that the first lunar landing crew be selected as soon as possible. (&lt;em&gt;George E. Mueller to Robert R. Gilruth, 5 June 1968&lt;/em&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(&lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4205/ch13-2.html&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;hr/&gt;&#xA;&lt;p&gt;When Houston began work on the two-man scheme, the planners used &lt;strong&gt;a 1964 concept that called for the lunar module pilot to emerge first&lt;/strong&gt;. Armstrong and Aldrin began concentrating on Apollo 11 as soon as they finished their backup duties for Apollo 8 in December. &lt;strong&gt;Almost immediately, on the 20th, a procedures document listed the commander as the first crewman to leave&lt;/strong&gt; the lunar module. On a summary minute-by-minute work chart, issued in &lt;strong&gt;January 1969&lt;/strong&gt;, the crew positions - commander and lunar module pilot - were &lt;strong&gt;crossed through&lt;/strong&gt; and the letters A and B were penciled in. A lunar surface operations chart, using these letters, was then published, but without any identification of either A or B.&lt;/p&gt;&#xA;&lt;p&gt;Collins wrote in Carrying the Fire that Armstrong had &amp;quot;exercised his commander's prerogative&amp;quot; and that Aldrin's &amp;quot;basic beef&amp;quot; was this switch in who crawled out first. But Slayton later took the credit (or blame) for making the change. &amp;quot;I observed the procedures under the old plan one day,&amp;quot; he said, &amp;quot;and they appeared awkward to me.&amp;quot; Slayton told Raymond G. Zedekar, in charge of preparing a lunar surface operations plan, to change the sequence. At the 15th lunar surface operations planning meeting on 14 February, Zedekar said that Aldrin would follow the commander to the lunar surface in less than the hour listed in the old plan, to assist Armstrong with the outside tasks, and that the lunar module pilot would return to the lander first. &amp;quot;If the CDR returns last,&amp;quot; Zedekar remarked, &amp;quot;the crewmen will be in their proper respective positions in the LM.&amp;quot; Since the portable life-sustaining backpacks were stored directly behind the lunar module pilot's crew station, getting out and then back in this sequence made crew movements in the cabin easier. (23)&lt;/p&gt;&#xA;&lt;p&gt;Surprisingly, Mueller did not inform Administrator Thomas Paine that the two men would take a 2-hour 40-minute walk, nor did he tell him that the order of exit had changed, until 7 April - at least, that was the date of his written report. Even more surprising was the fact that it was not until 14 April that a newsman asked Low, &amp;quot;Who will be the first out to the moon?&amp;quot; Low replied that, from &amp;quot;the present way that we're working, . . . the Commander gets out first.&amp;quot; The change later roused a small furor. Low was awakened in the middle of the night on 27 June by a call from an Associated Press reporter, who told the Apollo manager that the wire service had a story&amp;quot; that Neil Armstrong had pulled rank on Buzz Aldrin.&amp;quot; (Armstrong, incidentally, was a civilian and Aldrin a colonel in the Air Force.) (24)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(23) Collins, Carrying the Fire, p. 347; Robert Sherrod, &amp;quot;Men for the Moon,&amp;quot; in Edgar M. Cortright, ed., Apollo Expeditions to the Moon, NASA SP-350 (Washington, 1975), p. 160; Slayton and Raymond G. Zedekar, telephone interviews, 18 March 1976; Grumman Aircraft Engineering Corp., Space Div., &amp;quot;Anatomy of the NASA Grumman Apollo Lunar Module,&amp;quot; in Apollo Spacecraft News Reference (Bethpage, N.Y., 1969); MSC, &amp;quot;Results of 15th LSOP Meeting,&amp;quot; 14 Feb. 1969; L. J. Riche, G. M. Colton, and T. A. Guillory, &amp;quot;Apollo 11, Apollo AS-506/CSM-107/LM-5, Preliminary Flight Plan,&amp;quot; 15 April 1969, pp. 3-78, 3-79.&lt;/p&gt;&#xA;&lt;p&gt;(24) OMSF Weekly Report, 7 April 1969; Astronautics and Aeronautics, 1969: Chronology on Science, Technology, and Policy, NASA SP-4014 Washington, 1970 , pp. 68, 100; MSC, &amp;quot;Apollo Program History,&amp;quot; briefing, 14 April 1969, tape D-1; Low to Brian M. Duff, &amp;quot;Press Inquiry,&amp;quot; 27 June 1969, as cited in Ertel and Newkirk, Apollo Chronology, 4.&lt;/p&gt;&#xA;&lt;p&gt;(&lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4205/ch13-3.html&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-09T02:15:57.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1107" PostTypeId="2" ParentId="819" CreationDate="2013-08-08T23:38:03.247" Score="4" Body="&lt;p&gt;&lt;strong&gt;In simplest terms: The upper limit is the point at which the object asserts enough gravity to collapse itself, and the available limit on materaials available.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An object of sufficient mass should self-collapse into a spheroid; for silicates this is estimated to be several hundred miles diameter; note that Ceres is above the self-rounding limit, and is about 0.00015 Earths (895E18 kg), and about 590 miles diameter (about 1/13 earth diameter).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One cannot build a structure bigger than the total materials available; in the Sol System, assuming cannibalization of all rocky bodies, that limit is somewhere around 3-4 Earth Masses.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-08T23:38:03.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1108" PostTypeId="2" ParentId="1100" CreationDate="2013-08-09T00:10:06.690" Score="3" Body="&lt;p&gt;Any fuel sent to a depot has to be launched, and so needs to be in a sufficiently anti-slosh container.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sending up deflated tankage to be filled from depots presumes inflatable tankage capable of handling the fuels; the most common space fuels use cryogenic fluids, and the others tend to be corrosives. (Hydrazine, monomethylhydrazine and nitrogen tetroxide are all hazardous fuels; Liquid Hydrogen and Oxygen are cryogenic fluids.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since any spacecraft going anywhere of note at present is likely to be using a high thrust short burst to accumulate the needed vector to go  wherever, and another at the far end, the need to stress the fuel tankage remains; the use of separately launched tanks being thus preferable to inflation from a depot in orbit.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-09T00:10:06.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1109" PostTypeId="1" CreationDate="2013-08-09T01:29:38.810" Score="9" ViewCount="455" Body="&lt;p&gt;I was reading about the British &lt;a href=&quot;http://en.wikipedia.org/wiki/Skylon_%28spacecraft%29&quot;&gt;Skylon SSTO (Single Stage To Orbit)&lt;/a&gt; vehicle and its &lt;a href=&quot;http://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29&quot;&gt;SABRE engine&lt;/a&gt; described in Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engine technology looks remarkable, if not revolutionary. Is this a unique effort, or is this technology also being explored by other nations/corporations? If so, what sort of timelines are they on?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="12102" LastEditDate="2021-01-01T10:53:03.723" LastActivityDate="2021-01-01T10:53:03.723" Title="Any other countries/companies doing anything similar to British Skylon SSTO?" Tags="|engine-design|spacecraft-development|ssto|sabre-engine|ramjet|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1110" PostTypeId="5" CreationDate="2013-08-09T01:50:18.747" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-09T01:50:18.747" LastActivityDate="2013-08-09T01:50:18.747" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1111" PostTypeId="4" CreationDate="2013-08-09T01:50:18.747" Score="0" Body="Questions about air breathing jet engines that use forward motion to compress air at the intake, and their forward thrust used to accelerate already non-stationary spacecraft through atmospheres of celestial bodies, or interstellar medium." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-09T12:44:41.543" LastActivityDate="2013-08-09T12:44:41.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1112" PostTypeId="5" CreationDate="2013-08-09T01:59:19.917" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-09T01:59:19.917" LastActivityDate="2013-08-09T01:59:19.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1113" PostTypeId="4" CreationDate="2013-08-09T01:59:19.917" Score="0" Body="Questions about single-stage-to-orbit spacecraft or system designs, that don't dispose any of their hardware during flight and can be reused as a whole. They can be entirely self-propelled, or partially accelerated to initial velocity by systems that are not parts of the space vehicle." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-09T12:44:24.260" LastActivityDate="2013-08-09T12:44:24.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1114" PostTypeId="5" CreationDate="2013-08-09T02:14:03.187" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-09T02:14:03.187" LastActivityDate="2013-08-09T02:14:03.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1115" PostTypeId="4" CreationDate="2013-08-09T02:14:03.187" Score="0" Body="Questions about propellant depots placed in space or celestial bodies to allow propellant refueling of space vehicles or artificial satellites, to extend their range, payload capacity, or orbit sustainability." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-09T12:43:33.163" LastActivityDate="2013-08-09T12:43:33.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1116" PostTypeId="5" CreationDate="2013-08-09T02:22:13.223" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-09T02:22:13.223" LastActivityDate="2013-08-09T02:22:13.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1117" PostTypeId="4" CreationDate="2013-08-09T02:22:13.223" Score="0" Body="Questions regarding effects on health by activities, prolonged stay, or exposure to environmental elements in space, celestial bodies, or space stations. Predominantly questions pertaining to health of human inhabitants, or astronauts. Questions pertaining to health of other, non-human organisms should include additional tags for further categorization." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-09T12:43:52.977" LastActivityDate="2013-08-09T12:43:52.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1118" PostTypeId="2" ParentId="1096" CreationDate="2013-08-09T03:30:10.210" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.linkedin.com/company/isro&quot; rel=&quot;nofollow&quot;&gt;this link&lt;/a&gt; and &lt;a href=&quot;http://www.isro.org/pdf/HumanResources.pdf&quot; rel=&quot;nofollow&quot;&gt;this pdf&lt;/a&gt;, there are 14713 staff working under ISRO as of 2012.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="58" LastEditDate="2013-08-12T19:39:41.863" LastActivityDate="2013-08-12T19:39:41.863" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1123" PostTypeId="1" AcceptedAnswerId="1137" CreationDate="2013-08-09T07:46:35.527" Score="8" ViewCount="435" Body="&lt;p&gt;Currently there seem to be two efforts under way:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Bigelow_Aerospace&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bigelow Aerospace&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.popularmechanics.com/science/space/news/russia-is-building-an-inflatable-space-module-of-its-own-15706708&quot; rel=&quot;nofollow noreferrer&quot;&gt;RKK Energiya&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The latter source (an article in the Popular Mechanics by Anatoly Zak) intimates that structural creep is a major engineering hurdle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I'm not convinced it's the only one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the engineering problems that we have to watch out for while building and deploying inflatable structures in LEO and beyond?&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="12102" LastEditDate="2018-08-01T03:23:14.817" LastActivityDate="2018-08-01T03:23:14.817" Title="What are the practical engineering difficulties in building and launching large inflatable modules?" Tags="|engineering|habitat|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1124" PostTypeId="2" ParentId="1123" CreationDate="2013-08-09T10:12:36.383" Score="4" Body="&lt;p&gt;It can burst as did &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=12494.0&quot; rel=&quot;nofollow noreferrer&quot;&gt;one of the earlier, 1991 attempts&lt;/a&gt; to produce a large inflatable object. That has happened because the envelope hitched an antenna. during deployment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xcDYp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/YMcDb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/sYHsz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or as did the &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/irt.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;IRT -- an even earlier attempt (1984).&lt;/a&gt; Because &quot;&lt;em&gt;the lanyards pulled loose from the orbiter instead of pulling out of the canister. The nitrogen inflation cartridge did fire as scheduled, and the balloon burst as it began to inflate inside its canister.&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/O1rD6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also it can wrap around your spacecraft during inflation phase. See the &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=12494.0&quot; rel=&quot;nofollow noreferrer&quot;&gt;picture number three&lt;/a&gt; there. Note the Progress space craft completely covered by the thin envelope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wdip0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-09T21:06:28.107" LastActivityDate="2013-08-09T21:06:28.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1125" PostTypeId="1" AcceptedAnswerId="1131" CreationDate="2013-08-09T11:18:37.650" Score="3" ViewCount="1431" Body="&lt;p&gt;Is it possible to predict whether the orbital of the object is circular, elliptical or hyperbolic only with the help of potential and the kinetic energy of the object is given.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="157" LastEditDate="2013-08-09T13:07:28.290" LastActivityDate="2013-08-11T11:20:33.753" Title="Is it possible to predict the orbital shape with the given kinetic and potential energy?" Tags="|orbit|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1126" PostTypeId="1" CreationDate="2013-08-09T11:42:37.673" Score="11" ViewCount="693" Body="&lt;p&gt;On Earth (under the influence of gravity) a horizontal tube in which water is flowing may form a layer of air at the top of the tube (since less denser than water). But on the ISS (which experiences micro-gravity) it  does not happen.  In fact air may form a cone shape in the tube preventing the flow of water (as shown in the image) how does the ISS overcomes this situation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they use a motor to pump water?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KGG0R.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="25" LastEditDate="2015-12-17T22:56:22.987" LastActivityDate="2015-12-17T22:56:22.987" Title="Plumbing work of ISS?" Tags="|iss|microgravity|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1129" PostTypeId="2" ParentId="1125" CreationDate="2013-08-09T15:11:11.550" Score="0" Body="&lt;p&gt;If you are considering only two objects and want to determine whether one object is in an orbit with respect to the other, you need to know:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The combined mass of the two objects in question&lt;/li&gt;&#xA;&lt;li&gt;The position, at some point in time, of the object in question, with respect to the combined center of mass of the two objects&lt;/li&gt;&#xA;&lt;li&gt;The velocity (vector) of the object in question, with respect to the combined center of mass, at the same point in time you determined its position.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It's not really about knowing the kinetic and/or potential energy of an object as such.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2013-08-09T15:11:11.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1130" PostTypeId="2" ParentId="1125" CreationDate="2013-08-09T15:32:40.163" Score="2" Body="&lt;p&gt;Okay, so let's take a look at what you actually have with kinetic and potential energy. For simplicity's sake, let's assume you only have gravitational potential energy.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Kinetic Energy = $\frac{m v^2}{2}$&lt;/li&gt;&#xA;&lt;li&gt;Potential Energy = $m*g*height$.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, so given those two terms, what are you missing? What you really need to know to determine an orbit is a position and velocity, having both will give you the orbit. Alternatively, multiple positions can give you the same thing. So, what do you still need?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;You need to know the gravity to get the height.&lt;/li&gt;&#xA;&lt;li&gt;You need to know the direction to get velocity (Energy is scalar, it doesn't have a direction)&lt;/li&gt;&#xA;&lt;li&gt;You need the mass of the object.&lt;/li&gt;&#xA;&lt;li&gt;You need position other than height.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;You could form a partial orbit with two bits of information:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Gravity (Specifically, g(height)) If orbiting Earth, this is easily known.&lt;/li&gt;&#xA;&lt;li&gt;The mass of the object.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This will give you some, but not all, of the &lt;a href=&quot;http://www.amsat.org/amsat/keps/kepmodel.html&quot; rel=&quot;nofollow&quot;&gt;Keplerian elements&lt;/a&gt;. Specifically, you could get eccentricity, Mean Motion, and Mean Anomaly, but you can't get inclination, Right Ascension Node, or Argument of the Perigee.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="168" LastEditDate="2013-08-11T11:20:33.753" LastActivityDate="2013-08-11T11:20:33.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1131" PostTypeId="2" ParentId="1125" CreationDate="2013-08-09T16:16:42.910" Score="4" Body="&lt;p&gt;the difference between the kinetic and the potential energy is H&#xA;$$H = T - \left. {\left| \phi  \right.} \right| =  - {1 \over 2}{{{k^4}m} \over {{h^2}}}(1 - {e^2})$$&#xA;solving for e:&#xA;$$e = \sqrt {1 + {{2{h^2}H} \over {m{k^4}}}} $$&#xA;which shows that if H&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;negative then e&amp;lt;1 (the trajectory is ellipse)(kinetic energy &amp;lt; potential energy)&lt;/li&gt;&#xA;&lt;li&gt;positive then e&gt;1 (the trajectory is hyperbola)(kinetic energy &gt; potential energy)&lt;/li&gt;&#xA;&lt;li&gt;equal then e=1 (the trajectory is parabola)(kinetic energy = potential energy)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-08-11T09:41:56.530" LastActivityDate="2013-08-11T09:41:56.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1132" PostTypeId="1" CreationDate="2013-08-09T17:02:42.357" Score="13" ViewCount="2177" Body="&lt;p&gt;There are a few questions regarding the Alcubierre drive here, so I'll pose this one and see what I get.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, our space exploration vehicles, both manned and unmanned, are at the mercy of Kepler and Newton. We have some control over what direction we're pointed and how fast we're going, but our movement through space is governed by slingshotting around the Earth, Sun and other planets and moons, which allow us to gain, lose or maintain our velocity much more efficiently than any rocket could. There's no energy source nor thrust motor we can conceive of that's powerful or long-lived enough to allow us to ignore orbital motion and simply &lt;em&gt;push&lt;/em&gt;, against the Sun and any other nearby gravity wells, to take a relatively straight-line path anywhere in our neighborhood.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Editing based on Deer Hunter's comment...&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's now conceive of a drive system powerful enough that we no longer need to use gravity slingshotting as our primary means of propulsion outside LEO. The Alcubierre Drive is currently the most plausible future tech, but we could imagine virtually any drive system that would allow us to not only &quot;win&quot;, handily, against the straight-line pull of anything in the Solar system, but be efficient enough that such a tug-o-war is a drop in the bucket for the fuel capacity of the ship equipped with such a drive. Such a drive system, even if it were subluminal, would be the gateway to the solar system (even half-light speed, impractical for interstellar travel, would make landing on Mars and coming back a day trip).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the question; given a drive system, Alcubierre-based or otherwise, powerful enough that we no longer depend on gravity-based navigation maneuvers to get around, what role would gravity still play in our navigation and maneuvering? Obviously, where gravity helps us out, such as falling toward the Sun to cross the system, we could let it, and there's really no &quot;standing still&quot; (though even that could be possible with such a drive system, depending on your definition of &quot;still&quot;), but given that we'd have the ability to travel much faster than gravity could pull us, I imagine that gravity would be more hindrance than help; we'd simply adjust course to account for the effect of gravity wells as we pass by them, much as we currently do in airplanes to account for wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, would gravity really matter anymore in the post-reactionary thrust engine era of space travel? Or would it, much like the wind to an airplane, simply be something to correct for?&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="49" LastEditDate="2013-12-31T18:34:18.227" LastActivityDate="2016-06-12T18:02:14.210" Title="Alcubierre &quot;warp&quot; drive and gravitation/orbital considerations" Tags="|propulsion|gravity|spacecraft-development|advanced-propulsion|alcubierre-drive|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1133" PostTypeId="2" ParentId="1125" CreationDate="2013-08-09T18:01:26.980" Score="7" Body="&lt;p&gt;Yes you can, and it's really easy. Assuming that you have taken the reference level for 0 gravitational potential at infinity (this is quite standard), then, if KE+PE is positive, the pathis hyperbolic. If it is negative, it is elliptical (or circular&lt;sup&gt;1&lt;/sup&gt;). If it is zero, you are at escape velocity and the path is parabolic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a hyperbolic path, the object has enough total energy to reach &quot;infinity&quot;. So, KE+PE&gt;0. When it is equal to zero you get a parabolic path, because it still has enough energy to reach infinity, but &lt;em&gt;just&lt;/em&gt; enough (and a parabola can be looked at as a limiting case of a hyperbola lobe). If it is less than zero, it does not have enough energy and will eventually &quot;turn back&quot;, giving a elliptical orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a simulation (I've taken some of the quantities to be equal to 1 for simplicitly):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MDUKU.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note: The image seems to become a parabola before it really does at -0.04 total energy because there isn't enough space to show the large ellipse)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1. If you wish to determine whether or not the orbit is circular, you need to know the mass ratio and the angle the velocity makes with the radius vector&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="96" LastEditorUserId="96" LastEditDate="2013-08-09T18:45:09.343" LastActivityDate="2013-08-09T18:45:09.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1134" PostTypeId="1" AcceptedAnswerId="1136" CreationDate="2013-08-09T18:39:05.110" Score="27" ViewCount="3029" Body="&lt;p&gt;Are artificial satellites such as communication or weather satellites completely sealed or are they open to space? (In either case, why? How are their electronics (un)affected? Presumably they can be shielded.)&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="510" LastEditDate="2013-08-09T21:34:59.623" LastActivityDate="2023-06-05T08:31:11.587" Title="Are artificial satellites completely sealed or open to space?" Tags="|artificial-satellite|vacuum|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1135" PostTypeId="2" ParentId="1134" CreationDate="2013-08-09T18:53:10.063" Score="17" Body="&lt;p&gt;Generally open to space.  Maintaining a perfect seal a long time is hard.  If your equipment needs non-vacuum to operate, and you expect it to last, then it is pretty likely it will leak over time, so might as well design to operate in a vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One benefit of an atmosphere inside would be better cooling.  But since there is no difference between hot/cold air in weight, due to micro-gravity, you do not get natural convection anyway. Now you have to maintain a fan for active maintenance.  Another moving part, another hard thing to keep going long term with no humans around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each benefit and trade-off, usually lead towards just designing to operate in a vacuum.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="25" LastEditDate="2015-12-17T22:55:50.963" LastActivityDate="2015-12-17T22:55:50.963" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1136" PostTypeId="2" ParentId="1134" CreationDate="2013-08-09T19:19:42.067" Score="26" Body="&lt;p&gt;For the most part, satellites are opened loosely to space. The components are designed to work in a vacuum, and in fact, are tested to work in a vacuum on the ground. However, there's a few things of some note along with this:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;There are typically a few components which must be sealed in. The most common is propulsion tanks, also, some Reaction Wheels are designed to work in atmosphere, and there are probably some others that I don't know about as well. They have to be &lt;a href=&quot;http://en.wikipedia.org/wiki/Hermetic_seal&quot; rel=&quot;noreferrer&quot;&gt;hermetically sealed&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;The main box of the satellite is not air tight, but usually is completely closed in. This allows for better thermal management, as well as preventing &lt;a href=&quot;http://en.wikipedia.org/wiki/Electromagnetic_interference&quot; rel=&quot;noreferrer&quot;&gt;EMI&lt;/a&gt; hitting the antenna/receivers. Typically this results in a loose sealing, which will take some time to get to complete vacuum (On the order of hours typically). Note that the hours is for a near complete vacuum, a rough vacuum is obtained quite quickly.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-05-03T15:44:38.150" LastActivityDate="2014-05-03T15:44:38.150" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1137" PostTypeId="2" ParentId="1123" CreationDate="2013-08-09T20:55:06.520" Score="5" Body="&lt;h1&gt;Problem 1: Material Strength&lt;/h1&gt;&#xA;&lt;p&gt;Strong and Flexible generally means heavy. The material has to be strong enough to handle the stresses of both the internal pressure when inflated, but also the localized pressures during inflation.&lt;/p&gt;&#xA;&lt;h1&gt;Problem 2: Inflation Sequencing&lt;/h1&gt;&#xA;&lt;p&gt;As the Russians discovered, inflation sequencing can go awry. This can result in a variety of issues, from wrapping around the launching craft, to binding against itself into the wrong shape.&lt;/p&gt;&#xA;&lt;h2&gt;Problem 2.1: Vacuum effects on materials&lt;/h2&gt;&#xA;&lt;p&gt;Many projects tested in pressure react differently in the lack of pressure. It must be accounted for. It affects lubricants, expanding pressure-driven components, and a variety of other factors.&lt;/p&gt;&#xA;&lt;h2&gt;Problem 2.2: Microgravity effects on containers&lt;/h2&gt;&#xA;&lt;p&gt;Microgravity can allow some aspects to deform in ways that can be anticipated, but in gravity tend to be overlooked.&lt;/p&gt;&#xA;&lt;h1&gt;Problem 3: Utilization Based Requirements&lt;/h1&gt;&#xA;&lt;p&gt;At present, all such proposals are either for fuel depots or for habitat modules. The fuel depot proposals are for planetary use, either on Luna or Mars, and so have related considerations, but are not &amp;quot;inflatable space modules&amp;quot;, and thus beyond the question&lt;/p&gt;&#xA;&lt;p&gt;Which leaves habitation modules.&lt;/p&gt;&#xA;&lt;h2&gt;Problem 3.1: Radiation Resistance&lt;/h2&gt;&#xA;&lt;p&gt;Modules must provide protection for occupants from 2 forms of radiation, both high energy particles (solar wind) and electromagnetic radiation (especially US, Gamma, and X). Spacecraft at present do not provide adequate protection from Gamma, X, and particulate radiation, so lifetime space travel limits are imposed by NASA. The particulates are supposedly better stopped by liquids rather than solids, but shipping that much liquid up isn't cheap. It is, however, a 1-time deal for a long-term module.&lt;/p&gt;&#xA;&lt;h2&gt;Problem 3.2: Puncture Resistance&lt;/h2&gt;&#xA;&lt;p&gt;The resistance to microbody interactions, specifically, being hit by mirco-asteroidal bodies and space-junk, requires a specific set of engineering challenges. Note that space-suits share this requirement, and extensive NASA documentation is available for them.&lt;/p&gt;&#xA;&lt;h2&gt;Problem 3.3: Internal Fittings&lt;/h2&gt;&#xA;&lt;p&gt;An inflatable structure still may require additional fittings inside. A habitation module will, at a minimum, require air circulation fittings, while a science lab will require workspaces. How to attach and configure these is a significant issue.&lt;/p&gt;&#xA;&lt;h2&gt;Problem 3.4: Human Psychological Needs&lt;/h2&gt;&#xA;&lt;p&gt;Humans are not well adapted to enclosed spaces. This is part of the reason for inflatable modules; more open spaces are expected to be helpful in long duration spaceflight. Also, view-ports, privacy enclosures, designated work spaces, and similar considerations require engineering consideration.&lt;/p&gt;&#xA;&lt;h1&gt;Problem 4: Overall Launch Weight&lt;/h1&gt;&#xA;&lt;p&gt;The proposed inflatable modules are intended to reduce the launch weight per unit volume of habitation modules. Still, the limit on size is a function of weight lofted, and individual launchers still have relatively limited payloads.&lt;/p&gt;&#xA;&lt;h1&gt;Problem 5: Overall Cost&lt;/h1&gt;&#xA;&lt;p&gt;While not strictly an engineering consideration, no engineer can work on a project without considering the costs. All projects have budgets, and Engineers who can't keep designs within them don't see their work built.&lt;/p&gt;&#xA;&lt;h1&gt;Problem 6: Inflation &amp;amp; Assembly Crew Requirements&lt;/h1&gt;&#xA;&lt;p&gt;Spaceflights have very limited crews. If the inflation and assembly require 5 men, then it's going to be impractical for a soyuz launch to inflate, but the ISS could do so at a changeover.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-09T20:55:06.520" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1138" PostTypeId="2" ParentId="1126" CreationDate="2013-08-09T21:36:44.130" Score="7" Body="&lt;p&gt;There are two answers to the question of fluid flow in micro-G: heat pipes with capillary flow and pumps.&lt;/p&gt;&#xA;&lt;p&gt;Heat pipes are installed in several places on both American and Russian segments to take extra heat off pieces of avionics with largest loads. Heat-pipes are by definition two-phase, liquid and vapor (ammonia-based).&lt;/p&gt;&#xA;&lt;p&gt;The ISS has water loops in the life support system (including condensate collection), and cooling loops in the thermal control system.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Internal loops&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Water-filled (US)&lt;/li&gt;&#xA;&lt;li&gt;Filled with a mixture of water and ethylene glycol (Russia)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;External loops&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Anhydrous ammonia (US)&lt;/li&gt;&#xA;&lt;li&gt;Polymethyl Siloxane (Russia)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Life support system has electric and manual pumps. Plumbing work may include manually pumping through air blockages but the loops aren't critical.&lt;/p&gt;&#xA;&lt;p&gt;Cooling loops &lt;strong&gt;are&lt;/strong&gt; critical, and are &lt;strong&gt;single-phase&lt;/strong&gt; (liquid throughout). All the pumps are electrical.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;US: there are two internal water loops (low- and moderate temperature).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Russia: two redundant switch-on-fail Triol loops&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Each US water loop has a Pump Package Assembly with a centrifugal pump, an accumulator, a gas trap, filters, a flowmeter and T (temperature) and P (pressure) sensors.&lt;/p&gt;&#xA;&lt;p&gt;In the Russian segment, each loop has four pumps and one accumulator.&lt;/p&gt;&#xA;&lt;p&gt;So, to answer your question: it is the &lt;strong&gt;gas trap&lt;/strong&gt; that prevents air from accumulating in the loop, and the &lt;strong&gt;accumulator&lt;/strong&gt; that makes up for smaller leaks.&lt;/p&gt;&#xA;&lt;p&gt;In the external ammonia-filled loops (there are two of them), there is again &lt;strong&gt;an accumulator&lt;/strong&gt; which contains a fixed charge of nitrogen to maintain ammonia in the liquid state.&lt;/p&gt;&#xA;&lt;p&gt;For each of Russian external loops, there is one accumulator.&lt;/p&gt;&#xA;&lt;h1&gt;If redundancy measures in cooling loops fail, it's time to leave the station.&lt;/h1&gt;&#xA;&lt;p&gt;&lt;strong&gt;And now you know how important plumbing is...&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceref.com/iss/ops/sm.thermal.control.syst.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spaceref.com/iss/ops/sm.thermal.control.syst.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceref.com/iss/ops/iss.familiarization.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spaceref.com/iss/ops/iss.familiarization.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-17T12:02:15.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1139" PostTypeId="2" ParentId="1001" CreationDate="2013-08-09T22:27:49.917" Score="2" Body="&lt;p&gt;Answering my own question to broaden the context a little bit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 60 meter &lt;a href=&quot;http://www2.jpl.nasa.gov/srtm/mast.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ADAM mast&lt;/a&gt; was the larges &lt;em&gt;rigid&lt;/em&gt; structure of its time (STS-77, 16 February 2000). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DSrH2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote the JPL description: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Unique latches on the diagonal members of the truss allowed the mechanism to deploy bay-by-bay out of the mast canister to a length of 60 meters (200 feet). The canister housed the mast during launch and landing, and it also deployed and retracted the mast.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-08-09T22:27:49.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1140" PostTypeId="1" AcceptedAnswerId="1148" CreationDate="2013-08-10T01:32:19.113" Score="6" ViewCount="1584" Body="&lt;p&gt;There are many cameras currently orbiting Mars, taking pictures of rocks and sand and things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the highest (in the visible spectrum) sensor resolution of these? Are there higher res ones planned?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2014-03-07T09:47:13.520" LastActivityDate="2018-05-08T06:20:01.673" Title="What's the highest resolution camera orbiting Mars as of 2013?" Tags="|mars|camera|remote-sensing|mapping|topography|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1141" PostTypeId="5" CreationDate="2013-08-10T01:53:50.620" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-10T01:53:50.620" LastActivityDate="2013-08-10T01:53:50.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1142" PostTypeId="4" CreationDate="2013-08-10T01:53:50.620" Score="0" Body="Questions regarding the physics measurements of an object, such as length, width, and height." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-10T10:58:27.733" LastActivityDate="2013-08-10T10:58:27.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1143" PostTypeId="2" ParentId="1001" CreationDate="2013-08-10T03:48:22.933" Score="4" Body="&lt;p&gt;The satellites are classified, so details are speculative, but it is believed that the Magnum/Orion and Mentor SIGINT satellites have parabolic antennae around 100 m in diameter.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2013-08-10T03:48:22.933" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1144" PostTypeId="1" AcceptedAnswerId="1158" CreationDate="2013-08-10T05:06:01.733" Score="14" ViewCount="968" Body="&lt;p&gt;Has NASA tested any nuclear thermal rocket engine after dropping the NERVA program? Is there any current ongoing mission to use nuclear power to make a rocket fly?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="12102" LastEditDate="2019-01-07T10:55:30.147" LastActivityDate="2019-01-07T10:55:30.147" Title="Has there been any nuclear thermal rocket test program after NERVA?" Tags="|rockets|nuclear|ntr|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1145" PostTypeId="1" CreationDate="2013-08-10T05:09:37.453" Score="15" ViewCount="1948" Body="&lt;p&gt;At the very end, NERVA (&lt;em&gt;Nuclear Engine for Rocket Vehicle Application&lt;/em&gt;) had developed nearly flight-ready engines able to:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;operate for periods in excess of an hour while providing maximum thrust up to 220,000 lb (1000 kN),&lt;/li&gt;&#xA;&lt;li&gt;achieve reactor core temperatures of 2500 K,&lt;/li&gt;&#xA;&lt;li&gt;give a specific impulse over 850 seconds (roughly double that of chemical rockets),&lt;/li&gt;&#xA;&lt;li&gt;have a thrust-to-weight ratio of better than 3:1.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Despite these results, what made NASA-(AEC/DoE) drop the NERVA program?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="40257" LastEditDate="2022-05-05T11:30:07.063" LastActivityDate="2023-12-20T23:40:13.393" Title="Why was the NERVA program dropped?" Tags="|nasa|engine-design|ntr|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="1147" PostTypeId="1" CreationDate="2013-08-10T06:56:51.193" Score="6" ViewCount="447" Body="&lt;p&gt;I recently studied that life may exist on Europa, an icy moon of Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does NASA plan to land a spacecraft on Europa to study its environment in the near future?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-08-10T17:40:29.270" LastActivityDate="2015-12-07T17:50:24.627" Title="Does NASA plan to land on Europa?" Tags="|nasa|artificial-satellite|jupiter|landing|moons|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1148" PostTypeId="2" ParentId="1140" CreationDate="2013-08-10T09:12:45.367" Score="10" Body="&lt;p&gt;Mars Reconnaissance Orbiter has a camera called HiRISE (High Resolution Imaging Science Experiment), launched in 2005-08-12, which is the highest resolution camera in orbit of Mars at an altitude that varies from 200 to 400 kilometers (about 125 to 250 miles) above the martian surface by Carrier rocket Atlas V-401.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The High Resolution Imaging Science Experiment camera is a 0.5 m reflecting telescope, the largest ever carried on a deep space mission, and has a resolution of 1 microradian (μrad), or 0.3 m from an altitude of 300 km. In comparison, satellite images of Earth are generally available with a resolution of 0.5 m, and satellite images on Google Maps are available to 1 m.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;HiRISE collects images in three color bands, 400 to 600 nm (blue-green or B-G), 550 to 850 nm (red) and 800 to 1,000 nm (near infrared or NIR). Red color images are 20,264 pixels across (6 km wide), and B-G and NIR are 4,048 pixels across (1.2 km wide). HiRISE's onboard computer reads these lines in time with the orbiter's ground speed, and images are potentially unlimited in length. Practically however, their length is limited by the computer's 28 Gigabit (Gb) memory capacity, and the nominal maximum size is 20,000 × 40,000 pixels (800 megapixels) and 4,000 × 40,000 pixels (160 megapixels) for B-G and NIR images.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Each 16.4 Gb image is compressed to 5 Gb before transmission and release to the general public on the HiRISE website in JPEG 2000 format.To facilitate the mapping of potential landing sites, HiRISE can produce stereo pairs of images from which topography can be calculated to an accuracy of 0.25m.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;REFERENCE :&#xA;&lt;a href=&quot;http://marsmobile.jpl.nasa.gov/mro/mission/instruments/hirise/&quot;&gt;NASA&lt;/a&gt;  , &lt;a href=&quot;http://en.m.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter&quot;&gt;link&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-09-04T16:30:36.357" LastActivityDate="2013-09-04T16:30:36.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1149" PostTypeId="5" CreationDate="2013-08-10T11:03:31.223" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-10T11:03:31.223" LastActivityDate="2013-08-10T11:03:31.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1150" PostTypeId="4" CreationDate="2013-08-10T11:03:31.223" Score="0" Body="Questions on ascertaining, predicting, and controlling spatial orientation and rotation of spacecraft, and on forces that affect spatial orientation." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-10T15:07:35.897" LastActivityDate="2013-08-10T15:07:35.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1151" PostTypeId="1" AcceptedAnswerId="1575" CreationDate="2013-08-10T11:54:51.350" Score="13" ViewCount="338" Body="&lt;p&gt;Our knowledge of ballistics improves all the time:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Multiple Earth-Venus gravity assists have become quite standard.&lt;/li&gt;&#xA;&lt;li&gt;We keep on planning and sending spacecraft to &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrange_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lagrange points&lt;/a&gt;, and increasingly make use of &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Superhighway&quot; rel=&quot;nofollow noreferrer&quot;&gt;low-energy transfers&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;While such sophisticated mission designs allow us to cram more payload into limited scientific budgets, they also mean higher transit times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outer Solar System missions get longer (unless, as in New Horizons, payload mass is sacrificed to ensure higher escape velocities).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not many scientific instruments can yield sensible data while in transit, so this leads to the following problems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;PIs (Principal Investigators) who originally supervised the design and manufacture of the instrument get older and sometimes leave us all for grand exploration of heaven on their own. As leadership changes, team motivation may drop.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The original designers and programmers quit, and if glitches appear it may take their successors much more time to iron the bugs out.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Simply embarking on a decade (or &lt;strong&gt;decades&lt;/strong&gt;) -long crusade without realistic hope of getting valuable science results in &lt;strong&gt;own lifetime&lt;/strong&gt; or &lt;strong&gt;soon&lt;/strong&gt; may seem discouraging for a lot of bright researchers out there. I know we aren't dropping Mars from our sights any time soon, but Mars is not the only target.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Related: &lt;a href=&quot;https://space.stackexchange.com/questions/1078/why-will-juice-take-8-years-to-reach-jupiter-longer-than-any-earlier-spacecraft/1079#1079&quot;&gt;Why will JUICE take 8 years to reach Jupiter, longer than any earlier spacecraft?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is simple:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What can we do to keep motivation and tacit knowledge and team know-how in long-flight space probe research projects?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: Yes, larger $\Delta V$ budgets always help. However, it's not something most can afford.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-08-26T02:38:11.927" Title="How do long-flight space probe teams preserve the interest and the know-how?" Tags="|research|interplanetary|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1153" PostTypeId="1" AcceptedAnswerId="1156" CreationDate="2013-08-10T13:16:49.960" Score="9" ViewCount="1247" Body="&lt;p&gt;Space junk is a real threat to working satellites in the Earth's orbit. Is it theoretically possible to make this space junk escape from the Earth's gravity (by giving them some kinetic energy to overcome the Earth's gravity), make it travel towards the Sun, and let it burn in the intense heat environment of the Sun's coronasphere? The space junk may orbit the Earth at any altitude. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Are there any practical problems in doing this?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-08-11T12:14:39.860" LastActivityDate="2021-07-11T14:51:50.823" Title="Is it theoretically possible to dispose of the larger pieces of space junk into the Sun?" Tags="|orbital-mechanics|debris|" AnswerCount="6" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1154" PostTypeId="2" ParentId="1153" CreationDate="2013-08-10T14:38:24.740" Score="4" Body="&lt;p&gt;It is completely theoretically possible to dump things in to the sun, given enough energy. How much energy would it require? Well, there isn't a table I've seen that shows how much delta V is required to shoot to the Sun, but there is a &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/appmissiontable.php&quot; rel=&quot;nofollow&quot;&gt;convenient&lt;/a&gt; one for Mercury, which we can safely assume is less than would be required to get to the Sun. The Delta V requirements to directly get to Mercury from Earth are 48 km/s. That includes landing and returning to Earth, but even that would only add a factor of 2 or so. Given about 10 km/s to orbit, that's a huge rocket, just to get to Mercury. Real missions save some of this by doing flybys, but they require active course corrections for some time to get things just right. That's a huge amount, far more than any ship is likely to have!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what are some alternatives? There are basically 2 main goals for space junk removal, depending on where the item is. The first is to make it crash in to Earth (Or another suitable object). The second is to get it outside of a usable orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so how are these accomplished? The first can be done by simply slowing the object down to the point where it lightly touches Earth's atmosphere at it's minimum point. Given enough time, that will cause the object to re-enter Earth. Assuming it's not that large, this shouldn't pose any danger at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second possibility is to have it enter a useless orbit. GEO would require substantial fuel to re-enter Earth. Instead, what they do is raise the orbit by a bit such that it will never intersect the GEO orbit, at least, not for a very long time. This basically eliminates the possibility of impacts.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-10T19:48:31.193" LastActivityDate="2013-08-10T19:48:31.193" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1155" PostTypeId="2" ParentId="1147" CreationDate="2013-08-10T17:26:35.490" Score="9" Body="&lt;p&gt;&quot;Plan&quot; is a vague word...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has an orbiter mission in the works: &lt;a href=&quot;http://www.jpl.nasa.gov/missions/details.php?id=6002&quot;&gt;Europa Clipper&lt;/a&gt; as a predicate mission prior to any attempt to land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Europa Lander is a &quot;concept under study&quot; - that is, they are making plans to do the project, but haven't budgeted for it, nor have then given the actual project the proverbial &quot;green light.&quot; NASA does have a page about a &lt;a href=&quot;http://solarsystem.nasa.gov/missions/profile.cfm?MCode=EAL&quot;&gt;Europa Lander&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, at present, they intend to do so eventually, but a specific mission is not yet in the works.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-10T17:26:35.490" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1156" PostTypeId="2" ParentId="1153" CreationDate="2013-08-10T17:44:41.317" Score="10" Body="&lt;p&gt;It is possible to remove any and all space junk and put it into a solar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is highly impractical, though. The ∆V (Delta-Vee = total change in vector)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Attaining escape velocity requires (very roughly) about 3x the energy as attaining LEO. Plus you have to capture, and that eats ∆V. Further, launches are expensive in and of themselves, and generate micro-debris in orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further still, there is little reason to send it solarward; for the same ∆V, one could return the junk to Earth for reuse. Or, eventually, potentially take it to an orbital manufacturing facility for reuse in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The larger stuff isn't much of a concern, anyway. It's easily tracked, its orbits are known and predicted months in advance. (Note that LEO is still experiencing atmospheric drag, minimal as it is, and that such orbits can't be accurately predicted on the scope of years due to variations in drag. This is part of why Skylab came down earlier than originally predicted.) Small stuff is the problem. Orbital tracking can track objects of several centimeters; most of the damage to spacecraft in orbit has been from debris estimated at a handful of millimeters across.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-10T17:44:41.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1157" PostTypeId="2" ParentId="1153" CreationDate="2013-08-10T18:48:29.097" Score="-1" Body="&lt;p&gt;Theoretically, yes. Practically, I think it would depend on the object's orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some satellites are de-orbited when they reach the end of their useful life, causing them to descend into the atmosphere and burn up. I believe some satellites are actually boosted out of Earth orbit at end-of-life, presumably because they are in a relatively high orbit and the delta-V to escape Earth is less than the delta-V to bring them down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space junk in low orbit will eventually de-orbit all by itself, just from the cumulative effect of atmospheric drag. It's just a matter of time.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2013-08-10T18:48:29.097" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1158" PostTypeId="2" ParentId="1144" CreationDate="2013-08-10T18:55:13.820" Score="8" Body="&lt;p&gt;As of 2012 (Nuclear and Emerging Technologies in Space meeting at LPI) the outline of development program is as &lt;a href=&quot;http://www.lpi.usra.edu/meetings/nets2012/pdf/3088.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;follows&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;See &lt;a href=&quot;http://www.lpi.usra.edu/meetings/nets2012/pdf/program.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this pdf file&lt;/a&gt; for links to papers.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(note: only tangible test &amp;amp; evaluation items listed)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Manufacture and test fuel elements in two compositions: NERVA composite, and Uranium dioxide UO2 in tungsten ceramic-metallic &quot;cermet&quot;. Testing will be conducted at DOE facilities and at Marshall (NTR Element Environmental Simulator)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Conduct safety tests at Nevada Test Site by injecting hot gas with Krypton-85 tracer to find out if future testing will not leak radioactive particles.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;After that NASA and DOE will go for approval of testing by higher-ups in 2015.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If approved, full scale testing will be started at Nevada Test Site in late 2019.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Flight technology demonstration mission is scheduled for 2023.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vPu2P.png&quot; alt=&quot;timeline courtesy Stan Borowski&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-10-31T18:19:13.917" LastActivityDate="2015-10-31T18:19:13.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1159" PostTypeId="5" CreationDate="2013-08-10T19:31:08.857" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-10T19:31:08.857" LastActivityDate="2013-08-10T19:31:08.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1160" PostTypeId="4" CreationDate="2013-08-10T19:31:08.857" Score="0" Body="Questions regarding the design or design process for a propulsion system." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-10T21:00:07.407" LastActivityDate="2013-08-10T21:00:07.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1161" PostTypeId="2" ParentId="1153" CreationDate="2013-08-10T20:04:32.207" Score="3" Body="&lt;p&gt;There is a small issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of space junk that we have created are really tiny, like dust or other small things, but they are also travelling and stupendous speeds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you will either need to travel to it which requires fuel and energy, and then force it towards another orbiting body or indeed the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, you can have a net or surface to either capture or bounce the junk to it's destination, which would have to be pretty strong, because some space junk can be moving at 20,000 mph and faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Countries have been coming up with solutions of space junk, however it will be horrifically expensive until we create the &quot;space elevator&quot;.&lt;/p&gt;&#xA;" OwnerUserId="516" LastActivityDate="2013-08-10T20:04:32.207" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1162" PostTypeId="1" AcceptedAnswerId="1963" CreationDate="2013-08-10T23:22:16.147" Score="14" ViewCount="1745" Body="&lt;p&gt;The launch of the ISRO GSLV-D5 launching India's GSAT 14 advanced communications satellite (weighing 1,980 kg) is slated for 19th of August, 2013, flying in the GSLV Mk.2 configuration with an Indian-built cryogenic third stage (&lt;a href=&quot;http://www.astronautix.com/props/loxlh2.htm&quot; rel=&quot;noreferrer&quot;&gt;LH2/LOX&lt;/a&gt; propellants). This mission is delayed from October, December, January, February, April, July and Aug. 6.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 2010 GSLV-D3 mission failed due to the Cryogenic Upper Stage (CUS) not sustaining ignition for longer than 0.8 seconds after initial burn and the subsequent failure of the fuel booster turbo pump (FBTP):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;GSLV-D3 was launched from the Second Launch Pad (SLP) at Satish Dhawan&#xA;  Space Centre SHAR, Sriharikota on April 15, 2010. Flight testing of&#xA;  the Indigenous Cryogenic Stage was unsuccessful.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/gtmJv.jpg&quot; alt=&quot;GSLV-D3 Cryogenic Upper Stage (CUS)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;GSLV-D3 Cryogenic Upper Stage (CUS) (Quote and photograph source: &lt;a href=&quot;http://www.isro.org/gslv-d3/gslv-d3.aspx&quot; rel=&quot;noreferrer&quot;&gt;ISRO&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.thehindu.com/sci-tech/indigenous-cryogenic-engine-to-power-gslvd5-on-aug-19/article5008029.ece&quot; rel=&quot;noreferrer&quot;&gt;The Hindu&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is enormous focus on this mission, as the GSLV flight with an&#xA;  indigenous cryogenic engine failed in April 2010. The subsequent GSLV&#xA;  flight with a Russian cryogenic stage also failed in December of that&#xA;  year.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In April 2010, the indigenous cryogenic engine ignited; the steering&#xA;  engine and the gas generator ignited, but the ignition could not be&#xA;  sustained beyond 800 milliseconds and the fuel booster turbo pump&#xA;  (FBTP) stopped.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/QSEaX.jpg&quot; alt=&quot;ISRO launch vehicle range&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Various ISRO launch vehicles. The configuration to be launched on Aug. 19th is GSLV-Mk II with the indigenous Cryogenic&lt;br/&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Upper Stage (CUS) using LH2/LOX propellants as the 3rd stage (in black just below the payload) (Image: &lt;a href=&quot;http://www.isro.gov.in/hindi/launchvehicles/launchvehicles.aspx&quot; rel=&quot;noreferrer&quot;&gt;ISRO&lt;/a&gt;).&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;S. Ramakrishnan, Director of Vikram Sarabhai Space Centre (VSSC) in&#xA;  Thiruvananthapuram, brimmed with confidence when asked whether the&#xA;  indigenous cryogenic engine would perform well this time.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;“Of course,”&lt;/em&gt; he said. &lt;em&gt;“We are doing well. We have done all the&#xA;  possible tests. As of now, the [cryogenic] stage health is fine. All&#xA;  checks show that the health of the vehicle is all right.”&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Dr. Radhakrishnan elaborated on the steps taken to address the&#xA;  inadequacies of the 2010 flight. He said, &lt;em&gt;“Over the last three years…&#xA;  we have done a series of ground tests on the sub-systems and the&#xA;  cryogenic engine”&lt;/em&gt; at the LPSC at Mahendragiri, Tamil Nadu, &lt;em&gt;“after&#xA;  making the necessary design changes in the FBTP and the oxidiser turbo&#xA;  pump.”&lt;/em&gt; An important test done this year was testing the FBTP in&#xA;  operating conditions at cryogenic temperatures. Ignition of the engine&#xA;  in high-altitude conditions [simulating the vacuum in space] was also&#xA;  done. The duration of this test was 3.5 seconds, when the ignition of&#xA;  the main engine, the gas generator and the two steering engines should&#xA;  take place in a given sequence. &lt;em&gt;“This happened.”&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;All quoted excerpts except the first one copyright and courtesy of: &lt;a href=&quot;http://www.thehindu.com/sci-tech/indigenous-cryogenic-engine-to-power-gslvd5-on-aug-19/article5008029.ece&quot; rel=&quot;noreferrer&quot;&gt;The Hindu&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/E0feq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The indigenously built cryogenic engine being tested at the High Altitude Test Facility (HAT) of the Liquid Propulsion Systems &lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Centre (LPSC), Mahendragiri in early 2013. (Photograph source: &lt;a href=&quot;https://www.facebook.com/isro.org&quot; rel=&quot;noreferrer&quot;&gt;ISRO on Facebook&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Precisely which additional ground tests were conducted on CUS since the GSLV-D3 for the planned GSLV-D5 launch (short of those already described in the excerpt), &lt;/li&gt;&#xA;&lt;li&gt;what changes were made to its design to support this apparent confidence of Dr. Radhakrishnan, Director of VSSC in Thiruvananthapuram that the GSLV-D5 will complete its mission successfully (looking for more detailed explanation), &lt;/li&gt;&#xA;&lt;li&gt;how precise are high-altitude tests to test performance in near vacuum conditions (any previous references?), and&lt;/li&gt;&#xA;&lt;li&gt;what does &lt;em&gt;“testing the FBTP in operating conditions at cryogenic temperatures”&lt;/em&gt; involve?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In a nutshell, I expect answers to further substantiate the claims and confidence that the future GSLV-D5 mission will &quot;deliver&quot; (no pun intended) with a bit more detail than the linked to &lt;a href=&quot;http://www.thehindu.com/sci-tech/indigenous-cryogenic-engine-to-power-gslvd5-on-aug-19/article5008029.ece&quot; rel=&quot;noreferrer&quot;&gt;news article in The Hindu&lt;/a&gt;.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Primary focus should be on the reliability analysis of Cryogenic Upper Stage and its components (e.g. FBTP). Somewhat liberal interpretation of any test data provided and comments are not discouraged, if there isn't any other way, but please provide a link to your sources for reference.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-12T19:59:06.013" LastActivityDate="2013-09-15T01:33:33.687" Title="Changes made to ISRO GSLV-D5's CUS and FBTP since GSLV-D3's LH2/LOX CUS failure to sustain ignition?" Tags="|engine-design|fuel|isro|cryogenics|cus|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1163" PostTypeId="5" CreationDate="2013-08-11T01:37:37.197" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-11T01:37:37.197" LastActivityDate="2013-08-11T01:37:37.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1164" PostTypeId="4" CreationDate="2013-08-11T01:37:37.197" Score="0" Body="Questions regarding use, design, storage, consumption, or other considerations of materials, propellants, systems, or biological samples stored and/or used under very low temperatures, often under larger than ambient pressure." OwnerUserId="49" LastEditorDisplayName="user12" LastEditDate="2013-12-11T16:49:15.407" LastActivityDate="2013-12-11T16:49:15.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1165" PostTypeId="5" CreationDate="2013-08-11T01:45:47.767" Score="0" Body="&lt;p&gt;Chemical rockets use propellants in some form of chemical reaction to generate thrust.  Thermal rockets (Nuclear or otherwise) accelerate propellants by transferring heat to them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally a fuel (item to be reacted/burned) and an oxidizer are required, though some monopropellant examples exist, where the single propellant is both fuel and oxidizer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are liquid propellants and solid propellants.  Hybrid rockets will use a solid fuel, with a liquid oxidizer.&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="51" LastEditDate="2014-01-07T21:51:34.280" LastActivityDate="2014-01-07T21:51:34.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1166" PostTypeId="4" CreationDate="2013-08-11T01:45:47.767" Score="0" Body="Questions about chemicals and chemical mixtures used to propel spacecraft by changing state, composition, or other physical properties that through exhaustion produce thrust." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-11T11:03:58.533" LastActivityDate="2013-08-11T11:03:58.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1167" PostTypeId="5" CreationDate="2013-08-11T01:49:54.690" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-11T01:49:54.690" LastActivityDate="2013-08-11T01:49:54.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1168" PostTypeId="4" CreationDate="2013-08-11T01:49:54.690" Score="0" Body="Questions about Cryogenic Upper Stage (CUS) designs, testing, their use, advantages and disadvantages compared to other types of upper rocket / launch vehicle stages." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-11T11:03:29.203" LastActivityDate="2013-08-11T11:03:29.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1169" PostTypeId="1" CreationDate="2013-08-11T03:29:53.167" Score="-1" ViewCount="444" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/questions/88/total-mass-sent-into-orbit-over-all-history&quot;&gt;question&lt;/a&gt; asks how much mass has been launched into Space. I would like to know how much mass was required to launch that mass into Space. What is the ratio between mass launched and the resources required to make the worlds (successfull &amp;amp; unsuccessfull) launches since Sputnik?&lt;/p&gt;&#xA;" OwnerUserId="137" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-01-19T08:21:23.533" Title="How much total mass has humankind utilized to launch (successfully &amp; unsuccessfully) all the worlds satellites, probes, and manned missions?" Tags="|natural-resources|space-program|earth-observation|" AnswerCount="1" CommentCount="4" ClosedDate="2013-09-04T04:43:17.887" ContentLicense="CC BY-SA 3.0" />
  <row Id="1170" PostTypeId="2" ParentId="1169" CreationDate="2013-08-11T06:30:41.317" Score="3" Body="&lt;p&gt;If we assume around 4000 total launches, and make the wildly inaccurate assumption that the average is 3000Tons (Saturn V plus maximum LEO payload)... 12 million tons.&#xA;(Actual is less).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth Mass 5.972E21Tons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;12E6 tons versus just shy of 6E21 =0.5E15=5E14&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So under 1/4.976E14 or 2 quadrillionths. Given the stupidly high assumption of mass launched, and the ignoring the returned to planet mass, easily less than 1 quadrillionth has actually remained in space.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Saturn_V&lt;/a&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2013-08-12T23:17:36.220" LastActivityDate="2013-08-12T23:17:36.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1171" PostTypeId="1" AcceptedAnswerId="1190" CreationDate="2013-08-11T10:02:42.000" Score="14" ViewCount="19291" Body="&lt;p&gt;Is there any other type of nozzle other than common convergent-divergent nozzle (see picture below) which is used in many rockets? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ArfU5.gif&quot; alt=&quot;Convergent-divergent nozzle&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;A convergent-divergent nozzle (Source: &lt;a href=&quot;http://www.spiraxsarco.com/resources/steam-engineering-tutorials/control-hardware-el-pn-actuation/control-valve-sizing-for-steam-systems.asp&quot; rel=&quot;noreferrer&quot;&gt;Spirax Sarco&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is more efficient in increasing the velocity of exhaust gas?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Please attach pictures of nozzles cited into your answers.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-11T16:17:00.957" LastActivityDate="2013-12-06T10:52:41.430" Title="Efficient types of nozzles used in rockets" Tags="|propulsion|rockets|engine-design|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1172" PostTypeId="1" AcceptedAnswerId="1175" CreationDate="2013-08-11T10:32:51.993" Score="9" ViewCount="703" Body="&lt;p&gt;Under standard condition &lt;a href=&quot;http://en.wikipedia.org/wiki/Methane&quot;&gt;methane&lt;/a&gt; burns with oxygen in stoichiometric ratio of 1:2 to liberate &lt;em&gt;891 kJ/mol&lt;/em&gt; of energy. Why is the ISS not making use of the methane, which is expelled as waste instead? Why couldn't it be mixed with oxygen and this mixture used as fuel for reboosting, and reduce the amount of fuel that needs to be delivered to the ISS by rockets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the &lt;strong&gt;practical problems&lt;/strong&gt; in implementing &lt;strong&gt;methane&lt;/strong&gt; as a propellant?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-11T16:06:01.217" LastActivityDate="2013-08-11T16:06:01.217" Title="Could waste methane be recycled as propellant on the ISS?" Tags="|iss|fuel|waste|isru|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1173" PostTypeId="1" AcceptedAnswerId="3963" CreationDate="2013-08-11T14:22:18.163" Score="43" ViewCount="58498" Body="&lt;p&gt;Recently NASA shut down its Space Shuttle program. Between the first launch on April 12, 1981, and the final landing on July 21, 2011, NASA's space shuttle fleet --Columbia, Challenger, Discovery, Atlantis and Endeavour -- flew 135 missions, helped construct the International Space Station and inspired generations. What made NASA do this ?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Does NASA have plans for an even bigger reusable spacecraft?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorUserId="4660" LastEditorDisplayName="user29" LastEditDate="2015-07-11T14:38:22.800" LastActivityDate="2021-10-18T20:42:59.823" Title="What made NASA shut down the Shuttle program?" Tags="|nasa|space-shuttle|spacecraft-development|space-program|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1174" PostTypeId="2" ParentId="1173" CreationDate="2013-08-11T14:37:51.363" Score="23" Body="&lt;p&gt;The cost of running the space shuttle never met the predictions, and was more expensive (Largely due to the standing army of folk it took to prepare and manage it).&lt;/p&gt;&#xA;&lt;p&gt;Then after the Columbia broke up on re-entry the lack of options for aborts, bailouts, etc, and the inherent danger in the foam on the main tank hitting the wings the notion slowly morphed into the idea of replacing it.&lt;/p&gt;&#xA;&lt;p&gt;Initially the Constellation program was suggested as Shuttle SRB as the first stage for Ares-1, and then a heavy lift Ares-V with two SRB's and a stretched shuttle main tank, but with the payload on top, instead of on the side.&lt;/p&gt;&#xA;&lt;p&gt;The thinking was, SRB's were at this point well charecterized and mostly thought to be understood.  The equipment to make the main tank and stretch it was not a big deal.&lt;/p&gt;&#xA;&lt;p&gt;Retiring the complex shuttle component was thought to make it cheaper to run, since a capsule would be much simpler than the shuttle orbiter.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/DIRECT&quot; rel=&quot;nofollow noreferrer&quot;&gt;DIRECT&lt;/a&gt; guys, ex-NASA folk were pushing a similar plan, but focussing on commonality and minimal changes to keep costs down.&lt;/p&gt;&#xA;&lt;p&gt;However the shuttle program had turned out to be less about space access and more about a jobs program.  Tens of thousands are employed in the shuttle program in many states and thus there is a strong desire to keep the jobs going.  Alas, that was the truly expensive part of the program.&lt;/p&gt;&#xA;&lt;p&gt;Thus we got Constellation a compromise that started looking like the DIRECT plan, and slowly changed to be less and less of a good deal.&lt;/p&gt;&#xA;&lt;p&gt;Then it was cancelled and replaced with the Space Launch System, which sadly is probably a worse approach. (I wish it were not so).  The focus was changed as well to invest in and then hire commercial carriers (Like SpaceX (crewed Dragon), Boeing (CST-100), SNC (Dream Chaser)) for operations just to the space station.  NASA would then focus on the 'hard' missions, since it was becoming clear that commercial options might actually succeed.&lt;/p&gt;&#xA;&lt;p&gt;Thus the Orion capsule (remained mostly intact from Constellation) is designed for the radiation environment outside LEO, and for longer term missions than any of the other commercial providers.  Alas, to do so it is too heavy to launch on pretty much anything other than Delta-4 Heavy (and maybe one day a Falcon Heavy?  Who knows!).  So the single SRB approach was tossed out, and the SLS booster, which needed a stretched SRB for additional thrust was required (Constellation needed them as well, but DIRECT tried very hard to avoid needing modifications).&lt;/p&gt;&#xA;&lt;p&gt;Alas, the SLS appears to be as much of a jobs program, and is looking less and less likely to fly seriously.  Current plans are a launch of the Orion capsule on a Delta-4 Heavy in 2015 (This did happen, Dec 2014), and then the next launch on an actual SLS is 2018 (Which has since been delayed till at least 2022).  The next flight is possibly 4 years after that, which is of course, completely untenable.&lt;/p&gt;&#xA;&lt;p&gt;Time will tell, and the success of SpaceX with a Falcon Heavy will have a huge effect upon the program. If Falcon Heavy launches, and is available at the cost currently advertised it will be very hard to maintain the need for SLS at it Shuttle sized cost.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2021-10-18T20:42:59.823" LastActivityDate="2021-10-18T20:42:59.823" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="1175" PostTypeId="2" ParentId="1172" CreationDate="2013-08-11T14:55:22.660" Score="5" Body="&lt;p&gt;There are a few studies at NASA (and I don't doubt, under the auspices of RKA and ESA) that bring up designs for tying Sabatier and liquefaction equipment that potentially can be used in LEO and elsewhere:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://hdl.handle.net/2060/20020023715&quot; rel=&quot;noreferrer&quot;&gt;A Liquefier for Mars Surface Propellant Production&lt;/a&gt;. Salerno, Lou J.; Helvensteijn, B. P. M.; Kittel, P. 1999.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://hdl.handle.net/2060/20100011002&quot; rel=&quot;noreferrer&quot;&gt;Cryogenic Fluid Management Technology for Moon and Mars Missions&lt;/a&gt;. Doherty, Michael P.; Gaby, Joseph D.; Salerno, Louis J.; Sutherlin, Steven G. 2010.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://hdl.handle.net/2060/20120015764&quot; rel=&quot;noreferrer&quot;&gt;In-Space Propellant Production Using Water&lt;/a&gt;. Notardonato, William; Johnson, Wesley; Swanger, Adam; McQuade, William. 2012.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;strong&gt;Doherty et al., 2010&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;AES funding will be used primarily to develop the experiment package that will include interfaces with the ISS&#xA;power, data, cooling and GH 2 sources, the GH 2 liquefaction heat exchangers and J-T valves, the cryogenic&#xA;refrigerator (20 W at 20 K and 40W at 90K), the cryogenic fluid management payload (mixing pump, spray bar,&#xA;liquid acquisition device) , a small (&amp;lt;200 liter) liquid hydrogen tank, and the passive thermal control systems. The&#xA;demonstration project will be a collaboration between multiple NASA centers, including Kennedy Space Center&#xA;(KSC), Johnson Space Center (JSC), Glenn Research Center (GRC), and Goddard Space Flight Center (GSFC). As&#xA;envisioned, KSC will responsible for the design and fabrication of the experiment pallet, the passive thermal control&#xA;system, the fluid components, and the pallet integration and systems engineering; JSC will be responsible for&#xA;integration of the pallet to the ISS, the command and control system, and the orbital test operations; GRC will be&#xA;responsible for design, fabrication and integrating of the CFM components into the storage tank and the system&#xA;modeling; and GSFC will be responsible for the development of the 20W at 20K cryogenic refrigerator.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JyF4n.png&quot; alt=&quot;ISS experiment&quot; /&gt;&lt;/p&gt;&#xA;&lt;h1&gt;Why hasn't this been done before?&lt;/h1&gt;&#xA;&lt;p&gt;The simplest answer is that you need a bunch of liquefying machinery (or compressors) to transform methane into usable state, and that machinery was not fitted into the original Russian modules (which do the thrusting).&lt;/p&gt;&#xA;&lt;p&gt;Another consideration is that storable hypergolic propellants are much more studied &lt;strong&gt;at the time&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Methane-oxygen engines are very much in vogue nowadays, with Mars exploration and &amp;quot;clean&amp;quot; rocketry, but that was not always the case.&lt;/p&gt;&#xA;&lt;p&gt;A related question: &lt;a href=&quot;https://space.stackexchange.com/questions/67/why-does-the-international-space-station-produce-so-much-methane&quot;&gt;Why does the International Space Station produce so much methane?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-11T15:45:21.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1176" PostTypeId="2" ParentId="1172" CreationDate="2013-08-11T15:17:59.993" Score="6" Body="&lt;p&gt;The methane by itself isn't useful as a fuel in space. It would need additional oxygen. So not only would the station require the equipment to collect and store the methane, it would need additional supplies of liquid oxygen to make the methane useful as a fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides... hypergolics (which I assume the ISS uses) don't require an ignition system to initiate combustion (just open or close a valve), making the thrusters simpler and more reliable than any fuel+oxygen system.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2013-08-11T15:17:59.993" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1177" PostTypeId="5" CreationDate="2013-08-11T15:19:21.750" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-11T15:19:21.750" LastActivityDate="2013-08-11T15:19:21.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1178" PostTypeId="4" CreationDate="2013-08-11T15:19:21.750" Score="0" Body="In situ resource utilization aims to exploit materials found in the lithosphere and atmosphere of planetary bodies for life support, propulsion, and other functions of interplanetary missions." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-11T16:50:08.780" LastActivityDate="2013-08-11T16:50:08.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1179" PostTypeId="2" ParentId="1153" CreationDate="2013-08-11T15:23:45.377" Score="1" Body="&lt;p&gt;The point at which the force of gravity from the sun &amp;amp; earth balance out is L1 -- which is roughly one million miles from Earth.  (92 million miles from the sun).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to get space junk to the sun, you'd need sufficient energy to get it out of earth's orbit with enough momentum to get past L1.  For spacecraft, they use gravity assist from the earth &amp;amp; moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For earth orbiting space debris, it's much more cost effective to just slow it down so that it comes down to earth ... even with attempting to stear it so that it burns up in the atmosphere or comes down in a safe location it's going to be easier than getting it to the sun.&lt;/p&gt;&#xA;" OwnerUserId="46" LastActivityDate="2013-08-11T15:23:45.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1180" PostTypeId="2" ParentId="1145" CreationDate="2013-08-11T17:52:37.637" Score="8" Body="&lt;p&gt;Once its initial incarnation and the Mars program was cancelled for cost reasons there was no longer a need for anything like it.  As the upper stage of a Saturn 5 it would create a much more powerful rocket than what we used to go to the moon; but with Apollo being wound down and no successor in store there wasn't anything it would be beneficial for to resurrect it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure where your 3:1 thrust to weight ratio came from; while I've seen multiple claimed performance numbers for what Nasa achieved before the program was cancelled; but none of them had a rocket stage with a &lt;strong&gt;fueled&lt;/strong&gt; thrust to weight ratio greater than one, leaving them only usable as upper stages.  The highest I found was in &lt;a href=&quot;http://www.astronautix.com/engines/nerva2.htm&quot; rel=&quot;noreferrer&quot;&gt;Encyclopedia Astronautica&lt;/a&gt; (bottom of the page) which listed a fueled mass of ~178 tons and an 867kN thrust for a thrust to weight ratio of 0.5.  &lt;a href=&quot;http://en.wikipedia.org/wiki/NERVA#NERVA_rocket_stage_specifications&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;'s description of what otherwise appears to be the same system lists a much lower thrust of 330kN for a 0.2 ratio.  This leaves it unusable for anything but an upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if modern designs could get a thrust to weight ratio slightly above 1. the political impossibility of dropping reactors from spent stages would mean that to use it as first stage you'd need performance high enough to also carry hardware to make a safe landing afterward or to serve as an SSTO.  These days I suspect nuclear distrust would make even using it as an upper stage impossible under any but extraordinary circumstances.&lt;/p&gt;&#xA;" OwnerUserId="157" LastActivityDate="2013-08-11T17:52:37.637" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1182" PostTypeId="2" ParentId="911" CreationDate="2013-08-11T20:31:25.117" Score="5" Body="&lt;p&gt;Following up on Deer Hunter's answer showing the Russian device, we can make a mass on a spring oscillate; this is the equation of motion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$x \left( t \right) =A \cos \left( \sqrt{k \over m}t \right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(from &lt;a href=&quot;http://en.wikipedia.org/wiki/Harmonic_oscillator#Spring.2Fmass_system&quot;&gt;Wikipedia's harmonic oscillator page&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives the frequency to be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$f = \sqrt{k \over m}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if we move the mass away from the rest position, we can measure the frequency of its oscillation over a period of time. If we have carefully calibrated the spring and know the spring constant $k$ to high precision, we can  calculate the mass $m$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The benefits of this over a linear acceleration device are its simplicity, robustness and that it is contained; the mass oscillates in place, rather than moving in space, and you need only measure the frequency which can be done over a number of oscillations. By comparison, a linear acceleration device requires knowledge of the force involved, careful measurement of velocity and is single-shot operation. There's not a lot of space in a space station!&lt;/p&gt;&#xA;" OwnerUserId="519" LastActivityDate="2013-08-11T20:31:25.117" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1183" PostTypeId="2" ParentId="1173" CreationDate="2013-08-11T21:23:07.290" Score="10" Body="&lt;p&gt;The simple answer is that between operating shuttle and station, there were no funds left in the human spaceflight budget to develop anything new to go somewhere else or do something different, assuming a flat budget.  By all indications, a flat budget is an extremely good assumption.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They had to pick one to kill, and dropped the one that had been operational for a few decades, as opposed to the one that just became operational.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now they have money to do new and different things.  The saga of what exactly those new and different things should be is a whole 'nother story.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-08-11T21:23:07.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1184" PostTypeId="1" AcceptedAnswerId="1187" CreationDate="2013-08-11T21:30:43.770" Score="15" ViewCount="1695" Body="&lt;p&gt;Space ships are always launched vertically. Its reason is pretty obvious: vertical launch has the lowest distance between the desired height and surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, would horizantal launch be worth to experiment?  Since space shuttles always landed on a runway, why we wouldn't launch them also on a runway, just like every other kind of airplanes?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="-1" LastEditDate="2015-04-13T23:18:47.010" LastActivityDate="2015-07-03T05:10:00.733" Title="Horizontal space launch" Tags="|launch|travel|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1185" PostTypeId="5" CreationDate="2013-08-11T21:33:42.143" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-11T21:33:42.143" LastActivityDate="2013-08-11T21:33:42.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1186" PostTypeId="4" CreationDate="2013-08-11T21:33:42.143" Score="0" Body="Questions related to any kinds of travel where entering space is involved." OwnerUserId="306" LastEditorUserId="306" LastEditDate="2013-08-12T00:34:36.590" LastActivityDate="2013-08-12T00:34:36.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1187" PostTypeId="2" ParentId="1184" CreationDate="2013-08-11T23:19:24.810" Score="21" Body="&lt;p&gt;Actually the main reason most launches begin vertically is to get away from the atmosphere, and atmospheric drag/heating as quickly as possible. This minimizes the amount of energy lost to friction, and the amount of mass needed to be used for heat shielding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is at least one orbital rocket whose first stage is an aircraft: the &lt;a href=&quot;http://en.wikipedia.org/wiki/Pegasus_%28rocket%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pegasus&lt;/a&gt;.  It's carried by a Lockheed L-1011.  It's a small rocket, and, on a per kg basis, among the most expensive on the market.  It survives, however, because it allows operators to put small satellites into non-standard orbits that wouldn't be reachable if they piggybacked on a large rocket also carrying a more conventional payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Virgin Galactic's &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceShipTwo&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceShipTwo&lt;/a&gt; is also being piggybacked on an aircraft for its initial stage, but is only going for sub-orbital flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the main reason this approach isn't common is that a conventional, sub-sonic aircraft is able to impart only ~4% of the velocity needed to reach orbit.  Which means you're adding a lot of additional complexity for a relatively modest saving in fuel/oxidizer consumption.  Efficiency gains would be larger if higher speeds were achieved by the air breathing component; however, I believe the only manned ramjet engine aircraft to go beyond R&amp;amp;D prototypes into active service was the SR-71 Blackbird.  While there also were about a dozen cruise missiles/SAMs/etc built using ramjets, as military projects, tech transfer to civilian industry is restricted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible that if the &lt;a href=&quot;https://space.stackexchange.com/questions/1109/any-other-countries-companies-doing-anything-similar-to-british-skylon-ssto&quot;&gt;Sklyon&lt;/a&gt;, a winged SSTO with a hybrid ramjet/rocket engine is completed, the chicken and egg problem may be resolved; however as the Skylon is barely being funded at present, I suspect it will end up being nothing more than a paper study.  If successful, the scramjet research being funded by DARPA and the AFRL might increase interest in launchers with air-breathing first stages by increasing their maximum velocity from mach 5 to mach 10; however in addition to limited funding there could be problems in transferring the knowledge from military to civilian custody.&lt;/p&gt;&#xA;" OwnerUserId="157" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-04-14T14:02:25.590" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="1188" PostTypeId="1" AcceptedAnswerId="1412" CreationDate="2013-08-11T23:24:47.260" Score="14" ViewCount="2596" Body="&lt;p&gt;Can Monomethylhydrazine and Unsymmetrical dimethylhydrazine be used one in place of another for the class of the existing orbital maneuver engines (with reduced performance may be)? Such as Shuttle's OMS (uses MMH), Soyuz DPO (uses UDMH) and ATV's main engine (uses MMH).&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user29" LastEditDate="2013-08-12T01:09:28.817" LastActivityDate="2015-04-15T17:16:42.963" Title="Are MMH and UDMH interchangeable?" Tags="|propulsion|engine-design|fuel|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1189" PostTypeId="1" AcceptedAnswerId="1192" CreationDate="2013-08-11T23:39:34.597" Score="14" ViewCount="1215" Body="&lt;p&gt;Do the majority of astronauts (and cosmonauts) experience nausea, or symptoms of space sickness while adapting to micro-gravitational conditions? How severe are their symptoms and how much they vary by individual, e.g. does the majority throw up during the weightlessness adaptation period?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Canadian astronaut Chris Hadfield explains from the ISS that space sickness does occur and how astronauts experiencing it deal with it in &lt;a href=&quot;https://www.youtube.com/watch?v=Lin7Z5XWLHY&quot; rel=&quot;noreferrer&quot;&gt;this YouTube video&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="306" LastEditDate="2013-08-13T19:26:07.637" LastActivityDate="2013-08-13T19:26:07.637" Title="Do the majority of astronauts experience space sickness while adapting to micro-gravitational conditions?" Tags="|health|astronauts|microgravity|space-sickness|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1190" PostTypeId="2" ParentId="1171" CreationDate="2013-08-12T00:04:03.650" Score="17" Body="&lt;p&gt;&lt;strong&gt;Yes, there are several other nozzle types in use, being tested or patented. Let's first recap on the various nozzle types used in aeronautics&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Low ratio nozzle&lt;/strong&gt; is predominantly used on the civil aircrafts and also some low speed reconnaissance airplanes, and is convergent-divergent &lt;a href=&quot;https://en.wikipedia.org/wiki/De_Laval_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;de Laval nozzle&lt;/a&gt; with an extremely low inlet-outlet area pressure ratio that prevents choking at low air speeds, reduces generated noise, and is as reliable as they come:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/DEJcP.jpg&quot; alt=&quot;Boeing ecoDemonstrator&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Boeing ecoDemonstrator tested on American Airlines airplane&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Ejector nozzle&lt;/strong&gt; is the simpler of the variable exhaust nozzles, and is more commonly used on jet propelled aircrafts than the iris nozzles due to its simpler design of spring-loaded petals and are thus more reliable, but do produce more secondary airflow drag and are less efficient than some other, more advanced designs:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/C1fR8.jpg&quot; alt=&quot;Variable Exhaust Nozzle, on the GE F404-400 low-bypass turbofan installed on a Boeing F/A-18 Hornet&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Variable Exhaust Nozzle, on the GE F404-400 low-bypass turbofan installed on a Boeing F/A-18 Hornet&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Iris nozzle&lt;/strong&gt; is a variable exhaust nozzle commonly used on jet fighter airplanes and bombers and can adjust its contour by iris like petal design to maximize performance and avoid uneven pressure distribution (oblique shock). In some designs, they can also change the thrust vector (angle to aircraft), or add air brakes (e.g.: &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:MiG-23_afterburner_exhaust_airbrakes.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;MiG-23 afterburner exhaust air brakes&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/uKx53.jpg&quot; alt=&quot;Iris nozzle afterburners on the F-15 &amp;quot;Eagle&amp;quot; fighter&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Iris nozzle afterburners on the F-15 Eagle fighter&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;And now for the fun part - the nozzle types used in astronautics, hypersonic experimental airplanes,...&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Bell_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bell nozzle&lt;/a&gt;&lt;/strong&gt; is possibly the most commonly used nozzle type on rocket engines, for its simplicity, relatively low weight with advanced materials and in some designs even adjustability (see iris nozzle below) of the volume of its exhaust / expansion chamber:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/mIKon.jpg&quot; alt=&quot;Rocket nozzle on V2 showing the classic shape&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Rocket nozzle on V2 showing the classic shape&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Expansion_deflection_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expansion-deflection nozzle&lt;/a&gt;&lt;/strong&gt; (or &lt;em&gt;Pintle Injector&lt;/em&gt;) is a type of propellant injection device for a rocket engine that was first used on a flight vehicle during the Apollo Program in the Lunar Excursion Module's descent engine. Pintle injectors are currently used in SpaceX's Merlin engines:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/UIXOb.jpg&quot; alt=&quot;Patent application cross-section schematic diagram of the pintle injector&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Patent application cross-section schematic diagram of the pintle injector&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Plug_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Plug nozzle &quot;Aerospike&quot;&lt;/a&gt;&lt;/strong&gt; (or &lt;em&gt;Spike Nozzle&lt;/em&gt;) is an altitude compensating nozzle with the ideal contour a long, gradual pressure reducing 'spike', often with a wide (large volume) annular type combustion chamber at the base. This nozzle is self-compensating for atmospheric pressure, and the plug and the combustion chamber can vary in size for different applications (shorter convex shaped &quot;spike plugs&quot; are used also on civil aviation jet engines, and truncated/non-truncated or full-length concave spikes usually used for supersonic aircraft, rockets,...). Among main advantages is up to 30% reduction in propellant required at lower altitudes due to their self-compensating nature:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/lfnFJ.jpg&quot; alt=&quot;3D model of the Aerospike engine&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;3D model of the components of the Aerospike engine with a slightly convex shaped &quot;spike&quot;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Aerospike_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;Annular and Linear aerospike&lt;/a&gt;&lt;/strong&gt; are variants on the truncated aerospike nozzle design, commonly with several turbine combustion exhausts placed linearly, or annularly over exhaust nozzle. Spike nozzle is truncated and allows for additional thrust with subsonic recirculating flow field forming at the truncated part, as the gases expand over the nozzle's surface. Dynamics of a linear aerospike engine are explained in detail in &lt;a href=&quot;https://www.youtube.com/watch?v=-0Y0FS8Z1Qk&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Linear Aerospike Engine video&lt;/a&gt;:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/DcU2O.jpg&quot; alt=&quot;XRS-2200 linear aerospike engine for the X-33 program being tested&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;XRS-2200 linear aerospike engine for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_Martin_X-33&quot; rel=&quot;nofollow noreferrer&quot;&gt;X-33 program&lt;/a&gt; being tested&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/SERN&quot; rel=&quot;nofollow noreferrer&quot;&gt;SERN (Single Expansion Ramp Nozzle)&lt;/a&gt;&lt;/strong&gt; is essentially a single side linear aerospike nozzle, but can be accompanied by more complex pitch and elevation control systems due to momentum transfer that can be angular to the aircraft / spacecraft due to throttling:&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Many designs for space planes with scramjet engines make use of SERNs&#xA;  because of the weight reduction at large expansion ratios, or the&#xA;  additional lift at under-expansion. The X-43, a test vehicle in NASA's&#xA;  Hyper-X programme, is a flying example.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/NGN8H.jpg&quot; alt=&quot;Aurora Mach 5 and SR-71 Mach 3 reconnaissance aircraft flying in formation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Aurora Mach 5 (below) and SR-71 Mach 3 (above) reconnaissance aircraft flying in formation.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;b&gt;And then there are many other subtypes, combining several of same or different nozzle types into a single design&lt;/b&gt;:&lt;br /&gt;&lt;br /&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Expanding_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expanding nozzle&lt;/a&gt;&lt;/strong&gt; which is a type of rocket nozzle that, unlike traditional designs, maintains its efficiency at a wide range of altitudes. It is a member of the class of altitude compensating nozzles, a class that also includes the plug nozzle and aerospike. While the expanding nozzle is the least technically advanced and simplest to understand from a modeling point of view, it also appears to be the most difficult design to build.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Stepped_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stepped nozzle (dual-bell nozzle)&lt;/a&gt;&lt;/strong&gt;: a de Laval rocket nozzle which has altitude compensating properties.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://www.google.com/patents/US4220001&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dual-expander nozzle&lt;/a&gt;&lt;/strong&gt; that is a composite cycle rocket engine having an inner engine disposed to discharge directly into the nozzle of an outer engine.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://www.freepatentsonline.com/7000403.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dual-throat nozzle&lt;/a&gt;&lt;/strong&gt; (or dual fuel capability nozzle) which is a fuel nozzle and gas turbine combustor capable of operating on multiple fuels with reduced carbon build-up to the fuel nozzle and adjacent combustor components is disclosed. The fuel nozzle incorporates a reconfigured gas fuel assembly and mixing tube to eliminate known areas of recirculation. Furthermore, the liquid fuel assembly includes reconfigured spray characteristics to further reduce droplet interaction with the mixing tube.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Suggested additional reading: &lt;a href=&quot;http://www.k-makris.gr/RocketTechnology/Nozzle_Design/nozzle_design.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kostas Makris' blog post on Nozzle Design&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;It appears I've somewhat overstretched the capability of our contents parser to format hypertext, links, e.t.c., so I didn't add any photographs for the last group of nozzles. I've based my list somewhat loosely on Wikipedia, omitting some duplicated designs and adding a few more to those not described there. It should be noted however, that there isn't any official classification, short of their use in various industries (which might vary greatly), so I've adopted my own for this purpose.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-12T01:09:18.173" LastActivityDate="2013-08-12T01:09:18.173" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1191" PostTypeId="1" AcceptedAnswerId="1460" CreationDate="2013-08-12T07:53:09.143" Score="14" ViewCount="291" Body="&lt;p&gt;Known fact: &lt;a href=&quot;https://space.stackexchange.com/questions/91/why-is-it-that-during-reentry-phase-a-capsule-cannot-communicate-with-mission-co&quot;&gt;Heat causes a charge to accumulate on the hull of a craft at reentry.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Questions: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does the hull acquire a charge during the course of its &lt;em&gt;ascent&lt;/em&gt; too? &lt;/li&gt;&#xA;&lt;li&gt;What magnitude is this charge? (My guess is it would be a small fraction compared to the charge generated at reentry owing to the velocity difference.)&lt;/li&gt;&#xA;&lt;li&gt;How is this charge dissipated?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2021-09-18T21:28:15.947" Title="Does the hull of a spacecraft acquire a charge during its rapid ascent through Earth's atmosphere?" Tags="|energy|shielding|capsule|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1192" PostTypeId="2" ParentId="1189" CreationDate="2013-08-12T13:16:52.597" Score="12" Body="&lt;p&gt;&lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/16935570&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Space motion sickness: incidence, etiology, and countermeasures&lt;/strong&gt; (Martina Heer, William Paloski. Autonomic Neuroscience Vol.129, no.1-2, 2006. Pp. 77-79)&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Space motion sickness is experienced by &lt;strong&gt;60% to 80%&lt;/strong&gt; of space travelers during their first 2 to 3 days in microgravity and by a &lt;strong&gt;similar proportion&lt;/strong&gt; during their first few days after return to Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Since &lt;span class=&quot;math-container&quot;&gt;$60\% &amp;gt; 50\%$&lt;/span&gt; :), the answer to your question is an unequivocal &lt;strong&gt;YES&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Space motion sickness symptoms are similar to those in other forms of motion sickness; they include: pallor, increased body warmth, cold sweating, malaise, loss of appetite, nausea, fatigue, vomiting, and anorexia.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;Facts to remember:&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;NASA's STS program: &lt;strong&gt;up to 80% of the U.S. astronauts&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A study on 15 Russian cosmonauts: 13 out of 15.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;50% of 72 U.S. astronauts &lt;strong&gt;reported&lt;/strong&gt; various space motion sickness symptoms.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;It doesn't matter whether you have flown before&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Space sickness hits both males and females.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-13T05:51:06.413" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1193" PostTypeId="1" AcceptedAnswerId="1195" CreationDate="2013-08-12T15:21:57.320" Score="20" ViewCount="1520" Body="&lt;p&gt;Most of the energy we use on Earth in our daily activities is a derivative of &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_energy&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar&lt;/a&gt;; wind, hydroelectric, coal, gas (oil), wood fire, solar cells. While we might not think about it, all of these are directly dependent on the Sun and Earth's relative proximity to the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As PearsonArtPhoto's answer to this question shows; &lt;a href=&quot;https://space.stackexchange.com/questions/952/once-lightyears-out-of-the-solar-system-could-you-expect-to-get-feasible-propul/953#953&quot;&gt;Once lightyears out of the solar system, could you expect to get feasible propulsion with a solar sail?&lt;/a&gt; past Mars orbit, Sol's energy is significantly diminished.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What viable &amp;amp; sustainable energy source(s) are available beyond Mars? &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Where sustainability = &lt;strong&gt;no&lt;/strong&gt; possibility of expending the source prior to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Future_of_the_Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;suns natural transition to a red giant&lt;/a&gt; in about 7 billion years.&lt;/li&gt;&#xA;&lt;li&gt;Where viable = sufficient energy to sustain the culture and technology to continue the construction of new and replacement tools for harnessing the energy.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Example;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Solar cells are sustainable but not viable, insufficient surplus energy would be available to process raw materials and construct new cells.&lt;/li&gt;&#xA;&lt;li&gt;Nuclear - is viable but not sustainable as there is finite amount of fissionable material. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-10-29T14:54:28.227" Title="What types of sustainable energy are available beyond Mars?" Tags="|energy|solar-system|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1194" PostTypeId="2" ParentId="1193" CreationDate="2013-08-12T15:48:00.110" Score="8" Body="&lt;p&gt;Not entirely sustainable - but probably more so than Solar (which uses up nuclear fusion of the Sun) - and available pretty much everywhere in the universe, is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Bussard_ramjet&quot;&gt;Bussard ramjet&lt;/a&gt;. Collecting free hydrogen from the space and using it in nuclear fusion. There aren't any sustainable means of collecting meaningful amounts of energy while &lt;em&gt;staying&lt;/em&gt; in one place out there, but the space is big; there's a lot of free hydrogen to be collected while traveling, and it can be converted to energy &quot;on the fly&quot;, during the travel to some &quot;meaningful&quot; place.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-08-12T15:48:00.110" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1195" PostTypeId="2" ParentId="1193" CreationDate="2013-08-12T17:20:36.027" Score="22" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jovian magnetosphere&lt;/a&gt; is extremely active and stretches nearly all the way to Saturn. It is indeed in volume the second largest continuous structure in the Solar System, right after the heliosphere. Jupiter's radiation in the radio frequencies is in fact so strong, you can &lt;a href=&quot;https://astronomy.stackexchange.com/a/608/13&quot;&gt;tune in and listen to it&lt;/a&gt; between frequencies of other radio stations on AM/FM radio receivers. This is a bit of oversimplification, since what you'd hear is a composition of various other celestial bodies emitting radio frequencies (pulsars, the Sun,...), and also Earth's atmospheric charge, but the Jupiter is particularly loud and accounts for the large portion of musical notes in this Cosmic opera.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This radiation in the radio frequencies could potentially be tapped into and converted to electricity. Alternatively, oscillations in magnetic fields as Io orbits Jupiter cause a huge &lt;a href=&quot;https://en.wikipedia.org/wiki/Alfv%C3%A9n_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alfvén wave&lt;/a&gt; electric current carried by magnetized ring of ionized gas by between Io and Jupiter. The power of this field is estimated to be around two trillion watts and is the biggest DC electrical circuit in the Solar system known to man. So that are two renewable sources - the electric current, and the highly charged plasma itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Io_%28moon%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Io&lt;/a&gt; is also highly active itself, constantly being at the mercy of the the enormous gravitational pull of Jupiter. Due to it, it's geologically highly active, and is known to produce volcanic eruptions that eject large amounts of sulfur dioxide gas into space. As such, it is a strong source of plasma. There is also a potential to tap into the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geothermal_energy&quot; rel=&quot;nofollow noreferrer&quot;&gt;geothermal wells&lt;/a&gt; and convert that power to electrical current.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also other opportunities around Saturn. One idea (admittedly pretty wild, but why not?) that comes to mind is taking advantage of the electrostatic charge of the Saturn's rings. In theory, it's as huge of a collector as they come. Also, the &lt;a href=&quot;http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20130731/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn moon jets&lt;/a&gt;, that were detected by the Cassini Solstice Mission carry a potentially huge amount of kinetic, thermal and electric energy in the form of charged plasma.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thinking farther, all the way to Pluto, tidal forces created by gravitational interaction with Charon as they orbit each other (I can't really say Charon alone orbits Pluto, since they're tidally locked to the other and Pluto orbits a point outside itself) might be strong enough to produce &quot;cryo-geysers&quot; of ammonia hydrates and water crystals on the surface of Charon, as suggested by observations of the Gemini Observatory. And, of course, we already know of other methods of producing electricity by effects of tidal forces - i.e. &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_power&quot; rel=&quot;nofollow noreferrer&quot;&gt;tidal power&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So these are all sources that can be considered as sustainable. There are all kinds of other opportunities to collect energy in sustainable ways around gas giants, their moons, or even their interaction as they orbit each other. We just need to learn how, and develop technology to convert and/or store all this energy to our liking and needs.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2013-10-29T14:54:28.227" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1196" PostTypeId="5" CreationDate="2013-08-12T19:42:12.483" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-12T19:42:12.483" LastActivityDate="2013-08-12T19:42:12.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1197" PostTypeId="4" CreationDate="2013-08-12T19:42:12.483" Score="0" Body="Questions about Indian Space Research Organization (ISRO), its missions, research projects, and other activities financed, organized, planned, managed, or otherwise arranged under this organization's umbrella." OwnerUserId="49" LastEditorDisplayName="user12" LastEditDate="2013-08-13T15:19:35.823" LastActivityDate="2013-08-13T15:19:35.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1198" PostTypeId="2" ParentId="940" CreationDate="2013-08-13T00:35:29.463" Score="12" Body="&lt;p&gt;After days of Googling this, I am going to say that it &lt;em&gt;appears&lt;/em&gt; none have. It's worth noting that there are dozens of propulsion systems for cubesats (Google  'cubesat propulsion'), but I was able to turn up &lt;em&gt;nothing&lt;/em&gt; referencing any that have actually flown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One would think that if the systems had been used, be it privately or by a university, a result for an article, a paper, &lt;em&gt;something&lt;/em&gt; would have surfaced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll continue looking for evidence, but until someone finds it, the answer is '&lt;strong&gt;no, not to the best of our knowledge.&lt;/strong&gt;'&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-08-13T00:35:29.463" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1199" PostTypeId="1" AcceptedAnswerId="1202" CreationDate="2013-08-13T01:06:53.753" Score="20" ViewCount="542" Body="&lt;p&gt;Tonight (Aug 12 2013) is the annual peak of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Perseids&quot; rel=&quot;nofollow noreferrer&quot;&gt;Perseid meteor shower&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since meteors are chunks of rock flying through the atmosphere at &lt;a href=&quot;https://space.stackexchange.com/questions/287/what-maximum-speeds-can-be-expected-from-meteorites&quot;&gt;crazy high speeds&lt;/a&gt;, I'm wondering if there are any considerations for the launching of space flights on and around this time - it is generally considered detrimental to have your craft pierced by a flying rock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do space agencies take heightened meteor activity around this time into account when planning launches? If so, how?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-08-23T12:57:14.783" Title="Does the Perseid peak affect mission launches?" Tags="|launch|meteorites|perseid|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1200" PostTypeId="2" ParentId="1098" CreationDate="2013-08-13T02:01:52.767" Score="10" Body="&lt;p&gt;Jonathan McDowell's &lt;a href=&quot;http://planet4589.org/space/book/LOGS/logindex/geo.html&quot;&gt;geostationary orbit catalog&lt;/a&gt; lists &lt;strong&gt;1349&lt;/strong&gt; objects. The criteria for inclusion in this list is only that the period is between 23 and 25 hours (inclination is not a factor). It is worth noting that while a geostationary orbit is typically defined as circular, uninclined, and having a period equal to one sidereal day, actual satellites are only maintained &lt;em&gt;near&lt;/em&gt; this, as the unequal mass distribution of the Earth (specifically the tesseral harmonics) will perturb the satellite East/West, and the Sun and the Moon perturb it North/South. &quot;Near&quot;, here, is very flexible, but inclinations below 3-5 degrees and a mean altitude of +/- 50-75  km from GEO is common.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How &quot;crowded&quot; it is depends on the location you're looking at, as the distribution in longitude is not at all uniform. For instance, there will tend to be fewer satellites over the open Pacific than the Americas or Europe. As mentioned in geoffc's answer, the International Telecommunication Union (ITU) does indeed assign orbital slots, but how that occurs or even the size of these slots is &lt;a href=&quot;http://www.rpctelecom.com/filing.php&quot;&gt;complicated enough&lt;/a&gt; that there is no &quot;typical&quot; answer (but think on the order of tenths of a degree of East/West longitude). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An additional complicating factor is that operators may share slots with either other spacecraft that they own or spacecraft from other operators. In this case, the stationkeeping maneuvers alluded to above must be tightly coordinated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike the LEO environment, GEO is a single, well-defined orbit regime, and most countries and operators understand the need to be responsible with it. As such, when a satellite nears retirement, it usually raises its orbit to the GEO &lt;a href=&quot;http://en.wikipedia.org/wiki/Graveyard_orbit&quot;&gt;graveyard&lt;/a&gt;, which is (roughly) a circular orbit several hundred km above GEO (the IADC has guidelines which are discussed on the linked page). This does a lot to keep the amount of debris/dead satellites out of the GEO ring.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-13T02:01:52.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1201" PostTypeId="2" ParentId="1199" CreationDate="2013-08-13T02:18:29.480" Score="10" Body="&lt;p&gt;&lt;a href=&quot;http://www.space.com/7365-space-traffic-meteor-showers-threaten-shuttle-launch-date.html&quot;&gt;This article&lt;/a&gt; mentions the Leonids meteor shower as a worry for a Shuttle launch (the mission ended up launching near the beginning of the window, well before the Leonids peak). Toward the end, though, the article says &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA in August 1993 delayed a Discovery launch attempt by a day to avoid the peak of an extra active Perseid meteor shower.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-51.html&quot;&gt;archive&lt;/a&gt; entry for that mission, STS-51, does mention the delay, but does not offer much more detail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, &lt;strong&gt;yes&lt;/strong&gt;, meteor showers affect launches. How often this happens, I don't know, but it seems to be an easy way to mitigate a potentially great risk.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-13T02:18:29.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1202" PostTypeId="2" ParentId="1199" CreationDate="2013-08-13T03:08:58.973" Score="20" Body="&lt;p&gt;Yes, NASA definitely takes meteor showers seriously and has delayed one Space Shuttle mission due to colliding dates with predicted peaks of meteoritic activity, and was somewhat concerned about the future launch window for another mission, that later launched just a day shy of a peak of a meteor shower:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;In August 1993, the launch of Space Shuttle Discovery mission STS-51 was additionally delayed due to the same Perseid meteor shower peak, after the launch being scrubbed two times due to technical problems - on July 17th due to failed electrical switch, and on July 24th due to a fault in the steering system of the right solid rocket booster. The next launch was postponed to August 12th due to threat of &lt;a href=&quot;https://en.wikipedia.org/wiki/Perseids&quot;&gt;Perseids&lt;/a&gt; with peak activity on August 11th. News articles: &lt;a href=&quot;http://news.google.com/newspapers?nid=1665&amp;amp;dat=19930801&amp;amp;id=NjIaAAAAIBAJ&amp;amp;sjid=UyUEAAAAIBAJ&amp;amp;pg=6731,18028&quot;&gt;The Times&lt;/a&gt;, &lt;a href=&quot;http://news.google.com/newspapers?nid=1298&amp;amp;dat=19930731&amp;amp;id=KucyAAAAIBAJ&amp;amp;sjid=lAcGAAAAIBAJ&amp;amp;pg=5522,6117630&quot;&gt;The Free Lance Star&lt;/a&gt;, &lt;a href=&quot;http://news.google.com/newspapers?nid=1916&amp;amp;dat=19930730&amp;amp;id=UAchAAAAIBAJ&amp;amp;sjid=bHYFAAAAIBAJ&amp;amp;pg=3276,4423835&quot;&gt;The Hour&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In 2009, Space Shuttle Atlantis mission STS-129 was slotted just a day before the annual Leonids meteor shower that peaked on November 17th, that was followed in a less than a month's time by the Geminids meteor shower (peaked on December 13-14th. Atlantis mission &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-129&quot;&gt;STS-129&lt;/a&gt; to the ISS was launched on November 16th, 2009. According to &lt;a href=&quot;http://www.space.com/7365-space-traffic-meteor-showers-threaten-shuttle-launch-date.html&quot;&gt;Space.com news article&lt;/a&gt;, this was NASA's reasoning:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The showers will produce hundreds of meteoroids per hour. NASA&#xA;  wouldn't launch a shuttle into a cosmic shooting gallery, so managers&#xA;  likely would forego any launch opportunity at the peak of either&#xA;  shower.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It should also be noted that chances of colliding with a meteor are astronomically slim even during heightened meteoritic activity. Meteors hit the Earth's atmosphere (of even hit the surface, and if they survive this collision we call the rocks &lt;a href=&quot;https://en.wikipedia.org/wiki/Meteorite&quot;&gt;meteorites&lt;/a&gt;) all the time, albeit at most times less frequently than during annual meteor shower peaks. Meteor showers also happen unpredictably, e.g. the &lt;a href=&quot;https://en.wikipedia.org/wiki/June_Bootids&quot;&gt;June Boötids&lt;/a&gt; that are hard to predict their peak and frequency, usually in the week between June and July each year.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-08-13T03:08:58.973" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1204" PostTypeId="1" CreationDate="2013-08-13T07:33:17.203" Score="14" ViewCount="5225" Body="&lt;p&gt;When a geosynchronous/geostationary satellite is launched, how do its designers calculate its life expectancy? A random sampling of some communication satellites suggest that ~ten years might be the norm. How is this figure arrived at? Is it simply a question of propellant required for orbital corrections and eventual retirement to a graveyard orbit?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="25" LastEditDate="2013-08-13T22:15:42.033" LastActivityDate="2018-04-24T09:11:33.397" Title="What is the life expectancy of a geosynchronous satellite dependent on?" Tags="|artificial-satellite|geosynchronous|end-of-life|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1205" PostTypeId="2" ParentId="634" CreationDate="2013-08-13T09:03:16.883" Score="-1" Body="&lt;p&gt;I would say, probably &quot;no&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars has now a thin atmosphere because of the magnetic field, and because of its mass (1/10 the Earth one). &#xA;The gravity of the planet depends on its mass, and it is the gravity which keeps the atmosphere there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, if you want to have an earth-like atmosphere, you should have the magnetic field of the Earth, and its gravity. Otherwise you will progressively lose atmosphere. This means that the atmosphere needs to be artificially maintained, which is again a matter of costs (assuming that we have the technology to generate it...). &lt;/p&gt;&#xA;" OwnerUserId="529" LastEditorUserId="25" LastEditDate="2015-11-05T18:52:44.240" LastActivityDate="2015-11-05T18:52:44.240" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1206" PostTypeId="1" CreationDate="2013-08-13T10:49:32.143" Score="4" ViewCount="321" Body="&lt;p&gt;Is it possible to remove the space junk having orbital velocity with the help of a skyhook? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has any country in the world planned to place a skyhook in orbit?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="6816" LastEditDate="2015-08-07T22:30:52.627" LastActivityDate="2015-08-07T22:30:52.627" Title="Could a skyhook be used to remove space junk?" Tags="|debris|space-elevator|conceptual|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1207" PostTypeId="1" CreationDate="2013-08-13T11:01:20.193" Score="11" ViewCount="347" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/812/can-you-have-birthday-cake-in-space&quot;&gt;Mostly this question&lt;/a&gt; (and some earlier knowledge about food &amp;amp; drink behaviour in space) made me think that which are those foods that would mean a serious threat for people in space? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My two ideas are: foods that make a lot of crumbs (like chips) since those are floating in air and it may enter to the lungs of people. And drink, with the same reason with drops.&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-08-13T11:01:20.193" Title="Most unsafe foods in zero gravity" Tags="|microgravity|food|safety|" AnswerCount="0" CommentCount="11" ClosedDate="2013-08-13T20:31:36.980" ContentLicense="CC BY-SA 3.0" />
  <row Id="1209" PostTypeId="2" ParentId="1206" CreationDate="2013-08-13T12:27:09.707" Score="6" Body="&lt;p&gt;There is the &lt;a href=&quot;http://www.star-tech-inc.com/id121.html&quot;&gt;ElectroDynamic Debris Eliminator (EDDE)&lt;/a&gt; notion that seems to be pretty serious.  Not clear if they have realistic funding or not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is basically a multisegmented tether that using solar panels along its length for energy generation, rotates and grabs debris in a net at the end to then deorbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A key design feature is that if it gets severed the remaining elements can continue to operate as two smaller EDDE vehicles.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-08-13T12:27:09.707" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1210" PostTypeId="2" ParentId="1204" CreationDate="2013-08-13T12:33:14.857" Score="10" Body="&lt;h2&gt;Propellant&lt;/h2&gt;&#xA;&lt;p&gt;Typically propellant consumables for station keeping were the limiting factor.  Once a Geosat reaches near the end of its propellant life, they need to maneuver it out of GEO so as to free up the slot for its replacement.&lt;/p&gt;&#xA;&lt;p&gt;This has been mitigated somewhat by the use of electric propulsion, where Xenon or the like is used as propellant after being accelerated electrically for thrust.  The propellant in that case is much smaller in quantity due to the higher ISP but lower thrust.  I.e. If you use a low thrust very high ISP system, and have lots of time, it is more efficient long term.&lt;/p&gt;&#xA;&lt;h2&gt;Solar Panels&lt;/h2&gt;&#xA;&lt;p&gt;Solar panels used to degrade and power was thus reduced to the point where it could no longer function. (Or conversely, knowing how fast the panels would degrade, they would launch oversized panels, so that in 10 years, degraded it would still have enough power to operate).  Better panels have mitigated that somewhat.&lt;/p&gt;&#xA;&lt;h2&gt;Reaction Wheels&lt;/h2&gt;&#xA;&lt;p&gt;If there are strong pointing requirement, reaction control wheels (gyros) are well known failure points, since by definition they are moving parts.  (Kepler, Hubble, ISS, etc are examples where issues have come up).&lt;/p&gt;&#xA;&lt;h2&gt;Electronics&lt;/h2&gt;&#xA;&lt;p&gt;Electronics exposure to radiation in space, outside the Van Allen belts can degrade the electronics, but mostly this has been mitigated with better shielding and electronics.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-13T12:33:14.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1211" PostTypeId="1" CreationDate="2013-08-13T14:09:39.717" Score="9" ViewCount="2598" Body="&lt;p&gt;Solar panels must be always exposed in the direction of sun to produce maximum power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the ISS orbits the Earth, if the panels are fixed they will not produce maximum power (often fluctuating).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How does the ISS control its solar panels' direction ?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-08-13T15:15:18.817" LastActivityDate="2014-02-13T19:52:54.003" Title="How does the ISS adjust its solar panels?" Tags="|iss|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1212" PostTypeId="1" AcceptedAnswerId="1214" CreationDate="2013-08-13T14:43:29.417" Score="13" ViewCount="22713" Body="&lt;p&gt;The average life span of a LEO satellite is approximately 5 years, but the average life span for a GEO satellite is approximately 8 years. Why is this? &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="25" LastEditorDisplayName="user29" LastEditDate="2017-09-06T16:47:50.360" LastActivityDate="2018-10-25T14:20:20.620" Title="Why is the life span of a LEO satellite less than that of a GEO satellite?" Tags="|artificial-satellite|orbit|low-earth-orbit|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1213" PostTypeId="2" ParentId="1211" CreationDate="2013-08-13T14:49:46.117" Score="7" Body="&lt;p&gt;There are electric motors that are used to adjust the solar panels, but not necessarily to face the Sun. The solar panels are a major source of atmospheric drag for the station, as even at 200-300 KM above the Earth's surface there are enough air (nitrogen, mostly) molecules to cause slight drag at the speeds that the ISS passes. Every few months the ISS needs to be reboosted due to this effect. Therefore, the solar panels are turned with electric motors to reduce this drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reviewing the excellent &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station&quot;&gt;ISS Wikipedia article&lt;/a&gt; I see it mentioned that the solar arrays are only turned to reduce drag while on the night side of Earth, and on the day side they are turned toward the Sun. Other sources that I've seen mention that the solar arrays collect light from both sides and that the Earth reflects significant light so that facing the panels exactly towards the Sun is not a top priority. However no matter how much priority is put on where to face the panels, for aerodynamic or for power reasons, the panels are rotated with an electric motor.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="472" LastEditDate="2013-08-13T15:10:24.553" LastActivityDate="2013-08-13T15:10:24.553" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1214" PostTypeId="2" ParentId="1212" CreationDate="2013-08-13T14:58:49.783" Score="22" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot;&gt;Wikipedia on Low Earth orbit&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A low Earth orbit (LEO) is generally defined as an orbit below an&#xA;  altitude of approximately 2,000 kilometers (1,200 mi). Given the rapid&#xA;  orbital decay of objects below approximately 200 kilometers (120 mi),&#xA;  the commonly accepted definition for LEO is between 160 kilometers (99&#xA;  mi) (with a period of about 88 minutes) and 2,000 kilometers (1,200&#xA;  mi) (with a period of about 127 minutes) above the Earth's surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At this altitude, there are atmospheric molecules present, leading to increased drag that translates to orbital decay (more so during &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_maximum&quot;&gt;solar maxima&lt;/a&gt;, due to the expansion of gases in the atmosphere). Hence, these LEO orbited satellites need to be constantly reboosted to overcome this drag, otherwise their orbital velocity decreases, and they spiral into the lower atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, the geostationary orbit is a circular orbit 35,786 kilometres (22,236 mi) above the Earth's equator, and the atmospheric drag is comparatively smaller at these heights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in a nutshell, the difference in atmospheric drag is the main reason for longer orbital lifetime of GEO orbited satellites, than LEO orbited satellites.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-13T20:52:08.497" LastActivityDate="2013-08-13T20:52:08.497" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1215" PostTypeId="5" CreationDate="2013-08-13T15:06:55.363" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-13T15:06:55.363" LastActivityDate="2013-08-13T15:06:55.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1216" PostTypeId="4" CreationDate="2013-08-13T15:06:55.363" Score="0" Body="Questions regarding the absence of significant gravitational force." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-13T15:19:22.993" LastActivityDate="2013-08-13T15:19:22.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1217" PostTypeId="5" CreationDate="2013-08-13T15:12:20.197" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-13T15:12:20.197" LastActivityDate="2013-08-13T15:12:20.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1218" PostTypeId="4" CreationDate="2013-08-13T15:12:20.197" Score="0" Body="Questions regarding computer programs used in space exploration." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-13T15:19:19.820" LastActivityDate="2013-08-13T15:19:19.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1219" PostTypeId="5" CreationDate="2013-08-13T15:20:17.103" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-13T15:20:17.103" LastActivityDate="2013-08-13T15:20:17.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1220" PostTypeId="4" CreationDate="2013-08-13T15:20:17.103" Score="0" Body="Questions regarding the type of orbit in which the orbiting object stays in roughly the same location over Earth's surface." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-13T18:20:12.493" LastActivityDate="2013-08-13T18:20:12.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1221" PostTypeId="1" CreationDate="2013-08-13T15:20:50.817" Score="5" ViewCount="123" Body="&lt;p&gt;What is the difference in delta-V required for a launch from the highest natural point on Earth (&lt;a href=&quot;http://en.wikipedia.org/wiki/Mount_Everest&quot; rel=&quot;nofollow noreferrer&quot;&gt;about 8,800 m or 29,000 ft&lt;/a&gt;) as opposed to a sea level launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a completely &lt;a href=&quot;http://en.wikipedia.org/wiki/Launch_loop&quot; rel=&quot;nofollow noreferrer&quot;&gt;ground based launch system&lt;/a&gt;, air resistance would presumably be a consideration.  This similar question &lt;a href=&quot;https://space.stackexchange.com/questions/744/effect-of-atmospheric-drag-on-rocket-launches&quot;&gt;Effect of atmospheric drag on rocket launches and benefits of high altitude launch sites&lt;/a&gt; is specific to rockets, where lift is generated throughout the transition through the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-08-13T15:20:50.817" Title="How much cheaper would a high level launch be from a sea level launch?" Tags="|launch|atmospheric-drag|delta-v|" AnswerCount="0" CommentCount="7" ClosedDate="2013-08-18T17:25:19.873" ContentLicense="CC BY-SA 3.0" />
  <row Id="1222" PostTypeId="5" CreationDate="2013-08-13T15:22:43.127" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-13T15:22:43.127" LastActivityDate="2013-08-13T15:22:43.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1223" PostTypeId="4" CreationDate="2013-08-13T15:22:43.127" Score="0" Body="Questions regarding the craft that house humans or equipment during space exploration." OwnerDisplayName="user12" LastEditorUserId="6816" LastEditDate="2016-12-21T04:19:03.793" LastActivityDate="2016-12-21T04:19:03.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1224" PostTypeId="1" AcceptedAnswerId="2814" CreationDate="2013-08-13T15:53:01.027" Score="9" ViewCount="3495" Body="&lt;p&gt;Motivation is: I would like to be able to reproduce the part of &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:Orbital_Debris_Lifetime_Diagram_Low_Eccentricity.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;this graph&lt;/a&gt; for circular orbits from first-principles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Nhzpo.png&quot; alt=&quot;decay times&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The critical information to do this is the gas density as a function of altitude, which includes the range of about 100 km to 2000 km for all objects that will decay anytime within the next few eons.  This covers some parts of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thermosphere&lt;/a&gt; in the lower range, and the &lt;a href=&quot;http://en.wikipedia.org/wiki/Exosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;Exosphere&lt;/a&gt; in the higher part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: what is a good expression for the density in these two regions?  I want to avoid over-complicating, by going for the &lt;em&gt;most simple non-trivial&lt;/em&gt; approach.  If there's a first level and second level approximation, let's not bother with anything but the first one.  But saying the exosphere is &quot;approximately zero&quot; is not helpful because that would predict infinite lifetimes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia provides some guidance in the form of equations, but there's still some way to go.  For the Exosphere, they give a particle number per unit volume, but this volume is a variable volume, so you would have to change that, and &lt;em&gt;then&lt;/em&gt; convert to density.  I don't even have a good starting point for the Thermosphere.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="49" LastEditDate="2013-10-22T02:44:04.740" LastActivityDate="2013-11-15T22:27:49.313" Title="Expression for density in the Thermosphere and Exosphere" Tags="|low-earth-orbit|orbital-lifetime|exosphere|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1225" PostTypeId="1" AcceptedAnswerId="1229" CreationDate="2013-08-13T16:29:55.830" Score="4" ViewCount="3633" Body="&lt;p&gt;In zero gravity, the vestibular system (which is responsible for feeling the balance) doesn't work since the fluids in it don't move properly in absent of significant gravitational force. Does it also mean that we can't feel dizzy when we are spinning around (either vertically or horizontally)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Motion sickness also based on that the vestibular system signs that we are moving but we can't see it - which definitely occurs in space, where Earth  and its horizon move slowly because of its size and the distance. It may mean that if we are in space and the vestibular system works, then those who are sensitive to it, they should be sick all time - or, in my theory, at least they should feel themselves like on a ship. Fix me if it's false.&lt;/p&gt;&#xA;" OwnerUserId="306" LastActivityDate="2013-08-13T17:04:18.750" Title="Do we have motion sickness in zero gravity?" Tags="|microgravity|astronaut-lifestyle|humans|travel|space-sickness|" AnswerCount="1" CommentCount="4" ClosedDate="2013-08-14T01:50:28.157" ContentLicense="CC BY-SA 3.0" />
  <row Id="1226" PostTypeId="2" ParentId="1224" CreationDate="2013-08-13T16:32:47.927" Score="5" Body="&lt;p&gt;Since you're looking for the simplest non-trivial approach, I'd recommend the exponential model detailed &lt;a href=&quot;http://scipp.ucsc.edu/outreach/balloon/glost/environment3.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, which is very simplistic but is sometimes used as a first (zeroth?) order approximation in things like trade studies. Alternatively, the &lt;a href=&quot;http://www.pdas.com/atmos.html&quot; rel=&quot;noreferrer&quot;&gt;1976 Standard Atmosphere&lt;/a&gt; model is also fairly simple to use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond these, you're looking at models like the classic &lt;a href=&quot;http://en.wikipedia.org/wiki/Jacchia_Reference_Atmosphere&quot; rel=&quot;noreferrer&quot;&gt;Jacchia-70&lt;/a&gt;, &lt;a href=&quot;http://omniweb.gsfc.nasa.gov/vitmo/msis_vitmo.html&quot; rel=&quot;noreferrer&quot;&gt;MSIS&lt;/a&gt;, and the related &lt;a href=&quot;http://www.nrl.navy.mil/research/nrl-review/2003/atmospheric-science/picone/&quot; rel=&quot;noreferrer&quot;&gt;NRLMSISE&lt;/a&gt; models, which are significantly higher-fidelity and, as such, more difficult to implement on your own.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-13T16:32:47.927" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1227" PostTypeId="5" CreationDate="2013-08-13T16:35:28.617" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-13T16:35:28.617" LastActivityDate="2013-08-13T16:35:28.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1228" PostTypeId="4" CreationDate="2013-08-13T16:35:28.617" Score="0" Body="Motion sickness occurs when the vestibular system makes feel the move of the body, but movement can't be seen by eyes. Question with this tag are related to space traveling aspects and involvements of this condition." OwnerUserId="306" LastEditorUserId="306" LastEditDate="2013-08-13T18:20:44.983" LastActivityDate="2013-08-13T18:20:44.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1229" PostTypeId="2" ParentId="1225" CreationDate="2013-08-13T17:04:18.750" Score="5" Body="&lt;p&gt;To answer your question, Yes, almost everyone has motion sickness in zero gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your question has bits that are correct. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Humans rely on the vestibular system for balance and attitude, and with the lack of gravity the fluids don't move correctly. But they do still move, because inertia doesn't go away. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you turn your head the fluid will try to remain stationary, same as on earth. The problem is that there is no longer any 'down' so aspects of the vestibular system and eyesight that rely on correctly orienting with respect to gravity fail. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, being far away from a fixed location means the brain has trouble positioning itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For most astronauts (and similarly for fighter pilots) you can become accustomed to it. &lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-08-13T17:04:18.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1230" PostTypeId="2" ParentId="1211" CreationDate="2013-08-13T17:11:56.347" Score="12" Body="&lt;p&gt;There are two major joints that allow the solar arrays to rotate, the Alpha and Beta gimbals. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note this is a discussion of the main 4 arrays at the opposing ends of the main truss.  The Russian segment has its own solar arrays, and there used to be a plan for more. There have been a series of proposed Solar Power modules for the Russian segment that keep getting cancelled for budgetary reasons. The current plan calls for the Nakua modules to dock to where Pirs is (earth facing port of Zvezda) and then have the UM (Universal Module) with 6 docking ports, dock to the bottom, and then have the Solar Power module dock to the UM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These two joints allow the panels to rotate to track the sun as needed.  The alpha gimbal allows them to rotate as if in a circle, centered on the truss itself.  I.e. Imagine the panels windmilling with the center on the truss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Beta gimbals allow the panels to tilt around to accommodate inclination changes in the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also on the truss, are the thermal radiators to dump waste heat that rotate to AVOID direct sun exposure so they can better dump heat into space. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-02-13T19:52:54.003" LastActivityDate="2014-02-13T19:52:54.003" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1231" PostTypeId="1" AcceptedAnswerId="1761" CreationDate="2013-08-13T17:25:03.917" Score="17" ViewCount="1435" Body="&lt;p&gt;What range of orbital elements (&lt;em&gt;inclination, apogee&lt;/em&gt;, &lt;em&gt;eccenticity&lt;/em&gt;) was available for the space shuttle?&#xA;What range has it really achieved?&#xA;What were the engineered limits?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user12" LastEditDate="2014-07-05T22:37:16.463" LastActivityDate="2014-07-05T22:37:16.463" Title="What range of orbital elements was available for the Space Shuttle?" Tags="|orbit|orbital-maneuver|space-shuttle|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1232" PostTypeId="1" AcceptedAnswerId="1793" CreationDate="2013-08-14T08:19:18.903" Score="23" ViewCount="1768" Body="&lt;p&gt;The electronic &quot;heart&quot; of the International Space Station are the Multiplexers-Demultiplexers (MDMs) located within &amp;amp; outside the pressurized modules. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How often are they replaced? (An ideal answer would be based on actual statistics and would, hopefully, include a graph/table/link to detailed data.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Can we confidently say that the reliability of MDMs is increasing, that is, can we disentangle the effects of solar cycles from changes in manufacturing quality?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What are the lessons from the ISS avionics experience for our future missions beyond LEO?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tUBxi.png&quot; alt=&quot;MDM_External&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vMCjc.jpg&quot; alt=&quot;MDM DRAM SEU graph&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://wrmiss.org/workshops/seventeenth/Atwell.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://wrmiss.org/workshops/seventeenth/Atwell.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-14T13:15:05.713" LastActivityDate="2016-04-19T14:47:32.527" Title="How often are ISS computers (MDMs) replaced?" Tags="|iss|flight-computer|maintenance|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="1234" PostTypeId="1" AcceptedAnswerId="1240" CreationDate="2013-08-14T14:31:55.237" Score="43" ViewCount="18940" Body="&lt;p&gt;Yes, you read that title correctly.  I saw the claim in &quot;&lt;a href=&quot;http://www.nss.org/settlement/ColoniesInSpace/colonies_chap09.html&quot;&gt;Colonies in Space&lt;/a&gt;&quot;, hosted on the NSS site.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a weightless space farm, it may be possible to raise fish without water. On Earth, when a fish is taken from water, gravity makes its gills collapse so that it cannot get oxygen. In weightless space these same fish might easily &quot;swim&quot; through an atmosphere of 100 percent humidity, keeping comfortably moist: hydroponic fish, if you will.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This sounds completely bonkers.  Could astronauts aboard the ISS open up the fish tank and let them &quot;swim&quot; (or flail randomly) about the station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there not problems with Oxygen availability?  Even if the limit in air on Earth is collapse of the gills due to gravity, can a fish really survive getting air instead of water?  Are there any credible references, or a sound scientific case that this is possible?  Why hasn't it been attempted in experiments?  Do gills even &lt;em&gt;work&lt;/em&gt; that way?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="49" LastEditDate="2014-03-19T04:07:05.750" LastActivityDate="2020-05-30T01:14:00.920" Title="Can fish really live in microgravity without water?" Tags="|colonization|microgravity|environment|food|animals|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1235" PostTypeId="2" ParentId="50" CreationDate="2013-08-14T14:33:20.070" Score="10" Body="&lt;p&gt;The Roscosmos requirements for the professional cosmonaut candidates are &lt;a href=&quot;http://www.gctc.ru/media/files/Otbor_kosmonavtov/perecen..medizinskih.dokumentov.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;published here&lt;/a&gt;.&#xA;The &lt;a href=&quot;https://translate.google.com/translate?sl=ru&amp;amp;tl=en&amp;amp;u=http%3A%2F%2Fwww.gctc.ru%2Fmedia%2Ffiles%2FOtbor_kosmonavtov%2Fperecen..medizinskih.dokumentov.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;google translation&lt;/a&gt; seems to be intelligible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It includes the list of medical areas of inspection:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;internal organs inspection;&lt;/li&gt;&#xA;  &lt;li&gt;neuropsychiatric inspection;&lt;/li&gt;&#xA;  &lt;li&gt;surgical inspection;&lt;/li&gt;&#xA;  &lt;li&gt;ophthalmological inspection;&lt;/li&gt;&#xA;  &lt;li&gt;otorhinolaryngologic inspection;&lt;/li&gt;&#xA;  &lt;li&gt;dental inspection;&lt;/li&gt;&#xA;  &lt;li&gt;functional inspection;&lt;/li&gt;&#xA;  &lt;li&gt;gynecological examination (for women);&lt;/li&gt;&#xA;  &lt;li&gt;psychiatric inspection;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;as well as the list of assays required (all kinds of &lt;em&gt;blood tests, feces, urine, 12-lead ECG, etc etc etc&lt;/em&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, in particular, the set of general anthropometric requirements.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Anthropometric data (the maximal allowed values):&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;height (150-190 cm);&lt;/li&gt;&#xA;  &lt;li&gt;height in the sitting position (80-99 cm);&lt;/li&gt;&#xA;  &lt;li&gt;weight (50-90 kg);&lt;/li&gt;&#xA;  &lt;li&gt;foot length (29.5 cm);&lt;/li&gt;&#xA;  &lt;li&gt;shoulders (52 cm);&lt;/li&gt;&#xA;  &lt;li&gt;distance between the armpits (45 cm);&lt;/li&gt;&#xA;  &lt;li&gt;hip width in the sitting position (41 cm).&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2019-04-18T19:15:52.997" LastActivityDate="2019-04-18T19:15:52.997" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="1236" PostTypeId="1" CreationDate="2013-08-14T15:00:25.983" Score="7" ViewCount="425" Body="&lt;p&gt;Does the necessary delta v given to a satellite (earth orbiting) by the rocket during its launch depend on the positions of other planets? Please justify your answer.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-08-14T15:39:21.477" LastActivityDate="2013-08-15T00:38:39.580" Title="Does the delta v requirement of a rocket to place a satellite in orbit depend on the position of other planets?" Tags="|launch|orbit|artificial-satellite|delta-v|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1238" PostTypeId="1" CreationDate="2013-08-14T15:24:56.317" Score="15" ViewCount="1660" Body="&lt;p&gt;Inspired by the question &lt;a href=&quot;https://space.stackexchange.com/questions/1234/can-fish-really-live-in-microgravity-without-water&quot;&gt;Can fish really live in microgravity without water?&lt;/a&gt; I got wondering.  If I lived in zero G, could I have a dolphin for a pet?  Dolphins (and whales) breath normal air just like people, the two biggest reasons reasons they must be in water is for support and movement, neither of this would seem to be significant issue in zero gravity.  Assuming skin moisturization is addressed (and given sufficient area), is there any reason a dolphin (or whale) could not swim and survive just fine in space?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-08-16T20:48:22.837" Title="If I lived in space, could I have a dolphin for a pet?" Tags="|colonization|microgravity|animals|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="1239" PostTypeId="2" ParentId="1236" CreationDate="2013-08-14T20:23:57.237" Score="8" Body="&lt;p&gt;Not really. Here's the effect that a few major bodies would have, depending on the direction launched, expressed in the unit of Newtons/kg. Note that 9.8 N/kg is the equivalent of Earth's gravity. I used &lt;a href=&quot;http://www.ajdesigner.com/phpgravity/newtons_law_gravity_equation_force.php&quot; rel=&quot;noreferrer&quot;&gt;this calculator&lt;/a&gt;, and &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size&quot; rel=&quot;noreferrer&quot;&gt;this table&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Sun       0.00593&#xA;Moon      5.554E-9&#xA;Jupiter   1.352E-7 to 3.664E-7&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it might be beneficial to launch at noon, but there won't be much of a difference really no matter what you do. These are extremely small, and really just don't make much of a difference. 0.1% of the Earth's gravity at the surface is negligible. Put another way, Earth's gravity at sea level is about 9.8201024640745, 1km higher is 9.8170204389969, for a difference of 0.0030820250776. That's about half of the effect of the sun, a very small effect indeed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The requirements change considerably if you plan on leaving Earth Orbit, but I won't get too much in to that complexity.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-15T00:38:39.580" LastActivityDate="2013-08-15T00:38:39.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1240" PostTypeId="2" ParentId="1234" CreationDate="2013-08-15T01:08:31.667" Score="17" Body="&lt;p&gt;There have been several experiments with fish in space; as far as I can tell, all have used water rather than humid air as the habitat.  While there is no definitive answer available as of 2013 (lack of empirical research), present research suggests fish cannot really live in space without water.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears difficult to keep fish alive and healthy in a water environment in space.  This quote from the NASA article points to some of the challenges.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This facility includes an improved water circulation system that&#xA;  monitors water conditions, removing waste while ensuring proper&#xA;  pressure and oxygen flow rates. The system’s design upgrades are based&#xA;  on lessons learned from previous habitats that flew on space shuttle&#xA;  missions STS-47, STS-65, and STS-90.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If we imagine a humid air environment that accounts for all these known hurdles, it is easy to imagine that significant air movement would be required to keep a healthy environment.  It is difficult to imagine how you would move the air sufficiently without negatively impacting the fish. Air is about 784 times less dense then water, to me that means it is going to be 784 times harder to swim in air than it is in water. Without some kind of modification is difficult to imagine how a fish would be able to generate sufficient mobility in air to overcome the required air movement to keep the habitat healthy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Animals_in_space&quot; rel=&quot;noreferrer&quot;&gt;Animals in space (Wikipedia)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/14631634&quot; rel=&quot;noreferrer&quot;&gt;Life-cycle experiments of medaka fish aboard the international space station (NCBI)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.jaxa.jp/article/special/spaceshuttle/koyama02_e.html&quot; rel=&quot;noreferrer&quot;&gt;Japan's first experiments in space (JAXA)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.howfishbehave.ca/pdf/Fishes%20in%20space.pdf&quot; rel=&quot;noreferrer&quot;&gt;Fishes in Space&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/aquatic.html&quot; rel=&quot;noreferrer&quot;&gt;A Fish Friendly Facility for the International Space Station (NASA)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.theweatherprediction.com/habyhints/216/&quot; rel=&quot;noreferrer&quot;&gt;Comparing the density of air to water&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Related&lt;/strong&gt; &#xA;Swimming in air.  Humans can swim in water at about 5MPH (8KPH), this is faster then &lt;a href=&quot;https://www.nmri.go.jp/eng/khirata/fish/general/speed/speede.htm&quot; rel=&quot;noreferrer&quot;&gt;some fish can swim&lt;/a&gt; but floating in space people are essentially unable to swim in air.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/8073/can-you-swim-in-space&quot;&gt;Can you swim in space?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/19396/can-a-human-body-change-direction-when-floating-in-a-space-without-gravity&quot;&gt;Can a human body change direction when floating in a space without gravity?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/18386&quot;&gt;Tiny emergency propulsive device if stuck floating in a large volume in microgravity&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-01-23T13:39:44.157" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1241" PostTypeId="5" CreationDate="2013-08-15T02:45:24.170" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T02:45:24.170" LastActivityDate="2013-08-15T02:45:24.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1242" PostTypeId="4" CreationDate="2013-08-15T02:45:24.170" Score="0" Body="Questions regarding the use of living organisms from kingdom Animalia for space exploration." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-15T03:05:56.613" LastActivityDate="2013-08-15T03:05:56.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1243" PostTypeId="1" AcceptedAnswerId="1253" CreationDate="2013-08-15T02:51:33.600" Score="16" ViewCount="2242" Body="&lt;p&gt;Back in the days of the Apollo era, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot;&gt;Apollo–Soyuz Test Project&lt;/a&gt; had the two craft meeting each other in orbit and docking. The stated reason for this was so that if a spacecraft from one country was undergoing difficulties, the other craft could rescue the distressed one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I kind of like this idea. However, I can't imagine that docking mechanisms haven't changed since then, becoming more reliable, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do both countries' spacecraft use compatible docking apparatus? Does the idea of being able to 'rescue' the crew of another country' ship still hold true?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-25T16:22:29.820" LastActivityDate="2013-10-25T16:22:29.820" Title="Do Russian and American spacecraft use the same docking mechanism?" Tags="|nasa|apollo-program|docking|roscosmos|soyuz-spacecraft|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1244" PostTypeId="5" CreationDate="2013-08-15T03:08:58.440" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T03:08:58.440" LastActivityDate="2013-08-15T03:08:58.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1245" PostTypeId="4" CreationDate="2013-08-15T03:08:58.440" Score="0" Body="questions about the observation of the planet Earth from space, usually for scientific, technical, or other well-defined reasons, or about the spacecraft that do this. This tag is usually used for systematic observations, but it could also apply to a questions on specific images or photos observing something unusual requiring analysis." OwnerDisplayName="user12" LastEditorUserId="12102" LastEditDate="2019-05-28T01:20:05.333" LastActivityDate="2019-05-28T01:20:05.333" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1246" PostTypeId="5" CreationDate="2013-08-15T03:10:48.620" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T03:10:48.620" LastActivityDate="2013-08-15T03:10:48.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1247" PostTypeId="4" CreationDate="2013-08-15T03:10:48.620" Score="0" Body="Questions regarding the computers onboard spacecraft which handle mission-critical data.&#xD;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-15T09:32:15.770" LastActivityDate="2013-08-15T09:32:15.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1248" PostTypeId="5" CreationDate="2013-08-15T03:13:21.677" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T03:13:21.677" LastActivityDate="2013-08-15T03:13:21.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1249" PostTypeId="4" CreationDate="2013-08-15T03:13:21.677" Score="0" Body="Questions regarding the Global Positioning System's satellites and uses." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-15T09:11:06.573" LastActivityDate="2013-08-15T09:11:06.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1250" PostTypeId="5" CreationDate="2013-08-15T03:14:33.790" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T03:14:33.790" LastActivityDate="2013-08-15T03:14:33.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1251" PostTypeId="4" CreationDate="2013-08-15T03:14:33.790" Score="0" Body="Questions regarding a 'sail' that can be attached to spacecraft to harness the sun's radiation to propel a ship in one direction or another." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-15T09:32:51.960" LastActivityDate="2013-08-15T09:32:51.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1252" PostTypeId="2" ParentId="1243" CreationDate="2013-08-15T05:26:05.437" Score="1" Body="&lt;p&gt;&lt;strong&gt;Purpose&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot; rel=&quot;nofollow&quot;&gt;reference&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;April 1972, the United States and the USSR signed an &lt;a href=&quot;http://en.wikipedia.org/w/index.php?title=Agreement_Concerning_Cooperation_in_the_Exploration_and_Use_of_Outer_Space_for_Peaceful_Purposes&amp;amp;action=edit&amp;amp;redlink=1&quot; rel=&quot;nofollow&quot;&gt;Agreement Concerning Cooperation in the Exploration and Use of Outer Space for Peaceful Purposes&lt;/a&gt; committing both the USSR and the United States to the launch of the Apollo–Soyuz Test Project in 1975.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;APPARATUS&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Apollo Command/Service Module (CSM) was launched with a docking module specially designed to enable the two spacecraft to dock with each other, used only once for this mission. The Apollo was pressurized at 5.0 psi using pure oxygen, while the Soyuz used a nitrogen/oxygen atmosphere at sea level pressure . and an adapter, since the surplus Apollo hardware used for the ASTP mission was not equipped with the APAS docking collar jointly developed by NASA and the Soviet Academy of Sciences for the mission. One end of the docking module was attached to the Apollo using the same &quot;probe-and-drogue&quot; docking mechanism used on the Lunar Module and the Skylab space station, while its other end had the APAS docking collar, which Soyuz 19 carried in place of the standard Soyuz/Salyut system of the time. The APAS fitting with the Soyuz 19 was releasable, allowing the two spacecraft to separate.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user29" LastEditDate="2013-08-15T14:18:41.883" LastActivityDate="2013-08-15T14:18:41.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1253" PostTypeId="2" ParentId="1243" CreationDate="2013-08-15T06:38:59.107" Score="10" Body="&lt;p&gt;For the time being, no. That of course doesn't mean they can't be joint (docked or berthed, depending on the purpose of the port) by using various, mostly semi-permanent adapters, like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Pressurized_Mating_Adapter&quot; rel=&quot;noreferrer&quot;&gt;Pressurized Mating Adapter (PMA)&lt;/a&gt; or International Docking Adapter (IDA) that are planned to be used first in 2014-2015, converting &lt;a href=&quot;https://en.wikipedia.org/wiki/Androgynous_Peripheral_Docking_System#APAS-95&quot; rel=&quot;noreferrer&quot;&gt;APAS-95&lt;/a&gt; docking interface to the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Low_Impact_Docking_System&quot; rel=&quot;noreferrer&quot;&gt;NASA Docking System (NDS)&lt;/a&gt; that will be used on future NASA spacecraft:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An NDS interface will be available on the two International Docking&#xA;  Adapters (IDA), which are intended to be installed on two of the three&#xA;  Pressurized Mating Adapters (PMA) on the ISS. The Soft Capture&#xA;  Mechanism (SCM) on the Hubble Space Telescope is compatible with the&#xA;  NDS.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NDS is intended to be used by Commercial Crew, Orion, and all other US&#xA;  vehicles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/sfkS7.png&quot; alt=&quot;NASA Docking System (NDS) passive and active configuration&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;NASA Docking System (NDS) passive and active configuration (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Passive_and_active_NDS..png&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;List of past, current and future types of spacecraft docking and berthing mechanisms is &lt;a href=&quot;https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacecraft#List_of_types_of_spacecraft_docking_and_berthing_mechanisms&quot; rel=&quot;noreferrer&quot;&gt;available on Wikipedia&lt;/a&gt;. Among those currently in use on the ISS are three Pressurized Mating Adapters (PMA) with an APAS-95 docking port:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;PMA-1 is semi-permanently connected to the APAS interface of&#xA;  Functional Cargo Block (Zarya), connecting the US Orbital Segment&#xA;  (USOS) and the Russian Orbital Segment (ROS); The other two PMAs are&#xA;  free, possibly available for visiting spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/YdbAS.jpg&quot; alt=&quot;APAS-95 active side&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;APAS-95 in active configuration, its soft dock ring with the three petals extended (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:APAS-95_active_side.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;There is however the &lt;a href=&quot;http://www.internationaldockingstandard.com/&quot; rel=&quot;noreferrer&quot;&gt;International Docking System Standard (IDSS)&lt;/a&gt; in development&lt;/strong&gt;, that plans on creating a standard for docking and berthing:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.internationaldockingstandard.com/download/IDSS_IDD_RevA_Final_051311.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Docking System Standard (IDSS) Interface Definition Document (IDD)&lt;/a&gt;&lt;br /&gt;(available in PDF format)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The new standard allows for the use of APAS heritage design features&#xA;  while enabling technical advances made available during the past 40&#xA;  years since APAS was first designed. Specifically the low impact soft&#xA;  capture system is a step forward in delivering reduced energy&#xA;  dockings. Rather than reinventing the wheel, this standard&#xA;  incorporates proven APAS features while advancing the&#xA;  state-of-the-art.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-08-15T06:38:59.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1254" PostTypeId="1" CreationDate="2013-08-15T07:32:06.497" Score="6" ViewCount="184" Body="&lt;p&gt;Can a laser be used to measure the rate of descent of a re-entry probe during its re-entry into a planet's or satellite's atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any probes that used a laser to measure its descent?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2014-07-05T22:37:37.137" LastActivityDate="2014-07-05T22:37:37.137" Title="Can the rate of descent be measured with a laser?" Tags="|reentry|landing|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1255" PostTypeId="2" ParentId="1254" CreationDate="2013-08-15T08:04:44.380" Score="7" Body="&lt;p&gt;Yes. This technology is commonly referred to as &lt;a href=&quot;https://en.wikipedia.org/wiki/Lidar&quot;&gt;LIDAR&lt;/a&gt; (Laser Image Detection and Ranging) and is a combination (portmanteau) of words &lt;em&gt;&quot;light&quot;&lt;/em&gt; and &lt;em&gt;&quot;radar&quot;&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Lidar is popularly known as a technology used to make high resolution&#xA;  maps, geomatics, archaeology, geography, geology, geomorphology,&#xA;  seismology, forestry, remote sensing, atmospheric physics, airborne&#xA;  laser swath mapping (ALSM), laser altimetry, and contour mapping.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As far as the use of LIDARs as altimeters in astronautics is concerned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The general public became aware of the accuracy and usefulness of&#xA;  lidar systems in 1971 during the Apollo 15 mission, when astronauts&#xA;  used a laser altimeter to map the surface of the moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They are also used by Earth orbiting satellites to produce digital elevation maps, &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_Experiment&quot;&gt;measure the distance to reflectors placed on the Moon&lt;/a&gt;, Mars Global Surveyor uses LIDAR for precise global topographic surveys, Phoenix Lander to detect snow in the atmosphere of Mars,...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Lidar is increasingly being utilized for range finding and orbital&#xA;  element calculation of relative velocity in proximity operations and&#xA;  station keeping of spacecraft. Lidar has also been used for&#xA;  atmospheric studies from space. Using short pulses of laser light&#xA;  beamed from a spacecraft, some of that &quot;light reflects off of tiny&#xA;  particles in the atmosphere and back to a telescope aligned with the&#xA;  laser. By precisely timing the lidar 'echo,' and by measuring how much&#xA;  laser light is received by the telescope, scientists can accurately&#xA;  determine the location, distribution and nature of the particles. The&#xA;  result is a revolutionary new tool for studying constituents in the&#xA;  atmosphere, from cloud droplets to industrial pollutants, that are&#xA;  difficult to detect by other means.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is also used in robotic missions to determine distance to the ground, obstacles, e.t.c.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;American Lunar probe &lt;a href=&quot;https://en.wikipedia.org/wiki/Clementine_%28spacecraft%29&quot;&gt;Clementine&lt;/a&gt; (Deep Space Program Science Experiment) also used LIDAR system to measure the distance from the spacecraft to a point on the surface of the Moon.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-15T08:31:38.833" LastActivityDate="2013-08-15T08:31:38.833" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1257" PostTypeId="1" CreationDate="2013-08-15T08:44:29.767" Score="10" ViewCount="452" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_program#Missions&quot; rel=&quot;noreferrer&quot;&gt;Surveyor landers&lt;/a&gt; are the second generation of US lunar spacecraft following the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ranger_program#Mission_list&quot; rel=&quot;noreferrer&quot;&gt;Ranger series&lt;/a&gt;. They paved the way for the later soft landings of Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What type of onboard radars were used in the Surveyors and how did they work?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2015-08-17T17:14:07.320" LastActivityDate="2015-08-17T19:15:08.660" Title="What type of onboard radar did the Surveyor landers use?" Tags="|the-moon|lander|instrument|radar|surveyor-program|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1258" PostTypeId="2" ParentId="1257" CreationDate="2013-08-15T08:48:23.223" Score="8" Body="&lt;p&gt;Surveyor measured the rate of descent with radar by using &lt;a href=&quot;http://en.wikipedia.org/wiki/Doppler_effect&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Doppler shift&lt;/strong&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the book &lt;strong&gt;&lt;a href=&quot;http://books.google.com/books?id=cyehD1KsTKoC&quot; rel=&quot;noreferrer&quot;&gt;Planetary Landers and Entry Probes&lt;/a&gt;&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Surveyor_Program&quot; rel=&quot;noreferrer&quot;&gt;surveyor lander&lt;/a&gt; used a pulsed radar to generate an altitude reference at 100 km, a separate RADVS (Radar Altimeter and Doppler Velocity Sensor) turned on, using a four beam frequency modulated continuous wave technique (Figure 5.4). A central beam at 12.9GHz was used to measure the altitude, while the three outer beams at 13.3 GHz were used to determine the three components of the velocity. To accommodate the large variation in range (and therefore signal strength) amplifiers provided 40, 65, or 90 dB of gain. &#xA;  &lt;img src=&quot;https://i.stack.imgur.com/aYGzp.png&quot; alt=&quot;Image of Surveyor from the source book, via Google Books.&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorUserId="6816" LastEditorDisplayName="user29" LastEditDate="2015-08-17T19:15:08.660" LastActivityDate="2015-08-17T19:15:08.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1259" PostTypeId="1" AcceptedAnswerId="1281" CreationDate="2013-08-15T09:02:30.273" Score="15" ViewCount="1255" Body="&lt;p&gt;This question is more on the history side. What I want to know is what the differences between the USA and Germany before 1945 were that led to the States falling behind in space technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The United States could boast:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Robert H. Goddard, a pioneer of rocketry&lt;/li&gt;&#xA;&lt;li&gt;Theodore von Kármán and his associates at Caltech&lt;/li&gt;&#xA;&lt;li&gt;Huge industrial potential&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;More specifically, I'm interested in pre-war progress (the Manhattan project did divert a bit of resources during the war).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NOTE: I'm not asking for &lt;strong&gt;subjective opinions&lt;/strong&gt;. Please back your argument with historical documents, memoirs, reports or other evidence.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="12102" LastEditDate="2021-09-13T00:14:48.723" LastActivityDate="2021-09-13T00:14:48.723" Title="Why did the USA fail to be the first rocket power?" Tags="|history|spaceflight-firsts|" AnswerCount="3" CommentCount="7" ClosedDate="2013-08-17T12:29:54.613" ContentLicense="CC BY-SA 3.0" />
  <row Id="1260" PostTypeId="1" AcceptedAnswerId="1268" CreationDate="2013-08-15T09:05:52.147" Score="9" ViewCount="1955" Body="&lt;p&gt;What is the main function of high and low gain antennas?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do all satellites and space probes have high and low gain antennas?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user29" LastEditDate="2013-08-15T11:58:59.483" LastActivityDate="2016-10-12T11:12:57.007" Title="Do all satellites and space probes have high &amp; low gain antennas and what are their functions?" Tags="|communication|artificial-satellite|probe|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1262" PostTypeId="2" ParentId="1260" CreationDate="2013-08-15T12:16:38.697" Score="3" Body="&lt;p&gt;There are a couple of different uses. For looking at incredibly small radio sources, the idea of a high gain antenna is that it can detect smaller signals - effectively 'gain' is a type of amplification. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(As a quick example, your home wifi network - typically it is broadcasting at 100mW and you might be able to get decent reception round your house, or possibly in your garden, with the standard antennas fitted - usually 3dB. You can use the same wifi router to communicate over miles using a high gain antenna - eg 20dB - as it will put out a larger signal, and detect smaller signals, but is usually much more directional)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low gain antennas are typically used for communication with the Earth, as relatively speaking, signal strength from the Earth is massive compared to the incredibly weak signal from a stellar source. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @DeerHunter points out, you also use both types of antenna for comms. The high gain is very directional, so as long as the satellite attitude control is good, you can point it directly at the base station and communicate, A low gain antenna is generally much more forgiving of direction, so if the satellite drifts, you can send adjustment commands to the low gain antenna - at lower data rates.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2013-08-15T12:42:15.237" LastActivityDate="2013-08-15T12:42:15.237" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1263" PostTypeId="2" ParentId="1238" CreationDate="2013-08-15T14:58:25.100" Score="2" Body="&lt;p&gt;Dolphins require water to push against to swim, but that would be the only problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dolphin might panic and thrash around like a fool at first, but would eventually adapt. The other problem might be crashing into things thereby injuring the dolphin, if the dolphin could move at speed at all.&lt;/p&gt;&#xA;" OwnerUserId="319" LastActivityDate="2013-08-15T14:58:25.100" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1264" PostTypeId="1" CreationDate="2013-08-15T15:15:39.560" Score="6" ViewCount="1877" Body="&lt;p&gt;What are the most commonly used wave bands for communications between satellites in LEO and ground station, and what is the minimum frequency used?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user29" LastEditDate="2013-08-16T20:15:29.270" LastActivityDate="2013-08-24T19:32:57.527" Title="What frequency wavebands are used for communication in LEO?" Tags="|communication|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1265" PostTypeId="2" ParentId="463" CreationDate="2013-08-15T15:18:55.197" Score="7" Body="&lt;p&gt;One of the longest lasting (tiny) self-sustaining ecosystems is the &lt;a href=&quot;http://www.eco-sphere.com/about.html&quot; rel=&quot;noreferrer&quot;&gt;Ecosphere&lt;/a&gt;.  You can't eat it, but it's completely sealed and lasts for about two years, but can be as long as seven. It contains tiny shrimp, algae, some bacteria, and seawater. You need to keep it at about room temperature in indirect sunlight, but other than those requirements you could keep it anywhere(although most people have them as desk toys).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of thermodynamics and &lt;a href=&quot;http://en.wikipedia.org/wiki/Trophic_level&quot; rel=&quot;noreferrer&quot;&gt;trophic levels&lt;/a&gt;, you would probably need about 10 times the weight of the crew in plants at minimum, and probably more like 100. Humans would probably not provide adequate nutrient recycling on their own, so they probably couldn't be the only herbivore species. If nothing else, you'd need something that could eat our banana peels, etc. So that puts a lower bound on the size of such a farm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would expect the actual participants in the farm wouldn't make as much of a difference as you think, but I suppose things like the oxygen crisis are a counterargument. &lt;/p&gt;&#xA;" OwnerUserId="319" LastEditorUserId="319" LastEditDate="2013-08-15T22:55:07.380" LastActivityDate="2013-08-15T22:55:07.380" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1266" PostTypeId="2" ParentId="1264" CreationDate="2013-08-15T16:03:07.527" Score="3" Body="&lt;p&gt;There were several experiments (Model-1, Model-2, Krab) of large antennas deployment &lt;a href=&quot;http://journals.ru/attach/653/65203/974979.jpg&quot; rel=&quot;nofollow&quot;&gt;(two circles of 20 m diameter)&lt;/a&gt; for communication in what is reported to be the &lt;a href=&quot;http://en.wikipedia.org/wiki/Super-low_frequency&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;SLF&lt;/em&gt; range&lt;/a&gt; .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Model-1 (1980) and the early Model-2 (1982, 1983) experiments have failed to deploy properly the elastic antennas. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last of the Model-2 (1987, with an inflatable antenna) &#xA;and Krab (1989, shape-memory alloys) have been successfuly deployed and several communication sessions between the ground station and Progress spacecraft have been established.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://rutube.ru/video/ecf36b720490c4a5097a259e7727a940?bmstart=1920&quot; rel=&quot;nofollow&quot;&gt;this video @ 32:00&lt;/a&gt; for an unsuccessful and a successful Model deployments.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-24T19:32:57.527" LastActivityDate="2013-08-24T19:32:57.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1267" PostTypeId="2" ParentId="1264" CreationDate="2013-08-15T16:05:42.147" Score="4" Body="&lt;p&gt;I'm going to address this question as per the US FCC, because it seems similar rules apply anywhere. The FCC has a &lt;a href=&quot;http://www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf&quot; rel=&quot;nofollow&quot;&gt;chart&lt;/a&gt; which shows the various frequency allocations. For the first question, the lowest frequency signals appear to be &lt;a href=&quot;http://iaru-r1.org/index.php?option=com_content&amp;amp;view=article&amp;amp;id=175&amp;amp;Itemid=127&quot; rel=&quot;nofollow&quot;&gt;Amateur Satellites&lt;/a&gt;, namely, 29300-29510 kHz. For &lt;a href=&quot;http://www.spaceacademy.net.au/spacelink/radiospace.htm&quot; rel=&quot;nofollow&quot;&gt;commercial/government satellites&lt;/a&gt;, they seem to be limited to the VHF range, I see some in the 146-138 range, for instance, and that seems to be the highest supported. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, given that, why would there not be a lower frequency? And why don't Amateurs seem to care? Low frequency requires larger antennas, usually shaped in a Yagi configuration. Higher frequencies can use dish type antennas, and typically get higher gain than they would otherwise. Also, lower frequency= lower bandwidth, thus, they can't transmit data very fast. Finally, they tend to bounce around the Ionosphere, not always leaving the atmosphere, and can be confused with signals very far apart. Furthermore, they are noisier, the Sun puts out a lot of energy at lower frequencies. As a whole, they are just clunky for orbital operations. Amateur Radio Operators don't typically care, as they often have large antennas, and only want to do simple communication with others (Voice, small data, Morse Code, etc)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to what frequency is most commonly used, one of the goals to planning satellites is to get them to not use exactly the same frequency, so that they don't interfere with each other. Not withstanding, the most common band for communication satellites is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Ku_band&quot; rel=&quot;nofollow&quot;&gt;KU Band&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-16T20:27:50.613" LastActivityDate="2013-08-16T20:27:50.613" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1268" PostTypeId="2" ParentId="1260" CreationDate="2013-08-15T16:22:57.923" Score="10" Body="&lt;p&gt;First of all, not all satellites have both high gain and low gain antennas. Most inexpensive LEO satellites only have one, depending on the bandwidth requirements. This isn't an issue, as the satellites are so close to Earth that the link margin is considerably improved over further out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you get further away, you start to have more demanding RF requirements, due to the fact that you are much further away, and might need a higher gain antenna to accommodate the extra distance. These higher gain antennas will require a higher pointing accuracy. As you start to have a very small beamwidth, it becomes more important to have a back up method to communicate with the satellite in the event of an extreme off pointing of the primary antenna. This signal will be a much lower bandwidth than the high gain, and also can't take the full bandwidth receiving that the high gain can take, but it is something. Hopefully you've designed the system such that you can figure out from the limited data what's going on, and uplink the right commands to fix the satellite. In other words, low gain antenna are a contingency that you can't point your high gain antenna correctly.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-10-12T11:12:57.007" LastActivityDate="2016-10-12T11:12:57.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1269" PostTypeId="1" AcceptedAnswerId="1449" CreationDate="2013-08-15T16:30:31.540" Score="24" ViewCount="1974" Body="&lt;p&gt;According to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Venera-D&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article on Venera-D&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A lander, based on the Venera design, is also planned, capable of surviving for a long duration on the planet's surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The surface of Venus has temperatures of 462°C and pressures of 9.2 MPa.  How can any useful payload function for more than a couple of hours in such conditions?  In the near-vacuum of space, radiators can be used to get rid of excess heat, but on Venus, the atmosphere would just radiate back, so that wouldn't work.  The only solution I can envisions is a giant, powerful, active refrigerator — and that would only address one of the problems.  I suppose one would not only need to cool down the instruments, but also external components such as solar panels (if applicable).  On the plus side, one should be able to simulate Venus conditions in a lab somewhat accurately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have any payloads been demonstrated (in a lab) to function for a longer time in conditions similar to the Venus surface, using active cooling with a feasible power budget?  By a longer time, I mean long enough so that mission control can interact with the spacecraft, decide what to look for based on initial results, etc.; so let's say &lt;em&gt;at least one month&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similar question: &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/q/175/33&quot;&gt;What material properties would be necessary to shield a lander from the environment of the Venusian surface?&lt;/a&gt;&lt;/em&gt;.  The other question focusses on materials, whereas I specifically ask if it has been demonstrated in a lab, considering &lt;em&gt;all&lt;/em&gt; aspects.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-07-16T20:48:38.717" Title="Has any technology been demonstrated that allows a useful payload to survive at least one month on Venus' surface?" Tags="|technology|venus|environment|thermal|venera|" AnswerCount="1" CommentCount="19" ContentLicense="CC BY-SA 3.0" />
  <row Id="1270" PostTypeId="1" AcceptedAnswerId="1280" CreationDate="2013-08-15T16:40:17.923" Score="17" ViewCount="4625" Body="&lt;p&gt;Why did Curiosity choose to go with nuclear power? While solar panels have issues with Martian dust, this was a known factor before the 2004 Spirit and Opportunity mission. Why then did the 2004 rovers choose to use solar panels for power?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="80" LastEditDate="2013-08-15T17:14:55.663" LastActivityDate="2016-10-01T00:24:46.420" Title="Why does Curiosity use an RTG rather than solar (as Spirit and Opportunity do) for power?" Tags="|curiosity|nuclear|solar-power|spirit|opportunity|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1271" PostTypeId="5" CreationDate="2013-08-15T16:54:35.803" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T16:54:35.803" LastActivityDate="2013-08-15T16:54:35.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1272" PostTypeId="4" CreationDate="2013-08-15T16:54:35.803" Score="0" Body="Questions regarding the use of nuclear devices in space exploration, e.g. to power or propel spacecraft." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-15T16:56:20.120" LastActivityDate="2013-08-15T16:56:20.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1273" PostTypeId="5" CreationDate="2013-08-15T16:55:05.337" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T16:55:05.337" LastActivityDate="2013-08-15T16:55:05.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1274" PostTypeId="4" CreationDate="2013-08-15T16:55:05.337" Score="0" Body="Questions regarding the 2004 Spirit rover." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-15T16:56:26.277" LastActivityDate="2013-08-15T16:56:26.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1275" PostTypeId="5" CreationDate="2013-08-15T16:55:29.213" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T16:55:29.213" LastActivityDate="2013-08-15T16:55:29.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1276" PostTypeId="4" CreationDate="2013-08-15T16:55:29.213" Score="0" Body="Questions regarding equipment and techniques used to decrease temperature." OwnerDisplayName="user12" LastEditorUserId="406" LastEditDate="2013-08-15T16:57:03.750" LastActivityDate="2013-08-15T16:57:03.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1277" PostTypeId="5" CreationDate="2013-08-15T16:58:31.240" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-15T16:58:31.240" LastActivityDate="2013-08-15T16:58:31.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1278" PostTypeId="4" CreationDate="2013-08-15T16:58:31.240" Score="0" Body="Questions relating to sounds created by or experienced in space exploration." OwnerUserId="-1" LastEditorUserId="40257" LastEditDate="2022-05-17T16:25:27.000" LastActivityDate="2022-05-17T16:25:27.000" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1279" PostTypeId="2" ParentId="1270" CreationDate="2013-08-15T17:39:04.157" Score="6" Body="&lt;p&gt;While I've not yet found explicit back up to my recollection of this, the explanation I remember is that solar panels would not provide adequate power to operate the rover and the instruments on it and still fit within the mission profile.  Curiosity/Mars Science Laboratory is much larger and more massive and has more instruments than the previous Mars rovers.  Thus the power requirements are much higher.  Current solar technology is not adequate to power the rover.  Solar panels to provide adequate power likely would have increased the size and weight of the spacecraft so much that the mission would have become infeasible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following statement from &lt;a href=&quot;http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Alpha&amp;amp;Alias=Mars%20Science%20Laboratory/Curiosity&amp;amp;Letter=M&amp;amp;Display=ReadMore&quot; rel=&quot;noreferrer&quot;&gt;the MSL mission profile&lt;/a&gt; does not quite explicitly support the above, but it somewhat addresses the matter:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The rover will carry a radioisotope power system that generates&#xA;  electricity from the heat of plutonium's radioactive decay. This power&#xA;  source gives the mission an operating lifespan on Mars' surface of a&#xA;  full martian year (687 Earth days) or more, while also providing&#xA;  significantly greater mobility and operational flexibility, enhanced&#xA;  science payload capability, and exploration of a much larger range of&#xA;  latitudes and altitudes than was possible on previous missions to&#xA;  Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="36" LastEditorUserId="49" LastEditDate="2013-08-15T17:54:03.353" LastActivityDate="2013-08-15T17:54:03.353" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1280" PostTypeId="2" ParentId="1270" CreationDate="2013-08-15T18:39:27.397" Score="22" Body="&lt;p&gt;Two alternatives (RTG or solar) were investigated in the &lt;a href=&quot;http://science1.nasa.gov/media/medialibrary/2010/11/05/MSL-FEIS_Vol1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Mars Science Laboratory Environmental Impact Statement&lt;/a&gt;. In summary, solar arrays would limit the MSL to operate between 5°N and 20°N, and for one Martian Year only if at exactly 15°N, whereas RTGs would permit it to operate for at least one Martian year anywhere from 60°S to 60°N.&lt;/p&gt;&#xA;&lt;p&gt;From page 2-1 (PDF page 35):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Proposed Action (Alternative 1, NASA’s Preferred Alternative) (...) The proposed MSL rover would utilize a radioisotope power system as its primary source of electrical power to operate and conduct science on the surface of Mars. (...)&lt;/p&gt;&#xA;&lt;p&gt;Alternative 2 (...) The alternative MSL rover would utilize solar energy as its primary source of electrical power to operate and conduct science on the surface of Mars. (...)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Of particular interest is Table 2-5 on page 2-36 (PDF page 70):&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5pKxT.png&quot; alt=&quot;MSL alternatives&quot; /&gt;&#xA;&lt;br /&gt;&lt;sup&gt;MSL alternative capabilities as investigated by NASA.&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;Accompanying text below the table:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Alternative 1. The MMRTG-powered rover would be capable of achieving all of the target operational capabilities summarized in Table 2-1, including landing at a scientifically interesting location between 60° South and 60° North latitude, and operating and conducting science for at least one Mars year.&lt;/p&gt;&#xA;&lt;p&gt;Alternative 2. At most latitudes on Mars the amount of time that a solar-powered rover could perform science operations would be limited by the ability of the solar array to generate sufficient power for the rover to survive the extreme thermal environment. At latitudes on Mars between 60° South and 5° North and between 20° North and 60° North a solar-powered rover either would not have sufficient power to operate at all, or would not be able to survive long enough to accomplish even the minimum science. A&#xA;solar-powered rover could operate for at least one-half Mars year and achieve the minimum science capability only at latitudes ranging from slightly above 5° North to slightly below 20° North, and could operate for a full Mars year and accomplish the full science objectives only at 15° North latitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;For more details, including a detailed description of both (and other) alternatives as well as a consideration of non-scientific factors, see the Mars Science Laboratory Environmental Impact Statement, &lt;a href=&quot;http://science1.nasa.gov/media/medialibrary/2010/11/05/MSL-FEIS_Vol1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Volume 1&lt;/a&gt; and &lt;a href=&quot;http://science1.nasa.gov/media/medialibrary/2010/11/05/MSL-FEIS_Vol2.pdf&quot; rel=&quot;noreferrer&quot;&gt;Volume 2&lt;/a&gt;.  The non-scientific factors include environmental concerns, which speak for solar — considerably less risk in case of a catastrophic launch failure (see also &lt;a href=&quot;https://space.stackexchange.com/q/17/33&quot;&gt;this question on RTG alternatives&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-10-01T00:24:46.420" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1281" PostTypeId="2" ParentId="1259" CreationDate="2013-08-15T20:31:25.220" Score="11" Body="&lt;p&gt;The short answer is that Germany focused on rockets as part of their program of improving artillery. The focus being delivering munitions at long ranges. The initial work was done at Kummersdorf and had enough progress to keep on going resulting in the development of the A4 rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast Goddard and von Kármán worked on other types of projects, like assisted takeoff for aircraft, which had no clear path of development to a long range rocket. While Goddard tried to convince the US Army of the value of long range rockets ultimately the work they assigned him would not create any opportunity to develop that technology.&lt;/p&gt;&#xA;" OwnerUserId="151" LastEditorUserId="151" LastEditDate="2013-08-16T12:35:52.083" LastActivityDate="2013-08-16T12:35:52.083" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1282" PostTypeId="2" ParentId="1259" CreationDate="2013-08-15T22:20:25.823" Score="14" Body="&lt;p&gt;Mostly because China was the &lt;a href=&quot;http://en.wikipedia.org/wiki/History_of_rockets&quot;&gt;first rocket power&lt;/a&gt;, a few hundred years before the USA existed. &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-08-15T22:20:25.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1284" PostTypeId="1" AcceptedAnswerId="1285" CreationDate="2013-08-16T06:03:00.410" Score="23" ViewCount="2449" Body="&lt;p&gt;All the Mars rovers tend to move very slowly. For example, Curiosity moves at an average speed of &lt;a href=&quot;http://en.wikipedia.org/wiki/Curiosity_%28rover%29#Specifications&quot;&gt;30 metres per hour&lt;/a&gt; and a maximum speed of 90 metres per hour. Why is this so? Is it because of safety reasons? Does the time lag between Earth and Mars necessitate such caution? Then again, the rovers are equipped with &lt;a href=&quot;http://en.wikipedia.org/wiki/Hazcam&quot;&gt;hazcams&lt;/a&gt; to avoid accidents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, do they travel so slowly for reasons of stability? Does the science equipment on board have such requirements?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is it simply because there is no need for haste? How much time does the Curiosity spend idle as it moves from one location to another?&lt;/p&gt;&#xA;" OwnerUserId="80" LastActivityDate="2013-08-16T09:08:13.413" Title="Why do the Mars rovers move so cautiously?" Tags="|mars|rovers|curiosity|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1285" PostTypeId="2" ParentId="1284" CreationDate="2013-08-16T09:08:13.413" Score="20" Body="&lt;p&gt;The main reason for the MSL driving slower than physically possible is hazard avoidance (and also navigation/visual odometry):&lt;/p&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/43242/1/12-4334_A1b.pdf&quot; rel=&quot;noreferrer&quot;&gt;Introduction to mobility for MSL&lt;/a&gt; Heverly, Matthew C. 10-Sep-2012&lt;/p&gt;&#xA;&lt;h1&gt;Hazard avoidance and navigation&lt;/h1&gt;&#xA;&lt;p&gt;Drive rates (m/hour):&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Absolute top speed:   151.2 (On rigid terrain with zero slip)&#xA;Blind drive:          139.5 (top speed for straight driving&#xA;                             with pause between steps)&#xA;Blind drive&#xA;with slip checks:     116.0 (Averaged over a significant number&#xA;                             of slip checks)&#xA;Visual Odometry (VO):  64.5 (At every step, look around, &#xA;                             see how far the rover has driven)&#xA;Autonomous&#xA;navigation (Autonav):  40.1&#xA;Autonav &#xA;with slip checks:      38.5&#xA;Autonav with VO:       30.0 (Assumes VO driving rate with &#xA;                             additional time for map imaging &#xA;                             at the appropriate spacing)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;strong&gt;What is Visual Odometry?&lt;/strong&gt; Taking stereo images &amp;quot;before&amp;quot; and &amp;quot;after&amp;quot; the step to track true motion, calculate slip and embedding. Accuracy of VO is 3% of the step distance, so you wouldn't want steps to be too long.&lt;/p&gt;&#xA;&lt;p&gt;Why is it capable of moving slowly?&lt;/p&gt;&#xA;&lt;h1&gt;Actuator design constraints&lt;/h1&gt;&#xA;&lt;p&gt;The mission design logic (see &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38317/1/03-2974.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38317/1/03-2974.pdf&lt;/a&gt;) is as follows:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Interesting landing site =&amp;gt; Technically feasible landing error ellipse =&amp;gt; Traverse length&lt;/li&gt;&#xA;&lt;li&gt;Number of samples&lt;/li&gt;&#xA;&lt;li&gt;Nominal mission duration (one martian year)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Hence, &lt;span class=&quot;math-container&quot;&gt;$Nominal\ Speed = Traverse\ Length/(Mission\ Duration - Number\ of\ Samples * Time\ per\ Sample)$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y8SO4.png&quot; alt=&quot;Tradeoff logic&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rlkz5.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-16T09:08:13.413" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1286" PostTypeId="2" ParentId="1259" CreationDate="2013-08-16T10:56:46.203" Score="4" Body="&lt;p&gt;Choosing not to invest in long range rocketry during the war was the correct decision. Due to their high cost it wasn't until long range rockets grew powerful enough to carry miniaturized nuclear weapons that they became a militarily useful asset to the US. Bombers were a much more cost effective way to deliver high explosives on a target.  And while &gt;99% of bomber crews couldn't place their bombs any closer than a mile of their target except via sheer luck; the same was true of 100% of long range rockets during that time period.&lt;/p&gt;&#xA;" OwnerUserId="157" LastActivityDate="2013-08-16T10:56:46.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1287" PostTypeId="5" CreationDate="2013-08-16T13:38:05.743" Score="0" Body="&lt;p&gt;The JPL &quot;basics of spaceflight&quot; &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf13-1.php#&quot; rel=&quot;nofollow&quot;&gt;defines&lt;/a&gt; navigation as including these two topics.  One is estimation of velocity and prediction of course, and the other is actuating rocket motor action to fire and impart corrective impulse to the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-08-16T14:17:45.077" LastActivityDate="2013-08-16T14:17:45.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1288" PostTypeId="4" CreationDate="2013-08-16T13:38:05.743" Score="0" Body="Topics that relate to orbit determination and flight path control. May also be used for navigation on the surface of celestial objects." OwnerUserId="26" LastEditorUserId="25" LastEditDate="2013-10-28T12:49:12.237" LastActivityDate="2013-10-28T12:49:12.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1289" PostTypeId="5" CreationDate="2013-08-16T13:42:23.200" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-16T13:42:23.200" LastActivityDate="2013-08-16T13:42:23.200" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1290" PostTypeId="4" CreationDate="2013-08-16T13:42:23.200" Score="0" Body="Type of satellite in the shape of a cube 10 cm on each side.  Popular with academics and amateurs with a focus on off-the-shelf components." OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-08-16T13:54:43.227" LastActivityDate="2013-08-16T13:54:43.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1291" PostTypeId="5" CreationDate="2013-08-16T13:49:28.477" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-16T13:49:28.477" LastActivityDate="2013-08-16T13:49:28.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1292" PostTypeId="4" CreationDate="2013-08-16T13:49:28.477" Score="0" Body="Literal meaning is &quot;change in velocity&quot;.  The quantity has utility in rocketry as measure of the change in velocity that a reaction engine must produce to move into a given trajectory.  The rocket equation establishes that the mass ratio of payload to initial mass is an exponential function of delta-v divided by the propellant velocity." OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-08-16T13:54:50.930" LastActivityDate="2013-08-16T13:54:50.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1293" PostTypeId="1" AcceptedAnswerId="1294" CreationDate="2013-08-16T14:26:48.307" Score="11" ViewCount="2579" Body="&lt;p&gt;Many manned space exploration vehicles have some way to abort, to end the mission as quickly as possible and get back to relative safety.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering if the Apollo command module had an abort, a way to get back to Earth while enroute to the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, why wasn't it used on Apollo 13?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2014-06-03T10:29:51.883" Title="Did the Apollo command module have an abort mode?" Tags="|apollo-program|abort|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1294" PostTypeId="2" ParentId="1293" CreationDate="2013-08-16T14:40:48.310" Score="15" Body="&lt;p&gt;According to a &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_abort_modes&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; Apollo missions had multiple abort modes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pad abort - If an emergency occurred during the last few minutes before launch, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_spacecraft#Launch_Escape_System_.28LES.29&quot; rel=&quot;noreferrer&quot;&gt;launch escape system&lt;/a&gt; could pull the command module away from the rocket stack.&lt;/li&gt;&#xA;&lt;li&gt;Mode I - Would have been used during Stage I burn and would have used the lauch escape system to pull the command module out of the way.  It was subdivided into IA, IB, and IC, depending on altitude: Below 3000 meters, IA would have been used.  In it the command module would be moved out of the flight path of the rocket stack and pulled away by the launch escape system. IB would have been used between 3000 meters and 30.5 km.  In it, canards would be used to turn the command module into a &quot;bottom forward&quot; flight so that the parachutes could deploy properly.  IC was similar to IB, but would use motors to orient the CM.  When the rocket/spacecraft stack was too high for the launch escape system to be useful, that system was jettisoned.&lt;/li&gt;&#xA;&lt;li&gt;Mode II - In the event of a problem early in the second stage burn, the command and service modules would separate from the rest of the stack, using the service module's rocket.  Once clear, the command module could separate from the service module and splash down.&lt;/li&gt;&#xA;&lt;li&gt;Mode III (aka Contingency Orbit Insertion) - Later in the stage 2 burn, orbit was achievable, so the 3rd stage and service module engines would be used to get into orbit.&lt;/li&gt;&#xA;&lt;li&gt;S-IVB to orbit - Similar to Mode III, but only the SIV-B stage is needed to get into orbit.&lt;/li&gt;&#xA;&lt;li&gt;Mode IV - This was for a problem in the SIV-B stage and uses just the service module to achieve orbit.&lt;/li&gt;&#xA;&lt;li&gt;Direct Abort Trajectory - For problems that might occur during the cruise to the moon, the service module would be used to turn the spacecraft around and direct it back to Earth.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Obviously, Apollo 13 was well past the point where the launch aborts could be used.  The problem with using Direct Abort Trajectory was that the condition of the service module was unknown and &lt;a href=&quot;https://space.stackexchange.com/a/2145/415&quot;&gt;it would have required jettisoning the LM&lt;/a&gt;. Since the ship and crew were almost into the zone of lunar influence, they re-established a &lt;a href=&quot;http://en.wikipedia.org/wiki/Free-return_trajectory&quot; rel=&quot;noreferrer&quot;&gt;free return trajectory&lt;/a&gt; to use the moon's gravity to return them to Earth.  You can read more at &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_13#Crew_survival_and_return_journey&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia's write up&lt;/a&gt; about the successful return of the crew.&lt;/p&gt;&#xA;" OwnerUserId="36" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-06-03T10:29:51.883" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="1295" PostTypeId="1" AcceptedAnswerId="1310" CreationDate="2013-08-16T15:12:39.810" Score="115" ViewCount="19909" Body="&lt;p&gt;Every rocket launch has a countdown. But what purpose does it serve?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2015-02-01T00:29:44.300" LastActivityDate="2023-03-22T19:50:33.667" Title="What is the purpose of having a countdown during a rocket launch?" Tags="|launch|rockets|launch-sequence|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1296" PostTypeId="5" CreationDate="2013-08-16T15:16:46.793" Score="0" Body="&lt;p&gt;Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. It has no natural satellite. It is named after the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows. Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus is a terrestrial planet and is sometimes called Earth's &quot;sister planet&quot; because of their similar size, mass, proximity to the Sun and bulk composition. It is radically different from Earth in other respects. It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planet's surface is 92 times that of Earth's. With a mean surface temperature of 735 K (462 °C; 863 °F), Venus is by far the hottest planet in the Solar System, even though Mercury is closer to the Sun. Venus has no carbon cycle that puts carbon into rock, nor does it seem to have any organic life to absorb carbon in biomass. Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. It may have had oceans in the past, but these would have vaporized as the temperature rose due to a runaway greenhouse effect. The water has most probably photodissociated, and, because of the lack of a planetary magnetic field, the free hydrogen has been swept into interplanetary space by the solar wind. Venus's surface is a dry desertscape interspersed with slab-like rocks and periodically refreshed by volcanism.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venus&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-01T08:58:07.147" LastActivityDate="2015-08-01T08:58:07.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1297" PostTypeId="4" CreationDate="2013-08-16T15:16:46.793" Score="0" Body="Questions regarding the second planet from the Sun." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-16T15:45:26.927" LastActivityDate="2013-08-16T15:45:26.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1298" PostTypeId="2" ParentId="1295" CreationDate="2013-08-16T15:19:57.023" Score="7" Body="&lt;p&gt;How else would you do it? Choose a random time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Really, the purpose of a countdown is to make sure all the engineers know &lt;em&gt;exactly&lt;/em&gt; when liftoff should be, and how much time they have to make last minute adjustments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a surprising number of things that have to work together, and a countdown allows engineers to start those things working at the right time. A schedule/checklist follows these all, and interestingly, there is an allotment of &quot;time reserve&quot; outside the countdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In essence, normally when you hear &quot;the countdown has been stopped&quot; your gut feeling would be &quot;something went really bad&quot;, but the reality is each countdown is stopped a few times normally - these pauses are planned. They allow for these last-minute adjustments that tests have shown to be required; they allow re-synchronizing the countdown in case some tasks took longer than expected, delaying others in consequence. They often overlap lengthy procedures like filling the fuel tanks, but some of them are pauses on their own. The countdown sequence is not written in stone - sure the launch must fit in the launch window (a couple hours) and there's very little deviation allowed during the final seconds, but the countdown a couple hours before the launch is more of guidelines &quot;this should be done about now&quot; than &quot;T-16:36:58.07: flight captain uses a spoon to break the shell of the hard-boiled egg of his breakfast&quot; type of lists.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="103" LastEditDate="2013-08-16T23:41:26.313" LastActivityDate="2013-08-16T23:41:26.313" CommentCount="6" CommunityOwnedDate="2013-08-16T15:35:59.663" ContentLicense="CC BY-SA 3.0" />
  <row Id="1300" PostTypeId="1" CreationDate="2013-08-16T17:03:00.990" Score="13" ViewCount="1911" Body="&lt;p&gt;Technology has apparently advanced to the point some states in the Untied States are debating &lt;a href=&quot;http://en.wikipedia.org/wiki/Google_driverless_car&quot;&gt;AI managed cars&lt;/a&gt;. The thought comes to mind that space is relatively less crowded than most streets on Earth. An AI system could probably be tested out there with less risk than down here. Of course the same argument may be used to test AI systems in the oceans rather than on roads .. Apparently on most modern jetliners too in-flight controls are managed by a computer bank. But that's a different subject... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;AI having been proven to be mature enough to handle atleast traffic patterns in a city, the questions that come to mind are - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What systems/programs in a mission may be considered for delegation to AI?&lt;/li&gt;&#xA;&lt;li&gt;Is there any move to use AI onboard systems in the space program?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="12102" LastEditDate="2023-10-27T19:30:20.047" LastActivityDate="2023-11-30T23:03:04.543" Title="Do any systems in space use Artificial Intelligence yet?" Tags="|technology|flight-computer|uncrewed-spaceflight|artificial-intelligence|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="1301" PostTypeId="2" ParentId="1295" CreationDate="2013-08-16T17:25:19.723" Score="33" Body="&lt;p&gt;There is a long list of things that have to happen to launch a rocket after it gets to the pad. Fueling, engine gimbal tests, sensor checks, computer start ups, etc. There's a certain order these have to go in. A countdown provides the backbone for the planning of all this. Don't think of it so much as &amp;quot;time until launch,&amp;quot; but rather &amp;quot;progress through the checklist.&amp;quot; Sure, the launch does happen at T-0, but in some launches, when T-0 actually could change if they were to hit a problem earlier in the countdown.&lt;/p&gt;&#xA;" OwnerUserId="554" LastEditorUserId="46507" LastEditDate="2022-05-04T10:43:58.117" LastActivityDate="2022-05-04T10:43:58.117" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="1302" PostTypeId="1" AcceptedAnswerId="1305" CreationDate="2013-08-16T17:28:42.820" Score="45" ViewCount="7562" Body="&lt;p&gt;When Neil Armstrong and Buzz Aldrin descended to the moon in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Module&quot;&gt;Lunar Module&lt;/a&gt;, Michael Collins stayed up in lunar orbit in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot;&gt;Command Module&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did they need someone to stay behind?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point, rendezvous and docking with an unmanned spacecraft in orbit had &lt;a href=&quot;http://en.wikipedia.org/wiki/Agena_target_vehicle&quot;&gt;already been done&lt;/a&gt;.  Was there something else they needed someone up there for?&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2016-09-23T12:29:21.353" Title="Why did Apollo 11 need someone to stay behind in lunar orbit?" Tags="|apollo-program|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1303" PostTypeId="2" ParentId="1302" CreationDate="2013-08-16T17:30:10.823" Score="8" Body="&lt;p&gt;If the Lander had a problem, the CSM needed to be able to accommodate unusual docking circumstances. Sure, they had done unmanned docking before, but the computer on the CSM was not exactly great by today's standards.&lt;/p&gt;&#xA;" OwnerUserId="554" LastActivityDate="2013-08-16T17:30:10.823" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="1304" PostTypeId="2" ParentId="1300" CreationDate="2013-08-16T17:40:27.770" Score="8" Body="&lt;p&gt;There are already many systems that fall under the umbrella of Artificial Intelligence in use in space (and on earth). Here are two examples:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The Mars rovers (&lt;a href=&quot;http://marsrovers.jpl.nasa.gov/technology/is_autonomous_mobility.html&quot;&gt;http://marsrovers.jpl.nasa.gov/technology/is_autonomous_mobility.html&lt;/a&gt;) have rudimentary intelligence which allows them to navigate the planet autonomously, since the distance between Earth and Mars is too great to drive by remote control. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There is also a robot aboard the ISS right now, called Robonaut (&lt;a href=&quot;http://robonaut.jsc.nasa.gov/ISS/&quot;&gt;http://robonaut.jsc.nasa.gov/ISS/&lt;/a&gt;). This robot will be continuously improved and upgraded throughout the missions.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, if you are wanting to know when HAL 9000 will appear Soyuz spacecraft, the answer is probably not within our lifetimes...&lt;/p&gt;&#xA;" OwnerUserId="553" LastActivityDate="2013-08-16T17:40:27.770" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1305" PostTypeId="2" ParentId="1302" CreationDate="2013-08-16T18:33:26.213" Score="48" Body="&lt;p&gt;This is an outcome of several requirements and constraints:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &lt;strong&gt;crew size was fixed at three astronauts very early&lt;/strong&gt; (on 28-29 July 1960, in a large planning conference).&#xA;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Space Task Group's] &quot;Richard S. Johnston presented three demands: &quot;shirt-sleeve&quot; environment, &lt;strong&gt;three-man crew&lt;/strong&gt;, and radiation protection. He discussed the need of the crews for a safe environment and for atmospheric control.&quot; (&lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4205/ch1-6.html&quot;&gt;source&lt;/a&gt;)&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;The mission design switched to Lunar-Orbit Rendezvous (thanks to work by &lt;a href=&quot;https://en.wikipedia.org/wiki/John_Houbolt&quot;&gt;John Houbolt&lt;/a&gt;), and the Lunar Excursion Module &lt;strong&gt;could have the mass budget only for two astronauts&lt;/strong&gt;.&lt;/p&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;By the end of 1961, the newly named Manned Spacecraft Center had virtually swung over to the lunar-orbit rendezvous idea. &lt;a href=&quot;https://en.wikipedia.org/wiki/Robert_R._Gilruth&quot;&gt;Gilruth&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Maxime_Faget&quot;&gt;Faget&lt;/a&gt;, and the other Apollo planners conceded that this approach had drawbacks: a successful rendezvous with the mother craft after the bug left the lunar surface was an absolute necessity, and only two of the three crew members would be able to land on the moon.&quot; (&lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4205/ch3-5.html&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;The docking argument is not really valid&lt;/strong&gt; since CSM's Apollo Guidance Computer (AGC) could receive commands directly from Mission Control and hence, could maintain fixed attitude required for docking without any intervention from the lunar module crew. (&lt;em&gt;I read it somewhere in the handbooks&lt;/em&gt; - see &lt;a href=&quot;http://www.ibiblio.org/apollo/links.html&quot;&gt;http://www.ibiblio.org/apollo/links.html&lt;/a&gt;, see also letters from Grumman to ASPO cited &lt;a href=&quot;http://www.astronautix.com/craft/csmcking.htm&quot;&gt;here&lt;/a&gt;)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;However, the overall doctrine during the Apollo program hinged on using &lt;strong&gt;crew members as backup for computers&lt;/strong&gt;. If computers or communications failed, command module pilot would be instrumental in mission success. (A sextant, a slide rule, pencil and paper were provided for manual navigation).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Shea and his staff reviewed these studies and presented the results to the rest of the manned space flight organization early in October [1962]. The contractors agreed that either two-man direct flight or earth-orbit rendezvous was feasible but both were less attractive than lunar rendezvous because &lt;strong&gt;the probability for mission success was lower&lt;/strong&gt;, the first landing would be later, and the developmental complexity would be greater. &lt;strong&gt;The vote was still for three-man, lunar-orbit rendezvous&lt;/strong&gt;.&quot; (&lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4205/ch4-4.html&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-09-24T04:23:28.817" LastActivityDate="2014-09-24T04:23:28.817" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1306" PostTypeId="2" ParentId="1238" CreationDate="2013-08-16T18:59:53.330" Score="5" Body="&lt;p&gt;Your main problem is that dolphins breathe air. Normally they come to the surface or even jump out of it and take a breath before submerging again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How will they do this in space?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If you have a large capsule full of water, there will be no air.&lt;/li&gt;&#xA;&lt;li&gt;If you add in some air, will it stay in one place, or in one large blob so that the dolphin could breathe it? Or would it split up into bubbles each time your hapless pet swam around? &lt;/li&gt;&#xA;&lt;li&gt;If you have a very large ball of water in the middle of a larger sphere (and work out how to keep it from touching the sides and adhering) a dolphin could jump through the surface tension, take a breath, and...hit the side of the capsule!&lt;/li&gt;&#xA;&lt;li&gt;Maybe have a cylindrical capsule and spin it to provide centripetal force to 'stick' the water to the sides so the dolphin could jump. That would work, I guess. &lt;/li&gt;&#xA;&lt;li&gt;Or fit Flipper with SCUBA gear? Little bit extravagant though - how about a fish?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If you want instead to keep the dolphin in air you would need to sort a different range of problems, including how to moisturise their skin, provide them enough water to drink etc., but really the main issue will be protecting them from the major injuries they will incur when bashing into the walls all the time as it won't be able to change direction in air.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2013-08-16T20:48:22.837" LastActivityDate="2013-08-16T20:48:22.837" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1308" PostTypeId="1" AcceptedAnswerId="1311" CreationDate="2013-08-16T19:37:49.473" Score="49" ViewCount="26543" Body="&lt;p&gt;Spacecraft rotating to generate artificial gravity through &quot;centrifugal force&quot; are commonplace in science fiction but not in reality. Considering the problems in long missions (&lt;a href=&quot;https://en.wikipedia.org/wiki/Effect_of_spaceflight_on_the_human_body#The_effects_of_weightlessness&quot;&gt;among others:&lt;/a&gt; bone loss, muscle loss, fluid redistribution, permanent visual impairment, feet disease, lowered immune defence, increased growth of dangerous microbes like ecoli, increased exposure of eyes and skin and lungs to freefloating microbes and dust), why isn't this method used?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, wouldn't artificial gravity (radial acceleration) simplify the design and increase the reliability of technical equipment which use moving parts, flows of fluids or gasses or need to distribute heat? Wouldn't eliminating microgravity eliminate the cause of all these severe and very diverse problems which today have no other working solution?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that a large structure with moving parts as in &lt;em&gt;2001&lt;/em&gt; is currently unfeasible, but how about a simple tether with the craft and a counterweight at the ends, like bolas? Does it interfere with communications or require the craft/tether to be much stronger?&lt;/p&gt;&#xA;" OwnerUserId="555" LastEditorUserId="1457" LastEditDate="2014-11-25T10:48:10.427" LastActivityDate="2021-03-17T14:24:02.200" Title="Why are there no spacecraft rotating for artificial gravity?" Tags="|artificial-gravity|spacecraft|" AnswerCount="14" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="1309" PostTypeId="2" ParentId="1295" CreationDate="2013-08-16T20:01:41.540" Score="13" Body="&lt;p&gt;Countdown is more than the &quot;60&quot;, &quot;30 seconds&quot;, 10,9...1  we see on TV, or hear on the PA adjacent to the launch site. Countdown commences well before the launch date with regular checkpoints for various systems and parts on the schedule as Thomas Tarrants mentions in his answer. It is also a process that may be forked - more on that below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to verification countdown also allows resolution of issues identified. Say diagnostics on system X report an error at T-24 Hours. The error may be of such a nature as not to interfere with the rest of the systems. Then the system owner for X may recommend continuing verification (countdown till T-23 Hours i.e. 1 hour to identify, and fix the fault) to hold at T-23 Hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This page on the NASA site explains how countdowns work - &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/launch/countdown101.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/mission_pages/shuttle/launch/countdown101.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember that the rocket/spacecraft looks to us laymen to be a monolith; In reality it is thousands of parts, and systems working together. &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="7025" LastEditDate="2015-09-27T19:40:44.497" LastActivityDate="2015-09-27T19:40:44.497" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1310" PostTypeId="2" ParentId="1295" CreationDate="2013-08-16T20:35:24.073" Score="77" Body="&lt;p&gt;In addition to the answers that have been given, it should be noted that there is a window of opportunity for a launch to occur, known as a launch window. This certainly occurs when trying to get somewhere such as the International Space Station, but even earth orbiting satellites have a window of time where the rocket can meet its performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the exact time of launch is very important, and a complex sequence of events have to happen in just the right order to make it happen. &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-generic-countdown-timeline.html&quot;&gt;SpaceFlightNow&lt;/a&gt; provided a generic countdown timeline for the Falcon 9, I'll re-post a few of the highlights. Some of these things are done to keep the batteries topped off till the last minute. You don't want to put the fuel in until the last minute, to save it (Most rocket fuel is cryogenic). When the rocket has its fuel loaded, you want to keep non-essential personnel away from it, making it so any tasks they need to do is put off until the end. You don't want to arm the system to launch until you are completely ready, thus, it only happens right before. There are many more such examples of why things are done in a particular order, but hopefully this will give you a sense.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;L-13:30:00  Dragon Power Up     &#xA;L-7:30:00   Launch Vehicle Power Up     &#xA;L-3:50:00   Liquid Oxygen Loading   &#xA;L-3:40:00   Rocket Propellant 1 Loading     &#xA;L-1:00:00   Weather Briefing    &#xA;T-0:09:40   Verify that Terminal Countdown has started  &#xA;T-0:09:30   Merlin 1C: Lox Bleeder Valves Open  &#xA;T-0:07:00   Dragon to Internal Power        &#xA;T-0:04:46   Stage 1 &amp;amp; Stage 2 Auto Sequence starts  &#xA;T-0:04:30   Transfer to Internal Power  &#xA;T-0:04:10   Vehicle Release Auto Sequence   &#xA;T-0:03:40   TEA-TEB Ignition System Activation  &#xA;T-0:03:25   Flight Termination System to Internal Power     &#xA;T-0:03:11   Flight Termination System Armed     &#xA;T-0:03:02   LOX Topping Termination     &#xA;T-0:03:00   Second Stage Thrust Vector Actuator Test        &#xA;T-0:02:00   Range Verification  &#xA;T-0:01:30   Final Engine Chilldown, Pre-Valves/Bleeders Open    &#xA;T-0:01:00   Flight Computer to start-up         &#xA;T-0:00:50   First Stage Thrust Vector Actuator Test     &#xA;T-0:00:40   Propellant Tank Pressurization  &#xA;T-0:00:20   All Tanks at Flight Pressure    &#xA;T-0:00:15   Arm Pyrotechnics    &#xA;T-0:00:03   Merlin Engine Ignition  &#xA;T-0:00:00   LIFTOFF&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6214" LastEditDate="2014-11-06T08:18:58.957" LastActivityDate="2014-11-06T08:18:58.957" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1311" PostTypeId="2" ParentId="1308" CreationDate="2013-08-16T20:56:06.757" Score="33" Body="&lt;p&gt;First off, it really isn't artificial gravity, it is radial acceleration (often known as centripetal force) - &lt;a href=&quot;https://space.stackexchange.com/a/288/38&quot;&gt;AlanSE's answer&lt;/a&gt; on the 'Size and rotation of a station' question describes how radial acceleration is very different to gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The major reason this is unlikely for some time is that in order for radial acceleration to feel like gravity, you need a very large diameter. Over 200 metres according to his calculations. This presents a couple of major problems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Getting all the materials to orbit. A 200m diameter station or spacecraft will weigh a lot!&lt;/li&gt;&#xA;&lt;li&gt;Building it securely so punctures from space debris don't cause major problems&lt;/li&gt;&#xA;&lt;li&gt;Building a stationary (or contrarotating) hub for comms antenna/docking etc will require spinning seals...complex and likely to fail&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Basically, we could do it - it would just be incredibly expensive and dangerous.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-08-16T20:56:06.757" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1312" PostTypeId="2" ParentId="1300" CreationDate="2013-08-16T23:39:46.590" Score="5" Body="&lt;p&gt;It depends entirely on what you qualify as 'artificial intelligence'.  I'm not aware of any self-learning systems in space to the point of worrying about SkyNet, but there are plenty of ones that are built to be more autonomous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, on STEREO, if the spacecraft detects something interesting, it'll take additional images, and if there's sufficient time in the next DSN passes, it'll be downlinked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the earth-observing realm, there's the &lt;a href=&quot;http://ase.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Autonomous Sciencecraft Experiment&lt;/a&gt; which uses analysis software to detect interesting events (eg, erupting volcano) and focus instruments there.  Their website says the spacecraft does 'machine learning' so it's possible that it might qualify as AI.  The software from ASE (&lt;a href=&quot;http://nmp.nasa.gov/st6/TECHNOLOGY/sciencecraft_tech.html&quot; rel=&quot;noreferrer&quot;&gt;Sciencecraft&lt;/a&gt;) was also used on the Mars Rovers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, NASA stopped funding the AISR program years ago, which was for developing low TRL technology that could be used to do better science.  There's still an &lt;a href=&quot;http://trotsky.arc.nasa.gov/~pgazis/News/Workshops.html&quot; rel=&quot;noreferrer&quot;&gt;archive of past PI presentations&lt;/a&gt;, and you can try digging through &lt;a href=&quot;http://web.archive.org/web/%2a/http://aisrp.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;archive.org&lt;/a&gt; for more info.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(disclaimer: I work for the Solar Data Analysis Center which manages the STEREO Science Center, and I might have been on AISR grant review boards)&lt;/p&gt;&#xA;" OwnerUserId="46" LastActivityDate="2013-08-16T23:39:46.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1313" PostTypeId="2" ParentId="1308" CreationDate="2013-08-17T06:01:59.677" Score="7" Body="&lt;p&gt;There are paper missions that include a tether and simulated gravity in the hab, but the only long-term stays in space have been in LEO. The ISS was designed to be a microgravity research facility, so there's little motivation for a rotating design. There was a Japanese centrifuge module that was developed but never launched (&lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module&quot; rel=&quot;nofollow&quot;&gt;Centrifuge Accommodations Module&lt;/a&gt;). There was also an inflatable sleeping centrifuge that has been talked about.&lt;/p&gt;&#xA;" OwnerUserId="563" LastEditorUserId="1457" LastEditDate="2014-11-25T10:14:52.563" LastActivityDate="2014-11-25T10:14:52.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1314" PostTypeId="1" CreationDate="2013-08-17T08:37:06.510" Score="13" ViewCount="5244" Body="&lt;p&gt;While I realise that &quot;frequently&quot; is relative, why exactly do &lt;a href=&quot;http://en.wikipedia.org/wiki/Reaction_wheel&quot;&gt;reaction wheels&lt;/a&gt; fail so frequently? Is it simply wear and tear? What happens to their mechanisms?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the recent Kepler failure, how were NASA going about trying to fix the wheel remotely?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's possible to include redundancy by adding a fourth wheel in tetrahedral formation, can't more points be added for additional redundancy?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="80" LastEditDate="2013-08-17T08:44:01.887" LastActivityDate="2018-10-11T01:28:26.190" Title="Why do reaction wheels fail so &quot;frequently&quot;?" Tags="|failure|reaction-wheel|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1315" PostTypeId="2" ParentId="1314" CreationDate="2013-08-17T09:17:05.860" Score="9" Body="&lt;p&gt;Reaction wheels consist of an electric motor attached to a flywheel. There are two causes of failure: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;mechanical&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;electrical&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;MECHANICAL FAILURES&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both the flywheels and the motor can be damaged by the G-forces and the vibration caused during launch. Once in space, lubricating the bearing is almost impossible which leads to increase in friction and eventually to failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;ELECTRONIC FAILURE&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in orbit or in space radiation possesses a greater threat to the electronic circuits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Example:&lt;/strong&gt;&lt;br&gt;&#xA;In the case of Kepler, one of the four reaction wheels was lost because of &lt;a href=&quot;http://spaceflightnow.com/news/n1304/11kepler/&quot; rel=&quot;noreferrer&quot;&gt;erratic friction&lt;/a&gt; over several months.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In an exercise of caution, mission managers switched off Kepler's reaction wheels for 10 days in January, hoping the break would redistribute lubricant inside the wheel assemblies, reducing friction and allowing the units to cool down. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That did not happen: it actually showed increased friction after the shutdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But hopefully the two remaining reaction wheels are working properly.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="897" LastEditDate="2014-12-19T11:30:39.530" LastActivityDate="2014-12-19T11:30:39.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1316" PostTypeId="1" AcceptedAnswerId="1318" CreationDate="2013-08-17T10:32:30.753" Score="12" ViewCount="4670" Body="&lt;p&gt;Are all rocket launch sites surrounded by water bodies (such as sea)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, what is the purpose of having a rocket launch site near a water body?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-17T11:08:43.123" LastActivityDate="2018-02-03T17:45:20.577" Title="Are all rocket launch sites surrounded by water bodies?" Tags="|launch|rockets|launch-site|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1317" PostTypeId="2" ParentId="1316" CreationDate="2013-08-17T11:01:12.450" Score="5" Body="&lt;p&gt;The question may be re-phrased to read &#xA; - Where should a space launch site be located?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just like a car with an IC engine, a spacecraft is really riding a controlled explosion. Therefore it makes sense to reduce the risk at launch - hence, distant from centres of population. An example of what happens when control is lost may be &lt;a href=&quot;http://www.bbc.co.uk/news/world-us-canada-29812139&quot; rel=&quot;nofollow&quot;&gt;viewed here&lt;/a&gt; A water body such as a sea/ocean is a preference, but there are also space stations located elsewhere E.g. in a desert.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A list of active launch sites is available &lt;a href=&quot;http://www.spacetoday.org/Rockets/Spaceports/LaunchSites.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;&#xA;For instance, Baikonur &amp;amp; Jiquan are situated in the interior&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-10-29T05:10:12.690" LastActivityDate="2014-10-29T05:10:12.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1318" PostTypeId="2" ParentId="1316" CreationDate="2013-08-17T11:01:31.543" Score="20" Body="&lt;p&gt;No, not surrounded by water at least. There are of course advantages to having a body of water nearby, for example in an event of a larger fire the fire fighting helicopters and airplanes would have a shorter flight between the fire and where they scoop the water from. But that doesn't necessitate the water body completely surrounding the launch site. In fact, there are many launch sites that don't have their entire perimeter surrounded by water, for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikonur_Cosmodrome&quot; rel=&quot;noreferrer&quot;&gt;Baikonur Cosmodrome&lt;/a&gt; in Kazakhstan (which does have a large &lt;a href=&quot;https://en.wikipedia.org/wiki/Syr_Darya&quot; rel=&quot;noreferrer&quot;&gt;Syr Darya&lt;/a&gt; river relatively close by, but is not surrounded by it):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/uHZ12.jpg&quot; alt=&quot;Aerial photograph of the Baikonur Cosmodrome launch site area&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Aerial photograph of the Baikonur Cosmodrome launch site area (source: &lt;a href=&quot;https://maps.google.com/maps?q=Baikonur%20Cosmodrome&amp;amp;ll=45.846978,63.410339&amp;amp;spn=0.810236,1.290894&amp;amp;hnear=Baikonur%20Cosmodrome,%20Kazakhstan&amp;amp;t=h&amp;amp;z=10&quot; rel=&quot;noreferrer&quot;&gt;Google Maps&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a clear need though to have a wide degree arc of land oriented towards the East that is unpopulated, in case of an in-air launch failure, and to have easy access to first rocket stages and boosters falling / landing back to Earth. That is due to the Earth's rotation, and for the same reason the rockets that are launched appear to have a radial path moving away from the launch site towards the East.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's better having such area mostly of water surfaces, or merely unpopulated land, is however debatable and depends on launch operations' ability to retrieve objects that have fallen back to Earth. Obviously, you wouldn't want a large forest area, to avoid fires caused by falling debris and rocket stages, in case their landing parachute didn't deploy properly. You'd also want easy access to all areas of this arc, which might prove easier on water, than on land, if there is not enough access roads, the land is overgrown, marshy, or any other reason why you wouldn't be able to access sites of fallen debris easy.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-17T11:17:40.477" LastActivityDate="2013-08-17T11:17:40.477" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1319" PostTypeId="1" AcceptedAnswerId="1689" CreationDate="2013-08-17T14:54:26.933" Score="14" ViewCount="2063" Body="&lt;p&gt;According to the &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=3769&quot; rel=&quot;noreferrer&quot;&gt;article on the Cassini Solstice Mission website&lt;/a&gt; dating back to December 9, 2009:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The six-sided shape remains a mystery. Scientists think the hexagon is&#xA;  a meandering jet stream at 77 degrees north latitude, but they don't&#xA;  know what controls the path the stream takes. These images also show&#xA;  new phenomena for scientists to decipher, such as waves that can now&#xA;  be seen radiating from the corners of the hexagon where the jet takes&#xA;  its hardest turns. These images confirm the presence of a multi-walled&#xA;  structure in each of the hexagon's six sides, and the structure now&#xA;  can be seen extending to the top of Saturn's cloud layer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And the article is equipped with links to the movie of the hexagon jet stream:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Yo0Bm.gif&quot; alt=&quot;Saturn&amp;#39;s north pole persistent hexagon jet stream animation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;This movie from Cassini, made possible only as Saturn's north pole emerged from &lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;winter darkness, shows new details of a jet stream that follows a hexagon-shaped &lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;path and has long puzzled scientists. (Source: &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=3769&quot; rel=&quot;noreferrer&quot;&gt;NASA JPL&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the time passed, we now have more observational data and more photographs of this phenomena, than we had back in 2009. For example this photograph of Saturn's stormy north was &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=4875&quot; rel=&quot;noreferrer&quot;&gt;published&lt;/a&gt; on August 5, 2013:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/IPbKN.jpg&quot; alt=&quot;Saturn&amp;#39;s persistent hexagon jet stream&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;This view is centered on Saturn's north pole. North is up and rotated 33 degrees to the left. The image &lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;was taken with the Cassini spacecraft wide-angle camera on June 14, 2013 using a spectral filter &lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;sensitive to wavelengths of near-infrared light centered at 752 nanometers. (Source: &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=4875&quot; rel=&quot;noreferrer&quot;&gt;NASA JPL&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Saturn's north polar hexagon appears to be a long-lived feature of the&#xA;  atmosphere, having been spotted in images of Saturn in the early&#xA;  1980s, again in the 1990s, and then by the Cassini spacecraft in the&#xA;  past several years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Source: The Persistent Hexagon, October 8, 2008, &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=3254&quot; rel=&quot;noreferrer&quot;&gt;NASA JPL Cassini Solstice Mission&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, have we by now (since that article in 2009) been able to decipher the reason behind these mysterious hexagonal jet streams around Saturn's north pole, and what is it? Please back any possible theories with links to actual data and references.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="25" LastEditDate="2013-09-24T15:35:07.377" LastActivityDate="2013-09-24T15:35:07.377" Title="What is the cause of Saturn's north pole persistent hexagon jet streams?" Tags="|planetary-science|weather|saturn|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1320" PostTypeId="2" ParentId="894" CreationDate="2013-08-17T15:28:00.953" Score="23" Body="&lt;p&gt;The material used for the seal is silicone rubber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example materials considered for NDS iLIDS are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Parker S0383-70 (as part of Gask-O-Seal product)&lt;/li&gt;&#xA;&lt;li&gt;Esterline ELA-SA-401&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Silicone rubber is the only class of space flight-qualified elastomeric seal material that functions across the expected temperature range. NASA Glenn has tested three silicone elastomers for such seal applications: two provided by Parker (S0899-50 and S0383-70) and one from Esterline (ELA-SA-401). The effects of atomic oxygen (AO), UV and electron particle radiation, and vacuum on the properties of these three elastomers were examined. Critical seal properties such as leakage, adhesion, and compression set were measured before and after simulated space exposures. The S0899-50 silicone was determined to be inadequate for extended space seal applications due to high adhesion and intolerance to UV, but both &lt;strong&gt;S0383-70&lt;/strong&gt; and &lt;strong&gt;ELA-SA-401&lt;/strong&gt; seals were adequate.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://hdl.handle.net/2060/20100029591&quot;&gt;Space Environment Effects on&#xA;Silicone Seal Materials&lt;/a&gt; (PDF)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The metal for the interface is aluminum. NDS IDD specifies MIL-DTL-5541 Type 1, Class 3 chemical conversion coating (containing hexavalent chromium Cr (VI)) over the metal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;iLIDS is spec'ed to withstand 70 docking/undocking cycles in passive mode (20 on the ground and 50 in space), and 24 cycles in active mode (on a spacecraft) (20 on the ground, 4 in space).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Leak limit is specified as follows:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;iLIDS-to-iLIDS: maximum of 0.0011 kg dry air/day leakage at vestibule pressurization of 101 kPa and an external vacuum pressure when mated.&lt;/li&gt;&#xA;&lt;li&gt;iLIDS-to-vehicle: maximum of 0.0004 kg dry air/day leakage at vestibule pressurization of 101 kPa and an external vacuum pressure when mated.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The leakage rate is determined by two active factors:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Atomic oxygen in LEO&lt;/li&gt;&#xA;&lt;li&gt;Ultraviolet radiation from the Sun&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110020556_2011018028.pdf&quot;&gt;International Low Impact Docking System (iLIDS) Technical Requirements Specification&lt;/a&gt; (PDF)&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-17T15:38:20.933" LastActivityDate="2013-08-17T15:38:20.933" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1321" PostTypeId="1" AcceptedAnswerId="1322" CreationDate="2013-08-17T16:14:07.430" Score="8" ViewCount="899" Body="&lt;p&gt;According to &lt;a href=&quot;http://marsmobile.jpl.nasa.gov/msl/mission/technology/insituexploration/edl/guidedentry/&quot;&gt;this article&lt;/a&gt;, the landing site of the Curiosity rover was elliptical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why an ellipse?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would all planetary landers have elliptical landing sites?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-17T16:44:44.163" LastActivityDate="2013-11-24T05:13:40.883" Title="Why was Curiosity rover's landing site an ellipse?" Tags="|curiosity|landing|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1322" PostTypeId="2" ParentId="1321" CreationDate="2013-08-17T16:41:05.320" Score="7" Body="&lt;p&gt;Landing ellipse, or a &lt;a href=&quot;https://en.wikipedia.org/wiki/Landing_footprint&quot; rel=&quot;nofollow noreferrer&quot;&gt;landing footprint&lt;/a&gt; is a plot of the area encompassing possible landing sites when reentry parameters are entered into a simulation and accounted for their margin of error, and is a &lt;a href=&quot;http://mathworld.wolfram.com/BivariateNormalDistribution.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;bivariate normal distribution&lt;/a&gt;, a statistical distribution with probability density function. According to Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After atmospheric reentry, a non-powered spacecraft will land on an&#xA;  area depending upon entry angle, entry mass, atmosphere and drag. It&#xA;  is therefore impossible to know the spacecraft's landing point with&#xA;  absolute precision. By simulating varying re-entry courses, a numerical&#xA;  simulation will produce a plot resembling a footprint.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/kQ1tV.png&quot; alt=&quot;Probability density function&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;Probability density function&lt;/strong&gt;: Many samples from a multivariate normal distribution, shown along with the 3-sigma ellipse, the &lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;two marginal distributions, and the two 1-d histograms. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Multivariate_normal_distribution&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Multivariate normal distribution)&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A paper on &lt;a href=&quot;https://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4805259&quot; rel=&quot;nofollow noreferrer&quot;&gt;Determination of Spacecraft Landing Footprint for Safe Planetary Landing&lt;/a&gt; discusses this to greater detail:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A methodology is developed based on a ballistic analysis to estimate&#xA;  the landing footprint associated with the powered terminal descent&#xA;  phase of a spacecraft soft landing. The analysis is based on an&#xA;  idealized two-impulse thrust maneuver and leads to an analytical&#xA;  expression for the elliptical boundary of the landing footprint. The&#xA;  objective is to develop a computationally efficient method to estimate&#xA;  the landing footprint for use in an on-board fuzzy-logic based&#xA;  inference engine for real-time hazard avoidance. The inference engine&#xA;  combines an estimate of the landing footprint with information about&#xA;  the safeness of the landing terrain to construct an overall landing&#xA;  site quality index. The landing site quality index is a critical&#xA;  parameter that enables the spacecraft to make intelligent real-time&#xA;  decisions about landing safely on unknown and hazardous terrains. The&#xA;  footprint generated from the ballistic analysis is also compared with&#xA;  the footprint resulting from numerically integrating a representative&#xA;  guidance law.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/9ww0q.jpg&quot; alt=&quot;Apollo 11 landing ellipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Plot of the landing ellipse of the lunar landing  of the Apollo 11 Lunar Module. Actual landing spot is marked with the black rectangle.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, all the landing sites would have an elliptical plot of the area of possible landing sites, once the simulation is done and calculated for all the possible values and correcting for an accounted for margin of error.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-17T22:02:00.847" LastActivityDate="2013-08-17T22:02:00.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1323" PostTypeId="1" CreationDate="2013-08-17T18:18:29.237" Score="1" ViewCount="1404" Body="&lt;ul&gt;&#xA;&lt;li&gt;The approximate volume of water on Earth is &lt;a href=&quot;http://en.wikipedia.org/wiki/Water#cite_note-b1-5&quot; rel=&quot;nofollow&quot;&gt;1.338 Billion cu.km&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Earth regularly intercepts a number of space objects as meteors&lt;/li&gt;&#xA;&lt;li&gt;During the course of their descent through the atmosphere some burn totally to ash&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Larger bodies may make it through the atmosphere burning off a part of their content which may remain part of the atmosphere, or make it to the surface.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Does a meteor (subject to it's composition) add to, or deduct from the water volume on Earth? &lt;/li&gt;&#xA;&lt;li&gt;What is the approximate volume of water acquired/lost annually to such extra-terrestrial bodies by Earth?&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-08-17T18:18:29.237" Title="Approximately how much water does Earth acquire/lose annually from extra-terrestrial sources?" Tags="|water|atmosphere|" AnswerCount="0" CommentCount="7" ClosedDate="2013-08-18T04:20:04.777" ContentLicense="CC BY-SA 3.0" />
  <row Id="1324" PostTypeId="1" AcceptedAnswerId="1328" CreationDate="2013-08-17T18:41:23.380" Score="9" ViewCount="184" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How many technology demonstration/certification unmanned flights does it take to man-rate a spacecraft in the US, Russia, and China?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What Loss-of-Crew probabilities are considered acceptable in those countries? (The question explicitly excludes the Space Shuttle) (please, provide the numbers)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastActivityDate="2015-01-11T20:24:41.273" Title="Flight testing before manned flights: what are various countries' requirements?" Tags="|safety|reliability|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1325" PostTypeId="1" AcceptedAnswerId="1379" CreationDate="2013-08-17T20:15:27.183" Score="39" ViewCount="15931" Body="&lt;p&gt;Since the very beginning of space exploration, rockets had some sort of Launch Escape System (LES). From &lt;a href=&quot;http://en.wikipedia.org/wiki/Launch_escape_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Wikipedia article&lt;/a&gt;, we know that Mercury and Apollo had an escape tower, while Vostok and Gemini had ejection seats. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zPavQ.jpg&quot; alt=&quot;Apollo LES pad abort test&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Apollo LES pad abort test (source: wikimedia.org)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initially, the Space Shuttle had ejection seats but, (citing Wikipedia)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;were removed once the vehicle was deemed operational&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;practically leaving it without any LES. What are technical reasons for this choice? &lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="23891" LastEditDate="2019-06-19T21:18:48.610" LastActivityDate="2019-06-19T21:18:48.610" Title="Why didn't the Space Shuttle have a launch escape system?" Tags="|launch|space-shuttle|launch-escape-system|failure|" AnswerCount="4" CommentCount="18" ContentLicense="CC BY-SA 4.0" />
  <row Id="1326" PostTypeId="5" CreationDate="2013-08-17T20:46:36.430" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-17T20:46:36.430" LastActivityDate="2013-08-17T20:46:36.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1327" PostTypeId="4" CreationDate="2013-08-17T20:46:36.430" Score="0" Body="Questions regarding fields of magnetisms and their effects on Space Exploration, such as the Earth's magnetic field." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-17T21:05:20.257" LastActivityDate="2013-08-17T21:05:20.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1328" PostTypeId="2" ParentId="1324" CreationDate="2013-08-17T21:11:27.940" Score="7" Body="&lt;p&gt;In practice, there were the following unmanned flights before a first manned attempt for each of the manned programs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The Russians:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Vostok:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;5 unmanned flights &lt;em&gt;Sputnik 4,5,6,9,10&lt;/em&gt;      &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Voskhod (quite similar to Vostok):&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Kosmos-47&lt;/em&gt; unmanned test spacecraft.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Soyuz: &#xA;&lt;ul&gt;&#xA;&lt;li&gt;2 unmanned &lt;em&gt;Kosmos-133&lt;/em&gt;, &lt;em&gt;Kosmos-140&lt;/em&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Soyuz-1&lt;/em&gt; a tragic disaster during the first manned attempt&lt;/li&gt;&#xA;&lt;li&gt;6 more unmanned after that: &lt;em&gt;Kosmos-186,188,212,213,238; Soyuz-2&lt;/em&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Soyuz T:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Kosmos 1001, Kosmos 1074&lt;/em&gt; without docking to Salyut&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Soyuz T-1&lt;/em&gt; unmanned with docking to Salyut&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Soyuz TM:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Soyuz TM-1 unmanned&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Soyuz-TMA:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;all manned, new ideas are tested on similar Progress cargo spacecrafts.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Salyut (a space station, not intended for a manned ascend):&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Salyut-1&lt;/em&gt; flew for 5 days unmanned, then a manned &lt;em&gt;Soyuz&lt;/em&gt; docked, but the crew could non visit &lt;em&gt;Salyut-1&lt;/em&gt; (although they planned to do so). &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;TKS/VA: &#xA;&lt;ul&gt;&#xA;&lt;li&gt;3 unmanned &lt;em&gt;docked pairs of VA reentry capsules&lt;/em&gt; &lt;/li&gt;&#xA;&lt;li&gt;1 complete unmanned spacecraft&lt;/li&gt;&#xA;&lt;li&gt;3 complete spacecrafts launched without a crew, the crew came in from Salyut space stations.&lt;/li&gt;&#xA;&lt;li&gt;Never launched with a crew although certified for that.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mir&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The basic block is a continuation of Salyut lineage. The addon modules are essentially TKS without VA, so no additional unmanned flights before Mir have been performed.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Buran:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1 unmanned flight&lt;/li&gt;&#xA;&lt;li&gt;No flights after that.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The US manned programs:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1 successful &lt;em&gt;Big Joe&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;3 successful &lt;em&gt;Mercury-Redstone 1A,2(with a chimpanzee),BD&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;1 successful &lt;em&gt;Mercury-Atlas 2&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Gemini:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2 unmanned &lt;em&gt;Gemini 1,2&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2 unmanned &lt;em&gt;AS-201, AS-202&lt;/em&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;AS-204&lt;/em&gt; a tragic disaster during the first manned attempt&lt;/li&gt;&#xA;&lt;li&gt;3 more unmanned &lt;em&gt;Apollo 4,5,6&lt;/em&gt; after that&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Skylab:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Flew unmanned for 10 days, then it was visited by a manned Apollo.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The Chinese program:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Shenzhou:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;4 unmanned &lt;em&gt;Shenzhou 1 to 4&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Tiangong (a space station):&#xA;&lt;ul&gt;&#xA;&lt;li&gt;flew unmanned for 2 months&lt;/li&gt;&#xA;&lt;li&gt;visited by an unmanned &lt;em&gt;Shenzhou-8&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;flew unmanned for 6 more months, and then it was visited by a crew.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-18T00:18:48.477" LastActivityDate="2013-08-18T00:18:48.477" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1329" PostTypeId="2" ParentId="835" CreationDate="2013-08-17T22:04:04.737" Score="8" Body="&lt;p&gt;Talking about Jupiter, there was the &lt;a href=&quot;http://en.wikipedia.org/wiki/Galileo_%28spacecraft%29#Galileo_Probe&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo Probe&lt;/a&gt;, also referred to as Jupiter Atmosphere Probe, which was dropped into the atmosphere of Jupiter in 1995. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/flIj8.jpg&quot; alt=&quot;Galileo Probe mission&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its data is still the only data measured &lt;em&gt;in situ&lt;/em&gt; on any gas giant. There are profiles for wind speed, temperature and pressure, besides other parameters, up to a depth of about 160 km (compared to Jupiter's mean radius of almost 70,000 km). This is when the probe stopped working (@ 23 atm and 153 °C). The depth is given relative to a 1 bar pressure level, which was chosen as zero altitude. The message is, you can drop a probe into a gas giant's atmosphere, it was done before (once), but it is hell complicated and you can not go really deep before your probe dies of excessive heat and/or ambient pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides &lt;em&gt;in situ&lt;/em&gt; measurements, you have a fair number of options based on methods of remote sensing. Think of it in terms of what is done here on Earth for studying our atmosphere. Images in different spectral ranges can reveal processes at different depths, which is the most basic concept. The most relevant concepts in the context of your questions are radar or microwave soundings. Depending on the design of such an experiment, you can 'look' rather deep into an atmosphere. A classic idea in this field is the use of a &lt;a href=&quot;http://en.wikipedia.org/wiki/Radio_Science_Subsystem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radio Science Subsystem&lt;/a&gt; (RSS). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4CzhT.gif&quot; alt=&quot;Cassini RSS&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The picture illustrates one of the &lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instrumentscassinirss/&quot; rel=&quot;nofollow noreferrer&quot;&gt;most advanced experiments of this kind&lt;/a&gt; - the RSS on &lt;a href=&quot;http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini&lt;/a&gt;. A future experiment on Juno, the &lt;a href=&quot;http://juno.wisc.edu/spacecraft_instruments_MWR.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Microwave Radiometer&lt;/a&gt; (MWR), intends to directly look into the atmosphere. &lt;a href=&quot;http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012espc.conf..264J&amp;amp;link_type=EJOURNAL&quot; rel=&quot;nofollow noreferrer&quot;&gt;Depending on the source&lt;/a&gt;, it is designed to look as &quot;deep&quot; as 200 or even 1,000 bar. (It is a habit of atmospheric physicists to provide pressures instead of altitudes or depths. I can not find a good source for 'translating' this into a depth in metres for Jupiter.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is worth mentioning, that you could also work with &lt;a href=&quot;http://en.wikipedia.org/wiki/Lidar&quot; rel=&quot;nofollow noreferrer&quot;&gt;LIDARs&lt;/a&gt;, at least to some degree. Although &lt;a href=&quot;http://link.springer.com/chapter/10.1007/0-387-25101-4_13&quot; rel=&quot;nofollow noreferrer&quot;&gt;LIDARs are used on Earth satellites&lt;/a&gt;, such instruments have not fund their way into many space missions yet. Besides, you can always use measurements of the gravity field or magnetic field as 'secondary evidence' and build atmosphere models upon them. This gives you some insight on a rather large scale, if you do not have any other useful instruments. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Answering your question(s): &lt;strong&gt;Yes, there is technology. Yes, there is past and present research.&lt;/strong&gt; But, yes, there is also a lack of funding and therefore a lack of data. But you could basically investigate the spots as mentioned by you. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="12102" LastEditDate="2019-05-21T07:20:56.320" LastActivityDate="2019-05-21T07:20:56.320" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="1330" PostTypeId="1" AcceptedAnswerId="1331" CreationDate="2013-08-17T23:27:21.967" Score="10" ViewCount="3082" Body="&lt;p&gt;What kind of launch escape system will be aboard Dream Chaser?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="58" LastEditDate="2013-09-09T16:44:43.720" LastActivityDate="2014-09-18T20:26:46.547" Title="What kind of launch escape system (LES) does Dream Chaser have?" Tags="|launch|crewed-spaceflight|launch-escape-system|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1331" PostTypeId="2" ParentId="1330" CreationDate="2013-08-18T00:49:39.750" Score="8" Body="&lt;p&gt;Since &lt;a href=&quot;https://en.wikipedia.org/wiki/Dream_Chaser_%28spacecraft%29&quot;&gt;Dream Chaser&lt;/a&gt; is based on the &lt;a href=&quot;https://en.wikipedia.org/wiki/HL-20_Personnel_Launch_System&quot;&gt;HL-20&lt;/a&gt; lifting body design, I presume it would have same or similar Launch Escape System (LES). What follows are to the question the most relevant excerpts from the &lt;a href=&quot;http://www.researchgate.net/publication/24298984_Launch-pad_abort_capabilities_of_the_HL-20_lifting_body&quot;&gt;Launch Pad Abort of the HL-20 Lifting Body&lt;/a&gt; journal article (including diagrams). &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;JOURNAL OF GUIDANCE, CONTROL, AND DYNAMICS&lt;br /&gt;&#xA;Vol. 17, No. 6, November-December 1994&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://www.researchgate.net/publication/24298984_Launch-pad_abort_capabilities_of_the_HL-20_lifting_body&quot;&gt;Launch Pad Abort of the HL-20 Lifting Body&lt;/a&gt;&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The capability of the HL-20 lifting-body spacecraft to perform an&#xA;  abort maneuver from the launch pad to a horizontal landing was&#xA;  studied. This study involved both piloted and batch simulation models&#xA;  of the vehicle. A point-mass model of the vehicle was used for&#xA;  trajectory optimization studies. The piloted simulation was performed&#xA;  in a fixed-base simulator. A candidate maneuver was developed and&#xA;  refined for the worst-case launch-pad-to- landing-site geometry using&#xA;  an iterative procedure of off-line maneuver analysis followed by&#xA;  piloted evaluations and heuristic improvements to the candidate&#xA;  maneuver. The resulting maneuver demonstrates the launch site abort&#xA;  capability of the HL-20 and dictates requirements for nominal abort&#xA;  motor performance. The sensitivity of the maneuver to variations in&#xA;  several design parameters was documented. Introduction T 20,000-lb&#xA;  lifting body with a maximum subsonic lift-to-drag ratio of 4.3 (see&#xA;  Fig. 1) capable of being launched vertically into low Earth orbit with&#xA;  a crew of two and up to eight passengers using an expendable launch&#xA;  vehicle and of being landed horizontally following re-entry. Both&#xA;  manual and automatic landing capabilities are planned.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/mQfQO.png&quot; alt=&quot;Figure 1&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A 6700-1b adapter module will be used to connect the HL-20 to the&#xA;  launch vehicle. (See fig. 2.) This adapter design will include a&#xA;  launch escape system that is intended to thrust the HL-20 away from&#xA;  the booster in case of a malfunction either during the actual launch&#xA;  or on the pad prior to launch (on-pad abort). Acceleration levels on&#xA;  tile order of 8g's (lg = 32.2 ft/sec 2) would be required to propel&#xA;  the vehicle a safe distance away from a malfunctioning booster. After&#xA;  a specified time, the abort-motor thrust would drop to approximately&#xA;  lg for an additional specified amount of time to avoid excessive&#xA;  velocities and associated drag. The adapter module would be jettisoned&#xA;  following abort-motor burnout.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/w0VVI.png&quot; alt=&quot;Figure 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The capabilities of the HL-20 to successfully abort during the ascent&#xA;  phase of a launch have been treated in a separate study by Naftel and&#xA;  Taley; the on-pad-abort maneuver, performed in an emergency prior to&#xA;  ignition, is the subject of this paper. Since the initial lift-to-drag&#xA;  ratio precluded a glide to a nearby runway, original launch pad abort&#xA;  scenarios were similar to those used for earlier manned capsules,&#xA;  i.e., an abort to an ocean landing using a recovery parachute.&#xA;  Additional aerodynamic refinements of the HL-20 configuration led to&#xA;  increased subsonic lift-to-drag ratios and a higher performance launch&#xA;  escape system abort motor was specified.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Full text is available in the journal article: &lt;a href=&quot;http://www.researchgate.net/publication/24298984_Launch-pad_abort_capabilities_of_the_HL-20_lifting_body&quot;&gt;Launch-pad abort capabilities of the HL-20 lifting body&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-20T12:25:23.197" LastActivityDate="2013-08-20T12:25:23.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1332" PostTypeId="1" AcceptedAnswerId="1333" CreationDate="2013-08-18T03:35:14.783" Score="7" ViewCount="167" Body="&lt;p&gt;Recently, NASA &lt;a href=&quot;http://www.nasa.gov/content/nasa-ends-attempts-to-fully-recover-kepler-spacecraft-potential-new-missions-considered/&quot;&gt;declared the Kepler telescope 'unfixable'.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it doesn't seem to be the end of the telescope, as NASA appears to have asked the broader scientific community for suggestions on what to do with the disabled spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An engineering study will be conducted on the modifications required to manage science operations with the spacecraft using a combination of its remaining two good reaction wheels and thrusters for spacecraft attitude control.&#xA;  Informed by contributions from the broader science community in response to the call for scientific white papers announced Aug. 2, the Kepler project team will perform a study to identify possible science opportunities for a two-wheel Kepler mission.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Depending on the outcome of these studies, which are expected to be completed later this year, NASA will assess the scientific priority of a two-wheel Kepler mission. Such an assessment may include prioritization relative to other NASA astrophysics missions competing for operational funding at the NASA Senior Review board early next year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, it seems that the loss of 2 reactor wheels is significant. What kind of science could still be done with the Kepler telescope?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-18T04:47:23.717" LastActivityDate="2013-08-18T11:27:30.840" Title="What kind of science can be conducted with the crippled Kepler telescope?" Tags="|nasa|kepler-telescope|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1333" PostTypeId="2" ParentId="1332" CreationDate="2013-08-18T04:12:20.803" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.space.com/21173-kepler-alien-planet-mission-future.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler&lt;/a&gt; has successfully completed its three and half year primary mission. And even if two of its four reaction wheels are beyond help:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It's possible Kepler could still gather valuable data by switching to a scanning mode, as opposed to the&#xA;&amp;quot;point and stare&amp;quot; operations that defined its first four years in space. If neither failed reaction&#xA;wheel is recovered, NASA will carry out studies addressing possible new missions .&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.huffingtonpost.com/2013/07/25/kepler-space-telescope-nasa_n_3653455.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;We've got a lot of planetary candidates that we know when the transits occur,&amp;quot; he said. &amp;quot;We might be able to schedule a day or two days of precise pointing during that period and catch the next transit, and improve our knowledge of the planetary systems — maybe determine the mass of those systems, things like that.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;So it's not clear that planet hunting per se is off the table,&amp;quot; he added. &amp;quot;It's just a different style of hunting.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;In two or three weeks, the Kepler team aims to return the observatory to wheel control,&amp;quot; Sobeck said. The observatory's handlers will attempt to point Kepler using its reaction wheels, first coarsely and then, if that works, with much more precision.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;If all goes well, mission team members will then start considering seriously when to return the spacecraft to science operations — whatever goals those operations end up pursuing.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-18T11:27:30.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1334" PostTypeId="1" AcceptedAnswerId="1348" CreationDate="2013-08-18T08:10:29.003" Score="17" ViewCount="3990" Body="&lt;p&gt;&lt;a href=&quot;http://en.m.wikipedia.org/wiki/Atmosphere_of_Venus&quot;&gt;Atmosphere of Venus&lt;/a&gt; consists predominately of Carbon dioxide and Nitrogen. Is it possible to convert the atmospheric Carbon dioxide into Oxygen with the help of bacteria by photosynthesis, and make the planet habitable  also for humans, on top of other organisms that would be required to terraform it?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-18T11:43:38.163" LastActivityDate="2022-11-29T18:32:29.847" Title="Can Venus be made habitable?" Tags="|terraforming|venus|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1335" PostTypeId="1" AcceptedAnswerId="1337" CreationDate="2013-08-18T10:32:04.420" Score="3" ViewCount="310" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/questions/797/radiation-shielding-magnetic-or-mass-which-is-more-efficient/862#862&quot;&gt;answer&lt;/a&gt;, and I believe a few others, mention using water as a radiation shield. I looked around a bit, and did not find a good answer on the logistics of how you would do that. Ignoring the lift to space cost and assuming you have H&lt;sub&gt;2&lt;/sub&gt;O in sufficient quantity near your space station, a number of problems remain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would you position it for shielding? Unless you place your water in a heated and pressurized container, the water won't remain in liquid state and will freeze solid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could I erect a temporary pressure tent on the outside of my habitat, position the water to let it freeze, and move on to the section behind the raw and exposed block of ice serving as my radiation shield? Would I need to cover the ice with something, to keep it from boiling off?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-08-18T12:05:29.310" Title="Can you store exposed ice in the Earth's orbit?" Tags="|radiation|space-station|water|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1336" PostTypeId="1" AcceptedAnswerId="1826" CreationDate="2013-08-18T10:37:55.740" Score="33" ViewCount="59388" Body="&lt;p&gt;If using water or ice as a radiation shield how thick/deep would it need to be?  Would it make a difference if the water was frozen (water ice being less dense then liquid)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to know how much shielding would be required for an indefinite stay. Preferably at Terrestrial limits per Code of Federal Regulations (29 CFR 1910.96), but at least at levels per National Council on Radiation Protection and Measurements (NCRP) presented in its Report 98 A beginning place for research is the &lt;a href=&quot;http://srag.jsc.nasa.gov/Publications/TM104782/techmemo.htm&quot;&gt;Spaceflight Radiation Health Program at JSC&lt;/a&gt;  A listing of all different types of radiation &lt;a href=&quot;http://srag-nt.jsc.nasa.gov/SpaceRadiation/What/What.cfm&quot;&gt;NASA &quot;What is space radiation?&quot;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2013-09-06T10:12:10.323" LastActivityDate="2016-05-05T18:55:06.150" Title="What thickness/depth of water would be required to provide radiation shielding in Earth orbit?" Tags="|radiation|space-station|water|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1337" PostTypeId="2" ParentId="1335" CreationDate="2013-08-18T11:21:00.940" Score="4" Body="&lt;p&gt;If you leave ice exposed to vacuum then yes, it will sublime. The rate of sublimation will vary depending on whether it is in the dark or facing the sun, so you will probably want to have it in a plastic container to avoid it dissipating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, if you have enough then maybe the rate of sublimation won't be an issue for you, but if you have expended energy getting it there you might as well save it.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-08-18T11:21:00.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1338" PostTypeId="2" ParentId="1334" CreationDate="2013-08-18T11:24:53.143" Score="7" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could use various techniques to convert/fix all the harmful substances in Venus' atmosphere, but photosynthesis is not likely to be one of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is because Venus has an incredibly opaque atmosphere, so all you would be able to photosynthesise (assuming you had bacteria resistant to the acidic sulphur compounds found n the atmosphere) would be the top layer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, the technology we have now or for the foreseeable future just won't let us do this at a rate which will terraform Venus in time for humans to still be human. &lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-08-18T11:24:53.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1339" PostTypeId="1" CreationDate="2013-08-18T13:39:14.300" Score="10" ViewCount="468" Body="&lt;p&gt;During descent phase and entering Mars' atmosphere, was the rover's descent controlled by the Earth based ground station, or by the rover itself?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the technical challenges, when communicating with an Earth based ground station during descent phase entering Martian atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-18T14:06:39.070" LastActivityDate="2013-08-20T03:36:18.250" Title="Did Curiosity rover communicate to the ground station during its descent phase?" Tags="|mars|rovers|communication|reentry|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1340" PostTypeId="1" AcceptedAnswerId="1341" CreationDate="2013-08-18T14:21:18.387" Score="11" ViewCount="814" Body="&lt;p&gt;At present days, it's common to have two astronauts/cosmonauts in EVA at the same time during ISS maintenance (or experiments or when expanding the station). But there was a time when three or more people were in EVA at the same time? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If yes, what is the biggest number ever reached and when it happened?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastActivityDate="2016-11-30T08:54:35.053" Title="What is the biggest number of astronauts in EVA at the same time ever reached in space history?" Tags="|iss|record|eva|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1341" PostTypeId="2" ParentId="1340" CreationDate="2013-08-18T14:43:09.090" Score="11" Body="&lt;p&gt;The current record was set during &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-49&quot; rel=&quot;noreferrer&quot;&gt;STS 49&lt;/a&gt; on May 13th 1992: &lt;strong&gt;three people&lt;/strong&gt; at a time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&lt;a href=&quot;https://www.nasa.gov/audience/foreducators/spacesuits/facts/&quot; rel=&quot;noreferrer&quot;&gt;A NASA website states that a three man EVA was done only once&lt;/a&gt;. Virtually all EVA's today are performed in pairs.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="220" LastEditDate="2016-11-30T08:54:35.053" LastActivityDate="2016-11-30T08:54:35.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1343" PostTypeId="2" ParentId="1334" CreationDate="2013-08-18T16:09:40.827" Score="5" Body="&lt;p&gt;Kim Stanly Robinson had a few throwaway lines about the people trying to do so in his Mars trilogy; however it was with technology well advanced beyond our own.  At this point in the series Mars was well on the way to being terraformed with swarms of von Neumann (autonomous self replicating) robots available to do the mass industrial steps.&lt;/p&gt;&#xA;&lt;p&gt;The initial stages of the plan were to deploy a sunshade large enough to fully block out all the sunlight from reaching the planet (originally this was the mirror array used to increase light fall on Mars; the Venus project grew out of the question of what to do with the mirrors after the Martians voted to remove them).  Decades later, after the atmosphere cooled enough that all the CO&lt;sub&gt;2&lt;/sub&gt; was liquefied/frozen the plan was to seal off the tops of the CO&lt;sub&gt;2&lt;/sub&gt; seas with sheets of artificial diamond.&lt;/p&gt;&#xA;&lt;p&gt;A normal day/night cycle would then be provided by wrapping the planet in superconducting cables to spin it up like the solar systems largest electric motor and then adjusting the sunshade to allow an Earth normal amount of light to reach the planet.&lt;/p&gt;&#xA;&lt;p&gt;If there was insufficient water in the residual atmosphere the Kuiper belt would've been mined for comets similar to how the Martian hydrosphere was expanded.&lt;/p&gt;&#xA;" OwnerUserId="157" LastEditorUserId="7062" LastEditDate="2022-11-19T05:27:32.293" LastActivityDate="2022-11-19T05:27:32.293" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="1344" PostTypeId="1" AcceptedAnswerId="1346" CreationDate="2013-08-18T17:01:50.653" Score="9" ViewCount="889" Body="&lt;p&gt;A polar satellite orbits a planet over it's poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An East/West orbiter (I couldn't find the term for such satellites) may orbit anywhere from LEO up as high as GEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does the launch process (for want of a better word) differ other than that the former launches southward, and the latter eastward? &lt;em&gt;Is&lt;/em&gt; there a difference?&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-10-07T18:49:01.883" Title="How does the launch of a Polar Satellite differ from that of an East-West orbiter?" Tags="|launch|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1345" PostTypeId="1" CreationDate="2013-08-18T17:03:39.643" Score="8" ViewCount="1153" Body="&lt;p&gt;It would appear that &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_drag&quot;&gt;gravity drag&lt;/a&gt; has a exact formulation for an instantaneous moment in a rocket launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm tempted to think that some calculus could get you an exact number for Delta-v of an entire launch to orbit knowing only the starting and ending values.  For any given launch, we have know things like:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;altitude at the launch site&lt;/li&gt;&#xA;&lt;li&gt;altitude of the orbit (assuming circular)&lt;/li&gt;&#xA;&lt;li&gt;the time and position when it reached full orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It seems non-obvious whether this information is sufficient to get an expression for the gravity drag or not.  Given the same initial/final location and timing, do all approaches to orbits have the same gravity drag?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-08-19T19:15:17.220" Title="Gravity drag Delta-v calculation for launches to orbit" Tags="|launch|orbital-mechanics|mathematics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1346" PostTypeId="2" ParentId="1344" CreationDate="2013-08-18T17:35:26.547" Score="4" Body="&lt;p&gt;The key difference is that you don't get any rotational assistance from the Earth for a North-South launch if you try a direct launch into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice this means North-South orbits seem to be launched at a relatively steep angle to the North or South while still getting some of the Earth's rotation boost and then using thrust when in orbit to change from a steep angle to a North-South orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Haven't managed to find figures on optimum launch direction - will update as I find them.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-08-18T17:35:26.547" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1347" PostTypeId="1" CreationDate="2013-08-18T17:37:26.777" Score="15" ViewCount="400" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Satellite_insurance&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; speaks of satellite insurance. This is restricted as follows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It covers three risks: relaunching the satellite if the launch operation fails; &#xA;  replacing the satellite if it is destroyed, positioned in an improper orbit, or fails &#xA;  in orbit; and liability for damage to third parties caused by the satellite or the &#xA;  launch vehicle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But these are specific to satellites. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What insurance cover do/did long-term missions/projects receive? E.g. Chang'e, Mars One, Dawn&lt;/li&gt;&#xA;&lt;li&gt;What insurance cover is provided to entities on board such craft, other than State/Employee/Group Insurance?&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does it cover mixed nationalities? E.g. ISS&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Are the terms of insurance different during &#xA;&lt;ul&gt;&#xA;&lt;li&gt;launch/return &lt;/li&gt;&#xA;&lt;li&gt;whilst resident in orbit&lt;/li&gt;&#xA;&lt;li&gt;when back on surface after exposure &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="25" LastEditDate="2014-05-27T17:37:07.897" LastActivityDate="2014-05-29T13:47:02.343" Title="Do long term missions receive insurance coverage?" Tags="|launch|artificial-satellite|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1348" PostTypeId="2" ParentId="1334" CreationDate="2013-08-18T18:12:08.600" Score="26" Body="&lt;p&gt;To some extent, we can answer the question for the tools currently available to life, using basic physical principles, and the answer to that is &amp;quot;no&amp;quot;.  This needs consideration of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;atmosphere of Venus&lt;/a&gt; and the nature of &lt;a href=&quot;http://en.wikipedia.org/wiki/Photosynthesis&quot; rel=&quot;nofollow noreferrer&quot;&gt;photosynthesis&lt;/a&gt;.  The goal of terraforming would primarily be to sequester (or &amp;quot;lock away&amp;quot;) the extra atmosphere.  So we mainly need to take the CO&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt; and put it in compound.  Photosynthesis does this &lt;em&gt;in principle&lt;/em&gt; but the reaction could not trivially be complete on Venus.&lt;/p&gt;&#xA;&lt;p&gt;Photosynthesizing organisms on Earth basically operate in an open system.  Lots of reactions are involved, but you can &amp;quot;bound&amp;quot; the process to only look at what goes in and what goes out.  In the following notation, D is an electron donor, which is basically oxygen as far as we're concerned.&lt;/p&gt;&#xA;&lt;p&gt;Goes in:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2n CO&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt;;&lt;/li&gt;&#xA;&lt;li&gt;2n DH&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt;  - basically H&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt;O;&lt;/li&gt;&#xA;&lt;li&gt;photons;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;goes out:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2(CH&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt;O)&lt;span class=&quot;math-container&quot;&gt;$_n$&lt;/span&gt;;&lt;/li&gt;&#xA;&lt;li&gt;2n DO - basically O&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pG4ra.png&quot; alt=&quot;photosynthesis&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Ideally, we would like an environment where all the reactants are abundant, and where there's also a diverse environment of other elements that contains the intermediary species.  For instance, on Earth we like to use so-called &amp;quot;NPK&amp;quot; fertilizer, which stands for nitrogen, phosphorus, and potassium.&lt;/p&gt;&#xA;&lt;p&gt;Now let's look at how well the Venus environment fits that.  We will have to limit this to the upper atmosphere, because the surface is too hot for any life as we know it.  There are other concerns, like how much light it gets and other toxic species that would be less present in the upper atmosphere.  But Earth has some floating organisms itself (although not photosynthesizing).&lt;/p&gt;&#xA;&lt;p&gt;Venus &amp;quot;score&amp;quot; for plants:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;CO&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt; - great, too much even;&lt;/li&gt;&#xA;&lt;li&gt;photons - great, better than Earth;&lt;/li&gt;&#xA;&lt;li&gt;H&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt;O - fail;&lt;/li&gt;&#xA;&lt;li&gt;nitrogen - fail (the N&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt; form is not sufficient);&lt;/li&gt;&#xA;&lt;li&gt;phosphorous - fail;&lt;/li&gt;&#xA;&lt;li&gt;potassium - fail.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Here is the actual composition of the Venus atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/h4UsH.png&quot; alt=&quot;Atmosphere of Venus&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The nitrogen might not actually need to be removed.  Here is my calculation of the N&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt; mass:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ m_{N_{2}} = (4.8 \times 10^{20} \text{kg}) \times 0.035 = 1.68 × 10^{19} kg $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;In comparison, the mass of Earth's atmosphere is &lt;span class=&quot;math-container&quot;&gt;$ 5 \times 10^{18} kg$&lt;/span&gt; with about two-thirds nitrogen.  It's thinkable that something could live on a planet with 10x as much nitrogen.  Anyway, the primary concern is CO&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt;, which is the reason the surface is at huge pressure and temperature.&lt;/p&gt;&#xA;&lt;p&gt;The next question is: can't we just do photosynthesis and recycle the water?  The real reason you can't do this is the nature of the sugars, which generally have a formula like C&lt;span class=&quot;math-container&quot;&gt;$_x$&lt;/span&gt;H&lt;span class=&quot;math-container&quot;&gt;$_2$$_x$&lt;/span&gt;O&lt;span class=&quot;math-container&quot;&gt;$_x$&lt;/span&gt;.  This is the form that photosynthesis gives us to lock away the extra carbon.  But there's a problem here - it's not just carbon.  It also has oxygen, and more problematically hydrogen.  You will run out of hydrogen if you store the product as sugar.  Plants stop being helpful here.  You can't engineer a plant to produce pure carbon structures!&lt;/p&gt;&#xA;&lt;p&gt;The real process you would want is something that uses photons to turn CO&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt; into a solid structure, with only a small amount of oxygen gas produced as a byproduct and no water.  How?  You could use plants as a part of the process in a closed ecosystem floating in the Venus open atmosphere.  But then you would have to refine the plant products to an extraordinary purity so that you won't run out of hydrogen.  But whatever system you use would have to also be floating and self-replicating due to the sheer scale.  That's where the discussion gets crazy.&lt;/p&gt;&#xA;&lt;p&gt;The problem is &lt;em&gt;much&lt;/em&gt; more space-age than simply engineering some algae and dropping it into the atmosphere (there's no water).  Your options are limited to:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;bringing in material from elsewhere in the solar system;&lt;/li&gt;&#xA;&lt;li&gt;engineering artificial life to sequester carbon &lt;em&gt;from the ground up&lt;/em&gt;;&lt;/li&gt;&#xA;&lt;li&gt;some kind of self-replicating, autonomous, robotic, floating isolated ecosystem with amazing chemical processing performance.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;None of these are ideas that we can even intelligently speculate about at the present moment in time.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="7062" LastEditDate="2022-11-19T05:24:24.337" LastActivityDate="2022-11-19T05:24:24.337" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="1350" PostTypeId="1" AcceptedAnswerId="1836" CreationDate="2013-08-18T23:22:34.897" Score="11" ViewCount="958" Body="&lt;p&gt;The upper end of a space elevator is moving considerably faster than orbital speed at that distance from Earth.  As a result anything released from the top of elevator would get thrown away from the Earth instead of just entering into high Earth orbit.  How much would being released from the end of a space elevator at twice geosynchronous radius reduce travel times or the amount of rocket thrust needed during Earth departure for interplanetary probes.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm assuming the cable is capable of damping out the oscillations caused by releasing the payload.  Doing so is necessary to release a payload at any altitude other than geosynchronous altitude; due to the amount of activity currently occurring at significantly lower altitudes being able to do so seems to be a necessary requirement for constructing an elevator in the first place.&lt;/p&gt;&#xA;" OwnerUserId="157" LastEditorUserId="157" LastEditDate="2013-08-19T01:58:21.957" LastActivityDate="2013-09-07T13:23:14.343" Title="Benefit of sling shot effect with a space elevator" Tags="|space-elevator|interplanetary|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1351" PostTypeId="1" CreationDate="2013-08-19T06:41:01.550" Score="11" ViewCount="522" Body="&lt;p&gt;Imagine a huge fuel depot in geostationary orbit. Suppose further that it is tethered to the Earth and that there's a large hose snaking down all the way. Is it feasible to pump fuel up 35,786km? Can something based on capillary action work?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="288" LastEditDate="2013-08-20T18:47:40.460" LastActivityDate="2013-09-15T11:35:21.183" Title="Is it feasible to pump fuel into orbit?" Tags="|space-elevator|fuel-depots|geostationary|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1352" PostTypeId="1" AcceptedAnswerId="1360" CreationDate="2013-08-19T07:22:22.843" Score="8" ViewCount="4673" Body="&lt;p&gt;As there is zero gravity in space, how do astronauts drink liquid from a container? Do they suck the liquid or is there any mechanism for that? Are there any changes in oesophagus?&lt;/p&gt;&#xA;" OwnerUserId="579" LastEditorUserId="1028" LastEditDate="2013-11-21T11:10:13.353" LastActivityDate="2015-10-26T15:22:11.383" Title="How do astronauts drink or eat in space?" Tags="|food|astronauts|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1353" PostTypeId="1" AcceptedAnswerId="1405" CreationDate="2013-08-19T07:34:33.953" Score="16" ViewCount="8351" Body="&lt;p&gt;You see many pictures of spacecraft being handled by people with masks and hairnets. What's the reason for this?&lt;/p&gt;&#xA;" OwnerUserId="580" LastEditorUserId="49" LastEditDate="2013-09-10T02:46:03.753" LastActivityDate="2013-09-10T02:46:03.753" Title="Why are spacecraft assembled in cleanrooms?" Tags="|spacecraft-development|standards|construction|integration|clean-room|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1354" PostTypeId="2" ParentId="1353" CreationDate="2013-08-19T07:55:08.300" Score="-1" Body="&lt;p&gt;No specialist I, but I would argue the cleanliness becomes essential for at-least two reasons as follows - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Contamination: Even the air we breathe is filled with bacteria, and what-not. Spacecraft sent on scientific missions could potentially return a false positive at the least. Worse, assuming life existed on the target body, Earth bacteria could easily do something they ought not. &lt;/li&gt;&#xA;&lt;li&gt;Risk: Dust may be susceptible to charge. In gravity, such dust would settle at the point of largest attraction. In a vacuum, dust would be free to potentially short out a micro-chip, or a PCB track ... whatever. Thus posing a hazard to the mission.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Ergo, we err on the side of caution by clean-room assembly.&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-08-19T07:55:08.300" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1355" PostTypeId="2" ParentId="1352" CreationDate="2013-08-19T08:07:34.097" Score="6" Body="&lt;p&gt;As for the activity, &lt;a href=&quot;https://www.youtube.com/results?q=eating%20in%20space&quot;&gt;Youtube videos&lt;/a&gt; are probably the best answer. Drinking is done usually through straws, but that isn't a strict necessity, just convenience. Foods that would produce a lot of crumbs are avoided, but generally eating and drinking isn't much of a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for oesophagus - lie on your stomach on a bed, with your head hanging down from it, and have a drink through the straw - not only the liquid isn't propelled to your stomach through gravity, just opposite, gravity pulls it in opposite direction, towards your mouth. Still, the pushing action of oesophagus will get it to the stomach just right.&lt;/p&gt;&#xA;&#xA;&#xA;" OwnerUserId="103" LastEditorUserId="581" LastEditDate="2013-08-19T10:32:53.973" LastActivityDate="2013-08-19T10:32:53.973" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1356" PostTypeId="2" ParentId="1300" CreationDate="2013-08-19T10:09:15.080" Score="5" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Buran_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Buran&lt;/a&gt; spacecraft (  Snowstorm or Blizzard ), a Soviet orbital vehicle analogous in function and design to the US Space Shuttle, but &lt;strong&gt;it made unmanned spaceflight&lt;/strong&gt;, but again, only once in 1988.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was the first Unmanned Space Vehicle ( USV ) with humans on board, all of&lt;br&gt;&#xA;blast-off, flight, landing made by machine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it was destroyed in a 2002 hangar collapse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/n06YR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="581" LastEditorUserId="581" LastEditDate="2013-08-19T10:51:17.037" LastActivityDate="2013-08-19T10:51:17.037" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1358" PostTypeId="1" CreationDate="2013-08-19T11:31:57.740" Score="17" ViewCount="1887" Body="&lt;p&gt;Assuming unpowered flight, what were the significant differences between the STS orbiter and the Buran in:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;re-entry trajectory?&lt;/li&gt;&#xA;&lt;li&gt;re-entry guidance?&lt;/li&gt;&#xA;&lt;li&gt;navigation during re-entry (inputs and algorithms - there was no &lt;a href=&quot;https://en.wikipedia.org/wiki/GLONASS&quot;&gt;Glonass&lt;/a&gt; for the Russians back then)?&lt;/li&gt;&#xA;&lt;li&gt;terminal area energy management (TAEM)?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-09-02T08:40:26.593" LastActivityDate="2013-09-02T08:40:26.593" Title="What were the differences between Space Shuttle's and Buran's reentry guidance?" Tags="|space-shuttle|reentry|buran|guidance|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1359" PostTypeId="1" AcceptedAnswerId="1363" CreationDate="2013-08-19T11:57:15.310" Score="12" ViewCount="3427" Body="&lt;p&gt;What is the maximum mass of a satellite that can be placed in GEO? Or can satellites of any mass be placed in GEO?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2016-01-03T17:08:42.517" LastActivityDate="2019-05-21T14:44:54.397" Title="What is the maximum mass of a satellite that can be placed in GEO?" Tags="|orbital-mechanics|artificial-satellite|geostationary|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1360" PostTypeId="2" ParentId="1352" CreationDate="2013-08-19T12:38:06.233" Score="4" Body="&lt;p&gt;swallowing of food or water from mouth to stomach is done with the help of peristalsis action of oesophagus &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Then rhythmic,&#xA;  unidirectional waves of&#xA;  contractions will work to&#xA;  rapidly force the food into&#xA;  the stomach. This process&#xA;  works in one direction only&#xA;  and its sole purpose is to move&#xA;  food from the mouth into the&#xA;  stomach. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In earth it also happens but it is assisted with gravity but in space Peristalsis action is the only process by which food or water is taken to stomach .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qkUBF.gif&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-08-19T13:02:48.887" LastActivityDate="2013-08-19T13:02:48.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1362" PostTypeId="2" ParentId="1347" CreationDate="2013-08-19T13:36:58.270" Score="9" Body="&lt;p&gt;First of all, NASA typically doesn't insure its launches. Insured launches are typically for commercial entities. This, as you mentioned, is done in case of some sort of major satellite malfunction, or a poor launch. A common cost is around 10% of the cost of launch for such missions, give or take. Typically, they stop taking effect after some short period of time, maybe a few months to a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as manned space flight and insurance, it's a whole new ball game. Typically, being an astronaut excludes one from life insurance (at least, as a result of the spaceflight itself). Being high profile government missions, those astronauts who died were able to &lt;a href=&quot;http://www.npr.org/blogs/money/2012/08/30/160267398/what-the-apollo-astronauts-did-for-life-insurance&quot; rel=&quot;nofollow&quot;&gt;find other ways&lt;/a&gt; to get their funds, and &lt;a href=&quot;http://articles.latimes.com/2003/feb/10/nation/na-insure10&quot; rel=&quot;nofollow&quot;&gt;NASA provides some life insurance&lt;/a&gt;, but this won't necessarily happen in the private enterprise. There &lt;a href=&quot;http://moonandback.com/2012/12/13/insurance-company-to-provide-insurance-for-space-travelers/&quot; rel=&quot;nofollow&quot;&gt;are companies&lt;/a&gt; which are signing up to insure manned spaceflight missions, but each will have their own terms no doubt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, private manned space flight will be dangerous, and no doubt command a very high insurance premium. There are a few companies willing to sign up to provide such services, but this no doubt will be a point of major difficulty in private space flight. There is some similarity for high risk travel on Earth, such as insurance for &lt;a href=&quot;http://www.inglegroups.com/peakfreaks/&quot; rel=&quot;nofollow&quot;&gt;climbing Mount Everest&lt;/a&gt;. In fact, they probably will be quite similar to such things. The terms of these agreements have yet to be worked out, however.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="3427" LastEditDate="2014-05-29T13:47:02.343" LastActivityDate="2014-05-29T13:47:02.343" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1363" PostTypeId="2" ParentId="1359" CreationDate="2013-08-19T13:44:07.340" Score="14" Body="&lt;p&gt;Assuming orbital assembly (with separate launch vehicles, each delivering a portion of the total mass of the satellite), there is no real upper limit, short of technical limitations that we're currently struggling with. At least not until you orbit such a huge mass, that you literally start playing the game of pool with celestial bodies, permanently destabilize their orbits, cause extinction level event and terminate any life on the Earth as we know it due to gravitational pull between two objects (the Earth and the satellite) so huge, they start &quot;feeding&quot; on each other's mass by forming an &lt;a href=&quot;https://en.wikipedia.org/wiki/Accretion_disk&quot; rel=&quot;noreferrer&quot;&gt;accretion disk&lt;/a&gt; connecting the two, and suffocate their own atmospheres with it (among a plethora of other undesirable side effects).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GEO orbited mass could as well be larger than the total mass of the Earth, and we have Charon, locked in the skies of Pluto as a proof of that. &lt;a href=&quot;http://www.seasky.org/solar-system/pluto-charon.html&quot; rel=&quot;noreferrer&quot;&gt;Pluto and Charon&lt;/a&gt; are tidally locked in a synchronous orbit, meaning they always face each other with the same side (as do the Earth and our Moon), but also synchronously orbiting each other (Pluto orbits a point that is outside its body), meaning Charon is locked in the skies of Pluto as a rock that's been frozen in motion, and Pluto is locked in the skies of Charon. Charon has approximately 11.65% of the mass of Pluto, meaning Pluto has about 8.6 times the mass of Charon, yet it's still technically in a GEO orbit &quot;around&quot; it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/jLnTo.gif&quot; alt=&quot;enter image description here&quot;&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Erwut.gif&quot; alt=&quot;enter image description here&quot;&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/qbyPi.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;Cosmic dance, face to face&lt;/strong&gt;: Two bodies with different mass ratios orbiting around a common barycenter. Any of these three &lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;mass ratios could be tidally locked in synchronous orbits, always facing each other with the same side. (Animations: &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-body_problem&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similar was believed for Mercury orbiting the Sun, and that it might be &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_locking&quot; rel=&quot;noreferrer&quot;&gt;tidally locked&lt;/a&gt; with the Sun, always facing it with the same side, but that was later disproved by radar observations in 1965. Strictly theoretically speaking though, it would have been perfectly possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in short, there is no upper limit to total mass that can be put in GEO. If your mass comes mostly from the Earth itself, then it would obviously have to be smaller than the total mass of it (poor Earth, people should learn to travel light), but if you bring mass from elsewhere (other celestial bodies, maybe asteroids, or the Moon itself?), then &lt;strong&gt;your upper mass limit is the patience of the humankind to tolerate your endeavours and the problems they bring along&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having too large of a mass would, of course, change requirements of the GEO orbit itself, destabilize the Earth's rotation on it's axis (day/night and seasons of the year cycles) and around the Sun (length of year and seasons), change the tidal patterns (that we do depend upon), accelerate atmospheric loss, and all kinds of other &lt;em&gt;uncomfortable side effects&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-19T14:51:09.430" LastActivityDate="2013-08-19T14:51:09.430" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1364" PostTypeId="1" AcceptedAnswerId="1541" CreationDate="2013-08-19T14:08:11.343" Score="20" ViewCount="1109" Body="&lt;p&gt;Can spacecraft be made invisible to ground stations? For example, are spy satellites perhaps equipped with stealth technology to prevent radar tracking? Or is it pointless as they can always be optically tracked?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="14" LastEditDate="2013-08-19T17:36:21.800" LastActivityDate="2016-01-30T20:01:35.207" Title="Can spacecraft be made invisible to stations on Earth?" Tags="|technology|artificial-satellite|tracking|stealth|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1365" PostTypeId="1" AcceptedAnswerId="1372" CreationDate="2013-08-19T16:24:08.217" Score="15" ViewCount="359" Body="&lt;p&gt;How are planetary landers to low gravity bodies (such as the Moon, asteroids) designed to hold on to the surface without bouncing off after touching the surface?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="168" LastEditDate="2013-08-19T16:57:54.383" LastActivityDate="2013-08-20T06:03:15.437" Title="How are planetary landers to low gravity bodies (asteroids/comets) designed?" Tags="|lander|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1366" PostTypeId="2" ParentId="1324" CreationDate="2013-08-19T17:43:14.657" Score="4" Body="&lt;p&gt;This is a reference post for everybody to draw upon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://hdl.handle.net/2060/20110002949&quot; rel=&quot;nofollow&quot;&gt;NASA Astronauts on Soyuz: Experience and Lessons for the Future&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The RSC-Energia’s Reliability Laboratory provided NASA with a briefing on Mir/Shuttle Project quality, reliability, and safety assurance. The Russians mentioned that &lt;strong&gt;they did not have a safety program requirement&lt;/strong&gt;. Instead, they have a Quality and Reliability Program Requirement. In Russian standards, quality encompasses a very broad range of factors that defines the consumer value of a product. Quality includes reliability, safety, and a set of other factors described as fabrication quality, documentation quality, workmanship, etc. While reliability and safety relate to a significant extent to vehicle characteristics, other aspects of quality can describe both the hardware and other elements of the vehicle development process.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Of the two concepts, “reliability” and “safety,” the RSA narrowed the definition most for vehicle reliability. In its general meaning, dependability includes the following properties: reliability, longevity, preservability, and maintainability. &lt;strong&gt;Reliability is analyzed primarily by performing quantitative analysis on probability parameters&lt;/strong&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Russian experts indicated that they rely on four levels of technical standards. At the top level are RSA and government standards. The second level defines the enterprise Rocket Space Technology. The third level is composed of standards from facilities such as NPO-Energia. The fourth level is product standards.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Safety means roughly the same thing to RSA and NASA; specifically, it is the capability to prevent damage to the health of the crew and service personnel, along with major losses of material and property. The relationship between reliability and safety can be illustrated by an expression often used at RSA:&#xA;  “&lt;strong&gt;safety is assured primarily by reliability&lt;/strong&gt;.” Those safety assurance facilities and procedures with no relation to reliability are primarily geared toward controlling contingency (hazardous) situations, e.g., situations&#xA;  that arise due to a lack of hardware reliability. &lt;strong&gt;Russia makes far less use of quantitative indices for safety than for dependability (e.g., crew hazard probability, specific contingency occurrence probability).&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Russians consider this approach to reliability and safety as being close to the one taken by NASA. One of the main differences is in the methods and forms of reliability and safety analysis. For example, &lt;strong&gt;NASA emphasizes measures to prevent hazardous situations from arising in its safety analysis&lt;/strong&gt;. The RSA essentially &lt;strong&gt;examines those measures as part of a reliability analysis&lt;/strong&gt;, while &lt;strong&gt;focusing most of its attention on measures to control off-nominal situations&lt;/strong&gt; in its safety analysis.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA and the RSA have roughly identical principles for safety and reliability assurance to include: development in stages; establishing, implementing, and monitoring compliance with requirements; redundancy principles, etc. They also have similar approaches to problem resolution to include: tasks are similar in terms of goals and content; methods and procedures for task resolution vary; and there are significant differences with respect to formats for analyses and reports generated on their results.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;NASA man-rating standards (the most recent revision is 2012)&#xA;&lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?t=NPR&amp;amp;c=8705&amp;amp;s=2B&quot; rel=&quot;nofollow&quot;&gt;http://nodis3.gsfc.nasa.gov/displayDir.cfm?t=NPR&amp;amp;c=8705&amp;amp;s=2B&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Change history: &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8705_002B_&amp;amp;page_name=ChangeHistory&quot; rel=&quot;nofollow&quot;&gt;http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8705_002B_&amp;amp;page_name=ChangeHistory&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The standards are effective until 2016.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-01-11T20:24:41.273" LastActivityDate="2015-01-11T20:24:41.273" CommentCount="0" CommunityOwnedDate="2013-08-19T17:43:14.657" ContentLicense="CC BY-SA 3.0" />
  <row Id="1367" PostTypeId="1" AcceptedAnswerId="1392" CreationDate="2013-08-19T17:51:07.127" Score="12" ViewCount="262" Body="&lt;p&gt;Space stations like the ISS orbit at an elevation that puts them squarely in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermosphere&quot;&gt;Thermosphere&lt;/a&gt;, and while this has extremely rarefied gas (which is probably more accurately a plasma at many times), it still suffers orbital decay due to drag.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Has anyone seriously proposed a way to compress and sequester the gas at such an orbit, so that it could be used as propellant or for other applications?&lt;/li&gt;&#xA;&lt;li&gt;Is there anything that clearly rules the idea out?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It seems like a notable absence in places like the &lt;a href=&quot;http://www.niac.usra.edu/&quot;&gt;NASA Institute for Advanced Concepts&lt;/a&gt;.  It's quite common to see proposals for novel launch systems for delivery to propellant depots, but sequestering gas in orbit would accomplish the same thing.  You could produce cryogenic fuels right there in orbit, without the need for launches at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best counter-argument might be momentum balance.  If each molecule of gas you sequester hits you at 7.9 km/s, then your thrusters have to exceed that.  But ion thrusters &lt;a href=&quot;http://en.wikipedia.org/wiki/Spacecraft_propulsion#Example&quot;&gt;can do&lt;/a&gt; 50 km/s.  So that doesn't seem to kill the concept right away.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-08-26T15:17:28.390" LastActivityDate="2013-08-26T15:17:28.390" Title="Are there any proposals to vacuum up gases in orbit for use as propellant?" Tags="|fuel|conceptual|atmospheric-drag|fuel-depots|station-keeping|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1368" PostTypeId="2" ParentId="1359" CreationDate="2013-08-19T18:38:22.097" Score="2" Body="&lt;p&gt;To take TildalWave's answer a little farther:  Eventually you'll have enough mass up there that Earth enters the Roche limit of the object you built--goodbye Earth.  You'll need something well above Earth's mass for that to happen, though.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2013-08-19T18:38:22.097" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1369" PostTypeId="2" ParentId="1351" CreationDate="2013-08-19T18:45:10.440" Score="9" Body="&lt;p&gt;I can't imagine such a system would make any sense to build.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At it's heart you have a space elevator--something we don't have the materials to build on Earth yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A pipe full of fuel hanging on that elevator will be &lt;strong&gt;HEAVY&lt;/strong&gt;--meaning a lot more elevator.  Not to mention the weight of all the fuel in it.  The pressure requirements won't be a big deal--use booster pumps like other pipelines do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thing is there's little reason for it--if you have an elevator why not just haul fuel tanks up it?&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2013-08-19T18:45:10.440" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1370" PostTypeId="2" ParentId="1345" CreationDate="2013-08-19T19:15:17.220" Score="4" Body="&lt;p&gt;It's definitely not--Counterexample:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mission #1:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft is fired from a supergun.  At apogee a little booster fires to lift perigee out of the atmosphere.  It then deploys solar cells and uses a ion engine to complete the job of orbital insertion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mission #2, designed by a lunatic that hates g forces:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It lifts at 1.00001g until it's on the transfer orbit, then relights the same booster to circularize.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously mission #2 suffers an awful lot more gravity drag.  (That is, if you could built it at all.  I doubt anything less than antimatter has the Δv for the mission.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2013-08-19T19:15:17.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1371" PostTypeId="1" AcceptedAnswerId="1373" CreationDate="2013-08-19T19:35:20.030" Score="15" ViewCount="4629" Body="&lt;p&gt;What kind of career should I start if I want to be an &lt;em&gt;astronaut pilot&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First astronauts were military aircraft pilots. Since those days, automatic flying was present in the spacecraft so that someone considered astronauts like &lt;a href=&quot;http://en.wikipedia.org/wiki/Gordon_Cooper#.22Spam_in_a_can.22&quot;&gt;spam in a can&lt;/a&gt; for their supposed role as passenger. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is something changed today for &lt;strong&gt;astronauts that have &lt;em&gt;pilot&lt;/em&gt; role&lt;/strong&gt; (piloting a spacecraft) or alternative careers can lead to this position? According to this question context, also being a civil pilot is a different professional path. &lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="14" LastEditDate="2013-08-19T19:58:24.100" LastActivityDate="2017-05-19T15:07:22.080" Title="If I want to be an astronaut pilot, what career should I start?" Tags="|crewed-spaceflight|astronauts|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1372" PostTypeId="2" ParentId="1365" CreationDate="2013-08-19T21:21:06.147" Score="8" Body="&lt;p&gt;You need to divide between heavy bodies with a significant field of gravity and light bodies, around which you can experience &lt;a href=&quot;http://en.wikipedia.org/wiki/Micro-g_environment&quot;&gt;microgravity&lt;/a&gt; (also called µg-environment) only. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the surface of the Moon, you can experience about 0.17 &lt;a href=&quot;http://en.wikipedia.org/wiki/G-force&quot;&gt;g&lt;/a&gt;. This looks like a small number, but it is still fairly sufficient for a 'normal &lt;em&gt;landing&lt;/em&gt;'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of asteroids and comets, you are looking at fields of gravity, which are many orders of magnitude weaker than one g. Operating in the proximity of such a body or 'landing' on it is technically more some sort of rendezvous and docking manoeuvre. This basically means, that you need to &lt;em&gt;attach&lt;/em&gt; a lander to the surface of a celestial body. It also means, that based on current engineering thought, everything has to happen really careful and most importantly slow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rather prominent example is &lt;a href=&quot;http://en.wikipedia.org/wiki/Philae_%28spacecraft%29&quot;&gt;Philae&lt;/a&gt;, which is supposed to land on the comet &lt;a href=&quot;http://en.wikipedia.org/wiki/67P/Churyumov-Gerasimenko&quot;&gt;67P/Churyumov-Gerasimenko&lt;/a&gt;. It is part of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot;&gt;Rosetta mission&lt;/a&gt;. The lander uses a mix of harpoons, for final approach, and &lt;a href=&quot;http://en.wikipedia.org/wiki/File%3aPhilae_ESA_Model_closeup_feet.JPG&quot;&gt;screws inside the landing gears&lt;/a&gt;, for fixing it to surface (in the docking analogy, this is then &lt;em&gt;achieving hard dock&lt;/em&gt;). At the surface, Philae is expected to be confronted with about 5.35e-06 g or &lt;em&gt;5.35 µg&lt;/em&gt; (based on the comet's estimated mass of 3.14e12 kg and an approximate diameter of 4 km) - illustrating the difference to the Moon. &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-08-20T06:03:15.437" LastActivityDate="2013-08-20T06:03:15.437" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1373" PostTypeId="2" ParentId="1371" CreationDate="2013-08-19T21:35:44.957" Score="15" Body="&lt;p&gt;If you look at the qualifications of U.S. pilot astronauts (not mission or payload specialists) since the original Mercury 7 in the 60's, the overwhelming majority have been military (or ex-military) test pilots with advanced engineering or science degrees. Based on the bios of the most recent &lt;a href=&quot;http://www.nasa.gov/astronauts/2013astroclass.html#.UhKNv22bToM&quot; rel=&quot;noreferrer&quot;&gt;astronaut selectees&lt;/a&gt; (see picture), that's still true.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/kUGUW.jpg&quot; alt=&quot;2013 NASA Astronaut class&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your best bet would be to start with the Air Force or Naval Academy (or ROTC equivalent at a place like MIT), with an engineering major, followed by flight school, test pilot school, graduate degree in something like engineering or applied physics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/audience/forstudents/postsecondary/features/F_Astronaut_Requirements.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Astronaut Requirements&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, a &lt;em&gt;whole lot&lt;/em&gt; easier said than done.&lt;/p&gt;&#xA;" OwnerUserId="584" LastEditorUserId="168" LastEditDate="2013-08-20T18:23:51.610" LastActivityDate="2013-08-20T18:23:51.610" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1374" PostTypeId="1" AcceptedAnswerId="1376" CreationDate="2013-08-20T00:15:54.327" Score="13" ViewCount="6152" Body="&lt;p&gt;Did NASA use metric or English units for Project Mercury?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, did they measure delta-v in meters/second or in feet/second?&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="26446" LastEditDate="2020-05-22T21:31:45.640" LastActivityDate="2020-05-22T21:31:45.640" Title="Did NASA use metric units for the Mercury missions?" Tags="|nasa|terminology|project-mercury|unit-systems|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1375" PostTypeId="1" AcceptedAnswerId="2350" CreationDate="2013-08-20T00:41:04.593" Score="13" ViewCount="432" Body="&lt;p&gt;&lt;strong&gt;Integration&lt;/strong&gt; is the process of assembling or otherwise combining the separate subsystems into your final overall system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Testing&lt;/strong&gt; involves making sure your components, subsystems, and overall system meet their tolerances and perform as expected. This is typically done all along the integration process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The majority of CubeSat launches take place as rideshares, that is, they hitch a ride on a rocket purchased for some other, primary, customer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I'm interested in is the influence the primary payload on the launch has on the standards to which hitchhikers are held. For instance, can a very risk-averse primary disallow CubeSats with propulsion? Does a CubeSat operator need permission from the primary operator to, say, turn on its radios? Do some requirements get relaxed somewhat if the CubeSat is housed in a container, like the P-POD? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Or&lt;/strong&gt;, does the primary have no influence at all, in which case the CubeSats need only meet the requirements set by the launch vehicle provider?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&lt;strong&gt;Essentially, how closely do the CubeSat folks work with the primary payload folks?&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-08T21:05:16.207" LastActivityDate="2013-10-09T14:36:26.477" Title="Primary payload influence on hitchhiking CubeSat integration and testing?" Tags="|launch|cubesat|integration|testing|standards|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1376" PostTypeId="2" ParentId="1374" CreationDate="2013-08-20T01:13:16.350" Score="14" Body="&lt;p&gt;Project Mercury used &lt;a href=&quot;https://en.wikipedia.org/wiki/Imperial_units&quot; rel=&quot;noreferrer&quot;&gt;imperial units&lt;/a&gt; of measure. For example, the &lt;a href=&quot;http://history.nasa.gov/diagrams/mercury2.gif&quot; rel=&quot;noreferrer&quot;&gt;Mercury spacecraft main instrument panel&lt;/a&gt; indicated altitude in &lt;code&gt;FT&lt;/code&gt; (feet):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://history.nasa.gov/diagrams/mercury2.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s3pKe.gif&quot; alt=&quot;the Mercury spacecraft main instrument panel&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The Mercury spacecraft main instrument panel from Project Mercury Indoctrination, May 1959 (Source: &lt;a href=&quot;http://history.nasa.gov/diagrams/mercury.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;. Click for full size)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They used &lt;a href=&quot;https://en.wikipedia.org/wiki/Mile#Statute_mile&quot; rel=&quot;noreferrer&quot;&gt;statute miles&lt;/a&gt; as a measure of distance in schematics, diagrams and other vectors in the documentation, for example this &lt;a href=&quot;http://msquair.files.wordpress.com/2011/05/results-of-the-first-manned-sub-orbital-space-flight.pdf&quot; rel=&quot;noreferrer&quot;&gt;Proceedings of a Conference on Results of the first U.S. manned suborbital space flight&lt;/a&gt; from June 6, 1961:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/UspPg.png&quot; alt=&quot;First U.S. manned suborbital space flight - pictorial presentation of the overall flight&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;First U.S. manned suborbital space flight - pictorial presentation of the overall flight (Source: &lt;a href=&quot;http://msquair.files.wordpress.com/2011/05/results-of-the-first-manned-sub-orbital-space-flight.pdf&quot; rel=&quot;noreferrer&quot;&gt;Public archive (in PDF format)&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And even went at length to provide their personnel travelling abroad (in this case Mexico) with unit conversion tables from the imperial to the metric system of units in NASA's &lt;a href=&quot;http://www.scribd.com/doc/46186817/Introduction-to-Project-Mercury-And-Tracking-Sites-Handbook&quot; rel=&quot;noreferrer&quot;&gt;Introduction to project Mercury and site handbook&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KdLgE.png&quot; alt=&quot;Chapter 6, Page 13 extract from the Introduction to project Mercury and site handbook&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Page extract from the Introduction to project Mercury and site handbook, notes on personnel travels to Mexico (metric system of units)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imperial units of measure persisted throughout the Mercury program. This is the mission timeline for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Redstone_4&quot; rel=&quot;noreferrer&quot;&gt;Mercury-Redstone 4&lt;/a&gt;, launched on July 21, 1961:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;T+ Time     Event                Description&#xA;---------------------------------------------------------------------------------------&#xA;T+00:00:00  Liftoff              Mercury-Redstone lifts off, onboard clock starts.&#xA;T+00:00:16  Pitch Program        Redstone pitches over 2 deg/s from 90 deg to 45 deg.&#xA;T+00:00:40  End Pitch Program    Redstone reaches 45 deg pitch.&#xA;T+00:01:24  Max Q                Maximum dynamic pressure ~575 lbf/ft² (28 kPa).&#xA;T+00:02:20  BECO                 Redstone engine shutdown - Booster Engine Cutoff.&#xA;                                 Velocity 5,200 mph (2.3 km/s)&#xA;T+00:02:22  Tower Jettison       Escape Tower Jettison, no longer needed.&#xA;T+00:02:24  Craft Separation     Posigrade rockets fire for 1 s giving 15 ft/s (4.6 &#xA;                                 m/s) separation.&#xA;T+00:02:35  Turnaround Maneuver  Spacecraft (ASCS Automatic Stabilization and Control&#xA;                                 System) system rotates spacecraft 180 deg, to heat &#xA;                                 shield forward attitude. Nose is pitched down 34 deg &#xA;                                 to retro fire position.&#xA;T+00:05:00  Apogee               Apogee of about 115 mi (185 km) reached at 150 mi (240 &#xA;                                 km) downrange from launch site.&#xA;T+00:05:15  Retrofire            Three retro rockets fire for 10 s each. They are &#xA;                                 started at 5 s intervals, firing overlaps. Delta-V of&#xA;                                 550 ft/s (170 m/s) is taken off forward velocity.&#xA;T+00:05:45  Retract Periscope    Periscope is automatically retracted in preparation &#xA;                                 for reentry.&#xA;T+00:06:15  Retro Pack Jettison  One minute after retrofire retro pack is jettisoned,&#xA;                                 leaving heat shield clear.&#xA;T+00:06:20  Retro Attitude Man.  (ASCS) orients spacecraft in 34 deg nose down pitch, 0&#xA;                                 deg roll, 0 deg yaw.&#xA;T+00:07:15  .05 g Maneuver       (ASCS) detects beginning of reentry and rolls &#xA;                                 spacecraft at 10 deg/s to stabilize spacecraft during &#xA;                                 reentry.&#xA;T+00:09:38  Drogue Para. Deploy  Drogue parachute deployed at 22,000 ft (6.7 km) &#xA;                                 slowing descent to 365 ft/s (111 m/s) and stabilizing &#xA;                                 spacecraft.&#xA;T+00:09:45  Snorkel Deploy       Fresh air snorkel deploys at 20,000 ft (6.1 km). (ECS) &#xA;                                 switches to emergency oxygen rate to cool cabin.&#xA;T+00:10:15  Main Para. Deploy    Main parachute deploys at 10,000 ft (3.0 km). Descent &#xA;                                 rate slows to 30 ft/s (9.1 m/s)&#xA;T+00:10:20  Landing Bag Deploy   Landing Bag Deploys, dropping heat shield down 4 ft &#xA;                                 (1.2 m).&#xA;T+00:10:20  Fuel Dump            Remaining hydrogen peroxide fuel automatically dumped.&#xA;T+00:15:30  Splashdown           Spacecraft lands in water about 300 mi (480 km) &#xA;                                 downrange from launch site.&#xA;T+00:15:30  Rescue Aids Deploy   Rescue aid package deployed. The package includes &#xA;                                 green dye marker, recovery radio beacon and whip &#xA;                                 antenna.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Units are also converted into metric system for convenience, original mission communications transcript is available &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/MR04_TEC.PDF&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; (in PDF format).&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-20T01:33:30.460" LastActivityDate="2013-08-20T01:33:30.460" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1377" PostTypeId="2" ParentId="1339" CreationDate="2013-08-20T03:36:18.250" Score="2" Body="&lt;p&gt;The Curiosity Rover's descent stage was entirely autonomous. As mentioned in some of the comments, &quot;7 Minutes of Terror&quot; is a video by the MSL Team which visualizes the descent stage [watch it at &lt;a href=&quot;http://www.youtube.com/watch?v=Ki_Af_o9Q9s&quot; rel=&quot;nofollow&quot;&gt;http://www.youtube.com/watch?v=Ki_Af_o9Q9s&lt;/a&gt; ]. The significant delay in one-way communication (from 3-21 minutes) is the main challenge, and means that much of what the rover does is autonomous. In fact, by the time that mission control received the signal from MSL indicating it had reached the upper part of the atmosphere, the rover had already landed on the surface of Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more on Communication Delay, check out: &lt;a href=&quot;http://www.spaceacademy.net.au/spacelink/commdly.htm&quot; rel=&quot;nofollow&quot;&gt;http://www.spaceacademy.net.au/spacelink/commdly.htm&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="586" LastActivityDate="2013-08-20T03:36:18.250" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1378" PostTypeId="1" CreationDate="2013-08-20T04:48:31.560" Score="4" ViewCount="73" Body="&lt;p&gt;What communication protocols are used to communicate with probes that are many light minutes or even hours away?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not only the signal will be weak and badly damaged by background noise, we can't just go confirming each packet upon receiving it, or demanding a resend; time windows are limited (radio antennas on Earth facing given probe), and there's a plenty of data to be sent - plus broadcasting that from the probe towards Earth costs precious energy too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely the hardware plays a big role in getting as little losses as possible but what protocols handle this kind of transmission and how do they improve the communication - what mechanisms do they use to handle the problems?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-08-20T04:48:31.560" Title="What communication protocols are used with latencies going into hours?" Tags="|communication|" AnswerCount="0" CommentCount="0" ClosedDate="2013-08-20T13:22:29.543" ContentLicense="CC BY-SA 3.0" />
  <row Id="1379" PostTypeId="2" ParentId="1325" CreationDate="2013-08-20T06:50:28.303" Score="37" Body="&lt;p&gt;Providing crew escape for all phases of flight of the Space Shuttle, given its design architecture, was simply not practical.  Keep in mind you have up to 7 crew members on two decks.  Keep in mind that the flight regime consists of large ranges of altitude and velocities.  Keep in mind that it would have to cover launch, landing, and several abort modes.  Even the ejection seats used in the early flights couldn't cover the entire flight regime and only worked for the Commander and the Pilot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So to answer your question directly:&lt;/strong&gt; ejection seats would have only been useful for the crew on the flight deck and then only for a (very) limited part of the total flight regime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding the use of a LES on the Shuttle:  There were some studies done during early design phases of a separable crew compartment.  Not surprisingly, this added an unacceptable amount of mass to the Orbiter -- at least with the architecture selected.  Of course, this system looked nothing like the LES used on Apollo.  Also, even the LES on Apollo didn't cover all of Apollo's flight regime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another interesting fact: the Shuttle was initially designed to allow a shirt-sleeve environment for the astronauts.  This means that prior to STS-26, Shuttle astronauts didn't even wear parachutes (excepting the early qualification flights with ejection seats).  In fact, the seats were not designed to handle astronauts with a parachute.  By &quot;handle,&quot; I mean handle the additional weight and volume of a crew member with a parachute and other gear attached.  The seats were modified (and eventually entirely redesigned) for STS-26 and beyond for when the crew members did wear parachutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These new chutes were expected to be used for a very limited range of flight regimes/abort modes where the Orbiter could be put into a stable glide configuration that allowed all of the crew to bail out.  This means that you had to ride the solids for at least 2 minutes till they burned out, do some fancy flying to get rid of the External Tank, get the Orbiter into a stable auto-piloted glide, climb down (or over) to the hatch, blow the hatch, extend a pole (see next paragraph), and bail out (x7).  So this really wasn't a launch escape system -- once you light the SRBs, you are committed to at least 2 minutes of powered flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bailing out was to be done with the aid of a crew escape pole.  The pole kept the crew from hitting the leading edge of the wing.  There is some good test footage available here: &lt;a href=&quot;http://youtu.be/dfVTX25hH-I&quot;&gt;http://youtu.be/dfVTX25hH-I&lt;/a&gt;.  Note that none of this would have helped the astronauts we lost on 51-L or STS-107.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is the take-away here?  One (of many) of the big reasons the Shuttle was retired was its lack of provision for crew escape.  All of the likely new manned US systems (government contracted and private) include crew escape for most if not all of the flight regime.  This is one of the reason why most of these systems go with capsules too.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2013-08-20T08:10:20.067" LastActivityDate="2013-08-20T08:10:20.067" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="1380" PostTypeId="1" AcceptedAnswerId="1444" CreationDate="2013-08-20T07:55:11.173" Score="19" ViewCount="26839" Body="&lt;p&gt;How do you calculate the delta-v required to do a Hohmann transfer from a circular orbit around one body to a circular orbit around another?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm assuming you'd need to know the masses of the two bodies, and the mass of whatever they're orbiting around, also the radii of the different orbits.  Let's assume those are all known.&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:53:20.263" LastActivityDate="2017-05-18T20:50:03.730" Title="How to calculate delta-v required for a planet-to-planet Hohmann transfer?" Tags="|orbital-mechanics|orbital-maneuver|interplanetary|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1381" PostTypeId="1" AcceptedAnswerId="1384" CreationDate="2013-08-20T08:09:52.947" Score="0" ViewCount="126" Body="&lt;p&gt;I would like to put a spin on &lt;a href=&quot;https://space.stackexchange.com/questions/1359/what-is-the-maximum-mass-of-a-satellite-that-can-be-placed-in-geo&quot;&gt;What is the maximum mass of a satellite that can be placed in GEO?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity is a function of the mass of both (all) bodies, and their relative separation. &lt;a href=&quot;http://en.wikipedia.org/wiki/Minor-planet_moon#Main-belt_asteroids&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; lists several binary asteroids. The orbit may vary wildly from elongated elliptical to circular as a function of the masses involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say a cosmonaut on board a spherical satellite at GEO were to mislay a nut whilst outside the satellite. Say also this incident were to happen in close proximity to the satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Would the nut go into orbit around the satellite?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a table of the gravitational pull of launched/to-be-launched spacecraft? &#xA;Sort-of like the way we say Earth Gravity is 9.8 m/s²?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-08-20T09:28:47.313" Title="Is there a table of the gravitational pull of a spacecraft?" Tags="|orbital-mechanics|microgravity|gravity|" AnswerCount="2" CommentCount="1" ClosedDate="2013-08-20T13:23:01.263" ContentLicense="CC BY-SA 3.0" />
  <row Id="1382" PostTypeId="1" AcceptedAnswerId="1388" CreationDate="2013-08-20T08:23:07.010" Score="14" ViewCount="4845" Body="&lt;p&gt;Space is empty ... lots of empty. Apparently so empty that the risk of collision between a spacecraft of our times, and an asteroid in the asteroid belt is said to be infinitesimal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Running a search for asteroid collision brings up any number of talk about an asteroid/comet colliding with Earth. There doesn't appear to be much known about Asteroid-Asteroid collisions though&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How frequently does an asteroid collide with another? &lt;/li&gt;&#xA;&lt;li&gt;Are there any such asteroid-asteroid collisions calculated to be visible in the near future?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="14" LastEditDate="2013-08-20T08:55:35.363" LastActivityDate="2020-01-30T18:27:36.063" Title="How frequently do asteroids collide with each other?" Tags="|orbital-mechanics|asteroid|impact|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1383" PostTypeId="2" ParentId="1380" CreationDate="2013-08-20T08:26:18.593" Score="5" Body="&lt;p&gt;There is no closed form solution to the n-body problem, so you will probably start by using a patched-conic approach.  This will treat the trajectory as a simple Hohmann transfer between to spheres of influence (assuming your bodies are in coplanar orbits).  The delta-v for a Hohmann transfer is well known.  To this you will add (subtract) the delta-v for the &quot;patched&quot; trajectories inside the bodies spheres of influence which are solvable for two (n=2) bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point you will probably need a refined answer -- especially if you are going to be onboard this mission.  This means you will need to do some numerical integration with your patched-conic approximation as your starting point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck, and be sure to take enough food!&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="80" LastEditDate="2013-08-20T19:49:55.280" LastActivityDate="2013-08-20T19:49:55.280" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1384" PostTypeId="2" ParentId="1381" CreationDate="2013-08-20T08:39:32.250" Score="2" Body="&lt;p&gt;A &lt;em&gt;field of gravity&lt;/em&gt; depends on the &lt;em&gt;mass&lt;/em&gt; (kg) of an object. &lt;em&gt;Gravitational force&lt;/em&gt; (N) depends on the masses of both objects involved (as well as the gravitational constant and the square of the distance between the objects). However, what both objects experience, is called &lt;em&gt;acceleration&lt;/em&gt; (m/s²). So what you are really looking for is e.g. a table of the masses or weights of a spacecrafts. (This is the fundamental underlying flaw of your question.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the 'nut part' of your question, I would like to refer you to the following previously given answer: &lt;a href=&quot;https://space.stackexchange.com/questions/306/under-what-circumstances-would-an-object-falling-out-of-spacecraft-start-orbitin/321#321&quot;&gt;Under what circumstances would an object falling out of spacecraft start orbiting that spacecraft?&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-08-20T08:45:29.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1385" PostTypeId="1" AcceptedAnswerId="1414" CreationDate="2013-08-20T08:48:22.140" Score="13" ViewCount="965" Body="&lt;p&gt;Apart from testing Rogallo wing for Gemini capsules are there any cases of use of steerable parachutes/wings for final phase of re-entry of space vehicle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When landing in big steppe or ocean it doesn't really matter, but it would be useful to have ability to steer if landing takes place in some area with obstacles (for one reason or another).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be disadvantages of such system (I guess weight and complexity)?&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorDisplayName="user12" LastEditDate="2013-10-29T04:42:38.507" LastActivityDate="2018-10-16T05:51:32.340" Title="Use of steerable parachutes for re-entry" Tags="|reentry|parachute|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1386" PostTypeId="2" ParentId="1385" CreationDate="2013-08-20T08:58:29.267" Score="13" Body="&lt;p&gt;The X-38 (based on the outer mold line of the X-24 lifting body) was planning to use the largest parafoil parachute in history for its final deceleration.  Alas, she only flew as a test.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This parafoil was actually designed to autonomously deliver light armor to forward operating locations for the military.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/PZgHw.png&quot; alt=&quot;X-38 Parafoil&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-08-20T08:58:29.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1387" PostTypeId="2" ParentId="1381" CreationDate="2013-08-20T09:28:47.313" Score="2" Body="&lt;p&gt;The shape of the orbit, if the body is near enough significantly depends on roundness of the object orbited (and thus shape of its gravitational field), which is not the case with most satellites, but let's pick a satellite that is fairly spherical, like Sputnik-1 $M=83.6 kg;$ and pick a reasonable distance at which it can be treated roughly as a point mass; $r=10m$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the mass of the nut is negligible comparing to the mass of the satellite, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;source: &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;nofollow&quot;&gt;1&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;v = \sqrt{GM \over r} \\&#xA;s = 2 \pi r \\&#xA;s=vt \\&#xA;t={s \over v} = {2 \pi r \over v}\\&#xA;t={2 \pi} { \sqrt{ r^3 \over {GM}}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;t = 2 \pi \sqrt{10^3 \over 6.67384 × 10{^-11} \cdot 83.6  } = 2660000 seconds.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's about a month, at speed of 8cm/hour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course over this long time Earth will totally rain upon your parade, the orbits of the two crossing much more often due to orbiting Earth at different angles - only with orbital period significantly shorter than 1 day it could work, but that would pretty much bring the nut below the surface of the satellite. OTOH it might work when far enough from major bodies, say, a nut could easily orbit Voyager-1 at distance of few meters, with several weeks of orbital period.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-08-20T09:28:47.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1388" PostTypeId="2" ParentId="1382" CreationDate="2013-08-20T09:45:00.483" Score="12" Body="&lt;p&gt;Both questions are really hard to answer. You can only estimate how often this happens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no monitoring system, which constantly observes the entire sky. There is an unknown number of asteroids and comets in the solar system. Right now, the &lt;a href=&quot;http://www.youtube.com/watch?v=xJsUDcSc6hE&quot;&gt;rate of newly discovered objects increases exponentially&lt;/a&gt; (due to better and better technology). Today's number of uniquely identified objects is in the range of half a million to one million objects (620284 as of July 22 2013). They are &lt;a href=&quot;http://www.minorplanetcenter.net/iau/lists/ArchiveStatistics.html&quot;&gt;listed&lt;/a&gt; by the &lt;a href=&quot;http://www.minorplanetcenter.net/iau/mpc.html&quot;&gt;Minor Planet Center&lt;/a&gt;. While there actually are estimated values of the total amount of objects in our solar system, nobody can be sure about it at the current state of science, technology and monitoring. There are some attempts like &lt;a href=&quot;http://www.nasa.gov/centers/marshall/news/lunar/&quot;&gt;observing and counting present impacts&lt;/a&gt; and &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1111/maps.12054/abstract&quot;&gt;dating existing craters&lt;/a&gt; on the Moon, but this only helps for the class of objects, which intersect Earth's orbit around the sun. For instance, it excludes objects in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid_belt&quot;&gt;asteroid belt&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/Kuiper_belt&quot;&gt;Kuiper belt&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping in mind those numbers, inter-asteroid / asteroid-comet / inter-comet collisions are extremely rare. They are in fact so rare, that there is basically no statistics about it. It is hard or near impossible to put a number of that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are attempts to predict collisions. Keeping in mind that orbital parameters of small and distant objects can only be determined with some uncertainty, the idea is to go through the database of known objects and &lt;a href=&quot;http://malcolm.shute.free.fr/rastro.htm&quot;&gt;predict when a pair of objects comes into a certain proximity&lt;/a&gt;. This gives &lt;em&gt;candidates&lt;/em&gt; for collision events. They are rather interesting for researchers, who are studying the structure or chemical composition of such objects. Another however, the amount of collisions actually happening after such a prediction is still incredibly low. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;If you are interested in such events, it is a good idea to set a number of Google alerts on some relevant key words and follow as many astronomers mailing lists as possible. This may sound like stupid advice, but it is as good as it can get. Sometimes, there are announcements. You just need to find them at the right time.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-08-20T09:50:33.977" LastActivityDate="2013-08-20T09:50:33.977" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1392" PostTypeId="2" ParentId="1367" CreationDate="2013-08-20T14:50:12.877" Score="5" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Yes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1950 Wayne Proell proposed the use of &quot;fuelless box-ramjets&quot; to harness the energy of recombination of atomic oxygen at about 100 km altitude to propel space stations and satellites. (&lt;em&gt;The Significance of Monatomic Gases in Planetary Space Operations&lt;/em&gt;, J. of Space Flight, V.2, no.7, Sept. 1950 pp.1-8, V.2, no.8, Oct.1950, pp.1-9.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1958 Lionel Baldwin and Perry Blackshear from the Lewis lab (now &lt;a href=&quot;https://en.wikipedia.org/wiki/Glenn_Research_Center&quot; rel=&quot;nofollow&quot;&gt;GRC&lt;/a&gt;) published a &lt;a href=&quot;http://wayback.archive-it.org/all/20100515203820/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800018907_1980018907.pdf&quot; rel=&quot;nofollow&quot;&gt;report&lt;/a&gt; with detailed computations on the feasibility of this approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Another variation on the topic is, of course, &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;nofollow&quot;&gt;Bussard ramjets&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Baldwin and Blackshear's conclusion was that the contraption is of marginal utility. Too large, too inflexible, too costly to develop.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;EDIT: Transforming the ramjet into a fuel scoop would negate all the thrust advantage, since it would no longer be ejecting &lt;strong&gt;anything&lt;/strong&gt;. To counteract drag you will be forced to spend lots of fuel from the Earth. Essentially, producing oxygen and hydrogen here is much more economical (due to abundance of energy and raw inputs) than hauling the machinery into space and keeping it from falling on our heads.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-20T15:07:30.993" LastActivityDate="2013-08-20T15:07:30.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1393" PostTypeId="1" AcceptedAnswerId="1395" CreationDate="2013-08-20T15:07:36.977" Score="15" ViewCount="2051" Body="&lt;p&gt;In all rockets that launch satellites to orbit, the satellite is always placed at the top.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why are payloads always placed at the top of rockets?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-21T13:23:13.237" LastActivityDate="2022-10-13T22:08:06.107" Title="Why are satellites and other payloads always placed at the top of rockets?" Tags="|launch|rockets|artificial-satellite|payload|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="1394" PostTypeId="1" CreationDate="2013-08-20T15:59:49.817" Score="15" ViewCount="2491" Body="&lt;p&gt;Does rocket propellant (solid or liquid) get added to the rocket during the final preparation of the launch (count down) in all cases? If so, why is it filled at the final instant?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user29" LastEditDate="2013-08-20T16:18:55.883" LastActivityDate="2015-12-28T16:19:27.973" Title="Do all rockets get fueled during the countdown?" Tags="|launch|rockets|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1395" PostTypeId="2" ParentId="1393" CreationDate="2013-08-20T16:30:52.677" Score="21" Body="&lt;p&gt;First of all, let's look at what the limitations are for placing objects in a rocket.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The object must be aerodynamic, such that it does not influence the launch characteristics.&lt;/li&gt;&#xA;&lt;li&gt;You need to easily be able to get the objects separated from the rocket.&lt;/li&gt;&#xA;&lt;li&gt;The thrust must happen at the bottom, for maximum efficacy, straight down, and the center of gravity should align with the thrust/center of drag vectors..&lt;/li&gt;&#xA;&lt;li&gt;You don't want to overheat the satellite.&lt;/li&gt;&#xA;&lt;li&gt;Every launching system to date has multiple stages.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Okay, so given all of that, what are your options, aside from a satellite launch at the top, as you mentioned? There are two things of note which I will include.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The final booster could be made in to a satellite. This is rarely done, but was done with the &lt;a href=&quot;http://en.wikipedia.org/wiki/Explorer_1&quot;&gt;Explorer 1&lt;/a&gt;, the first US satellite. The reason this is rarely done is that for the most part, the satellite and rocket are built independently, and thus, the final rocket state can't contribute to the satellite much. In addition, this still fits the satellite being the top of the stack.&lt;/li&gt;&#xA;&lt;li&gt;The Space Shuttle was launched from the side. The Space Shuttle was meant to be aerodynamic, and could take the buffeting of the ride, but it would be difficult for a non-aerodynamic object like most satellites to take the force. It lost some efficiency by its design, although it was not a significant component of loss.&lt;/li&gt;&#xA;&lt;li&gt;Abort rockets are often placed at the top of the stack for manned spaceflight missions.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, putting the satellite at the top is convenient to dampen vibrations, reduce aerodynamic turbulence, improve performance, reduce heating of the spacecraft, etc. It doesn't have to be done that way, but it sure makes the design a lot easier, and there aren't any compelling cases that I'm aware of to do anything differently.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-20T17:51:37.823" LastActivityDate="2013-08-20T17:51:37.823" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1396" PostTypeId="2" ParentId="1394" CreationDate="2013-08-20T16:42:50.143" Score="17" Body="&lt;p&gt;The bottom line is, rockets are fueled when it is the least dangerous and most cost effective to do so. Let me start by talking about some of the major types of rocket fuels, and when they are typically fueled. For the sake of simplicity, I'm making no difference between fuel and oxidizer, even though they are technically different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid fueled rockets are usually filled when manufactured. Putting the fuel in can be quite dangerous, and needs to be done under exceptionally controlled circumstances. Furthermore, X-rays often need to be done on the fuel to ensure an even batch, cracking can cause serious issues during launch, up to and including total launch failure. Thus, putting it in when you build the rocket is usually the best path forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cryogenic fuels, such as are used by Falcon, Saturn V, Soyuz, the Space Shuttle, etc. These need to be loaded in at the last minute, because they will boil off of the rocket, and can potentially cause damage to the rocket if left for too long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other liquid fuels are highly toxic, and as a result, require special precautions when operating around them. Thus, they are usually not loaded until the last minute. The exception for this might lay in military grade missiles, which need to be fueled and ready to go at a moment's notice. These are usually placed in some silo, allowing for greater environmental control than a rocket exposed to the elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft fuels are typically loaded at the launch site, as many of them are toxic as well, and virtually all of them are explosive. They are usually loaded prior to the countdown commencing, however, but something like a week or two before, depending on how things work exactly. The primary exception is Xenon used for ion engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket_propellant#Current_Types&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-12-28T16:19:27.973" LastActivityDate="2015-12-28T16:19:27.973" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1397" PostTypeId="1" AcceptedAnswerId="1411" CreationDate="2013-08-20T18:41:10.253" Score="24" ViewCount="8458" Body="&lt;p&gt;I've noticed that some of the American astronauts on the ISS can speak (at least a smattering of) Russian and similarly, some of the Russian cosmonauts are fluent in English. Is such bilinguality a requirement for all/some of the occupants of the ISS? If not, how do the two parties communicate?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorDisplayName="user12" LastEditDate="2013-11-07T04:58:32.240" LastActivityDate="2018-03-22T21:26:58.540" Title="Do ISS astronauts have to be conversant in both English and Russian?" Tags="|iss|astronauts|astronaut-lifestyle|language|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1398" PostTypeId="1" CreationDate="2013-08-20T19:05:12.993" Score="31" ViewCount="8734" Body="&lt;p&gt;&lt;a href=&quot;http://www.newscientist.com/article/dn20664-cosmonaut-soviet-space-shuttle-was-safer-than-nasas.html&quot; rel=&quot;noreferrer&quot;&gt;In an interview with Oleg Kotov&lt;/a&gt;, a Russian cosmonaut, he makes an ambiguous statement on the military uses of the Space Shuttle:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;New Scientist:&lt;/em&gt; After the cold war, why didn't Russia maintain its shuttle programme?&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Oleg Kotov:&lt;/em&gt; We had no civilian tasks for Buran and the military ones were no longer needed. It was originally designed as a military system for weapon delivery, maybe even nuclear weapons. &lt;strong&gt;The American shuttle also has military uses.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Is this true? Did the American shuttle have &amp;quot;military uses&amp;quot;? What were these uses? Was it, as with Buran, designed to also function as a weapon delivery vehicle?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="-1" LastEditorDisplayName="user29" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2021-01-01T17:02:42.823" Title="Did the Space Shuttle have &quot;military uses&quot;?" Tags="|space-shuttle|military|" AnswerCount="6" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1399" PostTypeId="2" ParentId="1398" CreationDate="2013-08-20T19:37:37.340" Score="29" Body="&lt;p&gt;There were shuttle launches which had military tasks:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-4&quot;&gt;STS-4&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;24 Jun 1982, Columbia&lt;br&gt;&#xA;Crew: Thomas K. Mattingly II, Henry W. Hartsfield, Jr.&lt;br&gt;&#xA;Payload: Classified US Air Force payload of two missile launch-detection systems&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-51-C&quot;&gt;STS-51-C&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;24 Jan 1985, Discovery&lt;br&gt;&#xA;Crew: T. Mattingly, L. Shriver, E. Onizuka, J. Buchli - all military and G. Payton - military engineer (his name was a secret for 2 years).&lt;br&gt;&#xA;Payload: Magnum satellite - radio intelligence&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-51-J&quot;&gt;STS-51-J&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3 Oct 1985, Atlantis&lt;br&gt;&#xA;Crew: K. Bobko, R. Grabe, D. Hilmers, R. Stewart - all military and W. Pailes - military engineer.&lt;br&gt;&#xA;Payload: Two military communication satellites - DSCS-3&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-27&quot;&gt;STS-27&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 Dec 1988, Atlantis&lt;br&gt;&#xA;Crew: M. Mullane, R. Gibson, J. Ross, W. Shepherd&lt;br&gt;&#xA;Payload: Lacrosse satellite - radar intelligence (maybe?)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-28&quot;&gt;STS-28&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aug 1989, Columbia&lt;br&gt;&#xA;Crew: B. Shaw, R. Richards, J. Adamson, D. Leestma, M. Brown.&lt;br&gt;&#xA;Payload: photo intelligence satellite - Key Hole or SDS-2 (maybe?)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-33&quot;&gt;STS-33&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;23 Nov 1989, Discovery&lt;br&gt;&#xA;Crew: M. Carter, S. Musgrave, K. Thornton, F. Gregory, J. Blaha.&lt;br&gt;&#xA;Payload: Magnum(Orion) (maybe?)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-36&quot;&gt;STS-36&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;28 Feb 1990, Atlantis&lt;br&gt;&#xA;Crew: J. Creighton, J. Casper, D. Hilmers, P. Thout, M. Mallein.&lt;br&gt;&#xA;Payload: AFP-731 satellite (maybe stealth satellite)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-38&quot;&gt;STS-38&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;15 Nov 1990, Atlantis&lt;br&gt;&#xA;Crew:R. Covey, F. Culbertson, C. Meade, R. Springer, C. Gemar.&lt;br&gt;&#xA;Payload: AFP-658 satellite or SDS-2 satellite&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-39&quot;&gt;STS-39&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;28 Apr 1991, Discovery&lt;br&gt;&#xA;Crew: M. Coats, B. Hammond, G. Harbaugh, D. McMonagle, G. Bluford, L. Veach, R. Hieb&lt;br&gt;&#xA;Payload: AFP-675 satellite&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-44&quot;&gt;STS-44&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;24 Nov 1991, Atlantis&lt;br&gt;&#xA;Crew: T. Henricks, J. Voss, M. Runco, T. Hennen.&lt;br&gt;&#xA;Payload: DSP satellite (rocket attack alarm)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-53&quot;&gt;STS-53&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2 Dec 1992, Discovery&lt;br&gt;&#xA;Crew: D. Walker, R. Cabana, M. Clifford, G. Bludford, J. Voss.&lt;br&gt;&#xA;Payload: SDS-2 (retransmission satellite)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is from one Russian magazine. So I cannot say this is all true. But it's some useful info:)&lt;/p&gt;&#xA;" OwnerUserId="540" LastEditorUserId="80" LastEditDate="2013-08-21T08:05:29.590" LastActivityDate="2013-08-21T08:05:29.590" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1400" PostTypeId="2" ParentId="1398" CreationDate="2013-08-20T20:08:11.637" Score="41" Body="&lt;p&gt;There was originally supposed to be a &quot;blue&quot; (Air Force) Shuttle.  Military missions benefit from retrograde orbits, so there was originally supposed to be a west coast launch facility (Vandenberg) as well.&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/B4TkA.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle was saddled with large cross-range landing requirements which were entirely due to the DoD's involvement (and never used).  As chiron has pointed out, the STS was used for several DoD payloads as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cross-range requirement was particularly damaging to the whole system's life span.  It drove the size of the aerosurfaces &amp;mdash; which were already much larger than they had to be due to the uncertainty of the flight regime when they were designed.  The vertical stabilizer alone on the shuttle is probably twice as big as it needs to be to provide the control authority needed by the Orbiter.  So much dead weight&amp;hellip;.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="6816" LastEditDate="2017-02-13T19:24:22.583" LastActivityDate="2017-02-13T19:24:22.583" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="1401" PostTypeId="5" CreationDate="2013-08-20T20:49:14.560" Score="0" Body="&lt;p&gt;Questions regarding missions that failed to reach their goal and systems/procedures used during a mission abort, such as launch escape systems.&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:14:49.647" LastActivityDate="2013-08-20T21:14:49.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1402" PostTypeId="4" CreationDate="2013-08-20T20:49:14.560" Score="0" Body="Questions regarding missions that failed to reach their goal and systems/procedures used during a mission abort, such as launch escape systems." OwnerDisplayName="user55" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:14:04.940" LastActivityDate="2013-08-20T21:14:04.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1403" PostTypeId="5" CreationDate="2013-08-20T20:53:00.523" Score="0" Body="&lt;p&gt;A Launch Escape System (LES) is a system connected to the crew module of a manned spacecraft and it is used to quickly separate the crew module from the rest of the rocket in case of emergency (like an explosion of the rocket)&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-08-20T21:15:57.697" LastActivityDate="2013-08-20T21:15:57.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1404" PostTypeId="4" CreationDate="2013-08-20T20:53:00.523" Score="0" Body="A Launch Escape System (LES) is a system connected to the crew module of a manned spacecraft. It is used to quickly separate the crew module from the rest of the launch vehicle in case of emergency (such as an explosion of the rocket)." OwnerDisplayName="user55" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:15:40.697" LastActivityDate="2013-08-20T21:15:40.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1405" PostTypeId="2" ParentId="1353" CreationDate="2013-08-20T21:44:11.843" Score="11" Body="&lt;p&gt;&lt;a href=&quot;http://www.nature.com/news/2007/070904/full/news070903-5.html&quot;&gt;You're right&lt;/a&gt; about clean rooms not effectively protecting against bacteria. So why &lt;em&gt;are&lt;/em&gt; they used?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, there are &lt;a href=&quot;http://www.iest.org/StandardsRPs/ISOStandards/ISO14644Standards/tabid/10135/Default.aspx&quot;&gt;different levels&lt;/a&gt; of clean rooms. Typically the level is specified as a maximum number of particles per cubic meter (with different levels for different size particles). For instance, under the ISO standard, a class 6 clean room must not have more than 100,000 particles ≥0.1 µm, 23,700 particles ≥0.2 µm, and so on (for quick reference, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Cleanroom#Cleanroom_classifications&quot;&gt;Wikipedia&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;cleanest&quot; clean rooms (lowest number of particles per unit volume) are used when the satellite has sensitive optics or other instruments on board. Cryogenically-cooled instruments are particularly susceptible to particulate contamination. Obviously if you have components that are very sensitive to contamination you want to keep your entire spacecraft free from possible contaminants, thus the need for your entire environment to be as clean as you can get it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;less clean&quot; clean rooms (higher number of particles per unit volume) are for when you don't have components that are highly sensitive to contamination, and you can afford a few particles per square cm of surface area. While instrument performance may not be adversely affected by the presence of contaminants, you still want to keep the amount of dust and debris to a minimum to protect against possible electrical shorts or electrostatic discharge (ESD).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-20T21:44:11.843" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1406" PostTypeId="5" CreationDate="2013-08-20T21:47:00.227" Score="0" Body="" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:47:00.227" LastActivityDate="2013-08-20T21:47:00.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1407" PostTypeId="4" CreationDate="2013-08-20T21:47:00.227" Score="0" Body="Integration is the process of assembling or otherwise combining the separate spacecraft subsystems into your final overall system." OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:47:00.227" LastActivityDate="2013-08-20T21:47:00.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1408" PostTypeId="5" CreationDate="2013-08-20T21:51:24.553" Score="0" Body="" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:51:24.553" LastActivityDate="2013-08-20T21:51:24.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1409" PostTypeId="4" CreationDate="2013-08-20T21:51:24.553" Score="0" Body="Standards refer to formalized descriptions of &quot;best practices&quot; for any given process, design, or procedure. They allow systems to be developed such that &quot;reinventing the wheel&quot; is kept to a minimum, and they aid in compatibility between standardized systems." OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-20T21:51:24.553" LastActivityDate="2013-08-20T21:51:24.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1410" PostTypeId="1" CreationDate="2013-08-20T22:40:27.477" Score="20" ViewCount="590" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Luca_Parmitano&quot;&gt;Luca Parmitano&lt;/a&gt; wrote about his experience with &lt;a href=&quot;http://blogs.esa.int/luca-parmitano/2013/08/20/eva-23-exploring-the-frontier/&quot;&gt;water filling his helmet&lt;/a&gt; during EVA on the International Space Station.  It's a harrowing account and he explains part of his thought process:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I force myself to stay calm and, patiently locating the handles by touch, I start to move, all the while thinking about how to eliminate the water if it were to reach my mouth. The only idea I can think of is to open the safety valve by my left ear: if I create controlled depressurisation, I should manage to let out some of the water, at least until it freezes through sublimation, which would stop the flow. But making a ‘hole’ in my spacesuit really would be a last resort.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As I read the phrase &quot;eliminate the water&quot;, I wondered why he didn't consider the snorkel technique called &lt;a href=&quot;http://en.wikipedia.org/wiki/Snorkeling#Flooding_and_clearing&quot;&gt;&quot;blast clearing&quot;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The snorkeler expels water from the snorkel either with a sharp exhalation on return to the surface (&lt;em&gt;blast clearing&lt;/em&gt;) or by tilting the head back shortly before reaching the surface and exhaling until reaching or breaking the surface (&lt;em&gt;displacement method&lt;/em&gt;) and facing forward again before inhaling the next breath.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Obviously the displacement method won't work except in snorkeling, but blowing out a puff of air to push the water away seems like it would work.  I mean, the water will go off to some other part of the spacesuit rather than blocking the trachea and drowning the occupant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bonus question:&lt;/strong&gt; what are the odds that Luca's plan would have worked if he'd needed to exercise it?&lt;/p&gt;&#xA;" OwnerUserId="406" LastEditorUserId="406" LastEditDate="2013-08-21T00:21:05.217" LastActivityDate="2013-08-21T14:43:17.163" Title="Would the blast clearing technique used in snorkeling work in an EVA situation?" Tags="|microgravity|water|eva|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1411" PostTypeId="2" ParentId="1397" CreationDate="2013-08-21T05:41:33.460" Score="27" Body="&lt;p&gt;From the NASA &lt;a href=&quot;http://astronauts.nasa.gov/content/broch00.htm&quot;&gt;Astronaut Candidate Program&lt;/a&gt; brochure:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Applicants should be aware that selection as an Graduation from the Astronaut Candidate Program will require successful completion of the following: International Space Station systems training, Extravehicular Activity skills training, Robotics skills training, &lt;strong&gt;Russian language training&lt;/strong&gt; , and aircraft flight readiness training. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.jaxa.jp/projects/iss_human/astro/index_e.html&quot;&gt;JAXA's website&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After being selected, astronaut candidates undergo Basic Training. They obtain basic knowledge and technical skills in space engineering, space science, space medicine, the Space Shuttle and ISS systems, the &lt;strong&gt;English and Russian&lt;/strong&gt; languages. They also receive flight training and physical training. Upon completion of Basic Training, they are officially certified as astronauts …&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From the Russian Cosmonaut Training Center's &lt;a href=&quot;http://www.gctc.ru/main.php?id=1253&quot;&gt;Candidate Selection Criteria&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... знания &lt;strong&gt;русского языка&lt;/strong&gt; (письменно и устно) должны быть не ниже хороших, относительно требований средней школы Российской Федерации; знания &lt;strong&gt;английского языка&lt;/strong&gt; на оценку не ниже «хорошо» по требованиям вузов Российской Федерации; &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Translation:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Russian language&lt;/strong&gt; skills (written and oral) must not be less than &lt;em&gt;good&lt;/em&gt;, according to the requirements of high school in Russian Federation; the knowledge of the &lt;strong&gt;English language&lt;/strong&gt; assessment at least &lt;em&gt;good&lt;/em&gt; according to the requirements of higher education institutions of the Russian Federation &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-21T10:37:27.443" LastActivityDate="2013-08-21T10:37:27.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1412" PostTypeId="2" ParentId="1188" CreationDate="2013-08-21T07:37:48.470" Score="11" Body="&lt;p&gt;There are several considerations that lead to selecting a particular hypergolic fuel:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$I_{sp}$ specific impulse&lt;/li&gt;&#xA;&lt;li&gt;$\rho$ density&lt;/li&gt;&#xA;&lt;li&gt;$T_f$ freezing point&lt;/li&gt;&#xA;&lt;li&gt;thermal stability (rate of decomposition vs. temperature)&lt;/li&gt;&#xA;&lt;li&gt;vapor pressure&lt;/li&gt;&#xA;&lt;li&gt;chemical compatibility (rate of corrosion)&lt;/li&gt;&#xA;&lt;li&gt;presence (or absence) of well-characterized economical production capacity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The usual bi-propellant pairs have the following $I_{sp}$ (in seconds):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                    At 1    In &#xA;                    atm    vacuum&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/N2O4&quot; rel=&quot;nofollow&quot;&gt;Dinitrogen tetroxide&lt;/a&gt; ($\require{mhchem}\ce{N2O4}$)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;MMH                 288.5   341.5&#xA;Hydrazine           292.0   343.8&#xA;Aerozine (50/50)    288.9   341.7&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/IRFNA&quot; rel=&quot;nofollow&quot;&gt;Inhibited red fuming acid&lt;/a&gt; III-A (83.4% $\ce{HNO3}$, 14% $\ce{NO2}$, 2% $\ce{H2O}$, 0.6% $\ce{HF}$)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Hydyne&#xA; (UDMH/DETA 60/40)  269.2   318.7&#xA;MMH                 274.5   324.3&#xA;UDMH                272.2   322.1&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Inhibited red fuming acid IV, HDA (54.3% $\ce{HNO3}$, 44% $\ce{NO2}$, 1% $\ce{H2O}$, 0.7% $\ce{HF}$)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Hydyne&#xA; (UDMH/DETA 60/40)  274.4   325.2&#xA;MMH                 279.8   330.8&#xA;UDMH                277.4   328.6&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_rocket_propellants&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Liquid_rocket_propellants&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Diethylenetriamine&quot; rel=&quot;nofollow&quot;&gt;DETA&lt;/a&gt; = $\ce{NH{(C2H4NH2)}_2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/UDMH&quot; rel=&quot;nofollow&quot;&gt;UDMH&lt;/a&gt; = $\ce{{(CH3)}_2 NNH2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrazine&quot; rel=&quot;nofollow&quot;&gt;Hydrazine&lt;/a&gt; = $\ce{N2H4}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Density:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Hydrazine            1010 @ 20 deg. C&#xA;Hydyne                855 @ 15 deg. C&#xA;MMH                   878 @ 20 deg. C&#xA;UDMH                  789 @ 15 deg. C&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Source: Modern Engineering for Design of Liquid-Propellant Rocket Engines, Huzel, Huang. 1992. P.21.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I heartily recommend a great and really funny story of propellants research - &lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;nofollow&quot;&gt;John D. Clark. Ignition! (1972)&lt;/a&gt;. See, in particular, the &quot;Hunting of the Hypergol&quot; chapter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting one fuel for another leads to complications:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ballistics part of the design changes; &lt;/li&gt;&#xA;&lt;li&gt;you have to rethink thermal limits imposed on the tanks;&lt;/li&gt;&#xA;&lt;li&gt;MMH can be used to cool the nozzles due to higher thermal stability, UDMH cannot;&lt;/li&gt;&#xA;&lt;li&gt;launch infrastructure may have to be adapted (different densities and thermal brackets =&gt; changes in procedures and sensors)&lt;/li&gt;&#xA;&lt;li&gt;industrial production capacity cannot be switched to the new fuel at once&lt;/li&gt;&#xA;&lt;li&gt;re-certification is a pain&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All in all, an engine/propellant combination is a design unit, and you won't like changing parts of the combination without much thought and effort.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="8162" LastEditDate="2015-04-15T17:16:42.963" LastActivityDate="2015-04-15T17:16:42.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1413" PostTypeId="2" ParentId="1410" CreationDate="2013-08-21T10:43:55.883" Score="13" Body="&lt;p&gt;No the blast clearing technique would not work, it requires gravity and the closed environment filled with one substance (water) of the snorkel to work.  Neither are present in Luca's situation.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tidalwave's Video links in his comment, indicate that attempts to blow or shake the water away were counter productive, resulting in water entering Luca's ears and nose. While all the data is not in, it appears that water was from the drinking bag and arrived through the ventilation system. The video showing the Lucas re-entering the station, indicates they knew that his drinking water was empty and helmet was filling, so actually his best choice might have been to simply drink any water near his mouth (&lt;em&gt;hindsight in tranquility&lt;/em&gt;). This is of course based on no knowledge my part of another source for the water that would potentially be toxic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit News Update&lt;/strong&gt;&#xA;BBC News 21 August 2013 Last updated at 05:22 ET &quot;&lt;em&gt;Luca Parmitano, 36, expressed bewilderment about the source of the water, as it did not appear to have come from his drinking flask.&lt;/em&gt; &quot; &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-23777804&quot;&gt;http://www.bbc.co.uk/news/science-environment-23777804&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2013-08-21T14:43:17.163" LastActivityDate="2013-08-21T14:43:17.163" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1414" PostTypeId="2" ParentId="1385" CreationDate="2013-08-21T14:44:54.067" Score="8" Body="&lt;p&gt;In addition to Erik's answer I might be able to give you some insights on some disadvantages. I've been growing up around parachutes and until about 10 - 15 years ago you would often see people having a parafoil  as a main canopy and a round one as a reserve (today it's very different, but that's not really relevant). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Round canopies are much simpler than parafoils and their openings have a higher chance of being successful. When a parafoil opens, the object which is attached to it should be stable and still. This means that the object can not have any forward movement, side movement or up and down movement. The object should also have the parachute having on top. If a parafoil is opened with any of the above factors present, chances are high you will end up with a (small) malfunction. A person under a canopy can easily &quot;debug&quot; these malfunctions, machines however can't. While round canopies can be susceptible to these malfunctions, they tend to be a lot more forgiving than parafoils.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of the shape and nature of a parafoil, twists and line-overs can happen during deployment of the canopy (I'm not taking into account malfunctions generally associated with bad packing of the canopy). With a round canopy (line overs don't really exist)  this would not cause any trouble as the canopy would still be symmetrical. With a parafoil you will make one of the corners or sides unstable (this is how parafoils are steered), this will result in the canopy starting to pitch down to one corner and start turning. If the malfunction is not resolved the canopy will start spinning down and picking up vertical speed, ultimately resulting in a high speed crash. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In normal skydiving you would just cut away the canopy and deploy your reserve chute. I guess this could be implemented in spacecraft as well, but due to the weight of the spacecraft the vertical distance covered between start of the parachute deployment and deployed canopy might take a few thousand feet. Take into account that people want to reduce the weight of these things as much as possible, so another canopy this size might be around 100-200 kg in extra equipment for a reserve chute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in short:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Higher chance of successful deployment&lt;/li&gt;&#xA;&lt;li&gt;Less amount of possible malfunctions&lt;/li&gt;&#xA;&lt;li&gt;Round canopies are more forgiving than parafoils&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="521" LastActivityDate="2013-08-21T14:44:54.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1415" PostTypeId="1" AcceptedAnswerId="1425" CreationDate="2013-08-21T15:35:36.960" Score="33" ViewCount="22313" Body="&lt;p&gt;Many rockets use hydrogen- and oxygen-based propellants as fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why are hydrogen-fluorine fuels not used? It has a specific impulse of 390 seconds, higher than hydrogen-oxygen combustion (360 seconds).&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-22T00:12:14.320" LastActivityDate="2017-07-31T14:02:13.110" Title="Why are hydrogen-fluorine fuels not used for rockets more frequently?" Tags="|launch|rockets|fuel|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1416" PostTypeId="2" ParentId="1415" CreationDate="2013-08-21T15:46:59.987" Score="8" Body="&lt;p&gt;Fluorine is very toxic and tends to oxidize more than just the fuel.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorDisplayName="user12" LastEditDate="2013-09-04T17:45:05.907" LastActivityDate="2013-09-04T17:45:05.907" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1417" PostTypeId="5" CreationDate="2013-08-21T16:46:38.867" Score="0" Body="&lt;p&gt;Questions regarding medical aspects of space exploration, like effects of missions on human body during launch, flight and reentry. This tag can include also medical requirements for going into space.&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-08-21T16:47:15.990" LastActivityDate="2013-08-21T16:47:15.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1418" PostTypeId="4" CreationDate="2013-08-21T16:46:38.867" Score="0" Body="Questions regarding medical aspects of space exploration, like effects of missions on human body during launch, flight and reentry." OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-08-21T16:47:20.537" LastActivityDate="2013-08-21T16:47:20.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1419" PostTypeId="5" CreationDate="2013-08-21T16:51:24.843" Score="0" Body="&lt;p&gt;Artificial gravity is the varying (increase or decrease) of apparent gravity via artificial means, particularly in space, but also on Earth. &lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-08-21T16:55:39.550" LastActivityDate="2013-08-21T16:55:39.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1420" PostTypeId="4" CreationDate="2013-08-21T16:51:24.843" Score="0" Body="Artificial gravity is the varying (increase or decrease) of apparent gravity via artificial means, particularly in space, but also on Earth. " OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-08-21T16:55:33.580" LastActivityDate="2013-08-21T16:55:33.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1421" PostTypeId="5" CreationDate="2013-08-21T17:03:47.040" Score="0" Body="&lt;p&gt;Questions related to the movement of human-made objects as they enter atmosphere of Earth or other planetary bodies with atmospheres (Mars, Titan, Venus etc.) from space after being successfully launched. Such objects experience high thermal and aerodynamic loads. Reentry provides the means to dispose of space objects, or reduce propellants for successful landing. The latter necessitates special design measures to ensure survival of the object and its payload, provide precision landing/targeting capabilities and/or to enable controlled maneuvering in the atmosphere. Reentry is divided into ballistic and aerodynamic.&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="168" LastEditorDisplayName="user55" LastEditDate="2015-04-21T13:58:54.910" LastActivityDate="2015-04-21T13:58:54.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1422" PostTypeId="4" CreationDate="2013-08-21T17:03:47.040" Score="0" Body="Questions related to the movement of human-made objects as they enter atmosphere of Earth or other planetary bodies with atmospheres from space after being successfully launched." OwnerDisplayName="user55" LastEditorUserId="168" LastEditorDisplayName="user55" LastEditDate="2015-04-21T13:58:54.910" LastActivityDate="2015-04-21T13:58:54.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1423" PostTypeId="1" AcceptedAnswerId="1426" CreationDate="2013-08-21T17:18:06.440" Score="6" ViewCount="289" Body="&lt;p&gt;There do not appear to be a definite number of stages to launch a craft. Amongst others entries under the 'launch', 'spacecraft-development' tags mention as many as 4 stages. Engines and fuel vary too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are payload and impulse the sole criteria to decide the launch engines, and each stage engine? What factors are relevant to this decision?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="25" LastEditDate="2013-08-21T17:40:45.730" LastActivityDate="2013-08-23T06:52:02.210" Title="Are payload and impulse the sole criteria to decide the engine?" Tags="|launch|rockets|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1424" PostTypeId="2" ParentId="1423" CreationDate="2013-08-21T17:45:50.983" Score="4" Body="&lt;p&gt;Aside from the delta V requirements, there are a number of other things that are used to decide how to make a rocket. A few of the key things:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How flexible do you want the profile of the rocket to be? If you don't care, solid rockets are a great choice. If you do care, then maybe you need a liquid fuel.&lt;/li&gt;&#xA;&lt;li&gt;Where are you launching from? If you are forced to launch near other people, you will make choices based on the environment near where you are. For example, ICBMs and other large rockets launched from ships commonly have a small first stage using a &quot;safe&quot; rocket propellant to get it away from the ship.&lt;/li&gt;&#xA;&lt;li&gt;Availability of the fuel.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;As far as which engine to use, essentially you want to make a good engine that will work with your desired fuel, and at your desired attitude, be easy to make and test, and meet your weight requirements. More than that is probably beyond the scope of this answer.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-21T17:45:50.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1425" PostTypeId="2" ParentId="1415" CreationDate="2013-08-21T17:48:02.930" Score="52" Body="&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/props/lf2lh2.htm&quot;&gt;LF2/LH2&lt;/a&gt;, or liquid difluoride liquid dihydrogen bipropellant (binary cryogenic fuel) has a specific impulse of 410 seconds (by weight) at sea level, which is more than e.g. &lt;a href=&quot;http://www.astronautix.com/props/loxlh2.htm&quot;&gt;LOX/LH2&lt;/a&gt; (liquid oxygen liquid dihydrogen) with average specific impulse by weight of 391 seconds, also at sea level. As the oxidizer is in both cases on board and one of the binary cryogenic propellants, the difference in specific impulse (19 seconds) would remain the same in vacuum, too. I'm not sure where you got your numbers from, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this seems pretty straightforward, LF2/LH2 has a roughly 4 to 5% greater specific impulse per weight. But, as mentioned by many in this thread before me, there's a lot more to liquid difluoride (LF2) than meets the eye with a presentation of a few simple statistics on its performance as a rocket propellant. Mainly, it's toxic, a strong irritant and, as the strongest oxidizer (i.e. the most electronegative element) we know of, will react with anything that's ready to lend it a spare electron. That's not good, because it makes it - in practical terms - too reactive:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Reactions with elemental fluorine are often sudden or explosive. Many&#xA;  substances that are generally regarded as unreactive—such as powdered&#xA;  steel, glass fragments, and asbestos fibers—are readily consumed by&#xA;  cold fluorine gas. Wood and even water burn with flames when subjected&#xA;  to a jet of fluorine, without the need for a spark.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Excerpt source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Fluorine&quot;&gt;Wikipedia on Fluorine&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Needless to say, this makes it extremely difficult and rather costly to produce (NASA was paying \$ 6.00 per kg, compared to LOX that cost \$ 0.04 per kg in 1959), store, and use as a propellant component of any system, rockets being no exception. Combining it with liquid hydrogen in lower atmosphere also brings oxygen and water vapor to the equation, producing &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrofluoric_acid&quot;&gt;hydrofluoric acid&lt;/a&gt;. While it's a weak acid, it's not really a desirable byproduct of reaction of fluoride and hydrogen, while reaction of LOX with LH2 produces H&lt;sub&gt;2&lt;/sub&gt;O (water).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in short, LOX/LH2 is a lot cheaper, much safer, many times over easier to design (engines, storage tanks, e.t.c.) for, and because of all of these trades a preferred oxidiser for large space launchers. It does have a few drawbacks, but ones we've learned to design around, and has been used in many launches, with no need to evacuate a large radius around a launch site prior and during actually using LF2, of fear environmental catastrophe. Sacrificing 4-5% of performance is well worth lowering these risks, no question about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: But, it should be mentioned, 4-5% is a theoretical &lt;em&gt;best case&lt;/em&gt; difference between LF2/LH2 and LOX/LH2 specific impulse, and this difference would lower quite substantially, possibly even reverse in favor of LOX/LH2, when we account for design constraints and the added weight to stages and their engines when using LF2/LH2 instead. This is a bit argumentative, but should also be considered when interpreting theoretical &lt;em&gt;best case&lt;/em&gt; data.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-01T23:59:01.953" LastActivityDate="2013-09-01T23:59:01.953" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1426" PostTypeId="2" ParentId="1423" CreationDate="2013-08-21T17:52:01.163" Score="5" Body="&lt;p&gt;A rocket has different needs at different times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At liftoff, there is a need for a level of thrust that exceeds the weight of the vehicle, or it ain't going anywhere.  Solids are very good at high thrust, but usually have a kind of sucky $I_{sp}$ (&lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;nofollow&quot;&gt;specific impulse&lt;/a&gt;).  For example, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;nofollow&quot;&gt;SRBs&lt;/a&gt; from the shuttle produce 2.8 million lbs of thrust each which is an awe inspiring amount of thrust, but $I_{sp}$ is 269 seconds, really lousy.  In contrast the &lt;a href=&quot;http://www.astronautix.com/engines/f1.htm&quot; rel=&quot;nofollow&quot;&gt;F-1&lt;/a&gt; engine (the largest liquid fueled single chamber (Yes &lt;a href=&quot;http://www.astronautix.com/engines/rd170.htm&quot; rel=&quot;nofollow&quot;&gt;RD-170&lt;/a&gt; I am ignoring you!) ever flown) is only 1.5 million lbs thrust, but an $I_{sp}$ of 263? (What?  I thought it was in the 300's.)  Regardless the point is high thrust is key for a first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once out of the atmosphere, performance changes, so using a single rocket from ground level to orbit has its own penalties.  (Thus the aerospike, where air pressure (and then lack thereof) acts as a nozzle extension for performance reasons). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, once you are out of the atmosphere and working towards orbit, you want the highest $I_{sp}$ at that point.  Nozzle design is different for in atmosphere vs out of atmosphere. Usually want higher expansion ratio as the ambient pressure gets lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, denser propellants mean less tankage, which reduces size of the stage.  Hydrogen (the fluffiest of them all) is the least dense of the common propellants and thus needs huge tanks in comparison to RP-1 which is pretty dense and can get by with smaller tanks.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="168" LastEditDate="2013-08-23T06:52:02.210" LastActivityDate="2013-08-23T06:52:02.210" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1427" PostTypeId="1" AcceptedAnswerId="1432" CreationDate="2013-08-21T18:37:18.507" Score="8" ViewCount="1845" Body="&lt;p&gt;Just what the title says.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image below shows any number of displays, knobs, switches and what-not in the cockpit of the Space Shuttle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ldmz4.jpg&quot; alt=&quot;Space Shuttle cockpit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet all of them may not be &lt;em&gt;necessary&lt;/em&gt; throughout the mission. For instance, an aeroplane pilot can probably get by, in a pinch, with just the following&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;airspeed indicator&lt;/li&gt;&#xA;&lt;li&gt;altimeter&lt;/li&gt;&#xA;&lt;li&gt;artificial horizon&lt;/li&gt;&#xA;&lt;li&gt;compass&lt;/li&gt;&#xA;&lt;li&gt;fuel guage&lt;/li&gt;&#xA;&lt;li&gt;tachometer&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(Apparently the Wright brothers started out with just a tachometer, an anemometer, and a stopwatch)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say powered and manned interplanetary commute become possible on a regular basis &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What instruments would be necessary?&lt;/li&gt;&#xA;&lt;li&gt;What kind of instruments would need to be developed?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-08-22T05:53:37.057" LastActivityDate="2013-08-22T05:53:37.057" Title="What instruments are necessary aboard spacecraft?" Tags="|spacecraft-development|instrument|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1429" PostTypeId="5" CreationDate="2013-08-21T20:09:06.470" Score="0" Body="&lt;p&gt;A space probe is a scientific robotic spacecraft. It explores space toward other planetary bodies (planets, satellites, asteroids, ..). It can flyby, orbit or land on its target or approach interstellar space. Pioneer 10/11 and Vojager 1/2 are examples of space probe.&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorDisplayName="user55" LastEditDate="2013-08-21T20:45:33.667" LastActivityDate="2013-08-21T20:45:33.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1430" PostTypeId="4" CreationDate="2013-08-21T20:09:06.470" Score="0" Body="A space probe is a scientific robotic spacecraft. It explores space and/or other planetary bodies (planets, satellites, asteroids, ..). It can flyby, orbit, enter atmosphere of, land on or submerge into its target, or approach interstellar space. Pioneer 10/11 and Voyager 1/2 are examples of space probe." OwnerDisplayName="user55" LastEditorUserId="49" LastEditDate="2013-08-21T20:45:55.887" LastActivityDate="2013-08-21T20:45:55.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1432" PostTypeId="2" ParentId="1427" CreationDate="2013-08-21T21:09:59.403" Score="8" Body="&lt;p&gt;There is no minimum for cockpit instrumentation, as spacecraft can be completely automated and/or controlled from the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For practical purposes, the minimum craft controls would be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a translational and rotational controller; in theory, these could be combined.&lt;/li&gt;&#xA;&lt;li&gt;a fuel gage&lt;/li&gt;&#xA;&lt;li&gt;a two-way radio&lt;/li&gt;&#xA;&lt;li&gt;life support controls &#xA;&lt;ul&gt;&#xA;&lt;li&gt;Flow Rate (either a dial or an on/off switch)&lt;/li&gt;&#xA;&lt;li&gt;pressure&lt;/li&gt;&#xA;&lt;li&gt;temperature&lt;/li&gt;&#xA;&lt;li&gt;oxygen fraction&lt;/li&gt;&#xA;&lt;li&gt;oxygen level monitor&lt;/li&gt;&#xA;&lt;li&gt;carbon dioxide level monitor&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are based upon the combined controls for the NASA MMU and the NASA EVA suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A practical spacecraft should also have:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;attitude indicator &lt;/li&gt;&#xA;&lt;li&gt;additional internal environmental controls&#xA;&lt;ul&gt;&#xA;&lt;li&gt;lights&lt;/li&gt;&#xA;&lt;li&gt;radio controls&lt;/li&gt;&#xA;&lt;li&gt;antenna pointing controls&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;computer interface for guidance controls&lt;/li&gt;&#xA;&lt;li&gt;drive condition monitors&lt;/li&gt;&#xA;&lt;li&gt;power system monitors and controls&lt;/li&gt;&#xA;&lt;li&gt;door status monitors&lt;/li&gt;&#xA;&lt;li&gt;other mission equipment monitors.&lt;/li&gt;&#xA;&lt;li&gt;fuel condition and quantity monitors&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Most of the controls will be multiple devices, and many will have redundancies, so each redundant system will have its own controls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's worth noting that the Shuttle could have been controlled with far fewer manual controls, but that the NASA felt it important to retain manual controls rather than computerized ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the Shuttle, since it flies in US airspace, also has full aircraft instrumentation, including &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;altimeter&lt;/li&gt;&#xA;&lt;li&gt;airspeed&lt;/li&gt;&#xA;&lt;li&gt;landing gear operation&lt;/li&gt;&#xA;&lt;li&gt;landing gear status&lt;/li&gt;&#xA;&lt;li&gt;flight control surface status&lt;/li&gt;&#xA;&lt;li&gt;control hydraulic pressures&lt;/li&gt;&#xA;&lt;li&gt;Approach slope indicator&lt;/li&gt;&#xA;&lt;li&gt;turn and bank indicator.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-21T21:09:59.403" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1433" PostTypeId="2" ParentId="1423" CreationDate="2013-08-21T21:24:09.273" Score="3" Body="&lt;p&gt;No, but ISP and thrust are two of the most important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also considered are environmental concerns, fuel handling requirements, drive mass, fuel mass, and price of drives, fuel, and supporting structure. A few types of drive, namely ORION and NTR, are not used despite fairly high thrust and ISP, for reasons of politics, namely, extant treaties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ESA won't use certain types of power system (fission and/or radiothermal generator) due to the environmental risks in case of launch failure; this also makes the needed drives for maneuver more restricted, since ion drives require rather high power that cannot be provided in the far reaches of the solar system.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-21T21:24:09.273" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1434" PostTypeId="2" ParentId="1398" CreationDate="2013-08-21T21:58:39.717" Score="8" Body="&lt;p&gt;Any launcher that can carry a warhead payload into orbit has military uses. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In point of fact, the first civilian launch vehicles were in fact ICBMs with science payloads instead of warheads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, any orbital camera with a 1m or smaller surface object resolution has military uses; current civil cameras in orbit are below 20cm surface resolution. (I was able to see a roughly 15cm wide post on a 2013 orbital image on google maps, and it was more than 1 pixel wide. I can make out 18cm wide by 40cm long mailboxes on the same image. If I can make out individual mailboxes on civil orbital imaging, then I can identify most military vehicles, at least as to type and location.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-21T21:58:39.717" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1435" PostTypeId="1" AcceptedAnswerId="1557" CreationDate="2013-08-21T22:24:17.833" Score="5" ViewCount="90" Body="&lt;p&gt;It can be argued that first rocket in history was a steam powered pigeon in 400 BCE. Around 1200 CE, the Chinese were using rocket powered weapons of war. Beginning around 1960, both the Americans and the Russians had major government funded scientific rocket and space exploration programs in place. Presently (2013), we're seeing an increasing involvement of privately funded rocket research.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to report the funding for rocket research and/or construction by the three major sources: military, private and government?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;http://www.grc.nasa.gov/WWW/k-12/TRC/Rockets/history_of_rockets.html&quot;&gt;Brief History of Rockets (NASA)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-08-21T23:54:51.927" LastActivityDate="2013-08-25T09:41:36.593" Title="How is rocketry funding divided by sources throughout the ages?" Tags="|rockets|history|cost|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1436" PostTypeId="1" AcceptedAnswerId="1437" CreationDate="2013-08-21T22:33:04.697" Score="16" ViewCount="5488" Body="&lt;p&gt;If you had enough ion thrusters, could you take off from Earth's surface into low Earth orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, why not?&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorDisplayName="user29" LastEditDate="2013-08-23T00:51:32.947" LastActivityDate="2013-08-23T12:23:02.300" Title="Could a ship take off from Earth using ion thrusters?" Tags="|launch|propulsion|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1437" PostTypeId="2" ParentId="1436" CreationDate="2013-08-21T22:48:43.943" Score="10" Body="&lt;p&gt;The answer depends on how you define 'enough ion thrusters'.  Per some of the discussion on &lt;a href=&quot;https://space.stackexchange.com/questions/1423/are-payload-and-impulse-the-sole-criteria-to-decide-the-engine&quot;&gt;this question&lt;/a&gt; the key at launch is, can you generate more thrust, than your vehicle masses.  Else you sit still on the launch pad. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the Shuttle uses SRB's that have awesome amounts of thrust (&lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;14.7 MN (3.3M lbs) at sea level&lt;/a&gt;, vs the &lt;a href=&quot;http://arstechnica.com/science/2013/04/new-f-1b-rocket-engine-upgrades-apollo-era-deisgn-with-1-8m-lbs-of-thrust/&quot; rel=&quot;nofollow noreferrer&quot;&gt;F-1B which is in theory 8.0 MN (1.8M lbs)&lt;/a&gt;) but crappy ISP. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ion thrusters (accelerate a charged particle out the back very fast, and live off the momentum gain) have awesome ISP, but crappy thrust levels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So can you get 'enough' ion thrusters to generate enough thrust to exceed the mass of your vehicle? Today? No.  Tomorrow?  Who can know? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is known as thrust to weight ratio.  Since you cannot exceed 1.0 you will just sit on the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does seem like it will take a radical change in the technology to increase the thrust for this to ever work, but stranger things have happened. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2013-08-22T18:14:50.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1438" PostTypeId="1" AcceptedAnswerId="1842" CreationDate="2013-08-21T23:27:38.657" Score="14" ViewCount="3722" Body="&lt;p&gt;Does radiation danger &lt;em&gt;increase&lt;/em&gt; or &lt;em&gt;decrease&lt;/em&gt; as you travel farther from the Sun?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that there's a significant radiation hazard to astronauts. Say, traveling from Earth to Mars, due to radiation from the Sun and also from other sources.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this hazard decrease the farther you get from the Sun and into the &lt;strong&gt;outer solar system&lt;/strong&gt;? I'm not asking solely about the radiation dangers from the Sun. I'm asking about all radiation dangers, regardless of their source. Do &lt;strong&gt;levels of any and all radiation sources&lt;/strong&gt; increase or decrease the farther out you go?&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="49" LastEditDate="2013-09-07T18:05:04.890" LastActivityDate="2013-09-08T07:30:53.240" Title="Does the radiation danger in interplanetary space decrease the further you get from the sun?" Tags="|radiation|solar-system|cosmic-radiation|interstellar-medium|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1439" PostTypeId="2" ParentId="1436" CreationDate="2013-08-21T23:28:11.487" Score="15" Body="&lt;p&gt;I believe the strongest engine in this class is the &lt;a href=&quot;https://en.wikipedia.org/wiki/VASIMR&quot; rel=&quot;noreferrer&quot;&gt;VASIMR&lt;/a&gt; VX-50 -- which produced a whopping &lt;a href=&quot;http://www.adastrarocket.com/Jared_IEPC07.pdf&quot; rel=&quot;noreferrer&quot;&gt;0.5∓0.1 N (0.11∓0.02 lbf) of thrust&lt;/a&gt; in a test stand for a very short time.  That's less than the weight of the engine itself so I suspect we are a ways away from launching anything with an ion drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think these drives also need to be operated in a vacuum.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="168" LastEditDate="2013-08-23T12:02:17.657" LastActivityDate="2013-08-23T12:02:17.657" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1441" PostTypeId="1" AcceptedAnswerId="1831" CreationDate="2013-08-22T03:47:09.327" Score="16" ViewCount="4997" Body="&lt;p&gt;In 19th century texts on astronomy, the planets are often represented by &lt;a href=&quot;http://en.wikipedia.org/wiki/Astronomical_symbols&quot;&gt;symbols&lt;/a&gt;: ☿ for Mercury, ♀ for Venus, etc..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did NASA ever use these symbols, or were they already obsolete by then?  If they did use them, I'd expect it to be very early on.&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="552" LastEditDate="2013-08-22T05:39:27.793" LastActivityDate="2019-03-09T23:02:38.943" Title="Did NASA ever use astronomical symbols for planets?" Tags="|terminology|" AnswerCount="4" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="1442" PostTypeId="5" CreationDate="2013-08-22T06:11:12.820" Score="0" Body="&lt;p&gt;Questions about research and use of water in space environment&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="10273" LastEditorDisplayName="user55" LastEditDate="2018-09-29T10:16:05.723" LastActivityDate="2018-09-29T10:16:05.723" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1443" PostTypeId="4" CreationDate="2013-08-22T06:11:12.820" Score="0" Body="Questions about research and use of water in space environment" OwnerDisplayName="user55" LastEditorUserId="10273" LastEditorDisplayName="user55" LastEditDate="2018-09-29T10:16:10.973" LastActivityDate="2018-09-29T10:16:10.973" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1444" PostTypeId="2" ParentId="1380" CreationDate="2013-08-22T08:55:55.090" Score="21" Body="&lt;p&gt;Here is an approximate but quite accurate solution.  This assumes that the orbit about the departure body is aligned with the outgoing asymptote of the Hohmann orbit, which is the standard practice when putting a spacecraft in a parking orbit by a launch vehicle before its escape maneuver.  This also assumes that the bodies' orbits about the Sun are circular and that they are coplanar.  This is pretty close for the planets (as currently defined) about our Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We will also assume instantaneous maneuvers, which is a good approximation for chemical rockets, and what is implied by the use of the Hohmann solution.  The transfer would be quite different for low-thrust systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $\Delta V$ can be determined by repeated application of this equation that simply says that the total energy is the sum of the kinetic energy and the potential energy:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}=\frac{v^2}{2}-\frac{\mu}{r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mathcal{E}$ is the total energy per unit mass of the object or the &quot;specific energy&quot;, $v$ is the velocity of the object at the current position, $\mu$ is the GM of the central body, i.e. Newton's gravitational constant times its mass, and $r$ is the current distance from the center of the central body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key is that the total energy of the object is a constant of motion over the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We will also use the fact that orbits are ellipses, and this equation, which determines that constant of motion from the apses of the orbit, i.e. the radii of the closest and farthest points in the orbit, $r_1$ and $r_2$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}=-{\mu\over r_1+r_2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this problem we define:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mu_S$ = GM of the Sun.&lt;br&gt;&#xA;$\mu_1$ = GM of the departure body.&lt;br&gt;&#xA;$\mu_2$ = GM of the arrival body.&lt;br&gt;&#xA;$r_1$ = radius of departure body orbit about the Sun, assumed to be circular.&lt;br&gt;&#xA;$r_2$ = radius of arrival body orbit about the Sun, assumed to be circular.&lt;br&gt;&#xA;$a_1$ = circular orbit radius of the spacecraft about the departure body.&lt;br&gt;&#xA;$a_2$ = circular orbit radius of the spacecraft about the arrival body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hohmann transfer orbit has apses $r_1$ and $r_2$.  So its specific energy is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}_H=-{\mu_S\over r_1+r_2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We then compute the velocity of that orbit at $r_1$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}_H=-{\mu_S\over r_1+r_2}=\frac{v_{H1}^2}{2}-\frac{\mu_S}{r_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which gives:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{H1}=\sqrt{\frac{2\mu_S r_2}{r_1(r_1+r_2)}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the circular orbit of the departure body about the Sun, we have for its velocity about the Sun:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$-{\mu_S\over 2r_1}={v_1^2\over 2}-{\mu_S\over r_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_1=\sqrt{\mu_S\over r_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The geometry dictates that the Hohmann transfer orbit velocity at periapsis is in the same direction as the departure body velocity, and they are at the same radius from the Sun.  So the velocity change from the departure circular orbit to the Hohmann transfer orbit is just the difference (this will be squared later, so the sign doesn't matter):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{T1}=v_{H1}-v_1=\sqrt{\frac{2\mu_S r_2}{r_1(r_1+r_2)}}-\sqrt{\mu_S\over r_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is the velocity change to get from the departure body &lt;em&gt;orbit&lt;/em&gt; to the Hohmann transfer orbit, but without the departure body being there.  To do the maneuver from a circular orbit about the body, we will use the patched conic approximation that we need an escape velocity from the body equal to that transfer velocity.  Here is where the plane of the orbit about the body is important, since it needs to line up with the direction of the transfer velocity to minimize the $\Delta V$.  In this case, that is the plane shared by the orbits of the two bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The specific energy of an escaped object sufficiently far from the body (where the sufficiency is at the heart of the patched conic approximation), is the energy equation with the second term going to zero as the distance goes to infinity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E_{escape}}=\frac{v_{\infty}^2}{2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now to get the change in velocity for that escape and injection to the Hohmann transfer, we take the difference between the velocity of that escape energy at orbital radius of the spacecraft about the departure body and the orbital velocity of the spacecraft:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\frac{v_{T1}^2}{2}=\frac{v_{escape}^2}{2}-\frac{\mu_1}{a_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{escape}=\sqrt{v_{T1}^2+\frac{2\mu_1}{a_1}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{orbit}=\sqrt{\mu_1\over a_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{inject}=v_{escape}-v_{orbit}=\sqrt{v_{T1}^2+\frac{2\mu_1}{a_1}}-\sqrt{\mu_1\over a_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{inject}=\sqrt{\left(\sqrt{\frac{2\mu_S r_2}{r_1(r_1+r_2)}}-\sqrt{\mu_S\over r_1}\right)^2+\frac{2\mu_1}{a_1}}-\sqrt{\mu_1\over a_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is the actual $\Delta V$ required by the spacecraft to go directly from the circular orbit about the departure body to to the Hohmann transfer orbit with a single instantaneous maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can repeat all that for insertion into the arrival orbit to get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{insert}=\sqrt{\left(\sqrt{\frac{2\mu_S r_1}{r_2(r_1+r_2)}}-\sqrt{\mu_S\over r_2}\right)^2+\frac{2\mu_2}{a_2}}-\sqrt{\mu_2\over a_2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total $\Delta V$ is then the sum of the two maneuvers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V=\sqrt{\left(\sqrt{\frac{2\mu_S r_2}{r_1(r_1+r_2)}}-\sqrt{\mu_S\over r_1}\right)^2+\frac{2\mu_1}{a_1}}-\sqrt{\mu_1\over a_1}+\sqrt{\left(\sqrt{\frac{2\mu_S r_1}{r_2(r_1+r_2)}}-\sqrt{\mu_S\over r_2}\right)^2+\frac{2\mu_2}{a_2}}-\sqrt{\mu_2\over a_2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or simplified a little:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V=\sqrt{{\mu_S\over r_1}\left(\sqrt{\frac{2r_2}{r_1+r_2}}-1\right)^2+\frac{2\mu_1}{a_1}}-\sqrt{\mu_1\over a_1}+\sqrt{{\mu_S\over r_2}\left(\sqrt{\frac{2r_1}{r_1+r_2}}-1\right)^2+\frac{2\mu_2}{a_2}}-\sqrt{\mu_2\over a_2}$&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-08-23T01:45:11.197" LastActivityDate="2013-08-23T01:45:11.197" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1445" PostTypeId="1" CreationDate="2013-08-22T09:19:48.280" Score="8" ViewCount="442" Body="&lt;p&gt;Has any satellite proved the statement that the Sun's gravity could be used as a magnifying lens, focussing electromagnetic waves of distant galaxies or other celestial objects?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, is this idea, for the time being, only hypothetical?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-22T09:55:58.183" LastActivityDate="2013-08-22T12:57:05.650" Title="Has any satellite proved the hypothesis that the Sun could be used as a gravitational lens?" Tags="|telescope|artificial-satellite|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1446" PostTypeId="2" ParentId="1445" CreationDate="2013-08-22T09:29:04.997" Score="6" Body="&lt;p&gt;Hubble and various others have proven this (although this was already proven from ground observatories)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some excellent pictures of lensed objects online, such as this galaxy arc on &lt;a href=&quot;http://hubblesite.org/gallery/album/pr2012008a/&quot;&gt;Hubblesite - this one was found in 2011:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That was lensed from another stellar object, but there should be no difference between the sun and other stars etc.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-08-22T09:29:04.997" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1448" PostTypeId="2" ParentId="1438" CreationDate="2013-08-22T11:18:39.953" Score="3" Body="&lt;p&gt;Yes as you move away from our solar system the radiation from the sun decreases because it follows inverse square law.. But other source of radiation is&#xA;&lt;a href=&quot;http://srag-nt.jsc.nasa.gov/SpaceRadiation/What/What.cfm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galactic cosmic radiation &lt;/a&gt;:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/541rd.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Galactic Cosmic Radiation (GCR) Galactic cosmic radiation originates outside the solar system. It consists of ionized atoms ranging from a single proton up to an uranium nucleus. The flux (rate of flow) levels are the low. However, since they travel very close to the speed of light, and because some of them are composed of very heavy elements such as iron, they produce intense ionization as they pass through matter.&lt;/p&gt;&#xA;&lt;p&gt;For the most part, the Earth's magnetic field provides shielding for spacecraft from galactic cosmic radiation. However, cosmic rays have free access over the polar regions where the magnetic field lines are open to interplanetary space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9Ay3Y.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The flux of galactic cosmic radiation is isotropic outside heliosphere , inside it propagation effect results in an isotropic of approximately 1%&lt;/p&gt;&#xA;&lt;p&gt;The sun is not only the source of radiation to an spacecraft but there are countless stars like sun in and outside our solar system which emit radiation.&lt;/p&gt;&#xA;&lt;p&gt;The radiation effects decreases to greater extent ( after heliosphere ) unless the spacecraft is hit by the suddenout burst of galactic cosmic radiation.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-07T03:53:23.590" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1449" PostTypeId="2" ParentId="1269" CreationDate="2013-08-22T12:48:26.910" Score="25" Body="&lt;p&gt;There is one area of exploration on Earth that approximates conditions on Venus, namely that of &lt;a href=&quot;http://www.analog.com/library/analogdialogue/archives/46-04/high_temp_electronics.html&quot;&gt;deep oil and gas mining&lt;/a&gt;, and a few additional areas of technology, near avionics engines, and even auto engines. The stated goal for such electronics is to function at 200 C or higher. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most promising technology for surviving high temperatures is Silicon Carbide electronics, with hermetically sealed ceramic packages. These can be rated for as high as 600 C, but they aren't completely available yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it is just barely within our technology today to build something to survive on Venus (460 C), and operate at high temperatures/pressures. Likely, the solution will come from the Oil Mining Industry, where it is common to operate at high temperatures and pressures today. It will take significant engineering, but everything does in the space industry anyways.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-03-07T17:38:50.957" LastActivityDate="2014-03-07T17:38:50.957" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1450" PostTypeId="2" ParentId="1445" CreationDate="2013-08-22T12:57:05.650" Score="5" Body="&lt;p&gt;There kind of is a problem for doing that within the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can imagine the sun's gravitational field to be a lens, but this is an imperfect model.  The main issue with the analogy is that the deflection angle increases as you get closer.  This isn't a big issue, provided that we recognize that we're talking about the creation of an &lt;a href=&quot;http://en.wikipedia.org/wiki/Einstein_ring&quot; rel=&quot;noreferrer&quot;&gt;Einstein ring&lt;/a&gt;, and not &lt;em&gt;literally&lt;/em&gt; focusing something in the same way we would for a telescope.  For the ring, the same relationship between angle of deflection and focal length holds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All spherically symmetric bodies have the same gravitational field around them.  This is true in both classical mechanics and &lt;a href=&quot;http://en.wikipedia.org/wiki/Schwarzschild_metric&quot; rel=&quot;noreferrer&quot;&gt;general relativity&lt;/a&gt;, and the idea holds right up until you get to the surface of the body.  Now here's the problem:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you calculate it, the radius that gets you the right angle of deflection to form an Einstein ring (of a far-away object) with the sun will be &lt;em&gt;below the radius of the sun&lt;/em&gt;.  This obviously isn't useful because the light will not pass through the interior (defined by the photosphere) of the sun.  To walk through the logic a little bit more, imagine these steps:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pick some radius from the center of the sun&lt;/li&gt;&#xA;&lt;li&gt;With that radius and the mass of the sun, we can calculate an angle of deflection&lt;/li&gt;&#xA;&lt;li&gt;That angle of deflection will correlate to some focal length - this is where you would want to put the probe according to the proposal at hand&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For the minimal requirements to do such a mission, we will start out requiring that the radius at which the deflection takes place will be greater than the sun's radius itself.  That will give a relatively low angle of deflection.  That will then give a long focal length - certainly more than 1 AU, which is kind of the assumption implicit in the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could Voyager possibly see an Einstein ring produced of a distant star when looking at the sun?  I haven't done the calculation, but yes, that would be possible if the math checks out.  So my is that yes you can do this, but only at a huge distance, certainly not from the inner solar system where we reside.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-08-22T12:57:05.650" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1451" PostTypeId="2" ParentId="684" CreationDate="2013-08-22T13:19:20.163" Score="3" Body="&lt;p&gt;The key part to answer this question can be found at &lt;a href=&quot;http://www.dangermouse.net/gurps/science/ramjet.html&quot; rel=&quot;nofollow&quot;&gt;dangermouse.net&lt;/a&gt;. Specifically, this formula must be satisfied:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_t   &amp;gt;   \frac{dm}{dt} / ( π\cdot r2 \cdot\ ρ)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $V_t$= Velocity, $\frac{dm}{dt}$ is the minimum fusion rate of hydrogen, $ π\cdot r2 $ is the area of the funnel, and $ρ$ is the density of hydrogen in the interstellar medium. Right now, all of these are known except for one, namely what is the minimum rate to fuse hydrogen. This will very likely depend on the engine. I'm going to say that we need to collect 1 gram every second, just as a demonstration, feel free to insert the real requirement when you have it. Let's also assume a 1 km funnel, as that seems to be a popular size for a rotational sphere. Let's assume about 20 atoms/cm^3, per &lt;a href=&quot;http://en.wikipedia.org/wiki/Interstellar_medium&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;. That gives us a required speed of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$(1)/(10^6*pi*20e6/6.0221413e+23)=9584535558.93 m/s=31c$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which is impossible, being 31 times faster than the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, this would require a huge funnel, and the ability to fuse hydrogen at trace quantities, or operating in a dense cloud of hydrogen, in order to make it work. For the mean time, it seems extremely difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the same set of numbers, let's see what a more realistic 20 AU/year, or about 100 km/s. This chosen because it's the fastest conventional rocket I've heard of. That would require a funnel area of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$(1)/(100000 \cdot 20e6/6.0221413e+23)=301107064500 m^2$ That leaves a radius of 175 km, which is clearly large, but not impossible!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's try one more time, with the solar escape velocity from the sun's surface, and the particle density near the sun. &lt;a href=&quot;http://en.wikipedia.org/wiki/Corona&quot; rel=&quot;nofollow&quot;&gt;That density&lt;/a&gt; is about $10^9 particles/m^2$, &lt;a href=&quot;http://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow&quot;&gt;solar escape velocity&lt;/a&gt; is 617 km/s. Thus, the required size is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$(1)/(617000 \cdot 1e9/6.0221413e+23)=976035867.099m^2$ That leaves a radius of 10 km. Thus, the best thing to do with a ramjet is to somehow have it pass right by the sun. Activating the ram jet there would allow one to gain some speed. I'm not going to go through the math to determine the appropriate acceleration to make it work at that speed, but it could work, given appropriate amounts of mass.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-22T15:45:05.817" LastActivityDate="2013-08-22T15:45:05.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1452" PostTypeId="1" AcceptedAnswerId="1454" CreationDate="2013-08-22T15:42:00.103" Score="12" ViewCount="451" Body="&lt;p&gt;It is a well-known fact that it is quite noisy aboard the ISS. Fans and pumps working all the time, valves closing/opening etc. Crew members live through it with ear protection, but that doesn't mean we shouldn't try to make long-term stay in space more comfortable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much will it cost mass-, time-, and money-wise to reduce noise by any combination of means&lt;sup&gt;*&lt;/sup&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've long had an industry (building submarines) making considerable money on noise reduction, and loads of expertise. Can this expertise be applied to ECLSS (environment control and life support system) aboard manned spacecraft?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;*&lt;/sup&gt; &lt;sup&gt;Better bearings, noise cancellation, enhanced computer modelling of vibration &amp;amp; transient sounds, etc.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2013-08-23T15:44:45.590" Title="Combatting noise in spacecraft" Tags="|life-support|astronaut-lifestyle|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1453" PostTypeId="2" ParentId="1394" CreationDate="2013-08-22T16:02:36.293" Score="14" Body="&lt;p&gt;No. Two examples:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Solid rockets are &quot;fueled&quot; (not really the right term) during manufacture. The grain of a solid rocket motor contains a mix of fuel and oxidizer. Once manufactured it is integrated into the rocket. No additional propellant is added during the countdown. Lots of rockets use solid rockets, e.g. the shuttle. Most ballistic missiles are entirely solid motors, specifically because by the time they fueled the nuclear war would be over. Pegasus is also entirely solid (see #2 below regarding HAPS).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Rockets with storable propellants are loaded earlier than final countdown. Typically during launch integration or even by the propulsion system vendor. Examples of storable propellant systems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Hydrazine for monopropellant thrusters like the &lt;a href=&quot;http://en.wikipedia.org/wiki/Pegasus_%28rocket%29#Launch_profile&quot;&gt;Pegasus HAPS module&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Xenon Hall thrusters, where the non-cryogenic propellant is stored in pressure vessels. Not suitable for launch of course because they are low thrust, but they are used on satellites that sit on rockets.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Cryogenics (liquid hydrogen, liquid oxygen) are loaded during final countdown because they boil-off and thermally soak the structure causing potentially nasty things like icing and getting components below their operating temperatures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For LOX/RP rockets that use a mix of storable (RP) and cryogenic (LOX) propellants, the RP is often loaded well before launch. I know for example, that the RP on an Atlas V is often loaded  during the dry-run for the roll-out to the pad, and once loaded stays on the rocket. It's only the LOX that's loaded during the final countdown.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2013-08-22T16:02:36.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1454" PostTypeId="2" ParentId="1452" CreationDate="2013-08-22T16:28:22.287" Score="11" Body="&lt;p&gt;Yes, noise is a problem on the ISS - this graph shows noise levels in various parts of the station:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rdnoq.png&quot; alt=&quot;noise levels&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;https://www.google.com/search?q=spacecraft+noise+reduction&amp;amp;oq=spacecraft+noise+reduction&amp;amp;aqs=chrome.0.69i57j0l3j69i62l2.9877j0&amp;amp;sourceid=chrome&amp;amp;ie=UTF-8#fp=d7af98befdd717b2&amp;amp;q=iss+noise+reduction&quot; rel=&quot;noreferrer&quot;&gt;quick search&lt;/a&gt;  for spacecraft noise reduction revealed &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100039608_2010042539.pdf&quot; rel=&quot;noreferrer&quot;&gt;this rather interesting NASA technical note&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is important to control acoustic noise aboard the International Space Station &#xA;  (ISS) to provide a satisfactory environment for voice communications, crew productivity, &#xA;  and restful sleep, and to minimize the risk for temporary and permanent hearing loss.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The paper goes on to show what is being done on the station to combat the noise levels - such as 'acoustic lining' on air vents:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nZJm1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, an 'acoustic cover' was put over the air conditioning unit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pTrWz.png&quot; alt=&quot;T&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many more examples in the paper - it's a good read.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-22T19:54:34.930" LastActivityDate="2013-08-22T19:54:34.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1455" PostTypeId="1" AcceptedAnswerId="1671" CreationDate="2013-08-22T17:57:43.653" Score="14" ViewCount="1648" Body="&lt;p&gt;While the American segment of the ISS has data published on-line (&lt;a href=&quot;http://spacestationlive.nasa.gov&quot; rel=&quot;nofollow&quot;&gt;http://spacestationlive.nasa.gov&lt;/a&gt;), the same cannot be said about the Russian segment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is known that telemetry is transmitted to the Earth on 128 kHz-wide PCM (&lt;a href=&quot;https://en.wikipedia.org/wiki/PCM&quot; rel=&quot;nofollow&quot;&gt;Pulse-code modulated&lt;/a&gt;) FM (&lt;a href=&quot;https://en.wikipedia.org/wiki/Frequency_modulation&quot; rel=&quot;nofollow&quot;&gt;Frequency modulated&lt;/a&gt;) channels at 628, 630, 632, 634, 922, 928 MHz (source: &lt;a href=&quot;http://www.ea1uro.com/megasat.html&quot; rel=&quot;nofollow&quot;&gt;http://www.ea1uro.com/megasat.html&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How could one receive, demodulate, and decode the telemetry?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is Russian ISS telemetry encrypted?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Are there any high-frequency data items in the frames (transmitted more often than once a second - 1 Hz).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Historical reference: &lt;a href=&quot;http://www.svengrahn.pp.se/radioind/radioin1.htm&quot; rel=&quot;nofollow&quot;&gt;Sven Grahn's Space Radio page&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-05-03T22:07:23.100" LastActivityDate="2015-05-03T22:07:23.100" Title="Decoding Russian ISS telemetry" Tags="|iss|communication|russia|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1458" PostTypeId="5" CreationDate="2013-08-22T21:25:06.790" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-22T21:25:06.790" LastActivityDate="2013-08-22T21:25:06.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1459" PostTypeId="4" CreationDate="2013-08-22T21:25:06.790" Score="0" Body="Radio detection and ranging devices. Operate by detecting and processing return echoes from objects and surfaces of signals at radio frequencies. Can be used to ascertain spatial coordinates and velocities, surface characteristics, object orientation, aid in object identification. In space applications used for tracking, radar imaging, rendezvous, collision avoidance, and other purposes." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-22T21:42:47.127" LastActivityDate="2013-08-22T21:42:47.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1460" PostTypeId="2" ParentId="1191" CreationDate="2013-08-23T00:06:14.370" Score="5" Body="&lt;h2&gt;Yes.&lt;/h2&gt;&#xA;&lt;p&gt;How do I know that? Well, &lt;a href=&quot;http://www.columbiassacrifice.com/$C_hypersonic.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;multiple&lt;/a&gt; &lt;a href=&quot;https://www.grc.nasa.gov/WWW/BGH/hihyper.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;sources&lt;/a&gt; say that a layer of 'electrically charged plasma' develops around the ship.&lt;/p&gt;&#xA;&lt;p&gt;Since the plasma is electrically charged (&lt;em&gt;highly&lt;/em&gt; charged), some of that charge will be transferred to the hull of the vehicle.&lt;/p&gt;&#xA;&lt;p&gt;I couldn't find any info on the &lt;em&gt;amount&lt;/em&gt; of charge generated, but I'm sure it's quite high considering the forces involved.&lt;/p&gt;&#xA;&lt;p&gt;I found &lt;a href=&quot;http://arxiv.org/pdf/0906.3884.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;a paper&lt;/a&gt; on the subject of 'Spacecraft Charging and Hazards to Electronics in Space' - which seems to be exactly what we're looking for. It primarily deals with the effects of this plasma and charges in orbit, but much of the information is relevant to reentry also.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-23T00:06:14.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1461" PostTypeId="2" ParentId="1436" CreationDate="2013-08-23T06:42:16.803" Score="13" Body="&lt;p&gt;Ion thrusters of current designs would short out the charged grid in surface level atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current generation ion thrusters rely upon a difference in charges ionizing and then attracting the reaction mass. Since the electrical voltage difference between the charged grids is extremely high, and needs to remain so to operate, the presence of atmosphere could, and probably would, result in air arcing between them, and discharging the grids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, as an example, the NSTAR Ion Engine masses 8.2kg, and produces a peak of 92millinewtons, which, before accounting for anything else, would be about 11mm per second per second...  about 1/1100th the thrust needed to lift itself. Just the drive, power, and digital controller unit is about 25.5kg... for about 3.6mm per second per second... actual performance of the DS-1 probe, massing 373kg, was about 0.25mm per second per second... highway speed (88kph, or 24km/s) takes 97777.8 seconds or so... just over 27.1 hours (and almost 1200km). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been estimated that, at best, ion drives will peak at around 100 times the thrust to weight ratio they are currently at. (In part, this is because one can only accelerate the ions to just shy of light speed...) which would still leave them well below the performance shown in certain science fiction settings. After all, 1.1 N on a 25 kg package would still be only 4cm per second per second... far short of the 9.80665 Newtons per kg needed to attain the 9.80665 meters per second per second to merely hold against surface gravity... &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Deep_Space_1&quot;&gt;http://en.wikipedia.org/wiki/Deep_Space_1&lt;/a&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/vision/space/travelinginspace/future_propulsion.html&quot;&gt;http://www.nasa.gov/vision/space/travelinginspace/future_propulsion.html&lt;/a&gt;&#xA;&lt;a href=&quot;http://www.grc.nasa.gov/WWW/ion/past/90s/nstar.htm&quot;&gt;http://www.grc.nasa.gov/WWW/ion/past/90s/nstar.htm&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-23T06:42:16.803" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1462" PostTypeId="2" ParentId="646" CreationDate="2013-08-23T06:56:52.030" Score="18" Body="&lt;p&gt;As far as games/simulations go, I have stumbled upon &lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/index.html&quot;&gt;Orbiter&lt;/a&gt;. Seems to have quite a few &lt;a href=&quot;https://orbithangar.com&quot;&gt;add-ons&lt;/a&gt; and a &lt;a href=&quot;http://www.orbiter-forum.com/&quot;&gt;forum&lt;/a&gt;. Unfortunately, works under Windows only.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2013-08-23T06:56:52.030" CommentCount="2" CommunityOwnedDate="2013-08-23T06:56:52.030" ContentLicense="CC BY-SA 3.0" />
  <row Id="1464" PostTypeId="1" AcceptedAnswerId="1465" CreationDate="2013-08-23T08:55:06.263" Score="8" ViewCount="330" Body="&lt;p&gt;Recently, the Sun has ejected coronal mass particles towards the Earth at a rate of 3.3 million km per hour. Can these particles affect our artificial satellites' signals, and why coronal mass ejections happen on the Sun?&lt;/p&gt;&#xA;" OwnerUserId="536" LastEditorUserId="49" LastEditDate="2013-08-23T09:02:35.447" LastActivityDate="2013-08-23T10:16:42.960" Title="Why do coronal mass ejections happen on the Sun? Can they affect artificial satellites?" Tags="|artificial-satellite|the-sun|cme|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1465" PostTypeId="2" ParentId="1464" CreationDate="2013-08-23T09:16:19.603" Score="6" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot;&gt;Wikipeia on &lt;strong&gt;Coronal mass ejection (CME)&lt;/strong&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Recent scientific research has shown that the phenomenon of magnetic reconnection is responsible for CME and solar flares. Magnetic reconnection is the name given to the rearrangement of magnetic field lines when two oppositely directed magnetic fields are brought together. This rearrangement is accompanied with a sudden release of energy stored in the original oppositely directed fields. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Effects on artificial satellites&lt;/strong&gt; are best explained in &lt;a href=&quot;http://hesperia.gsfc.nasa.gov/sftheory/spaceweather.htm&quot;&gt;NASA Goddard Space Flight Center's article on &lt;strong&gt;Space weather and impact of solar flares on human activities&lt;/strong&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Communications satellites are generally in these high orbits. Either&#xA;  the satellite becomes highly charged during the storm and a component&#xA;  is damaged by the high current that discharges into the satellite, or&#xA;  a component is damaged by high-energy particles that penetrate the&#xA;  satellite.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Other effects of CME include increase in the atmospheric drag of the artificial satellites, so that their orbital velocity decreases, hence might need to re-boost more frequently. Previously mentioned NASA's article goes on to explain the reason for this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The x-rays from flares are stopped by our atmosphere well above the&#xA;  Earth's surface. They do disturb the Earth's ionosphere, however,&#xA;  which in turn disturbs some radio communications. Along with energetic&#xA;  ultraviolet radiation, they heat the Earth’s outer atmosphere, causing&#xA;  it to expand. This increases the drag on Earth-orbiting satellites,&#xA;  reducing their lifetime in orbit. Also, both intense radio emission&#xA;  from flares and these changes in the atmosphere can degrade the&#xA;  precision of Global Positioning System (GPS) measurements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But satellites in LEO (&lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot;&gt;Low Earth Orbit&lt;/a&gt;) are shielded from CMEs by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetosphere&quot;&gt;magnetosphere&lt;/a&gt; of the Earth, because of their proximity to it where the &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth%27s_magnetic_field&quot;&gt;Earth's magnetic field&lt;/a&gt; is still strong, and mostly experience drag due to expansion of gases in the Earth's upper atmosphere. They are also affected by &lt;a href=&quot;http://en.wikipedia.org/wiki/Geomagnetic_storm&quot;&gt;Geomagnetic storms&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-23T10:16:42.960" LastActivityDate="2013-08-23T10:16:42.960" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1466" PostTypeId="5" CreationDate="2013-08-23T09:51:33.657" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-23T09:51:33.657" LastActivityDate="2013-08-23T09:51:33.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1467" PostTypeId="4" CreationDate="2013-08-23T09:51:33.657" Score="0" Body="A coronal mass ejection (CME) is a massive burst of solar wind and magnetic fields rising above the solar corona or being released into space, or towards other celestial bodies of the Solar system. They are not to be confused with solar flares (a sudden brightening over the Sun's surface or the solar limb) or sunspots, even though they are often associated with such activities of the Sun. CME are more frequent during or near solar maxima." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-23T09:51:33.657" LastActivityDate="2013-08-23T09:51:33.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1468" PostTypeId="5" CreationDate="2013-08-23T10:03:15.183" Score="0" Body="&lt;p&gt;The Sun, also named Sol in Latin and Helios in Ancient Greek, is the star at the center of our Solar system. It should not be confused with the barycenter of the Solar system, that moves relative to the center of the Sun and can in fact be out of its body, or central stars of other celestial systems.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorDisplayName="user12" LastEditDate="2013-08-23T13:48:10.793" LastActivityDate="2013-08-23T13:48:10.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1469" PostTypeId="4" CreationDate="2013-08-23T10:03:15.183" Score="0" Body="Questions regarding the Sun, or Sol in Latin, the star at the center of the solar system Earth is in." OwnerUserId="49" LastEditorDisplayName="user12" LastEditDate="2013-08-23T13:48:16.810" LastActivityDate="2013-08-23T13:48:16.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1470" PostTypeId="2" ParentId="1436" CreationDate="2013-08-23T12:23:02.300" Score="2" Body="&lt;p&gt;Once we have a &lt;a href=&quot;https://en.wikipedia.org/wiki/Vactrain&quot; rel=&quot;nofollow&quot;&gt;Vactrain&lt;/a&gt; loop big enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Levitating on magnetic field, it could accelerate to nearly arbitrary speeds, before being allowed to leave the vacuum tubes, shoot through the atmosphere diagonally and head towards the orbit. The trick is enough of launch track without air and changing the horizontal acceleration vector to vertical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes; plain maglev propulsion would be much more efficient in these conditions but that doesn't make using the ion drive impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until then... well, others said it.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-08-23T12:23:02.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1471" PostTypeId="5" CreationDate="2013-08-23T13:36:48.687" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T13:36:48.687" LastActivityDate="2013-08-23T13:36:48.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1472" PostTypeId="4" CreationDate="2013-08-23T13:36:48.687" Score="0" Body="Questions regarding the money required for space exploration, as well as the raising of that money.&#xD;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-23T13:47:35.980" LastActivityDate="2013-08-23T13:47:35.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1473" PostTypeId="5" CreationDate="2013-08-23T13:39:52.557" Score="0" Body="&lt;p&gt;Buzz Aldrin, &lt;a href=&quot;https://www.nasa.gov/audience/foreducators/spacesuits/historygallery/apollo-index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;as photographed by Neal Armstrong&lt;/a&gt;, during their historic EVA to the Lunar surface&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qrCqq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qrCqq.jpg&quot; alt=&quot;Buzz Aldrin on the moon&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="22326" LastEditDate="2021-01-05T13:08:05.783" LastActivityDate="2021-01-05T13:08:05.783" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1474" PostTypeId="4" CreationDate="2013-08-23T13:39:52.557" Score="0" Body="Extra-Vehicular Activity (EVA, or sometimes called &quot;spacewalking&quot;) involves donning a space suit and leaving whatever vehicle you are in. Common reasons are repairing or maintaining said vehicle, but can include exploring the space or celestial object near your vehicle." OwnerDisplayName="user12" LastEditorUserId="22326" LastEditDate="2021-01-05T13:05:41.343" LastActivityDate="2021-01-05T13:05:41.343" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1475" PostTypeId="5" CreationDate="2013-08-23T13:41:23.360" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T13:41:23.360" LastActivityDate="2013-08-23T13:41:23.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1476" PostTypeId="4" CreationDate="2013-08-23T13:41:23.360" Score="0" Body="Questions regarding the process of bringing a mission to an early stop, often due to an emergency or unforeseen circumstances." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-23T13:47:44.153" LastActivityDate="2013-08-23T13:47:44.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1477" PostTypeId="5" CreationDate="2013-08-23T13:49:54.077" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T13:49:54.077" LastActivityDate="2013-08-23T13:49:54.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1478" PostTypeId="4" CreationDate="2013-08-23T13:49:54.077" Score="0" Body="Questions regarding the Perseid meteor shower, which peaks in mid August yearly." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-23T13:52:50.287" LastActivityDate="2013-08-23T13:52:50.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1479" PostTypeId="1" AcceptedAnswerId="3740" CreationDate="2013-08-23T14:58:10.183" Score="15" ViewCount="3193" Body="&lt;p&gt;The shuttle had a empty gross weight of &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle#Technical_data&quot; rel=&quot;noreferrer&quot;&gt;172,000 lb (78,000 kg)&lt;/a&gt;, and the only feasible way shedding the &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station#Orbit_and_mission_control&quot; rel=&quot;noreferrer&quot;&gt;27,724 kilometres (17,227 mi)&lt;/a&gt; per hour, of the relative speed of the ISS is &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerobraking&quot; rel=&quot;noreferrer&quot;&gt;Aerobraking&lt;/a&gt;.  A four passenger &lt;a href=&quot;http://en.wikipedia.org/wiki/Cessna_172&quot; rel=&quot;noreferrer&quot;&gt;Cessna 172&lt;/a&gt; has an empty weight of under 2,500 lb (1,136 kg).   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While actually using Cessna as a reentry vehicle would not be practical, the image allows for exploring the concept of an lite reentry vehicle (as opposed to &lt;a href=&quot;https://space.stackexchange.com/questions/511/how-possible-are-space-jumps&quot;&gt;How possible are &amp;#39;space jumps&amp;#39;?&lt;/a&gt; ). Assuming we swap out the gas engine of the Cessna, and replace the mass with fuel, then strap some rockets (&lt;em&gt;clearly an oversimplification&lt;/em&gt;) to the plane to decrease the orbital velocity, we have a nice glider with 2% of the mass and twice or more of the glide capacity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your pilot and passengers are in space suits for the trip, you won't need to add mass for keeping the vehicle pressurized. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So could a lite winged reentry vehicle use rockets to decelerate, and allow reentry with out having drag/heat issues?  Would the fuel requirements to decelerate safely outweigh the benefits of creating vehicle that does not need to stand up to high speed Aerobraking?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&#xA;This answer to this question comes down what what is more economical; The extra mass for aerobraking heat shielding or the mass for fuel to decelerate.  There was a comment addressing this but it has been deleted.  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-11-08T20:56:22.330" Title="Could you take a Cessna from the ISS to Earth?" Tags="|iss|reentry|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1480" PostTypeId="1" AcceptedAnswerId="2236" CreationDate="2013-08-23T14:58:49.773" Score="15" ViewCount="4908" Body="&lt;p&gt;The lunar surface visible to us is mostly regolith - space dust, asteroid remains, and what-not. The true lunar surface is below - at depths ranging from a few inches to a few metres. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that Luna survived the impact which created &lt;a href=&quot;http://en.wikipedia.org/wiki/Oceanus_Procellarum&quot;&gt;Oceanus Procellarum&lt;/a&gt; it may be as hard as bedrock. How hard is the Lunar Surface? &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="25" LastEditDate="2013-09-30T17:39:27.817" LastActivityDate="2013-10-06T07:06:18.467" Title="How hard is the Lunar surface?" Tags="|the-moon|planetary-science|dust|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1481" PostTypeId="2" ParentId="1479" CreationDate="2013-08-23T15:32:22.937" Score="7" Body="&lt;p&gt;Cessna is at the edge of the atmosphere where its potential energy is very high which must be dissipated in some form (which re-entry vehicles dissipate as heat) since energy in this case is conserved. It is not about gliding through the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;During re-entry the shuttle experience enormous stress which Cessna would collapse .&lt;/li&gt;&#xA;&lt;li&gt;Eventhough Cessna glides through it would eventually reach hypersonic speed (Apollo capsule reached Mach number nearly equal to 36) which leads to a bow shock wave which inturn causes increase in temperature near its front end which Cessna would not survive (would melt or it will become weak leads to failure). But re-entry vehicles are shielded from this intense heat (shuttles having tiles in the nose).&lt;/li&gt;&#xA;&lt;li&gt;If the Cessna is not pressurised it will leads to more  force (due to high pressure difference inside and outside the Cessna where the pressure would be very high) being exerted on the Cessna body which is not designed to withstand it .&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-08-23T15:32:22.937" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1482" PostTypeId="2" ParentId="1452" CreationDate="2013-08-23T15:44:45.590" Score="4" Body="&lt;h1&gt;What is being done?&lt;/h1&gt;&#xA;&lt;p&gt;Drawing on an excellent &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100039608_2010042539.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;find&lt;/a&gt; by @Undo, here are concrete technologies currently in use:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Replacing old fans by new ones (with &lt;a href=&quot;https://en.wikipedia.org/wiki/CNC&quot; rel=&quot;nofollow noreferrer&quot;&gt;CNC&lt;/a&gt;-machined blades that have been optimized through rigorous &lt;a href=&quot;https://en.wikipedia.org/wiki/Computational_fluid_dynamics&quot; rel=&quot;nofollow noreferrer&quot;&gt;CFD&lt;/a&gt; modelling).&lt;/li&gt;&#xA;&lt;li&gt;Putting mufflers, acoustic covers, vibration isolation, and acoustic lining into fans/air ducts, wrapping fluid lines in air conditioning loops.&lt;/li&gt;&#xA;&lt;li&gt;Putting doors onto crew quarters in the Zvezda service module.&lt;/li&gt;&#xA;&lt;li&gt;Fixing individual pieces of equipment across the station.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Unresolved issues remain:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Oxygen Generation System (in the US segment) and Vozdukh (in the Russian segment) are waaaay too noisy.&lt;/li&gt;&#xA;&lt;li&gt;Water recovery pumps/separators.&lt;/li&gt;&#xA;&lt;li&gt;Toilets.&lt;/li&gt;&#xA;&lt;li&gt;Exercise devices (&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1001.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARED&lt;/a&gt;, in particular).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;It is also apparent that RSC Energiya, the main designer of the Russian segment, was prodded by the Americans to reduce acoustic signatures, and that its technologies are somewhat archaic.&lt;/p&gt;&#xA;&lt;hr/&gt;&#xA;&lt;h1&gt;The framework&lt;/h1&gt;&#xA;&lt;p&gt;Goodman, Grosveld. &lt;a href=&quot;http://www.congrex.nl/08a11/presentations/day1_S03/S03_01_Goodman.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Acoustics in habitable space vehicles and enclosures&lt;/a&gt; (pdf). Rome, Italy, 2008.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;What’s required to ensure acceptable acoustic levels in crew compartments:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Good set of acoustic requirements&lt;/li&gt;&#xA;&lt;li&gt;Noise Control, compliance &amp;amp; verification&lt;/li&gt;&#xA;&lt;li&gt;Management relationship&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr/&gt;&#xA;&lt;h1&gt;What I am sad to see:&lt;/h1&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Noise cancellation isn't exactly on the menu&lt;/li&gt;&#xA;&lt;li&gt;Astronauts/cosmonauts do not often have the leverage to make noise control part of the design objective function&lt;/li&gt;&#xA;&lt;li&gt;Equipment manufacturers aren't keen on preventing the problem from popping up&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-23T15:44:45.590" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1483" PostTypeId="1" AcceptedAnswerId="1488" CreationDate="2013-08-23T15:45:58.333" Score="25" ViewCount="4149" Body="&lt;p&gt;&lt;a href=&quot;https://imageserv5.team-logic.com/mediaLibrary/93/Dust_Mitigation_Solutions_for_Lunar_and_Mars_Surface_Systems.pdf&quot; rel=&quot;noreferrer&quot;&gt;Dust mitigation in the context of manned landings&lt;/a&gt; is now a rather vital topic of research and testing, but back in the days of Apollo it was a fairly new problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TtR1Q.jpg&quot; alt=&quot;Gene Cernan on the Moon during Apollo 17&quot;&gt;&#xA;&lt;sub&gt;'Dusty' Gene Cernan inside the LM on the Moon during Apollo 17 (Harrison Schmitt / NASA, 1972)&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dust is a complex problem. It cloaks filters, reduces the efficiency of seals, can theoretically cause shorts, can be inhaled, electro-statically sticks to almost everything ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;What were the initial expectations with respect to dust prior Apollo 11? How was dust addressed for this mission? What were notable effects of dust on the spacecraft, instruments and suits? What was eventually changed for later missions and why?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bonus question: There are plenty of photos of how Apollo astronauts looked liked after their EVAs. Yet, I have not found any close-up photo of the floor of the LM or its hatch after EVA. This would be really interesting.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-08-25T20:30:13.183" LastActivityDate="2015-07-24T16:50:23.670" Title="How was dust-mitigation addressed during the Apollo program?" Tags="|the-moon|apollo-program|lunar-landing|dust|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1484" PostTypeId="1" CreationDate="2013-08-23T16:12:18.540" Score="6" ViewCount="420" Body="&lt;p&gt;There are many galaxies in the universe. Is there any other Earth-like planet out there?&lt;/p&gt;&#xA;" OwnerUserId="579" LastEditorUserId="80" LastEditDate="2013-08-24T06:52:30.957" LastActivityDate="2015-07-25T00:19:39.300" Title="Are there any other Earth-like planets in the universe?" Tags="|exoplanet|astronomy|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1485" PostTypeId="1" AcceptedAnswerId="1632" CreationDate="2013-08-23T16:31:01.113" Score="15" ViewCount="1871" Body="&lt;p&gt;The basic concept of an &lt;a href=&quot;http://en.wikipedia.org/wiki/Ares_I&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ares 1&lt;/a&gt; consisted of a five-segment &lt;a href=&quot;http://en.wikipedia.org/wiki/Solid_rocket_booster&quot; rel=&quot;nofollow noreferrer&quot;&gt;SRB&lt;/a&gt; and some liquid-fuelled upper stage. The SRB was derived from the four-segment &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle SRBs&lt;/a&gt;. A similar concept was later promoted under the name '&lt;a href=&quot;http://en.wikipedia.org/wiki/Liberty_%28rocket%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Liberty rocket&lt;/a&gt;' - my question applies equally to this one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OZrSm.jpg&quot; alt=&quot;Ares-1-X vehicle&quot;&gt;&lt;br&gt;&#xA;&lt;sub&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Ares_I-X&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ares-1-X vehicle, a &lt;em&gt;design concept demonstrator&lt;/em&gt;&lt;/a&gt;, sitting on launch pad 39B at the Kennedy Space Center (NASA, 2009)&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I keep wondering how this was supposed to work with respect to vibrations caused by the SRB. If you look at &lt;a href=&quot;http://www.youtube.com/watch?v=5WdQHQY0xJs&quot; rel=&quot;nofollow noreferrer&quot;&gt;cockpit views recorded during Space Shuttle launches&lt;/a&gt;, you can see that the astronauts are shaken rather violently. Even professional astronauts, flown in space multiple times, described it as &quot;highly unpleasant necessity&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Space Shuttle combined with its external tank had a rather high mass, thus absorbing significant portions of the vibrations. &lt;strong&gt;In case of the Ares 1, a rather small upper stage was supposed to be the only thing between a single massive SRB and the astronauts. How was this envisioned to work - allowing for an at least bearable ride into space?&lt;/strong&gt; Are there any good sources, which explain ideas, concepts, designs, numbers or tests in this context?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(At the end of the of the constellation program, shortly before it was cancelled, I had the opportunity to talk to engineers from Lockheed. By this time, Lockheed was the main-contractor for &lt;a href=&quot;http://en.wikipedia.org/wiki/Orion_spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion&lt;/a&gt; and its service module. I asked precisely the same question. They replied &quot;We are working on it ...&quot; In other words, it was an issue of significance.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="49" LastEditDate="2015-07-20T18:40:51.390" LastActivityDate="2022-08-04T16:43:09.470" Title="How were vibrations supposed to be handled in an Ares I?" Tags="|nasa|vibration|constellation-program|canceled-project|srb|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1486" PostTypeId="1" AcceptedAnswerId="1627" CreationDate="2013-08-23T17:02:09.040" Score="11" ViewCount="171" Body="&lt;p&gt;As per the title, I'm curious about technologies that would detect previously unknown objects like the &lt;a href=&quot;http://en.wikipedia.org/wiki/Chelyabinsk_meteor&quot;&gt;Chelyabinsk Meteor&lt;/a&gt; as they enter the atmosphere.  An answer that could satisfy some or all of the questions below would be ideal.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is there technology that is explicitly deployed for this task, or do we retrieve data from unrelated sources like seismographs or radar?  &lt;/li&gt;&#xA;&lt;li&gt;What sort of data is collected about the object? &lt;/li&gt;&#xA;&lt;li&gt;Would their data have any lead time as compared to observations from people on the ground (even a very small leadtime)?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="62" LastEditorUserId="168" LastEditDate="2013-08-23T17:25:16.647" LastActivityDate="2013-08-28T00:44:48.157" Title="What existing technologies record data on meteors?" Tags="|meteorites|space-surveillance|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1487" PostTypeId="2" ParentId="1484" CreationDate="2013-08-23T17:03:15.497" Score="7" Body="&lt;p&gt;&lt;strong&gt;YES&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ok, it is easy to say yes but the basic idea is statistics. There are more and more planets discovered outside of our solar system virtually every day. With better instruments, ever smaller planets can be discovered. Besides, it depends on your definition of 'earth-like'. If you restrict this to &lt;em&gt;size&lt;/em&gt;, the &lt;em&gt;distance to its star&lt;/em&gt; (with respect to the '&lt;a href=&quot;http://en.wikipedia.org/wiki/Circumstellar_habitable_zone&quot; rel=&quot;nofollow&quot;&gt;habitable zone&lt;/a&gt;') and an &lt;em&gt;potential atmosphere&lt;/em&gt; (thus ignoring the questions of atmosphere chemistry and life), it is a rather clear yes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;News (all referring to a discovery published December 2011): &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/kepler/news/kepler-20-system.html&quot; rel=&quot;nofollow&quot;&gt;NASA Kepler-mission news - The Kepler-20 system&lt;/a&gt;&lt;br&gt;&lt;em&gt;&quot;NASA's Kepler mission has discovered the first Earth-size planets orbiting a sun-like star outside our solar system.&quot;&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nature.com/news/kepler-discovers-first-earth-sized-exoplanets-1.9688&quot; rel=&quot;nofollow&quot;&gt;Nature News - Kepler discovers first Earth sized exoplanet&lt;/a&gt;&lt;br&gt;&lt;em&gt;&quot;Whizzing around the star Kepler-20, about 290 parsecs (946 light years) from Earth, is not only an Earth-sized planet, but also something just a touch smaller — a Venus.&quot;&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nytimes.com/2011/12/21/science/space/nasas-kepler-spacecraft-discovers-2-earth-size-planets.html?_r=0&quot; rel=&quot;nofollow&quot;&gt;NYTimes - NASA's Kepler spacecraft discovers 2 Earth-size planets&lt;/a&gt;&lt;br&gt;&lt;em&gt;&quot;[...] astronomers from NASA’s Kepler spacecraft announced Tuesday that they had discovered a pair of planets the size of Earth orbiting a distant star.&quot;&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A paper: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://iopscience.iop.org/0004-637X/749/1/15&quot; rel=&quot;nofollow&quot;&gt;Gautier, T.N., Charbonneau, D., Rowe, J.F., Marcy, G.W., Isaacson, H., Torres, G., Fressin, F., Rogers, L.A., Désert, J.-., Buchhave, L.A., Latham, D.W., Quinn, S.N., Ciardi, D.R., Fabrycky, D.C., Ford, E.B., Gilliland, R.L., Walkowicz, L.M., Bryson, S.T., Cochran, W.D., Endl, M., Fischer, D.A., Howell, S.B., Horch, E.P., Barclay, T., Batalha, N., Borucki, W.J., Christiansen, J.L., Geary, J.C., Henze, C.E., Holman, M.J., Ibrahim, K., Jenkins, J.M., Kinemuchi, K., Koch, D.G., Lissauer, J.J., Sanderfer, D.T., Sasselov, D.D., Seager, S., Silverio, K., Smith, J.C., Still, M., Stumpe, M.C., Tenenbaum, P. &amp;amp; Van Cleve, J. 2012, &quot;Kepler-20: A sun-like star with three sub-neptune exoplanets and two earth-size candidates&quot;, Astrophysical Journal, vol. 749, no. 1. &lt;/a&gt; (free access) &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Further reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Earth_analog#Extrasolar_Earth_analog&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Extrasolar Earth analog&lt;/a&gt; (in: Earth Analog)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Super-Earth&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Super-Earth&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-08-23T20:39:40.687" LastActivityDate="2013-08-23T20:39:40.687" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1488" PostTypeId="2" ParentId="1483" CreationDate="2013-08-23T17:21:15.583" Score="25" Body="&lt;p&gt;There was an expectation that there would be some dust, however, there were a number of surprises associated with this dust. Apollo astronauts tried to remove dust using vacuums and brushes. This failed to work because the dust is electrostaticly charged, it sticks to everything.  The summary of all of the mission in the NASA report quite quite interesting, namely:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Apollo 11 crew vacuumed suits, samples and equipment in the LM&lt;sup&gt;* &lt;/sup&gt;&#xA;  before transferring these items to the CM&lt;sup&gt;* &lt;/sup&gt;. Because the suction was&#xA;  low, this was a tedious job. The vacuum used in Apollo 12 did not&#xA;  remove dust from the space suits, but was used to brush dust off&#xA;  sample boxes. Dust and debris in the LM impeded transfer operations&#xA;  and although the gear that was transferred was filthy, very little&#xA;  dust was transferred to the CM. In Apollo 14, the vacuum performed&#xA;  satisfactorily and very little dust was transferred to the CM. The&#xA;  cabin fan filter cleaned the atmosphere well. The vacuum and the cabin&#xA;  fan used in Apollo 15 worked well. During Apollo 16, the vacuum&#xA;  cleaner and cabin fans failed resulting in severe CM contamination.&#xA;  This was exacerbated by a decision to postpone LM jettison. The CM&#xA;  filled up with dust and rocks immediately and dust coated all the&#xA;  surfaces within an hour. The Apollo 17 tunnel transfer went smoothly.&#xA;  A small amount of dust was transferred to the CM; however, the&#xA;  cleaning control system, combined with running the vacuum cleaner in&#xA;  the LM kept the CM air clean.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The main thing that was done differently in later missions was procedures. For instance, Apollo 15 reported the following in their mission report:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“The first order of business after we got repress was to go through&#xA;  the checklist and do the EVA post and try and come up with a plan on&#xA;  how to handle all the dirt in the cabin. We were pretty dusty. We had&#xA;  planned prior to the flight to take the jettison bags and step into&#xA;  them with the suits to keep the lower portion of the suit isolated&#xA;  from the rest of the cabin. Our legs from about thigh down were just&#xA;  completely covered with dirt. I guess the dust brush worked fairly&#xA;  well. It got most of it, but we were still pretty dirty.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Apollo 16 quotes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The dust floor was cleaned by wetting a rag, caking the dust into mud&#xA;  and picking it up in the rag; however, there was no way to remove the&#xA;  dust from the Velcro on the floor. Since the Velcro does not restrain&#xA;  the crew to the floor in zero gravity, it is not needed.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And Apollo 17 quotes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You knew you were in a very heavily infiltrated atmosphere in the LM&#xA;  because of the lunar dust. I don’t know how much lunar dust previous&#xA;  flights had, but I think we saved a great deal of grief by sweeping&#xA;  all the dust we could find in the floor into the holes and putting our&#xA;  tape covers over those holes. I think that had to help a great deal.&#xA;  There was an awful lot of dust on the floor that we didn’t see.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The bottom line, each mission was incrementally better, but for the most part, it didn't cause many issues, with the exception of an episode of Hay Fever. Long term no doubt it would have caused issues, but there are plans put in motion to address this for future missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Adverse_health_effects_from_lunar_dust_exposure#cite_note-Portree_and_Trevino_1997-47&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; and the &lt;a href=&quot;http://www.hq.nasa.gov/alsj/TP-2006-213726.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA constellation report&lt;/a&gt;, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;*) LM: Lunar Module; CM: Command Module&lt;/sub&gt; &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-24T16:50:23.670" LastActivityDate="2015-07-24T16:50:23.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1489" PostTypeId="1" AcceptedAnswerId="1490" CreationDate="2013-08-23T18:50:25.477" Score="36" ViewCount="46883" Body="&lt;p&gt;In many videos of the ISS, you can see tethered laptops all over the place. Presumably these are heavily customised for space. What kinds of customisation do they need? Are they off-the-shelf laptops with customisations for space-worthiness or custom-built hardware? What OS do they run?&lt;/p&gt;&#xA;" OwnerUserId="80" LastActivityDate="2021-10-14T19:46:53.840" Title="What kind of laptops do ISS astronauts use?" Tags="|iss|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1490" PostTypeId="2" ParentId="1489" CreationDate="2013-08-23T20:00:54.287" Score="32" Body="&lt;p&gt;Astronauts interact with the onboard systems on ISS via laptops, categorized into the Portable Computer System (PCS) and Station Support Computer (SSC).&lt;/p&gt;&#xA;&lt;p&gt;The firsts provide the interface with the ship’s computer systems while the others are used for office and personal tasks.&lt;/p&gt;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://blog.lenovo.com/products/thinkpad-laptop-nasa-youtube-spacelab&quot; rel=&quot;noreferrer&quot;&gt;Lenovo blog&lt;/a&gt; (march 20, 2012)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;• ThinkPad is the only laptop certified for use on the International Space Station (ISS).&lt;/p&gt;&#xA;&lt;p&gt;• ThinkPads have been used aboard the International Space Station since 1998.&lt;/p&gt;&#xA;&lt;p&gt;• ThinkPads have been on every NASA Shuttle space flight since 1995, when the ThinkPad 755 blasted off with the crew.&lt;/p&gt;&#xA;&lt;p&gt;• ThinkPad laptops are used by all the major world space agencies (e.g., JAXA, ESA) thanks to their partnership with NASA.&lt;/p&gt;&#xA;&lt;p&gt;• Some tests ThinkPads have to pass before they lift off: radiation testing, off-gas testing, thermal testing, fire &amp;amp; fire suppression.&lt;/p&gt;&#xA;&lt;p&gt;• Number of ThinkPads aboard the International Space Station: more than 60.&lt;/p&gt;&#xA;&lt;p&gt;• Number of NASA shuttle flights that have lifted off with ThinkPads aboard: more than 50.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and this link is &lt;a href=&quot;http://training.linuxfoundation.org/why-our-linux-training/training-reviews/linux-foundation-training-prepares-the-international-space-station-for-linux-migration&quot; rel=&quot;noreferrer&quot;&gt;about software&lt;/a&gt;: astronauts in future will use Debian 6 moving from  Windows and Scientific Linux&lt;/p&gt;&#xA;&lt;p&gt;If you wish more details you can read also:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www-03.ibm.com/ibm/history/exhibits/space/space_thinkpad.html&quot; rel=&quot;noreferrer&quot;&gt;IBM ThinkPads in space&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://paso.esa.int/3_Payload_Safety/JSC-29948%20RevB.pdf&quot; rel=&quot;noreferrer&quot;&gt;ISS IBM ThinkPad Series A31p ™ Laptop Hardware&#xA;Project Technical Requirements Specification&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="-1" LastEditorDisplayName="user55" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-25T06:31:21.653" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1491" PostTypeId="5" CreationDate="2013-08-23T20:36:00.083" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T20:36:00.083" LastActivityDate="2013-08-23T20:36:00.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1492" PostTypeId="4" CreationDate="2013-08-23T20:36:00.083" Score="0" Body="Questions regarding planets outside of Earth's solar system." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-23T20:55:19.900" LastActivityDate="2013-08-23T20:55:19.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1493" PostTypeId="5" CreationDate="2013-08-23T21:03:53.217" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:03:53.217" LastActivityDate="2013-08-23T21:03:53.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1494" PostTypeId="4" CreationDate="2013-08-23T21:03:53.217" Score="0" Body="Questions on obtaining electrical energy from the light of the Sun through either direct photovoltaic conversion or thermal conversion." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:26:48.820" LastActivityDate="2013-08-23T21:26:48.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1495" PostTypeId="5" CreationDate="2013-08-23T21:07:19.577" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:07:19.577" LastActivityDate="2013-08-23T21:07:19.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1496" PostTypeId="4" CreationDate="2013-08-23T21:07:19.577" Score="0" Body="Spacecraft propulsion obtained through frequent controlled explosions of specially modified nuclear or thermonuclear explosive devices under the spacecraft." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:25:18.373" LastActivityDate="2013-08-23T21:25:18.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1497" PostTypeId="5" CreationDate="2013-08-23T21:10:37.030" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:10:37.030" LastActivityDate="2013-08-23T21:10:37.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1498" PostTypeId="4" CreationDate="2013-08-23T21:10:37.030" Score="0" Body="A set of procedures executed to extend lifetime of a spacecraft or planetary vehicle by reducing and/or rearranging power consumption." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:26:35.083" LastActivityDate="2013-08-23T21:26:35.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1499" PostTypeId="5" CreationDate="2013-08-23T21:16:34.710" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:16:34.710" LastActivityDate="2013-08-23T21:16:34.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1500" PostTypeId="4" CreationDate="2013-08-23T21:16:34.710" Score="0" Body="Questions related to structure, operations, missions, and facilities of the European Space Agency, an intergovernmental organization of 20 European states and Canada." OwnerUserId="168" LastEditorUserId="14" LastEditDate="2013-08-23T21:55:48.860" LastActivityDate="2013-08-23T21:55:48.860" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1501" PostTypeId="5" CreationDate="2013-08-23T21:19:35.417" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:19:35.417" LastActivityDate="2013-08-23T21:19:35.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1502" PostTypeId="4" CreationDate="2013-08-23T21:19:35.417" Score="0" Body="Within the context of space exploration: recreational, privately funded cislunar, orbital and suborbital flights. Space tourists may perform scientific experiments during flights as a secondary activity." OwnerUserId="168" LastEditorUserId="53239" LastEditDate="2024-01-30T16:17:30.530" LastActivityDate="2024-01-30T16:17:30.530" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1503" PostTypeId="5" CreationDate="2013-08-23T21:27:28.053" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:27:28.053" LastActivityDate="2013-08-23T21:27:28.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1504" PostTypeId="4" CreationDate="2013-08-23T21:27:28.053" Score="0" Body="Very large base interferometry - high-precision measurements of Earth, stellar bodies' and spacecraft parameters based on combining signals received by an array of spatially separated radio telescopes." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:30:12.913" LastActivityDate="2013-08-23T21:30:12.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1505" PostTypeId="5" CreationDate="2013-08-23T21:30:18.393" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:30:18.393" LastActivityDate="2013-08-23T21:30:18.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1506" PostTypeId="4" CreationDate="2013-08-23T21:30:18.393" Score="0" Body="Questions about the use of inflatable balloons to cushion landing of probes/exploration vehicles on planetary bodies." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:33:21.693" LastActivityDate="2013-08-23T21:33:21.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1507" PostTypeId="5" CreationDate="2013-08-23T21:37:47.200" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:37:47.200" LastActivityDate="2013-08-23T21:37:47.200" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1508" PostTypeId="4" CreationDate="2013-08-23T21:37:47.200" Score="0" Body="Experimental and/or theoretical analysis aimed at correcting common misperceptions about space-, science-, and technology-related phenomena." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:48:58.127" LastActivityDate="2013-08-23T21:48:58.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1509" PostTypeId="5" CreationDate="2013-08-23T21:41:08.167" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:41:08.167" LastActivityDate="2013-08-23T21:41:08.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1510" PostTypeId="4" CreationDate="2013-08-23T21:41:08.167" Score="0" Body="Part of planetary protection activities involving isolation of samples, equipment, and crew to prevent backward contamination of the planet of origin (the Earth) by potentially harmful biological agents from space or other planetary bodies." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:47:36.917" LastActivityDate="2013-08-23T21:47:36.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1511" PostTypeId="5" CreationDate="2013-08-23T21:45:12.607" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:45:12.607" LastActivityDate="2013-08-23T21:45:12.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1512" PostTypeId="4" CreationDate="2013-08-23T21:45:12.607" Score="0" Body="Release of gases and volatiles from various materials under underpressure or vacuum conditions." OwnerUserId="168" LastEditorUserId="49" LastEditDate="2013-08-23T22:25:29.827" LastActivityDate="2013-08-23T22:25:29.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1513" PostTypeId="5" CreationDate="2013-08-23T21:52:08.277" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:52:08.277" LastActivityDate="2013-08-23T21:52:08.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1514" PostTypeId="4" CreationDate="2013-08-23T21:52:08.277" Score="0" Body="In the narrow sense: politically and militarily motivated rivalry in space exploration between the USA and the USSR from 1957 to 1991. In the broad sense: rivalry in space exploration between two or more states or groups of states." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:56:38.927" LastActivityDate="2013-08-23T21:56:38.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1515" PostTypeId="5" CreationDate="2013-08-23T21:56:38.453" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T21:56:38.453" LastActivityDate="2013-08-23T21:56:38.453" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1516" PostTypeId="4" CreationDate="2013-08-23T21:56:38.453" Score="0" Body="Upper stages of rockets employed to put payloads into mission-defined orbits. Usually fitted with space-restartable engines. Also called orbital transfer vehicles. Some concepts imply repeated orbital maneuvers." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T21:57:11.337" LastActivityDate="2013-08-23T21:57:11.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1517" PostTypeId="5" CreationDate="2013-08-23T22:02:25.243" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T22:02:25.243" LastActivityDate="2013-08-23T22:02:25.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1518" PostTypeId="4" CreationDate="2013-08-23T22:02:25.243" Score="0" Body="Questions about significant reduction in sensor signatures of aerospace vehicles (radar, optical, infrared, and others), leading to greatly reduced probabilities of detection by adversaries in the context of hostilities. " OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T22:03:10.943" LastActivityDate="2013-08-23T22:03:10.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1519" PostTypeId="5" CreationDate="2013-08-23T22:10:09.260" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-23T22:10:09.260" LastActivityDate="2013-08-23T22:10:09.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1520" PostTypeId="4" CreationDate="2013-08-23T22:10:09.260" Score="0" Body="Questions about orbits utilizing nodal precession so that spacecraft are able to maintain the same local mean solar time at any given point of an orbit. The effect is achieved through choosing inclination and altitudes of the apses." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-23T22:19:45.113" LastActivityDate="2013-08-23T22:19:45.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1521" PostTypeId="2" ParentId="1484" CreationDate="2013-08-23T23:48:07.820" Score="6" Body="&lt;p&gt;The answer is strongly defined by how one defines &quot;Earth Like.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The working definition has yet to be established for the scientific purposes that I've seen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The common minimum: terrestrial, within the liquid water zone, and not more than about 4 Earth masses; a lower bound of about 1/2000th Earth mass (large enough to be self rounding). In which case, there's one about 1 light second away... Luna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over a dozen extrasolar worlds fit that very broad definition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let us narrow it to worlds which have an atmosphere; this sets the mass to between 2 Earth masses and 1/4 Earth mass. There are several detected in the same range, but all are over Earth mass, and as far as I know, outside the &lt;a href=&quot;https://en.wikipedia.org/wiki/Circumstellar_habitable_zone&quot; rel=&quot;nofollow&quot;&gt;Goldilocks zone&lt;/a&gt;. &lt;a href=&quot;https://en.wikipedia.org/wiki/Gliese_581&quot; rel=&quot;nofollow&quot;&gt;Gliese 581&lt;/a&gt; has 3 candidate worlds (&lt;a href=&quot;https://en.wikipedia.org/wiki/Gliese_581_c&quot; rel=&quot;nofollow&quot;&gt;581 c&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gliese_581_d&quot; rel=&quot;nofollow&quot;&gt;581 d&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Gliese_581_e&quot; rel=&quot;nofollow&quot;&gt;581 e&lt;/a&gt;) that are within the correct mass and potentially have atmospheres. &lt;a href=&quot;http://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth&quot; rel=&quot;nofollow&quot;&gt;Kepler-452b&lt;/a&gt; is 1.6 earth diameters, and in the ecosphere of it's parent star; its atmosphere isn't yet detected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we narrow it further, to free atmospheric oxygen on such a world, only one so far is known, and we are on it. Until the &lt;a href=&quot;http://www.terrestrial-planet-finder.com/&quot; rel=&quot;nofollow&quot;&gt;Terrestrial Planet Finder&lt;/a&gt; mission (presuming it isn't cancelled before then; as of Jul 2015, it's indefinitely postponed), it's unlikely to be detected, as the current instruments are not optimized for that.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2015-07-24T12:54:35.143" LastActivityDate="2015-07-24T12:54:35.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1524" PostTypeId="1" AcceptedAnswerId="1526" CreationDate="2013-08-24T09:49:42.597" Score="10" ViewCount="779" Body="&lt;p&gt;I have been reading a bit about the &lt;a href=&quot;http://www.nasa.gov/exploration/systems/mpcv/index.html#.Uhh_-T_ldTg&quot;&gt;Orion spacecraft&lt;/a&gt;. I see they just did some sea recovery testing. It seems like in the post Space Shuttle era, NASA is going backwards. Is this a one off solution, or is there a solid rationale for moving away from controlled flight reentry?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-08-24T11:34:58.207" LastActivityDate="2020-04-04T04:28:35.250" Title="Post Space Shuttle, is NASA going from touchdown back to splashdown?" Tags="|nasa|reentry|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1526" PostTypeId="2" ParentId="1524" CreationDate="2013-08-24T11:00:30.460" Score="12" Body="&lt;p&gt;Please realize that capsules like the Orion fly a controlled aerodynamic re-entry. &lt;strong&gt;They are not dumb balls thrown into the atmosphere&lt;/strong&gt;. They have low Lift/Drag ratio compared to winged air/spacecraft, but they use their lift nonetheless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wings for a precision runway landing are all fine and dandy, but they are simply dead mass for missions beyond Low Earth Orbit (LEO).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same can be said for the wheels - unless you want a re-usable spacecraft, wheels are a nuisance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Missions to Mars, asteroids, or the Moon intend to bring back the crew and samples only. Unlike complex spacecraft, samples are insensitive to high g-loads/shocks at landing. This leaves you with the safety of the crew as the ultimate factor in mission design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those missions will be infrequent and not exactly time-sensitive as far as search and recovery go. Thus, you can allow an hour or two before the crew is recovered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final choice (water or land) depends on the country and the structural limits of the spacecraft. Say, Russia historically had almost no aircraft carriers, a sturdy Soyuz reentry capsule design, and a large territory without too many man-made landing hazards - rivers, railroads, transmission lines, mountains, marshes (okay it's technically Kazakhstan, but whatever). Not so for the United States. Anyway, making safe landing into water is easier at interplanetary re-entry speeds (terminal braking solid rockets/engines also take away precious mass budget). For instance, the Orion lost its capability to land on the ground after &lt;a href=&quot;http://www.nasaspaceflight.com/2013/08/orion-rallying-call-eft-1-closes-t-12-months/&quot; rel=&quot;noreferrer&quot;&gt;a mass reduction campaign&lt;/a&gt; scratched away its air bags.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-08-25T05:58:17.707" LastActivityDate="2013-08-25T05:58:17.707" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1527" PostTypeId="1" AcceptedAnswerId="1530" CreationDate="2013-08-24T11:28:15.867" Score="12" ViewCount="1348" Body="&lt;p&gt;Are there any &lt;strong&gt;humanoid robots&lt;/strong&gt; on board the International Space Station to help out its astronaut occupants? If not, are there any plans to send humanoid robots to the ISS?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-08-24T12:37:01.183" LastActivityDate="2013-08-24T14:37:24.977" Title="Are there any humanoid robots on board the ISS?" Tags="|iss|robotic-missions|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1530" PostTypeId="2" ParentId="1527" CreationDate="2013-08-24T12:29:51.010" Score="11" Body="&lt;p&gt;There are currently two &quot;humanoid robots&quot; on board the international Space Station (ISS). One was built by NASA with the help of various other government and non-government organisations and goes by the name of &lt;a href=&quot;http://robonaut.jsc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Robonaut 2&lt;/a&gt; and is &lt;em&gt;&quot;a humanoid robot designed to work side by side with humans, or go where the risks are too great for people.&quot;&lt;/em&gt; On 22nd of August 2013, it's been exactly 2 years since it was first powered up on the ISS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/crMm3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Robonaut R2 being powered on now 2 years ago, on Aug. 22nd 2011 (Source: &lt;a href=&quot;https://twitter.com/AstroRobonaut&quot; rel=&quot;noreferrer&quot;&gt;Robonaut Twitter page&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://robonaut.jsc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;NASA Johnson Space Center's official web page on Robonaut&lt;/a&gt; (freeform excerpts):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A Robonaut is a dexterous humanoid robot built and designed at NASA&#xA;  Johnson Space Center in Houston, Texas. Our challenge is to build&#xA;  machines that can help humans work and explore in space.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In the current iteration of Robonaut, Robonaut 2 or R2, NASA and&#xA;  General Motors are working together with assistance from Oceaneering&#xA;  Space Systems engineers to accelerate development of the next&#xA;  generation of robots and related technologies for use in the&#xA;  automotive and aerospace industries.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Robonaut 2 (R2) is a state of the art highly dexterous anthropomorphic&#xA;  robot. Like its predecessor Robonaut 1 (R1), R2 is capable of handling&#xA;  a wide range of EVA tools and interfaces, but R2 is a significant&#xA;  advancement over its predecessor.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/rwWsA.jpg&quot; alt=&quot;Robonaut 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted though, that Robonaut 2 currently on board the ISS doesn't yet have its own functioning legs. What we see in photographs is nothing more than a stand to which Robonaut 2 is mounted when helping with, for the time being, rather mundane tasks. Future plans include Robonaut 2 getting its own functional climbing legs (late 2013, if my sources are correct), enabling it to move more freely throughout the ISS, and to also perform tasks with slowly progressing autonomy. Climbing legs will be designed in such a way, to enable Robonaut 2 always having one of them stable on the ISS &quot;floor&quot;, allowing it more precise and stable movement.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;YouTube video: &lt;a href=&quot;https://www.youtube.com/watch?v=8ZqLNMGSRjY&quot; rel=&quot;noreferrer&quot;&gt;ISS Update: Robonaut 2&lt;/a&gt;&lt;br /&gt;&lt;br /&gt;&lt;sup&gt;(Published on 7 Jan 2013, ISS Update commentator Pat Ryan sits down with Dr. Ron Diftler, Robonaut 2 project lead, to discuss the tests going on aboard the International Space Station with Robonaut 2 and the future plans for the first dexterous humanoid robot in space.)&lt;/sup&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The other robot currently on board the ISS is &lt;a href=&quot;http://www.jaxa.jp/index_e.html&quot; rel=&quot;noreferrer&quot;&gt;JAXA&lt;/a&gt; (Japan Aerospace Exploration Agency) Tanegashima Space Center's &lt;a href=&quot;http://kibo-robo.jp/en/&quot; rel=&quot;noreferrer&quot;&gt;KIROBO&lt;/a&gt;. This is how &lt;a href=&quot;https://en.wikipedia.org/wiki/Kirobo&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia describes it&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Kirobo is Japan's first robot astronaut, developed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Tomotaka_Takahashi&quot; rel=&quot;noreferrer&quot;&gt;Tomotaka&#xA;  Takahashi&lt;/a&gt;, to accompany &lt;a href=&quot;https://en.wikipedia.org/wiki/Koichi_Wakata&quot; rel=&quot;noreferrer&quot;&gt;Koichi Wakata&lt;/a&gt;, the first Japanese commander of&#xA;  the International Space Station. Kirobo arrived on the ISS on August&#xA;  10th 2013 on JAXA's H-II Transfer Vehicle &lt;a href=&quot;https://en.wikipedia.org/wiki/Kounotori_4&quot; rel=&quot;noreferrer&quot;&gt;Kounotori 4&lt;/a&gt;, an unmanned&#xA;  resupply spacecraft launched August 4th 2013 from Japan's &lt;a href=&quot;https://en.wikipedia.org/wiki/Tanegashima_Space_Center&quot; rel=&quot;noreferrer&quot;&gt;Tanegashima&#xA;  Space Center&lt;/a&gt;. A twin to Kirobo, named Mirata, was created with the&#xA;  same characteristics. Mirata will stay on Earth as a backup crew&#xA;  member. The word &quot;kirobo&quot; itself is a portmanteau of &quot;kibō&quot;, which&#xA;  means &quot;hope&quot; in Japanese, and the word &quot;robo&quot;, used as a generic short&#xA;  word for any robot.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;KIROBO is a small (approximately 34 centimetres / 13 inches tall) robot with human speech  (voice and speech recognition, natural language processing, speech synthesis), telecommunications, face recognition and video recording capabilities. It is designed to navigate zero-gravity environments and assist Commander Wakata in various experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Nunae.jpg&quot; alt=&quot;Kirobo zero-gravity tests&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Kirobo was put through a series of zero-gravity tests before it was deemed ready for flight. (Source: &lt;a href=&quot;http://kibo-robo.jp/en/&quot; rel=&quot;noreferrer&quot;&gt;Kibo Robot Project&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Suggested further reading: News article by Geekquinox Yahoo! news blog, from 9 Aug, 2013 on &lt;a href=&quot;http://ca.news.yahoo.com/blogs/geekquinox/tiny-robot-astronaut-arrives-international-space-station-170731360.html&quot; rel=&quot;noreferrer&quot;&gt;Talking humanoid robot makes contact with International Space Station&lt;/a&gt;&lt;br /&gt;&lt;br /&gt;&lt;sup&gt;(includes two videos: Kirobo's arrival to the ISS, and a Japanese language Kirobo demonstration / interaction capabilities)&lt;/sup&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-24T14:37:24.977" LastActivityDate="2013-08-24T14:37:24.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1532" PostTypeId="1" AcceptedAnswerId="1533" CreationDate="2013-08-24T16:00:29.337" Score="7" ViewCount="602" Body="&lt;p&gt;The Apollo space capsules had a ton of velocity changes - from liftoff, to separations, to reentry to splashdown, there were a lot of sporadic movements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The capsules also needed to carry things that would be used in space. How were these things kept in place? Velcro was only a few decades old when the program started. Was Velcro used? Straps?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-08T03:15:07.007" LastActivityDate="2013-09-08T03:15:07.007" Title="What was the preferred method of holding things down in Apollo?" Tags="|apollo-program|velcro|fastening|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1533" PostTypeId="2" ParentId="1532" CreationDate="2013-08-24T17:27:36.523" Score="5" Body="&lt;p&gt;I'll borrow heavily from a September 1973 Apollo Experience report called &quot;&lt;a href=&quot;http://history.nasa.gov/alsj/tnD7434Stowage.pdf&quot;&gt;Crew Station Integration: Vol.IV - Stowage and the support team concept&lt;/a&gt;&quot; by Marion W. Hix from Houston.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Use of Velcro started during Mercury missions&lt;/li&gt;&#xA;&lt;li&gt;Gemini program introduced &lt;strong&gt;modular containers&lt;/strong&gt; with molded foam holders for particular items&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Duct tape&lt;/strong&gt; was used since Gemini missions&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Modular containers were fixed in place by &lt;strong&gt;pip pins&lt;/strong&gt; or &lt;strong&gt;Calfax fasteners&lt;/strong&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Anything not in containers could be held in place by &lt;strong&gt;Velcro&lt;/strong&gt; (#1), &lt;strong&gt;metal snaps&lt;/strong&gt; (#2), &lt;strong&gt;utility straps&lt;/strong&gt;, and &lt;strong&gt;bungee cords&lt;/strong&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;After Apollo 1 fire, nylon-based Velcro was banned, and its successor found to be less efficient&lt;/li&gt;&#xA;&lt;li&gt;Utility straps (12 inches long - 30.4 cm) were used as a makeshift restraint where Velcro was unavailable&lt;/li&gt;&#xA;&lt;li&gt;Bungee cords were suggested by Apollo 10 crew (Cernan, Stafford, Young) and implemented from Apollo 11 onwards&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr/&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://history.nasa.gov/alsj/WOTM/WOTM-Velcro.html&quot;&gt;http://history.nasa.gov/alsj/WOTM/WOTM-Velcro.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Velcro had its share of problems - fouling up by dust (3 times), tiny hooks floating in the air in the Apollo 11 Command Module and irritating Collins, Velcro patch failures a total of nine times in space and in lunar EVAs.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One of the more innovative uses of Velcro included putting a patch into EVA suits to assist in scratching the astronaut's nose.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2013-08-24T17:27:36.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1534" PostTypeId="1" AcceptedAnswerId="1537" CreationDate="2013-08-24T19:35:33.777" Score="10" ViewCount="1193" Body="&lt;p&gt;This is a follow-up building on &lt;a href=&quot;https://space.stackexchange.com/questions/967/does-voyager-1-do-anything-to-be-more-observable/979#979&quot;&gt;Does Voyager 1 do anything to be more observable?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps no later than 2025 AD, the Voyager twins are expected to become mute; the &lt;a href=&quot;http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;RTGs&lt;/a&gt; will no longer provide enough electricity to power their instruments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After that, anyone interested in locating either twin will have to rely on the last reported heading and velocity. There is no reason to believe it's path will change, though more definite knowledge of it's course and velocity would be helpful — but the availability of such information is not a reasonable expectation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At that point, how would one track either twin? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;E.g. &#xA;&lt;em&gt;Optically by Kepler or it's descendant&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-08-25T19:50:29.200" Title="How would the Voyagers be tracked after the RTGS runs out of fuel?" Tags="|voyager|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1535" PostTypeId="1" CreationDate="2013-08-24T20:36:48.257" Score="8" ViewCount="1452" Body="&lt;p&gt;&lt;em&gt;(Reposting from Physics.Stackexchange, as it is more appropriate here.)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit: I calculated it, but the results are so big, I'll write it as a small pop-sci article, then will post a summary and the data here. Lot's of funny findings and ideas.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I tried calculating this, but it gets too complicated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume, we have a Moon orbit station and ISS on Earth orbit. We have a Moon base. We want to send a tourist for a week on the Moon and back. We need to launch only the oxygen/fuel cells and the fuel for trans-lunar and trans-earth injection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lunar lander lands and takes off in one piece (we don't need a pile of used stages on the lunar base). It can be lighter than 10 metric tons of Apollo LEM. Fuel cells (200 kg each) may go, because now we have solar panels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, how low can we get with newer technologies? And the main question, how small can the launch weight be? Is this doable with conventional Soyuz or Proton rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume, we might have a coilgun to do trans-Earth injection right from the lunar surface. Delta-v of 2.7 km/s means 90 km of rail and accelerating @ 5g, if I remember my calculations correctly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some data from Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apollo Lunar Module&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Ascent stage 4,547 kg of which 2,353 kg is propellant&lt;/li&gt;&#xA;&lt;li&gt;Descent stage: 10,149 kg (8,200 kg of propellant for 2,500 m/s delta-v)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Command and Service module&#xA;&lt;ul&gt;&#xA;&lt;li&gt;command module 5,809 kg&lt;/li&gt;&#xA;&lt;li&gt;service module 24,523 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;all together 46,980 kg&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Saturn V&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1st stage with fuel: 2,300,000 kg&lt;/li&gt;&#xA;&lt;li&gt;2nd stage: 480,000 kg&lt;/li&gt;&#xA;&lt;li&gt;3rd stage: 120,800 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The third stage fired only partially (165 + 335 seconds) to get to LEO, and then was used for TLI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, 120 tons were sent to LEO, and only 47 tons left after TLI. How low can the latter get?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lunar module will be orbiting the Moon and reused. This means several tonnes less for TLI and Lunar orbit injection. But the fuel for it has to fly from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If CM gets smaller, this can also make the vehicle lighter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Service module can be reduced by using inflatable materials, and it can be reused and stored at ISS. So we save 25 tons x 9 km/s (launch from Earth), but add 3km/s of delta-v to park it after the way back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, all together, we need to launch&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the Earth landing module (CM analog)&lt;/li&gt;&#xA;&lt;li&gt;fuel for&#xA;&lt;ul&gt;&#xA;&lt;li&gt;TLI&lt;/li&gt;&#xA;&lt;li&gt;LOI&lt;/li&gt;&#xA;&lt;li&gt;lunar landing and takeoff&lt;/li&gt;&#xA;&lt;li&gt;TEI&lt;/li&gt;&#xA;&lt;li&gt;fuel to park SM in LEO&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If coilgun is used, we don't need takeoff and TEI fuel. (Hm... we need to launch the SM back too :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much does this weigh?&lt;/p&gt;&#xA;" OwnerUserId="612" LastEditorUserId="612" LastEditDate="2013-08-29T19:36:19.343" LastActivityDate="2013-08-29T21:15:51.170" Title="What would be the lightest possible moon launch vehicle?" Tags="|the-moon|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1536" PostTypeId="1" AcceptedAnswerId="24352" CreationDate="2013-08-24T20:44:02.917" Score="9" ViewCount="1258" Body="&lt;p&gt;Playing Kerbal Space Program, I wonder why they do several burns to get into orbit. Why can't a vehicle get smoothly to it, just pointing slightly off its course?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I saw a Soyuz launch at night from 1000 kms from Baikonur, faint exhaust was clearly visible, and it kept burning continuously, and got into an orbit. (At the cutoff moment, you could see a satellite-like moving star.) So, Soyuz get into in one single burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I studied the control theory (thanks for correcting my English), which solves problems like getting to some state by means of some parameters, and taking into account the trajectory. A problem like this is solved with a system of differential equations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, is such an optimal path smooth, or not? Is this difference just because of low precision of human flying by hand?&lt;/p&gt;&#xA;" OwnerUserId="612" LastEditorUserId="612" LastEditDate="2019-08-12T00:58:59.163" LastActivityDate="2019-08-12T00:58:59.163" Title="How do vehicles get smoothly to a circular low Earth orbit?" Tags="|launch|low-earth-orbit|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="1537" PostTypeId="2" ParentId="1534" CreationDate="2013-08-24T20:44:45.943" Score="15" Body="&lt;p&gt;The Voyagers will not &lt;strong&gt;deplete&lt;/strong&gt; their fuel for centuries. Within a few more years, however, the rest of the involved system will degrade to the point that the voyagers can no longer transmit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IR telescopes will be able to spot them, provided they don't have a course change, as the Plutonium in the RTG continues to decay, and it's decay products continue to decay, for many years, probably at least a century.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia shows that the RTG has about 2400W of thermal power, and the reason the three RTG's are out on the boom was to prevent excess heating of the rest of the probe. Internally, the RTG's are about 1000° C, and cooling slowly. Given the half-life of 87 years for the Plutonium, it will still be about or above 400°C in the 2050's. The junction couplings are likely to corrode to non-function before then. The relatively high temperature will generate a strong IR signature via the principle of blackbody radiation. Still, it's only a 400W signature at launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the distance regime (101-124 AU) where it's at, the IR signature should be detectable for at least the next 50 years, and probably considerably longer, tho', thanks to the Inverse Square Law of EM Radiation, the IR signature is a tiny source. You'll need to be looking for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The distance regime also means that optical is not particularly useful. The light from the sun is too dim to be readily detected with current telescopes, tho' a hubble image might be doable with sufficient exposure time. At 101.8 and 124 AU, they receiver 1/10363 and 1/15376 the energy as an object near earth. And with only around 13 square meters of cross sectional surface area, most of which is angled, that's VERY little being reflected back.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://voyager.jpl.nasa.gov/mission/weekly-reports/&quot;&gt;http://voyager.jpl.nasa.gov/mission/weekly-reports/&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/MHW-RTG&quot;&gt;http://en.wikipedia.org/wiki/MHW-RTG&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Plutonium-238&quot;&gt;http://en.wikipedia.org/wiki/Plutonium-238&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2013-08-25T08:12:16.233" LastActivityDate="2013-08-25T08:12:16.233" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1538" PostTypeId="2" ParentId="1536" CreationDate="2013-08-24T21:22:45.690" Score="10" Body="&lt;p&gt;Similar to a &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot;&gt;Hohmann transfer orbit&lt;/a&gt;, short high-thrust bursts are more energy-efficient than a continuous low-thrust burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the ideal trajectory has two burns: first the launch, which gets the vehicle on an elliptical orbit whose apogee just touches the final destination orbit, and an orbit-insertion burn, that puts the vehicle on the final orbit.&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2013-08-24T21:22:45.690" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1539" PostTypeId="5" CreationDate="2013-08-24T21:34:10.990" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-24T21:34:10.990" LastActivityDate="2013-08-24T21:34:10.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1540" PostTypeId="4" CreationDate="2013-08-24T21:34:10.990" Score="0" Body="Questions regarding the fastener known as Velcro, modeled after burrs sticking to a dog's fur." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-24T21:36:24.870" LastActivityDate="2013-08-24T21:36:24.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1541" PostTypeId="2" ParentId="1364" CreationDate="2013-08-24T21:34:56.500" Score="14" Body="&lt;p&gt;Not particularly, at least not long term. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take a hypothetical high earth orbit imaging satellite. We want it stealthy. Here are the iterations....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have to account for the blackbody radiation, which is the infrared light given off by all objects that are above 0 kelvins (-273.15° C). To hide this, we need to exactly match the backdrop. That backdrop is either 2.7 K (cosmic background), or anywhere between 20 K and 310 K (the moon), or 5778 K (the sun). Cooling anything to under 20 K is difficult even in the lab. It's impractical long term, and nearly impossible when the sun is trying to keep it around 250 K. Plus, every electronic device and the power source produces heat. (And it won't be using solar, because Solar is &lt;em&gt;VERY&lt;/em&gt; visible.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can get it down to &quot;barely noticeable&quot; by cooling it to about 50 K on one side, but the other side is then going to be considerably warmer. Anything flying behind it will reflect some of this increased IR (and also heat up). Further, if it winds up between a further object that wasn't designed for, it may occlude the object, which blips off the view briefly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also need to block visual - it needs to look like whatever is behind it. Black objects aren't going to stay cool, so we need to put it against the moon or sun. And moon, that's still gray, so it's going to need more cooling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we need to make it invisible to radar - stealth tech doesn't actually make it invisible, just reduces the return to lower than some threshold. Since the USAF tracked (until 2013) objects down to the size of a few square centimeters, any useful sized satellite will still be big enough to exceed the squelch setting on the orbital radars. Further, if radar absorbing materials are used, they add to the heat that needs to be eliminated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, we need to cope with the optics - they are reflective by their very nature. So, again, we have a problem. If the optics are aimed at earth, the optics can be detected from earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, any one is doable, but blocking all of these is hard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And we still have not gotten to a useful satellite; there's no data return. An antenna is, itself, a reflective item, and the focusing dish is by design a highly reflective surface in its operational wavelengths. Plus, when transmitting, the signal itself is a beacon. Too many private dishes and university dishes looking for off-earth signals. Someone gets a &quot;wow&quot;, it gets confirmed, and the object is detected. Then, others start looking at what's there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, several of these are directional - viewers from various ground stations may be looking at it side on. Which, from even just two places in the US, means it's not going to be up against the moon, but against the cosmic background.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Far easier is obfuscation - put the military mission on a non-military satellite. Release the wrong image detail specifications. But you still have the issue of secrecy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in the long run, no, you can't make a satelite invisible to a down-station and still observe earth.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2013-08-24T21:50:16.923" LastActivityDate="2013-08-24T21:50:16.923" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1542" PostTypeId="5" CreationDate="2013-08-24T21:38:00.600" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-24T21:38:00.600" LastActivityDate="2013-08-24T21:38:00.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1543" PostTypeId="4" CreationDate="2013-08-24T21:38:00.600" Score="0" Body="Questions regarding quick oscillations known as vibrations. " OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-24T21:38:27.910" LastActivityDate="2013-08-24T21:38:27.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1544" PostTypeId="2" ParentId="1536" CreationDate="2013-08-24T22:56:50.600" Score="4" Body="&lt;p&gt;The typical way that people get to LEO in real life is somewhat similar to the 1ish burn KSS insertion.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Get your rocket so that it's effective apogee is at the appropriate elevation.&lt;/li&gt;&#xA;&lt;li&gt;Change your momentum so it is horizontal, to get the perigee at the correct altitude.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This is the way that the space shuttle works, for instance. It is very close to the optimal orbit for very low LEO orbits, due to the fact that you have to get out of the atmosphere in order for it to work at all. It also makes things a lot simpler.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality, there is a trade off between a lot of factors to get the optimal orbit. These include:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Overall capacity of rocket.&lt;/li&gt;&#xA;&lt;li&gt;Range safety concerns&lt;/li&gt;&#xA;&lt;li&gt;Desired orbit, including inclination, apogee, perigee, etc.&lt;/li&gt;&#xA;&lt;li&gt;Coverage of downrange objects.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Depending on all of these, and more, the optimal mission might be 1 or 2 burns. In general, fewer burns is desired, due to less complex rocketry, missions, etc.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-24T22:56:50.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1545" PostTypeId="1" AcceptedAnswerId="1546" CreationDate="2013-08-24T23:59:23.730" Score="13" ViewCount="1696" Body="&lt;p&gt;The Russians were the first to launch a satellite into space - and it did one thing: beep. It would send out radio waves of nothing but beeps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, it was a huge technological achievement. My question: &lt;strong&gt;Is the Russian satellite still beeping? If so, how can I hear it?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="12102" LastEditDate="2021-09-13T00:18:26.387" LastActivityDate="2021-09-13T00:18:26.387" Title="Is the first Russian satellite still beeping?" Tags="|artificial-satellite|russia|spaceflight-firsts|roscosmos|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1546" PostTypeId="2" ParentId="1545" CreationDate="2013-08-25T00:04:27.063" Score="21" Body="&lt;p&gt;No.  Sputnik 1 reentered on Jan 4, 1958, a few months after launch.  The batteries died and it stopped beeping about three weeks after launch.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-08-25T00:04:27.063" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1547" PostTypeId="1" AcceptedAnswerId="1548" CreationDate="2013-08-25T00:26:22.580" Score="6" ViewCount="248" Body="&lt;p&gt;I was reading that recently the Dream Chaser completed a carry test. Seems like they hung it from a helicopter and flew around with it.  What benefit would this have over a wind tunnel test?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-08-25T01:27:39.403" LastActivityDate="2013-08-25T01:27:39.403" Title="What is the purpose of the Dream Chaser carry test?" Tags="|commercial|testing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1548" PostTypeId="2" ParentId="1547" CreationDate="2013-08-25T01:23:39.367" Score="4" Body="&lt;p&gt;What you describe are called &lt;em&gt;&quot;captive-carry&quot;&lt;/em&gt; or &lt;em&gt;&quot;captive-carriage&quot;&lt;/em&gt; tests. Their purpose is to verify integrated spacecraft performance under landing and rollout conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of these captive carry Dream Chaser tests at Nasa's Dryden Flight Research Centre in California, run by NASA partner Sierra Nevada Corporation (SNC) of Louisville, Colorado, an Erickson Air-Crane helicopter picked up a test version of the Dream Chaser and flew it a distance of three miles at a maximum altitude of 12,400 feet.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The tests ensure the Dream Chaser would operate properly upon landing&#xA;  and that the spacecraft will come to a controlled stop after touching&#xA;  down on the runway. SNC’s Dream Chaser team is now preparing for the&#xA;  next CCP tests including a captive carry test, and an approach and&#xA;  landing free flight test, all of which are scheduled for later this&#xA;  fall.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Press release: &lt;a href=&quot;http://www.prweb.com/releases/2013/8/prweb11023986.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sierra Nevada Corporation's Dream Chaser® Completes Ground Tow Tests for Upcoming Approach and Landing Test&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Jt5dN.jpg&quot; alt=&quot;Dream Chaser captive carriage test&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Dream Chaser flight vehicle lifted by an Erickson Air-Crane helicopter during a captive-carry test (Credit: &lt;a href=&quot;http://www.sncorp.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sierra Nevada Corp.&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Among tests performed were systems verifications, including flight computer and flight software, instrumentation, guidance, navigation and control, braking and steering performance, flight control surface actuation, mission control and remote commanding capability, and landing gear dynamics.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-08-25T01:23:39.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1549" PostTypeId="5" CreationDate="2013-08-25T01:35:20.020" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-25T01:35:20.020" LastActivityDate="2013-08-25T01:35:20.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1550" PostTypeId="4" CreationDate="2013-08-25T01:35:20.020" Score="0" Body="Questions about the procedure of using a piece of equipment to find imperfections or problems." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-25T01:36:18.590" LastActivityDate="2013-08-25T01:36:18.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1551" PostTypeId="1" AcceptedAnswerId="1552" CreationDate="2013-08-25T01:39:10.487" Score="54" ViewCount="49251" Body="&lt;p&gt;I have watched several STS launches, and noticed that there were always huge jets of water that would begin right before the rocket engines lit. They were right under the rocket, mounted to the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure they weren't trying to extinguish the flame, and it's kind of futile to try to keep the equipment cool - what was their purpose?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2013-10-31T16:17:25.527" LastActivityDate="2022-11-09T03:05:48.487" Title="What is the purpose of the jets of water often under rocket engines during launch?" Tags="|launch|space-shuttle|water|launchpad|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1552" PostTypeId="2" ParentId="1551" CreationDate="2013-08-25T01:59:52.287" Score="64" Body="&lt;p&gt;Those are jets of water released by the sound suppression systems installed on the pads and the mobile launcher platforms to protect orbiters and their payloads from being damaged by acoustical energy, reflected from the platform during the liftoff stage of a rocket launch.&lt;/p&gt;&#xA;&lt;p&gt;For example, this is the sound suppression system at the NASA Kennedy Space Center's Launch Pad 39A:&lt;/p&gt;&#xA;&lt;p&gt;    &lt;img src=&quot;https://i.stack.imgur.com/MN2Sg.jpg&quot; alt=&quot;Launch Pad 39A sound suppression system&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;    &lt;sup&gt;Water is released onto the Launch Pad 39A at the start of a water sound suppression test. (Credit: NASA/KSC)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;And a &lt;a href=&quot;https://www.youtube.com/watch?v=reE62UvBd8I&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video of the launchpad water sound suppression system&lt;/a&gt; tested for &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_%28rocket%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ares&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Here are a few excerpts from the &lt;a href=&quot;http://www.nasa.gov/missions/shuttle/f_watertest.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sound Suppression Test Unleashes a Flood&lt;/a&gt; NASA/KSP article from 2004, explaining this system's purpose:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The water system is designed to protect the Shuttle and its payloads&#xA;from any damage that may occur from acoustical energy reflected from&#xA;the Mobile Launcher Platform (MLP) during launch. The water is&#xA;released seconds before ignition of the orbiter's three main engines&#xA;and twin Solid Rocket Boosters (SRB), then flows through parallel&#xA;7-foot-diameter pipes to the Pad.&lt;/p&gt;&#xA;&lt;p&gt;The system includes a 290-foot-high water tank filled with 300,000&#xA;gallons of water, and it empties in 41 seconds during a launch. Water&#xA;pours from 16 nozzles on top of the flame deflectors and from outlets&#xA;in the Shuttle main engine exhaust hole in the MLP at main engine&#xA;ignition, which occurs approximately 7 seconds before liftoff.&lt;/p&gt;&#xA;&lt;p&gt;The system was first installed at the pad when reflective energy from&#xA;the top of the Mobile Launch Platform was causing minor damage to&#xA;thermal curtains on the SRBs and putting stress on the wings. After&#xA;adding the system, the sound pressure was reduced by half.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/szrWj.jpg&quot; alt=&quot;Launch Pad 39A sound suppression system&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Photograph and excerpts credit: &lt;a href=&quot;http://www.nasa.gov/missions/shuttle/f_watertest.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's John F. Kennedy Space Center&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Additional&lt;/strong&gt;: Here's an interesting related link: &lt;a href=&quot;https://www.google.com/maps/@28.6088672,-80.6041219,3a,75y,172.98h,107.23t/data=!3m8!1e1!3m6!1sAF1QipMqx08PNNqfHoULXpmLJGBLZWwxJP8rcP7_AKRK!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipMqx08PNNqfHoULXpmLJGBLZWwxJP8rcP7_AKRK%3Dw203-h100-k-no-pi0-ya270.48-ro-0-fo100!7i5300!8i2650&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google Street View of the bottom of the launch pad 39A&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="14254" LastEditDate="2020-11-26T00:10:58.053" LastActivityDate="2020-11-26T00:10:58.053" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="1553" PostTypeId="1" CreationDate="2013-08-25T03:49:53.003" Score="9" ViewCount="739" Body="&lt;p&gt;One of the main components of the Space Shuttle's thermal protection system for reentry was the layer of tiles on the outside of the orbiter. What, if any, changes were made to the composition of these tiles between the first and last flights of the Shuttle program? &lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-25T20:02:17.867" Title="Were any changes made to the composition of the Shuttle's thermal protection tiles over the lifetime of the program?" Tags="|space-shuttle|thermal|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1554" PostTypeId="2" ParentId="1553" CreationDate="2013-08-25T05:27:14.257" Score="5" Body="&lt;p&gt;In &lt;a href=&quot;http://www.airspacemag.com/how-things-work?shuttle_tiles.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;1981&lt;/a&gt;, each shuttle had more than 24,000 tiles of six by six inch blocks made of silica (high grade sand) and fibres. These are made of silica fibres and chemicals mixed,  molded and heated at microwave oven at 2350 degrees, at which temperature the silica fuses to form a high temperature insulator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In earlier days of the &lt;a href=&quot;http://pubs.acg.org/cen/coverstory/83/8344NASA.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;shuttle&lt;/a&gt; program, three types of spray on foams were used &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;polyurethane foam (BX_250)&lt;/li&gt;&#xA;&lt;li&gt;polyisocyanurate foam (NCFI 24_124,NCFI 24_57) &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Both of the foams are applied with the blowing agent HCHC-141b&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wjftq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;Previously&lt;/a&gt; spacecraft used ablative heat shield which usually burnt off and cannot be reused after re-entry and they are very heavy and they are also used in Apollo command module comprised of 0.333 of vehicles weight and they are fragile and can be crush a tiles by hand&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After Columbia accident the changes are made to the tiles:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;EMITTANCE WASH&lt;/strong&gt; - (polish) consist of silicon carbide fibres and mixed with strong glue which attachs them to the tiles leading to stiffer bonds and on ground testing indicated that the material has increased the amount of heat that damages the tiles by 70 to 160 degrees &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;RENFORCED CARBON_CARBON&lt;/strong&gt;-the light gray material capable of withstanding 1510 degree placed in the leading edges of the wing and the nose cap and 6.4 to 13 mm thick 22 RCC plates are attached to each orbitor &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;SHUTTLE TILE ABLATOR 54&lt;/strong&gt; -it is basically a mixture of room_temperature_vulcanizing rubber, glass microballoons , silicone oil , a fumed silica (Cab-O-Sil) and a catalyst . the technique uses silicone condensation reaction to create silicon rubber which pyrolised during re-entry to a ceramic state&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;RCC was made from laminated composite material made from graphite rayon cloth and impregnated with phenolic resins and exposed to high temperature (curing) in autoclave and pyrolised to convet the resin to carbon and then impregnated in furfural alcohol in vacuum chamber the heated till the desired properties are achieved&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="14" LastEditDate="2013-08-25T20:02:17.867" LastActivityDate="2013-08-25T20:02:17.867" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1555" PostTypeId="1" AcceptedAnswerId="1560" CreationDate="2013-08-25T08:59:38.247" Score="11" ViewCount="535" Body="&lt;p&gt;Water is a little buoyant which may be part of the reason a lot of commerce happens on the waves. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How much of a difference in terms of fuel would it make if the craft were launched from a water-body? &lt;/li&gt;&#xA;&lt;li&gt;Have all spacecraft to-date launched from land? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-08-26T22:27:57.423" Title="Have all spacecraft on record been launched from land?" Tags="|launch|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1556" PostTypeId="1" AcceptedAnswerId="1569" CreationDate="2013-08-25T09:07:34.963" Score="20" ViewCount="8468" Body="&lt;p&gt;Nowadays, many rockets transport more than one satellite in orbit. But how does a single rocket place multiple satellites into their orbits? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do all rockets which place multiple satellites into orbit follow the same mechanism? What is the mechanism behind this?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="9092" LastEditDate="2021-03-26T15:00:08.737" LastActivityDate="2021-03-26T15:00:08.737" Title="How does a single rocket place multiple satellites into orbit?" Tags="|rockets|artificial-satellite|payload|payload-deployment|secondary-payload|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1557" PostTypeId="2" ParentId="1435" CreationDate="2013-08-25T09:41:36.593" Score="0" Body="&lt;p&gt;Based on the comments this question has received, it does not appear possible to divide funding by the three categories given.  Both Military and scientific government funding are the same source, and the designation between them is merely a political preference of the time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Private funding, as a commercial speculation as of 2013 is essentially insignificant. &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-08-25T09:41:36.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1558" PostTypeId="2" ParentId="1555" CreationDate="2013-08-25T10:38:29.783" Score="5" Body="&lt;p&gt;There will not be a difference in fuel consumption  unless the launch site is placed at high altitude (above sea level). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Sea_Launch&quot; rel=&quot;noreferrer&quot;&gt;Sea launch&lt;/a&gt;&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thirty-one rockets have been assembled and&#xA;  launched so far, with three failures and one partial failure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://www.sciencepresse.qc.ca/clafleur/Spacecrafts-index.html&quot; rel=&quot;noreferrer&quot;&gt;Land launch&lt;/a&gt;&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.sciencepresse.qc.ca/clafleur/Spacecrafts-index.html&quot; rel=&quot;noreferrer&quot;&gt;this site&lt;/a&gt; 6851 launches have been conducted by various nations using this technique. &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-08-25T12:05:03.147" LastActivityDate="2013-08-25T12:05:03.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1559" PostTypeId="2" ParentId="1556" CreationDate="2013-08-25T11:58:21.727" Score="11" Body="&lt;p&gt;It really depends on the rocket, and mission, etc. But there is generally speaking a same common thread. The spacecraft are typically spring loaded on to the rocket, held in place by some sort of explosive bolts (Or other clamps) type technology. When the rocket is ready to release the spacecraft, it activates the explosive bolts, causing the spacecraft to be pushed away from the rocket at some speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A very simple example is the P-POD, used to deploy cubesats, pictured below, from &lt;a href=&quot;http://www.pe0sat.vgnet.nl/tag/lecture/&quot; rel=&quot;noreferrer&quot;&gt;PE0SAT&lt;/a&gt;.  The door to the POD is closed until it receives the signal from the spacecraft, where it opens. The spring slowly deploys the spacecraft until they are all released.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wJAHg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For larger satellites, the satellites are typically placed around some sort of a ring, such as the &lt;a href=&quot;http://www.moog.com/products/spacecraft-payload-interfaces/&quot; rel=&quot;noreferrer&quot;&gt;ESPA ring&lt;/a&gt; from Moog pictured below. Essentially, the spacecraft lay hiding in the nose code, the ring deploys, and one at a time the spacecraft are deployed from the espa ring until everything has been deployed. There are similar systems with other names by other providers, but they all follow the same principals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mCtlU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-25T11:58:21.727" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1560" PostTypeId="2" ParentId="1555" CreationDate="2013-08-25T12:07:48.697" Score="14" Body="&lt;p&gt;Spacecraft have been launched on Air, Land, and Sea. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Air Launch&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most popular of these is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Pegasus_%28rocket%29&quot;&gt;Pegasus Rocket&lt;/a&gt; from Orbital. The airplane carrying the rocket goes up to ~40,000 ft, and fires off the rocket from below. These have carried a number of missions, including NASA, commercial, and other sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sea Launch&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The name of a company which launches such rockets. They utilized the Zenit-3SL rockets for all of their launches, and primarily launched GEO satellites from the equator, which reduces considerably the cost to launch a GEO sat, due to the fact that the inclination doesn't have to be removed.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-25T12:07:48.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1561" PostTypeId="1" AcceptedAnswerId="1562" CreationDate="2013-08-25T12:11:46.747" Score="11" ViewCount="5277" Body="&lt;p&gt;During the Cold War, one of the main reasons for space exploration was military/propaganda purposes. One of the things that the US was worried about was the prospect of a nuclear bomb being dropped from a satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me, however, that the bomb would simply burn up in the upper atmosphere. Would it be possible to drop a nuclear bomb from space onto the Earth, and have it work as expected?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I'm looking for more of a 'how' answer - maybe citing research?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user29" LastEditDate="2013-08-25T12:44:32.690" LastActivityDate="2013-11-14T14:17:47.020" Title="Would it actually be feasible to drop a nuclear bomb from a satellite?" Tags="|artificial-satellite|nuclear|military|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1562" PostTypeId="2" ParentId="1561" CreationDate="2013-08-25T12:29:27.613" Score="13" Body="&lt;p&gt;Yes it is possible to develop such a weapon. Example: &lt;a href=&quot;https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment_System&quot; rel=&quot;nofollow&quot;&gt;Fractional Orbital&#xA;Bombardment System&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It consists of a rocket and a warhead. The rocket would place the warhead into LEO where small on-board thrusters would guide the the warhead into place for it's controlled descent to the target. There was also a thermal protection system required but overall, this system was not very precise. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are not operational now. Their development was halted after the &lt;a href=&quot;http://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;nofollow&quot;&gt;outer space&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/SALT_II&quot; rel=&quot;nofollow&quot;&gt;SALT 2&lt;/a&gt; treaties. These agreements prohibit weapons of mass destruction in outer space.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2013-11-12T19:24:55.063" LastActivityDate="2013-11-12T19:24:55.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1563" PostTypeId="2" ParentId="1561" CreationDate="2013-08-25T12:42:04.583" Score="8" Body="&lt;p&gt;Of course, you can't simply &quot;drop&quot; the bomb — it would just stay in the same orbit as the satellite. The bomb would need a propulsion system (rocket) to decelerate. But this would certainly be possible.&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2013-08-25T12:42:04.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1564" PostTypeId="1" AcceptedAnswerId="1565" CreationDate="2013-08-25T12:59:13.173" Score="23" ViewCount="24222" Body="&lt;p&gt;Wikipedia states that &lt;a href=&quot;http://en.wikipedia.org/wiki/Vanguard_1&quot;&gt;Vanguard I&lt;/a&gt; is the oldest artificial satellite still in orbit. What is the oldest that's still in use?&lt;/p&gt;&#xA;" OwnerUserId="79" LastActivityDate="2020-09-04T11:20:36.377" Title="What is the oldest artificial satellite still in use?" Tags="|artificial-satellite|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1565" PostTypeId="2" ParentId="1564" CreationDate="2013-08-25T13:15:20.133" Score="14" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/GOES_3&quot;&gt;GOES 3&lt;/a&gt; is most likely the oldest&#xA;satellite in operation as of early 2014. Launched in 16 June 1978 as a weather forecasting satellite, it was repurposed as a communications satellite when it became unusable for meteorological studies in 1989. &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="1142" LastEditDate="2014-03-16T01:20:55.437" LastActivityDate="2014-03-16T01:20:55.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1566" PostTypeId="1" CreationDate="2013-08-25T14:26:53.693" Score="7" ViewCount="108" Body="&lt;p&gt;I'm asking about standards promoted by ISO, IEEE, ITU, intergovernmental bodies, or simply de-facto standards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, &lt;a href=&quot;https://en.wikipedia.org/wiki/MIL-STD-1553&quot;&gt;MIL-STD-1553&lt;/a&gt; defines serial data bus used in avionics all over the world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the significant technologies applicable to spacecraft where international standardization has taken root? (The above example of 1553 buses also shows extensive use of the standard in decidedly non-NATO countries - e.g. Russia, are there any other standardization &quot;success stories&quot;?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the specific disadvantages of international standardization in space exploration?&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorDisplayName="user29" LastEditDate="2013-08-25T15:22:25.773" LastActivityDate="2013-08-27T09:46:02.610" Title="What spacecraft technologies are subject to international standards?" Tags="|standards|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1567" PostTypeId="1" AcceptedAnswerId="1646" CreationDate="2013-08-25T15:42:03.527" Score="10" ViewCount="472" Body="&lt;p&gt;Does man send any signal (such as electromagnetic radiation) to space to attract aliens?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditorDisplayName="user29" LastEditDate="2015-04-21T14:59:40.457" LastActivityDate="2015-04-21T15:17:12.440" Title="Does man send any signal to attract aliens?" Tags="|communication|astrobiology|seti|radio-communication|meti|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1568" PostTypeId="2" ParentId="1567" CreationDate="2013-08-25T16:15:00.950" Score="11" Body="&lt;p&gt;Assuming that by signal you mean radio signals, there were many attempts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this has already been described in detail on Wikipedia, so I'm not inclined to repeat it. What you are looking for is available in these two pages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Active_SETI&quot; rel=&quot;noreferrer&quot;&gt;Active Search for Extra-Terrestrial Intelligence&lt;/a&gt;&lt;/strong&gt; (Active SETI)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Communication_with_Extraterrestrial_Intelligence&quot; rel=&quot;noreferrer&quot;&gt;Communication with Extraterrestrial Intelligence &lt;/a&gt;&lt;/strong&gt; (CETI). &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Probably the most famous example of such signal is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Arecibo_message&quot; rel=&quot;noreferrer&quot;&gt;Arecibo Message&lt;/a&gt;, sent on November 16th 1974:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rDbsr.png&quot; alt=&quot;The 1974 Arecibo Message&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="49" LastEditDate="2013-12-27T15:50:33.577" LastActivityDate="2013-12-27T15:50:33.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1569" PostTypeId="2" ParentId="1556" CreationDate="2013-08-25T17:03:26.747" Score="14" Body="&lt;p&gt;Pearson's answer is technically correct for smaller satellites, which are flying &lt;em&gt;pickaback&lt;/em&gt; as &lt;em&gt;secondary payloads&lt;/em&gt;. But it is missing the big &lt;strong&gt;&lt;em&gt;primary payloads&lt;/em&gt;&lt;/strong&gt;. So for the sake of completeness, have a look at different &lt;a href=&quot;http://en.wikipedia.org/wiki/Ariane_5&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5&lt;/a&gt; configurations: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/byb3W.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&lt;sub&gt;&lt;a href=&quot;http://www.cnes.fr/web/CNES-en/5413-technical-data.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Different types of upper section for Ariane 5. Credits: Esa/D.Ducros&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rather common strategy is to 'pile' two or more satellites on top of each other. The upper satellite sits directly inside the &lt;a href=&quot;http://en.wikipedia.org/wiki/Payload_fairing&quot; rel=&quot;nofollow noreferrer&quot;&gt;payload fairing&lt;/a&gt;. Any further satellite below is sitting inside some sort of an inner fairing. The relevant systems for the Ariane 5 are called &lt;a href=&quot;http://www.astrium.eads.net/en/articles/how-does-ariane-launch-two-satellites-on-the-same-flight.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;'SPELTRA' and 'SYLDA' payload dispensers / dual launch systems&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the satellites' needs, the upper stage of the rocket (the one directly below the payload dispenser), can be ignited multiple times (it needs to be designed in such a way). This is for instance done with the 4th stage of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Vega_%28rocket%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vega rocket&lt;/a&gt;. Then, you can deploy the satellites on rather different orbits with only one launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For further reading, have a look at the &lt;a href=&quot;http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5 User's Manual&lt;/a&gt; (e.g. page 1-7) or the &lt;a href=&quot;http://www.arianespace.com/launch-services-vega/VEGAUsersManual.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vega User's Manual&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, 'piling' satellites is not the only technique. E.g. Russian Glonass satellites are launched right next to each other, virtually sharing a single payload adapter. This can be done if the satellites are build and designed in such a way, but it is usually not applicable if two totally different customers share a launch of e.g. an Ariane 5. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dm01T.jpg&quot; alt=&quot;3 Glonass&quot;&gt;&lt;br&gt;&#xA;&lt;sub&gt;Proton and Block DM upper stage mounted with 3 Glonass satellites&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;There are in fact &lt;a href=&quot;https://space.meta.stackexchange.com/a/319/14&quot;&gt;'user manuals' for virtually all commercially available rocket types&lt;/a&gt; and they are freely available. Buying a rocket is a bit like buying a house-hold-device, if you ignore the price tag - they come with manuals. You can google for them, they are amazing reads. There are various implementations for deploying multiple satellites from different rockets / launchers, so it is worth digging for them.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:04.403" LastActivityDate="2013-08-25T18:27:38.867" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1570" PostTypeId="1" AcceptedAnswerId="1582" CreationDate="2013-08-25T18:06:51.560" Score="13" ViewCount="6494" Body="&lt;p&gt;After ignition, if any problems occur such as fuel leakage, can a rocket motor can be shut off?&lt;/p&gt;&#xA;" OwnerUserId="579" LastEditorUserId="14" LastEditDate="2013-08-25T20:26:31.920" LastActivityDate="2019-04-03T12:54:16.493" Title="Can a rocket motor be shut off after ignition?" Tags="|propulsion|rockets|fuel|engine-design|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1572" PostTypeId="1" AcceptedAnswerId="1573" CreationDate="2013-08-25T23:10:18.347" Score="8" ViewCount="2528" Body="&lt;p&gt;Obviously a launch vehicle needs to be tracked quite precisely during launch, but how is this accomplished? Is it all done from the ground, or do launch vehicles normally fly with GPS receivers? Is there info available about typically attainable levels of precision during the launch phase?&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-26T00:22:33.350" Title="During launch, how is the launch vehicle tracked?" Tags="|launch|tracking|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1573" PostTypeId="2" ParentId="1572" CreationDate="2013-08-26T00:11:12.987" Score="6" Body="&lt;p&gt;Different systems are employed during a rocket launch to track the launcher position, but the most precise and relied upon is the &lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Telemetry&quot; rel=&quot;noreferrer&quot;&gt;telemetry data&lt;/a&gt;&lt;/strong&gt; that is transmitted from the launcher itself to the ground control. Of course, all such systems have redundancy and their accuracy and reliability is constantly controlled in real time by use of secondary, even tertiary systems, on top of on board sensors, sensor boards, and communications subsystems redundancy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia puts &lt;a href=&quot;https://en.wikipedia.org/wiki/Telemetry#Applications&quot; rel=&quot;noreferrer&quot;&gt;telemetry in rocketry&lt;/a&gt; thusly:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In rocketry, telemetry equipment forms an integral part of the rocket&#xA;  range assets used to monitor the position and health of a launch&#xA;  vehicle to determine range safety flight termination criteria (Range&#xA;  purpose is for public safety). Problems include the extreme&#xA;  environment (temperature, acceleration and vibration), the energy&#xA;  supply, antenna alignment and (at long distances, e.g., in&#xA;  spaceflight) signal travel time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Exact configurations of such tracking and tracking control systems would vary substantially with different launchers, even individual missions. For example, what NASA calls &lt;em&gt;Launch Vehicle Data Center&lt;/em&gt;, would be flooded with sensor information on temperature, speed, trajectory, and vibration from the launcher itself. On top of that, ground based telemetry data is fed into the tracking system for controls and redundancy in case of failures. Various ground based tracking systems might be deployed, such as radars, lidars, even optical/visual trackers (cameras, optical telescopes,...).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More specifically, for example this is what &lt;a href=&quot;http://www.sea-launch.com/&quot; rel=&quot;noreferrer&quot;&gt;Sea Launch&lt;/a&gt; says about &lt;a href=&quot;http://www.sea-launch.com/launch-q11143-Communications_and_Telemetry.aspx&quot; rel=&quot;noreferrer&quot;&gt;communications and telemetry&lt;/a&gt; for their marine segment &lt;a href=&quot;https://en.wikipedia.org/wiki/Odyssey_%28launch_platform%29&quot; rel=&quot;noreferrer&quot;&gt;launch platform Odyssey&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sea Launch uses line of sight telemetry tracking links between the&#xA;  launch vehicle and the Attitude Control System (&lt;a href=&quot;https://en.wikipedia.org/wiki/Attitude_control&quot; rel=&quot;noreferrer&quot;&gt;ACS&lt;/a&gt;) at lift off and then switches to the NASA&#xA;  Telemetry and Data Relay Support Systems (&lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite_System&quot; rel=&quot;noreferrer&quot;&gt;TDRSS&lt;/a&gt;) at fairing jettison&#xA;  for the remainder of flight - stage separations up to an including&#xA;  spacecraft separation into orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/gwH6D.jpg&quot; alt=&quot;White Sands Ground Terminal (WSGT)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;a href=&quot;http://wikimapia.org/2000668/White-Sands-Ground-Terminal-TDRSS-Ground-Station-in-Las-Cruces&quot; rel=&quot;noreferrer&quot;&gt;White Sands Ground Terminal (WSGT)&lt;/a&gt;, one of three ground segments of &lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite_System#Ground_segment&quot; rel=&quot;noreferrer&quot;&gt;TDRSS&lt;/a&gt; (Telemetry and Data Relay Support Systems)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And NASA Computer Technology Spinoff article from 2009, the &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090039415_2009040016.pdf&quot; rel=&quot;noreferrer&quot;&gt;Telemetry Boards Interpret Rocket, Airplane Engine Data&lt;/a&gt; (PDF) has this to say:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The remote measurement and transmission of systems  data—called&#xA;  telemetry—is essential to ensuring the safe  and successful launch of&#xA;  the Agency’s space missions.  When a launch is unsuccessful, as it was&#xA;  for this year’s  Orbiting Carbon Observatory satellite, telemetry data&#xA;  also provides valuable clues as to what went wrong and  how to remedy&#xA;  any problems for future attempts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Suggested additional reading: &lt;a href=&quot;http://www.brighthub.com/science/space/articles/116036.aspx&quot; rel=&quot;noreferrer&quot;&gt;Space Telemetry: An Overview of Spacecraft to Earth Communications&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-26T00:22:33.350" LastActivityDate="2013-08-26T00:22:33.350" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1574" PostTypeId="1" AcceptedAnswerId="1580" CreationDate="2013-08-26T02:19:58.583" Score="18" ViewCount="1723" Body="&lt;p&gt;On the third flight of SpaceX's Falcon 1, the following failure occurred &lt;a href=&quot;http://www.slate.com/blogs/quora/2013/08/14/elon_musk_what_is_it_like_to_work_for_the_spacex_tesla_chief.html&quot;&gt;according to one narrative&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We turned off the first-stage engine and then proceeded to separate the vehicle stages; however, when the stages uncoupled, there was still a little leftover &quot;kick,&quot; or thrust, in the first-stage engine. Our first stage literally rear-ended our second stage immediately after we had tried to separate the two sections of the vehicle. It was a devastating, emotional experience.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sounds logical.  You wouldn't want to wait until your prior stage fully putters out completely before separating because (among other things) you'll lose altitude.  Ideally, you'd want to be delivering as much thrust as possible as constantly as possible, and that might entail having some kick left in the prior stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the general sense (not specific to SpaceX), how is this avoided in vertically staged rockets?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-08-26T12:15:07.783" Title="How do rockets prevent rear-ending after stage seperation?" Tags="|rockets|stages|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1575" PostTypeId="2" ParentId="1151" CreationDate="2013-08-26T02:23:12.060" Score="6" Body="&lt;p&gt;Good question. This answer is going to focus on how the Jet Propulsion Lab handles this problem, as described mostly in &lt;a href=&quot;http://mediawiki.cite.hku.hk/images/b/b4/JPL.pdf&quot; rel=&quot;noreferrer&quot;&gt;&amp;quot;Managing Knowledge and Learning at NASA and the Jet Propulsion Laboratory (JPL)&amp;quot;, a Harvard Business School publication (2002)&lt;/a&gt; (if someone can find a more stable link, please edit it in), and also &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/13024/1/01-1492.pdf&quot; rel=&quot;noreferrer&quot;&gt;&amp;quot;Architecting an Approach to Knowledge Management&amp;quot;, NTRS (2001)&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;These are somewhat dated, but as far as I can tell the agency is following through on the initiatives laid out by in these documents. These (particularly the HBS publication) are quite good reads, and I'll try to summarize below.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Jeanne Holm, the &amp;quot;Chief Knowledge Architect&amp;quot; for NASA, identified three main challenges:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;How do we [(NASA)] successfully transfer information gleaned from one mission to other missions and across generations of people who come in and out of the agency?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How do we get hugely diverse, geographically distributed communities to work together on complex space missions?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How do we manage the knowledge that we already have?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Her response to these challenges was three-fold. &lt;strong&gt;First&lt;/strong&gt;, redesigning NASA's underutilized internal &lt;em&gt;lessons learned&lt;/em&gt; system. &amp;quot;Lessons learned&amp;quot; is a catch-all term for bits of information and advice a team captures during their experience working on the project (an oft-occurring example: &amp;quot;Lock the mission requirements down at a specific date, and do not allow the sponsor to change them after this date.&amp;quot;). Unfortunately capture of lessons learned is often a difficult process, as mission managers are reluctant to record all of the bad decisions that may have been made, for fear of them negatively affecting their reputation as a manager. The redesign of the system (the Lessons Learned Information System (LLIS)) addresses these concerns (unfortunately my source does not go into detail beyond that).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Second&lt;/strong&gt;, a variety of online tools were developed -- essentially what we today would consider a &lt;em&gt;wiki&lt;/em&gt;. The idea was to allow &amp;quot;staffers... to search efficiently for information useful to internal and external agency constituents.&amp;quot; The idea of these tools was to foster collaboration and aid in knowledge sharing in a system that could also serve as a sort of archive.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;strong&gt;third&lt;/strong&gt; initiative was something called the &lt;em&gt;&lt;a href=&quot;http://appel.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Academy of Program and Project Leadership (APPL)&lt;/a&gt;&lt;/em&gt;. I'll let the source speak for this one:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Through its Knowledge Sharing Initiative (KSI), APPL supported best-practice communities and a culture of sharing knowledge through grass-roots efforts across the agency. KSI worked with centers to identify best-of-best managers and bring them together to share project-related stories in a series of workshops. Once or twice a year, KSI brought a group of such managers from all over NASA to meet one another. The head of APPL, Edward Hoffman, contended that APPL was crucial to NASA's efforts to capture and share expertise that was impossible to load onto databases.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In addition to these, Holm suggests that a cultural shift needs to happen at the agency level, and initiatives &amp;quot;such as expanding our mentoring programs, creating oral histories, and creating the time and incentives for mentoring&amp;quot; are needed.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The above largely addresses how to preserve the &amp;quot;know-how&amp;quot;. Preserving the &amp;quot;interest&amp;quot; is, hopefully, relatively easy, as the missions tend to be extremely interesting in their own right (as anecdotally evidenced by the relatively large community of people who consider the missions interesting enough to discuss in their free time ;) ).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="-1" LastEditorDisplayName="user29" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-26T02:38:11.927" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1576" PostTypeId="2" ParentId="511" CreationDate="2013-08-26T06:45:44.687" Score="14" Body="&lt;p&gt;Sure.  Why not.  You'll want some sort of &lt;a href=&quot;http://www.patrawlings.com/detail.cfm?id=1295&quot; rel=&quot;noreferrer&quot;&gt;heatshield&lt;/a&gt; of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/81xSG.jpg&quot; alt=&quot;Skysurfing human weight-shift steered entry vehicle&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or this &lt;a href=&quot;http://www.astronautix.com/craft/paracone.htm&quot; rel=&quot;noreferrer&quot;&gt;more practical design&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vWlc3.png&quot; alt=&quot;Paracone&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or this &lt;a href=&quot;http://en.wikipedia.org/wiki/MOOSE&quot; rel=&quot;noreferrer&quot;&gt;earlier, less convincing concept&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SDJJf.jpg&quot; alt=&quot;General Electric’s one-man, orbital escape pod from the 1960’s&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the fall from a $100\,km$ tower is survivable in terms of G's.  I assumed a $100\,kg$ person and a $2\,m$, $100\,kg$ heatshield and other equipment.  Assuming a blunt body $C_D$, I get a ballistic coefficient of about $40{kg\over m^2}$.  Integrating that fall through a standard atmosphere with gravity properly varying with altitude, I get a maximum velocity of $900\,{m\over s}$, and a maximum acceleration of $2.8\,G$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fall from a $400\,km$ tower is problematic.  Then the maximum velocity is $2400\,{m\over s}$, with a maximum acceleration of $16\,G$.  For a ballistic entry, you can't really get it much below $14\,G$, at an optimum $C_D$ of about $7{kg\over m^2}$ (a much larger heatshield).  Perhaps with some lift you could mitigate the G forces, but then the fall wouldn't be straight down anymore.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-08-26T07:52:11.830" LastActivityDate="2013-08-26T07:52:11.830" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1578" PostTypeId="2" ParentId="1570" CreationDate="2013-08-26T10:04:52.977" Score="5" Body="&lt;p&gt;A qualified 'Yes'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Solid-fuel_rocket#Basic_concepts&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Solid fuel rockets&lt;/a&gt; states&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;More advanced solid rocket motors can not only be throttled but also be extinguished &#xA;  and then re-ignited by controlling the nozzle geometry or through the use of vent &#xA;  ports. Also, pulsed rocket motors that burn in segments and that can be ignited upon &#xA;  command are available.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Qualifier: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;The page says rockets, not necessarily rockets meant for space-craft (either launch, or for use in space)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-08-26T10:04:52.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1579" PostTypeId="1" AcceptedAnswerId="1602" CreationDate="2013-08-26T10:18:38.167" Score="17" ViewCount="650" Body="&lt;p&gt;Recently the ISRO GSLV launch was called off on account of a fuel leak. Other launches too have, on occasion, been called off for similar reasons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fuel used to launch a craft to orbit may be solid, or liquid. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has solid fuel leakage ever been reported? &#xA;Can solid fuel even leak? &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user12" LastEditDate="2013-08-26T12:29:27.527" LastActivityDate="2013-08-27T00:57:46.233" Title="Can solid fuel leak?" Tags="|launch|fuel|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1580" PostTypeId="2" ParentId="1574" CreationDate="2013-08-26T12:15:07.783" Score="9" Body="&lt;p&gt;This type of failure is known as separation failure which is caused due to timing error in separating the stages &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Musk said SpaceX would be releasing video of the staging event that clearly shows that the first and second stages separated as planned about 2 minutes and 20 seconds into the flight, but that unanticipated residual thrust from the redesigned Merlin engine caused the first stage to bump the second stage just as it began to fire&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is normally overcome in the rockets by &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;shutting down the rocket engine properly (cutting down the fuel source(liquid propellent)  in case of &lt;a href=&quot;http://en.wikipedia.org/wiki/Solid-fuel_rocket&quot;&gt;solid propellent&lt;/a&gt; it is  impossible to shut down so timing is very important the fuel is entirely burnt before shut down)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cctv.com/program/Nature&amp;amp;Science/20050531/101773.shtml&quot;&gt;Source&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The separation of large boosters uses direct staging. Each booster is connected to the rocket’s core through a bar mechanism in the front joint side and the round head mechanism in the back joint side. When the engine of the booster stops working, the connection bolt and separation nut will be unlocked separately. The small separation rockets that are bound to the boosters are ignited and separate the boosters from the core. The boosters fall down and separation is complete.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-08-26T12:15:07.783" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="1581" PostTypeId="2" ParentId="1561" CreationDate="2013-08-26T12:48:04.097" Score="3" Body="&lt;p&gt;Why would you want a satellite for that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean: launch a typical carrier rocket with a nuclear missile much smaller than your typical ICBM as the last stage. Once it's in orbit, cut the engine off, and possibly fire it every few months to compensate for orbital decay. If the war comes, deorbit into enemy territory. In case you're about to run out of fuel, deorbit without arming the warhead, into sea area where it would be easy to recover.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2013-11-13T08:51:29.813" LastActivityDate="2013-11-13T08:51:29.813" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1582" PostTypeId="2" ParentId="1570" CreationDate="2013-08-26T13:06:52.673" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Solid-fuel_rocket&quot; rel=&quot;noreferrer&quot;&gt;Once&lt;/a&gt; it is ignited it is impossible to shut down a solid propellent in a rocket because they have all the necessary things ( fire, oxidizer,fuel) to burn .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but in &lt;a href=&quot;http://en.wikipedia.org/wiki/Hybrid_rocket&quot; rel=&quot;noreferrer&quot;&gt;hybrid rocket&lt;/a&gt; is possible to have a controlled burn &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When&#xA;  thrust is desired, a suitable&#xA;  ignition source is introduced in&#xA;  the combustion chamber and&#xA;  the valve is opened. The liquid&#xA;  propellant (or gas) flows into&#xA;  the combustion chamber where&#xA;  it is vaporized and then&#xA;  reacted with the solid&#xA;  propellant. Combustion occurs&#xA;  in a boundary layer diffusion&#xA;  flame adjacent to the surface&#xA;  of the solid propellant.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;so in hybrid rocket burning can be controlled by controlling the valve (or) by controlling the nozzle geometry&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-08-26T13:06:52.673" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1583" PostTypeId="5" CreationDate="2013-08-26T13:41:27.690" Score="0" Body="&lt;p&gt;Landing refers the process of safely touching down a spacecraft on a surface. This includes all parts of the process including entry, descent, and landing.&lt;/p&gt;&#xA;" OwnerUserId="46507" LastEditorUserId="46507" LastEditDate="2023-03-20T23:57:17.887" LastActivityDate="2023-03-20T23:57:17.887" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1584" PostTypeId="4" CreationDate="2013-08-26T13:41:27.690" Score="0" Body="Questions regarding the specific part of flights, during which a spacecraft touches down on the solid ground of a celestial body." OwnerDisplayName="user12" LastEditorUserId="14" LastEditDate="2013-08-26T15:38:17.993" LastActivityDate="2013-08-26T15:38:17.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1585" PostTypeId="5" CreationDate="2013-08-26T13:42:49.540" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-26T13:42:49.540" LastActivityDate="2013-08-26T13:42:49.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1586" PostTypeId="4" CreationDate="2013-08-26T13:42:49.540" Score="0" Body="Questions regarding the effects and conditions in a 'partial vacuum' or space with a gaseous pressure significantly smaller than atmospheric pressure." OwnerDisplayName="user12" LastEditorUserId="14" LastEditDate="2013-08-26T15:38:21.713" LastActivityDate="2013-08-26T15:38:21.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1587" PostTypeId="5" CreationDate="2013-08-26T13:54:05.093" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-26T13:54:05.093" LastActivityDate="2013-08-26T13:54:05.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1588" PostTypeId="4" CreationDate="2013-08-26T13:54:05.093" Score="0" Body="Questions regarding the people or procedures of the center that manages space missions." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-26T14:13:10.907" LastActivityDate="2013-08-26T14:13:10.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1589" PostTypeId="5" CreationDate="2013-08-26T13:56:57.307" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-26T13:56:57.307" LastActivityDate="2013-08-26T13:56:57.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1590" PostTypeId="4" CreationDate="2013-08-26T13:56:57.307" Score="0" Body="Questions regarding words and abbreviations used in the fields of spaceflight and space exploration, and their meaning when used in those contexts." OwnerDisplayName="user12" LastEditorUserId="12102" LastEditDate="2019-12-14T04:08:12.200" LastActivityDate="2019-12-14T04:08:12.200" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1591" PostTypeId="5" CreationDate="2013-08-26T14:03:37.127" Score="0" Body="&lt;p&gt;Questions regarding the intentional process of removing or killing microbial life, including transmissible agents, on space vehicles prior to launch or in flight.&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="14" LastEditDate="2013-08-26T15:28:48.913" LastActivityDate="2013-08-26T15:28:48.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1592" PostTypeId="4" CreationDate="2013-08-26T14:03:37.127" Score="0" Body="Questions regarding the process of removing bacteria and other life forms from space vehicles prior to launch. " OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-26T14:10:22.397" LastActivityDate="2013-08-26T14:10:22.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1593" PostTypeId="1" CreationDate="2013-08-26T14:13:59.627" Score="10" ViewCount="2289" Body="&lt;p&gt;Would &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetorquers&quot; rel=&quot;nofollow&quot;&gt;magnetic torquers&lt;/a&gt; placed in a &lt;strong&gt;polar&lt;/strong&gt; satellite keep the satellite stable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it would work, would it be very &lt;em&gt;efficient&lt;/em&gt;?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="288" LastEditDate="2013-09-20T17:15:04.013" LastActivityDate="2013-09-20T17:15:04.013" Title="Would magnetic torquers placed in a polar satellite work?" Tags="|artificial-satellite|attitude|magnetic-field|magnetorquer|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1594" PostTypeId="1" AcceptedAnswerId="1598" CreationDate="2013-08-26T14:24:28.950" Score="12" ViewCount="1011" Body="&lt;p&gt;I can imagine a satellite with a solar sail it rolls when on the night side of Earth, then unrolls near &quot;afternoon&quot;, propelled on each past of its &quot;sunset side&quot; of travel. Would such system be a viable method of keeping the satellite in orbit indefinitely (or until actuators fail) or would some other factors (e.g. atmospheric drag) make it impossible?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="26" LastEditDate="2013-08-26T15:16:52.933" LastActivityDate="2024-01-11T19:26:13.127" Title="Can solar sail prevent orbital decay?" Tags="|artificial-satellite|solar-sail|station-keeping|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="1595" PostTypeId="2" ParentId="1579" CreationDate="2013-08-26T15:19:51.270" Score="8" Body="&lt;p&gt;Solid Fuel, per se, ought not to leak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is hydraulic fluid for Thrust Vector Systems (TVC) that can leak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hybrids, like Virgin Galactic's Space Ship 2 will use have a solid fuel, but a liquid oxidizer so they can be started and stopped.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be an APU (Aux Power Unit) like system that burns some kind of fuel to provide electrical power needs beyond what a battery could provide.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-08-26T15:19:51.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1596" PostTypeId="5" CreationDate="2013-08-26T15:20:00.243" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-26T15:20:00.243" LastActivityDate="2013-08-26T15:20:00.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1597" PostTypeId="4" CreationDate="2013-08-26T15:20:00.243" Score="0" Body="Operations done to keep a spacecraft in a particular orbit.  Generally done by by thruster burns.  In LEO this must be done to counteract the momentum loss to atmospheric drag.  In other orbits it may be needed to maintain a planned relative position over a long period of time." OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-08-26T15:22:51.157" LastActivityDate="2013-08-26T15:22:51.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1598" PostTypeId="2" ParentId="1594" CreationDate="2013-08-26T15:42:40.013" Score="13" Body="&lt;p&gt;First, there are several forces acting on a sailcraft.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Aerodynamic drag&lt;/li&gt;&#xA;&lt;li&gt;Solar radiation pressure&lt;/li&gt;&#xA;&lt;li&gt;Gravity field non-sphericity&lt;/li&gt;&#xA;&lt;li&gt;Electrodynamic drag&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If you go anywhere lower than 770 km, atmospheric drag is by far the largest force that pulls your apoapsis down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quoting &lt;a href=&quot;https://books.google.com/books?id=jaExBQAAQBAJ&amp;amp;pg=PA250#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow&quot;&gt;Vulpetti (2008)&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When unfurled in LEO, the drag on the sail produced by its flight through&#xA;  this residual atmosphere can be very high; larger in magnitude than the&#xA;  thrust the sail experiences by reflecting sunlight. Simply put, a sail flown in&#xA;  LEO will very quickly lose energy by interacting with the ionosphere&#xA;  (despite the fact that it is getting accelerated by reflected sunlight), and find itself on a reentry trajectory. It is easy to compute that a sail shall operate beyond 700 km (nominally); if one takes the upper-atmosphere changes into account, the previous lower limit increases to &lt;strong&gt;750 to 770&lt;/strong&gt; km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Second, deploying a sail is a risky and not fully understood maneuver. You don't want to stow and deploy it repeatedly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, when there's no solar pressure, you don't need to stow the sail. Provided you operate beyond the 770 km limit, the only thing you have to do is rotate the sailcraft. Now, ensuring necessary rotation rates (attitude control authority) is a problem if you take into account that the whole sailcraft must be very light to rely on solar radiation pressure.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="1142" LastEditDate="2016-02-20T01:56:30.283" LastActivityDate="2016-02-20T01:56:30.283" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1599" PostTypeId="1" CreationDate="2013-08-26T16:14:56.393" Score="19" ViewCount="3664" Body="&lt;p&gt;Specifically with the small satellite and cubesat people, there is an rather 'old' idea around: Passive attitude stabilization based on permanent magnets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/oXgBR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&lt;sub&gt;&lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/m/m3-mcubed#hHXWD12afHerb&quot; rel=&quot;nofollow noreferrer&quot;&gt;Schematic of the Earth magnetic field lines and camera orientation throughout polar orbit (image credit: University of  Michigan)&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concept is rather simple. A magnet, which is mounted on a satellite, will always attempt to align its magnetic field with Earth's magnetic field. This is intriguing, because it does not need computers, power, moving parts or just any form of control. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For me, there are two critical weak spots in this concept. First, if it works, it does stabilize a satellite only on a single axis. Second, Earth's magnetic field is not a clean dipole - with respect to both, its intensity and the orientation of magnetic field lines. But even if you treat it as a simple dipole, the relative rotation of the magnetic field lines along e.g. an polar orbit does not occur in one geometric plane or with a constant angular velocity. For me, intuitively, this whole thing should lead to a rather strange tumbling behaviour of such a satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is it supposed to work? &lt;strong&gt;Are there studies of this concept based on attitude data from satellites actually in space?&lt;/strong&gt; If yes, what was found? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Results from detailed computer simulations are interesting for me, too, but I clearly prefer results based on flight data. There are a fair number of projects, which intend to use magnets or used them (while the satellite failed), so I find it rather hard to find statements, which go beyond 'it will work'.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="16322" LastEditDate="2016-10-17T13:55:18.900" LastActivityDate="2017-10-09T18:40:35.447" Title="Passive attitude stabilization with magnets - are there studies based on actual flight data?" Tags="|artificial-satellite|attitude|cubesat|magnetic-field|passive-systems|" AnswerCount="5" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1600" PostTypeId="2" ParentId="1594" CreationDate="2013-08-26T19:08:44.143" Score="7" Body="&lt;p&gt;Between atmospheric drag out through almost 1000km, there's the issue that solar light and solar wind are both cut by the passing into the shadow of a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this puts more of the energy on the solar side, even for high orbits, the sail is, if passively deployed, making the orbit more ellipsoid and thrusting it planetward on the sunward lobe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume a flat sail perpendicular to wind: as you come sunward, it slows you, lowering your periapsis by both slowing you down and by direct thrust, and increasing your apoapsis slightly, but less, due to the shadow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, we tilt it, for outward tack on the inbound: you still slow, widening and shallowing the inbound arc. On the outbound, it's pulling you towards the planet, narrowing it and accelerating you back slightly. At the chunk closest to the sun, it's accelerating you counter-orbitally; that's slowing your orbit, and lowering your periapsis. At away-from sun, it's getting increased speed except in shadow - so it doesn't equal the amount from them sunward arc. This skews the orbit's long axis, but still lowers periapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tilting the other way drives the approach downward and anti-spinward, lowering periapsis more radically, and the sunward side and half the retreat is acceleration; the away-from sun is slowing but interrupted, slightly increasing apoapsis. It also skews the long axis of the ellipse of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, even if it's tacked properly (inbound arm outward, ourbound arm outward, solar-side arm spinward) it's still consistently pushing towards the planet. It might be able, if the orbit is high enough, to push the apoapsis up and maintain enough acceleration to overcome the outward force of solar energies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a passive system, it's going to eventually push you into atmosphere, and at that point, it is a liability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an active system, it can theoretically be used to increase orbital velocity for the majority of the orbit, and slowly accelerate one outward. This could even — in theory — be used to accelerate to breakaway, provided one starts in an orbit well above 1000km, and can tack the sail fast enough to not be a problem on the sail-turn at the sunward extreme.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://userpages.umbc.edu/~tokay/chapter9new.html&quot;&gt;http://userpages.umbc.edu/~tokay/chapter9new.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-26T19:08:44.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1601" PostTypeId="2" ParentId="1555" CreationDate="2013-08-26T22:17:07.823" Score="9" Body="&lt;p&gt;To add to the existing answers, &lt;a href=&quot;http://www.space.com/2440-russian-submarine-launches-russian-microsatellite.html&quot;&gt;this article&lt;/a&gt; describes a small satellite launched from a Russian submarine on a &lt;a href=&quot;http://en.wikipedia.org/wiki/Shtil%27&quot;&gt;converted&lt;/a&gt; submarine-launched ballistic missile (SLBM). The satellite launched was called &lt;em&gt;Kompass 2&lt;/em&gt;, and it was put into a ~ 400x480 km orbit at around 80 degrees inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia article linked above describes another submarine-based launch in 1998, which placed two payloads into a 400x776 km orbit, again near 80 degrees inclination.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-26T22:27:57.423" LastActivityDate="2013-08-26T22:27:57.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1602" PostTypeId="2" ParentId="1579" CreationDate="2013-08-27T00:57:46.233" Score="15" Body="&lt;p&gt;&quot;Solid&quot; fuels are not all completely solids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some are actually pastes — viscous liquids, often colloidal suspensions — these can (very slowly) ooze. Some are pastes until they are cured; if not properly cured, they can remain liquid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some are particulates with binder chemicals. If the binder ages, liquifies, or otherwise becomes less effective, the particles can fall out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some are themselves solids that can liquify at certain temperatures below ignition temperature; if heating happens, this can result in liquefaction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Potassium Nitrate and Sugar fuels can be liquified by humidity and/or temperature, and is crystalline and thus fragile and frangible. Vibration can render it cracked or even powdered, and it's hygroscopic; in wet enough conditions, it dissolves in water, and can result in sticky flammable messes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zinc-Sulfur propellant and black powder propellant are pressed powders - vibration can cause particles to come loose and &quot;leak&quot;. (corollary: Never store a black powder rocket next to your subwoofers...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ammonium-nitrate &amp;amp; Aluminum or Aluminium Perchlorate &amp;amp; aluminum fuels use a  binder, typically  Hydroxyl-terminated polybutadiene (HTPB); the binder can potentially degrade over time resulting in fuel particle losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydroxyl-terminated polybutadiene (HTPB) itself is used as a solid fuel itself in a hybrid design; it's oxidizer is Nitrous Oxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gunpowder &amp;amp; wax is a rarely used colloidal solid propellant; thin layers of wax are used to bind together the powder and reduce the powder's reaction rate. The wax can melt at not too terribly high a temperature, and a hot summer day can result in such propellant slumping or even running. Parafin, while melting at 122°F or so, will slump at about 80°F, so if experimenting with wax and powder mixtures, use beeswax, and even then, don't leave it in the car on a hot day - beeswax melts around 145°F, and a closed car on a hot summer day can hit 160°F readily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any hybrid rocket can have leaks of the liquid component, while still having the solid.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.waxwild.com/&quot;&gt;http://www.waxwild.com/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-27T00:57:46.233" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1603" PostTypeId="5" CreationDate="2013-08-27T01:50:24.390" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-27T01:50:24.390" LastActivityDate="2013-08-27T01:50:24.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1604" PostTypeId="4" CreationDate="2013-08-27T01:50:24.390" Score="0" Body="Questions regarding devices used to make a controlled descent and touchdown onto another object, usually either to transfer objects, or perform scientific observations." OwnerDisplayName="user12" LastEditorUserId="25" LastEditDate="2013-10-01T18:58:25.280" LastActivityDate="2013-10-01T18:58:25.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1605" PostTypeId="5" CreationDate="2013-08-27T01:53:26.120" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-27T01:53:26.120" LastActivityDate="2013-08-27T01:53:26.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1606" PostTypeId="4" CreationDate="2013-08-27T01:53:26.120" Score="0" Body="Questions regarding the solid rocket boosters used to propel spacecraft, often during launch." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-27T01:54:19.217" LastActivityDate="2013-08-27T01:54:19.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1610" PostTypeId="2" ParentId="1566" CreationDate="2013-08-27T09:46:02.610" Score="1" Body="&lt;p&gt;Your question seems to be very broad. &#xA;Different countries employ different types of Qualification Standards. Here are a few&#xA;NASA Technical Standards Program, Europe - ECSS (European Cooperation for Space Standardisation), JAXA parts and Standards Program etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on your broad Criteria, I would like to quote an example for your understanding.&#xA;If there is a company that is delivering a Ground Segment to say European Space Agency, then the company should follow all the Qualification standards mentioned in the ECSS Ground Systems and Operations document.&lt;/p&gt;&#xA;" OwnerUserId="131" LastActivityDate="2013-08-27T09:46:02.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1611" PostTypeId="2" ParentId="1029" CreationDate="2013-08-27T11:24:04.683" Score="5" Body="&lt;p&gt;Each instrument on Mars, or any other planet, has strengths and weaknesses. Used together, you get a whole picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HiRISE can take visible and near visible images, in very high resolution. There's a lot that can be learned from this that is harder to learn with poorer resolution. Visible images will allow you to see real objects, and in particular, how they change with time. It is easier to relate to these images than those with other images. Furthermore, it is extremely useful in seeing exactly what is on the surface for making landing attempts, for instance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;THEMIS, for instance, is infrared. One of Infrared's strengths is to determine the size of surface particles. It can tell the difference between sand, bedrock, and boulder fields, for instance. One of the things that HiRISE has done to address THEMIS data is to confirm some of the sand/boulder fields that THEMIS has seen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, it isn't that one or the other isn't reliable, it's just that the show very different things. In fact, it is extremely common to use multiple sources to publish papers. The knowledge of the digital elevation map is actually used in the planning process for all images, and at least when I worked at HiRISE, THEMIS images were used to help plan where to shoot HiRISE, as they were the highest global resolution images of the planet. Perusing the internet, I see many references to using THEMIS and HiRISE data together, such as &lt;a href=&quot;http://www.uahirise.org/PSP_010597_1470&quot; rel=&quot;noreferrer&quot;&gt;Possible Chloride Salt Deposits Observed in THEMIS&lt;/a&gt;, &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2761.pdf&quot; rel=&quot;noreferrer&quot;&gt;EXTENDING CRISM SPECTRAL COVERAGE IN GALE CRATER USING THEMIS VIS AND HIRISE.&lt;/a&gt;, and &lt;a href=&quot;http://www.spacegrant.hawaii.edu/reports/18_FA07/NVandeberg_FA07.pdf&quot; rel=&quot;noreferrer&quot;&gt;IDENTIFICATION AND ANALYSIS OF&#xA;ROOTLESS VOLCANIC CONES ON MARS&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-27T11:24:04.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1612" PostTypeId="1" AcceptedAnswerId="1615" CreationDate="2013-08-27T11:38:33.847" Score="10" ViewCount="327" Body="&lt;p&gt;I &lt;em&gt;think&lt;/em&gt; that the relevant naming 'authority' for e.g. &lt;em&gt;craters&lt;/em&gt; on the Moon (and Mars) is the &lt;a href=&quot;http://www.iau.org&quot; rel=&quot;nofollow&quot;&gt;International Astronomical Union&lt;/a&gt;. Besides weak spots in the system, the basic idea is that people or institutions can &lt;em&gt;propose&lt;/em&gt; names for a crater. This is usually done for paying respect and honour to important and/or deceased members of the space exploration and astronomy communities. Notable examples are the &lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovskiy_%28crater%29&quot; rel=&quot;nofollow&quot;&gt;Tsiolkovskiy crater&lt;/a&gt; (after &lt;a href=&quot;http://en.wikipedia.org/wiki/Konstantin_Tsiolkovsky&quot; rel=&quot;nofollow&quot;&gt;Konstantin Tsiolkovsky&lt;/a&gt;) or a group of seven lunar craters named after the crew of &lt;a href=&quot;http://news.google.com/newspapers?id=5v8lAAAAIBAJ&amp;amp;sjid=Q_wFAAAAIBAJ&amp;amp;pg=4112,2485024&quot; rel=&quot;nofollow&quot;&gt;STS-51-L&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have observed that the teams of e.g. the Mars Exploration Rovers and Mars Science Lab also give names to a lot of objects, rocks and mountains etc along their routes. Those names are then used in (scientific) literature, too. &lt;strong&gt;What are the official procedures for naming such objects, if there are any?&lt;/strong&gt; Do they accept proposals (from non-team-members) and/or are there examples of this happening in the past?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;As odd as this question may be, it is relevant with respect to 'exploration'. The first thing humans do, if they discover something new, is giving names.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14" LastEditorUserId="80" LastEditDate="2013-08-29T15:11:16.027" LastActivityDate="2013-08-29T15:11:16.027" Title="What is the naming procedure for rocks on Mars?" Tags="|mars|terminology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1613" PostTypeId="5" CreationDate="2013-08-27T12:51:41.170" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-27T12:51:41.170" LastActivityDate="2013-08-27T12:51:41.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1614" PostTypeId="4" CreationDate="2013-08-27T12:51:41.170" Score="0" Body="Questions regarding equipment or other material to be transported during space exploration." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-27T13:05:20.050" LastActivityDate="2013-08-27T13:05:20.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1615" PostTypeId="2" ParentId="1612" CreationDate="2013-08-27T13:25:10.043" Score="6" Body="&lt;p&gt;According to the &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_rocks_on_Mars&quot;&gt;wikepedia&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Names for Mars rocks are&#xA;  largely unofficial designations&#xA;  used for ease of discussion&#xA;  purposes, as the International&#xA;  Astronomical Union 's official&#xA;  Martian naming system&#xA;  declares that objects smaller&#xA;  than 100 m (330 ft) are not&#xA;  to be given official names.&#xA;  Because of this some less&#xA;  significant rocks seen in&#xA;  photos returned by Mars&#xA;  rovers have been named more&#xA;  than once, and others have&#xA;  even had their names changed&#xA;  later due to conflicts or even&#xA;  matters of opinion. Often&#xA;  rocks are named after the&#xA;  children or family members of&#xA;  astronauts or NASA employees.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;HOW DO THEY APPROVE THE &lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_nomenclature&quot;&gt;NAME&lt;/a&gt; :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When images are first&#xA;  obtained of the surface of a&#xA;  planet or satellite, a theme for&#xA;  naming features is chosen and&#xA;  a few important features are&#xA;  named, usually by members of&#xA;  the appropriate IAU task&#xA;  group (a commonly accepted&#xA;  planet-naming group). Later,&#xA;  as higher resolution images&#xA;  and maps become available,&#xA;  additional features are named&#xA;  at the request of&#xA;  investigators mapping or&#xA;  describing specific surfaces,&#xA;  features, or geologic&#xA;  formations. Anyone may&#xA;  suggest that a specific name&#xA;  be considered by a task group.&#xA;  If the members of the task&#xA;  group agree that the name is&#xA;  appropriate, it can be retained&#xA;  for use when there is a&#xA;  request from a member of the&#xA;  scientific community that a&#xA;  specific feature be named.&#xA;  Names successfully reviewed by&#xA;  a task group are submitted to&#xA;  the IAU Working Group for&#xA;  Planetary System&#xA;  Nomenclature (WGPSN). Upon&#xA;  successful review by the&#xA;  members of the WGPSN, names&#xA;  are considered provisionally&#xA;  approved and can be used on&#xA;  maps and in publications as&#xA;  long as the provisional status is&#xA;  clearly stated. Provisional&#xA;  names are then presented for&#xA;  adoption to the IAU's General&#xA;  Assembly, which met triennially&#xA;  in the past, and which now&#xA;  adopts nomenclature for&#xA;  planetary surface features as&#xA;  required. A name is not&#xA;  considered to be official —&#xA;  that is, &quot;adopted&quot; — until the&#xA;  General Assembly has given its&#xA;  approval.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-08-27T13:25:10.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1616" PostTypeId="5" CreationDate="2013-08-27T13:31:40.447" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-27T13:31:40.447" LastActivityDate="2013-08-27T13:31:40.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1617" PostTypeId="4" CreationDate="2013-08-27T13:31:40.447" Score="0" Body="Questions regarding the Opportunity rover (a.k.a. Mars Exploration Rover (MER) B), which was launched by NASA in 2003 and landed on Mars in 2004." OwnerDisplayName="user12" LastEditorDisplayName="user29" LastEditDate="2013-08-27T13:55:52.557" LastActivityDate="2013-08-27T13:55:52.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1618" PostTypeId="2" ParentId="1358" CreationDate="2013-08-27T14:48:52.510" Score="8" Body="&lt;p&gt;&lt;a href=&quot;http://www.buran.su/buranvssts-comparison.php&quot; rel=&quot;noreferrer&quot;&gt;ONBOARD COMPUTERS&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Buran's computer is cadenced à 4MHz (3 MHz for STS), consisting of 4 independent units and 5 independent units on the shuttles. the dead memory is stored on magnetic tapes, the memory of the Buran's computer is 819 200 words of 32 bits and for shuttle 106 496 words of 16 bits which give buran a better calculation power. Shuttles used FORTAN algorithms whereas for Buran new computer science languages were developed (high and down level). It's more powerful because it uses all the power of the hardware but needs more time to make the language and needs more time for the engineers and technicians to be fully ready to work on it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://science.howstuffworks.com/space-shuttle7.htm&quot; rel=&quot;noreferrer&quot;&gt;SHUTTLES RE-ENTRY&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ftyaY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During re-entry , the orbiter encounters the main air of the atmosphere and is able to fly like an airplane. It is designed as a lifting body design with swept back &quot;delta&quot; wings which can generate lift with a small wing area. flight computers fly the orbiter. The orbiter makes a series of S-shaped, banking turns to slow its descent speed as it begins its final approach to the runway. The commander picks up a radio beacon from the runway (Tactical Air Navigation System) when the orbiter is about 140 miles (225 km) away from the landing site and 150,000 feet (45,700 m) high. At 25 miles (40 km) out, the shuttle's landing computers give up control to the commander. The commander flies the shuttle around an imaginary cylinder (18,000 feet or 5,500 m in diameter) to line the orbiter up with the runway and drop the altitude. During the final approach, the commander steepens the angle of descent to minus 20 degrees &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.buran.ru/htm/algoritm.htm&quot; rel=&quot;noreferrer&quot;&gt;BURAN RE-ENTRY&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The difficulty in creating algorithms for automatic traffic control was the fact that the conditions orbiter should be able to point out any areas of admissible initial conditions of entry into the atmosphere at an altitude of 100 km in a time just over 30 minutes to land at a given airport located at a distance of more than 8000 km, while paying off the flight speed to 28,000 km / h to zero. &lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;part of the descent N = 100 to 20 km;&lt;/li&gt;&#xA;  &lt;li&gt;site of pre maneuvering H = 20 to 4 km &lt;/li&gt;&#xA;  &lt;li&gt;approach segment and landing N = 4 to 0 km.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jSEeQ.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user29" LastEditDate="2013-08-27T15:01:27.603" LastActivityDate="2013-08-27T15:01:27.603" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1619" PostTypeId="1" CreationDate="2013-08-27T15:20:36.983" Score="12" ViewCount="1530" Body="&lt;p&gt;Nuclear batteries are very small in size. Hence they find application in satellites or shuttles as a replacement to solar batteries. They also use large amounts of energy released naturally by tiny bits of radioactive materials.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Do the radioactive rays emitted by these materials cross the battery shell? &lt;/li&gt;&#xA;&lt;li&gt;Do they affect any components of the satellites? &lt;/li&gt;&#xA;&lt;li&gt;What material is the battery covering constructed with to block the radiation?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="579" LastEditorUserId="288" LastEditDate="2013-08-27T16:22:49.943" LastActivityDate="2016-07-11T17:43:56.353" Title="Do nuclear batteries emit radioactive rays while in use by satellites and shuttles?" Tags="|radiation|rtg|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1621" PostTypeId="1" AcceptedAnswerId="1623" CreationDate="2013-08-27T16:30:47.703" Score="38" ViewCount="32836" Body="&lt;p&gt;Pioneer 10, Pioneer 11, and the two Voyagers are all headed out of the solar system. Has anybody extrapolated where they will all end up (if at all)? Is there a graphic that depicts the direction of flight of all four spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorDisplayName="user29" LastEditDate="2013-08-28T00:51:45.573" LastActivityDate="2021-03-24T19:54:25.950" Title="Where are Pioneer 10, 11 and the Voyagers ultimately headed?" Tags="|voyager|probe|interstellar-travel|pioneer|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1623" PostTypeId="2" ParentId="1621" CreationDate="2013-08-27T16:46:29.487" Score="38" Body="&lt;p&gt;Besides &quot;beyond the &lt;a href=&quot;http://en.wikipedia.org/wiki/Heliopause_(astronomy)#Heliopause&quot; rel=&quot;noreferrer&quot;&gt;heliopause&lt;/a&gt;&quot;, they are not ultimately headed towards &lt;em&gt;anything&lt;/em&gt; in particular. The sparsity of the universe and even our galaxy means that they are unlikely to encounter much of anything without being specifically targeted for it (which is out of the question at this point).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://prd.aps.org/abstract/PRD/v65/i8/e082004&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; addresses this question more specifically (for Pioneer 10 and 11, at least). It states that Pioneer 10 is headed in the general direction of the star &lt;em&gt;Aldebaran&lt;/em&gt; (part of the Taurus constellation) (which it will &quot;reach&quot; in about 2 million years), and Pioneer 11 &quot;should pass close to the nearest star in the constellation Aquila in about 4 million years&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as &lt;a href=&quot;http://voyager.jpl.nasa.gov/mission/interstellar.html&quot; rel=&quot;noreferrer&quot;&gt;the Voyagers&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager 1 will drift within 1.6 light-years (9.3 trillion miles) of AC+79 3888, a star in the constellation of Camelopardalis which is heading toward the constellation Ophiuchus.  In about 40,000 years, Voyager 2 will pass 1.7 light-years (9.7 trillion miles) from the star Ross 248 and in about 296,000 years, it will pass 4.3 light-years (25 trillion miles) from Sirius, the brightest star in the sky . The Voyagers are destined—perhaps eternally—to wander the Milky Way.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/voyager/multimedia/pia14112.html&quot; rel=&quot;noreferrer&quot;&gt;This page&lt;/a&gt; gives a nice overview of the relative positions of these probes as of 2011:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager 1 is the most distant spacecraft, about 17.5 billion kilometers (10.9 billion miles) away from the sun at a northward angle. Pioneer 10, the next most distant, is about 15.4 billion kilometers (9.6 billion miles) away from the sun on the opposite side of the solar system. Voyager 2 is about 14.2 billion kilometers (8.8 billion miles) away from the sun on a southward trajectory, on the same side of the solar system as Voyager 1. Pioneer 11 is about 12.4 billion kilometers (7.8 billion miles) away from the sun.&#xA;  &lt;img src=&quot;https://i.stack.imgur.com/MXMPC.jpg&quot; alt=&quot;relative probe positions&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.heavens-above.com/SolarEscape.aspx?lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&quot; rel=&quot;noreferrer&quot;&gt;Heavens Above&lt;/a&gt; has a slightly better graphic, IMO:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dDSta.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-27T17:02:48.893" LastActivityDate="2013-08-27T17:02:48.893" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1624" PostTypeId="2" ParentId="1619" CreationDate="2013-08-27T16:53:40.010" Score="15" Body="&lt;p&gt;If we can limit the discussion to modern RTGs used for exploration probes, there really isn't much in the way of precautions.  There are precautions to prevent launch failure, and then contamination of Earth's environment.  But obviously you can stand next to them as long as the fuel is in solid ceramic form.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/3/3a/RTG_radiation_measurement.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kePEc.jpg&quot; alt=&quot;RTG&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Environmental Health Specialist Jamie Keeley performs an external radiation inspection of the Cassini-Huygens spacecraft's&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Radioisotope Thermoelectric Generators. NASA photo from &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/3/3a/RTG_radiation_measurement.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wiki Commons (click for full resolution photograph)&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not obvious from pictures like this, but that worker is wearing a radiation dosimetry tube filled with polymer gel which measures occupational dose. Interested in the specifics, I found that &lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/safety/chap2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Table 2-3 in this link&lt;/a&gt; gives the isotope concentration for the Cassini RTG - which is what we need to answer your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It contains Pu-236 through Pu-242.  Isotopes are almost completely chemically identical, and because of that it's not practical to separate these, and the impurities remain.  If isotope separation were trivial, we would leave nothing but Pu-238 but it's not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drawing the line between isotopes and activity requires looking them up in your &lt;a href=&quot;http://www.nndc.bnl.gov/chart/&quot; rel=&quot;nofollow noreferrer&quot;&gt;favorite chart of nuclides&lt;/a&gt;. Combining the relative activities and the decay branching ratio, you can observe two things that dominate the non-alpha radiation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pu-238 contributes around $9 \times 10^6 Bq$ of spontaneous fission activity&lt;/li&gt;&#xA;&lt;li&gt;Pu-241 contributes about $7 \times 10^{11} Bq$ of beta activity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is pretty comprehensive.  There's not much other radiation aside from the (intended) alpha radiation and the two above terms.  The main thing this shows is that Cassini's RTG is &lt;em&gt;extremely&lt;/em&gt; clean in terms of radiation.  Sure, you would probably restrict time your engineers spend next to it, but that's not really because it presents a biological danger, it's just so that you can report a lower workplace radiation dose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The beta radiation makes it a little further than the alpha radiation, but not by that much on a multi-ton spacecraft.  It will affect the electronics almost none.  It's pretty much the same for the spontaneous fission.  That could create some biological concern.  Fission produces a host of particles and the neutrons are particularly problematic.  However, their effects will be dwarfed by the overall radiation environment in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the term &lt;strong&gt;nuclear battery&lt;/strong&gt; could refer to other designs, even including fission reactors.  Spacecraft with those &lt;em&gt;do&lt;/em&gt; require shielding to be engineered into their design between the reactor and the scientific payload.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="49" LastEditDate="2013-08-27T22:33:03.300" LastActivityDate="2013-08-27T22:33:03.300" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1625" PostTypeId="1" AcceptedAnswerId="1626" CreationDate="2013-08-27T23:40:57.900" Score="16" ViewCount="2538" Body="&lt;p&gt;There has been a lot of press (&lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-23777804&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;example&lt;/em&gt;&lt;/a&gt;) and commentary about the near drowning in space of  Italian astronaut Luca Parmitano on July 16, 2013.  We have a question &lt;a href=&quot;https://space.stackexchange.com/questions/1410/would-the-blast-clearing-technique-used-in-snorkeling-work-in-an-eva-situation&quot;&gt;Would the blast clearing technique used in snorkeling work in an EVA situation?&lt;/a&gt; on how to clear the problem.  But for me the bigger question is, where did the water or fluid come from? You are in a space suit surrounded by vacuum, there are not a lot of options for sources.  All of the articles I have read give theories on where it did not come from.  Is there any hard data on what the cause of the fluid build up was?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="22326" LastEditDate="2020-03-07T15:25:38.300" LastActivityDate="2020-03-07T15:25:38.300" Title="What was the source of the fluid in Luca Parmitano's suit?" Tags="|spacesuits|water|eva|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1626" PostTypeId="2" ParentId="1625" CreationDate="2013-08-28T00:18:16.777" Score="16" Body="&lt;p&gt;We don't know yet exactly what caused it, but all tests so far ruled out everything else but the PLSS (Portable Life Support Unit). In fact, just today (Aug. 27, 2013) &lt;a href=&quot;https://en.wikipedia.org/wiki/Christopher_Cassidy&quot; rel=&quot;noreferrer&quot;&gt;Chris Cassidy&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Luca_Parmitano&quot; rel=&quot;noreferrer&quot;&gt;Luca Parmitano&lt;/a&gt; managed to recreate the water leak that terminated EVA-23:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://www.nasa.gov/content/robotics-science-and-spacesuit-tests-aboard-station/&quot; rel=&quot;noreferrer&quot;&gt;Robotics, Science and Spacesuit Tests Aboard Station - Aug. 27, 2013&lt;/a&gt;&lt;/strong&gt;:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Flight Engineer Chris Cassidy, along with Parmitano, performed a&#xA;  checkout of the spacesuit that Parmitano wore during a July 16&#xA;  spacewalk that was cut short when its helmet began to fill with water.&#xA;  After assembling and powering up the empty suit as if it were about to&#xA;  go out on another spacewalk, the two astronauts observed water once&#xA;  again leaking into the helmet. With the issue reproduced, NASA now has&#xA;  a baseline configuration for the crew to begin swapping out parts for&#xA;  additional tests to pinpoint the problem.  There are also&#xA;  opportunities to either launch replacement parts on upcoming cargo&#xA;  flights or return parts to Earth for further study once more is known&#xA;  about the cause of the issue.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/h7pKE.png&quot; alt=&quot;Chris Cassidy and Luca Parmitano recreate the water leak that terminated EVA-23&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=_LU60dA7gLg&amp;amp;feature=youtu.be&quot; rel=&quot;noreferrer&quot;&gt;YouTube video&lt;/a&gt; is available of NASA's ISS update on Aug. 27, 2013 including Luca's spacesuit test&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Previously, the NASA’s Anomaly Resolution Team (ART) evaluations on Luca Parmitano suit ruled out other causes and everything pointing towards the PLSS (Portable Life Support Unit). Among the remaining possible root cause are the sublimator, the gas trap, a filter clog and check valve failure, or a water separator failure.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: NASA made the full report on the Mishap Investigation Board investigating US EVA 23 mishap and its root causes available to the public in the &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/Suit_Water_Intrusion_Mishap_Investigation_Report.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Space Station (ISS) EVA Suit Water Intrusion, IRIS Case Number S–2013–199–00005&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In summary, the causes for this mishap evolved from (1) inorganic&#xA;  materials causing blockage of the drum holes in the EMU water&#xA;  separator resulting in water spilling into the vent loop; (2) the NASA&#xA;  team’s lack of knowledge regarding this particular failure mode; and&#xA;  (3) misdiagnosis of this suit failure when it initially occurred on&#xA;  EVA 22.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The source of the inorganic materials blocking the water separator&#xA;  drum holes had not been experienced  during an EVA before and is still&#xA;  undergoing a concurrent investigation. The results of this&#xA;  investigation  will ultimately lead to resolution of this issue;&#xA;  however, since the concurrent investigation into the source  of the&#xA;  debris is expected to continue for many months, the MIB does not yet&#xA;  have the required data to  determine the root causes of the&#xA;  contamination source, which must ultimately be determined to prevent &#xA;  future mishaps.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Related article on Space.com: &lt;a href=&quot;http://www.space.com/24835-spacesuit-water-leak-nasa-investigation.html&quot; rel=&quot;noreferrer&quot;&gt;Spacesuit Leak That Nearly Drowned Astronaut Could Have Been Avoided&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-03T19:39:13.573" LastActivityDate="2014-03-03T19:39:13.573" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1627" PostTypeId="2" ParentId="1486" CreationDate="2013-08-28T00:44:48.157" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://neo.jpl.nasa.gov/faq/#diff&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA defines&lt;/a&gt; meteors as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The light phenomena which results when a meteoroid enters the Earth's atmosphere and vaporizes; a shooting star.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So we'll stick with that (besides, meteoroids are generally too small to track otherwise, and the ones that are big enough to track are referred to as &lt;em&gt;asteroids&lt;/em&gt; anyway). If you &lt;em&gt;are&lt;/em&gt; interested in asteroid tracking, NASA's &lt;a href=&quot;http://neo.jpl.nasa.gov/programs/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Near Earth Object Program&lt;/a&gt; has got you covered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.amsmeteors.org/fireballs/fireball-tracking-system-analysis/&quot; rel=&quot;nofollow noreferrer&quot;&gt;American Meteor Society&lt;/a&gt; maintains a system for reporting &quot;fireballs&quot; and &quot;bolides&quot; - both terms for meteors that are bright enough to be seen from a very wide area. The link has a very good discussion of the details of this system, but a particularly interesting chart showing the number of event reports collected by the AMS is below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WTLqU.png&quot; alt=&quot;AMS trends&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They note that in 2010 the system was upgraded to a &quot;database Google maps&quot; system, which made it much easier for people to file reports (thus the drastic increase). They classify events by the number of reports received, ranging from &quot;&lt;strong&gt;Unconfirmed&lt;/strong&gt;&quot; (reported by only 1 witness) to &quot;&lt;strong&gt;Huge&lt;/strong&gt;&quot; (100+ witnesses). They also have a special class for fireballs large enough to produce a sonic boom (as confirmed by 2 or more witnesses).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space-based sensors owned by the U.S. Government (namely IR sensors) are able to detect and report particularly bright fireball events (the sensors are typically meant to detect space/missile launches). As the AMS article notes, the U.S. recently decided to start sharing data on fireball events. &lt;a href=&quot;http://neo.jpl.nasa.gov/fireballs/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; has set up a website for sharing these reports with the public. According to this, reports generally include &lt;strong&gt;position&lt;/strong&gt; (latitude, longitude, and altitude, presumably at the peak brightness point), &lt;strong&gt;date/time of peak brightness&lt;/strong&gt;, &lt;strong&gt;velocity&lt;/strong&gt;, and the estimated &lt;strong&gt;radiated energy&lt;/strong&gt; (from which NASA can estimate the total impact energy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably, events like this are picked up on ground-based sensors (such as radar) typically used for space surveillance, but this data does not appear to be reported. One main reason for this is that it's probably difficult to even confirm that the detected object was in fact a meteor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as lead time, there is really nothing significant to speak of, since the detection can only happen once the object begins interacting with the atmosphere.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-28T00:44:48.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1628" PostTypeId="1" CreationDate="2013-08-28T00:51:44.213" Score="6" ViewCount="452" Body="&lt;p&gt;They say, &lt;a href=&quot;http://en.wikipedia.org/wiki/Colonization_of_Mercury#Ice_in_polar_craters&quot; rel=&quot;nofollow&quot;&gt;Mercury polar areas have temperatures of around 0C&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What other areas in the Solar System (on the planets or on the moons, on the surface or under) in what seasons have temperatures suitable for human beings?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say, within the range of &lt;strong&gt;-50C to +80C&lt;/strong&gt;, for the duration &lt;strong&gt;no less than a week&lt;/strong&gt;. Let's say the pressure is within the range of 0 to 100MPa (1000 atm).&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-28T12:40:34.270" LastActivityDate="2013-08-28T15:54:26.400" Title="What places in the Solar System have temperatures suitable to humans?" Tags="|colonization|planet|climate|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1629" PostTypeId="5" CreationDate="2013-08-28T02:00:08.440" Score="0" Body="" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-28T02:00:08.440" LastActivityDate="2013-08-28T02:00:08.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1630" PostTypeId="4" CreationDate="2013-08-28T02:00:08.440" Score="0" Body="The use of sensors to detect, identify, and characterize space-borne objects." OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-28T02:00:08.440" LastActivityDate="2013-08-28T02:00:08.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1631" PostTypeId="1" AcceptedAnswerId="1638" CreationDate="2013-08-28T02:47:37.753" Score="18" ViewCount="2096" Body="&lt;p&gt;Do astronauts on the International Space Station have a backup spacesuit with them so that they could use it in case of any malfunction with one of the spacesuits?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, how many spacesuits does each astronaut have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not why they don't have a backup ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-08-28T03:57:03.197" LastActivityDate="2014-02-14T22:08:07.030" Title="Do astronauts on board the ISS have a backup spacesuit?" Tags="|iss|astronauts|spacesuits|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1632" PostTypeId="2" ParentId="1485" CreationDate="2013-08-28T03:39:24.367" Score="20" Body="&lt;p&gt;NASA was the most concerned with the thrust oscillation happening near and on depletion of the first stage fuel, roughly 105-115 seconds into the flight after liftoff and just before first stage separation (see &lt;em&gt;Ares I-X Mission Profile&lt;/em&gt; below), as it was shown by the computer modeling and early design analyses of the Ares 1 rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/SI4e2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This so-called thrust oscillation occurs as solid fuel in the first stage depletes, leaving a long, empty shell that takes on the characteristics of an organ pipe, and resonates at frequencies between 12 and 14 hertz. The second stage of the rocket and the Orion spacecraft atop it would naturally dampen the resulting pressure pulses, which would essentially jackhammer the astronauts and make it nigh impossible for them to read console displays and respond accordingly during that only roughly 5 seconds long, but crucial interval of the ascent phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was not ideal, so NASA engineers looked into many possible ways to mitigate this thrust oscillation at that time (probably matching the time of that &lt;em&gt;&quot;We are working on it ...&quot;&lt;/em&gt; reply that you got). Some of the proposed solutions included tuned mass damper around first stage parachutes (active tuned mass or harmonic absorbers / dampers), reaction control system (a spring-and-damper ring separating the first and second stage of the rocket), active pulse RCS (Reaction Control System, actuators acting like shock absorbers and added to the bell-shaped aft skirt at the bottom of the rocket), and so on:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KRKav.jpg&quot; alt=&quot;Ares thrust oscillation mitigation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Engineers were also looking to use a passive &lt;em&gt;&quot;compliance structure&quot;&lt;/em&gt;, a spring-loaded ring detuning the stack by softening the interface between the first and upper stages, while preserving lateral stability in the Ares 1 design concept. Proposals are described in more detail in this &lt;a href=&quot;http://www.nasaspaceflight.com/2008/04/ares-i-thrust-oscillation-mitigation-options-head-into-trade-study/&quot; rel=&quot;noreferrer&quot;&gt;Ares I Thrust Oscillation mitigation options head into trade study&lt;/a&gt; from April 29, 2008.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/dYFbf.jpg&quot; alt=&quot;enter image description here&quot;&gt; &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/qIT9F.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This concept was expected to reduce the G-forces by vibrations on the astronauts from about 0.7 G’s to 0.25 G’s (on top of 4 G’s from acceleration at that stage of the ascent). Needless to say, all this would add significant costs to designing and constructing Ares 1 first stage, stage engine, first and second interstage, and the crew module (MPCV, Multi-Purpose Crew Vehicle) adapter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/pvAgg.jpg&quot; alt=&quot;The Orion Multi-Purpose Crew Vehicle (MPCV)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The Orion Multi-Purpose Crew Vehicle (MPCV). The MPCV includes both crew and service modules, and a MPCV adaptor.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this was happening around the times of announced budget cuts for the next fiscal years, so they started looking at other, cheaper solutions. And good that they did, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA turned to the Human Factors Division, among them their asst. chief Human Performance, &lt;a href=&quot;http://human-factors.arc.nasa.gov/organization/personnel_view.php?personnel_id=17&quot; rel=&quot;noreferrer&quot;&gt;Brent R. Beutter&lt;/a&gt; and people from the &lt;a href=&quot;http://humansystems.arc.nasa.gov/groups/vibration/&quot; rel=&quot;noreferrer&quot;&gt;NASA Ames Vibration Lab&lt;/a&gt;. They have fast realized the vibrations were limited to a rather narrow frequency range between 12 and 14 Hz. This lead them to the idea of strobing crew displays, tuned to the frequency of launch vehicle's in-flight vibrations as detected by accelerometers attached to the chairs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/esdgL.png&quot; alt=&quot;Testing the oscillating display&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Oscillating display tested with vibrations of up to 0.5 G’s, by Brent R. Beutter, NASA asst. chief Human Performance and Brent&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Rose, Gizmodo stuff. &lt;a href=&quot;https://www.youtube.com/watch?v=dVf65shmWso&quot; rel=&quot;noreferrer&quot;&gt;Video of this demonstration is available on YouTube&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few cheap and cheerful circuits later, the flight displays was perfectly legible even at predicted vibration acceleration up to 0.7 G’s.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each operator was asked to locate the highlighted (magenta) box in a&#xA;  display consisting of a six-by-six matrix of boxes, read and process&#xA;  the contents of the middle row of digits, and then make a&#xA;  two-alternative forced-choice (2AFC) based on the row's content.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/bEeJc.jpg&quot; alt=&quot;enter image description here&quot;&gt; &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/SAwG9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Task error rates in the main condition, shown below, suggest that&#xA;  vibration levels above 0.3 g (0-to-peak) may meaningfully compromise&#xA;  the processing of alphanumeric symbology in the currently anticipated&#xA;  Orion display viewing conditions, particularly if the font size is&#xA;  small.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/3pUhg.jpg&quot; alt=&quot;Task error rates&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Source: &lt;a href=&quot;http://humansystems.arc.nasa.gov/groups/vibration/research.php&quot; rel=&quot;noreferrer&quot;&gt;NASA Ames Human Vibration Laboratory Research&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And this is the story of how NASA solved a multi-hundred-million dollars problem for maybe a few ten thousand bucks.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-28T14:14:32.100" LastActivityDate="2013-08-28T14:14:32.100" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1633" PostTypeId="2" ParentId="1570" CreationDate="2013-08-28T06:51:43.397" Score="17" Body="&lt;h1&gt;Liquid fueled&lt;/h1&gt;&#xA;&lt;p&gt;Most (almost all, but not all) are configured for both shut-off and restart.&lt;/p&gt;&#xA;&lt;p&gt;Some early liquid fueled rockets could not be shut off - they were &amp;quot;gravity fed,&amp;quot; and once under acceleration, could not be shut off, as the acceleration itself drove the fuel into the ongoing combustion.&lt;/p&gt;&#xA;&lt;p&gt;Likewise, some military tactical rockets are non-stoppable liquid fuel rockets.&lt;/p&gt;&#xA;&lt;h1&gt;Solid Fueled&lt;/h1&gt;&#xA;&lt;p&gt;Most solid fuel rockets are not stoppable. This is due to the fuel mass usually being both fuel and oxidizer mixed in a stable but ignitable configuration.&lt;/p&gt;&#xA;&lt;p&gt;A few theoretical designs use a solid oxidizer mass adjacent to a solid fuel mass, and one or the other can be ejected.&lt;/p&gt;&#xA;&lt;p&gt;A few theoretical designs use multiple linear cells, and can have the currently burning cell ejected by a mechanical or pyrotechnic method; in theory later cells could have igniters. I've only seen this used in model rocketry; it's theoretically possible to cap an ejection charge such that it ejects motor 1 without igniting motor 2, and then have, past the cap, a remote ignition for motor 2 that, when it fires, ejects the cap. (I saw someone try to launch just such a rig; I didn't see it fly, due to engine 1 failure.)&lt;/p&gt;&#xA;&lt;h1&gt;Hybrid Motors&lt;/h1&gt;&#xA;&lt;p&gt;In hybrid motors, the fuel is usually solid, and the oxidizer is liquid. The motor is able to be shut off by stopping the oxidizer flow.&lt;/p&gt;&#xA;&lt;p&gt;Some such motors use a hypergolic pair - that is, a pair that ignites on contact. Such motors are almost always able to be restarted.&lt;/p&gt;&#xA;&lt;p&gt;Other motors include a re-ignition system of some kind, so that they can be restarted.&lt;/p&gt;&#xA;&lt;h1&gt;Abort Methods&lt;/h1&gt;&#xA;&lt;p&gt;Solid Rocket Boosters have often been configured such that they can be detached after ignition but before launch; generally, this is similar to a normal separation event. While this does not turn off the engine, it does allow the engine to launch without the rest of the craft, and from a payload perspective, is preferential to losing the payload as well.&lt;/p&gt;&#xA;&lt;p&gt;Most NASA, ESA, and Russian Space Agency space launches also have charges to destroy the launch vehicle; manned launches also have an escape rocket that pulls the manned portion away prior to the rocket's destruction sequence. Again, this is not ideal, and isn't really a safe shutdown, but is used to protect downrange ground items from a failed launch.&lt;/p&gt;&#xA;&lt;p&gt;With liquid fuel and hybrid fuel rockets, the first step of abort is to cease fuel flow; if that fails, then the payload eject and detonate protocols are invoked. With mixed launchers, both SRB detach and then escape/eject and if needed, destruction of the launcher are public listed protocols.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-08-28T06:51:43.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1634" PostTypeId="2" ParentId="1625" CreationDate="2013-08-28T07:14:42.400" Score="4" Body="&lt;p&gt;There are several potential sources:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Drinking Water. Some suits are configured with a tube for drinking water, using a suction demand system - when you suck on the tube, the valve opens, and water under gentle pressure is released. A stuck valve could release drinking water into the helmet.&lt;/li&gt;&#xA;&lt;li&gt;Thermal Control Garment leak. Most TCGs use water in tubes affixed to the garment itself. A damaged or improperly connected TCG coolant loop could release water into the suit.&lt;/li&gt;&#xA;&lt;li&gt;Oral Moisture. It is possible that the astronaut may have had excess moisture in his mouth which escaped. In gravity, unintentional releases are called &quot;drool&quot; while intentional ones are expectoration/spittle.&lt;/li&gt;&#xA;&lt;li&gt;Dislodged condensate. If the astronaut is faced one direction for some time, it's possible for the helmet to be cool enough to condense moisture. That moisture is most likely to bead up and remain on the surface, but a sudden movement could dislodge such moisture. Note that moisture is lost to atmosphere with every breath, and suits need to regulate humidity to prevent condensation. &lt;/li&gt;&#xA;&lt;li&gt;Dislodged sweat. While astronauts should be thermally regulated by the TCG such that they aren't sweating from &lt;em&gt;insolation heating&lt;/em&gt;¹, they may still sweat from exertion or illness. As with condensate, it should just bead up on the individual, but a sharp movement might dislodge some.&lt;/li&gt;&#xA;&lt;li&gt;Micro-cometary penetration. A small fragment of comet might create a hole which freezes closed from vacuum thermal loss condensation, but have a chunk that melts in the helmet itself. This can be ruled out since no penetrations were found, and the astronaut would almost certainly have noticed a pressure loss and the impact.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Until the final report we really won't know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can, however, rule out oral moisture, dislodged sweat, and cometary penetration, since the replication has shown it occurs in an empty suit.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;¹: heating from direct sunlight. See &lt;em&gt;insolation&lt;/em&gt; at &lt;a href=&quot;http://www.thefreedictionary.com/insolation&quot; rel=&quot;nofollow&quot;&gt;the free dictionary&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-08-28T07:14:42.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1635" PostTypeId="1" AcceptedAnswerId="1641" CreationDate="2013-08-28T09:12:38.637" Score="49" ViewCount="40867" Body="&lt;p&gt;There are several (commercial) organisations looking into alternative means to launch space-bound rockets. One commonly proposed method is to use a large airplane as the launch platform. This method should, theoretically, reduce the cost of sending rockets into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Weather balloons can reach an altitude of 20 KM, or more. Could a balloon be used as a launch platform for space-bound rockets? I imagine skipping the first 20km of flight could significantly defray the cost of a launch.&lt;/p&gt;&#xA;" OwnerUserId="628" LastEditorUserId="80" LastEditDate="2013-08-28T14:07:14.797" LastActivityDate="2017-01-25T08:40:41.450" Title="Could it be possible to launch a rocket from a balloon?" Tags="|launch|spacecraft-development|balloons|" AnswerCount="11" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="1636" PostTypeId="1" AcceptedAnswerId="1643" CreationDate="2013-08-28T10:33:27.900" Score="9" ViewCount="660" Body="&lt;p&gt;&lt;a href=&quot;http://www.space.com/21364-moon-gravity-mascons-mystery.html&quot; rel=&quot;nofollow&quot;&gt;http://www.space.com/21364-moon-gravity-mascons-mystery.html&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...These geologic structures, called mascons (short for mass concentrations), are so &#xA;  dense they alter the moon's gravity field, causing perturbations that can tug a &#xA;  spacecraft lower in its orbit around the moon, or push it wildly off course...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Lacking an atmosphere, &lt;em&gt;perhaps&lt;/em&gt; Luna exhibits the acute form of this malaise. If a celestial body - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;lacks an atmosphere,&lt;/li&gt;&#xA;&lt;li&gt;has a superficial/non-existent magnetosphere, and &lt;/li&gt;&#xA;&lt;li&gt;has a near solid core &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;would it follow that it would have lumpy gravity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What celestial bodies are known to be lumpy in gravity?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2016-04-29T20:00:53.317" LastActivityDate="2016-04-29T20:00:53.317" Title="Besides Luna, what celestial bodies exhibit lumpy gravity?" Tags="|the-moon|gravity|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1637" PostTypeId="2" ParentId="1635" CreationDate="2013-08-28T10:44:03.977" Score="6" Body="&lt;p&gt;It has never been attempted, but there have been a few people to consider it. There actually was an extensive section on balloon launches in the Ansari X-Prize competition a decade ago. The most notable rocket to be designed was the &lt;a href=&quot;http://space.xprize.org/ansari-x-prize/the-da-vinci-project&quot;&gt;da Vinci Project&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this could work for suborbital, it is rather unlikely to work well for an orbital flight. The speed is the key factor for an orbital flight, and you get more from a plane. Plus planes are more flexible in general than a balloon. Overall, they are just easier to work with.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-28T10:44:03.977" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1638" PostTypeId="2" ParentId="1631" CreationDate="2013-08-28T13:12:30.057" Score="23" Body="&lt;p&gt;There is space for two full Extravehicular Mobility Units (EMUs), one short EMU, and two Orlan space suits (Russian equivalent to the EMU) aboard the ISS. They are stored in the Joint Airlock. Since the spacesuits are not part of the emergency plan, they do not store one for every astronaut (let alone a backup for each).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.quora.com/International-Space-Station/How-many-EVA-spacesuits-are-there-on-the-ISS&quot;&gt;Answer by Robert Frost, Engineer/Instructor at NASA, on Quora&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note: I wouldn't normally use another Q&amp;amp;A site like Quora as a source, but in this case a verified expert answer is given there by someone whose identity and qualifications are known.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2013-08-28T13:12:30.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1639" PostTypeId="2" ParentId="1635" CreationDate="2013-08-28T14:02:29.450" Score="3" Body="&lt;p&gt;Yes it is possible to launch rockets from balloon as &lt;a href=&quot;http://en.wikipedia.org/wiki/Rockoon&quot; rel=&quot;nofollow&quot;&gt;Reckon&lt;/a&gt;. But there are many disadvantages in launching from an balloon &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;balloons are very hard to steer hence precision in launch is almost impossible&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;the balloons required to take the rocket to high altitudes must have a very large surface&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;{&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;http://science.howstuffworks.com/science-vs-myth/everyday-myths/question185.htm&quot; rel=&quot;nofollow&quot;&gt;reference&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Helium has a lifting force of 1&#xA;  gram per liter.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So you will require very large balloon to lift the rocket (You need 1000 liters of helium to lift just 1 KG of load)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;}&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;it is almost impossible to launch a rocket over a balloon (launching straight out of earth)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;For orbiter its all about the velocity so the more the altitude you go the more acceleration you need to attain the required orbital velocity &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;launching from earth provides initial velocity but in air its simply not&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But still launching from a balloon (If possible) can save your cost on fuel (but not for orbiter) . &#xA;&lt;strong&gt;launch a rocket with a payload is impossible&lt;/strong&gt; (since there are many technical difficulty )&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="629" LastEditDate="2013-08-28T15:20:21.833" LastActivityDate="2013-08-28T15:20:21.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1640" PostTypeId="2" ParentId="1619" CreationDate="2013-08-28T14:20:39.643" Score="2" Body="&lt;p&gt;In most &lt;a href=&quot;http://en.wikipedia.org/wiki/RTG&quot; rel=&quot;nofollow&quot;&gt;RTG&lt;/a&gt; plutonium dioxide fuel is used in it’s heat-resistant, ceramic form which reduces its chance of vaporizing in fire. This ceramic-form fuel is also highly insoluble and has a low chemical reactivity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel is divided among many  small, independent modular units, each with its own heat shield and impact shell. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually most RTG's have multiple layers of protective materials consisting of iridium capsules and high-strength graphite blocks. Iridium is a metal that has a very high melting point and is strong, corrosion-resistant and chemically compatible with plutonium dioxide. These characteristics make iridium useful for protecting and containing each fuel pellet.&#xA;Graphite is used because it is lightweight and highly heat-resistant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the radiation is absorbed by these material but still some radioactive rays may escape (very very small amounts).&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="14549" LastEditDate="2016-07-11T12:08:57.633" LastActivityDate="2016-07-11T12:08:57.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1641" PostTypeId="2" ParentId="1635" CreationDate="2013-08-28T14:51:45.840" Score="19" Body="&lt;p&gt;In order to stay within the scope of this question, I will reference one idea that I believe fits the criteria, although that might be disputable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll call the idea &lt;a href=&quot;http://www.hindawi.com/journals/ijae/2012/830536/&quot; rel=&quot;noreferrer&quot;&gt;balloon-tether LAS&lt;/a&gt;, and it was published in a journal paper in 2012.  The reason this idea is notable is that it started from a study of previously proposed LAS (Launch Assistance Systems), and formalized the requirements for a realistic system.  Because of this, I would say it's one of the &quot;most possible&quot; proposals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Description&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;High altitude balloons would suspend large pulleys that are basically pulled by trains.  The system would be at a remote location and high altitude.  The value of the LAS itself is that it:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Increases altitude of the rocket&lt;/li&gt;&#xA;&lt;li&gt;Gives the rocket some initial vertical speed (order of 1 km/s)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;From the release point on, the rocket fires and attains orbit for a payload of around 7 kg.  This all sounds a little trite.  After all, it only accelerates the rocket to a fraction of orbital velocity, to an altitude only a fraction of LEO altitude, the payload is paltry, and the launch rate is only once per day.  But this is rocket science, by the rocket equation, these reductions make a bigger difference than you would think.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a picture, with the pink being the balloons, the blue is the rocket, and brown is the tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QWM7V.jpg&quot; alt=&quot;schematic&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Feasibility&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly you &lt;em&gt;can&lt;/em&gt; launch something to orbit from balloons, but if there's no economic case for doing it, it won't ever happen.  The balloon-tether LAS shows mastery of a couple of the issues that will come with the territory.  Mainly, there is a problem that balloons are very limited in their lifting capacity.  For more lift you need a larger balloon, and you quickly start to push the limit of what's possible.  That puts a lot of downward pressure on the payload sizes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of that size constraint, it's unlikely that any balloon system could compete with heavy lifting capacity or for manned flights.  Even for micro-satellites, you can't justify the production chain cost because the launch frequency demand isn't high enough.  That's why the balloon-tether LAS proposes a propellant depot model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are still some dubious parts to this proposal.  There are a couple of fields where exploratory engineering has been proposed using high-altitude tethered balloons.  Notably, solar energy, wind energy, and communication balloons.  There have been some historical precedents for tethered balloons &lt;a href=&quot;http://en.wikipedia.org/wiki/Radio_y_Televisi%C3%B3n_Mart%C3%AD&quot; rel=&quot;noreferrer&quot;&gt;flying at around 3 km&lt;/a&gt;.  Military technology bumps up on &lt;a href=&quot;http://en.wikipedia.org/wiki/Tethered_Aerostat_Radar_System&quot; rel=&quot;noreferrer&quot;&gt;7 km or so&lt;/a&gt;.  To get to the desirable weatherless regions you'll have to go much further, and we're also talking about using really large balloons.  There's still the option of not tethering the balloon, and just flying it up and launching a rocket.  But where's the re-usability in that?  That makes it a difficult equation to make a competitive launch system, although, that depends on the technology status for high altitude tethered systems.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="16942" LastEditDate="2016-09-13T20:45:37.247" LastActivityDate="2016-09-13T20:45:37.247" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1642" PostTypeId="2" ParentId="800" CreationDate="2013-08-28T15:13:53.403" Score="5" Body="&lt;p&gt;&quot;RKK Energia&quot; has a plan to take the new ISS modules and use them as the base for the new station. Those modules will be launched no earlier than 2014 and will still have enough of resource in 2020.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Nauka_%28ISS_module%29&quot; rel=&quot;nofollow&quot;&gt;MLM Nauka&lt;/a&gt; scheduled for the launch in April 2014&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Nodal_Module&quot; rel=&quot;nofollow&quot;&gt;UM&lt;/a&gt; the Nodal Module&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Science_Power_Platform&quot; rel=&quot;nofollow&quot;&gt;NEM&lt;/a&gt; the Science and Energy Module&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One of the design goals is an unlimited life time of the station (given the timely exchange of the old modules for the newer ones).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Among the proposed tasks of the station are a &lt;em&gt;fueling station&lt;/em&gt; and a &lt;em&gt;construction dock&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.militarynews.ru/excl.asp?ex=193&quot; rel=&quot;nofollow&quot;&gt;the interview with the President of RKK Energia&lt;/a&gt; on MAKS-2013. &lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-08-28T15:29:02.467" LastActivityDate="2013-08-28T15:29:02.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1643" PostTypeId="2" ParentId="1636" CreationDate="2013-08-28T15:14:28.393" Score="9" Body="&lt;p&gt;What you are looking for is a formal way to measure the &quot;lumpiness&quot; -- this is the spherical harmonic model of a body's gravity field.  These models are pretty well known for the earth and moon as they are critical for trajectory analysis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This paper gives some nice plots that visualize and compare the harmonic models of various planets: &lt;a href=&quot;http://www.ipgp.fr/~wieczor/MyPapers/WieczorekTOGSubmit.pdf&quot;&gt;http://www.ipgp.fr/~wieczor/MyPapers/WieczorekTOGSubmit.pdf&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The math can be daunting, so sometimes this is simplified by describing a body's geoid -- the shape that the surface of the oceans would take under the influence of Earth's gravity and rotation alone.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-08-28T15:14:28.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1644" PostTypeId="2" ParentId="1635" CreationDate="2013-08-28T15:19:38.113" Score="4" Body="&lt;p&gt;Let me give you a little of calculations:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;20 km is only a 5 percents of IIS's &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Perigee 412 km&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main resistance is not air, the main resistance is gravitation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even on IIS gravity exerts tremendous force, orbital period is 92.87 minutes across the Earth. By 1.5 hours!!! The same force attract IIS to the Earth.&#xA;This two forces, to down and to forward, are equal. Let propose, i'm going from one of the Earth's point, and after 1.5 hours, i'm crossing the Earth and coming back. It is a mad speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To fly rocket must go by diagonal to get orbit, rockets lurches sideways after start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By 20km may be saved only friction of air, because it is Troposphere (&lt;a href=&quot;https://en.wikipedia.org/wiki/Troposphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;80% of the atmosphere's mass&lt;/a&gt;), but the speed of the rocket not so big to feel the air friction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm risky to propose that air friction vs gravity force correlate like 1 vs 100 in this case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;20km(5 percents), cheaply overcome by fuel than keep spaceport on the dirigible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the other side, if there will be discovered newest fuel or method to get speed, this spaceport would appear to save from air friction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today - it is unnecessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See: &lt;a href=&quot;http://en.wikipedia.org/wiki/Coilgun&quot; rel=&quot;nofollow noreferrer&quot;&gt;Coilgun&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_gun&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space gun&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And look at this picture:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/qv6bi.jpg&quot; alt=&quot;barier&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="601" LastEditorUserId="601" LastEditDate="2013-08-29T11:38:59.577" LastActivityDate="2013-08-29T11:38:59.577" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1645" PostTypeId="2" ParentId="1628" CreationDate="2013-08-28T15:43:14.277" Score="4" Body="&lt;p&gt;I think the best answer here doesn't even rely on specifics of planets and other bodies.  The basic radiative physics of the universe do the job.  So let's look at the different ways we could produce room-temperature elsewhere in the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Criteria 1: Rotating blackbody in sun orbit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If something (like an asteroid) is rotating fast enough, then the temperature is pretty well evenly distributed over its surface.  So if you could burrow &lt;em&gt;just&lt;/em&gt; under the surface (strange hypothetical, I know), then the temperature you see there will be dictated by the equation for a blackbody's temperature in orbit around the sun.  My version of this equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ T_s = \left( \left( \frac{R_S}{4 R} \right)^2 T_S^4 \right)^{1/4} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here capital S is the sun and lowercase s is for the surface values.  R is the distance from the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can walk you through the logic behind this in more detail if needed.  But basically, you assume the temperature of space is zero, which covers the majority of the sphere around the body.  Then the fraction of area the sun covers is the visual area of the sun divided by the area of the sphere on which the sun lies, which is where the factor of 4 comes from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The value of this equation is about -78 degrees Celsius for Earth's location.  Now, if I set this equation to be equal to room temperature I can get a distance from the sun.  I find that at about 0.44 AU from the sun the surface temperature will be equal to room temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means that it'll be difficult to be cool enough to be livable closer than about 1/2 AU from the sun.  That can only happen with constant shielding.  I don't believe that any bodies exist which are truly tidally locked with our sun, so that limits it to polar caps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Criteria 2: Internal heat production&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the question, any place even &lt;em&gt;inside&lt;/em&gt; a body is sufficient.  Internal heat production keeps the insides of large bodies toasty even if its far from the sun.  You could derive the equation for the internal temperature, but I'm lazy, so I'm copying from &lt;a href=&quot;http://books.google.com/books?id=-nCHlVuJ4FoC&amp;amp;printsec=frontcover&quot; rel=&quot;nofollow&quot;&gt;a book&lt;/a&gt;.  Here, $a$ is the radius of the body (which we assume is spherical), and this is the equation for the temperature inside it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ T(r) = T_0 + \frac{\rho H }{6 k } \left( a^2 - r^2 \right) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ T(0) \approx \alpha a^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I assume any other body is made of the same stuff Earth is, then it has the same thermal conductivity and heat production term.  That means I can just use values for Earth to find that coefficient and not worry about the specifics.  See, I am a &lt;em&gt;very&lt;/em&gt; lazy physicist.  Note that the temperature of the center of the Earth is the same as the temperature of the surface of the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \alpha = \frac{ 5700 K }{ R_E^2 }$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If something is far from the sun, we can use this to find the minimum size needed to have a livable center temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ R = \left( \frac{293}{5700} \right)^{1/2} R_E = 1,500 km$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So any moon or asteroid with a radius larger than this has some room temperature area inside it.  The pressure would be too high though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How many are there&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to that, there are 14 objects that meet criteria 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Criteria 1 includes basically all asteroids around Venus orbit.  That could be a million, because there are lots of small asteroids.  I hope you weren't expecting a list!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you combine both criteria it will be many more.  The pressure requirement given combined with temperature is simply &lt;em&gt;way&lt;/em&gt; to complicated to address as a blanket query.  You can ask specifically for each body.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-08-28T15:54:26.400" LastActivityDate="2013-08-28T15:54:26.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1646" PostTypeId="2" ParentId="1567" CreationDate="2013-08-28T16:07:18.407" Score="11" Body="&lt;p&gt;The first message in the history of humanity was: &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Morse_Message_%281962%29&quot; rel=&quot;noreferrer&quot;&gt;&quot;Мир, Ленин, СССР&quot;&lt;/a&gt;. The English translation being, &quot;World, &lt;a href=&quot;https://en.wikipedia.org/wiki/Lenin&quot; rel=&quot;noreferrer&quot;&gt;Lenin&lt;/a&gt;, USSR&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of people will try to bring up that &quot;мир&quot; actually has two meanings, the second one being &quot;peace&quot;. My opinion is, that in this case, it only means &quot;world&quot;, or in stricto sensu &quot;Universe&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was long ago, in 1962, when this radio message was sent in Morse code towards Venus. Unfortunately, no extraterrestrials were found there.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This message is the first radio broadcast for extraterrestrial&#xA;  civilizations in the history of mankind. The signal reflected from&#xA;  surface of Venus and was received 4 minutes 32.7 seconds (Nov 19) and&#xA;  4 minutes 44.7 seconds (Nov 24) later.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xeYN1.jpg&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;" OwnerUserId="601" LastEditorUserId="49" LastEditDate="2013-12-27T15:41:38.917" LastActivityDate="2013-12-27T15:41:38.917" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1647" PostTypeId="2" ParentId="1635" CreationDate="2013-08-28T16:49:17.353" Score="11" Body="&lt;p&gt;You probably could, but it wouldn't help very much.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The reason it's hard to get to orbit isn't that space is high up.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;It's hard to get to orbit because you have to go so fast.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://what-if.xkcd.com/58/&quot;&gt;XKCD What If? #58&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="390" LastActivityDate="2013-08-28T16:49:17.353" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1648" PostTypeId="1" AcceptedAnswerId="1655" CreationDate="2013-08-28T17:38:10.373" Score="5" ViewCount="330" Body="&lt;p&gt;Recently, Air Force Space Command indicated that the space fence will be discontinued on October 1.[&lt;a href=&quot;http://www.af.mil/News/ArticleDisplay/tabid/223/Article/466832/air-force-space-command-to-discontinue-space-surveillance-system.aspx&quot; rel=&quot;nofollow&quot;&gt;source&lt;/a&gt;]&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Due to resource constraints caused by sequestration, Air Force Space Command officials have directed the 21st Space Wing to prepare to discontinue operations at the Air Force Space Surveillance System by Oct. 1. Final decisions on all fiscal 2014 budget issues will be made over the next few weeks.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How will this decision affect contractors who are involved in the development and maintenance of the space fence?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've read a lot of public information about contracts possibly getting canceled, but also about new systems possibly replacing the old ones. Has the Air Force released any plans for what new systems might be and how they will be divided between contractors?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More info on the space fence:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The AFSSS, which has been operational since 1961, is just one part of AFSPC’s global Space Surveillance Network. The system is designed to transmit a “fence” of radar energy vertically into space to detect all objects intersecting that fence. The operational advantage of the AFSSS is its ability to detect objects in an un-cued fashion, rather than tracking objects based on previous information.  The disadvantage is the inherent inaccuracy of the data, based on its dated design. The new operating modes at Cavalier AFS and Eglin AFB will provide more accuracy than the AFSSS and still collect un-cued observations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="49" LastEditorDisplayName="user29" LastEditDate="2015-07-20T18:37:45.177" LastActivityDate="2015-07-20T18:37:45.177" Title="How will the Air Force decision to discontinue the space fence due to sequestration affect contractors?" Tags="|cost|military|space-surveillance|canceled-project|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1649" PostTypeId="1" AcceptedAnswerId="1650" CreationDate="2013-08-28T17:49:38.590" Score="45" ViewCount="82318" Body="&lt;p&gt;While a rocket is waiting on the platform during the final countdown, it often seems to have clouds of steam or something similar escaping from it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this normal, and what are these clouds?&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="897" LastEditDate="2016-11-29T18:14:35.833" LastActivityDate="2016-11-29T18:14:35.833" Title="What is the vapour/smoke that comes from a rocket before launch?" Tags="|launch|rockets|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1650" PostTypeId="2" ParentId="1649" CreationDate="2013-08-28T18:07:10.773" Score="41" Body="&lt;p&gt;This occurs when cryogenic boosters are used (Liquid Oxygen / Liquid Hydrogen) - as these warm up on the launch pad, some of the liquids boil and to release pressure, bleeder valves allow this gas to escape. If it remained within the boosters, there is a risk of the pressure blowing seals or damaging other components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hH9h9.jpg&quot; alt=&quot;enter image description here&quot;&gt; (from &lt;a href=&quot;http://www.nasaspaceflight.com/2013/02/hot-fire-success-orbitals-antares/&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasaspaceflight.com/2013/02/hot-fire-success-orbitals-antares/&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of what you can see is water vapour as it condenses around the O2 and H2, which are still extremely cold despite being in gaseous form.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2013-08-28T18:19:08.053" LastActivityDate="2013-08-28T18:19:08.053" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1651" PostTypeId="2" ParentId="1649" CreationDate="2013-08-28T18:10:14.547" Score="18" Body="&lt;p&gt;Typical spacecraft fuels/oxidizers are super-cooled. The vapour visible is formed due to the extreme temperature difference. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://history.nasa.gov/ap11fj/01launch.htm&quot;&gt;Apollo 11 flight journal&lt;/a&gt; has this to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Filling these tanks with such cold contents requires a little finesse. Initially, the &#xA;  LOX is fed at a slow rate which furiously boils as it contacts the relatively warm tank structure. The vapourisation of the LOX takes heat away until a pool of liquid begins &#xA;   to form. When enough liquid has collected, filling steps up to a fast rate until the &#xA;   tanks are nearly full and the slow rate is reestablished to top them off. From then on, until three minutes before launch, the level is replenished as the volatile LOX &#xA;  continues to boil off from heat leaking into the tank.]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-08-28T18:10:14.547" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1652" PostTypeId="1" AcceptedAnswerId="1653" CreationDate="2013-08-28T18:21:45.910" Score="28" ViewCount="1597" Body="&lt;p&gt;In today's launch of the Delta IV heavy, it was stated that the starboard booster rocket would be ignited first, followed by the port and centre boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would have thought this would give some extra push which would tip the top of the rocket to port - my expectation was that they would ignite all three simultaneously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do they do it this way?&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="58" LastEditDate="2016-09-29T14:03:35.703" LastActivityDate="2020-03-26T17:46:18.780" Title="For a 3 booster rocket, why do they start with igniting the starboard booster, followed by the center and port?" Tags="|launch|rockets|booster|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1653" PostTypeId="2" ParentId="1652" CreationDate="2013-08-28T18:46:07.583" Score="27" Body="&lt;p&gt;The starboard booster was ignited two seconds earlier as an attempt to diminish the fireball effect noticed in earlier launches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;William Graham of NASASpaceflight.com says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The effect – caused by the rocket’s exhaust igniting hydrogen vented near the rocket – has scorched or even set fire to the first stage insulation on previous flights. While it has been observed on launches from both Vandenberg and Cape Canaveral, the effect is more severe on west-coast launches.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to Stewart Money of Innerspace.net:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Under the new sequence, the starboard engines fires first, creating a draft which directs the gases from center and port engines down towards the launch platform.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2013/08/ula-delta-iv-h-launch-nrol-65/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ULA Delta IV-H launches with NROL-65 - NASASpaceflight.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://innerspace.net/2013/08/28/delta-heavy-ariane-v-prepare-for-launch/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta Heavy, Ariane V Prepare for Launch - Innerspace.net&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Additional Resources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=jm2cbiN1wTs&quot; rel=&quot;nofollow noreferrer&quot;&gt;Slow Motion Video of Booster Ignition&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.space.com/22567-largest-rocket-delta-4-heavy-launch.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;America's Largest Rocket Set For Launch Today - Space.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.doobybrain.com/2008/05/18/space-shuttle-engine-ignition-in-slow-motion/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space shuttle engine ignition in slow motion [article] - Doobybrain.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2020-03-26T17:46:18.780" LastActivityDate="2020-03-26T17:46:18.780" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1654" PostTypeId="2" ParentId="1635" CreationDate="2013-08-28T18:59:36.543" Score="22" Body="&lt;p&gt;There are good reasons why balloons have not been used for launch systems.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Fragility of the balloons&lt;/li&gt;&#xA;&lt;li&gt;The highly energetic nature of rocket launches&lt;/li&gt;&#xA;&lt;li&gt;Limited control over balloon trajectory&lt;/li&gt;&#xA;&lt;li&gt;Expense of Helium&lt;/li&gt;&#xA;&lt;li&gt;Flammability of Hydrogen.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Balloons are inherently fragile. You have to have very thin, very light materials to make an effective high altitude balloon system. Baumgartener launched in a ballon that was 550 feet (168 metre) tall at launch, with 30 million cubic feet (850,000 m³) of helium at STP, to carry about 3150 pounds (1430 kg) of payload. The balloon itself could be easily punctured by a person intent on pushing a finger through it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, the Falcon 9 launch mass is about 735,000 pounds (333,000 kg) - about 233 times the weight, before accounting for it's up to 14,000 pound (6350 kg) payload capacity. Plus, since the balloons are so fragile, one would need to use at least three and a gondola system that keeps them well separated, and so one is looking at about 7.2 billion (7.2e9) cubic feet (204 million m³) of lift helium to save on about 10% of launch fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Helium isn't cheap. At 84 dollars per 1000 cubic feet (\$ 2.97/m³), that's \$6.048e8 (just over half a billion dollars) just in helium. The cost savings is not present for large launchers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrogen, a better lift gas, can be manufactured, but is still going to be about 4 billion cubic feet (113 million m³). But, if it catches fire, it will be a major flame issue. This will then drop any gondola and structure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that a rocket launch produces up to a half-kilometer long plume of highly energetic gasses. Even tho' combustion has ended, those gasses may still be hot enough to damage the fragile balloon envelope. If that envelope is ignited, the balloon suffers a sudden (and probably catastrophic) loss of lift; if it's filled with hydrogen, it is almost guaranteed to suffer a catastrophic loss of lift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Balloon light enough to launch a significant payload will experience a sudden and massive lift as the rocket clears it. Provided that the envelope isn't compromised, loss of 95% of the mass will result in sudden and rapid ascent; not as fast as the rocket, but fast enough that recovery will be an issue. The lack of trajectory control also means having to carefully monitor airflow aloft. In order to recover, either the balloon must be able to compress the envelope, vent the envelope, or detach from the envelope; any of these options adds mass, and two of them render the lift gas a loss. Given the low-thicknesses needed to get efficient lift, compression is unlikely. Therefore most of the lift gas will be unrecoverable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the long run, it's simply too expensive and risky to use balloons to overcome the initial liftoff.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.redbullstratos.com/technology/high-altitude-balloon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.redbullstratos.com/technology/high-altitude-balloon/&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Falcon_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Falcon_9&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.weldingandgasestoday.org/index.php/2012/04/a-look-at-rising-helium-prices/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.weldingandgasestoday.org/index.php/2012/04/a-look-at-rising-helium-prices/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="6816" LastEditDate="2017-01-25T08:40:41.450" LastActivityDate="2017-01-25T08:40:41.450" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1655" PostTypeId="2" ParentId="1648" CreationDate="2013-08-28T21:47:26.513" Score="4" Body="&lt;p&gt;As your source states, the Air Force Space Surveillance System (AFSSS, or &quot;the Fence&quot;, formerly NAVSPASUR) will be shut down come October 1 (really September). This means that the Air Force will not renew the contract for operating the AFSSS, which belongs to Five Rivers Services. So, obviously, those employees will be adversely affected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the planned follow on (confusingly called Space Fence) is an entirely new and separate system. Lockheed Martin and Raytheon have been competing for this new contract, but recently the Air Force &lt;a href=&quot;http://www.spacenews.com/article/military-space/36919with-current-system-slated-for-closure-air-force-defers-next-gen-space&quot; rel=&quot;nofollow&quot;&gt;deferred this decision&lt;/a&gt; following a Pentagon review of all major acquisitions, instead saying they plan to issue a new request for proposals. This would delay initial operating capability of the new system to at least 2018, which is one reason the (old) fence shutdown is worrying a lot of people in the industry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the AFSSS shutdown is ostensibly due to Sequestration (budget problems), it is widely believed that it's actually to provide leverage when it comes to getting the new Space Fence funded (a belief that is only made stronger by the fact that Gen. Shelton, commander of Air Force Space Command, &lt;a href=&quot;http://www.spacenews.com/article/military-space/36974sn-blog-gen-shelton-on-space-fence-closure-and-the-road-ahead&quot; rel=&quot;nofollow&quot;&gt;can't string two sentences together about the shutdown without mentioning the Space Fence&lt;/a&gt;). In this sense, Lockheed Martin and Raytheon are in an arguably better position, since this gaping hole in space surveillance capability highlights the need for the new system.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-28T22:03:43.540" LastActivityDate="2013-08-28T22:03:43.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1656" PostTypeId="1" AcceptedAnswerId="1658" CreationDate="2013-08-28T23:11:25.697" Score="2" ViewCount="765" Body="&lt;p&gt;&lt;a href=&quot;http://www.bbc.co.uk/news/world-asia-23870765&quot; rel=&quot;nofollow&quot;&gt;China is launching a rover&lt;/a&gt; to the moon this year, with more launches and returning samples coming soon. They also plan to have humans on the moon within the next decade.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon offers many benefits for exploration beyond Earth - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Abundant resources &lt;/li&gt;&#xA;&lt;li&gt;A stable launch platform with a much lower gravity well, and &lt;/li&gt;&#xA;&lt;li&gt;Easy access to shielding from the hazards of open space (i.e. radiation &amp;amp; impacts)  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Identifying the best locations to &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Harvest resources &lt;/li&gt;&#xA;&lt;li&gt;Set up factories, and &lt;/li&gt;&#xA;&lt;li&gt;Create habitats &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;would all seem to be basic logistical, political and economic necessities for exploration of the Moon and the Solar system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does China have the most aggressive moon exploration plan as of 2013?   &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="288" LastEditDate="2013-08-29T14:20:30.143" LastActivityDate="2019-01-11T19:57:19.767" Title="Does China have the most aggressive moon exploration plan as of 2013?" Tags="|the-moon|lunar-landing|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="1657" PostTypeId="2" ParentId="1635" CreationDate="2013-08-29T00:12:25.123" Score="12" Body="&lt;p&gt;Yes.  However the largest high-altitude balloons in operation can only lift 8,000 pounds (3600 kg).  Plot from &lt;a href=&quot;http://www.csbf.nasa.gov/balloons.html&quot; rel=&quot;noreferrer&quot;&gt;NASA's Columbia Scientific Balloon Facility&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wz8uR.png&quot; alt=&quot;plots of scientific balloon capability&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it would be a pretty small rocket.  For comparison, the airplane-launched Pegasus XL weighs about 50,000 pounds (23000 kg).&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="33" LastEditDate="2017-01-11T16:24:31.260" LastActivityDate="2017-01-11T16:24:31.260" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1658" PostTypeId="2" ParentId="1656" CreationDate="2013-08-29T00:51:15.233" Score="4" Body="&lt;p&gt;Well, American &lt;a href=&quot;http://ru.wikipedia.org/wiki/Lunar_Atmosphere_and_Dust_Environment_Explorer&quot; rel=&quot;nofollow&quot;&gt;LADEE is scheduled for the next week&lt;/a&gt;, but that doesn't count -- it doen't land, right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current Roscosmos plans are with &lt;a href=&quot;http://en.wikipedia.org/wiki/Luna-Glob&quot; rel=&quot;nofollow&quot;&gt;Luna-Glob&lt;/a&gt; scheduled for &lt;a href=&quot;http://www.ato.ru/content/lunnye-plany-npo-imeni-s-lavochkina&quot; rel=&quot;nofollow&quot;&gt;the second half of 2016&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Indian &lt;a href=&quot;http://en.wikipedia.org/wiki/Chandrayaan-2&quot; rel=&quot;nofollow&quot;&gt;Chandrayaan-2&lt;/a&gt; will not be ready until at least 2015.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Japanese &lt;a href=&quot;http://en.wikipedia.org/wiki/SELENE-2&quot; rel=&quot;nofollow&quot;&gt;SELENE-2&lt;/a&gt; is not to be expected earlier than 2017.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves Chang'e as the current winner candidate for the first soft landing  on the Moon in 37 years. :-)&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-08-29T00:51:15.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1659" PostTypeId="1" CreationDate="2013-08-29T00:58:06.073" Score="10" ViewCount="19475" Body="&lt;p&gt;What is the success rate of NASA launches since it was founded in 1958? I'd appreciate a decade by decade statistic. By successful I mean a successful launch into space and release of payload (if any). It's irrelevant if the payload itself fails later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; IOW, the Challenger disaster should be considered a failure while Columbia, &lt;em&gt;within this scope&lt;/em&gt;, should be considered a success as disaster struck only during re-entry. I'm only interested in official NASA launches into space. Sounding Rockets, ICBMs, etc. can be excused from this question.&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="80" LastEditDate="2013-08-29T16:56:32.520" LastActivityDate="2017-02-13T13:41:59.800" Title="What is the success rate of NASA launches?" Tags="|launch|nasa|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1661" PostTypeId="2" ParentId="1659" CreationDate="2013-08-29T13:42:22.553" Score="10" Body="&lt;p&gt;According to this &lt;a href=&quot;http://www.spaceref.com/news/viewpr.html?pid=8176&quot; rel=&quot;nofollow noreferrer&quot;&gt;site&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the Delta II has successfully launched more than 190 NASA missions and maintains a &lt;strong&gt;98 percent success rate&lt;/strong&gt;. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;success rate of &lt;a href=&quot;http://www.braeunig.us/space/specs/shuttle.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;SHUTTLE&lt;/a&gt; is &lt;strong&gt;98.25%&lt;/strong&gt; to end-2005&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3xPIo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/MGBlK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2015-11-19T22:35:53.610" LastActivityDate="2015-11-19T22:35:53.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1663" PostTypeId="1" CreationDate="2013-08-29T14:59:52.093" Score="10" ViewCount="724" Body="&lt;p&gt;Earth's magnetic field, and atmosphere shield the surface. Space debris that would otherwise cause damage lose a fair amount of mass and velocity in transit through these layers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Luna lacking such shields is relatively more susceptible - and has the scars to show it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have any missions (instrument, animal, or both) suffered damage from meteorites whilst on the Lunar surface? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;There may have been any volume of micrometeorites. It may be reasonable to consider only that damage which directly affected the on-surface mission profile.&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-08-29T17:17:38.387" LastActivityDate="2013-09-24T13:28:24.440" Title="Have any missions suffered damage from meteorites whilst on the Lunar surface?" Tags="|the-moon|meteorites|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1664" PostTypeId="1" CreationDate="2013-08-29T17:11:12.393" Score="24" ViewCount="7122" Body="&lt;p&gt;When a spacecraft performs a &lt;a href=&quot;http://en.wikipedia.org/wiki/Splashdown_%28spacecraft_landing%29&quot;&gt;splashdown&lt;/a&gt; maneuver, a recovery team is standing by to retrieve the capsule and its human contents quickly.  A flotation collar is deployed to increase buoyancy and prevent the spacecraft from sinking.  If, however, the spacecraft &lt;em&gt;did&lt;/em&gt; sink, would it be able to withstand the pressure of the deep?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Structurally speaking, what are the differences between a spaceship floating in the vacuum of space and a submarine floating underwater in an ocean?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorDisplayName="user12" LastEditDate="2013-08-30T11:46:55.250" LastActivityDate="2018-05-22T06:50:13.180" Title="Do spacecraft have similar structural integrity requirements as submarines?" Tags="|spacecraft|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="1665" PostTypeId="2" ParentId="1535" CreationDate="2013-08-29T20:41:46.040" Score="7" Body="&lt;p&gt;This is rather difficult, but let's see what we can do with it. First of all, let's make a few assumptions.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Let's use the Dragon capsule as our base platform. The manned version should be able to hold 7 people, and enough supplies for them to launch and land.&lt;/li&gt;&#xA;&lt;li&gt;Let's take the direct approach to the moon.&lt;/li&gt;&#xA;&lt;li&gt;Let's just assume that a Dragon could hold enough supplies for the 2 week trip to the Moon.&lt;/li&gt;&#xA;&lt;li&gt;Let's assume you enter LEO for an orbit. From there, there are 2 stages, one to get to the moon and land, and the second to get home. This violates the no littering principal, but it makes the math a lot easier...&lt;/li&gt;&#xA;&lt;li&gt;Let's just use Merlin 1-D engines. This is because it is the rocket enginue with the highest &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_rocket_engines&quot;&gt;thrust/weight ratio&lt;/a&gt;. It's vacuum thrust is about ~801 kN, and a weight of 500 kg roughly. The ISP is 310.&lt;/li&gt;&#xA;&lt;li&gt;Let's assume a tank/fuel ratio of about 3% (Tank weights 3% of fuel)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Okay, so how much does the dragon itself weigh? Cargo + dry mass is about 8,000 kg. Okay, so what are the delta V requirements? From &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot;&gt;Wikipedia&lt;/a&gt;, here's the stats:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;LEO to Moon- 5.93 km/s&lt;/li&gt;&#xA;&lt;li&gt;Moon to Earth- 2.74 km/s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So how do you figure this out? I made a spreadsheet which helps me calculate this, using the following formula: $\text{fuel}=[\text{payload}+\text{engine}+(1/2+0.03)\text{fuel}(\text{iter}-1)]/(I_{SP}*9.8)$. It's approximate, but should be fairly close. To verify it, I plugged in some of the apollo numbers and got something in the same ballpark (But slightly better).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, given this spreadsheet, let's crunch some numbers. Let's start with the return trip. You have a mass of 8,000 kg previously mentioned. Add 500 for the engine, and plugging it in to my magic spreadsheet gives 14700 kg fuel, 440 tank, and 23700 total.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so how do you get that to land on the moon from LEO? It turns out that the 5.93 km/s delta v is higher than is practical with a single stage. Let's try 2 stages, split down the middle (Unrealistically, but there it is). The second stage is still a single Merlin D engine, 46904 kg fuel, 1400 kg tank, and 71500 kg total. The first stage is then 145600 kg fuel, 4370 kg tank, and 222,000 kg total. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so if you can get 220,430 kg to LEO, you are on your way to the moon! How much is that? It turns out that is twice as large as the Saturn V was capable of lifting to LEO. Considering we are taking 7 people instead of 3, that isn't bad, but it does mean that technology isn't as far along as we would hope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, with some careful staging this could be reduced, but not significantly. I don't doubt that some careful physics could reveal that my analysis isn't perfect either, or that if we did an Apollo style mission we couldn't save a few kg anyways. Feel free to create your own spreadsheet to explore this if you want;-)&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user29" LastEditDate="2013-08-29T21:15:51.170" LastActivityDate="2013-08-29T21:15:51.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1666" PostTypeId="2" ParentId="55" CreationDate="2013-08-29T21:54:57.737" Score="7" Body="&lt;p&gt;The worst-case scenario includes tripping off early warning sensors of some nation (like the Norwegians did in &lt;a href=&quot;https://en.wikipedia.org/wiki/Norwegian_rocket_incident&quot; rel=&quot;noreferrer&quot;&gt;1995&lt;/a&gt;) and starting an unfortunate chain of events.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;World War III is pretty heavy on unwanted consequences, legal or not. I seriously doubt the survivors can be benevolent to those who launched the satellite.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2013-08-29T21:54:57.737" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1667" PostTypeId="1" AcceptedAnswerId="2012" CreationDate="2013-08-29T23:56:48.890" Score="17" ViewCount="2368" Body="&lt;p&gt;With &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Launch_vehicles/Ariane_6&quot; rel=&quot;noreferrer&quot;&gt;Ariane 6&lt;/a&gt; launch site selected, CNES (Centre National d’Etudes Spatiales, the French space agency) was aiming to freeze the design of the new rocket in May 24, 2013. A few days after and that same month (May 30, 2013), Europe’s Air &amp;amp; Space Academy &lt;a href=&quot;http://www.spacenews.com/article/launch-report/35546academy-urges-europe-to-halt-work-on-%E2%80%98wrong-choice%E2%80%99-ariane-6-design&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;“urged to halt work on the Ariane 6 design”&lt;/em&gt;&lt;/a&gt; and among reasons brought forth, in their view, the wrong choice for the first two stages propellant:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The academy is urging the agencies to stop work on the Ariane 6 they&#xA;  approved in November with a view to beginning full development in&#xA;  2014. The academy-favored rocket would use liquid propulsion instead of solid, and would face four more years of preparatory work before&#xA;  moving to full development in 2018.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On May 8, 2013 Europe’s Air &amp;amp; Space Academy sent a letter to the ESA director general Jean-Jacques Dordain, in which it urged ESA to &lt;em&gt;“urgently reopen the configuration studies”&lt;/em&gt; for Ariane 6.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/DU9en.jpg&quot; alt=&quot;Some of the Ariane 6 concepts under investigation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Some of the Ariane 6 concepts under investigation (Source: &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Launch_vehicles/Ariane_6&quot; rel=&quot;noreferrer&quot;&gt;ESA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Among &lt;a href=&quot;http://www.spacenews.com/article/launch-report/35546academy-urges-europe-to-halt-work-on-%E2%80%98wrong-choice%E2%80%99-ariane-6-design&quot; rel=&quot;noreferrer&quot;&gt;other objections&lt;/a&gt; to the proposed Ariane 6 design, Air &amp;amp; Space Academy gave the following reason for not using solid propellants as being &lt;em&gt;“the wrong choice”&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Two solid-fueled stages topped by a cryogenic upper stage is a&#xA;  dead-end design that does not permit the flexibility needed in a&#xA;  rocket that will serve as Europe’s main launcher for several decades.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This however seems rather argumentative. Granted, solid- versus liquid-fueled rockets debate among engineers is as old as the spage age, but besides most current launch vehicle designs using liquid and/or cryogenic rocket stages, is there any non-disputable evidence the choice of one is better over the choice of another?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ESA aims to develop a per launch cheaper, simpler design launch vehicle with Ariane 6, and the use of solid propellants seems rather obvious to me, which just so happens is also a firm belief of the ESA launcher director Antonio Fabrizi. ESA estimates a per launch cost of around €70 million, something the academy disagrees on, but still quotes significant savings compared to &lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5&quot; rel=&quot;noreferrer&quot;&gt;Ariane 5&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The academy estimates that today’s Ariane 5 ECA rocket costs 145&#xA;  million euros per launch. The solid-fueled Ariane 6 will likely cost&#xA;  98 million per launch, at least at the outset. It will be difficult to&#xA;  cut costs to arrive at the announced target.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Right. So the cost savings are still there and they are substantial regardless of whose quotes we believe. So this &lt;em&gt;“solid- versus liquid-fueled rockets”&lt;/em&gt; isn't as intuitive as I would've hoped, after reading so many articles on the Ariane 6 dispute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's see, if we can get some additional perspective, though. On July 12, 2013, it was announced that Johann-Dietrich Woerner, chairman of the &lt;a href=&quot;http://www.dlr.de/dlr/en/&quot; rel=&quot;noreferrer&quot;&gt;German Aerospace Center (DLR)&lt;/a&gt;, said the German government &lt;a href=&quot;http://www.spacenews.com/article/launch-report/36225dlr%E2%80%99s-woerner-remains-unconvinced-just-unveiled-ariane-6-design-is-right#.UeEMI6y3kZY&quot; rel=&quot;noreferrer&quot;&gt;remains in favor of continued development of the current Ariane 5 heavy-lift rocket&lt;/a&gt;, with possible evolutions including environmentally acceptable new fuels in place of the vehicle’s current solid-rocket boosters. He goes on to say:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... solid propellant carries other disadvantages, including the fact&#xA;  that a solid-fueled second stage adds vibration risks to sensitive&#xA;  satellite payloads and also pollutes the upper atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The statement regarding &quot;vibration&quot; again seems a bit argumentative, but is that even true? And while the environmental concerns seem a bit more fair, is that really such a problem, and why mention upper atmosphere pollution only?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, to recap, my questions are:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Are there any other technical reasons for favoring liquid propellants over solid ones, besides above mentioned reasons?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Can the argument that the solid-fueled second stage adds vibration risk be substantiated with hard facts for a design-stage rocket?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;What kind of upper atmosphere pollution are we talking of, if solid propellants are chosen over liquid ones, that don't even warrant mentioning launch site and lower atmosphere pollution?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In essence, I'm looking for answers that will provide more insight into this age old engineering debate on &lt;em&gt;“solid- versus liquid-fueled rockets”&lt;/em&gt;. Ideally, the ESA / CNES design choice for the Ariane 6 should be discussed, and if possible, avoid or completely dismiss / invalidate politically motivated arguments, such as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Woerner said that for this kind of scale economy to work, all the&#xA;  Ariane 6 solid-fueled boosters would need to be made in the same&#xA;  place. Given today’s European industrial landscape, he said, that&#xA;  would mean in Italy.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This quote just smells of local industry favoritism and should, without substantial evidence that &lt;em&gt;&quot;liquid&quot;&lt;/em&gt; is the way to go in favor of &lt;em&gt;&quot;solid&quot;&lt;/em&gt;, be easy enough to dismiss as such. Or is there more to it than meets the eye?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="58" LastEditDate="2018-09-27T14:41:11.560" LastActivityDate="2018-11-05T15:38:17.873" Title="Why are Europe’s Air &amp; Space Academy and DLR urging ESA and CNES to reconsider the use of solid fuels for the proposed Ariane 6?" Tags="|rockets|esa|planning|ariane|pollution|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1668" PostTypeId="1" AcceptedAnswerId="1741" CreationDate="2013-08-30T00:53:54.287" Score="17" ViewCount="862" Body="&lt;p&gt;I've never been to see a rocket launch. I'd like a checklist of what to consider in choosing where to go, what type of launch to pick, and what to allow for in terms of missed launch dates or other issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I probably would not try to photograph it - others can do a much better job and I've seen pics and videos. I just want to be close as possible, close enough to see it unaided, and hear and feel the roar.&lt;/p&gt;&#xA;" OwnerUserId="79" LastEditorUserId="49" LastEditDate="2014-03-26T20:29:54.567" LastActivityDate="2014-03-26T20:29:54.567" Title="What should I consider in choosing what launch to see live?" Tags="|launch|rockets|launch-site|observation|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1669" PostTypeId="1" AcceptedAnswerId="1670" CreationDate="2013-08-30T04:52:08.513" Score="7" ViewCount="256" Body="&lt;p&gt;America has planned to create an army outpost on the Moon. They called it &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Horizon&quot;&gt;Project Horizon&lt;/a&gt;. Did they succeed in that? How many satellites were launched for that project so far?&lt;/p&gt;&#xA;" OwnerUserId="536" LastEditorUserId="49" LastEditDate="2013-08-30T05:39:30.020" LastActivityDate="2013-08-30T06:13:20.357" Title="Has America launched any satellites for Project Horizon" Tags="|artificial-satellite|history|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1670" PostTypeId="2" ParentId="1669" CreationDate="2013-08-30T05:32:26.410" Score="6" Body="&lt;p&gt;Like the &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Horizon&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page on Project Horizon you link to&lt;/a&gt; states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Horizon never progressed past the feasibility stage in an official capacity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In a nutshell, it was merely a feasibility study for the U.S. Army that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...called for 147 early Saturn A-class rocket launches to loft&#xA;  spacecraft components for assembly in low Earth orbit at a spent-tank&#xA;  space station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7MsAb.jpg&quot; alt=&quot;Project Horizon&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;A depiction of the construction of the second, larger part of the 12-man Moon base proposed as the end goal for Project Horizon.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The initial part of the base, shown covered in lunar material at left, was to be built within two months of the first manned landing.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The proposed Lunar Landing Craft can be seen soft-landing in the background. Public domain image from:&lt;br/&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://www.history.army.mil/faq/horizon/Horizon_V1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;► Project Horizon: Volume I.&lt;/a&gt; (in PDF format)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_%28rocket_family%29#Launch_history&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn A never launched&lt;/a&gt;, so there were no satellites launched for the purpose of this project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a three-part blog post describing this project in detail:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://falsesteps.wordpress.com/2012/08/06/project-horizon-part-i-christmas-island-launch-facility/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Horizon (Part I): Christmas Island Launch Facility&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://falsesteps.wordpress.com/2012/08/29/project-horizon-part-ii-the-minimal-orbital-station-and-the-orbital-return-vehicle/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Horizon (Part II): The Minimal Orbital Station and the Orbital Return Vehicle&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://falsesteps.wordpress.com/2012/09/22/project-horizon-part-iii-landing-soldiers-on-the-moon-and-staying-there/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Horizon (Part III): Landing Soldiers on the Moon and Keeping Them There&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And a short excerpt from the mentioned blog post:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;What it was&lt;/strong&gt;: Project Horizon was the US Army’s full-blown proposal to&#xA;  put a man on the moon (and, in fact, start up a whole lunar base) by&#xA;  the end of 1966. While much of its focus was on the base itself, it&#xA;  also included extensive discussion of both the Earth-bound and orbital&#xA;  infrastructure they felt was necessary to reach their goal. The&#xA;  Minimal Orbital Station (MOS) was essentially a gas station in LEO for&#xA;  ships headed to the Moon, while the Orbital Return Vehicle was to be&#xA;  used to bring the gas station’s attendants back down to Earth when&#xA;  their tour of duty was done.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On a side note tho, at that time there were many overly optimistic projects proposed, that never made it and were utterly infeasible in the first place. Dreamy times, one could easily say they were the romantic decades of the space exploration and exploitation era. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-08-30T06:13:20.357" LastActivityDate="2013-08-30T06:13:20.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1671" PostTypeId="2" ParentId="1455" CreationDate="2013-08-30T07:29:26.907" Score="8" Body="&lt;p&gt;&lt;a href=&quot;http://rax.engin.umich.edu/?page_id=882&quot;&gt;RAX-2 telemetry decoder&lt;/a&gt; is the software that can do your work &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tutorial/How to use: &#xA;&lt;a href=&quot;http://pe0sat.vgnet.nl/2013/telemetry-decoding&quot;&gt;pe0sat.vgnet.nl/2013/telemetry-decoding&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="639" LastActivityDate="2013-08-30T07:29:26.907" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1672" PostTypeId="2" ParentId="1105" CreationDate="2013-08-30T11:44:42.977" Score="2" Body="&lt;p&gt;I really, really hate saying this, but I'm afraid not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My reasoning? I, and probably many others, have been Googling like crazy to find the answer to this for three weeks. If there were a pre-published time, then one would think that it would come up as one of the first results for a search like &lt;code&gt;iss ham radio contact&lt;/code&gt; or &lt;code&gt;iss ham radio time&lt;/code&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm very eager to be proven wrong, but it certainly seems that this information wouldn't be hidden deep in the 35th page of Google.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-08-30T11:44:42.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1673" PostTypeId="5" CreationDate="2013-08-30T11:48:17.070" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-08-30T11:48:17.070" LastActivityDate="2013-08-30T11:48:17.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1674" PostTypeId="4" CreationDate="2013-08-30T11:48:17.070" Score="0" Body="Questions regarding the temperature of the atmosphere in an area." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-08-30T11:51:58.477" LastActivityDate="2013-08-30T11:51:58.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1675" PostTypeId="2" ParentId="1105" CreationDate="2013-08-30T11:49:55.280" Score="1" Body="&lt;p&gt;It seems like the best thing that can be done for this is to look at the times they have in the past, and try to be on at the same time. Bottom line, you just have to keep trying, and hopefully some day you'll get lucky. But they don't publish schedules ahead of time, just after the fact.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-30T11:49:55.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1676" PostTypeId="2" ParentId="1664" CreationDate="2013-08-30T11:55:38.200" Score="14" Body="&lt;p&gt;Let's differentiate &quot;spacecraft&quot; and &quot;landing capsule&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are various phases of the flight with various prerequisites, and several different (considerably different) designs of spacecrafts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, let's take the capsule. It must withstand violent reentry, unequal air pressures - the exposed side heats up enormously due to air friction, which is connected with quite huge pressure. It doesn't need to be lean and aerodynamic, quite opposite, it's one big airbrake with a passenger cabin attached to the trailing side. The pressure is largely unidirectional and primarily overheated, so the construction concerns of submarine are really distantly related, with its omnidirectional pressure, ability to withstand unexpected pressure waves (explosions), lean shape to minimize resistance, and ubiquitous water cooling making overheating non-issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, the orbital craft. To get there it must be be light. A mild kick could have pierced Apollo lunar module walls, ISS is more sturdy but still it's to withstand  1 bar pressure difference towards the outside (that's equivalent of mere 10m submersion depth), and again - towards the outside, that means no need for cross-beams to prevent buckling; it has natural tendency to bloat like a balloon. It isn't lean or aerodynamic in the least, air resistance is not a concern. Its construction problems are much closer to these of airplane hull than to that of a submarine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And finally, the launch vehicle. This one does have to be lean, but again, it needs to be light. It is not resistant to lateral forces (its own acoustic wave reflected from the tower could damage it!), and again it doesn't have to withstand high outer pressures. The shuttle main fuel tank depends on internal fuel pressure; in other cases the structure is mostly unpressurized shell that allows air pressure to escape as it leaves the atmosphere, providing structural integrity but not isolating the inside - except for the relatively tiny human compartment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Really, submarines with a considerable fraction of a meter thick solid metal shell, and designed for battle conditions - stealth, maneuverability, durability, weapons, aquatic environment - face entirely different construction challenges. On the other hand, there's much more overlap with airplanes.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2018-05-22T06:50:13.180" LastActivityDate="2018-05-22T06:50:13.180" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="1677" PostTypeId="1" AcceptedAnswerId="1678" CreationDate="2013-08-30T13:34:29.750" Score="20" ViewCount="1206" Body="&lt;p&gt;The Delta IV Heavy recently launched &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_NRO_Launches&quot;&gt;NORL-65&lt;/a&gt; to a low Earth orbit.  Some of these missions will be in the range of &lt;a href=&quot;http://www.nasaspaceflight.com/2013/08/ula-delta-iv-h-launch-nrol-65/&quot;&gt;390 km&lt;/a&gt; altitude with circular orbits.  Plus, the Delta IV Heavy is a big rocket, with a LEO payload of &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta_IV_Heavy&quot;&gt;22 tons&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that it's a sensitive payload, how do you avoid it crashing into a random place on Earth when the orbit decays?  I presume they would strive to avoid that for obvious reasons.  Can such large masses burn up in the atmosphere?  Is that even an option for a large telescope?  Do people seriously entertain the idea of a controlled re-entry?  Or do they use a disposal orbit?  It's a classified mission so there's probably no official word on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the options that are seriously considered for that kind of mission?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="25" LastEditDate="2014-11-05T19:22:51.310" LastActivityDate="2014-11-05T19:22:51.310" Title="What are the end-of-life options for large classified satellites?" Tags="|artificial-satellite|low-earth-orbit|reentry|end-of-life|classified|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1678" PostTypeId="2" ParentId="1677" CreationDate="2013-08-30T14:21:51.683" Score="17" Body="&lt;p&gt;De-orbiting isn't too bad of an option really. While a really large satellite would still have some pieces intact, it by and large will not be completely intact. Furthermore, the most sensitive things on a spacecraft tend to be the computers, antennas, and cameras. Of those, the only remotely likely to survive re-entry is the cameras, most likely only the mirrors from said camera.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, most LEO satellites are reasonably well designed to break up. Unprotected aluminum doesn't survive reentry very well, and it is what most LEO satellites are composed of. Practically speaking, one has to design purposely to have anything survive re-entry, otherwise all you get is chunks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The destruction procedure for classified material for the US in general is explained in &lt;a href=&quot;http://www.dtic.mil/whs/directives/corres/pdf/522022m.pdf&quot; rel=&quot;noreferrer&quot;&gt;NISPOM&lt;/a&gt;, which states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;5-705. Methods of Destruction. Classified material may be destroyed by burning, shredding, pulping, melting, mutilation, chemical&#xA;  decomposition, or pulverizing (for example, hammer mills, choppers,&#xA;  and  hybridized disintegration equipment). Pulpers, pulverizers, or&#xA;  shredders may be used only for the destruction of paper products. High&#xA;  wet strength paper, paper mylar, durable medium paper substitute, or&#xA;  similar water repellent papers are not sufficiently destroyed by&#xA;  pulping; other methods such as disintegration, shredding, or burning&#xA;  shall be used to destroy these types of papers. Residue shall be&#xA;  inspected during each destruction to ensure that classified&#xA;  information cannot be reconstructed. Crosscut shredders currently in&#xA;  use capable of maintaining a shred size not exceeding 1/32 inch in&#xA;  width (with a 1/64 inch tolerance by 1/2 inch in length) may continue&#xA;  to be used. However, any crosscut shredders requiring replacement of&#xA;  the unit and/or rebuilding of the shredder blades assembly must be&#xA;  replaced by a crosscut shredder on the latest NSA Evaluated Products&#xA;  List of High Security Crosscut Shredders. The list may be obtained&#xA;  from the CSA. Classified material in microform; that is, microfilm,&#xA;  microfiche, or similar high data density material; may be destroyed by&#xA;  burning or chemical decomposition, or other methods as approved by the&#xA;  CSA.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While each classified program can add in additional standards, I believe that uncontrolled re-entry of most spacecraft falls within this destruction principal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, the re-entry for any satellite is usually many years after launch, at which point in time the classification parameters aren't as important (Most stuff of this nature can be declassified after 25 years). Of course, the mentioned 390 km orbit doesn't quite fit in to the 25 year range, but I would expect that the satellite's orbit will be raised if possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, the re-entry of a satellite can be controlled, it just takes a bit more work and a lot of planning. MIR was &lt;a href=&quot;https://en.wikipedia.org/wiki/Deorbit_of_Mir&quot; rel=&quot;noreferrer&quot;&gt;successfully deorbited&lt;/a&gt;, and I suspect any satellite can be deorbited in a controlled fashion.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-31T15:45:12.950" LastActivityDate="2013-10-31T15:45:12.950" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="1679" PostTypeId="5" CreationDate="2013-08-30T14:27:55.113" Score="0" Body="" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-30T14:27:55.113" LastActivityDate="2013-08-30T14:27:55.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1680" PostTypeId="4" CreationDate="2013-08-30T14:27:55.113" Score="0" Body="Relating to classified (By any government) missions, requirements, proposals, etc." OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-30T14:27:55.113" LastActivityDate="2013-08-30T14:27:55.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1681" PostTypeId="1" AcceptedAnswerId="1702" CreationDate="2013-08-30T14:41:25.640" Score="5" ViewCount="132" Body="&lt;p&gt;&lt;a href=&quot;http://innerspace.net/2013/07/29/the-grand-asteroid-challenge/&quot; rel=&quot;nofollow&quot;&gt;The Grand Asteroid Challenge&lt;/a&gt; is a project to capture an incoming asteroid, enter it into a stable Lunar orbit and send a manned mission to its surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That sounds... implausible at best. Do we have even hope of acquiring necessary technological means - like a vehicle capable of delivering enough delta-V to an asteroid to get it into an orbit and then prepare and send a manned mission (and this all until 2017)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do the scientists of NASA see the plausibility of this idea?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2013-08-30T14:43:01.677" LastActivityDate="2013-09-01T02:24:38.667" Title="Is The Grand Asteroid Challenge plausible or just a PR stunt?" Tags="|crewed-spaceflight|asteroid|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1682" PostTypeId="2" ParentId="1681" CreationDate="2013-08-30T15:09:54.063" Score="2" Body="&lt;p&gt;According to this &lt;a href=&quot;http://www.thespacereview.com/article/2318/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA associate administrator for human&#xA;  exploration and operations Bill&#xA;  Gerstenmaier said the use of “redirect”&#xA;  in the mission concept is deliberate,&#xA;  since they’re interested in objects that&#xA;  already would pass close to the Earth.&#xA;  “We take advantage of the fact that it’s&#xA;  already coming back so we don’t have&#xA;  to change the velocity in that&#xA;  direction,” he said. “All we need to do&#xA;  is essentially redirect it” into a lunar&#xA;  orbit, taking advantage of the Moon’s&#xA;  gravity. Left unstated was that&#xA;  redirection would also be useful if an&#xA;  object is passing too close to the Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hfhj1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I think is that it is possible since the asteroid is heading   towards  the earth the only thing we need to do is to catch it and redirect it to an orbit ( which may sometimes require adding or reducing velocity  which require fuel )&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-08-30T15:09:54.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1683" PostTypeId="2" ParentId="982" CreationDate="2013-08-30T15:11:46.687" Score="8" Body="&lt;p&gt;Exotic matter is an area that is worth looking at. The geometry of warp drive (or equivalently, Krasnikov tubes) is not really as interesting for FTL travel as people believe: Something that usually gets overlooked is the causality of the matter-geometry dependency. You need the matter to be deployed &lt;em&gt;first&lt;/em&gt; on a space-like region between a home and a destination, and then the space-time will change to allow FTL travel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is clear that if you are in Earth, even if you had the exotic matter in adequate quantities, you'll still need some way to deploy it along the road, otherwise there will be no warp!. So, in summary, you need a FTL drive in order to deploy the matter that will enable a warp drive. It still could find other applications though. For instance, could be applied (in principle) for exceeding the bounds of computation rates imposed by the speed of light on local information exchange between different regions of a computer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more reasonable proposition for spaceflight applications is researching micro-wormholes that might become stable and large enough to be observable if exotic-matter can be made to persist above the constraints imposed by the weak and dominant energy conditions. &lt;a href=&quot;http://www.icarusinterstellar.org/team/eric-davis/&quot; rel=&quot;nofollow&quot;&gt;Eric Davis&lt;/a&gt; is looking into parabolic mirrors that focus high quantum fluctuations in a confined volume. That should be interesting to test the general validity of the energy conditions&lt;/p&gt;&#xA;" OwnerUserId="638" LastEditorUserId="1142" LastEditDate="2014-11-28T17:54:24.450" LastActivityDate="2014-11-28T17:54:24.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1685" PostTypeId="2" ParentId="756" CreationDate="2013-08-30T16:29:50.927" Score="9" Body="&lt;p&gt;The best way to explore the subsurface oceans of Europa is to see what &lt;a href=&quot;http://www.space.com/20078-jupiter-moon-europa-ocean-surface.html&quot;&gt;comes out from the vents&lt;/a&gt;. Basically, along each crack will be a small amount of ocean water. Take a sample, and you could find out a lot about what is below the ocean. One simply has to land on a recent crack, and you should be able to find something out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually getting in to the ice is far more problematic. There are two primary challenges associated with this. The first is simply to get below the ocean layer to begin with. The second is how to communicate with the probe after you have gone deep. Both are problematic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's start with the first one, getting underneath the ocean. The easiest way would be to find a crack forming, and work your way in to it. This is actually quite difficult to do in practice, mostly because you have to know where the crack is going to form before you land, and these cracks change location every &lt;a href=&quot;http://news.bbc.co.uk/2/hi/science/nature/449135.stm&quot;&gt;3 days or so&lt;/a&gt;. And they are pretty small, for the most part, you might have difficulty fitting a probe in to one. Plus they make the communication more dangerous, as you'll see later on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second way is to take a portable heat source and melt the ice away. Basically, this would be a novel use for an RTG of sorts, radioactive heat could melt a hole. This could take a long time, but would work eventually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, a hole could be drilled. The most challenging factor with this is the fact that even &lt;a href=&quot;http://www.chegg.com/homework-help/questions-and-answers/life-span-oil-drilling-bits-depends-types-rocks-soil-drill-encounters-estimated-mean-lengt-q1618534&quot;&gt;the best drill bits&lt;/a&gt; need to be changed on a regular basis for mining operations. Let's say you could get 300 hours out of a drill bit, at a rate of  3 m/hour, leaves only about a km before the drill bit needs replacing, and that's probably optimistic. Who is there to replace the bit? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, assuming you can get through the surface, what do you do there? Basically, this is in the same realm as submarines, except through ice as well as water. There is a whole &lt;a href=&quot;https://en.wikipedia.org/wiki/Communication_with_submarines&quot;&gt;Wikipedia article&lt;/a&gt; on submarine communications. Method could include buoys to the surface, running a long cable, sound waves, or very low frequency waves. The cable or accustics are the best ideas, ideally there should be a combination of both of them, to give some redundancy in case something fails. Keeping the cable intact, would be difficult due to the moving ice, and acoustics would probably be off a bit as well, but they could be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the only cheap way to study the oceans of Europa is to see what is on the surface, along with technologies like RADAR to see what lies beneath. Truly exploring it would be amazing, but incredible difficult and expensive. Someday we'll get there, however.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-30T16:29:50.927" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1686" PostTypeId="2" ParentId="937" CreationDate="2013-08-30T16:35:51.140" Score="12" Body="&lt;p&gt;The highest frequency is gamma rays, dedicated to detecting nuclear explosions on Earth. These have provided some knowledge of space events as well, due to their low selectivity, but they are intended and dedicated to detecting nuclear explosions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first series of satellites to do this detection was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vela_%28satellite%29&quot; rel=&quot;noreferrer&quot;&gt;Vela satellites&lt;/a&gt;, which detected x-rays, gamma rays, neutrons, and looked for quick flashes of light. There have been many others over the years.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-08-31T00:10:27.453" LastActivityDate="2013-08-31T00:10:27.453" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1687" PostTypeId="2" ParentId="1593" CreationDate="2013-08-30T16:40:24.940" Score="10" Body="&lt;p&gt;&lt;strong&gt;Yes&lt;/strong&gt;, and in fact, magnetic torquers are &lt;em&gt;more efficient&lt;/em&gt; in polar orbits than in equatorial ones. This makes sense if you look at a picture of Earth's magnetic field lines:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZAu1q.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a spacecraft is in a polar orbit, it experiences higher magnetic flux (passes through more lines), and thus is exposed to a stronger magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The map below shows a map of the Earth's magnetic field strength (numbers are in nanoTeslas).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/m7krl.gif&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see (and as &lt;a href=&quot;http://denali.gsfc.nasa.gov/terr_mag/core.html&quot; rel=&quot;noreferrer&quot;&gt;this page&lt;/a&gt; confirms), the magnetic field strength at the poles is about twice that of the equatorial field strength.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, how does this apply to magnetic torquers on a spacecraft? Well, the torque $T$ produced by a magnetic torquer depends on the magnetic field strength $B$ and the dipole induced by the torquer $D$ (which is proportional to the current applied to the torquer):&#xA; $$T=DB$$&#xA;As you can plainly see, the higher the magnetic field strength ($B$), the larger the torque ($T$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going a step further, &lt;em&gt;altitude&lt;/em&gt; is much more of a factor. $B$ can be approximated by:&#xA;$$B=2M/R^3$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $M$ is the magnetic &lt;em&gt;moment&lt;/em&gt; of the Earth (about $7.96 \times 10^{15} \text{tesla} \cdot \text{m}^3$) and $R$ is the distance from Earth's dipole center and the spacecraft. As you can see here, higher altitudes decrease the field strength drastically, which in turn decreases the torque you'll see from your magnetic torquer. This is why you only see magnetic torquers on LEO spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Note&lt;/strong&gt;: for the last section I used &lt;em&gt;Space Mission Analysis and Design&lt;/em&gt; (3rd ed., Wertz, J.R. and Wiley, J.L., editors, 1999) as a reference.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-08-30T16:54:03.450" LastActivityDate="2013-08-30T16:54:03.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1688" PostTypeId="1" AcceptedAnswerId="1695" CreationDate="2013-08-30T17:18:23.857" Score="8" ViewCount="363" Body="&lt;p&gt;I saw a &lt;a href=&quot;http://www.space.com/3087-death-spacecraft-unknown-fate-cassini.html&quot;&gt;news article&lt;/a&gt; once that discussed having Cassini use gravity assists to crash land on Mercury to end it's life. Is this even possible, and is there a chance that this could actually happen? Or is it just a space.com crazy article with no basis in reality?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give a few relevant quotes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Should Cassini be directed back toward the inner solar system it is&#xA;  likely her final port of call would be Jupiter, Mitchell explained,&#xA;  &quot;One possibility is to escape Saturn and then most likely put the&#xA;  spacecraft on an impacting trajectory with Jupiter. This appears to be&#xA;  feasible, but the flight times get to be rather long.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Cassini's leisurely final journey need not end at Jupiter; rather the&#xA;  giant planet's gravity could fling Cassini into position for an impact&#xA;  on Mercury. Such an impact would provide valuable data on the&#xA;  composition of Mercury's surface and could feasibly occur around 2021&#xA;  to be observed by the BepiColombo spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-08-31T17:03:20.917" Title="Could Cassini crash land on Mercury?" Tags="|orbital-mechanics|end-of-life|gravity-assist|cassini|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1689" PostTypeId="2" ParentId="1319" CreationDate="2013-08-30T17:48:35.770" Score="8" Body="&lt;p&gt;After reading &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2010/2471.html&quot; rel=&quot;noreferrer&quot;&gt;this blog post&lt;/a&gt; and skimming through &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103509004382&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;&lt;sup&gt;1&lt;/sup&gt;, it seems like it's just the stabilization of a standing waves caused by the difference in angular velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, as you go outwards from the poles, there is an increase in the velocity as the system moves with a constant angular velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This creates some shear on the gases present there, and there is a chance for local eddy currents. While in a stable situation one would have winds just spinning with the gases, in the real world situation there are instabilities which eventually coalesce into eddy currents. These can be of any number, however 6 seems to be one number which accepts a wide range of initial parameters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The folks in the paper linked above tried to emulate the system by taking two concentric rings and spinning them at different frequencies&lt;sup&gt;2&lt;/sup&gt;. When a dyed liquid placed on top of it is disturbed, one can get polygonal patterns of varying number of size:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mY7jg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While Saturn has a much more complex atmosphere, this same principle may apply here to give a hexagonal structure around which the rest of the wind currents organize.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1.  &lt;em&gt;A laboratory model of Saturn’s North Polar Hexagon&lt;/em&gt; : Icarus, Volume 206, Issue 2, April 2010, Pages 755–763&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;2. While the zones have no difference in frequency on Saturn, that is on a curved surface. I believe that they have introduced the difference in frequency to accommodate for this.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="96" LastActivityDate="2013-08-30T17:48:35.770" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1690" PostTypeId="2" ParentId="811" CreationDate="2013-08-30T21:39:05.963" Score="10" Body="&lt;p&gt;Well, I can give you a couple data points. First, we can look at NASA's &lt;a href=&quot;http://www.nasa.gov/mission_pages/dawn/mission/index.html#.UiEM32TBKSw&quot; rel=&quot;noreferrer&quot;&gt;Dawn&lt;/a&gt; mission, which uses an ion propulsion system (IPS) based on the system demonstrated on Deep Space 1 (which we'll look at next).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38349/1/05-1854.pdf&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; gives details of Dawn's IPS. It notes that this system should give the spacecraft a total of about &lt;strong&gt;11 km/s&lt;/strong&gt;. Here's the break down:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/shkSy.png&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper notes that Dawn will carry &lt;strong&gt;450 kg&lt;/strong&gt; of xenon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a graphical representation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hs8Fz.png&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, &lt;a href=&quot;http://www.reshitmada.org/yNews/Ionic%20Propulsion/DS1_IPS_Report.pdf&quot; rel=&quot;noreferrer&quot;&gt;this report&lt;/a&gt; gives the breakdown for DS1. The total amount of xenon propellant on DS1 was &lt;strong&gt;81.5 kg&lt;/strong&gt;. Keep in mind, this would be a significantly less mature system:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pch4I.png&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer the title question more directly: the propellant contributes the most total mass by far. Of the hardware, it's typically the PPUs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acronyms list:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;PPU&lt;/strong&gt;: Power processor unit&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;DCIU&lt;/strong&gt;: Digital Control and Interface Unit&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;XCA&lt;/strong&gt;: Xenon Control Assembly&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;TGA&lt;/strong&gt;: Thruster Gimbal Assembly&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-08-30T21:39:05.963" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1691" PostTypeId="1" AcceptedAnswerId="1693" CreationDate="2013-08-04T04:08:54.897" Score="32" ViewCount="34190" Body="&lt;p&gt;In this picture, we can see the footprints near the Apollo 11 lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1123X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shouldn't the dust have been blown away when Apollo 11 landed?&lt;/p&gt;&#xA;" OwnerUserId="2833" OwnerDisplayName="Tom Brito" LastEditorUserId="897" LastEditDate="2015-02-16T15:26:04.910" LastActivityDate="2023-08-24T08:02:37.003" Title="Why didn't the Apollo 11 lander blow the dust away? (or why does it look like it didn't)" Tags="|the-moon|apollo-program|lunar-landing|myth-busting|dust|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1692" PostTypeId="2" ParentId="1691" CreationDate="2013-08-04T04:17:22.127" Score="2" Body="&lt;p&gt;Off the top of my head I think the reasons are &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The ground there is made of dust. It's not just a thin layer on the surface, so you could blow a huge quantity of it away and still have plenty left to stand on; and &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;On Earth, a thruster blows dust around by setting the air in motion, but the Moon has no atmosphere. Some dust will still be blown away by the exhaust gases themselves, but this will be a much weaker effect than on Earth, because the exhaust gases will rapidly expand and become diffuse, which makes them unable to move much dust.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Perhaps someone with more direct knowledge can add more details, but I think these are the basic reasons.&lt;/p&gt;&#xA;" OwnerUserId="4561" OwnerDisplayName="Nathaniel" LastActivityDate="2013-08-04T04:17:22.127" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="1693" PostTypeId="2" ParentId="1691" CreationDate="2013-08-04T06:16:48.463" Score="57" Body="&lt;p&gt;Two main reasons really:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The dust on the Moon, while it would be extremely fine-grain, is also highly charged due to Sun's radiation and solar winds, so it would stick quite good to the surface, grain to grain, but also cling to astronauts' space suits, something that was made quite apparent when they had fairly big problems getting it off and somewhat cleaning the space suits before going back to the Lunar Module (LM) during several Apollo missions. See for example &#xA;answers to &lt;a href=&quot;https://space.stackexchange.com/q/1483/49&quot;&gt;How was dust-mitigation addressed during the Apollo&#xA;program?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Dust on the moon is electrified, at least in part, by&#xA;  exposure to the solar wind. Earth is protected from the solar wind by&#xA;  our planet's magnetic field, but the moon has no global magnetic field&#xA;  to ward off charged particles from the sun. Free electrons in the&#xA;  solar wind interact with grains of moondust and, in effect, &quot;charge&#xA;  them up.&quot; &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/avCo3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Lunar surface charging and electric fields caused by sunlight and solar wind.&lt;br /&gt;Image and quote source: &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2008/10apr_moondustinthewind/&quot; rel=&quot;noreferrer&quot;&gt;NASA Science News&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;Lunar Module&lt;/a&gt; on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11&lt;/a&gt; mission, named Eagle, didn't land directly (see: &lt;a href=&quot;https://www.youtube.com/watch?v=2BvbD-1qZtc&quot; rel=&quot;noreferrer&quot;&gt;one shot video of the landing on YouTube&lt;/a&gt;), but was navigating horizontally to the Moon's plain at a fairly low altitude for quite some time (they even had a  fuel warning, but that was later determined to have been premature), trying to find a suitable landing site. This means, that the LM landed with engines at an angle to the surface of the Moon for the most part, and only straightened for the very last few meters. This would limit the amount of dust raised, and it being blown away mainly in one direction away from LM when it was landing on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mare_Tranquillitatis&quot; rel=&quot;noreferrer&quot;&gt;Sea of Tranquility&lt;/a&gt; (&lt;em&gt;Mare Tranquillitatis&lt;/em&gt;) on 20 July 1969 at 20:17,40 UTC.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now, the dust would be blown away as mentioned in my last point in the direction the astronaut on the photograph is shown (more or less in the direction towards the hatch where the navigation controls and windows were also installed), but if you look a bit more closer and on a photograph of higher resolution (included below), you'll actually notice the dust trail:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/9/98/Aldrin_near_Module_leg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/9/98/Aldrin_near_Module_leg.jpg/593px-Aldrin_near_Module_leg.jpg&quot; alt=&quot;dust trail at landing site&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Buzz standing just beyond the north strut of the Lunar Module&lt;/strong&gt; (20 July 1969)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Buzz_Aldrin&quot; rel=&quot;noreferrer&quot;&gt;Buzz&lt;/a&gt; is standing just beyond the north strut. Note the distinctive&#xA;  dust smudges on Buzz's legs. The photo also shows the furrows in the&#xA;  bulk sample area and the area to the left of the footpad that shows&#xA;  unmistakable signs of sweeping by the descent engine exhaust. In a&#xA;  &lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/alsj/a11/a11det5902.jpg&quot; rel=&quot;noreferrer&quot;&gt;detail&lt;/a&gt; Ulli Lotzmann notes a reflected image of the rendezvous radar.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Photograph and quote source: &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Aldrin_near_Module_leg.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;, Credit: &lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/alsj/a11/AS11-40-5902HR.jpg&quot; rel=&quot;noreferrer&quot;&gt;NASA History Office&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is the dust trail visible on the photograph, just look a bit closer. ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, not only that there is no evidence to support conspiracy theories to be found on this photograph, there would be perfectly good reasons why some dust would still remain on the surface of the landing site, even if there was no sign of the dust trail. There is, it just so happens that &lt;a href=&quot;https://en.wikipedia.org/wiki/Buzz_Aldrin&quot; rel=&quot;noreferrer&quot;&gt;Buzz Aldrin&lt;/a&gt; is walking back to LM where it isn't the most pronounced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is, however, an alien on this photograph:&lt;/p&gt;&#xA;&#xA;&lt;blockquote class=&quot;spoiler&quot;&gt;&#xA;  &lt;p&gt; &lt;strong&gt;Neither Neil Armstrong, nor Buzz Aldrin were born on the Moon :P&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Spoiler alert! Hover over the yellow bar above with your mouse cursor to reveal it.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="49" OwnerDisplayName="TildalWave" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-02-09T08:04:40.873" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1695" PostTypeId="2" ParentId="1688" CreationDate="2013-08-31T08:53:10.937" Score="5" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Some of the options examined include collision with Saturn atmosphere, icy satellite, or rings; another is departure from Saturn orbit to Jupiter, Uranus, Neptune, or a Centaur. Other options include leaving it in certain stable orbits around Saturn, or departure to a heliocentric orbit. Each plan requires certain amounts of time and changes in velocity.Another possibility was aerobraking into orbit around Titan.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;These images gives us the &lt;strong&gt;clear idea of the possible end-life of cassini&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xK9dk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DpTR9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And also the delta v required to reach mercury from Saturn is very large and cannot be obtained by gravity assist&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so getting cassini to mercury and crashing it , is &lt;strong&gt;time consuming&lt;/strong&gt; but still possible by gravity assist by a Jupiter Venus or Jupiter Venus earth flyby &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-08-31T17:03:20.917" LastActivityDate="2013-08-31T17:03:20.917" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1696" PostTypeId="2" ParentId="1664" CreationDate="2013-08-31T10:18:46.863" Score="8" Body="&lt;p&gt;As far as I am aware, only one landing vehicle was not reclaimed before it sank to the bottom of the see (Liberty Bell 7). It was recovered in 1999 after spending 38 years on the bottom of the sea. The Liberty Bell 7 sunk after the hatch blew away prematurely and the capsule filled with water. Due to the fact there was both water on the inside and the outside of the capsule, the recovery report will not give much information about deep sea pressure on the whole capsule. It will however give an insight about the effect of sea water on the hull, and the effect of sea water and pressure on various instruments.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An inspection of the Mercury capsule, Liberty Bell 7, and its contents&#xA;  was made on September 1 and 2, 1999. The condition of the capsule and&#xA;  its contents was consistent with long-term exposure to salt water and&#xA;  high pressures at the bottom of the ocean. Many of the metallic&#xA;  materials suffered corrosion, whereas the polymer-based materials seem&#xA;  to have survived remarkably well. No identifiable items or structures&#xA;  were found that appeared to have any scientific value. At this time,&#xA;  no further nondestructive evaluation appears to be justified.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Abstract &quot;Liberty Bell 7 Recovery Evaluation and&#xA;Nondestructive Testing&quot;&lt;/em&gt; [1]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is worked out better in the article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The external structure was in surprisingly good condition. (See fig.&#xA;  3.) The black object at the bottom of the capsule is a skirt that provided ballast support. The heat shield would have been below the&#xA;  skirt, but the heat shield was missing and had not been recovered. The&#xA;  bottom structures included the flexible skirt and various metal strips&#xA;  and springs, a covering that appeared to be a polymer-based material,&#xA;  and the capsule base behind that; all looked to be in good condition.&#xA;  The external covering of the cone-shaped sides of the capsule&#xA;  consisted of small sheet-metal corrugated panels approximately&#xA;  0.031 inch thick, which showed no signs of corrosion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And later&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The upper portion of the capsule, which was made of thick aluminum&#xA;  plates, experienced significant material loss caused by corrosion.&#xA;  Although some areas measured full thickness of approximately 0.220&#xA;  inch, most regions were less, including several areas where the&#xA;  material was completely lost. Attached to these aluminum plates were&#xA;  large nodules of corrosion products.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, when sea water meets the interior of the landing capsule, it will destroy almost anything, either by pressure or corroding.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;As a last note: The report mentions exact the exact thickness of various parts of the hull. While the heat shield will shield the 0.031 inch (0.7874 mm) thick metal sheet at the bottom from heat, I doubt the heat shield will shield it from the sea pressure. Eventually the heat shield will completely corrode away too. While I have no exact numbers, I doubt the (thick?) 0.220 inch (5.58 mm) aluminium sheets will protect against even the slightest sea pressure either if the hull was completely waterproof. I have however no exact numbers to proof this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While landing capsules have been improved a lot since then, I doubt there have been much improvements in &quot;the survivability of a landing capsule on the bottom of the ocean&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1] &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.35.4881&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;noreferrer&quot;&gt;Liberty Bell 7 Recovery Evaluation and&#xA;Nondestructive Testing&lt;/a&gt;, E. Madaras &amp;amp; W. Smith, December 1999. Accessed August 31, 2013.&lt;/p&gt;&#xA;" OwnerUserId="567" LastActivityDate="2013-08-31T10:18:46.863" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1701" PostTypeId="2" ParentId="108" CreationDate="2013-08-31T14:43:36.267" Score="11" Body="&lt;p&gt;The Apollo 10 Command Module is on display at the &lt;a href=&quot;http://www.sciencemuseum.org.uk/centenary/home/icons/apollo_10_capsule.aspx&quot; rel=&quot;noreferrer&quot;&gt;Science Museum in London&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Lyj1N.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(pic from &lt;a href=&quot;http://www.beenthere-donethat.org.uk/london/london-kensington212big.html&quot; rel=&quot;noreferrer&quot;&gt;beenthere-donethat.org.uk&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="260" LastEditorUserId="38" LastEditDate="2013-08-31T16:46:42.630" LastActivityDate="2013-08-31T16:46:42.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1702" PostTypeId="2" ParentId="1681" CreationDate="2013-08-31T16:37:21.653" Score="3" Body="&lt;p&gt;Yes, if you find a suitable asteroid in a suitable orbit, it can be done.  The asteroid would be meters in size, requiring a few hundred m/s $\Delta V$.  Calculations on asteroid populations indicate that we can find such an asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://arxiv.org/abs/1304.5082&quot; rel=&quot;nofollow&quot;&gt;paper&lt;/a&gt; referenced in the comment below (ERO = &quot;easily retrievable object&quot;, where &quot;easily&quot; must be meant in relative terms):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Indeed, the paper presents a list of 12 EROs, with a total of 25&#xA;  trajectories to periodic orbits near L2 and 6 near L1 below a cost of&#xA;  500 m/s, and the number of these objects is expected to grow&#xA;  considerably in the coming years. The lowest cost is of 58 m/s to&#xA;  transfer asteroid 2006 RH120 to a halo southern family with a single&#xA;  burn on 1st February 2021. All the capture transfer opportunities to&#xA;  Earth’s vicinity have been identified for the currently catalogued&#xA;  NEOs during the next 30 years, and enable capture of bodies within 2-5&#xA;  meters diameter with low propellant costs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As for your question on schedule, 2017 is extremely aggressive even just for the launch of the capture mission, so probably not.  I did not see that date in your link, so I don't know what it is referring to.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-01T02:24:38.667" LastActivityDate="2013-09-01T02:24:38.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1703" PostTypeId="1" AcceptedAnswerId="1711" CreationDate="2013-08-31T17:04:11.820" Score="13" ViewCount="1276" Body="&lt;p&gt;We have many long-term satellite observatories and probes in &lt;em&gt;outer space&lt;/em&gt; way above the Earth's magnetosphere, and some of them progressively &lt;em&gt;cruising&lt;/em&gt; out of our Solar system. Off the top of my head, some of such satellites, probes and observatories are the &lt;a href=&quot;http://chandra.harvard.edu/&quot;&gt;Chandra X-ray Observatory&lt;/a&gt;, &lt;a href=&quot;http://hubblesite.org/&quot;&gt;Hubble Space Telescope&lt;/a&gt; (HST), the &lt;a href=&quot;http://sohowww.nascom.nasa.gov/&quot;&gt;Solar and Heliospheric Observatory&lt;/a&gt; (SOHO), the two Voyager probes, Cassini–Huygens of the &lt;a href=&quot;http://saturn.jpl.nasa.gov/&quot;&gt;Cassini Solstice Mission&lt;/a&gt;, e.t.c.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess it could be easily misconceived that these man-made objects are passing through the emptiness of space, devoid of any mass particles hitting their outer hull or other equipment, but that simply cannot be true. Depending on their position in our Solar system, they would be &lt;em&gt;sailing&lt;/em&gt; through variously dense clouds of &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_dust&quot;&gt;cosmic dust&lt;/a&gt; (granted, still at environmental pressure near zero), whichever their origins (remains of comets' tails and comas, remains of the circumstellar disk not cleared by larger mass bodies orbiting the Sun from the accretion period when the Solar system was forming, ejected materials from Mass Coronal Ejections and asteroid impacts on atmosphereless celestial bodies, even constant &lt;em&gt;grinding&lt;/em&gt; of the colliding rocks in asteroid belts, and so on). So there would be some dust, and depending on its origins / formation, its electrostatic charge due to it being exposed to solar radiation (or other causes), and its relative velocity to our man-made objects &lt;em&gt;cruising&lt;/em&gt; through these &lt;em&gt;clouds&lt;/em&gt; possibly low enough in some circumstances to not simply bounce off, some of this cosmic dust could easily stick on their surface and gather in time, either on their outer hull, or even sensitive equipment (e.g. lenses) they carry aboard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, do we have any data on individual probes, satellites and space observatories on how much of this dust do they gather in long years of service, has it ever been a problem and caused equipment failure or degradation, and do any of our man-made objects in outer space employ dust mitigation techniques, perhaps in a similar fashion to professional grade photographic lenses using various &lt;a href=&quot;https://en.wikipedia.org/wiki/Dust_reduction_system&quot;&gt;dust reduction systems&lt;/a&gt; like piezo crystal ultrasonic vibration, coupled with &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrostatic_discharge&quot;&gt;electrostatic discharge&lt;/a&gt; systems and/or &lt;a href=&quot;https://en.wikipedia.org/wiki/Degaussing&quot;&gt;degaussing&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to be clear, I'm not asking about implications of micrometeorites, but dust particles that individually don't pose a threat to our man-made objects in outer space, but could gather in longer periods of time and either degrade or cause a failure of onboard experiments and scientific equipment.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="58" LastEditDate="2013-09-10T13:12:37.197" LastActivityDate="2022-05-23T21:24:06.577" Title="Does cosmic dust pose a problem for long-term satellites, telescopes and probes?" Tags="|artificial-satellite|probe|solar-system|space-dust|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1704" PostTypeId="1" AcceptedAnswerId="1710" CreationDate="2013-08-31T17:55:21.883" Score="14" ViewCount="5179" Body="&lt;p&gt;On the Earth, measuring elevation above &lt;em&gt;sea level&lt;/em&gt; is relatively easy. Throughout the years of evolving these measurements to not rely on different reference points of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sea_level&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mean Sea Level&lt;/a&gt; (MSL) of various seas and oceans, which would be the average height of some ocean or the halfway point between its mean high tide and its mean low tide, and since moving to &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_sea_level&quot; rel=&quot;nofollow noreferrer&quot;&gt;Standard Sea Level&lt;/a&gt; (SSL) on an international scale, we have agreed on a standard to measure elevation of geographic features and other objects that is acceptable to all and not too difficult to measure using all kinds of different equipment, from barometers to triangulating position by the use of satellites that all work with the same reference point (SSL). Having agreed on that, we can then also agree on how high &lt;a href=&quot;https://en.wikipedia.org/wiki/Mount_Everest&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mount Everest&lt;/a&gt; is, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how do we measure elevation on other celestial bodies, that might not have oceans and seas, or even any atmosphere to speak of? Measuring elevation relative to some surface is easy enough, we can use radars, lidars,... anything that will &lt;em&gt;bounce off&lt;/em&gt;, so to speak, and then measure the time it took for it to do the round trip. But where is the &lt;em&gt;sea level&lt;/em&gt; on such bodies? Is there any international standard, an agreement that defines methods for &lt;em&gt;zero sea level&lt;/em&gt; on other celestial bodies, or does simply each nation, agency, even observatory define their own reference points arbitrarily?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, here is an example of an elevation (topographical) map from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Global_Surveyor&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Global Surveyor&lt;/a&gt;'s &lt;a href=&quot;http://mola.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Orbital Laser Altimeter&lt;/a&gt; (MOLA): &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/4pQBQ.jpg&quot; alt=&quot;Elevation map of Mars&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or the topographical map of Venus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/hEPpC.jpg&quot; alt=&quot;Elevation map of Venus&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These elevation maps both use surface elevation relative to some reference zero elevation. How are these defined on celestial bodies with no atmosphere (e.g. the Moon), bodies with no large bodies of water or oceans of other liquids (e.g. Mars), or both?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, how do we define depth for gas giants like Jupiter and Saturn? Or even the Sun? We would also have to define some reference point first, no? Who decides where that reference point is and what methods are employed to determine it?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-07T09:26:12.447" LastActivityDate="2014-03-07T09:26:12.447" Title="How do we measure height on celestial bodies with no significant atmospheric pressure, no seas, or no surface to speak of?" Tags="|observation|measurement|standards|mapping|topography|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1705" PostTypeId="2" ParentId="1704" CreationDate="2013-08-31T18:57:18.510" Score="2" Body="&lt;p&gt;The reference datum is usually chosen to be about the average altitude. For example, on Mars, the datum, known as the Mars areoid, is very close to the average radius of Mars, as measured round the equator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(It was defined by the height at which the pressure corresponds to the triple point of water.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality it wouldn't matter very much what the reference was, as long as all interested parties agree on it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Still trying to find out who made the decisions- for Mars it appears to have been defined by NASA, but I can't find definite evidence)&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-08-31T18:57:18.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1706" PostTypeId="2" ParentId="1105" CreationDate="2013-08-31T19:16:56.263" Score="6" Body="&lt;p&gt;There is an international organisation of amateur radio societies from the ISS partner countries with a volunteer working group devoted to developing and putting into operation the ISS amateur radio station, and they go by the name &lt;a href=&quot;http://www.ariss-eu.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARISS&lt;/a&gt; (Amateur Radio on the International Space Station).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ARISS program is a cooperative venture of NASA, the &lt;a href=&quot;http://www.arrl.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARRL&lt;/a&gt; and &lt;a href=&quot;http://www.amsat.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMSAT&lt;/a&gt;&#xA;  and other international space agencies that organizes scheduled&#xA;  contacts via Amateur Radio between astronauts and cosmonauts aboard&#xA;  the ISS and classrooms and communities. With the help of experienced&#xA;  Amateur Radio volunteers from Amateur Radio clubs, and coordination&#xA;  from the ARISS Team, the ISS crewmembers speak directly with large&#xA;  group audiences in a variety of public forums such as school&#xA;  assemblies or at science museums, Scout camporees and jamborees and&#xA;  space camps, where students, teachers, parents, and communities learn&#xA;  about space and space technologies and  Amateur Radio.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Id6dJ.jpg&quot; alt=&quot;ARISS Live&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About pre-published timetables however, and according to &lt;a href=&quot;http://www.nasa.gov/audience/foreducators/teachingfromspace/students/ariss.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA page on ARISS&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;ARISS contacts are typically not a good fit for events that take place&#xA;  on a specific date. ARISS contacts are subject to real-time mission&#xA;  operations and often move around on the schedule. Host organizations&#xA;  must be flexible and prepared to reschedule their ARISS event.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But &lt;strong&gt;there are occasional announcements of scheduled amateur radio contacts with the ISS&lt;/strong&gt;, either on &lt;a href=&quot;http://www.ariss-eu.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARISS international&lt;/a&gt;, on the individual nations' ARISS pages, or other pages of other associated members. Not in any particular order, some of these are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Belgium: &lt;a href=&quot;http://www.amsat-on.be&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMSAT-Belgium&lt;/a&gt; and &lt;a href=&quot;http://www.uba.be&quot; rel=&quot;nofollow noreferrer&quot;&gt;UBA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Portugal: &lt;a href=&quot;http://www.amrad.pt/ariss.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMSAT-CT&lt;/a&gt; and &lt;a href=&quot;http://www.rep.pt&quot; rel=&quot;nofollow noreferrer&quot;&gt;REP&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Italy: &lt;a href=&quot;http://www.amsat.it/&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMSAT-Italy&lt;/a&gt; and &lt;a href=&quot;http://www.ari.it&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARI&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Sweden: &lt;a href=&quot;http://www.amsat.se&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMSAT-SM&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;United Kingdom: &lt;a href=&quot;http://www.uk.amsat.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMSAT-UK&lt;/a&gt; and &lt;a href=&quot;http://www.rsgb.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;RSGB&lt;/a&gt;    &lt;/li&gt;&#xA;&lt;li&gt;Germany: &lt;a href=&quot;http://www.darc.de&quot; rel=&quot;nofollow noreferrer&quot;&gt;DARC&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Malta: &lt;a href=&quot;http://www.9h1mrl.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;MARL&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Poland: &lt;a href=&quot;http://www.pzk.org.pl&quot; rel=&quot;nofollow noreferrer&quot;&gt;PZK&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Lebanon: &lt;a href=&quot;http://ral.org.lb&quot; rel=&quot;nofollow noreferrer&quot;&gt;RAL&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;France: &lt;a href=&quot;http://www.ref-union.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;REF-Union&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Switzerland: &lt;a href=&quot;http://www.uska.ch&quot; rel=&quot;nofollow noreferrer&quot;&gt;USKA&lt;/a&gt;      &lt;/li&gt;&#xA;&lt;li&gt;The Netherlands: &lt;a href=&quot;http://www.veron.nl&quot; rel=&quot;nofollow noreferrer&quot;&gt;VERON&lt;/a&gt;     &lt;/li&gt;&#xA;&lt;li&gt;Japan: &lt;a href=&quot;http://www.jarl.or.jp/ariss/&quot; rel=&quot;nofollow noreferrer&quot;&gt;JARL&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For example, following these pages around a bit more, I was able to find a &lt;a href=&quot;http://www.amsat.org/amsat/ariss/news/arissnews.rtf&quot; rel=&quot;nofollow noreferrer&quot;&gt;link to a published RTF document on Upcoming ARISS Contact Schedule (as of 2013-08-30 22:00 UTC)&lt;/a&gt; that is still somewhat current as of the time and date of this answer:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Quick list of scheduled contacts and events:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;&lt;strong&gt;Amicale Space Camp&lt;/strong&gt; organized by Sterrenlab for children of the European Patent Office/NL, Leiden, The Netherlands (Summer Space Camp,&#xA;  Noordwijk, Netherlands), Netherlands, via PA3GUO Contact was&#xA;  successful: Fri 2013-08-30 12:11:43 UTC 55 deg&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;&lt;strong&gt;Gwalior Glory High School, Gwalior, Madhya Pradesh, India&lt;/strong&gt;, telebridge via IK1SLD Contact is a go for: Sat 2013-08-31 11:22:58 UTC&#xA;  43 deg&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;&lt;strong&gt;Mill Springs Academy, Alpharetta, GA&lt;/strong&gt;, direct via KK4OVR Contact is a go for: Wed 2013-09-04 14:15:45 UTC 40 deg&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;&lt;strong&gt;Duluth Children's Museum, Duluth, MN&lt;/strong&gt;, direct via WØGKP Contact is a go for: Sat 2013-09-07 15:03:34 UTC 67 deg&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;ARISS is requesting listener reports for the above contacts.  Due to&#xA;  issues with the Kenwood radio that are not fully understood at&#xA;  present, the Ericsson radio is going to be used for these contacts. &#xA;  ARISS thanks everyone in advance for their assistance.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More information and a lot of useful links are &lt;a href=&quot;http://www.amsat.org/amsat/ariss/news/arissnews.rtf&quot; rel=&quot;nofollow noreferrer&quot;&gt;in the mentioned document&lt;/a&gt; and on the &lt;a href=&quot;http://www.arrl.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;American Radio Relay League&lt;/a&gt; (ARRL) &lt;a href=&quot;http://www.arrl.org/amateur-radio-on-the-international-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;page on Amateur Radio on the International Space Station&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in short, following ARISS contacts and pre-announced radio contact schedules with the ISS is a bit tricky for non-member of any of these organisations listed, but still possible. I imagine schools that are members of any of these amateur radio organisations receive notifications also through newsletters they're subscribed to, or other channels.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T20:39:06.353" LastActivityDate="2013-09-10T20:39:06.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1707" PostTypeId="1" AcceptedAnswerId="1708" CreationDate="2013-08-31T22:57:48.420" Score="14" ViewCount="12387" Body="&lt;p&gt;Are surface-scanning satellites able to show certain parts of Earth's surface real-time? For example, to follow a person who's escaping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Somewhere I heard that these satellites are able to make pictures only, but unable to make constant video stream. In fact, 20-30 pictures per seconds might be enough to create the feel of moving.&lt;/p&gt;&#xA;" OwnerUserId="306" LastActivityDate="2021-12-17T01:20:56.407" Title="Are satellites able to stream real-time videos about certain parts of Earth's surface?" Tags="|artificial-satellite|data-transmission|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1708" PostTypeId="2" ParentId="1707" CreationDate="2013-08-31T23:43:21.403" Score="17" Body="&lt;p&gt;Satellites are nowadays definitely not bandwidth starved, considering we've just had a few launches in the past month orbiting satellites capable of relaying live telecasts even in quad HD resolution. But the problem with tracking someone on the ground in real-time is more one of radial resolution, than the communications capabilities of satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You see, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;noreferrer&quot;&gt;Geostationary Orbit&lt;/a&gt; (GEO) is at an altitude of roughly 35,786 kilometres (22,236 mi) above the Earth's equator, so that's quite a long distance and you'd need a telescope in the range of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt; to see much detail on the ground. Obviously, we're talking of a huge mass satellite then, which doesn't make the task of orbiting at least 3 of them any easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/tZyJL.png&quot; alt=&quot;GEO&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So alternatively, you could use a cluster of satellites that orbit at a lot lower altitudes in &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;Low Earth Orbit&lt;/a&gt;, say around 500 km above the Earth's surface. Their radial resolution could be a lot better even by using fairly small (in relative terms) lenses, but they will pass one specific point on the surface of the Earth a lot faster, so you'd want to move the task of tracking a single point from one satellite to another to keep your target in their line of sight. They would also have to move the lense constantly, to adjust for the fast changing angle of a relatively stationary target on the surface of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ZcgPe.png&quot; alt=&quot;LEO&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer to your question is not quite as straightforward as you might have hoped it would be. It is possible, but would require either a huge telescope-like tracking satellite in the GEO (and even then the object you're tracking has to be observable from the position the satellite is), or you will have to delegate this task of tracking an object on the surface of the Earth in real-time to several satellites that follow each other's orbit at regular intervals, enabling covering of a single point on the Earth at all times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But regardless if you're using a few huge GEO satellites, or a large number of smaller LEO ones, they still can't make clouds move away from being over your target you're tracking. You could of course use different electromagnetic wavelengths than those in visible light spectrum, and hope they penetrate lower atmosphere weather formations easier, but it wouldn't really look like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orthophoto&quot; rel=&quot;noreferrer&quot;&gt;orthophoto&lt;/a&gt; images on Google Maps, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the most part, it's just easier, faster and cheaper to deploy &lt;a href=&quot;https://en.wikipedia.org/wiki/Reconnaissance_aircraft&quot; rel=&quot;noreferrer&quot;&gt;reconnaissance airplanes&lt;/a&gt; over your target, or if you're on a tight budget, maybe paint a &lt;a href=&quot;https://en.wikipedia.org/wiki/Blimp&quot; rel=&quot;noreferrer&quot;&gt;blimp&lt;/a&gt; with some advert nobody cares to give attention to, and have your cameras hover a lot closer to the target you'd like to track and avod the cloud cover. But it is certainly not impossible to reconfigure a large array of satellites &lt;em&gt;on the fly&lt;/em&gt;, although you better hope your boss didn't want you to read your target's newspaper through the clouds, or at an angle with direct line of sight.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-01T16:04:48.950" LastActivityDate="2013-09-01T16:04:48.950" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1709" PostTypeId="2" ParentId="1704" CreationDate="2013-09-01T01:53:40.193" Score="6" Body="&lt;p&gt;For Mars, the current definition of 0 km is derived from data from the Mars Orbiting Laser Altimeter (MOLA) data from Mars Global Surveyor.  In fact the altitude reference is referred to as &quot;MOLA altitude&quot;.  You would say for example: &quot;minus 1.4 km MOLA&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www-geodyn.mit.edu/mola.summary.pdf&quot; rel=&quot;nofollow&quot;&gt;the paper&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Zero elevation on Mars from MOLA is defined as the equipotential&#xA;  surface (gravitational plus rotational) whose average value at the&#xA;  equator is equal to the mean radius as determined by MOLA (cf. Table4).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also recognizing the established practice and officially setting the standard in &lt;a href=&quot;http://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2009reprint.pdf&quot; rel=&quot;nofollow&quot;&gt;this IAU paper&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The topographic reference surface of Mars is that specified in the&#xA;  final MOLA Mission Experiment Gridded Data Record (MEGDR) Products&#xA;  (Smith et al. 2003). In particular, the 128 pixels/◦ resolution,&#xA;  radius and topographic surfaces are recommended, although the lower&#xA;  resolution versions may be used where appropriate and documented, and&#xA;  for the areas poleward of ±88◦ latitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There was an older definition used for Mariner 9 and Viking data using the triple point of water as the definition for 0 km.  There was a short period of annoyance in converting from the old to the new system, since they were about 1.6 km apart from each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that even on Earth, we don't really use sea level anymore.  We use a reference datum that does not change with time, which was based on some average sea level measurement.  So if sea level rises, which in fact it's doing, the height of Mount Everest will not change.  (Except of course that Everest is still rising very slowly due to tectonic forces.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Venus, the planet is sufficiently spherical and has such a low rotation rate, and since the topographic data is of relatively coarse resolution, a sphere whose radius is the mean radius of the planet, 6051.881 km, is used.  See &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/18052/1/99-1509.pdf&quot; rel=&quot;nofollow&quot;&gt;this paper&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At Jupiter and the other gas giants, 0 km is defined as the radius of 1 bar pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;surface&quot; of the Sun is defined to be the uppermost visible layer, the top of the photosphere.  Layers above that, referred to as the Sun's &quot;atmosphere&quot;, are too thin or cool to be seen by the naked eye.  (Which shouldn't be looking at the Sun by the way.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-02T15:54:47.320" LastActivityDate="2013-09-02T15:54:47.320" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1710" PostTypeId="2" ParentId="1704" CreationDate="2013-09-01T02:37:13.117" Score="17" Body="&lt;p&gt;&lt;strong&gt;Introduction to selecting a reference surface&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The surface of any celestial body can be anything but uniform. The oceans, where existing, can be treated as reasonably uniform, but the surface or topography of the land masses can exhibit large vertical variations between mountains and valleys. These variations make it impossible to approximate the shape of the celestial body with any reasonably simple mathematical model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case of the Earth, two main reference surfaces have been established to approximate the shape of it. One reference surface is called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geoid&quot; rel=&quot;noreferrer&quot;&gt;geoid&lt;/a&gt;, the other reference surface is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Reference_ellipsoid&quot; rel=&quot;noreferrer&quot;&gt;ellipsoid&lt;/a&gt;. These are illustrated in the figure below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/td4kL.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The body's surface, and two reference surfaces used to approximate it: the geoid, and a reference ellipsoid.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Leaving further clarification to a few short excerpts from Wikipedia pages on &lt;a href=&quot;https://en.wikipedia.org/wiki/Geoid&quot; rel=&quot;noreferrer&quot;&gt;geoid&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Reference_ellipsoid&quot; rel=&quot;noreferrer&quot;&gt;reference ellipsoid&lt;/a&gt;, we get these descriptions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;The geoid is the shape that the surface of the oceans would take under the influence of Earth's gravity and rotation alone, in the&#xA;  absence of other influences such as winds and tides. All points on&#xA;  that surface have the same scalar potential—there is no difference in&#xA;  potential energy between any two.&lt;/li&gt;&#xA;  &lt;li&gt;In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or&#xA;  other planetary body. Because of their relative simplicity, reference&#xA;  ellipsoids are used as a preferred surface on which geodetic network&#xA;  computations are performed and point coordinates such as latitude,&#xA;  longitude, and elevation are defined.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So we have defined two possible candidates for how to establish solid body reference surface, or as it's sometimes called, particularly when in reference to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sea_level&quot; rel=&quot;noreferrer&quot;&gt;mean sea levels&lt;/a&gt;, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geodetic_datum&quot; rel=&quot;noreferrer&quot;&gt;vertical datum&lt;/a&gt;. For establishing the reference surface by using the ellipsoid method, this can be further differentiated to those measured over a local, or global area of the celestial body's surface:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/JPXAf.png&quot; alt=&quot;Reference surface methods&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The Geoid, a globally best fitting ellipsoid, and a regionally or locally best fitting ellipsoid, for a chosen region.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;OK, so we have established possible methods, possible areas over which they apply, and we've also learned a bit on the terminology used. But how does this translate to actual uses on celestial bodies of our Solar system? This is what I was able to find out;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Zero elevation on Mars&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Since Mars has no oceans and hence no 'sea level', it is convenient to&#xA;  define an arbitrary zero-elevation level or &quot;datum&quot; for mapping the&#xA;  surface. The datum for Mars is arbitrarily defined in terms of a&#xA;  constant atmospheric pressure. During the Mariner 9 mission, this was&#xA;  chosen as 610.5 Pa (6.105 mbar), on the basis that below this pressure&#xA;  liquid water can never be stable (i.e., the triple point of water is&#xA;  at this pressure). This value is only 0.6% of the pressure at sea&#xA;  level on Earth, which forms the zero elevation datum for our planet.&#xA;  Note that the choice of this value does not mean that liquid water&#xA;  does exist below this elevation, just that it could were the&#xA;  temperature to exceed 273.16 K.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eIzRM.jpg&quot; alt=&quot;Mars elevation map&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Quoted excerpt source and Mars elevation map above: &lt;a href=&quot;https://en.wikipedia.org/wiki/Geography_of_Mars#Zero_elevation&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Geography of Mars&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Wikipedia is however a bit outdated and more recently, the data from the &lt;a href=&quot;http://mola.gsfc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Mars Orbiting Laser Altimeter&lt;/a&gt; (MOLA) of the NASA's Mars Global Surveyor and a spherical-harmonic representation of an equipotential surface is recommended to be used as the reference for elevations by the Mars geodesy / cartography working group. MOLA transmits infrared laser pulses towards the Martian surface at a rate of 10 times per second, to produce a highly detailed map of the surface. This is an excerpt from &lt;a href=&quot;http://astropedia.astrogeology.usgs.gov/alfresco/d/d/workspace/SpacesStore/cff0ec29-ce29-4c39-a361-1500af5b696b/Duxbury.pdf&quot; rel=&quot;noreferrer&quot;&gt;Mars geodesy / cartography working group &lt;strong&gt;recommendations&lt;/strong&gt; on Mars cartographic constants and coordinate systems&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;MOLA topographic model will be used for accurate projection  of images&#xA;  and other remote-sensing data onto the planet, and a &#xA;  spherical-harmonic representation of an equipotential surface  will be&#xA;  used as the reference for elevations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/5JLDX.jpg&quot; alt=&quot;Mars MOLA Tracks&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Mars Orbiting Laser Altimeter (MOLA) Tracks&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Zero elevation on the Moon&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to dr. Simon O'Toole from the &lt;a href=&quot;http://www.aao.gov.au/&quot; rel=&quot;noreferrer&quot;&gt;Australian Astronomical Observatory&lt;/a&gt;, a similar instrument to the one employed to map elevation of Mars was used by NASA's &lt;a href=&quot;http://lro.gsfc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Lunar Reconnaissance Orbiter&lt;/a&gt; spacecraft to provide a precise topographic model of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because it has no ocean or significant atmosphere, the zero elevation point of the Moon is the average diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Qe1Lj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Colored global elevation map based on terrain data from the &lt;a href=&quot;https://en.wikipedia.org/wiki/SELENE&quot; rel=&quot;noreferrer&quot;&gt;SELENE / Kaguya orbiter&lt;/a&gt;, Japan Aerospace Exploration Agency (Source: &lt;a href=&quot;http://astrogeology.usgs.gov/maps/control-networks/mars&quot; rel=&quot;noreferrer&quot;&gt;USGS&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://pubs.usgs.gov/of/2006/1367/&quot; rel=&quot;noreferrer&quot;&gt;The Unified Lunar Control Network 2005&lt;/a&gt; article (PDF) published by USGS (U.S. Department of the Interior U.S. Geological Survey):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This report documents a new general unified lunar control network and&#xA;  lunar topographic model based on a combination of Clementine images&#xA;  and a previous network derived from Earth  based and Apollo&#xA;  photographs, and Mariner 10 and Galileo images. This photogrammetric&#xA;  network solution is the largest planetary control network ever&#xA;  completed. It includes the determination of the 3-D positions of&#xA;  272,931 points on the lunar surface and the correction of the camera&#xA;  angles for 43,866 Clementine images, using 546,126 tie point&#xA;  measurements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Reference surface area of gas giants&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a fair bit of digging around, I have finally stumbled upon one explanation to where the reference surface of gas giants is measured from, according to which we can actually measure depth and heights of clouds and other weather phenomena forming on them, how deep our atmospheric probes have penetrated these planets before they finally gave in and imploded, and similar. In a &lt;a href=&quot;http://www.iau.org/&quot; rel=&quot;noreferrer&quot;&gt;IAU&lt;/a&gt; (International Astronomical Union) / &lt;a href=&quot;http://www.iag-aig.org/&quot; rel=&quot;noreferrer&quot;&gt;IAG&lt;/a&gt; (International Association of Geodesy) working group &lt;a href=&quot;http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA471037&quot; rel=&quot;noreferrer&quot;&gt;report on cartographic coordinates and rotational elements from 2006&lt;/a&gt; (PDF), this quote can be found:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The radii and axes of the large gaseous planets, Jupiter, Saturn,&#xA;  Uranus, and Neptune in Table 4 refer to a one-bar-pressure surface.&#xA;  The radii given in the tables are not necessarily the appropriate&#xA;  values to be used in dynamical studies; the radius actually used to&#xA;  derive a value of J&lt;sub&gt;2&lt;/sub&gt; (for example) should always be used in&#xA;  conjunction with it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The table it refers to doesn't really matter for us, but the definition of the reference surface area does. This means that, by international standards these two bodies (IAU/IAG) represent, the reference surface area is defined by the point at one-bar-pressure of their atmosphere. This same paper gives us another clue:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Reference surface area of minor planets and comets&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For irregularly shaped bodies the ellipsoid is obviously useless,&#xA;  except perhaps for dynamical studies. For very irregular bodies, the&#xA;  concept of a reference ellipsoid ceases to be useful for most&#xA;  purposes. For these bodies, topographic shapes are usually represented&#xA;  by a grid of radii to the surface as a function of planetocentric&#xA;  latitude and longitude (when possible, or also by a set of vertices&#xA;  and polygons).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Another problem with small bodies is that two coordinates (i.e.&#xA;  spherical angular measures) may not uniquely identify a point on the&#xA;  surface of the body. In other words it is possible to have a line from&#xA;  the center of the object intersect the surface more than once.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It goes on to describe examples and cartographers' &lt;em&gt;ad hoc&lt;/em&gt; tricks for specific maps, and later gives a recommendation that longitudes on minor planets and comets should be measured positively from 0 to 360 degrees using a righthand system from a designated prime meridian, where the origin is the center of mass, to the extent known.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;As for the international authoritative bodies defining standards of measure for the reference surface areas of celestial bodies, it appears &lt;a href=&quot;http://www.iau.org/&quot; rel=&quot;noreferrer&quot;&gt;IAU&lt;/a&gt; (International Astronomical Union) and &lt;a href=&quot;http://www.iag-aig.org/&quot; rel=&quot;noreferrer&quot;&gt;IAG&lt;/a&gt; (International Association of Geodesy) are as good as it gets, and who's to argue that?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-02T13:22:23.983" LastActivityDate="2013-09-02T13:22:23.983" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1711" PostTypeId="2" ParentId="1703" CreationDate="2013-09-01T06:10:18.840" Score="11" Body="&lt;p&gt;&lt;strong&gt;YES&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Cosmic dust is a real threat to satellite that is operated over a long period of time.&lt;/p&gt;&#xA;&lt;p&gt;Dust particles vary in size from few molecules to 0.1 micrometer, but sometimes they get ionized by the Sun's radiation or by cosmic rays.&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://news.stanford.edu/news/2013/february/space-dust-threat-022613.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this test report about space dust threat&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;Since they travel at high velocity they get converted to ions (quasi neutral), commonly called plasma, which could interfere with the radio waves of the satellite or even have enough energy to create a radio wave so that it would shut down a satellite.&lt;/p&gt;&#xA;&lt;p&gt;So &lt;strong&gt;DOES plasma emit a radio waves? If so, what frequency and with what power?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;the researchers fired tiny dust particles at targets resembling satellites at a speed of 60 kilometre per second  &amp;quot;we found that when these particles hit, they create a plasma or quasi-neutral gas of ions and electron and that plasma can then emit in the radio frequency range&amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There are many satellites that failed electronically one of them is the European Space Agency's &lt;strong&gt;OLYMPUS communication satellite&lt;/strong&gt; in 1993.&lt;/p&gt;&#xA;&lt;p&gt;AND also it causes the deceleration of the satellites which lead to decrease in orbital velocity and their orbit radius decrease and then enter the Earth's atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;How does plasma interact with the electromagnetic waves?&lt;/p&gt;&#xA;&lt;p&gt;Plasma is &lt;strong&gt;electrically conductive&lt;/strong&gt;, so it strongly interacts with the EM rays.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Plasma_stealth&quot; rel=&quot;nofollow noreferrer&quot;&gt;PLASMA STEALTH&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;a proposed process to use ionized gas (plasma) to reduce the radar cross-section (RCS) of an aircraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There are many &lt;a href=&quot;http://oai.dtic.mil/oai/oai?verb=getRecord&amp;amp;metadataPrefix=html&amp;amp;identifier=AD0405535&quot; rel=&quot;nofollow noreferrer&quot;&gt;ways to reduce the effect of plasma&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;reduce the radar's cross section;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;use of dielectric plasma shell;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;have a conductive surface coat.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastEditorUserId="47149" LastEditDate="2022-05-23T21:24:06.577" LastActivityDate="2022-05-23T21:24:06.577" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="1712" PostTypeId="1" AcceptedAnswerId="1765" CreationDate="2013-09-01T09:33:36.167" Score="13" ViewCount="3654" Body="&lt;p&gt;Is it possible to use &lt;a href=&quot;http://en.wikipedia.org/wiki/Mechanical_pencil&quot;&gt;mechanical pencil&lt;/a&gt; instead of space pen and pencil in International Space Station ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-09-04T23:01:06.187" Title="is it possible to use mechanical pencil instead of space pen in ISS?" Tags="|iss|astronaut-lifestyle|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1713" PostTypeId="2" ParentId="1712" CreationDate="2013-09-01T09:40:39.227" Score="11" Body="&lt;p&gt;Not only is it possible, it was so done. Mechanical pencils were used earlier. They were phased out because of the danger they posed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Pen&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA programs previously used pencils (for example a 1965 order of mechanical pencils) but because of the substantial dangers that broken-off pencil tips and graphite dust pose in zero gravity to electronics and the flammable nature of the wood present in pencils,a better solution was needed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user12" LastEditDate="2013-09-04T23:01:06.187" LastActivityDate="2013-09-04T23:01:06.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1714" PostTypeId="2" ParentId="1712" CreationDate="2013-09-01T11:53:22.867" Score="11" Body="&lt;p&gt;Yes, it is &lt;em&gt;possible&lt;/em&gt;. In fact, it's a better solution except for graphite getting into expensive things that don't like graphite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we think about how a mechanical pencil works, it doesn't involve gravity at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z2Ep5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;From &lt;a href=&quot;http://i1.squidoocdn.com/resize/squidoo_images/800/draft_lens11858861module108380201photo_1277951447staedtler_diag1.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can kind of tell from the diagram, the only force that need be involved is that of the top button being depressed. The pencil is built in such a way that a clamp-like thing holds the lead in place when the pencil is in use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the button is depressed, the clamp releases the lead, and the lead is pushed down by another mechanism. After this, the clamp grabs the lead again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, gravity is not needed to operate a mechanical pencil, and a standard pencil from Stap-o-les or Wally-Mart would work just fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are concerns about the fine column of lead inside the pencil. If you've ever used a mechanical pencil (or a normal pencil, for that matter), you know that it's fairly easy to break the lead off the tip of the pencil. When this happens, the lead seems to fly at supersonic speeds, and hurts like crazy if it hits you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a huge problem in space - since there is no downward force to pull the lead to the floor, it will just keep bouncing around the station until it finds a home. Unfortunately, this home would more often than not be in a piece of rare and expensive equipment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So yes, it's certainly mechanically possible, but there are risks regarding the lead shooting off into something important.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-09-01T11:53:22.867" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="1715" PostTypeId="5" CreationDate="2013-09-01T11:59:40.920" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-01T11:59:40.920" LastActivityDate="2013-09-01T11:59:40.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1716" PostTypeId="4" CreationDate="2013-09-01T11:59:40.920" Score="0" Body="Questions regarding the use of an object's gravity to change the velocity of a spaceship." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-01T13:11:54.213" LastActivityDate="2013-09-01T13:11:54.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1717" PostTypeId="5" CreationDate="2013-09-01T12:04:37.360" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-01T12:04:37.360" LastActivityDate="2013-09-01T12:04:37.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1718" PostTypeId="4" CreationDate="2013-09-01T12:04:37.360" Score="0" Body="Questions regarding the study of life outside of Earth." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-01T13:12:02.137" LastActivityDate="2013-09-01T13:12:02.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1719" PostTypeId="5" CreationDate="2013-09-01T18:11:38.030" Score="0" Body="&lt;p&gt;All missions are limited by the amount of energy available to the vehicle, and mission instruments. This is particularly acute in spacecraft because of the large amount of energy needed to make it to the edge of Earth's atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources of energy may be &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Chemical E.g. Battery&lt;/li&gt;&#xA;&lt;li&gt;Nuclear E.g. RTGS&lt;/li&gt;&#xA;&lt;li&gt;Solar E.g. Photovoltaics&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;When a question or it's description addresses or attempts to address acquiring/fulfilling the energy requirements of &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mission propulsion&lt;/li&gt;&#xA;&lt;li&gt;Mission instruments &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;it may be suitable for this tag&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-09-01T18:47:17.003" LastActivityDate="2013-09-01T18:47:17.003" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1720" PostTypeId="4" CreationDate="2013-09-01T18:11:38.030" Score="0" Body="Questions addressing the energy requirement for space/orbit/interplanetary missions." OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-09-01T18:47:21.127" LastActivityDate="2013-09-01T18:47:21.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1721" PostTypeId="1" AcceptedAnswerId="1724" CreationDate="2013-09-02T05:23:04.043" Score="13" ViewCount="3068" Body="&lt;p&gt;Known facts: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The lunar surface is covered by &lt;a href=&quot;http://en.wikipedia.org/wiki/Regolith&quot;&gt;regolith&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Solar wind imparts a charge to the lunar surface&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_regolith&quot;&gt;Wikipedia&lt;/a&gt; says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the daylit side of the Moon, solar ultraviolet and X-ray radiation is energetic enough to knock electrons out of atoms and molecules in the lunar soil&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same article further states&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the night side, the dust is negatively charged by electrons in the solar wind. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Given both sides of Luna acquire an electrical charge, &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Was the magnitude of the acquired charge on either side ever quantified? &lt;/li&gt;&#xA;&lt;li&gt;What is the average charge magnitude on either side of the lunar soil?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user12" LastEditDate="2013-09-02T12:08:11.093" LastActivityDate="2013-09-02T22:28:56.497" Title="What is the average electrical charge on the lunar surface?" Tags="|the-moon|solar-wind|regolith|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1722" PostTypeId="2" ParentId="1707" CreationDate="2013-09-02T09:46:33.390" Score="9" Body="&lt;h1&gt;Short Answer&lt;/h1&gt;&#xA;&lt;p&gt;Yes.&lt;/p&gt;&#xA;&lt;h1&gt;Theory Answer&lt;/h1&gt;&#xA;&lt;p&gt;Satellite Television is broadcast at full 1080P high definition (1980x1080 px + sound + Closed Captioning + Program Metadata) data rates with many channels per satellite — 50+ — and is also receiving at a slightly higher rate (Data In = Data Out + control data on a separate channel).&lt;/p&gt;&#xA;&lt;p&gt;Likewise, HDTV quality signal is well within the bounds of fielded space-rated CCD  video cameras.&lt;/p&gt;&#xA;&lt;p&gt;Both can be operated on reasonable solar panel loads.&lt;/p&gt;&#xA;&lt;h1&gt;Practical Factor Answer&lt;/h1&gt;&#xA;&lt;p&gt;But, theory aside, It has been done in the past.&lt;/p&gt;&#xA;&lt;p&gt;The Space Shuttle has often broadcast live NTSC feeds to earth during it's operational tenure. Most, but not all, have been crew interviews. Some have been observations using outboard cameras, including the live feed of the Hubble Repair.&lt;/p&gt;&#xA;&lt;p&gt;By simply using a different lens set, namely changing to a narrow field telephoto lens, a mounted camera could be pointed at specific features on planet and used to provide live surveillance.&lt;/p&gt;&#xA;&lt;h1&gt;And History leading back to more Theory&lt;/h1&gt;&#xA;&lt;p&gt;Note that such cameras are a &amp;quot;open secret&amp;quot; - Military satelite video has been released showing roughly 0.5m per pixel NTSC video (29.97FPS with roughly 720x480) since the early 1990's. Given the slightly better than 0.1m resolution of civilian mapping photography (EG: Google Earth), it shows that the cameras can physically resolve that, so it's widely supposed that 0.1m per pixel video feed is within the capabilities of military spy satellites. And given that the broadcast rate is capable of far more, it's unlikely that the military does not have at least that level of capability.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-02T09:46:33.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1723" PostTypeId="1" CreationDate="2013-09-02T13:22:06.780" Score="14" ViewCount="566" Body="&lt;p&gt;It seems that if we had a lunar base going then we would be more likely to invent things that would improve space travel.  It also seems that private industry is faster at developing these improvements than if we leave it to governments alone.  So is there any commercially exploitable resources we know about on the moon that could be commercially viable as an industry?&lt;/p&gt;&#xA;" OwnerUserId="123" LastEditorUserId="6816" LastEditDate="2017-01-03T23:24:50.583" LastActivityDate="2017-01-03T23:24:50.583" Title="Are there any resources available on the moon that could make a base commercially viable?" Tags="|colonization|lunar-landing|commercial|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1724" PostTypeId="2" ParentId="1721" CreationDate="2013-09-02T13:29:22.440" Score="10" Body="&lt;p&gt;Yes, the &lt;a href=&quot;http://artemis.ssl.berkeley.edu/overview.shtml&quot;&gt;ARTEMIS&lt;/a&gt; satellite monitors the lunar surface (a &lt;a href=&quot;http://artemis.ssl.berkeley.edu/summary.shtml?autoload=1&quot;&gt;link to mission data&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lunar surface gets ionized by the plasma, cosmic rays, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot;&gt;coronal mass ejections&lt;/a&gt; from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar System Exploration Division, NASA Goddard Space Flight Center and &#xA;Space Sciences Laboratory, University of California paper on &lt;a href=&quot;http://sprg.ssl.berkeley.edu/adminstuff/webpubs/2008_lpsc_1365.pdf&quot;&gt;Lunar surface charging: Magnitude and implications as a function of space and time&lt;/a&gt; (PDF) suggests that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Theoretically, the Moon should charge to small positive values of ~+5-10V on the sunlit hemisphere (where photoemission dominates), and to larger negative values of ~-100V on the shad-owed hemisphere (where photoemission is absent)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Another paper on &lt;a href=&quot;http://sprg.ssl.berkeley.edu/adminstuff/webpubs/2007_workshop_181.pdf&quot;&gt;Lunar surface charging: A global perspective using Lunar Prospector data&lt;/a&gt; by that same group of researches states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are significant uncertainties in lunar surface charging&#xA;  processes, and very little is known about either spatial or temporal&#xA;  values.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dKSQY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-02T22:28:56.497" LastActivityDate="2013-09-02T22:28:56.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1725" PostTypeId="1" AcceptedAnswerId="1728" CreationDate="2013-09-02T13:43:41.467" Score="8" ViewCount="1304" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1721/what-is-the-average-electrical-charge-on-the-lunar-surface&quot;&gt;This question&lt;/a&gt; discusses the charges that exist on the Lunar surface. Would it be possible or practicable to use the differential between the surface and subsurface or different locations on the surface to power a station there?&lt;/p&gt;&#xA;" OwnerUserId="79" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2018-11-20T18:21:08.120" Title="Would it be practical to use the electrical charge of the Lunar surface as an energy source?" Tags="|the-moon|power|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1726" PostTypeId="5" CreationDate="2013-09-02T13:45:32.043" Score="0" Body="&lt;p&gt;Luna is Earth's closest celestial neighbor. This makes her the most approachable for study, and for verification of science &amp;amp; technology for missions further from Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To date, Moon landings appear on the agenda of several space-capable nations/organizations (USA, USSR/Russia, China, India)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tag may be applied when a question seeks to enquire about a mission (past, present, future) to the Lunar surface&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-09-02T13:46:56.850" LastActivityDate="2013-09-02T13:46:56.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1727" PostTypeId="4" CreationDate="2013-09-02T13:45:32.043" Score="0" Body="Questions on the subject of missions to the Lunar surface - past, present, and future" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-09-02T13:47:07.600" LastActivityDate="2013-09-02T13:47:07.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1728" PostTypeId="2" ParentId="1725" CreationDate="2013-09-02T15:28:44.807" Score="5" Body="&lt;p&gt;While you may have a static charge difference between night and day sides, 100V is not going to be that useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't connect them and use the moon as a battery, because it is a static charge- it will not flow. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your better option would be solar power, as there is no atmosphere to absorb the Sun's rays. &lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-09-02T15:28:44.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1729" PostTypeId="2" ParentId="1725" CreationDate="2013-09-02T15:29:14.317" Score="2" Body="&lt;p&gt;&lt;strong&gt;It is possible to use the electrostatic  charge on the lunar surface to generate the electricity&lt;/strong&gt;, but it's only theoretical. One such study was done by the NASA Langley Research Center and the National Institute of Aerospace, and presented in their paper on &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100032922_2010034552.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Electrostatic Power Generation from Negatively Charged, Simulated Lunar Regolith&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The electrostatic power generator was devised to alleviate the&#xA;  hazardous effects of negatively charged lunar soil by neutralizing the&#xA;  charged particles through capacitive coupling and thereby&#xA;  simultaneously harnessing power through electric charging . The amount&#xA;  of power generated or collected is dependent on the areal coverage of&#xA;  the device and hovering speed over the lunar soil surface. A thin-film&#xA;  array of capacitors can be continuously charged and sequentially&#xA;  discharged using a time-differentiated trigger discharge process to&#xA;  produce a pulse train of discharge for DC mode output. By controlling&#xA;  the pulse interval, the DC mode power can be modulated for powering&#xA;  devices and equipment. In conjunction with a power storage system, the&#xA;  electrostatic power generator can be a power source for a lunar rover&#xA;  or other systems. The negatively charged lunar soil would also be&#xA;  neutralized mitigating some of the adverse effects resulting from&#xA;  lunar dust.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vM5bQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-02T22:10:15.253" LastActivityDate="2013-09-02T22:10:15.253" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1730" PostTypeId="1" AcceptedAnswerId="1733" CreationDate="2013-09-02T16:09:27.227" Score="6" ViewCount="273" Body="&lt;p&gt;On Earth, our temperature scales are calibrated based on some universal (on Earth) values. For example, 212°F and 100°C are defined as the temperature water boils at sea level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How are these calibration values decided on other planets? Many planets don't have a 'sea level' and if they did, atmospheric pressure would still be different. How would thermometers be calibrated on other planets?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-09-02T19:07:54.180" Title="How are base temperature units found on other planets?" Tags="|measurement|temperature|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1731" PostTypeId="5" CreationDate="2013-09-02T16:17:41.987" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-02T16:17:41.987" LastActivityDate="2013-09-02T16:17:41.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1732" PostTypeId="4" CreationDate="2013-09-02T16:17:41.987" Score="0" Body="Questions regarding regolith which is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials." OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-02T16:52:09.380" LastActivityDate="2013-09-02T16:52:09.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1733" PostTypeId="2" ParentId="1730" CreationDate="2013-09-02T18:50:04.137" Score="10" Body="&lt;p&gt;I think you have it backwards - it doesn't matter where in the universe you are, absolute zero is still the same (-273.15°C) and the temperature increase marked by 1 Kelvin or 1 Centigrade is the same, so you could find a calibration point on any planet that came to the same temperature and list that as the way to measure 100°C etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Realistically though, you wouldn't need to do this, and you just wouldn't use a thermometer that had any gaseous or liquid connection to the outside environment. It is much easier to not have to worry about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calibrate your thermometers on earth before you go traipsing round other planets.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2013-09-02T19:07:54.180" LastActivityDate="2013-09-02T19:07:54.180" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1734" PostTypeId="1" AcceptedAnswerId="1748" CreationDate="2013-09-02T18:57:40.390" Score="15" ViewCount="2045" Body="&lt;p&gt;I've been watching &lt;a href=&quot;https://www.youtube.com/watch?v=2z_oi36O8tw&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA ScienceCasts video on YouTube&lt;/a&gt; (also published on &lt;a href=&quot;http://science.nasa.gov/science-news/sciencecasts/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Science&lt;/a&gt;), and towards the end the narrator mentiones the end of life for the &lt;a href=&quot;http://www.nasa.gov/mission_pages/ladee/main/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;LADEE&lt;/a&gt; (Lunar Atmosphere and Dust Environment Explorer) mission. This is a partial transcript from the mentioned video:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On April 15th of next year, the sunset colored shadow of Earth will&#xA;  envelop the Moon for a Lunar eclipse. It will be a grand sight from&#xA;  Earth, but bad news for LADEE. The spacecraft is solar powered, and&#xA;  requires sunlight to charge its batteries. An eclipse could end the&#xA;  mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It goes on to explain that NASA is considering an option to deorbit LADEE before the eclipse and crash it on the Lunar surface, which might provide additional opportunity to study Lunar surface and its dust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/wOuSD.jpg&quot; alt=&quot;Lunar eclipse&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Next Lunar eclipse will be on April 15th, 2014 when the Moon will pass directly behind the Earth into its shadow. (Image: myself :P)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But all this talk of LADEE not being able to survive roughly 2 hours of more or less total darkness sounds rather counterintuitive to me, considering it would (my assumption) have an orbit around the Moon with nearly half of it in the dark anyway. Or am I wrong on this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7UYZl.jpg&quot; alt=&quot;LADEE&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;LADEE, the Lunar Atmosphere and Dust Environment Explorer, will explore the Moon’s thin exosphere and dust environment to&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;answer questions on the formation and evolution of &lt;a href=&quot;https://en.wikipedia.org/wiki/Exosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;exospheres&lt;/a&gt;, that are thin, atmosphere-like volumes surrounding planetary&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;bodies, with molecules gravitationally bound to that body, but their density too low to behave as gas. (Image: NASA Ames)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some previous probes that were also at risk of failing on Lunar eclipses survived just fine, in fact, NASA was rather surprised how well they did, for example with the &lt;a href=&quot;http://www.nasa.gov/mission_pages/grail/main/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;GRAIL&lt;/a&gt; (Gravity Recovery And Interior Laboratory) twins. Quoting &lt;a href=&quot;http://www.spaceflight101.com/grail-mission-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101 GRAIL mission updates&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On June 4, the GRAIL twins had to endure a lunar eclipse during which&#xA;  their solar arrays received no sunlight to charge the vehicle’s&#xA;  batteries. Both spacecraft made it through the eclipse alive and in&#xA;  good condition. Originally, the GRAIL mission was planned to occur in&#xA;  between lunar eclipses because the spacecraft were not expected to&#xA;  survive two hours in darkness. After flying the two spacecraft for a&#xA;  while, teams saw that the twins were performing better than expected&#xA;  and could survive an eclipse.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So my question is, how well can the implications of this next Lunar eclipse be predicted for LADEE, learning from past missions that faced similar threats, and taking LADEE mission parameters and onboard equipment requirements into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Does LADEE really have no chance of continuing its mission past the April 15th, 2014 Lunar eclipse and should indeed be deorbited and crashed into Lunar surface before that event?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: I've found a bit more information on LADEE end-of-life, what NASA calls Decommissioning Phase, in the &lt;strong&gt;&lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/LADEE-Press-Kit-08292013.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Atmosphere and Dust Environment Explorer (LADEE) Launch Press Kit&lt;/a&gt;&lt;/strong&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The final phase of the LADEE mission is  the Decommissioning Phase.&#xA;  During this  phase, the LADEE observatory will  perform a final&#xA;  Ka-band calibration and will  continue to acquire science data, as &#xA;  power and thermal resources allow. The  Decommissioning Phase ends at&#xA;  the end  of the Science Phase and 100 days after  commissioning.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The design of the LADEE science orbit is  such that, when the onboard&#xA;  propellant is  almost gone at the end of the Science  Phase, LADEE's&#xA;  orbit will be managed  down to lower altitudes until it impacts the &#xA;  lunar surface. LADEE will continue to  acquire data until impact.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;It is unlikely there will be any attitude  requirements during&#xA;  decommissioning,  except perhaps keeping the antenna  pointed at the&#xA;  Earth for telemetry and  tracking until the last orbit. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;There are no plans to target the impact  points on the lunar surface.&#xA;  If LADEE's  propellant is depleted and orbital decay  occurs&#xA;  naturally, the point of impact may  not be in sight of Earth.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WvW4Z.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;&lt;strong&gt;Artist's concept of the LADEE end-of-mission lunar impact. Image credit: NASA Ames / Dana Berry&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So in short, no additional mention of the Lunar eclipse being the likely reason for LADEE's early retirement with a bang, but it does give more info on mission parameters that will inevitably lead to it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="25" LastEditDate="2013-09-26T12:36:06.587" LastActivityDate="2021-04-01T23:24:11.967" Title="How big of a problem is the Lunar eclipse in April 2014 for LADEE?" Tags="|the-moon|artificial-satellite|end-of-life|environment|ladee|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1735" PostTypeId="2" ParentId="1723" CreationDate="2013-09-02T19:53:09.690" Score="8" Body="&lt;p&gt;The moon's gravity well is deeper than most asteroids.  Therefore, materials that can be found on both asteroids and the moon are probably not going to be cost competitive when sourced from the moon.  However, there are some materials that might be on the moon that aren't on asteroids.  Water and Helium isotopes come to mind.  Also, the moon has a unique resource that other bodies don't: its relative position to the Earth.  In particular, the far side of the moon is a very quiet place electromagnetically speaking.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-09-02T19:53:09.690" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1736" PostTypeId="1" CreationDate="2013-09-03T00:18:41.323" Score="11" ViewCount="1524" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OOPZ7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I read this article: &lt;a href=&quot;http://www.interferencetechnology.com/astronauts-test-electromagnetic-propulsion-technology-aboard-iss/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astronauts Test Electromagnetic Propulsion Technology Aboard ISS&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here is a video: &lt;a href=&quot;https://www.youtube.com/watch?v=J2pJ6T3MSVI&quot; rel=&quot;nofollow noreferrer&quot;&gt;RINGS on a Reduced Gravity Flight&lt;/a&gt;.&#xA;Looks like they tested the &quot;rotate mechanism&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How exactly would these two devices propel themselves forward? Even if we assume they have an infinite energy supply from solar arrays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to use Earth's electromagnetic field to develop some similar propulsion technology?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean, Earth is a big magnet. Two magnets interact. These &lt;a href=&quot;http://www.sppl.umd.edu/projects/03-resonant-inductive.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;two devices&lt;/a&gt; change polarization to make motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qst0h.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="601" LastEditorDisplayName="user29" LastEditDate="2013-10-25T21:54:09.243" LastActivityDate="2013-10-25T21:54:09.243" Title="Electromagnetic Propulsion Technology" Tags="|propulsion|attitude|magnetic-field|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1737" PostTypeId="2" ParentId="925" CreationDate="2013-09-03T00:54:44.017" Score="16" Body="&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: First KickSat is now officially in orbit since today, April 18, 2014 and the 104 Sprites will be deployed in 16 days after the launch giving KickSat CubeSat enough time to complete its system checks. The 3U CubeSat unit was deployed onboard the SpaceX Falcon 9 during its &lt;a href=&quot;http://www.spaceflight101.com/dragon-spx-3-mission-updates.html&quot; rel=&quot;noreferrer&quot;&gt;Dragon SpX-3 mission&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The 3U CubeSat consists of a 1U avionics bus that provides electrical&#xA;  power, active attitude control, command and control systems and&#xA;  communications for telemetry downlink and command uplink. The&#xA;  remaining 2 CubeSat units are dedicated to the deployment mechanism&#xA;  that holds and secures the Sprites during launch and in orbit using a&#xA;  lid. The mechanism can hold up to 200 Sprites and uses a spring-loaded&#xA;  pusher and by a nichrome burn wire system to deploy all the Sprites.&#xA;  The prototype mission launching on SpX-3 carries 104 Sprites. The&#xA;  KickSat CubeSat weighs 5.5 Kilograms.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Source: &lt;a href=&quot;http://www.spaceflight101.com/dragon-spx-3-cargo-overview.html&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101 article on Dragon SpX-3 Cargo Overview&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And this is how each of these 104 Sprite ChipSat femtosatellites look like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/lMb0Z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/jkDTq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the whole 3U KickSat with the ChipSat trays, release mechanisms and main bus unit looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/wlWwr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All images credit: &lt;a href=&quot;https://zacinaction.github.io/kicksat/&quot; rel=&quot;noreferrer&quot;&gt;KickSat on HitHub&lt;/a&gt; (includes more info and a &lt;a href=&quot;https://www.youtube.com/watch?v=Mh9tdf8KmcY&quot; rel=&quot;noreferrer&quot;&gt;video of KickSat sprite deployment&lt;/a&gt;). All 104 Spite spacecraft will be deployed at intervals on a timer starting at 4:00 PM EDT (20:00 GMT) on May 4th.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Not yet &lt;em&gt;flying&lt;/em&gt; on their own (but already used since 2011 when they were &lt;a href=&quot;http://www.examiner.com/article/endeavor-to-deliver-stamp-sized-sprite-micro-satellite-experiment-to-iss&quot; rel=&quot;noreferrer&quot;&gt;attached to the ISS external platform for testing&lt;/a&gt;), so this is answer is a bit premature for this question, but in December 2013 the now successfully crowd-funded &lt;a href=&quot;http://www.kickstarter.com/projects/zacinaction/kicksat-your-personal-spacecraft-in-space&quot; rel=&quot;noreferrer&quot;&gt;KickSat project&lt;/a&gt; will launch first free-flying Sprite &lt;a href=&quot;http://en.wikipedia.org/wiki/Miniaturized_satellite#Femtosatellite&quot; rel=&quot;noreferrer&quot;&gt;femtosatellites&lt;/a&gt; in low orbit, presumably onboard the &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX_CRS-3&quot; rel=&quot;noreferrer&quot;&gt;SpaceX CRS-3&lt;/a&gt; ISS resupply mission:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/HqeFD.jpg&quot; alt=&quot;Sprite femtosatellite&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;a href=&quot;http://www.bis-space.com/what-we-do/projects/project-kicksat&quot; rel=&quot;noreferrer&quot;&gt;KickSat's&lt;/a&gt; &lt;a href=&quot;http://en.wikipedia.org/wiki/KickSat&quot; rel=&quot;noreferrer&quot;&gt;Sprite femtosatellite&lt;/a&gt; design prototype in hand, the size of a couple of postage stamps.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Three prototype &quot;chip satellites&quot; were launched to the &lt;a href=&quot;http://en.wikipedia.org/wiki/ISS&quot; rel=&quot;noreferrer&quot;&gt;ISS&lt;/a&gt; on &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Endeavor&quot; rel=&quot;noreferrer&quot;&gt;Space&#xA;  Shuttle Endeavor&lt;/a&gt; on its &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-134&quot; rel=&quot;noreferrer&quot;&gt;final mission&lt;/a&gt; in May 2011. They were attached&#xA;  to the ISS external platform &lt;a href=&quot;http://en.wikipedia.org/wiki/Materials_International_Space_Station_Experiment&quot; rel=&quot;noreferrer&quot;&gt;Materials International Space Station&#xA;  Experiment&lt;/a&gt; (MISSE-8) for testing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As far as future plans go, a bit confusingly also named KickSat (maybe &lt;em&gt;KickSat 1&lt;/em&gt; rather?) will be a 3U P-POD CubeSat (housing a 1U avionics bus and a 2U Sprite deployer) carrying 128 of Sprite femtosatellites into space, launched as auxiliary payloads through the &lt;a href=&quot;http://www.nasa.gov/home/hqnews/2012/feb/HQ_12-050_CubeSats.html&quot; rel=&quot;noreferrer&quot;&gt;NASA's ELaNa program&lt;/a&gt;, and will be deployed in low Earth orbit (between 300 and 350 kilometers).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/rXbsT.jpg&quot; alt=&quot;KickSat deploying Sprite femtosatellites&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Artist's impression of the KickSat deploying Sprite femtosatellites (source: &lt;a href=&quot;http://zacinaction.github.io/kicksat/&quot; rel=&quot;noreferrer&quot;&gt;KickSat on GitHub&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After being released from the P-POD, KickSat will perform a de-tumble&#xA;  maneuver and establish communication with Cornell’s ground station.&#xA;  After check-out, the spacecraft will be put in a sun-pointing attitude&#xA;  and spun up to maintain that attitude. A command signal from the&#xA;  ground station will then trigger the deployment and the Sprites will&#xA;  be released as free-flying spacecraft.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;After deployment, telemetry and sensor measurements from the&#xA;  individual Sprites will be received through Cornell’s ground station&#xA;  in Ithaca, NY, as well as several other amateur radio ground stations&#xA;  throughout the world.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More information on KickSat's Sprite femtosatellites can be found on the &lt;a href=&quot;http://www.kickstarter.com/projects/zacinaction/kicksat-your-personal-spacecraft-in-space&quot; rel=&quot;noreferrer&quot;&gt;KickSat's KickStarter&lt;/a&gt; and &lt;a href=&quot;http://zacinaction.github.io/kicksat/&quot; rel=&quot;noreferrer&quot;&gt;KickSat's GitHub&lt;/a&gt; pages.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-21T21:20:38.887" LastActivityDate="2014-04-21T21:20:38.887" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="1738" PostTypeId="5" CreationDate="2013-09-03T07:10:15.793" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-03T07:10:15.793" LastActivityDate="2013-09-03T07:10:15.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1739" PostTypeId="4" CreationDate="2013-09-03T07:10:15.793" Score="0" Body="NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) is a robotic lunar orbit mission  gathering detailed information about the lunar atmosphere, conditions near the surface and environmental influences on lunar dust." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-03T07:10:15.793" LastActivityDate="2013-09-03T07:10:15.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1740" PostTypeId="2" ParentId="892" CreationDate="2013-09-03T10:36:45.860" Score="7" Body="&lt;p&gt;This answer is really a wrap-up of comments from Deer Hunter, Gerrit and Hunter2:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your options include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Solar Power: effective when close to the sun. Beyond Mars' orbit this becomes less useful for power generation&lt;/li&gt;&#xA;&lt;li&gt;RTG - Radioisotope Thermoelectric Generator: this is used for long missions, and it really isn't very dangerous. These are not fission reactors - they simply generate relatively small amounts of heat through radioactive decay.&lt;/li&gt;&#xA;&lt;li&gt;Hydrothermal - would work on a body with sufficient liquid with a decent temperature differential (Europa?)&lt;/li&gt;&#xA;&lt;li&gt;Geothermal - would work on a geologically active body (Io?)&lt;/li&gt;&#xA;&lt;li&gt;Magnetic- Jupiter and Saturn have strong magnetic fields which may be usable. Between Io and Jupiter there is a plasma torus, which may also be a source of energy&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Additionally, we have a few related questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/301/38&quot;&gt;What power source options provide reliable and sustainable power generation for a Martian Colony&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/17/38&quot;&gt;Are there any safe-to-launch alternatives to RTG&amp;#39;s for outer solar system exploration?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/623/38&quot;&gt;How far from the Sun can solar power be used as a reliable energy source?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/1193/49&quot;&gt;What types of sustainable energy are available beyond Mars?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-06-05T12:15:47.923" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1741" PostTypeId="2" ParentId="1668" CreationDate="2013-09-03T14:23:46.123" Score="14" Body="&lt;h2&gt;Disclaimer&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;These are from my own personal experience, which only encompasses launches from the east coast of Florida.  Other US launch sites include Vandenberg AFB in California, which does have some decent viewing sites (though I do not know details), and Wallops Island, Virginia (about which I know next to nothing).  I also have no real knowledge regarding non-US launch sites, other than general location (Kwajalein, Centre spatial guyanais, Baikonur).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Location&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Depending on the launch details, Cape Canaveral Air Force Station offers several good viewing locations for a launch.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Kennedy Space Center Visitors Center offers tickets to various launches, which will typically let you watch from the closest point NASA and the Air Force are comfortable allowing the public.  This is typically either the KSC press site or the East NASA Causeway, which is about 7 miles from LC-39A (Shuttle), 5 miles from SLC-41 (Atlas), and about 4 miles from SLC-40 (SpaceX).  The farthest point on the causeway is only about 3.5 miles from SLC-37 (Delta), which is probably quite a bit closer than you would be allowed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to watch for free, your best bet would be either Space View Park in Titusville (13-15 miles from the active launch complexes) for high-azimuth launches, like SpaceX resupply missions to the ISS, or Jetty Park in Cape Canaveral for due-east launches (9-12 miles).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Other considerations&lt;/h1&gt;&#xA;&#xA;&lt;h2&gt;Photography&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;If you've never seen a launch in person before, don't focus too hard on getting good pictures.  You want to watch the launch with your eyes, not with the screen on your camera.  I was able to get decent video by zooming out and holding the camera at my chest -- pointing it in the general direction of the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Delays&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Launch schedules only move in one direction.  Plan to be there for at least two or three days after the planned launch date, plan to be there at least a couple hours after launch time, and be aware that the launch may not happen after all.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Environment&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;CCAFS and KSC are basically in a giant swamp.  Bring sunscreen (lots -- unless it's a night launch), plenty of water, and mosquito repellant (always!).  Also keep an eye out for alligators -- expect to find one in pretty much every ditch and puddle around you.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Keeping tabs on the launch&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;If you have a digital trunking scanner, program in the &lt;a href=&quot;http://www.radioreference.com/apps/db/?sid=808&quot;&gt;CCAFS/KSC radio frequencies&lt;/a&gt;.  You won't necessarily get the launch control loop, especially with military or proprietary launches, but you should get enough chatter to give you an idea of how long until liftoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have 4G service, you might try following streaming video of the launch (varies, depending on launch provider).  Keep in mind the stream will about 30 seconds behind real time.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2013-09-03T15:25:32.693" LastActivityDate="2013-09-03T15:25:32.693" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1742" PostTypeId="2" ParentId="1725" CreationDate="2013-09-03T15:01:49.947" Score="6" Body="&lt;p&gt;I'll throw some math at this problem.  Given that the negative charge is much greater than the positive charge, I'll treat the moon as basically an &lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capsph.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;isolated sphere capacitor&lt;/a&gt;.  This really only applies to the side facing away from the sun, so I'll make the egregious assumption of dividing it by half, but that is very approximate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ C \approx 2 \pi \epsilon_0 R = 1 \times 10^{-4} F = 100 \mu F $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The moon has about &lt;span class=&quot;math-container&quot;&gt;$100 V$&lt;/span&gt; stored within this capacitance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I &lt;em&gt;own&lt;/em&gt; capacitors that can store more energy than that.  You can go &lt;a href=&quot;http://www.goldmine-elec-products.com/prodinfo.asp?number=G8722&quot; rel=&quot;nofollow noreferrer&quot;&gt;pick up one for yourself for $2.50&lt;/a&gt;.  Your microwave probably has a better capacitor than the moon is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It still might be possible to get useful electricity this way, but it doesn't look good.  We know that the charge difference is continuously replenished by the sun's effect on the moon's regolith.  As you draw down the voltage, it's possible that will cause a huge current to flow from the sun side to the dark side.  That could be usable, but it's hard to imagine.  At the scale of &lt;span class=&quot;math-container&quot;&gt;$100 V$&lt;/span&gt; you need a large current to produce a decent amount of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of the electrical specs of the moon are impressive.  The voltage isn't impressive, the capacitance is pathetic, and we don't know much about the natural recharging rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I seriously doubt the idea has any merit.  If anything, I think the reverse is more likely &amp;mdash; that humans would control the charge of the moon's surface to help manage issues with solar wind, moon dust, and other things.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="6816" LastEditDate="2018-11-20T18:21:08.120" LastActivityDate="2018-11-20T18:21:08.120" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1743" PostTypeId="5" CreationDate="2013-09-03T15:12:13.087" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-03T15:12:13.087" LastActivityDate="2013-09-03T15:12:13.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1744" PostTypeId="4" CreationDate="2013-09-03T15:12:13.087" Score="0" Body="Questions regarding aerobot which are aerial robots used in the context of unmanned space probe" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-09-03T15:29:31.463" LastActivityDate="2013-09-03T15:29:31.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1746" PostTypeId="1" CreationDate="2013-09-03T18:25:54.400" Score="15" ViewCount="744" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967#Article_XV&quot;&gt;Article XV&lt;/a&gt; of the &lt;a href=&quot;http://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967&quot;&gt;Outer Space Treaty&lt;/a&gt; (henceforth OuST) is a provision for amendments to the original treaty. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The treaty dates back almost half-a-century (46 years to be a little more precise). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since that date, what amendments, if any, were proposed, voted upon, and passed? Is the OuST mostly dormant despite significant changes globally on both the technological, and political fronts?&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2018-05-23T20:45:52.287" Title="What amendments were made to the Outer Space Treaty?" Tags="|law|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1747" PostTypeId="1" CreationDate="2013-09-03T20:23:18.493" Score="14" ViewCount="790" Body="&lt;p&gt;I just read &lt;a href=&quot;http://arstechnica.com/science/2013/09/nasas-original-flight-director-calls-agencys-direction-a-tragedy/&quot;&gt;this article&lt;/a&gt;, in which former director Kraft is described as deriding NASA's current plan to use a single heavy-lift vehicle (Space Launch System, or SLS) as a primary launch platform. Why is NASA doing this? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A single failure destroys an enormous amount of work. Distributing the across rockets increases the possibly of a single failure, but a single failure results in less loss. Furthermore, the smaller rockets exist today, so it doesn't require a huge new new development effort. Since they exist today, a lot of the design flaws have already been discovered and corrected.&lt;/p&gt;&#xA;" OwnerUserId="79" LastEditorUserId="49" LastEditDate="2014-04-10T22:54:32.283" LastActivityDate="2014-04-11T13:29:04.760" Title="What's the advantage of using an SLS system rather than a number of small lift rockets?" Tags="|rockets|nasa|hllv|sls|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="1748" PostTypeId="2" ParentId="1734" CreationDate="2013-09-03T22:17:41.753" Score="5" Body="&lt;p&gt;Okay, so let's get a few numbers here straight, and then I'll explain why it's an issue, and why we can probably recover from it just fine, hopefully. Let's start with figuring out what the nominal eclipse time is for the LADEE satellite. &lt;a href=&quot;http://science1.nasa.gov/missions/ladee/&quot; rel=&quot;nofollow&quot;&gt;NASA quotes&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nominal science orbit will be a near-circular retrograde&#xA;  (clockwise) equatorial orbit with a period of 113 minutes at about 50&#xA;  km above the surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Okay then, how much time will be spent in eclipse at that range? I don't have the tools to make that calculation with the Moon, but I found &lt;a href=&quot;http://www.cdeagle.com/omnum/pdf/shadow1.pdf&quot; rel=&quot;nofollow&quot;&gt;an article&lt;/a&gt; about Earth orbits, with numbers that can conveniently be changed to Lunar parameters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\left \{ cos^{-1} \left ( \frac{\sqrt{1-R^2}}{cos (\beta)} \right )\right \}\frac{\tau}{\pi}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$R$ = radius ratio = $\frac{r_{eq}}{r_{sat}}$&lt;/li&gt;&#xA;&lt;li&gt;$r_{eq}$ = equatorial radius of the Earth&lt;/li&gt;&#xA;&lt;li&gt;$r_{sat}$ = geocentric radius of the satellite = $r_{eq} + h_{sat}$&lt;/li&gt;&#xA;&lt;li&gt;$h_{sat}$= altitude of the satellite &lt;/li&gt;&#xA;&lt;li&gt;$\beta$ = Sun-orbit-plane angle&lt;/li&gt;&#xA;&lt;li&gt;$\tau$ = orbital period of the satellite&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Using the lunar numbers instead of earth, assuming a worse case $\beta=0$, gives an eclipse time of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\left \{ cos^{-1} \left ( \frac{\sqrt{1-( 1,738/ 1,788)^2}}{1} \right )\right \}\frac{113}{\pi} = 50.46\ \text{minutes}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, one can expect that the batteries will last for at least that long. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why can't the satellite survive a lunar eclipse? The worse case lunar eclipse starts just before one leaves eclipse. That will give 1 normal eclipse, plus 2 hours eclipse time, plus possibly up to 1 more eclipse after. I'm going to just make it simple and say 1 orbit plus 1 eclipse time, or 113+51=164 minutes. That is 3 times what the nominal eclipse period is. Thus, if you were going to survive that, you have two options. First of all, reduce the power to 1/3rd of nominal eclipse power. The second is to put 3 times more battery capacity in. Of these, I suspect the first will be attempted, while using some surplus battery power to help keep things stable. Otherwise, your battery mass has increased by a factor of 2. As batteries are typically one of the heavier components on a spacecraft, that is significant!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why is this probably not a bit deal? Spacecraft are usually made with the idea that occasionally, something will happen, and they have a resulting safe mode as a result. This safe mode will keep the satellite from damaging its batteries, for instance, but might cause some damage as the thermal limits often are violated in such a mode. However, there are spacecraft which survive regularly without relying on batteries to survive eclipse. The best example I know of is the &lt;a href=&quot;http://www.arrl.org/news/ao-7-zombie-satellite-again-enjoying-its-time-in-the-sun&quot; rel=&quot;nofollow&quot;&gt;AO-7&lt;/a&gt;, without battery power for 30 years in LEO, yet it still keeps on working. Most likely, the lunar satellites employ such a technique, that while it might take a few days to recover, they will be able to eventually point back at Earth and recover communications. LADEE is equipped with omnidirectional antennas, so as long as it can point it's solar arrays at the sun, it can recover. I seriously doubt NASA will crash the satellite before the eclipse, unless there is something else wrong with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's worth mentioning that this is a very common scenario for Earth orbiting satellites. Many of them are actually designed to tumble slightly in the event of a power loss, which will ensure that eventually they will point at the sun, charging the batteries, and from there they can re-gain communications with the ground. It actually takes very careful work to make a satellite that won't tumble in the event of low power, and this will ensure eventual recovery of the satellite given enough time, although perhaps with the slight loss of lifetime or other such issues as a result from the uncontrolled period of time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2014-06-14T13:30:08.653" LastActivityDate="2014-06-14T13:30:08.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1749" PostTypeId="2" ParentId="1747" CreationDate="2013-09-04T00:12:12.963" Score="4" Body="&lt;p&gt;Just guesses but:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Avoiding rendezvous in orbit and the extra equipment that takes? &lt;/li&gt;&#xA;&lt;li&gt;Amortizing the lift cost of things that scale sub-linearly?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also, at first blush, it doesn't sound like they are thinking of putting multiple major payloads on a single super-sized booster.&lt;/p&gt;&#xA;" OwnerUserId="646" LastActivityDate="2013-09-04T00:12:12.963" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1750" PostTypeId="2" ParentId="1736" CreationDate="2013-09-04T00:20:49.967" Score="6" Body="&lt;p&gt;The technology referred to here is called &lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/793Miller.pdf&quot;&gt;electromagnetic formation flight&lt;/a&gt; (PDF warning). It's a project out of MIT, and Raymond Sedwick is continuing &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/1.45017&quot;&gt;development&lt;/a&gt; at the University of Maryland (the RINGS work above is from his team).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The important thing to remember here is that this system is meant for &quot;control of &lt;em&gt;relative&lt;/em&gt; separation, &lt;em&gt;relative&lt;/em&gt; attitude, and inertial attitude ... by creating electromagnetic dipoles on each spacecraft&quot; (from &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/1.2172&quot;&gt;here&lt;/a&gt;, emphasis is mine). So basically you need at least two spacecraft with this technology flying in formation, which is useful for all sorts of types of missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are details in the papers, but the system essentially uses the effect of the like poles of a pair of magnets repelling, while opposite poles attract. Electromagnets are used on-board to modify the magnetic fields of the two spacecraft relative to each other, while reaction wheels help with attitude control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;There is no inertial maneuvering capability here, only relative position control&lt;/strong&gt;. This gets close to the topic of using tethers to collect electricity which would then be used to power some traditional electric propulsion system, but here, there are &lt;strong&gt;no consumables&lt;/strong&gt; and &lt;strong&gt;no ability to change inertial orbits&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally, precision control of spacecraft formations requires a significant amount of maneuvering to overcome the effects of differential perturbations, such as nodal drift or drag. Even if you can do it very efficiently, eventually your propellant budget is going to limit your operational lifetime. That seems to be the motivation with this work: maintaining the relative formation without using consumable propellant. If you wish to raise your formation by 100km to overcome the effects of drag in LEO (for instance), that still must be done with a thruster or EP system of some kind.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-09-04T00:20:49.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1753" PostTypeId="1" AcceptedAnswerId="1755" CreationDate="2013-09-04T09:51:12.843" Score="19" ViewCount="18035" Body="&lt;p&gt;Does the orbit of the ISS decay? I don't know much about the ISS, but I heard some rumors.&#xA;The rumor I heard was that there is a very, very tiny atmosphere at the ISS orbit.&lt;/p&gt;&#xA;" OwnerUserId="648" LastActivityDate="2015-07-21T13:21:41.750" Title="Does the orbit of the ISS decay?" Tags="|iss|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1755" PostTypeId="2" ParentId="1753" CreationDate="2013-09-04T10:18:40.903" Score="27" Body="&lt;p&gt;Yes. The International Space Station is on average orbiting at an altitude of around 230 miles (370 km), but is currently roughly at an altitude of 260 miles (418 km) as it just completed an orbital reboost a few days ago (or maybe it was yesterday, my biorhythm is all messed up).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can track the current position relative to the ground (ground track), altitude, velocity, and a few other ISS telemetry data live via various websites, for example this &lt;a href=&quot;http://www.isstracker.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Tracker&lt;/a&gt;. The ISS is without this orbital reboosting constantly losing velocity and with it altitude due to slight atmospheric drag, as there indeed is some atmospheric pressure at altitudes it orbits. These low orbital altitudes are also referred to as &lt;a href=&quot;http://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low Earth Orbits&lt;/a&gt;, or LEO for short.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atmospheric pressure in LEO is around 10&lt;sup&gt;-8&lt;/sup&gt; &lt;em&gt;Pa&lt;/em&gt; (&lt;a href=&quot;http://en.wikipedia.org/wiki/Pascal_%28unit%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pascals&lt;/a&gt;) or 1.45*10&lt;sup&gt;-12&lt;/sup&gt; &lt;em&gt;psi&lt;/em&gt; (&lt;a href=&quot;http://en.wikipedia.org/wiki/Pounds_per_square_inch&quot; rel=&quot;nofollow noreferrer&quot;&gt;pounds per square inch&lt;/a&gt;), which is of course a few orders of magnitude smaller than the Earth's average sea level pressure at 101 &lt;em&gt;kPa&lt;/em&gt; (kilopascals) or 14.7 &lt;em&gt;psi&lt;/em&gt;, but still enough to cause some orbital decay due to atmospheric drag to the ISS that orbits at the average velocity of 4.791 miles/s (7.71 km/s). To compensate for this drop in orbital velocity, the ISS periodically has to reboost and regain velocity and orbital altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For easier appreciation of different values of (atmospheric or otherwise) particle density pressure: &lt;a href=&quot;http://en.wikipedia.org/wiki/Orders_of_magnitude_(pressure)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Orders of magnitude pressure values, expressed in Pascals&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: Note that since &lt;em&gt;assembly complete&lt;/em&gt; and retirement of the Space Shuttle fleet in 2011, ISS' average orbital altitude is a bit higher than the numbers I give above. For some plots of that, see &lt;a href=&quot;https://space.stackexchange.com/a/9088/49&quot;&gt;How often does ISS require re-boosting to higher orbit?&lt;/a&gt; Its current altitude (semi-major axis) is roughly 401 km (&lt;a href=&quot;http://www.heavens-above.com/orbit.aspx?satid=25544&quot; rel=&quot;nofollow noreferrer&quot;&gt;400 x 402 km&lt;/a&gt; at perigee and apogee, respectively). For information on specific orbital reboosts, see &lt;a href=&quot;https://blogs.nasa.gov/stationreport/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS On-Orbit Status Report&lt;/a&gt;. Last one (as of writing this update on 07/21/2015) was completed on the night of &lt;a href=&quot;https://blogs.nasa.gov/stationreport/2015/07/10/&quot; rel=&quot;nofollow noreferrer&quot;&gt;07/10/15&lt;/a&gt; to correct ISS orbital phase for the upcoming &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_44&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expedition 44&lt;/a&gt; launch to the station on July 22, aboard &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-17M&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-17M&lt;/a&gt; with the &lt;em&gt;expedited&lt;/em&gt; four-orbit rendezvous. Next ISS reboosts are scheduled for August 7 and August 24, 2015.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-07-21T13:21:41.750" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1756" PostTypeId="2" ParentId="980" CreationDate="2013-09-04T13:01:33.103" Score="2" Body="&lt;p&gt;&lt;strong&gt;YES&lt;/strong&gt; it is monitored by &lt;a href=&quot;http://en.m.wikipedia.org/wiki/Christopher_C._Kraft_Jr._Mission_Control_Center&quot; rel=&quot;nofollow&quot;&gt;ground control station &lt;/a&gt; but it is controlled by the astronaut in ISS or sometimes it may be automated &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Advanced_Space_Vision_System&quot; rel=&quot;nofollow&quot;&gt;space vision system&lt;/a&gt; which uses camera to capture 2D images and then convert it to 3D images by using computer . And with the help of this system astronaut uses the robotic arm . but this system has certain problem &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.m.wikipedia.org/wiki/TriDAR&quot; rel=&quot;nofollow&quot;&gt;TRIdar&lt;/a&gt; is a  relative navigation vision system which uses a 3D camera and thermal imaging (LASER based) &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This laser-&#xA;  based system tracks retro&#xA;  reflectors located on the ISS&#xA;  to provide bearing, range and&#xA;  closing rate information. While&#xA;  reliable, target based systems&#xA;  have operational limitations as&#xA;  targets must be installed on&#xA;  target payloads. This is not&#xA;  always practical or even&#xA;  possible.  For example,&#xA;  servicing existing satellites&#xA;  that don’t have reflectors&#xA;  installed would require a&#xA;  targetless tracking capability.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/TORU&quot; rel=&quot;nofollow&quot;&gt;TORU&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;TORU has two joysticks which&#xA;  can be used to manually fly&#xA;  the ship.The left joystick is&#xA;  used to control the movement&#xA;  of the ship and the right&#xA;  joystick is used to control its&#xA;  orientation. The system also&#xA;  includes a camera that is&#xA;  mounted on the docking&#xA;  spacecraft and provides visual&#xA;  feedback when the spacecraft&#xA;  is remotely controlled from&#xA;  the station to that it is&#xA;  docking. TORU also transfers&#xA;  sounds from the spacecraft&#xA;  that may provide indirect&#xA;  information about the docking&#xA;  process&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Kurs_%28docking_system%29&quot; rel=&quot;nofollow&quot;&gt;kurs docking system&lt;/a&gt; it is used in ISS and MIR when a vehicle approach the space station it will emit radar pulses from&#xA;multiple antennas and change in the strength between&#xA;the antennas allows the&#xA;system to compute relative&#xA;position, attitude, and&#xA;approach rate.&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-09-04T13:01:33.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1757" PostTypeId="2" ParentId="1232" CreationDate="2013-09-04T13:42:59.957" Score="4" Body="&lt;p&gt;I'm going to stick my neck out and say this: &lt;strong&gt;very rarely&lt;/strong&gt;. Why? After three weeks of me and several others trying every combination of ISS and MDM we could shove into Google, we still only know of Tildal's &lt;a href=&quot;http://www.nasaspaceflight.com/2008/05/sts-124-frr-debate-outstanding-issues-faulty-mdm-removed/&quot; rel=&quot;nofollow&quot;&gt;1 replacement&lt;/a&gt; and &lt;a href=&quot;http://www.nasaspaceflight.com/2013/04/iss-communications-overhaul-boost-scientific-output/&quot; rel=&quot;nofollow&quot;&gt;1 upgrade&lt;/a&gt; - and that's it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These things aren't classified, and it would seem that replacing them because of a failure would show up &lt;em&gt;somewhere&lt;/em&gt; in Google. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, here's the thing we have to remember: lack of evidence is just that: lack of evidence. Therefore, all we can say is that, &lt;strong&gt;to the best of our knowledge, MDM's aren't replaced often enough to make a graph and say things about it.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-09-04T13:42:59.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1758" PostTypeId="5" CreationDate="2013-09-04T14:55:30.037" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-04T14:55:30.037" LastActivityDate="2013-09-04T14:55:30.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1759" PostTypeId="4" CreationDate="2013-09-04T14:55:30.037" Score="0" Body="Questions regarding life which did not originate on Earth." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-04T15:31:12.113" LastActivityDate="2013-09-04T15:31:12.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1760" PostTypeId="2" ParentId="1746" CreationDate="2013-09-04T14:56:41.317" Score="12" Body="&lt;p&gt;According to Andrew Tingkang of Seattle University School of Law (in a paper from 2011), the Outer Space Treaty has never been amended. The amendment addressed in &quot;Common Ground in the Sky: Extending the 1967 Outer Space Treaty to Reconcile U.S. and Chinese Security Interests&quot; by Alex B. Engelhart was a proposed amendment that was never ratified.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://digitalcommons.law.seattleu.edu/cgi/viewcontent.cgi?article=2070&amp;amp;context=sulr&quot;&gt;These Aren’t the Asteroids You Are Looking For: Classifying Asteroids in Space as Chattels, Not Land - Andrew Tingkang, Seattle University School of Law&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://digital.law.washington.edu/dspace-law/bitstream/handle/1773.1/568/17PacRimLPolyJ133.pdf?sequence=1&quot;&gt;Common Ground in the Sky: Extending the 1967 Outer Space Treaty to Reconcile U.S. and Chinese Security Interests - Alex B. Engelhart, University of Washington School of Law&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2013-09-04T14:56:41.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1761" PostTypeId="2" ParentId="1231" CreationDate="2013-09-04T16:27:00.297" Score="7" Body="&lt;p&gt;&lt;strong&gt;ORBITAL INCLINATION&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;p&gt;All the shuttles launched from KSC has a maximum inclination of 57 &lt;a href=&quot;http://www.satobs.org/seesat/Jan-2000/0199.html&quot; rel=&quot;noreferrer&quot;&gt;degrees&lt;/a&gt;(latitudes between 60 N and 60 S) .&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Azimuths greater than 120 degrees to the southeast (i.e., inclinations greater than 39&#xA;degrees) are not allowed because they overfly the Bahamas just off the coast&#xA;of Florida, not to mention the extreme eastern tip of Cuba and Haiti.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;ORBIT&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;p&gt;usually circular (almost)with the the altitude of 200 km to 600 km&lt;/p&gt;&#xA;&lt;p&gt;but for &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-125&quot; rel=&quot;noreferrer&quot;&gt;STS-125&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Perigee             486 kilometres (302 mi)&lt;/p&gt;&#xA;&lt;p&gt;Apogee         578 kilometres (359 mi)&lt;/p&gt;&#xA;&lt;p&gt;Inclination        28.5°&lt;/p&gt;&#xA;&lt;p&gt;Period          97 min&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;external source&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.amsat.org/amsat/ftp/keps/sarex/sts-vec.txt&quot; rel=&quot;noreferrer&quot;&gt;http://www.amsat.org/amsat/ftp/keps/sarex/sts-vec.txt&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-06T10:45:24.770" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1762" PostTypeId="1" CreationDate="2013-09-04T16:36:46.923" Score="14" ViewCount="6588" Body="&lt;p&gt;Let's say that for the purpose of terraforming, we need to speed up the rotation of a planet (like Venus), a moon or an asteroid to near Earthian day. How would we go about doing this? Can this be accomplished from within the body itself?&lt;/p&gt;&#xA;" OwnerUserId="80" LastEditorUserId="6944" LastEditDate="2015-07-04T16:14:54.053" LastActivityDate="2020-12-10T16:05:12.130" Title="Changing the rotational rate of a natural body" Tags="|venus|planet|terraforming|rotation|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1763" PostTypeId="2" ParentId="955" CreationDate="2013-09-04T16:48:20.620" Score="16" Body="&lt;p&gt;In the pressure fed rocket engines the propellant (both the oxidizer and the fuel) is fed to the combustion chamber by the pressurized gas (usually Helium) and it does not contain any complexity such as fed pump or turbo pumps &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hZ5us.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The presence of the turbo pump prevents the pressure wave reaching the propellant  tank. But in a pressure fed rocket engine contains only the valve that opens and closes under pressure &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;strong&gt;valve opens&lt;/strong&gt; if the &lt;em&gt;pressure in the combustion chamber is less then the pressure in the propellant  tank&lt;/em&gt;(which is thick walled because they need to withstand high pressure)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;strong&gt;valve closes&lt;/strong&gt; if the &lt;em&gt;pressure in the combustion chamber is more than the pressure in the propellant  tank&lt;/em&gt; (to prevent the pressure waves reaching the propellant  tank )&lt;/p&gt;&#xA;&#xA;&lt;p&gt;with a direct pressure-fed cycle is that any variation in pressure will result in double the change in the whole loop, amplifying the oscillation. There's no turbine between the injectors and the containers to stop that oscillation from propagating &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the combustion process the pressure in the combustion chamber increases(flow velocity decreases) at the same time the amount of propellant  injection from the injector decreases and suddenly the flow velocity increases(combustion chamber pressure decreases) the injectors, injects more propellents that burns outside the nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be theoretical the fuel residence time in a combustion chamber is given by the &lt;strong&gt;Characteristic length&lt;/strong&gt;(usually denoted by L*)(minimum length that the fuel will remain in the  combustion chamber and nozzle for complete combustion to take place)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$L^*= {{q*V*t_s}\over A}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;q is the propellant mass flow rate, V is the average specific volume, and $t_s$ is the propellant stay-time A is the sonic throat area&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so the propellent flow rate is a function of the pressure difference between the combustion chamber and the propellant  tank and as the propellant  rate increases as a result of low pressure (when compared to the pressure in the fuel tank) in the combustion chamber the characteristic length also increases(since the nozzle length remains constant) result in the instabilities in the fuel combustion and the combustion occurs outside the nozzle&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-09-05T09:55:13.340" LastActivityDate="2013-09-05T09:55:13.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1765" PostTypeId="2" ParentId="1712" CreationDate="2013-09-04T21:10:25.603" Score="14" Body="&lt;p&gt;The primary consideration for pencils is the risk of breaking, and the need for sharpening producing waste products that can be hazards. But let's examine the major factors: Materials, mechanisms, and failure modes.&lt;/p&gt;&#xA;&lt;p&gt;Some terms:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;nib: the small tube at the working end, past the mechanism. Not all forms have one. The nib is usually a friction fit that aids the mechanism's function.&lt;/li&gt;&#xA;&lt;li&gt;mechanism: the part of the pencil which advances the lead&lt;/li&gt;&#xA;&lt;li&gt;lead: the material which rubs onto the surface and is abraded away, leaving a mark.&lt;/li&gt;&#xA;&lt;li&gt;shaft: the structure into which the mechanism fits, and to which is often affixed the nib.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h1&gt;Materials&lt;/h1&gt;&#xA;&lt;p&gt;Lead is a holdover term from the early pencils, many of which used actual lead. Currently, the metal is only used by reenactors and forgers.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Graphite — a shaft of pure graphite. Fragile, non-toxic, electroconductor.&lt;/li&gt;&#xA;&lt;li&gt;Graphite and binder — powdered graphite glued together into a shaft. Not overly fragile, usually non-toxic, can be electro-conductive.&lt;/li&gt;&#xA;&lt;li&gt;Wax &amp;amp; China Clay, with pigments — typical composition for colored pencils. depending upon which wax is used and what the pigment is, can be toxic or non-toxic. Non-conductor, usually. Tends to be fragile. Can write on many surfaces too smooth for other lead types.&lt;/li&gt;&#xA;&lt;li&gt;polymer and pigment — used for some small sized (0.3, 0.5, 0.7mm) colored leads. Specific polymer used can affect both fragility, and suitability for writing on various surfaces.&lt;/li&gt;&#xA;&lt;li&gt;soft metals - not common in modern uses. Gold, silver, copper and lead have all been used. Of these, only lead is soft enough for writing on modern papers reliably. &lt;em&gt;Lead (Pb)&lt;/em&gt; is toxic, electro-conductive, and leaves a light line. It is highly ductile, and not fragile at room temperatures.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h1&gt;Mechanisms&lt;/h1&gt;&#xA;&lt;p&gt;There are 3 common types of mechanical pencil mechanism, and several less common ones.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;twist action: the lead is affixed to a carrier which rides in a spiral slot and a straight slot; by twisting one or the other, the carrier is advanced. Skillcraft, Paper*Mate, and Cross have all used this for larger sizes; with graphite. This type is gravity-independent. Typically used for 0.9mm and 1.1mm leads; 0.7mm leads are a bit fragile, but are available.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://leadholder.com/&quot; rel=&quot;noreferrer&quot;&gt;Leadholders&lt;/a&gt;, commonly called Drafting Pencils, have a friction mouth that holds a fairly thick lead (often 2mm). The mouth is simple: a ring sporting 3 to 5 jaws as a single piece, thicker towards the ends of the jaws, pulled back into a tube by a spring. The lead is advanced by pushing the jaws forward with a button, and letting the lead fall forward. This is gravity dependent. Note that this type usually has no nib.&lt;/li&gt;&#xA;&lt;li&gt;jaw-pushed lead. A set of jaws is held, much like the leadholder above, but is configured so that the jaws don't open until past a significant part of the stroke by a floating collet, but past a certain point, the collet is blocked, and the jaws then open from their own springiness. On retraction the collet moves back with the jaws, but again, is more limited in range of motion, and the jaws retract back into the collet. This is the most common mecahnism. Typically, additional leads are in a magazine, which gravity feeds a new lead when the first is advanced far enough; this makes it gravity dependent, as the typical lead is short - 3 to 5 cm.&lt;/li&gt;&#xA;&lt;li&gt;friction and wheel — a friction nib holds the lead in place; a small wheel in the pencil body advances the lead. Not suitable for smaller leads, the lack of jaws often allows the lead to slip, and advancing the lead can be a problem. Very rare. Gravity-independent.&lt;/li&gt;&#xA;&lt;li&gt;Friction manual advance - a sliding component retracts alongside the lead, and when pushed down and forward, pushes the lead forward as well. Some versions have one-way teeth. A friction nib is typical. Prone to lead slippage. Gravity independent. Requires larger leads (1.5 to 2.5mm)&lt;/li&gt;&#xA;&lt;li&gt;magazine of nibs with non-advancing lead — the shaft is also the magazine, holding 15 to 30 short (1cm) nibs with 5-6mm of lead. When the lead nib is worn down sufficiently, it is pulled out and fed through the back of the shaft, advancing the other nibs forward. Gravity-independent. Fairly durable (1.5 to 2mm leads are used), but failure mode results in 20 to 40 small items loose.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;And, for comparison:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;lead bonded in shaft. The lead is bonded to the shaft at manufacture, and is revealed by removing material from the shaft. Shaft construction is typically wood, but can also be rolled and glued paper, or polymer. Sharpening results in both lead material and shaft material being generated in small pieces.&lt;/li&gt;&#xA;&lt;li&gt;wrapped lead — the lead is wide enough to be its own shaft. Messy if not wrapped; if wrapped, sharpening of some kind becomes needed; &amp;quot;china markers&amp;quot; are a wax lead wrapped in coiled perforated paper; the paper is removed in sequential spirals, each revealing about 5-7mm of the wax lead.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h1&gt;Failure Modes&lt;/h1&gt;&#xA;&lt;p&gt;Mechanical pencils have several means of failing.&lt;/p&gt;&#xA;&lt;p&gt;The mildest and most common is a lead break ahead of the advancement mechanism, and behind the nib. In single lead systems, especially screw advanced, this can render chunks of lead unusable, and chunks up to 1cm may need to be disposed of immediately.&lt;/p&gt;&#xA;&lt;p&gt;A second form is vibration damage breaking lead inside the pencil, either within the advancement mechanism, or in the magazine. Breaks at certain points can render the jaw-pushed lead mechanism impotent; a break behind the retracted jaw but ahead of the magazine can render the pencil non-functional. Vibration damage can break leads in the magazine into chunks too small to feed from the jaw to the nib.&lt;/p&gt;&#xA;&lt;p&gt;A third common failure mode is foreign debris in the mechanism. This can prevent jaw movement or prevent jaw closure.&lt;/p&gt;&#xA;&lt;p&gt;A fourth common failure is a damaged nib. This requires the user to adjust the writing angles and pressure, or in cases where it bends, prevent function. Plastic nibs can be removed with a sharp knife; metal nibs often can be removed with pliers or even teeth.&lt;/p&gt;&#xA;&lt;h1&gt;Space Hazards&lt;/h1&gt;&#xA;&lt;p&gt;Broken bits of lead can float in the environment. They are often small enough to not be visible, and large enough to be painful or even damaging if inhaled.&lt;/p&gt;&#xA;&lt;p&gt;Electro-conductive leads can cause shorts in circuitry.&lt;/p&gt;&#xA;&lt;p&gt;Wax and polymer leads can melt and then adhere to warmer components; this can result in potential for overheat of wiring.&lt;/p&gt;&#xA;&lt;p&gt;Multiple failure modes can result in being deprived of writing implements. The space pen, by comparison, has fewer failure modes, and is vibration resistant. Vibration is a major issue during launch.&lt;/p&gt;&#xA;&lt;p&gt;Gravity fed mechanisms, for example the magazine on typical 0.5 and 0.7mm lead jaw-advanced mechanisms, or the leadholder systems, do not operate in microgravity. Leadholders can be worked around.&lt;/p&gt;&#xA;&lt;p&gt;Non-mechanical pencils (Traditional wood, or modern plastic or paper ones alike) produce shavings which are themselves hazards. Wood and paper shavings are fire hazards, as well as potentially painful to breath.&lt;/p&gt;&#xA;&lt;p&gt;Any loose material, including the fairly common small flakes of the lead material generated when writing (that dust one normally blows off) is an inhalation issue, as well as potentially clogging the filters in the air recirculation system. When electro-conductive, it's also a risk of a short circuit.&lt;/p&gt;&#xA;&lt;h1&gt;Conclusion&lt;/h1&gt;&#xA;&lt;p&gt;With the right polymers, the right construction, and appropriate vibration reduction, certain types of mechanical pencils could be used safely; the space pen is already functional, flight ready, and vibration resistant, and can be made erasable if desired by the various space agencies.&lt;/p&gt;&#xA;&lt;p&gt;There is thus little need for pencils' most noted utility, erasability, and the hazards outweigh the benefits.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-04T21:10:25.603" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1766" PostTypeId="1" CreationDate="2013-09-04T22:12:30.477" Score="13" ViewCount="299" Body="&lt;p&gt;Say you start with a &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_nearest_stars&quot;&gt;list of stars&lt;/a&gt;. What properties do we believe to be critical in the present existence or future formation of earth-like life in those star systems? How can one reasonably pare down such a list, as to select targets for exploration/colonization?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be clear, I'm specifically interested in life as we understand it, on a planet, orbiting a star. Intelligence or complexity isn't necessary to the question. If a star could have planets resembling a young earth, with very simple life, that's fine.&lt;/p&gt;&#xA;" OwnerUserId="656" LastEditorDisplayName="user12" LastEditDate="2013-12-13T00:17:57.710" LastActivityDate="2014-05-13T21:41:18.860" Title="What properties are used to quantify the odds of a star harboring earth-like life?" Tags="|life|astrobiology|planet|" AnswerCount="4" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1767" PostTypeId="5" CreationDate="2013-09-04T22:43:17.840" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-04T22:43:17.840" LastActivityDate="2013-09-04T22:43:17.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1768" PostTypeId="4" CreationDate="2013-09-04T22:43:17.840" Score="0" Body="Questions regarding astronomical bodies orbiting stars which are of proper size to be called planets." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-05T00:58:44.113" LastActivityDate="2013-09-05T00:58:44.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1769" PostTypeId="5" CreationDate="2013-09-04T22:44:06.827" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-04T22:44:06.827" LastActivityDate="2013-09-04T22:44:06.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1770" PostTypeId="4" CreationDate="2013-09-04T22:44:06.827" Score="0" Body="Questions regarding the amount of heat energy in an environment, as well as how to measure it." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-05T00:59:11.897" LastActivityDate="2013-09-05T00:59:11.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1771" PostTypeId="5" CreationDate="2013-09-04T22:49:33.300" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-04T22:49:33.300" LastActivityDate="2013-09-04T22:49:33.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1772" PostTypeId="4" CreationDate="2013-09-04T22:49:33.300" Score="0" Body="Questions regarding systems of or relating to armed forces." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-05T00:58:56.910" LastActivityDate="2013-09-05T00:58:56.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1773" PostTypeId="2" ParentId="1766" CreationDate="2013-09-04T23:04:01.580" Score="2" Body="&lt;p&gt;Although there is an incredibly wide variety of stars that could have planets orbiting them that could contain life, we don't yet have any examples of life other than what we have on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And we orbit a yellow dwarf star (a G2 main sequence star) at a mean distance of about 93 million miles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which means our only real chance of finding intelligent life that is similar to our own would be to look for similar stars and narrow down the list by only choosing those G2 stars with planets as possible targets for exploration. Then look for ones with planets about the same distance from the sun as Earth. Once we have better instruments, we would then want to look for evidence of water, oxygen etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We just have no data about how life in any other environment may evolve. Even the weird and wonderful bacteria we find in extreme environments on Earth are still on Earth.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-09-04T23:04:01.580" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1774" PostTypeId="2" ParentId="1762" CreationDate="2013-09-05T02:17:19.887" Score="13" Body="&lt;p&gt;The key thing to doing this is to accelerate some of the mass of Venus outside of the planet, or alternatively bringing in some mass. Thus, there are 2 main things that could be done to alter the rotation speed. These ideas are explained in &lt;a href=&quot;https://en.wikipedia.org/wiki/Terraforming_of_Venus#Rotation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Carefully plan a series of asteroid impacts in a direction where they can increase the rotational energy of the planet.&lt;/li&gt;&#xA;&lt;li&gt;Shoot out some part of Venus, most likely it's thick atmosphere, in to space beyond the escape velocity.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In addition, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation&quot; rel=&quot;nofollow noreferrer&quot;&gt;wind speed of Venus&lt;/a&gt; is higher than the land speed, so putting up some large wind blockers would help to convert some of that energy into rotational energy of the planet. However, this would be a fairly minimal effect, as this is already being done in some scale, although not as quickly as is desired.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, how else might this be accomplished? &lt;a href=&quot;http://xkcd.com/162/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Xkcd&lt;/a&gt; has a nice comic showing one way, spinning around in circles can cause the motion of the planet to change. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CCYPJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, in a &lt;a href=&quot;http://what-if.xkcd.com/41/&quot; rel=&quot;nofollow noreferrer&quot;&gt;what if&lt;/a&gt; scenario, they gave one way to change the rotation speed of the Earth, driving around in circles. That would work as well, although very slowly. But in theory a train could be built that circled the planet, solar powered, that would slowly increase the rotational speed of the planet, although it would be quite slow.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="4660" LastEditDate="2015-07-04T14:59:58.887" LastActivityDate="2015-07-04T14:59:58.887" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1775" PostTypeId="1" CreationDate="2013-09-05T03:00:52.943" Score="11" ViewCount="1489" Body="&lt;p&gt;The Dutch have started the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_One&quot; rel=&quot;nofollow&quot;&gt;Mars One&lt;/a&gt; Mars colonization program. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the next 20-30 years, they plan to build a little colony. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Humans will be living in lower gravity than the Earth's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any research about growing children in low gravity? Is it possible without mutation problems? Will these new humans, Martians, be bigger than 2 meters?&lt;/p&gt;&#xA;" OwnerUserId="601" LastEditorUserId="17174" LastEditDate="2016-10-20T13:04:40.273" LastActivityDate="2020-09-25T06:09:00.390" Title="How exactly does the Dutch &quot;Mars One&quot; colonization program envision dealing with the difference in Earth's and Mars' gravity?" Tags="|mars|colonization|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1776" PostTypeId="1" CreationDate="2013-09-05T03:22:01.837" Score="9" ViewCount="816" Body="&lt;p&gt;Consider this thought process:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;In selecting the propellant for a reaction engine, lower atomic mass is better because equipment is largely temperature limited, and lighter gas will get higher specific impulse&lt;/li&gt;&#xA;&lt;li&gt;Nuclear thermal rockets beat liquid-hydrogen specific impulse (over twice) by using &lt;em&gt;only&lt;/em&gt; Hydrogen, which is the lightest you can ever hope for&lt;/li&gt;&#xA;&lt;li&gt;Why not just use a big vat of Hydrogen?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In fact, I've even heard of proposals that would use ground-based lasers to heat Hydrogen, used as the rocket's reaction mass.  Both laser-thermal and nuclear-thermal use a unique form of heat input.  You can't just open the bottom of a $H_2$ gas pressure tank and fly it into orbit, because you're also limited by the thermal velocity of the molecules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that's not very convincing.  Why can't we just heat $H_2$ gas to the temperature you &lt;em&gt;would have&lt;/em&gt; gotten from the nuclear thermal rocket, then load that into a tank / rocket, and blast to orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure, you lose a large amount of thrust as the depletion of the tank reduces P and T.  And you have lots of structural mass from the (now) pressurized rocket boosters.  But the advantage is going from $18 \text{amu}$ of the reaction mass to $2 \text{amu}$.  I mean, there are valid cases for nuclear thermal where that would justify the weight of &lt;em&gt;an entire nuclear reactor&lt;/em&gt;.  After all, the main engineering reason we don't use &lt;em&gt;those&lt;/em&gt; is because of the radiation nightmare.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even in &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/surfaceorbit.php&quot;&gt;very fanciful compilations&lt;/a&gt; of would-be rocket technologies, I've never seen anyone touch on this concept.  Putting hot Hydrogen gas in a pressure tank just doesn't seem that difficult (okay, there's H embitterment, but it won't stay there long).  And after all, if the Hydrogen you put in the tank has similar conditions to what the exit from a nuclear thermal rocket would, &lt;em&gt;it's not that much different&lt;/em&gt;.  I can only think of one possible thing that could kill it - which is the structural mass for pressurization.  You could do that calculation, and my bet would be on it maintaining viability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kept this line of design out of the discourse in classical rocketry?  It's probably a laughable idea, but with my knowledge I have no way to rule it out.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="6944" LastEditDate="2021-01-15T16:39:35.153" LastActivityDate="2021-01-15T16:39:35.153" Title="What are the deal-breakers with a preheated Hydrogen gas tank orbital rocket?" Tags="|launch|fuel|design-alternative|conceptual|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1777" PostTypeId="2" ParentId="1775" CreationDate="2013-09-05T04:54:31.650" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;If we ever manage to overcome the &lt;a href=&quot;http://www.livescience.com/33047-space-sex-pregnancy.html&quot; rel=&quot;nofollow&quot;&gt;fertility and sex troubles of&#xA;  space&lt;/a&gt; , we'll probably be popping out little humanoid children on&#xA;  other planets. But our little tykes might not stay little for very&#xA;  long.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Researchers are working on ways to make &lt;a href=&quot;http://spacecycle.hs.uci.edu/&quot; rel=&quot;nofollow&quot;&gt;artificial gravity&#xA;  possible&lt;/a&gt; in order to make long flights easier on human bodies.&#xA;  According to NASA, most astronauts grow about 2 inches while they're&#xA;  in space because the reduced gravity causes the fluid between&#xA;  vertebrae to expand. They lose the height within 10 days of returning&#xA;  to Earth's crushing gravity. Because of the growth, NASA uses space&#xA;  suits that have extra room to accommodate the additional height.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Souce:- &lt;a href=&quot;http://www.livescience.com/33082-would-humans-born-on-mars-grow-taller-than-earthlings-.html&quot; rel=&quot;nofollow&quot;&gt;http://www.livescience.com/33082-would-humans-born-on-mars-grow-taller-than-earthlings-.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And also this topic is little bit of described on wikipedia as &lt;a href=&quot;http://en.wikipedia.org/wiki/Colonization_of_Titan&quot; rel=&quot;nofollow&quot;&gt;Colonization of titans&lt;/a&gt;&#xA;now of-course its about &lt;strong&gt;The Titan&lt;/strong&gt; but I am sure you will get answer to your &quot;Is there any research about growing children in low gravity?&quot; will be given.&lt;/p&gt;&#xA;" OwnerUserId="639" LastEditorUserId="49" LastEditDate="2013-09-05T10:07:07.413" LastActivityDate="2013-09-05T10:07:07.413" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1778" PostTypeId="2" ParentId="1775" CreationDate="2013-09-05T04:55:04.413" Score="7" Body="&lt;p&gt;The plans of the Dutch &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_One&quot;&gt;Mars One&lt;/a&gt; non-profit organization to establish a permanent human colony on Mars by 2023 are ambitious, to say the least. We actually have no idea on the possible adverse effects of &lt;em&gt;living&lt;/em&gt; in lower-than-Earth's gravity (2.64 times smaller than gravity on the Earth, or simplifying it for the sake of argument - roughly one third), all data we have is only limited to adults being exposed to micro-gravitational conditions for (on average) a lot less than a year, with the current record holder being the Russian cosmonaut Valeri Polyakov that was in space for nearly 438 days. That's not even a year and a half!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that's not even touching the surface of all the problems a human colony on Mars would be facing, even when contemplating procreation, let alone later raising colonists' offspring. Needless to say, we have never even sent any children in space, and as far as I'm aware there were no experiments made to even attempt to have sex in space (and if two love birds had a hanky panky on their own, without supervision of scientists while in space, that's none of our business, is it?). Anyway, we're dealing with a lot of unknowns here, but one related &lt;a href=&quot;http://www.livescience.com/33047-space-sex-pregnancy.html&quot;&gt;article on LiveScience&lt;/a&gt; discusses exactly this problem, attempting to add some perspective from the strictly theoretical standpoint on how complicated would it be to put &lt;em&gt;a bun in the oven&lt;/em&gt; while on another planet:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Pretty difficult, actually. Space, it turns out, essentially acts as&#xA;  one big birth control system. One space-sex study published last fall&#xA;  predicts that cosmic radiation would bombard the body at such a high&#xA;  rate during a long space trip or an extended stay on Mars that sperm&#xA;  count would decrease and fetuses wouldn't be able to properly develop&#xA;  in a space environment. Even though existing and proposed space&#xA;  conveyances have improved radiation protection, they don't contain&#xA;  nearly enough shielding to allow zygotes to develop. And even if a&#xA;  baby was able to make it out of the womb, it would have a high&#xA;  probability of birth defects from the radiation damage.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;And that's not the end of the potential problems. Animal studies have&#xA;  shown that exposure to radiation can kill egg cells in a fetus as far&#xA;  along as the second or third trimester, so any babies born would be&#xA;  sterile making it hard to populate a new planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it might not even matter how tall children would grow on Mars, because there's a very big likelihood there wouldn't be any. But even if they were, let's consider first a few effects of Earth's gravity on our development, before we'll try to answer your last question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, you might have noticed some shoes that fit your feet perfectly in the mornings might seem a bit tight later that same day, if you were &lt;em&gt;on your feet&lt;/em&gt; for the longer part of the day. That's because your weight exerted on your feet due to gravity actually enlarged them a bit. But that's something that will &lt;em&gt;normalize&lt;/em&gt; on its own when you sleep, right? Yes, your foot probably won't be any larger the next morning, otherwise we'd be spending a lot more time in shoe shops, maybe more than some would like it to. But the more interesting thing about this daily process is why do your feet &lt;em&gt;shrink&lt;/em&gt; back? Because while you sleep in the horizontal position, the gravitational force pushing your body mass downwards isn't affecting your feet so much anymore, compared to when you were standing, so the body has the time to regenerate and negate that effect of your weight from the previous part of the day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did I describe all this? So it's hopefully a bit easier to appreciate that gravity affects our bodies a great deal. Now, we can try and answer the fun part of your question. Would children born and raised on Mars grow taller?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to NASA, most astronauts grow about 2 inches while they're&#xA;  in space because the reduced gravity causes the fluid between&#xA;  vertebrae to expand. They lose the height within 10 days of returning&#xA;  to Earth's crushing gravity.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Because of the growth, NASA uses space suits that have extra room to&#xA;  accommodate the additional height.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Does this quote sound familiar? It's nearly identical to what I've been describing what happens with our feet on a daily basis. It is also a well know fact that we grow taller while we sleep, not because there is any magic to it while lying horizontally, but because you give your body the time needed to reverse the effect of the gravity on it while in vertical position. We are simply genetically programmed to grow taller than we the naturally grow in Earth's gravity. So if we can rationalize this, and at the same time have actually data this &lt;em&gt;growing in height&lt;/em&gt; happens even to adult astronauts when staying in microgravity for longer time, we can safely assume that the answer to your question is - yes.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-05T04:55:04.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1779" PostTypeId="1" AcceptedAnswerId="1780" CreationDate="2013-09-05T04:58:50.010" Score="8" ViewCount="674" Body="&lt;p&gt;I recently came across this video of an impact on Jupiter in 2012:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://youtu.be/5-msU-YVb9E&quot;&gt;http://youtu.be/5-msU-YVb9E&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curious about its origin, I did &lt;a href=&quot;http://o.canada.com/2012/09/12/what-hit-jupiter-anatomy-of-an-impact/&quot;&gt;some research&lt;/a&gt; and found that the video was taken at 11:35 UTC on September 10th, 2012, by amateur astronomer George Hall.  Now that the video had a bit more credence, I became curious as to the circumstances of the event.  The linked article claims the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Currently, the theory is that a 10-metre asteroid hit Jupiter early&#xA;  Monday morning, just below a region known as the Northern Equatorial&#xA;  Belt.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This article was written in September, 2012.  Has anyone else looked into this since that time?  Were any other instruments able to record or document this event? I would like some additional details on what exactly happened, if there are any to be had.&lt;/p&gt;&#xA;" OwnerUserId="62" LastActivityDate="2013-09-05T05:40:03.653" Title="What were the circumstances of this impact event on Jupiter in 2012?" Tags="|jupiter|impact|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1780" PostTypeId="2" ParentId="1779" CreationDate="2013-09-05T05:32:20.447" Score="8" Body="&lt;p&gt;To start off, here's an excerpt from &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_Jupiter_impact_event#2012_impact&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on this September 2012 Jupiter impact event&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;2012 impact&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;On 10 September 2012 at 11:35 UT amateur astronomer Dan Petersen&#xA;  visually detected a fireball on Jupiter that lasted 1 or 2 seconds&#xA;  using a Meade 12″ LX200. George Hall had been recording Jupiter with a&#xA;  webcam on his 12&quot; Meade; upon hearing the news Hall checked the video&#xA;  to see if the impact was captured. Hall had indeed captured a 4-second&#xA;  clip of the impact and released the video to the public. The impact's&#xA;  estimated position in the system was longitude = 345 and latitude =&#xA;  +2. Dr Michael H. Wong estimated that the fireball was created by a meteoroid less than 10 meters in diameter. Several collisions of this&#xA;  size may happen on Jupiter on a yearly basis. The 2012 impact was the&#xA;  6th impact observed on Jupiter, and the fourth impact seen on Jupiter&#xA;  between 2009 and 2012. It was quite similar to the flash observed on&#xA;  20 August 2010.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In a report posted on the &lt;a href=&quot;http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5413921/Main/5413225&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cloudy Nights forum&lt;/a&gt;, Peterson said it wasn't clear if the impact would leave a scar on Jupiter much like those seen in 1994, when the &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;comet Shoemaker-Levy 9&lt;/a&gt; broke into pieces and peppered the planet with debris, creating visible impact marks in Jovian cloud tops. It is also not yet known if the object to strike Jupiter was a small asteroid or comet. Peterson wrote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;My best guess is that it was a small undetected comet that is now history, hopefully it will sign its name on Jupiter's cloud tops.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But, according to &lt;a href=&quot;http://cosmicdiary.org/fmarchis/2012/09/11/some-news-from-the-september-2012-jupiter-flash/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Franck Marchis' Cosmic Diary blog post on the 2012 Jupiter impact event&lt;/a&gt;, several images were taken before and after the flash by amateurs astronomers around the world, but none were able to detect any debris field visible at the impact site:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://cosmicdiary.org/fmarchis/files/2012/09/2012sep10_impact-beforeafter-hires_v2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/n3P6C.jpg&quot; alt=&quot;September 2012 Jupiter impact event before and afer observations&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Observations of Jupiter collected before and after the event by amateur astronomers. The arrow indicates the position of the flash.&lt;br/&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;No debris field can be seen. Click the image for a higher resolution. (credit: J.H. Rogers, &lt;a href=&quot;http://britastro.org/baa/&quot; rel=&quot;nofollow noreferrer&quot;&gt;British Astronomical Association&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Despite the large uncertainty on the generated light curve he [Mike Wong]&#xA;  predicted &lt;em&gt;“that this event is too small to create a visible impact&#xA;  scar“&lt;/em&gt;. The total amount of energy that was derived for this impact is&#xA;  very close to the &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_Jupiter_impact_event&quot; rel=&quot;nofollow noreferrer&quot;&gt;June 2010 event&lt;/a&gt; one which was analyzed and discussed&#xA;  by R. Hueso and his colleagues in &lt;a href=&quot;http://adsabs.harvard.edu/abs/2010ApJ...721L.129H&quot; rel=&quot;nofollow noreferrer&quot;&gt;arXiv:1009.1824&lt;/a&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;At the present, since no debris field has been detected and the total&#xA;  energy seems to be low, we can conclude that the flash was most likely&#xA;  a meteor due to a small (diameter &amp;lt;10m assuming a density of 2 g/cc)&#xA;  meteoroid. Even if the event may be 500 times less energetic than&#xA;  recent estimates of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Tunguska_event&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tunguska event&lt;/a&gt;, it is remarkable that an&#xA;  amateur astronomer equipped with a 12″ telescope and a webcam could&#xA;  record it at 730 million of km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I've included in the quote a link to the &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_Jupiter_impact_event&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page on 2010 Jupiter impact event&lt;/a&gt; that also lists a few other detected Jupiter impacts, but as you might be aware, there were so far 6 impacts observed on Jupiter, among which probably the most famous impact of the &#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;comet Shoemaker–Levy 9&lt;/a&gt;, in July 1994.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wasn't able to find any reference that any of the observatories around Jupiter, like maybe the &lt;a href=&quot;http://saturn.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini-Huygens&lt;/a&gt; detected this impact from 2012.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-05T05:40:03.653" LastActivityDate="2013-09-05T05:40:03.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1781" PostTypeId="1" AcceptedAnswerId="1791" CreationDate="2013-09-05T06:45:48.803" Score="32" ViewCount="12330" Body="&lt;p&gt;So far, all the space exploration related events that resulted in Loss of Crew (LOC) took a complete crew, and as far as we're aware, no accident in space that resulted in the loss of human life, be it during launch, descent, or even while on the launchpad (&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_1&quot; rel=&quot;noreferrer&quot;&gt;Apollo 1&lt;/a&gt;), left any surviving crew member astronauts/cosmonauts.&lt;/p&gt;&#xA;&lt;p&gt;This got me thinking that there must be some procedures in place that would be followed by surviving crew members in the eventuality of a loss of a single or a few members, but not a complete crew. I'm in particular interested in procedures in place while aboard space stations or manned spacecraft in space, but since that's possibly still a bit too broad, let's limit ourselves to how such events are supposed to be handled by surviving crew members aboard the International Space Station (ISS).&lt;/p&gt;&#xA;&lt;p&gt;A few points to my question that I'd like answers to address/discuss:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Body retrieval in the event of LOC during Extravehicular Activity (EVA)&lt;/li&gt;&#xA;&lt;li&gt;Body storage aboard the spacecraft/space station and transport to the Earth&lt;/li&gt;&#xA;&lt;li&gt;Procedures that are in place, and in effect for surviving crew of any nationality&lt;/li&gt;&#xA;&lt;li&gt;Notifying spouse and/or family members, and their further contact with the surviving crew&lt;/li&gt;&#xA;&lt;li&gt;How much do LOC procedures vary across nations participating in activities aboard the ISS&lt;/li&gt;&#xA;&lt;li&gt;Order of importance (priority) of any of these procedures&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;As I suspect such documentation might already be available online, I will accept answers covering most of these points above in freeform quotes and my own description, and referencing used literature (attaching links, where possible). The scope of the question is trying to get an impression of such procedures, with an emphasis on the effects of such an event on the surviving crew members and their ability to cope with it, and how much they can count on support from the ground crew.&lt;/p&gt;&#xA;&lt;p&gt;In a nutshell, how well are the ISS astronauts prepared for any such eventuality?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="39182" LastEditDate="2021-04-26T05:07:13.487" LastActivityDate="2021-04-26T05:07:13.487" Title="What are the procedures aboard the ISS in the eventuality of a loss of a crew member?" Tags="|iss|death|procedure|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="1782" PostTypeId="2" ParentId="1776" CreationDate="2013-09-05T11:55:47.390" Score="5" Body="&lt;p&gt;Actually, for gasses, higher atomic numbers are actually better than lower atomic numbers. This is the primary reason why Xenon is the gas of choice for ion drives, it is the most massive elemental non-radioactive gas around. The reason behind this logic falls under the Ideal Gas Law, $PV=nRT$. Essentially, as the temperature goes up, so does the pressure. However, as the atomic number goes up, the pressure goes down. As the most important part of spacecraft is the momentum, and momentum equals mass times speed, basically you want the most mass for your volume. Liquid hydrogen, however, can be stored much denser.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, for your specific idea, what pressure would the hydrogen need to be stored at to make it work? Well, the proposed temperature for &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;nofollow&quot;&gt;NTR rockets&lt;/a&gt; is around 3000k. Let's assume you want a density similar to liquid hydrogen as well. That density, according to google, is 70.85 kg/m³. Okay, so let's take those numbers and plug them into the ideal gas law, assuming a volume of 1 m³. That gives a total of 35425 mols of hydrogen. Using &lt;a href=&quot;http://www.chemicool.com/cgi-bin/gaslaws.pl&quot; rel=&quot;nofollow&quot;&gt;this calculator&lt;/a&gt;, that would provide a pressure of 9514 atmospheres, which would need to be maintained at 3000K. I don't know of any substance that can contain such pressure that wouldn't weight a ton.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just for the fun of it, let's assume a super dense gas, say, &lt;a href=&quot;https://en.wikipedia.org/wiki/Sulfur_hexafluoride&quot; rel=&quot;nofollow&quot;&gt;Sulfur Hexafluoride&lt;/a&gt;. I'm going to ignore the fact that this would almost certainly be banned due to it's extremely high greenhouse gas potential. It's atomic mass number is 146, thus the number of mols per 1 m³, assuming the same mass as hydrogen, would be 485.3. Putting it in to the same calculator, the pressure would be 130 atmospheres, which is manageable, but probably not at such high temperatures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line, storing high temperature, high pressure substances is pretty much impossible with today's technology. Perhaps sometime in the future someone will come up with a better way to make this happen, but for now, I wouldn't count on it.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="195" LastEditDate="2015-05-19T20:23:31.707" LastActivityDate="2015-05-19T20:23:31.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1783" PostTypeId="1" AcceptedAnswerId="1784" CreationDate="2013-09-05T13:45:46.993" Score="76" ViewCount="55146" Body="&lt;p&gt;Recently, Donald.McLean posted this image in &lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/the-pod-bay&quot;&gt;The Pod Bay&lt;/a&gt;, the site chat room:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eJy3b.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a rather disturbing image once you think about it for more than about half a second. &lt;strong&gt;Did it really happen? What consequences is it going to have for the mission?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2015-08-24T00:08:43.813" LastActivityDate="2019-11-06T23:42:07.260" Title="Was the NOAA-N Prime satellite really dropped on the floor?" Tags="|artificial-satellite|failure|myth-busting|integration|noaa|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1784" PostTypeId="2" ParentId="1783" CreationDate="2013-09-05T13:55:18.280" Score="62" Body="&lt;p&gt;Yes, it really happened.&lt;/p&gt;&#xA;&lt;p&gt;It took place at the Lockheed Martin Space Systems factory in Sunnyvale, California. As the team was turning the satellite into a horizontal position, they found out that the twenty-four bolts that were supposed to hold it in place had been removed by a technician - and the action was never documented.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/home/hqnews/2004/oct/HQ_n04158_noaa_n_mishap.html&quot; rel=&quot;noreferrer&quot;&gt;According to NASA&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The NASA Mishap Board investigating damage to the NOAA-N-Prime satellite has released its final report.&lt;/p&gt;&#xA;&lt;p&gt;On September 6, 2003, the National Oceanic and Atmospheric Administration (NOAA) N-Prime satellite fell to the Lockheed Martin Space Systems Company factory floor in Sunnyvale, Calif. Technicians were working on the spacecraft, when the accident happened. The spacecraft suffered significant damage.&lt;/p&gt;&#xA;&lt;p&gt;The Board, chaired by the NASA Deputy Associate Administrator of the Science Mission Directorate, had representatives from NASA research centers and the Air Force Space Command, as well as advisors from NOAA and the Federal Aviation Administration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The full report is &lt;a href=&quot;http://www.nasa.gov/pdf/65776main_noaa_np_mishap.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;From &amp;quot;Final Launch in NOAA Series&amp;quot; in &lt;em&gt;Aviation Week &amp;amp; Space Technology&lt;/em&gt;, 26 January 09:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;After it suffered significant damage when it tipped over on the factory floor in September 2003, the contractor used fees earned on the NOAA spacecraft it previously built to cover the cost of repairs.&lt;/p&gt;&#xA;&lt;p&gt;Fortunately, since NOAA-N Prime was the last in the series, there were backup instruments and components available, so the contractor required “only a few” new components to fill in gaps, said NASA’s Wayne McIntyre, program manager for the Polar-orbiting Environmental Satellite, during a briefing on Jan. 22.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The satellite was launched in February 2009.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-05T14:06:03.680" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1785" PostTypeId="2" ParentId="1783" CreationDate="2013-09-05T13:55:59.830" Score="16" Body="&lt;p&gt;This satellite, NOAA-19 or NOAA-N Prime was launched in February 2009. The pictured incident happened in 2003:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The spacecraft has had a troubled history at Lockheed Martin’s Sunnyvale, Calif., factory. After it suffered significant damage when it tipped over on the factory floor in September 2003, the contractor used fees earned on the NOAA spacecraft it previously built to cover the cost of repairs.&lt;/p&gt;&#xA;&lt;p&gt;Fortunately, since NOAA-N Prime was the last in the series, there were backup instruments and components available, so the contractor required “only a few” new components to fill in gaps, said NASA’s Wayne McIntyre, program manager for the Polar-orbiting Environmental Satellite, during a briefing on Jan. 22.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;From &amp;quot;Final Launch in NOAA Series&amp;quot; in &lt;em&gt;Aviation Week &amp;amp; Space Technology&lt;/em&gt;, 26 January 09&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-05T13:55:59.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1786" PostTypeId="1" AcceptedAnswerId="1789" CreationDate="2013-09-05T14:02:52.607" Score="9" ViewCount="938" Body="&lt;p&gt;NASA seems to have a mission planned to send a 2016 lander to Mars, and they say that they are in the process of choosing landing targets. What factors are considered when selecting these sites? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-05T14:10:33.997" LastActivityDate="2013-10-09T17:20:23.123" Title="What factors are considered when choosing a Mars lander landing site?" Tags="|mars|lander|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1787" PostTypeId="2" ParentId="1776" CreationDate="2013-09-05T14:05:53.810" Score="5" Body="&lt;p&gt;I found it.  The reason is because of the pressure vessel weight.  My intuition was wrong on that point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume a long cylindrical tank.  The weight of the tank itself is the following.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ M_t = 2 PV \frac{\rho}{\sigma} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass of gas in the tank is the following where $m$ is the formula mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ PV = N R T = \frac{ M_g }{m} RT $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combine these two to get the ratio of the gas weight to the tank weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{M_g}{M_p} = \frac{ m \frac{\sigma}{\rho} }{ 2 RT } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the best options for structural material is carbon fiber.  The strength to weight value for that is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{ \sigma}{\rho} = 2457 \frac{kN-m}{kg} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I assume that the gas is held at $1500 K$, then that limits the idea to a pathetic specific impulse of $440 s$.  This is useful for a conservative calculation.  If the tank to gas mass ratio isn't good for this value, it will never be a competitive idea. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{M_g}{M_p} = \frac{ ( 1 \text{amu} ) \left(2457 \frac{kN-m}{kg} \right) }{ 2 R (1500 K) } = 0.2 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tank would weight &lt;strong&gt;5 times&lt;/strong&gt; as much as the propellant it stores.  This is with the most optimistic assumptions.  This rules out the idea.  We conclude this idea is basically impossible because it is impossible to make a material light enough and strong enough to pull it off.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-09-05T14:05:53.810" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1788" PostTypeId="2" ParentId="1786" CreationDate="2013-09-05T14:49:06.540" Score="1" Body="&lt;p&gt;The landing site selected based on two factors &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;scientific factors  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;engineering factors &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Scientific Factors&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing site must be selected such that it should be very close to the testing area (such as habitable zone)&#xA;where experiments are to be conducted&#xA; It include geological , mineralogical, and &#xA;meteorological. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Engineering Factors&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is protect&#xA;the rover because of the engineering constraints. Includes  dust&#xA;factors (Mars’ ubiquitous red dust&#xA;can interfere with all sorts of&#xA;machine parts and instruments,&#xA;particularly optics which was a&#xA;problem experienced in the last rover&#xA;missions Spirit and the still working&#xA;Opportunity), atmospheric conditions&#xA;during landing, elevation of the&#xA;Martian surface, surface roughness&#xA;and inclination and finally solar&#xA;surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surface roughness and inclination is a&#xA;major factor for missions especially&#xA;those that use airbags to cushion&#xA;their landings.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="692" LastEditDate="2013-10-09T17:20:23.123" LastActivityDate="2013-10-09T17:20:23.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1789" PostTypeId="2" ParentId="1786" CreationDate="2013-09-05T14:50:25.760" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://webgis.wr.usgs.gov/mer/March_2002_presentations/Adler/adler_landing3-factors.pdf&quot; rel=&quot;nofollow&quot;&gt;This presentation for Mars Exploration Rover landing site selection&lt;/a&gt; gives a good overview of the typical engineering considerations for a robotic Mars science mission landing site.  &lt;a href=&quot;http://www.mars.asu.edu/christensen/docs/golombek_mersites_jgr.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; gives more details including the science considerations.  MER is a good example for the consideration of safety, mission success, and science objectives for landing site selection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find a large amount of material on the Curiosity landing site selection &lt;a href=&quot;http://marsoweb.nas.nasa.gov/landingsites/index.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;InSight is a little different from previous landing site selections, though similar to the one for Mars Pathfinder in some ways.  For InSight, the selection has fewer factors to consider, since the science mission doesn't care very much about the characteristics of the landing site.  All they really need for the science is good contact with firm ground for the transmission of seismic signals.  The InSight landing site selection is all about landing safety and solar power for the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;InSight recently narrowed down to four landing sites: Elysium, Elysium, Elysium, and Elysium.  They're all in the same place since it is a uniquely large, flat, low rock abundance site, near the equator for good solar power.  &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2013/pdf/1691.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; provides many more details on that site selection.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-05T19:54:34.237" LastActivityDate="2013-09-05T19:54:34.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1790" PostTypeId="1" AcceptedAnswerId="1792" CreationDate="2013-09-05T15:09:24.393" Score="8" ViewCount="677" Body="&lt;p&gt;I've heard somewhere that we have better maps of the surface of Mars than the deepest areas of the sea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how accurate are the maps really? Also, how much we have mapped about the surface of Mars?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="25" LastEditDate="2013-09-05T17:45:18.540" LastActivityDate="2013-09-05T17:45:18.540" Title="How many square kilometres of Mars is exactly mapped?" Tags="|mars|resolution|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1791" PostTypeId="2" ParentId="1781" CreationDate="2013-09-05T17:34:06.853" Score="34" Body="&lt;p&gt;During the Gemini missions, when we started sending more than one person at a time in a spacecraft and also doing thinks like EVAs, these eventualities were discussed in detail.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Body retrieval - wouldn't happen. SOP, if a crew member becomes separated from the ship (alive or dead) and cannot return to it, is to leave him behind. Energy, such as fuel, is a fundamental constraint of our spacecraft, and spending it to retrieve a stranded crewman may put the entire mission in jeopardy.&lt;/li&gt;&#xA;&lt;li&gt;Body storage - Only an issue if the person died while aboard the spacecraft (as in the previous point). Procedures would depend on the spacecraft, its current configuration, the agenda for the mission and the wishes of the family. If the family wishes it and it's possible, the body can be returned to Earth by simply strapping it into the person's seat, or storing it in their sleeping bag. Almost any situation involving a crew fatality, however, would likely be an emergency, and such concerns would be secondary. &#xA;&lt;ul&gt;&#xA;&lt;li&gt;A fatality of one crewmember on a Gemini mission would depend on whether an EVA was planned; the body may be released into space during the EVA. If an EVA was not planned or had already occurred, the deceased crewmember would likely be belted in their seat and would simply ride back down to Earth.&lt;/li&gt;&#xA;&lt;li&gt;For a fatality on an Apollo moon mission while in transit to the moon, the mission would likely be aborted in much the same manner Apollo 13 was. While in lunar orbit, if the CMP died with the LEM on the lunar surface, that may well be a fatal situation for the entire crew, as the CMP has to coordinate with the LEM as it returns. Either of the pair in the LEM, while it's on the lunar surface, could be left there if they died. During the return leg, the body could be placed in a spacesuit (to control the odor) and left in the LEM to be jettisoned. After LEM jettison, the body would simply be strapped into the person's seat and would ride down to Earth with the rest of the crew, likely being committed to the sea by the crew of the recovery vessel.&lt;/li&gt;&#xA;&lt;li&gt;A fatality aboard the Shuttle would either be committed to space from the airlock or would be stowed in their sleeping bag for return to Earth.&lt;/li&gt;&#xA;&lt;li&gt;A fatality aboard the ISS, or any other station, would likely be handled similarly, with the additional option of placing the body aboard a Soyuz or Dragon for return to Earth.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;As far as procedures for surviving crewmembers, if they're in contact with Mission Control, the Flight Director makes the call regarding whether to continue or abort, and what the surviving crewmembers should do with the body. If they're not in contact with the ground at the time, and regaining contact is unlikely, the ranking member of the crew makes the call. In almost all cases, while there are specialists on every crew, everyone can do anyone's job when it comes to keeping the spacecraft running and performing basic mission functions. This is in contrast to most space sci-fi, where it's often a critical plot point that only one guy knows how to do something mission-critical. Even the situation in &lt;em&gt;Apollo 13&lt;/em&gt; where Swigert's being counted on to dock with the LEM, was fictionalized a little; in real life, if Swigert couldn't have docked the CSM with the LEM, either of the other two people in that spacecraft could have, and everyone would have gotten a try before they scrubbed the mission.&lt;/li&gt;&#xA;&lt;li&gt;Notifying spouse/family members: This will happen as soon as the death is confirmed, because NASA's communications loop is available to the media, and so if there's a problem the media will know about it very quickly and tell the entire world about it.&lt;/li&gt;&#xA;&lt;li&gt;I wouldn't know how different any member agency's procedures could be; what's possible becomes sharply limited when you're in a tin can, hundreds of miles from help.&lt;/li&gt;&#xA;&lt;li&gt;The #1 priority in an LOC scenario is always the life and safety of the surviving crew, followed very closely by salvaging as much of the mission as is possible. These priorities may be reversed in the right circumstances; while humans don't like to discuss assigning a monetary value to life, it happens regularly for a variety of practical and legal reasons. By most such measurements (e.g. lifetime earnings potential), the vehicle and the mission, even in a &quot;routine&quot; trip to the ISS or deploying/maintaining a satellite, are many orders of magnitude more expensive than any one crewmember.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="411" LastEditorUserId="411" LastEditDate="2013-09-10T16:40:56.597" LastActivityDate="2013-09-10T16:40:56.597" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1792" PostTypeId="2" ParentId="1790" CreationDate="2013-09-05T17:44:03.910" Score="5" Body="&lt;p&gt;The highest global coverage of Mars belongs to the &lt;a href=&quot;http://themis.asu.edu/news/themis-camera-yields-best-mars-map-ever&quot;&gt;THEMIS camera&lt;/a&gt;, which has a resolution of 100m per pixel. CTX will likely have global coverage as well, it is currently at &lt;a href=&quot;http://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=3&amp;amp;ved=0CDUQFjAC&amp;amp;url=http://www.msss.com/news/index.php?id=43&amp;amp;ei=074oUp6vILal4APugoHQAg&amp;amp;usg=AFQjCNF-KvoIK9jBlBUZ9Tt1L8TpB8GlOQ&amp;amp;bvm=bv.51773540,d.dmg&quot;&gt;75% coverage&lt;/a&gt;, as of April 12, 2012. No doubt this is even higher since then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The highest resolution from orbit at Mars is the HiRISE camera, at 25 cm/pixel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's Oceans, for instance, don't have near the resolution. There is &lt;a href=&quot;http://www.nytimes.com/1995/10/24/science/map-makes-ocean-floors-as-knowable-as-venus.html&quot;&gt;an article&lt;/a&gt; from 1995 for instance that states that the Ocean Bottom was finally mapped to Venus's resolution, for instance. The &lt;a href=&quot;http://www.marine-geo.org/portals/gmrt/&quot;&gt;best map&lt;/a&gt; of the Ocean floor appears to be 100m resolution, with some areas of higher resolution.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-05T17:44:03.910" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1793" PostTypeId="2" ParentId="1232" CreationDate="2013-09-05T17:48:25.077" Score="11" Body="&lt;p&gt;&lt;sub&gt;&lt;em&gt;Note: This answer has changed as of July 23, 2014. Because of the April 11, 2014 external MDM failure and some additional corrections and improvements, both the data &lt;strong&gt;and&lt;/strong&gt; analysis portions of this answer have changed, so read carefully. If you are interested in the previous versions of this answer, please view the edit history.&lt;/em&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How often are they replaced?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/aJE0T.png&quot; alt=&quot;ISS MDM Replacements&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pOX1D.png&quot; alt=&quot;Replacements by Tier&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1011t.png&quot; alt=&quot;Primary vs. Backup&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UJRUT.png&quot; alt=&quot;Enhanced vs. Standard&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources: &lt;a href=&quot;http://lubbockonline.com/stories/042801/upd_shuttle.shtml&quot; rel=&quot;noreferrer&quot;&gt;[2001]&lt;/a&gt;, &lt;a href=&quot;http://spacese.spacegrant.org/uploads/images/ISS/ISS%20SE%20Case%20Study.pdf&quot; rel=&quot;noreferrer&quot;&gt;[2007]&lt;/a&gt;, &lt;a href=&quot;http://www.nasaspaceflight.com/2014/04/imminent-spacewalks-external-mdm-failure-iss/&quot; rel=&quot;noreferrer&quot;&gt;[2014]&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In previous analysis (see edit history), I erroneously included an MDM replacement on a Space Shuttle in 2008 as part of the ISS MDM replacements. I also skimmed over the 2007 replacement and did not realize that it involved replacing not one but &lt;em&gt;three&lt;/em&gt; MDMs. The MDMs (best described in &lt;a href=&quot;http://www51.honeywell.com/aero/common/documents/MDMs.pdf&quot; rel=&quot;noreferrer&quot;&gt;this infosheet&lt;/a&gt; from Honeywell) are divided into three tiers. There are three Tier 1 MDMs responsible for Command and Control. One is primary, and the other two are backups. There are six categories of Tier 2 MDMs which relay commands to various systems. Two MDMs exist for each category--one primary, one backup. Finally, there are three categories of Tier 3 MDMs which execute commands from the Tier 2 MDMs. In the Internal Systems category are 11. Twelve are Power Systems MDMs, and 8 are External Systems MDMs. In all, that's 46 MDMs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is notable that in the first decade of the life of the ISS three backup Tier 1 MDMs have had to be replaced, and one primary Tier 1 MDM had to be replaced at the same time as the backups.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Can we confidently say that the reliability of MDMs is increasing, that is, can we disentangle the effects of solar cycles from changes in manufacturing quality?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/h5wZt.gif&quot; alt=&quot;Sunspot Number Progression&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/12hhJ.gif&quot; alt=&quot;Radio Flux Progression&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0RAKy.gif&quot; alt=&quot;Solar Cycle Ap Progression&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;http://www.swpc.noaa.gov/SolarCycle/&quot; rel=&quot;noreferrer&quot;&gt;[Source of NOAA Charts]&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we only have three data points, there is a high margin of error for extrapolating from them. We can see that the first replacement coincided with a crest in solar activity, and the next three coincided with a trough, though there were momentary spikes in magnetic activity that may have coincided with the MDM failure. The most recent MDM failure &lt;em&gt;did&lt;/em&gt; coincide with a crest in activity, but unlike the previous failures this was an external MDM and not one of the Tier 1 MDMs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this it is difficult to derive a correlation between the solar cycle and the need to replace MDMs. &lt;strong&gt;However&lt;/strong&gt;, it is notable that both solar activity crests in the lifetime of the ISS coincided with MDM failures! We cannot say with any confidence that the reliability of MDMs is increasing. It seems that a few MDMs need to be replaced every 6 years or so. The Command and Control MDMs seem especially vulnerable, which is a serious issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What are the lessons from the ISS avionics experience for our future missions beyond LEO?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only five of the 46 MDMs have had to be replaced during the life of the ISS. A few do seem to need to be replaced every six years or so, so it may be necessary to take this into account for future missions beyond LEO. However, since we have little data on the effect of the solar cycle on MDMs, it is difficult to tell what effect increased radiation beyond LEO will have on MDMs. It may be that increased radiation results in a greater frequency of computer failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vulnerability of the Tier 1 MDMs is particularly troublesome if solar activity is connected to the failure, because the more extreme environment beyond LEO may make this untenable with the current technology. On the other hand, there hasn't been another Tier 1 failure since 2007, so perhaps the issue has already been handled.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-07-23T17:14:24.897" LastActivityDate="2014-07-23T17:14:24.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1794" PostTypeId="2" ParentId="1783" CreationDate="2013-09-05T20:43:48.863" Score="21" Body="&lt;p&gt;The story behind this is one of several layers of procedures, as is documented in &lt;a href=&quot;https://www.nasa.gov/pdf/65776main_noaa_np_mishap.pdf&quot; rel=&quot;noreferrer&quot;&gt;a review&lt;/a&gt; of NASA &quot;mishaps&quot;. Essentially, the story comes down to the following sequence of events:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A team prepared the vibration testing, including putting the bolts on.&lt;/li&gt;&#xA;&lt;li&gt;A second team removed the bolts, to use for another purpose. This removal was not documented at the time.&lt;/li&gt;&#xA;&lt;li&gt;The first team comes back to the satellite, doesn't see that the bolts were removed, and continues on with their test.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In addition, there were numerous steps that were not followed, such as not having a safety person in attendance for a potentially hazardous test. In fact, they came up with several suggested tips for anyone in this situation in said report. Here's the highlights&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Communicate all changes on the floor to all technicians and supervisors.&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Is this really working in your program now?&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;li&gt;Do not back stamp procedures&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Are your technicians doing this now?  This has been a factor in many mishaps!&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;li&gt;If an audit or an investigation identifies a problem, or a non-conformance, fix it!  – This has been a factor in previous mishaps&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-02-20T13:03:27.683" LastActivityDate="2017-02-20T13:03:27.683" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1795" PostTypeId="1" CreationDate="2013-09-05T22:36:36.553" Score="6" ViewCount="506" Body="&lt;p&gt;Lithium Ion Batteries are becoming more and more popular in spacecraft, due to their high power per mass ratio. There are several companies, such as &lt;a href=&quot;http://www.quallion.com/sub-ms-satellites.asp&quot;&gt;Quallion&lt;/a&gt;, advertising their space rated batteries. What is the proper way to charge them to optimize battery life, and prevent fires such as occurred in the Boeing 787 Dreamliner?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, &lt;a href=&quot;http://spaceflightnow.com/news/n1304/30issbattery/#.UikOi3-NPfM&quot;&gt;the ISS&lt;/a&gt; is planning on launching them in 2016, as are several other missions I am aware of.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-05T23:13:48.583" LastActivityDate="2013-09-06T06:16:04.867" Title="What is the proper way to charge Lithium Ion batteries?" Tags="|power|battery|lifetime|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1796" PostTypeId="2" ParentId="1795" CreationDate="2013-09-06T06:16:04.867" Score="3" Body="&lt;p&gt;Although this question belongs to physics or chemistry still following is the answer you are looking for.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The  typical charging protocol for the Li-Ion cells with layered&#xA;  cathodes includes a constant current  charge to a voltage of 3.9 V to&#xA;  4.2 V (depending on the metal oxide cathode and manufacturer’s  recommendations) and held at constant voltage until the current falls&#xA;  down to approximately  C/50 or C/100 (this can vary according to the&#xA;  manufacturer).  The term, “C” signifies the charge  or discharge rate,&#xA;  in Amperes, expressed as a multiple of the rated capacity in&#xA;  Ampere-hours  (Ah).  Due to the unique charging characteristic of the&#xA;  Li-Ion cells and batteries, charging  requires a dedicated charger&#xA;  that can keep the cells and batteries within their specified voltage &#xA;  limits.  This charger may be a “smart” charger in some cases.  The&#xA;  discharge of the cell depends  on the load used, but the end voltage&#xA;  during discharge should not go below 2.5 V.  Typical end  of discharge&#xA;  voltages for the batteries in different equipment has been 3.0 V/cell.&#xA;  Internal  resistance for the Li-Ion cells varies from 9 to 120 mΩ for&#xA;  small (1 to 3 Ah) cells to about 0.8  mΩ for large (190 Ah) cells&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;this the quote from the NASA document called &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090023862_2009023573.pdf&quot; rel=&quot;nofollow&quot;&gt;Guidelines on Lithium-ion Battery Use in Space &#xA;Applications&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="639" LastActivityDate="2013-09-06T06:16:04.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1797" PostTypeId="1" AcceptedAnswerId="1822" CreationDate="2013-09-06T09:43:31.327" Score="5" ViewCount="389" Body="&lt;p&gt;Space is apparently filled with lots of debris that did not make a star, system, or planet. In a pinch asteroids, and comets may serve to illustrate. Asteroids are typically flagged as carbon, stone, or metallic. Thus such debris varies widely in mass - size need not accompany mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A metallic asteroid is believed to be possessed of a Ni-Fe core. Could such debris agglutinate to form a core of higher/heavier elements? What if some asteroids were to be the debris of an ore-rich planet/satellite that got too close to it's primary? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is any small celestial body (other than a star, nebula, whatever) known to be a spontaneous radiator/emitter?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2016-03-02T03:34:46.473" LastActivityDate="2023-06-14T23:14:44.877" Title="Is any non-planetary celestial body smaller than a star known to be a spontaneous radiator?" Tags="|radiation|asteroid|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1798" PostTypeId="5" CreationDate="2013-09-06T10:13:52.357" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-06T10:13:52.357" LastActivityDate="2013-09-06T10:13:52.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1799" PostTypeId="4" CreationDate="2013-09-06T10:13:52.357" Score="0" Body="Questions regarding atmospheric drag which includes the resistance offerd by a moving object in  fluid" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-09-06T11:23:55.333" LastActivityDate="2013-09-06T11:23:55.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1800" PostTypeId="1" AcceptedAnswerId="1803" CreationDate="2013-09-06T10:35:05.760" Score="8" ViewCount="1141" Body="&lt;p&gt;Plastics are &lt;a href=&quot;https://en.wikipedia.org/wiki/Plastic&quot;&gt;made from organic solids&lt;/a&gt;, and &lt;a href=&quot;http://www.nbcnews.com/science/plastic-could-shield-astronauts-space-radiation-6C10326039&quot;&gt;recent studies indicate&lt;/a&gt; that pound for pound they are a better shield against &lt;a href=&quot;https://en.wikipedia.org/wiki/Galactic_cosmic_rays&quot;&gt;galactic cosmic rays&lt;/a&gt; then some metals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are lifting mass to orbit or moving it around, less mass is best. Asteroids and comets are obvious sources for metals. These may prove more economical sources for metals then sources launched from Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about source materials for making plastics, are there any sources in our solar system other then Earth for raw materials?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2014-01-19T08:19:03.210" LastActivityDate="2014-01-19T08:19:03.210" Title="What source of raw materials could be found in space for making plastics?" Tags="|materials|natural-resources|cosmic-radiation|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1801" PostTypeId="5" CreationDate="2013-09-06T12:25:47.077" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-06T12:25:47.077" LastActivityDate="2013-09-06T12:25:47.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1802" PostTypeId="4" CreationDate="2013-09-06T12:25:47.077" Score="0" Body="Questions regarding Radioactive decay, also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses its energy by emitting particles of ionizing radiation." OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-09-06T13:19:34.387" LastActivityDate="2013-09-06T13:19:34.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1803" PostTypeId="2" ParentId="1800" CreationDate="2013-09-06T12:36:42.463" Score="7" Body="&lt;p&gt;&lt;strong&gt;YES&lt;/strong&gt; there are raw materials for making plastics other than found on earth(as far as we explored the solar system). Since major of raw material of the plastics are from the petrochemicals (they are chain of carbons and other organic compounds).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible to grow genetically engineered &lt;a href=&quot;http://www.sciencedaily.com/releases/2010/11/101108140638.htm&quot; rel=&quot;noreferrer&quot;&gt;plants&lt;/a&gt; in space to extract raw material for making plastics &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We've engineered a new metabolic pathway in plants for producing a kind of fatty acid that could be used as a source of precursors to chemical building blocks for making plastics such as polyethylene&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Rich organic compounds are found in the meteorites and also in the space dust &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ethane and methane are found in the &lt;a href=&quot;http://www.en.wikipedia.org/wiki/Lakes_of_Titan&quot; rel=&quot;noreferrer&quot;&gt;lakes  of titan&lt;/a&gt; but they must &lt;a href=&quot;https://engineering.stanford.edu/news/how-methane-sourced-polymers-could-save-world&quot; rel=&quot;noreferrer&quot;&gt;processed&lt;/a&gt; to &lt;a href=&quot;http://www.news.psu.edu/story/140855/1998/09/01/research/converting-methane&quot; rel=&quot;noreferrer&quot;&gt;convert&lt;/a&gt; them into a polymer &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-09-06T16:56:21.607" LastActivityDate="2013-09-06T16:56:21.607" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1804" PostTypeId="1" AcceptedAnswerId="1841" CreationDate="2013-09-06T12:44:43.983" Score="9" ViewCount="4953" Body="&lt;p&gt;When I first heard of the Voyager missions I thought this: why not do the same, only make it a telescope? I thought it would surely see a lot of things we can't see from Earth or the inner Solar System. I just thought that when you're in a place that's sufficiently different, you start to see different things or the same things differently. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I'm not sure the outer reaches of the Solar System are far enough to produce enough change in what's visible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question(s):&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What are the most significant advantages of sending a telescope out of the solar system? &lt;/li&gt;&#xA;&lt;li&gt;Would it be possible to power a space telescope at such distance? I know Voyagers were still sending information to the Earth when they were extremely far from it, but I'm guessing a useful telescope might need significantly more power. &lt;/li&gt;&#xA;&lt;li&gt;Would such a device be so expensive as to become inviable?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="125" LastEditorUserId="12102" LastEditDate="2020-05-28T01:54:44.440" LastActivityDate="2020-05-28T01:54:44.440" Title="What are the advantages and disadvantages of sending a telescope into deep space?" Tags="|deep-space|space-telescope|" AnswerCount="5" CommentCount="13" ClosedDate="2013-09-23T15:09:59.377" ContentLicense="CC BY-SA 4.0" />
  <row Id="1805" PostTypeId="1" AcceptedAnswerId="1812" CreationDate="2013-09-06T13:33:57.723" Score="9" ViewCount="18402" Body="&lt;p&gt;Space Shuttle Columbia exploded during reentry due to missing heat shield tiles. When this happened, the shuttle would have had a great amount of speed, which would imply debris was strewn far down range. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was the furthest distance between discovered pieces of the shuttle?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="29286" LastEditDate="2020-08-31T18:04:16.843" LastActivityDate="2020-08-31T18:04:16.843" Title="About how far was debris scattered from the Columbia incident?" Tags="|space-shuttle|debris|failure|shuttle-columbia|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1806" PostTypeId="5" CreationDate="2013-09-06T13:51:21.917" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-06T13:51:21.917" LastActivityDate="2013-09-06T13:51:21.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1807" PostTypeId="4" CreationDate="2013-09-06T13:51:21.917" Score="0" Body="Questions regarding mission or spacecraft plans which are not material, but are only concepts." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-06T14:01:16.310" LastActivityDate="2013-09-06T14:01:16.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1808" PostTypeId="2" ParentId="1804" CreationDate="2013-09-06T13:59:49.337" Score="5" Body="&lt;p&gt;&lt;em&gt;Technologically,&lt;/em&gt; we could certainly do it. We could power such a telescope using nuclear cells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;However,&lt;/strong&gt; as you mention! you would have to send these things a &lt;em&gt;long ways&lt;/em&gt; just to produce the most minute change in our perspective. Since we are talking about hundreds of thousands of light years, the distance to the outside of the solar system wouldn't be close to enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the time you got a telescope far enough to produce a real change in what we can observe, it would be next to impossible to communicate with it, and latencies would be upwards of months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main advantage of such a telescope would be the ability to look through space without an atmosphere in the way - and the Hubble telescope already has almost all of that advantage.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-09-06T13:59:49.337" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1809" PostTypeId="1" AcceptedAnswerId="1813" CreationDate="2013-09-06T14:17:56.887" Score="12" ViewCount="561" Body="&lt;p&gt;I am watching a documentary called &quot;The Electric Comet,&quot; and the narrator claims that water has never been collected or detected from comets. This should be a surprising result as standard theory claims the Oort cloud as the origin of comets; an icy and rocky place. Whatchy'all think? Is this true?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2013-09-06T14:41:11.843" LastActivityDate="2013-09-08T01:31:10.973" Title="Has water ever been confirmed on comets?" Tags="|water|comet|oort-cloud|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1810" PostTypeId="1" AcceptedAnswerId="1814" CreationDate="2013-09-06T14:19:10.630" Score="5" ViewCount="189" Body="&lt;p&gt;As a layman, I like to read about space exploration and come up with ideas. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of them that I couldn't refute was a sort of firework that would scatter thousands or even millions of self-locating measurement devices in random directions across the sky. This could significantly lower the cost of information - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;First, the launch would be a cheap operation as the level of precision required would&lt;br&gt;&#xA;be near zero -- some of the devices would be &lt;em&gt;expected&lt;/em&gt; not to get anywhere near the&lt;br&gt;&#xA;space. &lt;/li&gt;&#xA;&lt;li&gt;And second, the devices themselves could be rather cheap probably. If we were worried &#xA;about creating an orbital mess, we could create a kind of bomb that would go off after &#xA;leaving the vicinity of Earth. (Again the launch of the bomb wouldn't have to be done&lt;br&gt;&#xA;with the level of precision that most of the devices sent into the space require.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A couple of months ago I read something that reminded me of the idea. I think it was pretty much the same thing, but I can't find it anywhere now. Could you please tell me where I can read about it and explain why this is not being done? (Or why it is being done, if it indeed is.)&lt;/p&gt;&#xA;" OwnerUserId="125" LastEditorUserId="288" LastEditDate="2013-09-06T14:49:22.713" LastActivityDate="2013-09-06T17:12:52.253" Title="Could we send lots of cheap measurement devices in random directions into the space?" Tags="|measurement|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1811" PostTypeId="2" ParentId="1805" CreationDate="2013-09-06T14:32:01.757" Score="1" Body="&lt;p&gt;Roughly 400km (250 miles) long and 65km (40 miles) wide. The debris scattered over a large part of Texas, so this is an estimate. One of the comments linked &lt;a href=&quot;http://caib1.nasa.gov/news/report/pdf/vol2/part10.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; which includes a picture of all the debris found in Texas. &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="6642" LastEditDate="2018-07-18T03:50:23.403" LastActivityDate="2018-07-18T03:50:23.403" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="1812" PostTypeId="2" ParentId="1805" CreationDate="2013-09-06T14:49:10.983" Score="9" Body="&lt;p&gt;When the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Columbia&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Columbia&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaster&quot; rel=&quot;noreferrer&quot;&gt;disintegrated&lt;/a&gt; and plummeted to earth on Feb. 1, 2003, the debris field extended from West Texas to Arkansas and Louisiana. According to an independent report on &lt;a href=&quot;http://www.columbiassacrifice.com/$B_breakup.htm&quot; rel=&quot;noreferrer&quot;&gt;Columbia's Breakup and Debris Field with Debris Trajectory&lt;/a&gt; (the source might be controversial in other points, but there is to my knowledge nothing controversial about where the debris were recovered from since it was an endeavour crossing three federal states, so I'm quoting that source for convenience alone since it went to great length in presenting the debris field data):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/QkRd6.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Location of Debris Field&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Location    N. Latitude    W. Longitude&#xA;Beginning:  32.6530        -96.9080&#xA;Center:     31.6484        -95.0238&#xA;End:        31.0315        -93.0742&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;  &#xA;  &lt;p&gt;Total length of debris field: 250 miles (402 kilometers).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-06T14:49:10.983" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1813" PostTypeId="2" ParentId="1809" CreationDate="2013-09-06T15:06:16.060" Score="14" Body="&lt;p&gt;Indeed the presence of water has been confirmed.&lt;/p&gt;&#xA;&lt;p&gt;The late 1980s mission &lt;a href=&quot;http://en.wikipedia.org/wiki/Giotto_mission#Scientific_results&quot; rel=&quot;noreferrer&quot;&gt;Giotto&lt;/a&gt; confirmed the existence of water on a comet well-known to mankind - Halley's Comet.&lt;/p&gt;&#xA;&lt;p&gt;Wikipedia states:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Measured volume of material ejected by Halley&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;80% water,&lt;/li&gt;&#xA;&lt;li&gt;10% carbon monoxide&lt;/li&gt;&#xA;&lt;li&gt;2.5% A mix of methane and ammonia.&lt;/li&gt;&#xA;&lt;li&gt;Other hydrocarbons, iron, and sodium were detected in trace amounts.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Many references on this subject from relatively reliable sources are available on the World Wide Web.&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-08T01:31:10.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1814" PostTypeId="2" ParentId="1810" CreationDate="2013-09-06T15:11:14.883" Score="4" Body="&lt;p&gt;The text you describe you read reminds me of the Cornell University's page on &lt;a href=&quot;http://www.spacecraftresearch.com/MII/MII_concepts.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sprite Concepts&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rather than consider the “tremendous mirrors of very thin sheets” they&#xA;  envisioned, we consider solar pressure acting on our extremely small&#xA;  Sprite spacecraft. We have shown that competitive lightness numbers&#xA;  can be achieved by printing the spacecraft onto ultra thin silicon,&#xA;  essentially integrating the sail and the spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/rXbsT.jpg&quot; alt=&quot;KickSat deploying Sprite femtosatellites&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Artist's impression of the KickSat deploying Sprite femtosatellites (source: &lt;a href=&quot;http://zacinaction.github.io/kicksat/&quot; rel=&quot;nofollow noreferrer&quot;&gt;KickSat on GitHub&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I'm sure a lot of articles were born out of this idea of Sprite &lt;a href=&quot;http://en.wikipedia.org/wiki/Miniaturized_satellite#Femtosatellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;femtosatellites&lt;/a&gt; (see &lt;a href=&quot;https://space.stackexchange.com/a/1737/49&quot;&gt;this answer&lt;/a&gt; for their current and near-future use in space), and while it might be a nice idea how to spread these tiny femtosatellites in a wide radius and let them &lt;em&gt;drift&lt;/em&gt; through space, the main problem is actually later picking up their communications. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, such &lt;em&gt;cheap and cheerful&lt;/em&gt; devices &lt;strong&gt;don't have a large communications antenna&lt;/strong&gt;, a power source needed, and they actually &lt;strong&gt;don't even have any means of propulsion to at least orient themselves to some close by receiver that could act as a relay&lt;/strong&gt; and send data back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the &lt;em&gt;major show stopper&lt;/em&gt; considering your thoughts of sending such tiny satellites to deep space and let them do their science on their own is, &lt;strong&gt;we either wouldn't be receiving their data, or we'd need a large number of communication relays in their vicinity&lt;/strong&gt; and amplify data transmissions. Needless to say, this would end up costing possibly more than having more independent, but larger and more expensive space probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But such femtosatellites could still be used, though on a scale that makes sense. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;For example:&lt;/em&gt; As atmospheric probes to other celestials, and even the Earth itself, with a relatively close proximity communications satellite in range (probably where they would be launched from in the first place) to pick their data transmissions and forward them to wherever they'd be later collated and analyzed.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-09-06T17:12:52.253" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1815" PostTypeId="2" ParentId="1809" CreationDate="2013-09-06T15:36:09.890" Score="6" Body="&lt;p&gt;The existence of water on comets is not really news, we can detect it by many means, for example by &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_spectrometry&quot; rel=&quot;nofollow noreferrer&quot;&gt;mass spectrometry&lt;/a&gt; analysis of comets' tails and coma using ground based or orbited observatories, so I'll rather point out maybe a bit more interesting fact that we have actual direct evidence through the collected and returned to Earth samples taken by the NASA's comet &lt;a href=&quot;https://en.wikipedia.org/wiki/81P/Wild&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wild 2&lt;/a&gt; chasing probe named &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stardust&lt;/a&gt; that even &lt;strong&gt;liquid water forms on comets&lt;/strong&gt;, something it was previously not believed possible and the prevailing thought was water molecules in ice form &lt;a href=&quot;https://en.wikipedia.org/wiki/Sublimation_%28phase_transition%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;sublimate&lt;/a&gt; from comet surface directly to water vapor when comets are passing by closer to the Sun and forming comet coma and tails:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In April 2011, scientists from the University of Arizona discovered&#xA;  evidence for the presence of liquid water in a Comet Wild 2. They have&#xA;  found iron and copper sulfide minerals that must have formed in the&#xA;  presence of water. The discovery shatters the existing paradigm that&#xA;  comets never get warm enough to melt their icy bulk.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_%28spacecraft%29#Sample_processing&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Sample processing of the Stardust spacecraft&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The samples were collected by the Stardust Sample Collection (SSC) during the sample return mission of NASA's robotic space probe &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stardust&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iCBsm.jpg&quot; alt=&quot;Stardust Sample Collection (SSC)&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The particle collector uses aerogel, a low-density, inert,&#xA;  microporous, silica-based substance, to capture dust grains as the&#xA;  spacecraft passes through the coma of Wild 2. After sample collection&#xA;  was complete, the collector receded into the Sample Return Capsule for&#xA;  entering the Earth's atmosphere. The capsule with encased samples&#xA;  would be retrieved from Earth's surface and studied.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-06T15:36:09.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1816" PostTypeId="5" CreationDate="2013-09-06T17:04:06.527" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-06T17:04:06.527" LastActivityDate="2013-09-06T17:04:06.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1817" PostTypeId="4" CreationDate="2013-09-06T17:04:06.527" Score="0" Body="Questions regarding an envelope of gasses surrounding a celestial body held in place by the body's gravity." OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-09-12T23:57:17.410" LastActivityDate="2013-09-12T23:57:17.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1818" PostTypeId="5" CreationDate="2013-09-06T17:09:54.033" Score="0" Body="&lt;p&gt;&lt;strong&gt;Cosmic rays&lt;/strong&gt; are immensely high-energy radiation, mainly originating outside the Solar System.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Cosmic background radiation&lt;/strong&gt; is electromagnetic radiation from the sky with no discernible source.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_ray&quot; rel=&quot;nofollow&quot;&gt;Wikipedia: Cosmic Ray&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_background_radiation&quot; rel=&quot;nofollow&quot;&gt;Wikipedia: Cosmic Background Radiation&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T03:33:14.633" LastActivityDate="2015-07-31T03:33:14.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1819" PostTypeId="4" CreationDate="2013-09-06T17:09:54.033" Score="0" Body="Questions about cosmic rays and cosmic background radiation." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T03:33:22.083" LastActivityDate="2015-07-31T03:33:22.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1820" PostTypeId="5" CreationDate="2013-09-06T17:12:19.917" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-06T17:12:19.917" LastActivityDate="2013-09-06T17:12:19.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1821" PostTypeId="4" CreationDate="2013-09-06T17:12:19.917" Score="0" Body="The theoretically , large collection of comets orbiting the sun far beyond the orbit of pluto (approximately  50000 AU from sun), proposed to be the source of most long term comets observed." OwnerUserId="414" LastEditorUserId="25" LastEditDate="2013-09-09T13:24:43.677" LastActivityDate="2013-09-09T13:24:43.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1822" PostTypeId="2" ParentId="1797" CreationDate="2013-09-06T18:18:24.823" Score="9" Body="&lt;p&gt;I think there is some denial of terms here, especially in the title question. Pedantically, anything that is not itself a star or stellar remnant, which orbits a star, is large enough that its gravity overcomes structural rigidity of its material to form a spheroid, and that has &quot;cleared the neighborhood&quot; of its orbit around the star, is a &quot;planet&quot;. There is an upper size limit to what makes a planet as well; get too big and it starts exhibiting star-like behavior. More on that in a sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The limitations of our technology make it impossible to prove categorically that there are no masses fitting your description &lt;em&gt;not&lt;/em&gt; orbiting a star (so-called &quot;rogue planets&quot; which, because they're not bathed in a star's light nor causing it to &quot;wobble&quot;, would be very difficult to discern from cosmic background noise even if they were spontaneous emitters), but what we do know about our universe tells us that such a proposition is extremely unlikely because of the way matter accretes around large gravity wells like stars and other stellar/superstellar objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Turning more directly to your question, Jupiter is a well-known source of radio noise; the storms raging across its outer atmospheric layers produce charge differences orders of magnitude larger than those that cause lightning on Earth, which result in bursts of radio activity between about 18 and 32MHz (lying within the CB radio spectrum, but too faint to pick up from Earth's surface with this class of equipment). NASA's Radio Jove project collects and studies this noise for clues as to what's going on deeper inside the planet (and it makes for interesting listening to some).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, Jupiter's definitely a planet. It orbits our Sun, and one could assert that most of the energy we detect from Jupiter is, in some form, reprocessed solar energy. Jupiter, in the grand scheme of things, is also relatively small; we have found exoplanets orbiting other stars which are dozens of times Jupiter's mass. If they have similar composition and other characteristics, it stands to reason these objects would be pretty strong radio emitters (if they weren't being drowned out by their star).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In between about 13 and 80M&lt;sub&gt;J&lt;/sub&gt; (Jupiter masses), there is a grey area, or perhaps more fitting, a brown area. We've found celestial masses in this range of mass that do not exhibit true H&lt;sup&gt;1&lt;/sup&gt; fusion and so are not &quot;stars&quot; (that starts happening around 75 to 80 M&lt;sub&gt;J&lt;/sub&gt;), but &lt;em&gt;do&lt;/em&gt; still produce self-sustaining EMR, through a sub-fusion process called deuterium combustion, which makes them more than planets. The most massive of these objects we have found to date (for which the mass can be reliably calculated) is Teide 1 at 55M&lt;sub&gt;J&lt;/sub&gt;. Not all of them orbit stars, though a few do, like Gliese 229B (somewhere north of 20M&lt;sub&gt;J&lt;/sub&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These masses are thus spontaneous EMR emitters, not planets and not stars; they are known as &quot;brown dwarves&quot;, and are the closest known objects to your description. But, they &lt;em&gt;still&lt;/em&gt; don't fit exactly, because they're made of lightweight materials that support fusion (or something like it), like hydrogen, helium and lithium. They're the true &quot;stars that never quite made it&quot; (Jupiter didn't even come close, despite many misguided statements to the contrary). Not all masses in this category are brown dwarves; for instance, USco1602-2401b, at 47M&lt;sub&gt;J&lt;/sub&gt;, is the most massive substellar object being called a planet (though we're not sure; it was only discovered this year, and at an estimated temperature of 4700*K while orbiting its binary system at a distance ten times the radius of anything we'd call part of our Solar System, there's &lt;em&gt;something&lt;/em&gt; going on). Brown dwarves are only classed as such when we can be reasonably sure that the planet is massive enough, hot enough and made of enough fusionable materials that these pre-fusion processes are the most viable explanation for their characteristics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the fundamental problem of aggregating enough mass to form a spontaneous emitter; it is extremely unlikely for there to be enough of anything in the neighborhood &lt;em&gt;but&lt;/em&gt; hydrogen to form a body that massive, and as is shown billions of times across the heavens, when you aggregate enough hydrogen you get a star. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take everything we've identified or think exists orbiting our Sun, that is made up largely of something other than hydrogen and helium (the Sun, Jupiter and Saturn are all primarily H/He with relatively small amounts of everything else), and put it all together. The two largest single non-H/He objects in the solar system are the Neptune and Uranus &quot;ice giants&quot;, at 17 and 15M&lt;sub&gt;e&lt;/sub&gt; (Earth masses) respectively. The largest collection of objects with an estimated mass is the Oort Cloud at about 40M&lt;sub&gt;e&lt;/sub&gt;. The asteroid belt and Kuiper belt's total combined mass is less than 1M&lt;sub&gt;e&lt;/sub&gt;, and the masses of all moons of the solar system are just over 1M&lt;sub&gt;e&lt;/sub&gt;. Earth, Venus and Mars together are not quite 2M&lt;sub&gt;e&lt;/sub&gt;. So, all together, these bodies would create a celestial mass orbiting the sun of roughly 75M&lt;sub&gt;e&lt;/sub&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter is 317M&lt;sub&gt;e&lt;/sub&gt;. Saturn is 95M&lt;sub&gt;e&lt;/sub&gt; (including rings). The Sun is 332,837M&lt;sub&gt;e&lt;/sub&gt;. Brown dwarves would range in the thousands to tens of thousands of Earth masses. If our solar system is typical of others in the universe, there just isn't enough of anything but hydrogen (and other lightweight fusionable matter like helium/lithium) anywhere near a main sequence star that could be aggregated to make a superplanet on the scale we're talking about.&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="411" LastEditDate="2013-09-06T19:23:38.840" LastActivityDate="2013-09-06T19:23:38.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1823" PostTypeId="1" CreationDate="2013-09-06T18:53:46.737" Score="6" ViewCount="1897" Body="&lt;p&gt;Although it's not very likely to be built, the idea of a centrifuge module attached to the International Space Station is taken seriously, and at least two have been proposed.  I have in mind the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nautilus-X#ISS_centrifuge_demonstration&quot; rel=&quot;noreferrer&quot;&gt;Nautilus-X&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How astronauts would move between the non-rotating segments and the rotating segment?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MpU6V.png&quot; alt=&quot;image from Wikipedia of Nautilus-X&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is this soft-berth, and mechanically how would this work?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-09-07T10:39:53.283" Title="How would a centrifuge module's berthing system work on the ISS?" Tags="|iss|artificial-gravity|berthing|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1824" PostTypeId="2" ParentId="1823" CreationDate="2013-09-06T19:22:36.137" Score="3" Body="&lt;p&gt;This is IMO a great question. I was a bit surprised to see &quot;soft-berthing&quot; used to describe a berthing system that can't possibly have anything in common with using rubber buckling or foam filled fenders when mooring ships, or even airbags on landers for cushioned, soft-berthing and/or landing. Well, I guess any new designs have a bit of a leeway with naming conventions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wasn't able to find any reference to official documentation describing how this &quot;soft-berthing&quot; is supposed to be achieved between the International Space Station and the Nautilus-X centrifuge module, perhaps this naming has something to do with it, so I'll give you my best guess:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Counter rotating nested cylinders&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nested magic cylinders are rotatable and coaxially arranged so&#xA;  that the bore of the inner cylinder provides a central working space.&#xA;  The predetermined magnetic orientation of each cylinder's sections is&#xA;  arranged to create a relatively strong and uniform magnetic field&#xA;  within the central working space or bore in which copper wires are&#xA;  imbedded [sic] to form a core.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A core having copper wires embedded within the working space is&#xA;  coaxially positioned within the bore to act as a rotor and when&#xA;  current flows within the copper wires an electric motor is therefore&#xA;  formed.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Alternatively, a rotating device can mechanically rotate the rotor or&#xA;  core, inducing a current in the copper wires in the rotor or core&#xA;  thereby forming an electric generator. The sections of the nested&#xA;  permanent magnet cylinders are arranged such that each of the sections&#xA;  have a magnetic orientation forming a pair of coaxial “magic”&#xA;  cylinders or rings that counter rotate with respect to each other.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This results in a relatively large magnetic field within the inner&#xA;  cylinder's central working space with minimal magnetic flux leakage&#xA;  exterior to the central working space. In a preferred embodiment the&#xA;  pair of magnetic cylinders counter rotate and the core remains&#xA;  stationary, which simplifies the required electrical connections with&#xA;  the core.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QKZtl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Quote and image source: &lt;a href=&quot;http://www.google.com/patents/US6304017&quot; rel=&quot;nofollow noreferrer&quot;&gt;Patent US6304017, application US 09/507,296&lt;/a&gt; filed on Feb. 18, 2000 by Herbert A. Leupold&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;After searching a bit more, it appears my initial thoughts are (at least somewhat) confirmed in the M.L. Holderman's (&lt;a href=&quot;http://www.nasa.gov/centers/johnson/home/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Johnson Space Center&lt;/a&gt;) presentation on the concepts of &lt;a href=&quot;http://thinkingscifi.files.wordpress.com/2013/01/holderman_1-26-11.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NAUTILUS - X (Non – Atmospheric Universal Transport Intended for Lengthy United States X-ploration) Multi-Mission Space Exploration Vehicle&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Page 11: Rotating hub/ transition tunnel&lt;/li&gt;&#xA;&lt;li&gt;Page 12: &lt;a href=&quot;https://en.wikipedia.org/wiki/Slip_ring&quot; rel=&quot;nofollow noreferrer&quot;&gt;Slip-rings&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Page 16: Hub design utilizes Liquid-metal seals with low-rumble/wobble thrust bearings&lt;/li&gt;&#xA;&lt;li&gt;Page 18: Nested cylinder &amp;amp; deployable drawer approach&#xA;for Transit Tunnel&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Sadly, none of it discusses possible solutions in detail, but these few hints do suggest the use of counter rotating nested cylinders, like mentioned before.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-06T19:47:56.567" LastActivityDate="2013-09-06T19:47:56.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1825" PostTypeId="2" ParentId="1809" CreationDate="2013-09-06T20:08:38.377" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;documentary called &quot;The Electric Comet,&quot; and the narrator claims that water has never been collected or detected from comets.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Obvious lies (or extremely outdated info) and the title indicates that it was about &quot;Plasma cosmology&quot; presented as viable (or maybe even the best) cosmological model. Relatively good info about this is available for example at Wikipedia (&lt;a href=&quot;https://en.wikipedia.org/wiki/Plasma_cosmology&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Plasma_cosmology&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="671" LastActivityDate="2013-09-06T20:08:38.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1826" PostTypeId="2" ParentId="1336" CreationDate="2013-09-06T20:58:21.457" Score="27" Body="&lt;p&gt;We know from the nuclear power industry that spent fuel storage pools are pretty safe places to be around, radiation-wise. They're actually safe to swim in, to a point, because they're serviced routinely by human divers. They just can't get too close to the spent fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We use these pools for short-term storage because water is a really good radiation shield. How good? Well, according to a report on the topic prepared for the DoE back in 1977, a layer of water 7 centimeters thick reduces the ionizing radiation (rays and particles) transmitted through it by half (the remainder is captured or moderated to non-ionizing energy levels, mainly heat). Freshly discharged nuclear fuel puts out about 100,000 R/hour as measured from one foot away in air (at that rate, certain death is about 5 minutes' exposure and you'd fall into a coma in about 10). Background ionizing radiation levels on Earth's surface are about .000001 R/hour (1 mSv/hr), while a &quot;safe dose&quot; to live with long-term is about .0004 R/hr. A halving represents about .3 of a power of 10, so in rough terms, to reduce a fresh fuel rod's radioactivity to safe levels, you would need about 2 meters (8/0.3 * 7 / 100), and through more than 2.5m the radioactivity of the fuel rods is indistinguishable from background radiation. In fact, according to the link from the comment, diving about 6 feet down would expose you to less radiation than at the surface of the pool.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia, the upper estimate for a dose equivalent received by unshielded astronauts operating outside Earth's magnetic field (such as a mission to Mars) is about 90,000 R/yr or about 10 R/hour. If we assume the energy levels are comparable, reducing that to lower than Earth background radiation would only require a layer of water around 1 meter thick.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, let's do some more math. Let's say that the Mars vehicle that will get them there and back is a cylinder roughly 3.5m by 20m (same as was used for the MARS-500 experiments; that is a &lt;em&gt;very&lt;/em&gt; small tin can in which to spend 3 1/2 years with 4 or 5 other people). With 1m of shielding water around all surfaces of that cylinder, the outer hull would be about 5.5m by 22m. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The volume of shielding water needed is the difference between those two cylinders, or $22\cdot\pi\cdot2.75^2 - 20\cdot\pi\cdot1.75^2 \approx 522.68 - 192.42 = 330.26 m^3$. As one cubic meter of water weighs 1 metric ton (1,000kg), that's 330,260kg to get into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting that in perspective, the current record holder for payload-to-LEO is the Saturn V rocket, which had a maximum LEO payload of 120,000kg (said payload being the S-IVb, including CSM, LEM and Earth departure stage, for most of its missions). To put the volume of water we'd need into orbit would require 3 Saturn V rockets. The planned-but-never-built Ares V was spec'ed to have 188 tonnes P2LEO capacity, which would have cut down the number of launches to only 2. Doing it with Space Shuttles (25 tonnes cargo to LEO) would take 14 missions. The SLS Block II (130 tonnes payload to LEO) would take just about 3 launches. Doing it with any orbital rocket currently in service, manned or unmanned (Soyuz II, Soyuz FG, Delta IV, Atlas V, Falcon 9) would require between 50 and 100 launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that we could achieve getting this much mass into LEO, getting it to break orbit and out into interplanetary space is that much harder; going to Mars using a Hohmann transfer orbit takes about as much delta-V as getting to LEO in the first place, so all the fuel expended to get the craft and its water shield into space has to be brought up into orbit, requiring many more launches. Using a gravity assist, say from Venus, would be a logistical nightmare requiring all three planets to be in alignment as of departure from LEO, and while it would save fuel it would require us to cover much more distance and take much more time, possibly placing the mission further beyond our current capabilities.&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="-1" LastEditDate="2016-05-05T18:55:06.150" LastActivityDate="2016-05-05T18:55:06.150" CommentCount="20" ContentLicense="CC BY-SA 3.0" />
  <row Id="1827" PostTypeId="1" AcceptedAnswerId="61290" CreationDate="2013-09-06T21:44:24.507" Score="14" ViewCount="4447" Body="&lt;p&gt;There is apparently some confusion as to why pump or pressure fed liquid rocket engines need to operate at high pressure to product significant thrust, based on the comments to this question about &lt;a href=&quot;https://space.stackexchange.com/q/955/127&quot;&gt;pressure-fed engine combustion instabilities&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I thought that the reaction increases pressure within the cone, but that doesn't propagate back up because the flow is choked. But there could be a lot I'm not understanding. I also thought that the Saturn V used a similar 60 psi Helium pusher balloon. — AlanSE&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If there is pressure already, why need for so much heat? I could understand counteracting the temperature drop as pressurized material expands, to prevent the whole thing freezing over, but normally temperature alone doesn't convert to velocity (or the Sun would be flying really fast), it can merely increase energy of a gas, either expanding it or increasing its pressure (and gas expanding out of an open chamber will provide thrust against the direction of the nozzle.) Still, you don't need heat for that, pressurized cold gas still suffices. — SF&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The answer is not obvious or trivial without a background in rocket propulsion, so I thought it made more sense to make this topic a separate question and get the community to weight in with how best to describe this. I'll plan to leave my own answer below, but have confidence others can do better. &lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="9092" LastEditDate="2021-03-15T10:21:37.463" LastActivityDate="2022-12-15T14:34:13.967" Title="Why do pump and pressure fed liquid engines need to operate at high pressures?" Tags="|rockets|engines|pressure|turbopump|pressure-fed-engine|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1828" PostTypeId="2" ParentId="439" CreationDate="2013-09-06T22:03:05.770" Score="11" Body="&lt;p&gt;The SSME is a &lt;a href=&quot;http://en.wikipedia.org/wiki/Staged_combustion_cycle_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;staged-combustion rocket engine&lt;/a&gt;, which means that some small fraction of the propellant flow into the main combustion chamber is first diverted into a small pre-burner (two actually). These preburners combust  (relatively) small amounts of fuel and oxidizer to produce hot exhaust gas which is expanded through a turbine, which is mechanically connected to a pump (hence turbopump), which is used to supply high-pressure fuel and oxidizer to the main combustion chamber. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual engine is significantly more complicated than this (even in a simplified schematic), but this is the basic premise of any turbopump-fed engine: hot gas turns a turbopump that pumps up the main propellant supply and injects it into the combustion chamber. The SSME has lots of other little details, like high and low pressure pumps, and feeding the combustion gasses back into the main chamber—rather than venting them overboard as the RS-68 does. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the bottom line is that the OP is right. The pressure is very high in the combustion chamber, you inject propellant into the combustion chamber by having an even higher pressure at the outlet of the turbopumps. Store in the tanks at lower pressure, pump up with the turbos, then ride the pressure gradient all the way to the bright blue light at the end of the nozzle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/z5KeF.png&quot; alt=&quot;SSME schematic&quot;&gt; courtesy Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want a better understanding of &lt;a href=&quot;https://space.stackexchange.com/q/1827/127&quot;&gt;&lt;em&gt;why&lt;/em&gt; the propellant needs to be at high pressure in the first place&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-09-06T22:03:05.770" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1829" PostTypeId="1" AcceptedAnswerId="1832" CreationDate="2013-09-06T23:29:33.863" Score="46" ViewCount="30006" Body="&lt;p&gt;I'm curious about whether or not the Space Shuttle could have been used as a lunar orbit vehicle - basically, would have it been technically capable of getting to the Moon, entering orbit, then leaving orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My reasoning is that since the Shuttle was designed to carry large payloads, you could easily stuff enough fuel in there to power the already-existing thrusters to do whatever you needed to. Also, the area not taken up by fuel could be used to carry food and supplies for the extended mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be possible?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2015-04-05T19:17:18.493" LastActivityDate="2021-05-20T07:40:17.750" Title="Would it have been possible to have sent the Space Shuttle around the Moon?" Tags="|the-moon|space-shuttle|feasibility|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="1830" PostTypeId="2" ParentId="1827" CreationDate="2013-09-07T00:20:17.833" Score="7" Body="&lt;p&gt;The most basic answer is that pressure gives you velocity, and velocity gives you energy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rocket engine becomes more efficient the faster the particles making up the thrust gases are leaving the rocket through the nozzle. Simple projectile physics: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E = \dfrac{1}{2}mv^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy of a mass, such as a stream of rocket exhaust (and, by Newton's third law, the rocket and whatever's strapped to it), is one half the product of the mass of that exhaust times the square of its velocity. If you need double the energy per second in a rocket (to accelerate faster), you can either double the rate of mass you're ejecting from the nozzle by doubling your fuel consumption, or you can increase the velocity of the same mass being ejected by $\sqrt{2} \approx 1.414$. By increasing the pressure inside the ignition chamber, you will increase the velocity of gases escaping it through the nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the fluid is incompressible and frictionless (it isn't, but we can make rough calculations under these assumptions), Bernoulli's Equation will give us the velocity of the rocket exhaust, if we know the pressure and density of the gas in the chamber and the pressure and density of the environment outside the nozzle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\dfrac{P_a}{\gamma} + \dfrac{v_a^2}{2g} + z_a = \dfrac{P_b}{\gamma} + \dfrac{v_b^2}{2g} + z_b$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where, for the subscripts $a$ and $b$ corresponding to the environment inside and outside the ignition chamber respectively, $P$ is the pressure in Pascals (N/m&lt;sup&gt;2&lt;/sup&gt;), $\gamma$ is the specific gravity (relative density to that of water) of the fluid, $g$ is the acceleration of gravity (we can effectively ignore it), $v$ is the velocity in meters per second, and $z$ is the relative elevation of the two fluids. In our circumstance, internal pressure and external velocity dominate the equation, to the point where we don't care much about internal velocity (effectively zero), external pressure (effectively zero), gravity or relative elevation, and so the equation simplifies to:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\dfrac{P_a}{\gamma} = \dfrac{v_b^2}{2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This equation rearranges very similarly to the energy equation - $P_a = \dfrac{1}{2}\gamma v_b^2$ - and for similar reasons (A pressure differential is a form of potential energy). We see from this equation that the higher the pressure, or the &lt;em&gt;less&lt;/em&gt; dense the gas, the higher the velocity. Because lowering the density would lower mass, thus lowering energy of the moving gas, increasing pressure is the way to go. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And how do you increase pressure? well, according to the ideal gas law:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P = \dfrac{nRT}{V}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $P$ is pressure in Pascals, $n$ is number of moles of gas, $R$ is the gas constant for the units of measure we're using (for SI units it's 8.3144621 J/(mol*K)), $T$ is temperature in degrees Kelvin and $V$ is volume in cubic meters. Assuming a constant volume (the &quot;combusion chamber&quot; of an SRB is in fact always increasing as fuel burns away to create a larger cavity inside the casing, but most liquid rockets have a fixed-volume combustion chamber), increasing either the amount of gas or its temperature are your options. Again, this mainly comes down to either reacting more fuel to produce more gas, or increasing the temperature at which the fuel burns (and better insulating the chamber to prevent heat loss).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given a specific design and a specific fuel, the thrust energy of the rocket is dictated by your fuel flow rate. By the same token, given a specific fuel and a &lt;em&gt;desired&lt;/em&gt; thrust energy, the required fuel flow rate is dictated by your design, so you'll design for the highest velocity exhaust, and that means the highest pressure you can get with the strength of the materials you're using (which you choose for the best tradeoff of high strength to low weight).&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="411" LastEditDate="2013-09-26T21:59:28.040" LastActivityDate="2013-09-26T21:59:28.040" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1831" PostTypeId="2" ParentId="1441" CreationDate="2013-09-07T00:34:47.190" Score="8" Body="&lt;p&gt;In the strictest sense of use &quot;for any purpose&quot; as you say in the comments, yes. Taken from &lt;a href=&quot;https://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=167&quot;&gt;NASA page on Solar System Symbols&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/E2XPF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbols for the planets, dwarf planet Pluto, Moon and Sun (along&#xA;  with the symbols for the zodiac constellations) were developed for use&#xA;  in both astronomy and astrology.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The astronomical symbol for the Sun is a shield with a circle inside.&#xA;  Some believe this inner circle, or &quot;boss&quot; represents a central sun&#xA;  spot.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for Mercury represents the head and winged cap of Mercury,&#xA;  god of commerce and communication, surmounting his caduceus (staff).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for Venus is designated as the female symbol, thought to be&#xA;  the stylized representation of the hand mirror of this goddess of&#xA;  love.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for Earth shows a globe bisected by meridian lines into&#xA;  four quarters.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for the Moon is a crescent.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for Mars represents the shield and spear of the god of war,&#xA;  Mars; it is also the male or masculine symbol.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for Jupiter is said to represent a hieroglyph of the eagle,&#xA;  Jove's bird, or to be the initial letter of Zeus with a line drawn&#xA;  through it to indicate its abbreviation.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for Saturn is thought to be an ancient scythe or sickle, as&#xA;  Saturn was the god of seed-sowing and also of time.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for Uranus is represented by combined devices indicating&#xA;  the Sun plus the spear of Mars, as Uranus was the personification of&#xA;  heaven in Greek mythology, dominated by the light of the Sun and the&#xA;  power of Mars.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for Neptune is the trident (long three-pronged fork or&#xA;  weapon) of Neptune, god of the sea.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The symbol for dwarf planet Pluto is a monogram made up of P and L in&#xA;  Pluto (and also the initials of Percival Lowell, who predicted its&#xA;  discovery).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Credit: Lunar and Planetary Institute&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All the planets of our Solar system have similar pages of their own, also using &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_symbols&quot;&gt;astronomical symbols&lt;/a&gt;, for example &lt;a href=&quot;https://solarsystem.nasa.gov/multimedia/display.cfm?Category=Planets&amp;amp;IM_ID=265&quot;&gt;Neptune's Symbol page&lt;/a&gt;. The credited &lt;a href=&quot;http://www.lpi.usra.edu/&quot;&gt;Lunar and Planetary Institute&lt;/a&gt; is a NASA funded institute in Houston, Texas, devoted to studying the solar system and sharing the excitement of space exploration with the public.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being somewhat time-constraint, I wasn't able to find any mission imagery (observation data, telemetry data readings, mission control software, or any other such media with possible notations) that would use these astronomical symbols for planets. But as the time goes by, I'm pretty sure someone will find something, and I don't see why NASA using astronomical symbols also for other uses besides acknowledging they exist and describing them would even be considered anything unusual. Press kits might be watered down for easier consumption of the general populace, but for their internal use and where as concise as possible notations would be required not to obstruct other meaningful data, I imagine they'd use them frequently.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-07T00:34:47.190" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1832" PostTypeId="2" ParentId="1829" CreationDate="2013-09-07T01:06:13.163" Score="53" Body="&lt;p&gt;This exact problem was presented in &lt;a href=&quot;http://www.asi.org/&quot;&gt;The Artemis Project&lt;/a&gt;'s &lt;em&gt;&lt;a href=&quot;http://www.asi.org/adb/05/sts-to-the-moon.html&quot;&gt;&quot;Why We Won't Fly a Space Shuttle to the Moon&quot;&lt;/a&gt;&lt;/em&gt; article, so I'll just quote a few short excerpts from the points it is making, and the rest is then in the article:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Vehicle Mass&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Dry weight of Shuttle Orbiter is about 250,000 lbs, compared to the&#xA;  space-only LTV's &lt;em&gt;[Lunar Transfer Vehicle]&lt;/em&gt; weight of perhaps 7500 lbs. We'd be accelerating a&#xA;  lot of mass that we don't really need in lunar orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Refueling the Orbiter&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Since the Shuttle Orbiter gets to LEO &lt;em&gt;[Low Earth Orbit]&lt;/em&gt; with near-zero extra fuel, we'd&#xA;  have to use other launches to get the fuel up there. You can work out&#xA;  the math. Use Isp=460 sec, delta-V=Isp*g*ln(Mo/Mf). The delta-V&#xA;  requirements are in the Mission Timeline. It's a lot of launches... &lt;em&gt;&lt;a href=&quot;http://www.asi.org/adb/05/sts-to-the-moon.html&quot;&gt;continues in the article&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Heat Shield&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Orbiter's heat shield is designed for entry from low Earth orbit,&#xA;  about 25,000 ft/sec. Coming home from the moon, we'd hit the&#xA;  atmosphere at about 36,000 ft/sec. Whether the Shuttle could take that&#xA;  increase in kinetic energy is an unknown.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Atmospheric Entry Loads&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Orbiter is designed for 3 g's max operational load. Assuming we&#xA;  didn't use rockets to decelerate while coming home from the moon, we'd&#xA;  hit up to 6 g's; even up to 12 if the piloting is just a bit off... &lt;em&gt;&lt;a href=&quot;http://www.asi.org/adb/05/sts-to-the-moon.html&quot;&gt;continues in the article&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Radiation, Vehicle Reliability, Operational Flexibility&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Shuttle wins all those issues. It has a proven track record. If we got&#xA;  to LEO and the Orbiter was fully operational (fuel cells and auxiliary&#xA;  power units all in the green), it would probably be as reliable for&#xA;  the rest of the mission as anything else we could design...&#xA;  &lt;em&gt;&lt;a href=&quot;http://www.asi.org/adb/05/sts-to-the-moon.html&quot;&gt;continues in the article&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.asi.org/adb/05/sts-to-the-moon.html&quot;&gt;Mentioned article&lt;/a&gt; then continues with another point that is limiting Space Shuttle STS (Space Transportation System) vehicle's actual landing on the Moon, so that's not applicable to our problem.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-07T01:06:13.163" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1833" PostTypeId="2" ParentId="1823" CreationDate="2013-09-07T10:39:53.283" Score="2" Body="&lt;p&gt;The simplest method of transfer to the rotation wheel is similar to what is shown in the movie &lt;strong&gt;&lt;em&gt;2001: A Space Odyssey.&lt;/em&gt;&lt;/strong&gt; The rotating ring is within a non-rotating outer hull. One side of the hub is an open ring through a bushing and into a tube; the other side is a central axle. Near by, behind the axle side, is the counterrotating flywheel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tube entry actually is long enough to float into; a meter, maybe 2, and on the ring-side wall, a series of ladders run down to the &quot;gravity deck.&quot; The ladder positions are marked on the insides of the transition tube, and have recessed handholds along the ladder lines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the airlock section, one floats into the transition module, stops using a handhold affixed to the non-rotation portion but extending into the transfer tube section. While stopped, one waits until just before a ladder mark is &quot;down&quot;, extends hands to grab a handhold, and then crawls, feet first, face to the side of the tube, into the rotation wheel. Swinging the feet down, one transitions to the ladder, maintaining downward pressure with the hands at first, but as one descends, acquiring more and more gravity, until one is on the deck. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To exit, one climbs the ladder, and at the hub, pulls out into the transfer tube, grabs the non rotating handle to stop rotation, and floats forward into the non-rotation section of the transfer tube.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the entire system is in a single pressure container, with no external moving parts, it's simple for the astronauts, and reasonably safe. It does, however, require on-orbit assembly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A ring that rotates with it's own vacuum capable rotation ring is mechanically more complex, but less massive, and easier to pack.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that items dropped from the hub will appear to acquire antispin velocity to a person on the deck, so baffles might be needed; anything dropped will hit the antispin baffle, and accelerate down it until it hits. The use of the ladder is as much to accelerate one as to allow egress. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that some form of dynamic load balancing is advisable, as well, since an astronaut climbing or descending the ladder changes the balance. Water is a likely candidate.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-09-07T10:39:53.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1834" PostTypeId="2" ParentId="1350" CreationDate="2013-09-07T11:30:09.803" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How much would being released from the end of a space elevator at&#xA;  twice geosynchronous radius reduce travel times or the amount of&#xA;  rocket thrust needed during Earth departure for interplanetary probes?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are a lot of variables in this question and the answer, even given the clarity of the length of the elevator. The major variable being the reduction of non-defined values for times or thrust. So I will just focus on the velocities and destinations obtainable at different release elevations, with no added fuel costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where &lt;a href=&quot;http://en.wikipedia.org/wiki/Geosynchronous_orbit#Orbital_characteristics&quot; rel=&quot;nofollow&quot;&gt;Earth geosynchronous orbits, whether circular or elliptical, have a semi-major axis of 42,164 km (26,199 mi)&lt;/a&gt;, Twice would equal 84,328 km (52,398 mi).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I personally don't have the math to define the exact velocity imparted at 84,328 km, and my research did not find a calculated value for this.  But values for just above and below this are available. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia lists a few &quot;free&quot; destinations at release heights just above GSO. &#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_elevator#Launching_into_deep_space&quot; rel=&quot;nofollow&quot;&gt;An object attached to a space elevator at a radius of approximately 53,100 km will be at escape velocity when released. Transfer orbits to the L1 and L2 Lagrangian points can be attained by release at 50,630 and 51,240 km, respectively, and transfer to lunar orbit from 50,960 km&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For destinations beyond the earth moon system, the calculations get a bit more complex, as you need to also consider the sun.  In the same manor the elevator gives velocity in relation to it's height above earth, its position in relationship to the sun also effects relative velocity. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An elevator cable of somewhat over 100, 000 km in length should&#xA;  suffice as a sling to launch spacecraft to Jupiter (at the outer end)&#xA;  and Mercury (at the inner end). Reaching Jupiter is critical, because&#xA;  we can take advantage of Jupiter’s gravity assist to send spacecraft&#xA;  further outward or even beyond the solar system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://users.wpi.edu/~paravind/Publications/PKASpace%20Elevators.pdf&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;Aravind, P. K. (2007). &quot;The physics of the space elevator&quot;. American Journal of Physics (American Association of Physics Teachers)&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As we see velocity or obtainable destinations is simply a matter of height.  Or said another way, the sky really is not the limit, the higher you go the faster and the farther you can fly.&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-09-07T11:30:09.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1835" PostTypeId="1" CreationDate="2013-09-07T12:15:23.050" Score="9" ViewCount="160" Body="&lt;p&gt;Do solid propellants have an optimum temperature to burn effectively?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, what is the range of temperature  needed?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="38" LastEditDate="2013-09-08T19:19:29.900" LastActivityDate="2013-09-08T19:19:29.900" Title="Do solid propellant engine needs optimum temperature?" Tags="|rockets|propulsion|" AnswerCount="0" CommentCount="3" ClosedDate="2013-09-09T16:57:13.083" ContentLicense="CC BY-SA 3.0" />
  <row Id="1836" PostTypeId="2" ParentId="1350" CreationDate="2013-09-07T13:08:28.687" Score="9" Body="&lt;p&gt;Your total energy is ${v^2\over 2}-{\mu\over r}$.  If that's negative, you're still in orbit.  If positive, you've escaped.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the point of release, $r$ is twice geosynchronous radius, $2\times 42164\,\mathrm{km}$, and $v$ is twice geosynchronous velocity, $2\times 3.075\,\mathrm{km\over s}$.  The $\mu$ for Earth is $398600\,\mathrm{km^3\over s^2}$. The result is $14.18\,\mathrm{km^2\over s^2}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have escaped.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The result is equal to your $v_\infty^2\over 2$, so your $C_3$, which is $v_\infty^2$, is $28.36\,\mathrm{km^2\over s^2}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not enough to get you directly to Jupiter, but you'll get well past Mars' orbit or well inside Venus' orbit, depending on your preference.  A judicious set of Venus and Earth flybys could then get you to Jupiter.  From there you could go pretty much anywhere, including potentially escaping the solar system.  Also a Venus flyby could get you to Mercury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will be constrained on the departure plane, but perhaps a lunar flyby could help get you going in the direction you want.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $\Delta V$ required to get to that $C_3$ from low-Earth orbit is about $4.5\,\mathrm{km\over s}$.  Or from GEO, about $3.3\,\mathrm{km\over s}$ (by lowering periapsis to $150\,\mathrm{km}$ altitude and escaping from there).  So it buys you a lot.  Though that's one hell of a cable you'll have to build.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-07T13:23:14.343" LastActivityDate="2013-09-07T13:23:14.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1837" PostTypeId="1" AcceptedAnswerId="1838" CreationDate="2013-09-07T14:44:20.823" Score="16" ViewCount="497" Body="&lt;p&gt;Often when launching deep-space probes, scientists will &quot;loop&quot; them around planets, &quot;stealing&quot; some of the planet's momentum and transferring it to the probe - giving the probe a huge speed boost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've heard of multiple assists being chained together, for example Jupiter, then Saturn, then flying off at enormous speed out of the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that getting more and more of these assists would be beneficial to the probe's mission. Which probe that we have launched has received the most gravitational assists? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2016-11-16T15:44:35.097" Title="What probe received the most gravitational assists?" Tags="|deep-space|probe|gravity-assist|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1838" PostTypeId="2" ParentId="1837" CreationDate="2013-09-07T18:43:52.163" Score="20" Body="&lt;p&gt;If you only count planets, then I believe it's &lt;a href=&quot;http://messenger.jhuapl.edu/the_mission/trajectory.html&quot; rel=&quot;noreferrer&quot;&gt;MESSENGER&lt;/a&gt; at six.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though your question was explicitly: &quot;Which probe that we have launched has received the most gravitational assists?&quot;  The winner there is &lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/flybys/&quot; rel=&quot;noreferrer&quot;&gt;Cassini&lt;/a&gt;, hands down.  It is on Titan flyby number &lt;s&gt;93&lt;/s&gt; 125, so far.  And it's flown by other moons of Saturn.  Plus the four planetary flybys on the way to Saturn.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-11-16T15:44:35.097" LastActivityDate="2016-11-16T15:44:35.097" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1839" PostTypeId="2" ParentId="1804" CreationDate="2013-09-08T02:07:11.000" Score="2" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jUY4Q.jpg&quot; alt=&quot;radio telescope in Arecibo, Puerto Rico&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no actual technology to receive data from such telescope. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at the picture. This is SETI@home's radio telescope. It is situated in Puerto Rico. There is no technology on the Earth, to move all amount of data by radio or wires to Berkeley, California, but physically, with whole of storage, magnetic tapes in that case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is about 6,000 km, only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sneakernet&quot; rel=&quot;nofollow noreferrer&quot;&gt;wiki: Sneakernet&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The SETI@home project uses a sneakernet to overcome bandwidth&#xA;  limitations: data recorded by the radio telescope in Arecibo, Puerto&#xA;  Rico is stored on magnetic tapes which are then shipped to Berkeley,&#xA;  California for processing. In 2005, Jim Gray reported sending hard&#xA;  drives and even &quot;metal boxes with processors&quot; to transport large&#xA;  amounts of data by postal mail.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, radio telescope in Arecibo, is a largest one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably, it is hard to win anything with deep space's telescope, because data received from it, will not be so preciseness.&lt;/p&gt;&#xA;" OwnerUserId="601" LastEditorUserId="601" LastEditDate="2013-09-08T02:14:23.590" LastActivityDate="2013-09-08T02:14:23.590" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1840" PostTypeId="1" CreationDate="2013-09-08T02:54:15.387" Score="11" ViewCount="1376" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Ionocraft&quot;&gt;ionocraft&lt;/a&gt; produces lift by accelerating ions in the air downward by the use of two meshes held at a large relative voltage difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a cursory look at the physical principle, it seems like it could have advantages over other methods of propulsion in sub-orbital high altitude flight.  It's not limited by having to take reaction mass on board, it doesn't have to deal with high drag because it can be functionally stationary, and higher altitude might not force it to scale up size like balloons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only problem is that the idea isn't practical.  Wikipedia notes power requirements of 1 Watt per gram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, ions are more available in the higher atmosphere.  Qualitatively, more ions for an ionocraft would seem to imply that you would get more force with less power input.  The lift equation for an ionocraft is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$     F = \frac{Id}{k} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$k$ is &quot;ion mobility coefficient of air&quot;.  I'm confused about the meaning of that so I can't answer this myself.  Should this be lower or higher at very high altitudes?  And considering the entire picture, might an ionocraft (which is non-viable at sea level) become viable if you flew it up to high altitudes first?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="414" LastEditDate="2013-09-08T06:29:09.463" LastActivityDate="2019-10-19T15:01:43.337" Title="Would an ionocraft have better or worse performance in the upper atmosphere?" Tags="|propulsion|ion-thruster|lift|sub-orbital|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1841" PostTypeId="2" ParentId="1804" CreationDate="2013-09-08T05:00:46.963" Score="5" Body="&lt;p&gt;The advantages of getting a single telescope into space might be.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The ability to see wavelengths normally absorbed by the atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Lack of atmospheric turbulence leads to a crisper, sharper image.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Very little of that changes though, between LEO and 'deep space'.&lt;/p&gt;&#xA;&lt;p&gt;Once a single telescope has gone far enough, it does enable us to see different perspectives on local objects.  E.G. This picture of Saturn taken by NASA’s Voyager 1 spacecraft in 1980.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6hKT0.jpg&quot; alt=&quot;Picture of Saturn taken by NASA’s Voyager 1 spacecraft in 1980&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;It does also raise the possibility to pass directly behind the planet and view the upper reaches of the atmosphere as they filter the light from the Sun.  I believe this can aid in determining the composition of the atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rM2wa.jpg&quot; alt=&quot;Saturn eclipsing the sun, seen from behind from the Cassini orbiter&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Saturn eclipsing the sun, seen from behind from the Cassini orbiter.&lt;/p&gt;&#xA;&lt;p&gt;Having said that, I suspect the atmospheres of most objects in the Solar System is pretty well quantified, and getting such images would add little to our existing knowledge.&lt;/p&gt;&#xA;&lt;h2&gt;Multiple telescopes&lt;/h2&gt;&#xA;&lt;p&gt;When it comes to &lt;em&gt;groups of telescopes&lt;/em&gt; the situation changes for at least one, possibly two, more abilities.&lt;/p&gt;&#xA;&lt;h3&gt;1. Parallax measurement&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Parallax&quot; rel=&quot;noreferrer&quot;&gt;Parallax&lt;/a&gt; measurements.  Sending a &lt;strong&gt;pair&lt;/strong&gt; of telescopes into deep space, or a single scope with an Earth(/Earth orbit) based counterpart, would allow us to view the 3-D nature of the galaxy to a greater distance/depth.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Pair with one in Earth orbit, the other in orbit about another planet&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Baselines for parallax measurements for pairs of telescopes, assuming the images are taken at the same moment as opposed to '6 months apart' as is done to get the maximum parallax possible from a single point on Earth.&lt;/p&gt;&#xA;&lt;p&gt;Maximum baseline advantage between Earth orbit and the orbits of other&#xA;planets, when in opposition.  Approximate, given the distances are the&#xA;average of the elliptical orbits.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Planet  Dist.   Total   Advantage&#xA;Mars    1.5     2.5     1.25&#xA;Jupiter 5.2     6.2     3.1&#xA;Saturn  9.54    10.54   5.27&#xA;Uranus  19.18   20.18   10.09&#xA;Neptune 30.06   31.06   15.53&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Minimum baseline when both planets are on the same side of the sun.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Planet  Dist.   Total   Advantage&#xA;Mars    1.5     0.5     0.25&#xA;Jupiter 5.2     4.2     2.1&#xA;Saturn  9.54    8.54    4.27&#xA;Uranus  19.18   18.18   9.09&#xA;Neptune 30.06   29.06   14.53&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;strong&gt;Lagrange points&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Lagrangian points&lt;/a&gt; formed by two massive objects.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tkL74.png&quot; alt=&quot;L1-L5 Lagrange points&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The L2/L3 Lagrange points would be most optimal.  Since these are on the opposite side of the sun by&#xA;definition, the baseline will change much less over time.  The baselines are (very) approximately as much larger than those of Earth's L2/L3 distance, directly in proportion to the orbital radii of the other planet.&lt;/p&gt;&#xA;&lt;p&gt;I wrote &amp;quot;(very) approximate&amp;quot; above because the L2 point is very much affected by the mass of the planet (bigger mass leads to further away from the planet) and distance from the Sun (larger orbital radius leads to larger planet/L2 distance).&lt;/p&gt;&#xA;&lt;h3&gt;2. Astronomical interferometry&lt;/h3&gt;&#xA;&lt;p&gt;WikiPedia on &lt;a href=&quot;http://en.wikipedia.org/wiki/Astronomical_interferometer&quot; rel=&quot;noreferrer&quot;&gt;Astronomical interferometry&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution by means of interferometry. The benefit of the interferometer is that the angular resolution of the instrument is nearly that of a telescope with the same aperture as a single large instrument encompassing all of the individual photon-collecting sub-components. The drawback is that it does not collect as many photons as a large instrument of that size. Thus it is mainly useful for fine resolution of the more luminous astronomical objects, such as close binary stars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;Other questions.&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Would it be possible to power such a device?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Quite possible with nuclear power generation.  The main power drains would be:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Realigning the scope(s) for different objects.&lt;/li&gt;&#xA;&lt;li&gt;Keeping the core areas where the circuitry is located warm enough to avoid failure (which can sometimes be done purely as a side effect of the waste heat produced by the power generator).&lt;/li&gt;&#xA;&lt;li&gt;The power to transmit high quality images (what's the point if we only get a grainy image?) over vast distances.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Would such a device be so expensive as to become inviable?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I have little idea of the cost, and even less of what is considered viable.&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-08T07:25:21.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1842" PostTypeId="2" ParentId="1438" CreationDate="2013-09-08T06:17:24.947" Score="4" Body="&lt;p&gt;Let's start with a few unspoken assumptions &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Radiation refers to emissions from any body in space. To name a few possible sources&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Star&lt;/li&gt;&#xA;&lt;li&gt;Nebula&lt;/li&gt;&#xA;&lt;li&gt;Gas giant&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Danger means danger to humans&lt;/li&gt;&#xA;&lt;li&gt;Sun refers to our very own Sol&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The answer may appear to contradict itself at times. This may be because my attempt here is to provide a ... holistic (for want of a better word) view of what little I comprehend, recall, and read.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Background:&lt;/strong&gt;&#xA;As already premised, Earth's atmosphere &amp;amp; magnetosphere serve as an umbrella. They shield surface bound Earth life from known sources of radiation. The Sun is perhaps the nearest source of radiation. However even the Solar wind serves to protect planets within the Solar System from Cosmic Rays&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Cosmic_ray#Cosmic-ray_flux&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At distances of ~94 AU from the Sun, the solar wind undergoes a transition, called the termination shock, from supersonic to subsonic speeds. The region between the termination shock and the heliopause acts as a barrier to cosmic rays, decreasing the flux at lower energies (≤ 1 GeV) by about 90%. However, the strength of the solar wind is not constant, and hence it has been observed that cosmic ray flux is correlated with solar activity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;An &lt;a href=&quot;http://www.space.bas.bg/SENS/Ses2005/a6.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;independent source&lt;/a&gt; for the above&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving onwards - another major source of radiation within the Solar System is Sol itself. The planetary fact sheets available from &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NSSDC&lt;/a&gt; show Solar irradiance fall as one moves within the Solar System from Mercury to Pluto. Of course, this is inclusive of the visible spectrum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So ...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Cosmic ray radiation &lt;strong&gt;within&lt;/strong&gt; the Solar System is affected by Sol's weather.&lt;/li&gt;&#xA;&lt;li&gt;Incident radiation from Sol is directly proportional to distance from the Sun&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Still staying within the Solar System, what are the &lt;em&gt;other possible sources of radiation&lt;/em&gt;? &lt;a href=&quot;https://space.stackexchange.com/users/168/deer-hunter&quot;&gt;DeerHunter&lt;/a&gt;'s comment of Aug 22 provides an answer. The Gas giants are audible in the radio bands; they may be a source, and therefore potentially dangerous. A person standing within a pressure vessel of a reactor would likely be in danger regardless of Cosmic Rays, and Solar radiation; replace 'reactor' with, say, Jupiter and the analogy fits to a 'T'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Between the known major sources (Cosmic Rays, and Sol) my impressions are &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The risk &lt;strong&gt;increases&lt;/strong&gt; as one moves away from the inner Solar System, and&lt;/li&gt;&#xA;&lt;li&gt;The risk is probably a function of proximity to any alternate sources.  &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-09-08T07:30:53.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1843" PostTypeId="1" AcceptedAnswerId="1845" CreationDate="2013-09-08T09:35:11.200" Score="12" ViewCount="1142" Body="&lt;p&gt;There is a lot of popular talk that &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;There may be huge deposits of Helium-3 on the Moon, and &lt;/li&gt;&#xA;&lt;li&gt;It could be used for very efficient fusion reactors. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Unfortunately in the bulk of popular talk actual resources become difficult to find.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Questions:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How is the He-3 contained in lunar soil - not escaping into space as soon as it is created? &lt;/li&gt;&#xA;&lt;li&gt;Considering the need to process vast quantities of lunar soil, how could it be harvested? &lt;/li&gt;&#xA;&lt;li&gt;How could it be separated (refined) from other elements especially relatively abundant He-4? &lt;/li&gt;&#xA;&lt;li&gt;Does it's use as a fuel require some resources continuously from Earth? &lt;/li&gt;&#xA;&lt;li&gt;Could the lunar industry be powered by locally mined He-3 self-sustainably (subject to it's availability on Luna) after the construction phase?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="103" LastEditorUserId="49" LastEditDate="2014-02-20T03:34:49.533" LastActivityDate="2014-02-20T03:34:49.533" Title="Practical Lunar He-3 mining approach?" Tags="|the-moon|energy|mining|natural-resources|helium-3|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1844" PostTypeId="1" AcceptedAnswerId="1847" CreationDate="2013-09-08T11:10:32.333" Score="6" ViewCount="990" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/questions/1350/benefit-of-sling-shot-effect-with-a-space-elevator&quot;&gt;Benefit of sling shot effect with a space elevator&lt;/a&gt; implies that you can get free $\Delta V$ from a space elevator launch. As highlighted in &lt;a href=&quot;https://scifi.stackexchange.com/questions/39639/who-wrote-about-an-energy-free-space-elevator/39672#39672&quot;&gt;this answer on SciFi&lt;/a&gt; quoting Sheffield book &quot;The Web Between the Worlds&quot;, and &lt;a href=&quot;https://space.stackexchange.com/questions/1762/how-would-one-go-about-speeding-up-the-rotation-of-a-planet/1774#1774&quot;&gt;this answer on Space&lt;/a&gt; The launch is not really free, it is stealing rotational energy from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So two questions, given a space elevator launch from 100,000 kilometers, ignoring the energy cost getting mass to and from the launch point, and assuming all mass involved goes up and down the space elevator:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;How much mass would need to be launched for the Earth to lose 1 second of rotational speed?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How much mass would need to be lowered to the Earth to gain 1 second of rotational speed?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;For prospective, on potential launch mass.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_orbiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Orbiter&lt;/a&gt; has a fully loaded weight of about 120 tons&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/USS_Enterprise_%28CVN-65%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;USS Enterprise (CVN-65)&lt;/a&gt; (&lt;em&gt;an American aircraft carrier&lt;/em&gt;) has a fully loaded weight of about 94,781 tons&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-09-09T02:16:02.437" Title="Can a &quot;free launch&quot; from a space elevator really be free?" Tags="|launch|planet|space-elevator|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1845" PostTypeId="2" ParentId="1843" CreationDate="2013-09-08T12:36:50.713" Score="11" Body="&lt;p&gt;Well we don't yet have the actual proof that there's an &lt;em&gt;&quot;abundance&quot;&lt;/em&gt; of &lt;sup&gt;3&lt;/sup&gt;He (&lt;a href=&quot;https://en.wikipedia.org/wiki/Helium-3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Helium-3&lt;/a&gt;) embedded in the Lunar &lt;a href=&quot;https://en.wikipedia.org/wiki/Regolith&quot; rel=&quot;nofollow noreferrer&quot;&gt;regolith&lt;/a&gt; that would make any &lt;em&gt;&quot;mining&quot;&lt;/em&gt; worth our trouble. All we have so far is the indication that there might be, as shown by the trace amounts of &lt;sup&gt;3&lt;/sup&gt;He detected in the regolith samples extracted by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo&lt;/a&gt; missions. Current estimates are that there might be anything from 1 ppm up to 50 ppm (parts per million) of &lt;sup&gt;3&lt;/sup&gt;He embedded in the Lunar regolith, and while that might not seem much, if true, it &lt;em&gt;might&lt;/em&gt; make any exploitation of it feasible. Of course, the higher ppm, the better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have to add here that Apollo sampling of Lunar regolith arguably isn't the most indicative of levels of &lt;sup&gt;3&lt;/sup&gt;He embedded in it, since much of the surface trapped &lt;sup&gt;3&lt;/sup&gt;He would likely escape due to mechanical disturbance and electrostatic discharge when Apollo missions sampled it by drilling. But it is a better representative of how much of &lt;sup&gt;3&lt;/sup&gt;He is contained within the topmost Lunar bedrock, with Apollo 11 having best success landing in the TiO&lt;sub&gt;2&lt;/sub&gt; (Titanium Dioxide) rich region of Mare Tranquillitatis. Surfaces with electro-conductive metallic minerals seem to be the best, &lt;sup&gt;3&lt;/sup&gt;He rich &lt;sup&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;(1)&lt;/a&gt;&lt;/sup&gt; candidate regions suitable for Lunar surface mining on top of permanently shadowed Lunar polar regions &lt;sup&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103509005235&quot; rel=&quot;nofollow noreferrer&quot;&gt;(2)&lt;/a&gt;&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What makes this rare isotope so tempting is the energy stored in it as two of &lt;sup&gt;3&lt;/sup&gt;He atoms coupled in a superheated plasma break apart into two protons and a &lt;sup&gt;4&lt;/sup&gt;He (Helium-4) alpha particle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\require{mhchem}\ce{^3_2He + ^3_2He \to ^4_2He + 2\!^1_1p^+ + } \text{12.86 MeV}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the energy density in mega &lt;a href=&quot;https://en.wikipedia.org/wiki/Electronvolt&quot; rel=&quot;nofollow noreferrer&quot;&gt;electronvolts&lt;/a&gt; per &lt;a href=&quot;https://en.wikipedia.org/wiki/Mole_%28unit%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;mole&lt;/a&gt; (3 grams) of &lt;sup&gt;3&lt;/sup&gt;He plasma that was reached by the third-generation Helium-3 fusion reactor, developed by &lt;a href=&quot;http://www.thespacereview.com/article/536/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gerald Kulcinski&lt;/a&gt; at the University of Wisconsin. This is a reaction that produces very high amounts of energy, and at the same time very little harmful byproduct. Second-generation fusion generators produce more energy per fuel mass, with Wikipedia putting them in the range of 18 MeV (493 &lt;a href=&quot;https://en.wikipedia.org/wiki/Kilowatt_hour&quot; rel=&quot;nofollow noreferrer&quot;&gt;megawatt-hours&lt;/a&gt; or 4.93×10&lt;sup&gt;8&lt;/sup&gt; W·h) per mole of &lt;sup&gt;3&lt;/sup&gt;He, but they do require &lt;a href=&quot;https://en.wikipedia.org/wiki/Deuterium&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deuterium&lt;/a&gt; ($\ce{^2_1H}$), which makes it less suitable for direct conversion of &lt;sup&gt;3&lt;/sup&gt;He resources on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far so good, at least as far as &lt;sup&gt;3&lt;/sup&gt;He's actual use is concerned. But, as I mentioned before, we don't actually have direct evidence of mineable amounts of &lt;sup&gt;3&lt;/sup&gt;He in the Lunar regolith, and all the &lt;em&gt;current&lt;/em&gt; evidence is indirect at best. The theory behind its forming seems sound enough, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While &lt;sup&gt;3&lt;/sup&gt;He doesn't form plasma easily, and huge temperatures are required for the two &lt;sup&gt;3&lt;/sup&gt;He atoms to have enough of kinetic energy to &lt;em&gt;couple&lt;/em&gt; and remain in a plasma state, we're kinda lucky that, it just so happens, our Sun does that really good and the main natural and renewable sources of &lt;sup&gt;3&lt;/sup&gt;He come from solar Coronal Mass Ejections (CME) and solar wind. As the solar wind can reach velocities up to 900 kilometers per second, and nothing stopping it on its way to the surface of the Moon, this &lt;sup&gt;3&lt;/sup&gt;He plasma can embed in the rather thick layer of Lunar dust (samples from up to 1 m deep were recovered by Apollo missions also retaining trace amounts of it), and Lunar surface is also bombarded by asteroids, that would possibly embed &lt;sup&gt;3&lt;/sup&gt;He even deeper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another point to possible &lt;em&gt;lingering&lt;/em&gt; of &lt;sup&gt;3&lt;/sup&gt;He in the Lunar regolith is that there isn't really any dense enough atmosphere on the Lunar surface to sustain what we would consider as &lt;em&gt;normal&lt;/em&gt; behavior of molecules in gases - frequently colliding with each other in the excited state. These thin enough atmosphere not supporting this behavior of gases are called &lt;a href=&quot;https://en.wikipedia.org/wiki/Exosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;exospheres&lt;/a&gt;. The point being, that they don't form enough of the atmospheric pressure to push lighter on the atomic scale and less dense gases upwards, displaced by denser and heavier gases (like we'd expect on the Earth). There is also no weather to speak of, except for the Solar weather, but that one, as we said earlier, is supposed to bring more &lt;sup&gt;3&lt;/sup&gt;He with it, not blow what's already embedded in the regolith away. Well, possibly both, but the total quantity embeded should stay at least equal, if not larger, than before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How long is this &lt;sup&gt;3&lt;/sup&gt;He contained within the Lunar surface is unknown though, and while there's a few working theories, we'll probably have to wait for more data returned by Chinese &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chang'e program&lt;/a&gt;, or even NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Atmosphere_and_Dust_Environment_Explorer&quot; rel=&quot;nofollow noreferrer&quot;&gt;LADEE&lt;/a&gt; (Lunar Atmosphere and Dust Environment Explorer) that was just launched (as of writing of this answer) and is on its way to a roughly 100 days long mission to study the Lunar &lt;a href=&quot;https://en.wikipedia.org/wiki/Exosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;exosphere&lt;/a&gt; and dusty environment. My money is on the temporary &lt;sup&gt;3&lt;/sup&gt;He plasma containment by the electrostatic charge it produces on impact with the surface regolith and that there's not enough of pressure to displace it faster than it is replenished, but that's just a somewhat educated assumption at best.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;3&lt;/sup&gt;He in a plasma state is so far easiest to contain using &lt;a href=&quot;https://en.wikipedia.org/wiki/Tokamak&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tokamak&lt;/a&gt; devices that use magnetic fields to confine plasma, but Kulcinski proposed the &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_electrostatic_confinement&quot; rel=&quot;nofollow noreferrer&quot;&gt;Inertial Electrostatic Confinement&lt;/a&gt; (IEC) reactor that can contain it and at the same time sustain Helium-3 fusion &lt;sup&gt;&lt;a href=&quot;http://www.thespacereview.com/article/536/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;(3)&lt;/a&gt;&lt;/sup&gt;. He believes that he can better collect the energy produced by these reactions by using a reverse particle accelerator that can convert the power directly into electricity without the need of turning a turbine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/8HyUb.jpg&quot; alt=&quot;Inertial Electrostatic Confinement reactor&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Inertial Electrostatic Confinement reactor (IEC) containing Helium-3 plasma and converting it to el. energy (Source: &lt;a href=&quot;http://iec.neep.wisc.edu/photopages/GeneralOpPicsII.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;U. of Wisconsin&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer your questions more directly:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;How is the He-3 be contained in lunar soil - not escaping into space as soon as it's created? How could harvesting it be performed, considering both the need to process vast quantities of lunar soil to be processed?&lt;/strong&gt;&lt;br /&gt;&lt;br /&gt; &#xA;I've already described one of the containment methods - the &lt;sup&gt;3&lt;/sup&gt;He plasma being contained within the electrostatically charged Lunar regolith, predominantly fine-grain dust, as it charges when the CME and solar wind particles hit it. There's another point to it though. Lunar &quot;atmosphere&quot; doesn't really behave like gases, as it's way too thin and its molecular density means the molecules, even when in their excited state, rarely hit each other. This is the main difference between an atmosphere, and what we call an exosphere - what Luna has. This means that there isn't enough pressure on the otherwise in-atmosphere easily pushed upwards (due to gravity) lighter gases by their heavier brethren. So when in exosphere, while these gases are &lt;em&gt;light&lt;/em&gt;, they're not pushed away by heavier gases and stay on surface for longer time.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;How could refining it be done - separating it from other elements, especially relatively abundant He-4?&lt;/strong&gt;&lt;br /&gt;&lt;br /&gt;&#xA;Thinking of the last point, probably relatively easy. One way would be the &lt;em&gt;passive&lt;/em&gt; approach of trapping upper layers of Lunar regolith in containers with a higher, true atmosphere pressure and simply skimming the top (there is still some gravity on the Moon, making &quot;up&quot; and &quot;down&quot; unambiguous, and &lt;sup&gt;3&lt;/sup&gt;He would be among the lightest molecules), then repeating the process for as long as it takes to get the required purity. Another approach, the &lt;em&gt;active&lt;/em&gt; one is by using centrifuge systems to separate molecules of different atomic mass, and then sucking &lt;sup&gt;3&lt;/sup&gt;He out from somewhere close to the center. This approach would work also in microgravtational conditions. Also common way to do that is by &lt;a href=&quot;https://en.wikipedia.org/wiki/Microfiltration&quot; rel=&quot;nofollow noreferrer&quot;&gt;microfiltration&lt;/a&gt;, or magnetic field containment using &lt;a href=&quot;https://en.wikipedia.org/wiki/Tokamak&quot; rel=&quot;nofollow noreferrer&quot;&gt;tokamak&lt;/a&gt; devices.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Could the lunar industry be powered by locally mined He-3 self-sustainably (...well, as sustainably as inexhaustible its deposits are) after the construction phase, or would it require some other resources to be continuously delivered from Earth?&lt;/strong&gt;&lt;br /&gt;&lt;br /&gt;&#xA;By using third-generation fusion reactors that don't require isotopes of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrogen&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hydrogen&lt;/a&gt; (Deuterium and &lt;a href=&quot;https://en.wikipedia.org/wiki/Tritium&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tritium&lt;/a&gt; $\ce{^3_1H}$) and solely depend on renewable resources available on the Moon and its regolith, then we can definitely think of this process as self-sustainable.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Of course, all of this by no easily dismissible amount depends on the amount of &lt;sup&gt;3&lt;/sup&gt;He (in the range of parts per million) on the Lunar surface. However, pockets of &lt;sup&gt;3&lt;/sup&gt;He plasma contained deeper within the Lunar surface and created by asteroid impacts aren't dismissed, if somewhat more difficult to mine.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;References and suggested reading&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Estimation of Helium-3 Probable Reserves in Lunar Regolith&lt;/a&gt;, Slyuta et al., Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, 2007  (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103509005235&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;sup&gt;3&lt;/sup&gt;He in permanently shadowed lunar polar surfaces&lt;/a&gt;, F. H. Cocks, Icarus&#xA;Volume 206, Issue 2, April 2010, Pages 778–779&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.thespacereview.com/article/536/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;A fascinating hour with Gerald Kulcinski&lt;/a&gt;, Eric R. Hedman, The Space Review, 2006&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://fti.neep.wisc.edu/pdf/fdm1119.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;D-&lt;sup&gt;3&lt;/sup&gt;He Fusion in an Inertial Electrostatic Confinement Device&lt;/a&gt;, Ashley et al., Wisconsin Fusion Technology Institute, 1999 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.itmindia.edu/images/ITM/pdf/Helium-3%20As%20An%20Alternate%20Fuel%20Technology%20-%20Satish%20Kumar.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Helium-3 As An Alternate Fuel Technology (for Producing Electricity)&lt;/a&gt;, S. Kumar, K. Gupta, The Journal of Department of Applied Sciences &amp;amp; Humanities, Vol. IV, 2006 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://fti.neep.wisc.edu/research/iec&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wisconsin Fusion Technology Institute: Collection of papers on Inertial Electrostatic Confinement&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-20T01:31:48.650" LastActivityDate="2014-02-20T01:31:48.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1846" PostTypeId="2" ParentId="1844" CreationDate="2013-09-08T14:36:37.440" Score="1" Body="&lt;p&gt;FWIW, I'm with Arthur C. Clarke on this whole &quot;space elevator&quot; business. There is no &quot;free lunch&quot; (or is it &quot;free launch&quot;?) to be had with any &quot;space elevator&quot; that is supposedly capable of taking a payload with non-negligible mass and with non-negligible air resistance in the lower atmosphere to the stratosphere and beyond. It doesn't really matter then where we put this alleged &quot;space elevator&quot;, so I'll take a few shortcuts to answer this as easy as possible. Take it as it is, I have no interests in discussing &quot;space elevators&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For your first question, there is no upper limit, short of removing the total of the Earth's mass. The remaining mass would still have radial velocity equal to the total mass from before, only it's kinetic potential will be lowered as the mass spinning decreases, so it would be more prone to changes in its rotation velocity, if we added some of that mass back later on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second question is also relatively easy, if we assume the kinetic potential of the added mass in the direction of the Earth's rotation is equal to zero, i.e. &lt;strong&gt;it's lowered directly perpendicular to the Earth's surface and &lt;em&gt;dropped&lt;/em&gt; at either of the poles&lt;/strong&gt; (another &lt;a href=&quot;https://space.stackexchange.com/a/1847/49&quot;&gt;answer by @MarkAdler&lt;/a&gt; already includes calculations for dropping the mass at the equator):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{duration\ of\ Earth\ day\ in\ seconds}{Earth's\ total\ mass} = x  \left(\frac{duration\ of\ Earth\ day\ in\ seconds + 1\ }{Earth's\ total\ mass}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this comes out as $1/86400$ &amp;nbsp;of the Earth's mass ($5.972 * 10^{23} kg$), which is $6.9123 * 10^{18} kg$, if my data input was correct. But please do check again. ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But assuming my &lt;em&gt;pocket calculator&lt;/em&gt; doesn't lie, this would come out as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;72,929,329,875&lt;/strong&gt; (nearly 73 billion) of USS Enterprise (CVN-65) aircraft carriers&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;57,602,623,456,791&lt;/strong&gt; (a good 57 and a half trillion) of fully loaded Space Shuttle Orbiters&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-09-08T16:36:43.967" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1847" PostTypeId="2" ParentId="1844" CreationDate="2013-09-08T15:00:17.680" Score="12" Body="&lt;p&gt;This is a simple calculation in the conservation of angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angular momentum of uniform sphere is ${2\over 5}MR^2\omega$.  We will assume that Earth is a uniform sphere (close enough for this question), so $M=5.972\times 10^{24}\,\mathrm{kg}$, $R=6371\,\mathrm{km}$ (mean), and $\omega=7.292\times 10^{-5}\,\mathrm{s}$.  So the angular momentum of the Earth is $L_e=7.071\times 10^{33}\mathrm{kg\,m^2\over s}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angular momentum of a small object rotating about a point is $mr^2\omega$.  $\omega$ does not change when climbing the elevator, just $r$.  So the change in angular momentum is $\Delta L=m\omega(r_f^2-r_i^2)$.  $\omega$ is that of Earth above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To change the Earth's rotation rate by $1\,\mathrm{s}$, we need to change its angular momentum by $1\over 86164$ of what it was, so $\Delta L=8.206\times 10^{28}\mathrm{kg\,m^2\over s}$.  Assuming a space elevator at the equator, and using the equatorial radius of the Earth, $6378\,\mathrm{km}$, and $100000\,\mathrm{km}$ above that, we get $\Delta L=8.206\times 10^{28}\mathrm{kg\,m^2\over s}=m\omega\left(\left(106378\,\mathrm{km}\right)^2-\left(6378\,\mathrm{km}\right)^2\right)$.  To the extent that $m$ reduces (or increases) $M$, there is also a small change in the rotation rate for the given angular momentum when it is released.  However that is negligible here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can be simplified and approximated, in order to better see the sensitivities, as: $m={2\over 5}M{\Delta T\over T}{R^2\over H^2}$, where $T$ is the rotation period of the Earth, $\Delta T$ is the small amount by which you want to change it, and $H$ is the radius of the release point from the elevator.  $m$ is the amount of mass to let go of at the release point to effect a change in rotation period of $\Delta T$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That gives $m\approx 10^{17}\mathrm{kg}$.  About a billion (US-style) of your aircraft carriers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If each aircraft-carrier size spaceship only carried a handful of people, then we could evacuate the entire human population from the Earth, only slowing Earth down by one second. So I would say, for all practical purposes, yes, a space elevator is free.  You just have to pay the energy to climb to that altitude, which is quite small compared to the energy from the velocity that you get.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-09T02:16:02.437" LastActivityDate="2013-09-09T02:16:02.437" CommentCount="0" CommunityOwnedDate="2013-09-08T19:47:06.450" ContentLicense="CC BY-SA 3.0" />
  <row Id="1848" PostTypeId="5" CreationDate="2013-09-08T15:34:38.307" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-08T15:34:38.307" LastActivityDate="2013-09-08T15:34:38.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1849" PostTypeId="4" CreationDate="2013-09-08T15:34:38.307" Score="0" Body="Questions regarding devices used to store energy for a duration of time." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-08T15:55:29.120" LastActivityDate="2013-09-08T15:55:29.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1850" PostTypeId="5" CreationDate="2013-09-08T15:36:00.453" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-08T15:36:00.453" LastActivityDate="2013-09-08T15:36:00.453" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1851" PostTypeId="4" CreationDate="2013-09-08T15:36:00.453" Score="0" Body="Questions regarding either the lifespan of an astronaut, or the amount of time that a device or instrument is usable." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-08T15:55:32.790" LastActivityDate="2013-09-08T15:55:32.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1852" PostTypeId="1" AcceptedAnswerId="1853" CreationDate="2013-09-08T18:19:29.857" Score="3" ViewCount="1397" Body="&lt;p&gt;Spacecraft typically launch close to the equator along Earth's rotation. This contributes some velocity to the craft. In addition Earth's gravity is weakest along the equator (bar mountainous regions).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/460/288&quot;&gt;This reply&lt;/a&gt; mentions retrograde, and inclined orbits favour a high latitude launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the hypothetical scenario where the logistics are suitable, what advantage or disadvantage accrue by launching the craft &lt;em&gt;through&lt;/em&gt; the axis of rotation i.e at the Geographic South Pole in case of Earth? Would angular velocity still play a role?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if the situation were reversed - why would one choose to land along the axis of rotation? One situation may be if the Celestial had a very fast rotation - E.g. Jupiter, or elsewhere faster yet. &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-09-09T17:24:40.530" Title="What advantage or disadvantage when the craft moves through the axis of rotation?" Tags="|launch|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1853" PostTypeId="2" ParentId="1852" CreationDate="2013-09-08T19:06:26.507" Score="5" Body="&lt;p&gt;Only disadvantages, unless you're launching in a retrograde orbit, I'm afraid. And no, angular velocity at the dead center to the axis of rotation (i.e. true axial poles the Earth spins around, not magnetic poles) wouldn't play a role, as there is no angular velocity to speak of. Unless you count no angular velocity as one, of course. Out of my head (I might add a few other points, or describe the ones I'll make better, at a later date tho):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Gravity is at the poles slightly bigger than the Earth's average surface gravity because of the centrifugal force slightly negating its effect elsewhere, and the most of course at the equator (up to 0.3%), so you'll need more thrust to lift payloads of the same mass. This might not seem much, but it will once you'll have to lower expectations of your clients by up to that amount. Constantly working against margins of error also means this small percentage can fast cascade into a more substantial one.&lt;/li&gt;&#xA;&lt;li&gt;No Earth's rotation assist, buying you roughly 1,670 kilometers (1,038 miles) per hour of free velocity at the equator (the radial velocity at which the Earth rotates around its axis there). This is a rough estimate, assuming the Earth was a perfect sphere tho. It isn't and the advantage to the equatorial launches should be even slightly bigger. Prograde launches use this to their advantage for a slight, but important assistance of the Earth's rotation and slightly less atmospheric drag. I.e. your initial velocity relative to the surface is already something, compared to nothing, and you don't get a benefit of moving in the direction the atmosphere rotates (this is blatant oversimplification though, it really depends on many more factors, but on average, it's true).&lt;/li&gt;&#xA;&lt;li&gt;Faster exposure to the solar radiation because of the Earth's magnetosphere being the weakest there (and somewhat also at the South Atlantic Anomaly). This might or might not matter though, depending on the tolerance of your onboard and launch equipment.&lt;/li&gt;&#xA;&lt;li&gt;It's freezing cold to say the least, which doesn't really help with your work conditions, but it might help keep the pressure in the cryogenic stages a bit more stable. It might. Some of the onboard equipment might also not be expected to work properly during and/or after such atmospheric conditions, if they need active heating by the onboard energy source (which might not be on at the time of the launch operations). Some propellants are also highly cryogenic with exposure to the air in the atmosphere, like e.g. liquid hydrogen (which is already stored in cryogenic temperatures), so subsequently more ice can form on your launch vehicle and cause damage to it (e.g. think - STS Columbia).&lt;/li&gt;&#xA;&lt;li&gt;You're launching in the direction more or less perpendicular to the position of pretty much anything of interest in the Solar system, so you'll possibly want to adjust your vector later on anyway. And if you're launching a communications satellite, you'll probably still want to later position it over some population, which tends not to be as dense near true poles.&lt;/li&gt;&#xA;&lt;li&gt;Awful weather conditions. You should be prepared to scrub your launch and have a possibly long wait for better conditions due to rough winds, blizzards, fog,... and before your launch vehicle &lt;em&gt;thaws&lt;/em&gt;. If it would be still in functional order, that is.&lt;/li&gt;&#xA;&lt;li&gt;International laws regarding exploitation of the polar regions for commercial purposes and your own pollutants associated with your launch might be against you. Be prepared to have environmental group activists camping at the edge of your launch facility's perimeter at all times. Have plenty of hot tea prepared at all times.&lt;/li&gt;&#xA;&lt;li&gt;Half of the year it's daylight, the other half of the year its dark. This is not really a problem for your launch vehicle, but it will make a difference with your crew, technicians, e.t.c. Well rested workers will be less prone to making mistakes, and with multiple hundreds of millions of dollars per launch, you might not want to risk them suffering insomnia, or have other adverse psychological effects due to day/night cycle stretching a whole year.&lt;/li&gt;&#xA;&lt;li&gt;Accessibility problems. There's no roads to speak of, and even if you made some, they wouldn't last for very long due to glacial movement. So the only available access is by  sea (you need icebreakers to get there, which is not cheap), or by air. Neither are optimal for sensitive and/or heavy transports.&lt;/li&gt;&#xA;&lt;li&gt;Nobody really lives there, except penguins and polar bears (depends at which axial pole you are), so you'll have higher commute costs and abroad working fees. You might also need more security guards with stun and tranquilizer guns. Of course, not being a populated area can be an advantage, tho there are unpopulated areas a lot closer to the equator and large enough for a launch facility.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As for other celestials, the faster they rotate the better actually. At least until they start spinning with big enough velocity to spew their own materials into outer space, reaching escape velocity. You wouldn't want to be anywhere near them, preferably not in the same solar system, if that was the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why would faster radial velocity be good? Because you're approaching them at higher than escape velocities anyway, unless you have enough propellant left aboard to slow yourself down with thrust against your vector (in which case, it would still help you burn less propellant, if it rotated. Remember, you are stopping to zero speed relative to the speed the celestial is rotating). So chances are, you'd want to use aerobraking, and reuse some of the radial momentum to your advantage, landing in the same direction the celestial rotates and have a shallower atmospheric reentry approach vector (which might even save your landing vehicle from disintegrating). The faster it rotates (but below its own escape velocity), the easier this will be. Plus, you save a lot of kinetic potential for the next takeoff, if that's in your schedule at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The single advantage to attempting to penetrate a gas giant through the vector following its axis is that their upper atmosphere tends to be somewhat recessed there, so you could hope for a bit longer descent with no weather. Depending on your atmosphere penetrating probe (not a landing at any stretch of imagination, for one, there's nothing to land on, and all the probes we sent so far merely achieved a rather short splash until they imploded due to immense atmospheric pressure, even if their parachutes deployed properly) and its mission, this might or might not be desirable. But probably for the most part, it wouldn't be. Especially, if you're set to study the celestial's weather from within its upper atmosphere. And from above it, there are safer ways to do that anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So these are a few points I could think of, some of them serious drawbacks, some a bit less serious or even an arguable advantage (I wouldn't call it as such, tho), but I'm sure there are many more. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-09T17:23:28.447" LastActivityDate="2013-09-09T17:23:28.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1854" PostTypeId="1" AcceptedAnswerId="1855" CreationDate="2013-09-08T21:43:12.783" Score="12" ViewCount="3363" Body="&lt;p&gt;I heard that Virgin Galactic's SpaceShipTwo just tested something called “feathering”.  What is it? How come I didn't hear about the shuttle doing this, it is unique to  Virgin Galactic's ship?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-09-12T12:19:58.813" LastActivityDate="2019-07-23T21:56:19.070" Title="What is feathering?" Tags="|technology|reentry|terminology|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1855" PostTypeId="2" ParentId="1854" CreationDate="2013-09-08T23:40:31.793" Score="15" Body="&lt;p&gt;This latest (second powered) test flight of the &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipTwo&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceShipTwo&lt;/a&gt; was really interesting to watch (&lt;a href=&quot;https://www.youtube.com/watch?feature=player_detailpage&amp;amp;v=eEmLOU8zTSE#t=134&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt;, the &quot;feathering&quot; aerobraking starts at 2:14 into it), but this currently unique to &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Spaceship_Company&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Spaceship Company&lt;/a&gt; designs (owned by its sister company &lt;a href=&quot;http://www.virgingalactic.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Virgin Galactic&lt;/a&gt; that's itself within the &lt;a href=&quot;https://en.wikipedia.org/wiki/Richard_Branson&quot; rel=&quot;nofollow noreferrer&quot;&gt;Richard Branson's&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Virgin_Group&quot; rel=&quot;nofollow noreferrer&quot;&gt;Virgin Group&lt;/a&gt;) approach to &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobraking&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerobraking&lt;/a&gt; was actually already introduced with the &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipOne&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceShipOne&lt;/a&gt;. With the help of a side-by-side image, this is &quot;feathering&quot; of the SpaceShipTwo:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/LsR0G.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Nominal and &quot;Feathering&quot; wing positions on SpaceShipTwo, side by side comparison (Source: &lt;a href=&quot;http://www.space.com/13296-photos-virgin-galactic-spaceshiptwo-test-flights.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space.com&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what Wikipedia has to say about &lt;a href=&quot;https://en.wikipedia.org/wiki/Spaceshiptwo#Design_overview&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceShipTwo &quot;Feathering&quot;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceShipTwo uses a feathered reentry system, feasible due to the low&#xA;  speed of reentry – by contrast, the Space Shuttle and other orbital&#xA;  spacecraft re-enter at orbital speeds, closer to 25,000 km/h (16,000&#xA;  mph), using heat shields. SpaceShipTwo is furthermore designed to&#xA;  re-enter the atmosphere at any angle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And here's another image, showing the movement of the side wings on the SpaceShipTwo (similar approach, source: &lt;a href=&quot;http://www.theverge.com/2012/7/11/3151861/virgin-galactic-first-space-tourism-flight-next-year&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Verge&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/vSJw5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?&amp;amp;v=tqYAQdNWQjQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;this YouTube video&lt;/a&gt;, Richard Branson explains this feathered reentry system as (starting 55 seconds into the video):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/tqYAQdNWQjQ?start=55&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The wing and tail section would take upwards, into what we call &quot;the&#xA;  feather position&quot;, allowing the vehicle to slow its rate of descent&#xA;  smoothly and stably, just like a &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttlecock&quot; rel=&quot;nofollow noreferrer&quot;&gt;shuttlecock&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And here's a technical snapshot of the Virgin Galactic's SpaceShipTwo:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/LkMOO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="6944" LastEditDate="2015-03-23T03:06:16.603" LastActivityDate="2015-03-23T03:06:16.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1856" PostTypeId="2" ParentId="1804" CreationDate="2013-09-09T04:21:32.577" Score="6" Body="&lt;p&gt;Yes, there is at least one advantage to a deep-space telescope.  That would be getting away from our Sun's dust cloud to avoid the reflected &lt;a href=&quot;http://en.wikipedia.org/wiki/Zodiacal_light&quot; rel=&quot;noreferrer&quot;&gt;Zodiacal light&lt;/a&gt;.  In fact, &lt;a href=&quot;http://www.popsci.com/technology/article/2011-12/proposed-telescope-would-hitchhike-outer-solar-system-escape-local-light-pollution&quot; rel=&quot;noreferrer&quot;&gt;such missions&lt;/a&gt; have &lt;a href=&quot;http://www.universetoday.com/94162/how-nasa-will-improve-its-telescopes-vision/&quot; rel=&quot;noreferrer&quot;&gt;been proposed&lt;/a&gt; in order to study the &lt;a href=&quot;http://en.wikipedia.org/wiki/Extragalactic_background_light&quot; rel=&quot;noreferrer&quot;&gt;Extra-Galactic Background Light&lt;/a&gt; which is blocked by Zodiacal light.  They would ideally like to get out to 5 AU and well above or below the ecliptic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such telescopes do not have to be very large, nor transmit very much data in order to improve on our current ability to see the EBL by a few orders of magnitude.  That sort of increase gets astronomers very excited.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-09T05:50:58.377" LastActivityDate="2013-09-09T05:50:58.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1857" PostTypeId="1" AcceptedAnswerId="1879" CreationDate="2013-09-09T05:43:37.350" Score="8" ViewCount="1978" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centripetal force required to orbit with them&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The major bulk of the Solar System is concentrated in Sol with planets gravitationally bound to Sol. &lt;strong&gt;Sol orbits the galactic centre&lt;/strong&gt; over a distance of some thousand light-years. The galactic centre is therefore massive enough to exert it's influence over such large separation.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Can Lagrangian points exist for a whole planetary system/star-system? E.g. Between Sol, and the Galactic Hole. Similarly between any other Star in the galaxy, and the Galactic Hole.&lt;/li&gt;&#xA;&lt;li&gt;How much must a &lt;strong&gt;star/planetary system&lt;/strong&gt; mass for such libration points to exist?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user12" LastEditDate="2013-09-10T23:01:53.050" LastActivityDate="2022-08-05T16:14:30.137" Title="Can a whole planetary system have Lagrangian points?" Tags="|lagrangian-points|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="1858" PostTypeId="1" CreationDate="2013-09-09T12:39:56.750" Score="11" ViewCount="3599" Body="&lt;p&gt;Is there any reason to send deep space probes (Pioneer and Voyager) along the plane of our solar system (maybe with a slight inclination) instead of perpendicular to the plane of our solar system?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so what is the reason ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="897" LastEditDate="2014-02-22T08:34:14.020" LastActivityDate="2020-12-17T17:15:01.760" Title="Is there any reason to send space probes along the plane of our solar system instead of perpendicular to the plane of our solar system?" Tags="|orbital-mechanics|probe|orbit-selection|" AnswerCount="5" CommentCount="2" ClosedDate="2019-07-31T17:12:16.760" ContentLicense="CC BY-SA 3.0" />
  <row Id="1859" PostTypeId="2" ParentId="1858" CreationDate="2013-09-09T13:29:11.070" Score="14" Body="&lt;p&gt;The main reason is it's a lot easier. It requires an enormous delta v to change the inclination of the spacecraft such that it would be perpendicular to the plane of the solar system. Furthermore, there isn't that much of interest beyond the ecliptic plane. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only reason to send a spacecraft perpendicular is to somehow study the poles of the Sun, or to orbit an object significantly outside of the ecliptic plane. This has been done once, with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ulysses_%28spacecraft%29&quot;&gt;Ulysses&lt;/a&gt; spacecraft. It achieved the required delta v by a carefully planned flyby of Jupiter. This flyby allowed it to have an inclination of 80 degrees around the sun. There aren't many objects worth visiting outside of the Ecliptic plane, so there just hasn't been many missions to make the change.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-11T21:10:28.440" LastActivityDate="2013-09-11T21:10:28.440" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1861" PostTypeId="1" CreationDate="2013-09-09T17:10:55.823" Score="11" ViewCount="369" Body="&lt;p&gt;Spacecraft meant to operate in both, an atmosphere &amp;amp; space, are typically &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Stream-lined (as in the case of the Space Shuttle, and Buran), or &lt;/li&gt;&#xA;&lt;li&gt;Spherical (Vostok comes to mind). &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In contrast spacecraft meant to work in a vacuum may dispense with the niceties of aerodynamics. Wikipedia displays the Apollo Lunar Module in all it's functional glory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jxgLT.jpg&quot; alt=&quot;Apollo Lunar Module&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Voyager craft are designed to work in a vacuum, and have the appropriate shape for it.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are there any studies on the ideal form for a spacecraft at large velocity in vacuum?&lt;/li&gt;&#xA;&lt;li&gt;Would velocity be relevant to the shape?&lt;/li&gt;&#xA;&lt;li&gt;What velocity must technology achieve for the shape of the craft to be relevant?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Alternately the last may be rephrased to read - &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the technology to achieve it becomes available (whenever that happens (+:) What kind of shape might contribute most to the craft approaching even a minimal 0.1c?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="58" LastEditDate="2013-09-09T17:47:42.013" LastActivityDate="2017-10-21T15:25:41.823" Title="Are there any studies on the ideal form for a spacecraft at large velocity in vacuum?" Tags="|spacecraft-development|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1862" PostTypeId="1" CreationDate="2013-09-09T17:21:46.457" Score="8" ViewCount="517" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/RD-180&quot;&gt;RD-180&lt;/a&gt; is a rocket engine produced by Russian NPO Energomash company. It is used in such US rockets as &lt;a href=&quot;http://en.wikipedia.org/wiki/Atlas_III&quot;&gt;Atlas III&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Common_Core_Booster&quot;&gt;Atlas V&lt;/a&gt;.&#xA;There are rumors, that Russia is considering to &lt;a href=&quot;http://rt.com/news/russian-rocket-engine-ban-039/&quot;&gt;ban RD-180 export to US&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would that affect the US space program? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does Lockheed have some other engine to use instead, or the ban means the end of Atlas rockets? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will NASA be able to perform the same tasks it does with Atlas using Delta and Falcon rockets, or does the ban open an empty niche?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user12" LastEditDate="2013-09-11T00:46:53.420" LastActivityDate="2013-09-12T16:23:38.397" Title="How will the RD-180 ban affect US space program?" Tags="|russia|engines|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1863" PostTypeId="2" ParentId="1804" CreationDate="2013-09-09T17:35:34.780" Score="3" Body="&lt;p&gt;For infra-red astronomy, operating anywhere near the sun can be problematic. Even the &lt;a href=&quot;http://en.wikipedia.org/wiki/JWST&quot; rel=&quot;nofollow&quot;&gt;James Webb Space Telescope&lt;/a&gt; design has issues in that the sun shield is expected to degrade over time causing a gradual increase in the telescope's operating temperature and corresponding decrease in the telescope's effectiveness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An IR telescope launched into deep space would be free of any meaningful source of heat and would be able to operate at optimal temperatures pretty much indefinitely. Indefinitely, in this case, being defined by the lifespan of the telescopes critical hardware components.&lt;/p&gt;&#xA;" OwnerUserId="113" LastActivityDate="2013-09-09T17:35:34.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1864" PostTypeId="2" ParentId="1858" CreationDate="2013-09-09T19:09:30.050" Score="10" Body="&lt;p&gt;Because of the slightly fuzzy understanding of planes demonstrated in this question and &lt;a href=&quot;https://space.stackexchange.com/q/834/49&quot;&gt;HyperAnthony's similar question&lt;/a&gt;, I want to clear up the matter of planes.&lt;/p&gt;&#xA;&lt;p&gt;First, keep in mind that planes are two-dimensional. They have zero thickness.&lt;/p&gt;&#xA;&lt;p&gt;The invariable (or invariant) plane is the plane that passes through the center of mass of the solar system perpendicular to the solar system's angular momentum vector.&lt;/p&gt;&#xA;&lt;p&gt;The ecliptic plane is the plane on which the Earth's orbit is located.&lt;/p&gt;&#xA;&lt;p&gt;Generally, most objects are going to be inclined (at an angle away from) these planes. In the case of planets, it is significantly so. Here is a handy chart from Wikipedia that shows the inclinations of each of the planets:&lt;/p&gt;&#xA;&lt;div class=&quot;s-table-container&quot;&gt;&#xA;&lt;table class=&quot;s-table&quot;&gt;&#xA;&lt;thead&gt;&#xA;&lt;tr&gt;&#xA;&lt;th&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;/th&gt;&#xA;&lt;th&gt;Inclination&lt;/th&gt;&#xA;&lt;th&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;/th&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/thead&gt;&#xA;&lt;tbody&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;strong&gt;Name&lt;/strong&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;strong&gt;Inclination to &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic&quot; rel=&quot;nofollow noreferrer&quot;&gt;ecliptic&lt;/a&gt;&lt;/strong&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;strong&gt;Inclination to &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun's equator&lt;/a&gt;&lt;/strong&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;strong&gt;Inclination to &lt;a href=&quot;http://en.wikipedia.org/wiki/Invariable_plane&quot; rel=&quot;nofollow noreferrer&quot;&gt;invariable plane&lt;/a&gt;&lt;/strong&gt;&lt;sup&gt;&lt;a href=&quot;https://web.archive.org/web/20130603143144/http://home.surewest.net/kheider/astro/MeanPlane.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;[1]&lt;/a&gt;&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Terrestrial_planet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Terrestrials&lt;/a&gt;&lt;/strong&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury_(planet)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mercury&lt;/a&gt;&lt;/td&gt;&#xA;&lt;td&gt;7.01°&lt;/td&gt;&#xA;&lt;td&gt;3.38°&lt;/td&gt;&#xA;&lt;td&gt;6.34°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus&lt;/a&gt;&lt;/td&gt;&#xA;&lt;td&gt;3.39°&lt;/td&gt;&#xA;&lt;td&gt;3.86°&lt;/td&gt;&#xA;&lt;td&gt;2.19°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth&lt;/a&gt;&lt;/td&gt;&#xA;&lt;td&gt;0°&lt;/td&gt;&#xA;&lt;td&gt;7.155°&lt;/td&gt;&#xA;&lt;td&gt;1.57°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars&lt;/a&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.85°&lt;/td&gt;&#xA;&lt;td&gt;5.65°&lt;/td&gt;&#xA;&lt;td&gt;1.67°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gas_giant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gas giants&lt;/a&gt;&lt;/strong&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter&lt;/a&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.31°&lt;/td&gt;&#xA;&lt;td&gt;6.09°&lt;/td&gt;&#xA;&lt;td&gt;0.32°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn&lt;/a&gt;&lt;/td&gt;&#xA;&lt;td&gt;2.49°&lt;/td&gt;&#xA;&lt;td&gt;5.51°&lt;/td&gt;&#xA;&lt;td&gt;0.93°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Uranus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Uranus&lt;/a&gt;&lt;/td&gt;&#xA;&lt;td&gt;0.77°&lt;/td&gt;&#xA;&lt;td&gt;6.48°&lt;/td&gt;&#xA;&lt;td&gt;1.02°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;/td&gt;&#xA;&lt;td&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Neptune&quot; rel=&quot;nofollow noreferrer&quot;&gt;Neptune&lt;/a&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.77°&lt;/td&gt;&#xA;&lt;td&gt;6.43°&lt;/td&gt;&#xA;&lt;td&gt;0.72°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/tbody&gt;&#xA;&lt;/table&gt;&#xA;&lt;/div&gt;&#xA;&lt;p&gt;Since all of our destinations in the solar system (and even in the galaxy) are inclined from the ecliptic plane, travel to those destinations involves plane transfer (changing the inclination of the space craft relative to the ecliptic).&lt;/p&gt;&#xA;&lt;p&gt;The greater the change in inclination, the greater delta v required.&lt;/p&gt;&#xA;&lt;p&gt;To change inclination to perpendicular to the invariable plane would require &amp;quot;an enormous delta v&amp;quot;, as PearsonArtPhoto said in his answer. Just how much would this be? Well, for a simple plane change (where the orbit size stays the same), &lt;span class=&quot;math-container&quot;&gt;$\Delta V = 2V_i * sin (\theta / 2)$&lt;/span&gt;. &lt;span class=&quot;math-container&quot;&gt;$V_i$&lt;/span&gt; is the initial velocity, and &lt;span class=&quot;math-container&quot;&gt;$\theta$&lt;/span&gt; is the angle change. That's nearly 1.4 times the initial velocity &lt;strong&gt;increase in velocity&lt;/strong&gt; to transfer from the ecliptic plane to a plane perpendicular to the invariable plane. And that's assuming you don't have to change the size of your orbit (which you will).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Invariable_plane&quot; rel=&quot;nofollow noreferrer&quot;&gt;Invariable plane - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.braeunig.us/space/orbmech.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Basics of Space Flight: Orbital Mechanics - Rocket &amp;amp; Space Technology&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2020-12-17T17:15:01.760" LastActivityDate="2020-12-17T17:15:01.760" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1865" PostTypeId="1" CreationDate="2013-09-09T19:14:28.610" Score="6" ViewCount="396" Body="&lt;p&gt;Suppose there are two asteroids, one very large (larger than a football field) and one very small (roughly the size of a refrigerator).  Assume they have similar compositions and are at equal distances from my current location.  My hypothetical goal is to begin collecting these smaller, &quot;refrigerator-sized&quot; chunks of material -- by any means -- and use them for some purpose that is unrelated to the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would I be better off going to the large asteroid and setting up a mining operation, or locating and collecting the asteroid that is already the size I'm interested in?  What if instead of a single chunk, I began identifying, locating, and retrieving smaller asteroids that are already near the size I am interested in?  Would the effort I am putting out depend on the asteroid's composition, and would the hypothetical mining operation cost less effort over time as compared to hunting down the appropriate chunks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some more specific parameters:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;For the first part of the question, assume I've identified the first two asteroids in different parts of the asteroid belt, but they're approximately the same distance from my location. Let's call it 1 AU.&lt;/li&gt;&#xA;&lt;li&gt;I would like to return any contents to an arbitrary point in open space, not under great influence of any nearby bodies.  The vehicle carrying the payload should be capable of either coming to a stop at this location, or slowing enough for another, larger device to capture it.  I'm not so much concerned about the details of this procedure for the question, more the actual retrieval of the material instead of the delivery.&lt;/li&gt;&#xA;&lt;li&gt;Primary candidates for retrieval would be &lt;a href=&quot;http://en.wikipedia.org/wiki/M-type_asteroid&quot; rel=&quot;noreferrer&quot;&gt;M-type asteroids&lt;/a&gt;.  They are desirable for identification and retrieval due to their moderate brightness and metal content.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="62" LastEditorUserId="62" LastEditDate="2013-09-09T19:53:07.167" LastActivityDate="2014-04-06T07:12:36.890" Title="Over time, would I exert less effort mining a large asteroid or collecting small asteroids, assuming both have similar compositions?" Tags="|asteroid|mining|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1866" PostTypeId="1" AcceptedAnswerId="1876" CreationDate="2013-09-09T20:15:34.603" Score="8" ViewCount="564" Body="&lt;p&gt;Besides the US, we focus a lot on Russia and China's space programs. India also gets a little attention. Brazil, Russia, India, and China form the BRIC group which is widely presumed to surpass G7 economically. As such, the BRIC nations are considered to be roughly on the same level in economy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of space, Brazil does have a growing space program that has been pretty successful so far. They've made several launches, as well as contributions to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What plans does AEB (the Brazilian Space Agency) have for missions beyond GEO?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-09-16T13:53:03.253" LastActivityDate="2021-05-10T14:15:20.690" Title="What plans does AEB (the Brazilian Space Agency) have for missions beyond GEO?" Tags="|interplanetary|future-missions|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1867" PostTypeId="2" ParentId="1861" CreationDate="2013-09-09T21:04:33.010" Score="11" Body="&lt;p&gt;The optimal form is the same as in atmosphere: long and slender, and pointed at the end, for the acceleration phase, and wide, flat and perpendicular to the path for the decelleration phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is because space is not a true absolute vacuum, but has at least a few atoms per cubic meter. This induces a small, but measurable, drag. The closer one is to a body, the thicker the &quot;interplanetary medium&quot;; the closer to a star system, the thicker the interstellar medium. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Densities¹&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Atmosphere at 1 Bar is roughly 3e19 molecules per cc.² Slightly more than double that to get density in atoms, and note that free subatomic particles are rare. further, in terms of mass, these roughy 6.1e19 atmos per cc are heavier, nearly 10 times more mass per atom on average.⁴&lt;/li&gt;&#xA;&lt;li&gt;The interplanetary medium is estimated to be about 5 particles per cc, but can hit peaks of 100 particles per cc.³ Those particles are mostly atomic nuclei, but some are subatomic particles. Note that Protium (¹H) is both an atomic nucleus and also a free proton, and Deuterium (²H) is a  proton neutron pair;  Hydrogen is the bulk (75%) of the IPM, mixed between Protium, and Deuterium nuclei, in various ionization states.&lt;/li&gt;&#xA;&lt;li&gt;The interstellar medium is about 1 atom per cc², again mostly hydrogen&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;¹: cc = cubic centimeter&lt;br&gt;&#xA;²: &lt;a href=&quot;http://www-ssg.sr.unh.edu/ism/what1.html&quot;&gt;http://www-ssg.sr.unh.edu/ism/what1.html&lt;/a&gt;&lt;br&gt;&#xA;³: &lt;a href=&quot;http://nineplanets.org/medium.html&quot;&gt;http://nineplanets.org/medium.html&lt;/a&gt;&lt;br&gt;&#xA;⁴: Air atomic mass per atom (0.78 * 14)+ (0.21 * 16) = 14.3 is a good approximation&lt;br&gt;&#xA;&lt;code&gt;&lt;/code&gt; ISM atomic mass per particle (0.75 * 1) + (0.25 * 4) = 1.5 is a workable approximation, and accounts for some heavier atoms scattered about.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2013-09-09T21:04:33.010" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1868" PostTypeId="2" ParentId="1865" CreationDate="2013-09-09T21:24:18.213" Score="5" Body="&lt;p&gt;This is more complicated than just how far away, or the size of the asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the material is on the surface and easily harvest-able then the smaller asteroids are going to be more efficient to mine.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if you need to extract large amounts of ore to get any usable amount of material, then a larger asteroid where you can set up something to mine and at least pre-process if not fully process the ore is going to be more efficient.  On small asteroids you would need to move the asteroid to someplace where it can be harvested for the ore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first asteroids that are likely to be targeted to set up to mine will be larger and have multiple mineral targets.  Since ever operation is going to require a certain base effort you will be able to leverage that base infrastructure to build out for multiple mining targets.  Smaller targets will require some medium to mine the ore, load the ore into carriers, and transport the ore to some place to be refined.  &lt;/p&gt;&#xA;" OwnerUserId="123" LastActivityDate="2013-09-09T21:24:18.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1869" PostTypeId="5" CreationDate="2013-09-10T03:36:07.547" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T03:36:07.547" LastActivityDate="2013-09-10T03:36:07.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1870" PostTypeId="4" CreationDate="2013-09-10T03:36:07.547" Score="0" Body="Any atmosphere with particle density low enough to not cause excited particles in gas form and/or electromagnetically charged particles to frequently collide and behave like gases in denser atmospheres, where heavier gas molecules displace the lighter ones and form substantial enough static pressure to cause it to behave like a low viscosity fluid. Exospheres can be permanent on celestial bodies, periodic on comets, and sporadic on other small bodies." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T03:36:07.547" LastActivityDate="2013-09-10T03:36:07.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1871" PostTypeId="5" CreationDate="2013-09-10T03:46:22.893" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T03:46:22.893" LastActivityDate="2013-09-10T03:46:22.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1872" PostTypeId="4" CreationDate="2013-09-10T03:46:22.893" Score="0" Body="Clean room environment of production, assembly and research facilities, with strict regime over airborne impurities or pollutants that could be introduced to worked on materials, equipment or experiments and cause permanent damage or long-term degrade their performance. These environments are fully sealed, airtight rooms with controlled access and air conditioning and filtering systems in place, with use related acceptable levels of contamination." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T03:46:22.893" LastActivityDate="2013-09-10T03:46:22.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1873" PostTypeId="5" CreationDate="2013-09-10T03:57:26.117" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T03:57:26.117" LastActivityDate="2013-09-10T03:57:26.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1874" PostTypeId="4" CreationDate="2013-09-10T03:57:26.117" Score="0" Body="Rosetta is a European Space Agency (ESA) built and launched, semi-autonomous robotic spacecraft on its mission to study the environment, physical properties and dynamics of the comet 67P/Churyumov–Gerasimenko as it passes close to the Sun and forms its coma and tails. It is a large orbiter with 12 onboard instruments, and a robotic lander Philae designed to touch down and grip to the comet's surface to study its nucleus. See also the [philae] tag." OwnerUserId="49" LastEditorUserId="415" LastEditDate="2014-11-15T17:40:26.100" LastActivityDate="2014-11-15T17:40:26.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1875" PostTypeId="1" AcceptedAnswerId="1877" CreationDate="2013-09-10T14:36:02.017" Score="4" ViewCount="320" Body="&lt;p&gt;Would launching space ships at the top of high places (like buildings or mountains) would have significant effect? What would be the height where launching would be less energy-/fuel-consuming than launching from the sea-level?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it even possible to create a building that is tall enough and even able to keep the weight of the space ship, the necessary rocket units (full of fuel) and the other necessary stuff?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="306" LastEditDate="2013-09-10T15:54:30.003" LastActivityDate="2013-09-10T15:54:30.003" Title="Launch from the top of high places" Tags="|launch|" AnswerCount="1" CommentCount="4" ClosedDate="2013-09-10T20:31:06.923" ContentLicense="CC BY-SA 3.0" />
  <row Id="1876" PostTypeId="2" ParentId="1866" CreationDate="2013-09-10T15:25:03.030" Score="7" Body="&lt;p&gt;Brazil is currently planning its first deep space mission to the triple asteroid 2001 SN263. The mission, named ASTER, is planned to launch in &lt;a href=&quot;https://www.sciencedirect.com/science/article/pii/S0273117720307729&quot; rel=&quot;nofollow noreferrer&quot;&gt;June 2022 or June 2025&lt;/a&gt;. The triple asteroid system will come within 150 million miles of Earth in 2019, w̶h̶e̶n̶ ̶t̶h̶e̶ ̶A̶S̶T̶E̶R̶ ̶m̶i̶s̶s̶i̶o̶n̶ ̶p̶r̶o̶b̶e̶ ̶s̶h̶o̶u̶l̶d̶ ̶a̶r̶r̶i̶v̶e̶ ̶a̶t̶ ̶t̶h̶e̶ ̶a̶s̶t̶e̶r̶o̶i̶d̶. ASTER would arrive later--if launched in 2022, it would arrive in 2025. The probe will orbit and then land on the main body of the triple asteroid to learn more about &amp;quot;the origins of the Solar system, asteroid-Earth collision protection mechanisms and the origin of life on Earth.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;The mission is currently considering the following instruments for exploring the target:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Imaging Camera&lt;/li&gt;&#xA;&lt;li&gt;Laser Rangefinder&lt;/li&gt;&#xA;&lt;li&gt;Infrared Spectrometer&lt;/li&gt;&#xA;&lt;li&gt;Synthetic Aperture Radar&lt;/li&gt;&#xA;&lt;li&gt;Mass Spectrometer&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The asteroid is a C-type asteroid which is difficult to study from Earth, but may hold clues to the formation of planets and life on Earth.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.iepc2013.org/get?id=318&quot; rel=&quot;nofollow noreferrer&quot;&gt;Development of a Pulsed Plasma Thruster for the ASTER Mission - INPE&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://adsabs.harvard.edu/abs/2012LPICo1667.6080W&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA ADS: ASTER: A Brazilian Mission to an Asteroid - The Smithsonian/NASA Astrophysics Data System&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://arxiv.org/pdf/1503.07546v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stable retrograde orbits around the triple system 2001 SN263 - Araujo, Winter, and Prado&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2021-05-10T14:15:20.690" LastActivityDate="2021-05-10T14:15:20.690" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="1877" PostTypeId="2" ParentId="1875" CreationDate="2013-09-10T15:49:49.343" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Would launching space ships at the top of high building would have significant effect?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Not any standing building or tower.  The highest of these come close to 1 km, so I'll use that.  There is a certain &quot;delta V&quot; needed to increase altitude by rockets that amount.  There are other factors, notably drag and gravity drag which I'll ignore for now.  The delta v from the elevation change itself is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \sqrt{ g h } = \sqrt{ 9.8 \frac{m}{s^2} 1,000 m } \approx 100 \frac{m}{s} = 0.1 \frac{km}{s} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The often quoted value for the delta v to get to space is around $10 \frac{km}{s}$.  So even with the tallest building, it would only save you on the order of 1% of your budget.  This is underselling the factor somewhat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The glaring flaw is that you could just go to higher altitude.  There are nice flat planes in plenty of areas of the world over 3 km high.  A tall building is obviously not a good alternative.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some people do talk about a &lt;a href=&quot;http://autogeny.org/tower/tower.html&quot; rel=&quot;nofollow&quot;&gt;space pier&lt;/a&gt;, for instance.  But these are built on technology that we've never come close to implementing and would require heights many times beyond what has been accomplished in building technology to date.  I've read some papers that talk about the engineering required for the supports.  They would necessarily entail active control.  Super-tall masts have a history of breaking due to a combination of wind forces and structural resonances.  In order to keep going higher, you first have to understand these very well, then you need to go into novel technology that will dampen modes of flexing and basically keep those members straight over long distances.  These could be gyroscopic, but there are several approaches that could be used (even airfoils).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These super-tall tower approaches (if ever reasonable) would have to compete with other approaches of high-altitude balloons, or possibly even rotating space tethers (a space elevator alternative).  I'm still asking around about high-altitude ionic levitation, but only because I can't yet figure out why it won't work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also consider the scaling factors involved.  High launch points are much more advantageous for smaller rockets.  With air drag, there is a scaling benefit.  The larger your rocket, the less air drag subtracts from your delta v budget.  A higher altitude of launch will make smaller rockets viable, which would not have been viable at sea level.  That's nice, considering that your structural limits will restrict the launch mass anyway.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-09-10T15:49:49.343" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1878" PostTypeId="2" ParentId="1865" CreationDate="2013-09-10T17:59:24.597" Score="7" Body="&lt;p&gt;Your best bet is mining the smallest &lt;strong&gt;single&lt;/strong&gt; rock that provides as much material as you need.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose you mine a bunch of little rocks--you have to expend Δv to move between each rock--energy that you don't expend when you're mining a single rock.  On the other hand the bigger the rock the more Δv you will expend lifting your cargo against it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the means by which you lift your cargo matters here, you very well might be better off with a rock big enough to mount some sort of throwing system.  Throwing your stuff home from Ceres is going to take a lot less work than bringing it home by rocket from your football field.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2013-09-10T17:59:24.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1879" PostTypeId="2" ParentId="1857" CreationDate="2013-09-10T19:50:38.530" Score="9" Body="&lt;p&gt;Lagrangian points would work for any system where the third object is &quot;small&quot; relative to the other two bodies. Since, in your question, the first two bodies are the galactic center and the Solar System, even a fairly large object, such as an M class red dwarf star could be used as the third body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the nature of this specific case could create two problem situations:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Unlike the area immediately around the Earth, the area around the Sun has lots of stuff in it: planets, asteroids, comets and the Oort Cloud. The placement for the third body could be close enough to Sol that it could be pelted by material from, say, the Oort Cloud.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Again, unlike the Solar System, where there are a moderate number of large objects (planets), the Milky Way is home to something like 100 billion stars and the area around Sol is relatively clear only in absolute terms, not in relative terms and this condition is not guaranteed to continue indefinitely. The placement for the third body could be far enough away from Sol that another passing star could yank it out of the Lagrangian point (in a few million years, say).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The exact circumstances would depend on the size of the object to be used. Do the math and see where it would have to be to satisfy the conditions of the Lagrangian point.&lt;/p&gt;&#xA;" OwnerUserId="113" LastEditorUserId="113" LastEditDate="2013-09-11T16:58:38.973" LastActivityDate="2013-09-11T16:58:38.973" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1880" PostTypeId="5" CreationDate="2013-09-10T20:53:32.217" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T20:53:32.217" LastActivityDate="2013-09-10T20:53:32.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1881" PostTypeId="4" CreationDate="2013-09-10T20:53:32.217" Score="0" Body="Launch of a space faring vehicles from high-altitude places, such as mountain ranges and high plains, advantages and disadvantages of high-altitude over sea-level launch sites." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T20:53:32.217" LastActivityDate="2013-09-10T20:53:32.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1882" PostTypeId="5" CreationDate="2013-09-10T21:04:48.480" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T21:04:48.480" LastActivityDate="2013-09-10T21:04:48.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1883" PostTypeId="4" CreationDate="2013-09-10T21:04:48.480" Score="0" Body="Lagrangian points (also Lagrange points, L-points, or libration points) are the five positions (L1 - L5) surrounding two celestial bodies where gravitational pull of the two large mass bodies provides the centripetal force required to orbit them. Such points are usually nominally unstable but somewhat periodic around celestial systems with stable orbits." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-10T21:04:48.480" LastActivityDate="2013-09-10T21:04:48.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1884" PostTypeId="1" AcceptedAnswerId="1895" CreationDate="2013-09-10T22:04:08.553" Score="11" ViewCount="414" Body="&lt;p&gt;Space isn't empty - there are tons of things floating around in it. Seem of these things are pockets/molecule of gas - and a space probe is very likely to encounter some of these at some point in its lifespan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there high enough concentrations of gasses to justify &quot;streamlining&quot; spacecraft? Are there any examples?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-10T23:15:29.303" LastActivityDate="2013-09-13T01:57:33.783" Title="Are there enough stray gasses in space to justify streamlining a probe?" Tags="|probe|deep-space|gasses|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1885" PostTypeId="5" CreationDate="2013-09-10T23:11:00.130" Score="0" Body="&lt;p&gt;Question regarding inflatable spheres filled with material lighter than the surrounding material used to lift objects, and their uses in space exploration.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="49" LastEditDate="2016-01-13T22:38:56.937" LastActivityDate="2016-01-13T22:38:56.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1886" PostTypeId="4" CreationDate="2013-09-10T23:11:00.130" Score="0" Body="Question regarding inflatable spheres filled with material lighter than the surrounding material used to lift objects, and their uses in space exploration." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-10T23:34:33.833" LastActivityDate="2013-09-10T23:34:33.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1887" PostTypeId="5" CreationDate="2013-09-11T00:16:28.243" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-11T00:16:28.243" LastActivityDate="2013-09-11T00:16:28.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1888" PostTypeId="4" CreationDate="2013-09-11T00:16:28.243" Score="0" Body="Questions regarding Chinese politics, people, and space programs and their relation to space exploration." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-11T00:32:29.883" LastActivityDate="2013-09-11T00:32:29.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1889" PostTypeId="5" CreationDate="2013-09-11T00:17:53.127" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-11T00:17:53.127" LastActivityDate="2013-09-11T00:17:53.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1890" PostTypeId="4" CreationDate="2013-09-11T00:17:53.127" Score="0" Body="Questions regarding small, clustered pieces of dirt/rock associated with the surface of a body, and its effect on space exploration. Use [tag:space-dust] for particulates in space." OwnerDisplayName="user12" LastEditorUserId="12102" LastEditDate="2024-02-03T01:16:06.233" LastActivityDate="2024-02-03T01:16:06.233" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="1891" PostTypeId="5" CreationDate="2013-09-11T00:19:50.663" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-11T00:19:50.663" LastActivityDate="2013-09-11T00:19:50.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1892" PostTypeId="4" CreationDate="2013-09-11T00:19:50.663" Score="0" Body="Questions regarding ongoing repairs made to spacecraft and satellites to keep them in working condition." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-11T00:32:01.947" LastActivityDate="2013-09-11T00:32:01.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1893" PostTypeId="5" CreationDate="2013-09-11T00:22:24.313" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-11T00:22:24.313" LastActivityDate="2013-09-11T00:22:24.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1894" PostTypeId="4" CreationDate="2013-09-11T00:22:24.313" Score="0" Body="Questions regarding systems used to continually locate objects, often radar." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-11T00:32:23.370" LastActivityDate="2013-09-11T00:32:23.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1895" PostTypeId="2" ParentId="1884" CreationDate="2013-09-11T01:32:51.373" Score="8" Body="&lt;p&gt;Let's just assume the interstellar density of about 10^6 particles/m^3. Let's also assume they are hydrogen. Plugging those numbers in to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Drag_equation&quot;&gt;drag formula&lt;/a&gt; $F_D\, =\, \tfrac12\, \rho\, v^2\, C_D\, A$, and let's assume that 1mN is the first force that is significant enough to merit correcting for. Putting all of that together, assuming a hideous coefficient of drag of 2, and an area of 100 m^2 (For solar panels), the appropriate velocity becomes 2,496,000, or almost 1% of light speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, Aerodynamics only makes a difference if you are going really fast, much faster than anything we've sent so far.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-11T01:32:51.373" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1896" PostTypeId="1" AcceptedAnswerId="1901" CreationDate="2013-09-11T04:37:57.700" Score="8" ViewCount="3087" Body="&lt;p&gt;Just what the title states please. This &lt;a href=&quot;http://web.nmsu.edu/~xiumin/project/satellite_orbit_design/&quot; rel=&quot;nofollow&quot;&gt;site&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Orbit epoch is the time at which the established orbital elements are true&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The site goes on to mention sub-elements - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Start Day The satellite's start date that we are interested. &lt;/li&gt;&#xA;&lt;li&gt;Start Month The satellite's start month that we are interested. &lt;/li&gt;&#xA;&lt;li&gt;Start Year The satellite's start year that we are interested. &lt;/li&gt;&#xA;&lt;li&gt;Step Size The interval between calculated output points. &lt;/li&gt;&#xA;&lt;li&gt;Propagate Duration The satellite's time period that we are interested. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'm less than nouveau at this stuff - my programming background tells me a suitable analogy for 'orbital epoch' may be 'timestamp'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this interpretation of the term correct?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-09-12T12:17:10.837" LastActivityDate="2013-09-12T12:17:10.837" Title="Is an orbital epoch merely a timestamp?" Tags="|orbital-mechanics|terminology|orbital-elements|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1897" PostTypeId="1" AcceptedAnswerId="1899" CreationDate="2013-09-11T05:49:57.650" Score="12" ViewCount="2363" Body="&lt;p&gt;&lt;a href=&quot;http://www.en.wikipedia.org/wiki/Hubble_Space_Telescope&quot;&gt;Wikipedia&lt;/a&gt; states that &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The telescope had to be kept&#xA;  in a clean room, powered up&#xA;  and purged with nitrogen, until&#xA;  a launch could be rescheduled.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Spacecraft are always kept in a clean room environment. My question is, why should it be kept &#xA;purged with nitrogen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would keeping it in a vacuum chamber instead of a clean room be a good idea?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-12T12:17:14.583" LastActivityDate="2016-01-09T18:25:53.113" Title="Why was the Hubble Space Telescope purged with nitrogen gas?" Tags="|telescope|spacecraft|engineering|clean-room|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1898" PostTypeId="2" ParentId="1896" CreationDate="2013-09-11T05:50:57.853" Score="4" Body="&lt;p&gt;Somewhat. The term &lt;em&gt;epoch&lt;/em&gt; denotes distinctive time at which an era of some significance or distinction has started, so while they are &lt;em&gt;some&lt;/em&gt; timestamps relative to an arbitrarily chosen and fixed date of the first epoch, they are &lt;em&gt;not just any&lt;/em&gt; timestamps (they have more weight), and in our case also &lt;em&gt;not merely&lt;/em&gt; timestamps (they also include two other data), so perhaps a better replacement term could be &lt;em&gt;a milestone&lt;/em&gt;, or even &lt;em&gt;a stage&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem of using &lt;em&gt;timestamp&lt;/em&gt; as a replacement term is that there might be several reference times defined, even for a single project, event, or as is in our case - a mission. For example, you could be using recorded times by the probe, and translate those to a local time. Both would then be &lt;em&gt;timestamps&lt;/em&gt;, only their reference time would be different. They might also not mark any time of greater significance for your project, or indeed say anything about whether it's a start, or a beginning of something. &lt;em&gt;Timestamp&lt;/em&gt; is also just that, a recording of date and time in a predefined format, relative to a predefined reference time frame, and the term doesn't denote the inclusion of some other descriptive parameters. &lt;em&gt;Timestamps&lt;/em&gt; might also be a lot more frequent than &lt;em&gt;epochal&lt;/em&gt; events.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And what you called &lt;em&gt;sub-elements&lt;/em&gt; of the &lt;em&gt;orbital epoch&lt;/em&gt; in your question, those merely define some arbitrary format to record each &lt;em&gt;epoch&lt;/em&gt;, and are specific to their use. The point of a static format being, that you can easily retrieve any individual pieces of the information it contains at a later date. You could easily define your own format, and a timestamp would be a part of it, just like the first three parameters in our example. But the last two parameters are possibly already outside the definition of &lt;em&gt;a timestamp&lt;/em&gt;, depending on your convention, especially if you'll be using &lt;em&gt;timestamps&lt;/em&gt; to also record other events, not just those of greater significance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to avoid confusion, I'd suggest using a different substitution term, or even simply &lt;em&gt;an epoch&lt;/em&gt;, now that we've hopefully described what it means in a way that's easy enough to understand.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-11T05:56:25.567" LastActivityDate="2013-09-11T05:56:25.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1899" PostTypeId="2" ParentId="1897" CreationDate="2013-09-11T06:32:14.857" Score="17" Body="&lt;p&gt;Nitrogen is a relatively inert diatomic (N&lt;sub&gt;2&lt;/sub&gt;) gas, but also importantly, while there are other gases that refuse to react with much anything even more so, like e.g. Helium, Nitrogen is cheap since it's often a byproduct of industrial processes. It is also frequently  readily available at scientific facilities in its liquid form for being an extremely effective coolant that doesn't damage electronic equipment or leave colored stains on optical equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since it's relatively light (atomic number 7 on the periodic table of elements) it will also expand pretty well in all directions and displace any impurities that might still linger suspended in air. It is also non-toxic, non-corrosive, non-hygroscopic, colorless, odorless, tasteless, and will displace Oxygen (O&lt;sub&gt;2&lt;/sub&gt;) and greatly reduce fire hazard and oxidation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I.e. it is the industry standard technique for the replacement of a hazardous or undesirable atmosphere with an inert dry atmosphere by displacement or dilution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case of the Hubble telescope however, the reason is also described in the same &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope#Spacecraft_systems&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; a bit later on:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A shroud of multi-layer insulation keeps the temperature within the&#xA;  telescope stable, and surrounds a light aluminum shell in which the&#xA;  telescope and instruments sit. Within the shell, a graphite-epoxy&#xA;  frame keeps the working parts of the telescope firmly aligned.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Because graphite composites are hygroscopic, there was a risk that&#xA;  water vapor absorbed by the truss while in Lockheed's clean room would&#xA;  later be expressed in the vacuum of space; the telescope's instruments&#xA;  would be covered in ice. To reduce that risk, a nitrogen gas purge was&#xA;  performed before launching the telescope into space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This should also answer your last question regarding storing the Hubble Space Telescope in a vacuum chamber instead. While a vacuum chamber could prevent in-air impurities by simply &lt;em&gt;sucking&lt;/em&gt; all the atmosphere out of it, that process wouldn't clean it of possible water molecules gathering on and around its hygroscopic multi-layer insulation, if they started condensing. It is also pretty hard to find a vacuum chamber large enough to hold the whole telescope, and a malfunction of its pump could result in a sudden gush of air, damaging the telescope and its shroud.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast to a vacuum chamber, using Nitrogen only results in a slightly changed atmospheric pressure when the clean room environment it was stored in would be accessed by the staff working on it. They also wouldn't need expensive and cumbersome pressure suits, and any Oxygen supplied Hazardous Environment (HEV) suits would do just fine for such tasks.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="4660" LastEditDate="2016-01-09T18:25:53.113" LastActivityDate="2016-01-09T18:25:53.113" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1900" PostTypeId="1" AcceptedAnswerId="1979" CreationDate="2013-09-11T07:57:06.923" Score="9" ViewCount="303" Body="&lt;p&gt;Having read a few articles on the suggested new uses for the crippled &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler space observatory&lt;/a&gt; with only two out of four functioning reaction wheels, several ideas suggest that Kepler’s pointing problems would be minimized, if it aimed its camera within the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic&quot; rel=&quot;nofollow noreferrer&quot;&gt;ecliptic&lt;/a&gt; plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/BUGoh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;a href=&quot;http://astrobites.org/2013/09/09/whats-next-for-kepler/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;What’s Next For Kepler?&lt;/strong&gt;&lt;/a&gt; is an &lt;a href=&quot;http://astrobites.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astrobites&lt;/a&gt; article listing most interesting proposals to repurpose the Kepler telescope.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the ideas advocating Kepler's use for the target objects within the ecliptic is this proposal for &lt;a href=&quot;http://keplerscience.arc.nasa.gov/docs/WhitePapers/Beichman_Kepler.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Survey of the Ecliptic Plane For Transiting Planets and Star Formation&lt;/a&gt; (PDF), that mentions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This White Paper calls for the use of Kepler to conduct a survey in the&#xA;  ecliptic plane to search for planet transits around stars at high&#xA;  galactic latitudes and to study star forming regions to investigate&#xA;  physics of very young stars not studied by Kepler in its prime mission.&#xA;  Recent analysis by the Kepler project indicates that the spacecraft's&#xA;  best pointing will be possible in the ecliptic plane.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The proposal seems otherwise sound enough. Searching for &lt;a href=&quot;https://en.wikipedia.org/wiki/Transit_%28astronomy%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;planet transits&lt;/a&gt; was within the Kepler's tasks anyway, with its original goal to determine the frequency of Earth-like planets around Sun-like stars and its wide-angle view, albeit monitoring them for years, instead of what would now be possible perhaps for weeks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I couldn't find any other reference as to why Kepler would be easier to control when pointing towards targets within the ecliptic plane. This seems to me rather non intuitive. What are the reasons behind this claim? Have any tests been performed on the current Kepler's attitude control capabilities that would officially support this theory?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorDisplayName="user12" LastEditDate="2013-09-15T14:40:09.820" LastActivityDate="2013-09-15T19:12:35.510" Title="Why would Kepler’s pointing problems be minimized if aimed within the ecliptic?" Tags="|telescope|attitude|reaction-wheel|kepler-telescope|ecliptic-plane|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1901" PostTypeId="2" ParentId="1896" CreationDate="2013-09-11T10:13:18.840" Score="4" Body="&lt;p&gt;The Epoch in the common Two Line Elements (TLEs) is in fact a record of when the TLEs are valid. While some of them only change very slightly with time most of the time (Period, eccentricity, perigee and apogee), at least one of the parameters changes frequently with time (Argument of Perigee). In this case, you have to know when they are valid from, in order to determine their work.&lt;/p&gt;&#xA;&lt;p&gt;If you take a TLE file, and change only the Epoch, you will change the location of the orbit, and possibly the plane of the orbit. The period, etc will remain the same, however.&lt;/p&gt;&#xA;&lt;p&gt;Quoting from the TLE definition page &lt;a href=&quot;https://celestrak.org/columns/v04n03/&quot; rel=&quot;nofollow noreferrer&quot;&gt;on CelesTrak&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The epoch defines the time to which all of the time-varying fields in&#xA;the element set are referenced.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2013-09-11T13:12:03.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1902" PostTypeId="1" AcceptedAnswerId="1903" CreationDate="2013-09-11T11:01:26.120" Score="12" ViewCount="29401" Body="&lt;p&gt;Neil Armstrong was an Eagle Scout and the first person to walk on the moon.  I have heard that everyone who has walked on the moon (through 2012), was an Eagle Scout.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it true? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so other then attributes of birth (country of origin, etc), are there any other accomplishment they all share?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-09-12T12:16:04.233" LastActivityDate="2015-12-18T04:37:38.273" Title="Do only Eagles walk on the moon?" Tags="|the-moon|astronauts|apollo-program|myth-busting|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1903" PostTypeId="2" ParentId="1902" CreationDate="2013-09-11T11:39:53.970" Score="17" Body="&lt;p&gt;No, they were not all &lt;a href=&quot;https://en.wikipedia.org/wiki/Eagle_Scout_%28Boy_Scouts_of_America%29&quot;&gt;Eagle Scouts&lt;/a&gt;. While at least forty astronauts earned the rank as a youth Eagle scouts, in fact only two &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot;&gt;Apollo&lt;/a&gt; moonwalkers did: Neil Armstrong and Charles Duke.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But most were Boy Scouts, with only James Irwin not being a scout at all. The ranks earned by the 12 moonwalkers (sorted by rank, then by Apollo mission):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Eagle Scouts&lt;/strong&gt;: Neil Armstrong (Apollo 11) and Charles Duke (Apollo 16)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Life Scouts&lt;/strong&gt;: Edgar Mitchell (Apollo 14) and David Scott (Apollo 15)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;First Class Scouts&lt;/strong&gt;: Alan Bean (Apollo 12) and Alan Shepard (Apollo 14)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Second Class Scouts&lt;/strong&gt;: John Young (Apollo 16) and Eugene Cernan (Apollo 17)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Tenderfoot Scouts&lt;/strong&gt;: Edwin &quot;Buzz&quot; Aldrin (Apollo 11) and Harrison Schmitt (Apollo 17)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Cub Scouts&lt;/strong&gt;: Charles &quot;Pete&quot; Conrad (Apollo 12)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Not A Scout&lt;/strong&gt;: James Irwin (Apollo 15)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/khTOw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Boy Scout rank badges&lt;/strong&gt;: Scout, Tenderfoot, Second Class, First Class, Star, Life, Eagle (image source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Ranks_in_the_Boy_Scouts_of_America&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Eagle_Scout_%28Boy_Scouts_of_America%29&quot;&gt;Eagle Scout (Boy Scouts of America)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ranks_in_the_Boy_Scouts_of_America&quot;&gt;Ranks in the Boy Scouts of America&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Additional notes&lt;/strong&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.baldeagleinfo.com/eagle/eagle9.html&quot;&gt;The Bald Eagle&lt;/a&gt; is however the USA's National Emblem and often depicted on badges, pins, buttons, insignia, and other emblems, so chances are they all wore on their person &lt;em&gt;&quot;The Eagle&quot;&lt;/em&gt; in one form or another while walking on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the first &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot;&gt;Lunar Module&lt;/a&gt; (LM) to ever have landed on the lunar surface during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11&quot;&gt;Apollo 11&lt;/a&gt; mission, with Neil Armstrong and Edwin &quot;Buzz&quot; Aldrin onboard being the first and the second moonwalkers, has been named Eagle. Perhaps the most memorable mention of the module's name is described in Wikipedia as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Armstrong acknowledged Aldrin's completion of the post landing&#xA;  checklist with &quot;Engine arm is off,&quot; before responding to Duke with the&#xA;  words, &quot;Houston, Tranquility Base here. The Eagle has landed.&quot;&#xA;  Armstrong's unrehearsed[24] change of call sign from &quot;Eagle&quot; to&#xA;  &quot;Tranquility Base&quot; emphasized to listeners that landing was complete&#xA;  and successful. Duke mispronounced his reply as he expressed the&#xA;  relief at Mission Control: &quot;Roger, Twan-- Tranquility, we copy you on&#xA;  the ground. You got a bunch of guys about to turn blue. We're&#xA;  breathing again. Thanks a lot.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Additionally, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11#Insignia&quot;&gt;the Apollo 11 insignia&lt;/a&gt;, designed by Michael Collins, the Command Module (CM) pilot of that mission, also depicted an eagle with an olive branch in its talons:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/oPLfq.jpg&quot; alt=&quot;Apollo 11 insignia&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in the strictest sense of the title of your question alone, it might have as well been that so far, only Eagles have walked on the Moon.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-11T12:01:56.033" LastActivityDate="2013-09-11T12:01:56.033" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1904" PostTypeId="1" AcceptedAnswerId="1919" CreationDate="2013-09-11T11:49:48.697" Score="39" ViewCount="57889" Body="&lt;p&gt;I would like to build some orbital mechanical software from scratch. I feel that this would be a great way to learn the steps required to calculate different Kepler orbital elements of an object, plot orbits, and predict where the object will be at some future time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, I want to start with calculating the Keplarian elements. The inputs I would give the program would be the position and velocity vectors, along with a time. These input vectors will be relative to the center of the Earth, so I may also need to do a coordinate transfer if I want to use a specific location on the surface as a reference point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have seen the math for calculating Kepler orbital elements from &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0486600610&quot;&gt;this book&lt;/a&gt;, and I know lots of software has been developed throughout the years to calculate them, but I am having a hard time bridging the two. The math in the book is slightly confusing, and I think it would be easier for me to understand if I saw the steps &quot;written out&quot; in a programming language.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2013-09-11T15:56:47.677" LastActivityDate="2020-08-26T05:57:17.733" Title="How to programmatically calculate orbital elements using position/velocity vectors?" Tags="|orbital-mechanics|mathematics|orbital-elements|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1905" PostTypeId="2" ParentId="1904" CreationDate="2013-09-11T15:34:20.280" Score="6" Body="&lt;p&gt;The first key to figuring this out is getting your coordinate system correct. There are two commonly used coordinate systems for such things. They are the Earth Centered Earth Fixed &lt;a href=&quot;https://en.wikipedia.org/wiki/ECEF&quot; rel=&quot;noreferrer&quot;&gt;(ECEF)&lt;/a&gt;, and Earth Centered Interial &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth_Centered_Inertial&quot; rel=&quot;noreferrer&quot;&gt;(ECI)&lt;/a&gt; frames. At midnight, these two line up exactly, but they diverge in other times, based on the rotation of the Earth. ECEF works best for things on the Earth (If you are not moving, you should have 0 velocity. ECEF takes this into account, ECI velocity will have you move with the rotation of the Earth), ECI works best for things in Orbit (Orbiting objects don't care about the rotation of the Earth, at least, the physics doesn't care). Make sure the coordinate systems are correct!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so you have a position and velocity in ECI coordinates, what do you do? There is an &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a289281.pdf&quot; rel=&quot;noreferrer&quot;&gt;excellent paper&lt;/a&gt; that describes the entire process, which I will copy the end formulas here. There is also a few good sources &lt;a href=&quot;http://sce.uhcl.edu/helm/SpaceNavHandbook/Chapter4.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;http://adsabs.harvard.edu/full/2003ala..conf...85B&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;http://adsabs.harvard.edu/full/1991SvA....35..428K&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. I highly recommend reading them carefully. Uncertainty is much more difficult, so let's just assume you have a perfect knowledge of velocity and position. Specifically, the 6 classical Keplarian Elements are eccentricity (e), inclination (i), right ascension of the ascending node ($\Omega$), argument of perigee ($\omega$), semi-major axis (a), and time of perigee passage ($T_O$). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should mention that I am primarily following the &lt;a href=&quot;http://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=4&amp;amp;cad=rja&amp;amp;ved=0CDwQFjAD&amp;amp;url=http://www.dtic.mil/dtic/tr/fulltext/u2/a126174.pdf&amp;amp;ei=eYYwUpuXLJHi9gSLgIGYBw&amp;amp;usg=AFQjCNHejs5P5iLM2Fctq3Bn6eW2gj3fzA&amp;amp;bvm=bv.51773540,d.eWU&quot; rel=&quot;noreferrer&quot;&gt;Laplace Method of Orbital Determination&lt;/a&gt;, there is a competing methodology known as the Gauss Method. But finally this came down to deciphering &lt;a href=&quot;http://www.mathworks.com/matlabcentral/fileexchange/31333-orbital-elements-from-positionvelocity-vectors/content/vec2orbElem.m&quot; rel=&quot;noreferrer&quot;&gt;Matlab code&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Semi-Major Axis&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$W_s = \frac{1}{2}*v^2s - \text{mus}./r;$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$a = -mus/2./W_s$; %semi-major axis&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Eccentricity&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;L = [rs(2,:).*vs(3,:) - rs(3,:).*vs(2,:);...&#xA;     rs(3,:).*vs(1,:) - rs(1,:).*vs(3,:);...&#xA;     rs(1,:).*vs(2,:) - rs(2,:).*vs(1,:)]; %angular momentum&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;$p = \sum{L^2}./mus;$ %semi-latus rectum&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$e = \sqrt{1 - p/a}; %eccentricity$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Inclination&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$I = atan(\frac{\sqrt{L(1,:)^2 + L(2,:)^2}}{L(3,:)});$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Arguments of Pericenter&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\omega = atan2(\frac{(vs(1,:).*L(2,:) - vs(2,:).*L(1,:))./mus - rs(3,:)./r)./(e.*sin(I))}{((\sqrt{L2s}.*vs(3,:))./mus - (L(1,:).*rs(2,:) - L(2,:).*rs(1,:))./(\sqrt{L2s}.*r))./(e.*sin(I)))}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Longitude of Ascending Node&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Omega = atan2(-L(2,:),L(1,:));$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Time of Perigee Passing:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$T_0 = -(E - e.*sin(E))./\sqrt{mus.*a.^-3}$&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-11T15:34:20.280" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1906" PostTypeId="1" AcceptedAnswerId="1907" CreationDate="2013-09-11T16:23:22.047" Score="10" ViewCount="230" Body="&lt;p&gt;The number of asteroids that have been &lt;a href=&quot;http://apod.nasa.gov/apod/ap100726.html&quot; rel=&quot;noreferrer&quot;&gt;directly photographed&lt;/a&gt; is very limited, because the probes that do so are very large budget and public.  That's why it's surprising to see images like this of &lt;a href=&quot;https://en.wikipedia.org/wiki/3554_Amun&quot; rel=&quot;noreferrer&quot;&gt;Amun&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/65evL.jpg&quot; alt=&quot;image from that site&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please correct me if I'm wrong (because this is important), but I believe the &lt;em&gt;original&lt;/em&gt; source for this is The Celestia Motherlode:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.celestiamotherlode.net/catalog/show_addon_details.php?addon_id=732&quot; rel=&quot;noreferrer&quot;&gt;http://www.celestiamotherlode.net/catalog/show_addon_details.php?addon_id=732&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photography of this object is not possible with any existing telescope.  The 3D model has a great deal of intentionality to it, so I don't think it is just made up, epically after looking at the rest of their library.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This claims to be the product of the &lt;a href=&quot;http://www.shatters.net/celestia/&quot; rel=&quot;noreferrer&quot;&gt;Celestia simulator&lt;/a&gt;.  But &lt;em&gt;what&lt;/em&gt; does this do?  Is there radio astronomy data that goes into such renderings?  What are these illustrations showing?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2018-09-05T09:40:39.923" Title="What data are used to do these 3D asteroid renderings?" Tags="|asteroid|imaging|radio-telescope|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1907" PostTypeId="2" ParentId="1906" CreationDate="2013-09-11T20:18:36.630" Score="6" Body="&lt;p&gt;The way these images are usually done are by imaging the object for an extended period of time, and seeing how they are different. This can be done either with radar or passive systems like visible and infrared light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is &lt;a href=&quot;https://web.archive.org/web/20151002030441/http://home.comcast.net/~eliws/ceres/files/LightCurveModelsOf20Asteroids.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;a paper&lt;/a&gt; detailing how it is done, but basically they incrementally rotate the object in simulation around and seeing what matches, and what doesn't. Improvements are then attempted to make it match better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This probably doesn't account for the texture, which is probably artificial, but it could be based off of radar returns from the area, which will show things like the texture.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-09-05T09:40:39.923" LastActivityDate="2018-09-05T09:40:39.923" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="1908" PostTypeId="1" AcceptedAnswerId="1918" CreationDate="2013-09-12T02:24:46.243" Score="8" ViewCount="9198" Body="&lt;p&gt;Since a black hole only has as much gravity as it did before it collapsed, it's perfectly safe to orbit it. Do any countries have plans to send a probe to orbit a black hole? What is the timeframe?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2016-05-02T20:46:39.703" Title="Are there any plans to send a probe to orbit a black hole?" Tags="|probe|black-hole|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1909" PostTypeId="1" AcceptedAnswerId="1914" CreationDate="2013-09-12T02:26:47.317" Score="12" ViewCount="4165" Body="&lt;p&gt;It's perfectly safe to orbit a black hole - as long as you don't cross the event horizon, you're fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions is this: is there anything we could learn by doing so? Since nothing can escape a black hole's event horizon, it seems pointless to me. How am I wrong?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2017-07-05T17:11:46.977" Title="Would there be any benefit to sending a probe to a black hole?" Tags="|probe|black-hole|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1910" PostTypeId="2" ParentId="1908" CreationDate="2013-09-12T06:46:38.510" Score="9" Body="&lt;p&gt;&lt;strong&gt;No&lt;/strong&gt; so far there is no plan to send probe to orbit black hole but we are more interested in finding the presence of black hole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the  reason for not sending probe to black hole is that they are not close to earth &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.en.wikipedia.org/wiki/V4641_Sagittarii&quot;&gt;Wikipedia&lt;/a&gt; article states as&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;V4641 Sagittarii ( V4641 Sgr ) is a&#xA;  variable X-ray binary star system in&#xA;  the constellation Sagittarius ......&#xA;  At the time, it was considered to be&#xA;  the closest known black hole to Earth,&#xA;  at a distance of approximately 1,600&#xA;  light-years (490 pc) &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So with it is approximately 15141 trillion kilometres from earth and with the speed of &lt;a href=&quot;http://www.en.wikipedia.org/wiki/Voyager_1&quot;&gt;voyager 1&lt;/a&gt; (17 Km/s). It will take 890.64706 trillion seconds (30 million years ) to get there . We don't have propulsion system to take a probe to black hole ( only way is with the help of gravity assist and warp drive in case it is developed successfully)&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-09-12T06:46:38.510" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1911" PostTypeId="1" AcceptedAnswerId="1920" CreationDate="2013-09-12T08:04:39.553" Score="27" ViewCount="16213" Body="&lt;p&gt;Is there any stable orbit around a black hole so that the spacecraft  will remain in orbit without any disturbance over a long period of time ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="58" LastEditDate="2013-09-12T21:06:13.627" LastActivityDate="2019-05-28T18:10:49.277" Title="Is there any stable orbit around a black hole?" Tags="|orbit|probe|black-hole|" AnswerCount="5" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="1912" PostTypeId="2" ParentId="1909" CreationDate="2013-09-12T10:40:35.413" Score="-2" Body="&lt;p&gt;It is theorized that &lt;a href=&quot;https://en.wikipedia.org/wiki/Hawking_radiation&quot; rel=&quot;nofollow&quot;&gt;Hawking radiation&lt;/a&gt; is emitted from black holes.  In fact they are believed to &lt;a href=&quot;http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/010703a.html&quot; rel=&quot;nofollow&quot;&gt;eventually evaporate&lt;/a&gt;.  So hypothetically you could send a signal using Hawking radiation, that could be received outside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your more pressing problem &lt;em&gt;(word play:)&lt;/em&gt; would be creating a probe that could survive the gravitational and tidal forces of the black hole.&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-09-12T10:40:35.413" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1913" PostTypeId="1" AcceptedAnswerId="1915" CreationDate="2013-09-12T10:53:21.223" Score="7" ViewCount="389" Body="&lt;p&gt;I don't think there is any reasonable doubt that comets have been ejected from ours and other solar systems and are drifting in the vast voids of space.  If there where larger bodies say the size of the Earth or Jupiter drifting in space would be be able to identify their presence?  If so how close to our sun would they need to be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question assumes that the bodies are not in orbit around a sun as part of a solar system.  Rather they are drifting between solar systems or galaxies, either in orbit around the galactic center, or other wise behaving as galactic or universal asteroids.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-09-12T11:04:09.700" LastActivityDate="2013-09-12T11:35:04.637" Title="If there are Earth size bodies drifting in space could we identify them?" Tags="|asteroid|deep-space|planet|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1914" PostTypeId="2" ParentId="1909" CreationDate="2013-09-12T10:56:29.683" Score="14" Body="&lt;p&gt;Various things:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Hawking Radiation&lt;/strong&gt;: It is theorized that black holes emit a kind of radiation known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Hawking_radiation&quot; rel=&quot;noreferrer&quot;&gt;Hawking Radiation&lt;/a&gt;. We could measure it with the probe, validating the theory.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Testing Relativity&lt;/strong&gt;: General Relativity makes many predictions about space close to a black hole. Accurately testing these predictions will tell us how accurate the theory is (and may also give insight to new physics). Some of the measurable things include:&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_force&quot; rel=&quot;noreferrer&quot;&gt;Tidal forces&lt;/a&gt;: When near a gravitating object, there are forces which stretch an object in one direction and squeeze it in another. These forces are much stronger near a black hole.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lense-Thirring_effect&quot; rel=&quot;noreferrer&quot;&gt;Gravitomagnetism/Frame Dragging/Lense-Thirring effect&lt;/a&gt;: Rotating gravitational bodies have a tendency to &quot;swirl&quot; space around them, giving rise to &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitomagnetism&quot; rel=&quot;noreferrer&quot;&gt;gravitomagnetism&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Electromagnetic properties of a black hole: &lt;a href=&quot;https://en.wikipedia.org/wiki/Reissner-Nordstr%C3%B6m_black_hole&quot; rel=&quot;noreferrer&quot;&gt;Reissner–Nordström&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Kerr-Newman_black_hole&quot; rel=&quot;noreferrer&quot;&gt;Kerr-Newman&lt;/a&gt; black holes are charged, and the surface charges have some remarkable and complex properties that mirror the behavior of normal charges on a conductor.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Gravitational_time_dilation&quot; rel=&quot;noreferrer&quot;&gt;Gravitational time dilation&lt;/a&gt;: Time behaves strangely near gravitating objects, and extremely strangely near black holes, with large relative differences in the &quot;rate&quot; at which time passes.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Figuring out how quasars work&lt;/strong&gt;: We yet are not very clear on the functioning of &lt;a href=&quot;https://en.wikipedia.org/wiki/Quasar&quot; rel=&quot;noreferrer&quot;&gt;quasars&lt;/a&gt;. One popular candidate theory is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Blandford%E2%80%93Znajek_process&quot; rel=&quot;noreferrer&quot;&gt;Blandford-Znajek process&lt;/a&gt;, which gives an explanation to how gas jets can arise from rotating black holes with accretion disks, giving rise to quasars. Analyzing the behavior of smaller black holes with accretion disks may help clarify the behavior of quasars for us.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Exploratory mission&lt;/strong&gt;: What if we wanted to build a &lt;a href=&quot;https://physics.stackexchange.com/q/20813/7433&quot;&gt;black hole power plant&lt;/a&gt; and attached colony around it?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="96" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2013-09-12T12:15:56.113" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1915" PostTypeId="2" ParentId="1913" CreationDate="2013-09-12T11:35:04.637" Score="8" Body="&lt;p&gt;Yes, it is possible to detect &lt;a href=&quot;https://en.wikipedia.org/wiki/Rogue_planet&quot;&gt;rogue planets&lt;/a&gt; by observing distant stars and measuring the  &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_microlensing&quot;&gt;microlensing&lt;/a&gt; effect when a big enough object passes in front of them and increases their apparent brightness. If the rogue planet passes one of the observed stars, this microlensing effect will be a lot shorter than when any of the planets orbiting the star would pass in front of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far, we have identified several candidate rogue planets, with the &lt;a href=&quot;https://en.wikipedia.org/wiki/CFBDSIR_2149-0403&quot;&gt;CFBDSIR 2149-0403&lt;/a&gt; being the closest to our Solar system, roughly 100 light years away. There are however huge  differences in estimates of how many are out there, these being somewhere between 2 to a 100,000 free-floating planets per each star in the galaxy. It should be noted though, that not all of these rogue planets are thought to have been ejected from the systems they formed in, but many might have been formed on their own, possibly in similar way to how stars are formed, and then orbit the galaxy independent of any other systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for how far they have to be for us to be able to observe their presence, it's rather hard to give an estimate, because the most suitable telescopes for such tasks probably wouldn't be the ones with the highest radial resolution, but powerful yet wider angle telescopes that could monitor many thousands of stars at the same time, like for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_%28spacecraft%29&quot;&gt;Kepler&lt;/a&gt; space observatory. Kepler's main mission was looking for habitable-zone Earth-like planets around distant star systems in our galaxy, but now that only two out of four reaction wheels are working, it might even end up being more useful for the search of rogue planets, sweeping the skies more frequently than it otherwise would have, if it could target more precisely. However, its future is yet to be determined, so let's not jump the gun on this one.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-12T11:35:04.637" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1916" PostTypeId="2" ParentId="1911" CreationDate="2013-09-12T11:54:30.337" Score="14" Body="&lt;p&gt;Well yes, and the best proof I can think of are the stars orbiting our galaxy's galactic center black hole, which is a supermassive black hole in the center of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Milky_Way&quot; rel=&quot;noreferrer&quot;&gt;Milky Way&lt;/a&gt; in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sagittarius_A*&quot; rel=&quot;noreferrer&quot;&gt;Sagittarius A*&lt;/a&gt; region:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7px0C.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&lt;sup&gt;Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way galactic centre. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Galactic_centre_orbits.svg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could also be argued, that we all orbit around this black hole anyway, since the whole galaxy orbits around its barycenter, where this supermassive black hole in the galaxy nucleus is located.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-12T11:54:30.337" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1917" PostTypeId="2" ParentId="1911" CreationDate="2013-09-12T12:29:22.763" Score="14" Body="&lt;p&gt;Yes, there are, to a reasonable approximation&lt;sup&gt;1&lt;/sup&gt;. For a nonspinning black hole, there are exactly 4 things that can happen (assuming that you haven't thrown the object directly at the black hole):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The object will be going too fast, and it will just continue to infinity with a slight deflection in path.&lt;/li&gt;&#xA;&lt;li&gt;The object will be going too slow, and it will spiral into the center&lt;/li&gt;&#xA;&lt;li&gt;The object goes at a perfect speed at an angle perpendicular to the position vector, giving you a circular orbit&lt;/li&gt;&#xA;&lt;li&gt;The object goes at a (different) perfect speed at an angle not perpendicular to the position vector, giving a &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28orbit%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;rosetta orbit&lt;/a&gt;:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2298e.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is similar to an elliptic orbit, except that the major/minor axes themselves rotate. Mercury also prominently displays this kind of orbit (nowhere near as much as one orbiting a black hole, though), as it exhibits a prominent precession of the perihelion  &amp;mdash; the ellipse itself seems to orbit around the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of a spinning black hole, things become more complicated as a spiraling object may actually &lt;em&gt;reverse&lt;/em&gt; the direction of rotation due to frame dragging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1. See &lt;a href=&quot;https://space.stackexchange.com/a/1948/96&quot;&gt;Mark Adler's answer&lt;/a&gt;. Similar to orbits arising from the electromagnetic force, two orbiting bodies emit gravitational waves, which leads to an eventual loss of energy and inspiral. However, this process is very slow except near the end.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="96" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-09-14T03:41:55.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1918" PostTypeId="2" ParentId="1908" CreationDate="2013-09-12T12:49:04.240" Score="21" Body="&lt;p&gt;No, for these reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;We're not yet entirely sure of any black holes being black holes. If you want to send a very expensive probe on a mission to inspect a black hole, you need to first be sure that it indeed is a black hole.&lt;/li&gt;&#xA;&lt;li&gt;No black hole candidates are close to us. It &lt;a href=&quot;https://i.stack.imgur.com/F4JO8.gif&quot;&gt;would take very, very long to get results&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;We don't have near-light-speed propulsion technology. Voyager 1 is a pithy 125 AU away from us. Propulsion technology has improved since then, but not the the extent that we can send something at near light speed. To reach such speeds one needs to accelerate, and for that the probe needs to carry lots of fuel, which in turn makes it harder to accelerate.&lt;/li&gt;&#xA;&lt;li&gt;There's no way to tell the probe what to do once it's gone. We have a 4 minute lag for the Mars rovers, which isn't too bad because we can still respond to new data. This can't be done for a probe that's going so far away. If there are any problems, it's sunk. This means that will have to set it on a very precise path and then just pray we aimed it right. This brings me to my next point:&lt;/li&gt;&#xA;&lt;li&gt;We don't have the technology to aim so well. We could include an onboard computer that is able to dynamically correct the path, but this requires extra fuel on the ship for thrusters. If the probe is going at near light speed, it will take a &lt;em&gt;lot&lt;/em&gt; of fuel to deflect its path.&lt;/li&gt;&#xA;&lt;li&gt;The cost. Getting to the technological levels listed above probably would cost many, many times more than the entire world economy put together.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;With all these hurdles, we don't even know where to start on making such a probe, (and if we did, we would have to wait a long time to make it and a lot more time to get results). In light of that, the cost is very hard to justify.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's far better to focus on things that are within our reach (the Moon, and Mars for manned missions, other planets and possibly our outer solar system for probes), and use these goals to improve out technology. When we are reasonably close to having the required technology, we can work on a black hole probe.&lt;/p&gt;&#xA;" OwnerUserId="96" LastEditorUserId="238" LastEditDate="2015-08-27T14:16:25.163" LastActivityDate="2015-08-27T14:16:25.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1919" PostTypeId="2" ParentId="1904" CreationDate="2013-09-12T14:02:43.737" Score="46" Body="&lt;p&gt;Given Earth-centered, inertial (ECI) position and velocity vectors $\vec{r}$ and $\vec{v}$, you can directly solve for the classical orbital elements $(a,e,i,\Omega,\omega,\nu)$ as follows (algorithms first, followed by pseudocode at the bottom):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, solve for the angular momentum $$\vec{h}=\vec{r} \times \vec{v}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;then the node vector $$\hat{n}=\hat{K} \times \vec{h}$$ which will be used later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The eccentricity vector is then $$\vec{e} = \frac{(v^2-\mu /r)\vec{r}-(\vec{r} \cdot \vec{v})\vec{v}}{\mu}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and $e=|\vec{e}|$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specific mechanical energy is $$E=\frac{v^2}{2}-\frac{\mu}{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If $e\neq 1$, then $$a = -\frac{\mu}{2E}$$&#xA;$$p=a(1-e^2)$$&#xA;Otherwise, $$p=\frac{h^2}{\mu}$$ $$a=\infty$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, $$i=\cos^{-1}{\frac{h_K}{h}}$$&#xA;$$\Omega=\cos^{-1}{\frac{n_I}{n}}$$&#xA;$$\omega=\cos^{-1}{\frac{\vec{n}\cdot\vec{e}}{ne}}$$&#xA;$$\nu=\cos^{-1}{\frac{\vec{e}\cdot\vec{r}}{er}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And you'll need to make the following checks:&#xA;If $n_J&amp;lt;0$, then $\Omega=360^{\circ}-\Omega$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If $e_K&amp;lt;0$, then $\omega=360^{\circ}-\omega$, and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If $\vec{r}\cdot\vec{v}&amp;lt;0$, then $\nu=360^{\circ}-\nu$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that you'll run into problems (singularities) for certain cases: circular orbits ($e\approx 0$), and equatorial orbits ($i\approx 0$), particularly. In these cases you normally introduce a new, less troublesome variable, like mean longitude or true longitude of perigee.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;h=cross(r,v)&#xA;nhat=cross([0 0 1],h)&#xA;&#xA;evec = ((mag(v)^2-mu/mag(r))*r-dot(r,v)*v)/mu&#xA;e = mag(evec)&#xA;&#xA;energy = mag(v)^2/2-mu/mag(r)&#xA;&#xA;if abs(e-1.0)&amp;gt;eps&#xA;   a = -mu/(2*energy)&#xA;   p = a*(1-e^2)&#xA;else&#xA;   p = mag(h)^2/mu&#xA;   a = inf&#xA;&#xA;i = acos(h(3)/mag(h))&#xA;&#xA;Omega = acos(n(1)/mag(n))&#xA;&#xA;if n(2)&amp;lt;0&#xA;   Omega = 360-Omega&#xA;&#xA;argp = acos(dot(n,evec)/(mag(n)*e))&#xA;&#xA;if e(3)&amp;lt;0&#xA;   argp = 360-argp&#xA;&#xA;nu = acos(dot(evec,r)/(e*mag(r))&#xA;&#xA;if dot(r,v)&amp;lt;0&#xA;   nu = 360 - nu&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note&lt;/strong&gt;: this follows from the method laid out in &lt;em&gt;Fundamentals of Astrodynamics and Applications&lt;/em&gt;, by Vallado, 2007.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-09-12T14:02:43.737" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="1920" PostTypeId="2" ParentId="1911" CreationDate="2013-09-12T14:10:43.593" Score="28" Body="&lt;p&gt;The answer is &quot;yes&quot;, and there are a surprising number of ways to argue this.  You will probably want to look at the question &lt;a href=&quot;https://physics.stackexchange.com/questions/46332/what-happens-to-orbits-at-small-radii-in-general-relativity/46527#46527&quot;&gt;about small orbits around black holes&lt;/a&gt; in Physics Stack Exchange.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;At sufficient distances, black holes are not special&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Black holes behave the same as any other spherical collection of matter.  This is a conclusion of the (intimidating) equations of general relativity (GR), but it's not a surprising result.  Newtonian gravity (&quot;classical&quot; gravity) states that any spherically symmetric collection of matter will behave the same.  This means that the &lt;em&gt;radial&lt;/em&gt; distribution of the sun's mass doesn't affect its tug on the Earth.  The vast majority of the sun's mass is contained below 1/4th of its visible radius.  But if it were uniform, it would behave exactly the same!  This is a very strong statement, and general relativity follows suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, we only &lt;em&gt;need&lt;/em&gt; general relativity when a combination of parameters puts the system in the regime of highly relativistic effects.  For gravity regarding a point mass orbiting a large mass, this is often dictated by the gravitational potential, which you should be familiar with as $\frac{G M }{r}$.  When this value starts to get close to $\frac{c^2}{2}$, then you have to worry about these strange relativistic distortions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The story of Mercury's precession is a common anecdote about GR, but this only deals with the accumulation of small change over a long period of time.  Specifically 43 arc-seconds over a century, which is something like 1/100th of a degree... in 100 years.  However, since the solar system has been around for billions of years, this can still have an impact on orbital stability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's a very minute correction to our conclusions.  Otherwise, it generally stands to say that whatever orbits are stable for any large body are also stable for a black hole.  Actually, a black hole will be even &lt;em&gt;more&lt;/em&gt; stable.  All the planets are stars have a complex gravitational tapestry due to density changes due to composition changes.  If you collapse any of these bodies to a black hole, they'll have to expel their angular momentum.  At long distances from a black hole, the gravitational field will be exceptionally constant, and this leads to greater orbital stability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we're talking about Sagittarius A* and things like that, we're generally still in this regime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Truly relativistic orbits are weird&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you get close enough such that the potential is close to the relativistic limits, the dynamics change.  You can't make a blanket statement that &quot;all orbits are stable/unstable&quot;.  Things you can say:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;obviously anything that passes within the event horizon is gone&lt;/li&gt;&#xA;&lt;li&gt;all orbits that pass within the &quot;IBCO&quot; are dead unless &quot;rockets&quot; are used&lt;/li&gt;&#xA;&lt;li&gt;unless you're beyond the &quot;ISCO&quot;, your orbit will be highly elliptical and precess&lt;/li&gt;&#xA;&lt;li&gt;If your orbit precession hits an angle that divides $2 \pi$ evenly, then it's &quot;stable&quot;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;My apologies for the jargon here.  It's fairly hard to say any of this concisely without the specific terminology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me just establish that IBCO (Innermost Bound Circular Orbit, which is 1.5 times the event horizon radius) is basically the line of death.  If you cross this point, you can only escape if you have &quot;powerful rockets&quot;.  I use this terminology because the physicists use it, but it's really a lie.  No rocket would be powerful enough to bring you back out unless you were &lt;em&gt;right&lt;/em&gt; next to the IBCO.  Nonetheless, there are other ways to get an exception - rotating black holes, for instance.  Without these, venturing beyond the IBCO &lt;em&gt;will&lt;/em&gt; plunge you to the event horizon.  But now, the crazy thing about the IBCO is that you can dance right on the edge of it for basically an unlimited amount of time, and hop right back out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, my points #3 &amp;amp; 4 are basically that orbits that come close to the IBCO have &quot;zoom-whirl&quot; behavior.  This can be avoided fully, but only beyond the &quot;ISCO&quot; (Innermost Stable Circular Orbit).  Beyond that point, you can orbit in a circle and it will be stability until infinity.  If you don't meet this criteria, the orbit moves over time like Mercury.  But the shift over every revolution can be &lt;em&gt;any angle&lt;/em&gt;.  That means that you can spin around the IBCO a few times, and then come back and retrace your previous path to the ISCO.  You can repeat this exact path for infinity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This demonstrates the types of stable orbits you can obtain, but real life will have a more complicated collection of parameters.  Most black holes are rotating, and they apparently rotate at a large fraction of their theoretical maximum.  Plus, there's other material around it.  But both of these open up avenues to get energy from it.  These complications will likely prevent &quot;stable&quot; orbits, but since they're non-conservative, you can sort-of &quot;surf&quot; the environment around the black hole until you deplete all its usable energy (hint: that's a lot).&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2014-06-03T21:06:19.653" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="1921" PostTypeId="2" ParentId="1884" CreationDate="2013-09-12T15:00:37.183" Score="4" Body="&lt;p&gt;Short answer is NO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Within interstellar space or within our local solar system, aerodynamics isn't going to make much of a difference. And from @PearsonArtPhoto's calculations, at the speed at which they DO make a difference, the particles are going to ram into the probe with enough energy to erode the outer-surface. Gases and particles in interstellar space are often ionized (i.e. charged) and some atoms are very hot. It is akin to driving a car at 100mph through a dust-cloud - your windshield is going to be poc-marked and if one drove long enough, it might crack and fail. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With that in mind, you'd want to make the cross-section of a ship small to deal with erosion more than aerodynamics. Probably made of thick and durable material, and ideally shielded somehow - possibly with a magnetic field to help push particles away from the ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have a look at: &lt;a href=&quot;http://en.wikipedia.org/wiki/Interstellar_medium&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Interstellar_medium&lt;/a&gt; for a bit more detail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a real-world example, Voyager 1 - traveling at a brisk 17 Kilometers per SECOND away from the sun isn't very aerodynamic at all. Big clunky right angles, a giant radio dish on the back, etc. &lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="692" LastEditDate="2013-09-13T01:57:33.783" LastActivityDate="2013-09-13T01:57:33.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1922" PostTypeId="2" ParentId="1766" CreationDate="2013-09-12T15:17:37.323" Score="3" Body="&lt;p&gt;The simplest answer is &lt;strong&gt;Complex Chemistry&lt;/strong&gt;. You and I can be thought of as bags of very complicated self-sustaining chemical reactions.&#xA;To that end, we need:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Lots of different chemicals: a ball of hydrogen gas isn't going to&#xA;do much on it's own. &lt;/li&gt;&#xA;&lt;li&gt;Density: chemical reactions happen faster when the reactants are densely packed. Liquids work great for this, but so might a gas-giant. A planet with solid, unchanging rock &lt;strong&gt;probably&lt;/strong&gt; wouldn't do much. &lt;/li&gt;&#xA;&lt;li&gt;Outside source of energy: Energy-input would allow more interesting chemical-reactions to take place. More specifically, sustainable reactions to occur.  &lt;/li&gt;&#xA;&lt;li&gt;Stability, but not too much stability: If a planet goes from 1000 K to 20K each day, it's going to be hard for life to form. Conversely, if a system is totally stable, there isn't going to be enough 'mixing' to get metabolism going.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The search for habitable planets, and for possible extra-terrestrial life-bearing planets is one fraught with HUGE amounts of guesswork. We have no real clue what alien life might look like, so we focus on looking for stuff similar to our own earth. So, things like:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Habitable Zone - the planet needs to be in the correct distance to maintain the right temperatures for liquid water to be present&lt;/li&gt;&#xA;&lt;li&gt;Magnetic Field - Something to shield life from intense radiation&lt;/li&gt;&#xA;&lt;li&gt;Presence of water - Again, we think life originated in water, so searches for life tend to focus on this.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Here's some fun food-for-thought:&#xA;The elements needed for carbon-based life: Carbon, oxygen, nitrogen, hydrogen - are all going to be very common on high-metallicity star-systems. They are readily formed by stellar neuclsynthesis, and -should- be present on most population I and II stars (have a read-through of : &lt;a href=&quot;http://en.wikipedia.org/wiki/Metallicity&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Metallicity&lt;/a&gt;) &lt;/p&gt;&#xA;" OwnerUserId="692" LastActivityDate="2013-09-12T15:17:37.323" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1923" PostTypeId="2" ParentId="1766" CreationDate="2013-09-12T15:43:47.020" Score="2" Body="&lt;p&gt;Generally, you would want to look at &lt;a href=&quot;http://en.wikipedia.org/wiki/Metallicity#Population_I_stars&quot; rel=&quot;nofollow&quot;&gt;Population I stars&lt;/a&gt;. Also, the nature of most &lt;a href=&quot;http://en.wikipedia.org/wiki/Variable_star&quot; rel=&quot;nofollow&quot;&gt;variable stars&lt;/a&gt; would tend to interfere with evolution on their planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, the star would have to be old enough for life to have evolved. For main sequence stars, &lt;a href=&quot;http://astrojourney.files.wordpress.com/2009/09/star_table.gif&quot; rel=&quot;nofollow&quot;&gt;smaller stars have longer lifetimes&lt;/a&gt;. As it took approximately 1 billion years &lt;a href=&quot;http://en.wikipedia.org/wiki/Timeline_of_evolutionary_history_of_life&quot; rel=&quot;nofollow&quot;&gt;for even basic single celled organisms to evolve&lt;/a&gt;, nothing larger than an F class star would be a candidate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible that larger stars, having a larger habitable zone, might have a higher chance of having a suitable planet, but this is pure conjecture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also some debate about the feasibility of life evolving on tidally-locked planets. This may or may not eliminate the smaller M class stars whose habitable zone would be so close to the sun as to cause any planets in that zone to be tidally locked.&lt;/p&gt;&#xA;" OwnerUserId="113" LastActivityDate="2013-09-12T15:43:47.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1924" PostTypeId="1" CreationDate="2013-09-12T15:53:41.833" Score="4" ViewCount="1269" Body="&lt;p&gt;Instead of placing an X-ray telescope in orbit or deep space, would building an X-ray telescope on the surface of the Moon an good idea, since it has no atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If no, please don't forget to mention the reasons.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-12T17:41:41.543" LastActivityDate="2013-09-12T21:35:22.780" Title="Would placing an X-ray telescope on the surface of the Moon be a good idea?" Tags="|the-moon|telescope|orbit|deep-space|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1925" PostTypeId="2" ParentId="1862" CreationDate="2013-09-12T16:14:20.967" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://www.spacenews.com/article/opinion/37019sn-blog-report-of-possible-rd-180-ban-could-mean-many-things-including-nothing&quot;&gt;This&lt;/a&gt; blog post does a pretty good job of answering this question. I'll pick out a couple of the most relevant points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the report itself should be taken with a grain of salt. It appeared on RT.com (which has a history of publishing stories with inflammatory headlines), and cites a story from a Russian news paper which quotes an unnamed source as saying the ban is &quot;under consideration&quot;. Hardly convincing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, the inventory of existing engines is enough to keep the Atlas program going for &quot;several years&quot;, according to ULA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, if necessary, production of the engines could (theoretically) be done in the U.S. The article notes that this was one of the conditions of allowing the use of a Russian engine in the EELV program. The article also notes, however, that there have long been doubts about whether Russia would actually hand over the blueprints.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-09-12T16:23:38.397" LastActivityDate="2013-09-12T16:23:38.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1926" PostTypeId="1" AcceptedAnswerId="1927" CreationDate="2013-09-12T16:37:49.613" Score="10" ViewCount="146" Body="&lt;p&gt;A station situated somewhere like EML-2 is really at an unstable point.  I've heard of crafts using rocket firing to maintain that position in response to small deviations.  Could you do this without using propellant?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder if this could be done by moving a long pole back and fourth, like a tightrope walker.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-09-12T17:29:31.353" Title="Are there propellant-less ways to balance at an unstable orbital point?" Tags="|lagrangian-points|station-keeping|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1927" PostTypeId="2" ParentId="1926" CreationDate="2013-09-12T17:29:31.353" Score="5" Body="&lt;p&gt;No, unless you have a rather massive and long pole. To be in a stable orbit, your center of mass&lt;sup&gt;1&lt;/sup&gt; must be at a particular point with a particular velocity. Rearranging your own mass without expelling any will not work as you cannot move your own center of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, with a long enough pole, the inverse square effects come into play, so one can change the net gravitational attraction by moving a pole back and forth, and thus regain the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1. The mass distribution will change your rotational properties, but not the revolutionary ones which depend on the position and velocity of the COM, assuming a near-homogeneous gravity field..&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="96" LastActivityDate="2013-09-12T17:29:31.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1928" PostTypeId="2" ParentId="1924" CreationDate="2013-09-12T17:35:05.230" Score="8" Body="&lt;p&gt;Compared to what we already have, for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandra_X-ray_Observatory&quot;&gt;Chandra X-ray Observatory&lt;/a&gt; that already has the orbital path one third as long as the distance from the Earth to the Moon (apogee 133,000 km and perigee 16,000 km, or 83,000 and 9,900 miles respectively), I don't think there's enough incentive to put a similar observatory on the surface of the Moon. There are a few advantages, but as you can probably guess, they don't outweigh all the disadvantages. At least not for the time being. Let's see how they stack up, assuming a Moon based observatory versus an orbited one:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Earth Interference&lt;/strong&gt;: In favor of the Moon based observatory, provided you build it on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Far_side_of_the_Moon&quot;&gt;far side of the Moon&lt;/a&gt;, or your subject of study is the Earth and its &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetosheath&quot;&gt;magnetosheath&lt;/a&gt; that is &lt;em&gt;visible&lt;/em&gt; in the X-ray glow caused by particles from the Sun colliding with the gas trapped within the Earth's magnetic shield, something that has actually already been proposed to the NASA for consideration before, in the form of the &lt;a href=&quot;http://gsfcir.gsfc.nasa.gov/authors/publication/13570/&quot;&gt;MagEX&lt;/a&gt; (Magnetosheath Explorer in X-rays). And since the Moon and the Earth are tidally locked, the Moon always shows the same face towards the Earth, and &lt;em&gt;the far side of the Moon&lt;/em&gt; is always facing the other way. This might prove beneficial to your sensitive equipment, and their measurements not being interfered with the Earth's own or reflected radiation. The Earth's magnetosheath emissions in the X-ray spectrum stretch way beyond the Earth's own size, so this is something orbited observatories might need to constantly adjust for, and possibly lose a great deal of resolution / vision in the process, as they &lt;em&gt;navigate&lt;/em&gt; through it. Think of it in a similar sense as areas of varying density fog in front of optical telescopes. You would be free of this interference by placing your observatory on &lt;em&gt;the far side of the Moon&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Cost factor&lt;/strong&gt;: Greatly in favor of the orbited observatory. The Moon based observatory, if placed on the &lt;em&gt;far side of the Moon&lt;/em&gt;, would require some communication relay station, possibly another satellite tasked with it and orbiting the Moon. Orbits around the Moon are not really stable, because of its &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitation_of_the_Moon&quot;&gt;gravity anomalies&lt;/a&gt;, so any long term orbits would be hard and costly to achieve. You would also need at least three, possibly more of communications satellites orbited around the Moon, if communications without interruptions are your requirement. It is also, of course, a lot more expensive to launch all the required equipment to build it first to the TLI (&lt;a href=&quot;https://en.wikipedia.org/wiki/Trans-lunar_injection&quot;&gt;Trans-Lunar Injection&lt;/a&gt;) trajectory, and later land it on the Moon, than orbit the same weight or possibly even lighter observatory in any high altitude orbits around the Earth. Your Moon based observatory will also have to be either self-assembled and anchored, or you'll need other equipment there, maybe even astronauts and/or robots, all adding to your total weight, and with it cost. If your Lunar observatory is placed on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Near_side_of_the_Moon&quot;&gt;near side of the Moon&lt;/a&gt;, then of course you don't require additional satellites orbited around it, but the total weight and your launcher requirements will still be much, much greater than those of an orbiting observatory.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Risk factor&lt;/strong&gt;: Again, greatly in favor of the orbited observatory. We have ample experience in lifting, orbiting, self-assembling, and controlling attitude of satellites in orbits around the Earth, but we don't have nearly as much of experience in doing that on the surface of the Moon. In case of a costly to ignore failure of the satellite, we could probably (currently not, but in the not-so-distant future by all means) send astronauts-technicians there to repair it a lot easier than sending them to the Moon. And since the Moon is a body with no &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetosphere&quot;&gt;magnetosphere&lt;/a&gt; or atmosphere (it has an &lt;a href=&quot;https://en.wikipedia.org/wiki/Exosphere&quot;&gt;exosphere&lt;/a&gt;, but that's irrelevant for this), it is constantly bombarded by &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind&quot;&gt;Solar wind&lt;/a&gt;, has no protection against CME (&lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot;&gt;Coronal Mass Ejections&lt;/a&gt;) and attracts asteroids with its mass that will impact on its surface, no matter how small they are. There is also plenty of dust mixed with &lt;a href=&quot;https://en.wikipedia.org/wiki/Regolith&quot;&gt;regolith&lt;/a&gt; on the Lunar surface, which tends to jump up seemingly on its own accord and completely random (there are several theories behind this phenomenon, the prevailing theories attributing it to static charge and/or Helium plasma, both due to Solar wind and CME), so there is a potential for your expensive equipment degradation due to dust particles a lot faster than with your orbited satellite observatory.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Targeted, long period observations&lt;/strong&gt;: This goes, for the most part, in favor of the orbited observatory. While you'd have to have constant attitude control in place to be able to point at a single location in space for a long time, your Lunar observatory would be completely hopeless regarding this, and rotate together with the Moon rotating on its axis. For shorter period sweeps, you'd be fine, but not so much for precise measurements over longer periods of time.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Self-sustained power&lt;/strong&gt;: This is a bit moot point and could go either way, favoring one or another, but it should be mentioned nonetheless. While the Moon is tidally locked with the Earth, it's not locked to the Sun, so it still has day/night cycles. But the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_day&quot;&gt;Lunar synodic period&lt;/a&gt; lasts on average roughly 29 days and a half Earth days. Half of this time, any stationary on the Moon's surface positioned equipment will be in complete darkness. This is not a good news, if you plan to power your observatory with solar cells, and you'll then require large and heavy batteries to store your electricity requirements for the &lt;em&gt;dark period&lt;/em&gt;. It is an easy enough task for your orbited observatory, though. It's something we have done many times before. You could of course bypass any of these drawbacks of RPG (&lt;a href=&quot;https://en.wikipedia.org/wiki/Renewable_energy&quot;&gt;Renewable Power Generation&lt;/a&gt;) and simply rely on the power provided by decaying radioisotopes in RTG (&lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;Radioisotope Thermoelectric Generator&lt;/a&gt;), but then you start dealing with the tree-hugging people or other political agendas trying to stop your endeavours, so you might spend more money on the medication required to numb yourself senseless, than on the actual mission. I said it's a &quot;moot&quot; point, didn't I?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So there we have it. There are some advantages to be had, but come with serious disadvantages (or maybe more drawbacks?) of placing an X-ray observatory on the surface of the Moon. To answer the question in the title more directly though:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Would placing an X-ray telescope on the surface of the Moon be a good idea?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are no obstacles that would explicitly prohibit us in doing so, merely challenges we have yet to learn to defy. Taking on all these challenges head-on in a single project would probably be a foolish waste of hard to come by funds for scientific research, but eventually we will get to the point when we'll look back and say, my, haven't we done well to have come thus far? But even now, when all the &lt;em&gt;pro et contra&lt;/em&gt; are weight, there might be projects that would greatly benefit by having an X-ray observatory placed on the surface of the Moon and are possibly worth taking the risk, like for example the &lt;a href=&quot;http://gsfcir.gsfc.nasa.gov/authors/publication/13570/&quot;&gt;MagEX&lt;/a&gt; one I mentioned earlier that neatly avoids all of the major hurdles by calling for its placement on the near side of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have however so far done pretty well without going that extra mile, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandra_X-ray_Observatory&quot;&gt;Chandra X-ray Observatory&lt;/a&gt; has an exemplary track record with planned lifespan of 5 years, yet it's still providing us with valuable data good 14 years after it was launched in the July 23, 1999. Granted though, completely free of interference - that it isn't.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-12T21:35:22.780" LastActivityDate="2013-09-12T21:35:22.780" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1929" PostTypeId="1" AcceptedAnswerId="1939" CreationDate="2013-09-12T18:06:19.877" Score="6" ViewCount="130" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter#Mission_progress&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once the spacecraft reached the far side of the Moon, its rocket motor was fired in order for it to be captured by the Moon's gravity into an elliptical lunar orbit.[27]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I vaguely recall reading the Apollo 11, and Apollo 13 slowed in the shadow of Luna, or just shy of the far side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Orbit_Insertion&quot; rel=&quot;nofollow&quot;&gt;LOI&lt;/a&gt; (lunar orbit insertion) usually performed on/close to the far side of Luna? Why?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user12" LastEditDate="2013-09-12T19:14:56.210" LastActivityDate="2013-09-12T22:31:31.950" Title="Is LOI usually performed on the far side of Luna?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1930" PostTypeId="1" CreationDate="2013-09-12T18:23:12.900" Score="6" ViewCount="74" Body="&lt;p&gt;Lunar missions are known to be the subject to the exceptionally disuniform gravity. &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_Recovery_and_Interior_Laboratory&quot; rel=&quot;nofollow&quot;&gt;GRAIL&lt;/a&gt; generated a very high resolution gravity map of Luna. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will any missions to Luna in the near future (say, upto 2017) use the GRAIL data?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2013-09-12T20:13:58.987" LastActivityDate="2013-09-12T21:13:30.007" Title="Will any near future lunar missions use GRAIL data?" Tags="|the-moon|gravity|future-missions|grail|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1931" PostTypeId="5" CreationDate="2013-09-12T20:04:27.417" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-12T20:04:27.417" LastActivityDate="2013-09-12T20:04:27.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1932" PostTypeId="4" CreationDate="2013-09-12T20:04:27.417" Score="0" Body="Questions regarding devices used to keep items in one place." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-12T20:06:29.480" LastActivityDate="2013-09-12T20:06:29.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1933" PostTypeId="2" ParentId="1930" CreationDate="2013-09-12T21:03:17.310" Score="2" Body="&lt;p&gt;According to &lt;a href=&quot;http://pds-geosciences.wustl.edu/dataserv/gravity_models.htm&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; site, the lunar gravity model from the GRAIL mission is set to be released next month(!!). That said, these models tend to be a fairly standard format (&quot;Spherical Harmonic ASCII Models&quot;), and typically are easy to &quot;swap out&quot; with one another. While I'm sure there is interesting science that can be done with the data, gravity field models are most often used in orbit design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, assuming the GRAIL data is as good as it is hoped to be, I would expect that the next lunar mission &lt;strong&gt;for which analysis begins after the model is released&lt;/strong&gt; would use the GRAIL model, since more precision is always a good thing from an orbit/mission design standpoint. The reason for that bolded part is that changing something as integral to the analysis as a gravity model is a bit risky, and might not be done in the middle of a development cycle.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-09-12T21:13:30.007" LastActivityDate="2013-09-12T21:13:30.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1934" PostTypeId="1" AcceptedAnswerId="1935" CreationDate="2013-09-12T21:12:03.017" Score="8" ViewCount="4712" Body="&lt;p&gt;We have a lot of man made objects flying around the universe.  Which one was the first one out of our solar system?  How far away from the sun was it when it became &quot;out of the solar system? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="12102" LastEditDate="2021-09-05T23:20:53.210" LastActivityDate="2021-09-05T23:20:53.210" Title="What was the first man made object to leave our solar system?" Tags="|probe|spaceflight-firsts|extra-solar-object|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1935" PostTypeId="2" ParentId="1934" CreationDate="2013-09-12T21:57:54.360" Score="12" Body="&lt;p&gt;The rather funny thing about it is, that we don't actually know for sure that any man-made objects have already left the Solar system. There were many speculations on where our Solar system actually ends and the interstellar space begins, free of any influence from the Sun and its orbiting celestials. This is also the reason for this &lt;a href=&quot;http://www.xkcd.com/1189/&quot; rel=&quot;nofollow noreferrer&quot;&gt;XKCD comic no. 1189 titled Voyager 1&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://www.xkcd.com/1189/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t8vD4.png&quot; alt=&quot;XKCD 1189 - Voyager 1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were many claims before, that the &lt;a href=&quot;http://voyager.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1&lt;/a&gt; has already exited the Solar system and entered the interstellar medium, and the &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2013-251&quot; rel=&quot;nofollow noreferrer&quot;&gt;latest one from NASA&lt;/a&gt;, dating at the time of writing merely a month ago, August 15, 2013, quotes NASA's Voyager project scientist, Ed Stone of the California Institute of Technology in Pasadena, that had this to say:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Details of a new model have just been published that lead the&#xA;  scientists who created the model to argue that NASA's Voyager 1&#xA;  spacecraft data can be consistent with entering interstellar space in&#xA;  2012. In describing on a fine scale how magnetic field lines from the sun and magnetic field lines from interstellar space can connect to&#xA;  each other, they conclude Voyager 1 has been detecting the&#xA;  interstellar magnetic field since July 27, 2012. Their model would&#xA;  mean that the interstellar magnetic field direction is the same as&#xA;  that which originates from our sun. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Other models envision the interstellar magnetic field draped around&#xA;  our solar bubble and predict that the direction of the interstellar&#xA;  magnetic field is different from the solar magnetic field inside. By&#xA;  that interpretation, Voyager 1 would still be inside our solar bubble.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The fine-scale magnetic connection model will become part of the&#xA;  discussion among scientists as they try to reconcile what may be&#xA;  happening on a fine scale with what happens on a larger scale. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Voyager 1 spacecraft is exploring a region no spacecraft has ever&#xA;  been to before. We will continue to look for any further developments&#xA;  over the coming months and years as Voyager explores an uncharted&#xA;  frontier.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eJvHx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;This artist's concept shows NASA's Voyager spacecraft against a field of stars in the darkness of space. The two Voyager spacecraft are traveling farther and farther away from Earth, on a journey to interstellar space, and will eventually circle around the center of the Milky Way galaxy. Image credit: NASA/JPL-Caltech &lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But assuming these latest findings are trustworthy, then it's probably relatively safe to say that Voyager 2 has either already exited the Solar system, or is just about to, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has published a convenient &lt;a href=&quot;http://voyager.jpl.nasa.gov/where/&quot; rel=&quot;nofollow noreferrer&quot;&gt;web page showing current distance from the Sun, the Earth and a few other details about the two Voyager probes&lt;/a&gt;. It has also this to say, indicating there might be a bit of a misnomer saying &lt;em&gt;&quot;out of the Solar system&quot;&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager 1 and 2 are currently in the &quot;Heliosheath&quot; -- the outermost&#xA;  layer of the heliosphere where the solar wind is slowed by the&#xA;  pressure of interstellar gas.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/d62as.jpg&quot; alt=&quot;Voyager probes on the Interstellar Mission&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The image above also gives a bit more perspective on this problem of defining our Solar system boundaries, and &lt;a href=&quot;https://space.stackexchange.com/a/1936/49&quot;&gt;@called2voyage is indeed correct in his answer&lt;/a&gt;, that this depends on how you define &lt;em&gt;&quot;out of our Solar system&quot;&lt;/em&gt;. So the proper answer to your question could perhaps be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Awaiting conclusive readings from the Voyager probes themselves&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can probably imagine, the only way to know they have left the area of certain influence of the Solar system is that they are incapable of detecting any change for a longer period of time. So this will be a waiting game, and while some would love to jump the gun with sensational news, the truth is, we simply don't know yet. We do have some indication this might have happened already, but come tomorrow, we might receive changes in readings and will have to consider that they might not have left the Solar system just yet. Again!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;This just in&lt;/strong&gt;: Quick, cue that fanfares: &lt;strong&gt;&lt;a href=&quot;http://www.space.com/22729-voyager-1-spacecraft-interstellar-space.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;It's Official! Voyager 1 Spacecraft Has Left Solar System&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seems everyone is reporting that as of September 12, 2013 (on the day of writing this answer), it is considered that Voyager 1 has officially been travelling through the interstellar space since the August 2012. For example, &lt;a href=&quot;http://www.space.com/22729-voyager-1-spacecraft-interstellar-space.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space.com reports&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Voyager has boldly gone where no probe has gone before, marking one&#xA;  of the most significant technological achievements in the annals of&#xA;  the history of science, and as it enters interstellar space, it adds a&#xA;  new chapter in human scientific dreams and endeavors,&quot; NASA science&#xA;  chief John Grunsfeld said in a statement. &quot;Perhaps some future&#xA;  deep-space explorers will catch up with Voyager, our first&#xA;  interstellar envoy, and reflect on how this intrepid spacecraft helped&#xA;  enable their future.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;http://edition.cnn.com/2013/09/12/tech/innovation/voyager-solar-system/&quot; rel=&quot;nofollow noreferrer&quot;&gt;so does CNN&lt;/a&gt;. And there are videos even! Well, it's official then. Or is it? This latest announcement sounds so strangely familiar, doesn't it? We can all add another mark to that XKCD Voyager 1 comic, though. It's now off by at least one more time the Voyager 1 has left the Solar system:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/c6j0C.png&quot; alt=&quot;XKCD 1189 updated&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-09-12T23:00:54.060" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1936" PostTypeId="2" ParentId="1934" CreationDate="2013-09-12T21:58:48.093" Score="5" Body="&lt;p&gt;Depending on how you define &quot;out of our solar system&quot;, no objects have gotten out yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pioneer 10 was the first object to cross the asteroid belt, pass Jupiter, pass the Kuiper belt, and eventually reach the heliosheath. It was launched on March 2, 1972. It, however, has not crossed the heliopause. We lost contact with Pioneer 10 on January 23, 2003 because of loss of power to its transmitter. It was 12 billion kilometers from Earth at that point, and it will eventually leave the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No man-made object has yet crossed the heliopause or escaped the Sun's gravitational influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On August 25, 2012, Voyager 1 reached interstellar space, but it is argued that it may not yet have crossed the heliopause.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Pioneer_10&quot; rel=&quot;noreferrer&quot;&gt;Pioneer 10 - Wikipedia&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;noreferrer&quot;&gt;Voyager 1 - Wikipedia&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-09-12T22:04:18.297" LastActivityDate="2013-09-12T22:04:18.297" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1938" PostTypeId="1" AcceptedAnswerId="2031" CreationDate="2013-09-12T22:22:19.783" Score="30" ViewCount="5614" Body="&lt;p&gt;This question will be controversial, so I want to start out by saying that I am not advocating any sort of conspiracy. I'm merely acknolwedging the fact that there are numerous claims that some astronauts have said they have seen UFOs or some other sign of extraterrestrial intelligence (ETI). Some of these claims are dubious, whereas others seem to have some substance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are five types of purported encounters:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;UFO sightings&lt;/li&gt;&#xA;&lt;li&gt;Extraterrestrial artifacts&lt;/li&gt;&#xA;&lt;li&gt;ETI sightings&lt;/li&gt;&#xA;&lt;li&gt;Abductions&lt;/li&gt;&#xA;&lt;li&gt;Contact&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Please be specific about what type of encounter happened.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good answer should:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;list astronaut/s by name, &lt;/li&gt;&#xA;&lt;li&gt;detail what occured in the encounter, and &lt;/li&gt;&#xA;&lt;li&gt;provide strong evidence for the account. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Many &quot;documentaries&quot; and websites have quote mined things that various astronauts have said, so be careful to bring the full context to bear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I would like to count all who have been to space, regardless of nationality or whether or not they were pilots, etc. But please provide detail on each astronaut's nationality and qualifications.)&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-09-17T13:23:00.620" LastActivityDate="2020-08-05T15:27:01.137" Title="How many astronauts have reported an alien encounter of some kind?" Tags="|astronauts|life|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1939" PostTypeId="2" ParentId="1929" CreationDate="2013-09-12T22:31:31.950" Score="2" Body="&lt;p&gt;Typically yes. Basically, it provides the best delta V to achieve an orbit. Let's see if I can sketch a picture of each case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WK8p4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what are the relative advantages of each? The first is closer to an actual orbit around the Moon. A short burn will lower the apogee of the moon such that it will be orbiting well. The second case is quite a bit more work. I'm struggling to fully understand what will happen, but basically you'd have to increase your speed somehow, and it's much less natural and thus would probably take more fuel than the alternative.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-12T22:31:31.950" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1940" PostTypeId="1" AcceptedAnswerId="9815" CreationDate="2013-09-12T22:52:15.070" Score="22" ViewCount="2538" Body="&lt;p&gt;I was reading &lt;a href=&quot;https://space.stackexchange.com/q/1934/25&quot;&gt;this question&lt;/a&gt;, and was trying to remember an article I read many years ago. It seemed the article was indicating that there is a chance that one of the boosters from one of these missions could be going faster than the probe it launched, and thus be leaving the solar system faster. I can't find the article in question, but it got me thinking, where are the upper stages to the Voyager and Pioneer rockets?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2020-01-10T18:57:48.720" Title="Where are the upper stages for the Voyager/Pioneer stages?" Tags="|rockets|voyager|pioneer|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1941" PostTypeId="1" AcceptedAnswerId="1945" CreationDate="2013-09-12T23:03:48.457" Score="11" ViewCount="608" Body="&lt;p&gt;It is thought that &lt;a href=&quot;http://en.wikipedia.org/wiki/Pluto#Atmosphere&quot;&gt;Pluto's atmosphere will freeze&lt;/a&gt; as it moves further from the Sun. We know that Pluto was it's closest to the Sun in the 1990's. Is it known if the atmosphere will freeze prior to New Horizon's reaching it?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-13T05:44:52.747" Title="Will Pluto's atmosphere collapse prior to New Horizon's visiting it?" Tags="|atmosphere|new-horizons|pluto|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1942" PostTypeId="2" ParentId="922" CreationDate="2013-09-13T00:04:03.657" Score="3" Body="&lt;p&gt;Yes, antimatter &lt;em&gt;can&lt;/em&gt; be used to propel a spacecraft. The concept of an &lt;a href=&quot;http://en.wikipedia.org/wiki/Antimatter_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;antimatter rocket&lt;/a&gt; is what we're after.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EzKTz.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Just a concept drawing, but it gives us an idea of what we're talking about.&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.technologyreview.com/view/427923/antimatter-propulsion-engine-redesigned-using-cerns-particle-physics-simulation-toolkit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Another article&lt;/a&gt; discusses such a rocket (highlights here):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Smash a lump of matter into antimatter and it will release a &lt;strong&gt;thousand times more energy&lt;/strong&gt; than the &lt;strong&gt;same mass&lt;/strong&gt; of fuel in a &lt;strong&gt;nuclear fission reactor&lt;/strong&gt; and some 2 billion times more than burning the equivalent in hydrocarbons.&lt;/p&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;p&gt;First, some basic rocket science. &lt;strong&gt;The maximum speed of a rocket depends on its exhaust velocity&lt;/strong&gt;, the fraction of mass devoted to fuel and the configuration of the rocket stages.&lt;/p&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;p&gt;So these guys focus on the exhaust velocity–the &lt;strong&gt;speed of the particles produced in matter-antimatter annihilations&lt;/strong&gt; as they leave the rocket engine.&lt;/p&gt;&#xA;&lt;p&gt;The thrust from these annihilations comes largely from &lt;strong&gt;using a magnetic field to deflect charged particles&lt;/strong&gt; created in the annihilation. These guys focus on the annihilation of protons and anti-protons to produce charged pions.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;In the past,&lt;/strong&gt; various physicists have calculated that the pions should travel at over 90 per cent the speed of light but that the nozzle would be only 36 per cent efficient. That translates into an average exhaust velocity of only a third of light-speed, barely relativistic and somewhat of a disappointment for antimatter propulsion fans.&lt;/p&gt;&#xA;&lt;p&gt;All that is set to change now, however. Keane and Zhang have come up with a different set of figures with the help of software developed by CERN that simulates the interaction between particles, matter and fields of various kinds.&lt;/p&gt;&#xA;&lt;p&gt;CERN uses this software, called GEANT4 (short for Geometry and Tracking 4), to better understand how particles behave at the Large Hadron Collider, which itself collides beams of protons and anti-protons. So it’s ideally suited to Keane and Zhang’s task.&lt;/p&gt;&#xA;&lt;p&gt;... &lt;em&gt;To continue reading, &lt;a href=&quot;http://www.technologyreview.com/view/427923/antimatter-propulsion-engine-redesigned-using-cerns-particle-physics-simulation-toolkit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;click here&lt;/a&gt;. I'm not going to copy the whole thing.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It's a decent report on the subject, and one that doesn't read like dry sandpaper.&lt;/p&gt;&#xA;&lt;p&gt;All in all, it's certainly possible - if you can get your hands on the antimatter.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-12-17T22:52:37.113" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1943" PostTypeId="1" AcceptedAnswerId="1944" CreationDate="2013-09-13T03:57:33.150" Score="10" ViewCount="1199" Body="&lt;p&gt;Currently, we're able to aim probes at various bodies in the solar system pretty well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we have the technology to send a probe to Alpha Centauri? If we're aiming at a star, approximately how far does it have to be (an order of magnitude answer is OK) before we lose precision in aiming and have an appreciable chance of missing altogether?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the probe is allowed to have mechanisms that correct its path&lt;/p&gt;&#xA;" OwnerUserId="96" LastEditorUserId="25" LastEditDate="2015-10-13T12:45:39.153" LastActivityDate="2015-11-11T15:18:22.963" Title="What's our limit in sending probes to distant bodies?" Tags="|probe|interstellar-travel|mission-design|long-duration|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1944" PostTypeId="2" ParentId="1943" CreationDate="2013-09-13T04:35:12.883" Score="9" Body="&lt;p&gt;We have the much of the technology needed, but not the engineering and patience. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sending a Voyager-like probe to Alpha-Centauri would take many, many, many years. Around 50,000 years given a 25 km/sec speed (that's a good deal faster than Voyager is going). Because of the time-frame, we'd need to engineer something that could last that long. Several of the Voyager sensors have already malfunctioned, and it's only been 40+ years - 1 one-thousandth of the interstellar time. Another example is power - solar-cells that far out in space don't give us much power, and batteries just ain't going to help.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the moment, we need a major technological jump before an interstellar probe is really feasible. We can certainly send something now, but it probably wouldn't make it.&lt;/p&gt;&#xA;" OwnerUserId="692" LastActivityDate="2013-09-13T04:35:12.883" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1945" PostTypeId="2" ParentId="1941" CreationDate="2013-09-13T05:44:52.747" Score="13" Body="&lt;p&gt;According to &lt;a href=&quot;http://arxiv.org/abs/1309.0841&quot;&gt;this paper&lt;/a&gt;, Pluto's atmosphere never collapses.  Though it was handy that we thought it did at one time, since that helped to get New Horizons approved as a mission, in order to try to get there before the imminent collapse.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-09-13T05:44:52.747" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1946" PostTypeId="1" AcceptedAnswerId="1947" CreationDate="2013-09-13T09:09:59.953" Score="14" ViewCount="4380" Body="&lt;p&gt;Firstly, not sure if this question ought to be in the physics SE site. Please let me know if it should.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, I don't know a whole lot about physics (I'm just inquisitive). So please try to keep answers simple (or at least dummed down to laymans terms).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inspired by &quot;A Hitchhikers Guide to the Galaxy&quot; movie (haven't read the book yet)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, for a non-spherical object (lets say a cube for now) of a size and mass like that of Earth, how would objects act in it's gravity? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aspects of this question lead to the following questions&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Would something weigh more/less in certain areas over other areas?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I'm aware that it's possible to 'slingshot' around massive spherical objects to change their directory. Would it be possible to slingshot objects around something non-spherical to change their velocity?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If this massive object was orbiting our sun, would it's orbit be as uniform as ours? if not what would that look like and why?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="703" LastActivityDate="2013-09-13T11:49:22.560" Title="How does the gravity of a massive non-spherical object act on things around it?" Tags="|orbital-mechanics|gravity|orbital-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1947" PostTypeId="2" ParentId="1946" CreationDate="2013-09-13T11:49:22.560" Score="17" Body="&lt;p&gt;Yes, a cubic celestial would most certainly have anomalous gravity depending on your position relative to its sides. Gravity anomalies are however a perfectly normal thing in space travel, and I'm not aware of any celestials (including the Earth) that would have a seamless gravitational field throughout its surface. Reasons for this could vary, but in simplest terms possible, we could say that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The body is not a perfect sphere, and&lt;/li&gt;&#xA;&lt;li&gt;The mass of the body is not equally distributed (inhomogeneous)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Any of these properties of a celestial will affect the surface gravity to vary depending on your position relative to its surface. For example, our moon - Luna - was mapped for gravitational fields over its surface to high detail by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Recovery_and_Interior_Laboratory&quot; rel=&quot;nofollow noreferrer&quot;&gt;GRAIL&lt;/a&gt; (Gravity Recovery and Interior Laboratory) probe, and this is an example of the resulting map:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/xWDkG.jpg&quot; alt=&quot;GRAIL map&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Gravity map of the Moon as mapped by the NASA's Discovery Program GRAIL probe (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Recovery_and_Interior_Laboratory&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we have established, that a non-varying gravity over the surface of celestial objects is a normal thing. Great. What we now have to do is convince you that when we say &lt;code&gt;x orbits y&lt;/code&gt; what we actually mean is that &lt;code&gt;x and y orbit each other&lt;/code&gt;, albeit it might appear to a casual observer that since gravitational pull of one object is much larger than that of the other, one is dominant, and the other submissive in their role in this cosmic dance. To somewhat show what I mean, here are 3 models of two celestial object orbiting one another:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/jLnTo.gif&quot; alt=&quot;enter image description here&quot;&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Erwut.gif&quot; alt=&quot;enter image description here&quot;&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/qbyPi.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;Cosmic dance&lt;/strong&gt;: Two bodies with different mass ratios orbiting around a common barycenter. (Animations: &lt;a href=&quot;https://i.stack.imgur.com/qbyPi.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both these explanations, hopefully demonstrated well enough for the purpose, will serve us later when we describe an orbit of the cube shaped celestial. We'll assume these three conditions are true, to simplify things a bit:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The body's is homogeneous, i.e. made of same material throughout,&lt;/li&gt;&#xA;&lt;li&gt;The body's material is non-compressible by its own weight, and&lt;/li&gt;&#xA;&lt;li&gt;The body's equator is going along the middle of the four of its faces&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This simplification is important, because we don't want the cube's own mass to flatten its  gravity along its surface closer to what it would be, if it was a perfect sphere, and we intentionally exaggerate the effects of its shape for the sake of this demonstration. The cube's gravitational pull would of course still point to a single cube's center of mass, like so:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/EvTZV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Gravity ﬁeld through a cube sliced in half through the faces.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can observe the slight distortion of the ﬁeld lines between the edges and the center of each face, and this is exactly what we want. If we compared the gravitational potential along the equator of the cube to the one of a perfect sphere of the same volume, we would get:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Cube's gravitational potential over its corners is higher than nominal, and&lt;/li&gt;&#xA;&lt;li&gt;Cube's gravitational potential over the center of any of its face is lower than nominal&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Where with &quot;nominal&quot;, I mean the gravitational potential of the perfect sphere that is equally distributed along its equator. If we transferred these gravity potential values into a graph, we would get something similar to a &lt;a href=&quot;https://en.wikipedia.org/wiki/Sine_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;sine wave&lt;/a&gt; for the cube, and a straight line for the sphere, as we move along its equator. So each cube's corner would translate to a higher gravitational potential, and each center of its sides to a lower one than our nominal gravitational potential of the sphere. Assuming our cube has the same mass and volume as the Earth does, we'd get such a diagram:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/eNRRo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The force of gravity on the cube surface, in Earth g's. (Source: &lt;a href=&quot;http://possiblywrong.wordpress.com/2011/09/09/if-the-earth-were-a-cube/&quot; rel=&quot;nofollow noreferrer&quot;&gt;If the Earth Were a Cube&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, I'm probably already losing your patience, but I believe we've established our parameters well enough and somewhat described why they might not equally apply in all cases, so let's press on with answering your questions more directly:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Would something weigh more/less in certain areas over other areas?&lt;/strong&gt;&lt;br /&gt;&lt;br /&gt;&#xA;Yes. Assuming all other parameters described above don't magically cancel each other out (and why wouldn't we, mountains as high to protrude the thickness of the Earth's atmosphere many times over and at the same time both themselves and an equal weight worth of their basin made out of marshmallow sitting in an otherwise chocolaty planet is sadly hard to come by in the real Universe), the certain areas of a cube shaped celestial would have higher gravity potential than others. Large part of this is already explained in the introductory text.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Would it be possible to slingshot objects around something non-spherical to change their velocity?&lt;/strong&gt;&lt;br /&gt;&lt;br /&gt;&#xA;Again - yes. But the difference in gravitational potential along the body's surface would somewhat affect your slingshot vector, so it's something that would need adjustment for. This is of course assuming the body doesn't rotate on its axis at approximately the same radial velocity as your average rate of the change of your vector when you slingshot past its equator (i.e. the body is more or less showing the same face towards your vehicle as it flies past it). You would also want to do your slingshot in as few passes around the body as possible, since your orbit will decay towards the direction the cube's body is moving. Eventually, you would hit it, and your orbit might look something like this:&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/6ev9o.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br /&gt;&lt;br /&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;A satellite orbiting around the equator of a cube. Eventually, it will hit the celestial.&lt;/sup&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;If this massive object was orbiting our sun, would it's orbit be as uniform as ours? if not what would that look like and why?&lt;/strong&gt;&lt;br /&gt;&lt;br /&gt;&#xA;Provided the body is not &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_locking&quot; rel=&quot;nofollow noreferrer&quot;&gt;tidally locked&lt;/a&gt; with the Sun that it orbits, always showing it same face and with it not change the gravitational potential oriented towards the Sun, then this periodic change in the gravitational potential will reflect in a slight &quot;wobble&quot; in the orbit for both bodies, more pronounced for the one with on average lower gravitational potential. As the cube celestial rotates on its own axis to face with one of its corners towards the body it rotates, this gravitational potential will slightly increase, moving the bodies slightly closer to one another, and the opposite as the cube body shows towards the other celestial it rotates one of its sides. If the masses of the two bodies orbiting each other would approximately match, this &quot;wobble&quot; would look a bit like one energetic drunkard dancing around another, perhaps slightly less energetic drunkard (depending on their mass distribution). &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So this is about it. Hopefully not too difficult to imagine and easy enough to understand, as per your request to keep the answers simple. If you're interested in reading slightly heavier to read sources on this same presented situation of a cubic celestial in orbit of another celestial (or vice versa), here are some external resources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://arxiv.org/pdf/1206.3857v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Gravitational Field of a Cube&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://possiblywrong.wordpress.com/2011/09/09/if-the-earth-were-a-cube/&quot; rel=&quot;nofollow noreferrer&quot;&gt;If the Earth Were a Cube&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-13T11:49:22.560" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1948" PostTypeId="2" ParentId="1911" CreationDate="2013-09-13T15:05:16.630" Score="13" Body="&lt;p&gt;Technically, in General Relativity, there are no stable orbits in any two-body system, period.  Regardless of mass or black holiness.  Bodies rotating about their mutual center of mass will emit gravitational radiation.  By conservation of energy, the orbits will get smaller, and given enough time, they will crash into each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Practically, this takes so long to happen in most circumstances that we find in the universe, that it is not a consideration on the time scales of age of the universe.  However in unusual circumstances with large masses orbiting each other tightly, in this case two neutron stars, &lt;a href=&quot;http://en.wikipedia.org/wiki/Hulse-Taylor_binary&quot;&gt;this has been observed&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The orbit has decayed since the binary system was initially&#xA;  discovered, in precise agreement with the loss of energy due to&#xA;  gravitational waves predicted by Einstein's general theory of&#xA;  relativity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-09-13T15:05:16.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1949" PostTypeId="1" CreationDate="2013-09-13T15:14:19.203" Score="6" ViewCount="374" Body="&lt;p&gt;This question is a bit more complex then it sounds at first.  On Earth when getting a massage you lay on a table and gravity and friction holds you in place while the therapist pushes, pulls and kneads your body.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Microgravity your body would tend to bounce, between the masseuse and the table.  If you try and hold yourself in place it would fail the &quot;relax&quot; part of the massage experience. Straps might be used but unless they were fairly tight and several of them, would seem to not a be a solution to the bounce issue.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I considered a table with a light suction to hold the massage receiver in place, but was not sure if that would be viable solution. You would need to mimic the friction a 1G body on table without causing undue sheer force at any one body location. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question focuses on the massage receiver, it is a given that the masseuse would need some way of anchoring themselves. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-09-15T12:00:16.357" LastActivityDate="2021-08-03T03:13:02.773" Title="Can you have a relaxing full body massage in zero G?" Tags="|microgravity|health|physical-activity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1950" PostTypeId="1" AcceptedAnswerId="1954" CreationDate="2013-09-13T16:02:20.193" Score="12" ViewCount="3752" Body="&lt;p&gt;&lt;a href=&quot;http://www.space.com/22783-voyager-1-interstellar-space-star-flyby.html&quot;&gt;This article&lt;/a&gt; says it will pass by the star in about 40,000 years. However, my quick and dirty calculation shows that Voyager will only be about a few light years away in 40,000 years, still quite a ways from any star. How should this be reconciled?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, When will Voyager 1 actually encounter AC +79 3888?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Upon further inspection, &lt;a href=&quot;http://voyager.jpl.nasa.gov/mission/interstellar.html&quot;&gt;this article&lt;/a&gt; appears to be the source.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-14T00:21:44.583" LastActivityDate="2023-11-10T21:26:02.717" Title="Did NASA provide the wrong year for Voyager 1's close approach to AC +79 3888?" Tags="|interstellar-travel|voyager|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1952" PostTypeId="2" ParentId="1949" CreationDate="2013-09-13T23:46:47.163" Score="2" Body="&lt;p&gt;Normal commercial massage technique usually involves placing a sheet over the parts of the body not being worked on at the moment.  This is obviously adjusted as they go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus why couldn't the person receiving the massage be strapped in instead of simply draped?&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2013-09-13T23:46:47.163" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1953" PostTypeId="1" AcceptedAnswerId="1959" CreationDate="2013-09-14T00:10:37.323" Score="10" ViewCount="631" Body="&lt;p&gt;Getting struck by &lt;a href=&quot;https://en.wikipedia.org/wiki/Upper-atmospheric_lightning&quot; rel=&quot;noreferrer&quot;&gt;lighting in Earths upper atmosphere&lt;/a&gt; is a real possibility.  It is often found in the 100km range, and can reach 400 km (250 mi) in diameter. The ISS has an &lt;a href=&quot;https://en.wikipedia.org/wiki/ISS&quot; rel=&quot;noreferrer&quot;&gt;orbit of 400km&lt;/a&gt;, and as pointed out below it appears that electrical disturbances can travel light years in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.popsci.com/technology/article/2011-06/strongest-current-universe-detected-some-2-billion-light-years-away?cmp=tw&quot; rel=&quot;noreferrer&quot;&gt;Popular Science&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Galaxy 3C303, Keeping Current Generated from a Very Large Array image, this image shows the huge jet of current stretching for 150,000 light years across galaxy 3C303. Philipp P. Kronberg, Richard V.E. Lovelace, Giovanni Lapenta, Stirling A. Colgate via arXiv&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.boeing.com/commercial/aeromagazine/articles/2012_q4/4/&quot; rel=&quot;noreferrer&quot;&gt;Boeing&lt;/a&gt; Commercial airplanes have some some protection built into them, this page also indicates that most lighting strikes to planes occur while the plane is a cloud and between altitudes of 5,000 feet (1,524 meters) and 15,000 feet (4,572 meters).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what are the odds of being struck by lightning while in Earth Orbit?  Are some orbits at more risk then others? Are lightning protection devices installed in satellites or the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison; according to &lt;a href=&quot;http://news.nationalgeographic.com/news/2004/06/0623_040623_lightningfacts.html&quot; rel=&quot;noreferrer&quot;&gt;National Geographic&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The odds of becoming a lightning victim in the U.S. in any one year is 1 in 700,000. The odds of being struck in your lifetime is 1 in 3,000.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2013-09-15T18:55:46.823" LastActivityDate="2013-09-15T18:55:46.823" Title="What are the chances of getting struck by lightning while in Earth orbit?" Tags="|orbit|safety|electronics|lightning|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1954" PostTypeId="2" ParentId="1950" CreationDate="2013-09-14T00:24:09.093" Score="19" Body="&lt;p&gt;No, as far as I can tell NASA got it right. Voyager 1 isn't going fast enough to reach AC +79 3888's current position in 40,000 years -- but the star is coming to meet it.&lt;/p&gt;&#xA;&lt;p&gt;Assuming the current speed is 17 km/sec (as stated in the first paragraph of &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Wikipedia article&lt;/a&gt;, and that it won't slow down much beyond that, it should cover approximately 1 light-year every 17,500 years.&lt;/p&gt;&#xA;&lt;p&gt;AC +79 3888, also known as &lt;a href=&quot;http://en.wikipedia.org/wiki/Gliese_445&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gliese 445&lt;/a&gt;, is currently about 17.6 light-years away. But the star itself is rapidly approaching us, with a current radial velocity of -119 km/sec -- nearly 7 times as fast as Voyager 1's (Sun-relative) motion.&lt;/p&gt;&#xA;&lt;p&gt;According to the above-cited Wikipedia article, in 40,000 years, Gliese 445 will be about 3.45 light-years away (but still too dim to see with the naked eye), and Voyager 1 will be 1.6 light-years from it.&lt;/p&gt;&#xA;" OwnerUserId="574" LastEditorUserId="574" LastEditDate="2023-11-10T21:26:02.717" LastActivityDate="2023-11-10T21:26:02.717" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="1955" PostTypeId="2" ParentId="1351" CreationDate="2013-09-14T01:01:02.600" Score="4" Body="&lt;p&gt;Capillary action will not get your fuel to space as this &lt;a href=&quot;https://en.wikipedia.org/wiki/Capillary_action#Height_of_a_meniscus&quot; rel=&quot;nofollow noreferrer&quot;&gt;article in Wikipedia&lt;/a&gt; points out the height of lift is related to the diameter of the tube, where a narrower tube gets you more lift measured in inches. Trees use capillary action combined with &lt;a href=&quot;https://en.wikipedia.org/wiki/Capillary_action#In_plants_and_trees&quot; rel=&quot;nofollow noreferrer&quot;&gt;other actions&lt;/a&gt; to raise fluids.  In no case would capillary action by itself get you anywhere near orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming you have a space elevator and a line running the length of it, you might think you could &quot;pump&quot; your fuel for &quot;free&quot;.  As seen in &lt;a href=&quot;https://space.stackexchange.com/questions/1350/benefit-of-sling-shot-effect-with-a-space-elevator&quot;&gt;Benefit of sling shot effect with a space elevator&lt;/a&gt; once you get to twice geosynchronous radius, you have more outward then downward pressure.  In theory you could &lt;a href=&quot;https://en.wikipedia.org/wiki/Siphon&quot; rel=&quot;nofollow noreferrer&quot;&gt;siphon&lt;/a&gt; off the oceans of the world without any cost above establishing the line and priming it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas a siphon will not work, the top fluid is pulling up the lower fluid, so you would need to exert the same amount of pull that would be required to push or pump the liquid up your elevation.  There are many variable; atmospheric pressure, fluid density, purity of the fluid and several other factors.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At distances measured in thousands of kilometer's, regardless of the other variables you would encounter, at some point you reach the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vapour_pressure&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vapor Pressure&lt;/a&gt; of the fluid, and it would turn from a liquid into a gas. At this point the siphon stops functioning, Wikipedia has &lt;a href=&quot;https://en.wikipedia.org/wiki/Siphon#Maximum_height&quot; rel=&quot;nofollow noreferrer&quot;&gt;some formulas&lt;/a&gt; for what point this will occur.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for directly pumping, there are a lot of variables, density of your fuel, diameter of your line, turbulence, resistance and such, but in the end a column of fluid 35,786km high would be VERY Heavy, it is difficult (&lt;em&gt;impossible?&lt;/em&gt;) to imagine a single pump performing the task.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could of course have pumping stations, with several pumps at whatever level your engineering solution desires, but this leaves you with the second huge problem with a space elevator which is getting power (presumably electrical) along a 35,786km stretch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your goal is fuel at GSO, go with Loren Pechtel's solution and haul tanks of fuel in the elevator.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-09-15T11:35:21.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1956" PostTypeId="2" ParentId="1943" CreationDate="2013-09-14T10:45:48.970" Score="16" Body="&lt;p&gt;In addition to what John provided, it's also worth noting that given the scale of tens of thousands of years, the stars will actually move. This is clearly shown in a &lt;a href=&quot;http://en.wikipedia.org/wiki/File:Near-stars-past-future-en.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt;. As can be show, Alpha Centauri will only be 3 Light Years away in about 30,000 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YgZAc.png&quot; alt=&quot;Nearby Star Movement&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so the fastest mission I've heard of using nearly obtainable technology is the method detailed in &lt;a href=&quot;https://space.stackexchange.com/q/829/25&quot;&gt;this question&lt;/a&gt;, graphic provided from &lt;a href=&quot;http://xkcd.com/1244/&quot; rel=&quot;nofollow noreferrer&quot;&gt;XKCD&lt;/a&gt;. It results in a speed of 20 au/year, or about 95 km / second. Give that speed, how long would it take to go to another star? Well, that speed is equivalent to 1 light year per 3000 years, give or take. Thus, given the fact that Alpha Centauri is getting closer, I'd say around 12,000 years or so would be possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/R1gLH.png&quot; alt=&quot;xkcd comic&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(image license: CC BY-NC 2.5)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the challenges associated with this? I'm going to include a very unscientific list, in my thoughts about what order they should be.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Power- Keeping the spacecraft powered in the void.&lt;/li&gt;&#xA;&lt;li&gt;Longevity- It's hard to make something that still works after 50 years, let alone 12,000... Furthermore, space is a difficult environment by it self, including a high amount of radiation, making it even more difficult.&lt;/li&gt;&#xA;&lt;li&gt;Future Faster Space Probe- A huge expense could be spent to launch this spacecraft, and have a spacecraft 100 years later that travels 10% faster reach the system well before. Or it could even be much faster, leading to even more discrepancies.&lt;/li&gt;&#xA;&lt;li&gt;Communications- There is a significant link margin required to make this work.&lt;/li&gt;&#xA;&lt;li&gt;Light Speed Delay- Being so far away, the spacecraft would need to decide 100% of what it did, there would be no way to take a few pictures and command a new stack&lt;/li&gt;&#xA;&lt;li&gt;Objects of Interest- Some work would need to be done to find objects of interest to photograph, such as planets&lt;/li&gt;&#xA;&lt;li&gt;Course correction- This should be easy enough to accomplish, but would require keeping fuel in the void for a long period of time. Fuel usually has requirements about what temperature to store it at, which might be difficult for such a long journey.&lt;/li&gt;&#xA;&lt;li&gt;Technology not getting out of date- It's hard to make something still compatible with 50 years of technology, let alone 10,000+..&lt;/li&gt;&#xA;&lt;li&gt;There's always a fear with such long term items that someone a thousand years later will find a better method to send the probe that only takes 8,000 years, thus making the probe you sent out entirely useless, as it will arrive 2,000 years too late.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;And there are no doubt quite a few other very serious issues. Bottom line is, I think the following minimum technology is required to make this feasible.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A way to make the journey in less than 100 years.&lt;/li&gt;&#xA;&lt;li&gt;Careful observation of the star system to ensure that targets of interest are identified.&lt;/li&gt;&#xA;&lt;li&gt;Perfecting Laser Communications, or similar, to allow for rapid far communications to occur.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-11-11T15:18:22.963" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1957" PostTypeId="2" ParentId="1109" CreationDate="2013-09-14T14:03:39.700" Score="2" Body="&lt;p&gt;There is an &lt;a href=&quot;http://en.wikipedia.org/wiki/Single-stage-to-orbit&quot; rel=&quot;nofollow&quot;&gt;entire Wikipedia article&lt;/a&gt; on the subject. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One could call the Apollo lunar module a SSTO vehicle, as it made it to lunar orbit with one stage. Probably not what you're after, but still.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, there appears to be only one country other than Britain looking into the idea - the Romanians with their &lt;a href=&quot;http://en.wikipedia.org/wiki/Single-stage-to-orbit#Haas_2C&quot; rel=&quot;nofollow&quot;&gt;Haas 2C&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On June 1, 2012, Romanian organization ARCA announced that they are constructing an expendable rocket, named Haas 2C that will attempt to reach orbit in one stage. The rocket has 520 kg empty weight and can carry 15.5 tons of fuel. It will use kerosene as fuel and liquid oxygen as oxidizer. In Spring 2012 they have successfully tested a lightweight composite kerosene fuel tank. The liquid oxygen tank is being designed and it will also be made of composite materials. The launch is expected to take place in Spring 2013.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.arcaspace.com/en/haas2c.htm&quot; rel=&quot;nofollow&quot;&gt;ARCA's site&lt;/a&gt; states that Haas 2C is an attempt at the &lt;a href=&quot;http://www.googlelunarxprize.org/&quot; rel=&quot;nofollow&quot;&gt;Google lunar X-Prize&lt;/a&gt;. Their &lt;a href=&quot;http://www.googlelunarxprize.org/teams/arca&quot; rel=&quot;nofollow&quot;&gt;team page&lt;/a&gt; says that one of their goals is to create the Haas 2C to be capable of placing 300kg in LEO - quite impressive for a single stage rocket.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2013-09-14T15:13:27.947" LastActivityDate="2013-09-14T15:13:27.947" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1958" PostTypeId="1" AcceptedAnswerId="1962" CreationDate="2013-09-14T15:48:14.510" Score="7" ViewCount="483" Body="&lt;p&gt;Recently, someone posted this picture in &lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/the-pod-bay&quot;&gt;The Pod Bay&lt;/a&gt;, our chatroom:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/NZJ8a.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It shows a frog being blown off of the launch of LADEE, a lunar probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It got me thinking: Would a frog, successfully holding on to the side of a rocket, be enough to steer the rocket substantially off course if the rocket didn't compensate for it? By how much?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-09-14T17:09:56.377" Title="Would a frog on the side of a rocket be enough to steer it substantially offcourse?" Tags="|ladee|launch|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="1959" PostTypeId="2" ParentId="1953" CreationDate="2013-09-14T16:48:18.490" Score="10" Body="&lt;p&gt;The probability is, simply, &lt;strong&gt;zero&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lightning is the heavy discharge between two electrically charged bodies that have enough electrostatic potential to ionize that medium. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Where lightning occurs:&lt;/h3&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Within the clouds&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Between two different clouds with different charge &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Clouds to earth&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h3&gt;Why cloud to earth not cloud to space?&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.en.wikipedia.org/wiki/Lightning&quot;&gt;Wikipedia&lt;/a&gt; article states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In order for an electrostatic discharge to occur, two things are necessary: &lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;a sufficiently high electric potential between two regions of space must exist; and &lt;br /&gt;&lt;/li&gt;&#xA;  &lt;li&gt;a high-resistance medium must obstruct the free, unimpeded equalization of the opposite charges.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So let us make few assumptions; Let the height of the cloud be 85 kilometres (which is the maximum height at which the clouds are present), the cloud develops a negative charge, the spacecraft is at 300 kilometres with respect to the cloud, the tree on the ground at 85 kilometres with respect to the cloud, and the spacecraft and the tree on the ground both develop equal and opposite charge in respect to the cloud.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For lightning to occur, it requires a breakdown voltage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Paschen's law states that a breakdown voltage is described by the equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${V={a*p*d}\over{Ln(p*d)+b}}$$ &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$a$ and $b$ are gas composition constants&lt;/li&gt;&#xA;&lt;li&gt;$p$ is pressure (in Atmospheres or Bar)&lt;/li&gt;&#xA;&lt;li&gt;$d$ is the gap distance (in meters)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For the cloud to the Earth, the values are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a=4.36*10^7, b=12.5, p=1, d=85*1000$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On substituting it, this becomes: $${{4.36*{10^7}*{1}*85*1000}\over Ln({1}*85*1000) + 12.8}&#xA;=1.5345497371054913*10^{11}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is for vacuum (I couldn't find the constant values), but the breakdown voltage should be very high, and the nature of the lightning is always choosing the simplest and the closest path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For lightning to occur and sustain its voltage, it depends on the levels of ionization, which in turn depends upon the electrons being able to hit other electrons (avalanche breakdown), and that probability is given by: $$P={N \sigma \over A}={x\over mean\ free\ path}$$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$N\sigma$ is the number of electrons&lt;/li&gt;&#xA;&lt;li&gt;$A$ is the area &lt;/li&gt;&#xA;&lt;li&gt;$x$ distance travelled by the electron &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The probability is inversely proportional to the mean free path, but in vacuum the mean free path is very large and hence very little ionization occurs (since the probability decreases). Hence, even though the discharge can occur towards the space, its intensity is very small (since the probability of electron collisions is very small), and the lightning dies out before reaching the spacecraft (since the electrons lose their energy as they travel). So the spacecraft are safe from lightning, but us on earth aren't. ;)&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-15T18:55:11.447" LastActivityDate="2013-09-15T18:55:11.447" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1960" PostTypeId="5" CreationDate="2013-09-14T16:52:46.413" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-14T16:52:46.413" LastActivityDate="2013-09-14T16:52:46.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1961" PostTypeId="4" CreationDate="2013-09-14T16:52:46.413" Score="0" Body="Questions regarding frogs, tailless stout-bodied amphibians with long hind limbs for leaping, semiaquatic and terrestrial species." OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-14T20:21:18.423" LastActivityDate="2013-09-14T20:21:18.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1962" PostTypeId="2" ParentId="1958" CreationDate="2013-09-14T17:04:47.277" Score="9" Body="&lt;p&gt;There won't be much of an effect, if at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this is an order of magnitude calculation. I'll be making approximations/assumptions left and right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drag force will be $\frac12\rho v^2c_D A$, and by taking some approximations (as there are too many variables) we get $\frac12 1.2 v^2 0.1 (\pi 0.05)^2 =0.00047v^2$. This can produce a torque up to $F\frac{L}2=0.0056v^2$, which in turn produces an angular acceleration of $\alpha=\frac\tau{m}=6\times 10^{-8}v^2 \:\mathrm{s^{-2}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say that the rocket accelerates constantly till it leaves the bulk of the atmosphere (after which point the drag doesn't apply). I'm taking the acceleration as 17 from the thrust values.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then,$\alpha(t)=6\times 10^{-8}v^2*17*17t^2\implies \theta(t)=6\times 10^{-8}17*17 frac{t^4}{12}$. For the first 10 seconds (when this constant-acceleration scheme holds --after this its velocity stabilizes and the atmosphere is thinner, so the net deviation should be of the same order of magnitude or less), this gives us a deviation of 0.01 radians. The total deviation will be of a similar order of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isn't much. Winds can generate a much larger deviation. Of course, the deviation of a couple hundredths of a radian is a lot when talking about the moon, but again, the deviation due to other effects is a lot more, so the onboard stabilizers can take care of this pretty well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&#xA;Values used in answer: Mass of rocket: $90,000\:\mathrm{kg}$, Acceleration: $17.8 \:\mathrm{m/s^2}$, Length: 24 m, Drag coefficient of frog: To the order 0.1, Radius of frog: $0.05 \:\mathrm{m}$. h/t @TildalWave for providing this info :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="96" LastEditorUserId="96" LastEditDate="2013-09-14T17:09:56.377" LastActivityDate="2013-09-14T17:09:56.377" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="1963" PostTypeId="2" ParentId="1162" CreationDate="2013-09-15T01:33:33.687" Score="6" Body="&lt;p&gt;The closest I could find to information on this came from &lt;a href=&quot;http://www.isro.org/gslv-d5/pdf/brochure.pdf&quot; rel=&quot;noreferrer&quot;&gt;this brochure&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h3&gt;Design Improvements in GSLV-D5&lt;/h3&gt;&#xA;&lt;p&gt;Based on its performance during the earlier missions, end-to-end design of GSLV as well as indigenous cryogenic stage systems have been re-examined. Design modifications are implemented wherever required along with rigorous ground testing and improvements are made with respect to the fabrication and quality control to enhance the reliability&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;The changes that were made:&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Redesign of Lower Shroud which protects the cryogenic engine during atmospheric flight of GSLV-D5&lt;/li&gt;&#xA;&lt;li&gt;Redesign of the wire tunnel of the cryo stage to withstand larger forces during flight&lt;/li&gt;&#xA;&lt;li&gt;Revised Aerodynamic characterisation of the entire launch vehicle&lt;/li&gt;&#xA;&lt;li&gt;Inclusion of Video Imaging System to monitor lower&#xA;shroud movement during various phases of flight&lt;/li&gt;&#xA;&lt;li&gt;Improvements in the Cryogenic upper Stage:&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Modified design of the Fuel Booster Turbo Pump (FBTP), taking care of the expansion and contraction of the bearings and casing at cryogenic temperatures&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Modification of Ignition Sequence to ensure the smooth, successful and sustained ignition for Main Engine (ME), Steering Engine (SE) and Gas Generator (GG)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;In addition, indigenisation of many critical systems including Liquid Hydrogen Propellant Acquistion System (to prevent the possibility of  contamination), Polyimide pipelines and Liquid Oxygen &amp;amp; Liquid Hydrogen Level Sensors has&#xA;been successfully accomplished.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Ground tests performed:&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In order to validate the design improvements, the following extensive qualification tests have been carried out on the engine at the Main Engine Test (MET) facility and the High Altitude Test (HAT)  facility:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Two acceptance tests for flight unit of FBTP&lt;/li&gt;&#xA;&lt;li&gt;High altitude tests to confirm the ignition sequence in flight under vacuum&lt;/li&gt;&#xA;&lt;li&gt;Cryogenic Main Engine (200 sec) and Steering Engine (100 sec) acceptance tests&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;All the improvements have been thoroughly reviewed by expert committees including eminent national experts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That's really all I could find on the subject. Of course, the launch &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=3&amp;amp;cad=rja&amp;amp;ved=0CDcQFjAC&amp;amp;url=http%3A%2F%2Fzeenews.india.com%2Fnews%2Fspace%2Fisro-calls-off-gslv-d5-launch-after-fuel-leak_870020.html&amp;amp;ei=sQs1Uv6BIcjRiAKlroCQDQ&amp;amp;usg=AFQjCNHmT-sIKPBOf8HkVgZ9M39OM7OMXw&amp;amp;sig2=wQGSn1t08HcFNF6SrowjFw&quot; rel=&quot;noreferrer&quot;&gt;was called off&lt;/a&gt; due to a fuel leak and delayed until December, so more info may come with time.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-15T01:33:33.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1964" PostTypeId="5" CreationDate="2013-09-15T01:38:00.363" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-15T01:38:00.363" LastActivityDate="2013-09-15T01:38:00.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1965" PostTypeId="4" CreationDate="2013-09-15T01:38:00.363" Score="0" Body="Questions regarding spacecraft, projects, or agencies of or relating to America." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-15T01:42:07.660" LastActivityDate="2013-09-15T01:42:07.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1966" PostTypeId="5" CreationDate="2013-09-15T03:25:34.473" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-15T03:25:34.473" LastActivityDate="2013-09-15T03:25:34.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1967" PostTypeId="4" CreationDate="2013-09-15T03:25:34.473" Score="0" Body="In aeronautics, lift is a component of aerodynamic force that acts perpendicular to drag, and balances the weight of the aircraft against the effects of gravity to achieve flight. In astronautics however, the lift component of a flight of the spacecraft is a force directly negating and greater of the effects of gravity and atmospheric drag on the body of the spacecraft as it gains altitude, a distance between itself and the body it is lifting off from." OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-15T06:32:32.900" LastActivityDate="2013-09-15T06:32:32.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1968" PostTypeId="1" AcceptedAnswerId="1976" CreationDate="2013-09-15T13:49:57.383" Score="9" ViewCount="1258" Body="&lt;p&gt;This &lt;a href=&quot;http://www.en.wikipedia.org/wiki/Lightening&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; article states that &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rocket exhaust plumes provided a pathway for lightning when it was witnessed striking the Apollo 12 rocket shortly after takeoff.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;My question is whether this was an accident or whether there were any valid reasons behind that incident &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-15T18:56:41.937" LastActivityDate="2014-12-19T14:50:02.177" Title="Do rocket exhaust plumes really provide a pathway for lightning?" Tags="|launch|rockets|lightning|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1969" PostTypeId="5" CreationDate="2013-09-15T15:00:53.713" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-15T15:00:53.713" LastActivityDate="2013-09-15T15:00:53.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1970" PostTypeId="4" CreationDate="2013-09-15T15:00:53.713" Score="0" Body="Questions regarding the design or process of design of a spacecraft. " OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-15T15:21:53.647" LastActivityDate="2013-09-15T15:21:53.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1971" PostTypeId="1" CreationDate="2013-09-15T15:23:02.413" Score="8" ViewCount="448" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Race&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...The Space Race sparked unprecedented increases in spending on education and pure research, which accelerated scientific advancements and led to beneficial spin-off technologies. ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A few such technologies may be&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Meteorology&lt;/li&gt;&#xA;&lt;li&gt;Remote Sensing&lt;/li&gt;&#xA;&lt;li&gt;Communications&lt;/li&gt;&#xA;&lt;li&gt;Science Missions&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ISS&lt;/li&gt;&#xA;&lt;li&gt;Hubble&lt;/li&gt;&#xA;&lt;li&gt;DSN&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Perhaps communications, and remote sensing Satellites form the bulk of any commercial launch payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early days few organizations/nations were capable of deploying to orbit. As of 2013 several nations may contract to deliver a payload to orbit ; USA, Russia, India, France, Japan to name a few. Similarly Lockheed, Virgin Galactic, SpaceX, Sea Launch form a small sample of private organizations headed into orbit - if not deep space, or transplanet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a virtual smörgåsbord of options to choose from, these few factors came to mind that may be relevant &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Track Record of the launcher&lt;/li&gt;&#xA;&lt;li&gt;Cost per kilogram to orbit&lt;/li&gt;&#xA;&lt;li&gt;Insurance/Liability in the event of failure to fulfil commitment&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Questions:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does cost to orbit make/break the sale for a satellite launch service? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="25" LastEditDate="2013-09-15T19:50:28.553" LastActivityDate="2013-09-15T19:50:28.553" Title="Does cost to orbit make the sale for a satellite launch service?" Tags="|launch|rockets|commercial|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1972" PostTypeId="5" CreationDate="2013-09-15T15:33:50.380" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-15T15:33:50.380" LastActivityDate="2013-09-15T15:33:50.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1973" PostTypeId="4" CreationDate="2013-09-15T15:33:50.380" Score="0" Body="Questions regarding death, cessation of all biological functions sustaining a living being, that are related to space exploration that can happen in space, on a space station, a spacecraft, or any ground based space operations." OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-15T16:29:14.300" LastActivityDate="2013-09-15T16:29:14.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1974" PostTypeId="5" CreationDate="2013-09-15T15:45:47.073" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-15T15:45:47.073" LastActivityDate="2013-09-15T15:45:47.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1975" PostTypeId="4" CreationDate="2013-09-15T15:45:47.073" Score="0" Body="Questions about meteoroids, small solid extraterrestrial bodies travelling through interplanetary space. Meteoroids that enter the Earth's atmosphere are called meteors, and remains of those that don't entirely burn or explode in the atmosphere and hit the ground are called meteorites." OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-15T16:32:59.300" LastActivityDate="2013-09-15T16:32:59.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1976" PostTypeId="2" ParentId="1968" CreationDate="2013-09-15T16:19:09.173" Score="9" Body="&lt;p&gt;It's a bit unclear what the question is, but I think it might be whether the lightning merely chose a path along the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V&lt;/a&gt;'s plume by chance, or did it facilitate the electric discharge by providing a grounded, more conductive pathway than its surroundings normally would?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is the question, then I would have to say the latter to be the cause of the path lightnings took in hitting the ground and causing the main lightning bolt's electrical discharge. I said lightnings (plural), because there were actually two lightnings that discharged through the Saturn V's body during the flight itself, which also traveled towards the ground hitting the crane and the platform of the mobile launcher, so that mission was really unfortunate in this sense.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The launch was made into a threatening gray sky with ominous cumulus&#xA;  clouds. &lt;a href=&quot;https://en.wikipedia.org/wiki/Pete_Conrad&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pete Conrad&lt;/a&gt;'s words 43 seconds after liftoff, electrified&#xA;  everyone in the Control Center: &lt;em&gt;&quot;We had a whole bunch of buses drop&#xA;  out&quot;&lt;/em&gt;, followed by &lt;em&gt;&quot;Where are we going?&quot;&lt;/em&gt; and &lt;em&gt;&quot;I just lost the&#xA;  platform.&quot;&lt;/em&gt; The spacecraft had been struck by lightning. Warning lights&#xA;  were illuminated, and the spacecraft guidance system lost its attitude&#xA;  reference.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/wQZ4D.jpg&quot; alt=&quot;Apollo 12 mobile launcher platform lightning&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Quote and photograph source: &lt;a href=&quot;http://history.nasa.gov/SP-350/ch-7-3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA History Program Office&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The way lightnings travel through the atmosphere during normal circumstances is a rather complex process of finding best conductive conduit from the negatively charged &lt;em&gt;source&lt;/em&gt; towards the positively charged &lt;em&gt;sink&lt;/em&gt;. Since the invention of extremely high speed cameras, we've been fortunate enough to study this process that, for the most part, happens roughly in one second in great detail. I suggest checking some videos explaining how the lightning forms in slow motion, for example &lt;a href=&quot;https://www.youtube.com/watch?v=W9xzU0xjlhE&quot; rel=&quot;nofollow noreferrer&quot;&gt;this YouTube video&lt;/a&gt; should serve the purpose well enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lightning is usually oriented cloud to ground, with the cloud having a negative electrical charge, and the ground having a positive one, but in some cases this charge can also be reversed. Before the main discharge can happen though, the lightning will form a conductive conduit from source towards the sink by forming many smaller lightning bolts in seemingly random direction and size towards the sink. They are not exactly random, but that is irrelevant to the question. These smaller &lt;em&gt;arm&lt;/em&gt; (or is it a tentacle?) bolts will discharge a smaller part of the total charge of the lightning, but enough to charge the surrounding air and form a &lt;a href=&quot;https://en.wikipedia.org/wiki/Plasma_%28physics%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;plasma&lt;/a&gt; conduit. Once this conduit reaches the sink, the full discharge happens that is usually a lot stronger, provided of course, the total charge didn't dissipate in the atmosphere during the forming of the conductive plasma conduit already. This is somewhat liberal simplification of the lightning discharge process, but let's for the time being say that the lightning will try and find the path of least &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance&quot; rel=&quot;nofollow noreferrer&quot;&gt;electrical resistance&lt;/a&gt;, and we'll apply this later on to our &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_12&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 12&lt;/a&gt; from the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll assume for the Apollo 12 lightning events to have been caused by the way more frequent negatively charged storm cloud discharging towards the ground, but as far as the plume conductivity goes, this doesn't even matter and would work both ways. This direction is also indicated by the description of the event in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_12&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page on Apollo 12&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thirty-six-and-a-half seconds after lift-off, the vehicle triggered a&#xA;  lightning discharge through itself and down to the earth through the&#xA;  Saturn's ionized plume. Protective circuits on the fuel cells in the&#xA;  Service Module (SM) falsely detected overloads and took all three fuel&#xA;  cells offline, along with much of the Command/Service Module (CSM)&#xA;  instrumentation. A second strike at 52 seconds after launch knocked&#xA;  out the &quot;8-ball&quot; attitude indicator. The telemetry stream at Mission&#xA;  Control was garbled. However, the vehicle continued to fly correctly;&#xA;  the strikes had not affected the Saturn V Instrument Unit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We have previously established, that the lightning's main discharge bolt will take the path of least &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrical_resistance_and_conductance&quot; rel=&quot;nofollow noreferrer&quot;&gt;electrical resistance&lt;/a&gt; during its &lt;em&gt;conduit search&lt;/em&gt;, starting from the source and discharging into the sink, that is usually the ground. Now, if we introduced along its path something that is already a pretty good electrical conductor, like for example a metallic rocket's body or also a good and grounded conductor like the mobile launch platform, this would save the lightning a lot of &lt;em&gt;effort&lt;/em&gt; into establishing its conduit and simply reuse what's already there and continue from then on towards the sink. But what's this got to do with the rocket's plume? Two things really.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, both lightning events were within the Saturn V's first stage burn (thanks to &lt;a href=&quot;https://space.stackexchange.com/users/429/anthony-x&quot;&gt;@AnthonyX&lt;/a&gt; for pointing this out in the comments), for which 5 of the &lt;a href=&quot;http://www.astronautix.com/engines/f1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocketdyne F-1&lt;/a&gt; engines were used, using &lt;a href=&quot;http://www.astronautix.com/lvs/satoxrp1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lox/RP-1&lt;/a&gt; propellant formulation (liquid diatomic oxygen or O&lt;sub&gt;2&lt;/sub&gt; as oxidizer and &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Propellant-1&lt;/a&gt;, a highly refined &lt;a href=&quot;https://en.wikipedia.org/wiki/Kerosene&quot; rel=&quot;nofollow noreferrer&quot;&gt;kerosene&lt;/a&gt; as fuel) with an oxidizer to fuel ratio of 1:2.27.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrocarbon&quot; rel=&quot;nofollow noreferrer&quot;&gt;hydrocarbon&lt;/a&gt;-based fuel, which is what kerosene is, would produce carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;), carbon monoxide (CO), hydrocarbon (HC), and oxides of nitrogen (NO&lt;sub&gt;x&lt;/sub&gt;) in the exhaust plume. While there wouldn't be much water vapor forming due to spare hydrocarbon hydrogen reacting with the oxidizer and atmospheric oxygen into water (H&lt;sub&gt;2&lt;/sub&gt;O) molecules, the weather conditions provided for raised humidity levels and with it environmental water added to the mix. Pure water itself isn't much of an electrical conductor (pure and deionized water has a conductivity of about 5.5 μS/m, &lt;a href=&quot;https://en.wikipedia.org/wiki/Siemens_%28unit%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;microsiemens&lt;/a&gt; per meter, which might be up to 10 times less than your average tap water due to additives), the rocket's exhaust gasses would provide additives into the mix of these by now vaporized water molecules within and surrounding the plume, and dramatically improve its electrical conductivity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that's not the whole story. This plume is also expelled through the rocket engine's nozzle at supersonic speeds and, of course, extremely hot. Velocity and temperature of the rocket's exhaust gases through the atmosphere are big enough for the exhaust and atmospheric molecules to highly &lt;a href=&quot;https://en.wikipedia.org/wiki/Ionization&quot; rel=&quot;nofollow noreferrer&quot;&gt;ionize&lt;/a&gt; and form a shroud of &lt;a href=&quot;https://en.wikipedia.org/wiki/Plasma_%28physics%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;plasma&lt;/a&gt; surrounding the exhaust plume. Plasma is of course exactly that conduit the lightning would form with its &lt;em&gt;arm&lt;/em&gt; bolts that we mentioned earlier, explaining how the lightning will try to find the most electrically conductive path from the source towards its sink where it discharges. So this plasma plume is what the lightning's main discharge bolt would reuse as well, on top of the rocket's metallic body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's why the two lightnings that hit Apollo 12 during flight through the lower atmosphere and on a cloudy day &lt;em&gt;found&lt;/em&gt; a path through the Saturn V's metallic body, through the rocket's exhaust plume, and eventually hitting the mobile launcher platform's mostly metallic construction sticking high above ground, before discharging into the earth. It was the pathway of the least electrical resistance, and the plume is hot and fast enough at the exhaust to ionize and form water vapor plasma at the edges of the plume that is an excellent electrical conductor in comparison to the surrounding air. Even if it was heavily raining, it would still be not much different than taking a long &lt;a href=&quot;https://en.wikipedia.org/wiki/Lightning_rod&quot; rel=&quot;nofollow noreferrer&quot;&gt;lightning rod&lt;/a&gt; and poking the storm cloud with it. Which is, in a way, what Saturn V on the Apollo 12 mission did.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good that they had Pete Conrad onboard, he was known to find humor in nearly all circumstances.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-12-19T14:50:02.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1977" PostTypeId="1" CreationDate="2013-09-15T16:38:17.287" Score="10" ViewCount="166" Body="&lt;p&gt;Since the automobile came into existence towards the end of the 19th century AD, motive technology has evolved rapidly &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Multiple Stroke Engines&lt;/li&gt;&#xA;&lt;li&gt;Rotary engines&lt;/li&gt;&#xA;&lt;li&gt;Square block&lt;/li&gt;&#xA;&lt;li&gt;Vee engines&lt;/li&gt;&#xA;&lt;li&gt;Hybrid engines&lt;/li&gt;&#xA;&lt;li&gt;Electric motors&lt;/li&gt;&#xA;&lt;li&gt;Fuel type  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Space technology has seen almost 60 years since 1957. It dates back still further ;  Goddard's patents for Solid/Liquid fuel rocket technology, Multi-stage launch ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the milestones in the evolution of space &lt;em&gt;launch&lt;/em&gt; engine/process technology for achieving LEO from 1957 till 2013?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-09-17T20:02:22.197" LastActivityDate="2013-11-21T13:57:46.953" Title="Milestones in LEO launch technology" Tags="|history|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1978" PostTypeId="1" CreationDate="2013-09-15T18:12:15.160" Score="5" ViewCount="1521" Body="&lt;p&gt;In the last 10 years what was the Highest and lowest, construction cost of satellites launched?  What are the mean and average costs of satellite construction over the same period?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where &quot;a&quot; Satellite is a single device capable of communicating with Earth or other satellites and is physically separated from other bodies in orbit by design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where cost and launch numbers are calculated based on both figures being available. &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2013-09-15T18:12:15.160" Title="What is the average cost of satellite construction over the last 10 years?" Tags="|launch|artificial-satellite|cost|" AnswerCount="0" CommentCount="5" ClosedDate="2013-09-17T10:18:09.427" ContentLicense="CC BY-SA 3.0" />
  <row Id="1979" PostTypeId="2" ParentId="1900" CreationDate="2013-09-15T19:12:35.510" Score="6" Body="&lt;p&gt;Kepler has four reaction wheels positioned in such a way that the attitude control system is tolerant to &lt;strong&gt;one&lt;/strong&gt; wheel failure (i.e. if one wheel fails, all three axes are still controllable). However, two of Kepler's wheels have failed, and so the need for off-nominal operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ball Aerospace, the prime contractor for Kepler, performed a preliminary study on two-wheel operations, the results of which are part of the &lt;a href=&quot;http://keplergo.arc.nasa.gov/docs/Kepler-2wheels-call-1.pdf&quot; rel=&quot;noreferrer&quot;&gt;call for white papers&lt;/a&gt; for alternative missions. &lt;a href=&quot;http://keplergo.arc.nasa.gov/docs/Kepler-2-Wheel-pointing-performance.pdf&quot; rel=&quot;noreferrer&quot;&gt;This supplement&lt;/a&gt; does a good job of pulling out the most relevant points:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;3) The reaction wheels can maintain pointing for 4 days while absorbing solar torque momentum in the Y &amp;amp; Z axes. This establishes a 4-day cycle for managing wheel momentum, although more frequent management is allowable.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;4) Momentum in the X axis must be absorbed through spacecraft roll. Low drift about the boresight limits the sun to the XY-plane.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;5) Maximum pointing stability is achieved when the spacecraft is pointed in the velocity- or anti-velocity-vectors, where the sun remains in the XY-plane throughout the 4-day period.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here is the body axis system they're talking about (from the call for white papers):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/YzRtB.png&quot; alt=&quot;Kepler axes&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So basically, any torque in the X axis will need to be absorbed. Therefore, to minimize the amount of roll, you want to balance the torque in that axis (i.e. keep the sun in the XY-plane, since solar radiation pressure is the main source of attitude disturbance). In order to do that, they recommend that the pointing target is in the ecliptic plane, since the sun essentially remains in that XY-plane for the entire imaging period (up to 4 days). This plot from the supplement shows the drift due to solar radiation pressure over a 4 day period:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/C0HwW.png&quot; alt=&quot;roll drift plot&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, they expect relatively little drift in the roll axis with the right attitude. The paper has plots like this for other attitude profiles, and they exhibit a much faster drift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combine the &quot;sun in the XY-plane&quot; requirement with the fact that you need sun on the solar arrays, and you have your optimal attitude defined as along either the velocity vector or the anti-velocity vector, both of which are in the ecliptic plane.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-09-15T19:12:35.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1980" PostTypeId="2" ParentId="1971" CreationDate="2013-09-15T19:45:30.950" Score="8" Body="&lt;p&gt;There are a number of factors when choosing who to launch your satellites. Some of the key factors are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Cost&lt;/li&gt;&#xA;&lt;li&gt;Time schedule (Will they keep their schedule to build your rocket?&lt;/li&gt;&#xA;&lt;li&gt;Payload capacity (Both size and weight)&lt;/li&gt;&#xA;&lt;li&gt;How closely they can get you to your desired orbit.&lt;/li&gt;&#xA;&lt;li&gt;ITAR or other export restrictions (It's easier to launch from the US if your satellite was built in the US)&lt;/li&gt;&#xA;&lt;li&gt;Track record (New rockets, or recently re-designed rockets, are usually cheaper for a few flights). This factors significantly in to the insurance cost as well.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Of these, you first select the rockets that meet your orbital insertion requirements for your mass, and then you pick either cost or launch schedule, depending on which is more important depends for the company or organization. I won't give any specifics away, but there are some launch providers which are known to have a lower cost, but you aren't guaranteed the launch window you want, while others have a higher cost, but they will launch closer to your launch day. The rockets launched with the cheaper rocket launch service tend to be small companies, whereas the ones launched more reliably tend to be larger organizations.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-15T19:45:30.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1981" PostTypeId="1" CreationDate="2013-09-15T20:29:33.093" Score="7" ViewCount="342" Body="&lt;p&gt;&lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html&quot;&gt;NASA&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As the electrical power becomes less and less, power loads on the spacecraft must be turned off in order to avoid having demand exceed supply. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A lead battery would 'die' &lt;strong&gt;if demand were to exceed supply&lt;/strong&gt; for any sustained duration ; What happens to an RTG in the same situation? &lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-09-15T21:28:38.850" Title="What happens to an RTG if the demand exceeds supply?" Tags="|voyager|rtg|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1982" PostTypeId="5" CreationDate="2013-09-15T21:07:49.560" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-15T21:07:49.560" LastActivityDate="2013-09-15T21:07:49.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1983" PostTypeId="4" CreationDate="2013-09-15T21:07:49.560" Score="0" Body="Space sickness is a form of motion sickness that occurs when the vestibular system makes feel the move of the body, but movement can't be seen by eyes. Question with this tag are related to space traveling aspects and involvements of this condition." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-15T21:07:49.560" LastActivityDate="2013-09-15T21:07:49.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1984" PostTypeId="5" CreationDate="2013-09-15T21:11:16.640" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-15T21:11:16.640" LastActivityDate="2013-09-15T21:11:16.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1985" PostTypeId="4" CreationDate="2013-09-15T21:11:16.640" Score="0" Body="Questions pertaining to natural satellites of planetary objects." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-15T21:11:16.640" LastActivityDate="2013-09-15T21:11:16.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1986" PostTypeId="2" ParentId="1981" CreationDate="2013-09-15T21:28:38.850" Score="14" Body="&lt;p&gt;As you demand more current, the voltage goes down.  You eventually brown-out the system.  The RTG doesn't care.  If you then reduce the load, the voltage goes back up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You cannot drain an RTG like a battery.  Its power output depends only on its radioactivity (which goes down over time), the efficiency of the thermocouples (which also goes down a little over time), and the temperature of the sink.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-09-15T21:28:38.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="1987" PostTypeId="1" AcceptedAnswerId="1992" CreationDate="2013-09-16T07:44:42.127" Score="12" ViewCount="667" Body="&lt;p&gt;What is the &quot;payload mass&quot; curve in relation to &quot;cost to orbit?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean: preparing a whole launch system for delivering a single cubesat to LEO would be an overkill, the cost to bring such a small payload up there would be excessive comparing to benefits. It's much better to bundle several smaller satellites in a single launch. On the other hand, ISS was assembled from parts on the orbit. While the shape might be unwieldy to launch as a whole, there was no fundamental reason other than economical (be it originating from technology development costs or from sheer material/fuel costs) to launch all parts in one flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where's the &quot;sweet spot&quot; of cost/weight ratio at bringing payload to orbit? What factors create the lower and upper bounds that make both too small and too big payloads not viable? Why there's no orbital supertruck or small private-owned cubesat launchpads?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-09-16T16:02:35.897" Title="Payload mass scale economy" Tags="|launch|economics|cost-to-orbit|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1988" PostTypeId="2" ParentId="1987" CreationDate="2013-09-16T15:25:38.513" Score="7" Body="&lt;p&gt;The incremental cost of a launch service in $/kg goes down as the vehicle size goes up.  So just based on that, you'd want the largest launch vehicle you can build.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However this is balanced by the overhead costs of maintaining the launch capability.  You need to maintain the launch site, the launch crews, the manufacturing capability, the supply chain for parts, etc.  This drives you in the direction of smaller launch vehicles to minimize the overhead costs amortized over the launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, you want a higher launch rate for reliability, though that's harder to factor into the costs, since insurance is usually not paid for by the launch provider, it's paid for by the payload provider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So ignoring for the moment what size payloads the customer needs, you do end up with a sweet spot in vehicle size based on the amount of business you have or expect to have.  Where that is depends on all the factors mentioned above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good example is the Delta II.  The Delta II had a steady stream of business driven by the launch of GPS satellites.  This allowed them to provide a very cost effective and highly reliable launch service that was used by many other customers, including NASA.  Mars Pathfinder, MGS, MPL, MCO, Stardust, Genesis, the MERs, Spitzer, and others were launched on the Delta II.  However when the GPS launches were done, the Delta II overhead costs didn't change much (they did try to reduce them), and as a result the cost of a single launch went up dramatically.  It was no longer cost effective, and NASA could no longer support maintaining the Delta II launch capability for their small number of launches.  NASA is a relatively small customer in the commercial market.  The Delta II was taken off the list of available launch vehicles for NASA missions.  NASA science missions now use mainly EELVs, whose overhead costs are subsidized by the Air Force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another good example may be SLS.  At a launch rate of once every two years, that vehicle seems too large to justify the overhead costs.  The resulting total cost per launch (not incremental) becomes, well, astronomical.  Also it is difficult to see how they would get to any level of acceptable, proven reliability with such a low launch rate.  Perhaps they are imagining a world with a much higher launch rate, but it is not clear where that funding would come from, either for the launch vehicles or the payloads.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-09-16T15:25:38.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1989" PostTypeId="1" AcceptedAnswerId="1991" CreationDate="2013-09-16T15:25:43.417" Score="30" ViewCount="59559" Body="&lt;p&gt;As a student of economics and finance I would love to learn more about the cost structures of sending a payload into orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More specifically -&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What is the cost-per-pound to send something into LEO for the handful of operating private and public space programs? &lt;/li&gt;&#xA;&lt;li&gt;How has the cost-per-pound to orbit changed over the years?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE&lt;/strong&gt;:&#xA;Has anyone come across newer costs?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2015-06-10T23:52:57.690" LastActivityDate="2015-10-02T16:01:36.993" Title="What is the current cost-per-pound to send something into LEO?" Tags="|launch|commercial|cost|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1990" PostTypeId="1" CreationDate="2013-09-16T15:36:24.123" Score="21" ViewCount="2580" Body="&lt;p&gt;The two primary documents regarding legal niceties on the Moon and often mentioned are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967&quot;&gt;Outer Space Treaty of 1967&lt;/a&gt;, which according to Wikipedia is essentially &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot;&gt;ratified by any country with a chance of getting there&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikisource.org/wiki/Moon_Treaty&quot;&gt;Moon Treaty&lt;/a&gt; which according to Wikipedia is &lt;strong&gt;NOT&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon_Treaty&quot;&gt;ratified by any country with a chance of getting there&lt;/a&gt; and such has negligible influence. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Outer Space Treaty of 1967, seems to prohibit land ownership (Article II; &quot;is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means.&quot;) and Article VI puts all non-governmental entities under the control and responsibility of their home country. This would seem to preclude land ownership.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Questions of a legal opinion are out of scope here. This question relates solely to the existence of other documents and treaties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any other documented agreements or treaties between a majority of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Human_spaceflight#National_spacefaring_attempts&quot;&gt;nations which have the technologies to travel into space&lt;/a&gt; which would allow for, or do not specifically prohibit owning land on the moon?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="38" LastEditDate="2013-09-16T16:48:40.783" LastActivityDate="2021-05-31T01:41:00.337" Title="Can you buy land on the moon?" Tags="|the-moon|law|" AnswerCount="5" CommentCount="19" ContentLicense="CC BY-SA 3.0" />
  <row Id="1991" PostTypeId="2" ParentId="1989" CreationDate="2013-09-16T15:41:57.097" Score="21" Body="&lt;p&gt;The cost depends on the rocket, and on the percent capacity of said rocket, but here's a few numbers for you, cost in U.S. dollars/kg to LEO (&lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;*). It should be noted that not all rocket prices are publicly available, in fact, most aren't.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon Heavy: \$2200&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 v 1.1- \$4,109&lt;/li&gt;&#xA;&lt;li&gt;DNEPR- \$3,784&lt;/li&gt;&#xA;&lt;li&gt;Ariane 5- \$10,476&lt;/li&gt;&#xA;&lt;li&gt;Delta IV- \$13,072&lt;/li&gt;&#xA;&lt;li&gt;Atlas V- \$13,182&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;*&amp;nbsp;Cost per kg to LEO column has since been removed from linked to Wikipedia page. Here is a link to the &lt;a href=&quot;https://en.wikipedia.org/w/index.php?title=Comparison_of_orbital_launch_systems&amp;amp;oldid=590182048&quot; rel=&quot;noreferrer&quot;&gt;latest archived version that still includes that column&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-02T16:01:36.993" LastActivityDate="2015-10-02T16:01:36.993" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="1992" PostTypeId="2" ParentId="1987" CreationDate="2013-09-16T16:02:35.897" Score="8" Body="&lt;p&gt;Some of the factors constraining launch vehicle size are the same as for any other technology-based product, namely fixed and incremental costs, the learning curve, time to market, and economies of scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A larger launch system will have larger fixed costs (not necessarily per unit of payload mass per launch). Gathering the funding for a larger research and development effort is more difficult, in part because the risk is higher (the payoff is likely over a longer period which increases the risk from inaccurate prediction and increases the required magnitude of the payoff by the compound interest effect).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Larger systems also have larger fixed costs with respect to manufacturing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a technology is relatively immature and has relatively limited demand (both from cost and lack of familiarity), fixed costs tend to dominate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With respect to the learning curve, since a larger launch system will have fewer launches, more payload will be attempted to be launched with a lesser understanding of the system. E.g., if the first two launches each have an 90% success probability, the next four 95%, the next sixteen 98%, then to launch 20 units of payload, a four-units per launch system will expect to require 5.37 launches, losing 0.37 vehicles and 1.5 units of payload while a single-unit system will expect to require 20.69 launches losing 0.69 vehicles and 0.69 units of payload. Just counting incremental costs, if the payload value is twice the single-unit system vehicle cost, then to break even at 20 units of payload launched the heavy lift vehicle must have an incremental cost only 3.56 times that of the single-unit vehicle (11% less expensive per unit of payload capacity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The learning curve also applies to manufacturing costs and even rocket efficiency (which can adjust payload size upward slightly or fuel costs down slightly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time to market interacts with funding and the learning curve. Being the first to begin making a profit (or even having a firm prospect of making a profit) reduces the cost of attracting capital (which funds additional R&amp;amp;D). Being the first to successfully launch will bring in more customers, increasing revenue (and profit) and increasing the number of launches. (A similar phenomenon for programming languages has been called &lt;a href=&quot;http://en.wikipedia.org/wiki/Worse_is_better&quot;&gt;&quot;Worse Is Better&quot;&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Economies of scale give a modest advantage to smaller systems beyond what would be expected from a simple model of the above factors. As volume increases, competition becomes more practical (increasing innovative pressure and opportunity [outsiders are more able/inclined to contribute] and reducing profit margins [but also risk]) and standards tend to develop which reduce production and design costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another factor favoring smaller systems is that multiple types of systems become more economical to run. If an unexpected failure mode grounds a particular type of system, other systems will be available for launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch systems have some specific features that constrain the minimization of payload size. The launch itself has costs associated with use of a given area that must be cleared as an aviation hazard and the ground area cleared for launch failure safety issues. The monitoring cost of a launch is also relatively fixed per launch rather than per unit of payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, a larger launch vehicle will tend to be somewhat more efficient due to area versus volume effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also limits on how many devices can be tracked, communicated with, and maintained in non-conflicting orbits, so there is some benefit to having a larger payload. Without in-space assembly, the size of a system that works effectively as a whole can be a limiting factor at the lower end of size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scheduling of launches is also a factor. With a multi-payload launch, all the cargo must be able to accommodate launch at the same time. With multiple launches, the delay of one launch would not necessarily impact the schedule of other launches. Furthermore, with more launches, there would likely be more launch sites, so localized effects like weather would have a reduced impact on launch capability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A multi-payload system would also tend to increase the number of failure modes and might make specialization for specific components less attractive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The economics of space launches is one of the arguments in favor of fuel depots, despite the fact that a significant fraction of cryogenic fuels would be lost due to heating while in orbit.)&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastActivityDate="2013-09-16T16:02:35.897" CommentCount="1" CommunityOwnedDate="2013-09-16T16:02:35.897" ContentLicense="CC BY-SA 3.0" />
  <row Id="1993" PostTypeId="2" ParentId="14" CreationDate="2013-09-16T18:51:07.713" Score="8" Body="&lt;p&gt;The technological solutions are still being tested, but it is possible.  NASA's &lt;a href=&quot;http://www.nasa.gov/exploration/analogs/hawaii_analog_2008_prt.htm&quot; rel=&quot;noreferrer&quot;&gt;ROxygen generator&lt;/a&gt; is being used in Hawaii on rocks that are similar to lunar dust which Erik points out contains oxygen.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/HGWhm.jpg&quot; alt=&quot;roxygen&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it doesn't produce enough for a crew of 4 to survive, it is a first step in developing a solution to a critical problem.&lt;/p&gt;&#xA;" OwnerUserId="446" LastActivityDate="2013-09-16T18:51:07.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1994" PostTypeId="1" CreationDate="2013-09-16T20:56:37.107" Score="8" ViewCount="462" Body="&lt;p&gt;Consider I have a 10 kg object I want to deliver from the ISS to &#xA;Tiangong-1 station. The object can not go through the &lt;em&gt;land and launch again&lt;/em&gt; cycle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can any of the present day spacecrafts do the delivery for me?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am okay with re-fueling on the ISS if possible, EVA if needed, installing alternative docking systems and other &quot;routine works&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's some info relating to the orbit of each:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Tiangong-1 42.78 inclination, 343x350 km orbit. &lt;/li&gt;&#xA;&lt;li&gt;ISS- 51.65 inclination, 413x418 km orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="12102" LastEditDate="2017-07-19T01:40:12.217" LastActivityDate="2017-07-19T01:40:12.217" Title="Cargo delivery from the ISS to Tiangong" Tags="|iss|orbital-maneuver|tiangong-1|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="1995" PostTypeId="2" ParentId="1994" CreationDate="2013-09-17T00:20:13.340" Score="11" Body="&lt;p&gt;Okay, first of all, let's assume that the two space stations are aligned properly. How often does this happen? The key thing is that each satellite will drift westward each day, due to what is known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Nodal_precession&quot; rel=&quot;noreferrer&quot;&gt;Nodal Precession&lt;/a&gt;. Wikipedia gives us an approximate formula:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$ \omega_{p} = -\frac{3}{2}\frac{R_E^2}{(a(1-e^2))^2} J_2 \omega \cos i $&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That gives each of them a precession value of (Using Heaven's Above values):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.heavens-above.com/orbit.aspx?satid=25544&quot; rel=&quot;noreferrer&quot;&gt;ISS&lt;/a&gt;- 4.98248196853&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.heavens-above.com/orbit.aspx?satid=37820&quot; rel=&quot;noreferrer&quot;&gt;Tiangong-1&lt;/a&gt; - 6.05226344588&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Thus, the relative differences are 1.06978147735 degrees. Thus, you can use the minimum solution once every 336 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so they line up properly, now what do you do? I don't have access to a simulator, so I'm going to break this in to two parts, the inclination change, and the raising of the orbit. In reality, these would be done together with an improved result, but it should give you an idea. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, let's assume that the change in eccentricity happens at the orbit of the ISS, as eccentricity changes are more effective in higher orbits. Also, let's assume a maximum efficiency transfer. Let's start with the harder of the two, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination_change&quot; rel=&quot;noreferrer&quot;&gt;inclination change&lt;/a&gt;. I'm going to use the simplified formula for circular orbits, as the orbit is almost so. Thus, it is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta{v_i}= {2v\, \sin \left(\frac{\Delta{i}}{2} \right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that, the delta v is 1.084 km/s. Likewise, we can calculate the delta V to raise or lower the orbit appropriately. The quick way is to assume twice the change in orbital velocity (Once to raise the apogee, once to raise the perigee). Using &lt;a href=&quot;http://www.stuegli.com/phyzx/calculators/calc-orbitvel.htm&quot; rel=&quot;noreferrer&quot;&gt;this calculator&lt;/a&gt;, I find the ISS velocity is 7662.5112 m/s, and Tiangong is 7694.277. Thus, there is a change in velocity of only about 70 m/s required to raise the orbit thus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the major requirement to raise the orbit is the inclination change. I'm confident that a delta v of 1.2 km/s would be enough to make the change, if you did it at the right time of the orbit. This is less than the delta v to get to the Moon, so there are plenty of rockets that can manage the change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the total momentum required to move the hypothetical 10 kg object between stations is 12 kN*s. Hydrazine, a very common fuel for on orbit purposes, has an ISP of about 220. Taking away the mass for fuel and engine, the Hydrazine required to do such a maneuver is only 5 kg. Virtually any reasonably sized spacecraft engine has that much maneuvering capability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there is also the added complexity of getting it exactly right, as is required in a rendezvous. The best bet would be something like the Dragon capsule, modified to make sure it had enough fuel. Any of the spacecraft currently used for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Unmanned_resupply_spacecraft&quot; rel=&quot;noreferrer&quot;&gt;ISS resupply&lt;/a&gt; could do that, possibly with slight modifications to increase the fuel load. In addition, given enough fuel, the Space Shuttle, Soyuz, and Apollo missions have demonstrated such capacity, although the first two would probably require more fuel than is typically done for a manned mission, they could be modified to have more fuel and run unmanned to make it happen.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-17T16:17:10.537" LastActivityDate="2013-09-17T16:17:10.537" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="1996" PostTypeId="1" AcceptedAnswerId="1998" CreationDate="2013-09-17T00:27:11.397" Score="10" ViewCount="560" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967&quot; rel=&quot;noreferrer&quot;&gt;Outer Space Treaty of 1967&lt;/a&gt;, according to Wikipedia is &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;noreferrer&quot;&gt;ratified by any country with a chance of getting there&lt;/a&gt; - Article VI Makes each member country responsible for it's citizens actions.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Article VI&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;States Parties to the Treaty shall bear international responsibility for national activities in outer space, including the moon and other celestial bodies, whether such activities are carried on by governmental agencies or by non-governmental entities, and for assuring that national activities are carried out in conformity with the provisions set forth in the present Treaty. The activities of non-governmental entities in outer space, including the moon and other celestial bodies, shall require authorization and continuing supervision by the appropriate State Party to the Treaty. When activities are carried on in outer space, including the moon and other celestial bodies, by an international organization, responsibility for compliance with this Treaty shall be borne both by the international organization and by the States Parties to the Treaty participating in such organization.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There are multiple opportunities, for non-citizens to ride with another nation to space.&lt;/p&gt;&#xA;&lt;p&gt;As of 2013, has a citizen of a country who has not signed and ratified the treaty, made it to orbit?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-17T18:56:12.320" Title="Has a non-space treaty citizen ever been in orbit?" Tags="|astronauts|law|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="1997" PostTypeId="1" AcceptedAnswerId="1999" CreationDate="2013-09-17T00:46:44.987" Score="18" ViewCount="943" Body="&lt;p&gt;Say we send a craft to Europa which would then launch a probe to land on the surface of Europa. Let's say for the sake of this question that the probe weighs more or less like the Opportunity rover. Given what we know about the moon's atmosphere and gravitational pull, how would a lander accomplish that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets like the moon landers? Parachutes like the Opportunity rover? Something else?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="49" LastEditDate="2015-04-22T23:20:24.830" LastActivityDate="2015-04-22T23:22:12.763" Title="How would a probe land on Europa?" Tags="|landing|europa|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="1998" PostTypeId="2" ParentId="1996" CreationDate="2013-09-17T01:21:16.363" Score="9" Body="&lt;p&gt;With some help from &lt;a href=&quot;http://en.wikipedia.org/wiki/Category%3aAstronauts_by_nationality&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, here's a few:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Category%3aIranian_astronauts&quot; rel=&quot;noreferrer&quot;&gt;Iran (Not ratified, but signed)&lt;/a&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Anousheh_Ansari&quot; rel=&quot;noreferrer&quot;&gt;Anousheh Ansari&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Category%3aLatvian_cosmonauts&quot; rel=&quot;noreferrer&quot;&gt;Latvia&lt;/a&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Oleg_Artemyev&quot; rel=&quot;noreferrer&quot;&gt;Oleg Atremyev&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Category%3aUzbekistani_cosmonauts&quot; rel=&quot;noreferrer&quot;&gt;Uzbekistan&lt;/a&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Vladimir_Dzhanibekov&quot; rel=&quot;noreferrer&quot;&gt;Vladimir Dzhanibekov&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line, two small former soviet nations, and Iran, who just didn't ratify it. But most of the astronauts are from countries which have ratified the treaty. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While these astronauts were citizens of non-ratifying countries, all were citizens of a ratifying country at the time of their launch (Dual citizenship).&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="109" LastEditDate="2013-09-17T18:56:12.320" LastActivityDate="2013-09-17T18:56:12.320" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="1999" PostTypeId="2" ParentId="1997" CreationDate="2013-09-17T02:50:15.577" Score="22" Body="&lt;p&gt;The &quot;atmosphere&quot; at $10^{-12}$ that of Earth's is entirely useless for landing.  It would simply be retrorockets as you would for our Moon.  Many types of landing gear could be considered, such as traditional legs, airbags, wheels a la skycrane, pallets, crushables, whatever.  But to get to that point at a survivable velocity, it's all about rockets.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-09-17T02:50:15.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2000" PostTypeId="5" CreationDate="2013-09-17T03:39:18.760" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-17T03:39:18.760" LastActivityDate="2013-09-17T03:39:18.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2001" PostTypeId="4" CreationDate="2013-09-17T03:39:18.760" Score="0" Body="Questions regarding space activities or services performed by private entities for commercial interests/the gain of money." OwnerDisplayName="user12" LastEditorUserId="40257" LastEditDate="2022-04-27T12:54:25.597" LastActivityDate="2022-04-27T12:54:25.597" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2002" PostTypeId="5" CreationDate="2013-09-17T03:42:43.840" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-17T03:42:43.840" LastActivityDate="2013-09-17T03:42:43.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2003" PostTypeId="4" CreationDate="2013-09-17T03:42:43.840" Score="0" Body="Questions regarding the amount of money required to send a mass into Earth orbit." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-17T06:51:10.400" LastActivityDate="2013-09-17T06:51:10.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2004" PostTypeId="1" CreationDate="2013-09-17T07:37:00.207" Score="6" ViewCount="427" Body="&lt;p&gt;Firstly, I want to say that I'd be the first person with their hand up when asked, &quot;So who wants to go to space?&quot;. There's nothing I'd love more than to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But from looking at the amount of Technology on the Curiosity rover it seems that we shouldn't bother using astronauts. Is it right to say that until mankind find/creates a habitable planet within our reach, shouldn't we just leave it to the robots to do the donkey work?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure in some cases a robot is even preferred. After all, the only thing that ages is their battery (which is solar powered) and they don't have to worry about things like breathing, mental disorders from prolonged isolation, biological issues from gravity changes or fear of the unknown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though a robot doesn't have intuition or common sense the person controlling it does. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why risk human lives when a robot can be made to do it instead?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So with all the above in mind:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;What can today's astronauts do better than the Curiosity rover?&lt;/h2&gt;&#xA;" OwnerUserId="703" LastEditorDisplayName="user12" LastEditDate="2013-11-28T01:03:15.373" LastActivityDate="2013-11-28T01:03:15.373" Title="What can an astronaut do better than the Curiosity rover?" Tags="|interstellar-travel|astronauts|rovers|instrument|" AnswerCount="0" CommentCount="7" ClosedDate="2013-09-17T09:04:35.593" ContentLicense="CC BY-SA 3.0" />
  <row Id="2005" PostTypeId="1" CreationDate="2013-09-17T08:15:01.057" Score="7" ViewCount="538" Body="&lt;p&gt;&lt;a href=&quot;http://www.space.com/22680-nasa-lunar-laser-communications-experiment-infographic.html&quot;&gt;Space.com&lt;/a&gt; writes to say (freeform improved quote):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;a href=&quot;http://esc.gsfc.nasa.gov/267/271.html&quot;&gt;LLCD&lt;/a&gt; (Lunar Laser Communication Demonstration) space terminal is designed to beam data from the moon at a rate six times faster than the best advanced radio communications systems available today. The system consists of a laser mount on the &lt;a href=&quot;http://www.nasa.gov/mission_pages/ladee/main/index.html&quot;&gt;LADEE&lt;/a&gt; (Lunar Atmosphere and Dust Environment Explorer) spacecraft which fires a 0.5-watt laser through a &lt;strong&gt;10-centimeter telescope...&lt;/strong&gt; The primary ground station, called the Lunar Lasercom Ground Terminal, is located in White Sands, New Mexico, near Las Cruces and features a set of &lt;strong&gt;four 40-centimeter telescopes&lt;/strong&gt; to receive the signal.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Optical_fiber&quot;&gt;Wikipedia&lt;/a&gt; indicates 4 db/km attenuation in optical fibres was achieved decades ago. In free space, the attenuation would be higher; which brings me to my questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How much power does the LLCD lose in transit from Lunar orbit to the ground station?&lt;/li&gt;&#xA;&lt;li&gt;Could the gas in the laser module freeze when in space?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2013-09-17T09:18:57.350" LastActivityDate="2013-09-17T18:03:53.053" Title="How much power loss does the LLCD suffer in transit?" Tags="|the-moon|artificial-satellite|communication|ladee|laser|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2006" PostTypeId="2" ParentId="2005" CreationDate="2013-09-17T12:46:24.027" Score="7" Body="&lt;p&gt;Pulling some info from the &lt;a href=&quot;http://www.nasa.gov/sites/default/files/llcdfactsheet.final_.web_.pdf&quot;&gt;Fact Sheet&lt;/a&gt; and &lt;a href=&quot;http://www.lexellaser.com/techinfo_wavelengths.htm&quot;&gt;this website&lt;/a&gt;. The formula, from &lt;a href=&quot;https://en.wikipedia.org/wiki/Gaussian_beam#Beam_width_or_spot_size&quot;&gt;Wikipedia&lt;/a&gt;, is &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\theta \simeq \frac{\lambda}{\pi w_0} \qquad (\theta \mathrm{\ in\ radians}). $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm going to assume $W_0$ is the aperture width provided. That gives a beam width, assuming 1.1 um wavelength, of about 3.5 uRads&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, at that value at the distance of the Earth will give a spot size of about 2.5 km, or an area of 4,900,000 m^2. The total aperture on Earth is about 0.5 m^2. The difference between them gives a maximum path loss of 70 dB. There will be some atmospheric path loss as well, and no doubt the system isn't quite this ideal, but I would expect the total path loss to be around 70-80 dB.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-17T18:03:53.053" LastActivityDate="2013-09-17T18:03:53.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2007" PostTypeId="5" CreationDate="2013-09-17T15:18:44.663" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-17T15:18:44.663" LastActivityDate="2013-09-17T15:18:44.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2008" PostTypeId="4" CreationDate="2013-09-17T15:18:44.663" Score="0" Body="Questions regarding devices which emit focused beams of electromagnetic energy." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-24T14:34:19.757" LastActivityDate="2013-09-24T14:34:19.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2009" PostTypeId="1" AcceptedAnswerId="38141" CreationDate="2013-09-17T18:57:24.453" Score="12" ViewCount="666" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Sultan_bin_Salman_bin_Abdulaziz_Al_Saud&quot; rel=&quot;noreferrer&quot;&gt;Prince Sultan bin Salman Al Saud&lt;/a&gt; was the first (and so far only) Saudi astronaut, and was included in the crew of &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-51-G&quot; rel=&quot;noreferrer&quot;&gt;STS-51-G&lt;/a&gt; as a payload specialist representing &lt;a href=&quot;http://en.wikipedia.org/wiki/Arab_Satellite_Communications_Organization&quot; rel=&quot;noreferrer&quot;&gt;Arabsat&lt;/a&gt;. Does that mean his role was strictly honorary and he had no explicit function within the mission (somewhat like the Teacher in Space Project)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In essence, what were his duties aboard the mission as a payload specialist?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="6944" LastEditDate="2019-08-15T02:09:50.550" LastActivityDate="2021-11-09T18:01:00.453" Title="What were Prince Sultan's duties in STS-51-G?" Tags="|space-shuttle|astronauts|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2010" PostTypeId="2" ParentId="646" CreationDate="2013-09-17T19:32:34.707" Score="19" Body="&lt;p&gt;Shameless plug for &lt;a href=&quot;https://github.com/tudat&quot; rel=&quot;noreferrer&quot;&gt;Tudat (TU Delft Astrodynamics Toolbox)&lt;/a&gt;...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're looking for something that allows you a lot of freedom to set up and play with simulations, you might want to consider an open-source C++ project I've been working on for the last few years as part of my PhD. Most of the graduate students in my group use it, so a lot of effort has gone into it.&lt;/p&gt;&#xA;" OwnerUserId="720" LastEditorUserId="10671" LastEditDate="2017-07-03T02:52:13.400" LastActivityDate="2017-07-03T02:52:13.400" CommentCount="4" CommunityOwnedDate="2013-09-17T19:32:34.707" ContentLicense="CC BY-SA 3.0" />
  <row Id="2011" PostTypeId="2" ParentId="2009" CreationDate="2013-09-17T20:46:18.673" Score="5" Body="&lt;p&gt;I would stay away from terms and sayings such as &quot;honorary&quot; and &quot;no explicit function&quot; -- especially when you are talking about Christa McAuliffe.  It's insulting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said that, there is a history of sending astronauts into space for indirect purposes.  Early astronauts were really not required for the mission other than to demonstrate survivability -- far from an &quot;honorary&quot; role...  McAuliffe's purpose was perfectly in line with the overall mission of NASA to build a scientific/technical American workforce by inspiring school children.  While the appropriateness of that mission for a government agency is subject to debate, her role in it is not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, if you are going to create a list like this, you probably should include &lt;a href=&quot;http://en.wikipedia.org/wiki/Jake_Garn&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jake Garn&lt;/a&gt;.  Even he managed to provide a data point on space sickness...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to answer your question directly, no.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2019-08-17T00:46:39.113" LastActivityDate="2019-08-17T00:46:39.113" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="2012" PostTypeId="2" ParentId="1667" CreationDate="2013-09-18T00:46:53.983" Score="10" Body="&lt;p&gt;Okay, here's the general break down in Solids vs Liquids debate, with a bit of help from &lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket_propellant&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, &lt;a href=&quot;http://www.esa.int/Education/Solid_and_liquid_fuel_rockets&quot; rel=&quot;noreferrer&quot;&gt;ESA&lt;/a&gt;, and &lt;a href=&quot;http://history.nasa.gov/conghand/propelnt.htm&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Solids&lt;/strong&gt;- Solid fueled rockets must be manufactured in a controlled environment. If not constructed properly, they can have impurities resulting in uncontrolled expansions (Explosions). They are safer generally speaking than Liquids, and don't require any special precautions. The on pad requirements are simpler. Most model rockets are solid fuel, and in fact, all they really need is a source of ignition, plus verification of the equipment. They do not have the ability to throttle on demand, although they can be built in such a way to throttle according to a pre-determined rate. They can be stopped at any time by ejecting the solid core. The rocket requirements around the engine are rather loose, and there aren't really any moving parts, aside from a usually throttleable nozzle and fins for atmospheric use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Liquids&lt;/strong&gt; Most liquid rockets are either cryogenic or toxic, requiring at least some component to be loaded at the rocket pad. They are throttleable upon demand, allow for more flexibility in the event of an issue, and can be stopped and restarted. Usually it is not safe to be near a fueled liquid rocket, usually requiring evacuation of the pad prior to fueling. This fueling requires extra work in the last minutes. Liquid rockets are generally speaking more complex, but usually offer better ISPs than solids. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so when you are wanting to launch a rocket to orbit, why would you want one vs the other?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Solid&lt;/strong&gt;- Usually slightly cheaper and easier to use, usually less massive overall. Could allow for quicker re-use of the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Liquid&lt;/strong&gt; More flexible in the event of an issue, allows for a broader range of missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using solid rockets for the first stages could improve the capabilities, as the Space Shuttle demonstrated, but the upper stage should be liquid or at least Hyrbid, or else you won't get the accuracy that is desired.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Vibration&lt;/strong&gt; Liquid fueled rockets burn at a very continuous rate, and thus have relatively few vibrations. Slight impurities in the solid fuel can cause the burn rate to vary, causing a higher level of vibrations. In particular, trying to get fine course adjustments with a solid fueled rocket is very difficult, causing extra vibrations in the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Toxicity&lt;/strong&gt; Most liquid rockets exhaust is either water or other natural chemicals, resulting in minimum damage from their exhaust. Solid Fueled rockets have slightly more exotic exhaust, which will cause more damage than water, but still are relatively benign. Bottom line here, it depends on which type of fuel you use, there are good and bad options for the environment among each. Liquids tend to be more extreme in the good/bad than solids, which tend to be more consistently middle-of-the-road.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-18T00:46:53.983" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2013" PostTypeId="2" ParentId="646" CreationDate="2013-09-18T01:31:09.223" Score="7" Body="&lt;p&gt;Here's a few other things out there as well depending on what you're looking for...&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;WEB&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;While not a simulator for orbital mechanics, I found this &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/index.php&quot; rel=&quot;noreferrer&quot;&gt;Trajectory Browser&lt;/a&gt; from Nasa to be interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More game-like is the &lt;a href=&quot;http://www.colorado.edu/physics/2000/applets/satellites.html&quot; rel=&quot;noreferrer&quot;&gt;LEO launcher app&lt;/a&gt; and the &lt;a href=&quot;http://mdj.dk/project/rocket-launch-simulator/&quot; rel=&quot;noreferrer&quot;&gt;launch simulator&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's the &lt;a href=&quot;http://space.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;JPL 3d simulator&lt;/a&gt; and the &lt;a href=&quot;http://neo.jpl.nasa.gov/orbits/&quot; rel=&quot;noreferrer&quot;&gt;Near-Earth-Object Simulator&lt;/a&gt; (both web based).  There is also a &lt;a href=&quot;http://ssd.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;JPL SSD simulator&lt;/a&gt; and here's some &lt;a href=&quot;http://mangsbatpage.433rd.com/2009/04/orbits-intro-to-nasas-orbit-simulator.html&quot; rel=&quot;noreferrer&quot;&gt;quick start instructions&lt;/a&gt;.  Like so:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/f6DLS.gif&quot; alt=&quot;system&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;*nix&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;For *nix (linux, unix) systems there is also the &lt;a href=&quot;http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicerone/Cicerone_Obs_Sim/orbit_simulation.html&quot; rel=&quot;noreferrer&quot;&gt;FERMI toolset&lt;/a&gt; with an &lt;a href=&quot;http://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/overview.html&quot; rel=&quot;noreferrer&quot;&gt;overview here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Windows-PC&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Popular and Free game is &lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/&quot; rel=&quot;noreferrer&quot;&gt;orbital simulator&lt;/a&gt; in 3-d mentioned by deer hunter.&lt;/p&gt;&#xA;" OwnerUserId="446" LastActivityDate="2013-09-18T01:31:09.223" CommentCount="0" CommunityOwnedDate="2013-09-18T01:31:09.223" ContentLicense="CC BY-SA 3.0" />
  <row Id="2014" PostTypeId="1" CreationDate="2013-09-18T01:45:44.773" Score="9" ViewCount="11546" Body="&lt;p&gt;Typically a cadence of one resupply mission every couple months is maintained. If this cadence were to be interrupted for whatever reason, how long could, say, the &lt;a href=&quot;http://www.howmanypeopleareinspacerightnow.com/&quot;&gt;current crew size&lt;/a&gt; of three members last onboard before needing to abandon ship due to lack of supplies? Have we ever come close to this limit?&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2017-03-30T10:37:12.777" Title="What's the maximum amount of time the ISS can go between resupply runs?" Tags="|iss|operations|supply|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2015" PostTypeId="1" AcceptedAnswerId="2018" CreationDate="2013-09-18T02:23:21.443" Score="17" ViewCount="37234" Body="&lt;p&gt;I was reading &lt;a href=&quot;https://space.stackexchange.com/questions/1989/what-is-the-current-cost-per-pound-to-send-something-into-leo&quot;&gt;this article&lt;/a&gt; and thought it was reasonably cheap, probably because I mistook LEO for GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the going rate to get something into GeoStationary Orbit and Geosynchronous Equatorial Orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do these prices exist?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-10-16T05:19:36.353" Title="What is the current cost per kg to send something into GSO/GEO?" Tags="|geosynchronous|geostationary|cost-to-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2016" PostTypeId="2" ParentId="2014" CreationDate="2013-09-18T02:35:31.520" Score="10" Body="&lt;p&gt;ISS operations plans re-boosts to have &lt;em&gt;roughly&lt;/em&gt; 90 days before the vehicle gets too low.  This doesn't mean that re-supply missions are 90 days apart since the ISS is often boosted much higher than this minimum level -- especially post-Shuttle.  But, it gives you a rough idea of the timeframes involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that other consumable margins are larger than this since there is always a Soyuz seat for each crew member.  This is especially true if you reduce the crew size to extend the supplies.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-09-18T02:35:31.520" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2017" PostTypeId="1" AcceptedAnswerId="2019" CreationDate="2013-09-18T07:05:53.023" Score="33" ViewCount="6614" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ChA3P.jpg&quot; alt=&quot;source Wikipedia- shuttle main engine firing&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Eg2vN.jpg&quot; alt=&quot;source Wikipedia -J2 engine&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do the exhaust flames from cryogenic stage engines appear to be separated from the nozzle?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-18T08:02:20.367" LastActivityDate="2017-04-17T07:27:45.353" Title="Why do the exhaust flames from cryogenic stage engines appear to be separated from the nozzle?" Tags="|rockets|fuel|engines|nozzle|burn|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2018" PostTypeId="2" ParentId="2015" CreationDate="2013-09-18T07:44:23.330" Score="12" Body="&lt;p&gt;We have a similar question in &lt;a href=&quot;https://space.stackexchange.com/q/1989/49&quot;&gt;What is the current cost-per-pound to send something into LEO?&lt;/a&gt; While launching to a &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low Earth Orbit&lt;/a&gt; (LEO) is of course expected to be a lot cheaper per kilogram than lifting same mass to a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Geosynchronous Orbit&lt;/a&gt; (GSO) or a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Geostationary Orbit&lt;/a&gt; (GEO), be it because of an individual GSO/GEO capable launcher's payload capacity to LEO being much larger than that to GSO/GEO due to smaller &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta-V&lt;/a&gt; requirements to achieve it and the cost per kilogram spreading over a theoretically much larger total payload mass, or due to LEO launchers being simpler and more cost effective, when altogether different launchers are used, that mentioned question might still prove relevant in the future, when the launch prices are updated to include new, yet to be developped launch systems. So think of it as future-proofing, and whichever thread updates faster will probably have more up to date information available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer your question however, some launch prices are available to the general public as some launch systems are commercial in nature, while others might be partially commercial or the costs of their space services completely funded out of budgets of various countries. So this information will be a subject to availability for various reasons. But there are some launch services that disclose the cost to GSO/GEO per launching system and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page on Comparison of orbital launch systems&lt;/a&gt; currently lists a single price per kilogram:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.ulalaunch.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;United Launch Alliance&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas V&lt;/a&gt; 541 at US\$ 27,063 per kg to GTO&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Comparing it with &lt;a href=&quot;https://space.stackexchange.com/a/1991/49&quot;&gt;prices per kilogram to LEO&lt;/a&gt; given in the @PearsonArtPhoto's answer, this seems to suggest a roughly double and higher the price to &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Geostationary Transfer Orbit&lt;/a&gt; (GTO) than to LEO. GTO is of course merely a transfer orbit on the way to GSO/GEO, so a large part of this mass will be your payload's thrusters, which is a dead weight once your destination orbit is achieved. Comparing GTO to GEO capabilities of most of the still operational orbital launchers comes down to a factor of roughly 2:1. So in short, your cost per kg to GEO will be roughly double that of the cost per kg to GTO, but as it depends on many variables and data we don't have, I'll use GTO to calculate the estimated price to GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Direct comparison with LEO launchers is a bit tricky though, as there are of course many more launchers available to LEO, they might be a lot simpler by design with fewer and/or more reliable stages, and smaller SRBs (Solid Rocket Boosters) strapped to their first stages. But let's try to infer from that Wikipedia page what other GSO/GEO capable launchers' prices might be, regardless. Adding &lt;s&gt;20%&lt;/s&gt; 10% (using Atlas 401 to 541 calculated difference of 6%, but let's round that up for good measure since it's a guesswork) to the cost to LEO approximating more powerful and expensive SRBs or additional upper stages needed on same launch systems to reach GSO/GEO (yes, this is blatant oversimplification, but should be close enough), we'd get:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas V 401&lt;/a&gt;: &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;$\left(\frac{13,812\ \mathrm{US\$}/\mathrm{kg}_{LEO}\ * 9,050\ \mathrm{kg}_{LEO}}{4,950\ \mathrm{kg}_{GTO}}\right) + 10\% = 27,777\ \mathrm{US\$}/\mathrm{kg}_{GTO}$&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta IV Heavy&lt;/a&gt;: $\left(\frac{13,072\ \mathrm{US\$}/\mathrm{kg}_{LEO}\ * 22,950\ \mathrm{kg}_{LEO}}{12,980\ \mathrm{kg}_{GTO}}\right) + 10\% = 25,424\ \mathrm{US\$}/\mathrm{kg}_{GTO}$&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5 ECA&lt;/a&gt;: &amp;nbsp;&amp;nbsp;$\left(\frac{10,476\ \mathrm{US\$}/\mathrm{kg}_{LEO}\ * 21,000\ \mathrm{kg}_{LEO}}{10,050\ \mathrm{kg}_{GTO}}\right) + 10\% = 24,079\ \mathrm{US\$}/\mathrm{kg}_{GTO}$&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5 ES&lt;/a&gt;: &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;$\left(\frac{10,476\ \mathrm{US\$}/\mathrm{kg}_{LEO}\ * 21,000\ \mathrm{kg}_{LEO}}{8,000\ \mathrm{kg}_{GTO}}\right) + 10\% = 30,249\ \mathrm{US\$}/\mathrm{kg}_{GTO}$&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Proton-M&quot; rel=&quot;nofollow noreferrer&quot;&gt;Proton-M&lt;/a&gt;: &amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;$\left(\frac{4,302\ \mathrm{US\$}/\mathrm{kg}_{LEO}\ * 21,600\ \mathrm{kg}_{LEO}}{6,150\ \mathrm{kg}_{GTO}}\right) + 10\% = 16,620\ \mathrm{US\$}/\mathrm{kg}_{GTO}$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And so on and you get your average price per kg to GTO at $\approx$ \$US 25,000. I'll repeat again though, these are &lt;em&gt;guesstimates&lt;/em&gt; for GTO, not GEO (you should roughly double the price to GEO per kilogram, depending on your configuration, as described before), and shouldn't be used for anything else than to compare price per kg to LEO to a ballpark price of GEO at $\approx$ \$US 50,000 per kg.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-10-31T13:46:15.493" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2019" PostTypeId="2" ParentId="2017" CreationDate="2013-09-18T12:35:52.057" Score="13" Body="&lt;p&gt;It's because of black body radiation; as the temperature increases the maximum light wavelength gets shifted to ultraviolet (near blue) regions which humans cannot see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.sbuv.com/Hydrogen/&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hydrogen flame, like alcohol flame emits little visible radiation but yet emits UV radiation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Flame&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With increasing oxygen supply, less black body-radiating soot is produced due to a more complete combustion and the reaction creates enough energy to excite and ionize gas molecules in the flame, leading to a blue appearance. The spectrum of a premixed (complete combustion) butane flame on the right shows that the blue color arises specifically due to emission of excited molecular radicals in the flame, which emit most of their light well below ~565 nanometers in the blue and green regions of the visible spectrum.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorUserId="6816" LastEditDate="2017-02-22T20:03:49.883" LastActivityDate="2017-02-22T20:03:49.883" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2020" PostTypeId="2" ParentId="2017" CreationDate="2013-09-18T12:46:59.297" Score="27" Body="&lt;p&gt;They might appear as the flame is detached from the nozzle, but that's in essence merely an illusion and the burn is there, all over the plume of the nozzle exhaust. It is however nearly translucent due to high purity of cryogenic propellants and by the chemical reaction producing molecules having high translucency. Visibility of the plume, unless you're launching at night, has actually little to do with its &lt;em&gt;burn&lt;/em&gt;. The &lt;em&gt;burn&lt;/em&gt; itself, as the propellant components react (fuel with the oxidizer) will be mostly in the non-visible spectrum with propellants that don't use additives / fuel impurities to shift the heat radiation to the visible spectrum. You have answered yourself why the flame appears invisible to the naked eye, but you didn't mention why it appears to be separated from the engine's nozzle, as per your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both images you attached to your question, judging by the color of the exhaust plume and light &lt;a href=&quot;https://en.wikipedia.org/wiki/Refraction&quot; rel=&quot;noreferrer&quot;&gt;refraction&lt;/a&gt;, use &lt;a href=&quot;http://www.astronautix.com/props/loxlh2.htm&quot; rel=&quot;noreferrer&quot;&gt;LOX/LH2&lt;/a&gt; or liquid diatomic oxygen O&lt;sub&gt;2&lt;/sub&gt; and liquid diatomic hydrogen H&lt;sub&gt;2&lt;/sub&gt; in 1:2 ratio as their oxidizer + fuel propellant components, producing superheated but pure water H&lt;sub&gt;2&lt;/sub&gt;O at the nozzle exhaust, and unless these plumes combine with atmospheric or propellant impurities, they will appear nearly translucent, especially given the pressure they're ejected from the nozzle injectors into the expansion chamber, where they combine. If you look closely at the images you attached, you'll however still notice a somewhat typical light refraction of water at the test facility railings behind them, more pronounced on the top photograph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, so you're probably asking yourself now, what about the more white &lt;em&gt;core&lt;/em&gt; of the exhaust plume? That is simply an area of negative pressure that forms different shapes depending on the nozzle size, shape, and of course the fuel injection rate. With &lt;a href=&quot;https://space.stackexchange.com/a/1190/49&quot;&gt;some nozzles&lt;/a&gt;, much of this can be adjusted depending on the current performance requirements of the engine, so its position might vary, or it might even repeat itself more frequently, as for example on this image of the alcohol fueled &lt;a href=&quot;https://en.wikipedia.org/wiki/Armadillo_Aerospace#Quad_vehicle&quot; rel=&quot;noreferrer&quot;&gt;Armadillo Aerospace's quad vehicle&lt;/a&gt; showing visible banding, or &lt;em&gt;shock diamonds&lt;/em&gt;, in the exhaust plume:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/QSMwo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short though, the areas of negative (back-) pressure will cause this banding and additional walls forming within the plume. With molecules high refraction index, the light will scatter at these edges in a zigzag pattern (more likely much more complex with vortices forming actually) and block any direct view of the background. To rather leave this explanation to &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_engine#Plume_physics&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Plume physics&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_plume&quot; rel=&quot;noreferrer&quot;&gt;Rocket plume&lt;/a&gt; varies depending on the rocket engine, design altitude,&#xA;  altitude, thrust and other factors.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Carbon rich exhausts from kerosene fuels are often orange in color due&#xA;  to the black body radiation of the unburned particles, in addition to&#xA;  the blue &lt;a href=&quot;https://en.wikipedia.org/wiki/Swan_band&quot; rel=&quot;noreferrer&quot;&gt;Swan bands&lt;/a&gt;. &lt;a href=&quot;https://en.wikipedia.org/wiki/High_test_peroxide&quot; rel=&quot;noreferrer&quot;&gt;Peroxide&lt;/a&gt; oxidiser based rockets and hydrogen&#xA;  rocket plumes contain largely &lt;a href=&quot;https://en.wikipedia.org/wiki/Steam&quot; rel=&quot;noreferrer&quot;&gt;steam&lt;/a&gt; and are nearly invisible to the&#xA;  naked eye but shine brightly in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ultraviolet&quot; rel=&quot;noreferrer&quot;&gt;ultraviolet&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Infrared&quot; rel=&quot;noreferrer&quot;&gt;infrared&lt;/a&gt;. Plumes&#xA;  from &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid_rocket&quot; rel=&quot;noreferrer&quot;&gt;solid rockets&lt;/a&gt; can be highly visible as the propellant frequently&#xA;  contains metals such as elemental &lt;a href=&quot;https://en.wikipedia.org/wiki/Aluminium&quot; rel=&quot;noreferrer&quot;&gt;aluminium&lt;/a&gt; which burns with an&#xA;  orange-white flame and adds energy to the combustion process.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Some exhausts, notably &lt;a href=&quot;https://en.wikipedia.org/wiki/Alcohol&quot; rel=&quot;noreferrer&quot;&gt;alcohol&lt;/a&gt; fueled rockets, can show visible &lt;a href=&quot;https://en.wikipedia.org/wiki/Shock_diamond&quot; rel=&quot;noreferrer&quot;&gt;shock&#xA;  diamonds&lt;/a&gt;. These are due to cyclic variations in the plume pressure&#xA;  relative to ambient creating shock waves that form 'Mach disks'.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The shape of the plume varies from the design altitude, at high&#xA;  altitude all rockets are grossly under-expanded, and a quite small&#xA;  percentage of exhaust gases actually end up expanding forwards.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In short, the answer to the question is not exactly in the &lt;em&gt;burn&lt;/em&gt; itself, since these bands, walls, and any other shapes that form within the plume as a result of a backpressure would do that just as well, if there wasn't any &lt;em&gt;burn&lt;/em&gt; to start with. It is due to light refraction and high pressure, high velocity ejecta forming walls travelling at the opposite direction each side of the wall (detached &lt;em&gt;burn&lt;/em&gt;), or because of the jets intersecting (diamonds).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-09-18T14:02:31.117" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2021" PostTypeId="1" AcceptedAnswerId="2022" CreationDate="2013-09-18T14:02:33.577" Score="12" ViewCount="1473" Body="&lt;p&gt;Following the parameters in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot;&gt;Wikipedia nuclear thermal rocket article&lt;/a&gt;, it seems to paint somewhat of a shaky world view.  Consider these quotes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Current (2010) 25,000 pound-thrust reference designs (NERVA-Derivative Rockets, or NDRs) are based on the Pewee, and have specific impulses of &lt;strong&gt;925 seconds&lt;/strong&gt;.[citation needed]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A solid-core nuclear thermal rocket's fuel elements are unlikely to spread over a wide area because the elements are designed to withstand very high temperatures (up to &lt;strong&gt;3500K&lt;/strong&gt;) and high pressures (up to 200 atm)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I seriously doubt any of these actual parameters are realistic, but that's not important, I just want to talk about them in an academic sense.  I think I understand the general idea of how a temperature can be turned into a specific impulse:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{3}{2} k T = \frac{1}{2} m v^2 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ I_{sp} = \frac{ \sqrt{  \frac{3 k T}{m}  } }{g} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I use the above temperature, I can reproduce their specific impulse.  &lt;a href=&quot;https://www.google.com/search?q=convert%20sqrt%283%2ak%2a%283500%20Kelvin%29/%281%20amu%29%29/%289.8%20m/s%5E2%29%20to%20s&quot;&gt;Like so&lt;/a&gt;, and Google gives 950 seconds.  I'm sure there are some other factors that could easily reduce that by 25.  But in order to get that, I had to plug in $m= 1 \text{ amu}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That clearly can't be right!  A nuclear thermal rocket heats cryogenic Hydrogen to produce Hydrogen &lt;em&gt;gas&lt;/em&gt;, a diatomic gas with the formula $H_2$, not $H_1$.  The molecular weight of the diatomic gas is obviously $2 \text{ amu}$, and there is no way to get that specific impulse (or anywhere close) using that mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what's going on here?  Did NASA engineers of the 60s demonstrate that heating of the Hydrogen gas would disassociate the molecule, or did some high school kid blindly plug numbers into the equation without thinking?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="4660" LastEditDate="2015-01-09T05:16:21.320" LastActivityDate="2015-01-09T05:16:21.320" Title="Nuclear thermal rocket specific impulse calculation uses 1 amu, is that wrong?" Tags="|fuel|nuclear|specific-impulse|ntr|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2022" PostTypeId="2" ParentId="2021" CreationDate="2013-09-18T14:58:46.623" Score="15" Body="&lt;p&gt;Yes&lt;/p&gt;&#xA;&#xA;&lt;p&gt;they considered the disassosciation of hydrogen&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;http://www.gizmag.com/nasa-nuclear-cryogenic-propulsion/25772/&quot;&gt;source&lt;/a&gt; :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Previous testing used a&#xA;  maximum temperature of 2,750° K,&#xA;  short of the 3000+° K design&#xA;  temperature for the NCPS.&#xA;  The NTREES facility is designed to&#xA;  test fuel elements and materials in hot&#xA;  flowing hydrogen, reaching pressures&#xA;  up to 1,000 pounds per square inch and&#xA;  temperatures of nearly 5,000° F&#xA;  (2,760° C) – conditions that simulate&#xA;  space-based nuclear propulsion systems&#xA;  to provide baseline data critical to the&#xA;  research team.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Under that conditions the hydrogen would disassociate into atomic hydrogen &lt;/p&gt;&#xA;&#xA;&lt;p&gt;external &lt;a href=&quot;http://www.infoplease.com/encyclopedia/science/dissociation.html&quot;&gt;source&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;at 5,000°K about 95% of the molecules in a sample of hydrogen are&#xA;  dissociated into atoms&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorUserId="265" LastEditDate="2013-09-18T21:36:08.407" LastActivityDate="2013-09-18T21:36:08.407" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2023" PostTypeId="5" CreationDate="2013-09-18T16:10:32.193" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-18T16:10:32.193" LastActivityDate="2013-09-18T16:10:32.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2024" PostTypeId="4" CreationDate="2013-09-18T16:10:32.193" Score="0" Body="Questions about specific impulse (Isp), which is a comparative measure of efficiency of rocket, jet engines and used propellants, measured either in seconds of thrust per unit weight of propellants, or effective exhaust velocity (thrust per unit mass of propellants) as a measure of launcher system's specific energy efficiency." OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-09-18T17:05:20.500" LastActivityDate="2013-09-18T17:05:20.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2025" PostTypeId="1" AcceptedAnswerId="2028" CreationDate="2013-09-18T16:16:14.330" Score="20" ViewCount="5796" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle&quot;&gt;Space Shuttle&lt;/a&gt; relied on a pair of &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot;&gt;SRB&lt;/a&gt;s, and fuel supplement in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_external_tank&quot;&gt;External Tank&lt;/a&gt; for it's launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming Payload, Service parts/consumables, Crews, SRBs, and ET were ready - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How long did it take, at a minimum, to prep a shuttle for launch after it landed? &lt;/li&gt;&#xA;&lt;li&gt;Were the shuttles capable of back-to-back missions?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2013-10-21T04:14:50.493" LastActivityDate="2018-03-18T17:36:41.763" Title="How long did it take, at a minimum, to prep a shuttle for launch after it landed?" Tags="|space-shuttle|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2026" PostTypeId="5" CreationDate="2013-09-18T16:21:53.290" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-18T16:21:53.290" LastActivityDate="2013-09-18T16:21:53.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2027" PostTypeId="4" CreationDate="2013-09-18T16:21:53.290" Score="0" Body="Questions regarding the rocket engine nozzles, semi-open compartments of various shapes and sizes into which propellant components are injected by their own pressure or with a help of a turbine, often preheated and ignited to start a thermochemical reaction in the nozzle's combustion chamber, producing thrust with expansion into gases and ejecting them through the nozzle's exhaust as plumes, accelerating the rocket to high-mach velocities." OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-18T16:38:04.077" LastActivityDate="2013-09-18T16:38:04.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2028" PostTypeId="2" ParentId="2025" CreationDate="2013-09-18T17:12:38.433" Score="25" Body="&lt;p&gt;It is hard to say what &lt;em&gt;might&lt;/em&gt; be possible, but we can certainly look at what was actually done.  The fastest turnaround time in the Program's history was 54 days by Atlantis in 1985.  I suspect that would not be possible with some of the new procedures put into place post-Challenger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems Columbia did it the fastest post-Challenger: 88 days.  I believe SSME inspection is the long-pole in this process.  Even if you just replace the SSMEs, you still have a lot to do.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="11721" LastEditDate="2018-01-03T16:22:37.397" LastActivityDate="2018-01-03T16:22:37.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2029" PostTypeId="5" CreationDate="2013-09-19T12:37:29.410" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-19T12:37:29.410" LastActivityDate="2013-09-19T12:37:29.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2030" PostTypeId="4" CreationDate="2013-09-19T12:37:29.410" Score="0" Body="Questions regarding Europa, a moon of Jupiter with an orbital period of 3.55 days, which has a thin atmosphere primarily of oxygen and whose surface is made up of ice. It is the smallest of the four Galilean satellites of Jupiter. " OwnerUserId="414" LastEditorUserId="10273" LastEditDate="2018-10-22T17:32:44.293" LastActivityDate="2018-10-22T17:32:44.293" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2031" PostTypeId="2" ParentId="1938" CreationDate="2013-09-19T15:47:33.620" Score="23" Body="&lt;p&gt;&lt;sub&gt;This is a community wiki. Please contribute any additional information you find. The following list is likely incomplete, so the total number of astronauts that follows is subject to change.&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;There are &lt;strong&gt;four&lt;/strong&gt; astronauts that have reported an encounter of the kind detailed in the question.&lt;/p&gt;&#xA;&lt;p&gt;In alphabetical order (by last name, then first name) they are:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Gordon Cooper&lt;/strong&gt; - a NASA astronaut and one of the original seven astronauts on Project Mercury&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;First Encounter - UFO sighting in the early 1950s, before his days as an astronaut&lt;/p&gt;&#xA;&lt;p&gt;According to a &lt;a href=&quot;http://web.archive.org/web/20100327122001/http:/www.space.com/sciencefiction/phenomena/cooper.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space.com article&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;On one occasion, I saw some strange vehicles that we assumed were UFOs,&amp;quot; he told Bell.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In the encounter, which took place over Germany in the early 1950s, Cooper saw &amp;quot;flights of fighters flying by in the same sort of formation we flew, moving east to west.&amp;quot; The U.S. Air Force base scrambled its own pilots, including Cooper, who remembers the objects as looking &amp;quot;just like saucers -- they were metallic looking, but we couldn't really get close enough to see more than that. You couldn't see any wings on them.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Second Encounter - UFO sighting in 1957, also before his days as an astronaut&lt;/p&gt;&#xA;&lt;p&gt;From the same Space.com article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;He also stood by his belief that he saw a UFO land at Edwards Air Force Base in California in 1957. Although Cooper had been filming the base with a camera crew at the time, the film, which he handed over to a high-ranking officer from Washington, has never emerged.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Pavel Popovich&lt;/strong&gt; - a Soviet cosmonaut, commander of Vostok 4 and Soyuz 14&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Sole Encounter - UFO sighting at an unspecified time&lt;/p&gt;&#xA;&lt;p&gt;Popovich was a UFO researcher for the Soviet government, and it is likely this account occurred during or after his work as a UFO researcher. The primary source for the account is the UFO documentary &lt;em&gt;Out of the Blue&lt;/em&gt;, and it is possible that his account may be misconstrued due to bias. However, without any record of Popovich contesting the presentation of his account in this documentary, this answer will tentatively assume it was accurately reported. Here are the English subtitles of his account in &lt;em&gt;Out of the Blue&lt;/em&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For some reason, I was compelled to look out the plane's window. I looked and my eyes grew real big. I saw next to us, about one and a half kilometers away, and about 1,000 meters higher, there is a flying object. Its shape is a perfect right triangle. It was white, white, a very bright white. I'm a pilot, a fighter pilot, and I know many different types of flying crafts. It was like nothing I've ever seen.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The &lt;em&gt;Out of the Blue&lt;/em&gt; narrator explains:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;[He was] flying home from Washington, D.C. with a delegation of scientists.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Pilots contacted ground control. Ground control did not detect anything on radar, even though the object could be seen by all occupants of the plane.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Popovich continues (English subtitles):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It travelled a parallel course. And its speed -- if we were travelling approximately 950 to 1,000 kph, it was travelling at about 1,500 kph, because it passed us rather quickly. In about 30 or 40 seconds it passed by, and overtook us. And that's all. And no academic, or anybody, could say what that object was.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Donald &amp;quot;Deke&amp;quot; Slayton&lt;/strong&gt; - a NASA astronaut and one of the Mercury Seven&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Sole Encounter - UFO sighting in 1951, before his days as an astronaut&lt;/p&gt;&#xA;&lt;p&gt;According to his autobiography &lt;a href=&quot;http://books.google.com/books?id=z_xE22Pp2P0C&amp;amp;pg=PA50&amp;amp;lpg=PA50&amp;amp;dq=%22Hynek%20contacted%20me%20once%20about%20my%20story%22%20Deke&amp;amp;source=bl&amp;amp;ots=FvJaAAel8N&amp;amp;sig=WqIQj59TK1ilsWwqF1G-Wg2OYds&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=ZNg5UtS_Ou6v4AO91oGgBQ&amp;amp;ved=0CCwQ6AEwAA#v=onepage&amp;amp;q=%22Hynek%20contacted%20me%20once%20about%20my%20story%22%20Deke&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Deke!&lt;/em&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It was when I was doing a maintenance test flight like this that I had my one and only encounter with an unidentified flying object.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;I was up about the middle of one afternoon--a nice sunny day--wringing out this particular 51. I had just come out of a spin at around ten thousand feet over the Mississippi River, near Prescott, where the Mississippi and the St. Croix meet, about twenty-five miles from the Twin Cities. I was heading back to Holman Field when all of a sudden I saw this white object about my altitude, at one o'clock.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;I didn't think anything about it. My first thought was that it looked like a kite. But logic said nobody's flying a kite at this altitude. So I started kind of watching to see what it was.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;I was closing on it, but I still didn't think too much about it. The closer I got, the more it looked like a weather balloon, and I'm thinking, that's what it's gotta be. Then I flew past it a little high, about a thousand feet off. It still looked like a three-foot-diameter weather balloon to me.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;My guess on the dimensions couldn't have been too far off. I had plenty of gas, so I figured I'd make a pass on it. Burn some gas and have a little fun. I pulled into a turn.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But when I came out of that turn and headed straight at it, all of a sudden it didn't look like a balloon anymore. It looked like a disk on edge!&#xA;I thought, that's strange. Then I realized I wasn't closing on that son of a bitch. A P-51 at that time would cruise at 280 miles an hour. But this thing just kept going and climbing at the same time at about a forty-five degree climb. I kept trying to follow it, but he just left me behind and flat disappeared.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;I wondered what that was, but I never saw it again. I turned around, headed back, and landed, and didn't tell anybody about it for two days. I was afraid they'd think I'd lost my mind.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A couple  of evenings later I was over in the O-club with my boss, a full colonel, and after I &lt;em&gt;[sic]&lt;/em&gt; couple of beers I thought I'd better tell him, and I did.&#xA;He said, &amp;quot;Get your ass over to Intelligence in the morning and give them a briefing.&amp;quot; So I did. They sat there and nodded and took notes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Then they told me: Just for your information, the day you saw this object a local company was flying high-altitude research balloons. They had a light airplane tracking it, and a station wagon on the ground. Both observers were watching this balloon and had seen this object come up beside the balloon. The object appeared to hover, then it took off like hell.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The guys on the ground tracked it with a theodolite, and they'd computed the speed at four thousand miles an hour.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Robert White&lt;/strong&gt; - a US Air Force astronaut who acheived suborbital spaceflight by X-15 on 17 July 1962&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Sole Encounter - UFO sighting in 1962&lt;/p&gt;&#xA;&lt;p&gt;From the book &lt;a href=&quot;http://books.google.com/books?id=jV_63sZBUg8C&amp;amp;pg=PA125&amp;amp;lpg=PA125&amp;amp;dq=time%20magazine%2027%20july%201962%20robert%20white&amp;amp;source=bl&amp;amp;ots=yZ8Fqn4nJ5&amp;amp;sig=wMRNbtYoct8MZXsHJRRNuSa7wuA&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=s9s5UoWeDtTF4AOOg4G4Cg&amp;amp;ved=0CEgQ6AEwBA#v=onepage&amp;amp;q=time%20magazine%2027%20july%201962%20robert%20white&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Beyond Belief&lt;/em&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;[T]est pilot Robert White is reported as exclaiming in flight 'There are things out there. There absolutely is'. This certainly sounds impressive. However, Good omits to describe what White was talking about. So I had to go to the reference (&lt;em&gt;Time&lt;/em&gt; magazine, 27 July 1962). This revealed that White was talking about an object looking 'like a piece of paper the size of my hand'. This is hardly strong evidence for UFOs...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;A previous version of this answer included this alleged encounter:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Frank Borman&lt;/strong&gt; - a NASA astronaut and commander of Gemini 7 and Apollo 8 missions&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Sole Encounter - UFO sighting in December 1965, in orbit during Gemini 7 mission&lt;/p&gt;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/GT07_061.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gemini 7 mission transcript&lt;/a&gt; (C and CC replaced with Borman and Capcom, respectively):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: Gemini VII here. Houston, how do you read?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Loud and clear, VII. Go ahead.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: I have a bogey at 10:00 o'clock high.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: This is Houston. Say again, VII.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: Said we have a bogey at 10:00 o'clock high.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Roger.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Gemini VII, is that the booster or is that an actual sighting?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Say again, VII.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: Said ... we have several - looks like ... actual sighting.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Do you have any more information, estimated distance, or size?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: We also have the booster in sight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Understand you also have the booster in sight. Roger.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: ... there are very many - looks like hundred of little particles going by from the left out about 3 or 4 miles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Understand you have many small particles going by on the left. At what distance?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: ... looks like ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Roger. Understand they're about 3 or 4 miles away?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: They're past now; they're in a polar orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Roger. And understand they were about 3 to 4 miles away?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: That's what it appeared like, or further.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Roger.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Gemini VII, Houston. Were these particles in addition to the booster and the bogey at 10:00 o'clock high?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Roger.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: Houston, this is VII.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Go ahead.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: I have the booster on my side and the brilliant body of the sun against a black background with billions of particles around it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Roger. What direction is it from you?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: It's about 2:00 o'clock position.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Does that mean it's ahead of you?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Borman: It's - ahead of us at 2:00 o'clock and it's slowly tumbling.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Capcom: Roger.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;Note: It is unclear whether Borman confirmed that the bogey was a separate entity from the particles.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;However, Borman later explained in a &lt;a href=&quot;https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/BormanF/Bormanff_4-13-99.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;1999 interview&lt;/a&gt; that he was referring to the second stage.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2020-08-05T15:27:01.137" LastActivityDate="2020-08-05T15:27:01.137" CommentCount="3" CommunityOwnedDate="2013-09-19T15:47:33.620" ContentLicense="CC BY-SA 4.0" />
  <row Id="2032" PostTypeId="1" CreationDate="2013-09-19T16:14:27.707" Score="8" ViewCount="1328" Body="&lt;p&gt;The way that the ISS manages it's attitude now is to use a set of reaction wheels for primary control, and occasionally firing small thrusters to allow the wheels to despin themselves. This isn't always a practical response, as &lt;strong&gt;it uses fuel&lt;/strong&gt; continually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way that many satellites manage to do this is with &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetorquers&quot; rel=&quot;nofollow&quot;&gt;Magnetorquers&lt;/a&gt;, which align themselves with the Earths Magnetic Field, albeit in an active way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I want to know is, &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Under what conditions would the management of the Space Station be too difficult using &lt;strong&gt;only&lt;/strong&gt; Magnetorquers? &lt;/li&gt;&#xA;&lt;li&gt;Could &lt;strong&gt;only&lt;/strong&gt; Magnetorquers be used to control it, and if only some of the time, when would the most difficult times to use them arise?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="288" LastEditDate="2013-09-20T17:12:16.727" LastActivityDate="2013-09-20T17:12:16.727" Title="Could Magnetorquers be used on the ISS?" Tags="|iss|attitude|magnetic-field|magnetorquer|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2033" PostTypeId="1" CreationDate="2013-09-19T19:55:33.540" Score="5" ViewCount="331" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_1&quot; rel=&quot;nofollow&quot;&gt;Vostok 1&lt;/a&gt; to date, &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry&quot; rel=&quot;nofollow&quot;&gt;atmospheric reentry&lt;/a&gt; has been essentially passive. Passive here means that retrorockets are used to de-orbit, and then &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobraking&quot; rel=&quot;nofollow&quot;&gt;aerobraking&lt;/a&gt; is provided by the atmosphere. In contrast, active reentry would mean reentry under engine power (which may lie somewhere in the future perhaps).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I understand it, this reduces the amount of fuel required to be carried for consumption only on the return journey. Indirectly, it allows more payload to be carried on the journey up &amp;mdash; thus good math.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any other advantages to a passive atmospheric reentry / aerobraking?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="6816" LastEditDate="2015-08-08T23:50:36.153" LastActivityDate="2019-05-03T13:25:02.547" Title="Any other advantages to passive atmospheric reentry besides reaction mass economy?" Tags="|reentry|atmosphere|aerobraking|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2034" PostTypeId="2" ParentId="2033" CreationDate="2013-09-19T21:38:11.090" Score="4" Body="&lt;p&gt;Depending on what you mean by &quot;passive,&quot; using the atmosphere to provide deceleration also provides larger cross range.  This means you have either more landing sites to select from or less time in orbit required to line up with any given landing site.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-09-19T21:38:11.090" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2035" PostTypeId="1" AcceptedAnswerId="2036" CreationDate="2013-09-19T22:05:37.707" Score="7" ViewCount="411" Body="&lt;p&gt;One way to align a spacecraft into a certain manner while in orbit around a body is the use a &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetorquer&quot;&gt;Magnetorquer&lt;/a&gt;. Does Mars have enough of a magnetic field to allow such a device to operate?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-09-24T17:46:06.920" Title="Could a magnetorquer be used in orbit around Mars?" Tags="|mars|magnetic-field|magnetorquer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2036" PostTypeId="2" ParentId="2035" CreationDate="2013-09-19T22:14:03.867" Score="5" Body="&lt;p&gt;&lt;strong&gt;No&lt;/strong&gt;, a magnetorquer would be useless in orbit around Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars has a surface field strength of maybe a few tens of nanoteslas. That's less of a magnetic field strength than a cassette tape. To make things worse, there isn't one single magnetic field, but a collection of weak remnant fields. Mars used to have a global magnetic field a few billion years ago, but today the remaining local fields barely even cover their relatively small areas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www-ssc.igpp.ucla.edu/personnel/russell/papers/mars_mag/&quot; rel=&quot;nofollow&quot;&gt;Mars: Magnetic Field and Magnetosphere - Luhmann and Russell&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Orders_of_magnitude_%28magnetic_field%29&quot; rel=&quot;nofollow&quot;&gt;Orders of magnitude (magnetic field) - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.exploremars.org/mls-picture-of-the-dayt18-days-maven&quot; rel=&quot;nofollow&quot;&gt;MLS Picture of hte Day:T+18 Days: MAVEN - ExploreMars&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-09-24T17:46:06.920" LastActivityDate="2013-09-24T17:46:06.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2037" PostTypeId="2" ParentId="2032" CreationDate="2013-09-19T22:15:44.537" Score="4" Body="&lt;p&gt;I can't give a definitive answer until I have numbers on how much torque that the devices can output, which I wasn't able to find on the Interwebs.&lt;/p&gt;&#xA;&lt;p&gt;However, &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetorquer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; paints a grim picture for the possiblity of their use on larger sattelites:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h2&gt;Disadvantages&lt;/h2&gt;&#xA;&lt;p&gt;The main disadvantage of magnetorquers is that &lt;strong&gt;very high magnetic flux densities would be needed if large craft had to be turned very fast&lt;/strong&gt;. This would either necessitate very high current in the coils, or &lt;strong&gt;much higher ambient flux densities than are available in Earth orbit&lt;/strong&gt;. Subsequently, the torques provided are very limited and &lt;strong&gt;only serve to accelerate or decelerate the change in a spacecraft's attitude by minute amounts&lt;/strong&gt;. Over time active control can produce very fast spinning even here, but for accurate attitude control and stabilization the torques provided often aren't enough.&lt;/p&gt;&#xA;&lt;p&gt;A broader disadvantage is the dependence on Earth's magnetic field strength, making this approach unsuitable for deep space missions, and also more suitable for low Earth orbits as opposed to higher ones like the geosynchronous. The dependence on the &lt;strong&gt;highly variable intensity of Earth's magnetic field is also problematic&lt;/strong&gt; because then the attitude control problem becomes highly nonlinear. It is also impossible to control attitude in all three axes even if the full three coils are used, since the torque can be generated only perpendicular to the Earth's magnetic field vector.&lt;/p&gt;&#xA;&lt;p&gt;Any spinning satellite made of a conductive material will lose rotational momentum in Earth's magnetic field due to generation of eddy currents in its body and the corresponding braking force proportional to its spin rate. Aerodynamic friction losses can also play a part. &lt;strong&gt;This means that the magnetorquer will have to be continuously operated, and at a power level which is enough to counter the resistive forces present&lt;/strong&gt;. This is not always possible within the energy constraints of the vessel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It's not a definitive 'no', but it certainly seems unlikely that the power requirements to keep such a large satellite as the ISS aligned in a certain way would be cheaper than the amount of propellent used to align the craft currently.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-19T22:15:44.537" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2038" PostTypeId="1" AcceptedAnswerId="2188" CreationDate="2013-09-19T23:04:27.820" Score="7" ViewCount="304" Body="&lt;p&gt;On Earth, small satellites often include &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetorquers&quot; rel=&quot;nofollow&quot;&gt;Magnetorquers&lt;/a&gt;. A magnetorquer allows the satellite to re-orient itself without using fuel in a way more permanent than magnets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While not every planet has a magnetic field as strong as Earth's, some do have one that is substantial. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Have Magnetorquers ever been used around another planet? &lt;/li&gt;&#xA;&lt;li&gt;Could Magnetorquers be so used?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2013-11-21T01:35:39.060" LastActivityDate="2013-11-21T01:35:39.060" Title="Have Magnetorquers been used anywhere outside of the Earth?" Tags="|attitude|magnetic-field|solar-system|magnetorquer|feasibility|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2039" PostTypeId="2" ParentId="2032" CreationDate="2013-09-20T01:43:44.417" Score="6" Body="&lt;p&gt;In addition to Undo's answer, I'll try to give a &lt;strong&gt;&quot;zeroth order&quot;, back of the envelope&lt;/strong&gt; analysis for this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, from my answer &lt;a href=&quot;https://space.stackexchange.com/a/1687/29&quot;&gt;here&lt;/a&gt;, we have $$T=DB=2DM/R^3$$ where $T$ is the torque applied by the torquer, $M$ is the magnetic moment of the Earth (about $7.96\times 10^{15}~\text{tesla}\cdot \text{m}^3$), $D$ is the dipole induced by your torquer, and $R$ is the distance from the center of Earth's dipole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, mechanics tells us that $$T=I\alpha$$ where $I$ is the moment of inertia and $\alpha$ is angular acceleration. We'll take $I_{xx}=127908568~\text{kg}\cdot\text{m}^2$ from &lt;a href=&quot;http://www.acsu.buffalo.edu/~johnc/bigsky03.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; for our purposes, and let's say you want to impart a modest angular rate of $0.05~^{\circ}/\text{sec}^2\approx 0.0009~\text{radians}/\text{sec}^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the magnetic dipole required in this case would be $$D=\frac{I\alpha R^3}{2M}=\frac{127908568\cdot 0.0009\cdot (6478*1000)^3}{2\cdot 7.96\times 10^{15}}\approx 1.966\times 10^9~\text{A}\cdot \text{m}^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a massive dipole. If we calculate the dipole moment of a wrapped coil with $N$ turns as $A\times I\times N$, and even assume a fairly massive 1 meter cross sectional area, and a generous 10 Amps of current, a torquer using 1 mil copper wire would be 1 meter long and have a layer of copper 13 cm thick. This would weigh about &lt;strong&gt;1200 kg&lt;/strong&gt;, which is pretty damn heavy. Consider this with Undo's more qualitative points, and it's starting to look like a bad idea, even if it's not physically impossible.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="-1" LastEditorDisplayName="user29" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-09-20T16:58:40.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2040" PostTypeId="1" CreationDate="2013-09-20T02:08:38.390" Score="8" ViewCount="240" Body="&lt;p&gt;Pretty good science has nailed down the fact that a significant fraction of asteroids have macro-porosity of 10% or more.  It seems this was more or less established as of a &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38610/1/03-2155.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;2003 paper&lt;/a&gt;, and &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0032063312000773&quot; rel=&quot;nofollow noreferrer&quot;&gt;here are some updated graphs in a 2012 paper&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding is that they establish the material density (including microporosity) from material characterization (including meteorites), and then get the mass of the body from other factors (like gravitational influence).  Since the average density (or &quot;bulk density&quot;) is found to be much lower, large empty spaces are predicted.  Words that have been used (in 2003 at least) to describe categories of low and high macroporosity are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;fractured&lt;/li&gt;&#xA;&lt;li&gt;rubble piles&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But what does that &lt;em&gt;mean&lt;/em&gt;?  &lt;a href=&quot;https://en.wikipedia.org/wiki/253_Mathilde&quot; rel=&quot;nofollow noreferrer&quot;&gt;253 Mathilde&lt;/a&gt; must be the poster child for a rubble pile, with a well-established macro-porosity of 50%, and it looks nothing like what I would colloquially call a rubble pile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/oJ0Ug.png&quot; alt=&quot;253 Mathilde&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Half of this is empty space?  How??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it filled with caves?  Are they connected?  Swiss cheese?  If we travel to such an asteroid, what kind of structures would we expect to encounter?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-12-27T14:17:38.293" Title="Theories of what asteroid macroporosity actually means" Tags="|asteroid|research|caves|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2041" PostTypeId="5" CreationDate="2013-09-20T03:23:30.797" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-20T03:23:30.797" LastActivityDate="2013-09-20T03:23:30.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2042" PostTypeId="4" CreationDate="2013-09-20T03:23:30.797" Score="0" Body="Questions regarding devices which use an external magnetic field to produce a torque, usually used to help align a spacecraft in a certain manner while in orbit." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-20T08:00:57.343" LastActivityDate="2013-09-20T08:00:57.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2043" PostTypeId="1" AcceptedAnswerId="48150" CreationDate="2013-09-20T03:37:29.040" Score="9" ViewCount="841" Body="&lt;p&gt;What is the smallest launch vehicle, by dry mass, to have delivered a payload to orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am guessing that this might be either a re-purposed sounding rocket or something from the high-end of the model rocketry space.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="8693" LastEditDate="2020-10-25T15:13:57.657" LastActivityDate="2020-10-25T16:50:15.327" Title="Smallest launch vehicle by dry mass" Tags="|launch-vehicle|record|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2044" PostTypeId="2" ParentId="2043" CreationDate="2013-09-20T05:30:11.680" Score="10" Body="&lt;p&gt;For dry mass, it appears to be &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Vanguard_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vanguard&lt;/a&gt; with a launch mass of 10,050 kg, and a dry mass of 1536 kg.&lt;/p&gt;&#xA;&lt;p&gt;At first I thought it was the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lambda_4S&quot; rel=&quot;nofollow noreferrer&quot;&gt;Japanese Lambda 4S&lt;/a&gt; weighing in at a lower 9400 kg at launch.  However it's &amp;quot;dry&amp;quot; mass (mass with the solid propellant subtracted) &lt;a href=&quot;http://www.isas.jaxa.jp/e/enterp/rockets/vehicles/l-4s/index.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;is 2408 kg&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="36698" LastEditDate="2020-10-25T15:53:25.550" LastActivityDate="2020-10-25T15:53:25.550" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="2045" PostTypeId="2" ParentId="2040" CreationDate="2013-09-20T09:41:50.433" Score="2" Body="&lt;p&gt;Correct me if I'm wrong, but I believe asteroid macroporosity is meant to represent the amount (usually a percentage of its total volume) of void space between separated or semi-attached pieces of asteroid's body, what we consider a single mass body but might in fact be a combination of several larger chunks of materials held together by their microgravity and low radial velocity (axial rotation), or finer grained materials, be it on its body or formed between the cracks at the time of the &lt;em&gt;impact&lt;/em&gt; (not necessarily a spectacular one, mind you) of one chunk of the asteroid into another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's why the word &lt;em&gt;rubble&lt;/em&gt; is used so frequently in such documents. If these individual chunks would be somehow embedded into one another, I guess the word &lt;a href=&quot;https://en.wikipedia.org/wiki/Conglomerate_%28geology%29&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;conglomerate&lt;/em&gt;&lt;/a&gt; would be more frequently used, but in geology, conglomerates are not necessarily highly porous, or not at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to rather leave this description to astrogeologists that study the geology of celestial bodies, here's an excerpt from the document on &lt;a href=&quot;http://www.lpi.usra.edu/meetings/metsoc99/pdf/5226.pdf&quot; rel=&quot;nofollow&quot;&gt;Asteroid rubble piles: How big are the pieces? (PDF)&lt;/a&gt; prepared by the Department of Geological Sciences, University of Tennessee, and Vatican Observatory, of the Vatican City State for their 62nd Annual Meteoritical Society Meeting:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Introduction&lt;/strong&gt;: Many lines of evidence lead to the conclusion that&#xA;  asteroids are strengthless bodies, piles of rubble held together only&#xA;  by their own self gravity. Among the indications leading to this&#xA;  conclusion are the observations that all asteroids spin slowly enough&#xA;  to be held together by their own self gravity; that near-Earth&#xA;  asteroids have odd shapes matching those predicted for strengthless&#xA;  bodies undergoing tidal distortion; and that the densities of&#xA;  asteroids are significantly lower than corresponding meteorite bulk&#xA;  densities. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This latter work allows one to put numerical estimates&#xA;  on the amount of macroporosity in asteroids. If S-type asteroids like&#xA;  Ida and Eros with a density on the order of 2.6 g/cm3 are made up of&#xA;  material similar to that in ordinary chondrites, then they must be at&#xA;  least 30% empty space. Given the 10% porosity typical of ordinary&#xA;  chondrites, these asteroids must have void spaces on a scale&#xA;  comparable to the meteorites representing another 20% of the volume of&#xA;  the asteroid. For C type objects like Mathilde, Phobos, and Deimos,&#xA;  the observed low density (1.5 g/cm3) implies a macroporosity of at&#xA;  least 20% if they are made of rocky material similar to low density&#xA;  hydrated carbonaceous chondrites. However, these dark asteroids are&#xA;  observed to be anhydrous; to make them out of water-free dark&#xA;  meteoritic material would require that they were 50% or more void&#xA;  space&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I believe this &lt;em&gt;Google-fu&lt;/em&gt; find describes well enough what macroporosity in asteroids is. So, sadly no Swiss cheese, or even grains of instant coffee. Just piles of rubble held together by various forces, and this macroporosity varying greatly from one celestial object to another. It is a percentage of void space between cracks, separate pieces held together by their own gravity and not perfectly matching with their joining sides, rubble and dust on their surface, etc. I.e. a measure of how solid/hollow an asteroid is, or its bulk, large-scale porosity&amp;nbsp;— macroporosity.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="1332" LastEditDate="2013-12-27T14:17:38.293" LastActivityDate="2013-12-27T14:17:38.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2046" PostTypeId="1" AcceptedAnswerId="2048" CreationDate="2013-09-20T11:49:25.293" Score="25" ViewCount="24172" Body="&lt;p&gt;Could someone explain me the mathematics behind the Delta-V charts one can find on internet such as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/nkeWa.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Delta-Vs for inner Solar System (Image converted from source &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Delta-Vs_for_inner_Solar_System.svg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia SVG&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How are they created?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say I would like to make an estimation of the delta-v required for putting an object on low earth orbit, how should I proceed in order to calculate that amount regardless of the rocket specifications or the payload? Is it even possible?&lt;/p&gt;&#xA;" OwnerUserId="732" LastEditorUserId="732" LastEditDate="2013-09-22T18:13:33.610" LastActivityDate="2019-02-14T13:25:41.557" Title="Delta-V chart mathematics" Tags="|orbital-mechanics|orbital-maneuver|mathematics|delta-v|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2047" PostTypeId="2" ParentId="646" CreationDate="2013-09-20T12:31:01.507" Score="3" Body="&lt;p&gt;You could try &lt;a href=&quot;http://www.stellarium.org/&quot; rel=&quot;nofollow&quot;&gt;Stellarium&lt;/a&gt; for locating most celestial objects from the earth frame. AFAIK, it works very well on Linux, and is available for OS X and Windows as well.&lt;/p&gt;&#xA;" OwnerUserId="733" LastEditorUserId="1225" LastEditDate="2013-12-08T11:48:48.393" LastActivityDate="2013-12-08T11:48:48.393" CommentCount="1" CommunityOwnedDate="2013-09-20T12:31:01.507" ContentLicense="CC BY-SA 3.0" />
  <row Id="2048" PostTypeId="2" ParentId="2046" CreationDate="2013-09-20T13:06:37.437" Score="24" Body="&lt;p&gt;This is a large question, but we can certainly boil it down.  You need several levels of requisite knowledge.  I'll break it down as so:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Relevance of Delta v for propellent budget&lt;/li&gt;&#xA;&lt;li&gt;Conversion between gravitational potential and its corresponding velocity&lt;/li&gt;&#xA;&lt;li&gt;The basic physics of Hohmann transfers&lt;/li&gt;&#xA;&lt;li&gt;Non-ideal factors going from surface to orbit&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Not all of these are fully necessary, so other people will take a different route to explaining it.  This sequence of points just reflects my own intuition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Propellent budget&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of a Delta-v map would be useless if it was not additive.  Think about it.  That's the entire point of a map.  If I can't add segments to calculate the total distance, then it's not a &quot;map&quot;.  But there is an interesting criticism of this point - that the &quot;distance&quot; on this map doesn't scale linearly with fuel needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We tend to think of fuel as proportional to distance driven.  But this is actually incorrect.  As your gas tank is full it cases more rolling friction with the road, so your car is more efficient with a nearly empty gas tank.  You and I neglect this because our energetic budget is small compared to the weight of the car in gas.  In rocketry, it matters to the extreme.  BUT, the calculation for the delta-v is still linear.  In that way, it is a strong mathematical analogy to distance traveled by car.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$     \Delta v = v_\text{e} \ln \frac {m_0} {m_1} $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the delta-v maps, you are counting &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;, and they faithfully add linearly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Gravitational potential&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From physics you should be familiar with the concept of &quot;potential&quot; as &lt;span class=&quot;math-container&quot;&gt;$GM/r$&lt;/span&gt;.  This has units of &lt;span class=&quot;math-container&quot;&gt;$m^2/s^2$&lt;/span&gt;.  In this form, you can apply energy balance.  If you think about energy conservation manifested in an equation, divide that equation by the mass of your test mass.  This is then the governing energy balance in a stationary reference frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we had a perfect reference frame, then we would apply all the energy balance equations with the naive gravitational potential above.  In other words, &lt;em&gt;that&lt;/em&gt; quantity would be additive.  But it's not because we care much more about the &lt;em&gt;rocket's&lt;/em&gt; reference frame than the Earth's or the sun's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the rocket's frame, units of &lt;span class=&quot;math-container&quot;&gt;$m/s$&lt;/span&gt; make a heck of a lot more sense.  Consider the situation:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A rocket lies at rest relative to the Earth in the upper atmosphere.  It fires its rockets toward the center of Earth, the burn finishes quickly, and imparts just enough momentum to get out of Earth's gravity well.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For this case, the problem is answerable with relative ease.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;energy balance:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \frac{1}{2} m v^2 = \frac{GM_E m}{R_E+(200 km)} $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;divide by m.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \frac{1}{2}  v^2 = \frac{GM_E }{R_E+(200 km)} $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the actual calculation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \Delta v = \sqrt{\frac{2 GM_E}{R_E+(200 km)}} = 10,925 \frac{m}{s} $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is an &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;noreferrer&quot;&gt;escape velocity&lt;/a&gt; calculation, and it is a very simple example of a delta v segment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Hohmann transfers&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In real life, of course we need to take the most efficient route possible.  It looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pZoRf.png&quot; alt=&quot;Hohmann transfer&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;You start in a circular orbit&lt;/li&gt;&#xA;&lt;li&gt;You burn to go into a elliptical &lt;em&gt;transfer&lt;/em&gt; orbit&lt;/li&gt;&#xA;&lt;li&gt;Once you reach the desired orbit, you burn again to make your path circular&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It never makes sense to &quot;stop along the way&quot;, and doing so will always cost you more propellant.  That might seem a little confusing to you considering the diagram, with many stop-points.  But those are basically transfer orbits, as in the above diagram, and then moving from one scale to another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that a Hohlmann transfer has two burns, you have to expressions for the Delta v.  Here they are with notation consistent with the above image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ &#xA;    \Delta v = \sqrt{\frac{GM}{R}} \left( \sqrt{\frac{2 R'}{R+R'}} - 1 \right),$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$    \Delta v' = \sqrt{\frac{GM}{R'}} \left( 1 - \sqrt{\frac{2 R}{R+R'}}\,\! \right) , $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These account for a large fraction of the numbers you see in the image.  These add together to get the ultimate requirement on your rocketry, so it makes sense to put them in a &quot;map&quot;.  For increase orbit from LEO to GEO, for instance, you could have 3 dots, and two segments, where first dot is LEO, second is the transfer orbit, and 3rd is GEO.  I think they didn't put this on the map (although they could have) because no one cares too much about LEO-to-GEO transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Non-ideal factors&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going from the surface of Earth to low Earth orbit includes other factors:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;gravity drag&lt;/li&gt;&#xA;&lt;li&gt;air resistance&lt;/li&gt;&#xA;&lt;li&gt;some additional minimum increase in elevation to avoid quick orbit decay&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These explain why going to LEO is about 10 km/s, instead of the &lt;em&gt;literal speed&lt;/em&gt; of LEO which is more like 7.9 km/s.  Gravity drag and elevation increase each contribute on the order of 1 km/s, so the final answer isn't surprising.  Not all bodies will have these same factors.  This is just an example of special considerations for that map.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also realize this answer isn't comprehensive.  It explains maybe half of the graph.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="29326" LastEditDate="2019-02-14T13:25:41.557" LastActivityDate="2019-02-14T13:25:41.557" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="2049" PostTypeId="2" ParentId="2040" CreationDate="2013-09-20T13:21:08.337" Score="9" Body="&lt;p&gt;This is an answer from email communication with the author of the 2012 paper.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The way we understand catastrophic collisions (= events that disrupt entirely the target body), the larger blocks will re-accrete first. They are the most massive, and will therefore have a) a lower ejection speed, b) have more gravity to be pulled together.&lt;/p&gt;&#xA;&lt;p&gt;So after the impact, you start first by gluing together the large blocks, with possible large voids between them. The smaller rock and dust fall back on the already rubble-pile structure, possibly generating smaller voids.&lt;/p&gt;&#xA;&lt;p&gt;Now, we could imagine that the small rocks and dust would fill in the voids... It's not obvious at all: 1) the deeper they enter into the structure the less gravity pull would they feel toward the center; 2) whatever happens, they need to be pulled stronger than the friction stops them from percolating inside.&lt;/p&gt;&#xA;&lt;p&gt;The (scarce) theoretical works on that point predict an internal structure where most of the void are deep inside, while the shell around is pretty-much packed. Unfortunately, we have absolutely NO measurements of mass distribution inside these bodies (e.g., Mathilde). It is definitively something we'd like to do on a future space mission to an asteroids (= orbit a high macroporosity asteroids and map its gravitational field as a proxy of 3D mass distribution inside).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Benoit Carry&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://benoit.carry.free.fr/&quot; rel=&quot;noreferrer&quot;&gt;http://benoit.carry.free.fr/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-20T13:21:08.337" CommentCount="0" CommunityOwnedDate="2013-09-20T13:21:08.337" ContentLicense="CC BY-SA 3.0" />
  <row Id="2050" PostTypeId="1" AcceptedAnswerId="2052" CreationDate="2013-09-20T15:33:29.857" Score="8" ViewCount="353" Body="&lt;p&gt;I was reading &lt;a href=&quot;http://www.bbc.co.uk/news/magazine-22904141&quot;&gt;Does gold come from outer space?&lt;em&gt;(BBC News)&lt;/em&gt;&lt;/a&gt;.  If the majority of gold on Earths surface does come from space would it tend to be equally distributed on comets and asteroids, or would tend to collect in pockets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I understand it, for the most part it seems that nuggets and pockets of gold on Earth tend to develop from erosion factors, it seems unlikely that these same effects would be present in Space, and that any catastrophic event (i.e. planetary destruction) would tend to disrupt any existing high density areas of gold.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any known process that would tend to create high density grouping of gold elements in space? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-10-08T18:22:32.687" LastActivityDate="2016-02-26T17:04:21.840" Title="Would gold in space tend to collect in pockets like it does on Earth?" Tags="|asteroid|mining|comet|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2051" PostTypeId="1" CreationDate="2013-09-20T16:19:14.077" Score="8" ViewCount="471" Body="&lt;p&gt;Spacecraft/robot are usually chock-a-block with instruments. These are usually devices with some electronics to them. These are probably semiconductor, or vacuum-tube devices - &lt;em&gt;I may be wrong&lt;/em&gt;! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Working in vacuum may be different from working in close proximity of a celestial. The operating profile (temperature, pressure, atmosphere ...) of an electronic device meant to work, say, on Luna may vary widely from that meant for, say, Io. This is unlike commercial satellites (which I believe) placed usually in Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are semiconductors ordered one-off for &lt;strong&gt;each&lt;/strong&gt; mission headed beyond Earth? &lt;/li&gt;&#xA;&lt;li&gt;How are these devices tested for compatibility to their field environment?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-09-20T17:37:44.617" LastActivityDate="2013-09-21T17:23:59.500" Title="Are custom semiconductors used on board spacecraft/robots?" Tags="|electronics|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2052" PostTypeId="2" ParentId="2050" CreationDate="2013-09-20T16:55:35.537" Score="13" Body="&lt;p&gt;&lt;strong&gt;Yes.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_differentiation&quot; rel=&quot;nofollow&quot;&gt;Planetary Differentiation&lt;/a&gt; is the key here.  When enough smaller asteroids smash together and form a big enough body, the heavy stuff sinks to the bottom (iron and other metals), middle layers tend to be formed of silicates (i.e. sand and rocks), and the lighter stuff floats on top (water, methane). (Mind you, the examples here are based on the composition we find in our solar-system. Other stars have significantly different make-ups.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once that's occurred, if a body were to be broken up (by a fast-moving asteroid, pulled apart by a gravity-well, etc), there'd be some chunks of metals, some rocky silicates, and some icy masses. Some might even be all three. And as it turns out, this is just the kinds of asteroids we find in space &amp;mdash; some are rocky, some are metal-rich, some (like comets) are icy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gold is HEAVY. Around twice as heavy as iron. Gold will be more strongly differentiated by gravity (particularly since it doesn't react chemically with much). Same sort of thing with other heavy metals &amp;mdash; lead, uranium, platinum, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is why there's some hope about asteroid mining &amp;mdash; the hard part of mining is often getting to the valuable metals through the dirt and waste-rock. Some asteroids are mostly metal &amp;mdash; no digging required.&lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="692" LastEditDate="2016-02-26T17:04:21.840" LastActivityDate="2016-02-26T17:04:21.840" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2053" PostTypeId="1" AcceptedAnswerId="2054" CreationDate="2013-09-20T19:41:54.527" Score="27" ViewCount="6418" Body="&lt;p&gt;I had recently learned that Voyager 1 uses magnetic tape to record data. Voyager 1 has been going on more or less strongly since 1977 (before I was even born!), and the wikipedia article states that the tape recorder is scheduled to terminate working in 2015. That's 38 years! I more or less need to replace my hard drives every 3 to 4 years. So what's the secret of the longevity of Voyager 1's tapes?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="25" LastEditDate="2013-09-26T12:35:14.423" LastActivityDate="2021-09-17T20:18:19.063" Title="How was magnetic tape decay prevented in Voyager 1?" Tags="|probe|voyager|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2054" PostTypeId="2" ParentId="2053" CreationDate="2013-09-20T22:41:38.570" Score="18" Body="&lt;p&gt;Voyager probes use 8-track Digital Tape Recorders (DTR) to record telemetry and scientific data, and they both have error-correction capabilities, with Voyager 2 having a stronger, more error resistant algorithms in place. Quoting &lt;a href=&quot;https://en.wikipedia.org/wiki/Error_detection_and_correction#Deep-space_telecommunications&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Error detection and correction in Deep-space telecommunications&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Development of error-correction codes was tightly coupled with the&#xA;  history of deep-space missions due to the extreme dilution of signal&#xA;  power over interplanetary distances, and the limited power&#xA;  availability aboard space probes. Whereas early missions sent their&#xA;  data uncoded, starting from 1968 digital error correction was&#xA;  implemented in the form of (sub-optimally decoded) &lt;a href=&quot;https://en.wikipedia.org/wiki/Convolutional_code&quot; rel=&quot;noreferrer&quot;&gt;convolutional codes&lt;/a&gt;&#xA;  and &lt;a href=&quot;https://en.wikipedia.org/wiki/Reed%E2%80%93Muller_code&quot; rel=&quot;noreferrer&quot;&gt;Reed–Muller codes&lt;/a&gt;. The Reed–Muller code was well suited to the&#xA;  noise the spacecraft was subject to (approximately matching a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gaussian_function&quot; rel=&quot;noreferrer&quot;&gt;bell&#xA;  curve&lt;/a&gt;), and was implemented at the Mariner spacecraft for missions&#xA;  between 1969 and 1977.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The mentioned article goes on to explain that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Concatenated_code&quot; rel=&quot;noreferrer&quot;&gt;concatenated error correction&lt;/a&gt; code is used combining &lt;a href=&quot;https://en.wikipedia.org/wiki/Convolutional_code&quot; rel=&quot;noreferrer&quot;&gt;convolutional code&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Binary_Golay_code&quot; rel=&quot;noreferrer&quot;&gt;binary Golay code&lt;/a&gt; providing for encoded data redundancy that is decoded by a &lt;a href=&quot;https://en.wikipedia.org/wiki/Viterbi_decoder&quot; rel=&quot;noreferrer&quot;&gt;Viterbi decoder&lt;/a&gt;. It also explains that the Voyager 2 is theoretically more error resistant with the use of an additional concatenated &lt;a href=&quot;https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_code&quot; rel=&quot;noreferrer&quot;&gt;Reed–Solomon code&lt;/a&gt; (Reed–Solomon–Viterbi) for a very powerful error correction.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/CCSDS&quot; rel=&quot;noreferrer&quot;&gt;CCSDS&lt;/a&gt; [Consultative Committee for Space Data Systems] currently recommends usage of error correction codes with&#xA;  performance similar to the Voyager 2 RSV code as a minimum.&#xA;  Concatenated codes are increasingly falling out of favor with space&#xA;  missions, and are replaced by more powerful codes such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Turbo_code&quot; rel=&quot;noreferrer&quot;&gt;Turbo codes&lt;/a&gt;&#xA;  or &lt;a href=&quot;https://en.wikipedia.org/wiki/LDPC_code&quot; rel=&quot;noreferrer&quot;&gt;LDPC codes&lt;/a&gt; [Low-density parity-check].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is possibly a bit overwhelming for anyone not familiar with how data parity, concatenated error correction or cyclic redundancy check algorithms work, though. But we can conclude that using data redundancy and encoding algorithms, data can be decoded and to some extent reconstructed error free with the use of these measures, and any still persisting errors would be detectable. This is however not the whole story. Again quoting Wikipedia, but this time on &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_program#Telemetry&quot; rel=&quot;noreferrer&quot;&gt;Telemetry of the Voyager Program&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The telemetry comes to the Telemetry Modulation Unit (TMU) separately&#xA;  as a &quot;low-rate&quot; 40-bit-per-second (bit/s) channel and a &quot;high-rate&quot;&#xA;  channel.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Low rate telemetry is routed through the TMU such that it can only be&#xA;  downlinked as uncoded bits (in other words there is no error&#xA;  correction). At high rate, one of a set of rates between 10 bit/s and&#xA;  115.2 kbit/s is downlinked as coded symbols.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The TMU encodes the high rate data stream with a convolutional code&#xA;  having constraint length of 7 with a symbol rate equal to twice the&#xA;  bit rate (k=7, r=1/2).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, while there isn't any error correction in place from the TMO to the DTR (there is however on the DTR itself, as we've mentioned previously), this quote suggests there's two different bit rate streams recorded on the same 8-track DTR. Since there is no way for any track moving at any different speed than the rest of them (being a physical part of the same tape), this means Voyagers actually use two tracks simultaneously and recorded at different data density per track and the telemetry data being the low bit rate recording (and thus lower data density on the tape itself), this should translate to a higher signal to noise ratio of the tape recording and be less prone to data errors that would need to be corrected by previously mentioned algorithms, or in failure of doing that, not deliver telemetry data at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia quoted bit rates for both telemetry and scientific data of 1400 and 7200 bit/s, respectively. I wasn't able to find any quote on the actual tape used and its declared data density, and the data rate per second doesn't really tell us anything of its limits (it could simply be spun faster over reading/recording magnetic tape heads, for all we know), but I'd suspect having a lot slower than nominal recording speed would have some part in measures taken to prevent data loss and somewhat increase signal to noise ratio on the recording tape itself. On top, of course, of using error checking and correction algorithms with Voyager's digital recordings.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="6214" LastEditDate="2014-10-27T18:50:06.167" LastActivityDate="2014-10-27T18:50:06.167" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2055" PostTypeId="1" CreationDate="2013-09-21T10:54:48.210" Score="8" ViewCount="3179" Body="&lt;p&gt;Anyone who has been in an email conversation where the chain split into different threads, or has been in chat where they &lt;strong&gt;receive&lt;/strong&gt; two diverging comments requiring a response &lt;strong&gt;before a reply&lt;/strong&gt; to the first is complete, know what is referred to here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now imagine there are light years separating the communications, not just your speed on the keyboard. There is no chance for a phone call to clarify things, and it can be years before you get back a response that the other party is lost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you communicate with someone who left Earth on a journey you may set up rules.  If you communicate with a previously unmet entity, you would not have an existing rule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do any studies or protocols give best practices to maintain clear communications with &lt;strong&gt;significant&lt;/strong&gt; time lag?  If yes, have there been any tests to validate their effectiveness? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="288" LastEditDate="2013-09-23T08:02:34.907" LastActivityDate="2013-09-23T10:08:53.813" Title="How would we communicate over light years?" Tags="|communication|time|logistics|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2056" PostTypeId="2" ParentId="2051" CreationDate="2013-09-21T11:54:36.653" Score="7" Body="&lt;p&gt;For general purpose electronics, such as the processors, memory, etc, there is actually a fairly broad market for radiation tolerant devices. These are often used for military electronics, people who just want to be cautious, weather balloons, etc. Of course, some of these don't meet the long term radiation requirements that satellites are forced to endure, but they are available for many missions. (&lt;a href=&quot;http://www.militaryaerospace.com/articles/2011/05/radiation-hardened-electronics.html&quot; rel=&quot;nofollow&quot;&gt;Source&lt;/a&gt;, &lt;a href=&quot;http://www.maxwell.com/products/microelectronics/docs/how_rad_hard.pdf&quot; rel=&quot;nofollow&quot;&gt;Source&lt;/a&gt;) These chips are more expensive, have limited selection, but they are much cheaper than making your own.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many missions have requirements which exceed any commercial electronics available. These usually occur in one shot things, huge cameras, new frequencies for observation, etc. These will often require special components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is usually done for most satellites is some sort of a combination, using radiation hardened technologies for the most sensitive parts, either off the shelf or custom made, and using radiation enhanced semiconductors, things not made to handle the rigor of space, but tolerant of a radiation. In addition, there are a a number of commercial parts which have a reasonable amount of radiation tolerance, which are often used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as temperature, and pressure, in general, most electronics will work when in a vacuum. Some mechanical work occasionally has difficulties, but that is generally easier to do than custom computer chips. Most chips work over a reasonable temperature, and the satellites simply ensure that they stay in their specified ranges. Most satellites are tested in a vacuum prior to launch (Thermal Vacuum testing), where they test out extreme temperatures and vacuum pressure, to ensure they work fine, but in general, satellites are designed to keep such concerns to a minimum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vibration is a bit trickier, but usually the chips aren't sensitive to them as much as the legs of chips are. There are best practices to ensure that they are able to survive the vibration analysis of launch. As vibration is only a concern at launch, they design them as best they can, and test them on the ground to verify they did everything correctly. Most consumer electronics can handle some level of vibration fairly well, due to people dropping them, etc. In general, portable type chips (Cell Phones, for example) work better than stuff to plug in the wall. The same thing applies to power.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-21T17:23:59.500" LastActivityDate="2013-09-21T17:23:59.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2057" PostTypeId="2" ParentId="2055" CreationDate="2013-09-21T14:48:49.287" Score="2" Body="&lt;p&gt;This is one of those things that, for the time being, everyone is looking at the possibility of the use of &lt;a href=&quot;https://en.wikipedia.org/wiki/Quantum_entanglement&quot; rel=&quot;nofollow&quot;&gt;quantum entanglement&lt;/a&gt; as means of relaying information error free and over enormous distances at greater than light speeds:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Repeated experiments have verified that this works even when the&#xA;  measurements are performed more quickly than light could travel&#xA;  between the sites of measurement: there is no lightspeed or slower&#xA;  influence that can pass between the entangled particles. Recent&#xA;  experiments have shown that this transfer occurs at least 10,000 times&#xA;  faster than the speed of light; this merely establishes a lower limit&#xA;  to the speed — according to the formalism of quantum theory, the&#xA;  effect of measurement happens instantly.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But short of quantum entanglement for purposes of communication, we currently have no means of relaying information faster than light, and they all come with various drawbacks, mainly the signal strength and with it required power for communications signals to traverse without loss of data over such vast distances. So, of course, for any missions at lightyear distances, you'd want your probe to be as autonomous as possible, possibly have some form of &lt;a href=&quot;https://en.wikipedia.org/wiki/Artificial_intelligence&quot; rel=&quot;nofollow&quot;&gt;Artificial Intelligence&lt;/a&gt; (AI) onboard, at least somewhat controlling the probe to avoid any obstacles it might encounter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similar semi-autonomous systems are already in use with several unmanned probes that we sent in space, for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Rosetta spacecraft&lt;/a&gt; has the ability to avoid close proximity obstacles, something that will likely need as it starts closing the distance to the &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow&quot;&gt;67P/Churyumov–Gerasimenko comet&lt;/a&gt; it is chasing and when it approaches the Sun increases its coma and tails, with possible debris chipping off it. Even being merely around 1 &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_unit&quot; rel=&quot;nofollow&quot;&gt;Astronomical Unit&lt;/a&gt; (AU) away is still distant enough to cause communications delay that is utterly unmanageable for avoiding collisions from the Earth in real time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So having more advanced AI in place and have our probes eventually 100% autonomous, not depend on having to communicate with us over distances in the magnitude of light years, and do their work completely independent of our constant control of their functions is, arguably, just a few decades shy from materializing and certainly within the reach of even our current technology. That ought to be a lot faster than we'll be able to develop any interstellar drive technology getting us there by the time our probes are still expected to operate and anyone here still ready to listen to data it transmits back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess what I'm trying to say is the communications delay might not matter so much, if we learn to design around this obstacle with the use of AI, which seems to advance a lot faster than drive technology needed to reach other stars in a reasonable timeframe is. And when quantum entangled communications will be possible, all these drawbacks in communications delay and power requirements needed to produce strong enough signals for us to eventually pick them up will be gone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the off-chance of encountering yet unmet sentient beings out there that would be technologically advanced enough to detect our presence - all of this is a matter of opinion, like with the rest of dealing with the unknown. My opinion (take it as such) is, that luckily for us, chances of encountering another civilisation in their counterpart to our 16th century development and though are extremely remote to say the least, so any &lt;em&gt;proper protocols&lt;/em&gt; that should be in place in the eventuality we'd encounter &lt;em&gt;previously unmet entities&lt;/em&gt; should be a lot easier to achieve than what popular culture would have us believe in science fiction movies and literature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they'd be any less technologically advanced, you'd likely pass by undetected, or our presence dismissed as something that the civilisation you encountered can relate to. And if they're same or more technologically advanced than we currently are, they will know well enough that it's a robotic probe they've detected, as such a proof of intelligent life elsewhere in the vastness of the Universe, and will likely rejoice and have a golden record with greeting from the Earth to place on top of their charts. And if they won't and are combatant in nature, they will most likely be involved with themselves too much to pay attention to some other civilisation, light years away, and by the time they could, their civilisation will be back to stone ages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think we should preoccupy ourselves with how advanced some theoretical civilisations that aren't ours and are light years away might be, and rather more frequently look back and reflect on ourselves in which of the two directions described we are heading. My 2 cents. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-21T14:48:49.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2058" PostTypeId="1" AcceptedAnswerId="2060" CreationDate="2013-09-21T16:34:30.740" Score="8" ViewCount="123" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/GRAIL&quot; rel=&quot;nofollow&quot;&gt;GRAIL&lt;/a&gt; recently constructed a complete gravity map of Luna's irregular gravity field.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are similar GRAIL missions on the anvil to any other Celestial bodies as yet? &lt;/li&gt;&#xA;&lt;li&gt;As a corollary (probably opinion-based), should future missions to any body include a similar GRAIL map exercise as a preliminary?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2016-04-29T20:18:25.350" LastActivityDate="2016-04-29T20:18:25.350" Title="To what celestial bodies are GRAIL-like missions planned?" Tags="|artificial-satellite|gravity|grail|remote-sensing|mapping|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2059" PostTypeId="2" ParentId="2055" CreationDate="2013-09-21T19:39:25.673" Score="7" Body="&lt;p&gt;Historically, we haven't had the instantaneous technology that we enjoy today. However, we have never had in place a way to communicate at a distance without setting up some sort of protocol beforehand. Let me discuss how some of this has been handled historically, how it is handled with distance spacecraft today, and a bit of how we could figure it out with an unknown entity in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Historically, when there was a months lag in communication between, say, Europe and the Americas, an ambassador was sent to another country, with some specific guidelines to follow, but given some lee-way to exercise judgement if something fundamentally different happened. This is the same way that explorers worked, and any other number of people making important journeys, they were given some direction, but had to exercise autonomy to fulfill said directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so how do we manage communication with distance spacecraft today? Essentially you have to set up an asynchronous communication protocol. You send a packet, each packet identifying where it is in the stream. Some packets will be lost, but as a whole, most will make it. You listen carefully to get as many of the packets as you can,  but you might miss something. Depending on the spacecraft, a re-transmit request might be made. It is actually fairly similar to how the internet works. The further you are, the less likely you will request a retransmit. In most instances, they listen on a different frequency than they transmit, allowing 2 way communication to happen. This is called Full Duplex communication, by the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what to do about really large distances? Fundamentally, it is the same as we communicate with Voyager. Send a packet, request a response, and be listening the entire time for said response. You should see the response come in eventually, especially for a machine, the time can be predicted when it will arrive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what about a human communication approach at a huge distance? You either need to set up a time scale to know when said communications can be expected, or you can have something listen all the time. Both ends will require asynchronous communications, as well as requiring pointing a large dish at each other, so you have to time it correctly, or else the other party won't be listening. You will have to allow them some time to send a response.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to how to communicate with an alien entity? The first step is to decode their message pattern, so you can understand it. At first, you are either going to have to do a simple (Return on the same frequency), or a close duplex, as you will not know where to send a response that they can listen to. You should be able to get the right thing figured out in the end. You will have to apply the lessons from above, but as you can't receive when you transmit on the same frequency, you will have to be very careful as to when you transmit. Or better yet, have a station on one part of the earth receive, and another part transmit.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-21T19:39:25.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2060" PostTypeId="2" ParentId="2058" CreationDate="2013-09-21T20:45:27.990" Score="6" Body="&lt;p&gt;There is an effort to map Earth's gravity, &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_Experiment&quot; rel=&quot;noreferrer&quot;&gt;GRACE&lt;/a&gt;, which has been ongoing since 2002. It's produced this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xJM3m.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Neat, right?&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several GRAIL-like missions are planned / in progress / completed.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Mars_Express/First_Mars_Express_gravity_results_plot_volcanic_history&quot; rel=&quot;noreferrer&quot;&gt;Mars Express&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://lisa.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;LISA&lt;/a&gt;, a probe designed to measure gravitational fields in space, with hopes of finding black holes.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet.html&quot; rel=&quot;noreferrer&quot;&gt;Magellan&lt;/a&gt; mapped Venus' gravitational anomalies.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/MESSENGER&quot; rel=&quot;noreferrer&quot;&gt;MESSENGER&lt;/a&gt; for Mercury&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;noreferrer&quot;&gt;Cassini&lt;/a&gt; mapping the gravity fields of Saturn and it's moons.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://works.bepress.com/cgi/viewcontent.cgi?article=1244&amp;amp;context=jeremy_straub&quot; rel=&quot;noreferrer&quot;&gt;GEORGE&lt;/a&gt;, a proposal for a probe to Uranus with one purpose being to '&lt;em&gt;characterize the gravitational field of Uranus and make a gravity map&lt;/em&gt;'.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As for your other question, I would say that it would certainly be &lt;em&gt;nice&lt;/em&gt; to have such data on gravity variations, but it's not strictly necessary. We can deal with such things as we come to them with just a few thrusters and make up for what variations in trajectory are caused.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-21T21:37:00.210" LastActivityDate="2013-09-21T21:37:00.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2061" PostTypeId="5" CreationDate="2013-09-21T20:50:53.020" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-21T20:50:53.020" LastActivityDate="2013-09-21T20:50:53.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2062" PostTypeId="4" CreationDate="2013-09-21T20:50:53.020" Score="0" Body="Questions regarding the coordination of people, machinery, or facilities to accomplish an end goal." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-21T20:52:43.547" LastActivityDate="2013-09-21T20:52:43.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2063" PostTypeId="5" CreationDate="2013-09-21T21:05:41.040" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-21T21:05:41.040" LastActivityDate="2013-09-21T21:05:41.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2064" PostTypeId="4" CreationDate="2013-09-21T21:05:41.040" Score="0" Body="Questions regarding discharge of electrical charges built up inside stormclouds to another object." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-21T21:24:59.270" LastActivityDate="2013-09-21T21:24:59.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2065" PostTypeId="5" CreationDate="2013-09-21T21:06:11.370" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-21T21:06:11.370" LastActivityDate="2013-09-21T21:06:11.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2066" PostTypeId="4" CreationDate="2013-09-21T21:06:11.370" Score="0" Body="Questions regarding equipment used to propel a spacecraft or a rocket. Questions about attitude control jets should be tagged &quot;attitude&quot;." OwnerDisplayName="user12" LastEditorUserId="168" LastEditDate="2015-02-16T18:10:18.337" LastActivityDate="2015-02-16T18:10:18.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2067" PostTypeId="5" CreationDate="2013-09-21T21:07:06.060" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-21T21:07:06.060" LastActivityDate="2013-09-21T21:07:06.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2068" PostTypeId="4" CreationDate="2013-09-21T21:07:06.060" Score="0" Body="Questions regarding spacecraft designed to travel between planets." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-21T21:25:16.617" LastActivityDate="2013-09-21T21:25:16.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2069" PostTypeId="5" CreationDate="2013-09-21T21:13:37.900" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-21T21:13:37.900" LastActivityDate="2013-09-21T21:13:37.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2070" PostTypeId="4" CreationDate="2013-09-21T21:13:37.900" Score="0" Body="Questions regarding the Gravity Recovery and Interior Laboratory (GRAIL), an American project to map the gravitational deviations of the Moon.  " OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-21T21:25:20.727" LastActivityDate="2013-09-21T21:25:20.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2071" PostTypeId="1" CreationDate="2013-09-21T21:30:12.633" Score="15" ViewCount="530" Body="&lt;p&gt;Already on earth the time is a bit of a problem with the time-zones. It is not that big a problem though, since we all &quot;travel&quot; at (nearly) the same speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not so in space. With the time dilatation (and probably some other factors) one cannot simply rely on ones watch. So, how would it be done?&lt;/p&gt;&#xA;" OwnerUserId="87" LastActivityDate="2019-06-06T07:59:21.640" Title="How would time measurement be done over great distances?" Tags="|time|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2072" PostTypeId="1" AcceptedAnswerId="2073" CreationDate="2013-09-21T22:53:00.873" Score="11" ViewCount="1211" Body="&lt;p&gt;According to Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus&lt;/a&gt; has a gravity of 9/10 Earth which would be very nice for a human to live on, but the temperature seems a bit high. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without &lt;a href=&quot;https://space.stackexchange.com/questions/1334/can-venus-be-made-habitable&quot;&gt;terrforming&lt;/a&gt; and with our current knowledge and technology, would it be possible to build a habitat on (or in) Venus that would support human life? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-09-22T01:01:44.193" Title="Would a human colony on Venus be possible?" Tags="|colonization|venus|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2073" PostTypeId="2" ParentId="2072" CreationDate="2013-09-22T01:01:44.193" Score="10" Body="&lt;p&gt;On the surface of the planet, without terraforming, it would be virtually impossible to construct a habitat where humans could live. If one was going to do such a thing, they would have to find a way to reflect an extraordinarily high amount of heat, somehow power the base, create fuel, and protect against the acidic compounds of Venus's atmosphere. Nothing conceivable could be done to last for any length of time in that environment, let alone build a habitat there. In addition, you would have to build weak points to get in and out, which would be difficult. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a few other possibilities, however. Below the surface of Venus, a habitat could be constructed which would work. Also, if one could build high enough supports, the atmosphere about 50 km up is actually quite similar to Earth's, one could live there, given the support structure. Other than that, I can't imagine any way to live there without some sort of terraforming.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The minimum terraforming required would be to cool the planet down. This actually would be relatively easy, essentially all one would need to do is build a giant tin foil shade, or something similar, in orbit and keep it there. If the atmosphere cooled sufficiently, it would start to thin out, eventually making it much more manageable. It would be somewhat easier to get Mars to be more of a habitable planet, but it could still be done to get Venus there, someday.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-22T01:01:44.193" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2074" PostTypeId="1" AcceptedAnswerId="2075" CreationDate="2013-09-22T08:45:12.873" Score="11" ViewCount="2905" Body="&lt;p&gt;I often read about future spacecraft gathering water ice to use as a propellant, yet this doesn't line up with any of the propulsion methods I know to exist today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What type of engine could use water as a propellant?  Does such an engine exist; has any development been done on it?&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="25" LastEditDate="2013-09-22T20:08:41.973" LastActivityDate="2020-12-08T17:44:34.770" Title="What kind of engine uses water as a propellant?" Tags="|propulsion|water|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2075" PostTypeId="2" ParentId="2074" CreationDate="2013-09-22T09:35:20.083" Score="11" Body="&lt;p&gt;Not water (H&lt;sub&gt;2&lt;/sub&gt;O) per se, at least not as a single propellant, but as a reaction mass, propellant component (&lt;a href=&quot;https://en.wikipedia.org/wiki/ALICE_%28propellant%29&quot;&gt;ALICE&lt;/a&gt; is a rocket propellant that consists of nanoaluminium powder and water, both mixed in ice form to keep it stable), or atomic components of water molecules that we could separate with the use of &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrolysis&quot;&gt;electrolysis&lt;/a&gt; and producing diatomic oxygen (O&lt;sub&gt;2&lt;/sub&gt;) as an oxidizer and a diatomic hydrogen (H&lt;sub&gt;2&lt;/sub&gt;) as fuel, sure that would work and components of water molecules are already used in many launch systems, albeit in their cryogenic liquid state to increase their density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/ALICE_%28propellant%29&quot;&gt;ALICE&lt;/a&gt; has &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot;&gt;specific impulse&lt;/a&gt; (Isp) of 210 seconds in theory, in practice though closer to 150 seconds, which is really not that much compared to cryogenic &lt;a href=&quot;http://www.astronautix.com/props/loxlh2.htm&quot;&gt;LOX/LH2&lt;/a&gt; with specific impulse (both at sea level) of 391 seconds, and it also requires an additional propellant component (nanoaluminium powder) to be taken onboard, so it wouldn't be the most suitable for missions to distant celestials that require refueling. But in case of both using water as a reaction mass that you could expel at your engine exhaust and produce thrust, for example with the use of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot;&gt;ion thrusters&lt;/a&gt;, or by separating molecular water into its constituent components and use that as your source of oxidizer and fuel components of your propellant, it doesn't require additional propellant components, but it would require strong source of electrical power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lucky for us though, taking onboard either &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;Radioisotope Thermoelectric Generators&lt;/a&gt; (RTGs) or later relying on &lt;a href=&quot;https://en.wikipedia.org/wiki/Photovoltaic_system&quot;&gt;Photovoltaic Arrays&lt;/a&gt; (where the source of light radiation is more or less continuous and strong enough, i.e. in the inner Solar system) provides for relatively light, strong and long duration energy source (RTGs), or sustainable source of it, where it's readily available and merely need collecting (Solar Arrays). There are, of course, other ways of producing electrical power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to recap:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Use molecular water as your reaction mass, for example with &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot;&gt;ion thrusters&lt;/a&gt; that could greatly accelerate its molecules as ions and &lt;a href=&quot;https://en.wikipedia.org/wiki/Plasma_%28physics%29&quot;&gt;plasma&lt;/a&gt; on the engine's exhaust and produce thrust, or&lt;/li&gt;&#xA;&lt;li&gt;Use &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrolysis&quot;&gt;electrolysis&lt;/a&gt; and separate water molecules into its constituent atomic components, oxygen and hydrogen, and use that as your oxidizer and fuel components to your &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid-propellant_rocket&quot;&gt;bipropellant&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For pure water in its liquid state, &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrolysis&quot;&gt;electrolysis&lt;/a&gt; requires a minimum &lt;a href=&quot;https://en.wikipedia.org/wiki/Voltage&quot;&gt;voltage&lt;/a&gt; of 1.229 V (&lt;a href=&quot;https://en.wikipedia.org/wiki/Volt&quot;&gt;Volt&lt;/a&gt;) and separates H&lt;sub&gt;2&lt;/sub&gt;O into negative oxygen ions (&lt;a href=&quot;https://en.wikipedia.org/wiki/Ion#Anions_and_cations&quot;&gt;anions&lt;/a&gt;) at its positive anode, and positive hydrogen ions (&lt;a href=&quot;https://en.wikipedia.org/wiki/Ion#Anions_and_cations&quot;&gt;cations&lt;/a&gt;) at its negative cathode ends as atomic ions that combine into diatomic gas, H&lt;sub&gt;2&lt;/sub&gt; and O&lt;sub&gt;2&lt;/sub&gt;, respectively:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$2H_2O → 2H_2 + O_2 -1.229 V$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These two oxidizer and fuel components in gas form could then be compressed and stored in their cryogenic liquid form for later use with &lt;a href=&quot;https://en.wikipedia.org/wiki/Cryogenic_rocket_engine&quot;&gt;cryogenic liquid propellant rocket engines&lt;/a&gt;. Such techniques could serve both for refueling stations, or be done onboard your spaceship itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking either strong electrical power sources or having solar panels onboard is still a lot cheaper in terms of your requirements to accelerate to required &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot;&gt;delta-v&lt;/a&gt; to reach escape velocities of whichever celestial body you'd like to make a distance from, than having to also take all your reaction mass with you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So having readily available sources of this reaction mass your rocket will be using to produce thrust and propel itself, be it on spaceship's own accord, or with the use of separate refueling stations, is imperative to our endeavours of exploring distant places and keep economy of it manageable, something we could afford on a larger scale.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-22T10:04:01.427" LastActivityDate="2013-09-22T10:04:01.427" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2076" PostTypeId="1" AcceptedAnswerId="2077" CreationDate="2013-09-22T12:10:22.983" Score="23" ViewCount="90840" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.latimes.com/science/sciencenow/la-sci-sn-usc-rocket-lab-space-20130920,0,2123551.story&quot;&gt;a recent news article&lt;/a&gt; a group of USC students are attempting to launch a rocket &quot;in to space&quot; with a planned height of 62 miles.  Making them &quot;the first group of students to successfully launch a rocket into space,&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS being at about the 250 mile height, and weather balloons reaching heights of 25 miles or more, space would seem to begin someplace between those two heights. Where does &quot;space&quot; begin?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2014-03-08T09:32:07.243" LastActivityDate="2021-09-17T13:21:37.550" Title="How far do you have to be from Earth to be &quot;in space&quot;?" Tags="|terminology|standards|altitude|" AnswerCount="6" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2077" PostTypeId="2" ParentId="2076" CreationDate="2013-09-22T13:13:59.043" Score="26" Body="&lt;p&gt;We are &lt;em&gt;&quot;in space&quot;&lt;/em&gt;, in fact everything that exists and has a physical presence is. But what we usually mean by it is to describe &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; conditions of near (or hard) vacuum, where atmospheric pressure is already low enough to affect matter differently than under true atmospheric conditions, for example at atmospheric pressure below &lt;a href=&quot;https://en.wikipedia.org/wiki/Triple_point&quot;&gt;triple point&lt;/a&gt; of water. But where &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; begins, as you might expect by now, is a matter of opinion and there isn't any clear line to be drawn. For example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_space&quot;&gt;Wikipedia on Outer space&lt;/a&gt; has this to say:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is no firm boundary where space begins. However the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot;&gt;Kármán&#xA;  line&lt;/a&gt;, at an altitude of 100 km (62 mi) above sea level, is&#xA;  conventionally used as the start of outer space in space treaties and&#xA;  for aerospace records keeping.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Kármán line, named after the Hungarian-American engineer and physicist &lt;a href=&quot;https://en.wikipedia.org/wiki/Theodore_von_K%C3%A1rm%C3%A1n&quot;&gt;Theodore von Kármán&lt;/a&gt;, is the theoretical altitude at which the Earth's atmosphere becomes too thin for aeronautical purposes. But this is merely one arbitrarily set boundary, and where &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; begins could as well be defined differently, for example by the mentioned atmospheric pressure where water can &lt;a href=&quot;https://en.wikipedia.org/wiki/Sublimation_%28phase_transition%29&quot;&gt;sublimate&lt;/a&gt; directly from ice to gas phase without the transition to a liquid first. Again quoting Wikipedia, this time on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Triple_point#Triple_points_of_water&quot;&gt;Triple points of water&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The gas–liquid–solid triple point of water corresponds to the minimum&#xA;  pressure at which liquid water can exist. At pressures below the&#xA;  triple point (as in outer space), solid ice when heated at constant&#xA;  pressure is converted directly into water vapour in a process known as&#xA;  sublimation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there it is again, mentioned as where the &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; begins. And it could, of course, be defined by other means as well. For example by what arbitrary atmospheric boundaries we set, at the edge of any altitude that we name atmospheric layers with a different term, designating different atmospheric conditions, maybe at the edge of &lt;a href=&quot;https://en.wikipedia.org/wiki/Stratosphere&quot;&gt;Stratosphere&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Mesosphere&quot;&gt;Mesosphere&lt;/a&gt; at 50 km? Yet again quoting &lt;a href=&quot;https://en.wikipedia.org/wiki/Mesosphere#Uncertainties&quot;&gt;Wikipedia on Uncertainties of Mesosphere&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The mesosphere lies above the maximum altitude for aircraft and below&#xA;  the minimum altitude for orbital spacecraft. It has only been accessed&#xA;  through the use of sounding rockets. As a result, it is the most&#xA;  poorly understood part of the atmosphere. The presence of red sprites&#xA;  and blue jets (electrical discharges or lightning within the lower&#xA;  mesosphere), noctilucent clouds and density shears within the poorly&#xA;  understood layer are of current scientific interest.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it again mentions the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot;&gt;Kármán line&lt;/a&gt;, but gives us an additional clue that &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; might as well be defined by where atmospheric weather phenomena stops. This could go on forever really, so I'll stop with these three examples.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can draw a few conclusions though: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;First, that it seems the internationally accepted standard for defining where &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; actually begins is the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot;&gt;Kármán line&lt;/a&gt;, at an altitude of 100 km (62 mi) above sea level, and that we have agreed on certain treaties, namely the &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot;&gt;Outer Space Treaty&lt;/a&gt;, based on this definition. &lt;/li&gt;&#xA;&lt;li&gt;Second, that any boundary where the &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; begins is exclusively arbitrary and might vary depending on your own definitions and use case. &lt;/li&gt;&#xA;&lt;li&gt;And lastly, that we have not yet agreed on any of these arbitrary notions of where &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; begins, that would be equally applicable to atmospheric conditions of other celestials. I.e. at what altitude is considered to be &lt;em&gt;&quot;outer space&quot;&lt;/em&gt; above the Earth will not equally apply to what might be this boundary for, say, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_atmosphere&quot;&gt;Atmosphere of Mars&lt;/a&gt; that has surface atmospheric pressure on average only slightly larger than the water triple point, or only about 0.6% of the Earth's mean sea level pressure.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-22T13:13:59.043" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2078" PostTypeId="2" ParentId="2076" CreationDate="2013-09-22T14:03:42.457" Score="9" Body="&lt;p&gt;Concurrent with &lt;a href=&quot;https://space.stackexchange.com/a/2077/288&quot;&gt;TildalWave's reply&lt;/a&gt; I too say 'Karman line'(&lt;em&gt;100 kilometres (62 mi) above the Earth's sea level&lt;/em&gt;). Apart from the fact the K-line is legally so recognized an alternative definition of 'in space' is covered in the same &lt;a href=&quot;http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... any vehicle at this altitude would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself, neglecting centrifugal force). ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-09-23T10:24:55.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2079" PostTypeId="2" ParentId="2014" CreationDate="2013-09-22T16:58:43.220" Score="5" Body="&lt;p&gt;I looked into this a while ago.  It's easy to get bogged down in the details, because although the data is very public, there are so many parameters for every mission that I found it hard to get a straight answer.  I used these points to &quot;dumb down&quot; the question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Seeking maximum time between launch of one mission and the next mission&lt;/li&gt;&#xA;&lt;li&gt;Manned and unmanned missions are counted&lt;/li&gt;&#xA;&lt;li&gt;Content of the mission is ignored&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Someone involved might hem and haw due to a number of inadequacies with this.  For example, the payload doesn't reach the station on the same orbit (and maybe not even the same day) of the launch.  The Wikipedia pages on the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spaceflights_to_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;list of space flights to the ISS&lt;/a&gt; is also quite concerned with the length of time that every spacecraft remains docked to the ISS.  There is some valid contention about how to handle &lt;em&gt;return&lt;/em&gt; trips to Earth, because over some interval it may have &lt;em&gt;received&lt;/em&gt; nothing from Earth but still &lt;em&gt;sent&lt;/em&gt; something to Earth.  I'm not interested in return trips because it doesn't supply the station &lt;em&gt;with new resources&lt;/em&gt;, nor do I care about the time taken to reach the station because all I care about is how long people survive in orbit &lt;em&gt;with resources already in orbit&lt;/em&gt;.  I find all of these details highly tangential to the straightforward query of how long have we had people up there without sending anything &lt;em&gt;more&lt;/em&gt; up there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the record holders:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Nov 25, 2002 to April 2, 2003 -&gt; for a total of 128 days&lt;/li&gt;&#xA;&lt;li&gt;April 28, 2003 to Oct 27, 2003 -&gt; for a total of 125 days&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As I understand it, these extended intervals were connected to &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaster&quot; rel=&quot;noreferrer&quot;&gt;the Space Shuttle Columbia disaster&lt;/a&gt;, which happened on Feb 1, 2003.  Now I could have potentially had a data processing error, but in a simplistic sense, these seem consistent with what you can read about those missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The story is that the ISS was, indeed, sort of struggling during those time frames.  Because of that, these records should be within striking distance of the maximum time they &lt;em&gt;could&lt;/em&gt; go without resupply.  However, there are other complications.  I would hope that since 2003 they've better prepared for a similar scenario.  Plus, there's the simple fact that as modules are added, the mass to area ratio increases which would give more time for station keeping.  The ISS also has experimented with plasma/ion thrusters, and this could keep it in orbit for longer.  Supplies are an issue, but I don't think there's any fundamental limit to how much could be stored.  If the time frame for those things was extended, they &lt;em&gt;could&lt;/em&gt; probably push the time they can go without resupply to close to the human records for time in microgravity, but I don't think there's any reasons for them to spend resources on this objective just for the sake of a record.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mir went for longer times without resupply, and I find a maximum of 240 days, which is almost twice what the ISS record was.  They had a smaller crew and experimented with keeping them up there for longer periods of time.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-09-22T16:58:43.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2080" PostTypeId="1" AcceptedAnswerId="2082" CreationDate="2013-09-22T17:54:54.823" Score="16" ViewCount="1661" Body="&lt;p&gt;A spacecraft may launch with multiple payload, as is evidenced by &lt;a href=&quot;https://space.stackexchange.com/questions/1556/how-does-a-single-rocket-place-multiple-satellites-into-orbit&quot;&gt;How does a single rocket place multiple satellites into orbit?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question however is &lt;strong&gt;not about multiple payload&lt;/strong&gt; carried in a single launch vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question is whether there have ever been multiple launches (E.g. Canaveral, Baikonur, Xichang) on the same date. Obviously the contracting client would be different in each case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So ...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Historically&lt;/em&gt;, Have multiple spacecraft launched on the &lt;strong&gt;same date&lt;/strong&gt; from geographically distinct locations? &lt;/li&gt;&#xA;&lt;li&gt;Is there a global body to coordinate such multiple spacelaunch events?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-12-19T21:52:18.827" Title="How are multiple launches handled?" Tags="|launch|launch-site|procedure|multi-launch|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2081" PostTypeId="2" ParentId="2046" CreationDate="2013-09-22T18:14:38.210" Score="6" Body="&lt;p&gt;I have been crunching some numbers and, with @AlanSE's answer, I think I’m on the right path. I’ll take the example of a Delta-V map calculation for the launch a rocket to LEO, followed by a Hohmann transfer to the moon. Correct me if I'm wrong:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gZFpO.jpg&quot; alt=&quot;Earth to moon delta v map&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;We have this equation for the delta-V required to lift off and to reach an altitude of 200km:&lt;/em&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}&#xA;\Delta v_{0} &amp;amp; = \sqrt{\frac{\mu}{R}} \\&#xA; &amp;amp; = \sqrt{\frac{6.673\times 10^{-11} * 5.97219 \times 10^{24}}{6371000}} \\&#xA; &amp;amp; = 7909.034~\text{m}/\text{s}&#xA;\end{align}$$&#xA;&lt;br&gt;&#xA;$$\begin{align}&#xA;v &amp;amp; = \sqrt{2 * Gh} \\&#xA; &amp;amp; = \sqrt{ 2 * 9.8 * 200000 } \\&#xA; &amp;amp; = 1979.899~\text{m}/\text{s}&#xA;\end{align}$$&#xA;&lt;br&gt;&#xA;$$\text{Where } \mu = GR_{earth}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we have ~9888.9 m/s  Δv to reach 200KM above the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Hohmann transfer to the moon&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}&#xA;r_{1} &amp;amp; = 6571000~\text{m}~\text{(Earth radius plus 200km)} \\&#xA;r_{2} &amp;amp; = 362600000~\text{m}~\text{(Moon average position at perigee)} \\&#xA;\end{align}$$&#xA;&lt;br&gt;&#xA;$$\begin{align}&#xA;\mu_{earth} &amp;amp; = 3.98524 \times 10^{14}~\text{m}^3/\text{s}^2 \\&#xA;\mu_{moon} &amp;amp; =  4.903120210000 \times 10^{12}~\text{m}^3/\text{s}^2&#xA;\end{align}$$&#xA;&lt;br&gt;&#xA;$$\begin{align}&#xA;\Delta v_{1} &amp;amp; = \sqrt{\frac{\mu}{r_{1}}} * \left(\sqrt{ \frac{2*r_{2}}{r_{1}+r_{2}} } - 1\right) \\&#xA; &amp;amp; = 3127.331~\text{m}/\text{s}&#xA;\end{align}$$&#xA;&lt;br&gt;&#xA;$$\begin{align}&#xA;\Delta v_{2} &amp;amp; = \sqrt{\frac{\mu}{r_{2}}} * \left(1 - \sqrt{ \frac{2*r_{1}}{r_{1}+r_{2}} }\right) \\&#xA; &amp;amp; = 94.345~\text{m/s}&#xA;\end{align}$$&lt;/p&gt;&#xA;" OwnerUserId="732" LastEditorDisplayName="user29" LastEditDate="2013-09-23T01:45:30.837" LastActivityDate="2013-09-23T01:45:30.837" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2082" PostTypeId="2" ParentId="2080" CreationDate="2013-09-22T19:39:21.390" Score="13" Body="&lt;p&gt;To answer the first part of your question, have historically multiple spacecraft launched on the same date from geographically distinct locations, yes. In fact, it just happened last Wednesday, September 18, 2013 when two rockets were launched on the same date, and both from different facilities in the United States:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Space_Launch_Complex_41&quot; rel=&quot;nofollow noreferrer&quot;&gt;Complex 41&lt;/a&gt; at &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cape Canaveral Air Force Station&lt;/a&gt;, Fla. on September 18, 2013 at at 4:10 a.m. EDT (08:10 GMT)&lt;/strong&gt;:&lt;br /&gt;&lt;a href=&quot;http://www.ulalaunch.com/site/default.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;United Launch Alliance&lt;/a&gt; (ULA) &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas 5&lt;/a&gt; rocket launched and deployed an ultra-secure, jam-resistant &lt;a href=&quot;https://en.wikipedia.org/wiki/Advanced_Extremely_High_Frequency&quot; rel=&quot;nofollow noreferrer&quot;&gt;AEHF 3&lt;/a&gt; (Advanced Extremely High Frequency) communications satellite built by &lt;a href=&quot;http://www.lockheedmartin.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lockheed Martin Corp.&lt;/a&gt; in orbit for the U.S. Air Force:&lt;br /&gt;&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/evtco.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://www.nasa.gov/centers/wallops/home/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wallops Island Flight Facility&lt;/a&gt;, Va. on September 18, 2013 at 10:58 a.m. EDT (14:58 GMT)&lt;/strong&gt;:&lt;br /&gt;&lt;a href=&quot;http://www.orbital.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Sciences&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Antares_%28rocket%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Antares rocket&lt;/a&gt; launched a &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cygnus&lt;/a&gt; unmanned resupply spacecraft on its inaugural resupply mission to the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;International Space Station&lt;/a&gt; (ISS):&lt;br /&gt;&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/cqN5E.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Since space launches are becoming rather frequent events in recent history, I'm pretty sure there were many other instances of two or more launches on the same day, so I'm just going to additionally mention the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo-Soyuz Test Project&lt;/a&gt; for its historical significance, during which both launch vehicles (Apollo CSM-111 and Soyuz 7K-TM) also launched on the same day, July 15, 1975, from Kennedy Space Center and Baikonur Cosmodrome, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the other part of your question, if there is a global body to coordinate such multiple space launch events, I would have to say no. Not a global body and not really coordinating it, but there would of course be local or nationwide bodies regulating airspace activities, and when rocket launches cover international airspace, coordination between all involved jurisdictions would have to take place and give clearance for the launch to proceed at any pre-designated time frame (&lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_window&quot; rel=&quot;nofollow noreferrer&quot;&gt;launch window&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, there is &lt;a href=&quot;http://www.oosa.unvienna.org/oosa/SORegister/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The United Nations Register of Objects Launched into Outer Space&lt;/a&gt;, set in motion under the &lt;a href=&quot;http://www.oosa.unvienna.org/oosa/SpaceLaw/gares/html/gares_29_3235.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Resolution 3235 of the UN General Assembly&lt;/a&gt;, and possibly other international / global registers that all launch systems under their jurisdiction would have to report to, but I'm not aware of any that would also coordinate various launch site activities on a global scale.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For what it's worth, we covered both of these two events mentioned in &lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/the-pod-bay&quot;&gt;The Pod Bay&lt;/a&gt;, our main chat room, where we posted links to live broadcasts for the events and had a chat observing them unfold. Our &lt;a href=&quot;http://chat.stackexchange.com/rooms/info/9682/the-pod-bay?tab=schedule&quot;&gt;main chat room's schedule&lt;/a&gt; is also the place to look at for future chat room events, on top of featured events also being displayed in the Community Bulletin box on the right side to every &lt;a href=&quot;https://space.stackexchange.com/&quot;&gt;Space Exploration SE&lt;/a&gt; main site page with a convenient countdown displayed next to each scheduled event. The chat room's &lt;a href=&quot;http://chat.stackexchange.com/rooms/info/9682/the-pod-bay?tab=schedule&quot;&gt;schedule&lt;/a&gt; also includes your local times for any currently scheduled events and an option to register for events with an email notification sent to your address a few hours beforehand as a reminder.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-12-19T21:52:18.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2083" PostTypeId="2" ParentId="2080" CreationDate="2013-09-22T19:55:03.833" Score="9" Body="&lt;p&gt;There are many examples of multiple launches on the same date - NASA has even done multiple launches on the same date. One of these was to practice docking with an &lt;a href=&quot;http://en.wikipedia.org/wiki/Agena_target_vehicle&quot;&gt;Agena target vehicle&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On &lt;a href=&quot;http://en.wikipedia.org/wiki/Gemini_11&quot;&gt;Gemini XI&lt;/a&gt;, the Agena target vehicle was launched Sepetember 12, 1966 at 8:05:01 a.m. EST. The Gemini vehicle itself was launched 10:42:26 am EST the same day.  &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-09-22T19:55:03.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2084" PostTypeId="2" ParentId="2080" CreationDate="2013-09-22T20:03:46.337" Score="6" Body="&lt;p&gt;Having multiple launches on the same day from the same location is difficult, as there are quite a few people needed to support each launch, and that becomes very difficult when there are two launches. It could be done in theory, but it isn't done in practice, there's just too much to coordinate. In general, two such objects from the same location would have to be related to each other somehow, or else it would simply add a huge amount of complexity. As @Undo mentioned, this has happened at least once, as a test for Gemini, but they usually depend on being related to the same mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From geographically different locations, there really isn't much that needs to be coordinated. Space is really really big, the odds of any kind of collision between the two are almost negligible. Most launches are done from an altitude where they are favorable, and makes it even less likely that they will have any issues. As a result, this has happened fairly frequently, as others have mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There no doubt is coordination if, say, both are intended for the ISS, but I don't think they actually would launch two vehicles at the same time for the ISS.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-23T10:05:03.630" LastActivityDate="2013-09-23T10:05:03.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2085" PostTypeId="2" ParentId="2074" CreationDate="2013-09-22T20:08:08.470" Score="7" Body="&lt;p&gt;Water is a pretty common substance across the universe. It also happens to be one of the lightest compounds that is found in a solid state that contains hydrogen. Hydrogen can be used in a fusion operation. The general idea is that you somehow separate the oxygen and hydrogen, and fuse the hydrogen together. Doing so will give you a burst of energy, which you can use somehow to accelerate your spacecraft, perhaps by accelerating the oxygen molecule, or otherwise. The exact mechanism depends on the imagination of the author, but in general, it depends on fusing hydrogen.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-22T20:08:08.470" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2086" PostTypeId="1" AcceptedAnswerId="2087" CreationDate="2013-09-22T20:29:12.543" Score="26" ViewCount="14658" Body="&lt;p&gt;In videos of astronauts walking on the Moon, it seems very hard to move around with any efficiency, and it seems impossible to run.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this? What could be done to mitigate these effects? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="12102" LastEditDate="2019-07-31T02:51:02.853" LastActivityDate="2024-02-25T21:14:24.197" Title="Why is it so hard to walk on the Moon?" Tags="|the-moon|reduced-gravity-sports|moonwalking|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2087" PostTypeId="2" ParentId="2086" CreationDate="2013-09-22T21:22:59.570" Score="26" Body="&lt;p&gt;More than anything, the reason for this were cumbersome EVA (Extravehicular Activity) suits used with the six &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo program&lt;/a&gt; missions that landed on the Moon. They were basically a one-piece suit, each made to fit (custom tailored) each of the astronauts.&lt;/p&gt;&#xA;&lt;p&gt;There were two basic configurations of the &lt;em&gt;space suits&lt;/em&gt; used to support Apollo Missions:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;An Intravehicular (IV) configuration designated as the CMP A7LB PGA and worn by the Command Module Pilot, Crew Commander and the Lunar Module Pilot during their stay in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo Command/Service Module&lt;/a&gt;, and&lt;/li&gt;&#xA;&lt;li&gt;An Extravehicular (EV) configuration identified as the EV A7LB PGA and worn by the Crew Commander and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo Lunar Module&lt;/a&gt; Pilot during Lunar landing, for extravehicular activities on the Lunar surface, and during Lunar Orbit Rendezvous of the Lunar Module with the Command/Service Module.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;We will be more interested in the latter, the Apollo EV A7LB PGA EV suit to answer the question though, since it was the suit used for extravehicular activities on the Moon's surface by &lt;a href=&quot;//space.stackexchange.com/a/1903/49&quot;&gt;the 12 moonwalkers&lt;/a&gt;. Here is a photograph of how it looked like:&lt;/p&gt;&#xA;&lt;p&gt;   &lt;img src=&quot;https://i.stack.imgur.com/E9Q3Y.jpg&quot; alt=&quot;Apollo EV A7LB PGA EV suit&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;   &lt;sup&gt;Apollo 11 A7LB space suit worn by &lt;a href=&quot;https://en.wikipedia.org/wiki/Gene_Aldrin&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gene Cernan&lt;/a&gt; on lunar surface. Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo/Skylab_A7L&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Apollo/Skylab A7L&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;So we mentioned they were custom tailored suits, but they also look pretty cumbersome on their own, let alone with the heavy &lt;a href=&quot;https://en.wikipedia.org/wiki/Primary_Life_Support_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Primary Life Support System&lt;/a&gt; (PLSS) with a separate &lt;a href=&quot;http://www.hq.nasa.gov/alsj/alsj-OPS.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Oxygen Purge System&lt;/a&gt; (OPS) mounted on top of the PLSS, both strapped on the astronaut's back. While the gravity is of course a lot smaller on the Moon compared to sea level gravity on the Earth (roughly 1/6), PLSS still weighed 6.4 kg (14 lb) and the OPS 3.1 kg (6.8 lb) on the Moon. That's additional 9.5 kg (20.8 lb) of bulk that each moonwalker had to carry around.&lt;/p&gt;&#xA;&lt;p&gt;But that's not the whole story. The EVA suit itself weighed 34.5 kg (76 lb) on the Earth, translating to roughly 5.7 kg (12.6 lb) on the surface of the Moon, and we still didn't add the helmet, and the boots, and any other equipment standard on Apollo lunar EVA to the bulk, and all of that amounted to a total of roughly an additional weight of 18-20 kg (40-44 lb) that needed to be carried around at all times during the lunar EVA.&lt;/p&gt;&#xA;&lt;p&gt;These roughly 20 kg (44 lb) might not sound as much, but try working with this bulk on your back for several hours, and you'll soon appreciate the weight. Quite literally. And that doesn't even account for the weight of any samples astronauts took from the lunar surface and needed to carry around in sample bags before returning them to the Lunar Module during each day's EVA. So while Apollo moonwalkers were rather happy with how their space suits were, with &lt;a href=&quot;https://en.wikipedia.org/wiki/Neil_Armstrong&quot; rel=&quot;nofollow noreferrer&quot;&gt;Neil Armstrong&lt;/a&gt; being quoted saying his Apollo 11 A7L space suit to be &lt;em&gt;&amp;quot;tough, reliable and almost cuddly&amp;quot;&lt;/em&gt;, they were also cumbersome to work in for longer periods of time, and astronauts weren't able to keep up with their packed tight schedules.&lt;/p&gt;&#xA;&lt;p&gt;Of course, as  &lt;a href=&quot;//space.stackexchange.com/questions/2086/why-is-it-so-hard-to-walk-on-the-moon/2087#comment3838_2087&quot;&gt;Loren Pechtel commented&lt;/a&gt;, it's not merely the weight of the EVA suits in Lunar gravity, but their mass too. &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertia&quot; rel=&quot;nofollow noreferrer&quot;&gt;Inertia&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's first law of motion&lt;/a&gt; will prevent the suit to change speed and direction without a sufficient force to move its mass where the moonwalker in it intended to move. So they might weigh less, but they still have (more or less, they did get rather dusty and PLSS/OPS were not closed-loop systems) same mass as they did on Earth.&lt;/p&gt;&#xA;&lt;p&gt;NASA soon realized this and included the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Roving Vehicle&lt;/a&gt; (LRV) in the last three missions of the Apollo program (Apollo 15-17) between the years 1971 to 1972 to help moonwalkers traverse distances needed to complete their scheduled duties:&lt;/p&gt;&#xA;&lt;p&gt;   &lt;img src=&quot;https://i.stack.imgur.com/04rGm.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;   &lt;sup&gt;The U.S. Apollo Lunar Roving Vehicle from &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 15&lt;/a&gt; on the moon in 1971. Photograph and following quote source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;LRVs were used for greater surface mobility during the Apollo &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Apollo_mission_types&quot; rel=&quot;nofollow noreferrer&quot;&gt;J-class&lt;/a&gt;&#xA;missions, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 15&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_16&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 16&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_17&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 17&lt;/a&gt;. The rover was first&#xA;used on 31 July 1971, during the Apollo 15 mission. This greatly&#xA;expanded the range of the lunar explorers. Previous teams of&#xA;astronauts were restricted to short walking distances around the&#xA;landing site due to the bulky space suit equipment required to sustain&#xA;life in the lunar environment. The range, however, was operationally&#xA;restricted to remain within walking distance of the lunar module, in&#xA;case the rover broke down at any point. The rovers were designed&#xA;with a top speed of about 8 mph (13 km/h), although &lt;a href=&quot;https://en.wikipedia.org/wiki/Eugene_Cernan&quot; rel=&quot;nofollow noreferrer&quot;&gt;Eugene Cernan&lt;/a&gt;&#xA;recorded a maximum speed of 11.2 mph (18.0 km/h), giving him the&#xA;(unofficial) lunar land-speed record.&lt;/p&gt;&#xA;&lt;p&gt;The LRV was developed in only 17 months and performed all its&#xA;functions on the Moon with no major anomalies. Scientist-astronaut&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Harrison_Schmitt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Harrison Schmitt&lt;/a&gt; of Apollo 17 said, &lt;em&gt;&amp;quot;The Lunar Rover proved to be the&#xA;reliable, safe and flexible lunar exploration vehicle we expected it&#xA;to be. Without it, the major scientific discoveries of Apollo 15, 16,&#xA;and 17 would not have been possible; and our current understanding of&#xA;lunar evolution would not have been possible.&amp;quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So there we have it, hopefully answering all your questions, except maybe why none of the moonwalkers was seen running. Not an official explanation at any length, but I'd presume Apollo astronauts not to be of the easily scared variety, and not inclined to running for no good reason. There was a fair deal of jumping going on on the lunar surface, though:&lt;/p&gt;&#xA;&lt;p&gt;   &lt;img src=&quot;https://i.stack.imgur.com/CZbfk.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;On a more serious note though, the PLSS and OPS systems had performance parameters that meant they will only be capable of sufficiently supplying oxygen, cooling down and circulating the air inside the space suit at a certain rate, to prevent the astronaut from overheating, excessive sweating, and using up the oxygen supply too quickly.&lt;/p&gt;&#xA;&lt;p&gt;Even if one of the astronauts tried running, they would most likely be advised against it by their Flight Director, a person they would most likely obey, if for no other reason than respect for &lt;a href=&quot;https://en.wikipedia.org/wiki/Gene_Kranz&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gene Krantz&lt;/a&gt;, one of the Apollo program Flight Directors that also directed team efforts to save the crew of &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 13&lt;/a&gt; and was in that position also for the first landing on the Moon by &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 11&lt;/a&gt; Lunar Module (LM) named &lt;em&gt;&amp;quot;Eagle&amp;quot;&lt;/em&gt;, on July 20, 1969.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2024-02-25T21:14:24.197" LastActivityDate="2024-02-25T21:14:24.197" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="2088" PostTypeId="2" ParentId="2086" CreationDate="2013-09-22T21:32:06.947" Score="3" Body="&lt;p&gt;The joints.  Bending your elbows, knees, finger joints -- anything -- required you to basically compress a balloon.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-09-22T21:32:06.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2089" PostTypeId="2" ParentId="2080" CreationDate="2013-09-23T01:50:31.053" Score="8" Body="&lt;p&gt;Major space launching facilities have multiple launch pads and thus the capacity to launch multiple vehicles at the same time. As already answered, this is not usually done these days for logistic reasons, but multiple consecutive launches were fairly common during the heyday of the space (and ICBM) race. People had already mentioned the american exercise, however, USSR did something similar as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider, for example, &lt;strong&gt;&lt;a href=&quot;http://www.astronautix.com/sites/baikonur.htm&quot;&gt;Baikonur Cosmodrome&lt;/a&gt;, September 28, 1971&lt;/strong&gt;. Two launches of heavy vehicles from the same  facility only 2 hours apart - and one of those going to the Moon:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;September 28, 1971, 07:40 GMT, Baikonur Cosmodrome, Launch Complex &lt;a href=&quot;http://www.astronautix.com/sites/baiurlc1.htm&quot;&gt;LC1&lt;/a&gt;&lt;/strong&gt;:&lt;br /&gt;Russian &lt;a href=&quot;http://www.astronautix.com/lvs/vos11a57.htm&quot;&gt;Voskhod 11A57&lt;/a&gt; orbital launch vehicle launches a 6,300 kg military surveillance type high resolution photo reconnaissance satellite named &lt;a href=&quot;http://www.astronautix.com/craft/zenit4m.htm&quot;&gt;Zenit-4M&lt;/a&gt; on a Cosmos 441 mission into &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot;&gt;Low Earth Orbit&lt;/a&gt; (LEO).&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;September 28, 1971, 10:00 GMT, Baikonur Cosmodrome, Launch Complex &lt;a href=&quot;http://www.astronautix.com/sites/baic8124.htm&quot;&gt;LC81/24&lt;/a&gt;&lt;/strong&gt;:&lt;br /&gt;Russian &lt;a href=&quot;http://www.astronautix.com/lvs/vos11a57.htm&quot;&gt;Voskhod 11A57&lt;/a&gt; orbital launch vehicle carries 5,820 kg &lt;a href=&quot;http://www.astronautix.com/craft/lunye8ls.htm&quot;&gt;Luna Ye-8-LS&lt;/a&gt; lunar orbiter on a Luna 19 mission to investigate the Moon and near-lunar space into an intermediate earth parking orbit and was then put on a translunar trajectory by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Blok_D&quot;&gt;Proton Block D stage&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="743" LastEditorUserId="49" LastEditDate="2013-09-23T02:44:16.303" LastActivityDate="2013-09-23T02:44:16.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2090" PostTypeId="5" CreationDate="2013-09-23T02:59:41.390" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-23T02:59:41.390" LastActivityDate="2013-09-23T02:59:41.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2091" PostTypeId="4" CreationDate="2013-09-23T02:59:41.390" Score="0" Body="Questions regarding extravehicular activity performed on the surface of the moon." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-23T09:32:43.927" LastActivityDate="2013-09-23T09:32:43.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2092" PostTypeId="1" CreationDate="2013-09-23T09:03:29.227" Score="7" ViewCount="298" Body="&lt;p&gt;A gasket is a precisely formed body of some (yielding) material placed at a mating surface usually to prevent leakage. It may also be constructed of an alloy as was done in early IC engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft work in a vacuum, typically with instruments, engine/s, and fuel/s. Anthropic habitats are restricted to &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Spacesuits, &lt;/li&gt;&#xA;&lt;li&gt;ISS, &lt;/li&gt;&#xA;&lt;li&gt;Crew/resupply missions&lt;/li&gt;&#xA;&lt;li&gt;Test flight&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The loss of a craft to a faulty gasket would be quite the anti-climax given the figures usually bandied against mere construction/testing of the spacecraft, to say nothing of launch/deployment. It may be worse to have it happen to a habitat.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Has any mission to-date &lt;strong&gt;in orbit, or beyond&lt;/strong&gt; suffered a damaged gasket? &#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are gaskets even used in spacecraft?&lt;/li&gt;&#xA;&lt;li&gt;What material are such gaskets constructed of?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;What procedures are in place for damage control against such an event?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-03-19T04:06:02.137" LastActivityDate="2014-06-28T12:19:33.333" Title="Has any spacecraft to date suffered catastrophic gasket degeneration in space?" Tags="|materials|engineering|design|vacuum|outgassing|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2093" PostTypeId="2" ParentId="2055" CreationDate="2013-09-23T10:08:53.813" Score="3" Body="&lt;p&gt;Of particular interest in scenarios such as this where the delay could be extremely large and the communication medium lossy is the combination of repetition and multiple options:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the craft is populated by humans you can allow a certain amount of autonomy, but you may still want to provide alternative messages. As a simplified example: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If scenario x occurs, follow the instructions in envelope A, otherwise follow the instructions in envelope B. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How this would translate into modern day comms would be to send a series of alternative instructions giving guidance/orders based on possible outcomes as predicted from Earth: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If that asteroid is iron based, take 4 extra days to analyse; or if it is water ice, replenish reaction mass; or if it is populated ask to see their leader&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A problem with providing multiple messages is that loss of packets could cause confusion, hence the requirement to repeat. Unlike TCP/IP where the recipient can state &quot;I missed packets 10405 and 11200&quot; the lag would be too long, so a continuous stream of message is likely to be needed. As you can imagine, this can lead to a requirement for multiplexing all the comms that may be needed to the explorer on one channel (or more) and all the return comms on another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You start with a range of predicted scenarios (back to the original messages in envelopes scenario - or store them in HAL's memory) and leave it up to the explorer.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-09-23T10:08:53.813" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2094" PostTypeId="1" CreationDate="2013-09-23T10:13:58.757" Score="9" ViewCount="463" Body="&lt;p&gt;The Cydonia region of Mars is littered with mesas, which are an unusual geographic feature on Mars.  A sample of a few of these, including the commonly known as &quot;The Face on Mars&quot;, is shown below. How were these mesas actually constructed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/3/38/Martian_face_viking.jpg&quot; alt=&quot;enter image description here&quot;&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.253" LastActivityDate="2013-09-24T17:28:30.243" Title="What caused Cydonia mesas, such as &quot;The Face on Mars&quot;, to be created?" Tags="|mars|planetary-science|geology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2095" PostTypeId="1" AcceptedAnswerId="2099" CreationDate="2013-09-23T10:42:48.647" Score="211" ViewCount="23865" Body="&lt;p&gt;According to Wikipedia there are &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle#Current_locations&quot;&gt;3 LRVs&lt;/a&gt; sitting on the moon, in very well defined locations. Would China, or whomever gets to the moon with a person next, be able to get in and drive around in an LRV left by the US?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/04rGm.jpg&quot; alt=&quot;Apollo Lunar Roving Vehicle (LRV)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The U.S. Apollo Lunar Roving Vehicle from &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15&quot;&gt;Apollo 15&lt;/a&gt; on the moon in 1971. Photograph source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot;&gt;Wikipedia on Lunar Roving Vehicle&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="9092" LastEditDate="2021-02-07T02:35:00.780" LastActivityDate="2023-03-23T01:12:29.797" Title="Can I borrow a lunar rover?" Tags="|the-moon|nasa|apollo-program|crewed-spaceflight|lunar-rover|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2096" PostTypeId="2" ParentId="2095" CreationDate="2013-09-23T11:39:06.980" Score="23" Body="&lt;p&gt;You probably can.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lunar rovers are powered by two 36-volt silver zinc batteries which cannot be recharged. The lunar rover has a productive life of nearly three lunar days (or it can travel a distance of 92 kilometres). Since it has not been used after its mission, it might have lost its power by now, because of &lt;a href=&quot;http://en.m.wikipedia.org/wiki/Self-discharge&quot; rel=&quot;noreferrer&quot;&gt;self discharging&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only way is to carry suitable batteries and replace the old one, which is a tedious process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.hq.nasa.gov/alsj/a17/A17_LunarRover2.pdf&quot; rel=&quot;noreferrer&quot;&gt;Lunar Roving Vehicle specs - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2016-01-28T02:09:55.243" LastActivityDate="2016-01-28T02:09:55.243" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2097" PostTypeId="1" CreationDate="2013-09-23T11:55:32.650" Score="10" ViewCount="821" Body="&lt;p&gt;Since the Moon has no atmosphere, is it possible to perform the &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo%27s_Leaning_Tower_of_Pisa_experiment&quot;&gt;Galileo's Leaning Tower of Pisa experiment&lt;/a&gt; on its surface? Has any astronaut performed anything similar?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-09-23T12:50:51.713" LastActivityDate="2019-07-31T23:50:52.863" Title="Is it possible to perform the Galileo's Leaning Tower of Pisa experiment on the Moon?" Tags="|the-moon|astronauts|gravity|experiment|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2098" PostTypeId="2" ParentId="2097" CreationDate="2013-09-23T12:15:02.903" Score="20" Body="&lt;p&gt;A very similar experiment was performed, dropping a feather and a hammer on the moon. They weren't tied together, as &lt;a href=&quot;http://en.wikipedia.org/wiki/Galileo%27s_Leaning_Tower_of_Pisa_experiment&quot; rel=&quot;noreferrer&quot;&gt;Galileo's experiment&lt;/a&gt; was done, but it is similar enough that it is worth mentioning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GmHJb.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GmHJb.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;From this &lt;a href=&quot;http://www.youtube.com/watch?v=5C5_dOEyAfk&quot; rel=&quot;noreferrer&quot;&gt;YouTube Video&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="18663" LastEditDate="2019-07-31T23:50:52.863" LastActivityDate="2019-07-31T23:50:52.863" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="2099" PostTypeId="2" ParentId="2095" CreationDate="2013-09-23T12:27:09.037" Score="173" Body="&lt;p&gt;Fun question! Provided the three &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;Lunar Roving Vehicles&lt;/a&gt; (LRV) left on the surface of the Moon by the last three &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;noreferrer&quot;&gt;Apollo program&lt;/a&gt; missions were not, tongue-in-cheek, towed away for unpaid parking, reckless driving and littering fines, or clamped by the &lt;a href=&quot;https://english.stackexchange.com/questions/127824/what-do-you-call-hypothetical-inhabitants-living-on-the-moon/&quot;&gt;Lunar people&lt;/a&gt; (not to be confused with &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunatic&quot; rel=&quot;noreferrer&quot;&gt;Lunatics&lt;/a&gt;), I don't see why not, provided you have brought along all the spare parts you might need and can make them work:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/sI5O0.jpg&quot; alt=&quot;Clamped Lunar Roving Vehicle (CLRV)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Clamped Lunar Roving Vehicle (CLRV). Seriously, NASA! (Photograph attribution: NASA and yours truly)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seriously though, while I might not be a &lt;em&gt;space lawyer&lt;/em&gt;, there would be nothing wrong in legal terms to &lt;em&gt;borrow and reuse a vehicle abandoned on &lt;a href=&quot;https://en.wikipedia.org/wiki/No_man%27s_land&quot; rel=&quot;noreferrer&quot;&gt;no man's land&lt;/a&gt;&lt;/em&gt;. Americans might have been sticking their national flags into the lunar surface, but that wasn't a territorial claim, just a display of national pride for making it there first. Nobody is a legal authority on the Moon, not yet at least, and even if there was someone, would they be able to stop you appropriating by now a defunct vehicle? I don't think so. It would be customary, a polite gesture, to ask their previous owners first. But it most certainly wouldn't be a &lt;a href=&quot;https://en.wikipedia.org/wiki/Motor_vehicle_theft&quot; rel=&quot;noreferrer&quot;&gt;Grand Theft Auto&lt;/a&gt;, even if you didn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would need quite a few spare parts on you to make it work, though. It has been &lt;a href=&quot;https://space.stackexchange.com/a/2096/49&quot;&gt;mentioned previously&lt;/a&gt; that the batteries these LRVs used to run their engines with would have been depleted by now. Partially because of them being used up to a great extent by the Apollo missions themselves, partially because of self-discharging, and partially because of exposure to solar radiation decaying their chemical composition. Either case, they would be utterly out of juice and possibly impossible to recharge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, there are the tires. They weren't inflatable, so merely bringing a hand air pump along wouldn't work, and is not merely a matter of the pump not working because there is no real atmosphere to speak of (there is a tiny bit of pressure though due to what we call an &lt;a href=&quot;https://en.wikipedia.org/wiki/Exosphere&quot; rel=&quot;noreferrer&quot;&gt;exosphere&lt;/a&gt;) on the surface of the Moon. So while the tires were made of zinc-coated and woven steel strands, they are still likely in rather poor condition with good 45 years of continuous &lt;em&gt;bombardment&lt;/em&gt; of highly charged particles in Coronal Mass Ejections (CME), Solar radiation and the lunar surface &lt;a href=&quot;https://en.wikipedia.org/wiki/Regolith&quot; rel=&quot;noreferrer&quot;&gt;regolith&lt;/a&gt; electromagnetic discharge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other parts of LRVs would also likely be in terrible condition, with maybe their aluminum alloy frame, fender extensions and the metallic parts of the seats still in functional order, but the rest would be decayed due to previously mentioned environmental conditions. You would likely need heavier equipment and spare parts to just get them running again, than your brand new vehicle you could bring along would weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yeah, while a fun question, the answer is not really in &lt;em&gt;can you&lt;/em&gt;, but more in &lt;em&gt;could you&lt;/em&gt;. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-09-23T16:18:37.813" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2100" PostTypeId="2" ParentId="2095" CreationDate="2013-09-23T12:31:35.930" Score="108" Body="&lt;p&gt;Probably, but you likely wouldn't drive it away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not only will the batteries be completely discharged, but the rovers have gone through a large number of lunar days since then, creating a large thermal cycle. The number of thermal cycles is on the order of 500 for each one, and the &lt;a href=&quot;http://www.lunarpedia.org/index.php?title=Lunar_Temperature&quot; rel=&quot;noreferrer&quot;&gt;cycles&lt;/a&gt; are brutal, going from -150C to as high as 120C each time. Even averaging that out a bit, that's still a 200 degree thermal swing, which will cause problems will all sorts of things. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wheels &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Roving_Vehicle#Wheels_and_power&quot; rel=&quot;noreferrer&quot;&gt;are composed&lt;/a&gt; of a variety of metals. Most metals have a slightly different expansion coefficient, most likely causing them to have expanded/contracted at different rates. No doubt they have some holes in them by now, the best bet would be to just replace them. Similarly, the seats are probably gone. The metal frame is probably mostly intact, but will contain some cracks in it, causing significant losses of structural capacity. No doubt there would be other problems as well. The exact types of problems are difficult to guess ahead of time, but it would require bringing a whole lot of spare parts, for technology from 40 years ago. I'm also not sure, but I would suspect that some of the items, such as the tires, have actually exploded, resulting in some sort of collateral damage to the rover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the thermal cycles, there are effects caused by the moon itself. While there isn't much of an atmosphere, the dust slowly flies across the moon, meaning that dust most likely covered it. Most likely, there are a few small impacts as well from micrometeorites. And the electronics are probably fried completely as a result of being exposed to high levels of radiation for years. The bearings have probably seized in the wheels, and overall, they are probably in really poor shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the images taken of the LRVs from orbit, it appears that they haven't completely fallen apart, but they are darker than I would expect them to be. I can't see the antenna mounted on the rover as is seen in the photograph above. In fact, the rover itself isn't visible, only the shadow. This indicates to me that it is either covered in dust, or most of the interior stuff has fallen off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/o4gG9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My personal thought, if you can get to the rovers, take them back to Earth to put in a museum. Making one of the lunar rovers work would be more akin to restoring a classic car that hasn't been taken care of at all for years, it would require a lot of time and energy. It seems more suited for a museum. This should fall under similar rules to the &lt;a href=&quot;http://www.cnn.com/2013/07/19/tech/innovation/amazon-apollo-engines/index.html&quot; rel=&quot;noreferrer&quot;&gt;Apollo Engines&lt;/a&gt; found on the Ocean Bed, NASA wants some say, but the person who finds them has a lot to say about what happens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZSO0L.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give you an idea of how well something can survive on the Moon, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Surveyor_3&quot; rel=&quot;noreferrer&quot;&gt;Surveyor 3&lt;/a&gt; lander was visited by Apollo 12. Some of the Surveyor landers survived multiple lunar nights, but Surveyor 3 failed to accomplish this task. There is actually &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720019081_1972019081.pdf&quot; rel=&quot;noreferrer&quot;&gt;an entire paper&lt;/a&gt; written on the subject. The results are that everything survived relatively well for the 2 years Surveyor was on the Moon. There was a failure in one of the legs, discoloration, dust contamination, and failure of the electronics, but otherwise the spacecraft appears to be in good overall condition. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/7/77/Surveyor_3_on_Moon.jpg&quot; alt=&quot;Surveyor 3&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, I suggest just bringing your own rover. It'll just work better that way. The lunar rovers weren't all that difficult to get to the moon 40 years ago, it shouldn't be all that difficult to make a better replacement for them.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.267" LastActivityDate="2015-04-16T18:03:46.680" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2101" PostTypeId="1" AcceptedAnswerId="2113" CreationDate="2013-09-23T13:41:09.053" Score="23" ViewCount="23584" Body="&lt;p&gt;We all know that for most people access to space is currently pretty limited because space flight services are expensive, as they are in short supply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose I wanted to bypass an expensive service and get into space as quickly as possible. Could I build a &lt;strong&gt;suborbital spaceflight vehicle&lt;/strong&gt; in my backyard with components that an individual could get access to?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the components are easy enough to access, but the number of people doing the construction is a limiting factor--could I do it with 5? 10? Any number less than 100 (at which point the line between an organization and a DIY effort is blurry)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is possible to get the components and construct the vehicle with a few people, would it be possible for this small effort to control the construction enough to keep the risk factor manageable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would there be any legal considerations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/nlVcAJFU-5E?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Video by Roy Dawson&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;For more information on the pictured DIY Saturn V replica rocket, check out &lt;a href=&quot;http://www.popularmechanics.com/home/how-to-plans/woodworking/4329771&quot;&gt;Cool DIY Engineering Projects at Popular Mechanics&lt;/a&gt;.&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; I'm not asking if this could be done on the ordinary man's &lt;strong&gt;budget&lt;/strong&gt;. Supposing the average person could get access to all the money that they needed to get it done, could they do it themselves or with a small group of people? The answer should still mention the cost, however; as it is an important practical determiner of whether or not it &lt;em&gt;will&lt;/em&gt; actually be done.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="1142" LastEditDate="2015-02-27T17:54:20.307" LastActivityDate="2017-12-18T23:18:19.250" Title="Could I build my own spaceship?" Tags="|rockets|law|spacecraft-development|crewed-spaceflight|sub-orbital|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2102" PostTypeId="1" CreationDate="2013-09-23T14:03:49.567" Score="23" ViewCount="1651" Body="&lt;p&gt;Is there a common definition of where the &lt;em&gt;space&lt;/em&gt; begins on other planets (and other celestial bodies)? An equivalent of the Karman line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How far one should go from the martian surface to get to the outer space? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I already in space jumping 2 meters above the Moon surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(please don't answer that anything beyond the earthly Karman line (including any other planets' surfaces) is in space by definition).&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2020-01-28T13:16:47.420" LastActivityDate="2020-04-14T16:47:34.397" Title="Karman line analog on other celestial bodies" Tags="|terminology|karman-line|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2103" PostTypeId="1" AcceptedAnswerId="2315" CreationDate="2013-09-23T14:08:55.097" Score="16" ViewCount="11834" Body="&lt;p&gt;As I understand the idea of a lunar flyby, you can pass a spacecraft by the far side of the moon without having to burn fuel to decrease your momentum and get to LLO (Low Lunar Orbit).  But it seems like multiple Hohmann transfer applications would be insufficient here.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would you go about this calc, and what are all of the things involved?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if you require that you come very close to the lunar surface?  Would that require more fuel?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="288" LastEditDate="2013-09-23T14:18:37.217" LastActivityDate="2014-06-27T02:02:09.417" Title="How to calculate Delta V for lunar flyby" Tags="|the-moon|delta-v|flyby|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2104" PostTypeId="2" ParentId="187" CreationDate="2013-09-23T17:40:49.767" Score="1" Body="&lt;p&gt;Gravity turns out to be an extremely convenient force when you want to exercise. Here on Earth, we mostly use it for two purposes. The more obvious of the two is that it provides resistance to work against: either your own bodyweight (when running, doing pull-ups, etc) or the weight of other objects (when lifting weights).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The less obvious use of gravity is that it holds objects in place or pulls them back into place. Without gravity, you'd never be able to run for more than one step: you'd launch into the air and never come back down. When you swim, gravity holds you on the surface of the water, and it holds the water in the pool. Even when you're lifting weights, you have to put the weights down from time to time, and while you do this, gravity is what holds you onto the bench or floor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you're in a microgravity environment, you can't use gravity for either of these things, but you still need to address both issues in some way. This is what we're still perfecting, though we have made progress.&lt;/p&gt;&#xA;" OwnerUserId="756" LastActivityDate="2013-09-23T17:40:49.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2105" PostTypeId="1" CreationDate="2013-09-23T18:29:07.237" Score="6" ViewCount="222" Body="&lt;p&gt;Is there a set of rules or guidelines for how one ought to behave whilst in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apart from all of the necessary rules in place for safety reasons, do astronauts and cosmonauts live by a unique etiquette, necessary only when in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing that stands out to me is conversing to others in zero-G. I'd find it especially annoying if the person I was talking was upside-down and refused to 'stand in line'(?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have there been any studies on how others react when a person defies both social norms and natural laws (&lt;em&gt;gravity&lt;/em&gt;) at the same time?  Are there any documented suggestions or protocols for how to orient yourself to others?  &lt;/p&gt;&#xA;" OwnerUserId="703" LastEditorUserId="109" LastEditDate="2013-09-24T10:31:42.943" LastActivityDate="2013-09-24T10:31:42.943" Title="Is there 'Space Etiquette' for orientation?" Tags="|microgravity|psychology|" AnswerCount="0" CommentCount="22" ClosedDate="2013-09-23T20:25:40.140" ContentLicense="CC BY-SA 3.0" />
  <row Id="2106" PostTypeId="2" ParentId="2086" CreationDate="2013-09-23T18:32:21.463" Score="10" Body="&lt;ul&gt;&#xA;&lt;li&gt;The EVA suits worn are big and bulky - leading to a odd distribution&#xA;of weight - throwing off the balance.&lt;/li&gt;&#xA;&lt;li&gt;Aside from the bulk of the suit - when you're in a pressure-suit in a&#xA;hard-vacuum environment, bending your arms/legs changes the volume of&#xA;the suit somewhat. For a practical example, inflate a latex glove and&#xA;try bending the fingers - they snap-back into position from&lt;br&gt;&#xA;air-pressure - the moonwalkers are dealing with the same thing.&lt;/li&gt;&#xA;&lt;li&gt;Low gravity is hard to walk in when one is accustomed to&lt;br&gt;&#xA;Earth-gravity. Push too hard off the surface and you'll accidentally &#xA;'jump' and possibly tumble off of your feet.&lt;/li&gt;&#xA;&lt;li&gt;The lunar surface is covered in very fine dust (known as Regolith)&lt;br&gt;&#xA;from billions of years of meteor hits and solar radiation. It tends&lt;br&gt;&#xA;to be 'slippery' sorta like trying to run/walk on beach-sand. It's&lt;br&gt;&#xA;hard to get good footing.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;How to fix this? Newer materials and tech could lighten a suit somewhat. There's also research into suits that don't use air-pressure, but instead strap-around the body like a compression-garment to counter the vacuum. (There's air for breathing of course) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving in low-gravity is something we don't have a practical fix for. Practice in actual low-grav is about the only way to do it, and it's hard to get to the moon to practice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the lunar surface goes, we're thinking of paving the place. (No really: &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2005-2510&quot; rel=&quot;noreferrer&quot;&gt;http://arc.aiaa.org/doi/abs/10.2514/6.2005-2510&lt;/a&gt; ) &lt;/p&gt;&#xA;" OwnerUserId="692" LastActivityDate="2013-09-23T18:32:21.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2107" PostTypeId="1" AcceptedAnswerId="2115" CreationDate="2013-09-23T20:19:34.990" Score="9" ViewCount="1054" Body="&lt;p&gt;Spacecraft in terrestrial Solar System typically use &lt;a href=&quot;http://en.wikipedia.org/wiki/Photovoltaics&quot; rel=&quot;nofollow noreferrer&quot;&gt;PV&lt;/a&gt; cells to power their instruments. As the referenced Wikipedia article writes, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;PV meant for operation in vacuum &lt;a href=&quot;https://physics.stackexchange.com/questions/31907/is-a-photovoltaic-cell-on-an-artificial-satellite-the-same-construction-as-one-u/&quot;&gt;differ&lt;/a&gt; from those meant for use on Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus is closer to Sol than Earth. In orbit it receives greater radiant energy from Sol. It follows a PV cell may deliver more power. This may vary from one cell design to another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.emcore.com/wp-content/uploads/ZTJ_Datasheet.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ZTJ datasheet&lt;/a&gt; writes to convey the cell is capable of providing 2.726V (open circuit), and 17.4mA (short-circuit).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How much more power could this cell deliver in Venerean orbit? &lt;/li&gt;&#xA;&lt;li&gt;Since all PV cells, regardless of design, would encounter similar solar radiance would it follow the power generation of all PV cells may increase in corresponding proportion?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2013-09-24T06:53:01.753" Title="How much more power does a PV cell generate in Venerean orbit?" Tags="|venus|solar-power|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2108" PostTypeId="1" AcceptedAnswerId="2109" CreationDate="2013-09-23T22:03:45.827" Score="15" ViewCount="1762" Body="&lt;p&gt;The basic idea behind a key for a vehicle is to keep an unauthorized person from using your car to go somewhere else - probably further away from you than you would like your transportation to be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would think that this would be true for spacecraft as well - I certainly wouldn't want someone stealing my Saturn V rocket!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it seems that there is much more required to launch these space vehicles. Has the system of a key ever been used on a spacecraft for security?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2020-07-04T23:42:54.520" Title="Do any spacecraft use carkeys?" Tags="|launch|security|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2109" PostTypeId="2" ParentId="2108" CreationDate="2013-09-23T23:56:13.230" Score="16" Body="&lt;p&gt;This is the Soyuz-27 start key:&lt;img src=&quot;https://i.stack.imgur.com/SsknK.jpg&quot; alt=&quot;This is the Soyuz start key&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is the keyhole: &lt;img src=&quot;https://i.stack.imgur.com/7LBlh.jpg&quot; alt=&quot;This is the keyhole&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz rocket is nee ICBM R-7. And that is exactly the ICBM key.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is all happening in a bunker. The &lt;em&gt;shooter&lt;/em&gt; gives a series of commands before the final countdown. One of them is &quot;key to start&quot;. It initiates the automated starting sequence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tradition prescripts to give the key to the cosmonauts after they return.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.youtube.com/watch?v=Wyrj0ibxt8c&quot; rel=&quot;noreferrer&quot;&gt;This recording of Yuri Gagarin radio communication&lt;/a&gt; with ground control @00:26 gives an example of &quot;key to start&quot; command. It is translated as &quot;ignition key to starting position&quot; in the subs.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-09-26T22:19:56.307" LastActivityDate="2013-09-26T22:19:56.307" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2110" PostTypeId="1" AcceptedAnswerId="10753" CreationDate="2013-09-24T01:34:58.050" Score="6" ViewCount="880" Body="&lt;p&gt;I'm thinking about something like this: Suppose you &lt;em&gt;really wanted&lt;/em&gt; to get into orbit and didn't care about getting back down / staying alive up there. You sneak into a launch facility at night, push some kind of button in a bunker somewhere, and run like crazy to get into the capsule. You then get blasted into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any rocket configuration that has been used that would allow you to do this? The main requirements would be, as far as I can tell:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;There must be some kind of master control which would initiate the launch sequence,&lt;/li&gt;&#xA;&lt;li&gt;It must be physically possible to be in the rocket by the time it lifted off,&lt;/li&gt;&#xA;&lt;li&gt;The rocket must not need any kind of ground tracking to be operational.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note that the only requirement for you is that you make it to orbit in one piece, and could survive there for a substantial (&gt;30 minutes) period of time. Also, let's say you don't have time to make any major adjustments to the systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any configuration used in the past for a mission that could conceivably allow this?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2017-07-27T03:03:11.263" Title="Are there any space vehicles in which it would be possible to launch yourself into space on your own?" Tags="|launch|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="2111" PostTypeId="5" CreationDate="2013-09-24T01:46:29.580" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-24T01:46:29.580" LastActivityDate="2013-09-24T01:46:29.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2112" PostTypeId="4" CreationDate="2013-09-24T01:46:29.580" Score="0" Body="Questions regarding scientific procedures performed in space to test a hypothesis or make a discovery.&#xD;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-24T01:50:56.210" LastActivityDate="2013-09-24T01:50:56.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2113" PostTypeId="2" ParentId="2101" CreationDate="2013-09-24T01:50:47.990" Score="16" Body="&lt;p&gt;Could you? Well, maybe. Let's take a look at one instance where people were encouraged to do that very feat, for $10 million. I'm of course talking about the &lt;a href=&quot;http://space.xprize.org/ansari-x-prize&quot; rel=&quot;noreferrer&quot;&gt;Ansari X-Prize&lt;/a&gt; of 10 years ago. This did a few things that were difficult for the time, giving a cash incentive, and helping with some of the permits and other required items to make it happen. Most of the teams were small, but the winner, Scaled Composites, was larger than your criteria. To date, no one has matched this feat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what could you do with a large budget, but on your own? Well, you would first need some sort of a life support module. These are fairly readily available, given some work. Next you would need a large rocket engine and fuel tank, again, fairly easy. The rocket fuel would be a problem, as is somewhat humorously demonstrated in the movie &quot;Astronaut Farmer&quot;. While I doubt that the project could have actually been built as depicted in the movie, I believe that the government wouldn't care much, until it came time to fuel and launch the rocket. The permit would be challenging as well, it's not easy to get a permit to fly a model rocket to 100,000 feet, and to reach space, you'd have to be even higher. Re-entry gear would be problematic as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, you might be able to do it, but it would be a fairly risky enterprise, especially considering the modeling that goes it to most vehicles today prior to launch. But it could be done, given some work, knowledge, and a whole lot of cash, and a bit of luck with the government not getting in the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has been done to an extent in the form of SpaceX, a fairly successful company founded by a billionaire who had to get to space. Elon Musk funded most of the early development, and the company no doubt will someday launch a person in to space, in to orbit. A similar feat has been tried by Jeff Bezos with his company Blue Origin, focusing on sub-orbital missions. It seems likely that Blue Origin will succeed someday as well.  Although both of these aim to be commercial companies, it seems more likely that both are just a way to get people to space by really rich people.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6537" LastEditDate="2017-12-18T23:18:19.250" LastActivityDate="2017-12-18T23:18:19.250" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2114" PostTypeId="1" CreationDate="2013-09-24T04:17:40.853" Score="8" ViewCount="286" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/MPPT&quot; rel=&quot;nofollow&quot;&gt;MPPT&lt;/a&gt; aka Maximum Power Point Tracking is an active mechanism to collect maximum power a &lt;a href=&quot;http://en.wikipedia.org/wiki/photovoltaics&quot; rel=&quot;nofollow&quot;&gt;PV&lt;/a&gt; cell may deliver. This is typically achieved by varying the resistive load connected to a cell.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Juno&lt;/a&gt; will perhaps be the most distant robot in interplanetary yet to rely upon Sol for it's power. Appropinquity to, nay, immersion in &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter&quot; rel=&quot;nofollow&quot;&gt;Jupiter's magnetosphere&lt;/a&gt;, and remoteness from Sol - are two factors (there may be others) that could serve to limit her mission due to loss of power. This may partially be mitigated by an on-board MPPT implementation, which brings us to the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does Juno use MPPT? &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="25" LastEditDate="2015-06-30T11:12:45.493" LastActivityDate="2015-07-30T11:38:28.037" Title="Does Juno use MPPT?" Tags="|probe|solar-power|jupiter|electronics|juno|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2115" PostTypeId="2" ParentId="2107" CreationDate="2013-09-24T05:55:40.617" Score="5" Body="&lt;p&gt;That depends on a lot of factors but if we just look at the irradiance factors we can make a quick comparison.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STC in an acronym for “Factory Standard Test Conditions,” which is 1,000 watts per square meter solar irradiance, 1.5 Air Mass and 77 degrees Fahrenheit cell temperature. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;PTC is an acronym for “Photo Voltaic USA Test Conditions,” which were developed at the PV USA test site at the University of California Davis. The PTC rating represents a more real life condition of 1,000 watts per square meter solar irradiance, 1.5 Air Mass, 68 degrees Fahrenheit ambient temperature at 33 feet above ground level and wind speed of 1 meter per second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a panel's rating is based on 1,000 watts per square meter.  In orbit this is a little stronger 1361 ⁠watts per square meter.  Venus's solar radiance is about 2613.9 watts per square meter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A panel rated at 100W on earth, would produce 136 watts in space and 261 watts around Venus.  This assumes the same environmental conditions, but basically you get about 2x the power.&lt;/p&gt;&#xA;" OwnerUserId="446" LastActivityDate="2013-09-24T05:55:40.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2117" PostTypeId="5" CreationDate="2013-09-24T07:05:11.340" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-24T07:05:11.340" LastActivityDate="2013-09-24T07:05:11.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2118" PostTypeId="4" CreationDate="2013-09-24T07:05:11.340" Score="0" Body="Questions about Cosmic-dust which is a dust found in the cosmos, capable of interacting with the electromagnetic radiation and they are formed by supernovae, birth of star. Use [tag:dust] for dust and particulates associated with the surface of a body." OwnerUserId="414" LastEditorUserId="12102" LastEditDate="2024-02-03T01:17:02.147" LastActivityDate="2024-02-03T01:17:02.147" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2120" PostTypeId="5" CreationDate="2013-09-24T12:20:36.883" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-24T12:20:36.883" LastActivityDate="2013-09-24T12:20:36.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2121" PostTypeId="4" CreationDate="2013-09-24T12:20:36.883" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-24T12:20:36.883" LastActivityDate="2013-09-24T12:20:36.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2122" PostTypeId="1" AcceptedAnswerId="2125" CreationDate="2013-09-24T12:26:53.140" Score="20" ViewCount="124426" Body="&lt;p&gt;The Moon has a large number of dark spots, as can be seen on the photo below (&lt;a href=&quot;http://en.wikipedia.org/wiki/File:FullMoon2010.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;), that differ significantly from the lighter areas. The dark areas often seem to be roundish, but not exclusively. What is the source of the darker areas of the moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qGp7Z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2023-02-23T02:56:45.620" Title="What are the dark areas on the moon?" Tags="|the-moon|planetary-science|geology|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2123" PostTypeId="1" AcceptedAnswerId="6564" CreationDate="2013-09-24T12:36:10.553" Score="13" ViewCount="273" Body="&lt;p&gt;I thought I was going to be able to answer this question for myself, after seeing the paper &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20050217416&quot;&gt;Determination of On-Orbit Cabin Air Loss from the International Space Station (ISS)&lt;/a&gt;.  But it turned out that it is pay-walled.  I don't care about the paper, I just want a number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say that you were planning a long term interplanetary manned mission, so you know the volume in $m^3$ of space filled with breathable air.  During the long leg of the flight, there will be no EVA or docking so the simplifications mentioned in that paper's abstract would be relevant.  What kind of number for leak rate in terms of $\frac{kg}{s}$ or $\frac{m^3}{s}$ per unit volume might be anticipated?  You could combine those units for ultimate units of $\frac{kg}{m^3 s}$ or just $\frac{1}{s}$.  The latter unit would simply represent what fraction of the total volume is lost per unit time.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-12-24T13:54:05.113" Title="What kind of leak rates do modern habitat modules have?" Tags="|space-station|habitat|outgassing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2124" PostTypeId="1" AcceptedAnswerId="2256" CreationDate="2013-09-24T12:46:29.187" Score="17" ViewCount="1161" Body="&lt;p&gt;Since deep space probes and satellites receive commands from Earth via radio, what prevents unauthorized commands from being accepted by such devices?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For deep space probes, the required power of radio transmissions would seem to provide some degree of security since sufficiently capable transmitters would have some security to prevent unauthorized use. (The cost of building such a transmitter would also make building one's own relatively unattractive for limited use even if the project site was in a country with weak enforcement of regulations about use of radio frequencies. The cost and risk of building a transmitter relative to the payoff would probably discourage most crackers [doing such for fun or to prove their cleverness] and terrorists.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lack of public documentation of the systems would tend to increase the difficulty of sending meaningful commands to such systems. (A denial of service attack might be somewhat easier than a hijacking attack given such limited information while still being noteworthy.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of processing power and energy use limits, it seems unlikely that early deep space probes used any form of encryption even for command and control transmissions (beyond a perhaps undocumented form of error detection and correction encoding), but this would seem to be less of an issue for more recently deployed systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While such systems are probably not considered high value targets by terrorists or system crackers (excluding military [and perhaps communications] satellites, which presumably do use encryption), even a temporary disruption of a scientific probe might be sufficiently newsworthy to attract some attempts.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This question was inspired by &lt;a href=&quot;https://space.stackexchange.com/questions/2108/do-any-spacecraft-use-carkeys&quot;&gt;Do any spacecraft use carkeys?&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-10-03T06:51:16.880" Title="What, if any, mechanisms prevent unauthorized commands?" Tags="|communication|security|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2125" PostTypeId="2" ParentId="2122" CreationDate="2013-09-24T12:48:05.913" Score="18" Body="&lt;p&gt;The dark and large areas are solidified lava beds from earlier periods of Lunar evolution, when it was still volcanically active. Notice that they might be somewhat less covered by still visible impact craters than on average the lighter areas are, as then still liquid lava would completely cover any traces of impacts from the early Lunar history. As the lava eventually solidified with the Moon cooling down, later impacts would of course still remain visible. These large lava beds are called Maria (Latin, &lt;em&gt;pl.&lt;/em&gt; for Seas, &lt;em&gt;sing.&lt;/em&gt; Mare), for example, Apollo 11 Lunar Module &quot;Eagle&quot; landed in the area called The Sea of Tranquility (&lt;em&gt;Lat.&lt;/em&gt; Mare Tranquillitatis), calling the landing site The Tranquility Base.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Smaller dark regions, but still large enough to be clearly observable on the attached to the question photograph and indeed also to the naked eye when observing Luna on the night's sky, are a product of larger meteor impacts that were energetic enough to literally melt the impact area's Lunar surface and penetrate deep enough to cause mixing of the molten rock from the Lunar surface with materials from the Lunar inner, denser layers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Denser areas of smaller impact craters are however just casting shadows on the Lunar surface, also making them appear darker than the average darkness of the Lunar surface, covered in regolith and with better light reflection properties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This answer is not based on any external reference. Please, let me know in the comments, if they are required.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: Another interesting thing about the darkness of the Lunar surface is that &lt;a href=&quot;https://en.wikipedia.org/wiki/Far_side_of_the_Moon&quot; rel=&quot;noreferrer&quot;&gt;the far side of the Moon&lt;/a&gt; has relatively few lunar maria, compared to its &lt;a href=&quot;https://en.wikipedia.org/wiki/Near_side_of_the_moon&quot; rel=&quot;noreferrer&quot;&gt;near side&lt;/a&gt; that faces the Earth at all times due to this Earth's natural satellite being in a tidally locked orbit with our planet. Knowing that, we could argue that the most commonly used term for Lunar far side, &lt;em&gt;the dark side of the Moon&lt;/em&gt;, is in fact a misnomer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/DOqJA.jpg&quot; alt=&quot;Side-by-side comparison of the near (left) and the far (right) sides of the Moon, taken by the NASA&amp;#39;s Lunar Reconnaissance Orbiter.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, and as luck would have it, NASA has just released a 360 degree view of the Moon, a video of stitched together still images taken by its Lunar Reconnaissance Orbiter, and published for example &lt;a href=&quot;http://www.cbsnews.com/video/watch/?id=50155485n&quot; rel=&quot;noreferrer&quot;&gt;on this CBS News page&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks go to Paul A. Clayton for pointing this out in the comments below!&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-09-24T16:07:55.500" LastActivityDate="2013-09-24T16:07:55.500" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2126" PostTypeId="2" ParentId="1663" CreationDate="2013-09-24T13:28:24.440" Score="2" Body="&lt;p&gt;I'm going to cheat a bit and include non-functioning satellites, just to give a flavor of what this does. The &lt;a href=&quot;http://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;ved=0CDEQFjAA&amp;amp;url=http://en.wikipedia.org/wiki/Surveyor_3&amp;amp;ei=upBBUuC5D8-y4APdmICgCA&amp;amp;usg=AFQjCNFH1ilKEbwN3XYx18Mon6qf3nDL0g&amp;amp;sig2=xg8BsGnzTEYMWgb3inPIZg&amp;amp;bvm=bv.52434380,d.dmg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Surveyor 3&lt;/a&gt; lander was visited by the Apollo 12 crew approximately 2 years after it landed on the moon. There was a c&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720019081_1972019081.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;omplete report&lt;/a&gt; done to analyze the status of the satellite after it's time on the moon, including micrometeor impacts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OX5H5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The report states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A major effort in the analysis of Surveyor 3 parts has been the search&#xA;  for hypervelocity impact features-an effort roughly analogous to the&#xA;  search for the needle in the haystack. A great number of low-velocity&#xA;  features exist that were caused by lunar particles striking the&#xA;  surfaces due to Surveyor and Apollo landing events, handling of the&#xA;  material, and natural phenomena. The 1- to 4.5-um size of the surface&#xA;  features prohibited the effective use of optical instruments. However,&#xA;  all participating investigators concluded that no material or surface&#xA;  features were found that definitely could be stated to be meteoritic&#xA;  in origin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Okay, so what does that mean? Basically, there were no &quot;Primary&quot; events discovered, where a micrometeorite hit the spacecraft directly from space. However, they also wanted to discover the secondary rate, where a dust particle from a far away impact hit the spacecraft. They looked at the optical fibers, and found:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Particles 1 pm and larger with velocities high enough to produce&#xA;  plastic flow in glass were found to be about l0^3 times the cratering&#xA;  rate expected for primary micrometeoroids. The rate is approximately&#xA;  800 impacts per square centimeter per year for impacts greater than 1 um.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, there was some slight meteoric damage to the spacecraft, but not significant. The rate is relatively low, and most spacecraft can easily handle that kind of impact. This is definitely a danger which should be carefully watched out for during lunar activities, especially of the secondary type. There is more importance to put shielding on the sides than above the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-24T13:28:24.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2127" PostTypeId="5" CreationDate="2013-09-24T14:01:29.797" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-24T14:01:29.797" LastActivityDate="2013-09-24T14:01:29.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2128" PostTypeId="4" CreationDate="2013-09-24T14:01:29.797" Score="0" Body="Questions regarding surface features of celestial bodies." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-24T14:22:15.710" LastActivityDate="2013-09-24T14:22:15.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2129" PostTypeId="5" CreationDate="2013-09-24T14:02:23.633" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-24T14:02:23.633" LastActivityDate="2013-09-24T14:02:23.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2130" PostTypeId="4" CreationDate="2013-09-24T14:02:23.633" Score="0" Body="Questions regarding flyby in which the spacecraft flies near celestial object in a hyperbolic path for scientific explorations. Such flybies may also be made to increase velocity or reduce the velocity via a gravitational assist." OwnerUserId="414" LastEditorUserId="25" LastEditDate="2013-09-24T16:11:36.843" LastActivityDate="2013-09-24T16:11:36.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2131" PostTypeId="5" CreationDate="2013-09-24T14:05:11.823" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-24T14:05:11.823" LastActivityDate="2013-09-24T14:05:11.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2132" PostTypeId="4" CreationDate="2013-09-24T14:05:11.823" Score="0" Body="Questions regarding the distance of an object above a surface." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-09-24T14:23:20.840" LastActivityDate="2013-09-24T14:23:20.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2133" PostTypeId="1" CreationDate="2013-09-24T14:07:53.180" Score="18" ViewCount="556" Body="&lt;p&gt;&lt;a href=&quot;http://www.sciencenews.org/view/generic/id/353355/description/Mars_rover_fails_to_find_methane&quot;&gt;ScienceNews&lt;/a&gt; states that: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Scientists had previously identified&#xA;  methane on Mars using Earth-based&#xA;  telescopes and spacecraft orbiting the&#xA;  Red Planet. In 2003, researchers&#xA;  detected a concentration as high as 45&#xA;  ppb. Since then, experiments have&#xA;  reported much lower levels of less than&#xA;  10 ppb.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This was in response to the Curiosity Rover not detecting methane on the surface of Mars, where previous Earth based studies had found it to exist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the possible reasons behind this miscalculation that were done based on ground observation, and why are they mostly searching for methane?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="1142" LastEditDate="2014-03-05T16:20:42.883" LastActivityDate="2014-03-05T16:20:42.883" Title="Why did the Earth based observations of methane on Mars go wrong?" Tags="|mars|curiosity|gasses|planetary-science|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2134" PostTypeId="1" AcceptedAnswerId="2136" CreationDate="2013-09-24T14:17:50.460" Score="21" ViewCount="11104" Body="&lt;p&gt;Mars, as I understand, has a very thin atmosphere. However, it still has weather like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lmFG7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What causes the thin atmosphere to move fast enough to cause such a monster wave of dust?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2013-09-26T13:38:26.427" LastActivityDate="2022-05-21T23:24:03.293" Title="How can Mars have dust storms with such a thin atmosphere?" Tags="|mars|planetary-science|weather|dust|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2135" PostTypeId="2" ParentId="2134" CreationDate="2013-09-24T14:49:08.510" Score="9" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HH0eR.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The answer is that the solar radiation heats the thin atmosphere of Mars but hot air rises up and it also lift its small dust (since Mars has low surface gravity of 3.711 &lt;span class=&quot;math-container&quot;&gt;$m/s^2$&lt;/span&gt; they are almost weightless) and dust-storms begin&#xA;to form, swirling at great speeds due&#xA;to the thin atmosphere probably due to formation of different pressure in the atmosphere (what we call in earth as &lt;a href=&quot;http://kids.earth.nasa.gov/archive/hurricane/creation.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;hurricane&lt;/a&gt;)&#xA;The greater the temperature, the better chance of dust storm.&lt;/p&gt;&#xA;&lt;h3&gt;External Sources&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://curiosity.discovery.com/question/mars-dust-storm-causes&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://curiosity.discovery.com/question/mars-dust-storm-causes&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.universetoday.com/14892/mars-dust-storms/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.universetoday.com/14892/mars-dust-storms/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-09-25T14:36:10.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2136" PostTypeId="2" ParentId="2134" CreationDate="2013-09-24T14:56:11.880" Score="18" Body="&lt;p&gt;Mostly a product of two environmental factors, I would wager:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lack of in-air humidity or liquid water on the ground keeping dust fine grained and not sticking together to form larger mass particles, and&lt;/li&gt;&#xA;&lt;li&gt;Relatively low surface gravity of roughly 1:2.6 when compared to the sea-level gravity on the Earth keeping them suspended in atmosphere for longer periods of time.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So while there might be low atmospheric pressure on the surface of Mars and winds not having much strength at all, even if they are moving fast over its surface, what we'd consider a mere breeze here on Earth, can produce in relative terms hurricane force long-lasting dust storms and local dust devils lifting the fine grain dust off the Mars' regolith and suspend them in the thin atmosphere for a long time, before they finally settle with the winds subduing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have observed similar phenomena here on the Earth with the Chelyabinsk Meteor's mostly silica based dusty plumes lingering in the upper atmosphere (with similar atmospheric conditions than that of Mars' surface, i.e. low humidity, low atmospheric pressure, low temperatures, e.t.c.) for months and circling the globe:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/zcJak.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Chelyabinsk's dust plume circled Earth in just four days, as shown in this image based on modelling. (Source: NASA Goddard)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-24T14:56:11.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2137" PostTypeId="2" ParentId="2133" CreationDate="2013-09-24T15:57:02.577" Score="12" Body="&lt;p&gt;The two measurements appear to be in disagreement, and so the magnificent thing that is scientific debate is now ensuing on this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 2003 observations would suggest, based on the believed lifetimes of methane in the atmosphere of Mars of several hundred years, that Curiosity should have detected it a 6 ppb.  Curiosity has ruled out concentrations greater than 1.3 ppb.  So either the original observation is wrong, or the lifetime of methane is wrong, or the models of how it distributes in the atmosphere are wrong.  Or all of the above.  &lt;a href=&quot;http://www.nature.com/news/missing-methane-gas-mystifies-mars-scientists-1.13779&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; provides an excellent summary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to &lt;em&gt;why&lt;/em&gt; we care so much about methane, it is because it is one signature of life.  Both extant life, in the form of, for example, cows on Earth or microbes on Mars (on Earth it's actually made by microbes in the cows), or even extinct life in the form of buried organics producing natural gas as it does on the Earth.  Methane is not however a definitive signature, since there are abiotic geological processes that produce methane as well.  But it would have been of great interest in the search for life on Mars to find methane in the atmosphere, since even several hundred years is incredibly fast in geological terms, indicating a nearly constant production of methane to maintain anything detectable in the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-24T23:10:55.483" LastActivityDate="2013-09-24T23:10:55.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2138" PostTypeId="1" AcceptedAnswerId="2144" CreationDate="2013-09-24T16:38:31.290" Score="11" ViewCount="5603" Body="&lt;p&gt;From the first Soyuz to the launch vehicles used today, how many launches have been deemed successful?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="13208" LastEditDate="2017-02-27T21:35:13.320" LastActivityDate="2021-01-11T23:17:37.180" Title="How many successful Soyuz launches have there been?" Tags="|launch|spacecraft|soyuz-launcher|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2139" PostTypeId="2" ParentId="2094" CreationDate="2013-09-24T17:28:30.243" Score="6" Body="&lt;p&gt;One theory is that long ago the mesas were craters that got filled with lava. Once the lava solidified, floods eroded the land around the lava trap, creating a mesa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ITrcm.png&quot; alt=&quot;Mesa Creation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Dr. James B. Garvin, former Chief Scientist for NASA, the mesas could also have been carved by glaciers or wind, or pushed up by tectonic activity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that we don't know yet, because we don't have enough data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2012/pdf/1117.pdf&quot; rel=&quot;noreferrer&quot;&gt;Mega-flood Erosion on Mars -- How a Lava-filled Crater Became a Mesa, Neil M. Coleman&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2001/ast24may_1/&quot; rel=&quot;noreferrer&quot;&gt;Unmasking the Face on Mars, NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2013-09-24T17:28:30.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2140" PostTypeId="1" AcceptedAnswerId="2145" CreationDate="2013-09-24T17:29:15.123" Score="15" ViewCount="3186" Body="&lt;p&gt;When Apollo 13 had most of its internal systems destroyed by an explosion, NASA decided that the best course of action would be to fire the main engine on the dark side of the moon to send the capsule back to Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, many sources mention that there was great apprehension as to whether the engine would successfully start and sustain a burn. From what I can tell, this apprehension was tied to the low supply of electrical power. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about the engine made the engineers distrust it so? Doesn't an engine keep burning until you shut off it's fuel, no electrical power required?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2019-11-29T12:50:24.493" Title="Why was the Apollo CM main engine thought to be unreliable on Apollo 13?" Tags="|apollo-program|engine-design|engines|apollo-13|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2143" PostTypeId="1" CreationDate="2013-09-24T19:36:56.213" Score="16" ViewCount="1567" Body="&lt;p&gt;Food is ubiquitous. All organisms known so far need food, and it is commonly believed all organisms unknown need it to. Size matters not - Eating &lt;em&gt;is&lt;/em&gt;.  Consumption continues ad infinitum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Failure to eat all your spinach (and other healthy/unhealthy eats on your plate) leaves it to &lt;a href=&quot;http://en.wikipedia.org/wiki/Mould&quot;&gt;go bad&lt;/a&gt; eventually. Cosmonauts (atleast on ISS) consume irradiated &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/astronauts/living-eating.asp&quot;&gt;food&lt;/a&gt;, or processed &amp;amp; packaged to last the duration of the mission. Left overs, if any, are promptly dispatched. In addition care is taken to maintain a sterile environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet &lt;a href=&quot;http://cfpub.epa.gov/si/si_public_file_download.cfm?p_download_id=491399&quot;&gt;http://cfpub.epa.gov/si/si_public_file_download.cfm?p_download_id=491399&lt;/a&gt;. writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... Dust was collected over a period of several weeks in 2007 from HEPA filters in the U.S. Laboratory Module of the International Space Station (ISS). The dust was returned on the Space Shuttle Atlantis, mixed, sieved, and the DNA was extracted. Using a DNA-based method called mold specific quantitative PCR (MSQPCR), 39 molds were measured in the dust. ... Molds like Penicillium chrysogenum were measured on the Mir Space Station from the beginning of its on-orbit service&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Even more recently &lt;a href=&quot;http://en.rian.ru/world/20130618/181723912.html&quot;&gt;http://en.rian.ru/world/20130618/181723912.html&lt;/a&gt; wrote to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Spaceflight101 portal said the delay was due to possible &quot;mold and bacteria contamination on three cargo bags that are inside the spacecraft” and that a decision is yet to be made on whether the crew should use anti-mold kits to clean ATV-4 cargo before taking it inside the ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Where should I look to find mouldy food in ISS?&lt;/li&gt;&#xA;&lt;li&gt;Could mould even grow on crumb/fluid/gravy there in ISS habitat? &lt;/li&gt;&#xA;&lt;li&gt;Would mould be more susceptible to mutation in the combination of hard radiation, and microgravity?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="25" LastEditDate="2013-11-01T19:49:13.493" LastActivityDate="2013-11-01T19:49:13.493" Title="Where should I look in ISS to find mouldy food?" Tags="|iss|food|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2144" PostTypeId="2" ParentId="2138" CreationDate="2013-09-24T20:55:56.963" Score="13" Body="&lt;p&gt;&lt;strong&gt;As of Sep 24 2013:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;TsSKB Progress&lt;/em&gt; site gives &lt;a href=&quot;http://en.samspace.ru/products/launch_vehicles/launch_vehicles_stat/&quot; rel=&quot;noreferrer&quot;&gt;the official numbers&lt;/a&gt; for all of the R-7 family.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, counting only rockets designated Soyuz, that is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz-ST,&#xA;Soyuz-2,&#xA;Soyuz-U,&#xA;Soyuz-FG,&#xA;Soyuz-U2,&#xA;Soyuz-M,&#xA;Soyuz-L,&#xA;Soyuz.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And not counting Vostok, Molnia, Voskhod, Sputnik and R7 MBR, we have &lt;/p&gt;&#xA;&#xA;&lt;p&gt;total: 5+23+778+44+70+8+3+32 = 963&lt;/p&gt;&#xA;&#xA;&lt;p&gt;failure: 1+21+2 = 24&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;success:&lt;/strong&gt; 963 - 24 = &lt;strong&gt;939&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2017-06-06T19:49:08.907" LastActivityDate="2017-06-06T19:49:08.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2145" PostTypeId="2" ParentId="2140" CreationDate="2013-09-24T21:15:19.827" Score="19" Body="&lt;p&gt;I think you are incorrect on your question. There were two main burns done using the Lunar Module descent engine. One was before reaching the moon, to position them into a free return trajectory, the other was about 2 hours after passing the moon, to allow them to land in the Atlantic Ocean instead of the Indian Ocean, a difference of about 19 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://history.nasa.gov/alsj/a13/A13_MissionOpReport.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 13 Mission Report&lt;/a&gt; the reason they couldn't initiate a immediate turn around using the Service Module propulsion system was because they would need to jettison the Lunar Module. It was determined that the Lunar Module was necessary to serve as a life boat for the astronauts, and a direct abort was not an option. (page 23)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the SPS was only an option with the removal of the Lunar Module (page 43)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It was obvious that we were committed to going around the moon&#xA;  rather than performing a direct abort because the large [vector deviation] could have&#xA;  been supplied by the SPS only if the LM were jettisoned, but that was out&#xA;  of the question.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Because the damage to the Service Module was uncertain, all systems relating to the Service Module, including the propulsion system, was determined to be unreliable and only to be used as a last resort. pages 40 and 49&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At this time the spacecraft was on a non-free-return trajectory with&#xA;  a 62 n.m. pericynthion. The crew had an abort pad onboard which required&#xA;  an SPS burn of 6079 fps at 60+00 to land in the Pacific Ocean at 118 hours.     &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;At 56+48, the Flight Dynamics team was advised to initiate all of the&#xA;  return-to-earth planning based on going around the moon, and assuming use&#xA;  of the LM descent engine and LM RCS, and that we would not use the SPS&#xA;  except as a last ditch effort.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Perform a PC+2 hour abort burn with the Service Module&#xA;  Propulsion System for a landing at 118+00 in the Pacific Ocean. &lt;strong&gt;This option&#xA;  was rejected unanimously as being too risky since no data was available&#xA;  as to the structural integrity of the Service Module. This option was&#xA;  put into the category of &quot;last ditch&quot;.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4" LastEditorUserId="23839" LastEditDate="2019-11-29T12:50:24.493" LastActivityDate="2019-11-29T12:50:24.493" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="2146" PostTypeId="1" AcceptedAnswerId="2147" CreationDate="2013-09-24T21:20:31.823" Score="41" ViewCount="26607" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_17&quot;&gt;Apollo 17&lt;/a&gt; was the last manned mission to the lunar surface. Since then every country or organization I heard of are planning to send motorized, unmanned and automated lunar probes (&lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_of_the_Moon#Plans&quot;&gt;list of plans&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The unmanned objects cannot bring back new samples or identify elements unknown on Earth. With an abundance of &lt;a href=&quot;https://en.wikipedia.org/wiki/Helium-3&quot;&gt;helium-3&lt;/a&gt;, traces of liquid water yet to be explored, and its poles yet to be studied, why are they sending just unmanned probes?&lt;/p&gt;&#xA;" OwnerUserId="784" LastEditorUserId="5946" LastEditDate="2014-08-12T23:00:27.890" LastActivityDate="2019-10-19T22:20:37.650" Title="Why are there no more manned missions to the Moon?" Tags="|the-moon|crewed-spaceflight|" AnswerCount="5" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="2147" PostTypeId="2" ParentId="2146" CreationDate="2013-09-24T21:42:49.087" Score="49" Body="&lt;p&gt;The short answer is that humans are heavy. For the weight of one human you can haul a big bunch of scientific instruments. Not to mention saving the weight of all the food, water and air that a human would need to eat, drink and breathe during the trip. Humans get cranky when they get too hot or cold. A well-built space probe can deal with much larger swings of temperature/radiation/etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, you're totally wrong about bringing back new samples. There's nothing particularly stopping any of the missions from bringing back samples — even much larger samples if they don't need to lug said heavy human back as well. It's just they find they it's easier to do the science in-situ. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All that said, human exploration of space &lt;em&gt;does&lt;/em&gt; have a certain appeal to it. Sadly, it tends to be really expensive and risky. Remember the uproar after the Challenger disaster? That's something NASA and other space agencies want to avoid. &lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorDisplayName="user12" LastEditDate="2015-05-12T14:23:59.833" LastActivityDate="2015-05-12T14:23:59.833" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="2148" PostTypeId="2" ParentId="2146" CreationDate="2013-09-24T22:27:51.433" Score="15" Body="&lt;p&gt;The US government has been divesting itself of basic R&amp;amp;D and funding that sort of activity less and less.  Initially it was all about beating the Russians to the moon and scoring a political victory. The science and technology was a benefit of that, but with the change in thrust by the US Government towards the military industrial complex, further investments weren't pursued. The thought was that commercial entities would pick up where the government left off.  As of yet, the cost benefit analysis doesn't make these sort of ventures economically viable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also arguments that we can accomplish the basic science with much less risk by using robotic probes instead of manned missions, at a much lower cost.&lt;/p&gt;&#xA;" OwnerUserId="727" LastEditorUserId="33" LastEditDate="2013-09-25T09:27:06.173" LastActivityDate="2013-09-25T09:27:06.173" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2149" PostTypeId="1" CreationDate="2013-09-25T04:17:01.853" Score="10" ViewCount="465" Body="&lt;p&gt;The Voyager 1 recently &lt;a href=&quot;http://www.nasaspaceflight.com/2013/09/the-unknown-voyager-1-begins-interstellar-space-adventure/&quot;&gt;entered the interstellar space&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Voyager group said that the Voyager 1 has left the Heliosphere and entered the interstellar space. Charge particles and plasma activities increased about 40X as compared to readings inside the Heliosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Proof of having crossed the Heliopause should be signaled by &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a sharp drop in the temperature of charged particles, &lt;/li&gt;&#xA;&lt;li&gt;a change in the direction of the magnetic field, and &lt;/li&gt;&#xA;&lt;li&gt;an increase in the amount of galactic cosmic rays&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Did it really cross the Heliopause?&lt;/p&gt;&#xA;" OwnerUserId="784" LastEditorUserId="692" LastEditDate="2013-09-25T12:07:46.063" LastActivityDate="2018-12-10T17:59:33.157" Title="Does the Heliopause really exist?" Tags="|voyager|interstellar-medium|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2150" PostTypeId="2" ParentId="2149" CreationDate="2013-09-25T04:38:59.477" Score="3" Body="&lt;p&gt;From the Nasa &lt;a href=&quot;http://www.nasa.gov/mission_pages/voyager/voyager20130912.html#.UjJLPZKR86s&quot; rel=&quot;nofollow&quot;&gt;missions&lt;/a&gt; page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;We literally jumped out of our seats when we saw these oscillations in our data -- they showed us the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble,&quot; Gurnett said. &quot;Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and in &lt;a href=&quot;https://en.wikipedia.org/wiki/Heliosphere#Heliopause&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In May 2012, Voyager 1 detected a rapid increase in such cosmic rays (a 9% increase in a month, following a more gradual increase of 25% from Jan. 2009 to Jan. 2012), suggesting it was approaching the heliopause. In the fall of 2013, NASA announced that Voyager 1 had crossed the heliopause as of August 25, 2012.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All the articles suggest that Voyager1 has crossed the heliopause, meaning there is no chance of Voyager encountering it again. But none of them have confirmed its existence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The current mission of the Voyager 1 and 2 spacecraft is to find and study the termination shock, heliosheath, and heliopause. Voyager 1 reached the termination shock on May 23–24, 2005, and Voyager 2 reached it on August 30, 2007 according to NASA. Meanwhile, the Interstellar Boundary Explorer (IBEX) mission is attempting to image the heliopause from Earth orbit within two years of its 2008 launch. Initial results (October 2009) from IBEX suggest that previous assumptions are insufficiently cognisant of the true complexities of the heliopause.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Change in the number of Charged particles and plasma has been gradual and not abrupt. So the fact denies the existence of Heliopause.&lt;/p&gt;&#xA;" OwnerUserId="784" LastEditorUserId="784" LastEditDate="2013-09-25T08:50:25.023" LastActivityDate="2013-09-25T08:50:25.023" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2151" PostTypeId="2" ParentId="2146" CreationDate="2013-09-25T08:02:33.030" Score="9" Body="&lt;p&gt;It is easy to send a probe instead of humans because humans need food, sufficient amount of air, living temperature (around 30&amp;nbsp;°C) and shelter, but probes only need electricity and they must be landed safely. They will do their work autonomously or controlled by a ground station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible to return a sample from the Moon without humans, and we can use a robot to drill a sample and place it in a reentry capsule and return that to Earth (example: &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_16&quot; rel=&quot;noreferrer&quot;&gt;Luna 16&lt;/a&gt;). There are many difficulties, but sending and returning a human is a lot more difficult. In addition, the humans must be trained.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's all about cost and safety. If we lose a probe, it's a loss of cost. But if we lose a crew, it's a loss of both life and cost.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="286" LastEditDate="2014-04-05T09:30:41.600" LastActivityDate="2014-04-05T09:30:41.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2153" PostTypeId="2" ParentId="2149" CreationDate="2013-09-25T08:56:49.287" Score="8" Body="&lt;p&gt;The heliopause is not expected to be a sharp cutoff, but instead a turbulent and fluctuating area, outside of which there is one norm, and inside another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The transition could be much wider than we expect, hence the IBEX mission has been sent to understand this further, but no one is doubting that the data shows something is happening at the heliopause. NASA have data indicating that various shock boundaries have been crossed - initially, expectations were that this may be well defined, but it makes equally as much sense for there to be any number of layers, vortices, shocks etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if your question is does it exist, yes, it does, by its very definition. Has Voyager crossed it - maybe not yet, but it is certainly at or in it.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-09-25T08:56:49.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2155" PostTypeId="1" AcceptedAnswerId="2167" CreationDate="2013-09-25T10:54:11.063" Score="12" ViewCount="572" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/questions/2072/would-a-human-colony-on-venus-be-possible&quot;&gt;Would a human colony on Venus be possible?&lt;/a&gt; received an answer showing how inhospitable the surface is. But combined with comments to the answer implies that a station in the upper atmosphere might be a bit more practical.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course &lt;a href=&quot;https://en.wikipedia.org/wiki/Around_the_World_in_Eighty_Days&quot; rel=&quot;nofollow noreferrer&quot;&gt;Around the World in Eighty Days&lt;/a&gt; by Jules Verne popped into my mind (&lt;em&gt;thought the balloon is from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Around_the_World_in_Eighty_Days_%281956_film%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;1956 film&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Around_the_World_in_Eighty_Days#Background_and_analysis&quot; rel=&quot;nofollow noreferrer&quot;&gt;not the book&lt;/a&gt;&lt;/em&gt;), on looking into it seems feasible in theory, and with Wikipedia giving wind speeds of &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus#Atmosphere_and_climate&quot; rel=&quot;nofollow noreferrer&quot;&gt;300 km/h (190 mph)&lt;/a&gt;, it seems like you might be able to circumnavigate Venus in 8 days or less. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saving the challenges of deploying, arriving and departing for another question...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a balloon colony/station in the Atmosphere of Venus be possible?  If so what elevations would it be at, and in Earth and/or Venus days how fast would it circumnavigate Venus?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2019-02-26T19:10:39.130" LastActivityDate="2019-02-26T19:10:39.130" Title="Around Venus in 8 days by Balloon?" Tags="|colonization|venus|weather|balloons|aerobot|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2156" PostTypeId="1" CreationDate="2013-09-25T11:41:31.800" Score="16" ViewCount="5139" Body="&lt;p&gt;Gravitational attraction is determined using &lt;a href=&quot;http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation&quot;&gt;G (m1m2)/r^2&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which indicates the force is inversely proportional to the square of separation of the bodies&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of the terrestrial planets Mercury, Venus, Earth, and Mars - Venus and Mars are immediate neighbours to Earth. Any quantity of missions have studied either. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venerean gravity is almost 90% of Earth's own. Venus is also closer to Sol than Earth - figuratively downhill that is. All three bodies (Venus, Earth, Mars) being in motion there may be occasions when Mars is perhaps physically closer. Yet Venus appears to have the following advantages - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Further downhill in the Solar System&lt;/li&gt;&#xA;&lt;li&gt;Masses almost the same as Earth; ergo larger sphere of influence&lt;/li&gt;&#xA;&lt;li&gt;Average separation from Earth ~25 M Miles (Mars average distance from Earth is ~140M Miles)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Does a mission to Venus orbit require less propellant than a similar mission to Mars with the same payload?&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-11-24T18:11:14.087" Title="Does a mission to Venus orbit require less propellant than a similar mission to Mars?" Tags="|orbital-mechanics|mars|orbital-maneuver|venus|interplanetary|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2158" PostTypeId="1" AcceptedAnswerId="3183" CreationDate="2013-09-25T12:58:48.557" Score="14" ViewCount="994" Body="&lt;p&gt;On one hand, the Moon has lower gravity, and the gravity from Earth also reduces the distance the it has to span.  This is particularly true for EML-1, and a little less so for EML-2.  I don't know if there are any other coherent ideas of a lunar space elevator, so restricting the discussion to EML-1 would be best.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, Mars has a rotation similar to Earth.  Its day length is nearly 30 times the moon, so it seems like this would very heavily favor Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which one would have the lower requirement for material specific strength?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A similar question has been asked on &lt;a href=&quot;http://www.quora.com/Space-Elevators/Would-it-be-easier-building-a-space-elevator-on-Mars-or-the-Moon&quot;&gt;Quora&lt;/a&gt;, but lacked any genuine empiricism.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2017-10-05T12:53:18.807" Title="Between a space elevator on Mars versus the Moon, which would have lower material strength requirement?" Tags="|the-moon|mars|materials|space-elevator|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2159" PostTypeId="2" ParentId="2156" CreationDate="2013-09-25T13:04:57.443" Score="16" Body="&lt;p&gt;I'm going to only start calculating delta v from Earth Escape, using this &lt;a href=&quot;http://clowder.net/hop/railroad/deltaveemap.html&quot;&gt;Delta V table&lt;/a&gt;. Furthermore, I'll assume we are starting from LEO, and we want to get in to a low orbit of the appropriate planet. The numbers turn out to be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Venus- 3.5 km/s to Venus, 3.3 km/s to Low Venus Orbit&lt;/li&gt;&#xA;&lt;li&gt;Mars- 3.6 km/s to Mars, 2.1 km/s to Low Mars Orbit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, it takes more Delta v to orbit Venus because of it's higher gravity. But what if you are willing to accept a higher orbit? The minimum orbit is a capture orbit. Venus has a delta V of 0.4 km/s, while Mars requires 0.7 km/s. Thus, the minimum orbit is slightly easier for Venus than Mars, but if you want to get in to a better orbit, Mars is easier. If you use aerobraking, then Venus becomes slightly easier.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="195" LastEditDate="2014-11-24T18:11:14.087" LastActivityDate="2014-11-24T18:11:14.087" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2160" PostTypeId="5" CreationDate="2013-09-25T13:14:31.077" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-25T13:14:31.077" LastActivityDate="2013-09-25T13:14:31.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2161" PostTypeId="4" CreationDate="2013-09-25T13:14:31.077" Score="0" Body="Questions about the combustion of a material and the material is consumed by the fire ." OwnerUserId="414" LastEditorUserId="25" LastEditDate="2013-09-25T13:20:45.880" LastActivityDate="2013-09-25T13:20:45.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2162" PostTypeId="5" CreationDate="2013-09-25T13:25:23.047" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-25T13:25:23.047" LastActivityDate="2013-09-25T13:25:23.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2163" PostTypeId="4" CreationDate="2013-09-25T13:25:23.047" Score="0" Body="Questions regarding Apollo 13 which is launched as a part of American Apollo space program by Saturn v from Kennedy Space Center in April 11, 1970 carrying three crew members but lunar landing was aborted because of rapture of oxygen tank and few technical problems after injection to orbit" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-09-25T13:34:07.663" LastActivityDate="2013-09-25T13:34:07.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2164" PostTypeId="2" ParentId="2155" CreationDate="2013-09-25T13:30:19.997" Score="7" Body="&lt;p&gt;In tradition with the mentioned Jules Verne's Around the World in Eighty Days, this would take a fair deal of imagination to achieve, but yes. Let's see what our &lt;a href=&quot;https://en.wikipedia.org/wiki/Cheat_sheet&quot;&gt;cheat sheet&lt;/a&gt; could look like:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The length of one &lt;a href=&quot;https://en.wikipedia.org/wiki/Venusian&quot;&gt;Venusian&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_day&quot;&gt;solar day&lt;/a&gt; is equal to 116 Earth solar days and 18 Earth hours, so 8 Venusian days translate to 22,416 Earth hours.&lt;/li&gt;&#xA;&lt;li&gt;Assuming we travel West to East and in the direction of the planet's rotation, which is the direction its atmosphere would move on average so a better choice for a balloon and also the direction Fogg and Passepartout travelled (London - Suez - Bombay - Calcutta - Hong Kong - Yokohama - San Francisco - New York City - London) for the authenticity factor, and we started and ended our circumnavigating journey like our protagonists did at or near the &lt;a href=&quot;https://en.wikipedia.org/wiki/Prime_meridian&quot;&gt;prime meridian&lt;/a&gt;, half way along your path you would cross the &lt;a href=&quot;https://en.wikipedia.org/wiki/180th_meridian&quot;&gt;180th meridian&lt;/a&gt; as the Venusian &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Date_Line&quot;&gt;date line&lt;/a&gt;, gaining a whole Venusian &lt;a href=&quot;https://en.wikipedia.org/wiki/Calendar_day&quot;&gt;calendar day&lt;/a&gt;, or 2,802 Earth hours.&lt;/li&gt;&#xA;&lt;li&gt;If you could assume a &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot;&gt;Sun-Synchronous Orbit&lt;/a&gt; in Venusian upper atmosphere, and it should be perfectly possible given Venusian slow &lt;a href=&quot;https://en.wikipedia.org/wiki/Rotation_period&quot;&gt;rotation period&lt;/a&gt; even though impossible for satellites due to the planet being simply too spherical, in theory and as far as you're concerned, not a single day would have passed (though one rotation of Venus around in axis in hours, or 2,802 Earth hours would) before the planet completes its rotation period, rotating a full 360° around its axis.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So I won't even bother plugging in all the numbers, and as you can see, there are many ways to argue that you've indeed circumnavigated Venus in eight or less days. It could take 25,218 Earth hours, and you could argue you've been counting Venusian days and crossed its date line into the previous calendar day in the process, you could simply assume an orbit that would further decrease or even completely stop the rate of the planet's day cycle from your vantage point, or even try tucking against the upper atmospheric winds to effectively stay &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot;&gt;geostationary&lt;/a&gt; relative to its ground and let the planets rotation do that for you in a mere Venusian day with its own rotation on its axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess what I'm trying to say is, that you're already cheating with the balloon, since it has so many ways to take advantage of the calendar systems that are usually established for the purpose of on the planet's surface living beings. Jules Verne was a visionary that could see that before most would think it's even possible, and described one way to achieve that in a most fabulous adventure novel from 1873 that is still exciting to read even in this day and age with spaceships and maglev trains and whatnot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So next time you think you're jet lagged, don't complain before you checked your calendar. You might have a whole day to fight it, all for free and courtesy of conventions. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-25T13:30:19.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2165" PostTypeId="1" AcceptedAnswerId="3614" CreationDate="2013-09-25T14:12:31.810" Score="13" ViewCount="244" Body="&lt;p&gt;After discussing the &lt;a href=&quot;https://space.stackexchange.com/questions/2108/do-any-spacecraft-use-carkeys&quot;&gt;&lt;em&gt;Key to start&lt;/em&gt; (Ключ на старт) command during Soyuz launch sequence&lt;/a&gt;, I decided to look at the historic lineage of the key and the command. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gradually moving backward through &lt;a href=&quot;http://www.youtube.com/watch?v=y4LVqi7sMVw#t=25m50&quot; rel=&quot;nofollow noreferrer&quot;&gt;R7 ICBM&lt;/a&gt; to &lt;a href=&quot;http://www.youtube.com/watch?v=rDhFdGLz2i4#t=14m47&quot; rel=&quot;nofollow noreferrer&quot;&gt;R1 (the first large Soviet ballistic missile, a replica of V2/A4)&lt;/a&gt; historic videos I see that the launch sequence doesn't change a lot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That made me go a bit further back in history... and voila:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.v2rocket.com/start/deployment/procedure.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;V2 site states&lt;/a&gt; that the A4 rocket launch sequence involved the &lt;strong&gt;&quot;Schlüssel auf Schießen!&quot;&lt;/strong&gt; order. That is &lt;em&gt;Key to launch&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, &lt;a href=&quot;http://en.wikipedia.org/wiki/PGM-11_Redstone&quot; rel=&quot;nofollow noreferrer&quot;&gt;the dear Wikipedia&lt;/a&gt; states that the &lt;em&gt;PGM-11 Redstone&lt;/em&gt; rocket is a direct descendant of V2. Moreover, it was designed by the group of german engineers led by von Braun himself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the question is: &lt;strong&gt;&lt;em&gt;does the Redstone launch sequence involve the key and an equivalent of &quot;Schlüssel auf Schießen!&quot; command&lt;/em&gt;&lt;/strong&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bonus question is: does the &lt;a href=&quot;http://en.wikipedia.org/wiki/Dongfeng_%28missile%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dongfeng&lt;/a&gt; launch sequence involve the key and the command? The first Dongfeng rockets were licensed copies of R2 and other Soviet rockets. &lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-02-06T16:01:02.743" Title="Does Redstone start sequence involve a key?" Tags="|launch|history|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2166" PostTypeId="1" CreationDate="2013-09-25T15:20:34.370" Score="29" ViewCount="14248" Body="&lt;p&gt;Currently, the closest known black hole to Earth is &lt;a href=&quot;http://en.wikipedia.org/wiki/V4641_Sagittarii&quot;&gt;between 1600-24000 light years away&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if we do send a drone travelling at the speed of light, it would reach the black hole long after we are all dead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, let's assume that NASA has some spare cash and decides to go forward with such an endeavour and send a miniature spaceship straight into the black hole's direction, so that future generations can study it more closely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What will the natural processes be as the drone approaches the black hole or enters it (is this even possible)? Will it just be smashed like a pancake due to the high gravity as it approaches?&lt;/p&gt;&#xA;" OwnerUserId="792" LastEditorUserId="25" LastEditDate="2013-09-25T17:19:55.210" LastActivityDate="2013-09-27T11:09:04.380" Title="What Would Happen if We (Theoretically) Send a Drone With a Camera Inside a Black Hole?" Tags="|black-hole|" AnswerCount="5" CommentCount="1" ClosedDate="2013-09-26T16:13:53.687" ContentLicense="CC BY-SA 3.0" />
  <row Id="2167" PostTypeId="2" ParentId="2155" CreationDate="2013-09-25T16:17:58.873" Score="9" Body="&lt;p&gt;It would be quite easy. As you mentioned, the &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Venus_Express/The_fast_winds_of_Venus_are_getting_faster&quot;&gt;wind speed&lt;/a&gt; is roughly 300 km/hour, in the clouds, where Venus is the most hospitable. The same article mentions that oscillations of the atmosphere happen every 4.8 Earth days. &lt;a href=&quot;http://www.fun-flying.com/weather.htm&quot;&gt;Hot air balloons&lt;/a&gt; move at about the same speed as the wind around them. Thus, a hot air balloon should be able to circumnavigate Venus in 8 Earth days, no problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be some architectural challenges, but the key question is, what is the turbulence like. In fact, there has been a balloon flight which, according to &lt;a href=&quot;http://en.wikipedia.org/wiki/Hot_air_balloon&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Designed to fly in the trans-oceanic jet streams, the Pacific Flyer&#xA;  recorded the fastest ground speed for a manned balloon at 245 mph (394&#xA;  km/h).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The atmosphere at that level will have a similar pressure to Earth's, but there will be sulfuric acid all around, and the heat would be too hot for traditional hot air balloons. I would recommend using some sort of a helium type balloon, but it could be possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was done once unmanned, by the Russians. &lt;a href=&quot;http://en.wikipedia.org/wiki/Venus#Exploration&quot;&gt;Wikipedia&lt;/a&gt; quotes the performance as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In 1985, the Soviet Union took advantage of the opportunity to combine&#xA;  missions to Venus and Comet Halley, which passed through the inner&#xA;  Solar System that year. En route to Halley, on 11 and 15 June 1985,&#xA;  the two spacecraft of the Vega program each dropped a Venera-style&#xA;  probe (of which Vega 1's partially failed) and released a&#xA;  balloon-supported aerobot into the upper atmosphere. The balloons&#xA;  achieved an equilibrium altitude of around 53 km, where pressure and&#xA;  temperature are comparable to those at Earth's surface. They remained&#xA;  operational for around 46 hours, and discovered the Venusian&#xA;  atmosphere was more turbulent than previously believed, and subject to&#xA;  high winds and powerful convection cells.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-25T16:17:58.873" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2168" PostTypeId="2" ParentId="2166" CreationDate="2013-09-25T16:18:50.387" Score="29" Body="&lt;p&gt;If by &quot;enters it&quot; you mean crossing the event horizon, then the easy answer is: &lt;em&gt;we'll never know&lt;/em&gt;.  No signal can reach us from the probe since photons can't escape.  By definition the &quot;material,&quot; if there is such a thing, at the center of a black hole is inside the event horizon.  So the last signal you will get from the probe will be just before it crosses the event horizon.  The time dilation noted in another answer here results in the signal being Doppler shifted to lower and lower frequencies, with the encoded data going to lower and lower data rates, and the intensity going down rapidly until you receive the last photon from the probe as it dims to nothing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For supermassive black holes, spaghettification will not occur until after crossing the event horizon, so you can survive an approach to the event horizon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, what happens when you cross the event horizon is a subject of great debate.  When considering the quantum mechanics and quantum information transfer at the event horizon of a black hole, it all becomes very complicated.  You might make it across uneventfully (pun intended), or you might be completely destroyed right at the event horizon.  See &lt;a href=&quot;http://quantumfrontiers.com/2012/12/03/is-alice-burning-the-black-hole-firewall-controversy/&quot;&gt;this excellent blog post&lt;/a&gt; for a discussion of the current debates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The event horizon is defined as the distance from the center of the black hole at which the gravitational escape velocity equals the velocity of light.  Therefore, given that the speed of light is a universal speed limit, nothing inside of the event horizon can ever escape.  That distance for a simple, non-rotating black hole is called the Schwarzschild radius, equal to $2\mu\over c^2$.  $\mu$ is the standard gravitational parameter for the mass of the black hole $M$, $\mu=GM$, and $c$ is the speed of light.  If the Earth were somehow compressed into a black hole, its Schwarzschild radius would be about 1 cm.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-26T08:30:09.990" LastActivityDate="2013-09-26T08:30:09.990" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2169" PostTypeId="1" AcceptedAnswerId="2170" CreationDate="2013-09-25T19:34:54.813" Score="10" ViewCount="1639" Body="&lt;p&gt;A follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/2107/how-much-more-power-does-a-pv-cell-generate-in-venerean-orbit&quot;&gt;How much more power does a PV cell generate in Venerean orbit?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1972-021A&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; writes to say about Venera 8&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... measured the light level as being suitable for surface photography, finding it to be similar to the amount of light on Earth on an overcast day with roughly 1 km visibility. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Was this corroborated by other probes to Venus?&lt;/li&gt;&#xA;&lt;li&gt;Is the average intensity of sunlight (even through the overcast) adequate for a PV cell to supply power, or did any instruments work off the charge stored in batteries during descent?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-09-26T05:51:11.437" Title="What is the average intensity of sunlight on the Venerean surface?" Tags="|venus|solar-power|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2170" PostTypeId="2" ParentId="2169" CreationDate="2013-09-25T21:34:22.950" Score="8" Body="&lt;p&gt;The Venera landers operated on batteries for an hour or two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spectrometer on Venera 11 provided direct data on the solar insolation at the surface as a function of wavelength.  &lt;a href=&quot;http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5411663&quot; rel=&quot;noreferrer&quot;&gt;This paper by Landis and Vo&lt;/a&gt; provides a plot of the insolation as a ratio of the intensity at the surface to the intensity in space at Venus, i.e. just outside the atmosphere.  Blue is gone completely, green is almost gone at about 0.02, red is better at 0.08 to 0.12, and infrared at about 0.1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, you can operate solar cells at very low intensities.  For example the Juno spacecraft will use them at Jupiter with an insolation of about 0.04 (relative to at Earth).  From the Landis paper, a single-junction cell panel at the surface of Venus would require $1.17\,{{\mathrm m}^2\over W}$.  That's a lot of panel for not a lot of power.  Also it would require a significant technology development to come up with cells that can survive the temperature at the surface.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-09-26T05:51:11.437" LastActivityDate="2013-09-26T05:51:11.437" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2171" PostTypeId="2" ParentId="2166" CreationDate="2013-09-25T22:45:44.717" Score="-3" Body="&lt;p&gt;Black hole has enormous density and gravity. So let's imagine we have a drone that can escape any gravitational forces and reach any place at any speed. As we reach the black hole we could  see materials that compressed to the black hole, and that materials would become hot. We can see them, because as materials become hot enough they start emit light. But even light can not escape the gravity of black hole, so we will not see anything inside. But if you have a microscope on that drone may be you will see that the atomic structure is compressed, electrons are not turning around the atom nucleus but glued by gravitational force to nucleus surface.&lt;/p&gt;&#xA;" OwnerUserId="800" LastActivityDate="2013-09-25T22:45:44.717" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2172" PostTypeId="2" ParentId="2166" CreationDate="2013-09-26T02:47:29.043" Score="5" Body="&lt;p&gt;Viewing far from the blackhole: As the probe falls in it will move slower and slower through time.  It will appear redder, colder, and dimmer.  As it approaches the event horizon their movement through time will halt, as they fade completely from view.  Technically, you’ll never actually see someone fall into a blackhole, you’ll just see them get really close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Probe:  First, torn apart and crushed, but let's skip that detail.  Things farther from the blackhole move through time faster, so the rest of the universe will speed up from your probe's point of view.  As a result the rest of the universe becomes bluer, hotter, and brighter.  The blue shift of the incoming light turns it into gamma rays.  So, right before the probe passes through the event horizon, it’ll get radiated with a universe’s lifetime worth of starlight and microwave background radiation.  Whereas the event horizon itself is only special from an outside perspective.  If you fall in you should pass right through it.  However, what you see in the moment that you pass through the horizon is dependent on things we don’t know yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The video:  very slow images until they stop and the data fades away.&lt;/p&gt;&#xA;" OwnerUserId="446" LastActivityDate="2013-09-26T02:47:29.043" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2173" PostTypeId="2" ParentId="2149" CreationDate="2013-09-26T03:42:06.083" Score="7" Body="&lt;p&gt;Yes.  On the 25th of August, 2012, Voyager 1 measured very abrupt changes in two out of three of the criteria listed in the question.  This was in fact Voyager 1 crossing the heliopause.  In a single day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The kicker was that the magnetic field direction only changed by 2°.  This was a puzzlement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By sheer luck, a large coronal mass ejection from the Sun tickled the plasma around Voyager 1 a few months later.  The resulting ringing allowed them to measure the plasma density, which matched what was expected for the interstellar space in the vicinity of our solar system.  They were able to find earlier, weaker oscillations, extrapolated back, and found that the density increased very rapidly in, you guessed it, August 2012.  That provided the confirmation they were looking for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnetic fields were expected to have different directions since they have different sources.  There would be no reason for them to have the same direction, so that would be unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, not impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, there was a very sharp change when crossing the very real heliopause.  The determination that that really was the heliopause was delayed by an unexpected approximate alignment of magnetic fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;News flash:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the 5th of November, 2018, &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?feature=7301&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 2 crossed the heliopause&lt;/a&gt; entering into interstellar space.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2018-12-10T17:59:33.157" LastActivityDate="2018-12-10T17:59:33.157" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2174" PostTypeId="2" ParentId="2166" CreationDate="2013-09-26T04:50:19.787" Score="2" Body="&lt;p&gt;well a black hole has so much gravity it doesn't even allow light to escape. So first of all the camera wouldn't see anything. Second of all the camera, once inside it can't send any information back because even the electromagnetic wave it sends cannot escape. Third of all ,as soon as the camera enters the black hole it would be stripped apart. Such is the gravity of a black hole&lt;/p&gt;&#xA;" OwnerDisplayName="user738" LastEditorDisplayName="user738" LastEditDate="2013-09-27T11:09:04.380" LastActivityDate="2013-09-27T11:09:04.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2175" PostTypeId="1" AcceptedAnswerId="2398" CreationDate="2013-09-26T06:03:11.873" Score="15" ViewCount="366" Body="&lt;p&gt;Extremes of temperature, and pressure serve to effectively constrict the life-span of a PV cell on the Venerean surface. Despite the cloud cover, the Venus' proximity to Sol makes for &lt;a href=&quot;https://space.stackexchange.com/questions/2169/what-is-the-average-intensity-of-sunlight-on-the-venerean-surface&quot;&gt;upto 1km visibility&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2155/around-venus-in-8-days-by-balloon&quot;&gt;Elsewhere on SEx.SE&lt;/a&gt; is discussed the possibility of using a balloon to circumnavigate (using the term loosely!) Venus. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But ... why restrict thought to a balloon? We have something much better! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.solarimpulse.com/en/airplane/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Impulse&lt;/a&gt; is a proven Solar powered craft flown in Earth's atmosphere. It could, in Venerean atmosphere, rise to the necessary altitude when power runs low. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Off the top of my heads, the arguments that come to mind are these&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Favourable:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Sunlight (PV panels probably &lt;a href=&quot;https://space.stackexchange.com/questions/2107/how-much-more-power-does-a-pv-cell-generate-in-venerean-orbit&quot;&gt;twice as effective&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Soup aka atmosphere (Almost tangibly thick)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Unfavourable:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Issue of getting the assembly to Venus&lt;/li&gt;&#xA;&lt;li&gt;Toxic atmosphere &#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Would an electric engine work in Venerean atmosphere? &lt;/li&gt;&#xA;&lt;li&gt;How long would it last?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;High Winds &#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Would Solar Impulse' aerofoil survive?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As a thought experiment and &lt;strong&gt;assuming the Solar Impulse assembly is capable of surviving the toxic/corrosive environment&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How long could it fly in the Venerean atmosphere?&lt;/li&gt;&#xA;&lt;li&gt;Would it be capable of flight into the lower atmosphere, and back upstairs for recharging?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-10-12T15:21:37.300" Title="How long could Solar Impulse fly in the Venerean atmosphere?" Tags="|venus|solar-power|environment|aerobot|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2177" PostTypeId="1" AcceptedAnswerId="2182" CreationDate="2013-09-26T11:45:17.157" Score="14" ViewCount="4332" Body="&lt;p&gt;According to the &lt;a href=&quot;https://space.stackexchange.com/questions/2134/how-can-mars-have-dust-storms-with-such-a-thin-atmosphere&quot;&gt;answers to this question&lt;/a&gt;, a dust storm on Mars can last for many days. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dust is a serious problem for mechanical joints if they use lubrication, because it may stick to the lubricant and can even impede its function. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do &lt;a href=&quot;http://en.wikipedia.org/wiki/Curiosity_%28rover%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity&lt;/a&gt; and other Mars rovers protect themselves from storms?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2022-05-20T22:12:42.293" Title="How does the Curiosity rover survive dust storms on Mars?" Tags="|mars|rovers|curiosity|environment|dust|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2178" PostTypeId="5" CreationDate="2013-09-26T12:22:28.867" Score="0" Body="" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-26T12:22:28.867" LastActivityDate="2013-09-26T12:22:28.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2179" PostTypeId="4" CreationDate="2013-09-26T12:22:28.867" Score="0" Body="The study of planets, asteroids, comets, etc, including weather, geology, composition, etc. This tag should be used when the focus of the question is on the the science of a non-Earth non-star natural object, and not used when designing spacecraft to cope with said challenges." OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-26T12:22:28.867" LastActivityDate="2013-09-26T12:22:28.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2180" PostTypeId="5" CreationDate="2013-09-26T12:24:48.533" Score="0" Body="" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-26T12:24:48.533" LastActivityDate="2013-09-26T12:24:48.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2181" PostTypeId="4" CreationDate="2013-09-26T12:24:48.533" Score="0" Body="How human made objects in space cope with the environment of space, including radiation, dust storms, thermal issues, magnetic fields, toxicity, etc." OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-26T12:24:48.533" LastActivityDate="2013-09-26T12:24:48.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2182" PostTypeId="2" ParentId="2177" CreationDate="2013-09-26T14:05:49.743" Score="15" Body="&lt;p&gt;The gear boxes are sealed, often a &lt;a href=&quot;http://en.wikipedia.org/wiki/Labyrinth_seal&quot;&gt;labyrinth seal&lt;/a&gt;.  Opportunity has been operating for almost ten years with many actuators still working fine.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-09-26T14:05:49.743" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2183" PostTypeId="1" CreationDate="2013-09-26T14:13:43.650" Score="11" ViewCount="496" Body="&lt;p&gt;My understanding is that no one owns space, but countries retain ownership of the vehicles they put in space.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I as a commercial entity launched a recovery mission to the moon, and brought back to earth a lunar vehicle ( a buggy, a launch platform, etc ) would that be an act of piracy occurring in space?  Would I be considered a bona fide space pirate?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does that work legally?  If something were adrift on the sea, that would be legally salvage.  But my understanding is that such laws don't apply to space.  Is that true?&lt;/p&gt;&#xA;" OwnerUserId="806" LastActivityDate="2013-09-27T07:52:28.787" Title="Would recovering a lunar vehicle without permission be space piracy?" Tags="|the-moon|law|debris|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2184" PostTypeId="2" ParentId="2166" CreationDate="2013-09-26T14:44:18.430" Score="1" Body="&lt;p&gt;The probe would be pulled apart from the tidal forces of the black hole as it approached it. As explained in this Minute Physics video (specifically time code 1:39, but watch the whole video to understand tidal forces). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.youtube.com/watch?v=gftT3wHJGtg&quot; rel=&quot;nofollow&quot;&gt;http://www.youtube.com/watch?v=gftT3wHJGtg&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4" LastActivityDate="2013-09-26T14:44:18.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2185" PostTypeId="1" AcceptedAnswerId="2227" CreationDate="2013-09-26T14:46:14.350" Score="8" ViewCount="1239" Body="&lt;p&gt;Plenty of spacecraft seem to have moving parts. Space shuttle doors open, robot arms manipulate things, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is perplexing, when we consider that lubricating oil probably won't remain liquid, and even solid-on-solid contact risks &lt;a href=&quot;https://en.wikipedia.org/wiki/Vacuum_cementing&quot; rel=&quot;noreferrer&quot;&gt;vacuum welding&lt;/a&gt;, which can (and apparently does) freeze up joints. If lubricating won't work, and not lubricating won't work, it seems like we've argued ourselves into a corner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Robotic arms often seem to be covered in a fabric. Maybe they're just &quot;canned&quot;, like a space suit so that the joint itself isn't exposed to vacuum? Excluding those that are canned, are there any types of joints that are actually used in space with the rotating part exposed to vacuum?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7hmn6.jpg&quot; alt=&quot;Shuttle arm&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-09-26T15:27:36.913" LastActivityDate="2013-09-30T14:55:11.853" Title="Is there any type of mechanical bearing that can be used in the full vacuum of space?" Tags="|spacecraft|vacuum|robotics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2186" PostTypeId="2" ParentId="2183" CreationDate="2013-09-26T15:56:03.240" Score="7" Body="&lt;p&gt;To my understanding, this is not much different than legal ramifications of appropriating once one's property that is now adrift defunct and abandoned in international waters. So, since my answer can not in any way be considered legally binding to any jurisdiction, I feel confident enough to give you my fair opinion on the matter. Do note though, I am not a lawyer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case of some equipment having been abandoned and serves no further use to its operator or owner, and it's not on anyone's territory, you will likely not meet any resistance and appropriation of it is left to one's own devices. You will most likely suffer costs far greater than having left such defunct equipment where it is and replacing its functionality  with your own replacements anyway. But the far more interesting question, in my opinion, is whether you could charge its previous owner for any costs incurred for &lt;em&gt;cleaning their mess up&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will likely become a hot topic in the near future, with so many defunct space debris in orbits around the Earth, and threatening future missions of any nations. For the time being, no tax is being collected to fill coffers of any projects to clean the mess we make on an international level in these no man's lands. Not directly at least, but there are several project funded by various national and international bodies that try and establish how much of this space junk really is out there and tracking them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On an international level, we do have some mechanisms in place by the United Nations (UN) resolutions calling for registration of space faring objects, the 1975 Convention on Registration of Objects Launched into Outer Space and the followed UN register collating this information as part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_law&quot; rel=&quot;nofollow&quot;&gt;Space Law&lt;/a&gt;, and some other conventions and agreements, but no real governing body short of the nations that signed these documents and agreed to respect them. For the time being, we have these treaties in place, that somewhat regulate our activities in outer space:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;nofollow&quot;&gt;The Outer Space Treaty&lt;/a&gt; - The 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Rescue_Agreement&quot; rel=&quot;nofollow&quot;&gt;The Rescue Agreement&lt;/a&gt; - The 1968 Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Liability_Convention&quot; rel=&quot;nofollow&quot;&gt;The Liability Convention&lt;/a&gt; - The 1972 Convention on International Liability for Damage Caused by Space Objects).&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Registration_Convention&quot; rel=&quot;nofollow&quot;&gt;The Registration Convention&lt;/a&gt; - The 1975 Convention on Registration of Objects Launched into Outer Space.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Moon_Treaty&quot; rel=&quot;nofollow&quot;&gt;The Moon Treaty&lt;/a&gt; - The 1979 Agreement Governing the Activities of States on the Moon and Other Celestial Bodies.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So we're only scratching the surface in terms of internationally agreeing on &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_law&quot; rel=&quot;nofollow&quot;&gt;Space Law&lt;/a&gt;, and much of this is left to one's own devices. A lot of our space exploration missions are international endeavours though, and might have their own set of conventions in place. To return back to the questions though:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short of having to respect a handful of international treaties and resolutions that draw rather broad strokes in painting the whole picture on legal ramifications of your actions in outer space, you're left free to do anything you want with defunct pieces of equipment left adrift or otherwise abandoned in international territories. You also report to the governing body of the nation your spaceship was registered to belong to, so even those treaties mentioned might apply differently to activities in outer space of different nations. Not all countries have signed all conventions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, pirates in space? Yes, we're certainly allowing that to happen as far as international laws go. It is however not even close to being as easy as arming a few men with guns, sticking them on a boat and telling them to go loot something. Lucky us.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="38" LastEditDate="2013-09-27T07:52:28.787" LastActivityDate="2013-09-27T07:52:28.787" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2187" PostTypeId="1" AcceptedAnswerId="2202" CreationDate="2013-09-26T16:27:50.177" Score="9" ViewCount="221" Body="&lt;p&gt;Me? I like roast aubergines cooked the way Mum &lt;a href=&quot;http://www.tarladalal.com/Baingan-Bharta-%28-Swadisht-Subzian-Recipe%29-4329r&quot;&gt;makes&lt;/a&gt; it with mashed potatoes deep-fried, omelette, and &lt;a href=&quot;http://en.wikipedia.org/wiki/Roti&quot;&gt;roti&lt;/a&gt;. If I travel in space that would be the staple in my &lt;a href=&quot;http://en.wikipedia.org/wiki/Tiffin&quot;&gt;tiffin&lt;/a&gt;. In my on-board farm would loom large - Aubergines, Potatoes, Wheat, and Onions, and Green Chillies, and Coriander, and ...  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I digress - the ISS, and the powers-that-be instead seem to prefer a random mix. Zucchini, Cress, Maize, Rye, Cotton, Wild Carrot, Lettuce, Radish, Rice, Onions - these are just a few &lt;a href=&quot;http://en.wikipedia.org/wiki/Plants_in_space&quot;&gt;plants studied/cultivated&lt;/a&gt; in various space missions. Fungus, and other micro-organisms have been cultivated too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For now, I'll focus on plants - and my favoured aubergines (+:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is/are the criteria to select plants for study in space?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-03-19T04:08:53.413" LastActivityDate="2014-03-19T04:08:53.413" Title="What are the criteria to select plants for study in space?" Tags="|iss|food|plants|experiment|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2188" PostTypeId="2" ParentId="2038" CreationDate="2013-09-26T18:09:44.620" Score="5" Body="&lt;p&gt;If they were going to be used anywhere else, it would have to be with one of the Gas Giants, because no other planet has enough magnetic field in the solar system to use them. Furthermore, it would be an orbiter of some kind, because it wouldn't make sense to use them for only the short period of time when flying by the planet of interest. Thus, the candidates are JUNO, Cassini, and Galileo. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Cassini - Uses Reaction Wheels and thrusters&lt;/li&gt;&#xA;&lt;li&gt;Galileo - Primarily managed via spinning the spacecraft and thrusters.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceflight101.com/juno-spacecraft-information.html&quot; rel=&quot;noreferrer&quot;&gt;Juno&lt;/a&gt; - Just uses thrusters, along with being spin stabilized. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, none have done it. They could probably be used, but as magnetorquers are somewhat limited in their ability to point specifically at something at a given time, and there isn't as much movement of spacecraft as they would be on Earth, and as the orbits are quite slow by Earth standards, I don't think it would gain much, thus, it hasn't been tried.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2013-11-20T12:22:28.400" LastActivityDate="2013-11-20T12:22:28.400" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2189" PostTypeId="1" CreationDate="2013-09-26T19:04:58.090" Score="8" ViewCount="502" Body="&lt;p&gt;When the weatherman on TV says 'Rain', I make it a point not to take brolly with me. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But jokes apart, meteorology here on Earth is developed enough to assist decision-making. This is demonstrated time-and-again as was the case with &lt;a href=&quot;http://en.wikipedia.org/wiki/Breitling_Orbiter_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Breitling Orbiter&lt;/a&gt;'s flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several planets in the Solar System exhibit an &lt;a href=&quot;https://space.stackexchange.com/questions/869/flying-on-dense-atmosphere-planets-moons&quot;&gt;atmosphere dense enough to fly&lt;/a&gt; an &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerobot&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerobot&lt;/a&gt; mission. Given climate, and weather are so relevant to flight here on Earth, it is reasonable to make a similar assumption about atmospheric flight on other planets too. Such flight may be unpowered as &lt;a href=&quot;https://space.stackexchange.com/questions/2155/around-venus-in-8-days-by-balloon&quot;&gt;a balloon&lt;/a&gt;/glider, or powered (Helicopter/Aeroplane)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atmospheric flight could provide the option of future missions - manned, and/or bot; perhaps capable of staying aloft in the thick Venerean atmosphere for sustained periods. At the least atmospheric flight may allow in-situ study of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus, despite it's proximity, is still several minutes from Earth even at radio-speed. The atmosphere is thick, toxic, and high-speed. Perhaps an aerobot mission would initially have to rely on local decisions augmented by intelligence from Earth - &lt;em&gt;assuming meteorological models of Venus developed thus far are so capable&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What institution/s have, thus far, developed/attempted to develop a meteorological model of Venus?&lt;/li&gt;&#xA;&lt;li&gt;At what stage of development are such models of Venusian climate? &lt;/li&gt;&#xA;&lt;li&gt;Are these models capable of generating a tentative map of Venerean climate several weeks into the future?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-10-02T18:24:04.983" Title="What stage of development are meteorology models of Venus?" Tags="|venus|aerobot|planetary-science|weather|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2190" PostTypeId="2" ParentId="1132" CreationDate="2013-09-26T20:05:37.427" Score="4" Body="&lt;p&gt;The Alcubierre drive requires exotic matter to contract space-time behind it. The contraction of space-time in the front requires equal amount of positive energy/matter. The negative mass/energy requirement as &lt;a href=&quot;https://en.wikipedia.org/wiki/Alcubierre_drive#Mass.E2.80.93energy_requirement&quot; rel=&quot;nofollow&quot;&gt;proposed by Ramon and Ford&lt;/a&gt; is about -10E64Kg. The same amount (10E64Kg) is required on the front. Now let us analyze this mass:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass of Sun=1.989E30 Kg=1 Solar Mass&#xA;Mass of Super Massive Blackhole=A Billion Solar Masses~1.0E40 Kg&#xA;which is 1.0E24 times less than a SMB.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now imagine the space time warp created by such amount of mass. We are creating such large warp to achieve FTL travel which is the purpose of our Alcubierre Drive. You can read more &lt;a href=&quot;http://www.nasa.gov/centers/glenn/technology/warp/ipspaper.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your Question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;given a drive system, Alcubierre-based or otherwise, powerful enough that we no longer depend on gravity-based navigation maneuvers to get around, what role would gravity still play in our navigation and maneuvering? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Currently Alcubierre drive is the only realizable approach for FTL which is powerful enough.  The utility and correctness of &lt;a href=&quot;http://www.nasa.gov/centers/glenn/technology/warp/ipspaper.html&quot; rel=&quot;nofollow&quot;&gt;other theories&lt;/a&gt; remains to be determined.&#xA;So, any ship using this drive system must obtain a region of space between source and destination, so that the affect of warp is nil. Otherwise the dominant warping of the ship w.r.t. other masses would attract the surrounding matter towards the warp bubble. So, yes Gravity/Matter would still matter but not in the way it mattered in the case of Gravitational Slingshots. &lt;/p&gt;&#xA;" OwnerUserId="784" LastActivityDate="2013-09-26T20:05:37.427" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2192" PostTypeId="5" CreationDate="2013-09-26T20:22:26.837" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-09-26T20:22:26.837" LastActivityDate="2013-09-26T20:22:26.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2193" PostTypeId="4" CreationDate="2013-09-26T20:22:26.837" Score="0" Body="FTL, an acronym for Faster Than Light travel, is a system of covering distances with speeds exceeding that of light." OwnerUserId="784" LastEditorDisplayName="user12" LastEditDate="2013-10-25T16:27:29.140" LastActivityDate="2013-10-25T16:27:29.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2197" PostTypeId="1" AcceptedAnswerId="2198" CreationDate="2013-09-27T03:23:10.800" Score="10" ViewCount="247" Body="&lt;p&gt;With how much delta-v it costs to reenter space from the surface of a planet or moon, why would anyone ever build a base/station on the surface, rather than in orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there some reason assembly would be easier on the ground, rather than in space?&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2013-09-27T04:36:28.293" Title="Why would we ever build a base on the surface of a planet?" Tags="|space-station|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2198" PostTypeId="2" ParentId="2197" CreationDate="2013-09-27T04:36:28.293" Score="11" Body="&lt;p&gt;We wouldn't build a base on another celestial because it's simpler than assembling a space station, but because it would be closer to valuable resources, have gravity, and a chance of at least partial self-sustainability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The list of advantages could stretch several pages long, but for the sake of argument, let's just say that's potentially a lot of resources that you don't have to continuously deliver to your space station and a ground based station could produce on its own. And gravity, even a lot smaller than that of the sea-level Earth gravity, is valuable for any long duration human missions to help reduce the adverse effects of prolonged stay in microgravity conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point to having a ground based station as opposed to a space based one is thus not one of merely delta-v requirements. But even looking at those alone, on the long run, and assuming you're building a station for any specific reason, you're likely to require far greater delta-v and far more launches for a space station. You don't stop your deliveries to a space station once it's built, but you might eventually stop delivering great number of things to a more self-sustainable ground based station, on whichever planet it may be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for space vs ground assembly in the last part of your question, assembling large support structures that can be cemented into ground is by far simpler and cost effective than building any space station. If for nothing else, and even if we assume each cubic meter of the stations total volume would be as difficult to assemble both on the ground or in space, your ground based structure would already have large portion of its outer shell built by the grounds of the planet you're stationed on itself. You might want to flatten it out and overlay with some more suitable surface, but unless you're building it directly on a tectonic ridge or a volcano, you need not worry too much about its integrity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've mentioned cementing the foundation of your ground based station, so you might have guessed by now that another major advantage to it, even if you don't really use cement, is that you have access to several building materials directly on your building site and all around it. They might vary in composition, but so do various cements. And we can learn to use new ground materials, we're pretty good at that. You can melt silicate rocks in smelters to produce tough glass, limestone as soft yet strong material to easily carve out bricks, dust and gravel to use in your cement mixtures,... you name it. On the other hand, for your space station, you need to deliver all the building materials with your launch vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also a lot easier for your workers, be it humans or robots, to construct things in gravity and have something to push against, than having to deal with microgravity and still be required to exert force in one direction while staying more or less stationary. We might have developed some tools to help with that, but they can't beat the simplicity and efficiency of a good old mallet and a chisel. This list of of advantages to having ground based station is miles long really, so I'll stop here.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-27T04:36:28.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2199" PostTypeId="1" CreationDate="2013-09-27T07:19:32.387" Score="12" ViewCount="277" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zDlKH.jpg&quot; alt=&quot;source:[astrobob](http://areavoices.com/2013/09/25/countdown-to-comet-ison-mars-flyby/)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Within a few days &lt;strong&gt;Comet ISON&lt;/strong&gt; is going to make a close &lt;em&gt;flyby of Mars&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will the rovers on Mars have a chance to take a look at the comet as it approaches Mars?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2016-04-29T21:12:25.987" LastActivityDate="2016-04-29T21:12:25.987" Title="Can the rovers on Mars see the comet flyby?" Tags="|mars|rovers|curiosity|spacecraft|comet|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2200" PostTypeId="1" AcceptedAnswerId="2201" CreationDate="2013-09-27T07:33:11.153" Score="27" ViewCount="2231" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rTSgn.jpg&quot; alt=&quot;Source:Discovery&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image above was taken from the Curiosity rover (I hope). There appears to be a coin in the image. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that really a coin or some kind of button? Is it really present on the rover?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is indeed a coin, why is it there?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="288" LastEditDate="2013-09-28T05:15:22.177" LastActivityDate="2023-11-02T09:30:44.227" Title="What's the coin on the Curiosity rover?" Tags="|mars|history|nasa|curiosity|camera|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2201" PostTypeId="2" ParentId="2200" CreationDate="2013-09-27T07:42:36.897" Score="26" Body="&lt;p&gt;Yes, this is a real &lt;a href=&quot;https://en.wikipedia.org/wiki/Lincoln_cent&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lincoln cent&lt;/a&gt; and it makes part of the camera calibration target for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Hand_Lens_Imager&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Hand Lens Imager&lt;/a&gt; (MAHLI) instrument.&lt;/p&gt;&#xA;&lt;p&gt;A geologist tradition is to place coins (or other object of a known size) besides photographed objects to show the scale of the photograph. This seems to be a homage to this tradition.&lt;/p&gt;&#xA;&lt;p&gt;For details see:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://mars.nasa.gov/resources/3803/lincoln-cent-on-mars-rover/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lincoln Cent on Mars Rover&lt;/a&gt; (NASA)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://science.nasa.gov/resource/lucky-penny-on-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lucky Penny on Mars&lt;/a&gt; (NASA)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.coinnews.net/2012/09/26/1909-v-d-b-lincoln-cent-on-nasas-mars-rover-curiosity/&quot; rel=&quot;nofollow noreferrer&quot;&gt;1909 V.D.B. Lincoln Cent on NASA’s Mars Rover Curiosity&lt;/a&gt; (CoinNews.net)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="810" LastEditorUserId="810" LastEditDate="2023-11-02T09:30:44.227" LastActivityDate="2023-11-02T09:30:44.227" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="2202" PostTypeId="2" ParentId="2187" CreationDate="2013-09-27T09:08:28.850" Score="8" Body="&lt;p&gt;Looking at the information on what plants were taken in to space, it appears that the criteria include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Size: initially small plants were taken. This makes sense from a weight perspective&lt;/li&gt;&#xA;&lt;li&gt;Well studied: the list of plants aligns well with those studied extensively in universities. Again, this makes sense - it may be easier to identify odd growth in a well studied plant type&lt;/li&gt;&#xA;&lt;li&gt;Growth speed: a large number of the listed plants have rapid growth, making them suitable for relatively short mission timescales, but also having potentially more use in a long term space mission&lt;/li&gt;&#xA;&lt;li&gt;Robust: I don't see any on the list that are considered difficult to grow on Earth. These all seem to be common, fertile, resilient plant types.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The breadth of plants gives us food production, materials (eg cotton), good CO2 scrubbing etc.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-09-27T09:08:28.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2204" PostTypeId="2" ParentId="2199" CreationDate="2013-09-27T10:22:42.600" Score="10" Body="&lt;p&gt;There are a total of 4 Martian spacecraft that have promised to attempt a photo, according to &lt;a href=&quot;http://au.ibtimes.com/articles/508822/20130925/2013-comet-ison-mars-spacecraft-capture-images.htm#.UkLheH9jHSg&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars Express- Will look for a total of 2 weeks, it started this week (On Monday)&lt;/li&gt;&#xA;&lt;li&gt;MRO- HiRISE camera will take 3 photos, at closest approach and the day before and after.&lt;/li&gt;&#xA;&lt;li&gt;Curiosity and Opportunity will both attempt a photo as well.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In fact, at least &lt;a href=&quot;http://hirise.lpl.arizona.edu/releases/ison.php&quot; rel=&quot;noreferrer&quot;&gt;HiRISE&lt;/a&gt; has released it's pictures. They are dim and noisy, but here they are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/60Ioa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-03T13:36:25.427" LastActivityDate="2013-10-03T13:36:25.427" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2205" PostTypeId="2" ParentId="2200" CreationDate="2013-09-27T14:32:10.370" Score="22" Body="&lt;p&gt;When calibrating a camera, there are typically 3 things that are used:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Some sort of a color pallet&lt;/li&gt;&#xA;&lt;li&gt;A line chart of some sort to identify the fine resolution&lt;/li&gt;&#xA;&lt;li&gt;A real object, to make sure there isn't something fundamentally wrong.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/msl/news/msl20120207.html&quot; rel=&quot;noreferrer&quot;&gt;NASA followed this same suit&lt;/a&gt;, choosing the penny to be the real object, claiming that it give homage to the practice of using coins to get a sense of scale in photographs. If you take a look below, you will see color calibration targets, resolution targets, and the penny, for the final touch of realism.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KnPKR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for why that particular coin, NASA states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The coin is from 1909. That was the first year Lincoln pennies were&#xA;  minted and the centennial of Abraham Lincoln's birth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2013-09-27T21:54:41.397" LastActivityDate="2013-09-27T21:54:41.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2206" PostTypeId="1" AcceptedAnswerId="2207" CreationDate="2013-09-27T16:18:35.663" Score="8" ViewCount="1007" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2200/whats-the-coin-on-the-curiosity-rover#2201&quot;&gt;PearsonArtPhoto's answer&lt;/a&gt; on &quot;What's the coin on the Curiosity rover?&quot; brought to mind the possibility of using a fractal-like image that could combine scale and color calibration functions &lt;strong&gt;and&lt;/strong&gt; would be more of a work of art (suitable for posters).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could such a pattern be a better (or at least equally good per unit area) calibration target? (Such would seem to provide greater information density and to provide more sensitive testing of color transition handling, but such might require more complex calibration software.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would such be excessively expensive to manufacture? (Ordinary printing technology might be excessively subject to fading or other damage and might not have the precise color control desired. The color samples on Curiosity appear to be plaques, presumably for damage resistance.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unifying function with aesthetics would seem to have at least some value in itself, even if the unification of specialized use with general appreciation did not increase the appreciation of the specialized use (i.e., it was useless as promotional material).&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-11-05T08:37:59.377" Title="Would a fractal-like pattern be a better calibration target?" Tags="|camera|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2207" PostTypeId="2" ParentId="2206" CreationDate="2013-09-27T17:26:15.433" Score="4" Body="&lt;p&gt;It's an interesting idea, and similar to fractal pattern objects are frequently used for testing and calibration of photographic / optical equipment, like for example inclusion of a bird's feather, a tip of a paintbrush, a furry toy, or even frequently included photograph of &lt;a href=&quot;https://en.wikipedia.org/wiki/Lenna&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lenna&lt;/a&gt; with a dark purple feather / fur like fashion accessory on her beige hat, e.t.c., but I don't think a fractal-like pattern would be sufficient on its own. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ITZ78.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Digital Photographic Review (dpreview.com) test target setup, including multiple &lt;em&gt;fractal&lt;/em&gt; targets&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photographic calibration targets test multiple aspects of photographic fidelity that require not only various levels of detail to test sharpness, &lt;a href=&quot;https://en.wikipedia.org/wiki/Metering_mode&quot; rel=&quot;nofollow noreferrer&quot;&gt;metering&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Compression_artifact&quot; rel=&quot;nofollow noreferrer&quot;&gt;compression artefacts&lt;/a&gt;, digital sensor noise, or a plethora of other things, but also larger blocks of single colors, &lt;a href=&quot;https://en.wikipedia.org/wiki/Monochrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;monochrome&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Halftone&quot; rel=&quot;nofollow noreferrer&quot;&gt;halftone&lt;/a&gt; pattern areas to test color reproduction, color transition &lt;a href=&quot;https://en.wikipedia.org/wiki/Colour_banding&quot; rel=&quot;nofollow noreferrer&quot;&gt;banding&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Moir%C3%A9_pattern&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moiré effect&lt;/a&gt;, test for dead or half-dead sensor cells, &lt;a href=&quot;https://en.wikipedia.org/wiki/Distortion_%28optics%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;distortion&lt;/a&gt;, and so on. As the list goes on, more and more of these tests require different approach to testing it, and with it either different test / calibration images or a single, complex and combining multiple tests one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/XeKsN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Aerial optical calibration targets from the photo resolution range, &lt;a href=&quot;https://en.wikipedia.org/wiki/Edwards_Air_Force_Base&quot; rel=&quot;nofollow noreferrer&quot;&gt;Edwards Air Force Base&lt;/a&gt;, Lancaster, CA (Source: &lt;a href=&quot;https://maps.google.com/maps?ll=34.8512,-117.657266&amp;amp;spn=0.001862,0.004109&amp;amp;gl=us&amp;amp;t=h&amp;amp;z=19&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google Maps&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, while fractal patterns would clearly be interesting to include in such calibration targets, and they indeed often are (&lt;a href=&quot;http://bldgblog.blogspot.com/2013/02/optical-calibration-targets.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;even buildings and patches of land&lt;/a&gt;), and you can &lt;a href=&quot;http://www.pointsinfocus.com/learning/diy/diy-autofocus-calibration-target/&quot; rel=&quot;nofollow noreferrer&quot;&gt;make your own&lt;/a&gt; too, they most likely wouldn't really suffice for all the fine-tuning and performance testing requirements of your digital and optical equipment.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-05T08:37:59.377" LastActivityDate="2013-11-05T08:37:59.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2208" PostTypeId="2" ParentId="205" CreationDate="2013-09-27T18:24:29.413" Score="8" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dlTq6.jpg&quot; alt=&quot;Modern Orion Suit ISS&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EVA suits need to meet the following requirements:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;A stable internal pressure. This can be less than earth's atmosphere,&#xA;as there is usually no need for the space suit to carry nitrogen&#xA;(which comprises about 78% of earth's atmosphere and is not used by&#xA;the body). Lower pressure allows for greater mobility, but requires&#xA;the suit occupant to breathe pure oxygen for a time before going into&#xA;this lower pressure, to avoid decompression sickness.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mobility. Movement is typically opposed by the pressure of the suit;&#xA;mobility is achieved by careful joint design. See the Theories of&#xA;space suit design section.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Supply of breathable oxygen and elimination of carbon dioxide; these&#xA;gases are exchanged with the spacecraft or a &lt;a href=&quot;http://en.wikipedia.org/wiki/Primary_Life_Support_System&quot; rel=&quot;noreferrer&quot;&gt;Primary Life Support&#xA;System (PLSS)&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Temperature regulation. Unlike on Earth, where heat can be&#xA;transferred by convection to the atmosphere, in space, heat can be&#xA;lost only by thermal radiation or by conduction to objects in&#xA;physical contact with the exterior of the suit. Since the temperature&#xA;on the outside of the suit varies greatly between sunlight and&#xA;shadow, the suit is heavily insulated, and air temperature is&#xA;maintained at a comfortable level.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A communication system, with external electrical connection to the&#xA;spacecraft or PLSS&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Means of collecting and containing solid and liquid bodily waste&#xA;(such as a &lt;a href=&quot;http://en.wikipedia.org/wiki/Maximum_Absorbency_Garment&quot; rel=&quot;noreferrer&quot;&gt;Maximum Absorbency Garment&lt;/a&gt;)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Advanced suits better regulate the astronaut's temperature with a&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_Cooling_and_Ventilation_Garment&quot; rel=&quot;noreferrer&quot;&gt;Liquid Cooling and Ventilation Garment&lt;/a&gt; (LCVG) in contact with the&#xA;astronaut's skin, from which the heat is dumped into space through an&#xA;external radiator in the PLSS.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Shielding against ultraviolet radiation&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Limited shielding against particle radiation&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Means to maneuver, dock, release, and/or tether onto a spacecraft&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Protection against small micrometeoroids, some traveling at up to&#xA;27,000 kilometers per hour, provided by a puncture-resistant &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermal_Micrometeoroid_Garment&quot; rel=&quot;noreferrer&quot;&gt;Thermal&#xA;Micrometeoroid Garment&lt;/a&gt;, which is the outermost layer of the suit.&#xA;Experience has shown the greatest chance of exposure occurs near the&#xA;gravitational field of a moon or planet, so these were first employed&#xA;on the Apollo lunar EVA suits.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To your question specifically:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What components of cosmic rays are dangerous? What sort of protection&#xA;  is used to shield from these as well as from the temperature issues?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Radiation:&lt;/strong&gt; High energy particles destroy organic matter at a fundamental level, breaking DNA chains and killing tissue.  Exposure greatly increases the risk that during the lifetime of the exposed person will develop some form of cancer.  To reduce exposure they sometimes position the craft to use it as additional shielding against exposure.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Despite considerable efforts, the cancer and the toxicity risks remain&#xA;  to be quantified: 1) the nature and the frequency of secondary heavy&#xA;  ions need to be better characterized in order to estimate their&#xA;  contribution to the dose and to the final biological response; 2) the&#xA;  diversity of radiation history of each astronaut and the impact of&#xA;  individual susceptibility make very difficult any epidemiological&#xA;  analysis for estimating hazards specifically due to space radiation&#xA;  exposure. 3) Cytogenetic data undoubtedly revealed that space&#xA;  radiation exposure produce significant damage in cells. However, our&#xA;  knowledge of the basic mechanisms specific to low-dose, to repeated&#xA;  doses and to adaptive response is still poor. The application of new&#xA;  radiobiological techniques, like immunofluorescence, and the use of&#xA;  human tissue models different from blood, like skin fibroblasts, may&#xA;  help in clarifying all the above items. (&lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/21436608&quot; rel=&quot;noreferrer&quot;&gt;http://www.ncbi.nlm.nih.gov/pubmed/21436608&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://srag-nt.jsc.nasa.gov/Publications/TM104782/intro.html&quot; rel=&quot;noreferrer&quot;&gt;number of parameters affect astronaut exposure to radiation&lt;/a&gt;. These parameters include the structure of the spacecraft, the materials used to construct the vehicle, the altitude and inclination of the spacecraft, the status of outer zone electron belts, the interplanetary proton flux, geomagnetic field conditions, solar cycle position, and EVA start time and duration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/168.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Study&lt;/a&gt; found:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The results from EVARM have shown that EVA doses are elevated from&#xA;  those inside the ISS, but not significantly. In addition, this time&#xA;  period recorded doses during a time of increased geomagnetic activity&#xA;  (October/November 2002). It was determined that during this event&#xA;  doses to EVA participants were increased due to elevated levels of&#xA;  electrons in Earth orbit. These electrons are easily shielded by&#xA;  spacecraft materials and thus not measured inside the ISS.&#xA;  Fortunately, proper positioning of the spacecraft can dramatically&#xA;  reduce the radiation field encountered during EVA missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Temperature:&lt;/strong&gt;  Requirement for protection here is pretty self-explanatory.  Suit for ISS - Low Earth Orbit (LEO), typically cover -250°F to +250°F for human protection.  This is accomplished by the &lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_Cooling_and_Ventilation_Garment&quot; rel=&quot;noreferrer&quot;&gt;Liquid Cooling and Ventilation system&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BLVsZ.jpg&quot; alt=&quot;LCVG&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="446" LastEditorUserId="446" LastEditDate="2013-09-28T02:19:59.670" LastActivityDate="2013-09-28T02:19:59.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2209" PostTypeId="2" ParentId="2206" CreationDate="2013-09-27T19:36:11.403" Score="4" Body="&lt;p&gt;The point of the middle image is to assist in calibration of lines and such. That's harder to do on a pure fractal. There is a commonly seen calibration pattern which follows some principals of fractals that is used, however, but it doesn't have the beauty that most fractals have. I found this one &lt;a href=&quot;https://www.fas.org/irp/imint/docs/rst/Sect10/Sect10_3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YusJD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-27T19:36:11.403" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2210" PostTypeId="2" ParentId="2177" CreationDate="2013-09-28T06:07:09.753" Score="2" Body="&lt;p&gt;The engineers at NASA have made sure the rover is tightly sealed. Everything is made up of tough metals from the outside like titanium, copper, aluminium etc. So there is no chance of damage even in high speed dust storms.&#xA;It also uses epoxies that are very strong and tightly seal the delicate instruments from the dust.Other joints are not problems as they are not affected by the dust&#xA;As for the cameras the lenses are very tough, made of high grade polymer glass.&#xA;There are no joints as you can see in the picture below, it is made of hydraulics which also is tightly sealed. So the hydraulic fluids are safe from the dust.&#xA;&lt;a href=&quot;https://www.synlube.com/2011images/Mars_Curiosity_right-side.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://www.synlube.com/2011images/Mars_Curiosity_right-side.jpg&quot; alt=&quot;Curiosity Rover&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user738" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-02-17T02:28:36.873" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="2212" PostTypeId="1" CreationDate="2013-09-28T13:36:39.843" Score="2" ViewCount="66" Body="&lt;p&gt;Why doesn't curiosity (or other) use caterpillar tracks (like tanks) instead of wheels?&#xA;It would be good for rough terrain and slow speed, and this is exactly the situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regards&lt;/p&gt;&#xA;" OwnerUserId="815" LastActivityDate="2013-09-28T13:36:39.843" Title="Curiosity with caterpillars?" Tags="|curiosity|" AnswerCount="0" CommentCount="3" ClosedDate="2013-09-28T15:29:09.377" ContentLicense="CC BY-SA 3.0" />
  <row Id="2213" PostTypeId="1" CreationDate="2013-09-28T13:46:52.880" Score="5" ViewCount="3113" Body="&lt;p&gt;Over the past few years, it's become somewhat of a trend to send up high-altitude balloons and post videos on internet. Well-known examples include &lt;a href=&quot;http://www.youtube.com/watch?v=XoMN-zg7r3M&quot;&gt;Toy Train in Space&lt;/a&gt; and &lt;a href=&quot;http://trekmovie.com/2012/05/22/picard-kirk-and-friends-journey-into-space-on-gigantic-balloon/&quot;&gt;Picard &amp;amp; Kirk in Space&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this is very cool, and a great way to show that space can be accessible to ordinary citizens. As such, I'd love to see more of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I can't help but feel that there have to be a fair number of risks involved (the most obvious being that if a part falls off a few hundred meters up, it'll fall fast enough to kill but not fast enough to disintegrate), which might become considerable if the number of experiments increase. As such, I'm not so sure I would want to see more of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, my question is really two-fold:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What are the risks involved with such experiments?&lt;/li&gt;&#xA;&lt;li&gt;What steps can be taken to insure safety in the event that such experiments do keep increasing in frequency?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="156" LastEditorUserId="49" LastEditDate="2013-09-28T15:40:57.167" LastActivityDate="2013-09-28T21:09:46.583" Title="What are the risks involved in amateur high-altitude ballooning, and how can we mitigate them?" Tags="|safety|balloons|uncrewed-spaceflight|amateur-radio|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2214" PostTypeId="2" ParentId="2213" CreationDate="2013-09-28T17:03:30.330" Score="3" Body="&lt;p&gt;&lt;strong&gt;How do you stop people acting irresponsibly?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no idea. I don't think anyone has. They have been main antagonists of our nightmares ever since we started dreaming.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why am I saying this? Because you're absolutely right and the &lt;em&gt;Toy Train in Space&lt;/em&gt; was let from a densely populated area, possibly without notifying and gaining permission from airspace authority of the jurisdiction, and has also landed in populated area. Yes, some will say, &lt;em&gt;&quot;but it landed in a cornfield&quot;&lt;/em&gt;. So it did, lucky neighborhood. And the next one might land on someone's head and &lt;a href=&quot;http://www.huffingtonpost.com/2012/09/21/toy-train-in-space_n_1903263.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Huffington Post&lt;/a&gt; won't have so cute photos and videos to report on. Let's all laugh about it, right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not right. And it's not even the toy train that is really a problem. Or action figures. The problem is, that common people don't realize that access to airspace is tightly controlled mainly to prevent accidents / disasters. The problem is, that there were largely loose metallic rods holding the camera and the toy, and the camera wasn't as light as the toy either. This is what you see in the videos. The toy. What you don't see is the darn thing that will kill you. A metallic rod travelling pointy side first at near &lt;a href=&quot;https://en.wikipedia.org/wiki/Mach_number&quot; rel=&quot;nofollow noreferrer&quot;&gt;mach one&lt;/a&gt; (340.29 m/s) towards the skulls of civilians. And that's only one skull. How about this &lt;em&gt;cute thingy&lt;/em&gt; together with the balloon and the frame being sucked into a jet engine? While the airplane is at low altitude, low airspeed and ready to land maybe? Still cute?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a way to do it properly tho:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Declare your intentions and gain access to airspace with your authorities, &lt;/li&gt;&#xA;&lt;li&gt;ask for aviation weather report before launch, &lt;/li&gt;&#xA;&lt;li&gt;launch into &lt;a href=&quot;https://en.wikipedia.org/wiki/Jet_stream&quot; rel=&quot;nofollow noreferrer&quot;&gt;jet streams&lt;/a&gt; that will push your balloon over unpopulated areas and from a &lt;a href=&quot;https://en.wikipedia.org/wiki/No-fly_zone&quot; rel=&quot;nofollow noreferrer&quot;&gt;no-fly zone&lt;/a&gt;, &lt;/li&gt;&#xA;&lt;li&gt;equip your balloon's frame with a self-activating parachute release system, &lt;/li&gt;&#xA;&lt;li&gt;have tracking devices in place and &lt;/li&gt;&#xA;&lt;li&gt;coordinate with authorities in case of unpredictable turn of events, so they will &lt;/li&gt;&#xA;&lt;li&gt;keep contact with your event's organizer and can &lt;/li&gt;&#xA;&lt;li&gt;notify local population to remain indoors, while &lt;/li&gt;&#xA;&lt;li&gt;you or the nearest airspace tracking radar can tell them which area to notify to do so.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Too difficult? No problem. Join your local model rocketry, balloon or airship club, or even your school's related extracurricular activities, and organizers of the events will do that for you. They will even help you build it, give advice, and maybe even provide snacks for the event. Then, it can be properly cute. Meet Camilla:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/VPZ6d.jpg&quot; alt=&quot;Camilla&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Camilla is a rubber chicken mascot of NASA's Solar Dynamics Observatory, launched by Bishop Union High School's Earth to Sky&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;student group, in mountains near Bishop, California on a helium balloon to the stratosphere, roughly 120,000 ft (36.6 km) high.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Camilla was launched from unpopulated mountain range near Bishop, California by a group of Bishop Union High School students for an astrobiology project:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During the two and a half hour flight, Camilla spent approximately 90&#xA;  minutes in the stratosphere where temperatures (-40 to -60 C) and air&#xA;  pressures (1% sea level) are akin to those on the planet Mars. The&#xA;  balloon popped, as planned, at an altitude of about 40 km and Camilla&#xA;  parachuted safely back to Earth. The entire payload was recovered&#xA;  intact from a landing site in the Inyo Mountains.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One week later, on March 10th, the storm was underway, and the students repeated the experiment. Sam Johnson (age 16) of Bishop Union High School's Earth to Sky student group mentions the second landing site:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The profile of the second flight was almost identical to the&#xA;  first--perfect for our experiment,&quot; adds Johnson. &quot;We recovered the&#xA;  payload from a landing site near Deep Springs, California.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As you will likely know, both these landing site areas are scarcely populated, if at all, and their balloon both times had a parachute properly deployed. More can be read in &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2012/19apr_camilla/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article on NASA Science&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So there is a way to have fun, learn from it, and keep everyone safe at the same time while launching a balloon with a toy on its travel towards &lt;a href=&quot;https://space.stackexchange.com/a/2077/49&quot;&gt;the edge of space&lt;/a&gt;.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, I don't enjoy being as serious as this answer might sound. I'd much, much rather see it completely unnecessary and self-understood, and we can all continue having fun launching odd things into space.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-09-28T17:03:30.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2215" PostTypeId="2" ParentId="2213" CreationDate="2013-09-28T18:49:09.843" Score="4" Body="&lt;p&gt;I assume most of these balloon flights follow the proper regulations, and thus are legal, and in theory safe. There are a number of regulations, which &lt;a href=&quot;http://www.stratostar.net/faq/&quot; rel=&quot;nofollow&quot;&gt;this site&lt;/a&gt; includes, including weight limits, not dropping objects, and limiting the weight/surface area ratio, to keep the terminal velocity low. There are also quite a few regarding communication, but I'll skip on those for now. The key points to legally do this in the US are are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Payloads cannot exceed a package weight/size ratio of three ounces per square inch. (Determined by taking the total weight of the package divided by the smallest area)&lt;/li&gt;&#xA;&lt;li&gt;No payload package can exceed 6 lb in weight. &lt;/li&gt;&#xA;&lt;li&gt;Entire weight of all payloads cannot exceed 12 lb in total weight (This doesn't include the weight of balloon).&lt;/li&gt;&#xA;&lt;li&gt;No rope or cable should be used which requires more than 50lb of force to separate payload packages from balloon.&lt;/li&gt;&#xA;&lt;li&gt;No one may launch a high-altitude weather balloon which creates a hazard to other people and property.&lt;/li&gt;&#xA;&lt;li&gt;No one may use a high-altitude weather balloon to drop objects (i.e. projectiles).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If all of these regulations are followed, then the process should be relatively safe. The package should also be structurally sound so it doesn't fall apart. There are also a few other issues, the primary one being how do you track the balloon after it hits the ground, and when it's in the air, for that matter. But it can be a lot of fun.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-09-28T21:09:46.583" LastActivityDate="2013-09-28T21:09:46.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2216" PostTypeId="1" CreationDate="2013-09-28T19:09:55.767" Score="9" ViewCount="472" Body="&lt;p&gt;A sort-of follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/67/why-does-the-international-space-station-produce-so-much-methane/77#77&quot;&gt;Why does the International Space Station produce so much methane?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2010 Sabatier hardware was integrated into the regenerative environment control. The Sabatier hardware implements the Sabatier process; this retrieves water out of the Carbon Dioxide, Hydrogen present ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\require{mhchem}\ce{CO2 + 4H2 -&amp;gt; CH4 + 2H2O}$$&#xA;With a crew of 6, the system is capable of &lt;a href=&quot;http://articles.courant.com/2010-05-25/business/hc-water-for-outer-space-0520-20100521_1_international-space-station-russian-spacecraft-space-shuttles&quot; rel=&quot;nofollow noreferrer&quot;&gt;recovering up to 2000 pounds of water annually&lt;/a&gt;. I could be wrong, but iirc the reaction liberates some heat-energy in the process. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How much heat did the Sabatier hardware add to the annual ISS heat profile? &lt;/li&gt;&#xA;&lt;li&gt;As a corollary, what is the overload capacity built into the thermal management system on the ISS? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-04-02T16:22:27.297" Title="How much heat did the Sabatier hardware add to the ISS heat profile?" Tags="|iss|thermal|temperature|climate|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2217" PostTypeId="2" ParentId="668" CreationDate="2013-09-28T20:37:32.827" Score="-1" Body="&lt;p&gt;I also like the idea of the caterpillar tracks. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For short distances, such as the distances covered by Curiosity, they don't need as much maintenance as suggested. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to that, tracks don't suffer from puctures or bursts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only serious problem is that they can derail if not properly designed. But at low speeds that shouldn't be a problem either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They could be made with kevlar or other materials to get light tracks. Though I know the additional weight would come from the multiple wheels needed to move it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regards&lt;/p&gt;&#xA;" OwnerUserId="815" LastActivityDate="2013-09-28T20:37:32.827" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2218" PostTypeId="1" AcceptedAnswerId="2221" CreationDate="2013-09-29T00:43:18.350" Score="22" ViewCount="6236" Body="&lt;p&gt;Plants produce oxygen, we know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This might make plants useful in the supply system of possible colonies on other planets or moons. But how efficient they are?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is the maximum amount of oxygens can be produced by a single entity and what is it enough for?&lt;/p&gt;&#xA;" OwnerUserId="306" LastActivityDate="2017-10-04T15:38:02.903" Title="Oxygen self-supply by plants" Tags="|colonization|life-support|plants|oxygen|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2219" PostTypeId="2" ParentId="2218" CreationDate="2013-09-29T02:07:09.187" Score="17" Body="&lt;p&gt;The higher plants are less efficient, so the &lt;em&gt;entity&lt;/em&gt; will probably be not a bush, but a jar of algae.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;30-40 liters of Сhlorella suspension&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qvIR7.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;could provide for a gaseous exchange of a single person.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://epizodsspace.no-ip.org/bibl/znan/1989/7/7-iss-kosm-ekos.html&quot; rel=&quot;noreferrer&quot;&gt;Grishin Yu. I. &amp;quot;Artificial Space Ecosystems&amp;quot;&#xA;&lt;em&gt;Cosmonautics, Astronomy&lt;/em&gt; 7/1989&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-04T15:38:02.903" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2220" PostTypeId="1" AcceptedAnswerId="6338" CreationDate="2013-09-29T03:35:46.037" Score="14" ViewCount="352" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Biosphere_2&quot;&gt;Biosphere 2&lt;/a&gt; operated as a closed entity for study of closed ecological systems for stints between 1991 to 1994.  Given that the ISS has been continuously inhabited from 2000 to the present day, it seems like any lessons learned from Biosphere 2 would have been timely and relevant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was there even a single discovery or clearly identifiable scientific product that resulted from Biosphere 2 and was later used for the ISS or other manned spaceflight?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="49" LastEditDate="2014-03-08T09:29:30.447" LastActivityDate="2014-12-12T02:49:47.440" Title="Did Biosphere 2 make any discoveries later used on manned space flight?" Tags="|crewed-spaceflight|research|habitat|science|sustainability|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2221" PostTypeId="2" ParentId="2218" CreationDate="2013-09-29T04:19:10.917" Score="14" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Jmaky.jpg&quot; alt=&quot;source:space.com&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are certain plants that are more efficient in removing and adding Oxygen to the surrounding environment, but the main disadvantage is that they need soil with comfortable acidity otherwise the roots of the plants would die. That prevents them from being used on other planets where the acidic contents in the soil is more (but there are a few exceptions). Also, plants add water to the surrounding environment through transpiration which helps to cool atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the other option would be microbes with chlorophyll (green pigments which helps photosynthesis) what we call as &lt;strong&gt;algae&lt;/strong&gt; and suspend them in suitable way in certain liquid (mostly water) and exposed to light so that they may carry out photosynthesis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://zeenews.india.com/news/space/china-looks-at-algae-for-oxygen-in-long-space-missions_772894.html&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Scientists at the Hydrology&#xA;  institute of China have developed&#xA;  a system that sees 1.5 cubic&#xA;  metres of algae produce enough&#xA;  oxygen for a man, weighing 70&#xA;  kilogrammes, to survive in an&#xA;  enclosed space for one day. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;External source &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://wiki.answers.com/Q/What_plant_produces_the_most_oxygen#page1&quot; rel=&quot;noreferrer&quot;&gt;http://wiki.answers.com/Q/What_plant_produces_the_most_oxygen#page1&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.mnn.com/health/healthy-spaces/stories/best-air-filtering-houseplants-according-to-nasa&quot; rel=&quot;noreferrer&quot;&gt;http://www.mnn.com/health/healthy-spaces/stories/best-air-filtering-houseplants-according-to-nasa&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://news.softpedia.com/news/Top-15-NASA-039-s-Plants-That-Can-Save-Your-Life-78345.shtml&quot; rel=&quot;noreferrer&quot;&gt;http://news.softpedia.com/news/Top-15-NASA-039-s-Plants-That-Can-Save-Your-Life-78345.shtml&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-12-12T16:59:12.677" LastActivityDate="2013-12-12T16:59:12.677" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2222" PostTypeId="2" ParentId="2218" CreationDate="2013-09-29T04:25:56.950" Score="4" Body="&lt;p&gt;Well it is not as easy as it seems, it needs a suitable environment to grow in. First of all it itself need some oxygen to grow. And you cannot just give oxygen to the plant directly, the oxygen would escape into the atmosphere.You need CO2. You need the right amount of sunlight. You need nutrient specific soil. You need bacteria to fix nitrogen into the soil. You need a thick atmosphere like earth has to prevent harmful UV radiation from the sun. This is just the basic conditions you need to grow a plant in other planets. rain forest plants produce the most oxygen. Algae also is a good oxygen producing plant but need more specific conditions to grow in. A single grown rain forest tree can produce oxygen required needed by 4 to 5 people.  100 liters of algae can produce oxygen for 2 to 3 people&lt;/p&gt;&#xA;" OwnerDisplayName="user738" LastActivityDate="2013-09-29T04:25:56.950" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2223" PostTypeId="1" AcceptedAnswerId="2224" CreationDate="2013-09-29T21:23:56.050" Score="11" ViewCount="1784" Body="&lt;p&gt;They are reporting that the three engine relight in the upper atmosphere worked. But the relight of a single engine, lower down for pretend landing on the water generated too much torque and the fuel (in the mostly empty tanks) centrifuged away from the feed line and the engine ran dry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did it torque out of control? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Momentum from the turbopump?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the RCS run out of fuel to counteract/control it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The three engine re-light, might make sense that was probably center and two side engines, so could gimbal two of them to counter any rotation.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="25" LastEditDate="2013-09-30T12:40:25.097" LastActivityDate="2013-12-02T21:14:44.717" Title="What went wrong, Falcon 9 1.1 first flight with first stage landing attempt?" Tags="|spacex|reuse|falcon-9|rockets|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2224" PostTypeId="2" ParentId="2223" CreationDate="2013-09-29T22:18:22.317" Score="9" Body="&lt;p&gt;For the time being, there likely won't be more information than what &lt;a href=&quot;https://twitter.com/elonmusk/status/384407846349062144&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elon Musk tweeted&lt;/a&gt; two hours ago:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;Rocket booster relit twice (supersonic retro &amp;amp; landing), but spun up&#xA;  due to aero torque, so fuel centrifuged &amp;amp; we flamed out.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This was an attempt at testing the rocket's ability to reignite its engine to reduce its velocity prior to hitting the Pacific Ocean, part of the reusable Falcon 9 rocket system to reduce the cost of future launches, but wasn't the launch's primary objective. The rest of the Falcon 9 launch went nominal though, and it successfully deployed its cargo into LEO (Low-Earth Orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Musk later &lt;a href=&quot;https://twitter.com/elonmusk/status/384410742675341312&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweeted&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;Between this flight &amp;amp; Grasshopper tests, I think we now have all the&#xA;  pieces of the puzzle to bring the rocket back home.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From the comments, thanks @Chris, &lt;a href=&quot;https://twitter.com/jeff_foust/status/384396898385526784&quot; rel=&quot;nofollow noreferrer&quot;&gt;another tweet&lt;/a&gt; from &lt;a href=&quot;https://twitter.com/jeff_foust&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jeff Foust&lt;/a&gt;, space industry analyst and writer, provides a bit more perspective on the matter:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;Musk: Lower stage 3-engine relight went well, reentered. Single-engine&#xA;  relight went well, but exceeded roll control of ACS.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, according to all of these tweets, after the first stage successfully launched and separated from Falcon 9 second stage, &lt;strong&gt;Attitude Control System (ACS) wasn't capable to correct for the roll of the first stage due to atmospheric torque centrifuging the fuel&lt;/strong&gt; and choking the ACS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; And at the 3:30EST press conference, they mentioned that the landing legs are designed to be aerodynamic for some extra control at landing time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since I don't have anything more, here's a nice photo of the Falcon 9 lifting off from SpaceX's pad at Vandenberg Air Force Base in California on September 29, 2013, carrying Canada's Cascade SmallSat and Ionospheric Polar Explorer satellites (or &lt;a href=&quot;https://en.wikipedia.org/wiki/CASSIOPE&quot; rel=&quot;nofollow noreferrer&quot;&gt;CASSIOPE&lt;/a&gt;) to orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/pFoCy.jpg&quot; alt=&quot;Falcon 9 lifting off from SpaceX’s pad at Vandenberg, carrying Cascade SmallSat and Canada&amp;#39;s CASSIOPE satellite to orbit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://youtu.be/RtDbDMRG3q8?t=2m40s&quot; rel=&quot;nofollow noreferrer&quot;&gt;Video of the launch is now available on YouTube&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-02T21:14:44.717" LastActivityDate="2013-12-02T21:14:44.717" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2226" PostTypeId="1" CreationDate="2013-09-30T12:39:15.060" Score="11" ViewCount="1753" Body="&lt;p&gt;Europa, moon of Jupiter, has a number of large cracks, as seen below. The cracks appear to be associated with a brownish color, while the base surface appears to be a whitish color. What is the cause of these cracks, and is there any interesting science that could be performed by sampling around these cracks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/i0J8c.jpg&quot; alt=&quot;Europa&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-30T17:01:24.677" Title="What causes the cracks on Europa to form?" Tags="|planetary-science|jupiter|europa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2227" PostTypeId="2" ParentId="2185" CreationDate="2013-09-30T14:55:11.853" Score="6" Body="&lt;p&gt;This is actually quite a serious issue, and one that has been solved in a variety of ways. The method I'm most familiar with is Reaction Wheels, and I'll try and cover some of the other key points as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are basically two kinds of reaction wheels. The first kind is those that are sealed up, done to make it easier to manage the lubrication. The second is those that use lubrication made specifically for a vacuum in a zero g environment. The second is considered the more reliable way, although there are a fair number of missions which choose the first, as it is made much easier in the end. &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/2.3422?journalCode=jsr&quot;&gt;Hubble&lt;/a&gt; chose the second, and while I can't read the paper about it, I can tell you that the reaction wheels used a grease qualified for space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also a fair number of &lt;a href=&quot;http://www.esmats.eu/esmatspapers/pastpapers/pdfs/2011/haslehurst.pdf&quot;&gt;dry lubrications&lt;/a&gt; out there. The linked paper details how it was done, but the end result was that it worked, although the maximum speed was reduced over other methods.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as why many mechanical joints have cloth, no doubt it is to help keep the thermal situation in better control. Direct light could heat the joint up, causing it to not work optimally, but the cloth works an an insulator. A great insulator isn't required, and it is required to move flexibly, cloth would work ideally for this situation.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-09-30T14:55:11.853" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2228" PostTypeId="2" ParentId="2226" CreationDate="2013-09-30T17:01:24.677" Score="9" Body="&lt;p&gt;&lt;sup&gt;Disclaimer: I'm going to attempt answering this question mostly &lt;em&gt;out of my head&lt;/em&gt; due to lack of time, but will later equip it with references and correct my assumptions, if anything goes wrong...&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Ever since Voyager 1's fly-by back in 1979 and providing us with detailed enough photographs of the Europa to show these cracks on its surface (earlier Pioneer 10 photographs weren't really showing them), there has been much speculation over what is or has been causing them in this Jupiter satellite's history. Prevailing theories were focusing on the possibility of Europa to have (or have had) oceans beneath its topmost layer, kept liquid due to tidal forces exerted upon Europa due to enormous gravitational influence of its orbiting partner, planet Jupiter. It was unclear though, whether these were fairly recent formations, or indeed just remnants of once more geologically active moon's core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Later, Galileo probe's many fly-byes during its Jupiter mission have provided even more detailed images of Europa's surface, its cracks in the topmost layer, and also their development over the course of roughly 8 (?) years. Some other spacecraft have also provided us with even more images, like the NASA's New Horizons and the Cassini Solstice missions. They have shown new development in these surface cracks, with the bigger, darker cracks remaining pretty much constant, but new, smaller and less dark cracks appearing that previously weren't there. This was a clear proof of Europa still being geologically active body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further proof of its activity was found in the evidence for this moon's atmosphere, with Galileo detecting an ionosphere forming above it due to Jupiter's enormous magnetosphere and also Solar radiation ionizing upper atmosphere. The presence of this tenuous and easily blown away by radiation pressure atmospheric layer showed evidence this moon is still active and continues to replenish its atmosphere with gases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact nature behind formation of these younger cracks is still somewhat of a mystery, but most of the theories suggest a combination of interactions between Europa and Jupiter, such as tidal forces, effects of changes in strength of Jupiter's magnetosphere as the satellite orbits it, Jupiter as a source of radiation causing greenhouse effect on Europa's liquid subsurface with the surface isolating the heat within its shroud, and also difference in radial velocity of the liquid oceans compared to its surface and possible geological activity beneath them to keep this oceanic mass warm enough to remain liquid, forming surface cracks similar to those found in the polar regions on the Earth, and perhaps even warm enough at its bottom to sustain some forms of life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in short, gravitational and magnetic interactions between Jupiter and its moon Europa keep this natural satellite geologically active, and provide warmth needed to sustain liquid oceans beneath its icy surface layer. There might even be some subsurface volcanic activity going on there, but we don't actually have any proof for that, short of speculating over the origins of its atmosphere. Yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some future missions planned, that would target Europa as one of its objects of interest, namely the ESA's (European Space Agency) JUICE mission, and of course many proposed missions that aren't yet agreed on. The most promising of those seems the Europa Jupiter System Mission or &quot;Laplace&quot; named after the French astronomer Pierre-Simon Laplace, with many other space agencies showing interest to join efforts for this presumably joint ESA/NASA mission.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-09-30T17:01:24.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2229" PostTypeId="2" ParentId="2189" CreationDate="2013-09-30T19:08:02.940" Score="8" Body="&lt;p&gt;Venus has been closely studied for a long time.  In fact in 1961 it was the second object (the moon was the first) to be radar mapped from Earth.  Also in 1961 the Soviets sent a probe that did a flyby but due to a malfunction it wasn't until 1962 when the American Mariner 2 spacecraft, which flew past Venus. A modified Ranger Moon probe, it established that Venus has practically no intrinsic magnetic field and measured the planet's temperature range as 490 to 590 K.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The initial environmental measurements of the surface temperature of Venus is what sparked the idea of &quot;Greenhouse Gases&quot; because the surface was much hotter than expected based on solar input.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first successful landing on Venus was by Venera 7 on December 15, 1970. It remained in contact with Earth for 23 minutes, relaying surface temperatures of 455 °C to 475 °C (855 °F to 885 °F). Venera 8 landed on July 22, 1972. In addition to pressure and temperature profiles, a photometer showed that the clouds of Venus formed a layer, ending over 22 miles above the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since then we have continued to measure and observe with a total of about 26 successful probes, however we don't do any sort of weather predicting similar to the extreme detail done on Earth.  The study of climate on other planets is done mostly by observing what is happening and then seeing if our models allow this to happen.  If not, then we have to figure out what was wrong with out understanding of that planet's mechanisms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently the &lt;a href=&quot;http://en.wikipedia.org/wiki/Venus_Express&quot; rel=&quot;noreferrer&quot;&gt;Venus Express&lt;/a&gt; is a long term probe that has been sending back data since 2005.  It has led to deeper understanding of the &lt;a href=&quot;http://www.lpi.usra.edu/vexag/nov_2007/presentations/schubert.pdf&quot; rel=&quot;noreferrer&quot;&gt;atmospheric dynamics&lt;/a&gt; that drives climates like Venus and Earth.  In addition Earth based observations like &lt;a href=&quot;http://ssed.gsfc.nasa.gov/hipwac/howitworks.html&quot; rel=&quot;noreferrer&quot;&gt;HIPWAC&lt;/a&gt; help to monitor the climates of other planets to better understand how they work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I get more time, I'll update this with some modeling information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:  Some of the more challenging aspects of Venus is the extreme thermal conditions.  This is the same engine that drives all the significant weather on Earth as well.  Winds on Venus travel at about 224 miles per hour and the velocity changing with altitude. Cloud wind speeds decrease at higher elevations, and the highest clouds are estimated to be traveling at a few miles per hour. The cause of cloud movement is still being studied at NASA. These clouds produce lightning bursts that are the result of sulfuric acid in Venus's atmosphere. The acid produces a constant electrical charge that is discharged to the planet's surface as lightning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Venus Express took a mosaic of images that has been helpful in simulating the extreme vortex type weather that has been observed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MUGM0.jpg&quot; alt=&quot;mosaic&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simulations of &lt;a href=&quot;http://venus.wisc.edu/Vortex_Model.html&quot; rel=&quot;noreferrer&quot;&gt;vortex models&lt;/a&gt; have achieved some likeness to observations made of Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iNOQ9.gif&quot; alt=&quot;vortex&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What institution/s have, thus far, developed/attempted to develop a meteorological model of Venus?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Some Venus global atmosphere models have been built by the CCSR/NIES (Center for Climate System Research, University of Tokyo; National Institute for Environmental Studies, Japan) GCM and the United Kingdom Meteorological Office (UKMO) Unified Model (UM).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can even get an open souce &lt;a href=&quot;http://planetwrf.com&quot; rel=&quot;noreferrer&quot;&gt;planet weather simulator&lt;/a&gt; that uses parts of these models for scientific research.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At what stage of development are such models of Venusian climate?&#xA;  Are these models capable of generating a tentative map of Venerean climate several weeks into the future?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Due to the lack of ground and atmospheric sensors Venusian weathermen are quite inaccurate.  The stage of their models are just in the &quot;can we get a simulation to match what we see on a planetary scale&quot;.  There is nothing done on a micro scale such as we are used to here on Earth.  No surface wind forecasts, storms, daily weekly forecasts.  We are just trying to emulate some of the global aspects that we observe.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth we have many sensors all over the planet and in orbit to calibrate and check our models against, without that real-time feedback there is no way to refine such models. So trying to model micro weather on Venus without being able to verify it is mostly a waste of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further Reading:&#xA;&lt;a href=&quot;http://books.google.com/books?id=3LEvFw_GIvoC&amp;amp;pg=PA115&amp;amp;lpg=PA115&amp;amp;dq=CCSR/NIES%20venus&amp;amp;source=bl&amp;amp;ots=9QryInRIwV&amp;amp;sig=SC7KcDkOpBleQ4gq1_-P_4Gg4rI&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=PdxLUtjzIoWtigKUvIHIBA&amp;amp;ved=0CDsQ6AEwAg#v=onepage&amp;amp;q=CCSR/NIES%20venus&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Towards Understanding the Climate of Venus: Applications of Terrestrial Models to Our Sister Planets.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="446" LastEditorUserId="446" LastEditDate="2013-10-02T18:24:04.983" LastActivityDate="2013-10-02T18:24:04.983" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2230" PostTypeId="1" AcceptedAnswerId="2231" CreationDate="2013-10-01T04:09:49.070" Score="31" ViewCount="6040" Body="&lt;p&gt;&lt;strong&gt;Curiosity is missing an arm, can you help us find it?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The self-portrait photograph of the Curiosity rover from the Gale Crater is probably one of the most well known ones of this particular rover of the NASA's Mars Science Laboratory mission:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/d/dc/PIA16239_High-Resolution_Self-Portrait_by_Curiosity_Rover_Arm_Camera.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2rQQf.jpg&quot; alt=&quot;Curiosity rover self-portrait&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Self-portrait of Curiosity in Gale Crater on the surface of Mars, taken with the rover's arm&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;camera on October 31, 2012. Click on image for higher resolution photo (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:PIA16239_High-Resolution_Self-Portrait_by_Curiosity_Rover_Arm_Camera.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But here's the thing. If this photograph was taken from the rover's robotic arm, then how come it is not visible? No arm from the camera's vantage point, no shadow beneath it yet there are shadows of its other limbs and main body, and absolutely no trace of the method of taking the shot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So here's my question: How come there is no trace of the Curiosity's robotic arm, from which this photograph is supposedly taken from, on the photograph itself?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this photograph really a composite image of many different angle ones that were stitched together? Was the robotic arm skillfully brushed out after it was transmitted back to Earth? Or did someone mislabel some previously taken photograph while the rover was still tested on Earth, it was uploaded to rover's memory and then sent back to Earth from the surface of the Gale Crater on Mars? Some other explanation, perhaps?&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2016-06-27T21:28:51.567" Title="Who took the Curiosity rover's self-portrait photo from the surface of Mars?" Tags="|mars|rovers|curiosity|myth-busting|photography|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2231" PostTypeId="2" ParentId="2230" CreationDate="2013-10-01T04:51:49.623" Score="41" Body="&lt;p&gt;Yes, it is a mosaic made of 55 images from the &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Hand_Lens_Imager&quot;&gt;MAHLI camera&lt;/a&gt; on the arm.  The arm was deliberately removed from the images where bits of it showed up.  See &lt;a href=&quot;http://news.nationalgeographic.com/news/2012/121204-curiosity-mars-rover-portrait-science-space/&quot;&gt;this description, including a video&lt;/a&gt; of how it was done.  The arm motions were very carefully planned to cover the rover and surface around it by changing the angle of the camera, but not the absolute position of the center of the lens.  Though on the next sol they did it all again with that lens position moved to the side by the distance between human eyes, to allow them to make a 3-D version.  In putting together the mosaic:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There were challenges galore, but none quite so great as getting the&#xA;  long arm of the rover out of the picture.  A close look at the final&#xA;  image shows the &quot;shoulder&quot; of the robotic arm on the front of the&#xA;  rover. But the rest of the five-jointed appendage has been removed and&#xA;  replaced by pixels from other frames that showed the ground or the&#xA;  rover behind the arm.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;Actually, there weren't that many images with the arm in them because&#xA;  of how we positioned the arm,&quot; Ravine explained. &quot;It's like if you&#xA;  hold a camera out in front of you with your elbow crooked and&#xA;  shoot—what you'll probably get is your face and top of your body&#xA;  including your shoulder, but most of your arm is out of the frame.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also you can go through &lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?showtopic=7522&quot;&gt;this thread on unmannedspaceflight.com&lt;/a&gt; to see amateurs putting the images together as they arrive.  I call them &quot;amateurs&quot;, only because they are not paid to do that.  They are just as good as the professionals.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-10-01T16:14:27.560" LastActivityDate="2013-10-01T16:14:27.560" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2233" PostTypeId="1" AcceptedAnswerId="2235" CreationDate="2013-10-01T11:13:48.127" Score="25" ViewCount="3368" Body="&lt;p&gt;The Moon has a very thin (practically non-existent) atmosphere so the foot print left by Neil Armstrong won't be eroded for centuries. But there are solar winds. Is it possible that solar wind would have enough strength to erode the legendary foot print? If so how many years will it take to completely erode?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The answer can be approximate.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/i5uvT.jpg&quot; alt=&quot;source: NASA &quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="58" LastEditDate="2013-10-01T13:10:35.627" LastActivityDate="2018-02-24T23:09:46.130" Title="Will the foot print of Neil Armstrong erode?" Tags="|the-moon|astronauts|history|apollo-program|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2234" PostTypeId="2" ParentId="2233" CreationDate="2013-10-01T12:34:06.920" Score="10" Body="&lt;p&gt;Wind, whether solar or not, is not going to be the main erosion issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will over time strip particles away, but over those sorts of timeframes there will be numerous impacts, whether dust or larger bodies, which may scatter dust over the footprints, or in the case of impact on or near the footprints, may scatter or destroy the footprints themselves.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2013-10-01T12:34:06.920" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2235" PostTypeId="2" ParentId="2233" CreationDate="2013-10-01T12:45:23.737" Score="19" Body="&lt;p&gt;Space.com has &lt;a href=&quot;http://www.space.com/12846-apollo-moon-landing-sites-flags-footprints.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;an interesting article&lt;/a&gt; that discusses this very question. Basically, it postulates that everything will eventually go away, due to micrometeorite bombardment, etc. It has the following to say about the rate of deterioration:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;From past studies of moon rocks collected by astronauts during the&#xA;  Apollo missions, researchers have learned that the rocks erode at a&#xA;  rate of about 0.04 inches [1.0 mm] every 1 million years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Given that rate, how long would it take the footprints to disappear? I'm going to guess that the footprint is about an inch (25 mm) deep at it's tallest, and the detail is about a quarter inch (6 mm) (The tread). Given those numbers, it'll take about 6 million years before you won't easily be able to recognize it as a footprint, and about 25 million years before it's completely gone. This assumes that the area doesn't get struck by a particularly large meteor, which would accelerate the rate, and that nothing is done to protect the area in the mean time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's worth noting that the provided erosion rate is for rock. The dust might have a higher rate, as it does on Earth, but it likely won't be that much higher. This will be slightly complicated as well that there is a small dust storm that happens at the terminator of the moon. I strongly suspect that the footprints will be gone in a million years, if nothing is done to prevent it from happening.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="33" LastEditDate="2018-02-24T23:09:46.130" LastActivityDate="2018-02-24T23:09:46.130" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2236" PostTypeId="2" ParentId="1480" CreationDate="2013-10-01T12:47:04.820" Score="7" Body="&lt;p&gt;&lt;strong&gt;I assume you mean the density of the lunar surface, which itself is made up of a few different rocks, the densities of which I analyse below:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.psrd.hawaii.edu/CosmoSparks/July12/Lunar-rock-densities.html&quot; rel=&quot;noreferrer&quot;&gt;Cosmo Sparks&lt;/a&gt; have a short article around the topic of the densities of lunar rock. If we compare that to the densities of Earth rocks, we should be able to come up with a rough estimate as to how &lt;em&gt;hard&lt;/em&gt; the rocks are. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some important quotes from the article rate the density of moon rocks to be:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The seven mare basalts range in grain density from 3270±10 to 3460±50 kg/m&lt;sup&gt;3&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The four feldspathic highlands rocks range in grain density from 2840±10 to 2910±10 kg/m&lt;sup&gt;3&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and also&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Grain densities of the two impact breccias are 3030±30 and 3050±10 kg/m&lt;sup&gt;3&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Converting these to g/cm&lt;sup&gt;3&lt;/sup&gt; for the sake of ease of use gives us:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Lunar rock          Grain density&#xA;─────────────────────────────────&#xA;Basalts:            ~ 3.365 g/cm³&#xA;Highland Rocks:     ~ 2.875 g/cm³&#xA;Breccias:           ~ 3.040 g/cm³&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://geology.about.com/cs/rock_types/a/aarockspecgrav.htm&quot; rel=&quot;noreferrer&quot;&gt;A quick check for densities of Earth rocks&lt;/a&gt; in g/cm&lt;sup&gt;3&lt;/sup&gt; gives us:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Rock             Density (g/cm³)              │             Rock             Density (g/cm³)&#xA;­­­­­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─┼­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­─­­&#xA;Andesite              2.5 - 2.8               │             Limestone             2.3 - 2.7&#xA;Basalt                2.8 - 3.0               │             Marble                2.4 - 2.7&#xA;Coal                  1.1 - 1.4               │             Mica schist           2.5 - 2.9&#xA;Diabase               2.6 - 3.0               │             Peridotite            3.1 - 3.4&#xA;Diorite               2.8 - 3.0               │             Quartzite             2.6 - 2.8&#xA;Dolomite              2.8 - 2.9               │             Rhyolite              2.4 - 2.6&#xA;Gabbro                2.7 - 3.3               │             Rock salt             2.5 - 2.6&#xA;Gneiss                2.6 - 2.9               │             Sandstone             2.2 - 2.8&#xA;Granite               2.6 - 2.7               │             Shale                 2.4 - 2.8&#xA;Gypsum                2.3 - 2.8               │             Slate                 2.7 - 2.8&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So comparatively, on the Moon:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Basalts are of similar density as Peridotite&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Highland Rocks are similar to many Earth Rocks, of strong similarity to Dolomite&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Breccias are similar to a dense Diabase, Diorite or Basalt on Earth&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If you instead meant the bearing strength of the lunar surface there is a paper on this, which is unfortunately paywalled: &lt;a href=&quot;http://link.springer.com/article/10.1007%2FBF00561845#page-1&quot; rel=&quot;noreferrer&quot;&gt;http://link.springer.com/article/10.1007%2FBF00561845#page-1&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="49" LastEditDate="2013-10-01T14:36:17.260" LastActivityDate="2013-10-01T14:36:17.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2237" PostTypeId="1" AcceptedAnswerId="2257" CreationDate="2013-10-01T13:29:00.670" Score="14" ViewCount="1593" Body="&lt;p&gt;The atmosphere of Earth bends light such that there is still light after the sun goes down for a period of time. How would such effects appear on Venus, given the thicker atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-01T13:44:37.740" LastActivityDate="2013-10-03T16:43:51.653" Title="How long will it be light on Venus at night?" Tags="|venus|planetary-science|atmosphere|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2238" PostTypeId="1" AcceptedAnswerId="2260" CreationDate="2013-10-01T15:10:21.900" Score="16" ViewCount="959" Body="&lt;p&gt;At midnight on October 1, 2013 the US government entered the state of government shutdown where all functions deemed non-essential are suspended. This shutdown lasted 16 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did this shutdown affect NASA operations?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-01-07T17:50:13.047" LastActivityDate="2019-08-04T22:59:31.767" Title="How did the US government shutdown of 2013 affect NASA operations?" Tags="|nasa|cost|operations|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2239" PostTypeId="1" AcceptedAnswerId="2312" CreationDate="2013-10-01T17:42:31.133" Score="17" ViewCount="11244" Body="&lt;p&gt;I'm trying to better understand the mathematics behind how torque rods work. I know that even if you have 3 axis of magnetic torquers, in effect there is only 2 axis of control, and I'm trying to figure out how all of this works exactly. Specifically, let's assume the following hypothetical situation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a location of a satellite in LEO, and the magnetic field vector for said satellite. I have magnetorquers on the satellite of a known strength, and I'm trying to determine how much control authority I have in various directions. How much control authority do I have, given a magnetic field in body coordinates, and 3 equal strength magnetorquers in orthogonal vectors lined up with the body axis?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-08-23T14:35:27.280" Title="What is the math behind Magnetorquers?" Tags="|artificial-satellite|attitude|magnetic-field|magnetorquer|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2240" PostTypeId="1" AcceptedAnswerId="2243" CreationDate="2013-10-01T18:48:12.120" Score="17" ViewCount="1444" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot;&gt;Rosetta&lt;/a&gt; is en route to rendezvous with &lt;a href=&quot;http://en.wikipedia.org/wiki/67P/Churyumov-Gerasimenko&quot;&gt;Chury&lt;/a&gt;. Briefly, the mission comprises an orbiter, and a lander. The latter named &lt;a href=&quot;http://en.wikipedia.org/wiki/Philae_%28spacecraft%29&quot;&gt;Philae&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Philae_lander&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... The lander is designed to touch down on the comet’s surface after detaching itself from the main spacecraft body and “falling” towards the comet along a ballistic trajectory. ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Why is Philae not provided with a propulsion system? &lt;/li&gt;&#xA;&lt;li&gt;Could a random event (say, a tumble even outgassing), potentially, deflect Philae's planned trajectory?&lt;/li&gt;&#xA;&lt;li&gt;Is it's mass of 100kg on Earth, and Chury's infinitesimal gravity adequate to ensure touch-down on the comet along the planned trajectory? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-08-08T22:17:12.097" LastActivityDate="2023-12-14T18:02:59.153" Title="Why is Philae not provided with a propulsion system?" Tags="|propulsion|rosetta|lander|philae|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2241" PostTypeId="2" ParentId="2240" CreationDate="2013-10-02T01:06:09.430" Score="8" Body="&lt;p&gt;It doesn't really need it and it would needlessly add to its mass. Rosetta will assume a relatively slow and probably highly elliptical orbit around 67P/Churyumov-Gera...aaagh! Chury!, with the perigee at only roughly a kilometer away from it. The orbit has not yet been determined though, see my &lt;a href=&quot;https://space.stackexchange.com/q/768/49&quot;&gt;related question&lt;/a&gt; and the answer there. It likely won't be until Rosetta transmits its own close proximity observations of the comet, and might even change in time as the comet's coma, tails and in general its surface activity increases during its flyby closer to the Sun. Rosetta of course also has its own semi-autonomous collision avoidance system onboard, that will enable it to react to any debris in its path and adjust its trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, such elliptical orbits leave mission control plenty of leeway to later (when the Rosetta will be in orbit) decide at what point to release Philae on its ballistic trajectory. My guess would be this trajectory will be attempted from the point of Rosetta's flying past the comet in the opposite direction of its movement, to reduce the chance of incoming debris to near zero. The lander also has a harpoon (see the spike in the middle of the legs frame) that will fire towards the comet immediately after the touchdown to hook itself on it. Additionally, each of the three legs have battery powered ice screws in between the two padded gears, to additionally grip to the comet and pads to cushion the collision / landing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/o0cpw.jpg&quot; alt=&quot;Philae lander&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Training sessions for the Philae comet lander (Source: &lt;a href=&quot;http://www.dlr.de/dlr/presse/en/desktopdefault.aspx/tabid-10308/471_read-6556/year-all/#gallery/9151&quot; rel=&quot;nofollow noreferrer&quot;&gt;DLR, National Aeronautics and Space Research Centre of Germany&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer your questions more directly tho, adding propulsion system to it would be needless weight. The Philae lander wouldn't be able to react fast enough with its Attitude Control System (ACS), if it had any onboard, and to avoid any possible obstacles. Instead, the lander will be put on a collision course (or ballistic trajectory, your pick) with the comet with an approximate velocity of 1 m/s. That is, I would say, higher relative velocity than lowest achievable with its release system, and the resulting kinetic potential should suffice to negate effects of any collisions with smaller debris or pressure in the opposite vector to its movement from outgassing, as you mention. The lander's mass is negligible in gravitational sense, but it still adds to its inertial mass. With a bit of smart decision making from the mission control, they ought to have plenty of opportunities to assure its touchdown. The harpoon, soft pads and powered drills should do the rest in assuring the lander stays put where it landed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, there are chances all of this will go sour. One of the main concerns I've seen mentioned online is, that the lander itself creates with the harpoon and the screws deep enough cracks for a part of the comet on which it landed to simply chip away from the comet's main body. That doesn't sound too serious as the lander could still do all of its experiments (some might become less conclusive tho, like for example the CONSERT comet nucleus radio wave sounding experiment), just on a smaller piece of the &lt;em&gt;icy rock&lt;/em&gt;, but it could cause the lander to completely unhook from any piece of the comet's surface, or the chipped away piece starts spinning, squashes the lander between the chipped away piece and the comet's main body and causes it to lose its grip to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess we'll have to wait and see, but adding propulsion system to it wouldn't have mitigated main concerns I've seen mentioned over its chance of success either. In fact, it would only add to the problem, with more parts that could malfunction and larger mass of the lander increasing its physical influence on the comet.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-10-02T01:26:07.243" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2242" PostTypeId="1" AcceptedAnswerId="2246" CreationDate="2013-10-02T07:56:19.633" Score="9" ViewCount="349" Body="&lt;p&gt;What kind of transfer orbit will MAVEN follow?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is a Hohmann Transfer Orbit which will be effective only when the launch point and destination of spacecraft are in straight line. Earth is its launch point and  if Mars is to be the destination point it will happen once in few days or years so that the Earth and Mars will be in suitable position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If MAVEN misses its launch (as planned by NASA) because of the shut down of the government, when will Earth and Mars align in a suitable position again so that it can be transferred to Mars orbit by a Hohmann transfer orbit ?&#xA;&lt;img src=&quot;https://i.stack.imgur.com/hKqFf.jpg&quot; alt=&quot;source:Wikipedia&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="265" LastEditDate="2013-10-02T14:07:33.070" LastActivityDate="2013-10-02T14:07:33.070" Title="When could MAVEN launch to get to Mars, if it misses its launch date?" Tags="|mars|orbital-mechanics|trajectory|cost|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2243" PostTypeId="2" ParentId="2240" CreationDate="2013-10-02T12:52:22.170" Score="15" Body="&lt;p&gt;Let me just expand a bit on @Tildalwave's answer. Most landers on airless bodies need a propulsion system, because they will be going too fast otherwise to land. But that's only because most landings have been done on objects with a lot of gravitational mass. Let's just try and figure out what the escape velocity would be. Wikipedia gives us the following clues, including some &lt;a href=&quot;http://en.wikipedia.org/wiki/Comet&quot;&gt;comet&lt;/a&gt; info:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Size: 4 km diameter&lt;/li&gt;&#xA;&lt;li&gt;Shape: Irregular, but roughly oval shaped.&lt;/li&gt;&#xA;&lt;li&gt;Average comet density: 0.6 g/cm^3&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, that's not a lot to go on, but what can we gleam from that? Well, not a lot, but let's just try and figure out a mass, and then escape velocity from that. Here's the numbers leading up to that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Volume: (4 km diameter sphere) $3.4 \times 10^{16}$ cm$^3$&lt;/li&gt;&#xA;&lt;li&gt;Mass: $2.0 \times 10^{13}$ kg&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.calctool.org/CALC/phys/astronomy/escape_velocity&quot;&gt;Escape Velocity&lt;/a&gt; (2 km from center of mass): 1.2 m/s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, that's a pretty low escape velocity, walking would more than do it, and you could easily jump off of the comet! So, what would you need to do if you were going to actively try to land on it?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Some sort of a radar system, to know how close you were to the ground&lt;/li&gt;&#xA;&lt;li&gt;Propulsion system&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Those take power and mass, adding much complexity. The alternative is to have Rosetta put you in to a ballistic trajectory, landing at exactly the escape velocity. It's not hard to absorb an impact of 1 m/s. In fact, the &lt;a href=&quot;http://phoenix.lpl.arizona.edu/phases04.php&quot;&gt;Mars Phoenix Lander&lt;/a&gt; landed at 2.4 m/s on Mars, making this even more realistic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thruster system makes the whole thing more complicated. The only real advantage would be some sort of an abort capability, and a slightly improved landing from Rosetta, but that's fairly insignificant compared to the added complexity of the mission. The delta v required to change from orbiting to ballistic is negligible for Rosetta, and it already requires such capabilities anyways. Why go through the bother of adding on another several complex systems?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="3065" LastEditDate="2014-10-06T03:30:35.137" LastActivityDate="2014-10-06T03:30:35.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2244" PostTypeId="1" CreationDate="2013-10-02T13:08:19.993" Score="6" ViewCount="678" Body="&lt;p&gt;Iapetus, one of the moons of Saturn, is known for having two distinct color regions, one bright and the other dark. This was noted as early as the 17th century. This region can be seen below, and even better in the map. What caused this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KEsWs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/H9TdG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-03T05:45:18.147" Title="What is responsible for the different colors of Iapetus?" Tags="|planetary-science|saturn|moons|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2245" PostTypeId="1" AcceptedAnswerId="2326" CreationDate="2013-10-02T13:25:52.817" Score="32" ViewCount="17533" Body="&lt;p&gt;Currently the ISS has two basic segments, the Russian and US sides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian side has 4 docking ports.  Usually occupied by 2 Soyuz, 1 or 2 Progress freighters or an ESA ATV vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZBnjd.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see in the picture the tail end of Zvezda has the ATV, the two ports on the front end of Zarya, and then the earth facing (Nadir) side of Zarya.  (I see in the manifest that the Pirs docking compartment will be discarded to make room for another MRM module in 2014-2015.  Which is another question I will ask, why not move Pirs to the top of the front end of Zarya?) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://historicspacecraft.com/ISS.html&quot; rel=&quot;noreferrer&quot;&gt;US side&lt;/a&gt; has three PMAs (Pressurized Mating Adapters).  One connects the US and Russian segment together and is permanently occupied.  One is on the far end of the Harmony node where the shuttle used it to dock.  The third has been shuffled a bit as modules are added, and is on Tranquility (Node 3) now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The PMAs adapt a CBM (&lt;a href=&quot;https://en.wikipedia.org/wiki/Common_Berthing_Mechanism&quot; rel=&quot;noreferrer&quot;&gt;Common Berthing Module&lt;/a&gt;) port, to an &lt;a href=&quot;https://en.wikipedia.org/wiki/Androgynous_Peripheral_Attach_System&quot; rel=&quot;noreferrer&quot;&gt;APAS port&lt;/a&gt; that the shuttle can use.  The current crop of cargo vessels, using the US side (&lt;a href=&quot;https://en.wikipedia.org/wiki/H-II_Transfer_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;HTV&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Cygnus&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Dragon&lt;/a&gt;, thus excluding the ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Automated_Transfer_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;ATV&lt;/a&gt;), use a CBM since the 'door' is wider and they are berthed by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mobile_Servicing_System&quot; rel=&quot;noreferrer&quot;&gt;Canadarm2&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The planned CCiCAP/Commercial crew vehicles all plan to use the PMA (after SpaceX delivers an adapter on a Dragon flight in 2014) for docking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as the image of 6 vehicles at the station shows, those ports are getting pretty full.  The ATV and Progress vehicles hang around for months, tying up a port, as they are used to dump waste to burn up on reentry.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far, Cygnus and Dragon has used the nadir (earth facing) CBM on Harmony (Node 2).  From the image, showing the HTV docked to the zenith (space facing) CBM on Harmony, it seems like both CBM ports are usable.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will there come a time where the available ports will not suffice?  It seems likely that a US crew vehicle will remain on station as a lifeboat on a regular basis.  (NASA seems to be thinking of launching with 4 crew instead of 7 that most plan to handle.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One imagines a cargo vehicle will remain for a while as well on the US side.  That quickly ties up two of the available 4 ports.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like ports at the station may get tight as the station gets busy, but maybe there is enough room.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course space is one issue, the other is the available time in the schedule for the crew to deal with visiting vehicles. Rotating a Progress every 3-4 months, an ATV every 18 months, an HTV every 18 months, Cygnus and Dragon a total of 20 visits over 4-5 years, and suddenly that is a lot of vehicle ops, and that seems to occupy the crew a fair bit when it happens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seems like automation is really important to get working, going forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In May of 2019, someone captured the ISS in this picture with 5 vehicles. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/3cb5Z.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3cb5Z.png&quot; alt=&quot;ISS May 2019&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2019-05-16T16:06:54.010" LastActivityDate="2019-05-16T16:06:54.010" Title="Will the ISS need more docking ports?" Tags="|iss|docking|dragon|cygnus|htv|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="2246" PostTypeId="2" ParentId="2242" CreationDate="2013-10-02T13:38:24.553" Score="7" Body="&lt;p&gt;Most missions to other planets, in particular Mars and Venus, use Hohmann transfer orbits, or something akin to it. The perfect Hohmann transfer orbit has an instantaneous point of time where it needs to be launched, in reality, there is sufficient fuel to use another orbit that is very close to the perfect Hohmann orbit for a period of a few weeks. Specifically, &lt;a href=&quot;http://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0CC4QFjAA&amp;amp;url=http://www.colorado.edu/news/features/mavens-three-week-launch-window-starts-nov-18&amp;amp;ei=1SBMUqmVMY_s9ASjn4GABQ&amp;amp;usg=AFQjCNHbYtQPNlQRAXh9AH8mho24ZIl1IA&amp;amp;sig2=V3V3JCqkq0TMi97-DoHbCA&amp;amp;bvm=bv.53371865,d.eWU&quot; rel=&quot;noreferrer&quot;&gt;MAVEN&lt;/a&gt; has a launch window of 3 weeks, starting November 18. If the government is shut down for only a week, then it probably won't make a difference, but there could be a significant difference in the ability to launch if it takes longer than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what would happen if the window is missed entirely? Well, the next opportunity to get to Mars is 26 months later, due to the geometry of the planets. The ideal launch window happens every 26 months or so. Bottom line is, if it doesn't launch this year, we have to wait until mid 2016 to launch, with added expense of keeping the satellite in storage, and taking it from its near launch configuration to the stored mode.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="265" LastEditDate="2013-10-02T13:54:50.127" LastActivityDate="2013-10-02T13:54:50.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2247" PostTypeId="1" AcceptedAnswerId="2249" CreationDate="2013-10-02T14:17:19.200" Score="21" ViewCount="6247" Body="&lt;p&gt;The plan for the ISS upon decomissioning is to deorbit the structure and &lt;a href=&quot;http://www.slashgear.com/international-space-station-to-get-watery-grave-in-2020-27167729/&quot;&gt;let it fall into the ocean&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I get that we do not want extra debris in orbit but I would think that leaving the structure in a stable orbit may provide an opportunity to recycle/reuse the infrastructure in future missions. It is expensive to get things of this size to orbit, but if it already exists then it makes more sense to me to leverage the existing assets where possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what would it cost to keep the ISS in orbit?  I am not looking for total operating costs, or what it would cost to keep it operational, just the amount that it costs to maintain orbit.  What would it cost to put it into an orbit that would be highly unlikely to create a problem for 100+ years? Would that even be possible?&lt;/p&gt;&#xA;" OwnerUserId="123" LastEditorUserId="25" LastEditDate="2013-10-02T17:00:08.393" LastActivityDate="2017-08-02T16:06:50.623" Title="What is the cost of keeping the ISS in orbit" Tags="|iss|cost|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2248" PostTypeId="2" ParentId="2247" CreationDate="2013-10-02T14:48:00.633" Score="12" Body="&lt;p&gt;NASA's 2014 budget for the ISS is about $3B and this covers about half the cost with the other partner nations covering the rest of the costs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mission has been &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station#International_co-operation&quot;&gt;extended to 2020&lt;/a&gt;; it could be further extended to 2028 but the station isn't really designed to last beyond that. The &lt;strong&gt;rapid day-night cycle in low earth orbit causes metal fatigue&lt;/strong&gt; and the solar panels have a limited lifespan as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Russian_Federal_Space_Agency#ISS_involvement&quot;&gt;Russian segments&lt;/a&gt; might be removed from station to build a new Russian station but there is no plan to keep any of the US components. End of life for the station will free the budget to build better stations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Chinese will be flying &lt;a href=&quot;http://en.wikipedia.org/wiki/Chinese_space_station&quot;&gt;their station&lt;/a&gt; by then. Hopefully there will be commercially operated stations as well. The lessons learned on ISS will be put to use.&lt;/p&gt;&#xA;" OwnerUserId="833" LastEditorUserId="288" LastEditDate="2013-10-02T16:08:30.717" LastActivityDate="2013-10-02T16:08:30.717" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2249" PostTypeId="2" ParentId="2247" CreationDate="2013-10-02T16:02:03.663" Score="21" Body="&lt;p&gt;Yes, it is unfortunate that the space station is not quite fully in space.  It is in Earth's upper atmosphere, which is why it requires such frequent reboosts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_Propulsion_Module&quot;&gt;this Wikipedia entry&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ISS requires an average 7,000 kg of propellant each year for&#xA;  altitude maintenance, debris avoidance and attitude control.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Multiple supply vehicles are required to satisfy the ISS's 7,000 kg&#xA;  annual average propellant need. The current plan for six Progress M1&#xA;  spacecraft per year meets that need.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.globalsecurity.org/space/world/russia/launch_services_cost_study.htm&quot;&gt;good working number&lt;/a&gt; for a Progress launch is around \$50M.  So about \$300M a year just for the propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be additional costs to execute the maneuvers.  Progress does the maneuver itself (you don't transfer propellant to the station for the reboost), but I don't know if the docking and reboost maneuver can be done without people on board ISS.  If you need people, then that vastly increases the orbit maintenance cost, since you need to resupply people as well, and their life support resources, and operate the whole station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it would likely be cheaper in the long run to send a whole bunch of Progress vehicles to boost the orbit, since the atmosphere decays exponentially.  But only if your only objective is to keep ISS from augering in.  You would then pay the penalty for less payload per launch to ISS at the higher altitude.  There is a sweet spot where you minimize the total cost of propellant for reboost and up mass to the station.  I suspect that that was a factor in determining the current ISS altitude.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-10-02T16:16:06.340" LastActivityDate="2013-10-02T16:16:06.340" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2250" PostTypeId="1" AcceptedAnswerId="2251" CreationDate="2013-10-02T16:37:33.600" Score="10" ViewCount="1936" Body="&lt;p&gt;According to a &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=32006.30&quot; rel=&quot;noreferrer&quot;&gt;ISS operations/launch manifest&lt;/a&gt; on May 2, 2014 the Progress docked to Pirs, will undock, with Pirs attached to deorbit it.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;May 2 - Progress M-22M undocking with Pirs module (from Zvezda nadir)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Now May 4th, Nakua (MRM-2 a second Russian segment Modular Research Module) is set to dock where Pirs was.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;May 4 - Nauka docking (to Zvezda nadir)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So Zvezda has 4 docking ports (sort of).  One to itself.  One to Zarya. One up/zenith, one facing down/nadir. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zarya, only has 3 (They removed two during its initial modifications)  It has the two obvious ones, one to itself, one to the PMA to Unity, and a nadir (earth facing) port with Rassvvet (MRM-1) attached.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It helps if you look at this picture, of a day, that the ISS was very occupied by vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZBnjd.jpg&quot; alt=&quot;Six visiting vehicles at the ISS&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does Zarya have a zenith docking port?  Seems like it would make more sense to move Pirs (which has an airlock) to the Zarya zenith port, and make room for a fifth Russian segment docking port.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-01-10T14:31:00.227" LastActivityDate="2018-01-10T14:31:00.227" Title="Why is the Pirs docking module being discarded?" Tags="|iss|docking|future-missions|zvezda|zarya|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2251" PostTypeId="2" ParentId="2250" CreationDate="2013-10-02T19:30:56.657" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why is the Pirs docking module being discarded?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.militarynews.ru/excl.asp?ex=193&quot; rel=&quot;noreferrer&quot;&gt;head of RKK Energia says&lt;/a&gt; Pirs lives longer than 2x of its initial planned resource.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wonderful ideas like &lt;em&gt;&quot;Save Pirs!&quot;&lt;/em&gt; don't seem to be based on solid grounds. MLM has incompatible docking adapters. Even if it had compatible docking adaptors it'd be cheaper to bring there another Pirs than to save the current one with any complex schemes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, one of the (seemingly obvious) ideas, is to use the Progress spacecraft needed to deorbit Pirs to just detach it from ISS, and fly near the station indefinitely until MLM or MLM+UM arrives. Then dock it back using the same Progress. The problem with this solution is that for deorbiting a regular Progress can be used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/osTVP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While for docking back, it needs to be like &#xA;the one that brought Pirs to the ISS. Otherwise the thrusters are too far away from the center of mass for fine maneuvering needed for the docking:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/q6OOk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, to keep Pirs afloat that way one needs to spare another customized Progress. And if that's a customized Progress, then there should be some design process to bring it to ISS without Pirs (with some other payload).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another, (seemingly simpler and even more seemingly obvious) idea is to hold it with Canadarm and dock back when possible. It faces obvious problems of not having the manipulator attachment fixture on Pirs, the manipulator ocuppied indefinitely. And less obvious problems, such as powering Pirs off indefinitely and then back on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After considering all different scenarios and costs of saving Pirs, it was decided to deorbit the old tin.&#xA;(This is rumours though, I can't cite any reliable sources, other than publicly stated deorbiting plans)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Does Zarya have a zenith docking port?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No it doesn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was planned initially, but then the plan has changed and a spherical cover has been placed instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://www.gctc.ru/main.php?id=322&quot; rel=&quot;noreferrer&quot;&gt;this scheme on the cosmonaut training center site&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TvuWU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Number twenty two says &quot; Пассивный стыковочный агрегат&quot; that is &quot;Passive docking unit&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While number five says &quot;Стыковочно-такелажный узел ERGF для захвата американским манипулятором.&quot; that is &quot;ERGF docking and rigging adaptor for American manipulator&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here you can see it from a better position:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/MvLxj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is how it works:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EIE02.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2013-10-03T14:21:19.643" LastActivityDate="2013-10-03T14:21:19.643" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2252" PostTypeId="2" ParentId="2244" CreationDate="2013-10-02T23:42:38.987" Score="5" Body="&lt;p&gt;&lt;strong&gt;Well, it's the sugarcoating blowing off the Saturn's doughnut, of course!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not far from the truth, but jokes aside still, the color dichotomy of Iapetus is due to the darker half, the &lt;em&gt;Cassini Regio&lt;/em&gt;, being a result of the moon's accumulation of the dust in the Saturn's largest, yet extremely tenuous, diffuse dust ring called the Phoebe ring depositing onto the moon's leading hemisphere, while the trailing hemisphere's surface remains uninfluenced by this source of dust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Phoebe ring was only observed first in late 2009 by the Spitzer telescope in the infrared range, and is believed to have been caused by bombardment of micrometeorites of Phoebe, another Saturn's moon that the prevailing theories suggest is a captured Kuiper belt originating planetesimal orbiting Saturn in a retrograde, 173° to the ecliptic inclined orbit. It is the largest discovered ring of the Saturn, stretching from calculated 59 to 300 Saturn's radii (observed between 128 and 207 Saturn's radii by the Spitzer telescope), is tilted 27° to Saturn's inner rings and with an orbital inclination of 175°, suggesting its origins and again retrograde orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/t8MJU.jpg&quot; alt=&quot;Spitzer&amp;#39;s infrared Phoebe ring observations&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The Phoebe ring was discovered by NASA's Spitzer Space Telescope, detecting it in the infrared spectrum. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Infrared_Ring_Around_Saturn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Iapetus, being in a prograde orbit with a 17.28° inclination to the ecliptic, is directly in the path of this retrograde dust ring at an average distance from Saturn of 215 Saturn radii. Since it's tidally locked to Saturn, it would always face same side towards any debris and dust in its path. Joseph A. Burns, Cornell's Irving Porter Church Professor of Engineering and professor of astronomy &lt;a href=&quot;http://www.news.cornell.edu/stories/2009/12/cassini-unveils-history-saturns-moon-iapetus&quot; rel=&quot;nofollow noreferrer&quot;&gt;said in a press release for Cornell Chronicle&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;The ring of collisional debris that has come off Phoebe and its&#xA;  companion moons is out there, and now we understand the process&#xA;  whereby the stuff is coming in. When you see the coating pattern on&#xA;  Iapetus, you know you've got the right mechanism for producing it.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So this is the theory. Iapetus is in a prograde orbit intersecting at inclination Phoebe dust ring in a retrograde orbit around Saturn, and since Iapetus is tidally locked and always hitting this dusty path &lt;em&gt;head on&lt;/em&gt;, only one hemisphere (the darker one, called &lt;em&gt;Cassini Regio&lt;/em&gt;) is covered in Phoebe's dust, while the other hemisphere (the lighter one, called &lt;em&gt;Roncevaux Terra&lt;/em&gt;) remains free from it, unveiling this Saturn moon's true surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/0YTGe.jpg&quot; alt=&quot;Saturn&amp;#39;s Phoebe ring&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Artist's conception showing nearly invisible and largest Saturn's Phoebe ring (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Saturn_largest_ring_Spitzer_telescope_20091006.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.news.cornell.edu/stories/2009/12/cassini-unveils-history-saturns-moon-iapetus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Same mentioned Cornell Chronicle press release&lt;/a&gt; explains why the transition from the dark hemisphere to the whiter parts of the moon isn't seamless, but rather a mottled, patchy array of bright and dark spots:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Small, white craters that dot Iapetus' darker half indicate a veneer&#xA;  of dark dust, only meters deep, covering a white, icy surface that&#xA;  matches the rest of the satellite. The imaging data also revealed that&#xA;  all the materials on the leading side are much redder than the&#xA;  shielded and brighter trailing side -- another indication that the&#xA;  leading side's dust came from elsewhere.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The pattern, the scientists say, supports a theory described in a&#xA;  companion paper in Science that the darker parts of the moon tend to&#xA;  heat up when struck by sunlight, encouraging the ice to evaporate&#xA;  underneath.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This causes any dark spots to get even darker, creating the mottled&#xA;  look.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/9xgnz.jpg&quot; alt=&quot;Iapetus as seen by the Cassini probe&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Mosaic of Iapetus taken by Cassini probe during NASA's Cassini Solstice mission, showing the bright trailing &lt;em&gt;Roncevaux&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Terra&lt;/em&gt; hemisphere with part of the dark area &lt;em&gt;Cassini Regio&lt;/em&gt; appearing on the right. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Iapetus_as_seen_by_the_Cassini_probe_-_20071008.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote class=&quot;spoiler&quot;&gt;&#xA;  &lt;p&gt; &lt;strong&gt;It's a Saturn Mooncake, sugarcoated, and with chocolate sprinkles on top! :-D&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-03T05:45:18.147" LastActivityDate="2013-10-03T05:45:18.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2253" PostTypeId="1" AcceptedAnswerId="2254" CreationDate="2013-10-02T23:49:58.623" Score="19" ViewCount="14329" Body="&lt;p&gt;I had read that the early astronauts were all 'short guys'.  This made a lot of sense.  A short man can have as much stamina and endurance as a large man, yet eat less, consume less oxygen &amp;amp; (mainly) is less grams to haul to 7+ Kilometers per second, &amp;amp; 200+ Kilometers up.&lt;/p&gt;&#xA;&lt;p&gt;Even for the small guys, the capsules did not seem to have a great deal of space inside (yeah, OK, they wanted to keep the 'space' outside, but you get my point, I hope).&lt;/p&gt;&#xA;&lt;p&gt;As such, when I watched the Apollo 13 movie, I'd presumed the movie makers had to 'scale up' the module, to allow the (lanky) actors to not look like they'd been stuffed in a sardine can.&lt;/p&gt;&#xA;&lt;p&gt;So I was about to post a question on what scale the set was, and thought I'd better get some hard data on the relevant people.&lt;/p&gt;&#xA;&lt;h2&gt;Movie&lt;/h2&gt;&#xA;&lt;p&gt;Tom Hanks - According to &lt;a href=&quot;http://www.imdb.com/name/nm0000158/bio?ref_=nm_ov_bio_sm&quot; rel=&quot;noreferrer&quot;&gt;IMDB&lt;/a&gt; he is &lt;strong&gt;1.83 meters&lt;/strong&gt; tall.  No surprises there.  It is quite common for actors to be tall.  When you see a tall person on film, they look 'average sized', whereas an 'average sized' person on film tends to look small.&lt;/p&gt;&#xA;&lt;p&gt;So let's move to the astronauts themselves..&lt;/p&gt;&#xA;&lt;h2&gt;Astronauts&lt;/h2&gt;&#xA;&lt;p&gt;Jim Lovell - according to &lt;a href=&quot;https://www.google.com.au/search?q=james+lovell+height&amp;amp;ie=utf-8&amp;amp;oe=utf-8&amp;amp;rls=org.mozilla:en-US:official&amp;amp;client=firefox-a&amp;amp;gws_rd=cr&amp;amp;ei=FalMUpruOIuSkgXD-YCgCw&quot; rel=&quot;noreferrer&quot;&gt;Google&lt;/a&gt; he is &lt;strong&gt;1.80 meters&lt;/strong&gt; in height.&lt;/p&gt;&#xA;&lt;p&gt;The IMDB(?!?) page on &lt;a href=&quot;http://www.imdb.com/name/nm0842641/bio&quot; rel=&quot;noreferrer&quot;&gt;Jack Swigert&lt;/a&gt; suggests he is &lt;strong&gt;1.83 meters&lt;/strong&gt; tall.&lt;/p&gt;&#xA;&lt;p&gt;Fred Haise, the 'little guy' of the 3, is &lt;strong&gt;1.78 meters&lt;/strong&gt; tall according to &lt;a href=&quot;http://www.talltask.com/Fred-Haise-height-7326.html&quot; rel=&quot;noreferrer&quot;&gt;this page&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;I myself am 1.68 meters tall.  I do not regard myself as either tall or short, but of average height.  All 3 of the Apollo 13 astronauts are at least 10cm taller than I am, so they do not seem to be 'little guys'.&lt;/p&gt;&#xA;&lt;p&gt;So that brings me to my question.  Did NASA &lt;em&gt;ever&lt;/em&gt; have either a policy or even a 'notable preference' for short astronauts?  If so, when did that end?&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-12-21T20:33:14.643" Title="Size of Apollo era astronauts" Tags="|astronauts|nasa|apollo-program|project-mercury|apollo-13|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2254" PostTypeId="2" ParentId="2253" CreationDate="2013-10-03T01:20:37.157" Score="21" Body="&lt;p&gt;Alan Shepard is 1.80&lt;/p&gt;&#xA;&#xA;&lt;p&gt;John Glenn is 1.79&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote &lt;a href=&quot;http://en.wikipedia.org/wiki/Mercury_Seven#Selection_process&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mercury Seven wikipedia page:&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Because of the small space inside the Mercury spacecraft, candidates&#xA;  could be no taller than 5 feet 11 inches (180 cm) and weigh no more&#xA;  than 180 pounds (82 kg).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, to your question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;when did that end?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;the answer is: it didn't. The requirements have been just relaxed a little bit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the answers to the question &lt;a href=&quot;https://space.stackexchange.com/questions/50/what-medical-conditions-can-impede-someone-from-going-into-space&quot;&gt;&quot;What medical conditions can impede someone from going into space?&quot;&lt;/a&gt; states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Pilots&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Height between 62 and 75 inches. (1.57 to 1.90)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;  &#xA;  &lt;p&gt;Mission Specialists&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Height between 58.5 and 76 inches. (1.49 to 1.93)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The common trend is to relax the limit.&#xA;And this is not NASA specific at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, as you probably remember, the &lt;em&gt;A&lt;/em&gt; in Soyuz-TMA stands for &lt;em&gt;Anthropometric&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Soyuz-TM allowed 1.64 to 1.82&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Soyuz-TMA allows  1.50 to 1.90&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Now, who was short in the end???&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yuri Gagarin was (ta-da!) 1.57.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first Soviet Cosmonaut Squad &lt;a href=&quot;http://ru.wikipedia.org/wiki/%D0%9F%D0%B5%D1%80%D0%B2%D1%8B%D0%B9_%D0%BE%D1%82%D1%80%D1%8F%D0%B4_%D0%BA%D0%BE%D1%81%D0%BC%D0%BE%D0%BD%D0%B0%D0%B2%D1%82%D0%BE%D0%B2_%D0%A1%D0%A1%D0%A1%D0%A0&quot; rel=&quot;nofollow noreferrer&quot;&gt;had the limit of 1.75 (or by some other sources even 1.70)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-10-03T01:20:37.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2256" PostTypeId="2" ParentId="2124" CreationDate="2013-10-03T06:51:16.880" Score="11" Body="&lt;p&gt;This question has been covered in great depth over on Security Stack Exchange: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://security.stackexchange.com/questions/6424/what-would-one-need-to-do-in-order-to-hijack-a-satellite&quot;&gt;https://security.stackexchange.com/questions/6424/what-would-one-need-to-do-in-order-to-hijack-a-satellite&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A very quick read of the answers there shows that while encryption is used for military satellites, security of many devices sent to space is mostly reliant on esoteric protocols, unlisted frequency bands, accurate location finding etc.- security by obscurity rather than strong security mechanisms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a threat model for deep space probes this could well be appropriate. For satellites, probably not, but read that question anyway.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2017-03-17T13:14:44.980" LastActivityDate="2013-10-03T06:51:16.880" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2257" PostTypeId="2" ParentId="2237" CreationDate="2013-10-03T08:14:01.863" Score="13" Body="&lt;p&gt;Many of the articles are from the late 1960's into the 1970's, around the time of the Mariner and Venera probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the first effects of the thicker atmosphere is that, according to the article &lt;a href=&quot;http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281968%29025%3C0666:OARROV%3E2.0.CO;2&quot; rel=&quot;noreferrer&quot;&gt;&quot;Optical and Radio Refraction on Venus&lt;/a&gt; (Stratton, 1968), standing on the surface, it would be somewhat like looking through a fish-eye lens.  Where (from the article):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;an observer on the surface of Venus seems to stand in the centre of a great depression since he must look up from the local horizontal to see the horizon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/naGjC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Refraction, critical angle and reflection of light at the interface between two media. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Refractive_index&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Refractive index)&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This suggests a very high refractive index.  More importantly, according to the author, is in a region of &lt;strong&gt;critical refractivity&lt;/strong&gt;, which he defined as occurring in a planetary atmosphere when&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the refractive bending of a ray is sufficient to give it a radius of curvature equal to the radius of a surface of constant refractive index in the atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/TiQWr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This notion is further discussed in &lt;a href=&quot;http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1971IAUS...40...36S&amp;amp;db_key=AST&amp;amp;page_ind=0&amp;amp;data_type=GIF&amp;amp;type=SCREEN_VIEW&amp;amp;classic=YES&quot; rel=&quot;noreferrer&quot;&gt;&quot;Some Consequences of Critical Refraction in the Venus Atmosphere&lt;/a&gt; (Snyder, 1971), stating that critical refraction occurs at an elevation of 35km, the consequences of this not only include the 'elevation' of the horizon, the article suggests that objects would be magnified and you would &lt;strong&gt;possibly be able to 'see' around the planet&lt;/strong&gt; (= panoramic view).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This article describes what the sky would be like at all times of the Venusian day (scaled to an Earth day), specifically relevant to this question is at 'midnight', the sun would appear as a sliver of light all around the visually-elevated horizon. Going to and from 'midnight' the sliver of light would start to coalesce at the point of where the sun was rising or setting:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Both the Mariner V mission and the Venera 4 mission demonstrate conclusively the existence of the phenomenon of critical refraction in the Venusian atmosphere. This means that the atmosphere is so dense that there is a level at which a horizontal ray of electromagnetic radiation would have a curvature equal to the distance from that level to the center of the planet and hence it would be &lt;strong&gt;bent into a circle and returned to its original point&lt;/strong&gt;. Horizontal rays above this level would be less refracted and would not meet the surface of the planet if extended in either direction, whereas horizontal rays below the critical level would encounter the planet's surface at both ends.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Not mentioned in the papers is the possibility of light reflecting from the sulphuric acid haze layer from the believed-surface volcanism, which may contribute a hazy-coloured 'glow'.  Another article &quot;Noctiluminescent clouds on Venus&quot; (Lazarev, 1999 - unfortunately behind a paywall), suggests that &lt;em&gt;these clouds could appear in a similar manner as &lt;a href=&quot;http://en.wikipedia.org/wiki/Noctilucent_cloud&quot; rel=&quot;noreferrer&quot;&gt;noctilucent&lt;/a&gt; clouds on Earth.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, in answer to the question (the TL:DR version)&lt;/strong&gt; - the view at night from the surface of Venus would be dim, but not completely dark, with a thin sliver of light on an visually-elevated horizon.  Possibly with noctiluminescent clouds.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="288" LastEditDate="2013-10-03T16:43:51.653" LastActivityDate="2013-10-03T16:43:51.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2258" PostTypeId="2" ParentId="2245" CreationDate="2013-10-03T11:50:33.623" Score="21" Body="&lt;p&gt;Yes, the ISS was a busy place recently. As you probably remember Cygnus had to wait for a week to proceed with its rendezvous to let Soyuz dock first. We can consider that a traffic jam in orbit. :-)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current Russian plans include docking MLM Nauka after deorbiting Pirs.&#xA;And a bit later to add a spherical Uzlovoy Module (i.e. Hub Module or Nodal Module). It will be attached to MLM from nadir, and it will have six ports.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5MalM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan is to deliver it on a specialized Progress vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lHENZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the &lt;a href=&quot;http://www.russianspaceweb.com/iss_node.html&quot; rel=&quot;noreferrer&quot;&gt;description of Uzlovoy Module&lt;/a&gt; on the wonderful &lt;em&gt;Russian Space Web&lt;/em&gt; site&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note the picture of MLM together with UM there. The Nauka's own docking compartment is ther smaller sphere and the UM is the bigger shpere.&#xA;In this picture UM has two large module and a brand new smaller module (similar to Pirs and Poisk) attached.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mBQh6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-10-03T11:50:33.623" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2259" PostTypeId="1" AcceptedAnswerId="25944" CreationDate="2013-10-03T13:49:05.593" Score="12" ViewCount="8924" Body="&lt;p&gt;It seems to be a given that gas giants have rings. However,Saturn has rings that are far larger than any others in the Solar System, and far more visible as well. What is unique to Saturn that it has such a large and complex ring system? Images from &lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_ring&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, in order of planet, Jupiter, Saturn, Uranus, and Neptune.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YFbQK.jpg&quot; alt=&quot;Jupiter&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MPD4l.jpg&quot; alt=&quot;Saturn&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lmsxk.png&quot; alt=&quot;Uranus Ring System&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Bco3E.jpg&quot; alt=&quot;Neptune Ring System&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2020-11-28T05:12:56.677" Title="Why are the Rings of Saturn so much brighter than the other planets?" Tags="|planetary-science|gas-giant|saturn|rings|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2260" PostTypeId="2" ParentId="2238" CreationDate="2013-10-03T14:36:12.087" Score="14" Body="&lt;p&gt;Here is a basic rundown of the effects of the shutdown:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;97% of NASA was furloughed. That is &lt;a href=&quot;http://www.slate.com/articles/news_and_politics/slate_labs/2013/10/federal_government_shutdown_agencies_with_the_most_furloughs.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;the highest percentage&lt;/a&gt; of any agency listed. The only known NASA direct operations are those related to Mission Control, and astronauts on the ISS. Everything else was shut down completely.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL&lt;/a&gt; and &lt;a href=&quot;http://www.jhuapl.edu/&quot; rel=&quot;nofollow noreferrer&quot;&gt;APL&lt;/a&gt; are technically NASA contractors. This is really good news as they are the primary contacts for the deep space probes. Since the money to keep things going has already been appropriated, they can operate normally.  (Source: Shutdown FAQ dated September 27, 2013 &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/ShutdownFAQs-09-27-2013-Update.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;[PDF]&lt;/a&gt;.) There is some evidence that things are still working in this department, for instance, HiRISE recently released pictures of the comet &lt;a href=&quot;http://www.universetoday.com/105223/this-is-comet-ison-seen-from-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISON&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The single hardest hit to NASA right now is that &lt;a href=&quot;http://spaceflightnow.com/atlas/av038/131001shutdown/#.Uk2ABn9jHSg&quot; rel=&quot;nofollow noreferrer&quot;&gt;MAVEN&lt;/a&gt; has stopped being prepared for launch in just a month an a half. Working in the space business, I can almost guarantee that the last month and a half are extremely critical. The launch window is 3 weeks long, so in theory, a 2 week delay won't have significant impact, but if the launch window is missed, the orbiter will have to be put in to storage and &lt;a href=&quot;https://space.stackexchange.com/a/2246/25&quot;&gt;wait another 26 months&lt;/a&gt; until launch. However, it was announced on &lt;a href=&quot;http://spaceflightnow.com/atlas/av038/131003resumption/#.Uk61jn9jHSg&quot; rel=&quot;nofollow noreferrer&quot;&gt;October 3rd&lt;/a&gt; that access to the MAVEN crew would be granted, allowing them to continue work, with only 3 lost days resulting.&lt;/li&gt;&#xA;&lt;li&gt;This site is showing huge amounts of panic as we can't seem to access &lt;a href=&quot;http://www.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nasa.gov&lt;/a&gt;. Luckily there's the internet Wayback Machine, which SE won't let me link to;-)&lt;/li&gt;&#xA;&lt;li&gt;LADEE seems to be fine, the orbital maneuvers will be performed according to schedule. Photos might be delayed, however. (&lt;a href=&quot;http://spectrum.ieee.org/tech-talk/aerospace/space-flight/government-shutdown-threatens-future-nasa-missions/?utm_source=techalert&amp;amp;utm_medium=email&amp;amp;utm_campaign=100313&quot; rel=&quot;nofollow noreferrer&quot;&gt;IEEE&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Hubble will receive enough attention to maintain health, but not perform any science. (&lt;a href=&quot;http://spectrum.ieee.org/tech-talk/aerospace/space-flight/government-shutdown-threatens-future-nasa-missions/?utm_source=techalert&amp;amp;utm_medium=email&amp;amp;utm_campaign=100313&quot; rel=&quot;nofollow noreferrer&quot;&gt;IEEE&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;All NASA PR activity has stopped. No twitter, press releases, etc.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-12-10T14:38:56.860" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2261" PostTypeId="2" ParentId="2259" CreationDate="2013-10-03T15:52:33.283" Score="4" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/k7L86.jpg&quot; alt=&quot;source:Wikipedia&quot; /&gt;&#xA;This image shows that the rear side of Saturn rings is dark.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wx9v7.jpg&quot; alt=&quot;source:astrobob&quot; /&gt;These images show the variation of the rings' brightness: the closer Saturn is to Earth, the brighter its rings appear.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Rings_of_Saturn&quot; rel=&quot;nofollow noreferrer&quot;&gt;rings of Saturn&lt;/a&gt; consist of 99% of water and the remainder are various impurities.&lt;/p&gt;&#xA;&lt;p&gt;The water is in the form of water ice. Since ice is a crystalline structure, it reflects and refracts light, making the rings appear bright.&lt;/p&gt;&#xA;&lt;p&gt;But the brightness also depends upon the intensity of incident light: the closer Saturn is to the Sun the bright its rings.&lt;/p&gt;&#xA;&lt;p&gt;The reason the rings of Jupiter appear dull (even though it is on average closer to Earth than Saturn is) is that they are composed of dust, which is darker than ice.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="37047" LastEditDate="2020-11-28T05:12:56.677" LastActivityDate="2020-11-28T05:12:56.677" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="2262" PostTypeId="1" AcceptedAnswerId="2270" CreationDate="2013-10-03T16:45:01.300" Score="7" ViewCount="121" Body="&lt;p&gt;In October 2010, the &lt;a href=&quot;http://www.nasa.gov/mission_pages/WISE/main/index.html&quot;&gt;Wide-field Infrared Survey Explorer&lt;/a&gt; (WISE) ran out of the coolant it needs for its IR detectors to work, so it's &lt;a href=&quot;http://www.space.com/10895-nasa-wise-space-telescope-mission-ends.html&quot;&gt;mission ended&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In August 2013, NASA powered up WISE again with a mission to look for asteroids headed towards Earth. How is WISE &lt;em&gt;seeing&lt;/em&gt; anything given it's IR detectors are out of coolant?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="288" LastEditDate="2013-10-03T17:04:48.060" LastActivityDate="2013-10-03T17:33:05.190" Title="How is WISE continuing to work without coolant?" Tags="|artificial-satellite|telescope|camera|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2263" PostTypeId="1" AcceptedAnswerId="2293" CreationDate="2013-10-03T16:45:01.760" Score="11" ViewCount="1535" Body="&lt;p&gt;NASA has been working on &lt;a href=&quot;http://www.space.com/22680-nasa-lunar-laser-communications-experiment-infographic.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;demonstrations of laser communication with satellites&lt;/a&gt;.  The motivation is simple, in that lasers see less power loss because they spread over a smaller angle.  That raises the question, what &lt;em&gt;is&lt;/em&gt; the angle over which they spread?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going by &lt;a href=&quot;https://physics.stackexchange.com/questions/13648/physics-of-focusing-a-laser&quot;&gt;laser physics fundamentals&lt;/a&gt;, it seems that for our purposes any Gaussian beam can be effectively taken to be waist-less.  By that I mean, the area of the beam at the target distance will be &lt;em&gt;literally&lt;/em&gt; the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid_angle&quot; rel=&quot;nofollow noreferrer&quot;&gt;solid angle&lt;/a&gt; time the distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MzxQg.png&quot; alt=&quot;laser spread with waist&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some information on the &lt;a href=&quot;http://web.archive.org/web/20130215161346/http://esc.gsfc.nasa.gov/267/271/Ground-Terminal.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Lasercom Ground Terminal (LLGT)&lt;/a&gt; (archived page, link isn't working due to govt shutdown?), but none of these specs relate to the divergence angle.  Laser pointers you can buy in stores can be &lt;a href=&quot;http://laserpointerforums.com/f49/typical-beam-divergence-rates-59627.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;1-3 mrad&lt;/a&gt;, which sounds small, but it's really not.  Such a beam is literally half the diameter of the moon.  A tighter beam introduces a tradeoff where it gets harder to aim, but preserves more of the beam intensity.  I'm interested to know where, along that spectrum, modern space laser communications fall.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="58" LastEditDate="2020-02-26T14:04:25.977" LastActivityDate="2020-02-26T14:04:25.977" Title="Long range laser communication beam divergence angle values?" Tags="|communication|probe|optical-communication|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2264" PostTypeId="1" CreationDate="2013-10-03T16:58:33.523" Score="14" ViewCount="2374" Body="&lt;p&gt;Earlier today a grotesque incident occurred. My attempt to navigate to &lt;a href=&quot;http://www.nasa.gov/mission_pages/soho/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHO&lt;/a&gt; was redirected instead to &lt;a href=&quot;http://notice.usa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this notice&lt;/a&gt; which wrote to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Due to the lapse in federal government funding, this website is not available. &#xA;  We sincerely regret this inconvenience.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;For information about available government services, visit USA.gov.&lt;/p&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;Este sitio web no se está disponible durante el cierre del Gobierno. &#xA;  Lamentamos profundamente las molestias que esto pueda causar.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Visite GobiernoUSA.gov para informarse sobre los servicios gubernamentales disponibles durante el cierre del Gobierno.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is also mentioned elsewhere on SEx.SE in - &lt;a href=&quot;https://space.stackexchange.com/questions/2238/how-did-the-government-shutdown-of-2013-affect-nasa-operations&quot;&gt;How did the US government shutdown of 2013 affect NASA operations?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An article on &lt;a href=&quot;http://abcnews.go.com/Technology/tweet-blackout-federal-agencies-organizations-government-shuts/story?id=20430932&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;While 97 percent of NASA will be furloughed during a shutdown, the exception to the rule will be astronauts in space, who keep working, shutdown or no shutdown. Mission Control, which supports them, will also be working. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;With the bulk of the US Government in shutdown, do Mission Control &amp;amp; Astronauts qualify to be calculated for overtime?&lt;/li&gt;&#xA;&lt;li&gt;Further, &lt;em&gt;regardless of the present shut-down&lt;/em&gt;, do astronauts &amp;amp; staff at any launch-pad (not restricted to NASA, or USA personnel) qualify for hazardous duty calculation?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-12-16T13:43:47.620" Title="Do astronauts qualify to be calculated for hazardous duty in their pay?" Tags="|astronauts|logistics|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2265" PostTypeId="1" AcceptedAnswerId="2269" CreationDate="2013-10-03T17:16:09.120" Score="1" ViewCount="850" Body="&lt;p&gt;This site says that NASA will release &quot;The Biggest Discovery That will shake the earth, It will never be the same again&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it hoax? Is it true? Does anyone here know something about it?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&lt;strong&gt;Warning: This website has a questionable domain registrant and is collecting email addresses from unsuspecting visitors for unexplained purposes and no legal explaining their intentions or information about the website's owners. Enter at your own risk, and remember to be careful with your email address and other personal information! Space Exploration Stack Exchange cannot give any reassurances regarding activities of third parties on the Internet, nor is it expected to.&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote class=&quot;spoiler&quot;&gt;&#xA;  &lt;p&gt; rememberthe13th.com&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Hover with your mouse over the yellow bar above to reveal the websites location. Please read our warning before opening it!&lt;/p&gt;&#xA;" OwnerUserId="844" LastEditorUserId="58" LastEditDate="2013-10-03T19:58:55.723" LastActivityDate="2013-10-03T19:58:55.723" Title="Will NASA really make a big announcement on November 13th, 2013?" Tags="|nasa|" AnswerCount="1" CommentCount="4" ClosedDate="2013-10-03T19:25:42.813" ContentLicense="CC BY-SA 3.0" />
  <row Id="2266" PostTypeId="1" AcceptedAnswerId="2271" CreationDate="2013-10-03T17:30:14.917" Score="8" ViewCount="503" Body="&lt;p&gt;On Mir, the docking module was at the front end of the station.  It technically had 6 ports (one was used to connect to Mir itself, so 5 exposed).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general the docking node had one active port that a Soyuz, Progress, or incoming module could dock to with Kurs or the like.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once docked they used the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lyappa_arm&quot; rel=&quot;nofollow&quot;&gt;Lyappa Arm&lt;/a&gt; to undock, rotate, and move the docked module to a side port or around to another side port.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I am wondering is, did they ever use Lyappa to move a Soyuz or Progress to a side port? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One clue would be, if a Soyuz could undock on its own from a side port, vs the active docking port.  (Soyuz was there as a lifeboat, and is why Dragon has to move to use the PMA/NDS system to dock vs the CBM and berthing since it cannot unberth without the CanadaArm2s help, but could undock from the PMA on its own).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it could not undock on its own from a side port, then it could never have been there for very long if at all. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6642" LastEditDate="2016-10-14T09:06:25.930" LastActivityDate="2016-10-14T09:06:25.930" Title="On Mir, did they ever move a Soyuz at the front end, to a side docking port using Lyappa?" Tags="|docking|mir|soyuz-spacecraft|lyappa|progress|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2267" PostTypeId="5" CreationDate="2013-10-03T17:31:26.920" Score="0" Body="&lt;p&gt;The docking module on the Soviet/Russian Mir space station had 6 ports, and used the Lyappa arm to move docked modules from the end port to the side ports. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was used to shuffle the various modules around, once they arrived, and then after they arrived, to accommodate incoming new modules. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-03T17:32:14.303" LastActivityDate="2013-10-03T17:32:14.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2268" PostTypeId="4" CreationDate="2013-10-03T17:31:26.920" Score="0" Body="The docking module on the Soviet/Russian Mir space station had 6 ports, and used the Lyappa arm to move docked modules from the end port to the side ports. " OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-03T17:32:00.707" LastActivityDate="2013-10-03T17:32:00.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2269" PostTypeId="2" ParentId="2265" CreationDate="2013-10-03T17:31:49.160" Score="4" Body="&lt;p&gt;NASA has never been known to give more than a few days notice prior to big announcements, and they have never set up a custom domain to give out the knowledge before. Here is the email that announced that Voyager 1 left the Solar System, which is one of the biggest announcements that NASA has made in some time. Note that this was done only a day in advance. The only way that anyone would know about something sooner is if they worked for NASA, or reviewed a big paper it was about to publish, in which case they would not be permitted to do anything about it. &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;JPL/NASA News&#xA;News advisory: 2013-276b                                                             Sept. 12, 2013&#xA;&#xA;NASA News Conference Today to Discuss Voyager Spacecraft&#xA;&#xA;The full version of this story with accompanying images is at:&#xA;http://www.jpl.nasa.gov/news/news.php?release=2013-276&amp;amp;cid=release_2013-276&#xA;&#xA;&#xA;PASADENA, Calif. - NASA will host a news conference today at 11 a.m. PDT (2 p.m. EDT), to discuss NASA's Voyager mission. It is related to a paper to be published in the journal Science, which is embargoed until 11 a.m. PDT (2 p.m. EDT).&#xA;&#xA;The briefing will be held at NASA Headquarters in Washington and air live on NASA Television and the agency's website.&#xA;&#xA;During the news conference, the public may send questions via Twitter to #AskNASA.&#xA;&#xA;For NASA TV streaming video, scheduling and downlink information, visit: http://www.nasa.gov/ntv .&#xA;&#xA;The event will also be streamed live on Ustream at: http://www.ustream.tv/nasajpl2 .&#xA;&#xA;For information about the Voyager mission, visit: http://www.nasa.gov/voyager .&#xA;&#xA;DC Agle/Jia-Rui Cook 818-393-9011/818-354-0850&#xA;Jet Propulsion Laboratory, Pasadena, Calif.&#xA;agle@jpl.nasa.gov/jccook@jpl.nasa.gov&#xA;&#xA;Dwayne Brown 202-358-1726&#xA;NASA Headquarters, Washington&#xA;dwayne.c.brown@nasa.gov &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Furthermore, let's look at the domain registry information for the site:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;[Querying whois.verisign-grs.com]&#xA;[Redirected to whois.enom.com]&#xA;[Querying whois.enom.com]&#xA;[whois.enom.com]&#xA;=-=-=-=&#xA;&#xA;Registration Service Provided By: Namecheap.com&#xA;Contact: support@namecheap.com&#xA;Visit: http://namecheap.com&#xA;Registered through: eNom, Inc.&#xA;&#xA;&#xA;Domain name: rememberthe13th.com&#xA;&#xA;Registrant Contact:&#xA;   WhoisGuard, Inc.&#xA;   WhoisGuard Protected ()&#xA;&#xA;   Fax: &#xA;   P.O. Box 0823-03411&#xA;   Panama, Panama NA&#xA;   PA&#xA;&#xA;Administrative Contact:&#xA;   WhoisGuard, Inc.&#xA;   WhoisGuard Protected (b8bd1b7f7bdf45ed942317298e44f729.protect@whoisguard.com)&#xA;   +507.8365503&#xA;   Fax: +51.17057182&#xA;   P.O. Box 0823-03411&#xA;   Panama, Panama NA&#xA;   PA&#xA;&#xA;Technical Contact:&#xA;   WhoisGuard, Inc.&#xA;   WhoisGuard Protected (b8bd1b7f7bdf45ed942317298e44f729.protect@whoisguard.com)&#xA;   +507.8365503&#xA;   Fax: +51.17057182&#xA;   P.O. Box 0823-03411&#xA;   Panama, Panama NA&#xA;   PA&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I have no idea why someone from Panama would possibly know anything about the workings of NASA, especially during a government shutdown. The odds of this being legitimate are &lt;a href=&quot;http://en.wikipedia.org/wiki/%28%CE%B5,_%CE%B4%29-definition_of_limit#Informal_statement&quot; rel=&quot;nofollow&quot;&gt;epilon&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-03T17:31:49.160" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2270" PostTypeId="2" ParentId="2262" CreationDate="2013-10-03T17:33:05.190" Score="6" Body="&lt;p&gt;In general IR instrumentation can work at higher temperatures, just poorly, in comparison when cooled properly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually when designing an instrument with a known coolant lifespan it makes sense to ensure it can at least function (even if degraded in quality) after the coolant runs out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space can be 'cold' if managed properly.  IR instruments like REALLY cold, which is hard to attain passively, but passive cooling is required anyway, to keep coolant use down as low as possible. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-10-03T17:33:05.190" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2271" PostTypeId="2" ParentId="2266" CreationDate="2013-10-03T18:02:47.377" Score="6" Body="&lt;p&gt;I think I found the answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the Lyappa arm, was not attached to the Mir node, rather the docking node had a connection point, that the modules launched with a Lyappa arm, could latch onto to rotate themeselves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus since Soyuz did not launch with such an arm attached it could not be moved to a side port.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is because I am interested in how the UM for the ISS being proposed for the Russian segment will work, and if the side ports will be useful for moving Soyuz's around.  Looks like not.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-10-03T18:02:47.377" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2272" PostTypeId="1" AcceptedAnswerId="2273" CreationDate="2013-10-03T18:50:54.247" Score="4" ViewCount="4174" Body="&lt;p&gt;How does it? I have a turning hard disk drive in my pc, but I think it would break if I put it on a rocket. Also, what speeds do those things have?&lt;/p&gt;&#xA;" OwnerUserId="648" LastActivityDate="2013-10-04T14:24:50.483" Title="On which device data gets saved on spacecrafts?" Tags="|probe|spacecraft|" AnswerCount="2" CommentCount="3" ClosedDate="2013-10-04T13:17:01.623" ContentLicense="CC BY-SA 3.0" />
  <row Id="2273" PostTypeId="2" ParentId="2272" CreationDate="2013-10-03T22:17:56.683" Score="6" Body="&lt;p&gt;Like it was mentioned in the comments, this would in great extent depend on the evolution of non-volatile storage technology itself. A few things can be taken for granted though with any space exploration mission: Spacecraft active memory, main bus system components and non-volatile memory are built to last with redundant units in place, and all of them physically hardened to meet operational requirements for the environment they'll work in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does that mean? Well, let's describe a few examples throughout the history of space exploration to gain some perspective. For example, Voyager probes use what was at the time of their development the cutting edge of digital storage technology. Quoting myself from another &lt;a href=&quot;https://space.stackexchange.com/a/2054/49&quot;&gt;answer on how was magnetic tape decay prevented in Voyager 1&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager probes use 8-track Digital Tape Recorders (DTR) to record&#xA;  telemetry and scientific data, and they both have error-correction&#xA;  capabilities, with Voyager 2 having a stronger, more error resistant&#xA;  algorithms in place.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So we see there is redundancy in the way the data is written on the magnetic tape, and the use of error correction algorithms assures data integrity. Moving on to more current times, the Curiosity rover exploring the surface of Mars has a fully redundant set of electronics in two main system buses called &lt;em&gt;A-side&lt;/em&gt; and &lt;em&gt;B-Side&lt;/em&gt;. It has actually happened to this particular rover, that an unexplained cause resulted in what NASA called the &lt;em&gt;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/03041633-curiosity-sol-205-memory-anomaly.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sol 200 memory anomaly&lt;/a&gt;&lt;/em&gt;, and the rover was put into safe mode and later switched to using its &lt;em&gt;B-Side&lt;/em&gt; while the NASA engineers were working to resolve the problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is more to it than merely using systems built for redundancy, ensuring data integrity and hardening against environmental conditions like solar radiation, mechanical damage, or even faulty or slowly wearing out and equipment. And no, it is not the use of cutting edge technology and investing a great deal of money into their development. That one should be taken for granted and I probably don't have to throw at you all kinds of examples, Googling for &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_spin-off_technologies&quot; rel=&quot;nofollow noreferrer&quot;&gt;space exploration spin-off technology&lt;/a&gt; should produce all kinds of interesting results that we all might be using now everyday as a direct product of throwing a lot of money into space exploration technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I mean is that there are elaborous decision making process involved with designing each and every space exploration mission, establishing operational parameters and matching them with suitable equipment that the mission's success will depend on. This goes the same also for choosing non-volatile storage technology, how many redundant systems are required, and indeed can be afforded. This decision making process that everything depends on is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Risk_management&quot; rel=&quot;nofollow noreferrer&quot;&gt;risk management&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Risk management is the identification, assessment, and prioritization&#xA;  of risks (defined in ISO 31000 as the effect of uncertainty on&#xA;  objectives, whether positive or negative) followed by coordinated and&#xA;  economical application of resources to minimize, monitor, and control&#xA;  the probability and/or impact of unfortunate events or to maximize the&#xA;  realization of opportunities.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Part of risk management is also deciding when some equipment should be active, deactivated, or put in other modes of operation. For example, if the spacecraft or a rover would be predicted (or detected &lt;em&gt;in situ&lt;/em&gt;) to experience sudden movement or other contributing factors to the safety of operations and exceed its previously determined operational limits, parts that could potentially be negatively affected by it could be put into safe mode automatically or on remote issued command, hard disks could despin their platters, or be shut down completely to be awaken at a later date.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such risk management procedures are an integral part of any stage of mission planning, from designing it through to deciding its main objectives. On top of &lt;a href=&quot;https://en.wikipedia.org/wiki/Quality_assurance&quot; rel=&quot;nofollow noreferrer&quot;&gt;quality assurance&lt;/a&gt;, these two management and engineering functions will be working hand in hand to guarantee mission success the best they can, technology and budget permitting. This can be applied to any other industries, and space exploration is no exception.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, as you can see, it is not merely about what type of hardware is used, but equally importantly how it is used. Often, the parts selected are not on the cutting edge of technology either, but more lasting parts with proven track record will be used instead. That's why you can still read about NASA putting some 5 year old computer processors aboard their spacecraft and you might laugh about having a faster, newer one in your own computer. They have been determined to be sufficient for handling their tasks, and have 5 years of quality assurance and evolution in manufacturing techniques behind them. Something the newer part you might use doesn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And same goes for non-volatile storage technology. They would use parts that are generally more robust, environmentally hardened, heavy-duty rated, e.t.c., but any part can fail no matter how well they have proven to work beforehand, so they will also employ redundancy and integrity systems both in hardware, as well as software and the way the data itself is written to ensure its integrity. As there are literally hundreds of individual techniques to assure that, I'll avoid listing them here. But if you have any more specific question, don't hesitate to ask it in a new question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing can be deducted though, that consumer products like the ones you might have in your own computer don't tend to come as hardened, failure resilient as specialized use equipment does. Your hard disk drive (HDD) would still use a lot of technologies that were developed with this mindset described, but they tend to trickle from the edge use case into consumer grade products slowly. Your disk would, for example, employ &lt;a href=&quot;https://en.wikipedia.org/wiki/Cyclic_redundancy_check&quot; rel=&quot;nofollow noreferrer&quot;&gt;cyclic redundancy check&lt;/a&gt; and other &lt;a href=&quot;https://en.wikipedia.org/wiki/Error_detection_and_correction&quot; rel=&quot;nofollow noreferrer&quot;&gt;error detection and correction&lt;/a&gt; code, some of which might have been developed exclusively for space exploration purposes maybe a few decades back, and have expired patents or don't come with royalties, something consumer grade equipment manufacturers would be avoiding as much as possible to keep the costs down and keep your equipment's selling price cheap and cheerful. I.e. - affordable.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-10-03T23:04:33.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2274" PostTypeId="1" AcceptedAnswerId="2277" CreationDate="2013-10-03T22:21:54.143" Score="9" ViewCount="723" Body="&lt;p&gt;NASA has been working on a glider aircraft they wanted to send to Mars - this is &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerial_Regional-scale_Environmental_Survey&quot; rel=&quot;nofollow&quot;&gt;the Aerial Regional-scale Environmental Survey. (ARES)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How long would it be able to travel? What would be the possible maximum distance for ARES to travel through?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="6214" LastEditDate="2015-01-11T08:39:42.970" LastActivityDate="2015-01-11T16:29:30.567" Title="How effective would the ARES aircraft be?" Tags="|mars|nasa|aerobot|future-missions|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2275" PostTypeId="1" AcceptedAnswerId="2276" CreationDate="2013-10-03T05:23:20.683" Score="15" ViewCount="2103" Body="&lt;p&gt;What is the most distant planet from the earth that a spacecraft visited has visited so far? What was the mission and when did it happen?&lt;/p&gt;&#xA;" OwnerUserId="637" OwnerDisplayName="M.Tarun" LastEditorUserId="414" LastEditDate="2013-10-04T01:32:18.127" LastActivityDate="2019-03-14T12:11:35.263" Title="What is the most distant planet from the Earth that a spacecraft has visited to date?" Tags="|history|spacecraft|planet|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2276" PostTypeId="2" ParentId="2275" CreationDate="2013-10-03T05:48:54.927" Score="19" Body="&lt;p&gt;In 1989, &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 2&lt;/a&gt; did a fly-by of the planet Neptune. The orignal plan for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_Grand_Tour&quot; rel=&quot;nofollow noreferrer&quot;&gt;Grand Tour&lt;/a&gt; would have included Pluto, but the Voyager series was reduced from four craft to just two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons&lt;/a&gt; craft, launched in 2006, made it's closes approach to Pluto on July 14, 2015, at 11:49 UTC (2,500 km/7,800 mi). Technically, Neptune is still be the most distant visited &quot;planet&quot; since Pluto has been reclassified as a &lt;em&gt;dwarf planet&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, regarding the furthest explored astronomical &quot;feature&quot;, in 2013, it was determined that &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1&lt;/a&gt; crossed the &lt;a href=&quot;http://en.wikipedia.org/wiki/Heliopause_%28astronomy%29#Heliopause&quot; rel=&quot;nofollow noreferrer&quot;&gt;heliopause&lt;/a&gt; and entered interstellar space. The nature of the heliopause is more subtle, though, and probably not directly observable without instrumentation.&lt;/p&gt;&#xA;" OwnerUserId="113" OwnerDisplayName="Donald.McLean" LastEditorUserId="113" LastEditDate="2019-03-14T12:11:35.263" LastActivityDate="2019-03-14T12:11:35.263" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="2277" PostTypeId="2" ParentId="2274" CreationDate="2013-10-03T23:35:20.163" Score="7" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4086.pdf&quot;&gt;this documentation&lt;/a&gt; (PDF) from 2012 on mission implementation of The Aerial Regional-scale Environmental Surveyor (ARES) Concepts and Approaches for Mars Exploration by Levine et al., NASA Langley Research Center:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Transition, extraction, and  deployment of the ARES aerial vehicle use&#xA;  proven methods demonstrated during the airplane technology &#xA;  development. After release, the aerial vehicle &lt;strong&gt;will fly a 500 km&#xA;  flight path&lt;/strong&gt; to perform the aeromagnetic survey, determine local&#xA;  radiation levels, search for the presence and concentration of&#xA;  chemically active gases,  measure atmospheric dust and electric&#xA;  fields, and  measure the local density and wind. All data will be &#xA;  transmitted to either the carrier spacecraft as it flies by Mars or to&#xA;  an existing orbital asset.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the mission goal was for this aerobot to fly 500 kilometers. As for its capabilities to sustain flight in Martian atmosphere, since its proposal mentions the use of a rocket system to achieve flight, its total range would be limited by the amount of the rocket propellant it could still fly with, and was in no way meant as a permanent addition to the Martian skies. I wasn't able to find any theoretical limits to this proposed aerobot's maximum range, but since the last (2012) proposal dropped any idea of using other propulsion technologies, I would expect the mission objectives as described before to match its theoretical total range well enough. So, 500 kilometers ± an acceptable margin of error?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since I wasn't able to answer your question better (I was stuck at those 500 kilometers with my search, and other references I could find were even older), here's a nice image of artist's impression on how the ARES aerial platform could look like while performing its survey of the red planet:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/03oHK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-10-03T23:35:20.163" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2278" PostTypeId="2" ParentId="2274" CreationDate="2013-10-03T23:35:34.737" Score="8" Body="&lt;p&gt;The concept was not a glider, but powered.  It used storable biprop propellant rocket thrusters to fly at best for 81 minutes at 140 m/s.  That comes out to a 680 km flight.  With margins applied for mass growth during development, the range would be at or above the science requirement of 500 km.  Once the propellant runs out, then it &lt;em&gt;is&lt;/em&gt; a glider, but not for long.  It's mission would end impacting the surface at high speed.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-01-11T16:29:30.567" LastActivityDate="2015-01-11T16:29:30.567" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2279" PostTypeId="5" CreationDate="2013-10-03T23:38:42.107" Score="0" Body="&lt;p&gt;The Mir (usually translated as Peace) space station was launched Feb 28, 1986 and was deorbited Mar 23, 2001 to free up resources for the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/0/06/Mir_diagram.svg/545px-Mir_diagram.svg.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;Mir had a rear docking port, occupied by the Kvant module. The front end had 5 exposed docking ports that held a variety of modules in different configurations over its lifespan in orbit.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Kvant 2 &lt;/li&gt;&#xA;&lt;li&gt;Spectr&lt;/li&gt;&#xA;&lt;li&gt;Priroda&lt;/li&gt;&#xA;&lt;li&gt;Kristall&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To accommodate the Space Shuttle docking, a docking module was attached to the far end of the Kristall module.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="10273" LastEditDate="2018-10-16T14:28:45.223" LastActivityDate="2018-10-16T14:28:45.223" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2280" PostTypeId="4" CreationDate="2013-10-03T23:38:42.107" Score="0" Body="The Mir space station was launched by the Soviet Union/Russians and eventually was a complex of many modules." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-03T23:40:25.503" LastActivityDate="2013-10-03T23:40:25.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2281" PostTypeId="2" ParentId="584" CreationDate="2013-10-03T23:54:17.070" Score="9" Body="&lt;p&gt;The website report &lt;a href=&quot;http://ares.jsc.nasa.gov/HumanExplore/Exploration/EXLibrary/DOCS/EIC039.HTML&quot;&gt;An International Lunar Farside Observatory and Science Station&lt;/a&gt; based on a design project by the &lt;a href=&quot;http://www.isunet.edu/&quot;&gt;International Space University&lt;/a&gt; looked at one aspect of a lunar-farside operation, that being an array of observatories.  The website draws on existing examples.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of management, the findings of this project emphasised that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Previous international collaborations have taught that projects should be well-defined with self-contained phases, should have explicit technical and political objectives, and should be true partnerships with equitable sharing of decision-making.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They state that while initial management responsibilities in the construction and logistics would be in the hands of the various space agencies; however, the final phases&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;must be maintained by some supranational entity to maintain the sense of international cooperation. An appropriate structure might be similar to the Interagency Consultative Group (IACG), which coordinated studies of Halley's Comet and now oversees the Solar Terrestrial Science Project. The IACG coordination is largely informal, &lt;strong&gt;taking advantage of the collaborative culture of the international scientific community&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Perhaps, even a Space.SE type collaboration).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The installation, use and management of the array would also need to be under the auspices of an international group&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Scientific utilization of the preliminary phase instruments probably will be done through the Principal Investigator (PI) model. However, the primary phase instruments will be regarded as international facilities similar to observatories. A managing organization will be set up to evaluate scientific proposals for observing time, to schedule the instruments, and to operate the facility. The model for this organization is the &lt;a href=&quot;http://www.stsci.edu/portal/&quot;&gt;Space Telescope Science Institute&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-10-03T23:54:17.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2282" PostTypeId="2" ParentId="2272" CreationDate="2013-10-04T00:28:40.837" Score="6" Body="&lt;p&gt;Nowadays it's usually flash memory.  Curiosity has 2 GB of flash per redundant computer.  Each camera (both Mastcams and the hand lens camera) have 8 GB of flash each.  The computers also have a small amount of &lt;a href=&quot;http://en.wikipedia.org/wiki/EEPROM&quot; rel=&quot;nofollow noreferrer&quot;&gt;EEPROM&lt;/a&gt; for storing flight software, but those are not used for data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible to get things like tape drives to survive launch and work in space, and it &lt;a href=&quot;http://caltech.typepad.com/.a/6a0105349b8251970b0120a5393e57970b-pi&quot; rel=&quot;nofollow noreferrer&quot;&gt;has been done&lt;/a&gt;.  However today solid state memory is lower mass and more reliable than other approaches, and can be made &lt;a href=&quot;http://www.3d-plus.com/product.php?type=1&amp;amp;fm=20&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiation tolerant&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TbTEq.jpg&quot; alt=&quot;Galileo tap recorder&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA372706&quot; rel=&quot;nofollow noreferrer&quot;&gt;Two 545 MB hard drives in a single pressurized enclosure were flown and operated successfully on MSTI-3 in 1996&lt;/a&gt;.  That is the only example I've found of hard drives operating on a spacecraft, outside of those used inside human-occupied pressure vessels, e.g. inside the ThinkPads on ISS.  The STEP-3 spacecraft had one, but that one never made it to space due to   a launch failure.  Hard drives require air or some gas for the head to literally fly over the hard disk surface. So a hard drive used in space would need its own pressure vessel, or needs to already be hermetically sealed and rated for the vacuum of space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I doubt that a hard drive will ever be used on a spacecraft again, since the cost benefit of smaller, lighter, and lower power solid-state memory far, far outweighs the cost difference for the same amount of storage.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-10-04T14:24:50.483" LastActivityDate="2013-10-04T14:24:50.483" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2283" PostTypeId="2" ParentId="2239" CreationDate="2013-10-04T01:36:18.683" Score="4" Body="&lt;p&gt;Basically, most magnetorquers function something like bar magnets that can be dialed to select how powerful, and what direction, they pull in. The math behind them turns out to be really simple. $\mathbf{\tau}=\mathbf{\mu}\times\mathbf{B}$, where $\tau$ is the torque on the satellite, $\mathbf{B}$ is the ambient magnetic field, and $\mu$ is the magnetic field of the satellite. Usually, there are 3 axis of magnetorquers, which means that $\mu$ can effectively be set to anything within a certain range, although the torque will be applied differently depending on exactly where the magnetorquer is placed. The full analysis will be easier to apply by measuring one magnetorquer at a time, but I'm going to assume that their torque lines up with the rotational vectors reasonably well, allowing for all of them to be handled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, what happens when you turn on the magnetorquer is that there is a torque applied to move the spacecraft to line up with the magnetic field vector. Like a compass needle, the magnet will line up with the north pole of the magnet lining up to the south magnetic field. Without resistance, it will overshoot the pole, making it difficult to exactly line up on an axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, one must consider that there are in effect only 2 axis of rotation. If you are lined up perfectly with the magnetic field, you cannot control your rotation about the field. There is always one axis that you cannot rotate about, although you might be able to mitigate it with some effort.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-04T01:36:18.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2284" PostTypeId="1" AcceptedAnswerId="2286" CreationDate="2013-10-04T03:02:59.003" Score="7" ViewCount="305" Body="&lt;p&gt;I know we've obtained some images of asteroids from probes.  Since each of those constitutes a fairly costly and public mission, I wonder if it would be possible to get a list of all such objects that have been imaged, as of this year, 2013?  Just a text-based list of the IAU names would be fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this would be more than 10 or 20, then I'll gladly withdraw this question.  I ask because I don't think that's the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from probes equipped with a camera, is there any other source of direct images of asteroids?  I know there is radio astronomy as well as other computational methods to approximate the shape from varying intensity over time.  The number &quot;imaged&quot; by those methods would be more difficult to pin down, so let's exclude those.  But did Hubble image near Earth asteroids?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-10-04T15:21:37.913" Title="Full list of asteroids that have been optically imaged as of 2013" Tags="|probe|asteroid|imaging|observation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2285" PostTypeId="1" AcceptedAnswerId="2287" CreationDate="2013-10-04T03:47:02.430" Score="17" ViewCount="1827" Body="&lt;p&gt;I've been to Kennedy Space Center in Florida, where I saw the &lt;strong&gt;huge&lt;/strong&gt; Saturn V rocket...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qp7Yd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was used to get to people to the moon. It had a Lunar Module to propel itself on and off the moon's surface once the rest of the rocket had been ejected and dropped to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as far as the moon goes, the Saturn V was an ideal machine because the Moon has a relatively small acceleration due to gravity on its surface. $g = 1.624 m/s^2$, so the Lunar Module doesn't have to be too big or powerful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how can NASA apply the same technology to getting people to Mars? Not only is Mars a lot farther, needing more fuel, and an even bigger rocket, but Mars also a higher acceleration due to gravity amounting to $g = 3.741 m/s^2$. This would not only need a &lt;strong&gt;huge&lt;/strong&gt; rocket, but also a pretty big (and I dare say unfeasible) Martian Module. So, even if we could make the journey &lt;em&gt;to&lt;/em&gt; Mars, how would we get back to Earth? &lt;/p&gt;&#xA;" OwnerUserId="733" LastEditorUserId="-1" LastEditDate="2016-04-29T22:19:17.603" LastActivityDate="2018-10-27T23:50:26.770" Title="How do we make a round-trip journey to Mars?" Tags="|mars|rockets|crewed-spaceflight|interplanetary|feasibility|" AnswerCount="2" CommentCount="6" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2286" PostTypeId="2" ParentId="2284" CreationDate="2013-10-04T03:56:47.993" Score="7" Body="&lt;p&gt;Emily Lakdawalla has this &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2012/12181404-asteroids-comets-color-toutatis.html&quot;&gt;fantastic montage of images of asteroids and comets from probes&lt;/a&gt;.  That's all of them, except for Vesta.  The numbers are on there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble can only resolve large asteroids, like &lt;a href=&quot;http://www.newswise.com/articles/hubble-images-of-asteroids-help-astronomers-prepare-for-spacecraft-visit&quot;&gt;Ceres and Vesta&lt;/a&gt;.  (Actually Ceres is now a &quot;dwarf planet&quot;, but she's always an asteroid to me.  Cue Billy Joel music.)  I presume what you mean by &quot;image&quot; is actually resolve the disk, as opposed to just seeing a point of light.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-10-04T03:56:47.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2287" PostTypeId="2" ParentId="2285" CreationDate="2013-10-04T04:09:40.323" Score="20" Body="&lt;p&gt;If the US Government ever gets back in gear, you can read NASA's &lt;a href=&quot;http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf&quot;&gt;Design Reference Architecture 5.0&lt;/a&gt; (DRA 5) for human missions to Mars.  (I have a copy, but I don't see a way to upload it here.)  Here is a &lt;a href=&quot;http://journalofcosmology.com/Mars105.html&quot;&gt;summary&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic idea is to use several Saturn V's or equivalent, and put up pieces of the mission.  Some parts are assembled in Earth orbit, some meet up on the surface of Mars or in Mars orbit.  The document shows variations that require seven to twelve Ares V launch vehicles, where each Ares V is about equal to 1.5 Saturn V's.  In the chemical propulsion variant, about 2/3 of the launches are rocket stages with propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for how to make getting back appear somewhat feasible, a few tricks are applied.  First, Mars orbit rendezvous is used, just as we used Lunar orbit rendezvous for Apollo.  Then the humans only need a rocket on Mars big enough to get them into Mars orbit, and a capsule that they only need to survive in for hours or days.  Their return vehicle in orbit around Mars, with the propellant to leave Mars and the supplies to survive the trip, didn't have to land, vastly reducing the mass required.  Second, the rocket on Mars is mostly fueled there using carbon and oxygen extracted from the CO&lt;sub&gt;2&lt;/sub&gt; atmosphere.  (They bring hydrogen to make methane with.)  Third, the orbiting return vehicle got into orbit around Mars by using aerocapture, replacing a lot of propellant with an aeroshell, saving some mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though I would ask: why do we need to provide a way to get back?  The ability to get back is something like 2/3 of the mission cost.  It would be far cheaper to send them on a one-way trip, and follow that with much smaller and inexpensive resupply missions to keep them alive and productive indefinitely.  There would be no shortage of highly-qualified volunteers.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-10-21T14:06:17.963" LastActivityDate="2013-10-21T14:06:17.963" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2288" PostTypeId="1" AcceptedAnswerId="2745" CreationDate="2013-10-04T04:12:08.817" Score="16" ViewCount="465" Body="&lt;p&gt;According to the article &lt;a href=&quot;http://news.nationalgeographic.com/news/2013/13/131002-mars-super-volcano-space-ancient/&quot; rel=&quot;noreferrer&quot;&gt;&amp;quot;Supervolcanoes Rocked Early Mars&amp;quot;&lt;/a&gt; (Vergano, 2013), there is evidence that supervolcanic activity has occurred on the red planet.  Also, it is believed some features identified as craters are in fact the collapsed remains of ancient supervolcanic eruptions (calderas).&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eamcQ.jpg&quot; alt=&quot;A false-color view of Eden Patera, a possible supervolcano on Mars&quot; /&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A false-color view of Eden Patera, a possible supervolcano on Mars.&lt;/p&gt;&#xA;&lt;p&gt;Image courtesy ASU/GSFC/JPL/NASA&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As, from the article, these eruptions would have resulted in&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;volcanic pools spread across its surface like fuming, open wounds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;On Earth, these calderas can be a rich source of mineral deposits, which leads me to the question - given the differences in geochemistry between Earth and Mars:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;What are the minerals and other raw materials left on Mars by supervolcanoes and how would these be useful for future missions and colonists to Mars?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-01-19T08:20:05.370" Title="What are the resource implications of the prevalence of supervolcanoes on Mars?" Tags="|mars|colonization|natural-resources|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2289" PostTypeId="1" AcceptedAnswerId="2290" CreationDate="2013-10-04T12:32:16.360" Score="34" ViewCount="43865" Body="&lt;p&gt;Venus has an extremely slow rotation rate, to the point that it actually rotates slower than it revolves around the sun. In other words, its day is longer than its year. With the exception of Mercury, which has some resonance with the sun, the Venus day is by far the slowest of any planet, and for that matter, non-tidally locked object in the Solar System. Why is that?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-04-30T19:54:44.567" Title="Why is the rotation rate of Venus so slow?" Tags="|planetary-science|venus|rotation|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2290" PostTypeId="2" ParentId="2289" CreationDate="2013-10-04T12:57:41.097" Score="30" Body="&lt;p&gt;Not only is the rotation slow, it is &lt;strong&gt;retrograde&lt;/strong&gt;, thus rotates in the opposite direction to the other planets - which may be linked to the slow rotation.  The atmosphere also rotates much faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several theories as to why Venus has a slow retrograde rotation, according to &lt;a href=&quot;https://www.scientificamerican.com/article/why-venus-spins-the-wrong/&quot; rel=&quot;noreferrer&quot;&gt;Why Venus Spins the Wrong Way&lt;/a&gt; (Franzen, 2001), states that a theory of this phenomenon states that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Venus initially spun in the same direction as most other planets and, in a way, still does: it simply flipped its axis 180 degrees at some point. In other words, it spins in the same direction it always has, just upside down, so that looking at it from other planets makes the spin seem backward.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The mechanism behind this, according to the theory, is that &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;sun's gravitational pull on the planet's very dense atmosphere could have caused strong atmospheric tides. Such tides, combined with friction between Venus's mantle and core, could have caused the flip in the first place. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, in the same article, some researchers argue that the planet simply slowed to a standstill and reversed due to the same factors as the previous theory as with tidal effects of other planets - that it simply settled into its own stable rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To add to the mystery, the article &lt;a href=&quot;http://news.nationalgeographic.com.au/news/2012/02/120214-venus-planets-slower-spin-esa-space-science/&quot; rel=&quot;noreferrer&quot;&gt;Venus Spinning Slower Than Thought—Scientists Stumped&lt;/a&gt; (Major, 2012) suggest that the rotation is slowing down even further, with&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to the new data, Venus is rotating 6.5 minutes slower than it was 16 years ago, a result that's been found to correlate with long-term radar observations taken from Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;2 possible mechanisms for this slow down are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;friction from the thick atmosphere&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;an exchange of angular momentum with Earth&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;With the new data, there is still no definitive answers, but very interesting theories.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="10259" LastEditDate="2018-04-30T19:54:44.567" LastActivityDate="2018-04-30T19:54:44.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2291" PostTypeId="2" ParentId="2289" CreationDate="2013-10-04T13:03:01.613" Score="21" Body="&lt;p&gt;Nobody really knows for sure. And there's two, nay three more odd things about it's rotation on its own axis, namely:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It is the only planet in the Solar system that &lt;a href=&quot;http://www.universetoday.com/14299/retrograte-rotation-of-venus/&quot;&gt;rotates retrograde&lt;/a&gt;, i.e. clockwise, when all other planets rotate prograde, or anticlockwise, on their axes, &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.sott.net/article/265781-Scientists-baffled-to-discover-that-Venus-spin-is-slowing-down&quot;&gt;Latest findings&lt;/a&gt; (merely a good month ago as of writing this answer) revealed that its rotation period is actually increasing, i.e. its axial rotation velocity is slowing down, and&lt;/li&gt;&#xA;&lt;li&gt;Its sidereal rotation period, or &lt;a href=&quot;http://www.universetoday.com/47898/length-of-day-on-venus/&quot;&gt;duration of one Venusian day&lt;/a&gt;, is longer than it takes for it to complete one orbit around the Sun, or one Venusian year.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Venus is indeed an oddball. Some theories explaining this strange Venusian day cycle go like this:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;Astronomers think that Venus was impacted by another large planet&#xA;  early in its history, billions of years ago. The combined momentum&#xA;  between the two objects averaged out to the current rotational speed&#xA;  and direction.&#xA;  &lt;a href=&quot;http://www.universetoday.com/36687/rotation-of-venus/#ixzz2gl1kpOUB&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;One possibility is that Venus rotated normally when it first formed&#xA;  from the solar nebula, and then the tidal effects from its dense&#xA;  atmosphere might have slowed its rotation down.&#xA;  &lt;a href=&quot;http://www.universetoday.com/14299/retrograte-rotation-of-venus/#ixzz2gl1uPO6v&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;It is difficult to find a mechanism that will cause the average&#xA;  rotation rate to change this much in only 16 years,&quot; said Venus&#xA;  Express project scientist Håkan Svedhem. &quot;The origin of this could lay&#xA;  in the solar cycle or in long-term weather patterns that modify the&#xA;  atmospheric dynamics. But this puzzle is not yet solved.&quot; &lt;a href=&quot;http://www.sott.net/article/265781-Scientists-baffled-to-discover-that-Venus-spin-is-slowing-down&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;The rotation period of Venus may represent an equilibrium state between tidal locking to the Sun's gravitation, which tends to slow rotation, and an atmospheric tide created by solar heating of the thick Venusian atmosphere. The 584-day average interval between successive close approaches to the Earth is almost exactly equal to 5 Venusian solar days, but the hypothesis of a spin–orbit resonance with Earth has been discounted. &lt;a href=&quot;http://adsabs.harvard.edu/abs/1979ApJ...230L.123S&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;And so on, ad infinitum ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&lt;strong&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/en/4/42/Shrug.jpg&quot;&gt;shrug&lt;/a&gt;&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-04T13:47:31.617" LastActivityDate="2013-10-04T13:47:31.617" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2292" PostTypeId="2" ParentId="2284" CreationDate="2013-10-04T14:17:27.460" Score="2" Body="&lt;p&gt;Here is a list of the images in the Planetary Society image:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Ceres&lt;/li&gt;&#xA;&lt;li&gt;Vesta&lt;/li&gt;&#xA;&lt;li&gt;Lutetia&lt;/li&gt;&#xA;&lt;li&gt;Mathilde&lt;/li&gt;&#xA;&lt;li&gt;Ida&lt;/li&gt;&#xA;&lt;li&gt;moon of Ida: Dactyl&lt;/li&gt;&#xA;&lt;li&gt;Eros&lt;/li&gt;&#xA;&lt;li&gt;Gaspra&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Halley%27s_Comet&quot; rel=&quot;nofollow&quot;&gt;Halley&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Steins&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Annefrank&quot; rel=&quot;nofollow&quot;&gt;Annefrank&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Tempel 1&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Wild_2&quot; rel=&quot;nofollow&quot;&gt;Wild 2&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/19P/Borrelly&quot; rel=&quot;nofollow&quot;&gt;Borrelly&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/4179_Toutatis&quot; rel=&quot;nofollow&quot;&gt;Toutatis&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/103P/Hartley&quot; rel=&quot;nofollow&quot;&gt;Hartley 2&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/9969_Braille&quot; rel=&quot;nofollow&quot;&gt;Braille&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Itokawa&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is a total of 18.  It's 17 if you don't count Ceres, which might not be classified as an asteroid.  Here is a list of the missions that the images came from, their nation, and year of launch.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_2&quot; rel=&quot;nofollow&quot;&gt;Vega 2&lt;/a&gt;, Soviet, 1984&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Galileo&lt;/a&gt;, NASA, 1989&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NEAR_Shoemaker&quot; rel=&quot;nofollow&quot;&gt;NEAR Shoemaker&lt;/a&gt;, NASA, 1996&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_1&quot; rel=&quot;nofollow&quot;&gt;Deep Space 1&lt;/a&gt;, NASA, 1998&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Stardust&lt;/a&gt;, NASA, 1999&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow&quot;&gt;Hayabusa&lt;/a&gt;, Japan, 2003&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Impact_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Deep Impact&lt;/a&gt;, NASA, 2005&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Rosetta&lt;/a&gt;, ESA, 2004&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Dawn&lt;/a&gt;, NASA, 2007&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_2&quot; rel=&quot;nofollow&quot;&gt;Chang'e 2&lt;/a&gt;, China, 2010&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-10-04T15:21:37.913" LastActivityDate="2013-10-04T15:21:37.913" CommentCount="2" CommunityOwnedDate="2013-10-04T14:17:27.460" ContentLicense="CC BY-SA 3.0" />
  <row Id="2293" PostTypeId="2" ParentId="2263" CreationDate="2013-10-04T16:37:26.867" Score="4" Body="&lt;p&gt;From @PearsonArtPhoto's answer &lt;a href=&quot;https://space.stackexchange.com/a/2006/29&quot;&gt;here&lt;/a&gt;, he estimates a beamwidth of about &lt;strong&gt;$3.5~\text{$\mu$rad}$&lt;/strong&gt;, or roughly $0.002~\text{deg}$ (unfortunately, his &lt;a href=&quot;http://www.nasa.gov/sites/default/files/llcdfactsheet.final_.web_.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;main source&lt;/a&gt; is also offline because of the government shutdown). He goes on to point out that this corresponds to an area on the Earth of about $4.9~\text{km}^2$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this does indeed levy a tighter pointing requirement, it's still about 10 times larger than typical absolute pointing accuracy seen in &lt;a href=&quot;http://books.google.com/books?id=PBN5XZjagbsC&amp;amp;lpg=PA253&amp;amp;ots=M_6oIyK6Qr&amp;amp;dq=ground%20telescope%20pointing%20accuracy&amp;amp;pg=PA253#v=onepage&amp;amp;q=ground%20telescope%20pointing%20accuracy&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;ground-based telescopes&lt;/a&gt; (about 1 arcsec).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-10-04T16:37:26.867" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2294" PostTypeId="1" AcceptedAnswerId="2298" CreationDate="2013-10-04T17:11:15.517" Score="10" ViewCount="802" Body="&lt;p&gt;Following &lt;a href=&quot;https://space.stackexchange.com/questions/869/flying-on-dense-atmosphere-planets-moons&quot;&gt;question about flight on other planets&lt;/a&gt; an answer contained a quote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Jupiter: Flight on Jupiter is unrealistic. Jupiter's gravity is much too strong. The power required to maintain flight is about 3x that of Earth making flight there highly unrealistic.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Fighter jet planes use engines that are much more than 3x the power required for flight. Sure that's wasteful and expensive, but not nearly impossible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, what is expensive in terms of energy, is rocket engine that contains its own fuel and oxidizer. On Earth we commonly use jet engines that use atmospheric oxygen for oxidizer, and own fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, what if we reverse that? If the engine were to fly in hydrogen (or methane for that matter!) atmosphere, it would need its own oxidizer but it could burn the atmospheric gas for power... or am I missing something? Would an aerobot on Jupiter, Neptune or any of the planets with enough burnable material for an atmosphere, using own oxidizer to burn it for propulsion, be viable?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-03-08T19:06:46.313" Title="&quot;Hydrogen-breathing&quot; jet engine?" Tags="|aerobot|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2295" PostTypeId="1" AcceptedAnswerId="2330" CreationDate="2013-10-04T17:12:06.597" Score="14" ViewCount="710" Body="&lt;p&gt;We hear a lot about the usefulness of plants and bacteria in human colonization efforts in space, whether interstellar or within our solar system. Other organisms, on the other hand, seem to receive less attention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Animals are frequently studied in space, and there is a long history of the use of &lt;a href=&quot;http://en.wikipedia.org/wiki/Animals_in_space&quot;&gt;animals in space&lt;/a&gt;. I am wondering if there have been any studies into the use of non-plant, non-bacteria organisms in the &lt;em&gt;colonization&lt;/em&gt; of space, specifically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, what uses have been studied? Have any been determined to be of definite value for future colonization efforts?&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2013-10-07T02:17:53.120" Title="What non-plant, non-bacteria organisms have been studied for their use in colonization?" Tags="|colonization|life|animals|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2296" PostTypeId="1" AcceptedAnswerId="2301" CreationDate="2013-10-04T17:25:57.587" Score="9" ViewCount="822" Body="&lt;p&gt;The dark side of the Moon (i.e., the side not currently illuminated by the Sun) has high static potential. But I have heard that hairs can raise if humans are exposed to such situation. But if I managed to land on the Moon, will my hair raise on my head or on my body?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/h2sm9.jpg&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="574" LastEditDate="2013-10-04T19:16:53.763" LastActivityDate="2013-10-05T10:32:51.727" Title="Will my hair raise if I land on the dark side of the moon?" Tags="|the-moon|life|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2297" PostTypeId="1" AcceptedAnswerId="2311" CreationDate="2013-10-04T17:46:33.837" Score="18" ViewCount="1315" Body="&lt;p&gt;There is a whole bunch of &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_junk&quot; rel=&quot;nofollow noreferrer&quot;&gt;space junk&lt;/a&gt;; Wikipedia says there are about 19,000 pieces of junk being tracked.  With no clouds, solar energy is readily available, so the biggest missing ingredient for gaining altitude once you are in orbit is &lt;a href=&quot;http://en.wikipedia.org/wiki/Reaction_mass&quot; rel=&quot;nofollow noreferrer&quot;&gt;reaction mass&lt;/a&gt;.  Normally you have to launch any mass with you that you want to use; the &lt;a href=&quot;https://space.stackexchange.com/questions/1989/what-is-the-current-cost-per-pound-to-send-something-into-leo&quot;&gt;current cost-per-Kg to send mass into LEO&lt;/a&gt; is between 4,000 and 13,000 US$.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There must be tons of space junk floating around in orbit.  Have there been any studies or experiments using space junk as reaction mass to gain orbital lift or modify orbit/travel?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-08-07T22:54:22.183" Title="Can I get higher with Space Junk?" Tags="|debris|orbital-maneuver|conceptual|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2298" PostTypeId="2" ParentId="2294" CreationDate="2013-10-04T17:53:15.257" Score="10" Body="&lt;p&gt;Sure.  That would save a fair bit of weight, avoiding the tankage, insulation, and cooling systems to maintain liquid hydrogen, or alternatively, the mass of the pressure tank for gaseous hydrogen.  You'd need to bring the oxygen.  You actually don't save much weight due to the hydrogen itself, which is only $1\over 9$ of the total propellant mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also consider bringing &lt;a href=&quot;http://ia600806.us.archive.org/29/items/nasa_techdoc_19660024058/19660024058.pdf&quot;&gt;flourine&lt;/a&gt; instead of oxygen to burn with the hydrogen.  Nasty stuff though.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-10-04T18:28:22.640" LastActivityDate="2013-10-04T18:28:22.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2299" PostTypeId="2" ParentId="2297" CreationDate="2013-10-04T18:16:28.080" Score="5" Body="&lt;p&gt;If we are ever able to scale up the effect of tractor beams beyond the microscopic level, using a tractor beam on a massive object--in theory--should alter your trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tractor beam research is currently in its infancy, but has been effective on the microscopic level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-15535115&quot;&gt;Nasa examines 'tractor beams' for sample gathering - BBC News&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/topics/technology/features/tractor-beam.html&quot;&gt;NASA Studying Ways to Make 'Tractor Beams' a Reality - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-10-04T18:27:02.340" LastActivityDate="2013-10-04T18:27:02.340" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2300" PostTypeId="2" ParentId="2297" CreationDate="2013-10-04T18:35:40.190" Score="13" Body="&lt;p&gt;Short answer: No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could use some of the mass in LEO, except that most of the space-junk is metal. We can't burn that easily to produce any thrust. Some sort of magnetic accelerator might be productive, but that's WAAAYYY in the realm of science-fiction, not to mention hard to use once you get past low-earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, while the 19,000 number seems high, it's not like there's a huge cloud of stuff hanging in orbit - it's REALLY diffuse and hard to find, let alone capture and use. Keep in mind, the stuff in orbit is generally going at orbital speeds - in the ballpark of 5-10 km/second. You'd end-up chasing-down a defunct satellite or something and it just wouldn't work well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if there was some space-junk that happened to be a full tank of rocket fuel, then it might be worth the effort. I don't think there's too many of those at the moment. &lt;/p&gt;&#xA;" OwnerUserId="692" LastActivityDate="2013-10-04T18:35:40.190" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2301" PostTypeId="2" ParentId="2296" CreationDate="2013-10-04T20:49:24.507" Score="12" Body="&lt;p&gt;Not if you wanna survive. I joked a bit in the comment, but slightly more seriously, your &lt;strong&gt;Extra Vehicular Activity (EVA) suit would have to be electrically insulated to prevent electrostatic charge buildup and assume a role of one giant capacitor&lt;/strong&gt;. And that goes for both ends, not just the outside layer. For one, your life support system would use all kinds of electrically powered equipment to control your internal environment, each of those would have to be insulated and/or grounded to prevent internal charge buildup and shock hazard upon contact with your body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Merely designing charge neutral outer layers of the EVA suit likely wouldn't be enough for three reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The lunar dust is electrostatically charged,&lt;/li&gt;&#xA;&lt;li&gt;it is magnetic due to the accumulation of nanometer sized iron (Fe) dust particles, and&lt;/li&gt;&#xA;&lt;li&gt;your own movement will cause friction between dust particles and cause &lt;a href=&quot;https://en.wikipedia.org/wiki/Triboelectric_effect&quot;&gt;triboelectric charge&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Your life support system and parts of suit internal components would likely be partially or in whole metallic and magnetic. The EVA suit could also &lt;em&gt;photo-electrostatically&lt;/em&gt; charge due to photoemission of electrons from the surface of the dust particles by UV radiation, though granted, we're supposed to be on the &lt;em&gt;dark side of the Moon&lt;/em&gt;, so the last one with your suit being exposed to UV radiation at different angles, forming regions of differently charged surface wouldn't really be in effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the remaining three processes on the list would however cause fine grain dust to adhere strongly to your suit due to magnetism and electrostatic charge. Additionally, this dust is fairly sharp due to the lack of weathering processes on the Moon, as it's mostly fractured silica surfaces and could lodge itself to your suit's surface, nearly permanently embedding itself into its top layer. The Apollo astronauts had quite a problem with brushing dust off before returning to the Lunar Module (LM), and were reporting smell similar to burning electronics in LM due to the dust's strong electrostatic charge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, even if your suit's outer layer was electrostatically neutral on its own, dust clinging to it wouldn't be, will act as the carrier of strong electrostatic charge or help in the process of its formation causing triboelectric charge on your EVA suit's top layer to form, and &lt;strong&gt;could discharge &lt;em&gt;through&lt;/em&gt; the suit&lt;/strong&gt;, cause high voltage electric shock or even rip through the fabric of your spacesuit and kill you otherwise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So all this insulation, grounding, or even actively discharging / degaussing would play a vital role in keeping you alive, your life support system functioning and in the process preventing your hair to raise due to electrostatic buildup, regardless of how big your helmet is and actually allowing that to happen space-wise. If you'd feel much of electrical current anywhere on your skin, your equipment has likely malfunctioned and without intervention, your hair raising and ruining your hairstyle should be the least of your worries!&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-05T10:32:51.727" LastActivityDate="2013-10-05T10:32:51.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2302" PostTypeId="1" AcceptedAnswerId="2303" CreationDate="2013-10-04T21:29:03.897" Score="14" ViewCount="1253" Body="&lt;p&gt;Both &lt;a href=&quot;http://csep10.phys.utk.edu/astr161/lect/mercury/surface.html&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; and &lt;a href=&quot;https://web.archive.org/web/20140630173546/http://www.uapress.arizona.edu:80/onlinebks/Mercury/MercuryCh11.pdf&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt; say that Mercury's tectonic activity is mostly in the past, but the first article does have this to say:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;After the volcanic activity subsided, the planet has been very quiet geologically except for the occasional meteor (&lt;strong&gt;however, it is probably geologically more active than the Moon&lt;/strong&gt;).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;sub&gt;emphasis added&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;What exactly does it mean to say that Mercury is more active than the Moon in this context? How do we measure Mercury's tectonic activity today?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P4564.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P4564.jpg&quot; alt=&quot;Mercury volcanic deposit&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://astrobob.areavoices.com/astrobob/images/Mercury_volcano.jpg&quot; rel=&quot;noreferrer&quot;&gt;areavoices.com&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="10273" LastEditDate="2020-12-29T00:00:02.070" LastActivityDate="2022-07-28T08:11:33.547" Title="How tectonically active is Mercury?" Tags="|planetary-science|geology|mercury|" AnswerCount="2" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2303" PostTypeId="2" ParentId="2302" CreationDate="2013-10-04T22:49:34.420" Score="11" Body="&lt;p&gt;As we don't have any direct measuring tools to measure the effects of tectonics (e.g. Mercury-quakes) and not have observed volcanism in the short while that we have been properly observing the planet - we can only surmise what tectonics have and are occurring by observing the surface features.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To understand the past tectonic features and dynamics is a means to understanding the present.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, an important distinction, it appears, according to &lt;a href=&quot;http://people.earth.yale.edu/sites/default/files/korenaga12b.pdf&quot; rel=&quot;nofollow&quot;&gt;Plate tectonics and planetary habitability: current status&#xA;and future challenges&lt;/a&gt; (Korenaga, 2012), that &lt;em&gt;plate&lt;/em&gt; tectonics is a feature unique to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the chapter &lt;a href=&quot;http://www.es.ucsc.edu/~fnimmo/website/Mercury_Chapter.pdf&quot; rel=&quot;nofollow&quot;&gt;Tectonics of Mercury&lt;/a&gt;, the mostly &lt;strong&gt;compressional&lt;/strong&gt; tectonic features &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A combination of tidal despinning and thermal contraction may&#xA;  account for equatorial N–S and polar E–W trending lobate scarp thrust faults&#xA;  in the regions by reactivation of normal faults.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and that the&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Local preferred orientations of the lobate scarps and uniform thrust slip dip&#xA;  directions suggest regional-scale stresses influenced the formation of the thrust&#xA;  faults. The wrinkle ridges in smooth plains outside of the Caloris basin are&#xA;  likely due to loading and subsidence of volcanic material that flooded lowland&#xA;  areas.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Recent observations discussed in &lt;a href=&quot;http://blaustein.eps.jhu.edu/~jhr/research/Michel_etal_2013_JGR.pdf&quot; rel=&quot;nofollow&quot;&gt;Thermal evolution of Mercury as constrained by MESSENGER observations&lt;/a&gt; (Michel et al. 2013), determined that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;that the planet’s core is larger than previously estimated. As Mercury’s&#xA;  mantle layer is also thinner than previously thought, this result gives greater likelihood to the&#xA;  possibility that mantle convection is marginally supercritical or even that the mantle is not&#xA;  convecting.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Using this data in simulations, they&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;demonstrate that mantle convection can persist in such a thin mantle for a substantial&#xA;  portion of Mercury's history, and often to the present, as long as the mantle is thicker than&#xA;  ~300 km. We also find that magma generation in Mercury’s convecting mantle is capable of&#xA;  producing widespread magmas by large-degree partial melting&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;MESSENGER also observed large, compressional fold structures and further systems of thrust faults.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to summarise, it appears that the tectonic activity on Mercury has been and is from a number of dynamic sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Tidal de-spinning&lt;/li&gt;&#xA;&lt;li&gt;Contraction as the core cools - reactivating older faults borne from past impacts and earlier tectonic activity.&lt;/li&gt;&#xA;&lt;li&gt;Conversely, possibly due to some mantle convection that might exist, hot spot activity.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="58" LastEditDate="2013-10-21T21:22:20.900" LastActivityDate="2013-10-21T21:22:20.900" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2304" PostTypeId="1" AcceptedAnswerId="2305" CreationDate="2013-10-05T01:12:39.623" Score="21" ViewCount="1249" Body="&lt;p&gt;Usually if an upper stage experiences a failure before it can &quot;passivate&quot; itself (empty tanks, drain batteries, etc.), it's only a matter of time before it explodes. For recent examples, see any of &lt;a href=&quot;http://www.spacenews.com/article/launch-report/32825proton-failure-confirms-need-to-fully-review-breeze-m-upper-stage&quot; rel=&quot;noreferrer&quot;&gt;three&lt;/a&gt; Breeze-M upper stages (pictured below), including one that resulted in &lt;a href=&quot;http://www.spacesafetymagazine.com/2013/01/17/upper-stage-explosion-places-leo-satellites-risk/&quot; rel=&quot;noreferrer&quot;&gt;thousands of new debris pieces in relatively energetic orbits&lt;/a&gt; -- the U.S., of course, has also had its fair share of upper stage explosions, although not as many in recent decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, &lt;strong&gt;what is (usually) the failure mechanism that leads to these explosions&lt;/strong&gt;? Why is it (seemingly) inevitable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I am &lt;em&gt;not&lt;/em&gt; asking what leads to the initial failure that strands the upper stage in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QA4GF.jpg&quot; alt=&quot;breeze-m&quot; title=&quot;Breeze-M upper stage (lower half) attached to a satellite (upper half).&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;I suspect there is a common underlying reason for all of these events, but if this is considered too general, then please answer the question specifically for the recent Breeze-M mishaps.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="4660" LastEditDate="2016-04-19T23:26:12.263" LastActivityDate="2016-04-21T02:11:19.597" Title="What exactly causes stranded upper stages to explode?" Tags="|rockets|propulsion|debris|failure|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2305" PostTypeId="2" ParentId="2304" CreationDate="2013-10-05T01:39:33.287" Score="14" Body="&lt;p&gt;Most upper stages need to be flexible to some degree. This requires liquid propulsion of some kind. There are two primary families of propellant for these stages, Cryogenic and Corrosive. Let's take a look at each independently:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Cryogenic&lt;/strong&gt;- Eventually, the fuel will heat up, to something around room temperature. Room temperature on Earth results because the amount of light hitting the Earth and the thermal energy/ light leaving the Earth sit in a balance. A similar balance will exist in the space near Earth. When the temperature rises, the pressure will quickly rise. Essentially, the limit of pressure will be reached, and like a balloon that is too full, it will explode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Corrosive&lt;/strong&gt;- I can't work out a method that will guarantee explosion every time, but here's what I suspect will happen. Eventually, the tank will be penetrated, either via a leak or otherwise. Then eventually the two types of fuel will mix together in an uncontrolled way, causing an explosion. I would say that this isn't guaranteed, but could easily happen. The thermal cycles will also cause stress, which might lead to this happening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, empty your fuel tanks!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-06T14:38:43.500" LastActivityDate="2015-10-06T14:38:43.500" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2306" PostTypeId="1" AcceptedAnswerId="2313" CreationDate="2013-10-05T06:27:32.353" Score="10" ViewCount="379" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Phobos_%28moon%29&quot;&gt;Phobos&lt;/a&gt;, the larger moon of Mars, was once theorized to be hollow by &lt;a href=&quot;http://en.wikipedia.org/wiki/Iosif_Samuilovich_Shklovsky&quot;&gt;Iosif Shklovsky&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plenty of publications reference the idea (such as this &lt;a href=&quot;http://books.google.com/books?id=DSEDAAAAMBAJ&amp;amp;pg=PA101&amp;amp;dq=hollow%20phobos&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=b69PUvyOD-LCigLLg4D4CA&amp;amp;ved=0CDgQ6AEwAg#v=onepage&amp;amp;q=hollow%20phobos&amp;amp;f=false&quot;&gt;snippet&lt;/a&gt; from Popular Science), but did anyone take it seriously?  What did astronomers of the day (other than Shklovsky) think of the idea?&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="25" LastEditDate="2013-10-05T17:01:12.390" LastActivityDate="2013-10-06T13:20:08.267" Title="How seriously did astronomers take the idea of a hollow Phobos?" Tags="|mars|history|planetary-science|myth-busting|moons|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2307" PostTypeId="2" ParentId="2304" CreationDate="2013-10-05T07:06:55.037" Score="2" Body="&lt;p&gt;&lt;strong&gt;CRYOGENIC LIQUID PROPELLANTS&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nowadays the most commonly used cryogenic fuel is the Hydrogen($H_2$) (That too in the Para form). It has a boiling point at a temperature of 20.37 K and a freezing point of $\approx$ 13 k so one must be very careful in storing Hydrogen because below certain temperature it would freeze and has a chance of blocking the pipeline(But this is impossible because that the Turbopumps used in the cryogenic rockets are more powerful such that it can break the icy Hydrogen) . Another thing is that all metal(including alloys except few) has an increased performance (such as yield strength, Ultimate strength ) but still the &lt;strong&gt;fatique strength decrease as temperature reached cryogenic temperature, which means that if the metals at the cryogenic temperatures are exposed to harmonic{cyclic} stress will break&lt;/strong&gt;.&#xA;The hydrogen would evaporate at very low temperature and so the pressure in the fuel tank is going to increase so much such that it can explode(very high insulation is required)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;SOLID PROPELLANTS&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are quite reliable but their disadvantage is that it cannot be turn off once it is ignited . &lt;/p&gt;&#xA;&#xA;&lt;p&gt;coming to the &lt;strong&gt;Breeze-M&lt;/strong&gt; they suspect that the bearing in the Turbopump (S5.98) could have caused the problem to the cut off of the engine.I think the reason is since they operate at the cryogenic temperatures the metal would have  become very cold (near the cryogenic temperature) since it runs continuously cycle at high rpm . Their strength would has decreased (since the fatique strength decrease at cryogenic temperature) and metals contract at low temperatures(but in this case metals with very low co-efficient of thermal expansion would be used) as the metal shrinks it causes more friction so it would have jammed as it was too brittle there is a possibility of even fracture&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(the last paragraph of this answer is just only my anologly of possible reasons)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-10-05T07:06:55.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2308" PostTypeId="2" ParentId="2297" CreationDate="2013-10-05T11:48:22.243" Score="3" Body="&lt;p&gt;JSPOC is currently tracking around 22,000 objects, at least, unclassified objects. That number is fairly low actually, compared to the vastness of space. As of writing this post, &lt;a href=&quot;http://www.flightradar24.com/0.22,-27.43/2&quot; rel=&quot;nofollow&quot;&gt;Flight Radar&lt;/a&gt; says there are over 5000 planes in the air, and it's quite early on an East Coast Saturday morning. Also adding in the fact that most planes are larger than satellites, and that satellites are uniformly spread across the globe, while planes aren't, and you've got somewhat of a picture as to how rare space junk is in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, how likely is it that you will have a close encounter with space junk? Not very likely. From my experience, a single satellite will pass within a few hundred meters of a trackable piece of junk only once maybe a few times a year. Then keeping in mind that you actually have to be much closer in velocity to take advantage, and, well, you've got a lot of problems. You might be able to latch on to a larger piece of space junk and use it for some purpose, but the material wouldn't be ideal, and the orbital rendezvous problem is difficult still, although it is manageable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You probably could de-orbit a spacecraft by sticking out some large wings meant to withstand the impact of small space junk, which would slow you down gradually. But the same effect will be had by air resistance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, you're more likely to find use out of fixing old satellites, rather than using them for fuel. Use them for spare parts, putting in new batteries, fuel, etc. But you're probably not going to get anything significant out of them in terms of propulsion.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-05T11:48:22.243" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2309" PostTypeId="1" AcceptedAnswerId="2317" CreationDate="2013-10-05T11:57:21.090" Score="9" ViewCount="1122" Body="&lt;p&gt;I've heard that counting impact craters is a great way to figure out how old the surface of a planet is, but I've never heard what the actual rates are. Do they vary from planet to planet? Anything else that should be noted in crater counting?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-05T22:40:59.673" Title="How can one find the cratering rate for different parts of the Solar System" Tags="|planetary-science|geology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2310" PostTypeId="1" AcceptedAnswerId="3280" CreationDate="2013-10-05T13:01:41.927" Score="13" ViewCount="659" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_debris&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Currently, about 19,000 pieces of debris larger than 5 cm (2.0 in) are tracked, with another 300,000 pieces smaller than 1 cm below 2000 km altitude. For comparison, the International Space Station orbits in the 300–400 km range and both the 2009 collision and 2007 antisat test events occurred at between 800 and 900 km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there is some debris out there - most of it unused artifacts. As definitions go, it is debris because it is not in active use. As most users on this site may be aware - the amount of debris is &lt;a href=&quot;http://www.google.co.in/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;frm=1&amp;amp;source=web&amp;amp;cd=4&amp;amp;cad=rja&amp;amp;ved=0CEUQFjAD&amp;amp;url=http://spectrum.ieee.org/aerospace/satellites/weve-already-passed-the-tipping-point-for-orbital-debris&amp;amp;ei=BgpQUtTID8TirAed8oCgBA&amp;amp;usg=AFQjCNHvSIq2gIhJQ68niMfcG_ognjZtkA&amp;amp;sig2=qqQ4ergibnFRKBSL3Yvk0Q&amp;amp;bvm=bv.53537100,d.bmk&quot; rel=&quot;nofollow noreferrer&quot;&gt;approaching the point&lt;/a&gt; we are concerned about it. This is reflected by questions on the subject on this very site. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1009/how-is-the-international-space-station-protected-against-orbital-debris&quot;&gt;This&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/926/reason-for-space-debris-clustering-in-leo&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/999/space-debris-half-life&quot;&gt;this&lt;/a&gt; - and others. In fact there is a whole tag devoted to debris! The thing is ... what do we do about it? A commonly suggested option is to &lt;a href=&quot;https://space.stackexchange.com/questions/930/any-possible-setbacks-in-deorbiting-larger-space-junk-and-let-it-burn-up-upon-re&quot;&gt;burn it&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I have in mind may be considered naïve, half-baked uh ... lots of adjectives, and adverbs may be here used. There may be logistical issues, and practical problems with the idea - some of which I hope to see in the replies to this post.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, here goes - debris put to use becomes an asset. After all we have &lt;a href=&quot;https://space.stackexchange.com/questions/78/is-it-more-efficient-to-assemble-spacecraft-in-orbit-or-assemble-on-the-ground&quot;&gt;discussed orbital assembly&lt;/a&gt; some on this site.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Can orbital debris be collected, and assembled into a module?&lt;/strong&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;With &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=32006.30&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pir due to be modified&lt;/a&gt;, and &lt;a href=&quot;https://space.stackexchange.com/questions/237/why-decommission-the-iss&quot;&gt;ISS approaching the end of it's operational life&lt;/a&gt; could such eventual debris be modified and made into an &lt;em&gt;unmanned&lt;/em&gt; craft?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Not part of the main question, but perhaps the module, depending upon what we get, (after all, form isn't relevant to space travel at our present level of technology) could be provided with fuel, and other necessary artifacts to augment &lt;a href=&quot;http://www.icarusinterstellar.org/projects/project-icarus/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Icarus&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-08-31T07:24:45.060" Title="Can orbital debris be assembled into a module for space exploration?" Tags="|debris|interstellar-travel|interplanetary|orbital-assembly|wet-workshop|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2311" PostTypeId="2" ParentId="2297" CreationDate="2013-10-05T14:35:48.550" Score="12" Body="&lt;p&gt;Contrary to what seems to be a popular belief in this thread, I'm going to say yes, you could use space junk as your reaction mass. Technically, it would of course be rather challenging, but assuming you could gain more thrust from, oh don't know, maybe superheating metal into plasma and ejecting it by your ion propulsion system, than the momentum you would lose by capturing this mass, you'd be eventually on your way faster than you started with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You assert correctly in your question that reaction mass is expensive to get to orbit, and since there is rather large mass of it there without any function whatsoever, it is natural to ask if it could be reused to any advantage. Indeed, we have many similar questions on our site, and some rather impressive answers for them, too. Now, thinking of how we could transfer all this useless mass into something useful like reaction mass to propel your spacecraft, here are a few options:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Use it as is or break into smaller pieces, and either accelerate metallic pieces you gathered with the help of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Railgun&quot; rel=&quot;nofollow noreferrer&quot;&gt;railgun&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/q/416/49&quot;&gt;hook onto it with tether&lt;/a&gt; to achieve momentum transfer, or repel from the whole piece of the space junk magnetically with the use of strong electrically powered magnetic field generator.&lt;/li&gt;&#xA;&lt;li&gt;Superheat it with the use of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_furnace&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar powered furnace&lt;/a&gt; and use superheated metallic gases as your reaction mass at your exhaust. Alternatively, you could ionise superheated gases and use them as your reaction mass for your &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;ion thrusters&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Reuse remains of propellants in defunct and abandoned upper stages directly and replenishing your fuel and oxidizer tank levels. This is of course assuming same propellant components can be salvaged from space debris that your thrusters are built to use.&lt;/li&gt;&#xA;&lt;li&gt;Launch lighter from the start, and count on collecting some of these space debris to use as your radiation shield, harden your outer shell or otherwise find use for this mass you didn't have to bother launching into orbit and saved a great deal on your payload mass.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Not a complete list at any length, and there are countless other ways provided you have technology and ways to generate required energy, limited by our imagination and technical capabilities to actually exploit this space junk and find some use for it. None of these techniques mentioned would be easy, and you would likely want to identify and track specific pieces of space debris you'd later target to reuse, but short of having to spend more of your inertia to capture this mass than you can later produce trust from it, and of course not unwittingly deorbiting in the process, yes, you could &lt;em&gt;get higher with space junk&lt;/em&gt;. And I don't mean this in psychotropic sense.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-10-05T14:35:48.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2312" PostTypeId="2" ParentId="2239" CreationDate="2013-10-05T16:25:04.247" Score="16" Body="&lt;p&gt;A set of three orthogonally aligned torque rods wired up so they can generate a magnetic dipole field of either sign (i.e. flip the North and South poles) can generate a magnetic field of arbitrary orientation (up to the maximum vector sum of the dipole moment of each rod individually). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This artificial field interacts with the Earth's magnetic field to produce a net external torque on the vehicle that will tend to line up the fields. Mathematically, the torque is provided in the direction of:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$\mathbf{\tau}=\mathbf{\mu}\times\mathbf{B}$, where $\tau$ is the torque on the satellite, $\mathbf{B}$ is the ambient magnetic field, and $\mu$ is the magnetic field of the satellite&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This torque only has two degrees of freedom, i.e. instantaneously the torque rods will tend to align the vehicle and Earth fields, with no control of the rotation of the vehicle around its magnetic poles. (see below for proof)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, remember that the magnetic field lines around the Earth are themselves a dipole field, which is a toroidal shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/G93lD.jpg&quot; alt=&quot;From the Physics page at San Jose State University&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means as the spacecraft orbits the Earth it encounters a diversity of Earth magnetic field orientations and in general the time-averaged effect of this field diversity enables full 3-axis control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the field is weak, so the actual torque produced by torque rods is very small. It's completely unsuitable for agile spacecraft (e.g. imagers), for which attitude control effectors like reaction wheels or control moment gyros are typically used. Torque rods are used to desaturate these effectors which accumulate momentum due to disturbance torques like atmospheric drag, and to de-spin satellites (e.g. due to tip-off rates at launch vehicle separation). &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h3&gt;Proof of the ineffectiveness of torque rods in the absence of magnetic field diversity&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Torque rods rely on the change in direction of the Earth's magnetic field, which is especially problematic in equatorial orbits because (to first order) the field has a constant inertial direction. Equating the control torque with the rigid-body rotational equations of motion:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$$&#xA;\mathbf{\mu}\times\mathbf{B}=\mathbf{\tau}=\mathbf{I\alpha+\omega}\times\mathbf{I\omega}&#xA;$$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;where $\mathbf{I}$ is the moment of inertia tensor, $\mathbf{\omega}$ is the vector body rates, and $\mathbf{\alpha}$ is the vector of body accelerations (i.e. $\dot{\omega}$), all in an arbitrary inertial reference frame. &lt;a href=&quot;http://en.wikipedia.org/wiki/Moment_of_inertia#Principal_axes&quot; rel=&quot;noreferrer&quot;&gt;Moment of inertia matrices are always real and symmetric and can thus be decomposed / rotated into a principle reference frame&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$$&#xA;\mathbf{\mu}\times\mathbf{B}=\mathbf{Q\Lambda Q}^{-1}\mathbf{\alpha}+\mathbf{\omega}\times\mathbf{Q\Lambda Q}^{-1}\mathbf{\omega}&#xA;$$ $$&#xA;\mathbf{RQ\Lambda}^{-1}\mathbf{Q}^{-1}(\mathbf{\mu}\times\mathbf{B})=\mathbf{R\alpha}+\mathbf{RQ}(\mathbf{\Lambda}^{-1}\mathbf{Q}^{-1}\mathbf{\omega}\times\mathbf{Q}^{-1}\mathbf{\omega})&#xA;$$ $$&#xA;\mathbf{R\mu'}\times\mathbf{RB'}=\mathbf{R\alpha}&#xA;$$ $$&#xA;\mathbf{\mu''}\times[0, 0, Bz]=\mathbf{\alpha'}&#xA;$$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;where $\mathbf{\mu}$ and $\mathbf{B}$ are rotated into the principle axes and the matrix math has shown there is no gyroscopic coupling between the axes of rotation in the principle frame because the $ \mathbf{\omega}\times\mathbf{I\omega}$ term cancels. (We could have started at this step by choosing the principle reference frame to start, but many seem comforted by starting with the full EOM.) We've additionally rotated by $\mathbf{R}$, selected so that the Earth's magnetic field only acts in the z-axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vector $\mathbf{\mu}$ is our torque rod control input, which we may point in any direction by combining the effect of three orthogonal rods. This means the double rotation of $\mathbf{\mu}$ to $\mathbf{\mu''}$ has to be accounted for by the control system, but has no bearing on controllability. We can now expand the cross product and show despite the ability to point $\mathbf{\mu}$ in an arbitrary direction, the resultant acceleration (in the frame where the z-axis is aligned to magnetic North) has the form $[kx, kz, 0]$, which shows we can only control the angular accelerations about the x- and y-axes, but never z. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the initial equation $\mathbf{\mu}\times\mathbf{B}$ we know a toque cannot be generated in the direction of $\mathbf{B}$; thus, any initial angular momentum in that direction is uncontrollable, but spacecraft kinematics can be counterintuitive, so sometimes the math is helpful.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="127" LastEditDate="2015-05-24T22:59:17.687" LastActivityDate="2015-05-24T22:59:17.687" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2313" PostTypeId="2" ParentId="2306" CreationDate="2013-10-05T17:00:50.873" Score="8" Body="&lt;p&gt;They took it seriously enough to publish a rebuttal to it 3 years later, as is documented in &lt;a href=&quot;http://en.wikipedia.org/wiki/Phobos_%28moon%29#Shklovsky.27s_.22Hollow_Phobos.22_hypothesis&quot;&gt;Wikipedia&lt;/a&gt;. As is well known today, there was an error in the mathematics. Specifically, Wikipedia quotes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Singer's critique was justified when earlier studies were discovered&#xA;  to have used an overestimated value of 5 cm/yr for the rate of&#xA;  altitude loss, which was later revised to 1.8 cm/yr.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, it was taken quite seriously, but it was discounted rather quickly afterwards.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-06T13:20:08.267" LastActivityDate="2013-10-06T13:20:08.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2314" PostTypeId="2" ParentId="2103" CreationDate="2013-10-05T17:32:56.760" Score="6" Body="&lt;p&gt;You can't send something directly in to an orbit around something without some change in velocity being applied.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, the fuel required to get in to Lunar Orbit, and for that matter, a Lunar Flyby, has been well calculated. &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space_.E2.80.94_high_thrust&quot;&gt;Wikipedia&lt;/a&gt; has these values as they were calculated. LEO to LLO requires 4.04 km/s delta V. A lunar flyby only would require slightly less. I'm not quite sure exactly, but the difference will be less than the delta V required to land after achieving that orbit, so no more than 1.6 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How you would really calculate it is to find the speed of your LEO orbit, then find one such that your new orbit just barely goes beyond the moon after you've done your flyby. This equation is $v=\sqrt{\mu\left({2\over{r}}-{1\over{a}}\right)}$, as is documented at &lt;a href=&quot;http://en.wikipedia.org/wiki/Elliptic_orbit#Velocity&quot;&gt;Wikipedia&lt;/a&gt;. For a circular LEO orbit of, say, 500 km, the initial velocity is $\sqrt{\frac{\mu}{r}}$, or 7.617 km/s. The velocity at the same point for a Lunar orbit is 10.7 km/s. Thus, the delta V required to do a lunar flyby is 10.7-7.617=3.084 km/s&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-05T17:32:56.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2315" PostTypeId="2" ParentId="2103" CreationDate="2013-10-05T19:11:20.050" Score="14" Body="&lt;p&gt;Here's how, using an approximate patched conic technique.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $\Delta V$ using instantaneous (e.g. chemical propulsion) maneuvers can be determined by repeated application of this equation that simply says that the total energy is the sum of the kinetic energy and the potential energy:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}=\frac{v^2}{2}-\frac{\mu}{r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mathcal{E}$ is the total energy per unit mass of the object or the &quot;specific energy&quot;, $v$ is the velocity of the object at the current position, $\mu$ is the GM of the central body, i.e. Newton's gravitational constant times its mass, and $r$ is the current distance from the center of the central body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key is that the total energy of the object is a constant of motion over the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We will also use the fact that orbits are ellipses, and this equation, which determines that constant of motion from the apses of the orbit, i.e. the radii of the closest and farthest points in the orbit, $r_1$ and $r_2$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}=-{\mu\over r_1+r_2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an escape or inbound from escape trajectory:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}={v_\infty^2\over 2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $v_\infty$ is the velocity at infinity relative to the body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this problem we define:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mu_E$ = GM of the Earth.&lt;br&gt;&#xA;$\mu_M$ = GM of the Moon.&lt;br&gt;&#xA;$r_E$ = low Earth orbit radius.&lt;br&gt;&#xA;$r_M$ = low Moon orbit radius.&lt;br&gt;&#xA;$a_M$ = semi-major axis (mean radius) of Moon orbit around Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For simplicity, we will assume that the Moon's orbit is circular, which is not far from the truth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the above, in low-Earth orbit we have for the orbital velocity $v_{LEO}$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$-{\mu_E\over 2r_E}=\frac{v_E^2}{2}-\frac{\mu_E}{r_E}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which gives:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{LEO}=\sqrt{\mu_E\over r_E}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hohmann transfer from the Earth to the Moon is half of an elliptical orbit about the Earth with periapsis $r_E$ and apoapsis $a_M$.  For the velocity $v$ at any radius $r$ in that orbit, we have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$-{\mu_E\over r_E+a_M}=\frac{v^2}{2}-\frac{\mu_E}{r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity in that transfer orbit at the radius of a low-Earth orbit, i.e. its periapsis, is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_p=\sqrt{2a_M \mu_E\over r_E\left(a_M+r_E\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity to leave low-Earth orbit to get on the transfer orbit is then:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V_{inject}=v_p-v_{LEO}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's all you need to fly by the Moon, which is the question title.  Though you ask in the question body how to get into low-Lunar orbit.  You need another maneuver and more propellant to slow down and insert into an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Applying what was done for LEO, the low-lunar orbit velocity $v_{LLO}$ is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{LLO}=\sqrt{\mu_M\over r_M}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly the velocity of the Moon in its orbit around the Earth is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_M=\sqrt{\mu_E\over a_M}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity in the transfer orbit at the radius of the Moon, i.e. its apoapsis, is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_a=\sqrt{2r_E \mu_E\over a_M\left(a_M+r_E\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity relative to the Moon on approach, if the Moon weren't there, is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_\infty=v_M-v_a$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That gives for the velocity on approach when the Moon &lt;em&gt;is&lt;/em&gt; there, for any radius from the Moon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${v_\infty^2\over 2}=\frac{v^2}{2}-\frac{\mu_M}{r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the radius of a low-lunar orbit, that velocity is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_L=\sqrt{\left(v_M-v_a\right)^2+{2\mu_M\over r_M}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To insert into orbit, we need to slow down relative to the Moon by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V_{insert}=v_L-v_{LLO}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total $\Delta V$ is then:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V_{total}=v_p-v_{LEO}+v_L-v_{LLO}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in the numbers, and assuming 200 km LEO and 100 km LLO altitudes, we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V_{inject}=3.13\,\mathrm{km\over s}$&lt;br&gt;&#xA;$\Delta V_{insert}=0.82\,\mathrm{km\over s}$&lt;br&gt;&#xA;$\Delta V_{total}=3.95\,\mathrm{km\over s}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is close to the answer you get when doing a full integration, allowing the lunar gravity to start pulling on the spacecraft well before it gets to the distance of the Moon.  That increases the speed of the spacecraft relative to the Moon a little, increasing the $\Delta V$ to insert a little.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a direct transfer that can complete in days.  If you're willing to take a few months, &lt;a href=&quot;http://astrogatorsguild.com/wp-content/papers/0800_wsb.pdf&quot; rel=&quot;noreferrer&quot;&gt;there are lower $\Delta V$ paths that go through generalized Lagrange points&lt;/a&gt;.  You can save on the order of $0.1\,\mathrm{km\over s}$.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Since this is community wiki, we will include things from &lt;a href=&quot;http://www.braeunig.us/apollo/free-return.htm&quot; rel=&quot;noreferrer&quot;&gt;the relevant link&lt;/a&gt; for a lunar flyby and return.  The mission trajectory goes as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rBYl2.png&quot; alt=&quot;Moon flyby&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the Delta v value:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Prior to TLI the spacecraft is in a low circular parking orbit around Earth. In this example, we have assumed a parking orbit altitude of 185 kilometers and a TLI delta-v of &lt;strong&gt;3,150 m/s&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note, also that this has a certain Lunar altitude associated with it.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Pericynthion is the point in the spacecraft's trajectory that is nearest the Moon. For a free return trajectory, the altitude at pericynthion is typically about 100 to 1,500 nautical miles (185 to 2,800 km)&amp;nbsp;— see diagram. The pericynthion altitude in this example is 1,446 kilometers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you wanted to graze the surface of the moon, then you would need to adjust the parameters.  Fortunately, there are enough to do that.  Variables you can control include:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;the velocity you gain by the burn in LEO&lt;/li&gt;&#xA;&lt;li&gt;the timing at which you do this.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This is assuming everything is in a 2D plane, as in the above image.  Things you need to control in order to do the mission (and avoid death) include:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;the minimum altitude above the moon&lt;/li&gt;&#xA;&lt;li&gt;the impact location on Earth&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I list these because the number of variables you can control are equal to the number of variables you need to control.  This proves that any given flyby distance is attainable, but it also proves you need to adjust the burn time to achieve that.  So in order to approach the moon more closely, you would need to change your propellant requirement.  Whether that would make it more or less, I do not know.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-03-09T03:45:26.493" LastActivityDate="2014-03-09T03:45:26.493" CommentCount="11" CommunityOwnedDate="2013-10-05T20:41:26.807" ContentLicense="CC BY-SA 3.0" />
  <row Id="2316" PostTypeId="2" ParentId="2310" CreationDate="2013-10-05T21:28:19.227" Score="7" Body="&lt;p&gt;They could be, in theory. Let's talk about some of the levels by which this could be done:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;If you had the right equipment, you could melt it and create whatever you wanted. However, if you are just using it a raw resources, why not go to an asteroid?&lt;/li&gt;&#xA;&lt;li&gt;If you want to use some of the resources as is, you could probably have some use. What would it take? Well, you might get something like solar cells, but they usually are fairly cheap, and as a whole, not that difficult to launch. More complex things would be difficult to re-use in orbit, but some things could.&lt;/li&gt;&#xA;&lt;li&gt;Large structures have been discussed. The ISS could no doubt be in large part recovered, for instance. Tanks have been discussed for a habitat, as &lt;a href=&quot;http://www.permanent.com/ext-tank.htm&quot;&gt;@john3103 mentioned&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-05T21:28:19.227" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2317" PostTypeId="2" ParentId="2309" CreationDate="2013-10-05T22:40:59.673" Score="9" Body="&lt;p&gt;In terms of using cratering as a means to determine at least an estimate of the age of the planet, there are some theorised commonalities.  In particular, according to the Lunar and Planetary Sciences webpage &lt;a href=&quot;http://www.lpi.usra.edu/education/explore/shaping_the_planets/impact_cratering.shtml&quot; rel=&quot;noreferrer&quot;&gt;Shaping the Planets: Impact Cratering&lt;/a&gt;, state a couple of general principles:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;older impact basins are larger than the more recent craters. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They are also more eroded/destroyed on planets that have appreciable atmospheres or volcanic activity, such as Venus, Earth, Titan and Triton (there are more examples of course).  On planets that a don't have these, a general rule is &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;older surfaces have more impact craters. Mercury and the Moon are covered with impact craters; their surfaces are very old. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On planets where there has been, for whatever reason, some resurfacing, &lt;a href=&quot;http://www.geoinf.fu-berlin.de/publications/EPSL/EPSL_Michael.pdf&quot; rel=&quot;noreferrer&quot;&gt;Planetary surface dating from crater size–frequency distribution measurements:&#xA;Partial resurfacing events and statistical age uncertainty&lt;/a&gt; (Michael and Neukum, 2010):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The concept is to fit the observed crater size–frequency distribution&#xA;  (SFD) of a given surface unit to a known crater production&#xA;  function (PF), and to use the crater frequency for certain crater sizes&#xA;  together with a calibrating chronology function (CF) to obtain an&#xA;  absolute age.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The absence of a particular diameter crater(s) can also determine when a resurfacing event has occurred, particularly volcanism and even the presence of a stronger erosion/depositional cycle at certain times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To clarify this, an example about the cratering around the time and after the eruptions of Mars's Olympus Mons have been summarised in the webpage &lt;a href=&quot;http://nixolympica.weebly.com/&quot; rel=&quot;noreferrer&quot;&gt;Olympus Mons&lt;/a&gt;, specifically:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The theory behind crater retention age dating is that more densely cratered surfaces are older than sparsely cratered surfaces because of differences in exposure to bolide impacts (Hartmann, 2005). &lt;strong&gt;Numerical ages from craters have been determined from cratering rate frequencies and dated Lunar rock samples&lt;/strong&gt; (Hartman, 2005). From this lunar data, isochrones for various bins of crater diameters over a given areal extent have been established for Mars (e.g. Hartmann, 2005).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As shown in the diagram (and example below):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/j4Y0O.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reference used for this explanation and the image is from:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hartmann, W.K., 2005. Martian cratering 8: Isochron refinement and the chronology of Mars. Icarus, 174, 294-320.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why base it on the lunar samples?  It is because we have been there and have sampled the data and made the age-size-frequency graph in the diagram above.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-10-05T22:40:59.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2318" PostTypeId="1" CreationDate="2013-10-06T01:11:03.813" Score="12" ViewCount="1516" Body="&lt;p&gt;How much time is needed to an astronaut on ISS to prepare herself for an EVA? Is this time so much different from time needed during Apollo missions?&lt;/p&gt;&#xA;" OwnerDisplayName="user55" LastEditorUserId="49" LastEditDate="2013-10-06T13:19:55.697" LastActivityDate="2023-05-30T13:58:19.293" Title="Time to prepare for EVA out of ISS" Tags="|iss|apollo-program|spacesuits|eva|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2319" PostTypeId="2" ParentId="1480" CreationDate="2013-10-06T07:06:18.467" Score="2" Body="&lt;p&gt;The rocks on the surface of the Earth are formed by the sedimentary (because of the erosion of the rocks by the water and the atmospheric winds) while in the moon it is only igneous (&quot;fire-formed rocks&quot; which are not eroded by anything since it has no atmosphere) so it must be so hard when compared to that on earth&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://csep10.phys.utk.edu/astr161/lect/moon/moon_surface.html&quot; rel=&quot;nofollow&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The bulk density of the Moon is 3.4 g/cc, which is comparable to that of (volcanic) basaltic lavas on the Earth (however, the bulk density of the Earth is 5.5 g/cc, because of the dense iron/nickel core). The Moon is coverered with a gently rolling layer of powdery soil with scattered rocks that is called the regolith; it is made from debris blasted out of the Lunar craters by the meteor impacts that created them. Each well-preserved Lunar crater is surrounded by a sheet of ejected material called the ejecta blanket. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;1.The Maria are mostly composed of dark basalts, which form from rapid cooling of molten rock from massive lava flows. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;2.The Highlands rocks are largely Anorthosite, which is a kind of igneous rock that forms when lava cools more slowly than in the case of basalts. This implies that the rocks of the Maria and Highlands cooled at different rates from the molten state and so were formed under different conditions. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.Breccias, which are fragments of different rocks compacted and welded together by meteor impacts, are found in the Maria and the Highlands, but are more common in the latter. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;4.Lunar Soils contain glassy globules not commonly found on the Earth. These are probably formed from the heat and pressure generated by meteor impacts. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Anorthosites that are common in the Lunar Highlands are not common on the surface of the Earth (The Adirondack Mountains and the Canadian Shield are exceptions). They form the ancient cores of continents on the Earth, but these have largely been obliterated by overlying sedimentary deposits and by plate tectonic activity. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;External source&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Moon_rock&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Moon_rock&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-10-06T07:06:18.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2320" PostTypeId="1" AcceptedAnswerId="37136" CreationDate="2013-10-06T10:31:05.903" Score="9" ViewCount="1556" Body="&lt;p&gt;I've recently seen movie Gravity. The protagonist detaches from the shuttle and is hurled into space. The mission commander then asks her for GPS coordinates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This happenned while they were fixing Hubble Space Telescope. But later they board ISS. I'm not really sure about height of those two objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So is it possible to use GPS there? Disregarding the fact, that coordinates would be probably quite off...&lt;/p&gt;&#xA;" OwnerUserId="853" LastActivityDate="2019-07-11T11:50:47.383" Title="Does GPS work at ISS?" Tags="|iss|gps|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2321" PostTypeId="2" ParentId="2318" CreationDate="2013-10-06T10:53:17.013" Score="2" Body="&lt;p&gt;Some of it depends on which space suit they will use.  There are two sets of suits at the station.  The Russian Orlan suits and the US suits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US suits require customization to lengthen/shorten bits so it fits properly.  Earlier suits were customized per use on Shuttle flights and took hours or days by trained technicians to adjust. The current suits on the station are upgrades that are easier and faster but still take some time to adjust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So first EVA by an astronaut in a US suit takes longer than the second, due to this issue.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="414" LastEditDate="2013-10-06T12:58:16.950" LastActivityDate="2013-10-06T12:58:16.950" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2323" PostTypeId="1" AcceptedAnswerId="2740" CreationDate="2013-10-06T12:03:07.277" Score="10" ViewCount="429" Body="&lt;p&gt;The Nili Patera region has enjoyed fairly good media exposure recently, ever since high detail &lt;a href=&quot;http://hirise.lpl.arizona.edu/&quot; rel=&quot;nofollow noreferrer&quot;&gt;HiRISE&lt;/a&gt; (High Resolution Imaging Science Experiment) photographs hit the Internet, showing fabulous sand dunes to the south of the region in all their glory. Just to refresh your memory, or in case these photographs didn't reach you, here's a small preview of these aeolian formations, areology-wise short-lived migratory topological features of the Syrtis Major Planum:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/u8lkd.jpg&quot; alt=&quot;Nili Patera dunes&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nili Patera dunes, while indeed lovely, are however hardly an unexplained phenomena.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at results of some &lt;a href=&quot;https://en.wikipedia.org/wiki/2001_Mars_Odyssey&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Odyssey&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_Emission_Imaging_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;THEMIS&lt;/a&gt; (Thermal Emission Imaging System) observations though, I've noticed something rather peculiar within the impact craters in the otherwise sandless North by Northeastern part of the Nili Patera region. Seemingly regular shaped protrusions, straight or only slightly bent ridges nearly parallel to each other that appear almost as if, tongue-in-cheek, Martians were keeping a hand-drawn score on how many times Voyager 1 was believed to have left the Solar system. Smaller craters have one straight ridge in them, slightly bigger ones maybe two to three, and larger ones - albeit still relatively small craters on global Martian scale - three or more of these ridges. This is what I'm trying to describe:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/VLfDK.png&quot; alt=&quot;Nili Patera craters&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Cropout of the larger THEMIS image of the Nili Patera dunes. Source: &lt;a href=&quot;http://themis.mars.asu.edu/zoom-20101126a&quot; rel=&quot;nofollow noreferrer&quot;&gt;Arizona State University THEMIS data, Image ID V39109006&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These within impact crater &lt;em&gt;ripples&lt;/em&gt; only appear in the roughly 2 km depressed region of Nili Patera caldera, and the craters on the top plain surrounding this volcanic caldera bed appear clean from this phenomena. My first impression was these are smaller sand dunes, remains of aeolian sedimentation from the dust blowing from the higher plain onto Nili Patera and keeping some of this sand trapped within the even more depressed impact craters, but I later realized THEMIS images are actually recording heat signature, and sand would have to appear on average darker, just like the sand dunes to the South do on the same &lt;a href=&quot;http://themis.mars.asu.edu/zoom-20101126a&quot; rel=&quot;nofollow noreferrer&quot;&gt;source image&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did a bit of digging around to answer my question, and indeed found somewhat related &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2012/11081607-hiwish-butterfly-crater.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;blog post by Emily Lakdawalla&lt;/a&gt;, after one of her HiWish requests for HiRISE to focus on one of the Nili Patera butterfly (low-angle impact) elliptical craters has been granted and results came in. Sadly though, that particular crater doesn't show same ridges in it like the ones I'm inquiring about. While there is a central &lt;em&gt;scar&lt;/em&gt; in it along the path of the impacting meteor, it is not nearly as seamless as the ones from the THEMIS image appear to be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KubJ0.jpg&quot; alt=&quot;Nili Patera butterfly crater&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;HiRISE imaging of the butterfly crater in the Nili Patera region. Credit: NASA / JPL / UA / Emily Lakdawalla&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, failing to find an answer on my own, here's my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is causing these seemingly seamless and regular shaped protrusions within the impact craters on the Nili Patera caldera? What is the story behind their formation, and do we have any physical models that would explain their shape and number?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/5VN4d.png&quot; alt=&quot;Nili Patera dunes and craters&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorDisplayName="user12" LastEditDate="2013-11-04T22:48:02.137" LastActivityDate="2013-11-08T22:06:33.077" Title="Any explanation for morphology of within crater formations on the northern Nili Patera caldera of the Syrtis Major volcanic plain of Mars?" Tags="|planetary-science|impact|mars|crater|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2324" PostTypeId="2" ParentId="2294" CreationDate="2013-10-06T14:42:49.363" Score="3" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wwcg9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Hydrogen breathing jet engines&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the gravity of of Jupiter is three time stronger than Earth so we need to create three time the lift that we create in earth to lift the craft. But hydrogen fueled aircraft are more powerful and we use then to attain hypersonic flight in Earth. The main problem with this is that materials at such high temperature would make the metals brittle. But creating lift in high Reynolds number flow is very hard and  because lift coefficient is inversely proportionaly to Reynolds number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Hydrogen breathing rocket engine&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One can save the amount of materials such as insulation, tank but that it will be compensated to some extent by the following&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Instruments required to Increase  of Hydrogen and simultaneously reducing the temperature of Hydrogen&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If you use Fluorine tho it may react with Hydrogen to produce Hydrogen Fluoride which is very toxic and burn to burn one mole of Hydrogen requires one mole of Fluorine (Oxygen requires one mole to burn two mole of Hydrogen )&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;you need lot of thrust to take off which means we need to have solid booster and burn more hydrogen to accelerate .&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The exhaust velocity is directly proportional to the ratio of pressure inside and outside of  chamber pressure . Since the atmospheric pressure of  Jupiter is high, so the pressure of combustion must be very high  so chamber must be heavy to withstand such high pressure &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-11-21T15:46:57.047" LastActivityDate="2013-11-21T15:46:57.047" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2325" PostTypeId="1" AcceptedAnswerId="2332" CreationDate="2013-10-06T19:34:05.113" Score="10" ViewCount="272" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29&quot;&gt;Juno&lt;/a&gt; is flying by by the planet Earth on October 9, 2013. Will it be visible to the naked eye anywhere on Earth, and if so, where and when?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-07T00:08:07.550" LastActivityDate="2013-10-08T19:14:43.793" Title="Will Juno be visible during its flyby?" Tags="|observation|flyby|juno|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2326" PostTypeId="2" ParentId="2245" CreationDate="2013-10-06T19:49:24.027" Score="19" Body="&lt;p&gt;Horsh answered for the Russian side of the station very.  But the question was more about the US side of the station.&lt;/p&gt;&#xA;&lt;p&gt;The US side of the station does need more ports.  It looks like a fair bit of reconfiguration will be performed of the current modules to free up spots. (Edit: Early 2016 this reconfiguration was complete. CRS-7 with IDA-1 was lost on launch. IDA-2 will be launched by CRS-9).&lt;/p&gt;&#xA;&lt;p&gt;NASA desires to have two of each type of port available as a backup.  Thus they would like two CBM ports available when cargo flights arrive, in case of an issue with one port, to allow berthing regardless.  The same is true for the docking ports for manned flights.&lt;/p&gt;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.nasaspaceflight.com/2013/07/nasa-planning-module-relocations-future-vehicles/&quot; rel=&quot;noreferrer&quot;&gt;great article discussing the reconfiguration&lt;/a&gt; explains how this will be handled.&lt;/p&gt;&#xA;&lt;p&gt;The PMM (Permanent Multipurpose Module) which is currently on the earth facing (nadir) side of Node 1 will be moved to the forward port of Node 3.  (Which would put it parallel to the US Lab module).  Interestingly, the Bigelow Expandable Activity Module (BEAM) will be on the aft port of Node 3 (facing the Russian segment).   (Edit: This has happened).&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/45iA2.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;That is, the PMM, facing down in the photo above, will move onto Node 3 such that it is parallel to the US Lab (photo below). (Edit: This has happened.) You can also see in the image below the 2 PMAs on Node 2 (one facing forward, one facing upwards):&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ci3PK.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;That will leave the Node 1 nadir, Node 2 (where the PMA is currently docked facing forward) nadir (earth facing) and zenith (space facing) CBM ports available.&lt;/p&gt;&#xA;&lt;p&gt;Then PMA-3 which is currently unusable on Node 3, will be moved to Node 2 Zenith. (Edit: This has happened).&lt;/p&gt;&#xA;&lt;p&gt;Thus the ports available will be:&lt;/p&gt;&#xA;&lt;h1&gt;Docking ports&lt;/h1&gt;&#xA;&lt;p&gt;Node 2 (where the shuttle used to dock, at the front end of the station) will have a PMA facing away from Earth, and a PMA facing forward.  Those will be available for manned crew vehicles, which the current schedule only ever has a single one at station at a time. (Indirect handoff, instead of perhaps both docked at once, and one leaving after a week for the handoff). These PMA's will be upgraded with a new docking ring, called an IDA (International Docking Adapter) to meet the NDS (Nasa Docking System) standard that all currently planned manned vehicles will be using.  (Amusingly, Dragon will be delivering the two IDA adapters, and the BEAM module to the station).&lt;/p&gt;&#xA;&lt;h1&gt;Berthing Ports&lt;/h1&gt;&#xA;&lt;p&gt;Then the Node 2 nadir and Node 1 nadir will be available as CBM ports for berthing cargo vehicles. This is preferred over using the nadir and zenith ports on Node 2, as using the top and bottom of the same module would require the CanadaArm2 to berth only to the nadir side (Capture box is below the station, not above it) disconnect, change its grip on the station to then re-position the vehicle from nadir port to the zenith port.  This would then require that the vehicles depart in order as well, restricting access more than NASA would like.&lt;/p&gt;&#xA;&lt;p&gt;A Japanese HTV used the Zenith Node 2 CBM port. It was moved there when a shuttle mission was scheduled and the tail of the shuttle would get too close to the HTV for comfort.&lt;/p&gt;&#xA;&lt;p&gt;Currently the lack of ports is being handled by restricting access to the station to one vehicle at a time. The reconfiguration will allow two cargo vehicles and two crew vehicles at once, but may not be used in the crew configuration.&lt;/p&gt;&#xA;&lt;p&gt;But the restriction on access is already starting to cause issues as Dragon, Cygnus, and HTV all share the CBM on Node 2 nadir right now for cargo, and thus scheduling becomes a complex dance with many dependencies and a delay in one flight ripples down the manifest for years.&lt;/p&gt;&#xA;&lt;p&gt;Before the CRS-7 loss of mission, the plan was to have a Cygnus and Dragon on station at the same time, using the two nadir CBM ports.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-13T12:27:53.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2327" PostTypeId="1" CreationDate="2013-10-06T22:29:34.167" Score="8" ViewCount="715" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_spacecraft#Galileo_Probe&quot;&gt;Galileo Probe&lt;/a&gt; did not include any imaging equipment to return images or video as it descended into Jupiter's atmosphere. Are there missions underway or planned that will include visual data transmitted during a descent into the atmosphere of Jupiter or any other gas giant?&lt;/p&gt;&#xA;" OwnerUserId="79" LastActivityDate="2014-01-08T08:18:22.677" Title="Are there any gas giant probes planned that will return images or video?" Tags="|probe|jupiter|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2328" PostTypeId="1" CreationDate="2013-10-06T22:51:41.367" Score="13" ViewCount="543" Body="&lt;p&gt;According to Geology.com's web-article &lt;a href=&quot;http://geology.com/articles/active-volcanoes-solar-system.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Active Volcanoes of Our Solar System&lt;/a&gt;, there are several volcanically active regions in the solar system asides from the Earth.  In particular, the satellites Europa and Ganymede (Jupiter), Enceladus and Titan (Saturn) and Triton (Neptune) exhibit a type of volcanism known as &lt;strong&gt;cryovolcanism&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cryovolcanism is defined by NASA as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Eruption of water or other liquid or vapor-phase volatiles (collectively referred to as &quot;cryomagma&quot;), together with gas-driven solid fragments, onto the surface of a planet or moon due to internal heating. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/J1OW7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www4.nau.edu/meteorite/Meteorite/Book-GlossaryC.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Image and definition source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any research that investigates how viable cryovolcanism would be as a power source to a future colony?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does the energy output of cryovolcanoes compare to 'normal' magmatic volcanoes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note: I have read the thread &lt;a href=&quot;https://space.stackexchange.com/questions/1193/what-types-of-sustainable-energy-are-available-beyond-mars&quot;&gt;What types of sustainable energy are available beyond Mars?&lt;/a&gt; and are looking at cryovolcanism in particular&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="-1" LastEditorDisplayName="user838" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-10-27T05:07:54.697" Title="Is solar system active cryovolcanism a potential viable power source for future colonies?" Tags="|colonization|energy|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2329" PostTypeId="2" ParentId="1940" CreationDate="2013-10-07T00:27:24.887" Score="9" Body="&lt;p&gt;It isn't known exactly, as we don't have a good way of tracking them. However, this question was brought up in the form &lt;a href=&quot;http://www.unmannedspaceflight.com/lofiversion/index.php/t3375.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;unmannedspaceflight.com&lt;/a&gt; as @Hash mentioned, which mentions a &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_artificial_objects_escaping_from_the_Solar_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt;. Bottom line, we believe there are 11 artifacts that are leaving the Solar System that we launched:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5 probes (Pioneer 10, 11, Voyager 1, 2, New Horizons), and 4 booster stages (All but Pioneer 11), plus two &lt;a href=&quot;http://en.wikipedia.org/wiki/Yo-yo_de-spin&quot; rel=&quot;nofollow noreferrer&quot;&gt;spin weights&lt;/a&gt; from New Horizons. Of these, we can confirm that all of the spacecraft will leave the solar system, and only one of the booster stages (New Horizons), and the two spin weights. The other booster stages can't be confirmed, as we lost contact prior to the flyby that would put them on a solar system eject route.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of the unknown 3 booster stages, we can make a few guesses. Pioneer 10 was about 132,000km above Jupiter's atmosphere. If the booster stage passed further, then it would be further away, and closer, it could be closer. My guess is that it passed further away, which would put it on a fairly slow path leaving the solar system, but roughly in the same direction as Pioneer 10.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Voyager 2 rocket stage should be leaving the solar system, but at a much slower pace than Voyager 2 itself. It will be headed most likely at a bit of an angle, but in general will be tending towards the direction that Voyager 2 left from Jupiter, albeit much slower. The same thing can be said of Voyager 2's upper stage as well.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6834" LastEditorDisplayName="user29" LastEditDate="2020-01-10T18:57:48.720" LastActivityDate="2020-01-10T18:57:48.720" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2330" PostTypeId="2" ParentId="2295" CreationDate="2013-10-07T01:06:56.040" Score="9" Body="&lt;p&gt;A chapter from the reference 'Colonies in Space' (Heppenheimer, 1977, 2007): &lt;a href=&quot;http://www.nss.org/settlement/ColoniesInSpace/colonies_chap09.html&quot;&gt;Up on the Farm&lt;/a&gt; discusses as the chapter title suggests - farming the food needs of any colony.  Specifically, they discuss which type of livestock is the most viable given constraints of space and feed, specifically for meat,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The colonists will also want meat and this poses a problem. Is it feasible to raise meat in the space farm, when space in the farm is at such a premium? The ideal animal must have high productivity. For instance, if you have a herd of cattle, only 20 percent of its mass can be harvested as meat per year. But chickens and rabbits reproduce so fast and grow so rapidly that a herd of either animal can produce five times its initial weight in edible meat over a year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And for dairy:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is the question of which ruminant to choose. The two most common milk-producing ruminants are the cow and the goat. Cows weigh ten times as much as goats and eat ten times as much feed. But a cow will produce only four times as much milk as a goat. For a given amount of feed, a goat will produce more than twice as much milk as a cow.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So essentially, cows are out, goats chickens and rabbits are in.  This is mainly due to the space required to not only house them, but the space required to grow specific food for them.  This in a way, rules out domesticated pets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there is a twist to the final point about space colony pets, the article &lt;a href=&quot;http://ezinearticles.com/?Spiders-Will-Make-Great-Pets-for-Space-Colonists&amp;amp;id=6408683&quot;&gt;Spiders Will Make Great Pets for Space Colonists&lt;/a&gt; (Winslow), suggests spiders may make great pets - they've been taken in space before and did fine (maybe not for everyone).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of waste plant material could be decomposed by the use of fungi (which are neither plant nor algae), namely mushrooms, according to the thesis &lt;a href=&quot;http://digital.library.adelaide.edu.au/dspace/bitstream/2440/61315/3/02chapters1-4.pdf&quot;&gt;Colonization: A Permanent Habitat for the Colonization of Mars&lt;/a&gt;, which in itself would provide another food source.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorDisplayName="user838" LastEditDate="2013-10-07T02:17:53.120" LastActivityDate="2013-10-07T02:17:53.120" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2331" PostTypeId="2" ParentId="2328" CreationDate="2013-10-07T12:13:53.023" Score="4" Body="&lt;p&gt;I think it's viable to produce power from cryovolcanism since they eject very cold liquid or plumes at high pressure(or we can drill a pipes deep and connect the output to turbine but the turbine will  become brittle and break) we can make a turbine to rotate as the fluid is ejected and the turbine is connected to a generator which generates electricity by &lt;a href=&quot;http://en.wikipedia.org/wiki/Electromagnetic_induction&quot; rel=&quot;nofollow&quot;&gt;electromagnetic induction&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the main problem is that it needs to work at cryogenic temperatures. So the materials would become brittle and may fracture under such cyclic force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But using the temperature and pressure  under the ground we can make the geothermal energy by drilling deep pipes to heat the water and convert it to steam and run the turbine. But the main issue here is that water would freeze before it is pumped to hot rocks so it must be preheated and another thing is that even though it's heated to steam as it moves up its losses it temperature and eventually become water so you cannot run the turbine. The pipes must be insulated to maximum extent. &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-10-07T15:53:18.990" LastActivityDate="2013-10-07T15:53:18.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2332" PostTypeId="2" ParentId="2325" CreationDate="2013-10-07T12:40:29.937" Score="6" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/i51WQ.png&quot; alt=&quot;source:universetoday&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/AoVB5.png&quot; alt=&quot;Source:Universetoday&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image shows the trajectory of the Juno spacecraft. It can be seen that the closest approach takes place in the shroud of Earth's shadow, so most of the world is going to miss this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.universetoday.com/104695/spotting-juno-nasas-jupiter-bound-spacecraft-gets-a-boost-from-earth-on-october-9th-2013/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Two minutes before closest approach&#xA;  Juno will experience the only eclipse of&#xA;  its mission, passing into the umbra of&#xA;  Earth’s shadow for about 20 minutes.&#xA;  Chris Peat at Heavens-Above also told&#xA;  Universe Today that observers in India&#xA;  are also well-placed to catch sight of&#xA;  Juno with binoculars after it exits the&#xA;  Earth’s shadow. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user29" LastEditDate="2013-10-07T16:51:17.417" LastActivityDate="2013-10-07T16:51:17.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2333" PostTypeId="1" CreationDate="2013-10-07T13:53:43.207" Score="8" ViewCount="145" Body="&lt;p&gt;Juno will fly by Earth on October 9, 2013. There is &lt;a href=&quot;http://www.jpl.nasa.gov/hijuno/&quot;&gt;an event&lt;/a&gt; to encourage Amateur Radio Operators to say high to the satellite. What I'm trying to do is figure out if I have any chance of pulling it off. In order to do that, I need to know the distance to Juno at a particular moment in time. How might I best be able to figure this out?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-07T14:56:37.740" Title="How can I figure out the distance to Juno at any point in time?" Tags="|orbital-mechanics|trajectory|amateur-radio|flyby|juno|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2334" PostTypeId="2" ParentId="2333" CreationDate="2013-10-07T14:16:58.693" Score="3" Body="&lt;p&gt;The event web-site you posted shows the&lt;a href=&quot;http://www.jpl.nasa.gov/hijuno/images/EFB_publicmap1.png&quot; rel=&quot;nofollow&quot;&gt;ground-track&lt;/a&gt;, and a &lt;a href=&quot;http://missionjuno.swri.edu/hijuno/static/pdf/Juno_EFB_Waves_Ham_Radio_Headings_00.pdf&quot; rel=&quot;nofollow&quot;&gt;list of cities&lt;/a&gt; to orient a directional antenna . &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://missionjuno.swri.edu/news/status-20130916&quot; rel=&quot;nofollow&quot;&gt;This site&lt;/a&gt; also writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;See Juno’s current position, speed and more via NASA’s Eyes on the Solar System 3D interactive. &lt;a href=&quot;http://bit.ly/junoeotss&quot; rel=&quot;nofollow&quot;&gt;Launch the Juno module&lt;/a&gt; or view Juno in the standard &lt;a href=&quot;http://eyes.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;Eyes on the Solar System&lt;/a&gt; interface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Hope this helps! I'm QRU unfortunately )+: &lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-10-07T14:16:58.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2335" PostTypeId="2" ParentId="2328" CreationDate="2013-10-07T14:19:20.767" Score="7" Body="&lt;p&gt;Harnessing the Cryovolcano itself would be rather difficult - just like trying to generate power from a terrestrial volcano - the area is usually unstable, eruptions are not continuous or consistently the same force, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Geothermal heat might be useful - we drill down through the icey-crust into the warm(er) watery 'mantle', run some pipe, and pump down liquid helium/nitrogen. That liquid boils, and can power a turbine in much the same way steam-turbines work. The big problem is finding a way to cool/condense the gas into a liquid effectively. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's LOTS of issues with finding materials that are stable at cryogenic temperatures, but the basic concept should be workable.  The mechanical aspect would need to be designed for the phase-change temp-range of whatever liquid you chose, which in-turn, depends on just how cold the target planet is. I'm not that smart, and the suggestion of liquid helium/nitrogen is not much more than a guess on my part. (Both are non-corrosive and well-understood industrial products, so they'd be good candidates.)&lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="692" LastEditDate="2013-10-07T15:27:28.720" LastActivityDate="2013-10-07T15:27:28.720" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2336" PostTypeId="2" ParentId="2333" CreationDate="2013-10-07T14:56:37.740" Score="5" Body="&lt;p&gt;The &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL HORIZONS&lt;/a&gt; website is what you want for precise, tabulated values. Using the web interface, you can specify your position, and get all the info you could need for a given body (in your case, observer-centric (?) range to JUNO). It will also give you relevant parameters if you wanted to try and (visually) observe it, such as pointing angles, and the Sun-Spacecraft-Observer angle.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-10-07T14:56:37.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2337" PostTypeId="1" AcceptedAnswerId="2341" CreationDate="2013-10-07T17:42:03.493" Score="15" ViewCount="6754" Body="&lt;p&gt;I heard somewhere about a possible way to mantain power plants outside Earth. To transport energy, we can send it into space as electromagnetic waves through satellites to reuse them later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may work with solar panels in space, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it even possible? How can we convert electromagnetic waves into electric energy - if we can?&lt;/p&gt;&#xA;" OwnerUserId="306" LastActivityDate="2017-12-12T20:05:59.100" Title="Can we transport energy through space?" Tags="|energy|transport|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2338" PostTypeId="2" ParentId="2337" CreationDate="2013-10-07T17:56:04.757" Score="3" Body="&lt;p&gt;Solar-Power cells do exactly this. Light is an electromagnetic wave that can be converted into electricity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a larger scale, we could build a giant space-laser to beam concentrated light at a particular point. As others have pointed-out, this is solidly in the realm of Sci-Fi right now. There are major hurdles preventing such a project, engineering a &quot;Giant Space Lazer&quot; isn't easy, and even with the best focusing, any sort of interplanetary transmission will get diffused given the huge distances. Not to mention you don't want to miss-aim the thing and fry a small city. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not to mention it might be easier to simply generate power on-site rather than trying to 'beam' it somewhere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Microwave_transmission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia has an article with details on the actual real-world use of Microwave transmissions (another electromagnetic wave) for power&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="6816" LastEditDate="2017-09-12T19:36:45.457" LastActivityDate="2017-09-12T19:36:45.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2339" PostTypeId="1" CreationDate="2013-10-07T18:41:13.060" Score="26" ViewCount="5315" Body="&lt;p&gt;Many systems have been discussed where you have a rotating spacecraft for artificial gravity purposes, with a portion of the spacecraft that doesn't rotate, perhaps the engine for instance. I've been struggling in my mind to picture how you can put in an interface between the two sections that will allow people to go through without being in a vacuum suit between the two sections, and still be air tight. How would this work in a real operation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For bonus points, if you could add in how to get power/other cables between the two sections, I would be interested in that as well.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-07T18:53:27.197" LastActivityDate="2020-10-01T05:14:08.630" Title="How can you make a vacuum proof rotatable seal?" Tags="|space-station|artificial-gravity|rotation|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2340" PostTypeId="2" ParentId="2337" CreationDate="2013-10-07T19:40:17.153" Score="12" Body="&lt;p&gt;Yes, you can transport energy through space.  That's how we get energy from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe that what you saw was referring to the use of solar power satellites to provide electrical energy for use in Earth power grids.  Solar energy collected in space has the potential advantage to be independent of the day-night cycle, as opposed to solar energy plants on Earth.  That energy would be sent to receivers on Earth using microwaves.  The approach usually promoted would limit the energy density of the beams to be on the order of that of sunlight, and so would not be terribly dangerous.  The collectors on Earth would then need to be about the same size as a solar collector for the same power, but would be much cheaper and would collect the same power all the time.  All that is needed is a mesh of wires closer to each other than the microwave wavelength (a few cm).  In fact such a mesh could be mounted above a ground solar power plant, or crops, or most anything and would not significantly block the sunlight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether a space solar power scheme could ever be cost effective is an entirely different question.  But the power beaming part is technically feasible with existing technology, and at low energy density could plausibly survive regulatory approval.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-10-07T21:00:55.833" LastActivityDate="2013-10-07T21:00:55.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2341" PostTypeId="2" ParentId="2337" CreationDate="2013-10-07T19:49:33.890" Score="13" Body="&lt;p&gt;There are several ways to transfer energy through long distances of hard vacuum and even the atmosphere wirelessly. What Nikola Tesla was doing back in the pioneering days of &lt;a href=&quot;https://en.wikipedia.org/wiki/Wireless_power&quot; rel=&quot;noreferrer&quot;&gt;wireless power&lt;/a&gt; wouldn't really work, because &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrodynamic_induction&quot; rel=&quot;noreferrer&quot;&gt;electrodynamic induction&lt;/a&gt; transfers lose too much of their efficiency over distances much larger than one sixth of the wavelength. And accounting for atmospheric resistance, these distances should probably be even a lot shorter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But short of actually transporting stored energy physically in charged &lt;a href=&quot;https://en.wikipedia.org/wiki/Galvanic_cell&quot; rel=&quot;noreferrer&quot;&gt;galvanic cells&lt;/a&gt; (i.e. batteries) even if those came at far greater than currently achievable capacity per their mass, maybe by using carbon nanotubes as charge carriers similar to what is currently being developed for mobile applications, or even transporting charged plasma, all of that would most definitely incur net loss in required energy to establish such pathways compared to gains in transported energy. So here are a few other ways, with no transport required (short of building the transmitter and receiver end, of course):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Microwave&lt;/strong&gt;: Powerful microwave wavelength radio transmitters could be orbited in GEO (Geostationary Orbit) and target a ground based &lt;a href=&quot;https://en.wikipedia.org/wiki/Rectenna&quot; rel=&quot;noreferrer&quot;&gt;rectenna&lt;/a&gt; (rectifying antenna) that would convert microwave energy into direct current electricity.&lt;br /&gt;&lt;br /&gt;Problem with this approach is, that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Diffraction&quot; rel=&quot;noreferrer&quot;&gt;diffraction&lt;/a&gt; is too great due to interference pattern forming at the transmitter end increasing the radius of the receiver end with distance and angle of diffraction. The shorter the wavelength however, the smaller the required size of the transmitter and the rectenna, their required size ratio, and lower absorption and refraction of the microwave beam in the atmosphere. The biggest problem however is with the required rectenna size increasing by the distance from the beam's source, so while GEO sounds simple enough in terms of targeting a particular area on the surface, it is too far away and would require an enormous size collector, and stationing the transmitter in lower orbits means you'll have targeting problems. A bit off, and you'll either be scorching its surroundings if the power density is too high (see: feasibility*), or you'll require too big rectenna to lower power density to human safe levels (1 mW/cm&lt;sup&gt;2&lt;/sup&gt;).&lt;br /&gt;&lt;br /&gt;So while this method of transmitting wireless power might be used for some other space applications, maybe from orbits of less massive celestials, preferably with less dense atmosphere, between space stations or to power up satellites wirelessly, i.e. it is feasible for shorter distances or through hard vacuum alone, Earth based rectenna targeted by a satellite in Earth's orbit simply isn't. As much*. Quoting &lt;a href=&quot;https://en.wikipedia.org/wiki/Wireless_power#Microwave_method&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, for a station that would transmit 750 megawatts of total power (one modern electric power plant's worth of power), a 10 km diameter receiving array would be needed to transmit human safe power density to Earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Laser&lt;/strong&gt;: This is far more feasible for Earth orbit to Earth (earthbound) application, or indeed the other way around, Earth to Earth orbit (space bound), than microwave wavelength electromagnetic radiation due to narrower beam requiring a smaller receiving photovoltaic cell array radius per distance, but suffers higher &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_refraction&quot; rel=&quot;noreferrer&quot;&gt;atmospheric refraction&lt;/a&gt; and with it loss in efficiency, power conversion rates are far smaller (best of class photovoltaic cells achieve up to 50% efficiency, and they are terribly expensive to manufacture) and you end up with a &lt;a href=&quot;https://en.wikipedia.org/wiki/Death_ray&quot; rel=&quot;noreferrer&quot;&gt;death ray&lt;/a&gt; scorching the Earth beneath it, if the targeting is off by a tiny amount at the transmitting end.&lt;br /&gt;&lt;br /&gt;Cloud cover is also a big problem for laser solutions, increasing atmospheric absorption and scattering to the point of infeasibility. While converting electricity into a laser beam might be highly efficient and relatively easy to achieve at the transmitter end, it would be almost impossible to provide continuous power with all the atmospheric formations obscuring the beam.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Solar concentrators&lt;/strong&gt;: Earth orbit stationed &lt;a href=&quot;https://en.wikipedia.org/wiki/Parabolic_reflector&quot; rel=&quot;noreferrer&quot;&gt;parabolic mirror&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Fresnel_lens&quot; rel=&quot;noreferrer&quot;&gt;fresnel lens&lt;/a&gt; based solar concentrators could be used to focus a narrow beam of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Total_spectrum_solar_concentrator&quot; rel=&quot;noreferrer&quot;&gt;total spectrum solar concentrator&lt;/a&gt; towards the Earth based &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_thermal_collector&quot; rel=&quot;noreferrer&quot;&gt;solar thermal collector&lt;/a&gt;, and either convert its power to electricity, or additionally concentrate captured beams into a narrower spot and use its energy to power a &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_furnace&quot; rel=&quot;noreferrer&quot;&gt;solar furnace&lt;/a&gt;.&lt;br /&gt;&lt;br /&gt;While I honestly haven't a clue as to how narrow of a spot one could achieve by using optics alone over such distances, I presume it could be narrow enough to be perfectly feasible, given proper atmospheric conditions. With dynamic spot focusing, it could even be used to disperse cloud cover first, then refocus to an Earth based total spectrum solar concentrator and do that continuously, if cloud cover is on the move (which it usually is). Of course, you really wouldn't want to miss your target with this highly concentrated sunbeam when its focal length is close or equal to its source distance from the surface of the Earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All of these methods require a direct line of sight and are affected to varying degree by atmospheric absorption and scattering. Not just the cloud cover, but also fog, pollutants, even pollen count. Microwave and laser links are however perfectly acceptable for long distance communication needs (magnitudes less powerful transmissions than power beams), but our own planet's environmental constraints make wireless transmission through the atmosphere infeasible* and couldn't provide continuous power without relaying it through several satellite transmitters and ground based receivers, with each transfer decreasing the total system's efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For in-space application though, with no atmosphere in between to negatively affect your power transmission efficiency, sure. All of these methods would work, as long as the distances aren't too great to require too large collectors.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Has this been done before?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes. It is believed that the anti-ballistic missile (ABM) testing range in Kazakhstan, named &lt;a href=&quot;https://en.wikipedia.org/wiki/Terra-3&quot; rel=&quot;noreferrer&quot;&gt;Terra-3&lt;/a&gt; and built by Soviets in the late sixties was (on top ABMs) using a laser &lt;a href=&quot;https://en.wikipedia.org/wiki/Directed-energy_weapon&quot; rel=&quot;noreferrer&quot;&gt;directed-energy weapon&lt;/a&gt; powerful and accurate enough to disintegrate or at least completely damage targeting electronics of Intercontinental Ballistic Missiles (ICBM) in flight, before they would get the chance to hit their targets. If they were really powerful enough to do that remains a mystery, and this laser array might have only been used for tracking, but according to the &lt;a href=&quot;http://www.fas.org/spp/military/program/track/okno.pdf&quot; rel=&quot;noreferrer&quot;&gt;report on the Okno (Window) and Krona (Crown) Space Surveillance Sites&lt;/a&gt; (multilingual PDF, including English translation):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On 11 October 1984, the US defence secretary reported to the president&#xA;  that equipment on the Challenger shuttle [&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-41-G&quot; rel=&quot;noreferrer&quot;&gt;STS-41-G&lt;/a&gt;] broke down&#xA;  [malfunctioned] and crew felt unwell when it passed over Lake&#xA;  Balkhash, near Norak, suggesting that the Soviets were testing a new&#xA;  anti-satellite weapon. Indeed, the Terra-3 experimental laser radar&#xA;  was used on Defence Minister Dmitriy Ustinov's orders. After a US&#xA;  protest, the Soviets promised not to use it against manned spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Dmitriy_Ustinov&quot; rel=&quot;noreferrer&quot;&gt;Dmitriy Ustinov&lt;/a&gt; later that month contracted pneumonia and died two months later. This incident is similarly reported in other media, but if true, this would clearly show capability of transferring large power density energy beam (laser) through the Earth's atmosphere and into Low Earth Orbit (LEO).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For tracking, measurements and communication purposes however, similar but with smaller power density was done multiple times, for example with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment&quot; rel=&quot;noreferrer&quot;&gt;Lunar Laser Ranging Experiment&lt;/a&gt; all the way back in 1962 aiming at retroreflectors planted on the Moon during the Apollo program with Earth based lasers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, but for communication purposes, &lt;a href=&quot;https://en.wikipedia.org/wiki/LADEE&quot; rel=&quot;noreferrer&quot;&gt;Lunar Atmosphere and Dust Environment Explorer&lt;/a&gt; (LADEE) that just successfully entered lunar orbit uses Lunar Laser Communication Demonstration (LLCD) that, quoting liked to Wikipedia page on LADEE (NASA pages are down due to US government shutdown):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...will use a laser to transmit and receive data as pulses of light,&#xA;  much the same as data is transferred in a fiber optic cable, to three&#xA;  ground stations. This method of communication has the potential to&#xA;  provide five times the current data return, as compared to radio&#xA;  frequency communication, from both LADEE and future missions. The&#xA;  technology is a direct predecessor to NASA's Laser Communication Data&#xA;  Relay (LCDR) satellite due to launch in 2017.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;European Space Agency (ESA) also successfully demonstrated &lt;a href=&quot;http://www.esa.int/esapub/bulletin/bullet91/b91lutz.htm&quot; rel=&quot;noreferrer&quot;&gt;Semiconductor-laser Inter-satellite Link EXperiment&lt;/a&gt; (SILEX) in 2001 that will be used by planned &lt;a href=&quot;https://en.wikipedia.org/wiki/European_Data_Relay_System&quot; rel=&quot;noreferrer&quot;&gt;European Data Relay System&lt;/a&gt; (EDRS) for space-to-Earth and space-to-space communication purposes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of these systems, be it for targeting, tracking, measuring or communication purposes demonstrate feasibility of laser based wireless power transfers through the Earth's atmosphere in either direction, space bound or earthbound. Of course, for such applications, power density would have to be magnitudes greater, and that comes with its own disadvantages (safety, environmental issues, cost,...) as well as advantages (lower atmospheric diffraction, i.e. effectiveness for one).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;* If you're confident in your targeting not to scorch the living daylight out of the nearby populace, I guess you could up the ante and go with a high enough power density to achieve reasonable levels of effectiveness on a smaller radius area and even with mild weather formations over them. There will be much opposition, and you'd have likely unwittingly started a war with some country threatened by your newly acquired capability, but apparently, so I was corrected in the comments, that is &lt;em&gt;feasible&lt;/em&gt; (possible to do easily or conveniently).&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-08T01:37:58.793" LastActivityDate="2013-10-08T01:37:58.793" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="2342" PostTypeId="2" ParentId="2339" CreationDate="2013-10-07T19:53:01.730" Score="17" Body="&lt;p&gt;The power and data part is easy.  The Galileo spacecraft that went to Jupiter was a &amp;quot;dual spinner&amp;quot;, which had spun and stationary sections.  The rotating joint had 48 &lt;a href=&quot;http://en.wikipedia.org/wiki/Slip_ring&quot; rel=&quot;noreferrer&quot;&gt;slip-rings&lt;/a&gt; over which power and data were transmitted between the sections.&lt;/p&gt;&#xA;&lt;p&gt;As for connecting pressure vessels with a rotating joint, there are &lt;a href=&quot;http://web.archive.org/web/20170630114842/http://www.duffnorton.com/productmenu.aspx?id=7925&quot; rel=&quot;noreferrer&quot;&gt;similar industrial applications requiring a seal at a rotating joint&lt;/a&gt;.  I'm sure that something could be worked out with an acceptable leak rate.  All spacecraft have such a rate defined.  It's never zero.&lt;/p&gt;&#xA;&lt;p&gt;By the way, it's not clear that you need a despun section for your application.  You just need to thrust along your spin axis.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="11262" LastEditDate="2020-10-01T05:14:08.630" LastActivityDate="2020-10-01T05:14:08.630" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="2343" PostTypeId="1" CreationDate="2013-10-07T21:46:49.200" Score="4" ViewCount="1045" Body="&lt;p&gt;NASA's &lt;a href=&quot;https://www.spacecomm.nasa.gov/spacecomm/programs/tdrss/default.cfm&quot; rel=&quot;nofollow&quot;&gt;Tracking and Data Relay Satellite System&lt;/a&gt; (TDRSS) is a space-based relay system at geostationary orbit. If one has, for instance, a LEO mission, and one's coverage requirements can not be met by one's ground system, TDRSS offers an alternative - the ability to communicate with your satellite at any point in its orbit via one of their satellite relays).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is simple: &lt;strong&gt;are there any &lt;em&gt;commercial&lt;/em&gt;, space-based satellite relay systems besides TDRSS&lt;/strong&gt;? If so, do they offer essentially the same performance/coverage? Is pricing competitive with what TDRSS costs?&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-10-14T15:52:36.173" Title="Commercial TDRSS alternatives?" Tags="|communication|low-earth-orbit|tdrss|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2344" PostTypeId="2" ParentId="2343" CreationDate="2013-10-07T22:49:10.117" Score="3" Body="&lt;p&gt;I don't think there are. There are several other ground based tracking and satellite relay systems, for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; (DSN), &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Indian Deep Space Network&lt;/a&gt; (IDSN), &lt;a href=&quot;https://en.wikipedia.org/wiki/Chinese_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chinese Deep Space Network&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Soviet_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soviet Deep Space Network&lt;/a&gt; and of course the &lt;a href=&quot;https://en.wikipedia.org/wiki/ESTRACK&quot; rel=&quot;nofollow noreferrer&quot;&gt;European Space Tracking&lt;/a&gt; (ESTRACK), but these are all ground based, and none of them exclusively commercial in nature. I wasn't able to find any mention of a commercial, or even a non-commercial and already existing &lt;em&gt;space-to-space tracking and relay system&lt;/em&gt; similar to NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tracking and Data Relay Satellite System&lt;/a&gt; (TDRSS)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA Goddard Space Flight Center (GSFC) however has another study active, called Space Based Relay Study (SBRS), under the umbrella of the Goddard's Exploration and Space Communications Division on behalf of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Communications_and_Navigation_Program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Communications and Navigation Program&lt;/a&gt; (SCaN), tasked with identifying future space-based relay communication and navigation architectures to support NASA missions in the 2022 and beyond timeframe. Since the NASA pages are down due to US government shutdown, I wasn't able to find any information if this is meant to replace TDRSS, or co-exist alongside it, or if it is meant to also provide commercial services. But I would presume not, and it will be a gradual replacement of NASA's own &lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tracking and Data Relay Satellites&lt;/a&gt; (TDRS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, European Space Agency (ESA) has its own plans to build a &lt;a href=&quot;https://en.wikipedia.org/wiki/European_Data_Relay_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;European Data Relay System&lt;/a&gt; (EDRS) with a constellation of GEO (Geosynchronous Orbit) satellites intended to relay information and data between satellites, spacecraft, Unmanned Aerial Vehicles (UAV) and ground stations, currently under implementation within the &lt;a href=&quot;https://en.wikipedia.org/wiki/ARTES&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced Research in Telecommunications Systems Programme&lt;/a&gt; (ARTES-7):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After a successful demonstration of the Semiconductor-laser&#xA;  Inter-satellite Link EXperiment (SILEX) in 2001 between Artemis GEO&#xA;  satellite and SPOT-4 LEO satellite, EDRS will become the first&#xA;  commercial European data relay system providing a wide range of&#xA;  operational services (both optical and Ka-band based).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/AdxSG.png&quot; alt=&quot;European Data Relay System&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Source: &lt;a href=&quot;http://icsos2012.nict.go.jp/pdf/1569603307.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Status of the European Data Relay Satellite System&lt;/a&gt;, ICSOS, October 9, 2012 (PDF)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;First customer of the EDRS service (using Sentinel 1A/B and 2A/B LEO satellites) will be &lt;a href=&quot;http://ec.europa.eu/enterprise/policies/space/copernicus/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Copernicus - The European Earth Observation Programme&lt;/a&gt;. ESA has also prepared a proposal for an expanded data relay service, called GlobeNet (nothing to do with the same named submarine telecommunications cable system linking the USA, Bermuda, Venezuela and Brazil), that would &lt;strong&gt;provide support to the adoption by customers beyond Copernicus&lt;/strong&gt; (previously GMES) and has proposed a third node in GEO orbit to provide redundancy and increase the availability and the flow of data from Earth observation satellites and UAVs to users, as well as to enhance near real-time services, &lt;strong&gt;including GEO-GEO O-ISL&lt;/strong&gt; (optical inter-satellite link) capabilities. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it looks like in a few years time, ESA's own and commercially available space-to-space capable tracking and data relay system, called either like it is now (EDRS - European Data Relay System), or by the already taken name of &lt;em&gt;GlobeNet&lt;/em&gt;, will join TDRSS.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-07T23:02:47.443" LastActivityDate="2013-10-07T23:02:47.443" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2345" PostTypeId="1" CreationDate="2013-10-08T02:59:16.283" Score="7" ViewCount="131" Body="&lt;p&gt;I just finished reading the Japanese hard science fiction novel &lt;a href=&quot;http://www.haikasoru.com/the-ouroboros-wave/&quot;&gt;Ouroboros Wave&lt;/a&gt;.  In the afterword, the author writes the following.  For context, the book goes into quite a large amount of detail of the regolith of comets and other small bodies (in-world).  It was also published in 2002, although the date of the English translation on my book is 2010.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In closing, I would like to thank Atsushi Noda, Naru Hirata, Masao Hirota, Tomohiro Araki, Jun Fukue, and Masahiro Maeno, as well as the members of the Minor Body Exploration Forum (&lt;a href=&quot;http://www.as-exploration.com/mef/index.html&quot;&gt;http://www.as-exploration.com/mef/index.html&lt;/a&gt;), for their invaluable suggestions and guidance. I would also like to thank the members of the Osaka Chapter of the Space Authors Club, espcially Housuke Nojiri and Yasumi Kobayashi, for many stimulating discussions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Given the time frames involved, it's somewhat unsurprising that the link doesn't work anymore.  &lt;a href=&quot;http://web.archive.org/web/20070806133331/http://www.as-exploration.com/mef/index.html&quot;&gt;The archived version&lt;/a&gt; isn't very helpful as to a simple &quot;about&quot; description, although it makes me think that it is primarily a Japanese organization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What did this organization do, and does is it still operating?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2017-08-18T03:11:55.190" Title="What was/is the Minor Body Exploration Forum?" Tags="|asteroid|comet|japan|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2346" PostTypeId="1" AcceptedAnswerId="2348" CreationDate="2013-10-08T03:02:43.427" Score="15" ViewCount="951" Body="&lt;p&gt;What techniques have been employed for the Juno spacecraft to successfully transmit data packets back to Earth once she's in a polar orbit around Jupiter? Jupiter is a strong source of radio wavelength interference, at the same time it is of course also rather large distance away from the Earth (currently roughly 5.135 AU).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/o1HXf.jpg&quot; alt=&quot;Juno&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;An artist's concept of Juno at Jupiter (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Juno_Mission_to_Jupiter_%282010_Artist%27s_Concept%29.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please describe Juno's communication subsystem, its predicted efficacy, and any techniques in place to thwart against possible radio wavelength interference, direct line of sight occlusion, transmission carrier waves intersecting with Jupiter's enormous magnetosphere, and other possible reasons for loss of transmission, regardless the source. What levels of data redundancy will be in place (package repetition, multiple carrier wavelengths, e.t.c.), how will data transmissions be encoded (error detection and correction algorithms used), at what rate and orbital position will they be transmitted to help assure the success of this mission critical subsystem component?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If possible, please compare the predicted data transmission failure rates for Juno with observed failure rates from previous missions to Jupiter, what lessons have we learned by then and learned to build around them with Juno, making her more data transmission error resilient.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="51" LastEditDate="2016-07-08T13:38:31.943" LastActivityDate="2016-07-18T12:31:56.607" Title="Resilience to data transmission errors of the Juno spacecraft" Tags="|probe|communication|jupiter|juno|electronics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2347" PostTypeId="2" ParentId="2345" CreationDate="2013-10-08T03:58:02.383" Score="6" Body="&lt;p&gt;According to &lt;a href=&quot;http://translate.google.com/translate?sl=auto&amp;amp;tl=en&amp;amp;js=n&amp;amp;prev=_t&amp;amp;hl=en&amp;amp;ie=UTF-8&amp;amp;u=http://web.archive.org/web/20070806133331/http://www.as-exploration.com/mef/index.html&quot; rel=&quot;noreferrer&quot;&gt;Google Translate version of The Wayback Machine cached copy&lt;/a&gt; of the now expired domain:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Small bodies exploration Forum&quot; is a group that joins us from various&#xA;  positions for the next small bodies exploration of &quot;Hayabusa&quot; mission,&#xA;  followed by the public debate by using the Internet, etc.. In this&#xA;  site, I would like to introduce appropriate ones can be exposed from&#xA;  those discussed in the group, and we have been featured information&#xA;  various other related to solar system exploration further. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;Small&#xA;  bodies exploration Forum Official page&quot; is ::: Www.Minorbody.Com/ is&#xA;  :::.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;OK, the machine translation is slightly awkward, as expected, but the clue is there, the new location of this same web forum is: &lt;a href=&quot;http://www.minorbody.com/&quot; rel=&quot;noreferrer&quot;&gt;www.minorbody.com&lt;/a&gt;. Of course, that again lands us on a Japanese language only website, but at least it is still maintained. Well, kind of. Translating it, it reads as a referral website advertising pregnancy and ovulation test products. LOL. I'm not sure if it's some form of web domain hijacking, or also the newer domain registration expired, but it certainly doesn't have anything to do with where we started at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the initial description is so far all we have. It is web forum for what basically says on the box:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;A community of people of various professions interested in the (then) next minor body exploration mission of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;noreferrer&quot;&gt;JAXA's Hayabusa spacecraft&lt;/a&gt; (&lt;a href=&quot;http://spaceinfo.jaxa.jp/hayabusa/&quot; rel=&quot;noreferrer&quot;&gt;JAXA Hayabusa page in Japanese&lt;/a&gt;).&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This group is, according to its bulletin board, still active in the &lt;a href=&quot;http://groups.yahoo.co.jp/group/minorbody/&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;minorbody&lt;/em&gt; Yahoo Japan group&lt;/a&gt;. Translating its front page post, I got this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Small bodies exploration forum (Minor Body Exploration Forum: MEF)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt; &lt;strong&gt;Description of the group&lt;/strong&gt;: &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;In your hand, do not send out spacecraft&#xA;  to the home of The Little Prince?&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Hayabusa spacecraft JAXA Institute of Space and Astronautical Science&#xA;  (ISAS) was launched in May 2003 , was brought back to earth a dazzling&#xA;  return capsule in June 2010 . This group , that for small bodies&#xA;  exploration of Japan following the falcon, to solicit ideas from all&#xA;  over the country , and to consider the feasibility and engineering&#xA;  approaches scientific significance , to create a powerful mission&#xA;  candidate of scale that can be conducted by JAXA / ISAS it is a&#xA;  volunteer organization for the purpose .&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Since its inception in May 2000 , I looked at the exploration plan of&#xA;  seven . In the Science Council of Japan and overseas , including the&#xA;  Space Science Symposium of four times since 2001 , and has been&#xA;  announced the examination result of integration plan to achieve it and&#xA;  Science aim is next small bodies exploration . Issue a final version&#xA;  in January &quot; MEF report &quot; Preliminary , '04 , was distributed at the&#xA;  HP and the release on to the end of 2002 . Held a &quot; small bodies&#xA;  exploration WG &quot; application launched preparatory meeting immediately&#xA;  after , it was submitted to JAXA / ISAS Space Science Committee in&#xA;  March , launched a petition that a starting point the same report .&#xA;  Result of deliberation , &quot; small bodies exploration WG &quot; establishment&#xA;  is approved , the purpose of the establishment of the MEF was achieved&#xA;  .&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So apparently, the purpose of this group was expanded to include planning of future candidate JAXA / ISAS missions to minor celestial bodies, and collect and promote ideas from all over Japan. It was also upgraded to a status of a volunteer based organization, and attends space exploration symposiums and some of its members might have a say in the Science Council of Japan? Don't know, this machine translation is somewhat hard to read.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The translation later reads as this group being open to members of any nations, but the language used therein is Japanese. Right. I really need find the time to learn Japanese someday, they seem to do pretty cool stuff as well.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-10-08T03:58:02.383" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2348" PostTypeId="2" ParentId="2346" CreationDate="2013-10-08T04:39:50.233" Score="17" Body="&lt;p&gt;&lt;a href=&quot;http://descanso.jpl.nasa.gov/DPSummary/Juno_DESCANSO_Post121106H--Compact.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; describes the Juno telecomm system in detail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is a standard deep-space X-band system with a 2.5 m high-gain antenna, a 25 W traveling-wave tube amplifier, and concatenated convolutional and Reed-Solomon or Turbo 1/6 rate error-correcting codes.  It will get 18,000 bits per second down to a 34-m antenna on Earth at maximum range (6.459 AU) at a $10^{-6}$ bit error rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the noise in the transmission comes from space plasma between Jupiter and Earth, and Earth's atmosphere.  Not from Jupiter.  Jupiter is noisy in the MHz frequencies, but not in the GHz frequencies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is done to assure data integrity is forward error correction.  Normally a &lt;a href=&quot;https://en.wikipedia.org/wiki/Turbo_code&quot; rel=&quot;nofollow&quot;&gt;Turbo code&lt;/a&gt; is used, and the rate is chosen to assure less than a $10^{-6}$ bit error rate, with 3 db of margin.  For data that doesn't make it due to unexpected interference, e.g. a storm over Canberra or some such, that data can be retransmitted on another pass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is just standard procedure for all deep-space direct-to-Earth communication.  There's nothing special for Jupiter, once you plug in the range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only lesson learned I know of from Galileo might be to &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(spacecraft)#Main_antenna_problem&quot; rel=&quot;nofollow&quot;&gt;not use a deployable high-gain antenna&lt;/a&gt; again.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="415" LastEditDate="2016-07-18T12:31:56.607" LastActivityDate="2016-07-18T12:31:56.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2350" PostTypeId="2" ParentId="1375" CreationDate="2013-10-08T18:57:26.710" Score="9" Body="&lt;p&gt;I was able to find a few requirements for CubeSats that are based upon its integration into a larger system which includes a primary payload (all instances of &lt;strong&gt;primary payload&lt;/strong&gt; are bolded in quotes below, emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;CubeSats shall not present any danger to neighboring CubeSats in the P-POD,&#xA;  the LV, or &lt;strong&gt;primary payloads&lt;/strong&gt;:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;2.2.1.1 All parts shall remain attached to the CubeSats during launch, ejection and&#xA;  operation. No additional space debris shall be created.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;2.2.1.2 CubeSats shall be designed to minimize jamming in the P-POD.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;2.2.1.3 Absolutely no pyrotechnics are allowed inside the CubeSat.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-- from &lt;a href=&quot;http://www.ok1mjo.com/all/ostatni/space_aircraft/CubeSat_specifikace_CDS_rev11.pdf&quot;&gt;CubeSat Design Specification rev 11 - California Polytechnic State University&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;No electronics shall be active during launch to prevent any electrical or RF&#xA;  interference with the launch vehicle and &lt;strong&gt;primary payloads&lt;/strong&gt;. CubeSats with&#xA;  rechargeable batteries shall be fully deactivated during launch or launch with&#xA;  discharged batteries.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-- from &lt;a href=&quot;http://browncubesat.org/wp-content/uploads/2013/01/Cubesat-Reqs.pdf&quot;&gt;CubeSat Design Specification rev 12 - California Polytechnic State University&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Placard: The payload and canisters may be integrated several months or years ahead of launch. A placard informs the LV of the contents, provides traceability, and reaffirms conformance to a specification giving the LV and &lt;strong&gt;primary payload&lt;/strong&gt; assurance of mission success.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-- from &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1111&amp;amp;context=smallsat&quot;&gt;An Advanced Standard for CubeSats - Hevner, Holemans, Puig-Suari, and Twiggs&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This brings up&#xA;  another requirement, which is the CubeSat must be powered down while in the P-POD so it will&#xA;  not interfere with the &lt;strong&gt;primary payload&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-- from &lt;a href=&quot;http://www.spacegrant.hawaii.edu/reports/15_FA05-SP06/Kikugawa_Tyson_FA.pdf&quot;&gt;Systems Integration and Stabilization of a CubeSat - Kikugawa&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &quot;Placard&quot; section in &quot;An Advanced Standard for CubeSats&quot;, we get the impression that the primary payload team and the CubeSat team do not necessarily work closely together, but that the CubeSat teams are given requirements to ensure that the CubeSat does not interfere with the primary payload.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-10-09T14:36:26.477" LastActivityDate="2013-10-09T14:36:26.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2351" PostTypeId="2" ParentId="2325" CreationDate="2013-10-08T19:14:43.793" Score="1" Body="&lt;p&gt;You may visit &lt;a href=&quot;http://www.heavens-above.com/ManualLocationInput.aspx?lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&quot; rel=&quot;nofollow&quot;&gt;Heavens Above&lt;/a&gt; to configure your current location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next step is to visit &lt;a href=&quot;http://www.heavens-above.com/JunoFlyby.aspx?lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&quot; rel=&quot;nofollow&quot;&gt;this page on the same site&lt;/a&gt;. It renders a map showing your configured location against the Juno Ground-track. The page also renders a table to guide your observation of the spacecraft in the skies.&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-10-08T19:14:43.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2352" PostTypeId="1" AcceptedAnswerId="2353" CreationDate="2013-10-08T19:50:17.427" Score="7" ViewCount="237" Body="&lt;p&gt;Just a fun question (+: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno is &lt;a href=&quot;http://www.nasa.gov/mission_pages/juno/earthflyby.html&quot; rel=&quot;nofollow&quot;&gt;approaching Earth&lt;/a&gt; for a gravity assist on her way to Jupiter. Her approach will bring her, at the nearest point, a mere 559KM above Earth's surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow&quot;&gt;ISS&lt;/a&gt; orbits just a little lower down, an average of 415KM above Earth. &#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Tiangong-1&quot; rel=&quot;nofollow&quot;&gt;Tiangong-1&lt;/a&gt; is still lower down, an average of 357KM above Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All three craft have nearly comparable velocity around 7KM/s (I may be wrong on this one!), although Juno's will increase a fair bit doing the fly-by (Why do we call it fly-by? She isn't even in air!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Between Earth, ISS, and Tiangong, which will be the &lt;strong&gt;first habitat to get closest to Juno&lt;/strong&gt; during the fly-by?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="12102" LastEditDate="2017-07-19T01:40:37.950" LastActivityDate="2017-07-19T01:40:37.950" Title="Will Earth, ISS, or Tiangong be the first habitat closest to Juno during Earth Flyby 2013?" Tags="|iss|juno|flyby|conjunction|tiangong-1|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2353" PostTypeId="2" ParentId="2352" CreationDate="2013-10-08T20:21:49.163" Score="3" Body="&lt;p&gt;Looking at STK and the public elements for each object. Unfortunately I can't do a conjunction report, as I can't seem to get JUNO into STK unfortunately... But here's a few photos:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno Flyby:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rrCY2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS/Tiangong at the time of closest approach:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/l4MfD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given this, hands down the answer is Tiangong. I don't have an exact distance for you, but it will be within 4 track minutes of being in the same area as Juno, and they actually fly along a similar trajectory for most of the closest approach period. I'm guessing they will pass within about 1000 km of each other, maybe closer, whereas the ISS won't pass within 5000 km, or so I would expect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the Earth will no doubt beat both of them, being only 500 km or so away from the surface of the Earth. Still, Tiangong will fly surprisingly close.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-08T20:21:49.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2355" PostTypeId="1" AcceptedAnswerId="2356" CreationDate="2013-10-09T01:02:31.780" Score="13" ViewCount="1850" Body="&lt;p&gt;I am looking for some software that will help me figure out what different satellites are as I spot them gliding across the sky. I have tried using Stellarium, but I think the satellite package has some bugs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best thing would be to see just a sky map of my current location, and watching the objects move across the map in real time with what I observe in the sky. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyone know any options that are similar? Is Stellarium my best bet, or are there other options?&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2015-07-13T23:13:59.770" Title="Satellite tracking software" Tags="|artificial-satellite|tracking|software|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2356" PostTypeId="2" ParentId="2355" CreationDate="2013-10-09T09:23:59.117" Score="5" Body="&lt;p&gt;You have several options (please note: I have not used any of these myself, thus can not verify how good they are), a directory exists on the website &lt;a href=&quot;http://www.satobs.org/orbsoft.html&quot; rel=&quot;noreferrer&quot;&gt;Satellite Related Software.&lt;/a&gt; for a range of platforms, this website is attached to the &lt;a href=&quot;http://www.satobs.org/&quot; rel=&quot;noreferrer&quot;&gt;Visual Satellite Observer's Home Page&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further information and links can be obtained from the linked &lt;a href=&quot;http://www.satobs.org/tletools.html&quot; rel=&quot;noreferrer&quot;&gt;Tracking Programs and TLE Resources&lt;/a&gt; page.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;A recommendation is to research reviews of these programs before downloading and using them&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-10-09T09:23:59.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2357" PostTypeId="1" AcceptedAnswerId="2361" CreationDate="2013-10-09T09:58:46.307" Score="12" ViewCount="446" Body="&lt;p&gt;Juno is approaching her gravity assist. During the course of the maneuver, several on-board systems will be powered up - in all probability they are nominal, having already been checked after launch.&lt;/p&gt;&#xA;&lt;p&gt;Assuming, say, a new algorithm has been developed, or an instrument is made more efficient by a code update, could Juno's mission control opt to schedule such an update during the course of the maneuver?&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are there any software updates scheduled for Juno during the maneuver (either before or after perigee)?&lt;/li&gt;&#xA;&lt;li&gt;What is the process of performing an instruction update on spacecraft instruments?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="37597" LastEditDate="2023-10-08T01:04:16.187" LastActivityDate="2023-10-08T01:04:16.187" Title="Are any software updates scheduled for Juno?" Tags="|juno|flyby|software|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2358" PostTypeId="1" CreationDate="2013-10-09T11:21:56.027" Score="20" ViewCount="2531" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;At a distance of about 125.97 AU (1.884 × 10&lt;span class=&quot;math-container&quot;&gt;$^{10}$&lt;/span&gt; km) from the Sun as of September 9, 2012,  it is the farthest manmade object from Earth&lt;/p&gt;&#xA;&lt;p&gt;As of 2013, the probe was moving with a relative velocity to the Sun of about 17 km/s.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;How did NASA determine the distance and velocity of Voyager 1 ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditorDisplayName="user29" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-10-09T20:39:19.577" Title="How did NASA determine the distance and velocity of voyager 1?" Tags="|orbital-mechanics|nasa|voyager|navigation|orbit-determination|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2359" PostTypeId="2" ParentId="2355" CreationDate="2013-10-09T11:53:48.870" Score="7" Body="&lt;p&gt;The list on the Visual Satellite Observer's Homepage is a good option.&lt;/p&gt;&#xA;&lt;p&gt;I've experienced those software/web pages with a lot of fun:&lt;/p&gt;&#xA;&lt;p&gt;Try the Heavensat tracking software (freeware) it's easy to use and you can easily download the TLE's directly with the software. Another one is &lt;a href=&quot;http://www.stoff.pl/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbitron&lt;/a&gt; (also freeware).&lt;/p&gt;&#xA;&lt;p&gt;On line you can try &lt;a href=&quot;http://www.heavens-above.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heavens-Above.com&lt;/a&gt; web page or (I recommend) &lt;a href=&quot;https://celestrak.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak.com&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In any case you have to set the software/web page with your position coordinates&lt;/p&gt;&#xA;" OwnerUserId="865" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2015-02-12T03:51:13.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2360" PostTypeId="2" ParentId="2358" CreationDate="2013-10-09T12:41:07.393" Score="9" Body="&lt;p&gt;The distance is quite easy for something so far out there. You send a command to the satellite, and see how long it takes to respond. The time it takes to respond times the speed of light is the distance that it is away from the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, the velocity is likewise easy. If one day it is 17 light hours exactly, and the next day it is 17 hours and 10 seconds, you have a distance, and the amount of time between the two points. Divide one by the other, and you have the speed that the spacecraft is going.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, if you are worried about those measurements being accurate, there's another frequently used method, the Doppler shift. The faster something is moving away from you, the lower it's frequency will appear to be. If the receiver is stable enough, this can be used to determine it's relative speed, which one can use to find the velocity.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-09T12:51:23.687" LastActivityDate="2013-10-09T12:51:23.687" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2361" PostTypeId="2" ParentId="2357" CreationDate="2013-10-09T15:36:45.557" Score="13" Body="&lt;p&gt;During the maneuver?  Certainly not.  You don't mess with the spacecraft configuration around or anywhere near critical events unless it is absolutely necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern spacecraft are all designed to permit their flight software to be updated.  A complete image of the new flight software is uploaded the spacecraft.  The image's integrity is verified, and then it is set to be the load used on just the next reboot.  The previous image is kept in place in case you need to fall back on it.  A subsequent reboot will go back to the old one as a safety net.  You boot on the new image, check it out, and if all is well, you make the new image the default for future reboots.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-10-09T15:36:45.557" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2362" PostTypeId="2" ParentId="2358" CreationDate="2013-10-09T15:55:15.893" Score="20" Body="&lt;p&gt;Here is a &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf13-1.php&quot;&gt;good tutorial&lt;/a&gt; on the navigation of deep space vehicles.  The two main data types used are two-way Doppler (using an atomic clock reference at the DSN station, with the frequency locked to and sent back to Earth by the spacecraft), which gives the velocity component along the Earth-spacecraft line to better than 0.1 mm/s, and ranging (sending a pseudo-noise signal which is immediately reflected back by the spacecraft radio), which gives the distance from the DSN station to the spacecraft to better than one meter.  Though this only gives you one component of position and one component of velocity, you can use many of these high-accuracy measurements over time combined with the constraints of motion in the gravity field of the Sun and planets to solve for the other components of position and velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is another data type, used less frequently, which is to use two widely separated DSN stations (i.e. at two different complexes on Earth) and a quasar of known position to determine the apparent position of the spacecraft with interferometry.  This can determine the angular position of the spacecraft on the projection of the baseline between the two stations to an accuracy of a few nanoradians, letting you solve immediately for one more component.  It is more expensive, and so is generally used when higher precision is required over a shorter time, e.g. on approach to Mars to accurately target a landing site.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-10-09T16:00:40.480" LastActivityDate="2013-10-09T16:00:40.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2363" PostTypeId="1" CreationDate="2013-10-09T23:43:28.513" Score="3" ViewCount="768" Body="&lt;p&gt;I am making a multiplayer video game, and am wondering how I should scale the in-game universe. The game has pseudo-realistic travel, but I want the scaling to be large enough to offer challenging gameplay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have already determined the Sci-Fi portions of my game (in which an evil Monsanto-like corporation got nuked out of Mars and escaped using an Orion-like ship), and the technology in the game is fairly advanced. I would like a reasonable scale keeping this in mind, perhaps for travels up to a month or two for crazy explorers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How should I scale the Solar System for my video game?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user871" LastEditorUserId="25" LastEditDate="2015-07-16T13:14:05.030" LastActivityDate="2015-07-16T13:14:05.030" Title="How should I scale the Solar System for my video game?" Tags="|propulsion|solar-system|simulation|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="2364" PostTypeId="1" AcceptedAnswerId="2365" CreationDate="2013-10-10T07:52:30.823" Score="22" ViewCount="2792" Body="&lt;p&gt;I often think about the future of humanity and how to achieve it. Despite criticism, the USA and USSR achieved significant scientific breakthroughs, such as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;putting the first man in space&lt;/li&gt;&#xA;&lt;li&gt;landing the first man on the moon&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But we have to move on. And one of the ways, of course, is deep space exploration. The main problem seems to be the engine for spacecraft, which could give us the opportunity to reach deep space in acceptable time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is nuclear fission the right approach?&lt;/p&gt;&#xA;" OwnerUserId="873" LastEditorUserId="709" LastEditDate="2013-10-11T05:20:50.370" LastActivityDate="2018-10-25T07:50:59.463" Title="Are nuclear-powered engines the way to go for space exploration?" Tags="|deep-space|engine-design|nuclear|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2365" PostTypeId="2" ParentId="2364" CreationDate="2013-10-10T08:26:12.917" Score="21" Body="&lt;p&gt;The problem with using nuclear fission reactors as means of power to propel spacecraft is twofold: our own aversion to anything &lt;em&gt;nuclear&lt;/em&gt; due to environmental hazards and the problem of reaction mass still persisting, regardless of your energy source longevity and power density per its own mass. Let's explain these points a bit more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reaction mass problem comes from the fact that there isn't anything to propel against in the vacuum of space, unless you bring along some reaction mass that could be expelled at high velocities and result in thrust, a kinetic momentum transfer of thrust equaling mass flow rate of exhaust by exhaust velocity: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T = {dm \over{dt}}*v$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while your nuclear reactor might be perfectly capable of having great energy density per its mass, you'd still have to have some &lt;em&gt;consumable&lt;/em&gt; mass on you that you can energize and increase this mass excitation state to either greatly reduce its density (superheating, chemical reaction, e.t.c.), or otherwise be able to accelerate it in the opposite direction to your vector of travel (e.g. ion thrusters, direct hot nuclear fission products, e.t.c.), and later expel it at you rocket engine's nozzle, resulting in acceleration of your spacecraft in the opposite direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some proposed nuclear powered rockets would thus either carry onboard reaction mass that is a rocket engine consumable, or alternatively use nuclear fuel byproducts as their reaction mass in a bit more direct nuclear propulsion system, like for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Fission-fragment_rocket&quot; rel=&quot;noreferrer&quot;&gt;fission-fragment rocket&lt;/a&gt;. The latter however achieve theoretically low &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;noreferrer&quot;&gt;specific impulse&lt;/a&gt; per time, even if they might operate for a long time (high overall specific impulse $I_{sp}$ over its total lifespan). Similar goes for &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;noreferrer&quot;&gt;ion thrusters&lt;/a&gt; that could also be nuclear powered of course, and indeed their electrical power requirements are usually provided by an onboard &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;RTG&lt;/a&gt; (Radioisotope Thermoelectric Generator), although it may also be by solar panels, depending on mission parameters like proximity to the Sun, its duration, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another problem with using direct-nuclear power propulsion is designing for these reactors' heat producing characteristics. Direct heat exchange with another mass surrounding the spacecraft simply isn't possible in the vacuum of space with not nearly high enough particle density even in hard vacuum of inner Solar system. So enormous heat radiators would be needed to cool down liquids responsible for cooling down the fission reactor's core and dissipate heat through radiation alone. That would require tremendous mass to be first lifted to outer space, for any spacecraft to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Fission-fragment_rocket&quot; rel=&quot;noreferrer&quot;&gt;fission-fragment rocket&lt;/a&gt; engines (FFRE).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/CYjW5.jpg&quot; alt=&quot;NASA&amp;#39;s HOPE Fission Fragment Rocket&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;HOPE spacecraft is a NASA concept fission-fragment rocket engine (FFRE) powered vehicle (Source: &lt;a href=&quot;https://i.stack.imgur.com/CYjW5.jpg&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Power density of nuclear power generators is thus, as explained, not really the only factor in the rocket propulsion equation, and the simplicity of chemical reaction powered rocket stages using highly reactive component propellants is still by far the prefered method of achieving thrust. Since this problem of reaction mass either persists and you still have to carry it with you, or the resulting specific impulse of direct-nuclear engines isn't high enough for most space exploration applications, and nuclear generators come with their own non-negligible mass, the chemical rocket is still prefered, especially when used for Earth based rocket launches that come with an additional risks if nuclear powered engines were used instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another factor already mentioned are of course environmental concerns. Several modes of propulsion using nuclear powered engines were already proposed to achieve lift as well, not merely thrust big enough to have in-space application but also achieve lifting capability (thrust big enough per launcher mass to achieve in-atmosphere and in-gravity escape velocity), however they were all much opposed to and never saw the light of day, actually materializing beyond laboratory experiments, theoretical calculations, or perhaps some top secret military experiments. A few of such experiments were done that are know to public, for example the Space Nuclear Propulsion Office (SNPO) project, a joint program of the U.S. Atomic Energy Commission and NASA, called &lt;a href=&quot;https://en.wikipedia.org/wiki/NERVA&quot; rel=&quot;noreferrer&quot;&gt;NERVA&lt;/a&gt; (Nuclear Engine for Rocket Vehicle Application):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/LnumU.jpg&quot; alt=&quot;first ground experimental nuclear rocket engine (XE) assembly&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;1 December 1967: The first ground experimental nuclear rocket engine (XE) assembly, is shown here in &quot;cold flow&quot; configuration,&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;as it makes a late evening arrival at Engine Test Stand No. 1 at the Nuclear Rocket Development Station, in Jackass Flats, Nevada.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;(Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Nuclear thermal rocket&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So &lt;strong&gt;TL;DR&lt;/strong&gt; - The problem of reaction mass over the total mass of the launcher still persists, and direct nuclear fission-fragment rocket engines don't achieve thrust required for reaching escape velocity. It also doesn't make environmentalists particularly happy.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-10T10:00:39.777" LastActivityDate="2013-10-10T10:00:39.777" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2366" PostTypeId="1" AcceptedAnswerId="2369" CreationDate="2013-10-10T12:10:24.720" Score="10" ViewCount="612" Body="&lt;p&gt;A sort of followup to &lt;a href=&quot;https://space.stackexchange.com/questions/2357/are-any-software-updates-scheduled-for-juno&quot;&gt;Are any software updates scheduled for Juno?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;For the purpose of illustration, seeing that Juno has most recently encountered such an issue, I mention Juno here. However the questions at the end may apply universally.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Juno got a gravity assist from Earth  ... and then drew the veil.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.universetoday.com/105405/nasas-juno-probe-gets-gravity-speed-boost-during-earth-flyby-but-enters-safe-mode/&quot; rel=&quot;nofollow noreferrer&quot;&gt;news website&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... the Juno spacecraft unexpectedly entered ‘safe mode’ during the fly by maneuver and the mission teams are assessing the situation. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Let's assume a worst case condition &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;that it is a mechanical fault, say, caused by debris&lt;/li&gt;&#xA;&lt;li&gt;that safe mode is here to stay&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Going by this assumption &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Can Juno Mission Control bypass safe mode?&lt;/li&gt;&#xA;&lt;li&gt;Can Juno brake &amp;amp; enter Jupiter orbit in safe mode?&lt;/li&gt;&#xA;&lt;li&gt;What studies may still be performed in safe mode?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-12-02T20:35:12.383" Title="Can safe mode on a spacecraft be bypassed?" Tags="|juno|flyby|operations|software|safe-mode|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2367" PostTypeId="2" ParentId="2366" CreationDate="2013-10-10T12:35:55.753" Score="9" Body="&lt;p&gt;Most satellites have some sort of a safe mode, a mode the satellite enters when it is having difficulties with certain things. The exact nature of the safe mode varies from satellite to satellite, but in general, it is a lower power state, thermally stable and power positive, turns off science instruments, and relies less on absolute pointing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can say that most safe modes can be by-passed in some way or another. For instance, &lt;em&gt;New Horizons&lt;/em&gt; will disable it's safe mode for a few weeks around the Pluto flyby, and if it doesn't succeed in doing that, the mission was pointless. However, safe mode did trip during the Jupiter flyby, which was deemed non-critical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as what caused Juno's safe mode during flyby, I can't help but suspect it's related to this statement, as quoted &lt;a href=&quot;http://www.denverpost.com/breakingnews/ci_24276963/juno-mission-flies-by-earth-but-anomaly-triggers&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Juno's flyby was the first and only time it would enter a solar eclipse and be forced to rely on its battery for power&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I suspect that the satellite determined it didn't have enough power to make it through the eclipse with all of it's instruments on full power, which is one of the most common safe mode triggers for solar-powered satellites. If this is the case, the probe should be up and running again in no time. This is the only eclipse of the mission, most likely the batteries aren't specified to last much longer than this eclipse, and perhaps either the batteries degraded with time, or else they weren't specified correctly. In either case, the spacecraft still is working and overall positive, so it should be fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tildalwave provided me with the &lt;a href=&quot;http://descanso.jpl.nasa.gov/DPSummary/Juno_DESCANSO_Post121106H--Compact.pdf&quot; rel=&quot;noreferrer&quot;&gt;system requirements&lt;/a&gt;, which includes a nice section about safe mode. Essentially what happens is the data rate drops, the attitude may be compromised, and as a result, lower gain antennas are used for communication.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-10T13:21:11.963" LastActivityDate="2013-10-10T13:21:11.963" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2368" PostTypeId="2" ParentId="2363" CreationDate="2013-10-10T13:08:06.103" Score="6" Body="&lt;p&gt;Okay, let's just take a few hypothetical situations here and see what we can come up with. You specifically mentioned going from Mars to Jupiter, and want something realistic, but quick. Let's assume you have a 1g acceleration capability. How you do that is up to your game really. To make things a bit simpler, I'm going to have two modes of operation, which should bound pretty well how long it will take. I'm going to assume you want to stop there, and not just flyby. They are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Mars/Jupiter at closest approach. Will add in a bit of time to match the different relative velocities, and ignore inclination, etc.&lt;/li&gt;&#xA;&lt;li&gt;Mar/Jupiter at furthest apart. Will assume that one losses all Martian speed, heads straight to Jupiter, then gains all of the Jupiter speed.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;These aren't quite accurate, but they are close enough. Okay, so here's a few key facts:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Jupiter- Distance= 778,500,000 km, orbital velocity= 13.07 km/s&lt;/li&gt;&#xA;&lt;li&gt;Mars- Distance= 227,900,000 km, Orbital velocity= 24.125 km/s.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The time given a 0 starting speed, 0 ending speed, straight line, and half accel, half decel can be found by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\sqrt{2*\text{dist}/a}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so, what do we have? Let's start with the maximum distance case. Distance is added, so 1,006,400,000 km. Plugging that in to the above formula, that gives us 453197 s, or about 125.9 hours. Adding in the relative speed difference will take another 3795 seconds, or a bit more than an hour. Thus, I'll say 127 hours should be sufficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How about the best case scenario? The distance is reduced to 550,600,000 km. Plugging it in to the same formula gives 335212 s, or 93 hours. The relative speed difference will be 1128 s, or only a fraction of an hour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you use a higher speed, the time will decrease by the same formula, just use a different value for a other than $9.8 m/s^2$. It seems likely that humans could accelerate at 3 g for some time, which would reduce the time by almost a factor of 2 to make this work. Beyond that, you need some sort of an inertial dampener.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-10T13:08:06.103" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2369" PostTypeId="2" ParentId="2366" CreationDate="2013-10-10T16:07:37.050" Score="10" Body="&lt;p&gt;Safe mode is a software state that is initiated by a detection of some inability of the system to do what it has been asked to do.  There are many, many ways that can happen, such as insufficient power, inability to determine attitude, detection of a failed component, such as a reaction wheel, or a software fault or crash.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In safe mode, only the minimum activities are supported to keep the spacecraft power-positive and in communication with Earth.  The communication with Earth is usually on a low-gain antenna at low rate to minimize the need for accurate pointing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;You can always command the spacecraft out of safe mode.&lt;/strong&gt;  But before you do that, you want to make sure that you figured out why it went into safe mode in the first place, and that you have fixed or worked around that problem so that it won't immediately drop back into safe mode again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During critical events, such as a orbit insertion, a flyby, or landing where you only get one chance, many fault protection responses are disabled to avoid losing the mission due to going into safe mode.  We sometimes call this &quot;battle shorting&quot;, analogous to military equipment that can disable protections such as over current or over temperature when in battle, when you care more about other things than damage to the equipment. The original form of battle shorting was removing fuses and shoving pennies into the fuse sockets.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="288" LastEditDate="2013-10-13T15:57:16.087" LastActivityDate="2013-10-13T15:57:16.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2370" PostTypeId="1" AcceptedAnswerId="2371" CreationDate="2013-10-10T16:50:40.050" Score="30" ViewCount="3838" Body="&lt;p&gt;Ever since Jules Verne wrote &quot;From the Earth to the Moon&quot; &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_cannon&quot;&gt;space guns&lt;/a&gt; have long been proposed as a method of launching objects into space. And for non-human, non-fragile payloads they make sense as the payload doesn't have to carry much of it's fuel with it. But apart from a few experiments no successful space gun has been developed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'm wondering, what technological/engineering hurdles that need to be overcome before we can see a working and efficient space gun?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="1142" LastEditDate="2014-02-26T21:55:32.497" LastActivityDate="2022-02-15T15:56:30.293" Title="What technological hurdles prevent the development of a space gun?" Tags="|launch|space-gun|" AnswerCount="7" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2371" PostTypeId="2" ParentId="2370" CreationDate="2013-10-10T16:57:15.487" Score="14" Body="&lt;p&gt;Scaling is probably the biggest issue.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, you always need an upper stage, as a gun cannot get you into a reasonable orbit.  It can get you high enough, and maybe fast enough, but not into an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now your gun has to be large enough to launch a payload, including at least some kind of upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you scale up to those kind of sizes to deliver meaninful payloads, these guns get VERY large and much more complex than would seem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many issues that come up with scaling.  As things get larger, their frontal area must get longer, air resistance gets higher, and now more energy is needed to push all the way through the atmosphere, with sufficient velocity at the end, to be close enough to orbital velocity so that a small upper stage can circularize the orbit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as you trade off upper stage size/payload/initial speed, the initial G load gets higher and higher in order to deliver a useful payload.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-10T22:26:04.740" LastActivityDate="2013-10-10T22:26:04.740" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2372" PostTypeId="1" AcceptedAnswerId="2425" CreationDate="2013-10-10T18:16:28.107" Score="45" ViewCount="7341" Body="&lt;p&gt;Historically, &lt;a href=&quot;http://en.wikipedia.org/wiki/Mutiny&quot;&gt;mutiny&lt;/a&gt; has often occurred in close-quarter situations. While the International Space Station (ISS) is not a military operation, the term &lt;em&gt;mutiny&lt;/em&gt; is often used to refer any sort of rebellion against the prevailing authorities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the people chosen to crew the ISS are thoroughly vetted and selected for their outstanding behavior and skill, it is not completely beyond the realm of possibility that two or more individuals (maybe under the influence of cabin fever) could stage a mutiny on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What laws and/or procedures are in place in the event of such a mutiny?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-07-09T16:34:07.870" LastActivityDate="2020-05-26T14:58:24.433" Title="What would happen if a mutiny occurred on the International Space Station?" Tags="|iss|astronauts|law|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2373" PostTypeId="1" AcceptedAnswerId="2375" CreationDate="2013-10-10T18:30:07.477" Score="11" ViewCount="661" Body="&lt;p&gt;Looking at the &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Scientific_instruments&quot;&gt;scientific instruments&lt;/a&gt; of JUNO, there doesn't appear to be a camera. Furthermore, no mention is made of any photos being returned in the s&lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Scientific_objectives&quot;&gt;cientific objectives&lt;/a&gt;. Will anything resembling a photo be returned from JUNO?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-10T19:07:20.503" Title="Will JUNO be able to return any photographs?" Tags="|instrument|juno|jupiter|imaging|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2374" PostTypeId="2" ParentId="2373" CreationDate="2013-10-10T18:50:03.993" Score="7" Body="&lt;p&gt;Yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Scientific_instruments&quot;&gt;The list you cited&lt;/a&gt; includes a link to &lt;a href=&quot;http://en.wikipedia.org/wiki/JunoCam&quot;&gt;JunoCam&lt;/a&gt;, which is planned to return a relatively limited number of images.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's described as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A visible light camera/telescope, included in the payload to&#xA;  facilitate education and public outreach. It will operate for only&#xA;  seven orbits around Jupiter because of the planet's damaging radiation&#xA;  and magnetic field.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It appears that photography isn't Juno's primary mission, but as long as it's in the neighborhood ...&lt;/p&gt;&#xA;" OwnerUserId="574" LastActivityDate="2013-10-10T18:50:03.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2375" PostTypeId="2" ParentId="2373" CreationDate="2013-10-10T18:50:32.040" Score="7" Body="&lt;p&gt;Actually, there is a visible light camera/telescope onboard. From a &lt;a href=&quot;http://www.lockheedmartin.com/us/products/juno.html&quot;&gt;page on Juno by Lockheed Martin&lt;/a&gt; that built it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Junocam&lt;/strong&gt;: An education and public outreach visible-light camera&#xA;  provides first pictures of Jupiter’s poles (Malin Space Science&#xA;  Systems).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Going through &lt;a href=&quot;http://descanso.jpl.nasa.gov/DPSummary/Juno_DESCANSO_Post121106H--Compact.pdf&quot;&gt;Juno Telecommunications Design and Performance documentation&lt;/a&gt; (PDF) to find more explanations on JunoCam, I've found these requirements, capabilities, and constraints during the Science Orbits phase (page 2):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Primary science observations will be obtained within 3 hours of&#xA;  closest approach to Jupiter during each orbit. Calibrations,&#xA;  occasional remote sensing, and magnetospheric science  observations&#xA;  are planned throughout the orbits. JunoCam will provide visible-light&#xA;  three-color  images for educational and public outreach purposes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;later on page 79:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;MWR [Microwave Radiometer], JIRAM [Jovian Infrared Auroral Mapper],&#xA;  and JunoCam will be off during Grav [Gravity Science] passes, although we will consider&#xA;  using JIRAM and JunoCam as allowed by power, thermal, and other Flight&#xA;  System and mission operations constraints.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and on that same page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Later orbits are also constrained by data rate and volume, but we no&#xA;  longer expect to use MWR, JIRAM, and JunoCam.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So JunoCam will provide visible wavelength photographs from Jupiter before and during Juno's insertion into Jupiter's polar orbit, will provide first pictures of Jupiter's polar regions, and they expect it to operate on close approach and up to expected 7 orbits, since Grav will be active on Orbit 4 and later from Orbit 9 to Orbit 32-33, when JunoCam cannot be operational due to bandwidth, power, thermal, and other Flight System and mission operations constraints.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the JunoCam is not expected to make it for many more orbits due to Jupiter's damaging radiation and magnetic field, those 7 orbits of operation are all anyone is hoping to achieve with it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-10T19:07:20.503" LastActivityDate="2013-10-10T19:07:20.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2376" PostTypeId="2" ParentId="2363" CreationDate="2013-10-10T18:51:45.307" Score="6" Body="&lt;p&gt;In terms of interplanetary travel-time, there's a couple of different limits that are overcome as the presumptive technology-level increases (AKA sci-fi):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1.) Limited by Energy:&lt;/strong&gt;  spacecraft use a boost-and-coast method with gravity-assists where possible from planetary bodies along the way. (this is where human space-travel is at right now). Limited to speeds of 5-15 kilometers/second, depending on how large a craft and how much one can spend on fuel and what gravity-assists we can get.&#xA;Travel from Earth to Jupiter (~4 AU) takes many months. Voyager 1 did it in 16 months. Humans traveling in space have HUGE limitations and it's terribly expensive and dangerous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2.) Limited by Inertia:&lt;/strong&gt; If we were able to get LOTS of energy into a spacecraft, i.e. fusion-power or antimatter, etc. and a way to generate lots of thrust with little reaction-mass (aka a plasma-drive), we'd be able to constantly accelerate a craft on a trip. With humans on-board, we'd generally be limited to 1-2 g's of acceleration (to avoid injuring or killing passengers). Travel between planets in our solar system would generally be a matter of days. Unmanned craft could go even faster, accelerating at 5-10 g's - depending on how well-build the craft is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on how good our power and thrust generation is, we could get to nearby stars in a matter of a few generations (i.e. ~100 years to get to Tau Ceti) - so interstellar colonies are a possibility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;3.) Limited by Lightspeed&lt;/strong&gt;: If we then developed some way to reduce or eliminate inertia (i.e. manipulating the Higgs boson ) and could accelerate as much as we wanted so long as speeds were below lightspeed. Travel between planets would be a matter of 5-25 minutes between inner planets (Mars to Venus for example), and outer planets are a matter of hours (Jupiter to Neptune and the like) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interstellar travel would still take 5-20 years to get between the near star systems (5-10 light-years) depending on exactly what percent of c we were able to achieve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;4.) Faster-Than-Light travel:&lt;/strong&gt; Wormholes and Warp-Drive - the sky's the limit here. Travel could be instantaneous or still take elapsed time. It all depends on how you want your sci-fi universe to work.&lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="692" LastEditDate="2013-10-10T20:06:59.867" LastActivityDate="2013-10-10T20:06:59.867" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="2377" PostTypeId="1" AcceptedAnswerId="2390" CreationDate="2013-10-10T19:28:09.773" Score="13" ViewCount="1751" Body="&lt;p&gt;There are 20 objects in orbit associated with the Falcon 9 v 1.1 launch. So far only 1 has been identified. 5 were payloads of the launch, plus the booster and some debris. Bottom line, &lt;a href=&quot;http://moonandback.com/2013/10/02/analysis-of-newest-falcon-9%E2%80%B2s-first-flight-continues/&quot;&gt;11 objects were expected&lt;/a&gt;, and 9 are unexpected. What are the other objects? &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="709" LastEditDate="2013-10-11T09:24:37.110" LastActivityDate="2013-10-13T15:56:09.937" Title="What are the mystery Falcon 9 v 1.1 objects?" Tags="|launch|debris|spacex|falcon-9|rockets|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2378" PostTypeId="2" ParentId="2377" CreationDate="2013-10-10T20:05:25.707" Score="10" Body="&lt;p&gt;Here's what's known about them, from a bit of analysis. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Object G is the booster. It has the largest RCS, and JSPOC has identified it as such.&lt;/li&gt;&#xA;&lt;li&gt;A-F would be likely candidates for the primary payloads. They are grouped closely to the rocket booster, occupying a similar orbit. There were 5 payloads, plus there might have been some sort of a payload adapter. JSPOC seems to think they might be important, as it put them ahead of the object booster. Normally the order is payloads, separation hardware, booster stages, and debris.&lt;/li&gt;&#xA;&lt;li&gt;There appears to generally speaking be 2 groups of objects, in 2 distinct orbits. Even early on, as seen below, this separation occurred. Objects H, J, K, L, and U all seem to be in a similar orbit to the booster and payloads, while objects M, N, P, Q, R, S, T, and V appear clustered together behind. &lt;/li&gt;&#xA;&lt;li&gt;On October 1, Object U is shown to be decaying significantly. I suspect that this object should have been grouped with the earlier objects, perhaps paired with Object K, both being a &lt;a href=&quot;http://www.spacex.com/falcon-heavy#payload&quot; rel=&quot;noreferrer&quot;&gt;payload fairing&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;Objects M, N, P, and V have a higher perigee than the payloads. I don't know what this indicates, but it is curious.&lt;/li&gt;&#xA;&lt;li&gt;Objects R, S, and T look like they are rapidly decaying, indicating that they are light. They are on a rapid return course, looking to re-enter no later than this weekend.&lt;/li&gt;&#xA;&lt;li&gt;Object I became U, O became V, to avoid confusion (&lt;a href=&quot;http://chat.stackexchange.com/transcript/message/11621489#11621489&quot;&gt;Chris in chat&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tJF3f.jpg&quot; alt=&quot;Separation of satellites&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/qfVNg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A-F are primary payload or related.&lt;/li&gt;&#xA;&lt;li&gt;G is the rocket booster&lt;/li&gt;&#xA;&lt;li&gt;U and K are likely payload fairings&lt;/li&gt;&#xA;&lt;li&gt;R, S, T are light quickly decaying objects. These are likely additional debris created along the way, before the rocket stopped firing, or when the rocket stopped firing (If given some impulse from the rocket as it stopped)&lt;/li&gt;&#xA;&lt;li&gt;H, J, and L are likely objects separated during the deployment of the satellites.&lt;/li&gt;&#xA;&lt;li&gt;M, N, P, Q, and V are a mystery.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-10T20:17:43.697" LastActivityDate="2013-10-10T20:17:43.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2379" PostTypeId="1" AcceptedAnswerId="2400" CreationDate="2013-10-11T03:32:12.343" Score="11" ViewCount="207" Body="&lt;p&gt;This image of the &lt;a href=&quot;http://mars.jpl.nasa.gov/mgs/msss/camera/images/aug_1_00/index.html&quot; rel=&quot;noreferrer&quot;&gt;Sirenum Fossae Trough - MGS MOC Release No. MOC2-248&lt;/a&gt; of Mars shows spots on the northern lip of the crater that draw my attention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the &lt;em&gt;Mars Global Surveyor - Mars Orbiter Camera&lt;/em&gt; page linked above for the original 768x1536 px GIF, here are some zooms and crops of the images to highlight the 3 dark spots.  Most zooms are showing the image from further away (smaller) but the last shows the section at double size (2x zoom in).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/fuQvd.gif&quot; alt=&quot;1/4 Zoom&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/90S4f.gif&quot; alt=&quot;1/2 Zoom&quot;&gt;&lt;br&gt;&lt;img src=&quot;https://i.stack.imgur.com/908zB.gif&quot; alt=&quot;1to1 Zoom (crop)&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/k60lB.gif&quot; alt=&quot;2 x Zoom&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the spots are on the border between the crater and ravine.  One is near the right of the last image, while the other two are over to the left.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only things I could think them to be are either:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Glitches in the image (unlikely in such a pattern).&lt;/li&gt;&#xA;&lt;li&gt;Craters from smaller meteorites (unlikely pattern of hits, and they seem far too dark).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Does anyone know what they are?&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="238" LastEditDate="2013-10-13T08:24:04.423" LastActivityDate="2013-10-13T08:24:04.423" Title="Features on border of a crater &amp; ravine on Mars" Tags="|mars|planetary-science|geology|crater|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2381" PostTypeId="1" AcceptedAnswerId="2451" CreationDate="2013-10-11T15:00:34.820" Score="8" ViewCount="298" Body="&lt;p&gt;Iran is soon to launch two satellites which will reportedly take images &quot;with a high degree of accuracy of less than 10 meters&quot; (&lt;a href=&quot;http://www.azernews.az/region/60600.html&quot; rel=&quot;nofollow&quot;&gt;azernews&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What will these satellites be imaging?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorDisplayName="user29" LastEditDate="2013-10-22T03:00:05.377" LastActivityDate="2013-11-30T22:37:36.743" Title="What will the new Iranian satellites be imaging?" Tags="|artificial-satellite|future-missions|optical|iran|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2382" PostTypeId="1" AcceptedAnswerId="2413" CreationDate="2013-10-11T17:02:20.033" Score="5" ViewCount="195" Body="&lt;p&gt;It seems like the primary focus of JUNO is to get a better idea of how Jupiter works. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Scientific_objectives&quot; rel=&quot;nofollow&quot;&gt;Science Objectives&lt;/a&gt; listed on Wikipedia don't seem to include anything about Jupiter's moons, other than a desire to not impact them. Will JUNO contribute in any way to the understanding of Jupiter's moons?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2013-10-11T23:36:15.623" LastActivityDate="2013-10-14T09:39:40.793" Title="Will JUNO do any science of Jupiter's Moons?" Tags="|jupiter|juno|moons|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2383" PostTypeId="1" AcceptedAnswerId="2385" CreationDate="2013-10-11T18:21:28.877" Score="21" ViewCount="5003" Body="&lt;p&gt;In several NASA animations of missions, I've noticed that the spacecraft is rotating while traveling in deep space.  This was true of &lt;a href=&quot;http://www.youtube.com/watch?v=P4boyXQuUIw&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity&lt;/a&gt; when that video came out.  Now the &lt;a href=&quot;http://www.youtube.com/watch?v=NNt9DTw7nzk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno&lt;/a&gt; mission demonstrates the same behavior.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I saw one &lt;a href=&quot;https://space.stackexchange.com/questions/867/how-is-spin-stability-used-in-spacecraft&quot;&gt;related question&lt;/a&gt;, but there, stability of rocket boosting is given as a justification.  In the videos I'm referencing, they're mid-travel, just floating in space, so that doesn't really answer it for me.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-10-15T10:48:00.103" Title="Why does NASA intentionally spin space probes?" Tags="|probe|attitude|station-keeping|rotation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2385" PostTypeId="2" ParentId="2383" CreationDate="2013-10-11T19:22:33.497" Score="22" Body="&lt;p&gt;It's done in the interest of &lt;strong&gt;stability&lt;/strong&gt;, both inertial and thermal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inertial stability: basically if something is spinning, it resists attempts to change its spin axis. So, if you align your spin axis with, say, the axis along which your thrusters point, it's easier to keep yourself pointed in the right direction while you burn. After your burn, you still want to keep some attitude profile (say, to keep your antenna pointing at Earth), and spin-stabilization is an easy way of doing that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermal stability: on your spacecraft, whichever surface can see the Sun is hot, and whichever surface cannot is cold. This can introduce thermal gradients, which are generally undesirable.  However, if you introduce a spin, now you have a symmetry around your spin axis, and thus less of a thermal gradient -- this is often referred to as a &quot;barbecue roll&quot;.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-10-11T19:22:33.497" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2386" PostTypeId="1" CreationDate="2013-10-11T20:40:41.197" Score="12" ViewCount="462" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/juno/spacecraft/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Juno page&lt;/a&gt; says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Juno spacecraft launched aboard an Atlas V-551 rocket from Cape Canaveral, Fla., on Aug. 5, 2011, and will reach Jupiter in July 2016. The spacecraft will orbit Jupiter 32 times, skimming to within 3,100 miles (5,000 kilometers) above the planet's cloud tops, for approximately one year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The environment around Jupiter itself is reportedly corrosive thanks to Io. In addition, the craft may suffer mechanical/tidal stresses from Jupiter &amp;amp; her family of moons.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How were Juno's solar arrays tested for mechanical/tidal stresses from Jupiter, and her family of moons?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Will Juno be able to do science if a single array remains functional? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;What is the typical power redundancy built into a spacecraft?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="6816" LastEditDate="2017-09-16T09:42:49.783" LastActivityDate="2017-09-17T06:26:24.037" Title="What is the typical power redundancy built into a spacecraft?" Tags="|jupiter|power|juno|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2387" PostTypeId="1" CreationDate="2013-10-11T21:40:56.573" Score="63" ViewCount="9419" Body="&lt;p&gt;I&amp;nbsp;saw &lt;em&gt;Gravity&lt;/em&gt; the other day and while visually and emotionally compelling, and besides the liberties taken with orbits, something else struck me as unrealistic, but I'm not sure if it is or isn't: the apparently lack of physical security on spacecraft.  As for its counterpart, I have a vague notion there is ferocious information security so someone with a transmitter can't deorbit a satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While getting into orbit is something of an obstacle, are there no locks on airlocks?  Is space like Canada?&lt;/p&gt;&#xA;" OwnerUserId="881" LastEditorUserId="6471" LastEditDate="2017-03-02T10:49:29.983" LastActivityDate="2017-03-02T10:49:29.983" Title="Is there no physical security in space, other than being in space?" Tags="|iss|space-station|security|quest-joint-airlock|airlock|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2388" PostTypeId="1" AcceptedAnswerId="2389" CreationDate="2013-10-12T00:59:32.800" Score="11" ViewCount="734" Body="&lt;p&gt;With the possible exception of, for instance, a fleet of GEO communications satellites, most spacecraft are going to have very unique requirements. I would imagine it's difficult to use the same software on different missions, if it's possible at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To what extent is flight software unique to the mission&lt;/strong&gt;? Are large swaths of code shared within institutions? What about between different institutions? Has any mission just thrown their code onto SourceForge?&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2014-01-24T19:01:53.360" Title="Is flight software shared among different spacecraft?" Tags="|spacecraft-development|software|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2389" PostTypeId="2" ParentId="2388" CreationDate="2013-10-12T02:53:30.400" Score="9" Body="&lt;p&gt;I only know of a few examples, but yes, there are examples of large swaths of code being shared between spacecraft and sometimes between institutions.  Mars Pathfinder is at the root of one such family tree, with much of that code inherited for Mars Exploration Rover and Mars Science Laboratory, and another offshoot of the Mars Pathfinder code taking root and evolving in Lockheed-Martin spacecraft from Mars Polar Lander to Mars Reconnaissance Orbiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have seen cubesat open-source software, but I am not aware of any large spacecraft software being made open source.  United States companies and institutions would not be allowed to put flight software on any sort of open repository due to ITAR regulations.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-10-12T02:53:30.400" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2390" PostTypeId="2" ParentId="2377" CreationDate="2013-10-12T03:04:14.893" Score="9" Body="&lt;p&gt;&lt;strong&gt;UPDATE 2013-10-13&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=32946.msg1108277#msg1108277&quot;&gt;The latest update names&lt;/a&gt; B as CUSat 1 and C as DANDE. All payloads have now been identified. Interestingly, they say CUSat-2 is attached to the upper stage (object G). Everything after object G is identified as &lt;strong&gt;debris&lt;/strong&gt;. &lt;strong&gt;Additionally, there are now 26 total objects cataloged, with debris pieces labeled H-AB&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;It helps to understand what was expected in the first place. Here's a list of the payloads:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://www.asc-csa.gc.ca/eng/satellites/cassiope.asp&quot;&gt;Cassiope&lt;/a&gt; (1 piece expected)&lt;/strong&gt; - the primary payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://spacegrant.colorado.edu/boulderstudents/boulderprojects/dande&quot;&gt;DANDE&lt;/a&gt; (2 pieces expected)&lt;/strong&gt; - According to &lt;a href=&quot;http://spacegrant.colorado.edu/about-dande/spacecraft&quot;&gt;their website&lt;/a&gt;, DANDE separates from the upper stage &lt;em&gt;with a lightband adapter bracket&lt;/em&gt;, which then separates from the spacecraft itself. Apparently, this separation has not yet occurred, according to the DANDE team.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/popacs.htm&quot;&gt;POPACS&lt;/a&gt; (7 pieces expected - 3 spheres, 4 spacers)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://cornellsun.com/blog/2013/09/23/cornell-satellite-to-launch-after-delays/&quot;&gt;CUSat&lt;/a&gt; (only 1 piece expected - NOT 2 as reported elsewhere)&lt;/strong&gt; - the linked article has details about the decision to keep them mated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Total&lt;/strong&gt;: 11 objects associated with the payloads. We can add one for the rocket body, bringing us to &lt;strong&gt;12 expected objects&lt;/strong&gt;, and &lt;strong&gt;8 unknowns&lt;/strong&gt;. The fairing was separated before the vehicle reached orbit, as is typically the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @PearsonArtPhoto stated, Objects A-F would be the payloads, and G has been identified as the upper stage. In fact, it has recently been &lt;a href=&quot;http://www.satobs.org/seesat/Oct-2013/0115.html&quot;&gt;confirmed&lt;/a&gt; that A is Cassiope, and D-F are the three POPACSS spheres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that that is all cleared up, it brings us to your actual question: what's everything else? To clarify, it is somewhat rare for a so-called successful launch to result in more tracked objects than expected, so the fact that there's even one extra object is noteworthy. @PearsonArtPhoto did a good job looking at the orbits of the unknown objects, so instead I'll focus on speculating about the source of the pieces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has not yet released much information about the upper stage test fire, other than &lt;a href=&quot;http://www.parabolicarc.com/2013/10/02/ses-awaits-answer-falcon-9-anomaly-explosion-claim-denied/&quot;&gt;there was an anomaly, and no explosion&lt;/a&gt;. The spread of the unknown objects is troubling - energy &lt;em&gt;was&lt;/em&gt; added to them in some way - either through a short second burn (before it shut down) or via an explosion of some kind - even just a tank rupture. &lt;a href=&quot;http://planet4589.org/space/jsr/back/news.687&quot;&gt;According to Jonathan McDowell&lt;/a&gt;, the delta-V for the scatter is about 60 m/s, which is quite significant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A couple of the objects have/had very large decay rates, indicating that they are likely the pieces of insulation that SpaceX posited. However, many of the unknown objects have &lt;em&gt;not&lt;/em&gt; yet decayed, so they likely have a higher ballistic coefficient (mass/cross sectional area), i.e. they are more dense than typical insulation. It's possible that there were other pieces associated with the secondary payloads, but something &quot;coming loose&quot; from an upper stage (and not causing other, bigger problems) would be a pretty egregious fault, and I think it's unlikely. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;In my opinion, there &lt;em&gt;was&lt;/em&gt; some sort of energetic event immediately following the upper stage firing attempt, resulting in several debris pieces.&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bottom line&lt;/strong&gt;: SpaceX may not be talking much, but there are lots of people paying attention and trying to figure this out (including the Air Force... &lt;a href=&quot;http://www.aviationweek.com/Article.aspx?id=/article-xml/asd_10_10_2013_p01-01-625424.xml&quot;&gt;eventually&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-13T15:56:09.937" LastActivityDate="2013-10-13T15:56:09.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2391" PostTypeId="5" CreationDate="2013-10-12T03:24:03.597" Score="0" Body="" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-12T03:24:03.597" LastActivityDate="2013-10-12T03:24:03.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2392" PostTypeId="4" CreationDate="2013-10-12T03:24:03.597" Score="0" Body="Questions pertaining to Iran's involvement in space." OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-12T03:24:03.597" LastActivityDate="2013-10-12T03:24:03.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2393" PostTypeId="5" CreationDate="2013-10-12T03:26:15.233" Score="0" Body="" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-12T03:26:15.233" LastActivityDate="2013-10-12T03:26:15.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2394" PostTypeId="4" CreationDate="2013-10-12T03:26:15.233" Score="0" Body="A close approach between two orbiting objects." OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-12T03:26:15.233" LastActivityDate="2013-10-12T03:26:15.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2395" PostTypeId="5" CreationDate="2013-10-12T03:27:28.143" Score="0" Body="" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-12T03:27:28.143" LastActivityDate="2013-10-12T03:27:28.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2396" PostTypeId="4" CreationDate="2013-10-12T03:27:28.143" Score="0" Body="NASA's Tracking and Data Relay Satellite System - a network of satellites in geosynchronous orbit used to facilitate communications with and tracking of spacecraft in lower orbits." OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-12T03:27:28.143" LastActivityDate="2013-10-12T03:27:28.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2397" PostTypeId="2" ParentId="2387" CreationDate="2013-10-12T05:15:23.897" Score="41" Body="&lt;p&gt;First, I have to mention I haven't seen the movie yet. I've resisted temptation after seeing so many technical inaccuracies mentioned on Twitter by the International Space Station (ISS) astronauts that I follow. No, not the astrophysicist &lt;a href=&quot;https://en.wikipedia.org/wiki/Neil_deGrasse_Tyson&quot; rel=&quot;noreferrer&quot;&gt;Neil DeGrasse Tyson&lt;/a&gt;, even though he was mentioned most frequently in the mass media for taking it a personal quest of his to debunk all the inaccuracies that were apparently littered throughout the movie, about its physics, technical details, even visuals. I mean people that were actually there, and most of them actually enjoyed the movie (I've read many of their reviews of the movie so this is my impression), albeit mostly for other reasons than its accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could give you detailed technical specification of all the airlocks, hatches and whatnot that has access to and from the ISS (yes, there are several, although I suspect the movie tried to present the &lt;a href=&quot;https://en.wikipedia.org/wiki/Quest_Joint_Airlock&quot; rel=&quot;noreferrer&quot;&gt;Quest joint airlock&lt;/a&gt;?), So I think this is best left to be explained by those that were both there, and have seen the movie. For example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Leroy_Chiao&quot; rel=&quot;noreferrer&quot;&gt;Leroy Chiao&lt;/a&gt; (&lt;a href=&quot;https://twitter.com/AstroDude&quot; rel=&quot;noreferrer&quot;&gt;@AstroDude on Twitter&lt;/a&gt;), a former NASA astronaut and commander of the International Space Station (ISS) said in &lt;a href=&quot;http://www.space.com/23105-gravity-film-review-astronaut-leroy-chiao.html&quot; rel=&quot;noreferrer&quot;&gt;his review of the movie for Space.com&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... &lt;strong&gt;&lt;em&gt;existing spacecraft are depicted with hatches and other controls&#xA;  that are in the wrong places.&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... &lt;strong&gt;&lt;em&gt;there is no way to enter an airlock from the outside, unless it&#xA;  had already been prepared for such an entry. The inner hatch would&#xA;  have to already be sealed.&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mw5tj.jpg&quot; alt=&quot;Astronaut Leroy Chiao in a Russian Sokol spacesuit.&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Astronaut Leroy Chiao in a Russian Sokol spacesuit (Source: &lt;a href=&quot;http://www.space.com/23105-gravity-film-review-astronaut-leroy-chiao.html&quot; rel=&quot;noreferrer&quot;&gt;Space.com&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Additionally, again from my experience on Twitter, I've seen it mentioned that the repressurisation process in the movie took perhaps 3 seconds. That is again highly inaccurate and the repressurisation of the crew lock takes roughly 45 minutes, but can be somewhat hurried in case of an emergency. It would most certainly not be 3 seconds from 0 psi (0 kPa) to 14.7 psi (101 kPa) that is the ISS internal atmospheric pressure on astronauts' return from their EVA (Extravehicular Activity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, when &lt;a href=&quot;https://space.stackexchange.com/a/1626/49&quot;&gt;Luca Parmitano's helmet started filling with water from his PLSS&lt;/a&gt; (Portable Life Support Unit) and he rushed back to the airlock, it took many agonizing minutes to repressurize it, as he wrote in &lt;a href=&quot;http://blogs.esa.int/luca-parmitano/tag/water/&quot; rel=&quot;noreferrer&quot;&gt;one of his blog posts&lt;/a&gt;. If it took 3 seconds, or thereabouts, that would literally feel like being inside an explosion. This repressurisation process can be somewhat appreciated by looking at the reverse, the depressurisation process for the Quest joint airlock:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The usual pressure inside the ISS is 1 bar, though in the Quest&#xA;  airlock this is 0.7 bar during depressurisation in connection with&#xA;  nitrogen purging. When the astronauts are in the crewlock ready to&#xA;  start their EVA this pressure is reduced first to 0.35 bar when a leak&#xA;  check is performed on the suits. If this is ok the crewlock is reduced&#xA;  in pressure down to 0.2 bar. The final depressurisation to vacuum&#xA;  occurs through venting through a valve in the EVA hatch. The hatch can&#xA;  now be opened and the EVA can begin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The hatch remains open for the duration of the EVA for several reasons, among which also for astronauts to be able to return in an emergency, and also to conserve the volumes of the nitrogen and oxygen in the external tanks that are used for nitrogen purge,   repressurisation and also to recharge the EMUs (&lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit&quot; rel=&quot;noreferrer&quot;&gt;Extravehicular Mobility Unit&lt;/a&gt;, of which there are &lt;a href=&quot;https://space.stackexchange.com/a/1638/49&quot;&gt;two types onboard the ISS&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And no, none of the ISS external airlocks or berthing hatches open from the outside, so no keys there. Not the Russian docking ports on &lt;a href=&quot;https://en.wikipedia.org/wiki/Poisk_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Poisk&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Pirs_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Pirs&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Rassvet_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Rassvet&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Zvezda&lt;/a&gt; service module, not on &lt;a href=&quot;https://en.wikipedia.org/wiki/Leonardo_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Leonardo&lt;/a&gt; cargo bay, &lt;a href=&quot;https://en.wikipedia.org/wiki/Tranquility_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Tranquility&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Harmony_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Harmony&lt;/a&gt;, the JAXA's &lt;a href=&quot;https://en.wikipedia.org/wiki/JEM-PM&quot; rel=&quot;noreferrer&quot;&gt;Kibō&lt;/a&gt; that has an airlock with a &lt;em&gt;key release&lt;/em&gt; of the CYCLOPS (Kinetic Launcher for Orbital Payload Systems) small satellite launch system but from within the ISS, and not on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Quest_Joint_Airlock&quot; rel=&quot;noreferrer&quot;&gt;Quest joint airlock&lt;/a&gt;. The only hatches that have a sliding opening door that opens from the outside are on the resupply vehicles (there's many, so I'll spare you with the list), that are mostly tightened in place with a set of bolts that need to be removed with a torque wrench.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, I didn't see the movie yet, but I guess its script likely wouldn't work, if they tried to make it 100% accurate. The preparations for EVA take roughly one day with all kinds of, in terms of motion pictures, rather boring procedures that I don't expect anyone would be too interested in. Not in an action movie at least. Maybe in a documentary, but even there it likely wouldn't take longer than perhaps 10 minutes to go through it? So my advice would be that if you've forked out for the tickets, then make the best of it and enjoy the ride. ;)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;P.S.: Oh, and about information security in space that you also mention in your question, here's a nice relevant thread from &lt;a href=&quot;https://security.stackexchange.com/&quot;&gt;Information Security Stack Exchange&lt;/a&gt; on &lt;a href=&quot;https://security.stackexchange.com/q/6424/20074&quot;&gt;What would one need to do in order to hijack a satellite?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-10-19T15:52:13.350" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2398" PostTypeId="2" ParentId="2175" CreationDate="2013-10-12T07:03:58.833" Score="9" Body="&lt;p&gt;Given your condition that for this thought exercise, we are&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;assuming the Solar Impulse assembly is capable of surviving the toxic/corrosive environment &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to the article &lt;a href=&quot;http://www.universetoday.com/15495/cruising-the-cloud-tops-of-venus-with-a-solar-powered-airplane/&quot; rel=&quot;noreferrer&quot;&gt;Cruising the Cloud Tops of Venus With a Solar-Powered Airplane&lt;/a&gt; (Atkinson, 2008), there is a reasonably strong chance that a especially adapted solar impulse flyer could fly in the Venusian atmosphere. According to the studies that it reports, the glider could be covered in solar panels and 'dropped' into the Venusian atmosphere, such as in the diagram below (from the website linked):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5ehdv.jpg&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking lessons from a similar thought exercise performed by NASA in the slides &lt;a href=&quot;http://www.lpi.usra.edu/vexag/oct2009/presentations/hunterVenusLanderTechnologies.pdf&quot; rel=&quot;noreferrer&quot;&gt;Long-Lived Venus Lander Technologies:&#xA;A Brief Discussion Of Technologies Relevant To&#xA;Long-lived Landers For Venus Exploration&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note: I obviously are aware that the slides are for a long lived lander, but some of the adaptations can be applied to a glider).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They state in the slides that the solar efficiency is almost zero, but as you linked, the amount of light present can be useful.  Given that for this question, the assembly can survive the atmospheric chemistry.  The issue is largely based on temperature, wind speed and pressure (atmospheric buoyancy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A key challenge is the electronics, for the motor assembly and for communication with the glider, according to the link above, there already exists technologies that can withstand temperatures of the Venusian atmosphere, specifically, Using &lt;strong&gt;silicon carbide&lt;/strong&gt; as the basis of the electronics as it has been shown to be able to maintain long term operation at temperatures up to 500C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest problem in the having a solar glider on Venus is the wind, the challenge specifically according to the article &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.195.172&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;noreferrer&quot;&gt;Atmospheric Flight on Venus&lt;/a&gt; (Landis et al. 2002) is that &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In order to remain on the sunlit side of&#xA;  Venus, an exploration aircraft will have to be capable of sustained flight at or above the wind&#xA;  speed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A major limitation according to Landis et al. (2002), is that the&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the minimum flight altitude&#xA;  for remaining stationary at the subsolar point was&#xA;  about 70 km. Below this altitude the combination&#xA;  of higher atmospheric density, lower solar energy,&#xA;  higher temperature (and hence lower solar cell&#xA;  performance), and high wind speed made it not&#xA;  possible for this design to indefinitely remain&#xA;  stationary at the subsolar point.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, this is not envisaged as being a major problem, as this could be adapted to be used as part of the aerial exploration, specifically from the article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For exploration of lower altitudes, it is feasible&#xA;  to glide down to low altitudes for periods of several&#xA;  hours, accepting the fact that the airplane ground&#xA;  track will blow downwind, and then climb back to&#xA;  higher altitudes and fly upwind to the original&#xA;  point, allowing both high and low altitudes to be&#xA;  probed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And an alternative is suggested (but not discussed in detail by Landis et al.):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An alternate technique might be to mount solar&#xA;  arrays on the vertical surfaces of the airplane, and&#xA;  to fly in near-polar latitudes. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This adaptation could be used anywhere in the Venusian atmosphere, potentially even on the night side (depending on the sensitivity of the solar panels), as in the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/2237/how-long-will-it-be-light-on-venus-at-night/2257#2257&quot;&gt;How long will it be light on Venus at night?&lt;/a&gt; &lt;em&gt;(yes, it is my own answer, but it is relevant here)&lt;/em&gt;, the refraction of sunlight in the Venusian atmosphere (due to the atmospheric pressure) potentially means that it is never fully dark on the planet (day or night).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A combination of the wind and if the sensitivity of the solar panels is sufficient, then atmospheric exploration by a solar glider on Venus has a potential not to be limited to the day-side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One major factor that comes up in all references, is that amount of 'moving parts' must be kept to a minimum for any long term exploration of Venus (or anywhere realistically), this design fulfils that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, according to studies, it is very feasible for a Venusian-atmospheric glider, not only that, according to Landis et al. 2002 and Atkinson, 2008, it is possible for a fleet of these gliders to profile the Venusian atmosphere and surface.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="-1" LastEditorDisplayName="user838" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-10-12T15:21:37.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2399" PostTypeId="1" AcceptedAnswerId="2401" CreationDate="2013-10-12T16:45:39.990" Score="6" ViewCount="432" Body="&lt;p&gt;How will a satellite powered by RTG be terminated at the end of its  life? &#xA;Is there any set of rules to be followed? What's the general procedure?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-10-14T03:02:42.063" LastActivityDate="2018-11-20T06:00:52.163" Title="How would a satellite powered by RTG be terminated?" Tags="|artificial-satellite|rtg|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2400" PostTypeId="2" ParentId="2379" CreationDate="2013-10-12T17:03:06.217" Score="4" Body="&lt;p&gt;I did further processing on a 80x40 sub-image covering the anomalous pixels:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All images are seen at a browser based 8X zoom scale (FF will dither it smoothly, which makes the lines less sharp, but almost as clear).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The information in pixel x/y, RGB &amp;amp; 'gray scale difference' between that pixel and the surrounding pixels was calculated and shown in a sortable table (of 3200 rows).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each titled section shows an image, as well as the first six rows of the table, indicating both the sort keys used, and the rows selected.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Image of interest - nothing selected.&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/z8U5w.png&quot; width=&quot;640&quot; height=&quot;320&quot; title=&quot;unchanged&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 3 darkest pixels seem obvious, without any further prompt..&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;X   Y   RGB     Diff.   Select&#xA;0   0   192     20.75   -&#xA;1   0   186     21.625  -&#xA;2   0   174     25.625  -&#xA;3   0   177     23.375  -&#xA;4   0   175     24.375  -&#xA;5   0   181     24.25   -&#xA;6   0   189     19.75   -&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;h3&gt;Darkest Pixels&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gJVnw.png&quot; width=&quot;640&quot; height=&quot;320&quot; title=&quot;Darkest&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But here are the darkest pixels high-lit with green cross-hairs just to check our logic. &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;X   Y   RGB ▲   Diff.   Select&#xA;16  25  0       240.625 TRUE&#xA;6   30  0       240.125 TRUE&#xA;74  7   6       230.875 TRUE&#xA;78  31  112     37.875  -&#xA;79  34  112     22.75   -&#xA;75  37  115     31.375  -&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;h3&gt;Next Darkest Pixels&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ttb9M.png&quot; width=&quot;640&quot; height=&quot;320&quot; title=&quot;Darkest Next&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference in gray scale between the 'next darkest' pixels and their immediate neighbors is much less obvious.  Not nearly as obvious as the darkest 3 pixels.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;X   Y   RGB ▲   Diff.   Select&#xA;16  25  0       240.625 -&#xA;6   30  0       240.125 -&#xA;74  7   6       230.875 -&#xA;78  31  112     37.875  TRUE&#xA;79  34  112     22.75   TRUE&#xA;75  37  115     31.375  TRUE&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;h3&gt;Next Biggest Difference&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QlqM0.png&quot; width=&quot;640&quot; height=&quot;320&quot; title=&quot;Diff&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next biggest difference in pixels by gray scale is similarly nondescript.  The difference drops from over 200 pixels to less than 50.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;X   Y   RGB     Diff. ▼ Select&#xA;16  25  0       240.625 -&#xA;6   30  0       240.125 -&#xA;74  7   6       230.875 -&#xA;37  18  205     47.875  TRUE&#xA;31  20  209     45.375  TRUE&#xA;59  13  196     44.875  TRUE&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;h2&gt;Conclusion&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Based on the data it seems those 3 darkest pixels are outliers &amp;amp; anomalous, so I'd say &quot;image glitch&quot; failing a more definitive answer.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Code&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://stackoverflow.com/a/19337751/418556&quot;&gt;this answer on SO&lt;/a&gt; for the Java code used to generate the images.  It hot-links to the first, cropped image.&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="-1" LastEditDate="2017-05-23T12:37:21.217" LastActivityDate="2013-10-12T18:41:40.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2401" PostTypeId="2" ParentId="2399" CreationDate="2013-10-12T17:11:20.150" Score="8" Body="&lt;p&gt;Some examples:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Viking 1 was inadvertently terminated by overwriting the antenna pointing software.  Contact was never regained.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Viking 2's rechargeable battery died, ending that mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So neither Viking lander was deliberately terminated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Galileo orbiter was sent into Jupiter's atmosphere, where it was destroyed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cassini will be sent into Saturn's atmosphere in 2017.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are planetary protection considerations in systems that might harbor biology, such as the Jupiter and Saturn systems, leading to some sort of deliberate disposal of any vehicle that has not been sterilized.  You want to avoid contamination by an inadvertent impact of an unsterilized vehicle on, for example, Europa or Enceladus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no general procedure.  If the spacecraft does not need to be disposed of, like the Voyagers or Curiosity, I expect that they will be operated until they run out of power, fail, or until they are no longer funded.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-10-12T17:11:20.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2402" PostTypeId="5" CreationDate="2013-10-13T02:10:09.587" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-13T02:10:09.587" LastActivityDate="2013-10-13T02:10:09.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2403" PostTypeId="4" CreationDate="2013-10-13T02:10:09.587" Score="0" Body="Questions regarding the JUNO spacecraft, a NASA mission to Jupiter." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-13T02:10:09.587" LastActivityDate="2013-10-13T02:10:09.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2405" PostTypeId="1" CreationDate="2013-10-13T06:07:54.950" Score="13" ViewCount="800" Body="&lt;p&gt;&lt;a href=&quot;http://www.spaceflightnow.com/station/exp25/101007launch/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflightnow.com&lt;/a&gt; writes to say&#xA;(Edited to apply my highlights)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;The improvements are rather significant,&amp;quot; Kelly said in a NASA interview. &amp;quot;The displays that the cosmonauts and myself ... use to control the vehicle have been upgraded to make &lt;strong&gt;flying it easier. It's less operator intensive. But the main and most important change is they have a new, what we would refer to as a flight control computer.&lt;/strong&gt;&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This was in context of the (then) new &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-M#Soyuz_TMA-M&quot; rel=&quot;nofollow noreferrer&quot;&gt;TMA-M craft&lt;/a&gt; which carries a crew of three.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-M#Design&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; further writes to say about TMA-M&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It has an automatic docking system. The ship can be operated automatically, or by a pilot independently of ground control.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In all probability the spacecraft from launch through docking, is controlled by computers. The pilot (or whatever designation applies) is responsible for managing the flight - applying his mind to ensure the flight is nominal, go/no-go decisions. In the absence of dynamic flight surfaces (as in the Space Shuttle) the pilot may, in an emergency, only control the duration &amp;amp; intensity that various rockets fire.&lt;/p&gt;&#xA;&lt;p&gt;In mind of the cost involved, it seems reasonable to assume the crew must cross-train.&lt;/p&gt;&#xA;&lt;p&gt;Are all crew on board a space-bound craft (Soyuz, Shenzhou, other craft of that ilk) trained to monitor, and control flight systems in &amp;amp; out of Earth's atmosphere?&lt;/p&gt;&#xA;&lt;p&gt;For instance, say, an emergency de-orbit (God forbid!) or launch abort, or  ...&lt;/p&gt;&#xA;&lt;p&gt;EDIT:&#xA;The situation was a little different with the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS&lt;/a&gt; aka Space Shuttle which was possessed of a true cock-pit with flight-controls. &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/launch/landing101.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; writes about this to say&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Landing-5 minutes&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The orbiter's velocity eases below the speed of sound about 25 statute miles from the runway. As the orbiter nears the Shuttle Landing Facility, the commander takes manual control, piloting the vehicle to touchdown on one of two ends of the SLF.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Operations in orbit though (dock/undock/maneuver) would still have been computer controlled, and astronaut managed though.&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-18T19:39:50.833" Title="Are all crew on board a spacecraft trained to handle flight-systems?" Tags="|astronauts|flight-computer|training|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2406" PostTypeId="1" AcceptedAnswerId="2530" CreationDate="2013-10-13T11:22:44.503" Score="21" ViewCount="2501" Body="&lt;p&gt;Between ground control and the ISS there is the occasional short communications loss which they are obviously able to deal with, but is there a procedure to follow if the communication is lost for extended periods of time? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that ground control does relay information such as impending debris in the path of the ISS so that the orbit can be changed slightly, this is obviously important enough for the safety of the crew alone to warrant a procedure for downed communications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;e.g If communications went down for 24 hours or more what actions would ground control and the astronauts in the ISS take?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be more specific &lt;strong&gt;this isn't just about procedure to regain communications&lt;/strong&gt;, but all procedure, including &lt;strong&gt;what must be done to ensure the continued health and safety of the crew of the ISS&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastEditorUserId="288" LastEditDate="2013-10-13T15:19:09.587" LastActivityDate="2013-10-25T01:15:06.140" Title="What is the procedure if communication with the ISS is lost for an extended period?" Tags="|iss|communication|safety|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2407" PostTypeId="2" ParentId="2388" CreationDate="2013-10-13T13:18:37.523" Score="12" Body="&lt;p&gt;Much of flight software is done according to a &quot;clone &amp;amp; own&quot; principle - that is, code from a previous and similar missions are adopted and extended for the new spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The entire source code for spacecrafts are not freely available, though NASAs Goddard Space Flight Center have made their &lt;a href=&quot;http://sourceforge.net/projects/coreflightexec/files/cFE-6.1.1/&quot; rel=&quot;noreferrer&quot;&gt;Core Flight Executive&lt;/a&gt; available, a base platform for flight software used in a few recent missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an &lt;a href=&quot;http://www.youtube.com/watch?v=00b9b2xn8R0&quot; rel=&quot;noreferrer&quot;&gt;overview&lt;/a&gt; of cFE, and if you're interested in the software and heritage on the Curiosity rover, see &lt;a href=&quot;http://www.youtube.com/watch?v=9jVt5vb68xA&quot; rel=&quot;noreferrer&quot;&gt;this presentation&lt;/a&gt;, and the &lt;a href=&quot;http://flightsoftware.jhuapl.edu&quot; rel=&quot;noreferrer&quot;&gt;Flight Software Workshop&lt;/a&gt; have many other insights into the world of flight software.&lt;/p&gt;&#xA;" OwnerUserId="52" LastEditorUserId="52" LastEditDate="2014-01-24T19:01:53.360" LastActivityDate="2014-01-24T19:01:53.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2408" PostTypeId="2" ParentId="787" CreationDate="2013-10-13T20:30:26.817" Score="6" Body="&lt;p&gt;I'll only give a small overview if this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Or do deep space exploration probes need this in general? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Inherently, they don't - but often they do perform time synchronization, or clock re-calibration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The clock on spacecrafts is a counter, often maintained in software, derived from a hardware oscillator. An absolute(1) (earth) time can be derived from that counter when we know when it started and the period of which it is incremented. So time on the spacecraft is a relative time, just a counter from when it was started. (This is conceptually the same as how time is kept on a normal PC).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, all oscillators drift, the counter wraps around, and there's other problems with this kind of timekeeping, but carrying atomic clock for timekeeping is normally infeasible on spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as long as one knows the value of the onboard clock (the counter), when it was started, how much that clock has drifted, all data is available to calculate the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculating the absolute (earth) time can be done by ground systems derived from the relative timestamps of data from the spacecraft clock to earth time. Commands sent to the spacecraft can be translated the other way, from the absolute time used on earth to the future relative time used on the spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ofcourse, the translating to/from earth time to the spacecraft time can be done onboard the spacecraft as well instead of on ground systems - in such cases&#xA;periodical time synchronization is needed so correct time can be maintained. Correct time in this case can conceptually just be to transmit the current absolute time to the spacecraft, and have it maintain the difference between that time and its onboard counter, so it can just add an offset from the onboard clock to get the absolute time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether absolute time is kept on the spacecraft or the ground system depends on the spacecraft and mission - often both are done in order to ease and prevent errors in timekeeping from coming up. In either case telemetry from the spacecraft about the onboard clock, its value, whether it has been reset, overflowed, and other parameters are very important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some references:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://messenger.jhuapl.edu/the_mission/publications/Hudson_Colwell.2011.pdf&quot; rel=&quot;noreferrer&quot;&gt;Spacecraft Clock Maintenance for MESSENGER&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/43244/1/12-3587_A1b.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Effects of Clock Drift on the Mars Exploration Rovers&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf11-1.php&quot; rel=&quot;noreferrer&quot;&gt;Basics of space flight&lt;/a&gt; chapter 11 in particular&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sourceforge.net/projects/coreflightexec/files/cFE-6.1.1/&quot; rel=&quot;noreferrer&quot;&gt;Core Flight Executive&lt;/a&gt; source code and accompanying documents&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(1) Ofcourse, the &quot;absolute&quot; time we use is really also a relative time from an epoch and according to some more or less arbitrary calendar.&lt;/p&gt;&#xA;" OwnerUserId="52" LastEditorUserId="52" LastEditDate="2013-10-13T20:36:35.700" LastActivityDate="2013-10-13T20:36:35.700" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2409" PostTypeId="1" AcceptedAnswerId="2410" CreationDate="2013-10-14T03:00:34.740" Score="15" ViewCount="1303" Body="&lt;p&gt;I'm currently reading through a book covering the early US space program - up through Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the book, it mentions that one of the crew on one of the flights was sick. He didn't tell mission control about it until later, though. When he finally did, the author made a comment like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why he waited so long, I don't know. If he had reported it earlier, the doctors could have prescribed &lt;strong&gt;pills from the on-board supply&lt;/strong&gt; to cure the sickness.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What, exactly, did this onboard supply contain?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2014-09-26T09:46:12.710" Title="What medicines were onboard the Apollo flights?" Tags="|apollo-program|medical|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2410" PostTypeId="2" ParentId="2409" CreationDate="2013-10-14T03:41:44.047" Score="12" Body="&lt;p&gt;Yes medicines are carried on all space flights.&lt;/p&gt;&#xA;&lt;p&gt;The astronauts will communicate with doctors in the ground station, and doctors will monitor the astronauts periodically.&lt;/p&gt;&#xA;&lt;h2&gt;HEADACHE&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Mercury crews carried nothing for&#xA;pain, Gemini missions carried Aspirin&#xA;and injectable Demerol, and Apollo&#xA;added &lt;strong&gt;Tylenol and Darvon&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;BACKPAIN&lt;/h2&gt;&#xA;&lt;p&gt;The astronauts are advised to stand with the support of the wall for supporting the spinal cord, and pain killers are not often used.&lt;/p&gt;&#xA;&lt;h2&gt;INFECTION&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Minor infections of the skin, eyes and&#xA;respiratory tract were reported 13&#xA;times in Apollo (including stomatitis,&#xA;pharyngitis, recurrent inguinal and&#xA;axillary infections) (2), and 8 times in&#xA;Skylab (3), despite carrying&#xA;Tetracycline, Ampicillin and Neosporin&#xA;antibiotics (4).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://chapters.marssociety.org/usa/oh/aero5.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;  and other &lt;a href=&quot;http://www.doctorzebra.com/drz/s_medhx.html&quot; rel=&quot;noreferrer&quot;&gt;link&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-09-26T09:46:12.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2411" PostTypeId="2" ParentId="3" CreationDate="2013-10-14T07:54:32.013" Score="16" Body="&lt;p&gt;The Voyager probes are currently further away from earth than any object before. Knowing their position alone gives interesting scientific data, because it tells us their speed and direction, which gives us information about the forces which act on objects so far outside of the solar system (microgravity, acceleration through solar winds, friction with particles in interstellar space...).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are also some scientific sensors on board of the Voyager probes which still give interesting data&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most instruments on board of the Voyager probes were designed for observing the planets and their moons during the fly-by's (optical cameras, infrared spectrometer etc). These instruments haven't got many applications in open space, so they are powered down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are other instrument which are still active and give very interesting readings of the heliosphere, the area where solar winds interact with interstellar space. Most of what we know and are still learning about this area of space is from these instruments on the Voyager probes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Cosmic Ray System:&lt;/strong&gt; A particle detector for high-energetic particles &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Low-Energy Charged Particles:&lt;/strong&gt; A particle detector for low-energetic particles&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Triaxial Fluxgate Magnetometer:&lt;/strong&gt; Measures the density of magnet fields in three different axises&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Voyager probes are currently planned to continue their mission until 2025 when their RTGs will become too weak to still supply these last instruments.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2013-10-14T08:38:18.813" LastActivityDate="2013-10-14T08:38:18.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2412" PostTypeId="1" CreationDate="2013-10-14T08:37:19.563" Score="20" ViewCount="2164" Body="&lt;p&gt;The &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/European_Astronaut_Selection/FAQs_Health_and_physical_condition&quot;&gt;European Space Agency&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;◾The applicant must have the normal range of motion and functionality in all joints.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Historically, IMHO, astronauts came from a flight/test-pilot background. Physical fitness may have been mandatory for orbital flight, and for the lunar programme. It may even be necessary for any missions that involve actually landing on a body exhibiting significant gravitational attraction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA and other Space Agencies have similar physical mobility requirements from their astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Astronaut_training&quot;&gt;Wikipedia&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What began as the selection of military fighter and test pilots in the 1960s, with a considerable focus on physical capability, has evolved into a selection that now selects for aptitude in engineering, sciences, life sciences, and mathematics&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Say instead the astronaut is headed to the &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station&quot;&gt;ISS&lt;/a&gt; where she/he is expected to be in free-fall most of the time. A person with a reduced range of motion (&lt;strong&gt;I'm specifically of the mind of restricted movement of the lower-limb&lt;/strong&gt;) might actually be better suited for free-fall - &lt;strong&gt;as long as other physical, social, mental health parameters are met&lt;/strong&gt;. About the only time I imagine full-range of motion in the lower-limb/s might be necessary would be at launch/landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've never been an astronaut, or even close to medicine let alone space-medicine so I may be wrong. Feel free to poke me in the rib if my premise is flawed!&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Do astronauts aboard the International Space Station (ISS) actually use their legs? (Goes without saying they're in Earth orbit since they &lt;em&gt;are&lt;/em&gt; aboard the ISS)&lt;/li&gt;&#xA;&lt;li&gt;Is the physical fitness requirement that an astronaut prove full-range of mobility in all limbs relevant to astronauts headed to the ISS, or low-gravity bodies such as Phobos/asteroids?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-10-16T01:40:32.403" LastActivityDate="2022-07-19T00:59:08.303" Title="Do astronauts aboard the International Space Station (ISS) actually use their legs?" Tags="|iss|nasa|medical|physical-activity|esa|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2413" PostTypeId="2" ParentId="2382" CreationDate="2013-10-14T09:39:40.793" Score="3" Body="&lt;p&gt;According to the &lt;a href=&quot;http://missionjuno.swri.edu/mission&quot; rel=&quot;nofollow&quot;&gt;Mission Juno mission&lt;/a&gt; page, the only scientific data that will be done will be on Jupiter itself, there is no mention whatsoever about any studies that would be performed on the moons.  In fact, Juno will be spending a lot of its time in very close proximity to the planet - well within the orbits of all known satellites.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-10-14T09:39:40.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2414" PostTypeId="1" AcceptedAnswerId="2415" CreationDate="2013-10-14T13:08:34.020" Score="3" ViewCount="869" Body="&lt;p&gt;Does any documented/official footage from another planet or from the outskirts of our solar system exist?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scientists working on the Mars Curiosity Rover mission have compiled a large number of images to create &quot;footage&quot; (even though, technically, footage is precisely that - a collection of images taken within a very short time of one another which have been strung together) but for the sake of this question, this doesn't qualify as the images weren't recorded continuously - they were taken image by image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there ever been any footage from the atmosphere or surface of Jupiter, for example, where Galileo (purposely) crash-landed?  Or perhaps a number of the Venera spacecraft which landed on Venus and were able to transmit data for approximately 50 minutes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am aware that the spacecraft in my examples may not have been fitted with video capture devices, but would like to know whether anything ever has been (with, perhaps, the exception of the MCR)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or how about the Voyage 1 spacecraft (which has now left our solar system)?  In 1990 it transmitted an image of Earth as seen from 3.7 billion miles away, which appeared as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Pale_Blue_Dot&quot; rel=&quot;nofollow&quot;&gt;pale blue dot&lt;/a&gt;; has any further footage ever been transmitted back to Earth?  I understand that lots of the equipment has since been retired in order to prolong the life of Voyager 1, but am curious to know whether or not anything else ever made it back to Earth.&lt;/p&gt;&#xA;" OwnerUserId="864" LastEditorUserId="58" LastEditDate="2013-10-14T14:21:43.753" LastActivityDate="2013-10-14T14:21:43.753" Title="Does Footage from Another Planet or the Outskirts of Our Solar System Exist?" Tags="|voyager|imaging|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2415" PostTypeId="2" ParentId="2414" CreationDate="2013-10-14T13:44:27.113" Score="5" Body="&lt;p&gt;Yes, &quot;footage&quot; is nothing more than pictures stringed together in time.  For an example from the &quot;outskirts&quot; of our solar system, the &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;New Horizons&lt;/a&gt; probe has captured a time-sequence of Jupiter's moon &lt;a href=&quot;http://en.wikipedia.org/wiki/Io_%28moon%29&quot; rel=&quot;noreferrer&quot;&gt;Io&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gM3Jn.gif&quot; alt=&quot;New Horizons&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this isn't the only example.  NASA has &lt;a href=&quot;http://www.nasa.gov/centers/goddard/multimedia/.../EduVideoGallery.html&quot; rel=&quot;noreferrer&quot;&gt;lots of short clips&lt;/a&gt; of the Jovian system, assembled from a collection of images, which I assume will be available again after the US government shutdown is over.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The distinction of video capture devices is something I doubt would be taken seriously.  Obviously manned missions are a different story, and have carried with them devices that would clearly be identified as video capture devices.  Of course, manned missions have never gone anyway remotely close to the outer solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concept of our conventional notion of video capture with probes has several issues.  Notably:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The time scale probes are interested in is much longer than we're used to.  If you were to &lt;em&gt;toss&lt;/em&gt; something on Mars, you would need sub-second scale image capture.  But we don't do that.&lt;/li&gt;&#xA;&lt;li&gt;Even if something interesting was happening on these time scales, probes have limited power, and in some cases rapid capture isn't going to be very viable, and it might not be very good in the cases it is viable.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Since you're looking for something &lt;em&gt;intended&lt;/em&gt; to have rapid capture of images, the best example would probably be the &lt;a href=&quot;http://www.spaceflight101.com/msl-edl-reconstruction.html&quot; rel=&quot;noreferrer&quot;&gt;Mars Curiosity EDL camera&lt;/a&gt;.  Since the decent was a relatively rapid process, this was designed to take a &quot;movie&quot;, at a frame rate that actually looks like a movie.  You might be able to find other cases where a probe's frame rate rivals this, but this is the best example I know of.  This was also a fairly large probe with a good amount of power available, and the video is still choppy by TV/youtube standards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than the frame rate, I don't think there's any other meaningful distinction for classifying something as a video camera.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-10-14T13:44:27.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2416" PostTypeId="1" AcceptedAnswerId="2418" CreationDate="2013-10-14T14:56:43.487" Score="25" ViewCount="5244" Body="&lt;p&gt;A recent news article on &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-24477667&quot;&gt;BBC News&lt;/a&gt; indicates that &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;1,000 tonnes of diamonds a year are being created on Saturn&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is like science fact that screams it must be Science Fiction!  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the science is true and there really are diamonds raining in 6,000km, to 30,000km range; &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could they be harvested?  &lt;/li&gt;&#xA;&lt;li&gt;Is the pressure too great? &lt;/li&gt;&#xA;&lt;li&gt;Would the friction of diamonds on your recovery device wear it away before you get clear? &lt;/li&gt;&#xA;&lt;li&gt;Would the gravity be so great that you could not bring enough fuel to reach escape velocity?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2014-01-19T08:21:36.877" LastActivityDate="2020-04-11T18:03:17.503" Title="Can I harvest diamonds from the rain of Saturn and Jupiter?" Tags="|planetary-science|jupiter|natural-resources|saturn|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2417" PostTypeId="2" ParentId="3" CreationDate="2013-10-14T15:20:06.477" Score="9" Body="&lt;p&gt;The two Voyager spacecraft are now conducting the &lt;a href=&quot;http://voyager.jpl.nasa.gov/mission/interstellar.html&quot;&gt;Voyager Interstellar Mission&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The mission objective of the Voyager Interstellar Mission (VIM) is to&#xA;  extend the NASA exploration of the solar system beyond the&#xA;  neighborhood of the outer planets to the outer limits of the Sun's&#xA;  sphere of influence, and possibly beyond. This extended mission is&#xA;  continuing to characterize the outer solar system environment and&#xA;  search for the heliopause boundary, the outer limits of the Sun's&#xA;  magnetic field and outward flow of the solar wind. Penetration of the&#xA;  heliopause boundary between the solar wind and the interstellar medium&#xA;  will allow measurements to be made of the interstellar fields,&#xA;  particles and waves unaffected by the solar wind.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Recently it was &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2013-278&quot;&gt;announced that Voyager 1 had in fact entered interstellar space&lt;/a&gt; on August 25, 2012.  The heliopause boundary was sharp, going from lower to higher density plasma and a change in the direction of the magnetic field (though a smaller change than expected).  However it took a year to figure that out, waiting for the assistance of our Sun with a coronal mass ejection that excited the local plasmas, and the interpretation of that data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 2 has not reached interstellar space yet, but it is expected to while it is still operational.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Voyagers should remain scientifically operational through 2020, with continued operations with reduced science possibly as far as 2025.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-10-14T15:20:06.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2418" PostTypeId="2" ParentId="2416" CreationDate="2013-10-14T15:40:15.497" Score="21" Body="&lt;p&gt;Could they be harvested? Sure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could they be harvested in an economically profitable way? &lt;strong&gt;Probably not&lt;/strong&gt;, for all the reasons you listed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a lot of unknowns to this question &amp;mdash; while the chemistry they mention certainly makes sense, there's lots of stuff about the atmosphere we don't know. A harvesting ship would need to find exactly what altitudes the diamonds formed, but haven't yet been crushed into the core of the planet. Remember, it's not like there's millions of tons of diamonds just sitting on the surface, there's an altitude range where they're formed and haven't yet been destroyed. This range appears after a lightning storm, so you'd need to send your harvester underneath the graphite 'cloud' at the right time-frame to harvest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pressures high enough to turn graphite into diamond means that any ship would need to be ridiculously strong to not also be crushed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we could get to the point technologically where asteroid mining is feasible, there might be more opportunity to harvest materials from the gas-giants. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other trick is if you were able to harvest them effectively and in large quantities, selling tons of diamonds would likely destabilize the market, and probably cause a price-crash. On the flip side, 'Saturn Diamonds' might become a huge status symbol and command a high-enough price to actually make such a venture worthwhile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what we know, carbon is a very common element in the universe &amp;mdash; &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-11942451&quot; rel=&quot;noreferrer&quot;&gt;there might be entire planets that are carbon-dominant instead of silicon-dominant, meaning thick-swaths of the planet's crust could be solid diamond&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that we've been able to create lab-grown diamonds on an industrial scale for decades, even with huge leaps in space-exploration technology, it is still going to be much easier to synthesize diamond than it will be to harvest or mine them from off-earth sources.&lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="6816" LastEditDate="2017-02-27T01:55:33.953" LastActivityDate="2017-02-27T01:55:33.953" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2419" PostTypeId="1" AcceptedAnswerId="3655" CreationDate="2013-10-14T15:44:15.270" Score="20" ViewCount="2832" Body="&lt;p&gt;In &quot;&lt;a href=&quot;https://space.stackexchange.com/questions/1334/can-venus-be-made-habitable&quot;&gt;Can Venus be made habitable?&lt;/a&gt;&quot; we see that Venus's atmosphere would have to be substantially altered in order to terraform Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;&lt;a href=&quot;https://astronomy.stackexchange.com/questions/639/why-has-venuss-atmosphere-not-been-stripped-away-by-solar-wind&quot;&gt;Why has Venus's atmosphere not been stripped away by solar wind?&lt;/a&gt;&quot; tells us that even though Venus has no magnetic field, it does have an ionosphere generated by the interaction between solar wind and the thick atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If we altered Venus's atmosphere substantially for terraforming, would it weaken this effect thereby destroying Venus's ionosphere?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;In other words, if we made Venus have the same atmospheric thickness and composition as Earth, would its ionosphere be weaker than the Earth's?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is the case, then Venus's atmosphere would be at risk of being stripped by solar wind like Mars's was (see &quot;&lt;a href=&quot;https://space.stackexchange.com/questions/634/would-terraforming-mars-be-possible&quot;&gt;Would terraforming Mars be possible?&lt;/a&gt;&quot;). This would mean we would also need to create a magnetic field for Venus to truly terraform it.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2017-05-06T07:14:23.633" Title="Would terraforming Venus destroy its ionosphere?" Tags="|venus|atmosphere|terraforming|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2420" PostTypeId="2" ParentId="2343" CreationDate="2013-10-14T15:52:36.173" Score="0" Body="&lt;p&gt;Please take a look at the following link &lt;a href=&quot;http://avantiplc.com/news-media/pressreleases&quot; rel=&quot;nofollow&quot;&gt;http://avantiplc.com/news-media/pressreleases&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Amongst the text of the two articles describing the recent bond offering, you should see details of the planned handover of the ARTEMIS data rely satellite from the Euroean Space Agency to Avanti Communications, a UK satellite operator.  ARTEMIS has similar capabilities to the TDRSS system.  Please contact the company if you have any questions.&lt;/p&gt;&#xA;" OwnerUserId="893" LastActivityDate="2013-10-14T15:52:36.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2421" PostTypeId="5" CreationDate="2013-10-14T17:49:55.103" Score="0" Body="&lt;p&gt;Constituted in 1992 by President Yeltsin after the dissolution of the USSR.&#xA;Roscosmos is responsible for space-research &amp;amp; aerospace activities in the Russian Republic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This tag should be applied to any reference to programs of the erstwhile USSR. If possible a tag for the particular mission in context should accompany this tag&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-10-14T18:40:31.793" LastActivityDate="2013-10-14T18:40:31.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2422" PostTypeId="4" CreationDate="2013-10-14T17:49:55.103" Score="0" Body="The Russian Federal Space Agency is commonly referenced by the name Roscosmos" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-10-14T18:40:38.170" LastActivityDate="2013-10-14T18:40:38.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2423" PostTypeId="1" AcceptedAnswerId="2426" CreationDate="2013-10-14T22:04:16.910" Score="51" ViewCount="82668" Body="&lt;p&gt;Jumping from &lt;a href=&quot;https://space.stackexchange.com/questions/634/would-terraforming-mars-be-possible&quot;&gt;this question&lt;/a&gt; about terraforming Mars, how would one mitigate the main obstacle, namely Mars's lack of a magnetic field? Are there any hypothetical processes that might restart Mars's magnetic field (assuming it had one in the past)?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2018-10-11T00:46:11.320" Title="How would it be possible to kick start Mars's magnetic field?" Tags="|mars|magnetic-field|terraforming|" AnswerCount="7" CommentCount="19" ContentLicense="CC BY-SA 3.0" />
  <row Id="2424" PostTypeId="2" ParentId="2405" CreationDate="2013-10-14T22:39:46.163" Score="11" Body="&lt;p&gt;I can't speak for current procedures aboard a Soyuz or Shenzhou. From what I've seen, during launch the middle chair has pretty much all the access to various in-capsule systems (and thus the power and responsibility), though the two side chairs do have some tasks to perform.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aboard Apollo spacecraft, absolutely; each of the three astronauts trained and cross-trained until they had a basic level of proficiency with the flight controls of all of the associated vehicles of the mission. While each of them specialized in their own mission profile (the CMP, for instance, wasn't expected to spend much time in the LEM), certain mission-critical maneuvers such as S-IVb transposition and docking, LEM/CSM rendezvous, re-entry etc were practiced by all three crewmembers, so that if any crewmember found themselves unable to perform their mission duties, either of the other two could.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Shuttle era, roles became more specialized; the Mission Commander, Shuttle Pilot, and a mix of up to 5 more crewmembers divided between mission specialists and payload specialists, depending on the goals of the flight. Between the Mission Commander and Shuttle Pilot, either of the two could do anything that was needed with regard to the Shuttle vehicle itself; if you look at the list of shuttle mission crews, virtually all the MCs, especially of later missions, were the Shuttle Pilot of one or more previous missions, and so these two are effectively pilot and copilot during orbiter maneuvers. The mission specialists are usually exactly that, however these crewmembers were typically chosen from among the astronaut corps; they're astronauts first and scientists second, and they trained for the duties of the specific mission to which they were assigned. As such, I would posit that these crewmembers would be able to perform at least some of the key functions of the orbiter, though I can't find any instance of a mission specialist &quot;graduating&quot; to pilot or MC. Payload specialists, on the other hand, were scientists first and astronauts second; these guys were typically technical leads from the ground staff that designed or built the payload in question, and their only business on the Shuttle is to ensure a successful deployment of that payload. I would not expect any of these guys to get much time in the pilot's seat of the trainer.&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="411" LastEditDate="2013-10-14T22:46:36.427" LastActivityDate="2013-10-14T22:46:36.427" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2425" PostTypeId="2" ParentId="2372" CreationDate="2013-10-14T23:54:50.130" Score="37" Body="&lt;p&gt;There aren't very many places a mutineer could go...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, for a combination of practical and logistical reasons, the ISS has never held its original planned crew of 7, and instead has been crewed by 3 people for virtually all of its service life, the exceptions being Shuttle and Soyuz visits, either for crew changes or to install or maintain parts of the station. The upshot is that, while privacy is still at a premium, it is possible, if necessary, for a crewmember to distance themselves from the rest of the crew for a while to cool off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If an actual FTFO should occur, as a violation of the ISS Code of Conduct, the ISS Commander has the authority, under the ISS Crew Disciplinary Policy, to deal with the situation as he or she sees fit. Certain doors in the ISS can only be opened from one side, so the other two crewmen could, after subduing the mutineer, lock him in the Progress capsule or a similar small, outlying module of the station while a longer-term plan is hashed out with the ground. As far as actually subduing a lone mutineer, the ISS drug cabinet, if it's anything like previous space missions, will have Demerol available, which is one option; another is simple brute force, though a zero-G wrestling match aboard the ISS is likely to damage some very expensive equipment, and there really isn't any good way for any combatant to get the necessary leverage needed for something like a punch, to say nothing of a body slam. As far as we know there are no weapons aboard the station, and it would make little sense to have them; one errant shot could depressurize the entire station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further to another question - &lt;a href=&quot;https://space.stackexchange.com/questions/2431/how-is-the-iss-equipped-to-deal-with-the-outbreak-of-a-contagion&quot;&gt;How is the ISS equipped to deal with the outbreak of a contagion?&lt;/a&gt; - I found that the &lt;a href=&quot;http://spaceref.com/iss/medical.ops.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS operations guide&lt;/a&gt; indicates that &quot;acute psychosis&quot; and &quot;suicidal thoughts or actions&quot;, each of which would create a potential situation for mutiny and threat to the entire crew, are considered medical emergencies. For exactly these types of emergencies, the ISS medical locker contains Haldol and Valium in both oral and IM-injectable forms. Procedures further recommend restraining the crewmember by wrapping his extremities with duct tape and/or bungee cords, then stowing the person in their sleeping bag. There are no cases on record of these drugs or procedures ever being needed in orbit, but someone obviously thought ahead and considered the possibility of someone unexpectedly going off the deep end in a closed capsule hundreds of miles from help.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the entire crew were to mutiny, and attempt to do something that jeopardized the station, Mission Control can lock the crewmembers out of most station systems and run the thing on autopilot. There really isn't anything else &lt;em&gt;to&lt;/em&gt; do, until the next Soyuz heads to the station with the crew replacement. That would likely not happen until Mission Control has regained some semblance of control over the crewmembers aboard the station, and that in turn would involve measures up to and including the withholding of critical supplies, such as food and water (and water is used to make oxygen by electrolysis). The ISS crew simply must cooperate with the ground in order to do almost anything of value to the crewmembers or the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall, the options for dealing with a crazy person aboard the space station are very limited, and for that reason, astronauts are very carefully chosen, and team chemistry monitored closely during training. Dealing with multiple crazy people, however, is rather simple; lock down the station and wait them out. I can't imagine anything approaching a true mutiny by the crew of the ISS ending well for any of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://download.esa.int/docs/ECSL/ISS_Crew_Code_of_Conduct.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Crew Code of Conduct&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://download.esa.int/docs/ECSL/ISS_Crew_Disciplinary_Policy.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Crew Disciplinary Policy&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceref.com/iss/medical.ops.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Operations Guide - Routine and Emergency Medical Operations&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="411" LastEditorUserId="11721" LastEditDate="2020-05-26T14:58:24.433" LastActivityDate="2020-05-26T14:58:24.433" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="2426" PostTypeId="2" ParentId="2423" CreationDate="2013-10-15T10:20:20.470" Score="29" Body="&lt;p&gt;Since the question states that the answer can be &quot;hypothetical&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the core of Mars does not have enough heat to start the convection process, we can drill a hole to the solid core and connect them to a source of electricity, and pass a huge current so it heats up ($\textrm{Heat}~=I^2*\textrm{Resistance of core}*\textrm{Time of passage of current}$) the core until the core melts because of the heat, so the convection process becomes self-sustainable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way is to hit it with a huge asteroid (as agweber said in a comment)&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="58" LastEditorDisplayName="user29" LastEditDate="2016-04-28T21:53:08.570" LastActivityDate="2016-04-28T21:53:08.570" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2428" PostTypeId="1" AcceptedAnswerId="2430" CreationDate="2013-10-15T13:50:19.270" Score="10" ViewCount="3072" Body="&lt;p&gt;Can we convert the motion energy of bodies (such as satellites) moving around a high-gravity objects (like Earth or the Sun) to some kind of usable energy?&lt;/p&gt;&#xA;" OwnerUserId="306" LastActivityDate="2013-10-16T10:02:53.160" Title="Can we get energy from the motion of orbiting bodies?" Tags="|energy|technology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2429" PostTypeId="2" ParentId="2423" CreationDate="2013-10-15T16:44:05.843" Score="10" Body="&lt;p&gt;Yes, this is purely speculation (+: (apart from the content available from web-references)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mars is the fourth planet from the Sun and the second smallest planet in the Solar System. Named after the Roman god of war, it is often described as the &quot;Red Planet&quot; because the iron oxide prevalent on its surface gives it a reddish appearance&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Iron Oxide is commonly known as rust; typically caused when iron well, oxidizes. So depending upon the oxidation it may/may not be ferromagnetic. That's one part of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other part is that all of the Solar System is &lt;a href=&quot;http://www.nasa.gov/mission_pages/voyager/heliosphere-surprise.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;steeped in Sol's magnetic field&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The sun's magnetic field extends all the way to the edge of the solar system,&quot; explains Opher.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Assuming the iron-oxide exists in adequate quantity on the surface of Mars (which it should, seeing as we're able to discern the colour way out here!) here's what I'd do&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Determine just how ferro-magnetic the iron oxide out there is&lt;/li&gt;&#xA;&lt;li&gt;Come up with a mechanism to align all that iron oxide in a circular band around the Martian surface pole-to-pole&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Let the repeated caresses of Sol's magnetic field magnetize the band (That's how we magnetize razors as kids anyway - move the magnet above the razor blade again &amp;amp; again in the same direction)&lt;/li&gt;&#xA;&lt;li&gt;This has the advantage that rust being a semi-conductor will hopefully prevent demagnetization as the band spins away.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;If necessary&lt;/strong&gt; &#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Reduce the iron-oxide so it is adequately ferromagnetic for this scheme to work&lt;/li&gt;&#xA;&lt;li&gt;Drill into the the Core, and connect the band there (Then mix this scheme with &lt;a href=&quot;https://space.stackexchange.com/a/2426/288&quot;&gt;Hash's answer&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Again (+: this is hypothetical&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-10-15T16:44:05.843" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2430" PostTypeId="2" ParentId="2428" CreationDate="2013-10-15T17:42:28.983" Score="13" Body="&lt;p&gt;Consider the Earth and moon to start out with.  Obviously we can extract energy through tidal power (which is a real thing, although the economic question is non-trivial).  This is possible because the Earth is spinning faster than the moon orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this tidal power is rate-limited.  As long as we're using water as the working fluid, the theoretical maximum rate of power conversion is pretty close to what we'd get by damming up all the world's oceans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What other kinds of systems can we get energy from?  To answer this, I would suggest the following general checklist:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Identify the initial system&lt;/li&gt;&#xA;&lt;li&gt;Identify a final system that is possible (angular momentum conserved), and is lower energy than the starting system&lt;/li&gt;&#xA;&lt;li&gt;Does the idea make any practical sense?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I can use this to answer for a number of configurations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Elliptical orbits&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will imagine a pair of binary asteroids in a highly elliptical orbit about each other.  If you consider the burn necessary to circularize the orbit at perigee (the closest point), you are trying to throw away kinetic energy by burning your engines.  Obviously you could, instead, convert that extra energy into useful energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I use asteroids as an example because the scales and forces can be handled by materials we can actually construct.  You could &lt;em&gt;literally&lt;/em&gt; connect the two bodies by some ropes designed for space.  Then the energy conversion process consists of pulleys and generators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To compensate for the change of angular momentum, you would simply speed up the rotation of the asteroids themselves.  Even if this resulted in negative effective gravity on its equator, we know that some asteroids can still hold together.  There are certainly some binary asteroid systems where this is possible within some bounds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Circular, but rotating&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the case of the Earth and the moon.  If you're interested in a more directly mechanical (absurdist) process than tidal power, then you can imagine some.  Just build space elevators all the way around Earth's equator.  Then make the counter-weight station a full ring.  Then build a space elevator from the moon's surface to the Earth-moon L1 point, with a fiber extending down to Earth's space elevator's counterweight ring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you have a system with relative motion between the Earth's and moon's space elevators that can be essentially converted on contact to energy.  The speeds would be too great for some kind of direct (space-train) mechanical connection, so we would have to postulate an electromagnetic connection, but this is not outrageous since we're already in space and (apparently) have the technology to build space elevators to begin with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The initial system consists of 1 body tidally locked into the orbit with the other body rotating too fast.  This is an interesting property of the Earth-moon system.  The final state is that Earth throws the moon out into space.  Our energy extraction system would hasten this process.  Tidal power does too, but not as fast.  The length of an Earth day would shorten faster the more power we produce.  If we did it long enough the moon would fly away to play gravitational billiard balls with the rest of our solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Completely tidally locked&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This situation was the real challenge for me to think about.  You can't get energy from the motion of the orbit or rotation of the bodies, unless you can grab hold of another reference frame.  By the formulation of the problem, that's not allowed (it's also not very practical).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Turns out, we &lt;em&gt;could&lt;/em&gt; get energy out of this system if one body is larger than the other, but only &lt;em&gt;gravitational&lt;/em&gt; energy.  Here's how:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine the Earth-moon system.  The moon's gravity-well is quite small compared to Earth's.  So on the moon, lob rocks at the Earth with a railgun, aiming the orbit so they graze LEO.  In LEO, have some catcher system that converts the energy.  This could be a simple orbital ring (much easier than a space elevator), although the &quot;catching&quot; part will be difficult, it's still physically defensible.  This system will work totally irrelevant of the rotation of Earth.  You could throw rocks into East-West orbits or West-East orbits, and use this to balance the momentum changes from catching.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the final state of this system, you're left with a major part of the moon's mass orbiting close to Earth's surface (safe? who cares).  The low circular orbit is lower energy than what we started out with, and so we conclude that the energy gained by the catcher &lt;em&gt;could&lt;/em&gt; be more than it took to launch rocks from the railgun on the moon.  But importantly, this &lt;em&gt;does not&lt;/em&gt; come from orbital motion.  It comes from &lt;em&gt;gravitational&lt;/em&gt; energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final system will (again) be tossing the moon into space.  In LEO, you'd have to dispel the extra momentum, which you would do by the East/West alternation, but to sustain the scheme as long as possible you'd prefer to park the caught mass in the West-&gt;East orbit because that contains the right direction of angular momentum.  However, every payload you launch, regardless of its destination on Earth, will push the moon further out.  The moon's gravity itself isn't a deal-breaker because the &lt;a href=&quot;https://www.google.com/search?q=3/5%2aG%2a%28mass%20of%20the%20moon%29/%28radius%20of%20the%20moon%29&quot;&gt;averaged gravitational binding energy of the moon&lt;/a&gt; is small compared to &lt;a href=&quot;https://www.google.com/search?q=%28G%2a%28mass%20of%20Earth%29/%28radius%20of%20Earth%20%2b200%20km%29%29&quot;&gt;the energy from dropping something from infinity into a LEO orbit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So at the end, one giant orbital ring of moon-rock is in LEO, a leftover moon is about to go rogue, and a bunch of politicians are asking &quot;&lt;em&gt;now&lt;/em&gt; what?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The transition I'm describing is a sort of &lt;a href=&quot;http://en.wikipedia.org/wiki/Ostwald_ripening&quot;&gt;Ostwald ripening&lt;/a&gt; of planets, where energy is minimized by combining all small bodies into larger bodies, like corporate consolidation.  Within orbiting systems, however, you still have to discharge a small part into space in order to preserve the angular momentum condition.  After all, that's basically why we have a moon in the first place!  It would be impossible to combine the Earth and moon into a single body because gravity is not strong enough to hold that body together with its present angular momentum value.  At the end of the process, the larger planet may be left with some angular momentum (by design), and the smaller object may overshoot the escape velocity by some amount, dictated by the optimization of the problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Tidally locked, equal masses&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the &lt;em&gt;hardest&lt;/em&gt; system to get energy out of, because it can't be done &lt;em&gt;differentially&lt;/em&gt;.  You can throw some of the mass from one object into the other one, but even with 100% efficiency you'll exactly break even.  For practical purposes, this makes the scheme impossible.  You'll have to move planetary-scale masses before you could start getting a payoff, and that's nonsense... unless maybe we're talking about a binary asteroid system, which could transfer mass by using a (literal) conveyor belt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for large-scale systems, the only way you'd make energy from this is with another reference frame in the picture.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-10-15T20:08:27.587" LastActivityDate="2013-10-15T20:08:27.587" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2431" PostTypeId="1" AcceptedAnswerId="2432" CreationDate="2013-10-15T21:58:23.820" Score="32" ViewCount="2884" Body="&lt;p&gt;Suppose one of the astronauts was a carrier for a disease that did not show up in the medical testing prior to flying up to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would the outbreak of this contagion be dealt with? &lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-03-18T15:48:20.113" Title="How is the ISS equipped to deal with the outbreak of a contagion?" Tags="|iss|medical|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2432" PostTypeId="2" ParentId="2431" CreationDate="2013-10-15T23:21:58.280" Score="35" Body="&lt;p&gt;NASA, ESA, and RSA follow very conservative policies towards sending a potentially infected person into space. All the blood tests and other screenings in the days, weeks, and months leading up to a launch would make it virtually impossible for an astronaut to carry a communicable disease up to the ISS without showing symptoms or being detected as an asymptomatic carrier (a &lt;a href=&quot;http://en.wikipedia.org/wiki/Typhoid_Mary%E2%80%8E&quot;&gt;Typhoid Mary&lt;/a&gt;), either of which would have the astronaut grounded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, astronauts have gotten sick in space. Obviously, motion sickness (or zero-g sickness) is a common problem, but it's not caused by an infection. It's more akin to seasickness, caused by a combination of your lunch constantly churning around in your stomach as you move, and conflicting inputs from sensory organs like your inner ear that are a gravity-dependent indicator of &quot;up&quot;. For that, there are and always have been barf bags aboard spacecraft, of continually increasing sophistication. Antiemetic drugs are a newer additional measure, but as the primary side effect of most such drugs like phenergan is drowsiness, they're saved for the worst of the worst.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first, worst, and only publicly-known episode of a communicable disease spreading aboard ship was Apollo 7, in which the mission commander, &lt;a href=&quot;http://en.wikipedia.org/wiki/Wally_Schirra%E2%80%8E&quot;&gt;Wally Schirra&lt;/a&gt;, gave a rather nasty head cold to his two crewmates. The resulting breakdown in order aboard that craft (it was an eleven-day flight, the second-longest thus far and the first manned flight of the Apollo capsule, so there were several SNAFUs relating to the crew's supplies and accommodations on top of the illness) ultimately resulted in all three astronauts being permanently grounded (it was Schirra's last flight anyway, but Cunningham and Eisele had been in line for Moon landings). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo 7 also made NASA flight doctors &lt;em&gt;very&lt;/em&gt; skittish about infections; the original prime crew of Apollo 13 included &lt;a href=&quot;https://en.wikipedia.org/wiki/Alan_Shepard&quot;&gt;Alan Shepard&lt;/a&gt;, but an ear infection caused NASA medical staff to switch his entire crew with Jim Lovell's. &lt;a href=&quot;http://en.wikipedia.org/wiki/Ken_Mattingly%E2%80%8E&quot;&gt;Ken Mattingly&lt;/a&gt; was famously scrubbed from the Apollo 13 crew three days before the launch due to exposure to measles. Ironically, he never got sick, while &lt;a href=&quot;http://en.wikipedia.org/wiki/Fred_Haise%E2%80%8E&quot;&gt;Fred Haise&lt;/a&gt;, as was popularized in the movie, brought up a non-contagious urinary tract infection on that same flight, and without any antibiotics in the medical kit to treat the infection, he was in considerable pain and suffering during most of the mission and for weeks after landing. It's not known whether the mission would have been scrubbed anyway due to his illness; by the time the worst of his symptoms manifested, the accident that crippled the Odyssey had already occurred.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have not been able to uncover any other instances of astronauts contracting an infectious disease while en route to or aboard the station, but the ISS does have measures in place for illnesses and other ailments, similar to other U.S. space missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a link to the &lt;a href=&quot;http://spaceref.com/iss/medical.ops.html&quot;&gt;Medical Operations section&lt;/a&gt; of the ISS &quot;user guide&quot;. In it are the following tidbits:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The various procedures in this guide infer that the ISS has a small pharmacy aboard in its drug cabinet, containing several options each for analgesic pain relievers, antihistamines, decongestants, antiemetics, anti-diarrheals, sleep aids, etc up to the &quot;hard stuff&quot; like cortizone and albuterol steroids, opioid painkillers including hydrocodone and possibly morphine (it's mentioned in drug interaction warnings but no procedure says to administer it), antibiotics, antifungals, and even a local anesthetic (Lidocane/Xylocaine) and two anti-psychotics/anti-anxietals (Haldol and Diazepam/Valium), each of those in oral and injectable form. Procedures detail the dispensing of these drugs for a variety of symptoms and presentations including those associated with common communicable illnesses (sinus blockage, cough, vomiting/nausea, fever, headache, rash, etc).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;As stays aboard the ISS are long-term, and 31 of the total 88 ISS crewmembers to date have been women, the medical kit includes such sundries as the anti-fungal drug fluconazole for yeast infections, and - hand to God - &lt;em&gt;pregnancy tests&lt;/em&gt;. Apparently this made the news in 2001, when this medical procedures document was first leaked, as it represented NASA's first tacit acknowledgment that some of its astronauts might try to join the &quot;200-mile-high club&quot;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Similarly, due to the long duration of ISS missions, various conditions that would otherwise be treated very primitively with first-aid measures on a moon shot or Shuttle flight instead have procedures and supplies for a more lasting fix; these conditions include everything from broken bones and concussions to kidney stones and dental cavities. The kit includes tools and materials specifically to perform dental repairs such as crown replacements, cavity fillings, bracing/securing of cracked teeth, and extractions.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="411" LastEditorUserId="411" LastEditDate="2015-03-18T15:48:20.113" LastActivityDate="2015-03-18T15:48:20.113" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2433" PostTypeId="1" CreationDate="2013-10-15T23:30:26.460" Score="-1" ViewCount="23094" Body="&lt;p&gt;&lt;strong&gt;Background&lt;/strong&gt;: I am 40 years old, and in my entire life the only face to face eyewitness event that I can remember where humans actually observed an attempt to fly into outer space, the craft exploded and showered its fiery remains - including 5 dead astronauts - down over the Earth as soon as it reached the atmosphere. I have seen no other real eyewitness evidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an Australian, I also live really far away from any launch or landing sites, with all of them too far to be directly observable even through a powerful telescope. I have never witnessed any space exploration endeavors in person and all I have available to me is accounts by other people that I have never met in real life and videos on the Internet or recordings and interviews on the television or radio stations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question&lt;/strong&gt;: How could I go about finding proof of mankind ever reaching outer space first hand, with astronauts doing moonwalks, spacewalks, whatnot when all of it seems unreal, happening somewhere else and never directly touching the grounds of my own country, which happens to be a whole continent of the planet Earth that I live on, while only having indirect and anecdotal evidence of it? I would take a word from any of you that have been a direct eyewitness to human space exploration, but preferably ask of you for any idea how I could prove that by holding something in my own hands that could only exist if that was true, or by doing my own direct observations, be an eyewitness myself and have proof beyond reasonable doubt?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there any direct evidence of human space exploration that nobody on our planet could dismiss as circumstantial?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="900" LastEditorUserId="49" LastEditDate="2013-10-17T14:03:22.037" LastActivityDate="2018-07-02T08:36:41.510" Title="How would one go about proving humans have really been to outer space?" Tags="|astronauts|nasa|atmosphere|cost|travel|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="2434" PostTypeId="2" ParentId="2433" CreationDate="2013-10-15T23:39:39.887" Score="21" Body="&lt;p&gt;Yes, humans &lt;strong&gt;definitely&lt;/strong&gt; have been to outer space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The easiest proof I can think of immediately is &lt;a href=&quot;https://youtu.be/o8TssbmY-GM?t=1m22s&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; of commander &lt;a href=&quot;https://en.wikipedia.org/wiki/Chris_Hadfield&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chris Hadfield&lt;/a&gt; demonstrating wringing of a wet towel on the &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or &lt;a href=&quot;http://www.youtube.com/watch?v=Uk9ptueTDwY&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt; — it contains more than 200 seconds long uncut shot of a̶b̶s̶e̶n̶t̶ &lt;em&gt;micro&lt;/em&gt;-gravity. It also shows vast interiors of the ISS, which are technically challenging to simulate in a &quot;vomit comet&quot;, due to theirs volume and structure.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt; &lt;a href=&quot;https://astronomy.stackexchange.com/questions/666&quot;&gt;it isn't impossible&lt;/a&gt; to &lt;em&gt;see astronauts spacewalking near the ISS&lt;/em&gt; from the ground, given enough time, skill, and equipment. It has been done before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, you can still claim that the photographs could've been edited and/or made up. In this case, you'll have to repeat the observations yourself.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update2:&lt;/strong&gt; I can even add the following praise of modern communication technology (and publicity of the space program). &lt;strong&gt;&lt;a href=&quot;http://www.nasa.gov/connect/hangout.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;You can talk live&lt;/a&gt;&lt;/strong&gt; with astronauts inhabiting the ISS! With enough perseverance, you can even ask them to provide whatever proofs you'd find satisfactory. Just have enough patience and don't forget to take your son to the chat.&lt;/p&gt;&#xA;" OwnerUserId="338" LastEditorUserId="338" LastEditDate="2018-07-02T08:36:41.510" LastActivityDate="2018-07-02T08:36:41.510" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="2435" PostTypeId="1" CreationDate="2013-10-16T01:37:35.143" Score="2" ViewCount="516" Body="&lt;p&gt;On F9 flight 6, the first demo of the V1.1 configuration, the putative rationale for the extra second stage burn was to demonstrate relight capabilities for Geosync, and really, any orbit that requires more than one burn to achieve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But one thing Elon Musk said in his press conference after the flight has me wondering. (I cannot find a link to a transcript).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He described the burn they attempted on the second stage as &quot;&lt;a href=&quot;http://www.spaceflightnow.com/falcon9/007/131005shipment/#.Ul32w1Cfi1U&quot; rel=&quot;nofollow&quot;&gt;sideways&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they were really trying to demo relight for a GeoSync launch profile, you would expect a more representative burn attempt, which would be generally to a higher orbit, and probably not described as sideways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if they were going to take a shot at de-orbiting the second stage, just to get some data on it, a sideways burn to slow down would make some sense.    &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorDisplayName="user29" LastEditDate="2013-10-16T12:25:57.857" LastActivityDate="2013-10-16T12:51:44.017" Title="Falcon 9 1.1 demo flight (flight 6) - Was the second stage burn meant to test deorbit?" Tags="|orbital-maneuver|spacex|falcon-9|reuse|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2436" PostTypeId="2" ParentId="2435" CreationDate="2013-10-16T01:49:17.467" Score="2" Body="&lt;p&gt;This was a demonstration of the restart capability of the engine. To verify this, the direction of the burn effectively does not matter - you just want to measure how much the orbit changed and gather telemetry from the engine. In fact, a GTO-like burn would have (if successful) put apogee much higher, and thus extended the decay time for the engine. This would have been irresponsible and dangerous, as SpaceX obviously recognized the risk of failure, and the worst case scenario - an upper stage with full fuel tanks in a high orbit - would've been quite a &lt;a href=&quot;http://www.flightglobal.com/news/articles/debris-fears-for-international-space-station-after-russian-briz-m-upper-stage-explodes-378285/&quot; rel=&quot;nofollow noreferrer&quot;&gt;bad situation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;sideways&quot; burn likely refers to a burn vector that is out of the orbital plane - in the cross-track direction (see below). This type of burn has the effect of just changing the inclination and/or the &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;nofollow noreferrer&quot;&gt;node&lt;/a&gt; of the orbit. It's about as close to a &lt;a href=&quot;http://en.wikipedia.org/wiki/NOP&quot; rel=&quot;nofollow noreferrer&quot;&gt;no-op&lt;/a&gt; as you can get while still performing a burn - it doesn't add or remove any energy from the orbit (of course, it was a no-op anyway, since it failed).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iQJKR.jpg&quot; alt=&quot;RIC frame&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not know why they didn't choose to burn to lower their orbit - possibly just because they didn't want to decay that quickly (i.e. they wanted a little more time on orbit to check things out). Then again, there's &lt;a href=&quot;https://space.stackexchange.com/q/2377/29&quot;&gt;quite a bit&lt;/a&gt; we don't know about this launch.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="-1" LastEditorDisplayName="user29" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-10-16T12:51:44.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2437" PostTypeId="2" ParentId="2412" CreationDate="2013-10-16T02:35:34.143" Score="7" Body="&lt;p&gt;I would love to see/hear about amputees in space, first off.  Gives hope to all of us who are not the NORM...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, you would still need the ability to move around the ship without bouncing into controls and knocking into others aboard.  In my mind that means the ability to use your legs full range of motion to kick off or slow your momentum down in addition to the use of hands/arms.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if you need to move a sensitive electronic from one side of ISS to the other? I'm sure there are all sorts of things that could come up where having a fully agile individual is needed to deal with the problem. Saying that however, if we put a few NASA heads together we could come up with solutions to most anything that would arise... and I do imagine that in the future amputees will be as useful and indispensable as any other crew memeber.  &lt;/p&gt;&#xA;" OwnerUserId="901" LastEditorUserId="288" LastEditDate="2013-10-16T02:56:11.723" LastActivityDate="2013-10-16T02:56:11.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2438" PostTypeId="1" CreationDate="2013-10-16T02:47:42.443" Score="7" ViewCount="301" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/Sprint.html&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt; says (emphasis mine)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Current exercise countermeasures on the International Space Station (ISS) are insufficient to prevent muscle atrophy, cardiovascular deconditioning and bone loss associated with long-duration space flight&lt;/strong&gt;. Despite crew allocation of approximately 2.5 hours per day to exercise, decrements in fitness are observed following flights averaging 180 days in duration. Muscle strength is decreased 11% to 17%, muscular endurance approximately 10% and bone mineral density 2% to 7%. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is thus that NASA commissioned Sprint also linked in the article above&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Integrated Resistance and Aerobic Training Study (Sprint) evaluates the use of high intensity, low volume exercise training to minimize loss of muscle, bone, and cardiovascular function in ISS crewmembers during long-duration missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is the regimen under the Sprint program?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-03-08T09:33:16.827" LastActivityDate="2015-09-07T19:01:37.420" Title="What is the exercise regimen under the SPRINT program?" Tags="|iss|nasa|medical|health|humans|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2439" PostTypeId="1" CreationDate="2013-10-16T04:21:42.800" Score="7" ViewCount="127" Body="&lt;p&gt;A follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/2438/what-is-the-exercise-regimen-under-the-sprint-program&quot;&gt;What is the exercise regimen under the SPRINT program?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/Sprint.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sprint&lt;/a&gt; program is briefly described as &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Integrated Resistance and Aerobic Training&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now this may just be dogma on my part but it puts me in mind of &lt;a href=&quot;http://en.wikipedia.org/wiki/Hatha_yoga&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hatha Yoga&lt;/a&gt;. The latter may crudely be described as calisthenics with aerobics often applying muscles to oppose each other.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;What traditional forms of exercise have been studied for the space program, and in orbit? &#xA;E.g. Yoga, Tai Chi&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Are there any papers specifically on the use of Hatha Yoga aboard ISS or it's predecessors?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-10-16T10:04:44.627" Title="Have traditional forms of exercise such as Yoga been studied in orbit?" Tags="|medical|physical-activity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2440" PostTypeId="1" CreationDate="2013-10-16T07:26:03.277" Score="14" ViewCount="1342" Body="&lt;p&gt;There are many dangers involved in space exploration, as we all know, there are always chances that an astronaut would be seriously injured or fall ill.  This would be especially true for continued space exploration, particularly on long duration journeys.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would imagine that there are considerable challenges of surgery in low or zero-gravity environments.  Should an astronaut have an accident or suffer a life threatening ailment, what is the procedure to perform surgery in near zero-gravity?&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorDisplayName="user838" LastEditDate="2013-10-16T07:50:06.737" LastActivityDate="2014-11-06T02:01:59.120" Title="What procedures are there to perform emergency surgery on the space station or space shuttle?" Tags="|medical|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2442" PostTypeId="2" ParentId="2439" CreationDate="2013-10-16T10:04:44.627" Score="7" Body="&lt;p&gt;This is a part answer about the connection between yoga (in general) and space travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A hypothesis about yoga and space travel is discussed in the article &lt;a href=&quot;http://www.yogaforspacehealth.org/downloads/paper4.pdf&quot;&gt;Yoga Therapy as a Complement to Astronaut Health and Emotional Fitness –&#xA;Stress Reduction and Countermeasure Effectiveness Before, During, and in&#xA;Post-Flight Rehabilitation: a Hypothesis&lt;/a&gt; (Vernikos et al. 2012), where the basis of their hypothesis is&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;yoga practices – breathing maneuvers, relaxation,&#xA;  meditation, and specific muscular movement –&#xA;  may serve this purpose by addressing as a whole,&#xA;  both the effects of microgravity and those of nongravitational&#xA;  stressful conditions, before, during,&#xA;  and in post-flight rehabilitation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Specifically, by allowing self-directed stress reduction/mitigation techniques (both conscious and unconscious), as well as allowing a quicker and more effective adaptation between the Earth and space environments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Our&#xA;  hypothesis acknowledges that stress and the&#xA;  microgravity of space disturb the balance of the&#xA;  adrenocortical, autonomic, and immune systems&#xA;  resulting in a generalized inflammatory response.&#xA;  Our hypotheses also reflect the need for more&#xA;  evidence that yoga practices can be used to restore&#xA;  balance and thereby complement the effectiveness&#xA;  of current CMs used during spaceflight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is not the first time that yoga techniques have been considered for space travel, according to the article &lt;a href=&quot;http://www.yoga-science.rs/eng/sciarticles2011/12-arun-raghuwanshi.pdf&quot;&gt;A Review: History of Revival of Yoga in 20th Century&#xA;and Establishment of Yoga as a Science in the 21st century&lt;/a&gt; (Raghuwanshi, 2011), state&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In early sixties Dhirendra Brahmachari (1924 – 1994) propagated yoga researches, with the help of modern medical science,... In late sixties and early seventies he took part in promotion of scientific approach in yoga. &lt;strong&gt;He was asked by the Government of USSR to teach yoga to the astronauts as a technique to adjust and equip to the unknown situations of the space journey.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-10-16T10:04:44.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2443" PostTypeId="1" CreationDate="2013-10-16T12:20:19.470" Score="22" ViewCount="3615" Body="&lt;p&gt;I know from a few previous questions that some pretty strong drugs are available aboard different spacecraft and the ISS. Have any officials ever determined, on record, that drugs have been abused?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To clarify, by abused, I mean taken when not needed or more taken than needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information about drug abuse, see the &lt;a href=&quot;http://www.easyread.drugabuse.gov/drugs-of-abuse.php&quot;&gt;National Institute on Drug Abuse&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-08-19T15:16:07.667" LastActivityDate="2016-08-19T15:16:07.667" Title="Have any astronauts ever abused drugs in space?" Tags="|medical|astronaut-lifestyle|policy|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2444" PostTypeId="1" CreationDate="2013-10-17T08:22:19.130" Score="14" ViewCount="2080" Body="&lt;p&gt;There are any quantity of spin-offs from the Space Program. Developments by the erstwhile &lt;a href=&quot;http://en.wikipedia.org/wiki/Soviet_space_program%E2%80%8E&quot; rel=&quot;nofollow&quot;&gt;Soviet Space Program&lt;/a&gt;, and others are probably not well known, or easy to locate online. Yet &lt;a href=&quot;http://www.nasa.gov&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt;, and it's affiliates are more visible. A few of their contributions to the quality of life are as listed &lt;a href=&quot;http://www.isset.org/nasa/tss/aerospacescholars.org/scholars/earthstationmoon/Unit2/Spinoffs.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scrolling the pages reveals screen after screen of technologies that have contributed to the quality of life, and health-care. Yet it is strangely disappointing - all I see listed are &lt;em&gt;&lt;strong&gt;technologies&lt;/em&gt;&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What &lt;em&gt;medicines&lt;/em&gt; (stuff like penicillin, statins; therapeutic formulations) come from the space program? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: At the moment I'm looking not at commercial production, but active/proposed research into formulations regardless whether such formulation were meant for use by astronauts, or whether they were (for whatever reason) short-listed for study in microgravity, hard radiation ... and whatever else orbit environment may provide.&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-12-26T13:54:42.037" LastActivityDate="2023-12-22T00:26:06.283" Title="What medicines have come from the Space Program?" Tags="|medical|spinoff|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2446" PostTypeId="1" AcceptedAnswerId="2485" CreationDate="2013-10-17T13:40:50.133" Score="29" ViewCount="8169" Body="&lt;p&gt;In zero gravity, the liquid in a fuel tank tends to form a blob in a random location, so you'd have to take precautions to make sure the fuel pump can draw fuel. In &lt;a href=&quot;https://space.stackexchange.com/questions/2383/why-does-nasa-intentionally-spin-space-probes&quot;&gt;a recent question about spinning spacecraft&lt;/a&gt;, @Erik suggested that spinning can be used to get the fuel in place for an engine start.&lt;br&gt;&#xA;But if you spin a spacecraft along its thrust axis, the fuel ends up towards the outside of the craft, or at 90 degrees relative to the thrust line. So when you start the engine, the fuel will move violently towards the engine, potentially (if the fuel tank is less than 50% filled) moving away from the fuel intake.&lt;br&gt;&#xA;How is this done in practice? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2021-11-03T23:28:55.733" Title="Starting a liquid fuel engine in zero gravity" Tags="|propulsion|orbit|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2447" PostTypeId="2" ParentId="2446" CreationDate="2013-10-17T16:08:54.933" Score="10" Body="&lt;p&gt;The final position of the liquid fuel and/or oxidizer in a spinning fuel tank depends on the shape of the walls of the tank.  if the tank tapers towards the top, the fuel will spin out to the bottom. Google &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Erlenmeyer_flask&quot;&gt;Erlenmeyer flask&lt;/a&gt;&quot;, chemistry glassware designed to allow  swirling of liquids without the risk of spilling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can recall from the early days of the US space program, reference to what was called an &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Ullage#Rocketry&quot;&gt;ullage&lt;/a&gt; burn&quot;. This involves firing a very small rocket (solid or gas fuel).  The small thrust encourages the main liquid fuel to settle to the &quot;bottom&quot; of the tank, where the fuel intakes are located.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember this particularly since the explanation of the term included getting the last drops out of a wine or whiskey barrel!&lt;/p&gt;&#xA;" OwnerUserId="895" LastEditorUserId="288" LastEditDate="2013-10-17T16:26:36.930" LastActivityDate="2013-10-17T16:26:36.930" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2448" PostTypeId="2" ParentId="2412" CreationDate="2013-10-17T16:55:50.740" Score="17" Body="&lt;p&gt;&lt;strong&gt;Yes, astronauts do use their legs&lt;/strong&gt;.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In weightlessness, any movement you make (even tiny things like pressing a button) will push you in the opposite direction. At many workstations, footholds are provided. The astronauts &lt;strong&gt;use the footholds to hold themselves in place&lt;/strong&gt;, so they can use both hands for the work they're doing instead of continually needing one hand for stationkeeping.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://observationdeck.io9.com/it-never-clicked-before-that-those-were-footholds-1252790243&quot;&gt;Photos of footholds being used&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://science.howstuffworks.com/weightlessness3.htm&quot;&gt;Another photo&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.youtube.com/watch?v=lc8BcBZ0tAI&quot;&gt;Chris Hadfield demonstrating use of footholds in his music video&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the robotic arms are used during a spacewalk, the astronaut is generally attached to the arm by his boots. &lt;a href=&quot;https://www.google.nl/search?q=astronaut%20on%20canadarm&amp;amp;num=50&amp;amp;client=firefox-a&amp;amp;rls=org.mozilla%3aen-US%3aofficial&amp;amp;channel=fflb&amp;amp;source=lnms&amp;amp;tbm=isch&amp;amp;sa=X&amp;amp;ei=JRZgUuzTFsrDswaMmoGADA&amp;amp;ved=0CAkQ_AUoAQ&amp;amp;biw=1280&amp;amp;bih=599&quot;&gt;lots of photos&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="288" LastEditDate="2013-10-18T05:06:20.313" LastActivityDate="2013-10-18T05:06:20.313" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2449" PostTypeId="1" AcceptedAnswerId="2450" CreationDate="2013-10-17T17:22:50.820" Score="33" ViewCount="4060" Body="&lt;p&gt;Wikipedia article states as&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;STS-1 at liftoff. The&#xA;  External Tank was painted&#xA;  white for the first two&#xA;  Space Shuttle launches.&#xA;  From STS-3 on, it was&#xA;  left unpainted.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5GOj0.jpg&quot; alt=&quot;source Wikipedia&quot;&gt;&#xA; &lt;img src=&quot;https://i.stack.imgur.com/HqQHe.jpg&quot; alt=&quot;source:Wikipedia&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder why the early shuttles External tanks are not painted and after NASA  started painting the ET's why it painted it in color of rusted iron why not other color's ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="6944" LastEditDate="2016-06-07T17:07:32.453" LastActivityDate="2018-01-08T03:23:18.123" Title="Why do the Shuttle's external tanks appear to be rusted?" Tags="|launch|rockets|nasa|space-shuttle|engineering|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2450" PostTypeId="2" ParentId="2449" CreationDate="2013-10-17T17:30:54.443" Score="42" Body="&lt;p&gt;From STS-3, the tank was left unpainted, the brown color is the natural color of the insulation foam, &lt;a href=&quot;http://www.nasa.gov/sites/default/files/114022main_TPS_FS.pdf&quot;&gt;see this PDF&lt;/a&gt;. The color would get darker from exposure to the sun.&lt;br&gt;&#xA;The tanks of STS-1 and 2 were painted white, but this took several hundred kg of paint, and after STS-1 and 2 &lt;a href=&quot;http://www.space.com/2282-columbias-white-external-fuel-tanks.html&quot;&gt;tests showed&lt;/a&gt; that the paint wasn't necessary. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-12-29T08:54:01.400" LastActivityDate="2015-12-29T08:54:01.400" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2451" PostTypeId="2" ParentId="2381" CreationDate="2013-10-18T02:20:53.877" Score="6" Body="&lt;p&gt;Since the best information we have is quoted from a news article, this is going to be an exercise in speculation and deduction. First, let's assume &quot;a high degree of accuracy of less than 10 meters&quot; puts the possible spatial resolution range at 10-30 meters or so (note: I'm going on the assumption that the use of the term &quot;accuracy&quot; is just typical misreporting, and what they're really referring to is &quot;resolution&quot;). What does this buy you? The below image from &lt;a href=&quot;http://www.satimagingcorp.com/characterization-of-satellite-remote-sensing-systems.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this nifty website&lt;/a&gt; will help visualize:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HzfTv.png&quot; alt=&quot;spatial resolution&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, while there are certainly things larger than a house that are of military interest, this will not exactly provide you with detailed intelligence - you could do much better with Google Maps, for instance, whose high-resolution imagery is &lt;a href=&quot;https://gis.stackexchange.com/a/11410&quot;&gt;&amp;lt;1 meter&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So... what's the point? Well, there are plenty of other things you can do with satellite imagery. The SPOT-5 satellite has similar resolution, and &lt;a href=&quot;http://www.satimagingcorp.com/satellite-sensors/spot-5.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;is marketed as&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a key asset for applications such as medium-scale mapping (at 1:25 000 and 1:10 000 locally), urban and rural planning, oil and gas exploration, and natural disaster management. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you happen to be collecting &lt;a href=&quot;http://en.wikipedia.org/wiki/Multispectral_image&quot; rel=&quot;nofollow noreferrer&quot;&gt;multispectral imagery&lt;/a&gt;, you can do even more interesting things, particularly in the name of science. NASA's &lt;a href=&quot;http://glcf.umd.edu/data/landsat/&quot; rel=&quot;nofollow noreferrer&quot;&gt;LANDSAT&lt;/a&gt; imagery is of even lower spatial resolution, but is very information-rich, as it does not include only visible light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The small size of Iran's satellite (&amp;lt;50 kg according to the article) indicates that it may be more of a technology demonstration mission as opposed to a large, mature, operational system, which makes sense. While a domestic satellite imagery capability is obviously of interest to a country like Iran, there are no doubt several other components, systems, and processes that will be demonstrated, all in the name of technological maturation.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="-1" LastEditDate="2017-04-13T12:34:15.620" LastActivityDate="2013-11-30T22:37:36.743" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2452" PostTypeId="2" ParentId="2440" CreationDate="2013-10-18T06:29:01.007" Score="11" Body="&lt;p&gt;The Med.Ops section of the ISS User's Guide is apparently &lt;a href=&quot;http://spaceref.com/iss/medical.ops.html&quot; rel=&quot;noreferrer&quot;&gt;available online&lt;/a&gt;.&#xA;It is generally split into the following sections &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceref.com/iss/medical.ops.html#routine&quot; rel=&quot;noreferrer&quot;&gt;Crew Health Procedures&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceref.com/iss/medical.ops.html#routine&quot; rel=&quot;noreferrer&quot;&gt;Routine Non-emergency procedures&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceref.com/iss/emergency&quot; rel=&quot;noreferrer&quot;&gt;Emergency procedures&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'm far from being a medic, or even para-medic so my take may be wrong. A quick gander at the sections on &lt;a href=&quot;http://www.spaceref.com/iss/medical/4037.nosebleed.pdf&quot; rel=&quot;noreferrer&quot;&gt;Nosebleeds&lt;/a&gt;, and &lt;a href=&quot;http://www.spaceref.com/iss/medical/7976.eye.penetration.pdf&quot; rel=&quot;noreferrer&quot;&gt;Penetration of the Eye by a foreign object&lt;/a&gt;, and &lt;a href=&quot;http://www.spaceref.com/iss/medical/4022.collapsed.lung.pdf&quot; rel=&quot;noreferrer&quot;&gt;Collapsed Lung&lt;/a&gt; revealed nothing beyond algorithms. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My concerns on the subject of surgery in microgravity&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;containment of bodily fluids,&lt;/li&gt;&#xA;&lt;li&gt;organ relocation upon ingress &lt;/li&gt;&#xA;&lt;li&gt;post-op&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;remained unaddressed in the Med.Ops manual itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However a further search brought up &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3752909/&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;this article&lt;/em&gt;&lt;/a&gt; specifically on the subject of surgery in space was published in Acta Astronaut. 2012; 79: pp.61–66&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thus far, no surgical procedure has ever been performed on a human during spaceflight. Despite that, surgery involving anaesthesia, interventions, and survival was successfully performed in rodents for the first time on the STS-90 Neurolab Shuttle mission demonstrating that minor surgical procedures may be feasible in humans&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;https://web.archive.org/web/20061205192016/http://neurolab.jsc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;https://web.archive.org/web/20061205192016/http://neurolab.jsc.nasa.gov/&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specific to the topic of containment of bodily fluids is &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/8342090&quot; rel=&quot;noreferrer&quot;&gt;this article (paywalled methinks)&lt;/a&gt; - at-least relevant to bleeding in microgravity&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Simulations of bleeding using dyed fluid and citrated bovine blood, as well as actual arterial and venous bleeding in rabbits, were examined. The high surface tension property of blood promotes the formation of large fluid domes, which have a tendency to adhere to the wound. The use of sponges and suction will be adequate to prevent cabin atmosphere contamination with all bleeding, with the exception of temporary arterial droplet streams. The control of the bleeding with standard surgical techniques should not be difficult.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As a layman, the general impression I receive is that &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Surgery may be done in micro-gravity&lt;/li&gt;&#xA;&lt;li&gt;Minimally invasive aka laparoscopy may be the way to proceed&lt;/li&gt;&#xA;&lt;li&gt;Closing &amp;amp; post-op might require specialized techniques, and tools. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For instance &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;getting collected fluids back into the body reducing/breaking surface-tension&lt;/li&gt;&#xA;&lt;li&gt;bodily organs apparently expand, and relocate themselves around the thoracic cavity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Finally, having spouted all that (+:, research on micro-gravity parabolic flights, and simulations may have a long way to go before a procedure may be planned/executed on board the ISS, or in space. Until then, let's hope no emergency arises.&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="1142" LastEditDate="2014-11-06T02:01:59.120" LastActivityDate="2014-11-06T02:01:59.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2453" PostTypeId="2" ParentId="2433" CreationDate="2013-10-18T08:15:54.127" Score="18" Body="&lt;p&gt;The problem here is that your standard of proof is ridiculous. By the same token, you'd have to doubt the existence of Moscow because you've never met anyone from that city. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the only face to face eyewitness event that I can remember where humans actually observed an attempt to fly into outer space...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Then your memory must not be very good. There have been hundreds of manned launches that have been witnessed by &lt;a href=&quot;https://www.google.com/search?tbm=isch&amp;amp;q=shuttle%20launch%20crowd&quot;&gt;millions of people&lt;/a&gt;. The Apollo and early Shuttle launches had massive audiences that were able to see the spacecraft launch, and reach an altitude of dozens of km in a few minutes. For launches to higher inclinations, the Shuttle was visible &lt;a href=&quot;http://www.nytimes.com/library/national/science/120198sci-shuttle.html&quot;&gt;all the way up the US East Coast&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These days, a few thousand dollars will buy you a VIP visit to Baikonur where you can &lt;a href=&quot;http://rusadventures.com/tour1.shtml&quot;&gt;witness a launch yourself&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you're saying basically is &quot;I'd rather believe all of the millions of photos and thousands of hours of video are hoaxes and that millions of people are involved in a vast conspiracy to lie about every manned space program.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back in the days of the Space Race, the US and USSR would not have hesitated to expose any attempt by the other to lie about sending men to space. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="338" LastEditDate="2013-10-18T15:45:36.450" LastActivityDate="2013-10-18T15:45:36.450" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="2454" PostTypeId="2" ParentId="2053" CreationDate="2013-10-18T09:44:48.050" Score="11" Body="&lt;p&gt;From a comment by @user6972:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;My parents were lucky to get 2 years out of their 8 tracks, much less 30+ years...error correction wouldn't fix the knot they would spew out.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The 8-track 1/4&amp;quot; cartridge audio tape this refers to was notorious for being badly engineered. Many modern tape formats use the same general principle:  a single spool that contains an endless loop of tape. &lt;a href=&quot;http://stweb.peelschools.org/pcsweb/pc_tut/15DLT.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;DLT&lt;/a&gt;, for example. None of them are as &lt;a href=&quot;https://en.wikipedia.org/wiki/8-track_tape#Design&quot; rel=&quot;nofollow noreferrer&quot;&gt;unreliable&lt;/a&gt; as 8-track 1/4&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/McS1y.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/McS1y.jpg&quot; alt=&quot;8-track cartridge&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Voyager DTR used 1/2&amp;quot; tape in a format more like an open-reel tape recorder than an 8-track cartridge. So this avoids the single-spool design and many of its potential problems entirely. &lt;a href=&quot;https://commons.wikimedia.org/wiki/Voyager_Program&quot; rel=&quot;nofollow noreferrer&quot;&gt;This&lt;/a&gt; is what the DTR looks like:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WnDJg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WnDJg.jpg&quot; alt=&quot;Voyager DTR&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;According to this &lt;a href=&quot;https://ntrs.nasa.gov/citations/19810001583&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager Backgrounder, October 1980 (PDF)&lt;/a&gt;&lt;sup&gt;1&lt;/sup&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;at each encounter, the flight data subsystem can handle the imaging data at six downlink rates from 115.2 to 19.2 kbps. The 115.2-kbps rate represents the standard full-frame readout (at 48 seconds per frame) of the TV vidicon. Under normal conditions,&#xA;that rate was used at Jupiter. Full-frame, full-resolution TV from Saturn can be obtained by increasing the frame readout time to 144 seconds (3:1 slow scan) and transmitting the data at 44.8 kbps. A number of other slow scan and frame-edit options are available to match the capability of the telecommunications link.&lt;br /&gt;&#xA;The data-storage subsystem can record at two rates: TV pictures, general science and engineering at 115.2 kbps; general science and engineering at 7.2 kbps; and engineering only at 7.2 kbps (engineering is acquired at only 1,200 bps, but is formatted with filler to match the recorder input rate). The tape transport is belt-driven. Its 1/2 in. magnetic tape is 328m (1,076 ft.) long and is divided into eight tracks that are recorded sequentially one track at a time. Total recycleable storage capacity is about 536 million bits -- the equivalent of 100 TV pictures. Playback is at four speeds-- 57.6; 33.6; 21.6 and 7.2 kbps.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So only one track is recorded to at a time.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;NASA-NEWS-RELEASE-80-160, P80-10172&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="12102" LastEditDate="2021-07-10T06:27:25.343" LastActivityDate="2021-07-10T06:27:25.343" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="2455" PostTypeId="2" ParentId="2444" CreationDate="2013-10-18T11:10:49.643" Score="1" Body="&lt;p&gt;It's not surprising that no new medicines have come directly from the space program. The one thing that the space environment can contribute to the production of medicines is microgravity, but for now, access to space is much too expensive to consider commercially producing any medicines in space.&lt;br&gt;&#xA;Similarly, I get the impression that pharmaceutical research is mostly a matter of repeating similar experiments thousands of times with small variations, so this is more suited to a large lab than a tiny experiments cabinet on the ISS.&lt;br&gt;&#xA;What has been done instead is fundamental research into all kinds of biological and chemical processes. This knowledge feeds into research programs on earth that may lead to new medicines etc. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-10-18T11:10:49.643" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2456" PostTypeId="1" CreationDate="2013-10-18T12:35:34.003" Score="30" ViewCount="22987" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.nasa.gov/returntoflight/system/system_SSME.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each Space Shuttle Main Engine&#xA;  operates at a liquid oxygen/liquid&#xA;  hydrogen mixture ratio of 6 to 1 to&#xA;  produce a sea level thrust of 179,097&#xA;  kilograms (375,000 pounds) and a&#xA;  vacuum thrust of 213,188 (470,000&#xA;  pounds). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why does a rocket engine provide more thrust in vacuum than in atmosphere? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this hold true for all rocket engines?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="716" LastEditDate="2015-06-01T17:47:52.063" LastActivityDate="2023-02-19T17:08:52.477" Title="Why does a rocket engine provide more thrust in a vacuum than in atmosphere?" Tags="|launch|rockets|nasa|construction|engines|" AnswerCount="7" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2457" PostTypeId="2" ParentId="2456" CreationDate="2013-10-18T13:34:29.183" Score="33" Body="&lt;p&gt;Rocket thrust is given by the equation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;F = \dot{m}v_{exit} + A_e(P_1 - P_2)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\dot{m}$ is the mass flow rate, $v_{exit}$ is the average exit flow velocity across the exit plane, $A_e$ is the cross-sectional area of the exhaust jet at the exit plane, $P_1$ is the static pressure inside the engine just before the exit plane, and $P_2$ is the ambient static pressure (&lt;em&gt;i.e.&lt;/em&gt; atmospheric pressure).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Provided that the nozzle is not overexpanded and flow separation does not occur, $A_e$ remains constant, and the thrust difference is realized primarily from the change in $P_2$.  If nozzle is overexpanded to the point that flow separation occurs, however, the exhaust jet area drops as well, causing further losses.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2013-10-18T13:34:29.183" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2458" PostTypeId="2" ParentId="2456" CreationDate="2013-10-18T16:02:05.080" Score="11" Body="&lt;p&gt;In addition to the answer of Tristan I would like to add few more points&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust in the rocket is equal to $T=\dot m V$ (Assuming the rocket nozzle is operating at its optimum condition)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Thrust is a strong function of the exhaust velocity&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V=\sqrt{\frac{2 \gamma  R_{{}^{\circ}} T_{{}^{\circ}}}{(\gamma -1) \mu&#xA;   }\left(1-\left(\frac{P_e}{P_c}\right){}^{\frac{\gamma -1}{\gamma&#xA;   }}\right)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This equation gives the exhaust velocity of the rocket &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exhaust velocity is a function of $\left(\frac{P_e}{P_c}\right){}^{\frac{\gamma -1}{\gamma }}$ and for vacuum the $P_e$ is almost equal to zero so the above term reduces to zero hence the exhaust velocity is maximum &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For sealevel the above term does not reduces to zero so the exhaust velocity is less compared to that in the vacuum&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence the thrust in the vacuum is more than that of in sea level (within atmosphere)&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="105" LastEditDate="2013-11-04T11:20:25.717" LastActivityDate="2013-11-04T11:20:25.717" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2459" PostTypeId="1" CreationDate="2013-10-18T20:37:12.423" Score="8" ViewCount="502" Body="&lt;p&gt;Following on from the question &lt;a href=&quot;https://space.stackexchange.com/questions/2237/how-long-will-it-be-light-on-venus-at-night&quot;&gt;How long will it be light on Venus at night?&lt;/a&gt;, which is essentially about the visual properties of the Venusian atmosphere, this question is concerned about the sound-related properties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that the properties of sound 'change' when it passes through different materials, densities and temperatures, &lt;strong&gt;what would the atmospheric conditions of Venus sound on its surface?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Bonus question&lt;/em&gt; what would an opera on Venus sound like?&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="-1" LastEditorDisplayName="user838" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2013-10-19T10:16:10.770" Title="What would the atmospheric conditions of Venus sound like on its surface?" Tags="|venus|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2460" PostTypeId="1" CreationDate="2013-10-18T21:01:19.087" Score="13" ViewCount="183" Body="&lt;p&gt;The issue of the effects of radiation on medication potency is not only important for long term stays in the ISS, but also very important for longer duration journeys and even for colonization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing is certain, the chances are that astronauts/colonists will fall ill, not necessarily to the extent asked in &lt;a href=&quot;https://space.stackexchange.com/questions/2431/how-is-the-iss-equipped-to-deal-with-the-outbreak-of-a-contagion&quot;&gt;How is the ISS equipped to deal with the outbreak of a contagion?&lt;/a&gt;, but with the more mundane illnesses mentioned in the answers.  Some would require regular medication as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the question is, in the increased radiation environment of space, &lt;strong&gt;how does increased environmental radiation affect medication potency, in the long and short term?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, are medications on the ISS shielded to protect it from radiation?&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="30164" LastEditorDisplayName="user838" LastEditDate="2019-04-16T01:46:04.260" LastActivityDate="2021-11-11T05:00:17.620" Title="How does increased environmental radiation affect medication potency, long and short term?" Tags="|radiation|space-station|medical|colonization|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="2461" PostTypeId="1" AcceptedAnswerId="2463" CreationDate="2013-10-19T01:09:31.497" Score="20" ViewCount="3263" Body="&lt;p&gt;We know that being in space has negative effects on bones. If you had a broken bone is space would it heal ok?  Has there ever been a broken bone healed in space? Other then &lt;a href=&quot;https://en.wikipedia.org/wiki/Spaceflight_osteopenia&quot;&gt;Spaceflight osteopenia&lt;/a&gt; would there be any concerns like the bone not knowing how to grow because of the lack of gravity?&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2022-07-29T16:21:34.610" Title="How well would a broken bone heal in zero gravity?" Tags="|microgravity|medical|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2463" PostTypeId="2" ParentId="2461" CreationDate="2013-10-19T03:32:02.287" Score="19" Body="&lt;p&gt;Studies, simulations and observations reported in the National Space Biomedical Research Institute's document &lt;a href=&quot;http://www.dsls.usra.edu/20090528Midura.pdf&quot;&gt;Fracture healing in spaceflight and in a ground-based hypogravity analog&lt;/a&gt; conclude that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Fibular fracture healing in a chronically&#xA;  unloaded condition is impaired when&#xA;  compared to the healing response in a&#xA;  weight weight-bearing condition.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Where the unloaded condition represents zero or a low gravity environment.  The conclusions were based on the observations on test rats, finding that in low gravity:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;healing rate was more variable&lt;/li&gt;&#xA;&lt;li&gt;the rate and extent of callus size was diminished to 20-30%&lt;/li&gt;&#xA;&lt;li&gt;Non union incidence (meaning, union of both sides of the fracture) was 46%.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Further, in the article &lt;a href=&quot;http://journalofcosmology.com/Mars147.html&quot;&gt;Effects of Long-Duration Spaceflight, Microgravity, &#xA;and Radiation on the &#xA;Neuromuscular, Sensorimotor, and Skeletal Systems &#xA;&lt;/a&gt; (Carpenter et al. 2010), it is reported that in simulated tests using rats, bone fracture repair was delayed significantly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, no, unless you had some means to simulate the mechanical loading (Carpenter et al. 2010) necessary for the healing process, in a microgravity environment, the healing process would be delayed and not complete (according to the simulations studied).&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-10-19T03:32:02.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2465" PostTypeId="2" ParentId="2459" CreationDate="2013-10-19T10:16:10.770" Score="4" Body="&lt;p&gt;Using &lt;a href=&quot;http://en.wikipedia.org/wiki/Speed_of_sound&quot; rel=&quot;nofollow&quot;&gt;this formula&lt;/a&gt;, I get a speed of sound of about 400 m/s for Venus' surface, so a bit higher than on Earth. But that doesn't change how it sounds by much, just your sense of acoustics (being able to gauge the size of a room by how long it takes for reflections to arrive).&lt;br&gt;&#xA;Ignoring the whole 'being able to breathe' issue, the higher density (~150 times more dense than air) dampens the vocal cords more, so your opera would sound a few octaves lower.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.youtube.com/watch?v=uvvzItzLSxw&quot; rel=&quot;nofollow&quot;&gt;video: demonstration using helium and sulphur hexafluoride&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-10-19T10:16:10.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2466" PostTypeId="1" AcceptedAnswerId="2467" CreationDate="2013-10-19T16:09:35.363" Score="28" ViewCount="107570" Body="&lt;p&gt;In Rocket Launch countdowns I noticed That they count with respect to some parameter T as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;T-10 seconds&#xA;T-9 seconds&#xA;.&#xA;.&#xA;.&#xA;T-0 seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not simply count as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;10&#xA;9&#xA;8&#xA;.&#xA;.&#xA;0&lt;/p&gt;&#xA;&#xA;&lt;p&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mp3/590318main_ringtone_135_launch.mp3&quot;&gt;http://www.nasa.gov/mp3/590318main_ringtone_135_launch.mp3&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="784" LastEditorUserId="414" LastEditDate="2013-10-19T16:51:18.927" LastActivityDate="2015-06-05T23:47:28.583" Title="What is meaning of T in rocket launch countdown?" Tags="|launch|rockets|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2467" PostTypeId="2" ParentId="2466" CreationDate="2013-10-19T16:44:15.360" Score="31" Body="&lt;p&gt;The &quot;T&quot; stands for the time at which the rocket is scheduled to be launched. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;T minus 4 minutes and 49 second  indicate 4 minutes and 49 second  before the launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eventhough the last ten seconds are counted in most of the case as 10,9,8,7,....,3,2,1 &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This method is also be used as &quot;T-PLUS&quot; which shows the time after the launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;http://en.wikipedia.org/wiki/Countdown&quot;&gt;also&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;even &quot;E-&#xA;  minus&quot; for events that involve&#xA;  spacecraft that are already in&#xA;  space,..... the &quot;E&quot;&#xA;  stands for &quot;Encounter&quot;, as with a&#xA;  comet or some other space object.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;External source&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/epoxi/epoxi20101004.html&quot;&gt;http://www.nasa.gov/mission_pages/epoxi/epoxi20101004.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-10-19T16:44:15.360" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2468" PostTypeId="2" ParentId="2466" CreationDate="2013-10-19T16:54:09.623" Score="8" Body="&lt;p&gt;&lt;em&gt;T&lt;/em&gt; is just a designator for the main sequence countdown time that serves as the synchronisation device for the majority of systems and procedures that have to take place before, exactly on and even after the countdown sequence reaches &lt;em&gt;T-0&lt;/em&gt;. Countdown technically ends with &lt;em&gt;T-0&lt;/em&gt; (&lt;em&gt;T minus zero&lt;/em&gt;) and the count then continues in the positive range (&lt;em&gt;T plus 1, 2,...&lt;/em&gt;), but we usually refer to all of it as &lt;em&gt;the countdown&lt;/em&gt; for simplicity sake, even after continuing to count upwards with other sequences taking place post launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;T-0&lt;/em&gt; does not necessarily mean the exact time in this sequence when the liftoff happens, and can also stand for the time of main engine ignition, or anything similar, though usually close to the actual liftoff and rocket's clearing of the launch tower. There are other time designators used, for example &lt;em&gt;L&lt;/em&gt; can stand for time before the launch day, &lt;em&gt;E&lt;/em&gt; for the time to some event, encounter or ETA (Estimated Time of Arrival), and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in short, &lt;em&gt;T&lt;/em&gt; it's just a convention, a single letter abbreviated designator for the system clock (mostly quoted to stand for &lt;em&gt;Time&lt;/em&gt;) that a lot of events are synchronised with throughout the duration of the launch. More about its purpose can be read in this thread: &lt;a href=&quot;https://space.stackexchange.com/questions/1295/what-is-the-purpose-of-having-a-countdown-during-a-rocket-launch&quot;&gt;What is the purpose of having a countdown during a rocket launch?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have to mention that &lt;em&gt;T&lt;/em&gt; might not be used at all, for example with the JAXA's launch of the Epsilon rocket, no designator was used since there was only a single system clock to synchronize with (something that didn't go quite according to plans with the first launch attempt of this new launcher and the timestamp discrepancy between the onboard timekeeper and the mission control of a few milliseconds caused the launch sequence to be halted and postponed to a later date). Second attempt was successful though, but they still didn't use the &lt;em&gt;T&lt;/em&gt; designator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some ILS (International Launch Services) rocket launches also didn't use any designator at all, for example the last ILS Proton launch, and even though the English reporter covering the launch still used it during live streams (I presume out of habit), the official announcer in Russian language didn't use it at all. They also didn't say &lt;em&gt;&quot;Liftoff&quot;&lt;/em&gt; like we'd hear some other announcers say. With the last Soyuz launch, for example, the official announcer said &lt;em&gt;&quot;Fly now!&quot;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So all this &lt;em&gt;T business&lt;/em&gt; is a sort of convention that seems to stick best to western launch systems presumably for historical reasons, but it could really be any other single letter abbreviation (or whatever is easy to pronounce and won't be confused for other designators) of the main system's clock, or even none at all when a single system clock is used to synchronise all the launch subsystems against.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-10-19T18:13:48.770" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2469" PostTypeId="2" ParentId="973" CreationDate="2013-10-19T17:51:56.047" Score="3" Body="&lt;p&gt;The Low Cost Access to Space Project data on &lt;a href=&quot;http://lcas.otaski.org/index.php/Main_Page&quot; rel=&quot;nofollow noreferrer&quot;&gt;OTASKI&lt;/a&gt; analyzes trade-offs between mass and rocket thrust to launch a rocket from a balloon or an aircraft with payloads of less than 10kg. The LCAS aims to provide low cost and innovative methods for low cost orbital launch system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Diagram of delta-V vs Rocket Mass&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HvvgD.png&quot; alt=&quot;Graph of delta-V vs mass&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://lcas.otaski.org/index.php/File:Rocket_mass.c&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Mass model&lt;/a&gt;(on which the graph is based) using the rocket equation is provided in the form of &lt;a href=&quot;http://lcas.otaski.org/images/3/3e/Rocket_mass.c&quot; rel=&quot;nofollow noreferrer&quot;&gt;C Program&lt;/a&gt; takes various parameters such as payload mass, overall thrust, length to diameter ratio, etc. etc. as inputs and produces dry/wet mass as result. It is modeled on SSTO rocket with one engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Rocket Mass Model:&lt;/strong&gt; Rough Sketch of the rocket&#xA;&lt;img src=&quot;https://i.stack.imgur.com/LYYbl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, the Vactrain model is of-course viable. One point worth noting while using the Vactrain would be to separate the communal segment with the rocket segment by means of an air-lock once the rocket takes its course. Without an airlock, In case of launch failure or accident inside the rocket segment, A rupture or flaw might propagate to the whole tunnel and would be disastrous.&lt;/p&gt;&#xA;" OwnerUserId="784" LastActivityDate="2013-10-19T17:51:56.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2470" PostTypeId="1" CreationDate="2013-10-19T20:52:48.000" Score="11" ViewCount="799" Body="&lt;p&gt;The ISS is using the Control Moment Gyroscopes to maintain its attitude (the Cupola pointing towards Earth). That means the ISS is slowly rotating with respect to distant stars. What happens during docking of a visiting spacecraft (Soyuz, Progress, ATV, Dragon, etc)? These spacecraft should approach the ISS from one direction towards a designated docking port. If ISS would be keeping its nominal attitude, it would be very difficult for the incoming craft to keep the port in its approach trajectory. Is the ISS attitude changed during docking? Does the ISS have to &quot;catch-up&quot; (rotate to point the Cupola again towards the Earth) after the docking?&lt;/p&gt;&#xA;" OwnerUserId="921" LastActivityDate="2016-01-25T23:48:24.173" Title="ISS attitude during docking" Tags="|iss|attitude|docking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2472" PostTypeId="2" ParentId="2444" CreationDate="2013-10-20T08:33:53.553" Score="10" Body="&lt;p&gt;I am aware of one such scientific program that spun off something useful: the Salmonella vaccine research by Arizona State University. The virulence of the Salmonella bacteria was altered in microgravity environment, generally making it more aggressive. These changes in bacteria behavior helped identify new possibilities for vaccine development. The research happened aboard Space Shuttles and the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/benefits/vaccine_development_prt.htm&quot; rel=&quot;nofollow&quot;&gt;International Space Station Plays Role in Vaccine Development&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="1142" LastEditDate="2014-12-26T16:44:38.667" LastActivityDate="2014-12-26T16:44:38.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2473" PostTypeId="1" AcceptedAnswerId="2489" CreationDate="2013-10-20T16:39:11.297" Score="24" ViewCount="3358" Body="&lt;p&gt;In this &lt;a href=&quot;https://twitter.com/pbdes/statuses/377729523308965888&quot; rel=&quot;noreferrer&quot;&gt;Tweet&lt;/a&gt; (dated 11 Sept 2013) Peter de Selding reported that Gwynne Shotwell of SpaceX said:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX's Shotwell: We're building 7 Falcon 9 fairings/yr now, 15 in&#xA;  2014, &amp;amp; 4 engines/week rising to at least 5/week by January.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But I was wondering what they mean by 'engine'.  First thought is Merlin 1D since they need 10 per Falcon 9 launch (9 on the first stage, 1 on the second/upper stage).  But 4/week is 208/year.  5/week is 260/year.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is vaguely 20 to 26 cores a year, and they are clearly not anywhere close to using that many engines a year.  At best they are aiming for 8 launches in 2014, and maybe one Heavy, which is only 80 to 108 engines in 2014.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some point, stockpiling Merlin 1D's gets to be counter productive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, each Dragon has 4 quads of Draco thrusters, with a total of 18 Draco total on a Dragon.  Dragonrider (manned Dragon) is reported to have 8 Super Draco thrusters. Which seem would be on top of the 18 regular Dracos. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that manner, a single Dragon would need 18 engines, ignoring the launcher, and a manned Dragonrider would likely needed 26 engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2014 they are aiming at 3-4 Dragon flights, plus a pad abort (Of a Dragonrider config), which would mean 54-72 Draco's just for the CRS missions and 8 Super Dracos + 18 Dracos for the pad abort test.  So about 98 non-Merlin engines just for the manifested Dragon flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they aim for 8 Falcon 9 launches (all V1.1 so all Merlin 1D's) that is 80 Merlin 1Ds and possibly as many as 98 Draco/SuperDracos.  That nets out around 178 engines used up in 2014. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, it is kind of cheesy to classify a Draco (90 lbs (not kilo-lbs, just simply 90 lbs) with a 145,000 lb thrust Merlin 1D or a 75,000 lb Super Draco. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="897" LastEditDate="2015-10-04T13:51:16.157" LastActivityDate="2018-01-03T20:16:52.150" Title="When SpaceX claims to build 4 engines/week, which engines do they mean?" Tags="|spacex|falcon-9|dragon-v2|draco|merlin-1d|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2474" PostTypeId="5" CreationDate="2013-10-20T17:08:02.337" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-10-20T17:08:02.337" LastActivityDate="2013-10-20T17:08:02.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2475" PostTypeId="4" CreationDate="2013-10-20T17:08:02.337" Score="0" Body="Maneuvering thrusters used on SpaceX's Dragon and DragonRider capsules.  This hypergolic thruster produces 90 lbs of force.  Each Dragon capsule has 18 Draco thrusters." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-20T17:48:42.827" LastActivityDate="2013-10-20T17:48:42.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2476" PostTypeId="1" AcceptedAnswerId="2484" CreationDate="2013-10-20T17:20:37.533" Score="14" ViewCount="2815" Body="&lt;p&gt;It is &lt;a href=&quot;http://en.wikipedia.org/wiki/Gemini_4#Rendezvous_attempt&quot; rel=&quot;noreferrer&quot;&gt;said in Wikipedia&lt;/a&gt;, that &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the first orbit, McDivitt attempted to rendezvous with the spent&#xA;  Titan second stage. This was unsuccessful for a number of reasons:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA engineers had not yet worked out the idiosyncrasies of orbital&#xA;  mechanics involved in rendezvous,[citation needed] which are&#xA;  counter-intuitive. Simply thrusting the spacecraft toward the target&#xA;  changed its orbital altitude and velocity relative to the target. When&#xA;  McDivitt tried this, he found himself moving away and downward, as the&#xA;  retrograde thrust lowered his orbit, increasing his speed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't understand this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any explanation, given in local reference frame? Referring &quot;orbital altitude&quot; referrers global reference frame and is OK. But any set can be possibly regarded in any reference frame. Local reference frame should be inertial with tidal, Coriolis and other forces. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How to describe the situation with this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I need explanation WITHOUT notion of &quot;orbit&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE 2&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose we are inside giant closed spacecraft like &lt;a href=&quot;http://en.wikipedia.org/wiki/Rama_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Rama&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/O%27Neill_cylinder&quot; rel=&quot;noreferrer&quot;&gt;O'Neill cylinder&lt;/a&gt;. This spacecraft is on Earth orbit, but we are inside and don't know this. We feel weightlessness. Now, If Rama is rotating, we can feel some non-inertial effects like centrifugal or Coriolis forces. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But suppose Rama is not rotating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, the only strange thing we will feel is Earth tidal force. The tidal force mean that all objects will periodically distracted along axis, directed to (invisible) Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, you want to say, that McDivitt failed due to tidal forces?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hard to believe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE 3&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The possibility to regard the task from within any reference frame I like -- is the basic physical principle. You won't convince me it is wrong in the case of orbital movement.&lt;/p&gt;&#xA;" OwnerUserId="924" LastEditorUserId="924" LastEditDate="2013-10-21T07:29:05.063" LastActivityDate="2020-05-13T20:16:40.173" Title="Why did the rendezvous attempt fail on Gemini 4?" Tags="|orbital-mechanics|project-gemini|rendezvous|" AnswerCount="6" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2477" PostTypeId="1" CreationDate="2013-10-20T18:08:39.343" Score="18" ViewCount="704" Body="&lt;p&gt;&lt;a href=&quot;http://www.spacedaily.com/news/russia-97h.html&quot;&gt;Spacedaily&lt;/a&gt; writes to say about President. Kennedy's Sept 20th 1963 address to the UN General Assembly&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;I include among these possibilities,&quot; he added, &quot;a joint expedition to the Moon.&quot; Why, the President asked, should the United States and the Soviet Union conduct parallel efforts that would include &quot;duplication, of research, construction, and expenditure?&quot; &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The article referenced suggests the hint of joint-cooperation in space may have sprung from a view to share/reduce cost &amp;amp; effort. Assuming this to be true brings one to the question - &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In what areas was the relevant Soviet technology more developed than that of the United States at that time? Or would such collaboration simply have shared the burden of research, development, implementation, and execution in a field equally new to both parties in all aspects?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user56" LastEditDate="2013-10-21T13:51:41.620" LastActivityDate="2013-10-21T13:51:41.620" Title="What technology could the Soviets have contributed to the Apollo program?" Tags="|apollo-program|technology|russia|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2478" PostTypeId="2" ParentId="2476" CreationDate="2013-10-20T18:24:53.337" Score="28" Body="&lt;p&gt;This is more of a physics question, but here goes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both the piloted spacecraft and the rendezvous target are objects each in their own orbit, even though they may be separated by minimal distance and have minimal relative velocity one to the other. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An object in an ideal orbit always travels in a plane of fixed absolute orientation around the center of mass of the object it is orbiting. The orbit is always an ellipse, such that the center of mass of the orbited object occupies one of the focal points of the ellipse. A perfectly circular orbit is simply a special case where both focal points coincide. As articulated by Kepler, an orbiting object covers equal areas in equal times - travelling most slowly at the &quot;highest&quot; point, most rapidly at its &quot;lowest&quot; point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any &quot;push&quot; given to an orbiting object, no matter the direction, will change its orbit in some way - increasing or decreasing its mean altitude, increasing or decreasing eccentricity (how elliptical or circular the orbit is), or changing the orientation of the plane in which it is travelling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider an object in a perfectly circular orbit. Giving it an extra push along its orbit will increase its orbital energy, increasing its mean altitude. That will cause its orbit to become elliptical, &quot;climbing&quot; to some greater altitude and losing speed until it arrives at point opposite to where it got the push, then descending and accelerating back to the altitude and speed it just after the push.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, imagine you've successfully placed yourself on exactly the same orbital path as an object you want to rendezvous with, just some distance &quot;behind&quot;. You fire your thrusters to push you &quot;forward&quot;. Well, you start to move toward your target, but because you are moving faster, you start to gain altitude and move &quot;up&quot; above your target... but moving up slows you down and causes you to fall behind!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, you could fire a thruster &lt;em&gt;&quot;downward&quot;&lt;/em&gt;. Forcing yourself into a &quot;lower&quot; point with the same orbital energy causes you to move faster along your orbit, so you catch up. Then you fire a thruster &quot;upward&quot; to return you to your original energy state (and orbit), closer to your target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Counter-intuitive, but it comes down to working with orbital energy states.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2013-10-20T18:24:53.337" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2479" PostTypeId="5" CreationDate="2013-10-20T20:08:46.343" Score="0" Body="&lt;p&gt;SpaceX has claimed that the Dragon capsule was designed to be manned from its initial design.  The first flights of Dragon were however unmanned. In order to support manned operations a launch escape system was required, and the key differentiator between a Dragon and DragonRider model would be the escape system. The design is using a pusher escape system, powered by 8 75Klb thrust SuperDraco engines integrated in the sidewalls of the capsule. When not used for launch escape the intention is to use the Super Dracos at the end of the mission to propulsively land.  Initial launch is expected in 2015 or 2016, with a pad abort planned in 2014 and a Max-Q abort sometime later.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-20T20:17:57.643" LastActivityDate="2013-10-20T20:17:57.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2480" PostTypeId="4" CreationDate="2013-10-20T20:08:46.343" Score="0" Body="Putative name for manned version of the SpaceX Dragon capsule." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-20T20:17:52.110" LastActivityDate="2013-10-20T20:17:52.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2481" PostTypeId="5" CreationDate="2013-10-20T20:55:18.523" Score="0" Body="&lt;p&gt;Russia, the country is the largest remaining component of the former Soviet Union. The space program of the Soviet Union and of Russia has generated many space firsts, often goaded on by political aims to attain those firsts at all costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main launcher used for Soyuz manned missions to the space station is fundamentally the same basic design as the launcher used for some of the very first space missions.  If it works, why change it, appears to be a Russian model. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-20T22:00:17.900" LastActivityDate="2013-10-20T22:00:17.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2482" PostTypeId="4" CreationDate="2013-10-20T20:55:18.523" Score="0" Body="The country of Russia is what remains of the former Soviet Union (Of which Russia was a province/state).  The Russian/Soviet space program has been quite expansive and capable over many decades.  Currently it appears to be suffering from lack of funding leading to quality control issues at times." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-20T22:00:21.293" LastActivityDate="2013-10-20T22:00:21.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2483" PostTypeId="2" ParentId="2477" CreationDate="2013-10-20T21:07:21.793" Score="12" Body="&lt;p&gt;At the very start of Apollo? Very little, beyond a basic launcher.  Everything else was mostly developed on the fly for Apollo.  Pretty much all of Gemini was developing ideas to use for Apollo. At the time of the announcement by JFK very little existed in either sides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basic engine design might have helped, but in the end, the US did a better job at large liquid fueled rockets (F-1 at 1.5 Million lbs thrust vs the NK-33 at 300 Klbs (and needing 30 of them on the first stage!)). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps, a different outcome, favouring Earth Orbit Rendezvous would resulted from such a collaboration.  The ability to build the Saturn V with its very impressive payload capability helped make Lunar Orbit Rendezvous the better choice for faster delivery of a mission.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-10-20T21:07:21.793" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2484" PostTypeId="2" ParentId="2476" CreationDate="2013-10-20T21:19:43.110" Score="8" Body="&lt;p&gt;The explanation in the frame of the body T that McDivitt was trying to approach is this. When he turned the thrust on, the spacecraft acquired velocity $\mathbf v$ towards the body. The Coriolis force $-2m\mathbf{\Omega}\times \mathbf v$ acted on the spacecraft, where $\mathbf \Omega$ is the angular velocity of the rotation of the body T in the orbit around the Earth. If the two bodies were initially at the same height, the Coriolis force acted upwards, thus giving the spacecraft some velocity in the direction away from the Earth. This necessarily got the spacecraft to higher altitude and there the tidal force plus Coriolis force changed the velocity of the body further so it actually began to recede from the body T (tidal force vector at higher altitude points away from the body T).&lt;/p&gt;&#xA;" OwnerUserId="925" LastActivityDate="2013-10-20T21:19:43.110" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2485" PostTypeId="2" ParentId="2446" CreationDate="2013-10-20T22:35:23.143" Score="20" Body="&lt;p&gt;Starting liquid-fueled rocket engines in zero-g conditions is a non-trivial problem that should be explicitly addressed during design. There are a number of common approaches:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Use an auxiliary propulsion system to perform what is often called a settling burn. The purpose of this maneuver is to provide just enough thrust to force the propellant towards the sump, or feed-line from the tank(s) to the engine, and the ullage to the opposite end. The trick is ensuring the system provides enough positive thrust for enough time to ensure good wetting of the sump. After ignition, the main engine thrust / acceleration should keep the propellant in the right place. (see below)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If there is a reaction or attitude control system on the stage that produces axial thrust this is a good option. &lt;/li&gt;&#xA;&lt;li&gt;The Saturn V used solid &lt;a href=&quot;http://en.wikipedia.org/wiki/Ullage_motor&quot;&gt;ullage motors&lt;/a&gt; for a similar purpose. &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Maintain positive acceleration during coast. For engines that shut down and then restart (e.g. after an in-space coast phase), a very small amount of thrust can be used, not to settle propellants per se, but to keep them settled. In this case, the thrust can be micro-g levels. The downside is that positive thrust should be maintained for the entire coast, so even though the thrust is small it has to be active for a long time. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Propellant management devices. Non-rocket systems, for example liquid propellant systems on satellites, typically use other methods to keep enough propellant near the sump to get the system started. Often these can be screens or other internal tank features that use capillary or surface tension forces to keep at least some of the liquid from floating away.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Lastly would be if the propulsion system was robust to the having liquid, gas, or two-phase flow enter the engine. This is pretty rare, but if it was the case then the design could theoretically ignore this issue.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="127" LastActivityDate="2013-10-20T22:35:23.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2486" PostTypeId="1" AcceptedAnswerId="2490" CreationDate="2013-10-21T02:48:55.760" Score="12" ViewCount="1082" Body="&lt;p&gt;Ever since &lt;a href=&quot;https://space.stackexchange.com/questions/2428/can-we-get-energy-from-the-motion-of-orbiting-bodies&quot;&gt;a recent orbital mechanics question&lt;/a&gt;, lunar flybys have been nagging me in the sense that there is obviously energy available in the system. I'm interested if you could get a repeatable process that is energy-positive and could be done at the same cost as an ordinary NASA mission. This requirement limits the possible approaches to a spacecraft trajectory, along with the familiar toolset.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is precedent that deep space probes get gravity assists from the Moon. Accumulated over time, these are demonstrated to be useful for helping to get probes out of Earth's gravity well. However, that's a 1-way process, so it has no obligation to preserve angular momentum, and it's non-trivial whether it could be tailored to make a repeatable orbit or not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lunar flybys are certainly &lt;a href=&quot;https://space.stackexchange.com/questions/2103/how-to-calculate-delta-v-for-lunar-flyby&quot;&gt;tunable to some degree&lt;/a&gt; - in that you can hit a bullseye on Earth after the flyby. You could graze LEO instead, and you're off into another elliptical orbit. The problem is that the Moon has left you behind in that time, and you won't come close to it at next apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the system is rife with orbital energy! More than likely, the lunar flyby itself gave the probe kinetic energy. With extra juice to spend, why not spend some of that in an aero-brake with Earth, allowing you to tune your orbital parameters for the next pass?  Something like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cNlEv.png&quot; alt=&quot;maneuver diagram&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I'm asking this question because there's something obviously ill-conceived about the above scheme. I definitely can't get the energetic arguments to work. Notice that I used East-West (retrograde) orbits above. That's because angular momentum arguments seem to rule out West-East orbits. The tidal evolution of the Earth-Moon system is that Earth's rotational energy is transferred to the Moon, so &lt;em&gt;if&lt;/em&gt; this scheme is energy-positive, I expect it to slow down Earth's rotation (marginal amounts).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My arguments in &lt;em&gt;favor&lt;/em&gt; of the scheme are so-far highly suspicious. While the Earth's rotation contains energy, it seems unlikely that skimming it at 11 km/s could ever possibly extract energy from that rotation. From the probe's perspective, the Earth could probably be taken to not be rotating at all. That logic ties me in knots, because it suggests that energy could be extracted from the Moon by slowing it down, which we know to be totally wrong. Energy can only be extracted from the Moon &lt;em&gt;by speeding it up&lt;/em&gt;. Something is obviously wrong with that simplification.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course &quot;simple&quot; aerobraking won't work. That would only lower apogee, which isn't what we want. However, if NASA outfits it with some purpose-built movable flaps, it could do something like &quot;&lt;a href=&quot;https://space.stackexchange.com/questions/1854/what-is-feathering&quot;&gt;feathering&lt;/a&gt;&quot; to do controlled aerobraking before perigee, affecting the other orbital parameters. You could even possibly &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist#Limits_to_slingshot_use&quot; rel=&quot;noreferrer&quot;&gt;use lift during the pass&lt;/a&gt; to &quot;aim&quot; for the Moon's new position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But all of those arguments are useless, unless the energy and momentum transfer can be clearly articulated. If possible, it could be proven beyond a doubt in an orbital simulator. If not possible, I expect that orbital physics arguments could falsify it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To add more detail, I'm asking if a system is possible that uses:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A trajectory-based approach&lt;/li&gt;&#xA;&lt;li&gt;converts Earth-Moon kinetic energy into some other form of energy&lt;/li&gt;&#xA;&lt;li&gt;Doesn't rely on Earth's quadrupole moment (you could say tidal power does this now)&lt;/li&gt;&#xA;&lt;li&gt;Doesn't require direct force transference on &quot;space elevator&quot; scales&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That last point is the killer here. If you could get around that, you could do this with something the size of a cubesat. But that might also kill the entire idea.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2021-07-21T04:37:32.163" Title="Could Earth-lunar flybys be repeated consecutively without the use of propellant?" Tags="|the-moon|propulsion|orbital-maneuver|energy|aerobraking|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2487" PostTypeId="2" ParentId="2476" CreationDate="2013-10-21T09:45:04.963" Score="7" Body="&lt;p&gt;Sorry, but it's impossible to explain this without referring to orbits. When you are in orbit, your altitude and your linear velocity (speed in the direction of the orbit) are &lt;a href=&quot;http://www.physicsclassroom.com/class/circles/u6l4c.cfm&quot; rel=&quot;noreferrer&quot;&gt;inextricably linked&lt;/a&gt;: your linear velocity is proportional with the square root of the altitude (radius).&lt;br&gt;&#xA;As a result, any change in speed inevitably changes your orbit.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia article does not say this explicitly, but logically, the Gemini would have been ahead of the Titan stage in the same orbit. Then the rest of the description makes sense: McDivitt used the thrusters pointed in the direction of the orbit to try and reduce the distance (i.e. retrograde thrust). So he reduced his linear speed, which had to result in a lower orbit. &#xA;In a lower orbit, it doesn't take as long to complete one orbit (360 degrees), so as a result he moves away from the Titan stage. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-10-21T09:45:04.963" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2488" PostTypeId="2" ParentId="2486" CreationDate="2013-10-21T13:33:25.767" Score="7" Body="&lt;p&gt;I don't know if there was propellant used in between, but I know of at least one mission that had a spacecraft use two lunar assists: &lt;a href=&quot;http://stereo.gsfc.nasa.gov/&quot;&gt;STEREO&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at the first video on their &lt;a href=&quot;http://stereo.gsfc.nasa.gov/orbit.shtml&quot;&gt;web page explaining the orbits&lt;/a&gt;, you'll see that B (STEREO-Behind) gets a second gravity assist from the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Disclaimer: I work for the &lt;a href=&quot;http://stereo-ssc.nascom.nasa.gov/&quot;&gt;STEREO Science Center&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="46" LastActivityDate="2013-10-21T13:33:25.767" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2489" PostTypeId="2" ParentId="2473" CreationDate="2013-10-21T17:04:29.790" Score="14" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.aviationweek.com/Article.aspx?id=/article-xml/AW_09_23_2013_p40-617797.xml&amp;amp;p=2&quot; rel=&quot;noreferrer&quot;&gt;Aviation Week&lt;/a&gt;, it's 4 Merlin 1D engines per week. The same article says they're planning 12 Falcon 9 launches in 2014.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In October 2014, SpaceX announced they'd completed the &lt;a href=&quot;http://www.spacex.com/news/2014/10/16/spacex-completes-100th-merlin-1d-engine&quot; rel=&quot;noreferrer&quot;&gt;100th Merlin 1D&lt;/a&gt;. According to that article, build tempo was 4 engines per week at that point, and 80 Merlin 1Ds had flown by then.&lt;br&gt;&#xA;This engine was slated to be flown on a flight in early 2015. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-04T13:52:17.167" LastActivityDate="2015-10-04T13:52:17.167" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2490" PostTypeId="2" ParentId="2486" CreationDate="2013-10-21T18:00:44.430" Score="8" Body="&lt;p&gt;What you are referring to is a &lt;a href=&quot;http://en.wikipedia.org/wiki/Free_return_trajectory&quot;&gt;Free Return Trajectory&lt;/a&gt;. It references a paper called &lt;a href=&quot;http://babel.hathitrust.org/cgi/pt?id=uiug.30112106596973;view=1up;seq=10&quot;&gt;Trajectories in the Earth-Moon Space with Symmetrical Free Return Properties&lt;/a&gt;, which states that the right path can lead to flying between the Earth and Moon with no further fuel required. But you won't gain any momentum from that.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-21T18:00:44.430" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2491" PostTypeId="1" AcceptedAnswerId="2492" CreationDate="2013-10-21T21:59:05.053" Score="19" ViewCount="60615" Body="&lt;p&gt;Many of us have watched the launch of a Space Shuttle space ship, I think: a giant red rocket, some smaller, and the way smaller space ship on the red one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It reaches Mach 23 to be able to leave Earth. But what's the amount of fuel that is used overall for this purpose? How much fuel is burnt in the first phases? (so it doesn't include the fuel burnt by the space shuttle itself; only the rockets)&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="58" LastEditDate="2016-06-07T17:14:34.003" LastActivityDate="2016-06-07T17:14:34.003" Title="How much fuel was used for a Space Shuttle launch?" Tags="|launch|rockets|space-shuttle|fuel|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2492" PostTypeId="2" ParentId="2491" CreationDate="2013-10-21T23:07:01.233" Score="26" Body="&lt;p&gt;NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot;&gt;Space Transportation System&lt;/a&gt; (STS) vehicle, better known as the Space Shuttle, used two single engine &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot;&gt;Solid Rocket Boosters&lt;/a&gt; (SRB) as Stage 0, an engineless &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle_External_Tank&quot;&gt;external tank&lt;/a&gt; providing propellant for the three &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Main_Engine&quot;&gt;Space Shuttle Main Engines&lt;/a&gt; (SSME) on the orbiter as stage 1, and additional two &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_Maneuvering_System&quot;&gt;Orbital Maneuvering System&lt;/a&gt; (OMS) hypergolic liquid-propellant rocket engines on the Space Shuttle orbiter as stage 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two solid rocket boosters used roughly 500,000 kg (1.1 Mlb) of a 11-star perforated solid propellant &lt;em&gt;cake&lt;/em&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ammonium_Perchlorate_Composite_Propellant&quot;&gt;Ammonium Perchlorate Composite Propellant&lt;/a&gt; (APCP - a mixture of of ammonium perchlorate, aluminium, iron oxide, PBAN or HTPB polymers, and an epoxy curing agent) each, that provided 124 seconds of burn time with a specific impulse (&lt;em&gt;Isp&lt;/em&gt;) of 269 s that provided 12.5 MN of thrust per SRB and the external tank that came in three different configurations (mostly progressively reducing tank's own weight) capacity was  629,340 kg (1,387,457 lb) of cryogenic liquid oxygen (LOX) as the oxidizer and 106,261 kg (234,265 lb) of cryogenic liquid hydrogen (LH2) as the fuel components of the bipropellant &lt;a href=&quot;http://www.astronautix.com/props/loxlh2.htm&quot;&gt;LOX/LH2&lt;/a&gt; that provided 480 seconds of burn time with specific impulse of 455 seconds, resulting in 5.45 MN of thrust at sea-level (for the Super Lightweight Tank or SLWT, the last and most advanced of the three versions used with STS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to answer your question directly, not counting the OMS propellant as per the specifics of your question, the total mass of all propellants of the SRBs (stage 0) and the external tank (stage 1) was at launch of the STS 1,735,601 kg (3,821,722 lb). The solid rocket boosters provided roughly 83% of liftoff thrust for the Space Shuttle and were the largest, most powerful solid-propellant motors flown to date.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-21T23:12:01.410" LastActivityDate="2013-10-21T23:12:01.410" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="2493" PostTypeId="1" AcceptedAnswerId="30208" CreationDate="2013-10-22T02:58:38.527" Score="17" ViewCount="2720" Body="&lt;p&gt;A quick follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/2491/how-much-fuel-is-used-for-a-space-shuttle-launch&quot;&gt;How much fuel was used for a Space Shuttle launch?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My memory is a little foggy; I seem to recall reading the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_11&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11&lt;/a&gt; mission &lt;a href=&quot;http://en.wikipedia.org/wiki/Neil_Armstrong&quot; rel=&quot;noreferrer&quot;&gt;commander&lt;/a&gt; depleted his &lt;em&gt;reserve&lt;/em&gt; fuel landing &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;Eagle&lt;/a&gt;. Apparently the landing was manual because the location designated was rocky. Most missions (unmanned, or otherwise) do a correction burn at some point. The duration of the burn being determined by the correction required - which will probably be known only after the mission itself is off-surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;noreferrer&quot;&gt;Apollo missions&lt;/a&gt; carrying human cargo, I would assume the volume of fuel to carry in reserve was calculated conservatively rather than optimistically. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was the process to calculate reserve fuel to carry on-board the Apollo missions in the CSM, and LM? Was it merely a rule of thumb - &quot;Let's have 1% over-and-above the calculated figure&quot; ?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2018-11-19T18:36:48.817" Title="How was reserve fuel calculated for the Apollo missions?" Tags="|apollo-program|fuel|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2494" PostTypeId="1" CreationDate="2013-10-22T12:11:04.100" Score="11" ViewCount="7463" Body="&lt;p&gt;The nice wikipedia  &lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_space_station_cargo_vehicles&quot;&gt;comparison table&lt;/a&gt; of space station cargo vehicles doesn't provide any hints on the price per kilogram for the cargo delivery to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As it is usual with price per kilogram questions, the hard numbers can be hard to formulate. Yes.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="22203" LastEditDate="2024-03-09T16:02:48.303" LastActivityDate="2024-03-09T16:02:48.303" Title="Price per kg of cargo delivery to ISS" Tags="|iss|cost|economics|cost-to-orbit|supply|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2495" PostTypeId="2" ParentId="2494" CreationDate="2013-10-22T12:54:18.000" Score="6" Body="&lt;p&gt;I suspect the core reason is that there is almost no way to come up with a meaningful number.  Only Dragon and Cygnus have a chance of having real hard numbers, since they are actual commercial contracts, that are a pay for service.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whereas ATV, HTV, Progress, and Shuttle are all government/national programs so it is next to impossible to reliably decide what should be included in their costs.  In the case of Russia they are fairly opaque on real costs anyway. Consider how hard it was to get a real/meaningful cost for a shuttle launch.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simplistically, for the initial CRS contract, NASA tendered a request for 20,000K to the station by each of the two bidders.  Cygnus at 1.2 billion/20,000K is about 60 thousand dollars a kilo.  Dragon at 1.6 billion for 20,000K would be around 80,000 dollars a kilo.  But of course, CRS was also meant to pay back for some of the development of the systems.   (Even so, 2.8 billion for 40,000K of cargo plus two complete launch systems and cargo vessels is a literal steal.  That is probably less than one year of the SLS budget, for nothing real till 2021) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the contract AFTER CRS will be the one of interest.  Once SpaceX/Orbital have recouped development costs through a lucrative first contract, how much will the second services contract be worth?  Presumably a lot less. (One hopes!) &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-22T14:09:17.573" LastActivityDate="2013-10-22T14:09:17.573" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2496" PostTypeId="1" AcceptedAnswerId="2499" CreationDate="2013-10-22T14:09:35.163" Score="20" ViewCount="23657" Body="&lt;p&gt;The LEM was comprised of an ascent, and a descent stage. Whilst the descent stage was in most cases left behind on Luna, the ascent stage ferried the astronauts upstairs to the CSM. Where are these vehicles now? Did they crash into the Earth/Moon, or are they still in orbit around the Moon/Earth/Sun?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-23T01:09:24.470" LastActivityDate="2016-06-25T19:02:42.543" Title="Where were the various Apollo Lunar Modules (LMs) discarded?" Tags="|apollo-program|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2498" PostTypeId="2" ParentId="2496" CreationDate="2013-10-22T15:50:22.093" Score="8" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot;&gt;Wikipedia&lt;/a&gt; the ascent stage (i.e. the upper half) for most of the Lunar Modules either:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Were crashed into the moon deliberately&lt;/li&gt;&#xA;&lt;li&gt;burned-up in Earth's atmosphere shortly after the dust-off from the moon&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;LM-4 &quot;Snoopy&quot; - the ascent stage is still in solar-orbit - the only surviving flown Lunar-module. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The descent-stage (i.e. the lower half) in most cases is still on the lunar surface.&lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="692" LastEditDate="2013-10-22T16:08:22.060" LastActivityDate="2013-10-22T16:08:22.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2499" PostTypeId="2" ParentId="2496" CreationDate="2013-10-22T15:51:52.213" Score="27" Body="&lt;p&gt;After digging around, I found &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/lunar/apolloloc.html&quot;&gt;this article&lt;/a&gt;, which states the final locations are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apollo 5- Destroyed in Earth's Atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Apollo 9- Destroyed in Earth's Atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Apollo 10- Heliocentric orbit (Around Sun)&lt;/li&gt;&#xA;&lt;li&gt;Apollo 11- &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=9091.0&quot;&gt;Released around Moon&lt;/a&gt;, impacted, location unknown. &lt;/li&gt;&#xA;&lt;li&gt;Apollo 12- Impacted Moon 20 November 1969 at 22:17:17.7 UT (5:17 PM EST)&#xA;3.94 S, 21.20 W &lt;/li&gt;&#xA;&lt;li&gt;Apollo 13- Burned up in Earth's atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Apollo 14- Impacted Moon 07 February 1971 at 00:45:25.7 UT (06 February, 7:45 PM EST)&#xA;3.42 S, 19.67 W &lt;/li&gt;&#xA;&lt;li&gt;Apollo 15- Impacted Moon 03 August 1971 at 03:03:37.0 UT (02 August, 11:03 PM EDT)&#xA;26.36 N, 0.25 E &lt;/li&gt;&#xA;&lt;li&gt;Apollo 16- Released around moon, impact site unknown. Orbited for about a year.&lt;/li&gt;&#xA;&lt;li&gt;Apollo 17- Impacted Moon 15 December 1972 at 06:50:20.8 UT (1:50 AM EST)&#xA;19.96 N, 30.50 E &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It seems that Apollo 11 was left unguided, and after that they seemed to want to guide the LMs to hit the Moon at a specific spot and time to use for testing lunar properties.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="10442" LastEditDate="2015-06-11T05:17:25.953" LastActivityDate="2015-06-11T05:17:25.953" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2500" PostTypeId="1" AcceptedAnswerId="4103" CreationDate="2013-10-22T17:19:17.123" Score="38" ViewCount="93030" Body="&lt;p&gt;Apollo 1 was a ground test that ended in a fire. The next Apollo mission I have any reference to is Apollo 7. What happened to Apollo 2,3,4,5 and 6?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2018-05-09T18:14:40.027" LastActivityDate="2018-05-09T18:14:40.027" Title="Why did Apollo mission numbering skip 2,3,4,5 and 6?" Tags="|apollo-program|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="2501" PostTypeId="2" ParentId="2500" CreationDate="2013-10-22T17:37:54.837" Score="10" Body="&lt;p&gt;They were either unmanned test-flights, manned test-flights (Apollo 7), or not-quite-numbered as Apollo missions (AS-201 - AS-203)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_Apollo_missions&quot;&gt;http://en.wikipedia.org/wiki/List_of_Apollo_missions&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="692" LastActivityDate="2013-10-22T17:37:54.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2502" PostTypeId="2" ParentId="2500" CreationDate="2013-10-22T17:40:13.163" Score="8" Body="&lt;p&gt;&lt;a href=&quot;http://burro.astr.cwru.edu/stu/20th_close_apollo.html&quot;&gt;This site&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Apollo 2 and 3: There were no craft named Apollo 2 or 3.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Apparently after the Apollo-1 craft was destroyed during a pre-flight test at Cape Canaveral, the first few mission (through Apollo-6) were unmanned missions to test various aspects of the Apollo program - Launch vehicle, CSM, LM, and their inter-play.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo-7 was the first manned mission after the Apollo 1 disaster; which seems to dovetail with the data you have. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A brief summary of the Apollo program is also listed &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo.html&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-10-22T17:40:13.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2503" PostTypeId="1" AcceptedAnswerId="3696" CreationDate="2013-10-22T22:54:26.010" Score="8" ViewCount="167" Body="&lt;p&gt;Regardless of the &lt;a href=&quot;http://www.nasaspaceflight.com/2013/09/cygnus-cots-graduation-iss-berthing/&quot;&gt;GPS discrepancy&lt;/a&gt;, Cygnus completed its mission. How well did it do, and, does SpaceX now have a proper competitor? &lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2014-11-11T03:25:26.140" Title="How well did the Orbital Sciences' Cygnus cargo craft perform?" Tags="|iss|docking|testing|cygnus|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2504" PostTypeId="1" CreationDate="2013-10-23T00:59:16.513" Score="11" ViewCount="1701" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_11&quot;&gt;Wikipedia's article on Apollo 11&lt;/a&gt; indicates that this first mission left behind a flag, an Apollo 1 mission patch, a plaque (on the lander), and a bag with a gold olive branch and a message disk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over all the Apollo missions, what memorial items were left on the lunar surface?&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastEditorDisplayName="user12" LastEditDate="2013-10-23T01:38:28.620" LastActivityDate="2013-10-23T07:23:01.033" Title="What memorial items were left on the Moon by the Apollo missions?" Tags="|apollo-program|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2505" PostTypeId="1" AcceptedAnswerId="2507" CreationDate="2013-10-23T06:36:46.973" Score="24" ViewCount="3663" Body="&lt;p&gt;It goes without saying that training/conditioning one's reflexes for a situation is highly relevant to some professions. Pilots train in a computer simulator.&lt;/p&gt;&#xA;&lt;p&gt;During the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Project&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo missions&lt;/a&gt; computers were not as ubiquitous as they now are. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Command_Module#Command_Module_.28CM.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;CSM&lt;/a&gt; could be, and was checked-out in Earth orbit.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Module (LM)&lt;/a&gt; however was meant for a non-Earth environment; one which could probably not be simulated - yet it would have been necessary to give the astronauts a 'feel' of the craft in reduced gravity.&lt;/p&gt;&#xA;&lt;p&gt;The mission commander for &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_11&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo-11&lt;/a&gt; apparently did such a great job with his manual landing the &lt;a href=&quot;http://www.popsci.com/military-aviation-amp-space/article/2009-06/40-years-later-ten-things-you-didnt-know-about-apollo-ii-moon-landing&quot; rel=&quot;nofollow noreferrer&quot;&gt;shock-absorbers failed to depress&lt;/a&gt;. This may be merely an urban legend ... but it does draw one's attention to the question -&lt;/p&gt;&#xA;&lt;p&gt;How did the Apollo astronauts train to land the LM in the 1/6th lunar gravity?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="51070" LastEditDate="2023-04-21T18:51:30.580" LastActivityDate="2023-04-21T18:51:30.580" Title="How did the Apollo astronauts train for the 1/6G lunar landing?" Tags="|apollo-program|gravity|lunar-landing|training|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2507" PostTypeId="2" ParentId="2505" CreationDate="2013-10-23T06:47:27.013" Score="44" Body="&lt;p&gt;They used the &lt;a href=&quot;http://history.nasa.gov/alsj/LLTV-952.html&quot; rel=&quot;noreferrer&quot;&gt;Lunar Landing Training Vehicle&lt;/a&gt;, as pictured below. This had a jet engine to provide 5/6 of the lift needed to hover the vehicle, plus rocket engines that simulated the LM's engines.&lt;br&gt;&#xA;With the jet running, the LLTV felt like it weighed 1/6 of its actual weight, so it came pretty close to simulating moon gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/L5rhm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="25" LastEditDate="2013-10-23T14:02:38.463" LastActivityDate="2013-10-23T14:02:38.463" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2508" PostTypeId="2" ParentId="2504" CreationDate="2013-10-23T07:23:01.033" Score="7" Body="&lt;p&gt;According to the NASA's history office's document &lt;a href=&quot;http://history.nasa.gov/FINAL%20Catalogue%20of%20Manmade%20Material%20on%20the%20Moon.pdf&quot; rel=&quot;noreferrer&quot;&gt;Catalogue of Manmade Material on the Moon&lt;/a&gt; (Apollo materials are on the 2nd page), produced as a&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;resource offers a&#xA;  comprehensive catalogue of human artefacts on the Moon based on the currently available data.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As at the time of the document's publication, a lot of artefact, from the badges you mentioned, to urine collection assemblies, to a pair of tongs are on the lunar surface.  According to the &lt;a href=&quot;http://spacegrant.nmsu.edu/lunarlegacies/artifactlist.html&quot; rel=&quot;noreferrer&quot;&gt;Lunar Legacy Project&lt;/a&gt; page, there are well over 100 objects ('artefacts') on the Moon from Apollo 11 alone. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I know some of these objects do not seem like memorials in the traditional sense, however, consider that these objects are relics of our forays to another world, specifically&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The cultural landscape of space includes both sites and objects on and off Earth,&quot; said Beth O'Leary, an associate professor of anthropology at the University of New Mexico in Las Cruces. &quot;It is necessary to evaluate the significance of the latter and treat them as important objects and places worthy of legitimate archaeological inquiry.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://www.space.com/20743-space-archaeology-artifacts-preservation.html&quot; rel=&quot;noreferrer&quot;&gt;Space Archaeologists Call for Preserving Off-Earth Artifacts&lt;/a&gt; (David, 2013).&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-10-23T07:23:01.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2509" PostTypeId="1" CreationDate="2013-10-23T08:58:43.330" Score="15" ViewCount="404" Body="&lt;p&gt;What was the heaviest artificial object to reach orbit of Mars?&#xA;Delivering the heaviest object of all times should have been quite an achievement in itself. So I'd like to understand the details of stages, masses and fuels of its launch and its flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How it was launched? On which rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How it was accelerated? What was the upper stage(s)? When has it separated from its upper stage(s)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the same rocket stages and spacecraft engines deliver any heavier object, or was it close to the limit for the given rocket configuration?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was the dry mass of the spacecraft, its full mass?&#xA;How has it utilized its engine after separation from the rocket stages?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How it was braking (an aero-brake, multiple aero-brakes)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What martian orbit it ended up at?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-10-23T17:42:57.623" Title="The heaviest martian spacecraft" Tags="|launch|mars|history|upper-stage|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2510" PostTypeId="2" ParentId="2509" CreationDate="2013-10-23T17:42:57.623" Score="10" Body="&lt;p&gt;Starting with this &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_missions_to_Mars&quot;&gt;list of missions to Mars&lt;/a&gt;, I first looked at the launchers used. The Proton-K, Ariane 5 and Titan 3 Centaur are the heaviest in the list. That eliminates most candidates.&lt;br&gt;&#xA;I initially expected Curiosity to lead at &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Science_Laboratory&quot;&gt;3893 kg&lt;/a&gt;. But the Russians have launched some heavier probes, e.g. Mars 3 at &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_3&quot;&gt;4650 kg&lt;/a&gt; for the orbiter plus lander. The Mars 96 weighed &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_96&quot;&gt;6180 kg&lt;/a&gt;, but failed to reach Mars. Other Proton launches were lighter. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-10-23T17:42:57.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2512" PostTypeId="1" AcceptedAnswerId="2513" CreationDate="2013-10-23T19:28:33.583" Score="7" ViewCount="5346" Body="&lt;p&gt;I'm trying to correlate a behavior on LEO satellites with the position it is over the Earth. The behavior takes place over a period of time. I'm trying to come up with a metric that will identify something about the position relative to the position on the equator, and not vs the actual long/lat at the moment. The reason is that the ground track of the orbit could have put it in a very different position. For instance, here is a few ISS ground tracks from STK. These two ground tracks for my purposes are significantly different, and any metric should give different values for each, despite the lat/long of the ISS being nearly identical in each case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ORd8G.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/PP6ox.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any metric I find must be easily computable, either with a python library, or something that can be directly computed. One can assume that I have the TLE files for the satellite. The reason being is that I'm doing statistical data on a lot of these, and I really don't have the time to plop each of these instances in to a stand alone program. Using a web API is acceptable as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I've been thinking about using is some sort of a metric of the longitude of the ascending node, especially if it can be tracked in a smooth manner, but I'm finding it difficult to figure out what could calculate that. Any other suggestions out there?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, anything that would work for the ISS would work for my needs. My satellites are in LEO, occasionally thrust, mostly circular, but are fairly stable overall.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-19T17:24:01.047" LastActivityDate="2016-07-19T17:24:01.047" Title="LEO metric to determine the ground track of a satellite" Tags="|artificial-satellite|low-earth-orbit|orbital-elements|ground-track|analysis|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2513" PostTypeId="2" ParentId="2512" CreationDate="2013-10-23T22:24:04.610" Score="7" Body="&lt;p&gt;I &lt;em&gt;think&lt;/em&gt; I understand what you're looking for. You mention the longitude of the ascending node, but that you're not sure how to actually implement this. Here's how.&lt;/p&gt;&#xA;&lt;p&gt;First, the longitude of the ascending node (a.k.a. right ascension of the ascending node) &lt;span class=&quot;math-container&quot;&gt;$\Omega$&lt;/span&gt; for an Earth-orbiting satellite is defined as the angle between some reference vector  (typically the &lt;span class=&quot;math-container&quot;&gt;$X$&lt;/span&gt; axis in an Earth-centered, inertial frame, which in turn typically coincides with a vector from the center of the Earth to the &lt;a href=&quot;http://www.glyphweb.com/esky/concepts/firstpointofaries.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;first point of Aries&lt;/a&gt;) and the point at which your orbit crosses the equator while ascending (going South to North).&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QrEFu.png&quot; alt=&quot;RAAN&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;You're in luck, because the two-line element set file type includes this angle directly (&lt;a href=&quot;https://celestrak.org/columns/v04n03/&quot; rel=&quot;nofollow noreferrer&quot;&gt;columns 18-25 on the second line&lt;/a&gt;). Unfortunately, this has no connection (yet) to the orientation with respect to Earth's surface.&lt;/p&gt;&#xA;&lt;p&gt;For the conversion from an &amp;quot;inertial&amp;quot; &lt;span class=&quot;math-container&quot;&gt;$\Omega$&lt;/span&gt; to something relative to the Earth, you'll need to know what time it is. See below&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/NSjap.png&quot; alt=&quot;Hour angles&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here, the green vector is pointing towards the object of interest (here, the ascending node), the grey vector is your reference axis (first point of Aries), and the yellow vector is the prime meridian (0 deg. longitude).&lt;/p&gt;&#xA;&lt;p&gt;So, to get the angle you're interested in (Earth-referenced longitude of the ascending node, or the angle between the yellow and green vectors), you need to subtract GMST from &lt;span class=&quot;math-container&quot;&gt;$\Omega$&lt;/span&gt;. Your TLE gives you Universal Time (UT) for the &lt;em&gt;epoch&lt;/em&gt;, but you may wish to propagate your orbit and calculate this for other times (just remember that in LEO your node will drift). Going from UT to GMST isn't particularly difficult, but since you mentioned Python in the question &lt;a href=&quot;http://infohost.nmt.edu/tcc/contrib.html/lang/python/examples/sidereal/SiderealTime-fromDatetime.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here is a package that will do that conversion for you&lt;/a&gt;. To convert hours to a longitude, it's just &lt;span class=&quot;math-container&quot;&gt;$\lambda = \frac{\textrm{GMST}}{24}\cdot360^{\circ}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Now, the quantity &lt;span class=&quot;math-container&quot;&gt;$\Omega - \lambda$&lt;/span&gt; will be the longitude (with respect to Earth's surface) of your ascending node.&lt;/p&gt;&#xA;&lt;p&gt;One caveat is that only using GMST here ignores some other effects (precession and nutation of Earth's poles), but generally those effects are very small compared to the GMST correction, and considering you're just using TLEs anyway, it's probably precise enough for what you want.&lt;/p&gt;&#xA;&lt;p&gt;Another thing to keep in mind - in STK, that ground track is going to &amp;quot;step&amp;quot; over every period, but that's just because of how it's displayed. The actual longitude of the ascending node is going to change continuously in time as the Earth rotates beneath your orbit (and as your plane precesses), so just remember that.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2013-10-23T22:24:04.610" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2515" PostTypeId="1" CreationDate="2013-10-24T02:23:05.747" Score="25" ViewCount="32085" Body="&lt;p&gt;As the picture below (kind of) shows, there are lots of satellites in geostationary orbit. Couple this with the fact that certain longitudes are more desirable than others (like above North America or Europe, for example), and the question soon arises: how close do these satellites typically get?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would think that different operators would leave some (very) safe amount of distance between their spacecraft, but an operator with a fleet of satellites (like, say, &lt;a href=&quot;http://www.intelsat.com/&quot; rel=&quot;noreferrer&quot;&gt;Intelsat&lt;/a&gt;) might risk a much more closely-spaced cluster if they trusted their tracking data enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what's standard practice with respect to spacing? Are there exceptions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EPIBu.jpg&quot; alt=&quot;GEO belt&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2014-03-02T17:55:18.903" Title="How closely spaced are satellites at GEO?" Tags="|orbit|operations|geostationary|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2516" PostTypeId="2" ParentId="2515" CreationDate="2013-10-24T05:43:14.590" Score="12" Body="&lt;p&gt;In theory, GEO locations are subdivided into fixed &quot;slots&quot; of 2 degrees longitude (or 1470km of orbital extent per slot). Within a slot, the individual satellite must confine itself to a &quot;box&quot; of 0.1 degrees longitude (or something like 70km).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Naturally, this is not always a case in reality. Satellites do drift across slot boundaries every now and then. There's also an issue of uncontrolled debris (there's more debris fragments than working satellites). And, finally, there exist an interesting procedure of &quot;handover&quot;, whereupon a new satellite is expected to replace an existing aging satellite in the same slot, but without causing any service interruption (so the old and new satellite must co-exist for some time in roughly the same orbital locality).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nice article to read (has plenty of details on topic):&#xA;&lt;a href=&quot;http://thespacereview.com/article/1634/1&quot;&gt;http://thespacereview.com/article/1634/1&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="743" LastActivityDate="2013-10-24T05:43:14.590" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2517" PostTypeId="2" ParentId="2515" CreationDate="2013-10-24T05:50:33.443" Score="20" Body="&lt;p&gt;Regarding proximity, we'll need a good database first. For example, Eric Johnston publishes on a &lt;a href=&quot;http://www.satsig.net/sslist.htm&quot; rel=&quot;noreferrer&quot;&gt;SatSig.net page&lt;/a&gt; a list of satellites in Geostationary Orbit (GEO) that was last updated on 21 Feb 2014, so fairly recent. I couldn't find more complete list, but this individual has been upkeeping the list for a fairly long time (some older pages where he collated these lists of GEO satellites go as far back as 2004). I wouldn't know how complete this list is and I can imagine how some of the satellites wouldn't make it there and their orbits being classified, but the total number of satellites on the list is 414 (the page says 416 but an import to a spreadsheet actually comes out with two in minus, wherever they've gone?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going by that number of listed satellites (414), and being utterly simplistic about their orbits calculating only theoretical minimum satellite separation by longitude, that on average separates them by 0.87° (or 52'10&quot;) and at distance of 42,164 km (26,199 mi) from the center of the Earth, that puts them at an average distance of 440 km (273 mi). With a bit of data import wizardry (I've prepared &lt;a href=&quot;http://pastebin.com/Ncrx7WAK&quot; rel=&quot;noreferrer&quot;&gt;this fixed column width file&lt;/a&gt; to make it easier), the minimum distance listed is only 0.0014576° (or merely 5.25&quot;) between &lt;a href=&quot;https://en.wikipedia.org/wiki/Astra_2C&quot; rel=&quot;noreferrer&quot;&gt;Astra 2C&lt;/a&gt; (catalog number 01025A) and &lt;a href=&quot;https://en.wikipedia.org/wiki/Astra_1KR&quot; rel=&quot;noreferrer&quot;&gt;Astra 1KR&lt;/a&gt; (06012A) that were both at longitude of 19.2° East in 2006-2007 and 2010-2013. Astra 2C is now slotted 28.2° East. With fairly similar inclination between each other (0.083 and 0.089°respectively), those simplistic maths of mine from before make more sense now and we can calculate that they were during those two time periods at the minimum distance to each other at roughly 1.1 km (0.666 mi).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a side note, Wikipedia also lists &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_satellites_in_geosynchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;satellites in Geosynchronous Orbit&lt;/a&gt; (GSO) with 204 active, 5 in transit and 6 retired GSO satellites. GEO is a circular geosynchronous equatorial orbit, and GSO satellites would have their &lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-major_axis&quot; rel=&quot;noreferrer&quot;&gt;semi-major axis&lt;/a&gt; at exact distance to Earth as GEO has. So while not exactly the same thing, they are of course still relevant to you question, and indeed they all repeat also in the list of GEO satellites by Eric Johnston. Conveniently though, that Wikipedia page already does some maths for us:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Note that some of these satellites are separated from each other by as&#xA;  little as one tenth of a degree longitude. While that may seem like&#xA;  very little separation, and it is, at geosynchronous altitude (nearly&#xA;  36,000 km), this corresponds to an inter-satellite spacing of&#xA;  approximately 73 km. The major consideration for spacing of&#xA;  geostationary satellites is the beamwidth at-orbit of uplink&#xA;  transmitters, which is primarily a factor of the size and stability of&#xA;  the uplink dish, as well as what frequencies the satellite's&#xA;  transponders receive; satellites with discontiguous frequency&#xA;  allocations can be much closer together.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In all fairness though, the functioning satellites whose GEO/GSO orbits are depicted in the picture you attach to the question don't account for the majority of &lt;em&gt;The GEO Belt&lt;/em&gt; clutter. Far bigger concern are misbehaving satellites like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Galaxy_15&quot; rel=&quot;noreferrer&quot;&gt;Galaxy 15&lt;/a&gt; &lt;em&gt;&quot;Zombiesat&quot;&lt;/em&gt; and orbital debris. The picture you attach also doesn't do justice to the size of the GEO area, so here's another one that defines it as a toroidal volume (the green toroid is the GEO region, the blue one surrounding it the GSO region, and the red shroud around the Earth is the LEO region, all of them protected regions as defined by the &lt;a href=&quot;http://www.iadc-online.org/&quot; rel=&quot;noreferrer&quot;&gt;Inter-Agency Space Debris Coordination Committee&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/xzORe.jpg&quot; alt=&quot;GEO Belt&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;em&gt;The GEO Belt&lt;/em&gt; showing the orbital region as a toroidal volume defined by an angle of +/- 15° around the geostationary orbit and an&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;altitude of approximately +/- 300 km. (Source: &lt;a href=&quot;http://debris-spatiaux.cnes.fr/english/mesure_prevention_eng.html&quot; rel=&quot;noreferrer&quot;&gt;CNES Centre National d'Etudes Spatiales&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, the &lt;a href=&quot;http://www.iadc-online.org/&quot; rel=&quot;noreferrer&quot;&gt;Inter-Agency Space Debris Coordination Committee&lt;/a&gt; (IADC) defines this geosynchronous region in the &lt;a href=&quot;http://www.iadc-online.org/Documents/IADC-2002-01,%20IADC%20Space%20Debris%20Guidelines,%20Revision%201.pdf&quot; rel=&quot;noreferrer&quot;&gt;Space Debris &#xA;Mitigation Guidelines&lt;/a&gt; (PDF) from 2007 with the same inclination range of +/- 15°, but an altitude range of +/- 200 km. IADC is an international governmental forum for the worldwide coordination of activities related to the issues of man-made and natural debris in space. From their home page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The primary purposes of the IADC are to exchange information on space&#xA;  debris research activities between member space agencies, to&#xA;  facilitate opportunities for cooperation in space debris research, to&#xA;  review the progress of ongoing cooperative activities, and to identify&#xA;  debris mitigation options.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The 12 IADC member agencies include: &lt;a href=&quot;http://www.asi.it/en&quot; rel=&quot;noreferrer&quot;&gt;ASI&lt;/a&gt; (Agenzia Spaziale Italiana), &lt;a href=&quot;http://www.cnes.fr/web/CNES-en/7114-home-cnes.php&quot; rel=&quot;noreferrer&quot;&gt;CNES&lt;/a&gt; (Centre National d'Etudes Spatiales), &lt;a href=&quot;http://www.cnsa.gov.cn/n615709/cindex.html&quot; rel=&quot;noreferrer&quot;&gt;CNSA&lt;/a&gt; (China National Space Administration), &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/&quot; rel=&quot;noreferrer&quot;&gt;CSA&lt;/a&gt; (Canadian Space Agency), &lt;a href=&quot;http://www.dlr.de/dlr/en/&quot; rel=&quot;noreferrer&quot;&gt;DLR&lt;/a&gt; (German Aerospace Center), &lt;a href=&quot;http://www.esa.int/ESA&quot; rel=&quot;noreferrer&quot;&gt;ESA&lt;/a&gt; (European Space Agency), &lt;a href=&quot;http://www.isro.org/&quot; rel=&quot;noreferrer&quot;&gt;ISRO&lt;/a&gt; (Indian Space Research Organisation), &lt;a href=&quot;http://www.jaxa.jp/index_e.html&quot; rel=&quot;noreferrer&quot;&gt;JAXA&lt;/a&gt; (Japan Aerospace Exploration Agency), &lt;a href=&quot;http://www.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt; (National Aeronautics and Space Administration), &lt;a href=&quot;http://www.roscosmos.ru/&quot; rel=&quot;noreferrer&quot;&gt;ROSCOSMOS&lt;/a&gt; (Russian Federal Space Agency)&#xA;&lt;a href=&quot;http://www.ssau.ru/english/&quot; rel=&quot;noreferrer&quot;&gt;SSAU&lt;/a&gt; (State Space Agency of Ukraine), and &lt;a href=&quot;http://www.ukspace.org/&quot; rel=&quot;noreferrer&quot;&gt;UKSpace&lt;/a&gt; (UK Space Agency).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IADC website publishes a public &lt;a href=&quot;http://www.iadc-online.org/index.cgi?item=docs_pub&quot; rel=&quot;noreferrer&quot;&gt;Document Registration List&lt;/a&gt; with many documents detailing agreed on procedures in place to control GEO/GSO satellite attitudes, their minimum separation, re-orbiting, disposal in higher orbits and so on and &lt;a href=&quot;https://space.stackexchange.com/a/2516/49&quot;&gt;@user16653 has already mentioned in another answer&lt;/a&gt; a few of these procedures that the 12 member agencies agreed to respect, and to give you one additional example, the &lt;a href=&quot;http://www.iadc-online.org/Documents/IADC%20reorbit%20recommendation199704.pdf&quot; rel=&quot;noreferrer&quot;&gt;Reorbit Procedure for GEO Preservation&lt;/a&gt; (PDF) demonstrates well enough that even simple procedures like re-orbiting of satellites are quite complicated in a bid to avoid any issues with other GEO/GSO orbited satellites, avoiding collisions with orbital debris, interference with other satellite frequency ranges, and so on. Satellite operators' own wish to have a certain longitude slot always populated with a properly functioning satellite doesn't make this any easier either. I.e. the dynamics of these processes are fairly complicated.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-03-02T17:05:00.100" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2518" PostTypeId="1" AcceptedAnswerId="2519" CreationDate="2013-10-24T08:47:58.890" Score="4" ViewCount="768" Body="&lt;p&gt;Apropos Project A119 &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_A119&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The aim of the project was to detonate a nuclear bomb on the Moon which would help in answering some of the mysteries in planetary astronomy and astrogeology, and had the explosive device not entered into a lunar crater, the flash of explosive light would have been faintly visible to people on earth with their naked eye, &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One's attention is drawn to the grammar in the opening paragraph quoted above &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;had the explosive device not entered into a lunar crater&lt;/strong&gt;, the flash of explosive light would have been faintly visible to people on earth with their naked eye&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same article goes on to state &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Neither the Soviet nor the US Project A119 were ever carried out,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Was a nuclear device detonated over Earth's Moon?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-10-24T10:20:23.700" Title="Was a nuclear device detonated over Earth's Moon?" Tags="|the-moon|nuclear|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2519" PostTypeId="2" ParentId="2518" CreationDate="2013-10-24T10:20:23.700" Score="5" Body="&lt;p&gt;The grammar is a bit convoluted, because it's a nesting of two conditions. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;If the project had been carried out&lt;/li&gt;&#xA;&lt;li&gt;the explosion would only be visible if the explosive device is not detonated inside a lunar crater.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If condition 1 is false, then condition 2 is never tested. The sentence &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Neither the Soviet nor the US Project A119 were ever carried out&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;unequivocally states that condition 1 is false. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-10-24T10:20:23.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2520" PostTypeId="1" AcceptedAnswerId="2525" CreationDate="2013-10-24T12:10:43.013" Score="10" ViewCount="1844" Body="&lt;p&gt;Coming from a question about &lt;a href=&quot;https://space.stackexchange.com/questions/2515/how-closely-spaced-are-satellites-at-geo&quot;&gt;distances between satellites in GEO&lt;/a&gt;, what liquid propulsion systems do a majority of GEO satellites carry to maintain their slot positions? It is easy to find information about Merlin engines, SSME, Rocketdyne F-1, etc., but smaller purpose engines are harder to find.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Thanks for the info about the propellents used. What about the engines?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-14T10:35:48.690" Title="What kind of liquid propulsion systems do GEO satellites use?" Tags="|artificial-satellite|propulsion|orbit|geostationary|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2521" PostTypeId="1" AcceptedAnswerId="2526" CreationDate="2013-10-24T18:27:03.040" Score="11" ViewCount="1931" Body="&lt;p&gt;NASA's Apollo modules, the ones that went to the moon, had a fairly standard-for-the-time reentry method: Hit the atmosphere at the right angle, deploy a parachute after things are done burning up, and land gently upon the ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the main problem I can see: That parachute &lt;em&gt;has&lt;/em&gt; to work. Otherwise, you have some fairly serious problems - such as water being like concrete if you hit it going very fast. And the Apollo modules where going &lt;em&gt;very&lt;/em&gt; fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did any of the Apollo missions include a backup parachute in case the main one didn't deploy properly?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-10-24T21:53:49.883" Title="Did any Apollo missions have backup parachutes?" Tags="|apollo-program|reentry|parachute|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2522" PostTypeId="1" AcceptedAnswerId="2528" CreationDate="2013-10-24T18:32:59.600" Score="7" ViewCount="371" Body="&lt;p&gt;NASA's Curiosity rover &lt;a href=&quot;http://mars.nasa.gov/msl/mission/rover/&quot;&gt;is a '&lt;em&gt;car-sized, six-wheeled robot&lt;/em&gt;'&lt;/a&gt; on the surface of Mars. The phrase &lt;em&gt;car-sized&lt;/em&gt; caught my eye - car... Hmm...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could future Mars explorers potentially find the rover and use it for utility purposes? By utility purposes I mean anything that would leverage the rover for transportation of people/supplies in a way that would be more efficient than just having people do it.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:56:50.960" LastActivityDate="2013-10-28T19:56:50.960" Title="Could Curiosity be used as a transportation device by future Mars explorers?" Tags="|mars|colonization|rovers|transport|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2523" PostTypeId="5" CreationDate="2013-10-24T18:43:16.033" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-24T18:43:16.033" LastActivityDate="2013-10-24T18:43:16.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2524" PostTypeId="4" CreationDate="2013-10-24T18:43:16.033" Score="0" Body="Questions regarding Mercury, the planet closest to the Sun. Use [project-mercury] for questions about the NASA missions." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-24T18:43:16.033" LastActivityDate="2013-10-24T18:43:16.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2525" PostTypeId="2" ParentId="2520" CreationDate="2013-10-24T19:45:56.907" Score="7" Body="&lt;p&gt;Typically GEO satellites use either mono-prop (often hydrazine) or bi-prop (often Mono Methyl Hydrazine and Nitrogen Tetroxide) storable propellant engines for station keeping.  Electric propulsion is a technology that is also used for GEO spacecraft station keeping (&lt;a href=&quot;http://spaceflightnow.com/news/n1203/19boeing702sp/&quot;&gt;http://spaceflightnow.com/news/n1203/19boeing702sp/&lt;/a&gt;).  &lt;/p&gt;&#xA;" OwnerUserId="943" LastActivityDate="2013-10-24T19:45:56.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2526" PostTypeId="2" ParentId="2521" CreationDate="2013-10-24T19:54:51.273" Score="13" Body="&lt;p&gt;The apollo command module had three parachutes and two of the three would have been sufficient to slow the capsule sufficiently for a survivable landing.  However, this redundancy would not have protected against certain failure scenarios where the parachutes failed to deploy properly or multiple chutes failed due to a common cause, such as collision with debris. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/lxuGm.jpg&quot; alt=&quot;Apollo 15 descends with two good parachutes into the Pacific Ocean on August 7, 1971&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Apollo 15 descends with two good parachutes into the Pacific Ocean on August 7, 1971 (source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15&quot; rel=&quot;noreferrer&quot;&gt;Wiki on Apollo 15&lt;/a&gt;, image: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional reference: &lt;a href=&quot;http://books.google.com/books?id=ZwD8uTpXet4C&amp;amp;pg=PA291&amp;amp;lpg=PA291&amp;amp;dq=apollo%20command%20module%20parachute%20redundancy&amp;amp;source=bl&amp;amp;ots=l_s4t0g_cs&amp;amp;sig=q5BqxWTq_BzJLXYLXWwlZoz1fPE&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=8nlpUoyaN4HW2AXkrID4DA&amp;amp;ved=0CEAQ6AEwAg#v=onepage&amp;amp;q=apollo%20command%20module%20parachute%20redundancy&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Chapter 8 - Collision Avoidance Systems, 8.2 - Descent and Landing Systems, &lt;strong&gt;Safety Design for Space Systems&lt;/strong&gt;&lt;/a&gt;, 2009 Butterworth-Heinemann, by Gary E. Musgrave Ph.D, Axel Larsen, Tommaso Sgobba&lt;/p&gt;&#xA;" OwnerUserId="943" LastEditorUserId="49" LastEditDate="2013-10-24T21:53:49.883" LastActivityDate="2013-10-24T21:53:49.883" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2527" PostTypeId="2" ParentId="2406" CreationDate="2013-10-24T20:00:49.860" Score="4" Body="&lt;p&gt;One of the advantages of the design of the International Space Station is that the Russian and US segments offer dissimilar, redundant systems.  In the event that there is a failure of the communication link between NASA mission control and the US segment, the ISS can be safely operated (and communicated with) via the Russian systems and mission control center.  &lt;/p&gt;&#xA;" OwnerUserId="943" LastActivityDate="2013-10-24T20:00:49.860" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2528" PostTypeId="2" ParentId="2522" CreationDate="2013-10-24T20:49:59.117" Score="8" Body="&lt;p&gt;Short Answer: &lt;strong&gt;No.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The maximum speed of curiosity is around 90 meters per hour with a 'cruising' speed of around 30 meters per hour. Average human walking speed is around 5000 meters per hour (that's 5km/h or around 3 miles per hour).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The various Mars-rovers simply aren't built for long-range travel, aiming more for a lifetime max of a few kilometers of travel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_Rover&quot;&gt;https://en.wikipedia.org/wiki/Curiosity_Rover&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, there might be some chance of salvaging parts from the probably long-dead rover(s). Precision-machined metal-parts might be hard to come by on Mars, but that brings up it's own set of problems. A permanent human colony on Mars is going to have it's own set of needs, and trying to set-up shop nearby any of the rovers is going to limit options far too much. It is probably not worth the effort to get the rover.&lt;/p&gt;&#xA;" OwnerUserId="692" LastActivityDate="2013-10-24T20:49:59.117" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2529" PostTypeId="1" AcceptedAnswerId="2566" CreationDate="2013-10-24T22:54:51.667" Score="10" ViewCount="2005" Body="&lt;p&gt;While I was answering another question on &lt;a href=&quot;https://space.stackexchange.com/a/2517/49&quot;&gt;How closely spaced are satellites at GEO&lt;/a&gt; I came to realize I can't really word it with much conviction how two close proximity satellites in GEO/GSO orbits approach each other and at what point would they theoretically come the closest. I will try to explain my current understanding:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When observing GEO/GSO orbited satellites from the surface of the Earth, they would appear to make a circle on the night sky whose radius is defined by the satellite's inclination to the equator. The circle would have its center on the equatorial plane at the longitude the satellite is stationed at, usually defined as an angle to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Prime_meridian&quot; rel=&quot;nofollow noreferrer&quot;&gt;prime meridian&lt;/a&gt; in the Eastern to Western direction (but could be differently). If this is more or less correct, then since these satellites are &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;geosynchronous&lt;/a&gt; to the rotation of the Earth on its own axis this &lt;em&gt;circle on the night sky&lt;/em&gt; (or as a projection for their &lt;a href=&quot;https://en.wikipedia.org/wiki/Ground_track&quot; rel=&quot;nofollow noreferrer&quot;&gt;ground track&lt;/a&gt; on the Earth's surface) they make in their orbit should translate to a &lt;a href=&quot;https://en.wikipedia.org/wiki/Wave_function&quot; rel=&quot;nofollow noreferrer&quot;&gt;wave function&lt;/a&gt; with the inclination being the wave's amplitude and frequency at the same time, if the Earth didn't rotate. I can draw both these orbits and distances of two satellites with similar longitude and inclination well enough from &lt;em&gt;the ground up&lt;/em&gt; or the &lt;em&gt;bird's-eye view&lt;/em&gt;, that's not a problem and so far so good, I hope. Correct me if I'm wrong though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But here's the thing; If two GEO/GSO satellites have inclined orbit that by definition has to intersect the equatorial plane split in the middle (I'm not thinking here of &lt;a href=&quot;http://www.sciencedaily.com/releases/2010/07/100726094749.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;levitated orbits&lt;/a&gt;, just the &lt;em&gt;vanilla&lt;/em&gt; GEO/GSO ones), if they were at the same altitude, wouldn't their orbits intersect too, preferably exactly on the equatorial plane so their orbits are stable? So all this got me thinking, which orbital elements (or a combination of them) exactly defines the minimum separation between two GEO/GSO orbital bodies? My intuition says it would have to be the longitudinal separation, since it defines their orbital position at any time, but would I be correct in assuming that, or should I correct my calculations in the mentioned answer?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, if there is some way to show this in pictures, I'd appreciate if you could include some in your answer. Hand drawings will do just fine, if you can't find any online that were already prepared. In short though, I would like you to describe the orbital path two close proximity GEO/GSO satellites make in relation to one another, and not from the frame of reference of the body they orbit (in our case the Earth's surface) and at which point would these two satellites come the closest to each other?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculations would be appreciated, and if you need an example to work with, then you can use orbital elements of &lt;a href=&quot;https://en.wikipedia.org/wiki/Astra_2C&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astra 2C&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Astra_1KR&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astra 1KR&lt;/a&gt; that were both slotted at longitude of 19.2° East during 2006-2007 and 2010-2013:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Name:      Common name:     Orbit:         Inclination:&#xA;-------------------------------------------------------&#xA;01025A     ASTRA 2C         19.2174576°    0.078°&#xA;06012A     ASTRA 1KR        19.2189152°    0.083°&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;My simple maths tell me they were theoretically separated by mere 1.1 km at their closest approach, and I based that assumption on their longitudinal separation of 0.0014576° (5.25&quot;) and took their altitude from the GSO semi-major axis at exact altitude of GEO, 42,164 km (26,199 mi) from the center of the Earth.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2013-10-27T22:12:21.300" Title="At which point would two GSO/GEO satellites with similar orbital elements be closest to each other?" Tags="|orbital-mechanics|geostationary|geosynchronous|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2530" PostTypeId="2" ParentId="2406" CreationDate="2013-10-25T01:04:59.287" Score="7" Body="&lt;p&gt;I did some searching, and I'm going to say that there does not exist a specific procedure for continued operations during some extraordinarily long communications outage, and here's why.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, communications possibilities with the ISS are varied and robust. There are &lt;a href=&quot;https://www.spacecomm.nasa.gov/spacecomm/programs/tdrss/system_description.cfm&quot; rel=&quot;noreferrer&quot;&gt;nine TDRSS&lt;/a&gt; satellites on orbit. There are a large number of &lt;a href=&quot;http://en.wikipedia.org/wiki/Near_Earth_Network&quot; rel=&quot;noreferrer&quot;&gt;NASA ground stations&lt;/a&gt;, which are geographically varied (i.e. one big hurricane won't take them all out). As user943 pointed out, there are redundant and dissimilar systems provided by the Russian segment that provides further robustness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most likely scenario, I think, would be a problem on-board, such as &lt;a href=&quot;http://www.usatoday.com/story/tech/2013/02/19/nasa-regains-space-station-contact-after-outage/1930787/&quot; rel=&quot;noreferrer&quot;&gt;what happened earlier this year&lt;/a&gt;. The ISS suffered a 3-hour communications loss due to a software problem - they couldn't point their antennas correctly to talk to TDRSS. As the article notes, however, they were able to contact Mission Control during a pass over Russian ground stations. They went on to essentially reboot the necessary systems, and restored comms. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretically, the ISS could operate in some &quot;emergency&quot; mode indefinitely while just maintaining comms via direct passes over ground stations while the problems with TDRSS were sorted out. Low-rate antennas typically don't require pointing, and while ground passes are short, the communications could be restricted to absolutely necessary commands, etc. (including planned debris avoidance maneuvers, although I think it might be considered too high-risk to perform one of these maneuvers during a comms blackout).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the unlikely event that all communications were lost, the astronauts would just follow their normal procedures to regain communications, just like they did during the 3 hour outage. Basically, communications are so vital to operations, there would not be anything &lt;em&gt;to&lt;/em&gt; do besides trying to regain them.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorDisplayName="user29" LastEditDate="2013-10-25T01:15:06.140" LastActivityDate="2013-10-25T01:15:06.140" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2531" PostTypeId="2" ParentId="2143" CreationDate="2013-10-25T08:38:13.263" Score="10" Body="&lt;p&gt;In the ISS, you can find mould &lt;a href=&quot;http://www.our-space.org/materials/states-of-matter/metro-mould-in-space&quot;&gt;basically everywhere&lt;/a&gt;. Mould is quite a problem in space. Tiny bits of food that get away or are exhaled, plus all of the biomatter that humans lose (skin cells, etc) spread everywhere (thanks to zero gravity) and can go mouldy over time. There are lots of places in the ISS that are difficult to clean (e.g. the space inside and behind the equipment racks), and where mould will accumulate. Mir also had &lt;a href=&quot;http://www.rense.com/general8/mir.htm&quot;&gt;severe mould problems&lt;/a&gt;. (&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2007/11may_locad3/&quot;&gt;more info on Mir&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-10-25T08:38:13.263" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2532" PostTypeId="1" AcceptedAnswerId="2606" CreationDate="2013-10-25T09:38:49.740" Score="20" ViewCount="1720" Body="&lt;p&gt;In this image of the Ariane 4, you can see a blue shroud around the second stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/av1m3.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least I think it's a shroud. It is present in many photos of Ariane 4 launches, but the drawings I've seen omit it. What is the purpose of this object? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:03:42.340" LastActivityDate="2013-11-08T17:02:55.500" Title="Ariane 4: shroud around stage?" Tags="|launch|ariane|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2533" PostTypeId="5" CreationDate="2013-10-25T16:24:07.077" Score="0" Body="&lt;p&gt;The Soyuz spacecraft has been the workhorse of the Soviet and then Russian space program since the 1960's.  The launch vehicle is also known as Soyuz. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current Soyuz models can carry three astronauts to low earth orbit, and the ISS.  &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="51" LastEditDate="2014-01-07T21:52:43.220" LastActivityDate="2014-01-07T21:52:43.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2534" PostTypeId="4" CreationDate="2013-10-25T16:24:07.077" Score="0" Body="Questions regarding the Soyuz manned spacecraft used by the Soviet and Russian space program." OwnerDisplayName="user12" LastEditorUserId="4606" LastEditDate="2014-06-19T00:45:18.593" LastActivityDate="2014-06-19T00:45:18.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2535" PostTypeId="1" AcceptedAnswerId="2536" CreationDate="2013-10-25T18:40:05.850" Score="16" ViewCount="3185" Body="&lt;p&gt;Earlier today a Proton-M rocket took off, carrying a Sirius FM6 satellite radio payload destined for geosynchronous orbit. Nothing super exciting, Briz-M upper stage for transfer burns and then the payload itself has some small engines on it for final geosync insertion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The weird bit is the orbital inclination. Check this out:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/aQ6VJ.jpg&quot; alt=&quot;Sirius FM6 orbital track&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It launches on a 51º inclination, then does a plane change to 0º once it hits geosynchronous altitude! Plane changes aren't cheap; I would have expected it to be much more efficient to just launch as close to 0º as possible and make whatever minor corrections are needed to get into geosync. At first I thought it might be because it was dropping off other satellites enroute, but the &lt;a href=&quot;http://spaceflightnow.com/proton/siriusfm6/launchtimeline.html&quot; rel=&quot;noreferrer&quot;&gt;mission timeline&lt;/a&gt; doesn't list anything like that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why would they use such a high-inclination orbit?&lt;/p&gt;&#xA;" OwnerUserId="890" LastActivityDate="2013-10-25T21:29:48.427" Title="Why did the recent Sirius FM6 launch use such a high inclination?" Tags="|launch|orbit|orbital-maneuver|geosynchronous|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2536" PostTypeId="2" ParentId="2535" CreationDate="2013-10-25T19:07:01.307" Score="19" Body="&lt;p&gt;Launches from &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikonur_Cosmodrome&quot; rel=&quot;noreferrer&quot;&gt;Baikonur Cosmodrome&lt;/a&gt; are tilted to a higher inclination than the site's azimuth to the equator (roughly 46° North depending on the individual launchpad used) to avoid the early launch ground track going over the territory of China in case of an aborted or failed launch, so any booster rockets don't fall on the Chinese territory and can be  retrieved easier, and to simply avoid violating the China's airspace or requiring permission for an overflight even with successful launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To prevent any of such complications, the inclination is elevated to 51.65° and the ground track the launcher makes in the ascent phase doesn't go over that territory at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/eEWBl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Rockets coming from the Baikonur Cosmodrome, Kazakhstan are sent into orbits inclined 51.65 degrees to avoid overflying China.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is also one of the reasons for the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;International Space Station&lt;/a&gt;'s (ISS) orbital inclination at 51.65 degrees, since Baikonur Cosmodrome is a site for most of the ISS related launches, at least in recent years. Quoting &lt;a href=&quot;http://astrobob.areavoices.com/tag/inclination/&quot; rel=&quot;noreferrer&quot;&gt;one of Astro Bob's blog posts&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The U.S. worked with Russia to pick the best orbital tilt. Since&#xA;  Russia launches Progress and Soyuz from Baikonur (latitude 46 degrees&#xA;  N), a high inclination orbit made economic sense. It also lets&#xA;  astronauts study more of the Earth’s surface compared to an orbit&#xA;  closer to the equator. 75 percent of the planet and 95 percent of its&#xA;  inhabited lands are open to view.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-25T21:29:48.427" LastActivityDate="2013-10-25T21:29:48.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2537" PostTypeId="1" CreationDate="2013-10-25T20:21:15.773" Score="12" ViewCount="670" Body="&lt;p&gt;Just reading about manned space-craft. Space travel on record is listed briefly on Wikipedia. The section on &lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Manned_Space_Programs#Successful_programs&quot; rel=&quot;nofollow noreferrer&quot;&gt;successful manned missions&lt;/a&gt;&lt;/strong&gt; is of particular interest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Successful &lt;em&gt;manned&lt;/em&gt; programs&lt;/strong&gt; have, for the most part, carried &lt;strong&gt;upto/exactly 3 crew&lt;/strong&gt;. For instance &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Tier One - 3&lt;/li&gt;&#xA;&lt;li&gt;Shenzhou 10, 9, 7 - 3&lt;/li&gt;&#xA;&lt;li&gt;TKS         - 3&lt;/li&gt;&#xA;&lt;li&gt;Soyuz-T     - 3&lt;/li&gt;&#xA;&lt;li&gt;Soyuz-TM    - 3&lt;/li&gt;&#xA;&lt;li&gt;Soyuz-TMA   - 3&lt;/li&gt;&#xA;&lt;li&gt;Soyuz-TMA-M - 3&lt;/li&gt;&#xA;&lt;li&gt;First Gen Soyuz - Upto 3&lt;/li&gt;&#xA;&lt;li&gt;Apollo/Moon - 3&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The following programs being the &lt;strong&gt;exception&lt;/strong&gt; to that rule of 3. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Vostok&lt;/li&gt;&#xA;&lt;li&gt;Mercury&lt;/li&gt;&#xA;&lt;li&gt;Gemini&lt;/li&gt;&#xA;&lt;li&gt;Voskhod&lt;/li&gt;&#xA;&lt;li&gt;Space Shuttle&lt;/li&gt;&#xA;&lt;li&gt;Early Shenzhou craft&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The impression one receives is that preliminary craft carried none, one, or two astronauts. More mature missions/technologies carried 3. The space-shuttle is notable in carrying a still larger crew/mission-specialists; as is the proposed ACTS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is/was the rationale behind a crew of 3 astronauts? How did the systems evolve to reach this figure of a crew of 3?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="15906" LastEditDate="2019-07-24T13:54:09.270" LastActivityDate="2019-07-24T13:54:09.270" Title="Why have so many space missions had a crew of 3?" Tags="|crewed-spaceflight|crew-size|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="2538" PostTypeId="2" ParentId="2537" CreationDate="2013-10-25T21:27:41.640" Score="16" Body="&lt;p&gt;From a mathematical perspective rather than historical, I would expect that it stems from the natural constraints of space travel. By far, the biggest constraint is weight. Every pound is expensive, and human flesh and its sustenance is really heavy stuff. So astronauts are expected to keep trim, but also, you want to minimize the size of the crew. This is why unmanned missions are such a bargain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But why three? Why not one or two more often? I suspect it boils down to psychology mostly. A single crewman in a tin can in the middle of space - well, I don't want to do it. I suspect it carries a high risk of madness of one kind or another, and with something as expensive as human space flight, we don't want to put all our eggs in one basket-case. And two - well, there's a word for when two people are placed in a highly stressful situation together. It's called a fight. I'm exaggerating, but I think it is true that when two people are completely alone under stress for a long time, the potential for conflict is excessive. When you throw that third person in, you have an objective witness, a mediator, a tie-breaker... I think a lot of things go a lot better with three or more. And four doesn't make enough difference to justify the weight.&lt;/p&gt;&#xA;" OwnerUserId="945" LastEditorUserId="945" LastEditDate="2013-11-18T19:43:00.293" LastActivityDate="2013-11-18T19:43:00.293" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2539" PostTypeId="1" CreationDate="2013-10-26T06:19:53.280" Score="24" ViewCount="27480" Body="&lt;p&gt;The International Space Station being an artificial habitat for humans; it seems reasonable to assume the air temperature, and humidity levels are maintained at well-defined levels. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What are the air temperature and humidity inside the International space station? &lt;/li&gt;&#xA;&lt;li&gt;Have those levels been adjusted over the years? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A link to a page or document on some official website with this info will be appreciated.&lt;/p&gt;&#xA;" OwnerUserId="947" LastEditorUserId="49" LastEditDate="2015-09-20T21:13:22.987" LastActivityDate="2018-12-31T00:22:26.727" Title="Air temperature and humidity inside the ISS" Tags="|iss|life-support|environment|temperature|eclss|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2540" PostTypeId="2" ParentId="2539" CreationDate="2013-10-26T07:20:43.310" Score="28" Body="&lt;p&gt;Temperature and Humidity Control (THC) system is a part of the International Space Station's (ISS) &lt;a href=&quot;https://en.wikipedia.org/wiki/ISS_ECLSS&quot; rel=&quot;noreferrer&quot;&gt;Environmental Control and Life Support System&lt;/a&gt; (ECLSS). &lt;a href=&quot;http://spacestationlive.nasa.gov/coresys/index.html#thermal_control&quot; rel=&quot;noreferrer&quot;&gt;Thermal Control System&lt;/a&gt; (TCS) is a component part of the THC system and subdivides into the Active Thermal Control System (ATCS, &lt;a href=&quot;http://www.nasa.gov/pdf/473486main_iss_atcs_overview.pdf&quot; rel=&quot;noreferrer&quot;&gt;PDF document&lt;/a&gt;) and Passive Thermal Control System (PTCS). From &lt;a href=&quot;http://www.nasa.gov/centers/marshall/pdf/104840main_eclss.pdf&quot; rel=&quot;noreferrer&quot;&gt;a PDF document on NASA facts&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Temperature and Humidity and Control subsystem helps maintain a&#xA;  habitable environment in the Station by removing heat and humidity,&#xA;  and circulating the cool dry air. Circulation of the atmosphere&#xA;  minimizes the temperature variations, ensures a well-mixed, breathable&#xA;  atmosphere and supports smoke detection;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/vuIRo.gif&quot; alt=&quot;International Space Station&amp;#39;s Environmental Control and Life Support System (ECLSS)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Regenerative Environmental Control and Life Support System (ECLSS) Diagram&lt;/strong&gt; (Source: &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2000/ast13nov_1/&quot; rel=&quot;noreferrer&quot;&gt;NASA Science News&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ECLSS is however a complex system and many of its parts failed on multiple occasions, for example the Oxygen Generation System (OGS) triggered several alarms, both &lt;a href=&quot;http://www.wired.com/science/space/news/2004/09/65026&quot; rel=&quot;noreferrer&quot;&gt;the old one&lt;/a&gt; and &lt;a href=&quot;http://www.spaceflightnow.com/station/exp13/060918elektron.html&quot; rel=&quot;noreferrer&quot;&gt;its replacement&lt;/a&gt; that was &lt;a href=&quot;http://www.nasaspaceflight.com/2007/07/oxygen-generating-system-activated-onboard-iss/&quot; rel=&quot;noreferrer&quot;&gt;activated in 2007&lt;/a&gt;. Now, this wasn't so critical because of redundancy in life support systems onboard the ISS, and e.g. the &lt;a href=&quot;http://suzymchale.com/ruspace/issrslss.html&quot; rel=&quot;noreferrer&quot;&gt;Russian segment has a Life Support Systems&lt;/a&gt; (LSS) too. So this should hopefully answer your question if the air temperature and humidity levels have ever been adjusted. With replacements and parts of individual systems failing these levels would obviously vary substantially at least on some segments of the ISS. And with many isolated systems, they would also vary segment to segment, module to module. Leaving the description of the complexity of all these systems to another &lt;a href=&quot;http://wsn.spaceflight.esa.int/docs/Factsheets/30%20ECLSS%20LR.pdf&quot; rel=&quot;noreferrer&quot;&gt;document on ECLSS from the European Space Agency (ESA) from 2010&lt;/a&gt; (also PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Several Life Support systems are accommodated at the ISS to safely&#xA;  sustain its crew in conditions similar to those on the Earth.  They&#xA;  are distributed amongst the ISS modules. Specific differences exist&#xA;  between the equipment located in the US and Russian  Segment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ScGws.png&quot; alt=&quot;ECLS Hardware&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;On-Orbit ISS ECLS Hardware Distribution as of February 2010&lt;/strong&gt; (Source: &lt;a href=&quot;http://wsn.spaceflight.esa.int/docs/Factsheets/30%20ECLSS%20LR.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA/ISEC&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the document quotes these values:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Cabin pressure nominal range: T = 14.2 to 14.9 psi&lt;/li&gt;&#xA;  &lt;li&gt;Oxygen and nitrogen distribution:   P = 93 - 120 psia, T = 60 - 113°F from Node 1&lt;/li&gt;&#xA;  &lt;li&gt;Oxygen Recharge:  P = &amp;lt; 1050 psia (max 16 lb/h), T = 25 - 113 °F from PMA3&lt;/li&gt;&#xA;  &lt;li&gt;Nitrogen recharge:  P = &amp;lt; 3400 psia (max 3 lb/h), T = 25 - 113 °F from PMA3&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Cabin Temperature nominal range:  T = 65 to 80˚F (18.3 to 26.7°C)&lt;/strong&gt;&lt;/li&gt;&#xA;  &lt;li&gt;Cabin air velocity: 10 to 40 feet per minute&lt;/li&gt;&#xA;  &lt;li&gt;Atmospheric heat removal and humidity control:  CCAA CHX&lt;/li&gt;&#xA;  &lt;li&gt;Airborne particulate and microbes removal and disposal:  CCAA Filters&lt;/li&gt;&#xA;  &lt;li&gt;Waste water collection and distribution&lt;/li&gt;&#xA;  &lt;li&gt;Fuel cell water and potable water distribution&lt;/li&gt;&#xA;  &lt;li&gt;Pre-treated urine:  1 line from W&amp;amp;HC to WRS2, P &amp;lt; 5 psig, T = 65 - 105 °F&lt;/li&gt;&#xA;  &lt;li&gt;Process Water: 4 lines from WRS1 to WRS2&lt;/li&gt;&#xA;  &lt;li&gt;Cabin smoke detection: 2 Area smoke detectors&lt;/li&gt;&#xA;  &lt;li&gt;External venting lines: CO2 /CH4 , H2 , MCA, Cabin air&lt;/li&gt;&#xA;  &lt;li&gt;Air sampling: from Zen, Port, Stbd, Fwd ports to ARS rack  (flow rate 100 - 400 scc/min)&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Finding actual nominal performance quotes is a bit more tricky though with possibly many standalone systems active at the same time in different segments of the ISS. For example for the Russian segment, these are the quoted values:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Basic values of parameters of the atmospheric composition of the Russian segment, taking into account the gas exchange between the human body and the environment, fall within the following limits:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;total (absolute) pressure in habitable pressurized compartments – 660-860 mm Hg;&lt;/li&gt;&#xA;  &lt;li&gt;ppO2 of inhaled air – 150-200 mmHg&lt;/li&gt;&#xA;  &lt;li&gt;ppCO2 – 6 mmHg, max.;&lt;/li&gt;&#xA;  &lt;li&gt;ppH2O – 10 ± 5 mmHg;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;air temperature – 20-25°C (68-77°F);&lt;/strong&gt;&lt;/li&gt;&#xA;  &lt;li&gt;air circulation rate – 0.1-0.4 m/sec (0.33-1.31 ft/sec);&lt;/li&gt;&#xA;  &lt;li&gt;noise level – 60 dB, max.;&lt;/li&gt;&#xA;  &lt;li&gt;contaminants – within maximum permissible concentrations as determined by the ГОСТ, the Russian contaminants standard&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The humidity control is explained in the &lt;a href=&quot;http://suzymchale.com/ruspace/issrslss.html#ch4&quot; rel=&quot;noreferrer&quot;&gt;4.2. Atmospheric Condensate Water Regeneration System section&lt;/a&gt; of that document and is a bit too complex to describe here, so I'll just recommend reading it. The document also quotes maximum and minimum values that trigger certain procedures, or alerts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section%20III.1.2%20Cabin%20Environment%20and%20EVA%20Environment.doc&quot; rel=&quot;noreferrer&quot;&gt;another document by Kira Bacal, MD, PhD, MPH&lt;/a&gt; (direct Word file download) though that explains it in a bit more general sense (but is still 22 pages long):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The cabin environment is usually maintained at &lt;strong&gt;about 60% relative&#xA;  humidity&lt;/strong&gt; (corresponding to approximately 0.2 psi of water vapor&#xA;  pressure).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'll let others chip in with their findings on more exact values for other segment ECLS systems and their nominal performance. I wouldn't expect all of them to have the same requirements though, this is certainly not the case for scientific modules that would have stricter environmental control parameters, like for example is described in the sadly paywalled document on &lt;a href=&quot;http://papers.sae.org/2005-01-2801/&quot; rel=&quot;noreferrer&quot;&gt;Centrifuge Accommodation Module (CAM) Cabin Air Temperature and Humidity Control Analysis&lt;/a&gt; (the &lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module&quot; rel=&quot;noreferrer&quot;&gt;module never made it to the ISS&lt;/a&gt; tho), so you'd be looking at a long list of per module, per sensor values.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding all the readings and changes in environmental temperature and humidity, intentional or accidental to the list would possibly stretch the limits of this Q&amp;amp;A, though. Just to give you a fair impression, these are some of the operational values for Node 1 Heater sensors alone, taken from &lt;a href=&quot;http://www.spaceref.com/iss/ops/iss.TCS.Final.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Space Station Thermal Control Group (TCG) Thermal Control System documentation&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Heater       Availability   Failure   Upper      Lower      Failure   Cyclic&#xA;(Sensor)                    Upper     Setpoint   Setpoint   Lower     Load&#xA;                            Limit                           Limit     Delta&#xA;-----------------------------------------------------------------------------&#xA;1A (Snsr 1)  Enable BU      35.6      29.7       26.1       20.6      1.7&#xA;1A (Snsr 2)                 35.6      29.7       26.1       20.6      1.7&#xA;1B (Snsr 1)  Enable Opr     35.6      32.7       26.1       20.6      1.7&#xA;1B (Snsr 2)                 35.6      32.7       26.1       20.6      1.7&#xA;2A           Enable BU      35.6      26.1       23.3       17.8      1.7&#xA;2B           Enable Opr     35.6      26.1       23.3       17.8      1.7&#xA;3A (Snsr 1)  Enable BU      35.6      27.5       22.7       17.2      1.7&#xA;3A (Snsr 2)                 35.6      27.5       22.7       17.2      1.7&#xA;3B (Snsr 1)  Enable Opr     35.6      25.1       22.7       17.2      1.7&#xA;3B (Snsr 2)                 35.6      25.1       22.7       17.2      1.7&#xA;4A           Enable BU      35.6      25.5       22.7       17.2      1.7&#xA;4B           Enable Opr     35.6      25.5       22.7       17.2      1.7&#xA;5A (Snsr 1)  Enable BU      35.6      26.2       23.8       18.3      1.7&#xA;5A (Snsr 2)                 35.6      26.2       23.8       18.3      1.7&#xA;5B (Snsr 1)  Enable Opr     35.6      26.2       23.8       18.3      1.7&#xA;5B (Snsr 2)                 35.6      26.2       23.8       18.3      1.7&#xA;6A (Snsr 1)  Enable BU      35.6      26.8       24.4       18.9      1.7&#xA;6A (Snsr 2)                 35.6      26.8       24.4       18.9      1.7&#xA;6B (Snsr 1)  Enable Opr     35.6      26.8       24.4       18.9      1.7&#xA;6B (Snsr 2)                 35.6      26.8       24.4       18.9      1.7&#xA;7A (Snsr 1) Enable BU       35.6      28.5       22.7       17.2      1.7&#xA;7A (Snsr 2)                 35.6      28.5       22.7       17.2      1.7&#xA;7B (Snsr 1) Enable Opr      35.6      26.3       22.7       17.2      1.7&#xA;7B (Snsr 2)                 35.6      26.3       22.7       17.2      1.7&#xA;8A          Enable BU       35.6      24.9       22.2       16.7      1.7&#xA;8B          Enable Opr      35.6      24.9       22.2       16.7      1.7&#xA;9A          Enable BU       35.6      25.5       22.7       17.2      1.7&#xA;9B          Enable Opr      35.6      25.5       22.7       17.2      1.7&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;All Temperatures listed are in °C. To my knowledge, there is no archived or live telemetry data stream of all the sensor readings from the ISS available to public.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt; - It's ... complicated. ;)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: Some of the ETHOS (Environmental and Thermal Operating Systems) Life Support, Thermal System and Regenerative Life Support data on &lt;a href=&quot;http://spacestationlive.nasa.gov/displays/ethosDisplay1.html&quot; rel=&quot;noreferrer&quot;&gt;The Space Station Live&lt;/a&gt; NASA's web page is finally available, so it should help you get an overview. It should be mentioned tho, that it doesn't include any telemetry data on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Russian_Orbital_Segment&quot; rel=&quot;noreferrer&quot;&gt;Russian segment&lt;/a&gt; of the ISS, and excludes large number of individual sensor readings, presenting per module current average values (with not all modules listed).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an animation of all three pages (&lt;a href=&quot;http://spacestationlive.nasa.gov/displays/ethosDisplay1.html&quot; rel=&quot;noreferrer&quot;&gt;Life Support&lt;/a&gt;, &lt;a href=&quot;http://spacestationlive.nasa.gov/displays/ethosDisplay2.html&quot; rel=&quot;noreferrer&quot;&gt;Thermal System&lt;/a&gt; and &lt;a href=&quot;http://spacestationlive.nasa.gov/displays/ethosDisplay3.html&quot; rel=&quot;noreferrer&quot;&gt;Regenerative Life Support&lt;/a&gt;) of data available to public, valid for Nov 7, 2013 at 4:30 pm UTC:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/2rUJ7.gif&quot; alt=&quot;ETHOS data display from The Space Station Live&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="10273" LastEditDate="2018-12-31T00:22:26.727" LastActivityDate="2018-12-31T00:22:26.727" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="2541" PostTypeId="1" CreationDate="2013-10-26T13:27:46.017" Score="3" ViewCount="1024" Body="&lt;p&gt;I often wonder about fixing the carbon the atmosphere of Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whilst there is no water meaning this is unlikely to be possible using biological means, is it possible that some kind of machine could do this?.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer to &lt;a href=&quot;https://space.stackexchange.com/questions/1334/can-venus-be-made-habitable&quot;&gt;this&lt;/a&gt; question doesn't rule out carbon fixing without water, but does suggest it is beyond our current capability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose that fixing is possible by some kind of floating machine in the upper atmosphere, energy should not be a problem, sunlight and wind should be plentiful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further suppose that the machine fixes the carbon into hollow shapes, essentially carbon balloons.  With enough machines operating over a long enough time period, probably a geological era even with self replicating machines, what would be the effect on the Venusian atmosphere?.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I like to imagine Venus being blanketed with these carbon balloons, they could even be made in shapes that naturally interlock to create a kind of surface high above the real surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the effect of such a blanket 20cm thick?, I often speculate that the temperature underneath the blanket would plummet, perhaps causing the CO2 to liquify, further reducing the atmospheric pressure at the (real) surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the atmospheric pressure and temperatures reduced, would we see renewed volcanic activity?, would Venus expand noticeably?.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would such a scenario be likely to end?, would the carbon blanket be weathered away or turned back into CO2 some other way?.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting aside the scale of such a venture, how feasible is any of this?.&lt;/p&gt;&#xA;" OwnerUserId="948" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2018-12-31T15:04:06.023" Title="Fixing Carbon in Venus's Atmosphere" Tags="|venus|terraforming|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2542" PostTypeId="2" ParentId="2537" CreationDate="2013-10-26T14:00:18.750" Score="12" Body="&lt;p&gt;Mercury was essentially an X-vehicle.  The payload of an X-vehicle is a test pilot with a radio, and not much more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gemini was a two-man bird.  One man flew the Gemini spacecraft, the other guy controlled the Agena docking target.  There was other work that had to be done as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo was a three-man bird in part because of the rendezvous requirement, which dictated that you needed pilots in both vehicles, and because NASA wanted two guys in the Lunar Module.&lt;/p&gt;&#xA;" OwnerUserId="949" LastActivityDate="2013-10-26T14:00:18.750" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2543" PostTypeId="5" CreationDate="2013-10-26T15:06:16.413" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:06:16.413" LastActivityDate="2013-10-26T15:06:16.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2544" PostTypeId="4" CreationDate="2013-10-26T15:06:16.413" Score="0" Body="Questions regarding the building or style of building used to create a spacecraft." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:06:16.413" LastActivityDate="2013-10-26T15:06:16.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2545" PostTypeId="5" CreationDate="2013-10-26T15:07:36.107" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:07:36.107" LastActivityDate="2013-10-26T15:07:36.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2546" PostTypeId="4" CreationDate="2013-10-26T15:07:36.107" Score="0" Body="Questions regarding underground chambers, usually naturally occurring." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:07:36.107" LastActivityDate="2013-10-26T15:07:36.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2547" PostTypeId="5" CreationDate="2013-10-26T15:10:59.717" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:10:59.717" LastActivityDate="2013-10-26T15:10:59.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2548" PostTypeId="4" CreationDate="2013-10-26T15:10:59.717" Score="0" Body="Questions regarding the amount of money required to perform or sustain space exploration." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:10:59.717" LastActivityDate="2013-10-26T15:10:59.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2549" PostTypeId="5" CreationDate="2013-10-26T15:12:26.440" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:12:26.440" LastActivityDate="2013-10-26T15:12:26.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2550" PostTypeId="4" CreationDate="2013-10-26T15:12:26.440" Score="0" Body="Questions regarding visual impressions of objects or instruments used to capture visual impressions." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T15:12:26.440" LastActivityDate="2013-10-26T15:12:26.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2551" PostTypeId="5" CreationDate="2013-10-26T17:53:33.100" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T17:53:33.100" LastActivityDate="2013-10-26T17:53:33.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2552" PostTypeId="4" CreationDate="2013-10-26T17:53:33.100" Score="0" Body="Questions regarding an object or spacecraft rotating around an axis." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-26T17:53:33.100" LastActivityDate="2013-10-26T17:53:33.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2553" PostTypeId="1" AcceptedAnswerId="2554" CreationDate="2013-10-26T21:58:19.637" Score="26" ViewCount="5421" Body="&lt;p&gt;I was watching the movie &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13_%28film%29&quot;&gt;Apollo 13&lt;/a&gt;, and one thing struck me as quite odd. According to Wikipedia,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Movie director Ron] Howard went to great lengths to create a technically accurate movie, employing NASA's technical assistance in astronaut and flight controller training for his cast, and even obtaining permission to film scenes aboard a reduced gravity aircraft for realistic depiction of the &quot;weightlessness&quot; experienced by the astronauts in space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, around 63 minutes into the movie, during the command module shutdown scene, the movie shows the command module's guidance computer being shut down followed by mission control stating &quot;computer off&quot; and specifically confirming shutdown of the CM computer with indications on one of the big screens in the background that would appear to state that the telemetry values are out of range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I find weird is that even after the DSKY digital display is turned off and during the &quot;Odyssey signing off&quot; scene immediately following it, the CM computer &lt;a href=&quot;https://en.wikipedia.org/wiki/DSKY#DSKY_interface&quot;&gt;DSKY&lt;/a&gt; shows the &lt;code&gt;STBY&lt;/code&gt; light seemingly &lt;em&gt;turned on.&lt;/em&gt; According to Wikipedia, &lt;a href=&quot;https://en.wikipedia.org/wiki/DSKY#Standby_mode&quot;&gt;standby mode&lt;/a&gt; was originally designed for use in mid-flight and to reduce the CM computer's power consumption from 70 W to 60-65 W, though &quot;in practice ... this feature was never used&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously some things were changed for dramatic effect in the movie (for example, the apparent proximity in time between the cryo tank fans being turned on and the explosion, which in the movie is depicted as being on the order of a few seconds whereas in reality it took considerably longer), but this seems like a really silly mistake to make if one sets out to make a technically accurate movie.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Was the CM guidance computer completely shut down following the accident on Apollo 13, or was it simply set to standby mode?&lt;/li&gt;&#xA;&lt;li&gt;If it was set to standby mode and the computer was designed to allow standby mode for long stretches of mid-course flight, why was this seemingly so much of a problem if (presumably) when needed the computer could relatively easily be powered back up for only a marginal increase in power drain for the short period of time it would be needed?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bonus question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Even considering NASA's desire to make it possible for the crew to operate the spacecraft (relatively) autonomously, was the Apollo CM even designed so that it was possible for the astronauts to fully power off such a critical system as the main guidance computer in the middle of the flight?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="415" LastEditorUserId="49" LastEditDate="2013-10-27T10:21:11.673" LastActivityDate="2017-03-01T18:39:27.850" Title="Was the Apollo 13 CM guidance computer fully shut down?" Tags="|crewed-spaceflight|apollo-program|flight-computer|myth-busting|apollo-13|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2554" PostTypeId="2" ParentId="2553" CreationDate="2013-10-27T02:36:43.387" Score="33" Body="&lt;p&gt;If I get you correctly, this should be the moment from the movie you're talking about:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Q6Gqg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is the corresponding part of the &lt;a href=&quot;https://sfy.ru/?script=apollo13&quot; rel=&quot;noreferrer&quot;&gt;movie script&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;JACK SWIGERT&#xA;- Now... Do we know for sure that we can power this thing&#xA;back up?... It's going to get awfully cold in here.&#xA;&#xA;ANDY (CAPCOM - WHITE)&#xA;- Copy that, Jack. We'll just have to deal with that later.&#xA;&#xA;INCO - WHITE&#xA;- Computer off.&#xA;&#xA;TELMU - WHITE&#xA;- We're clear.&#xA;&#xA;CONTROL - WHITE&#xA;- We're go on the LM.&#xA;&#xA;ANDY (CAPCOM - WHITE)&#xA;- We confirm shutdown, Jack. Lunar Module now in control.&#xA;&#xA;JACK SWIGERT&#xA;- Roger that, Houston. This is Odyssey. Signing off.&#xA;&#xA;JIM LOVELL&#xA;- Freddo, we're gonna have to execute some sort of a burn&#xA;here, it's just a matter of when.&#xA;&#xA;JIM LOVELL (to SWIGERT)&#xA;- Did they shut us all down in there?&#xA;&#xA;JACK SWIGERT&#xA;- Yeah.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;To not bore you with too many details, yes, the DSKY warning lights stayed on after the power down of most of the Command Module systems and before moving to the Lunar Module. Here's the part from the &lt;a href=&quot;https://www.jsc.nasa.gov/history/mission_trans/apollo13.htm&quot; rel=&quot;noreferrer&quot;&gt;Apollo 13 Technical Air-to-Ground Voice Transcription&lt;/a&gt; that should clear all doubts:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Mission Time:&lt;/strong&gt; 02d 17h 39m 52s&lt;br /&gt;&lt;br /&gt;&lt;strong&gt;Joe Kerwin (CAPCOM)&lt;/strong&gt;: &lt;em&gt;Okay, Jim, since we're in LOW BIT RATE now, we cannot monitor the DSKY for&#xA;  program alarms, et cetera, and we recommend that, in order for you to&#xA;  do so on board, you push in the following circuit breakers: On panel&#xA;  11 and panel 16 the ANNUNCIATOR / DOCK / COMPONENT circuit breakers. That&#xA;  will allow you to monitor your DSKY warning lights. Over.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same transcript is also available in a bit more convenient (searchable) format on &lt;a href=&quot;https://apollo13.spacelog.org/02:17:39:52/#log-line-236392&quot; rel=&quot;noreferrer&quot;&gt;spacelog.org&lt;/a&gt;. And this is how the real Apollo 13 DSKY looked like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/SAzl3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;DSKY panel on the Main Display Console of Odyssey, the surviving Command Module of Apollo 13&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;(Source: Bruce Yarbro, &lt;a href=&quot;http://www.papertrainer.com/&quot; rel=&quot;noreferrer&quot;&gt;Papertrainer.com&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if you checked the Apollo 13 mission transcript, you probably noticed that the movie script actually has little to do with the real mission's transcript, but I guess that's OK since the procedures involved with shutting down CM systems were really nothing like flicking a &lt;em&gt;standby allowed&lt;/em&gt; switch and setting &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Guidance_Computer&quot; rel=&quot;noreferrer&quot;&gt;AGC&lt;/a&gt; in a power-saving mode, like the movie depicts. This was in no way a &lt;em&gt;normal&lt;/em&gt;  flight computer to a standby mode procedure. It was a shutdown, as much as possible and involved a lot of on site reprograming, turning off circuit breakers and following exact sequence order so the computer could be later turned back on and still consume as little of power as possible while the crew occupied the LM on their way back to the Earth. The mission transcript attests to this with dozens of pages of comms lasting several hours, something that happened in the movie in a maybe a few seconds. Probably for the better too, I doubt we'd be having a discussion on the accuracy of this movie, if it actually was more accurate. Rare few would ever go and see it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This hopefully answers your first two questions, now for the last one, if the Apollo CM was even designed in such a way that it was possible for the astronauts to fully power off such a critical system as the main guidance computer in the middle of the flight? No, this was certainly not something that the Apollo missions were designed for but it obviously wasn't impossible either. A lot of ingenuity went into saving the crew of Apollo 13 and getting them safely back home. The ground team was working around the clock to think of ways to solve problems the crew was faced with, and powering down the CM merely being one of them. Apollo missions of course had all kinds of contingency plans (emergency power down being one of them), but with this one, everyone was &lt;a href=&quot;https://apollo13.spacelog.org/03:07:34:23/#log-line-286463&quot; rel=&quot;noreferrer&quot;&gt;thinking on their feet&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;03 07 34 23 LMP Okay. Is it going to be better to write this&#xA;                on a blank page, Vance, or can we use some portion&#xA;                of the power down list there in the contingency&#xA;                book that already exists.&#xA;03 07 34 33 CC  Stand by 1.&#xA;03 07 36 03 CC  Aquarius, Houston. Over.&#xA;03 07 36 07 CDR Go ahead.&#xA;03 07 36 08 CC  Slight delay here, Jim. It will be a couple of&#xA;                minutes before we read that up to you, and we're&#xA;                looking at the contingency checklist power down&#xA;                and that's on page 5. You might be getting that&#xA;                out while we get all ready to give it to you.&#xA;03 07 37 03 LMP Could you give us that page number again. Page 5&#xA;                doesn't make sense.&#xA;03 07 37 14 CC  Okay. Make that power 5 in the contingency&#xA;                checklist, Fred. And it's the - -&#xA;03 07 37 24 LMP - - that's better.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt;: Yes, this shot makes sense in the Apollo 13 movie, even if the script didn't follow the mission transcript exactly to the letter:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/lMhOY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="415" LastEditDate="2017-03-01T18:39:27.850" LastActivityDate="2017-03-01T18:39:27.850" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2555" PostTypeId="5" CreationDate="2013-10-27T02:59:46.183" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-27T02:59:46.183" LastActivityDate="2013-10-27T02:59:46.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2556" PostTypeId="4" CreationDate="2013-10-27T02:59:46.183" Score="0" Body="Questions regarding missions or parts of missions which were not successful." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-27T02:59:46.183" LastActivityDate="2013-10-27T02:59:46.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2557" PostTypeId="2" ParentId="2539" CreationDate="2013-10-27T05:22:41.660" Score="4" Body="&lt;p&gt;Quoting from the document posted at &lt;a href=&quot;http://www.faa.gov/&quot; rel=&quot;nofollow&quot;&gt;http://www.faa.gov/&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Section III.1.2 Cabin Environment and EVA Environment.doc&#xA;authored by Kira Bacal&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(direct download link is provided in TildalWave's answer):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ambient temperatures on current spacecraft are usually kept at “shirtsleeve” conditions, i.e. around 75°F. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The cabin environment is usually maintained at about 60% relative humidity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is not clear where these figures come from, how up-to-date they are, or whether they are applicable to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(for reference: 75°F equals 24°C)&lt;/p&gt;&#xA;" OwnerUserId="947" LastEditorUserId="947" LastEditDate="2013-10-28T01:10:34.203" LastActivityDate="2013-10-28T01:10:34.203" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2558" PostTypeId="1" CreationDate="2013-10-27T05:23:23.600" Score="18" ViewCount="8458" Body="&lt;p&gt;Given that deep craters on Mercury's polar regions could provide protection from the searing heat and radiation of the closer Sun, a colony on Mercury could be possible. For any colony to be sustainable, it would need to be as self sufficient as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, would there be enough natural resources on Mercury itself to sustain the needs of a human colony?&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2015-05-18T21:43:56.087" Title="Does Mercury have the necessary natural resources to sustain a human colony?" Tags="|colonization|mercury|natural-resources|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2559" PostTypeId="2" ParentId="2558" CreationDate="2013-10-27T06:26:49.250" Score="8" Body="&lt;p&gt;Well, one large issue is that on Mercury we can't make plastics. Petrochemicals are needed as a raw material, and as far as I can tell, there were no dinosaurs on Mercury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bioplastics exist, but we would need a large plantation to be able to sustainably make plastics. We already would need large plantations for food and for sustaining the atmosphere, so this might get infeasible due to space considerations. Also, we need to consider the recycling of organic matter. Plastics are a dead end in the organic matter chain, they cannot be easily degraded to manure even if they are created from organic matter. On a planet where the building blocks must be shipped from the Earth, this can be a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, no plastic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mercury seems to have tons of iron though. That's good.&lt;/p&gt;&#xA;" OwnerUserId="96" LastActivityDate="2013-10-27T06:26:49.250" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2560" PostTypeId="2" ParentId="2541" CreationDate="2013-10-27T09:47:10.070" Score="2" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;take a fairly big metallic asteroid in the 25+ kilometer range from the inner NEA asteroid belt and by means of slow acceleration and slingshotting haul it in to a venus orbit.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;keep adding solar sells and create an automated industrial infrastructure on that asteroid&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;produce reflective material and start engineering a microfilm think layer around the orbital plane of venus. Reflect sunlight away from the equatorial zone around venus. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Reduce incoming sunlight reaching the venusian clouds gradually to under 10% over a few decades. Given sufficient means of production this can easily be achieved with a several 10km asteroid. More than enough raw materials. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Wait. Temperature will drop on venus. Venus is baked hot through and through, and the cooling down, i.e. radiating away heat, will take an absurd long time. There simply isn't a sensible way to cool Venus down. The only way that will do anything is adding the solar shielding (in a polar orbit probably) to well over 95% reflectivity. At that rate it makes even more sense to saturate the upper atmosphere with a reflective fillament &quot;floating&quot; on top of the atmosphere. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This will take a few centuries. I think we'll colonize and industrialize the Kuyper belt and Mercury before we tackle Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Early stages of this project may resemble the world depicted in Larry Niven's &quot;plateau&quot;, where the planet proper is toxic and uninhabitable, but &quot;a few mountain plateau's stick high above the atmosphere. As the terraforming advances you may see high tower structures erected on the surface, in some cases 50 kilometers or more high, where there's cities and industrial activity. Compare the norwegian troll platform - it's pillars are hollow down to the ocean floor - a full hundreds of meters. With active, assertive cooling (?) and hyper-advanced heat resistant materials it may &lt;strong&gt;eventually&lt;/strong&gt; become feasible to start erecting meaningful real estate. Right now such towers would have to resist extreme pressures, corrosive winds, blast furnace heat and that would require various types of uhm, unobtaneous constructs.&lt;/p&gt;&#xA;" OwnerUserId="951" LastEditorUserId="951" LastEditDate="2018-12-31T15:04:06.023" LastActivityDate="2018-12-31T15:04:06.023" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2561" PostTypeId="1" AcceptedAnswerId="2563" CreationDate="2013-10-27T11:55:30.830" Score="8" ViewCount="6116" Body="&lt;p&gt;Theoretically MPD thrusters can provide high thrust with extremely high specific impulse. What problems currently prevent this technology from being used in space exploration?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any planned mission which will use MPD thrusters?&lt;/p&gt;&#xA;" OwnerUserId="105" LastEditorUserId="105" LastEditDate="2013-11-11T18:00:32.040" LastActivityDate="2017-05-18T14:16:35.830" Title="What are the main problems with magneto plasma dynamic (MPD) thrusters?" Tags="|propulsion|magnetic-field|magnetoplasmadynamic|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2562" PostTypeId="1" AcceptedAnswerId="2590" CreationDate="2013-10-27T17:18:43.957" Score="13" ViewCount="4399" Body="&lt;p&gt;Given a planet of mass $M$ at the origin, and the state vectors $\textbf{r}$ (in meters) and $\textbf{v}$ (in $m/s$) at time $t = 0$ for a satellite, what are are the equations for:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Distance $d(t)$ between the satellite and the planet at time $t$, and&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Distance $d(\theta)$ when the satellite is at an angle $\theta$ from &quot;horizontal&quot; (the vector $&amp;lt;1, 0&amp;gt;$)?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Edit: I can already compute certain values, such as the semi-major axis $a$, and the eccentricity $e$. I could even make do with just being able to compute the angle from the horizontal of periapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm writing a simple 2d orbital simulator, and I would really like to draw a projected trajectory for the satellite/spaceship that the player is controlling, but I need these formulas to be able to do that. Thanks!&lt;/p&gt;&#xA;" OwnerUserId="953" LastEditorUserId="953" LastEditDate="2013-10-28T03:42:57.890" LastActivityDate="2020-03-10T15:00:31.103" Title="2D Orbital Path from State Vectors" Tags="|orbital-mechanics|orbit|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2563" PostTypeId="2" ParentId="2561" CreationDate="2013-10-27T19:49:01.167" Score="6" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ygiXz.jpg&quot; alt=&quot;MPD thruster shot&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;An MPD thruster in action&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.princeton.edu/%7Eachaney/tmve/wiki100k/docs/Magnetoplasmadynamic_thruster.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Princeton page&lt;/a&gt;, MPD's could prove an exhaust velocity of up to and beyond 110,000 m/s - which is triple the value of current xenon-based ion thrusters. So yes, it would be great for space applications.&lt;/p&gt;&#xA;&lt;p&gt;Here's the problem: The crazy things take &lt;em&gt;a lot of power&lt;/em&gt;. As in a &lt;em&gt;lot&lt;/em&gt;. Here's the gist of the problem, from &lt;a href=&quot;http://alfven.princeton.edu/papers/Encyclopedia.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The MPDT has demonstrated its capability of providing specific impulses in the range of 1500–8000 sec with thrust efficiencies exceeding 40%. High efficiency (above 30%) is &lt;strong&gt;typically reached only at high power levels&lt;/strong&gt; (above &lt;strong&gt;100 kW&lt;/strong&gt;); consequently, &lt;strong&gt;the steady-state version of the MPDT is regarded as a high-power propulsion option&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;When the thruster is operated &lt;strong&gt;below 200 kW&lt;/strong&gt;, the self-induced magnetic field becomes &lt;strong&gt;only marginally sufficient&lt;/strong&gt; to provide the desired body force, and external fields are frequently added to enhance performance in this range.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Of course, spacecraft wouldn't be able to add the external field in an efficient manner - at this point, it would be more power efficient to use another option.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In addition to the present unavailability of high power in space, the cathode erosion rates of the steady-state MPDT (which can be as high as &lt;span class=&quot;math-container&quot;&gt;$0.2\frac{\mu g}{C}$&lt;/span&gt;), have slowed the evolution of steady-state MPDTs toward flight applications.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There is still hope, though, in the form of a varied system - a steady-state MPDT, called the lithium Lorentz-force accelerator (Li-LFA):&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lQvvc.png&quot; alt=&quot;Li-LFA shot&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;An Li-LFA thruster.&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The lithium Lorentz-force accelerator (&lt;span class=&quot;math-container&quot;&gt;$\rm{Li}$&lt;/span&gt;-LFA) uses a multichannel hollow cathode and lithium propellant to substantially reduce the cathode erosion problem while significantly raising the thrust efficiency at moderately high power levels. For example, a 200-kW &lt;span class=&quot;math-container&quot;&gt;$\rm{Li}$&lt;/span&gt;-LFA has demonstrated essentially erosion-free operation over 500 hr of steady thrusting at 12.5 N, 4000 sec &lt;span class=&quot;math-container&quot;&gt;$I_{sp}$&lt;/span&gt;, and 48% efficiency. Since no other electric thruster has yet shown such a high power processing capability, the &lt;span class=&quot;math-container&quot;&gt;$\rm{Li}$&lt;/span&gt;-LFA is at the forefront of propulsion options for nuclear-powered deep-space exploration and heavy cargo missions to the outer planets.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://alfven.princeton.edu/papers/Encyclopedia.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The paper&lt;/a&gt; is a great resource on the subject - there are many more nice little gems in it that I didn't cover here. Highly recommended!&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-11-11T09:20:25.207" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2564" PostTypeId="5" CreationDate="2013-10-27T20:12:22.150" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-27T20:12:22.150" LastActivityDate="2013-10-27T20:12:22.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2565" PostTypeId="4" CreationDate="2013-10-27T20:12:22.150" Score="0" Body="Questions regarding magnetoplasmadynamic thrusters (MPD's), a thruster system with a very large specific impulse." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-27T20:12:22.150" LastActivityDate="2013-10-27T20:12:22.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2566" PostTypeId="2" ParentId="2529" CreationDate="2013-10-27T22:12:21.300" Score="7" Body="&lt;p&gt;Ok, there's quite a bit to cover here. First off, let's look at the groundtrack of a satellite in GEO. Obviously, a satellite in a perfectly geostationary orbit will project as just a point on the Earth's equator. Because of natural perturbations (mass distribution of Earth and the combined effect of Lunar and Solar gravity, specifically), a satellite in a perfect geostationary orbit will not stay there for long. If we assume only a change in inclination (i.e. still at the ideal GEO altitude), the ground track looks like the red track here (from the ever-helpful &lt;a href=&quot;https://celestrak.org/columns/v04n07/&quot; rel=&quot;nofollow noreferrer&quot;&gt;T.S. Kelso&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nE7El.gif&quot; alt=&quot;GEO groundtracks&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The maximum latitude of this figure-8 is equal to the inclination of the orbit. If you consider deviations in the altitude, you start drifting with respect to the Earth. Consider a slightly lower-than-GEO altitude: your orbital period is now less than a sideral period of Earth rotation, and so in time, you begin drifting &lt;strong&gt;Eastward&lt;/strong&gt; (by the same token, you drift West if you're at a higher altitude).&lt;/p&gt;&#xA;&lt;p&gt;Now, getting back to relative distance... if you assume you have two satellites separated by some (mean) longitude, and at the ideal circular GEO altitude, then the minimum distance will in fact be the arc length for the difference in longitude. Another way of showing this is by inspecting &lt;a href=&quot;http://courses.ae.utexas.edu/ase366k/cw_equations.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hill's equations for relative orbit motion&lt;/a&gt;. They describe the motion of an object with respect to another object, assuming the object is in a nearly circular orbit. That assumption fits nicely here.&lt;/p&gt;&#xA;&lt;p&gt;Without getting too far into it, just take one of your inclined objects, and look at its motion with respect to a &amp;quot;virtual&amp;quot; satellite with the same properties but at zero degrees inclination. If you look at the closed-form solution of the three equations in that paper, the important thing to realize is that cross-track (&lt;span class=&quot;math-container&quot;&gt;$z$&lt;/span&gt;) motion is totally decoupled from in-track (&lt;span class=&quot;math-container&quot;&gt;$y$&lt;/span&gt;) and radial (&lt;span class=&quot;math-container&quot;&gt;$x$&lt;/span&gt;) motion. Adding inclination is essentially only adding a cross-track component to the orbit - no matter what inclination you add, it in itself does not have any effect on radial or in-track motion. Therefore, if you assume circular orbits and look at only the longitudinal separation, no matter what the inclination is, your minimum separation is still given by the arc length of the difference in longitude.&lt;/p&gt;&#xA;&lt;p&gt;Of course, in reality, orbits are not this perfect. However, taking away that assumption makes things significantly more complex - now you need to worry about where the argument of perigee is with respect to the other object, where the node is, etc. For the situation you name, it's likely that they control the relative motion of these two satellites by controlling just those two things - argument of perigee and the right ascension of the ascending node (or something equivalent). This way, you can ensure one satellite is at perigee while the other is at apogee (thus ensuring some radial distance), and choose the node such that when the altitudes &lt;em&gt;are&lt;/em&gt; equal (twice per period), they're at their maximum cross-track separation.&lt;/p&gt;&#xA;&lt;p&gt;If you're interested in the more general problem of relative motion, I suggest reading up on Hill's equations, as they make describing relative motion fairly straightforward. Given the full state of two spacecraft, though, it would probably be easier to just propagate both over one period and find the point of closest approach that way, because it really does depend on all six orbital elements (and time).&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2013-10-27T22:12:21.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2567" PostTypeId="2" ParentId="2562" CreationDate="2013-10-28T00:11:46.927" Score="3" Body="&lt;p&gt;With $M$, $\textbf{r}$, $\textbf{v}$, you can calculate the specific energy, $\mathcal{E}$ and specific angular momentum of the object, $\mathcal{M}$, which are constants of the orbital motion.  From those you can get the semi-major axis, $a$, and the eccentricity, $e$.  That is all you need in two dimensions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That won't get you a closed form solution for $d(t)$.  What you &lt;em&gt;can&lt;/em&gt; get are parametric solutions in the form $t(\tau)$, $r(\tau)$, and $\phi(\tau)$ (or $x(\tau)$ and $y(\tau)$), which can be used for making plots.  $\tau$ is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Eccentric_anomaly&quot; rel=&quot;nofollow&quot;&gt;eccentric anomaly&lt;/a&gt;, which for an elliptical orbit is the angle of the position from the center of the ellipse (&lt;em&gt;not&lt;/em&gt; the focus of the ellipse, where the planet is).  When you run $\tau$ from $0$ to $2\pi$, you get a complete orbit.  The equations are (where $r$ is the magnitude of $\textbf{r}$ (what you're calling &quot;$d$&quot;), and $\mu$ is $GM$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$r(\tau)=a\left(1-e\cos{\tau}\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\phi(\tau)=\tan^{-1}\left(\cos{\tau}-e,\sqrt{1-e^2}\sin{\tau}\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t(\tau)=\sqrt{a^3\over\mu}\left(\tau-e \sin{\tau}\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The argument order for the two-parameter inverse tangent above is $\tan^{-1}\left(x,y\right)$.  Many programming languages have an &lt;code&gt;atan2(y,x)&lt;/code&gt; function with the arguments in the other order, so be careful lest you cause havoc in the heavens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will need to solve for $\tau_0$, e.g. using $r(\tau_0)=r_0$ to know where your starting point is in the orbit, if that matters for your plot.  You may also want to add an offset to $\phi$ to get the orbit rotated to some specific starting location, again if that matters to you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, it is common to use $\mu$ instead of $M$ because we can usually measure $\mu$ to much higher accuracy than we currently know the fundamental physical constant $G$.  So how well we know $M$ for a body is usually limited by how well we know $G$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-10-28T17:54:13.123" LastActivityDate="2013-10-28T17:54:13.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2568" PostTypeId="2" ParentId="2562" CreationDate="2013-10-28T00:53:12.157" Score="2" Body="&lt;p&gt;I won't solve 100% of this for you, but I'll give you enough to figure it out. From &lt;a href=&quot;http://en.wikipedia.org/wiki/Elliptic_orbit&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v=\sqrt{\mu\left({2\over{r}}-{1\over{a}}\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mu=GM$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From these, G is a constant, M is the mass. You provide then every term in that equation except for $a$. Simply back out what $a$ is, and you know the orbital velocity at for any given distance. The next thing to know is the orbital period. That can be found, according to &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_period&quot; rel=&quot;nofollow&quot;&gt;Wikipedia/ Kepler's Third Law&lt;/a&gt;, to be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$T = 2\pi\sqrt{a^3/\mu}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You know all of these values already. Last piece of information is &lt;a href=&quot;http://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion&quot; rel=&quot;nofollow&quot;&gt;Kepler's Second Law&lt;/a&gt;. Put it all together, and you should be able to piece together the formula.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-28T00:53:12.157" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2569" PostTypeId="1" AcceptedAnswerId="2578" CreationDate="2013-10-28T12:43:38.773" Score="17" ViewCount="3286" Body="&lt;p&gt;Does Mars contain the materials required to make plastic? If not, can they relatively easily be made, or an adequate substitute found?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:48:54.727" LastActivityDate="2018-10-09T14:30:17.687" Title="Can materials be found to make plastic on Mars?" Tags="|mars|colonization|materials|natural-resources|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2570" PostTypeId="1" CreationDate="2013-10-28T12:48:43.110" Score="32" ViewCount="5175" Body="&lt;p&gt;There are two primary systems of navigation on Earth today, using maps/compass, and using GPS satellites. It seems unlikely there will be a full constellation of GPS satellites prior to the first humans landing on Mars. In addition, Mars doesn't have a magnetic field like Earth, so a compass is out. It also seems like many long trips will be taken on Mars, even by the first group landing there, requiring accurate navigation. How will this be accomplished?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-20T11:00:58.540" Title="How could one navigate on Mars?" Tags="|mars|colonization|navigation|" AnswerCount="8" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="2571" PostTypeId="1" AcceptedAnswerId="2574" CreationDate="2013-10-28T13:02:00.113" Score="24" ViewCount="1605" Body="&lt;p&gt;Mars has a different axial tilt than Earth. It stands to reason that the constellations will remain identical from Mars, but their locations will be somewhat different. On Earth, the North Star is used as a navigational aide. What is Mars's &quot;North Star&quot;, &quot;South Star&quot;, and if there isn't a star for either, what would the region look like over the North/South poles?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-28T18:06:45.880" LastActivityDate="2019-08-07T23:30:44.777" Title="Is there a Martian &quot;North Star&quot;, usable as Earth's North Star?" Tags="|mars|navigation|astronomy|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2572" PostTypeId="2" ParentId="2570" CreationDate="2013-10-28T13:22:10.040" Score="18" Body="&lt;p&gt;If long-range navigation is necessary, a combination of &lt;a href=&quot;http://en.wikipedia.org/wiki/Inertial_navigation_system&quot;&gt;inertial navigation&lt;/a&gt; with occasional position fixes using stars gives reasonably accurate navigation. Inertial navigation has a few drawbacks: &lt;a href=&quot;http://www.vectornav.com/support/library?id=76&quot;&gt;high cost&lt;/a&gt; and these systems become less accurate over time (hence the need for occasional position fixes).&lt;br&gt;&#xA;For short-range navigation, dead reckoning may be sufficient. For dead reckoning, you use sensors on the wheels of your vehicle to keep track of how far it travels and which way it turns. This was used on the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot;&gt;lunar rover&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-10-28T13:22:10.040" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2573" PostTypeId="2" ParentId="2570" CreationDate="2013-10-28T13:44:08.317" Score="17" Body="&lt;p&gt;There are a few things that could be done (Using pretty heavily &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/145160811X&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;A Case for Mars&quot;&lt;/a&gt; second edition):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;There will not be full constellation of satellites, but undoubtedly there will be satellites orbiting Mars at that time that are functioning. Each Mars orbiter since Mars Global Surveyor has included some sort of a beacon which could be heard. Measuring Doppler shift and listening to the time you hear it can give you your latitude, and approximate longitude. This could be easily found via a look up table of some sort on the rover.&lt;/li&gt;&#xA;&lt;li&gt;Stars could be observed at night, which gives you a reference point. In addition, if you are low enough in latitude, you can use Deimos and Phobos to give you a pretty good latitude/longitude estimate. If you keep careful time, you can use the same trick that mariners have used for a long time to find longitude, in addition, you can use the moons as an additional reference point. In fact, given 2 of the 3 objects (Sun, Phobos, Deimos), a sextant, a clock, and an almanac, one could determine one's location exactly.&lt;/li&gt;&#xA;&lt;li&gt;Some sort of a inertial navigation system could be used, as Hobbes mentioned.&lt;/li&gt;&#xA;&lt;li&gt;Use the &lt;a href=&quot;https://space.stackexchange.com/q/2571/25&quot;&gt;North Star&lt;/a&gt;, or best equivalent.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-10-28T14:44:26.607" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2574" PostTypeId="2" ParentId="2571" CreationDate="2013-10-28T14:23:00.047" Score="21" Body="&lt;p&gt;The location of the North Pole is 21.18 RA, 52.89 North. There is no &quot;North Star&quot; located there, although it is about half-way between Deneb and Alderamin. (&lt;a href=&quot;http://earthsky.org/space/mars-north-south-star&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;). In the sourthern sky, Kappa Velorum is a good South Pole Star. Here's also a few screenshots as to what the poles would look like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.eknent.com/etc/mars_sp.png&quot; rel=&quot;noreferrer&quot;&gt;Mars North Pole&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/O8Sgk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.eknent.com/etc/mars_sp.png&quot; rel=&quot;noreferrer&quot;&gt;Mars South Pole&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/t0koZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've also added a bit of video showing how these stars will appear in the following video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/_c11z54x1Ik?start=112&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-09-14T11:40:17.200" LastActivityDate="2018-09-14T11:40:17.200" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2575" PostTypeId="2" ParentId="2570" CreationDate="2013-10-28T14:27:51.800" Score="7" Body="&lt;p&gt;You could try old school navigation. Like the one they did in the past when sailing ships. They dint have navigation back then so they used a &quot;tool&quot; to calculate the angle of the horizon and the sun. And then according to the specific time they took the angle they could calculate there exact position. Now this is still ment to work only on earth. Because of all the calculation specific to earth and at the beginning it did took a lot of time to calculate where you are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This video could explain a little more: &lt;a href=&quot;http://youtu.be/AGCUm_jWtt4&quot;&gt;http://youtu.be/AGCUm_jWtt4&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="958" LastActivityDate="2013-10-28T14:27:51.800" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2576" PostTypeId="1" AcceptedAnswerId="2577" CreationDate="2013-10-28T15:43:01.657" Score="11" ViewCount="3186" Body="&lt;p&gt;The International Space Station tries to maneuver to avoid debris, but sometimes (and it &lt;a href=&quot;http://www.space.com/15031-space-junk-station-astronaut-threat.html&quot;&gt;has actually happened&lt;/a&gt;) they discover something that could hit them too late to move, so they shelter in Soyuz spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What exactly is the logic behind this action, and what do they expect to happen to pressure levels if the ISS is hit?  I imagine the danger is that debris hits the ISS and cause depressurization.  But that doesn't do any good if the Soyuz has its airlock completely connected.  Isn't that the normal state of affairs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they isolate the Soyuz's atmosphere from the rest of the station when sheltering?  Or do they just have a hatch that will only close fully if depressurization starts happening?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-10-28T19:13:54.877" Title="What would happened if the ISS was depressurized while astronauts were sheltering?" Tags="|iss|debris|astronaut-lifestyle|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2577" PostTypeId="2" ParentId="2576" CreationDate="2013-10-28T16:33:43.473" Score="9" Body="&lt;p&gt;A Soyuz has two (three?) compartments.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Orbital module at the top which has the docking probe and the main door.&lt;/li&gt;&#xA;&lt;li&gt;The Descent module in the middle that is where the crew resides for launches and landings.&lt;/li&gt;&#xA;&lt;li&gt;Propulsion/Service module at the bottom.  No room for people, but fuel, equipment, etc. (So is it really a compartment?  Or more of a 'module', whatever you want to call it).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There is a hatch between the Orbital and Descent module, of course, since the Orbital module is dumped before reentry, the hatch on the Descent module must be sufficient for exposure to the atmosphere, space, everything in between plus reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus I imagine, if they are sheltering in the Soyuz they probably close at least the Orbital modules hatch to the station, since they would need it closed to depart the station anyway.  But they have an extra hatch if needed, in the descent module.  This of course is a distinct advantage that Soyuz has over ATV, HTV, Dragon, Cygnus, CST-100, and Dream Chaser (Who the heck knows what Blue Origin is really doing) in terms of other craft visiting the station. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is unclear if in an emergency they could discard the orbital module and leave it attached to the station and depart in just the Descent and propulsion modules.  Nor is it clear, what they would do after that to recover.  I imagine, worst case they could fashion a grapple point for the CanadaArm to the Orbital module to 'throw' it away.  Of course, if it ever got that bad, who knows if there would be a recovery for the ISS.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-10-28T19:13:54.877" LastActivityDate="2013-10-28T19:13:54.877" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2578" PostTypeId="2" ParentId="2569" CreationDate="2013-10-28T16:40:33.503" Score="14" Body="&lt;p&gt;&lt;strong&gt;Yes&lt;/strong&gt; - for silicone plastics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars does have plenty of silica (aka sand) so there are the basic materials to synthesize silicone - which can be used in place of most organic plastics. (Often performing better than conventional carbon-based plastics.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methane, chlorine, water, and carbon-dioxide all exist on the surface as well. It should be stated that this is speculative - the exact industrial processes to create plastics in any quantity given the Martian resources is far beyond me at this point. However, the raw elements seem to be available. &lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="25234" LastEditDate="2018-04-12T02:51:16.330" LastActivityDate="2018-04-12T02:51:16.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2579" PostTypeId="2" ParentId="2570" CreationDate="2013-10-28T18:11:03.923" Score="11" Body="&lt;p&gt;If you can afford to get people to Mars, then you can &lt;em&gt;easily&lt;/em&gt; afford a navigational satellite or two.  Since you asked how it &lt;em&gt;will&lt;/em&gt; be accomplished (as opposed to how it perhaps &lt;em&gt;could&lt;/em&gt; be accomplished, with lots of interesting answers to that question here), then I can answer with some confidence that they will use navigational satellites for safety and reliability.  It won't be much different than GPS here.  The main difference will be satellite availability and accuracy.  There probably won't be 31 of them flying around Mars.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-10-28T18:11:03.923" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2580" PostTypeId="2" ParentId="2569" CreationDate="2013-10-28T19:39:57.837" Score="16" Body="&lt;p&gt;The key material to produce to make plastics is the production of ethylene, which is $C_2H_4$. According to &lt;em&gt;The Case for Mars&lt;/em&gt;, this can be produced by the reaction $2CO+4H_2 \rightarrow C_2H_4+2H_2O$, with the presence of an Iron catalyst.. And the carbon monoxide comes from $6H_2+2CO_2 \rightarrow 2H_2O+2CO+4H_2$. Thus, the key to making plastics on Mars is Carbon Dioxide and Water. We know both materials exist on Mars, thus, it should be easy enough to make plastics, given the right catalysts. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user29" LastEditDate="2013-10-29T01:55:13.567" LastActivityDate="2013-10-29T01:55:13.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2581" PostTypeId="5" CreationDate="2013-10-28T19:48:29.703" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:48:29.703" LastActivityDate="2013-10-28T19:48:29.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2582" PostTypeId="4" CreationDate="2013-10-28T19:48:29.703" Score="0" Body="Questions regarding resources available on another astronomical object." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:48:29.703" LastActivityDate="2013-10-28T19:48:29.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2583" PostTypeId="5" CreationDate="2013-10-28T19:51:16.013" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:51:16.013" LastActivityDate="2013-10-28T19:51:16.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2584" PostTypeId="4" CreationDate="2013-10-28T19:51:16.013" Score="0" Body="Questions regarding the number of crew members onboard a mission." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-28T19:51:16.013" LastActivityDate="2013-10-28T19:51:16.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2585" PostTypeId="1" AcceptedAnswerId="2586" CreationDate="2013-10-28T19:51:06.363" Score="11" ViewCount="1769" Body="&lt;p&gt;I saw the movie &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_%28film%29&quot; rel=&quot;noreferrer&quot;&gt;Gravity&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;strong&gt;Spoiler alert, hover with your mouse over the yellow bar below to reveal a part of the movie's plot:&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote class=&quot;spoiler&quot;&gt;&#xA;  &lt;p&gt; In the movie some astronomers die just because they were unable to hold on to the satellites or space stations and by hard luck they are left alone in the space with a going-to-empty oxygen supply tanks which empties and they die.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why can't they have a gun kind-of thing with a string/rope? One end of the rope is tied to them and the other end to the dart that is inside the gun. If they shoot pointing to the space station, the dart kind of thing goes and sticks to space station. Thereby they can use that string/rope to get back to the station. For me it looks a simple thing to invent and produce.&lt;/p&gt;&#xA;" OwnerUserId="980" OwnerDisplayName="Ravishankar Rajendran" LastEditorUserId="26446" LastEditDate="2021-07-05T04:07:12.590" LastActivityDate="2021-07-05T04:07:12.590" Title="Could a harpoon-like gun be used by an astronaut to stop drifting away from a ship?" Tags="|iss|eva|microgravity|astronaut-propulsion|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2586" PostTypeId="2" ParentId="2585" CreationDate="2013-10-28T21:38:21.433" Score="17" Body="&lt;p&gt;Problem with what you propose, a kind of harpoon with probably a soft-tipped and magnetic head tethered projectile so it doesn't damage / penetrate the station's hull yet still holds onto it once it would reach it, is that the shooting of a kinetic projectile in one direction would propel you with equal force in the opposite direction since there's nothing to &lt;em&gt;hold on to&lt;/em&gt; in a freefall orbit around the Earth and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Momentum#Conservation&quot; rel=&quot;noreferrer&quot;&gt;law of conservation of momentum&lt;/a&gt; as implied by &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;noreferrer&quot;&gt;Newton's third law of motion&lt;/a&gt; states that: &lt;em&gt;&quot;when one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body&quot;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/O9ZHl.jpg&quot; alt=&quot;Newton’s 3rd Law of Motion&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Newton’s 3rd Law of Motion: &lt;strong&gt;For every action there is an equal and opposite reaction&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the projectile would probably be a lot lighter than your own and the extravehicular suit's total &lt;a href=&quot;https://en.wikipedia.org/wiki/Invariant_mass&quot; rel=&quot;noreferrer&quot;&gt;rest-mass&lt;/a&gt;, so the &lt;em&gt;dart&lt;/em&gt; would still propel relative to your initial point faster towards the station than you would be pushed the other way (with magnitude of &lt;a href=&quot;https://en.wikipedia.org/wiki/Momentum&quot; rel=&quot;noreferrer&quot;&gt;linear momentum&lt;/a&gt; being $p = mv$, where $m$ is the mass of the object and $v$ its velocity), but since you're already moving with some velocity away from the station, otherwise you wouldn't have these problems to start with, the difference might not be enough and you possibly wouldn't have a tether long enough to reach the station. And you should really try not to tug on the tether while still flying towards the space station and unwittingly reduce its velocity and make it twist and bend.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Targeting wouldn't be easy either, since you'd have to fire it from the center of your mass perpendicular to the remaining two axes than the one you're firing in, not to merely end up in an uncontrollable spin. At the exactly the right time too, since you'd most likely already be spinning relative to the station as a result of your unexpected separation from it. To put it differently, for example, if you were standing on ice and fired a gun from your right hand at your waist, even if you managed to keep your balance after the gun recoils (something you wouldn't be able to in microgravity of space, remember, there's nothing to really hold on to), you would end up in a clockwise spin on the ice with only a small drag of the ice on your shoes eventually stopping you. There is no meaningful drag in outer space (at least not meaningful for our case), so you would spin indefinitely or until some other body with mass &lt;em&gt;absorbs&lt;/em&gt; your &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_momentum&quot; rel=&quot;noreferrer&quot;&gt;angular momentum&lt;/a&gt;. The physics behind this are explained a bit more in the LiveScience article on &lt;a href=&quot;http://www.livescience.com/18588-shoot-gun-space.html&quot; rel=&quot;noreferrer&quot;&gt;What Would Happen If You Shot a Gun In Space?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, there are better ways though and a lot of research went into developing various &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronaut_propulsion_unit&quot; rel=&quot;noreferrer&quot;&gt;astronaut propulsion units&lt;/a&gt; that are constantly improved on throughout the manned space exploration history. What is common to all of them though is that they try to control the relative movement on all three axes, and your &lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_dynamics&quot; rel=&quot;noreferrer&quot;&gt;pitch, roll and yaw&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/TxWhH.jpg&quot; alt=&quot;Edward H. White II during Gemini-Titan 4 egress from the spacecraft carrying a Hand-Held Maneuvering Unit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Edward_Higgins_White&quot; rel=&quot;noreferrer&quot;&gt;Edward H. White II&lt;/a&gt; during &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_4&quot; rel=&quot;noreferrer&quot;&gt;Gemini-Titan 4&lt;/a&gt; egress from the spacecraft carrying a &lt;a href=&quot;https://en.wikipedia.org/wiki/Hand-Held_Maneuvering_Unit&quot; rel=&quot;noreferrer&quot;&gt;Hand-Held Maneuvering Unit&lt;/a&gt; (Source: Wikimedia)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hand-Held_Maneuvering_Unit&quot; rel=&quot;noreferrer&quot;&gt;Early versions&lt;/a&gt; of astronaut propulsion units were handheld and that didn't prove too intuitive to control and had too limiting amount of propellant to be really of much use except for providing short entertainment for some and mild annoyance for others of its early astronaut testers (they were only ever tested while the astronauts were tethered to the space vehicle), but later versions (e.g. the &lt;a href=&quot;https://en.wikipedia.org/wiki/Manned_Maneuvering_Unit&quot; rel=&quot;noreferrer&quot;&gt;Manned Maneuvering Unit&lt;/a&gt;) evolved to be easier, more intuitive to maneuver and carry useful amounts of onboard propellant to allow for shorter excursions in the space vehicle's vicinity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The latest development in astronaut propulsion units (&lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_Aid_for_EVA_Rescue&quot; rel=&quot;noreferrer&quot;&gt;Simplified Aid for EVA Rescue&lt;/a&gt; or SAFER) is even more advanced propulsive backpack with ability to automatically stabilize uncontrolled rotation of its carrier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/bUK1C.jpg&quot; alt=&quot;Astronaut Rick Mastracchio working with a SAFER system attached.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Astronaut Rick Mastracchio working with a SAFER system attached. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_Aid_for_EVA_Rescue&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on SAFER: Simplified Aid for EVA Rescue&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is now a standard equipment carried by every spacewalking astronaut in an &lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit&quot; rel=&quot;noreferrer&quot;&gt;Extravehicular Mobility Unit&lt;/a&gt; (EMU) during their Extravehicular Activity (EVA) at the International Space Station (ISS). It is however a self-rescue device and would only be used in an emergency. It is not meant as a mobility device to assist astronauts during normal EVA conditions, and ISS spacewalkers are tethered to the space station and use safety grips on its outer hull to push against.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-29T00:17:32.457" LastActivityDate="2013-10-29T00:17:32.457" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2587" PostTypeId="5" CreationDate="2013-10-28T23:21:00.360" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-28T23:21:00.360" LastActivityDate="2013-10-28T23:21:00.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2588" PostTypeId="4" CreationDate="2013-10-28T23:21:00.360" Score="0" Body="Questions regarding energy used to operate a mechanical or electrical device." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-28T23:21:00.360" LastActivityDate="2013-10-28T23:21:00.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2589" PostTypeId="1" CreationDate="2013-10-29T01:06:54.010" Score="3" ViewCount="1027" Body="&lt;p&gt;I'm curious about the initiatives out there to regulate the space colonization, once it becomes affordable for most countries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the government in the colonies be like some sort of federation with representatives of countries of the Earth, for instance?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the democracy be superseded by a benevolent dictatorship of such federation?&lt;/p&gt;&#xA;" OwnerUserId="962" LastEditorDisplayName="user12" LastEditDate="2013-10-29T04:45:04.030" LastActivityDate="2013-10-29T04:45:04.030" Title="What form of government would there be in space colonies?" Tags="|colonization|law|" AnswerCount="0" CommentCount="6" ClosedDate="2013-10-29T17:07:09.863" ContentLicense="CC BY-SA 3.0" />
  <row Id="2590" PostTypeId="2" ParentId="2562" CreationDate="2013-10-29T04:11:35.600" Score="9" Body="&lt;p&gt;Well, there are quite a few values to compute for the equations. Here is a list of what the symbols mean:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$\mu$&lt;/span&gt; is the standard gravitational parameter&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;r&lt;/strong&gt; is the position vector, whose magnitude &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; is the current distance from the origin&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;v&lt;/strong&gt; is the velocity vector, whose magnitude &lt;span class=&quot;math-container&quot;&gt;$v$&lt;/span&gt; is the current speed&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;h&lt;/strong&gt; is the angular momentum vector, whose magnitude &lt;span class=&quot;math-container&quot;&gt;$h$&lt;/span&gt; is the angular momentum&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;e&lt;/strong&gt; is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Eccentricity_vector&quot; rel=&quot;nofollow noreferrer&quot;&gt;eccentricity vector&lt;/a&gt;, whose magnitude is the orbital eccentricity&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; is the semimajor axis&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt; is argument of periapsis (which in the 2d case is the angle from the &lt;span class=&quot;math-container&quot;&gt;$x$&lt;/span&gt;-axis of periapsis, measured counterclockwise)&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$\theta$&lt;/span&gt; is the true anomaly&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$\tau$&lt;/span&gt; is the eccentric anomaly&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; is the mean anomaly&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$M_0$&lt;/span&gt; is mean anomaly at periapsis&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$d(\theta)$&lt;/span&gt; is distance of a point on the orbit with a given true anomaly &lt;span class=&quot;math-container&quot;&gt;$\theta$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$d(t)$&lt;/span&gt; is the distance of a point on the orbit at a particular time &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt; (with &lt;span class=&quot;math-container&quot;&gt;$t = 0$&lt;/span&gt; giving the periapsis distance)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now here are the equations to use/solve:&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\mu = MG$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\textbf{h} = \textbf{r}\times\textbf{v} = \left&amp;lt;0, 0, h\right&amp;gt;, h = x v_y - y v_x$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\textbf{e} = \frac{\textbf{v}\times\textbf{h}}\mu - \frac{\textbf{r}}{|\textbf{r}|} = \left&amp;lt;\frac{v_y h}{\mu} - \frac{x}{r}, -\frac{v_x h}{\mu} - \frac{x}{r}\right&amp;gt;$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$e = |\textbf{e}|$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$a = \frac{h^2}{\mu(1-e^2)}$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\omega = tan^{-1}\frac{\textbf{e}_y}{\textbf{e}_x}$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$d(\theta) = \frac{a(1 - e^2)}{1 + e \cos(\theta + \omega)}$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\tau = \tan^{-1}\frac{\sqrt{1-e^2} \sin\theta}{e+\cos\theta}, \ \theta = 2 \tan^{-1}\left(\sqrt\frac{1+e}{1-e} \tan\frac{\tau}{2}\right)$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$M(\tau) = \tau - e \sin \tau$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$M_0 = M|_{\tau = \tau(\omega)}$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$M(t) = t\sqrt{\frac{G(M + m)}{a^3}}+M_0$$&lt;/span&gt;&#xA;There isn't a closed form solution for &lt;span class=&quot;math-container&quot;&gt;$d(t)$&lt;/span&gt;; to compute &lt;span class=&quot;math-container&quot;&gt;$d(t)$&lt;/span&gt; at a specific &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt;, one must:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;compute mean anomaly &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; for the given time &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;li&gt;solve for eccentric anomaly &lt;span class=&quot;math-container&quot;&gt;$\tau$&lt;/span&gt; in &lt;span class=&quot;math-container&quot;&gt;$M = \tau - e \sin \tau$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;li&gt;compute true anomaly &lt;span class=&quot;math-container&quot;&gt;$\theta$&lt;/span&gt; from &lt;span class=&quot;math-container&quot;&gt;$\tau$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;li&gt;and then plug &lt;span class=&quot;math-container&quot;&gt;$\theta$&lt;/span&gt; into &lt;span class=&quot;math-container&quot;&gt;$d(\theta)$&lt;/span&gt; to get the distance at the angle that corresponds to the given time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="953" LastEditorUserId="953" LastEditDate="2020-03-10T15:00:31.103" LastActivityDate="2020-03-10T15:00:31.103" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="2591" PostTypeId="1" AcceptedAnswerId="2596" CreationDate="2013-10-29T04:32:47.717" Score="19" ViewCount="821" Body="&lt;p&gt;Mercury, being right around than 55,000,000 km away from the Sun, gets hot. As in &lt;em&gt;very&lt;/em&gt; hot - 700 °K hot (426 °C and 800 °F). Not a place you would want to live.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the flip side, the side of Mercury &lt;em&gt;not&lt;/em&gt; facing the Sun tends to be very cold - bottoming out at around 100 °K (-173 °C and -279 °F). Nice and frosty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These temperature shifts seem to present a major issue for colonization - one would have to build a structure that could keep a fairly constant internal temperature while the outside temperature varied tremendously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One potential solution to this problem I can think of is the use of caves - Mammoth cave in Kentucky &lt;a href=&quot;http://www.nps.gov/maca/faqs.htm&quot;&gt;tends to stick right around 54°F&lt;/a&gt; (12°C and 285°K).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a way we could leverage the tendency of a cave to normalize temperatures for a colony? How deep would a Mercurial cave need to be to achieve this effect?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-29T05:07:13.297" LastActivityDate="2016-02-18T09:30:20.830" Title="Would temperatures be more stable under the surface of Mercury?" Tags="|colonization|caves|mercury|temperature|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2592" PostTypeId="5" CreationDate="2013-10-29T04:42:13.363" Score="0" Body="&lt;p&gt;A parachute is an object used to slow down objects. It's often found in either a round or rectangular shape. In regards to space travel it's often used to slow down capsules after re-entry allowing for safe landing or to slow down space shuttles after landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are different types of parachutes, mostly in the case of spacecraft they are of the round type. These are not steerable and will be subject to the external flow of air. They are built to slow down an aircraft. Parachutes are also found in smaller forms, these mini-parachutes are called &quot;drogues&quot; and used for high speed descelaration, often at a certain altitude they are then used to pull the parachute out from it's container.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="521" LastEditDate="2013-12-17T18:38:36.367" LastActivityDate="2013-12-17T18:38:36.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2593" PostTypeId="4" CreationDate="2013-10-29T04:42:13.363" Score="0" Body="A parachute is an object used to slow down objects. It's often found in either a round or rectangular shape. In regards to space travel it's often used to slow down capsules after re-entry allowing for safe landing or to slow down space shuttles after landing." OwnerDisplayName="user12" LastEditorUserId="521" LastEditDate="2013-12-17T18:38:40.640" LastActivityDate="2013-12-17T18:38:40.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2594" PostTypeId="2" ParentId="2570" CreationDate="2013-10-29T05:52:53.187" Score="-1" Body="&lt;p&gt;Basically this is a non-problem because any place on Mars is as good as any other. You don't achieve anything by moving to a specific location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose people settle on Mars. At first, the settlements will probably be close together. Either within sight of each other or nearly so, so that people can go from one to the other by landmarks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need a way to explore such that you can find your way to the base, even if you cannot see it and don't recognize landmarks. Building a very tall tower to serve as a beacon could help. It could give off light, or perhaps also a long range radio signal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simple navigation device would tell you what direction the beacon signal is coming from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for me, being lazy, I will just download the Google Mars Navigation application from the Play Store for my Marsphone.&lt;/p&gt;&#xA;" OwnerUserId="965" LastActivityDate="2013-10-29T05:52:53.187" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2595" PostTypeId="2" ParentId="2591" CreationDate="2013-10-29T12:41:56.873" Score="15" Body="&lt;p&gt;Yes, they would be more stable, but there is a big issue as to where you'd want it to be. There's &lt;a href=&quot;http://einstein-schrodinger.com/Mercury_temperature.pdf&quot;&gt;a paper&lt;/a&gt; on this very subject. The paper states that at specific rings around Mercury near the poles, that just below the surface the temperature is constant and near room temperature, making them quite ideal for colonization. Bottom line, it is entirely possible, if you choose the right spot, to have a constant desirable temperature.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-10-29T12:41:56.873" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2596" PostTypeId="2" ParentId="2591" CreationDate="2013-10-29T14:48:26.063" Score="17" Body="&lt;p&gt;I've done a fair deal of spelunking (caving) and one of the first things you realize when you've visited several caves is that their inner air temperature tends to be at roughly the yearly average of the outside temperatures. They might produce their own climatic system if they're deep enough or have any connection with the outside temperatures, like both end openings or a connected body of water, but the deeper you go, the more isolated from the more rapid outside changes you are and the temperatures stabilize. This could still mean different areas of the caves might have different temperatures, even have the &lt;em&gt;heater&lt;/em&gt; or the &lt;em&gt;freezer&lt;/em&gt; effect in some places, but deep enough, they tend to be stable all year long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if the caves here on the Earth have anything to teach us is that the thermal dynamics of a well isolated system do indeed work the way the &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermodynamics&quot;&gt;thermodynamic theories&lt;/a&gt; teach us; that any body of mass would try to be in a constant &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermodynamic_equilibrium&quot;&gt;thermodynamic equilibrium&lt;/a&gt; with its surroundings, and the more mass you throw at it (or surround it with) the longer the temperatures will be, for all intents and purposes, considered stable. This should go the same for any celestial body, Mercury not being any exception.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the celestial body might have a lot longer (or shorter) days, years, e.t.c. and a lot more dynamic external environment than the relatively mild changes here on Earth (comparatively speaking), so the depths at which you would find reasonably stable environment might vary substantially, even cave to cave depending on their topography and connection with outside environment, but given enough caves to choose from and perhaps some reengineering of their less stable climatic influencers, this should be perfectly doable. If that would be considered convenient enough, that's another matter tho.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is however one big reservation in all of this. If the celestial body or the area where you wanted to occupy a cave is still geologically active, the caves might be even more active than their surroundings. Caves can form by tectonic shifts, geothermal activity, erosion, impacts or even just by cooling down and solidifying. Most of these processes last a long time and are difficult to predict, let alone control in any way. And trust me, caves are not a friendly environment to be in even with mild &lt;em&gt;disturbance&lt;/em&gt;. I've been roughly one kilometer deep in one cave (we have thousands in my country) once, when a really small tremor happened that likely wasn't even noticed by most people outside. And even though I'm not claustrophobic, I did experience first hand then what it must mean to be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Walls of caves here on Earth (at least the interesting to visit ones) are mostly made of limestone sedimentation, which is rather brittle even if it doesn't look like that with millions of years of being &lt;em&gt;polished&lt;/em&gt; by wind and water, or cave topography slowly grows by sedimentation. And the walls of volcanic &lt;em&gt;chimneys&lt;/em&gt; are often glass of once molten silicates. The problem is also what you usually don't see, which is the porosity of the whole cave system you're in. The larger passages are obvious, but most caves that were first formed by tectonic shifts of upper crust layers might literally have millions of smaller cracks around them and even enormous rock boulders &lt;em&gt;suspended&lt;/em&gt; above the larger passages by brittle, unstable rock that might give in at any time and close access. So the caves anywhere else might be just as fragile and the processes that made them still active. And isolation comes with its own disadvantages as well. No radio communication will reach the surface unless you laid a cable with a transmitter on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, I thought to describe a bit what else might be a problem, because the stability of inner climate really isn't the most problematic. It could possibly be even artificially controlled a whole lot easier than engineering around the problem of the stability of the cave system you'd build your base (spaceport?) in.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-10-29T14:48:26.063" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2597" PostTypeId="1" AcceptedAnswerId="4239" CreationDate="2013-10-29T18:44:01.563" Score="8" ViewCount="625" Body="&lt;p&gt;In the book &quot;The Case for Mars&quot;, cited in &lt;a href=&quot;https://space.stackexchange.com/a/25/25&quot;&gt;this answer&lt;/a&gt;, there is a study cited that states that 20-50 REM radiation occurs due to cosmic rays in deep space. It seems likely that the atmosphere of Mars would reduce that somewhat. By how much will the atmosphere of Mars reduce the radiation?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2022-05-18T09:13:30.173" Title="What, if any, reduction in radiation exposure does the atmosphere of Mars provide?" Tags="|mars|colonization|radiation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2598" PostTypeId="1" CreationDate="2013-10-29T19:50:54.220" Score="16" ViewCount="2191" Body="&lt;p&gt;A spacesuit is a fairly easily identifiable piece of equipment in spacecraft. It's purpose is to keep it's interior distinct from the exterior in terms of the pressure, and temperature differentials that apply in space, in vacuum. To an extent it must also serve as a radiation shield. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia lists the requirements a spacesuit &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_suit#Requirements&quot; rel=&quot;noreferrer&quot;&gt;should&lt;/a&gt;, and &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_suit#Optional_requirements&quot; rel=&quot;noreferrer&quot;&gt;may&lt;/a&gt; fulfil. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like so many other technology products, spacesuits too have evolved over time. The wikipedia article referenced above lists several variations categorized as the Soviet/Russian, American, and Chinese.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given all that a spacesuit must be capable of, what is the energy/power requirement of the following space-suits &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Orlan&lt;/li&gt;&#xA;&lt;li&gt;EMU&lt;/li&gt;&#xA;&lt;li&gt;Shuguang&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="1142" LastEditDate="2014-12-18T23:36:52.747" LastActivityDate="2019-11-08T01:36:18.927" Title="What is the power requirement of a spacesuit?" Tags="|power|spacesuits|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2599" PostTypeId="1" CreationDate="2013-10-29T20:27:01.990" Score="14" ViewCount="1355" Body="&lt;p&gt;A quick follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/2598/what-is-the-power-requirement-of-a-space-suit&quot;&gt;What is the power requirement of a spacesuit?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo 11 crew were in a vacuum out on Luna; their space-suits fulfilled the purpose of a basic space-suit about which &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_suit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A space suit is a garment worn to keep a human alive in the harsh environment of outer space, vacuum and temperature extremes. Space suits are often worn inside spacecraft as a safety precaution in case of loss of cabin pressure, and are necessary for extra-vehicular activity (EVA), work done outside spacecraft. Space suits have been worn for such work in Earth orbit, on the surface of the Moon, and en route back to Earth from the Moon. Modern space suits augment the basic pressure garment with a complex system of equipment and environmental systems designed to keep the wearer comfortable, and to minimize the effort required to bend the limbs, resisting a soft pressure garment's natural tendency to stiffen against the vacuum. A self-contained oxygen supply and environmental control system is frequently employed to allow complete freedom of movement, independent of the spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Colonization would probably require some manned missions to do the basic spade-work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say a manned mission heads out to Mars - &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could the suits evaluated for Luna/Earth Orbit be used on Mars as-is?&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;E.g. Orlan&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Say the same, or another mission trots over to ... oh, Titan&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could the Titan mission possibly use the suits designed/constructed for Mars?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is it necessary to design/configure an EVA-suit for each type of Celestial atmosphere?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2020-09-25T05:21:16.323" Title="Could an EVA Suit meant for Mars be used for EVA on another celestial, or vice-versa?" Tags="|colonization|spacesuits|life-support|environment|reuse|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2600" PostTypeId="2" ParentId="2558" CreationDate="2013-10-29T21:08:24.873" Score="4" Body="&lt;p&gt;The question to ask is not whether Mercury has the resources we need, but what we can do with the resources Mercury has.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given a large enough initial colonization payload, &lt;em&gt;in theory&lt;/em&gt;, any colony could continue almost indefinitely without additional input of matter. If we perfect the technology, we could colonize the space between Earth and Venus. Take this planet for example: We started with a, uh, REALLY big colonization payload, and we just keep going using the same matter, mostly. If we manage to destroy ourselves with some disaster like global warming, that will be a matter of management, not lack of resources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To sustain a colony at or below its initial population, the only resource that is absolutely required is energy, which Mercurial colonists would get from the sun just like we do, in much greater abundance, if they can harvest it. If we want the colony to be able to grow beyond its initial size, it will need the materials of bio-matter, like hydrogen, carbon, oxygen, and nitrogen. As far as I can tell, we don't have enough information to assess those resources on Mercury yet. For example, although there is no significant atmospheric oxygen, we will probably find metal oxides in sufficient quantity on the surface. If not, the colony can only grow at the rate that we can throw comets at it. In any case, the main challenge is not lack of resources at all, but of technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given only technology that already exists today, I don't think those colonists would have a snowball's chance on the day-time surface of Mercury of surviving the first Mercurial year. But once we have probed Mercury enough to have a strong sense of the variety of materials available, which will take decades or longer at our current rate, we need to determine if we can use those materials to build atmospheric containment structures, light reflection, dispersion and filtration materials, atmospheric replacement, soils, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we remain stuck on the idea that we need plastic or glass domes to contain our atmosphere and let in light, then a lack of silicon and petroleum will present a serious problem, but if we find a way to make a strong translucent material out of mostly hydrogen, sulfur, and magnesium, which we already know to be plentiful on Mercury, or out of something else we happen to find with our probes in sufficient quantity, then we're in business. We have barely scraped the surface of what is possible in materials sciences. We have only explored a handful of options that we have found to be interesting and useful for the challenges we face on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit in response to paul23:&lt;/strong&gt;&#xA;&lt;br&gt;&#xA;I am not a materials scientist or chemist, but just off the cuff, I can see that water (H&lt;sub&gt;2&lt;/sub&gt;O) is transparent as a gas, liquid or crystalline solid. Sulfur has the same outer shell as Oxygen, so what about H&lt;sub&gt;2&lt;/sub&gt;S? There is plenty of Hydrogen and Sulfur on the surface of Mercury. So I decided to Google the properties of that chemical. It is most familiar to us on Earth as something that smells like rotten eggs, and readily dissolves in water to form a powerful acid. But I eventually found some interesting data &lt;a href=&quot;http://sulphur.atomistry.com/hydrogen_sulphide.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; about it's chemical properties when isolated. In the absence of water, it is pH neutral and is transparent as a gas and a liquid. I can't immediately find any information about its opacity as a crystalline solid, but I know it can form a crystalline structure because it is a relatively high-temperature &lt;a href=&quot;http://www.scientificamerican.com/article/superconductivity-record-broken-with-rotten-egg-smelling-compound/&quot; rel=&quot;nofollow&quot;&gt;superconductor&lt;/a&gt;, which if anything makes it a lot more interesting as a potential resource for a Mercurial colony. Of course, the fact that it is a gas at low pressure and human-livable temperatures is an immediate engineering challenge. But look, I'm a layman giving it a few minutes of creative thought and doing a quick search on the internet. My goal is only to point out that you do not know what kinds of material properties researchers may discover until you look, and I suspect that we haven't even spent our first million on Mercurial materials science yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, I'm certainly not buying the idea that sulfur just &quot;won't be capable of creating&quot; the right kind of crystal roster. We haven't done the science to find out yet! Some things, perhaps this thing even, may not have an easy solution. But other things may have extraordinarily awesome solutions. I mean, maybe a Mercurial colony would have to live deep under ground where they use their H&lt;sub&gt;2&lt;/sub&gt;S superconductive technology to operate at high enough efficiency to reduce the burden of harvesting solar energy at the surface. There is a lot of research to do before we know what the pivotal challenges will be. I don't think it's time yet to throw up our hands and say those materials cannot possibly work.&lt;/p&gt;&#xA;" OwnerUserId="945" LastEditorUserId="945" LastEditDate="2015-05-18T21:43:56.087" LastActivityDate="2015-05-18T21:43:56.087" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2601" PostTypeId="2" ParentId="2599" CreationDate="2013-10-29T21:30:53.237" Score="13" Body="&lt;p&gt;I can see four major factors that determine where a spacesuit can be used: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Temperature issues. The temperature on &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Titan&quot;&gt;Titan&lt;/a&gt; is much lower than on the Moon or Mars, so you'll need more insulation or heating. conversely, closer to the sun you need to reflect the sunlight more to prevent overheating.&lt;br&gt;&#xA;If you use the suit in a vacuum, you need to get rid of excess heat using radiation, or maybe evaporation if you can compensate for the water loss.&lt;br&gt;&#xA;If you use the suit inside an atmosphere, you get convection cooling/heating added to the mix. &lt;/li&gt;&#xA;&lt;li&gt;Pressure. Current space suits are made to function in a vacuum. The low atmospheric pressure of Mars probably won't be much of a problem, but Titan is another matter. Current space suits are designed to limit inflation, they have structures inside that keep the suit together. On Titan you could omit these. &lt;/li&gt;&#xA;&lt;li&gt;Corrosive chemicals in the atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Dust control. The moon has a significant dust problem - lunar dust being very fine-grained with a tendency to stick to everything. Martian dust is worse, as there are frequent dust-storms on the planet. Suits would need to be resistant to dust-infiltration, and be easy-to-clean. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2013-10-31T12:53:28.927" LastActivityDate="2013-10-31T12:53:28.927" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2602" PostTypeId="1" AcceptedAnswerId="2605" CreationDate="2013-10-29T23:10:52.863" Score="13" ViewCount="1125" Body="&lt;p&gt;Let's say we have a colony on Europa (or any other Jovian moon really). Jupiter exerts a lot of tidal forces on its satellites. Could this force be used to generate energy for our colonists?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="49" LastEditDate="2013-10-30T06:30:02.657" LastActivityDate="2021-01-06T20:48:09.243" Title="Could Jupiter's tidal forces be used to generate energy?" Tags="|colonization|planetary-science|jupiter|power|europa|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2603" PostTypeId="5" CreationDate="2013-10-29T23:28:35.240" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-29T23:28:35.240" LastActivityDate="2013-10-29T23:28:35.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2604" PostTypeId="4" CreationDate="2013-10-29T23:28:35.240" Score="0" Body="Questions regarding the path of a point directly below an object in space." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-10-30T02:28:27.380" LastActivityDate="2013-10-30T02:28:27.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2605" PostTypeId="2" ParentId="2602" CreationDate="2013-10-30T06:22:44.063" Score="8" Body="&lt;p&gt;We don't really know how the tidal energy that &lt;a href=&quot;https://en.wikipedia.org/wiki/Europa_(moon)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Europa&lt;/a&gt; gets from its orbit around Jupiter translates exactly to its surface or subsurface activity, so it would be hard to speculate on any possible power plant types that we might employ to produce electrical power, but Europa's orbit is kept elliptical (&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_eccentricity&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital eccentricity&lt;/a&gt; of 0.0094) due to the resonance with the other &lt;a href=&quot;https://en.wikipedia.org/wiki/Galilean_moons&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galilean moons&lt;/a&gt;, especially the &lt;a href=&quot;https://en.wikipedia.org/wiki/Io_(moon)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Io&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Ganymede_(moon)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ganymede&lt;/a&gt; that are in lower and higher orbits than Europa is, respectively:&lt;/p&gt;&#xA;&lt;p&gt;                                        &lt;img src=&quot;https://i.stack.imgur.com/dfVrC.gif&quot; alt=&quot;Io-Europa-Ganymede Laplace resonance&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;   &lt;sup&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Laplace_resonance#The_Laplace_resonance&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laplace resonance&lt;/a&gt; exhibited by three of the Galilean moons. The ratios in the figure are of orbital periods. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Galilean_moon_Laplace_resonance_animation.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;This translates into an enormous gravitational pull that is not synchronised with Europa's surface because of the &lt;a href=&quot;https://astronomy.stackexchange.com/questions/16/why-is-only-one-side-of-the-moon-visible-from-earth/22#22&quot;&gt;libration effects&lt;/a&gt;, even though Europa is &lt;a href=&quot;https://astronomy.stackexchange.com/questions/16/why-is-only-one-side-of-the-moon-visible-from-earth/22#22&quot;&gt;tidally locked&lt;/a&gt; to Jupiter. If this is hard to imagine, this is, for example, how Lunar librations look like:&lt;/p&gt;&#xA;&lt;p&gt;                         &lt;a href=&quot;https://web.archive.org/web/20201007012740/https://upload.wikimedia.org/wikipedia/commons/b/ba/Lunar_libration_with_phase_Oct_2007_450px.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://web.archive.org/web/20201007012740/https://upload.wikimedia.org/wikipedia/commons/b/ba/Lunar_libration_with_phase_Oct_2007_450px.gif&quot; alt=&quot;Tidal locking of the Moon with the Earth&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;                         &lt;sup&gt;Lunar librations in latitude and longitude over a period of one month (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase_Oct_2007_450px.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;https://en.wikipedia.org/wiki/Laplace_resonance#The_Laplace_resonance&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laplace resonance&lt;/a&gt; of the Io-Europa-Ganymede system of Jovian moons (their relation locking is described by &lt;span class=&quot;math-container&quot;&gt;$\Phi_L = \lambda_{Io} - 3\lambda_{Eu} + 2\lambda_{Ga} = 180°$&lt;/span&gt;, where &lt;span class=&quot;math-container&quot;&gt;$\lambda$&lt;/span&gt; are &lt;a href=&quot;https://en.wikipedia.org/wiki/Mean_longitude&quot; rel=&quot;nofollow noreferrer&quot;&gt;mean longitudes&lt;/a&gt; of the moons), on top of a small Europa's orbital inclination with respect to the Jovian equatorial plane (0.470°) results in also relatively small orbital eccentricity, but enough to result in tidal bulges, tidal dragging and torque (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_locking#Mechanism&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page on tidal locking mechanisms&lt;/a&gt; for description) that translate into thermal energy and keep Europa a geologically active body.&lt;/p&gt;&#xA;&lt;p&gt;Observations of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo spacecraft&lt;/a&gt; have proven that &lt;a href=&quot;https://space.stackexchange.com/questions/2226/what-causes-the-cracks-on-europa-to-form&quot;&gt;Europa's surface forms fresh cracks and sustains a tenuous ionosphere&lt;/a&gt; that needs to be constantly replenished not to be blown away by radiation pressure, so we know for sure that Europa has dynamic processes that might result in usable, easily convertible into electrical power geothermal activity, perhaps in a form of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrothermal_vent&quot; rel=&quot;nofollow noreferrer&quot;&gt;seafloor hydrothermal vents&lt;/a&gt; like some theories suggest or even periodic and stable surface geysers (perhaps like the renowned Yellowstone &lt;a href=&quot;https://en.wikipedia.org/wiki/Old_Faithful_Geyser&quot; rel=&quot;nofollow noreferrer&quot;&gt;Old Faithful Geyser&lt;/a&gt;) like the &lt;a href=&quot;https://web.archive.org/web/20160305043556/http://www.examiner.com/article/jupiter-s-moon-europa-scientists-believe-alien-life-possible&quot; rel=&quot;nofollow noreferrer&quot;&gt;presence of surface magnesium&lt;/a&gt; seems to suggest:&lt;/p&gt;&#xA;&lt;p&gt;       &lt;img src=&quot;https://i.stack.imgur.com/d14hQ.jpg&quot; alt=&quot;Europa's surface activity&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;         Jupiter’s moon Europa has an ocean under its frozen surface. The ocean is thought to be around 62 miles deep and is likely&lt;br /&gt;         made up of &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnesium_chloride&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnesium chloride&lt;/a&gt;, which leaks to the surface of the moon. (Source: &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?release=2013-082&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL-Caltech&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The finding, based on some of the first data of its kind since NASA's&#xA;Galileo mission (1989–2003) to study Jupiter and its moons, suggests&#xA;that there is a chemical exchange between the ocean and surface,&#xA;making the ocean a richer chemical environment, and implies that&#xA;learning more about the ocean could be as simple as analyzing the&#xA;moon's surface.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;&amp;quot;We now have evidence that Europa's ocean is not isolated—that the&#xA;ocean and the surface talk to each other and exchange chemicals,&amp;quot;&lt;/em&gt; says Brown, professor of planetary astronomy at Caltech.&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Source: &lt;a href=&quot;https://web.archive.org/web/20181129032040/http://www.caltech.edu:80/content/window-europas-ocean-lies-right-surface&quot; rel=&quot;nofollow noreferrer&quot;&gt;California Institute of Technology&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;If the tidal energy would be easier to convert into electrical power with the use of &lt;a href=&quot;https://en.wikipedia.org/wiki/Geothermal_electricity&quot; rel=&quot;nofollow noreferrer&quot;&gt;geothermal power plants&lt;/a&gt;, tapping into the kinetic energy of subsurface waves with &lt;a href=&quot;https://en.wikipedia.org/wiki/Wave_power&quot; rel=&quot;nofollow noreferrer&quot;&gt;wave power plants&lt;/a&gt;, or if it sustains constant currents that could power &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydroelectricity&quot; rel=&quot;nofollow noreferrer&quot;&gt;hydroelectric power plants&lt;/a&gt;, that remains to be established though. Perhaps the planned ESA's (European Space Agency) &lt;a href=&quot;https://sci.esa.int/web/juice/&quot; rel=&quot;nofollow noreferrer&quot;&gt;JUICE&lt;/a&gt; (JUpiter ICy moons Explorer) mission will give some hints on that.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="10273" LastEditDate="2021-01-06T20:48:09.243" LastActivityDate="2021-01-06T20:48:09.243" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="2606" PostTypeId="2" ParentId="2532" CreationDate="2013-10-30T07:34:51.713" Score="20" Body="&lt;p&gt;Not a complete answer (update in the footer!) because I wasn't able to find any official explanation, but I did find enough that I thought you might be interested in my findings so far. It also gets more and more intriguing, check this out:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KhY78.jpg&quot; alt=&quot;Ariane 4 launch&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Ariane 44L launching &lt;a href=&quot;http://www.army-technology.com/projects/sicral/&quot; rel=&quot;noreferrer&quot;&gt;SICRAL 1&lt;/a&gt; and &lt;a href=&quot;http://www.army-technology.com/projects/skynet/&quot; rel=&quot;noreferrer&quot;&gt;Skynet 4F&lt;/a&gt; military satellites on its &lt;a href=&quot;http://spaceflightnow.com/ariane/v139/&quot; rel=&quot;noreferrer&quot;&gt;V139 mission&lt;/a&gt; (Source: &lt;a href=&quot;http://www.capcomespace.net/dossiers/espace_europeen/ariane/1996-2001/2001.htm&quot; rel=&quot;noreferrer&quot;&gt;CapComEspace&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this second stage shroud must be some sort of thermal insulation that is only added to the launcher on fueling up the stage tanks (it uses nitrogen tetroxide oxidizer and UH25 that is a mixture of UDMH with 25% hydrazine as fuel) and this shroud is collapsed from the launcher a few seconds into flight. Here is &lt;a href=&quot;http://www.esa.int/esatv/Videos/1995/04/Ariane_V72_Video_Transmission_-_ERS-2&quot; rel=&quot;noreferrer&quot;&gt;a video of Ariane V72 launch sequence&lt;/a&gt; (relevant part starts at 30 minutes into the video) from ESA pages that shows this process in motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also worth mentioning that this &lt;em&gt;shroud&lt;/em&gt; was used on all &lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_%28rocket_family%29&quot; rel=&quot;noreferrer&quot;&gt;Ariane rocket family&lt;/a&gt; from 1 to 4, since they all used same second stage bipropellant that was precooled before the tanks are fueled:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Second stages of Ariane are nitrogen tetroxide and unsymmetrical&#xA;  dimethyl hydrazine (UDMH), which, unlike  cryogenic fuel, do not&#xA;  require thermal insulation. But  because the propellants occupy a&#xA;  larger tank volume when  they are warm, the nitrogen tetroxide and&#xA;  UDMH are precooled before being loaded. First and second-stage&#xA;  fuelling  takes place within the tower, where the air-conditioning &#xA;  helps to keep the propellants cool.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;Source: &lt;a href=&quot;http://www.flightglobal.com/FlightPDFArchive/1979/1979%20-%200488.PDF&quot; rel=&quot;noreferrer&quot;&gt;Archived copy of FLIGHT International magazine from 17 February 1979&lt;/a&gt; (PDF)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So this must be something similar to what is used on Minotaur 1 and referred to as &lt;em&gt;the banana&lt;/em&gt; because of the color and since it is peeled away in sections during launch and while the rocket is clearing the launch tower:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/xMORv.jpg&quot; alt=&quot;Minotaur I NROL-66 launch&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_I&quot; rel=&quot;noreferrer&quot;&gt;Minotaur I&lt;/a&gt; rocket during launch on its &lt;a href=&quot;https://en.wikipedia.org/wiki/USA-225&quot; rel=&quot;noreferrer&quot;&gt;NROL-66&lt;/a&gt; mission showing the first stage &lt;em&gt;banana&lt;/em&gt; (Source: &lt;a href=&quot;http://spaceflightnow.com/minotaur/nrol66/gallery/&quot; rel=&quot;noreferrer&quot;&gt;SpaceflightNow&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are a few other things I was able to learn about the mysterious Ariane 4 second stage insulation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Used second stage N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;/UDMH propellant was precooled on all Ariane 1-4 launchers&lt;/li&gt;&#xA;&lt;li&gt;The shroud is not yet attached to the second stage once it leaves the assembly tower before the launch (based on photographic evidence)&lt;/li&gt;&#xA;&lt;li&gt;The &lt;em&gt;shroud&lt;/em&gt; seems to be attached in tiles and connected by a release cable during rocket's clearing of the launch tower&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The material these tiles are made of remains a mystery to me, but they do appear light and the front face is highly reflective so I'd imagine some low-density &lt;a href=&quot;https://en.wikipedia.org/wiki/EPDM_rubber&quot; rel=&quot;noreferrer&quot;&gt;EPDM&lt;/a&gt;-based thermal insulation covered with layers of aluminum foil and lightly framed might be used. This all seems to be related to second stage propellant stability and performance, keeping it thermally stable, preventing water vapor condensation and similar potential problems before the launch in the hot and humid climate of the Kourou, &lt;a href=&quot;https://en.wikipedia.org/wiki/Guiana_Space_Centre&quot; rel=&quot;noreferrer&quot;&gt;Guiana Space Centre&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's one more photo from a daytime launch of Ariane 4 (configuration 40, no stage 0 SRBs) on Flight 75 (Source: &lt;a href=&quot;http://www.arianespace.com/news-image_library/ariane_heritage.asp&quot; rel=&quot;noreferrer&quot;&gt;Arianespace Ariane heritage multimedia library&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/HDqby.jpg&quot; alt=&quot;Liftoff of Ariane 40 on Flight 75&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, I wanted to post my findings here to hopefully help someone find official explanation behind this &lt;em&gt;shroud&lt;/em&gt; (my French-fu is rusty and weak), what it's made of, when exactly is it attached to the second stage, how it is released (the use of the pull-cable is my educated guess based on photographs) and if it was indeed used on all launches of the Ariane family of rockets from 1 to 4.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: Thanks to @Hobbes and his superior French-fu skills we now have a confirmation (machine translation, a bit rough around the edges):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Before takeoff, while waiting on the launch platform, the tanks of the&#xA;  second stage are protected by a thermal cover made of polystyrene,&#xA;  ventilated with cold air, which limits the heat exchange between the&#xA;  propellant and the external environment. The evaporated N2O4 is above&#xA;  20 °C. This thermal cover is dropped off during the launch, pulled by&#xA;  cables attached to the umbilical tower.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;Source: &lt;a href=&quot;http://www.capcomespace.net/dossiers/espace_europeen/ariane/ariane1/caracteristiques.htm&quot; rel=&quot;noreferrer&quot;&gt;CapComEspace.net&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there we have it, it is indeed a thermal shroud, or a cover made of light insulator (polystyrene) that must be covered by a reflective foil or paint on the outer side (all the photographs show it in colors of the foreground it reflects), and serves as a protection against environmental heat and keeping the precooled second stage propellants nicely chilled to prevent in-tank thermal expansion of the oxidizer (N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; starts evaporating above 20°C). The tiles are indeed collapsed off the launcher by a cable that is attached to the umbilical tower on rocket launch. As a side note and perhaps another interesting information that isn't mentioned there, that the umbilical lines are released by explosive charges.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-10-30T17:10:11.530" LastActivityDate="2013-10-30T17:10:11.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2607" PostTypeId="1" AcceptedAnswerId="2608" CreationDate="2013-10-30T10:47:34.633" Score="10" ViewCount="778" Body="&lt;p&gt;The problem with Mars is that it's surface is not very rich in minerals important to plants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a significant lack of water on Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean that the only viable solution to plant growth on Mars will be to melt the ice, mix with minerals brought over from Earth and then directly distributed to the plants through hydroponics? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This itself has the issue of requiring the minerals to be brought in externally, and isn't very sustainable in the long run.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a more sustainable method that could be investigated to allow initial plant growth for a sustainable colony on Mars?&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastActivityDate="2013-10-30T14:51:32.047" Title="Is hydroponics the only viable solution to plant growth on Mars?" Tags="|mars|colonization|plants|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2608" PostTypeId="2" ParentId="2607" CreationDate="2013-10-30T12:19:17.590" Score="6" Body="&lt;p&gt;From &quot;The Case for Mars&quot;, using Viking data, it can be shown that the soil is similar, although we don't know about the nitrogen content. Iron in particular is higher, Potassium is lower. They also contain smectite clays. Bottom line is, there are probably parts of Mars that have enough minerals to grow plants, and those things that are lacking could probably be found in other places.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The proposal from &quot;The Case for Mars&quot; is to simply build greenhouses, with the frame of them buried deep under ground, and use a CO2 rich, but not excessively rich, environment to grow the plants. If any minerals needs to be added, they can be found somewhere on Mars, so they should be found there.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-10-30T14:51:32.047" LastActivityDate="2013-10-30T14:51:32.047" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2609" PostTypeId="1" AcceptedAnswerId="2612" CreationDate="2013-10-30T15:21:55.697" Score="9" ViewCount="852" Body="&lt;p&gt;In the early 90's I remember seeing a program where they were discussion new designs in space suits that would be more flexible and allow for less restrictive movements of the occupant.  These suits were not pressurized at the body like the suit commonly used today but rather formed to the body much like a wet suit but had a pressurized helmet that was actually smaller than the helmets used today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What advances have there been?  Have any of the advanced designs been tested in space yet?&lt;/p&gt;&#xA;" OwnerUserId="123" LastEditorDisplayName="user12" LastEditDate="2013-10-30T15:29:16.230" LastActivityDate="2013-10-31T14:41:13.117" Title="What progress has there been in the design of modern spacesuits?" Tags="|technology|spacesuits|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2610" PostTypeId="2" ParentId="2609" CreationDate="2013-10-30T16:00:20.290" Score="4" Body="&lt;p&gt;I believe you're referring to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_activity_suit&quot; rel=&quot;nofollow&quot;&gt;Space Activity Suit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's been a number of versions and approaches to the problem over the years, but they all are based on mechanical pressure (i.e. compression through elastic fabrics) to help support the skin against vacuum. The trick is to engineer them to give sufficient support while not constricting movement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most recent iteration is the &lt;a href=&quot;http://www.nasa.gov/offices/oce/appel/ask/issues/45/45s_building_future_spacesuit.html&quot; rel=&quot;nofollow&quot;&gt;BioSuit&lt;/a&gt;, being developed by Dava Newman at MIT. Her design incorporates a deeper study of biomechanics - knowing how the human body actually moves, and accurately building tension into the suit at the appropriate points. &lt;/p&gt;&#xA;" OwnerUserId="692" LastEditorUserId="49" LastEditDate="2013-10-31T14:37:17.537" LastActivityDate="2013-10-31T14:37:17.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2611" PostTypeId="1" AcceptedAnswerId="2634" CreationDate="2013-10-30T20:59:59.717" Score="16" ViewCount="3279" Body="&lt;p&gt;Past manned space missions teach us that the loss in gravity gradient has an adverse effect on the blood volume of astronauts. Our bodies are simply built to expect pressure gradient and in gravity, that pressure would be highest in our lower body and smaller in our upper body at least for the most part of the day (might not be exactly true during sleep). Problem is, that especially the higher than expected blood pressure in the head than the rest of the body sends to the body a signal that it has too much blood, or at least so it is described in &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2001/ast02aug_1/&quot; rel=&quot;noreferrer&quot;&gt;this NASA Science News article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Higher blood pressure in the head raises an alarm: The body has too&#xA;  much blood! Within two to three days of weightlessness, astronauts can&#xA;  lose as much as 22 percent of their blood volume as a result of that&#xA;  errant message. This change affects the heart, too. &lt;em&gt;&quot;If you have less&#xA;  blood,&quot;&lt;/em&gt; explains Dr. Victor Schneider, research medical officer for&#xA;  NASA headquarters, &lt;em&gt;&quot;then your heart doesn't need to pump as hard.&#xA;  It's going to atrophy.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This might result in potentially unwanted side effects, possibly more severe on astronauts' return to gravity (some of you might remember &lt;a href=&quot;https://en.wikipedia.org/wiki/Heidemarie_Stefanyshyn-Piper&quot; rel=&quot;noreferrer&quot;&gt;Heidemarie Stefanyshyn-Piper&lt;/a&gt; collapsing during the speech after returning from the &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-115&quot; rel=&quot;noreferrer&quot;&gt;STS-115 Atlantis mission&lt;/a&gt; in September 2006 - &lt;a href=&quot;https://www.youtube.com/watch?v=G2uSsgOBHDI&quot; rel=&quot;noreferrer&quot;&gt;YouTube video&lt;/a&gt;) than while in microgravity, and astronauts returning from missions in space are now carried and sat on chairs so they can adjust a bit before other duties come calling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/9Q46j.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Expedition 33 crew members sit in chairs outside the Soyuz Capsule just minutes after they landed. (Photo: &lt;a href=&quot;https://secure.flickr.com/photos/nasahqphoto/8203062371/in/set-72157632047078212/&quot; rel=&quot;noreferrer&quot;&gt;NASA HQ PHOTO&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;Each of the parameters have their own normal recovery time,&quot;&lt;/em&gt; says&#xA;  Schneider. Blood volume, for example, is typically restored within a&#xA;  few days. &lt;em&gt;&quot;Astronauts get thirsty when they come back,&quot;&lt;/em&gt; Schneider&#xA;  explains, &lt;em&gt;&quot;because their body says, you don't have enough blood in&#xA;  your blood vessels, and that causes the messengers to say, drink more.&#xA;  [Also, the body doesn't] urinate as much.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But while this readjustment to gravity period might last only a few days to restore the lost blood volume (it might last a lot longer to restore bone and muscle mass though), it still can not be easily dismissed and this brings me to my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How do astronauts nowadays fight blood volume loss? Do they take any medication to help them with the loss in pressure gradient while in space, on their return, or both? Do they do any exercises that would at least temporarily restore normal pressure gradient within the body? And have any astronauts reported longer lasting side effects or developed other cardiovascular atrophy that might take longer to recover?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-16T22:05:24.320" LastActivityDate="2018-12-12T20:53:36.253" Title="How do astronauts battle loss in blood volume in microgravity?" Tags="|microgravity|medical|humans|cardiovascular|pressure-gradient-loss|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2612" PostTypeId="2" ParentId="2609" CreationDate="2013-10-30T22:32:31.777" Score="12" Body="&lt;p&gt;The &quot;trick&quot; that @john3103 mentions in &lt;a href=&quot;https://space.stackexchange.com/a/2610/49&quot;&gt;his answer&lt;/a&gt;, that the suits have to be engineered to provide sufficient support, is actually pretty important, and belies some additional difficulties which are the primary reasons this type of suit hasn't yet been adopted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pretty much any spacesuit, whether mechanical-counter-pressure (MCP) or gas-pressurized, is required to provide uniform pressure across all surfaces of the body. While human skin is gas-tight and so will not dehydrate or shrivel in a vacuum (as many sci-fi authors incorrectly illustrate), it will swell due to the lack of external pressure, and this is painful, restricts movement and can be permanently damaging over a large enough area. To prevent this, the suit must provide counter pressure on every square inch of skin. Gas-pressure suits are basically a bag that holds a low-pressure atmosphere around the wearer (current US suit designs simulate an atmosphere at about 8000 feet, which when filled with pure oxygen provides a similar partial pressure of oxygen as you'd find at sea level), and gas, being a gas, will then expand uniformly to put equal pressure over all surfaces of its container, including the wearer's skin. As such, as long as the suit is airtight, the wearer is fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, an MCP suit must simulate this pressurized-gas environment by applying elastic tension over every surface of the body. While that alleviates the need to maintain a perfectly airtight membrane (which is easily damaged by anything from a tool to a toenail, rendering the entire suit inoperable until it's patched), thus dramatically increasing safety and reliability, it creates several additional engineering problems for an MCP suit, which a current gas-pressurized suit more or less avoids: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Each suit must be custom-fit to the wearer, and can only be worn by them.&lt;/strong&gt; That wasn't as big a deal with the two versions of the SAS, because at the time all spacesuits had to be custom-made to fit their wearer. Nowadays, NASA's EMU suits come in small, medium and large, and cover the 95th percentile of both male and female body types; Russia's ORLAN and Chinese knockoffs are one-size-fits-all with various adjustments possible. However, these suits are also \$12 million a pop; one of the BioSuit's big advantages is that the actual bodysuit can be constructed for a few thousand dollars, and the helmet-and-backpack system would be interchangeable between any two suits, so even though you're making more suits you still come out ahead.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;The suit, by design, must be skin-tight everywhere&lt;/strong&gt;. First, this makes getting into the suit more difficult; the wearer of the original SASes pretty much had to grease up from the neck down in order to squeeze into it, and both SAS designs required assistance from at least one other person, which would make suiting up quickly in an emergency a practical impossibility. EMUs have similar problems due to their bulk, and also require an assistant; an ORLAN can theoretically be donned by one person but still takes the better part of an hour. The BioSuit was designed to be donned by the wearer in less than ten minutes without assistance, and in situations where it's likely to be needed, the bodysuit is thin and breathable enough to be worn long-term, as if it were a pair of long johns. The helmet, backpack, gloves and boots are relatively quick to put on once the bodysuit is in place.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Not every square inch of the body can have the necessary mechanical counter pressure applied uniformly.&lt;/strong&gt; Such pressure is practically impossible to give to the surfaces on and in the head, and is notoriously difficult to provide where small joints are involved, namely hands and feet, and on the insides of &quot;tight corners&quot; like the armpits and groin. While the BioSuit solves the armpit/groin problem through its design, the current functional version still uses a pressurized &quot;fishbowl&quot; helmet and gas-pressurized gloves and boots, though alternate options like neoprene-foam gloves and boots are being considered.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Mechanical counter pressure must be carefully applied to maximize freedom of movement and minimize energy cost of movement.&lt;/strong&gt; These issues were the main problems with the first incarnation of the SAS; the thick neoprene construction meant that range of motion and energy cost were only slightly improved over the contemporary Mercury-era pressure suit, and the SAS was just as bulky. The second-generation SAS was better, but its multi-layer construction was still cumbersome, and the uniform flexibility of the material over large areas provided an increased cost of movement. The BioSuit was specifically designed to be superior in both these respects; the spandex is reinforced by elastic bands that traverse &quot;lines of non-extension&quot; on the body, so you aren't working against the primary source of mechanical tension when you move, and the bands in turn allow the thinner spandex to provide the needed pressure in a relatively thin layer.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Climate control can be a problem&lt;/strong&gt;. The original SAS, as mentioned, was not breathable (though not completely gas-tight), and so the suit had to be fitted with a liquid cooling system. The second-generation SAS and the BioSuit, both more or less breathable (the BioSuit more than the SAS), expose sweat to the vacuum, so it will evaporate more or less normally and with the same function as in air. However, that's the limit of climate control in such a suit; there's no air convection to either warm or cool the body, so you're relying on the body's natural temperature control mechanisms to perform in a vacuum the same way they do in an atmosphere. There is evidence from tests of the second-generation SAS that this won't be a major problem; the BioSuit hasn't gotten that far yet.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Thin and form fitting is better for mobility, but poses modesty concerns.&lt;/strong&gt; As wearers of a wetsuit or leotard can attest, they are among the least flattering things that clothing designers have ever produced. They also do not leave much to the imagination, either male or female (the SAS cheated a bit by using strategically-placed polyurethane foam to transfer pressure from the suit to the groin, which also reduced the... definition in that area). While such concerns are ultimately rather petty in an environment like space, in the interests of crew morale, they're not to be ignored. The BioSuit is thin enough to wear under a standard-issue NASA jumpsuit, retaining a modest outward appearance, at least while in the spacecraft. The only full suit in existence is for the program's director, Professor Dava Newman, so we don't yet know what a male BioSuit will look like.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So in summary, MCP suits have interested NASA since the dawn of the U.S. manned space program; the safety issues of a gas-pressure suit and subsequent costs and procedures inherent in avoiding them have long been known. However, until recently, the best options available had their own significant drawbacks which made the gas suits the better option. I personally have high hopes for the Bio-Suit; it's a dramatic leap forward in wearability, and even with the need to custom-fit them it should be far cheaper overall to have a BioSuit (or two) for each individual astronaut than to buy and maintain the current generation of EMU suits.&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-10-31T14:41:13.117" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2613" PostTypeId="1" CreationDate="2013-10-31T10:39:17.337" Score="11" ViewCount="193" Body="&lt;p&gt;Has there been any practical investigations into potential resources to make colonisation as waste less as possible? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously I don't expect colonisers to be throwing crisp packets on the floor or tucking empty Starbucks cups into small crevasses in the structures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been investigation into the biodegradability of potential colonisation materials to ensure minimum corruption or contamination of the target landing area?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example in the resources used to build, or package, the materials used for construction or delivery of said materials. I presume this would be important because it would be a bad start to land on a planet and have to start a rubbish dump on day 1!&lt;/p&gt;&#xA;" OwnerDisplayName="user106" LastActivityDate="2013-11-01T18:00:05.810" Title="What considerations have been made for waste produced during colonisation?" Tags="|colonization|waste|sustainability|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2614" PostTypeId="1" AcceptedAnswerId="2617" CreationDate="2013-10-31T16:14:00.657" Score="9" ViewCount="1223" Body="&lt;p&gt;Today, when discussing life in zero gravity this idea popped into my head. At first I was curious to know if any experiments have been done with birds in low/zero gravity environments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anyone ever attempted/discussed bringing a bird onto one of those zero gravity flights or even brought one to the ISS?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My next thought was that of a man-powerd winged flight (something akin to the Icarus folklore but also open to the wackey machines in Red-bull competition!). Are there any current or proposed plans/ideas for anything like this?&lt;/p&gt;&#xA;" OwnerUserId="703" LastActivityDate="2013-11-07T03:29:28.220" Title="Is winged flight possible in low/zero G environment?" Tags="|crewed-spaceflight|microgravity|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2616" PostTypeId="2" ParentId="2456" CreationDate="2013-10-31T19:11:18.483" Score="5" Body="&lt;p&gt;There are many reasons for that...&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;One is the atmospheric resistance which is very obvious&lt;/li&gt;&#xA;&lt;li&gt;Expansion of gases passed by the nozzle exit is very important in deciding the thrust produced. In normal atmosphere, pressure of the gas at the exit is negative gauge and hence nozzle is under-expanded which produces minimum thrust. In vacuum, it is over-expanded which produces higher thrust.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="976" LastEditorUserId="105" LastEditDate="2013-11-01T00:03:03.707" LastActivityDate="2013-11-01T00:03:03.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2617" PostTypeId="2" ParentId="2614" CreationDate="2013-10-31T20:49:58.170" Score="14" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Has anyone ever attempted/discussed bringing a bird onto one of those&#xA;  zero gravity flights or even brought one to the ISS?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Not ISS. Mir and Salyut.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a long sequence of experiments on &lt;a href=&quot;http://en.wikipedia.org/wiki/Japanese_Quail&quot; rel=&quot;noreferrer&quot;&gt;Japanese Quails&lt;/a&gt; on Salyut-6 and Mir stations. It is described in a lot of details in the very dramatic &lt;a href=&quot;http://finchwench.wordpress.com/2011/09/06/cosmoquails/&quot; rel=&quot;noreferrer&quot;&gt;Birds in Space II: Quails in the Cosmos&lt;/a&gt; story.&#xA;Also a video search for &lt;a href=&quot;http://www.youtube.com/watch?v=hEPv0lrc10w&quot; rel=&quot;noreferrer&quot;&gt;Quails in space&lt;/a&gt; gives several &lt;a href=&quot;http://www.youtube.com/watch?v=U0NS9bmbmOk&quot; rel=&quot;noreferrer&quot;&gt;amazing videos&lt;/a&gt;. You will have to judge for yourself if that can be qualified as &lt;em&gt;winged&lt;/em&gt; flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early days, US DoD experimented with &lt;a href=&quot;http://www.youtube.com/watch?v=hY3RmKmEMsI&quot; rel=&quot;noreferrer&quot;&gt;Pigeons in weightlessness&lt;/a&gt;. On that video you can see that pigeons definitely utilize their wings for powerfull motion, but completely disoriented.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think if there ever were &lt;strong&gt;adult&lt;/strong&gt; birds in zero-G long enough to adapt a little to their condition, it should have been four adult quails delivered to Mir on board  Soyuz TM-10. But I am not sure they have ever taken off their costumes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/s4TKC.jpg&quot; alt=&quot;adult&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user12" LastEditDate="2013-11-01T13:29:42.733" LastActivityDate="2013-11-01T13:29:42.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2618" PostTypeId="1" AcceptedAnswerId="2623" CreationDate="2013-11-01T04:02:41.433" Score="9" ViewCount="558" Body="&lt;p&gt;During the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot;&gt;Apollo-Soyuz test program&lt;/a&gt;, the Russians and Americans worked together on missions - and one big objective was to dock Apollo and Soyuz craft together. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Afterward, however, there seem to be very few, if any, instances of two spacecraft docking directly (not through a space station). Are there any occurrences of this after Apollo-Soyuz?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-01T04:10:12.090" LastActivityDate="2014-01-22T18:03:37.340" Title="Have two individual ships been docked directly since Apollo-Soyuz?" Tags="|apollo-program|docking|soyuz-spacecraft|apollo-soyuz|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2619" PostTypeId="5" CreationDate="2013-11-01T04:05:38.663" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-01T04:05:38.663" LastActivityDate="2013-11-01T04:05:38.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2620" PostTypeId="4" CreationDate="2013-11-01T04:05:38.663" Score="0" Body="Questions regarding the Apollo-Soyuz Test Program, a joint effort between Russia and the United States." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-01T04:05:38.663" LastActivityDate="2013-11-01T04:05:38.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2621" PostTypeId="1" AcceptedAnswerId="2622" CreationDate="2013-11-01T07:54:50.160" Score="35" ViewCount="6981" Body="&lt;p&gt;In a recent question, &lt;a href=&quot;https://space.stackexchange.com/questions/2599/could-an-eva-suit-meant-for-mars-be-used-for-eva-on-another-celestial-or-vice-v&quot;&gt;'Could an EVA Suit meant for Mars be used for EVA on another celestial, or vice-versa?'&lt;/a&gt;, there was some discussion in the comments about the effects of a dust storm on Mars versus sandstorms on Earth.&lt;br&gt;&#xA;Mars' atmosphere is much thinner, its gravity is lower but wind speeds can be quite high. How do these factors combine? Would a Mars dust storm be abrasive? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-12-02T20:58:34.873" Title="What would it feel like to be in a Martian dust storm?" Tags="|mars|colonization|environment|dust|weather|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2622" PostTypeId="2" ParentId="2621" CreationDate="2013-11-01T10:08:24.633" Score="27" Body="&lt;p&gt;According to the Mars-One webpage &lt;a href=&quot;http://www.mars-one.com/en/faq-en/22-faq-mission-features/250-how-dangerous-are-dust-storms-and-sand&quot;&gt;What are the risks of dust and sand on Mars?&lt;/a&gt;, that even though the wind speeds of the dust storms can be quite high - hurricane force, due to the far thinner atmosphere (1% of Earth), it would feel like a slight breeze.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is dusty, with dust storms that can cover the planet (rarely), and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Airborne dust on Mars is about as fine as cigarette smoke. Particle abrasion will be limited due to the small size of the particles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is another side effect of a dust storm, particularly, a large one, is that, according to the NASA page &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2001/ast11oct_2/&quot;&gt;The Perfect Dust Storm Strikes Mars&lt;/a&gt; causes significant atmospheric heating (up to 80F) - so thermal effects need to be considered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fine dust in the storms will pose another problem according to &lt;a href=&quot;http://www.space.com/4051-future-mars-explorers-face-dusty-challenges.html&quot;&gt;Future Mars Explorers Face Dusty Challenges&lt;/a&gt; (Mosher, 2007), is that it would electrostatically attach to everything, with&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;If you walk through, pick up or simply touch the dust, it would gather charge and stick to you. We've already seen this on the rovers' wheels,&quot; said Geoffrey Landis, a physicist with the NASA Glenn Research Center in Cleveland, Ohio. &quot;Things get even more interesting when winds come by and separate the charge.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and that, as the air is dry on Mars, the charge can build and potentially arc, damaging sensitive electronics if not properly shielded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But more information and studies would be needed to fully determine the effects of the Martian dust.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-11-01T10:08:24.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2623" PostTypeId="2" ParentId="2618" CreationDate="2013-11-01T11:19:29.437" Score="6" Body="&lt;p&gt;Gemini tested docking with an Agena Target Vehicle, and Soyuz earlier had demonstrated docking as each program needed to prove docking for future operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The docking practice/technology was needed for the eventual moon mission, as the Lunar Module and Command module would dock, soon after TLI, to remove it from atop the third stage.  Then of course once in lunar orbit, and after the lunar surface mission, the LEM would dock back with the Command Module to transfer crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;China being the next on the scene decided to use a docking target as well, just a bigger one, that could also act as a basic space station, Tiadong-1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the simple answer is no.  Turns out, it was not really needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ATSP docking was entirely a political stunt, which in hindsight did prove to be of technical value, as the module Apollo brought with it to dock provided information for the basis of the docking module added to the Spekctr module on Mir for use by the space shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next possible opportunity might be the proposed asteroid missions using two docked Orion that has been vaguely suggested.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-11-03T19:48:03.273" LastActivityDate="2013-11-03T19:48:03.273" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2624" PostTypeId="2" ParentId="2621" CreationDate="2013-11-01T11:37:55.867" Score="32" Body="&lt;p&gt;Part of this is also discussed in the &lt;a href=&quot;https://space.stackexchange.com/q/2134/49&quot;&gt;How can Mars have dust storms with such a thin atmosphere?&lt;/a&gt; thread, but yes it would be &lt;em&gt;abrasive&lt;/em&gt;, and direct evidence for this can be appreciated in all kinds of aeolian formations on the Martian surface, like the &lt;a href=&quot;https://space.stackexchange.com/q/2323/49&quot;&gt;dunes of the Nili Patera&lt;/a&gt; region or the ones in the &lt;a href=&quot;http://hirise.lpl.arizona.edu/ESP_027864_2295&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lyot Crater&lt;/a&gt;, just to name a few of the most well know examples:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/WgtyK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;High detail HiRISE image of the Lyot Crater dunes on the move due to aeolian processes (Source: &lt;a href=&quot;http://hirise.lpl.arizona.edu/ESP_027864_2295&quot; rel=&quot;nofollow noreferrer&quot;&gt;HiRISE / University of Arizona&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, while the atmospheric pressure and gravity are lower than on the surface of the Earth, and dust particles being on average lighter because of the lack of humidity preventing them from sticking grain to grain and forming larger dust particles, they would also be (for this same reason) finer grain and sharper. Not as sharp as the regolith dust on the surface of the Moon, since there is obviously still weather on the surface of Mars that would help &lt;em&gt;polish&lt;/em&gt; these grains to rounder spheres, but still a lot sharper than the dust we mostly deal with on Earth. Because of the lack of humidity, the size of these dust particles would be as small as the size of crystalline structures of various rock salts, and those tend to be fairly sharp. On top of that, wind gusts of 500 to 600 km/hr (or 300-375 miles/hr) have been recorded by &lt;a href=&quot;https://en.wikipedia.org/wiki/Mariner_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mariner 9&lt;/a&gt; and later orbited spacecraft, probes, landers and rovers, with typical wind speeds in the Martian atmosphere exceeding 200 km/hr (or 125 miles/hr).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while you'd think that Martian dust would have less kinetic potential due to it being smaller, finer grain and the smaller gravity making them additionally lighter, the dust storms can be quite &lt;em&gt;abrasive&lt;/em&gt; even if it would feel like you're in the middle of a mild breeze with merely visibility sharply decreasing. Some of this dust could also have corrosive properties in contact with different materials the space suit would be made of, with the dust being of all kinds of chemical compositions, originating either from Martian own bedrock erosion, silicate minerals and other dust of magmatic origins from its calderas, or even &lt;em&gt;imported&lt;/em&gt; to Martian surface by impacted asteroids. For example, the Phoenix lander has found &lt;a href=&quot;http://www.newscientist.com/article/dn14474-phoenix-soil-results-released-after-days-of-speculation.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;evidence of perchlorate&lt;/a&gt;, but other, more corrosive chemicals are not excluded with such low water vapor content of Martian atmosphere. And &lt;a href=&quot;http://www.space.com/7102-lightning-detected-mars.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Martian storms also produce lightnings&lt;/a&gt;, another hazard that means they would be best avoided, and perhaps needless to say, visibility would be near zero, so even easier to lose footing or worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On top of that, the dust would tend to cling on the space suit (or perhaps better said environmental suit) not unlike the &lt;a href=&quot;https://space.stackexchange.com/q/1483/49&quot;&gt;Lunar dust did on Apollo missions&lt;/a&gt; due to it being electrostatically charged or even magnetic (also discussed &lt;a href=&quot;https://space.stackexchange.com/a/2301/49&quot;&gt;here&lt;/a&gt;), and its sharpness could lead to it lodging deep into the suit's fabric and causing &lt;a href=&quot;https://en.wikipedia.org/wiki/Triboelectric_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;triboelectric charge&lt;/a&gt; (small electric charge carrying particles rubbing against each other, for example the iron oxide, or rust, that can be found in top layers of Martian soil) or tearing the fabric by increasing suit's local friction, so pioneers on Mars will have to find ways to mitigate this dust problem. Using many layers and threading could help mitigate this and increase their durability, but with increased thickness and with it its surface, all these problems increase too, so any environmental suits used on Mars will likely be custom fit and skin-tight, perhaps like this &lt;a href=&quot;http://www.nasa.gov/offices/oce/appel/ask/issues/45/45s_building_future_spacesuit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;BioSuit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional hazard with longer exposure to Martian environment comes on removing the suit after entering a protected environment, with the tiny and sharp &lt;a href=&quot;http://www.newscientist.com/article/dn23505-toxic-mars-dust-could-hamper-planned-human-missions.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;silicate dust posing health risk to humans&lt;/a&gt;, if it enters their lungs, causing &lt;a href=&quot;https://en.wikipedia.org/wiki/Silicosis&quot; rel=&quot;nofollow noreferrer&quot;&gt;silicosis&lt;/a&gt;, among other health problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So &lt;strong&gt;TL;DR&lt;/strong&gt; - Martian dust storms would be best avoided, and they're hardly harmless even if their force isn't such to throw you off your feet if you found yourself in the middle of it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-12-02T20:58:34.873" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2625" PostTypeId="1" CreationDate="2013-11-01T16:41:47.947" Score="6" ViewCount="490" Body="&lt;p&gt;It seems like the greatest challenge of a colony on the moon is the fact that the temperature between night and day is so different. I would assume that this difference would not be as great near the poles. Is the thermal situation at the poles of the moon such that a colony there would have manageable thermal swings? What would the thermal swings be for a base located at the poles? &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-01T19:57:34.120" Title="Are the lunar poles easier to manage thermally than the equator?" Tags="|the-moon|colonization|thermal|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2626" PostTypeId="2" ParentId="2613" CreationDate="2013-11-01T18:00:05.810" Score="2" Body="&lt;p&gt;I don't expect there's been 'investigation into the biodegradability of potential colonisation materials' because I haven't seen serious colonisation plans yet. There's a lot of research going on into biodegradability, but that's all for use on Earth. Every place you could colonise has its own environment, so you'd have to know where the colony will be to do any useful research.&lt;br&gt;&#xA;Also, biodegradation isn't the only way to reduce environmental impact. Reuse is another, and probably much easier to achieve for many of the materials we will need for colonisation (metals, for instance).&lt;br&gt;&#xA;Biodegradation depends on there being a biosphere, and we're a long way away from being able to create one on e.g. Mars. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-11-01T18:00:05.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2628" PostTypeId="5" CreationDate="2013-11-01T19:33:19.863" Score="0" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Comet%E2%80%8E&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A comet is an icy small Solar System body that, when passing close to the Sun, heats up and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If your question is to do with origin, composition, period, decomposition of a comet &lt;strong&gt;use this tag!!&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-11-01T19:35:20.067" LastActivityDate="2013-11-01T19:35:20.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2629" PostTypeId="4" CreationDate="2013-11-01T19:33:19.863" Score="0" Body="Questions to do with celestial comets." OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-11-01T19:35:22.677" LastActivityDate="2013-11-01T19:35:22.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2630" PostTypeId="2" ParentId="2625" CreationDate="2013-11-01T19:42:42.797" Score="4" Body="&lt;p&gt;Yes, definitely. The Lunar poles are actually so little exposed to direct sunlight that the Solar terminator barely has any meaning there reaching near zero speed with either constant day or constant night (imagine really long shadows with a shallow angle sunlight). In case of the Moon, this is perhaps most significant because that could theoretically sustain water ice deposits close to surface that wouldn't be immediately ionized into hydrogen and oxygen and evaporate. Quoting myself from another &lt;a href=&quot;https://astronomy.stackexchange.com/a/454/13&quot;&gt;answer on Astronomy.SE&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The most likely place where surface or near subsurface water could&#xA;  exist on the Moon and be suitable for mass extraction are its polar,&#xA;  permanently dark regions. Indeed, the ISRO (Indian Space Research&#xA;  Organisation) Chandrayaan-1 spacecraft has detected evidence for water&#xA;  locked in surface lunar regolith minerals in lunar south polar region,&#xA;  water that likely originates from asteroid and comet impacts embedding&#xA;  it deep within the lunar core and released as magmatic water closer to&#xA;  the surface. Any free-form water in other regions of the Moon that are&#xA;  exposed to sunlight and Solar radiation would sublimate to its gas&#xA;  form directly and with ionisation lose hydrogen atoms, so while&#xA;  hydrogen and oxygen atoms might still be present to some extent&#xA;  embedded into the surface layer minerals, extraction would likely be&#xA;  too laborious there.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/cD6iy.jpg&quot; alt=&quot;Chandrayaan-1 Lunar South Pole region with traces of surface water ice&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Chandrayaan-1 Lunar South Pole region with traces of surface water ice&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, Lunar poles would be both thermally more stable regions, and also have reacher deposits of water in pockets immediately below the surface where they'd be easier to extract from, so would likely be better suited for a permanent Lunar base as such. We would certainly lose the opportunity to mine for Helium-3 as a power source, since it would be mostly embedded into Lunar regolith closer to the Moon's equator that faces the Sun more directly, but as far as thermal stability goes, that would be my choice too.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2013-11-01T19:42:42.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2631" PostTypeId="1" AcceptedAnswerId="2637" CreationDate="2013-11-01T19:55:08.207" Score="21" ViewCount="21492" Body="&lt;p&gt;Here's a photo of the &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Back_side_of_the_Moon_AS16-3021.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Far Side&lt;/a&gt; of the Moon&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KKREl.jpg&quot; alt=&quot;Luna - Far side&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare that to the &lt;a href=&quot;https://en.wikipedia.org/wiki/File:FullMoon2010.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Near Side&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HnRCI.jpg&quot; alt=&quot;Luna - Near side&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why the difference between the two? The difference is quite significant.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2015-08-17T06:13:13.527" LastActivityDate="2015-08-17T06:13:13.527" Title="Why is the Far Side of the Moon so different from the Near Side?" Tags="|the-moon|planetary-science|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2632" PostTypeId="2" ParentId="2625" CreationDate="2013-11-01T19:57:34.120" Score="6" Body="&lt;p&gt;The moon has almost no atmosphere whatsoever. On Earth, the when the sun is low in the sky it delivers far less heat to the surface than when directly overhead. One reason is that it goes through much more air as it passes through the atmosphere at that angle. This factor would have almost no affect at all on the sunlight heating the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other reason is that the as the landscape is slanted with respect to the solar exposure, the same amount of light is spread over a larger area of the surface. This affects the heating of the landscape, but that all changes when we start talking about structures. If we think about a cubical building, for example, the sun directly overhead would hit the roof of that cube squarely, but would barely hit the sides at all. When the sun is right on the horizon and one of the cube sides is oriented directly toward the sun, then on the moon, assuming there are no obstacles in between, that sunlight is hitting that wall just as squarely, but barely touching the roof. There is no atmosphere to filter out some of the light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if we are dealing with structures that stick out of the ground, generally speaking, you are always either IN the sunlight or OUT of it. And this doesn't change on the poles. The one thing you could do on the poles however is more easily stabilize the temperature of a structure at the high level of being in the light by elevating the structure above all obstacles so that it is always in the light (until the next eclipse), or at the low temperature of being in the dark, by putting your structure in a pit or crater.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heat that dissipates through the ground at the poles is certainly more stable, but direct sunlight on a machine, structure, or person is still just as harsh. Perhaps the ideal way to take advantage of the situation would be to sink the core structures below the sunlight, but to extend conductive heat sinks into the light to gather the heat. These could then be raised and lowered to adjust the temperature to your liking.&lt;/p&gt;&#xA;" OwnerUserId="945" LastActivityDate="2013-11-01T19:57:34.120" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2634" PostTypeId="2" ParentId="2611" CreationDate="2013-11-01T20:51:07.463" Score="12" Body="&lt;p&gt;The &lt;a href=&quot;http://www.asc-csa.gc.ca/index.html&quot; rel=&quot;noreferrer&quot;&gt;Canadian Space Agency&lt;/a&gt; (CSA) was helpful on Twitter (&lt;a href=&quot;https://twitter.com/csa_asc&quot; rel=&quot;noreferrer&quot;&gt;@csa_asc&lt;/a&gt; in English and &lt;a href=&quot;https://twitter.com/asc_csa&quot; rel=&quot;noreferrer&quot;&gt;@asc_csa&lt;/a&gt; in French language) and answered a couple of my questions. This is in response to a general question in the title:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Me&lt;/strong&gt;: How do astronauts battle loss in blood volume in microgravity?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;CSA&lt;/strong&gt;: Astronauts typically combat the effects by wearing compression&#xA;  garments and with adequate hydration. &lt;a href=&quot;http://t.co/o2T2JZSe9i&quot; rel=&quot;noreferrer&quot;&gt;http://t.co/o2T2JZSe9i&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And their response on asking about ongoing cardiovascular research projects under their umbrella:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Me&lt;/strong&gt;: Do ISS astronauts take any medication to help combat the loss in blood pressure gradient? Any progress on BP REG/VASCULAR/CCISS?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;CSA&lt;/strong&gt;: BPReg &amp;amp; Vascular are still underway on the &lt;a href=&quot;https://twitter.com/search?q=#ISS&amp;amp;src=hash&quot; rel=&quot;noreferrer&quot;&gt;#ISS&lt;/a&gt; and going well! Here's one of 4 papers from CCISS &lt;a href=&quot;http://t.co/Z3C2fPFLq1&quot; rel=&quot;noreferrer&quot;&gt;http://t.co/Z3C2fPFLq1&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-06-14T13:06:16.660" LastActivityDate="2014-06-14T13:06:16.660" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2635" PostTypeId="2" ParentId="596" CreationDate="2013-11-01T21:29:13.880" Score="0" Body="&lt;p&gt;If General Fusion achieves their goal, an electric propulsion probe powered by nuclear fusion could not be that far-off as we thought:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/g6oAx.jpg&quot; alt=&quot;Piston-driven fusion prototype&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea seems to be worthy of consideration: a vortex of melted lead-lithium mixture, some spheromaks of D-T plasma are injected in the vortex, and then the pistons create a spherically convergent shock wave. The numbers say that fusion conditions could be reached, but in this case, the big if are the hydrodynamic instabilities, instead of the plasma&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nextbigfuture.com/2013/10/general-fusion-targeting.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://nextbigfuture.com/2013/10/general-fusion-targeting.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="638" LastEditorUserId="638" LastEditDate="2013-11-01T21:45:50.773" LastActivityDate="2013-11-01T21:45:50.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2636" PostTypeId="2" ParentId="952" CreationDate="2013-11-01T21:45:10.300" Score="1" Body="&lt;p&gt;Relying on solar radiation alone is ineffectual for interstellar missions. As &lt;a href=&quot;https://space.stackexchange.com/a/953/638&quot;&gt;PearsonArtPhoto commented&lt;/a&gt;, the right way is relying on lasers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you see the &lt;a href=&quot;http://en.wikipedia.org/wiki/Laser_propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page for laser propulsion&lt;/a&gt; you'll read the main references to the literature. Robert Forward even came up with a scheme using multi-staged sails and a huge Fresnel lens that enabled the mission to brake up at destination&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the problem with such missions is not only the huge sails required because of diffraction-limited beam divergence, but the huge optical accuracies required to point a laser correctly so that it is kept aimed where the sail is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've &lt;a href=&quot;http://www.academia.edu/4135938/Laser_Starways_light_bridges_to_the_closest_stars&quot; rel=&quot;nofollow noreferrer&quot;&gt;recently published an alternative proposal&lt;/a&gt; for &lt;a href=&quot;http://www.centauri-dreams.org/?p=28727&quot; rel=&quot;nofollow noreferrer&quot;&gt;using multiple self-stabilised optical beam relays for beam refocusing&lt;/a&gt;. It doesn't solve all the problems, but at least enables one to consider additional possibilities in the design space of beamed propulsion concepts, basically trading-off interstellar-scale beam divergences and optical accuracies with only interplanetary-scale beam divergences and optical accuracies. The downside is that the relay mirrors need to have outstanding optical performance (400 ppm scattering losses or less) and they need to be maintained and repaired in-situ in interstellar space. We are not there yet&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if that approach could be workable, then you could have beamed power &lt;em&gt;all the way&lt;/em&gt; between the two stars. Since the structure will not be stable unless there is also a laser at destination, you will be able to brake as quickly as you were able to accelerate&lt;/p&gt;&#xA;" OwnerUserId="638" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-11-01T22:13:35.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2637" PostTypeId="2" ParentId="2631" CreationDate="2013-11-01T22:46:12.720" Score="22" Body="&lt;p&gt;The shielding of the near side of the Moon by the Earth is in all reality quite insignificant, with merely 0.01% of the Moon's sky obstructed by the Earth. If we look at this animation of the speed of light from the Earth to the Moon that is in scale, we can appreciate to what degree the Earth shields it from asteroid impacts:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/EFMIq.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Animation of the time it takes for light from the surface of the Earth to reach the surface of the Moon, with Earth and Moon in scale&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This notion that the Earth shields the Moon's near side could be further negated by Earth's by far larger mass and with it gravitationally attracting more asteroids, changing orbits of some sufficiently enough to miss the Earth and later hit the Moon instead. In fact, the Moon shields the Earth from impact craters far more than the other way around, but not so much from impact from the other direction and the Moon being in the way for asteroids to hit the Earth, but because the Moon would intercept many asteroids in an elliptical approach that would be otherwise captured by the Earth's gravity and might eventually hit it by precession, closing in on approach to Earth with each orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/DOqJA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in reality, the crater count on the lighter parts of the near side of the Moon is more or less consistent with the crater count of the lighter, far side of the Moon. &lt;a href=&quot;https://space.stackexchange.com/a/2125/49&quot;&gt;The darker regions of the Moon are calderas&lt;/a&gt;, the lava beds of Lunar maria (seas) that have filled the older impact craters and the whole side is indeed on average younger than the far side of the Moon that has less of these maria. To leave this to experts, here's an answer on &lt;a href=&quot;http://sservi.nasa.gov/?question=3318&quot; rel=&quot;nofollow noreferrer&quot;&gt;near-side/far-side impact crater counts&lt;/a&gt; by experts of NASA's SSERVI (Solar System Exploration Research Virtual Institute):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Earth partially shields the near side of the Moon from incoming&#xA;  asteroids, but that is not a large enough effect to influence crater&#xA;  densities. Just using simple straight-line geometry, you can calculate&#xA;  how much of the lunar sky is obscured by the Earth, about 4 square&#xA;  degrees out of 41,000 sq degrees for the whole sky. This makes the&#xA;  Earth negligible as a shield for the Moon. The real reason there are&#xA;  more impact craters on the far side of the Moon is that the near side&#xA;  has a much thinner crust which has allowed volcanoes to erupt and fill&#xA;  in ancient large basins (or large impact craters). These large lava&#xA;  flows have covered craters that were formed early in the Moon’s&#xA;  history through the late heavy bombardment, which is when the largest&#xA;  percentage of impacts were occurring in the inner solar system. It is&#xA;  likely that each side of the Moon has received equal numbers of&#xA;  impacts, but the resurfacing by lava results in fewer craters visible&#xA;  on the near side than the far side, even though the both sides have&#xA;  received the same number of impacts. Further, the oldest areas in both&#xA;  near and far side are saturated, meaning that they have reached&#xA;  equilibrium (each new crater, on average, destroys one old one). In&#xA;  this case, the density of craters is no longer an accurate measure of&#xA;  the number of hits the surface has received.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;David Morrison, Senior Scientist Brad Bailey, Staff Scientist&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The question that seems to persist though is, why are these maria more frequent and larger in size on the near side of the Moon? There are several theories, the one that seems most reasonable to me is the combination of tidal effects of the Earth-Moon system and the resulting tidal bulging and tidal friction keeping the Moon's face we see from the Earth geologically active for longer (in similar way to &lt;a href=&quot;https://space.stackexchange.com/a/2605/49&quot;&gt;what's keeping Europa a geologically active body&lt;/a&gt;), on top of tidal locking of our natural satellite being established early enough in Lunar history that it would exchange some of the heat radiation with Earth while it was still closer to it, while the hidden side radiated its surface heat away faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another plausible explanation for the far side being more mountainous and the near side appearing younger, flatter and with more maira is called &lt;a href=&quot;http://io9.com/5827021/did-earth-once-have-two-moons&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Big Splat&lt;/a&gt; that, if the theory of the origin of our Moon being a product of a Mars-sized body (called Theia) slamming into the Earth is correct, suggests that the product of such a collision would produce many massive debris in orbit about the Earth besides the Moon itself, and later slow collisions splatted their materials all over the face of the Moon as we see it today:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/xF4uW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Visualisation of a theoretical lunar Big Splat (Source: &lt;a href=&quot;http://io9.com/5827021/did-earth-once-have-two-moons&quot; rel=&quot;nofollow noreferrer&quot;&gt;io9&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or that it could be attributed to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Late_Heavy_Bombardment&quot; rel=&quot;nofollow noreferrer&quot;&gt;Late Heavy Bombardment&lt;/a&gt; (interesting NASA's video on &lt;a href=&quot;https://www.youtube.com/watch?v=UIKmSQqp8wY&quot; rel=&quot;nofollow noreferrer&quot;&gt;Evolution of the Moon&lt;/a&gt;). This could as well be merely by chance though, and I don't think we'll really know until we at least establish &lt;a href=&quot;http://www.psi.edu/epo/moon/moon.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;our Moon's origins&lt;/a&gt; (with some theories being that it's a product of an impact of a planetesimal with the Earth, some that it's a sister formation and formed parallel with the Earth, another one that it is a captured asteroid, or even that the Earth was spinning so fast that it literally ejected some of its mass and lost a lot of its angular momentum like that).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2013-11-07T23:57:13.307" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="2638" PostTypeId="1" CreationDate="2013-11-02T07:54:49.377" Score="18" ViewCount="508" Body="&lt;p&gt;According to the NASA website &lt;a href=&quot;http://ares.jsc.nasa.gov/humanexplore/exploration/exlibrary/docs/isru/06energy.htm&quot;&gt;Energy from the Moon&lt;/a&gt;, that assuming we perfect the helium-3 ($^3\text{He}$) nuclear fusion technology required, that $^3\text{He}$&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;would generate only a very slight amount of radioactivity, equivalent in nature to that produced by hospitals in their nuclear medicine areas. When used in this plant,$^3\text{He}$ would have so much energy that it would require only 20 tons-less than one Shuttle load-to supply all the electricity used in the United States in a year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Thus, it would be very beneficial for colonists on other worlds.  So, my question is &lt;strong&gt;has there been proven sufficient $^3\text{He}$  reserves on the Moon (and other inner solar system worlds) that would make developing and implementing this power source viable?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorDisplayName="user12" LastEditDate="2013-11-07T04:57:26.130" LastActivityDate="2017-06-14T19:10:34.873" Title="Are there proven sufficient $^3\text{He}$ reserves in the inner Solar System to sustain fusion reactors for colonisation?" Tags="|colonization|energy|helium-3|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2641" PostTypeId="1" AcceptedAnswerId="2642" CreationDate="2013-11-02T08:02:31.857" Score="9" ViewCount="2804" Body="&lt;p&gt;As a fun side project I'm writing a turn-based game that tries to portrait solar system travel little bit more realistically then most of games do. I want to calculate the cost of traveling between planets based on delta-v differences graph like this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/bNAIZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I read up on delta-v budget and read through some of the Kerbal Space Program tutorials (and played some too) but I'm still not sure what &quot;planet intercept&quot; is. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I start from low Kerbin orbit and make delta-v steps of: 950 (Kerbin intercept), 80 (Eve intercept) am I at high Eve orbit? If I stop accelerating will I stay on that orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And more importantly, does this model make any sense at all?&lt;/p&gt;&#xA;" OwnerUserId="985" OwnerDisplayName="atok" LastActivityDate="2013-11-02T14:58:32.713" Title="What is a planet intercept?" Tags="|solar-system|orbit|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2642" PostTypeId="2" ParentId="2641" CreationDate="2013-11-02T14:58:32.713" Score="6" Body="&lt;p&gt;There's two parts to this question (and answer), KSP and real life. I'll get KSP out of the way first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;KSP uses a simplified two-body physics model rather than full N-body physics, for performance reasons. So, each celestial body has a &lt;em&gt;sphere of influence&lt;/em&gt;, a region in which any spacecraft are affected by its gravity and nothing else. An &lt;em&gt;intercept&lt;/em&gt;, for the purposes of a delta-v chart, means getting close enough that you enter its sphere of influence. This is represented on the orbital map as a circular icon with the words &quot;Mun Encounter&quot; (or similar); leaving a sphere of influence shows up as &quot;Mun Escape&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that just because you are in something's sphere of influence doesn't mean you are in a stable orbit around it. That 850dv to intercept the Mun will get you a flyby of the Mun, entering its SoI, passing it on a hyperbolic trajectory, and then exiting its SoI to return to Kerbin orbit. If you want a stable orbit, you'll need to expend additional dv circularizing. Whether this flyby is in &quot;high&quot; or &quot;low&quot; orbit depends on the precise details of your intercept burn; for bodies with atmosphere, such as Eve, you can even tune your intercept to fly through the planet's atmosphere, allowing you to aerobrake.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In real life, things are a bit fuzzier. The concept of a &lt;a href=&quot;http://en.wikipedia.org/wiki/Sphere_of_influence_%28astrodynamics%29&quot;&gt;sphere of influence&lt;/a&gt; still exists, but even within something's SoI you are still affected by the gravity of other bodies. What &quot;intercept&quot; means will depend on the mission - if you're trying to dock with a space station, you need to get within touching distance, whereas for a flyby of another planet you just need to get close enough for the cameras.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the purposes of your game, I would say:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Yes, it does make sense to model travel costs using delta-v.&lt;/li&gt;&#xA;&lt;li&gt;If you want to distinguish between flybys and orbits, separating &quot;delta-v to intercept&quot; and &quot;delta-v to enter a stable orbit after intercept&quot; makes sense; if you assume all intercepts will be for orbits you can combine the two.&lt;/li&gt;&#xA;&lt;li&gt;Don't forget to give craft that can aerobrake a discount, when intercepting things with atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Also don't forget that there are &lt;a href=&quot;http://www.braeunig.us/space/interpl.htm&quot;&gt;a variety of transfer orbits you can use&lt;/a&gt; with different delta-v/time/return trajectory tradeoffs, including ones not mentioned on that page like the &lt;a href=&quot;http://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot;&gt;bi-elliptic transfer&lt;/a&gt; and cunning gravity slingshot manouvers off other planets&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="890" LastActivityDate="2013-11-02T14:58:32.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2643" PostTypeId="1" AcceptedAnswerId="2644" CreationDate="2013-11-02T16:01:09.167" Score="8" ViewCount="804" Body="&lt;p&gt;I was reading the &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2012/30jul_skycrane/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA page about Curiosity's crazy landing scheme&lt;/a&gt;, and I ran across this sentence:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With a payload this size, the rockets could kick up enough dust to compromise the rover and its instruments&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Wait, what? Curiosity has to survive these things:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZHzV9.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving at around &lt;a href=&quot;https://space.stackexchange.com/a/2624/12&quot;&gt;300-375 mi/hr tops&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it true that a simple rocket blast for a few seconds would risk comprimising the rover, while the rover can survive 375 mi/hr gusts of the same sand?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-11-02T19:05:29.127" Title="Curiosity: Rockets kick up more dust than a sandstorm?" Tags="|mars|curiosity|dust|skycrane|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2644" PostTypeId="2" ParentId="2643" CreationDate="2013-11-02T17:05:33.570" Score="6" Body="&lt;p&gt;Retrorockets could accelerate dust particles and smaller stones to a lot higher velocities than highest recorded wind gusts on the surface of Mars that reached 500 to 600 km/h (300 to 375 mi/h). For example, the Apollo Lunar Modules' descent engines &lt;a href=&quot;http://www.space.com/4956-lunar-landers-sandblasted-moon.html&quot;&gt;blew out high-velocity lunar particles&lt;/a&gt; estimated to have reached speeds of &lt;strong&gt;between 0.6 and 1.5 miles per second (up to 5,400 mi/h or 8,690 km/h)&lt;/strong&gt;. Those are over one order of magnitude greater speeds than Martian dust devils or storms were recorded to reach!&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;The smallest particles were seen by the Apollo astronauts to fly&#xA;  right out over the horizon and keep on going,&quot;&lt;/em&gt; said Philip Metzger of&#xA;  NASA's Kennedy Space Center (KSC). &lt;em&gt;&quot;Depending on the actual velocity&#xA;  they may have gone halfway around the moon or more. In most cases they&#xA;  would only travel until they hit a natural terrain feature, such as a&#xA;  crater rim or a mountain range.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Of course, that means that it wouldn't merely be fine grain dust that would be &lt;em&gt;disturbed&lt;/em&gt; but the exhaust plumes of retrorockets would also be accelerating higher mass particles, like gravel and pebbles. That would be a whole lot more of &lt;em&gt;sandblasting&lt;/em&gt; than rovers would receive even if in the middle of a Martian sand storm. Additionally, a lot of these flying particles of different sizes would be heated up by the retrorocket's exhaust plume at low altitudes, and this heat could be also trapped in an &lt;em&gt;excavated&lt;/em&gt; by exhaust plumes crater in the Martian soil, so there'd be even more danger from extreme temperatures that the Curiosity rover wasn't designed for. Sure, the rover could be protected from this heat, but that would add to the total lander's weight, and it was already reaching limits of our capabilities as it is, with the launch weight of roughly 900 kg (2000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were also other dangers with using retrorockets on the lander itself (they were of course used on the Sky Crane, but higher above the landing site and the rover was lowered on the ground by a set of cables once the crane reached nearly zero vertical velocity), some of it discussed later in the article you linked to, like the possibility of tipping over or burying the lander too deep in the Martian soil for the rover to safely drive off its ramp, and so on.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-02T19:05:29.127" LastActivityDate="2013-11-02T19:05:29.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2645" PostTypeId="1" AcceptedAnswerId="2646" CreationDate="2013-11-02T17:30:21.267" Score="14" ViewCount="760" Body="&lt;p&gt;When I drive in cold/wet weather the state of the roads is cause for concern. But more worrying is the risk of the wind-screen fogging over when least it should. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space/Earth orbit is a different kettle of fish; an astronaut doesn't run the risk of rear-ending, or tail-gating. An astronaut doesn't drive a car either; not in space anyway - not yet. The astronaut has a space-suit, and a helmet. The risk of fogging &lt;em&gt;probably&lt;/em&gt; does not exist - given that the suit is either in the sun ( &gt; 100 C ), or in shade ( &amp;lt; -100C ). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about when we get to a celestial body with even a modicum of an atmosphere - &#xA;E.g. Venus, Mars, Titan just to name a few.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could an astronaut (nominally, celestonaut) suffer helmet fogging during the course of exploration activities? &lt;/li&gt;&#xA;&lt;li&gt;How would helmet-fogging in a potentially hostile environment be avoided?&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;As an aside&lt;/em&gt;, were any incidents of space-suit helmet fogging recorded in the annals of human spaceflight? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2016-03-21T10:30:02.963" LastActivityDate="2016-03-21T10:30:02.963" Title="Could helmet-fogging occur on Mars, or on any other celestial body with an atmosphere?" Tags="|mars|colonization|atmosphere|spacesuits|humans|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2646" PostTypeId="2" ParentId="2645" CreationDate="2013-11-02T18:18:23.197" Score="15" Body="&lt;p&gt;Yes, of course the fogging could occur, if not mitigated. You'd be exhaling and sweating pretty much same contents of water vapor wherever you are, and in a completely closed system like a helmet used in non-breathable atmospheres like on the surface of Mars would be, in the vacuum of space, or hazardous environments (hazmat helmets, full face gas masks, e.t.c.), exhaled water vapor could condense on inner side of your visor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The front of the visor, the outer side that is exposed to the environment, could also condense vapor, gather electrostatically charged dust, or even ice particles obscuring your vision, but that should be easy enough to wipe off on properly treated front glass / plastic element (faceplate), usually involving some &lt;a href=&quot;https://en.wikipedia.org/wiki/Antistatic_agent&quot;&gt;antistatic coating&lt;/a&gt; on top of filter coating to protect from harmful rays (gold filters are often used).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wiping off the inside of the visor isn't as easy though, so to combat this a good in-helmet ventilation will have to be provided, like it is in all current helmets used in spaceflight, or I would hope any properly designed fully closed helmets. Quoting &lt;a href=&quot;http://www.nasa.gov/audience/foreducators/spacesuits/home/clickable_suit_nf.html&quot;&gt;NASA's page on spacesuits and spacewalks&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Besides covering a spacewalker's head, the helmet has a Vent Pad. This&#xA;  pad directs oxygen from the Primary Life Support Subsystem and Hard&#xA;  Upper Torso to the front of the helmet. The helmet keeps the oxygen at&#xA;  the right pressure around the head. The main part of the helmet is the&#xA;  clear plastic bubble.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The inner visor glass / plastic element could also be treated with &lt;a href=&quot;https://en.wikipedia.org/wiki/Anti-fog&quot;&gt;anti-fogging chemicals&lt;/a&gt;, and its sides designed to allow for sufficient air circulation. If more than a single glass / plastic elements are used, the space between them could be filled with a low thermal conductivity gas, to prevent heat exchange directly on the visor itself.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-02T19:11:11.873" LastActivityDate="2013-11-02T19:11:11.873" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2647" PostTypeId="1" AcceptedAnswerId="2649" CreationDate="2013-11-02T21:56:12.353" Score="19" ViewCount="1579" Body="&lt;p&gt;The opening paragraph of &lt;a href=&quot;http://spaceflightnow.com/station/exp37/131101relocation/#.UnVyqpRga_g&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; caught my eye*:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Three space station crew members put on spacesuits and boarded their Soyuz spacecraft Friday for a short trip to relocate the capsule to a new docking port on the orbiting outpost, clearing the way for the arrival of three new residents next week.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4OFTo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to indicate that three cosmonauts are required to move a Soyuz - is this true?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, why?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/conversation/tildals-idea&quot;&gt;*Well, it caught Tildal's eye.&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2013-11-03T19:17:20.967" LastActivityDate="2013-11-03T19:17:20.967" Title="Why are three cosmonauts required to move a Soyuz?" Tags="|iss|crewed-spaceflight|docking|soyuz-spacecraft|procedure|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2648" PostTypeId="1" CreationDate="2013-11-02T21:57:45.773" Score="19" ViewCount="5128" Body="&lt;p&gt;Why did the Space Shuttle have an indicator to alert the crew to the arming of the range safety destruct system? Quoting from &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html#srb-safety&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first message, called arm, allows the onboard logic to enable a&#xA;  destruct and illuminates a light on the flight deck display and&#xA;  control panel at the commander and pilot station. The second message&#xA;  transmitted is the fire command.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This seems, er, rather grim. Why is this light there?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Perhaps it is used for testing purposes - can anyone provide a reference for whether this indicator was used by the crew or ground personnel during pre-flight checkout? Second idea is that it was to alert the crew to accidental arming, however since the command decoders appear to have used some type of encryption this seems unlikely to have occurred accidentally by remote.  If the range saftey system had become armed by an internal vehicle fault, is there anything the crew could have done about it?&lt;/p&gt;&#xA;" OwnerUserId="988" LastEditorDisplayName="user12" LastEditDate="2013-11-08T14:29:17.590" LastActivityDate="2019-08-26T00:56:19.517" Title="Space Shuttle range safety system - Why is there a caution light?" Tags="|space-shuttle|flight-termination-system|range-safety|" AnswerCount="7" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2649" PostTypeId="2" ParentId="2647" CreationDate="2013-11-02T23:07:59.210" Score="28" Body="&lt;p&gt;There are usually 6 crew members, and 2 Soyuz spacecraft on station.  Each Soyuz carries 3 astronauts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there is an issue redocking during the move procedure, the backup plan is to return to earth.  If they cannot redock and someone is left on the station, they would be stranded and cause all sorts of downstream issues. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, you do not need 3 to move a Soyuz, but because it is the ride home for 3 they need to be in it, in case they need to use it. For safety during launch and landing, each astronaut has a custom made seat liner, and they usually leave it in the Soyuz they intend to use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will get more interesting when they start using the commercial vendors, which are designed for 7 passengers (CST-100, DragonRider, Dream Chaser) but NASA is only planning on launching with 4 passengers (For a crew size of 7).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-11-03T13:44:19.990" LastActivityDate="2013-11-03T13:44:19.990" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2650" PostTypeId="5" CreationDate="2013-11-03T03:22:04.697" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-03T03:22:04.697" LastActivityDate="2013-11-03T03:22:04.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2651" PostTypeId="4" CreationDate="2013-11-03T03:22:04.697" Score="0" Body="Questions regarding systems in place to terminate the launch of a spacecraft to prevent the destruction of endangered people &amp; property." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-03T03:22:04.697" LastActivityDate="2013-11-03T03:22:04.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2652" PostTypeId="2" ParentId="2648" CreationDate="2013-11-03T05:10:04.227" Score="12" Body="&lt;p&gt;I think I may have found a possible answer.  The control panel for Columbia had the indicator:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5rZnh.png&quot; alt=&quot;Columbia panel&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The control layout for Discovery (and presumably the other orbiters as well) was somewhat different, and does not have the indicator:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CNkiF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that Columbia was originally fitted with ejection seats, in that context the indicator seems to make more sense.&lt;/p&gt;&#xA;" OwnerUserId="988" LastActivityDate="2013-11-03T05:10:04.227" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2653" PostTypeId="1" AcceptedAnswerId="2751" CreationDate="2013-11-03T13:37:57.600" Score="28" ViewCount="3897" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Space_Research_Organisation&quot; rel=&quot;noreferrer&quot;&gt;Indian Space Research Organisation&lt;/a&gt; (ISRO) is launching a &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;Polar Satellite Launch Vehicle&lt;/a&gt; (PSLV) this Tuesday (5 Nov 2013) from &lt;a href=&quot;https://en.wikipedia.org/wiki/Satish_Dhawan_Space_Centre&quot; rel=&quot;noreferrer&quot;&gt;Satish Dhawan Space Center&lt;/a&gt;, Sriharikota, India, flying on the PSLV-C25 mission with &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;noreferrer&quot;&gt;Mars Orbiter Mission&lt;/a&gt; (MOM), informally called &lt;em&gt;Mangalyaan&lt;/em&gt; onboard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/9PXAj.jpg&quot; alt=&quot;MOM&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The Mars Orbiter Mission spacecraft is enclosed inside the Polar Satellite Launch Vehicle fairing. Credit: &lt;a href=&quot;http://www.isro.org/&quot; rel=&quot;noreferrer&quot;&gt;ISRO&lt;/a&gt; / &lt;a href=&quot;http://www.spaceflightnow.com/pslv/c25/131024update/&quot; rel=&quot;noreferrer&quot;&gt;SpaceflightNow&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking through the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission#Objectives&quot; rel=&quot;noreferrer&quot;&gt;mission objectives&lt;/a&gt;, it seems the scientific payload is of secondary nature and ISRO's primary objectives are limited to building the spacecraft, getting there, and communicating with the spacecraft by extending the &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Deep_Space_Network&quot; rel=&quot;noreferrer&quot;&gt;India's Deep Space Network&lt;/a&gt; (the launch was delayed from late October to early November due to, I believe, one of the launch tracking ships not being able to get to Fiji in time because of a tropical storm in the Pacific Ocean).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-24772147&quot; rel=&quot;noreferrer&quot;&gt;BBC's interview with K. Radhakrishnan&lt;/a&gt;, chair of ISRO further emphasises main objectives:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Essentially in this Mars mission, which we are planning, the prime&#xA;  objective is to demonstrate India's capability to capture the Martian&#xA;  orbit and then to conduct a few meaningful scientific experiments.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/qj2M8.png&quot; alt=&quot;MOM scientific payload&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Proto-flight model of the payloads for Indian Mars Mission (Source: &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2760.pdf&quot; rel=&quot;noreferrer&quot;&gt;PDF, 44th Lunar and Planetary Science Conference&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft's 15 kg (33 lb) scientific payload still includes five instruments to conduct atmospheric, particle environment and surface imaging studies, with &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-24772147&quot; rel=&quot;noreferrer&quot;&gt;focus on establishing the presence of methane&lt;/a&gt; in the Martian environment. But all this sounds more like being a preliminary mission paving the way for future ISRO's missions to Mars, so it naturally begs the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What plans does ISRO have for future exploration of Mars? What are the India's Mars exploration campaign goals, which future missions are approved, in development, under consideration or on the drawing boards that are known to public?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: I see there's some confusion on where my question is coming from. This is not a non-researched question asking about what information is so far presented to the general public. To quote Dr. K. Radhakrishnan, Chairman of the Indian Space Research&#xA;Organisation (ISRO) when asked about India's efforts to explore Mars:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Would India have an ongoing Mars programme or would the 2013 mission&#xA;  be a one-mission agenda? If not, then has ISRO developed any well&#xA;  articulated Mars agenda? Has ISRO started preparations for any&#xA;  missions for the launch windows in 2016 and 2018 respectively?&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;br&gt;&#xA;  Of course, while exploring the launch opportunities in the year 2013,&#xA;  2016 and 2018, we have also studied the possible launch opportunities,&#xA;  corresponding velocity requirements and payload capabilities for&#xA;  Orbiter missions to planet Mars beyond 2018 as well.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;Source: &lt;a href=&quot;http://link.springer.com/content/pdf/bbm%3A978-81-322-1521-9%2F1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Ajey Lele, Mission Mars, SpringerBriefs in Applied Sciences and Technology,  Appendix E&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This statement has to have some grounds somewhere, in actual planned missions, draft proposals, and so on. A chairman of ISRO wouldn't be discussing future launch windows to Mars, if there wasn't a good reason for it. What are these reasons? In essence, what comes next, after &lt;em&gt;Mangalyaan&lt;/em&gt;, for ISRO and their endeavors to explore the red planet?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="12102" LastEditDate="2021-08-12T23:05:16.047" LastActivityDate="2021-08-12T23:05:16.047" Title="Does India have plans to explore Mars after Mangalyaan?" Tags="|mars|isro|mars-orbiter-mission|space-program|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2655" PostTypeId="5" CreationDate="2013-11-03T14:55:22.617" Score="0" Body="&lt;p&gt;For more info on the Mars Orbiter Mission, see its corresponding &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="58" LastEditDate="2013-11-07T13:25:34.270" LastActivityDate="2013-11-07T13:25:34.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2656" PostTypeId="4" CreationDate="2013-11-03T14:55:22.617" Score="0" Body="Questions regarding ISRO's Mars Orbiter Mission, informally known as Mangalyaan." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-03T14:55:22.617" LastActivityDate="2013-11-03T14:55:22.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2657" PostTypeId="1" AcceptedAnswerId="2658" CreationDate="2013-11-03T15:03:50.050" Score="7" ViewCount="187" Body="&lt;p&gt;There are already many great booster designs out there - take for example the Saturn V, which has enough power to reasonably put almost anything into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do new, developing space programs often use old, proven designs, or do they try to create their own booster from scratch?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-03T20:49:55.683" LastActivityDate="2013-11-03T20:49:55.683" Title="Do developing space programs generally use an existing booster design, or make their own from scratch?" Tags="|rockets|developing-space-programs|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2658" PostTypeId="2" ParentId="2657" CreationDate="2013-11-03T19:57:55.153" Score="5" Body="&lt;p&gt;It depends (when is that NOT the answer?).  There are many examples of technology researched and/or developed in the past being reused in modern spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has explained that the prior work, available to them through NASA greatly decreased development time. (I recall references to the Apollo parachute design work, and the use of the basic PICA heat shield formulation, updated to modern times as PICA-X). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Merlin engine uses a Pintle injector, used on the lunar module ascent engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like all innovators, they stand upon the shoulders of those who preceded them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However it is unclear that the resources made available to SpaceX, Orbital, Boeing, Lockheed-Martin, and Sierra Nevada Corp would be similarly made available to India, China or South Korea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is much in the public domain, as information, which might be available, but there are also more physical resources, (engine test stands, wind tunnels, research facilities) only available in the US that exist due to previous work by NASA.  The Michoud facility that made the Saturn V first stage, reused some of the tooling to make the Shuttle external tank, and is now slated to make components for the SLS (Senate Launch System).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely, look at the engines often employed by nascent space programs.  Often they start with Solid rockets or else MMDH/MMH/Nitrogen Textroxide, which usually indicates that they have learned that LOX/LH engines are really hard to build.  So start with something easier. Often, if they had a ballistic missile program, they will use that as the starting point for development. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch pads at Kennedy/Canaveral and Vandenberg AFB were developed at much cost, that the various launch providers in the US get access to for less than the real cost.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX was able to buy a used LOX storage ball that NASA was getting rid of, for very reduced pricing.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sierra Nevada is using HL-20 research work for the design of their Dream Chaser vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital seems to love reusing solid booster designs in their launchers (Minotaur, Taurus, Antares (upper stage), Pegasus, Pegasus II (For Stratolaunch)) that were developed for ICBM work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So many examples of things, US companies can get access to, that would be very hard for anyone else to get access to.  Which is very likely as it should be. (Return on investment?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not to discount the benefits of information, and the generally available information on engine design, cannot harm a developing space program.  You can get amazing information on very specific details of the development of the F-1, SSME, and other engines that can only help anyone approaching the task.  Some of the take away might be that it is 'very hard and expensive', from which SpaceX's approach of a simple design, scaled up through iterations might be the logical outcome.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-11-03T20:40:08.447" LastActivityDate="2013-11-03T20:40:08.447" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2659" PostTypeId="1" CreationDate="2013-11-03T21:06:30.917" Score="9" ViewCount="394" Body="&lt;p&gt;I was reading &lt;a href=&quot;http://www.spacenews.com/article/isros-future-plans-will-focus-low-cost-exploration&quot;&gt;this article&lt;/a&gt; (terrible layout, sorry). This paragraph caught my eye:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;While Japan is hoping to send humans  to the  Moon sometime in the future, Keiji Tachikawa, president of the Japan Aerospace Exploration Agency (JAXA), said Japan's initial focus in the years ahead will be &lt;strong&gt;to probe the Moon for &quot;potential utilization.&quot;&lt;/strong&gt; He said the Japanese spacecraft Kaguya, which was launched on a trajectory to the Moon Sept. 14, will begin its scientific explorations in October.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is meant by 'probing the moon for potential utilization'? What are the 'potential utilizations'? How will the probing be done?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2015-01-17T00:46:09.357" Title="What is JAXA looking for on the Moon in the way of 'potential utilization'?" Tags="|the-moon|probe|japan|jaxa|developing-space-programs|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2660" PostTypeId="1" AcceptedAnswerId="3645" CreationDate="2013-11-03T23:39:37.013" Score="11" ViewCount="718" Body="&lt;p&gt;Impacting the Moon with ballistic projectiles to study its surface characteristics through debris plume sounds fantastic to me, especially with the success of previous such missions (or better said their ending) in mind, like for example impacting &lt;a href=&quot;https://en.wikipedia.org/wiki/Kaguya_%28SELENE%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;SELENE&lt;/a&gt; spacecraft on the lunar surface near the crater Gill, &lt;a href=&quot;https://en.wikipedia.org/wiki/LCROSS&quot; rel=&quot;nofollow noreferrer&quot;&gt;LCROSS&lt;/a&gt; observing Centaur upper stage (and data collecting Shepherding Spacecraft) striking the crater Cabeus, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Recovery_and_Interior_Laboratory&quot; rel=&quot;nofollow noreferrer&quot;&gt;GRAIL Twins&lt;/a&gt; impacting between Philolaus and Mouchez, and so on. There are also planned impacts of other spacecraft, &lt;a href=&quot;https://space.stackexchange.com/q/1734/49&quot;&gt;LADEE&lt;/a&gt; to name one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But these are all spacecraft, and they weren't really designed to withstand impact forces and penetrate deep into the Lunar regolith, ejecting deeper materials we could then study with orbiting and ground based observatories. So having a mission with actual ballistic projectiles specifically designed to penetrate as deep and with as much force as possible is a whole new ball game. Meet the &lt;a href=&quot;https://en.wikipedia.org/wiki/MoonLITE&quot; rel=&quot;nofollow noreferrer&quot;&gt;MoonLITE project&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;strong&gt;Moon Lightweight Interior and Telecoms Experiment (MoonLITE)&lt;/strong&gt;, is a&#xA;  proposed British space mission to explore the Moon and develop&#xA;  techniques for future space exploration. If funded, it will be&#xA;  built by a consortium of UK industry likely including &lt;a href=&quot;https://en.wikipedia.org/wiki/Surrey_Satellite_Technology_Ltd&quot; rel=&quot;nofollow noreferrer&quot;&gt;Surrey Satellite&#xA;  Technology Ltd&lt;/a&gt;, and it is planned to be launched into lunar orbit in&#xA;  2014. The mission concept emerged out of a study run by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Particle_Physics_and_Astronomy_Research_Council&quot; rel=&quot;nofollow noreferrer&quot;&gt;Particle Physics and Astronomy Research Council&lt;/a&gt; (now the &lt;a href=&quot;https://en.wikipedia.org/wiki/Science_and_Technology_Facilities_Council&quot; rel=&quot;nofollow noreferrer&quot;&gt;Science and&#xA;  Technology Facilities Council&lt;/a&gt;) in 2006. In December 2008, the&#xA;  &lt;a href=&quot;https://en.wikipedia.org/wiki/British_National_Space_Centre&quot; rel=&quot;nofollow noreferrer&quot;&gt;British National Space Centre&lt;/a&gt; (part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/UK_Space_Agency&quot; rel=&quot;nofollow noreferrer&quot;&gt;UK Space Agency&lt;/a&gt; since April&#xA;  2010) announced that the project was moving to Phase A study. This&#xA;  is a 12 month study of the mission system, the penetrators and the&#xA;  penetrator descent systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No mention of what happened between 2009 when the mentioned study should finish and now, late 2013? What happened? The mentioned launch in 2014 should be around the corner. Didn't they fund it? With UK parliament recently bragging that the &lt;a href=&quot;http://www.parliament.uk/business/committees/committees-a-z/commons-select/science-and-technology-committee/news/131028-space-agencies-report/&quot; rel=&quot;nofollow noreferrer&quot;&gt;establishment of UK Space Agency has helped make UK's space sector a multi-billion pound success story&lt;/a&gt;, I would hope that they put some of those moneys aside for actual scientific missions, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/EpOJT.jpg&quot; alt=&quot;MoonLITE render&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Render of MoonLITE spacecraft with four penetrators that would be deployed to near side, far side and both poles of the Moon.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, did they fund it and is the MoonLITE project still on? Has the 12 month long &lt;em&gt;Phase A&lt;/em&gt; study from 2008 concluded favorably, giving the project a green light and it will indeed be carried out in 2014? What stage is the project in and when exactly will it be launched, onboard which rocket, from where and when can we expect any science done by it?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2023-08-26T18:31:37.900" Title="Is the UK Space's MoonLITE project still on?" Tags="|the-moon|future-missions|impact|uk-space|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2661" PostTypeId="1" CreationDate="2013-11-04T02:00:38.080" Score="17" ViewCount="22263" Body="&lt;p&gt;What was the velocity of the Command Module after it had penetrated through the Earth's atmosphere at the point where the parachutes were deployed?&lt;/p&gt;&#xA;" OwnerUserId="992" LastEditorDisplayName="user12" LastEditDate="2013-11-04T15:05:28.640" LastActivityDate="2018-11-24T15:09:01.383" Title="What was Apollo 11's reentry speed at parachute deployment?" Tags="|apollo-program|reentry|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2662" PostTypeId="2" ParentId="2661" CreationDate="2013-11-04T04:19:47.963" Score="9" Body="&lt;p&gt;The command module's speed is going to be terminal velocity &amp;mdash; the speed at which the force exerted by aerodynamic drag equals the force exerted by gravity &amp;mdash; because it entered the atmosphere going at a crazy high speed, and it's been falling for tens of thousands of meters already. So we know it's going to be terminal velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we can't stop there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was curious about this too, so I decided to run the numbers. The formula for &lt;a href=&quot;http://en.wikipedia.org/wiki/Terminal_velocity&quot; rel=&quot;nofollow&quot;&gt;terminal velocity&lt;/a&gt; is as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_T=\sqrt{\frac{2mg}{\rho AC_D}}$$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The mass of the command module, $m$, was 5809kg, from &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;The force of gravity on Earth, $g$ is right around $9.8~\textrm{m/s}^2$.&lt;/li&gt;&#xA;&lt;li&gt;The drag coefficient, $C_D$, seems to be about 1.3 (judging from the graph on pg 48 of &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670027745_1967027745.pdf&quot; rel=&quot;nofollow&quot;&gt;this NASA paper&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The projected area of the CM's heat shield, $A$, was $11.631~\textrm{m}^2$ ($125.2 ~\textrm{ft}^2$), according to page 9 of &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670027235_1967027235.pdf&quot; rel=&quot;nofollow&quot;&gt;another NASA paper&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Here I'll assume you're talking about the drogue parachutes, the first ones out. They were deployed at 24,000 feet (7315.2m). The density of the air at that altitude is, &lt;a href=&quot;http://m.wolframalpha.com/input/?i=air+density+24000+feet&quot; rel=&quot;nofollow&quot;&gt;according to Wolfram|Alpha&lt;/a&gt;, $0.57~\textrm{kg/m}^3$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now we have all our numbers. We just need to do the calculations &amp;mdash; which I performed, getting a terminal velocity at that altitude of (drumroll please)&amp;hellip; $103.71~\textrm{m/s}$ (or about 230 mph).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is&amp;hellip;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;About the maximum speed of a swing by a professional golf player,&lt;/li&gt;&#xA;&lt;li&gt;Almost as fast as a Ferrari F50 GT1 (about 2% slower),&lt;/li&gt;&#xA;&lt;li&gt;And about a third as fast as sound in 15°C dry air @ 1 atmosphere: Mach 0.33 &lt;a href=&quot;http://m.wolframalpha.com/input/?i=sound+speed+at+25000+feet+altitude&amp;amp;x=0&amp;amp;y=0&quot; rel=&quot;nofollow&quot;&gt;at that altitude&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Thanks, &lt;a href=&quot;http://m.wolframalpha.com/input/?i=103m%2Fs&quot; rel=&quot;nofollow&quot;&gt;Wolfram|Alpha&lt;/a&gt;!&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="6816" LastEditDate="2015-08-08T23:35:53.710" LastActivityDate="2015-08-08T23:35:53.710" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2663" PostTypeId="2" ParentId="2661" CreationDate="2013-11-04T14:18:48.077" Score="17" Body="&lt;p&gt;If you're asking about the deployment of the three main parachutes of &lt;a href=&quot;http://nassp.sourceforge.net/wiki/Earth_Landing_System&quot; rel=&quot;noreferrer&quot;&gt;C/M-ELS&lt;/a&gt; (Apollo Command Module Earth Landing System), then this is simple enough to answer, the pilot chutes are deployed at about 10,000 feet (3.05 km) by a barometric switch, pulling the three main parachutes from their containers. The ELS was designed so the &lt;strong&gt;&lt;a href=&quot;http://www.theregister.co.uk/Print/2013/05/21/reentry_tech/&quot; rel=&quot;noreferrer&quot;&gt;drogue chutes slow the descent down to roughly 200 km/h&lt;/a&gt; (124 mi/h)&lt;/strong&gt; before the pilot chutes pull the main chutes, eventually slowing down the CM to 22 miles per hour (35 km/h) for splashdown and to &lt;a href=&quot;http://www.spaceaholic.com/index.php/Detail/Object/Show/object_id/26&quot; rel=&quot;noreferrer&quot;&gt;roughly 24.5 mi/h&lt;/a&gt; (39.5 km/h) with only two main chutes properly deployed, as it happened during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15&quot; rel=&quot;noreferrer&quot;&gt;Apollo 15&lt;/a&gt; splashdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ug3r0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Earth Landing System sequence of events (Source: &lt;a href=&quot;http://nassp.sourceforge.net/wiki/Earth_Landing_System&quot; rel=&quot;noreferrer&quot;&gt;Project Apollo - NASSP&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the drogue deployment though (thanks go to @MarkAdler in the comments!), we now have this diagram of the &lt;a href=&quot;http://www.yumpu.com/en/document/view/4000016/tp-131-the-apollo-parachute-nding-system-nasa-cr-&quot; rel=&quot;noreferrer&quot;&gt;parachute deployment envelope&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/sFQQI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in normal atmospheric entry (not launch abort), the diagram for &lt;strong&gt;manual deployment&lt;/strong&gt; of the drogues describes the region in altitudes between 40,000 and 25,000 feet (12.2 - 7.6 km) and CM velocity between mach 0.7 and 0.3. Translating that to &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Comparison_US_standard_atmosphere_1962.svg&quot; rel=&quot;noreferrer&quot;&gt;US standard atmosphere in 1962&lt;/a&gt; figures, 0.7 mach at 40,000 feet equals roughly 206 m/s (743 km/h or 461 mi/h) and 0.3 mach at 25,000 feet equals roughly 94 m/s (338 km/h or 210 mi/h). That averages out at 32,500 ft (9.9 km) and velocity of 150 m/s (540 km/h or 336 mi/h).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The normal entry region for drogue chute &lt;strong&gt;deployment by barometric switch&lt;/strong&gt; (such as was the case with &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11&lt;/a&gt;) is described at altitudes between 25,000 and 20,000 feet (7.6 - 6.1 km) and CM velocity between mach 0.225 and 0.475 that come out at roughly 70.65 - 147.25 m/s (158 - 329 mi/h or 254 - 530 km/h). That averages out &lt;strong&gt;at 22,500 feet and 109 m/s (243.5 mi/h or 392 km/h)&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-04T17:35:57.000" LastActivityDate="2013-11-04T17:35:57.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2664" PostTypeId="1" AcceptedAnswerId="5828" CreationDate="2013-11-04T17:58:55.340" Score="7" ViewCount="693" Body="&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;noreferrer&quot;&gt;मंगलयान&lt;/a&gt; (Maṅgalayāna)&lt;/strong&gt; is due to launch on November 05, 2013 AD of the Gregorian Calendar at 1436H (GMT+0530). &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission#Launch.2C_transfer_and_orbit&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The November 2013 launch will place the Mars Orbiter Mission into Earth orbit, then six engine firings will raise that orbit to one with an apogee of 215,000 km and a perigee of 600 km, where it will remain for about 25 days. A final firing in 30 November 2013 will send MOM onto an interplanetary trajectory.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same article goes on to write &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The spacecraft's dry mass is planned to be 500 kg, and it will carry 850 kg of propellant and oxidiser. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm no rocket-man; what fraction of the 850KG is expected to be consumed during the course of transfer, and correction burn? Put another way, how much fuel will remain available to enter Mars' orbit?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user12" LastEditDate="2013-11-04T23:11:08.150" LastActivityDate="2014-11-06T01:19:30.300" Title="Approximately how much fuel will ISRO Mars Orbiter have available when it enter Mars' orbit?" Tags="|mars|orbital-maneuver|fuel|isro|mars-orbiter-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2665" PostTypeId="5" CreationDate="2013-11-04T23:09:04.953" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-04T23:09:04.953" LastActivityDate="2013-11-04T23:09:04.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2666" PostTypeId="4" CreationDate="2013-11-04T23:09:04.953" Score="0" Body="Questions regarding the United Kingdom's space program and its missions." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-04T23:09:04.953" LastActivityDate="2013-11-04T23:09:04.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2667" PostTypeId="5" CreationDate="2013-11-04T23:09:51.940" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-04T23:09:51.940" LastActivityDate="2013-11-04T23:09:51.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2668" PostTypeId="4" CreationDate="2013-11-04T23:09:51.940" Score="0" Body="Questions regarding vertical rocket launch platforms and facilities in their immediate vicinity supporting final preparations for a rocket launch." OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2016-01-17T21:06:37.680" LastActivityDate="2016-01-17T21:06:37.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2669" PostTypeId="1" AcceptedAnswerId="2672" CreationDate="2013-11-05T04:17:20.070" Score="7" ViewCount="339" Body="&lt;p&gt;I was reading &lt;a href=&quot;http://www.spacenews.com/article/isros-future-plans-will-focus-low-cost-exploration&quot;&gt;this article&lt;/a&gt; (sorry for the terrible formatting), and I noticed this statement made by an ISRO official:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nair said no decision has yet been made about sending Indian astronauts into space. &quot;&lt;strong&gt;We are assessing the feasibility of manned missions and these studies will take another year to complete,&lt;/strong&gt;&quot; he said. &quot;Once approved by the government, it will take seven to eight years for ISRO to put an Indian in orbit.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The part about feasibility studies caught my eye - hasn't it kind of already been proven feasible to put people in space? It's been done countless times before. What is feasibility studies is ISRO conducting? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="4660" LastEditDate="2015-07-11T14:24:24.050" LastActivityDate="2015-07-14T05:27:22.910" Title="What feasibility studies are being conducted by ISRO concerning manned flight?" Tags="|crewed-spaceflight|isro|research|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2670" PostTypeId="1" CreationDate="2013-11-05T04:30:46.980" Score="13" ViewCount="789" Body="&lt;p&gt;Fuel weight is a major limitation on the range of manned spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might make sense to pre-position fuel tanks (establish depot/depots) in orbit with different planets and moons to serve as docking stations to refuel manned craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any plans/studies on establishing depots in space? &lt;/p&gt;&#xA;" OwnerUserId="997" LastEditorUserId="168" LastEditDate="2013-11-05T08:07:14.223" LastActivityDate="2016-12-05T02:42:05.413" Title="Are there any plans to establish depots in space?" Tags="|mars|crewed-spaceflight|spacecraft|fuel-depots|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2671" PostTypeId="2" ParentId="2670" CreationDate="2013-11-05T05:16:15.243" Score="12" Body="&lt;p&gt;Yes.  This is considered in human interplanetary missions in the form of pre-placement of chemical rocket stages by solar-electric propulsion (SEP) systems.  SEP is very efficient, but slow, whereas chemical rockets are inefficient, but provide fast transit times.  You want fast transit times for humans to minimize radiation problems and to reduce the required supplies.  SEP lets you get the chemical stages out there efficiently to wait for the humans to go use them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for why stages, a rocket engine is a small fraction of the mass of a stage.  So if you're going to pre-position propellants, you might as well attach an engine to the tanks to have a ready-to-go stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The chemical propellants considered are usually cryogenic, which adds the difficulty of keeping the propellant cold.  There are &lt;a href=&quot;http://www.nasa.gov/mission_pages/tdm/cpst/&quot;&gt;technology development efforts underway&lt;/a&gt; to address that problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nicadd.niu.edu/~mwiesner/Landau_Strange.pdf&quot;&gt;Landau and Strange&lt;/a&gt; found that pre-positioning chemical stages in high-Earth orbit actually gets the propellant most of the way anywhere energetically, is convenient for stable storage and access (with the help of our Moon), and the propellant can be used very efficiently during a perigee pass close to Earth.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-05T18:04:47.120" LastActivityDate="2013-11-05T18:04:47.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2672" PostTypeId="2" ParentId="2669" CreationDate="2013-11-05T07:29:58.390" Score="5" Body="&lt;p&gt;As @Everyone said it's not regarding feasibility but it's matter of budget and absence of human rated space vehicle and components&lt;/p&gt;&#xA;&lt;h3&gt;Budget&lt;/h3&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Development of a fully autonomous&#xA;orbital vehicle to carry a two-&#xA;member crew into a low-Earth orbit&#xA;(LEO) has already begun. ISRO&#xA;sources said the flight is likely to be&#xA;in 2016. Government had allocated&#xA;950 million (US \$14.5 million) for pre-&#xA;project initiatives for 2007 through&#xA;2008. A manned mission into space&#xA;would require about 124 billion (US&#xA;\$1.9 billion) and a period of seven&#xA;years. Planning Commission estimates&#xA;that a budget of 50 billion (US&#xA;\$765.0 million) is required for initial&#xA;work on the manned mission during&#xA;the eleventh five-year plan (2007–&#xA;12). A project report prepared by&#xA;ISRO has been cleared by space&#xA;commission. In February 2009&#xA;the Government of India gave the&#xA;green light for the manned space&#xA;flight programme, due to &lt;a href=&quot;http://en.m.wikipedia.org/wiki/Indian_human_spaceflight_programme&quot; rel=&quot;noreferrer&quot;&gt;launch&lt;/a&gt; in&#xA;2016.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h3&gt;Human rated vehicle&lt;/h3&gt;&#xA;&lt;p&gt;The failure of GSLV  in 2010 one made in India and other imported from Russia made the human space program to loss it's momentum.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The NASA CCP human-rating&#xA;standards require that the&#xA;probability of loss on ascent is no&#xA;more than 1 in 500, and that the&#xA;probability of loss on descent is no&#xA;more than 1 in 500. The overall&#xA;mission loss risk, which includes&#xA;vehicle risk from micrometeorites&#xA;and orbital debris while in orbit for&#xA;up to 210 days is no more than 1 in&#xA;270. [2] Maximum sustained G-loads&#xA;are limited to three Earth-&lt;a href=&quot;http://en.m.wikipedia.org/wiki/Human-rating_certification&quot; rel=&quot;noreferrer&quot;&gt;standard&lt;/a&gt;&#xA;g's&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;ISRO must ensure that these conditions are satisfied&lt;/p&gt;&#xA;&lt;h3&gt;Space accessories&lt;/h3&gt;&#xA;&lt;p&gt;ISRO has planned to make everything indigenouly instead of buying from overseas countries. So lots of research must done to ensure &lt;a href=&quot;http://articles.economictimes.indiatimes.com/2012-09-17/news/33902713_1_cryogenic-engine-gslv-mk-iii-radhakrishnan-today&quot; rel=&quot;noreferrer&quot;&gt;reliability&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Radhakrishnan, however, said ISRO&#xA;already has a budget of Rs 150 crore&#xA;for conducting pre-project studies and&#xA;developing critical technologies including&#xA;environmental control, space suit,&#xA;reentry and crew escape system. The&#xA;rocket must have man-rating, he&#xA;noted.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;And another important thing I think is training astronauts and ISRO is now building astronaut training facilities in Bangalore and still it's under construction and it will become functional by &lt;a href=&quot;http://www.siliconindia.com/shownews/_ISRO_to_set_up_astronaut_training_institute-nid-48055-cid-2.html&quot; rel=&quot;noreferrer&quot;&gt;2013&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-11-05T14:57:09.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2673" PostTypeId="1" CreationDate="2013-11-05T10:19:05.907" Score="6" ViewCount="297" Body="&lt;p&gt;This &lt;a href=&quot;http://www.thehindu.com/sci-tech/india-scales-down-mars-mission/article4279757.ece&quot;&gt;news-report&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;India’s bid to explore Mars will be a scaled down affair with the space agency flying experimental payload of less than 15 kg as against 25kg planned originally&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Was the Mars mission really scaled down?&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Why?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;What instruments in the original payload did &lt;em&gt;not&lt;/em&gt; make it to the scaled down mission?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2013-11-05T10:53:51.173" LastActivityDate="2013-11-05T18:22:04.700" Title="Was the ISRO Mars mission really scaled down?" Tags="|mars|isro|pslv|mars-orbiter-mission|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2674" PostTypeId="1" AcceptedAnswerId="2709" CreationDate="2013-11-05T11:35:16.513" Score="11" ViewCount="947" Body="&lt;p&gt;Do any of the developing space programs (i.e. not NASA, ESA or Roscosmos) use horizontal launcher assembly in the launcher integration building and launchpad loading like the Soyuz at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikonur_Cosmodrome&quot; rel=&quot;noreferrer&quot;&gt;Baikonur Cosmodrome&lt;/a&gt; does and it will also be done with future &lt;a href=&quot;https://en.wikipedia.org/wiki/Guiana_Space_Centre&quot; rel=&quot;noreferrer&quot;&gt;Guiana Space Centre&lt;/a&gt; launches of the Soyuz rocket family?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;/sup&gt;&lt;img src=&quot;https://i.stack.imgur.com/yriYA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Soyuz rocket assembly: the first and second stages are in the background, already joined together; the third stage is in the lower&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;left corner of the image. The Soyuz spacecraft, covered by its launch shroud, is in the lower right corner. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_%28rocket_family%29&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;/sup&gt;&lt;img src=&quot;https://i.stack.imgur.com/ljZid.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Soyuz TMA-16 launch vehicle being transported to launchpad at Baikonur. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikonur_Cosmodrome&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;/sup&gt;&lt;img src=&quot;https://i.stack.imgur.com/1NWK6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;A Russian Soyuz TMA-02M rocket is mounted at a launch pad in a Russian leased Kazakh Baikonur cosmodrome on June 5, 2011.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;(Source: &lt;a href=&quot;http://www.theepochtimes.com/n2/world/astronauts-depart-for-the-international-space-station-photos-57370.html&quot; rel=&quot;noreferrer&quot;&gt;The Epoch Times&lt;/a&gt;, Copyright: Vyacheslav Oseledko/AFP/Getty Images)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do, for example, Chinese or perhaps Japan's JAXA use horizontal launcher assembly and launchpad loading? If they do, please add for which rocket families they use such assembly,  where are they assembled and how are they transported and lifted to the launchpad.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-24T18:57:02.343" LastActivityDate="2013-11-24T18:57:02.343" Title="Do any of the developing space programs use horizontal launcher assembly and launchpad loading?" Tags="|developing-space-programs|launch-site|launchpad|launcher-integration|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2675" PostTypeId="1" AcceptedAnswerId="2682" CreationDate="2013-11-05T11:18:24.003" Score="35" ViewCount="8762" Body="&lt;p&gt;I'm curious to know why any satellites that need to be sent to other planets should be moving around the Earth rather than directly going towards their destination? Today, India launched one satellite to Mars, and from the news I got to know these lines:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://www.ndtv.com/article/india/india-s-mission-to-mars-successfully-completes-first-stage-441799?pfrom=home-lateststories&quot;&gt;The launch vehicle will stay in the Earth's orbit for nearly a month,&#xA;  building up the necessary velocity to break free from our planet's&#xA;  gravitational pull.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So my question is, why can't we directly send satellites to another planet?&lt;/p&gt;&#xA;" OwnerUserId="1006" OwnerDisplayName="javaBeginner" LastEditorUserId="49" LastEditDate="2013-11-05T14:05:31.250" LastActivityDate="2019-08-19T04:46:18.550" Title="Why should space probes have to orbit the Earth before being launched at other planets?" Tags="|mars|orbital-mechanics|orbital-maneuver|isro|mars-orbiter-mission|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2676" PostTypeId="2" ParentId="2675" CreationDate="2013-11-05T13:40:53.460" Score="14" Body="&lt;p&gt;In the case of India's &lt;a href=&quot;http://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;PSLV&lt;/a&gt;, the launch vehicle &lt;strong&gt;is not powerful enough to insert the Mars Orbiter Mission probe directly on a route to Mars&lt;/strong&gt;. In contrast &lt;a href=&quot;http://en.wikipedia.org/wiki/MAVEN&quot; rel=&quot;noreferrer&quot;&gt;MAVEN&lt;/a&gt; is to be launched using the much more powerful Atlas V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The probe has to use &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;noreferrer&quot;&gt;gravity assist&lt;/a&gt; to gain enough velocity to travel to Mars. I.e. the probe first goes into a highly elliptical orbit, and uses specially timed short burns to achieve an acceleration due to Earth's gravity and motion around the sun. Especially the motion of the Earth relative to the sun and the probe is here of help, since this impulse is &lt;em&gt;added&lt;/em&gt; to the probe's existing impulse. The picture in the Wikipedia article is quite telling:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mXAPP.png&quot; alt=&quot;Gravitational slinghot&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="999" OwnerDisplayName="Arne" LastEditorUserId="288" LastEditDate="2013-11-05T16:56:46.177" LastActivityDate="2013-11-05T16:56:46.177" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2678" PostTypeId="2" ParentId="2675" CreationDate="2013-11-05T14:26:48.977" Score="24" Body="&lt;p&gt;As @Ame mentioned, the rocket didn't have enough fuel to put it there in one shot, like most US/Russian rockets do. However, the actual physics behind the orbital maneuver is slightly different than described. Specifically, the physics is called the &lt;a href=&quot;http://en.wikipedia.org/wiki/Oberth_effect&quot;&gt;Oberth effect&lt;/a&gt;. The short explanation of this is that a rocket thrust is more effective if done at perigee. Firing the rocket during successive perigee passes will improve the efficiency, ultimately allowing one to have maximum efficiency in one's rocket bursts. A true gravity assist does not use any thrusters, but this effect essentially magnifies the effectiveness of the thrust.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-05T14:26:48.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2679" PostTypeId="2" ParentId="2101" CreationDate="2013-11-05T14:37:42.720" Score="19" Body="&lt;p&gt;One interesting answer to your question is &lt;a href=&quot;http://www.copenhagensuborbitals.com/&quot;&gt;Copenhagen Suborbitals&lt;/a&gt;. They're almost a back-yard project. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Our mission is very simple. We are working towards launching a human&#xA;  being into space.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This is a non-profit suborbital space endeavour founded and led by&#xA;  Kristian von Bengtson and Peter Madsen, based entirely on sponsors,&#xA;  private donators and part time specialists. Since May 2008 we have&#xA;  been working full time to reach our goal of launching ourselves into&#xA;  space and to show the world that human space flight is possible&#xA;  without major government budgets and administration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Their interesting solution to the legal problem is to launch from international waters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.wired.com/wiredscience/author/kvonbengtson/&quot;&gt;Blog&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1001" LastActivityDate="2013-11-05T14:37:42.720" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2680" PostTypeId="2" ParentId="2674" CreationDate="2013-11-05T15:10:26.797" Score="4" Body="&lt;p&gt;I wonder if there is a difference in approach today, that may affect design choices like this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a national prestige program, versus a commercial launch provider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ultimate goals are somewhat different.  You would expect both are aimed at launching things into space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But consider Orbital or SpaceX vs JAXA (Japanese space agency). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;JAXA is very much a prestige program. Just look at the cost of the H2 launch vehicles and the fact they continue to launch them. No rational consumer would ever buy a launch on such an expensive launcher when cheaper options exist.  Yet JAXA continues to use this launcher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When building a prestige space program, a tall building like the VAB is fairly straightforward to build (Nothing rocket science in building design anymore) and it looks mighty impressive when it is covered in your national flag. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are building a cost effective launch site, you look at what is cheaper and more effective. Look at the Orbital and SpaceX facilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russia is probably an outlier in this model. They kept everything secret till it succeeded, then it became a prestige program.  I suspect they had some inferiority issues compared to the US at the time and thus the need to show them up, so they were more focused on success than direct prestige, but prestige came as a secondary feature.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-11-05T15:10:26.797" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2681" PostTypeId="2" ParentId="2670" CreationDate="2013-11-05T15:18:20.317" Score="7" Body="&lt;p&gt;There are lots of studies.  Are any serious?  Probably not at this time.  That is, nothing is funded. The US is unlikely to do so, due to the fixation on the SLS which is basically a design that says &quot;We don't need no stinkin depots&quot;.  The Russians seem to have no great ambitions at this time. The Chinese are not yet at a sufficient state to consider this.  Probably no one else in the world has a sufficiently serious program to be worth considering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is the direct answer.  As an aside, there are many benefits to depots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One that is important is that it opens a market for competition.  That is, if someone needs a depot for a mission (be it a government or a company) they have just created a market.  They need something.  They are willing to pay for it. Now, companies can compete to provide payload to it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depots are great examples for this, since really the cargo being launched is dirt cheap. LOX, LH2, Kerosene, or water or whatever. So any launcher can deliver it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cheapest one will win. Reliability interestingly is not that important if the payload is cheap and the launcher cost is cheap, and there is no immediate time dependency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus if the US for example, eschewed the SLS approach and went with a depot approach some interesting things could happen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They could stimulate an industry for launch.  Doesn't matter if it takes 100 Falcon 1e launches to fill the depot, if they are cheap enough, and launch frequently enough.  Nor does it matter if you fill the depot with a Delta 4-Heavy launch.  What matters is cost, and ability to deliver in time. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-11-05T15:18:20.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2682" PostTypeId="2" ParentId="2675" CreationDate="2013-11-05T16:54:45.303" Score="45" Body="&lt;p&gt;All interplanetary probes that I am aware of were launched into a parking orbit, and then waited some time in that orbit before restarting a stage or igniting another stage to inject on the desired outgoing asymptote.  This is done for convenience to allow long launch windows on days in the launch period.  It is possible and slightly more efficient to launch directly from the launch pad to the interplanetary trajectory, but you will have an instantaneous launch window and your launch pad needs to cross the plane of the outgoing trajectory to get that efficiency.  Instantaneous launch windows are risky, since if there's a problem such as a boat where it shouldn't be, you don't launch that day.  With the longer window offered by going first into a parking orbit, often an hour or more, you have time to remedy the problem and still launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(As an aside, I worked on a project, which was later cancelled, that was going to use an Ariane 5 to launch to Mars.  The problem was that its upper stage did not at the time have a qualified restart capability.  As a result, we had to plan to inject directly from the launch pad with no parking orbit.  Though the French Guiana launch site was fortuitously close to where we would want it for that opportunity, it wasn't exactly right so we had to incur some inefficiency with a dog-leg maneuver to get going the right way.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In most cases the wait in the parking orbit is measured in tens of minutes.  You are often restarting a second stage with limited battery life, so you don't want to wait for more than an orbit.  In order to have a two week or longer launch period, you need to accept some inefficiency in higher injection energies before and after the optimal day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;India is doing something a little different, which has some risk but offers higher efficiency.  They have a fixed departure day from Earth orbit with the smallest injection energy.  Whenever they launch, they wait in Earth orbit until that blessed day, and then they go.  That means they need an upper stage with longer life.  On the plus side, breaking up the injection burn into several pieces, executed at each perigee, allows the engine to be much smaller and lighter than a typical upper stage engine that's trying to flow out all the fuel in a single burn near the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As noted in the other answers, the injection burns whether done 20 minutes later or 20 days later are done at perigee for the maximum energy change per unit of $\Delta V$.  I'll throw an equation in here just for fun.  In basic physics you learn the equation for kinetic energy: $K={m v^2\over 2}$.  If you differentiate that with respect to $v$, you get $dK=m v\,dv$.  So your change in energy is proportional to your velocity times the $\Delta V$.  The faster you're going, the more change in energy you get for a fixed $\Delta V$.  You're going the fastest at perigee.  This is known as the Oberth effect.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-06T05:05:30.210" LastActivityDate="2013-11-06T05:05:30.210" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="2683" PostTypeId="2" ParentId="2673" CreationDate="2013-11-05T18:22:04.700" Score="3" Body="&lt;p&gt;Originally, MOM was going to &lt;a href=&quot;http://isp.justthe80.com/planetary-exploration/mars-obiter&quot; rel=&quot;nofollow&quot;&gt;use the GSLV&lt;/a&gt; to launch. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Geosynchronous_Satellite_Launch_Vehicle&quot; rel=&quot;nofollow&quot;&gt;GSLV&lt;/a&gt; has about 50% more capacity than the &lt;a href=&quot;http://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;nofollow&quot;&gt;PSLV&lt;/a&gt;. As a result, a few things had to be cut back from the mission, including the payload, removal of a direct to Mars orbit, among others.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-05T18:22:04.700" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2684" PostTypeId="1" AcceptedAnswerId="2688" CreationDate="2013-11-06T03:01:33.810" Score="13" ViewCount="999" Body="&lt;p&gt;I was reading recently about the near-loss of the Solar and Heliospheric Observatory (SOHO) spacecraft back in 1998, in &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory#Near_loss_of_SOHO&quot;&gt;this Wikipedia article&lt;/a&gt;. This article mentions that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Only one gyro remained operational after this recovery, and on December 21 that gyro failed. Attitude control was accomplished with manual thruster firings that consumed 7 kg of fuel weekly, while ESA developed a &lt;strong&gt;new gyroless operations mode&lt;/strong&gt; that was successfully implemented on February 1, 1999.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is this gyroless operations mode? What does it depend on to determine the spacecraft's attitude? Are there other spacecraft that use it?&lt;/p&gt;&#xA;" OwnerUserId="653" LastActivityDate="2013-11-07T00:00:49.680" Title="How is gyroless operation possible?" Tags="|attitude|spacecraft|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2685" PostTypeId="1" AcceptedAnswerId="2692" CreationDate="2013-11-06T04:54:54.967" Score="9" ViewCount="432" Body="&lt;p&gt;In March of this year, &lt;a href=&quot;http://www.usatoday.com/story/news/nation/2013/03/20/apollo-rocket-engines-moon-bezos/2003533/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jeff Bezos' team found Saturn V engines on the ocean floor&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yg2LN.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;While they will probably be placed in a museum, I'm wondering if there could possibly be a more practical use for the engines - could any parts be salvaged off of them? (And used in a new Saturn V with minimal restoration, I.e. no melting and recasting?)&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="195" LastEditDate="2023-05-04T02:30:51.343" LastActivityDate="2023-05-04T02:30:51.343" Title="Could parts be salvaged off the Apollo engines found on the ocean floor?" Tags="|rockets|apollo-program|reuse|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2686" PostTypeId="5" CreationDate="2013-11-06T05:01:23.087" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-06T05:01:23.087" LastActivityDate="2013-11-06T05:01:23.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2687" PostTypeId="4" CreationDate="2013-11-06T05:01:23.087" Score="0" Body="Questions regarding the reuse of parts or equipment." OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2013-12-04T16:48:56.183" LastActivityDate="2013-12-04T16:48:56.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2688" PostTypeId="2" ParentId="2684" CreationDate="2013-11-06T05:17:01.843" Score="9" Body="&lt;p&gt;SOHO as well as most spacecraft use star trackers for attitude determination, aided by Sun sensors for recovery and fault protection operations.  Gyroless operation is &lt;em&gt;normal&lt;/em&gt;.  In fact it is common to turn them off when you don't need them to preserve lifetime.  Gyros can only tell you your roll rates and how your attitude is changing over time.  They can't tell you your attitude by themselves.  They are used to improve your attitude knowledge over short time scales.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-07T00:00:49.680" LastActivityDate="2013-11-07T00:00:49.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2689" PostTypeId="1" AcceptedAnswerId="2693" CreationDate="2013-11-06T05:46:27.790" Score="7" ViewCount="805" Body="&lt;p&gt;After successfully completing the placement of the Mangalyaan into orbit, is there any plan for ISRO to launch a spacecraft to Venus ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="6723" LastEditDate="2014-11-27T03:01:12.413" LastActivityDate="2014-11-27T03:01:12.413" Title="Is there any plan for ISRO to reach Venus?" Tags="|launch|isro|venus|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2690" PostTypeId="2" ParentId="2684" CreationDate="2013-11-06T05:47:46.563" Score="8" Body="&lt;p&gt;The issue, and it's resolution are listed in detail on the &lt;a href=&quot;http://sohowww.nascom.nasa.gov/about/Recovery/blogs/updatesnogyros.html&quot;&gt;NASA web-site&lt;/a&gt;. The web-site links to the exact maneuver appear to be broken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However &lt;a href=&quot;http://sohowww.estec.esa.nl/operations/Recovery/olive.pdf&quot;&gt;this paper&lt;/a&gt; sums up the gyro-less operation as follows&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On December 21 1998, the third and last gyro was lost, which put SOHO into ESR mode.&#xA;  With new solutions to:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;reduce the orbit perturbation of the ESR mode (by manual yaw braking from the&#xA;  ground),&lt;/li&gt;&#xA;  &lt;li&gt;accurately measure the roll rate (wheels spun-up so that there is a net momentum on&#xA;  pitch, which combined with the roll rate creates a yaw disturbance torque),&lt;/li&gt;&#xA;  &lt;li&gt;patch the Attitude Control Software to fly SOHO without gyro control,&lt;/li&gt;&#xA;  &lt;li&gt;implement new or updated procedures, including the one to recover from ESR (now&#xA;  done without gyroscope control),&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;SOHO spacecraft was put back into normal mode on February 1, 1999.&#xA;  Since then it is the first three-axis-stabilized ESA spacecraft to be operated without a&#xA;  gyro.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="288" LastActivityDate="2013-11-06T05:47:46.563" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2691" PostTypeId="2" ParentId="2675" CreationDate="2013-11-06T06:06:53.423" Score="20" Body="&lt;p&gt;There are several reasons why Satellites need to orbit Earth before they go interplanetary...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first reason:&#xA;The launch site is very rarely in the right position to start an interplanetary flight. Earth rotates on a tilt, so a launch has to be timed when Kennedy Space Center crosses the ecliptic plane (the general plane that most planets orbit on). Also, it has to be in the right season, so that when the probe goes out to orbit it ends up headed the right direction when it leaves Earth's SOI. All of this is possible with a direct ascent profile, it just takes really good timing - but these perfect windows come very rarely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any probe that makes ecliptic orbit around Earth, first, has a launch window pretty much every 45 minutes, as apposed to a couple times per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second reason:&#xA;The amount of Delta-V required to escape Earth's SOI is pretty big. While it is possible to build rockets large enough to do so - the limiting factor is really the efficiency of typical rocket fuel and rocket engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To lift a probe out of Earth's orbit with a rocket takes a pretty heavy rocket. That heavy rocket must be lifted into Low Earth orbit, which takes a massive rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to improve this fact is to make your rocket much more efficient - but we are already close to the theoretical efficiency limit of a chemical rocket. So NASA started using ION propulsion, which is way more efficient than a chemical rocket - it's also very weak - which is the thirdreason...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third reason:&#xA;Now that most probes use Ion propulsion, they don't have the thrust to just eject themselves from earth in a direct ascent - they spend weeks with the Ion thruster thrusting for a little while (a few minutes) at a key point in the orbit. Each time the Ion engine does this, their orbit gets closer and closer to Earth escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the probe is out of Earth's SOI, it can basically turn the Ion engine On and leave it there for as long as it wants to complete interplanetary maneuvers. Usually, most maneuvers between planets are minor course corrections to take advantage of a planetary fly-by to sling-shot to much higher velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TL;DR?&#xA;Many reasons:&#xA;Timing - launch position and orbital orientation mean few good launch windows for direct ascent, reaching orbit first allows for many more options.&#xA;Too much fuel needed - Getting a probe way from the earth takes a lot of chemical propellant, so we now use Ion engines instead.&#xA;Ion engines are weak - it takes a long time (weeks!) for these highly efficient engines to do their job.&lt;/p&gt;&#xA;" OwnerUserId="700" LastActivityDate="2013-11-06T06:06:53.423" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2692" PostTypeId="2" ParentId="2685" CreationDate="2013-11-06T07:58:01.200" Score="15" Body="&lt;p&gt;The photos show they're in no state to be used for anything. It's all badly corroded on the outside, and the inside will be similar. For example, the nozzle is surrounded by hundreds of tubes. Fuel was circulated through them to cool the nozzle. There's no way to clean the inside of all these tubes, so you'd have to build a new nozzle.&lt;br&gt;&#xA;Similarly, the turbopump (which has to be absolutely straight and balanced or it'll tear itself apart when running at tens of thousands of rpm) will be bent by the impact. Even of you could clean it all up, removing the rust means the part is now thinner than before, and would be out of spec.&lt;br&gt;&#xA;You'd be better off manufacturing new ones; then you'd also have the chance to improve on the original design.&lt;br&gt;&#xA;An F-1 that was stored properly was used for some tests &lt;a href=&quot;http://www.nasa.gov/exploration/systems/sls/f1_sls.html&quot;&gt;earlier this year&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-11-06T07:58:01.200" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2693" PostTypeId="2" ParentId="2689" CreationDate="2013-11-06T09:59:37.700" Score="8" Body="&lt;p&gt;&lt;a href=&quot;http://articles.timesofindia.indiatimes.com/2012-02-17/hyderabad/31070866_1_venus-mission-chandrayaan-1-isro&quot;&gt;There were plans for a Venus mission in 2015&lt;/a&gt; (&lt;a href=&quot;http://www.asianscientist.com/topnews/isro-indian-mission-to-planet-venus-2012/&quot;&gt;alternate article&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An Indian scientific mission to Venus is being considered by the space&#xA;  community. This was announced by M S Anurup of Isro at the 17th&#xA;  National Space Science Symposium which has been jointly organised by&#xA;  Isro and Sri Venkateshwara University.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;According to Anurup, a preliminary study had indicated that if this&#xA;  mission gets the go ahead and depending upon the orbital parameters,&#xA;  it could be launched on May 20, 2015 arriving in Venus in October&#xA;  2015.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't see any indication of this mission being scrapped, but I don't see any further announcements either. I think it's pretty OK to assume that it is still ongoing.&lt;/p&gt;&#xA;" OwnerUserId="96" LastActivityDate="2013-11-06T09:59:37.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2694" PostTypeId="2" ParentId="2685" CreationDate="2013-11-06T15:44:51.777" Score="4" Body="&lt;p&gt;As @Hobbes mentioned, using the parts directly would be a bad idea. However, there might still be something practical that could be done with them. The main thing of interest would be trying to figure out what went right in the design, and any issues that might have resulted from a poor design. But such things would probably be pretty minimal. They might be useful in the case of Apollo 13, which had an engine flame out on it's way to orbit, but aside from that, not much. And the engines that went out were the J-2 engines anyways, not the F-1 engines recovered!&lt;/p&gt;&#xA;&lt;p&gt;In general, however, the rocket has far more sentimental value than anything practical, and as such, should be restored and put in a museum.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2023-05-04T02:16:17.777" LastActivityDate="2023-05-04T02:16:17.777" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2695" PostTypeId="2" ParentId="2675" CreationDate="2013-11-06T16:03:33.143" Score="8" Body="&lt;p&gt;Before you get to all other advantages, let's start with the single, essential one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rocket floating in place (not flying up at all, just hovering over the launchpad)  needs to generate enough thrust that would otherwise give it $9.8 m/s^2$ acceleration (1g) in horizontal flight. That's quite a bit, and that's all wasted thrust. If you launch directly up, earth gravitational acceleration is what you have to substract from your acceleration at all times. Every second, no matter what else you spend your fuel on, you need to spend a hefty dose on just not falling down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, a rocket moving horizontally on orbit doesn't use any fuel to maintain altitude. Every single gram of it is used to increase its kinetic energy. That way, your first priority as soon as you reduce air friction to manageable levels is to enter orbit and stop wasting fuel trying to prevent falling down. The energy you put in accelerating to orbital speed is still usable, you will add it to your speed in the interplanetary travel, and now is safely stored as your kinetic energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, once in orbit you can do all kinds of useful maneuvers at your leisure, switch to engines of better specific impulse but lower thrust, and not waste any more fuel.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-11-06T16:03:33.143" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2696" PostTypeId="1" AcceptedAnswerId="2705" CreationDate="2013-11-06T16:10:07.620" Score="11" ViewCount="1312" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/giO7l.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recently noticed the comic above, and wondered what, exactly, makes it so hard to tell whether Voyager 1 has left the solar system. I can think of three reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Scientists can't agree on where the solar system ends&lt;/li&gt;&#xA;&lt;li&gt;It's hard to tell just exactly how far out the probe is&lt;/li&gt;&#xA;&lt;li&gt;Or both.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Which is it?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2013-11-07T19:42:05.300" Title="Why is it so hard to figure out if Voyager 1 has left the solar system?" Tags="|probe|voyager|solar-system|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2697" PostTypeId="2" ParentId="2696" CreationDate="2013-11-06T17:08:13.007" Score="1" Body="&lt;p&gt;It is number 1.  No one entirely knows where the Solar System ends, and interstellar space begins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or put another way, this is the first time we have had a chance to look at conditions that far out, and they are constantly revising their ideas of of what is out there, and where it ends.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The data from Voyager keeps coming in, and it is changing the definition of the edge of the Solar System.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-11-06T17:08:13.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2698" PostTypeId="1" AcceptedAnswerId="4588" CreationDate="2013-11-06T20:32:36.230" Score="9" ViewCount="1445" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISRO&quot;&gt;Wikipedia&lt;/a&gt; lists both Hindi and English as the official languages of ISRO. But what language does ISRO use for radio communication? Say they launch a manned mission, which language would they use to communicate with their astronauts? &lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorDisplayName="user12" LastEditDate="2013-11-07T04:55:09.153" LastActivityDate="2014-12-08T22:39:25.533" Title="What language does ISRO use for radio communication?" Tags="|communication|isro|language|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2699" PostTypeId="1" AcceptedAnswerId="2701" CreationDate="2013-11-06T20:08:29.780" Score="14" ViewCount="1941" Body="&lt;p&gt;I've seen an hypothetical &lt;a href=&quot;http://www.isec.org/sec/&quot;&gt;&quot;Space elevator&quot;&lt;/a&gt; referenced in various media the last couple of years. Is this something thats really planned in the making, or just a hypothetical scenario? How will a space elevator work, if it's even theoretically possible to make one?&lt;/p&gt;&#xA;" OwnerUserId="416" OwnerDisplayName="Haaakon" LastEditorUserId="51" LastEditDate="2015-07-02T20:51:07.147" LastActivityDate="2015-09-27T23:12:31.143" Title="What is a &quot;space elevator&quot;?" Tags="|launch|space-elevator|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2700" PostTypeId="2" ParentId="2699" CreationDate="2013-11-06T20:11:51.823" Score="3" Body="&lt;p&gt;A space elevator on Earth is, for now, science fiction. We have the technology to build one on the Moon, though. Building one on any body would greatly reduce costs to launch payloads into orbit or beyond, so this is definitely a goal and not just a hypothetical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to how it would work, imagine a cross between an elevator and a train. Most of the elevator would be outside of the atmosphere (if the body has one at all), so the cars could move very quickly without encountering air resistance.&lt;/p&gt;&#xA;" OwnerUserId="1385" OwnerDisplayName="Sparr" LastActivityDate="2013-11-06T20:11:51.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2701" PostTypeId="2" ParentId="2699" CreationDate="2013-11-06T20:53:41.430" Score="14" Body="&lt;p&gt;The idea behind a Space Elevator is to build a structure, usually from the surface of the Earth (Though there are notions of ones that just touch the lower atmosphere) to Geosynchronous orbit (22,000Miles) and then to balance the weight out, a similar distance of structure continuing on a further 22,000 miles (or else a large counterweight)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It needs to be GeoSync, so that it stays in orbit over the same spot on Earth, if you wish to attach it to the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now attach a platform that is powered and can climb up and down this structure and you have a functional elevator to GeoSync orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In principal solar or beamed laser power is all you need to power the climb and descending.  Consider speed for a moment?  How fast can you build a climbing device?  It has to go 22,200 Miles.  That can take a long time, particularly if you are touching something (the elevator structure itself) and are friction limited. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What should it be made of?  Well right now we do not have anything strong enough, that could be manufactured (I note that some carbon nanotube structures have the appropriate strength, but we cannot manufacture large enough structures of them (yet)) to carry the needed load.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Sparr answered, the moon or Mars are easier targets due to lower LunaSync or AreoSync orbits and lower gravity, thus lowering the structural strength required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lots and lots of great Sci Fi stories about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kim Stanley Robinson in his &quot;Mars&quot; series has one at Mars that eventually is destroyed by terrorists, and it wraps around the planet several times in the process.  (Consider, that the circumference of the Earth is about 24,000 miles, which is pretty darn close to the height of GeoSync.  So almost twice around the Earth (Remember the extension out the back side for balance))&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-11-06T22:10:59.420" LastActivityDate="2013-11-06T22:10:59.420" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2702" PostTypeId="2" ParentId="2699" CreationDate="2013-11-06T20:54:47.910" Score="11" Body="&lt;p&gt;A &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_elevator&quot; rel=&quot;nofollow noreferrer&quot;&gt;space elevator&lt;/a&gt; is a hypothetical mode of transportation between Earth's surface and space. While the idea was popularized by science fiction author Arthur C. Clarke, the idea is much older.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, a space elevator is exactly what it sounds like. An elevator that takes you from the Earth surface up to a geostationary satellite or space station. In theory, it would make lifting payloads up to orbit cheaper than using rockets as we do today. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to do this is to capture an asteroid and place it in a geostationary orbit above the tropic. A cable is then constructed stretching from a &lt;em&gt;station&lt;/em&gt; on the asteroid to another station down on Earth. The asteroid thus becomes a &lt;em&gt;counter weight&lt;/em&gt; for the cable. Imagine twirling a yo-yo around yourself. Transporting things between the two stations could be done via using &lt;em&gt;climber&lt;/em&gt; vehicles that climb the cable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can imagine there are significant engineering hurdles that need to be tackled before something like a space elevator can be constructed. One significant hurdle is the cable itself, or rather the material used in constructing it. The only natural substance strong enough to be used for the space elevator is diamond. Of synthetic materials carbon nanotubes are a possible candidate, though we still do not have a way of constructing long lengths of it yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of now I know of only two groups attempting a space elevator: The &lt;a href=&quot;http://en.wikipedia.org/wiki/LiftPort_Group&quot; rel=&quot;nofollow noreferrer&quot;&gt;LiftPort Group&lt;/a&gt; are focusing on carbon nanotube research and are contemplating a &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_space_elevator&quot; rel=&quot;nofollow noreferrer&quot;&gt;lunar elevator&lt;/a&gt; as a preview for a terrestrial one, and the &lt;a href=&quot;http://en.wikipedia.org/wiki/Obayashi_Corporation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Obayashi Corporation&lt;/a&gt; which has set a goal date of 2050 for it's space elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are plenty of other &lt;a href=&quot;https://space.stackexchange.com/questions/tagged/space-elevator&quot;&gt;good questions (and answers)&lt;/a&gt; here in Space.SE about space elevators, so you may want to read those as well.&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2013-11-06T21:06:41.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2703" PostTypeId="1" CreationDate="2013-11-06T21:12:36.987" Score="3" ViewCount="91" Body="&lt;p&gt;In the movie &lt;em&gt;Gravity&lt;/em&gt;, there is a scene where the protagonist aims a Soyuz space craft towards a space station and engages thrust and eventually reaches that station. Many scientists have chimed in that this impossible, and that thrusting towards the target in the way depicted in the movie would actually make you go farther away from it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is that?&lt;/p&gt;&#xA;" OwnerUserId="122" LastActivityDate="2013-11-06T21:12:36.987" Title="Why does thrusting towards a target in orbit make you go farther away from it?" Tags="|orbital-mechanics|" AnswerCount="0" CommentCount="0" ClosedDate="2013-11-06T21:22:01.383" ContentLicense="CC BY-SA 3.0" />
  <row Id="2704" PostTypeId="2" ParentId="2696" CreationDate="2013-11-06T22:30:21.877" Score="6" Body="&lt;p&gt;Basically, because there is no &lt;em&gt;read ahead&lt;/em&gt;. What I mean by it is that Voyagers were never designed to make measurements of particle density, their magnetic charge, temperature and velocity with any instrument ahead of its path or even around it, so the spacecraft would first have to get there, do measurements &lt;em&gt;in situ&lt;/em&gt;, and not detect any significant change for a long time so we could say with any degree of certainty that the last change detected was indeed due to crossing over the heliopause, where the interstellar medium pressure prevails over the solar wind of the Solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that is not all there is. You see, the Voyager's plasma instrument stopped working around the time it flew by last planets back in 1980, so any of the measurements of the pressure of the interstellar space on our heliosphere could only be inferred indirectly through measuring magnetic field, or more precisely measuring magnetic orientation of the extremely tenuous particle density medium (we're talking of up to a couple of particles in a cubic centimeter), as has &lt;a href=&quot;https://space.stackexchange.com/a/2173/49&quot;&gt;Mark Adler already answered in one of his previous answers&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put this into a perhaps easier to understand analogy, imagine you're on your bicycle pedalling through a fog. You can't see ahead at all, because we've tied your eyes so you'll be our Voyager 1 trying to detect the heliopause, or in our case, where the city fog clears and you've entered a different, countryside fog. You have never before been to the countryside, so you're not exactly sure how it will feel like getting there. You've been doing fairly good for a while, sticking your tongue out and detecting humid air, got a fair impression of its temperature, velocity, and so on. You're fairly confident that if you entered a different fog, you'd be able to tell that by some of your sensors. Its temperature might change, or the density, velocity, wind direction, and so on. But to make matters worse, just as you started approaching the city suburbs, your tongue went all numb, so all you were left to use is your nose, and hope the rural countryside fog will smell slightly differently than the city one does. You've detected some change, or at least you thought you might have, but did you leave the city limits already, or did you just pass a rather stinky dumpster? You press on, and for a while you don't smell anything. Is that it? You press on. After a long while, you realize that the smell didn't change at all, so you start believing you're already at the countryside. But can you be sure?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is about why it's not easy to determine when exactly did (if at all) Voyager 1 leave our Solar system, and why NASA believes this time it really did back in August 2012. They didn't detect any significant change in magnetic orientation of the medium the probe is going through now in a good year. Now, I've again said it, didn't I? I've said &lt;em&gt;if it at all did&lt;/em&gt;. Why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, you see even though I personally have no doubt that it did leave our Solar system and entered the interstellar medium, the area of our Sun's influence isn't constant and this area would inflate and deflate like a balloon due to solar cycles, which is about 11 years long periodic change in Sun's activity. Solar maximum when the Sun is the most active, and Solar minimum when its activity subsides. Now, Voyager 1's velocity relative to the Sun is roughly 17.26 km/s, but the fastest solar winds can reach velocity of 750 km/s, which is faster than the solar escape velocity of 618 km/s. We could say, in a sense, that our Solar system is breathing, the Sun is inhaling and exhaling. Increased activity produces faster solar winds and they reach escape velocity, inflating the heliosphere. And there's the unpredictability of the density of the interstellar medium as our Solar system moves through it. We know it's not uniformly dense, and we call areas with greater density our local interstellar clouds:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/EhNkZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any change in density our Solar system moves through could also decrease or increase the influence area of our Sun. The truth though is that we don't know with what speed these heliosphere inflations and deflations happen, how much they vary in size, or how frequently the interstellar medium density fluctuates, so it's hard to predict if Voyager 1 could eventually be caught up again by the influence region of our Sun and overtaken by the fastest of solar winds, having it go through it all over again. But this is another reason why it is so hard to figure it out if Voyager 1 left the Solar system. The heliosphere isn't a static point in space. So this question isn't easily answerable with any of the options you gave in your question. In fact, I would lean towards saying neither are the most responsible for why it is difficult and there are other factors we need to consider, such as what sensors Voyager 1 still has left to its disposal, and dynamics of Solar and interstellar &lt;em&gt;weather&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://what-if.xkcd.com/38/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FmxCg.png&quot; alt=&quot;What If XKCD #38 - Voyager&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-11-07T19:42:05.300" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2705" PostTypeId="2" ParentId="2696" CreationDate="2013-11-07T01:01:33.987" Score="17" Body="&lt;p&gt;None of the above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Voyager project science team is in agreement over what is meant by leaving the solar system, knows exactly where the spacecraft is, and has been &lt;em&gt;very&lt;/em&gt; careful what they say about all this in their declarations over time.  Nevertheless the news reporting would simplify the findings to something like &quot;Voyager 1 has left our Solar System!&quot;, since that's easier to understand than &quot;The Voyager 1 spacecraft has reached a distant point at the edge of our solar system where there is no outward motion of solar wind.&quot; or &quot;The stagnation of this solar wind has continued through at least February 2011, marking a thick, previously unpredicted 'transition zone' at the edge of our solar system.&quot;  So there have been many news reports of Voyager 1 leaving the solar system over the past three years, when in fact the &lt;a href=&quot;http://voyager.jpl.nasa.gov/news/&quot;&gt;actual press releases&lt;/a&gt; were rather more specific.  In their &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2013-278&quot;&gt;September 12, 2013 press release&lt;/a&gt;, they said for the first time &quot;Voyager 1 Has Entered Interstellar Space&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 1 has left the solar system, defined as crossing the boundary between the plasma that comes from our Sun and the plasma in interstellar space.  It turns out that there is a clear boundary where the pressure from the solar wind and pressure from the local interstellar plasma wind balance.  At that boundary, there is a significant jump &lt;em&gt;up&lt;/em&gt; in plasma density when you move into the interstellar plasma.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the 25th of August, 2012, Voyager 1 measured very abrupt changes in the local environment. This was in fact Voyager 1 crossing the heliopause. In a single day.  Unfortunately, they only saw two out of the three expected signatures of having crossed the heliopause.  So being the conservative sorts they are, they did not declare a crossing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The kicker was that the magnetic field direction only changed by 2°.  They were expecting a much larger change, based on models of the direction of the local interstellar magnetic field. This was a puzzlement.  Since they did not have a direct measurement of plasma density (due to the failure of an instrument much earlier in the mission), the science team was left in a state of uncertainty about whether or not they were really in interstellar space.  From an August 2012 press release:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Scientists eagerly awaiting the entry of the two Voyagers into&#xA;  interstellar space have recently seen changes from Voyager 1 in two of&#xA;  the three observations that are expected to be different in&#xA;  interstellar space. The prevalence of high-energy particles streaming&#xA;  in from outside our solar system has jumped, and the prevalence of&#xA;  lower-energy particles originating from inside our solar system has&#xA;  briefly dipped, indicating an increasing pace of change in Voyager 1's&#xA;  environment. Voyager team scientists are now analyzing data on the&#xA;  direction of the magnetic field, which they believe will change upon&#xA;  entry into interstellar space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;By sheer luck, a large coronal mass ejection from the Sun tickled the plasma around Voyager 1 a few months later. The resulting ringing allowed them to indirectly measure the plasma density, which matched what was expected for the interstellar space in the vicinity of our solar system. They were able to find earlier, weaker oscillations, extrapolated back, and found that the density increased very rapidly in, you guessed it, August 2012. That provided the confirmation they were looking for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnetic fields were expected to have different directions since they have different sources. There would be no reason for them to have the same direction, so that would be unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, not impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, there was a very sharp change when crossing the heliopause. The determination that that really was the heliopause was delayed by an unexpected approximate alignment of magnetic fields.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-07T01:10:18.423" LastActivityDate="2013-11-07T01:10:18.423" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2706" PostTypeId="1" CreationDate="2013-11-07T01:03:49.567" Score="11" ViewCount="514" Body="&lt;p&gt;NASA is &lt;a href=&quot;http://www.spacesafetymagazine.com/2013/02/26/nasa-investigate-blurred-vision-space/&quot; rel=&quot;noreferrer&quot;&gt;pretty active&lt;/a&gt; in investigating deteriorated vision issues of astronauts that, I guess, are most frequently linked to the loss of intracranial blood pressure due to microgravity with their &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/204.html&quot; rel=&quot;noreferrer&quot;&gt;Ocular Health&lt;/a&gt; project. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/TP2so.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;NASA astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Michael_S._Hopkins&quot; rel=&quot;noreferrer&quot;&gt;Michael Hopkins&lt;/a&gt; performs ultrasound eye imaging in the Columbus laboratory of the International&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Space Station. European Space Agency astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Luca_Parmitano&quot; rel=&quot;noreferrer&quot;&gt;Luca Parmitano&lt;/a&gt; assists Hopkins. (Credit: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But watching &lt;a href=&quot;http://www.nasa.gov/multimedia/nasatv/&quot; rel=&quot;noreferrer&quot;&gt;NASA TV&lt;/a&gt; today, I've heard an interesting suggestion that the rate of deterioration in astronauts' vision might also be linked to what the interviewed scientist Scott Smith, a nutritionist and one of the authors of the recent study from &lt;a href=&quot;http://www.nasa.gov/centers/johnson/slsd/about/divisions/hacd/laboratories/nutritional-biochemistry-personnel.html&quot; rel=&quot;noreferrer&quot;&gt;NASA's Johnson Space Center's Nutritional Biochemistry Lab&lt;/a&gt;, called &lt;em&gt;dependent one-carbon metabolic pathway&lt;/em&gt;. I've been curious to see what that could mean, so I've ended up on this NASA's page on &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/vision_changes.html&quot; rel=&quot;noreferrer&quot;&gt;New Findings on Astronaut Vision Loss&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;All About the 1C Pathway&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Let's break this down: The human body is&#xA;  like a machine, with many chemical processes taking place to keep&#xA;  cells functioning, which in turn keep the body functioning. These&#xA;  chemical processes use specific pathways to do their job. Everyone has&#xA;  these pathways. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The one-carbon pathway is part of the chemical process the body uses&#xA;  to make DNA, or genetic instructions. The pathways are similar to an&#xA;  assembly line in a factory. If the pathway is not working properly, it&#xA;  gets behind or backs up. This is what happens if there is a vitamin&#xA;  deficiency. Since astronauts participate in dietary and nutritional&#xA;  assessments, researchers were able to rule out common vitamin and&#xA;  nutritional deficiencies. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Enzymes are the workers on the assembly line, and can have different&#xA;  forms -- little changes or variations that occur -- between&#xA;  individuals. These multiple forms are known as polymorphisms. As&#xA;  humans evolve, tiny changes in DNA occur resulting in multiple forms&#xA;  of enzymes. A good example of polymorphisms is blood types -- though&#xA;  blood is blood, there are differences that account for a variety of&#xA;  blood types. Lactose intolerance is another example of polymorphisms&#xA;  in the human body.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is interesting because, as Smith said:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;We now need to go another step forward to assess whether it is a&#xA;  small piece among many others, or a large piece that is a primary&#xA;  cause of this problem. Not only may the results have significant&#xA;  implications for NASA and future astronauts, but the implications for&#xA;  the general population could be profound.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But he also points out that these results, while very exciting, are preliminary and said that the next step will be to directly test for the presence of enzyme polymorphisms to verify whether this relates to the vision changes. The mentioned article is from May 2012, and as we see from the attached photograph, new expedition crew members (&lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_38&quot; rel=&quot;noreferrer&quot;&gt;Expedition 38&lt;/a&gt;) continue to perform ultrasound eye imaging tests and I imagine all kinds of other tests pre, during and post flight to support research.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question here is, has 1C pathway been successfully linked to ocular health of astronauts or have any other findings come out of NASA's research in vision deterioration of astronauts onboard the ISS since May 2012?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-16T22:06:15.777" LastActivityDate="2018-05-13T20:01:15.687" Title="Has dependent one-carbon metabolic pathway been successfully linked to ocular health of astronauts?" Tags="|iss|health|research|ocular|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2707" PostTypeId="1" AcceptedAnswerId="2711" CreationDate="2013-11-07T01:23:35.630" Score="9" ViewCount="1529" Body="&lt;p&gt;Going by known record the humans have, over the last few decades, started to look around in the Solar System. Apart from astronomy, and allied sciences - there have been space landing missions to study the various planets in-situ e.g. &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_program&quot;&gt;the Apollo Program&lt;/a&gt;,&lt;a href=&quot;http://en.wikipedia.org/wiki/Venera_program&quot;&gt;the Venera Program&lt;/a&gt;, proposed &lt;a href=&quot;http://en.wikipedia.org/wiki/Chang%27e_3&quot;&gt;Chang'e&lt;/a&gt;, various Mars landers (&lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Curiosity&quot;&gt;Curiosity&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Opportunity_%28rover%29&quot;&gt;Opportunity&lt;/a&gt;, to name a couple)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Movement on &lt;a href=&quot;http://en.wikipedia.org/wiki/Venus&quot;&gt;Venus&lt;/a&gt; at a pressure in excess of 9.2MPa is difficult; this constraint does not apply to other celestials that have seen lander missions - &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon&quot;&gt;Luna&lt;/a&gt;, and &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars&quot;&gt;Mars&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Movement however is not only about motive power, and direction. It is also about acceleration and deceleration &lt;strong&gt;as &amp;amp; when necessary&lt;/strong&gt;. Motive power provides acceleration, and maintains velocity to these rover missions. I can't recall encountering a brake system in reading about rovers on the internet. Ergo, the brake system is friction, and gravity.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How do rovers brake? &lt;/li&gt;&#xA;&lt;li&gt;Was a brake system designed for extra-terrestrial use?&lt;/li&gt;&#xA;&lt;li&gt;What would go into design/construction of a brake system for extra-terrestrial use?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2017-03-12T15:26:23.683" Title="How do rovers brake/decelerate?" Tags="|rovers|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2708" PostTypeId="2" ParentId="2707" CreationDate="2013-11-07T01:29:21.853" Score="14" Body="&lt;p&gt;The rovers I know of (Sojourner, MER, MSL) brake by not driving the wheels.  The wheels do not free spin -- they are connected to the motors by a high ratio gear box.  If you tried to turn a rover wheel with your hands, you simply would not be able to, due to the high gear ratio and the resistance of the electric motor to motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When power is applied to the motor, the wheel turns.   When power is removed, the wheel stops.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is important to note that a wheel may turn without moving the rover, and it is possible for the rover to move without the wheel turning.  There are many examples of the former during our rover missions, where wheels have dug in or were lifted off the ground.  In the latter category is the one busted wheel on Spirit that the other five wheels dragged around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo Lunar Roving Vehicle had actual mechanical brakes.  See @user16653's answer on those.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-03-12T15:26:23.683" LastActivityDate="2017-03-12T15:26:23.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2709" PostTypeId="2" ParentId="2674" CreationDate="2013-11-07T03:06:49.610" Score="7" Body="&lt;p&gt;SpaceX Falcon 9 is assembled and loaded from horizontal position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, it's all about engineering trade offs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Horizontal assembly is easier for the personnel and involves lower capital costs, but puts a lot of stress on the vehicle. Plus, the vehicle needs to be specifically designed to be able to sustain this sort of manipulation - considering the engineering margins of the vehicle design, this is a very non-trivial feat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Horizontal assembly also allows for a faster transportation to a launch pad, but this is mostly important for ICBMs and derivatives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, vertical assembly stresses the vehicle (and its designers) to a much lesser extent at the expense of a more complicated (and lower throughput) assembly set-up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nothing to do with prestige, really.&lt;/p&gt;&#xA;" OwnerUserId="743" LastActivityDate="2013-11-07T03:06:49.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2710" PostTypeId="2" ParentId="2614" CreationDate="2013-11-07T03:29:28.220" Score="1" Body="&lt;p&gt;Aerodynamic flight is obviously possible in zero-g, but not with traditional wings (which are specifically designed to push the air &quot;down&quot;, wherever this may be; poor bird will have to bend its neck rather uncomfortably to see where it's flying with this sort of wing design). Rather, you will need a creature (or suit) with something like large fish-style fins, designed for a lower density medium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a sense, you can think of fish as &quot;flying&quot; in zero-g because their body density is more or less equal to surrounding medium, so gravity is cancelled by the buoyant force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They even train astronauts this way. :)&lt;/p&gt;&#xA;" OwnerUserId="743" LastActivityDate="2013-11-07T03:29:28.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2711" PostTypeId="2" ParentId="2707" CreationDate="2013-11-07T04:02:53.033" Score="9" Body="&lt;p&gt;Curiously enough, the venerable Lunokhod rovers also had proper brakes, same as LRV. In both cases they were of usual mechanical type (friction plates pressing against the wheel), however in Lunokhod case the brakes were operated by electromagnetic actuator, while on LRV astronauts were supposed to use muscle power (same as with simple bicycle brakes).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, friction stays the same, no matter if you're on Earth or some place else. The only additional requirement for friction brakes operating in vacuum is that you may want to avoid using pneumatic/hydraulic actuators (even though they can be designed for use in vacuum), and use electromagnetic actuators instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional option is to use &quot;true&quot; electromagnetic brakes instead of friction ones: &lt;a href=&quot;http://en.wikipedia.org/wiki/Eddy_current_brake&quot;&gt;http://en.wikipedia.org/wiki/Eddy_current_brake&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="743" LastActivityDate="2013-11-07T04:02:53.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2712" PostTypeId="1" AcceptedAnswerId="2713" CreationDate="2013-11-07T04:24:53.567" Score="9" ViewCount="3355" Body="&lt;p&gt;In school, I learned that if a satellite moves at a speed $\sqrt{gR}$ (the square root of the product of the acceleration due to gravity and radius of the Earth), then it will remain in Earth's orbit. As we know, there are no opposing forces to a satellite, so its speed will stay constant, as per Newton's 3rd law of motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But today I saw &lt;a href=&quot;http://www.ndtv.com/article/world/satellite-will-fall-to-earth-but-no-one-s-sure-where-442621?pfrom=home-topstories&quot; rel=&quot;noreferrer&quot;&gt;this news article&lt;/a&gt;, saying that a European satellite is falling back to Earth. Why will the satellite fall back to Earth, and why it will move around in space? If the satellite is falling due to Earth's gravitational force, then why it did not fall when it was in working condition?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="49" LastEditDate="2013-11-07T06:07:06.483" LastActivityDate="2013-11-12T05:35:05.370" Title="Why do malfunctioning satellites come back to Earth?" Tags="|orbital-mechanics|artificial-satellite|low-earth-orbit|gravity|goce|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2713" PostTypeId="2" ParentId="2712" CreationDate="2013-11-07T04:40:36.513" Score="14" Body="&lt;p&gt;In short, the European Space Agency's &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Field_and_Steady-State_Ocean_Circulation_Explorer&quot; rel=&quot;noreferrer&quot;&gt;GOCE&lt;/a&gt; (Gravity Field and Steady-State Ocean Circulation Explorer) satellite was last orbited in extremely low altitude elliptical &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;Low Earth Orbit&lt;/a&gt; (LEO) with perigee of 195 kilometres (121 mi) and apogee of 201 kilometres (125 mi) altitude above the sea-level. At that altitude, there is still some atmospheric pressure, albeit extremely tenuous, but given the satellite's velocity and surface area still enough to result in meaningful drag and decreasing its orbital velocity. Additionally, the &lt;a href=&quot;http://www.spaceflightnow.com/news/n1311/02goce/&quot; rel=&quot;noreferrer&quot;&gt;Sun's increased activity&lt;/a&gt; during the last weeks resulted in increased particle density solar winds reaching the Earth through (flares and coronal mass ejections) and have somewhat increased the satellites rate of orbital decay due to increased atmospheric drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/WLXue.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Artist's depiction of the European Space Agency's Gravity Field and Steady-State Ocean Circulation Explorer satellite in orbit.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GOCE is even an aerodynamically streamlined satellite to reduce that drag to minimum possible and require less of its propellants to constantly do orbital &lt;a href=&quot;https://en.wikipedia.org/wiki/Reboost&quot; rel=&quot;noreferrer&quot;&gt;reboosts&lt;/a&gt; to increase its velocity and with it altitude, but since it ran out of its propellant (GOCE used Xenon &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_propulsion&quot; rel=&quot;noreferrer&quot;&gt;ion propulsion&lt;/a&gt; thrusters powered by 1,300 W fixed solar panels), its orbit will slowly decay due to atmospheric drag and it will end its life falling towards the lower and denser atmosphere where it will presumably completely burn up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These orbital reboosts are a standard procedure also for the International Space Station (ISS) that currently orbits at the altitude of 230 miles (370 km), and each new visiting spacecraft (such as today's Soyuz TMA-11M that was just launched from Baikonur Cosmodrome) lends it a hand with its own thrusters after the spacecraft has successfully docked with the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some values and a graph of atmospheric pressure at LEO altitudes and beyond are listed on the Wikipedia page on &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Standard_Atmosphere&quot; rel=&quot;noreferrer&quot;&gt;International Standard Atmosphere&lt;/a&gt;. Quoting a Cornell University document on &lt;a href=&quot;http://gps.mae.cornell.edu/AIAA-2010-8258_hinks&amp;amp;psiaki_asc2010.pdf&quot; rel=&quot;noreferrer&quot;&gt;Simultaneous Orbit and Atmospheric Density Estimation for a Satellite Constellation&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For many satellites in low earth orbit (LEO), the largest dynamic&#xA;  model uncertainty stems from atmospheric drag. Acceleration due to&#xA;  atmospheric drag $a_D$ is related to atmospheric density $p$ by the&#xA;  equation:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$$a_D = - {1\over2}({C_D{A_v(t)\over{m_s}}})\ {pv_r}^2e_v$$&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;where $C_D$ is a drag coefficient, $A_v(t)$ is the cross-sectional&#xA;  area of the satellite in the direction of travel, $m_s$ is the  total&#xA;  spacecraft mass, $v_r$ is the velocity  magnitude relative to the&#xA;  ambient atmosphere, and $e_v$ is a unit vector in the  relative&#xA;  velocity direction. Uncertainty enters this equation in three ways.&#xA;  First, the scalar product $({C_D{A\over{m_s}}})$, known as the inverse&#xA;  ballistic coefficient, is generally uncertain and may be time-varying.&#xA;  Second, the relative velocity  may be uncertain, either because it has&#xA;  not yet been estimated accurately or because the local wind does not&#xA;  rotate perfectly with the Earth. Finally, atmospheric density is very&#xA;  difficult to determine. Three basic paradigms exist for dealing with&#xA;  drag uncertainty: It can be modeled, measured directly or indirectly,&#xA;  or estimated in conjunction with  satellite orbits.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And this is a calculated density and reference density vs. altitude graph for &lt;a href=&quot;https://en.wikipedia.org/wiki/Circular_orbit&quot; rel=&quot;noreferrer&quot;&gt;Circular Equatorial Orbit&lt;/a&gt; (Direct PDF download: &lt;a href=&quot;http://www.dtic.mil/get-tr-doc/pdf?AD=ADA392479&quot; rel=&quot;noreferrer&quot;&gt;Determination of Atmospheric Density in Low-Earth Orbit Using GPS Data&lt;/a&gt;, United States Naval Academy, John L. Young III):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/IekMh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites orbited above any atmospheric drag like the ones in &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;Geosynchronous&lt;/a&gt; (GSO) or &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;noreferrer&quot;&gt;Geostationary&lt;/a&gt; Orbit (GEO), usually communications satellites, however aren't disposed of by deorbiting them into the atmosphere where they could burn, but are placed in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Graveyard_orbit&quot; rel=&quot;noreferrer&quot;&gt;graveyard or junk disposal orbit&lt;/a&gt; a few hundred kilometers above the GEO belt.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-07T14:08:08.717" LastActivityDate="2013-11-07T14:08:08.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2714" PostTypeId="5" CreationDate="2013-11-07T05:00:03.713" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-07T05:00:03.713" LastActivityDate="2013-11-07T05:00:03.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2715" PostTypeId="4" CreationDate="2013-11-07T05:00:03.713" Score="0" Body="Questions regarding methods or forms of communication, either spoken or written." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-07T05:00:03.713" LastActivityDate="2013-11-07T05:00:03.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2716" PostTypeId="1" AcceptedAnswerId="2719" CreationDate="2013-11-07T05:18:13.613" Score="9" ViewCount="434" Body="&lt;p&gt;I was reading &lt;a href=&quot;http://www.ndtv.com/article/world/satellite-will-fall-to-earth-but-no-one-s-sure-where-442621?pfrom=home-topstories&quot;&gt;this article&lt;/a&gt;, and noticed this statement:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The chances that a chunk of GOCE or any other space debris will injure anyone are tiny, but not zero. Floberghagen said the debris &lt;strong&gt;will endanger about 15 to 20 square yards of the Earth's surface.&lt;/strong&gt; &quot;If you compare that to the surface of the planet, it's a very small number,&quot; he said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Fifteen to twenty square yards? How can that be true? I'm assuming that by endangered, he means 'stuff might fall on it'. But twenty square yards is a twelve x fifteen foot square - not a very big place. How is it that a 1-ton satellite can deorbit, burn in the atmosphere until medium-rare, and fall for thousands of feet - and still have all its pieces end up in an area no bigger than the room I'm in right now?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2016-06-07T07:28:53.337" Title="How can all of GOCE's pieces land in a 20 square yard area?" Tags="|artificial-satellite|reentry|goce|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2717" PostTypeId="5" CreationDate="2013-11-07T05:22:25.310" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-07T05:22:25.310" LastActivityDate="2013-11-07T05:22:25.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2718" PostTypeId="4" CreationDate="2013-11-07T05:22:25.310" Score="0" Body="Questions regarding the European Space Agency's Gravity Field and Steady-State Ocean Circulation Explorer, also known as GOCE." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-07T05:22:25.310" LastActivityDate="2013-11-07T05:22:25.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2719" PostTypeId="2" ParentId="2716" CreationDate="2013-11-07T06:50:16.543" Score="16" Body="&lt;p&gt;I don't think that's what they mean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you describe incredulously is in fact not possible.  The varying ballistic coefficients of the pieces would result in a spread of impacts over tens to hundreds of miles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article also states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;About 25 to 45 fragments of the 1-ton spacecraft are expected to&#xA;  survive all the way to the surface, with the largest perhaps weighing&#xA;  200 pounds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So in the extreme we have 45 fragments endangering 15 square yards, or about a third of a square yard or three square feet per fragment on average.  So you have 45 of these little two-foot diameter circles threatened on Earth, spread over a hundred miles.  That's what I think they mean.  I could almost believe that.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-11-07T06:50:16.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2720" PostTypeId="5" CreationDate="2013-11-07T10:36:20.740" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-11-07T10:36:20.740" LastActivityDate="2013-11-07T10:36:20.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2721" PostTypeId="4" CreationDate="2013-11-07T10:36:20.740" Score="0" Body="Questions about the light isotope of helium not found in any significance on Earth, described as a potential source of nuclear fusion fuel on the Moon and other celestial solar system bodies." OwnerDisplayName="user838" LastEditorDisplayName="user838" LastEditDate="2013-11-07T12:40:23.097" LastActivityDate="2013-11-07T12:40:23.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2722" PostTypeId="2" ParentId="2716" CreationDate="2013-11-07T14:36:34.200" Score="6" Body="&lt;p&gt;ESA want's to spin this down to &lt;em&gt;&quot;nothing to see here, move along&quot;&lt;/em&gt;. The truth is that GOCE controllers haven't a clue over which area the satellite will enter the atmosphere, how long and wide of a debris field it will produce or indeed how many pieces it will disintegrate into. Those are all just somewhat &lt;a href=&quot;http://www.esa.int/For_Media/Press_Releases/ESA_s_GOCE_mission_comes_to_an_end&quot; rel=&quot;nofollow&quot;&gt;educated guesstimates&lt;/a&gt; that they quote in several press releases, interviews of GOCE scientists by the press, and &lt;a href=&quot;http://blogs.esa.int/rocketscience/2013/11/04/goce-at-192-km-update-from-esoc/&quot; rel=&quot;nofollow&quot;&gt;updates in ESA's blog&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They're hopeful that most of it will burn in the lower atmosphere during reentry, annihilated by heat generated from friction, with only small fragments reaching the ground. But since they lost any ability to control this as the satellite ran out of Xenon that its ion thrusters used, there's absolutely nothing they can do to control that, while the satellite is still operational and sending telemetry back to the ESA ground control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So those calculated numbers are really just musings of scientists over what are the chances that some of these falling debris cause problems, presenting it as utterly insignificant, with the total affected area being &lt;em&gt;&quot;a very small number&quot;&lt;/em&gt; when comparing that to the total surface area of the planet. Perhaps, but the comparison is really ridiculous. It's a bit like punching someone with your fist and then telling him that it's OK, because you've only hit an insignificant surface area of his entire body, and that this area gets even smaller if we calculate only the surface area of your fist's bones, where it would be most dense and impact with the strongest force. A notion so ridiculous it might not even be accepted as a valid argument in some courts, and that should say a lot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is, as usual, in what is not being mentioned. That GOCE will be a 1004 kg aerodynamically shaped kinetic projectile once it will enter the lower atmosphere. That it is streamlined with a pointed front and wing-shaped solar panels covering its body and it looks like a proper dart, almost like a &lt;a href=&quot;http://www.mssl.ucl.ac.uk/planetary/missions/Penetrators_site/Moon_Page.htm&quot; rel=&quot;nofollow&quot;&gt;planetary penetrator&lt;/a&gt;. It has even got a nickname &lt;em&gt;&quot;Ferrari of space&quot;&lt;/em&gt; due to its appearance. Its shape might keep it in low drag nosedive trajectory and it might not even disintegrate as much as some estimate, presumably based on how well other similar size and weight satellites did, but none of them were as streamlined before (to our knowledge, there might have been some military surveillance satellites though) to reduce atmospheric drag (GOCE was always meant to orbit low enough for atmospheric drag to be significant enough they even shaped its body like wings to passively help it stabilize its attitude).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the real question is, how come nobody mentions some sort of barometric switch or remote activated self-destruct &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Range_safety&quot; rel=&quot;nofollow&quot;&gt;range safety&lt;/a&gt;&lt;/em&gt; mechanism onboard, that would help it disintegrate into smaller pieces, assure it burns up completely on reentry? Could it be they don't mention it because GOCE simply doesn't have it, and they're now realizing it was particularly shortsighted of them to omit such a mechanism on a satellite that was built with as small area of friction with the atmosphere (cross-sectional area of the satellite in the direction of travel) as possible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess we'll have to wait and see what really happens. GOCE's orbit is now rapidly decaying with &lt;a href=&quot;http://blogs.esa.int/rocketscience/2013/11/04/goce-at-192-km-update-from-esoc/&quot; rel=&quot;nofollow&quot;&gt;last reported altitude&lt;/a&gt; of roughly 192 km high, and an expected drop by about 4 km during the next day. This rate of orbital decay will only increase as the atmosphere gets denser, and Sun being in solar maxima with its increased activity sending more ejecta our way as a product of flares and coronal mass ejections, the atmospheric drag will increase even faster.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: OK, this is an interesting &lt;a href=&quot;http://www.scpr.org/programs/airtalk/2013/11/07/34550/european-satellite-running-on-empty-to-fall-back-t/&quot; rel=&quot;nofollow&quot;&gt;interview with Stephen Clark&lt;/a&gt;, a reporter for Spaceflight Now answering a few questions regarding GOCE reentry on KPCC Southern California Public Radio (partial transcript):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Stephen Clark: &lt;em&gt;... and it's going to be falling back to Earth sometime&#xA;  between late tonight and Tuesday night.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;KPPC: &lt;em&gt;How do these bits behave? How big are these fragments?&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Stephen Clark: &lt;em&gt;Of course, when the spacecraft reenters the Earth, it&#xA;  will be disintegrating and about 550 pounds of its total 2,200 pound&#xA;  mass will actually survive to the surface, and the biggest fragment,&#xA;  that is predicted in these computer models that are specially designed&#xA;  to forecast these events, will be about 200 pounds.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;KPPC: &lt;em&gt;Oho! Anybody's been tracking where it's likely to hit?&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Stephen Clark: &lt;em&gt;It actually orbits over the entire planet, so it's&#xA;  a,... if you're in Los Angeles or if you're in Mongolia, the odds are&#xA;  equal.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;KPPC: &lt;em&gt;When will you have better idea than that?&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Stephen Clark: &lt;em&gt;These reentries are actually very hard to predict, in&#xA;  fact, you might not know when it will reenter until moments before or&#xA;  perhaps even they will not realize that it has reentered until they're&#xA;  no longer receiving signals from it.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;KPPC: &lt;em&gt;As far as I read, nothing of our space junk that we've created&#xA;  that has served us so well and it turns into junk, none of it has&#xA;  really hit a person or private property in, what, the last forty years&#xA;  of space junk?&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Stephen Clark: &lt;em&gt;Right, this has never happened. In fact, you're more&#xA;  likely to be hit by a falling meteor like the one that &lt;a href=&quot;http://losangeles.cbslocal.com/2013/11/06/large-meteor-shower-lights-up-night-sky-across-the-west/&quot; rel=&quot;nofollow&quot;&gt;streaked over&#xA;  Southern California last night&lt;/a&gt; than space junk, but as more&#xA;  satellites reenter and more satellites are launched they're more&#xA;  likely to fall down and odds are, as the space age continues, that&#xA;  eventually something will be damaged or someone will be hurt.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;KPPC: &lt;em&gt;So someone will be in &quot;the Guinness book&quot; to be the first person&#xA;  injured by space junk...&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Stephen Clark: &lt;em&gt;Right.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;KPPC: &lt;em&gt;How does space junk coming back to Earth behave differently&#xA;  than, say, these meteors that might become meteorites?&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Stephen Clark: &lt;em&gt;Well, the biggest difference is that it travels much,&#xA;  much more slowly. It reenters the atmosphere at about 5 miles per&#xA;  second and some of the meteors that are coming in from deep space on&#xA;  much more high velocity trajectory travel sometimes at an order of&#xA;  magnitude faster than that.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;KPPC: &lt;em&gt;Hmmm...&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Stephen Clark: &lt;em&gt;So, in fact, it makes them more likely, for many of&#xA;  these fragments, to reach the surface because they're travelling so&#xA;  much more slowly and they're heated much less than an actual&#xA;  meteorite.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The interview continues and it lasts in total roughly 6 minutes, but the parts most relevant to our thread here are in the transcript. I probably need not tell anyone here that the staff of &lt;a href=&quot;http://spaceflightnow.com/&quot; rel=&quot;nofollow&quot;&gt;Spaceflight Now&lt;/a&gt; is &lt;a href=&quot;http://spaceflightnow.com/news/n1311/02goce/&quot; rel=&quot;nofollow&quot;&gt;well informed on the matter&lt;/a&gt; and likely with access to information we don't have. I wanted to add this here as it re-emphasizes the unpredictability of such atmospheric reentries, and that GOCE is compact and aerodynamically streamlined enough for one quarter of its mass to likely survive the reentry and hit the surface, as predicted by computer models. Where? How big of an area? Nobody knows.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="10791" LastEditDate="2016-02-02T19:47:50.240" LastActivityDate="2016-02-02T19:47:50.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2723" PostTypeId="1" AcceptedAnswerId="2729" CreationDate="2013-11-07T15:24:50.020" Score="14" ViewCount="1227" Body="&lt;p&gt;Colonization of Mars (for example) has a pretty obvious and clear obstacle: The enormous distance. The Moon is kinda &quot;close&quot;. Are there any theoretical (or technological) obstacles that would prevent us from living on the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, would we be able to maintain a permanent life support system? It was successful in several space station projects, such as Mir and obviosuly the ISS, but the Moon is farther away  - a life support system needs a constant supply, but bringing water, oxygen, and food would take some days, so immediate intervention is not possible, if something goes wrong.&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="8383" LastEditDate="2015-02-12T15:35:54.133" LastActivityDate="2015-02-12T15:35:54.133" Title="Would it be possible to live on the Moon with our current technology?" Tags="|the-moon|colonization|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2724" PostTypeId="1" AcceptedAnswerId="2728" CreationDate="2013-11-07T15:33:35.773" Score="5" ViewCount="3790" Body="&lt;p&gt;Regular spaceship missions are effective but also incredibly expensive. The most discussed alternative of them would be the space elevator, but with our current technology, it isn't possible to construct it yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about fast acceleration, like shooting a satellite into orbit by a railgun?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Railguns use super strong electromagnetic fields to accelerate things to high speed. They have military application, but would it be possible to shoot satellites to an altitude and at velocity to achieve orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretically, if we can accelerate a body to Mach 23, that MAY be enough to get it into outer space orbit (since this is the lowest velocity to achieve it). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Can we reach required speeds with structures similar to railguns?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Do we know of any materials that can &quot;survive&quot; a quick acceleration to such velocities without disintegrating, melting or exploding into pieces due to atmospheric drag and acceleration forces involved?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="306" LastEditorUserId="49" LastEditDate="2013-11-07T15:48:04.567" LastActivityDate="2013-11-07T17:46:59.863" Title="Is it possible to literally shoot something into space?" Tags="|launch|materials|rail-gun|" AnswerCount="1" CommentCount="4" ClosedDate="2013-11-07T19:23:51.397" ContentLicense="CC BY-SA 3.0" />
  <row Id="2725" PostTypeId="2" ParentId="1059" CreationDate="2013-11-07T15:51:57.947" Score="-2" Body="&lt;p&gt;Using the definition of &quot;Satellite&quot; as something designed and capable of communicating with ground independently, on April 28, 2008 a C9 mission &lt;a href=&quot;http://en.wikipedia.org/wiki/PSLV&quot; rel=&quot;nofollow&quot;&gt;PSLV&lt;/a&gt; rocket of &lt;a href=&quot;http://en.wikipedia.org/wiki/Indian_Space_Research_Organisation&quot; rel=&quot;nofollow&quot;&gt;Indian Space Research Organisation&lt;/a&gt; successfully placed 10 satellites in orbit&lt;/p&gt;&#xA;" OwnerUserId="1015" LastEditorUserId="288" LastEditDate="2013-11-07T19:00:32.610" LastActivityDate="2013-11-07T19:00:32.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2726" PostTypeId="5" CreationDate="2013-11-07T16:09:56.673" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-07T16:09:56.673" LastActivityDate="2013-11-07T16:09:56.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2727" PostTypeId="4" CreationDate="2013-11-07T16:09:56.673" Score="0" Body="Questions regarding railguns, electromagnetically powered launchers." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-07T16:09:56.673" LastActivityDate="2013-11-07T16:09:56.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2728" PostTypeId="2" ParentId="2724" CreationDate="2013-11-07T16:58:11.250" Score="4" Body="&lt;p&gt;Sure.  See &lt;a href=&quot;http://research.lifeboat.com/ieee.em.pdf&quot; rel=&quot;nofollow&quot;&gt;this paper&lt;/a&gt;.  If the rail is long enough, the acceleration doesn't have to be that high, say 1000 Gs over a two-mile track.  You can design and test spacecraft hardware to survive those Gs.  That makes it much heavier, but with the incredibly low launch costs promised by such a rail, the mass doesn't matter.  You can also use the launch capability simply for materials, e.g. water, propellants, which have no problem with the Gs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need a pretty slender, aerodynamic projectile shape with thermal protection to make it through the atmosphere.  No matter what, you will need a rocket motor and some guidance in your projectile to circularize the orbit at apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also imagine much longer, more expensive tracks to reduce the Gs.  To get to human levels, you would something like a 400 to 700 mile track.  Kind of a big order.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-07T17:46:59.863" LastActivityDate="2013-11-07T17:46:59.863" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2729" PostTypeId="2" ParentId="2723" CreationDate="2013-11-07T18:16:35.740" Score="8" Body="&lt;p&gt;I don't think technology is preventing us from living on the Moon or even Mars at this point. I think budget and time are the main limitations--the time it takes to execute a mission to start a base on another world, not time to wait for new technology. Improved technology could help us come up with solutions to some of the budget and time hurdles--and make the entire enterprise a safer bet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the longer we wait, the more data we will have about those environments which will allow us to be more efficient in where and how we build a base.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that neither lunar nor martian colonies have to be self-sustainable. Resupply missions can take place, and improved technology will also help our resupply missions be more efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;For more information:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nss.org/settlement/mars/zubrin-promise.html&quot; rel=&quot;noreferrer&quot;&gt;The Promise of Mars - Robert Zubrin&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-11-11T13:59:56.960" LastActivityDate="2013-11-11T13:59:56.960" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2730" PostTypeId="2" ParentId="2723" CreationDate="2013-11-07T18:44:58.133" Score="4" Body="&lt;p&gt;This could easily be done. The proof is in the ISS. There has never once been an incident that required immediate evacuation of the ISS, nor an immediate re-supply. The main difference between the two is the distance, and therefore the cost. However, there are a few advantages to being on the moon in terms of cost/distance, namely that there are resources available on the ground. In particular, Oxygen would be fairly easily available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, if someone fronted the cost, I think we could have a colony on the Moon (Or Mars) within a decade. The cost would probably be in the $30-40 billion for either project, with a fairly high ongoing cost as well. Of course, don't quote me on the cost figures, but I'm pretty sure it wouldn't be less than that figure.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-07T18:44:58.133" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2731" PostTypeId="1" AcceptedAnswerId="2732" CreationDate="2013-11-07T19:47:26.680" Score="14" ViewCount="3517" Body="&lt;p&gt;The &lt;a href=&quot;http://www.nss.org/settlement/space/kalpana.htm&quot; rel=&quot;noreferrer&quot;&gt;Kalpana One&lt;/a&gt; presents itself as a practical modern design for a space station with artificial gravity.  One of the tweaks that it made was to limit the length to diameter ratio.  This leaves it looking somewhat stubby compared to the old &lt;a href=&quot;http://en.wikipedia.org/wiki/O%27Neill_cylinder&quot; rel=&quot;noreferrer&quot;&gt;O'Neil cylinder type&lt;/a&gt; or other &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_colony_%28Gundam%29&quot; rel=&quot;noreferrer&quot;&gt;fictional stations&lt;/a&gt;.  Their justification:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The radius is the minimum necessary to provide 1g at the hull when rotating at no more than 2rpm. The length is the longest possible while ensuring rotational stability.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/uA8Id.jpg&quot; alt=&quot;Kalpana One Station&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't expect that this would absolutely rule out the slender designs forever, but there is some concern about disturbances that longer stations would be subject to, and I can't quite understand what.  You'll have some wobble control either way, but what is it exactly that would make longer designs more challenging to manage?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorDisplayName="user12" LastEditDate="2013-11-08T00:52:25.347" LastActivityDate="2020-02-12T16:56:31.047" Title="What stability issues plague long artificial gravity cylinders?" Tags="|space-station|artificial-gravity|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2732" PostTypeId="2" ParentId="2731" CreationDate="2013-11-07T21:43:09.227" Score="14" Body="&lt;p&gt;If you can have dissipation of energy by any means, i.e. the body is not completely rigid (and there will be plenty of stuff moving around inside a space station, making it not rigid), then the angular energy will be minimized by shifting the angular momentum axis to the principal axis that has the greatest moment of inertia.  The axis of a long cylinder has the least moment of inertia, so it is not stable under energy dissipation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, exactly this happened to the first US satellite, Explorer 1, where the energy dissipation was via its flexible antennas.  From &lt;a href=&quot;http://en.wikipedia.org/wiki/Explorer_1&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The elongated body of the spacecraft had been designed to spin about&#xA;  its long (least-inertia) axis but refused to do so, and instead&#xA;  started precessing due to energy dissipation from flexible structural&#xA;  elements. Later it was understood that on general grounds, the body&#xA;  ends up in the spin state that minimizes the kinetic rotational energy&#xA;  for a fixed angular momentum (this being the maximal-inertia axis).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iJfwK.jpg&quot; alt=&quot;Explorer 1&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-08T00:06:27.960" LastActivityDate="2013-11-08T00:06:27.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2733" PostTypeId="2" ParentId="2731" CreationDate="2013-11-08T04:16:45.270" Score="3" Body="&lt;p&gt;Probably you have seen &lt;a href=&quot;https://physics.stackexchange.com/a/7012/955&quot;&gt;this answer&lt;/a&gt; already on physics.SE, but I'm going to quote the relevant part:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Another potential problem is that if the mass distribution of the ship&#xA;  is not symmetric, the ship would wobble.  Suppose that the ship is&#xA;  balanced at some time, but then an object of mass $m$ is moved a&#xA;  distance $d$ along the side of the cylinder towards the end.  Then the&#xA;  frequency of the wobble should be on the order of $\omega md/MR$ and&#xA;  the amplitude should be on the order of $md/M$.  The jerk you feel&#xA;  would then be on the order of $\omega^3 m^2d^2/M^2 R$, which should be&#xA;  small as long as it's a fairly small mass moving a small distance.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;How small can these effects be?  That relies mostly on the size of the&#xA;  ship, as we saw earlier.  If the ship is going to simulate one Earth&#xA;  gravity, it can't be too big.  Eventually, the tension in the sides of&#xA;  the ship would be so great the ship would tear itself apart.  If your&#xA;  ship were a loop, that critical radius would be $T/\lambda g$, with&#xA;  $T$ the tension and $\lambda$ the linear mass density of the wall. &#xA;  For carbon nanotubes this gets you a ship size of up to $10^7 {\rm&#xA;&amp;gt; m}$, a figure so large (bigger than  Earth) that all effects could be&#xA;  made inconsequential.  For steel it's about $5*10^3 {\rm m}$, meaning&#xA;  that the effects on to human-sized might be in general $.001$ or&#xA;  $.0001$ of gravity&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="638" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2013-11-08T04:16:45.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2734" PostTypeId="1" CreationDate="2013-11-08T05:13:31.853" Score="5" ViewCount="1159" Body="&lt;p&gt;ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Field_and_Steady-State_Ocean_Circulation_Explorer&quot; rel=&quot;nofollow noreferrer&quot;&gt;GOCE&lt;/a&gt; (Gravity Field and Steady-State Ocean Circulation Explorer) satellite is receiving a lot of attention these days, ever since it was announced it ran out of Xenon propellant used by its ion thrusters to keep it in orbit, and &lt;a href=&quot;https://space.stackexchange.com/q/2712/49&quot;&gt;it will inevitably reenter the lower atmosphere&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/2716/49&quot;&gt;parts of it might survive all the way to the surface&lt;/a&gt;, with no way to predict over which area it will partially disintegrate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/2722/49&quot;&gt;My main quarrel&lt;/a&gt; with that is, that GOCE is an aerodynamically streamlined 1 ton (2,200 lb) Low Earth Orbit (LEO) satellite that doesn't seem to have any flight termination system onboard to help it disintegrate into small enough pieces on reentry to completely burn up in atmosphere due to heat generated from friction. GOCE, as of writing this question, didn't yet fall to Earth, so it is still impossible to say how well that turned out, but European Space Agency (ESA) seems to be sending another set of aerodynamically streamlined satellites into LEO &lt;a href=&quot;http://www.esa.int/For_Media/Press_Releases/Call_for_Media_Follow_the_launch_of_ESA_s_Swarm_magnetic_field_mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;in two weeks time&lt;/a&gt; (22 November 2013) on its &lt;a href=&quot;http://www.esa.int/Our_Activities/Observing_the_Earth/The_Living_Planet_Programme/Earth_Explorers/Swarm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Swarm constellation mission&lt;/a&gt; studying the Earth's magnetic field:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/GWnvV.jpg&quot; alt=&quot;ESA&amp;#39;s Swarm Constellation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Artist's view of the three &lt;a href=&quot;https://en.wikipedia.org/wiki/Swarm_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Swarm spacecrafts&lt;/a&gt; being deployed in Low Earth Orbit by a &lt;a href=&quot;https://en.wikipedia.org/wiki/Briz-M&quot; rel=&quot;nofollow noreferrer&quot;&gt;Breeze M&lt;/a&gt; upper stage (Source: &lt;a href=&quot;http://blogs.esa.int/rocketscience/2013/11/07/swarm-lift-off-set-for-22-november/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA Blog&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eventually, these three 468 kilograms (1,032 lb) &lt;em&gt;streamlined beauties&lt;/em&gt; will run out of propellant for orbital maneuvering too, and will just as inevitably reenter the atmosphere and fall back on Earth. And we will again be wondering if any of their pieces will ruin our hairstyle. Or worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So my question here is, is ESA really launching another set of potential &lt;em&gt;&lt;a href=&quot;http://www.mssl.ucl.ac.uk/planetary/missions/Penetrators_site/Moon_Page.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;planetary penetrators&lt;/a&gt;&lt;/em&gt; into LEO? Or do these spacecraft have some self-destruct &lt;a href=&quot;https://en.wikipedia.org/wiki/Range_safety&quot; rel=&quot;nofollow noreferrer&quot;&gt;range safety / flight termination system&lt;/a&gt; installed, or are they otherwise designed to disintegrate easier on reentry than GOCE, the &lt;em&gt;&quot;Ferrari of space&quot;&lt;/em&gt;, seems to be with computer model predicted up to one quarter of its total mass impacting the Earth on reentry? Any other threat mitigation procedures, like assuring reentry over the middle of an ocean perhaps?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-03-13T05:08:26.407" Title="Swarm: Is ESA launching another set of aerodynamically streamlined satellites into LEO without any self-destruct system onboard?" Tags="|artificial-satellite|low-earth-orbit|esa|range-safety|swarm|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2737" PostTypeId="2" ParentId="2731" CreationDate="2013-11-08T10:26:53.483" Score="4" Body="&lt;p&gt;The biggest point about the instability of the longer cylinders is that it's inherent to an object spinning around a long axis moment of inertia, and dynamically increasingly unstable.&#xA;Unless constantly corrected against the slightest movements causing it to wobble further off axis, it must get worse.&#xA;The Kalpana chooses to limit the spin axis to keep it inherently stable, like a gyroscope -which inherently resists being pushed off axis. Hence coming out more like a torus with the massive spinning rim.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reducing the spin rate will make things slower, taking longer, but the inherent instability remains.&lt;/p&gt;&#xA;" OwnerUserId="1018" LastEditorUserId="1018" LastEditDate="2013-11-08T10:44:54.980" LastActivityDate="2013-11-08T10:44:54.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2738" PostTypeId="1" CreationDate="2013-11-08T12:29:15.037" Score="18" ViewCount="3254" Body="&lt;p&gt;Ceres is a dwarf planet in the midst of the Asteroid Belt between Mars and Jupiter - its location in the Asteroid Belt makes it a good base to mine the riches believed to exist in the asteroids.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there sufficient resources, specifically fuel and water, to allow this dwarf planet to be a feasible location for a self sustained colony?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/d6Zpbm.jpg&quot; alt=&quot;Ceres&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="58" LastEditDate="2015-06-26T14:10:53.133" LastActivityDate="2023-03-21T16:00:05.297" Title="Feasibility of colonising Ceres?" Tags="|colonization|planetary-science|mining|natural-resources|ceres|" AnswerCount="5" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2739" PostTypeId="2" ParentId="2738" CreationDate="2013-11-08T16:15:03.227" Score="16" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;This answer is outdated. The Dawn flyby in February 2015 added a lot of information which was not known at the writing of this answer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We don't actually know much about Ceres. All we know about it is from earth-based or earth-orbit-based observation. Until now it wasn't visited by a probe, but the Dawn spacecraft will do so in February 2015. This will likely give us much more insight into its composition. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What we do know, is that Ceres is quite small. Its mass is estimated to be only about 1.3% of that of the Moon. Its surface gravity is estimated to be only $0.27 m/s^2$. A low surface gravity makes colonization harder because astronauts will suffer almost as much from muscle atrophy, osteoporosis and other health problems like they do in microgravity in low earth orbit. But when it comes to researching and/or exploiting nearby asteroids, low gravity makes things easier, because this means it wouldn't require much energy to escape its gravity well and perform a landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ceres surface seems to contain ice. At least that's what it's infrared signature implies. Ice can potentially be melted and filtered into drinking-water, or split into oxygen for breathing and hydrogen for generating energy from nuclear fusion (which is still science fiction, but so is asteroid mining and space colonization). Another application of hydrogen and oxygen is that it can be liquidized and used as bipropellant for liquid-fuel rockets. That's an important plus-point for exploration. The surface also seems to contain some iron-rich minerals which could potentially be suitable for creating building material for habitats or maybe even space vessels, but that's hard to tell without an actual soil sample.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ceres seems to be quite warm for an object so far away from the sun. Its maximum temparature seems to be about -38° C. It also rotates quite fast (rotation period of 9 hours) which meanst that the temperature differences on the surface won't be very large. This is also not that bad of a thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in the end Ceres doesn't seem to be much more or less of a candidate for colonization than other large ice-asteroids. But maybe Dawn will tell us more.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2016-01-01T15:35:33.607" LastActivityDate="2016-01-01T15:35:33.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2740" PostTypeId="2" ParentId="2323" CreationDate="2013-11-08T22:06:33.077" Score="7" Body="&lt;p&gt;The following is one &lt;em&gt;possible&lt;/em&gt; explanation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These interesting looking features, when viewed differently have the appearance of being fluvial deposits (like from a river or stream).  Using the 'true-colour' image, adapted from the HiRISE IRB you provided in chat:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/PG9eA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One can see that it appears to be a larger version of the following landform:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vXURV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image above are described as &lt;strong&gt;fossilised ripple marks&lt;/strong&gt; from the &lt;a href=&quot;http://www.pc.gc.ca/eng/pn-np/ab/waterton/natcul/natcul1/a.aspx&quot; rel=&quot;noreferrer&quot;&gt;Waterton Lakes National Park&lt;/a&gt; page of Parks Canada (also the image source).  However, the ripples in the Martian crater are considerably larger than the example in Canada.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the HiRISE page &lt;a href=&quot;http://hirise.lpl.arizona.edu/PSP_007925_1990&quot; rel=&quot;noreferrer&quot;&gt;Channel into Jezero Crater Delta&lt;/a&gt;, the environment around the Nili Fossae region, that have similar geomorphological features (as seen in the image below):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FUOOu.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;suggesting that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;this water-carved channel in Nili Fossae region transported sediments across the Martian surface and deposited them on the floor of an impact crater just south of this image. The sediments were deposited in a delta-like mound on the floor of Jezero Crater, suggesting that the crater may have contained a lake at the time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Essentially meaning that the features in these craters could be the remnants of a lake, that had water draining into it.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-11-08T22:06:33.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2741" PostTypeId="1" AcceptedAnswerId="2742" CreationDate="2013-11-09T00:58:47.927" Score="26" ViewCount="20425" Body="&lt;p&gt;According to &lt;a href=&quot;https://space.stackexchange.com/questions/2738/feasibility-of-colonising-ceres/2739#2739&quot;&gt;this answer&lt;/a&gt; the surface gravity of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29&quot; rel=&quot;noreferrer&quot;&gt;Ceres&lt;/a&gt; is estimated to be only $0.27 m/s^2$.  With a rotation period of 9 hours.  The gravity seems light enough to overcome by leg muscle alone, and if you add in centrifugal force at the equator, it would seem even easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could an average human in a NASA space suit jump off of Ceres?  If not, could they achieve orbit by Jumping? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2022-01-06T09:27:40.900" Title="Could a Human reach escape velocity by jumping from the surface of Ceres (a dwarf planet)?" Tags="|launch|crewed-spaceflight|gravity|ceres|escape-velocity|" AnswerCount="7" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2742" PostTypeId="2" ParentId="2741" CreationDate="2013-11-09T01:12:36.800" Score="32" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; gives $0.51 {km \over s}$ or $510 {m \over s}$ escape velocity, so, no, no leaving Ceres by jumping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Following my &lt;a href=&quot;https://physics.stackexchange.com/questions/46318/is-there-a-small-enough-planet-or-asteroid-you-can-orbit-by-jumping/46325#46325&quot;&gt;earlier calculations&lt;/a&gt;, an asteroid of the radius of Ceres would have orbital speed at near-surface orbit of about $336 {m\over s}$, which is way beyond jump strength of anyone as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravitational acceleration on the Moon is $1.6249 {m\over s^2}$, barely 6 times more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With 9h4m day long (32640s) and 3061km equator length, the rotation adds only about $94{m\over s}$ to orbital speed of whatever is standing there - $242 {m\over s}$ remain for orbital speed, so still quite a bit out of human's body reach.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2013-11-12T16:02:42.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2745" PostTypeId="2" ParentId="2288" CreationDate="2013-11-09T14:54:02.327" Score="12" Body="&lt;p&gt;&lt;strong&gt;Existence of Essential Minerals on Mars&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Formation&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mineral deposits are made through Volcanology. Under the crust of Mars, there is a massive collection of magma, consisting of an amalgamation of elements. Over thousands of years, the heaviest of these elements sink to the bottom. These include copper, chromium, nickel and iron, which get concentrated at the bottom of the magma. After even more time, the most of the magma solidifies, and it's in this cooling process that elements bind together to form minerals. The remaining magma consists of elements that do not bind together as minerals, and which are thus called &lt;em&gt;incompatible elements.&lt;/em&gt; This collection includes Niobium, which is used in superconductors, and Neodymium, used in powerful magnets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Distribution&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you now have two groups: &lt;em&gt;incompatible elements&lt;/em&gt;, and &lt;em&gt;minerals&lt;/em&gt;. The incompatible minerals usually solidify as-are and are mined with some difficulty. An example on Earth is the niobium mine in South Dakota. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the minerals follow a different path. A small amount stays liquefied underground, to solidify years later as deposits of lead, tin, and bismuth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another amount of minerals is superheated in the intense temperature, and either forms a gas or forms a solution with water and sulphur. The superheated matter is forced up through cracks in the crust and forms &lt;em&gt;veins&lt;/em&gt; of minerals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest of the minerals form &lt;em&gt;mineral ore&lt;/em&gt;. The minerals that don't stay liquefied or superheat, they form solutions underground, and then crystallise when the solution cools down.  These massive deposits are called &lt;em&gt;hydrothermal deposits&lt;/em&gt;, and things like gold, silver and lead form here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Useful Materials&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a combined study of the Viking probes and of CNC meteorites which originated from Mars and stuck Earth, it has been conclusively proven that the following are confirmed to exist on Mars:  Magnesium, Aluminium, Titanium, Iron, Chromium, Lithium, Cobalt, Nickel, Copper, Zinc, Niobium, Molybdenum, Lanthanum, Europium, Tungsten, and Gold. Breaking this long list down, we can see that a lot of these must have super-volcanic origins (niobium, gold, iron etc.). Furthermore, probes have detected bunches of hematite and nickel-iron on the surface of Mars. Whether these originated from volcanoes or not is unknown, but they can definitely be used to make steel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Conclusion&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now that you know &lt;em&gt;what&lt;/em&gt; all is on Mars, and &lt;em&gt;where&lt;/em&gt; it came from, it's uses are up to us. For a new colony being established, the Iron and Titanium deposits will be of great interest, as they can be used as building materials on the colony.  All the other resources will have their uses, from Neodymium in magnets to Gold in high-speed cables. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is an unexplored land, and new discoveries are certain. But even now, with the information we have, we can conclusively say that there are essential minerals on Mars as a result of super-volcanic activity, and they would be useful to future colonists on the Red Planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2012-341&quot;&gt;http://www.jpl.nasa.gov/news/news.php?release=2012-341&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nytimes.com/2013/03/13/science/space/mars-could-have-supported-life-nasa-says.html?_r=0&quot;&gt;http://www.nytimes.com/2013/03/13/science/space/mars-could-have-supported-life-nasa-says.html?_r=0&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://marsprogram.jpl.nasa.gov/gallery/atlas/olympus-mons.html&quot;&gt;http://marsprogram.jpl.nasa.gov/gallery/atlas/olympus-mons.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/content/goddard/mars-crater-may-actually-be-ancient-supervolcano/#.Un5zFJQY0z0&quot;&gt;http://www.nasa.gov/content/goddard/mars-crater-may-actually-be-ancient-supervolcano/#.Un5zFJQY0z0&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.theguardian.com/science/2013/oct/03/mars-once-home-giant-volcanoes&quot;&gt;http://www.theguardian.com/science/2013/oct/03/mars-once-home-giant-volcanoes&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://news.discovery.com/space/history-of-space/mars-prospecting-ores-gold.htm&quot;&gt;http://news.discovery.com/space/history-of-space/mars-prospecting-ores-gold.htm&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2013-11-10T06:05:20.200" LastActivityDate="2013-11-10T06:05:20.200" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2746" PostTypeId="5" CreationDate="2013-11-09T20:48:39.230" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-09T20:48:39.230" LastActivityDate="2013-11-09T20:48:39.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2747" PostTypeId="4" CreationDate="2013-11-09T20:48:39.230" Score="0" Body="Questions regarding eyes or vision of astronauts, mostly from a medical and health perspective, about degradation of vision in microgravity due to loss of blood pressure gradient, exposure to higher levels of environmental radiation, and other environmental  changes like chemical composition of breathable air, and so on. Not to be confused with an alternative term for an eyepiece, usually in front of Barlow lenses in telescopes and microscopes." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-09T20:48:39.230" LastActivityDate="2013-11-09T20:48:39.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2748" PostTypeId="1" AcceptedAnswerId="2750" CreationDate="2013-11-09T23:29:54.503" Score="8" ViewCount="364" Body="&lt;p&gt;The KY-171 Audio Frequency Coder was used for the NASA &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Mercury&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Mercury&lt;/a&gt; (1958-1963) to encode radio traffic between ground stations, as well as between ground stations and the space capsule. It was used with the T-560B Radio Transmitter to send encoded telemetry data.  The R-669A Radio Receiver was paired with a KY-172 Audio Frequency Decoder and the C-1669 Coder Control to decipher the messages. All of these devices were part of the AN/FRW-2 Radio Set.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/A9R4e.jpg&quot; alt=&quot;KY-171 Audio Frequency Coder&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now… I have been looking around the internet for weeks now, but I can not seem to find any specification or paper describing the encryption/decryption algorithms/techniques used. The only thing I've been able to dig up is the schematic, which is printed on the inside of the bottom plate of the coder itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/q14RO.jpg&quot; alt=&quot;schematic printed on the inside of the bottom plate of the coder&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone confirm if any written documentation related to the used crypto is available to the general public at all? If it is, where can I find relevant papers describing the cryptographic algorithm(s) and/or techniques used within KY-171 and KY-172?&lt;/p&gt;&#xA;" OwnerUserId="417" LastEditorUserId="417" LastEditDate="2013-11-10T19:50:48.003" LastActivityDate="2013-11-10T19:50:48.003" Title="Where can I find relevant papers describing the cryptographic algorithm(s) and/or techniques used within KY-171 and KY-172?" Tags="|communication|project-mercury|security|reference-request|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2749" PostTypeId="1" CreationDate="2013-11-10T04:13:26.040" Score="11" ViewCount="289" Body="&lt;p&gt;&lt;a href=&quot;http://xkcd.com/618/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6d0sK.png&quot; alt=&quot;XKCD comic on asteroids&quot;&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;^ A Bad Idea. (You will now have a whole bunch of little asteroid pieces headed for you).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, has any government or space program issued an official statement on what it would do if a large object were found to be on a collision course with Earth?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-10T14:52:03.163" LastActivityDate="2014-04-09T16:09:26.573" Title="Does any space program / government have an official stance on a procedure for handling collision-trajectory comets?" Tags="|asteroid|comet|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2750" PostTypeId="2" ParentId="2748" CreationDate="2013-11-10T16:32:05.433" Score="7" Body="&lt;p&gt;To my knowledge, Mercury Project used keyed CWI (Continuous Wave Interrupted) FM (frequency modulated) system, or FMCWI, capable of receiving Morse code over the UHF (Ultra High Frequency) and voice over HF (High Frequency) channels. This would be a completely analogue system, so we can't really speak of cryptographic algorithms. That doesn't mean comms were not tamper-resistant, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/d62xI.gif&quot; alt=&quot;AM and FM carrier waves&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Sinusoidal FM Modulation identifies range by measuring the amount of spectrum spread produced by&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;propagation delay. AM is not used with FMCW. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Continuous-wave_radar#Modulated_continuous-wave&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Modulated Continuous-wave radar&lt;/a&gt;).&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would come with anti-jamming capability in the form of multiple frequency modulated narrow-band tuned filter carrier waves, and since its operation is similar to Continuous Wave (CW) radars, with interrupted carrier system (think of it as &lt;a href=&quot;https://en.wikipedia.org/wiki/Duplex_%28telecommunications%29&quot; rel=&quot;noreferrer&quot;&gt;half-duplex&lt;/a&gt;, requiring only a single antenna in contrast to non-interrupted CW) in place, the system would have ranging capability, so any attempt at transmitting false commands from a different location would, to my understanding, likely be detected and the operator could switch comms device / channel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two coder units were used, the master and the standby unit, both used simultaneously during system operation, with a monitoring device detecting failure and causing switchover to the remaining coder in case of a detected lack of modulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/tojCi.png&quot; alt=&quot;FMCW Intruder Detection&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Doppler signature of Frequency Modulated Continuous Wave (FMCW) radars (Source: &lt;a href=&quot;http://gs.flir.com/surveillance-products/surveillance-technology/frequency-modulated-continuous-wave-radars&quot; rel=&quot;noreferrer&quot;&gt;FLIR Surveillance Technologies&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More additional information on the comms system can be read in the &lt;a href=&quot;ftp://ftp.jaist.ac.jp/pub//sourceforge/m/ms/mscorbaddon/Research/Mercury/MercuryTrackingSites.pdf&quot; rel=&quot;noreferrer&quot;&gt;INTRODUCTION&#xA;TO PROJECT MERCURY AND SITE HANDBOOK&lt;/a&gt;, National Aeronautics and Space Administration (NASA) document from September, 1960 (PDF).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-10T16:45:40.017" LastActivityDate="2013-11-10T16:45:40.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2751" PostTypeId="2" ParentId="2653" CreationDate="2013-11-10T18:00:09.120" Score="14" Body="&lt;p&gt;&lt;a href=&quot;http://www.isro.org/scripts/futureprogramme.aspx&quot;&gt;Their future plans&lt;/a&gt; does not have Mars as a target at the moment.  The ISRO is working manned mission technology in addition to the Moon, Venus, Sun and asteroid probes.  They are also working to collaborate more with NASA on Mars and Moon missions after their first successful NASA/ISRO success:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“To this end the working group agreed to continue discussions in&#xA;  planetary science and Heliophysics to identify areas of potential&#xA;  cooperation,” the statement said, adding, “the value of bilateral&#xA;  cooperation was well reflected, for instance, through the inclusion of&#xA;  two NASA instruments on the successful Chandrayaan-1 lunar mission,&#xA;  which led to significant discoveries about lunar surface&#xA;  characteristics,” NASA Administrator Charles Bolden said Thursday at&#xA;  the fourth US-India Civil Space Joint Working Group meeting in&#xA;  Washington.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Future Mars ISRO missions will probably be a collaborative effort to accomplish more advanced research. &lt;/p&gt;&#xA;" OwnerUserId="446" LastActivityDate="2013-11-10T18:00:09.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2752" PostTypeId="1" CreationDate="2013-11-10T21:30:43.603" Score="6" ViewCount="170" Body="&lt;p&gt;Soon, ESA is planning to deorbit GOCE, letting it burn up in the atmosphere. It seems this has been the plan for a long time - so why won't the satellite be completely annihilated into pieces small enough to not pose any threat at all?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was breaking up in the atmosphere a design criteria for GOCE?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2014-03-27T18:17:40.693" Title="Is GOCE designed to break up in the atmosphere?" Tags="|artificial-satellite|reentry|construction|goce|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2753" PostTypeId="1" AcceptedAnswerId="2754" CreationDate="2013-11-11T00:20:44.813" Score="10" ViewCount="1333" Body="&lt;p&gt;Many artificial satellites are going to do a destructive reentry in the atmosphere and some will reach the ground, constituting a significant threat (e.g. GOCE, UARS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Often I heard that for those objects the reentry trajectory is not easy to calculate, or even impossible to predict. But why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the main uncertainties and difficulties to model such a motion?&lt;/p&gt;&#xA;" OwnerUserId="105" LastEditorUserId="6816" LastEditDate="2015-08-09T00:31:55.427" LastActivityDate="2015-08-09T03:48:51.493" Title="Why is it so difficult to predict the exact reentry location and time of a very low earth orbit object?" Tags="|orbital-mechanics|artificial-satellite|low-earth-orbit|reentry|trajectory|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2754" PostTypeId="2" ParentId="2753" CreationDate="2013-11-11T01:07:31.210" Score="12" Body="&lt;p&gt;Basically, you don't know exactly what drag the spacecraft at such low orbits will be a subject to. There are two main points to this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Unpredictability of the atmospheric density that is a subject to larger changes the lower you get and space weather also playing an important role in influencing particle density, for example the Sun is now in the period of &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_maximum&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar maximum&lt;/a&gt;, resulting in more ejecta hitting our upper atmosphere, increasing drag on the spacecraft that isn't constant in its orbital path. See more in &lt;a href=&quot;https://space.stackexchange.com/a/2713/49&quot;&gt;this answer&lt;/a&gt;, but just to back this up with a nice quote from Cornell University's document on &lt;a href=&quot;http://gps.mae.cornell.edu/AIAA-2010-8258_hinks&amp;amp;psiaki_asc2010.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Simultaneous Orbit and Atmospheric Density Estimation for a Satellite Constellation&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For many satellites in low earth orbit (LEO), the largest dynamic&#xA;  model uncertainty stems from atmospheric drag. Acceleration due to&#xA;  atmospheric drag $a_D$ is related to atmospheric density $p$ by the&#xA;  equation:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$$a_D = - {1\over2}({C_D{A_v(t)\over{m_s}}})\ {pv_r}^2e_v$$&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;where $C_D$ is a drag coefficient, $A_v(t)$ is the cross-sectional&#xA;  area of the satellite in the direction of travel, $m_s$ is the  total&#xA;  spacecraft mass, $v_r$ is the velocity  magnitude relative to the&#xA;  ambient atmosphere, and $e_v$ is a unit vector in the  relative&#xA;  velocity direction. Uncertainty enters this equation in three ways.&#xA;  First, the scalar product $({C_D{A\over{m_s}}})$, known as the inverse&#xA;  ballistic coefficient, is generally uncertain and may be time-varying.&#xA;  Second, the relative velocity  may be uncertain, either because it has&#xA;  not yet been estimated accurately or because the local wind does not&#xA;  rotate perfectly with the Earth. Finally, atmospheric density is very&#xA;  difficult to determine. Three basic paradigms exist for dealing with&#xA;  drag uncertainty: It can be modeled, measured directly or indirectly,&#xA;  or estimated in conjunction with  satellite orbits.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Inability to predict spacecraft's flight stability, how it will react to increased drag and orient itself without a functioning Attitude Control System (ACS) as it used up all of its Xenon propellant for its ion thrusters. GOCE is aerodynamically streamlined, so it might be pretty stable as &lt;a href=&quot;http://www.esa.int/Our_Activities/Observing_the_Earth/GOCE/Track_GOCE&quot; rel=&quot;nofollow noreferrer&quot;&gt;current telemetry&lt;/a&gt; indicates, and is not losing altitude fast even at its perigee now below the official outer space altitudes, or &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kármán line&lt;/a&gt; of 100 km (62 mi) above the Earth's sea level. It still seems to have no problems reaching apogee of over 130 km, as it did during last orbit at the time of writing this answer. It is however not an aeroplane and eventually it will lose control, spin and fall apart due to immense atmospheric pressure (its current velocity is nearly 8 km/s) affecting front-facing side more than its tail while it spins in the atmosphere.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/mAp6a.png&quot; alt=&quot;GOCE ground track&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Latest GOCE ground track as it heads towards China airspace during one of its last orbits (Source: &lt;a href=&quot;http://www.esa.int/Our_Activities/Observing_the_Earth/GOCE/Track_GOCE&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA GOCE Tracking&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-11-11T01:07:31.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2755" PostTypeId="1" CreationDate="2013-11-11T09:15:28.207" Score="5" ViewCount="1050" Body="&lt;p&gt;The gravitational potential of an object around the sun, relative to some point far far away (from the sun), should be smaller zero and proportional to $V_{escape}^2$. Hence the speed one needs should be $V_{mission}=\sqrt{V_{escape1}^2-V_{escape2}^2}$ with $V_{escape1}$ The escape velocity at the starting point relative to the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity#List_of_escape_velocities&quot; rel=&quot;noreferrer&quot;&gt;list of escape velocities&lt;/a&gt; states the Escape Velocity at the Earth Orbit relative to the Suns gravity as 42,1 km/s, at Jupiter (relative to the Sun) 18,5 km/s. So I would expect the lowest possible Delta V for a mission to fly to Jupiter would be the difference, or 37.6 km/s without taking into account leaving LEO or braking inside Jupiters gravity well.&lt;br&gt;&#xA;Now, the NASA trajectory browser typically gives Mission delta Vs around 5km/s. So obviously my quick and dirty calculation above is wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is a working, quick way to calculate the lowest possible mission delta v to reach a point higher in a gravity well, using the data in the list of Escape velocities?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="102" LastActivityDate="2013-11-11T17:40:44.850" Title="How do I calculate lowest possible mission delta V from Escape velocities?" Tags="|gravity|delta-v|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2756" PostTypeId="2" ParentId="2755" CreationDate="2013-11-11T14:41:08.443" Score="5" Body="&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/a/1444/265&quot;&gt;this answer&lt;/a&gt; for how to do a simplified Hohmann transfer calculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It depends on where you're starting from and where you want to end up.  Are you starting from low Earth orbit?  From the surface of the Earth?  Do you want to end up in Jupiter orbit?  What orbit?  Or do you just want to fly by?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your 42.1 km/s is going from a dead stop at Earth's distance from the Sun to escaping the solar system.  The Earth is not at a dead stop -- it's going around the Sun at 30 km/s.  So if you're coming from the orbit of the Earth (without the Earth being there), to escape the solar system takes 12.1 km/s.  Similarly, Jupiter is going around at 13 km/s, so escape from the orbit of Jupiter (without Jupiter being there) is 5.5 km/s.  That's still not how you do the calculation though, since Earth and Jupiter &lt;em&gt;are&lt;/em&gt; there at departure and arrival.  So you need to look at the Hohmann transfer calculation in that other answer.  By the way, the fact that the escape speed over the orbital speed in both those cases is about 1.4, or $\sqrt{2}$, is not a coincidence.  This is left as an exercise for the reader.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can generally do much better than a Hohmann transfer if you're willing to take longer and use the inner planets for gravity assists.  Juno just had its &lt;a href=&quot;http://www.nasa.gov/mission_pages/juno/multimedia/Juno20110727-3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;one Earth flyby&lt;/a&gt;.  Galileo did &lt;a href=&quot;http://www.jpl.nasa.gov/jplhistory/captions/galileotrajectory-t.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;one Venus and two Earth flybys&lt;/a&gt; to get to Jupiter.  Those calculations are much more complicated, and requires software that tracks the relative positions of the planets.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-11-11T17:40:44.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2757" PostTypeId="1" AcceptedAnswerId="2767" CreationDate="2013-11-11T14:47:54.010" Score="13" ViewCount="5280" Body="&lt;p&gt;While watching Soyuz exp. 37/1 land, I was struck with a thought: that capsule looks an awful lot like a submarine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/n2EYm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems to be a problem if the crew ever needed to make an emergency landing onto water - water is generally softer than permafrost for emergency landings. But submarines tend to sink, after all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would happen if a Soyuz landed in water?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2018-03-07T04:04:55.673" Title="Could a Soyuz land in water?" Tags="|water|landing|soyuz-spacecraft|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2758" PostTypeId="2" ParentId="2757" CreationDate="2013-11-11T15:11:42.837" Score="-1" Body="&lt;p&gt;Almost certainly it would sink.  Not that that would be a problem depending on how long it took to recover it.  One early mission, Soyuz 23, did land in a lake and it took until the next day to recover the craft and extract the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US Apollo capsules (similar in size) would have sunk.  They had 3 floatation bags that inflated on splashdown so it would stay upright and afloat until the recovery craft could arrive. (Wikipedia)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russia doesn't have the convienent access to oceans for recovery that the US has.  And probably in the early days, the Soviets perfered the security of landing the capsules in their interior where they could control the news of the outcome (which wasn't always positive -- Soyuz 1 &amp;amp; 11, for example).&lt;/p&gt;&#xA;" OwnerUserId="1033" LastEditorUserId="1033" LastEditDate="2013-11-11T15:20:34.667" LastActivityDate="2013-11-11T15:20:34.667" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2759" PostTypeId="1" AcceptedAnswerId="2760" CreationDate="2013-11-11T17:13:27.553" Score="4" ViewCount="194" Body="&lt;p&gt;Swarm satellites, &lt;a href=&quot;http://phys.org/news/2013-11-satellites-sardines.html&quot; rel=&quot;nofollow&quot;&gt;like in this article&lt;/a&gt;, are satellites being released at the same time. Do they enter different orbits? If so, how do they power themselves to get into unique orbits?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorDisplayName="user12" LastEditDate="2013-11-11T20:37:54.440" LastActivityDate="2013-11-11T20:37:54.440" Title="What pattern is made by the satellites of ESA's Swarm Mission?" Tags="|orbital-mechanics|artificial-satellite|propulsion|attitude|swarm|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2760" PostTypeId="2" ParentId="2759" CreationDate="2013-11-11T17:42:00.367" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/swarm&quot; rel=&quot;nofollow noreferrer&quot;&gt;eoPortal on ESA's Swarm constellation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Swarm mission architecture is driven by the requirement for&#xA;  separation of the various sources contributing to the Earth's magnetic&#xA;  field. Hence, the space segment concept employs a three-minisatellite&#xA;  constellation with the following characteristics:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;Three spacecraft in two different orbital planes, with two satellites in a plane of 84.7º inclination and with one satellite in a&#xA;  plane of 88º inclination&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;The two satellites in the 87.4º inclination orbit will fly at a mean altitude of 450 km, their east-west separation will be 1-1.5º,&#xA;  and the maximum differential delay in orbit will be about 10 s.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;The satellite in the higher inclination orbit (88º) will fly at a mean altitude of 530 km.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;The spacecraft require some degree of active orbit maintenance to control the relative positions in the constellation (this is an&#xA;  element of formation flight to support flight operations).&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vmAhc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;Overview of the Swarm orbit configuration (Quote and image source: &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/swarm&quot; rel=&quot;nofollow noreferrer&quot;&gt;eoPortal on ESA's Swarm constellation&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The trio of spacecraft will use cold gas (N&lt;sub&gt;2&lt;/sub&gt;) powered Orbit Control Subsystem (OCS) propulsion system, &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetorquer&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetorquers&lt;/a&gt;, and attitude sensing is provided by a star tracker assembly (3 star tracker heads), 3 magnetometers, and a CESS (Coarse Earth and Sun Sensor) assembly.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-11-11T17:42:00.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2761" PostTypeId="1" CreationDate="2013-11-11T18:05:09.477" Score="1" ViewCount="470" Body="&lt;p&gt;Are there private banks or companies that have paid for access to up-to-date satellite imagery, whether through financing the satellite or purchasing the raw data? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Out side of pure research, what are some notable names? I am more interested in companies that financed the satellites, and trying to learn what they use the information for. Why did they make the investment? I am curious if wall street companies are using satellite data to predict market movements, based on the image data of ports around the world. This, coupled with shipping traffic data, might &quot;give them an edge&quot; on world production and market activity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably not a very appropriate question for Space Exploration, so I am backing out a bit and wondering about companies that have financed satellite operation, which I understand may be too broad to give any answer to.  &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2013-11-12T15:01:36.593" LastActivityDate="2013-11-12T15:01:36.593" Title="What private companies have launched a satellite to gather satellite imagery?" Tags="|artificial-satellite|commercial|raw-data|" AnswerCount="0" CommentCount="2" ClosedDate="2013-11-11T23:05:15.843" ContentLicense="CC BY-SA 3.0" />
  <row Id="2762" PostTypeId="1" CreationDate="2013-11-11T20:49:56.523" Score="2" ViewCount="128" Body="&lt;p&gt;What is the temperature inside the ISS? How is the temperature regulated?&#xA;I have seen a few clips of the ISS and the crew was wearing just T-shirts and shorts, so the temperature must be normal.&lt;/p&gt;&#xA;" OwnerUserId="1037" OwnerDisplayName="user33571" LastEditorDisplayName="user12" LastEditDate="2013-11-11T22:27:59.790" LastActivityDate="2013-11-11T22:27:59.790" Title="What is the temperature inside the International Space Station?" Tags="|iss|temperature|" AnswerCount="0" CommentCount="1" ClosedDate="2013-11-11T22:27:20.243" ContentLicense="CC BY-SA 3.0" />
  <row Id="2764" PostTypeId="2" ParentId="2757" CreationDate="2013-11-12T01:52:35.927" Score="13" Body="&lt;p&gt;In fact, the Soyuz &lt;a href=&quot;http://www.spaceref.com/iss/spacecraft/soyuz.tm.html&quot; rel=&quot;noreferrer&quot;&gt;is designed&lt;/a&gt; to make an emergency water landing, and has done so once by accident, when Soyuz 23 landed by accident on a frozen lake. The landing didn't go particularly well, the parachute filled with water and caused the escape hatch to be covered in water. A complete account can be found &lt;a href=&quot;http://www.americaspace.com/2013/10/27/a-watery-yarn/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, but let's just say the experience was less than pleasant.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="13761" LastEditDate="2018-03-07T04:04:55.673" LastActivityDate="2018-03-07T04:04:55.673" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2765" PostTypeId="1" CreationDate="2013-11-12T01:54:31.357" Score="7" ViewCount="612" Body="&lt;p&gt;Comets can come from two sources, the Oort Cloud, or the Kuiper Belt. Is there a significant compositional difference between the two?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-12T20:08:18.507" Title="Is there a significant compositional difference between Oort Cloud comets and Kuiper Belt comets?" Tags="|planetary-science|comet|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2767" PostTypeId="2" ParentId="2757" CreationDate="2013-11-12T04:44:58.250" Score="9" Body="&lt;p&gt;Soyuz was designed and tested for water landing capability (the unfortunate lake landing notwithstanding):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WFMBX.jpg&quot; alt=&quot;Zond 5 splash down in the Indian ocean, 1968&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The photograph is attributed to Zond 5 unmanned mission within the lunar L1-Zond effort:&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_7K-L1&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Soyuz_7K-L1&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Some additional curious bits of info on Zond 5: there were some life turtles riding this probe around the moon and back. All survived, despite reentry at near escape velocity. Also, the weather at the recovery site was rather bad and there was an US military vessel nearby trying to take a look at what's going on and possibly snatch the probe. Despite this, the recovery vessel &quot;Borovichi&quot; was able to locate and recover the probe in just 10 hours overnight.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There also existed a special water landing facility at Feodosia to develop this capability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water landing tests are still performed, occasionally:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/d8Dse.jpg&quot; alt=&quot;Water landing test Southern Ural reservoir, 2011&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="743" LastEditorUserId="743" LastEditDate="2013-11-12T05:00:08.807" LastActivityDate="2013-11-12T05:00:08.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2768" PostTypeId="2" ParentId="2712" CreationDate="2013-11-12T05:28:38.067" Score="11" Body="&lt;p&gt;It is incorrect that there are no opposing forces on an Earth-orbiting satellite. &lt;strong&gt;In Low Earth Orbit, drag is a significant factor that will cause most satellites to reenter within years&lt;/strong&gt; to decades of launch. In fact, LEO satellites have a &lt;a href=&quot;http://www.hq.nasa.gov/office/codeq/doctree/871914.pdf&quot;&gt;25-year maximum life&lt;/a&gt; requirement. In order to obtain US government permission to launch a satellite the operator must conduct an analysis to show the satellite is expected to re-enter with high probability within 25 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact altitude at which re-entry is certain is hard to state because it is a function of many variables (hence the need for an analysis and not a strict altitude cut-off). The most important variable is when during the approximately 11-year &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_cycle&quot;&gt;solar cycle&lt;/a&gt; the satellite will operate. The solar cycle is correlated to changes in high-altitude atmospheric density, which changes the drag forces on a satellite by &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2000/ast30may_1m/&quot;&gt;more than an order of magnitude&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Increased solar heating makes the thermosphere puff out as denser layers from lower altitudes expand upward. The density of the thermosphere can soar by a factor of 50 during solar maximum, with a commensurate increase in atmospheric drag on satellites.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Solar pressure can also serve to de-orbit a satellite; however, the net effects are less obvious because solar pressure is applied in the direction of incident sunlight and thus for an arbitrary orbit does not have a deterministic direction with respect to the spacecraft's velocity vector. In contrast, drag always serves to slow down a satellite and thus lower its orbit. Furthermore, solar pressure is typically a much smaller force than drag: during low solar cycles solar pressure is typically on par with drag, whereas during high solar cycles drag is a factor of 10 or more higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, anything below 500 km is definitely coming home; it's only a matter of when. Above that and it could be several decades before re-entry. Remember that because the density profile as a function of altitude is roughly exponential, re-entry happens in the same way &lt;a href=&quot;http://www.goodreads.com/quotes/102579-how-did-you-go-bankrupt-two-ways-gradually-then-suddenly&quot;&gt;a man goes bankrupt&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Two ways. Gradually, then suddenly.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Geosynchronous satellites, which operate near 42,000 km have effectively zero drag. They will not re-enter in any time horizon worth considering. That is why GEO birds are supposed to be placed in a &lt;a href=&quot;http://en.wikipedia.org/wiki/Graveyard_orbit&quot;&gt;graveyard orbit&lt;/a&gt; when they have reached end-of-life but before they are no longer capable of responding to ground commands. GPS satellites operate at half-GEO; they're not coming home either. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, the rationale for a LEO orbit could be an entire post, but the short answer is that many missions don't make sense at GEO. For example, it's hard to take high-resolution pictures of the Earth from that far away. Almost exclusively GEO satellites are used for telecommunications. LEO satellites are used for all sorts of other missions, from Earth remote sensing, to measuring Earth's magnetic field, to specialized science payloads. &lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="127" LastEditDate="2013-11-12T05:35:05.370" LastActivityDate="2013-11-12T05:35:05.370" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2769" PostTypeId="1" AcceptedAnswerId="2770" CreationDate="2013-11-12T09:21:57.207" Score="19" ViewCount="5452" Body="&lt;p&gt;To-date who has flown farthest from the center of the Earth?&#xA;Is he alive now?&lt;/p&gt;&#xA;" OwnerUserId="105" LastEditorUserId="25" LastEditDate="2013-11-12T16:29:09.283" LastActivityDate="2017-05-01T08:05:02.760" Title="Which astronaut travelled farthest from Earth?" Tags="|crewed-spaceflight|apollo-program|record|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2770" PostTypeId="2" ParentId="2769" CreationDate="2013-11-12T14:01:43.563" Score="26" Body="&lt;p&gt;Currently, the crew of Apollo 13 holds the record for highest altitude above earth with 400,171 kilometers (248,655 mi) on 7:21 pm EST, April 14, 1970 (source: &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_13#Spacecraft_location&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;). That would be 406,542 km when measured from the center of the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew members were:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Jim_Lovell&quot; rel=&quot;noreferrer&quot;&gt;Jim Lovell&lt;/a&gt; (still alive)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Jack_Swigert&quot; rel=&quot;noreferrer&quot;&gt;Jack Swigert&lt;/a&gt; (died 1983)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Fred_Haise&quot; rel=&quot;noreferrer&quot;&gt;Fred Haise&lt;/a&gt; (still alive)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It should, however, be noted that this record wasn't intentional. The original mission plan was to enter a low Moon orbit, send the lander to the surface and back, and then return to earth. But due to a technical malfunction of the service module during the flight from earth to moon, the moon landing was canceled and the mission returned back to earth on a &lt;a href=&quot;http://en.wikipedia.org/wiki/Free_return_trajectory&quot; rel=&quot;noreferrer&quot;&gt;free return trajectory&lt;/a&gt; which lead it further away from earth than any of the other Apollo flights.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2013-11-14T21:42:21.357" LastActivityDate="2013-11-14T21:42:21.357" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2771" PostTypeId="1" AcceptedAnswerId="3184" CreationDate="2013-11-12T14:35:56.183" Score="10" ViewCount="467" Body="&lt;p&gt;Back in 2005, &lt;a href=&quot;http://en.wikipedia.org/wiki/Hayabusa_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Hayabusa&lt;/a&gt; carried a mini-rover '&lt;a href=&quot;http://en.wikipedia.org/wiki/Hayabusa_%28spacecraft%29#MINERVA_mini-lander&quot; rel=&quot;nofollow&quot;&gt;MINERVA&lt;/a&gt;'. MINERVA was a Solar Powered entity designed to hop around on &lt;a href=&quot;http://en.wikipedia.org/wiki/25143_Itokawa&quot; rel=&quot;nofollow&quot;&gt;Itokawa&lt;/a&gt;, and capture images. Unfortunately (perhaps due to the time-lag) MINERVA was lost at deployment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Rosetta&lt;/a&gt; bears &lt;a href=&quot;http://en.wikipedia.org/wiki/Philae_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Philae&lt;/a&gt;. Compared to MINERVA's miniscule 591 grams, Philae masses around 100 Kilograms. It is loaded with oodles of instruments - with a mass of over 25 Kilograms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet, like &lt;a href=&quot;http://en.wikipedia.org/wiki/Hayabusa_%28spacecraft%29#MINERVA_mini-lander&quot; rel=&quot;nofollow&quot;&gt;MINERVA&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Philae_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Philae&lt;/a&gt; too is apparently to be deployed on a ballistic trajectory by it's parent craft towards the target body; in this case the comet &lt;a href=&quot;http://en.wikipedia.org/wiki/Churyumov-Gerasimenko&quot; rel=&quot;nofollow&quot;&gt;Chury&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What lessons were learnt from the loss of Hayabusa's MINERVA?&lt;/li&gt;&#xA;&lt;li&gt;Are any precautions in place to prevent loss of the Rosetta Lander the way Hayabusa's MINERVA was?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-08-08T22:17:51.487" LastActivityDate="2014-08-08T22:17:51.487" Title="Are any precautions in place to prevent loss of the Rosetta Lander the way Hayabusa's MINERVA was?" Tags="|orbital-mechanics|rosetta|comet|philae|hayabusa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2772" PostTypeId="5" CreationDate="2013-11-12T15:47:24.357" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-12T15:47:24.357" LastActivityDate="2013-11-12T15:47:24.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2773" PostTypeId="4" CreationDate="2013-11-12T15:47:24.357" Score="0" Body="Questions regarding Hayabusa, an unmanned spacecraft developed by JAXA to return a sample from 25143 Itokawa to Earth." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-12T15:47:24.357" LastActivityDate="2013-11-12T15:47:24.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2774" PostTypeId="1" AcceptedAnswerId="2776" CreationDate="2013-11-12T18:07:15.027" Score="12" ViewCount="794" Body="&lt;p&gt;The probes sent by &lt;a href=&quot;http://en.wikipedia.org/wiki/NASA&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt; generate a lot of data for scientists to study, but how available is it to non-NASA affiliated scientists? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, would I (a layperson) be able to get the raw data collected by the Mars rovers?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="288" LastEditDate="2013-11-13T12:40:15.677" LastActivityDate="2013-11-13T12:40:15.677" Title="What's the public availability of data gathered by NASA probes?" Tags="|probe|nasa|raw-data|scientific-data|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2775" PostTypeId="2" ParentId="450" CreationDate="2013-11-12T18:50:27.093" Score="9" Body="&lt;p&gt;I think for colonization within the Sol system, the genetic diversity question is not a significant problem as new colonists will likely continue to arrive for a long time. Sending additional colonists is easy compared to the initial challenge of sending everything needed to establish a sustainable colony.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a scenario like interstellar colonization, where the group will potentially be on their own for a very long time, I don't think there is a solid threshold that should not be crossed. It's a matter of degree. More is better. Less is riskier. Relatively small gene pools have survived for long periods as we have seen in some remote island populations, royal lines, or in the &lt;a href=&quot;http://www.archives.com/genealogy/family-heritage-blue-people.html&quot; rel=&quot;noreferrer&quot;&gt;Fugates of Kentucky&lt;/a&gt;. The smaller your gene pool though, the more you see the emergence of long dormant recessive traits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many of these are very benign, but if you're unlucky, you could end up with something life-threatening, like Sickle Cell Anemia. And even if you specifically select for the absence of known dangerous recessive genes, you never really know what's going to turn up, because these recessive genes may have gone unexpressed for many centuries until inbreeding reveals them. So you are basically rolling the dice if your population is very small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are looking for a lower limit, as stated in the comments, one woman and a sperm bank could theoretically populate a world, or if we assume for some reason that we are limited to only natural conception, one man and one woman is enough. But it's very risky. It's really a ridiculous risk to take. An interstellar colonization would already be so expensive and require such a large payload that it's hard to imagine a scenario where you wouldn't want to include many thousands of genomes at least in the form of sperm on ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many exactly should be included as an absolute minimum? Again, it's entirely a matter of degree. There is no threshold value that makes a critical difference. How much risk are you willing to accept for this really really expensive venture? A whole lot? Then two genomes will do it - or even just one male if you're willing to make the female a cloned twin with two copies of the X instead of a Y. Would you rather have zero risk? There's no such thing. Somewhere in between? Well, take more than 2. And don't take so many that they occupy more than 10% of your payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately cost/benefit per pound is the equation you will be working. That's what everything usually comes down to in space exploration.&lt;/p&gt;&#xA;" OwnerUserId="945" LastEditorUserId="945" LastEditDate="2013-11-13T15:39:50.260" LastActivityDate="2013-11-13T15:39:50.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2776" PostTypeId="2" ParentId="2774" CreationDate="2013-11-12T19:28:02.840" Score="5" Body="&lt;p&gt;According to the NASA webpage &lt;a href=&quot;http://www.nasa.gov/open/data.html&quot; rel=&quot;noreferrer&quot;&gt;Open Government Initiative&lt;/a&gt;, there are several repositories of raw data from NASA missions.  These are sorted by categories, however, as this site is a bit dated, the links are sometimes broken - but is included to give you an idea what is actually there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The official US government website &lt;a href=&quot;http://catalog.data.gov/organization/about/nasa-gov&quot; rel=&quot;noreferrer&quot;&gt;Data.gov&lt;/a&gt; also has over 3000 data sets from NASA alone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;As a specific example&lt;/strong&gt;, about the Mars rovers, an example of the data available is from NASA's &lt;a href=&quot;http://atmos.pds.nasa.gov/data_and_services/atmospheres_data/Mars/Mars.html&quot; rel=&quot;noreferrer&quot;&gt;PDS: The Planetary Atmospheres Data Node&lt;/a&gt; (Scroll down the page and you'll see the Mars rover data for this context.&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastActivityDate="2013-11-12T19:28:02.840" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2777" PostTypeId="2" ParentId="2741" CreationDate="2013-11-12T19:51:52.073" Score="12" Body="&lt;p&gt;You'd have to go a lot smaller to achieve escape velocity with musclepower alone. According to &lt;a href=&quot;http://xkcd.com/681_large/&quot;&gt;xkcd&lt;/a&gt;, you can escape from Deimos using a bike and a ramp...&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-11-12T19:51:52.073" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2778" PostTypeId="2" ParentId="2774" CreationDate="2013-11-12T19:53:41.033" Score="3" Body="&lt;p&gt;The &lt;a href=&quot;http://archive.stsci.edu/&quot; rel=&quot;nofollow&quot;&gt;Mikulski Archive for Space Telescopes (MAST)&lt;/a&gt; contains a considerable amount of raw and normalized astronomy data including that from the Hubble Space Telescope, GALEX, FUSE, and Kepler.&lt;/p&gt;&#xA;" OwnerUserId="113" LastActivityDate="2013-11-12T19:53:41.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2779" PostTypeId="2" ParentId="2765" CreationDate="2013-11-12T19:58:59.467" Score="4" Body="&lt;p&gt;A way of looking at this question is to look at the processes that affect comets in these two regions. (Hence, this is a 'part answer')&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason, according to the article &lt;a href=&quot;http://www.boulder.swri.edu/AR2003/Nature_comets.pdf&quot;&gt;The evolution of comets in the Oort cloud&#xA;and Kuiper belt&lt;/a&gt;, is because recent data and research &lt;em&gt;suggest&lt;/em&gt; that the comets in these regions are not pristine examples of the early solar system, to paraphrase the main ideas in the article which relate to compositions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comets from the Oort cloud would have some of their supervolatiles lost due to heating from passing stars and supernovae events.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whereas the more densely packed Kuiper Belt would have more cratering and UV damage, from collisions of relative proximity to the Sun respectively.  Specifically for Kuiper belt objects, the authors suggest that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It also now seems inevitable that most comets&#xA;  from the Kuiper belt, although they are constructed of ancient&#xA;  material, cannot themselves be ancient—instead they must be&#xA;  ‘recently’ created chips off larger Kuiper belt objects, formed in&#xA;  violent ($km/s$ class) impacts, rather than the gentle environment&#xA;  required for cometary accretion in the early solar nebula.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Both types would be affected by cosmic rays and dust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One major conclusion of the article &lt;a href=&quot;http://techdigest.jhuapl.edu/TD/td2702/dellorusso.pdf&quot;&gt;Comet Chemistry: Obtaining Clues to the&#xA;Formation and Evolution of the Solar System&#xA;with High-Resolution Infrared Spectroscopy&lt;/a&gt; (Dello Russo et al. 2006) is that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Data suggest compositional differences between&#xA;  long-period (Oort cloud) comets and Jupiter-family (mostly Kuiper Belt) comets, with a greater percentage&#xA;  of Jupiter-family comets showing depleted&#xA;  carbon-chain and volatile abundances.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is based on their assertion that Oort cloud cometary material was originally with the gas giant planets and were expelled outwards, from the article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Other comets formed in the giant planets region&#xA;  were ejected out to great distances and formed the Oort&#xA;  cloud, a spherical reservoir of comets thousands of astronomical&#xA;  units from the Sun. On the other hand, comets&#xA;  that formed outside the orbit of Neptune are thought to&#xA;  have remained near the plane of the solar system close&#xA;  to their formation region. These comets make up a reservoir&#xA;  known as the Kuiper Belt.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user838" LastEditorDisplayName="user838" LastEditDate="2013-11-12T20:08:18.507" LastActivityDate="2013-11-12T20:08:18.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2781" PostTypeId="2" ParentId="2774" CreationDate="2013-11-12T23:44:26.687" Score="4" Body="&lt;p&gt;All NASA data is to be released to the public within 1 year of it's collection, and is released to the &lt;a href=&quot;http://pds.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;Planetary Data System&lt;/a&gt;. The 1 year is to give the scientists a chance to publish on the research they have worked long and hard to colelct.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-12T23:44:26.687" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2783" PostTypeId="2" ParentId="2741" CreationDate="2013-11-13T01:34:55.733" Score="14" Body="&lt;p&gt;FYI, the relevant equation is to set the kinetic energy equal to the gravitational potential energy.  This is written on a per-mass basis, because both kinetic energy and gravitational potential energy is proportional to mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{ G M m }{ r }  = \frac{ 1 }{ 2} m v_{\infty}^2 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Ceres:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ v_{\infty} = \sqrt{ \frac{ 2 G M }{r} } = \sqrt{ \frac{ 2 G ( 9.43 \times 10^{20} ) }{4.87 \times 10^5 m} } = 508.2 \frac{m}{s} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So obviously this is too high for someone to accomplish with human capabilities.  It's also interesting to note that this follows a different mathematical form than the surface gravity.  You could jump nearly 37 times as high on Ceres than on Earth, assuming the initial mechanics are fairly similar.  This still isn't sufficient because Ceres' gravitational sphere of influence extends much further than this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the &lt;em&gt;orbital&lt;/em&gt; velocity is lower by a factor of $\sqrt{2}$.  So to obtain orbit you only need to reach $360 m/s$.  While this is still not obtainable, you have a better shot at indefinitely departing from the ground if you exert force to the side, as opposed to straight up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, note that Ceres has a relatively fast rotation.  I calculate the equatorial velocity to be $93.7 m/s$, which would help you a good deal.  If you get on the equator and jump in the direction of rotation, then you're down to $265 m/s$ to obtain orbit.  Again, this is &lt;em&gt;still&lt;/em&gt; unobtainable but it's the best shot you've got.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-11-13T01:34:55.733" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2784" PostTypeId="2" ParentId="1990" CreationDate="2013-11-13T13:55:19.373" Score="8" Body="&lt;p&gt;In simple words, you cannot buy land on the Moon, because there is no &lt;em&gt;no one to buy from&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, examine the term 'ownership'. Yes, you cannot have a sale deed or a mortgage paper for your little patch of the Moon, so you technically cannot &lt;em&gt;own&lt;/em&gt; it. However, no law stops you from (with sufficient funds) building a rocket, flying over, and camping out in a spot of the Moon, thereby &lt;em&gt;occupying&lt;/em&gt; it. So, you can or cannot own land on the Moon, it just depends on your definition of 'own'.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2013-11-13T13:55:19.373" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2786" PostTypeId="2" ParentId="1990" CreationDate="2013-11-13T17:55:57.537" Score="9" Body="&lt;p&gt;In 2013, Bigelow Aerospace Inc. sought permission from the Federal Aviation Administration (FAA) to claim property on the moon for lunar mining. Bigelow's legal staff had advised that the FAA is the place to start asking for permission, while the FAA insisted that they have no jurisdiction over spacecraft--despite their involvement in launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;James Dunstan of Mobius Legal Group claimed that the FAA has no authority over the moon, and likely would not make a determination on the matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in December 2014, &lt;strong&gt;they did&lt;/strong&gt;. In a letter to Bigelow, the FAA stated that they intend to &quot;leverage the FAA’s existing launch licensing authority to encourage private sector investments in space systems by ensuring that commercial activities can be conducted on a non-interference basis.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Others have pointed out that both officially and practically speaking the FAA has no jurisdiction outside of the U.S. To clarify what role the FAA is playing, the author of the letter to Bigelow told the press:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We didn’t give (Bigelow Aerospace) a license to land on the moon. We’re talking about a payload review that would potentially be part of a future launch license request. But it served a purpose of documenting a serious proposal for a U.S. company to engage in this activity that has high-level policy implications&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the long run, then, the FAA will probably not exert much more control over space than they already do, but in the short term they are going to do what they can to work with commercial companies to bring greater visibility to the issues of commercial space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://news.nationalgeographic.com/news/2013/11/131113-lunar-property-rights-bigelow-nasa/#&quot; rel=&quot;noreferrer&quot;&gt;Moon Mining Rush Ahead? - National Geographic&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.reuters.com/article/2015/02/03/us-usa-moon-business-idUSKBN0L715F20150203&quot; rel=&quot;noreferrer&quot;&gt;Exclusive - The FAA: regulating business on the moon - Reuters&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-06-05T14:05:04.550" LastActivityDate="2015-06-05T14:05:04.550" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2788" PostTypeId="2" ParentId="4" CreationDate="2013-11-14T00:02:53.687" Score="2" Body="&lt;p&gt;Given the money and the time, most of the required technology is already available.  The one exception is that we do not have the technology today to land very large payloads on Mars, specifically in the 20 tonne to 40 tonne range, as is required in published human Mars mission architectures.  &lt;a href=&quot;http://www.nasa.gov/pdf/501326main_TA09-EDL-DRAFT-Nov2010-A.pdf&quot; rel=&quot;nofollow&quot;&gt;This report&lt;/a&gt; states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These interactions are most evident for the development and evolution&#xA;  of human class EDL for Mars. The approach that will be used to land&#xA;  tens of metric tons of payload on Mars is not known today, and cannot&#xA;  be known purely through paper studies. Only through technology&#xA;  development can the large uncertainties that exist today be reduced in&#xA;  the design of such a mission in order to begin to converge on the most&#xA;  cost effective and reliable combination of systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-11-14T00:02:53.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2789" PostTypeId="2" ParentId="450" CreationDate="2013-11-14T11:19:28.660" Score="3" Body="&lt;p&gt;The Amish are a genetically separated group. They could survive hundreds of years from a core population of 200 people. Yes, they can have genetic disorders, but it is not so bad as to threaten their long-term survival.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a &lt;a href=&quot;http://isogenic.info/html/outbred_stocks.html#definition&quot; rel=&quot;nofollow noreferrer&quot;&gt;genetic study&lt;/a&gt; saying that roughly 25 mouse pairs are needed to build a stable population on the long term. With humans, this number may be roughly in the same order.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think that the real answer is somewhere between the two.&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="1054" LastEditDate="2017-06-14T19:17:30.163" LastActivityDate="2017-06-14T19:17:30.163" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2790" PostTypeId="1" AcceptedAnswerId="2796" CreationDate="2013-11-14T13:46:12.887" Score="14" ViewCount="1633" Body="&lt;p&gt;Launched in 1985, ESA's &lt;a href=&quot;http://en.wikipedia.org/wiki/Giotto_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Giotto&lt;/a&gt; became the first spacecraft to visit a comet. Since then it has done at least some Earth flybys/slingshots to visit other comets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/kJvNF.png&quot; alt=&quot;Giotto orbit&quot;&gt;&lt;br&gt;&#xA;This image from the Wiki article suggests that Giotto was in a Sun-centric orbit that roughly coincides with Earth's orbit. Is it still there, or has the last Earth flyby altered its orbit? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorDisplayName="user12" LastEditDate="2013-11-14T15:19:47.280" LastActivityDate="2019-11-14T22:54:12.897" Title="Where is Giotto now?" Tags="|probe|comet|esa|giotto|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2791" PostTypeId="2" ParentId="1561" CreationDate="2013-11-14T14:17:47.020" Score="3" Body="&lt;p&gt;A system similar to what you would need for orbital bombardment was developed by the US for espionage satellites. The KH-9 Hexagon carried four film capsules, which would be ejected from the satellite to &lt;a href=&quot;http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=19110&quot; rel=&quot;nofollow&quot;&gt;reenter the atmosphere&lt;/a&gt; so the film could be recovered.&lt;br&gt;&#xA;Basically you need two things for an orbital bombardment system: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A heat shield that allows the bomb to survive reentry. This should be well within current technology, we can get (fragile) humans back from space in one piece after all. Ballistic missiles already use heat shield for their warheads, these are rated for the lower reentry speed of an ICBM. &lt;/li&gt;&#xA;&lt;li&gt;A trajectory control system that allows reentry in a small target area. Again, the reentry of manned space capsules shows that this is doable. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;As others have said, a satellite isn't strictly necessary, you could launch each bomb with reentry vehicle by itself (or deploy a group of them from a single launch).   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A final issue is how the fissile material would stand up to space radiation. Would it cause extra decay of the uranium, making the bomb less effective over time?  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-11-14T14:17:47.020" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2792" PostTypeId="1" AcceptedAnswerId="2797" CreationDate="2013-11-14T14:26:38.047" Score="9" ViewCount="833" Body="&lt;p&gt;When playing around with the &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;NASA trajectory browser&lt;/a&gt;, I noticed that practically every mission to the Jupiter system followed the mold (order of magnitude of $\Delta V$):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Start in Earth Orbit (of course), into Orbit around the Sun (5 km/s)  &lt;/li&gt;&#xA;&lt;li&gt;Deep-Space Maneuver (DSM) to change course for an Earth flyby (500 m/s)  &lt;/li&gt;&#xA;&lt;li&gt;Earth flyby with further change in velocity (100 m/s)  &lt;/li&gt;&#xA;&lt;li&gt;Onward!&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&amp;amp;NECs=on&amp;amp;chk_maxMag=on&amp;amp;maxMag=25&amp;amp;chk_maxOCC=on&amp;amp;maxOCC=4&amp;amp;chk_target_list=on&amp;amp;target_list=Jupiter%0D%0A&amp;amp;mission_class=roundtrip&amp;amp;mission_type=rendezvous&amp;amp;LD1=2015&amp;amp;LD2=2030&amp;amp;maxDT=20&amp;amp;DTunit=yrs&amp;amp;maxDV=7.0&amp;amp;min=DV&amp;amp;wdw_width=365&amp;amp;submit=Search#a_load_results&quot; rel=&quot;nofollow&quot;&gt;examples&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I understand that acceleration is more efficient when done in a gravity well, and that changes in direction require less fuel when the craft is moving fast - as is the case during the flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I can't figure out why the $\Delta V$ applied in the flyby can't be applied the same way with the first acceleration, as the position in the gravity well and the speed relative   to Earth will be almost the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why Earth flybys?&lt;/p&gt;&#xA;" OwnerUserId="102" LastEditorUserId="49" LastEditDate="2013-11-14T19:16:58.903" LastActivityDate="2013-11-14T21:09:46.923" Title="Why Earth flybys?" Tags="|orbital-mechanics|trajectory|delta-v|gravity-assist|flyby|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2793" PostTypeId="5" CreationDate="2013-11-14T15:20:31.050" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-14T15:20:31.050" LastActivityDate="2013-11-14T15:20:31.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2794" PostTypeId="4" CreationDate="2013-11-14T15:20:31.050" Score="0" Body="Questions regarding the Giotto ESA probe, which flew to within 600km of Halley's comet." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-14T15:20:31.050" LastActivityDate="2013-11-14T15:20:31.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2795" PostTypeId="2" ParentId="2792" CreationDate="2013-11-14T17:54:10.927" Score="3" Body="&lt;p&gt;The way to think about this is the way that all &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/grav/primer.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity slingshots&lt;/a&gt; work. NASA has provided a nice graphic to illustrate what happens:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/AVqTE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order for speed to be gained, you have to have an eternal reference, otherwise speed simply cannot be gained. As you can see, the relative speed of the ball will be 130 mph in this example. If you throw the ball from the train, you will have a similar speed. If you could imagine a situation by which you then bounce off of a train going the opposite direction, and you could have one go even further. But you have to start from a solar orbit first.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-14T17:54:10.927" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2796" PostTypeId="2" ParentId="2790" CreationDate="2013-11-14T18:16:36.843" Score="4" Body="&lt;p&gt;Well, to know that question, someone would have had to track it in the last 20 years, and that hasn't been done. It is fair to say it will still be somewhere in the vicinity of Earth, and I tracked through &lt;a href=&quot;http://www.unmannedspaceflight.com/lofiversion/index.php/t2383.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;a few rumors&lt;/a&gt; that it might be heading back to Earth around 2016, but the general consensus is that it isn't well known where the spacecraft in fact is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I've learned a lot more about this kind of thing since I originally made this answer, and so let me add a bit more. Many, if not most of the missions pre-internet did a very poor job of publicizing their data. There may exist hard copy printouts of the location of the satellite at a particular point in time. For very short lived, and particularly for non-US missions, this is even more likely to be the case. For Giotto, in particular, the only publicly available trajectory is a &lt;a href=&quot;ftp://spiftp.esac.esa.int/data/SPICE/GIOTTO/kernels/spk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SPICE kernel&lt;/a&gt; that only shows a few days of data around the closest approach to the comet, not even from launch. I briefly show this in a video I made, included below. What could be done is to make a fit to the provided data that matches with its launch time, and project that information forward. Even a small error, however, could result in a major change if it flies by a planet in that time period, as Giotto has no doubt done several times. The orbital period seems to be around 1.1 Earth years, so about every 10 (Wide guess!) years it passes close to Earth. When it does, if it approaches particularly close, the orbit could be massively changed. If ESA released enough data, it could be determined, but that would be difficult, to say the least, and is far beyond the value of a few fake internet points;-)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just for an example, I couldn't even get a reliable trajectory if it for its second cometary flyby;-)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/7ybPsMltyO8?start=175&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-11-14T22:54:12.897" LastActivityDate="2019-11-14T22:54:12.897" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="2797" PostTypeId="2" ParentId="2792" CreationDate="2013-11-14T21:09:46.923" Score="7" Body="&lt;p&gt;I believe what's confusing you is that the &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;NASA trajectory browser&lt;/a&gt; is applying your query parameters more literally than you're likely reading them. You see, &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&amp;amp;NECs=on&amp;amp;chk_maxMag=on&amp;amp;maxMag=25&amp;amp;chk_maxOCC=on&amp;amp;maxOCC=4&amp;amp;chk_target_list=on&amp;amp;target_list=Jupiter%0D%0A&amp;amp;mission_class=roundtrip&amp;amp;mission_type=rendezvous&amp;amp;LD1=2015&amp;amp;LD2=2030&amp;amp;maxDT=20&amp;amp;DTunit=yrs&amp;amp;maxDV=7.0&amp;amp;min=DV&amp;amp;wdw_width=365&amp;amp;submit=Search#a_load_results&quot; rel=&quot;noreferrer&quot;&gt;your query&lt;/a&gt; has, among other parameter values, two values that would likely make it seem that your observations are indeed correct:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Max duration: 20 years&lt;/strong&gt; (Maximum acceptable total mission duration.)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Minimize: $\Delta V$&lt;/strong&gt; (Parameter to minimize if more than one trajectory per destination is found to meet above constraints.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I.e. because your query selection was to optimize for $\Delta V$, the tool took as many maneuvers into account to help lower $\Delta V$ requirements as it fit the mission profile (duration), even if the benefits were really small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This appearance that the additional Earth flybys are necessary changes drastically, if you simply &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&amp;amp;NECs=on&amp;amp;chk_maxMag=on&amp;amp;maxMag=25&amp;amp;chk_maxOCC=on&amp;amp;maxOCC=4&amp;amp;chk_target_list=on&amp;amp;target_list=Jupiter%0D%0A&amp;amp;mission_class=roundtrip&amp;amp;mission_type=rendezvous&amp;amp;LD1=2015&amp;amp;LD2=2030&amp;amp;maxDT=20&amp;amp;DTunit=yrs&amp;amp;maxDV=7.0&amp;amp;min=DT&amp;amp;wdw_width=365&amp;amp;submit=Search#a_load_results&quot; rel=&quot;noreferrer&quot;&gt;change the Minimize query parameter to Duration&lt;/a&gt;. It also seems that the query results are limited in number (I wasn't able to get more than 13 results with any query), so that would further complicate matters, by not even listing more direct trajectory options with your example query.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fairness to NASA Ames Research Center though, they do provide the &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/user_guide.php&quot; rel=&quot;noreferrer&quot;&gt;Trajectory Browser User Guide&lt;/a&gt; that seems easy enough to read. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-11-14T21:09:46.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2799" PostTypeId="1" AcceptedAnswerId="2811" CreationDate="2013-11-15T11:57:24.613" Score="8" ViewCount="639" Body="&lt;p&gt;Zvezda, the third ISS module, (I think it was Zarya (FGB), Node 1, and then Zvezda) known as the Service Module was actually built as a spare for Mir and was originally to be the core of Mir-2. But with the merging of the US and Russian station plans, it became the key building block of the initial ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a Mir spare, the docking module had 5 ports. 6 if you count its connection to the Service Module itself. As delivered to the ISS it only has nadir (Earth facing) and zenith (space facing) ports, and of course the port it uses to dock to Zarya.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did they remove two docking ports? It seems like the flexibility would have been useful, and it was already built that way.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorDisplayName="user12" LastEditDate="2013-11-15T16:33:45.980" LastActivityDate="2013-11-15T17:21:46.940" Title="Why were the docking ports removed from Zvezda? (Service Module)" Tags="|iss|zvezda|docking-port|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2800" PostTypeId="1" CreationDate="2013-11-15T12:32:18.523" Score="14" ViewCount="648" Body="&lt;p&gt;How high is the density of asteroids in the rings of Saturn? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be possible for a spacecraft to fly through the thicker rings without colliding with asteroids? Would it be necessary to avoid a collision through careful maneuvering between the larger and smaller rocks? Or would it actually be a coincidence to even see an asteroid when flying through the rings?&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="49" LastEditDate="2013-11-16T19:11:43.680" LastActivityDate="2013-11-16T21:37:56.567" Title="How dense are Saturn's rings?" Tags="|navigation|saturn|gas-giant|rings|collision-avoidance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2801" PostTypeId="5" CreationDate="2013-11-15T12:46:17.060" Score="0" Body="&lt;p&gt;For space exploration questions about the planet Saturn, the 6th planet of our solar system.&lt;/p&gt;&#xA;&lt;p&gt;For the rockets with the same name, use the tag &lt;a href=&quot;/questions/tagged/saturn-v&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;saturn-v&amp;#39;&quot; rel=&quot;tag&quot;&gt;saturn-v&lt;/a&gt; or &lt;a href=&quot;/questions/tagged/saturn-i&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;saturn-i&amp;#39;&quot; rel=&quot;tag&quot;&gt;saturn-i&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10273" LastEditorUserId="10273" LastEditDate="2021-08-05T21:36:30.513" LastActivityDate="2021-08-05T21:36:30.513" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2802" PostTypeId="4" CreationDate="2013-11-15T12:46:17.060" Score="0" Body="For space exploration questions about the planet Saturn, the 6th planet of our solar system. For the rockets with the same name, use the tag [saturn-v] or [saturn-i]." OwnerUserId="335" LastEditorUserId="10273" LastEditDate="2021-08-05T21:35:14.830" LastActivityDate="2021-08-05T21:35:14.830" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2803" PostTypeId="1" CreationDate="2013-11-08T16:31:33.910" Score="16" ViewCount="3038" Body="&lt;p&gt;I've been making a Mathematica script whereby a spacecraft is transferred from Earth to Mars using a &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann interplanetary transfer&lt;/a&gt; by means of a &lt;a href=&quot;http://en.wikipedia.org/wiki/Patched_conic_approximation&quot; rel=&quot;nofollow noreferrer&quot;&gt;patched conic approximation.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I can tell all of the equations and numbers should be correct, yet when running the code the spacecraft doesn't make it to Mars. Without getting into any detail of the code (that will have to be for the Mathematica SX forums), I wanted to ask how accurate the patched conic approximation really is. If all of the numbers are correct should I be able to run the code and the spacecraft will hit Mars, or is it supposed to be used to find a rough trajectory to another planet? I understand the inherent inaccuracies with the approximation since it considers everything in terms of individual 2-body problems, and so thought I might be a little too hopeful when I thought it would get me to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MsUue.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Okay, I think my initial post used an incorrect angle for applying the delta-v impulse which I have since fixed (I think!). I made a second program that looked at just the Earth and spacecraft upon departure to confirm whether my angle was correct and this is what I got (the spacecraft doesn't actually pass through the green circle (Earth). The green circle is just the position of Earth at the start of the simulation):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ksnv0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, the spacecraft's escape velocity is parallel to the Earth's velocity which is a good start. I got the correct angle by finding the angle of the Earth's velocity vector using phi = arctan(v_y/v_x), the &quot;impulse angle&quot; (not too sure what to call it) by using beta = arccos(1/e) and then using theta = phi + beta + pi when finding the x- and y-components for delta-v = v_p - v_c (where v_x and v_y are the x- and y-components of the Earth's velocity respectively, e is the escape hyperbola's eccentricity and v_p and v_c are the spacecraft's periapse speed and circular orbit speed about Earth respectively).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, when running the full program again in the Sun's frame of reference I found the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Roughly half-period of spacecraft orbit about Sun&#xA;&lt;img src=&quot;https://i.stack.imgur.com/QhYba.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Full period of spacecraft orbit about Sun&#xA;&lt;img src=&quot;https://i.stack.imgur.com/67CUu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. The equations I'm using are from Orbital Mechanics for Engineering Students by Howard Curtis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE: Solution to Earth-Mars interplanetary transfer using Lambert solver (some much needed coding help was given my the Mathematica SX forums).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/AkYxq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1064" OwnerDisplayName="user7388" LastEditorUserId="12102" LastEditDate="2021-10-05T03:25:41.413" LastActivityDate="2021-10-05T03:25:41.413" Title="How accurate is the patched conic approximation when performing a Hohmann interplanetary transfer?" Tags="|orbital-mechanics|hohmann-transfer|trajectory-design|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2805" PostTypeId="2" ParentId="2638" CreationDate="2013-11-15T14:42:20.520" Score="3" Body="&lt;p&gt;Although the exact value is not known, on various estimations (like in &lt;a href=&quot;https://arxiv.org/pdf/astro-ph/9508086.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt;), the local abundance of ${}^3\mathrm{He}$ is in the order of $10^{-5}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a more close experimental data. On &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890005477.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA paper&lt;/a&gt;, we get roughly a 1mg ${}^3\mathrm{He}$ by heating a ton of Lunar regolith to $700 ^\circ \mathrm{K}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper also shows, a built up infrastructure would require $2253 \mathrm{GJ}$ energy to transfer ${}^3\mathrm{He}$ into the Earth what can produce $600000 \mathrm{GJ}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note, the mining infrastructure on the Moon wouldn't be cheap. According to &lt;a href=&quot;https://www.csis.org/analysis/costs-international-lunar-base&quot; rel=&quot;nofollow noreferrer&quot;&gt;this analysis&lt;/a&gt;, a moon base could be built in \$35billion, while its operative costs would be \$7billion yearly. It is roughly like the Russian section of the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe the development of the robotics and the private space industry could significantly lower the price.&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="1054" LastEditDate="2017-06-14T19:10:34.873" LastActivityDate="2017-06-14T19:10:34.873" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2807" PostTypeId="2" ParentId="2803" CreationDate="2013-11-15T15:53:18.567" Score="7" Body="&lt;p&gt;It's pretty hard to tell what's going on with just a plot, especially since you show an entire spacecraft orbit instead of just the half that you intended to be the transfer.  In any case, one thing that I'm seeing that doesn't look right is that the spacecraft trajectory goes inside Earth's orbit.  That shouldn't happen (at least not to the degree shown).  That indicates that your departure velocity from Earth ($V_\infty$) has the wrong direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are you integrating from a low Earth orbit?  It takes some care to time that correctly so you get the right $V_\infty$ direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the patched conic is just an approximation, if done correctly it should get you much closer to the orbit of Mars than what you show.  You should not be able to see a gap at the resolution of that plot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you say in the comments &quot;I was hoping to see the spacecraft either get pulled into an orbit about Mars ...&quot;, that's not possible.  Without a propulsive maneuver at Mars, the spacecraft will fly by, as indicated in the rest of your sentence &quot;... or have its orbital path highly perturbed by coming into Mars' sphere of influence&quot;.  That will happen if you get the timing right for when you depart Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Update:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ok, that's an improvement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you need to account for the eccentricity of the orbit of Mars.  The spacecraft should have an aphelion that corresponds to the distance of Mars from the Sun at the planned time of the encounter.  It will help to also take into account the eccentricity of the Earth's orbit, in particular the distance of the Earth from the Sun at the time of departure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Side note:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beware the perils of the numerical derivative.  Take care to not use &lt;em&gt;too&lt;/em&gt; small of a $\Delta T$.  Here is an example in Mathematica when trying to determine the velocity vector of Earth using Earth position data.  $\Delta T$ is on the x-axis in seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;f[dt_?NumericQ] := &#xA;  AstronomicalData[&quot;Earth&quot;, {&quot;Position&quot;, {2001, 3, 30, 0, 0, 1 + dt}}] - &#xA;  AstronomicalData[&quot;Earth&quot;, {&quot;Position&quot;, {2001, 3, 30, 0, 0, 1}}]&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;LogLinearPlot[f[dt][[1]]/dt, {dt, 0.001, 100}]&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/uc3Ms.png&quot; alt=&quot;numerical derivative noise plot&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-28T17:07:09.557" LastActivityDate="2013-11-28T17:07:09.557" CommentCount="25" ContentLicense="CC BY-SA 3.0" />
  <row Id="2808" PostTypeId="2" ParentId="2386" CreationDate="2013-11-15T16:15:44.527" Score="8" Body="&lt;p&gt;Solar arrays are wired as many strings of cells, where each string is a series of solar cells with the proper number to provide the required voltage.  Those strings connected to each other in parallel to add their currents to provide the required total power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An individual cell failure will take out one entire string, so the resiliency against cell failures can be seen in the number of strings.  The Juno spacecraft has a &lt;a href=&quot;https://trs.jpl.nasa.gov/handle/2014/42755&quot; rel=&quot;nofollow noreferrer&quot;&gt;complicated arrangement of short, medium, and long strings&lt;/a&gt; to handle the large range of solar distances that the mission operates over.  What matters for your question are the short strings which are used at Jupiter.  Juno has 848 short strings, so it is quite resilient to a few individual cell failures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are no significant stresses on the solar panels from tidal forces.  Juno's polar orbit keeps it out of the radiation belts (e.g. Io's) most of the time.  However radiation tolerance was still a driving design factor for that mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8Pd9x.png&quot; alt=&quot;Juno&amp;#39;s orbit and the radiation belts&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-09-17T06:26:24.037" LastActivityDate="2017-09-17T06:26:24.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2809" PostTypeId="1" CreationDate="2013-11-15T17:02:30.560" Score="3" ViewCount="590" Body="&lt;p&gt;Initially, the Russians did not wish to launch the FGB (Functional Block), now named Zarya, which was the first ISS component launched. But the US wanted it, so they could launch a US component sooner. That is, the FGB could provide control, life support, storage, a Canadarm grapple point, so that the first US node could be immediately attached, rather than waiting for the Service Module to be ready (Zvezda). Thus, the US sort of funded it. (The complete story appears to be very complicated, so saying the US paid for it, is reasonably close to the end result).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a docking 'ball' (module? Compartment?) at the front end that has a PMA (Pressurized Mating Adapter) linking it to the CBM port on Node 1 (Unity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zvezda was built with 6 ports (one of course attached to Zvezda itself) and reduced down to 4 total (Up, down, one to itself, and one to dock to Zarya). With Pirs, and Poisk consuming the ports, and Pirs is being discarded to be replaced with the MRM Nakua.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But Zarya only came with one, now Rassvet, that MLM-1 is docked to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did they not supply more ports? (Similar to the question &lt;a href=&quot;https://space.stackexchange.com/questions/2799/why-were-the-docking-ports-removed-from-zvezda-service-module&quot;&gt;Why were the docking ports removed from Zvezda&lt;/a&gt; but for a different module).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-02-14T03:13:05.237" Title="Why did Zarya (FGB) come with a single docking port?" Tags="|iss|zarya|docking-port|fgb|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2810" PostTypeId="1" AcceptedAnswerId="18027" CreationDate="2013-11-15T17:07:59.753" Score="10" ViewCount="548" Body="&lt;p&gt;I have been asking about why &lt;a href=&quot;https://space.stackexchange.com/questions/2809/why-did-zarya-fgb-come-with-only-1-docking-port&quot;&gt;Zarya&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/questions/2799/why-were-the-docking-ports-removed-from-zvezda-service-module&quot;&gt;Zvezda&lt;/a&gt; have the number of docking ports they have, and whether &lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;more will be needed&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This got me thinking about what the inside of the docking module looks like.  I watched a great video showing a Soyuz departing the station.  I noticed, that from the Soyuz, they closed a hatch, that had the docking probe atop it.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then on the ISS side they closed a cover over it, sort of a cone sticking into the station.  (Shape reminded me of Gemini capsules.  Thin tube at the top, widening into a more traditional cone shape below it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering if having a 6 port docking port, like Mir did, eats a lot of space in the docking compartment? But that clearly worked on Mir, and having two or three on Zarya/Zvezda seem like 2-3 more would not matter that much?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-09-02T05:44:05.800" Title="On Mir, Zarya, or Zvezda, do docking ports consume space inside the module?" Tags="|iss|zvezda|zarya|docking-port|mir|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2811" PostTypeId="2" ParentId="2799" CreationDate="2013-11-15T17:21:46.940" Score="6" Body="&lt;p&gt;While I can't speak with authority on the motivation behind design decisions made years ago, it's worth pointing out that port and starboard docking ports on Zvezda would create conflicts with the Zvezda solar arrays, the FGB solar arrays, the HRS radiators, and the solar array wings.  In fact, if you look throughout the station, the only docking and berthing ports are either forward, zenith, nadir, or aft.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2013-11-15T17:21:46.940" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2812" PostTypeId="1" AcceptedAnswerId="8722" CreationDate="2013-11-15T19:16:18.940" Score="12" ViewCount="758" Body="&lt;p&gt;I recall reading, that when Mir would get a new module added, after it was docked, a cosmonaut would stand in, feet on one wall, hands on the other, and try to shake it, to ensure the docking seal was completed properly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The maneuver was called the &lt;strong&gt;Rezonans&lt;/strong&gt; (or Resonance) experiment. It occurred on Soyuz 26 / Salyut 6 / Soyuz 27 &lt;sup&gt;[1][2]&lt;/sup&gt; and on MIR EO-1 with Soyuz T-15 / MIR / Progress 25 &lt;sup&gt;[3][4]&lt;/sup&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they still do this for new modules at the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it differ US vs Russian side, perhaps due to the different nature of the docking/berthing connectors?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, compare the CBM vs PMA vs Russian Active vs Russian Passive docking ports.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-27.htm&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 27 Mission&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://books.google.com/books?id=dbGchpi1HP8C&amp;amp;pg=PA226&amp;amp;lpg=PA226&amp;amp;dq=Rezonans+spacecraft&quot; rel=&quot;noreferrer&quot;&gt;Soyuz: A Universal Spacecraft By Rex Hall, David Shayle pp.226&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://books.google.com/books?id=2wIoqbYm0lIC&amp;amp;pg=PA507&amp;amp;lpg=PA507&amp;amp;dq=Rezonans+spacecraft&quot; rel=&quot;noreferrer&quot;&gt;Space Technologies, Materials and Structures&#xA;edited by B. Paton, pp. 507&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astronautix.com/flights/mireo1.htm&quot; rel=&quot;noreferrer&quot;&gt;Encyclopedia Astronautica: Mir EO-1&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-04-10T18:13:23.267" LastActivityDate="2015-04-11T14:02:07.843" Title="When a new module is added to the ISS, do they still do the &quot;Shake&quot; test?" Tags="|iss|docking-port|mir|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2813" PostTypeId="1" AcceptedAnswerId="4247" CreationDate="2013-11-15T19:46:23.323" Score="10" ViewCount="738" Body="&lt;p&gt;When reading space exploration related articles, I often come across the use of &lt;em&gt;&quot;former&quot;&lt;/em&gt; to classify astronaut's status. NASA even puts &lt;em&gt;&quot;former astronauts&quot;&lt;/em&gt; as a separate classification &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/astrobio_former.html&quot; rel=&quot;noreferrer&quot;&gt;on their pages&lt;/a&gt;, describing the status as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Former astronauts are those who have left NASA, including those who&#xA;  are deceased.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This use of &lt;em&gt;&quot;former&quot;&lt;/em&gt; seems a bit awkward to me, especially since NASA doesn't really hold any exclusive rights to the use of the word &lt;em&gt;&quot;astronaut&quot;&lt;/em&gt;. For example, we usually don't call &lt;a href=&quot;https://en.wikipedia.org/wiki/Ernest_Shackleton&quot; rel=&quot;noreferrer&quot;&gt;Ernest Shackleton&lt;/a&gt; a &lt;em&gt;&quot;former explorer&quot;&lt;/em&gt;. Deceased yes, but he was a polar explorer. So my question is: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is &lt;em&gt;&quot;astronaut&quot;&lt;/em&gt; an official title, a job description, or a life achievement? Should we use the word &lt;em&gt;&quot;former&quot;&lt;/em&gt;, and when would it be appropriate to use it?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-04-11T19:01:24.397" Title="Is the use of &quot;former&quot; with astronauts really necessary?" Tags="|astronauts|terminology|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2814" PostTypeId="2" ParentId="1224" CreationDate="2013-11-15T22:27:49.313" Score="4" Body="&lt;p&gt;This link (which is posted in the comments) covers the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://omniweb.gsfc.nasa.gov/vitmo/msis_vitmo.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://omniweb.gsfc.nasa.gov/vitmo/msis_vitmo.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently the thermosphere exhibits more variation due to a host of factors.  These include season, location on Earth, solar activity, and so on.  Because of that, anyone applying a single formula for this must realize that it might be up to an order of magnitude off.  However, the density profile varies by &lt;em&gt;many&lt;/em&gt; orders of magnitude, so I still think it's meaningful to talk about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a sample output.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;   km      O/cm3     N2/cm3     g/cm3&#xA;    1          2          3         4&#xA;  0.0  0.000E+00  2.120E+19 1.304E-03&#xA; 50.0  0.000E+00  1.361E+16 8.373E-07&#xA;100.0  3.995E+11  8.467E+12 5.173E-10&#xA;150.0  1.907E+10  3.236E+10 2.190E-12&#xA;200.0  4.918E+09  3.538E+09 3.100E-13&#xA;250.0  1.696E+09  5.676E+08 7.348E-14&#xA;300.0  6.293E+08  1.009E+08 2.181E-14&#xA;350.0  2.402E+08  1.873E+07 7.388E-15&#xA;400.0  9.331E+07  3.582E+06 2.723E-15&#xA;450.0  3.679E+07  7.029E+05 1.065E-15&#xA;500.0  1.471E+07  1.413E+05 4.394E-16&#xA;550.0  5.959E+06  2.907E+04 1.926E-16&#xA;600.0  2.446E+06  6.118E+03 9.136E-17&#xA;650.0  1.017E+06  1.317E+03 4.793E-17&#xA;700.0  4.279E+05  2.896E+02 2.815E-17&#xA;750.0  1.823E+05  6.506E+01 1.840E-17&#xA;800.0  7.861E+04  1.492E+01 1.308E-17&#xA;850.0  3.429E+04  3.494E+00 9.863E-18&#xA;900.0  1.513E+04  8.346E-01 7.717E-18&#xA;950.0  6.750E+03  2.033E-01 6.177E-18&#xA;1000.0  3.045E+03  5.048E-02 5.015E-18&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;It's also meaningful to note that the atmosphere becomes &lt;em&gt;highly&lt;/em&gt; differentiated at high altitudes.  So basically, the other elements fall to nearer to the surface and almost only Hydrogen is left at super high altitudes.  This is what the numerical models spend a lot of their time on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a plot of the density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/h9C7h.png&quot; alt=&quot;atmosphere density&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that I had to give different units of kg/m3 here.  Just because Excel would muck up the formatting with smaller numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's actually rather interesting that an exponential trend just won't fit this data.  A power fit makes a decent approximation.  I don't know why.  I can't explain why the atmosphere would better fit a power law than an exponential trend, since the exponential trend comes from the ideal gas state equation and the force balance.  That's actually a rather intriguing statement.  Of course, even the power law isn't fantastic, and I'm sure a point or two are off by a factor of 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nevertheless, this is usable for the query in the question.  The density figure could be combined with orbital mechanics to ballpark the orbit's lifetime.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-11-15T22:27:49.313" CommentCount="0" CommunityOwnedDate="2013-11-15T22:27:49.313" ContentLicense="CC BY-SA 3.0" />
  <row Id="2816" PostTypeId="1" AcceptedAnswerId="2817" CreationDate="2013-11-16T07:29:30.487" Score="9" ViewCount="2009" Body="&lt;p&gt;I was reading &lt;a href=&quot;http://timesofindia.indiatimes.com/home/science/Mars-mission-Isro-performs-last-orbit-raising-manoeuvre/articleshow/25862187.cms&quot;&gt;this news article&lt;/a&gt;, and it was mentioned that orbit raising is done in early hours. In all the five orbit raising operations, they are raising them only during night. What could be the reason behind this? Do they do them at night time, to minimize signal disturbance, or is there any other reason?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="49" LastEditDate="2013-11-16T09:47:59.170" LastActivityDate="2013-11-17T02:17:34.993" Title="Why are orbit raising maneuvers for Mangalyaan done in the early hours?" Tags="|orbital-mechanics|low-earth-orbit|orbital-maneuver|isro|mars-orbiter-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2817" PostTypeId="2" ParentId="2816" CreationDate="2013-11-16T09:43:42.250" Score="13" Body="&lt;p&gt;ISRO calls these &lt;em&gt;&quot;Midnight Maneuvers&quot;&lt;/em&gt;. This question, why do these maneuvers fall at around midnight Indian Standard Time (IST), has been asked by some of their readers on &lt;a href=&quot;https://www.facebook.com/pages/ISROs-Mars-Orbiter-Mission/1384015488503058&quot; rel=&quot;noreferrer&quot;&gt;Mars Orbiter Mission's Facebook page&lt;/a&gt; before, and ISRO answered:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;“Firing has to happen near the perigee and in the visibility from&#xA;  ISTRAC ground stations. All these orbits have argument of perigee of&#xA;  ~285 deg. When all these constraints are put together, firings time&#xA;  will almost always fall in to midnights of Indian sub continent.”&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Ra8ra.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Graphic showing trajectories for the Midnight Maneuvers of ISRO’s Mars Orbiter Mission Spacecraft (Credit: ISRO)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality though, not all of these orbital maneuvers were done at midnight IST:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;First Earth-bound maneuver: 7 Nov. 2013, 01:17 am IST (completed successfully)&lt;/li&gt;&#xA;&lt;li&gt;Second Earth-bound maneuver: 8 Nov. 2013, 02:18 am IST (completed successfully)&lt;/li&gt;&#xA;&lt;li&gt;Third Earth-bound maneuver: 9 Nov. 2013, 02:10 am IST (completed successfully)&lt;/li&gt;&#xA;&lt;li&gt;Fourth Earth-bound maneuver: 11 Nov. 2013, 02:06 am IST (achieved apogee of 78,722 km, planned 117,724 km)&lt;/li&gt;&#xA;&lt;li&gt;Fourth Earth-bound maneuver, supplemental: 12 Nov. 2013, 05:00 am IST (completed successfully)&lt;/li&gt;&#xA;&lt;li&gt;Fifth Earth-bound maneuver: 16 Nov. 2013, 01:27  IST (completed successfully)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All sources: &lt;a href=&quot;https://www.facebook.com/pages/ISROs-Mars-Orbiter-Mission/1384015488503058&quot; rel=&quot;noreferrer&quot;&gt;Mars Orbiter Mission on Facebook&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2013-11-17T02:17:34.993" LastActivityDate="2013-11-17T02:17:34.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2818" PostTypeId="2" ParentId="2800" CreationDate="2013-11-16T21:37:56.567" Score="6" Body="&lt;p&gt;No, you can't fly through the rings you can see without hitting lots and lots of dust-sized ice particles at high velocity.  Your vehicle will not fare well.  The ring material is not sparse in that sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a wide distribution of particle sizes from boulders to dust.  There's a lot more dust-sized, so that's what you need to worry about.  (I wouldn't really call the particles &quot;asteroids&quot;.  They're almost entirely water ice.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You shouldn't even fly through what appear to be gaps without at least looking at the occultation data and doing a calculation for what you might hit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-11-16T21:37:56.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2819" PostTypeId="1" CreationDate="2013-11-16T22:02:25.840" Score="9" ViewCount="701" Body="&lt;p&gt;This is a follow-up on a few similar questions of mine regarding effects of the loss of intracranial pressure gradient of astronauts that work in microgravity for extended periods of time, and how does that translate in their cardiovascular and ocular health, like for example &lt;a href=&quot;https://space.stackexchange.com/q/2611/49&quot;&gt;How do astronauts battle loss in blood volume in microgravity&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/2706/49&quot;&gt;Has dependent one-carbon metabolic pathway been successfully linked to ocular health of astronauts&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In essence, I was wondering if there are any studies, or merely reports of change in frequency of intracranial pain, like plain headaches, migraines, inflammation of the paranasal sinuses, or throbbing, aching, or stabbing sensation in the eyes by astronauts? Do these complaints in general increase, or decrease in frequency when astronauts are exposed to microgravity environment for longer times, compared to the pre- and post-flight occurrence rate, when astronauts are presumably under similar, equivalent dietary regimen? If change in occurrence rate has been detected, what cause is it most attributed to?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-12-22T23:05:06.847" Title="How often do astronauts suffer headaches, migraines, etc. in microgravity?" Tags="|microgravity|health|cardiovascular|pressure-gradient-loss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2820" PostTypeId="1" AcceptedAnswerId="2837" CreationDate="2013-11-18T16:07:51.463" Score="13" ViewCount="8194" Body="&lt;p&gt;For a manned mission to Mars that isn't a one-way trip, we'll need an ascent stage that can reach orbit from Mars' surface. How big would this need to be to transport e.g. 3 astronauts to orbit?&lt;br&gt;&#xA;We've seen examples for Earth (Soyuz comes to mind as a sort-of minimal solution at 600 tons loaded) and the Moon (LM ascent stage was roughly the size of a car). A Mars ascent stage would fall somewhere between these extremes, but where?&lt;br&gt;&#xA;Would it be feasible to launch the descent + ascent stages from Earth on a single launcher, or would it need to be assembled in orbit?  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="49" LastEditDate="2013-11-21T01:55:05.557" LastActivityDate="2015-10-26T16:08:30.830" Title="How big would a manned ascent stage for Mars need to be?" Tags="|mars|rockets|propulsion|engineering|stages|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2821" PostTypeId="2" ParentId="2820" CreationDate="2013-11-18T18:07:07.133" Score="8" Body="&lt;p&gt;Okay, so let's start from a ship somehow constructed and fueled on Mars, and work our way back from that. This convenient delta v chart from &lt;a href=&quot;http://en.wikipedia.org/wiki/File:Delta-Vs_for_inner_Solar_System.svg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; shows us a good starting point:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nSmp3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so the delta v to Low Mars Orbit is about 4.1 km/s. The delta V to get to Earth from there is 2.9 km/s (Earth C3 to Low Mars Orbit). Bottom line is, you need to lift 3 people and supplies and provide it with a delta v of about 6.0 km/s. I'm not an expert in the supplies category, so let's just pick a number, say, let's carry a Dragon capsule fully loaded. &lt;a href=&quot;http://en.wikipedia.org/wiki/Dragon_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; gives about 7500 kgs, which is probably a fair starting point. That's not quite as much as required for an Earth Launch, but would be similar to a Falcon 9 launch from Mars to Earth. Let's call it half of one. In fact, &lt;em&gt;The Case for Mars&lt;/em&gt; states that around 96 tons of fuel is required to make the trip for 4. Few (None existing) rockets can take that to LEO, let alone to the Martian surface. Still, there is an alternative that doesn't involve orbital assembly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key focus is 2 points. First of all, fuel the rocket on the surface of Mars. If you take hydrogen from Earth, you can easily combine it with carbon dioxide and have methane and oxygen, which is a very good rocket fuel. And that requires only 16 tons of fuel, not too bad to manage. Secondly, send the return rocket to the surface of Mars first, let it fuel itself for the return trip home, and you simply have to land close enough to the ship to get there on the surface, which shouldn't be too difficult, as you will have a large range with on board capabilities to get where you need to go.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-18T18:07:07.133" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2822" PostTypeId="1" CreationDate="2013-11-18T18:09:04.290" Score="9" ViewCount="283" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2820/how-big-would-a-manned-ascent-stage-for-mars-need-to-be&quot;&gt;How big would a manned ascent stage for Mars need to be?&lt;/a&gt; assumes 3 astronauts. That question appears to draw inspiration from the Apollo programme, and the Soyuz evolution. Part of the rationale behind the 3-count is mentioned in &lt;a href=&quot;https://space.stackexchange.com/questions/2537/why-have-so-many-space-missions-had-a-crew-of-3&quot;&gt;Why have so many space missions had a crew of 3?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet a manned mission to Mars, or any celestial body &lt;strong&gt;other than Earth's Moon&lt;/strong&gt; would probably be significantly different from the Apollo Program; if only because of the time-scale involved given contemporary technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would one come up with an optimal team-size for a manned mission from Earth to a celestial body &lt;strong&gt;other than Earth's Moon&lt;/strong&gt;?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-11-22T05:46:28.313" Title="How would one come up with an optimal team-size for a manned interplanetary mission?" Tags="|crewed-spaceflight|astronauts|interplanetary|crew-size|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2823" PostTypeId="1" AcceptedAnswerId="2824" CreationDate="2013-11-18T18:24:43.943" Score="10" ViewCount="5826" Body="&lt;p&gt;On 11/15/1961, John Houbolt, an engineer at the Langley Research Center, wrote a nine-page letter to Robert Seamans, associate administrator of NASA.  (&lt;a href=&quot;http://history.nasa.gov/monograph4.pdf#page=34&amp;amp;zoom=auto,0,510&quot;&gt;Referenced in this NASA retrospective.&lt;/a&gt;)  This letter is considered instrumental in promoting the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_orbit_rendezvous&quot;&gt;lunar orbit rendezvous&lt;/a&gt; mode for the Apollo missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the text of this letter available online?&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2018-08-15T01:14:30.127" Title="Text of John Houbolt's letter proposing Lunar Orbit Rendezvous for Apollo?" Tags="|apollo-program|nasa|history|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2824" PostTypeId="2" ParentId="2823" CreationDate="2013-11-18T18:42:48.877" Score="11" Body="&lt;p&gt;Yes, the original letter is available on &lt;a href=&quot;http://ntrs.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;NASA Technical Reports Server&lt;/a&gt; (NTRS) on page 55 of the &lt;em&gt;Key Documents&lt;/em&gt; section to the &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960014824_1996007704.pdf&quot; rel=&quot;noreferrer&quot;&gt;Enchanted Rendezvous, John C. Houbolt and the Genesis of the Lunar-Orbit Rendezvous Concept, Monographs in Aerospace History #4, NASA History Office, NASA-TM-111236&lt;/a&gt; (PDF).&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-11-18T18:42:48.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2825" PostTypeId="1" AcceptedAnswerId="6092" CreationDate="2013-11-18T19:00:34.920" Score="27" ViewCount="1053" Body="&lt;p&gt;Has anyone outside of the &lt;a href=&quot;http://www.mars-one.com/&quot;&gt;Mars One&lt;/a&gt; foundation done an independent evaluation that concept in terms of the following heads? &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Initial Cost&lt;/li&gt;&#xA;&lt;li&gt;Ongoing Cost&lt;/li&gt;&#xA;&lt;li&gt;Technical feasibility? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="288" LastEditDate="2013-11-23T12:00:26.817" LastActivityDate="2015-02-23T20:41:27.820" Title="Is there an independent cost estimate of the Mars One proposal?" Tags="|mars|crewed-spaceflight|cost|mars-one|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2826" PostTypeId="1" AcceptedAnswerId="2828" CreationDate="2013-11-18T19:19:03.910" Score="21" ViewCount="1487" Body="&lt;p&gt;I was watching the United Launch Alliance (ULA) Atlas 5 rocket launching NASA's MAVEN (Mars Atmosphere and Volatile EvolutioN) to Mars today, and the countdown contained a planned 10 minute pause at T-4:00.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1295/what-is-the-purpose-of-having-a-countdown-during-a-rocket-launch?lq=1&quot;&gt;This other answer&lt;/a&gt; touches on some of the general reasons for having planned pauses, and they generally make sense. However, I'd think that with only four minutes left on the clock, there shouldn't be any need or room for a &lt;em&gt;planned&lt;/em&gt; pause.  Otherwise, why not just schedule T for 10 minutes later?&lt;/p&gt;&#xA;" OwnerUserId="898" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-11-19T10:58:52.207" Title="Why have a 10 minute pause at T-4 minutes?" Tags="|launch|nasa|procedure|ula|launch-sequence|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2828" PostTypeId="2" ParentId="2826" CreationDate="2013-11-18T20:58:31.317" Score="17" Body="&lt;p&gt;A planned hold adds flexibility to the schedule. The hold at T-4 was planned to be 10 minutes long &lt;em&gt;if everything went without a hitch&lt;/em&gt;. If a problem had cropped up (e.g. weather suddenly deteriorates and goes outside the launch limits), the hold would have been longer. If there were delays earlier in the countdown, the T-4 hold might have been shorter.&lt;br&gt;&#xA;The sum of all planned holds plus the countdown time is calculated to get to the start of the launch window. The planned length of the final hold varies (&lt;a href=&quot;http://spaceflightnow.com/atlas/av004/status.html&quot;&gt;25 minutes at this Atlas launch in 2005&lt;/a&gt;, &lt;a href=&quot;http://www.interspacenews.com/FeatureArticle/tabid/130/Default.aspx?id=612&quot;&gt;Shuttle launch transcript showing a variable-length hold at T-9 minutes&lt;/a&gt;).&lt;br&gt;&#xA;The hold at T-4 has these purposes: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;NAM and NLM final launch polls -- go to continue countdown&lt;/li&gt;&#xA;&lt;li&gt;Spacecraft transfers to internal power&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(see the &lt;a href=&quot;http://www.nasa.gov/mission_pages/launch/atlas_V_count_101.html&quot;&gt;Atlas V launch overview&lt;/a&gt;). I watched the MAVEN launch coverage, and I don't remember the final launch poll taking up all of the 10 minutes allocated. So they finish the poll, then wait until they can restart the count so that T-0 coincides with the start of the launch window. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the clock is restarted at T-4, these events happen: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Automatic computer sequencer takes control for all critical events through liftoff&lt;/li&gt;&#xA;&lt;li&gt;Atlas first stage LO2 replenishment is secured, allowing the tank to be pressurized for flight&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So when the clock ticks past -4:00, you're committed to launch within a reasonable time, before your LO2 tank boils off too much O2.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is speculation: the reason for the planned hold at T-4 might be psychological. Nobody likes to make big decisions under a looming countdown. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2013-11-19T10:58:52.207" LastActivityDate="2013-11-19T10:58:52.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2830" PostTypeId="1" AcceptedAnswerId="2833" CreationDate="2013-11-18T23:10:30.880" Score="8" ViewCount="1943" Body="&lt;p&gt;It appears during most launches that the only engines working are the main first stage engines. It was my understanding that these engines are fixed in place and cannot turn the rocket, but I must be mistaken. What I am looking for is the mechanical system in place that slightly shift the engine direction so that the rocket can turn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a rocket launches and it begins to tilt into its orbit, how does it control this? Are the first stage engines able to move slightly to move the vehicle down range, or is there some other type of control that keeps the launch vehicle from heading directly upwards?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="49" LastEditDate="2013-11-21T01:57:27.567" LastActivityDate="2013-11-21T01:57:27.567" Title="How does a launch vehicle control its trajectory during the first stage?" Tags="|launch|attitude|engine-design|stages|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2831" PostTypeId="2" ParentId="2830" CreationDate="2013-11-19T00:11:54.510" Score="-3" Body="&lt;p&gt;In the ideal case of launch in a vacuum, all that is needed is an infinitesimal nudge to start a gravity turn.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2013-11-19T00:11:54.510" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2833" PostTypeId="2" ParentId="2830" CreationDate="2013-11-19T04:18:01.553" Score="9" Body="&lt;p&gt;Most rockets use gimbaled engines. They generally turn a few degrees and redirect their thrust. Not unlike driving a front wheeled car backwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle is a good example of this. You can even see the gimbal test in the count down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Gimbal#Rocket_engines&quot;&gt;http://en.wikipedia.org/wiki/Gimbal#Rocket_engines&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1076" LastActivityDate="2013-11-19T04:18:01.553" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2834" PostTypeId="2" ParentId="2830" CreationDate="2013-11-19T05:01:33.067" Score="8" Body="&lt;p&gt;Note that even in the absence of any need to turn, there &lt;strong&gt;&lt;em&gt;must&lt;/em&gt;&lt;/strong&gt; be some method to &lt;strong&gt;&lt;em&gt;stop&lt;/em&gt;&lt;/strong&gt; the rocket from turning.  The thrust of the rocket is directed along a specific line from aft to forward.  This line must pass through the center of mass of the rocket;  otherwise, the thrust will exert a torque around the CofM, and the rocket will tumble.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the fuel and oxidizer are consumed,  the CofM moves from side to side and the rocket thrust no longer acts through the center of mass, and the rocket begins to tumble out of control. The rocket nozzle twists slightly in just the right direction to stop the rotation, rotate back to eliminate the accumulated rotation, and then stop to direct the thrust through the new CofM.  Repeat multiple times per second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Picture balancing a looong broom stick on your finger tip.  Now imagine that the broomstick is alive, hurricane winds are blowing, and an earthquake is occurring...&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2013-11-19T05:01:33.067" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2835" PostTypeId="1" CreationDate="2013-11-19T13:36:07.703" Score="12" ViewCount="601" Body="&lt;p&gt;For about a century, humanity has broadcasted noise to space in the form of TV broadcasts, radio, etc...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_nearest_stars&quot;&gt;nearest star&lt;/a&gt; is 4.242 light-years away, so for about &lt;strong&gt;90 years&lt;/strong&gt;, aliens -- if they exist in the Alpha Centauri system -- have been able to listen to this noise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is humanity capable of eavesdropping on similar noise from aliens?  I mean, &lt;strong&gt;low quality&lt;/strong&gt; noise from a civilization similar to ours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example, some civilization, on the back side of the Milky Way, could probably catch some information after a couple of hundreds centuries with very sensitive receiver. Can we?&lt;/p&gt;&#xA;" OwnerUserId="1002" LastEditorUserId="49" LastEditDate="2013-11-21T01:40:21.757" LastActivityDate="2013-11-21T17:09:26.360" Title="Interstellar eavesdropping" Tags="|communication|technology|deep-space|astronomy|astrobiology|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2836" PostTypeId="2" ParentId="231" CreationDate="2013-11-19T15:43:22.467" Score="5" Body="&lt;p&gt;To plug the numbers given into a simple calculation: The asteroid belt has a volume of 4,35E25 km^3. If we assume for a moment that the asteroids are evenly spaced, and there are 15 billion asteroids (estimate given above for sizes 0,1km+), we arrive at roughly 180km distance from asteroid to asteroid. 3 Orders of magnitude in the number of asteroids give us one order of magnitude in the distance - if there were only 15 million asteroids, we'd have a mean distance of 1800 km.&lt;br&gt;&#xA;Actually, a few hundred kilometers between every object &gt;100m does not seem that much.&lt;/p&gt;&#xA;" OwnerUserId="102" LastActivityDate="2013-11-19T15:43:22.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2837" PostTypeId="2" ParentId="2820" CreationDate="2013-11-19T16:07:31.970" Score="4" Body="&lt;p&gt;I plugged the data provided by @PearsonArtPhoto into the rocket equation, with Isp=304 seconds (Merlin 1C in vacuum), and I get a launch mass of 262 tons to get a Dragon into Mars orbit. A Falcon 9v1.0 weighs 333 tons.&lt;br&gt;&#xA;To get from Mars surface directly to Earth C3, I get a launch mass of 3713 tons, or 1.3 times the weight of a Saturn V. So you really need a Mars Orbit rendezvous on the return trip.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-11-19T16:07:31.970" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2838" PostTypeId="2" ParentId="2835" CreationDate="2013-11-19T17:39:39.303" Score="8" Body="&lt;p&gt;Short answer - it's theoretically possible but extremely improbable. I found &lt;a href=&quot;http://www.straightdope.com/columns/read/3058/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth&quot; rel=&quot;nofollow&quot;&gt;this very well referenced article&lt;/a&gt; explaining some of the challenges involved. Basically, we would need a REALLY huge receiver and we would need to know exactly where to point it. And we would need the extraordinary luck of pointing it during a time exactly like this one when a civilization has radio technology, but has not refined it enough to stop wasting huge amounts of energy pouring their signals into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, this perspective is also taken from our current vantage point in a civilization that is barely advanced enough to start turning down the noise a bit. Fractal antennas, for example, were a huge game-changer in this area in some ways. Cell phones are much smaller now AND have better range, and we haven't even been using this tech for 20 years yet. So what might be possible in 100 years?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are more and more antennas on this planet every day. I haven't found any data on the exact exponential growth rate, but I would venture a guess that the quantity of similar quality antennas has been doubling every 5 or 10 years recently. They are not directional, but who's to say that some new breakthrough in 50 years won't allow &lt;strike&gt;the NSA&lt;/strike&gt; SETI-at-home to tap into every device with an antenna and use the whole planet as if it was one big ear and with precise timing circuits, quantum computing, intelligent noise cancellation, and crazy new signal processing technologies, be able to hear the entire sky at once with sufficient sensitivity to hear the birth of a new technologically active civilization of cuddle-fish people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until some of these sci-fi ideas become a reality, no, I don't think there's any real chance of hearing anything. But let's just wait and see what somebody comes up with next. I think it might become possible one day. And if so, maybe someone will hear us as our signals continue to dissipate out into deep space over the next several centuries.&lt;/p&gt;&#xA;" OwnerUserId="945" LastEditorUserId="945" LastEditDate="2013-11-21T17:09:26.360" LastActivityDate="2013-11-21T17:09:26.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2839" PostTypeId="2" ParentId="2835" CreationDate="2013-11-19T17:45:22.140" Score="10" Body="&lt;p&gt;Because radio waves in space follow the &lt;a href=&quot;http://en.wikipedia.org/wiki/Inverse-square_law&quot;&gt;inverse square law&lt;/a&gt;, it would require an exceptionally large antenna to pull in enough of an ordinary radio signal to actually notice it, let alone get any useful data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider that a typical radio signal is designed for distances measured in miles. TV broadcasts disappear into the noise within about 100 miles from the transmitter. One light year is about 5.9 trillion miles, or about 59 billion times farther than that, so the power of the a TV station at that distance is 59 billion &lt;em&gt;squared&lt;/em&gt; times as weak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now a signal to a geostationary relay satellite is designed to be received at a distance of about 30,000 miles. One light year is about 200 million times that distance, so the signal would be 40 quadrillion times weaker at one light year, or 640 quadrillion times weaker at 4 light years distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might be possible to use techniques such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Autocorrelation&quot;&gt;autocorrelation&lt;/a&gt; to turn the problem from a near impossibility into something merely very difficult; at the present time, Earth does not possess any capability for receiving anything short of deliberate attempts to contact us.&lt;/p&gt;&#xA;" OwnerUserId="113" LastEditorUserId="288" LastEditDate="2013-11-19T18:22:07.850" LastActivityDate="2013-11-19T18:22:07.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2840" PostTypeId="1" AcceptedAnswerId="4723" CreationDate="2013-11-20T10:24:22.323" Score="15" ViewCount="1647" Body="&lt;p&gt;According to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Mass_concentration_%28astronomy%29&quot;&gt;mascon entry on wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At that time, one of NASA's highest priority &quot;tiger team&quot; projects was&#xA;  to explain why the Lunar Orbiter spacecraft being used to test the&#xA;  accuracy of Project Apollo navigation were experiencing errors in&#xA;  predicted position of ten times the mission specification (2&#xA;  kilometers instead of 200 meters). This meant that the predicted&#xA;  landing areas were 100 times as large as those being carefully defined&#xA;  for reasons of safety. Lunar orbital effects principally resulting&#xA;  from the strong gravitational perturbations of the MASCONS were&#xA;  ultimately revealed as the cause. William Wollenhaupt and Emil&#xA;  Schiesser of the NASA Manned Spacecraft Center in Houston then worked&#xA;  out the &quot;fix&quot; that was first applied to Apollo 12 and permitted its&#xA;  landing within 163 meters of the target, the previously-landed&#xA;  Surveyor 3 spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I've been unable to find any reliable information on the fix mentioned here. What was the fix, exactly? Is there a reliable reference for this information?&lt;/p&gt;&#xA;" OwnerUserId="1079" LastEditorUserId="49" LastEditDate="2013-11-21T01:38:05.097" LastActivityDate="2023-04-27T11:59:47.490" Title="What was the mascon &quot;fix&quot; used by Apollo 12?" Tags="|the-moon|apollo-program|lunar-landing|gravity-fields|site-selection|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2841" PostTypeId="1" AcceptedAnswerId="4025" CreationDate="2013-11-20T19:27:41.390" Score="5" ViewCount="313" Body="&lt;p&gt;In the &lt;a href=&quot;http://www.mars-one.com/en/mission/summary-of-the-plan&quot; rel=&quot;nofollow&quot;&gt;Mars One roadmap&lt;/a&gt;, they are scheduled to land rovers on Mars in 2018 to search for possible colonization spots. But once that is done and the first settlers arrive will these rovers be usable as a means of transportation, or are they strictly automated exploratory vehicles?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="49" LastEditDate="2014-03-05T09:31:04.237" LastActivityDate="2014-03-18T01:58:35.897" Title="Could the Mars One rovers be used to carry personnel?" Tags="|mars|colonization|rovers|mars-one|transport|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2842" PostTypeId="1" CreationDate="2013-11-20T19:59:54.607" Score="6" ViewCount="686" Body="&lt;p&gt;How is Mars One planning on choosing a landing site? Will they be able to use NASA assets (Mars Reconnaissance Orbiter, etc) to assist with choosing a site, or will they have to make do with public sources?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2013-11-21T01:32:20.903" LastActivityDate="2013-11-21T17:31:47.160" Title="How will the Mars One landing site be chosen?" Tags="|mars|landing|mars-one|planning|site-selection|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2843" PostTypeId="1" AcceptedAnswerId="2844" CreationDate="2013-11-20T22:53:27.550" Score="4" ViewCount="335" Body="&lt;p&gt;As far as I can tell, there are dozens of aerospace companies and contractors, which partner with various space programs around the world to develop, build and maintain space vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are any of the non-US based aerospace companies and/or contractors actively, and with any noticeable actual progress, independently (or in partnership with other companies/governments, but with a non-government company or organization in the lead) pursuing manned space flight or exploration? &lt;/p&gt;&#xA;" OwnerUserId="1079" LastEditorUserId="49" LastEditDate="2013-11-21T01:31:28.963" LastActivityDate="2014-10-04T17:42:27.537" Title="Are there any non-government owned, non-US based, companies that are in the process of developing manned spaceflight?" Tags="|crewed-spaceflight|cost|commercial|developing-space-programs|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2844" PostTypeId="2" ParentId="2843" CreationDate="2013-11-20T23:06:05.127" Score="2" Body="&lt;p&gt;The only one that I can think of is &lt;a href=&quot;http://www.mars-one.com/en/&quot; rel=&quot;nofollow&quot;&gt;Mars One&lt;/a&gt; that is Netherlands based and plans to establish a permanent human settlement on Mars by 2023. From their front page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A reliable surface habitat will be set up before the first crew lands;&#xA;  more settlers and cargo will follow every two years. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Our plan is realistic because the technology needed already exists and&#xA;  can be purchased from the private space industry. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The first footprint on Mars will fascinate and inspire generations; it&#xA;  is this public interest that will help finance this human mission to&#xA;  Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'll leave it to readers to decide how realistic this project really is, but it does fit the bill based on your question's requirements. It is not government funded, and it isn't US based. As for &lt;em&gt;actual progress&lt;/em&gt;, I'll let you be the judge of that too, their planned &lt;a href=&quot;http://www.mars-one.com/en/mission/summary-of-the-plan&quot; rel=&quot;nofollow&quot;&gt;roadmap is published here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-11-20T23:06:05.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2845" PostTypeId="1" AcceptedAnswerId="2847" CreationDate="2013-11-20T23:07:44.010" Score="6" ViewCount="790" Body="&lt;p&gt;I was studying &lt;a href=&quot;http://www.nasa.gov/sites/default/files/styles/946xvariable_height/public/iss-infographic-22a.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA infographic&lt;/a&gt;, and noticed something interesting on the timeline portion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MwlFD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the station was originally assembled in late '98, while the 'first crew' didn't arrive until late 2000. Why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why wasn't the station inhabited from day 1? What wasn't ready? How was the station maintained during this timeframe?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2013-11-21T01:49:50.613" LastActivityDate="2018-07-02T06:44:57.837" Title="Why such a large gap between ISS construction and inhabitation?" Tags="|iss|crewed-spaceflight|construction|habitat|orbital-assembly|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2846" PostTypeId="2" ParentId="2845" CreationDate="2013-11-20T23:20:47.487" Score="7" Body="&lt;p&gt;What is missing from your timeline in when Zvezda, the Service Module arrived. It was launched July 12, 2000 and docked on July 26, 2000.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual living quarters in the station, initially were all in Zvezda.  Zarya could maintain orbital control, provide minimal power, and life support, but it is Zvezda that is really the main module of the station, at least at first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is basically a copy of the Mir space station module, modified.  Until the US Hab module was launched, it was the main living space on the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the gap is only 3 months and 4 days.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-11-20T23:20:47.487" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2847" PostTypeId="2" ParentId="2845" CreationDate="2013-11-20T23:47:42.327" Score="7" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_1&quot; rel=&quot;noreferrer&quot;&gt;Expedition 1&lt;/a&gt; in 2000 was the first long duration stay on the International Space Station, but in 1998 the construction of the station began, with several spacewalks and using Space Station Remote Manipulator System (SSRMS) installed on the Space Shuttle used to assemble and prepare for operation first three ISS modules, Russian &lt;a href=&quot;https://en.wikipedia.org/wiki/Zarya&quot; rel=&quot;noreferrer&quot;&gt;Zarya&lt;/a&gt; and and &lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Zvezda&lt;/a&gt;, and American &lt;a href=&quot;https://en.wikipedia.org/wiki/Unity_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Unity&lt;/a&gt;. Quoting &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Assembly&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on International Space Station, Assembly&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The assembly of the International Space Station, a major endeavour in&#xA;  space architecture, began in November 1998. Russian modules launched&#xA;  and docked robotically, with the exception of Rassvet. All other&#xA;  modules were delivered by the Space Shuttle, which required&#xA;  installation by ISS and shuttle crewmembers using the SSRMS and EVAs;&#xA;  as of 5 June 2011, they had added 159 components during more than&#xA;  1,000 hours of EVA. 127 of these spacewalks originated from the&#xA;  station, while the remaining 32 were launched from the airlocks of&#xA;  docked Space Shuttles.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The first module of the ISS, Zarya, was launched on 20 November 1998&#xA;  on an autonomous Russian Proton rocket. It provided propulsion,&#xA;  attitude control, communications, electrical power, but lacked&#xA;  long-term life support functions. Two weeks later a passive NASA&#xA;  module Unity was launched aboard Space Shuttle flight STS-88 and&#xA;  attached to Zarya by astronauts during EVAs. This module has two&#xA;  Pressurized Mating Adapters (PMAs), one connects permanently to Zarya,&#xA;  the other allows the Space Shuttle to dock to the space station.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;On 12 July 2000 Zvezda was launched into orbit. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And this is how the ISS looked like when occupied by its first, &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_1&quot; rel=&quot;noreferrer&quot;&gt;Expedition 1&lt;/a&gt; crew in 2000:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KnKY5.jpg&quot; alt=&quot;International Space Station (ISS), against darkness of space, photographed by STS-97 crew from the approaching Space Shuttle Endeavour&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;International Space Station photographed from approaching &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-97&quot; rel=&quot;noreferrer&quot;&gt;STS-97&lt;/a&gt; on 2 December 2000 (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:S97e5009.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More about the assembly of the ISS is available in the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Assembly&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia link&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-11-20T23:47:42.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2848" PostTypeId="1" CreationDate="2013-11-21T00:11:23.447" Score="15" ViewCount="3937" Body="&lt;p&gt;Working with liquid nitrogen all day, I am constantly bothered by their boiling away, forcing me to go get more from the storage dewar. Likewise should be the case with any cryogen, e.g. LH2 and LOX that are the most common rocket propellants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I've read, nearly all robotic probes use propellants that don't require any refrigeration: hydrazine, MMH, N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;, and so on, as they can sit happily in a steel ball for years. Driven by another question that brings up the proposition to &quot;bring hydrogen to Mars&quot; to produce rocket fuel, my query is how practical is that?  Hydrogen is not liquid at any (reasonable) pressure unless cooled below ~33 K, a difficult feat to maintain that requires lots of insulation and/or evaporative cooling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is the longest that any mission (manned or unmanned) has stored cryogens?&lt;/strong&gt; (used for fuel; &lt;a href=&quot;http://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer&quot; rel=&quot;noreferrer&quot;&gt;WISE&lt;/a&gt; doesn't count).  Apollo at a couple weeks?  I can't find what Soyuz/Progress use, or the ISS's engine, the Shuttle's OMS seems to use MMH/N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;&#xA;" OwnerUserId="881" LastEditorUserId="25" LastEditDate="2015-06-24T14:58:26.717" LastActivityDate="2015-06-25T12:13:35.743" Title="How long is it feasible to store cryogenic fuels?" Tags="|fuel|materials|thermal|cryogenics|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2849" PostTypeId="1" AcceptedAnswerId="2851" CreationDate="2013-11-21T02:17:01.033" Score="5" ViewCount="1228" Body="&lt;p&gt;After reading the question: &lt;a href=&quot;https://space.stackexchange.com/questions/296/is-artificial-gravity-feasible-in-manned-long-term-space-exploration&quot;&gt;Is artificial gravity feasible in manned long-term space exploration?&lt;/a&gt;, I am having a hard time imagining what experiencing the &quot;continuous acceleration&quot; method of gravity generation would be like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, what direction on the ship would the gravity be generated in?&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Imagine the following spaceship travelling from left to right:&#xA;         UP&#xA;BACK  #&amp;gt;-----O&amp;gt;  FRONT&#xA;        DOWN&#xA;&#xA;DOWN:  (earth like) as depicted in most movies, like driving a car&#xA;BACK:  (makes most logical sense to me) like spinning a bucket of water or getting&#xA;       pulled back in a car from accelerating quickly&#xA;UP:    like walking on the ceiling&#xA;FRONT: opposite of back&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Second, how long would you get the artificial gravity, and would it mess up half way through  the flight? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/296/is-artificial-gravity-feasible-in-manned-long-term-space-exploration/485#485&quot;&gt;This answer&lt;/a&gt; on that question states that you would travel at 1g until you reach the halfway point, then you turn retrograde and de-accelerate for the rest of the trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So would you have 1g throughout the entire trip? Including the retrograde phase?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the retrograde phase, would the gravity be reversed? Causing you to have to walk on the ceiling for the remainder of the trip?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My assumption is that future spaceships that travel in this manner would essentially be an RV, standing on it's nose on the back of a truck. The truck would be travelling so fast that you would be able to stand upright in the RV. Then for the last part of the journey (retrograde), you would stand on the ceiling for a while until slowly loosing gravity (and while changing to retrograde you would be in zero g breifly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see I am having a little trouble flying on one of these spaceships in my head. Can anyone help?&lt;/p&gt;&#xA;" OwnerUserId="1087" LastEditorUserId="-1" LastEditorDisplayName="user29" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2022-12-09T17:57:14.127" Title="Where is the gravity generated on a spaceship in super fast space travel?" Tags="|crewed-spaceflight|gravity|artificial-gravity|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2850" PostTypeId="2" ParentId="2849" CreationDate="2013-11-21T02:42:54.607" Score="7" Body="&lt;p&gt;The apparent direction of gravity depends on the direction in which the spaceship is accelerating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a constant 1G boost (which is far beyond our current capabilities), &quot;down&quot; would be in the direction of the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such a ship would have to be designed with its decks perpendicular to the direction of flight. During the initial part of the journey, while the ship is accelerating away from Earth, Earth and the Sun would be &quot;down&quot; from the perspective of the crew, and the destination would be &quot;up&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To arrive at the destination at low speed, you'd need to rotate the ship 180° halfway through the journey, and then accelerate &lt;em&gt;away&lt;/em&gt; from the destination (i.e., &lt;em&gt;decelerate&lt;/em&gt;) for the second half of the trip. During that part of the journey, Earth and the Sun would be &quot;up&quot;, and the destination would be &quot;down&quot; from the perspective of the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The change could be done by shutting off the engines, rotating the ship, and the restarting the engines. Or, if you want to maintain on-board gravity at all times, you can rotate the ship as it continues to thrust (which might require a bit of calculation to avoid messing up the trajectory).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most spaceships we see in fiction (like, say, the USS Enterprise and the Battlestar Galactica) have on-board artificial gravity, and are designed more like modern surface ships, with &quot;up&quot; and &quot;down&quot; perpendicular to the direction of travel. A ship that gets its on-board gravity by constant thrust acceleration would have to be designed differently, with &quot;up&quot; and &quot;down&quot; &lt;em&gt;parallel&lt;/em&gt; to the direction of travel -- or, more precisely, to the direction of acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're unable to accelerate at a constant 1G for years at a time, the other obvious way to generate on-board gravity is by rotating the ship. In that case, the decks would be cylindrical, and &quot;up&quot; would be toward the axis of rotation. We see this in the Discovery in &quot;2001: A Space Odyssey&quot;. Most likely the axis of rotation would be parallel to the direction of travel, so the Earth and Sun would be &quot;behind&quot; you along the axis, and the destination would be &quot;ahead&quot; of you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With current technology, we can accelerate a ship at 1G for no more than a few minutes. Rotational artificial gravity is probably feasible, but as far as I know it's never actually been tried. Artificial gravity by means other than accelerating or rotating the ship (as on Star Trek, Battlestar Galactica, and so forth) is still in the realm of &lt;strike&gt;magic&lt;/strike&gt; technology beyond any current theories I'm aware of.&lt;/p&gt;&#xA;" OwnerUserId="574" LastActivityDate="2013-11-21T02:42:54.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2851" PostTypeId="2" ParentId="2849" CreationDate="2013-11-21T03:10:59.940" Score="3" Body="&lt;p&gt;First of, it wouldn't be true &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitation&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity&lt;/a&gt;, but since we experience gravitational force same as a constant acceleration, there wouldn't be any apparent difference in its effect on anything you'd experience inside an accelerating spacecraft. You wouldn't experience constant speed as acceleration, no matter how fast your spaceship goes. It would have to be constant acceleration, otherwise you're &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertia&quot; rel=&quot;nofollow noreferrer&quot;&gt;inertial&lt;/a&gt; with your frame of reference (your spaceship) and you wouldn't feel any &lt;a href=&quot;https://en.wikipedia.org/wiki/Weight&quot; rel=&quot;nofollow noreferrer&quot;&gt;weight&lt;/a&gt;. The vector of this &lt;em&gt;artificial gravity&lt;/em&gt; would be in the opposite direction to the constant acceleration of your spaceship, as per &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's laws of motion&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Momentum#Conservation&quot; rel=&quot;nofollow noreferrer&quot;&gt;conservation of momentum&lt;/a&gt;, so in your graph that would be towards the &lt;em&gt;back&lt;/em&gt; of the spaceship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7EEDK.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Rocket principles and Newton's third law (Source: &lt;a href=&quot;http://exploration.grc.nasa.gov/education/rocket/TRCRocket/rocket_principles.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The strength of this force, its uniformity and duration would be exactly equal and opposite to the acceleration achieved by your spaceship, its ability to keep it constant and for the duration you could sustain it. So these are already technical limitations of your spacecraft, like e.g. how much of propellants it could carry, engine performance, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For deceleration, or any change in your acceleration vector, with any conventional means of propulsion at least (chemical rockets, ion thrusters, etc.), you would have to rotate your thrusters in the direction opposite to the one you want to apply force to, so as far as you're concerned, being inside the spaceship, your up/down orientation likely wouldn't change for long, if you rotated the whole spaceship to decelerate by applying thrust in the opposite vector. Deceleration is nothing else than a negative acceleration, so we have changed its vector by 180°, but have also rotated the spaceship for exact same amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In your common reference frame, your up and down would stay the same, even though you'd swap your zenith and nadir in an extended reference frame, looking at you in the spaceship from the outside. I.e., if you stood before with your feet towards the Earth you departed from, you'd now be facing it upside down on deceleration, with your feet towards your destination. For a moment though, while you rotate your spaceship (likely using smaller, side-mounted thrusters), you would experience zero gravity, i.e. no acceleration in relation to your spaceship, and your common, &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_frame_of_reference&quot; rel=&quot;nofollow noreferrer&quot;&gt;inertial frame of reference&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope that explains it well enough, and now, of course, an obligatory &lt;a href=&quot;http://xkcd.com/1115/&quot; rel=&quot;nofollow noreferrer&quot;&gt;XKCD&lt;/a&gt; (barely relevant, just cute... :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://xkcd.com/1115/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0UqTc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-21T03:16:16.413" LastActivityDate="2013-11-21T03:16:16.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2853" PostTypeId="1" CreationDate="2013-11-21T07:44:58.587" Score="22" ViewCount="5365" Body="&lt;p&gt;A question related to &lt;a href=&quot;https://space.stackexchange.com/questions/301/what-power-source-options-provide-reliable-and-sustainable-power-generation-for&quot;&gt;What power source options provide reliable and sustainable power generation for a Martian Colony&lt;/a&gt;, particularly the answer that solar and nuclear based energy sources being the most likely for any Martian colony.  This is also discussed on the &lt;a href=&quot;http://www.mars-one.com/en/mission/technical-feasibility&quot; rel=&quot;noreferrer&quot;&gt;Mars One website&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This leads me to the question, given how windy Mars is, would wind power be a viable 'backup' or supplementary power source for the Mars One colony?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edited to add: has there been any developments in making wind vanes sensitive enough to the conditions on Mars (or similar)?&lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="-1" LastEditorDisplayName="user838" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2022-06-07T16:55:46.507" Title="Is wind-power a viable backup power source for Mars One?" Tags="|mars|energy|atmosphere|mars-one|sustainability|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2854" PostTypeId="2" ParentId="2853" CreationDate="2013-11-21T08:50:03.307" Score="24" Body="&lt;p&gt;No, not really.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;The atmosphere of Mars is very thin.&lt;/strong&gt; &lt;a href=&quot;http://www.universetoday.com/22587/atmosphere-of-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;It has below 1% of the pressure on Earth&lt;/a&gt;. That means it has less than 1% of the force of wind on Earth with the same speed.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Wind only occurs at dawn or dusk.&lt;/strong&gt; Wind happens when there is a pressure gradient between two areas of an atmosphere. Pressure gradients are caused by temperature and humidity gradients. On Earth, these gradients happen mostly at the coasts, because when the Sun shines on an ocean, humidity raises a lot faster and temperature a lot slower than when it shines on the dryland. The oceans are one main reason why weather on Earth is so unpredictable. But on Mars, there are no humidity gradients to speak of and temperature gradients only happen at sunset or sunrise, so you only get wind two times a sol.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="335" LastEditorUserId="47149" LastEditDate="2022-06-01T09:22:51.360" LastActivityDate="2022-06-01T09:22:51.360" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="2855" PostTypeId="1" AcceptedAnswerId="2856" CreationDate="2013-11-21T09:54:51.163" Score="5" ViewCount="2574" Body="&lt;p&gt;I have one issue coming out of one conversation between me and my roommates (none of us professional in the subject of space)!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one had a ship which could approach light speed, could one reach intergalactic space?  (Suppose there are no Food, Water and Oxygen issues!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose this person leaves the earth at his 20s and lives an average life time.&lt;/p&gt;&#xA;" OwnerUserId="1092" LastEditorUserId="25" LastEditDate="2013-11-22T14:33:21.140" LastActivityDate="2014-05-24T03:46:04.393" Title="Would it be possible with a light-speed ship to reach intergalactic space in a human lifetime?" Tags="|interstellar-travel|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2856" PostTypeId="2" ParentId="2855" CreationDate="2013-11-21T10:18:32.447" Score="4" Body="&lt;p&gt;With current technology: no.&lt;br&gt;&#xA;The Milky Way is about 1000 light years thick, and we're near the center plane so you'd need to travel at least 500 light years. The fastest spacecraft, Voyager 1, needs ca. 17,000 years to travel 1 light year, so you'd need 8.5 million years at that speed to reach the nearest edge.&lt;br&gt;&#xA;(note: answer was given before the question was edited to add the 'light speed' condition)&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2013-11-21T17:52:49.257" LastActivityDate="2013-11-21T17:52:49.257" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2858" PostTypeId="2" ParentId="2820" CreationDate="2013-11-21T11:12:34.213" Score="0" Body="&lt;p&gt;&lt;strong&gt;Would it be feasible to launch the descent + ascent stages from Earth on a single launcher, or would it need to be assembled in orbit?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Hobbes and PearsonArtPhoto calculated, it is not feasible to start from Earth as there are no rockets ready, not even from United Launch Alliance or SpaceX, BlueOrigin or Bigelow Space Exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;The question is, &lt;strong&gt;why&lt;/strong&gt; not to assemble in space?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore taking Hydrogen from Earth to Mars distorts the fact that 90% of known visible matter in the universe is hydrogen (H2), the lightest element we know. Why take it from Earth where hydrogen resources are really limited. It's interesting seeing demand for Hydrogen cars on our planet - extracting the H2 out of water H2O via Electrolysis - and at the same time fear that the next war will be about water resources ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No doubt Hydrogen is a good choice for energy storage in space (where there is plenty of H2).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back to space assembly, there is a factory in orbit originally planned for 2014, now unfortunately postponed to 2015: &lt;a href=&quot;http://www.washingtonpost.com/national/health-science/international-space-station-to-receive-inflatable-module/2013/01/16/8a102712-5ffc-11e2-9940-6fc488f3fecd_story.html&quot; rel=&quot;nofollow&quot;&gt;Bigelow Inflatable Space Module&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bigelow BA330 project originally should be plugged together to yield a huge factory habitat. I now remember it's been called Complex Alpha for 16 and Complex Bravo for 32 workers. &lt;a href=&quot;http://bigelowaerospace.com/orbital-complex-construction.php&quot; rel=&quot;nofollow&quot;&gt;manufacturer website, also mentions year 2014 for launch&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1093" LastEditorUserId="1093" LastEditDate="2013-11-21T11:24:13.553" LastActivityDate="2013-11-21T11:24:13.553" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2860" PostTypeId="2" ParentId="1977" CreationDate="2013-11-21T13:45:23.423" Score="2" Body="&lt;p&gt;In one sense, rocket engine technology is easy. You have a simple process (burn fuel), only a few moving parts and very few concessions to longevity (you're throwing the engine away after a couple minutes' use). This means there are very few parts you can optimise.&lt;br&gt;&#xA;On the other hand, the engine will run at the ragged edge of what's technologically possible. Advances in rocket engine technology have been limited by materials science: you need materials that won't melt at very high temperatures, and won't react to pure oxygen, and can stand tremendous forces, etc.&lt;br&gt;&#xA;As new materials became available, new rocket fuels became usable. &lt;a href=&quot;http://www.lr.tudelft.nl/?id=29269&amp;amp;L=1&quot; rel=&quot;nofollow&quot;&gt;New materials&lt;/a&gt; also enabled the shift from open-cycle engines (which waste some of their fuel to run the pumps) to &lt;a href=&quot;http://en.wikipedia.org/wiki/Staged_combustion_cycle_%28rocket%29&quot; rel=&quot;nofollow&quot;&gt;closed-cycle engines&lt;/a&gt; (which reuse the pump fuel so they're more efficient).&lt;br&gt;&#xA;The first successful closed-cycle engine was the &lt;a href=&quot;http://en.wikipedia.org/wiki/RD-253&quot; rel=&quot;nofollow&quot;&gt;RD-253&lt;/a&gt; for the Proton. The first closed-cycle engine in the West was the SSME. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2013-11-21T13:57:46.953" LastActivityDate="2013-11-21T13:57:46.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2861" PostTypeId="2" ParentId="2855" CreationDate="2013-11-21T15:00:20.677" Score="12" Body="&lt;p&gt;Yes, kind-of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you move faster, according to special relativity, time actually slows down. Let's take the 500 light year number mentioned by Hobbes answer, and work with that. So let's assume you have a 60 year lifetime. That would require a time dilation factor of about 500/60, or 8.333. I found an &lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html&quot;&gt;online calculator&lt;/a&gt; that states you would have to go 0.99277c to achieve that dilation factor. For a smaller lifetime, the speed would have to increase, but it is possible, if you can achieve near light speed somehow, that you could reach any distance. Of course, anyone on Earth would have the full 500 years pass (Well, 500/speed, but close enough)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Practically speaking, achieving such speeds is virtually impossible, and then there's the chance of an impact, which would be quite serious. Still, it is possible.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-11-21T15:34:36.547" LastActivityDate="2013-11-21T15:34:36.547" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2862" PostTypeId="1" CreationDate="2013-11-21T16:41:44.373" Score="4" ViewCount="122" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/715/does-mars-space-suit-have-to-be-pressurized&quot;&gt;Does Mars space suit have to be pressurized?&lt;/a&gt; makes it pretty clear the atmospheric pressure on Mars is a miniscule fraction of Earth's own. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In it's turn &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; writes to say that the atmosphere of Mars is mostly &lt;a href=&quot;http://en.wikipedia.org/wiki/Carbon_Dioxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;Carbon Dioxide&lt;/a&gt;. This is at odds with Earth orbit (where the space-walks happen). It is also significantly different from the vacuum experienced by the Apollo astronauts on Luna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Primary_Life_Support_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;PLSS&lt;/a&gt; together with the &lt;a href=&quot;http://en.wikipedia.org/wiki/Extra-vehicular_mobility_unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;EMU&lt;/a&gt; form a closed system relative to the Martian atmosphere, is there any reason to change the &lt;strong&gt;breathing mixture&lt;/strong&gt; in the PLSS for use in atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-12-21T18:32:45.810" Title="Can PLSS in atmosphere use enriched oxygen?" Tags="|spacesuits|oxygen|plss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2863" PostTypeId="5" CreationDate="2013-11-21T16:49:39.033" Score="0" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Primary_Life_Support_System&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A Primary (or Portable or Personal) Life Support System (or /Subsystem) (PLSS), is a device connected to an astronaut's or cosmonaut's spacesuit, which allows extra-vehicular activity with maximum freedom, independent of a spacecraft's life support system&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Considering applying this tag when a question is in the context of &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Suit mobility&lt;/li&gt;&#xA;&lt;li&gt;Suit atmosphere&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Temperature&lt;/li&gt;&#xA;&lt;li&gt;Breathing mixture&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Communication &lt;/li&gt;&#xA;&lt;li&gt;Mobile health telemetry&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-11-21T16:59:06.840" LastActivityDate="2013-11-21T16:59:06.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2864" PostTypeId="4" CreationDate="2013-11-21T16:49:39.033" Score="0" Body="Acronym for 'Portable Life Support System'&#xD;&#xA;&#xD;&#xA;Allows astronaut activity independent of the spacecraft" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-11-21T16:59:13.817" LastActivityDate="2013-11-21T16:59:13.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2865" PostTypeId="2" ParentId="2862" CreationDate="2013-11-21T17:26:31.223" Score="1" Body="&lt;p&gt;The oxygen mixture depends on the pressure. The pressure limits on a space suit are basically limited to the fact that an inflated space suit tends to become stiff, much as a balloon that is blown up is stiff. The pressure on Mars isn't different enough where there will be a significant pressure difference, thus, if a current generation space suit were used on Mars, the pressure/ oxygen mixture would most likely remain nearly identical.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-11-21T17:40:09.297" LastActivityDate="2013-11-21T17:40:09.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2866" PostTypeId="2" ParentId="2842" CreationDate="2013-11-21T17:31:47.160" Score="6" Body="&lt;p&gt;From the &lt;a href=&quot;http://www.sen.com/feature/mars-one-plans-for-human-settlement-on-mars.html&quot;&gt;Mars One&lt;/a&gt; Website:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to the Mars One roadmap, a demonstration mission carrying&#xA;  proof of concept for some of the technologies and a communication&#xA;  satellite would be launched in January 2016. The communication&#xA;  satellite would arrive in orbit in October 2016 and would be able to&#xA;  relay images and videos from the surface of the Red Planet back to&#xA;  Earth.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The first settlement rover should arrive on the Red Planet in 2018,&#xA;  tasked to find the right location to establish the Martian village.&#xA;  Mission planners believe that the idea settlement site would be far&#xA;  enough north for the soil to contain water, equatorial enough for&#xA;  maximum solar power and flat enough for construction of the village.&#xA;  Once the ideal site is located, the rover would prepare the ground for&#xA;  the arrival of cargo supplies.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, a launch in 2016 to scout out potential areas to land will happen, followed by a settlement rover to better determine a landing spot, and finally cargo supplies will be sent out ahead of time. I strongly suspect that publicly available data will assist in landing site selection, and perhaps they will even use the &lt;a href=&quot;http://www.uahirise.org/hiwish/&quot;&gt;HiWish&lt;/a&gt; system to request a few images, but as a whole, it seems like they are prepared to launch their own equipment to determine their landing site.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-21T17:31:47.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2867" PostTypeId="1" CreationDate="2013-11-21T19:00:42.503" Score="10" ViewCount="697" Body="&lt;p&gt;Unlike Earth &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars&quot;&gt;Mars&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;does not have an atmosphere with oodles of Carbon Dioxide and water-vapour.&lt;/li&gt;&#xA;&lt;li&gt;lacks a planet-wide magnetic field&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Even alone these two factors make a significant difference to the amount of radiation incident on the Martian Surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronauts in Earth orbit space-missions are subjected to &lt;a href=&quot;http://en.wikipedia.org/wiki/Galactic_cosmic_rays&quot;&gt;cosmic rays&lt;/a&gt;, as also to &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_energetic_particles&quot;&gt;charged particles&lt;/a&gt; expelled by Sol. &lt;a href=&quot;http://en.wikipedia.org/wiki/Health_threat_from_cosmic_rays&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Exposures on the ISS average 150 mSv per year, although frequent crew rotations minimize individual risk&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.mars-one.com/en/faq-en/19-faq-health/185-will-the-astronauts-suffer-from-radiation&quot;&gt;Mars One program website&lt;/a&gt; sums up the exposure to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The 210-day trip results in radiation exposure of the crew of 386 +/- 61 mSv. On the surface, they will be exposed to about 11 mSv per year during their excursions on the surface of Mars. This means that the settlers will be able to spend about sixty years on Mars before reaching their career limit, with respect to ESA standards&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does this figure of 11mSv per annum over a duration of 60 years incorporate the cumulative nature of such exposure? &lt;/li&gt;&#xA;&lt;li&gt;What is the projected death-rate displacement for colonists to Mars relative to the corresponding age-specific rate on Earth?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user29" LastEditDate="2013-11-21T23:09:24.793" LastActivityDate="2013-11-21T23:09:24.793" Title="What is the projected death-rate displacement for colonists to Mars?" Tags="|mars|radiation|health|mars-one|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2868" PostTypeId="2" ParentId="2867" CreationDate="2013-11-21T19:37:10.347" Score="8" Body="&lt;p&gt;You should quickly find sources converge on a coefficient to convert dose into cancers.  Modern science has quantified this rather well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://rationalwiki.org/wiki/Linear_no-threshold&quot;&gt;http://rationalwiki.org/wiki/Linear_no-threshold&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;LNT estimates that the risk of premature death from radiation-induced cancer is around 5% per sievert or 0.5% per 100 mSv of exposure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The units are easy to mess up, so I'll rewrite the above coefficient:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0.00005 cancers / mSv&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are more variables you could put into this.  I mean, our linear (as in the LNT model) assumption is, in effect a model.  You could replace that model with a sophisticated computer code that takes in all kinds of adjustments, but I'm not interested in that accuracy here.  One very notable correction is fractionated versus non-fractionated dose.  Nuclear industry workers who get a large dose through a single work operation get something like twice the corresponding cancer risk, as opposed to getting that same dose distributed over a longer period of time.  For Mars colonization, I would imagine the dose is pretty constant.  But anyway, moving on:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(0.00005 cancers / mSv) x (11 mSv per year) = 0.00055 cancers / year&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know how long you'd expect to live on Mars.  Let's say 40 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(0.00055 cancers / year) x (40 years) = 0.022 cancers --&gt; 2.2% chance of getting cancer&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Had this number been much closer to 1.0, then different statistical equations would be needed.  For reference, these days the &quot;normal&quot; chance of someone getting cancer and dying from it is something like 23%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This answers half your question, and I'm going to leave it here.  I don't know if I believe that a year on Mars will result in 11 mSv.  With some &lt;em&gt;very&lt;/em&gt; cursory looking online about the subject, I'm starting to get a picture that it depends almost entirely on assumptions about how the colonists spend their time.  Assume that they're under a rock all day, and the dose could drop to near zero.  This isn't physically impossible.  Just put them under 10 tons/m^2 of dirt.  Finished.  Radiation requirement met.  On Earth this would be workable, even if unpleasant.  Design of the facilities on Mars would be subject to... more constraints.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-11-21T19:37:10.347" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2869" PostTypeId="1" CreationDate="2013-11-21T23:54:32.047" Score="3" ViewCount="8012" Body="&lt;p&gt;A satellite mission goes through different phases: planning, building, launch, operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering what percentage of the whole cost is consumed by the operation and maintenance of a satellite that is already in orbit. Let's say for a small satellite (like a cubesat) for scientific purpose? Someone has to maintain a receiving ground station, check the satellites health systems and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any sources or publicly available analyses on the topic?&lt;/p&gt;&#xA;" OwnerUserId="1096" LastActivityDate="2013-11-22T12:27:45.553" Title="How much does it cost to operate a satellite in orbit?" Tags="|low-earth-orbit|cost|ground-station|" AnswerCount="1" CommentCount="1" ClosedDate="2013-11-22T22:42:38.427" ContentLicense="CC BY-SA 3.0" />
  <row Id="2871" PostTypeId="1" AcceptedAnswerId="2884" CreationDate="2013-11-22T03:01:29.083" Score="10" ViewCount="1752" Body="&lt;p&gt;&lt;a href=&quot;http://www.isro.org/&quot;&gt;ISRO&lt;/a&gt; (Indian Space Research Organisation) so far never before used additional marine vessels to help track their launch vehicles, as an extension to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Deep_Space_Network&quot;&gt;Indian Deep Space Network&lt;/a&gt; (IDSN). So why did it require additional tracking stations in the South Pacific for the launch of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle#PSLV_flight_C25&quot;&gt;PSLV C25&lt;/a&gt; (Polar Satellite Launch Vehicle) on its &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot;&gt;Magalyaan / Mars Orbiter Mission&lt;/a&gt; (MOM)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia writes to say about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission#Tracking_and_command&quot;&gt;MOM tracking and command&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Space_Research_Organisation_Telemetry,_Tracking_and_Command_Network&quot;&gt;Indian Space Research Organisation Telemetry, Tracking and Command&#xA;  Network&lt;/a&gt; performed navigation and tracking operations for the launch&#xA;  with ground stations at Sriharikota, Port Blair, Brunei and Biak in&#xA;  Indonesia, and after the spacecraft's apogee becomes more than&#xA;  100,000 km, two large 18-metre and 32-metre diameter antennas of the&#xA;  &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Deep_Space_Network&quot;&gt;Indian Deep Space Network&lt;/a&gt; will be utilised.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA's Deep Space&#xA;  Network will provide position data through its three stations located&#xA;  in Canberra, Madrid and Goldstone on the U.S. West Coast during the&#xA;  non-visible period of ISRO's network. Additional monitoring is&#xA;  provided by technicians on board two leased ships from &lt;a href=&quot;https://en.wikipedia.org/wiki/Shipping_Corporation_of_India&quot;&gt;Shipping&#xA;  Corporation of India&lt;/a&gt;, SCI Nalanda and SCI Yamuna which are currently&#xA;  in position in the South Pacific near Fiji.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How did they track their previous launchers, like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandrayaan-1&quot;&gt;Chandrayaan-1&lt;/a&gt;, without these additional tracking stations, and why was this extension to the IDSN necessary for Magalyaan, if they already used NASA's &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsn/&quot;&gt;Deep Space Network&lt;/a&gt; with facilities in Canberra, Australia?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-11-22T13:36:07.757" LastActivityDate="2013-12-06T09:53:16.263" Title="Why did ISRO require additional tracking ships in the South Pacific for its MOM mission?" Tags="|isro|pslv|mars-orbiter-mission|ground-station|tracking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2872" PostTypeId="2" ParentId="2822" CreationDate="2013-11-22T05:46:28.313" Score="3" Body="&lt;p&gt;This question boils down to the objective and length of the specific mission. If the mission consisted of 30 days on the surface and the primary objective was simply to &lt;em&gt;get there&lt;/em&gt; and lay the framework for future missions, three or four crew members would suffice. But if you are opting for a longer mission with specific scientific objectives, you are going to need more people. Just how many &quot;more&quot; depends. To come up with this number, you must decide first what all you need people to do. List everything that will need to be completed by a human on the mission. From there, you can delegate how many people and -- more importantly -- what each person will do (Hint: cross-training is a magnificent resource saver.)&#xA;I hope this was helpful.&lt;/p&gt;&#xA;" OwnerUserId="1099" LastActivityDate="2013-11-22T05:46:28.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2874" PostTypeId="2" ParentId="2869" CreationDate="2013-11-22T12:01:22.417" Score="3" Body="&lt;p&gt;A satellite itself which is on an orbit high enough to not interact with the atmosphere is essentially maintainance-free. Without atmospheric drag it won't need any course corrections to maintain its orbit. It just stays there indefinitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now about it's operation: This really depends on what the satellite is doing. Do the instruments need regular instructions from earth? How difficult is it to come up with these instructions? Does it send signals back to earth? How much effort is needed for analyzing its transmissions? How strong is its transmission power and how much data does it transmit? Is it OK when it can only be reached from time or does it need constant contact to a ground station? When the latter, do its orbital characteristics allow to use existing ground stations or will it be necessary to build and maintain new ones? Are we talking about one satellite or a satellite system consisting of multiple ones (in that case you could likely reduce cost-per-satellite significantly through synergy effects)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on these factors, the cost can vary between zero and several million per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One random figure I could find: &lt;a href=&quot;http://europa.eu/rapid/press-release_IP-11-42_en.htm&quot; rel=&quot;nofollow&quot;&gt;The European Galileo and EGNOS navigation systems together are estimated to cost 800 million Euro in maintainance per year after completion&lt;/a&gt;. These systems consist of 48 satellites. Does that mean one satellite of the system would cost 16.7 million per year? Unlikely, because you would need almost the same number of operators and ground equipment. Would it be possible to do it cheaper when it wouldn't be an EU government project hampered by countless political factors? Likely.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2013-11-22T12:27:45.553" LastActivityDate="2013-11-22T12:27:45.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2875" PostTypeId="1" CreationDate="2013-11-22T15:33:59.963" Score="5" ViewCount="242" Body="&lt;p&gt;I was looking for a map of Chandrayaan-1's ground track, but I wasn't able to find one. I'm looking for something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7vuao.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;MOM's ground track (credit &lt;a href=&quot;http://www.spaceflight101.com/pslv-mom-ascent-data.html&quot; rel=&quot;noreferrer&quot;&gt;SpaceFlight101&lt;/a&gt;).&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know what the ground track looked like?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="12102" LastEditDate="2021-08-26T00:25:12.190" LastActivityDate="2021-08-26T00:25:12.190" Title="What was Chandrayaan-1's ground track?" Tags="|tracking|isro|ground-track|chandrayaan-spacecraft|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2876" PostTypeId="1" AcceptedAnswerId="2878" CreationDate="2013-11-22T22:47:19.433" Score="12" ViewCount="3430" Body="&lt;p&gt;OK. Hear me out. I know that blimps float because of the buoyancy of air, and there is no air in space. But think of the way a boat floats on the top of water even though the material that the boat is made of is generally much heavier than water. The amount of water displaced is heavier than the boat and all of its contents, so the buoyancy is sufficient to hold the top of the boat all the way above the surface of the water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So is it possible to float a huge blimp on top of the ionosphere? An ordinary blimp is filled with the lightest air possible - hydrogen or helium, which maintains the structure of the blimp with its air pressure. But could such a structure be made so large that it could float on top of the ionosphere and be filled with &lt;i&gt;nothing&lt;/i&gt;, so that it doesn't even need a top to keep out stray ions? (Maybe I should say space &lt;i&gt;boat&lt;/i&gt; rather than space blimp.) Is there any material that could be light enough while still maintaining a structure that wouldn't leak? How big would it have to be to displace enough ionosphere to float up there?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And to preempt the obvious question, &quot;Why?&quot;, I'm thinking in the same general direction as a partial space elevator. The energy cost of getting to space seems to be a primary barrier to space travel in significant volume. And every little bridge that we can come up with to lower the energy cost might put space within reach for a few more of us little earthlings. The edge of the atmosphere is only a small percentage of the way toward the altitude of geosynchronous orbit and beyond, where a space elevator could ultimately fling stuff into space. But the atmosphere is also where all of the turbulence will hit the structure, so maybe a support structure at that altitude could provide some marginally significant strength to such a structure. Imagine a space blimp/boat thing in a shape sort of like a Bundt pan with support struts extending to the elevator shaft.&lt;/p&gt;&#xA;" OwnerUserId="945" LastActivityDate="2017-12-03T07:40:18.423" Title="Can we make a space blimp?" Tags="|atmosphere|space-elevator|geophysics|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2877" PostTypeId="1" CreationDate="2013-11-23T04:46:58.450" Score="17" ViewCount="1941" Body="&lt;p&gt;It is often said that the $L_4$ and $L_5$ points are &quot;60 degrees ahead and behind&quot; a planet like Jupiter. Clearly this is true only in the case of circular orbits. In more elliptical orbits, I assume the rule of thumb would be that the $L_4$ and $L_5$ are at the points on the orbit that are equidistant from the planet and barycenter. Would that be a safe assumption?&lt;/p&gt;&#xA;" OwnerUserId="1101" LastEditorDisplayName="user12" LastEditDate="2013-11-23T16:10:23.267" LastActivityDate="2014-02-10T13:02:08.600" Title="How does orbital eccentricity affect positions of Lagrange points $L_4$ and $L_5$?" Tags="|orbital-mechanics|orbit|lagrangian-points|gravity-fields|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2878" PostTypeId="2" ParentId="2876" CreationDate="2013-11-23T08:58:18.813" Score="14" Body="&lt;p&gt;Water has a well-defined surface. Everything below it is water, everything above it is air. The top of the atmosphere isn't like that; you just get a very gradual decrease of pressure until you're several hundred kilometers above Earth's surface.&lt;br&gt;&#xA;You could try to build a cylinder with a closed bottom and open top, and place this upright, so it'll float like a bottle in water. But you'd have to build this cylinder hundreds of km high to prevent it filling up at the top, and strong enough to withstand the air pressure at the bottom because there will be a vacuum inside.&lt;br&gt;&#xA;At an &lt;a href=&quot;http://www.isas.jaxa.jp/e/special/2003/yamagami/03.shtml&quot;&gt;altitude of 50 km&lt;/a&gt;, the air pressure is about 16 mbar, so your structure would have to withstand 16 gram/cm^2.&lt;br&gt;&#xA;The structure also has to be big, because the thin air has little &lt;a href=&quot;http://www.engineeringtoolbox.com/air-density-specific-weight-d_600.html&quot;&gt;buoyancy&lt;/a&gt;. At 16 mbar, air weighs 20 grams per cubic meter, so that's all it will support. It'd be very difficult to build a structure lighter than this that is strong enough to withstand the air pressure, let alone have it carry any useful weight. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-11-23T08:58:18.813" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2879" PostTypeId="2" ParentId="2876" CreationDate="2013-11-23T12:04:22.667" Score="8" Body="&lt;p&gt;An object floats (in water or in atmosphere) when it weights less than the water/atmosphere it displaces with its volume. That means the whole trick of building a blimp is building an object which has a lower density than air. The higher you get, the lower the density of the atmosphere, so you need an even lower density to ascend higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A vacuum has a density of 0. So when you could build an object which is only a vacuum with nothing to contain it, it would float till the edge of the atmosphere. But unfortunately this is impossible. You need something to contain the vacuum in. This container must be rigid enough to withstand the pressure-difference between inside and outside. Otherwise it would collapse, which reduces the volume. To provide this rigidity, it must have mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, the amount of force the container needs to withstand reduces with height, because the pressure-gradient between inside and outside of the hull decreases. That means that after a certain height you could drop some reinforcements of the hull to reduce your weight and ascend even higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, the maximum height you can reach through buoyancy is an engineering problem: Build an air-tight object with the lowest ratio of mass to volume possible. The better the ratio, the closer it will get to the edge of the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2013-11-23T12:31:37.440" LastActivityDate="2013-11-23T12:31:37.440" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2880" PostTypeId="1" AcceptedAnswerId="2883" CreationDate="2013-11-23T13:37:15.233" Score="6" ViewCount="1561" Body="&lt;p&gt;I just saw &lt;a href=&quot;https://space.stackexchange.com/questions/2877/the-l4-and-l5-points-in-elliptical-orbits-libration-amplitude&quot;&gt;How does orbital eccentricity affect positions of Lagrange points $L_4$ and $L_5$?&lt;/a&gt; and it questions the difference between circular and elliptical orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know the Moon does not have a circular &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_of_the_Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbit&lt;/a&gt;. Do we know of any natural bodies that are actually in a circular orbit?  Is there any possible way for a circular orbit to occur naturally? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-11-23T20:37:32.443" Title="Are there any natural circular orbits?" Tags="|orbital-mechanics|orbit|astronomy|moons|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2881" PostTypeId="1" AcceptedAnswerId="18469" CreationDate="2013-11-23T14:21:52.217" Score="8" ViewCount="548" Body="&lt;p&gt;Is there a way to calculate the libration amplitude of a test object that orbits the $L_4$ or $L_5$ Lagrange points?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By amplitude I mean the distance along the orbit between the two extremes of libration. I.e. the point in the tadpole orbit (as examined in a co-rotating frame) where the orbiting test object is farthest from the planet, and then the point at the other extreme where the test object most closely approaches the planet before turning back toward the $L_5$ (or $L_4$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without working out any equations, on the surface it seems that the amplitude (as well as whether the object is in a tadpole or horseshoe orbit) will depend on the energy the test object has when it is captured. Is that a safe assumption?&lt;/p&gt;&#xA;" OwnerUserId="1101" LastEditorUserId="8693" LastEditDate="2016-10-03T18:20:39.957" LastActivityDate="2016-10-03T18:20:39.957" Title="How to calculate libration amplitude of objects orbiting the $L_4$, $L_5$?" Tags="|orbital-mechanics|orbit|lagrangian-points|gravity-fields|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2882" PostTypeId="1" CreationDate="2013-11-23T14:40:11.267" Score="6" ViewCount="743" Body="&lt;p&gt;It's easy to get spectral information about asteroids, &lt;a href=&quot;http://ssd.jpl.nasa.gov/sbdb_query.cgi&quot; rel=&quot;noreferrer&quot;&gt;from the JPL database&lt;/a&gt; for instance.  They often have an albedo estimation, but almost never have a mass estimate (if you have albedo and mass, then you roughly have density).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the natural question is if we can just estimate density generally for the spectral class of an asteroid.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_asteroid_physical_characteristics#Density&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia has to say&lt;/a&gt; on the subject:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, a better guess can be obtained by taking into account the asteroid's spectral type. A recent paper gives calculations for the mean densities of C, S, and M class asteroids as 1.38, 2.71, and 5.32 g/cm3.[9] (Here &quot;C&quot; included Tholen classes C, D, P, T, B, G, and F, while &quot;S&quot; included Tholen classes S, K, Q, V, R, A, and E). Assuming these values (rather than the present ~2 g/cm3) is a better guess.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://iau-comm4.jpl.nasa.gov/KPVY.pdf&quot; rel=&quot;noreferrer&quot;&gt;The reference is from 2002&lt;/a&gt;, and if you follow it, you can get &lt;em&gt;something&lt;/em&gt; that does the job like I'm talking about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SUvuu.png&quot; alt=&quot;density estimate on Tholen spectral type&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even with their revised estimate, I imagine that this is pretty out of date.  Have there been any updated density estimation methods based on spectral information published?  Or (even better) is there a code available somewhere online that will do this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can relatively easily get up-to-date tables of meteorite densities, but this doesn't translate into what I'm looking for, because you would have to a) connect meteorite type to spectral class, and b) it's definitely proven that this method is wrong anyway because of porosity.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-11-23T14:40:11.267" Title="Method to estimate asteroid density based on spectral type" Tags="|asteroid|meteorites|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2883" PostTypeId="2" ParentId="2880" CreationDate="2013-11-23T15:07:11.007" Score="12" Body="&lt;p&gt;If my understanding in correct, the orbit is perfectly circular if the dimensionless orbital parameter of eccentricity (e) is zero.  However, I'm not sure how the inclination works with this, and that comes down to the question &quot;circular in &lt;em&gt;what plane&lt;/em&gt;?&quot;  Anyway, I present to you:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Asteroid 113474 (2002 ST57).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=113474&quot; rel=&quot;noreferrer&quot;&gt;http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=113474&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Element     Value   Uncertainty (1-sigma)    Units &#xA;e   9.338379075815981E-5    1.3966e-07&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/aEScE.gif&quot; alt=&quot;orbit diagram&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sadly, however, the uncertainty is sufficiently lower than the value itself to show that it &lt;em&gt;does&lt;/em&gt;, in fact, have some eccentricity.  If you are satisfied with any object that &lt;em&gt;could&lt;/em&gt; theoretically be circular with our current understanding then that's easy.  There are lots of objects with an uncertainty enough such that a perfectly circular orbit is possible.  Actually, this turns up a huge number of candidates due to the simple fact that we don't know the the orbital parameters in the first place!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I argue that this above orbit is pretty darn circular.  If you want a theoretically perfect circle, then the answer is probably &quot;no&quot;.  If you want further navel-gazing, I will question whether the concept of a circle is valid in non-Euclidean geometry.  Since we have the spacetime distortion of Jupiter thrown in there, perhaps circles don't exist?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;In case it wasn't obvious, my example was possible because I used a large sample set and just searched for a candidate that fit my criteria&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nonetheless, there is a very good reason that things are circular-ish.  Our solar system's plane exists for very good physical reasons, which is that a collapsing gas cloud has an angular momentum value that it can't get rid of, and can't be held in a single body by gravity.  Then, the planets are &lt;em&gt;relatively&lt;/em&gt; circular, because if they were not, the solar system would not be as stable.  Orbital resonances (which is what the long term dynamics depend upon) apply to relatively circular orbits, and like to keep things relatively circular.  There are certain to be some exoplanet systems that more strongly favored circularizing the orbits.  Because of that, I'm sure that one day we'll discover some exquisitely circular orbit.  But that still doesn't make it a literal circle by the theoretical standard.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2013-11-23T17:33:01.677" LastActivityDate="2013-11-23T17:33:01.677" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2884" PostTypeId="2" ParentId="2871" CreationDate="2013-11-23T15:30:34.037" Score="13" Body="&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/GLCfq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;ISRO's MOM lifting off (credit &lt;a href=&quot;http://www.universetoday.com/106127/indias-first-mars-mission-launches-flawlessly-on-historic-journey-to-the-red-planet/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Universe Today&lt;/a&gt;).&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What it comes down to is that this launch went over the ocean - thereby creating a need for man-made buoyant pieces of steel equipped with tracking devices to collect telemetry data. It's ground track looked like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7vuao.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;MOM's ground track (credit &lt;a href=&quot;http://www.spaceflight101.com/pslv-mom-ascent-data.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceFlight101&lt;/a&gt;).&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, much of the track was over water, in places where ISRO didn't have access to tracking stations. &lt;a href=&quot;https://space.stackexchange.com/questions/2875/what-was-chandrayaan-1s-ground-track&quot;&gt;I'm looking for the ground track of Chandrayaan-1&lt;/a&gt;, but so far haven't been able to find it. I'm willing to bet, though, that either:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The ground track was slightly different, perhaps going more directly over Australia, or&lt;/li&gt;&#xA;&lt;li&gt;The launch vehicle was designed in such a way for that mission as to negate the need for continued tracking.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All in all, the extra ships were needed because of the way MOM's ground track was - right over an ocean.&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2013-11-23T16:24:28.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2886" PostTypeId="1" AcceptedAnswerId="2896" CreationDate="2013-11-23T22:47:16.780" Score="5" ViewCount="597" Body="&lt;p&gt;What are some techniques/ways to provide enough oxygen for some manned mission that would last, say, years?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edited&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean some arbitrary mission departing from earth  to infinity and beyond... No particular destination, just in the opposite direction from the sun, and for a very, very long time. The passengers would remain inside the space ship, which would continue to wonder away from the earth indefinitely...&lt;/p&gt;&#xA;" OwnerUserId="1104" LastEditorUserId="1104" LastEditDate="2013-11-24T21:28:12.627" LastActivityDate="2013-11-24T21:33:38.383" Title="Oxygen for manned missions" Tags="|future-missions|oxygen|mission-control|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2887" PostTypeId="1" AcceptedAnswerId="2888" CreationDate="2013-11-24T01:57:48.937" Score="5" ViewCount="455" Body="&lt;p&gt;I am trying to gain some intuition about Delta-V requirements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding is if the Delta-V budget was doubled, the difficulty of the mission is more than double because of the exponential nature of having to carry more fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is - &lt;strong&gt;how are Delta-V requirements influenced by the planet's gravity?&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if Earth's gravity were to double, would Delta-V (to an orbit of similar height) double as well?  Or is this a non-linear effect?&lt;/p&gt;&#xA;" OwnerUserId="1106" LastEditorUserId="288" LastEditDate="2013-11-24T11:43:07.197" LastActivityDate="2013-11-24T11:43:07.197" Title="Are Delta-V requirements for leaving the surface of a planet proportional to gravity?" Tags="|orbit|gravity|delta-v|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2888" PostTypeId="2" ParentId="2887" CreationDate="2013-11-24T03:03:00.583" Score="2" Body="&lt;p&gt;What do you mean by &quot;Earth's gravity&quot;?  Do you mean the mass of the Earth?  Do you mean the acceleration of gravity at the surface of the Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The acceleration of gravity is $g={GM\over r^2}$, where $M$ is the mass, and $r$ is the radius of the surface.  $G$ is Newton's gravitational constant.  The escape velocity from Earth is $v_e=\sqrt{2GM\over r}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer to your question depends on what you mean by &quot;gravity&quot;, and how you plan to increase the &quot;gravity&quot;.  Do you plan to increase the mass of the Earth but not the radius?  Do you plan to increase the mass of the Earth and keep its density constant?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would also need to know where you're going.  $\Delta V$ to where?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, the rocket equation is exponential in $\Delta V$.  For a single stage, ${m_i\over m_f}=e^{\Delta V\over v_e}$, where $m_i$ is the initial mass, $m_f$ is the final mass (the difference being the mass of the propellant expended), and $v_e$ is the exhaust velocity of the engine.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-24T03:31:48.907" LastActivityDate="2013-11-24T03:31:48.907" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2889" PostTypeId="2" ParentId="2886" CreationDate="2013-11-24T03:14:27.350" Score="4" Body="&lt;p&gt;Sure.  Just replicate the ecosystems of the Earth and the power source of the Sun sufficiently to provide a closed cycle.  The Earth and Sun are such a spaceship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can buy such closed systems (Sun not included) with plants, animals and bacteria.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4eOFE.jpg&quot; alt=&quot;ecosphere&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are called &quot;ecospheres&quot;.  They work thusly:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8V9v2.jpg&quot; alt=&quot;eco cycle&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you really want something &quot;indefinite&quot;, biological systems are self-replicating, self-repairing, and will even evolve over time to better meet your needs if you cull the populations that don't meet your needs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for replicating the Sun if you're far from any star, that would require nuclear power in any practical system of long duration.  You could use existing fission systems, or, we can hope, future fusion systems.  And bring enough fuel.  You could in theory collect interstellar hydrogen, but that is not likely to be practical.  Truly &quot;indefinite&quot; is not possible, and is limited by the fuel you can bring or find.  The same is true of our Sun.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-11-24T21:33:38.383" LastActivityDate="2013-11-24T21:33:38.383" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="2890" PostTypeId="1" AcceptedAnswerId="2897" CreationDate="2013-11-24T04:03:17.133" Score="14" ViewCount="1688" Body="&lt;p&gt;All manned spacecraft to date have come back to Earth eventually, and when they do it's through a flaming ball of plasma.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems too probable that this plasma would have a way of breaking things like parachutes, which are kind of important so that you don't go splat upon contact with the water/ground. Has any spacecraft had a means for the crew to escape upon the event of a parachute failure or other catastrophic failure?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2013-11-24T16:07:45.190" LastActivityDate="2024-03-17T16:42:55.890" Title="Has any spacecraft had a way for the crew to escape during reentry?" Tags="|crewed-spaceflight|reentry|safety|failure|procedure|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2891" PostTypeId="2" ParentId="2855" CreationDate="2013-11-24T04:57:57.993" Score="2" Body="&lt;p&gt;Hypothetically, if you could travel sufficiently close to the speed of light, you could not only exit the galaxy, but circumnavigate the observable universe in what would be less than a human lifespan in ship-time, due to time dilation. Since you'd be travelling no faster than light, and the observable universe is billions of light-years across, it would take you billions of years (Earth time) to complete such a voyage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practical terms, accelerating the required mass of ship, payload, and all to a such a large fraction of the speed of light, and then decelerating it at the end of a voyage would be practically impossible by any known means.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2013-11-24T04:57:57.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2892" PostTypeId="2" ParentId="1321" CreationDate="2013-11-24T05:13:40.883" Score="4" Body="&lt;p&gt;The descent trajectory would have a sort of &quot;cone of uncertainty&quot;. The cone starts from some initial point and expands as you progress along the &quot;ideal&quot; path. The path is not vertically downward toward the surface, but at an angle, so the cone of uncertainty will intersect the surface at an angle, describing an ellipse.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2013-11-24T05:13:40.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2893" PostTypeId="1" AcceptedAnswerId="2901" CreationDate="2013-11-24T09:57:12.500" Score="8" ViewCount="694" Body="&lt;p&gt;What I had in mind is that China's territory is rather large and they could place a launch site deep within its territory. Flow of information is tightly controlled as it is with the &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Golden_Shield_Project&quot; rel=&quot;nofollow noreferrer&quot;&gt;Great Firewall of China&lt;/a&gt;&lt;/em&gt; in place, and the communist &lt;em&gt;Big Brother&lt;/em&gt; having eyes and ears at every corner of its territory, leaving West in the cold without direct means of establishing their mission's purpose, if China chose to keep it secret.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least any information that could be gathered via intelligence agencies of the West would likely not leak to the public (or so they would try), to avoid diplomatic repercussions. So my question actually is, how likely would any information about China's secret space missions find its way to the general public? To keep this question from being primarily opinion based, I would like you to provide examples of past missions that were launched under wraps. Who disclosed what information and when (how long after the launch) it became public knowledge? How reliable have these past reports proved to be? Are there any instances of faked reports or additional counterespionage measures to throw the West off course?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could we &lt;strong&gt;infer, via remote sensing&lt;/strong&gt; or other means of tracking the Chinese territory launch site's activity and actual rocket launches, what they're up to? E.g., could we establish what orbit would some Chinese military satellite assume, or its purpose?&lt;br&gt;&#xA;Or, if the purpose of the mission was something else than orbiting a remote sensing satellite, do we have the means to &lt;strong&gt;analyze potential threat&lt;/strong&gt; such missions could pose if China chose to keep it a secret?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="3029" LastEditDate="2019-01-20T23:27:04.563" LastActivityDate="2019-01-20T23:27:04.563" Title="If China wanted to hush the purpose of a space launch, can it be inferred outside of China?" Tags="|spaceport|launch-site|china|remote-sensing|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="2894" PostTypeId="1" AcceptedAnswerId="13901" CreationDate="2013-11-24T10:27:27.820" Score="7" ViewCount="390" Body="&lt;p&gt;Recently, news hit the press about a possible new launch facility, a spaceport on the territory of the United Kingdom. The &lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2507637/Cape-Canaveral--Wales-British-space-industry-reveals-plans-base-aim-putting-satellites-orbit-years.html&quot; rel=&quot;noreferrer&quot;&gt;article in Daily Mail&lt;/a&gt; that I read is however rather vague, and even though it is, in the best of traditions of sensationalist journalism of course, naming it the future &lt;strong&gt;&lt;em&gt;&quot;Cape Canaveral ... in Wales&quot;&lt;/em&gt;&lt;/strong&gt;, I actually read it more as a possibility of building &lt;em&gt;a rather long runway&lt;/em&gt;, than also running a vertical launch platform. Here's a few bullet points out of the mentioned news article:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Locations are also being considered in the West Country and Scotland&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Would be the launching base for space tourism programmes&lt;/strong&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Plans enthusiastically backed by Science Minister David Willetts&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Could become the home of Richard Branson's Virgin Galactic programme&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/FLa5P.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Virgin Galactic &lt;a href=&quot;https://en.wikipedia.org/wiki/VSS_Enterprise&quot; rel=&quot;noreferrer&quot;&gt;SpaceShipTwo&lt;/a&gt; carried by &lt;a href=&quot;https://en.wikipedia.org/wiki/VMS_Eve&quot; rel=&quot;noreferrer&quot;&gt;WhiteKnightTwo&lt;/a&gt; on landing approach at &lt;a href=&quot;https://en.wikipedia.org/wiki/Mojave_Air_and_Space_Port&quot; rel=&quot;noreferrer&quot;&gt;Mojave Air and Space Port&lt;/a&gt;, CA (Source: &lt;a href=&quot;http://www.spacesafetymagazine.com/2012/10/30/federal-aviation-administration-grants-burgeoning-spaceports/&quot; rel=&quot;noreferrer&quot;&gt;SSM&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reading a report by Dan Lewis, Chief Executive of the Economic Policy Centre, Chief Executive of Future Energy Strategies and Energy Policy Adviser to the IoD on &lt;strong&gt;&lt;a href=&quot;http://www.iod.com/mainwebsite/resources/document/space-britains-new-infrastructure-frontier-may12.pdf&quot; rel=&quot;noreferrer&quot;&gt;Space: Britain’s New Infrastructure Frontier&lt;/a&gt;&lt;/strong&gt; (PDF), it merely confirms my initial suspicions that this is not so much about science and exploration, than it is primarily to support space tourism and Virgin Galactic in particular. Most relevant excerpts from the document's executive summary read:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;A spaceport would be a key piece of infrastructure for the UK’s space sector,  operating as a hub for space tourism, research and&#xA;  development. Space tourists  are willing to pay \$200,000 for a mere&#xA;  three hours in space, and will have  considerable disposable income&#xA;  that would help the wider local economy. The  private sector could&#xA;  help fund the costs of a spaceport.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;A spaceport would have several requirements, including a long runway and its  own undisturbed high altitude air corridor, which&#xA;  narrow down the location  options. Lengthening the runway of an RAF&#xA;  base in Scotland or Northern  Ireland would be a possibility, while&#xA;  the South West of England could represent  an alternative prospect.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This report is however from May 2012, so the news article is a good year and a half more recent. My question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Are there any confirmed plans or negotiations between &lt;a href=&quot;http://www.bis.gov.uk/ukspaceagency&quot; rel=&quot;noreferrer&quot;&gt;UK Space Agency&lt;/a&gt; and &lt;a href=&quot;https://www.gov.uk/government/ministers/minister-of-state-universities-and-science-department-for-business-innovation-and-skills&quot; rel=&quot;noreferrer&quot;&gt;UK's Minister of State for Universities and Science&lt;/a&gt;, The Rt. Hon. David Willetts M.P., to build a future vertical launch facility, or is this all about building a long runway suitable for Virgin Galactic's space tourism programme? Would building a launch facility for satellite launch vehicles in UK even make any economic sense, considering its geographic location and population density?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-24T13:59:08.197" LastActivityDate="2016-02-04T23:19:06.433" Title="Any chance of seeing a vertical launch of an orbital launch vehicle in UK in the not-too-distant future?" Tags="|developing-space-programs|spaceport|facility|uk-space|launch-site|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2895" PostTypeId="1" AcceptedAnswerId="2934" CreationDate="2013-11-24T11:16:48.073" Score="4" ViewCount="332" Body="&lt;p&gt;It seems that Russians and Roscosmos were given a green light, a go ahead to build the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostochny_Cosmodrome&quot; rel=&quot;nofollow&quot;&gt;Vostochny Cosmodrome&lt;/a&gt;, or Eastern Spaceport by 2018. They planned on spending \$1 billion on its construction in 2013 alone, and the project is estimated at a total of \$10 billion. At least according to &lt;a href=&quot;http://www.huffingtonpost.com/2012/04/12/russia-vostochny-cosmodrome_n_1421276.html&quot; rel=&quot;nofollow&quot;&gt;this article in Huffington Post&lt;/a&gt;, while &lt;a href=&quot;http://en.ria.ru/russia/20091015/156475196.html&quot; rel=&quot;nofollow&quot;&gt;some older articles&lt;/a&gt; put it at est. \$13.5 billion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the whole project seems to have a lot of opposition, well, questioning its usefulness at least, and even &lt;a href=&quot;http://www.russianspaceweb.com/zak.html&quot; rel=&quot;nofollow&quot;&gt;Anatoly Zak&lt;/a&gt; has an article on his webpage titled &lt;a href=&quot;http://www.russianspaceweb.com/vostochny_soyuz.html&quot; rel=&quot;nofollow&quot;&gt;The real rocket to nowhere: The strange case of Soyuz in Vostochny&lt;/a&gt;. To sum up his arguments:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Vostochny was designed to free Russia from dependency upon the Baikonur Cosmodrome, which is located in a foreign country, and to provide an anchor in the sparsely populated but strategically important Far East;&lt;/li&gt;&#xA;&lt;li&gt;The new Rus-M rocket was to launch commercial and the larger, next generation human spacecraft from the new spaceport;&lt;/li&gt;&#xA;&lt;li&gt;Roscosmos head Vladimir Popovkin canceled Rus-M after taking over the agency last year because it was redundant to the Angara rocket, which was finally nearing its first flights after nearly two decades in development;&lt;/li&gt;&#xA;&lt;li&gt;Popovkin decided to bring the Soyuz-2 rocket to Vostochny in order to meet a political deadline of flying something by 2015;&lt;/li&gt;&#xA;&lt;li&gt;This plan makes no sense for commercial launches because the Soyuz rocket has less lifting capacity from Vostochny than from the brand new launch complex at Kourou in French Guiana;&lt;/li&gt;&#xA;&lt;li&gt;The Soyuz spacecraft lacks the ability to make high-precision emergency landings in the rugged, heavily-wooded terrain of the Far East, making Vostochny useless for human missions at present;&lt;/li&gt;&#xA;&lt;li&gt;The 2015 deadline is unrealistic, with the first launch from Vostochny unlikely to occur until 2018 at the earliest due to construction delays;&lt;/li&gt;&#xA;&lt;li&gt;The Russian military has no interest in the civilian spaceport, preferring to launch Angara rockets from its newly constructed launch complex at Plesetsk;&lt;/li&gt;&#xA;&lt;li&gt;Just about every other government ministry — including the aviation agency, Rosaviatsiya — have tried to avoid any involvement in Roscosmos’ new spaceport.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is a lot to process, so I won't be asking about all these points. The sentiment of the story is rather clear though, and most of Anatoly's opinions seem well substantiated that the proposed spaceport, now already in construction, might not be as useful as some of the Russian politicians and Roscosmos are hoping it will be (even though there's &lt;a href=&quot;http://voiceofrussia.com/search/?filter-tp=&amp;amp;text=Vostochny%20Cosmodrome&amp;amp;sort=date&quot; rel=&quot;nofollow&quot;&gt;many news articles&lt;/a&gt; writing that Russians will remain open to European space programmes, and possibly others too with their new spaceport).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Anatoly mostly discusses the new spaceport from the perspective of what was known for the proposed 2015 start of operations. This date has by now changed to 2018, and plans for the spaceport's purpose have likely changed substantially by now, too. My question is, for the time being (I might have follow-up questions later on): &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What launch vehicle families is Roscosmos hoping to operate from the future Eastern Spaceport starting from 2018? Are there any LV families in development that would find their new home at Vostochny Cosmodrome, perhaps the &lt;a href=&quot;http://www.spacedaily.com/reports/Russia_starts_ambitious_super_heavy_space_rocket_project_999.html&quot; rel=&quot;nofollow&quot;&gt;rumoured revival of the Energia super-heavy booster rocket&lt;/a&gt;?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-11-24T13:27:34.543" LastActivityDate="2013-11-26T18:39:32.640" Title="Which LV families is Roscosmos hoping to operate from the future Eastern Spaceport?" Tags="|future-missions|russia|spaceport|roscosmos|launch-site|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2896" PostTypeId="2" ParentId="2886" CreationDate="2013-11-24T11:28:08.493" Score="6" Body="&lt;p&gt;Providing sufficient oxygen for a long journey is not the challenge.  If you have enough to meet the demands for a week, you have enough to meet the demands forever. Oxygen is not destroyed or created in &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecosystem_respiration&quot; rel=&quot;nofollow noreferrer&quot;&gt;respiration&lt;/a&gt; Animals (people) take in &lt;a href=&quot;https://en.wikipedia.org/wiki/Oxygen&quot; rel=&quot;nofollow noreferrer&quot;&gt;oxygen&lt;/a&gt; where there are two atoms joined together and exhale &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbon_dioxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;Carbon Dioxide&lt;/a&gt; where there are is a carbon between the two oxygen atoms.  On earth plants take in Carbon Dioxide keep the carbon, and release oxygen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The challenge is two fold.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Having enough oxygen to replace any that escapes from your vehicle &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Having enough energy to power the recyling system, which can be mechanical or biological.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;See related questions&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2218/oxygen-self-supply-by-plants&quot;&gt;Oxygen self-supply by plants&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/924/what-would-have-happened-if-the-skin-of-an-apollo-lunar-module-was-ruptured&quot;&gt;What would have happened if the skin of an Apollo lunar module was ruptured?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/449/would-a-biological-resource-system-be-feasible-for-long-term-manned-flights&quot;&gt;Would a biological resource-system be feasible for long-term manned flights?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/14/is-converting-moon-dust-into-oxygen-a-viable-option-for-sustainability&quot;&gt;Is converting moon dust into oxygen a viable option for sustainability?&lt;/a&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-11-24T11:28:08.493" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2897" PostTypeId="2" ParentId="2890" CreationDate="2013-11-24T14:37:50.013" Score="21" Body="&lt;p&gt;In &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/001523.html&quot; rel=&quot;noreferrer&quot;&gt;some Mercury flights&lt;/a&gt;, the astronaut had a personal parachute with them. The &lt;a href=&quot;http://www.ejectionsite.com/gemini.htm&quot; rel=&quot;noreferrer&quot;&gt;Gemini program&lt;/a&gt; used ejection seats, which could be used during launch and reentry. Later on, &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/001523.html&quot; rel=&quot;noreferrer&quot;&gt;they realized&lt;/a&gt; that igniting a rocket-propelled ejection seat wasn't a good idea in a pure-oxygen atmosphere.&lt;br&gt;&#xA;The &lt;a href=&quot;http://www.ejectionsite.com/vostok.htm&quot; rel=&quot;noreferrer&quot;&gt;Vostok capsule&lt;/a&gt; also had an ejection seat, but not as a backup: the designers hadn't yet developed a soft-landing system they were confident in. In both cases, the ejection seats were deleted in the successors (Apollo, Soyuz).&lt;br&gt;&#xA;During the reentry phase where plasma occurs, the parachutes are safely stowed inside the spacecraft. If the plasma can get at them, it has penetrated the hull and the crew are in imminent danger as well. They can't eject at this stage: if they eject, the heat shield no longer protects them so they're exposed to the same plasma they were trying to escape from.&lt;br&gt;&#xA;The backup system for landing generally consists of having 3 parachutes, any 2 of which can safely land the capsule. See &lt;a href=&quot;https://space.stackexchange.com/questions/2521/did-any-apollo-missions-have-backup-parachutes&quot;&gt;this earlier question&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-11-24T14:37:50.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2898" PostTypeId="1" CreationDate="2013-11-24T17:35:49.697" Score="20" ViewCount="1296" Body="&lt;p&gt;Mention of &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_escape_system&quot;&gt;Launch Escape System&lt;/a&gt; (LES) brings to mind images of the crew on board a spacecraft ejecting at launch; the crew being the most valuable &lt;em&gt;cargo&lt;/em&gt; on board.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch vehicles may also be unmanned, e.g. PSLV/GSLV, Ariane, etc. In this case, the most valuable cargo on board is probably the satellite(s) it carries to orbit. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Was a case ever made to preserve the pay-load on board in the event of a failed launch by means analogous to LES?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Why don't unmanned launch vehicles at present include LES for their pay-load?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-11-25T08:34:11.010" LastActivityDate="2021-12-26T17:11:57.270" Title="Why don't unmanned launch vehicles include launch escape system for payload?" Tags="|failure|launch-escape-system|range-safety|uncrewed-spaceflight|" AnswerCount="6" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2899" PostTypeId="2" ParentId="2898" CreationDate="2013-11-24T20:48:46.640" Score="18" Body="&lt;p&gt;Proposed Russian spacecraft, for Mars, Lunar, or orbital tug missions that would have used a nuclear reactor, did provide escape tower like functionality for the reactor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the issue is that the entire payload stack, is usually too big to save with an escape tower (and still have any payload left to go to orbit, due to weight of escape system).  In the manned case, usually everything BUT the minimum to save the people is discarded.  Similarly in the reactor case, it would save the reactor, but not the rest of the payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way of looking at, is Launch Escape systems are typically used, when Flight Termination Systems are not really good options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you terminate a flight with people on it, that really would suck.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you terminate a flight with a nuclear reactor on it, as it flies over land, that would really suck as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In those case, it can be worth having an escape system, to avoid those consequences.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-11-25T16:59:41.387" LastActivityDate="2013-11-25T16:59:41.387" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2900" PostTypeId="2" ParentId="2898" CreationDate="2013-11-25T03:55:59.603" Score="9" Body="&lt;p&gt;Weight is a major factor... every item on the vehicle has to have a good reason to be there. Rather than trying to save an expensive payload in the event of a launch failure, it may be simpler/cheaper to maximize the reliability of the launch system, and accept the cost of losing the entire vehicle and payload in the (presumably rare) event of failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even in the manned case, there is still the possibility of a failure for which no escape is viable. It becomes a calculated risk... minimizing but never really completely eliminating the possibility of losing crew/passengers to some mode of catastrophic failure.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2013-11-25T03:55:59.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2901" PostTypeId="2" ParentId="2893" CreationDate="2013-11-25T04:39:19.900" Score="15" Body="&lt;p&gt;Yes, there are methods to track and estimate what a payload is up to. Most methods rely on visual or radar to identify the orbit and its parameters. There are many amateurs that do this. Once an object has been located and its orbit calculated, you can then monitor changes in the orbit. Generally these things are public.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imaging the object is generally the next step. You can see many telescopic pictures of the ISS online. This is a bit harder for the average amateur but there are a few that can do this specifically. Sometimes the images are inconclusive, but generally a guesstimate is possible. Measuring the electronic emissions from the object can also tell you what it might be doing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final piece of the puzzle is the rocket that launched it. This will give you a size and mass estimate (or rather an upper and lower bounds). This is generally not something that will be made public except for the launching nation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good example of this is the X-37B. The USAF provide some launch info, but most is left to the amateur astronomy community to discover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tools we can probably assume the big boys have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HD Radar and images of the Sat. With the right telescopes, you can get good images of the sat. Might not tell you everything, but something that looks like a space telescope, probably is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/United_States_Space_Surveillance_Network#Ground-based_Electro-Optical_Deep_Space_Surveillance&quot;&gt;http://en.wikipedia.org/wiki/United_States_Space_Surveillance_Network#Ground-based_Electro-Optical_Deep_Space_Surveillance&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Air_Force_Maui_Optical_and_Supercomputing_observatory&quot;&gt;http://en.wikipedia.org/wiki/Air_Force_Maui_Optical_and_Supercomputing_observatory&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Passive Electronic Sensors. Basic things like a spectrum analyser and a directional antenna can give you its transmissions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't believe there are any public releases of examples of this with regards to Chinese Sats. The best is probably the sat they destroyed a number of years ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically like all intelligence, guessing is sometimes the best we have.&lt;/p&gt;&#xA;" OwnerUserId="1076" LastEditorUserId="1076" LastEditDate="2013-11-26T08:52:55.593" LastActivityDate="2013-11-26T08:52:55.593" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2902" PostTypeId="2" ParentId="2898" CreationDate="2013-11-25T09:55:50.870" Score="9" Body="&lt;p&gt;Besides reasons mentioned by others - humans are more durable against &quot;impact&quot; type stress than usual orbital payloads are. The satellites are calculated to withstand the sustained several &lt;em&gt;g&lt;/em&gt; of rocket acceleration, but not a single ~80g shock of hitting the ground. That means their emergency system would have to be far more advanced - provide much more gentle fall - than the one meant to save humans, or the satellites themselves would need to be engineered with impact shock durability in mind, adding to their weight and complexity in a way that is quite unlikely to ever be of any use. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2013-11-25T09:55:50.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2903" PostTypeId="1" AcceptedAnswerId="2904" CreationDate="2013-11-25T10:08:47.620" Score="12" ViewCount="2354" Body="&lt;p&gt;How much greenhouse gases are released per kg launched into space, what other energy is needed? For the scope of this question, let's ignore the embedded energy in all the machinery and just look at the rocket fuel (meaning both oxidizer and reducer) - then we'd have three things to consider. For the first we should be able to find a clear figure for released $CO_2$:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The burning of the fuel itself&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For the other two points along the production chain we'd need figures for the energy demand, as the actual carbon footprint strongly depends on the actual production method.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the production of the fuel from primary resources&lt;/li&gt;&#xA;&lt;li&gt;(possibly) compression and cooling of gasous fuels&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="102" LastEditorUserId="102" LastEditDate="2013-11-25T12:25:27.493" LastActivityDate="2013-11-25T14:39:16.390" Title="What is a typical energy demand and carbon footprint of a space launch?" Tags="|launch|fuel|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2904" PostTypeId="2" ParentId="2903" CreationDate="2013-11-25T12:08:29.347" Score="11" Body="&lt;p&gt;Most modern rockets run on liquid oxygen and liquid hydrogen which reacts to water vapor. There is no carbon in this reaction, so the carbon-monoxide and carbon-dioxide output of the reaction is zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding the CO&lt;sub&gt;2&lt;/sub&gt; footprint of the production of liquid oxygen and liquid hydrogen: These are made by splitting water into oxygen and hydrogen through electrolysis and then compressing it. There are different methods to do this, but all of them need more or less of electric energy. The CO&lt;sub&gt;2&lt;/sub&gt; footprint depends vastly on how this energy is created. When you use energy from regenerative resources or nuclear energy, it would be possible to have a completely CO2-neutral rocket launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some other ways to split water into its constituent components (hydrogen as fuel and oxygen as oxidizer) are &lt;a href=&quot;http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/production-solar.htm&quot;&gt;direct solar water splitting&lt;/a&gt; process in which the solar energy is directly used to produce hydrogen from water without going through the intermediate electrolysis step:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Photoelectrochemical water splitting&lt;/strong&gt; – this technique uses semiconducting electrodes in a photoelectrochemical cell to convert light energy into chemical energy of hydrogen. There are essentially two types of photoelectrochemical systems – one using semiconductors or dyes and another using dissolved metal complexes.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Photobiological&lt;/strong&gt; – these involve the generation of hydrogen from biological systems using sunlight. Certain algae and bacteria can produce hydrogen under suitable conditions. Pigments in algae absorb solar energy, and enzymes in the cell act as catalysts to split water into its hydrogen and oxygen constituents.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;High temperature thermochemical cycles&lt;/strong&gt; – these cycles utilize solar heat to produce hydrogen by water splitting using thermochemical steps.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Biomass gasification&lt;/strong&gt; – this uses heat to convert biomass into a synthetic gas rich In hydrogen.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;None of these require additional electric energy and their carbon emissions are zero. This does not include any electricity used for compression and cooling of cryogenic &lt;a href=&quot;http://www.astronautix.com/props/loxlh2.htm&quot;&gt;Lox/LH2&lt;/a&gt; propellants. In case of using electrolysis though, by using the lower heating value of hydrogen, the electrical &lt;a href=&quot;http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/production-solar.htm&quot;&gt;energy needed to generate one kg of hydrogen is 51 kWh&lt;/a&gt;, assuming electrolyzer efficiency of 65%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are still some older designs in use where the first or even all stages run on liquid oxygen and kerosene, like the Russian Soyuz which supply the ISS. Kerosene has a CO&lt;sub&gt;2&lt;/sub&gt; footprint of &lt;a href=&quot;http://micpohling.wordpress.com/2007/05/08/math-how-much-co2-released-by-aeroplane/&quot;&gt;about 2.5 kg per liter&lt;/a&gt;. A Soyuz-2 burns &lt;a href=&quot;http://www.spaceflight101.com/soyuz-2-1b.html&quot;&gt;about 82 tons of kerosene per launch&lt;/a&gt;. With &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot;&gt;RP-1 fuel density at 0.81–1.02 g/ml&lt;/a&gt;, this comes out roughly between 67-84 tons of CO&lt;sub&gt;2&lt;/sub&gt; per Soyuz 2-1B (using 4 boosters) launch.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="49" LastEditDate="2013-11-25T14:39:16.390" LastActivityDate="2013-11-25T14:39:16.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2905" PostTypeId="2" ParentId="2898" CreationDate="2013-11-25T12:14:12.407" Score="9" Body="&lt;p&gt;It's simple economy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unmanned payloads can and usually are insured against loss due to launch complications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The insurance costs less than the additional cost to install a launch escape system.&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2013-11-25T12:14:12.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2906" PostTypeId="2" ParentId="2898" CreationDate="2013-11-25T12:43:24.673" Score="10" Body="&lt;p&gt;@Philipp has nailed it in &lt;a href=&quot;https://space.stackexchange.com/a/2905/49&quot;&gt;his answer&lt;/a&gt; in my opinion, and we've, in a sense, discussed the &lt;a href=&quot;http://chat.stackexchange.com/transcript/message/12328249#12328249&quot;&gt;economic viability of such payload escape systems versus insurance cost&lt;/a&gt; before in our main chat room, too. Technical and economic feasibility studies of such payload escape systems have however been done before, one such good example is Fred E. Wagner's thesis on &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a189494.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;EXPECTED VALUE ANALYSIS FOR AN UNMANNED EXPENDABLE LAUNCH VEHICLE PAYLOAD ESCAPE SYSTEM&lt;/a&gt; (PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Truth of the matter is, that if the cost to launch the additional mass of such a payload escape system is at least near or even more than the insurance for your entire payload (might be many satellites, mind you), it's simply not worth it. And that's not even accounting for the cost to design, manufacture, and test such a system. So it was never really attempted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA runs a Payload Safety Working Group (PSWG) via its &lt;a href=&quot;http://www.hq.nasa.gov/office/codeq/codeqs1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Office of Safety and Mission Assurance&lt;/a&gt;, that is mostly involved with preparing procedures to assure safety of the payload and its handling crew during transport, payload integration, and so on, and publishes them in &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/npg_img/N_PR_8715_0007_/N_PR_8715_0007_.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expendable Launch Vehicle Payload Safety Program&lt;/a&gt; documentation (PDF). Here's a cool chart of the Safety Review Process (Source: NASA's Powerpoint presentation on &lt;a href=&quot;http://kscsma.ksc.nasa.gov/ELVPayloadSafety/documents/30thpresentation_04_13_10.ppt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expendable Launch Vehicle Payload Safety Program&lt;/a&gt;, prepared by Cal Staubus):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/gGMg0.png&quot; alt=&quot;Safety Review Process&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to my knowledge, no launch system offers to its customers a payload escape system that would be mounted to the payload fairing and land all of the payload safely in case of in-flight launch failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This of course doesn't exclude that some individual payloads might be designed for later retrieval at the end of the mission they were designed for, or could activate some custom launch escape and landing system in case of failed launch. I'm however not aware of any that would be designed for that, or if it is even technically feasible to jettison the payload out of the payload fairing during failed launch, even if you connect it to the range safety system and would receive jettison command prior to activation of the launcher's self-destruct mechanism. In such scenarios, there simply isn't any time to waste. You're trying to save human lives first, then worry about some fancy gadget that wanted to go to space.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-11-25T12:52:41.770" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2907" PostTypeId="1" AcceptedAnswerId="2909" CreationDate="2013-11-25T15:25:18.287" Score="18" ViewCount="11633" Body="&lt;p&gt;I'm curious as to what launchpad bases are made of - it seems to always be made of concrete, but wouldn't the concrete disintegrate rather quickly with jets of fire pounding on it?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2019-04-11T22:56:24.150" Title="What is the base of a launchpad made of?" Tags="|launch|materials|launchpad|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2908" PostTypeId="2" ParentId="2907" CreationDate="2013-11-25T16:07:12.057" Score="6" Body="&lt;p&gt;There are different concrete formulations that allow for better heat resistance and whatnot. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same as there are better steel alloys to resist heat as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, modern launchpads, in the US at least, have a water deluge system that dumps amazingly large amounts of water onto the pad, starting a few seconds before launch and through the launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This serves many purposes, but there are two main ones are to protect the pad from damage.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The heat must vaporize the water, before doing serious damage to the pad, thus reducing the effective heat load the pad must take.  (100 degree C water is not very damaging.  And when converted to steam, it dissipates/rises quickly, taking the energy with it).&lt;/li&gt;&#xA;&lt;li&gt;Protects from sonic damage, as again, the water absorbs the energy, vaporizes, and takes the energy away with it, or at least absorbs some of it.  (It is ok to be mean to water it would appear).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Russian pads (Soyuz, Proton, etc) seem to be built into hillsides, where the pad juts out into a valley, and thus there is no sort of bottom to the exhaust trench nearby, rather it is quite a distance downwards, probably buying some dispersion of the thermal load.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where this becomes a bigger issue is for reusable launchers, where they wish to land and take off from a concrete pad.  XCOR, Armadillo, and no doubt SpaceX (with Grasshopper) have run into erosion issues.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With XCOR/Armadillo, the overall thrust, and heat load is pretty low, and they usually found a disposable steel plate would suffice to protect the pad for long enough, and cheap enough to replace that it was not worth much more work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One imagines Grasshopper, with a full 147,000lb thrust Merlin 1D must have some kind of stronger concrete in use, since they do NOT have a deluge system in McGregor (&lt;a href=&quot;https://maps.google.com/maps?hl=en&amp;amp;ll=31.388197,-97.468525&amp;amp;spn=0.002033,0.002484&amp;amp;t=k&amp;amp;z=19&quot; rel=&quot;noreferrer&quot;&gt;look on Google Maps, for SpaceX in McGregor, TX&lt;/a&gt;, it is cool, Grasshopper is sitting on the pad, you can see, north and west of it, the BFT test stand, and where they are building the sideways test pad for the F-Heavy.  If you look to the east and north, there is a dumping area, where some first stage segments, and a fairing piece seem to be sitting out). &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2013-11-25T16:20:35.070" LastActivityDate="2013-11-25T16:20:35.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2909" PostTypeId="2" ParentId="2907" CreationDate="2013-11-25T16:08:21.407" Score="10" Body="&lt;p&gt;The majority of the heat of the exhaust plume can be redirected away from the actual launchpad where the rocket is sat, be it by building exhaust plume tunnels or trenches beneath the launchpad, using jets of water that also double as &lt;a href=&quot;https://space.stackexchange.com/a/1552/49&quot;&gt;acoustic shock suppressors&lt;/a&gt; and flame deflectors, by using concrete flame deflectors, or as is the case with some launches (e.g. Soyuz), the rocket itself is held suspended in air with launch arms and the exhaust plume escapes in the quarry beneath it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/S5etF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Large gantry mechanisms on either side of the Soyuz spacecraft are raised into position to secure the rocket (Source: &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Soyuz_TMA-05M_rocket_at_the_launch_pad_at_the_Baikonur_Cosmodrome.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/q/1162/49&quot;&gt;some rocket engine test facilities&lt;/a&gt;, they're also suspended or sat over an opening of a higher floor of the test building, with concrete walls surrounding it, and with sea launches, the rocket is usually sat over a steel reinforced concrete mesh that lets most of the exhaust plume into the sea beneath the launch platform.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/VUpv4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Zenit-3SL carrying Atlantic Bird 7 lifts off from Odyssey platform (Source: &lt;a href=&quot;http://exploringbeyondfinalfrontiers.wordpress.com/2011/09/25/sea-launch-comes-back-to-life-with-successfull-launch-of-atlantic-bird-7/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Way Up!&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess what I'm saying is that your solution might be even unique, but you'll likely be looking for ways to redirect most of the heat away from the launch platform, whatever it may look like. This then lets you use more conventional building materials, such as reinforced concrete floor slabs coated with fire suppressants, or any other commonly used materials for some structures like tunnels and alike.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be also interested in checking this &lt;a href=&quot;https://www.google.com/streetview/gallery/#nasa/bottom-of-launch-pad-39a&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google Street View of the bottom of the launch pad 39A&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="30102" LastEditDate="2019-04-11T21:19:14.977" LastActivityDate="2019-04-11T21:19:14.977" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2910" PostTypeId="1" CreationDate="2013-11-25T16:14:48.653" Score="9" ViewCount="985" Body="&lt;p&gt;With the launch of the SpaceX Falcon 9 V1.1 Nov 25th (Assuming it succeeds), it will apparently be the first US Commercial launch (I assume to GEO, since Orbital, with all the commercial small sats they have been launching, would otherwise count) in four years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically the entire market went to ILS with Proton, and ArianeSpace with Ariane V. (Some to China, if you could be ITAR free)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happened to the US commercial launch market?  I know that the Atlas V and Delta IV are 'expensive' but Ariane V is not particularly cheap, nor is Proton. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gwynne Shotwell of SpaceX had noted that in 2012, every commercial satellite launch competition (that they competed for) had been won by SpaceX.  So SpaceX will very clearly aim to change this situation. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-07-09T21:09:25.267" LastActivityDate="2015-07-09T21:09:25.267" Title="Why did the US lose almost all commercial launch opportunities?" Tags="|launch|spacex|falcon-9|ariane|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2911" PostTypeId="1" AcceptedAnswerId="9449" CreationDate="2013-11-25T17:09:24.667" Score="11" ViewCount="1030" Body="&lt;p&gt;Previous manned spaceflight vehicles, that had a Launch Escape System (Excluding you space shuttle!) usually seemed to favor the notion of a tractor (Puller) style LES.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, the manned module is at the top of the stack, there is a motor (often in a tower) that contains solid or liquid engines for an abort, to drag the manned module away from the stack in times of woe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the newcomers to the manned space business (SpaceX, SNC, Blue Origin) and the old school approaches (Boeing and Lockheed Martin) we start to see different approaches, where SpaceX (DragonRider), Boeing (CST-100), SNC (Dream Chaser), and Blue Origin (SV?  Darned if anyone knows what they plan to call it.  New Sheppard is the sub orbital vehicle name, and Space Vehicle, SV is the current name I last saw for their orbital version) are working towards the idea of pusher escape systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA with Mercury, Apollo, and now Orion seem to favor tractor LES.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New guys are seeming to favor pusher systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soviets have used tractor systems for Soyuz, and while some of the fun proposals in Anatoly Zak's book suggested pusher systems, none of those have really seen the light of day (Looking at you Klipper, NPK-TP, etc). What is neat is that even though the manned module is the middle of three in Soyuz, they still went with the tractor system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chinese with Shenzou seem to have gone with tractor systems as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there examples of actually flown hardware, using pusher escape systems, or will the new guys be the first, assuming any of them actually get to launching a manned system?  (I hope they all succeed, but who knows!  If you are reading this in 2020 and in hindsight laughing at my naivete, please enjoy). &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6312" LastEditDate="2014-11-12T15:28:28.803" LastActivityDate="2019-02-27T22:35:25.640" Title="Are there any Pusher Launch Escape Systems in actual use?" Tags="|launch-escape-system|tractor-les|pusher-les|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="2912" PostTypeId="1" AcceptedAnswerId="4223" CreationDate="2013-11-25T20:37:20.700" Score="4" ViewCount="1049" Body="&lt;p&gt;The follow is from the &lt;a href=&quot;http://books.google.com/books/about/Mining_the_Sky.html?id=k9hwi3ktye8C&quot; rel=&quot;noreferrer&quot;&gt;book summary&lt;/a&gt; of &lt;em&gt;Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;hollow out asteroids to transform them into livable habitats for billions of space-bound homesteaders&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Since the book is not publicly searchable, I can not easily find any more specificity on this idea.  As a reference request, I would just like to ask what the general concept prescribed by this book is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By that I mean, is the author of this book proposing to use asteroids as radiation shielding, a construction material source, as the vessel for a pressurized rotating habitat, or something else?  All of these concepts have been proposed in other contexts, and there's quite a lot of difference between them.  I would like to what is proposed in that book.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-04-09T01:16:58.073" Title="What is the hollow asteroid concept in &quot;Mining the Sky&quot;" Tags="|colonization|asteroid|habitat|reference-request|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2913" PostTypeId="2" ParentId="2910" CreationDate="2013-11-25T21:30:05.960" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www.spaceflightnow.com/falcon9/007/131124commercial/#.UpPAyeLCf2M&quot;&gt;http://www.spaceflightnow.com/falcon9/007/131124commercial/#.UpPAyeLCf2M&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is one of the better answers&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, the Atlas V and the Delta IV are significantly more expensive than an Ariane V or Proton. Also the former were designed mainly for the USAF and their payloads and they have structured their business that way. &lt;/p&gt;&#xA;" OwnerUserId="1076" LastActivityDate="2013-11-25T21:30:05.960" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2914" PostTypeId="1" AcceptedAnswerId="2988" CreationDate="2013-11-25T22:19:04.433" Score="13" ViewCount="12282" Body="&lt;p&gt;Specifically for the SpaceX SES-8 mission going on right now. But, generally, how long does it take for a satellite to reach GEO at a minimum?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2013-11-26T18:11:39.997" LastActivityDate="2013-12-05T04:54:34.260" Title="How long does it take for a satellite to reach GEO?" Tags="|artificial-satellite|orbit|geostationary|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2915" PostTypeId="2" ParentId="2912" CreationDate="2013-11-25T22:34:29.240" Score="1" Body="&lt;p&gt;A combination of radiation shielding and the pressure vessel, methinks. I'm not familiar with that specific book, but you're right; the concept's not exactly new. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with long-term habitation of space is that we're the product of billions of years of evolution in a very specific environment, which so far we've been unable to find anywhere else in the universe. We've found some possibilities, but so far none have been definitively confirmed to possess the key ingredients; a mass no more than about double that of Earth, an oxygen-containing atmosphere, and liquid water (which in turn requires the planet to orbit in the &quot;Goldilocks zone&quot; of its star). Any good possibilities would be several hundred years' journey using current propulsion technologies; most orbital masses in our own neighborhood are either too light, too heavy, too hot, too cold, or didn't have the right mix of stuff. So, instead of trying to terraform nearby space rocks to accumulate an atmosphere (a pointless exercise), or mining our own finite resources on Earth to create habitats, we dig into a rock and make habitable spaces inside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea would basically be to mine the asteroid from the inside out, creating first a network of tunnels and then larger caverns within the rock structure. The entry point into the asteroid can be sealed with an airlock relatively early in this process, and the hollow space filled with an atmospheric mix. The only problem would be finding asteroids with a dense, nonporous composition that won't bleed out the atmosphere we try to introduce over time, located in an area of space that's easy to get to and where collisions are unlikely (the asteroid belt isn't a terrible idea as it's had some 4 billion years to settle down, but it can still be dangerous), and then find some way to get the necessary equipment and atmospheric gases out to it. Nearby asteroids might be found with interior reserves of water ice, or we might harvest comets or even the outer layers of the &quot;ice giants&quot; for water and inert gases. Electrolyze the water to produce oxygen, add sufficient amounts of a relatively inert gas like nitrogen or argon, and add some lights powered by solar panels on the 'roid's surface, and you have a rudimentary space habitat. Build traditional enclosures, or just carve what you like directly out of the rock. As you dig, anything valuable as a building material or for export can be sent back to the Mother Planet. By the time the digging's done, you'll have large cavernous spaces suitable for zero-G heavy industry using materials from other minable asteroids, and hopefully a &quot;freight elevator&quot; airlock system that was originally used to get mining equipment in and rock out, and can now be used to get raw materials in and finished goods out. The asteroid becomes a city in space, developed much the way the American West grew up.&lt;/p&gt;&#xA;" OwnerUserId="411" LastActivityDate="2013-11-25T22:34:29.240" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2916" PostTypeId="1" AcceptedAnswerId="2918" CreationDate="2013-11-25T22:47:54.353" Score="8" ViewCount="1017" Body="&lt;p&gt;There have been many candidates stated as potential habitats for life outside of Earth, but what are considered to be the most likely to harbor life?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorDisplayName="user12" LastEditDate="2013-12-13T00:18:30.510" LastActivityDate="2020-04-21T13:08:04.040" Title="What other bodies in the solar system are most likely to contain forms of life?" Tags="|life|solar-system|astrobiology|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2917" PostTypeId="1" AcceptedAnswerId="2919" CreationDate="2013-11-25T23:13:13.520" Score="6" ViewCount="458" Body="&lt;p&gt;Why is the apogee of the Falcon 9 SES launch going clear out to 80,000 km? GEO is only 36,000 km or so, it seems like a waste to go so far beyond it.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="51" LastEditDate="2015-08-07T23:15:38.443" LastActivityDate="2015-08-07T23:15:38.443" Title="Why is the SES launch going to an apogee of 80,000 km?" Tags="|orbital-mechanics|spacex|falcon-9|geostationary|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2918" PostTypeId="2" ParentId="2916" CreationDate="2013-11-25T23:23:11.987" Score="12" Body="&lt;p&gt;For my money, I'd go with Jupiter's moon Europa. First, it has a thin oxygen atmosphere, and oxygen, as we've found, is not something normally found hanging around by itself. Given any chance, it bonds with natural &quot;oxygen sinks&quot; like iron and silicon. Europa has quite a bit of silica as well, and yet there's free oxygen in the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have deduced from our own planet's geology that the relatively abundant oxygen in our air is in fact a byproduct of early anaerobic microorganisms, which respirated in a similar way as modern plants; taking in the abundant carbon dioxide, and producing excess oxygen more or less as waste. With anaerobes being the dominant life form on this early Earth, the only consumers of oxygen were chemical; iron deposits, the water oceans, etc. Once those &quot;oxygen sinks&quot; were filled, atmospheric oxygen built up to toxic levels tfor the dominant anaerobes, causing the Great Oxygen Catastrophe, the largest mass extinction event in Earth's history.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, we're pretty sure that a planet with a significant percentage of diatomic oxygen in its atmosphere means that there was some active process to produce it, and the one we like best is life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also importantly, its surface, composed mostly of water ice, is very smooth and regular. This means several things of significance. First, it means the ice most likely formed when a layer of liquid later froze, meaning that Europa was, however briefly, in the &quot;Goldilocks&quot; temperature band (most likely a result of its own residual heat). Second, the fact that the ice is still smooth and hasn't been noticeably chewed up by meteorite impacts and exposure to low pressures over hundreds of millions of years, along with features like the Conamera Chaos which are indicative of movement of plates of ice over the surface, like our own mountain ranges are a result of tectonic shifts, both point to a liquid ocean under the ice. Where there's liquid water, there is the potential for life.&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="411" LastEditDate="2013-11-26T15:32:49.503" LastActivityDate="2013-11-26T15:32:49.503" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2919" PostTypeId="2" ParentId="2917" CreationDate="2013-11-26T00:01:04.353" Score="9" Body="&lt;p&gt;Short answer: to save the satellite propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch vehicle upper stage only delivers the satellite to a transfer orbit, typically with  a low perigee and apogee near GEO - in this case, the target orbit is 295x80000 km at 20.75$^\circ$ inclination. After that, it's up to the satellite itself to use its onboard propulsion system to get itself into geostationary orbit, which is circular at 35,786 km and as near 0$^\circ$ inclination as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $\Delta V$ for an impulsive combined maneuver to change altitude and inclination is given by the following equation: &#xA;$$\Delta V=\sqrt{V_i^2+V_f^2-2 V_i V_f \cos \Delta i}$$ Where $V_i$ is the initial orbit velocity, $V_f$ is the final, and $\Delta i$ is the inclination change. Since the inclination  change is what's interesting in this problem, let's just look at a pure inclination change with no change to the orbit's semi-major axis. Note that in the above equation, if $V_i = V_f$, it simplifies to $$\Delta V = 2V_i\sin\left(\frac{\Delta i}{2}\right)$$&#xA;So for a given $\Delta i$, the required $\Delta V$ changes linearly with $V_i$. For a circular orbit, $V=\sqrt{\frac{\mu}{r}}$, where $\mu$ is the standard gravitational parameter for the central body (Earth in our case), and $r$ is the circular orbit radius. From this, we see that $V$ &lt;em&gt;decreases&lt;/em&gt; as $r$ &lt;em&gt;increases&lt;/em&gt; (for non-circular orbits, we get the same effect). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting these together: increasing $r$ decreases your velocity $V$, which in turn decreases the amount of $\Delta V$ to reorient your orbital plane. So this means if you do your maneuver(s) to take out your inclination, you stand to save a lot of propellant (thus adding potentially months or years to your life) by doing it at a higher inclination. In this specific case, it's a no-brainer for SES, since the Falcon 9 &lt;em&gt;easily&lt;/em&gt; has the performance to inject to the higher orbit.&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastActivityDate="2013-11-26T00:01:04.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2920" PostTypeId="1" AcceptedAnswerId="2937" CreationDate="2013-11-26T00:17:46.620" Score="5" ViewCount="5360" Body="&lt;p&gt;The launch facilities at Cape Canaveral, FL are located at a latitude of about 28.5° N, so that puts a hard lower bound on inclinations from that site. I assume there are well-defined azimuth limits for launches from the Cape, but what is the highest inclination mission that has actually launched from here? If it was high enough, was it visible along the East coast of the U.S.?&lt;/p&gt;&#xA;" OwnerDisplayName="user29" LastEditorUserId="25" LastEditDate="2018-01-05T13:47:49.770" LastActivityDate="2018-01-05T13:47:49.770" Title="What is the highest inclination orbit ever attained from a Cape Canaveral launch?" Tags="|launch|launch-site|orbital-elements|cape-canaveral|launch-trajectories|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2921" PostTypeId="2" ParentId="2917" CreationDate="2013-11-26T00:25:33.877" Score="3" Body="&lt;p&gt;It actually saves satellite fuel, and for more than just the inclination. But first of all, let's start with the overall delta v budget, from &lt;a href=&quot;http://www.satsig.net/orbit-research/delta-v-geo-injection-calculator.htm&quot; rel=&quot;nofollow&quot;&gt;this calculator&lt;/a&gt;. Using an inclination of 20.5, and a normal GTO (300km perigee, 35686 apogee) orbit, a calculated delta v is required of 1666 m/s. With no inclination, the change required is 1466 m/s. With the 80,000 km orbit, 20.5 the delta v required is 1414 m/s, and without inclination, 1334 m/s. So, the fuel saved on the spacecraft is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;20.5 inclination- 252 m/s&lt;/li&gt;&#xA;&lt;li&gt;0 inclination- 52 m/s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-11-26T00:25:33.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2922" PostTypeId="1" AcceptedAnswerId="2923" CreationDate="2013-11-26T05:29:54.387" Score="11" ViewCount="1661" Body="&lt;p&gt;I was reading &lt;a href=&quot;https://space.stackexchange.com/a/2909/12&quot;&gt;yet another awesome Tildal answer&lt;/a&gt;, and this picture caught my eye:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/VUpv4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Zenit-3SL carrying Atlantic Bird 7 lifts off from Odyssey platform (Source: &lt;a href=&quot;http://exploringbeyondfinalfrontiers.wordpress.com/2011/09/25/sea-launch-comes-back-to-life-with-successfull-launch-of-atlantic-bird-7/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Way Up!&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that launching a vehicle directly over water would have several benefits:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;You could launch from that 70% of the world that isn't solid land&lt;/li&gt;&#xA;&lt;li&gt;You don't need to build a launch site&lt;/li&gt;&#xA;&lt;li&gt;You just let the water take care of the cooling part&lt;/li&gt;&#xA;&lt;li&gt;You don't need to perform nearly as much maintenance on your 'launchpad' afterward.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It seems that one potential problem would be the stability of such a surface - but aren't rockets able to make corrections in-flight for being launch a degree or so off? You wouldn't launch during the yearly tsunami, of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why aren't sea launches used more often?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2020-01-01T15:35:01.137" Title="Why aren't sea launches used more often?" Tags="|launch|water|launch-site|launchpad|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2923" PostTypeId="2" ParentId="2922" CreationDate="2013-11-26T07:09:27.123" Score="16" Body="&lt;p&gt;1) Startup cost. Buying and converting a boat is much more complicated and time consuming than pouring some concrete and welding some steel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Logistics. You will need at least 2 boats. One as the launcher and one as the command centre. You also need to have a reliable comms link to the rocket. You also need to house the personnel and maintain both boats. Both these costs are higher than on land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Stability. Yes rockets can correct for small errors but a rolling sea is not conducive to doing precision work. A good example is Aircraft Carrier landings. The pilots are compensating for the pitching deck and are always adjusting their exact landing time. This is not good for rocket launches&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) The use of water on rocket pads is less cooling, more shockwave reduction. The ocean as a whole is a nice flat plane to bounce shockwaves off. It won't be as severe as concrete, but you would still need the spray systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the concept is very sound, but by the time you get into the engineering of it, it is many times easier to launch from land.&lt;/p&gt;&#xA;" OwnerUserId="1076" LastActivityDate="2013-11-26T07:09:27.123" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2924" PostTypeId="2" ParentId="2920" CreationDate="2013-11-26T10:22:28.343" Score="2" Body="&lt;p&gt;Based on this &lt;a href=&quot;http://snebulos.mit.edu/projects/reference/NASA-Generic/EWR/99ewr-c1.pdf&quot; rel=&quot;nofollow&quot;&gt;document&lt;/a&gt; the maximum inclination possible is 37 degrees (from north) and the minimum is 114 degrees (from north).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know if how close any mission got to the limits, but most polar launches occur from Vandenberg&lt;/p&gt;&#xA;" OwnerUserId="1076" LastActivityDate="2013-11-26T10:22:28.343" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2925" PostTypeId="1" AcceptedAnswerId="2928" CreationDate="2013-11-26T11:36:29.743" Score="10" ViewCount="1010" Body="&lt;p&gt;If a satellite is to be inserted into a retrograde orbit (opposite to Earth rotation direction) is it accelerated straight in the desired &quot;westwards&quot; direction until (after overcoming Earth rotation speed) orbital speed is attained, or is it first inserted into a prograde orbit and then some engines of lower thrust but better specific impulse are used to gradually turn its orbit, through polar, into retrograde - or maybe is some other trick used?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2019-04-16T09:59:42.890" Title="How are retrograde orbits attained?" Tags="|orbital-maneuver|retrograde-orbit|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="2926" PostTypeId="1" AcceptedAnswerId="2927" CreationDate="2013-11-26T11:48:36.547" Score="11" ViewCount="507" Body="&lt;p&gt;I'm wondering, if it's possible for moons to have their own natural satellites, something big enough to be spherical? Stars have planets, planets have moons. Is it possible that moons have moons too? &lt;/p&gt;&#xA;" OwnerUserId="1116" LastEditorUserId="49" LastEditDate="2013-11-26T14:30:55.163" LastActivityDate="2013-11-26T14:30:55.163" Title="Is it possible for moons to have their own moons?" Tags="|orbital-mechanics|orbit|moons|astronomy|astrodynamics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2927" PostTypeId="2" ParentId="2926" CreationDate="2013-11-26T12:43:03.547" Score="8" Body="&lt;p&gt;It is theoretically possible, but such a satellite would probably not be in a stable orbit. Such a system is not known in the Solar system and due to gravitational perturbations it would not last long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.popsci.com/scitech/article/2008-04/could-moon-have-moons&quot;&gt;Popular Science&lt;/a&gt;&#xA;&lt;a href=&quot;http://curious.astro.cornell.edu/question.php?number=679&quot;&gt;Astro&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="921" LastActivityDate="2013-11-26T12:43:03.547" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2928" PostTypeId="2" ParentId="2925" CreationDate="2013-11-26T12:51:43.013" Score="11" Body="&lt;p&gt;For retrograde launches, the launch azimuth is westwards from the beginning. It would not be viable to first go into prograde orbit and then change the inclination into retrograde trajectory. Such huge inclination change would require an awful amount of fuel, because effectively it would be like killing the speed in the prograde direction (over 7 km/s) and gaining the same speed in the opposite direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://scienceworld.wolfram.com/physics/RetrogradeOrbit.html&quot;&gt;Scienceworld&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="921" LastActivityDate="2013-11-26T12:51:43.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2929" PostTypeId="1" AcceptedAnswerId="2952" CreationDate="2013-11-26T13:23:05.777" Score="9" ViewCount="472" Body="&lt;p&gt;&lt;a href=&quot;http://sci.esa.int/gaia/&quot;&gt;Gaia&lt;/a&gt; is a planned European Space Agency's (ESA) space observatory that will aim to chart a three-dimensional map of our Galaxy, the Milky Way. It is planned to operate around the Sun–Earth $L_2$ (SEL2) &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot;&gt;Lagrangian point&lt;/a&gt;. What is the flight plan to get Gaia in orbit around SEL2? We can break this down into a few sub-questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How many Earth flybys are planned?&lt;/li&gt;&#xA;&lt;li&gt;Will the Moon be used as gravity assist?&lt;/li&gt;&#xA;&lt;li&gt;Does it need lots of fuel to slow down near L2?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="603" LastEditorUserId="49" LastEditDate="2013-11-26T14:00:02.513" LastActivityDate="2013-12-15T02:21:03.377" Title="What is the flight plan to get Gaia in orbit around the Sun–Earth $L_2$ Lagrangian point?" Tags="|orbit|orbital-maneuver|lagrangian-points|esa|gaia|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2930" PostTypeId="2" ParentId="2922" CreationDate="2013-11-26T13:39:37.543" Score="9" Body="&lt;p&gt;The major reasons for Sea Launch's approach is to be able to launch on the equator, and not to have to worry about overflying any territory, where stages might drop. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cooling aspect is probably the least interesting part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the main argument against it is the infrastructure costs, and salt water corrodes everything, it would seem.  No matter how well you protect against salt water, it gets all over, when you are out in the ocean. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-11-26T13:39:37.543" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2931" PostTypeId="2" ParentId="2355" CreationDate="2013-11-26T13:59:36.817" Score="3" Body="&lt;p&gt;Nice tracker that runs in modern browsers (that support WebGL).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Useful because you do not need to install any additional software on computer from which you are accessing it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://apps.agi.com/SatelliteViewer/?Status=Operational&quot; rel=&quot;nofollow&quot;&gt;http://apps.agi.com/SatelliteViewer/?Status=Operational&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Built by AGI as demo of their Cesium virtual globe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another one with more features, but less satellites built on same technology:&#xA;&lt;a href=&quot;http://www.agsattrack.com/?view=3d&quot; rel=&quot;nofollow&quot;&gt;http://www.agsattrack.com/?view=3d&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorUserId="98" LastEditDate="2015-02-13T10:03:54.100" LastActivityDate="2015-02-13T10:03:54.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2932" PostTypeId="1" AcceptedAnswerId="2935" CreationDate="2013-11-26T16:03:54.977" Score="5" ViewCount="494" Body="&lt;p&gt;How does a space port differ from a launch pad in construction and what makes them different?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="58" LastEditDate="2013-11-26T16:53:28.810" LastActivityDate="2013-11-26T16:56:04.373" Title="How does a spaceport differ from a launch pad?" Tags="|launch|rockets|launchpad|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2933" PostTypeId="1" AcceptedAnswerId="2936" CreationDate="2013-11-26T16:34:18.880" Score="14" ViewCount="3403" Body="&lt;p&gt;When the Nov 25th, 2013 SpaceX Falcon 9 v1.1 launch was scrubbed on a Monday, next launch opportunity was reported as Thursday (Thanksgiving) since the Tues/Wed days are the busiest day in the year for air travel and the FAA did not wish to restrict traffic on those days as seen here: &lt;a href=&quot;https://twitter.com/pbdes/status/405335955608854530&quot;&gt;Tweet about air space issues&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the question is, Cape Canveral/Air Force Station/Kennedy Space Center is right on the coast.  Orlando is relativly far straight west of the cape.  Miami and Ft. Lauderdale are both pretty far south. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like rerouting around Kennedy/Cape Canveral is not that big an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless the exclusion zone is literally monstrous?  Why such a concern about the air traffic exclusion zone? How big is it really?  How many flights would it affect?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="10791" LastEditDate="2016-01-20T21:28:13.103" LastActivityDate="2016-01-20T21:28:13.103" Title="What is involved in clearing air space for an orbital launch?" Tags="|launch|spacex|launch-site|range-safety|faa|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2934" PostTypeId="2" ParentId="2895" CreationDate="2013-11-26T16:35:43.123" Score="2" Body="&lt;p&gt;According to Anatloy Zak's website, with the cancellation of Rus-M it will be the Angara family, with first to fly in March 2014, which is of course, to be taken with a grain of salt.  This is after all, rocket science.&lt;/p&gt;&#xA;&lt;p&gt;From: &lt;a href=&quot;http://www.russianspaceweb.com/vostochny_angara.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Angara at Vostochny&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On Oct. 23, 2013, Roskosmos announced a tender for the development of the launch complex 371SK32 for the Angara rocket in Vostochny. The agency allocated 813 million rubles for the work on the project code-named SK-Vostok-A (which stood for Launch Complex-Vostochny-Angara) until Nov. 25, 2014. This phase of the project was characterized as Opytno-Konstruktorskie Raboty, OKR, which can be translated as research and development work. In the Russian industrial terminology it usually refers to a second phase of development following the preliminary design.&lt;/p&gt;&#xA;&lt;p&gt;As it was expected, Roskosmos essentially ordered to replicate in Vostochny the launch facility for the Angara-A5 that had been previously developed for Russia's northern launch site in Plesetsk. The agency's paperwork called for a two-pad launch facility to be developed into two phases. The first pad would be built to support three-stage Angara-A5 rockets with DM and KVTK upper stages. The pad would have to be modifiable for a three-stage Angara-A5 rocket carrying manned PTK NP spacecraft, the tender documentation said. The second pad could accommodate both manned and unmanned versions of the rocket.&lt;/p&gt;&#xA;&lt;p&gt;At the time, Russian space officials still promised the first manned launch from Vostochny in 2018.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-11-26T18:39:32.640" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2935" PostTypeId="2" ParentId="2932" CreationDate="2013-11-26T16:48:37.173" Score="8" Body="&lt;p&gt;Space port usually means cosmodrome: that is a framework of buildings and technical infrastructure dedicated to launching rockets into the space. A launch pad is part of the technical infrastructure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So launch pad and space port are not 2 things of the same category, but rather a launch pad is part of a space port. See &lt;a href=&quot;http://en.wikipedia.org/wiki/Spaceport&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Launch_pad&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="414" LastEditDate="2013-11-26T16:56:04.373" LastActivityDate="2013-11-26T16:56:04.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2936" PostTypeId="2" ParentId="2933" CreationDate="2013-11-26T17:46:26.217" Score="16" Body="&lt;p&gt;The size of the restricted area is defined in TFR (Temporary Flight Restrictions) and is announced by NOTAM (notice to airmen).&lt;/p&gt;&#xA;&lt;p&gt;An example of a NOTAM for a space shuttle launch (STS-115 Atlantis) is &lt;a href=&quot;http://www.aero-news.net/index.cfm?do=main.textpost&amp;amp;id=22eadbfe-5667-4498-8ecb-f80b7ec12ec7&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; - the size is 30-40 nautical miles. Some additional information can be found on &lt;a href=&quot;http://www.nasa.gov/centers/kennedy/news/releases/2008/release-20080201.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt; web as well.&lt;/p&gt;&#xA;&lt;p&gt;The reason for the size is safety of the aircraft (falling debris in case of launch failure) and of course safety of the launch vehicle. In case of an intrusion there needs to be enough buffer to contact / intercept the approaching aircraft.&lt;/p&gt;&#xA;&lt;p&gt;Per &lt;a href=&quot;http://spaceflightnow.com/falcon9/007/131126faa/&quot; rel=&quot;noreferrer&quot;&gt;SpaceFlight Now's article on the delay&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SpaceX's launch site at Cape Canaveral, Fla., lies under busy East Coast air routes. Restrictions in place for the Falcon 9 launch prohibit unauthorized aircraft in a zone stretching nearly 30 miles offshore.&lt;/p&gt;&#xA;&lt;p&gt;The region closed off to civilian airliners and private planes covers about 1,500 square miles, larger than the state of Rhode Island, according to the boundaries of a temporary flight restriction released by the FAA.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="921" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-11-27T15:10:18.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2937" PostTypeId="2" ParentId="2920" CreationDate="2013-11-26T17:49:27.107" Score="10" Body="&lt;p&gt;Due to range safety requirements, which preclude launch trajectories that fly over populated areas, the maximum inclination by a standard launch from CCAFS/KSC is approximately 57 degrees.  There was one mission, however, that exceeded that.  STS-36, a classified shuttle mission, was launched to an inclination of 62 degrees, through the use of a &quot;dog-leg&quot; maneuver after SRB separation.  This did result in an overflight of portions of eastern Canada, but this occurred at a sufficient altitude and velocity not to endanger the overflown region.  This maneuver, in which the vehicle turned significantly northward after staging, came at a steep performance penalty, however.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2013-11-26T17:49:27.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2938" PostTypeId="5" CreationDate="2013-11-27T19:58:41.730" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-27T19:58:41.730" LastActivityDate="2013-11-27T19:58:41.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2939" PostTypeId="4" CreationDate="2013-11-27T19:58:41.730" Score="0" Body="Questions regarding the Federal Aviation Administration in the US, the governing body of US airspace." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-27T19:58:41.730" LastActivityDate="2013-11-27T19:58:41.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2940" PostTypeId="2" ParentId="925" CreationDate="2013-11-27T20:13:35.330" Score="7" Body="&lt;p&gt;The Pocketqube 50dollarsat (eagle-2) was successfully launched as part of the UNISAT-5 mission on Nov. 21, 2013. The satellite is in orbit now and can be heard on 437.505 MHz. This femtosatellite is 2x2x3 inches and can be received by anyone with a handheld radio and a small directional antenna. More info is available &lt;a href=&quot;http://www.50dollarsat.info&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This satellite is in a 600+km orbit and will be around for a while (we hope).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Howie AB2S&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1122" LastEditorDisplayName="user12" LastEditDate="2013-11-28T02:57:45.907" LastActivityDate="2013-11-28T02:57:45.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2941" PostTypeId="1" AcceptedAnswerId="2942" CreationDate="2013-11-28T01:39:31.750" Score="11" ViewCount="473" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.nasa.gov/jpl/msl/mars-rover-curiosity-20131125.html#.UpaeY2RDs1A&quot;&gt;this NASA announcement&lt;/a&gt;, there seems to have been some kind of trouble with Curiosity's electrical system:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The decision to resume science activities resulted from the success of work to diagnose the likely root cause of a Nov. 17 change in voltage on the vehicle. The voltage change itself did not affect the rover safety or health. The vehicle's electrical system has a &quot;floating bus&quot; design feature to tolerate a range of voltage differences between the vehicle's chassis -- its mechanical frame -- and the 32-volt power lines that deliver electricity throughout the rover. This protects the rover from electrical shorts. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, I wasn't able to find anything on what actually caused the jolt. What is thought to have caused it?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-28T02:29:33.337" LastActivityDate="2013-11-28T02:29:33.337" Title="What caused Curiosity's electrical trouble?" Tags="|mars|curiosity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2942" PostTypeId="2" ParentId="2941" CreationDate="2013-11-28T01:56:21.893" Score="14" Body="&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/658tl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Curiosity before launch (img credit &lt;a href=&quot;http://www.theatlantic.com/technology/archive/2012/08/curiosity-lands-on-mars-know-what-youre-watching-when-youre-watching-7-minutes-of-terror/260725/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Atlantic&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.space.com/23735-mars-rover-curiosity-glitch-recovery.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this space.com article&lt;/a&gt; (and several other sources), the most likely cause was an internal short in Curiosity's radioisotope thermoelectric generator (RTG) - the thingy that converts the heat of radioactive decay into electricity to power the rover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Zi1Hu.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Curiosity's RTG (img credit &lt;a href=&quot;http://www.extremetech.com/extreme/150756-to-the-stars-after-a-25-year-hiatus-nasa-restarts-plutonium-production&quot; rel=&quot;nofollow noreferrer&quot;&gt;Extreme Tech&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Luckily, it appears that the rover won't be negatively affected in the long run - from the same article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Due to resiliency in design, this short does not affect operation of the power source or the rover,&quot; NASA officials wrote in a Curiosity status update today (Nov. 25). &quot;Similar generators on other spacecraft, including NASA's Cassini at Saturn, have experienced shorts with no loss of capability.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to many sources (like &lt;a href=&quot;http://economictimes.indiatimes.com/news/politics-and-nation/mars-rover-curiosity-bounces-back-from-electrical-glitch/articleshow/26474534.cms&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt;), the rover has a 'floating bus' between the 32-volt main power lines in the vehicle and the metal frame - thus protecting it from huge shorts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://electronics.stackexchange.com/a/92086/29392&quot;&gt;this wonderful answer to my question&lt;/a&gt;, a floating bus&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;doesn't share grounds between the two systems it is connecting. It does so by using either optical or magnetic links instead of voltage levels to transfer data.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HTHX9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This allows two systems to be connected, but not be compromised if one fails - it's much like a wall made of bulletproof glass: You can't shoot whatever's on the other side, but you can still pound on the wall or perform another 'safe' action to get a message to the other side.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt;: the rover is going to be okay - there was an internal short in the RTG, but history has shown that this kind of issue doesn't tend to break things too badly.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:33:08.970" LastActivityDate="2013-11-28T02:18:59.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2943" PostTypeId="5" CreationDate="2013-11-28T02:59:38.857" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-28T02:59:38.857" LastActivityDate="2013-11-28T02:59:38.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2944" PostTypeId="4" CreationDate="2013-11-28T02:59:38.857" Score="0" Body="Questions regarding devices used to join two spacecraft together during flight." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-11-28T02:59:38.857" LastActivityDate="2013-11-28T02:59:38.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2945" PostTypeId="1" CreationDate="2013-11-28T03:50:15.870" Score="7" ViewCount="276" Body="&lt;p&gt;What kind of technique does the Curiosity rover employ to find the safest (probably not the fastest) way to its destination that was given to it by NASA from the Earth?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-12-02T18:11:33.650" LastActivityDate="2013-12-02T18:11:33.650" Title="How does Curiosity find its own way to selected destination?" Tags="|mars|nasa|curiosity|navigation|collision-avoidance|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2948" PostTypeId="2" ParentId="2925" CreationDate="2013-11-28T16:06:08.870" Score="3" Body="&lt;p&gt;A common example of a launcher that always launches retrograde, is &lt;a href=&quot;http://en.wikipedia.org/wiki/Shavit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Israel's Shavit&lt;/a&gt; booster.  Due to the nature of its neighbours to the east, and the fact most space launches closely resemble ICBM style launches (which might cause some concern on launch) and no where safe to drop empty stages, they launch west over the Mediterranean Sea. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more on that see answer(s) to &lt;a href=&quot;https://space.stackexchange.com/q/18295/12102&quot;&gt;The strange orbit of Ofeq 11 - how does it (actually) do this?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="12102" LastEditDate="2019-04-16T08:01:50.153" LastActivityDate="2019-04-16T08:01:50.153" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="2949" PostTypeId="1" AcceptedAnswerId="2950" CreationDate="2013-11-28T22:44:58.330" Score="12" ViewCount="1160" Body="&lt;p&gt;SES-8 had its launch aborted at T+2 seconds. What, exactly, caused the flight computer to decide the engines weren't ready?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="3029" LastEditDate="2016-09-02T23:43:06.557" LastActivityDate="2016-09-02T23:43:06.557" Title="What caused SES-8's Falcon 9 launch computer to abort at T+2 seconds?" Tags="|launch|spacex|falcon-9|abort|launch-sequence|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2950" PostTypeId="2" ParentId="2949" CreationDate="2013-11-28T23:09:23.823" Score="12" Body="&lt;p&gt;The report via Tweet from Elon Musk initially is that a slower than usual ramp up of thrust caused the flight computers to abort.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/406194506011525120&quot;&gt;Elon Musk via Twitter&lt;/a&gt;:&#xA;  Launch aborted by autosequence due to slower than expected thrust&#xA;  ramp. Seems ok on closer inspection. Cycling countdown.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is reported leeway in relaxing some of the sensitivity on this issue for another attempt. What that means was a bit later &lt;a href=&quot;https://twitter.com/elonmusk/status/406205240040771584&quot;&gt;tweeted by Musk&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Increasing helium spin start pressure. Probably &amp;lt;50% chance of passing&#xA;  all aborts, but worth a try. Countdown resuming ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Another try was however aborted on T-00:00:48 due to engineers not completing data review of the previous abort and the launch was scrubbed for the day.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2013-11-29T00:00:54.720" LastActivityDate="2013-11-29T00:00:54.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2951" PostTypeId="2" ParentId="2949" CreationDate="2013-11-29T08:41:11.967" Score="10" Body="&lt;p&gt;When the rocket is in design phase the designers usually design the the graph known as  &lt;strong&gt;thrust-time graph&lt;/strong&gt;.  Nowadays the flight computers integrated to the rockets  measures the thrust build up during lift off (which is important phase)  if the thrust build up is not satisfactory the flight computers close the fuel and oxidizer inlet valve and shut downs  the turbopump. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://www.spaceflight101.com/spacex-falcon-9-v11-ses-8-launch-updates.html&quot;&gt;According to SPACEFLIGHT101&lt;/a&gt;, SpaceX CEO and Chief&#xA;  Designer Elon Musk, some of the&#xA;  Merlin 1D engines were slow to ramp-&#xA;  up thrust. As part of Falcon’s tight&#xA;  red-line limits, the computer aborted&#xA;  when the engine parameters during&#xA;  ignition were not within those limits.&#xA;  The launch team then looked at the&#xA;  data that was collected during the&#xA;  ignition sequence and determined&#xA;  that the engines appeared to the in&#xA;  good condition and were just slow to&#xA;  reach their nominal thrust &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-11-29T12:57:26.660" LastActivityDate="2013-11-29T12:57:26.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2952" PostTypeId="2" ParentId="2929" CreationDate="2013-11-29T09:48:43.973" Score="7" Body="&lt;p&gt;&lt;a href=&quot;http://www.spaceflight101.com/gaia-mission-and-orbit-design.html&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt; has a detailed overview of Gaia's launch sequence. There don't seem to be any Earth flybys or gravity assists planned. Just a single orbit around Earth after launch, then the burn to inject Gaia into its L2 transfer trajectory.&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/XSRh0.jpg&quot; alt=&quot;Gaia launch sequence&quot;&gt;&#xA;When it arrives at L2, a delta-V of 180 meters per second inserts Gaia into its orbit around L2.&lt;br&gt;&#xA;Note that seems to disagree with &lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/Gaia&quot; rel=&quot;noreferrer&quot;&gt;ESA's Gaia website&lt;/a&gt;, which states that Gaia will spend 4 days in Earth orbit for systems checks before the L2 injection burn. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-11-29T09:48:43.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2953" PostTypeId="2" ParentId="2945" CreationDate="2013-11-29T16:44:17.457" Score="5" Body="&lt;p&gt;The terrain around the rover is scanned with &lt;a href=&quot;http://mars.nasa.gov/msl/mission/rover/eyesandother/&quot; rel=&quot;noreferrer&quot;&gt;two sets of cameras&lt;/a&gt;: the navigation cameras and the hazard avoidance cameras. These cameras are installed in pairs: each pair generates stereoscopic images, so the rover can measure distances to and dimensions of obstacles. The images from these cameras is converted to a 3D map of the surrounding terrain.&lt;br&gt;&#xA;An &lt;a href=&quot;http://mars.nasa.gov/msl/mission/rover/brains/&quot; rel=&quot;noreferrer&quot;&gt;inertial measurement unit&lt;/a&gt; provides information on the rover's position and attitude.&lt;br&gt;&#xA;The rover combines this information to plot possible paths through the photographed terrain, then compares these paths to &lt;a href=&quot;http://marsrover.nasa.gov/technology/is_autonomous_mobility.html&quot; rel=&quot;noreferrer&quot;&gt;choose the safest one&lt;/a&gt; (link about Spirit and Opportunity). (&lt;a href=&quot;http://marsrovers.jpl.nasa.gov/home/posters/OpportunityPosterBack.pdf&quot; rel=&quot;noreferrer&quot;&gt;another link with lots of images in a PDF&lt;/a&gt;). The navigation system used on &lt;a href=&quot;http://mars.jpl.nasa.gov/msl/mission/technology/insituexploration/planetarymobility/&quot; rel=&quot;noreferrer&quot;&gt;Curiosity&lt;/a&gt; is derived from the work done on Spirit and Opportunity. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-11-29T16:44:17.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2954" PostTypeId="1" AcceptedAnswerId="2970" CreationDate="2013-11-29T17:34:36.743" Score="27" ViewCount="13325" Body="&lt;p&gt;A recent question back in Physics asks &lt;a href=&quot;https://physics.stackexchange.com/questions/88322/is-there-a-way-for-an-astronaut-to-rotate&quot;&gt;whether there is a way for an astronaut to rotate&lt;/a&gt; when in microgravity and without touching anything else, while still conserving angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to do this is easily demonstrated using a heavy book and a swivelling office chair. You hold out the book in front of you and rotate it about a vertical axis, bringing it closer to, and away from, your body when it is going to your left and right, respectively. To conserve angular momentum, your body also rotates slightly, but due to the difference in moment of inertia of the book when close/far from your body, the angular displacement of your body is different for the two stages and the final state is a displaced attitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This approach, and other similar ones - including the proverbial cat turning in midair -, have been worked to bits in &lt;a href=&quot;https://physics.stackexchange.com/&quot;&gt;Physics&lt;/a&gt; and most other outlets. However, the usual approaches sound too cumbersome to be used in space, but there may be cleverer ways to move one's body to achieve the same effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In practice&lt;/strong&gt;, how &lt;em&gt;do&lt;/em&gt; astronauts change their orientation in space? Do they regularly perform free-body manoeuvres while within their spacecraft, or do they simply grab onto the craft? During EVAs, do they use their thrusters, grab onto something, or just wave around until they're in position? If the latter, what are common ways to achieve such rotations? Does this depend on what axis you want to rotate about?&lt;/p&gt;&#xA;" OwnerUserId="653" LastEditorUserId="26446" LastEditDate="2021-07-05T03:54:33.357" LastActivityDate="2021-07-05T03:54:33.357" Title="How do astronauts turn in space?" Tags="|attitude|eva|microgravity|rotation|astronaut-propulsion|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2955" PostTypeId="2" ParentId="2954" CreationDate="2013-11-29T17:52:54.863" Score="-5" Body="&lt;p&gt;Conservation of angular momentum would apply if the astronaut was a still rod.  Which to a first approximation they might be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, since they can rotate their body, similar to how a falling cat can rotate to land feet down, they can rotate their upper half, while attempting to keep lower half still, and when they capture some stationary point, would then release the momentum to the lower half.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if they never capture anything stationary, then all the twisting in the world is just whistling in the wind.  This of course is inside the vehicle.  Should they be outside, this is whistling in the vaccum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While outside the vehicle they are ALWAYS attached to something.  (&lt;a href=&quot;http://en.wikipedia.org/wiki/Manned_Maneuvering_Unit&quot; rel=&quot;nofollow&quot;&gt;MMU&lt;/a&gt;'s on one or two shuttle flights being the exceptions that make the rule.  They stopped using them after a few uses).  Letting go, is a horribly bad idea while on EVA.  They are also clipped via a cable.  (There is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Simplified_Aid_for_EVA_Rescue&quot; rel=&quot;nofollow&quot;&gt;SAFER pod&lt;/a&gt; they wear, that are like low performance, baby MMU's for emergency fly back if they did get disconnected). &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-11-29T17:52:54.863" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2956" PostTypeId="1" AcceptedAnswerId="2957" CreationDate="2013-11-29T18:18:14.380" Score="17" ViewCount="2095" Body="&lt;p&gt;One of the comments on &lt;a href=&quot;https://space.stackexchange.com/questions/2954/how-do-astronauts-turn-in-space&quot;&gt;this recent question&lt;/a&gt; touched on the use of the Manned Maneuvering Unit. It seemed like a good idea at the time: a system that would allow astronauts to do EVA without being tethered.&lt;br&gt;&#xA;The wikipedia page on the MMU suggests that use of the MMU was stopped after a few flights because it was deemed too risky, but it doesn't expand on this. Were there incidents on the missions that they were used, or other technical issues? Or was it purely due to the fear of being marooned? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="26446" LastEditDate="2021-07-05T03:48:22.693" LastActivityDate="2021-07-05T03:48:22.693" Title="Why did NASA discontinue use of the MMU?" Tags="|space-shuttle|astronauts|eva|astronaut-propulsion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2957" PostTypeId="2" ParentId="2956" CreationDate="2013-11-29T19:53:11.100" Score="18" Body="&lt;p&gt;They didn't have an application that couldn't be done better by maneuvering the shuttle itself and using the shuttle arm and tethered astronauts (sometimes with astronauts serving as the end effector on the arm).  There was little to no benefit to the MMU, so even a small risk outweighs a non-existent benefit.  Also the things are big and heavy.  In space, if you don't need it, you don't bring it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were cool though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From an upcoming &lt;a href=&quot;http://www.smithsonianmag.com/science-nature/story-nasas-jet-propulsion-backpack-180950190/&quot; rel=&quot;noreferrer&quot;&gt;Smithsonian article&lt;/a&gt; by Andy Chaikin:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But after the Challenger disaster in 1986, NASA re-evaluated shuttle&#xA;  missions, including spacewalks, and the MMU was deemed unnecessary.&#xA;  “It became pretty obvious that you didn’t need it,” explains Nelson.&#xA;  “The shuttle had such an amazing capability to fly right up to&#xA;  something, and it made more sense to just reach out and grab it,&#xA;  either with the [robotic] arm or just with a person, that the MMU&#xA;  became a really cool piece of technology that didn’t quite have a&#xA;  purpose.”&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;“Too bad,” Nelson adds, “because it’s a very cool machine.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-03-23T15:30:31.137" LastActivityDate="2014-03-23T15:30:31.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2958" PostTypeId="1" AcceptedAnswerId="2961" CreationDate="2013-11-29T20:20:19.873" Score="60" ViewCount="34089" Body="&lt;p&gt;I noticed four(?) towers around the SpaceX launch pad during yesterday's aborted launch&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gggHD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Image credit &lt;a href=&quot;http://www.nbcnews.com/science/last-minute-glitch-aborts-spacexs-thanksgiving-rocket-launch-2D11672066&quot; rel=&quot;noreferrer&quot;&gt;NBC News&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the towers used for? Tracking? Radio communications?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="23839" LastEditDate="2018-04-12T15:05:18.910" LastActivityDate="2023-07-26T14:10:57.483" Title="What are the towers around the SpaceX launch pad used for?" Tags="|spacex|launch|launch-site|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2959" PostTypeId="1" AcceptedAnswerId="2962" CreationDate="2013-11-29T20:26:31.223" Score="16" ViewCount="2186" Body="&lt;p&gt;During yesterday's aborted SpaceX launch, someone was giving instructions to the controllers about abort procedures:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To abort the launch, a controller will say 'abort, abort, abort'.... &lt;strong&gt;controllers will not call an abort in the final ten seconds of launch&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;sup&gt;Paraphrased from memory&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;In fact, the forcefulness with which this was announced provoked &lt;a href=&quot;https://twitter.com/Pillownaut/status/406188310512422912&quot; rel=&quot;noreferrer&quot;&gt;this tweet&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The dude giving abort instructions for #SpaceX #Falcon9 #SES8 kinda sounds like he is reciting the rules for Fight Club.&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Tweeted by Pillownaut on &lt;a href=&quot;https://twitter.com/Pillownaut/status/406188310512422912&quot; rel=&quot;noreferrer&quot;&gt;November 28, 2013 at 10:30 PM&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;My question is this: Why aren't controllers allowed to call an abort in the final ten seconds? It seems that if I were a controller and I found something that was going to cause something to blow up, it would be preferrable for others to know - even if in the last ten seconds.&lt;/p&gt;&#xA;&lt;p&gt;What is the rationale behind this rule?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-11-29T21:15:52.840" Title="Why aren't SpaceX controllers allowed to abort during the final 10 seconds?" Tags="|launch|spacex|falcon-9|abort|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2960" PostTypeId="1" AcceptedAnswerId="2963" CreationDate="2013-11-29T20:37:11.760" Score="16" ViewCount="1961" Body="&lt;p&gt;&lt;a href=&quot;http://www.nas.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;NASA Advanced Supercomputing&lt;/a&gt; (NAS) Division has apparently &lt;a href=&quot;http://googleresearch.blogspot.com/2013/05/launching-quantum-artificial.html&quot; rel=&quot;noreferrer&quot;&gt;partnered with Google&lt;/a&gt; and acquired the world's largest quantum computer, &lt;a href=&quot;http://www.dwavesys.com/d-wave-two-system&quot; rel=&quot;noreferrer&quot;&gt;D-Wave Two™&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a partnership with Google and independent, nonprofit research&#xA;  corporation Universities Space Research Association (USRA), Ames has&#xA;  established the Quantum Artificial Intelligence Laboratory (QuAIL) at&#xA;  its NASA Advanced Supercomputing (NAS) facility. The laboratory houses&#xA;  a 512-qubit D-Wave Two™ quantum computer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KvTp2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Support structure for installation of the D-Wave Vesuvius processor, which is cooled to 20 millikelvin (Source: &lt;a href=&quot;http://www.nasa.gov/ames/nasa-experts-available-for-interviews-about-quantum-computing/&quot; rel=&quot;noreferrer&quot;&gt;NASA / J. Hardman&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pages I came across describing D-Wave Two are however rather scarce with information on what NASA actually plans to do with it. Some vague explanation goes like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA researchers will use this system to investigate quantum&#xA;  algorithms that might someday dramatically improve the agency's&#xA;  ability to solve difficult optimization problems in aeronautics, Earth&#xA;  and space sciences, and space exploration. Applications relevant to&#xA;  NASA include machine learning, pattern recognition, mission planning&#xA;  and scheduling, distributed navigation and coordination, and system&#xA;  diagnostics and anomaly detection.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But apparently, &lt;a href=&quot;http://www.nasa.gov/ames/nasa-experts-available-for-interviews-about-quantum-computing/&quot; rel=&quot;noreferrer&quot;&gt;NASA experts are available for interviews about quantum computing&lt;/a&gt;, so my question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Have these &lt;a href=&quot;http://www.nas.nasa.gov/quantum/&quot; rel=&quot;noreferrer&quot;&gt;NASA QuAIL&lt;/strong&gt; (Quantum Artificial Intelligence Laboratory) &lt;strong&gt;team&lt;/a&gt; experts been more descriptive about what they plan on using this quantum computer for during some  interview? In particular, I mean if they have revealed any specific function it would serve, for some specific project, and when is it expected to prove useful? What role has Google in all of this?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="12102" LastEditDate="2023-10-27T19:32:43.527" LastActivityDate="2023-10-27T19:32:43.527" Title="What does NASA plan to do with the world's largest quantum computer?" Tags="|nasa|computing|artificial-intelligence|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2961" PostTypeId="2" ParentId="2958" CreationDate="2013-11-29T20:59:28.287" Score="81" Body="&lt;p&gt;&lt;strong&gt;TL;DR: They're lightning rods.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;They are used to redirect lightning in the immediate area. This essentially creates a faraday cage, shielding the rocket from being fried by lightning. You can see how high the towers reach, high enough to ensure there is no risk of lightning hitting the craft.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Update by @highonrope:&lt;/em&gt; The rectangle which the rocket launches through is &lt;em&gt;huge&lt;/em&gt;...from the ground to the bottom of the &amp;quot;candle stick&amp;quot; is 250 feet tall. The candle stick is the big white thing at the top. It is about another 150 feet tall. They look a lot smaller in a picture, but it takes several minutes for an experienced tower climber to go from ground to the top. They say that inside the lightning suppression system there, you are 100% safe from lightning. The towers are all interconnected with stainless steel cables which form a rectangle through which the rockets are launched from the rectangle, the cables run to the ground and are fully grounded. @highonrope was part of a team of people who were tasked with cleaning and repainting the top of the tower and testing the &amp;quot;ground&amp;quot; of each cable where it attached at the top and at the ground, when SpaceX first started at Kennedy, before the first launch there.  We got to see first hand how huge the SpaceX rocket was and what it does to the pavement in the &amp;quot;exhaust&amp;quot; tube, where the rocket exhaust is redirected at ground level. It's an awesome complex. It was designed for the Saturn rockets and is one of the oldest launch pads at Kennedy.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="51781" LastEditDate="2023-07-26T14:10:57.483" LastActivityDate="2023-07-26T14:10:57.483" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="2962" PostTypeId="2" ParentId="2959" CreationDate="2013-11-29T21:09:27.083" Score="14" Body="&lt;p&gt;It seems the most likely reason is that at T-10 seconds, anything that is a serious enough issue to abort should be caught by the computer, and aborting mid-stream could cause larger issues. Take a look at &lt;a href=&quot;http://www.space.com/23778-spacex-thanksgiving-launch-aborted-one-second-before-lift-off-video.html&quot;&gt;this video&lt;/a&gt; of the last 10 seconds. Here's what I can see happen:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;T-6 Water comes out for sound suppression on surface&lt;/li&gt;&#xA;&lt;li&gt;T-3 Engine ignites &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The last seconds, essentially everything is on auto. The automatic aborts have been tested, in fact, this is at least the second Falcon rocket to abort automatically right before launch. A manual abort would need to be tested further along in the system. I suspect an abort could still fairly easily happen at T-8, but at T-6, it could potentially only be partially aborted, which could be quite a bit more dangerous than automatically aborting. Give a couple of seconds to react, and the fact that it's just easier to remember 10 than 8, and that's probably why an abort can't be called at the last second.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-11-29T21:15:52.840" LastActivityDate="2013-11-29T21:15:52.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2963" PostTypeId="2" ParentId="2960" CreationDate="2013-11-30T02:12:29.220" Score="13" Body="&lt;p&gt;From &lt;a href=&quot;http://www.wired.co.uk/news/archive/2013-11/15/quantum-nasa&quot;&gt;wired&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...at Nasa's Ames Research Center, not far from Google headquarters,&#xA;  there's a machine that could go even faster. This is the&#xA;  multimillion-dollar quantum computer shared by Google and Nasa, and&#xA;  Google engineers are already pitting the thing against the company's&#xA;  existing hardware and software. It's a race between the quantum&#xA;  computer and the classic computers.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;We set up a blue team and a red team that race each other,&quot; Google&#xA;  spokesman Jason Freidenfelds tells Wired. &quot;The blue team throws up new&#xA;  problem classes they believe favor the [quantum] hardware, and the red&#xA;  team refines classical algorithms to match or outperform the&#xA;  hardware.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Google isn't ready to publish any results, so it can't say where the&#xA;  quantum computer is outperforming its more conventional digital&#xA;  computers. But the company is &quot;optimistic that we can find challenges&#xA;  where the [quantum] hardware is superior.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Built by a Canadian company called D-Wave, this quantum machine is one&#xA;  of only two in use around the world. Early research involving the&#xA;  system took a bit of a hit during the government shutdown last month,&#xA;  but things are now back up and running, with both Nasa and Google&#xA;  running tests to better understand what the machine is actually&#xA;  capable of doing.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;As Google runs its races, Nasa is running simulations that could feed&#xA;  the International Space Station project and various supercomputing&#xA;  efforts. It's an exciting time, says Rupak Biswas, the deputy director&#xA;  of the Exploration Technology Directorate at Ames: the dawn of the&#xA;  quantum computing age.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nasa's first tests will study techniques for scheduling supercomputer&#xA;  resources -- figuring out which supercomputer nodes should be used at&#xA;  what times if you're running a thousand supercomputing jobs. They'd&#xA;  also like to use the machine to better schedule work on the&#xA;  International Space Station.Today, Nasa solves these kinds of problems&#xA;  using heuristics. &quot;You make some educated guesses and you pare down&#xA;  your search space so that the search space becomes manageable,&quot; says&#xA;  Biswas. That means the space agency may not get definitive answers to&#xA;  its really complex problems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="446" LastActivityDate="2013-11-30T02:12:29.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2964" PostTypeId="1" AcceptedAnswerId="2966" CreationDate="2013-11-30T03:47:36.253" Score="10" ViewCount="22800" Body="&lt;p&gt;How much protection do space EVA suits offer against radiation?  Do they shield certain kinds of radiation more than others?  Any idea how many sieverts an individual is exposed to wearing a space suit on an EVA for 1 hour?&lt;/p&gt;&#xA;" OwnerUserId="901" LastActivityDate="2017-05-04T17:58:55.477" Title="How much protection do space EVA suits offer against radiation for the wearer?" Tags="|radiation|spacesuits|eva|" AnswerCount="2" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2965" PostTypeId="2" ParentId="2964" CreationDate="2013-11-30T03:52:05.413" Score="4" Body="&lt;p&gt;They do very little.  The suit will stop alpha and I believe beta radiation.  However, gamma and x-ray radiation go right through.  The only thing you can use to stop those is lots of mass, and spacesuits are as light as possible.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most 'radiation' suits, by the way, do little to stop actual radiation.  They are generally intended to contain radioactive materials on the outside of the suit for easy decontamination.  The dose is then monitored with dosimeters and handheld detectors.  Areas of high radioactivity are simply not entered at all or only entered for short periods of time.  This is done in spaceflight as well - astronauts do not perform EVAs during solar flares, for example.  &lt;/p&gt;&#xA;" OwnerUserId="1128" LastEditorUserId="1128" LastEditDate="2013-11-30T03:57:38.533" LastActivityDate="2013-11-30T03:57:38.533" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2966" PostTypeId="2" ParentId="2964" CreationDate="2013-11-30T07:03:02.507" Score="7" Body="&lt;p&gt;There are three ways to protect ourselves from radiation: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Keeping a distance from the source &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Minimizing the time of exposure to radiation&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Using shielding .&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, in space the only way to protect from radiation is by using the last two points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space suits provide protection from the UV rays, but they provide limited protection from particle radiation and gamma and x ray.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If  particles have enough energy they simply pass through the space suit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So EVAs are planned during low solar activity or they try to make the EVA short.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="1142" LastEditDate="2016-02-24T14:22:27.597" LastActivityDate="2016-02-24T14:22:27.597" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2967" PostTypeId="2" ParentId="2960" CreationDate="2013-11-30T13:36:07.640" Score="6" Body="&lt;p&gt;Some of the application of that quantum computer  that i found &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;If you're trying to schedule or&#xA;  plan a whole bunch of tasks on the&#xA;  International Space Station, you&#xA;  can do certain tasks only if&#xA;  certain preconditions are met,&quot; he&#xA;  explains. &quot;And after you perform&#xA;  the task you end up in another&#xA;  state where you may or may not&#xA;  be able to perform another task.&#xA;  So that's considered a hard&#xA;  optimization problem that a&#xA;  quantum system could potentially&#xA;  solve.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;......Kepler&#xA;  search for exoplanets . NASA&#xA;  astronomers use their various&#xA;  telescopes to look at light curves to&#xA;  understand whether any&#xA;  noticeable dimming represents a&#xA;  potential exoplanet as it moves&#xA;  across its host star. This is a&#xA;  massive search problem — one&#xA;  that D-Wave could conceivably&#xA;  help with.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;These computers would also come&#xA;  in handy if we had multiple rovers&#xA;  on Mars or other planets,&quot; he&#xA;  added. &quot;And if you wanted to&#xA;  manage the way all those rovers&#xA;  coordinate with one another,&#xA;  that's a good candidate for&#xA;  wanting to apply quantum&#xA;  computers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;source: &lt;a href=&quot;http://io9.com/what-will-nasa-be-doing-with-its-new-quantum-computer-1468333514&quot; rel=&quot;noreferrer&quot;&gt;http://io9.com/what-will-nasa-be-doing-with-its-new-quantum-computer-1468333514&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-11-30T13:36:07.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2968" PostTypeId="1" AcceptedAnswerId="2978" CreationDate="2013-11-30T19:18:52.557" Score="5" ViewCount="840" Body="&lt;p&gt;I often see it &lt;a href=&quot;http://www.spacenews.com/article/launch-report/35469with-ariane-6-launch-site-selected-cnes-aims-to-freeze-design-of-the-new&quot; rel=&quot;nofollow noreferrer&quot;&gt;mentioned&lt;/a&gt; that the selection of the launch site for Ariane 6 was one of the preliminary requirements that froze its final design. That does make sense, especially since there was some &lt;a href=&quot;https://space.stackexchange.com/q/1667/49&quot;&gt;disagreement between DLR and CNES whether Ariane 6 should have a solid fueled first two stages or not&lt;/a&gt;, with solid core likely leaving less of a leaway in launcher's configurations, depending on which launch site it would operate from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/zyzaF.png&quot; alt=&quot;Map of the Guiana Space Centre&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Map of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Guiana_Space_Centre&quot; rel=&quot;nofollow noreferrer&quot;&gt;Guiana Space Centre&lt;/a&gt;, Kourou, French Guiana (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Plan_Centre_Spatial_Guyanais-en.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch site selected was, expectedly so, French Guiana and a new launch pad will be build for this carrier rocket in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Guiana_Space_Centre&quot; rel=&quot;nofollow noreferrer&quot;&gt;Guiana Space Centre&lt;/a&gt;, Kourou. What I didn't find anywhere is however which other launch sites were considered for this, &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Ariane_6_moves_to_next_stage_of_development&quot; rel=&quot;nofollow noreferrer&quot;&gt;now with finalized preliminary design&lt;/a&gt;, ESA's next generation launcher? So to my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Which other launch sites (except Kourou) were considered by ESA for Ariane 6, and what arguments eventually tipped the scale in favor of Kourou?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-12-01T09:34:10.473" Title="Which launch sites were considered for Ariane 6?" Tags="|launch-site|esa|ariane|site-selection|space-program|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2969" PostTypeId="1" AcceptedAnswerId="2981" CreationDate="2013-11-30T23:38:51.420" Score="9" ViewCount="1303" Body="&lt;p&gt;I was going through the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_rocket_launch_sites#Europe&quot; rel=&quot;noreferrer&quot;&gt;list of European rocket launch sites on Wikipedia&lt;/a&gt;, and I can't find a single reference for an orbited satellite launched from any of them. Honestly, I wasn't even aware of most of the launch sites listed, even if they're marked as still operational.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found some sounding rocket launches, like for example from &lt;a href=&quot;https://en.wikipedia.org/wiki/Esrange&quot; rel=&quot;noreferrer&quot;&gt;Esrange&lt;/a&gt; in Kiruna, Sweden. Or &lt;a href=&quot;https://en.wikipedia.org/wiki/Salto_di_Quirra#Vega_test_firings&quot; rel=&quot;noreferrer&quot;&gt;test firings&lt;/a&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Zefiro_9#Zefiro_9&quot; rel=&quot;noreferrer&quot;&gt;Zefiro 9&lt;/a&gt; rocket engine for &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_%28launcher%29&quot; rel=&quot;noreferrer&quot;&gt;Arianespace Vega&lt;/a&gt; launch vehicle at &lt;a href=&quot;https://en.wikipedia.org/wiki/Salto_di_Quirra&quot; rel=&quot;noreferrer&quot;&gt;Salto di Quirra&lt;/a&gt;, Italy. Or launches of suborbital rockets for atmospheric soundings from &lt;a href=&quot;https://en.wikipedia.org/wiki/El_Arenosillo&quot; rel=&quot;noreferrer&quot;&gt;El Arenosillo&lt;/a&gt;, Spain. Or...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/BsEJc.jpg&quot; alt=&quot;Andøya Rocket Range&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Andøya Rocket Range, Andøya island, Norway launched since 1962 over 1,200 sounding rockets (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/And%C3%B8ya_Rocket_Range&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But none of these are actually orbited spacecraft, reaching at least &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;Low-Earth Orbit&lt;/a&gt; (LEO) altitudes at velocities to stay in orbit. Individual European private enterprises, space agencies, and also as members of the European Space Agency (ESA), have of course launched into Earth's orbit and beyond multitude of spacecraft and satellites. But...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Have Europeans, excluding the west Russia that still geographically belongs to the European subcontinent, launched a single satellite launch vehicle from the territory of Europe? If there were any, which one was the last, from where, and what orbit did it achieve?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-02T22:33:15.010" LastActivityDate="2013-12-02T22:33:15.010" Title="Excluding Russia, has Europe launched a single satellite or spacecraft into Earth's orbit and beyond from its own continent?" Tags="|launch-site|esa|spaceport|space-program|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2970" PostTypeId="2" ParentId="2954" CreationDate="2013-12-01T01:42:04.777" Score="28" Body="&lt;p&gt;This video published on YouTube on &lt;a href=&quot;https://www.youtube.com/watch?feature=player_detailpage&amp;amp;v=OvXKTOjaVQ4#t=130&quot; rel=&quot;noreferrer&quot;&gt;Zero-G: &quot;Movement in Microgravity: Skylab to Space Shuttle&quot; 1988 NASA Weightlessness Footage&lt;/a&gt;, starting at 2:10 into it, shows a &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot; rel=&quot;noreferrer&quot;&gt;Skylab&lt;/a&gt; astronaut doing a &lt;em&gt;front roll&lt;/em&gt; and a &lt;em&gt;spiral roll&lt;/em&gt; in the Skylab Orbital Workshop without touching anything to push against to change his orientation. And the same video from 5:45 to 6:00 shows astronauts &lt;em&gt;wiggling&lt;/em&gt; from one direction to another to attention (fun video!):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/OvXKTOjaVQ4?start=345&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you will see in it, astronauts did all kinds of zero-g / microgravity stunts like that, here's one such fun photograph:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/L9S0j.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Astronaut Gerald P. Carr, Commander for the Skylab 4 mission, jokingly demonstrates weight training in zero-gravity as he&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;balances astronaut William R. Pogue, pilot, upside down on his finger. (Source: &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Carr_and_Pogue_in_Skylab_-_GPN-2000-001946.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've also frequently seen International Space Station (ISS) astronauts use such movement to change their orientation on the station, for example by watching &lt;a href=&quot;http://spacestationlive.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Space Station Live&lt;/a&gt; or  &lt;a href=&quot;https://www.youtube.com/results?search_query=space+station+live&amp;amp;filters=long&amp;amp;lclk=long&quot; rel=&quot;noreferrer&quot;&gt;video recordings of it on YouTube&lt;/a&gt;, albeit while they would mostly first push against some surface to gain velocity towards their next destination. These movements wouldn't be much different than what swimmers do on a turn in a swimming pool, or as previously mentioned, a cat falling and reorienting to land on its feet. For a more direct demonstration, here's a &lt;a href=&quot;https://www.youtube.com/watch?v=VJcno_XL4RU&quot; rel=&quot;noreferrer&quot;&gt;Smarter Every Day video #85 on How Astronauts Turn In Space&lt;/a&gt; from March 2013 with ISS crew demonstrating change of orientation while not touching anything and of course preserving angular momentum:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/VJcno_XL4RU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During Extravehicular Activity (EVA) though, I doubt that they have much need for such stunts, or that they would be an easy feat to do after donning their EVA gear, with mobility units (latest one is &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_Aid_for_EVA_Rescue&quot; rel=&quot;noreferrer&quot;&gt;Simplified Aid for EVA Rescue&lt;/a&gt; or SAFER) somewhat impairing their ability to change orientation like that, prohibiting free flexing of the body, while at the same time making them unnecessary, since the change in orientation can be provided by the mobility unit itself, if there isn't any surface to push against.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronauts are now also tethered to the space station and use on the station's outer hull mounted safety grips during EVA, so not only would such movement be cumbersome due to their EVA suit, but could result in the astronaut entangling in the tether. Some more about the astronaut propulsion and mobility units is described in the second half of &lt;a href=&quot;https://space.stackexchange.com/a/2586/49&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/bUK1C.jpg&quot; alt=&quot;Astronaut Rick Mastracchio working with a SAFER system attached.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Astronaut Rick Mastracchio working with a SAFER system attached. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_Aid_for_EVA_Rescue&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on SAFER: Simplified Aid for EVA Rescue&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the ISS itself, astronauts use footholds to fix themselves at a work location so their own body movement doesn't continuously move them around, and they push against all kinds of surfaces with their feet and hands (and sometimes, for fun, even tips of their hair, like I believe Sunita Williams did first) to make their way through the station. For some examples, I recommend watching some video tour of the ISS, like for example &lt;a href=&quot;https://www.youtube.com/watch?v=tqAXYNL32B8&quot; rel=&quot;noreferrer&quot;&gt;this Sunita Williams one&lt;/a&gt;, or an &lt;a href=&quot;https://www.youtube.com/watch?v=QF2w2Dx_QMs&quot; rel=&quot;noreferrer&quot;&gt;ISS tour by André Kuipers&lt;/a&gt;. Footholds will also be used by &lt;a href=&quot;https://space.stackexchange.com/a/1530/49&quot;&gt;Robonaut 2&lt;/a&gt; once it gets its legs, which I believe should be this month or at the latest in January 2014.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/uQXwr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;One of the footholds on the International Space Station. (Source: &lt;a href=&quot;http://observationdeck.io9.com/it-never-clicked-before-that-those-were-footholds-1252790243&quot; rel=&quot;noreferrer&quot;&gt;Ghost In The Machine on Observation Deck&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-01-13T16:13:02.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2971" PostTypeId="1" CreationDate="2013-12-01T03:58:07.787" Score="12" ViewCount="1335" Body="&lt;p&gt;I'm primarily looking for comparison of currently operational orbital launch vehicles by &lt;a href=&quot;https://en.wikipedia.org/wiki/System-specific_impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;system-specific impulse&lt;/a&gt;, or &lt;em&gt;I&lt;sub&gt;ssp&lt;/sub&gt;&lt;/em&gt;, and to get a fair impression of achievable rocket propellant performance and not merely theoretical maximum sea-level and vacuum specific impulse of propellants, or &lt;em&gt;I&lt;sub&gt;sp&lt;/sub&gt;&lt;/em&gt;, per weight. For the sake of having some direct quotes also in answers, please provide the following information:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Highest &lt;em&gt;I&lt;sub&gt;ssp&lt;/sub&gt;&lt;/em&gt; achieved by a complete launch vehicle configuration&lt;/strong&gt;, and &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Highest &lt;em&gt;I&lt;sub&gt;ssp&lt;/sub&gt;&lt;/em&gt; achieved by a single rocket stage&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For completeness, please also quote most relevant technical information about them, e.g. total impulse delivered by the mass of contained propellant, total (wet) mass of the propulsion system, propellants used, etc. To keep the length of answers manageable, we shall limit ourselves to only the highest performing launch vehicles and stages, but if you can also provide links to any such external launch vehicle and rocket stage comparisons by &lt;em&gt;I&lt;sub&gt;ssp&lt;/sub&gt;&lt;/em&gt; that is more complete, then even better.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: Some related lists that might help you answer my question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_rocket_engines&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Comparison of orbital rocket engines&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Comparison of orbital launch systems&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_launchers_families&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Comparison of orbital launchers families&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Kudos to &lt;a href=&quot;https://space.stackexchange.com/users/25/pearsonartphoto&quot;&gt;@PearsonArtPhoto&lt;/a&gt; for the first link that also enables sorting of rocket engines by their &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust-to-weight_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;thrust-to-weight ratio&lt;/a&gt;, which is essentially what this question is about and that does nearly all the work to answer the second part of my question. As same rocket engines might be used on different launch systems and their stage configurations, some additional work would have to be done, but it is an excellent start.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-08-24T13:20:27.213" Title="Which currently operational orbital launch vehicle achieves highest system-specific impulse?" Tags="|launch|rockets|fuel|stages|specific-impulse|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2972" PostTypeId="5" CreationDate="2013-12-01T04:18:35.633" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-01T04:18:35.633" LastActivityDate="2013-12-01T04:18:35.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2973" PostTypeId="4" CreationDate="2013-12-01T04:18:35.633" Score="0" Body="Questions regarding the steps and procedures taken prior and during the launch of a space vehicle." OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2015-02-01T00:35:32.163" LastActivityDate="2015-02-01T00:35:32.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2974" PostTypeId="5" CreationDate="2013-12-01T04:20:21.393" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-01T04:20:21.393" LastActivityDate="2013-12-01T04:20:21.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2975" PostTypeId="4" CreationDate="2013-12-01T04:20:21.393" Score="0" Body="Questions regarding the procedures in place to protect the area around a launch from damage in case of launch failure." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-01T04:20:21.393" LastActivityDate="2013-12-01T04:20:21.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2976" PostTypeId="1" AcceptedAnswerId="2977" CreationDate="2013-12-01T04:40:15.933" Score="7" ViewCount="298" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.facebook.com/isromom/posts/1402478439990096&quot; rel=&quot;noreferrer&quot;&gt;ISRO's Mars Orbiter Mission&lt;/a&gt; (MOM):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Forward rotation of the spacecraft&#xA;  has been completed successfully.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What kind of maneuver is that and what is its purpose?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-12-01T14:13:20.713" LastActivityDate="2013-12-01T14:13:20.713" Title="What is &quot;forward rotation&quot; of a spacecraft?" Tags="|attitude|spacecraft|isro|mars-orbiter-mission|rotation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2977" PostTypeId="2" ParentId="2976" CreationDate="2013-12-01T05:12:06.563" Score="8" Body="&lt;p&gt;According to a follow-up question in the comments to &lt;a href=&quot;https://www.facebook.com/isromom/posts/1402478439990096&quot; rel=&quot;noreferrer&quot;&gt;this ISRO's announcement on Facebook&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Rajesh B Nataraja&lt;/strong&gt;: Basic question? The forward rotation is to alter&#xA;  the thrusters in the right direction for the sling shot?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;ISRO's Mars Orbiter Mission&lt;/strong&gt;: You are absolutely right!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So this maneuver was about positioning the spacecraft in the direction for the last Trans-Mars injection burn, which, &lt;a href=&quot;https://www.facebook.com/photo.php?fbid=1402500009987939&amp;amp;set=a.1384034005167873.1073741827.1384015488503058&amp;amp;type=1&amp;amp;theater&quot; rel=&quot;noreferrer&quot;&gt;according to ISRO on Facebook&lt;/a&gt;, has been completed successfully:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/jfOjn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-12-01T05:12:06.563" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2978" PostTypeId="2" ParentId="2968" CreationDate="2013-12-01T09:34:10.473" Score="5" Body="&lt;p&gt;I suspect you're referring to articles like &lt;a href=&quot;http://www.spacenews.com/article/launch-report/35469with-ariane-6-launch-site-selected-cnes-aims-to-freeze-design-of-the-new&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;With Ariane 6 Launch Site Selected, CNES Aims To Freeze Design of the New Rocket in July&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Taking advantage of work done years ago on what was then a quarry, CNES officials have selected a site to the north of the Ariane 5’s launch site for Ariane 6, an area called Roche Nicole. Quarry construction left a large pit, now filled with water, that will be used for the Ariane 6 flame trench.&lt;/p&gt;&#xA;&lt;p&gt;CNES officials say that because of the quarry work, done to support launches of the now-retired Ariane 4 rocket, the flame trench is now the equivalent of 70 percent complete even though no work has begun on it.&lt;/p&gt;&#xA;&lt;p&gt;CNES managers at the Guiana Space Center spaceport, on the northeast coast of South America, recall the difficulties they had in completing the flame pit for Russia’s Soyuz rocket, which is operated from the European spaceport under an agreement with the Russian government that expires at the end of 2016.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Given the way it's worded, I think the launch site was always going to be at Kourou. The only question was where within the spaceport the new platforms would be built. Because Ariane 6 was conceived as a cost-saving measure, it'd make little sense to develop an entirely new spaceport, especially as Kourou is ideally located for launches to equatorial orbits.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-12-01T09:34:10.473" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="2979" PostTypeId="1" AcceptedAnswerId="2980" CreationDate="2013-12-01T11:36:04.447" Score="8" ViewCount="4295" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/2978/414&quot;&gt;This answer&lt;/a&gt; to a question states that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;.... construction left a large pit,&#xA;  now filled with water, that will be used&#xA;  for the Ariane 6 flame trench.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is a flame trench, and what is its purpose?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="58" LastEditDate="2020-12-01T16:51:01.797" LastActivityDate="2020-12-01T16:51:01.797" Title="What is a flame trench?" Tags="|launch|launchpad|construction|spaceport|flame-trench|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2980" PostTypeId="2" ParentId="2979" CreationDate="2013-12-01T11:42:03.523" Score="13" Body="&lt;p&gt;The Flame Trench is the big hole used to manage the flame from the rocket initially. Here's a shot of the &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/launch/Flame-trench-deflector.html&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle's Flame Trench&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z1xce.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A closer look at the flame trench at LC-39A from the &lt;a href=&quot;http://spaceksc.blogspot.com/&quot; rel=&quot;noreferrer&quot;&gt;Space KSC&lt;/a&gt; blog:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HB1HU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that essentially it deflects the flame so that it won't damage the rocket. All flame trenches have similar objectives. My favorite example comes actually from a missile (RIM-161 Standard Missile 3 or SM-3) test, in which the flame trench essentially works like an exhast pipe, which is easily seen below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://commons.wikimedia.org/wiki/File:USS_Lake_Erie_(CG-70)_SM-3_start.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/n958Z.jpg&quot; alt=&quot;Launching SM3&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="1142" LastEditDate="2015-02-17T02:06:30.157" LastActivityDate="2015-02-17T02:06:30.157" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2981" PostTypeId="2" ParentId="2969" CreationDate="2013-12-01T12:01:21.983" Score="8" Body="&lt;p&gt;I've studied this out, and found a few additional close calls. But there hasn't been any launches out of Europe itself, except for Russia. This is probably because Europe is too population dense to make a launch safe. Some information taken from &lt;a href=&quot;http://space.skyrocket.de/directories/launchsites.htm&quot;&gt;Gunter's space page&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;France has launched from Centre interarmées d’essais d’engins spéciaux (CIEES), Hammaguir, Algeria (Ha)&lt;/li&gt;&#xA;&lt;li&gt;Italy has launched from the &lt;a href=&quot;http://en.wikipedia.org/wiki/Broglio_Space_Centre&quot;&gt;San Marco Platform&lt;/a&gt;, located near Kenya.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, only Russia can claim to have launched from Europe itself to orbit.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-12-01T12:01:21.983" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="2982" PostTypeId="1" AcceptedAnswerId="2983" CreationDate="2013-12-01T20:37:41.577" Score="16" ViewCount="3480" Body="&lt;p&gt;While watching today's live stream of the launch of the &lt;a href=&quot;https://en.wikipedia.org/wiki/China_National_Space_Administration&quot; rel=&quot;noreferrer&quot;&gt;China National Space Administration&lt;/a&gt; (CNSA) &lt;a href=&quot;https://en.wikipedia.org/wiki/Long_March_3B&quot; rel=&quot;noreferrer&quot;&gt;Long March 3B&lt;/a&gt; rocket carrying &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_3&quot; rel=&quot;noreferrer&quot;&gt;Chang'e 3&lt;/a&gt; lunar exploration probe and the Yutu rover to the Moon, &lt;a href=&quot;https://space.stackexchange.com/users/288/everyone&quot;&gt;@Everyone&lt;/a&gt; has noticed (&lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/conversation/launch-of-the-long-march-3b-on-change-3-mission&quot;&gt;chat room transcript with to launch relevant links and video screengrabs&lt;/a&gt;) that the rocket exhaust appeared unusually reddish during early launch phase while the rocket was clearing the launch tower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rewatching some launch videos (&lt;a href=&quot;http://english.peopledaily.com.cn/202936/8471536.html&quot; rel=&quot;noreferrer&quot;&gt;with English commentary&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/watch?v=lgZslWEQZHY&quot; rel=&quot;noreferrer&quot;&gt;and without&lt;/a&gt;), I've established that this red gas only appears at the first few seconds of the burn of the first stage and booster engines, and the exhaust plume later appears &lt;em&gt;normal&lt;/em&gt;. Some cameras show it slightly more yellowish than others, but that could be, I guess, put down to calibration and color profile of individual cameras used for the transmission. Knowing what I was looking for, I found this nice photograph of the early launch phase of Long March 3B on Chang'e 3 mission:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/EnnAA.png&quot; alt=&quot;enter link description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Long March 3B rocket on Chang'e 3 mission lifting off and clearing the launch tower (Image credit: CCTV / CNSA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;em&gt;red gas&lt;/em&gt; somewhat reminds me of the cold expansion gas used to launch ballistic and cruise missiles, and if that's it, can also be seen during some rocket launches of into orbital launch vehicle converted ICBM (Intercontinental Ballistic Cruise Missile) systems, for example the silo-launched &lt;a href=&quot;https://en.wikipedia.org/wiki/Dnepr-1&quot; rel=&quot;noreferrer&quot;&gt;ISC Kosmotras' Dnepr-1&lt;/a&gt; rocket that pops out of an underground silo with the help of some red and presumably cold expansion gas and ignites its main engine mid-air (also seen in e.g. &lt;a href=&quot;https://www.youtube.com/watch?v=jsfL19FV3qg&quot; rel=&quot;noreferrer&quot;&gt;this YouTube video&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/0QhER.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Silo-launched Dnepr-1 rocket on &lt;a href=&quot;https://en.wikipedia.org/wiki/TerraSAR-X&quot; rel=&quot;noreferrer&quot;&gt;Terra SAR-X&lt;/a&gt; mission (&lt;a href=&quot;http://www.b14643.de/Spacerockets_1/East_Europe_3/Dnepr/Gallery/Dnepr_1.htm&quot; rel=&quot;noreferrer&quot;&gt;Source &amp;amp; gallery&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is the red gas at the exhaust of the Long March 3B rocket during liftoff, and does it have anything to do with that red expansion gas used for silo-launched and likely submarine-launched ICBMs and converted into orbital launch vehicle ICBMs? What is it, and why would the first stage or perhaps booster rockets of the Long March 3B use it?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="6944" LastEditDate="2020-09-12T18:03:54.300" LastActivityDate="2020-09-12T18:03:54.300" Title="What is this red gas at the exhaust of the Long March 3B rocket during liftoff?" Tags="|fuel|china|chang-e|long-march|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2983" PostTypeId="2" ParentId="2982" CreationDate="2013-12-01T21:02:22.663" Score="19" Body="&lt;p&gt;That's unburned &lt;span class=&quot;math-container&quot;&gt;$\mathrm{N}_2\mathrm{O}_4$&lt;/span&gt;.  The zeroth and first stages of the Long March use &lt;a href=&quot;http://www.astronautix.com/n/n2o4udmh.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;span class=&quot;math-container&quot;&gt;$\mathrm{N}_2\mathrm{O}_4$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\mathrm{UDMH}$&lt;/span&gt;&lt;/a&gt;.  The Dnepr uses the same propellants.  I doubt that the red gas you see is from the cold launch system.  That system uses a black powder mortar.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="3029" LastEditDate="2019-01-21T00:20:36.690" LastActivityDate="2019-01-21T00:20:36.690" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="2985" PostTypeId="1" AcceptedAnswerId="2986" CreationDate="2013-12-02T02:34:14.960" Score="15" ViewCount="2141" Body="&lt;p&gt;On the Mars Curiosity rover, there are black and white circular patterns or marks spaced around the rover, as seen here marked in lime green: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Vg8Le.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are these?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I had to guess, I'd wager they're points of reference, like to determine alignment of the rover and its parts, and/or the relative size/position/distance/focus of things, whether on-rover or in the surrounding environment. They also look very similar to patterns found on &quot;focus cards&quot; used for cameras.&lt;/p&gt;&#xA;" OwnerUserId="1131" LastEditorUserId="49" LastEditDate="2013-12-02T03:14:40.123" LastActivityDate="2013-12-02T03:24:31.613" Title="What are the black and white circular patterns on the Curiosity rover?" Tags="|mars|rovers|curiosity|photography|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2986" PostTypeId="2" ParentId="2985" CreationDate="2013-12-02T02:57:12.430" Score="14" Body="&lt;p&gt;They are called &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Fiducial_marker&quot; rel=&quot;nofollow noreferrer&quot;&gt;fiducial markers&lt;/a&gt;&lt;/em&gt;, or &lt;em&gt;fiducials&lt;/em&gt; for short, and serve as &lt;a href=&quot;https://space.stackexchange.com/a/2207/49&quot;&gt;calibration targets&lt;/a&gt; for onboard cameras and at the same time to allow the rover to measure itself and distance on photographs and other measuring devices, sensors, and mechanisms on the Curiosity rover by referencing two or more such marks as reference points, since their position and distance between them is known. The pattern itself can be also used to check for sharpness, focus and contrast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look carefully on the &lt;a href=&quot;https://space.stackexchange.com/q/2230/49&quot;&gt;self-portrait photograph of the Curiosity rover&lt;/a&gt;, you will also notice various color markers at the back, left from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;RTG&lt;/a&gt; (Radioisotope thermoelectric generator) that serve as color calibration targets, one such set of many installed on the rover. As an interesting fact, other such targets also include a &lt;a href=&quot;https://space.stackexchange.com/q/2200/49&quot;&gt;Lincoln cent&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-12-02T03:24:31.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2987" PostTypeId="1" AcceptedAnswerId="2990" CreationDate="2013-12-02T03:15:12.863" Score="9" ViewCount="741" Body="&lt;p&gt;Most are aware of the fact that the &lt;a href=&quot;http://www.thehindu.com/news/national/isro-calls-off-gslv-launch-after-fuel-leak/article5038402.ece&quot; rel=&quot;noreferrer&quot;&gt;launch of GSLV&lt;/a&gt; has been aborted due to a fuel leakage, but how was the cryogenic fuel leak discovered? I'm not interested in this case only, but in general.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What techniques are employed to find fuel leakages?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="66" LastEditDate="2014-01-21T16:48:50.873" LastActivityDate="2014-01-21T16:48:50.873" Title="How are fuel leaks discovered on a rocket?" Tags="|launch|rockets|fuel|construction|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2988" PostTypeId="2" ParentId="2914" CreationDate="2013-12-02T04:05:15.300" Score="11" Body="&lt;p&gt;In theory, the shortest practical time from spacecraft separation would be about five and a quarter hours, which is half of a geosynchronous transfer orbit.  At apogee, which is carefully placed to occur over the equator, a single burn raises the perigee and changes the inclination of the orbit, and you're there.  All early GEO birds used this approach, and many launches still do, especially from near the equator (Ariane, Sea Launch).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SES-8 went to a supersynchronous transfer orbit, which allows for much lower $\Delta V$ to change the inclination, since the spacecraft is going so much slower at apogee.  It would be 1800 m/s from GTO, but it's 1500 m/s from SSTO.  That includes some inclination assistance from the Falcon upper stage, which reduced the inclination from 28° in the parking orbit to 20.75° in SSTO.  The SSTO in this case is 295 km x 80000 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSTO approach takes longer, and there are several maneuvers required.  But it is usually well worth it to save that fuel for extending the useful life of the spacecraft.  Lifetime is money.  A supersynchronous transfer orbit was first used in 1992.  SES-8 will perform five maneuvers over two weeks to get into GEO.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-05T04:54:34.260" LastActivityDate="2013-12-05T04:54:34.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="2990" PostTypeId="2" ParentId="2987" CreationDate="2013-12-02T06:43:12.437" Score="10" Body="&lt;ul&gt;&#xA;&lt;li&gt;Pressure Drops. Generally there are multiple sensors which means you&#xA;can localise the area. &lt;/li&gt;&#xA;&lt;li&gt;Thermal Camera. Rocket Fuel is generally kept&#xA;cold. So any leaks are going to show up. &lt;/li&gt;&#xA;&lt;li&gt;Unexplained Gas Emissions.&#xA;It is quite easy to see venting of something from where it should not&#xA;be.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1076" LastActivityDate="2013-12-02T06:43:12.437" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2991" PostTypeId="2" ParentId="2969" CreationDate="2013-12-02T10:09:05.843" Score="6" Body="&lt;p&gt;During the time of USSR, Bulgaria participated in a space program &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_33&quot;&gt;Soyuz 33&lt;/a&gt;. Then the Bulgarian cosmonaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Georgi_Ivanov&quot;&gt;Georgi Ivanov&lt;/a&gt; together with his Russian colleague  &lt;a href=&quot;https://en.wikipedia.org/wiki/Nikolai_Rukavishnikov&quot;&gt;Nikolai Rukavishnikov&lt;/a&gt; were launched into space in 1979. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The scientific program for the flight was prepared entirely by&#xA;  Bulgarian scientists, along with some of the equipment&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;as said per Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch was successful, however, a severe technical problem prevented success of the entire mission as the space craft had to return back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having in mind this was not a successful operation, I am not sure if it answers your question, but the information might seem curious and probably less known. Besides, I am a Bulgarian :)&lt;/p&gt;&#xA;" OwnerUserId="1134" LastEditorUserId="1134" LastEditDate="2013-12-02T10:52:31.090" LastActivityDate="2013-12-02T10:52:31.090" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="2992" PostTypeId="1" CreationDate="2013-12-02T10:57:50.833" Score="11" ViewCount="2910" Body="&lt;p&gt;In &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_sail&quot;&gt;this Wikipedia article&lt;/a&gt; about solar sails, I read the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The force on a sail and the actual acceleration of the craft vary by&#xA;  the inverse square of distance from the sun (unless close to the&#xA;  sun), and by the square of the cosine of the angle between the&#xA;  sail force vector and the radial from the sun&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This was followed by the equations:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$F = \frac{F_0 \cos^2 \theta} {R^2}$ (perfect sail)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$F = \frac{F_0 (0.349 + 0.662 \cos{2\theta} − 0.011 \cos{4\theta}) } {R^2}$ (realistic sail)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that the second equation is an expansion of the first, taking into account realistic factors that affect the performance of the sail. However I have two doubts here.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What does &quot;angle between the sail force vector and the radial from the sun&quot; mean?&lt;/li&gt;&#xA;&lt;li&gt;What exactly are the realism factors the second equation adds to the first?&lt;/li&gt;&#xA;&lt;li&gt;BONUS: What's the best way to graph these equations in order to&#xA;better understand them?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;UPDATE: Could I also get clarification on the variables in the equations and their sources?&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2013-12-02T14:14:16.687" LastActivityDate="2015-02-11T04:13:43.710" Title="Solar sail force calculation: Angle vector" Tags="|mathematics|solar-sail|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="2993" PostTypeId="1" AcceptedAnswerId="2995" CreationDate="2013-12-02T14:19:36.400" Score="104" ViewCount="108244" Body="&lt;p&gt;In 1969, NASA not only went to the moon, but broadcast the whole thing live on TV.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did they achieve the TV broadcast? What technology did they need to use to send a video and audio signal from the moon to earth? Was there much of a lag?&lt;/p&gt;&#xA;" OwnerUserId="1136" LastEditorUserId="12102" LastEditDate="2019-02-17T22:16:42.023" LastActivityDate="2019-02-17T22:16:42.023" Title="How did NASA achieve their live TV broadcast in 1969?" Tags="|the-moon|crewed-spaceflight|apollo-program|communication|honeysuckle-creek|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="2994" PostTypeId="2" ParentId="2958" CreationDate="2013-12-02T14:36:01.603" Score="24" Body="&lt;p&gt;The big top rods are insulators. Such cages are grounded through wires sloping out to ground points several tens of meters away. Also the ground points are connected to each other by a potential equalization loop.  The metal lattices of the towers are grounded too, and also connected to the loop - but they are insulated from the tips, so in a strike they do not act as conductors.  There was one such insulator on the top of the Shuttle launch tower.  It was a spun fiber in polymer matrix structure, ad the time the biggest such cylinder in the world.  The tip was connected to three ground points. &lt;/p&gt;&#xA;" OwnerUserId="1137" LastActivityDate="2013-12-02T14:36:01.603" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="2995" PostTypeId="2" ParentId="2993" CreationDate="2013-12-02T14:40:11.057" Score="73" Body="&lt;p&gt;Apollo 11 mission had two modules&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;Lunar module&lt;/a&gt; - which descent to moon carrying two astronauts&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot; rel=&quot;noreferrer&quot;&gt;command / service module&lt;/a&gt;- CSM was designed to return astronauts from the lunar surface on a direct-descent mission to earth and splash down.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Direct telecast from the Command service module is not possible but CSM stored the recording of conversation which is transmitted by LM (which occur once in a rotation because the LM is stationary while CSM is orbiting the moon) then CSM sents to earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8LDDw.png&quot; alt=&quot;source:Wikipedia&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this image you can see the high gain antennas which are used in transmission of live data&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eZEar.jpg&quot; alt=&quot;source Wikipedia&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the actual camera used in &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_TV_camera&quot; rel=&quot;noreferrer&quot;&gt;Apollo&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Originally, these slow-scan television (SSTV) cameras, running at 10 frames-per-second (fps), produced only black and white pictures and first flew on the Apollo 7 mission in October 1968. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/apollo/40th/apollo11_audio.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt; states as&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The equipment onboard the Apollo&#xA;  Command Module that was used to&#xA;  make the recordings was called the&#xA;  Data Storage Equipment (DSE). Its&#xA;  contents were transmitted to the&#xA;  ground periodically during the mission.&#xA;  Also, the Command Module DSE had the&#xA;  capability to record data live during&#xA;  certain periods from the Lunar Module&#xA;  as it flew separately in lunar orbit.&#xA;  The equipment used aboard the Lunar&#xA;  Module to make the recordings was&#xA;  called the Data Storage Electronics&#xA;  Assembly (DSEA). It made recordings&#xA;  onboard the Lunar Module, but the&#xA;  DSEA flown on the Eagle during Apollo&#xA;  11 malfunctioned. As a result, many of&#xA;  its recordings are barely, if at all,&#xA;  audible, with a constant high-pitched&#xA;  background tone. In the attached&#xA;  database, the recordings that are&#xA;  virtually inaudible are highlighted, but&#xA;  they are available on the web to&#xA;  ensure a complete release of the&#xA;  recordings made during the mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.wired.com/wired/archive/15.01/nasa.html&quot; rel=&quot;noreferrer&quot;&gt;and&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The portion of&#xA;  the broadcast spectrum traditionally&#xA;  used for video was sending vital ship&#xA;  data to Earth, and there was no room&#xA;  left for the standard black-and-&#xA;  white video format of the era: 525&#xA;  scan lines of data at 30 frames per&#xA;  second, transmitted at 4.5 MHz. So&#xA;  Lebar helped devise a smaller &quot;oddball&#xA;  format&quot; – 320 scan lines at 10 fps,&#xA;  transmitted at a meager 500 kHz.&#xA;  Tracking stations back on Earth would&#xA;  take this so-called slow-scan footage,&#xA;  convert it for TV broadcast, and beam&#xA;  it to Mission Control, which would send&#xA;  it out for the world to see. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Transmission stations&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To ensure&#xA;  a direct transmission signal from the&#xA;  moon, NASA had to maintain stations&#xA;  in three continents – two in Australia&#xA;  (the Honeysuckle Creek Tracking&#xA;  Station near Canberra and the Parkes&#xA;  Radio Observatory surrounded by sheep&#xA;  paddocks west of Sydney); one at the&#xA;  Goldstone Deep Space Communications&#xA;  Complex in the Mojave Desert of&#xA;  California; and one at the Madrid&#xA;  Manned Flight Tracking Site in Spain........ the tracking stations with a direct line on the Apollo 's signal were the ones in Australia. The 200-foot-diameter radio dish at the Parkes facility managed to withstand freak 70 mph gusts of wind and successfully captured the footage, which was converted and relayed to Houston. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qgO8q.jpg&quot; alt=&quot;source: www.honeysucklecreek.net/Apollo_11/Earth_from_Moon_A11EVA.jpg &quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;location of receiver in earth&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="19089" LastEditDate="2017-03-27T19:10:55.727" LastActivityDate="2017-03-27T19:10:55.727" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="2996" PostTypeId="2" ParentId="2993" CreationDate="2013-12-02T14:59:31.583" Score="38" Body="&lt;p&gt;From &lt;a href=&quot;http://www.theatlantic.com/video/archive/2012/07/1-small-step-for-a-cam-how-astronauts-shot-video-of-the-moon-landing/260122/&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;, and &lt;a href=&quot;http://www.nasa.gov/mission_pages/apollo/40th/apollo11_conference_prt.htm&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Live television was transmitted from the moon to 3 grounds stations,&#xA;  two in Australia and one in California. The signal was converted to a&#xA;  standard broadcast signal and then sent to Houston, via, satellite,&#xA;  landline or microwave antenna. These graphics show the path of the&#xA;  television feed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iCLEZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the video was transmitted to one of the large dishes that the DSN uses, and then re-broadcast to the world using standard worldwide broadcasting methods. Specifically, a format was used called slow scan, which according to &lt;a href=&quot;http://www.nasa.gov/mission_pages/apollo/40th/apollo11_conference_prt.htm&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The SSTV system used in NASA's early Apollo missions transferred ten&#xA;  frames per second with a resolution of 320 frame lines using less&#xA;  bandwidth than a normal TV transmission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-12-02T14:59:31.583" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="2997" PostTypeId="1" AcceptedAnswerId="3024" CreationDate="2013-12-02T15:15:43.057" Score="14" ViewCount="5232" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rgcoC.jpg&quot; alt=&quot;Artists depiction of the Japanese solar sail IKAROS&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;http://www.planetary.org/explore/projects/lightsail-solar-sailing/what-is-solar-sailing.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; on &lt;a href=&quot;http://www.planetary.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Planetary Society&lt;/a&gt; website, I read this about solar sails:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;At an acceleration rate of 1 millimeter per second per second (20&#xA;times greater than the expected acceleration for Cosmos 1), a solar&#xA;sail would increase its speed by approximately 310 kilometers per hour&#xA;(195 mph) after one day, moving 7500 kilometers (4700 miles) in the&#xA;process. After 12 days it will have increased its speed 3700&#xA;kilometers per hour (2300 mph).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I find these numbers quite staggering, as they open up a variety of avenues in interstellar travel.&lt;/p&gt;&#xA;&lt;p&gt;Now I know that this acceleration won't quite be constant, becuase as the craft travels away from the Sun, the force exerted by light decreases. &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;This article&lt;/a&gt; highlights this by saying:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The force on a sail and the actual acceleration of the craft vary by&#xA;the inverse square of distance from the sun (unless close to the&#xA;sun), and by the square of the cosine of the angle between the&#xA;sail force vector and the radial from the sun, so&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$F = F_0 \frac{\cos^2 θ}{R^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;(perfect sail) and&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$F = F_0 \frac{(0.349 + 0.662 \cos{2θ} − 0.011 \cos{4θ})}{R^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;(realistic sail)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I have asked for a detailed explanation of this equation in &lt;a href=&quot;https://space.stackexchange.com/questions/2992/solar-sail-force-calculation-angle-vector&quot;&gt;my question&lt;/a&gt;, but their general meaning is that the light force on the sail is inversely proportional to the distance between the sail and the Sun.&lt;/p&gt;&#xA;&lt;p&gt;Finally, this same article calculates the light force on a solar sail at 1 AU from the Sun thusly:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The momentum of a photon or an entire flux is given by &lt;span class=&quot;math-container&quot;&gt;$p = \frac{E}{c}$&lt;/span&gt;, where &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt; is the photon or flux energy, &lt;span class=&quot;math-container&quot;&gt;$p$&lt;/span&gt; is the momentum, and &lt;span class=&quot;math-container&quot;&gt;$c$&lt;/span&gt; is the speed of light. Solar radiation pressure is calculated on an irradiance (solar constant) value of &lt;span class=&quot;math-container&quot;&gt;$1361\ W/m^2$&lt;/span&gt; at 1 AU (earth-sun distance), as revised in 2011: An actual sail will have an overall efficiency of about 90%, about &lt;span class=&quot;math-container&quot;&gt;$8.25\ μN/m^2$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;How can I combine these three factors: Force imparted, distance to the sun, and the resulting acceleration, and derive an equation giving &lt;span class=&quot;math-container&quot;&gt;$v$&lt;/span&gt; = velocity of craft, with all the factors accounted for?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE for clarity:&lt;/strong&gt; I am NOT looking for a differential/integrated equation which actually calculates the final velocity as the craft moves. I am looking for an equation with velocity on one side, and the force, distance and initial velocity etc. on the other, an equation into which I can plug the values in manually to get the velocity &lt;em&gt;at a given time&lt;/em&gt;. So no calculus please, I want a simple velocity vector formula.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-06-15T11:23:48.053" Title="Can I derive a combined equation for velocity of solar sail?" Tags="|propulsion|solar-sail|mathematics|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3000" PostTypeId="1" CreationDate="2013-12-02T18:42:59.927" Score="2" ViewCount="91" Body="&lt;p&gt;Where does space start? If space starts right beyond the solid part of a planet, then wouldn't Jupiter not be a planet? And if it starts at the end of the atmosphere, at what point does one escape the atmosphere? Most of all, if it is where the gravitational influence ends, at what point of effect does it &quot;end?&quot;&lt;/p&gt;&#xA;" OwnerUserId="933" LastActivityDate="2013-12-02T18:42:59.927" Title="Where does a planet end and space begin?" Tags="|altitude|" AnswerCount="0" CommentCount="2" ClosedDate="2013-12-02T18:54:27.173" ContentLicense="CC BY-SA 3.0" />
  <row Id="3001" PostTypeId="2" ParentId="2993" CreationDate="2013-12-02T21:03:22.127" Score="19" Body="&lt;p&gt;The slow-scan signal mentioned in the other replies had to be converted to broadcast video. This was done by &lt;a href=&quot;http://www.hq.nasa.gov/alsj/SMPTE-79-7-1970.pdf&quot;&gt;playing back&lt;/a&gt; (pdf) the slow-scan video on a kinescope, and reading the image with a broadcast TV camera. The setup also involved a magnetic video disc recorder which briefly stored each frame of video (so that one recorded frame could be used to fill 3 frames of broadcast video).&lt;br&gt;&#xA;This was a bit of a Rube Goldberg setup, and accounts for the mediocre quality of many of the early images we have, esp. of Apollo 11, where the output of the scan converter survived but the input (with potentially higher quality, if converted on a better system) &lt;a href=&quot;http://www.apollo11video.com/&quot;&gt;was lost&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-12-02T21:03:22.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3002" PostTypeId="1" AcceptedAnswerId="3003" CreationDate="2013-12-02T21:27:20.440" Score="20" ViewCount="7358" Body="&lt;p&gt;Based on the aborted launch on Thanksgiving 2013 of the first Falcon 9 v1.1 from Florida, we know that an issue occurred where &lt;a href=&quot;https://twitter.com/elonmusk/status/406806983023820800&quot; rel=&quot;noreferrer&quot;&gt;oxygen got into the ground based TEA-TEB sources&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that TEA-TEB are pyrophoric , which means they combust on contact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX reported they &lt;a href=&quot;https://twitter.com/elonmusk/status/407313331247398912&quot; rel=&quot;noreferrer&quot;&gt;replaced the gas generator on the center engine.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This sounds like they might have experienced the issue on the center engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I was wondering, do they ignite the center engine with pyrophorics, and then the other 8 all ignite, since there is a 145,000lb thrust fire going on just feet away?  Or do they use TEA-TEB on all 9 engines during ignition?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-03-27T03:07:17.437" LastActivityDate="2017-03-27T03:07:17.437" Title="How does SpaceX ignite their engines?" Tags="|spacex|falcon-9|engines|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3003" PostTypeId="2" ParentId="3002" CreationDate="2013-12-02T21:45:59.937" Score="29" Body="&lt;p&gt;Yes, they use TEA-TEB (&lt;a href=&quot;https://en.wikipedia.org/wiki/Triethylaluminium&quot; rel=&quot;noreferrer&quot;&gt;Triethylaluminum&lt;/a&gt;-&lt;a href=&quot;https://en.wikipedia.org/wiki/Triethylborane&quot; rel=&quot;noreferrer&quot;&gt;Triethylborane&lt;/a&gt;) as their first stage ignitor and this mixture is &lt;a href=&quot;https://en.wikipedia.org/wiki/Pyrophoricity&quot; rel=&quot;noreferrer&quot;&gt;pyrophoric&lt;/a&gt; (i.e. &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030068416.pdf&quot; rel=&quot;noreferrer&quot;&gt;hypergolic in contact with oxygen&lt;/a&gt;, be it atmospheric or LOX), but no, they don't ignite one engine and let that one ignite all the others, they would want to ignite them all at once. Problem with one engine igniting others is that doing so would create a pressure spike that would likely damage the engine and, according to Ben Brockert, CEO of &lt;a href=&quot;http://ablesc.com/&quot; rel=&quot;noreferrer&quot;&gt;Able Space Co.&lt;/a&gt;, Denver:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The fire would then travel up into the engine where it would create a&#xA;  massive pressure spike, definitely destroying the engine and possibly&#xA;  destroying engines nearby. In propulsion testing the euphemism for&#xA;  this is a &quot;hard start&quot; leading to &quot;rapid unplanned disassembly&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Quote source and more information via Joseph Abbott's Joe Science blog post: &lt;a href=&quot;http://www.wacotrib.com/blogs/joe_science/the-tea-teb-glitch-can-t-light-a-falcon-without/article_1b7c4ae6-5a16-11e3-afbb-0019bb2963f4.html&quot; rel=&quot;noreferrer&quot;&gt;The TEA-TEB glitch: Can't light a Falcon 9 without a spark&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-03-19T12:38:12.437" LastActivityDate="2016-03-19T12:38:12.437" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3004" PostTypeId="1" AcceptedAnswerId="23897" CreationDate="2013-12-03T00:43:21.700" Score="27" ViewCount="20806" Body="&lt;p&gt;Linear &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerospike_engine&quot;&gt;aerospike engines&lt;/a&gt; are an old idea that seem so full of promise.  Why are they not widely used today by the likes of Boeing, SpaceX, etc.?&lt;/p&gt;&#xA;" OwnerUserId="1165" LastEditorUserId="49" LastEditDate="2015-07-09T12:23:28.453" LastActivityDate="2019-10-21T10:07:16.653" Title="Why aren't linear aerospike engines in common use?" Tags="|propulsion|engine-design|spacecraft-development|feasibility|advanced-propulsion|" AnswerCount="8" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3005" PostTypeId="1" AcceptedAnswerId="3006" CreationDate="2013-12-03T01:38:54.753" Score="5" ViewCount="302" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/VentureStar&quot;&gt;VentureStar&lt;/a&gt; seemed to make a bit of progress before being cancelled in 2001. Did any of the technology developed for this program lead anywhere? For example, its aerospike engines or high-tech fuel tanks? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that pretty much all rocket technology after then didn't deviate too far from previous generations. &lt;/p&gt;&#xA;" OwnerUserId="1164" LastEditorUserId="49" LastEditDate="2015-07-20T18:29:34.467" LastActivityDate="2015-07-20T18:29:34.467" Title="Did VentureStar technology lead anywhere?" Tags="|rockets|nasa|ssto|lockheed-martin|canceled-project|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3006" PostTypeId="2" ParentId="3005" CreationDate="2013-12-03T02:37:23.553" Score="5" Body="&lt;p&gt;Many lessons learned from that activity are being applied to &lt;a href=&quot;http://www.nasa.gov/press/2013/july/nasa-tests-game-changing-composite-cryogenic-fuel-tank/&quot; rel=&quot;nofollow&quot;&gt;ongoing composite tank work&lt;/a&gt; at NASA.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-12-03T02:37:23.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3008" PostTypeId="2" ParentId="3004" CreationDate="2013-12-03T05:09:17.210" Score="4" Body="&lt;p&gt;The answer is simple &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;lack of proven reliability&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;lack of proven flight experience&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;lack of performance validation..&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;more surface area exposed to hot gas . So it must be cooled which is difficult ..&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastEditorUserId="414" LastEditDate="2013-12-03T05:21:41.147" LastActivityDate="2013-12-03T05:21:41.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3009" PostTypeId="2" ParentId="3004" CreationDate="2013-12-03T05:28:11.307" Score="16" Body="&lt;p&gt;Money. Most engine designs we're now using are an evolution of the space race era in one form or another, from the times when financing research in rocket engine / nozzle design wasn't such an issue. Linear spike fundamentally changes rocket design, for one engine support structure, and would as such require a revolution in engineering if someone expects it on the cheap. Steady evolution simply isn't good enough, the changes required are too big.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So either rocket engine research &amp;amp; development would get a huge injection of money from somewhere, or we reach some engineering breakthrough that doesn't require so much of it, like maybe more reliable engineering simulators (&lt;a href=&quot;http://aerorocket.com/MOC/MOC.html&quot;&gt;one fine example&lt;/a&gt;, but not enough). Short of that, designing, building and testing cycles are simply too expensive. Engineering side of it is challenging, but no engineer will ever tell you it's impossible.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-12-03T05:28:11.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3010" PostTypeId="1" AcceptedAnswerId="3011" CreationDate="2013-12-03T08:31:29.443" Score="14" ViewCount="21328" Body="&lt;p&gt;I know there are some awesomely far spacecraft that have been travelling for decades away from earth, but which is the farthest? And has it discovered something amazing?&lt;/p&gt;&#xA;" OwnerUserId="1180" LastEditorUserId="58" LastEditDate="2013-12-03T21:07:47.830" LastActivityDate="2015-12-10T16:52:32.887" Title="What is the farthest a spacecraft has traveled away from earth?" Tags="|spacecraft|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3011" PostTypeId="2" ParentId="3010" CreationDate="2013-12-03T08:44:43.727" Score="30" Body="&lt;p&gt;Wikipedia states that &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1&lt;/a&gt; is currently farthest from Earth; it gives the distance as of Dec 3 2013 as 126.95 AU ($1.899 \cdot 10^{10}\text{ km}$) from Earth. At that distance, the speed of light delay is approximately 17 hours 36 minutes. By Dec 7 2013 it was listed as at a distance of 127.03 AU from Earth, which means it is moving away from Earth at a speed of 0.02 AU per day, corresponding to a speed (relative to Earth) of roughly 35 km/s. The speed relative to the sun is stated as approximately 17 km/s. The probe was launched, along with its sister &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 2&lt;/a&gt;, in August 1977; Wikipedia states that Voyager 2's current speed relative to the sun is slightly above 15.4 km/s at a distance from Earth of 103.00 AU as of Dec 7 2013. Since you're asking about the &lt;em&gt;farthest&lt;/em&gt;, I focus on Voyager 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exactly what might count as &quot;amazing discoveries&quot; is of course a matter of personal opinion rather than a strict scientific assessment, but Voyager 1 does seem to have contributed to &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1#Interstellar_medium&quot; rel=&quot;nofollow noreferrer&quot;&gt;our understanding of the outer limits of the heliosphere&lt;/a&gt;. As &lt;a href=&quot;https://space.stackexchange.com/users/49/tildalwave&quot;&gt;TildalWave&lt;/a&gt; pointed out in a comment (since deleted), it has also provided evidence of many other things, including that it has:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Provided evidence of &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Vulcanic_Explosion_on_Io.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;active volcanoes on celestial bodies other than Earth&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Sent us close-up pictures of Saturn's ring system, including detail pictures of &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Voyager1-saturn-f-ring.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn's F ring&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Proven to still be working (although &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1#Scientific_instruments&quot; rel=&quot;nofollow noreferrer&quot;&gt;much of the on-board equipment has either been deactivated or malfunctioned&lt;/a&gt;) 36 years after launch, and possible to communicate with across such vast distances, and sending useful scientific data &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1#Exit_from_the_heliosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;having exited the solar system&lt;/a&gt;; it is expected to remain operational until some time around 2025, for a nearly 50 years long active service life, whereas &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2#Power&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 2 is expected to remain operational until around 2020&lt;/a&gt; for 40-45 years of service&lt;/li&gt;&#xA;&lt;li&gt;Sent us the &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Pale_Blue_Dot.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Pale Blue Dot&quot;&lt;/a&gt; picture, showing Earth &lt;a href=&quot;http://www.planetary.org/explore/space-topics/earth/pale-blue-dot.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;from 6.4 billion km&lt;/a&gt; (42.8 AU) away&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/q/13227/14091&quot;&gt;The feat of communicating with the spacecraft&lt;/a&gt; (&lt;a href=&quot;https://space.stackexchange.com/q/958/415&quot;&gt;also here&lt;/a&gt;) might actually be quite significant in and of itself; Voyager 1 communicates with Earth (through the Deep Space Network) on &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1#Communication_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;frequencies near 2.3 GHz and 8.4 GHz&lt;/a&gt;. Unless I'm getting the calculations wrong (although the numbers do appear to be within the ballpark), &lt;a href=&quot;https://ham.stackexchange.com/a/353/29&quot;&gt;free space propagation losses alone&lt;/a&gt; is in the neighborhood of 300 dB on 2.3 GHz and 320 dB on 8.4 GHz. Communications with Voyager &lt;a href=&quot;https://space.stackexchange.com/q/9824/415&quot;&gt;is done&lt;/a&gt; with about 20 kW uplink into the DSN's large antennas.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:20.687" LastActivityDate="2015-07-10T22:18:36.207" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3012" PostTypeId="1" AcceptedAnswerId="3013" CreationDate="2013-12-03T11:08:05.070" Score="43" ViewCount="77508" Body="&lt;p&gt;On &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon_landing&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia page on Moon landing&lt;/a&gt;, Neil Armstrong is shown stepping down on the surface of the Moon using the spaceship's ladder:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;      &lt;img src=&quot;https://i.stack.imgur.com/UxIuW.jpg&quot; alt=&quot;One small step for Man&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Still frame from a video transmission, taken moments before Neil Armstrong became the first human to step onto the surface of the Moon, at 02:56 UTC on 21 July 1969. An estimated 500 million people worldwide watched this event, the largest television audience for a live broadcast at that time. (Photograph and caption source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon_landing&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Moon landing&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The camera is installed outside the spaceship, on the Moon's surface. Who put the camera there?&lt;/p&gt;&#xA;" OwnerUserId="568" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-06-19T20:02:46.060" Title="Who installed the camera on the Moon to film the &quot;First step&quot;?" Tags="|the-moon|apollo-program|camera|myth-busting|moonwalking|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3013" PostTypeId="2" ParentId="3012" CreationDate="2013-12-03T11:30:26.887" Score="67" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Neil_Armstrong&quot; rel=&quot;noreferrer&quot;&gt;Neil Armstrong&lt;/a&gt;'s &quot;First step on the Moon&quot; was filmed by a camera installed on the MESA (Modularized Equipment Stowage Assembly) at the side of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;Apollo Lunar Module&lt;/a&gt; (LM) descent stage that Neil Armstrong had to pull a lanyard to unlock the pallet and make it drop open. A switch inside the LM, operated by &lt;a href=&quot;https://en.wikipedia.org/wiki/Buzz_Aldrin&quot; rel=&quot;noreferrer&quot;&gt;Buzz Aldrin&lt;/a&gt;, then activated the TV camera which was installed there:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/n7if4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Modularized Equipment Stowage Assembly installed and deployed open on the Apollo Lunar Module Descent Stage (Photo: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information and photo galleries on Apollo TV are available on:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.myspacemuseum.com/mesa.htm&quot; rel=&quot;noreferrer&quot;&gt;Apollo Lunar Module Descent Stage &#xA;Modularized Equipment Stowage Assembly page&lt;/a&gt; of &lt;a href=&quot;http://www.myspacemuseum.com/&quot; rel=&quot;noreferrer&quot;&gt;My Little Space Museum&lt;/a&gt;, &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_TV_camera&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Apollo TV camera&lt;/a&gt;, &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.lpi.usra.edu/lunar/missions/apollo/apollo_11/photography/index.shtml#cameras&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11 Mission Photography page&lt;/a&gt; by &lt;a href=&quot;http://www.lpi.usra.edu/&quot; rel=&quot;noreferrer&quot;&gt;Lunar and Planetary Institute&lt;/a&gt;, &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.hq.nasa.gov/alsj/ApolloTV-Acrobat5.pdf&quot; rel=&quot;noreferrer&quot;&gt;Apollo Television&lt;/a&gt; (PDF) by Bill Wood, former Apollo &lt;a href=&quot;https://en.wikipedia.org/wiki/Manned_Space_Flight_Network&quot; rel=&quot;noreferrer&quot;&gt;MSFN&lt;/a&gt; (Manned Space Flight Network) station engineer,&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;and of course in answers to the question on &lt;a href=&quot;https://space.stackexchange.com/q/2993/49&quot;&gt;How did NASA achieve their live TV broadcast in 1969&lt;/a&gt;, here in our &lt;a href=&quot;https://space.stackexchange.com/about&quot;&gt;Space Exploration Q&amp;amp;A&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-12-03T13:07:49.873" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3014" PostTypeId="2" ParentId="1171" CreationDate="2013-12-03T16:20:51.403" Score="4" Body="&lt;p&gt;The explanation of nozzles working by decreasing the velocity of flow is not correct. Nozzles ALWAYS accelerate the flow passing through them. One must first understand what kind of flow is passing through the nozzle. If the nozzle inlet flow is moderately subsonic, the flow will accelerate at the nozzle throat (the minimum area) and impart the highest momentum to the gas. Since thrust is based largely on momentum, no expansion downstream of the throat is necessary (just look at commercial jet engine nozzles) and the gas is expelled to the atmosphere. It also holds that the gas pressure will be its lowest at the throat. Now if the nozzle inlet flow speed is high enough, the flow will accelerate to Mach 1 at the throat. In this case, the flow becomes supersonic and things happen in reverse. In supersonic flow, expanding area causes acceleration and pressure drop, opposite to that for subsonic flow. So after the flow reaches Mach 1 at the throat, an expanding area is needed downstream to further accelerate it. This is why nozzles expand and do so in such a way so as to achieve a desired exit velocity. As for the internal pressure distribution inside the supersonic nozzle, it continues to drop over the length of the nozzle downstream of the throat. Again, this is opposite to what happens for subsonic flow. The pressure forces provide some thrust, but not much. The pressure component of thrust is relatively insignificant compared to the momentum contribution from the accelerated flow. And incidentally, except for detonation-based engines, the nozzle inlet flow from any engine (jet or rocket) is always subsonic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This statement about nozzle expansion, however, is correct enough for this audience:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In designing the nozzle of real rockets, there is a balance to be struck; at lower altitude, the atmospheric pressure is higher, so the best nozzle shape would widen less than a nozzle for use in space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1193" LastEditorUserId="58" LastEditDate="2013-12-05T20:18:04.823" LastActivityDate="2013-12-05T20:18:04.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3015" PostTypeId="1" AcceptedAnswerId="3016" CreationDate="2013-12-03T16:25:00.667" Score="25" ViewCount="15243" Body="&lt;p&gt;There was a recent update on &lt;a href=&quot;https://www.facebook.com/photo.php?fbid=1403573953213878&amp;amp;set=a.1384034005167873.1073741827.1384015488503058&amp;amp;type=1&quot;&gt;ISRO's Mars Orbiter Mission's (MOM) Facebook page&lt;/a&gt; that it has crossed the Earth's Sphere of Influence (SOI):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;MOM moves on!&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;MOM is going to cruise out of the sphere of Influence of Earth in&#xA;  about 4 hours from now! we will keep you posted. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Sphere of Influence Explained:&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Sun is much more massive than any of the planets and its gravity&#xA;  dominates the Solar System. Only very near to the planets, the&#xA;  planetary gravity become stronger than that of the Sun. This region&#xA;  around the planet is referred to as the Sphere of Influence (SOI) of&#xA;  that planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;My questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;How to calculate the Earth's gravitational Sphere of Influence (SOI)?&lt;/strong&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;How far does the Earth's SOI reach from the center of the Earth?&lt;/strong&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2013-12-03T16:41:51.967" LastActivityDate="2017-10-07T07:36:54.233" Title="How large is the Earth's gravitational Sphere of Influence and how can it be calculated?" Tags="|orbital-mechanics|isro|mars-orbiter-mission|soi|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3016" PostTypeId="2" ParentId="3015" CreationDate="2013-12-03T17:12:00.373" Score="15" Body="&lt;p&gt;There are a couple of definitions, but the most useful one is called the &lt;a href=&quot;http://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;noreferrer&quot;&gt;Hill Sphere&lt;/a&gt;. Essentially this is the area around which one can orbit around an object, and not be pulled away by another object (Such as the Sun). As the linked article states, it can be &lt;strong&gt;calculated for objects that have one object much more massive than the other&lt;/strong&gt; (Almost every case of interest) by using the following formula:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$r \approx a (1-e) \sqrt[3]{\frac{m}{3 M}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $e$ is the eccentricity of the orbit, $m$ is the less massive object, and $M$ is the more massive object, and $a$ is the semi-major axis (Distance between objects).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a circular orbit, which makes the math easier, gives that distance to be :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$r \approx a  \sqrt[3]{\frac{m}{3 M}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's Hill Sphere is around 1,500,000 km, as shown on this graph from &lt;a href=&quot;http://en.wikipedia.org/wiki/File:Hill_sphere_of_the_planets.png&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FPxQJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2013-12-04T16:21:50.197" LastActivityDate="2013-12-04T16:21:50.197" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3018" PostTypeId="1" CreationDate="2013-12-03T19:14:57.440" Score="7" ViewCount="221" Body="&lt;p&gt;Rereading my answer &lt;a href=&quot;https://space.stackexchange.com/questions/2237/how-long-will-it-be-light-on-venus-at-night/2257#2257&quot;&gt;here&lt;/a&gt; and in particular, rereading the article &lt;a href=&quot;http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469%281968%29025%3C0666:OARROV%3E2.0.CO;2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Optical and Radio Refraction on Venus&lt;/a&gt; (Stratton, 1968) it is my understanding that there is significant refraction of radio waves, as with visible light - leading to critical refraction (explained in the linked answer).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is then, how would this radio wave refraction be corrected to be useful for potential future communications? &lt;/p&gt;&#xA;" OwnerDisplayName="user838" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2013-12-04T14:00:49.257" Title="How would critical refractivity of radio waves on Venus be overcome?" Tags="|communication|venus|atmosphere|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3019" PostTypeId="1" AcceptedAnswerId="3022" CreationDate="2013-12-03T21:04:57.993" Score="5" ViewCount="1163" Body="&lt;p&gt;What is the furthest an animal (humans included) has been sent away from Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not counting bacterial contamination or similar things, because it will make Voyagers the hands down winners again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect it should have been the Soviet moon turtles, but surely there can be something else.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-12-04T17:42:26.453" Title="What is the furthest an animal has been sent away from Earth?" Tags="|history|record|animals|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3021" PostTypeId="2" ParentId="3018" CreationDate="2013-12-03T21:54:36.717" Score="5" Body="&lt;p&gt;One could use &quot;very low frequency&quot;$^1$ communications from surface to satellite like the Venera 9 lander used to communicate with the orbiter, then have the Venusian satellite relay information back to Earth. Venera 9 lander used 122.8 and 138.6 MHz. I'm not a communications expert, but it is my understanding that lower frequencies experience less refraction. According to my source, it appears that Venera 9 &lt;em&gt;had&lt;/em&gt; to use these very low frequencies; I would assume for this very reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.svengrahn.pp.se/radioind/MVradio/MVradio.htm&quot; rel=&quot;nofollow&quot;&gt;Radio systems of Soviet Mars and Venus probes - Sven Grahn&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;$^1$My source refers to the communications as very low frequency but says that the numbers cited in Russian papers are 122.8 and 138.6 MHz which are in the range typically referred to as Very High Frequency (VHF).&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-12-04T14:00:49.257" LastActivityDate="2013-12-04T14:00:49.257" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3022" PostTypeId="2" ParentId="3019" CreationDate="2013-12-03T22:22:36.730" Score="5" Body="&lt;p&gt;The animals sent farthest from the Earth were the tortoises (and other creatures, see below) on Zond 7. They were sent roughly 407,000 km from the center of the Earth (possibly more depending on whether the distance of Zond 7 from the moon was calculated from the surface or the center).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding what was on board, there is an overview paper &quot;Results of Biological Investigations Undertaken on the Zond-5, Zond-6 and Zond-7 Stations&quot; Gazenko, O. G.; Antipov, V. V.; Parfenov, G. P. Cosmic Research, Vol. 9, p.548 which is a translation from Russian &quot;Kosmicheskie Issledovania&quot; journal. It states that Zond 5 and 7 were identical by animal selection. It included: Turtles, Drosophila, Tradescantia, Allium cepa (onion), dry seeds of Rye, different strains of Chlorella, and E. coli and other biological objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Human tissue samples were also on board according to another paper (Zhukov-Verezhnikov, et al.).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://46blyz.com/2012/02/05/tortoises-in-space&quot; rel=&quot;nofollow&quot;&gt;Tortoises in Space - 46BLYZ&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1969-067A&quot; rel=&quot;nofollow&quot;&gt;Zond 7 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.fourmilab.ch/earthview/pacalc.html&quot; rel=&quot;nofollow&quot;&gt;Lunar Perigee and Apogee Calculator - John Walker&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/11942358&quot; rel=&quot;nofollow&quot;&gt;Experiments with micro-organisms and human cell cultures in the Zond 5 and Zond 7 flights - Zhukov-Verezhnikov NN, Volkov MN, Maisky IN, Rybakov NI, Guberniev MA, Podoplelov II, Kulagin AN, Aniskin ED, Rybakova KD, Sharyi NI, Voronkova IP, Saxonov PP, Kopyev VY, Antipov VV, Kozlov VA, Parfyonov GP, Orlovsky VI&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-12-04T17:42:26.453" LastActivityDate="2013-12-04T17:42:26.453" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3023" PostTypeId="1" AcceptedAnswerId="3025" CreationDate="2013-12-03T22:51:39.963" Score="12" ViewCount="1920" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/VentureStar&quot; rel=&quot;nofollow noreferrer&quot;&gt;VentureStar&lt;/a&gt; was (IMHO) a pretty cool design; a reusable space plane designed for low running costs and rapid turn-around seems like just what the world needs. However, it was cancelled in 2001 due to (as I remember it) leaky tanks and excessive weight. Much of the technology was a bit too ambitious for the time.  Given the technology of today and the &lt;a href=&quot;https://space.stackexchange.com/questions/3005/did-venturestar-technology-lead-anywhere&quot;&gt;progress made since then&lt;/a&gt; does the design still make sense and could it be built?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, was the design a product of NASA being obsessed with space planes rather than simpler, less sexy rockets?&lt;/p&gt;&#xA;" OwnerUserId="1164" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-08-11T22:20:23.760" Title="Was VentureStar a bad design or simply ahead of its time?" Tags="|rockets|ssto|lockheed-martin|canceled-project|venturestar|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3024" PostTypeId="2" ParentId="2997" CreationDate="2013-12-04T00:37:08.480" Score="12" Body="&lt;p&gt;First, we need an equation for the acceleration:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$F = \frac{F_0}{R^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;One thing to note is that this equation is missing a factor (the units don't work out). Technically, &lt;span class=&quot;math-container&quot;&gt;$F_0$&lt;/span&gt; needs to be multiplied by &lt;span class=&quot;math-container&quot;&gt;$1\ AU^2$&lt;/span&gt;, that way the units cancel out properly.&lt;/p&gt;&#xA;&lt;p&gt;Assuming an optimal angle, so:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$A = \frac{F_0 \cdot 1AU^2}{M\cdot R^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Where &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; is the mass of our probe. Your cited &lt;span class=&quot;math-container&quot;&gt;$F_0$&lt;/span&gt; is 8.25 µN per square meter of sail. so:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$A = \frac{8.25\cdot 10^{-6} N/m^2 \cdot \text{Sail Area} \cdot 1 AU^2}{\text{Mass} \cdot R^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;We will need to convert our &lt;span class=&quot;math-container&quot;&gt;$\text{Sail Area}$&lt;/span&gt; to &lt;span class=&quot;math-container&quot;&gt;$AU^2$&lt;/span&gt; in order to work with that force equation and have units properly cancel.&lt;/p&gt;&#xA;&lt;p&gt;However, we can rewrite this as:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$A = \frac{K}{R^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Where &lt;span class=&quot;math-container&quot;&gt;$K$&lt;/span&gt; is just a constant based on the &lt;span class=&quot;math-container&quot;&gt;$\text{Sail Area}$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\text{Mass}$&lt;/span&gt; (I would have used &lt;span class=&quot;math-container&quot;&gt;$C$&lt;/span&gt;, but worried people might mix it up with the speed of light).&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$K = \frac{8.25*10^{-6} N/m^2 \cdot \text{Sail-Area} \cdot 1 Au^2}{\text{Mass}}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;This means we are going to have to deal with differential equations.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$A = \frac{d^2R}{dt^2} = \frac{K}{R^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$A = \frac{dV}{dT}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{d^2R}{dt^2} = \frac{dV}{dT} = \frac{dV}{dR} \cdot \frac{dR}{dT} = \frac{dV}{dR} \cdot V$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{dV}{dR} \cdot V = \frac{K}{R^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;so&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$dv*V = \frac{K}{R^2} * dR$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Indefinitely integrate&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{1}{2} V^2 = \frac{-k}{R} + P$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$V = \sqrt{\frac{-2k}{R} + 2P}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$P$&lt;/span&gt; is an additive constant, once again, trying to avoid using &lt;span class=&quot;math-container&quot;&gt;$C$&lt;/span&gt;.&#xA;Our additive constant &lt;span class=&quot;math-container&quot;&gt;$P$&lt;/span&gt; will depend on our starting velocity at &lt;span class=&quot;math-container&quot;&gt;$1 AU$&lt;/span&gt;. If our initial velocity is &lt;span class=&quot;math-container&quot;&gt;$0$&lt;/span&gt; for example,&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{1}{2} 0^2 = \frac{-k}{1 AU} + P$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$P = \frac{k}{1 AU}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;But it will vary based on that initial condition.&lt;/p&gt;&#xA;&lt;p&gt;So we have an expression for &lt;span class=&quot;math-container&quot;&gt;$V$&lt;/span&gt; based on distance from the Sun. However, I suspect that you want an answer in terms of time traveled, not distance traveled (if not we are done).&lt;/p&gt;&#xA;&lt;p&gt;so &lt;span class=&quot;math-container&quot;&gt;$V=\frac{dR}{dt}$&lt;/span&gt; so&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{dR}{dt} = \sqrt{\frac{-2k}{R} + 2P}$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\frac{dR}{\sqrt{\frac{-2k}{R} + 2P}} = dT$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Integrate&lt;/p&gt;&#xA;&lt;p&gt;At this point it is horrible. I tried doing an arctan substitution but, nothing seemed to work. So I dumped it into Mathematica.&lt;/p&gt;&#xA;&lt;p&gt;&lt;code&gt;T = (2*Sqrt[p]*Sqrt[p + k/r]*r - k*Log[k + 2*p*r + 2*Sqrt[p]*Sqrt[p + k/r]*r])/ (2*Sqrt[2]*p^(3/2))&lt;/code&gt;&lt;/p&gt;&#xA;&lt;p&gt;At this point you would need to solve for &lt;span class=&quot;math-container&quot;&gt;$R$&lt;/span&gt; in terms of &lt;span class=&quot;math-container&quot;&gt;$P$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$K$&lt;/span&gt;, and &lt;span class=&quot;math-container&quot;&gt;$T$&lt;/span&gt;. Something that does not seem like a fun proposition. You plug that back into your velocity equation to get the velocity as a function of time.&lt;/p&gt;&#xA;&lt;p&gt;If anyone sees an easier way to end up that math, please let me know. Maybe someone else can take it from here.&lt;/p&gt;&#xA;&lt;p&gt;Actually it basically boils down to &lt;span class=&quot;math-container&quot;&gt;$T = R-\ln{(R)}$&lt;/span&gt; as a horrible approximation.&lt;/p&gt;&#xA;&lt;p&gt;This actually makes a lot of sense. Initially we are going to have something that looks somewhat quadratic, however as we get beyond &lt;span class=&quot;math-container&quot;&gt;$10 AU$&lt;/span&gt; or so (will really depend on constants), it really becomes more or less linear as the acceleration drops to nearly zero and we are just going at a cruising speed.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://encrypted.google.com/search?q=x-ln%28x%29&quot; rel=&quot;noreferrer&quot;&gt;Google &lt;span class=&quot;math-container&quot;&gt;$x-\ln(x)$&lt;/span&gt;&lt;/a&gt; and zoom the graph out a bit to see.&lt;/p&gt;&#xA;&lt;p&gt;So distance is more or less linear with time, and &lt;span class=&quot;math-container&quot;&gt;$V$&lt;/span&gt; hits some constant, which we can find looking at the velocity equation.&lt;/p&gt;&#xA;&lt;p&gt;We can take the limit of &lt;span class=&quot;math-container&quot;&gt;$V = \sqrt{\frac{-2k}{R} + 2P}$&lt;/span&gt; as &lt;span class=&quot;math-container&quot;&gt;$R$&lt;/span&gt; goes to infinity. It becomes&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$V = \sqrt{2P}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;is what our velocity asymptotically approaches. Using the value I came up with for &lt;span class=&quot;math-container&quot;&gt;$P$&lt;/span&gt; earlier using the starting conditions.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\sqrt{\frac{-2k}{R} + \frac{k}{1AU}} = \sqrt{k \left(\frac{1}{1AU} - \frac{2}{R}\right)}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Once &lt;span class=&quot;math-container&quot;&gt;$\frac{2}{R}$&lt;/span&gt; is less than a hundredth of &lt;span class=&quot;math-container&quot;&gt;$\frac{1}{AU}$&lt;/span&gt; we have more or less reached our final velocity (we are only 1/10th away from it).&lt;/p&gt;&#xA;&lt;p&gt;So&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{2}{R} = \frac{1}{100 AU}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$R = 50 AU$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Which happens to be just beyond Pluto's Orbital distance.&lt;/p&gt;&#xA;&lt;p&gt;Kind of disappointing. However, this is in line with literature that I have read that shows outside of our solar system solar sails are not an efficient means of propulsion. This of course changes if you have some sort of laser system shooting at it.&lt;/p&gt;&#xA;" OwnerUserId="1200" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-06-15T11:23:48.053" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="3025" PostTypeId="2" ParentId="3023" CreationDate="2013-12-04T00:54:13.550" Score="13" Body="&lt;p&gt;Answers to this question are opinions.  Assuming that the stackexchange police permit that, here is my opinion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real objective is lowering cost through reusability.  In my opinion, the VentureStar approach made the unnecessary and suicidal leap that reusability required single-stage-to-orbit.  It does not.  It was single-stage-to-orbit that killed VentureStar, because of how thin the margins on everything had to be.  The SSTO assumption, and the use of chemical propellants, leads inevitably to a very fragile architecture where any noticeable mass growth will push you off the cliff.  It is simply an accident of nature that the size and mass of the Earth and the energy of chemical reactions conspire to make single-stage-to-orbit tantalizingly close to, but not quite practical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want low cost and reliable, you want robust engineering solutions with lots of margin.  The way to do that is with staging.  If you can recover one stage, then there's no reason you can't recover two.  A single or a second stage has the more difficult environment to deal with, which is entry from orbit.  The first stage will have an easier time surviving to be recovered, coming in at lower speeds from lower altitudes.  SpaceX is now developing the capability to recover their first stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you went nuclear thermal, that's a different story.  Then you could do SSTO, if they let you.  But I don't think nuclear anything would be low cost, even if reusable.  If you can do reusable chemical, that will be cheaper.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-04T01:21:25.333" LastActivityDate="2013-12-04T01:21:25.333" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3026" PostTypeId="5" CreationDate="2013-12-04T01:52:30.033" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-04T01:52:30.033" LastActivityDate="2013-12-04T01:52:30.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3027" PostTypeId="4" CreationDate="2013-12-04T01:52:30.033" Score="0" Body="VentureStar was a single-stage-to-orbit (SSTO) launch system proposed by Lockheed Martin with the intention of replacing the Space Shuttle." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-04T01:52:30.033" LastActivityDate="2013-12-04T01:52:30.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3028" PostTypeId="1" AcceptedAnswerId="3029" CreationDate="2013-12-04T04:15:52.893" Score="10" ViewCount="1106" Body="&lt;p&gt;Recently &lt;a href=&quot;https://space.stackexchange.com/questions/3015/how-large-is-the-earths-gravitational-sphere-of-influence-and-how-can-it-be-cal&quot;&gt;we had a question on gravitational SOI (Sphere of Influence) of planets&lt;/a&gt; but the image in answer has a plot showing that the SOI of Jupiter is less than that of Neptune. However,  Jupiter's mass is 317 times Earth's mass, but Neptune has a mass of 17.15 times Earth's mass. Therefore, it would seem Jupiter's SOI would be larger than that of Neptune &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FPxQJ.png&quot; alt=&quot;source:Wikipedia&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a mistake in the graph, or is it in my justifiration ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-06-23T14:20:00.700" Title="Why is the SOI of Jupiter less than that of Neptune?" Tags="|orbital-mechanics|planetary-science|soi|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3029" PostTypeId="2" ParentId="3028" CreationDate="2013-12-04T04:52:27.357" Score="16" Body="&lt;p&gt;Let's look at what, exactly, SOI is: &lt;strong&gt;the &lt;a href=&quot;http://en.wikipedia.org/wiki/Sphere_of_influence_%28astrodynamics%29&quot; rel=&quot;noreferrer&quot;&gt;Sphere Of Influence&lt;/a&gt;, the boundary inside of which the object has more force on other objects than the Sun does.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the weaker the force is from the Sun, the larger your sphere of influence is. Since gravity decreases as an inverse square ratio, the Sun's gravity is &lt;em&gt;much&lt;/em&gt; weaker at &lt;a href=&quot;http://en.wikipedia.org/wiki/Neptune&quot; rel=&quot;noreferrer&quot;&gt;Neptune&lt;/a&gt; (Perihelion 4,452,940,833 km) than it is at &lt;a href=&quot;http://en.wikipedia.org/wiki/Jupiter&quot; rel=&quot;noreferrer&quot;&gt;Jupiter&lt;/a&gt; (Perihelion 740,573,600 km). To clarify, that's 4.4 billion versus 740 million. A big difference, and one that makes an impact in the SOI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;ts;du&lt;sup&gt;*&lt;/sup&gt;:&lt;/strong&gt; &lt;em&gt;(from geoffc's comment)&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SOI is not about just the mass of the object, rather it is the comparison of the Sun's influence vs the mass of the object. Which clearly really makes a bit difference.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;terribly stated; didn't understand&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="690" LastEditDate="2014-06-23T14:20:00.700" LastActivityDate="2014-06-23T14:20:00.700" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3030" PostTypeId="5" CreationDate="2013-12-04T05:03:33.600" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-04T05:03:33.600" LastActivityDate="2013-12-04T05:03:33.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3031" PostTypeId="4" CreationDate="2013-12-04T05:03:33.600" Score="0" Body="Questions regarding the Sphere Of Influence, a region around a celestial body where it is the primary gravitational influence on orbiting objects." OwnerDisplayName="user12" LastEditorUserId="12102" LastEditDate="2020-08-23T22:16:32.117" LastActivityDate="2020-08-23T22:16:32.117" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3033" PostTypeId="1" AcceptedAnswerId="3034" CreationDate="2013-12-04T12:31:06.187" Score="20" ViewCount="1229" Body="&lt;p&gt;Suppose for a moment that cost would be no issue (&lt;a href=&quot;http://xkcd.com/697/&quot;&gt;and noone brought pruning shears&lt;/a&gt;) - could we build any of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Non-rocket_spacelaunch&quot;&gt;non-rocket spacelaunch systems from earth&lt;/a&gt; listed here or elsewhere? &lt;strong&gt;Which one is the least unfeasable given current technology?&lt;/strong&gt; &lt;/p&gt;&#xA;" OwnerUserId="102" LastActivityDate="2016-11-08T19:46:11.447" Title="Is there any non-rocket spacelaunch concept within reach of current material science and technology?" Tags="|launch|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3034" PostTypeId="2" ParentId="3033" CreationDate="2013-12-04T13:33:08.257" Score="17" Body="&lt;p&gt;A light gas gun firing a G-force hardened booster seems like the best bet for lofting some sorts of payloads. John Hunter gave a presentation on the state of the art for this technology at this summer's Icarus Interstellar Starship Congress in Dallas: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/BRCGylNe_mM?start=2263&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hunter's concept called for launching projectiles that fired a rocket near the apex of the ballistic trajectory to achieve insertion into LEO. His payload was fuel, to be collected in orbit and stored in orbital &quot;filling stations.&quot; He formed a company to develop this commercially, Quicklaunch (now called TekLaunch), but was apparently forced out by his partners.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also Wikis for &lt;a href=&quot;http://en.wikipedia.org/wiki/John_Hunter_%28scientist%29&quot;&gt;John Hunter (scientist)&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Super_High_Altitude_Research_Project&quot;&gt;SHARP&lt;/a&gt; (Hunter's research project at LLNL), and &lt;a href=&quot;http://en.wikipedia.org/wiki/Quicklaunch.&quot;&gt;Quicklaunch&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="49" LastEditDate="2015-01-23T04:25:05.183" LastActivityDate="2015-01-23T04:25:05.183" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3035" PostTypeId="1" AcceptedAnswerId="3052" CreationDate="2013-12-04T16:08:49.730" Score="5" ViewCount="924" Body="&lt;p&gt;My son really was upset he missed the launch live, so we re-watched the video at breakfast and I noticed starting &lt;a href=&quot;http://youtu.be/cXEJLhAh-Kg?t=3m27s&quot; rel=&quot;nofollow noreferrer&quot;&gt;at 3:27 in the video&lt;/a&gt; a few seconds after MECO, and stage separation, you can see the first stage firing thruster till it goes out of frame around 3:34.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I am sure they use the thrusters to make sure they DO not repeat the stage collision from flight 3 of the Falcon 1 launch, but these firings were already after the stage was pretty clear of the second stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am happy to see them using the thrusters, since it means they are doing 'something' with it, even if full recovery testing is not an option due to lack of reserve propellant.  (SpaceX said they would not test recovery per se, on flight 2 and 3 of the F9 1.1, since they had booked full reserves to the customers). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am curious if we know what they were testing. I would be happy to see that they are just exercising the thrusters to determine limits of control at different altitude densities.  Every bit of data helps, and the ability to test on each launch means they learn faster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lousy screen grabs from said video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JGUBj.png&quot; alt=&quot;Immediately after stage separation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Immediately after stage separation, (The right side of the split screen showed the M1-D Vac being exposed to space as the stages separated.) Oddly, looking at just the picture, the second stage is actually the object at the bottom in this view.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JwJq8.png&quot; alt=&quot;Second or two later, you can see the distances apart&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second or two later, you can see the stages separate from each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xd7RY.png&quot; alt=&quot;First firing from first stage&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we have a visible thruster firing from the first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WXrO7.png&quot; alt=&quot;Second firing from first stage&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And another firing off to the right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mtNzK.png&quot; alt=&quot;Third firing from first stage&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And another this time, sort of down and to the left.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TXNNz.png&quot; alt=&quot;This one was pulsing&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This one was hard to capture, but it pulsed its firing 3-4 times.  The same direction as the previous one.  Down and to the left.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An &lt;a href=&quot;http://www.nasaspaceflight.com/2013/12/ses-8-success-trajectory-future-spacex-possibilities/&quot; rel=&quot;nofollow noreferrer&quot;&gt;article at NASASpaceflight.com&lt;/a&gt; reported that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The CASSIOPE mission also involved the first “boost back” test of the first stage, while sources note there was also a boost back test during the SES-8 mission, or at least the restart of the first stage post staging.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So sources (unidentified in the article) suggest something was tested.  That is the info I am looking for in this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NasaSpaceflight.com &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=32998.msg1128797#msg1128797&quot; rel=&quot;nofollow noreferrer&quot;&gt;thread&lt;/a&gt; suggests that they did use the ACS to test orienting the stage, but did not actually relight the engines.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-13T01:08:24.223" LastActivityDate="2013-12-13T01:08:24.223" Title="Was any first stage recovery system tested on SpaceX Falcon 9 flight seven?" Tags="|spacex|reuse|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3037" PostTypeId="1" AcceptedAnswerId="3040" CreationDate="2013-12-04T16:41:20.153" Score="8" ViewCount="365" Body="&lt;p&gt;To-date the humans have dispatched surface missions to &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Mars&quot;&gt;Mars&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Moon&quot;&gt;Luna&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Venus&quot;&gt;Venus&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Mars has a thin atmosphere. Luna lacks one. Venus has a soup-thick atmosphere.&#xA;Landing a static craft on Venus is one thing. Exploration, rather &lt;strong&gt;mobile&lt;/strong&gt; exploration would be a different cup of tea altogether given the alien (pardon the pun!) atmosphere - high pressure, high wind force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Mars, and Luna electricity serves as the motive power - generated either by an &lt;a href=&quot;http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;RTG&lt;/a&gt;, or by &lt;a href=&quot;http://en.wikipedia.org/wiki/Photovoltaic_module&quot;&gt;PV&lt;/a&gt; arrays, or some form of batteries. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the &lt;a href=&quot;http://en.wikipedia.org/wiki/Curiosity_rover&quot;&gt;Curiosity&lt;/a&gt; rover is capable of surviving the Venerean atmosphere, could electricity serve as it's motive power at 93 bar? [ (+: Or a rover equaling Curiosity in terms of mass, and payload. ]&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-12-04T18:45:32.437" LastActivityDate="2013-12-05T17:13:03.013" Title="Could electricity propel Curiosity on Venus?" Tags="|rovers|venus|power|rtg|movement|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3038" PostTypeId="5" CreationDate="2013-12-04T16:52:14.167" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-04T16:52:14.167" LastActivityDate="2013-12-04T16:52:14.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3039" PostTypeId="4" CreationDate="2013-12-04T16:52:14.167" Score="0" Body="Variant of the SpaceX Falcon 9 rocket with reusable first and second stages using recovery systems demonstrated by the SpaceX Grasshopper and F9R-Dev1." OwnerUserId="49" LastEditorUserId="51" LastEditDate="2015-02-11T17:07:37.827" LastActivityDate="2015-02-11T17:07:37.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3040" PostTypeId="2" ParentId="3037" CreationDate="2013-12-04T17:58:12.527" Score="10" Body="&lt;p&gt;Not a good assumption.  Curiosity would die a &lt;em&gt;very&lt;/em&gt; quick thermal death on the surface of Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to answer your question, Curiosity's MMRTG would work on Venus and provide power.  The smaller temperature delta reduces the efficiency of conversion, but its not too bad.  See &lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?act=Attach&amp;amp;type=post&amp;amp;id=23200&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmospheric density and wind are not a factor at all for rover speeds and Venus wind speeds.  The wind at the surface is $0.1$ to $1\,\mathrm{m\over s}$.  The density is $67\,\mathrm{kg\over m^3}$.  So maybe $100\,\mathrm{N}$ over $3\,\mathrm{m}^2$ in a $1\,\mathrm{m\over s}$ wind.  The torque of the wheels is enormous, and wouldn't even notice that.  &lt;em&gt;Each&lt;/em&gt; of the six wheels of Curiosity generates almost $3000\,\mathrm{N}$ of force when fully engaged.  $100\,\mathrm{N}$ at $0.05\,\mathrm{m\over s}$ is just $5\,\mathrm{W}$ of motive power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd need to be careful to protect your soil samples so they don't blow away on the way from the ground to the instrument.  I'd worry more about the viscous drag when your wheels melt ...&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-05T17:13:03.013" LastActivityDate="2013-12-05T17:13:03.013" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3041" PostTypeId="2" ParentId="3037" CreationDate="2013-12-04T19:05:33.040" Score="6" Body="&lt;p&gt;The equation for &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_%28physics%29&quot; rel=&quot;nofollow&quot;&gt;aerodynamic drag&lt;/a&gt; is: &#xA;$$&#xA;        P_d = \mathbf{F}_d \cdot \mathbf{v} = \tfrac12 \rho v^3 A C_d&#xA;$$&#xA;&lt;a href=&quot;http://www.jpl.nasa.gov/education/marsrover.cfm&quot; rel=&quot;nofollow&quot;&gt;Curiosity&lt;/a&gt; has a max. speed of 5 cm/s (0.18 km/h). Air density $\rho$ of Venus is 67 $kg/m^3$, where on Earth it's 1.2 $kg/m^3$.&lt;br&gt;&#xA;I'll guesstimate frontal area A = 4 $m^2$ and $C_d$ = 1, then at 5 cm/s the drag force due to the rover's own speed is 0.016 N. The drag from wind speed (as @Mark calculated) is much higher, but still nothing to worry about.&lt;br&gt;&#xA;Curiosity has up to &lt;a href=&quot;http://jalopnik.com/5930877/the-ford-raptor-goes-head-to-head-against-mars-rover&quot; rel=&quot;nofollow&quot;&gt;4000 Nm&lt;/a&gt; of torque available, or 16 kN of force. That's with 110 W per wheel. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2013-12-05T11:12:10.343" LastActivityDate="2013-12-05T11:12:10.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3043" PostTypeId="2" ParentId="3033" CreationDate="2013-12-05T00:13:21.157" Score="7" Body="&lt;p&gt;Nuclear pulse propulsion, aka Orion.  There's little doubt it would work.  The limits are not technological, but the fact that boosting an Orion means you're detonating a lot of atomic bombs in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you'll accept launches from other bodies the mass driver is the best answer.  You can make the track as long as you like.  Many years ago on TV I saw one that could boost a projectile to 100mph in six feet.  If my understanding of the math is right that means you could easily scale it up to lunar escape velocity with a 3.2 mile track.  Punching through the atmosphere is quite another matter, while a track a few hundred miles long could safely boost an astronaut to Earth escape velocity he wouldn't survive the passage through the atmosphere and the capsule would fall back due to drag anyway.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2013-12-05T00:22:39.050" LastActivityDate="2013-12-05T00:22:39.050" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3044" PostTypeId="2" ParentId="2914" CreationDate="2013-12-05T00:38:36.500" Score="-3" Body="&lt;p&gt;&lt;strong&gt;Less than 30 minutes&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Falcon 9’s second stage single Merlin vacuum engine ignited at 185&#xA;  seconds after launch to begin a five minute, 20 second burn to deliver&#xA;  SES-8 into a temporary parking orbit. Eighteen minutes after injecting&#xA;  SES-8 into that orbit, the second stage engine reignited for just over&#xA;  a minute to carry the satellite to its final GTO.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Source: &lt;a href=&quot;http://www.redorbit.com/news/space/1113019268/spacex-launches-ses-8-satellite-geostationary-transfer-orbit-120413/&quot; rel=&quot;nofollow&quot;&gt;RedOrbit&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Best performance..&lt;/p&gt;&#xA;" OwnerUserId="1206" LastEditorUserId="49" LastEditDate="2013-12-05T01:24:24.173" LastActivityDate="2013-12-05T01:24:24.173" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3045" PostTypeId="2" ParentId="2893" CreationDate="2013-12-05T05:08:27.643" Score="7" Body="&lt;p&gt;A lot of information could be gleaned simply from determining the orbit and characterizing the type of electromagnetic radiation it emits. Getting a decent picture of the satellite would also help considerably.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, a GEO satellite is almost assuredly going to be used for communication or very wide field-of-view remote sensing (e.g. weather, missile warning). The longitude of its orbital slot would then help determine what geographic region it was assigned to. Over China, probably communications or weather. Over the US, perhaps not. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Coupling this information with how often at what frequency and at what power level it emits radiation (ostensibly for communications) would help further identify the mission. GEO communication satellites, like those that broadcast TV and radio can have kW of power and their primary mission is to relay large amount of data. In contrast, a weather or early-warning satellite's primary mission is to gather and downlink the imagery. I would generally assume such missions would transmit with less power and less regularly. The frequency might also shed light on military vs. civilian use, as different portions of the EM spectrum is often allocated to each. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For LEO satellites the orbit and constellation size will help. Sun synchronous orbits are often used for remote sensing because of repeatable lighting conditions over the same targets. Polar orbits are common for weather satellites. A couple of satellites flying in close tandem where one was constantly blasting land with radar sounds like a synthetic aperture radar mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/12730/1235&quot;&gt;Molniya orbits&lt;/a&gt; are used almost exclusively for high-latitude communications because of their long loiter times over high latitudes.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="1235" LastEditDate="2018-05-23T03:17:56.877" LastActivityDate="2018-05-23T03:17:56.877" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3047" PostTypeId="1" AcceptedAnswerId="3050" CreationDate="2013-12-05T18:57:57.360" Score="7" ViewCount="499" Body="&lt;p&gt;Are there any plans to launch a sample return mission to Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is such mission possible, and what would be approximate max weight of returned sample?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to return sample to Earth orbit and retrieve it there and save on thermal shielding on return vehicle?&lt;/p&gt;&#xA;" OwnerUserId="98" LastActivityDate="2013-12-05T22:41:32.867" Title="Sample return mission to Mars" Tags="|mars|sample-return|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3048" PostTypeId="5" CreationDate="2013-12-05T18:58:44.597" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2013-12-05T18:58:44.597" LastActivityDate="2013-12-05T18:58:44.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3049" PostTypeId="4" CreationDate="2013-12-05T18:58:44.597" Score="0" Body="Mission to some extraterrestrial object which results in return of materials to Earth for further examination." OwnerUserId="98" LastEditorUserId="98" LastEditDate="2013-12-05T23:13:49.623" LastActivityDate="2013-12-05T23:13:49.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3050" PostTypeId="2" ParentId="3047" CreationDate="2013-12-05T19:41:21.270" Score="5" Body="&lt;p&gt;It is possible and there are several such missions planned or proposed:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Russian Roscosmos proposed &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars-Grunt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars-Grunt&lt;/a&gt; mission that should be based on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Fobos-Grunt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phobos-Grunt&lt;/a&gt; sample return technology (the first Phobos-Grunt went sour, but I hear they started planning &lt;a href=&quot;http://www.russianspaceweb.com/phobos_grunt2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phobos-Grunt 2&lt;/a&gt;). The sample return weight would be about 0.2 kilograms (0.44 lb) according to this &lt;a href=&quot;http://www.thespacereview.com/archive/1980.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Roscosmos Space Missions&lt;/a&gt; (PDF) document.&lt;/li&gt;&#xA;&lt;li&gt;China has &lt;a href=&quot;http://news.xinhuanet.com/english/china/2012-10/10/c_123806897.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;plans&lt;/a&gt; to explore Mars in a similar fashion as they're now doing with Lunar exploration, so orbiters first, then landers, followed by sample return missions by 2030, and so on. China tends to build two identical craft instead of just one, so that adds quite a bit to mission robustness in case of unexpected failure. The mass and size of the return samples they would target is yet unknown.&lt;/li&gt;&#xA;&lt;li&gt;Joint NASA-ESA plans via &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Joint_Exploration_Initiative&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Joint Exploration Initiative&lt;/a&gt; were targeting 2020-2022 time frame but that's now uncertain due to cancellation of the &lt;a href=&quot;https://en.wikipedia.org/wiki/MAX-C&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Astrobiology Explorer-Cacher&lt;/a&gt; (MAX-C) concept rover mission. MAX-C project targeted return of 20-sample cache, likely under half a kilogram in weight.&lt;/li&gt;&#xA;&lt;li&gt;NASA has many proposals via its &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Exploration_Program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Exploration Program&lt;/a&gt;, for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Next_Generation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Next Generation&lt;/a&gt; mission with planned sample return in the 2018-2024 timeframe (&lt;a href=&quot;http://www.nasa.gov/pdf/691580main_MPPG-Integrated-v13i-Summary%20Report-9-25-12.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Summary of the Final Report, Mars Program Planning Group&lt;/a&gt; (PDF)), &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020_rover_mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars 2020 rover mission&lt;/a&gt; that is also a concept under study with target launch date in 2020 with ambitions to return samples, and so on. Mars 2020 rover mission is proposed to collect up to 31 samples of rock cores and soil, each 1 cm in diameter.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And there might be other such planned mission in different project stages. As for possibility to do so, there are many possible ways to go about it, and some such concepts are listed on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_sample_return_mission#NASA_proposals&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Mars sample return mission - NASA proposals&lt;/a&gt;, listing one-launch architecture using &lt;a href=&quot;https://en.wikipedia.org/wiki/Hall_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hall effect thruster&lt;/a&gt;, two-launches architecture with orbiter and lander with a return stage and I presume Mars Orbit Rendezvous, and three-launches architecture with a sample collecting rover, an orbiter, and a Mars Ascent Vehicle (MAV).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the last part of your question, if it is possible to capture returned samples in Earth's orbit, I believe that it would be, but likely not needed if you're using two- or even three-launch architecture with a Mars orbiter safely storing returned samples in its body. The sample return capsule also needs not be large and can be built to withstand Earth aerobraking and deploy a parachute once it has sufficiently slowed down its descent on reentry. E.g. sample cache could look something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ccGJJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Prototype for hardware to cache samples of cores drilled from Martian rocks for possible future return to Earth (Source: &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA17277&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this is not such a big sample cache that would be held in a return capsule, and can hold 31 samples with each core 0.4 inch (1 centimeter) in diameter. Problem is, you'd still need a spacecraft to deliver the sample capsule to Earth and inject it in reentry trajectory that is shallow enough for the capsule not to disintegrate in the atmosphere, and without constant corrections, that would be nigh impossible to do from Martian orbit. Simply &lt;em&gt;hurling&lt;/em&gt; samples off Mars and towards the Earth wouldn't work. Miss by a nanometer at your departure and you won't arrive at your destination where you planned to, either by far missing the Earth or hitting it too direct and your capsule will burn on reentry. And that's assuming your calculations were absolutely precise to start with, with no margin of error to speak of, and no trajectory corrections required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you have a powered vehicle with thrusters onboard delivering the capsule, like your Mars orbiter or in one-launch architecture an all-in-one spacecraft, it's simply easier (and cheaper) to just drop it into a ballistic trajectory from near Earth than to change its orbit to rendezvous with another craft or a space station in Earth's orbit. Especially, since you'd need powerful retrorockets and propellants for them, all adding to total weight of your spacecraft, to slow down to a more circular orbit suitable for rendezvous from what would be a highly elliptical orbit on your return to Earth.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-05T21:27:58.277" LastActivityDate="2013-12-05T21:27:58.277" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3051" PostTypeId="2" ParentId="3047" CreationDate="2013-12-05T21:04:19.390" Score="3" Body="&lt;p&gt;There have been plans to return samples from the surface of Mars for about the past 46 years.  There was a project initiated in 1998 to return samples by 2008, but it was cancelled in 2000 due to other Mars mission failures in 1999.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently the first part of a Mars Sample Return (MSR) mission is in serious planning, which is a sample caching missing to be launched in 2020.  It is not an approved mission yet, but it is well on its way.  That first step would collect rock cores and other samples and store them in a can, called the cache, on the rover for later retrieval.  This is the closest we've gotten to a &quot;real&quot; MSR project since 2000.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 2020 mission could be followed by another lander, this time with a rocket to launch the samples into Mars orbit.  Then a return orbiter could pick up the samples in Mars orbit, bring them to Earth, and send an entry vehicle in with the samples to land.  Those missions being further in the future are in much more nascent stages of planning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Returning samples from Mars, while difficult and complicated, is certainly possible.  One thing that has been relatively consistent over the years is that the sample collected and returned would be on the order of 500 grams of material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the requirements on MSR is to not inadvertently release Martian samples into Earth's environment on entry or landing.  The samples have to go through an isolated receiving facility on Earth first and be investigated there before they can be released into Earth's environment for more open use in other laboratories.  As a result, the safest approach appears to be direct entry and landing, since that can be done with the simplest, most robust vehicle with large margins.  It doesn't even need a parachute.  Going into Earth orbit, being retrieved by another vehicle, and then safely entering and landing that vehicle adds more failure modes for inadvertent entry and breakup.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole point is to get the samples into Earth laboratories, so sooner or later they have to enter and land.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-05T22:41:32.867" LastActivityDate="2013-12-05T22:41:32.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3052" PostTypeId="2" ParentId="3035" CreationDate="2013-12-05T21:58:34.053" Score="7" Body="&lt;p&gt;From Elon Musk, via &lt;a href=&quot;http://www.nasaspaceflight.com/2013/10/musk-plans-reusability-falcon-9-rocket/&quot; rel=&quot;noreferrer&quot;&gt;NasaSpaceFlight&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Musk indicated that SpaceX would not attempt to recover the first&#xA;  stage for the SES and Thaicom flights, instead “(for) the next two&#xA;  launches, we are going to gather data from the first stage but we are&#xA;  not going to attempt to recover it because we’ve committed to give the&#xA;  customers on the next two flights maximum performance of the rocket.&#xA;  The next recovery attempt for the first stage will be the fourth&#xA;  flight of this version.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, if they tried anything, it was a very small attempt, and they won't try it completely until CRS-3 again. There are rumors that I've heard that there was an attempt to simply restart the engine multiple times, but that there was only a few seconds of fuel in the rockets. Not sure what else if anything there was today on the matter.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2013-12-11T21:41:25.793" LastActivityDate="2013-12-11T21:41:25.793" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3053" PostTypeId="1" AcceptedAnswerId="3087" CreationDate="2013-12-05T22:15:55.300" Score="7" ViewCount="149" Body="&lt;p&gt;The previous launch of the Falcon 9 included &lt;a href=&quot;https://space.stackexchange.com/q/2377/25&quot;&gt;a number of mysterious objects&lt;/a&gt;. Has there been any indication that the recent SES launch produced similar objects?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-12-10T17:53:47.147" Title="Were there any extra objects in the latest Falcon 9 launch?" Tags="|launch|debris|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3054" PostTypeId="1" AcceptedAnswerId="3057" CreationDate="2013-12-06T07:35:20.980" Score="20" ViewCount="4906" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_hydrogen&quot;&gt;Cryogenic hydrogen&lt;/a&gt; can react with the atmospheric Oxygen to produce an explosion:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;as well as being cold enough to liquefy (and possibly solidify) atmospheric oxygen, which can be an explosion hazard.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If we start pumping fuel in when the tanks are made (I believe the inside chamber is not vacuum), it will explode. If the inner chamber is kept in a vacuum, the atmospheric pressure would crush it. So I assume that they do not suck the inside atmospheric air (Am I wrong?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is this: What process is currently used by space agencies to fill tanks with cryogenic hydrogen?  &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="25" LastEditDate="2013-12-06T15:27:00.640" LastActivityDate="2019-03-21T18:49:59.527" Title="How are fuel tanks filled with cryogenic hydrogen?" Tags="|fuel|engineering|rockets|cryogenics|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3057" PostTypeId="2" ParentId="3054" CreationDate="2013-12-06T14:29:40.600" Score="14" Body="&lt;p&gt;NASA has done &lt;a href=&quot;http://history.nasa.gov/SP-4230.pdf&quot; rel=&quot;noreferrer&quot;&gt;an extensive report&lt;/a&gt; on this, and in fact, cryogenic hydrogen tanks are considered to be one of the greatest technical achievements that NASA managed. Much of this is specific to the Centaur upper stage, but here's a few interesting quotes from the article:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Page 38&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Bossart led Mrazek out into the factory yard, where a Centaur tank&#xA;  stood gleaming in the sunlight. Mrazek asked, “What’s inside it?” To&#xA;  which Bossart responded, “Nitrogen.” Nitrogen was used for&#xA;  pressurization until the rocket was filled with its&#xA;  liquid-hydrogen/liquid-oxygen propellants just prior to launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Page 200&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One of the most onerous was a fluid systems interface and the special&#xA;  requirements for dumping the liquid-hydrogen propellant if the mission&#xA;  had to be aborted. In late 1984, Johnson released a Centaur Safety&#xA;  Waiver (WACR-1A) that requested a redesign of the fill, drain, and&#xA;  dump vent system. The problem it wanted addressed was the placement of&#xA;  the liquid-hydrogen and liquid-oxygen vent line inlets below the&#xA;  propellant liquid level in case of a mission abort.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the rocket was filled with Nitrogen prior to filling. I can't find a source for it. They might have put Helium in it's place as well, or gaseous hydrogen, as a stop gap when filling it, but I can't find any sources for that bit of information.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-04T16:29:55.953" LastActivityDate="2016-07-04T16:29:55.953" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3058" PostTypeId="1" AcceptedAnswerId="3059" CreationDate="2013-12-06T15:00:22.303" Score="18" ViewCount="26751" Body="&lt;p&gt;The Apollo missions, like most all missions since, used a heat shield to keep from disintegrating in the atmosphere. This approach had its flaws, however.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For one, if your approach was too shallow, you could do this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/drGxw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;This is know as a &lt;em&gt;bad day&lt;/em&gt;. Img credit me.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what would happen if it was too steep, but as I remember that scenario isn't fun either. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is this: For Apollo 13, what was the ideal sliver of atmosphere they needed to hit? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2014-01-20T13:28:44.163" LastActivityDate="2015-06-17T23:45:29.587" Title="At what angle did Apollo 13 need to reenter?" Tags="|apollo-program|reentry|apollo-13|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3059" PostTypeId="2" ParentId="3058" CreationDate="2013-12-06T15:13:58.773" Score="22" Body="&lt;p&gt;From &lt;a href=&quot;http://books.google.com/books?id=LDJ43xYxK5YC&amp;amp;pg=PA258&amp;amp;lpg=PA258&amp;amp;dq=Apollo%2013%20reentry%20angle&amp;amp;source=bl&amp;amp;ots=3Cc9mzZNJR&amp;amp;sig=sLWSkkivIHidkiZ_qY3YjyKypaQ&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=BemhUuf2NI6OkAfNioCoCg&amp;amp;ved=0CJYBEOgBMAw#v=onepage&amp;amp;q=Apollo%2013%20reentry%20angle&amp;amp;f=false&quot;&gt;Apollo 13 by Jim Lovell, Jeffrey Kluger&lt;/a&gt;, the following is stated:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In order to reenter Earth's atmosphere safely, Apollo 13 had to&#xA;  approach at an inclination no shallower than 5.3 degrees, and no&#xA;  steeper than 7.7 degrees. Come in at 5.2 degrees or below, and the&#xA;  blunt-ended command module would skip off the top of the atmosphere&#xA;  and boing straight back into space, entering a permanent orbit around&#xA;  the sun. Come in at a 7.8 degree or above, and the spacecraft would be&#xA;  able to reenter all right, but at so steep an angle and with such a&#xA;  high g force that the crew would probably be crushed well before they&#xA;  ever hit the water.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The quote is not quite correct about going into a &quot;permanent orbit about the sun&quot;.  They were in orbit about the Earth before entry, and would remain in orbit about the Earth if they skipped out or missed completely.  Apollo 13's entry velocity is documented in &lt;a href=&quot;http://georgetyson.com/files/apollostatistics.pdf&quot;&gt;Apollo by the Numbers&lt;/a&gt;.  With that and the entry interface of 400,000 ft altitude, it is straightforward to compute that the specific energy of the return trajectory was negative, about $-0.4\,\mathrm{{km}^2\over s^2}$.  It was therefore in orbit and not on an escape trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One could argue that Earth is in orbit about the Sun, so then anything in orbit around the Earth is as well.  However in this case, the orbit is definitely not permanent.  If they skipped out the first time, they would reencounter Earth's atmosphere on the next pass, since orbits are closed, and enter a final time.  That would eliminate the possibility of a lunar gravity assist (which might result in an escape), since the moon wouldn't be there on the next orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=KUAo_T__J2A&quot;&gt;This video&lt;/a&gt; provides an interesting account of what would have happened had they not been able to target an Earth entry at all, missing by 2500 miles.  In that case, perturbations from the Moon cause the vehicle to very steeply reenter Earth's atmosphere five weeks later.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="265" LastEditDate="2013-12-09T05:30:34.987" LastActivityDate="2013-12-09T05:30:34.987" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3060" PostTypeId="1" AcceptedAnswerId="3061" CreationDate="2013-12-06T19:29:49.050" Score="11" ViewCount="1164" Body="&lt;p&gt;Early spacesuits (like the &lt;a href=&quot;http://en.wikipedia.org/wiki/SK-1_spacesuit&quot;&gt;SK-1&lt;/a&gt; or the &lt;a href=&quot;http://en.wikipedia.org/wiki/Navy_Mark_IV&quot;&gt;Mark IV&lt;/a&gt;) were designed before anyone had gone into space.  How did they test that the suits would perform well in a vacuum?  Did they send them into space with sensors inside?  Did they have large vacuum chambers to put them in?&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2015-03-20T07:30:34.460" Title="How did they test early spacesuits in a vacuum?" Tags="|history|spacesuits|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3061" PostTypeId="2" ParentId="3060" CreationDate="2013-12-06T19:56:29.550" Score="13" Body="&lt;p&gt;They were tested in vacuum chambers. They actually still do, it's by far simpler than sending something in Low Earth Orbit (LEO) and beyond to later realize they need to make changes to it. Vacuum chambers also allow for a controlled depressurisation and repressurisation, which is another advantage to having them tested that way. Some interesting related videos:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.liveleak.com/view?i=1cb_1275963881&quot; rel=&quot;nofollow&quot;&gt;Early NASA Vacuum Chamber Test Gone Wrong on LiveLeak&lt;/a&gt; (caption on &lt;a href=&quot;http://www.universetoday.com/66061/early-nasa-vacuum-chamber-test-gone-wrong/&quot; rel=&quot;nofollow&quot;&gt;Universe Today&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=7yNoOYjR0_g&quot; rel=&quot;nofollow&quot;&gt;Trying on an Orlan space suit in a vacuum chamber on YouTube&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2015-03-20T07:30:34.460" LastActivityDate="2015-03-20T07:30:34.460" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3063" PostTypeId="1" AcceptedAnswerId="3066" CreationDate="2013-12-06T23:31:22.303" Score="8" ViewCount="5014" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/V-2_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, the german V-2 rocket from WW2 had a max altitude of 206 km when launched straight up.  That's clearly above the 100 km requirement to be considered '&lt;a href=&quot;https://space.stackexchange.com/questions/2076/how-far-do-you-have-to-be-from-earth-to-be-in-space&quot;&gt;in space&lt;/a&gt;.'  The V-2 was in service from 1944-1952. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there earlier examples than this?&lt;/p&gt;&#xA;" OwnerUserId="1165" LastEditorUserId="12102" LastEditDate="2021-09-13T00:13:28.077" LastActivityDate="2021-09-13T00:13:28.077" Title="What was the first man-made object to enter space?" Tags="|rockets|history|spaceflight-firsts|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3064" PostTypeId="1" AcceptedAnswerId="3065" CreationDate="2013-12-07T02:50:26.457" Score="10" ViewCount="713" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Earth_landing_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo command module&lt;/a&gt; had a diameter of about 12.8 ft (which I assume is pretty close to the diameter of its heat shield). However, the heat shield for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Orion_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;new Orion spacecraft&lt;/a&gt; is a little bigger, at 16.5 ft (again, the diameter of the vehicle itself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/mxI0O.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Orion's heat shield. Img credit &lt;a href=&quot;http://www.space.com/22046-nasa-orion-spacecraft-heat-shield.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;space.com&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know that for Apollo 13, &lt;a href=&quot;https://space.stackexchange.com/a/3059/12&quot;&gt;the ideal angle of attack&lt;/a&gt; was no shallower than 5.3 degrees, and no steeper than 7.7 degrees. I assume that the changed dimensions of the Orion spacecraft make some differences to this - so my question: &lt;strong&gt;What difference does the surface area of the heat shield make to the reentry angle?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-12-07T07:55:04.820" Title="How does the surface area of a heat shield impact the ideal angle of attack for reentry?" Tags="|reentry|heat-shield|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3065" PostTypeId="2" ParentId="3064" CreationDate="2013-12-07T07:48:58.763" Score="11" Body="&lt;p&gt;It's not the surface area by itself.  There are two main factors.  The first is the ballistic coefficient, $\beta$, which is the mass divided by the product of the surface area and the coefficient of drag: $\beta={m\over C_D A}$.  The second is the lift over drag (L/D) of the vehicle, which for these blunt vehicles is usually obtained with an offset center of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As $\beta$ goes up, the shallowest allowable entry angle goes steeper.  Also as $\beta$ goes up, the heating rate goes up, making the steepest angle shallower.  At some point you would improve the thermal protection system to allow higher heating at steeper angles to keep that corridor open.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an approximation, as a vehicle gets larger, the mass goes up as the cube of the size, whereas the area goes up as the square.  So you would expect that $\beta$ goes &lt;em&gt;up&lt;/em&gt; as the vehicle goes up in size, even though the surface area is increasing.  So Orion should have a higher $\beta$ than Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Higher L/D permits a wider entry flight path angle range, since you can go lift down on a shallow entry or lift up on a steep entry, and in general guide the vehicle down the desired corridor to prevent skip out or overheating.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-07T07:55:04.820" LastActivityDate="2013-12-07T07:55:04.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3066" PostTypeId="2" ParentId="3063" CreationDate="2013-12-07T07:59:09.943" Score="13" Body="&lt;p&gt;No earlier examples are known.  The first that could be argued to reach outer space was the &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_V-2_test_launches&quot;&gt;fourth V-2 launch&lt;/a&gt;, on October 3, 1942.  Its maximum altitude of 85-90 km or 97 km (depending on the source) meets the old NACA definition of outer space of about 50 miles (80 km).  The US Air Force awarded astronaut wings to X-15 pilots who went above 50 miles in the 1960's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However it does not meet the modern definition of outer space as starting at 100 km.  You only have to wait a few years though for another V-2 to crack 100 km, no later than 1944.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-07T19:57:08.727" LastActivityDate="2013-12-07T19:57:08.727" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3067" PostTypeId="1" AcceptedAnswerId="3071" CreationDate="2013-12-07T10:36:14.133" Score="6" ViewCount="163" Body="&lt;p&gt;How rigorously is conjunction analysis (finding out the chances of hitting an asteroid or another &lt;strong&gt;known object&lt;/strong&gt;) conducted for missions beyond Earth orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to larger computational resources available has there been (in recent years or decades) a corresponding increase in attention devoted to ensuring a deep space probe won't hit another body accidentally? I know the space is VAST, but so is human complacency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any fixed rules that are obeyed by mission planners to avoid conjuctions? &lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2013-12-07T21:28:56.963" LastActivityDate="2013-12-07T21:28:56.963" Title="Conjunction analysis for deep space missions" Tags="|probe|conjunction|ballistics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3068" PostTypeId="1" AcceptedAnswerId="3072" CreationDate="2013-12-07T12:25:25.050" Score="22" ViewCount="17120" Body="&lt;p&gt;I searched the web and found a couple entries on Wikipedia (&lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skip_reentry&quot; rel=&quot;noreferrer&quot;&gt;Skip reentry&lt;/a&gt; &amp;amp; &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry&quot; rel=&quot;noreferrer&quot;&gt;Atmospheric entry&lt;/a&gt;&lt;/em&gt;) that kind brush at the topic.  The article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Stone_skipping&quot; rel=&quot;noreferrer&quot;&gt;Stone skipping&lt;/a&gt; has some science and physics involved (speed, etc) but I don't find any about skipping off the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recall seriously worded concerns during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;noreferrer&quot;&gt;Apollo Program&lt;/a&gt; of the returning ships skipping off the atmosphere with dire results. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is fairly easy to image the stone skipping scenario, when the stone brushes across the surface of the water as it moves at an angle.  But the atmosphere does not have a &quot;surface&quot; it is more of a continual thinning as you get farther from the planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do you skip off of a boundary that &lt;a href=&quot;https://space.stackexchange.com/questions/2076/how-far-do-you-have-to-be-from-earth-to-be-in-space&quot;&gt;does not have a solid scientific existance&lt;/a&gt;? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-05-05T09:59:10.837" Title="How does skipping off the atmosphere work?" Tags="|reentry|atmospheric-drag|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3069" PostTypeId="2" ParentId="3068" CreationDate="2013-12-07T14:17:42.607" Score="3" Body="&lt;p&gt;Skipping is actually a bad word choice, since it would indicate that the skip would be a change in your trajectory and the reentry would be your &quot;non-interrupted&quot; trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity can be considered a conservative force (for non-relativistic speeds) so your energy would be conserved while in orbit. This means that once in orbit, you will stay in orbit (assuming only one gravity well will dominate in exerting a force on you).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However once you get into the atmosphere you will experience drag, which is a non-conservative force, so your orbital energy will decrease. But if your energy is not decreased enough, you can leave the atmosphere again. This is what they call a skip. This can be due to a not deep enough trajectory into the atmosphere, since the density and therefore the drag decreases approximately exponentially with height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if reentry is too deep you will experience too much drag, which will cause a lot of heat from &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmospheric_entry#Shock_layer_gas_physics&quot; rel=&quot;nofollow&quot;&gt;shock layer gas&lt;/a&gt;, and you might burn up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you have a too-high orbital energy, you might have to do a skip reentry, since otherwise you will burn up. But if you come in from an hyperbolic trajectory you do want to decrease your orbital energy enough such that you will get at least in an orbit around the celestial body. Otherwise I think you will have to get captured with the help or rockets (or plan another intercept trajectory).&lt;/p&gt;&#xA;" OwnerUserId="1086" LastEditorUserId="1086" LastEditDate="2013-12-07T21:35:12.257" LastActivityDate="2013-12-07T21:35:12.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3071" PostTypeId="2" ParentId="3067" CreationDate="2013-12-07T18:41:02.307" Score="3" Body="&lt;p&gt;For deep space missions, there usually is a conjugation analysis, but not for the reasons you predicted. In fact, they do it to see if there's any secondary objects that can be studied while on the way to their destination. The majority of asteroids that have been photographed up close, in fact, were such accidents. From the &lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid#Exploration&quot; rel=&quot;nofollow&quot;&gt;Wikipedia Article&lt;/a&gt;, here's a few examples:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first true asteroid to be photographed in close-up was 951 Gaspra&#xA;  in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which&#xA;  were imaged by the Galileo probe en route to Jupiter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The chances of impact, however, are so tiny that it isn't really worth discussing. Think of it like this. Earth gets hit by a rock large enough for us to see is in the thousands of years. The odds of them hitting a spacecraft are even lower.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2013-12-07T18:41:02.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3072" PostTypeId="2" ParentId="3068" CreationDate="2013-12-07T20:11:48.597" Score="30" Body="&lt;p&gt;First off, a stone does not skip off the surface of the water.  It skips off the water.  The stone has to bite into the water to use its lift to come back out.  A skipping stone can, and sometimes does, go completely &lt;em&gt;under&lt;/em&gt; the surface in the process of skipping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The air is a fluid as well, and a lifting body can in fact skip in the same way as a stone does in water.  Air has a very substantial &quot;existence&quot;, especially as you move faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However lift is not required for &quot;skipping&quot; out of the atmosphere, since unlike the apparent surface of a pond, the atmosphere is curved.  All that is really meant in the case of a skip entry is that the entry flight path angle was not steep enough to prevent the object from leaving the atmosphere again.  The trajectory of the ballistic skip flight does not curve up like that of a skipping stone.  It still curves down.  However the radius of curvature is greater than that of the radius of curvature of the atmosphere, so it departs back into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We get back to needing a lifting body if you want the skip to have a predictable set of exit conditions.  While you can skip ballistically with no lift, the uncertainty in the entry angle gets amplified to large uncertainty on the exit conditions, which is not good if you plan to reenter or insert into an orbit.  Lift and/or drag on the entry vehicle is modulated through the atmospheric flight of the skip in order to control the exit conditions.  That is what &lt;a href=&quot;http://www.astronautix.com/details/ski16333.htm&quot; rel=&quot;noreferrer&quot;&gt;Apollo was qualified&lt;/a&gt; to do, in order to control the conditions of the &lt;em&gt;second&lt;/em&gt; reentry and allow a safe landing at a predictable site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Skip entries can also be used to spread out the total heat load of an entry over multiple passes, allowing cooling between passes.  The aerobraking approach used by several &lt;a href=&quot;https://dx.doi.org/10.2514/6.2002-4821&quot; rel=&quot;noreferrer&quot;&gt;Mars orbiters&lt;/a&gt; is an extreme example of many high-altitude skip entries.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="168" LastEditDate="2015-05-05T09:59:10.837" LastActivityDate="2015-05-05T09:59:10.837" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3073" PostTypeId="2" ParentId="646" CreationDate="2013-12-07T23:27:09.093" Score="6" Body="&lt;p&gt;&lt;a href=&quot;https://itunes.apple.com/tr/app/itraject/id724557478?mt=8&quot; rel=&quot;nofollow&quot;&gt;iTraject&lt;/a&gt; might be very useful for learning orbital mechanics. Its numerical solver makes it very flexible. It also uses very precise astronomical algorithms for celestial positions. You can actually set your initial date, predict when your vehicle will be in Moon's SOI with analytical calculations, and make a flyby around the Moon. Moreover, you can get ground station, epoch and keplerian elements parameters with current time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;here a video:&#xA;&lt;a href=&quot;http://www.youtube.com/watch?v=msCEdOq5WhI&quot; rel=&quot;nofollow&quot;&gt;http://www.youtube.com/watch?v=msCEdOq5WhI&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1029" LastEditorUserId="4691" LastEditDate="2014-07-06T10:52:21.540" LastActivityDate="2014-07-06T10:52:21.540" CommentCount="1" CommunityOwnedDate="2013-12-07T23:27:09.093" ContentLicense="CC BY-SA 3.0" />
  <row Id="3074" PostTypeId="1" AcceptedAnswerId="3075" CreationDate="2013-12-08T00:51:43.717" Score="11" ViewCount="1134" Body="&lt;p&gt;As per &lt;a href=&quot;http://historical.whatitcosts.com/facts-space-station.htm&quot; rel=&quot;nofollow&quot;&gt;this link&lt;/a&gt;, the International Space Station (ISS) continues to provide research in such fields as &lt;strong&gt;biology&lt;/strong&gt;. What kind of biological research is being done on the ISS, considering there is no life in hard vacuum and microgravity of outer space to study? Is it just the research on effects of microgravity on human body, or also biological specimen being brought to the station to conduct laboratory environment experiments on?&lt;/p&gt;&#xA;" OwnerUserId="1228" LastEditorUserId="49" LastEditDate="2013-12-12T02:28:47.950" LastActivityDate="2013-12-12T16:18:05.327" Title="What kind of biological research does the ISS do?" Tags="|iss|research|life|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3075" PostTypeId="2" ParentId="3074" CreationDate="2013-12-08T01:56:57.723" Score="9" Body="&lt;p&gt;All kinds of research really, but let's start by quoting NASA's own page on &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/biology_research/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Station Boosting Biological Research in Orbit&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Studying the science of biology in microgravity opens a world of&#xA;  possibilities! Research ranges from plant growth to cell growth and&#xA;  from bacterial virulence to strength in human bones. The scope of&#xA;  biology research provides scientists from many disciplines with&#xA;  opportunities to express and explore their area of interest,&#xA;  translating findings into treatments and applications for use on Earth&#xA;  and in space exploration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And as one actual example mentioned on the same page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/5QdmS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The study of Arabidopsis thaliana, pictured here, will be a&#xA;  continuation of previous research to understand how&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;the effects of&#xA;  hypobaric environments on the International Space Station determine&#xA;  plant growth in microgravity&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;for long-duration space missions. (Image credit: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One thing to remember though is that NASA isn't the only space agency that has astronauts onboard the International Space Station (ISS), the station has scientific laboratories of many nations, and each may run their own or joint experiments through various scientific research institutions. In US alone, NASA quotes involvement from 21 institutions in 13 states. But to name a few examples, off the top of my head, some such &lt;em&gt;biological research&lt;/em&gt; includes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Virus and protein crystal (macromolecular crystallization) growth that has big potential and e.g. they have managed to grow &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/1998/notebook/msad22jul98_1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;extremely potent insulin crystals&lt;/a&gt; a lot larger than what's possible in gravity, a research that might in time help millions of people that suffer from diabetes.&lt;/li&gt;&#xA;&lt;li&gt;Medical research in effects of prolonged stay in microgravity on human health (bone structure, ocular health, body mass loss, cardiovascular health and so on due to e.g. loss of intracranial pressure gradient).&lt;/li&gt;&#xA;&lt;li&gt;Plant and cell growth in microgravity, resistance of microbiological specimens to increased radiation environment, studying cell mitosis, capillary action and metabolism processes, formation of cell structures (like roots, stems, etc.).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And many others, and the list would be too long to include here. But to somewhat shorten this by giving you a better overview, here's an excerpt from SpaceRef article following a press release on &lt;a href=&quot;http://spaceref.com/news/viewpr.html?pid=41987&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Spaceflight Research Opportunities in Space Biology&lt;/a&gt; that is fairly recent:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This NASA Research Announcement (NRA) solicits hypothesis-driven&#xA;  space-flight research in Space Biology (SB) to be conducted on ISS.&#xA;  NASA SB experiments have one or more of the following primary goals:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;to effectively use microgravity and the other characteristics of&#xA;  the space environment to enhance our understanding of basic biological&#xA;  processes;&lt;/li&gt;&#xA;  &lt;li&gt;to develop the scientific and technological foundations&#xA;  for a safe, productive human presence in space for extended periods&#xA;  and in preparation for exploration; and &lt;/li&gt;&#xA;  &lt;li&gt;to apply this knowledge and&#xA;  technology to improve our nation's competitiveness, education, and the&#xA;  quality of life on Earth.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, as you can see, this would cover a pretty wide range of scientific research in space biology. If it interests you, we also have many related questions on our site with interesting answers, for example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1074/can-plants-grow-in-microgravity&quot;&gt;Can plants grow in microgravity?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2187/what-are-the-criteria-to-select-plants-for-study-in-space&quot;&gt;What are the criteria to select plants for study in space?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2444/what-medicines-have-come-from-the-space-program&quot;&gt;What medicines have come from the Space Program?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/219/does-long-term-residence-on-the-iss-affect-eyesight&quot;&gt;Does long-term residence on the ISS affect eyesight?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/187/is-it-possible-to-prevent-muscular-atrophy-in-microgravity-with-proper-exercises&quot;&gt;Is it possible to prevent muscular atrophy in microgravity with proper exercises?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2611/how-do-astronauts-battle-loss-in-blood-volume-in-microgravity&quot;&gt;How do astronauts battle loss in blood volume in microgravity?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3112/have-extremophiles-and-anaerobic-extremophiles-been-studied-in-space&quot;&gt;Have extremophiles and anaerobic extremophiles been studied in space?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And some suggested tags to help you search our site are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;/questions/tagged/biology&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;biology&amp;#39;&quot; rel=&quot;tag&quot;&gt;biology&lt;/a&gt; &lt;a href=&quot;/questions/tagged/astrobiology&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;astrobiology&amp;#39;&quot; rel=&quot;tag&quot;&gt;astrobiology&lt;/a&gt; &lt;a href=&quot;/questions/tagged/biochemistry&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;biochemistry&amp;#39;&quot; rel=&quot;tag&quot;&gt;biochemistry&lt;/a&gt; &lt;a href=&quot;/questions/tagged/extremophile&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;extremophile&amp;#39;&quot; rel=&quot;tag&quot;&gt;extremophile&lt;/a&gt; &lt;a href=&quot;/questions/tagged/anaerobic-organism&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;anaerobic-organism&amp;#39;&quot; rel=&quot;tag&quot;&gt;anaerobic-organism&lt;/a&gt; &lt;a href=&quot;/questions/tagged/medical&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;medical&amp;#39;&quot; rel=&quot;tag&quot;&gt;medical&lt;/a&gt; &lt;a href=&quot;/questions/tagged/health&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;health&amp;#39;&quot; rel=&quot;tag&quot;&gt;health&lt;/a&gt; &lt;a href=&quot;/questions/tagged/cardiovascular&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;cardiovascular&amp;#39;&quot; rel=&quot;tag&quot;&gt;cardiovascular&lt;/a&gt; &lt;a href=&quot;/questions/tagged/pressure-gradient-loss&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;pressure-gradient-loss&amp;#39;&quot; rel=&quot;tag&quot;&gt;pressure-gradient-loss&lt;/a&gt; &lt;a href=&quot;/questions/tagged/intracranial&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;intracranial&amp;#39;&quot; rel=&quot;tag&quot;&gt;intracranial&lt;/a&gt; &lt;a href=&quot;/questions/tagged/microgravity&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;microgravity&amp;#39;&quot; rel=&quot;tag&quot;&gt;microgravity&lt;/a&gt; &lt;a href=&quot;/questions/tagged/radiation&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;radiation&amp;#39;&quot; rel=&quot;tag&quot;&gt;radiation&lt;/a&gt; &lt;a href=&quot;/questions/tagged/humans&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;humans&amp;#39;&quot; rel=&quot;tag&quot;&gt;humans&lt;/a&gt; &lt;a href=&quot;/questions/tagged/plants&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;plants&amp;#39;&quot; rel=&quot;tag&quot;&gt;plants&lt;/a&gt; &lt;a href=&quot;/questions/tagged/animals&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;animals&amp;#39;&quot; rel=&quot;tag&quot;&gt;animals&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And many such projects on the ISS are now run via the &lt;a href=&quot;http://www.iss-casis.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Center for the Advancement of Science in Space&lt;/a&gt; (CASIS) that in July 2011 NASA chose to be the sole manager of the International Space Station U.S. National Laboratory, which would be a good place to find more information on current biological research on the ISS. A lot of human cardiovascular health research is also done, for example, by the &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Canadian Space Agency&lt;/a&gt; (CSA) via their &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/iss/bpreg.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;BP Reg&lt;/a&gt;, &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/sciences/vascular.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;VASCULAR&lt;/a&gt; and &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/sciences/cciss.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCISS&lt;/a&gt; projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may also find this recent Forbes article interesting: &lt;a href=&quot;http://www.forbes.com/sites/alexknapp/2013/02/13/the-space-station-is-the-final-frontier-of-biological-research/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Space Station Is The Final Frontier Of Bio Research&lt;/a&gt; (includes a video ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-12-12T16:18:05.327" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3076" PostTypeId="2" ParentId="646" CreationDate="2013-12-08T11:33:56.090" Score="11" Body="&lt;p&gt;&lt;a href=&quot;http://rhodesmill.org/pyephem/&quot; rel=&quot;noreferrer&quot;&gt;PyEphem&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;PyEphem&lt;/strong&gt; provides scientific-grade astronomical computations for the Python programming language. Given a date and location on the Earth’s surface, it can compute the positions of the Sun and Moon, of the planets and their moons, and of any asteroids, comets, or earth satellites whose orbital elements the user can provide. Additional functions are provided to compute the angular separation between two objects in the sky, to determine the constellation in which an object lies, and to find the times at which an object rises, transits, and sets on a particular day.&lt;/p&gt;&#xA;&lt;p&gt;The numerical routines that lie behind PyEphem are those from the wonderful &lt;a href=&quot;http://www.clearskyinstitute.com/xephem/&quot; rel=&quot;noreferrer&quot;&gt;XEphem&lt;/a&gt; astronomy application, whose author, Elwood Downey, generously gave permission for us to use them as the basis for PyEphem.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://rhodesmill.org/pyephem/examples.html&quot; rel=&quot;noreferrer&quot;&gt;jovian_moon_chart.py&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This script prints out where the Jovian moons are around Jupiter for the next few days.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;import ephem&#xA;&#xA;moons = ((ephem.Io(), 'i'),&#xA;         (ephem.Europa(), 'e'),&#xA;         (ephem.Ganymede(), 'g'),&#xA;         (ephem.Callisto(), 'c'))&#xA;&#xA;# How to place discrete characters on a line that actually represents&#xA;# the real numbers -maxradii to +maxradii.&#xA;&#xA;linelen = 65&#xA;maxradii = 30.&#xA;&#xA;def put(line, character, radii):&#xA;    if abs(radii) &amp;gt; maxradii:&#xA;        return&#xA;    offset = radii / maxradii * (linelen - 1) / 2&#xA;    i = int(linelen / 2 + offset)&#xA;    line[i] = character&#xA;&#xA;interval = ephem.hour * 3&#xA;now = ephem.now()&#xA;now -= now % interval&#xA;&#xA;t = now&#xA;while t &amp;lt; now + 2:&#xA;    line = [' '] * linelen&#xA;    put(line, 'J', 0)&#xA;    for moon, character in moons:&#xA;        moon.compute(t)&#xA;        put(line, character, moon.x)&#xA;    print str(ephem.date(t))[5:], ''.join(line).rstrip()&#xA;    t += interval&#xA;&#xA;print 'East is to the right;',&#xA;print ', '.join([ '%s = %s' % (c, m.name) for m, c in moons ])&#xA;3/2 12:00:00                         g e     J   i                    c&#xA;3/2 15:00:00                        ge       J    i                    c&#xA;3/2 18:00:00                      g e        J     i                   c&#xA;3/2 21:00:00                     g e         J    i                    c&#xA;3/3 00:00:00                    g  e         J  i                       c&#xA;3/3 03:00:00                   g   e         Ji                         c&#xA;3/3 06:00:00                  g    e       i J                          c&#xA;3/3 09:00:00                  g     e   i    J                          c&#xA;3/3 12:00:00                 g       e i     J                          c&#xA;3/3 15:00:00                 g        ie     J                          c&#xA;3/3 18:00:00                 g         i e   J                          c&#xA;3/3 21:00:00                 g           i e J                          c&#xA;3/4 00:00:00                 g             i e                          c&#xA;3/4 03:00:00                  g              Jie                        c&#xA;3/4 06:00:00                  g              J  ie                      c&#xA;3/4 09:00:00                   g             J    ie                   c&#xA;East is to the right; i = Io, e = Europa, g = Ganymede, c = Callisto&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="1225" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-12-30T11:05:34.447" CommentCount="4" CommunityOwnedDate="2013-12-08T11:33:56.090" ContentLicense="CC BY-SA 3.0" />
  <row Id="3077" PostTypeId="1" AcceptedAnswerId="3081" CreationDate="2013-12-09T04:39:08.710" Score="6" ViewCount="2145" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Boeing_787_Dreamliner&quot;&gt;Wikipedia&lt;/a&gt; says this about the Boeing Dreamliner:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The 787 was designed to be the first production airliner with the fuselage assembled with one-piece composite barrel sections instead of the multiple aluminum sheets and some 50,000 fasteners used on existing aircraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Weight is a major problem in space/launch activities. In this context, these questions come to mind:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Can composites be used by space programs for launch vehicles, the spacecraft itself, or both?&lt;/li&gt;&#xA;&lt;li&gt;What arguments are there against the use of composites in this context?&#xA;&lt;ul&gt;&#xA;&lt;li&gt;E.g. Flammable, Porous&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Is there any launch vehicle/spacecraft that presently uses composites extensively?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorDisplayName="user12" LastEditDate="2013-12-09T04:49:35.780" LastActivityDate="2013-12-14T23:43:22.283" Title="Are composites used by space programs?" Tags="|rockets|spacecraft-development|materials|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3078" PostTypeId="1" AcceptedAnswerId="3082" CreationDate="2013-12-09T04:54:39.510" Score="12" ViewCount="1201" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/LLTV&quot; rel=&quot;noreferrer&quot;&gt;Lunar Landing Training Vehicle&lt;/a&gt; was used during the Apollo missions to simulate flying the lunar lander at 1/6th the force of Earth's gravity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/r3EyT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many sources have indicated a conflict between astronauts and management about the trainer - with the management deeming it dangerous and trying to get rid of it, and the astronauts wanting to keep it, calling it a vital training device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that astronauts had to bail out of failing LLTVs more than once - which would certainly cause me to think it's slightly dangerous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/stfk8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Stuart Present ejecting from a failing LLTV&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I've never seen anything which mentions the exact reason the trainer was unreliable - was it simply designed or built too quickly? Was there a vital design flaw? A certain piece of hardware that continually failed?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-22T13:34:07.560" LastActivityDate="2015-11-20T06:00:13.660" Title="What caused the Lunar Landing Training Vehicle to be unreliable?" Tags="|apollo-program|reliability|training|lltv|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3079" PostTypeId="5" CreationDate="2013-12-09T05:00:26.357" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-09T05:00:26.357" LastActivityDate="2013-12-09T05:00:26.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3080" PostTypeId="4" CreationDate="2013-12-09T05:00:26.357" Score="0" Body="Questions regarding the Lunar Lander Training Vehicle used to simulate flying the Lunar Module in 1/6 gravity." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-09T05:00:26.357" LastActivityDate="2013-12-09T05:00:26.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3081" PostTypeId="2" ParentId="3077" CreationDate="2013-12-09T05:45:50.483" Score="9" Body="&lt;p&gt;From what launcher components are currently using composite materials, this slide from NASA's &lt;a href=&quot;http://www.raes.org.au/assets/Presentations/Presentation-Dr-Mark-Shuart.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced Composite Structures and Materials Technologies for Launch Vehicles presentation&lt;/a&gt; (PDF) tells the story pretty well on composites on current launch vehicles (presentation is from March 2011):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/zfTzF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, some suborbital space planes also use composites, like for example the all-carbon-composite body &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipTwo&quot; rel=&quot;nofollow noreferrer&quot;&gt;Virgin Galactic SpaceShipTwo&lt;/a&gt; and to a various degree likely all of the Virgin Galactic suborbital fleet, including &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipOne&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceShipOne&lt;/a&gt; and their motherships that they're launched in-air from, &lt;a href=&quot;https://en.wikipedia.org/wiki/White_Knight_Two&quot; rel=&quot;nofollow noreferrer&quot;&gt;White Knight Two&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Scaled_Composites_White_Knight&quot; rel=&quot;nofollow noreferrer&quot;&gt;White Knight&lt;/a&gt;, respectively. They're all built by a California based &lt;a href=&quot;https://en.wikipedia.org/wiki/Northrop_Grumman&quot; rel=&quot;nofollow noreferrer&quot;&gt;Northrop Grumman corporation&lt;/a&gt; aerospace company &lt;a href=&quot;https://en.wikipedia.org/wiki/Scaled_Composites&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scaled Composites&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/5pyYi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Virgin Galactic's future orbital launch vehicle LauncherOne will use any of its parts made of composites is however unclear from what little of &lt;a href=&quot;http://www.bbc.co.uk/blogs/thereporters/jonathanamos/2009/11/launcherone-virgins-galactics.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;its proposed design&lt;/a&gt; is known to me, but it will be also launched in air from its mothership (Scaled Composites WhiteKnightTwo) that is a composite based body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA is also looking into the use of composites for future space exploration missions (not merely launchers and crew modules but also probes, landers, rovers etc.) and has announced &lt;a href=&quot;http://www.nasa.gov/press/2013/september/nasa-announces-advanced-composite-research-partnership/&quot; rel=&quot;nofollow noreferrer&quot;&gt;composites research partnership&lt;/a&gt; with Bell, GE, Lockheed Martin, Northrop Grumman, Boeing, United Technologies Corporation and Pratt &amp;amp; Whitney in September 2013.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has previously been working on a design for an &lt;a href=&quot;http://www.nasa.gov/mission_pages/constellation/orion/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion crew exploration vehicle&lt;/a&gt; pressure shell &lt;a href=&quot;http://www.nasa.gov/offices/nesc/press/070205.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;made entirely of composite materials&lt;/a&gt; to gain in-house composites experience, &lt;a href=&quot;http://www.nasa.gov/press/2013/july/nasa-tests-game-changing-composite-cryogenic-fuel-tank/&quot; rel=&quot;nofollow noreferrer&quot;&gt;testing composite cryogenic fuel tank&lt;/a&gt; and can use a lighter &lt;a href=&quot;http://www.compositesworld.com/articles/optimizing-the-ares-v-payload-shroud&quot; rel=&quot;nofollow noreferrer&quot;&gt;composite payload shroud&lt;/a&gt; on some of the launch vehicles if needs be, so they clearly have intentions in using composites more in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX Falcon 9 also uses what looked to me a carbon-fibre reinforced polymer payload fairing to me (judging by images presented in a webcast stream before its latest &lt;em&gt;flight seven&lt;/em&gt;), but it is certainly made of composites, as &lt;a href=&quot;http://www.spacex.com/falcon9&quot; rel=&quot;nofollow noreferrer&quot;&gt;stated on SpaceX page on Falcon 9&lt;/a&gt;. Together with NASA, they also built a &lt;a href=&quot;http://www.nasa.gov/offices/oce/appel/ask/issues/40/40s_space-x_prt.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;carbon-composite carrier structure for the first Dragon spacecraft heat shield&lt;/a&gt; (Photo Credit: SpaceX/Roger Gilbertson):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Z8wcw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-09T05:51:00.037" LastActivityDate="2013-12-09T05:51:00.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3082" PostTypeId="2" ParentId="3078" CreationDate="2013-12-09T11:33:33.030" Score="14" Body="&lt;p&gt;There were &lt;a href=&quot;http://en.wikipedia.org/wiki/Spaceflight_non-fatal_training_accidents&quot;&gt;3 LLTV crashes&lt;/a&gt;. One was caused by &lt;a href=&quot;http://www.clavius.org/techlltv.html&quot;&gt;running out of the helium&lt;/a&gt; that pressurized the attitude control system, in the second crash the LLTV was flying in &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/000153.html&quot;&gt;more wind than it was designed for&lt;/a&gt;, the third was a &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/000153.html&quot;&gt;malfunction in the flight control system&lt;/a&gt;, while testing a &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Landing_Research_Vehicle&quot;&gt;major modification to the LLTV's computer system&lt;/a&gt;.&lt;br&gt;&#xA;Like the Harrier, it was rather unforgiving. If the aircraft's attitude goes too far off the horizontal, you end up not having enough thrust in the manauvering thrusters to get it upright again. You basically fall off the thrust column and will crash.&lt;br&gt;&#xA;Cernan called it a &quot;&lt;a href=&quot;http://www.hq.nasa.gov/alsj/a17/a17-lltv.html&quot;&gt;very unstable vehicle&lt;/a&gt;&quot;. &#xA;So, 3 crashes in a couple hundred flights. It sounds like a lot, but the LLTV was a very experimental vehicle, in a flight regime where any malfunction would lead to a crash. With 3 different causes, I don't see much evidence for bad design or build quality. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-12-09T11:33:33.030" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3083" PostTypeId="1" AcceptedAnswerId="3084" CreationDate="2013-12-06T09:16:42.277" Score="28" ViewCount="30503" Body="&lt;p&gt;All the artificial satellites launched in space have a series of solar cells attached to them. When in use, these panels absorb a lot of radiation from the Sun and put it to use. So, do these panels get hot with exposure to radiation as on Earth? Since there is no medium available in space to dissipate the heat, how is it relieved?&lt;/p&gt;&#xA;" OwnerUserId="1231" OwnerDisplayName="Sylar" LastEditorDisplayName="user12" LastEditDate="2013-12-09T16:19:30.683" LastActivityDate="2017-08-05T04:11:41.337" Title="How is heat dissipated from a satellite or any metal in space?" Tags="|temperature|thermal-control|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3084" PostTypeId="2" ParentId="3083" CreationDate="2013-12-06T09:23:53.747" Score="33" Body="&lt;p&gt;Convection isn't a usable heat dissipating function in space, because there is no convecting medium. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiation, however, works extremely well. Any body in space will radiate in a wide range of wavelengths, and will also absorb radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So any part of a space craft or satellite facing the sun will absorb heat (a net gain) and any facing away will radiate heat (a net loss). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellite designers minimise the absorption of energy by the use of reflective foil wherever possible, and provide heat radiating fins in order to dissipate excess heat on the dark side of the satellite.&lt;/p&gt;&#xA;" OwnerUserId="38" OwnerDisplayName="Rory Alsop" LastActivityDate="2013-12-06T09:23:53.747" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3085" PostTypeId="2" ParentId="3077" CreationDate="2013-12-09T16:40:23.327" Score="5" Body="&lt;p&gt;Some additions to TildalWave's answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS solar arrays are supported by telescoping masts that use bent fiberglass rods for the telescoping capability and to provide structural rigidity. (Source: &lt;a href=&quot;http://web.mscsoftware.com/support/library/conf/wuc95/p02895.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the final Hubble servicing mission, the carriers that mounted the new instruments in Atlantis's payload bay were made entirely of composite materials. (Source: &lt;a href=&quot;http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SLIC_FS_HTML.html&quot; rel=&quot;nofollow&quot;&gt;This NASA page&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2013-12-09T16:40:23.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3087" PostTypeId="2" ParentId="3053" CreationDate="2013-12-10T00:34:20.893" Score="2" Body="&lt;p&gt;According to the latest JSPOC reports, there were only 2 objects in the launch, one which is SES, and another which looks like a booster. As it's been almost a week, it looks like in fact this launch did not include any extra objects.&lt;/p&gt;&#xA;&lt;p&gt;Source is &lt;a href=&quot;https://celestrak.org/NORAD/elements/tle-new.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt;, viewed Dec 10 2013. Here's the current TLE's for launch 71 (SES/Falcon 9). Only the two objects there.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;2013-071A               &#xA;1 39460U 13071A   13343.67928412 -.00000371  00000-0  00000+0 0   102&#xA;2 39460   1.2047 242.7304 3426221 179.1602  40.9644  0.53381995    40&#xA;2013-071B               &#xA;1 39461U 13071B   13343.10381580  .00000511  00000-0  11828-1 0    51&#xA;2 39461  20.4397 242.1309 8535840 180.6297 176.0625  0.87328304    54&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2013-12-10T17:53:47.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3088" PostTypeId="1" AcceptedAnswerId="3089" CreationDate="2013-12-10T09:05:51.280" Score="13" ViewCount="824" Body="&lt;p&gt;Now that the Space Shuttle has been retired, are there any spacecraft available that can retrieve an object from orbit and return it safely to Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the maximum size of such an object?&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorUserId="1142" LastEditDate="2015-08-25T13:06:45.693" LastActivityDate="2019-07-17T18:55:18.553" Title="Are there any remaining spacecraft that can retrieve objects from Earth orbit?" Tags="|orbital-mechanics|orbit|orbital-maneuver|payload|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3089" PostTypeId="2" ParentId="3088" CreationDate="2013-12-10T14:29:06.087" Score="10" Body="&lt;p&gt;There are several issues here.  Volume, and mass for return through the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is of course the next question of, how do I get this object? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz&lt;/a&gt; can return about 100-150 lbs of mass.  Volume wise it is very limited, since the descent capsule is very cramped and needs room for three cosmonauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Dragon_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dragon&lt;/a&gt; from &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt; is the only other vendor publicly available.  (X-37B is a classified system, so who knows).  Internally it has more volume, but has a 'door' that is volume limiting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has reported about 2500KG downmass.  In principle, they could probably fill the capsules entire volume.  But the door on the cargo Dragon is the size of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Common_berthing_mechanism&quot; rel=&quot;nofollow noreferrer&quot;&gt;CBM&lt;/a&gt; port which is 50 inches in diameter.  So the ultimate size is limited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of 'getting' the object to be retrieved, right now, the answer is via the space station, or via something a Dragon or Soyuz can dock/berth too.  Aside from unknown capabilities of the X37-B, there is no system available to approach, grapple, and the return with an object free floating. If you want to return something from the ISS, Dragon or Soyuz might be able to help.  If it is a satellite in orbit on its own, not many options at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CRS-2 contract includes funding for a cargo version of Dream Chaser that will be able to dock at the ISS and return measurable amounts of cargo. But same issues of docking, grappling, bay doors, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is proposing a cargo/satellite version of Starship, expecting it to capture the vast majority of commercial orbital lift. If they can meet their price point or even close, they will be able to undercut every other launch provider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That will require a vehicle that can disgorge a payload, and will have tons of performance, and likely could recover something if someone was willing to pay for it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2019-07-17T17:52:19.673" LastActivityDate="2019-07-17T17:52:19.673" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="3090" PostTypeId="2" ParentId="2218" CreationDate="2013-12-10T17:45:41.783" Score="1" Body="&lt;p&gt;This TED talk addresses the issue, but not from the standpoint of space habitats, just looking at oxygen in indoor environments:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ted.com/talks/kamal_meattle_on_how_to_grow_your_own_fresh_air.html&quot; rel=&quot;nofollow&quot;&gt;http://www.ted.com/talks/kamal_meattle_on_how_to_grow_your_own_fresh_air.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He gives estimates of number of plants needed per person.  You might want to look into the biodome projects as well:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Biosphere_2&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Biosphere_2&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1234" LastActivityDate="2013-12-10T17:45:41.783" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3091" PostTypeId="2" ParentId="3077" CreationDate="2013-12-10T19:20:47.623" Score="8" Body="&lt;ul&gt;&#xA;&lt;li&gt;Are composites used (extensively) by launch vehicles or spacecraft?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Yes, almost since NASA's beginning.  For example, the &lt;a href=&quot;https://conservationsolutionsinc.com/projects/view/63/saturn-v-rocket-conservation-johnson-space-center&quot; rel=&quot;nofollow&quot;&gt;Saturn V used composites.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that &quot;composite&quot; is a general term, meaning fibers held in a matrix.  Carbon fiber is only one kind of fiber, albeit a fashionable one with a high Young's modulus.  Fiberglass, basalt, and aramid (Kevlar) are other fibers.  Matrices include thermoset plastics like polyester, and many kinds of epoxy. Even plywood is technically a composite: wood fibers bound in a glue matrix.  Non-carbon-fiber composites are plenty strong enough for primary structure: Rutan's airplanes, countless sailboats, the plywood De Havilland Mosquito.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What arguments are there against the use of composites in this context?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If manufacturing isn't tightly controlled (temperature-pressure schedule during cure of the matrix, for instance), variations will be found in the composite's properties: density, thermal conductivity, stress-strain curves, resistance to crack propagation, resistance to delamination, yada yada.  Risk-averse engineers prefer metal: if you've seen one ingot of aluminum, you've seen them all.  (Back in the Saturn V days, a business-world saying was that nobody ever got fired for recommending IBM.)&lt;/p&gt;&#xA;" OwnerUserId="1235" LastEditorUserId="1235" LastEditDate="2013-12-13T18:49:29.230" LastActivityDate="2013-12-13T18:49:29.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3092" PostTypeId="2" ParentId="2102" CreationDate="2013-12-10T19:28:36.287" Score="16" Body="&lt;p&gt;Kármán himself would have defined it for nonterrestrial bodies just like he did for Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The height where the atmosphere becomes so thin that aerodynamic lift strong enough to support an aircraft happens only at airspeeds faster than orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you might as well call that the boundary between atmosphere and space, as far as propulsion is concerned.  &lt;a href=&quot;http://books.google.com/books?id=dTwIDun4MroC&amp;amp;pg=PA84#v=onepage&amp;amp;q&amp;amp;f=false&quot;&gt;http://books.google.com/books?id=dTwIDun4MroC&amp;amp;pg=PA84#v=onepage&amp;amp;q&amp;amp;f=false&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1235" LastActivityDate="2013-12-10T19:28:36.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3093" PostTypeId="1" AcceptedAnswerId="3876" CreationDate="2013-12-10T20:27:36.063" Score="18" ViewCount="3343" Body="&lt;p&gt;How does the Voyager 1 star tracker lock onto a specific star and how reliable is it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there approaches in more modern trackers used such as pattern recognition or other methods which would enable to track more stars to give more reliable fix and avoid problems if tracker mistakes one star for another?&lt;/p&gt;&#xA;" OwnerUserId="98" LastEditorUserId="98" LastEditDate="2013-12-10T21:08:46.487" LastActivityDate="2014-03-05T09:41:26.877" Title="How does the star tracker in Voyager 1 lock onto a specific star?" Tags="|voyager|navigation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3094" PostTypeId="1" AcceptedAnswerId="3104" CreationDate="2013-12-10T21:34:20.083" Score="11" ViewCount="13685" Body="&lt;p&gt;While reading up on the &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_(rocket)&quot; rel=&quot;noreferrer&quot;&gt;Soyuz rocket&lt;/a&gt;, I noticed this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Soyuz rockets are assembled horizontally in the MIK Building at the launch site. The rocket is then rolled out, and erected on the launch pad.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;/sup&gt;&lt;img src=&quot;https://i.stack.imgur.com/yriYA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Soyuz rocket assembly: the first and second stages are in the background, already joined together; the third stage is in the lower&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;left corner of the image. The Soyuz spacecraft, covered by its launch shroud, is in the lower right corner. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_%28rocket_family%29&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/2674/do-any-of-the-developing-space-programs-use-horizontal-launcher-assembly-and-lau&quot;&gt;Tildal's question&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is it assembled horizontally, and only righted on the launch pad? What are the advantages and disadvantages to building a vehicle this way?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-10-15T12:52:00.620" Title="What are the advantages and disadvantages of assembling a launch vehicle horizontally?" Tags="|rockets|launch-site|launchpad|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3095" PostTypeId="1" AcceptedAnswerId="3099" CreationDate="2013-12-10T21:44:43.300" Score="15" ViewCount="1951" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Sokol_space_suit&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article on the Sokol space suit&lt;/a&gt; seems to suggest that all personel traveling on a Soyuz vehicle must wear a Russian made spacesuit:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Each Soyuz crew member is provided with a made-to-measure suit for flight although, from the numbers made, it appears that the suits provided for ground training are re-used.&lt;/p&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;p&gt;The suit is worn during launch and re-entry of the Soyuz spacecraft—the gloves are attached and the visor is sealed at these times.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;                                                   &lt;img src=&quot;https://i.stack.imgur.com/WdOB6.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;                                        &lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Peggy_Whitson&quot; rel=&quot;noreferrer&quot;&gt;Peggy Whitson&lt;/a&gt; suited up in pressurized Sokol-KV2 suit.&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;From this, it appears that American astronauts travelling on the vehicle would also wear the Soviet suit. Is this true? If so, why aren't the American suits adequate?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2013-12-10T22:55:45.627" Title="Do American astronauts wear Sokol space suits when flying on a Soyuz?" Tags="|spacesuits|russia|soyuz-spacecraft|sokol-spacesuit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3096" PostTypeId="1" AcceptedAnswerId="3100" CreationDate="2013-12-10T21:49:59.223" Score="17" ViewCount="1671" Body="&lt;p&gt;According to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sokol_space_suit&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article on the Sokol spacesuit&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Sokol space suit (Russian: Cокол, Falcon) is a type of Russian space suit, worn by all who fly on the Soyuz spacecraft. It was introduced in 1973 and is still used as of 2013. &lt;strong&gt;The Sokol is described by its makers as a rescue suit, and it is not capable of being used outside the spacecraft in a spacewalk or extra-vehicular activity&lt;/strong&gt;. Instead, its purpose is to keep the wearer alive in the event of an accidental depressurisation of the spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/P6G5H.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The KV-2 variant of the Sokol space suit.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about the suit makes it not usable during EVA? Would wearing the suit during EVA mean instant death, or would it just mean increased risk?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-11T00:26:18.027" LastActivityDate="2013-12-11T00:26:18.027" Title="Why is the Sokol spacesuit considered inadequate for EVA?" Tags="|spacesuits|eva|sokol-spacesuit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3097" PostTypeId="5" CreationDate="2013-12-10T21:52:28.410" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-10T21:52:28.410" LastActivityDate="2013-12-10T21:52:28.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3098" PostTypeId="4" CreationDate="2013-12-10T21:52:28.410" Score="0" Body="Questions regarding the Sokol spacesuit developed by the Soviets." OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2013-12-10T21:52:28.410" LastActivityDate="2013-12-10T21:52:28.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3099" PostTypeId="2" ParentId="3095" CreationDate="2013-12-10T22:54:24.997" Score="16" Body="&lt;p&gt;Yes.  Sokol suits are suits that are highly specialized to the Soyuz spacecraft.  An easy example of the specialization is the extra room in the hindquarters region to accommodate how far they have to bring their knees in when sitting in the couches.  The American suits (by which I assume you mean the ACES suits), were not designed for this purpose -- they were designed to sit in a normal seat and to facilitate bailout (hence the parachute and the international orange color).&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2013-12-10T22:54:24.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3100" PostTypeId="2" ParentId="3096" CreationDate="2013-12-10T23:08:45.430" Score="18" Body="&lt;p&gt;The suit does not contain its own life support system.  It is ventilated from a blower or, in case of a decompression, pressurized bottles within the spacecraft.  It also does not have any meteoroid protection, and it tends to balloon when pressurized, restricting movement.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2013-12-10T23:08:45.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3101" PostTypeId="2" ParentId="2102" CreationDate="2013-12-11T00:59:02.187" Score="18" Body="&lt;p&gt;I am not aware of anyone using a Kármán line on another planet, but it's not hard to calculate.  I get about 88 km for Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equation is &lt;span class=&quot;math-container&quot;&gt;$r\,\rho(r)={2m\over A\,C_L}$&lt;/span&gt;, where &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; is the radius of the Kármán line from the center of the body, &lt;span class=&quot;math-container&quot;&gt;$\rho$&lt;/span&gt; is the density as a function of the radius, &lt;span class=&quot;math-container&quot;&gt;$m$&lt;/span&gt; is the mass of the aircraft, &lt;span class=&quot;math-container&quot;&gt;$A$&lt;/span&gt; is the planform area, and &lt;span class=&quot;math-container&quot;&gt;$C_L$&lt;/span&gt; is the coefficient of lift.  von Kármán picked some representative values for &lt;span class=&quot;math-container&quot;&gt;$m\over A$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$C_L$&lt;/span&gt;, which I don't know.  But I don't need to know.  I can calculate &lt;span class=&quot;math-container&quot;&gt;${2m\over A\,C_L}$&lt;/span&gt; by plugging the Kármán line for Earth into &lt;span class=&quot;math-container&quot;&gt;$r\,\rho(r)$&lt;/span&gt;.  I get that &lt;span class=&quot;math-container&quot;&gt;${2m\over A\,C_L}=3.6\,\mathrm{kg\over m^2}$&lt;/span&gt;.  Use that and the density profile of any other body to get its Kármán line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By this definition, you are already in space lying on your back on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though I don't know of anyone using the Kármán line outside of Earth, what &lt;em&gt;is&lt;/em&gt; in use is the entry interface altitude.  By definition, the entry phase has begun when this altitude is crossed.  It is sufficiently high to have no perceptible accelerations from the atmosphere, but low enough that you don't have to wait very long to see such accelerations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At Earth for Apollo it was defined as 400,000 feet, or about 122 km.  At Mars it has been defined as 125 km.  That is when the &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; minutes of terror begins.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="1235" LastEditDate="2020-01-28T17:29:02.543" LastActivityDate="2020-01-28T17:29:02.543" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="3102" PostTypeId="2" ParentId="3094" CreationDate="2013-12-11T03:41:01.693" Score="2" Body="&lt;p&gt;&lt;strong&gt;Advantage&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;It doesn't require as tall assembly facilities (in case is vertical assemblies requires tall assembly towers and the tools are stacked, starting with the&#xA;first stage (includes strap on boosters) and often ending with&#xA;the attachment of the spacecraft to&#xA;the launch vehicle.)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;one needs vehicle access and high lifting  cranes to lift heavy weight to their respective floors in vertical assemblies but in horizontal assemblies requires large tower(instead of tall) and the building is structurely less complex&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Disadvantages&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;You need proper support for the rocket (you cannot build a rocket by placing it in ground)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;there may be some nuts and bolts or welds which one needs to climb ladder  to do that..&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-12-11T15:14:27.983" LastActivityDate="2013-12-11T15:14:27.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3103" PostTypeId="1" CreationDate="2013-12-11T05:06:41.263" Score="24" ViewCount="3667" Body="&lt;p&gt;After the problem realized when trying to put up three astronauts for an extended stay in the Apollo 13 LEM, namely that the square CM CO₂ scrubbers didn't fit in the LEM's slots, were they redesigned?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rsfjx.png&quot; alt=&quot;Apollo 13 &amp;quot;Mailbox&amp;quot; CO₂ scrubber adapter rig&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="881" LastEditorUserId="881" LastEditDate="2016-12-16T20:08:59.723" LastActivityDate="2016-12-16T22:36:17.697" Title="Were the Apollo CO₂ scrubber canister designs unified after Apollo 13?" Tags="|apollo-program|life-support|apollo-13|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3104" PostTypeId="2" ParentId="3094" CreationDate="2013-12-11T09:04:32.823" Score="9" Body="&lt;p&gt;It's just easier access and more convenient to work on a tall thing that's on its side, rather than having to pick stuff up with cranes to put on top of the tall thing, and require lots of gantries for access.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However for the most efficient structure, i.e. the lightest, you would like to not add load cases for ground handling that are substantially different from flight.  Launch vehicles are designed to handle loads predominantly in the longitudinal direction.  The orthogonal loads are much smaller (unless you lose attitude control, in which case you see spectacular breakups as large aerodynamic forces are applied in other directions).  If you build it up vertically, then the ground handling loads are in the same direction as the flight loads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If on the other hand you build it horizontally, now there are lots of new load cases for the structure to handle that it doesn't need for flight, where parts of the structure are supported on one side to take the weight of the whole structure.  There are even more load cases in the process of erecting the launch vehicle, taking it through all angles between 0° and 90°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These load cases might not be too bad, since you never load propellant horizontally, only vertically.  Far and away most of the mass is in the propellant.  Still, you can make a more efficient structure if it is kept vertical from the get go.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-11T17:27:58.123" LastActivityDate="2013-12-11T17:27:58.123" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3105" PostTypeId="1" CreationDate="2013-12-11T10:43:40.770" Score="5" ViewCount="648" Body="&lt;p&gt;Is the Falcon 9 a &lt;em&gt;launch vehicle&lt;/em&gt; or a &lt;em&gt;spacecraft&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the key design features and why did NASA decide to use it for cargo instead of its own spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="1141" LastEditorUserId="709" LastEditDate="2013-12-12T16:56:07.207" LastActivityDate="2014-02-28T14:28:03.560" Title="What are the basic concepts of the SpaceX Falcon 9?" Tags="|spacex|falcon-9|" AnswerCount="2" CommentCount="0" FavoriteCount="0" ClosedDate="2014-03-06T13:08:25.920" ContentLicense="CC BY-SA 3.0" />
  <row Id="3107" PostTypeId="2" ParentId="3105" CreationDate="2013-12-11T12:44:45.100" Score="11" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Falcon_9&quot;&gt;Falcon 9&lt;/a&gt; is a launch vehicle. There have been two versions so far, with a third variant and a heavy lift version expected in 2014-2015 timeframe.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon 9 V1.0 was the original design, using Merlin 1C engines and the engines were arranged in a tic-tac-toe, 3X3 grid on the first stage.  Five of these were launched.&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 v1.1 is the new model going forward.  It uses a more powerful Merlin 1D engine, and arranges them in the Octaweb configuration.  Eight engines in a circle, directly in line with the sidewall of the tanks, and a ninth engine in the center.  It is longer, due to higher fuel usage by the upgraded engine.&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9R is a tentative name for a reusable first stage F9 v1.1.  It may become a synonym for the 1.1 and is not clear at this time.&lt;/li&gt;&#xA;&lt;li&gt;Falcon Heavy will be a launcher, with three side by side cores, that are basically Falcon 9 first stages on their own, and thus 27 engines at liftoff.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The upper stages for all models, to date, has been a similar diameter stage, with a single Merlin engine, adapted for vacuum use.  I.e. Has a much larger bell to take advantage of reduced pressure for better performance.  (F9 1.0 used the Merlin 1C Vac, and F9 v1.1, F9R, and FH will all use Merlin 1D Vac versions initially).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29&quot;&gt;Merlin&lt;/a&gt; engines burn RP1 and LOX as fuel and oxidizer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA is using it as a fully commercial means of transportation to low Earth orbit (LEO). Development of these commercial vehicles is part of NASA plan to transfer the routine LEO operations towards ISS to commercial sector so that NASA can focus on more challenging tasks in deep space (development of SLS, Orion, etc). Falcon 9 is launching the Dragon cargo capsule to ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is not only Falcon 9 rocket (by SpaceX), but also Antares (by Orbital Sciences) - another commercial launch vehicle, carrying the Cygnus cargo spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 is a 2 stage rocket using liquid oxygen and rocket grade kerosene as fuel. It can carry 29,000 lb to LEO or 10,700 lb to geostationary transfer orbit.&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="51" LastEditDate="2013-12-11T18:24:56.180" LastActivityDate="2013-12-11T18:24:56.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3108" PostTypeId="2" ParentId="3083" CreationDate="2013-12-11T15:26:46.450" Score="17" Body="&lt;p&gt;To add to Rory's answer, satellites also use heat pipes to transfer heat from the hot side to the cold side; they are highly thermally conductive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a cube-shaped satellite, with one side facing the sun. By using heat pipes or otherwise distributing the heat, the radiative area can be increased to up to 6 times the sun-facing side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping the temperature difference between different parts of the craft low is important, as a higher gradient induces greater mechanical stress (one side expands without the other side doing so). &lt;/p&gt;&#xA;" OwnerUserId="519" LastActivityDate="2013-12-11T15:26:46.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3109" PostTypeId="1" AcceptedAnswerId="3110" CreationDate="2013-12-11T15:52:02.077" Score="14" ViewCount="822" Body="&lt;p&gt;It is looking like Mars One might continue to go forward, and even if it doesn't it is fairly likely that we will have people on Mars sooner or later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have there been any scholarly estimations of the possible effects of manned missions to Mars on the search for life there?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, I would appreciate a good summary of the main benefits and disadvantages addressed.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-10-21T19:03:08.843" Title="How would the search for life on Mars be affected by having boots on the ground?" Tags="|mars|astrobiology|crewed-spaceflight|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3110" PostTypeId="2" ParentId="3109" CreationDate="2013-12-11T17:31:20.997" Score="10" Body="&lt;p&gt;On &lt;a href=&quot;http://www.space.com/6972-steve-squyres-robot-guy-humans-mars.html&quot; rel=&quot;nofollow&quot;&gt;good authority&lt;/a&gt; (Steve Squyres, principal investigator for the Mars Exploration Rovers):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You know, I'm a robot guy, that's what I have spent most of my career&#xA;  doing, but I'm actually a very strong supporter of human spaceflight.&#xA;  I believe that the most successful exploration is going to be carried&#xA;  out by humans, not by robots.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;What Spirit and Opportunity have done in 5 1/2 years on Mars, you and&#xA;  I could have done in a good week. Humans have a way to deal with&#xA;  surprises, to improvise, to change their plans on the spot. All you've&#xA;  got to do is look at the latest Hubble mission to see that.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On the other hand, there may be some constraints which derive from planetary protection concerns, in particular protecting Earth from putative Martian organisms that the crew might bring back outside of sealed sample containers.  From &lt;a href=&quot;http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf&quot; rel=&quot;nofollow&quot;&gt;NASA's Design Reference Architecture&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In order for humans to explore Mars and return to Earth safely, it&#xA;  will be necessary to identify sites on Mars that are free of hazards&#xA;  to the Earth’s biosphere. This is because astronauts on the martian&#xA;  surface inevitably would be exposed to local martian materials such as&#xA;  dust, and the plan is to return the astronauts to Earth at the end of&#xA;  the mission. The astronauts are therefore a potential vector for the&#xA;  transport of martian dust, which must be shown in advance to be&#xA;  sufficiently safe. The Space Studies Board has recommended the&#xA;  designation of &lt;a href=&quot;http://www.nap.edu/openbook.php?record_id=10360&amp;amp;page=6&quot; rel=&quot;nofollow&quot;&gt;Zones of Minimum Biological Risk (ZBRs)&lt;/a&gt; that are&#xA;  regions demonstrated to be safe for humans. That is, astronauts would&#xA;  only be allowed in areas that are demonstrated to be safe. For the&#xA;  initial landing site, such testing would probably have been performed&#xA;  as a part of the precursor mission activities, which may include&#xA;  analysis on Earth of returned martian samples, particularly wind-blown&#xA;  dust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same planetary protection concerns require the protection of Mars habitats from human-borne microorganisms:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The strategy that was adopted for the current DRA envisions targeting&#xA;  the human landing site that would be located within an area that is&#xA;  already known to be safe to humans (a zone of minimum biological risk)&#xA;  and in which microbial contamination would be permitted.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If humans have to avoid the very thing we might be looking for, it would put into question why we would send them.  If such constraints cannot be avoided, a human expedition might be limited to the search for evidence of life in the fossil record.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-11-16T17:24:56.767" LastActivityDate="2014-11-16T17:24:56.767" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3111" PostTypeId="1" AcceptedAnswerId="3547" CreationDate="2013-12-11T20:23:06.610" Score="11" ViewCount="1021" Body="&lt;p&gt;I was reading this article on New Scientist titled &lt;a href=&quot;http://www.newscientist.com/article/dn24715-fiercest-meteor-shower-on-record-to-hit-mars-via-comet.html&quot; rel=&quot;noreferrer&quot;&gt;Fiercest meteor shower on record to hit Mars via comet&lt;/a&gt;. Long story short, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Oort_cloud&quot; rel=&quot;noreferrer&quot;&gt;Oort cloud&lt;/a&gt; comet &lt;a href=&quot;https://en.wikipedia.org/wiki/C/2013_A1&quot; rel=&quot;noreferrer&quot;&gt;C/2013 A1 Siding Spring&lt;/a&gt; is predicted to near-miss Mars to within 173,000 kilometres from its surface and can get as close as 89,000 kilometres on 19 October 2014.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In astronomical terms, this is &lt;em&gt;a close shave&lt;/em&gt; and cometary debris field radius can easily stretch many times this distance, putting debris of various size and composition on a collision course with Martian surface and intersecting orbits of man-made satellites of Mars, such as &lt;a href=&quot;http://www.isro.org/pslv-c25/mission.aspx&quot; rel=&quot;noreferrer&quot;&gt;ISRO's Mars Orbiter Mission&lt;/a&gt; and &lt;a href=&quot;http://www.nasa.gov/mission_pages/maven/main/&quot; rel=&quot;noreferrer&quot;&gt;NASA's MAVEN orbiter&lt;/a&gt; that will be both there for the &lt;em&gt;occasion&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, some of this cometary debris might be large enough to survive their entry into Martian tenuous atmosphere and impact the Martian surface. This would depend on how large chunks of the comet would chip off it prior to Periareion (Martian periapsis) and their composition, but it wouldn't be unheard of that a comet breaks up in large chunks due to gravitational stresses as it approaches another celestial body. Comet &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9&quot; rel=&quot;noreferrer&quot;&gt;Shoemaker–Levy 9&lt;/a&gt; springs to mind, but there are numerous other examples.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/GN0mn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image of Comet Shoemaker–Levy 9 fragments (total: 21), taken on May 17, 1994 (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Comet Shoemaker–Levy 9&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while Bill Cooke of NASA's Marshall Space Flight Center in Huntsville, AL said for the mentioned article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;If I could have my druthers, I'd love to be on the surface of Mars&#xA;  for this event,&quot;&lt;/em&gt; says Cooke. &lt;em&gt;&quot;It will probably be the most intense&#xA;  meteor storm on record.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Speaking of personal preferences, I honestly rather wouldn't. I guess it's easy enough to say something like that when you know you won't be there, but if the debris will be large enough to impact the ground as numerous micrometeorites, it's not really a matter of simply remembering to bring along one's umbrella. Sentences like &lt;em&gt;&quot;potentially millions of meteors per hour&quot;&lt;/em&gt; and &lt;em&gt;&quot;between 1000 and 10,000 times the density of space rocks that are normally present in low Earth orbit&quot;&lt;/em&gt; don't sound too good either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, with some such astronomical events clearly presenting a significant risk to ongoing missions to other Solar system celestials that it might be worth doing something about it, and since some of these events can be predicted to a certain degree, my question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How could NASA and other space agencies that have or will have orbiters and rovers on and around Mars at the time of such events mitigate risks involved? What risk management options do they have available to them in an event of heightened risk of impacting with cometary debris? Would it be possible to redirect orbiters into less risky orbits, reorient them to face the impacting debris with hardened side of the orbiter, or instruct rovers on land to drive to a sheltered area?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="12102" LastEditDate="2019-12-18T06:55:42.190" LastActivityDate="2019-12-18T06:55:42.190" Title="What risk management options are available for Mars orbiters and rovers to mitigate threat of impacting with Siding Spring cometary debris?" Tags="|mars|rovers|impact|risk-management|maven|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3112" PostTypeId="1" AcceptedAnswerId="3114" CreationDate="2013-12-11T22:26:55.460" Score="3" ViewCount="772" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Extremophiles&quot; rel=&quot;nofollow&quot;&gt;Extremophiles&lt;/a&gt; are organisms that can survive in physically and geothermally extreme conditions. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Anaerobe&quot; rel=&quot;nofollow&quot;&gt;anaerobic&lt;/a&gt; variety do not require oxygen in order to survive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has extremophiles' survival, in particular of anaerobic extremophiles, been experimented with in space in general?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="49" LastEditDate="2013-12-12T02:37:22.607" LastActivityDate="2013-12-12T03:37:04.343" Title="Have extremophiles and anaerobic extremophiles been studied in space?" Tags="|research|astrobiology|life|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3113" PostTypeId="2" ParentId="3112" CreationDate="2013-12-12T02:49:53.313" Score="2" Body="&lt;p&gt;Check out this &lt;a href=&quot;http://biosputnik.imbp.ru/download/pdf/BION.pdf&quot; rel=&quot;nofollow&quot;&gt;bilingual description of &lt;em&gt;Bion-M&lt;/em&gt; program&lt;/a&gt;. It included such objectives as&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Perform astrobiology-related investigations to see&#xA;whether thermophilic microbes and lichens in minerals&#xA;will remain viable after exposure to open space and&#xA;whether microbial associations will remain stable in&#xA;natural loci&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Examine biological properties of microbes exposed&#xA;to open space and study the viability of thermophilic&#xA;cultures after their mineral carriers penetrate dense&#xA;layers of Earth’s atmosphere&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-12-12T02:49:53.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3114" PostTypeId="2" ParentId="3112" CreationDate="2013-12-12T03:37:04.343" Score="5" Body="&lt;p&gt;There were several such experiments conducted on the International Space Station (ISS), but perhaps most notable one is &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Human_Spaceflight_Research/Expose&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA's EXPOSE&lt;/a&gt; that includes sample trays installed outside the Columbus laboratory:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/GACho.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;ESA's Expose-E Exposed Pallet is handed off between the Japanese robotic arm and the Station robotic arm (Image credit: &lt;a href=&quot;http://spaceinimages.esa.int/Images/2009/10/Exposed_Pallet_is_handed_off_between_the_Japanese_robotic_arm_and_the_Station_robotic_arm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information on Expose-E experiments is available on &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/696.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA page&lt;/a&gt; and on &lt;a href=&quot;https://en.wikipedia.org/wiki/EXPOSE&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page on EXPOSE&lt;/a&gt; in general. EXPOSE-R2 is also planned to be installed on the ISS (this &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0273117713006042&quot; rel=&quot;nofollow noreferrer&quot;&gt;abstract from Advances in Space Research&lt;/a&gt; gives some clues what it will be about), some articles claiming that to happen in 2013, but since I can't find any information that it has been brought to the ISS and it's already mid December 2013, I'll presume that will happen in 2014 then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For previous research on extremophiles and anaerobic extremophiles, this article on &lt;a href=&quot;http://www.nss.org/adastra/volume14/rothschild.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;National Space Society: Life in Extreme Environments&lt;/a&gt; should have you covered what experiments have been conducted till 2002 (the date of the article), and for somewhat more recent (2010) research &lt;a href=&quot;http://www.cosmonline.co.uk/blog/2010/02/21/extremophiles-give-clues-alien-life-%E2%80%93-cosm-feb-05-2010&quot; rel=&quot;nofollow noreferrer&quot;&gt;CosmOnline: Extremophiles give clues to alien life&lt;/a&gt;. Quoting from the latter:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;European Space Agency in 2008 sent 12 boxes containing 664 examples of&#xA;  the best of the best of these so-called extremophiles to the&#xA;  International Space Station. For 18 months, two thirds of the samples&#xA;  sat exposed to the vacuum, massive temperature swings and desiccating&#xA;  conditions of open space. The rest had the luxury of enjoying a thin&#xA;  carbon dioxide atmosphere that simulated the Martian environment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wouldn't know if samples included any anaerobic extremophiles in particular, but I would presume so. This &lt;a href=&quot;http://webcache.googleusercontent.com/search?q=cache:_c1qPtV7MzUJ:ibb.kangwon.ac.kr/board/dl.asp?fn=Abstract%2520seminar%2520at%2520Kangwon%2520University%2520Nov%25202013.doc%20&amp;amp;cd=4&amp;amp;hl=en&amp;amp;ct=clnk&quot; rel=&quot;nofollow noreferrer&quot;&gt;EXTREMOPHILES AND EXTREME ENVIRONMENTS:&#xA;OPPORTUNITIES IN BIOTECHNOLOGY FROM DEEP-SEA TO SPACE by Dr. Francesco Canganella&lt;/a&gt; (Google cached copy) specifically mentions anaerobic thermophilic prokaryotes and bacteria as subjects of interest, and also mentions biocontaminants onboard the ISS (non-experiment related microbial life):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Regarding the microbial communities inside the International Space&#xA;  Station (ISS), research on the environmental microbiota has been&#xA;  conducted at a regular basis. Among the 70 species of microorganisms&#xA;  found in the environment (air and surfaces) of the ISS, 36 were&#xA;  bacteria and 34 fungi. Opportunistic pathogens were usually found and&#xA;  the most numerous group (15 species) was made of fungi with&#xA;  biocorrosive activities. Biocontamination is not only important for&#xA;  the ISS but in the future this would be one of the main issue for the&#xA;  success of Lunar and Mars bases, particularly for the biosafety of&#xA;  manned modules but also for the maintenance of space greenhouses.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Regarding biocontaminants, this &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2007/11may_locad3/&quot; rel=&quot;nofollow noreferrer&quot;&gt;fungus on the ISS internal wall panel&lt;/a&gt; in the gym area is particularly famous one, but there are other contaminants present, from protozoa to amoeba:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/TdqF5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Fungi on the ISS, growing on a panel where exercise clothes were hung to dry (Credit: &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2007/11may_locad3/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Granted, they might not be anaerobic extremophiles, but I thought it's worth mentioning not all lifeforms onboard the ISS are there to conduct research or research be conducted on (or eaten). ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-12-12T03:37:04.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3115" PostTypeId="1" AcceptedAnswerId="3120" CreationDate="2013-12-12T05:43:31.070" Score="3" ViewCount="295" Body="&lt;p&gt;Looking at one of the latest images of Titan's north pole as taken by Cassini probe made me curious how well equipped this NASA's probe actually is to detect non-water-based exobiology?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Rmp2y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Titan's lakes from T-96 pass: cb3, grn, bl filter. Processing: 2di7, titanio44 (Image credit: &lt;a href=&quot;http://www.ridingwithrobots.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Riding With Robots&lt;/a&gt;, raw data: &lt;a href=&quot;http://saturn.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA/JPL&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current theories for possible life on Titan are conveniently presented on &lt;a href=&quot;https://en.wikipedia.org/wiki/Life_on_Titan&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page on Life on Titan&lt;/a&gt;, and while Titan is in Earth terms extremely cold (surface temperature is about 90 K, or −179 °C, or −290 °F), possibility of its environment sustaining non-water-based life isn't dismissed:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Titan appears to have lakes of liquid ethane or liquid methane on its surface, as well as rivers and seas, which some scientific models&#xA;  (still tentative and debated) suggest could support non-water-based&#xA;  life.&lt;/li&gt;&#xA;  &lt;li&gt;It has also been suggested that life may exist in a sub-surface ocean consisting of water and ammonia. Recent data from NASA's Cassini&#xA;  spacecraft have strengthened evidence that Titan likely harbors a&#xA;  layer of liquid water under its ice shell.&lt;/li&gt;&#xA;  &lt;li&gt;Titan is the only known natural satellite (moon) in the Solar System that is known to have a fully developed atmosphere that&#xA;  consists of more than trace gases. Titan's atmosphere is thick,&#xA;  chemically active, and is known to be rich in organic compounds; this&#xA;  has led to speculation about whether chemical precursors of life may&#xA;  have been generated there.&lt;/li&gt;&#xA;  &lt;li&gt;The atmosphere also contains hydrogen gas, which is cycling through the atmosphere and the surface environment, and which living things&#xA;  comparable to Earth methanogens could combine with some of the organic&#xA;  compounds (such as acetylene) to obtain energy.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So this naturally begs the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How well equipped is Cassini to detect non-water-based life, traces of metabolic byproduct gases in Titan's atmosphere of such life forms, and other local environmental changes of non-water-based biota? Which of its many sensors are particularly suitable for this and have any observations to date shown evidence of unexplained local fluctuations in atmospheric composition, fluorescence or surface thermal characteristics, perhaps around areas of Solar terminator or edges of its Saturn-facing side during Titan's librations?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I.e. what would we be looking for with sensors Cassini has available to it during one of its many flybys of Titan, if we were looking for possible signs of non-water-based life on it?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-07T09:18:36.120" LastActivityDate="2014-03-07T09:18:36.120" Title="Is Cassini equipped for detection of non-water-based exobiology?" Tags="|probe|planetary-science|astrobiology|cassini|titan|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3116" PostTypeId="1" CreationDate="2013-12-12T08:53:13.503" Score="11" ViewCount="1148" Body="&lt;p&gt;International Space Station (ISS) is in trouble again. One of the two Thermal Control System (TCS) loops (Loop A) is malfunctioning due to a troublesome Flow Control Valve (FCV). This is nothing new for the ISS and such malfunctions have happened before (e.g. in 2010 during ISS Expedition 24).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole current situation is nicely explained (as nice as such complex systems can be, see for example &lt;a href=&quot;https://space.stackexchange.com/a/2540/49&quot;&gt;this try of mine to describe US segment TCS&lt;/a&gt; for perhaps half a page shorter version) in this &lt;a href=&quot;http://www.spaceflight101.com/space-station-encounters-thermal-control-system-failure.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight101 article titled Space Station encounters Thermal Control System Failure (ISS Expedition 38)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/4P83W.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Pump Module (PM) NH&lt;sub&gt;3&lt;/sub&gt; Quick Disconnects &amp;amp; Electrical Interfaces (Image credit: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/ISS101&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Updates on Twitter&lt;/a&gt; assures there is no immediate danger to the crew onboard, but answers a question on contingency procedures onboard the ISS should Loop B fail as well:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://twitter.com/ISS101/status/410974255153569794&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;ISS Updates&lt;/strong&gt;, 4:27 AM - 12 Dec 13&lt;/a&gt;: Power down US Systems to the bare minimum &amp;amp; move crew to Russian&#xA;  segment until repairs can be made. Otherwise - de-crew ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So while there is no reason to worry about the safety and wellbeing of &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition38/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expedition 38&lt;/a&gt; crew, and I have no doubt they will manage to convince the misbehaving FCV to respect its operational limits, or replace it with another during an unplanned spacewalk if the ground controllers fail to tame it (pump module in question in on the external Temperature and Humidity Control, or THC loop), I do have two questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;How long can a single US segment TCS provide adequate thermal control and some of its less mission critical client equipment and/or whole ISS modules need to be shut down?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Have any scientific experiments suffered permanent damage or serious delay during past such failures of the US segment TCS?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Additionally, is there any predetermined order in which equipment is to be shut down or unplugged from ISS's Temperature and Humidity Control, and how far in the list are ISS laboratories and their experiments?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: For quick reference, here's a diagram of the ECLSS (Environmental Control and Life Support System):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ScGws.png&quot; alt=&quot;ECLS Hardware&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;On-Orbit ISS ECLS Hardware Distribution as of February 2010&lt;/strong&gt; (Source: &lt;a href=&quot;http://wsn.spaceflight.esa.int/docs/Factsheets/30%20ECLSS%20LR.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA/ISEC&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And current readings of certain TCS sensors are available on &lt;a href=&quot;http://spacestationlive.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Station Live&lt;/a&gt; displays: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spacestationlive.nasa.gov/displays/ethosDisplay1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Life Support&lt;/a&gt;, &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spacestationlive.nasa.gov/displays/ethosDisplay2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thermal System&lt;/a&gt; and &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spacestationlive.nasa.gov/displays/ethosDisplay3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Regenerative Life Support&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-09-20T21:12:49.610" Title="With one out of two Thermal Control Systems on ISS malfunctioning, how long can Loop B keep up with increased load?" Tags="|iss|procedure|experiment|thermal-control|eclss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3119" PostTypeId="1" AcceptedAnswerId="3121" CreationDate="2013-12-12T14:10:47.953" Score="6" ViewCount="255" Body="&lt;p&gt;When NASA launched &lt;a href=&quot;http://en.wikipedia.org/wiki/Rassvet_%28ISS_module%29&quot;&gt;Rassvet&lt;/a&gt; (DCM - Docking and Cargo Module) on a shuttle, it carried a small airlock attached.  NASA was under contract to deliver 1.4 tonnes of equipment for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Nauka_%28ISS_module%29&quot;&gt;MLM Nauka&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Nauka_%28ISS_module%29&quot;&gt;Nauka&lt;/a&gt;, which is the replacement for Pirs, and is really FGB-2 rebuilt for this purpose will have a docking node with an airlock module attached.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering if the airlock launched on Rassvet is meant for the port on Nauka.  (Which is due for launch in March-April 2014, if all goes well).  Or is it actually in use on Rassvet these days? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-12T15:25:56.430" LastActivityDate="2013-12-12T15:28:04.813" Title="What is the airlock on Rassvet meant for?" Tags="|iss|rassvet|pirs|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3120" PostTypeId="2" ParentId="3115" CreationDate="2013-12-12T15:20:07.727" Score="2" Body="&lt;p&gt;It's possible that the Huygens probe already delivered us some data about the possibility of methanogenic life on Titan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &quot;Possibilities for methanogenic life in liquid methane&#xA;on the surface of Titan&quot; by McKay and Smith, they discuss many effects that methanogenic life might have on the surface of Titan that the Huygens probe would pick up. One of these is a reduction in the levels of $H_2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the paper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If bacteria are consuming complex hydrocarbons at the surface of Titan, the observable effects might include: complete consumption of $C_2H_2$ at the surface, reduction in $C_2H_6$ and organic solids at the surface compared to the accumulation expected from photolysis alone, and a sink of hydrogen at the surface creating a gradient in the hydrogen mixing ratio with altitude. All of these effects may be detected and measured by the Huygens probe.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Huygens probe, in fact, did detect a reduction in $H_2$ at the surface. The $H_2$ signal hovered between 10&lt;sup&gt;4&lt;/sup&gt; and 10&lt;sup&gt;5&lt;/sup&gt; counts per second throughout the atmosphere, but at the surface it was clearly below 10&lt;sup&gt;4&lt;/sup&gt; counts per second. I'm not qualified to assess whether this reduction is significant, but I imagine there are many professionals poring over this data, trying to extrapolate what they can from it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering this data gathered by the Huygens probe that Cassini already dispatched and ongoing data gathered by Cassini on each flyby, I would say that Cassini is certainly well equipped to help us detect life on Titan. As with any instrument, however, it requires a substantial degree of human interpretation of data so it may be a while before we can draw any real conclusions. And it may be that we determine that we need to send other instruments to bolster our results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astro.sunysb.edu/astro/seminars/archive/JS09/jcl27Feb09-1.pdf&quot; rel=&quot;nofollow&quot;&gt;Possibilities for methanogenic life in liquid methane&#xA;on the surface of Titan - McKay and Smith&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://solarsystem.nasa.gov/docs/6_Niemann_Huygens_GCMS.pdf&quot; rel=&quot;nofollow&quot;&gt;The Huygens Probe Gas Chromatograph Mass &#xA;Spectrometer experiment, &#xA;results and lessons learned - Niemann, et al.&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-12-12T16:30:37.387" LastActivityDate="2013-12-12T16:30:37.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3121" PostTypeId="2" ParentId="3119" CreationDate="2013-12-12T15:28:04.813" Score="6" Body="&lt;p&gt;And in updating the question, I found my answer.  Yes.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia page on &lt;a href=&quot;http://en.wikipedia.org/wiki/Nauka_%28ISS_module%29&quot;&gt;Nauka&lt;/a&gt; says: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Outfitting equipment launched in 2010 with the Rassvet (Mini-Research Module 1) on NASA's STS-132 will also be used for Nauka, including the spare elbow joint for the European Robotic Arm, internal hardware and an experimental airlock to be positioned on one of the side-facing ports at the bottom of the module. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the arm joint, the airlock and some extra stuff in Rassvet is meant for Nauka.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have been reading &lt;a href=&quot;http://www.russianspaceweb.com/book_future.html&quot;&gt;Anatoly Zak's Russian Space book&lt;/a&gt;, which is great, and this detail is not made clear.  At one point it looks like the airlock module would launch on its own Progress based module.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2013-12-12T15:28:04.813" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3122" PostTypeId="1" AcceptedAnswerId="3129" CreationDate="2013-12-12T16:50:22.463" Score="11" ViewCount="566" Body="&lt;p&gt;Is the effect of micro-gravity on bone structure similar to osteoporosis? If so, do they often take simple pills like calcium or more advance medicine such as ones listed &lt;a href=&quot;http://nof.org/articles/22&quot;&gt;here&lt;/a&gt;? Lastly, are they effective?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am aware that daily workouts on the ISS are crucial. Just wondering if a medication alone could be enough to help with bone density.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="51" LastEditDate="2014-04-20T11:19:52.483" LastActivityDate="2014-04-20T11:19:52.483" Title="Do astronauts take any pills to battle bone density loss?" Tags="|iss|microgravity|humans|life|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3123" PostTypeId="1" AcceptedAnswerId="3127" CreationDate="2013-12-12T18:10:48.507" Score="8" ViewCount="983" Body="&lt;p&gt;Recently, &lt;a href=&quot;http://www.cnn.com/2013/12/10/tech/innovation/mars-one-plan/index.html?hpt=hp_t3&quot; rel=&quot;noreferrer&quot;&gt;CNN&lt;/a&gt; informed us that Mars One seems to still be on track for landing on Mars in 2025.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is the case, we may have humans celebrating the first Christmas on Mars of all time. Since this is such a big milestone in the history of Christmas, it would be unfortunate if Santa Claus couldn't deliver. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may recall that in &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/1998/ast24dec98_1/&quot; rel=&quot;noreferrer&quot;&gt;Santa's 1998 interview with NASA&lt;/a&gt; he discussed his plans for delivering presents to any human colonies on other planets in the solar system. How is Santa's space program coming? Will he be ready to deliver to Mars in 2025?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2013-12-13T15:45:29.023" LastActivityDate="2013-12-13T15:45:29.023" Title="Will Santa Claus deliver presents to Mars in 2025?" Tags="|mars|mars-one|developing-space-programs|santa-claus|" AnswerCount="1" CommentCount="2" ClosedDate="2013-12-14T14:06:04.120" ContentLicense="CC BY-SA 3.0" />
  <row Id="3124" PostTypeId="1" AcceptedAnswerId="3126" CreationDate="2013-12-12T18:17:42.433" Score="9" ViewCount="692" Body="&lt;p&gt;Say that one of NASA's probes discovers definite evidence of life outside of Earth. Does NASA have any procedures in place on how to deal with such a situation? For instance, do they have to inform someone first before the go the press? Do they even &lt;em&gt;go&lt;/em&gt; to the press?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="58" LastEditDate="2013-12-12T18:19:28.560" LastActivityDate="2013-12-12T23:05:38.957" Title="Does NASA have any documented procedures in the event of discovery of extraterrestrial life?" Tags="|nasa|astrobiology|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3125" PostTypeId="2" ParentId="3109" CreationDate="2013-12-12T18:45:32.993" Score="18" Body="&lt;p&gt;If the search for life on Mars is a particular concern, I think putting boots on the ground would be devastating (or bring smashing success, depending on your perspective). When we send robotic probes to Mars, we do our best to send them sterile so as to not contaminate our search. If we send human beings, &lt;a href=&quot;http://www.scientificamerican.com/article.cfm?id=strange-but-true-humans-carry-more-bacterial-cells-than-human-ones&quot; rel=&quot;nofollow&quot;&gt;which contain far more microbial cells than human cells&lt;/a&gt;, there is no chance of avoiding contamination. We would wear space suits while out on the Marian surface, but we can't put those suits on without smudging millions of bacteria all over them. Maybe before we step out, we bathe the outside of the suit with bleach and radiation, but the chances are approximately zero that we will always be able to guarantee a clean suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, if we go there ourselves with all of our dirty biology, I can guarantee you that we will find life. Even if Mars is barren today, once we arrive, life will find a vector to escape from its human host to the Martian soil. Now, in the long term, I think that's a good thing. I don't think a barren planet is as valuable to the Universe as one where life is growing and evolving, but of course, I'm very biased, being alive. But I do think that if we want a clear-cut answer to whether life already exists on Mars, we need to conclude our search before we send living explorers. It's certainly possible even after our arrival to find fossil evidence that predates our arrival or to prove distinction of a Martian genetic strain, but without a doubt, the core question of life on Mars becomes more complicated and difficult once we contaminate the site, and it casts doubt on anything that we might hope to find. And I can just imagine the articles that our descendants may one day write about how our generation hastily flubbed the search for life on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit (yes, everything from here down):&lt;/strong&gt; Mark Adler has pointed out in comments below that I have not made clear how contamination would be a problem. Thank you Mark. You're right. I see 4 varieties of life that we might discover on Mars which, by their nature, would each have vastly different degrees of similarity to Earth-life: 1) Completely independent Martian evolution. Probably not even DNA-based. 2) Seeded by galactic or intergalactic &lt;a href=&quot;http://en.wikipedia.org/wiki/Panspermia&quot; rel=&quot;nofollow&quot;&gt;panspermia&lt;/a&gt;. If this turns out to be a real thing, Earth would have been similarly seeded. 3) Branched from Earth during the last couple of billion years (Think impact ejection dispersal; not Cambrian spaceships.) 4) Brought by humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, depending on your philosophy, you may be tempted to scoff at one or two of the possibilities I have mentioned. But I ask you to consider the fact that nothing has been or can be proven yet about the feasibility of these proposed sources for life, or even which is most likely. So far we have no scientifically admissible and conclusive evidence that life exists outside of our little spheroid, which is precisely why this question is so important on Mars and anywhere else we explore. So if you scoff, it is a philosophical scoff, not a scientific one. (Please correct me if you can prove anything contrary.) I'm not as worried about missing a variety 1 discovery through contamination, but missing varieties 2 and 3 would be every bit as tragic a loss, and we just don't know how easy it might be to miss. Imagine &lt;em&gt;actually finding them&lt;/em&gt;, but then later in peer review not being able to prove that they didn't come from Earth through contamination, because they are so similar to Earth life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the event of contamination, whatever little Earth bug manages to thrive in a place like Mars will likely be pretty unique. The creatures that we have found in the least Earth-like places on Earth, places like the Yellowstone caldera, are not your standard garden variety e coli. They are still recognizable as Earth life, but remember, we need to be able to distinguish them from Martian life variety 3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to hope that our Earth muck would not spread nearly as successfully as any existing Martian bugs growing in their home habitat. Our bacteria ought to need millions of years to adapt as well as the Martian native life. And if some Terran extremophile did manage to start spreading relatively quickly through the subsurface of Mars, we could take that as evidence that there was nothing native to Mars before our arrival, because clearly if any lifeform can spread like this, why wasn't Martian life all over the place and easy to find before this contamination event? &lt;em&gt;OR&lt;/em&gt;, it may be that our biota is simply so wealthy with genetic variety due to the ideal planet we developed on that it is better armed to crush the sparse native Martian population under it's heel, and we will never even know it was there. If we could have found it, yes, we might have been able to distinguish it from Earth life, but it will be exterminated in short order by our invading bacteria in this scenario. If something like this did occur, I tend to favor the probability of the former explanation to the latter, but again, no scientific experiment can actually &lt;em&gt;prove&lt;/em&gt; that. We have only philosophical conjecture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The worst case scenario is that there is life on Mars and the life we are looking for is of variety 3 - It's originally from Earth and therefore somewhat similar to potential sources of contamination. Then contamination occurs and doesn't look like anything we've seen before, because it's a rare little beastie. And then worse, it ends up spreading much more successfully than the uncommon Martian life that is there already. Finding native life at this point would be worse than looking for fireflies while the forest is on fire.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how, you may ask, could the contaminant species possibly be more successful than the Martian species that have the advantage of eons in the Martian environment, but are otherwise similar (from Earth)? I would expect that it would not be, from my perspective in an absolute vacuum of actual scientifically tested knowledge on the subject. However it &lt;em&gt;could&lt;/em&gt; be that the advantage of eons on Mars is dwarfed by the advantage of the species that hitched a ride to Mars not on the molten embers of an exploded planetary surface at the point of some massive impact event which then cooled to temperatures that made Mars seem tropical by comparison and then drifted, degrading gradually, for many millions of years before then barely surviving &lt;em&gt;another&lt;/em&gt; hot impact event on the surface of its new home, but instead inside a nice temperate wet human with a spaceship that provides such an incredibly safe and quick ride that even the over-sized eukaryote manages to survive and then do &lt;em&gt;SCIENCE&lt;/em&gt; after his/her arrival. Maybe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, you may notice that I have claimed that there is little we can prove, that we can only guess at what might be out there and what could happen during contamination, then I painted a relatively specific picture of what I would hope to avoid. So recognize that I'm not warning that this great bad thing &lt;em&gt;will&lt;/em&gt; happen if we send people too early. I'm pointing out the risk of what &lt;em&gt;could&lt;/em&gt; happen. And I see no good way to accurately measure that risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://phys.org/news/2016-10-year-old-viking-life-mars.html&quot; rel=&quot;nofollow&quot;&gt;This article&lt;/a&gt; was published at phys.org today (Oct 21, 2016). The final paragraph corroborates the primary point of my answer, as these experts in the field express concern about the spoiling of the pristine Martian environment even by remote landers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concerns I raised here would also be significantly validated if what they found with the LR experiments turns out to truly be biological: They describe the hypothesized microbe as fragile (very sensitive to temperature fluctuation at least). Thus, if it does exist, it may be easy to inadvertently exterminate.&lt;/p&gt;&#xA;" OwnerUserId="945" LastEditorUserId="945" LastEditDate="2016-10-21T19:03:08.843" LastActivityDate="2016-10-21T19:03:08.843" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="3126" PostTypeId="2" ParentId="3124" CreationDate="2013-12-12T19:28:43.527" Score="7" Body="&lt;p&gt;I am not aware of any official procedure, and I don't really think one is needed or useful.  The NASA administrator will go the White House first, and they will decide who to talk to first, how to assess the veracity of the evidence, what other government agencies should be consulted, and when and how to announce the findings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason that I don't think that a procedure is useful, other than talking to the White House, is that the White House will decide what to do in real time without looking at some procedure that someone wrote a while back, knowing nothing about what the nature of the discovery might be or the current state of affairs.  In other words, they will ignore the procedure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can look at the one example so far, which is &lt;a href=&quot;http://en.wikipedia.org/wiki/Allan_Hills_84001&quot;&gt;ALH84001&lt;/a&gt;.  The White House certainly did get involved, and &lt;a href=&quot;http://www2.jpl.nasa.gov/snc/clinton.html&quot;&gt;Clinton even spoke to the press about it&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-12-12T19:28:43.527" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3127" PostTypeId="2" ParentId="3123" CreationDate="2013-12-12T22:58:02.660" Score="5" Body="&lt;p&gt;Let's first get a few boring details out of the way, then we'll see if we need to tell Santa to add a detour to Mars to his 2025 schedule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There won't be any children on Mars by 2025 with the &lt;a href=&quot;http://www.mars-one.com/en/faq-en/21-faq-selection/251-do-i-qualify-to-apply&quot; rel=&quot;noreferrer&quot;&gt;minimum applicant age requirement of 18 years&lt;/a&gt; by end of August 2013, &lt;a href=&quot;http://www.mars-one.com/en/mars-one-news/press-releases/11-news/495-deadline-approaching-ten-days-remain-to-join-aspiring-martians-from-140-countries&quot; rel=&quot;noreferrer&quot;&gt;deadline for Mars One online application program&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The astronaut selection program will be open for applicants who are 18&#xA;  years or older. This is the age by which children become legal adults&#xA;  in most countries in the world. Mars One believes it is important that&#xA;  applicants who enter the astronaut selection program are capable of&#xA;  entering into a legal contract without the supervision of others.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This means that even if Mars One is capable of following its own &lt;a href=&quot;http://www.mars-one.com/en/mission/summary-of-the-plan&quot; rel=&quot;noreferrer&quot;&gt;road map&lt;/a&gt;, the youngest first permanent settlers on Mars will be no less than 30 years of age (in Earth years, see below) by Christmas 2025.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to answer more directly, if &lt;em&gt;Santa Claus is coming to Mars in 2025&lt;/em&gt;, we have no way of confirming that, short of establishing that so far, till December 2013, &lt;a href=&quot;http://www.mars-one.com/&quot; rel=&quot;noreferrer&quot;&gt;Mars One project&lt;/a&gt; is &lt;a href=&quot;http://www.mars-one.com/en/mission/summary-of-the-plan&quot; rel=&quot;noreferrer&quot;&gt;on schedule according to set roadmap milestones&lt;/a&gt;. The application program has ended and here is a &lt;a href=&quot;https://applicants.mars-one.com/&quot; rel=&quot;noreferrer&quot;&gt;searchable list of all the applicants&lt;/a&gt; with links to their profiles and introduction videos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, Santa Claus will have to keep in shape until 2025, with 12 complete and worldwide Christmas deliveries of presents to all the well behaved children around the globe first, then the same, plus a &lt;em&gt;quick (&lt;a href=&quot;http://www.wolframalpha.com/input/?i=Distance%20from%20Earth%20to%20Mars%20on%20December%2025%202025&quot; rel=&quot;noreferrer&quot;&gt;at least 40 minutes long&lt;/a&gt;&lt;/em&gt;)* detour to Mars to deliver present there in 2025 to a bit older children, too. How old? Since Martian year lasts whole 2.135 &lt;a href=&quot;https://en.wikipedia.org/wiki/Julian_year_%28astronomy%29&quot; rel=&quot;noreferrer&quot;&gt;Julian years&lt;/a&gt;, some of the first settlers might be as young as 14 years of age in Martian terms by Christmas 2025. So it looks like we will have to make sure to notify Santa of this fact before planning his worldwide presents delivery route for 2025. It should indeed be the very first &lt;a href=&quot;http://www.noradsanta.org/&quot; rel=&quot;noreferrer&quot;&gt;Santa's Interplanetary Presents Delivery&lt;/a&gt; (SIPD). ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/n61IW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Approximate location along the Earth's terminator (dividing line between day and night) when Santa might have 40 minutes to spare&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;* Assuming the &lt;a href=&quot;https://en.wikipedia.org/wiki/Speed_of_light&quot; rel=&quot;noreferrer&quot;&gt;speed of light&lt;/a&gt; in vacuum, the theoretical maximum speed at which information  in the universe can travel. Since we know that Santa Claus is real and has a physical presence, he cannot move any faster to Mars and back (20 minutes in each direction). That shouldn't stop him from delivering all the Christmas presents to all the children in the world too, tough. He does have &lt;a href=&quot;https://en.wikipedia.org/wiki/Category:Santa%27s_helpers&quot; rel=&quot;noreferrer&quot;&gt;the best helpers&lt;/a&gt; available to him to help him achieve that. And there is just about 40 minutes of one Earth day available in his schedule during Christmas, because not all the Earth's globe is populated by children and there might be a bit of space in his schedule when the Christmas evening reaches Pacific.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-13T00:56:53.790" LastActivityDate="2013-12-13T00:56:53.790" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3128" PostTypeId="1" AcceptedAnswerId="3182" CreationDate="2013-12-13T00:15:14.603" Score="9" ViewCount="197" Body="&lt;p&gt;&lt;a href=&quot;http://news.discovery.com/space/astronomy/hubble-discovers-water-plumes-over-europa-131212.htm&quot;&gt;Hubble has detected water vapor plumes shooting out of Europa&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Europa_%28moon%29&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;it is predicted that the outer crust of solid ice is approximately 10–30 km (6–19 mi) thick&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Getting to liquid water through ice that thick may not be feasible given contemporary technology. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a hypothetical fly-by probe collect samples from the plume? If life does indeed exist beneath the ice, could we detect it by analyzing this sample?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="122" LastEditDate="2013-12-13T17:54:31.077" LastActivityDate="2013-12-21T10:14:57.360" Title="Would we be able to detect signs of life from the contents of a water plume?" Tags="|astrobiology|europa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3129" PostTypeId="2" ParentId="3122" CreationDate="2013-12-13T03:59:31.573" Score="9" Body="&lt;p&gt;This is commonly known as &lt;a href=&quot;http://en.wikipedia.org/wiki/Spaceflight_osteopenia#Countermeasures&quot;&gt;Spaceflight osteopenia&lt;/a&gt; . This results in loss of average bone density. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Increasing dietary calcium and vitamin&#xA;  D is a standard countermeasure for&#xA;  osteoporosis. Clay is reportedly used&#xA;  by NASA for retaining calcium. &#xA;  A variety of drug remedies currently&#xA;  used or proposed for osteoporosis may&#xA;  work for spaceflight, including hormone&#xA;  therapy (estrogen and/or progestin),&#xA;  selective estrogen receptor modulators,&#xA;  bisphosphonates , teriparatide , and&#xA;  others. Whether they can provide the&#xA;  same benefits for spaceflight as they&#xA;  do for osteoporosis is not yet known.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;External source &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://web.archive.org/web/20080403060558/http://research.calacademy.org/calwild/2005winter/stories/mud.html&quot;&gt;http://web.archive.org/web/20080403060558/http://research.calacademy.org/calwild/2005winter/stories/mud.html&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://gravitationalandspacebiology.org/index.php/journal/article/view/337/338#page=46&quot;&gt;http://gravitationalandspacebiology.org/index.php/journal/article/view/337/338#page=46&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="414" LastActivityDate="2013-12-13T03:59:31.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3130" PostTypeId="1" AcceptedAnswerId="3132" CreationDate="2013-12-13T16:26:47.663" Score="2" ViewCount="477" Body="&lt;p&gt;The question, &lt;a href=&quot;https://space.stackexchange.com/questions/3123/will-santa-claus-deliver-presents-to-mars-in-2025&quot;&gt;Will Santa Claus deliver presents to Mars in 2025?&lt;/a&gt; got me thinking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 24 well documented zones on earth, of when Christmas occurs. Does Christmas on Mars, come before, during or after Christmas on Earth? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/1998/ast24dec98_1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Santa's 1998 interview with NASA&lt;/a&gt; Suggests there is only one Christmas per Mars year, but the specific date is not mentioned.  Presumably it would be timed near the martian &lt;a href=&quot;https://en.wikipedia.org/wiki/Winter_solstice&quot; rel=&quot;nofollow noreferrer&quot;&gt;Winter solstice&lt;/a&gt;, but I really have no idea.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-08-03T13:45:06.953" Title="When is Christmas on Mars?" Tags="|mars|santa-claus|" AnswerCount="1" CommentCount="2" ClosedDate="2013-12-14T02:25:24.437" ContentLicense="CC BY-SA 3.0" />
  <row Id="3131" PostTypeId="2" ParentId="881" CreationDate="2013-12-13T16:41:12.997" Score="12" Body="&lt;p&gt;This has happened in space before, as a matter of fact, on Mir.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During tests of a new manual control system for cargo ships, the cosmonaut piloting the ship managed to crash it into the Spektr module. The crew determined the location of the leak, and after some tense moments trying to sever the various hoses and cables leading into the module, they managed to close the hatch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.russianspaceweb.com/mir_close_calls.html&quot;&gt;http://www.russianspaceweb.com/mir_close_calls.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, that module was unusable for the rest of Mir's life. After some time, engineers were able to send up a modified hatch that allowed the power from Spetkr's solar panels to flow to the rest of the station, gaining some use from it. There was an attempt to find the hole and seal it via an Internal Vehicular Activity (IVA, wearing a spacesuit inside the station as opposed to outside), but it was unsuccessful.&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2013-12-13T16:41:12.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3132" PostTypeId="2" ParentId="3130" CreationDate="2013-12-13T17:50:00.943" Score="5" Body="&lt;p&gt;Christmas on Earth is essentially a winter solstice holiday. Its date was fixed based on the Roman winter solstice (a southern solstice) which was December 25th.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(Note: Southern solstice is the time of the year when the sun is at its southernmost point in the sky. It is independent of hemisphere, whereas winter/summer solstices are north/south dependent. All winter solstices in the Northern hemisphere are southern solstices.)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It only makes sense that if Santa is doing Christmas on Mars once a Martian year, that he would also use the Martian southern solstice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://web.archive.org/web/20130516125740/http://pweb.jps.net/~tgangale/mars/mst/WinterSolstice.htm&quot; rel=&quot;nofollow&quot;&gt;Here&lt;/a&gt; is a chart of winter solstice times on Mars from 1875 to 2127. It appears that the next winter solstice on Mars after Mars One is scheduled to arrive is April 25, 2026. I assume that these are southern solstices.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is when Santa plans on delivering gifts to Mars, this will give him plenty of time after Earth's Christmas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there is also the possibility that Santa will continue to use December 25 because of its convention. If this is the case, he will have to abandon his plans of only visiting Mars once per Martian year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Timekeeping_on_Mars#Martian_year&quot; rel=&quot;nofollow&quot;&gt;More on Martian timekeeping on Wikipedia&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-08-03T13:45:06.953" LastActivityDate="2015-08-03T13:45:06.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3134" PostTypeId="1" CreationDate="2013-12-14T03:05:03.120" Score="11" ViewCount="455" Body="&lt;p&gt;In about one day China's Chang'e 3 will land on the moon in a fully autonomous way, which is pretty exciting and I think especially the autonomous hazard avoidance system is just amazing. However, I noticed today China's official media quotes the chief commander of the lunar mission saying:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;The landing will be a black-750-seconds and ground tracking/control will be useless as there is no time to make judgement, the lander will&#xA;  have to land on its own through preset commands&quot;.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is certainly the case for Mars missions due to the significant signal delay. However, the Moon should not be such a big problem since the delay looks acceptable. I understand that this fully autonomous landing will be a great achievement and good way to demonstrate China's ability, and perhaps will be used to collect information for a future planetary mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I wonder is it true, as the media reported, that there is no way to control the attitude maneuver for a lunar lander during its descent and landing phases? How did the early unmanned Surveyor missions land on the moon?&lt;/p&gt;&#xA;" OwnerUserId="1262" LastEditorUserId="49" LastEditDate="2013-12-14T03:17:57.587" LastActivityDate="2014-01-27T04:31:15.533" Title="Do lunar landers communicate with ground stations during descent and landing?" Tags="|the-moon|communication|china|lander|chang-e|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3135" PostTypeId="2" ParentId="3134" CreationDate="2013-12-14T04:21:30.283" Score="10" Body="&lt;p&gt;Surveyor Program landers (1-7) made robotic soft-landing on the lunar surface. The major difference between Surveyor landers and Chang'e 3 is that they didn't orbit the Moon, but were put on a ballistic trajectory and the autopilot computer did the rest. &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1966-045A&quot; rel=&quot;noreferrer&quot;&gt;NASA page on Surveyor 1&lt;/a&gt; puts the soft-landing procedure like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the landing sequence, an altitude marking radar initiated the&#xA;  firing of the main retrorocket for primary braking. After firing was&#xA;  complete, the retrorocket and radar were jettisoned and the doppler&#xA;  and altimeter radars were activated. These provided information to the&#xA;  autopilot which controlled the vernier propulsion system to touchdown.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, all other Surveyor landers used the same landing procedure. Chang'e will however attempt landing from a low-inclination elliptical lunar orbit with perilune (lunar perigee) at only 15 kilometers. &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/12130915-change-3-landing-tomorrow.html&quot; rel=&quot;noreferrer&quot;&gt;Emily Lakdawalla describes Chang'e 3 landing phase in The Planetary Society blog post&lt;/a&gt; like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The descent engine will be ignited at 15 kilometers to decelerate,&#xA;  above 2 kilometers it'll have pointed its main engine downward, below&#xA;  2 kilometers it'll be slowly descending. At 100 meters the payload&#xA;  will be hovering without receiving control from Beijing. It'll utilize&#xA;  its camera and computer to identify the surface, and automatically&#xA;  select a plain to land on. At 4 meters the descent engine will turn&#xA;  off, and the payload will land with a free fall.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Robotic landing computer of Surveyor landers was programmed to target free-fall soft landing from about &lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_Program#Launch_and_lunar_landing&quot; rel=&quot;noreferrer&quot;&gt;3.4 meters&lt;/a&gt; (11 feet), so this is also fairly similar to what Chang'e 3 carrying Yutu lunar rover will attempt. The major difference being the angle of the approach vector, enabling the lander to autonomously decide on the most suitable landing site. Surveyor landers didn't utilize anything like this (technology in 60's wasn't really capable of autonomous decisions like that, and since they landed from a ballistic trajectory, there wouldn't be much leeway for maneuvering anyway).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About the communications delay though, I don't think the distance to the Moon is the reason they didn't decide for a remote-controlled landing. The majority of the delay would come from the communications network (&lt;a href=&quot;https://en.wikipedia.org/wiki/Chinese_Deep_Space_Network&quot; rel=&quot;noreferrer&quot;&gt;CDSN&lt;/a&gt;) relaying communications one station to another, and also from the controllers themselves to react to fast changing situation fast enough, while keeping their action synchronised with whatever communications delay would there be. This would be similarly hard than conducting an orchestra, without having any previous experience with any of its musicians. And since humans aren't infallible either, they must have decided it's simply easier to leave it to the onboard computer. One more thing about it is that if they attempted a remote controlled descent, it would complicate matters anyway, since the vehicle would have to keep its communications antenna pointed towards the Earth at all times, while changing orientation. This would also constantly shift vehicle's center of mass. So accounting for all these complications, autonomous landing just sounds a lot simpler.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, since you asked about remote operated hardware at a distance to the Moon, it is not impossible and has been done before. Some Apollo missions (the ones with lunar rovers) left TV cameras on the surface to transmit live pictures of ascent module liftoffs, and cameras of some of these Apollo missions were remote operated from Earth to provide a better view of the ascent. This required great skill from the camera operator, because he had to issue commands to the camera in advance and with communications delay in mind, while also anticipating the time when the ascent module will actually fire its ascent engines. During the Apollo 17 mission, this succeeded perfectly, as can be seen in this &lt;a href=&quot;https://www.youtube.com/watch?v=cOdzhQS_MMw&quot; rel=&quot;noreferrer&quot;&gt;video on YouTube&lt;/a&gt; (notice the zoom-out first, then slow tilt of the camera to keep the module inside the camera's frame).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-27T04:31:15.533" LastActivityDate="2014-01-27T04:31:15.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3136" PostTypeId="1" CreationDate="2013-12-14T10:33:09.650" Score="3" ViewCount="558" Body="&lt;p&gt;Would required $\Delta V$ for a lunar soft landing differ if the landing site is selected either on the near or the far side of the Moon? If there is a difference, how big would it be and what would be the reasons for different $\Delta V$ requirements?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorDisplayName="user12" LastEditDate="2013-12-14T17:37:14.003" LastActivityDate="2013-12-22T15:48:04.443" Title="Is there any difference in $\Delta V$ requirements for soft landing on near and far side of the Moon?" Tags="|orbital-mechanics|the-moon|landing|delta-v|site-selection|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3137" PostTypeId="2" ParentId="3136" CreationDate="2013-12-14T14:32:47.333" Score="6" Body="&lt;p&gt;There should be no discernible difference - the radius of the Moon compared to the distance to the earth means that at touchdown, it's effect is negligible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon is in a stable orbit at around 385,000 km. It's radius is around 1800 km, so the gravitational effect of the Earth will change by about 0.002%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Andrew Thompson pointed out, moon landers typically go into a low lunar orbit around the moon prior to landing - around 100 km (62 mi) altitude or less above the mean surface of the Moon.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="49" LastEditDate="2013-12-22T14:59:51.307" LastActivityDate="2013-12-22T14:59:51.307" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3138" PostTypeId="2" ParentId="3136" CreationDate="2013-12-14T18:39:58.843" Score="2" Body="&lt;p&gt;If we assume soft-landing from a direct ballistic trajectory and do a trans-lunar injection (TLI) burn immediately following the launch with no intermediate parking orbit around Earth or the Moon (latter might be useful to target a landing site further from lunar equator, Like Chang'e 3 did), then your launch window will be slightly earlier for a landing site on the far side of the moon, but your delta-v doesn't change because of it, as has Rory already answered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why? Because your $\Delta V$ requirements stem from the need to break free of Earth's gravitational influence (&lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hill sphere&lt;/a&gt;) and this has little to do with your target orbit around the Moon that would change depending on which side of it you'd like to land, but a bit more with your launch window, i.e. timing. Once the Moon's gravitational influence prevails, you're Δv-free and only need to worry about not crashing into it or missing it enough to later crash back into the Earth. By keeping the Moon with your launch window on average &lt;em&gt;on your side&lt;/em&gt; as it rotates around the Earth (in direction of your vector), you'd be able to somewhat lower your minimum $\Delta V$, since you'd be faster in region where its own gravity prevails over Earth's. But delta-v isn't really the most interesting aspect of launching to the Moon as far as total thrust (force per wet mass) requirements are concerned, what matters a whole lot more is your total payload mass that will define them. Let's split hairs a bit;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Strictly technically speaking, you will require either two launches or a single one launching a heavier two module lander-orbiter for a landing on the far side of the Moon, since you'd need an additional orbiter to relay communications between your lander and the Earth. Since the &lt;a href=&quot;https://astronomy.stackexchange.com/a/22/13&quot;&gt;Moon is tidally locked with Earth&lt;/a&gt;, this means that what we call &lt;em&gt;the far side of the Moon&lt;/em&gt; is always turned away from the Earth with no direct communications possible. So you would require either an additional orbiter in lunar orbit to relay communication prior and post each occultation when the orbiter would have both the lander and the Earth in sight, or an additional communications satellite stationed in a $L_2$ (Lagrange point) &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;halo orbit&lt;/a&gt; relaying communications to the Earth via another satellite, &lt;em&gt;parked&lt;/em&gt; in a Geostationary Orbit (GEO) when the two would be in direct line of sight to each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Far side of the Moon is also &lt;a href=&quot;https://space.stackexchange.com/a/2637/49&quot;&gt;much more mountainous&lt;/a&gt; than the near side, so you should likely assume you'd require more propellant for any additional landing trajectory course corrections. All this adds weight to your launch vehicle, but there's arguably a plus side that your lander wouldn't require as strong communications antenna, since it would only have to reach a much closer orbiter / L&lt;sub&gt;2&lt;/sub&gt; halo orbit satellite with it, so some of its onboard equipment would be lighter, and with it also requiring a bit less fuel for the deorbit burn. So all in all, your $\Delta V$ requirements mightn't change by much, but you'd be launching a lot heavier payload for the far side of the Moon soft landing. At least, if you also want to communicate with it. Anyway, I did warn you I'll be splitting hair, didn't I?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2013-12-22T15:48:04.443" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3139" PostTypeId="1" AcceptedAnswerId="3140" CreationDate="2013-12-14T19:24:20.963" Score="10" ViewCount="1765" Body="&lt;p&gt;What would happen when there is the warping of space occupied by matter? For example, if there is a ship propelled by the Alcubierre Drive, what would happen if it collided with something with mass? And how (if it does) would that differ from normal physics?&lt;/p&gt;&#xA;" OwnerUserId="410" LastEditorUserId="25" LastEditDate="2013-12-14T20:23:58.070" LastActivityDate="2018-11-15T03:19:54.657" Title="Alcubierre Drive and interacting with matter" Tags="|propulsion|alcubierre-drive|advanced-propulsion|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3140" PostTypeId="2" ParentId="3139" CreationDate="2013-12-14T20:24:22.150" Score="11" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Baryon#Baryonic_matter&quot; rel=&quot;noreferrer&quot;&gt;Baryonic matter&lt;/a&gt; by necessity occupies spacetime, but since the theoretical Alcubierre drive &lt;em&gt;warps&lt;/em&gt; spacetime, there wouldn't actually be any travel &lt;em&gt;through&lt;/em&gt; it and no additional interaction with baryonic matter would occur due to it. &lt;a href=&quot;http://iopscience.iop.org/0264-9381/11/5/001&quot; rel=&quot;noreferrer&quot;&gt;Miguel Alcubierre's proposal&lt;/a&gt; for warp drive does call for exotic matter to create a distortion in spacetime, when perceived as &lt;a href=&quot;https://en.wikipedia.org/wiki/Euclidean_space&quot; rel=&quot;noreferrer&quot;&gt;Euclidean space&lt;/a&gt;, but it's often misunderstood as &lt;a href=&quot;https://en.wikipedia.org/wiki/Faster-than-light&quot; rel=&quot;noreferrer&quot;&gt;faster-than-light&lt;/a&gt; (FTL) travel. Theoretical Alcubierre drive does not violate general relativity, but achieves &lt;em&gt;perceived FTL&lt;/em&gt; via local expansion and contraction of spacetime, behind and in front of the spacecraft, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we simplify it with an analogy, if we'd have to fold a sheet of paper to demonstrate theoretical travel through a wormhole, Alcubierre drive would analogously create a local &lt;em&gt;ripple&lt;/em&gt; (pardon my choice of words, some call it a &lt;em&gt;bubble&lt;/em&gt;) in this sheet of paper, our 2-dimensional representation of spacetime. This could look something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/o2RY7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Two-dimensional visualization of the Alcubierre drive, showing the opposing regions of expanding and contracting spacetime&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;that displace the central region (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Alcubierre_drive&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Alcubierre drive&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This 2-dimensional representation of spacetime shows that it would exclude interaction with any baryonic matter between &lt;em&gt;outside&lt;/em&gt; and &lt;em&gt;inside&lt;/em&gt; of the &lt;em&gt;ripple&lt;/em&gt; while such drive would be able to &lt;em&gt;warp&lt;/em&gt; spacetime around it, but at the same time does not prohibit &lt;em&gt;normal&lt;/em&gt; interaction with matter inside it. This means that, unless you've created a &lt;em&gt;ripple&lt;/em&gt; large enough to include in it an object already on collision course with your spaceship before you turned your Alcubierre drive on, or turned your drive off at an inconvenient place, you wouldn't have to fear collision. Well, that's to my understanding at least, it's not an easy idea to &lt;em&gt;wrap&lt;/em&gt; one's head around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suggested additional reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://io9.com/5963263/how-nasa-will-build-its-very-first-warp-drive&quot; rel=&quot;noreferrer&quot;&gt;How NASA might build its very first warp drive&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015936_2011016932.pdf&quot; rel=&quot;noreferrer&quot;&gt;Warp Field Mechanics 101 by Dr. Harold White&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-12-14T20:24:22.150" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3141" PostTypeId="2" ParentId="3077" CreationDate="2013-12-14T23:43:22.283" Score="4" Body="&lt;p&gt;I'd like to add ceramic-matrix-composites to the list of possible composite-materials!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several programs where these advanced ceramics are tested (e.g. SHEFEX I to III from the German Aerospace Center). Positive properties are e.g. heat-resistance, corrosion behaviour and insulation (electrical and thermal) at a relatively low density...&lt;/p&gt;&#xA;" OwnerUserId="1265" LastActivityDate="2013-12-14T23:43:22.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3142" PostTypeId="2" ParentId="2929" CreationDate="2013-12-15T02:21:03.377" Score="1" Body="&lt;p&gt;Arianespace’s sixth Soyuz launch from the Guiana Space&#xA;Center will carry the Gaia space telescope into orbit on L2 next 19th of December at 09:12:18 UTC. &#xA;More detail about the launch sequence can be read from ESA site &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cosmos.esa.int/web/gaia/news_20131206&quot; rel=&quot;nofollow&quot;&gt;http://www.cosmos.esa.int/web/gaia/news_20131206&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and arianespace launch kit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.arianespace.com/images/launch-kits/launch-kit-pdf-eng/VS06-launchkit-GB2.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.arianespace.com/images/launch-kits/launch-kit-pdf-eng/VS06-launchkit-GB2.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Day - 4&lt;/p&gt;&#xA;&#xA;&lt;p&gt;E.&lt;/p&gt;&#xA;" OwnerUserId="1266" LastActivityDate="2013-12-15T02:21:03.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3143" PostTypeId="2" ParentId="2264" CreationDate="2013-12-16T10:20:39.567" Score="2" Body="&lt;p&gt;Uh... It seems that if they are military astronauts (We still have those?), then they are being paid on the basis that they are part of the military.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://books.google.com/books?id=UiANV4PH5k4C&amp;amp;pg=PA4315&amp;amp;lpg=PA4315&amp;amp;dq=astronauts+hazardous+duty+pay&amp;amp;source=bl&amp;amp;ots=lCtj4nBvfg&amp;amp;sig=4I5AAxZpO3mcBSsgeHzV3VHPRUI&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=PdKuUufhBMHTkQeZ9oHQBQ&amp;amp;ved=0CGcQ6AEwCA#v=onepage&amp;amp;q=astronaut&amp;amp;f=false&quot; rel=&quot;nofollow&quot;&gt;IRS code&lt;/a&gt; seems to imply that it is, however, they are again, speaking strictly about military astronauts, there is not a single mention of civilian astronauts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't honestly know about Civilian astronauts, but they are at &lt;em&gt;least&lt;/em&gt; qualified to be called civilian pilots, and there were a lot of those who qualified for hazardous duty pay, so they probably did as well.&lt;/p&gt;&#xA;" OwnerUserId="1271" LastActivityDate="2013-12-16T10:20:39.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3144" PostTypeId="2" ParentId="2" CreationDate="2013-12-16T10:33:41.257" Score="3" Body="&lt;p&gt;The issues at hand, at least tensile strength, can be lessened by building something else, which is also cool and sufficiently &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceShaft&quot; rel=&quot;nofollow&quot;&gt;spacey&lt;/a&gt;. The Spaceshaft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean, I like it, but again, what we're probably going to be looking at in the next ten years is more like a railgun launch. Sadly, There is quite a consensus that carbon nanotubes aren't strong enough, that the space debris is terrifying (I wonder why?), and that in general, it could unbalance the rotation of the earth(pfft, totally worth it) &lt;/p&gt;&#xA;" OwnerUserId="1271" LastActivityDate="2013-12-16T10:33:41.257" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3145" PostTypeId="1" CreationDate="2013-12-16T12:45:35.513" Score="7" ViewCount="1324" Body="&lt;p&gt;Will the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_One&quot; rel=&quot;nofollow&quot;&gt;Mars One&lt;/a&gt; mission team be testing samples to know for sure that there are no deadly viruses on Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there are no viruses, will this pave the way for Mars travel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the consequences of introducing some of our viruses and bugs to Mars?&lt;/p&gt;&#xA;" OwnerUserId="1273" LastEditorUserId="286" LastEditDate="2014-04-05T09:31:38.240" LastActivityDate="2018-04-28T15:49:58.230" Title="Is 'Mars One' a one-way mission for fear of infecting Earth with unknown pathogens?" Tags="|mars|life|mars-one|forward-contamination|contamination|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3146" PostTypeId="2" ParentId="2264" CreationDate="2013-12-16T13:43:47.620" Score="3" Body="&lt;p&gt;I wouldn't know for the first part of your question, but I did research a bit the second half. Still limiting it to US astronauts alone, I would say that their qualification for &lt;em&gt;hazard pay&lt;/em&gt; depends on who they work for, their contract, and also what they're currently doing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will however be really hard to give fair estimate on what's &lt;em&gt;common practice&lt;/em&gt;, how often and for how much time they qualify for hazard pay. E.g. US military pilots / astronauts don't receive &lt;em&gt;&lt;a href=&quot;http://aoprals.state.gov/content.asp?content_id=177&amp;amp;menu_id=78&quot; rel=&quot;nofollow&quot;&gt;Danger Pay&lt;/a&gt;&lt;/em&gt; unless they work at posts where they're in imminent danger or under threat of physical harm due to &lt;em&gt;civil insurrection, terrorism, or war conditions&lt;/em&gt;. They would however qualify for &lt;em&gt;&lt;a href=&quot;http://www.military.com/benefits/military-pay/special-pay/hazardous-duty-incentive-pay.html&quot; rel=&quot;nofollow&quot;&gt;Hazardous Duty Incentive Pay&lt;/a&gt;&lt;/em&gt; while in flight, for which I doubt the International Space Station counts, its status would likely be defined as &lt;em&gt;a base&lt;/em&gt;, but actual flight to it and back to Earth might.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For non-military astronauts though, this would be nigh impossible to say, with so many possible employers, and even then, different responsibilities would come with different arrangements. You can even hire astronauts nowadays, for example via &lt;a href=&quot;http://www.astronauts4hire.org/p/about.html&quot; rel=&quot;nofollow&quot;&gt;Astronauts 4 Hire&lt;/a&gt; (A4H) program that &lt;em&gt;recruits and trains qualified scientists and engineers for the rigors of spaceflight&lt;/em&gt;. And there are other similar organisations, and I probably need not mention all the &lt;em&gt;payload specialist&lt;/em&gt; astronauts, but their hazard pay would likely depend on who would they be hired by, what benefits would they sign for (on an individual basis), and under which organisation would their employment, responsibilities, schedule, etc. be managed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In days when even US intelligence community outsources en mass to government contractors, such hires will likely be ever more frequent also in the future. And since these are individual contracts with private enterprises, involved companies aren't obliged to disclose any such information to the public (they don't fall under the &lt;a href=&quot;http://www.foia.gov/&quot; rel=&quot;nofollow&quot;&gt;Freedom of Information Act&lt;/a&gt; like the federal government does), so we can only guess and the amount they're paid for hazardous duty would stay between them, their employer, and the taxman.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-12-16T13:43:47.620" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3147" PostTypeId="2" ParentId="3145" CreationDate="2013-12-16T14:07:53.150" Score="19" Body="&lt;p&gt;Quarantine was always a standard procedure for the astronauts which landed on the Moon. The Apollo astronauts were kept in quarantine just in case they contracted &lt;em&gt;something&lt;/em&gt; on the Moon's surface. This turned out to be unnecessary, but better safe than sorry. But how likely is it that there are deadly pathogens on another planet? I would say it is pretty unlikely. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Infectuous pathogens (viruses, bacteria, parasites, fungi or any other microbiological stuff which makes macrobiological stuff feel sick) evolve to coexist with a particular host. But considering that it is practically impossible that there is higher life on mars right now and that there is currently no hint that it ever existed, there are no hosts for which pathogens could have evolved. When there are microbes on Mars it is unlikely that they evolved to infect and harm other life-forms. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even when we presume that there once were other higher life-forms on Mars &lt;strong&gt;and&lt;/strong&gt; pathogens evolved to infect them &lt;strong&gt;and&lt;/strong&gt; these pathogens somehow survived millenias and now lie dormant, it is still unlikely that they are harmful to humans. Most diseases on Earth only affect one particular species (they might be carried by more than one, but they are only harmful to one). When they affect more than one species, these are usually closely related. It is very unlikely that a pathogen which evolved in a completely separate ecosphere would be able to infect us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No, the reason for Mars One being a one way trip is a different one: &lt;strong&gt;Money&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A trip between Earth and Mars takes a lot of fuel per kg of payload. When you want to return, you need to carry the fuel for the return-trip with you. To move that fuel you need even more fuel. And you need to carry THAT fuel too. That means a ship for a two-way trip would have to be a lot larger than one for a single voyage. This is referred to as &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;the tyranny of the rocket equation&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then there are other complexities. Building a capsule which just has to land on Mars is much simpler than one which is equipped with a rocket-engine powerful enough to return into orbit. And it's heavier, which means that even more mass is needed.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="19335" LastEditDate="2018-04-28T15:49:58.230" LastActivityDate="2018-04-28T15:49:58.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3148" PostTypeId="2" ParentId="3145" CreationDate="2013-12-16T14:19:58.437" Score="6" Body="&lt;p&gt;There's no need to worry about any &lt;em&gt;viruses from Mars&lt;/em&gt;. If there are any, there would also have to be host organisms through which these viruses would multiply and &lt;em&gt;survive&lt;/em&gt; (it's still debatable whether viruses even count as &lt;em&gt;life&lt;/em&gt;). And host organisms are easier to detect through e.g. their metabolic process byproducts, and also more difficult to contaminate life support of any future colonists due to their size being much larger. And in the eventuality that these theoretical &lt;em&gt;Martian viruses&lt;/em&gt; could stay dormant for millennia and endure extreme conditions present there, then they would already have been introduced to the Earth through &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_meteorite&quot; rel=&quot;nofollow&quot;&gt;meteorites originating from Mars&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A common belief that meteorites are on impact glowing hot all the way to their inner core due to heat produced through friction on their fast travel through the atmosphere simply isn't true, and any expert on the subject will tell you that they're at most mildly warm to cold to the touch immediately after impacting on Earth. Or ice, as it is the case in polar regions where a lot of them are collected off the snowy / icy surface since they're easier spotted dark on all white, and the ice or snow under them doesn't melt much at all, icy surface might merely be cracked due to the impact force. This is mostly due to asteroids being cooled to near absolute zero (perhaps at most a few degrees Kelvin, or roughly -450 degrees Fahrenheit) in the vacuum of space before impacting to Earth, and also since they would lose most of the heat generated via friction in the atmosphere through ablation of its outermost layer, then cool again when they slow down in the lower, and still cold atmosphere. All of it lasts perhaps a few seconds, so there simply isn't enough time for their cores to &lt;em&gt;sterilize&lt;/em&gt; any hypothetical living microorganisms and viruses in them through extreme heat (i.e. thermal expansion or thermochemical change in levels of free radicals that would disintegrate biological samples).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there's two options left, either there aren't any, or they're localized to places of local biota which we're still struggling to find. And if there is indigenous life on Mars, chances that any of their viruses would be chemically and, ultimately, genetically compatible with life on Earth as it is, all prerequisites for it to have any effect on us, is extremely remote. And even if they would be, there's not much chance they'd be much of a bother to us because of our better evolved &lt;a href=&quot;https://en.wikipedia.org/wiki/Immune_system&quot; rel=&quot;nofollow&quot;&gt;immune system&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You see, viruses can multiply really fast (can go through new generations in hours, some even faster), and with each new generation they can adapt to host organism's defensive mechanisms through natural selection - some mutate and develop new trades that let them survive, and the survivals successfully multiply, preserving the trade that enabled them to survive and creating more resistant, better adapted new generation. Viruses that didn't go through such evolution wouldn't stand a chance, especially from a planet that's not evolving new lifeforms and new, better evolved breeds  as fast as here on Earth, i.e. has inferior biodiversity and is less likely to match or adapt pathogen-to-hosts with superior, more diverse biota.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I'm saying is we're safe from &lt;em&gt;space viruses&lt;/em&gt;, because the Earth is the fastest evolving habitat that we know of and viruses already introduced to us have had a chance to adapt through the millions of years of evolution of life and its own natural (and in recent years artificial) defenses. And if we really have to worry about new, yet unknown strains of viruses, then we ought to look no further than under the melting ice of the polar regions here on Earth. And we're safe from higher organisms because our technology is by now sufficiently evolved to detect them even in trace quantities, and we do follow decontamination procedures that are guaranteed to terminate any water-based life compatible with one that strives here on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other way, what we call forward-contamination, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_protection&quot; rel=&quot;nofollow&quot;&gt;planetary protection&lt;/a&gt; in general is a bit more interesting though and is lately much discussed in scientific and popular literature. History teaches us that we've caused much sorrow to indigenous population wherever we were discovering and inhabiting new lands and that it is by far more common to introduce a deadly pathogen causing havoc to an isolated, less biodiverse habitat than the other way around. One of the most infamous such examples is the &lt;a href=&quot;http://www.yale.edu/ynhti/curriculum/units/2009/5/09.05.03.x.html#b&quot; rel=&quot;nofollow&quot;&gt;introduction of smallpox, measles and influenza viruses to the indigenous populations of North America&lt;/a&gt;, but there are many other similar cases throughout the human history of exploration, which is nicely explained on &lt;a href=&quot;https://en.wikipedia.org/wiki/Social_history_of_viruses&quot; rel=&quot;nofollow&quot;&gt;Wikipedia page on Social history of viruses&lt;/a&gt;, but the havoc doesn't stop at microorganisms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem with any such discussions is of course that they might be irrelevant, if it turns out that the planet we're colonizing is indeed a dead rock, but to know that for sure might be nearly impossible without risking contamination first and exploring all of its distinct environment regions. In semi-official capacity, the &lt;em&gt;highest authority&lt;/em&gt; on the matter is considered to be the &lt;a href=&quot;https://en.wikipedia.org/wiki/COSPAR&quot; rel=&quot;nofollow&quot;&gt;Committee on Space Research&lt;/a&gt; (COSPAR) that is preparing recommendations to avoid interplanetary contamination for national space agencies that are members of the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Planetary_Data_Alliance&quot; rel=&quot;nofollow&quot;&gt;International Planetary Data Alliance&lt;/a&gt; (IPDA) and including NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But since this is an utterly broad topic and not to beat around the bush too much, I'll just refer you to Wikipedia sections on &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_protection#Containment_and_quarantine&quot; rel=&quot;nofollow&quot;&gt;Containment and quarantine&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_protection#Decontamination_Procedures&quot; rel=&quot;nofollow&quot;&gt;Decontamination Procedures&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_protection#Impact_prevention&quot; rel=&quot;nofollow&quot;&gt;Impact prevention&lt;/a&gt; where it's all described in greater detail. It is definitely a hot subject, though. Perhaps worth mentioning too is that we have obviously already managed to forward-contaminate another celestial - &lt;a href=&quot;https://en.wikipedia.org/wiki/Reports_of_Streptococcus_mitis_on_the_Moon&quot; rel=&quot;nofollow&quot;&gt;the Moon&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-18T08:41:33.250" LastActivityDate="2013-12-18T08:41:33.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3149" PostTypeId="1" AcceptedAnswerId="3150" CreationDate="2013-12-16T15:50:00.950" Score="7" ViewCount="1308" Body="&lt;p&gt;On Saturday (&lt;em&gt;December 14, 2013&lt;/em&gt;), the first soft landing on the Moon &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-25384057&quot; rel=&quot;nofollow&quot;&gt;since 1976 occurred&lt;/a&gt;.  The rover is named &quot;Jade rabbit&quot;, what is the significance of that name?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2016-02-02T19:24:14.710" LastActivityDate="2016-02-02T19:24:14.710" Title="What is the significance of the name &quot;Jade rabbit&quot;?" Tags="|the-moon|landing|terminology|chang-e|yutu|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3150" PostTypeId="2" ParentId="3149" CreationDate="2013-12-16T16:18:49.473" Score="9" Body="&lt;p&gt;Emily Lakdawalla was covering this naming process for the Yutu (玉兔) rover in her Planetary Society blog posts for the majority of us Westerners that don't speak Chinese (yours truly included). &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/11262206-change-3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;In one of her posts&lt;/a&gt;, she describes it like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Yesterday the Chinese space agency held a press briefing about the&#xA;  Chang'e 3 lunar lander. They announced that the rover has been named&#xA;  Yutu (or &quot;Jade Rabbit,&quot; a legendary companion of the goddess Chang'e).&#xA;  Yutu was selected from a list of 10 names, receiving 649,956 votes out&#xA;  of nearly 3.5 million votes cast. In a written statement that I'm&#xA;  decoding with the help of Google Translate, they remark that the name&#xA;  Yutu embodies Chinese traditional culture and reflects their intent&#xA;  for peaceful use of space, and that they're looking forward to the&#xA;  rover rolling across the Bay of Rainbows. Chang'e 3 will launch on a&#xA;  Long March III B onto a direct lunar transfer orbit (an orbit with&#xA;  perigee 200 kilometers and apogee 380,000 kilometers).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Since then, Wikipedia has also updated their &lt;a href=&quot;https://en.wikipedia.org/wiki/Jade_Rabbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;page on Moon rabbit&lt;/a&gt;, describing this name from a Chinese folklore in greater detail:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Moon rabbit in folklore is a rabbit that lives on the Moon, based&#xA;  on pareidolia that identifies the markings of the Moon as a rabbit.&#xA;  The story exists in many cultures, prominently in East Asian folklore&#xA;  and Aztec mythology. In East Asia, it is seen pounding in a mortar and&#xA;  pestle, but the contents of the mortar differ among Chinese, Japanese,&#xA;  and Korean folklore. In Chinese folklore, it is often portrayed as a&#xA;  companion of the Moon goddess Chang'e, constantly pounding the elixir&#xA;  of life for her; but in Japanese and Korean versions, it is pounding&#xA;  the ingredients for rice cake.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And specifically on Jade Rabbit (Yutu):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Han Dynasty poets call the rabbit on the Moon the &quot;Jade Rabbit&quot; (玉兔)&#xA;  or the &quot;Gold Rabbit&quot; (金兔), and these phrases were often used in place&#xA;  of the word for the Moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/LjmYc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The image of a rabbit and mortar delineated on the Moon's surface (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Jade_Rabbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Moon Rabbit&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to my understanding, Yutu / Jade Rabbit is to Chinese people what &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Man_in_the_Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Man in The Moon&lt;/a&gt;&lt;/em&gt; (or to some &lt;em&gt;&lt;a href=&quot;http://www.universetoday.com/26683/the-basketball-player-in-the-moon-catch-it-tonight/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Basketball Player in the Moon&lt;/a&gt;&lt;/em&gt;) is to us Westerners.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here's a cute reference to &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang&amp;#39;e&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chang'e&lt;/a&gt; (aka Chang'o) and the Jade Rabbit from the &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/apollo11.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 11 mission transcripts&lt;/a&gt; (page 270, 272 in the PDF, of the &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/AS11_TEC.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 11 Technical Air-to-Ground Voice Transcription&lt;/a&gt;, July 1969, 626 pages):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DlZxu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-02T17:14:29.817" LastActivityDate="2016-02-02T17:14:29.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3152" PostTypeId="1" CreationDate="2013-12-16T18:56:03.790" Score="9" ViewCount="246" Body="&lt;p&gt;The onus for Spectrum management lies with &lt;a href=&quot;http://en.wikipedia.org/wiki/ITU&quot;&gt;ITU&lt;/a&gt;. What little I know - Space communications (as against Satellite communication) are allocated frequencies out of the &lt;a href=&quot;http://en.wikipedia.org/wiki/S_band&quot;&gt;S&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/X_band&quot;&gt;X&lt;/a&gt; bands. Other bands may be allocated too - the ITU Space Service document was unavailable/pay-walled&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I'm curious about&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are space communication frequencies allocated per-mission, or per-agency?&lt;/li&gt;&#xA;&lt;li&gt;If allocation is per-mission, are the frequencies recycled after the mission is over? E.g. &lt;a href=&quot;http://en.wikipedia.org/wiki/Giotto_%28spacecraft%29&quot;&gt;Giotto&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;What is the rationale behind the allocation of micro-wave bands to space communication? Instead of, for instance, HF/VHF why are some of the micro-wave bands used? Is it merely a matter of convenient antennas, or is there more to it?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2013-12-18T14:24:48.440" LastActivityDate="2014-08-19T13:31:02.330" Title="What is the rationale behind the allocation of micro-wave bands for space communication?" Tags="|communication|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3153" PostTypeId="1" CreationDate="2013-12-17T04:21:00.993" Score="13" ViewCount="3125" Body="&lt;p&gt;All manned spacecraft have had seats in an &quot;on-back&quot; angle for launch. What is the benefit of this seating position for a manned spacecraft launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wbM7b.jpg&quot; alt=&quot;This a example of the &amp;quot;on-back&amp;quot; seating angle&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1283" LastEditorDisplayName="user12" LastEditDate="2013-12-17T04:30:20.467" LastActivityDate="2022-07-02T08:54:11.723" Title="If a human is being sent to space in a rocket, does the seating angle matter?" Tags="|crewed-spaceflight|astronauts|spacecraft-development|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3154" PostTypeId="2" ParentId="3153" CreationDate="2013-12-17T07:02:14.237" Score="4" Body="&lt;p&gt;let us take the shuttle&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Space Shuttle (takeoff): 29 m/s2&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;which means 29/9.8 &lt;span class=&quot;math-container&quot;&gt;$\approx$&lt;/span&gt; 3g In such a case the force acting on the body would be tremendous, so if your seating position is in the opposite direction, the force is strong enough to crush your ribs (chest) and the broken bones from the ribs can even penetrate leading to death or it can cause breathing problem and even cardiac attack.&lt;/p&gt;&#xA;&lt;p&gt;If the seating is &amp;quot;on-back&amp;quot; angle for launch much of the force during the acceleration would be transferred to your back side of the body to the chair so less force is felt by your heart.&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="28487" LastEditDate="2022-07-02T08:49:45.480" LastActivityDate="2022-07-02T08:49:45.480" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="3155" PostTypeId="2" ParentId="3153" CreationDate="2013-12-17T12:04:31.387" Score="17" Body="&lt;p&gt;The human body tolerates G-forces best if they're &lt;a href=&quot;http://en.wikipedia.org/wiki/G-force&quot;&gt;perpendicular to the spine&lt;/a&gt;.&lt;br&gt;&#xA;When you apply a G-force along the spine, the blood will gravitate towards the feet, leaving the head starved of blood, which induces loss of consciousness. Having the head at the lowest position helps to prevent these blackouts.&lt;br&gt;&#xA;3G is tolerable even if you're facing down, by the way. The seating position in capsules is chosen to support the body during landing (the forces in reentry and landing are larger than during launch). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-12-17T12:04:31.387" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3161" PostTypeId="1" AcceptedAnswerId="3166" CreationDate="2013-12-18T14:24:13.393" Score="71" ViewCount="55231" Body="&lt;p&gt;Currently, SpaceX has developed the &lt;a href=&quot;http://en.wikipedia.org/wiki/Merlin_%28rocket_engine%29&quot; rel=&quot;noreferrer&quot;&gt;Merlin 1&lt;/a&gt; family (1B (Falcon 1), 1C (Falcon 9 v1.0), 1D (Falcon 9 v1.1/F9-R/Falcon Heavy), vacuum versions and sealevel versions) which are LOX/RP1 based.   (75-205Klbs thrust)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They developed (and abandoned) the &lt;a href=&quot;http://en.wikipedia.org/wiki/Kestrel_%28rocket_engine%29&quot; rel=&quot;noreferrer&quot;&gt;Kestrel&lt;/a&gt; engine used on the second stage of the Falcon 1, also LOX/RP1.  (6.9Klbs thrust)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They developed the &lt;a href=&quot;http://en.wikipedia.org/wiki/Draco_%28rocket_engine_family%29&quot; rel=&quot;noreferrer&quot;&gt;Draco and SuperDraco&lt;/a&gt; engines using hypergolics (mono-methyl hydrazine fuel and nitrogen tetroxide oxidizer).  (90 lbs and 15K lbs thrust)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next engine on their list is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Raptor_%28rocket_engine%29&quot; rel=&quot;noreferrer&quot;&gt;Raptor&lt;/a&gt;, which is expected to be Methane (CH4) and LOX based.  (660Klbs thrust)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we know whey they chose methane over hydrogen?  Performance wise, LH2 is usually the go to propellant.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-08-28T19:08:16.340" LastActivityDate="2019-03-25T05:29:31.963" Title="Why is SpaceX considering Methane as fuel for their next engine, the Raptor?" Tags="|launch|spacex|engine-design|raptor|" AnswerCount="10" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="3162" PostTypeId="2" ParentId="3153" CreationDate="2013-12-18T15:31:07.273" Score="6" Body="&lt;p&gt;Interestingly, the Soyuz capsule has seats that change orientation at landing time.&lt;/p&gt;&#xA;&lt;p&gt;They are mounted appropriately for absorbing G-Loads the best at launch. Then seconds before landing, the seats change orientation to prepare for better shock absorption during landing.&lt;/p&gt;&#xA;&lt;p&gt;Apollo's seats had shock absorbers for landing, and required certain depths for 'travel' which was an issue they had to consider when they designed the Skylab rescue Apollo for the worst case of a disabled Apollo at Skylab.  They would need to carry at least 4 people, (one pilot up, 3 down, more likely 2 up so they could work as a team and three down) which meant 2 rows of seats, which the travel requirement caused issues with making it all fit.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="7062" LastEditDate="2022-07-02T08:54:11.723" LastActivityDate="2022-07-02T08:54:11.723" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3164" PostTypeId="1" AcceptedAnswerId="3198" CreationDate="2013-12-18T18:06:16.920" Score="4" ViewCount="944" Body="&lt;p&gt;NASA's &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsn/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt;, a collection of antennae throughout the globe, is sometimes used to support earth-orbiting missions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The NASA Deep Space Network - or DSN - is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. &lt;strong&gt;The network also supports selected Earth-orbiting missions.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/TTPGE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;One of the DSN's 70-meter antenae&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which earth-orbiting missions are currently using the DSN? What benefit do they receive by doing so?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/conversation/a-conversation-about-questions&quot;&gt;thanks to @Everyone for letting me steal his question idea&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="168" LastEditDate="2013-12-19T11:33:53.007" LastActivityDate="2013-12-27T16:58:51.620" Title="Which LEO missions are currently supported by NASA's DSN (and why)?" Tags="|artificial-satellite|communication|deep-space-network|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3165" PostTypeId="2" ParentId="3161" CreationDate="2013-12-18T19:35:24.510" Score="23" Body="&lt;p&gt;Logistically, methane can be easier to work with than hydrogen.  Methane's boiling point is about 110K, compared to hydrogen's 20K.  This means that both fuel and oxidizer lines can be purged with gaseous nitrogen.  Liquid hydrogen lines can only be purged with helium, as hydrogen's boiling point is below the melting point of other inert gases.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2013-12-18T19:35:24.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3166" PostTypeId="2" ParentId="3161" CreationDate="2013-12-18T20:34:11.873" Score="85" Body="&lt;p&gt;&lt;strong&gt;Methane has the benefit of being easier to store than hydrogen&lt;/strong&gt;. Mostly &lt;strong&gt;passive cooling can suffice to keep it cryogenic&lt;/strong&gt;, whereas hydrogen needs active cooling, and will still vent over time.  Which makes Methane much closer to 'storable' than hydrogen can be.  This would make it useful for deep space missions, with long mission durations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methane is less bulky than hydrogen.  Which means tankage is smaller for the same mission.  (The Shuttle external tank is mostly hydrogen tanks with a small oxygen tank (at the top?)). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methane should be simpler to use in an engine due to its higher density than hydrogen, less needs to be pumped by volume. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Methane is potentially manufacturable on Mars&lt;/strong&gt;.  With imported Hydrogen (or native water), CO2 (Carbon dioxide) can be converted to CH4 reasonably straight forwardly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are ideas for ISRU (In-Situ Resource Utilization) and demonstrations on Mars. (&lt;a href=&quot;http://en.wikipedia.org/wiki/The_Case_for_Mars&quot;&gt;Robert Zubrin&lt;/a&gt;'s model is launch the return vehicle that uses ISRU to fill its fuel tanks, and do not launch the manned mission until the return vehicle is fully fueled and ready to go.  Then you launch the manned mission, along with a second return vehicle, that uses ISRU over the duration of the surface mission to fuel itself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is focused on developing re-usability technology for their rocket lines. Traditional rocket-grade kerosene produces residue (a process known as &quot;coking&quot;) when it burns. Methane fuel burns cleaner so there is no residue build-up which means engines can be re-used more times without refurbishment.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2014-10-31T13:52:16.670" LastActivityDate="2014-10-31T13:52:16.670" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="3167" PostTypeId="1" AcceptedAnswerId="3171" CreationDate="2013-12-19T11:43:19.780" Score="7" ViewCount="1542" Body="&lt;p&gt;A bit of spin on &lt;a href=&quot;https://space.stackexchange.com/questions/3164/which-missions-are-currently-supported-by-nasas-dsn&quot;&gt;Which LEO missions are currently supported by NASA&amp;#39;s DSN (and why)?&lt;/a&gt; is what I have in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsn/antennas/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network home-page&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each complex consists of at least four deep space stations equipped with ultrasensitive receiving systems and large parabolic dish antennas. There are: &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;• One 34-meter (111-foot) diameter High Efficiency antenna.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;• One 34-meter Beam Waveguide antenna.&#xA;   (Three at the Goldstone Complex and two in Madrid)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;• One 26-meter (85-foot) antenna.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;• One 70-meter (230-foot) antenna.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Apparently each station is commanded remotely as mentioned on that page &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;All the stations are remotely operated from a centralized Signal Processing Center at each complex. The Centers house the electronic subsystems that point and control the antennas, receive and process the telemetry data, transmit commands, and generate the spacecraft navigation data.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are any number of space-craft operating in space &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Voyager 1 &lt;/li&gt;&#xA;&lt;li&gt;Voyager 2&lt;/li&gt;&#xA;&lt;li&gt;Rosetta  &lt;/li&gt;&#xA;&lt;li&gt;Dawn&lt;/li&gt;&#xA;&lt;li&gt;Spirit&lt;/li&gt;&#xA;&lt;li&gt;Curiosity &lt;/li&gt;&#xA;&lt;li&gt;Juno&lt;/li&gt;&#xA;&lt;li&gt;Kepler&lt;/li&gt;&#xA;&lt;li&gt;Herschel, to name just a few ...&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What I'm curious about &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Can a station communicate with more than one space-craft using simple time-slicing &lt;em&gt;without&lt;/em&gt; re-orientation of the antenna/s?&lt;/li&gt;&#xA;&lt;li&gt;What algorithm (is that the proper term?) is followed to orient an antenna? Surely any given antenna isn't moved from zenith to nadir then back &amp;amp; around at random ...&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-04-26T06:25:19.883" Title="How many space-craft can a station on the DSN communicate with concurrently?" Tags="|communication|deep-space-network|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3170" PostTypeId="2" ParentId="3164" CreationDate="2013-12-19T14:01:06.410" Score="2" Body="&lt;p&gt;None. NASA uses &lt;a href=&quot;http://www.nasa.gov/content/near-earth-network/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Near Earth Network&lt;/a&gt; (NEN) for telemetry, commanding, ground-based tracking, data and communications with satellites in low Earth orbit (LEO), that is a part of NASA's &lt;a href=&quot;http://www.nasa.gov/directorates/heo/scan/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Communications and Navigation&lt;/a&gt; (SCaN) network, including NEN, Space Network (SN) and Deep Space Network (DSN) managed by NASA's Networks Integration Management Office (NIMO).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NEN currently consists of these NASA owned, contracted and commercial ground stations:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Alaska Satellite Facility, Fairbanks&lt;/li&gt;&#xA;&lt;li&gt;USN Alaska, Poker Flat and North Pole&lt;/li&gt;&#xA;&lt;li&gt;Gilmore Creek, Alaska (National Oceanic and Atmospheric Administration)&lt;/li&gt;&#xA;&lt;li&gt;Wallops, Virginia Ground Station&lt;/li&gt;&#xA;&lt;li&gt;KSAT Svalbard, Norway&lt;/li&gt;&#xA;&lt;li&gt;SSC Kiruna, Sweden&lt;/li&gt;&#xA;&lt;li&gt;USN Germany, Weilheim&lt;/li&gt;&#xA;&lt;li&gt;KSAT Singapore, Malaysia&lt;/li&gt;&#xA;&lt;li&gt;USN Australia, Dongara&lt;/li&gt;&#xA;&lt;li&gt;McMurdo, Antarctica Ground Station&lt;/li&gt;&#xA;&lt;li&gt;South African National Space Agency, Hartebeesthoek, South Africa&lt;/li&gt;&#xA;&lt;li&gt;KSAT TrollSat, Antarctica&lt;/li&gt;&#xA;&lt;li&gt;SSC Santiago, Chile&lt;/li&gt;&#xA;&lt;li&gt;White Sands Ground Station, New Mexico&lt;/li&gt;&#xA;&lt;li&gt;USN Hawaii, South Point&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(KSAT = Kongsberg Satellite Services, SSC = Swedish Space Corporation, USN = Universal Space Network)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/pdv6D.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Map of Near Earth Network Sites (Source: &lt;a href=&quot;http://www.nasa.gov/content/near-earth-network/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Near Earth Network&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NEN also uses:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Satellites in low Earth orbit (LEO)&lt;/li&gt;&#xA;&lt;li&gt;Satellites in geosynchronous orbit (GEO)&lt;/li&gt;&#xA;&lt;li&gt;Satellites in highly elliptical orbit&lt;/li&gt;&#xA;&lt;li&gt;Satellites in Lunar orbit&lt;/li&gt;&#xA;&lt;li&gt;Satellites with multiple frequency bands&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The spacecraft that are orbiting the closest to Low Earth Orbit (LEO), which stretches from an altitude of 160 to 2,000 kilometers (99 to 1,200 miles) above sea-level, and that are supported by DSN are currently (including one near-future mission scheduled for 2014):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sci.esa.int/cluster/31258-summary/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA Cluster II&lt;/a&gt; of two spacecraft currently in elliptical polar orbit with 57 hours period, perigee of 19,000 km, and apogee of 119,000 km&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mms.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Magnetospheric Multiscale (MMS) Observatory&lt;/a&gt; is a near-future mission of two spacecraft that will occupy an elliptical orbit with inclination of 28.0°, perigee of 7,640 km, and apogee of 76,400 km&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://pwg.gsfc.nasa.gov/geotail.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISAS/NASA/JAXA GEOTAIL&lt;/a&gt; mission with a single spacecraft currently in highly elliptical orbit with inclination of 22°, perigee of 47,824 km, and apogee of 193,019 km&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Other current space missions supported by DSN are even further away from Earth, either interplanetary probes, orbiters, rovers, etc., deep space probes, a few space observatories and perhaps the next closest - Lunar orbiters. DSN for missions closer to Earth often serves as an extension of other NASA operated SCaN networks, NEN and SN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current and planned future missions supported by SCaN are listed on following pages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/directorates/heo/scan/services/missions/txt_solarmissions.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar System Missions&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/content/earth-missions-supported-by-scan/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth Missions&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/content/universe-missions-supported-by-scan/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Universe Missions&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/directorates/heo/scan/services/missions/txt_launchmissions.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Other Missions&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Pages list which NASA communications networks they are supported by.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-21T11:10:46.810" LastActivityDate="2013-12-21T11:10:46.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3171" PostTypeId="2" ParentId="3167" CreationDate="2013-12-19T15:00:57.730" Score="11" Body="&lt;p&gt;Yes, a station can communicate with more than one spacecraft, but not the way you suggest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each DSN complex consists of several antennas, each of which is called a Deep Space Station.  A &quot;station&quot; is one antenna.  Each antenna has a single transmitter, so any one station can only transmit to one spacecraft at a time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However if more than one spacecraft is in the antenna beam, which is often the case at Mars, then a station can &lt;em&gt;receive&lt;/em&gt; data from more than one spacecraft at a time.  This capability is called Multiple Spacecraft Per Aperture (MSPA).  Of course, the spacecraft have to be on different frequencies.  So one station can uplink to only one spacecraft, but it can downlink from more than one spacecraft in the beam.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is the DSN display showing MRO and Odyssey on the same antenna (DSS-43).  You will sometimes see the opposite, where there is one spacecraft on two or more antennas, like Maven on DSS-34 and DSS-45, also in the picture below.  When at the same complex, that is called &quot;arraying&quot;, where the signal from two or more antennas are combined to simulate a larger antenna for increased data rate.  That also only works for receiving.  Antennas cannot (today) be arrayed for transmitting.  You can also see the same spacecraft on two different stations at two different complexes.  In that case either the spacecraft is in the middle of a handoff from one complex to another to make for one long communication pass, or a special navigation data type is being collected using very long baseline interferometry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jFLeQ.jpg&quot; alt=&quot;DSN display showing MRO and Odyssey on one antenna&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time multiplexing is not useful, since it can take quite some time to lock on to and acquire a signal.  Once you have it, you don't want to lose it and then have to find it again.  It is better to have multiple receivers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose that in effect we &lt;em&gt;do&lt;/em&gt; use time multiplexing of the antennas, but on the scale of hours.  Since it takes around half an hour to lock on to a signal, you stay with it for at least a few hours, usually eight to ten hours, and then switch to another spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the &quot;algorithm&quot; used to point an antenna, we have very accurate models of the position of Earth, the position of any given antenna on Earth, and of the target spacecraft.  You just compute the vector to the target and point the antenna in that direction.  The direction changes with time as the Earth rotates (and as the Earth and target move relative to each other), so the vector is continuously updated for the antenna to track the target.  (I'm not clear on your question there, so if this doesn't answer it, then you need to clarify the question.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-19T20:51:35.847" LastActivityDate="2013-12-19T20:51:35.847" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3172" PostTypeId="1" CreationDate="2013-12-19T15:58:39.110" Score="3" ViewCount="433" Body="&lt;p&gt;Some nuclear powered spacecraft designs have been entertained where the radiator is arranged in a plate geometry which rotates.  For example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.projectrho.com/public_html/rocket/realdesigns.php#id--Mars_Umbrella_Ship&quot; rel=&quot;noreferrer&quot;&gt;http://www.projectrho.com/public_html/rocket/realdesigns.php#id--Mars_Umbrella_Ship&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/craft/stus1957.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/craft/stus1957.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This doesn't make any sense to me, because the nuclear reactor would have to be situated at the center.  All the designs are consistent with this.  So that means that hot coolant flows &lt;em&gt;out&lt;/em&gt; and cold coolant flows back &lt;em&gt;in&lt;/em&gt;.  Doesn't this mean that you'll be fighting natural circulation?  If the hot coolant is less dense than the cold, then there's a driving force in the reverse direction of flow due to the centripetal force being stronger on the cold fluid than on the hot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that, why do any of these designs propose to rotate in the first place?  For natural circulation, the reactor is in the wrong place.  So what was ever the logic behind that?  Is the idea to use some exotic fluid that decreases density with rising temperature, or is there some other motivation?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2013-12-21T06:10:25.043" Title="What is the point of rotation of an &quot;umbrella ship&quot;" Tags="|thermal|rotation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3173" PostTypeId="2" ParentId="3172" CreationDate="2013-12-19T18:20:31.933" Score="2" Body="&lt;p&gt;The only reason for it that is explained elsewhere and I can find is in the &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/realdesigns.php#id--Mars_Umbrella_Ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;first link&lt;/a&gt; that you give, and suggests that the rotation creates centrifugal force for the silicon oil coolant to be drawn through the &quot;umbrella&quot; radiator from two feed pipes rotating around the inner edge and moving it towards the radiator's outer edge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would wager though that there isn't a single reason for it but that it is a systemic solution that helps shift mass towards the &lt;em&gt;parasol&lt;/em&gt; radiator and enables the use of a longer boom, so the reactor stays sufficiently distanced from the habitat ring, which helps add to the total inertial mass that rotates the electricity generator required for the ion drive, and radially distributes the coolant along the radiator feed in the opposite direction to the radiator (they're fixed to the boom which is rotated in opposite direction so the boom on which the ion drive is mounted stays directionally fixed, at the center of mass that is clear of obstacles and so usable for the &quot;Mars Umbrella Ship&quot; propulsion). At least, this is what I understood from this image and its caption:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/XfJZW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;code&gt;(1) Hot silicon oil from reactor enters (2) turbine, spinning it. Turbine spin drives (3) generator,&lt;/code&gt;&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;code&gt;creating electricity for ion drive. (4) Hot silicon leaves turbine and enters (5) radiator feed ring.&lt;/code&gt;&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;code&gt;The ring feeds the hot oil into inner edge of (6) heat radiator, where the oil is drawn through&lt;/code&gt;&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;code&gt;radiator by centrifugal force and cooled. Cool oil enters (7) collector at rim of radiator and is fed&lt;/code&gt;&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;code&gt;into the (8) return pipes and sent to the center. There the cool oil enters the (9) return manifold&lt;/code&gt;&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;code&gt;where all the cool oil is (10) sent back to the reactor to start the cycle again.&lt;/code&gt;&lt;/sup&gt;&#xA;  &lt;br /&gt;&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image and caption source: &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/realdesigns.php#id--Mars_Umbrella_Ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atomic Rockets: Mars Umbrella Ship&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Interestingly though, rotation of the habitat ring and the radiator umbrella isn't mentioned in &lt;a href=&quot;https://www.youtube.com/watch?v=iEg7dF5rg8Y#t=2408&quot; rel=&quot;nofollow noreferrer&quot;&gt;Walt Disney's Mars and Beyond (YouTube video)&lt;/a&gt; and the umbrella ships aren't animated to rotate their radiator part either, merely that the hot coolant exiting the nuclear reactor rotates the generator that produces electricity for the ion drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to me, it doesn't seem there would be any point in rotating the umbrella even if that would help distribute the coolant (there are other, simpler ways of achieving that, and you already mention circulation of it through convective heat transfer, which does seem should do the job), especially since all these drawings and animations include an auxiliary landing vehicle mounted on the thrust chamber assembly, that would shift the center of mass off the axis the umbrella part would rotate, making the whole system rotationally unstable and introduce a wobble. For example, even if this is solved by counterweights, it unnecessarily adds weight to the whole system, and it doesn't explain what happens when the auxiliary mounted landing ship departs and ascent stage later docks with the umbrella ship either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Frankly, all of it seems way too complicated, at least how it is explained in sources claiming the umbrella radiator and the habitat ring to rotate, all of it merely to gain centrifugal force equal to 0.05 g (roughly 1.5 revolutions per minute) and radially distribute coolant along its surface, and it still doesn't answer questions regarding system's stability when some of this mass is detached from it or later (less than we started with) reattached. Odd indeed.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-21T06:10:25.043" LastActivityDate="2013-12-21T06:10:25.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3176" PostTypeId="1" CreationDate="2013-12-20T14:54:14.490" Score="5" ViewCount="467" Body="&lt;p&gt;We know that in zero gravity; you can simulate gravity with &lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifugal_force&quot; rel=&quot;nofollow&quot;&gt;centrifugal force&lt;/a&gt;.  This works great for people, but if you had fish in a body of water, would the fish be subject to simulated gravity?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="25" LastEditDate="2013-12-20T20:54:53.093" LastActivityDate="2013-12-20T20:54:53.093" Title="Would centrifugal force simulate gravity for fish?" Tags="|water|artificial-gravity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3178" PostTypeId="2" ParentId="3176" CreationDate="2013-12-20T18:21:02.383" Score="5" Body="&lt;p&gt;There are many processes in aquatic life that work, based on gravity.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Air bubbles float up.  &lt;/li&gt;&#xA;&lt;li&gt;Water levels separate based on temperature, because hotter water is lighter than colder water. &lt;/li&gt;&#xA;&lt;li&gt;Oxygen moves from the surface down to lower levels as the water circulates.&lt;/li&gt;&#xA;&lt;li&gt;Detritus sinks to the bottom, where it can decompose, allow plants to grow in it, etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Both these processes cause turn over of the body of water and create circulation patterns.  Centrifugal gravity simulation would help with this. Some of these matter more in large bodies of water vs small bodies of water, and pumps could be used to help simulate some of it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-20T18:44:54.987" LastActivityDate="2013-12-20T18:44:54.987" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3179" PostTypeId="2" ParentId="3103" CreationDate="2013-12-20T21:24:44.807" Score="17" Body="&lt;p&gt;No, the scrubber design was not modified. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.ibiblio.org/apollo/Documents/1APOLLO14.pdf&quot; rel=&quot;noreferrer&quot;&gt;Apollo 14 press kit&lt;/a&gt; has a list of the changes made after Apollo 13:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The major changes to the command/service modules include&#xA;  adding a third cryogenic oxygen tank installed in a heretofore&#xA;  empty bay of the service module, addition of an auxilliary [sic]&#xA;  battery in the service module as a backup in case of fuel cell&#xA;  failure, removal of destratification fans in the cryogenic&#xA;  oxygen tanks and removal of thermostat switches from the&#xA;  oxygen tank heater circuits. Provision for stowage of an&#xA;  emergency five-gallon supply of drinking water has been&#xA;  added to the command module. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(and several more paragraphs)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The file doesn't mention any changes to the CO₂ scrubbers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://answers.yahoo.com/question/index?qid=20070811130240AAnyAMd&quot; rel=&quot;noreferrer&quot;&gt;thread on Yahoo&lt;/a&gt; claims no changes were made, but offers no references. One of the posters there does say he has an Apollo 16 LM CO₂ canister, and it looks identical to the Apollo 13 items.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a14/a14-43939523-LM10-LM14-Fam-Manual.pdf&quot; rel=&quot;noreferrer&quot;&gt;Lunar Module, LM 10 Through LM 14, Vehicle Familiarization Manual&lt;/a&gt; (large PDF)  shows a drawing of the Environmental Control System (page 3-55/page 89 of the PDF). LM 10 flew with Apollo 15. The drawing still has round covers for the scrubber canisters, like the photo below.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/EwAoA.jpg&quot; alt=&quot;LM ECS with captions&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="881" LastEditDate="2016-12-16T22:36:17.697" LastActivityDate="2016-12-16T22:36:17.697" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3182" PostTypeId="2" ParentId="3128" CreationDate="2013-12-21T10:14:57.360" Score="4" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZZEJe.jpg&quot; alt=&quot;Rendition of plumes on Europa&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply, it is &lt;em&gt;possible&lt;/em&gt;, but whether or not we can build a probe like that is up for discussion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This geyser occurs when Europa is at it's apoapse around Jupiter due to the weakening of the tidal forces on it's surface. All the pent up pressure under the surface shoots outward as a geyser, reaching heights of up to 125 miles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, to quote NASA, these plumes jet out of the surface with &lt;em&gt;seven tons&lt;/em&gt; of material a second! As it get's higher it will diffuse and slow down slightly, but still, it's a really fast cloud of matter hurtling towards the probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the probe could time it's orbit and altitude to pass through the geyser at the right time, I'm pretty sure it won't be able to survive the impact with a regular design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However it would be pretty interesting to research ways to accomplish this mission. You could possible use a probe with a heat-shield and a cargo bay. The heat-shield will protect the probe and store the geyser material in the bay. The probe can then dock with a command module in high orbit and study the materials at it's leisure.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2013-12-21T10:14:57.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3183" PostTypeId="2" ParentId="2158" CreationDate="2013-12-21T10:26:02.127" Score="7" Body="&lt;p&gt;This &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_space_elevator&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; says that The Lagrange points L1 and L2 are the only places where you could place a lunar elevator.  L1 is 56,000 km away from the Earth-facing side of the Moon, (at the lunar equator) and L2 is 67,000 km from the center of the Moon's far side. Moon gravity is 1.6 $m/s^2$.&lt;br&gt;&#xA;A stationary orbit &lt;a href=&quot;https://en.wikipedia.org/wiki/Areostationary_orbit&quot; rel=&quot;noreferrer&quot;&gt;around Mars&lt;/a&gt; is 17,000 km. Mars gravity is 3.7 $m/s^2$.&lt;br&gt;&#xA;Getting from this data to a tensile strength requirement requires more &lt;a href=&quot;http://www.zadar.net/space-elevator/&quot; rel=&quot;noreferrer&quot;&gt;complex&lt;/a&gt; math than I can handle right now.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/1032Pearson.pdf&quot; rel=&quot;noreferrer&quot;&gt;study&lt;/a&gt; done for NASA's Institute of Advanced Concepts states &quot;Current composites have characteristic heights of a few hundred kilometers, which would require taper ratios of about 6 for Mars, 4 for the Moon, and about 6000 for the Earth. This suggests that the Moon elevator is has lower tensile strength requirements. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2013-12-21T10:26:02.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3184" PostTypeId="2" ParentId="2771" CreationDate="2013-12-21T11:15:17.593" Score="4" Body="&lt;p&gt;Minerva was fired towards the asteroid by Hayabusa, but it did not have its own propulsion system. The problem was that Minerva was &lt;a href=&quot;http://www.nbcnews.com/id/10013920&quot; rel=&quot;nofollow noreferrer&quot;&gt;released at the wrong moment&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;While awaiting the ground command to deploy MINERVA, the Hayabusa mothership was under autopilot control to maintain a set distance above the surface. As it drifted up or down to a range limit, Hayabusa fired small gas thrusters to stay within the designated interval.&lt;/p&gt;&#xA;&lt;p&gt;According to Kawaguchi, the deployment command happened to arrive at Itokawa during a period when Hayabusa was drifting away from the surface. Since the escape velocity associated with the asteroid's faint gravitational pull is so small — about one-half of a mile per hour (20 centimeters per second) — MINERVA was sent on a flight path that took it away from the asteroid.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Philae, on the other hand, has its &lt;a href=&quot;https://solarsystem.nasa.gov/docs/p395.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;own propulsion system&lt;/a&gt;. The Active Descent System is a &lt;a href=&quot;http://en.wikipedia.org/wiki/Cold_gas_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;cold-gas thruster&lt;/a&gt;. This &lt;a href=&quot;http://adsabs.harvard.edu/full/2004ESASP.548..449B&quot; rel=&quot;nofollow noreferrer&quot;&gt;can be used during the descent&lt;/a&gt;, and it is used after landing to make sure that Philae doesn't bounce off. The lander also has &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/The_Rosetta_lander&quot; rel=&quot;nofollow noreferrer&quot;&gt;shock absorbers on its landing legs&lt;/a&gt;. During the landing, threaded spikes are extended from the landing legs to screw the legs to the surface.&#xA;After the landing, a harpoon is fired into the ground, again to hold the lander down. Philae contains cameras that will start imaging the landing site before touchdown, so ESA can compare before and after and detect any damage to the landing area.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=1oEaGjgOB0M&quot; rel=&quot;nofollow noreferrer&quot;&gt;Video explanation using a Lego model of Philae.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-01-22T21:19:30.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3185" PostTypeId="1" AcceptedAnswerId="9857" CreationDate="2013-12-21T12:25:28.117" Score="8" ViewCount="1923" Body="&lt;p&gt;On December 28, 2013 two Russian SKRL-756 calibration sphere satellites will be launched during the maiden flight of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz-1_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;Soyuz-2.1v&lt;/a&gt;, a brand new lightweight derivative of the Soyuz launch vehicle, using &lt;a href=&quot;http://www.russianspaceweb.com/volga.html&quot; rel=&quot;noreferrer&quot;&gt;Volga upper stage&lt;/a&gt;. I wasn't able to find much information on them, but based on a few short descriptions I presume they're passive hollow metallic (likely aluminum) calibration targets, and spheric shape should assure constant radar signature.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/q9HPF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;&lt;strong&gt;Above&lt;/strong&gt;: The payload configuration in the first mission of the Soyuz-2-1v rocket. Credit: TsSKB Progress&lt;/sup&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;During its first test launch, the Soyuz-2-1v rocket with the Volga&#xA;  third stage carried an experimental version of the 50-kilogram Aist&#xA;  satellite &lt;strong&gt;and a pair of SKRL-756 radar-calibration spheres for the&#xA;  Ministry of Defense&lt;/strong&gt;. All three payloads were to be released from a&#xA;  special adapter mounted on the Volga upper stage after it enters a&#xA;  near-polar orbit.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;&lt;strong&gt;Emphasis mine. Image and quote source&lt;/strong&gt;: &lt;a href=&quot;http://www.russianspaceweb.com/soyuz1_lv_aist.html&quot; rel=&quot;noreferrer&quot;&gt;RussianSpaceWeb.com / Soyuz-2-1v lifts off successfully&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How do these radar calibration spheres look like, what is the material they're made of and their diameter (radar signature), how do they function (passive / active), which radars are they supposed to help calibrate, what is the purpose of these radars and what precision are they aiming for?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If available, please also provide information on their targeted orbit (altitude, period) and how long will they serve as radar calibration targets before their orbit decays enough for them to burn in the lower atmosphere on reentry.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-30T17:50:24.740" LastActivityDate="2022-04-21T17:14:20.250" Title="How does radar calibration using LEO orbited calibration spheres work?" Tags="|communication|low-earth-orbit|soyuz-spacecraft|radar|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3186" PostTypeId="1" AcceptedAnswerId="3190" CreationDate="2013-12-21T12:49:38.710" Score="7" ViewCount="838" Body="&lt;p&gt;I'm currently following US EVA-24 with Rick Mastracchio (EV1) and Mike Hopkins (EV2) tasked to replace a failed Pump Module (S1 PM), one of two external pump modules of the US section of the International Space Station (ISS). The schedule is also visible on this screen taken off live stream a few minutes before the EVA, scheduled to start on December 21, 2013 at 12:10 UTC (which it did):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Ua0Nj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Spacewalks overview for the US EVA-24 as broadcast on &lt;a href=&quot;http://www.nasa.gov/multimedia/nasatv/&quot; rel=&quot;noreferrer&quot;&gt;NASA TV&lt;/a&gt; (credit: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, good half an hour into EVA 1, Mike and Rick are already 15 minutes ahead of schedule, and it seems they were even asked to perform a few additional checks (e.g. gloves check) that were not on the checklist as read to them by Koichi Wakata inside the station. So this naturally begs the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why did NASA assess that they will require so much time (3 spacewalks each over 6 hours long) to replace a single Pump Module? Is this merely scheduling procedures with a &lt;em&gt;good measure&lt;/em&gt; so astronauts don't have to hurry? Or do they always prepare flexed schedules like this for some other reason?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: A good hour into EVA, the two astronauts are roughly 25 minutes ahead of schedule. Here's another screen captured off the live stream, showing Pump Module design:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/3qvUk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="897" LastEditDate="2013-12-21T16:34:44.687" LastActivityDate="2013-12-22T08:27:05.667" Title="Why did NASA assess 3 separate and over 6 hours long EVAs are needed to replace a single failed Pump Module (S1 PM)?" Tags="|iss|nasa|eva|crewed-spaceflight|maintenance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3187" PostTypeId="1" AcceptedAnswerId="3188" CreationDate="2013-12-21T13:16:11.627" Score="60" ViewCount="20589" Body="&lt;p&gt;Curiosity rover's wheels seem a bit &lt;em&gt;tired&lt;/em&gt; and show signs of wear and tear, as reported for example in &lt;a href=&quot;http://news.discovery.com/space/rough-roving-curiositys-wheels-show-wear-and-tear-130522.htm&quot; rel=&quot;noreferrer&quot;&gt;this Discovery News article&lt;/a&gt; from May 22, 2013. This damage only increased since then and wheels now show large holes. It doesn't look pretty, and since picture tells a thousand words, here's a photograph showing difference in wear and tear between Sol 34 and Sol 488 (Solar days on Mars since the start of the mission) so you can appreciate the extent of the damage for yourself:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/pJJJq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Close up and comparison of the gash in the aluminum wheel skin as seen on sol 488 on Dec. 20, 2013 (Source: &lt;a href=&quot;http://news.discovery.com/space/rough-roving-curiositys-wheel-damage-accelerated-131220.htm&quot; rel=&quot;noreferrer&quot;&gt;Discovery News&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This of course got me curious, if wear and tear on aluminum wheel skin is so extensive in under a year and a half (Earth years), why didn't NASA simply use classical car pressure tires? Mars has some atmosphere, so the rover could have a compressor onboard to help control tire pressure at all times, despite Martian days and nights having much larger temperature difference than what we're used to having here on Earth. And some tire designs are good for years in atmosphere with oxygen and used on heavy-duty industrial machinery here on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, while admittedly not too knowledgeable on tire technology, I still believe it would be feasible to simply use more conventional technology that has some 100 years of evolution and we trust every day to get us from point A to point B, and perhaps extend it with whatever support technology would be needed to keep them in good health, like a few tire pressure sensors, a compressor with 6 outputs one for each tire and so on. It is possible in automobile industry with current tech to have such automatic tire pressure monitoring and inflation/deflation system (know as Self-inflating Tires, or more technically correct - Central Tire Inflation System or CTIS) work on the go, and use threaded kevlar for outer skin or maybe some other synthetic materials instead of rubber for tire skin and inner tube, so...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What was the reason that NASA decided that Curiosity needs non-conventional, solid metallic tires / tire skin? And, given current rate of wear and tear, how long is the rover estimated to be able to continue driving around? Could this threaten rover's planned longevity?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: Damage to the wheels is now apparently much larger than what I initially found through related news articles good 100 Martian days ago. Here's the latest photograph of the left-front wheel of NASA's Curiosity Mars rover showing dents and holes on Sol 490, not even a day old as of the time of this update (red circle around the largest hole is my own addition):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/lHRDv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;NASA's Curiosity Mars rover showing dents and holes on Sol 490 (Source: &lt;a href=&quot;http://mars.jpl.nasa.gov/msl/multimedia/raw/?rawid=0490MH0262000001E1_DXXX&amp;amp;s=490&quot; rel=&quot;noreferrer&quot;&gt;NASA Mars Science Laboratory MAHLI Raw Images&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA's Mars rover Curiosity acquired this image using its Mars Hand&#xA;  Lens Imager (MAHLI), located on the turret at the end of the rover's&#xA;  robotic arm, on December 22, 2013, Sol 490 of the Mars Science&#xA;  Laboratory Mission, at 13:46:19 UTC.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Looks like Curiosity isn't making the &lt;code&gt;·--- ·--· ·-··&lt;/code&gt; (JPL) signature Morse code pattern tracks any longer with its wheels in the sand as it navigates the Martian surface. That's a pretty large additional gap there, on top of its designed traction pattern:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/0Vp7v.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Reading the Rover's Tracks (Image Credit: NASA/JPL-Caltech)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The straight lines in Curiosity's zigzag track marks are Morse code&#xA;  for JPL, which is short for NASA's Jet Propulsion Laboratory in&#xA;  Pasadena, Calif., where the rover was built and the mission is&#xA;  managed. The &quot;footprint&quot; is more than an homage to the rover's&#xA;  builders, however. It is an important reference mark that the rover&#xA;  can use to drive more precisely via a system called visual odometry. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Morse code, imprinted on all six wheels, is: &lt;code&gt;.---&lt;/code&gt; (J), &lt;code&gt;.--.&lt;/code&gt; (P),&#xA;  and &lt;code&gt;.-..&lt;/code&gt; (L), as indicated in this image.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;Quote and image source: &lt;a href=&quot;http://mars.jpl.nasa.gov/msl/multimedia/images/?ImageID=4586&quot; rel=&quot;noreferrer&quot;&gt;NASA JPL / MSL&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wonder what is Curiosity scribbling now on the Martian surface. I hope it's nothing offensive! :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-23T02:03:29.550" LastActivityDate="2022-04-18T15:54:23.320" Title="Curiosity: Why did they &quot;reinvent the wheel&quot; instead of using pressure tires?" Tags="|mars|nasa|rovers|curiosity|technology|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="3188" PostTypeId="2" ParentId="3187" CreationDate="2013-12-21T14:01:14.490" Score="92" Body="&lt;p&gt;Mars' &lt;a href=&quot;https://en.wikipedia.org/wiki/Climate_of_Mars&quot; rel=&quot;noreferrer&quot;&gt;temperature range&lt;/a&gt; is rather large, with temperatures down to -107 °C measured by the Viking landers.&#xA;This is below rubber's &lt;a href=&quot;https://en.wikipedia.org/wiki/Glass_transition_temperature&quot; rel=&quot;noreferrer&quot;&gt;glass transition temperature&lt;/a&gt; of -70 °C, below which rubber becomes brittle. So you can't use rubber.&lt;br /&gt;&#xA;A rubber tire is heavy. The tread of a normal road tire is more than 1 cm thick, off-road tires are thicker and heavier. It also needs a rim, adding more weight. A wheel that only consists of a rim (roughly speaking) is much lighter.&lt;/p&gt;&#xA;&lt;p&gt;The wheels &lt;a href=&quot;https://www.planetary.org/articles/08190630-curiosity-wheel-damage&quot; rel=&quot;noreferrer&quot;&gt;had to be as light as possible&lt;/a&gt;. So adding a rubber tire (inflatable or not) simply wasn't an option.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;There were several factors that drove them to design the wheels to be as lightweight as possible. The large size of the wheels means that very slight design changes add a substantial amount of mass. Increasing wheel thickness by one millimeter would add 10 kilograms to the rover's total mass. But total system mass wasn't the only constraint. Erickson explained that a major constraint arose from a tricky moment in the landing sequence, at the moment that the wheels deployed, while the rover was suspended from the bridle underneath the descent stage. The wheels' sudden drop imparted substantial forces on the mobility system, and keeping wheel mass as light as possible reduced those forces to manageable ones. There were other factors that made it important to keep wheel mass low.&lt;/p&gt;&#xA;&lt;p&gt;So the wheels needed to be as light as possible while still being able to do their job, but as to their job: &amp;quot;We misunderstood what Mars was,&amp;quot; Erickson said. &amp;quot;Strongly cemented ventifacts are not something that we saw on Mars before.&amp;quot; They designed Curiosity to handle all the challenges that Spirit and Opportunity had experienced, especially sand, which Curiosity traverses substantially better than her predecessors. &amp;quot;This vehicle is able to get itself out of situations that MER couldn't; it's got more flotation than MER had by a substantial margin.&amp;quot; They designed Curiosity to handle the sand traps, flat bedrock, and rocks-perched-on-sand landscapes seen by all the previous landers. They just didn't imagine the possibility of the peculiar and never-before-seen terrain type that they found in Gale crater. &amp;quot;There are [places] on Earth that do have these sharp ventifacts, but we hadn't seen them on Mars and we didn't test against them,&amp;quot; Erickson said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Punctures are a liability when you can't change the tire. A punctured tire soon disintegrates, and you end up dragging a mess of rubber and steel along, potentially jamming the wheel or ending up underneath the next wheel in line.&lt;br /&gt;&#xA;&lt;a href=&quot;https://web.archive.org/web/20150915101524/http://www.continental-tyres.co.uk/www/tyres_uk_en/themes/extended_mobility/contiseal_en.html&quot; rel=&quot;noreferrer&quot;&gt;Self-healing tires&lt;/a&gt; are available. But like puncture repair kits, these rely on having a viscous substance inside the tire that can seal a small hole. Again, Mars' temperature range makes this difficult to apply (the sealant must stay viscous at -100 °C). And no sealant will be able to close a gash as large as the one shown in the photo.&lt;/p&gt;&#xA;&lt;p&gt;Curiosity's wheel design is based on the earlier Mars rovers, which were studied in detail &lt;a href=&quot;https://ieeexplore.ieee.org/document/659282&quot; rel=&quot;noreferrer&quot;&gt;(wheel abrasion experiment on Sojourner)&lt;/a&gt;.&#xA;Some deformation, denting and rupture was &lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?act=attach&amp;amp;type=post&amp;amp;id=30104&quot; rel=&quot;noreferrer&quot;&gt;expected and designed for&lt;/a&gt; (entire presentation on MSL design &lt;a href=&quot;https://web.archive.org/web/20161109032437/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/42424/1/12-0690.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;).&lt;br /&gt;&#xA;&lt;a href=&quot;https://dougellison.smugmug.com/Machines/Mars-Yard-2012/i-CvPQRVR&quot; rel=&quot;noreferrer&quot;&gt;Photos of a prototype on Earth&lt;/a&gt; being put through its paces, dents and holes in the wheels clearly visible.&#xA;&lt;a href=&quot;https://astrogeology.usgs.gov/news/astrogeology/sol-469471-update-on-curiosity-from-usgs-scientist-ken-herkenhoff-tearing-on-wheels&quot; rel=&quot;noreferrer&quot;&gt;MSL team response to photos:&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;There was some concern about tears in the rover wheels, but today we got the go-ahead to drive, with no restrictions on distance or drive mode. The tears in the wheels were expected based on testing, and the wheels are designed to survive such damage without affecting mobility.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So, changing the design to rubber or other pliable materials wasn't necessary.&lt;/p&gt;&#xA;&lt;p&gt;Emily Lakdawalla of the Planetary Society &lt;a href=&quot;https://www.planetary.org/articles/08190630-curiosity-wheel-damage&quot; rel=&quot;noreferrer&quot;&gt;has an extensive blog post&lt;/a&gt; with more details on wheel wear, its causes, mitigation strategies and projected lifetime.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;At the end, I asked Erickson to put the wheel problem into context with his experience on many other missions. He said that the problem of damage to Curiosity's wheels has definitely had a significant impact on the mission, and mentioned for comparison the wheel failure on Spirit, when they had to start dragging the right front wheel behind them and drive exclusively backwards. But the Curiosity problem is not as bad as Spirit's because Curiosity is no less mobile than it was before. They can choose to accept wheel damage if they determine the scientific value to be worth it. So while Spirit's mobility problems limited the scope of what the rover could do, Curiosity's mobility problems do not -- at least, not directly. The biggest effect of the wheel damage problem is to slow the mission down. And that's what will limit how much Curiosity accomplishes. By not traveling as fast, and by having to limit their path choices, the amount of exploration that they can do is necessarily less than if they could go gallivanting across the bedrock outcrops at will.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: in June 2016, &lt;a href=&quot;https://www.planetary.org/articles/06151502-curiosity-update-sols-1311-1369&quot; rel=&quot;noreferrer&quot;&gt;the wheels were imaged again&lt;/a&gt;, showing only a slight increase in damage compared to last year.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="10273" LastEditDate="2022-04-18T15:54:23.320" LastActivityDate="2022-04-18T15:54:23.320" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="3189" PostTypeId="2" ParentId="3187" CreationDate="2013-12-21T14:16:41.647" Score="31" Body="&lt;p&gt;&lt;strong&gt;Resilience:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no way that an inflatable tyre of any kind can be used for remote missions, because they are incredibly unreliable. Think about car or bike tyres here on Earth, where we have a fairly small range of temperatures. They puncture easily, they burst, they lose pressure, the rubber degrades etc. When they have a puncture, fixing them is complex - we can't automate it on this planet, let alone sort out puncture repairs on another world with greater temperature extremes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Weight:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tyres are heavy. A thin aluminium shell wheel, or even grid wheel is light. And mass is all important when it comes to delta-v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and most importantly&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Lifespan&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity is only expected to run for 2 years. So far it has successfully survived and carried out useful scientific study. I think these wheels have performed extremely well.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2013-12-21T21:37:20.007" LastActivityDate="2013-12-21T21:37:20.007" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3190" PostTypeId="2" ParentId="3186" CreationDate="2013-12-21T16:24:37.797" Score="10" Body="&lt;p&gt;NASA scheduled these three EVA based on previous experience. In 2010, the International Space Station (ISS) suffered similar problems that also required one of the two Pump Modules (PM) be replaced. Douglas Wheelock and Tracy Caldwell Dyson of Expedition 24 took three EVA to complete that, each lasting over 7 hours. To be more precise:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Expedition 24 EVA 2 on 7 August 2010 lasted 8 hours and 3 minutes (11:19 - 19:22 UTC)&lt;/li&gt;&#xA;&lt;li&gt;Expedition 24 EVA 3 on 11 August 2010 lasted 7 hours and 26 minutes (12:27 - 19:53 UTC)&lt;/li&gt;&#xA;&lt;li&gt;Expedition 24 EVA 4 on 16 August 2010 lasted 7 hours and 20 minutes (10:20 - 17:40 UTC)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Reason for such long EVAs was stuck Quick Disconnects (QD) on the PM that would not release.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The QD task was considered an easy one as the hardware was built for&#xA;  frequent and fast mating/demating, but when Doug Wheelock tried to&#xA;  remove the QDs from the S1 Pump Module, the QDs showed how hard it was&#xA;  to get them detached. Teams battled with the hardware for hours before&#xA;  calling it a day and heading back inside the airlock to re-group for a&#xA;  second EVA that used modified procedures to get the stubborn QDs&#xA;  demated.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And from the same source:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After the trouble encountered when performing a Pump Module R&amp;amp;R in&#xA;  2010, this task is now known as a major undertaking and NASA expects&#xA;  that two to four EVAs will be needed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source of both quotes: &lt;a href=&quot;http://www.spaceflight101.com/iss-expedition-38-us-eva-24-updates.html&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight101: ISS Expedition 38 - US EVA-24 Updates&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problems during the first US EVA-15 egress by Doug Wheelock and Tracy Caldwell-Dyson are described in detail in this NASA's &lt;a href=&quot;http://www.nasa.gov/directorates/heo/reports/iss_reports/2010/08072010.html&quot; rel=&quot;noreferrer&quot;&gt;ISS On-Orbit Status 08/07/10&lt;/a&gt; update.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as Rick Mastracchio (EV1) and Mike Hopkins (EV2) have an added advantage of being able to study in detail the 2010 PM R&amp;amp;R EVAs, and they also had videos of those previous EVAs available, this schedule is computed based on previous NASA's experience with replacement of PM and they're even shortened a bit to account for better preparedness of the two Expedition 38 spacewalkers.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-22T08:27:05.667" LastActivityDate="2013-12-22T08:27:05.667" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3192" PostTypeId="2" ParentId="729" CreationDate="2013-12-22T19:08:27.183" Score="8" Body="&lt;p&gt;As @nos said, the decision to go with SPARC was made in 1991. There were &lt;a href=&quot;http://indico.cern.ch/getFile.py/access?contribId=101&amp;amp;sessionId=1&amp;amp;resId=0&amp;amp;materialId=slides&amp;amp;confId=11994&quot; rel=&quot;nofollow&quot;&gt;several reasons&lt;/a&gt; (presentation on history of ERC and Leon): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;ESA performed two architectural studies, evaluating processors such as MIPS, THOR, MC68020, I386, NS32. ESA also invited industry for round-table discussions. &#xA;  Finally, SPARC was selected due to:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Open architecture without &lt;a href=&quot;http://www.leox.org/docs/faq_MLleon.html&quot; rel=&quot;nofollow&quot;&gt;patents or license fees&lt;/a&gt; &lt;/li&gt;&#xA;  &lt;li&gt;Well designed and documented &lt;/li&gt;&#xA;  &lt;li&gt;Easy to implement &lt;/li&gt;&#xA;  &lt;li&gt;Established software standard &lt;/li&gt;&#xA;  &lt;li&gt;Available design (CY601)&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If those were selection criteria, you can see that a custom ISA would not satisfy the criteria.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://microelectronics.esa.int/erc32/misc/ERC32-ChipsetDevelopment-FinalReport-SaabEricsson-1997-05-27.pdf&quot; rel=&quot;nofollow&quot;&gt;final report for the ERC32 program&lt;/a&gt; (as found by @nos) said this: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Furthermore, it was requested to &quot;reuse&quot; an existing processor architecture in order to minimize both software and hardware development cost. Performed ESA and industrial studies resulted, at that time, in the selection of SPARC instruction set architecture as the baseline. This was in order to e.g. simplify bread-boarding and software development.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, ESA specifically required an existing ISA be used.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you choose a custom ISA, you have to create everything yourself:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the entire chip design (instead of being able to use existing building blocks as with SPARC),&lt;/li&gt;&#xA;&lt;li&gt;compiler, &lt;/li&gt;&#xA;&lt;li&gt;all software including the OS,&lt;/li&gt;&#xA;&lt;li&gt;cross-compilation and other equipment you need to develop software for this new ISA&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You also lose the convenience of having off-the-shelf hardware for the thousands of systems you need on earth, for development, testing etc. And you lose the thousands of man-years already invested in testing the SPARC architecture and software. By 1991, any bugs in SPARC were well-known; SPARC software was mature and in use everywhere. &#xA;A custom ISA might gain a bit more efficiency, but you increase the difficulty of the project by several orders of magnitude.&lt;br&gt;&#xA;(&lt;a href=&quot;http://www.eetimes.com/document.asp?doc_id=1214267&quot; rel=&quot;nofollow&quot;&gt;good article on Leon&lt;/a&gt;, &lt;a href=&quot;http://www.zdnet.com/how-the-european-space-agency-protects-chips-from-cosmic-radiation-7000009533/&quot; rel=&quot;nofollow&quot;&gt;Leon and radiation hardening&lt;/a&gt;, &lt;a href=&quot;http://ramp.eecs.berkeley.edu/Publications/LEON3%20SPARC%20Processor,%20The%20Past%20Present%20and%20Future.pdf&quot; rel=&quot;nofollow&quot;&gt;Leon development&lt;/a&gt;)&lt;br&gt;&#xA;The FAQ for LEON states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why does LEON implement a SPARC and not an ARM or MIPS?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Designing SPARC processors can be done without any licenses what so ever. &#xA;  This is indeed why Jiri Gaisler has selected SPARC for the&#xA;  development of LEON, just see how many times Intel, MIPS and ARM have&#xA;  sued companies that developed processors using their architecture.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;For example, in mid-February 2002 the legal battles between Lexra/MIPS &#xA;  and picoTurbo/ARM have both ended with a complete defeat of&#xA;  the two cpu-cloning companies (Lexra and picoTurbo). Both companies&#xA;  have been shut-down and their clients transferred to MIPS/ARM.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Jiri Gaisler said: &quot;More than ever, I'm happy with the decision to go SPARC. &#xA;  :-) And many thanks to Sun and SPARC International for the&#xA;  open license!&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="25" LastEditDate="2015-12-17T22:48:29.213" LastActivityDate="2015-12-17T22:48:29.213" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3193" PostTypeId="1" CreationDate="2013-12-22T23:25:08.330" Score="6" ViewCount="250" Body="&lt;p&gt;With this I mean using elastic energy to land and possibly reuse this energy at takeoff. So that the &quot;spring&quot; will remain in the compressed state between landing an takeoff by locking it in to place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I refer to storing energy in the landing gear, instead of just absorbing and dissipating it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The advantages I can think of are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;No/less need rockets at touchdown, so less dust will be blown from the surface into a cloud at the landing site.&lt;/li&gt;&#xA;&lt;li&gt;Less fuel is needed to land and takeoff.&lt;/li&gt;&#xA;&lt;li&gt;Mechanical potential energy seems more reliable that rocket engines, which have to pump fuel, ignite, ect.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The disadvantages I can think of are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If it fails it can have more devastating results as a leak in a fuel tank, since the fuels needs to be combined and given activation energy for it to release its potential chemical energy.&lt;/li&gt;&#xA;&lt;li&gt;The fuel needed to bring the &quot;spring&quot; with you might be more than you gain from it.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1086" LastEditorUserId="4660" LastEditDate="2015-04-22T23:26:34.567" LastActivityDate="2015-04-22T23:26:34.567" Title="Has there ever been done research on mechanical assisted landing and takeoff?" Tags="|launch|landing|landing-gear|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3194" PostTypeId="1" CreationDate="2013-12-23T14:21:45.777" Score="15" ViewCount="589" Body="&lt;p&gt;To my knowledge, most governments throughout the world seem to run institutions that give recommendations regarding safety and wellbeing of their citizens when travelling abroad in one form or another, issuing public safety warnings, health recommendations, and alike. With Mars One project having completed the application proceedings and already moved past &lt;a href=&quot;http://www.mars-one.com/en/faq-en/21-faq-selection/204-how-will-the-astronauts-be-chosen&quot;&gt;round one of the selection process&lt;/a&gt;, it seems a bit too late to give any official government recommendations  regarding application now, so my question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Have any governments issued public warning, recommendation, or an official statement regarding further involvement of its citizens with this venture, or announced their intention to do so in the near future?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that I'm not excluding the possibility of any such recommendations issued in the future, but my question is, if any of them have done this already, or plan to do so. If yes, then please provide links to most relevant excerpts from any such official statements or government order and a list of public offices tasked with preparing them.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For background, here's a relevant blog article explaining this issue in a bit more detail: &lt;a href=&quot;http://cibusmentis.ie/2013/12/23/mars-one-should-i-stay-or-should-i-go/&quot;&gt;Mars One: Should I Stay or Should I Go?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-13T21:31:27.923" LastActivityDate="2015-03-07T07:10:48.497" Title="Has any government published an official recommendation regarding Mars One?" Tags="|mars|law|mars-one|government|crewed-spaceflight|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3195" PostTypeId="1" AcceptedAnswerId="3201" CreationDate="2013-12-23T14:28:35.553" Score="7" ViewCount="604" Body="&lt;p&gt;Looking at &lt;a href=&quot;http://www.noradsanta.org/&quot; rel=&quot;nofollow&quot;&gt;http://www.noradsanta.org/&lt;/a&gt; implies a Satellite based tracking system for the annual NORAD Santa tracking report.  I looked at the &lt;a href=&quot;https://en.wikipedia.org/wiki/NORAD_Tracks_Santa&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article&lt;/a&gt; for some hint of the technology that allows NORAD to identify Santa's location, but did not find any clues.  I am not sure, if NORAD has an agreement with Santa so there is some tracking device placed on the sleigh, or if they are able to identify Rudolph's nose, or in some other way identify Santa and his sleigh flying through the night sky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, other then NORAD are there any other satellite systems that would have the ability to track Santa, and if so are any of them used to track him?  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="58" LastEditDate="2017-05-30T14:16:37.130" LastActivityDate="2017-05-30T14:16:37.130" Title="Does NORAD have the only system that can track Santa?" Tags="|artificial-satellite|santa-claus|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3196" PostTypeId="2" ParentId="3193" CreationDate="2013-12-23T16:37:36.400" Score="1" Body="&lt;p&gt;An integrated single vehicle &lt;a href=&quot;https://en.wikipedia.org/wiki/Regenerative_braking&quot; rel=&quot;nofollow noreferrer&quot;&gt;regenerative brake&lt;/a&gt; would not be feasible.  In short because you would not be able to gain any more lift then you could capture at landing.  So if you have spring loaded landing legs 100 feet long, the most lift you you get is about 100, which on a journey to orbit is insignificant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several ground (&lt;em&gt;semi ground&lt;/em&gt;) based solutions that have been considered for recapturing energy between launch and landing.  A good overview of one solution is described in &lt;a href=&quot;https://space.stackexchange.com/questions/1844/can-a-free-launch-from-a-space-elevator-really-be-free&quot;&gt;Can a &amp;quot;free launch&amp;quot; from a space elevator really be free?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pretty much any ground based launch system, can in theory be used to recapture energy from returning vehicle if designed with that in mind.  But there are difficulties; consider an example with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_gun&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Gun&lt;/a&gt; using a &lt;a href=&quot;https://en.wikipedia.org/wiki/Maglev&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetic propulsion similar to trains&lt;/a&gt; could recapture some of the energy, but this would require that a incoming vehicle enter the &quot;barrel&quot; of the gun at high speeds.  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-12-23T16:37:36.400" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3197" PostTypeId="1" CreationDate="2013-12-23T20:26:49.980" Score="8" ViewCount="765" Body="&lt;p&gt;Call this a thought experiment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft to Earth orbit and beyond are traditionally launched by means of one or more rocket engines. The fundamental principle is that a rocket moves by expelling the vapour from a high-energy combustion. This energy is adequate to move the rocket as well as anything (within reason!) in physical proximity&amp;hellip;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious limitation of this approach is that the spacecraft a rocket may carry is a miniscule fraction of the combined mass of the rocket and its fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say that a spacecraft launched is capable of human-rated re-entry. What is the typical proportion of its re-entry propellant and heat-shield as a fraction of&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;total payload, and &lt;/li&gt;&#xA;&lt;li&gt;launch vehicle propellant mass?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(The figures will probably vary from one launch engine to another; an approximate range will suffice in the answer. E.g. 3% to 5% of payload, and 0.9% to 1.2% of propellant mass.)&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="1142" LastEditDate="2015-11-22T16:37:15.153" LastActivityDate="2015-11-22T17:33:34.767" Title="Proportions of a reentering spacecraft as compared to fuel mass" Tags="|reentry|fuel|payload|heat-shield|mass|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3198" PostTypeId="2" ParentId="3164" CreationDate="2013-12-24T00:40:19.417" Score="3" Body="&lt;p&gt;Several Earth orbiting missions currently use the DSN, all of which are either in highly elliptical orbits or in lunar orbit (which is, in turn, in Earth orbit).  On &lt;a href=&quot;http://www.cdscc.nasa.gov/Pages/trackingtoday.html&quot; rel=&quot;nofollow&quot;&gt;this page&lt;/a&gt;, you can see many of them being tracked by the DSN antennas in Canberra.  Some of the ones there, with their orbits, are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ACE, 179 km x 1256768 km&lt;/li&gt;&#xA;&lt;li&gt;GEOTAIL, 47824 km x 193019 km&lt;/li&gt;&#xA;&lt;li&gt;Chandra, 16000 km x 133000 km&lt;/li&gt;&#xA;&lt;li&gt;Cluster, 19000 km x 119000 km&lt;/li&gt;&#xA;&lt;li&gt;LADEE, currently in a 250 km orbit about the Moon&lt;/li&gt;&#xA;&lt;li&gt;LRO, 50 km orbit about the Moon&lt;/li&gt;&#xA;&lt;li&gt;THEMIS, three spacecraft in highly elliptical Earth orbit, two in lunar orbit&lt;/li&gt;&#xA;&lt;li&gt;WIND, highly elliptical Earth orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The benefit they receive from the DSN are high data rates at relatively large distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your title question differs from your body question, and is not supported by the quote.  There are &lt;em&gt;no&lt;/em&gt; LEO missions supported by the DSN.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-24T04:33:49.333" LastActivityDate="2013-12-24T04:33:49.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3200" PostTypeId="1" AcceptedAnswerId="3231" CreationDate="2013-12-24T10:32:55.123" Score="13" ViewCount="2859" Body="&lt;p&gt;I've noticed this discoloration / stains / burn marks along the edges of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Quest_Joint_Airlock&quot; rel=&quot;noreferrer&quot;&gt;Quest Joint Airlock&lt;/a&gt; thermal cover on the hatch before, but they seem even more pronounced and contrasting with the cover's natural color now, during the last US EVA-24 of the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition38/&quot; rel=&quot;noreferrer&quot;&gt;Expedition 38&lt;/a&gt; crew (Rick Mastracchio as EV1, Mike Hopkins as EV2, and Koichi Wakata as Canadarm2 operator and support inside the station). For example, this is one of photographs taken during EVA 24 as the two spacewalkers egress:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/0lcPB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Rick Mastracchio as EV1 (red stripes) and Mike Hopkins as EV2 (no stripes) leaving the airlock during US EVA-24 (Credit: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thermal cover is visibly discolored along its edge, gradually pronounced towards the outermost edge. I'm not sure if this is some sort of oxidation, chemical burn of thermal insulation materials as the airlock gases escape, discoloration of some materials due to exposure to the Sun as it remains open during the EVAs, or something else?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is the cause of discoloration along the edges of the Quest Joint Airlock thermal cover?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-12-30T01:58:02.717" Title="What is the cause of discoloration along the edges of the Quest Joint Airlock thermal cover?" Tags="|iss|materials|eva|thermal|quest-joint-airlock|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3201" PostTypeId="2" ParentId="3195" CreationDate="2013-12-24T15:05:15.267" Score="8" Body="&lt;p&gt;One &lt;a href=&quot;https://itunes.apple.com/de/app/where-is-santa-lite-santa/id486270850?mt=8&quot; rel=&quot;noreferrer&quot;&gt;Santa Tracker&lt;/a&gt; app for iOS claims to use &quot;millions of Santa tracking stations&quot;, &lt;a href=&quot;https://itunes.apple.com/de/app/santa-tracker-lite-north-pole/id660842333?mt=8&quot; rel=&quot;noreferrer&quot;&gt;another one&lt;/a&gt; is using the &quot;latest Elfish satellite technology&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google is also &lt;a href=&quot;http://www.google.com/santatracker/#/tracker/dashboard&quot; rel=&quot;noreferrer&quot;&gt;tracking Santa&lt;/a&gt;. I haven't found any information, except that they are using &lt;a href=&quot;http://www.google.com/santatracker/about.html&quot; rel=&quot;noreferrer&quot;&gt;insider information&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We’ve received this special preview from one of Santa’s many developer elves, who are hard at work in the North Pole helping Santa prepare for his big day&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So I guess there are quite a few Santa tracking technologies in use.&lt;/p&gt;&#xA;" OwnerUserId="548" LastEditorUserId="548" LastEditDate="2013-12-24T20:38:19.450" LastActivityDate="2013-12-24T20:38:19.450" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3202" PostTypeId="1" AcceptedAnswerId="3203" CreationDate="2013-12-26T08:08:16.480" Score="17" ViewCount="16962" Body="&lt;p&gt;Have any spacecraft been lost transitioning between Mars and Jupiter? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do spacecraft navigate that field unscathed as well?&lt;/p&gt;&#xA;" OwnerUserId="35" LastEditorUserId="49" LastEditDate="2013-12-26T11:56:34.593" LastActivityDate="2016-02-20T05:44:31.643" Title="How do spacecraft navigate through the Asteroid belt to avoid collision?" Tags="|artificial-satellite|asteroid|navigation|collision-avoidance|asteroid-belt|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3203" PostTypeId="2" ParentId="3202" CreationDate="2013-12-26T11:54:18.460" Score="28" Body="&lt;p&gt;No spacecraft has been yet lost to the asteroid belt. In fact, we have the opposite example of missing an asteroid when it was even targeted, like was the case with &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;MINERVA lander of the JAXA's Hayabusa&lt;/a&gt; deep space probe, missing the &lt;a href=&quot;https://en.wikipedia.org/wiki/25143_Itokawa&quot; rel=&quot;nofollow noreferrer&quot;&gt;25143 Itokawa&lt;/a&gt; asteroid. Why haven't we lost any spacecraft due to collision with asteroids in the asteroid belt is also pretty straightforward once you start appreciating its volume and its average particle density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Asteroid belt is a huge area, sparsely populated with asteroids. It is incredibly difficult to estimate it's mean density considering we most certainly haven't detected all its objects, with most simply being too small to easily detect and track from up to a &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;few astronomical units&lt;/a&gt; away, but we did detect most of its largest to medium size objects and that should help us guesstimate its density a bit. We already have some answers to similar questions in other threads, for example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/231/whats-the-particle-density-of-the-asteroid-belt&quot;&gt;What&amp;#39;s the (particle) density of the asteroid belt?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1382/how-frequently-do-asteroids-collide-with-each-other&quot;&gt;How frequently do asteroids collide with each other?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But for the sake of argument, let's crunch some numbers here too, and I'll take a bit different approach not to leech on work of others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we define the asteroid belt region as a torus with the inner boundary at $2.06 AU$ (4:1 orbital resonance with Jupiter), outer boundary at $3.215 AU$ (keeping the &lt;a href=&quot;https://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;snow line&lt;/a&gt; from the formation of the asteroid belt at $2.7 AU$, nicely in the geometric center of the inner and outermost boundaries), and its thickness follows &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Main_belt_i_vs_a.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;average inclination to the invariable plane of all the known asteroids in the asteroid belt&lt;/a&gt; at $\approx 4 °$, multiplied by two so we get its total thickness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/vfccz.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Plot of inclination $i$ vs semi-major axis $a$ for numbered asteroids inward of about $6 AU$. The main belt region is shown in red,&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;and contains $93.4\%$ of all the objects. For reference, Mars orbits out to $1.666 AU$, and Jupiter between $4.95-5.46 AU$.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The diagram was created by &lt;a href=&quot;https://en.wikipedia.org/wiki/User:Deuar&quot; rel=&quot;nofollow noreferrer&quot;&gt;Piotr Deuar&lt;/a&gt; using orbit data for 120437 numbered minor planets from the &lt;a href=&quot;http://www.minorplanetcenter.net/iau/mpc.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Minor Planet Center&lt;/a&gt; orbit&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;database, dated 8 Feb 2006. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Main_belt_i_vs_a.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging these parameters in equation for the volume of a toroid (which is actually a leftover of subtracting the volume of the outer sphere with the inner one and then dividing it to a $8 °$ thick &lt;em&gt;slice&lt;/em&gt;), we get the total volume of $4.56 AU^3$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's a much lower volume than $16 AU^3$ quoted in the similar estimate of the asteroid belt volume in an &lt;a href=&quot;https://physics.stackexchange.com/a/26714/21740&quot;&gt;answer to a related question on Physics.SE&lt;/a&gt;, but keep in mind that I only accepted the area where majority of its asteroids are known to be, not its extremes. I'm doing this so we can appreciate highest risk of colliding with an asteroid in the asteroid belt, so the purpose is somewhat different. I also estimated the distance to the Sun boundaries somewhat differently, following same idea of keeping the area limited to where most asteroids would be (highest density).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, so now that we have this volume of highest particle density, let's try and describe it with to us a bit easier to imagine figures; Earth has a &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;volume of $108.321 * 10^{10}\ km^3$&lt;/a&gt;. Given calculated volume of the main density region of the main belt:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;We could fit a good &lt;strong&gt;14 &lt;a href=&quot;https://en.wikipedia.org/wiki/Sextillion&quot; rel=&quot;nofollow noreferrer&quot;&gt;sextillion&lt;/a&gt; ($14 * 10^{21}$) Earths&lt;/strong&gt; in it. &lt;/li&gt;&#xA;&lt;li&gt;Or, since &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun&quot; rel=&quot;nofollow noreferrer&quot;&gt;1,300,000 Earths could fit the volume of the Sun&lt;/a&gt;, we could fit nearly &lt;strong&gt;11 quadrillion ($10.84 * 10^{15}$) Suns&lt;/strong&gt; in the region. &lt;/li&gt;&#xA;&lt;li&gt;Or &lt;strong&gt;10.667 quintillion ($10.67 * 10^{18}$) &lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiters&lt;/a&gt;&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Which comes out at &lt;strong&gt;nearly 705 sextillion ($704.69 * 10^{21}$) Moons&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is regarding its volume. Trying to estimate its particle density though, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_belt#Characteristics&quot; rel=&quot;nofollow noreferrer&quot;&gt;total mass of the asteroid belt&lt;/a&gt; is estimated to be $2.8 * 10^{21}$ to $3.2 * 10^{21}\ kg$, which is just $4\%$ of the mass of the Moon. We'll use $93.4\%$ of the upper estimated mass, describing our main density, so $2.9888 * 10^{21}\ kg$. That's nearly exactly $3 * 10^{21}\ kg$, so let's round it up to mean of the estimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if we could assume similar composition, then if an average size object would be only 1 liter (1 cubic decimeter, or 10 * 10 * 10 cm) in volume and it would have a density of Moon rock (&lt;a href=&quot;https://en.wikipedia.org/wiki/Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;$3.3464 g/cm^3$&lt;/a&gt;, or to put it differently, would have a mass of $3.3464\ kg$), that would then be $896.49 * 10^{18}$ of such objects and they would occupy $1.965 * 10^{-11}\%$, or 0.00000000001965% of the total volume of the main density area of the asteroid belt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are just some guesstimates and I do hope I got all the decimal points correctly, but as you see, chances of colliding with an object in the asteroid belt are indeed astronomical. There is no reason to consume spacecraft's power to try and track asteroids potentially on a collision course with a passing-by spacecraft, especially since these objects aren't stationary to the vector of the spacecraft's travel and it would be incredibly difficult to detect them &lt;strong&gt;and&lt;/strong&gt; infer their orbits to try and avoid them with the use of onboard thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, no spacecraft to date tried to create a detailed radar picture of objects in the asteroid belt ahead of the spacecraft to also try and avoid them in an extremely remote chance of being on a collision course. Navigation and collision avoidance of the spacecraft in relation to a single object on its path yes, for example the mentioned Hayabusa, and Rosetta and its lander Philae will try something similar in 2014. And some other deep space probes (I believe also Pioneer and Voyager probes) did use wide-angle radar imaging to try and detect objects in the asteroid belt for the purpose of cataloging and studying them, but to avoid collision, you'd have to establish their orbit extremely precise, otherwise you wouldn't know if you're avoiding the object, or actually helping your spacecraft to collide with it. A bit like deciding that running in the rain will get you less wet, even if you don't know its angle or frequency.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-12-26T12:11:35.060" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3205" PostTypeId="1" AcceptedAnswerId="4296" CreationDate="2013-12-26T20:37:55.773" Score="12" ViewCount="2071" Body="&lt;p&gt;In this context, I mean, a way to get out of the station, without depressurizing the entire station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious ones are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Quest airlock on Unity (Node 1).&lt;/li&gt;&#xA;&lt;li&gt;Pirs (DC-1) airlock&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Does Poisk (DC-2?  Or is MRM 1?) have an airlock?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When Nauka launches, the small airlock attached to Rassvet will be moved over.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kibo (JAXA model) has a small airlock for experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can either of the 2 PMA's be depressurized and compartmentalized, so that they could be used as an airlock?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CBM ports cannot. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can any other Russian segment components provide airlock style services?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-04-13T13:52:36.577" LastActivityDate="2014-04-18T09:09:10.513" Title="How many airlocks does the ISS have?" Tags="|iss|pirs|airlock|quest-joint-airlock|poisk|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3207" PostTypeId="1" AcceptedAnswerId="3210" CreationDate="2013-12-27T01:28:23.557" Score="7" ViewCount="3540" Body="&lt;p&gt;In the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_17&quot;&gt;Apollo 17 Wikipedia article&lt;/a&gt;, the coordinates of the descent stage are given: 20.19080°N 30.77168°E. What are the specifics of the Moon's coordinate system? Where is 0°?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2018-08-05T06:13:23.353" Title="How is the Moon's coordinate system defined?" Tags="|the-moon|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3208" PostTypeId="1" AcceptedAnswerId="3209" CreationDate="2013-12-27T01:45:59.267" Score="12" ViewCount="1078" Body="&lt;p&gt;In &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/9/94/521430main_iss026e030308_hires.jpg&quot; rel=&quot;noreferrer&quot;&gt;this image&lt;/a&gt; of Discovery, two openings above the main windows are visible:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/NPW1c.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are they used for? Sightseeing?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2018-02-14T18:38:58.777" Title="What are the two openings above the Shuttle's cockpit?" Tags="|space-shuttle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3209" PostTypeId="2" ParentId="3208" CreationDate="2013-12-27T02:17:31.390" Score="13" Body="&lt;p&gt;When docking with Mir, the International Space Station, Hubble, etc., the docking port is just below those windows. For Hubble, they would approach, grab it with the Shuttle Remote Manipulator System (SRMS) a.k.a. Canadarm 1, and attach it to a platform further down the payload bay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/C37bB.jpg&quot; alt=&quot;Julie Payette at Overhead Window&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Canadian Space Agency astronaut Julie Payette, STS-127 mission specialist, looks through an overhead window while operating controls on the aft flight deck of Space Shuttle Endeavour during flight day two activities (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-127&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on STS-127&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The overhead aft flight deck windows were used for visual sighting of the station as they approach, but were mostly used for better visibility when maneuvering the shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/19Ga2.jpg&quot; alt=&quot;Atlantis Astronauts at Overhead Window&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;The five STS-74 astronauts aboard the space shuttle Atlantis look out overhead windows on the aft flight deck toward their counterparts aboard the Mir Russian space station, with which they had just rendezvoused in this NASA handout photo dated November 24, 1995. (Source: &lt;a href=&quot;http://www.ibtimes.com/atlantis-shuttle-prepares-final-flight-marks-end-era-707349&quot; rel=&quot;nofollow noreferrer&quot;&gt;IBTimes&lt;/a&gt;, Credit: REUTERS/NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="11721" LastEditDate="2018-02-14T18:38:58.777" LastActivityDate="2018-02-14T18:38:58.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3210" PostTypeId="2" ParentId="3207" CreationDate="2013-12-27T03:06:04.863" Score="9" Body="&lt;p&gt;0° is the line of longitude that contains the point on the surface closest to Earth.  In other words, (0,0) is at about the center of the face of the Moon we see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The details are a little more complicated, since the &quot;closest point&quot; is really a mean position.  &lt;a href=&quot;http://ssd.jpl.nasa.gov/dat/lunar_cmd_2005_jpl_d32296.pdf&quot;&gt;This document&lt;/a&gt; has much more information on the coordinate systems and other lunar standards.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-12-27T03:06:04.863" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3211" PostTypeId="1" AcceptedAnswerId="4629" CreationDate="2013-12-27T11:47:18.503" Score="12" ViewCount="1611" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/questions/2699/what-is-a-space-elevator&quot;&gt;space elevator&lt;/a&gt; is an interesting concept, allowing for very &lt;a href=&quot;https://space.stackexchange.com/questions/1844/can-a-free-launch-from-a-space-elevator-really-be-free&quot;&gt;low cost transit to orbit&lt;/a&gt;. Currently we don't seem to have a technology that will provide a &lt;a href=&quot;https://space.stackexchange.com/questions/2/what-technological-barriers-do-we-need-to-overcome-to-build-a-space-elevator&quot;&gt;strong enough cable to make the climb from Earth to Orbit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we have the technology to build a space elevator anyplace in the solar system? If so where?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-08-12T16:38:43.160" Title="Where could we build a space elevator today (2014)?" Tags="|technology|materials|solar-system|space-elevator|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3213" PostTypeId="1" AcceptedAnswerId="3214" CreationDate="2013-12-27T15:32:25.220" Score="1" ViewCount="4556" Body="&lt;p&gt;I read somewhere that Israel launched satellites in retrograde orbit. &#xA;America also has some satellites in retrograde orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am new to this concept, so I am curious:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt; 1. What are the benefits/objectives of retrograde orbit? What could the objectives be that a prograde orbit would not fulfill? &lt;/ul&gt; &#xA;&#xA;&lt;ul&gt;2. More power is required to keep the satellite in retrograde orbit (and to launch it there in the first place), right? &lt;/ul&gt;&#xA;&#xA;&lt;ul&gt;3. Do natural satellites ever orbit in retrograde? What factors influence whether a satellite is in retrograde or prograde orbit (i.e. distance from primary)? &lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Thank you. &lt;/p&gt;&#xA;" OwnerUserId="1340" LastEditorUserId="1340" LastEditDate="2015-05-29T09:18:36.927" LastActivityDate="2015-05-29T09:18:36.927" Title="Satellites in retrograde vs prograde orbit" Tags="|orbital-mechanics|artificial-satellite|orbit|moons|" AnswerCount="1" CommentCount="2" ClosedDate="2013-12-27T17:32:23.763" ContentLicense="CC BY-SA 3.0" />
  <row Id="3214" PostTypeId="2" ParentId="3213" CreationDate="2013-12-27T15:49:34.133" Score="5" Body="&lt;p&gt;1) Good question, I can't think of any off the top of my head. Maybe if you needed to snap photos of the Earth really quickly (and you had cameras that could take good photos considering the speed of the satellite relative to the Earth) it would make sense, but most imaging satellites are launched into polar orbits, meaning the orbit makes a right angle with the equator, or it's slightly prograde or slightly retrograde. Polar orbits ensure you can cover every bit of the globe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the reason Israel launches to retrograde orbits from its own launch sites is for safety concerns. There are a lot of inhabited areas underneath a trajectory for a rocket going to a prograde orbit, but for retrograde you just have the Mediterranean. Given high tensions in that region, launching prograde could easily be misinterpreted as an attack.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) More power is required to launch into a retrograde orbit because you have to go against the Earth's rotation. When launching into a prograde orbit, the Earth's rotation gives you a little benefit, so you can use a little less propellant getting to orbit. For retrograde, you need to work against that boost, which, of course, requires more propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does not require more power to keep the satellite in retrograde orbit. Although I suppose if it's in a fairly low orbit it might be more affected by atmospheric drag since the relative velocity of the atmosphere against the satellite will be higher in a retrograde orbit, but I would venture to guess that the difference would not be significant, probably not even by one order of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Yes, one of Saturn's moons orbits retrograde: &lt;a href=&quot;http://en.wikipedia.org/wiki/Phoebe_(moon)#Orbital_characteristics&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Phoebe_(moon)#Orbital_characteristics&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2013-12-27T15:49:34.133" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3216" PostTypeId="2" ParentId="2752" CreationDate="2013-12-27T17:14:42.680" Score="2" Body="&lt;p&gt;I'm not entirely sure what you're asking, but de-orbiting satellites and letting them burn up over a large uninhabited area (I can't think of any spacecraft that weren't de-orbited into the Pacific Ocean) is a very standard procedure for all types of satellites and even space station. All of the Russian progress cargo ships that resupply the space station do this, and have been doing this since the late 1970's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Breaking up in the atmosphere is rarely (almost never) a design criteria because the heat generated by flying through the atmosphere at near-orbital speeds is higher than the melting point of any metal. Some pieces can still survive if they're buried deep enough into the satellite and the heat doesn't have time to conduct through the entire spacecraft, which is why they are de-orbited over uninhabited land masses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the heating, atmospheric drag helps to break apart a spacecraft. Think about sticking your hand out a car window going ~60 mph. If you face your palm towards the airflow you feel a much stronger force than if your hand is parallel to the airflow. Now image that kind of force at ~17000 mph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GOCE was more aerodynamic than other spacecraft because of its mission requirements, but it still burned up when it re-entered just a few weeks ago. However, they didn't plan things out quite carefully enough and they ran out of propellant before re-entry, and so they did not have the ability to target the re-entry into the Pacific. Luckily, the spacecraft disintegrated mostly in the Atlantic over the Falkland Islands.&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2013-12-27T17:14:42.680" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3217" PostTypeId="1" AcceptedAnswerId="3218" CreationDate="2013-12-28T20:28:04.573" Score="7" ViewCount="2227" Body="&lt;p&gt;I know that cats, dogs, apes, guinea pigs, and a few other animals and insects have been sent into outer space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what was the first animal which has been sent into space (or better: into “orbit”)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking for the answer online returned contradicting answers. My instincts tell me it might have been insects, but I would like to know for sure.&lt;/p&gt;&#xA;" OwnerUserId="1353" LastEditorUserId="12102" LastEditDate="2021-09-05T23:14:34.313" LastActivityDate="2021-09-05T23:14:34.313" Title="What was the first animal sent into space?" Tags="|orbit|research|spaceflight-firsts|animals|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3218" PostTypeId="2" ParentId="3217" CreationDate="2013-12-28T20:47:02.837" Score="15" Body="&lt;p&gt;There's a whole &lt;a href=&quot;https://en.wikipedia.org/wiki/Animals_in_space&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page&lt;/a&gt; dedicated to this exact topic, so I'm just going to quote the first instance, and the rest is then available on the page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first animals sent into space were fruit flies aboard a&#xA;  U.S.-launched V-2 rocket on February 20, 1947. The purpose&#xA;  of the experiment was to explore the effects of radiation exposure at&#xA;  high altitudes. The rocket reached 68 miles (109 km) in 3 minutes and&#xA;  10 seconds, past both the U.S. 50-mile and the international 100 km&#xA;  definitions of the edge of space. The Blossom capsule was ejected and&#xA;  successfully deployed its parachute. The fruit flies were recovered&#xA;  alive. Other V2 missions carried biological samples, including moss.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And on the same page regarding first animals that were in orbit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On November 3, 1957, the second-ever orbiting spacecraft carried the&#xA;  first animal into orbit, the dog Laika, launched aboard the Soviet&#xA;  Sputnik 2 spacecraft (nicknamed 'Muttnik' in the West). Laika died&#xA;  during the flight, as was intended because the technology to return&#xA;  from orbit had not yet been developed. At least 10 other dogs were&#xA;  launched into orbit and numerous others on sub-orbital flights before&#xA;  the historic date of April 12, 1961, when Yuri Gagarin became the&#xA;  first human in space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You mention in your question &quot;contradicting answers&quot;, but you don't really explain what you found contradicting about them, or where. One possible source of confusion that comes to mind is &lt;a href=&quot;https://space.stackexchange.com/a/2077/49&quot;&gt;different definitions of &quot;outer space&quot;&lt;/a&gt; and at what altitude above the sea-level we actually count something as being &quot;in space&quot;, but it's nowadays generally considered at 100 km (62 mi), i.e. the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kármán line&lt;/a&gt; which is also used in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;nofollow noreferrer&quot;&gt;Outer Space Treaty&lt;/a&gt;. I can't think of other sources of confusion though, so please clarify what you meant in your question, if that's not it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2013-12-28T20:47:02.837" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3219" PostTypeId="1" AcceptedAnswerId="3229" CreationDate="2013-12-29T13:05:14.433" Score="9" ViewCount="1409" Body="&lt;p&gt;It is sometimes confusing that the Russians call the booster rocket Soyuz, and the payload Soyuz.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a reason for this naming model? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reading Anatloy Zak's book he notes several proposed launchers/payloads that would have followed the same model (Rus-M was one I think) but does not propose an explanation as to why they would do that.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="13208" LastEditDate="2017-02-27T21:47:23.893" LastActivityDate="2017-02-27T21:47:23.893" Title="Why do the Russians call the Soyuz launcher the same name as the Soyuz capsule?" Tags="|soyuz-spacecraft|terminology|russia|soyuz-launcher|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3220" PostTypeId="1" AcceptedAnswerId="3226" CreationDate="2013-12-29T13:09:28.510" Score="16" ViewCount="3368" Body="&lt;p&gt;The Soyuz is made up of three major components/modules.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GmW0i.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;The Orbital module on top, basically a sphere, which is  basically extra room, and contains the toilet, which allows for some modicum of privacy.  It is discarded when reentering.  (The Shenzou approach leaves its analog to this module orbiting with experiments, which is quite clever) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Descent module, in the middle, sort of gumdrop shaped, is where the astronauts reside for launch and reentry. This is reported to be very cramped and tight on space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Propulsion module at the bottom contains the engines, solar panels, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, during a launch abort, the first two modules need to be pulled away from the launch vehicle.  Why is the manned component in the middle?  Did it not make more sense to have the people at the very top, thus allowing a simpler abort (less mass to pull away)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am pretty sure Soyuz uses a tractor (not pusher) abort system.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2013-12-29T23:12:28.233" LastActivityDate="2014-01-02T02:29:10.770" Title="Why is the descent module of the Soyuz in the middle of its stack?" Tags="|crewed-spaceflight|soyuz-spacecraft|launch-escape-system|abort|design|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3221" PostTypeId="1" AcceptedAnswerId="3224" CreationDate="2013-12-29T13:12:03.663" Score="9" ViewCount="1422" Body="&lt;p&gt;The original Vostok and Voshkod space vehicles were mostly spherical shapes.  The Americans went with vaguely conical/capsule shapes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But Soyuz from the beginning has had a sort of gumdrop shape for the Descent module.  Clearly it works, having only killed 2 crews (?) in many hundreds of launches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QknSn.jpg&quot; alt=&quot;Soyuz Descent module&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What were the influences that led to that design choice? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2013-12-29T23:14:17.257" LastActivityDate="2014-01-24T11:25:14.107" Title="What influenced the shape of the Soyuz descent module?" Tags="|crewed-spaceflight|reentry|soyuz-spacecraft|design|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3223" PostTypeId="1" AcceptedAnswerId="3228" CreationDate="2013-12-29T14:42:07.340" Score="31" ViewCount="10602" Body="&lt;p&gt;Just to clarify: I've made some research and generally know what the case is, didn't mean to make the question sound stupid. :)&lt;/p&gt;&#xA;&lt;p&gt;I know there is a thing called Hohmann transfer orbit, named after Walter Hohmann, who came up with the idea of it, and each and every spacecraft we've sent to Mars so far, has used this very method to reach Mars.&#xA;On one side, the Hohmann transfer orbit is quite fuel efficient, compared to other methods, and for a unmanned probe, it wouldn't matter if the voyage to Mars is 7 months.&lt;/p&gt;&#xA;&lt;p&gt;However, we are all aware of the hazardous environment during the trip, if you're a human. So a round-trip of 5 months is better than year and a half.&lt;/p&gt;&#xA;&lt;p&gt;I know that in order to travel to Mars in a straight line we need to achieve a direct Earth-solar escape velocity. The probe New Horizons already did that.&lt;/p&gt;&#xA;&lt;p&gt;If we launch in a straight line, and we've calculated the position of Mars in the time frame of arrival, we'll rendezvous with Mars.&lt;/p&gt;&#xA;&lt;p&gt;However, we wouldn't be able to get into orbit around Mars in order to land, because our speed relative to Mars would be too high. So we need to do a deceleration fuel burn, to reduce speed.&lt;/p&gt;&#xA;&lt;p&gt;The two combined (direct Earth-solar escape velocity and Mars orbit deceleration fuel burn) would require very large amounts of fuel, especially for a manned mission, which is heavy to lift. But it is achievable either with a Mars manned spacecraft assembled in LEO (much like the ISS), or with some other means.&lt;/p&gt;&#xA;&lt;p&gt;So why not travel to Mars in a straight line, or am I missing something?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE (to further clarify the question):&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;If the only obstacle in achieving this is a large amount of required fuel, how large would it be, and what techniques could we apply to minimize this amount of required fuel? What about the assembly of a Mars manned vehicle in LEO? How would that change things?&lt;/p&gt;&#xA;" OwnerUserId="1357" LastEditorUserId="40767" LastEditDate="2021-12-20T22:02:14.923" LastActivityDate="2021-12-21T22:27:25.470" Title="Why not travel to Mars in 2 months?" Tags="|launch|mars|low-earth-orbit|interplanetary|escape-velocity|" AnswerCount="5" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="3224" PostTypeId="2" ParentId="3221" CreationDate="2013-12-29T15:14:13.973" Score="12" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_%28spacecraft%29#Reentry_Module&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One of the design requirements for the reentry module was for it to have the highest possible volumetric efficiency (internal volume divided by hull area). The best shape for this is a sphere, but such a shape can provide no lift, which results in a purely ballistic reentry. Ballistic reentries are hard on the occupants due to high deceleration and cannot be steered beyond their initial deorbit burn. That is why it was decided to go with the &quot;headlight&quot; shape that the Soyuz uses—a hemispherical forward area joined by a barely angled conical section (seven degrees) to a classic spherical section heat shield. This shape allows a small amount of lift to be generated due to the unequal weight distribution. The nickname was thought up at a time when nearly every headlight was circular. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="548" LastActivityDate="2013-12-29T15:14:13.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3225" PostTypeId="2" ParentId="3223" CreationDate="2013-12-29T15:54:32.977" Score="8" Body="&lt;p&gt;Like @oefe noted in the comments, you've already covered the reasons why not use direct launch to Mars pretty well in your question, so I'm going to assume you just haven't taken enough time for all these rather difficult concepts to clarify in your own mind, and I'll point you to a nicely written and relatively easy to understand description for it, that should help with that. From &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf4-1.php&quot; rel=&quot;noreferrer&quot;&gt;NASA Jet Propulsion Laboratory's Basics of Space Flight, Section I, Chapter 4. Interplanetary Trajectories&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When travelling among the planets, it's a good idea to minimize the&#xA;  propellant mass needed by your spacecraft and its launch vehicle. That&#xA;  way, such a flight is possible with current launch capabilities, and&#xA;  costs will not be prohibitive. The amount of propellant needed depends&#xA;  largely on what route you choose. Trajectories that by their nature&#xA;  need a minimum of propellant are therefore of great interest.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Hohmann Transfer Orbits&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;To launch a spacecraft from Earth to an outer planet such as Mars&#xA;  using the least propellant possible, first consider that the&#xA;  spacecraft is already in solar orbit as it sits on the launch pad.&#xA;  This existing solar orbit must be adjusted to cause it to take the&#xA;  spacecraft to Mars: The desired orbit's perihelion (closest approach&#xA;  to the sun) will be at the distance of Earth's orbit, and the aphelion&#xA;  (farthest distance from the sun) will be at the distance of Mars'&#xA;  orbit. This is called a Hohmann Transfer orbit. The portion of the&#xA;  solar orbit that takes the spacecraft from Earth to Mars is called its&#xA;  trajectory.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/8lupF.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Earth to Mars via Least Energy Orbit&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;Quote and image source: &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf4-1.php&quot; rel=&quot;noreferrer&quot;&gt;NASA Jet Propulsion Laboratory's Basics of Space Flight, Section I, Chapter 4. Interplanetary Trajectories&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And so on and I suggest reading the whole lot. It's the easiest and still sufficiently complete description of &lt;em&gt;interplanetary spaceflight economics&lt;/em&gt; that I can think of, and sometimes all it takes is reading or hearing it put in different words to really understand it. One other resource that I suggest is also reading Emily Lakdawalla's blog post on &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/11220947-maven-mom-trajectory-explainer.html&quot; rel=&quot;noreferrer&quot;&gt;Why are MAVEN and Mars Orbiter Mission taking such different paths to Mars?&lt;/a&gt; But I'd start with the NASA JPL one first, since it deals with the concepts, then move on to an actual example as described in Emily's blog.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, as you see, this all boils down to economics. For example, the Mars One project is expected to use 4 separate launches of Delta IV Heavy (four of the most powerful rockets currently available) to launch required parts into Low-Earth Orbit (LEO), use in-orbit assembly and then launch all of that into a Hohmann transfer orbit towards Mars. If they attempted to launch all required parts into a more direct trajectory, they would have to use many more launches (it's the lifting capacity that's &lt;em&gt;dragging you down&lt;/em&gt; and you'd require more fuel to reach escape velocity for a more direct trajectory, requiring greater delta-v, which adds more mass which requires more thrust, ad nauseum), and do that at the same time for in-trajectory assembly, otherwise you end up with parts indefinitely chasing each other. To put it differently, we simply don't have lifting capacity available for a more direct trajectory to Mars launch. And even if we had, we'd still be able to launch a lot greater mass towards Mars by requiring less delta-v by using Hohmann transfer trajectory.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2013-12-29T19:52:30.550" LastActivityDate="2013-12-29T19:52:30.550" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3226" PostTypeId="2" ParentId="3220" CreationDate="2013-12-29T16:25:56.400" Score="13" Body="&lt;p&gt;From &lt;a href=&quot;http://www.astronautix.com/fam/soyuz.htm&quot;&gt;Encyclopedia Astronautica&lt;/a&gt; (emphasis by me):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This configuration was selected only after considerable engineering&#xA;  angst. From the point of view of pulling the capsule away from the&#xA;  rocket in an emergency, positioning the capsule at the top of the&#xA;  spacecraft was ideal. &lt;strong&gt;But to use this layout with the living module&#xA;  concept, a hatch would have to be put through the heat shield to&#xA;  connect the two living areas.&lt;/strong&gt; Korolev's engineers just could not&#xA;  accept the idea of violating the integrity of the shield (and would&#xA;  later get in bitter battles with other design bureaus when competing&#xA;  manned spacecraft - Kozlov's Soyuz VI and Chelomei's TKS - used such&#xA;  hatches&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are a couple of other designs which come to mind, but have also significant problems. I don't know whether such ideas have ever been considered: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Put the orbital module below the reentry module during startup,&#xA;without any way to enter it at this time. After reaching orbit, the&#xA;configuration could then be changed to the present configuration,&#xA;similar to how the Apollo lunar module was moved to the front of the&#xA;spacecraft at this time. His would have been rather complex and&#xA;therefore more prone to failure. Also it would probably require some&#xA;redundant systems, and additional fuel for this maneuver.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Reverse the orientation of the reentry module during launch, so that&#xA;the top hatch is pointing downward. But this would have required a&#xA;way to rotate the seats, and everything pointing to them.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="548" LastEditorUserId="548" LastEditDate="2013-12-29T16:44:07.860" LastActivityDate="2013-12-29T16:44:07.860" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3227" PostTypeId="1" AcceptedAnswerId="3230" CreationDate="2013-12-29T18:21:09.320" Score="18" ViewCount="8191" Body="&lt;p&gt;It is very easy to spot LEO satellites during dusk or dawn. I am wondering if satellites further out in a geosynchronous orbit are also visible. Of course, if even possible, these would appear more stationary than any LEO satellites.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="49" LastEditDate="2013-12-29T23:07:01.040" LastActivityDate="2022-04-02T09:33:56.137" Title="Are any geosynchronous satellites visible with the naked eye?" Tags="|artificial-satellite|geostationary|tracking|observation|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3228" PostTypeId="2" ParentId="3223" CreationDate="2013-12-29T18:37:32.120" Score="56" Body="&lt;p&gt;The answer to your title question &quot;Why not travel to Mars in 2 months?&quot; has already been answered.  Money.  Lots and lots of money.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We should first note that the answer to a different question: &quot;Why haven't people gone to Mars?&quot; is much the same.  Money.  However the amount of money to go there in two months is many times greater than the money to go there in eight months.  Or even six months.  To go there in two months, you are riding up the very steep part of the exponential rocket equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assembling the vehicle in LEO does not substantially change the total mass required in LEO.  All that does is allow you to use more smaller launch vehicles instead of fewer large launch vehicles.  (I say &quot;all&quot;, but in fact it is a big cost improvement to not have to develop new giant launch vehicles.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that the &quot;only&quot; in your &quot;If the only obstacle in achieving this would be large amounts of fuel required&quot; hides the important fact that you have to get the fuel to where it needs to be used.  That's where all the money is spent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer the different question in the body on how much more fuel and how to minimize it, see &lt;a href=&quot;http://smad.com/wp-content/uploads/rirtme.pdf&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt; for some investigations of fast transfers and minimizing their costs.  In that paper, you can see for just the $\Delta V$ to leave Earth orbit and leave Mars orbit (so not including landing and taking off from Mars, or Earth for that matter), we go from $5.6\,\mathrm{{km}/s}$ to $28.6\,\mathrm{{km}/s}$!  And that only goes down to about a 2.5 month trip time, not your two months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the velocity is in the &lt;em&gt;exponent&lt;/em&gt; of the rocket equation.  You are not even multiplying here, but raising the exponential term &lt;em&gt;to the fifth power&lt;/em&gt; in order to get closer to your two months.  What that exponential term was in the first place depends on the specific impulse of the rockets you're using.  For typical chemical propulsion, the term was about 3.5 for a Hohmann transfer.  It goes up to about 580 for a 2.5 month transfer!  About 170 times the mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means about &lt;em&gt;170 times the cost&lt;/em&gt;.  There would be learning curve savings in the mass production of that many, that large launch vehicles, but we're still talking around one hundred times the cost.  Even if I assume nuclear thermal rockets with a specific impulse of $1000\,\mathrm{s}$, then to get 2.5 month transfers would cost &lt;em&gt;10 times&lt;/em&gt; as much.  Even if I imagine trying to mine Phobos for return fuel, we're still talking many times the cost.  And this all glosses over the mass of the propulsion systems -- I assumed zero mass for them, where taking them into account, with staging, would reduce the effective specific impulse.  I have also glossed over the cost of arriving at both bodies at &lt;em&gt;much&lt;/em&gt; higher velocities, requiring much more massive entry systems or orbit insertion systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are no problems with the longer transfer time that can't be solved with much, much less mass.  E.g. more supplies, more radiation shielding, centrifuge for gravity, etc.  So bottom line, it would be many times the cost to go there significantly faster, with no apparent benefit other than reduced boredom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that we haven't sent anyone at even &lt;em&gt;one&lt;/em&gt; times the cost, the answer to your question should now be crystal clear.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-01-30T07:33:36.723" LastActivityDate="2016-01-30T07:33:36.723" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3229" PostTypeId="2" ParentId="3219" CreationDate="2013-12-29T19:22:57.370" Score="12" Body="&lt;p&gt;It is a common situation of the early Soviet program. The rockets themselves didn't have names at all initially. Only top secret internal indexes and project designations. (The spacecrafts often didn't have names too, btw. The public names have often been assigned only after a successful launch. See the stories of Vostok/Zenit, Almaz/Salyut etc).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole &lt;a href=&quot;http://en.wikipedia.org/wiki/R-7_%28rocket_family%29&quot; rel=&quot;noreferrer&quot;&gt;R7 rocket family&lt;/a&gt; (such as Molnia, Soyuz, Vostok) is named after their spacecrafts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Universal_Rocket&quot; rel=&quot;noreferrer&quot;&gt;UR-500 rocket&lt;/a&gt; have got its name only after serving as the vehicle for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Proton_satellite&quot; rel=&quot;noreferrer&quot;&gt;Proton satellite&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-12-29T19:22:57.370" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3230" PostTypeId="2" ParentId="3227" CreationDate="2013-12-29T19:30:41.763" Score="16" Body="&lt;p&gt;No, and the reason is simple enough. GEO is at an altitude of 35,786 kilometres (22,236 mi) above the Earth's equator and no satellites in geostationary or geosynchronous (GSO) orbit are large enough to reflect sufficient amounts of light towards the observer with their truss and solar panels to be visible to the naked eye on the surface of the Earth. They're simply too far away and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_diffraction&quot;&gt;atmospheric diffraction&lt;/a&gt; doesn't help either, further blurring small and faint objects of high &lt;a href=&quot;https://en.wikipedia.org/wiki/Apparent_magnitude&quot;&gt;apparent magnitude&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're extremely lucky with weather and other conditions from where you're observing (especially the light pollution, described e.g. by &lt;a href=&quot;https://en.wikipedia.org/wiki/Bortle_scale&quot;&gt;Bortle scale&lt;/a&gt;, should be as low as possible to detect such faint objects), you might be able to see some with powerful binoculars or a hobbyist-grade telescope, as claimed on e.g. &lt;a href=&quot;http://www.satobs.org/geosats.html&quot;&gt;this website&lt;/a&gt;. I'd imagine though that it would only be possible from high altitudes where you'd deal with a lot smaller atmospheric effects and shouldn't be much light pollution. Observing them when transiting a brighter object in the background shouldn't help much either, again due to diffraction. If you're incredibly lucky though (just musing with infinitesimally remote chances here), a foreground object in lower orbit or upper atmosphere would momentarily align with a GEO satellite, and you might, might be able to observe slight lensing effect on its body, if the foreground object had magnifying optical properties, say a burst of translucent propellants ejected out of a rocket's nozzle. But what are the chances of that happening?&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-12-29T19:30:41.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3231" PostTypeId="2" ParentId="3200" CreationDate="2013-12-29T23:49:17.827" Score="11" Body="&lt;p&gt;OK, I found an answer on my own in a caption to the &lt;a href=&quot;http://www.nasa.gov/multimedia/imagegallery/image_feature_1740.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Image of the Day #1740&lt;/a&gt; (11 August, 2010):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/images/content/475466main_145248362_full.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oel9U.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Into the Light&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Reflecting on his experience as he emerged from the craft into the&#xA;  daylight on the Expedition 24 mission's second spacewalk, astronaut&#xA;  Doug Wheelock said &quot;the colors of the Earth just explode at you as you&#xA;  exit toward the planet. &lt;strong&gt;Notice what looks like scorch marks on the&#xA;  hatch thermal cover, the effect of vacuum and atomic oxygen on the&#xA;  threads and thread sealant used on the thermal cover. The 'smell' of&#xA;  space follows suit, I’ve heard it described like burnt cake or&#xA;  cookies, or like the smell of an extinguished match.&lt;/strong&gt;&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Wheelock and fellow astronaut Tracy Caldwell Dyson completed the&#xA;  second of three spacewalks to remove a failed ammonia coolant pump&#xA;  module on the station's S1 truss on Wednesday, Aug. 11.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;Image Credit: NASA&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Emphasis is mine. So this is indeed due to oxidation of the sealant materials used on the thermal cover, caused by monatomic oxygen coming from the tenuous atmosphere at ISS orbital altitude that the hatch is exposed to when it remains open during Extravehicular Activity (EVA), some of them over 7 hours long, and also when closed along its edge where the seal isn't hermetically tight with the rim of the airlock. At ISS orbital altitude between 330 km and 435 km (205 - 270 mi), 90% of atmosphere is monatomic oxygen due to solar UV radiation breaking bivalent molecular bonds of diatomic oxygen: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\require{mhchem}\ce{O_2} + \text{UV 116 kcal} \to \ce{2O} $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The more pronounced oxidation is on the Space Station's ram-facing side of the thermal cover, where interaction with the atmosphere is the greatest. Kudos to &lt;a href=&quot;https://space.stackexchange.com/users/265/mark-adler&quot;&gt;Mark Adler&lt;/a&gt; for this explanation!&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2013-12-30T01:58:02.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3232" PostTypeId="1" AcceptedAnswerId="3233" CreationDate="2013-12-30T01:12:12.647" Score="14" ViewCount="3518" Body="&lt;p&gt;Soyuz Backup Periscope (ВСК-4) is used in Soyuz spacecraft to align itself for orbital corrections, deorbit burn, and rendezvous with the International Space Station. Here is a cute closeup photograph of one such unit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/99lpg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;NASA's &lt;em&gt;poultrynaut&lt;/em&gt; Camilla looks through the Soyuz Backup Periscope (ВСК-4) used to align the Soyuz spacecraft. (Image: &lt;a href=&quot;http://sdo.gsfc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;SDO&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And its function is nicely explained by ESA astronaut &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Astronauts/Samantha_Cristoforetti&quot; rel=&quot;noreferrer&quot;&gt;Samantha Cristoforetti&lt;/a&gt; in one of her &lt;a href=&quot;https://plus.google.com/photos/+SamanthaCristoforetti/albums/5955004512408861265/5955004522705378658?pid=5955004522705378658&amp;amp;oid=112628950566484266163&quot; rel=&quot;noreferrer&quot;&gt;logbook entries on Google+&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Most of the time the view of interest is the one &quot;straight out&quot; along&#xA;  the axis of the periscope. Whenever there is a need to fire the&#xA;  engines, the reference orientation in which the Soyuz puts itself is&#xA;  along the local vertical: in this orientation the periscope faces&#xA;  towards Earth. The optical setup is such that the commander sitting in&#xA;  the center seat will see the entire globe symmetrically in the field&#xA;  of view if the Soyuz is properly oriented. Also, any feature on the&#xA;  surface of the Earth will be running from top to bottom (or from&#xA;  bottom to top if it's a braking attitude).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;When we approach the Station for rendez-vous the mirror is turned so&#xA;  that the commander can now have a forward view. This is the view we&#xA;  would use to align the Soyuz manually if we had to fly a manual&#xA;  docking.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/w2gK4.jpg&quot; alt=&quot;Soyuz periscope&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Quote and photograph source: &lt;a href=&quot;https://plus.google.com/+SamanthaCristoforetti/posts&quot; rel=&quot;noreferrer&quot;&gt;Samantha Cristoforetti on Google+&lt;/a&gt;, image credit: NASA&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But this periscope is a part of the landing module (the middle section of the Soyuz vehicle) and the periscope doesn't seem to retract into the module at all, so...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is this Soyuz Backup Periscope (ВСК-4) disposed of after the deorbit burn and before the atmospheric reentry, when the crew landing module separates from utility and instrument-assembly modules of the Soyuz spacecraft, and if it is, how is the remaining gap in the landing module's external hull sealed / secured to thermally insulate the capsule for the reentry?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2013-12-30T14:50:01.067" Title="How is the periscope port on Soyuz spacecraft secured for the atmospheric reentry?" Tags="|crewed-spaceflight|reentry|safety|soyuz-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3233" PostTypeId="2" ParentId="3232" CreationDate="2013-12-30T14:50:01.067" Score="14" Body="&lt;p&gt;Regarding the jettisoning of the periscope check out &lt;a href=&quot;http://www.youtube.com/watch?v=-l7MM9yoxII#t=9m34s&quot;&gt;this simulation in the video explaining Soyuz reentry sequence at 9:40.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'll notice the periscope being jettisoned at the same time the spacecraft propulsion and orbital modules separate from the landing capsule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think what left is just a small window of heatproof transparent material. In the same video @11:40 you can see the main windows are not covered during re-entry, so it is not a problem. The plasma just burns the outside layer of the window.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2013-12-30T14:50:01.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3234" PostTypeId="1" AcceptedAnswerId="4074" CreationDate="2013-12-30T16:13:01.230" Score="11" ViewCount="5724" Body="&lt;p&gt;When you look at a Soyuz, it seems like the descent and orbital modules are covered in some kind of fabric-like material. You can see wrinkles and it does not look like traditional metal as you might expect, for something that needs to survive reentry. Surely the heat shield takes the brunt of the heat load, but some heat clearly leaks over the edge to the sides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/b/bc/Soyuz_TMA-7_spacecraft2edit1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JmaKe.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;TMA version of the Soyuz spacecraft (Photograph source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Soyuz_TMA-7_spacecraft2edit1.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;, credit: NASA, &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/b/bc/Soyuz_TMA-7_spacecraft2edit1.jpg&quot; rel=&quot;noreferrer&quot;&gt;click on photo for full resolution&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2013-12-30T16:41:07.167" LastActivityDate="2014-03-23T19:22:56.413" Title="What is the outside of the Soyuz capsule covered in?" Tags="|crewed-spaceflight|reentry|materials|soyuz-spacecraft|heat-shield|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3235" PostTypeId="1" CreationDate="2013-12-30T18:02:53.920" Score="8" ViewCount="727" Body="&lt;p&gt;For man made object like the ISS, what are the chance that may get hit ,  How often or how many times might it get hit? &lt;/p&gt;&#xA;" OwnerUserId="1362" LastEditorUserId="1362" LastEditDate="2013-12-31T01:22:55.503" LastActivityDate="2023-11-16T05:33:33.673" Title="What are the chances of getting hit by meteoroids or micrometeoroids in space?" Tags="|artificial-satellite|space-station|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3236" PostTypeId="1" CreationDate="2013-12-30T18:07:07.867" Score="13" ViewCount="2896" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/2397/288&quot;&gt;This answer&lt;/a&gt; seems to indicate the air-lock pressurization/de-pressurization is a drawn out process. Pressurisation is said to consume 45minutes (somewhat less in case of an emergency). Depressurisation may be a tad quicker. In case of both there almost certainly are check-lists, and protocol to follow. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I'm curious about&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How long does &lt;strong&gt;depressurization&lt;/strong&gt; of the Quest Joint Air-lock take?&lt;/li&gt;&#xA;&lt;li&gt;Apart from check-lists, what do the astronaut/s do whilst the air-lock cycles?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-01T22:59:50.093" Title="What do the astronauts do whilst the air-lock cycles?" Tags="|airlock|quest-joint-airlock|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3237" PostTypeId="2" ParentId="3235" CreationDate="2013-12-30T18:10:24.580" Score="2" Body="&lt;p&gt;The probability is effectively 0.  Unless you are the Deep Impact probe.  Then it is 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pdWEp.jpg&quot; alt=&quot;moment of impact&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might really mean meteoroids or micrometeoroids, not asteroids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is difficult to intentionally hit an asteroid with a spacecraft, requiring autonomous guidance (as was done for Deep Impact).  On the other hand, it is very easy to know about and avoid an incoming or approaching asteroid, even with ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth, on the other hand, would be very difficult to move out of the way of an asteroid.  Hence ongoing discussions on how to move the asteroid instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(In this context, comets are in the class of asteroids.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2013-12-30T21:49:37.173" LastActivityDate="2013-12-30T21:49:37.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3238" PostTypeId="1" CreationDate="2013-12-31T01:13:35.680" Score="7" ViewCount="1650" Body="&lt;p&gt;I am looking for a diagram that shows the different diameters of various nozzle openings, including Rocketdyne's F1 (150 inches) SpaceX's Merlin (66 inches), the Air Force's Delta IV Heavy, and NPO Energomash's Proton-M (59 inches). Those ones are rather popular and easier to find, but I am wondering if anyone has a list of different engines and their different characteristics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My theory is that engine size compared to launch vehicle weight is going to decrease over time, especially as efficiency rises. If I had some more data I might be able to show this to be true. Thanks for any help. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me clarify a little bit. I want to see if, for a given launch vehicle weight, the diameter of the engine nozzle has decreased slowly over time, as engines become more efficient and use lighter materials. As such, if I have different nozzle diameters at different time periods of high engine usage, there may be a trend through time. Plus, it would just be cool to see this information, and see how different engine characteristics have changed through time. I want to see what sort of trends like this exist among other engine characteristics, to see how the technology evolves. I would also make this comparison against thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, I am really interested in engine design, and understanding how technological advancements change the dimensions and components will help a lot. &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-03-17T14:48:44.863" LastActivityDate="2016-06-11T18:14:26.493" Title="Is there a diagram of different nozzle diameters to scale?" Tags="|engine-design|engineering|nozzle|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="3239" PostTypeId="2" ParentId="3223" CreationDate="2013-12-31T01:29:41.213" Score="15" Body="&lt;p&gt;In another question - &lt;a href=&quot;https://space.stackexchange.com/questions/3197/proportions-of-a-space-craft-as-compared-to-fuel/3204#3204&quot;&gt;Proportions of a reentering spacecraft as compared to fuel mass&lt;/a&gt; - I explored the Tsiolkovsky rocket equation and the tyranny thereof. In &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;noreferrer&quot;&gt;the related NASA link&lt;/a&gt;, Don Pettit explains that, even using the Hohmann transfer orbit between Earth and Mars which is the most fuel-efficient, it takes about as much change in velocity to break LEO and head out toward Mars as it does to go from the launchpad to LEO. Where does that energy come from? Well, it comes from shoving superhot gases out your tailpipe. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, given a spaceship roughly the mass of the Space Shuttle orbiter (just as an example; the Mars vehicle may be larger, maybe smaller), which is 110 tonnes plus up to 24 tonnes cargo, to get that into LEO we normally burn about 725 tonnes of LH2/LOX fuel through the main engines, plus about a thousand tonnes of perchlorate fuel in the SRBs. Gross liftoff weight of the Shuttle is 2 thousand tonnes, of which the orbiter vehicle and cargo is a maximum of 6.7% of gross liftoff mass. There's some mass inherent in the support structure of the tank and SRBs than is accounted for here, but not nearly as much as you'd think; the SLWT external tank, for instance, has an empty mass of only 3.7% of its loaded mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the Shuttle's in LEO, that's typically it; it carries a small amount of fuel for its deorbit burn and for maneuvering, but the main engines are cold for the entire remainder of the trip. However, we're now talking about getting that mass out of LEO and on a transfer orbit to Mars. That actually requires about the same delta-V as getting to LEO and therefore about the same amount of fuel (we have to burn a little more fuel to get out of the atmosphere because of drag; the Tsiolkovsky equation assumes an &quot;ideal&quot; - drag-less - rocket). So, take that 135 tonnes, strap another 1725 tonnes of fuel and maybe 140 tonnes of support structure on it, and blast off again from LEO to parts unknown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;... But wait, where'd that fuel come from? We have to get the fuel out of Earth's gravity well. That requires us to lift 1725 tonnes of payload into LEO on subsequent launches in order to &quot;refuel&quot; our orbiting craft. Assuming we can use a Shuttle-derived vehicle, like the Space Launch System, to do that with a similar per-launch total payload as launching the Shuttle (it will eventually be possible; SLS Block II is spec'ed to carry 130 tonnes of payload to LEO and we can probably improve on that), and also assuming the spacecraft we launch can keep its fuel tank instead of jettisoning it as the Shuttle does, it would take about 13 launches (those launches consisting of basically a big fuel tank on top of a bigger fuel tank) to get enough fuel into orbit to send our craft on its way to Mars. Each of those launches would burn 1725 tonnes of fuel to get 135 tonnes into orbit, for a total fuel cost of 22,425 tonnes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;... But wait, we want to be able to get back from Mars. Well, that requires a similar delta-V as getting there in the first place. So, we need 1725 tonnes of fuel to get our 135-tonne spacecraft out of Mars orbit and back down to Earth. How does that fuel get to Mars? It rides with the spacecraft. And that means that we need more than 1725 tonnes of fuel to break Earth orbit. In fact, it needs the same 22,425 tonnes that we calculated we'd need in order to get the 1725 tonnes of fuel from Earth to LEO, which will now be used to break Mars orbit. This 22,425 tonnes will be used to break Earth orbit and get the 1860 tonnes of vehicle and return fuel out to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that fuel now needs to be lifted out of Earth's gravity into LEO so it can be strapped on the back of our much-larger rocket. To lift 22,425 tonnes of fuel, 135 tonnes at a time, up to LEO would require 166 more launches, in addition to the 13 needed for the return fuel, plus the one launch for the actual vehicle, for a total of 180 launches from Earth's surface to LEO in order to get this ship in orbit and fueled for its departure. In other words, we'd need more SLS launches carrying just fuel than total combined Saturn V and Shuttle launches (which are the only two vehicles we've ever launched with the lift capacity even close to getting the job done). Each of those launches needs 1725 tonnes of fuel, for 310,500 tonnes of fuel burned just getting the vehicle and fuel into LEO. Then the ship itself will burn 22,425 tonnes of fuel getting to Mars, then 1725 tonnes getting back, for a total fuel expenditure of about 335,000 tonnes. So, fuel costs aren't too bad; for LH2/LOX, at an 11%-89% mixture and today's prices (\$5.50/kg LH2, \$.20/kg LOX), we're looking at around \$250 million in raw fuel cost, plus off-gas losses (liquid hydrogen and oxygen don't just sit around in liquid form at room temp). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, total launch costs are a big deal. The SLS, if it hits its cost goals, will be about half a billion dollars a launch. 180 launches to get the vehicle and fuel into LEO represents a cost of about \$90 billion just to get the materiel into space. Actually designing and building what we're launching could exceed a trillion dollars, given that it &lt;em&gt;can not fail&lt;/em&gt;; if the crew has a &quot;problem&quot; halfway out to Mars, like the one Apollo 13 did, the chances of them getting back to Earth safely are nil.&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2013-12-31T01:38:31.807" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3240" PostTypeId="1" AcceptedAnswerId="3250" CreationDate="2013-12-31T05:27:45.730" Score="13" ViewCount="543" Body="&lt;p&gt;A sort-of follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/3236/what-do-the-astronauts-do-whilst-the-air-lock-cycles&quot;&gt;What do the astronauts do whilst the air-lock cycles?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;fact-sheet&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Pressurized Volume: 32,333 cubic feet (916 cubic meters)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/search?q=ISS%20maneuver&quot;&gt;Elsewhere&lt;/a&gt; on the stack are mentions of ISS maneuver to avoid space debris. So I let my mind free-wheel and came up with this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;If the crew on-station were to suit-up, and vent the station atmosphere ... how much velocity change could they achieve?&lt;/em&gt;&lt;/strong&gt; Sort-of the way a sub-marine pumps fluid for buoyancy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answerer will necessarily have to make some assumptions like the vent rate ...&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2013-12-31T19:00:31.137" Title="How much delta-v can be achieved by venting the ISS atmosphere?" Tags="|iss|propulsion|delta-v|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3241" PostTypeId="1" AcceptedAnswerId="4610" CreationDate="2013-12-31T17:08:55.547" Score="21" ViewCount="3726" Body="&lt;p&gt;With three CBC (&lt;a href=&quot;http://en.wikipedia.org/wiki/Common_Booster_Core&quot; rel=&quot;noreferrer&quot;&gt;Common Booster Core&lt;/a&gt;) modules firing their &lt;a href=&quot;http://en.wikipedia.org/wiki/RS-68&quot; rel=&quot;noreferrer&quot;&gt;RS-68&lt;/a&gt;A engines simultaneously, a &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta_4-H#Delta_IV_Heavy&quot; rel=&quot;noreferrer&quot;&gt;Delta 4 Heavy&lt;/a&gt; launch is an impressive sight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ys3f1.jpg&quot; alt=&quot;Charred D4 Heavy&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RS-68 is a LOX/LH2 engine. As part of the launch sequence, a fair bit of hydrogen is run through the engine, prior to actual ignition to cool down pumps, components, etc and condition the engine.  The &lt;a href=&quot;http://en.wikipedia.org/wiki/SSME&quot; rel=&quot;noreferrer&quot;&gt;SSME&lt;/a&gt; (Space Shuttle Main Engine, RS-25) does the same thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta_4-H#Delta_IV_Heavy&quot; rel=&quot;noreferrer&quot;&gt;D4H&lt;/a&gt; however, there is sufficient hydrogen run through, that at ignition, it ignites, and actually starts an awesome to see fire, running up the sides of the core modules, and the insulation on the outside gets quite charred.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to minimize this they have tried to start one engine first, then the other two, so as to burn off some of the hydrogen first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus my question.  On the Space Shuttle, there are sparklers that ignite as the SSME's go through their conditioning cycle to burn off the excess hydrogen gas.  Why not use the same on the D4H launches? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="288" LastEditDate="2014-01-01T18:14:32.147" LastActivityDate="2018-01-14T07:24:18.427" Title="Why not use Sparklers on Delta-4 Heavy launches to burn off the hydrogen?" Tags="|delta-iv|delta-iv-heavy|ssme|rs-68|hydrogen|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3242" PostTypeId="5" CreationDate="2013-12-31T17:16:02.723" Score="0" Body="&lt;p&gt;The Space Shuttle Main Engines (SSME) are one of the higher performing rocket engines with a vacuum ISP of 452.3s burning liquid oxygen and liquid hydrogen.  Each Space Shuttle launch used three SSME engines, drawing their fuel and oxidizer from the External Tank.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-31T17:16:02.723" LastActivityDate="2013-12-31T17:16:02.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3243" PostTypeId="4" CreationDate="2013-12-31T17:16:02.723" Score="0" Body="SSME is the acronym for the Space Shuttle Main Engine. Sometimes called the RS-25, or RS-25D." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-31T17:16:02.723" LastActivityDate="2013-12-31T17:16:02.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3244" PostTypeId="5" CreationDate="2013-12-31T17:17:56.590" Score="0" Body="&lt;p&gt;The Delta 4 Heavy is one of the most powerful boosters commercially available, with one of the largest operational payloads (until the Falcon 9 Heavy becomes operational).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It uses three CBC boosters, each by itself a Delta 4 first stage powered by RS-68 engines burning LOX/LH2.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-31T17:17:56.590" LastActivityDate="2013-12-31T17:17:56.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3245" PostTypeId="4" CreationDate="2013-12-31T17:17:56.590" Score="0" Body="Delta 4 Heavy uses three of the Delta 4 Common Booster Cores side by side to launch heavy payloads." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-31T17:17:56.590" LastActivityDate="2013-12-31T17:17:56.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3246" PostTypeId="5" CreationDate="2013-12-31T17:19:59.693" Score="0" Body="&lt;p&gt;The Delta 4 booster family comes in many configurations.  The most basic is the single core with no solid booster strapons.  The core booster uses an RS-68 engine burning LOX/LH2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Versions with different numbers of solid booster strap ons are available to provide additional payload capacity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The largest version is the Delta-4 Heavy, which uses three CBC (Common Booster Cores), each a full Delta 4 first stage, strapped together for one of the largest available payloads available.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-31T17:19:59.693" LastActivityDate="2013-12-31T17:19:59.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3247" PostTypeId="4" CreationDate="2013-12-31T17:19:59.693" Score="0" Body="The Delta 4 booster family is an evolution of the Delta boosters but shares little in common at this point." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-31T17:19:59.693" LastActivityDate="2013-12-31T17:19:59.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3248" PostTypeId="5" CreationDate="2013-12-31T17:20:47.700" Score="0" Body="&lt;p&gt;The RS-68 is one of the newest LOX/LH2 engines in use, used on the Delta 4 family of boosters.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-31T17:20:47.700" LastActivityDate="2013-12-31T17:20:47.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3249" PostTypeId="4" CreationDate="2013-12-31T17:20:47.700" Score="0" Body="The RS-68 engine is used on the Delta 4 family and burns LOX/LH2." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2013-12-31T17:20:47.700" LastActivityDate="2013-12-31T17:20:47.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3250" PostTypeId="2" ParentId="3240" CreationDate="2013-12-31T19:00:31.137" Score="12" Body="&lt;p&gt;That amount of air has a mass of about $1000\,\mathrm{kg}$.  The mass of ISS is about $400000\,\mathrm{kg}$.  The theoretical best specific impulse you could hope for is around $60\,\mathrm{s}$ from a cold gas thruster designed for the purpose.  So about $600\,\mathrm{m/s}\times 1000/400000=1.5\,\mathrm{m/s}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However to actually get $1.5\,\mathrm{m/s}$ would be very problematic.  The time for most of the gas to leave would be extremely long due to the very low starting pressure.  Water would freeze, clogging the thruster.  Probably a myriad of other problems I'm not thinking of.  You would not be able to just open the back door and expect to get that amount of $\Delta V$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2013-12-31T19:00:31.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3251" PostTypeId="1" CreationDate="2013-12-31T21:16:20.013" Score="10" ViewCount="247" Body="&lt;p&gt;I was traveling through Nevada when I stopped to eat at &lt;a href=&quot;http://www.wind-mill-ridge.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Windmill Ridge&lt;/a&gt; - a motel / resturaunt combo thing. I noticed this plaque on the wall:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9mAFDl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;In recognition of your contributions to the success of the Boeing Commercial Crew Transportation System program to develop a U.S. capability to carry crews to LEO destinations and return them to earth&quot; Thank you.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What 'contributions' could the private company have make? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="10132" LastEditDate="2015-07-07T03:19:00.640" LastActivityDate="2019-01-13T10:54:16.260" Title="What 'contributions' could this private company have made to Boeing?" Tags="|boeing|cst-100|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3252" PostTypeId="1" AcceptedAnswerId="3335" CreationDate="2013-12-31T21:25:34.943" Score="3" ViewCount="138" Body="&lt;p&gt;According to the Wikipedia article on the &lt;a href=&quot;http://en.wikipedia.org/wiki/Orlan_space_suit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orlan space suit&lt;/a&gt;, all models of the suit after and including the Orlan-DM were designed to be self-sustaining:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first Orlan suits were attached to the spacecraft by an umbilical tether that supplied power and communications links. The Orlan-DM, and all models since, were designed to be self-sustaining.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/4pjoq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;A spacewalker in an Orlan-DMA suit.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it seems that there are many advantages to using an umbilical - you could stay outside much longer, for one. Could the suit use an umbilical if needed?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="49" LastEditDate="2013-12-31T23:59:42.863" LastActivityDate="2014-01-09T04:24:41.710" Title="Could the Orlan-MK use an umbilical if need be?" Tags="|spacesuits|eva|russia|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3253" PostTypeId="1" AcceptedAnswerId="3347" CreationDate="2013-12-31T22:13:38.063" Score="5" ViewCount="236" Body="&lt;p&gt;There are so many proposed Soyuz booster upgrades, I cannot keep them straight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering if there is a good comparison chart between the various proposed upgrades, including engine information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example: The traditional Soyuz, used to launch manned and unmanned flights to the ISS.&#xA;The Soyuz-ST used to launch satellites out of Korou by ESA.&#xA;The Soyuz 2-1-V just launched. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a variety of proposed Soyuz 2 variants proposed as well.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="13208" LastEditDate="2017-02-27T21:22:55.330" LastActivityDate="2022-10-07T08:09:01.543" Title="Is there a good chart, comparing the various Russian engines proposed for Soyuz (Booster) upgrades?" Tags="|engines|booster|soyuz-launcher|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3255" PostTypeId="2" ParentId="3220" CreationDate="2014-01-02T02:29:10.770" Score="7" Body="&lt;p&gt;Consider the Soyuz spacecraft vs the Apollo spacecraft.  Both are 3 man spacecraft originally designed for the lunar mission.  Both use tractor rocket escape systems for launch abort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Apollo spacecraft, the reentry craft, living area and docking functions are combined into the same module.  The Soyuz separates those functions into two modules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soyuz design, by minimizing the size of the reentry module also minimizes the mass of heatshielding and parachute system needed.  The lightweight structure of the orbital module allows it to be made larger, providing the Soyuz a greater total volume of living space for the crew than the Apollo.  Because of the lightweight design the combined 4,300 kg mass of the Soyuz descent module and orbital module is less than the 5,800 kg mass of the Apollo command module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During launch abort, either spacecraft must jettison the launch escape tower and payload shroud before parachutes can be deployed for landing.  With the Soyuz, the tower/shroud separates along with the orbital module.  For all practical purposes the launch abort sequence is not any more complicated with the Soyuz than with the Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary reason the Soyuz orbital module is ahead of the descent module is so the docking equipment can be placed on the orbital module.  That helps to minimize the mass of the descent module, and also allows the orbital module to function as an airlock. &lt;/p&gt;&#xA;" OwnerUserId="1370" LastActivityDate="2014-01-02T02:29:10.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3256" PostTypeId="2" ParentId="2364" CreationDate="2014-01-02T03:21:07.623" Score="4" Body="&lt;p&gt;Short answer: yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Medium answer: the United States developed nuclear rocket engine technology to the point that by 1969 the only obstacle remaining to full flight status was a near term mission for the rocket engine to apply to.  Since spending for deep space exploration began to be cut back by Congress in 1967, and the Nixon administration cut back future NASA manned spaceflight projects to only the Space Shuttle, there was no mission and therefore no need for the nuclear rocket engine program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has often been said that getting to Low Earth Orbit is halfway to anywhere; in other words getting to Low Earth Orbit is half the effort needed to reach anywhere interesting to us in the Solar System.  On the other hand, Low Earth Orbit is only halfway which leaves the other half.  Nuclear rocket engine technology tremendously eases the effort needed for that other half, with performance double the efficiency of the best available chemical rocket engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket engine technology for reaching orbit is very well understood and very mature.  The primary boundaries of that technology today is reducing the costs, of which SpaceX is leading the way.  To do so SpaceX uses oxygen + kerosine liquid propellant rocket engines for its Falcon 1, and Falcon 9 two-stage launch vehicles, and for the proposed Falcon Heavy launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even though that kind of engine is very good at achieving low Earth orbit missions, the middling efficiency of that engine (vacuum ISP 330 seconds) drops the payload down tremendously for deeper space missions.  For example even though the 53 ton payload to low orbit of the proposed Falcon Heavy swamps the 27 ton payload of the Delta IV Heavy by more than 95%, the Falcon Heavy drops to only 21 tons to Geosynchronous Transfer Orbit while the Delta IV Heavy drops to 13 tons (only a 62% advantage).  That is because the upper stage of the Delta IV uses a oxygen + hydrogen liquid rocket engine of almost the highest possible efficiency for a chemical rocket, an ISP of 465 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To send manned spacecraft to the moon and beyond and have them return to Earth using only chemical rocket engines requires rockets of titanic size even to send very small spacecraft.  In fact it is so difficult to use chemical rockets that well regarded science/science-fiction writer Arthur C. Clarke thought back in the 1950's that landing on the moon would only happen after the introduction of nuclear rocket engine technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the book &quot;Interplanetary Flight: an introduction to astronautics&quot;.  It's an old book but still relevant today.  I also recommend the book &quot;To the end of the solar system: the story of the nuclear rocket&quot;.  That book was a real eye opener.&lt;/p&gt;&#xA;" OwnerUserId="1370" LastEditorUserId="6816" LastEditDate="2018-10-25T07:50:59.463" LastActivityDate="2018-10-25T07:50:59.463" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3258" PostTypeId="2" ParentId="2820" CreationDate="2014-01-02T07:26:31.353" Score="2" Body="&lt;p&gt;The Mars ascent stage would need to mass approximately 22,000 kg, assuming an ascent vehicle very similar to the complete Apollo lunar lander (LM), a two-stage vehicle using hypergolic propellant rocket engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I base this on the roughly 4 km/sec delta v astronautix.com describes for the LM, and multiply the mass of the LM by 1.5 to account for the increase of crew to three.  &lt;/p&gt;&#xA;" OwnerUserId="1371" LastActivityDate="2014-01-02T07:26:31.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3259" PostTypeId="2" ParentId="3161" CreationDate="2014-01-02T08:06:31.820" Score="6" Body="&lt;p&gt;Excellent question.  But twinned to that is also the question of the thrust level which is more than three times greater than the current Merlin engine used in the second stage of the Falcon 9.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I admit this is pure speculation.  But based on prior SpaceX practices with the Merlin engine, I believe the Raptor might be intended for use in larger launch vehicles than the Falcon 9.  Perhaps as a common engine used in the lower and upper stages of a upgraded Falcon Heavy, where the Raptor replaces the Merlin engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I think the most likely possibility is a major change in the Falcon 9 design (and maybe the Falcon Heavy too) spurred by recent successes with the Grasshopper vertical landing test vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Raptor would be a good engine for an oversized upper stage for the Falcon 9 or Falcon Heavy.  By transferring more of the effort to reach orbit to the upper stage, the first stage has greater performance margin and also stages at a lower altitude and speed making powered recovery of the first stage to the launch site easier.  Recovery and reuse of the 1st stage would save SpaceX a lot of money (particularly for the Falcon Heavy) and permit lower pricing.&lt;/p&gt;&#xA;" OwnerUserId="1372" LastActivityDate="2014-01-02T08:06:31.820" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3260" PostTypeId="1" CreationDate="2014-01-02T20:11:06.293" Score="8" ViewCount="777" Body="&lt;p&gt;When you compare the upper stage of an Atlas 5/Delta 4, which use an RL-10 upper stage engine vs a Falcon 9 upper stage that uses a Merlin 1D Vac version, you see really big performance differences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RL-10 is a LOX/LH2 engine with great specific impulse ($I_\text{sp}$ of 430-450 s depending on version) but crappy thrust of only 20-25 Klbs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Merlin 1D vac is an LOX/RP1 engine with so-so $I_\text{sp}$ (good for RP-1 though) at 304 s, but much better thrust at 180 Klbs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet, the RL-10 is considered a much better upper stage than a Merlin 1D.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure, that is a pretty big $I_\text{sp}$ gap (25% more for RL-10), but with so much more thrust (8-9 times more for Merlin), it seems like it should not be such a big deal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that $I_\text{sp}$ is a measure of efficiency, in terms of how many seconds of &lt;em&gt;'use'&lt;/em&gt; you get per unit of fuel. Thus an RL-10 is 25% more efficient than a Merlin 1D.  But as long as you are carrying sufficient fuel/oxidizer and have the thrust, why would the 8-9 times different thrust values matter more?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately this is probably a question of why $I_\text{sp}$ matters so much in this context? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="195" LastEditDate="2016-01-14T20:58:26.553" LastActivityDate="2016-01-15T04:22:01.447" Title="Why does $I_\text{sp}$ on an upper stage matter so dramatically?" Tags="|merlin-1d|upper-stage|rl-10|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3261" PostTypeId="2" ParentId="3260" CreationDate="2014-01-02T20:29:13.223" Score="15" Body="&lt;p&gt;The mass required to deliver the required $\Delta\mathrm{V}$ depends on the I&lt;sub&gt;sp&lt;/sub&gt; and the dry mass ratio of the stage.  It does not depend on the thrust, except to the extent that a high thrust would increase the mass required due to a large engine and feeds.  For a given I&lt;sub&gt;sp&lt;/sub&gt;, the thrust just determines how long it takes to use all the propellant.  For an upper stage, you're not in as much of a rush as for lower stages.  You are already on a trajectory that gives you some time in vacuum to complete your orbit insertion.  Once in orbit you have all the time in the world for the next burn of the upper stage, if there is one, to go to a different orbit or escape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You just need enough thrust to get the burn done before you start falling back, if it's completing an orbit insertion.  For the next burn, you need enough thrust so that the burn can be completed before a lot of altitude is gained, to maximize the Oberth effect (more $\Delta$energy per unit $\Delta\mathrm{V}$).  More thrust than that doesn't help an upper stage.  It just hurts with more mass that isn't needed.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-01-03T03:34:32.537" LastActivityDate="2014-01-03T03:34:32.537" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3262" PostTypeId="1" AcceptedAnswerId="3529" CreationDate="2014-01-03T01:23:41.900" Score="33" ViewCount="7007" Body="&lt;p&gt;Obviously, I mean any living organisms from the animal kingdom except humans, sent to the International Space Station (ISS) for the purpose of scientific research. I also don't mean if they did the &lt;em&gt;Chicken Run&lt;/em&gt; on their own. Clearly, that wouldn't be possible, so fear not, while I realize this might so far sound like an attempt at trolling, my motivation for asking this question isn't to impose my moral views, ask stupid questions, or anything like that. I'm asking this question to understand better the extent of medical and biological research performed onboard the ISS, and whether studies also continue post-exposure to microgravity and increased radiation conditions on the station, to study readjustment to gravity, the ability of subjects to regenerate under different medication regimes, and so on.&lt;/p&gt;&#xA;&lt;p&gt;Where does my question come from? Well, I just watched a &lt;a href=&quot;https://www.youtube.com/watch?v=zoUEhbEGLiQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;video interview with Dr. Julie Robinson&lt;/a&gt;, NASA's International Space Station Chief Scientist explaining some of the upcoming science experiments planned to be conducted on the ISS in 2014. And among other set goals for the year that barely started, she mentions that more rodents will be sent to the station onboard the third SpaceX Dragon operational cargo delivery mission to the ISS, currently scheduled for February 22, 2014. And we know that &lt;a href=&quot;https://en.wikipedia.org/wiki/Animals_in_space#1990s&quot; rel=&quot;nofollow noreferrer&quot;&gt;rodents and other live animals were already studied onboard Space Shuttle or sent to the ISS before&lt;/a&gt;. But here's the thing. Since the retirement of the Space Shuttle, I don't recall any other cargo vehicle also delivering experiments back to Earth, with the only notable exception being the Soyuz spacecraft, but since out of the three of its modules only the crew module actually lands in a controlled manner suitable for delivery of live beings, this means there's only a really limited space available for any experiments to be delivered back alive, where observations could continue. And live creatures tend to require specific conditions to stay alive.&lt;/p&gt;&#xA;&lt;p&gt;So, since it would obviously be of great scientific value to have at least some of these tiny visitors to the space station return back alive to study their ability to regenerate from their excursion to the outer space (e.g. regrow fractured bones, study effectiveness of medication to regain body and bone mass, blood volume, etc.), I suspect this has been done before, so here's my question:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Which laboratory animals were brought back to Earth alive after being studied in microgravity on the International Space Station, how often was that one of the goals of the scientific research on them, how often did it succeed, when did it last happen, and how severely has the retirement of the Space Shuttle and lack of other suitable cargo delivery vehicles delayed or otherwise affected such research on living animals? Can we expect such research to pick up in pace once commercial cargo vehicles or their capsules (like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dragon CRS&lt;/a&gt;) will also be retrieved on landing and not merely left to burn up in the atmosphere on re-entry?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Perhaps a bit too broad, but covering current success and future plans of a single larger space agency participating in research on live animals (NASA, Roscosmos, ESA, JAXA) onboard the ISS should answer most of my answers sufficiently. I don't expect list answers, an overview should do just fine.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="39182" LastEditDate="2021-04-26T05:05:03.953" LastActivityDate="2021-04-26T05:05:03.953" Title="Have any animals that have been studied onboard the ISS come back alive?" Tags="|iss|research|animals|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="3263" PostTypeId="1" AcceptedAnswerId="3533" CreationDate="2014-01-03T02:28:12.093" Score="17" ViewCount="18842" Body="&lt;p&gt;This NASA's image of a Mercury-Redstone rocket Freedom 7 carrying Alan Shepard, the first American in space, is titled &lt;strong&gt;Light the Candle&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/qzsuf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Light the Candle&lt;/strong&gt; (Image Credit: &lt;a href=&quot;http://www.nasa.gov/multimedia/imagegallery/image_feature_1203.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember this phrase also mentioned in the movie &lt;a href=&quot;http://www.imdb.com/title/tt0086197/&quot; rel=&quot;noreferrer&quot;&gt;The Right Stuff&lt;/a&gt;, with Alan Shepard (Scott Glenn) waiting for the launch to commence for a rather long time, having to urinate in his suit before launch, because he was sat in his capsule with no easy way to exit it for so long (and the movie portrays many cups of coffee that he had that morning before leaving home, in a somewhat jocular manner) that he couldn't hold it any longer. If I remember correctly from the movie, one of his fellow astronauts in the control center says: &lt;em&gt;&quot;Let's light the candle!&quot;&lt;/em&gt;, to which control center crew looks at each other and Wernher von Braun agrees: &lt;em&gt;&quot;Yes. Light the candle!&quot;&lt;/em&gt;. &lt;a href=&quot;https://www.youtube.com/watch?v=AHVsszW7Nds&quot; rel=&quot;noreferrer&quot;&gt;Gustav Holst's Op. 32 &lt;em&gt;&quot;The Planets&quot;&lt;/em&gt;&lt;/a&gt; starts playing in the background (I believe a passage from act 4, &lt;em&gt;Jupiter, the Bringer of Jollity&lt;/em&gt;) and seconds after, the countdown resumes and the the first American manned suborbital spaceflight launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, that's only a movie and they often dramatize events differently than they really happened, so here's my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Who coined the phrase &lt;em&gt;&quot;Light the Candle!&quot;&lt;/em&gt; to commence with a rocket launch, and how does the story really go? Did The Right Stuff movie accurately portray events before the Freedom 7 launch, or is there some other story behind this phrase?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please provide references for claims in your answers.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2017-06-11T19:17:36.203" Title="Who really coined the phrase &quot;Light the Candle!&quot; to commence with a rocket launch?" Tags="|crewed-spaceflight|nasa|history|terminology|project-mercury|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3264" PostTypeId="1" AcceptedAnswerId="3282" CreationDate="2014-01-03T04:41:19.640" Score="19" ViewCount="1884" Body="&lt;p&gt;After reading the question &lt;a href=&quot;https://space.stackexchange.com/questions/3262/have-any-animals-that-have-been-studied-onboard-the-iss-came-back-alive&quot;&gt;Have any animals that have been studied onboard the ISS come back alive?&lt;/a&gt;, my first thought was this: what about the ones that &lt;em&gt;don't&lt;/em&gt; live?&lt;/p&gt;&#xA;&lt;p&gt;It would seem that this wouldn't be an uncommon occurrence at least at first - humans don't do very well in microgravity, and animals even more so.&lt;/p&gt;&#xA;&lt;p&gt;Obviously, one wouldn't want deceased animals floating around the station. Are these unlucky critters sent back with normal trash, or in a different way?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="39182" LastEditDate="2021-04-26T05:06:00.910" LastActivityDate="2021-04-26T05:06:00.910" Title="How are dead animals disposed of on the ISS?" Tags="|iss|animals|death|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3265" PostTypeId="1" AcceptedAnswerId="6541" CreationDate="2014-01-03T17:04:26.660" Score="15" ViewCount="641" Body="&lt;p&gt;In the Soyuz program, quite a few accidents have been caused by malfunctions of the explosive bolts:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_11&quot;&gt;Soyuz 11&lt;/a&gt;: Explosive bolts designed to fire sequentially fired simultaneously instead, causing a fatal malfunction of a valve.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-T_No.39&quot;&gt;Soyuz 7K-T No.39&lt;/a&gt;: Second and third stages of the booster did not separate properly.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_5&quot;&gt;Soyuz 5&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-10&quot;&gt;TMA-10&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-11&quot;&gt;TMA-11&lt;/a&gt;: The service module did not separate from the reentry module&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Most other Soyuz accidents seem to have been one time occurrences, but problems with the explosive bolts misfiring seem to be recurring. One would think that explosive bolts are a quite simple and robust technique, so this is a bit unexpected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an explanation for these frequent issues with explosive bolts?&lt;/p&gt;&#xA;" OwnerUserId="548" LastEditorUserId="12102" LastEditDate="2019-06-07T05:34:53.993" LastActivityDate="2019-06-07T05:34:53.993" Title="Is there an explanation for repeating Soyuz accidents involving misfiring of explosive bolts?" Tags="|crewed-spaceflight|reentry|soyuz-spacecraft|failure|stage-separation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3267" PostTypeId="1" AcceptedAnswerId="3268" CreationDate="2014-01-03T22:21:52.183" Score="8" ViewCount="2312" Body="&lt;p&gt;Wikipedia lists the atmospheric pressure of Mars to be about 600 pascals, or just 0.6% of Earth's atmosphere.  I'm wondering, can anyone shed any light on what that means, in practical terms?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, I'm looking for what might happen if a habitat on the surface of Mars, pressurized for Human occupants, got a leak.  Not huge, like opening a door, but something along the lines of a bullet hole in the wall or a small meteorite impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would that kind of pressure difference lead to an explosive decompression, or just the equivalent of a strong wind blowing out the hole?  If they were outfitted with oxygen face-masks like Everest climbers, would the occupants be able to survive for long, or is that pressure quickly lethal?  There are so many images of depressurization in space, that I'm having a hard time visualizing an accurate picture of depressurization into the Martian atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks in advance for any insight you have!  This is my first question here, but I've already fallen in love with this particular Stack site.&lt;/p&gt;&#xA;" OwnerUserId="1216" LastActivityDate="2014-01-06T20:34:54.760" Title="Depressurization of a habitat on Mars" Tags="|mars|atmosphere|life-support|habitat|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3268" PostTypeId="2" ParentId="3267" CreationDate="2014-01-03T23:13:38.297" Score="5" Body="&lt;p&gt;Whether or not a decompression will be explosive depends on the size of the hole and the shape and material of the wall it's made in.  In terms of human factors, 600 Pa is not very different from a vacuum.  To give a comparison, it is equivalent to an altitude of about 34 km -- compare that with Mt. Everest's 8.8 km.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-01-03T23:13:38.297" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3269" PostTypeId="2" ParentId="3235" CreationDate="2014-01-03T23:29:25.953" Score="4" Body="&lt;p&gt;Answering the now-edited question:  It depends on how small you want to consider.  If you want to get down to dust-sized particles, the ISS is pelted with hundreds, if not thousands, every day.  As you start looking at larger particles, the flux drops off dramatically.  The environment that the station was designed to is described in the document &lt;a href=&quot;http://www.everyspec.com/NASA/NASA-JSC/NASA-SSP-PUBS/SSP-30425B_29660/&quot; rel=&quot;nofollow&quot;&gt;SSP 30425&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-01-03T23:29:25.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3270" PostTypeId="1" AcceptedAnswerId="3272" CreationDate="2014-01-04T00:27:26.993" Score="2" ViewCount="289" Body="&lt;p&gt;What approximate size range, considering the different construction and composition, must meteoroids be to become meteorites on Earth, Mars, and the Moon?&lt;/p&gt;&#xA;" OwnerUserId="1384" LastEditorUserId="49" LastEditDate="2014-03-07T09:21:32.913" LastActivityDate="2014-03-07T09:21:32.913" Title="Minimum size meteoroids for meteorites on Earth, Mars, and the Moon?" Tags="|mars|the-moon|atmosphere|impact|meteorites|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3272" PostTypeId="2" ParentId="3270" CreationDate="2014-01-04T18:45:53.463" Score="2" Body="&lt;p&gt;There isn't any, because we know of homogenous materials that have molecular and atomic bonds stronger than maximum dynamic pressure per their surface area that they would have to endure as they enter the Earth's atmosphere, are subject to gravitational forces and chemical interaction with the atmosphere. I'm specifically saying this for Earth, since it has the densest atmosphere and is the most massive object of the three in your question, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Mars#Composition&quot; rel=&quot;nofollow&quot;&gt;Martian atmosphere&lt;/a&gt; doesn't have any chemicals not already present in Earth's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, we know that in celestials of sufficient mass and carbon content, &lt;a href=&quot;http://www.astronomy.com/news/2013/10/planetary-scientists-find-jupiter-and-saturn-are-awash-in-diamonds&quot; rel=&quot;nofollow&quot;&gt;solid diamonds form&lt;/a&gt; in regions of required internal pressure and temperature. For example inside Jupiter, regions of its interior sustain tremendous pressure that is sufficient to form diamond lattice crystal of carbon atoms. In fact, Jupiter is even too massive and these diamonds are later subject to even stronger pressure they can no longer withstand, and they melt into a tar-like material. Anyway, chances that any of these diamonds would be free-floating in space are rather remote, but not impossible, if e.g. there was a cataclysmic collision between two such celestials tearing them apart, sending some of its innards at escape velocities on trajectories towards other celestials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Science fiction? Perhaps, but there are other homogenous materials of sufficient strength too that would be perfectly capable of entering Earth's atmosphere and not lose a single atom through ablation as they are subject to tremendous surface friction with the atmosphere, being torn apart by tidal forces, evaporate or interact chemically. Crystal structures are particularly strong and inert as the atoms form perfect of near perfect bonds between each other, but some polymer molecular bonds can be nearly as strong and don't change state to evaporate easily. Of course, most meteoroids aren't homogenous, and they might have structural weaknesses that would cause them to disintegrate, but the point is that there isn't any minimum size all the way down to a single atom. Not with Earth's atmosphere, and even less so on Mars and its tenuous atmosphere, and no atmospheric restrictions on the Moon whatsoever, since its exosphere isn't dense enough to even act like gasses do (molecules colliding with each other, and forming surface pressure).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing about meteoroids that is often misunderstood is that they tend to &quot;burn&quot; in the atmosphere. This is a simplification, and there isn't any burn in conventional sense, and bolides and meteoroids don't form flames. What happens is that they enter the atmosphere at such immense speeds that the friction with it causes its surface to heat up, ablate or gas out and all of these high energy particles ionize the atmosphere surrounding it into a gaseous plasma. The level of such interaction would depend on the total surface area of all its parts, velocity and the rate of ablation (macro scale) and evaporation (micro scale). Chemical burn is actually extremely remote. Meteoroids of strong enough surface materials and internal structure wouldn't lose any of their mass during such interaction with the atmosphere at all, even though the bolide / meteoroid would still appear as if it's &quot;burning&quot;, as it would still create an ionized trail along its path. How they would fragment on impact is another matter, but that depends on the hardness of the surface they impact with, and their velocity would be greatly reduced on their way through the atmosphere (where present).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this boils down to structural integrity of the impacting meteoroid, not so much its size. Sure, as the size increases, the surface area subject to friction, tidal forces and chemical interaction with the atmosphere will increase, and so will internal stretching and pulling on bonds holding it together, but we're already going in the opposite direction now, describing their maximum size before being torn apart (still not preventing impact, mind you), not their minimum one.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-04T20:13:29.393" LastActivityDate="2014-01-04T20:13:29.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3273" PostTypeId="1" AcceptedAnswerId="3274" CreationDate="2014-01-04T20:00:11.237" Score="8" ViewCount="599" Body="&lt;p&gt;My understanding is that for us to confirm that a signal is of artificial origin, we have to get the same signal at least twice. However, as far as I can tell, most of the signals we've sent out to contact extraterrestrials have only been sent once, and thus any extraterrestrial civilizations receiving them would likely dismiss them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, have we ever sent a signal at least twice, to make sure that anyone receiving it would realize it is of artificial origin?&lt;/p&gt;&#xA;" OwnerUserId="1133" LastEditorUserId="49" LastEditDate="2014-01-05T03:02:55.613" LastActivityDate="2014-01-05T03:02:55.613" Title="Has Earth ever sent out a repeating signal into space?" Tags="|communication|history|astrobiology|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3274" PostTypeId="2" ParentId="3273" CreationDate="2014-01-04T20:38:53.523" Score="5" Body="&lt;p&gt;Yes, and one such list is compiled on the Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_messages&quot; rel=&quot;noreferrer&quot;&gt;List of interstellar radio messages&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_Call&quot; rel=&quot;noreferrer&quot;&gt;Cosmic Call&lt;/a&gt; 1 (1999), four transmissions to nearby Sun-like stars&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Teen_Age_Message&quot; rel=&quot;noreferrer&quot;&gt;Teen Age Message&lt;/a&gt; (2001), six transmissions&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_Call&quot; rel=&quot;noreferrer&quot;&gt;Cosmic Call&lt;/a&gt; 2 (2003), five transmissions&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.naic.edu/science/wow_signal_reply2012.html&quot; rel=&quot;noreferrer&quot;&gt;Wow! Reply&lt;/a&gt; (2012), three transmissions to Hipparcos 34511, Hipparcos 33277 and Hipparcos 43587 in reply to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Wow!_signal&quot; rel=&quot;noreferrer&quot;&gt;Wow! signal&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And I'm sure there were others we sent, wittingly or unwittingly. To just think how many times some TV programmes we watch were retransmitted, is mind boggling on its own. But that is beyond the point, because a whole message repetition isn't the only measure by which one can establish its artificial origins. Repetition is important, but it can be already present in a single signal session, inherent to message encoding and its contents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, SETI's &lt;a href=&quot;https://en.wikipedia.org/wiki/Lone_Signal&quot; rel=&quot;noreferrer&quot;&gt;Lone Signal&lt;/a&gt; project is transmitting 144-character messages, sort of interstellar Twitter feed, towards Gliese 526. All these messages might be different when decoded as a whole, but their headers would repeat in some logical structure, following message contents. For example, you might want to send message length first, followed by the message itself, and then terminate all the messages equally. This should be sufficient to infer its artificial (but not necessarily intelligent) origins. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-01-04T20:38:53.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3275" PostTypeId="2" ParentId="264" CreationDate="2014-01-04T21:58:16.840" Score="7" Body="&lt;p&gt;AlenSE, Erik has the heart of the answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply put, at the time of the Shuttle External Tank separation the ET has full orbital velocity along with the rest of the Shuttle.  But because the ET is not part of the Shuttle when the Shuttle circularizes its orbit with an OMS burn, the orbit of the ET intersects with the surface of the Earth at the point of the Indian Ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a fully laden Shuttle tried to circularize its orbit with the ET still attached, the OMS would have to expend approximately 35% more propellant for the burn than normal.  Off hand I do not know if the OMS has that kind of excess capacity though I believe it does.  But at the very worst I expect that if the payload in the Shuttle bay is ejected into orbit after the circularization burn that the OMS would have the total capacity necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the short answer is, I believe a fully loaded Shuttle could have carried the ET up to LEO, provided the mission of the payload required it be left behind in orbit.&lt;/p&gt;&#xA;" OwnerUserId="1390" LastActivityDate="2014-01-04T21:58:16.840" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3276" PostTypeId="1" AcceptedAnswerId="3277" CreationDate="2014-01-05T00:30:16.017" Score="8" ViewCount="950" Body="&lt;p&gt;Humanity has received &lt;a href=&quot;https://en.wikipedia.org/wiki/Wow!_signal&quot; rel=&quot;noreferrer&quot;&gt;some odd signals in the past&lt;/a&gt;. &lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_messages&quot; rel=&quot;noreferrer&quot;&gt;We've also sent out some signals ourselves&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How could we determine that a signal we'd received was artificial in origin? Or of course inversely, how could an extraterrestrial civilization determine a signal we had sent out was was artificial?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember that when I was growing up and reading space books and playing with space software on my dad's 386, the big rule was that we had to get the exact same signal twice to prove it wasn't randomly generated. However, I've heard claims that we could demonstrate artificial origin via repeated patterns or something like placing the exact message length at the start of the message, but how would we know that we hadn't just uncovered a signal that happened to start with the message length? Sure, it would be unlikely, but given enough signals set off we should get one like that eventually, particularly if you fiddle around with the base used and such.&lt;/p&gt;&#xA;" OwnerUserId="1133" LastEditorUserId="49" LastEditDate="2014-01-05T03:06:42.657" LastActivityDate="2015-12-19T07:43:23.667" Title="What evidence would be needed to determine a signal was artificial in origin?" Tags="|communication|astrobiology|data-transmission|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3277" PostTypeId="2" ParentId="3276" CreationDate="2014-01-05T01:57:25.093" Score="8" Body="&lt;p&gt;I'm going to assume radio frequency transmissions were meant in the question, since there are other possible electromagnetic wavelength signal carriers, e.g. laser communication, and all kinds of other &lt;a href=&quot;https://en.wikipedia.org/wiki/Technosignature&quot; rel=&quot;nofollow noreferrer&quot;&gt;technomarkers&lt;/a&gt;, but repetition on itself is not the best indication of intelligent communication. And neither is periodic repetition, because we know of natural radio frequency emitters that can be both repeating and periodic - &lt;a href=&quot;https://en.wikipedia.org/wiki/Pulsar&quot; rel=&quot;nofollow noreferrer&quot;&gt;pulsars&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first detected pulsar in 1967 was actually designated &lt;a href=&quot;https://en.wikipedia.org/wiki/PSR_B1919%2B21&quot; rel=&quot;nofollow noreferrer&quot;&gt;LGM-1&lt;/a&gt; (Little Green Men) by Burnell and Hewish, because it wasn't yet clear at that time that the radio signals received were a naturally occurring phenomena, yet the pulses followed sidereal time so it was confirmed they're not of local origins. And there are other naturally occurring radio signals, perhaps better said radio interferences since they don't carry any structured data and are just more-or-less random noise, like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Whistler_%28radio%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;whistlers&lt;/a&gt;, shorter of which can be difficult to distinguish one from another, and some other such noise can be periodic too, e.g. radio interferences or intermissions caused by occulting natural satellites. I explain some others in this &lt;a href=&quot;https://astronomy.stackexchange.com/a/608/13&quot;&gt;answer to the &lt;em&gt;Jupiter FM&lt;/em&gt; question on Astronomy.SE&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Better demonstrators of signals being of intelligent origins are signal clarity (strength limited to a narrow band, i.e. spectral width), if they are carrier waves (&lt;a href=&quot;http://whatis.techtarget.com/definition/polarization-wave-polarization&quot; rel=&quot;nofollow noreferrer&quot;&gt;polarized&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Frequency_modulation&quot; rel=&quot;nofollow noreferrer&quot;&gt;modulated&lt;/a&gt;) and can contain readable data. This is perhaps better explained by SETI in their FAQ answering the question &lt;a href=&quot;http://www.seti.org/faq#obs9&quot; rel=&quot;nofollow noreferrer&quot;&gt;How do you know if you've detected an intelligent, extraterrestrial signal?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The main feature distinguishing signals produced by a transmitter from&#xA;  those produced by natural processes is their spectral width, i.e. how&#xA;  much room on the radio dial do they take up? Any signal less than&#xA;  about 300 Hz wide must be, as far as we know, artificially produced.&#xA;  Such narrow-band signals are what all SETI experiments look for. Other&#xA;  tell-tale characteristics include a signal that is completely&#xA;  polarized or the existence of coded information on the signal.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;These are just some markers that the signal &lt;em&gt;might&lt;/em&gt; have been sent by some intelligent civilisation however, and the signal origins would first have to be confirmed to have originated from outside our Solar system (or in the frequency range and position within the Solar system where we wouldn't expect any of our deep space probes to communicate). This can be done like already mentioned in the story about the first pulsar being detected, with an extrasolar signal following &lt;a href=&quot;https://en.wikipedia.org/wiki/Sidereal_time&quot; rel=&quot;nofollow noreferrer&quot;&gt;sidereal time&lt;/a&gt;, and using &lt;a href=&quot;https://en.wikipedia.org/wiki/Parallax&quot; rel=&quot;nofollow noreferrer&quot;&gt;parallax&lt;/a&gt; (triangulation) to establish distance to the signal origins. And signal strength can tell us direction it is being transmitted from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally of course, the signal would be decoded and we would detect some intelligent structure in it even before we'd be able to actually understand its message, for example use of metadata in the packet headers that precede actual data being sent and describe some of its properties (e.g. message length), use of BOF/EOF (Beginning of File / End of File) markers, use of large primes that might be used for data encryption, and so on. Any such data structures would clearly be of intelligent origins and all we'd have to establish is that they're not being transmitted by us. Which includes radio signal echo of past transmissions as they &lt;em&gt;bounce&lt;/em&gt; back towards us off some celestials in our cosmic neighborhood.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Single such transmission with only a few such signatures might not be enough though, you're right in your question that many of such signatures could be put down to chance, a random fluke radio transmission that was of natural origins, and somehow managed to confuse us into believing it might be of some distant civilisation. It wouldn't be the first time (LGM-1) either, so we would be looking for a certain total complexity, either in a single message, or many successive messages. At some point, we'd be able to either decode / decrypt the message, parts of it, or conclude it can't possibly be random noise, because its total signature's &lt;a href=&quot;https://en.wikipedia.org/wiki/Entropy_%28information_theory%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shannon entropy&lt;/a&gt; is too small in relation to its total complexity (internal repetition, i.e. its compressibility is a good measure of that) and it simply must be a data carrier signal.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;With regards to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Wow!_signal&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wow! signal&lt;/a&gt; specifically, since we've never been able to detect any repeat transmissions and no other sense has yet been made out of the single one, besides that it was a curiously narrow-band and strong &lt;a href=&quot;https://en.wikipedia.org/wiki/Signal-to-noise_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;SNR&lt;/a&gt; signal, if we were able to obtain more data, that would obviously help determine its nature with greater certainty. For the time being though we don't have any conclusive evidence to explain it, and Jerry R. Ehman that circled that print-out and wrote &lt;em&gt;Wow!&lt;/em&gt; next to it said it best: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://www.bigear.org/wow20th.htm#vast&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;&quot;I choose not to draw vast conclusions from 'half-vast' data&quot;&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2015-12-19T07:43:23.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3278" PostTypeId="2" ParentId="3145" CreationDate="2014-01-05T23:31:47.460" Score="4" Body="&lt;p&gt;First, no that's not the reason it is one way, though it is something that is a matter of concern that has to be thought about. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's for reasons of cost and technology. Mars is a much harder place to land on and take off from than the Moon. That's because of the rocket equation&#xA;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Delta-vs_between_Earth.2C_Moon_and_Mars&quot; rel=&quot;nofollow&quot;&gt;delta v&lt;/a&gt; to escape from Mars to orbit is 4.1 km / sec. To escape from the Moon is 1.6 km / sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's only 2.5625 times faster. But because of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equatio&quot; rel=&quot;nofollow&quot;&gt;rocket equation&lt;/a&gt; it makes a huge difference. You have to multiply the mass you need, to get into orbit by e^(deltaV/ExhaustV) to get the mass ratio. So if you multiply deltaV by 2.5625, keeping everything else the same, that means the mass ratio is not just multiplied by 2.5625, it's actually taken to the power of 2.5625.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example, lunar module ascent stage had dry mass of 2.15 tons and including fuel, 4.7 tons. So the ratio was 2.186. For Mars you have to take that to the power 2.5625 so that would be (4.7/2.15)^2.5625 or 7.412 approx. That would make the total including fuel a little short of 16 tons. That's a rough first idea of course. Nothing like as difficult as a Saturn V but much much harder than a lunar module.&#xA;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Rocket_mass_ratio_versus_delta-v.svg/640px-Rocket_mass_ratio_versus_delta-v.svg.png&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Critics of Mars One say their budget to land on Mars is already unrealistically low given the number of new technologies that would need to be developed for the mission. Landing on Mars is far harder than landing anywhere in the solar system - because of the high gravity again. The thin atmosphere lets you slow down from Mach 4 down to Mach 1 but is not enough to get you to a comfortable soft landing, as even with parachutes, at least the largest tested so far, you hit the ground at over 200 mph. That's why you have the need for technologies like the sky crane. It also makes landings far less accurate than they are on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So their idea is to make it easier by leaving out the ability to return, but it's not clear that that does reduce the level of technology that significantly as it's landing on the surface safely that's by far the hardest part of a journey to Mars. But it is one less thing to develop.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However - there is indeed significant risk of contamination both ways. Several studies and papers back this up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some back contamination issues are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Pathogens can evolve independently of any higher animal host e.g. as a disease of microbes (as is true for instance for Legionaire's disease - a disease of amoeba that jumped straight to humans quite recently).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;DNA of archaea can be shared via gene transfer agents - if those got into the sea, then one study found that almost half the &#xA;micro-organisms in a sample of ordinary sea water took up the genes in a GTA that conferred antibiotic resistance by the next day, even with no selection advantage to take it up.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A new micro-organism could disrupt natural cycles, displace other microbes in an ecosystem, be a disease of our food, or a disease of other organisms on land or in the sea. So it doesn't need to be a disease of humans to be a problem.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In the most interesting case, independently evolved, it may not be based on DNA. In that case, then the XNA (so called) could be hazardous if it out competes DNA or can occupy niches not occupied by DNA. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Also if it is XNA, then our immune systems and those of other creatures may not recognize the chemicals it produces as the signature of life. This was a concern raised by the Nobel prize winning microbiologist Joshua Lederberg.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In that last case, it could just colonize parts of our bodies, our skin, intestines, or whatever and our bodies only respond to any physical trauma it causes and are not adapted to try to do anything about their cause. &#xA;It could harm us directly, or it could produce chemicals that are harmful to us as byproducts. It's in the interest of microbes that colonize our bodies to keep us alive, as their host, indeed many eventually adapt to be symbiotes, e.g. the microbes we use to digest food, essential to our survival.  So it's not in a microbe's interest to kill us, but could easily do so just because it is not adapted to us, or us to them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - life from Mars - could co-exist peacefully, be so harmless we could eat kgs with no harm, could be unable to survive on Earth - but it could also be hazardous to Earth life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has been meteorite exchange between Mars and Earth but most of that happened billions of years ago and most microbes probably can't be transmitted between planets on meteorites. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is believed that some could (though not known for sure if any did) - but there is plenty of room for the possibility that life on Mars is significantly different from Earth life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other way - if we introduce Earth life to Mars then we risk contaminating a pristine planet irreversibly. This closes off any future options that might involve a Mars with no Earth life or with different lifeforms there. I would argue strongly that we shouldn't do that until we know very well exactly what Mars is like and have a good idea of the effect of our actions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might make any Martian present day life extinct and confuse the study of past life on Mars. Also, it could cause problems for future terraforming or whatever we might decide to do with Mars, if anything.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read a lot more about it in my articles on the subject:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.science20.com/robert_inventor/case_for_moon_new_positive_future_for_humans_in_space_open_ended_with_planetary_protection_at_its_heart-169902&quot; rel=&quot;nofollow&quot;&gt;Case For Moon - New Positive Future For Humans In Space - Open Ended With Planetary Protection At Its Heart&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.science20.com/robert_inventor/need_caution_early_mars_sample_return_opinion_piece-113913&quot; rel=&quot;nofollow&quot;&gt;Need For Caution For An Early Mars Sample Return - Opinion Piece&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.science20.com/robert_inventor/trouble_terraforming_mars-126407&quot; rel=&quot;nofollow&quot;&gt;Trouble with terraforming Mars&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and other articles in my column.&#xA;In forward direction, introducing any life to Mars would hugely confuse attempts at scientific investigation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://www.science20.com/robert_inventor/blog/could_microbes_transferred_spacecraft_harm_mars_or_earth_zubrins_argument_revisted-121110&quot; rel=&quot;nofollow&quot;&gt;Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisted&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On searching for life by looking for changes in the environment caused by life - that's okay if life is abundant. But it's no good if it is rare.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Life on Mars is likely to be rarer than life in the Atacama deserts or McMurdo dry valleys - and there - life is so hard to spot that in some places life was only detected for the first time in the last decade.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are talking about microbes that may have lifetimes of a thousand years or more, slowly metabolizing, hardly making any changes to their environment, just a few of them eeking out a living very much on the edge of what's possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have instruments that can detect life even in such challenging environments as those, and they could work on Mars also - able to detect a single amino acid in a gram of soil for instance, and find its chirality. For that matter the Viking labelled release - updated to take account of the confusing chemistry on Mars - is also exquisitely sensitive able to detect metabolic activity of a few non reproducing microbes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But could be a long search as there are many places on Mars where we can search, at least four different main types of habitat - the deliquescing salts (within top two cms of soil), the warm seasonal flows (whatever they are, nobody knows for sure but seems pretty convincing is water in some form), the solid state greenhouse effect melting layers of ice - it is same thing that causes the Martian geysers by evaporating dry ice - same thing could also melt ice and may be a habitat and may explain the dark dune spots - and then also life may be able to live using photosynthesis and just the humidity of the night time air even in equatorial regions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;May be life, very low concentration, in many places. Or may be very rare just here and there. And you may have habitats that life could survive in which it just hasn't reached  yet, because on Mars it would surely take longer for life to spread to new habitats, and instead of colonizing within weeks or months of habitat formation, may not happen for thousands or even millions of years depending how the life spreads and reproduces and if it can e.g. be spread in the Mars dust storms, say, or needs some other rarer or slower method of transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.science20.com/robert_inventor/will_nasas_sample_return_answer_mars_life_questions_need_for_comparison_with_in_situ_search-155893&quot; rel=&quot;nofollow&quot;&gt;On reverse contamination&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.science20.com/robert_inventor/how_to_keep_earth_safe_samples_from_mars_sterilized_or_returned_to_above_geostationary_orbit_op_ed-166448&quot; rel=&quot;nofollow&quot;&gt;More on reverse contamination&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.science20.com/robert_inventor/case_for_moon_new_positive_future_for_humans_in_space_open_ended_with_planetary_protection_at_its_heart-169902&quot; rel=&quot;nofollow&quot;&gt;On the general approach of Moon first&lt;/a&gt; and taking an open ended approach with planetary protection and reversible biological exploration as core principles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edited some more, Robert)&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="3038" LastEditDate="2016-04-14T18:10:05.567" LastActivityDate="2016-04-14T18:10:05.567" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3280" PostTypeId="2" ParentId="2310" CreationDate="2014-01-06T11:55:22.753" Score="7" Body="&lt;p&gt;The debris ranges from complete, defunct satellites to flecks of paint. Satellites are put into a graveyard orbit or deorbited at the end of their lives, so they're not a concern. That leaves random crap (bits of insulation etc. coming off during stage separation, and the wreckage of a couple of collisions), which would have to be reduced to raw material to be useful again. See @Pearson's answer.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves the defunct satellites. They have been decommissioned usually because their useful life is over due to malfunctioning components etc. Once their power supply fails and the interior can no longer be kept warm at night, they're subject to huge thermal cycles which quickly break all sorts of components.&lt;br&gt;&#xA;This means that at best, you have individual systems that could possibly be reused. You'd have to disassemble the entire satellite, test everything for cracks, etc. You're effectively building a new satellite, with the added difficulty of having to build it in zero gravity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a famous example of a satellite being reused: the Hubble telescope. The servicing missions extended Hubble's lifetime, but at such a huge cost that we could have launched a new telescope for the cost of one or two servicing missions. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-01-06T11:55:22.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3281" PostTypeId="1" AcceptedAnswerId="3334" CreationDate="2014-01-06T12:36:45.430" Score="10" ViewCount="788" Body="&lt;p&gt;The LLRV (Lunar Landing Research Vehicle) was used to develop the LLTV (Lunar Landing Training Vehicle), but were there design differences between the two? Were any major lessons learned from the LLRV that resulted in design changes for the LLTV?&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="49" LastEditDate="2014-01-06T14:26:44.713" LastActivityDate="2014-01-08T17:46:28.230" Title="What were the differences between the LLRV and LLTV?" Tags="|crewed-spaceflight|apollo-program|nasa|training|lltv|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3282" PostTypeId="2" ParentId="3264" CreationDate="2014-01-06T15:02:14.593" Score="12" Body="&lt;p&gt;All laboratory animals are sent and returned together, so far as I've ever heard. The animals are part of an experiment, and the experiment is not brought back piecemeal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, though the dead animals won't be 'floating around the station' I do wonder how the body is preserved before shipment down. Presumably, decomposition could be an issue.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-01-06T15:02:14.593" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3283" PostTypeId="2" ParentId="3267" CreationDate="2014-01-06T20:34:54.760" Score="2" Body="&lt;p&gt;Even with the most generous assumptions, a certain minimum partial pressure of Oxygen is needed to keep someone alive.  Formally, I believe this is &lt;a href=&quot;http://en.wikipedia.org/wiki/Hypoxia_%28environmental%29&quot; rel=&quot;nofollow&quot;&gt;Hypoxia&lt;/a&gt;.  There's obviously a certain range that we can tolerate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mount Everest has about 7 kPa of Oxygen partial pressure, and this is almost universally deadly.  However, the total air pressure is still around 33.7 kPa.  That means that we can up the partial pressure of Oxygen by increasing its ratio compared to other gases.  This is effectively what Oxygen tanks do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is different, because its atmospheric pressure is lower than even our minimum required partial pressure of Oxygen.  You are correct that the habitat will eventually reach equilibrium with the outside, so gas will only transfer through diffusion around the hole after that.  But even if you completely saturated the habitat with Oxygen, this would not be sufficient for a person to survive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For places with sufficient pressure, you can make due with a suit that isn't a pressure suit and tanks.  Unfortunately, the Martian atmosphere is too thin, by over an order of magnitude.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-01-06T20:34:54.760" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3284" PostTypeId="1" AcceptedAnswerId="3338" CreationDate="2014-01-06T21:56:20.633" Score="4" ViewCount="1701" Body="&lt;p&gt;While listening to the webcast for flight 8 of Falcon 9 (Thiacom-8 mission), they did a poll of all controllers/stations at T-13:00.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could guess a number of them, but wondering what the full set is. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I caught &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;FTS  Flight Termination System&lt;/li&gt;&#xA;&lt;li&gt;MM  Mission Manager&lt;/li&gt;&#xA;&lt;li&gt;GNC  Guidance and Navigation Control&lt;/li&gt;&#xA;&lt;li&gt;GC Ground Control&lt;/li&gt;&#xA;&lt;li&gt;VC ??&lt;/li&gt;&#xA;&lt;li&gt;CE  ??&lt;/li&gt;&#xA;&lt;li&gt;OSM ??&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What were the rest, and what do they stand for?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-01-09T11:55:17.827" Title="On a Falcon9 launch at T-13 they poll all stations, what are all the stations they poll?" Tags="|spacex|falcon-9|launch-sequence|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3285" PostTypeId="1" AcceptedAnswerId="3287" CreationDate="2014-01-06T22:13:07.490" Score="10" ViewCount="3892" Body="&lt;p&gt;On the SpaceX mission I hear periodic &quot;&lt;em&gt;Downrange distance: N kilometers&lt;/em&gt;&quot; for various values of N (initially 1, then 3, then 383, 920, 1500...).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the meaning of this value?&lt;/p&gt;&#xA;" OwnerUserId="1399" LastEditorUserId="51" LastEditDate="2017-03-30T13:56:36.107" LastActivityDate="2017-04-29T02:50:43.123" Title="What is &quot;downrange distance&quot;?" Tags="|launch|spacex|falcon-9|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3286" PostTypeId="1" CreationDate="2014-01-06T22:23:04.783" Score="3" ViewCount="1060" Body="&lt;p&gt;When I was watching the F-9 Flight 8 they showed some footage of the launcher on the pad. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I grabbed this screen grab that looks like they might have added to the T/E as as shoulders for the F-H launches. (This is from the SES launch campaign).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jlgcZ.jpg&quot; alt=&quot;Screen grab of wider T/E&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though I found a shot of the base of the T/E in the hangar, and there is not yet 'room' for two more cores on the blast pad at the base.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/NPhQW.jpg&quot; alt=&quot;Base of TE in hangar&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually that is a Dragon on the top (You can see the solar panels) so that is a much earlier launch and in fact, the shoulders you can see above in the picture are missing.  So a useful picture still.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;F-9 CRS-3 mission photo, focusing on the landing legs, shows the lower shoulders really clearly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4YR32.jpg&quot; alt=&quot;F9 CRS3 on the pad&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you look at the base, you can also see the wider base.  So looks like much work has been done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the Jun 20, 2014 ORb2 launch attempt, SpaceX published a couple of shots, where you can see the T/E, and it looks a bit different again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/q1nAm.jpg&quot; alt=&quot;F9 flight 9, Orb2 mission on the pad&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/7srES.jpg&quot; alt=&quot;F9 flight 9, Orb2 mission in the hangar&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that the side shoulders now bend back in, whereas in the earlier shots that is not obvious.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-06-20T15:01:12.377" LastActivityDate="2017-02-05T14:33:16.510" Title="Is the LC-40 SpaceX Transport/Erector updated for Falcon Heavy already?" Tags="|spacex|falcon-9|launcher-integration|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3287" PostTypeId="2" ParentId="3285" CreationDate="2014-01-06T22:32:33.580" Score="13" Body="&lt;p&gt;This is simply the distance across the ground from the launch site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the mission has reached orbit these distances are less relevant so they typically are not announced.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2014-01-06T22:32:33.580" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3288" PostTypeId="1" AcceptedAnswerId="5109" CreationDate="2014-01-06T22:41:47.023" Score="13" ViewCount="861" Body="&lt;p&gt;In the web cast for the Falcon 9 Flight 8 launch campaign they showed an alternate hangar where spacecraft can be prepared and fueled, before being brought to the SpaceX hangar on LC-40 for integration with the launcher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I grabbed a screen shot of the facility they were referring to as 'down the road'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/39U6y.jpg&quot; alt=&quot;Other Hangar for SpaceX at the Cape&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is this alternate hangar they are referring too, and what are they using it for?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-07-23T11:50:46.390" LastActivityDate="2020-12-06T01:56:13.517" Title="What is the alternate hangar SpaceX has available at the Cape?" Tags="|spacex|falcon-9|launch-site|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3289" PostTypeId="1" AcceptedAnswerId="3302" CreationDate="2014-01-06T23:11:33.517" Score="8" ViewCount="973" Body="&lt;p&gt;A quick follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/3281/what-were-the-differences-between-the-llrv-and-lltv&quot;&gt;What were the differences between the LLRV and LLTV?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;These craft were provided with a &lt;a href=&quot;http://en.wikipedia.org/wiki/Ejection_seat#Zero-zero_ejection_seat&quot; rel=&quot;nofollow noreferrer&quot;&gt;zero-zero ejection&lt;/a&gt; seat. The above referenced article on Wikipedia mentions Neil Armstrong apparently had to exercise that option back in 1968 too. &lt;em&gt;I vaguely recall mention that a pilot experiencing that kind of acceleration, and relatively hard landing is usually not put back in the cock-pit in a hurry&lt;/em&gt;; I could be wrong though ...&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How many minutes of &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Landing_Research_Vehicle#Lunar_Sim_Mode&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Sim&lt;/a&gt; did Neil Armstrong get in total before, and after his ejection?&#xA;&lt;ul&gt;&#xA;&lt;li&gt;In comparison, did the other crews get significantly more time in these vehicles?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;What was the policy on graduating the back-up crew if the planned crew had to eject from these contraptions?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2024-03-11T15:04:03.393" LastActivityDate="2024-03-11T15:04:03.393" Title="How many minutes of Lunar Sim did Neil Armstrong get?" Tags="|crewed-spaceflight|apollo-program|nasa|training|lltv|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3290" PostTypeId="1" AcceptedAnswerId="3305" CreationDate="2014-01-07T09:31:18.573" Score="9" ViewCount="7601" Body="&lt;p&gt;What is the approximate temperature and what factors cause this variation?&lt;/p&gt;&#xA;" OwnerUserId="1401" LastEditorUserId="68" LastEditDate="2014-01-07T23:27:19.553" LastActivityDate="2014-10-20T18:30:27.367" Title="What is the temperature variation between the lunar day and night on the moon?" Tags="|the-moon|temperature|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3291" PostTypeId="1" AcceptedAnswerId="3304" CreationDate="2014-01-07T09:39:37.140" Score="6" ViewCount="1082" Body="&lt;p&gt;There are articles that state that silver has been found on the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cosmosmagazine.com/news/moons-treasure-chest-includes-silver/&quot;&gt;moons treasure chest includes silver&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://news.nationalgeographic.com/news/2010/10/101021-science-space-moon-crash-water-silver-mercury-nasa-lcross/&quot;&gt;Moon's Silver Hints at Lunar Water Origins&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.independent.co.uk/news/science/silver-found-on-moon-ndash-and-you-thought-it-was-made-of-cheese-2113338.html&quot;&gt;Silver found on Moon – and you thought it was made of cheese&lt;/a&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How could such resources be estimated?&lt;/p&gt;&#xA;" OwnerUserId="1401" LastActivityDate="2016-10-02T18:40:19.543" Title="What is an approximation of how much silver could exist on the moon?" Tags="|the-moon|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3293" PostTypeId="2" ParentId="3234" CreationDate="2014-01-07T14:44:04.730" Score="10" Body="&lt;p&gt;The outside of the Soyuz is covered in multilayer &lt;a href=&quot;http://www.russianspaceweb.com/soyuz_tech.html&quot; rel=&quot;noreferrer&quot;&gt;vacuum-screen thermal insulation&lt;/a&gt;. Its Russian acronym is &lt;a href=&quot;http://www.collectspace.com/ubb/Forum31/HTML/000340.html&quot; rel=&quot;noreferrer&quot;&gt;ЭВТИ&lt;/a&gt;, long for экранно-вакуумная теплоизоляция or Shield Vacuum Thermal Insulation. It apparently is in many layers of metalized film and fiberglass cloth.&lt;br&gt;&#xA;Its purpose is to protect the modules during ascent and orbit. It doen't seem to be designed to survive reentry: in photos of the descent module post touchdown, this material is not intact.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Dcnq1.jpg&quot; alt=&quot;Soyuz descent module post touchdown&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-01-07T14:44:04.730" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3294" PostTypeId="1" AcceptedAnswerId="5936" CreationDate="2014-01-07T15:44:10.843" Score="9" ViewCount="878" Body="&lt;p&gt;Looking at the first six months of 2014's launch schedule for spacecraft visiting the ISS we have:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Jan 8 - Orbital Cygnus - Node 2 zenith CBM&lt;/li&gt;&#xA;&lt;li&gt;Feb 5 - Progress - Russian segment port&lt;/li&gt;&#xA;&lt;li&gt;Feb 22 - SpaceX Dragon - Node 2 zenith CBM&lt;/li&gt;&#xA;&lt;li&gt;Mar 25 - Soyuz - Russian segment&lt;/li&gt;&#xA;&lt;li&gt;Apr 9 - Progress - Russian segment port&lt;/li&gt;&#xA;&lt;li&gt;May 1 - Orbital Cygnus - Node 2 zenith CBM&lt;/li&gt;&#xA;&lt;li&gt;May 28 - Soyuz - Russian segment&lt;/li&gt;&#xA;&lt;li&gt;Jun 6 - SpaceX Dragon - Node 2 zenith CBM&lt;/li&gt;&#xA;&lt;li&gt;Jun 17 - ESA ATV - Russian segment (aft Zvezda port)&lt;/li&gt;&#xA;&lt;li&gt;Jul 1 - JAXA HTV - Node 2 zenith CBM&#xA;Jul 24 - Soyuz - Russian segment&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That is a fair bit of traffic.  (Now to be fair, these are probably the last ATV missions, and who knows how long JAXA will keep up the HTV flying, it is so expensive).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know the Russian segment uses an automated docking system (&lt;a href=&quot;http://en.wikipedia.org/wiki/Kurs_%28docking_system%29&quot;&gt;Kurs&lt;/a&gt;) and the ATV buys Kurs from the Russians for the ATV.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet someone still has to be on hand in case a manual override is required. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Berthing of the Dragon, HTV, and Cygnus is very manpower intensive, since the spacecraft gets to about 30 meters (?  feet?) away, and the CanadaArm2 reaches out to grab, and then retracts/rotates/etc as needed to berth the craft to the CBM port on the bottom of Node 2 module.  (Turns out the CBM port on the top, is the backup, but requires that CanadaArm2 relocate itself first so it can move a craft from the earth facing to space facing port.  The plan is to move the PLM on the zenith port of Node 1 to forward facing CBM port on Node 3 to free up a real spare berthing port.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember reading Allen Steele's book &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0441498515&quot;&gt;Orbital Decay&lt;/a&gt;, where there is a scene where a module docks, and people sneak aboard Freedom (The ISS predecessors name, when he first wrote that book in the 80's or 90's) and the astronauts aboard do not 'notice' initially.  I remember reading how unlikely that seemed.  But now I wonder, with so many craft arriving, each year, and no doubt automation being pushed as much as possible, if it will become a real possibility. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, the specific question is perhaps two fold. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Russian segment it is clearly different than the US Segment.  Additionally, all current US Segment cargo craft berth, which requires CanadaArm2 time.  But future manned spacecraft to the US segment will all dock (at a PMA, on the forward end of Node 2, and the backup will be the PMA from Node 3 move to the space facing CBM port on Node 2) which in principle should be more automated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how does a visiting craft disrupt operations on the station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does a Russian segment visitor disrupt much? (I.e. Perhaps only one crew member needs to be dedicated to watching the incoming craft, ready for manual override).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does a US segement visitor disrupt much?  (Clearly at least one crew member is required to drive the CanadaArm2 over several hours of work).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-14T13:38:44.177" Title="How much crew time is required for visiting craft, at the ISS?" Tags="|iss|docking-port|visiting-craft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3295" PostTypeId="2" ParentId="2327" CreationDate="2014-01-07T15:44:43.140" Score="1" Body="&lt;p&gt;The list of planned deep space missions (&lt;a href=&quot;http://www.jpl.nasa.gov/missions/&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt;, &lt;a href=&quot;http://sci.esa.int/sre-fp/31586-sre-fp-solar-system-and-robotic-exploration-missions-section/&quot; rel=&quot;nofollow&quot;&gt;ESA&lt;/a&gt;) does not contain any missions that will descend into a gas giant's atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest thing is the joint NASA/ESA &lt;a href=&quot;http://en.wikipedia.org/wiki/Titan_Saturn_System_Mission&quot; rel=&quot;nofollow&quot;&gt;Titan Saturn System Mission&lt;/a&gt;, which is still at the proposal stage. This will explore Titan's atmosphere. The &lt;a href=&quot;http://en.wikipedia.org/wiki/TiME_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Titan Mare Explorer&lt;/a&gt; lander will include cameras that would work during the descent. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-01-08T08:18:22.677" LastActivityDate="2014-01-08T08:18:22.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3296" PostTypeId="5" CreationDate="2014-01-07T16:19:41.177" Score="0" Body="&lt;p&gt;The RL-10 has a long and storied history. It is almost always used as an upper stage engine.  However it was used as the main engines of the DC-X vertical takeoff/landing technology demonstrator. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engine uses LOX/LH2 and is fairly high performing compared to other upper stage engines.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T16:19:41.177" LastActivityDate="2014-01-07T16:19:41.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3297" PostTypeId="4" CreationDate="2014-01-07T16:19:41.177" Score="0" Body="LOX/LH2 engine, used as on the upper stage on many US launchers." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T16:19:41.177" LastActivityDate="2014-01-07T16:19:41.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3298" PostTypeId="5" CreationDate="2014-01-07T16:23:39.853" Score="0" Body="&lt;p&gt;The ISS is serviced by many space craft, this term addresses them all, as a group.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Cargo&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Russian Progress&lt;/li&gt;&#xA;&lt;li&gt;ESA ATV (last launch in 2014)&lt;/li&gt;&#xA;&lt;li&gt;JAXA HTV&lt;/li&gt;&#xA;&lt;li&gt;SpaceX Dragon&lt;/li&gt;&#xA;&lt;li&gt;Orbital Cygnus&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Crew&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Russian Soyuz&lt;/li&gt;&#xA;&lt;li&gt;US Space Shuttle (Retired)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Future crew vehicles&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Blue Origin Space Vehicle&lt;/li&gt;&#xA;&lt;li&gt;Boeing CST-100 &lt;/li&gt;&#xA;&lt;li&gt;SNC Dream Chaser&lt;/li&gt;&#xA;&lt;li&gt;SpaceX Dragon Rider&lt;/li&gt;&#xA;&lt;li&gt;US Orion&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T16:23:39.853" LastActivityDate="2014-01-07T16:23:39.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3299" PostTypeId="4" CreationDate="2014-01-07T16:23:39.853" Score="0" Body="A generic term for all the various spacecraft that are used to service the ISS. Some for cargo, some for crew.  Many more such craft are under development." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T16:23:39.853" LastActivityDate="2014-01-07T16:23:39.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3300" PostTypeId="5" CreationDate="2014-01-07T16:24:06.370" Score="0" Body="" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T16:24:06.370" LastActivityDate="2014-01-07T16:24:06.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3301" PostTypeId="4" CreationDate="2014-01-07T16:24:06.370" Score="0" Body="Boeing is a very large manufacture of aerospace and space equipment." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T16:24:06.370" LastActivityDate="2014-01-07T16:24:06.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3302" PostTypeId="2" ParentId="3289" CreationDate="2014-01-07T18:54:37.860" Score="11" Body="&lt;p&gt;Well, we can make some rough approximation based on the information in &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Landing_Research_Vehicle#Lunar_Landing_Training_Vehicle.5B5.5D&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The test program of 18 flights, all flown by Bud Ream, was successfully completed on June 2, and the Board finally gave approval on June 30, 1969 for Neil Armstrong to resume LLTV flights. In the 16 days remaining before the Apollo 11 launch Armstrong was able to complete his LLTV flight training. He commented after his return:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;“Eagle (the Lunar Module) flew very much like the Lunar Landing Training Vehicle which I had flown &lt;strong&gt;more than 30 times&lt;/strong&gt; at Ellington Air Force Base near the Space Center. I had made from 50 to 60 landings in the trainer, and the final trajectory I flew to the landing was very much like those flown in practice. That of course gave me a good deal of confidence – a comfortable familiarity.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Wikipedia also says that the LLRV had an &quot;endurance&quot; of 10 minutes. Assuming that this means a single flight could take up 10 minutes, and that the same number holds for the LLTV as well, this gives about 300 minutes. &lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2014-01-07T18:54:37.860" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3303" PostTypeId="2" ParentId="2327" CreationDate="2014-01-07T19:23:59.643" Score="4" Body="&lt;p&gt;As noted, there are no planned gas giant probes at the moment, at least not beyond an unfunded conceptual stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Images taken on approach and just before entry would be pretty cool and might even have some science value to provide weather context for the entry location.  However you would need a much higher rate telecom system than what the probe itself would have.  Such probes are designed to return data down to several bars pressure, where the data rates through the atmosphere are highly constrained.  I could imagine a separate imaging stage for this purpose that is discarded just before entry, or that approaches in parallel with the probe with its own high-rate relay radio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's not much point in images during or after entry.  They wouldn't see anything, other than radiation from the entry plume, and after that completely uniform nothingness.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-01-07T19:23:59.643" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3304" PostTypeId="2" ParentId="3291" CreationDate="2014-01-07T19:30:58.777" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Abundance_of_elements_on_Earth#Abundance_of_elements_in_the_Earth&quot; rel=&quot;nofollow&quot;&gt;This Wikipedia page&lt;/a&gt; shows the abundance of silver on earth as $50$ parts per billion by mass.  One could assume that the moon is not too far off that.  Multiply by the mass of the moon $7.3477 \cdot 10^{22} \text { kg } \cdot 5 \cdot 10^{-8}\approx 4\cdot 10^{15}$kg&lt;/p&gt;&#xA;" OwnerUserId="176" LastEditorUserId="176" LastEditDate="2014-01-08T00:06:31.523" LastActivityDate="2014-01-08T00:06:31.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3305" PostTypeId="2" ParentId="3290" CreationDate="2014-01-07T19:57:22.687" Score="10" Body="&lt;p&gt;&lt;a href=&quot;http://www.space.com/18175-moon-temperature.html&quot; rel=&quot;nofollow&quot;&gt;Temperatures on the Moon&lt;/a&gt; vary between extremes of 120 °C during the lunar day and -150 °C during the lunar night. Some craters near the poles have areas that are in permanent shadow, these can reach temperatures down to -247 °C.&lt;br&gt;&#xA;The moon does not have a permanent &quot;dark side&quot;. The moon rotates on its axis once every 28 days, so every part of the moon has a day-and-night pattern. The American expression &quot;the dark side of the Moon&quot; refers to the &lt;em&gt;far&lt;/em&gt; side of the moon, which we never see from Earth.&lt;br&gt;&#xA;The temperature range is so different from Earth due to these factors: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the Moon has no atmosphere. Earth's atmosphere reflects some of the Sun's energy during the day and traps the heat during the night. &lt;/li&gt;&#xA;&lt;li&gt;the slow rotation results in very long days and nights, i.e. a long period of uninterrupted heating followed by a long period of uninterrupted cooling. &lt;/li&gt;&#xA;&lt;li&gt;The moon consists mostly of rock with a low heat capacity, which will heat up and cool down quickly. Earth's oceans have a high heat capacity, so they take much longer to heat up. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As on Earth, the surface temperature changes with the &lt;a href=&quot;http://www.lunarpedia.org/index.php?title=Lunar_Temperature&quot; rel=&quot;nofollow&quot;&gt;latitude&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The temperature below the Moon's surface &lt;a href=&quot;http://www.asi.org/adb/m/03/05/average-temperatures.html&quot; rel=&quot;nofollow&quot;&gt;quickly stabilizes&lt;/a&gt;. According to this link, only a meter below the surface, the temperature is a constant -35 °C. Note that this contradicts the previous link, which says that the subsurface temperature also changes with the latitude.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-01-10T09:47:35.733" LastActivityDate="2014-01-10T09:47:35.733" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3306" PostTypeId="5" CreationDate="2014-01-07T21:19:05.467" Score="0" Body="&lt;p&gt;The Russians built a space shuttle look alike, which only flew once, Buran.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core difference between the US Shuttle and Buran is the location of the main engines.  The US Shuttle carries SSME's on the shuttle itself, into orbit and returns them to earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Buran model does not have major engines on the orbiter itself, rather they are on the core of the Energia booster rocket.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:19:05.467" LastActivityDate="2014-01-07T21:19:05.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3307" PostTypeId="4" CreationDate="2014-01-07T21:19:05.467" Score="0" Body="Questions about the Russian space shuttle named Buran." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:19:05.467" LastActivityDate="2014-01-07T21:19:05.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3308" PostTypeId="5" CreationDate="2014-01-07T21:21:06.083" Score="0" Body="&lt;p&gt;The FGB (Known as Zarya) was the first module of the ISS launched.  Node 1 (Unity) was attached on a space shuttle mission on the front end, with the Service Module (known as Zvezda) docked to the aft port.  There forward element has one addition earth facing docking port, currently occupied by the Rassvet module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The FGB provided orbital control for the original ISS configuration.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:21:06.083" LastActivityDate="2014-01-07T21:21:06.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3309" PostTypeId="4" CreationDate="2014-01-07T21:21:06.083" Score="0" Body="Questions about the Functional Block Unit (FGB a Russian acronym), that is the first module of the ISS launched." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:21:06.083" LastActivityDate="2014-01-07T21:21:06.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3310" PostTypeId="5" CreationDate="2014-01-07T21:27:00.527" Score="0" Body="&lt;p&gt;Launch Escape Systems (LES) come in two basic modes.  Tractor and pusher.  A tractor LES has the rockets atop and they pull the craft away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The alternate would be a pusher LES that has the rockets below, and pushes the craft away when needed.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:27:00.527" LastActivityDate="2014-01-07T21:27:00.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3311" PostTypeId="4" CreationDate="2014-01-07T21:27:00.527" Score="0" Body="Launch Escape Systems (LES) come in two basic modes.  Tractor and pusher.  A tractor LES has the rockets atop and they pull the craft away." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:27:00.527" LastActivityDate="2014-01-07T21:27:00.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3312" PostTypeId="5" CreationDate="2014-01-07T21:29:17.550" Score="0" Body="&lt;p&gt;In the context of the ISS there are two methods for visiting craft to connect to the station. Berthing is used by the Dragon, Cygnus, and HTV cargo space craft. They approach the station, hold their location and the CanadArm2 picks up the vehicle and berths it to a CBM (Common Berthing Port) which has a wider door than a PMA (Pressurized Mating Adapter). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The manned vehicles for the US Segment planned will all dock to a PMA, without requiring intervention from the station in the same way. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:29:17.550" LastActivityDate="2014-01-07T21:29:17.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3313" PostTypeId="4" CreationDate="2014-01-07T21:29:17.550" Score="0" Body="Questions related to berthing a craft or module to the ISS.  A distinction is made between berthing and docking." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:29:17.550" LastActivityDate="2014-01-07T21:29:17.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3314" PostTypeId="5" CreationDate="2014-01-07T21:31:31.247" Score="0" Body="&lt;p&gt;The Russian segment of the ISS has 4 docking ports available for visiting craft.  Poisk is a docking compartment docked to the nadir (up/space facing) port of the Zvezda module. This provides additional storage space and an additional airlock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is mostly an identical clone of the Pirs module, which is docked to the zenith (earth facing) port of Zvezda. (Rassvet is docked to the zenith port of Zarya and is a different module design)&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:31:31.247" LastActivityDate="2014-01-07T21:31:31.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3315" PostTypeId="4" CreationDate="2014-01-07T21:31:31.247" Score="0" Body="The Poisk module is a docking compartment on the nadir port of Zvezda on the Russian segment of the ISS." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:31:31.247" LastActivityDate="2014-01-07T21:31:31.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3316" PostTypeId="5" CreationDate="2014-01-07T21:33:03.707" Score="0" Body="&lt;p&gt;SpaceX is developing a new engine, using Methane and LOX, with a thrust of around   2,200 kN (500,000 lbf). The engine will be used in SpaceX's upcoming Starship rocket, in both lower and upper stage (with different nozzles).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="37129" LastEditDate="2020-08-18T11:52:43.330" LastActivityDate="2020-08-18T11:52:43.330" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3317" PostTypeId="4" CreationDate="2014-01-07T21:33:03.707" Score="0" Body="Questions about an engine SpaceX has been developing using Methane/LOX as fuel, which has a 2,200 kN (500,000 lbf) thrust range." OwnerUserId="51" LastEditorUserId="37129" LastEditDate="2020-08-18T11:52:46.173" LastActivityDate="2020-08-18T11:52:46.173" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3318" PostTypeId="5" CreationDate="2014-01-07T21:37:02.113" Score="0" Body="&lt;p&gt;Rassvet is a research module on the Russian segment of the ISS docked to the zenith (earth facing) port of the Zarya (FGB) module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the extra space it provides, it has a docking port at its end so that Soyuz or Progress craft can dock. When the PLM was added to the zenith port of Node 1 (Unity) it blocked the use of the Russian segments docking port.  This module extends the port further out so that it can be used again.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:37:02.113" LastActivityDate="2014-01-07T21:37:02.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3319" PostTypeId="4" CreationDate="2014-01-07T21:37:02.113" Score="0" Body="Rassvet is a research module on the Russian segment of the ISS docked to the zenith (earth facing) port of the Zarya (FGB) module." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:37:02.113" LastActivityDate="2014-01-07T21:37:02.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3320" PostTypeId="5" CreationDate="2014-01-07T21:56:41.940" Score="0" Body="&lt;p&gt;A moon is an object that orbits another object. Satellites are artificial moons, but this tag is more directed at the natural body kind. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:56:41.940" LastActivityDate="2014-01-07T21:56:41.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3321" PostTypeId="4" CreationDate="2014-01-07T21:56:41.940" Score="0" Body="Questions about celestial bodies that orbit another celestial body." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:56:41.940" LastActivityDate="2014-01-07T21:56:41.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3322" PostTypeId="5" CreationDate="2014-01-07T21:57:58.163" Score="0" Body="&lt;p&gt;Meteorites are objects that reenter the Earth's atmosphere, usually burning up due to friction with the atmosphere before reaching the surface of the Earth. When they do not entirely burn up the results can be catastrophic, if they remain large enough.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:57:58.163" LastActivityDate="2014-01-07T21:57:58.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3323" PostTypeId="4" CreationDate="2014-01-07T21:57:58.163" Score="0" Body="Questions about objects that enter the Earth's atmosphere from space." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:57:58.163" LastActivityDate="2014-01-07T21:57:58.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3324" PostTypeId="5" CreationDate="2014-01-07T21:59:38.310" Score="0" Body="&lt;p&gt;The ISRO (Indian Sparce Research Organization) PSLV (Polar Space Launch Vehicle) launcher is the current workhorse of the ISRO space program.  It is being superceded by the GSLV (Geosynchronous Space Launch Vehicle) with greater payload and performance.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:59:38.310" LastActivityDate="2014-01-07T21:59:38.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3325" PostTypeId="4" CreationDate="2014-01-07T21:59:38.310" Score="0" Body="Questions about the ISRO PSLV (Polar Space Launch Vehicle) launch vehicle." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T21:59:38.310" LastActivityDate="2014-01-07T21:59:38.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3326" PostTypeId="5" CreationDate="2014-01-07T22:06:45.243" Score="0" Body="&lt;p&gt;The Zvezda (Service Module) of the Russian segment of the ISS was the third major element launched and provides crew accommodations for the Russian crew. It provides control functionality and orbital control of the entire ISS complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zvezda is a backup of the Mir core module modernized and forms the base of the ISS.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T22:06:45.243" LastActivityDate="2014-01-07T22:06:45.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3327" PostTypeId="4" CreationDate="2014-01-07T22:06:45.243" Score="0" Body="Questions about the Zvezda (Service Module) of the Russian segment of the ISS." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T22:06:45.243" LastActivityDate="2014-01-07T22:06:45.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3328" PostTypeId="5" CreationDate="2014-01-07T22:08:06.547" Score="0" Body="&lt;p&gt;The ESA (European Space Agency) has developed a family of launchers named Ariane. The Ariane 5 is currently active with the Ariane 6 under discussion for development. The Ariane 4 was the previous workhorse of the ESA and has been retired.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T22:08:06.547" LastActivityDate="2014-01-07T22:08:06.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3329" PostTypeId="4" CreationDate="2014-01-07T22:08:06.547" Score="0" Body="Questions about the ESA launch Ariane.  There were multiple versions, with Ariane 5 being active currently." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T22:08:06.547" LastActivityDate="2014-01-07T22:08:06.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3330" PostTypeId="5" CreationDate="2014-01-07T22:09:05.410" Score="0" Body="&lt;p&gt;The Zarya module was the first component of the ISS launched.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T22:09:05.410" LastActivityDate="2014-01-07T22:09:05.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3331" PostTypeId="4" CreationDate="2014-01-07T22:09:05.410" Score="0" Body="Questions about the Zarya (FGB) module of the Russian segment of the ISS." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-07T22:09:05.410" LastActivityDate="2014-01-07T22:09:05.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3333" PostTypeId="2" ParentId="3291" CreationDate="2014-01-08T08:15:54.997" Score="3" Body="&lt;p&gt;Earlier missions put silver at a concentration of 100 parts per billion (ppb). The &lt;a href=&quot;http://www.newscientist.com/article/mg20827844.600-lcross-mission-may-have-struck-silver-on-the-moon.html&quot; rel=&quot;nofollow&quot;&gt;LCROSS mission found a possibly higher concentration&lt;/a&gt;, but the &lt;a href=&quot;http://www.sciencemag.org/content/330/6003/468.full&quot; rel=&quot;nofollow&quot;&gt;original publication&lt;/a&gt; (subscription only) does not contain an estimate of the concentration. In fact, the article only says this: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This (spectral lines found in images of the crater) suggests a concentration of volatile Na (and perhaps Ag) near but not on the surface. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It appears that the popular-science press has been hyping this up beyond what's supported by actual data. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if LCROSS found a higher concentration of silver, that measurement is only valid for the impact site. It'd be very difficult to extrapolate this to the entire Moon. On Earth, the concentration of silver and other materials varies wildly, and it's only high enough to economically mine the silver in a few places. To give an idea, &lt;a href=&quot;http://silverinvestingnews.com/3545/what-makes-a-world-class-silver-deposit.html&quot; rel=&quot;nofollow&quot;&gt;one silver mine&lt;/a&gt; has a silver concentration of 0.3 %. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="10671" LastEditDate="2016-10-02T18:40:19.543" LastActivityDate="2016-10-02T18:40:19.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3334" PostTypeId="2" ParentId="3281" CreationDate="2014-01-08T17:46:28.230" Score="6" Body="&lt;p&gt;Essentially there was no difference – the LLRV was the &quot;prototype&quot;, the LLTV the &quot;production&quot; version.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Bearing new names, the LLTVs were nearly identical to the Dryden vehicles except for a few minor improvements to more closely match the LMs. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://www.nasa.gov/centers/dryden/about/Organizations/Technology/Facts/TF-2004-08-DFRC.html&quot; rel=&quot;noreferrer&quot;&gt;Dryden Flight Research Center - Lunar Landing Research Vehicle&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As is not uncommon with prototypes, they made modifications to the LLRV after delivery, most prominently:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Meanwhile, to make the Dryden LLRVs more comparable with the real Apollo lunar modules, controls for the reaction control systems were incorporated into a single three-axis side arm controller and the control panels were also relocated from the center of the pilot stations to the right sides. Later, the cockpits were enclosed and the window visibility was restricted to match the LMs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Ditto)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Initially the controls were like those of a helicopter: a center stick, a side stick (cyclic), and rudder pedals. Within three months, however, engineers and mechanics at the FRC had replaced these with two side sticks that better emulated the controls of the emerging lunar module. The left stick still controlled the lift thrusters and the jet engine (when not in lunar simulation) while the right stick now controlled all the maneuvering thrusters, eliminating the rudder pedals&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060047783_2006249450.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Lunar Landing Research Vehicle; Prelude to the Arrival at Tranquility Base&lt;/a&gt;, page 12)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And these changes were then incorporated into the LLTV:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Changes to the LLRV were incorporated into the plans for three LLTV, or Training Vehicles, which Bell was asked to construct and deliver to the Manned Spacecraft Center in Houston, starting in 1966. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Ditto, page 14)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;LLRV no. 1 was modified into LLTV and sent to Houston for astronaut training; although modified likes its sibling, LLRV No. 2 it remained at the FRC where it was cannibalized for parts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Ditto)&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2014-01-08T17:46:28.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3335" PostTypeId="2" ParentId="3252" CreationDate="2014-01-08T18:54:40.993" Score="2" Body="&lt;p&gt;In the book &quot;Russian Spacesuits&quot; by Isaac Abramov, Isaak Pavlovich Abramov, Ingemar Skoog (ISBN-13: 978-1852337322, &lt;a href=&quot;http://books.google.de/books?id=f7pZosHqkbEC&amp;amp;dq=S-901J%20suit&amp;amp;q=Umbilical#v=snippet&amp;amp;q=Umbilical&amp;amp;f=false&quot; rel=&quot;nofollow&quot;&gt;Google books preview&lt;/a&gt;), there is a page comparing the different Orlan models on page 319. It says &quot;umbilical&quot; for the older models, and &quot;(umbilical)&quot; in parentheses for the newer models. So this probably means that they can still be connected to an umbilical, probably for recharging on board of the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, just as with the older models, this &lt;strong&gt;cannot&lt;/strong&gt; be used to extend the duration of the EVA, as it provides only power and communications. It doesn't extend the capacity of the suit for oxygen, fresh water, carbon dioxide removal, etc.&lt;/p&gt;&#xA;" OwnerUserId="548" LastEditorUserId="49" LastEditDate="2014-01-09T04:24:41.710" LastActivityDate="2014-01-09T04:24:41.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3336" PostTypeId="1" AcceptedAnswerId="3337" CreationDate="2014-01-09T05:55:49.227" Score="3" ViewCount="199" Body="&lt;p&gt;According to Wikipedia, the &lt;a href=&quot;http://en.wikipedia.org/wiki/LLTV&quot; rel=&quot;nofollow&quot;&gt;Lunar Landing Training Vehicle&lt;/a&gt; had a &quot;service ceiling&quot; of 6,000 ft (1,800 m). I assume this is the maximum altitude that it can perform at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was the limiting factor? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2015-12-19T07:57:11.723" Title="Why did the LLTV have a service ceiling of 6,000 ft?" Tags="|training|lltv|altitude|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3337" PostTypeId="2" ParentId="3336" CreationDate="2014-01-09T06:20:12.250" Score="4" Body="&lt;p&gt;Blimey, that number surprised me since it only ever flew up to about 500 ft! Service ceiling is the maximum altitude (air pressure) at which an aircraft is designed to perform with expected stability that's constrained within its &lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_envelope&quot; rel=&quot;nofollow&quot;&gt;flight envelope&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In aerodynamics, the flight envelope, service envelope, or performance&#xA;  envelope of an aircraft refers to the capabilities of a design in&#xA;  terms of airspeed and load factor or altitude. The term is somewhat&#xA;  loosely applied, and can also refer to other measurements such as&#xA;  maneuverability. When a plane is pushed, for instance by diving it at&#xA;  high speeds, it is said to be flown &quot;outside the envelope&quot;, something&#xA;  considered rather dangerous.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So this is about rating the vehicle according to its design limitations, altitude being one of them, defined more specifically by &lt;a href=&quot;https://en.wikipedia.org/wiki/Density_altitude&quot; rel=&quot;nofollow&quot;&gt;density altitude&lt;/a&gt;. Still, 6,000 ft? Whoa!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is 6,000 ft (1.8 km) an altitude they would have never flown the Lunar Landing Training Vehicle (also dubbed &lt;em&gt;The Flying Bedstead&lt;/em&gt;) becomes more apparent when watching &lt;a href=&quot;https://www.youtube.com/watch?v=OlJGQ92IgFk&quot; rel=&quot;nofollow&quot;&gt;this Discovery Channel's video of the Neil Armstrong's LLTV accident&lt;/a&gt; when one of the thrusters failed and he had to eject before LLTV crashed and exploded on impact:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/OlJGQ92IgFk?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding more unpredictable weather at higher altitudes to the already rather unstable craft that had to be 100% operational to sustain flight would only mean such accidents would have happened more frequently, unnecessary risking lives of Apollo astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, one thing to keep in mind is that these 6,000 ft are altitude above mean sea-level. While the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Landing_Research_Facility&quot; rel=&quot;nofollow&quot;&gt;Lunar Landing Research Facility&lt;/a&gt; in Hampton, VA has an elevation of about 10 ft (3 m), they were tested at &lt;a href=&quot;https://en.wikipedia.org/wiki/Edwards_Air_Force_Base&quot; rel=&quot;nofollow&quot;&gt;Edwards Air Force Base&lt;/a&gt; that's at an altitude of 2,303 ft (702 m). That constrained service ceiling there to a maximum downrange of 3,697 ft (1,127 m), which is a whole lot more reasonable given maximum flight altitude above ground they ever achieved.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-19T07:57:11.723" LastActivityDate="2015-12-19T07:57:11.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3338" PostTypeId="2" ParentId="3284" CreationDate="2014-01-09T10:49:40.403" Score="3" Body="&lt;p&gt;For those acronyms in your list that are missing an explanation:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;CE    -   Chief Engineer&#xA;OSM   -   Operations Safety Manager&#xA;VC    -   Vehicle Control&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;And some other acronyms of the SpaceX parlance, in alphabetical order (repeating top ones):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;AVI   -   Avionics Operator&#xA;CE    -   Chief Engineer&#xA;CC    -   Control Center&#xA;CFR   -   Code of Federal Regulations&#xA;FRC   -   Falcon Recovery Coordinator&#xA;FTS   -   Flight Termination System&#xA;FS    -   Flight Software&#xA;FSPO  -   Flight Safety Project Officer&#xA;GC    -   Ground Control&#xA;GNC   -   Guidance, Navigation, &amp;amp; Control&#xA;GS    -   Ground Station&#xA;GSO   -   Ground Safety Officer&#xA;LAM   -   Launch Area Manager &#xA;LC    -   Launch Control&#xA;LD    -   Launch Director&#xA;LDA   -   Launch Decision Authority&#xA;LPM   -   Launch Pad Manager&#xA;MCC   -   Mission Control Center&#xA;MCC-X -   SpaceX Mission Control Center in Hawthorne, CA&#xA;MECO  -   Main Engine Cut Off&#xA;MM    -   Mission Manager&#xA;OD    -   Orbital Director&#xA;OSM   -   Operations Safety Manager&#xA;OSS   -   Operational Safety Supervisor&#xA;PAO   -   Public Affairs Office&#xA;PLDA  -   Pre-Launch Danger Area&#xA;POC   -   Point of Contact &#xA;Prop  -   Propulsion&#xA;RC    -   Range Coordinator&#xA;RCO   -   Range Control Officer&#xA;RMMO  -   Range and Mission Management Office &#xA;ROC   -   Range Operations Coordinator&#xA;RSO   -   Range Safety Officer&#xA;SE    -   Systems Engineer&#xA;SECO  -   Second Stage Engine Cut Off&#xA;SSO   -   Site Safety Officer&#xA;TVC   -   Thrust Vector Control&#xA;VC    -   Vehicle Control&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Most of these are taken from &lt;a href=&quot;http://www.reddit.com/r/spacex/comments/1rns03/rspacex_falcon_9_v11_ses8_official_launch/cdp2as3&quot; rel=&quot;nofollow&quot;&gt;reddit /r/SpaceX, compiled by &lt;em&gt;retiringonmars&lt;/em&gt;&lt;/a&gt;, with some additions by me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The station polling on T-13 minutes for the latest SpaceX launch of Falcon 9 deploying Thaicom 6 satellite on January 6, 2014 went as follows (transcription from the recorded launch video, in the polling order):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;FTS    -   Flight Termination System&#xA;Prop   -   Propulsion&#xA;AVI    -   Avionics Operator&#xA;GNC    -   Guidance, Navigation, &amp;amp; Control&#xA;Ground -   Ground Safety Officer (GSO)&#xA;VC     -   Vehicle Control&#xA;GC     -   Ground Control&#xA;RC     -   Range Coordinator&#xA;CC     -   Control Center&#xA;OSM    -   Operations Safety Manager&#xA;ROC    -   Range Operations Coordinator&#xA;MM     -   Mission Manager&#xA;CE     -   Chief Engineer&#xA;LD     -   Launch Director (gives go-no-go to LC for terminal count)&#xA;LC     -   Launch Control (resumes countdown)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-09T11:55:17.827" LastActivityDate="2014-01-09T11:55:17.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3339" PostTypeId="1" AcceptedAnswerId="3340" CreationDate="2014-01-09T18:05:49.250" Score="8" ViewCount="628" Body="&lt;p&gt;Watching the Antares launch for Orb-1 mission of Cygnus, I was thinking about the booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First stage is built in Ukraine by the company that makes the Zenit. They use the NK-33 engine, (Called an AJ-26 in the US) built in the 1960's (!! Yes, engines could be older than most of the people working on the project) for the Soviet N-1 Moon launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second stage is stranger, to me, if you can believe it! They use ATK Castor solid rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the question, if the first stage has a 'burp' for some reason and say first stage thrust is low, or high, or 'wrong', and you fire your second stage solid, how do you correct?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the SpaceX Falcon 9 'burped' an engine on a flight, they throttled up the other 8 engines and basically burned a little bit longer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second stage had some level of throttle control and timing control to adjust for that to keep the mission on track.  (The secondary payload for Orbcomm that was lost, was not deployed, because of NASA rules on orbits that approach the ISS, where they are super picky and strict). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would Orbital do, with a solid second stage in that case? You get two options. On, and burned out. Do you have to use an upper (third or payload based) stage to correct any variations?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="25" LastEditDate="2014-01-09T18:38:22.707" LastActivityDate="2014-01-09T18:38:22.707" Title="How do you handle variations in launches with a solid second stage?" Tags="|launch|rockets|cygnus|antares|solid-fuel|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3340" PostTypeId="2" ParentId="3339" CreationDate="2014-01-09T18:16:42.413" Score="9" Body="&lt;p&gt;A solid rocket can stop early, and typically in a situation where they rely on it, they will have extra fuel, and just cut off the rocket early. This is very commonly done for missile systems, which have to be very flexible, and be able to fire with little time, and near other people. In addition, they can correct for other wrongness by moving the thrust nozzle for thrust vector control. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust is stopped early by blowing out the engine. Usually there are port covers near the top that reduce the pressure, reducing the effectiveness of the thrust. Also, you can blow out the engine, or a number of alternatives to stop the engine immediately, the key is to lose the containment of the pressure, and the fuel will suddenly leave the rocket at minimal thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they can achieve orbit, they can use the payload to make up for some slight differences as well.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-01-09T18:23:29.753" LastActivityDate="2014-01-09T18:23:29.753" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3341" PostTypeId="5" CreationDate="2014-01-09T18:55:41.910" Score="0" Body="&lt;p&gt;Orbital Sciences developed the Cygnus spacecraft for cargo delivery to the ISS under the NASA COTS program.  They received a contract for 8 launches of cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cygnus has a pressurized module, built by Alenia in Italy (same folk who made the MPLM's and most US Segment space station pressure vessels), and the propulsion/control module is based upon an Orbital satellite bus.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-09T18:55:41.910" LastActivityDate="2014-01-09T18:55:41.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3342" PostTypeId="4" CreationDate="2014-01-09T18:55:41.910" Score="0" Body="Questions about the Cygnus cargo space craft designed to service the ISS." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-09T18:55:41.910" LastActivityDate="2014-01-09T18:55:41.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3343" PostTypeId="5" CreationDate="2014-01-09T18:57:19.527" Score="0" Body="&lt;p&gt;The Antares launcher (Formerly known as Taurus II) is built by Orbital Sciences, but little is built in house.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first stage is built in the Ukraine.  The engines (NK-33/AJ-26) are leftovers from the Soviet N-1 booster built during the space race in the 1960's.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second stage uses an ATK provided Castor 30 (later versions will use Castor 30XL engines). &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-09T18:57:19.527" LastActivityDate="2014-01-09T18:57:19.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3344" PostTypeId="4" CreationDate="2014-01-09T18:57:19.527" Score="0" Body="Questions about the Antares launcher, built by Orbital Sciences." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-09T18:57:19.527" LastActivityDate="2014-01-09T18:57:19.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3345" PostTypeId="5" CreationDate="2014-01-09T19:01:29.713" Score="0" Body="&lt;p&gt;&lt;em&gt;Solid-fuel&lt;/em&gt; (or &lt;em&gt;solid-propellant&lt;/em&gt;) rockets were the first type of &lt;a href=&quot;/questions/tagged/rockets&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;rockets&amp;#39;&quot; rel=&quot;tag&quot;&gt;rockets&lt;/a&gt; to be developed, being used in (for instance) early fireworks.  As the name suggests, they use a solid mass of propellant; this is usually an intimate mixture of finely-divided particles of &lt;a href=&quot;/questions/tagged/fuel&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;fuel&amp;#39;&quot; rel=&quot;tag&quot;&gt;fuel&lt;/a&gt; and &lt;a href=&quot;/questions/tagged/oxidizer&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;oxidizer&amp;#39;&quot; rel=&quot;tag&quot;&gt;oxidizer&lt;/a&gt; held together by a rubbery binder (the most common recipe is &lt;em&gt;ammonium perchlorate composite propellant&lt;/em&gt; (APCP), which uses ammonium perchlorate [duh!] as the oxidizer, and - usually - aluminium powder as the fuel), although other varieties exist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid-fuel rockets have a few things going for them:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;They are relatively cheap.&lt;/li&gt;&#xA;&lt;li&gt;They can be built to reliably provide vast amounts of thrust (far more than even the most powerful &lt;a href=&quot;/questions/tagged/liquid-fuel&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;liquid-fuel&amp;#39;&quot; rel=&quot;tag&quot;&gt;liquid-fuel&lt;/a&gt; rockets) without much risk of a &lt;a href=&quot;/questions/tagged/rocket-explosion&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;rocket-explosion&amp;#39;&quot; rel=&quot;tag&quot;&gt;rocket-explosion&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;They can be stored essentially forever, even out in the open, without their propellant degrading to any significant degree.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, they also have considerably lower &lt;a href=&quot;/questions/tagged/performance&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;performance&amp;#39;&quot; rel=&quot;tag&quot;&gt;performance&lt;/a&gt; than most liquid-fuel rockets.  The &lt;a href=&quot;/questions/tagged/specific-impulse&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;specific-impulse&amp;#39;&quot; rel=&quot;tag&quot;&gt;specific-impulse&lt;/a&gt; of even the best solid-fuel rockets can barely break 300 seconds even in a vacuum; this compares to &lt;em&gt;sea-level&lt;/em&gt; specific impulses of 340 seconds or more for even relatively-low-performance liquid-fuel rockets, and of over 450 seconds for &lt;em&gt;high&lt;/em&gt;-performance liquid-fuel rockets.  Also, unlike liquid-fuel rockets, most solid-fuel rockets cannot be throttled or shut down once ignited (although it &lt;em&gt;is&lt;/em&gt; possible to design a solid-fuel rocket that can be throttled, extinguished, and - potentially - restarted; the first extinguishable solid-fuel rocket was the U.S. &lt;a href=&quot;https://en.wikipedia.org/wiki/LGM-30_Minuteman&quot; rel=&quot;nofollow noreferrer&quot;&gt;Minuteman&lt;/a&gt; missile, dating to the mid-&lt;em&gt;1950s&lt;/em&gt;), which can pose problems should an &lt;a href=&quot;/questions/tagged/abort&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;abort&amp;#39;&quot; rel=&quot;tag&quot;&gt;abort&lt;/a&gt; become necessary.  Finally, although solid-fuel rockets don't fail often, when they &lt;em&gt;do&lt;/em&gt; fail (often due to flaws in the chunk of solid fuel and/or the motor casing), they have a tendency to do so rather... spectacularly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a result of all this, solid-fuel rockets are widely used as strap-on &lt;a href=&quot;/questions/tagged/booster&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;booster&amp;#39;&quot; rel=&quot;tag&quot;&gt;booster&lt;/a&gt; engines to help get heavy launch vehicles off the pad (the &lt;a href=&quot;/questions/tagged/titan-rocket&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;titan-rocket&amp;#39;&quot; rel=&quot;tag&quot;&gt;titan-rocket&lt;/a&gt; versions III/IV, &lt;a href=&quot;/questions/tagged/space-shuttle&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;space-shuttle&amp;#39;&quot; rel=&quot;tag&quot;&gt;space-shuttle&lt;/a&gt;, &lt;a href=&quot;/questions/tagged/ariane&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;ariane&amp;#39;&quot; rel=&quot;tag&quot;&gt;ariane&lt;/a&gt; 3 and later, and the higher-capacity versions of the &lt;a href=&quot;/questions/tagged/atlas-v&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;atlas-v&amp;#39;&quot; rel=&quot;tag&quot;&gt;atlas-v&lt;/a&gt; and the &lt;a href=&quot;/questions/tagged/delta4&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;delta4&amp;#39;&quot; rel=&quot;tag&quot;&gt;delta4&lt;/a&gt; Medium all use or used solid-fuel boosters, as will the under-development &lt;a href=&quot;/questions/tagged/vulcan&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;vulcan&amp;#39;&quot; rel=&quot;tag&quot;&gt;vulcan&lt;/a&gt;), as ICBMs (the military doesn't really care that using solid fuels rather than liquids means they need a bigger rocket to throw the same amount of warhead; what matters is that the missile can be stored fully-fuelled in its silo for decades and then launched on a moment's notice, all without exploding), and as small, cheap kickmotors for payloads that only need a little more &lt;a href=&quot;/questions/tagged/delta-v&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;delta-v&amp;#39;&quot; rel=&quot;tag&quot;&gt;delta-v&lt;/a&gt;; they are &lt;em&gt;not&lt;/em&gt; generally used as the main engines for launching &lt;a href=&quot;/questions/tagged/artificial-satellite&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;artificial-satellite&amp;#39;&quot; rel=&quot;tag&quot;&gt;artificial-satellite&lt;/a&gt;s (this task is usually left to a higher-performance liquid-fuel core and/or upper stage), although there are exceptions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solid-propellant_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;See Wikipedia for more information.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="23891" LastEditDate="2019-06-05T23:21:30.443" LastActivityDate="2019-06-05T23:21:30.443" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3346" PostTypeId="4" CreationDate="2014-01-09T19:01:29.713" Score="0" Body="Questions about solid fuels.  The rockets that use them, the fuels that are burned, and the vendors that make them." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-09T19:01:29.713" LastActivityDate="2014-01-09T19:01:29.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3347" PostTypeId="2" ParentId="3253" CreationDate="2014-01-09T20:22:53.170" Score="5" Body="&lt;p&gt;Based on a comment, there is a table at &lt;a href=&quot;http://www.spacelaunchreport.com/soyuz.html&quot; rel=&quot;noreferrer&quot;&gt;Space Launch Report&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-01-09T20:22:53.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3348" PostTypeId="1" CreationDate="2014-01-10T11:30:02.200" Score="10" ViewCount="330" Body="&lt;p&gt;&lt;a href=&quot;http://www.xconomy.com/boulder-denver/2014/01/09/space-flight-launched-today-includes-cu-studies-of-antibiotics-ants/&quot;&gt;An experiment launched with the Cygnus 'craft&lt;/a&gt; brought my attention to another difference between Earth and space environments: human infections may become more lethal, more virulent, and less prone to antibiotic treatment in space due to microgravity (and also possible increase in rate of mutations).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A search through PubMed yielded &lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pubmed/?term=antibiotic+microgravity&quot;&gt;a few interesting abstracts/articles&lt;/a&gt;, in two 'waves' (2004-2006 and 2012-2013).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How will a prolonged Beyond Earth Orbit (BEO) mission deal with antibiotic-resistant pathogens in terms of standard operating procedures, extra equipment and supplies required?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What are the implications of infections complicated by microgravity for in-flight surgical procedures?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How many researchers are active in this area across major space powers (NASA, the Chinese etc.)?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastActivityDate="2016-10-03T03:52:26.180" Title="Pathogenic microorganisms and antibiotics in space" Tags="|medical|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3349" PostTypeId="1" CreationDate="2014-01-10T11:53:18.880" Score="6" ViewCount="316" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot;&gt;Solar sails&lt;/a&gt; are not fast and acceleration is not going to provide meaningful simulated gravity.  For a long journey, spinning a ship to provided &lt;a href=&quot;https://en.wikipedia.org/wiki/Simulated_gravity&quot;&gt;simulated gravity&lt;/a&gt; can provide health benefits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially the ship and sail configuration is a large weight (&lt;em&gt;ship&lt;/em&gt;), hanging under a parachute. Except that in this case the parachute (&lt;em&gt;solar sail&lt;/em&gt;) is providing lift away from the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping the system stable will be important; to conserve expendable fuel/energy, maximize thrust from the sail, and maintain the general welfare (&lt;em&gt;homeostasis&lt;/em&gt;) of the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What configuration of ship and sail provides the most stability, where the ship is providing 1G of centrifugal force at it's outer most deck? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2014-02-16T15:15:54.700" Title="What is the most stable configuration for a centrifugal ship towed by a solar sail?" Tags="|solar-sail|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3350" PostTypeId="1" AcceptedAnswerId="3352" CreationDate="2014-01-10T16:07:49.350" Score="7" ViewCount="527" Body="&lt;p&gt;&lt;a href=&quot;http://www.shimz.co.jp/english/theme/dream/pdf/lunaring_e.pdf&quot;&gt;A Japanese paper on beaming energy&lt;/a&gt; from solar installation on the moon to earth via laser and microwave recently made the rounds in the media. Would this, or any space-based energy system cause a net increase of atmospheric temperatures?  To this layperson, it would seem like any such plan would pump energy into the atmosphere that otherwise would not have been there.  Would this be a real concern, or would the &quot;clean&quot; nature of the source offset it?&lt;/p&gt;&#xA;" OwnerUserId="1414" LastActivityDate="2016-06-05T21:34:28.337" Title="Will space-based energy beamed to earth increase heat on earth?" Tags="|energy|solar-power|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3351" PostTypeId="2" ParentId="3350" CreationDate="2014-01-10T16:15:12.580" Score="0" Body="&lt;p&gt;In principle, where is that 'heat' coming from?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any solar area that collects energy intercepts photons, which excite electrons, which get converted to microwaves to be beamed to earth.  Incidental molecules that get excited by the beam vibrate and generate heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each step along the way has efficiency losses. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All those photons would probably have hit earth anyway.  Instead, heat is being radiated at each stage, in high orbit, before a small portion of the energy is lost as heat in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I would say, not even worth worrying about.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-01-10T16:15:12.580" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3352" PostTypeId="2" ParentId="3350" CreationDate="2014-01-10T16:26:04.007" Score="6" Body="&lt;p&gt;Yes, since they put it on the Moon, it would increase the total energy or &quot;heat&quot; as you call it being deposited on Earth.  By a small amount.  17 toe is about $7\times 10^{20}\,\mathrm{J}$.  The total solar insolation on the Earth from the Sun in a year is about $5\times 10^{24}\,\mathrm{J}$.  So the (incredible) 400 km wide swath of solar collectors on the Moon would add 0.01% energy to Earth's input.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That does not necessarily mean that the average temperature of the Earth would be increased, depending on what other activities it offsets and how the added energy is dissipated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that nuclear energy also adds to the solar insolation.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-01-10T18:12:25.150" LastActivityDate="2014-01-10T18:12:25.150" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3353" PostTypeId="1" CreationDate="2014-01-10T19:35:50.053" Score="2" ViewCount="145" Body="&lt;p&gt;ESA committed to launching 5 ATV vehicles. All 5 were named.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;JAXA has committed to a limited number of HTV launches.  And all have been named.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital has contracted for 8 launches, and seems to be naming the Cygnus modules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russia has been launching Progress and Soyuz vehicles by the hundreds for close 40-50 years, and has not named any of them beyond a mission number/identifier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has launched several Dragons, with 10 more to go under the current contract and none have been given names.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can one infer a philosophical difference in the goals, based on the naming pattern?  It seems like SpaceX has a more open ended plan like the Russians as compared to the ones with more limited scopes.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-10T19:43:04.990" LastActivityDate="2014-01-10T19:43:04.990" Title="Does naming cargo space craft indicate intentions?" Tags="|soyuz-spacecraft|esa|cygnus|dragon|htv|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3354" PostTypeId="1" AcceptedAnswerId="3355" CreationDate="2014-01-10T21:56:20.630" Score="7" ViewCount="1713" Body="&lt;p&gt;Given that the majority of the operation of a chemical rocket is at different altitudes / pressures than at ground level, how are hot fire ground tests conducted to gather useful information about efficiency, thrust, etc?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine there is some process of 'truing' the thrust, as well - making sure the vector points in a particular direction, like a bike wheel. How is this confirmed?&#xA;Additionally...how do you gather information from a stream of hot exhaust that is several thousand degrees?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe my assumptions are way off-base. Just curious what some of the actual engineering processes are.&lt;/p&gt;&#xA;" OwnerUserId="794" LastActivityDate="2015-12-19T08:01:32.870" Title="How are rockets tested (hot fire)?" Tags="|rockets|propulsion|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3355" PostTypeId="2" ParentId="3354" CreationDate="2014-01-10T23:57:15.317" Score="7" Body="&lt;p&gt;I'm gonna say it straight off, I'm no expert. But the first part of your question seems relatively straightforward to answer. The easiest is by building test facilities at different altitudes, get the performance data you're after at each altitude you can, and then extrapolate for whichever atmospheric pressure you later require.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;E.g. ISRO has a high-altitude test facility at &lt;a href=&quot;https://en.wikipedia.org/wiki/Mahendragiri&quot; rel=&quot;nofollow&quot;&gt;Mahendragiri, India&lt;/a&gt; (1,654 m / 5,427 ft), JAXA can simulate atmospheric conditions of an altitude of approximately 30 km at their &lt;a href=&quot;http://www.rocket.jaxa.jp/kspc/english/tf/hats.html&quot; rel=&quot;nofollow&quot;&gt;High Altitude for Rocket Engine Test Facility&lt;/a&gt;, SpaceX is leasing a launch pad in Las Cruces, New Mexico (4,000 ft / 1,219 m), DLR has an &lt;a href=&quot;http://eucass.eu/files/u1/2-P41operation.pdf&quot; rel=&quot;nofollow&quot;&gt;Altitude Simulation Test facility in Lampoldshausen&lt;/a&gt;, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust itself is usually measured by &lt;a href=&quot;https://en.wikipedia.org/wiki/Load_cell&quot; rel=&quot;nofollow&quot;&gt;load cells&lt;/a&gt;, which have multiple &lt;a href=&quot;https://en.wikipedia.org/wiki/Strain_gauge&quot; rel=&quot;nofollow&quot;&gt;strain gauges&lt;/a&gt; oriented at different angles (usually four of them, two on each side oriented perpendicular to each other) and convert deformations to them (strain / load) into electric signal. Placing multiple load cells to measure force loads on the rigid rocket engine frame it is mounted to during test-fire should be sufficient to measure the thrust vector, since you can measure forces each side of the frame is a subject to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exhaust plume itself is usually measured with multiple infrared cameras that record its heat signature from various angles in real-time. All this data would be combined with data from a variety of sensors built into the rocket engine itself, such as e.g. measuring propellants pressure in its injectors (injection rate), and so on.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-19T08:01:32.870" LastActivityDate="2015-12-19T08:01:32.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3357" PostTypeId="2" ParentId="3354" CreationDate="2014-01-11T03:03:41.070" Score="6" Body="&lt;p&gt;In a &lt;a href=&quot;http://facilities.grc.nasa.gov/b2/index.html&quot; rel=&quot;nofollow&quot;&gt;vacuum chamber in Ohio&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA's Spacecraft Propulsion Research Facility (B-2) is the world's&#xA;  only facility capable of testing full-scale upper-stage launch&#xA;  vehicles and rocket engines under simulated high-altitude conditions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is also the &lt;a href=&quot;http://sscwebpub.ssc.nasa.gov/etd/ETDTestFacilitiesA3.asp&quot; rel=&quot;nofollow&quot;&gt;A3 test stand at NASA Stennis Space Center&lt;/a&gt; which is capable of flight duration (up to 550 seconds) hot fire test of engines or (with modifications) of stages at a 100,000 foot simulated altitude.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-04-23T22:50:34.107" LastActivityDate="2014-04-23T22:50:34.107" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3358" PostTypeId="1" CreationDate="2014-01-11T07:26:53.570" Score="22" ViewCount="1680" Body="&lt;p&gt;Shuttle's SSME uses cryogenic Hydrogen(LHX) and Oxygen(LOX) as their propellent to give hot water vapour. If I somehow manage to get the exhaust plume and condense it to liquid, Can I drink it directly(or is that toxic) ?&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2014-01-17T17:45:39.750" Title="Can I drink the water from the plumes of SSME?" Tags="|space-shuttle|humans|cryogenics|ssme|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3359" PostTypeId="2" ParentId="3358" CreationDate="2014-01-11T08:57:10.810" Score="17" Body="&lt;p&gt;I wouldn't recommend it, and there's several reasons why.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, even if we assumed you'd get chemically pure water out of the exhaust plume, in essence distilled water, it would lack any dissolved minerals and cause you to lose electrolytes. If that was all that was wrong with it, then small quantities wouldn't hurt you, but you'd have to substitute lost intake of trace minerals before your electrolyte levels become dangerous to your health. Distilled water would essentially cause you to require more liquids in your body, since it would reduce levels of your body fluid solvents and all your cells would swell up through a process called &lt;a href=&quot;https://en.wikipedia.org/wiki/Osmosis&quot;&gt;osmosis&lt;/a&gt;, trying to match levels of solvents with their surroundings and in turn requiring more water. While on itself not life threatening, you'd actually be undergoing faster dehydration, funny enough, and combined with dropped levels of electrolytes can lead to complications like high blood pressure, arrhythmia, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, such condensed and cooled exhaust plume water would be highly ionized. Depending on the levels of ionisation and amounts of such water that you'd drink, it could go from a mildly alkaline irritant to corroding through and dissolving your internals. It might also form hydrogen peroxide with atmospheric oxygen, although it isn't really stable so that wouldn't last too long. Either case, it wouldn't be a bad idea to first test its &lt;a href=&quot;https://en.wikipedia.org/wiki/PH&quot;&gt;pH levels&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, those propellants went through a lot of metallic machinery at extremely high pressure to get to the rocket's exhaust. They also changed their temperature from one extreme (LOX and LH2 are both cryogenic liquids) to the other as they burn and result in mostly molecular water at the exhaust. It might contain heavy metals, perhaps in trace amounts, or perhaps enough to be toxic or even kill you. Other impurities, such as lubricants aren't excluded either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are likely other reasons why you shouldn't drink it that I can't think of now, but let's say that these three reasons I mention should suffice to discourage anyone in trying it out, at least without thoroughly testing the liquid first before drinking it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should probably note that even though this sounds deadly toxic and less than friendly to the environment, it actually isn't that bad and these exhaust plumes would naturally deionize long before they would fall back to earth during a normal rocket launch using LOX/LH2 propellants as they condense and combine with atmospheric vapor. They however wouldn't purify too, so yes, even such &lt;em&gt;green&lt;/em&gt; rockets can pollute. By far not as much as most other alternatives though.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-01-11T08:57:10.810" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3361" PostTypeId="2" ParentId="3354" CreationDate="2014-01-11T20:50:33.790" Score="3" Body="&lt;p&gt;The effect of external pressure on a rocket engine is backpressure at the exit plane of the nozzle.  The combustion chamber is well insulated from that by the sonic flow at the throat o the nozzle.  High altitude engines have large nozzles that expand the gas to a pressure below ground atmosphere.  It is quite a good approximation to put a smaller nozzle on engines that will operate at high altitudes and test them in atmospheric pressure.  The effect on performance can then be modeled.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-01-11T20:50:33.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3362" PostTypeId="1" AcceptedAnswerId="3366" CreationDate="2014-01-12T11:31:45.097" Score="10" ViewCount="8151" Body="&lt;p&gt;Current knowledge suggests there is &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_polar_ice_caps&quot; rel=&quot;nofollow noreferrer&quot;&gt;plentiful water ice&lt;/a&gt; at the Martian poles.  The same knowledge says it gets cold enough at the poles for carbon dioxide to freeze out of the air.  If creating a habitat towards the equator of Mars (to have slightly warmer temperatures) we will need to get these gases somehow closer to the habitat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oxygen and hydrogen combine to form water. These are probably the most critical elements required in large quantities to sustain human life.  Oxygen to breathe and hydrogen to combine with oxygen to make drinking water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/614/does-mars-contain-materials-that-could-be-used-to-build-a-space-colony/617#617&quot;&gt;This answer&lt;/a&gt; suggests there is plentiful oxygen trapped in the soil as &lt;a href=&quot;https://en.wikipedia.org/wiki/Iron_oxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;iron oxide &amp;amp; hydroxides&lt;/a&gt; which contain both oxygen and hydrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much soil would need to be processed to extract a gallon of water?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much soil would need to be processed to fill a 10' x 10' X 10' room with the same amount of oxygen in the same room on Earth?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2018-11-20T02:37:35.913" Title="How much oxygen and hydrogen are in the soil of Mars?" Tags="|mars|water|habitat|oxygen|regolith|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3363" PostTypeId="1" AcceptedAnswerId="3365" CreationDate="2014-01-12T11:54:08.510" Score="40" ViewCount="22929" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Earth&quot;&gt;air on Earth&lt;/a&gt; is about 20% oxygen and about 78% nitrogen. Best case, there seems to be sufficient oxygen (in water) to support colonization in the polar ice caps.  What about the other 4/5 of the atmosphere?  Too much oxygen &lt;a href=&quot;https://en.wikipedia.org/wiki/Oxygen_toxicity&quot;&gt;can be harmful&lt;/a&gt;. On Earth, we achieve balance by the ratio of oxygen to nitrogen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to NASA the Martian Atmosphere is &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&quot;&gt;2.7% nitrogen (N2) and 0.13% oxygen (O2)&lt;/a&gt;  is it reasonable to assume that we would be able to harvest sufficient nitrogen from the atmosphere or would it need to be imported, or obtained someplace else?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that nitrogen plays &lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrogen_cycle&quot;&gt;other important roles&lt;/a&gt; in the ecosystem and should be considered as well.  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="1457" LastEditDate="2015-10-30T14:37:53.030" LastActivityDate="2021-12-12T12:28:36.957" Title="Would a settlement on Mars need to import Nitrogen?" Tags="|mars|atmosphere|" AnswerCount="8" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3364" PostTypeId="2" ParentId="3363" CreationDate="2014-01-12T12:32:46.060" Score="16" Body="&lt;p&gt;Nitrogen is usually considered the key missing element to terraforming Mars. There might be some in existence somewhere, but yes, Mars would need a lot more nitrogen than it currently has in it's atmosphere. According to &lt;a href=&quot;http://newmars.wikispaces.com/Nitrogen+on+Mars&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;, it is very difficult to detect Nitrates via spectroscopy, and they typically would only exist at least 1 meter below the surface. Until we start digging on Mars, we won't know if it has enough nitrogen to be self-sufficient. But yes, Nitrogen will be the key to life as we know it on Mars, everything else life needs we know we can find there.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6599" LastEditDate="2015-02-23T10:41:25.927" LastActivityDate="2015-02-23T10:41:25.927" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3365" PostTypeId="2" ParentId="3363" CreationDate="2014-01-12T13:47:08.727" Score="28" Body="&lt;p&gt;I think it's fairly safe to assume that Mars has plenty of nitrogen locked in its mineral deposits, since it's one of the most &lt;a href=&quot;https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements&quot;&gt;abundant elements&lt;/a&gt; of the Solar system and the planet formed out of the same protoplanetary disk as Earth has. Some &lt;a href=&quot;http://webmineral.com/chem/Chem-N.shtml&quot;&gt;nitrogen rich minerals&lt;/a&gt; are a safe bet, especially the ones of magmatic origins that might be easily accessible in top layers of the Martian soil in regions that were volcanically more active. Since we know of many such regions, this shouldn't be a problem. For example, silicate minerals are known to lock substantial amounts of environmental nitrogen when superheated (lava / magma), and chondrites can lock up to 27.96 % of nitrogen per weight (e.g. &lt;a href=&quot;http://webmineral.com/data/Sinoite.shtml&quot;&gt;sinoite Si2N2O&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such nitrogen rich mineral deposits could be used either directly as a fertilizer and let the plants and bacteria slowly enrich the atmosphere with it through their nitrogen cycle, or extract it chemically, with superheating, or other processes, possibly as a byproduct of extracting other sought after minerals and ores, perhaps titanium mineral &lt;a href=&quot;http://webmineral.com/data/Osbornite.shtml&quot;&gt;osbornite&lt;/a&gt; (TiN) that consists of 22.63 % of nitrogen per weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this extraction of nitrogen depends on how much of it you'd need. As an atmospheric gas, it's not really essential and could be substituted with other non-toxic inert gases (perhaps &lt;a href=&quot;https://en.wikipedia.org/wiki/Argon&quot;&gt;argon&lt;/a&gt; that's already roughly 2% of the Martian atmosphere?) to, e.g. increase the atmospheric pressure. Plants naturally don't take nitrogen from air (although &lt;a href=&quot;http://www.sciencedaily.com/releases/2013/07/130725125024.htm&quot;&gt;we now have the technology&lt;/a&gt; to enable that for them), so it isn't essential for that, and the industry needs could be satisfied through already mentioned nitrogen gas byproduct of other processes, such as ore mining.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-01-12T13:47:08.727" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3366" PostTypeId="2" ParentId="3362" CreationDate="2014-01-12T14:52:47.247" Score="10" Body="&lt;p&gt;Assuming you could extract all the water from the soil, which apparently has &lt;a href=&quot;http://www.theguardian.com/science/2013/sep/26/nasa-curiosity-rover-mars-soil-water&quot; rel=&quot;noreferrer&quot;&gt;roughly 2% of it by weight&lt;/a&gt; (by the way, that's really dry and even the driest of desert sands contain more), then you'd need at least 189.27 kg (417.27 lb) of Martian soil to produce one gallon (≈ 3.79 liters) of water. This would be extremely difficult to do though and would require a lot of power, probably to first grind it as fine as you can, then superheat, separate vapor through microfiltration and condense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we assumed this same water would later be used to extract oxygen through electrolysis (88.8% of its molecular mass is oxygen), and you'd want about the same oxygen contents as in average sea-level Earth atmosphere (&lt;a href=&quot;http://www.engineeringtoolbox.com/molecular-mass-air-d_679.html&quot; rel=&quot;noreferrer&quot;&gt;oxygen at 23,14% of molecular mass of air&lt;/a&gt; at its total weight of &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Density_and_mass&quot; rel=&quot;noreferrer&quot;&gt;1.2 kg/m&lt;sup&gt;3&lt;/sup&gt;&lt;/a&gt;), you'd require ≈ 6.98 liters (1.84 US gal) of water to fill a 10x10x10' (28.3168 m&lt;sup&gt;3&lt;/sup&gt;) room with it. Assuming I didn't misplace some decimal point in my unit conversions, but it seems about right.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-01-12T14:52:47.247" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3367" PostTypeId="1" AcceptedAnswerId="3369" CreationDate="2014-01-12T23:05:56.333" Score="5" ViewCount="149" Body="&lt;p&gt;If one was going to use a &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_sails&quot;&gt;solar sail&lt;/a&gt; on a long-term space mission, it seems as though &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_debris&quot;&gt;space debris&lt;/a&gt; would be very hard to avoid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a solar sail was perforated by a piece of space debris, would its usefulness be greatly reduced?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2014-01-13T00:26:05.410" Title="How much would a solar sail's usefulness be reduced by a perforation?" Tags="|propulsion|solar-sail|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3368" PostTypeId="1" AcceptedAnswerId="3387" CreationDate="2014-01-12T23:09:22.147" Score="15" ViewCount="753" Body="&lt;p&gt;The Wikipedia article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_sails#Satellites&quot; rel=&quot;noreferrer&quot;&gt;solar sails&lt;/a&gt; mentions that they could have a use in modyfing a spacecraft's orbit:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Robert L. Forward pointed out that a solar sail could be used to modify the orbit of a satellite around the Earth. In the limit, a sail could be used to &amp;quot;hover&amp;quot; a satellite above one pole of the Earth. Spacecraft fitted with solar sails could also be placed in close orbits about the Sun that are stationary with respect to either the Sun or the Earth, a type of satellite named by Forward a statite. This is possible because the propulsion provided by the sail offsets the gravitational potential of the Sun. Such an orbit could be useful for studying the properties of the Sun over long durations.&lt;/p&gt;&#xA;&lt;p&gt;Such a spacecraft could conceivably be placed directly over a pole of the Sun, and remain at that station for lengthy durations. Likewise a solar sail-equipped spacecraft could also remain on station nearly above the polar terminator of a planet such as the Earth by tilting the sail at the appropriate angle needed to just counteract the planet's gravity.&lt;/p&gt;&#xA;&lt;p&gt;In his book The Case for Mars, Robert Zubrin points out that the reflected sunlight from a large statite placed near the polar terminator of the planet Mars could be focussed on one of the Martian polar ice caps to significantly warm the planet's atmosphere. Such a statite could be made from asteroid material.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Have solar sails ever been used for such a purpose, outside of proof-of-concept testing?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2020-05-12T02:52:19.387" Title="Has a solar sail been used on an orbital mission?" Tags="|orbital-mechanics|orbit|solar-sail|static-satellite|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3369" PostTypeId="2" ParentId="3367" CreationDate="2014-01-13T00:26:05.410" Score="6" Body="&lt;p&gt;No.  They degrade gracefully with small punctures, with the degradation approximately proportional to the area ratio of the pinholes to the total sail.  So almost not at all.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-01-13T00:26:05.410" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3370" PostTypeId="2" ParentId="3368" CreationDate="2014-01-13T00:32:38.873" Score="11" Body="&lt;p&gt;As far as I know, no solar sail has been used for any purpose, other than demonstration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, except for MESSENGER.  It was not designed as a solar sail, but since it flies so close to the Sun, they &lt;a href=&quot;http://messenger.jhuapl.edu/the_mission/publications/AAS09-014.pdf&quot; rel=&quot;nofollow&quot;&gt;ended up using it like one&lt;/a&gt;.  They were able to use solar sailing instead of firing rockets for trajectory correction maneuvers for Mercury flybys.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sunjammermission.com/&quot; rel=&quot;nofollow&quot;&gt;Sunjammer&lt;/a&gt; plans to demonstrate pole-sitting at Earth, as well as maintaining position at L1, which is unstable otherwise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Update a few years later ...&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, Sunjammer was cancelled due to development issues.  However the Planetary Society's citizen-funded &lt;a href=&quot;http://sail.planetary.org/&quot; rel=&quot;nofollow&quot;&gt;LightSail&lt;/a&gt; successfully tested a solar sail deployment in May.  That was only a deployment test, since the planned orbit was too low to permit solar sailing (too much drag).  Another test is planned for next year in a higher orbit that will permit solar sailing.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-07-22T16:53:29.520" LastActivityDate="2015-07-22T16:53:29.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3371" PostTypeId="1" CreationDate="2014-01-13T01:23:20.233" Score="8" ViewCount="1834" Body="&lt;p&gt;What is the temperature at the lunar center? What is the highest temperature inside Luna?&lt;/p&gt;&#xA;" OwnerUserId="812" LastActivityDate="2016-03-18T09:45:36.197" Title="What is the temperature of the lunar core?" Tags="|the-moon|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3373" PostTypeId="1" AcceptedAnswerId="5935" CreationDate="2014-01-13T05:29:46.557" Score="14" ViewCount="801" Body="&lt;p&gt;Generally the CBM ports were meant to connect new space station nodes together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nadir port on Harmony (Node 2) has been used for more berthings than any other that I am aware of. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of Jan 2014, it has supported:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;4 HTV berthings (plus one swap of the HTV from nadir to zenith to make room for a space shuttle docking) for a total of 5.&lt;/li&gt;&#xA;&lt;li&gt;3 Dragon berthings (#4 due in Feb 2014)&lt;/li&gt;&#xA;&lt;li&gt;2 Cygnus berthings&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For a total of ten berthing events to date of writing this question, with 16 more expected at a minimum (duration of Orbital (8 flight) and SpaceX's (12 flights) current contracts with NASA).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the operational lifetime, in units of berthing events for a CBM port on an ISS module?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Contrast with the aft port of Zvezda which has supported dozens of docking operations, but is of a very different design. Or the forward port on Mir. Or aft port on Kvant which was attached to the aft port of Mir).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-03-19T04:03:59.693" LastActivityDate="2014-11-14T13:30:19.213" Title="How many docking/berthing operations can a CBM port tolerate?" Tags="|iss|docking|docking-port|cbm|pma|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3374" PostTypeId="1" CreationDate="2014-01-13T09:17:55.983" Score="3" ViewCount="133" Body="&lt;p&gt;The &lt;em&gt;Progress&lt;/em&gt; cargo vessels are often given  additional tasks of technical or scientific value. Typically performed after they undock from the station. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many of them participated in the series of &lt;a href=&quot;http://www.energia.ru/en/iss/researches/study/06.html&quot; rel=&quot;nofollow&quot;&gt;Radar-Progress&lt;/a&gt; and &lt;a href=&quot;http://www.energia.ru/en/iss/researches/geophis/13.html&quot; rel=&quot;nofollow&quot;&gt;Plazma-Progress&lt;/a&gt; experiments. It takes like 5 days of free flight before the reentry. &lt;em&gt;Progress M-13M&lt;/em&gt; has been used as an orbit-to-orbit tug to bring &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/chibis-m.htm&quot; rel=&quot;nofollow&quot;&gt;Chibis-M&lt;/a&gt; to 500 km orbit. &lt;em&gt;Progress M-04M&lt;/em&gt; took part in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Progress_M-04M&quot; rel=&quot;nofollow&quot;&gt;Reflection&lt;/a&gt; experiment, and so spent 6 weeks in autonomous flight. Etc etc. The tradition goes well into Mir (and may be even Salyut) times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are &lt;em&gt;ATV&lt;/em&gt;, &lt;em&gt;HTV&lt;/em&gt;, &lt;em&gt;Dragon&lt;/em&gt; and &lt;em&gt;Cygnus&lt;/em&gt; also used as something more than just cargo delivery?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2014-01-13T09:32:46.167" LastActivityDate="2014-01-13T13:50:54.747" Title="Additional tasks for ATV, HTV, Dragon and Cygnus" Tags="|uncrewed-spaceflight|experiment|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3375" PostTypeId="1" AcceptedAnswerId="3712" CreationDate="2014-01-13T11:21:08.703" Score="10" ViewCount="409" Body="&lt;p&gt;If a solar sail is to be of any use, it needs to be a rigid structure and maintain its ideal curvature and orientation, otherwise it would eventually fold-up over its center of mass like an umbrella does in a strong wind due to torque increasing radially with distance to its geometric center. Furthermore, for a solar sail to effectively translate weak momentum force of radiation pressure onto its rigid structure, it needs to be absolutely enormous, further increasing its radius, and ideally, the center of its mass would be along the axis of movement at its geometric center on a 2-dimensional plane perpendicular to the radiation source, otherwise its total torque translates into a spin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, this means that even though we're talking of relatively small force applied per its square area, the larger it is (to increase net force on it), the larger its radial distance towards its central axis, and the torque differential with it. So the larger it gets, the stronger its frame holding it together would have to be, which, with large area useful for any missions of &lt;em&gt;reasonable length of time&lt;/em&gt; means, we're fast approaching limits of maximum stress loads on any materials known to man. Either that, or we're increasing its ability to cope with structural loads by adding more mass to it (reinforcing the structure), negating the point in having enormous solar sail in the first place. Thus my question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How does one maintain flatness of a large solar sail, so it doesn't bend into a less-than-ideal concave / convex dish or fold onto itself? What kind of support structures are proposed to maintain this flatness, what are the limitations (maximum size) of such materials proposed, and could these limits be somewhat stretched by use of, say, &lt;a href=&quot;http://electrochem.cwru.edu/encycl/art-p02-elact-pol.htm&quot;&gt;Electroactive Polymers (EAP)&lt;/a&gt;, e.g. Carbon Nanotubes can be used for ionic EAP that are also some of the strongest materials known to science, to maintain its flatness / ideal curvature by applying voltage to the frame?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-13T13:57:16.420" LastActivityDate="2015-07-12T22:23:10.870" Title="Maintaining ideal curvature (flatness) of a solar sail" Tags="|materials|solar-sail|construction|design|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3376" PostTypeId="2" ParentId="3374" CreationDate="2014-01-13T13:34:22.490" Score="1" Body="&lt;p&gt;Dragon is being used to reenter cargo, which is pretty important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest of course remove garbage to free up space on the station, via destructive reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Progress will be used in April of 2014 to dock to Pirs, and when it undocks, take Pirs with it for destructive reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another example would be Shenzou, where the orbital module remains in orbit and conducts additional experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon has launched with secondary payloads that release before docking with the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A stretch answer as well might be all the Cubesats that Cygnus is bringing on the Orb-1 mission, that will be launched over time out the small airlock in the JAXA Kibo module. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-13T13:50:54.747" LastActivityDate="2014-01-13T13:50:54.747" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3377" PostTypeId="5" CreationDate="2014-01-13T15:05:43.327" Score="0" Body="&lt;p&gt;The ISS has several different types of docking connectors.  The CBM is the largest diameter, used to connect modules on the US Segment of the station.  Unmanned cargo vehicles (HTV, Cygnus, Dragon) use the CBM port on the nadir (earth facing) side of the Harmony (Node 2) module for berthing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Manned US craft (previous: Space Shuttle and future: DragonRider, Dream Chaser, CST-100, Orion) are meant to use the PMA (Pressurized Mating Adapaters).  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-13T15:05:43.327" LastActivityDate="2014-01-13T15:05:43.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3378" PostTypeId="4" CreationDate="2014-01-13T15:05:43.327" Score="0" Body="Questions about the Common Berthing Modules on the ISS.  (Not about Commodore Business Machines)" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-13T15:05:43.327" LastActivityDate="2014-01-13T15:05:43.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3379" PostTypeId="1" CreationDate="2014-01-13T16:56:55.787" Score="12" ViewCount="208" Body="&lt;p&gt;I know of 2 plant growing mediums: soil and liquid filled with vital components for plant growth. On Earth, when soil dries a bit, small particles of it will be carried by wind, moved by vibrations and otherwise misplaced. I suppose in zero G it could cause dust floating around. I'm not that experienced in hydroponics to take an educated guess at what would happen, but from what I've seen, liquids tends to create small droplets even from lightest vibrations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this a problem for plants in space? If not, how?&lt;/p&gt;&#xA;" OwnerUserId="990" LastActivityDate="2015-12-19T08:09:49.930" Title="How does plant growing medium not scatter around?" Tags="|plants|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3380" PostTypeId="2" ParentId="3379" CreationDate="2014-01-13T17:09:16.280" Score="9" Body="&lt;p&gt;Generally, without any external disturbance, cohesive forces of the materials in use will be sufficient to keep them together, since they all share the same inertial frame. But sure, to prevent soils from scattering around when moved around, worked with or perhaps due to air circulation, they can be contained in growing pots or similar containers that would additionally hold the soil due to surface tension.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/koQqn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Canadian astronaut Bob Thirsk holds plants while on board the ISSn in 2009. (Credit: Canadian Space Agency)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding water will only increase this cohesion because of increase in surface tension it would provide to the growth medium. So in general, there's no need to additionally &quot;secure&quot; the culture any more than you'd want to do in gravity. That soil isn't going anywhere unless it'll be eventually pushed out by growing roots, something that would happen in gravity just as well if you used too small pot when planting seeds or rooting cuttings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps this is better seen than read about, so I suggest for example &lt;a href=&quot;https://www.youtube.com/watch?v=o8TssbmY-GM&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; with astronaut Chris Hadfield performing a science experiment with a soaking wet washcloth. You'll appreciate water's surface tension at work in microgravity better than any words can describe:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/o8TssbmY-GM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the plants are in racks however, they would also use reflective covers over their growth mediums since they're grown under artificial light and illumination doesn't come cheap without increasing by plants less well tolerated heat. For soils, they mostly use nutrition gels or soil substitutes (e.g. coco fibers with vermiculite for minerals, clay pellets,...). Biological soils are rare nowadays in hydroponic gardens for various reasons, from poor ability to contain water, root rot, to harboring pests and infections.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-19T08:09:49.930" LastActivityDate="2015-12-19T08:09:49.930" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3381" PostTypeId="5" CreationDate="2014-01-13T19:09:54.210" Score="0" Body="&lt;p&gt;The ISS has three PMA's installed.  PMA-1 is the connector between Unity (node 1) and Zarya, bridging the US and Russian segments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PMA-2 is attached to the forward CBM port of the Harmnoy (node 2) module. It was used by the Space Shuttle to dock, for most of the stations life.  It was shuffled around the station as modules were added to allow continued docking and construction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PMA-3 is currently attached to the forward CBM port of Tranquility (node 3) and will be moved to the zenith (Space facing) port of Harmony (node 2) to act as a backup docking port for future manned vessels (Dream Chaser, Dragon Rider, CST-100, and Orion).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-13T19:09:54.210" LastActivityDate="2014-01-13T19:09:54.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3382" PostTypeId="4" CreationDate="2014-01-13T19:09:54.210" Score="0" Body="Questions about PMA (Pressurized Mating Adapters), three of which are deployed on the ISS." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-13T19:09:54.210" LastActivityDate="2014-01-13T19:09:54.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3383" PostTypeId="5" CreationDate="2014-01-13T19:10:53.137" Score="0" Body="&lt;p&gt;Space missions can be manned or unmanned.  This tag is useful to imply that humans are not involved in the mission.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-13T19:10:53.137" LastActivityDate="2014-01-13T19:10:53.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3384" PostTypeId="4" CreationDate="2014-01-13T19:10:53.137" Score="0" Body="Questions about space flight activities without humans onboard." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-13T19:10:53.137" LastActivityDate="2014-01-13T19:10:53.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3385" PostTypeId="1" AcceptedAnswerId="3388" CreationDate="2014-01-13T23:47:48.300" Score="12" ViewCount="2372" Body="&lt;p&gt;The external tank for the shuttle contained &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_external_tank#Liquid_oxygen_tank&quot;&gt;tanks of liquid Oxygen&lt;/a&gt; and Hydrogen for the main engines to use.  These two have similar (though not equal) volumes, but the Oxygen is heavier in an obvious stochastic sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that, I just assumed that the heavier fuel (Oxygen) tank would be on the bottom and the lighter one would be on top, to make the center of mass lower.  It seems like a lower center of mass would make it less likely to tip over during the climb.  But that's not how NASA did it.  The Oxygen tank was on top.  Why?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2014-01-15T13:59:13.657" LastActivityDate="2015-08-14T16:54:04.983" Title="Why was the Shuttle's LOX tank on top of the LH2 tank, since that makes it more top-heavy?" Tags="|space-shuttle|cryogenics|design|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3386" PostTypeId="2" ParentId="2238" CreationDate="2014-01-14T04:25:45.120" Score="6" Body="&lt;p&gt;Besides the issues already mentioned, it's also caused some morale problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike what user6972 claims, we did not have 'vacation'.  Some were designated as essential employees, and were required to work through the shutdown.  Some were sent home, but unlike with a vacation, you couldn't really go anywhere as you might be called back at any time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my case, I was designated as essential-on-call.  This meant that I was required to check in daily to see if there were any emergencies that I needed to respond to, and in one case, I had to actually go into the office to correct a problem that I couldn't deal with remotely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has also resulted in some significant morale issues, as we kept getting conflicting reports from our contract management about what we were or weren't allowed to do remotely.  As our task is staffed by personnel who work for three different companies, everyone got different information; even within the same task, some people's duties were considered essential, others not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we're sitting in a waiting game -- only the civil servants were paid for their time off.  Those of us that are contracts are still waiting to hear months later if we're going to have to burn through weeks of leave to make up our time, or if the government's going to pay us for the time.  We've been told that it depends on the terms of the contract, and that each contract will have to be evaluated separately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The long-term affects won't be known for some time ... and that is, how many people decide to leave NASA and federal work in general so that they don't have to go through this again?&lt;/p&gt;&#xA;" OwnerUserId="46" LastEditorUserId="17425" LastEditDate="2019-08-04T22:59:31.767" LastActivityDate="2019-08-04T22:59:31.767" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="3387" PostTypeId="2" ParentId="3368" CreationDate="2014-01-14T16:33:34.817" Score="12" Body="&lt;p&gt;A few years ago the Japanese spacecraft Ikaros traveled from Earth to Venus using a solar sail.  See &lt;a href=&quot;http://en.wikipedia.org/wiki/IKAROS&quot;&gt;http://en.wikipedia.org/wiki/IKAROS&lt;/a&gt;.  From the article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The IKAROS probe is the world's first spacecraft to use solar sailing as the main propulsion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Other spacecraft (noteably Hayabusa) have used solar pressure for attitude control.  Again, quoting wikipedia (&lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_sail&quot;&gt;http://en.wikipedia.org/wiki/Solar_sail&lt;/a&gt;) :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Both the Mariner 10 mission, which flew by the planets Mercury and Venus, and the MESSENGER mission to Mercury demonstrated the use of solar pressure as a method of attitude control in order to conserve attitude-control propellant.&#xA;  Hayabusa also used solar pressure as a method of attitude control to compensate for broken reaction wheels and chemical thruster&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, no spacecraft has so far used solar sailing as a technique for hovering outside traditional orbital paths, as you have described in your question.&lt;/p&gt;&#xA;" OwnerUserId="1165" LastEditorUserId="2699" LastEditDate="2014-02-20T14:17:45.430" LastActivityDate="2014-02-20T14:17:45.430" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3388" PostTypeId="2" ParentId="3385" CreationDate="2014-01-14T19:13:37.787" Score="25" Body="&lt;p&gt;While in the atmosphere, a forward center of gravity contributes to stability.  You want the center of gravity to be forward of the center of pressure for aerodynamic stability.  A longer distance between the center of gravity and the gimbaling engines gives them more control authority as well.&lt;/p&gt;&#xA;" OwnerUserId="176" LastEditorUserId="574" LastEditDate="2014-01-15T02:33:46.820" LastActivityDate="2014-01-15T02:33:46.820" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3389" PostTypeId="2" ParentId="3371" CreationDate="2014-01-14T21:38:27.957" Score="6" Body="&lt;p&gt;There is an article at &lt;a href=&quot;http://www.spiegel.de/wissenschaft/weltall/apollo-daten-mondbeben-enthuellen-inneres-des-erdtrabanten-a-738206.html&quot; rel=&quot;nofollow&quot; title=&quot;Spiegel Online&quot;&gt;Spiegel Online&lt;/a&gt; in German.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They write about quakes on the moon during the Apollo mission. The astronauts left several sensors on the moon which were sending data to Earth from 1969 to 1977. The quality was (and still is) quite poor, but with modern methods they have been able to analyse the data that allows a glance into past.&lt;br&gt;&#xA;Scientists from NASA used a method called &quot;Double Array Stacking&quot; to intensify the signals. It provides information about the centre of the moon:&lt;br&gt;&#xA;It contains an iron sphere with a diameter of about 480km. Around that sphere there was a liquid mass out of iron and sulphur.&lt;br&gt;&#xA;Over time the moon cooled down - the liquid became more viscous, but it's still not solid.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The melting point of iron is about 1.800K (~1.500°C), so let's assume that the core temperature is in a range between 1.270K and 1.800K (1.000-1.500°C).&lt;br&gt;&#xA;The German site of the &lt;a href=&quot;http://de.wikipedia.org/wiki/Mond&quot; rel=&quot;nofollow&quot; title=&quot;moon-wiki&quot;&gt;moon-wiki&lt;/a&gt; comes to the same result.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anybody is interested in registering for an article in English from the magazine &lt;a href=&quot;http://www.sciencemag.org/content/331/6015/309.abstract&quot; rel=&quot;nofollow&quot; title=&quot;English article&quot;&gt;Science&lt;/a&gt; to share any further information I'd be grateful.  &lt;/p&gt;&#xA;" OwnerUserId="1265" LastEditorUserId="8693" LastEditDate="2016-03-18T09:45:36.197" LastActivityDate="2016-03-18T09:45:36.197" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3390" PostTypeId="5" CreationDate="2014-01-14T21:57:45.407" Score="0" Body="&lt;p&gt;Airlocks are used to separate two areas of different environments, without letting one leak into the other.  In aquatic underwater environments an airlock allows transit between the aquatic environment and the normal air based environment. In space, this allows the transit between the space craft and the vacuum of space. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T21:57:45.407" LastActivityDate="2014-01-14T21:57:45.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3391" PostTypeId="4" CreationDate="2014-01-14T21:57:45.407" Score="0" Body="Questions about airlocks, used to exit a space craft, without venting all the interior atmosphere." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T21:57:45.407" LastActivityDate="2014-01-14T21:57:45.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3392" PostTypeId="5" CreationDate="2014-01-14T22:14:19.737" Score="0" Body="&lt;p&gt;For smaller spacecraft, the Ericson SkyCrane is useful as a heavy lift helicopter to drop the vehicle for landing tests.  Dragon, CST-100, and Dream Chaser have all used it to date.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T22:14:19.737" LastActivityDate="2014-01-14T22:14:19.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3393" PostTypeId="4" CreationDate="2014-01-14T22:14:19.737" Score="0" Body="Questions about Ericson SkyCranes, often used to drop spacecraft for landing tests." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T22:14:19.737" LastActivityDate="2014-01-14T22:14:19.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3394" PostTypeId="5" CreationDate="2014-01-14T22:22:59.647" Score="0" Body="&lt;p&gt;JAXA is the acronym for the Japanese Space Agency. Currently JAXA has three active launch vehicles (Epsilon (light launcher), H-2A for regular payloads, H2-B just for the HTV cargo vehicle) and a research module active at the ISS (Kibo).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T22:22:59.647" LastActivityDate="2014-01-14T22:22:59.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3395" PostTypeId="4" CreationDate="2014-01-14T22:22:59.647" Score="0" Body="Questions about JAXA, the Japanese Space Agency." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T22:22:59.647" LastActivityDate="2014-01-14T22:22:59.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3396" PostTypeId="5" CreationDate="2014-01-14T22:24:27.607" Score="0" Body="&lt;p&gt;The heat shield is used to protect spacecraft that reenter the atmosphere from the heat generated by the friction with the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Capsules generally use a material that either ablates (burns off, slowly) or else is able to withstand the heat load.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The term may also refer to spacecraft structures that are used to protect the spacecraft and manage thermal load while on-orbit in the vacuum of space. An example of this use would be the ceramic cloth heat shield on the MESSENGER Mercury orbiter.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="22352" LastEditDate="2018-01-07T21:44:54.303" LastActivityDate="2018-01-07T21:44:54.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3397" PostTypeId="4" CreationDate="2014-01-14T22:24:27.607" Score="0" Body="Questions about technologies used to protect reentering space craft from the heat of friction from the atmosphere. May also refer to technologies used to block heat from solar radiation while in the vacuum of space." OwnerUserId="51" LastEditorUserId="25" LastEditDate="2018-01-07T21:45:44.570" LastActivityDate="2018-01-07T21:45:44.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3398" PostTypeId="5" CreationDate="2014-01-14T22:26:38.067" Score="0" Body="&lt;p&gt;Pirs was delivered to the ISS by a modified Progress propulsion module. It is docked to the nadir (earth facing) port of the Zvezda (Service Module) module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In April-May of 2014 it will be removed by a departing Progress cargo ship, for destructive reentry, and will be replaced will the Nakua research module (an FGB derivative).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T22:26:38.067" LastActivityDate="2014-01-14T22:26:38.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3399" PostTypeId="4" CreationDate="2014-01-14T22:26:38.067" Score="0" Body="Questions about the Pirs docking module at the ISS." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T22:26:38.067" LastActivityDate="2014-01-14T22:26:38.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3400" PostTypeId="5" CreationDate="2014-01-14T22:27:29.507" Score="0" Body="&lt;p&gt;JAXA builds and launches the HTV on an H2-B launcher, carrying cargo to the ISS.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T22:27:29.507" LastActivityDate="2014-01-14T22:27:29.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3401" PostTypeId="4" CreationDate="2014-01-14T22:27:29.507" Score="0" Body="Questions about the H2 Transfer Vehicle (HTV) launched by the Japanese Space Agency (JAXA) on an H2-B booster as a cargo ship to the ISS." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-14T22:27:29.507" LastActivityDate="2014-01-14T22:27:29.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3402" PostTypeId="1" AcceptedAnswerId="3403" CreationDate="2014-01-15T03:12:56.943" Score="10" ViewCount="653" Body="&lt;p&gt;I was watching the most recent &lt;a href=&quot;http://en.wikipedia.org/wiki/Antares_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;Antares&lt;/a&gt; rocket launch. While I was watching it, I saw extreme close up video when it was even T+5 or T+7 minutes, when it was very far away from Earth. This was not the video from the rocket, it was of the rocket. I was wondering how they recorded this video. Was it an extreme close up from a camera on Earth? And what type of camera did they use for it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is one of these views, recorded at T+9:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UupZU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1301" LastEditorDisplayName="user12" LastEditDate="2014-01-15T04:42:32.353" LastActivityDate="2014-01-15T09:50:42.240" Title="How do they get up-close views of far away spacecraft after launch?" Tags="|launch|camera|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3403" PostTypeId="2" ParentId="3402" CreationDate="2014-01-15T04:42:37.397" Score="13" Body="&lt;p&gt;This will be somewhat different for each launch site, but at most launch facilities, among its buildings, there would also be some that resemble an astronomy observatory with a rotating dome on top (cupola).&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/3KBiu.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;br /&gt;&#xA;Inside, it would have a proper telescope, or even a few of them, most likely refractors (glass type) since they offer better clarity at the center of the image, but that's not necessarily the case and they might as well use off-center image from a reflector (Dobsonian / Newtonian) or hybrid telescopes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/eZM9A.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Observatory buildings along the coast next to the NASA's Wallops Flight Facility (Image credit: &lt;a href=&quot;http://www.nasa.gov/centers/wallops/home/&quot; rel=&quot;noreferrer&quot;&gt;NASA Wallops&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whichever telescope type they use, they would also have to use a motorized &quot;Go-To&quot; telescope mount with a reprogrammed star tracker to only follow the brightest spot, the rocket's exhaust plume. Additional image stabilization could also be done in software, along with correcting for atmospheric refraction, sharpness and all kinds of other things related to image quality, and they would often use multiple cameras / sensors and filter combinations to record the rocket's ascent in different wavelengths, with IR (infrared) wavelength being one of the most important ones on top of visible wavelength that they would mostly transmit for live streams during broadcasts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps you'll find it interesting, here's a few links to vintage videos of the Kennedy Space Center Observatory from 1958 that I found while searching for convincing photographs. Sadly, they're watermarked, but they do loop in the preview:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://footage.shutterstock.com/clip-2543891-stock-footage-the-observatory-in-kennedy-space-center-in-cape-canaveral-florida-circa-mgm-pictures.html&quot; rel=&quot;noreferrer&quot;&gt;The observatory in Kennedy Space Center in Cape Canaveral&lt;/a&gt;, Florida circa 1958 - MGM PICTURES, UNIVERSAL-INTERNATIONAL NEWSREEL, USA, filmed in 1958&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://footage.shutterstock.com/clip-2549456-stock-footage-an-observatory-telescope-rotates-in-cape-canaveral-florida-circa-mgm-pictures-universal.html?src=rel/2543891:0&quot; rel=&quot;noreferrer&quot;&gt;An observatory telescope rotates in Cape Canaveral&lt;/a&gt;, Florida circa 1958 - MGM PICTURES, UNIVERSAL-INTERNATIONAL NEWSREEL, USA, filmed in 1958&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/m0rYU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, I thought they're rather cool, and the second one shows how such a telescope would approximately move to track a rocket during launch. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-15T09:50:42.240" LastActivityDate="2014-01-15T09:50:42.240" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3405" PostTypeId="1" CreationDate="2014-01-15T08:32:53.990" Score="10" ViewCount="821" Body="&lt;p&gt;When we think of solar sails, I guess we mostly think of using radiation pressure of our closest star to achieve thrust, gracefully leaning on it with a large area sail of incredibly light and thin &lt;em&gt;reflective&lt;/em&gt; materials. The idea, we now know, works and there's no doubt about it. We're now way past the early days of testing torque on a tiny carbon fiber sail by &quot;light&quot; alone in laboratory environment, something I still vividly remember watching &lt;em&gt;nearly live&lt;/em&gt; (in those days that meant with months of delay, but it still felt as &lt;em&gt;live&lt;/em&gt; if the science was cutting edge).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the emissions from our Sun aren't exactly &lt;em&gt;clean&lt;/em&gt;, and there's much more to it than what &lt;em&gt;meets the eye&lt;/em&gt;, apparent to most of us only through photographs (and if we're lucky enough also seeing them in person) of polar auroras, a display of lights as the highly charged particles of solar winds hit the upper layers of our atmosphere and ionize the air molecules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such proton flux is however everyday's &lt;em&gt;business as usual&lt;/em&gt; in the outer space, beyond protective shroud of Van Allen radiation belt, with not much in the way of decreasing kinetic energy of these highly charged, high velocity mass particles from solar emissions, so it's hardly radiation pressure alone that actually interacts with whatever we send there. Solar sails being no exception. But as they're meant to be extremely light and thin, I imagine most of such proton emissions would just push through them, with not much matter in their way to decrease their kinetic potential, and would slowly &lt;em&gt;decay&lt;/em&gt; the sail's fabric. So here's my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;With current materials used for solar sails, how fast would proton flux decay them and lower their performance, if used in regions outside of Van Allen belt? What is their life expectancy, and does this mean they wouldn't be suitable for long duration missions, say as means to regain some of the velocity of Aldrin Cyclers as it would be lost during taxiing?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this question on &lt;a href=&quot;https://space.stackexchange.com/q/3367/49&quot;&gt;How much would a solar sail's usefulness be reduced by a perforation?&lt;/a&gt; and its answer, we know that the expected performance is directly proportional to the area ratio of damage it would sustain. Obviously, this proton decay would also be relative to total proton flux hitting it, its density depending on the distance to the source of emissions. But as far as I understand it, it should be directly proportional to solar sail's expected performance (total insolation), and also a function of time, so there should be some way to infer that regardless of where in the outer space it would be used. Additionally, would this decay rate be any different above and below the solar equator, where I expect proton emissions from the Sun are less frequent?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-01-30T08:53:36.943" Title="Durability of solar sails. How big of a problem is high-energy proton flux?" Tags="|materials|solar-sail|long-duration|solar-wind|the-sun|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3406" PostTypeId="5" CreationDate="2014-01-15T10:48:54.370" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-15T10:48:54.370" LastActivityDate="2014-01-15T10:48:54.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3407" PostTypeId="4" CreationDate="2014-01-15T10:48:54.370" Score="0" Body="Procedures followed or measures taken to ensure safety of individuals, assets, or information." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-15T10:48:54.370" LastActivityDate="2014-01-15T10:48:54.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3408" PostTypeId="2" ParentId="3385" CreationDate="2014-01-15T17:27:54.040" Score="8" Body="&lt;p&gt;An additional reason that the forward position is advantageous for the LOX tank is the pressure head created by the distance from the tank to the engines.  The turbopumps that drive the engines need sufficient inlet pressure to prevent cavitation.  Moving the LOX tank forward allows the weight of the LOX to contribute to that pressure, which reduces the necessary ullage pressure within the tank.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-01-15T17:27:54.040" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3409" PostTypeId="1" AcceptedAnswerId="3414" CreationDate="2014-01-16T08:40:25.883" Score="20" ViewCount="5804" Body="&lt;p&gt;I am looking for a cost estimate for a manned (return) mission to Mars. There is a similar question asking for the &lt;a href=&quot;https://space.stackexchange.com/questions/2825/is-there-an-independent-cost-estimate-of-the-mars-one-proposal&quot;&gt;cost of the Mars One proposal&lt;/a&gt;, but I would be happy with a cost estimate for any type of human mission to mars. I need something substantiated and ideally citable, e.g. from someone like NASA. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Manned_mission_to_Mars&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; on this says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The cost of sending people to Mars has been the main obstacle of any mission. Estimates of cost have ranged from \$6 billion to \$500 billion for various crewed programs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are sources given in the article, but I am not sure how substantiated they are. &lt;/p&gt;&#xA;" OwnerUserId="99" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-04-22T11:32:06.833" Title="What is the cost estimate for a manned Mars mission?" Tags="|mars|cost|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3411" PostTypeId="1" AcceptedAnswerId="3413" CreationDate="2014-01-15T20:00:45.483" Score="17" ViewCount="1052" Body="&lt;p&gt;Even after going straight inside the sun's &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_corona&quot;&gt;corona&lt;/a&gt;, how can &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_Probe_Plus&quot;&gt;Solar Probe Plus&lt;/a&gt; be safe at the one million degree Centigrade temperature of the sun's corona ?&lt;/p&gt;&#xA;" OwnerUserId="1441" OwnerDisplayName="AmitG" LastEditorUserId="12102" LastEditDate="2017-05-31T16:07:49.403" LastActivityDate="2017-05-31T16:07:49.403" Title="How can any probe be safe at one million degree Centigrade temperature?" Tags="|artificial-satellite|probe|temperature|the-sun|parker-solar-probe|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3412" PostTypeId="2" ParentId="3411" CreationDate="2014-01-15T21:09:35.473" Score="15" Body="&lt;p&gt;&lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/index.php&quot;&gt;Spacecraft Overview&lt;/a&gt;: &quot;preliminary designs include an 8-foot-diameter, 4.5-inch-thick, carbon-carbon carbon foam solar shield atop the spacecraft body...,  radiators for the solar array cooling system, ... actively cooled solar arrays&quot;. Low &lt;a href=&quot;http://en.wikipedia.org/wiki/Albedo&quot;&gt;albedo&lt;/a&gt; for the heat shield isn't mentioned explicitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A highly elliptical orbit leads to relatively short periods of very high heat exposure, details in &lt;a href=&quot;http://solarprobe.jhuapl.edu/mission/docs/SolarProbeME.pdf&quot;&gt;this paper&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1454" OwnerDisplayName="Gerald" LastActivityDate="2014-01-15T21:09:35.473" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3413" PostTypeId="2" ParentId="3411" CreationDate="2014-01-16T12:35:58.397" Score="23" Body="&lt;p&gt;While the solar corona is very hot, it also has very low density: Wikipedia gives a ballpark figure of about 10&lt;sup&gt;15&lt;/sup&gt; particles per cubic meter, which, at 1 million Kelvins, translates to a pressure of about 0.01 Pa.  That's a pretty good vacuum, comparable to that in low Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The low pressure means that the coronal plasma doesn't hold much heat that it could transfer to the spacecraft.  In practice, a much bigger issue for heat management so close to the Sun is the intense sunlight that transfers a lot of heat to anything that absorbs it.  As Gerald notes in his answer, the main way in which the Solar Probe Plus deals with that issue is by placing a highly reflective and heat-resistant shield on the Sun-facing side of the probe to shield it from sunlight, combined with an efficient cooling system and an elliptical orbit that only takes the probe close to the Sun for relatively short periods at a time.&lt;/p&gt;&#xA;" OwnerUserId="189" OwnerDisplayName="Ilmari Karonen" LastActivityDate="2014-01-16T12:35:58.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3414" PostTypeId="2" ParentId="3409" CreationDate="2014-01-16T15:59:45.213" Score="13" Body="&lt;p&gt;No one today can credibly provide a cost estimate for a crewed mission to the surface of Mars.  That doesn't mean that you can't find estimates, but rather that they will not be useful.  This is reflected quite well in the range of numbers in your quote.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason is that there has not been sufficiently detailed design and production planning for such a mission.  Sufficient in this case would be spending on the order of 5% to 15% of the final mission cost on planning and cost estimation.  That won't happen until someone says go.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-01-16T15:59:45.213" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3417" PostTypeId="2" ParentId="2738" CreationDate="2014-01-17T03:09:19.253" Score="0" Body="&lt;p&gt;There is enough water ice on Ceres. Regarding fuel, what do you mean? One can produce fuel on its surface from water by separating it into oxygen and hydrogen, but this needs energy. And the only energy source there is solar power.&lt;/p&gt;&#xA;" OwnerUserId="812" LastActivityDate="2014-01-17T03:09:19.253" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3418" PostTypeId="1" CreationDate="2014-01-17T11:59:49.283" Score="5" ViewCount="265" Body="&lt;p&gt;Astronauts are highly trained specialists, carefully selected for many characteristics, among them their personality. In many science fiction movies or novels, whole populations travel in space ships for extended periods of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any research, or theoretical extrapolations grounded in research, on how the average person or random samples from the populace deal with prolonged residence in an artificial interior, especially one that does not allow them the oportunity to step &quot;outside&quot; under an open sky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bunkers after a nuclear attack, mining accidents, or submarine habitats may fall into the same category, though all situations contain the possibility of reaching the earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am pretty sure even healthy people would develop serious symptoms of claustrophobia, making space travel impossible for most of us.&lt;/p&gt;&#xA;" OwnerDisplayName="user1448" LastActivityDate="2014-01-25T13:34:50.960" Title="Effects of prolonged space-travel on the human psyche" Tags="|interstellar-travel|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3419" PostTypeId="1" AcceptedAnswerId="3420" CreationDate="2014-01-17T12:14:40.990" Score="5" ViewCount="2075" Body="&lt;p&gt;On a non-scheduled flight, would an interstellar space ship need an active crew consisting of a captain, several officers and a number of technicians, similar to the bridge of the Enterprise, or would one pilot, with maybe a replacement for emergencies, be enough, similar to yachtspeople navigating earth's lakes and oceans alone?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question only pertains to the flying of the ship, not catering to the passengers, maintenance etc.&lt;/p&gt;&#xA;" OwnerDisplayName="user1448" LastActivityDate="2015-01-23T04:16:41.440" Title="How many men or women does it take to fly an interstellar space ship?" Tags="|interstellar-travel|navigation|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="3420" PostTypeId="2" ParentId="3419" CreationDate="2014-01-17T13:09:40.330" Score="18" Body="&lt;p&gt;Zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1&lt;/a&gt; has left the solar system, and is therefore an interstellar spacecraft.  I'm not sure what qualifies a &lt;em&gt;ship&lt;/em&gt;, but there is no reason why it would take any humans.  Nor do yachts (or cars) need humans for navigation, for that matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interstellar space is particularly dull.  You'll be travelling for many years with absolutely nothing happening.  To employ a human to fly a ship would quite literally mean letting him or her sit there from birth to death with no events happening for his or her entire life.  You can't do that to people.  That's what computers are for.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/questions/3419/how-many-men-or-women-does-it-take-to-fly-an-interstellar-space-ship/3420?noredirect=1#comment7131_3420&quot;&gt;comment by Mike Adler&lt;/a&gt; is a good addition, and I will quote it here in its entirety:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Autonomy is always with respect to a time scale. By necessity, spacecraft have always been autonomous on the minutes time scale. Modern spacecraft are autonomous on the days to weeks time scale. Rosetta took care of itself for more than two years. I would expect an interstellar ship would be designed to be autonomous over much longer periods, but would be designed for Earth to check in once every decade or so to see how things are going, and reprogram around any issues uncovered. The &quot;crew&quot; may very well be frozen embryos, awaiting blastulation upon arrival.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="33" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-02-05T16:40:16.580" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="3421" PostTypeId="1" AcceptedAnswerId="3422" CreationDate="2014-01-17T14:38:00.220" Score="12" ViewCount="463" Body="&lt;p&gt;There are &lt;a href=&quot;http://en.wikipedia.org/wiki/Artificial_gravity&quot;&gt;many ways&lt;/a&gt; to produce artificial gravitational effects, such as a rotating spacecraft, linear acceleration, or harnessing magnetism. Possibly there are other ways to produce gravity that have yet to be discovered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we talk about sending people to places beyond Mars, the lack of gravity quickly becomes a problem. It seems that this (along with faster transportation) is one of the biggest obstacles to outer solar system and deep space exploration by humans. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what work is currently being done to explore, develop, and test different methods of producing real or artificial gravity?&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2014-02-03T13:25:20.593" Title="What work is being done to develop and test artificial gravity?" Tags="|gravity|life-support|experiment|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3422" PostTypeId="2" ParentId="3421" CreationDate="2014-01-17T15:48:56.427" Score="12" Body="&lt;p&gt;&lt;a href=&quot;http://ssi.org/&quot; rel=&quot;noreferrer&quot;&gt;Space Studies Institute&lt;/a&gt; started by Gerald Oneil (Of Oneil colonies fame) has been pushing hard for a large centrifuge module in orbit somehow, somewhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point they make is that we know lots about 0G, lots of 1G and literally nothing about in between.  I.e. To know the answer of how much gravity is needed to minimize changes, we need to experiment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And he points out you need two centrifuges. One at 1G, and one at the different level, so you run all your experiments 3 times at the same time.  At 1G, at 0G, and different levels of G.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gary Hudson, president of SSI did a &lt;a href=&quot;http://thespaceshow.wordpress.com/2012/05/30/gary-hudson-tuesday-5-29-12/&quot; rel=&quot;noreferrer&quot;&gt;great podcast&lt;/a&gt; with David Livingstone of the Space Show, on this topic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the short term, the answer is very little research has been done.  The &lt;a href=&quot;http://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module&quot; rel=&quot;noreferrer&quot;&gt;Centrifuge Accomodation Module (CAM)&lt;/a&gt; for the ISS was cancelled in 2005.  It was designed to work on these sorts of experiments on a large scale. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nanoracks.com/products/centrifuge/&quot; rel=&quot;noreferrer&quot;&gt;Nanoracks&lt;/a&gt; has a centrifuge on the ISS right now. From that page it notes &quot; The centrifuge, previously flown on a number of STS missions, can simulate Earth, Moon and Mars gravity.&quot; Looking at the image they show on that page, it is pretty small.  Too small for a mouse even.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4BYhC.jpg&quot; alt=&quot;NanoRacks Centrifuge&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-01-17T18:02:19.877" LastActivityDate="2014-01-17T18:02:19.877" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3423" PostTypeId="2" ParentId="3358" CreationDate="2014-01-17T17:45:39.750" Score="12" Body="&lt;p&gt;The biggest risk I foresee is nitric acid. A hot open flame in the atmosphere will cause N2 and O2 to react, creating a number of different molecules such as NO and NO2. These dissolve reasonably well in water and will form acids.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2014-01-17T17:45:39.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3424" PostTypeId="1" AcceptedAnswerId="6530" CreationDate="2014-01-18T01:43:28.753" Score="10" ViewCount="271" Body="&lt;p&gt;If you are flying to Mars or elsewhere, and you want to use a centrifuge to 'make 1G' to keep the passengers healthy, why not increase it (&lt;em&gt;slowly&lt;/em&gt;) so they are actually at 2G's? Seems like this would make the passengers stronger.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does higher gravity = more healthy? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2014-12-22T14:12:42.310" LastActivityDate="2015-09-24T17:48:33.537" Title="Have there been any studies on the health impacts of prolonged high G?" Tags="|health|artificial-gravity|hypergravity|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3425" PostTypeId="1" AcceptedAnswerId="3427" CreationDate="2014-01-18T17:20:28.290" Score="7" ViewCount="923" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow&quot;&gt;Space Launch System&lt;/a&gt;, which recently secured funding from Congress, uses many features that were developed from the Shuttle program, including the first-stage engines and boosters, and it's overall design looks strikingly similar to the Saturn-V launch vehicle. Obviously this is fine because these technologies have been proven to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the capabilities or technical differences between the SLS boosters and those used by the Shuttle? The wiki page describes the boosters used by the SLS, but I would like to know more about what the upgrades are. &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="49" LastEditDate="2014-04-10T22:53:22.533" LastActivityDate="2014-04-10T22:53:22.533" Title="How will the SRBs used by the SLS differ from those used by the Shuttle?" Tags="|launch|nasa|technology|solid-fuel|sls|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3427" PostTypeId="2" ParentId="3425" CreationDate="2014-01-19T02:23:15.900" Score="7" Body="&lt;p&gt;There are at least three major differences.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;No parachutes for recovery. Saves a bunch of mass.&lt;/li&gt;&#xA;&lt;li&gt;One extra segment. Shuttle used 4 segments, SLS will have 5 segments.&lt;/li&gt;&#xA;&lt;li&gt;Fuel change. The details are murky but they upgraded the specifics of the fuel being used.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Sadly, when you change from 4 to 5 segments, you lose most commonality with the shuttle boosters. A Solid Rocket has no real thrust control (you can steer the nozzle a bit, but at 2.8 million lbs of thrust, not that much) so  most of the control is done by profiling the shape the of propellant on the inside, to control the pattern in which it burns. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus a 4 segments internal profiling of the propellant does not really resemble the 5 segments internal profiling. Thus a lot of the flight history is thrown out by this change.  The external stuff is similar (same casings, joints, nozzle, etc) but the tricky part is the internal profile to control the thrust. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-01-19T02:23:15.900" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3428" PostTypeId="1" AcceptedAnswerId="3432" CreationDate="2014-01-19T03:58:02.830" Score="8" ViewCount="7649" Body="&lt;p&gt;India has recently sent a &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot;&gt;small spacecraft to Mars&lt;/a&gt;, as a technology demonstration mission. It has also subsequently successfully demonstrated its much-delayed cryogenic upper stage engine with the launch of its &lt;a href=&quot;http://en.wikipedia.org/wiki/GSLV_D5&quot;&gt;GSLV D5&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Cryogenic_rocket_engine&quot;&gt;Cryogenic propulsion&lt;/a&gt; is a prerequisite for India's next generation of rocket, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Geosynchronous_Satellite_Launch_Vehicle_Mk_III&quot;&gt;GSLV-Mark-III&lt;/a&gt;. The planned payload capacity of the Mk-3 is much higher than the PSLV rocket which launched India's Mars Orbiter Mission, giving rise to calls to send a more substantive mission to Mars; this in the form of a lander mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on the projected launch capacity of the Mk-3 -&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How massive a lander could India send to Mars? &lt;/li&gt;&#xA;&lt;li&gt;Would there be adequate margin to accommodate a rover?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1456" LastEditorUserId="49" LastEditDate="2015-07-09T12:29:18.750" LastActivityDate="2015-07-09T12:29:18.750" Title="Could India Send a Lander to Mars?" Tags="|mars|propulsion|interplanetary|isro|feasibility|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3429" PostTypeId="2" ParentId="2689" CreationDate="2014-01-19T04:09:09.043" Score="0" Body="&lt;p&gt;From what I've read, it should be RISAT mission - ie. the spacecraft that would be sent to Venus would be similar to the radar imaging satellite RISAT-1 which India put in orbit around Earth. Otherwise, Venus' thick clouds make surface observation impossible.&lt;/p&gt;&#xA;" OwnerUserId="1456" LastActivityDate="2014-01-19T04:09:09.043" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3430" PostTypeId="2" ParentId="2669" CreationDate="2014-01-19T04:16:56.800" Score="2" Body="&lt;p&gt;According to ISRO, the maiden flight of the GSLV-Mark-III, with mission designation of LVM3-X1, is scheduled for April 2014, and will be carrying the Indian crew capsule. This will be a suborbital test flight, and will test the aerothermal characteristics of the capsule during atmospheric re-entry, and likely also communication blackout.&lt;/p&gt;&#xA;" OwnerUserId="1456" LastActivityDate="2014-01-19T04:16:56.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3431" PostTypeId="2" ParentId="2653" CreationDate="2014-01-19T04:54:14.750" Score="9" Body="&lt;p&gt;There are proposals for a follow-up mission to Mars, involving a lander:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.newindianexpress.com/states/kerala/VSSC-Chief-Hints-at-Second-Mars-Mission-Using-GSLV/2013/12/21/article1957410.ece&quot;&gt;http://www.newindianexpress.com/states/kerala/VSSC-Chief-Hints-at-Second-Mars-Mission-Using-GSLV/2013/12/21/article1957410.ece&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This mission would be launched using either the GSLV-Mk2 or Mk3.&lt;/p&gt;&#xA;" OwnerUserId="1456" LastActivityDate="2014-01-19T04:54:14.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3432" PostTypeId="2" ParentId="3428" CreationDate="2014-01-19T11:24:23.697" Score="5" Body="&lt;p&gt;Strictly from the lifting capacity perspective, yes. ISRO now &lt;a href=&quot;http://isro.org/Launchvehicles/GSLVMARKIII/mark3.aspx&quot; rel=&quot;noreferrer&quot;&gt;quotes&lt;/a&gt; expected &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_Satellite_Launch_Vehicle#GSLV_Mk_III&quot; rel=&quot;noreferrer&quot;&gt;GSLV Mk. III&lt;/a&gt; lifting capacity to GTO (Geosynchronous Transfer Orbit) at 4,000 kg, which is greater than, for example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_II&quot; rel=&quot;noreferrer&quot;&gt;Delta II-H 7925H-9.5&lt;/a&gt; that was used for &lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_%28rover%29&quot; rel=&quot;noreferrer&quot;&gt;MER-B&lt;/a&gt; (&lt;em&gt;Opportunity&lt;/em&gt;) which had lifting capacity to GTO at 2,170 kg (1,265 kg to HCO). &lt;em&gt;Spirit&lt;/em&gt; (&lt;a href=&quot;https://en.wikipedia.org/wiki/Spirit_%28rover%29&quot; rel=&quot;noreferrer&quot;&gt;MER-A&lt;/a&gt;) was launched on an even slower transfer orbit (Heliocentric or HCO) on Delta II 7925-9.5 with lifting capacity of up to 1,819 kg to GTO and (1,508 kg to HCO).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's of course assuming ISRO can pull off something similar to MER-A and MER-B and GSLV Mk. III cryogenic upper stage can follow required mission profile (here's a &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/12837/1/01-1164.pdf&quot; rel=&quot;noreferrer&quot;&gt;PDF of NASA's MER project mission profiles&lt;/a&gt;). GTO are otherwise somewhat comparable to HCO, since they require upper stage engine restarts to successfully launch satellites into GTO and achieve desired inclination, something that would also be required to perform &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;noreferrer&quot;&gt;Oberth maneuvers&lt;/a&gt; to reach escape velocity and achieve &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;noreferrer&quot;&gt;Hohmann Transfer Orbit&lt;/a&gt; to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it's very likely that ISRO could hurl, guesstimating here, up to around 3 metric tonnes heavy cargo into HCO. But even expected Mk. III performance (even if it was &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot; rel=&quot;noreferrer&quot;&gt;5,000 kg to GTO&lt;/a&gt;, as Wikipedia quotes it) wouldn't be enough to do something similar to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Science_Laboratory&quot; rel=&quot;noreferrer&quot;&gt;MSL&lt;/a&gt;, that was launched on an &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;noreferrer&quot;&gt;Atlas V 541&lt;/a&gt; with lifting capacity of 8,240 to GTO.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-01-19T11:24:23.697" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3433" PostTypeId="2" ParentId="2670" CreationDate="2014-01-19T13:04:52.220" Score="4" Body="&lt;p&gt;I can think of three uses for fuel depots:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;For a specific mission where payload and fuel are launched&#xA;    separately and dock in LEO. Either to achieve a larger spacecraft,&#xA;    or to make a safer smaller launch for a crew or other irreplacable&#xA;    payload.&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Prelaunched and placed on the surface of, or in the&#xA;        orbit around, the mission target, like Mars.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In geostationary&#xA;        orbit, in order to refill multiple satellites there.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The idea of having gas stations cruise around between different orbits cannot be economical.&lt;/strong&gt; It takes too much fuel to alter orbital inclination in LEO or to reach different GPS-satellites. For such orbits refueling must be satellite specific. &lt;strong&gt;&lt;em&gt;Even a placement in some orbit between Earth and Mars won't work, because if the launch is delayed by a day because of weather, it would miss the depot.&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In geostationary orbit, however, the delta-v between different satellites might be small enough for one fuel depot to service multiple satellites which have been designed to be refueled. The gain would be the satellite owner's option to prolong the lifetime or not, depending on how the technical lifetime and competition and whatnot develops in the future. This could be commercially feasible, the other two alternatives would be for scientifical purposes and governmentally financed. #2 is described in &lt;a href=&quot;http://en.wikipedia.org/wiki/NASA_Design_Reference_Mission_3.0&quot; rel=&quot;nofollow&quot;&gt;NASA's Design Reference Mission 3.0&lt;/a&gt; for Mars, but it is not on any schedule or budget. To some degree #1 is what is done with the ISS.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="288" LastEditDate="2014-01-19T14:43:56.087" LastActivityDate="2014-01-19T14:43:56.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3434" PostTypeId="2" ParentId="3348" CreationDate="2014-01-19T14:01:02.197" Score="1" Body="&lt;p&gt;This SETI Talk by one active researcher gives a &quot;nice&quot; overview of the health problems with space flight:&#xA;&lt;a href=&quot;http://www.seti.org/weeky-lecture/every-body-ark-how-microorganisms-we-carry-will-impact-long-term-space-travel&quot; rel=&quot;nofollow&quot;&gt;http://www.seti.org/weeky-lecture/every-body-ark-how-microorganisms-we-carry-will-impact-long-term-space-travel&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is very discouraging for microgravity. It makes me wonder how humans have been able to survive a year in a space station. And why centrifugal spacecrafts have not been researched at all, they are obviously the only way to go. Anti-biotic resistance caused by mutations from the increased radiation, one would have anyway. But at least bacteria would not grow as fast and float around everywhere as they do in microgravity.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-01-19T14:01:02.197" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3435" PostTypeId="1" CreationDate="2014-01-19T16:04:12.883" Score="7" ViewCount="1046" Body="&lt;p&gt;Last year the &lt;a href=&quot;http://www.mars-one.com/&quot; rel=&quot;noreferrer&quot;&gt;Mars One Foundation&lt;/a&gt; invited volunteers for a &lt;strong&gt;one-way trip&lt;/strong&gt; to take up residence with the War-Lord. Over 200,000 humans volunteered; amongst them close to 20,000 Indians. A fraction of this number - 62 to be precise, have since made the cut for further participation as is evinced from this &lt;a href=&quot;http://articles.timesofindia.indiatimes.com/2014-01-02/india/45798462_1_permanent-human-settlement-mars-one-bas-lansdorp&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt;. Space is known to be a hostile environment; the journey and destination environment even more so.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.advocatekhoj.com/library/bareacts/indianpenalcode/309.php?Title=Indian%20Penal%20Code,%201860&amp;amp;STitle=Attempt%20to%20commit%20suicide&quot; rel=&quot;noreferrer&quot;&gt;Section 309 of the Indian Penal Code&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;309. Attempt to commit suicide&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Whoever attempts to commit suicide and does any act towards the commission of such offence, shall he punished with simple imprisonment for a term which may extend to one year 151[or with fine, or with both].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The hazards involved may be considered a trade-off for a journey that attempts to take man to mars, and return safely. A one-way trip, on the other hand, may be construed suicidal.&lt;/p&gt;&#xA;&lt;p&gt;Will any Indian in the final crew of the Mars One missions invite action under Section 309 of the Indian Penal Code?&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;POST NOTICE: If you are looking for definitive legal answers, seek advice of legal&#xA;professionals. There is no legal advice presented in this question, its answers, &#xA;or 3rd party information referenced herein.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-25T02:59:02.690" Title="Will any Indian in the final crew of the Mars One missions be in violation of Section 309 of the Indian Penal Code?" Tags="|mars|law|mars-one|feasibility|crewed-spaceflight|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3436" PostTypeId="2" ParentId="3435" CreationDate="2014-01-19T16:23:42.070" Score="8" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/IANAL&quot; rel=&quot;nofollow noreferrer&quot;&gt;IANAL&lt;/a&gt; (I Am Not A Lawyer), and this is an unqualified opinion alone, not a legal advice, but I'd say no, since there's no proven intent to harm oneself, so it couldn't be interpreted under section 309.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I asked &lt;a href=&quot;https://space.stackexchange.com/questions/3194/has-any-government-published-an-official-recommendation-regarding-mars-one-appli&quot;&gt;a related question here&lt;/a&gt; if any government has issued an official recommendation regarding Mars One application, but sadly it didn't yet result in any answers. I've also been digging around on my own and found some &lt;a href=&quot;http://cibusmentis.ie/2013/12/23/mars-one-should-i-stay-or-should-i-go/&quot; rel=&quot;nofollow noreferrer&quot;&gt;criticism of the Mars One project from a legal perspective&lt;/a&gt;, but so far it seems no government has decided to issue an official position regarding this project, let alone passed any laws that would specifically list it as means of self-harm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, unless there's a clear declaration from the government that they would consider any such application as a suicidal act, and pass that as a law, I'd wager that they legally can't even consider it as such in the courts of law. And human exploration of space clearly isn't listed as such in broader terms either, if the government sponsors ISRO (Indian Space Research Organisation) a Human Flight Initiative program.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;However, searching again for any government issued warning regarding Mars One project, I stumbled upon this news article, dated February 20, 2014:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://www.independent.co.uk/news/world/middle-east/gulf-imams-issue-fatwa-warning-muslims-not-to-live-on-mars-as-it-would-pose-a-real-risk-to-life-9141631.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gulf imams issue fatwa warning Muslims not to live on Mars as it&#xA;  would pose 'a real risk to life'&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;According to reports in the Khaleej Times, a fatwa committee under the&#xA;  General Authority of Islamic Affairs and Endowment in the UAE,&#xA;  prohibits Muslims from being involved in such a journey as it would&#xA;  pose &quot;a real risk to life&quot; and is tantamount to suicide.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The report emerged after more than 500 people from Saudi Arabia and&#xA;  other Gulf countries were said to have signed up for a spot on the&#xA;  Mars One mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If this fatwa (a legal opinion or ruling issued by Islamic scholars) extends to Muslims in India, or if it was later adopted by their muftis, that I wouldn't know.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;One thing I want to add is that Mars One candidate selection and training processes should prevent inclusion of candidates with suicidal tendencies or those that are otherwise unstable or unfit for the tasks ahead of them. Mars One's candidate selection process is &lt;a href=&quot;https://www.mars-one.com/faq/selection-and-preparation-of-the-astronauts/how-will-the-astronaut-selection-proceed&quot; rel=&quot;nofollow noreferrer&quot;&gt;fairly complex&lt;/a&gt;, so I won't describe it in detail, but suffice to say that candidates will be under supervision of trained professionals while undergoing years long selection and later extremely demanding training. Round 3 and round 4 of the selection process will also be public events, broadcast on TV and internet, with fierce competition between candidates themselves, which should help expose any current or past self-harming tendencies of individual candidates, too.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-12-24T02:13:16.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3438" PostTypeId="2" ParentId="3193" CreationDate="2014-01-19T20:22:38.447" Score="1" Body="&lt;p&gt;The &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/The_Rosetta_lander&quot; rel=&quot;nofollow&quot;&gt;Philae&lt;/a&gt; lander, part of ESA's Rosetta mission to land on a comet, uses the elastic energy from the landing to drive spikes into the ground. Each landing leg contains a threaded spike, the landing compresses the landing leg, this motion is converted to a rotation and extension of the spike. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.simpack.com/fileadmin/simpack/doc/usermeeting04/um04_maxplanck_hilch.pdf&quot; rel=&quot;nofollow&quot;&gt;Presentation containing image of the spikes.&lt;/a&gt;&#xA;&lt;a href=&quot;https://www.youtube.com/watch?v=4BBcoRJ2Jh8&quot; rel=&quot;nofollow&quot;&gt;Video explanation using a Lego model of Philae.&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-01-19T20:22:38.447" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3441" PostTypeId="2" ParentId="3435" CreationDate="2014-01-20T12:57:29.953" Score="7" Body="&lt;p&gt;I don't know how the legal system of India officially defines &quot;suicide&quot;, but usually suicide is considered an act with the sole intention to end ones life. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While Mars One is definitely not a risk-free endeavour, the people who partake in it do not have the intention to die. They expect to survive the trip and then spend the rest of their life on Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a certain risk of lethal accidents involved, but so is in skydiving or freeclimbing, which are, as far as I know, not illegal in India. One could also argue that their life expecancy will be shortened due to the high radiation on Mars, but so does living near a coal power plant or in one of the many slums in India, which isn't illegal either.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-01-20T22:42:19.743" LastActivityDate="2014-01-20T22:42:19.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3442" PostTypeId="2" ParentId="3428" CreationDate="2014-01-20T14:20:08.033" Score="1" Body="&lt;p&gt;India has the lift capacity, as evidenced  by their recent &lt;a href=&quot;http://en.wikipedia.org/wiki/Chandrayaan-2&quot; rel=&quot;nofollow&quot;&gt;mars probe&lt;/a&gt; launch. A small rover could be less mass than the orbiter they sent, which has a laden mass of about 515kg. &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;nofollow&quot;&gt;❸&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, India has built and deployed landers on the moon. &lt;a href=&quot;http://www.pbs.org/newshour/bb/world/jan-june14/indiaspace_01-18.html&quot; rel=&quot;nofollow&quot;&gt;❶&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, it's proveably possible for them to do so. Whether or not the financial and political issues &lt;a href=&quot;http://en.wikipedia.org/wiki/Chandrayaan-2&quot; rel=&quot;nofollow&quot;&gt;❶&lt;/a&gt; will allow them to proceed, is another matter, but the technical capability is already in evidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;❶ Chandrayaan-2 &lt;a href=&quot;http://en.wikipedia.org/wiki/Chandrayaan-2&quot; rel=&quot;nofollow&quot;&gt;en.wikipedia.org/wiki/Chandrayaan-2&lt;/a&gt; &lt;br&gt;&#xA;❷ Indian Mars Mission (PBS) &lt;a href=&quot;http://www.pbs.org/newshour/bb/world/jan-june14/indiaspace_01-18.html&quot; rel=&quot;nofollow&quot;&gt;www.pbs.org/newshour/bb/world/jan-june14/indiaspace_01-18.html&lt;/a&gt; &lt;br&gt;&#xA;❸ Mars Orbiter Mission &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;nofollow&quot;&gt;en.wikipedia.org/wiki/Mars_Orbiter_Mission&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2014-01-21T07:08:23.053" LastActivityDate="2014-01-21T07:08:23.053" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3443" PostTypeId="1" AcceptedAnswerId="3444" CreationDate="2014-01-20T14:57:10.323" Score="21" ViewCount="835" Body="&lt;p&gt;Rosetta has been in hibernation mode for most of the last three years. As far as I am aware, &quot;sleep&quot; means using minimum resources since there is nothing for the probe to do during its long transfer orbit. When sleeping, it must still be able to receive commands so that it can power back up.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does this process work? What electronics on Rosetta are considered crucial and kept running throughout the entire trip? &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="12102" LastEditDate="2021-03-31T10:16:01.740" LastActivityDate="2021-03-31T10:16:01.740" Title="How does Rosetta &quot;wake-up?&quot;" Tags="|electronics|rosetta|deep-space-hibernation|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3444" PostTypeId="2" ParentId="3443" CreationDate="2014-01-20T15:54:41.780" Score="25" Body="&lt;p&gt;From the &lt;a href=&quot;http://download.esa.int/esoc/shares/rosetta_wake-up_faq_20_jan_2014.pdf&quot;&gt;Rosetta wake-up FAQ&lt;/a&gt; (pdf):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Q. Was Rosetta completely shut down?&lt;br&gt;&#xA;  Almost. Only the computer and several heaters remained active. These have been automatically controlled to ensure that the entire satellite doesn't freeze as its orbit took it from 660 million km from the Sun out to 790 million km and back between 2011 and 2014. Everything else on board was shut down, including the transmitters. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The wake-up is controlled by Rosetta's computer. It does not receive an external command, but starts the wake-up sequence based on its internal clock. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wake-up sequence: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... includes switching on the star trackers, slowing the spacecraft's hibernation spin and switching on and warming up certain systems. Once it has exited from hibernation, the spacecraft will switch itself into 'safe mode' - a standard mode of operation that ensures the basic functionality of the satellite. This, too, comprises several procedures, at the end of which Rosetta will orient itself into Earth-pointing mode and switch the transmitter on.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-01-20T15:54:41.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3446" PostTypeId="1" AcceptedAnswerId="3447" CreationDate="2014-01-20T18:57:45.387" Score="10" ViewCount="619" Body="&lt;p&gt;Today, January 20th 2014 AD of the Gregorian Calendar, is the wake-up day of &lt;a href=&quot;http://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot;&gt;Rosetta&lt;/a&gt;.&#xA;Expected AOS window commenced at 1830 UTC before which Rosetta was expected to wake-up, and perform basic house-keeping activities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AOS itself happened over 45min into the window. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Rosetta had failed to wake-up of her own accord, was there a provision to awaken her by signal from Earth?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="12102" LastEditDate="2021-03-31T10:14:42.310" LastActivityDate="2021-03-31T10:14:42.310" Title="Could Rosetta have been awakened from Earth if necessary?" Tags="|rosetta|deep-space-hibernation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3447" PostTypeId="2" ParentId="3446" CreationDate="2014-01-20T19:15:01.287" Score="12" Body="&lt;p&gt;Rosetta was spin-stabilized during her hibernation, which means that the solar panels were not optimally aligned to produce all required power for its transceiver, so in a nutshell - no. Part of this is explained in &lt;a href=&quot;http://download.esa.int/esoc/shares/rosetta_wake-up_faq_20_jan_2014.pdf&quot;&gt;ESA's Rosetta Hibernation Wake Up FAQ&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Q. How will ESA re-establish contact on 20 January? Will mission control send a signal?&lt;/strong&gt;   &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;No, just the opposite. Mission control will&#xA;  'listen' for a signal that will be transmitted by Rosetta. This will&#xA;  be triggered by an 'alarm clock' timer that has been running&#xA;  throughout hibernation. The alarm clock will kick off an on-board&#xA;  wake-up procedure that will culminate with sending the first signal to&#xA;  Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So since Rosetta would have to have her antenna pointed towards the Earth and sufficient power to listen in to any incoming signals, she had to do this wake-up procedure on her own.  This includes her thrusters firing, star tracker functioning, antenna correctly aligning with the Earth,... for all of it to work. The contingency in case she couldn't complete these procedures was to simply repeat them until successful.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-01-20T19:15:01.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3448" PostTypeId="1" AcceptedAnswerId="3449" CreationDate="2014-01-20T20:35:59.417" Score="17" ViewCount="16694" Body="&lt;p&gt;Around the launch of the Hubble Space Telescope, I remember comments in the media that the Hubble may be related to certain espionage satellites. The Hubble was designed and built around the time that the KH-9 series was operated by the NRO. In 2012, a KH-9 was &lt;a href=&quot;http://spaceref.com/onorbit/kh-9-hexagon-spy-satellite-makes-a-rare-public-outing-%28photos-and-video%29.html&quot; rel=&quot;noreferrer&quot;&gt;shown to the public&lt;/a&gt; for the first time.&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/S8uTX.png&quot; alt=&quot;KH-9 during assembly&quot;&gt;&lt;br&gt;&#xA;This shows the KH-9 to be rather different from the Hubble. The optical path is different (the KH-9 looks through an opening in its side, Hubble looks through its top), the camera technology is not even remotely related (film versus digital).&lt;br&gt;&#xA;Now that we know a bit more about Hubble's contemporaries, can a definitive answer be given as to the relationship between the two programs?&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="25" LastEditDate="2014-11-05T19:22:27.913" LastActivityDate="2023-04-26T01:23:58.230" Title="Was Hubble really related to spy satellites?" Tags="|artificial-satellite|telescope|optical|hubble|military|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3449" PostTypeId="2" ParentId="3448" CreationDate="2014-01-21T00:39:05.990" Score="15" Body="&lt;p&gt;It was, in the sense that all spacecraft have relationship to each other. Okay, beyond that, let's see how they are related. &lt;a href=&quot;http://en.wikipedia.org/wiki/Hubble_Space_Telescope#Construction_and_engineering&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; talks about the design of Hubble:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once the Space Telescope project had been given the go-ahead, work on&#xA;  the program was divided among many institutions. Marshall Space Flight&#xA;  Center (MSFC) was given responsibility for the design, development,&#xA;  and construction of the telescope, while Goddard Space Flight Center&#xA;  was given overall control of the scientific instruments and&#xA;  ground-control center for the mission. MSFC commissioned the optics&#xA;  company Perkin-Elmer to design and build the Optical Telescope&#xA;  Assembly (OTA) and Fine Guidance Sensors for the space telescope.&#xA;  Lockheed was commissioned to construct and integrate the spacecraft in&#xA;  which the telescope would be housed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Okay, so let's take that apart. Marshall Space Flight Center designed it, Goddard build the instruments.  Both of those are branches of NASA. So, who builds the spy satellites? Well, that's a bit tricky, but &lt;a href=&quot;http://en.wikipedia.org/wiki/Template:US_Reconnaissance_Satellites&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; again takes a stab, and based on the fact that Hubble was designed so long ago, this should be fairly accurate. We're interested in the IMINT missions, Image Intelligence, and more specifically Electra-optical. Of these, the most similar is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Misty_%28satellite%29&quot; rel=&quot;noreferrer&quot;&gt;Misty satellites&lt;/a&gt;, which, like Hubble, were launched from a Space Shuttle. The Misty satellites are believed to be built by Lockheed Martin, which would indicate a different design path entirely. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so the primary systems don't seem to bear a resemblance, but, as has been mentioned in comments, the optics were built by &lt;a href=&quot;https://en.wikipedia.org/wiki/Perkin-Elmer&quot; rel=&quot;noreferrer&quot;&gt;Perkin Elmer&lt;/a&gt;, for both Spy Satellites and Hubble. So, let's look at that a bit. If they were exactly the same design, then spy satellites would have suffered from the same problem that Hubble did, a blurring of the images that shouldn't be there. But I have little doubt that the basic technology is the same, and there's even a good chance that the same machinery was used to make the mirror that is made for spy satellites. They probably use similar materials as well, and similar processes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My feeling is that there are definitely some commonalities, especially in the optics, but they are fundamentally different. Hubble will have to be more sensitive than a spy satellite, because the nebulas it is designed to see are too dim to see otherwise, while Earth is kind of bright. They probably have similar optics, but Hubble will be forced at infinity focus, while a spy satellite might adjust slightly. Both have similar pointing requirements, and wavelength requirements. The biggest difference, however, is the way sensors are designed. Hubble uses a framing camera, not much different that a camera you own. Most spy satellites use broom cameras, which are more like a fax machine, the Earth moves beneath them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last point of interest is that the question of if Hubble can image Earth has been asked, and &lt;a href=&quot;http://hubblesite.org/reference_desk/faq/answer.php?id=78&amp;amp;cat=topten&quot; rel=&quot;noreferrer&quot;&gt;the answer&lt;/a&gt; states that it can't, because the exposure would blur the image beyond recognition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were going to compare a spacecraft imager to a spy satellite, I would take a look at MRO's HiRISE Camera. The design is quite similar to what I would expect from a spy satellite, although the optics would probably be a bit bigger. But Hubble doesn't really have all that much in common with a spy satellite, although they probably share similar technology.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6214" LastEditDate="2014-11-05T23:36:01.197" LastActivityDate="2014-11-05T23:36:01.197" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3451" PostTypeId="1" CreationDate="2014-01-21T08:45:03.277" Score="6" ViewCount="593" Body="&lt;p&gt;A brief follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/3446/could-rosetta-have-been-awakened-from-earth-if-necessary&quot;&gt;Could Rosetta have been awakened from Earth if necessary?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://download.esa.int/esoc/shares/rosetta_wake-up_faq_20_jan_2014.pdf&quot; rel=&quot;noreferrer&quot;&gt;Rosetta FAQ&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Q. Was Rosetta completely shut down?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Almost. Only the computer and several heaters remained active. These have been automatically controlled to ensure that the entire satellite doesn't freeze ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The same source further writes to say (italics mine!)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Q. What has to happen before Rosetta can transmit its first signal?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The spacecraft has been programmed to only transmit its first signal once the on-board wake-up procedure has reached a certain stage. The internal alarm clock will go off at 10:00 UTC on 20 January, triggering the exit from hibernation. This process that includes switching on the star trackers, slowing the spacecraft's hibernation spin and switching on and &lt;em&gt;&lt;strong&gt;warming up certain systems&lt;/strong&gt;&lt;/em&gt;...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Having a heater to keep the systems from entering a deep-freeze makes sense. If anything such heating would be mandatory given contemporary technology, and the cold of space. &lt;em&gt;Heat transfer, however, requires a medium; something that space has little of.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;Closed body craft, such as &lt;a href=&quot;http://en.wikipedia.org/wiki/ISS&quot; rel=&quot;noreferrer&quot;&gt;ISS&lt;/a&gt;, possess an atmosphere which aids air-conditioning.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How do heaters on Rosetta work?&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are the electronics embedded in some thermally conductive medium?&lt;/li&gt;&#xA;&lt;li&gt;How are mechanical systems heated in such space-bound systems? Hydraulics heating?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-02-14T11:40:16.467" Title="How do space-bound heaters work?" Tags="|rosetta|thermodynamics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3452" PostTypeId="2" ParentId="3451" CreationDate="2014-01-21T10:37:02.860" Score="13" Body="&lt;p&gt;The spacecraft's own truss and elements are thermally conductive, so there's no need for hot air heat distribution. Air (or gas) is actually not such a good heat conductor due to its low density, unless it's highly compressed, at which point you risk mechanical damage to parts it would be heating up. It would also unnecessarily add mass to the spacecraft and unless it was a hermetically tight system, these gases would be just another consumable, limiting your spacecraft's longevity. High-pressure containers also tend to be rather heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most commonly, various shape heating elements are attached to parts of the spacecraft that are thermally sensitive and require specific temperature to function, and the heat they generate is conducted through the spacecraft's own body, or better thermally conductive materials (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Kapton&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kapton&lt;/a&gt;), piping with heat conductive liquids, etc. are used to reach places that might heat up slower or require too much heat to be applied at one point to be later transferred to another. So we'd usually talk of a combination of multiple heating (heat generating) and heat distributing (thermally conductive) elements, applied wherever they're needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically with Rosetta, the supplier of flexible heaters used was RICA that is a part of &lt;a href=&quot;http://www.zoppasindustries.com/en&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zoppas Industries&lt;/a&gt; (here's their &lt;a href=&quot;http://www.zoppasindustries.com/en/product/italiano-resistenze-elettriche-flessibili/&quot; rel=&quot;nofollow noreferrer&quot;&gt;catalog for flexible heating elements and heat distributors&lt;/a&gt;). They supply heating elements for the space industry using silicone, polyimide, polyester, &lt;a href=&quot;https://en.wikipedia.org/wiki/Mica&quot; rel=&quot;nofollow noreferrer&quot;&gt;mica&lt;/a&gt;,... constructions in all kinds of shapes and sizes, including, e.g., &lt;a href=&quot;http://www.zoppasindustries.com/en/?product=resistenze-a-fascia-per-ugello&quot; rel=&quot;nofollow noreferrer&quot;&gt;band heaters for nozzles&lt;/a&gt;. Some other flexible heaters and heat distributors might look like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/iD7dI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Polyimide (Kapton) thermofoil heaters are for use in applications with space and weight limitations, or where the heater will be exposed to vacuum, oil, or chemicals, such as in medical devices, aerospace, aviation,... (source: &lt;a href=&quot;http://machinedesign.com/archive/new-products-thin-flexible-heaters&quot; rel=&quot;nofollow noreferrer&quot;&gt;MachineDesign.com&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-14T11:40:16.467" LastActivityDate="2016-02-14T11:40:16.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3453" PostTypeId="1" CreationDate="2014-01-21T14:51:37.623" Score="13" ViewCount="1655" Body="&lt;p&gt;I'm trying to do some research on this but very much struggling to work it all out. I'm looking for a cutaway diagram of said rocket engine (ideally one used on a first-stage), but I can't for the life of me work out what rocket uses what engine. Can anyone help?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of &quot;widely used&quot;, I'm looking for either (or perhaps both) the engine used on the most number of launches per year, or the engine used on the most varied number of rockets (e.g. is there an engine used on many different rockets?).&lt;/p&gt;&#xA;" OwnerUserId="1470" LastEditorUserId="6944" LastEditDate="2024-03-02T01:43:40.110" LastActivityDate="2024-03-02T01:43:40.110" Title="What is the most widely used first-stage rocket engine in operation today?" Tags="|rockets|launch|record|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3454" PostTypeId="2" ParentId="3453" CreationDate="2014-01-21T15:58:14.347" Score="15" Body="&lt;p&gt;The most numerous launcher is the Soyuz-U, with &lt;a href=&quot;http://en.wikipedia.org/wiki/R-7_%28rocket_family%29&quot;&gt;more than 700 launched&lt;/a&gt;. Its 4 boosters use the &lt;a href=&quot;http://en.wikipedia.org/wiki/RD-107&quot;&gt;RD-107 engine&lt;/a&gt; (&lt;a href=&quot;http://www.npoenergomash.ru/eng/engines/rd107/&quot;&gt;manufactured by Energomash&lt;/a&gt;), so you'd have ca. 2800 engines of a single type.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.raketenspezialisten.de/pdf/jbisdruckvorlage.pdf&quot;&gt;PDF with some drawings of RD-107&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RD-107 is maybe not the best engine to find details on, being a soviet-era design. Other rocket engines may have been built in smaller quantities, but more information is available on them. A Google Image search for 'rocket engine cutaway' gave me a pile of results, the first two being &lt;a href=&quot;http://heroicrelics.org/info/f-1/f-1-cut-away.html&quot;&gt;nice diagrams&lt;/a&gt; of the F-1 engine used in the Saturn V. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-01-21T20:40:33.580" LastActivityDate="2014-01-21T20:40:33.580" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3455" PostTypeId="2" ParentId="3453" CreationDate="2014-01-21T16:27:53.863" Score="8" Body="&lt;p&gt;The NK-33 has been used successfully on two different rockets' first stages (Antares and Soyuz-1), and was planned to be used on several more, including the Soviet N-1F, and I think for Rocketplane Kistler as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RD-170 family of engines have been used on a number of different launch vehicles as a first stage. It was used for the boosters of the Energiya rocket, it's used as the first stage of the Zenit rocket (which is very similar to an Energiya booster).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A half size version called the RD-180 is used on the American Atlas V and is being considered for use on Antares. A much smaller version called RD-150 was used on South Korea's Korea Space Launch Vehicle, and a quater sized version called the RD-190 is being developed for use on Russia's Angara series of rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All those engines - RD-150/170/180/190 are of the same basic design, but will have different flowrates and a different number of nozzles. Everything was essentially scaled down from RD-170 to some degree.&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="1255" LastEditDate="2014-01-21T19:35:40.207" LastActivityDate="2014-01-21T19:35:40.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3456" PostTypeId="1" CreationDate="2014-01-21T20:42:05.567" Score="8" ViewCount="336" Body="&lt;p&gt;Landing rockets take away precious mass from the space system mass budget. Recovering capsules in the air from helicopters and planes equipped with hooks and winches has a long tradition since after WWII (&lt;a href=&quot;https://en.wikipedia.org/wiki/Genesis_%28spacecraft%29&quot;&gt;Genesis&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/CORONA&quot;&gt;CORONA&lt;/a&gt;, etc.)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How large can the capsule be (as a proportion of the mass and size of the recovery aircraft, and in absolute numbers for the largest aircraft out there - including helos and tiltrotor craft)?&lt;/li&gt;&#xA;&lt;li&gt;What are the mass savings (approximately) compared to the conventional landing mode choices (using retrorockets for soft landing, parachute all the way down without retrorockets, parachute and splash on water, parachute and use inflatable cushions)?&lt;/li&gt;&#xA;&lt;li&gt;How does re-entry targeting accuracy depend on the speed and maneuverability of the recovery aircraft?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-01-22T06:33:18.450" LastActivityDate="2014-04-16T10:33:36.610" Title="What is the maximum mass for a reentry capsule that can be recovered in the air?" Tags="|reentry|landing|mission-design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3457" PostTypeId="1" CreationDate="2014-01-21T22:27:15.213" Score="3" ViewCount="1913" Body="&lt;p&gt;A &lt;a href=&quot;https://en.wikipedia.org/wiki/Hickey&quot; rel=&quot;nofollow noreferrer&quot;&gt;hickey&lt;/a&gt; can cause the blood to raise to the top levels of skin, leaving you violet mark on the skin. However, articles about space exposure (&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_exposure#Ebullism.2C_hypoxia.2C_hypocapnia_and_decompression_sickness&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, &lt;a href=&quot;http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;nasa.gov&lt;/a&gt;) do not talk about bleeding at all.&lt;/p&gt;&#xA;&lt;p&gt;So does the blood slowly leak when you happen to be in the vacuum environment, or not? And if not, how come you can suffer such effect with much higher pressure?&lt;/p&gt;&#xA;&lt;p&gt;I always imagine a man that survived vacuum exposure to be all purple-brown.&lt;/p&gt;&#xA;" OwnerUserId="1477" LastEditorUserId="47149" LastEditDate="2022-06-01T00:16:29.957" LastActivityDate="2022-06-01T00:16:29.957" Title="Won't an astronaut exposed to the vacuum of space bleed from everywhere?" Tags="|medical|vacuum|humans|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="3458" PostTypeId="2" ParentId="3453" CreationDate="2014-01-21T22:33:36.213" Score="6" Body="&lt;p&gt;If you have some patience, I will bet on Merlin engines, flown on SpaceX Falcon 9 and Falcon Heavy launchers.  They use 10 per launch.  9 on the first stage, one on the upper stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In only the first 8 launches (as I write this in early 2014) that is 80 engines.  With 50 flights on their manifest to clear, they will hit several hundred engines flown in the next year or two. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Updating this at the end of 2019, there were 76 Falcon 9 flights (760 engine flights), and 3 Falcon Heavy with 28 per flight (84), ignoring the Falcon 1's 5 engines, that is 844 with 20+ flights planned for 2002. That exceeds the SSME and SRB combined numbers already. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Burn time would be fun to compare:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SSME burn time, 480 s (405 burns) &lt;/li&gt;&#xA;&lt;li&gt;SRB burn time, 124 s (270 burns)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Total: 227,880s, 63 hours, 18 min&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;First stage Merlin burn time, 158 s (765 burns)&lt;/li&gt;&#xA;&lt;li&gt;Second stage Merlin burn time, 397 s  (79 burns)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Total: 152,233s, 42 hours, 17 min.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But with 75,000 seconds difference, and about 1819s per mission, Merlin only needs 41 more flights to catch up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course first and second stage burn times vary somewhat based on mission flight profile, this is meant only as an approximation.&#xA;However, as of the current time, the Soyuz booster with the RD-107 is the clear winner with 700+ launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SSME with 135 launches X 3 engines per launch is 405 full duration firings (Maybe -3 for Challengers last flight).  Then of course the SRB's with 270 full duration firings on those same launches.  Now, are the SSME's still in use?  Well not on the Space Shuttle, but in theory on the SLS now known as RS-25D or RS-25E engines.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2019-12-12T14:36:14.493" LastActivityDate="2019-12-12T14:36:14.493" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="3460" PostTypeId="2" ParentId="3457" CreationDate="2014-01-22T14:14:38.537" Score="14" Body="&lt;p&gt;Vacuum on the skin will not normally cause blood to flow through the skin. It can cause the skin to distend (push outward) from the fluid pressure behind it, and to stretch the capillaries under the skin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actual vacuum exposures have shown that even 20-30 minutes of exposure do not result in external bleeding - but they do result in massive bruising. Joseph Kittinger was exposed to near vacuum for an extended period due to a lost glove during his &quot;skydive from space&quot;; his hand swelled, and he had bruising upon return to surface, but did not lose blood through the skin. It took 3 hours for his hand to recover to functional.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible that extremely dry skin could crack and bleed from the distention caused by vacuum. This has not been seen in the handful of known vacuum exposures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Loss of moisture through the eyes, mucus membranes, and lungs are more concerning. Likewise, the very thin membrane of the lining of the nose is much easier to penetrate, and so vacuum exposure could cause nosebleed much more easily than skin surface exposure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Accordining to the Wikipedia entry, 6 people have suffered high quality vacuum exposure - counting Kittinger - and of them, the three fatalities were Soyuz 11's 3-man crew, who were depressurized for an extended period during reentry. &lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-01-22T14:14:38.537" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3461" PostTypeId="1" AcceptedAnswerId="3464" CreationDate="2014-01-22T14:15:49.343" Score="12" ViewCount="1343" Body="&lt;p&gt;Inspired by &lt;a href=&quot;https://space.stackexchange.com/questions/925/what-is-the-smallest-operational-artificial-satellite&quot;&gt;this question&lt;/a&gt;, besides the ISS and Tiangong modules, what is the largest operational, artificial, and uninhabited satellite, by either mass or volume? &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-05-20T13:54:01.657" Title="What is the largest operational artificial satellite?" Tags="|artificial-satellite|record|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3462" PostTypeId="2" ParentId="2618" CreationDate="2014-01-22T18:03:37.340" Score="4" Body="&lt;p&gt;Should also mention that mission plans were drawn up to do this on &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-3xx&quot; rel=&quot;nofollow&quot;&gt;an emergency basis during the Shuttle program&lt;/a&gt;. The post-Columbia plans included a docking at the ISS, and there is mention in the Wikipedia article (without much in the way of references though) to plans before the Columbia accident for direct rendez-vous between the two shuttles.&lt;/p&gt;&#xA;" OwnerUserId="1482" LastActivityDate="2014-01-22T18:03:37.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3463" PostTypeId="2" ParentId="3457" CreationDate="2014-01-22T18:17:17.100" Score="6" Body="&lt;p&gt;I read an article on Soyuz 11. Kind of relates to your question. Due to depressurization, &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_11#Death_of_crew&quot; rel=&quot;noreferrer&quot;&gt;crew of Soyuz 11 died&lt;/a&gt;. The statement given by the chair of state commission was:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Outwardly, there was no damage whatsoever. They knocked on the side,&#xA;  but there was no response from within. On opening the hatch, they&#xA;  found all three men in their couches,motionless, with dark-blue&#xA;  patches on their faces and trails of blood from their noses and ears.&#xA;  They removed them from the descent module.Dobrovolski was still warm.&#xA;  The doctors gave artificial respiration. Based on their reports, the&#xA;  cause of death was suffocation...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_11&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Soyuz 11&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Another &lt;a href=&quot;http://www.space.com/11353-human-spaceflight-deaths-50-years-space-missions.html&quot; rel=&quot;noreferrer&quot;&gt;article on Space.com&lt;/a&gt; also has a list of space hazards involving fatalities.&lt;/p&gt;&#xA;" OwnerUserId="1417" LastEditorUserId="49" LastEditDate="2014-01-24T22:52:14.713" LastActivityDate="2014-01-24T22:52:14.713" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3464" PostTypeId="2" ParentId="3461" CreationDate="2014-01-22T19:15:36.773" Score="12" Body="&lt;p&gt;Let's start with area first. That would probably be Rosetta, with an area of $64\text{ m}^2$. Okay, so how about the most massive? Let's try taking that apart some.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two currently existing &lt;a href=&quot;http://en.wikipedia.org/wiki/Heavy_lift_launch_vehicle&quot; rel=&quot;noreferrer&quot;&gt;heavy launch vehicles&lt;/a&gt;, the Delta IV Heavy, and the Ariane 5 Heavy. Heavier previous vehicles include the Saturn V, N1, the Space Shuttle, and Energia (&lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;). The most massive object will be low in orbit for sure, so let's look to see what LEO missions were launched with these orbiters.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Saturn V- 12 Apollo missions, 1 Skylab project. All manned, and not in orbit. No candidates.&lt;/li&gt;&#xA;&lt;li&gt;Energia- One successful launch, Buran, deorbited and destroyed.&lt;/li&gt;&#xA;&lt;li&gt;N1- No candidates.&lt;/li&gt;&#xA;&lt;li&gt;Space Shuttle- Most likely the heaviest unmanned object it delivered that is still in orbit is &lt;a href=&quot;http://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble&lt;/a&gt;. Wikipedia says it was 11,100 kg at launch, it's probably grown a fair bit since that time. I'm having a bit of a difficult time figuring out the mass change per servicing mission, but I do know the number of instruments went up over time, let's guess it's around 15,000 kg now.&lt;/li&gt;&#xA;&lt;li&gt;Delta IV Heavy- Primarily classified US missions. The latest launch is a likely candidate. The mass would have to be more than the Medium, so it's mass is between 13.5 and 23 metric tons, most likely. &lt;/li&gt;&#xA;&lt;li&gt;Ariane 5- Heaviest I can find is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Albert_Einstein_ATV&quot; rel=&quot;noreferrer&quot;&gt;Albert Einstein ATV&lt;/a&gt;, launched to the ISS but unmanned, 21 metric tons of payload. Of course, that being destroyed, and all ATVs, the largest operational would probably be a GEO satellite, the largest just over 10 metric tons.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, take your guess among these which is the most massive payload. My guess is some Delta IV heavy launched classified payload, the exact mass of which wouldn't be known. Or maybe Hubble.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @Tildalwave mentioned, most likely the largest by volume is the spend &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_18-25_S-IVB_Solar_Trajectory.htm&quot; rel=&quot;noreferrer&quot;&gt;S-IVB stages of the Saturn 5&lt;/a&gt;, with a whopping $326\text{ m}^3$ volume.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-12-19T11:41:33.683" LastActivityDate="2015-12-19T11:41:33.683" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3465" PostTypeId="1" CreationDate="2014-01-23T01:21:37.180" Score="4" ViewCount="644" Body="&lt;p&gt;Is there an established algorithm to determine the subset of the orbital parameters that will place an earth satellite in a given cone during some time window?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say I have a half-cone with its apex on the surface of the earth and opening outwards towards space. Given some range of time $[t_0,t_1]$, I'd like to determine the subset of orbital parameters that would place a vehicle in this cone during this window.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should add that I am interested in initial orbit parameters for two-body system and that the finer points such as precession and nutation can be ignored.&lt;/p&gt;&#xA;" OwnerUserId="1485" LastEditorUserId="49" LastEditDate="2014-01-25T22:54:52.267" LastActivityDate="2014-03-28T19:14:10.413" Title="Is there an algorithm to determine the subset of orbital parameters for vehicles in a cone?" Tags="|orbital-mechanics|orbit|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3467" PostTypeId="2" ParentId="2146" CreationDate="2014-01-23T08:18:35.167" Score="37" Body="&lt;p&gt;The political goals of the Manned Moon Landing program were implicit in President Kennedy's &amp;quot;We choose to go to the moon&amp;quot; speech.&lt;/p&gt;&#xA;&lt;p&gt;Taking  a chunk ending with the infamous quote:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Those who came before us made certain that this country rode the first waves of the industrial revolutions, the first waves of modern invention, and the first wave of nuclear power, and this generation does not intend to founder in the backwash of the coming age of space. We mean to be a part of it--we mean to lead it. For the eyes of the world now look into space, to the moon and to the planets beyond, and we have vowed that we shall not see it governed by a hostile flag of conquest, but by a banner of freedom and peace. We have vowed that we shall not see space filled with weapons of mass destruction, but with instruments of knowledge and understanding.&lt;/p&gt;&#xA;&lt;p&gt;Yet the vows of this Nation can only be fulfilled if we in this Nation are first, and, therefore, we intend to be first. In short, our leadership in science and in industry, our hopes for peace and security, our obligations to ourselves as well as others, all require us to make this effort, to solve these mysteries, to solve them for the good of all men, and to become the world's leading space-faring nation.&lt;/p&gt;&#xA;&lt;p&gt;We set sail on this new sea because there is new knowledge to be gained, and new rights to be won, and they must be won and used for the progress of all people. For space science, like nuclear science and all technology, has no conscience of its own. Whether it will become a force for good or ill depends on man, and only if the United States occupies a position of pre-eminence can we help decide whether this new ocean will be a sea of peace or a new terrifying theater of war. I do not say the we should or will go unprotected against the hostile misuse of space any more than we go unprotected against the hostile use of land or sea, but I do say that space can be explored and mastered without feeding the fires of war, without repeating the mistakes that man has made in extending his writ around this globe of ours.&lt;/p&gt;&#xA;&lt;p&gt;There is no strife, no prejudice, no national conflict in outer space as yet. Its hazards are hostile to us all. Its conquest deserves the best of all mankind, and its opportunity for peaceful cooperation many never come again. But why, some say, the moon? Why choose this as our goal? And they may well ask why climb the highest mountain? Why, 35 years ago, fly the Atlantic? Why does Rice play Texas?&lt;/p&gt;&#xA;&lt;p&gt;We choose to go to the moon. We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard, because that goal will serve to organize and measure the best of our energies and skills, because that challenge is one that we are willing to accept, one we are unwilling to postpone, and one which we intend to win, and the others, too.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;—John F. Kennedy&lt;/em&gt; &lt;a href=&quot;http://er.jsc.nasa.gov/seh/ricetalk.htm&quot; rel=&quot;noreferrer&quot;&gt;❶&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The goals are clear:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;to ensure that no one else weaponizes space&lt;/li&gt;&#xA;&lt;li&gt;to put the US into the leadership role in space technology&lt;/li&gt;&#xA;&lt;li&gt;to give a common goal to the US people&lt;/li&gt;&#xA;&lt;li&gt;to provide new technologies to improve life in the US&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The unspoken goals add:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;to make the USSR take notice that the US wasn't going to back off from confrontations with them.&lt;/li&gt;&#xA;&lt;li&gt;to establish clearly to world leaders that the US could deliver a warhead anywhere in the Earth-Moon orbital envelope.&lt;/li&gt;&#xA;&lt;li&gt;to fuel federal funding into new jobs created by the space program&lt;/li&gt;&#xA;&lt;li&gt;to distract people from the ongoing war in Vietnam&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;By the mid 1970's, NASA had been too successful at launches to the moon. Even Apollo 13 successfully returned. NASA made it look easy, which was good for geopolitical reasons, but not so good for continued funding. The military goals had been shown. The technology goal had resulted in a lot of spinoff technologies improving life in the military, and in civilian life.&lt;/p&gt;&#xA;&lt;p&gt;Moon shots are expensive. The cost to the moon was measured in hundreds of millions of dollars per launch, aboard the singularly most expensive vehicle to date, and it was a one-use vehicle. Further, the actual science goals could be achieved in near orbit for around 10% of the costs of moon shots. Skylab was a reuse of already built hardware intended for Apollo 18 - science on a budget.&lt;/p&gt;&#xA;&lt;p&gt;As for the USSR, the war in Vietnam was too big to not be noticed; it even crossed into the consciousness of elementary school children. (This author included.) The brinksmanship continued, and President Johnson had no personal commitment to continuing the space race, having already &amp;quot;won&amp;quot; it; President Nixon and later Carter likewise saw uses for most of NASA's budget planetside.&lt;/p&gt;&#xA;&lt;p&gt;Only the actual science goals were left. And that didn't require lunar missions, only LEO missions. The post-lunar missions tended to be longer, and still cost less to launch, reducing further the budget draw.&lt;/p&gt;&#xA;&lt;p&gt;Since then, there has been no compelling &amp;quot;race to the moon&amp;quot; and no compelling mineral exploitation opportunity. All of which adds up to &amp;quot;No compelling reason to spend that much on disposable rockets.&amp;quot; And smaller, non-manned-mission lunar rockets with probes are, pound for pound, less expensive than the manned lunar missions.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://er.jsc.nasa.gov/seh/ricetalk.htm&quot; rel=&quot;noreferrer&quot;&gt;❶&lt;/a&gt; NASA, Text of John F. Kennedy's Rice University Moon Speech. &lt;a href=&quot;http://er.jsc.nasa.gov/seh/ricetalk.htm&quot; rel=&quot;noreferrer&quot;&gt;er.jsc.nasa.gov/seh/ricetalk.htm&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-04-03T05:51:45.703" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3468" PostTypeId="1" CreationDate="2014-01-23T19:26:55.850" Score="9" ViewCount="855" Body="&lt;p&gt;What methods keep the internal liquids on lunar rovers from freezing? I understand that temperatures on the Moon are extremely low, way past freezing, especially in places where the Sun does not shine, yet fuels, pneumatic fluids and other liquids essential to rover operations stay functional. How is this done, besides the obvious heating coils and thermal wrappings that may be used? Are these liquids chemically endowed with certain &amp;quot;anti-freeze&amp;quot; components?&lt;/p&gt;&#xA;" OwnerUserId="1493" OwnerDisplayName="user32016" LastEditorUserId="7062" LastEditDate="2022-06-08T11:53:46.267" LastActivityDate="2022-06-08T11:53:46.267" Title="Vehicle liquids freeze on the Moon -- how has this problem been solved?" Tags="|the-moon|physics|temperature|environment|mechanisms|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="3469" PostTypeId="2" ParentId="3468" CreationDate="2014-01-23T20:11:43.043" Score="5" Body="&lt;p&gt;I'm not aware of any liquids essential to rover operations. Motion can be performed by electric stepper motors and gears. Instead of fuel, electricity from solar panels or RTGs (radionuclide thermic generators) is used, stored in lithium ion batteries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If liquids are needed for scientific purposes, they have to be kept at the proper temperature by heating/cooling and thermic isolation, as you state.&lt;/p&gt;&#xA;" OwnerUserId="1454" OwnerDisplayName="Gerald" LastActivityDate="2014-01-23T20:11:43.043" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3470" PostTypeId="2" ParentId="3468" CreationDate="2014-01-23T23:06:15.747" Score="10" Body="&lt;p&gt;If by rovers you mean the Lunokhod and Yutu rovers, they both use (used) nuclear radioisotope sources to assist in thermal control through the lunar nights.  Applicable fluids would be battery electrolytes and gear lubricants.  Batteries cannot be allowed to freeze, and so will depend on the radioisotope sources for survival.  Gear lubricants can be allowed to freeze, so long as they are warmed up before use.  That can be done with electrical heaters once the Sun comes up.&lt;/p&gt;&#xA;&lt;p&gt;As noted in the comments, Yutu uses Li-ion batteries.  They probably need to be kept above -20 °C.&lt;/p&gt;&#xA;&lt;p&gt;Lunokhod had a big turtle-like lid that had the solar panels on the inside.  During the lunar day, the lid was open for the solar panels to collect energy.  During the lunar night, the lid was closed, providing thermal insulation to help retain the heat from the radioisotope source.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="47149" LastEditDate="2022-06-06T20:23:00.807" LastActivityDate="2022-06-06T20:23:00.807" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3471" PostTypeId="2" ParentId="3221" CreationDate="2014-01-24T11:25:14.107" Score="3" Body="&lt;p&gt;Phillip Clark and Ralph Gibbons claim that design and configuration was influenced by a proposal from General Electric for a potential Apollo spacecraft in 1961, which was available to the public (&quot;The Evolution of the Soyuz Program&quot;, 1983).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though the orbital module as well as the propulsion module have a different shape than soyuz' you'll see that the descent module looks very familiar: &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_D-2#Similarity_to_Soyuz&quot; rel=&quot;nofollow&quot;&gt;Apollo D-2&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1146" LastActivityDate="2014-01-24T11:25:14.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3472" PostTypeId="2" ParentId="3468" CreationDate="2014-01-24T13:27:37.750" Score="4" Body="&lt;p&gt;We often think of the freezing point as being the temperature where water freezes (273.15 K).  There are several scales for measuring temperature, for simplicity let's use &lt;a href=&quot;https://en.wikipedia.org/wiki/Kelvin&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kelvin&lt;/a&gt; here.  If you have watched a pot of water boil away you are aware the water has three states: vapor (steam), liquid (water), and solid (ice).  The point where a solid turns to a liquid is often called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Melting_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;melting point&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;According to Wikipedia, temperatures on &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Moon&lt;/a&gt; range from 26 K to 390 K.  If we look at a &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_elements_by_melting_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;list of melting points&lt;/a&gt; we see that element like hydrogen and neon have melting points below 26 K but they also boil around 20 - 27 K, so they would not be a good liquid for using on the Moon, as they would vaporize in all but the coldest lunar winter day.&lt;/p&gt;&#xA;&lt;p&gt;The lunar equator has temperatures ranging from 100 k to 390 K.  As we look at the list of melting points there are some that are liquid around 100 K but they all have fairly short liquid ranges oxygen has one of the bigger ranges of about 45 degrees, but that is between 54 K and 90 K.  It seems our biggest risk then is not in finding an element that will flow during lunar cold spells, but rather one that will not boil during the warm spells.  Even water, which boils at 373.16 K, can boil on the Moon.&lt;/p&gt;&#xA;&lt;p&gt;There are also considerations for pressure, as it can modify the melting and boiling points.  In short, there is no single element that will work as liquid on the Moon, so any fluid would either need to be a compound or a solution (antifreeze) to keep it from freezing, some heat would likely be required for the coldest periods, and pressures maintained to prevent boiling from extremes of temperature or exposure to vacuum.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="47149" LastEditDate="2022-06-06T17:48:32.827" LastActivityDate="2022-06-06T17:48:32.827" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="3473" PostTypeId="1" AcceptedAnswerId="3474" CreationDate="2014-01-24T15:02:19.823" Score="36" ViewCount="5454" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Orion&lt;/a&gt; reentry vehicle will have a parachute system. Like Apollo, they'll have 3:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://www.nasa.gov/exploration/systems/mpcv/gallery/parachute_testing/jsc2013e028102.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yBbeH.jpg&quot; alt=&quot;Orion Parachute Drop Test&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Orion Parachute Drop Test on May 1, 2013&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A model of NASA's Orion spacecraft glides to a successful touchdown&#xA;  during a test of its parachute system on Wednesday, May 1. Orion's&#xA;  three main parachutes, which slow it gradually down for landing, weigh&#xA;  300 pounds each and can cover almost an entire football field.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Photo and caption source: &lt;a href=&quot;http://www.nasa.gov/exploration/systems/mpcv/gallery/parachute_testing/jsc2013e028102.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They seem so nicely separated in pictures.  But from a physics standpoint, the aerodynamic force seems like it should be upward, and that would tend to mush all the chutes together.  That would seem quite dangerous, by decreasing the area or making them crumple.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there anything about the design of the parachutes and tethers that prevents this?  Is there something about the aerodynamics that keeps this from happening?  Or is some bumping just not a big deal?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorDisplayName="user12" LastEditDate="2014-01-24T19:17:09.010" LastActivityDate="2020-01-19T18:29:34.227" Title="Why don't 3-parachute descent systems collide and collapse?" Tags="|nasa|reentry|parachute|separation|orion-spacecraft|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3474" PostTypeId="2" ParentId="3473" CreationDate="2014-01-24T15:21:45.213" Score="39" Body="&lt;p&gt;It's just the aerodynamics.  There is high pressure where the air spills out the side that tends to push them apart more than the forces that you mention that pulls them together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good thing too.  A giant parachute with the same drag would take too long to open.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clustering is very commonly used for cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JbXKD.png&quot; alt=&quot;cluster&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/XnDGO.jpg&quot; alt=&quot;another cluster&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/18Q1s.jpg&quot; alt=&quot;a third cluster&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-01-24T15:21:45.213" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3475" PostTypeId="2" ParentId="3435" CreationDate="2014-01-24T16:44:57.833" Score="3" Body="&lt;p&gt;I would say that any endeavor which has no precedent cannot be legally shown to either result or not result in a given conclusion. For example, if you choose to drink a glass full of an unknown liquid and there is no known previous attempt by anyone to drink it before you, then it cannot be said with total certainty that you will die or not die as a result of drinking it. Hence, since you will be the first one ever to drink this liquid, what happens to you will become the first precedent and it will show whether or not it is a poisonous substance or a harmless one. Once your precedent action is done, then and only then can one say that the next one to drink it will either die or not die, i.e., will or will not commit suicide. Therefore, since no one has yet ever attempted to colonize Mars, or even to go there, no one, in my opinion, can legally say that the endeavor will be a suicidal attempt or not. By this theory, the Indian Penal Code 309, as it applies to the Mars endeavor, does not yet have the experiential legal power to describe the voyage as either suicidal or harmless. &lt;/p&gt;&#xA;" OwnerUserId="1493" LastActivityDate="2014-01-24T16:44:57.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3476" PostTypeId="2" ParentId="3465" CreationDate="2014-01-24T16:51:07.980" Score="0" Body="&lt;p&gt;no i did not find any algorithm. but their is this article on Wikipedia which shows that we don't have some kind of algorithm, instead the half cone that you mentioned has a different value at different position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot; the orbital state vectors (en.wikipedia.org/wiki/Orbital_state_vectors) of an orbit are cartesian vectors of position and velocity that together with their time (epoch ) uniquely determine the trajectory of the orbiting body in space. The body does not actually have to be in orbit for its state vector to determine its trajectory; it only has to move ballistically , i.e., solely under the effects of its own inertia and gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because even satellites in low earth orbit experience significant perturbations (primarily from the non-spherical shape of the earth), the Keplerian elements computed from the state vector at any moment are only valid at that time. Such element sets are known as osculating elements (en.wikipedia.org/wiki/Osculating_elements) because they coincide with the actual orbit only at that moment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An osculating orbit and the object's position upon it can be fully described by the six standard Keplerian orbital elements (osculating elements), which are easy to calculate as long as one knows the object's position and velocity relative to the central body. The osculating elements would remain constant in the absence of perturbations . However, real astronomical orbits experience perturbations that cause the osculating elements to evolve, sometimes very quickly. In cases where general celestial mechanical analyses of the motion have been carried out, the orbit can be described by a set of mean elements with secular and periodic terms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Put in more general terms, a perturbed trajectory can be analysed as if assembled of points, each of which is contributed by a curve out of a sequence of curves. Variables parameterising the curves within this family can be called orbital elements . Typically (though not necessarily), these curves are chosen as Keplerian conics, all of which share one focus. In most situations, it is convenient to set each of these curves tangent to the trajectory at the point of intersection. Curves that obey this condition (and also the further condition that they have the same curvature at the point of tangency as would be produced by the object's gravity towards the central body in the absence of perturbing forces) are called osculating, while the variables parameterising these curves are called osculating&#xA;elements. In some situations, description of orbital motion can be simplified and approximated by choosing orbital elements that are not osculating. Also, in some situations, the standard (Lagrange-type or Delaunay-type) equations furnish orbital elements that turn out to be non-osculating.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My answer : So, as per the above statements you have to make many no. of cones for many instants. Hence you cannot have a single algoritm for complete orbit but at each instant you can apply some formula. I did not calculate it, you can. And may be those computer programmings for orbital calculations also use this method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;plz look these articles on wikipedia, then you will understand better, i have mentioned the links.&lt;/p&gt;&#xA;" OwnerUserId="1417" LastEditorUserId="1417" LastEditDate="2014-01-24T16:56:12.797" LastActivityDate="2014-01-24T16:56:12.797" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3477" PostTypeId="1" AcceptedAnswerId="3478" CreationDate="2014-01-24T18:11:26.377" Score="6" ViewCount="706" Body="&lt;p&gt;Are there any operating SSTO (single stage to orbit) vehicles? Are there others being tested such as Reaction Engine's &lt;a href=&quot;http://en.wikipedia.org/wiki/Skylon_%28spacecraft%29&quot;&gt;Skylon&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2015-07-13T23:34:11.020" Title="Which launch vehicles are considered SSTO?" Tags="|launch|ssto|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3478" PostTypeId="2" ParentId="3477" CreationDate="2014-01-24T18:18:47.053" Score="8" Body="&lt;p&gt;Operating?  No.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under development?  Maybe. Define under development. Someone is always working on an SSTO, but none have ever delivered (not to say it is impossible, just that the track record to date is null). Is Skylon seriously under development?  Hard to say.  People WANT to develop it, but the question is, do they have the needed funding.  As of now, no.  Maybe in the future. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several rocket stages have been shown to possibly be capable of SSTO performance numbers.  (Saturn V second stage, maybe an Atlas without sustainer engines).  But generally the payload delivered would be miniscule, or they have some other limitation.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-01-24T18:18:47.053" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3479" PostTypeId="2" ParentId="3473" CreationDate="2014-01-25T00:44:18.757" Score="6" Body="&lt;p&gt;Much more air flows out the top of a round parachute than out the sides - round canopies without vents oscillate rather violently, notice all the openings in that picture. If you see a canopy with no vents it's probably made of porous fabric, nylon woven for the purpose leaks a LOT of air - you could tape it over your mouth and still breathe comfortably. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Round canopies with drive vents ( search for &quot;Para-Commander&quot;, a last-generation steerable round parachute) do not cluster as their forward speed will cause collisions, and modern sport-skydiving canopies are very unfriendly to their peers. Any jumper with both canopies out has to land &lt;em&gt;very&lt;/em&gt; carefully as the system can collapse with little provocation, and that's usually fatal for the person under it.&lt;/p&gt;&#xA;" OwnerUserId="1497" LastActivityDate="2014-01-25T00:44:18.757" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3480" PostTypeId="1" AcceptedAnswerId="3481" CreationDate="2014-01-25T02:00:02.247" Score="3" ViewCount="424" Body="&lt;p&gt;Apparently a number of (what appear to be) cave entrances on the moon have been identified.  There is a long list of technical advantages to setting up a base in a cave, as opposed to surface modules.  However, all the US proposals for a moon base seem to call for surface modules.  &lt;a href=&quot;https://en.wikipedia.org/wiki/JSC_Moon_Base&quot; rel=&quot;nofollow noreferrer&quot;&gt;One&lt;/a&gt; after the &lt;a href=&quot;http://www.nss.org/settlement/moon/LUNOX.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;other&lt;/a&gt;, after the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_outpost_%28NASA%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;other&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking for more information, I saw an article, &lt;a href=&quot;http://www.universetoday.com/90031/russia-eyes-caves-on-moon-for-setting-up-a-lunar-base/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Russia Eyes Caves on Moon for Setting Up a Lunar Base&lt;/a&gt;.  But a more accurate headline would probably be &lt;em&gt;Boris Kryuchkov thinks that moon cave discoveries will influence moon base plans&lt;/em&gt;.  The article doesn't give indication that the Russian space agency is looking into it (with the possible exception of the title, as often happens with headlines).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to ask: has any space agency done a study of a lunar outpost using in-place cave walls?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The task would still involve taking inflatable modules that would expand to fill the space, and these would still have to be extremely strong.  Even if it's located in a long &quot;lava tube&quot;, the ends still need to be sealed and rated to full atmosphere pressure.  If it's not deep enough, then the walls can't resist the pressure in the first place.  However, one pictures is said to indicate a depth of 100 m, and the surrounding soil pressure would exceed a full atmosphere below 60 m (1/6th gravity x density x height).  That means that (depending on the cave) you could rely on the rock's strength.  But maybe redundancy would rule that out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't mentioned yet, the air delivery systems would be different.  Maybe a &lt;a href=&quot;https://en.wikipedia.org/wiki/Chemical_oxygen_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;chemical Oxygen generator&lt;/a&gt; would be more mass-efficient than a pressure tank?  There are so many things that would affect the assumptions for the mission funding and planning, I couldn't even begin to list them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are NASA plans just outdated because our knowledge of the moon has improved?  To be more straightforward, has there been any study by a space agency that attempts the quantify the huge differences in mission parameters?  Or is this idea just too far out there, or too new for something like that to exist?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="6816" LastEditDate="2017-01-03T23:24:21.370" LastActivityDate="2017-01-03T23:24:21.370" Title="Has any space agency studied a lunar outpost architecture inside of a cave?" Tags="|the-moon|colonization|planning|caves|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3481" PostTypeId="2" ParentId="3480" CreationDate="2014-01-25T02:48:49.100" Score="5" Body="&lt;p&gt;NASA has a long history of considering the possibility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has apparently done some studies, as Daga's paper on Lunar lava tubes implies considerations having already been laid out. ❶ How much and how detailed those studies are is not entirely apparent, but it's clear that NASA is looking for uncollapsed lava tubes on the moon. ❷&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can find plenty of references to NASA having some considerations; I cannot find specific studies of the potential construction explicitly of lava tube bases. Benaroya et al mention the potential use of cast regolith and lunar concrete in passing, for example. ❸ Other such en passant references abount in NASA documents not cited here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Oregon L-5 Society has participated in simulations of lava tube bases using terrestrial lava tubes. ❹ The OL5S has a history of cooperative projects with NASA, and their research is available to NASA, and presented at conferences which NASA partially sponsors, so, it is clear NASA has taken some interest in their work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;❶ Daga, Andrew,  &lt;a href=&quot;http://www.lpi.usra.edu/decadal/leag/AndrewWDagaFINAL.pdf&quot;&gt;http://www.lpi.usra.edu/decadal/leag/AndrewWDagaFINAL.pdf&lt;/a&gt; &lt;br&gt;&#xA;❷ Coulter, Dauna, &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2010/12jul_rabbithole/&quot;&gt;http://science.nasa.gov/science-news/science-at-nasa/2010/12jul_rabbithole/&lt;/a&gt; &lt;br&gt;&#xA;❸ Benaroya, Haym, Leonhard Bernold, and Koon Meng Chua,&#xA;&lt;a href=&quot;http://mvl.mit.edu/jclub/spr04/building-lunar-bases.pdf&quot;&gt;http://mvl.mit.edu/jclub/spr04/building-lunar-bases.pdf&lt;/a&gt; &lt;br&gt;&#xA;❹ Billings, Thomas L, Bryce Walden, and Jan Dabrowski, &lt;a href=&quot;http://www.oregonl5.org/lbrt/l5lbi88.html&quot;&gt;http://www.oregonl5.org/lbrt/l5lbi88.html&lt;/a&gt; &lt;br&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-01-25T02:48:49.100" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3483" PostTypeId="1" CreationDate="2014-01-25T04:30:43.313" Score="18" ViewCount="27174" Body="&lt;p&gt;I assume there is some reason we don't use magnets to launch off Earth.&#xA;Are they not strong enough? Isn't magnetic force technically stronger than gravity??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would either of these work:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;A tube tunnel deep in Earth, lined with electromagnets, leading up into a tower of more, to launch upward or across.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;An electromagnetic ring 'track' around the Earth, using multiple 'orbits' to accelerate, then launch, craft. Like a combination of Tesla's idea &amp;amp; the Hadron collider, perhaps even using 'particle-sized' ships.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Also, is there any way to use Earth's magnetic field or gravity to help?&#xA;For example, could the Earth's poles help at all, if launching from Antarctica?&lt;/p&gt;&#xA;" OwnerUserId="1500" LastActivityDate="2014-10-11T23:46:03.780" Title="Can magnets be used to launch spacecraft?" Tags="|launch|spacecraft-development|spacecraft|magnetic-field|magnetorquer|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3485" PostTypeId="2" ParentId="3483" CreationDate="2014-01-25T05:25:29.487" Score="23" Body="&lt;p&gt;Earth's magnetic field is way too weak to repel against with the force required to launch anything into orbit. Actually, it's really easy to demonstrate that. Take one fridge magnet, place it on the kitchen scale, and write down its weight. Then turn the magnet around and weigh it also with its polarity reversed. You shouldn't see any difference and the scale will show identical weight in both cases, something you'd expect to be much different if Earth's magnetic field was so strong that it would substantially negate gravitational acceleration in one case, and increase attractive force when the magnet's polarity would be flipped the other way around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding &lt;em&gt;tube tunnel with electromagnets&lt;/em&gt;, Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;noreferrer&quot;&gt;escape velocity&lt;/a&gt; on its surface is 11.2 km/s (that's 40,320 km/h or 25,053.7 mph!). Some of this velocity is already provided by the Earth's own rotation, depending on how far from equator the launch site would be (1670 km/hr at equator, 0 at true poles), but the majority would still have to be provided by the launch system. That's a lot to ask of a maglev / railgun system and would require either tremendous power on a relatively short track (a few kilometers perhaps) for accelerations that no human could survive, or roughly 1,472 km (914 mi) long track to keep acceleration down to 4 g, if the launch system would be at equator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To achieve Low Earth Orbit (LEO), required target velocity should reach approximately 8 km/s (28,800 km/h or 17,895 mph) once in LEO, but since there would of course still be all the atmosphere in the way once it would exit the launching system, the exit velocity would have to be much larger than that to negate atmospheric drag. The initial supersonic shock would also require incredibly tough materials to withstand the shock forces and aerodynamic heating, and the &lt;em&gt;projectile&lt;/em&gt; would likely still require some means of propulsion of its own, if for nothing else, then for orbital maneuvers, trajectory corrections, and deorbit burn. Oh, and in case of a human rated system, it would have to have launch escape and safe descent systems too, all of it adding to its size and weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as you see, there are some major drawbacks to such horizontal launching systems using electromagnetic propulsion, and you'd still have to carry most of the systems that are used with rocket launches onboard, even if you exited the &lt;em&gt;launch ramp&lt;/em&gt; at high velocities. A bit more about the hurdles to using railguns as space launch systems is also discussed in two similar questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2370/what-technological-hurdles-prevent-the-development-of-a-space-gun&quot;&gt;What technological hurdles prevent the development of a space gun?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2724/is-it-possible-to-literally-shoot-something-into-space&quot;&gt;Is it possible to literally shoot something into space?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Earth's magnetic field can however be useful as an in-orbit attitude control, mainly to stabilize satellite's alignment along the long axis and/or keep a particular part pointing towards nadir, using what we commonly refer to as &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetorquer&quot; rel=&quot;noreferrer&quot;&gt;magnetorquers&lt;/a&gt;. They work in a similar way to how a magnetized needle self-orients towards magnetic poles, only using powered electromagnetic coils. But, as you probably expect by now, they wouldn't really be reliable over the Earth's magnetic poles due to oscillation in the magnetic field, so they would leave their coils unpowered over those areas if the satellite is in a polar orbit. For more information on how they work, I suggest searching our site using the &lt;a href=&quot;/questions/tagged/magnetorquer&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;magnetorquer&amp;#39;&quot; rel=&quot;tag&quot;&gt;magnetorquer&lt;/a&gt; tag.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-01-27T16:40:33.313" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3486" PostTypeId="1" CreationDate="2014-01-25T06:19:59.523" Score="6" ViewCount="1056" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Metallic_hydrogen#Superconductivity&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on superconductivity of metallic hydrogen&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In 1968, Neil Ashcroft put forward that metallic hydrogen may be a&#xA;  superconductor, up to room temperature (~290 K), far higher than any&#xA;  other known candidate material. This stems from its extremely high&#xA;  speed of sound and the expected strong coupling between the conduction&#xA;  electrons and the lattice vibrations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If a planet like Jupiter has lots of hydrogen, is it likely it could be a giant superconductor?&#xA;What effects could this have &amp;amp; how can we look for or detect such a thing?&lt;/p&gt;&#xA;" OwnerUserId="1500" LastEditorUserId="49" LastEditDate="2014-01-25T07:15:54.537" LastActivityDate="2020-12-06T19:39:58.677" Title="Can a planet or other large body be a superconductor?" Tags="|planetary-science|jupiter|gas-giant|hydrogen|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3487" PostTypeId="1" CreationDate="2014-01-25T06:28:21.403" Score="4" ViewCount="516" Body="&lt;p&gt;I had just read &lt;a href=&quot;http://science.howstuffworks.com/electromagnetic-propulsion1.htm&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; about how they were trying to figure out how to make a vibrating superconductor move in 1 direction, for propulsion, but it didn't sound too promising. So I thought of dust-shaking. Is vibration useful in shaking off Mars dust?&lt;/p&gt;&#xA;" OwnerUserId="1500" LastEditorUserId="1500" LastEditDate="2014-01-25T21:15:29.933" LastActivityDate="2014-01-25T21:15:29.933" Title="Is vibration useful in shaking off Mars dust?" Tags="|mars|rovers|mars-one|dust|vibration|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3488" PostTypeId="2" ParentId="3487" CreationDate="2014-01-25T07:10:45.313" Score="5" Body="&lt;p&gt;Yes, it would be, of course. I'm not sure why you'd need a superconductor for that though, when a lot simpler technologies such as piezoelectric, electrostatic, sonic (basically a &quot;subwoofer&quot;), compressed air, or other types of mechanical drives could be used. Also, simple rotation of solar panels should help too, and let the gravity do the rest, if that's permissible by the rover's design. And sometimes, even weather helps. With MER (Spirit and Opportunity) rovers, when they were caught into a dust storm, solar panels of one (don't remember which one) were cleaned by it, and the other one's were made even worse. So that's perhaps a bit moot point, but it goes to show that there's sufficient atmospheric pressure on Mars to somehow take advantage of winds there too with suitable design, say by designing them to be more flexible, e.g. by using thin-film solar cells.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-01-25T07:10:45.313" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3489" PostTypeId="2" ParentId="3486" CreationDate="2014-01-25T08:12:25.850" Score="4" Body="&lt;p&gt;Yes, and that's believed to be the source of &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter's gigantic magnetosphere&lt;/a&gt;. It is also a possible explanation for &lt;a href=&quot;http://www.astronomy.com/News-Observing/News/2011/02/Superfluid%20and%20superconductor%20discovered%20in%20stars%20core.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;bizarre cooling of Cassiopeia A&lt;/a&gt;. So that's at least two immediate effects of large quantities of metallic hydrogen present in a celestial body; formation of a magnetosphere and faster cooling of their outer cores. Both of these effects are measurable. Magnetosphere of Jupiter was and still is measured by deep space probes, orbiters, any flyby spacecraft. Its side-effects also cause magnetic reconnection events with the Sun's magnetic field, causing polar aurorae and radio frequency emissions that can be observed remotely, some of them even &lt;a href=&quot;https://astronomy.stackexchange.com/a/608/13&quot;&gt;with amateur radio astronomy equipment&lt;/a&gt;. And cooling of the outer core is measurable through Jupiter's density (its mass to volume ratio) that can also be inferred through data obtained with observational astronomy.&#xA;In 2020 Snider et al&lt;a href=&quot;https://www.nature.com/articles/s41586-020-2801-z&quot; rel=&quot;nofollow noreferrer&quot;&gt;4&lt;/a&gt; metalized hydrogen sulfide at 267 GPa and detected superconductivity at a critical temperature of 287K or 15C, the highest so far and validating previous theoretical work on high pressure superconductors&lt;a href=&quot;https://www.sciencedirect.com/science/article/pii/S221137971500073X&quot; rel=&quot;nofollow noreferrer&quot;&gt;5&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="38508" LastEditDate="2020-12-06T19:39:58.677" LastActivityDate="2020-12-06T19:39:58.677" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="3490" PostTypeId="1" CreationDate="2014-01-25T09:14:01.067" Score="4" ViewCount="144" Body="&lt;p&gt;Iapetus, satellite of Saturn, has a huge mountain range along about half its equator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_tallest_mountains_in_the_Solar_System&quot; rel=&quot;nofollow&quot;&gt;this list&lt;/a&gt; of our solar system's biggest mountains, it is the only entry with uncertain origin.&lt;br&gt;&#xA;..The unmeasured peaks of the nameless range are  estimated nearly as high as Olympus on Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are 4 hypotheses for the equatorial ridge's formation: &lt;a href=&quot;http://en.wikipedia.org/wiki/Iapetus_%28moon%29#Equatorial_ridge&quot; rel=&quot;nofollow&quot;&gt;wiki again&lt;/a&gt;&lt;br&gt;&#xA;(&lt;em&gt;To me, the 4th sounds right but incomplete, &amp;amp; the rest seem unlikely&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.theguardian.com/environment/2009/aug/12/mountains-equator&quot; rel=&quot;nofollow&quot;&gt;This Guardian article&lt;/a&gt; says Earth's equatorial mountains are taller than lower-latitude mountains because cold-erosion has a huge effect on mountain height &amp;amp; snowlines etc. &lt;BR&gt;&#xA;..Is this part of the reason for Iapetus' high equatorial mountain range? &lt;BR&gt;&#xA;...Iapetus IS mostly ice...Does/Did it have glaciers?&#xA;&lt;BR&gt;Is Iapetus an extreme case of cold-erosion seen on Earth?&lt;/p&gt;&#xA;" OwnerUserId="1500" LastEditorUserId="8693" LastEditDate="2015-12-16T11:01:45.230" LastActivityDate="2016-02-14T11:38:46.427" Title="Is the equatorial mountain range of Iapetus due to cold erosion?" Tags="|planetary-science|moons|saturn|cassini|geology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3491" PostTypeId="2" ParentId="3487" CreationDate="2014-01-25T09:15:28.890" Score="2" Body="&lt;p&gt;The Mars rovers do not have any dust removing technology. And Opportunity worked fine for 10 years, so the dust problem should not be exaggerated. It is worse on the Moon (and on any asteroid) where there's no atmosphere which has eroded the dust. The regolith on the Moon is very fine, sharp as glass and sticky because of its electrostatic charge. The Apollo astronauts could not avoid bringing it into their lander, and it was even found in their lungs when they had returned to Earth. Thanks to the atmosphere of Mars the dust there is much less dangerous. (But could be full of poisonous chloride salts).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Moon the electrostatically charged (by the cosmic and solar radiation) dust which hoovers above ground might very well we removed/collected by a charged &quot;broom&quot; which sweeps over the ground. But the dust on Mars is more like the sand we know on earth.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-01-25T09:15:28.890" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3492" PostTypeId="2" ParentId="3418" CreationDate="2014-01-25T11:07:03.683" Score="0" Body="&lt;p&gt;Biosphere experiments with people &amp;amp; plants are self-sustained in enclosed environments for months or years. (China can even do city-size projects like that with entire green cities)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Antarctica seems like the closest thing to another planet we have ON our planet.&lt;BR&gt;&#xA;...You can go outside there, but in harsh conditions...even the sky is dangerous.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.bigdeadplace.com/&quot; rel=&quot;nofollow&quot;&gt;Big Dead Place&lt;/a&gt;  ..is a humorous but well-documented site about Antarctica life by people stuck there long periods, working &amp;amp; drinking &amp;amp; dealing with bureaucracy.&lt;/p&gt;&#xA;" OwnerUserId="1500" LastEditorUserId="1500" LastEditDate="2014-01-25T12:23:33.303" LastActivityDate="2014-01-25T12:23:33.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3493" PostTypeId="1" CreationDate="2014-01-25T12:37:30.893" Score="-1" ViewCount="410" Body="&lt;p&gt;I have no specific sources to cite, but years ago it seemed we had more general knowledge &amp;amp; study of the Moon than of Antarctica...&lt;BR&gt;&#xA;...Now however Antarctica is more studied than the Moon, &amp;amp; relevant to general space exploration...&lt;/p&gt;&#xA;" OwnerUserId="1500" LastEditorUserId="1500" LastEditDate="2014-02-04T00:54:58.580" LastActivityDate="2014-02-04T00:54:58.580" Title="Do we still know more about the Moon than Antarctica?" Tags="|the-moon|" AnswerCount="1" CommentCount="3" ClosedDate="2014-01-27T16:33:01.877" ContentLicense="CC BY-SA 3.0" />
  <row Id="3494" PostTypeId="2" ParentId="3418" CreationDate="2014-01-25T13:34:50.960" Score="1" Body="&lt;p&gt;There is no lack in Western history of small crews spending years isolated onboard small ships. Even under very dangerous conditions where most of the people died, and they were fully aware about that when they applied to join! It seems to have worked very well, it colonized the world. Humans are very adaptable, I think that the psychology problems of space travel are vastly overrated. Once you're locked up in that tin can you'll deal with the detailed practicalities to huzzle up a way to survive it.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-01-25T13:34:50.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3495" PostTypeId="2" ParentId="3493" CreationDate="2014-01-25T14:10:18.130" Score="11" Body="&lt;p&gt;This is unlikely. After World War 2, dozens of countries opened research stations on Antarctica. There are &lt;a href=&quot;https://en.wikipedia.org/wiki/Research_stations_in_Antarctica&quot;&gt;more than 50 stations&lt;/a&gt; at the moment, many of them operate year-round. Hundreds of scientists get the opportunity to visit and study Antarctica each year.&lt;br&gt;&#xA;This is in sharp contrast with 6 manned Moon missions totaling a few weeks of human presence on the Moon, plus some unmanned probes. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-01-25T14:10:18.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3500" PostTypeId="2" ParentId="2423" CreationDate="2014-01-26T08:45:08.960" Score="11" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Dynamo_theory#Formal_definition&quot;&gt;Wikipedia on formal definition of the dynamo theory&lt;/a&gt;, which itself paraphrases &lt;a href=&quot;https://www.springer.com/astronomy/astronomy,+observations+and+techniques/book/978-1-4419-1683-9&quot;&gt;The Earth as a Distant Planet&lt;/a&gt;, Vázquez et al.:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are three requisites for a dynamo to operate: &lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;&lt;p&gt;An electrically conductive fluid medium &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Kinetic energy provided by planetary rotation &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;An internal energy source to drive convective motions within the fluid.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All conditions are already met by Mars. But, if we can somehow increase the speed of its rotation, more powerful convection process will be observed due to increased kinetic energy and cause a dynamo action similar to Earth's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can be done in three ways:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Hitting it tangentially with a large asteroid or a number of smaller asteroids.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Slingshooting a bigger body of about one-fourth the mass of Mars around it, increasing its rotation due to the gravitational pull. Could be also achieved with a number of smaller asteroids, but would take longer time to sufficiently increase frequency of its rotation.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Giving Mars a new moon of about one-fourth its mass close enough to increase friction due to tidal forces (tidal friction), increasing the temperature inside its core.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Most people tend to think that the first option is the easiest. But such asteroid impact could also cause Mars to leave its present orbit, destabilizing orbits of terrestrial planets like Earth, in lower orbits around the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm in favour of the second option, as we will be capable of capturing and re-orbiting asteroids in the coming decades.&lt;/p&gt;&#xA;" OwnerUserId="1417" LastEditorUserId="49" LastEditDate="2014-01-26T15:00:57.810" LastActivityDate="2014-01-26T15:00:57.810" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3501" PostTypeId="2" ParentId="5" CreationDate="2014-01-26T12:33:50.990" Score="55" Body="&lt;p&gt;According to &lt;a href=&quot;http://commons.erau.edu/cgi/viewcontent.cgi?article=1055&amp;amp;context=space-congress-proceedings&quot;&gt;History of the Shuttle Landing Facility  at Kennedy Space Center&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;JSC wanted the water moat (located around the runway) since it would&#xA;  serve as a visual aid to identify the  runway. It is also required by&#xA;  the Environmental Protection Agency (EPA) as a retention pond for&#xA;  storm  water runoff from the concrete runway, and provides a barrier&#xA;  against the animals for the Merritt Island  National Wildlife Refuge&#xA;  (MINWR).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Wikipedia's &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Morpheus&quot;&gt;Project Morpheus&lt;/a&gt; page mentions this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As can be seen in the photograph the Kennedy's wide open spaces permit&#xA;  the entire flight path including runway and hazard field to be&#xA;  surrounded by a fire break consisting of a moat filled with water&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1506" LastActivityDate="2014-01-26T12:33:50.990" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3502" PostTypeId="1" AcceptedAnswerId="3518" CreationDate="2014-01-26T14:53:01.583" Score="9" ViewCount="813" Body="&lt;p&gt;NASA is working on an asteroid retrieval mission. A small asteroid will be moved into Lunar orbit where it will be visited by space walking astronauts. Since it will be only max seven meters in diameter, I prefer to call it a meteoroid. The one which exploded over Chelyabinsk last year was about 20 meters in diameter and hence about almost nine times more massive. The retrieved meteoroid will be too small to pose any danger to Earth. Also, I suppose it will have a lower speed relative to Earth, than would an average interplanetary asteroid. They will look for a low delta-v asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it be easier to visit it in low Earth orbit and maybe use the ISS to investigate it?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2014-02-16T00:22:51.480" LastActivityDate="2021-08-12T23:01:51.003" Title="Why does NASA plan to put a meteoroid in Lunar orbit instead of Earth orbit?" Tags="|nasa|asteroid|multi-launch|near-earth-object|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3503" PostTypeId="2" ParentId="3502" CreationDate="2014-01-26T15:19:30.320" Score="6" Body="&lt;p&gt;Safety of our blue planet. Eventually, &lt;a href=&quot;https://en.wikipedia.org/wiki/Bouguer_anomaly&quot;&gt;gravity anomalies&lt;/a&gt; would cause even a perfectly orbited object (a &lt;a href=&quot;https://en.wikipedia.org/wiki/Moonlet&quot;&gt;moonlet&lt;/a&gt;?) to &lt;a href=&quot;https://en.wikipedia.org/wiki/Precession&quot;&gt;preces&lt;/a&gt; and hit the body it orbits around. Since orbiting an asteroid means reducing large fraction of its momentum to bring it closer to celestials it naturally orbits (NASA's plans involve capturing a near-Earth asteroid, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Near-Earth_object&quot;&gt;NEO&lt;/a&gt;, as part of its &lt;a href=&quot;http://www.nasa.gov/mission_pages/asteroids/initiative/&quot;&gt;Asteroid Initiative&lt;/a&gt; program) it wouldn't impact with such velocity as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Chelyabinsk_meteor&quot;&gt;Chelyabinsk meteor&lt;/a&gt; did (required LEO velocity is &lt;em&gt;only&lt;/em&gt; about 8 km/s, which would have to be reduced to deorbit, while the Chelyabinsk meteor entered Earth's atmosphere at an estimated velocity of 18.6 km/s), which caused it to disintegrate to smaller fragments in the atmosphere due to friction and aerodynamic heating. So it would still pose a significant threat to Earth, even if it was much smaller than the Chelyabinsk superbolide was. I'm myself much more in favor of this happening on the Moon than on the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also wouldn't make much difference for the safety of the astronauts studying this asteroid (while it's much bigger than about 1 m in diameter, it can't be considered a &lt;a href=&quot;https://en.wikipedia.org/wiki/Meteoroid&quot;&gt;meteoroid&lt;/a&gt;) or much more difficult to reach, since it couldn't be orbited in Low Earth Orbit (LEO) due to still sufficient atmospheric drag decaying its orbit, and where astronauts would still be somewhat protected from &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind&quot;&gt;solar wind&lt;/a&gt; and high-energy proton particles of &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_ray&quot;&gt;cosmic radiation&lt;/a&gt; by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot;&gt;Van Allen radiation belt&lt;/a&gt;, or they could be reached easier or sent supplies by currently available orbital launch vehicles in case of something going sour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also easier (smaller required &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot;&gt;delta-v&lt;/a&gt;) to later haul the rock back into a safe orbit in &lt;a href=&quot;https://en.wikipedia.org/wiki/Cislunar_space#Cislunar_space&quot;&gt;cis-lunar space&lt;/a&gt; once you're done studying it, if it's orbited around a smaller mass body that's already a significant portion away from the Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot;&gt;Hill sphere&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-26T15:55:33.520" LastActivityDate="2014-01-26T15:55:33.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3504" PostTypeId="2" ParentId="889" CreationDate="2014-01-26T15:41:45.713" Score="1" Body="&lt;p&gt;The options for good gravity assists are limited. The most one can gain is the relative velocity of the object one passes by. The Moon's orbital velocity is only 1 km/s. Objects with low mass are less efficient to use too. Some probes staying close to Earth, like the STEREO probes, used the &lt;a href=&quot;https://space.stackexchange.com/q/4027/4660&quot;&gt;Moon&lt;/a&gt; for gravity assist. &lt;a href=&quot;http://%20nasa.gov/mission_pages/dawn/news/dawnf-20090212.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dawn&lt;/a&gt; and &lt;a href=&quot;http://www.esa.int/spaceinvideos/Videos/2013/10/Rosetta_s_twelve-year_journey_in_space&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta&lt;/a&gt; used Mars. But Jupiter is perfect with 13 km/s in orbit. All six probes which went beyond Jupiter used it for gravity assist (not counting Rosetta which temporarily is a bit beyond Jupiter's orbit). And one might get an extra push from Io which orbits Jupiter at 17 km/s. But it would of course only be helpful for the second half of a trip to Saturn 10 AU away. However, manned travel that far is very futuristic. Today there exists no plans at all to ever send humans beyond the Moon. Several necessary technologies like space nuclear power and centrifugal &quot;gravity&quot; remain completely unresearched and off the agenda.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-07-01T17:33:34.587" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3505" PostTypeId="1" CreationDate="2014-01-26T16:22:00.993" Score="12" ViewCount="737" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Does the CUS of ISRO have restart capability? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If so why was it not tested in the recent GSLV-D5 flight? Maybe by restarting the stage after S/C release?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What enhancements are required to make a Cryogenic engine restartable?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1507" LastEditorUserId="49" LastEditDate="2014-01-26T17:13:52.197" LastActivityDate="2017-10-28T19:47:08.883" Title="Does ISRO's Cryogenic Upper Stage have restart capability?" Tags="|isro|cryogenics|cus|upper-stage|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3506" PostTypeId="2" ParentId="3505" CreationDate="2014-01-26T17:05:57.793" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Indian_Space_Research_Organisation&quot; rel=&quot;nofollow&quot;&gt;ISRO&lt;/a&gt;'s indigenous CUS (&lt;a href=&quot;http://isp.justthe80.com/launchers/isro-s-cryogenic-upper-stage-cus&quot; rel=&quot;nofollow&quot;&gt;Cryogenic Upper Stage&lt;/a&gt;) does have restart capability, but this wasn't tested as it wasn't a requirement of the &lt;a href=&quot;http://www.isro.org/gslv-d5/pdf/brochure.pdf&quot; rel=&quot;nofollow&quot;&gt;GSLV-D5 flight profile&lt;/a&gt; (PDF), so I can't comment on its reliability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISRO did test variable thrust though, which is half-way there already. A full restart would require MECO (Main Engine Cut-Off), completely cutting supply of propellants (&lt;a href=&quot;http://www.astronautix.com/props/loxlh2.htm&quot; rel=&quot;nofollow&quot;&gt;Lox/LH2&lt;/a&gt;) as they're fed to the injectors, as the name suggests, and then repeating ignition. The restart wasn't tested after the stage separation because the flight profile of the GSLV-D5 mission called for a complete stage burnout until propellants were exhausted, so there were no propellants left in the stage to try reigniting its engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third part of your question is difficult to answer, because ISRO doesn't seem to particularly like sharing their designs with the public. &lt;a href=&quot;http://isp.justthe80.com/launchers/isro-s-cryogenic-upper-stage-cus&quot; rel=&quot;nofollow&quot;&gt;Indian Space Projects&lt;/a&gt; mentions this about the CUS propellant feeds and ignitors:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Liquid Oxygen (LOX) and Liquid Hydrogen (LH2) from the respective&#xA;  tanks are fed by individual booster pumps to the main turbo-pump,&#xA;  which rotates at 39,000 rpm to ensure a high flow rate of 16.6 kg/sec&#xA;  of propellants into the combustion chamber. The main turbine is driven&#xA;  by the hot gas produced in a pre-burner. Thrust control and mixture&#xA;  ratio control are achieved by two independent regulators. LOX and&#xA;  Gaseous Hydrogen (GH2) are ignited by pyrogen type igniters in the&#xA;  pre-burner as well as in the main and steering engines during initial&#xA;  stages.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;With &lt;em&gt;main turbo-pump&lt;/em&gt; the article is referring to the CUS FBTP (Fuel Booster Turbo Pump). The article also mentions that CUS uses pyro valves (e.g. here's a &lt;a href=&quot;http://cs.astrium.eads.net/sp/brochures/valves/Pyrotechnic%20Valve.pdf&quot; rel=&quot;nofollow&quot;&gt;PDF document on Astrium built pyro valves&lt;/a&gt; that are Ariane 5 qualified), but sadly isn't specific on their type and if they are Normally Open (NO), Normally Closed (NC), or where in the feed line they're used. Neither it mentions specifics on the pyrogen type igniters, though I presume they would use electric current heated &lt;a href=&quot;https://en.wikipedia.org/wiki/Bridgewire&quot; rel=&quot;nofollow&quot;&gt;bridgewire&lt;/a&gt; pyrotechnic initiators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, sadly, short of finding more descriptive documents on how ISRO implements this, there are just too many possibilities and I'm not confident in speculating over it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-01-29T23:38:19.530" LastActivityDate="2014-01-29T23:38:19.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3507" PostTypeId="1" CreationDate="2014-01-25T21:25:28.337" Score="5" ViewCount="461" Body="&lt;p&gt;Are there any possibilities to counter the negative effects of low or micro gravity by increasing the atmospheric pressure?&lt;/p&gt;&#xA;" OwnerDisplayName="Pradheep" LastEditorUserId="49" LastEditDate="2014-01-26T20:34:57.943" LastActivityDate="2014-01-26T20:34:57.943" Title="Could increased atmospheric pressure act as a countermeasure for effects of microgravity?" Tags="|crewed-spaceflight|microgravity|gravity|health|atmosphere|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3508" PostTypeId="2" ParentId="3507" CreationDate="2014-01-25T22:20:51.777" Score="6" Body="&lt;p&gt;Not all adverse effects of prolonged stay in microgravity environment, for example, it wouldn't make the slightest of difference regarding gravity gradient loss or decalcification, but it could increase atmospheric drag equally on all limbs and effectively help combat muscle atrophy. Problem is, it wouldn't be really practical, nor safe to additionally increase atmospheric pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mixture of breathable air would also have to use some other filler gas than nitrogen to prevent nitrogen narcosis, so perhaps argon that is equally inert. But regardless the filler gas used, increasing atmospheric pressure would additionally increase the load on the structure and further complicate extravehicular excursions (egress and ingress), and depending on the mixture used, might increase fire hazard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other, much simpler techniques with similar effects could be used instead, such as e.g. elastic compression garments, like &lt;a href=&quot;http://www.redorbit.com/news/space/1113044449/astronaut-skinsuit-to-protect-against-back-problems-011114/&quot;&gt;ESA's Skinsuit&lt;/a&gt;, that offer resistance with the use of stretchable fabrics. They might be rather uncomfortable though, and still require strict exercise and dietary regime to help alleviate gravity gradient, body mass and blood volume loss.&lt;/p&gt;&#xA;" OwnerUserId="49" OwnerDisplayName="TildalWave" LastActivityDate="2014-01-25T22:47:12.533" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3509" PostTypeId="1" AcceptedAnswerId="3511" CreationDate="2014-01-27T00:15:34.633" Score="13" ViewCount="15925" Body="&lt;p&gt;Launching from Cape Canaveral has many benefits, including launching over the ocean (in case something needs to be dropped off) and saving energy by being at a lower latitude than most of the United States. Like NASA, why doesn't the Air Force also have a launch complex in Florida instead of southern California? &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-01-27T00:47:07.610" LastActivityDate="2021-12-19T22:30:41.980" Title="Are there any benefits to launching from Vandenberg AFB instead of Cape Canaveral?" Tags="|launch-site|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3510" PostTypeId="2" ParentId="3509" CreationDate="2014-01-27T00:41:28.420" Score="8" Body="&lt;p&gt;It depends on what your goal is. In the case of the Air Force, many of their satellites are designed to be in polar orbits. In that case, California offers a few benefits:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;You can launch north/south&lt;/li&gt;&#xA;&lt;li&gt;You don't have to take as much equatorial spin out of the satellite.&lt;/li&gt;&#xA;&lt;li&gt;The launch trajectory is better for polar, which launch north-north-east. That would go over the East Coast if launched from Florida.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;As far as launching things from Florida, they actually do have a facility, it's called &lt;a href=&quot;http://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cape Canaveral&lt;/a&gt;. They do launch things from there when it make sense to, but a lot of their launches need to be more polar, and thus make sense to launch them from Florida.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-11-16T12:39:32.833" LastActivityDate="2017-11-16T12:39:32.833" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3511" PostTypeId="2" ParentId="3509" CreationDate="2014-01-27T01:18:07.883" Score="11" Body="&lt;p&gt;Good question. Air Force does have a launch facility on East Coast, &lt;a href=&quot;https://space.stackexchange.com/a/3510/49&quot;&gt;like @PearsonArtPhoto mentions&lt;/a&gt;, in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cape Canaveral Air Force Station&lt;/a&gt;, and I believe they could also launch from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Wallops_Flight_Facility&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wallops Flight Facility&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Kennedy_Space_Center&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kennedy Space Center&lt;/a&gt;. But I'd like to focus on whether there's any advantages in launching from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vandenberg_Air_Force_Base&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vandenberg Air Force Base&lt;/a&gt;, i.e. from the US West Coast in particular.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For commercial launches to prograde, low-inclination orbits, I can't think of any convincing advantages except that it &lt;em&gt;might&lt;/em&gt;, in some cases, be easier to retrieve booster and core stages from land than from water, or slightly easier to assume highly inclined or polar orbits over specific territories (e.g. overflying less densely populated areas). This would depend on launch vehicle used though and their possible flight profiles, but in case of launching into retrograde or polar orbits, Vandenberg is more suitable. For example, for Space Shuttle launches from Vandenberg AFB, &lt;a href=&quot;http://history.nasa.gov/shuttleoverview1988/part1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA quotes&lt;/a&gt; advantages as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Launches from Vandenberg have an allowable launch path suitable for&#xA;  polar insertions south, southwest and southeast.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/JwFty.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The launch limits at Vandenberg are 201 and 158 degrees. At a&#xA;  201-degree launch azimuth, the spacecraft would be orbiting at a&#xA;  104-degree inclination. Zero degrees would be due north of the launch&#xA;  site, and the orbital trajectory would be within 14 degrees east or&#xA;  west of the north-south pole meridian. At a launch azimuth of 158&#xA;  degrees, the spacecraft would be orbiting at a 70 degree inclination,&#xA;  and the trajectory would be within 20 degrees east or west of the&#xA;  polar meridian. Like KSC, Vandenberg has allowable launch azimuths&#xA;  that do not pass over habitable areas or involve safety, abort,&#xA;  separation and political considerations.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Quote and image source: &lt;a href=&quot;http://history.nasa.gov/shuttleoverview1988/part1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA History Office&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For military launches though, the large part of the initial ground track during a rocket launch would go over the territory of the United States in case of Vandenberg AFB, where with East Coast launch sites, it is soon over the international waters of the North Atlantic. That might be an advantage with launches where it's absolutely imperative to leave all the tracking to US owned and operated tracking stations (including the ones in North Atlantic that would extend this range due East) and, sort of speak, also &lt;em&gt;cover the tracks&lt;/em&gt; with any staging where the locations where the core stages fall could reveal intended target orbit of the space bound vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, not a U.S.A.F. related incident, but I remember reading about Russian &lt;a href=&quot;https://en.wikipedia.org/wiki/Spy_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;spy ships&lt;/a&gt; disguised as fishing trawlers mooring too close to the Guiana Space Center and even delaying some launches until the French navy managed to chase them away (I'll try to find some links). Why is that potentially a problem becomes more apparent when you consider that assumed orbits of satellites can reveal their purpose. For example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Molniya orbits&lt;/a&gt; are particularly interesting as the satellite lingers over certain locations on the ground for a long period of time (relatively speaking) and at a low altitude perfect for &lt;a href=&quot;https://en.wikipedia.org/wiki/Remote_sensing&quot; rel=&quot;nofollow noreferrer&quot;&gt;remote sensing&lt;/a&gt;. But these orbits are also highly inclined and highly eccentric, something that can be inferred through tracking the flight paths of launched rockets, especially if the launch vehicle's performance is fairly well understood (which can be inferred via previous launches or other means of espionage, like e.g. stolen documents).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;http://articles.latimes.com/1986-07-06/opinion/op-23298_1_u-s-air-force&quot; rel=&quot;nofollow noreferrer&quot;&gt;trying to interfere with communications&lt;/a&gt; with the launched vehicle to sabotage the launch isn't excluded either, so there's a clear advantage to keeping initial ground track over US terra firma, or at least also Canada and later Alaska or Greenland with retrograde or polar orbits, where any such espionage attempts would be easier to detect and deal with, while they might prove impossible or risking an incident in the international waters.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-01-27T05:00:17.880" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3512" PostTypeId="1" CreationDate="2014-01-27T06:16:26.173" Score="5" ViewCount="166" Body="&lt;p&gt;A quick follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/3505/does-isros-cryogenic-upper-stage-have-restart-capability&quot;&gt;Does ISRO&amp;#39;s Cryogenic Upper Stage have restart capability?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What temperature difference do the injectors in a cryo engine experience between the propellant tank, and the ignition chamber?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-03-20T22:07:58.147" Title="What temperature difference do the injectors in a cryo engine experience?" Tags="|propulsion|fuel|cryogenics|fuel-system|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3513" PostTypeId="1" AcceptedAnswerId="3516" CreationDate="2014-01-27T14:06:02.913" Score="8" ViewCount="344" Body="&lt;p&gt;Twitter &lt;a href=&quot;https://twitter.com/search?q=%23Yutu&amp;amp;src=hash&quot; rel=&quot;noreferrer&quot;&gt;is abuzz&lt;/a&gt; with rumors of the Yutu rover being dead. &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2014/01251527-bad-news-for-yutu-rover.html&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; mentions that it might not have been able to prepare for night properly:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;More details are available from the unofficial Twitter account of the mission. Links take you to original Chinese, with Google translation below it. It sounds like the rover is not responding properly to commands from Earth, and &lt;strong&gt;couldn't prepare for the oncoming night properly&lt;/strong&gt;. It doesn't sound good; Yutu appears to be saying farewell to us.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y3jHn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Artist's rendition of a lonely Yutu. Img credit &lt;a href=&quot;https://twitter.com/ShuttleAlmanac/status/427249228306341888/photo/1&quot; rel=&quot;noreferrer&quot;&gt;@ShuttleAlmanac&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What, exactly, does the rover need to do to prepare? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2014-01-27T16:03:30.573" Title="What does the Yutu rover need to do to prepare for night?" Tags="|the-moon|rovers|china|yutu|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3514" PostTypeId="5" CreationDate="2014-01-27T14:11:59.923" Score="0" Body="" OwnerDisplayName="user12" LastEditorDisplayName="user12" LastEditDate="2014-01-27T14:11:59.923" LastActivityDate="2014-01-27T14:11:59.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3515" PostTypeId="4" CreationDate="2014-01-27T14:11:59.923" Score="0" Body="Questions regarding the unmanned Yutu lunar rover, part of China's Chang'e 3 mission to the Moon, launched 1 December 2013." OwnerDisplayName="user12" LastEditorUserId="7062" LastEditDate="2022-01-11T03:06:36.890" LastActivityDate="2022-01-11T03:06:36.890" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3516" PostTypeId="2" ParentId="3513" CreationDate="2014-01-27T16:03:30.573" Score="6" Body="&lt;p&gt;Like &lt;a href=&quot;https://space.stackexchange.com/questions/3513/what-does-the-yutu-rover-need-to-do-to-prepare-for-night#comment6980_3513&quot;&gt;@LocalFluff mentions in the comment&lt;/a&gt;, much of what is circulating the western media in the last few days since news broke out that Yutu is in trouble is unsourced information, but those speculations, however substantiated they might or mightn't be, are regarding its fate and possible procedures to mitigate problems that it encountered, not what the rover would actually have to do to prepare for Lunar night, like I read the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Procedures to prepare the rover for the Lunar night, that lasts roughly 354 hours, are well known and understood. For example, &lt;a href=&quot;http://www.spaceflight101.com/change-3-mission-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt; puts it like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For lunar night, the Yutu rover retracts its mast that carries&#xA;  panoramic cameras and the high gain antenna. In is retracted position,&#xA;  the mast is protected by a box located on the rover deck. Then, one of&#xA;  the two solar panels of the rover would be folded up to cover the mast&#xA;  and prevent heat from radiating into space.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Radioisotope Heater Units are mounted in the vicinity to keep the&#xA;  equipment warm during lunar night, but they are only effective when&#xA;  the mast is shielded by the solar panel. The other solar panel would&#xA;  be oriented towards sunrise to trigger the wake up sequence when the&#xA;  next lunar day begins.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is in accord with reports from other sources, and also how it was explained on China's national broadcasting service &lt;a href=&quot;http://english.cntv.cn/01/index.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;CNTV&lt;/a&gt; during live webcast of the Chang'e 3 lander's Lunar descent, which some of us followed and commented on in our main chat room. Some of these webcast streams might have been recorded and hosted online somewhere, but I wouldn't hold my breath, since the live event lasted for several hours, and most videos I can find aren't longer than perhaps a quarter of an hour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, no &lt;em&gt;rumours&lt;/em&gt; are actually required to understand what the little rover would have to do to protect its equipment during long Lunar nights, but there isn't any end to speculations on what it is to do now, when it experienced problems that aren't well understood at all. But that wasn't the question. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-01-27T16:03:30.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3517" PostTypeId="2" ParentId="3483" CreationDate="2014-01-27T18:34:45.807" Score="4" Body="&lt;p&gt;To answer this portion of the question: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Isn't magnetic force technically stronger than gravity??&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The magnetic force is stronger than gravity, but only at close range.&lt;br&gt;&#xA;Magnetic field strength is proportional to $1/r^3$, where r is the distance to the magnet.&lt;br&gt;&#xA;Earth's gravity is proportional to $1/r^2$, where r is the distance to the center of the earth.&lt;br&gt;&#xA;As a result, when you get further away from the magnet, gravity quickly becomes the dominant force. You can see this in action in &lt;a href=&quot;http://en.wikipedia.org/wiki/Maglev&quot; rel=&quot;noreferrer&quot;&gt;Maglev trains&lt;/a&gt;: they use powerful magnets to hover a few mm above their track. If you want the train to hover further away, the energy required quickly becomes impractical.&lt;br&gt;&#xA;Similarly, if you use the magnets for propulsion, the propulsive force drops quickly when you move away from the magnet, and you're left with an unpowered projectile with the drawbacks explained by @aramis and @tildalwave. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-01-30T14:24:04.817" LastActivityDate="2014-01-30T14:24:04.817" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3518" PostTypeId="2" ParentId="3502" CreationDate="2014-01-28T06:33:58.543" Score="13" Body="&lt;p&gt;It would take vastly more $\Delta V$ to get it to a low-Earth orbit.  The targets selected are close enough to Earth's orbit about the Sun that it only takes around $200\,\mathrm{m/s}$ to get it into a distant retrograde orbit about the Moon.  To get the thing to a low Earth orbit would be around $3\,\mathrm{km/s}$.  The tyranny of the rocket equation makes that infeasible.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-01-28T06:33:58.543" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3519" PostTypeId="1" AcceptedAnswerId="5148" CreationDate="2014-01-28T08:55:56.150" Score="14" ViewCount="1744" Body="&lt;p&gt;Nazareth Bedrossian (from Draper Laboratory) et al. describe the ZPM in &lt;a href=&quot;http://issfd.org/ISSFD_2007/15-4.pdf&quot;&gt;this paper&lt;/a&gt;. What I want to know is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Why is planning and executing the maneuver so difficult it had not been done before 2007?&lt;/li&gt;&#xA;&lt;li&gt;What are the sources of errors that can occur during the maneuver?&lt;/li&gt;&#xA;&lt;li&gt;How does the fact that the ISS is not a rigid body complicate the attitude control problem for large maneuvers?&lt;/li&gt;&#xA;&lt;li&gt;Assuming loss of propulsive attitude control (i.e. failure of Russian segment's thrusters), can ZPMs result in off-nominal situations (e.g. the Station spinning beyond control authority of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Control_moment_gyroscope&quot;&gt;CMGs&lt;/a&gt;)?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Notes and fun facts:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The ISS is equipped with four 4760 Nms Double Gimbal Control Moment Gyroscopes&lt;/li&gt;&#xA;&lt;li&gt;Each CMG has a flywheel nominally spinning at 691 rad/s (6600 rpm)&lt;/li&gt;&#xA;&lt;li&gt;Each CMG can produce a maximum output torque of 258 Nm&lt;/li&gt;&#xA;&lt;li&gt;There are software limitations on gimbal rates of max. $0.014 \frac{rad}{s}$ ($0.8 \frac{deg}{s}$) and gimbal acceleration of max. $0.0007 \frac{rad}{s^2}$ ($0.04 \frac{deg}{s^2}$) to extend CMG lifetime and lower the probability of failure.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100021932_2010023824.pdf&quot;&gt;Gurrisi, Seidel, Dickerson et al. (2010)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-01-28T10:30:24.793" LastActivityDate="2018-03-28T02:48:58.140" Title="What is so difficult about the ISS zero propellant maneuver?" Tags="|iss|attitude|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3520" PostTypeId="1" AcceptedAnswerId="3521" CreationDate="2014-01-28T11:18:00.123" Score="73" ViewCount="71729" Body="&lt;p&gt;Basically, what the title asks. To my understanding, &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot;&gt;New Horizons&lt;/a&gt; is currently the fastest spacecraft that's moving away from the Sun, with arguably only &lt;a href=&quot;https://en.wikipedia.org/wiki/Helios_probes&quot;&gt;Helios-A and Helios-B&lt;/a&gt; possibly faster still, but in an elliptical orbit around the Sun, so they won't make it out of the heliosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So I'm curious, when will New Horizons traverse larger distance from the Sun than the &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1&quot;&gt;Voyager 1&lt;/a&gt; will in that time, and at approximately what distance?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not entirely sure this can yet be calculated with any precision, since we don't know which Kuiper belt object will be selected for New Horizons visit after its Pluto flyby (or do we?), so feel free to make broad assumptions in your calculations.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="68" LastEditDate="2014-01-28T15:57:46.400" LastActivityDate="2015-07-29T01:16:11.530" Title="When will New Horizons overtake Voyager 1 in distance from the Sun?" Tags="|probe|voyager|navigation|new-horizons|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3521" PostTypeId="2" ParentId="3520" CreationDate="2014-01-28T11:38:42.643" Score="86" Body="&lt;p&gt;&lt;strong&gt;New Horizons will never overtake Voyager 1.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although New Horizons is &lt;em&gt;currently&lt;/em&gt; faster than any other man-made object, it won't be by the time it reaches the outer corners of the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://pluto.jhuapl.edu/news_center/news/081706.php&quot;&gt;John Hopkins University/Applied Physics Laboratory New Horizons page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Though New Horizons will also reach 100 AU, it will never pass Voyager 1, because Voyager was boosted by multiple gravity assists that make its speed faster than New Horizons will travel. Voyager 1 is escaping the solar system at 17 kilometers per second. When New Horizons reaches that same distance 32 years from now, propelled by a single planetary swingby, it will be moving about 13 kilometers per second. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="33" LastActivityDate="2014-01-28T11:38:42.643" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3522" PostTypeId="2" ParentId="3520" CreationDate="2014-01-28T13:00:28.587" Score="50" Body="&lt;p&gt;Here's a nice graph of Voyager 2's speed, and the difference made by the gravitational assists:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/qiWvW.png&quot; alt=&quot;Voyager speed graph&quot;&gt;&lt;br&gt;&#xA;You can see that the probe slowed down between assists. New Horizons would follow a similar graph, but with fewer assists its speed will end up below Voyager 2's, as @gerrit said. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-01-28T13:00:28.587" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="3523" PostTypeId="1" CreationDate="2014-01-28T18:30:11.670" Score="12" ViewCount="1491" Body="&lt;p&gt;Luna orbits Earth which in turn orbits around Sol. The period of the Lunar rotation is around 28 Earth days; i.e. one lunar day + one lunar night.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike Earth, Luna is not possessed of an atmosphere. This leads to extremely sharp temperature, and illumination gradients. The side of Luna facing away from the Sun is as dark as the other is bright.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a clear winter new-moon night starlight alone is enough to make out the general landscape, flora &amp;amp; fauna here on Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the average illumination incident on the side of the moon facing away from the Sun?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;p.s. I seek the approximate average amount of light incident per unit area on the dark side in terms of a ratio/percentage/fraction of the sun-illuminated side&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-01-28T19:12:37.397" LastActivityDate="2022-06-08T16:18:07.407" Title="How dark is the side of the Moon in shadow as compared to the side facing the Sun?" Tags="|the-moon|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3524" PostTypeId="1" AcceptedAnswerId="5627" CreationDate="2014-01-29T04:38:15.160" Score="10" ViewCount="705" Body="&lt;p&gt;The 4th orbit raising manoeuvre of ISRO's Mars probe (MOM or &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;noreferrer&quot;&gt;Mars Orbiter Mission&lt;/a&gt;, informally  called Mangalyaan) did not go as planned, as the &lt;a href=&quot;http://www.isro.gov.in/pressrelease/scripts/pressreleasein.aspx?Nov11_2013&quot; rel=&quot;noreferrer&quot;&gt;LAM (Liquid Apogee Motor) shut down prematurely&lt;/a&gt;. According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Indian_Space_Research_Organisation&quot; rel=&quot;noreferrer&quot;&gt;ISRO&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, when both primary and redundant coils were energised&#xA;  together, as one of the planned modes, the flow to the Liquid Engine&#xA;  stopped.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;What was the root cause of this failure?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Was this redundant mode tested on ground before launch?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Will ISRO be uploading a software patch to ensure redundancy at MOI (Mars Orbit Insertion)?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Assuming that both primary and redundant coils can be energized independently, but not together, for operation of the valve, &lt;em&gt;one cause of this anomaly which &lt;strong&gt;I could think of&lt;/strong&gt; is, that the coils are wound in opposite directions&lt;/em&gt;. Or the current to one of the coils is being fed in opposite direction, maybe because the terminals got connected in the wrong order, thus resulting in cancellation of magnetic flux / force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;update ( to expand the question )&lt;/strong&gt;&#xA;Why have redundant solenoid coils but not redundant valves? Is it not more likely that mechanical components ( valves ) are more likely to fail than the electrical components ( coils )? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also what is the intention of energizing the coils together:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Extra force in case the valve is stuck or sluggish?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;or&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;To open the valve wider for more thrust?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1507" LastEditorUserId="1507" LastEditDate="2014-02-05T09:03:06.493" LastActivityDate="2014-10-17T07:33:05.230" Title="Cause of ISRO MOM's LAM shutdown during the 4th orbit raising manoeuvre" Tags="|mars|probe|orbital-maneuver|isro|mars-orbiter-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3525" PostTypeId="5" CreationDate="2014-01-29T10:04:49.810" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-01-29T10:04:49.810" LastActivityDate="2014-01-29T10:04:49.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3526" PostTypeId="4" CreationDate="2014-01-29T10:04:49.810" Score="0" Body="New Horizons is NASA's space probe launched in early 2006 towards the dwarf planet Pluto as the first spacecraft to fly by and study this Kuiper Belt Object (KBO) and its moons, Charon, Nix, Hydra, Kerberos, and Styx, in July 2015. Spacecraft's operational constraints permitting, it will be redirected to study another KBO, preferably a white body (icy plutoid) with its own moon(s) that's close on the path of New Horizon's current trajectory." OwnerUserId="298" LastEditorUserId="49" LastEditDate="2014-02-02T19:20:13.767" LastActivityDate="2014-02-02T19:20:13.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3527" PostTypeId="1" AcceptedAnswerId="3528" CreationDate="2014-01-29T10:06:20.137" Score="4" ViewCount="319" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/3522/472&quot;&gt;this terrific answer&lt;/a&gt; user Hobbes presents a graph of escape velocity as a function of distance from the Sun:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qiWvW.png&quot; alt=&quot;Voyager speed graph&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I notice that at about 14 AU the graph goes from decreasing exponentially to decreasing linearly. This looks to me like there must be some significant mass distributed in space more or less evenly starting at about ~14 AU which would account for the increase in speed compared to a continued exponential decline.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My first thought was &quot;dark matter&quot; but I doubt this is the reason for the change in graph profile. So far as I know, the significant large outer-solar-system objects start past Neptune (hence the term TNO), out in the Kupier belt and Oort cloud.  &lt;strong&gt;What is responsible for the change in graph profile (blue line) at ~14 AU?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-01-29T12:34:19.193" Title="Belt of matter at ~14 AU?" Tags="|solar-system|escape-velocity|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3528" PostTypeId="2" ParentId="3527" CreationDate="2014-01-29T12:34:19.193" Score="7" Body="&lt;p&gt;It looks to be merely an approximation in the &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/2/2c/Voyager_2_velocity_vs_distance_from_sun.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;SVG graph&lt;/a&gt; (in vector format) that was prepared for Wikipedia and later used in a &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Voyager_2_velocity_vs_distance_from_sun.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;converted to a PNG format&lt;/a&gt; (raster). If you check the SVG source, the &lt;code&gt;path&lt;/code&gt; was defined as:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;M 8 -400 C 10 -250 60 -85 140 -68 Q 150 -65 400 -50&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Where &lt;code&gt;M&lt;/code&gt; is &lt;code&gt;moveto&lt;/code&gt; moving the starting point to &lt;code&gt;x = 8, y = -400&lt;/code&gt;, &lt;code&gt;C&lt;/code&gt; is &lt;a href=&quot;http://www.w3.org/TR/SVG/paths.html#PathDataCurveCommands&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;code&gt;curveto&lt;/code&gt;&lt;/a&gt; drawing a Bézier curve from the previously defined starting point with curve control points &lt;code&gt;x1 = 10, y1 = -250&lt;/code&gt; and &lt;code&gt;x2=60, y2=-85&lt;/code&gt; to &lt;code&gt;x=140, y=-68&lt;/code&gt; and then continues with a &lt;a href=&quot;http://www.w3.org/TR/SVG/paths.html#PathDataQuadraticBezierCommands&quot; rel=&quot;nofollow noreferrer&quot;&gt;quadratic Bézier curve&lt;/a&gt; &lt;code&gt;Q&lt;/code&gt; from &lt;code&gt;x=400, y=-50&lt;/code&gt; to a control point at &lt;code&gt;x1=150, y1=-65&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it is not linear at all, even though it might appear so (it might even have something to do with browser specific interpretations of SVG format Bézier curves, but I didn't test that). And neither is entirely precise, nor I believe it was meant to be. It's not too hard to find different graphs of Solar system escape velocities required at different distances to the Sun neither, for example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/GpTmZ.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Heliocentric velocity vs. radial distance from the Sun for a constant thrust (Source: &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2011/varey_proj/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Dawn Mission, J. Varey, U. of Colorado&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, that's just musing with the impossibile, since IPS on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dawn spacecraft&lt;/a&gt; can't provide constant thrust for so long, but the escape velocity curve is not linear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why was the graph for Wikipedia prepared with such a shallow curve beyond 14 AU however remains unclear, as there isn't any explanation for it published on &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Voyager_2_velocity_vs_distance_from_sun.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;its page&lt;/a&gt;. My guess - because it aligned the two curves used nicer and it &quot;looks&quot; better like so. But I personally wouldn't start drawing conclusions that there must be some additional mass in the Solar system because of it. It has little to do with reality. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-01-29T12:34:19.193" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3529" PostTypeId="2" ParentId="3262" CreationDate="2014-01-29T15:20:59.847" Score="23" Body="&lt;p&gt;&lt;strong&gt;YES, They have come back alive at least once, aboard Space Shuttle Discovery in the Mice Drawer System (made in Italy).&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UbggB.jpg&quot; alt=&quot;MDS picture&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-128&quot; rel=&quot;noreferrer&quot;&gt;STS-128&lt;/a&gt; - Source: &lt;a href=&quot;http://www.space.com/7208-mouse-hotel-opens-space-station.html&quot; rel=&quot;noreferrer&quot;&gt;Mouse Hotel Opens on Space Station&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;MDS was carried to the ISS on August 28, 2009, and returned on November 27th, 2009, setting the longest permanence in space for rodents of 91 days. Three wild type (Wt) and three PTN-transgenic male mice were housed in the MDS during this experiment. A ground replica mimicking the flight experiment was performed at the University of Genova to evaluate the effects of microgravity and compare these effects to different Earth controls.&lt;/p&gt;&#xA;&lt;p&gt;Unfortunately during the mission, one PTN-Tg and two Wt mice died. The necropsy revealed that mouse Wt3 had a major spinal cord lesion that possibly occurred during the shuttle lift off. An analysis performed on feces present in the cage of the PTN-Tg3 mouse suggested that the animal could have developed a liver pathology. The post-landing check performed on the mice cages revealed that Wt1 mouse died in consequence of a failure of the food cassette system. The remaining mice showed a normal behavior throughout the experiment and appeared in excellent health conditions after landing. During the mission, the mice health conditions and their water and food consumption were checked daily. &lt;strong&gt;Upon landing, mice were sacrificed&lt;/strong&gt;, blood parameters were measured and tissues were dissected for subsequent analysis.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/665.html&quot; rel=&quot;noreferrer&quot;&gt;MDS experiment page&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-01-29T16:05:00.670" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3531" PostTypeId="1" AcceptedAnswerId="4190" CreationDate="2014-01-29T20:24:01.157" Score="9" ViewCount="1947" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipThree&quot;&gt;SpaceShip Three&lt;/a&gt; could be a point-to-point transportation method between two locations on Earth.  The types of routes being seriously considered include:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As of 2008, the SpaceShipThree concept spacecraft will be used for transportation through point-to-point suborbital spaceflight. This service could provide, for example, a two-hour trip &lt;strong&gt;from London to Sydney or Melbourne&lt;/strong&gt;. (Kangaroo Route)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This simple example indicates that the furthest separated locations on Earth are possibilities for such a suborbital flight.  I'm wondering about the parameters of such a suborbital flight...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How elliptic would these jumps be?  Would they be elliptic at all?  It could just be a LEO which will decay in half a turn, but is this the most energy-efficient?  Could you save on your delta v budget by making the arc higher?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe apogee and perigee are often given in terms of altitude above sea level, so if you &lt;em&gt;almost&lt;/em&gt; circularized the orbit (but not entirely) for a flight from London to Sydney, the perigee might still be above sea level (since the orbit decays when it gets into heavy atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If such a flight would be &lt;em&gt;almost&lt;/em&gt; circularized to LEO, then the claims of Scaled Composites seem to not make any sense.  Suborbital flights are a scaled down version of their plans, where they were previously considering orbital flights.  How can a suborbital flight to the other side of the planet have a significant energy advantage over reaching orbit?  I'm wondering if maybe this is possible by using a more highly elliptical orbit, where the perigee would remain below sea-level.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-04-04T18:19:34.447" Title="Would a point-to-point suborbital spaceflight have a &quot;negative&quot; perigee?" Tags="|sub-orbital|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3532" PostTypeId="2" ParentId="3483" CreationDate="2014-01-29T20:29:19.197" Score="13" Body="&lt;p&gt;Given the question, as written (rather than the misleading title) - yes, a magnetic accelerator could be used to launch a spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issues with so doing are several.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;the contents of the craft need to survive the magnetic fields needed for reasonable length accelerators&lt;/li&gt;&#xA;&lt;li&gt;the track &#xA;&lt;ol&gt;&#xA;&lt;li&gt;the linear track is extremely long&lt;/li&gt;&#xA;&lt;li&gt;the circular track causes fairly high G-forces upon the occupants&lt;/li&gt;&#xA;&lt;li&gt;the high accelerations needed to reduce track lengths to reasonable levels.&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;li&gt;the deceleration of atmospheric friction&lt;/li&gt;&#xA;&lt;li&gt;the lack of abort options&#xA;&lt;ol&gt;&#xA;&lt;li&gt;past the halfway point, insufficient distance to bring to a stop&lt;/li&gt;&#xA;&lt;li&gt;ability to rotate payload in the tube isn't a good use of mass except for failure modes, where the inability to do so moves the safe abort point below halfway.&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;NASA has looked into the use of magnetic accelerators for launching - and rejected as too unreliable for what NASA is launching. Not that it won't consistently put things into orbit, but that those things may not survive the trip in working order. The IEEE has as well; feasible lengths result in too high an acceleration &quot;for humans and fragile cargos.&quot;  (&lt;a href=&quot;http://research.lifeboat.com/ieee.em.pdf&quot;&gt;McNab, 2003&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, this leaves a number of items where it would be feasible. Food, water, fuel, life support supplies (filters, scrubbers, gasses), sample return containers, structural components, hardened electronics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The overall costs are not yet truly finalizable, but the 2003 cost estimate, without overruns, is in excess of $1.3 billion for a railgun, and the rails have a limited lifespan. Cost per kilogram should be comparable to some current high-efficiency launcher methods proposals.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;References&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;McNab, Ian (2003), &lt;em&gt;Launch to Space With an Electromagnetic Railgun&lt;/em&gt;,  &lt;a href=&quot;http://research.lifeboat.com/ieee.em.pdf&quot;&gt;http://research.lifeboat.com/ieee.em.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-01-29T20:29:19.197" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3533" PostTypeId="2" ParentId="3263" CreationDate="2014-01-29T21:18:46.887" Score="15" Body="&lt;p&gt;I am unsure if this reference is tight enough for your criteria, but &lt;a href=&quot;http://www.americaspace.com/?p=18436&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; written by Ben Evans at AmericaSpace appears to have a detailed accounting of the launch. Unfortunately, I cannot find what Mr. Evans's references for the accounting are. In any case, the article is highly entertaining.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a paragraph towards the bottom of the article that deals specifically with the origin of the &quot;light this candle&quot; phrase.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The clock was now marching rapidly towards 9:00 am. Two minutes remained on the countdown. Then, another halt was called. Pressures inside the Redstone’s liquid oxygen tank had climbed unacceptably high. NASA had two options. It could either reset the pressure valves – which would necessitate a launch scrub – or bleed off some of the pressure by remote control. An irritable Shepard, after almost four hours on his back and now lying in dried-up urine, obviously preferred the second option. “I’m cooler than you are!” he barked. “Why don’t you fix your little problem and light this candle?” Those final three words have since gained immortality and truly epitomise the ‘right stuff’ from which Shepard was cut.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This account is supported by Gene Kranz in chapter 3 of &lt;em&gt;Failure Is Not an Option&lt;/em&gt;, though he gives the quote as &quot;Why don't you guys fix your problems and light this candle?&quot;&lt;/p&gt;&#xA;" OwnerUserId="1531" LastEditorUserId="-1" LastEditDate="2017-06-11T19:17:36.203" LastActivityDate="2017-06-11T19:17:36.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3534" PostTypeId="1" CreationDate="2014-01-30T05:02:36.667" Score="6" ViewCount="677" Body="&lt;p&gt;I'm doing a lot of research on amateur sattelites and I am trying to understand everything.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm starting with the first amateur sattelite, Oscar 1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Oscar 1 was the first sattelite to be ejected as a secondary payload&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does the term &quot;secondary payload&quot; mean when referring to amateur sattelites?&lt;/p&gt;&#xA;" OwnerUserId="1545" LastEditorUserId="9092" LastEditDate="2021-03-26T14:24:36.853" LastActivityDate="2021-03-26T14:24:36.853" Title="Secondary Payload definition" Tags="|artificial-satellite|secondary-payload|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3535" PostTypeId="2" ParentId="3531" CreationDate="2014-01-30T05:28:01.550" Score="6" Body="&lt;p&gt;Perigee (and apogee) can be defined in two ways - by orbital radius around the barycenter, or by distance above surface.&lt;/p&gt;&#xA;&lt;p&gt;from the &lt;a href=&quot;http://www.thefreedictionary.com/perigee&quot; rel=&quot;noreferrer&quot;&gt;Free Online Dictionary&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;per·i·gee  (pĕr′ə-jē) &lt;br&gt;n.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The point nearest the earth's center in the orbit of the moon or a satellite.&lt;/li&gt;&#xA;&lt;li&gt;The point in any orbit nearest to the body being orbited.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;If the perigee is defined as orbital radius, then no, a negative perigee is impossible; any such perigee has a minimum of 0 (as one's distance from the barycenter begins to climb once one crosses it).&lt;/p&gt;&#xA;&lt;p&gt;If using the above surface measure, then a negative perigee would be a body impacting orbit, and yes, it could be used for portions of an orbit.&lt;/p&gt;&#xA;&lt;p&gt;Both modes of measure are in common enough use.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-01-30T05:28:01.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3536" PostTypeId="2" ParentId="3534" CreationDate="2014-01-30T05:32:12.257" Score="2" Body="&lt;p&gt;Another term is 'piggyback payload': launchers usually have some performance margin. When this margin is comfortably large, it may be profitable to sell the opportunity to ride 'piggyback', or the paying agency may want to subsidize university research. The price is quoted sometimes as \$12K to \$18K per sub-1 kg payloads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondary payloads will be deployed into the same orbital plane, often before separation of the main payload. Insertion accuracy is much inferior to normal payload's requirements.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-01-30T05:32:12.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3537" PostTypeId="2" ParentId="999" CreationDate="2014-01-30T06:04:08.103" Score="15" Body="&lt;p&gt;Anything below 300km will be out of orbit within a month or so, anything over 10,000km will be up there for essentially for ever. Dense objects will stay up for longer than low density objects (a sheet of tinfoil will slow faster than the same foil rolled up in a ball) and eccentric orbits decay more slowly than circular ones (because drag only happens at the low point of the orbit and does not greatly affect the height of the low point, it just tends to make the orbit more circular).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pretty much most stuff under 600km will be clear in 40 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How this stacks up as a half life, well you could guess a distribution for the densities for the debris and work it out that way or you could just guess, i.e. mostly gone  after five half lives (95% gone roughly) and 40/5 = a half life of 8 years for debris below 600km. But what's left will be your small dense really dangerous objects, i.e. bolts not flecks of paint. Plus there will be more stuff falling down from higher up and debris generated from collisions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a piece of free software here: &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/mitigate/das.html&quot;&gt;http://orbitaldebris.jsc.nasa.gov/mitigate/das.html&lt;/a&gt; that calculates orbital lifetimes if you are interested. &lt;/p&gt;&#xA;" OwnerUserId="1546" LastActivityDate="2014-01-30T06:04:08.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3538" PostTypeId="2" ParentId="3534" CreationDate="2014-01-30T07:23:29.357" Score="4" Body="&lt;p&gt;We already have a similar question in &lt;a href=&quot;https://space.stackexchange.com/questions/1556/how-does-a-single-rocket-place-multiple-satellites-into-orbit/1569#1569&quot;&gt;How does a single rocket place multiple satellites into orbit?&lt;/a&gt; with answers that might help you get an overview of how secondary payloads can be deployed, but &lt;a href=&quot;https://en.wikipedia.org/wiki/OSCAR&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSCAR&lt;/a&gt; (Orbiting Satellite Carrying Amateur Radio) satellites were actually piggybacked on rocket launches as ballast weight. &lt;a href=&quot;http://www.gma.org/surfing/sats.html#oscar&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gulf of Maine Research Institute article&lt;/a&gt; explains it as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is simpler and cheaper to ballast a rocket with dead weight than to&#xA;  reduce the thrust. As a result, it is possible to add secondary&#xA;  payloads of homemade satellites to multimillion-dollar NASA missions&#xA;  at minimal costs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wasn't able to find any information on where exactly they were mounted and how did their adapters look like, but I'd presume they were mounted on some custom made secondary payload adapter and used some simple deployment mechanism, since these satellites wouldn't really be considered first class passengers. Perhaps they used something similar to this DemoSat design that was meant to deploy nanosatellites:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/b35Be.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The craft are mounted atop the black cone-shaped adapter that mounts to the rocket&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;as seen in this image (Image credit: U.S. Air Force, Source: &lt;a href=&quot;http://spaceflightnow.com/delta/d310/041201demosat.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This deployment design mounts secondary payload nanosatellites on the primary payload adapter. They could be jettisoned with explosive bolts, compressed gas, or even by spring release or levers, and initiated by jettisoning the payload fairing, remote or onboard computer triggered, or simply by centrifugal force, if the primary payload or the launching vehicle are spin-stabilized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These secondary / auxiliary payloads might even serve as primary payload ballasts for the duration of the launch by positioning them where needed and/or changing the length and weight of their adapters to balance the mass distribution of the whole payload along the long axis during &lt;a href=&quot;http://www.popsci.com/science/article/2013-09/big-pic-nasa-lunar-satellite-undergoes-spin-testing-tonights-flight&quot; rel=&quot;nofollow noreferrer&quot;&gt;spin testing&lt;/a&gt;. Or any combination of mentioned options, or perhaps something I didn't even think of here. For example, more complex launch adapter for nanosatellites could use multiple launch cassettes and might look like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/26qY1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;NASA's Nanosatellite Launch Adapter System (Image credit: NASA Ames, Source: &lt;a href=&quot;http://spaceref.com/nasa-hack-space/nanosatellite-launch-adapter-system-ready-for-flight.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceRef&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or they could even be delivered in resupply vehicles to the International Space Station and deployed using JAXA's &lt;a href=&quot;http://iss.jaxa.jp/en/kiboexp/jssod/&quot; rel=&quot;nofollow noreferrer&quot;&gt;J-SSOD small satellite deployment system&lt;/a&gt; installed in the Kibo module's airlock, and of course when Space Shuttle was operational, they could be deployed from there too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there's probably thousands of ways to piggyback small, &lt;em&gt;feather-weight&lt;/em&gt; micro- or nano- or femto-satellites on a launch vehicle into orbit. For example, NASA is running one such program under the name &lt;a href=&quot;http://www.nasa.gov/directorates/heo/home/CubeSats_initiative.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;CubeSat Launch Initiative&lt;/a&gt; where you should find more information how these things are handled nowadays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're probably asking yourself by now, where am I going with all these broad descriptions of what constitutes an auxiliary payload? Well, I guess that's the point, that there isn't any simple definition for it. Perhaps the easiest would be saying that the major difference between main payloads (can be more of those, so I'll avoid calling them &lt;em&gt;primary&lt;/em&gt;, &lt;em&gt;secondary&lt;/em&gt;,...) and auxiliary payloads is that the latter don't cover the cost of launching a vehicle into orbit, and all subsequent differences stem from this fact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like, for example, that the launch vehicle design and main payload integration processes most certainly won't suffer huge additional costs to integrate auxiliary payloads, but they might be piggybacked if they either come useful (i.e. some dead weight would have to be used for ballast anyway), or adding them to the launch manifest costs less than the value they bring to the whole project (e.g. public exposure). Since &lt;em&gt;value&lt;/em&gt; can be a subjective term, the definition for these auxiliary payloads will vary too.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-01-30T22:30:18.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3539" PostTypeId="1" AcceptedAnswerId="3540" CreationDate="2014-01-30T14:21:43.353" Score="7" ViewCount="2546" Body="&lt;p&gt;We have high resolution images of the the surface of the Earth taken by satellites (e.g. the images from Google Earth). Why have none of the orbiters going to any other planet or celestial body in our Solar system have this technology? Is there a high cost, or are circumstances that limit this technology in any other place but earth. Or maybe we do have this but I haven't heard of it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they sent this technology to the Moon or Mars, they could get high resolution shots of interesting features such as the Moon landings, or plot potential landing sites and places to visit with rovers. It may also be a good publicity ploy, seen as pictures from space can be spread easier nowadays.&lt;/p&gt;&#xA;" OwnerUserId="65" LastEditorUserId="49" LastEditDate="2014-01-30T16:42:16.353" LastActivityDate="2014-01-30T16:42:16.353" Title="High resolution images of surfaces of other planets and moons" Tags="|orbit|planetary-science|solar-system|imaging|resolution|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3540" PostTypeId="2" ParentId="3539" CreationDate="2014-01-30T15:21:13.253" Score="9" Body="&lt;p&gt;This has been done for the Moon, Mars and Mercury.&lt;br&gt;&#xA;The &lt;a href=&quot;http://lro.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Reconnaisance Orbiter&lt;/a&gt; mission aims to map the moon. It has two narrow-angle cameras which make black-and-white images of the surface, capturing images with resolutions down to 1 meter (about 3.3 feet). This has yielded tons of &lt;a href=&quot;http://www.nasa.gov/mission_pages/LRO/multimedia/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;nice images&lt;/a&gt;, including the &lt;a href=&quot;http://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/apollosites.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo landing sites&lt;/a&gt; and many other probes.&lt;br&gt;&#xA;This is the Apollo 11 site:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/k80dO.jpg&quot; alt=&quot;Apollo 11 landing site&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Google_Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google Earth&lt;/a&gt; has spoiled us a bit, it uses aerial photos in addition to satellite photos, with no indication which you're looking at.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://mars.jpl.nasa.gov/mro&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Reconnaisance Orbiter&lt;/a&gt; has a similar mission on Mars. It has captured photos of the &lt;a href=&quot;http://mars.jpl.nasa.gov/mro/multimedia/images/?ImageID=5894&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity rover&lt;/a&gt;, including one of its landing:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/tcA4A.jpg&quot; alt=&quot;Curiosity landing&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://messenger.jhuapl.edu/the_mission/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Messenger&lt;/a&gt; has mapped Mercury.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image quality is limited by the size of the camera. The LRO weighed 1800 kg at launch, espionage satellites (which give the best image quality we've seen) weigh 10 tons or more, and would be prohibitively expensive to send to the Moon or Mars.&lt;br&gt;&#xA;We haven't had full visual mapping missions to other planets. Venus is permanently obscured by clouds, so we resorted to radar there. The gas giants have a similar problem.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-01-30T15:21:13.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3541" PostTypeId="1" AcceptedAnswerId="3542" CreationDate="2014-01-30T15:25:37.390" Score="9" ViewCount="1339" Body="&lt;p&gt;I saw &lt;a href=&quot;https://space.stackexchange.com/questions/3468/vehicle-liquids-freeze-on-moon#comment6997_3472&quot;&gt;a comment&lt;/a&gt; that made me curious.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth's axis tilt is about 23.4°, whereas Luna's is only 1.54°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is negligible atmosphere on Luna, and as far as I know although signs of water molecules have been found, there are no bodies of liquids in any way similar to Earth's oceans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know that the temperature swings between day and night on Luna are huge (approaching 400 K between the extremes).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there anything resembling Earth's seasons on the moon?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2019-03-18T16:12:24.637" Title="Are there seasons on Luna?" Tags="|the-moon|planetary-science|temperature|exosphere|climate|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3542" PostTypeId="2" ParentId="3541" CreationDate="2014-01-30T16:28:13.490" Score="11" Body="&lt;p&gt;There are seasonal variations, but they are negligible. The biggest driver of seasonal variation on Earth is the 23 degree tilt. The Moon, however, with very little tilt compared to the Sun (1.54 degrees to the ecliptic), would have much smaller seasonal variations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest variation on the Moon's surface, as you pointed out, are the temperature swings between day and night. These diurnal variations dwarf any seasonal variations and make them unnoticeable, and improbable to measure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's important to also understand the importance of atmosphere for controlling these diurnal and seasonal variations. If the Earth had no atmosphere, it would experience violent temperature swings similar to that on the Moon, making it very difficult to measure the seasonal changes of even a 23 degree tilted body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the negligible exosphere and small tilt of the Moon do not allow for seasonal variations that resemble those here on Earth.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="-1" LastEditDate="2019-03-18T16:12:24.637" LastActivityDate="2019-03-18T16:12:24.637" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3543" PostTypeId="1" CreationDate="2014-01-30T21:13:02.763" Score="11" ViewCount="4771" Body="&lt;p&gt;Wikipedia on its page on &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination_change#Efficiency&quot; rel=&quot;noreferrer&quot;&gt;efficiency of orbital inclination change&lt;/a&gt; writes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, maximum efficiency of inclination changes are achieved at&#xA;  apoapsis, (or apogee), where orbital velocity V, is the lowest.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Could somebody elaborate on this statement?&lt;/p&gt;&#xA;" OwnerUserId="1552" LastEditorUserId="9092" LastEditDate="2021-03-26T08:24:27.563" LastActivityDate="2021-03-26T08:24:27.563" Title="Why is it best to perform an orbital inclination change at apoapsis?" Tags="|orbital-mechanics|inclination-change|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3544" PostTypeId="2" ParentId="3543" CreationDate="2014-01-30T21:27:22.750" Score="2" Body="&lt;p&gt;I may be rusty in my understanding of orbital mechanics, but here's a try:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At apoapsis, an orbiting object will be travelling at the slowest velocity of its orbit. Hence, if you make a plane change at this point, assuming you are not changing any other orbital parameters, the velocity at your target orbit will also be the slowest velocity along that orbit. All you're really doing is changing direction, and so that's going to be easier when you're moving more slowly, as opposed to periapsis, at which point the the orbiting object will be at the fastest velocity of its orbit.&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2014-01-30T21:27:22.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3545" PostTypeId="2" ParentId="3543" CreationDate="2014-01-30T21:29:26.680" Score="11" Body="&lt;p&gt;To change the inclination by an angle $\alpha$, you need to apply a velocity change of $\Delta v = v \times sin \alpha$, so the higher the velocity, the higher the velocity change that you need to apply, and thus the energy that you need.&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2014-01-30T21:29:26.680" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3546" PostTypeId="1" CreationDate="2014-01-31T04:35:46.677" Score="9" ViewCount="760" Body="&lt;p&gt;During Apollo 11, the first manned lunar landing, a &quot;contingency sample&quot; was taken by Neil Armstrong. From &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_11&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;About seven minutes after stepping onto the Moon's surface, Armstrong collected a contingency soil sample using a sample bag on a stick. He then folded the bag and tucked it into a pocket on his right thigh. This was to guarantee there would be some lunar soil brought back in case an emergency required the astronauts to abandon the EVA and return to the LM&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Where is this sample currently located?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2014-01-31T13:44:56.640" Title="Where is the first Lunar soil sample currently located?" Tags="|the-moon|apollo-program|planetary-science|lunar-landing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3547" PostTypeId="2" ParentId="3111" CreationDate="2014-01-31T05:28:52.307" Score="9" Body="&lt;p&gt;According to Rich Zurek, &lt;a href=&quot;http://mars.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Exploration Program&lt;/a&gt; chief scientist at NASA's &lt;a href=&quot;http://www.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jet Propulsion Laboratory&lt;/a&gt; (JPL), Pasadena, CA whose statement was quoted in recent NASA's press release titled &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2014-026&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Preparing for 2014 Comet Watch at Mars&lt;/a&gt; (January 28, 2014):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Our plans for using spacecraft at Mars to observe comet Siding Spring&#xA;  will be coordinated with plans for how the orbiters will duck and&#xA;  cover, if we need to do that.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And JPL's Soren Madsen, Mars Exploration Program chief engineer adds to it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It's way too early for us to know how much of a threat Siding Spring&#xA;  will be to our orbiters. It could go either way. It could be a huge&#xA;  deal or it could be nothing -- or anything in between.&#xA;  How active will Siding Spring be in April and May? We'll be watching&#xA;  that, but if the red alarm starts sounding in May, it&#xA;  would be too late to start planning how to respond. That's why we're&#xA;  doing what we're doing right now.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So NASA is taking potential threat that comet &lt;a href=&quot;https://en.wikipedia.org/wiki/C/2013_A1&quot; rel=&quot;nofollow noreferrer&quot;&gt;C/2013 A1 Siding Spring&lt;/a&gt; might pose to Mars orbiters seriously, keeping close eye on it. At the same time, the mentioned press release dismisses chance that the cometary debris could pose a risk to two NASA's Mars rovers that are active on the surface, saying that while Martian atmosphere is thin, it is &lt;em&gt;&quot;dense enough to prevent dust from the comet from becoming a hazard&quot;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the three orbiters currently in Martian orbit (&lt;a href=&quot;http://mars.jpl.nasa.gov/mro/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Reconnaissance Orbiter&lt;/a&gt;, &lt;a href=&quot;http://mars.jpl.nasa.gov/odyssey/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Odyssey&lt;/a&gt;, and &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Mars_Express&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Express&lt;/a&gt;) and two on their way (&lt;a href=&quot;http://www.nasa.gov/mission_pages/maven/main/&quot; rel=&quot;nofollow noreferrer&quot;&gt;MAVEN&lt;/a&gt; and &lt;a href=&quot;http://www.isro.org/pslv-c25/mission.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Orbiter Mission&lt;/a&gt;) that will be there before the comet Siding Spring flyby though, threat might be more significant:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Two key strategies to lessen risk are to get orbiters behind Mars&#xA;  during the minutes of highest risk and to orient orbiters so that the&#xA;  most vulnerable parts are not in the line of fire.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Orbiters are designed with the risk of space-dust collisions in mind.&#xA;  Most such collisions do not damage a mission. Design factors such as&#xA;  blanketing and protected placement of vulnerable components help. Over&#xA;  a five-year span for a Mars orbiter, NASA figures on a few percent&#xA;  chance of significant damage to a spacecraft from the background level&#xA;  of impacts from such particles, called meteoroids. Whether the Siding&#xA;  Spring level will pack that much hazard -- or perhaps greater than 10&#xA;  times more -- into a few hours will depend on how active it becomes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Part of the risk management strategy is also assessing required maneuvers as soon as possible, to consume less fuel for any such orbit adjustments. That will depend on comet's reaction to crossing the Solar system's &lt;a href=&quot;https://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;snow line&lt;/a&gt; at a distance of roughly 3-4 AU to the Sun, which will happen in April to May 2014:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/K30tV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This graphic depicts the orbit of comet C/2013 A1 Siding Spring as it&#xA;  swings around the sun in 2014. On Oct. 19, the comet will have a very&#xA;  close pass at Mars, just 86,000 miles (138,000 kilometers) from the&#xA;  planet. Although the nucleus will miss the planet, the comet's coma of&#xA;  dust particles might envelop the Red Planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there we have it. NASA is considering the threat for its Mars orbiters coming from comet C/2013 A1 Siding Spring significant enough that it has started preparing risk mitigation procedures for the comet's flyby in October 2014, when the comet will shoot past Mars much closer than the Moon is to the Earth, but it assesses that it won't pose any risk to its two rovers on the Martian surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actual procedures that NASA will engage in will of course depend on the risk assessment and how early that can be established (no sooner than as the comet crosses the Solar system's snow line somewhere in April to May 2014), and they plan to keep a watchful eye on how the comet behaves and how much dust it releases as it approaches Mars. Latest NEOWISE images do show it to be quite a dusty fella, though:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JcLwN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA's &lt;a href=&quot;http://neo.jpl.nasa.gov/programs/neowise.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NEOWISE mission&lt;/a&gt; captured images of comet C/2013 A1 Siding&#xA;  Spring, which is slated to make a close pass by Mars on Oct. 19, 2014.&#xA;  The infrared pictures reveal a comet that is active and very dusty&#xA;  even though it was about 355 million miles (571 million kilometers)&#xA;  away from the sun on Jan. 16, 2014, when this picture was taken.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2014-026&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA JPL press release: NASA Preparing for 2014 Comet Watch at Mars&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA17833&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA JPL Space Images: NEOWISE Spies Comet C/2013 A1 Siding Spring&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mars.nasa.gov/comets/sidingspring/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Mars Exploration Program: Comets: Siding Spring&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mars.nasa.gov/multimedia/images/?ImageID=5932&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Mars Exploration Program: Multimedia: Comet Siding Spring C/2013 A1&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt; (August 15, 2014): &lt;a href=&quot;http://www.cometcampaign.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CIOC&lt;/a&gt; (Coordinated Investigations Of Comets) hosted on August 11, 2014 a workshop titled &lt;a href=&quot;http://www.cometcampaign.org/workshop&quot; rel=&quot;nofollow noreferrer&quot;&gt;Comet Siding Spring and Its Close Approach to Mars: Observer's Workshop&lt;/a&gt;, and all four of its sessions were &lt;a href=&quot;http://new.livestream.com/viewnow/comets&quot; rel=&quot;nofollow noreferrer&quot;&gt;webcast live via LiveStream&lt;/a&gt;. During the &lt;a href=&quot;http://new.livestream.com/viewnow/comets/videos/58947411&quot; rel=&quot;nofollow noreferrer&quot;&gt;Session 1: Setting the Stage; Spacecraft Observations&lt;/a&gt;, roughly 34 minutes into the recorded video, Tony Farnham (University of Maryland, College Park, Maryland) presented detailed Comet Siding Spring C/2013 A1 Hazard Overview, and shared this slide:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/BZdOH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In words, it was assessed that the comet doesn't pose much of a hazard to orbiters around Mars, and none to rovers with tenuous Martian atmosphere still providing sufficient protection to them from potential cometary dust and debris entering the atmosphere. Orbiters will still utilize a few simple risk management options available to them as a precaution, in particular they will phase their orbits to be on the opposite side of Mars as the planet enters comet's dust trail ahead of its orbit, which will buy orbiters additional protection for roughly 30 to 40 minutes during the time when the hazard is evaluated to be the greatest (impact with dust and debris, and destructive electrostatic discharge as they move through higher density dust and gas cloud that can potentially carry charge as plasma or provide a discharge path for orbiters' own electrostatic charge).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an additional note, Tony Farnham mentioned that &lt;a href=&quot;http://www.nasa.gov/mission_pages/maven/main/&quot; rel=&quot;nofollow noreferrer&quot;&gt;MAVEN&lt;/a&gt; might also turn off its high-voltage instruments as an additional precautionary measure, but for the time being wasn't sure on this point. I will update if and when I can confirm it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More up to date information as the comet approaches Mars will be available on the &lt;a href=&quot;http://www.cometcampaign.org/comet-siding-spring&quot; rel=&quot;nofollow noreferrer&quot;&gt;CIOC Comet Siding Spring&lt;/a&gt; pages. Also see the rest of the &lt;a href=&quot;http://new.livestream.com/viewnow/comets&quot; rel=&quot;nofollow noreferrer&quot;&gt;recorded workshop videos&lt;/a&gt; for everything we currently know about Comet Siding Spring C/2013 A1, past and planned observations, including by hardware we currently have (and will have, MAVEN is still on its way as of the time of this update with a planned orbital insertion date of September 22, 2014) on and around Mars. More on this can also be read in &lt;a href=&quot;http://www.nasa.gov/press/2014/july/nasa-s-mars-spacecraft-maneuvers-to-prepare-for-close-comet-flyby/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA’s Mars Spacecraft Maneuvers to Prepare for Close Comet Flyby&lt;/a&gt; news release from July 25, 2014.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-08-16T00:20:13.173" LastActivityDate="2014-08-16T00:20:13.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3548" PostTypeId="2" ParentId="3531" CreationDate="2014-01-31T06:57:01.100" Score="10" Body="&lt;p&gt;Yes, the minimum $\Delta V$ ballistic trajectory between two points on the Earth's surface will have a negative altitude periapsis, which of course is never reached.  For London to Sydney, that trajectory has a periapsis about $800\,\mathrm{km}$ below the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That &quot;orbit&quot; is elliptic with an eccentricity of 0.12.  The $\Delta V$ at the surface is $7.85\,\mathrm{km/s}$.  So you are correct that it would need to get quite close to orbital speed.  A more elliptic trajectory, i.e. one that lofts higher, would require more $\Delta V$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-01-31T06:57:01.100" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3549" PostTypeId="1" AcceptedAnswerId="3550" CreationDate="2014-01-31T13:05:24.307" Score="8" ViewCount="217" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.space-travel.com/reports/Constellation_is_Back_999.html&quot;&gt;this article&lt;/a&gt; the &lt;a href=&quot;https://en.wikipedia.org/wiki/Constellation_program&quot;&gt;Constellation program&lt;/a&gt; is funded again.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Finally, just last Friday President Obama signed a new federal spending bill into law. This bill includes money to continue NASA's human space exploration program. Surprisingly, the exploration program being funded is a program long thought dead by the space community.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Where can there one find an official confirmation of this milestone in US human space exploration? &lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2014-01-31T13:26:17.087" Title="Is the constellation program back on track?" Tags="|constellation-program|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3550" PostTypeId="2" ParentId="3549" CreationDate="2014-01-31T13:26:17.087" Score="6" Body="&lt;p&gt;The article you link to seems to be shooting from the hip, with no evidence to back it up.&lt;br&gt;&#xA;Spacepolitics has a more &lt;a href=&quot;http://www.spacepolitics.com/2014/01/14/omnibus-bill-gives-nasa-17-65-billion-and-launch-indemnification-extension-as-well/&quot;&gt;comprehensive overview&lt;/a&gt; of NASA's budget. They list \$1.6 billion for the Space Launch System and nearly \$1.2 billion for Orion. No mention of Constellation.&lt;br&gt;&#xA;Feel free to read the official text of the &lt;a href=&quot;http://thomas.loc.gov/cgi-bin/bdquery/z?d113%3ah.r.03547%3a&quot;&gt;2014 spending bill&lt;/a&gt;, but it's 600+ pages of legalese to wade through (and no mention of Constellation, at least not by name).&lt;br&gt;&#xA;&lt;a href=&quot;http://www.space.com/11411-nasa-2011-budget-cuts-constellation-funding.html&quot;&gt;Space.com&lt;/a&gt; says that NASA is now free to end the remaining Constellation contracts. Since 2011, the odd situation existed that NASA could not legally end some contracts, despite Constellation being cancelled. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-01-31T13:26:17.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3551" PostTypeId="2" ParentId="3546" CreationDate="2014-01-31T13:39:56.577" Score="13" Body="&lt;p&gt;Most of the contingency sample is located at the &lt;a href=&quot;http://curator.jsc.nasa.gov/lunar/lun-fac.cfm&quot;&gt;Lunar Sample Laboratory Facility&lt;/a&gt; (LSLF) in the building 31N at the Johnson Space Center in Houston, Texas. They are listing it in their &lt;a href=&quot;http://curator.jsc.nasa.gov/lunar/lsc/index.cfm&quot;&gt;catalog&lt;/a&gt;. The contingency samples are items number 10010 and 10021 - 10033, as you can see in &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a11/a11SampleList-1.jpg&quot;&gt;this&lt;/a&gt; list (see the last column - Returned Container - items &quot;Cont.Bag&quot; [Contingency Bag]).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only the items 10030 and 10033 are missing in the LSLF catalog. Where are these remaining parts of the contingency bag, I don't know. &lt;a href=&quot;http://www.dvidshub.net/image/704024/lunar-samples-apollo-11-100305#.Uuukp_M1h9M&quot;&gt;This&lt;/a&gt; seems to suggest that the item 10030 could be in Washington, DC. At least partially, because the item 10030 was apparently split into 5 pieces, as indicated &lt;a href=&quot;http://www.lpi.usra.edu/lunar/samples/atlas/misc_view/?mission=Apollo%2011&amp;amp;sample=10030&amp;amp;source_id=S76-21142&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="921" LastEditDate="2014-01-31T13:44:56.640" LastActivityDate="2014-01-31T13:44:56.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3552" PostTypeId="2" ParentId="3519" CreationDate="2014-01-31T21:59:23.413" Score="1" Body="&lt;p&gt;Why is planning and executing the maneuver so difficult it had not been done before 2007? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm just an armchair astronomer but having read the papers it sounds to me like this is an optimization. It's a little like a few other inventions that evoke the &quot;Well of course that's obvious&quot; feeling. But like many great ideas it might not have been so obvious before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was stated in the published paper that programming for rotating using propellant was the simplest thing to do. So they did that first. Now they must have realized there was a better way and they did it.&lt;/p&gt;&#xA;" OwnerUserId="332" LastActivityDate="2014-01-31T21:59:23.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3553" PostTypeId="2" ParentId="3519" CreationDate="2014-02-01T08:38:46.623" Score="6" Body="&lt;p&gt;I believe the ZPM was first performed before 2007.  The paper seems to be about using the ZPM to flip the ISS completely around (&quot;reorient the&#xA;International Space Station 180 degrees&quot;), not just using ZPM for day-to-day attitude control.  From the summary on the first page (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The flight test established the breakthrough capability to&#xA;  simultaneously perform a &lt;strong&gt;large angle&lt;/strong&gt; attitude maneuver and momentum&#xA;  desaturation without the need to use thrusters.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and later, on page, 2,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the early 00’s, the ZPM concept was used...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;i.e. the &quot;breakthrough&quot; in the paper you linked was &lt;em&gt;how much&lt;/em&gt; the ZPM moved the station, not just &lt;em&gt;that&lt;/em&gt; the ZPM moved the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My copy of the &lt;a href=&quot;http://www.apogeespacebooks.com/Books/InternationalSS.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Reference Guide to the International Space Station&lt;/em&gt;&lt;/a&gt;, published in 2006, has a page (p.67) on &quot;Guidance, Navigation, and Control&quot; that states &quot;The preferred method of attitude control is the use of gyrodynes, Control Moment Gyroscopes (CMGs)....&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That page goes on to say that &quot;CMGs are, however, limited in the amount of angular momentum they can provide and the rate at which they can move the Station. When CMGs &lt;strong&gt;can no longer provide the requisite energy&lt;/strong&gt;, rocket engines are called upon,&quot; (emphasis mine) which implies to me that CMGs are &lt;em&gt;not&lt;/em&gt; sufficient to permanently maintain the orientation of the ISS in the event that thrusters were lost, which maybe sort of answers your last question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't help with your second or third questions.&lt;/p&gt;&#xA;" OwnerDisplayName="user1556" LastEditorDisplayName="user1556" LastEditDate="2014-02-02T05:35:20.340" LastActivityDate="2014-02-02T05:35:20.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3554" PostTypeId="1" AcceptedAnswerId="3555" CreationDate="2014-02-01T09:02:15.863" Score="9" ViewCount="976" Body="&lt;p&gt;ESA:s mission to Europa will produce first data 17 years after it was decided. It will take 8 years from launch to arrive at Jupiter. Since technology advances rapidly, I wonder if it would not be much more efficient to put less money and time on the instruments and more money on rocket fuel to get their earlier? A 10 year faster mission would have much cheaper instruments with several times higher sensitivy and data rates. Won't the instruments get obscolete during the decade it takes to build them and the other decade it takes to get them to Jupiter? Another example is the mirror for James Webb telescope, which certainly could've been produced better and cheaper if it hade been produced with 5 years or so newer technology, than sitting in a storage room for 5 years before launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about diversifying to several cheap fast flyby missions, passing by Jupiter just 1 year after launch, like New Horizons? Producing data 15 years earlier with 15 years more modern technology than what the planned Europa mission will. The data it gives could be used for designing a follow up mission within a couple of years, rather than in the 2050s for following up the Europa mission with another one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are the time value of money, and the risk of concentrating 2 decades of Jupiter research to one single mission, really considered? Is it just a matter of political disorganization, or are there rational reasons for slow space travel and using obscolete technology on single high risk missions?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-02-08T11:07:40.027" Title="How are long space travel times motivated? (17 year Europa mission)" Tags="|future-missions|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3555" PostTypeId="2" ParentId="3554" CreationDate="2014-02-01T16:41:24.050" Score="10" Body="&lt;p&gt;Your question touches on several factors that influence the length of a mission: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mission preparation time&lt;/li&gt;&#xA;&lt;li&gt;Flyby versus orbital missions&lt;/li&gt;&#xA;&lt;li&gt;Transit time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Mission preparation time&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;You're trying to apply the rapid advances in mass-produced computing to other fields. Yes, if you wait five years, computers will be much faster. But Moore's Law is an exception: advances on most fields are much slower. &#xA;For instance, mirrors don't get cheaper if you wait five years, because it still takes the same amount of time and effort to fabricate them.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are a few factors that cause spacecraft design to take a long time:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Space-rated components. Every part of the probe must be able to withstand the space environment: &lt;a href=&quot;http://nmp.nasa.gov/st8/tech/eaftc_tech1.html&quot; rel=&quot;nofollow&quot;&gt;extreme temperatures and radiation&lt;/a&gt; mean that you can't use off-the-shelf components, everything has to be designed for space. For instance that ESA has developed its own microprocessor. To make sure all your components will work correctly, everything has to be tested thoroughly. This takes time. If you swap the microprocessor for a new, faster design, you have to start the tests all over again. The radiation also means you want your integrated circuits to use larger components, because they are less likely to be disrupted by radiation. This alone means you use chips that are several generations behind what is available commercially.  &lt;/li&gt;&#xA;&lt;li&gt;The science instruments. These have to be developed specially for the mission. You can't just stick a commercial camera in a probe.  &lt;/li&gt;&#xA;&lt;li&gt;Reliability. You have just one chance, the probe has to work perfectly when it arrives or all your work and money is down the drain. Cutting corners to save money would be counterproductive. &lt;/li&gt;&#xA;&lt;li&gt;Uniqueness. Each probe is a one-off, because each mission has very different requirements. So reuse is limited.   &lt;/li&gt;&#xA;&lt;li&gt;Obsolescence. In my opinion, this isn't relevant. The question is, is the mission worthwhile with the level of technology we have now? If the answer is yes, then the mission should go ahead.&lt;br&gt;&#xA;So on every level you're not just building the probe, but you're doing all the research and development that comes before. The only way to speed up the design phase, would be to simplify the design. See 'General observations' below.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Flyby versus orbital missions&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Flyby missions may be cheaper, but they also severely limit the amount of useful science you can do. You're only near the planet for a couple of days, versus several years for an orbiting mission.&lt;br&gt;&#xA;There's a lot of value in being able to observe a planet for a long time: weather patterns, seasons, influence of variations in the solar wind. An orbital mission also gives the opportunity to do high-resolution mapping of the entire planet.&lt;br&gt;&#xA;The Pioneer and Voyager missions were successful. They vastly expanded our knowledge of the outer solar system. But they were followed up by orbital missions that were designed to 'zoom in' on phenomena only glimpsed briefly.  &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Transit time&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The launch is a significant fraction of the total cost. We're talking more than a hundred million dollars. Heavier launchers (to shorten the transit time) cost even more, so your simple probes are still going to be expensive to launch. The rational thing to do is spend more on instruments and less on the launcher. The same goes for slow trajectories using gravity assists. So what if it takes 8 years for the spacecraft to reach Jupiter? If it halves your launch cost, that means you get an extra mission out of your budget. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one political factor that comes into play: ESA prefers to use their own launcher for its space probes. At the moment, that means using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5&quot; rel=&quot;nofollow&quot;&gt;Ariane 5&lt;/a&gt;, with the limits on spacecraft weight that entails. If they wanted to shorten the transit time, they'd have to use an American &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV&quot; rel=&quot;nofollow&quot;&gt;Delta IV Heavy&lt;/a&gt;. The heaviest variant can launch about 2 tons more to a geostationary orbit (12 vs. 10 tons), but it'd mean paying \$150 million to an American company instead of benefiting the European space sector. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Using commercial electronics in space&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;There's been a lot of research into the feasibility of using commercial electronics in space (COTS). Most of this is focused on Low Earth orbit applications, where radiation is less of a problem than in outer space.&lt;br&gt;&#xA;One study concluded that it's &lt;a href=&quot;https://archive.org/stream/nasa_techdoc_20010020284/20010020284#page/n5/mode/2up&quot; rel=&quot;nofollow&quot;&gt;not as straightforward as hoped&lt;/a&gt;. A big problem is that the change cycle in commercial electronics is so short that by the time you've finished verifying that a part will work in space, the part will have been obsoleted and replaced. This makes it &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/8081/1/02-0035.pdf&quot; rel=&quot;nofollow&quot;&gt;difficult to predict&lt;/a&gt; what the chance will be of the satellite still working after a year.&lt;br&gt;&#xA;Another &lt;a href=&quot;http://www.cosmiac.org/Radiation%20Data/Total%20Dose%20Test%20Results%20for%20CubeSat%20Electronics_final.doc&quot; rel=&quot;nofollow&quot;&gt;study&lt;/a&gt; concludes that even in LEO, many commercial parts fail due to radiation damage within a year.&lt;br&gt;&#xA;Redundancy by itself is not enough; a &lt;a href=&quot;https://en.wikipedia.org/wiki/Single_event_upset&quot; rel=&quot;nofollow&quot;&gt;single particle strike&lt;/a&gt; (single event upset, SEE) can change the behavior of a chip permanently (or temporarily). Commercial electronics don't allow you to detect this or route around it, so that means the first SEE can end the mission. This means you have to design the electronics to be fault-tolerant from the ground up.&lt;br&gt;&#xA;A few years ago, an SEE corrupted Voyager 2's memory. Thanks to a fault-tolerant design, NASA was able to &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2010-151&quot; rel=&quot;nofollow&quot;&gt;correct the error and resume normal operations&lt;/a&gt;. This problem could have ended the mission had commercial electronics been used. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;General observations&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;NASA tried your approach. The Mars Scout program consisted of missions budgeted at \$480 million each. Phoenix and MAVEN were part of this program. NASA decided to end this, because it wanted to focus on surface missions (landers and rovers), and they wouldn't &lt;a href=&quot;http://news.softpedia.com/news/NASA-s-Scout-Program-Discontinued-149866.shtml&quot; rel=&quot;nofollow&quot;&gt;fit within this budget&lt;/a&gt;.&lt;br&gt;&#xA;The &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=MESURPR&quot; rel=&quot;nofollow&quot;&gt;Pathfinder&lt;/a&gt; rover was another low-cost mission, at \$260 million. It traveled about &lt;a href=&quot;http://spacepioneers.msu.edu/robot_rovers/sojourner.html&quot; rel=&quot;nofollow&quot;&gt;100 m and was active for 80 days&lt;/a&gt;. Spirit and Opportunity cost &lt;a href=&quot;http://www.space.com/18766-spirit-rover.html&quot; rel=&quot;nofollow&quot;&gt;\$400 million each&lt;/a&gt;. &lt;a href=&quot;http://marsrovers.jpl.nasa.gov/mission/&quot; rel=&quot;nofollow&quot;&gt;Spirit&lt;/a&gt; covered 7.7 km in 6 years, Opportunity is still active after 10 years and has covered 38 km so far, with lots more instruments than Pathfinder. In this case, the additional cost of the mission offered a more-than-proportional increase in the amount of science that could be done.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think we can discount &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-25125672&quot; rel=&quot;nofollow&quot;&gt;political disorganization on ESA's part&lt;/a&gt;. They're deliberately taking the &lt;a href=&quot;http://sci.esa.int/cosmic-vision/46510-cosmic-vision/?fbodylongid=2152&quot; rel=&quot;nofollow&quot;&gt;long view&lt;/a&gt;, and have the political room to plan missions that take two decades. More so than NASA, it seems.&lt;br&gt;&#xA;ESA missions are divided into &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_Vision&quot; rel=&quot;nofollow&quot;&gt;3 classes&lt;/a&gt;: Large (~€1B or \$1.2B), Medium (~€400M or \$480M), and Small (~€50M).&lt;br&gt;&#xA;A final factor that makes the lead time &lt;a href=&quot;http://futureplanets.blogspot.nl/2013/09/europe-plans-its-next-large-missions.html&quot; rel=&quot;nofollow&quot;&gt;seem very long&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;While NASA typically selects missions after detailed design studies&#xA;  and then launches in four to five years, ESA selects concepts far in&#xA;  advance of launch and fills in the details after selection.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-02-08T11:07:40.027" LastActivityDate="2014-02-08T11:07:40.027" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3556" PostTypeId="1" AcceptedAnswerId="3558" CreationDate="2014-02-01T19:53:08.650" Score="38" ViewCount="5207" Body="&lt;p&gt;This dwarf planet reaches perihelion in the summer of 2076. At a distance of 76 au, is it feasible to launch a probe that would reach Sedna in time? With an orbital period of over 11,000 years, this may be humanity's (as we know it) only opportunity. Repeated internet searches have failed to reveal any nation or group making plans for this mission yet, but is this because of lack of vision, or is the task itself impossible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears that Neptune may be in a position to help provide a gravity assist around 2056. If at all possible, such a mission should aim for an orbital capture and/or landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides the scientific goals &amp;amp; boasting rights, such a mission would also serve as an interstellar precursor. Any orbiter/lander, logically, would need to be designed to last as long as we can make it last, centuries if not millenia. Imagine having an interstellar beacon at the edge of our solar system!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/aXssC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-03-15T14:19:19.730" LastActivityDate="2020-03-08T13:01:04.480" Title="What is the feasibility of launching a probe to Sedna?" Tags="|trajectory|future-missions|deep-space|gravity-assist|sedna|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="3557" PostTypeId="1" CreationDate="2014-02-02T01:06:36.530" Score="5" ViewCount="190" Body="&lt;p&gt;Got my two LR-101's, found some reasonably sized surplus LOX/RP-1 and pressurization tanks, found plans for fabricating valves, designed my vehicle, made an excel based back of the envelope sizing program. Now I need trajectory analysis software to see how high and fast this combination will fly. I looked at some free-ware offers most were dead-end links, codes for high end operating systems, out of business or bogus download sites. Down loaded Orbital and freaked out on the learn curve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any windows based trajectory analysis software out there other than soild fuel model rocket type that is reasonably priced (under $300 USD). Not a game just a basic program. Input basic vehicle parameters push the enter button and get graphic flight path results???? &lt;/p&gt;&#xA;" OwnerUserId="1559" LastEditorUserId="49" LastEditDate="2014-02-02T05:45:10.067" LastActivityDate="2014-02-02T05:45:10.067" Title="Economical Suborbital Trajectory Analysis Software" Tags="|trajectory|software|sub-orbital|analysis|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3558" PostTypeId="2" ParentId="3556" CreationDate="2014-02-02T02:54:52.013" Score="26" Body="&lt;h1&gt;First order analysis&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Given that we have practical ion thrusters, it's time to look at them.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Deep Space 1&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The DS1 probe massed 387kg, had 83kg of fuel, operated for 162 days, and generated 92mN. So, it generated about 0.2mm/s^2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The craft is not tanks-dry, either. It has approximately 6 months (180 days) of fuel per design. That's a roughly 20% fuel design, and my estimate on the mass of the thruster itself is 10kg - about 0.01N per kg, and linearly scaling, with about 16x thruster mass in fuel per year. (These numbers are rough, but provide a baseline)&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Powering a 2 kW thruster...&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;In the inner solar system, solar power is viable for an electric thruster; out past the asteroids, it becomes pretty much non-viable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radio-thermal generators, likewise, are measured in kilograms per watt... one of the most efficient was on the voyagers, at around 40W of electricity out per kilogram... to get a reasonable 0.2mm/s^2 acceleration, they become impractical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which pushes us into the range of nuclear fission reactors. Which also means large masses - the SNAP-10A was 290kg and 30kW. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Into the hypothesizing&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;We need a multi-ton spacecraft. &#xA;There is a design for a 100kW Electrical output, ~520kg nuclear reactor. This would be adequate to power 50-some NSTAR units at 91mN each; assuming only 20 such units, and 80kg each per 6 months in reaction mass, and 10kg each, plus a 200kg science payload, we can get a good first order hypothesis. I will assume for now a 5 year plant duration, since the SAFE400 has been in testing for several years, and I cannot find documentation for its fuel use.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;  kg    kW   Item&#xA; 200    40   NSTAR x20, giving 2N&#xA; 520  (100)  SAFE-400 400kW/100kWe nuclear reactor.&#xA;6400     0   2 years NSTAR fuel for 20 units.&#xA; 200    10   science package comparable to a mars orbiter.&#xA;7400    --   mission mass.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;This would give a mission thrust at launch of 0.00027m/s^2. Almost directly comparable to DS1... and a 720 day thrust, using a turn and flip, is roughly 3.4 AU covered, and peak speed of 8.3km/s, or 17861396s per AU or about 209 days per AU ... and 71 AU to cover. This would mean about 41 additional years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the actual acceleration would increase over the mission, and the mass of fuel being the largest proportion, we can use the average mass of around 4000kg for figuring overall - nearly doubling the engine-off speed, and cutting the coast time to about 20 years. The remaining issues are fuel for the power plant, which I lack the data to calculate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A larger fuel mass could be used, increasing duration, but decreasing initial acceleration. A 4 year fuel duration, for example,&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;   kg    kW   Item&#xA;  200    40   NSTAR x20, giving 2N&#xA;  520  (100)  SAFE-400 400kW/100kWe nuclear reactor.&#xA;12800     0   2 years NSTAR fuel for 20 units.&#xA;  200    10   science package comparable to a mars orbiter.&#xA;21800    --   mission mass. (probably about 1050kg tanks dry)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Initial would be about 0.00009m/s^2, with a peak of about 0.0019m/s^2, and an average of about 0.001m/s^2... and would cover about 51 AU under thrust, and peak velocity of about 62km/s... or about 28 days per AU, for about 2 years coasting time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would put a rough mission travel time on the order of 6 years, and about 1/2 of it thrusting outbound, 1/3 coasting, and 1/6 decelerating into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the technologies are not all fully proven. By not fully proven, I mean (1) we don't know that they actually will survive a 4-year constant &quot;burn&quot;... tho' we know they will last at least 160 days, and (2) the fission system hasn't been in existence long enough to establish that it will in fact last the 4-10 years needed for a mission&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Speculative answer&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Yes, a first order analysis indicates it is plausible that a mission could be made, and with a flight time of under 10 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of vagaries, however, in the available data. Structural mass is simply estimated; fuel mass may be insufficient for the indicated duration, etc.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://solarsystem.nasa.gov/missions/profile.cfm?MCode=DS1&amp;amp;Display=ReadMore&quot;&gt;http://solarsystem.nasa.gov/missions/profile.cfm?MCode=DS1&amp;amp;Display=ReadMore&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Deep_Space_1&quot;&gt;http://en.wikipedia.org/wiki/Deep_Space_1&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nmp.nasa.gov/ds1/&quot;&gt;http://nmp.nasa.gov/ds1/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.world-nuclear.org/info/Non-Power-Nuclear-Applications/Transport/Nuclear-Reactors-for-Space/&quot;&gt;http://www.world-nuclear.org/info/Non-Power-Nuclear-Applications/Transport/Nuclear-Reactors-for-Space/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://members.shaw.ca/bru_b/index/nkmoslikfly2.htm&quot;&gt;http://members.shaw.ca/bru_b/index/nkmoslikfly2.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://web.archive.org/web/20050321055406/http://www.spacetransportation.com/ast/presentations/7b_vandy.pdf&quot;&gt;http://web.archive.org/web/20050321055406/http://www.spacetransportation.com/ast/presentations/7b_vandy.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2014-02-03T22:18:35.537" LastActivityDate="2014-02-03T22:18:35.537" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3559" PostTypeId="2" ParentId="3556" CreationDate="2014-02-02T04:26:19.560" Score="18" Body="&lt;p&gt;Considering that Voyager 1 is already 126 AU from the Sun 36 years since launch, there should be no reason that it would not be possible energetically using a normal launch, small maneuvers, and planetary flybys.  Just a Jupiter flyby should be sufficient.  Jupiter will also provide the necessary change in inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Designing a probe that is assured to operate for that long would however be quite difficult.  (The Voyager spacecraft were not assured to work past Saturn.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-02T04:31:25.563" LastActivityDate="2014-02-02T04:31:25.563" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="3560" PostTypeId="1" CreationDate="2014-02-02T19:50:49.620" Score="23" ViewCount="3208" Body="&lt;p&gt;Other than fuel, what factors limit the lifetime of a spacecraft? For example, if someone wanted to send a spacecraft on a 100 year trip round the solar system and back to Earth, could it be done using today's technology, ignoring the orbital mechanics and merely focussing on power management and other similar issues?&lt;/p&gt;&#xA;" OwnerUserId="1562" LastActivityDate="2014-02-05T21:46:15.670" Title="What factors limit the lifetime of an unmanned spacecraft?" Tags="|probe|long-duration|" AnswerCount="8" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3561" PostTypeId="2" ParentId="3560" CreationDate="2014-02-02T20:43:39.577" Score="13" Body="&lt;p&gt;There are probably a bunch of problems, but lets start with some easy ones:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Thermal cycling&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Each side of the vehicle will differ in temperature a great deal, since one side will be in sunlight, the other in shade. You can do a rotisserie roll, as Apollo did on the way to the moon, but that makes maintaining lock on earth with an antenna harder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless, whatever materials the vehicle is made out of will expand and contract, a lot due to wide swings in temperatures and this will generate wear and tear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Circuit boards do not take well to this as welds and solder can crack. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Radiation damage&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Electronics without adequate shielding will get overwhelmed by the high energy protons or beta radiation that will naturally be encountered. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless of how good the shielding is, over a 100 year time frame, it is likely some damage will occur.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar panels degrade over time, so whatever power source you chose, must survive the 100 year time span which is currently unlikely.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Consumables&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Whatever is used for station keeping/orbit correction, will eventually run out.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-02-02T22:23:57.513" LastActivityDate="2014-02-02T22:23:57.513" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3562" PostTypeId="2" ParentId="3560" CreationDate="2014-02-02T22:47:29.587" Score="6" Body="&lt;p&gt;In addition to Geoffc's answer, let me give a few others.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Batteries- These eventually wear out. This is a bit deal for LEO missions, not as much for other missions.&lt;/li&gt;&#xA;&lt;li&gt;Mechanical failure- Reaction wheels are the most common, you just can't keep things spinning forever.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-02-02T22:47:29.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3563" PostTypeId="2" ParentId="3560" CreationDate="2014-02-02T23:56:21.833" Score="15" Body="&lt;p&gt;Money.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over how many administrations, regimes, revolutions, wars, etc. will a 100-year spacecraft operations need to be funded?  If you expect to get data back, you will need to keep antennas and/or photon buckets (for laser comm) pointed at the thing, and you have to maintain and likely replace those assets over time.  You are unlikely to design something completely autonomous over a 100-year time span, so you will also need to maintain a team to command the spacecraft.  Whatever the science is, you'll need someone to point the instruments and say what data to collect when.  You will likely need to navigate the spacecraft, and maintain a team for orbit determination and maneuver generation.  If it is a sample return, then you will need to pay for someone to find the thing, pick it up, and conduct the science on the returned samples.  100 years later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many perfectly good spacecraft have been shut down due to lack of money.  &lt;a href=&quot;http://www.spacenews.com/article/civil-space/36338nasa%E2%80%99s-planetary-science-budget-crunch-expected-to-pit-curiosity-vs-cassini&quot;&gt;The threats to do so in the future continue.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-03T00:30:53.827" LastActivityDate="2014-02-03T00:30:53.827" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3564" PostTypeId="2" ParentId="3560" CreationDate="2014-02-03T06:26:57.077" Score="7" Body="&lt;p&gt;Mark Adler makes a good case for this being a matter of financing, but like always, there's two sides to the coin. If we put it in analogy, if we built a jukebox that could play for a hundred years, we'd also need someone to keep feeding it dimes. If it was really as durable, you'd get as much music out of it, as you'd throw money in it. But you also get one hundred years of amortizing your initial investment, and that's a lot of dimes. And that's the other part of the coin. If we could agree to listen to its music for one hundred years, it would make whoever built it rich. Going away from the analogy, rich in our case meaning lots and lots of valuable science done with a single space probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in economic sense, this should clearly be desirable. In political sense, well we get tired of one single tune rather fast (read: there will be other politicians signing bills for it), so the device should cater for many different tastes. Meaning, there should be some incentive, a political motivation for it that would excuse its financing for a century. That's likely tougher to do than building it to last that long. But assuming we could do that, and opt to amortize huge initial costs of building and launching the spacecraft in a hundred years time, we'd get invaluable science out of it, and all the spin-off technologies developed for it a lot sooner than that even.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, assuming it would work for that long. And that on its own wouldn't be an easy feat. As you make distance away from the Sun, renewable energy sources become scarce and infeasible (solar irradiance reduces with an inverse square to the distance from the source), so you'd have to pack a supply of power that would last. For the first leg of the journey, solar panels could be used while the solar radiative energy incident is still high, but eventually you'd have to rely on stored energy sources, say, &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;RTG&lt;/a&gt;s. At that point, you start producing excess heat, be it by battery discharge, radioisotope decay, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of this heat could be used to heat the spacecraft itself and keep all its equipment within the thermal envelope it was designed for. And the solar panels could now double as excess heat radiators, but you already have one more consumable on top of propellants, and you're now producing local heat pockets that can't be easily dealt with by spinning the space probe like before in the inner Solar system due to proximity to external sources of radiation (your greatest worry now comes from within), so now all the spacecraft is a subject to wear and tear due to thermal expansion, and not merely its moving parts (which you might use less frequently to reduce damage through operation, but that option isn't really available for thermal expansion).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could these &lt;em&gt;problems&lt;/em&gt; be dealt with? Probably, but assuring a one hundred year long operation is a tall order and it would either require advances in material science we're not yet privy to, or the space probe would have to be built with redundancy in mind, one that could switch from a failed component to a working one, while keeping the rest of the standby units operational even though they're exposed to the same unforgiving environment. We could build a space probe with redundancy in mind today too, without major breakthroughs in material science (and other fields), but it would be heavy, difficult to launch, it would consume even more of its own propellants, take longer to get to places of interests (i.e. what we usually refer to as &lt;em&gt;the tyranny of the rocket equation&lt;/em&gt;), be even more complex with increased chance of its individual parts failing (e.g. a good example are RAID disk arrays that, while their operational lifetime might be extended due to redundancy, have shorter life expectancy per unit simply for sharing the same enclosure and thus producing more heat), and perhaps more importantly, it would essentially be many space probes stuffed into a single one, with no advantage over simply launching many in various directions, each with expected operation of 10-20 years. And if something unpredictable happens with one or a few of them, others were at a safe distance and somewhere else during the incident.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this brings me to the point I wanted to make. That even though there would be tremendous challenges, we likely could do it, but for the investment, you could simply have multiple, simpler and less durable space probes that could be more specialized for the targets they'd study, instead of having one that would outlast them all, but carry one hundred years old science equipment onboard. There just isn't any benefit to a space probe lasting for a hundred years. So this is what in my opinion is the ultimate limit of a space probe's lifetime. Our own patience with it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-03T07:13:56.383" LastActivityDate="2014-02-03T07:13:56.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3565" PostTypeId="1" AcceptedAnswerId="3573" CreationDate="2014-02-03T09:49:37.100" Score="23" ViewCount="2769" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/3561/33&quot;&gt;This answer mentioned thermal cycling&lt;/a&gt; made me think of this question:  Are there any points in the solar system, such as Lagrange points, where a spacecraft could reside in perpetual shade, protected from heat and perhaps even from the worst of solar radiation damage?  I mean specifically in space, excluding the interior of solid bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/L2_point#L2&quot; rel=&quot;noreferrer&quot;&gt;L₂&lt;/a&gt; would be a candidate, however, &lt;a href=&quot;https://en.wikipedia.org/wiki/L2_point#L2&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia points out&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is, however, slightly beyond the reach of Earth's umbra, so solar radiation is not completely blocked.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Are there any points in the Solar system where solar radiation &lt;em&gt;would&lt;/em&gt; be completely blocked?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbits would be fine, too, but I can't imagine how any orbit could be in perpetual shade.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; &lt;em&gt;in an earlier version, this question mentioned&lt;/em&gt; stable &lt;em&gt;points in particular, but I realised later I don't mean the technical definition of stable at all&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2022-04-15T23:22:25.043" Title="Are there any (Lagrange) points in the Solar System in perpetual shade?" Tags="|orbital-mechanics|lagrangian-points|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3566" PostTypeId="2" ParentId="3560" CreationDate="2014-02-03T10:58:38.700" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;could it be done using today's technology, ignoring the orbital mechanics and merely focussing on power management and other similar issues?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes. It's a question of feasibility more than possibility. Solar panels degrade, batters decrease their depth of discharge, reaction wheels need momentum burns, but all of these things just state the requirement for bigger/higher start of life performance. For example, if we need 5W of power from the solar panels at end of life and we know the panels degrade by 1% each year they will be operating at (1-0.01)^100 = 0.634 * original power output by the end of life. So to find the start of life power required we can simply do 5W/0.634 = 7.88W. Therefore we need appropriately sized solar panels to give 7.88W. None of these numbers are really appropriate though, the degradation may be much higher and the power requires are likely to be much higher also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mechanical failure is a different matter. The typical way to deal with this would be to calculate the likelihood of failure and provide sufficient redundancy so that the likelihood is below an acceptable threshold. This will increase the mass and physical size of the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To sum up it's perfectly possible to build a spacecraft for a period of 100 years. However the feasibility of such a large spacecraft is unlikely to be favourable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One final point, with some overwhelmingly precise orbit mechanics you could set a spacecraft on a trajectory that would give it a tour of the solar system and bring it crashing back into Earth. This spacecraft need not be operational at any point of this journey or the subsequent crash. This does fulfil your requirements though.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-02-03T10:58:38.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3567" PostTypeId="2" ParentId="999" CreationDate="2014-02-03T11:16:22.423" Score="5" Body="&lt;p&gt;The rate of decay of space debris is driven primarily by drag from the Earth's atmosphere. The degree to which the atmosphere slows and object is based on the objects ballistic coefficient. This is because the kinetic energy of an object is the half objects mass and it's velocity^2, and the drag force is proportional to the wetted surface area, or the cross section area of the debris item. However this is only half the story.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space debris problem is such a troublesome problem because objects can collide with one another. When these collisions happen objects are broken into smaller pieces. Generally this has a positive effect on the decay rate since the total wetted area of the pieces is greater than the wetted area of a intact object. However not all pieces are the same and hence you will get some very dense and some very low density objects. This will cause some object to have high ballistic coefficient and some to have low BCs, the pieces with the higher BCs will stay in orbit from longer than the other pieces and potentially longer than the original intact spacecraft. The net effect over time is an increase in the total number of objects, even if the total mass in orbit is decreasing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this is a complex problem Monte Carlo simulations are often used to predict the rate of increase in debris flux (number of objects). However the uncertainties involved in these simulations leave a lot of unknown factors and different models predict different outcomes. Please add a comment if you'd like more specific information - a half life is not something that can accurately be predicted here.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-02-03T11:16:22.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3568" PostTypeId="2" ParentId="3111" CreationDate="2014-02-03T12:54:35.330" Score="6" Body="&lt;p&gt;The Curiosity rover covers an area of a little less than 8 m². Spirit and Opportunity only cover about 4 m² each. The whole mars has a surface area of 144,798,500 km². or 144,798,500,000,000 m².&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you think of an asteroid as a point-impact which only affects the exact position where it lands, and you assume that each asteroid can hit anywhere on Mars with equal probability, the chance of each asteroid hitting one of the rovers is one in 18 trillion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means even when millions of small asteroids go down on Mars, it is still very unlikely that one will hit the rovers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is, however, the risk of a larger impact causing damage in a much larger area due to the impact shockwave and ejecta launched away from the impact crater. An impact event like the one which created the &lt;a href=&quot;http://en.wikipedia.org/wiki/Victoria_Crater&quot; rel=&quot;noreferrer&quot;&gt;Victoria crater&lt;/a&gt; would definitely be a danger for any rover in the vicinity. But such large impacts are rather the exception than the norm. And even when they occur, the destruction would just be locally.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-02-03T14:15:02.297" LastActivityDate="2014-02-03T14:15:02.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3569" PostTypeId="1" AcceptedAnswerId="3571" CreationDate="2014-02-03T13:16:32.910" Score="9" ViewCount="1277" Body="&lt;p&gt;Is there any specific, practical reason why we couldn't have a probe swing by Saturn, Uranus, Jupiter multiple times, over and over, before getting hurled into interstellar space at insane speed, overtaking Voyager and reaching neighbor solar systems within a reasonable timeframe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I understand, while probes use their fuel for accelerating during gravitational loops, that's not essential - they could just use minimal amount of it to adjust route, to enter and leave the slingshot at optimal angle, and repeat it pretty much indefinitely, with a significant speed gain each time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any specific reason we don't have some probe accelerating to speeds that would leave Voyagers far behind, currently? What are the practical limitations of using this method repeatedly to bring probes to extreme speeds?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="49" LastEditDate="2014-03-05T09:26:08.750" LastActivityDate="2016-04-09T09:41:50.367" Title="What are the practical limitations of gravitational assists?" Tags="|orbital-mechanics|gravity-assist|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3570" PostTypeId="2" ParentId="3421" CreationDate="2014-02-03T13:25:20.593" Score="2" Body="&lt;p&gt;Some engineering studies done by Joseph Carroll at &quot;Tether Application Inc&quot;. The big space players don't seem to be very interested. Small enterpreneurs and enthusiasts seem to lead the way for space travel, now as 100 years ago. At least someone does something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 2 hour sound presentation and discussion of Joe Carroll's ideas, including a tether station with the centrifugal gravity of Mars, Moon and what he thinks is the smallest somewhat useful gravity at 0.06g:&#xA;&lt;a href=&quot;http://archived.thespaceshow.com/shows/1552-BWB-2011-05-03.mp3&quot; rel=&quot;nofollow&quot;&gt;http://archived.thespaceshow.com/shows/1552-BWB-2011-05-03.mp3&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Powerpoints to the talk above:&#xA;&lt;a href=&quot;http://spaceshowclassroom.files.wordpress.com/2011/05/artificial-gravity-iac2010-presentation-revised-2011april30.pdf&quot; rel=&quot;nofollow&quot;&gt;http://spaceshowclassroom.files.wordpress.com/2011/05/artificial-gravity-iac2010-presentation-revised-2011april30.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-02-03T13:25:20.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3571" PostTypeId="2" ParentId="3569" CreationDate="2014-02-03T13:44:33.390" Score="10" Body="&lt;p&gt;The easiest explanation would be that eventually you'd gain enough momentum to reach escape velocity, at which point you'd actually have to decelerate to return to orbit from what would be a hyperbolic trajectory describing your maximum delta-v gained from an &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Oberth maneuver&lt;/a&gt; (powered flyby). With unpowered flyby, your theoretical maximum delta-v gain from a slingshot would be the body's own orbital velocity, there simply isn't anything more to be gained from it in terms of accelerating the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It then doesn't matter if you can achieve that during your first flyby, or do a flyby past  another body to later return back to the same one many times over. You could only increase efficiency of an unpowered flyby up to 100% of the momentum that the gravity assist body can lend to your spacecraft, which is your initial velocity, plus body's orbital velocity. And with Oberth maneuvers, you'd be consuming your propellants with each flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There actually exist such interchanging orbits between two celestial bodies, for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_cycler&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aldrin Cyclers&lt;/a&gt; (here's a &lt;a href=&quot;https://www.youtube.com/watch?v=qCVfUlFZQ4U&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video of one such example&lt;/a&gt;). And the reason they work is because of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_momentum#Conservation_of_angular_momentum&quot; rel=&quot;nofollow noreferrer&quot;&gt;conservation of angular momentum&lt;/a&gt;, which is another way of explaining why multiple such flybys you describe wouldn't work. Well, they would work, but not in the way of gaining some additional momentum out of nothing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The optimal trajectory for leaving the Solar System is known as the &lt;a href=&quot;https://space.stackexchange.com/questions/834/would-leaving-the-ecliptic-plane-offer-any-benefit-to-interstellar-travel/971#971&quot;&gt;Krafft von Ehricke trajectory&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-02-03T19:18:40.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3572" PostTypeId="2" ParentId="3565" CreationDate="2014-02-03T13:56:28.223" Score="5" Body="&lt;p&gt;First thought.  We need to compare the radius of the L2 from object centre to the length of the Umbra.  The Umbra is determined (partly) by diameter of the object while the mass determines the L2 radius. So mass rises with the cube of object radius, while area is a square function of radius.  So, density is be key.  see &lt;a href=&quot;http://www.merlyn.demon.co.uk/gravity4.htm#Ap1&quot; rel=&quot;nofollow&quot;&gt;http://www.merlyn.demon.co.uk/gravity4.htm#Ap1&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1569" LastEditorUserId="4660" LastEditDate="2016-03-09T15:11:50.697" LastActivityDate="2016-03-09T15:11:50.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3573" PostTypeId="2" ParentId="3565" CreationDate="2014-02-03T15:39:48.933" Score="24" Body="&lt;p&gt;To the extent of my creativity, L2 is the only location that can satisfy this for a given planet.  We would also need to constrain the discussion to planets because, again, my creativity fails me to think of any other option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, we must use the method described in the other answer to ask &quot;does any planetary L2 fall within the planet's umbra?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To start with, the location of the L2 point &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point#L2&quot;&gt;can be approximated by the following&lt;/a&gt;, where M1 is the sun's mass and M2 is the planet's mass.  R is the distance of the planet from the sun.  r is the distance of the L2 point from the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ M_2 \ll M_1 \\&#xA;  r \approx R \sqrt[3]{\frac{M_2}{3 M_1}} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for L2 to be close to the planet, we would want the planet's mass to be small.  However, a smaller planet has a smaller radius.  Let's move on to calculating the termination point of the umbra.  This can be done with geometry, using similar triangles defined by the end of the umbra with the planet and with the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ r_u \approx R \frac{ D_{planet} }{ D_{sun} } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our basic criteria for an eternally shaded L2 point is then simple, that $r&amp;gt;r_u$ in the above equations.  Here are my calculations of the ratio between the two values using &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/&quot;&gt;planetary data&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt; MERCURY    0.920628441&lt;/li&gt;&#xA;&lt;li&gt; VENUS  0.931131053&lt;/li&gt;&#xA;&lt;li&gt; EARTH  0.916884469&lt;/li&gt;&#xA;&lt;li&gt; &lt;strong&gt;MARS&lt;/strong&gt;   1.02662275&lt;/li&gt;&#xA;&lt;li&gt; &lt;strong&gt;JUPITER&lt;/strong&gt;    1.505844946&lt;/li&gt;&#xA;&lt;li&gt; &lt;strong&gt;SATURN&lt;/strong&gt;     1.897836869&lt;/li&gt;&#xA;&lt;li&gt; &lt;strong&gt;URANUS&lt;/strong&gt;     1.505423974&lt;/li&gt;&#xA;&lt;li&gt; &lt;strong&gt;NEPTUNE&lt;/strong&gt;    1.382215047&lt;/li&gt;&#xA;&lt;li&gt; &lt;strong&gt;PLUTO&lt;/strong&gt;  1.322002729&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For all values above 1, the L2 point is fully shaded.  If the value is very close to 1, there are likely to be some other complications, since you might not be in &lt;em&gt;exactly&lt;/em&gt; that point, so some sunlight will still get through.  You'll also still need some station-keeping at that point to hold in place.  Also, note that a space elevator is possible (in theory) for a point like the sun-Mercury L2 point, but the counter-weight would be further away than the L2 point itself, meaning that it would not be perfectly shaded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I repeated these calculations for planet's perihelion and aphelion.  They were exactly the same, so there must be some definitional reason why (unless I messed it up).  But managing the L2 point within an elliptical orbit is a complicated maneuver, so this couldn't be so simple in real life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But otherwise, the conclusion is pretty clear.  Once you get out of the inner solar system, the sun-planet L2 points lie fully within the planet's umbra.  I lack a general technical explanation for why this should be.  I just used the data.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-02-03T15:39:48.933" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3574" PostTypeId="1" AcceptedAnswerId="3577" CreationDate="2014-02-03T16:58:54.907" Score="10" ViewCount="879" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_2020_rover_mission&quot;&gt;Wikipedia page&lt;/a&gt; on the plans for NASA's 2020 Mars rover says that it will be designed similarly to &lt;em&gt;Curiosity&lt;/em&gt; but will differ in scientific instrumentation because of its goal:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;to investigate an astrobiologically relevant ancient environment on Mars, investigate its surface geological processes and history, including the assessment of its past habitability and potential for preservation of biosignatures within accessible geological materials.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The concept sketch of the rover also mentions that there will be some sample return.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any plans or papers yet detailing what difference in instrumentation the 2020 rover will have from &lt;em&gt;Curiosity&lt;/em&gt;?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="1427" LastEditDate="2014-02-04T13:05:19.190" LastActivityDate="2015-10-05T22:51:56.510" Title="How will the NASA 2020 Mars rover's instrumentation differ from Curiosity?" Tags="|mars|nasa|rovers|instrument|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3575" PostTypeId="1" AcceptedAnswerId="3576" CreationDate="2014-02-03T17:53:03.497" Score="2" ViewCount="762" Body="&lt;p&gt;I came across this page - &lt;a href=&quot;http://www.hq.nasa.gov/alsj/alsjcoords.html&quot; rel=&quot;nofollow&quot;&gt;http://www.hq.nasa.gov/alsj/alsjcoords.html&lt;/a&gt; that mentions the various landing coordinates of of NASA missions on the Earth's moon. The cordinates are given in the same way as that of the Earth N/S &amp;amp; E/W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that the moon does not have a molten core and therefore cannot have a magnetic North or South. So how is the coordinate system created?&lt;/p&gt;&#xA;" OwnerUserId="1571" LastActivityDate="2014-02-03T19:06:01.537" Title="How are coordinates and cardinal points defined on on Earth's Moon?" Tags="|the-moon|navigation|landing|lunar-landing|" AnswerCount="1" CommentCount="2" ClosedDate="2014-02-04T02:50:35.720" ContentLicense="CC BY-SA 3.0" />
  <row Id="3576" PostTypeId="2" ParentId="3575" CreationDate="2014-02-03T18:23:35.120" Score="4" Body="&lt;p&gt;They used &lt;a href=&quot;http://en.wikipedia.org/wiki/Selenographic_coordinates&quot; rel=&quot;nofollow&quot;&gt;selenographic coordinates&lt;/a&gt;. A small yet well-preserved crater called &lt;a href=&quot;http://en.wikipedia.org/wiki/M%C3%B6sting_%28crater%29&quot; rel=&quot;nofollow&quot;&gt;Mösting&lt;/a&gt; is used as a fundamental reference point, and given these coordinates:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Latitude:   3° 12' 43.2&quot; South&#xA;Longitude:  5° 12' 39.6&quot; West&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;All coordinates are defined relative to this location, so there's no need to rely on a magnetic field, although &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetic_field_of_the_Moon&quot; rel=&quot;nofollow&quot;&gt;the Moon does have one&lt;/a&gt;. For more information, please read &lt;a href=&quot;http://www.rand.org/pubs/notes/N2664.html&quot; rel=&quot;nofollow&quot;&gt;this RAND corporation report of 1987&lt;/a&gt;, and the fundamental &lt;a href=&quot;http://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2009reprint.pdf&quot; rel=&quot;nofollow&quot;&gt;IAU/IAG paper&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="168" LastEditDate="2014-02-03T19:06:01.537" LastActivityDate="2014-02-03T19:06:01.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3577" PostTypeId="2" ParentId="3574" CreationDate="2014-02-03T19:27:51.753" Score="11" Body="&lt;p&gt;The instrumentation will be completely different.  The &lt;a href=&quot;http://mars.nasa.gov/files/m2020/SDT_July2013.zip&quot; rel=&quot;nofollow noreferrer&quot;&gt;2020 SDT report&lt;/a&gt; covers in detail the science objectives and types of instrumentation needed to meet those objectives.  In short, the instrumentation will be focused on in situ microscopic observations and sample collection and caching for possible return to Earth.  Curiosity on the other hand is focusing on laboratory experiments on powdered samples, i.e. mass spectrometer and X-Ray diffraction experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What the instruments are exactly will be determined by the selection in process on the &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2014-017&quot; rel=&quot;nofollow noreferrer&quot;&gt;58 proposals received&lt;/a&gt; in response to the &lt;a href=&quot;http://nspires.nasaprs.com/external/solicitations/summary.do?method=init&amp;amp;solId=%7BC49E4810-6DE9-9509-E896-EBC006101A9E%7D&quot; rel=&quot;nofollow noreferrer&quot;&gt;Announcement of Opportunity&lt;/a&gt;, where the latter goes into great detail on what was requested of the proposers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Update:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars 2020 instruments were selected in July 2014:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k74nD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k74nD.jpg&quot; alt=&quot;rover callout with instruments&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The selected instruments are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Mastcam-Z, an advanced camera system with panoramic and stereoscopic&#xA;imaging capability with the ability to zoom. The instrument also will&#xA;determine mineralogy of the Martian surface and assist with rover&#xA;operations. The principal investigator is James Bell, Arizona State&#xA;University in Tempe.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;SuperCam, an instrument that can provide imaging, chemical&#xA;composition analysis, and mineralogy. The instrument will also be&#xA;able to detect the presence of organic compounds in rocks and&#xA;regolith from a distance. The principal investigator is Roger Wiens,&#xA;Los Alamos National Laboratory, Los Alamos, New Mexico. This&#xA;instrument also has a significant contribution from the Centre&#xA;National d’Etudes Spatiales,Institut de Recherche en Astrophysique et&#xA;Plane’tologie (CNES/IRAP) France.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Planetary Instrument for X-ray Lithochemistry (PIXL), an X-ray&#xA;fluorescence spectrometer that will also contain an imager with high&#xA;resolution to determine the fine scale elemental composition of&#xA;Martian surface materials. PIXL will provide capabilities that permit&#xA;more detailed detection and analysis of chemical elements than ever&#xA;before. The principal investigator is Abigail Allwood, NASA's Jet&#xA;Propulsion Laboratory (JPL) in Pasadena, California. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Scanning Habitable Environments with Raman &amp;amp; Luminescence for&#xA;Organics and Chemicals (SHERLOC), a spectrometer that will provide&#xA;fine-scale imaging and uses an ultraviolet (UV) laser to determine&#xA;fine-scale mineralogy and detect organic compounds. SHERLOC will be&#xA;the first UV Raman spectrometer to fly to the surface of Mars and&#xA;will provide complementary measurements with other instruments in the&#xA;payload. The principal investigator is Luther Beegle, JPL.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Mars Oxygen ISRU Experiment (MOXIE), an exploration technology&#xA;investigation that will produce oxygen from Martian atmospheric&#xA;carbon dioxide. The principal investigator is Michael Hecht,&#xA;Massachusetts Institute of Technology, Cambridge, Massachusetts.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mars Environmental Dynamics Analyzer (MEDA), a set of sensors that&#xA;will provide measurements of temperature, wind speed and direction,&#xA;pressure, relative humidity and dust size and shape. The principal&#xA;investigator is Jose’ Antonio Rodriguez-Manfredi, Centro de&#xA;Astrobiologia, Instituto Nacional de Tecnica Aeroespacial, Spain.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Radar Imager for Mars' Subsurface Experiment (RIMFAX), a&#xA;ground-penetrating radar that will provide centimeter-scale&#xA;resolution of the geologic structure of the subsurface. The principal&#xA;investigator is Svein-Erik Hamran, the Norwegian Defence Research&#xA;Establishment (FFI), Norway.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-10-05T22:51:56.510" LastActivityDate="2015-10-05T22:51:56.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3578" PostTypeId="2" ParentId="3207" CreationDate="2014-02-03T19:41:29.210" Score="2" Body="&lt;p&gt;They used &lt;a href=&quot;http://en.wikipedia.org/wiki/Selenographic_coordinates&quot; rel=&quot;nofollow&quot;&gt;selenographic coordinates&lt;/a&gt;. A small yet well-preserved crater called &lt;a href=&quot;http://en.wikipedia.org/wiki/M%C3%B6sting_%28crater%29&quot; rel=&quot;nofollow&quot;&gt;Mösting&lt;/a&gt; is used as a fundamental reference point, and given these coordinates:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Latitude:   3° 12' 43.2&quot; South&#xA;Longitude:  5° 12' 39.6&quot; West&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;All coordinates are defined relative to this location, so there's no need to rely on a magnetic field, although &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetic_field_of_the_Moon&quot; rel=&quot;nofollow&quot;&gt;the Moon does have one&lt;/a&gt;. For more information, please read &lt;a href=&quot;http://www.rand.org/pubs/notes/N2664.html&quot; rel=&quot;nofollow&quot;&gt;this RAND corporation report of 1987&lt;/a&gt;, and the fundamental &lt;a href=&quot;http://astropedia.astrogeology.usgs.gov/download/Docs/WGCCRE/WGCCRE2009reprint.pdf&quot; rel=&quot;nofollow&quot;&gt;IAU/IAG paper&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2014-02-03T19:41:29.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3579" PostTypeId="2" ParentId="3161" CreationDate="2014-02-03T22:45:33.710" Score="9" Body="&lt;p&gt;Methane would allow for around 380 second specific impulse (~3.8km/s exhaust velocity), depending on the chamber pressure, expansion ratio, and other design parameters for the engine, while LH2/LOX engines have demonstrated ~450 second specific impulse (~4.5km/s exhaust velocity). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite this lower efficiency though, methane has a couple of major advantages. It is significantly higher density as a liquid than LH2 (0.42 g/cc, vs 0.07 for liquid hydrogen), so it requires far less tank volume and smaller piping for the same mass of propellant. It also doesn't need to be stored as cold as liquid hydrogen, which reduces the insulation and cooling requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has traditionally favored dense, easy to handle propellant (LOX/RP1) and simple engine designs (the Merlin is a simple gas generator cycle, rather than the more efficient [but more complex] staged combustion design used by most other modern rockets). As such, it makes sense that they would go for the easier to handle and simpler solution of a methane rocket rather than the high performance but difficult and complicated liquid hydrogen motor, so long as the methane provides them with the performance that they require (which will remain to be seen). &lt;/p&gt;&#xA;" OwnerUserId="1572" LastEditorUserId="49" LastEditDate="2014-05-15T15:09:49.080" LastActivityDate="2014-05-15T15:09:49.080" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3580" PostTypeId="2" ParentId="3560" CreationDate="2014-02-03T23:13:53.397" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;nofollow&quot;&gt;Voyager 1 and 2&lt;/a&gt; have both been operational for 36 years; I believe they're our longest-lasting space missions to date. They're still in good working order. Their radiothermal power generators will probably give out in another ten years or so, but I don't see any fundamental reason that a Voyager-like probe couldn't be built with a 100-year lifetime. More/bigger RTGs would last longer, but entail adding weight to the craft, which would have to be paid for by reducing sensor payload or using a bigger launcher. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-02-03T23:13:53.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3581" PostTypeId="2" ParentId="3560" CreationDate="2014-02-04T01:23:35.067" Score="9" Body="&lt;ul&gt;&#xA;&lt;li&gt;Power System duration&lt;/li&gt;&#xA;&lt;li&gt;Mechanical wear&#xA;&lt;ul&gt;&#xA;&lt;li&gt;thermal cycling&lt;/li&gt;&#xA;&lt;li&gt;impact damage&lt;/li&gt;&#xA;&lt;li&gt;operational vibration&lt;/li&gt;&#xA;&lt;li&gt;friction wear&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;radiation damage&lt;/li&gt;&#xA;&lt;li&gt;Material choices&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h1&gt;Power System Duration&lt;/h1&gt;&#xA;&lt;p&gt;There is no more fundamental need on a spacecraft than a source of power. Except for some of the earliest satellites, spacecraft are fundamentally electronic systems.&lt;/p&gt;&#xA;&lt;p&gt;While the radio-thermal generator is a long duration device, their primary failure point is the corrosion of the thermocouples. The 470We output of the Voyager probe is now significantly reduced - it should, based upon the fuel decay, still be around 350We... but it's under 300 watts at present. Many earlier generation probes have ceased function due to RTG thermocouple corrosion.&lt;/p&gt;&#xA;&lt;p&gt;Solar panels also degrade over time, tho' how fast is a combination of other factors. Still, when the solar fails, the satellite is dead.&lt;/p&gt;&#xA;&lt;p&gt;A full nuclear reactor is one of the most compact and powerful sources of electricity for spacecraft. It has the issues of the thermal to electric conversion systems, not dissimilar to those of RTGs, of corrosion and radiation damage, plus the limit of fuel. Once the fuel is used, it's used. Further, the fuel decays whether or not it is used; for uranium-233, 234, 235, 238, the decay is trivial, but can itself cause damage to other systems.&lt;/p&gt;&#xA;&lt;h1&gt;Mechanical Wear&lt;/h1&gt;&#xA;&lt;p&gt;Several sources of mechanical wear are important. The most important is the thermal cycling - as an object heats, it expands (with a few exceptions, like water below 4°C), and as it cools, contracts. This can cause flexion and brittleness, and even cracking of components. The sources of this include operation of instruments, received sunlight, and venting of operational fluids (intentional or not).&lt;/p&gt;&#xA;&lt;p&gt;Most spacecraft are in motion. This means also that they are likely to encounter other objects in the local space environment. Impact with such objects is a primary cause of failures of solar panels, and can cause failures of a variety of sensors as well. It's also both quite predictable for low level events (like solar wind damage), and quite unpredictable for high energy events (what people might term &amp;quot;impacts&amp;quot;).&lt;/p&gt;&#xA;&lt;p&gt;Operational vibration can be generated by instrument operational cycling, motors to move instruments, and minor impacts. As with thermal expansion, this results in flexion damage of the same kinds. It's also noted for loosening connectors if they are not properly designed; most space agencies account for this.&lt;/p&gt;&#xA;&lt;p&gt;Friction wear is any wear caused by part A moving over part B. It's usually predictable, but is also a source of heating, and thus the mechanical wear of thermal expansion. Most friction wear is predictable for spacecraft; for landers/rovers, it's less so, due to environmental variables.&lt;/p&gt;&#xA;&lt;h1&gt;Radiation Damage&lt;/h1&gt;&#xA;&lt;p&gt;Radiation has multiple effects on spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;Radiation can directly set values in solid-state electronics; the smaller the microcircuitry, the more impact this has. This effect is usually not significant in terms of long-term lifespan, but can result in sudden terminations due to altered code. It's also routinely shielded against.&lt;/p&gt;&#xA;&lt;p&gt;Radiation on materials, especially alpha and beta versus metals, can result in isotopic changes and/or fission (even of normally &amp;quot;non-fissionable&amp;quot; materials); over time, this can progressively result in changes in conductivity, brittleness, and tensile strength. These changes are particularly slow, but are part of the considerations needed for RTGs and Nuclear Reactors.&lt;/p&gt;&#xA;&lt;p&gt;Radiation can also do direct damage to circuitry by removing bits of circuit (particulate), or by overloading circuitry (EMF and electron radiation).These can be shielded against, and are generally predictable, but subject to variations.&lt;/p&gt;&#xA;&lt;p&gt;The largest issue with radiation is that spacecraft carry multiple radiation sources - antennae, RTGs and/or nuclear reactors - and must be designed to account for both onboard and offboard sources.&lt;/p&gt;&#xA;&lt;h1&gt;Material Choices&lt;/h1&gt;&#xA;&lt;p&gt;Not all materials used are long-term stable. Some are chosen for their very reactivity - this is true of reagents in chemical experiments, and of fuels - and others for specific properties during the expected operational lifespan.&lt;/p&gt;&#xA;&lt;p&gt;Examples of reactivity by choice include hydrazine for thrusters, and the many chemistry experiments on various martian landers.&lt;/p&gt;&#xA;&lt;p&gt;Examples of specific desired properties include gasket materials on fuel systems - the gaskets are a weak point and decay over time, but are stable enough for their expected durations. The gaskets used on the NASA SRB's are a different material than the gaskets on, say, a set of compressed gas valves, or on an ISS airlock door.&lt;/p&gt;&#xA;&lt;h1&gt;A Hundred Year Mission?&lt;/h1&gt;&#xA;&lt;p&gt;Radio-thermal generators probably could be designed that would be useful that long - provided the thermocouples can be replaced in flight.&lt;/p&gt;&#xA;&lt;p&gt;Useful drive systems are going to be extremely limited, due to both fuel leakage and the inherent problems of limited delta-V.&lt;/p&gt;&#xA;&lt;p&gt;Mechanical wear of that duration is a problem - modern materials have not been adequately tested for such durations, and semi-modern materials seldom have the durability for this type of duration.&lt;/p&gt;&#xA;&lt;p&gt;A dead-fall target probe could easily pull off a 100 year mission - that is, a probe that simply is a target for spotting, and lacks active requirements - barring impacts, but as durations increase, impacts become considerably more likely.&lt;/p&gt;&#xA;&lt;p&gt;And, likewise, the Pioneer 10 and 11 missions are still considered to be operating, even tho the probes themselves are unable to respond due to low power (suspected to be mostly from corrosion losses in the thermocouples); a small part of their mission was to be dummy targets. They are able to be found with extant telescope technology, and thus their position tracked, allowing for continuation of their mission. Their active mission phases, however, ended in the 1990's.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;References&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2013/09/the-unknown-voyager-1-begins-interstellar-space-adventure/&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasaspaceflight.com/2013/09/the-unknown-voyager-1-begins-interstellar-space-adventure/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Voyager_1&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-04T01:42:10.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3582" PostTypeId="1" AcceptedAnswerId="3584" CreationDate="2014-02-04T04:07:35.673" Score="8" ViewCount="398" Body="&lt;p&gt;Understanding Kepler's laws, a body with speed exceeding escape speed of a major body will move in a hyperbolic trajectory, entry and exit vector, a &quot;less than ellipsis&quot; curve. You don't get closed (self-intersecting) trajectories other than ellipses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I correct that an inert body will not naturally pass by a major body along &#xA;a route shaped like the Roman gamma letter $ɣ$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, what powered flight prerequisites would the body need to fulfill to achieve exit along such a trajectory (and preferably exit with more speed than it had while entering, and possibly profit from gravitational assist and/or Oberth maneuver)?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorDisplayName="user12" LastEditDate="2014-02-04T04:11:49.530" LastActivityDate="2014-02-04T07:24:00.847" Title="What are the requirements for $ɣ$ shape flyby?" Tags="|orbital-mechanics|orbital-maneuver|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3583" PostTypeId="2" ParentId="3582" CreationDate="2014-02-04T07:04:45.683" Score="3" Body="&lt;p&gt;Can you make a loop?  Yes and no.  It depends on your point of view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only inertial trajectories possible are an ellipse, parabola, and hyperbola.  None of those can make a loop like a $\gamma$.  (An ellipse closes, but does not &quot;cross&quot; itself.)  From the &lt;em&gt;point of view of the gravitational body&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that body is orbiting another, e.g. our Moon about the Earth, then from the point of view of Earth, the trajectory of an object about the Moon &lt;em&gt;can&lt;/em&gt; cross itself and look like a $\gamma$ for a hyperbolic trajectory, or look like curly cues for an elliptical orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, the only way for an object to leave a body with higher hyperbolic velocity than it arrives with is to measure that velocity in a frame of reference in which the body is moving.  In the frame of reference of the body, the departing velocity always has exactly the same magnitude as the arriving velocity -- just different directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for powered flight, with sufficient thrust you can, of course, have the object follow any trajectory you like.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-02-04T07:04:45.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3584" PostTypeId="2" ParentId="3582" CreationDate="2014-02-04T07:08:59.327" Score="5" Body="&lt;p&gt;Yes, but for the unpowered flight it will take a bit of &lt;em&gt;cheating&lt;/em&gt; and it will be a three body orbit. We could argue that any &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28orbit%29&quot; rel=&quot;noreferrer&quot;&gt;rosetta orbit&lt;/a&gt; (a.k.a. rosette orbit, like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Klemperer_rosette&quot; rel=&quot;noreferrer&quot;&gt;Klemperer rosette&lt;/a&gt;) makes one such intersecting loop on each flyby: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/BmNWx.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Apsidal precession of a rosette pattern orbit&lt;/strong&gt; (Credit: &lt;a href=&quot;http://davidpratt.info/sacredpl.htm&quot; rel=&quot;noreferrer&quot;&gt;David Pratt&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could this be used to gain momentum with one gravity assist flyby, and not merely maintain orbital equilibrium? Yes, I believe so. Imagine a retrograde gravity assist slingshot from the direction of the assisting body's barycenter (or parallel to it). If the gravity assisting body of which we want to gain additional momentum from during the time of the full u-turn slingshot moves in orbit for more than its own breadth (diameter), then the slingshooting body would have prescribed a &lt;em&gt;gamma-shaped&lt;/em&gt; loop, theoretically intersecting at the gravity assisting body's barycenter it orbits around. Hopefully that wouldn't be in the middle of another body, but in theory it would work. If loop entering and exiting vectors are only slightly shallower than parallel to the imaginary line between the gravity assisting body and barycenter it orbits around, then you should gain a small portion of this body's orbital momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For powered flight performing Oberth maneuver, this is somewhat easier. The vehicle doing such gravity assisting flyby should enter into steep trajectory that would technically result in orbital precession and eventually hit the assisting body, and delay the Oberth burn for a bit longer, then using its own additional thrust to regain momentum and escape the gravitational grip of the assisting body. With such maneuver, it wouldn't matter if it's prograde or retrograde, but it would be crucial that the thrusters work and fire exactly when they need to. You also wouldn't achieve maximum efficiency of such a maneuver, but it is possible to exit the flyby faster than you entered it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-04T07:24:00.847" LastActivityDate="2014-02-04T07:24:00.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3585" PostTypeId="1" AcceptedAnswerId="3586" CreationDate="2014-02-04T07:20:21.400" Score="28" ViewCount="12422" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Loctp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space probes Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 are the first man-made devices to have escaped from our solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main problem they face is the lack of power source in them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So they are not able to contact Earth correctly....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there any way to find or track these probes with any of the newest technologies?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1427" LastEditorUserId="-1" LastEditDate="2016-06-04T07:10:06.077" LastActivityDate="2019-08-25T08:41:09.243" Title="Is there any way to find and track Pioneer 10, 11, and Voyager 1, 2?" Tags="|artificial-satellite|probe|voyager|interstellar-travel|pioneer|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3586" PostTypeId="2" ParentId="3585" CreationDate="2014-02-04T07:52:59.000" Score="33" Body="&lt;p&gt;Voyagers are &lt;a href=&quot;http://voyager.jpl.nasa.gov/where/&quot; rel=&quot;noreferrer&quot;&gt;still active&lt;/a&gt;, and albeit they don't have the power required to run all the scientific equipment onboard and some of it stopped working by now, they still transmit telemetry data streams towards the Earth that is picked up by NASA JPL's &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsn/antennas/70m.html&quot; rel=&quot;noreferrer&quot;&gt;70-meter antenna&lt;/a&gt; at Goldstone, California, part of the &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsn/&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Network&lt;/a&gt;. Quoting from &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_program&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Voyager program&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The two Voyager spacecraft continue to operate, with some loss in&#xA;  subsystem redundancy, but retain the capability of returning&#xA;  scientific data from a full complement of Voyager Interstellar Mission&#xA;  (VIM) science instruments.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Both spacecraft also have adequate electrical power and attitude&#xA;  control propellant to continue operating until around 2025, after&#xA;  which there may not be available electrical power to support science&#xA;  instrument operation. At that time, science data return and spacecraft&#xA;  operations will cease.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_program&quot; rel=&quot;noreferrer&quot;&gt;Pioneer probes&lt;/a&gt; are however not transmitting anything any more. Well, they might be but their signal is too weak at the distance to the Earth they are at (currently, Pioneer 10 is roughly 110 AU, and Pioneer 11 roughly 89 AU away) and due to deterioration of the thermocouple junctions in their &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;RTG&lt;/a&gt;s, they don't generate enough power required to operate their transceivers and their signals being picked up by even the most powerful radio antennas. &lt;a href=&quot;http://www.nasa.gov/centers/ames/news/releases/2003/03_25HQ.html&quot; rel=&quot;noreferrer&quot;&gt;According to NASA&lt;/a&gt;, Pioneer 10's last, very weak signal was received on Jan. 22, 2003, and the last time a Pioneer 10 contact returned telemetry data was April 27, 2002. For Pioneer 11, NASA assessed that its transmitter will fall silent altogether some time in late 1996 and in September 29, 1995 &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/text/pioneer-11_endops.txt&quot; rel=&quot;noreferrer&quot;&gt;announced they're ending Pioneer 11 operations&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before Pioneer probes went completely silent on us, &lt;a href=&quot;http://www.seti.org/&quot; rel=&quot;noreferrer&quot;&gt;SETI&lt;/a&gt; used to use Pioneer 10 as a test target during their &lt;a href=&quot;http://www.seti.org/seti-institute/project/details/pioneer-10&quot; rel=&quot;noreferrer&quot;&gt;Phoenix Project&lt;/a&gt;, serving as a good approximation target for an &lt;a href=&quot;http://www.setileague.org/askdr/decode.htm&quot; rel=&quot;noreferrer&quot;&gt;ETI signal&lt;/a&gt;. Problem with detecting Pioneer probes now is their distance and the power of their transmissions, if they still transmit at all (likely not, and their RTG thermocouple junctions deteriorated beyond supplying required power to transmit anything by now). So even moving much closer to the probes and listening in likely wouldn't result in success.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, for omnidirectional transmissions at 120 AU, the &lt;a href=&quot;http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html&quot; rel=&quot;noreferrer&quot;&gt;required transmission power is 1.6 kW&lt;/a&gt; using &lt;a href=&quot;https://en.wikipedia.org/wiki/Equivalent_isotropically_radiated_power&quot; rel=&quot;noreferrer&quot;&gt;EIRP&lt;/a&gt; isotropic antennas, defining detection range of a 305 meter diameter circular aperture receiver antenna, similar to the &lt;a href=&quot;http://www.naic.edu/public/the_telescope.htm&quot; rel=&quot;noreferrer&quot;&gt;Arecibo radio telescope&lt;/a&gt;. Pioneer probes of course used directional 2.74 meter diameter parabolic dish high-gain transceiver antennas, so the total power required would be much smaller, but their 8 W output simply wouldn't be enough for the very faint signals not to be lost in the background noise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Direct optical imaging would of course also be impossible from Earth or its vicinity, with &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt; (HST) resolving Pluto at &lt;a href=&quot;https://space.stackexchange.com/a/183/49&quot;&gt;roughly 16 x 16 pixels&lt;/a&gt;, and all these probes are far more distant to us than Pluto is, and of course, Pluto is a dwarf planet, that's orders of magnitude larger imaging target.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-02-05T10:28:40.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3589" PostTypeId="2" ParentId="601" CreationDate="2014-02-04T17:29:48.547" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://people.eecs.berkeley.edu/%7Emesser/&quot; rel=&quot;nofollow noreferrer&quot;&gt;David G. Messerschmitt&lt;/a&gt; has been looking at this issue. He argues that &amp;quot;energy consumption should take priority in interstellar communication, as distinct from most terrestrial systems that primarily conserve scarce spectrum.&amp;quot; His analysis suggests &amp;quot;transmit signals should have wide bandwidth and consist of energy concentrated sparsely in both time and frequency.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://www.icarusinterstellar.org/2013-starship-congress-speaker-announcement-prof-david-messerschmitt-power-efficiency-in-interstellar-communication/&quot; rel=&quot;nofollow noreferrer&quot;&gt;abstract&lt;/a&gt; to &amp;quot;Power Efficiency in Interstellar Communication&amp;quot;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A major obstacle to communicating with other civilizations at interstellar distances at radio wavelengths is the lack of coordination in transmitter/receiver design. We propose to deal with this by optimization with respect to relevant resource consumption in light of the observable interstellar impairments, which include interstellar propagation effects (noise, plasma dispersion, and scattering) and motion effects. In communication there are two primary resources, the transmitter’s energy requirement for radiated power and the signal bandwidth, and there is a direct tradeoff between the two.&lt;/p&gt;&#xA;&lt;p&gt;In view of the large distances and the large microwave window available, we argue that energy consumption should take priority in interstellar communication, as distinct from most terrestrial systems that primarily conserve scarce spectrum. The fundamental limit on energy consumption for interstellar communication is a wakeup call that the types of signals currently anticipated in SETI searches are inefficient by multiple orders of magnitude. We briefly review a set of five principles of transmit signal design that collectively can asymptotically approach that fundamental limit. These principles teach us that transmit signals should have wide bandwidth and consist of energy concentrated sparsely in both time and frequency.&#xA;Although signals with these characteristics will not be discovered by current SETI search methodologies, we review the discovery challenge and discuss how current searches can be modified to seek these energy-conserving signals. Information-free beacons as well as information-bearing signals can be sought simultaneously.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From his 2013 paper, &amp;quot;&lt;a href=&quot;http://arxiv.org/pdf/1305.4684v2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;End-to-end interstellar communication system design for power efficiency&lt;/a&gt;&amp;quot;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Abstract&lt;/strong&gt;&lt;br /&gt;&#xA;Radio communication over interstellar distances is studied, accounting for noise, dispersion, scattering and motion. Large transmitted powers suggest maximizing power efficiency (ratio of information rate to average signal power) as opposed to restricting bandwidth. The fundamental limit to reliable communication is determined, and is not affected by carrier frequency, dispersion, scattering, or motion. The available efficiency is limited by noise alone, and the available information rate is limited by noise and available average power. A set of five design principles (well within our own technological capability) can asymptotically approach the fundamental limit; no other civilization can achieve greater efficiency. Bandwidth can be expanded in a way that avoids invoking impairment by dispersion or scattering. The resulting power-efficient signals have characteristics very different from current SETI targets, with wide bandwidth relative to the information rate and a sparse distribution of energy in both time and frequency. Information-free beacons achieving the lowest average power consistent with a given receiver observation time are studied. They need not have wide bandwidth, but do distribute energy more sparsely in time as average power is reduced. The discovery of both beacons and information-bearing signals is analyzed, and most closely resembles approaches that have been employed in optical SETI. No processing is needed to account for impairments other than noise. A direct statistical tradeoff between a larger number of observations and a lower average power (including due to lower information rate) is established. The ”false alarms” in current searches are characteristic signatures of these signals. Joint searches for beacons and information-bearing signals require straightforward modifications to current SETI pattern recognition approaches.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There is also video on YouTube of Prof. Messerschmitt's presentation to the Starship Congress this summer:&lt;br /&gt;&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ef0aZ3zp1rk?start=11602&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-06-13T22:36:07.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3590" PostTypeId="2" ParentId="3419" CreationDate="2014-02-04T17:57:27.717" Score="4" Body="&lt;p&gt;Technically, no human presence is required, as elucidated by the previous answer. However, if one considers the subset of manned interstellar missions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming &lt;a href=&quot;http://en.wikipedia.org/wiki/Harold_G._White_%28NASA%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sonny White&lt;/a&gt; doesn't figure out FTL in the next couple of years, current and near-term technology in propulsion systems dictate an extremely long duration for any interstellar flight. Even the &lt;a href=&quot;http://www.icarusinterstellar.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Icarus Interstellar&lt;/a&gt; team, the folks redesigning the Daedalus probe, assume a flight time to Alpha Centauri of 100 years. Thus the question of crew size for a manned interstellar mission is either determined by progress in suspended animation (sleeper ship), or minimum population size for genetic viability after a given number of generations (world ship).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some light on the second option can be found here: &lt;a href=&quot;https://space.stackexchange.com/questions/450/what-is-the-smallest-number-of-humans-required-for-colonization&quot;&gt;What is the smallest number of humans required for colonization?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-01-23T04:16:41.440" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3591" PostTypeId="1" AcceptedAnswerId="3595" CreationDate="2014-02-04T19:20:21.487" Score="17" ViewCount="2851" Body="&lt;p&gt;The comment chain on &lt;a href=&quot;https://space.stackexchange.com/questions/3556/what-is-the-feasibility-of-launching-a-probe-to-sedna/3559?noredirect=1#comment7120_3559&quot;&gt;What is the feasibility of launching a probe to Sedna?&lt;/a&gt; indicates RTG thermocouples decay. With the passage of time the thermocouple may even have the ability to transduce an infinitesimal fraction of the power it could originally even when the RTG pile itself is still good.&lt;/p&gt;&#xA;&lt;p&gt;At the other end this &lt;a href=&quot;http://www.space.com/16648-pioneer-anomaly-spacecraft-mystery-solved.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;news-report&lt;/a&gt; writes to say&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The cause of the so-called &amp;quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Pioneer_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pioneer Anomaly&lt;/a&gt;,&amp;quot; it turns out, is heat coming from the electrical current flowing through the probes' instrument and power systems. This heat pushed back on the spacecraft, causing them to decelerate slightly, according to a new study&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;The Pioneer Anomaly was/is an inadvertent side-effect. It may be possible to improve on it by &lt;strong&gt;designing&lt;/strong&gt; to use heat as a propellant.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;For instance: Surface asymmetry may cause a surface to heat in a dis-uniform fashion.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;Assuming the report is in order&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are there any studies on radiating heat to propel space-craft in outer space?&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How close are these studies (if any) to experimental confirmation?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;How much heat would be necessary to accelerate a 1 kg body through 0.01G in outer-space?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="7062" LastEditDate="2023-05-18T11:03:58.267" LastActivityDate="2023-05-18T13:13:00.660" Title="Could radiated heat propel space-craft in outer space?" Tags="|propulsion|rtg|pioneer|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3592" PostTypeId="2" ParentId="601" CreationDate="2014-02-04T19:56:27.553" Score="1" Body="&lt;p&gt;Sending a memory card (or anything else) back to Earth would add a huge amount of weight to the probe, because of the engine and all the fuel you need to decelerate the memory card to 0, then accelerate it to a decent speed in the direction of Earth. Also, you'd need a heat shield, a radio transmitter so you can find it once back at Earth, which in turn means a power system, etc. etc. This would make the mission hugely more expensive. For the same reason, we've had quite a few spacecraft land on Mars but no return missions yet.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-02-04T19:56:27.553" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3593" PostTypeId="2" ParentId="3591" CreationDate="2014-02-04T20:21:12.577" Score="3" Body="&lt;p&gt;Heat is radiated to space in form of infra-red radiation. Infra-red radiation is nothing else than long-wavelength light. And there is a theoretical concept for a spacecraft accelerated through a beam of light, called photon propulsion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The great thing about photon propulsion is that it requires no propellant, that means that when you have access to an external energy source (like solar panels), you never run out of fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately it has a very, very low energy efficiency. &lt;a href=&quot;http://pdf.aiaa.org/preview/CDReadyMSPACE07_1808/PV2007_6131.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; talks about just 20 µN of thrust per Watt, and that was for a directed laser beam under lab conditions, not infra-red radiation aimed away from the general direction where you want to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, &lt;em&gt;theoretically&lt;/em&gt; you can propel a spacecraft through directing its waste heat, but &lt;em&gt;practically&lt;/em&gt; the effect is negligible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a larger-scale concept for a photonic rocket which is closer to your idea of using heat, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Nuclear_photonic_rocket&quot; rel=&quot;nofollow&quot;&gt;Nuclear Photonic Rocket&lt;/a&gt;. The idea is to use the heat generated by a nuclear reactor to generate thrust. But it's still not very efficient. A much better use of that much energy would be to use it to accelerate the reactor waste to generate thrust.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-02-04T20:29:36.453" LastActivityDate="2014-02-04T20:29:36.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3594" PostTypeId="2" ParentId="3591" CreationDate="2014-02-04T20:28:53.627" Score="11" Body="&lt;p&gt;The Pioneer Anomaly gives a first-order answer.&lt;br&gt;&#xA;The Pioneer Anomaly was an acceleration of $(8.74±1.33)×10^{−10} m/s^2$. According to the &lt;a href=&quot;http://arxiv.org/pdf/1204.2507v1.pdf&quot; rel=&quot;noreferrer&quot;&gt;paper&lt;/a&gt; in which the solution for the Anomaly was published, this acceleration was caused by about 50 W of heat output.&lt;br&gt;&#xA;So you get $10^{-11} m/s^2W$, or 100 GW for 1 $m/s^2$, for a spacecraft that weighs ~250 kg. So 40 MW for 1kg at 0.01G. There are better ways to utilize that power. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-02-04T21:06:56.113" LastActivityDate="2014-02-04T21:06:56.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3595" PostTypeId="2" ParentId="3591" CreationDate="2014-02-04T20:29:08.830" Score="12" Body="&lt;p&gt;This is propulsion via infrared photons, which is similar to other photon propulsion methods.  It's more common to hear about gamma (antimatter drives) and x-ray propulsion because we have mechanisms to drive higher power through it, whereas thermal photons are limited by the Stefan-Boltzmann law.&lt;/p&gt;&#xA;&lt;p&gt;To the question:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;How much heat would be necessary to accelerate a 1 kg body through 0.01G in outer-space?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The specific impulse for all photons is the same.  After all, they all travel at the same speed (but I guess the limit-based argument is a little more nuanced).  But you still need to correct for &lt;strong&gt;angle&lt;/strong&gt; of emission.  They're not perfectly collimated, so our best assumption is to assume isotropic emission over a half-sphere.  Details come out to the same as analogs here:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Relativistic_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Relativistic_rocket&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In electron-positron annihilation, the gamma rays are emitted in a spherically symmetric fashion, and they almost cannot be reflected with current technology. Therefore they cannot be directed towards the rear. A simple solution would be to have a gamma ray absorber absorbing all the gamma rays moving in the forward direction, delivering part of the thrust; and letting the rest be emitted without any deflection (therefore with an angle of divergence of 180°), which cuts in half the (average) useful momentum of the gamma rays, resulting in the specific impulse being less of what it would be in the idealized case.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I think that's a factor of 1/2, going by memory from classes on transport.&lt;/p&gt;&#xA;&lt;p&gt;With such a high specific impulse, we can trash the rocket equation in favor of an approximation for impulse.  You seem to want (1 kg)x(0.01)x(9.8 m/s&lt;sup&gt;2&lt;/sup&gt;)=0.1 Newtons.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \text{ Power} \approx 2.0 \cdot \text{Force} \cdot c \approx 30 MW $$&lt;/span&gt;&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="7062" LastEditDate="2023-05-18T11:04:27.663" LastActivityDate="2023-05-18T11:04:27.663" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="3596" PostTypeId="2" ParentId="3419" CreationDate="2014-02-04T22:49:49.043" Score="5" Body="&lt;h1&gt;Defining a ship&lt;/h1&gt;&#xA;&lt;p&gt;If defining a ship as having a minimum of one human crewman...&lt;/p&gt;&#xA;&lt;h1&gt;No FTL&lt;/h1&gt;&#xA;&lt;p&gt;In the absence of FTL drives, a minimum for an active human crew is however many are required to perform the maintenance tasks on the life support, drives, and power systems.&lt;/p&gt;&#xA;&lt;p&gt;Since this is extremely variable based upon automation and durability, what can be said is merely &amp;quot;you need engineering crew.&amp;quot;&lt;/p&gt;&#xA;&lt;h2&gt;Human need to interact&lt;/h2&gt;&#xA;&lt;p&gt;Given human need to interact, this sets a further minimum crew standard. The military has had a number of small outposts for years. Minimum crew size for these correlates directly to maximum duration of assignment. A 4-5 person crew tends to start experiencing issues between 2 and 6 months deployed in isolation, even with routine work contact via radio with outside persons. A 20-30 person crew can go for 4-6 months before similar issues. After this initial period of troubles, many crews are able to work things out, and a stable long term status quo evolves; but when it doesn't, the results are often tragic. NASA long term missions have typically been 3-5 person crews, and the crews have been put in isolation for the requisite several months during training; those who can't handle it have been weeded before flight.&lt;/p&gt;&#xA;&lt;p&gt;In looking at small communities, the smallest psychologically stable communities appear to be in the range of 20-30 people; smaller communities (such as McGrath, Alaska) tend to be stable until some stress, then violence happens; usually a new stable evolves shortly thereafter. The problem being that a spacecraft can not afford such violence.&lt;/p&gt;&#xA;&lt;p&gt;So, for an STL ship, you'll want at least 20 active persons, rotating which 20 if practical, in order to maintain the hamlet level of community which seems the minimum for long term psychological stability.&lt;/p&gt;&#xA;&lt;h2&gt;Colonial Genetic Stability&lt;/h2&gt;&#xA;&lt;p&gt;Since any mission to another star system in an STL craft is likely to be one-way, the overall mission crewing needs to be considered as well. And the limit there is the genetic viability of the colony. If completely unrelated individuals and absolute breeding authority can be presumed, as low as 30 individuals may be viable. Long term, the lowest genetically stable populations in isolation seem to be about 100 individuals - a small village level, or several hamlets. At about 200, consanguinity selection for the colonists sent is a low priority.&lt;/p&gt;&#xA;&lt;h1&gt;FTL - Alcubierre-White Warp Drive&lt;/h1&gt;&#xA;&lt;p&gt;Assuming Dr. White's assumption of 10C capability is correct, this puts many nearby stars under a year away. Within a year, the technical needs of the hardware trump the psychology of the crew. Keep them busy, and each with private space, and a crew of 3-5 should be functional long term. Even up to 2 years, to allow for a round trip, a Warp Drive at 10C can reach any of a dozen stars, specifically, any of  9 systems.&lt;/p&gt;&#xA;&lt;p&gt;We can't yet estimate the requisite engineering/maintenance times.&lt;/p&gt;&#xA;&lt;p&gt;We can note that, unlike STL ships, not all are one-way trips. Therefore, the colonial need is also able to be ignored.&lt;/p&gt;&#xA;&lt;h1&gt;FTL - other&lt;/h1&gt;&#xA;&lt;p&gt;There is no other FTL drive thought feasible at this time. Therefore, no practical considerations can be made.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-04T22:49:49.043" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3597" PostTypeId="1" AcceptedAnswerId="44419" CreationDate="2014-02-04T23:28:55.120" Score="19" ViewCount="4544" Body="&lt;p&gt;I was just reading &lt;a href=&quot;http://news.discovery.com/space/rough-roving-curiositys-wheel-damage-accelerated-131220.htm&quot;&gt;this article&lt;/a&gt; on how Curiosity's aluminum wheels are getting pretty dinged up. Usually titanium alloy is considered an upgrade from aluminum alloys in most applications, from aerospace to bikes to sporks. The &quot;usual&quot; downsides are that it is expensive and difficult to machine. However, those seem like less important factors when you're NASA and the plan is to send an object to another planet and drive over sharp rocks.&lt;/p&gt;&#xA;" OwnerUserId="2581" LastActivityDate="2020-05-30T23:19:11.757" Title="Why are Curiosity's wheels aluminum rather than titanium?" Tags="|mars|rovers|curiosity|engineering|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3598" PostTypeId="2" ParentId="3597" CreationDate="2014-02-05T00:25:05.140" Score="14" Body="&lt;p&gt;Aluminum is less dense than titanium.  For the same mass, the 0.75mm thick aluminum would have to be replaced with 0.45mm thick titanium.  Although the titanium sheet would be stronger in tension, it would probably be more susceptible to tearing from a point load like driving over a sharp rock.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-02-05T00:25:05.140" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3599" PostTypeId="1" AcceptedAnswerId="3600" CreationDate="2014-02-05T03:41:39.310" Score="5" ViewCount="1037" Body="&lt;p&gt;What's missing for everyday people to be able to launch his own satellite (or any other space objects) into space? I mean, I know that mostly money and technology, but what does it actually mean?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of equipment should I have to be able to launch something into orbit?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="306" LastEditDate="2014-02-05T21:02:20.723" LastActivityDate="2019-03-15T23:36:55.807" Title="Which are the main obstacles of launching my own home-made space object?" Tags="|launch|orbit|technology|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3600" PostTypeId="2" ParentId="3599" CreationDate="2014-02-05T04:53:07.957" Score="8" Body="&lt;p&gt;The infrastructure you are missing is a-plentyful:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A launch range and vast swaths of land/sea for the spent stages to fall (unless you are using a SSTO/reusable craft). Unless you have lots of money or the right of eminent domain, this is going to be prohibitive.&lt;/li&gt;&#xA;&lt;li&gt;A satellite control center (can be rented/outsourced, though).&lt;/li&gt;&#xA;&lt;li&gt;Assigned frequencies for uplink/downlink, telemetry and commands.&lt;/li&gt;&#xA;&lt;li&gt;Ground crew (to assemble/service launchers, mate with spacecraft, supervise and conduct actual launches).&lt;/li&gt;&#xA;&lt;li&gt;A license from a government authority.&lt;/li&gt;&#xA;&lt;li&gt;Insurance coverage.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Actual launchers might be easier to come by especially if you are launching a small payload. DARPA/US Army/etc. have made great progress in the &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Operationally_Responsive_Space_Office&quot;&gt;Operationally Responsive Space&lt;/a&gt;&quot; program; other countries may be willing to take your payloads on surplus ballistic missiles.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-02-05T04:53:07.957" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3601" PostTypeId="2" ParentId="3560" CreationDate="2014-02-05T20:27:07.463" Score="5" Body="&lt;p&gt;Intra solar travel is far more predictable and less harsh than low earth orbit, so we have the materials and technology to make something survive 100 years. The problem is chemical decomposition of the electrical power systems, primarily the solar arrays and batteries. Obviously we need backup components. But backups cannot solve the problem of chemical degradation that takes place &lt;strong&gt;before&lt;/strong&gt; the backups are deployed. Therefore, the limiting factor is how fast the electrical power components degrade when they are not in use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other problems are solvable considering the problems engineers have already faced keeping the International Space Station aloft for 15 years and counting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Thermal&lt;/strong&gt; considerations: solved through testing and analysis. Consider the fact that the thermal environment on the International Space Station is far more complicated than an unmanned intra solar probe because it &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station#Orbit&quot; rel=&quot;noreferrer&quot;&gt;cycles in and out of sunlight once every 90 minutes&lt;/a&gt;.  The ISS also endures days of direct radiation at a high orbital angles. Human living quarters necessitate thermal shielding much more protective than would be required to shield inorganic equipment inside of an unmanned probe. Large habitable modules are grouped closely with other large pieces of equipment and cast shadows over mechanical joints and other systems, contributing to the complexity. Our probe would be orders of magnitude smaller, and require much less analysis to cover all thermal cases due to the relatively static thermal environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mechanical&lt;/strong&gt; considerations: solved through testing and analysis.  The lifetime of the mechanical equipment shouldn't be measured by time in service. There is a reason we measure an automobile's value based on the number of miles traveled and not age, and that 'highway miles' are considered less distressing than 'city miles'. The same concept applies to space travel; intra solar travel represents 'highway miles', planetary orbit 'city miles'. A mechanical joint might have a requirement of &quot;degrees turned&quot; or &quot;open/close&quot; cycles required. These numbers would be known in advance, especially for a piece of equipment meant to glide inertly across the solar system for years on end. The ISS has many rotary joints, &lt;a href=&quot;http://en.wikipedia.org/wiki/Integrated_Truss_Structure#Solar_alpha_rotary_joint&quot; rel=&quot;noreferrer&quot;&gt;two of which&lt;/a&gt; have been traveling ~360 degrees every 90 minutes for the last several years. Basically, we have the technology to create mechanisms with a service life of 100 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Other Environment&lt;/strong&gt; solved through analysis: avoid asteroid belts and you'll probably be ok. There &lt;em&gt;is&lt;/em&gt; a degree of uncertainty about whether the probe would be struck by an asteroid or degrade from space debris, but my guess is the debris flux in interplanetary space is much lower than in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_debris&quot; rel=&quot;noreferrer&quot;&gt;low earth orbit currently occupied by the ISS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can build an object that will last 100 years drifting through empty space. This seems like a long time on the human scale, but is nothing in terms of the well-understood materials used in spacecraft. The big question is what we ask the probe to do.&lt;/p&gt;&#xA;" OwnerUserId="2586" LastEditorUserId="2586" LastEditDate="2014-02-05T21:46:15.670" LastActivityDate="2014-02-05T21:46:15.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3604" PostTypeId="1" AcceptedAnswerId="3605" CreationDate="2014-02-05T11:15:49.360" Score="7" ViewCount="632" Body="&lt;p&gt;I remember reading here that it is more difficult (cost? fuel?) to send a satellite towards the Sun compared to sending a satellite away from it. Is this true? And if it is, how come?&lt;/p&gt;&#xA;" OwnerUserId="12947" OwnerDisplayName="taintedromance" LastEditorUserId="49" LastEditDate="2014-02-06T09:51:43.710" LastActivityDate="2014-02-06T09:51:43.710" Title="Sending a satellite towards or away from the Sun" Tags="|orbital-mechanics|artificial-satellite|solar-system|delta-v|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3605" PostTypeId="2" ParentId="3604" CreationDate="2014-02-05T13:34:47.400" Score="6" Body="&lt;p&gt;The ratio of orbital velocities of &lt;a href=&quot;http://en.wikipedia.org/wiki/Mercury_%28planet%29&quot;&gt;Mercury&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Earth&quot;&gt;Earth&lt;/a&gt; is about&#xA;$$(47.87 km/s) / (29.78 km/s) = 1.607.$$&#xA;&lt;a href=&quot;http://www.pha.jhu.edu/~broholm/l24/node1.html&quot;&gt;Potential energy (negative) plus orbital kinetic energy&lt;/a&gt; is propotional to the square of the ratio of the orbital velocities, hence to $$1.607^2 = 2.58$$ for the Earth - Mercury example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you need more energy (the 1.58-fold) to slow down from Earth to Mercury relative to the sun than to escape from Earth orbit out of the solar system.&lt;/p&gt;&#xA;" OwnerUserId="1454" OwnerDisplayName="Gerald" LastActivityDate="2014-02-05T13:44:51.820" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3606" PostTypeId="1" CreationDate="2014-02-05T22:45:37.897" Score="13" ViewCount="3550" Body="&lt;p&gt;I see it as an awful waste of energy in not only creating the International Space Station (ISS) components, but also in getting them into space to only decommission the station by letting it fall back to Earth. Why not move the station either as a whole, or perhaps preferably in parts to the surface of the Moon, so that these materials and components are available to be recycled for future Lunar manned missions in order to help set up a Lunar base?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would even go as far as saying that we should get one of the usable Space Shuttles back up into space and send it to the Moon to land there as well. All of these parts have been built to be space worthy, why not keep them out in space to be recycled later on? As I understand it, one of the most significant costs of space instruments is getting them into space and off the Earth's surface in the first place. Why waste the effort we have made in doing this already?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it be a great science experiment and learning experience in trying to move the station or its components to the Moon also? We have everything to gain by doing this.&lt;/p&gt;&#xA;" OwnerUserId="2588" LastEditorUserId="49" LastEditDate="2014-02-06T06:25:45.977" LastActivityDate="2020-01-12T05:38:29.173" Title="How feasible would it be to move the ISS to the surface of the Moon to recycle its components in future?" Tags="|iss|the-moon|low-earth-orbit|future-missions|feasibility|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3607" PostTypeId="2" ParentId="3606" CreationDate="2014-02-05T23:17:03.103" Score="22" Body="&lt;p&gt;It wouldn't be at all. Let's look at it in a couple of different ways:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta-v_budget&quot;&gt;delta v&lt;/a&gt; required is 5.93 km/s. That's not quite, but a similar level of difficulty as launching the station in the first place!&lt;/li&gt;&#xA;&lt;li&gt;Landing either the Shuttle or the ISS on the moon would be difficult. They just weren't meant for it!&lt;/li&gt;&#xA;&lt;li&gt;There might be some parts that are usable, but in large part, the end of life ISS is going to be because the parts are worn out. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it's not a whole lot cheaper to land on the Moon than it is to get to orbit in the first place, and there's questionable value of what would survive the process. Overall, it just isn't going to happen.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-02-05T23:17:03.103" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="3608" PostTypeId="2" ParentId="3604" CreationDate="2014-02-06T03:06:23.213" Score="6" Body="&lt;p&gt;Well, the way you said it isn't quite right.  You could spend the same $\Delta V$ to escape Earth in one direction as the other, and in one direction you would move away from the Sun, and in the other direction you would move towards it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I think you're referring to is the fact that it would take far less $\Delta V$ from Earth to escape the Solar System than it would to reach the Sun and touch its surface.  From low-Earth orbit, it takes $8.75\,\mathrm{km/s}$ to escape the Solar System completely, but it takes $16.8\,\mathrm{km/s}$ to dive into the Sun from low-Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, you wouldn't do that $\Delta V$ yourself.  In either direction, you would use Jupiter to send you in or out (or perhaps even in for the out case, if you want to do an escape maneuver near the Sun, which can be very efficient).&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-02-06T03:06:23.213" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="3609" PostTypeId="1" AcceptedAnswerId="3623" CreationDate="2014-02-06T09:11:30.687" Score="36" ViewCount="7089" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/U07OE.jpg&quot; alt=&quot;An illustration of relative positions&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The position of Pioneer 11 is between Voyager 1 and 2, that means Pioneer 11 is closer to Voyager 1 and 2 than to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_11&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Wikipedia article on Pioneer 11&lt;/a&gt; we lost communication with it on November 30, 1995.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any way to communicate with &lt;strong&gt;Pioneer 11&lt;/strong&gt; through &lt;strong&gt;Voyager 1&lt;/strong&gt; or &lt;strong&gt;2&lt;/strong&gt; ?&lt;/p&gt;&#xA;" OwnerUserId="1427" LastEditorDisplayName="user10509" LastEditDate="2018-11-13T09:50:24.750" LastActivityDate="2018-11-13T09:50:24.750" Title="Is there any way to communicate with Pioneer 11 through Voyager 1 or 2?" Tags="|probe|communication|voyager|pioneer|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="3610" PostTypeId="2" ParentId="3609" CreationDate="2014-02-06T10:45:32.650" Score="24" Body="&lt;p&gt;No, absolutely none. Even assuming Pioneer 11's transceiver still works, and &lt;a href=&quot;https://space.stackexchange.com/questions/3585/is-there-any-way-to-find-and-track-pioneer-10-11-and-voyager-1-2&quot;&gt;there's no reason to believe that&lt;/a&gt;, that image you attach of distances between the three probes isn't showing it proportionally correct, since it's merely an approximate slice of euclidean space and is not axially aligned with anything to correctly appreciate distances involved. So here's NASA's own depiction of relative positions of the mentioned space probes that should provide a better view:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rHBVx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Relative Positions of Distant Spacecraft&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This graphic shows the relative positions of NASA’s most distant&#xA;  spacecraft in early 2011, looking at the solar system from the side.&#xA;  Voyager 1 is the most distant spacecraft, about 17.5 billion&#xA;  kilometers (10.9 billion miles) away from the sun at a northward&#xA;  angle. Pioneer 10, the next most distant, is about 15.4 billion&#xA;  kilometers (9.6 billion miles) away from the sun on the opposite side&#xA;  of the solar system. Voyager 2 is about 14.2 billion kilometers (8.8&#xA;  billion miles) away from the sun on a southward trajectory, on the&#xA;  same side of the solar system as Voyager 1. Pioneer 11 is about 12.4&#xA;  billion kilometers (7.8 billion miles) away from the sun. New Horizons&#xA;  is about 3 billion kilometers (2 billion miles) away from the sun, on&#xA;  its way to Pluto.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Image and quote source: &lt;a href=&quot;http://www.nasa.gov/mission_pages/voyager/multimedia/pia14112.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As you see from the right hand side part of the image, the three probes are all moving in direction away from the Sun at different &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination&quot; rel=&quot;nofollow noreferrer&quot;&gt;inclinations to the ecliptic&lt;/a&gt;, for example Pioneer 11 at 17° and Voyager 2 at 79° relative to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic&quot; rel=&quot;nofollow noreferrer&quot;&gt;ecliptic plane of the Solar system&lt;/a&gt;. To calculate their relative distances between each other, we'd have to first calculate their absolute positions, their cartesian coordinates $x, y, z$ in three dimensions, and then use formula to calculate their relative distances in 3-dimensional space:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I don't have exact numbers and extracting them from other equations would take too long, so let's just take rough approximations from the NASA's image, and make this dimensionless calculation. We can probably agree that both Voyager 1 and Voyager 2 are at least 0.7 units away from Pioneer 11 on our $y$ axis (let's say that $x, y$ are depicted on the right part of the image, and $x, z$ on the left part of it), that Pioneer 11 is 1 unit away from the Sun on the $x$ axis, putting Voyager 2 at about the same distance, while Voyager 1 is perhaps 1.1 units away. And on our $z$ axis, let's say that Pioneer 11 and Voyager 2 are separated by 0.5 of our relative, dimensionless units. OK, let's plug these numbers in our calculator. For distance between Pioneer 11 and Voyager 2 that seem to be the closest to each other, we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\sqrt{(1 - 1)^2 + (-0.7 - 0.2)^2 + (1.5 - 1)^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or roughly 1.03 of our units, where we said that 1 of these units is the distance between Pioneer 11 and the Sun. This means that Pioneer 11 and Voyager 2 are at roughly the same distance to each other as Pioneer 11 is to the Earth. So this necessarily means that communication between the probes wouldn't be possible even if one of the two probes involved carried the Arecibo antenna and the power required for it with it. Clearly, none of them do, so the answer to your question is a resounding no.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-02-06T10:45:32.650" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3611" PostTypeId="2" ParentId="3609" CreationDate="2014-02-06T11:15:06.617" Score="8" Body="&lt;p&gt;No. And here's why:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To establish radio communication between two points in space successfully, one has to achieve the Signal-to-Noise ratio (SNR) greater than some threshold which varies depending on the type of signal modulation and error correction used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is done through several means:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pointing the antennas' &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pattern&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiation patterns&lt;/a&gt;) of the receiver and transmitter towards each other (accounting for the finite &lt;a href=&quot;https://en.wikipedia.org/wiki/Speed_of_light&quot; rel=&quot;nofollow noreferrer&quot;&gt;speed of light&lt;/a&gt;, of course). The illustration below shows a typical 2D radiation pattern on a logarithmic scale.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/uwtI0.png&quot; alt=&quot;Radiation pattern - by Timothy Truckle&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Pioneers were equipped with three antennas, but the tradeoff here is obvious: accuracy vs. &lt;a href=&quot;https://en.wikipedia.org/wiki/Antenna_gain&quot; rel=&quot;nofollow noreferrer&quot;&gt;antenna gain&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To achieve precise pointing you have to know exactly where the other end of the link will be by the time your signal gets there. After losing comms with P-11, the position uncertainty has grown large.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Voyagers are very limited in their ability to point precisely in a given direction, and by changing their attitude to hunt for the Pioneer you risk losing the chance to re-establish Voyagers' own link to the Earth.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Usually, to compensate for sub-optimal antenna positioning, you increase the power output through the transmitter. Unfortunately, Voyagers don't have much power available now. This power is abundant here on Earth, so &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; can pour megawatts into space through large-gain parabolic dishes with very fine servo-mechanisms.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You can lower noise in the receiver. Shutting off sources of interference, lowering the temperature of receiver components (to get rid of extra &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_noise&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermal noise&lt;/a&gt;) may help, but this is not really feasible for a spacecraft which doesn't know when to do that. DSN transceivers, again, are much better in this regard.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You can lower the SNR threshold by accumulating and processing more of the incoming signal (that is, you lower the communication bitrate) and improving the processing algorithms. A spacecraft flying on its own deep in space has no ability to download better code.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;While the main question can be confidently answered negatively, the idea of relaying data through other spacecraft is currently in wide use - e.g., in Mars rovers. Certainly, here the main enabling factor is much lower distance between the rover and the Mars-orbiting craft.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-02-06T11:15:06.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3612" PostTypeId="1" AcceptedAnswerId="3613" CreationDate="2014-02-06T12:53:26.643" Score="16" ViewCount="32993" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;,&#xA;at the Earth/Moon distance from the sun, the escape velocity is 42.1 km/s.&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;,&#xA;the average orbital speed of Earth is 29.78 km/s.&lt;/p&gt;&#xA;&lt;p&gt;The difference is 42.1 km/s - 29.78 km/s = 12.32 km/s.&lt;/p&gt;&#xA;&lt;p&gt;The escape velocity from Earth surface is about 11.2 km/s.&#xA;The velocity in lowest Earth orbit is about 8 km/s. Take a delta-v of 9.5 km/s to become realistic.&#xA;Hence an additional 11.2 km/s - 9.5 km/s = 1.7 km/s is needed to escape the gravitational field of the Earth.&lt;/p&gt;&#xA;&lt;p&gt;The sum of the two deltas is 1.7 km/s + 12.32 km/s = 14.02 km/s.&lt;/p&gt;&#xA;&lt;p&gt;To dive into the sun, from the orbital speed of 29.78 km/s of Earth the probe needs to be slowed down to zero.&#xA;The gravity from lower Earth orbit has to be overcome by additional 1.7 km/s.&#xA;Together 29.78 km/s + 1.7 km/s = 31.48 km/s.&lt;/p&gt;&#xA;&lt;p&gt;Are there fundamental errors (besides minor aphelion-perihelion discrepancies)? If so, which ones?&lt;/p&gt;&#xA;" OwnerUserId="1454" LastEditorUserId="12180" LastEditDate="2022-05-14T08:14:23.950" LastActivityDate="2022-05-14T08:14:23.950" Title="Calculating solar system escape and and sun-dive delta V from lower Earth orbit" Tags="|orbital-mechanics|delta-v|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="3613" PostTypeId="2" ParentId="3612" CreationDate="2014-02-06T14:34:56.303" Score="18" Body="&lt;p&gt;One thing you're missing seems to be Oberth's effect.  To go from LEO to solar system escape velocity, you have to counter Earth's escape velocity, but after that, you get an additional multiplier by doing the burn at a higher initial velocity (at LEO).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your method here also has a problem:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hence an additional 11.2 km/s - 9.5 km/s = 1.7 km/s is needed to escape the gravitational field of the Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To get to LEO, the 9.5 or 10 km/s is the delta v you need delivered by the engines.  But that doesn't mean you're that much closer to escape from Earth's gravity.  That's because air drag and gravity drag are both &quot;wasted&quot; impulse.  Ultimately, they just go to friction.  So if you're in LEO:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It took 9.5 km/s to get there (we'll say)&lt;/li&gt;&#xA;&lt;li&gt;It will take an additional 11.2-7.9 = 3.3 km/s to escape the gravity well&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now, going from LEO to &lt;a href=&quot;http://en.wikipedia.org/wiki/Hyperbolic_trajectory&quot;&gt;hyperbolic orbits&lt;/a&gt; is a bit more difficult.  I'll use the energy balance, because I find it easiest to understand.  The specific orbital energy is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$     \epsilon={v^2\over2}-{GM\over{r}} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After it's escaped Earth's sphere of influence, the energy balance will be simply:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \epsilon = \frac{ v_{\infty}^2 }{2 } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For either escape or dropping into the sun, we have a final velocity in mind.  This is at 1 AU from the sun, after we get out of Earth's sphere of influence.  Earth is moving at 29.78 km/s.  so we need:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;To get to the sun, we need 0 km/s net velocity, so moving 29.78 km/s relative to Earth&lt;/li&gt;&#xA;&lt;li&gt;To get out of the solar system, we need 42.1 km/s in the direction of Earth's motion, so 42.1-29.78 = 12.32 km/s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now we need to use those above energy equations to get those velocities &lt;em&gt;after&lt;/em&gt; getting out of Earth's sphere of influence.  Now let's imagine we're half way through the burn, and are at LEO altitude with escape velocity.  So we've spent exactly 9.5+3.3 = 12.8 km/s so far.  We need to figure out how &lt;em&gt;much&lt;/em&gt; more we need in this same burn to shoot for our destination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ v_{\infty} = 29.78 \text{ km/s or } 12.32 \text{ km/s} = \sqrt{ 2 \left( \frac{v^2}{2} - \frac{GM}{r} \right) } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solve this for both cases in terms of $v$ now.  For completeness, I use $r=6,354.82 \text{ km}$.  Everything else is known.  Now the results are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;for getting to the sun v = 31.8 km/s&lt;/li&gt;&#xA;&lt;li&gt;for getting out of the solar system v = 16.65 km/s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are the numbers for the total velocity you need &lt;em&gt;at LEO altitude&lt;/em&gt;.  In the story I'm telling, you're at 11.2 km/s at the end of the previous leg, so subtract that number in order to calculate the final burn.  Again, the trip is broken up into 3 segments according to my organization, but the last 2 are really the same burn.  Let me focus on the escape from our solar system.  The three legs are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A launch that requires a &lt;strong&gt;9.5 km/s&lt;/strong&gt; burn&lt;/li&gt;&#xA;&lt;li&gt;The first part of the burn at LEO to get to escape velocity is &lt;strong&gt;3.3 km/s&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Continuing that same burn, an additional 16.65-11.2=&lt;strong&gt;5.45 km/s&lt;/strong&gt; to get the solar system's escape velocity &lt;em&gt;after&lt;/em&gt; you're out of Earth's sphere of influence&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The total of all these comes out to &lt;strong&gt;18.25 km/s&lt;/strong&gt;.  If your propellant exhaust velocity is 4 km/s, then your ultimate mass fraction on the launchpad will be about 96-to-1.  So a million pound rocket could get 10,436 pounds out of the solar system with this method (I'm not saying it's a good method for this purpose).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this clears up the &quot;from lower Earth orbit&quot; part.  It's not as simple as adding things up, because you're trying to get the velocity to escape &lt;em&gt;the sun's&lt;/em&gt; gravity well, while you're still in &lt;em&gt;Earth's&lt;/em&gt; gravity well.  To do that, you have to include the Oberth effect due to your location within Earth's potential well.  I hope I've demonstrated that correctly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: here is a different set of numbers that starts with the radius of the Earth, instead of the &quot;11.2&quot; and &quot;7.9&quot; numbers, which I only used because they were in prior discussion.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Base Earth radius   6378.1  km&lt;/li&gt;&#xA;&lt;li&gt;LEO altitude    300 km&lt;/li&gt;&#xA;&lt;li&gt;LEO radius  6678.1  km&lt;/li&gt;&#xA;&lt;li&gt;V at LEO    7.725529305 km/s&lt;/li&gt;&#xA;&lt;li&gt;Escape V from LEO   10.92554832 km/s&lt;/li&gt;&#xA;&lt;li&gt;Burn from LEO to escape 3.200019015 km/s&lt;/li&gt;&#xA;&lt;li&gt;V needed at LEO to get 29.78 km/s   31.81648047 km/s&lt;/li&gt;&#xA;&lt;li&gt;Extra burn needed past escape V 20.89093216 km/s&lt;/li&gt;&#xA;&lt;li&gt;Oberth ratio    1.425498861 unitless&lt;/li&gt;&#xA;&lt;li&gt;V needed at LEO to get 12.32 km/s   16.64999789 km/s&lt;/li&gt;&#xA;&lt;li&gt;Extra burn needed past escape V 5.724449572 km/s&lt;/li&gt;&#xA;&lt;li&gt;Oberth ratio    2.152171985 unitless&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="26" LastEditorUserId="265" LastEditDate="2014-02-07T04:41:11.877" LastActivityDate="2014-02-07T04:41:11.877" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="3614" PostTypeId="2" ParentId="2165" CreationDate="2014-02-06T16:01:02.743" Score="5" Body="&lt;p&gt;The Redstone missile firing procedures are described &lt;a href=&quot;http://www.myarmyredstonedays.com/Photos/page8/fm_6_36.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. According to &lt;a href=&quot;http://www.myarmyredstonedays.com/pdf_folder/fm_6_36_08.pdf&quot; rel=&quot;noreferrer&quot;&gt;Table XVII&lt;/a&gt;, p.125, the operator shall &quot;At X-0 minutes, depress FIRE &lt;strong&gt;switch&lt;/strong&gt; and hold for 1 second&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, there's no key.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-02-06T16:01:02.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3616" PostTypeId="1" AcceptedAnswerId="3618" CreationDate="2014-02-06T18:35:44.350" Score="14" ViewCount="8122" Body="&lt;p&gt;During the launch of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot; rel=&quot;noreferrer&quot;&gt;STS vehicle&lt;/a&gt;, the two stage 0 Solid Rocket Boosters (SRB) were jettisoned and recovered after they gently parachuted into the ocean:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SAM29.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what happened with the STS External Tanks after separation as shown in figure 3 on the image above? Were they recoverable and reusable, or were they a single use only? And did they stay in orbit, or splash down into the ocean some miles downrange after stage separation?&lt;/p&gt;&#xA;" OwnerUserId="1427" LastEditorUserId="49" LastEditDate="2014-02-06T19:49:25.373" LastActivityDate="2019-01-11T15:53:09.127" Title="Were Space Shuttle External Tanks recoverable and reusable?" Tags="|space-shuttle|reuse|fuel-system|external-tank|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3618" PostTypeId="2" ParentId="3616" CreationDate="2014-02-06T19:30:57.823" Score="22" Body="&lt;p&gt;&lt;strong&gt;No&lt;/strong&gt;, the previously used External Tanks (ETs) disintegrated in the atmosphere before they fell into the sea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OfGh4.jpg&quot; alt=&quot;External Tank&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notably, &lt;a href=&quot;http://web.archive.org/web/20080509062142/http://www.spacer.com/news/tourism-02b.html&quot; rel=&quot;noreferrer&quot;&gt;Buzz Aldrin&lt;/a&gt; and others proposed different ideas for reuse of the tank in orbit, and allegedly &lt;a href=&quot;http://www.permanent.com/ext-tank.htm&quot; rel=&quot;noreferrer&quot;&gt;NASA said&lt;/a&gt; that they would be willing to take external tanks to orbit if a private company would use them. No private effort ever stepped up to the plate. Now that the Shuttle has been retired no more external tanks will be taken up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/returntoflight/system/system_ET.html&quot; rel=&quot;noreferrer&quot;&gt;NASA - The External Tank&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-02-06T19:44:47.297" LastActivityDate="2014-02-06T19:44:47.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3619" PostTypeId="2" ParentId="3616" CreationDate="2014-02-07T00:30:03.947" Score="8" Body="&lt;p&gt;A big issue with boosting the tank to orbit, would be the foam insulation.  It was believed it would come off in chunks, like popcorn, causing immense amounts of orbital debris, potentially in the orbit you wished to store it at.  Which could be really bad news.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-02-07T00:30:03.947" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3622" PostTypeId="2" ParentId="3616" CreationDate="2014-02-07T01:27:31.030" Score="14" Body="&lt;h1&gt;No reuse, but...&lt;/h1&gt;&#xA;&lt;p&gt;NASA did not at any point actually reuse an external tank in any way. However...&lt;/p&gt;&#xA;&lt;h1&gt;They made plans to allow reuse in orbit.&lt;/h1&gt;&#xA;&lt;p&gt;NASA did have tentative plans for utilization of the tanks in orbit. These plans were scrapped, however. The primary factors being (1) decreased payload capacity to stable orbit †, (2) risk of insulating foam falling off and becoming space debris, and (3) lack of need for large tanks in orbit during the majority of the Space Shuttle Program era.&lt;/p&gt;&#xA;&lt;p&gt;The tanks could have been worked to allow for attaching an airlock, and using the tank volume for storage or habitation, or melted for metals in orbit. The primary issue remains the insulation, and preventing it from becoming a hazard of its own.&lt;/p&gt;&#xA;&lt;p&gt;Note that for reasons of weight, there is no external skin, and even the paint (which may have helped bind the insulation) was dropped for weight savings in later flights; a considerable mass (several hundred kilograms) of fabric or foil as an outer layer would suffice, as would a complete redesign of the tank to put the insulation between the inner tanks and the outer structural shell.&lt;/p&gt;&#xA;&lt;p&gt;This also would not have been the first time a fuel tank was converted to a space habitation - Skylab was a modification of a Saturn S-IVB's tankage into a space habitat, albeit the work was done on the ground.&lt;/p&gt;&#xA;&lt;p&gt;Also, keep in mind that the shuttle's normal operating regime isn't a stable height for stations - most potential customers looking to take advantage of the &amp;quot;free tanks&amp;quot; would have to have shipped up other components to make use of them, and had NASA deliver the tank to a more stable, higher orbit.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;Notes&lt;/h1&gt;&#xA;&lt;p&gt;† while the OMS burn to ensure the tank falls would no longer be needed, the mass being carried to orbit means the OMS burn is including the tank mass, and thus proportionately longer for the same change in craft vector. This means, for the same orbit, more OMS fuel being used overall.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/returntoflight/system/system_ET.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/returntoflight/system/system_ET.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.permanent.com/ext-tank.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.permanent.com/ext-tank.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://history.nasa.gov/SP-400/ch2.htm&quot; rel=&quot;noreferrer&quot;&gt;http://history.nasa.gov/SP-400/ch2.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-07T01:27:31.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3623" PostTypeId="2" ParentId="3609" CreationDate="2014-02-07T02:06:28.180" Score="30" Body="&lt;h1&gt;No, it's not feasible.&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The fundamental problems that prevent this are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The Pioneers do not have enough power to operate the transmitter, due to corrosion of the thermocouples&lt;/li&gt;&#xA;&lt;li&gt;The Voyagers and Pioneers, even at full power, use very low power transmissions&lt;/li&gt;&#xA;&lt;li&gt;The Pioneers would need to be able to receive and obey instructions to aim for a Voyager &lt;/li&gt;&#xA;&lt;li&gt;The craft are not significantly closer to each other than to Earth.&lt;/li&gt;&#xA;&lt;li&gt;Reprogramming of Voyager would likely remove its ability to use its own instruments&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;h3&gt;1 - Power&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Pioneers 10 and 11 both have shut down due to insufficient power to operate both the computer and the radio at the same time.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;2 - Low Power Transmissions&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;NASA uses huge antennae to capture the weak signals from the probes. The smallest dishes used are around 10m across, and in recent years, they've been using the larger (around 30m) dishes. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;3 - Finding Voyager?&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;The Pioneer probes, in order to make this work, would have to be &quot;effectively closer&quot; to the Voyagers than to Earth, be able to power up, and be able to aim at the Voyager probe being used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means the Pioneer would need to receive, process, and obey instructions to point at the Voyager, and the only way such a relay would be known would be after it worked. But this would require that we be able to send those instructions in the first place, and if it's operational at all, it's aimed at Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But further, it's not just the Pioneer that would need to point... the Voyager would have to aim at the Pioneer to make it work. And that means it would not be aimed at Earth. See #5...&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;4 - Range&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Radio follows an inverse square law. In order to pick up the 8w signal from 36 AU, NASA needed the 36m antenna on the ground. Voyager's antenna is under 3m.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Pioneer 11 for Thu 13 February 2014&#xA;  Right Ascension: 18h 49m 45.2s  Declination: -8° 48' 07.3&quot; (J2000)&#xA;  Sun dist: 13,202,867,763 km [11.4 km/s] Earth dist: 13,319,310,346 km [27.4 km/s]&#xA;  Constellation: Sct Magnitude: N.A.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Voyager 1 Spacecraft, ephemeris for Thu 13 February 2014, 00:00 UTC&#xA;  Right Ascension: 17h 11m 59s Declination: +12° 02’ 31” (J2000) [HMS|00:00:00|Dec]&#xA;  Distance from Sun: 18,977.42 Million Km Distance from Earth: 19,021.59 Million Km&#xA;  Magnitude: N.A. Constellation: Oph&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Voyager 2 Spacecraft, ephemeris for Thu 13 February 2014, 00:00 UTC&#xA;  Right Ascension: 20h 00m 25s Declination: -55° 57’ 34” (J2000) [HMS|00:00:00|Dec]&#xA;  Distance from Sun: 15,563.42 Million Km Distance from Earth: 15,663.78 Million Km&#xA;  Magnitude: N.A. Constellation: Tel&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This gives us a minimum of 20° difference in angle, and the distances can be roughly calculated based upon a 100AU triangle...&#xA;Given 100 x Sin(20°) that's 34 AU and change. Voyager's dish isn't big enough, at 3.6m; it pulls in roughly 1/100 the signal that the 36m dish on earth would. (other factors make this extremely simplistic an analysis, but establish the point well enough.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply put, the receiver is not sensitive enough, and the antenna gain not sufficient, to enable voyager to notice the energy from a pioneer transmitter.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;5 - Reprogramming Voyager&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Voyager probes don't have a whole lot of memory. And much of that memory is of dubious reliability now. The reprogramming would need to revise the whole mission programming, even if Voyager could &quot;hear&quot; the  Pioneer 11 transmission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would scrap ongoing data collection, possibly cripple the Voyager, and still not assure that the data would relay.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Conclusion &amp;amp; Discussion&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Given that any one of the five reasons is sufficient for a no-go on such an attempt, it's a fairly certain thing that no space probe is going to be used to contact the Pioneers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether a later probe might be used to relay to/from the Voyagers is another matter.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h1&gt;Selected References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://theskylive.com/pioneer11-tracker&quot; rel=&quot;noreferrer&quot;&gt;http://theskylive.com/pioneer11-tracker&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://theskylive.com/ephemerides-computation?obj=voyager1&amp;amp;date=2014-02-13&amp;amp;h=00&amp;amp;m=00&quot; rel=&quot;noreferrer&quot;&gt;http://theskylive.com/ephemerides-computation?obj=voyager1&amp;amp;date=2014-02-13&amp;amp;h=00&amp;amp;m=00&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://theskylive.com/ephemerides-computation?obj=voyager2&amp;amp;date=2014-02-13&amp;amp;h=00&amp;amp;m=00&quot; rel=&quot;noreferrer&quot;&gt;http://theskylive.com/ephemerides-computation?obj=voyager2&amp;amp;date=2014-02-13&amp;amp;h=00&amp;amp;m=00&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Pioneer_11&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Pioneer_11&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_2&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Voyager_2&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftTelemetry.do?id=1973-019A&quot; rel=&quot;noreferrer&quot;&gt;http://nssdc.gsfc.nasa.gov/nmc/spacecraftTelemetry.do?id=1973-019A&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastEditorUserId="11721" LastEditDate="2018-11-13T03:50:41.777" LastActivityDate="2018-11-13T03:50:41.777" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3624" PostTypeId="2" ParentId="3585" CreationDate="2014-02-07T02:19:40.700" Score="8" Body="&lt;h1&gt;&quot;Current&quot; positional data:&lt;/h1&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager 1 Spacecraft, ephemeris for Fri 7 February 2014, 02:13 UTC&#xA;  Right Ascension: 17h 11m 54s Declination: +12° 01’ 12” (J2000) [HMS|00:00:00|Dec]&#xA;  Distance from Sun: 18,968.76 Million Km Distance from Earth: 19,024.36 Million Km&#xA;  Magnitude: N.A. Constellation: Oph&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Voyager 2 Spacecraft, ephemeris for Fri 7 February 2014, 02:13 UTC&#xA;  Right Ascension: 20h 00m 03s Declination: -55° 56’ 58” (J2000) [HMS|00:00:00|Dec]&#xA;  Distance from Sun: 15,555.76 Million Km Distance from Earth: 15,662.57 Million Km&#xA;  Magnitude: N.A. Constellation: Tel&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Pioneer 11 Spacecraft, ephemeris for Fri 7 February 2014, 02:13 UTC&#xA;  Right Ascension: 18h 49m 45s Declination: -08° 48’ 07” (J2000) [HMS|00:00:00|Dec]&#xA;  Distance from Sun: 13,202.88 Million Km Distance from Earth: 13,319.30 Million Km&#xA;  Magnitude: N.A. Constellation: Sct&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Given that they are (1) powered by radio-thermal generators, (2) therefore above background temperature, and (3) on known trajectories, they are pretty easy to track. An orbital infrared telescope should be able to find them pretty easily. They are a small warm object on a known course.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://theskylive.com/ephemerides-computation?obj=voyager1#lat|77.9799442515|lon|102.02461516200002|fov|80&quot;&gt;http://theskylive.com/ephemerides-computation?obj=voyager1#lat|77.9799442515|lon|102.02461516200002|fov|80&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://theskylive.com/ephemerides-computation?obj=voyager2&quot;&gt;http://theskylive.com/ephemerides-computation?obj=voyager2&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://theskylive.com/ephemerides-computation?obj=pioneer11&quot;&gt;http://theskylive.com/ephemerides-computation?obj=pioneer11&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-02-07T02:19:40.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3625" PostTypeId="2" ParentId="3010" CreationDate="2014-02-07T02:26:56.853" Score="5" Body="&lt;p&gt;As of 7 February:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager 1 Spacecraft, ephemeris for Fri 7 February 2014, 02:13 UTC&#xA;  Right Ascension: 17h 11m 54s Declination: +12° 01’ 12” (J2000) [HMS|00:00:00|Dec]&#xA;  Distance from Sun: 18,968.76 Million Km Distance from Earth: 19,024.36 Million Km&#xA;  Magnitude: N.A. Constellation: Oph&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;given an AU as 149.597 million km... 127.1 AU and climbing. Note that distance will vary by ±2 AU over the year, and is travelling about 1AU per 100.5 days, or about 3.6 AU per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://theskylive.com/ephemerides-computation?obj=voyager1#lat|77.9799442515|lon|102.02461516200002|fov|80&quot;&gt;http://theskylive.com/ephemerides-computation?obj=voyager1#lat|77.9799442515|lon|102.02461516200002|fov|80&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-02-07T02:26:56.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3626" PostTypeId="2" ParentId="1599" CreationDate="2014-02-07T02:37:55.667" Score="13" Body="&lt;p&gt;Check with Kentucky Space. They have KySat-2 on orbit using passive magnetic control.&lt;/p&gt;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;http://ssl.engineering.uky.edu/missions/orbital/kysat-2/about-kysat-2/&quot; rel=&quot;noreferrer&quot;&gt;description of the control system from their website&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Passive Magnetic Stabilization:  KySat-2 is equipped with a passive attitude control scheme known as Passive Magnetic Stabilization. This passive control technique uses permanent magnets and magnetic hysteresis material fixed to the chassis of KySat-2. The permanent magnets will provide torque in attempts to align with Earth’s magnetic field (in the same fashion that a magnetic compass needle points to magnetic North). The permanent magnets will be mounted to the spacecraft’s chassis in such a way that when aligned with Earth’s magnetic field, KySat-2′s camera will point at Earth while over Northern Hemisphere, and out into space while over the Southern Hemisphere.&lt;/p&gt;&#xA;&lt;p&gt;Permanent magnets provide the small amount of torque needed to keep the spacecraft oriented correctly, but in the vacuum of space, there is no damping effect (such as air resistance as we have on the surface). Because of this, KySat-2 would oscillate around it’s [sic] targeted orientation rather than settle into it smoothly. Also, permanent magnets can only provide control over two axes while the third axis is uncontrolled and free to rotate. To alleviate these problems, KySat-2 is also equipped with hysteresis (memory) material. This material “memorizes” the current magnetic field and thus resists changes in KySat-2′s orientation. This effect is small, and much less assertive than the permanent magnets, but it provides the needed damping effect to stabilize KySat-2 on the two control axes, and resist changes on the uncontrolled axis as well.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2607" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-02-10T04:51:42.267" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3628" PostTypeId="1" AcceptedAnswerId="3629" CreationDate="2014-02-07T06:20:27.247" Score="11" ViewCount="510" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Giotto_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;The Wikipedia article on Giotto&lt;/a&gt; states that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Giotto was the first spacecraft to be re-activated from hibernation mode.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How was the reactivation accomplished? &lt;/li&gt;&#xA;&lt;li&gt;Does the statement mean the spacecraft restarted after a shutdown, or something else?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1427" LastEditorUserId="12102" LastEditDate="2021-03-31T10:13:33.963" LastActivityDate="2021-03-31T15:56:51.153" Title="How was the Giotto spacecraft reactivated from hibernation mode?" Tags="|spacecraft|probe|deep-space-hibernation|giotto|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3629" PostTypeId="2" ParentId="3628" CreationDate="2014-02-07T09:26:34.913" Score="7" Body="&lt;p&gt;Giotto woke up from hibernation &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Giotto_overview&quot; rel=&quot;nofollow noreferrer&quot;&gt;after a signal from ground control&lt;/a&gt;. This meant that at least the radio receiver and parts of the computer still had to be active. The science instruments and most of the spacecraft management systems (e.g. attitude control) were switched off.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.newscientist.com/article/mg13418211.700-space-probe-wakes.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The wakeup took several days&lt;/a&gt;. After the wakeup, Giotto &lt;a href=&quot;http://www.science-bbs.com/16-astro/732e95ff5988c9b1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;aimed its main antenna at Earth&lt;/a&gt; for new instructions.&lt;br&gt;&#xA;A &lt;a href=&quot;https://space.stackexchange.com/questions/3446/could-rosetta-have-been-awakened-from-earth-if-necessary&quot;&gt;recent question about Rosetta&lt;/a&gt; shows that a different approach was used there. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-02-07T09:26:34.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3630" PostTypeId="2" ParentId="3585" CreationDate="2014-02-07T12:08:27.247" Score="18" Body="&lt;p&gt;We currently cannot track Pioneer 10 or 11. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Someone on the &lt;a href=&quot;http://forums.xkcd.com/viewtopic.php?f=9&amp;amp;t=77319&quot; rel=&quot;nofollow noreferrer&quot;&gt;XKCD forum&lt;/a&gt; calculated how much radiation the Voyager probes emit besides their radio transmissions. The heat they give off is emitted as infrared light: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We have a power source of 420 W at 1.78×10&lt;sup&gt;10&lt;/sup&gt; km, which gives a brightness of 1×10&lt;sup&gt;-19&lt;/sup&gt; Wm&lt;sup&gt;-2&lt;/sup&gt;, or an apparent magnitude of 28. That is just visible for a big ground-based telescope....&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note that the radiation we're talking about would be in the infrared frequencies, not visible light. Still, an &lt;a href=&quot;https://en.wikipedia.org/wiki/Apparent_magnitude&quot; rel=&quot;nofollow noreferrer&quot;&gt;apparent magnitude of 30&lt;/a&gt; is about the limit for the biggest ground-based IR telescopes.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this discounts the angular resolution of a telescope. The Pioneers and Voyagers are much too small to be resolved. For example, here's a &lt;a href=&quot;http://hubblesite.org/newscenter/archive/releases/solar%20system/pluto/2012/32/image/c/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hubble image of Pluto and its moons&lt;/a&gt;. Pluto is only a few pixels across:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/2T97R.jpg&quot; alt=&quot;Hubble image of Pluto and its moons&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a space probe at that distance would fill only a fraction of a pixel. Here's a Spitzer IR image of a &lt;a href=&quot;https://en.wikipedia.org/wiki/%2855565%29_2002_AW197&quot; rel=&quot;nofollow noreferrer&quot;&gt;dwarf planet&lt;/a&gt;:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/ojHLx.jpg&quot; alt=&quot;Spitzer image of a dwarf planet&quot;&gt;&lt;br&gt;&#xA;I suspect a space probe would be lost in the noise. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-11-21T07:10:24.083" LastActivityDate="2017-11-21T07:10:24.083" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3631" PostTypeId="1" AcceptedAnswerId="3638" CreationDate="2014-02-07T13:03:43.013" Score="24" ViewCount="25717" Body="&lt;p&gt;Let's say a spacecraft is in an orbit like this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7caj5.png&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the red arrows point to prograde and retrograde, and the blue arrows point to normal and antinormal, &lt;strong&gt;what do the green arrows point to?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, what does one call the orientations that are perpendicular to both the orbit prograde and the orbit normal?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that it's not necessarily correct to say &quot;towards the planet&quot; or &quot;away from the planet.&quot;  In highly eccentric orbits like the one pictured above, both orientations can point &quot;away.&quot;&lt;/p&gt;&#xA;" OwnerUserId="499" LastEditorUserId="499" LastEditDate="2014-02-07T15:20:18.227" LastActivityDate="2018-11-30T20:01:28.830" Title="What are these orientations called in orbit?" Tags="|orbital-mechanics|orbit|terminology|" AnswerCount="7" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3632" PostTypeId="2" ParentId="3631" CreationDate="2014-02-07T13:51:48.790" Score="11" Body="&lt;p&gt;There are 3 directions in any orbit. The typical convention is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Nadir- This is the direction towards the center of the planet, straight down. Oposite to NADIR is the Zenith. &lt;/li&gt;&#xA;&lt;li&gt;Velocity Vector- Direction of movement, Prograde/retrograde are a common, Prograde is the direction of orbit, retrograde opposite&lt;/li&gt;&#xA;&lt;li&gt;Normal direction to plane of orbit. This is often referred to as the angular momentum vector.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;See also &lt;a href=&quot;http://www.dept.aoe.vt.edu/~cdhall/courses/aoe4140/intro2adcs.pdf&quot; rel=&quot;noreferrer&quot;&gt;this PDF&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="1255" LastEditDate="2014-02-10T16:45:30.057" LastActivityDate="2014-02-10T16:45:30.057" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3633" PostTypeId="1" AcceptedAnswerId="3636" CreationDate="2014-02-07T13:53:56.893" Score="12" ViewCount="1466" Body="&lt;p&gt;I've heard of many satellites talk about a &quot;Safe Mode&quot;. What is safe mode, and what are some common methods to entering safe mode?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-02-07T17:10:49.630" Title="What is &quot;Safe Mode&quot;?" Tags="|software|safe-mode|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3635" PostTypeId="2" ParentId="3633" CreationDate="2014-02-07T14:33:20.917" Score="9" Body="&lt;p&gt;With no one around to hit the power switch on site, they need a way to reboot, reset, recover, from any potential error.  A Blue Screen of Death in space, can really mean death.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Safe mode, one assumes is a pretty low level hardware thing (I am sure it is implemented different by every manufacturer), where when all else fails, or certain errors occurs, the system can get itself into a state, that the ground can tell it to recover from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Often this involves pointing the antenna back at Earth, or listening for a specific frequency for a reboot/restart/reset command.  It is usually a pretty simple, but specific command (would not want to trigger it by accident). Simple so the remote device can pick it up with minimal functionality, and specific to make it hard to happen when not desired.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may involve a command to restart on the backup hardware (B side?) or it may just trigger a reboot of the software.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The level of error handling we can tolerate on our devices is no where close to the level you can tolerate on a 100 million dollar device, where no one can reach it directly.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-02-07T14:33:20.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3636" PostTypeId="2" ParentId="3633" CreationDate="2014-02-07T14:59:50.377" Score="14" Body="&lt;p&gt;As mentioned by geoffc, it all depends on the manufacturer and especially the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the geostationary spacecrafts, the vehicle will usually switch the safe mode only in a contingency situation and after not receiving any ground signal for a given timeout (often 24 hours). In this case, the spacecraft will turn on the omni-directional antenna, start looking for the sun in order to both charge its batteries via the solar panels* and attempt to determine the position of Earth based on its current date time knowledge and ephemeris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once Earth is found, the vehicle will stay locked on Earth (with either Earth sensors or using only Star Trackers, like on the latest iteration of the Orbital Sciences Corp. Star 2 platform). The vehicle will usually require manual intervention before resuming normal operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Concerning geostationary spacecrafts, it's important to note that a software glitch will not put the vehicle in safe mode. Instead it will switch-over to another flight computer and continue operations, often times without service disruption (although there will be a telemetry and telecommand disruption, this will not affect transponder functionality). In fact, spacecraft are designed in such a way that each subsystem can be relatively independent (as long as the vehicle is powered of course). Each subsystem is connected to a Remote Unit Interface, and that RUI is connected to the on-board computer which will perform TM/TC processing, baseband commands, etc. In other words, the on-board computer (which hosts the flight software) is mainly used to relay telecommands to the appropriate Remote Unit Interface and to aggregate RUI telemetry, generate the correct bit sequence, encrypt the frames and send the frames as such to the TTX in order for the appropriate signal to be generated, amplified, routed to the output multiplexer and emitted through the appropriate antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this answers your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(*)  The spacecraft will charge the batteries no matter the current state of the charge, recent vehicles equipped with Lithium Ion batteries only rarely require trickle charge, which is not the case for previous generations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources: my previous job (Operations Architecture, SES), my passion and my very much loved copy of &lt;em&gt;The New SMAD&lt;/em&gt; by Wertz and co.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="1391" LastEditDate="2014-02-07T17:10:49.630" LastActivityDate="2014-02-07T17:10:49.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3637" PostTypeId="1" CreationDate="2014-02-07T15:43:37.040" Score="15" ViewCount="982" Body="&lt;p&gt;Sometimes, despite all ground testing, a new build of software can break a satellite. How does one put in protections to keep this failure from being a permanent failure? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, what do you do if for some reason the new software isn't 100% updated? Or if there was an unintended consequence of the new build? How can you ensure you can always fix a problem caused by a software bug?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-19T17:26:01.047" LastActivityDate="2016-07-19T17:26:01.047" Title="How does one build software for a Satellite such that a new build doesn't break it?" Tags="|artificial-satellite|failure|software|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3638" PostTypeId="2" ParentId="3631" CreationDate="2014-02-07T15:51:05.920" Score="15" Body="&lt;p&gt;I'm not sure there's any generally agreed on convention, but spaceships are often regarded as, well, ships, so similar terms will be often used. E.g. for the red arrows that you describe as &lt;em&gt;prograde&lt;/em&gt; and &lt;em&gt;retrograde&lt;/em&gt; orientation vectors, from a spaceship's point of view and relative to its movement, also &lt;em&gt;ram-facing&lt;/em&gt; and &lt;em&gt;wake-facing&lt;/em&gt; is often used to describe sides, but could be also &lt;em&gt;forward&lt;/em&gt; and &lt;em&gt;aft&lt;/em&gt;, or even &lt;em&gt;bow&lt;/em&gt; and &lt;em&gt;stern&lt;/em&gt;. For the blue orientation vectors, these could then be &lt;em&gt;port&lt;/em&gt; for &lt;em&gt;left&lt;/em&gt; and &lt;em&gt;starboard&lt;/em&gt; for &lt;em&gt;right&lt;/em&gt;, relative to the vehicle's movement, facing forward. The green ones are most commonly called the &lt;em&gt;nadir&lt;/em&gt; and &lt;em&gt;zenith&lt;/em&gt; facing sides, but as the terminology varies depending on who's referring to it, I'd guess there's all kinds of other terms used too, from obvious &lt;em&gt;down&lt;/em&gt;, &lt;em&gt;downward&lt;/em&gt;, and &lt;em&gt;up&lt;/em&gt;, &lt;em&gt;upward&lt;/em&gt;, to &lt;em&gt;deck&lt;/em&gt; and &lt;em&gt;overhead&lt;/em&gt;, or even &lt;em&gt;towards&lt;/em&gt; and &lt;em&gt;away&lt;/em&gt; from something, in our case the body it orbits around. So for orientation relative to the ship's movement we have:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ram-facing, forward, bow,...&lt;/li&gt;&#xA;&lt;li&gt;wake-facing, aft, stern,...&lt;/li&gt;&#xA;&lt;li&gt;starboard, right,...&lt;/li&gt;&#xA;&lt;li&gt;port, left,...&lt;/li&gt;&#xA;&lt;li&gt;nadir, deck, down, downward, towards &lt;em&gt;sth&lt;/em&gt;,...&lt;/li&gt;&#xA;&lt;li&gt;zenith, overhead, up, upward, away &lt;em&gt;from sth&lt;/em&gt;,...&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For example, from &lt;a href=&quot;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&quot; rel=&quot;noreferrer&quot;&gt;Reference guide to the International Space Station, Assembly complete edition, NASA 2010&lt;/a&gt; (PDF), four of these sides are described in the definitions section as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;strong&gt;nadir&lt;/strong&gt;: Direction directly below (opposite zenith)&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;port&lt;/strong&gt;: Direction to the left side (opposite starboard)&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;starboard&lt;/strong&gt;: Direction to the right side (opposite port)&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;zenith&lt;/strong&gt;: Directly above, opposite nadir&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And the remaining two sides mentioned in text as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;ram&lt;/strong&gt; (forward) or &lt;strong&gt;wake&lt;/strong&gt; (aft) pointing&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA's &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/multimedia/iss_labs_guide.html&quot; rel=&quot;noreferrer&quot;&gt;Guide to the International Space Station Laboratory Racks Interactive&lt;/a&gt; however names direction towards nadir as &lt;strong&gt;deck&lt;/strong&gt; and direction towards zenith &lt;strong&gt;overhead&lt;/strong&gt;, and alternatively also the +/- axial values that follow the &lt;a href=&quot;https://en.wikipedia.org/wiki/Right-hand_rule&quot; rel=&quot;noreferrer&quot;&gt;right-hand rule&lt;/a&gt; more commonly used by astronaut pilots during navigation or to describe station's attitude (such as during docking):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/c5qqK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image above: The International Space Station’s coordinate system. Credit: &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/multimedia/iss_labs_guide.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, movement relative to these three axes could be described using &lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_dynamics&quot; rel=&quot;noreferrer&quot;&gt;aviation terms&lt;/a&gt; &lt;em&gt;roll&lt;/em&gt;, &lt;em&gt;pitch&lt;/em&gt; and &lt;em&gt;yaw&lt;/em&gt; to describe attitude of a satellite, but these don't really denote the sides, merely rotation of the body with respect to the x, y, z (in your case red, blue, green) axes in Cartesian coordinate system, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There might be other terms I didn't think of though, but as always, it will depend on who's using them and if they're referring to the sides from the perspective of the vessel and relative to its movement, or relative to some wider frame of reference, for example with respect to the body it orbits, in which case, alas, I fail to think of other ways these orientation sides could be named, short of describing them with respect to &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;noreferrer&quot;&gt;orbital elements&lt;/a&gt; in any of the various coordinate systems used, such as Keplerian, Cartesian,... like you did with &lt;em&gt;prograde&lt;/em&gt; and &lt;em&gt;retrograde&lt;/em&gt;, which are essentially broadly describing orbital inclination with respect to the body's plane of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With specific spacecraft, often its sides are also named by its components or modules, which works irrespective of spacecraft's own movement relative to another object.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-09-12T03:02:03.477" LastActivityDate="2014-09-12T03:02:03.477" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3639" PostTypeId="1" CreationDate="2014-02-07T16:14:44.390" Score="7" ViewCount="390" Body="&lt;p&gt;Classified, as in, a government doesn't share the basic parameters of the launch, its purpose, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I vaguely remember this cropping up in &lt;a href=&quot;https://space.stackexchange.com/questions/88/total-mass-sent-into-orbit-over-all-history&quot;&gt;a previous question&lt;/a&gt;, but I can't easily find the numbers for this.  Not much on Google, so it deserves a question.  Any representative time frame would be good enough.  For instance, &quot;from 2000 to 2013 there were X launches and Y were classified&quot;.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-02-19T00:11:03.040" Title="What fraction of launches are classified?" Tags="|launch|classified|government|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="3640" PostTypeId="1" AcceptedAnswerId="3642" CreationDate="2014-02-07T16:16:34.467" Score="15" ViewCount="1976" Body="&lt;p&gt;I am wondering if there is any visual documentation of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Sputnik_%28rocket%29&quot;&gt;Sputnik launcher&lt;/a&gt; and/or launch, either video or photographs. &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="758" LastEditDate="2015-07-28T22:56:01.693" LastActivityDate="2015-07-28T22:56:01.693" Title="Is there any footage of the rocket that launched Sputnik 1?" Tags="|launch|artificial-satellite|history|sputnik|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3641" PostTypeId="2" ParentId="3637" CreationDate="2014-02-07T17:06:07.647" Score="22" Body="&lt;p&gt;A typical approach as used on Mars rovers is to keep the previous build on the spacecraft as the default to boot to on a reset.  You load the new build and command a boot to the new one.  Then you test it.  If something goes south, it will reboot to the old build.  Or you can command a reboot to the old one.  Once you're confident in the new build, you make it the default.  Even then you can keep the old build on-board just in case.  It can be booted to by setting a bit to use that one and commanding a reset  (using &quot;hardware&quot; commands decoded by the radio).&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-07T18:09:30.790" LastActivityDate="2014-02-07T18:09:30.790" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3642" PostTypeId="2" ParentId="3640" CreationDate="2014-02-07T20:26:08.383" Score="12" Body="&lt;p&gt;I found a photos, after searching around a bit. I haven't found any video yet. See &lt;a href=&quot;http://www.space.com/17852-sputnik-space-race-first-satellite-photos.html&quot; rel=&quot;noreferrer&quot;&gt;Space.com&lt;/a&gt; for more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/G9dsj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-02-07T20:26:08.383" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3644" PostTypeId="2" ParentId="3443" CreationDate="2014-02-08T14:19:32.880" Score="13" Body="&lt;p&gt;The wake up is triggerred by the onboard timer in Rosetta.&#xA;It is something like an alarm clock. When the timer triggers the start command, the system wakes up and runs a set of procedures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an &lt;a href=&quot;http://www.youtube.com/watch?v=UqcDtRmJbKY&quot; rel=&quot;nofollow&quot;&gt;&lt;strong&gt;illustrative video&lt;/strong&gt;&lt;/a&gt; which explains Rosetta's wake-up process. You can easily learn how the wake-up process works in the probe from this video.&lt;/p&gt;&#xA;" OwnerUserId="1427" LastEditorUserId="603" LastEditDate="2014-11-14T16:47:30.283" LastActivityDate="2014-11-14T16:47:30.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3645" PostTypeId="2" ParentId="2660" CreationDate="2014-02-08T15:36:25.743" Score="6" Body="&lt;p&gt;Apparently not, but the penetrator system seems to have survived:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The space penetrator has itself been in development for almost 10 years, and was originally proposed for a British lunar mission called Moonlite.&lt;/p&gt;&#xA;&lt;p&gt;That venture was eventually shelved, but the idea of a &amp;quot;hard lander&amp;quot; was so compelling, the European Space Agency decided to pick up the concept.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-23281423&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.bbc.co.uk/news/science-environment-23281423&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There are a number of other articles online from July and August of last year.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-08T15:36:25.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3646" PostTypeId="2" ParentId="3640" CreationDate="2014-02-08T15:56:04.680" Score="11" Body="&lt;p&gt;Yes, there are videos of the launch of Sputnik 1, there was no way that the Soviet propaganda machinery of those times would have missed the opportunity, and I've seen at least two separate sources, one filmed on a color camera, and another black &amp;amp; white from a slightly different angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://encrypted.google.com/webhp#q=sputnik+launch+video&amp;amp;tbm=vid&quot; rel=&quot;noreferrer&quot;&gt;what's available online&lt;/a&gt;, sadly none are of quality we've become accustomed to, but they should do:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.dailymotion.com/video/x4ytmk_launch-of-sputnik-1_tech&quot; rel=&quot;noreferrer&quot;&gt;Color version from what seems to be a Russian featurette&lt;/a&gt; (&lt;a href=&quot;https://www.youtube.com/watch?v=qcex_MuBT7Y&quot; rel=&quot;noreferrer&quot;&gt;alternative source via YouTube&lt;/a&gt;):&lt;br /&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.dailymotion.com/video/x4ytmk_launch-of-sputnik-1_tech&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3056Y.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.ted.com/talks/david_hoffman_shares_his_sputnik_mania.html&quot; rel=&quot;noreferrer&quot;&gt;Black &amp;amp; White clips featured in Sputnik Mania film by David Hoffman&lt;/a&gt; (can be also seen in &lt;a href=&quot;http://www.military.com/video/space-technology/artificial-satellites/soviet-union-launches-sputnik-1957/877326134001/&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt;):&lt;br /&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.ted.com/talks/david_hoffman_shares_his_sputnik_mania.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4JyjD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These footages appear to be legit copies of the launch of &lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_1&quot; rel=&quot;noreferrer&quot;&gt;Sputnik 1&lt;/a&gt; in 4 October 1957 from &lt;a href=&quot;https://en.wikipedia.org/wiki/Gagarin%27s_Start&quot; rel=&quot;noreferrer&quot;&gt;Site №1&lt;/a&gt;, a pad that is now part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikonur_Cosmodrome&quot; rel=&quot;noreferrer&quot;&gt;Baikonur Cosmodrome&lt;/a&gt; launch complex in Kazakhstan.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Another interesting thing about Sputnik 1 is how the news was reported in the western media, as seen for example in &lt;a href=&quot;http://www.dailymotion.com/video/x3hkv2_flight-of-sputnik-i_news&quot; rel=&quot;noreferrer&quot;&gt;video of CBS special report&lt;/a&gt;. :/&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-27T11:15:15.773" LastActivityDate="2014-02-27T11:15:15.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3647" PostTypeId="1" AcceptedAnswerId="4277" CreationDate="2014-02-08T18:13:22.077" Score="12" ViewCount="923" Body="&lt;p&gt;Most discussions of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;nofollow&quot;&gt;Van Allen radiation belts&lt;/a&gt; focus on the necessity to shield sensitive electronics and biomass from radiation hazards. But are there any possible uses for this cache of highly energetic particles? For instance, can they be used as an energy source for spacecraft, or might it be possible to mine the antiprotons discovered in the belts?&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="2699" LastEditDate="2014-04-16T16:46:23.097" LastActivityDate="2015-12-30T01:29:55.477" Title="Can the Van Allen radiation belts serve any useful functions for space flight?" Tags="|radiation|power|antimatter|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3648" PostTypeId="2" ParentId="3637" CreationDate="2014-02-08T18:55:49.147" Score="2" Body="&lt;p&gt;There are 2 main methods I'm aware of that spacecraft employ. One is what Mark mentioned, that two images are kept on the spacecraft. Usually one of these images is called the &quot;Gold&quot; image, and is never actually updated during flight. This image is thoroughly tested to ensure proper functionality on the ground prior to launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second way is to have 3 layers of software build on top of each other. The Mon 1 image is just a bootloader, which loads Mon 2. Mon 1 and Mon 2 never change during flight, and are thoroughly tested prior to launch. They contain the minimum code required to talk to the satellite, so it can be fixed. The Mon 3 code contains the useful code, and is updated in flight.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-02-08T18:55:49.147" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3649" PostTypeId="1" AcceptedAnswerId="3650" CreationDate="2014-02-08T21:48:32.790" Score="11" ViewCount="745" Body="&lt;p&gt;I'm trying to write a Mathematica application that allows users to choose their date of departure and time-of-flight for an interplanetary transfer mission (currently only working for Earth-Mars transfers), which is then animated (similar to &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's trajectory browser&lt;/a&gt; except not as cool).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far I've managed to get the &lt;a href=&quot;https://space.stackexchange.com/q/2803/49&quot;&gt;Lambert solver&lt;/a&gt; and 3-body Sun-Earth-Mars numerical model working, but now I need to figure out how to get ephemeris data automatically from &lt;a href=&quot;http://ssd.jpl.nasa.gov/?horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL's Horizons ephemeris&lt;/a&gt; when a user selects their departure date and TOF. I thought about trying to incorporate the Horizons telnet service into the application, but it might be quite tough to get that working, and the position and velocity data that is given doesn't seem to be in a Mathematica friendly format which might require lots of string matching/replacing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another alternative would be to somehow download and tabulate years worth of ephemeris data into a spreadsheet and extract it from there upon request, but this also seems like a laborious task. My question is therefore this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How do normal trajectory planners/plotters get their ephemeris data?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-02-08T23:57:25.057" Title="Ephemeris data usage in interplanetary trajectory application" Tags="|orbital-mechanics|trajectory|interplanetary|software|ephemeris|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3650" PostTypeId="2" ParentId="3649" CreationDate="2014-02-08T23:23:59.740" Score="10" Body="&lt;p&gt;Using the &lt;a href=&quot;http://naif.jpl.nasa.gov/naif/toolkit.html&quot;&gt;SPICE toolkit&lt;/a&gt; and a recent ephemeris file like &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/de430.bsp&quot;&gt;DE430&lt;/a&gt; (a 120 MB download).  You will also need a recent &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/generic_kernels/lsk/naif0010.tls&quot;&gt;leap seconds kernel&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The toolkit provides interfaces for Fortran, C, IDL, and Matlab, but not Mathematica.  You can use the C version of the toolkit, and Mathematica's MathLink or LibraryLink to access the toolkit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/index.html&quot;&gt;many, many functions in the toolkit&lt;/a&gt;, but you'll only need a handful.  For example, you can use &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/furnsh_c.html&quot;&gt;&lt;code&gt;furnsh_c()&lt;/code&gt;&lt;/a&gt; to load the ephemeris file and the leap seconds file, and &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/spkezr_c.html&quot;&gt;&lt;code&gt;spkezr_c()&lt;/code&gt;&lt;/a&gt; to get the position and velocity of the specified body at the specified time, in the specified reference frame.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-08T23:57:25.057" LastActivityDate="2014-02-08T23:57:25.057" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3651" PostTypeId="2" ParentId="1691" CreationDate="2014-02-09T15:34:53.227" Score="26" Body="&lt;p&gt;In this &lt;a href=&quot;https://www.youtube.com/watch?v=k_OD2V6fMLQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;video of the Apollo 11 landing&lt;/a&gt;, you can see dust being blown away during the approach (from ca. 4:30). Some of the particles &lt;a href=&quot;http://www.space.com/4956-lunar-landers-sandblasted-moon.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;were blasted clear over the horizon&lt;/a&gt; and may have ended up halfway across the moon.&lt;br /&gt;&#xA;The 'ground' below the dust isn't smooth. Like new basalt, it has a grainy texture, as you can see here directly below the exhaust:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Eq24m.jpg&quot; alt=&quot;LEM exhaust&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Apollo missions found that the lunar surface has a thin layer of dust (up to 5 cm), and the size of the fragments progressively gets larger the farther you go down. Because it's all the same material (rock fragmented on impact), most of the Apollo photos show no visual difference between places that have dust and places where the dust has been blow away. You'd need a closeup to see grain sizes clearly.&lt;/p&gt;&#xA;&lt;p&gt;The moon has no atmosphere, so only the dust directly in the path of the rocket exhaust will be disturbed. This is unlike Earth, where the exhaust would drag along the surrounding air, creating a much larger area of influence. So anyplace that's even a bit shielded from the exhaust would remain undisturbed. In the photo posted by @Tildalwave, you can see how uneven the ground was. In this photo, you can see that areas where the dust is intact are shielded, and other areas appear smoother:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://history.nasa.gov/ap11ann/kippsphotos/5927.jpg&quot; alt=&quot;Area around LEM&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Robert Braeunig did a &lt;a href=&quot;http://www.braeunig.us/apollo/LMcrater.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;detailed analysis&lt;/a&gt; of lunar soil erosion by the LEM engine. His conclusion:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Of all our assumptions, perhaps the one that most dramatically affects the results is the final velocity of the gas and soil. For instance, if we used the middle of the velocity range, i.e. 1,700 m/s, rather than 1,000 m/s, we get a maximum crater depth of just 10.6 mm (0.42 inch). In reality, the exhaust stream blew away very little soil because there wasn't enough energy in the gas to move a large volume of surface material.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The dust that &lt;a href=&quot;http://www.space.com/4956-lunar-landers-sandblasted-moon.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;did get blown&lt;/a&gt; away &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2007/23nov_flyingmoondust/&quot; rel=&quot;nofollow noreferrer&quot;&gt;had a remarkable effect&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In November 1969, the Apollo 12 Lunar Module (LM, pronounced &amp;quot;lem&amp;quot;) landed about 200 meters from Surveyor 3, a robotic probe that had landed on the Moon in April 1967. The Apollo 12 astronauts walked over to Surveyor 3 to photograph it and to retrieve some pieces for return to Earth. Right away, they noticed that most of Surveyor 3, which at launch was pristine white, had darkened to brown--a result of two-and-a-half years' exposure to extreme lunar conditions.&lt;/p&gt;&#xA;&lt;p&gt;But the side of Surveyor 3 facing the LM had been sandblasted back to white. In fact, &amp;quot;every bolt, cable, or bracket blocking the spray of fine grit from Apollo 12 left permanent shadows etched onto Surveyor,&amp;quot; Metzger says. From examining the returned artifacts, scientists later calculated the sandblasting resulted primarily from finest dust particles only 1 to 10 micrometers (0.00004 to 0.0004 inch) across.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This image shows a difference in color between the &lt;a href=&quot;http://historicspacecraft.com/Probes_Lunar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;two sides of the camera&lt;/a&gt; (the camera was retrieved by Apollo 12):&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JFUWA.jpg&quot; alt=&quot;Surveyor 3 camera&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Problem with this image is, in every Surveyor 3 photo I could find, the darker side of the camera was turned towards the LEM:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MxOtn.jpg&quot; alt=&quot;Surveyor 3 and LEM&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Now, it seems the Apollo 12 crew took only a few photos of Surveyor, it's possible they turned the camera (preparing to remove it from the probe) before they took the photo.&#xA;Photos taken on the Moon don't show the color difference very well, any subtle gradients are washed out by the harsh light.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://nssdcftp.gsfc.nasa.gov/miscellaneous/documents/b16023.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;original NASA report on Surveyor 3&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2023-08-24T08:02:37.003" LastActivityDate="2023-08-24T08:02:37.003" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3652" PostTypeId="1" AcceptedAnswerId="4493" CreationDate="2014-02-10T02:50:39.250" Score="18" ViewCount="1250" Body="&lt;p&gt;One of the very interesting things about the Soviet (and now Russian) space program is how some things seem almost eternal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first Sputnik launch was on an R-7 ICBM, modified for space launch. If you were to look at a picture of it, and compare it with a Soyuz launcher you saw launching last week to carry a Progress or Soyuz capsule to the ISS, it looks remarkably similar. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure the electronics/computers are upgraded over the years, and they added an upper stage, but what other major changes were made?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been many many launchers (over 1700) of the variants.  But the amazing thing is how similar to the original R-7 booster they remain. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(A different and interesting (but likely unanswerable) question would be to consider the design philosophies and decisions that allowed such a long run).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="13208" LastEditDate="2017-02-27T21:22:08.000" LastActivityDate="2017-02-27T21:22:08.000" Title="What is different from the R-7 that launched Sputnik, to the Soyuz launcher used for Soyuz/Progress launches?" Tags="|launch-vehicle|progress|sputnik|soyuz-launcher|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3653" PostTypeId="2" ParentId="2419" CreationDate="2014-02-10T12:43:39.350" Score="9" Body="&lt;p&gt;The Venus ionosphere would definitely not be destroyed. Ionospheres are formed by interaction between the atmosphere and the ultraviolet radiation from the Sun. The UV radiation is more intense closer to the Sun, so Earth-like atmosphere on Venus would probably have even &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1233&amp;amp;context=physics_facpub&quot;&gt;denser&lt;/a&gt; ionosphere than actual Earth has. Venus receives 1.9-times the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html&quot;&gt;irradiance&lt;/a&gt; in compare to the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ionosphere on terraformed Venus could be even thicker than it is now. The UV radiation does not currently penetrate deep into the Venerian atmosphere due to its immense density. It would get much deeper if the atmosphere was less dense. See &lt;a href=&quot;http://www.dlr.de/me/Portaldata/25/Resources/dokumente/strahlenbiologie/astrobook/P3_14.pdf&quot;&gt;here&lt;/a&gt; on page 229. This could ionize thicker layer of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is of course what would be the actual electron density (and its altitude profile) and whether it would be greater or smaller than currently on Venus. Today Venus has maximum electron &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Venus#Upper_atmosphere_and_ionosphere&quot;&gt;density&lt;/a&gt; in its ionosphere about $ 3.10^{11} m^{-3} $. &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/22203/1/97-0670.pdf&quot;&gt;Earth ionoshpere&lt;/a&gt; electron density peak is about $ 1.5.10^{11} m^{-3} $ which is half of Venus, so in fact it is comparable. We may be tempted to think that closer to the Sun the density in Earth-like ionosphere could be bigger. But density varies wildly over altitude and what exactly would happen with Earth-like ionoshpere electron density profiles at Venus is hard to estimate - I am not aware of any such study. Anything could happen, because there are many effects: atmosphere density, depth of UV penetration, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a side note, Venus does have an &lt;a href=&quot;http://www-ssc.igpp.ucla.edu/personnel/russell/papers/InducedMagnetosphere.pdf&quot;&gt;induced magnetosphere&lt;/a&gt;. Of course it is weaker than Earth magnetosphere, but it &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmospheric_escape#Significance_of_solar_winds&quot;&gt;does&lt;/a&gt; provide partial protection against the solar wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this would still not stop stripping of the terraformed Venus atmosphere by Solar wind.&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="921" LastEditDate="2014-02-10T13:15:58.547" LastActivityDate="2014-02-10T13:15:58.547" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3654" PostTypeId="2" ParentId="2877" CreationDate="2014-02-10T13:02:08.600" Score="7" Body="&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the more general case of elliptical orbits, there are no longer&#xA;  stationary points in the same sense: it becomes more of a Lagrangian&#xA;  “area”. The Lagrangian points constructed at each point in time, as in&#xA;  the circular case, form stationary elliptical orbits which are similar&#xA;  to the orbits of the massive bodies.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A more complete answer is given in &lt;a href=&quot;http://adsabs.harvard.edu/full/1992AN....313..315T&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;. It concludes that yes, L4 and L5 are at the points on the orbit that are equidistant from the planet and barycenter.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-02-10T13:02:08.600" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3655" PostTypeId="2" ParentId="2419" CreationDate="2014-02-10T13:08:56.117" Score="10" Body="&lt;p&gt;A short answer is no, Venus wouldn't lose its ionosphere if the target state of terraforming is matching Earth's atmospheric composition, temperature and pressure. The long answer is a bit more interesting though, particularly why that is a problem, and not a solution to Venusian lack of magnetic field. Let's first address the first point, and I'll slowly progress towards the problem parts;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ionosphere isn't anything special, it's something we should expect on any celestial body that's close enough to its star and holds lighter elements exposed on their surface, if it's initially having atmosphere or not, for its topmost layers to interact with the star's high energy proton flux and electromagnetic radiation. Even &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2011/14nov_lunarionosphere/&quot; rel=&quot;noreferrer&quot;&gt;the Moon has ionosphere&lt;/a&gt; because, while its exosphere isn't dense enough for its molecules to interact with each other, they still interact with the solar (photo and/or ionizing radiation induced) charged lunar surface. We also see this interaction with comets and their surface activity increasing as they cross the Solar system's snow line roughly somewhere in the middle of the main asteroid belt's distance to the Sun. Actually, it doesn't even take being close to any star and this interaction will in the long run be sufficiently provided by cosmic rays, it will only take longer for much of the pressure above the celestial body's surface to form, and the total pressure won't be sufficient for us to call it &lt;em&gt;an atmosphere&lt;/em&gt;, if it's then constantly being blown away by solar winds or the body isn't massive enough to hold onto these ions gravitationally or by strong force, say surface tension or electrostatically. E.g. we'd likely call it &lt;em&gt;an exosphere&lt;/em&gt; since its pressure wouldn't be sufficient for released particles to interact between each other like we'd expect them to in &lt;em&gt;proper&lt;/em&gt; atmospheres (where density is high enough that its constituent elements, atoms, molecules or more complex and heavier particles frequently collide with each other).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whichever source of interaction with high energy atomic and subatomic particles, or source of strong electromagnetic radiation like e.g. UV, gamma, et al., or even beta particle radiation, when they hit a molecule of weaker valence, they will cause their molecular bonds to break and atoms themselves to lose electrons, or rarer, even lose protons. In essence, they will ionize, and area of atmosphere that holds this proton gradient, where lighter, more energetic positive charge particles float over the remaining byproduct of such interaction, their heavier negative charge anions, is what we'd usually call an ionosphere. This is where Venus gets its weak electromagnetism of its ionosphere from. Now, most of its atmosphere is CO&lt;sub&gt;2&lt;/sub&gt;, and there isn't much in a sense of extremely light ions forming, such as we'd expect if there were molecules of water in the upper atmosphere, at first splitting into H&lt;sup&gt;+&lt;/sup&gt; and OH&lt;sup&gt;-&lt;/sup&gt;. And that's a problem, if terraformed Venus wouldn't have its own magnetic field. Let's inspect how Venusian interaction with solar wind looks like now:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Ji58L.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Interaction of Venus with the solar wind&lt;/strong&gt; (Source: &lt;a href=&quot;http://ase.tufts.edu/cosmos/print_images.asp?id=7&quot; rel=&quot;noreferrer&quot;&gt;Venus: The veiled planet&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see from the image, Venus is capable of sustaining thick enough plasma sheet in its magnetotail to act with its proton gradient as weak magnetic field, holding onto most of the now charged / ionized upper atmosphere that's been pushed away from it by the solar winds. This is possible because most of these ions (cations for positive charge ions and anions for negative charge ones) are heavier atoms with sufficient number of electrons orbiting their nuclei for this proton gradient to be relatively strong and countering their kinetic force as they're accelerated by interaction with high velocity proton flux of the solar winds. So Venus is able to hold onto most of its atmosphere and the rate at which it loses it to strongest interactions, such as being directly hit by a coronal mass ejection or a solar flare is slower than it is able to replenish it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This becomes a huge problem with magnitude thinner Earth-like atmosphere that also contains light elements, like hydrogen in water molecules. The magnetotail would compose of fewer and a lot lighter elements, including H&lt;sup&gt;+&lt;/sup&gt; and they would be blown away from Venus, losing great deal of its terraformed to be suitable for us atmosphere. So unless you can sustain this terraforming process that made it habitable in the first place and match loss of atmosphere, or kickstart a magnetic field on Venus to lower this interaction rate, or even somehow block or deflect the inbound solar wind, lower atmosphere would weaken and lose pressure to outer space. Ionosphere would still be there though, slowly sucking the denser lower atmosphere until it's own depletion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's the source of my now deleted comment that this question can only be sufficiently answered if you included means of terraforming Venus. One of the main problems with that is getting rid of most of its CO&lt;sub&gt;2&lt;/sub&gt;, and one way to do that is by pumping enormous amounts of hydrogen into its atmosphere, preferably with addition of metallic (iron, cobalt or nickel) catalyst to &lt;em&gt;force&lt;/em&gt; it into &lt;a href=&quot;https://en.wikipedia.org/wiki/Bosch_reaction&quot; rel=&quot;noreferrer&quot;&gt;Bosch reaction&lt;/a&gt;, where end result is atomic carbon and water: CO&lt;sub&gt;2&lt;/sub&gt; + 2 H&lt;sub&gt;2&lt;/sub&gt; → C + 2 H&lt;sub&gt;2&lt;/sub&gt;O. But whichever mode of terraforming would be chosen, if end state is to match Earth's own atmosphere (or come close to), and the planet to contain bodies of water, significant part of its atmosphere would be water vapor. And hydrogen bonds aren't strong, and water &lt;em&gt;likes&lt;/em&gt; to lose one of its hydrogen atoms even with relatively weak interaction with colliding high energy particles and UV radiation. So the ionosphere would end up being too big for comfort, magnetotail would be weaker (less dense than now and with weaker, single electron carrier ions, less electromagnetically attractive) and planet would start losing its atmosphere faster. But it wouldn't lose its ionosphere, it would replenish through lower atmosphere while there is any.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For an interesting fresh insight on Venusian ionospheric dynamics, here's a NASA Goddard article (inc. a video): &lt;a href=&quot;http://www.nasa.gov/content/goddard/nasa-research-helps-unravel-mysteries-of-the-venusian-atmosphere/index.html&quot; rel=&quot;noreferrer&quot;&gt;Research Helps Unravel Mysteries Of The Venusian Atmosphere&lt;/a&gt;. Reproducing a quick, most relevant excerpt from it here for easier reference:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Interpreting what is happening in Venus's ionosphere requires&#xA;  understanding how Venus interacts with its environment in space. This&#xA;  environment is dominated by a stream of electrons and protons – a&#xA;  charged, heated gas called plasma -- which zoom out from the sun. As&#xA;  this solar wind travels it carries along embedded magnetic fields,&#xA;  which can affect charged particles and other magnetic fields they&#xA;  encounter along the way. Earth is largely protected from this&#xA;  radiation by its own strong magnetic field, but Venus has no such&#xA;  protection.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;What Venus does have, however, is an ionosphere, a layer of the&#xA;  atmosphere filled with charged particles. The Venusian ionosphere is&#xA;  bombarded on the sun-side of the planet by the solar wind.&#xA;  Consequently, the ionosphere, like air flowing past a golf ball in&#xA;  flight, is shaped to be a thin boundary in front of the planet and to&#xA;  extend into a long comet-like tail behind. As the solar wind plows&#xA;  into the ionosphere, it piles up like a big plasma traffic jam,&#xA;  creating a thin magnetosphere around Venus – a much smaller magnetic&#xA;  environment than the one around Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Video of Glyn Collinson, NASA's Goddard Space Flight Center in Greenbelt, MA, explaining these processes at work is published on &lt;a href=&quot;https://www.youtube.com/watch?v=9MVRMzmwubM&quot; rel=&quot;noreferrer&quot;&gt;Goddard Multimedia&lt;/a&gt; and &lt;a href=&quot;https://www.youtube.com/watch?v=9MVRMzmwubM&quot; rel=&quot;noreferrer&quot;&gt;YouTube&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-25T14:16:10.520" LastActivityDate="2015-01-25T14:16:10.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3656" PostTypeId="1" AcceptedAnswerId="3660" CreationDate="2014-02-10T15:42:28.777" Score="5" ViewCount="879" Body="&lt;p&gt;It's going to take a lot of money, time and resources to design &amp;amp; launch interstellar probes. It would certainly be helpful if we could develop a mission profile that would lead to an orbital capture rather than a flyby. There may be many exotic trajectories to insert a probe into an orbit, but it seems logical to me to start with the simplest: thread the needle right between the two stars with a speed less than escape velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: if my somewhat simple grasp of orbital mechanics is in error &amp;amp; this is not the simplest trajectory to achieve orbital insertion, I would appreciate an explanation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, for the purposes of this question, let's ignore the various schemes proposed for deceleration.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2014-02-10T19:13:54.457" Title="What is the escape velocity of the Alpha Centauri system at the binary's barycenter" Tags="|orbital-mechanics|interstellar-travel|trajectory|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3657" PostTypeId="2" ParentId="3631" CreationDate="2014-02-10T16:39:38.957" Score="5" Body="&lt;p&gt;Green arrows are the &quot;radius&quot; vectors, the inner one points to the center of the planet.&#xA;Red arrows are velocity vectors. One of them will point in the direction of motion of the satellite, the other one, of course, in the opposite direction.&#xA;Blue arrows point in the direction of angular momentum, a quantity often labelled as &quot;h&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radius vector is often called the R-bar and the velocity vector the V-bar. You see this often in discussing about vehicles docking with the International Space Station. When they say a vehicle is making an R-bar approach, it is essentially &quot;climbing up&quot; the green arrow from below (so the ship is between the Earth and the station). V-bar approaches, if I'm not mistaken, typically take place from behind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: AAE 532 at Purdue University, graduate level course in orbital mechanics. &lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2014-02-10T16:39:38.957" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3658" PostTypeId="2" ParentId="3631" CreationDate="2014-02-10T18:16:54.593" Score="3" Body="&lt;p&gt;During the &lt;a href=&quot;https://www.youtube.com/watch?v=k_OD2V6fMLQ&quot; rel=&quot;nofollow&quot;&gt;Apollo moon landings&lt;/a&gt;, the astronauts referred to 'forward' and 'down' for the red and green vectors.&lt;br&gt;&#xA;In the &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/GT12_TEC.PDF&quot; rel=&quot;nofollow&quot;&gt;Gemini 12 voice comms transcript&lt;/a&gt; (page 29 of a 500-page PDF), a maneuver is referred to as 'Posigrade up south' (in @Maxpm's diagram these directions refer to red, green, blue in that order).&lt;br&gt;&#xA;The spacecraft attitude is described as 'yaw 1 right, pitch 4 up', and they refer to the 'thrusters aft'.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-02-10T18:16:54.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3659" PostTypeId="1" CreationDate="2014-02-10T18:38:00.223" Score="10" ViewCount="670" Body="&lt;p&gt;I've found that using a Lambert solver together with the equations of the patched conic approximation for an interplanetary flight from Earth to Mars is quite inaccurate, when taking Earth's gravitational field into consideration. I guess this is to be expected since it is an &lt;em&gt;approximation&lt;/em&gt;, but this has got me thinking: How does one accurately calculate an interplanetary trajectory between two planets if the perturbative effects of the target, and more importantly, source planet are taken into account? I suppose one way to fix the trajectory once out of the Earth's sphere of influence would be to run the Lambert solver again and apply the required &lt;code&gt;delta-v&lt;/code&gt; to the spacecraft, but this seems like a very inefficient thing to do when fuel is limited. Are there more advanced methods than the Lambert + patched conic approach out there, and if so, does anyone have any links to papers written on these topics? Any help would be greatly appreciated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. This is the output I get from a Lambert solution when disregarding the gravitational effects of Earth and Mars (inner semi-circle is the orbit of Earth, outer semi-circle is the orbit of Mars, and the line joining the two is the spacecraft's trajectory):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Kuq21.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is the output I get from a Lambert + patched conic solution when taking the gravitational effects of Earth and Mars into accout:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/zud5c.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="1064" LastEditDate="2014-02-10T19:36:05.807" LastActivityDate="2014-02-10T20:04:17.460" Title="3-Body Simulation and Accuracy of Lambert Interplanetary Solver" Tags="|orbital-mechanics|interplanetary|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3660" PostTypeId="2" ParentId="3656" CreationDate="2014-02-10T18:49:55.850" Score="3" Body="&lt;p&gt;This is a good question, but I'm afraid that the answer isn't anything particular, it's only  constrained by the direction vector. In case of a vector exactly perpendicular to both bodies of the binary system, escape velocity is combined escape velocity of each of the two bodies at the distance to them. So:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{e}={\sqrt  {{\frac  {2GM_\alpha}{r_\alpha}}}} + {\sqrt  {{\frac  {2GM_\beta}{r_\beta}}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $G$ is the universal &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow&quot;&gt;gravitational constant&lt;/a&gt; $6.67384(80)\cdot 10^{{-11}}\ {\mbox{m}}^{3}\ {\mbox{kg}}^{{-1}}\ {\mbox{s}}^{{-2}}$. This function of ours is describing combined &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_potential&quot; rel=&quot;nofollow&quot;&gt;gravitational potential&lt;/a&gt; of two bodies at their common barycenter that we need to escape from, and is smallest with a direction vector exactly perpendicular to the axis between them, keeping their gravitational potential equipotent. Changing vector even slightly in direction of either one will only result in greater required escape velocity, since you start approaching one of them and decreasing radius $r$, while losing gravitational potential in the opposite vector by the other one, but it would be somewhat more forgiving in direction towards the less massive body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In our case, the common barycenter is the &lt;em&gt;saddle point&lt;/em&gt;, or $L_1$ Lagrange point at which the gravitational potential of the two bodies is equipotent. But to get that distance relative to both bodies, since &lt;a href=&quot;https://en.wikipedia.org/wiki/Alpha_Centauri&quot; rel=&quot;nofollow&quot;&gt;Alpha Centauri&lt;/a&gt; binary stars α Cen A and α Cen B (our binary system Alpha Centauri AB) aren't exactly of the same mass, we'll have to first calculate it. &lt;a href=&quot;https://en.wikipedia.org/wiki/Alpha_Centauri&quot; rel=&quot;nofollow&quot;&gt;Wikipedia on Alpha Centauri&lt;/a&gt; tells us of their distance between each other this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During the pair's 79.91-year orbit about a common center, the distance&#xA;  between them varies from about that between Pluto and the Sun to that&#xA;  between Saturn and the Sun.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is not too encouraging with respect to precision, but since we're lowering the bar already and calculating averages, it allows us to neglect gravitational effects of Proxima Centauri (Alpha Centauri C). So the α Cen AB are distant to each other from 778,547,200 km (5.204267 AU) up to 5,874,000,000 km (39.264 AU), which are semi-major axes of &lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter&quot; rel=&quot;nofollow&quot;&gt;Jupiter&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto&quot; rel=&quot;nofollow&quot;&gt;Pluto&lt;/a&gt;, respectively. Let's split this in geometric half, at 3,326,273,600 km (22.2341335 AU). Now, α Cen A has a mass of $M_\alpha = 1.100\ \mbox{M}_{\odot }$ and α Cen B $M_\beta = 0.907\ \mbox{M}_{\odot }$, where the unit is in &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_mass&quot; rel=&quot;nofollow&quot;&gt;solar masses&lt;/a&gt; $\mbox{M}_{\odot }=(1.98855\ \pm \ 0.00025)\ \cdot 10^{{30}}{\hbox{ kg}}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To find the distance to common barycenter, we'll have to solve a &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-body_problem&quot; rel=&quot;nofollow&quot;&gt;two-body problem&lt;/a&gt;. For the distance to the primary star, α Cen A, our solver looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$r_{\alpha}=a\cdot {M_{\alpha} \over M_{\alpha}+M_{\beta}}={a \over 1+M_{\alpha}/M_{\beta}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $a$ is the distance between the centers of the two bodies (3,326,273,600 km). Solving this, we get $r_{\alpha} = 1,823,069,735.92\ \mbox{km}$ and since $r_{\beta} = a - r_{\alpha}$, we get $r_{\beta} = 1,503,203,864.08\ \mbox{km}$. OK, we now have all the information we require to plug it in our first solver, and we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{e\alpha}={\sqrt  {{\frac  {2 \cdot 6.67384\cdot 10^{{-11}}\ {\mbox{m}}^{3}\ {\mbox{kg}}^{{-1}}\ {\mbox{s}}^{{-2}} \cdot 1.100 \cdot 1.98855 \cdot 10^{{30}}{\hbox{ kg}}}{1,823,069,735,920\ \mbox{m}}}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{e\beta}={\sqrt  {{\frac  {2 \times 6.67384\times 10^{{-11}}\ {\mbox{m}}^{3}\ {\mbox{kg}}^{{-1}}\ {\mbox{s}}^{{-2}} \cdot 0.907 \cdot 1.98855 \cdot 10^{{30}}{\hbox{ kg}}}{1,503,203,864,080\ \mbox{m}}}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or $v_{e\alpha} = 17,064.14\ {\mbox{m s}}^{{-1}}$ and $v_{e\beta} = 12,655.11\ {\mbox{m s}}^{{-1}}$. Assuming I plugged all the right numbers at all the right places, and there isn't any major flaw in my approach, we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{e} = v_{e\alpha} + v_{e\beta} = 29,719.25 \pm 469.90\ {\mbox{m s}}^{{-1}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are of course all just ballpark averages, and the escape velocity would lower when α Cen A and B are more distant to each other, and increase as they approach. It also doesn't include position of Proxima Centauri with respect to the Alpha Centauri binary and approach vector, so don't send anything there just yet. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-10T19:13:54.457" LastActivityDate="2014-02-10T19:13:54.457" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3661" PostTypeId="2" ParentId="3659" CreationDate="2014-02-10T20:04:17.460" Score="8" Body="&lt;p&gt;The solutions are found using fully integrated trajectories.  (Our Moon can be quite important too, if it's close to the outgoing trajectory at the time.  Though you always need to take into account the total GM of the Earth-Moon system when departing.  Jupiter is included as well, though I don't know how important that is.  Also the center of the Sun has a small orbit about the Solar System barycenter ...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The patched conic solutions provide a good starting point for finding the integrated solutions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll see if I can find any published references for the techniques used.  There are not a lot of people in the world who do this for a living.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-02-10T20:04:17.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3662" PostTypeId="2" ParentId="531" CreationDate="2014-02-10T20:20:22.807" Score="5" Body="&lt;p&gt;I highly recommend that you read a book.  Just search for &quot;astrodynamics&quot; or &quot;orbital mechanics&quot; on Amazon and read the reviews.  Pick one that best matches what you're looking for.  Which one doesn't matter that much -- what matters is getting a book, as opposed to what seems to be the fashion nowadays which is to use google, wikipedia, and ask questions on the interwebs.  The book approach is much more efficient and reliable.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-11T17:23:41.007" LastActivityDate="2014-02-11T17:23:41.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3663" PostTypeId="1" AcceptedAnswerId="3745" CreationDate="2014-02-11T15:27:24.653" Score="16" ViewCount="2082" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/646/what-are-the-choices-today-for-orbital-mechanics-simulation-software&quot;&gt;What are the choices today for orbital mechanics simulation software?&lt;/a&gt;, an entire range of software was listed for many platforms. I'd like to narrow this down a bit. Specifically, two questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Which of these software packages, if any, can perform trajectory analysis for deep-space probes, including calculating gravity assists? &lt;/li&gt;&#xA;&lt;li&gt;For those of you familiar with &lt;a href=&quot;http://www.nasa.gov/centers/johnson/copernicus/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Copernicus trajectory analysis package&lt;/a&gt;, how do these compare?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&quot;Deep-space&quot;, for the purposes of this question, includes currently known Trans-Neptunian Objects (TNO). Also, you need not limit yourself to the specific packages mentioned in the above referenced question.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-10-21T11:16:13.147" Title="What software can be used to determine deep-space trajectories?" Tags="|trajectory|deep-space|software|gravity-assist|mission-design|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3665" PostTypeId="1" AcceptedAnswerId="3666" CreationDate="2014-02-11T22:11:40.753" Score="6" ViewCount="1633" Body="&lt;p&gt;Is NASA developing a new generation os human transportation ship, or are they doing any kind of research and/or development which final aim is to invent a new space ship model?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I know well, NASA wants to change the Space Shuttle model into something else, but I'm not sure if are they even working on creating one.&lt;/p&gt;&#xA;" OwnerUserId="306" LastActivityDate="2014-02-14T16:14:48.393" Title="Do NASA currently (early 2014) have any development projects for a better space ship?" Tags="|nasa|space-shuttle|spacecraft-development|humans|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3666" PostTypeId="2" ParentId="3665" CreationDate="2014-02-11T22:22:52.440" Score="8" Body="&lt;p&gt;NASA is funding &lt;a href=&quot;http://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion&lt;/a&gt; for their program of deep space exploration. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/9taG7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 4 person capsule designed for deep space missions (Beyond Low Earth Orbit) service module fluctuates between NASA and ESA (based on ATV). It will launch on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Launch System&lt;/a&gt;, a vaguely shuttle derived booster, that is set to fly once every 2 to 4 years, so is likely to not succeed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JD9et.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA is also funding the &lt;a href=&quot;http://en.wikipedia.org/wiki/CCiCap#Commercial_Crew_integrated_Capability&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCiCAP&lt;/a&gt; program, in which private companies design spaceships on which NASA will purchase seats (as opposed to Orion, which will be owned and operated by NASA). These ships are mostly designed to service destinations in LEO (Low Earth Orbit) like the International Space Station and potential future commercial space stations, such as the ones being developed by Bigelow Aerospace.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CCiCAP is currently funding 3 companies: SpaceX, Sierra Nevada Corporation (SNC), and Boeing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt; with &lt;a href=&quot;http://en.wikipedia.org/wiki/Dragon_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;DragonRider&lt;/a&gt; (maybe to be called Dragon 2.0) is probably the leading contender, since the launcher (&lt;a href=&quot;http://en.wikipedia.org/wiki/Falcon_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9&lt;/a&gt;) and the basic capsule design (&lt;a href=&quot;http://en.wikipedia.org/wiki/Dragon_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dragon&lt;/a&gt;) have flown a number of times now. (F-9 8 times, Dragon 4 times) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/48RyR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a vertical take off capsule, on a potentially reusable booster (Falcon 9-R) that will land propulsively on land. It will carry a crew of 7. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Sierra_Nevada_Corporation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sierra Nevada Corp (SNC)&lt;/a&gt; with &lt;a href=&quot;http://en.wikipedia.org/wiki/Dream_Chaser&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dream Chaser&lt;/a&gt; is the only horizontal lander in the bunch launching atop an Atlas 5 402 booster. (It is worth noting, the 402 variant has never flown.  The 4 means 4 meter fairing, 0 strap on boosters, and 2 RL-10 engines in the second stage. The 401 variant has flown more than 20 times, but never the 402 variant).  They have test dropped a full scale model, which executed the landing approach perfectly, but one of the landing gear did not deploy, so on landing on a runway it flipped over.  The landing gear were not the ones they intend to use for real, it was a temporary stand in just for this test.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0A1gD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Boeing&quot; rel=&quot;nofollow noreferrer&quot;&gt;Boeing&lt;/a&gt; is developing the &lt;a href=&quot;http://en.wikipedia.org/wiki/CST-100&quot; rel=&quot;nofollow noreferrer&quot;&gt;CST-100&lt;/a&gt;, a 7 person capsule design, launching on a Atlas 5 401 variant, and landing using parachutes and air bags on land. They have tested air dropping a capsule to land by parachute and air bags.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/V2tdR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Blue_Origin&quot; rel=&quot;nofollow noreferrer&quot;&gt;Blue Origin&lt;/a&gt; was funded during an earlier portion of CCiCAP (known as CCDev), proposing a &quot;biconic&quot; capsule which would land vertically, but would generate some extra lift on descent, providing a smoother ride for the occupants. Initially they planned on an Atlas V as well, but they intend to build their own reusable launcher at some point.  Their vehicle is shrouded in mystery with very few reliable details available at this time. They also did not submit a proposal for CCiCAP, so they are effectively out of the running for the near to medium term.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="1255" LastEditDate="2014-02-14T16:14:48.393" LastActivityDate="2014-02-14T16:14:48.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3667" PostTypeId="1" AcceptedAnswerId="3669" CreationDate="2014-02-11T23:03:17.240" Score="4" ViewCount="1081" Body="&lt;p&gt;There's a running Kickstarter to build a HARP-like gun that is multi-chambered and can fire a payload into &quot;space&quot;.  I don't see where this person's criteria for space is, except for the fact that it's intended to be suborbital.  I would assume this means that the payload gets over 100 km high.  As I understand it, HARP did &lt;em&gt;not&lt;/em&gt; fire up to the Karman line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.kickstarter.com/projects/1682852725/the-starfire-space-cannon&quot; rel=&quot;nofollow&quot;&gt;https://www.kickstarter.com/projects/1682852725/the-starfire-space-cannon&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thinking about the numbers, the minimum delta v should be determined by high school level kinematics:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ h = v_0 t - \frac{1}{2} g t^2 \\&#xA;v = 0 = v_0 - g t \\&#xA;v = \sqrt{ 2 h g } \approx 1.4 \frac{km}{s} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this would be difficult to achieve with a single chamber, it is on the very very far end of &lt;a href=&quot;http://hypertextbook.com/facts/1999/MariaPereyra.shtml&quot; rel=&quot;nofollow&quot;&gt;common muzzle velocities&lt;/a&gt;.  Searching around for it, &lt;a href=&quot;http://defense-update.com/products/digits/120ke.htm&quot; rel=&quot;nofollow&quot;&gt;tank guns look like they can get&lt;/a&gt; 1,750 m/sec.  Of course, I've left out the hard part - air resistance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any existing weapons that, if aimed upward, would reach the Karman line?  And are there any documented examples of this?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-02-13T17:44:44.377" Title="Could any existing gun reach the Karman Line?" Tags="|sub-orbital|space-gun|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3668" PostTypeId="2" ParentId="2670" CreationDate="2014-02-12T00:49:18.520" Score="4" Body="&lt;p&gt;Lots of plans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) The Canadians are apparently &lt;a href=&quot;http://www.popsci.com/technology/article/2011-03/first-space-gas-station-launch-2015-servicing-geosynchronous-satellites&quot; rel=&quot;nofollow noreferrer&quot;&gt;working on a spacecraft&lt;/a&gt; that is part space tug, part robo-mechanic &amp;amp; part filling station:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After years of planning, Canadian company MacDonald Dettwiler and Associates (MDA) announced it is building the first space gas station, with plans for a 2015 launch. The Space Infrastructure Servicing vehicle will fly in geosynchronous orbit, where it can reach several key commercial and government satellites. It will be able to bring them extra fuel as well as reposition them or perform basic maintenance, according to MDA.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The communications satellite company Intelsat, which has the most geosynchronous satellites, will be the first client.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Early last year &lt;a href=&quot;http://en.wikipedia.org/wiki/Dextre&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dextre&lt;/a&gt; tested a package sent to the ISS with (it looks like) dozens of different satellite parts: &lt;a href=&quot;http://www.spaceflight101.com/robotic-refueling-mission.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spaceflight101.com/robotic-refueling-mission.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Lots of &lt;a href=&quot;http://en.wikipedia.org/wiki/Propellant_depot#History_and_plans&quot; rel=&quot;nofollow noreferrer&quot;&gt;plans and studies&lt;/a&gt; from NASA and others:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In August 2011, NASA made a significant contractual commitment to the development of propellant depot technology by funding four aerospace companies to &quot;define demonstration missions that would validate the concept of storing cryogenic propellants in space to reduce the need for large launch vehicles for deep-space exploration.&quot; These study contracts for storing/transferring cryogenic propellants and cryogenic depots were signed with Analytical Mechanics Associates, Boeing, Lockheed Martin and Ball Aerospace. Each company will receive US$600,000 under the contract.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;3) One of the main rationales behind &lt;a href=&quot;https://space.stackexchange.com/questions/3033/is-there-any-non-rocket-spacelaunch-concept-within-reach-of-current-material-sci/3034#3034&quot;&gt;John Hunter's light gas gun&lt;/a&gt; was to shoot fuel cylinders into orbit (with the help of a rocket assist near the top of the projectile's parabola). These would be collected at depots at a predetermined orbit and then be available to craft to refuel for injection burns and/or to satellites for enhanced stationkeeping.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-12-05T02:42:05.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3669" PostTypeId="2" ParentId="3667" CreationDate="2014-02-12T01:21:08.457" Score="7" Body="&lt;p&gt;Yes, &lt;a href=&quot;http://en.wikipedia.org/wiki/Spacegun&quot; rel=&quot;nofollow&quot;&gt;it's been done&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the Project HARP a U.S. Navy 16 in (410 mm) 100 caliber gun was used to fire a 180 kg (400 lb) slug at 3600 m/s or 12,960 km/h (8,050 mph), reaching an apogee of 180 km (110 mi), hence performing a suborbital spaceflight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Or, if you follow &lt;a href=&quot;http://www.astronautix.com/articles/abroject.htm&quot; rel=&quot;nofollow&quot;&gt;this link&lt;/a&gt; from the Kickstarter campaign you mention (Graf's excellent history of the HARP project):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The third and final 16-inch gun of the HARP program was installed at the Yuma proving grounds in Arizona. . . . . The Yuma Gun's sole claim to fame was that on November 18, 1966 it lofted a Martlet 2 vehicle to a world record altitude of 180 km, which still stands today.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-02-12T14:49:26.433" LastActivityDate="2014-02-12T14:49:26.433" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3670" PostTypeId="1" AcceptedAnswerId="3671" CreationDate="2014-02-12T11:33:09.883" Score="9" ViewCount="6806" Body="&lt;p&gt;In the question &lt;a href=&quot;https://space.stackexchange.com/questions/768&quot;&gt;Will Rosetta have to adjust its orbit around Chury due to the comet&amp;#39;s coma and tails?&lt;/a&gt; and related answers it is implied that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta spacecraft&lt;/a&gt; will actually orbit &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow noreferrer&quot;&gt;67P/Churyumov–Gerasimenko&lt;/a&gt;.  When I think of orbits I think of the high speeds and large bodies as a gravity base for the orbit; Sun, planets and such.  A &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet#Tails&quot; rel=&quot;nofollow noreferrer&quot;&gt;comet has a tail&lt;/a&gt; because of all of the stuff falling off and falling behind the body of the comet as it orbits the sun. Anything orbiting it seems counter intuitive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will Rosetta actually orbit the comet or is that just a figure of speech?  Is there some minimum size of body (&lt;em&gt;in our solar system&lt;/em&gt;) that can be orbited? Is the smallest size based on the relationship between the two bodies size?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2019-12-03T23:48:39.720" Title="What is the smallest body that has sufficient gravity for another body to orbit it?" Tags="|orbital-mechanics|comet|rosetta|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3671" PostTypeId="2" ParentId="3670" CreationDate="2014-02-12T11:48:11.573" Score="16" Body="&lt;p&gt;There isn't any limit to how small bodies can orbit each other (gravity-wise) until you get to atomic scale where one of the remaining three fundamental forces (&lt;a href=&quot;https://en.wikipedia.org/wiki/Weak_interaction&quot; rel=&quot;nofollow noreferrer&quot;&gt;weak force&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Strong_interaction&quot; rel=&quot;nofollow noreferrer&quot;&gt;strong force&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Electromagnetism&quot; rel=&quot;nofollow noreferrer&quot;&gt;electromagnetism&lt;/a&gt;) take over and &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitation&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity&lt;/a&gt; becomes largely irrelevant. With smaller bodies, gravitational potential will only be that much smaller and required centrifugal force to precisely counter it and achieve orbit (&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital speed&lt;/a&gt;) will equally decrease.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta&lt;/a&gt; will orbit &lt;em&gt;Churi&lt;/em&gt; but its orbital speed relative to the parent body won't be much. Orbital velocity of two bodies where mass of the orbiting body is non-negligible relative to the parent body it orbits around is given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{o}\approx {\sqrt  {(M_{2})^{2}G \over (M_{1}+M_{2})r}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $M_1$ is the mass of the orbiting body and $M_2$ mass of the parent body, $G$ is &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational constant&lt;/a&gt;, and $r$ is the distance between their centers of mass (or their &lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-major_axis&quot; rel=&quot;nofollow noreferrer&quot;&gt;semi-major axis&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There might be other interactive forces at play though (mentioned weak force, strong force and electromagnetism), say atmospheric drag (or should that be &lt;em&gt;exoatmospheric&lt;/em&gt;?) due to comet's outgassing when selected orbit is close enough to pass through its tails and coma, but in case of &lt;em&gt;Churi&lt;/em&gt; and Rosetta &lt;a href=&quot;https://space.stackexchange.com/a/1005/49&quot;&gt;we don't know to what extent yet&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-02-12T12:09:07.077" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3672" PostTypeId="2" ParentId="1308" CreationDate="2014-02-12T11:49:47.423" Score="4" Body="&lt;p&gt;Actually it's not dangerous if the tether breaks, at least: the spacecraft would fly apart at only a few mph and would be easy to get back together again. And the tether only needs to be strong enough to hold the weight of the habitat under gravity; not a super material, just like the sort of cables you have on cranes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plus you could have multiple smaller stays like a suspension bridge, I suggested idea here of a tube you can roll up with internal stays so also doubles as ways to connect the two modules and perhaps also if wide enough, greenhouse space for plants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might like &lt;a href=&quot;http://www.science20.com/robert_inventor/blog/can_spinning_habs_solve_zero_g_health_issues_can_humans_live_mars_or_lunar_g_so_far_nobody_knows-129424&quot; rel=&quot;nofollow noreferrer&quot;&gt;my blog post about all this&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="6816" LastEditDate="2016-12-07T04:54:51.563" LastActivityDate="2016-12-07T04:54:51.563" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="3673" PostTypeId="2" ParentId="3670" CreationDate="2014-02-12T12:47:50.243" Score="7" Body="&lt;p&gt;Agreed, orbiting can happen at pretty much any scale. To give an idea, 67P/Churyumov–Gerasimenko has an &lt;a href=&quot;http://phys.org/news/2014-01-philae-probe-67p-churyumov-gerasimenko-comet.html&quot; rel=&quot;noreferrer&quot;&gt;escape velocity&lt;/a&gt; of about 1 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(removed reference to ISS experiment because that didn't rely on gravity)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quick experiment: for bodies that weigh 1 kg each and a radius of 1 m, the formula given by @Tildalwave yields an orbital speed on the order of $10^{-11} m/s$. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-02-12T16:21:16.887" LastActivityDate="2014-02-12T16:21:16.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3674" PostTypeId="2" ParentId="3670" CreationDate="2014-02-12T13:23:40.897" Score="6" Body="&lt;p&gt;(Consider this a spaceholder until I get around to the hard part of the answer)&lt;br&gt;&#xA;Principally there's no lower size above atom scale. But I would say that for stable orbiting, the gravitational force should be the dominant force compared to solar wind etc. This would depend qualitavly on these factor:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mass of the 'central' body - the heavier the less relative influence from solar wind and light&lt;/li&gt;&#xA;&lt;li&gt;Size of the orbiting body - the smaller and less dense, the bigger the impact of solar wind and light&lt;/li&gt;&#xA;&lt;li&gt;Position within the solar system - the closer to the sun, the bigger the influence of solar wind and light&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'll see if I quantify these assertions (or someone beats me to it!).&lt;/p&gt;&#xA;" OwnerUserId="102" LastActivityDate="2014-02-12T13:23:40.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3675" PostTypeId="1" AcceptedAnswerId="3677" CreationDate="2014-02-12T13:59:55.087" Score="7" ViewCount="1640" Body="&lt;p&gt;A recent answer included this graphic of projected SLS versions:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/JD9et.png&quot; alt=&quot;SLS versions&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even the smaller version is capable of lifting 70 tons to orbit. Is there a reason other than politics to use a rocket that powerful, for launching a crew capsule that weighs in the region of &lt;a href=&quot;http://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;20 tons&lt;/a&gt;? At first glance, this seems hugely wasteful, not to mention possibly problematic: they'd have to throttle back the main engines to avoid excessive g-loads. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-02-12T15:26:24.363" Title="What is the rationale behind the manned versions of SLS?" Tags="|launch|rockets|nasa|orion-spacecraft|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3676" PostTypeId="1" AcceptedAnswerId="3682" CreationDate="2014-02-12T14:01:11.710" Score="14" ViewCount="3171" Body="&lt;p&gt;One of the set goals for NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Frontiers_program&quot;&gt;New Frontiers&lt;/a&gt; Juno mission is studying Jupiter's interior and determining if this gas giant has a solid core and if, how large it is. This is mentioned several times on &lt;a href=&quot;http://www.nasa.gov/mission_pages/juno/overview/&quot;&gt;NASA's Juno page&lt;/a&gt;, for example:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Juno will measure the amount of water and ammonia in Jupiter's&#xA;  atmosphere and determine if the planet actually has a solid core,&#xA;  directly resolving the origin of this giant planet and thereby the&#xA;  solar system. By mapping Jupiter's gravitational and magnetic fields,&#xA;  Juno will reveal the planet's interior structure and measure the mass&#xA;  of the core.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But assuming Jupiter does indeed have a solid core, how will the size of it be established? Will this be down to statistical data from its vector magnetometer (MAG) and determining where the magnetic reconnection occurs? Or by any other means?&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-03-02T18:42:35.293" Title="How will Juno establish existence of solid core within Jupiter and determine its size?" Tags="|nasa|planetary-science|magnetic-field|jupiter|juno|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3677" PostTypeId="2" ParentId="3675" CreationDate="2014-02-12T14:25:12.777" Score="5" Body="&lt;p&gt;The 70ton capacity of the SLS Block 1 is measured for low-earth orbit. But getting people just into LEO isn't the goal of the SLS. It is designed for missions beyond earth orbit to destinations like Moon or Mars. The Orion capsule itself hasn't got enough fuel to get to the moon and back on its own. It has a delta-v capacity of less than 2000 m/s. &lt;a href=&quot;https://space.stackexchange.com/questions/2103/how-to-calculate-delta-v-for-lunar-flyby&quot;&gt;When you want to get from low earth orbit to the moon, you need about twice as much&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The additional 50 tons of capacity are to be used for a transfer stage to get the SLS into lunar orbit. This is planned for the &lt;a href=&quot;http://www.nasaspaceflight.com/2012/02/exploration-mission-1-sls-orion-debut-mission-moon-outlined/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Exploration Missions 1 and 2&lt;/a&gt;. Which upper stage will be used for this hasn't been decided yet. NASA lists the requirements as follows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Requirements: Human Rated or Human Ratable for Space Systems. Burns&#xA;  (accelerating Orion/MPCV) following placement of ICPS and MPCV at&#xA;  insertion point by SLS. Three engine ignitions to achieve greater than&#xA;  3050 m/s delta-V. ICPS Lift Capability total weight of 24224 kg (53404&#xA;  lbs). ICPS Mass less than 71400 lbs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;When the transfer stage weights 32 tons max and the requirements say it needs to push 24 tons, that still leaves 14 tons of unused LEO capacity on the SLS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not sure if the payload fairings and truss structures to connect the transfer stage to the rocket are already accounted for, but when they aren't that would likely be another ton or two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other uses for the excess payload capacity could be secondary payloads in form of unmanned satellites. But even when you do make use of this, keep in mind that manned missions, and especially those using a new launch vehicle for the first time, should always have some  tolerance.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-02-12T15:26:24.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3678" PostTypeId="1" AcceptedAnswerId="3687" CreationDate="2014-02-12T15:09:09.760" Score="4" ViewCount="365" Body="&lt;p&gt;Most launches from the Kennedy Space Center and Cape Canaveral Air Force Base drop their lower stages into the Atlantic ocean. The majority of these stages is not recovered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the environmental impact on the marine ecosystem near the Florida coast?&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2014-02-12T20:09:58.540" Title="Environmental impact of the Kennedy Space Center" Tags="|debris|environment|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3679" PostTypeId="2" ParentId="3675" CreationDate="2014-02-12T15:12:50.493" Score="5" Body="&lt;p&gt;Orion, the official current, manned payload for SLS is pretty heavy. Heavy enough that a Delta-4 Heavy is about the only American vehicle currently flying that could otherwise launch it.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But Orion is not meant for missions to LEO, everyone is agreed that would be a stupendous waste of money.  It is meant for BEO (Beyond Earth Orbit) missions, like Lagrange points, asteriods, the Moon, and maybe Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you launch to areas in BEO, you need more propellant to get there.  The Service Module of the Orion is quite heavy as well, and clearly can carry more or less propellant, within its capacity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus a third stage of sorts, to propel the vehicle to its destination is needed.  Perhaps that is the Service Module, perhaps it is a second (or more) firing of the SLS upper stage.  (What do the SRB's count as?  Stage 0 usually..) &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-02-12T15:12:50.493" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3680" PostTypeId="1" AcceptedAnswerId="3684" CreationDate="2014-02-12T15:40:34.427" Score="10" ViewCount="839" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/3667/could-any-existing-gun-reach-the-karman-line/&quot;&gt;Could any existing gun reach the Karman Line?&lt;/a&gt;, Richard Graf's Kickstarter campaign for his Starfire space-gun is referenced &amp;amp; discussed. Setting aside my native skepticism for a project with such scanty technical details, I have another problem: anytime I hear the word &quot;suborbital,&quot; the words that follow come out &quot;blah blah blah.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's face it, most useful science that can be accomplished through suborbital flights has been done years ago, and, unless you're going to make your space-gun many, many kilometers long, it's not going to be suitable for space tourism. From &lt;a href=&quot;https://www.youtube.com/watch?v=BRCGylNe_mM&amp;amp;feature=c4-overview&amp;amp;list=UUjJdNqYgYW7WL5U7oEtJRZA&quot; rel=&quot;nofollow noreferrer&quot;&gt;John Hunter's presentation this past summer at the Starship Congress&lt;/a&gt; (starting at 37:40), we hear that you cannot fire a projectile high enough for it to not eventually come down somewhere. So, if space-gun technology is ever to reach LEO, a boost near the top of the projectile's parabola needs to be employed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume two things to make it simple:&#xA;1) The max G-force sustained by the booster/projectile will be 10,000 Gs (Graf's Starfire claim),&#xA;2) To reduce moving parts, let's make it a solid fuel rocket.&#xA;Can such a booster be designed to strap on the back of our projectile with current technology? &lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-08-25T18:50:58.563" Title="Can a booster be designed to withstand 10,000 Gs?" Tags="|orbital-mechanics|rockets|engine-design|space-gun|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3681" PostTypeId="2" ParentId="3680" CreationDate="2014-02-12T16:01:05.267" Score="3" Body="&lt;p&gt;In general, all proposals I have ever seen require a kick motor to circularize the orbit, and they all assume solid rockets because of the G loading. A launch gun, can never launch to orbit, without something to provide that kick, into an actual orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is very hard to imagine a liquid fueled engine surviving that kind of load.  Heck it is hard to imagine any structure surviving 10K G's well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some interesting new low thrust designs out there, that are MEMS or 3-D printed with the goal of being able to use non-toxic propellants.  Having very few moving parts make it more likely this might be able to survive.  (Things scale funny between micro and macro levels.  Consider the case of ants ability to lift things, compared to their mass, but would crush them at a larger scale).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-02-12T16:01:05.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3682" PostTypeId="2" ParentId="3676" CreationDate="2014-02-12T16:28:49.650" Score="7" Body="&lt;p&gt;Turns out the way they're going to do that is quite a bit different than my initial thoughts, all neatly explained by Bill Nye in one of his &lt;em&gt;Why With Nye&lt;/em&gt; videos he did for NASA's Juno mission:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://www.youtube.com/watch?v=Ynzfjgtdius&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OgTZP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Gravity perturbed orbit of the Juno spacecraft as it orbits planet Jupiter&lt;/strong&gt; (video: &lt;a href=&quot;https://www.youtube.com/watch?v=Ynzfjgtdius&quot; rel=&quot;noreferrer&quot;&gt;Bill Nye Asks Does Jupiter Have A Core?&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The video is quite fun, so click on the image above and all will be revealed. For the textophiles among you though, the explanation goes like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Core or not&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;We don't know what's inside Jupiter. Jupiter could have a core made of&#xA;  heavy metal, that's sunk to the center. Or it might be, that Jupiter&#xA;  has this exotic material that's hydrogen, squeezed under such a&#xA;  pressure, that the hydrogen acts like a metal. &lt;a href=&quot;https://en.wikipedia.org/wiki/Metallic_hydrogen&quot; rel=&quot;noreferrer&quot;&gt;Metallic hydrogen&lt;/a&gt;. Now,&#xA;  we can't cut Jupiter in half. We don't have that capability. Instead,&#xA;  we have a happy little spacecraft in orbit around Jupiter.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;[I'm a little spacecraft ...]&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Now, as it orbits, we strongly believe there will be slight variations&#xA;  in its path due to tiny variations in the gravity of Jupiter. And that&#xA;  will help us figure out what's inside. Now, how would we measure the&#xA;  tiny variations of the orbit of a spacecraft around a great big&#xA;  planet, millions of kilometers from here? We would do it by measuring&#xA;  the frequency of radio waves. Radio waves coming from the spacecraft,&#xA;  and radio waves coming from Earth. We can measure those variations&#xA;  using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Doppler_effect&quot; rel=&quot;noreferrer&quot;&gt;Doppler effect&lt;/a&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;[Sets alarm to go off on an old alarm clock and spins it around on a string]&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Listen to the sound, as the alarm clock moves towards you. Now listen&#xA;  to the sound as the alarm clock moves away. As it's moving towards&#xA;  you, the sound gets higher-pitched. As it's moving away, the sound&#xA;  gets lower-pitched. Now this effect, or this phenomenon was described&#xA;  at first by a man named &lt;a href=&quot;https://en.wikipedia.org/wiki/Christian_Doppler&quot; rel=&quot;noreferrer&quot;&gt;Christian Doppler&lt;/a&gt;. And to this day, my&#xA;  brother would like to have a rock'n'roll group called &lt;em&gt;Christian&#xA;  Doppler and The Effects&lt;/em&gt;. But my brother's first name isn't Christian.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So, we believe that precisely measuring the path of Juno as it orbits&#xA;  Jupiter, using the Doppler effect, we'll be able to determine from&#xA;  Jupiter's gravity the nature of what's inside. Is it heavy metal? Is&#xA;  it metallic hydrogen? Or is it ...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;[Raaaaaaaaaaaaa!!!!!!]&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, I just transcribed the video!! Raaaaaaaaaaaaa!!!!!!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(A bit more &lt;em&gt;conventional&lt;/em&gt; explanation is also available &lt;a href=&quot;http://juno.wisc.edu/science_interior.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-12T16:42:26.123" LastActivityDate="2014-02-12T16:42:26.123" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3683" PostTypeId="2" ParentId="3670" CreationDate="2014-02-12T17:07:26.740" Score="0" Body="&lt;p&gt;I would say that it is possible for an object (call it A) in the gravity well of another, more massive object (B) to be too small to allow any third object (C) to orbit A, because any object C small enough for the mutual centre of gravity of A and C to lie outside A would be captured by B.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know enough about it to be able to give you the numbers though, or to calculate whether this applies to the Rosetta/Chury/Churyumov–Gerasimenko situation.&lt;/p&gt;&#xA;" OwnerUserId="2647" LastActivityDate="2014-02-12T17:07:26.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3684" PostTypeId="2" ParentId="3680" CreationDate="2014-02-12T17:18:37.183" Score="13" Body="&lt;p&gt;I'd expect so.  You should be able to design and test a solid rocket to take those G's.  The tricky part is to &lt;a href=&quot;http://www.google.com/patents/US3664133&quot;&gt;design a structure that keeps the grain from trying to detach&lt;/a&gt;.  It would just take money, time, and lots of testing.  &lt;strong&gt;The rocket motor should be facing backwards so the propellant is pushed back into the casing where it's bonded.&lt;/strong&gt;  Interestingly, that is the correct direction if the projectile is spin stabilized.  By the time it gets to the other side of the planet it will be pointing the right way to circularize the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are already &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a443252.pdf&quot;&gt;electronics that can survive those and higher accelerations&lt;/a&gt;, e.g. 30,000 G's.  The applications are, of course, for artillery.  There is even &lt;a href=&quot;http://www.militaryaerospace.com/articles/2012/08/hyper-velocity-weapon.html&quot;&gt;work on guidance systems that can survive those G's&lt;/a&gt;.  It's not that hard to design structures to take those G's.  It just takes mass, and a lot of testing to find the weak points.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-12T20:11:26.843" LastActivityDate="2014-02-12T20:11:26.843" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3685" PostTypeId="1" AcceptedAnswerId="3686" CreationDate="2014-02-12T18:44:53.457" Score="10" ViewCount="2150" Body="&lt;p&gt;Apropos the RD-180, &lt;a href=&quot;http://en.wikipedia.org/wiki/RD-180&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It features a dual-combustion chamber, dual-nozzle design and is fueled by a kerosene/liquid oxygen mixture&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What is a dual-combustion chamber design?&lt;/li&gt;&#xA;&lt;li&gt;What advantage/s if any accrue from such multiple combustion chambers?&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are more than 2 combustion chambers in an engine practical?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2019-03-25T00:37:52.443" Title="What is a dual-combustion chamber design?" Tags="|propulsion|engines|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3686" PostTypeId="2" ParentId="3685" CreationDate="2014-02-12T19:06:38.597" Score="13" Body="&lt;p&gt;The RD-180 is really half an RD-170/RD-171 which has 4 combustion chambers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Cg8vi.jpg&quot; alt=&quot;RD-170 with 4 thrust chambers&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For scale, here is an RD-180 with a person next to it.  The RD-170 is basically the same size, just 4 instead of 2 thrust chambers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jlXUF.jpg&quot; alt=&quot;RD-180 with person for scale&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RD-17x family is vaguely in the same thrust class as the F-1.  Combustion stability issues plagued the F-1 design, and took a lot of work to get going.  With great success, in the end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians/Soviets did not have such success, and they took a short cut.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were capable of building a turbo pump for a 1.5 Million lb thrust engine, but could not deliver a sufficient single thrust chamber.  Thus the shortcut of one turbopump (or two, one for propellant other for oxidizer) feeding into 4 (or 2 in the case of the RD-180) chambers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At a smaller scale, I think the &lt;a href=&quot;http://en.wikipedia.org/wiki/RD-107&quot; rel=&quot;noreferrer&quot;&gt;RD-107&lt;/a&gt; used on the Soyuz launcher is a much smaller engine, 183KLbs thrust that uses one set of turbo pumps and 4 thrust chambers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xb2PQ.jpg&quot; alt=&quot;RD-107 engine with 4 thrust chambers&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And a RD-107A with a person next to it for scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BRqsM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see on the bottom of the Soyuz booster, that launches Soyuz/Progress spacecraft, what looks like 20 engines, but is realy 20 thrust chambers, and 5 engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RBIlz.jpg&quot; alt=&quot;Bottom of Soyuz booster&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stability issues were much easier to resolve in many smaller chambers, than in one huge one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just for fun, compare the size of an F-1 engine (Which alas has good scale markers (people) but my other images do not really have them).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5lR0O.jpg&quot; alt=&quot;F-1 with people in front of it&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-02-12T23:50:49.857" LastActivityDate="2014-02-12T23:50:49.857" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3687" PostTypeId="2" ParentId="3678" CreationDate="2014-02-12T20:09:58.540" Score="4" Body="&lt;p&gt;Generally projects and programs need to publicly file Environmental Impact Statements or Environmental Assessments, since the environmental impact of the activities at a place like Kennedy Space Center change over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One example is the &lt;a href=&quot;http://www.nasa.gov/pdf/207909main_Cx_PEIS_final.pdf&quot; rel=&quot;nofollow&quot;&gt;EIS written for Constellation&lt;/a&gt;, which planned to drop some pretty big stuff into the drink.  One paragraph from that document states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA has consulted with the National Marine Fisheries Service (NMFS)&#xA;  on essential fish habitat regarding launches of Ares vehicles from KSC&#xA;  (KSC 2007e).  NASA has indicated to NMFS that over more than 25 years&#xA;  of Space Shuttle operations, there have been no documented long-term&#xA;  impacts to marine life or marine habitats.  Similarly, the proposed&#xA;  Constellation Program launches are not expected to produce any&#xA;  measurable impacts to marine species or habitats.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-02-12T20:09:58.540" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3688" PostTypeId="1" AcceptedAnswerId="3690" CreationDate="2014-02-13T00:17:13.087" Score="5" ViewCount="267" Body="&lt;p&gt;The landing procedure of Curiosity on Mars has been described as a real challenge in part because of the powered descent stage and sky crane. Indeed the separation of Curiosity with the backshell + parachute occurred at an altitude of 1.8km at a velocity of 100m/s. The powered descent stage has then brought the sky crane with Curiosity to a stop at an altitude of a couple of meters and then hovered for about 15 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any tests on Earth of the powered descent stage's drop and hovering before sending it to Mars ? and powered descent stage + sky crane tests ? Are there any videos of these tests?&lt;/p&gt;&#xA;" OwnerUserId="2650" LastActivityDate="2014-02-13T01:16:16.900" Title="Curiosity's powered descent stage test" Tags="|mars|curiosity|landing|testing|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3689" PostTypeId="2" ParentId="3688" CreationDate="2014-02-13T00:50:01.813" Score="5" Body="&lt;p&gt;If the sky crane was ever tested on Earth, there's no public footage of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are test videos for other parts of the descent phase:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.youtube.com/watch?v=YasCQRAWRwU&quot; rel=&quot;noreferrer&quot;&gt;Landing System Drop Test (lowering the rover)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.youtube.com/watch?v=O7vf2HUMMdo&quot; rel=&quot;noreferrer&quot;&gt;Testing Curiosity's Parachute&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="499" LastActivityDate="2014-02-13T00:50:01.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3690" PostTypeId="2" ParentId="3688" CreationDate="2014-02-13T01:16:16.900" Score="7" Body="&lt;p&gt;No, there were no tests of a descent stage with rockets firing on (or slightly above) Earth.  That may have been one part of the reason that there was so much cheering, jumping, hugging, and crying when the thing actually worked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The system as a whole was validated and verified through many subsystem tests and analyses, tied together in a detailed computer simulation of the event.  @Maxpm's answer has links for two of those.  Others such tests include radar tests on aircraft in the desert, heatshield coupon testing in arcjets, various separation tests, etc.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-02-13T01:16:16.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3692" PostTypeId="2" ParentId="3667" CreationDate="2014-02-13T17:44:44.377" Score="2" Body="&lt;p&gt;I was hoping that I could say this is physically feasible, but after running the numbers, it unfortunately seems not.  Let me start by listing the various claims on the Kickstarter:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Bore diameter: 8 inches&lt;/li&gt;&#xA;&lt;li&gt;Maximum pressure: 60,000 psi&lt;/li&gt;&#xA;&lt;li&gt;Tube length: 45 feet&lt;/li&gt;&#xA;&lt;li&gt;Altitude: 274.9 m above sea-level&lt;/li&gt;&#xA;&lt;li&gt;Muzzle velocity: 1500 m/s (mentioned in comments, I may relax this later)&lt;/li&gt;&#xA;&lt;li&gt;Maximum altitude of the payload is taken to be 100 km by the &quot;space&quot; qualifier&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The bore diameter kills it.  You can't do it with these parameters.  I'll show this by establishing a maximum and minimum mass for the shot based on obvious physical constraints.  I will show that these requirements conflict, and can not be satisfied by this device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Maximum mass&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is simple, because we're given a maximum pressure from combustion.  The core concept of the project is that it will be multi-chambered.  All this does is allow them to sustain that driving pressure over a longer segment of the barrel than a single combustion gun would allow.  So the actual pressures might be lower than this, but they can't be higher.  This easily constrains the kinetic energy that the projectile can have.  The calculation below is simple, but this only introduces conservatism.  I'm not introducing back-pressure from the air or anything - so I'm being as generous as possible here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ E = F \times d = \left( P A \right) \times d = (60,000 \text{ psi})\pi(4\text{ in})^2(45 \text{ ft}) = 184 MJ $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems simple enough.  If we take the muzzle velocity to be 1500 m/s, then we find that the maximum mass that can be shot is &lt;strong&gt;163.5 kg&lt;/strong&gt;.  The payload must be lighter than this, or else the gun can not accelerate it to the required speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Minimum mass&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the OP I gave the required velocity needed to make it to the Karman line in the absence of air resistance.  Note that the currently quoted muzzle velocity is 100 m/s higher than this.  Earth's atmosphere is clustered mostly around the first 10 km of sea-level.  So one approximation we can make it to imagine that atmosphere as a thin sheet that the bullet must make it through, in other words, we don't credit the loss of speed due to gravitational potential along this track.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we take the mass-thickness of the atmosphere to be $\mu$, we can write the energy lost to drag as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \Delta E =\frac{1}{2} \mu v^2 A C_d = 150.2 MJ  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A generous assumption I made was to take the $v$ in this equation to be the velocity needed to reach the Karman line, 1.4 km/s.  That's the lower of the options, so this will underestimate the impact of drag.  The area (again) comes from the bore diameter.  The drag coefficient can be looked up.  I'll take it to be a perfect sphere, with a coefficient of 0.47.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is only the energy lost to drag.  The projectile has to get past this and then still have enough energy to make it to the Karman line.  This is straightforward to set up mathematically.  With this, we establish a &lt;em&gt;minimum&lt;/em&gt; mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{1}{2} m v_1^2 - \Delta E = \frac{1}{2} m v_2^2  \\ &#xA;m  = \frac{ \Delta E }{ \frac{1}{2} ( v_1^2 - v_2^2 ) } = 1,035.9 kg$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Projectile must be more massive than this in order to preserve its momentum as it makes it past the atmosphere.  The contradiction is obvious.  It &lt;em&gt;needs&lt;/em&gt; to be this massive, but the gun can't supply the energy in order to get it there because the barrel simply isn't large enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't fix this by tweaking the parameters either.  In the above equation, I used 1500 and 1400 m/s for the v1 and v2.  But make v1 larger.  It doesn't reach parity with the other requirement until well over 2,000 m/s.  We all agree that the kinetic energy of the explosives aren't enough the achieve this.  It's outright physically impossible.  See many &lt;a href=&quot;https://space.stackexchange.com/questions/815/can-gunpowder-get-you-to-the-moon&quot;&gt;other questions&lt;/a&gt; on this site about the subject for elaboration to why.  This is why HARP moved on to light gas guns - to get closer to orbital velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Compared to others&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tank gun example I cited also fails.  Its parameters come out even worse than the Starfire.  With the possible exception some heavy munitions of battleships, the answer to the question I asked is more or less &quot;no&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The accepted answer has detail on the HARP firing that did what this project is trying to do.  So it's possible, but there is a critical difference - they used a &lt;strong&gt;16 inch&lt;/strong&gt; bore.  The theme of my calculations is that an 8 inch bore is too small.  If you double the bore diameter, you quadruple the maximum energy you can put into it.  I believe it was also longer, which puts it even further away from the energetic constraint here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the Starfire project might be able to launch the payloads a few kilometers up, but it can't go further than this.  It can't physically happen.  I'm eager to hear any challenges to the math I've used and the assumptions therein.  But as it looks now, this isn't getting anywhere close to space.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-02-13T17:44:44.377" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3693" PostTypeId="1" AcceptedAnswerId="3694" CreationDate="2014-02-13T17:52:51.110" Score="12" ViewCount="4465" Body="&lt;p&gt;Looking at the &lt;a href=&quot;http://en.wikipedia.org/wiki/Rocketdyne_J-2&quot;&gt;Wikipedia page for the J-2&lt;/a&gt; (one engine was used on the Saturn V third stage, and 5 engines on the second stage) it gets about 232Klbs thrust in vacuum, but at sea level, only 109KLbs thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is a pretty wild performance swing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why such a difference? If you compare the Vacuum Merlin's vs the sea level Merlin's performance you get a much smaller swing in performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am guessing it is because there never was a sea level optimized J-2 engine (it was meant as an upper stage engine) so the sea level number may be spurious or only from test stands.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-02-13T22:06:43.807" LastActivityDate="2016-08-31T00:11:56.793" Title="Why the huge thrust difference between a Sea level and Vacuum J-2 engine?" Tags="|launch|engine-design|j-2|saturn-v|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3694" PostTypeId="2" ParentId="3693" CreationDate="2014-02-13T18:48:51.433" Score="9" Body="&lt;p&gt;Designed for high altitude low ambient pressure, I assume. The primary &lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket_engine#Back_pressure_and_optimal_expansion&quot; rel=&quot;noreferrer&quot;&gt;optimization is in the nozzle&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For optimal performance the pressure of the gas at the end of the nozzle should just equal the ambient pressure: if the exhaust's pressure is lower than the ambient pressure, then the vehicle will be slowed by the difference in pressure between the top of the engine and the exit; on the other hand, if the exhaust's pressure is higher, then exhaust pressure that could have been converted into thrust is not converted, and energy is wasted.&lt;/p&gt;&#xA;&lt;p&gt;To maintain this ideal of equality between the exhaust's exit pressure and the ambient pressure, the diameter of the nozzle would need to increase with altitude, giving the pressure a longer nozzle to act on (and reducing the exit pressure and temperature). This increase is difficult to arrange in a lightweight fashion, although is routinely done with other forms of jet engines. In rocketry a lightweight compromise nozzle is generally used and some reduction in atmospheric performance occurs when used at other than the 'design altitude' or when throttled.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-13T18:48:51.433" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3695" PostTypeId="2" ParentId="3264" CreationDate="2014-02-13T20:02:21.030" Score="7" Body="&lt;p&gt;There are many freezers on the station for storing samples for experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things that need to be disposed of, such as garbage are placed into a visiting cargo craft (Progress, ATV, HTV, Cyngus) which burn up on reentry.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things that need to be returned to earth intact are stored until there is a vehicle capable of returning it.  Previously that was the Space Shuttle, which could returns many thousands of kilos of cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently Soyuz can return on the order of 100 kilos of cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon is about the only real option, which can return about 5000 lbs of cargo. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-02-13T20:02:21.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3696" PostTypeId="2" ParentId="2503" CreationDate="2014-02-13T20:10:10.763" Score="4" Body="&lt;p&gt;Cygnus is an interesting choice as a competitor to Dragon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is something of an apples to oranges in the comparison.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cygnus cannot return cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cygnus is using a booster (Antares) that uses engines built in the 1960/1970s (NK-33) of which there are a limited number left, and once they are gone, there is currently no way to produce more. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cygnus and Antares have the feel of a thrown together design for just 11 flights, with little thought on what to do next. Some of this is outside Orbitals control, since the RD-180 they would like to use, ULA is blocking access to, legally. It appears to be for monopolistic purposes.  (I.e. ULA uses the RD-180 from Russia to power the Atlas V, and do not wish there to be a competitor using the same engine).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Amusingly, the main body of Antares first stage is built by the same company that makes the Zenit booster, which also uses the RD-180, at which point, Orbital would basically be building a US Zenit booster, which is what Sea Launch/Land Launch is doing, with real Zenit boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon, by contrast, is just the first entry on a long roadmap of capabilities with a clear future. Dragon can reenter, and SpaceX has said they intend to reuse the capsule. The booster (Falcon 9) is working towards reusability, and the engines are made in house, so there is an active production line. (Claims of 400 engines a year have been made, but it is unclear if that includes Draco/SuperDraco engines in that count).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon is planned to carry humans as a future option (2015?). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for the very simplistic comparison of cargo only, upmass only, ignoring cost, Cygnus is a competitor.  Cost wise, it is more expensive, it cannot return cargo, and no plans for manned flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even in that model, not a good competitor.  But in the crazy world of NASA, it is still potentially cheaper than other choices (SLS for example).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2014-11-11T03:25:26.140" LastActivityDate="2014-11-11T03:25:26.140" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3697" PostTypeId="2" ParentId="1631" CreationDate="2014-02-13T20:19:09.990" Score="15" Body="&lt;p&gt;At the moment all astronauts on the ISS are launched on a Soyuz capsule/booster.  Thus they wear the &lt;a href=&quot;http://en.wikipedia.org/wiki/Sokol_space_suit&quot; rel=&quot;noreferrer&quot;&gt;Sokol&lt;/a&gt; space suit, for launch.  It is similar to the ACES suit worn by Shuttle astronauts during launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sokol is not meant for EVA, but is meant to provide support in case the capsule depressurizes on launch or descent. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus everyone on board currently has a 'spacesuit', but not one in which they could survive outside the station for very long.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="574" LastEditDate="2014-02-14T22:08:07.030" LastActivityDate="2014-02-14T22:08:07.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3698" PostTypeId="1" AcceptedAnswerId="3699" CreationDate="2014-02-13T23:21:33.610" Score="7" ViewCount="1652" Body="&lt;p&gt;I'm just curious, do exercise machines for weight lifting and cycling have electric generators built in them to charge International Space Station batteries, or it's all going to waste?&lt;/p&gt;&#xA;" OwnerUserId="2656" LastEditorUserId="49" LastEditDate="2014-02-14T00:24:33.173" LastActivityDate="2014-02-14T04:35:45.783" Title="Do exercise machines on the ISS generate electric energy?" Tags="|iss|power|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3699" PostTypeId="2" ParentId="3698" CreationDate="2014-02-14T00:22:28.837" Score="13" Body="&lt;p&gt;All the electrical power requirements of the International Space Station are provided by its solar array wings and excess is stored in batteries &lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Electrical_system_of_the_International_Space_Station&quot;&gt;(1)&lt;/a&gt;&lt;/sup&gt; to be used when the station isn't exposed to direct sunlight, so there isn't any &lt;em&gt;waste&lt;/em&gt; per se, the worst it could be said about it is that some of it isn't utilized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And it wouldn't be all that much anyway, for example, a professional athlete could produce short, 6-second long &lt;em&gt;cycling sprint&lt;/em&gt; bursts producing up to 1.2 kW &lt;sup&gt;&lt;a href=&quot;http://www.researchgate.net/profile/David_John_Bishop/publication/6839829_Reproducibility_of_a_6-s_maximal_cycling_sprint_test/file/72e7e51f68f750d5c5.pdf&quot;&gt;(2)&lt;/a&gt;&lt;/sup&gt; and minutes long sustained 120 W of electrical power, assuming there aren't any system losses and all of it could be utilized. In terms of ISS total electrical power consumption (solar arrays are capable of 120 kW of generating capacity &lt;sup&gt;&lt;a href=&quot;http://www.space.com/6400-big-space-station-piece-heads-tonight.html&quot;&gt;(3)&lt;/a&gt;&lt;/sup&gt; most of the time during one of its ~ 90 minutes long orbit), those few seconds of 1.2 kW or minutes of 120 W would be meaningless and all additional changes to its electrical system would needlessly complicate it for barely measurable benefits. Of course, ISS astronauts aren't professional cyclists, and their prolonged stay in microgravity environment further reduces their fitness, so consider those numbers highly exaggerated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality, the second generation of exercise equipment used for daily exercise on board the ISS even consumes electrical power, either to provide resistance such as on the treadmill, or to run all the required electrical equipment that collects information on protocols and forces that are used as supplemental data for studies of muscle and bone loss and cardiovascular health during long-duration space flight &lt;sup&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&quot;&gt;(4)&lt;/a&gt;&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.researchgate.net/profile/David_John_Bishop/publication/6839829_Reproducibility_of_a_6-s_maximal_cycling_sprint_test/file/72e7e51f68f750d5c5.pdf&quot;&gt;Wikipedia on Electrical system of the International Space Station&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.space.com/6400-big-space-station-piece-heads-tonight.html&quot;&gt;Reproducibility of a 6-s maximal cycling sprint test&lt;/a&gt;, A. Mendez-Villanuevaa, D. Bishopa, P. Hamer, Journal of Science and Medicine in Sport, 2007 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.space.com/6400-big-space-station-piece-heads-tonight.html&quot;&gt;Space.com: Last Big U.S. Space Station Piece Heads Up Tonight&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&quot;&gt;Reference guide to the International Space Station&lt;/a&gt;, Assembly complete edition, NASA 2010 (PDF)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-14T00:33:45.990" LastActivityDate="2014-02-14T00:33:45.990" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3700" PostTypeId="2" ParentId="2809" CreationDate="2014-02-14T03:13:05.237" Score="3" Body="&lt;p&gt;As far as I can tell, because of the Z-1 Truss (or S0 truss) on top of Unity (Node 1) there is no safe path for a module or vehicle to approach.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rassvet became important as an extension of the earth facing port of Zarya, as the PLM was berthed to the earth facing port on Unity, effectively blocking direct access to the port.  But Rassvet provides sufficient offset to allow Progress and Soyuz vehicles to dock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now Rassvet was delivered by shuttle, which meant the CanadArm was used to hand it off from the shuttle to the station arm, which docked it, so no concerns with approaching under power.  Also, I am not sure if the PLM arrived before or after Rassvet, but the truss has been attached to Unity on top since 2000 and STST-92 delivered it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-02-14T03:13:05.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3701" PostTypeId="2" ParentId="3698" CreationDate="2014-02-14T04:35:45.783" Score="8" Body="&lt;p&gt;Another way to look at it:  It's &lt;strong&gt;VERY&lt;/strong&gt; expensive to lift stuff to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's going to produce more power, a pound of generator on a treadmill or a pound of solar cells?  Which do you think NASA would lift if they want more power?&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-02-14T04:35:45.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3702" PostTypeId="1" AcceptedAnswerId="3705" CreationDate="2014-02-14T11:55:42.280" Score="9" ViewCount="7524" Body="&lt;p&gt;While reading the recent question '&lt;a href=&quot;https://space.stackexchange.com/questions/3698&quot;&gt;Do exercise machines on the ISS generate electric energy?&lt;/a&gt;', I got thinking all of the exerciser machines generate heat (&lt;a href=&quot;http://www.powermasters.com/heat_energy.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;heat energy&lt;/em&gt;&lt;/a&gt;).  So yes all exercise machine generate energy for the ISS.  But is that energy a positive attribute or a negative attribute to the homeostasis?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have an exceptional answer on '&lt;a href=&quot;https://space.stackexchange.com/questions/2539&quot;&gt;Air temperature and humidity inside the ISS&lt;/a&gt;' about how many different air control systems there are on the ISS and how they all work.  It is very complex, and it seems that as there is no central control they are probably working at cross purposes on occasion (&lt;em&gt;heating vs cooling&lt;/em&gt;).  Obviously being in orbit around the earth the ISS will alternately be exposed to the sun, or in the shadow of the earth.  I would assume that heating and cooling requirements fluctuate during the 92 minute orbit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the ISS require a net heat input, or does it need to shed more heat than it makes?  If it needs to shed heat (&lt;em&gt;cool&lt;/em&gt;) is there any attempt to limit heating producing activities to the period when the ISS is in earths shadow? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2018-07-17T18:36:20.813" Title="Does the ISS need more heating or more cooling?" Tags="|iss|energy|power|life-support|environment|" AnswerCount="3" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3703" PostTypeId="1" AcceptedAnswerId="3750" CreationDate="2014-02-14T11:57:27.380" Score="8" ViewCount="829" Body="&lt;p&gt;One of the intriguing facts about Uranus is that somewhere within the gravity is about 0.9g. Now, if one wants to daydream about living on Uranus in slightly chilly balloons, it would be interesting to have a side by side view of local gravity and atmospheric density/pressure at different altitudes above Uranus' center. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where do I find this?&lt;/p&gt;&#xA;" OwnerUserId="102" LastEditorUserId="49" LastEditDate="2014-02-14T16:57:43.953" LastActivityDate="2014-02-18T16:54:06.777" Title="Is there a profile of atmospheric pressure and gravity of Uranus?" Tags="|planetary-science|gravity|atmosphere|mapping|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3704" PostTypeId="1" CreationDate="2014-02-14T13:33:40.087" Score="3" ViewCount="699" Body="&lt;p&gt;HARP fired projectiles from a gun into suborbital trajectories.  This type of projectile carried a scientific payload, but did not have any reaction engines (although later proposals did).  Their designs evolved over the years, but it seems like the general shape of the projectile was always in a genetic rocket shape, similar to &lt;a href=&quot;https://www.kickstarter.com/projects/1682852725/the-starfire-space-cannon&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Beqyo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does a rocket shape make any sense, considering that this didn't have engines?  The &lt;a href=&quot;http://en.wikipedia.org/wiki/Drag_coefficient&quot; rel=&quot;nofollow noreferrer&quot;&gt;drag coefficient&lt;/a&gt; of this shape looks to be about 0.75.  On the other hand, alternative shapes have lower coefficients:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/M1YiB.png&quot; alt=&quot;drag coefficients&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we were to superficially accept these numbers, HARP could have achieved a significant reduction in atmospheric losses by switching to a droplet shape, possibly by a full order of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fins obviously have some role, but I don't see how they impact this discussion.  If you have a tapered end, that could still have fins.  Stability of its orientation is obviously important - does that restrict the shape?  Or, does the modeling of drag coefficient fundamentally change at several mach numbers, so that there would be no benefit to the tapered end?  To my knowledge, all projectiles had a disposable portion that held it in place as it moved through the barrel, so it seems like that would make any shape practically workable.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-02-15T09:08:48.117" Title="Why did HARP make projectiles in the rocket shape?" Tags="|atmospheric-drag|sub-orbital|space-gun|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3705" PostTypeId="2" ParentId="3702" CreationDate="2014-02-14T16:23:47.643" Score="13" Body="&lt;p&gt;The ISS has a number of thermal management systems. The most visible part are the radiators attached to the main truss (they are the two sets of 3 white panels just to the left and right of middle, and yes, there's a tear towards the end of the first one on the right side)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OeoRz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those panels are part of the External Thermal Control System (ETCS), if I'm not mistaken, which deals with heat loads generated by equipment inside the station. You also see radiators below each set of solar panels. Those are part of the Photo Voltaic Thermal Control System (PVTCS), which is a separate system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ammonia runs through the panels on the ETCS, taking in heat from various equipment and heat collectors on the station, and transporting the heat to the panels, which then lose heat via radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a serious problem with one of the pumps for this system in May 2013, which required the astronauts on board to power down some systems, before eventually donning their spacesuits and going outside to repair it. Here's a great article on that problem and repair: &lt;a href=&quot;http://www.nasaspaceflight.com/2013/05/ammonia-leak-iss-contingency-spacewalks/&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasaspaceflight.com/2013/05/ammonia-leak-iss-contingency-spacewalks/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2014-02-14T16:23:47.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3706" PostTypeId="1" CreationDate="2014-02-14T18:01:24.563" Score="8" ViewCount="940" Body="&lt;p&gt;&lt;a href=&quot;http://www.thehindubusinessline.com/news/science/human-space-flight-prog-hal-hands-over-crew-module-assembly-to-isro/article5685195.ece&quot;&gt;http://www.thehindubusinessline.com/news/science/human-space-flight-prog-hal-hands-over-crew-module-assembly-to-isro/article5685195.ece&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hindustan Aeronautics Ltd (HAL) has handed over the first ‘Crew Module Structural Assembly’ for the ‘Human Space Flight Program’ to ISRO's Vikram Sarabhai Space Centre (VSSC). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm assuming some specifications are universal (and rigid) &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Temperature tolerance &lt;/li&gt;&#xA;&lt;li&gt;Heat absorption/dissipation&lt;/li&gt;&#xA;&lt;li&gt;Pressure tolerance&lt;/li&gt;&#xA;&lt;li&gt;Viewports&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Other specifications are soft (design/fabrication trade-off)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Internal volume&lt;/li&gt;&#xA;&lt;li&gt;Crew member sizes&lt;/li&gt;&#xA;&lt;li&gt;Subassembly/systems&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Which brings me to my question &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How many crew can the ISRO crew module accommodate? &lt;/li&gt;&#xA;&lt;li&gt;For how long? &lt;/li&gt;&#xA;&lt;li&gt;Is the crew module provided with an air-lock, dock, and escape mechanism?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-02-14T18:12:25.377" LastActivityDate="2020-04-25T16:04:59.443" Title="What specifications is the ISRO crew module fabricated to?" Tags="|crewed-spaceflight|isro|developing-space-programs|crew-module|human-rating|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3707" PostTypeId="2" ParentId="3704" CreationDate="2014-02-15T09:08:48.117" Score="6" Body="&lt;p&gt;The &quot;rocket shape&quot; dart has a significantly lower drag coefficient than the cylinder or the shown 45° cone.  It's still above the teardrop, but those drag coefficients are for subsonic regimes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once one gets into transonic and supersonic flight regimes, the major issue isn't atmospheric flow, but shockwave turbulence. HARP's projectiles were well into the supersonic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As with all supersonic bodies, the shockwave has to remain clear of the body for minimum turbulence, and the longer and narrower shape does that. A gentle continuous curve also allows the maximum volume at designed optimal speed. As an unpowered projectile, the shockwave can be kept clear of the entire body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a reason to have the very back less aerodynamic - stability. The area inside the shockwave isn't free of motion, but is much reduced airflow. So, a slightly higher drag at the aft keeps the nose pointed forward without spin stabilization. Further, it can allow for control surfaces, though that isn't always utilized in research projectiles.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-02-15T09:08:48.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3708" PostTypeId="1" AcceptedAnswerId="3742" CreationDate="2014-02-15T10:52:28.220" Score="8" ViewCount="2384" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Barometric_formula&quot;&gt;barometric formula&lt;/a&gt; describes the atmospheric pressure depending on height and a host of other things. This formula assumes a constant gravitational acceleration over the whole height of the gas column - a reasonable assumption for Earth, as the atmosphere is thin compared to to the size of the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if we have a gas giant with far smaller mean density, this assumption may hold no longer. How does one calculate the density/pressure over height? I would assume that below a certain height, the pressure is above the critical pressure for the gas mixture and one can assume a constant density, down to the solid core. But above?&lt;/p&gt;&#xA;" OwnerUserId="102" LastEditorUserId="49" LastEditDate="2014-02-15T11:10:16.900" LastActivityDate="2014-02-19T08:02:57.903" Title="What is the barometric formula for a gas giant?" Tags="|planetary-science|atmosphere|gas-giant|physics|thermodynamics|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3709" PostTypeId="1" AcceptedAnswerId="3713" CreationDate="2014-02-15T16:06:23.373" Score="6" ViewCount="587" Body="&lt;p&gt;To me, it's a no-brainer that the Earth-Moon L1 location is a strategic locale, sort of a &lt;a href=&quot;http://en.wikipedia.org/wiki/Forks_of_the_Ohio#French_and_Indian_War&quot;&gt;forks of the Ohio&lt;/a&gt; in local space. I've seen some proposals to put some long-term station there, but has anyone really &quot;done the math&quot; and come up with a fully thought out plan?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such a plan should encompass most of the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A compelling rationale.&lt;/li&gt;&#xA;&lt;li&gt;Drawings backed up by preliminary engineering calculations.&lt;/li&gt;&#xA;&lt;li&gt;Some idea as to how it would be launched/assembled/positioned at L1, and&lt;/li&gt;&#xA;&lt;li&gt;Some notion of how it would be financed.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Manned, unmanned, or hybrid (i.e., visit occasionally) is immaterial, so long as the plan involves a long-term presence.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2014-12-31T15:21:09.313" Title="Does any group have a serious proposal to build &amp; maintain a station at L1?" Tags="|space-station|future-missions|lagrangian-points|fuel-depots|mission-design|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3710" PostTypeId="2" ParentId="3706" CreationDate="2014-02-15T16:20:10.017" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISRO_Orbital_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/ISRO_Orbital_Vehicle&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Indian manned spacecraft temporarily named Orbital Vehicle is intended to be the basis of the indigenous Indian human spaceflight program. The capsule will be designed to carry three people, and a planned upgraded version will be equipped with rendezvous and docking capability. In its maiden manned mission, ISRO's largely autonomous 3-ton capsule will orbit the Earth at 248 miles (400 km) altitude for up to seven days with a two-person crew on board.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/x9Mk5.jpg&quot; alt=&quot;ISRO Orbital Vehicle&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2014-02-15T16:20:10.017" CommentCount="0" CommunityOwnedDate="2014-02-15T16:20:10.017" ContentLicense="CC BY-SA 3.0" />
  <row Id="3711" PostTypeId="2" ParentId="3349" CreationDate="2014-02-15T16:54:13.890" Score="4" Body="&lt;p&gt;You have to consider instabilities caused by moving masses in your rotating structure: basically, you're better off with a torus than a cylinder, or at most a cylinder with length no more than its diameter:&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/2731/what-stability-issues-plague-long-artificial-gravity-cylinders/&quot;&gt;What stability issues plague long artificial gravity cylinders?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, rather than try to rotate only the vehicle, you should get the rotation up to speed before deploying the sail. Not only are the engineering requirements simpler than rotating vehicle/non-rotating sail, but also the centrifugal force working on the sail &lt;a href=&quot;http://deploytech.eu/science-technology/folding-sails/&quot; rel=&quot;nofollow noreferrer&quot;&gt;will help unfold it and maintain its shape after deployment&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most designs for solar sails involve enormous sails, often several kilometers in diameter, so sheer forces on the fabric of any rotating sail are not trivial as one approaches the circumference. This could be mitigated by allowing the sail to rotate at a slower rate than the vehicle, and/or by employing cutouts in the sail:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Uihcm.gif&quot; alt=&quot;star sail&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suggested reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.isas.jaxa.jp/e/forefront/2013/tsuda/index.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Realization of Centrifugal-Deployment Technology&lt;/a&gt;, Juichi Tsuda, ISAS / JAXA Institute of Space and Astronautical Science, 2008&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen2/docs/aiaa-2005-2045.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deployment Characteristics of Rotationally Skew Fold Membrane for Spinning Solar Sail&lt;/a&gt;, H. Furuya, Y. Inoue, T. Masuoka, Tokyo Institute of Technology, 2005 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://wiki.umn.edu/pub/SolarSail/WebHome/Deployment_Charateristics_of_Creasing_Membrane_For_Solar_Sail_on_Geometrical_Configuration_and_Fold_Pattern.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deployment Characteristics of Creased Membrane for Solar Sail on Geometrical Configuration and Fold Pattern&lt;/a&gt;, H. Furuya, Y. K. Miyazaki, H. Takeuchi, Tokyo Institute of Technology, 2003 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3349/what-is-the-most-stable-configuration-for-a-centrifugal-ship-towed-by-a-solar-sa&quot;&gt;DeployTech: Folding Sails, Centrifugal Deployment and Stabilisation&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-02-16T15:15:54.700" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3712" PostTypeId="2" ParentId="3375" CreationDate="2014-02-15T17:00:13.377" Score="6" Body="&lt;p&gt;Rotate the sail. Centrifugal force will help maintain flatness. Unless there's some engineering requirement that dictates otherwise, it would probably work better to spin both craft and sail. Getting the rotation on the craft up to speed before deploying the sail should do the trick. JAXA's &lt;a href=&quot;http://en.wikipedia.org/wiki/IKAROS#Design&quot; rel=&quot;nofollow noreferrer&quot;&gt;IKAROS&lt;/a&gt; solar sail demonstrator used rotational deployment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there is a major disconnect between the popular conception of solar sails and the designs of missions flown or contemplated. As C. Town Springer mentioned in his comment above, many people think of solar sails deployed by lines in front of the ship, as in Arthur C. Clarke's &lt;a href=&quot;http://en.wikipedia.org/wiki/Sunjammer&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Sunjammer&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GTJ2z.jpg&quot; alt=&quot;Boys&amp;#39;Life cover&quot;&gt;&lt;br&gt;&#xA;Robert Forward's design of &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight&quot; rel=&quot;nofollow noreferrer&quot;&gt;interstellar beam-driven solar sails&lt;/a&gt; also featured tow lines. In practice, however, solar sail projects have looked more like this:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/c/cf/Nano_Sail_D.jpg/1920px-Nano_Sail_D.jpg&quot; alt=&quot;NanoSail-D sail&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;IKAROS, &lt;a href=&quot;http://en.wikipedia.org/wiki/NanoSail-D2&quot; rel=&quot;nofollow noreferrer&quot;&gt;NanoSail-D&lt;/a&gt;, and the Planetary Society's &lt;a href=&quot;http://en.wikipedia.org/wiki/LightSail-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;LightSail-1&lt;/a&gt; all feature this design, a square-rigged sail deployed from the body of the craft by unfolding booms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one thinks about the way an atmospheric parachute deploys, the impracticality of mimicking this design for a proof-of-concept solar sail becomes obvious. Typically a drogue chute catches the air and proceeds to pull the main chute out, which (hopefully) expands out to its desired shape and pulls the lines taut. No such mechanism of an equivalent order of magnitude is available to a solar sailcraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flat-surface-normal-to-axis design, in this light, is mechanically easier to deploy. More sophisticated sailcraft may very well use other forces to deploy their sales, such as electrostatic or centrifugal force, as in this &lt;a href=&quot;https://space.stackexchange.com/questions/3349/what-is-the-most-stable-configuration-for-a-centrifugal-ship-towed-by-a-solar-sa/&quot;&gt;answer&lt;/a&gt;, but expect designs that owe more to the umbrella than to the traditional parachute in the near term.    &lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-07-12T22:23:10.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3713" PostTypeId="2" ParentId="3709" CreationDate="2014-02-15T17:49:21.933" Score="9" Body="&lt;p&gt;There's this proposal floated by Boeing:&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Exploration_Gateway_Platform&quot;&gt;http://en.wikipedia.org/wiki/Exploration_Gateway_Platform&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And Russia has had some similar plans, although it was for a station in lunar orbit: &lt;a href=&quot;http://www.russianspaceweb.com/los.html&quot;&gt;http://www.russianspaceweb.com/los.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Boeing proposal would use some structural elements left over from ISS construction, while also including modules similar to those planned for the Russian segment of the ISS. By placing a station there, you can take advantage of the orbital mechanics around Lagrange points and host a reusable lunar lander. If you did any sort of materials processing on the moon (i.e. extracting water from lunar dust or using lunar ice to make liquid hydrogen and liquid oxygen), then you could also use the lander to send those materials up to the L1/L2 waypoint so it could act as a propellant depot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's another link that describes the Boeing plans in more detail (it's referenced by the wikipedia article as well): &lt;a href=&quot;http://www.nasaspaceflight.com/2011/12/exploration-gateway-platform-hosting-reusable-lunar-lander-proposed/&quot;&gt;http://www.nasaspaceflight.com/2011/12/exploration-gateway-platform-hosting-reusable-lunar-lander-proposed/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="1255" LastEditDate="2014-02-15T19:28:28.473" LastActivityDate="2014-02-15T19:28:28.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3714" PostTypeId="2" ParentId="3708" CreationDate="2014-02-15T19:12:51.097" Score="4" Body="&lt;p&gt;For a spherically symmetric mass distribution in hydrostatic equilibrium:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${dP\over dr}=-g\rho$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $P$ is the pressure, $r$ is the radius, $g$ is the gravitational acceleration as a function of $r$, and $\rho$ is the density of the gas as a function of $r$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you integrate up or down from some known conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$g$ as a function of $r$ is straightforward since we are assuming a spherically symmetric mass distribution.  $g$ only depends on the mass below $r$.  So $g={Gm(r)\over r^2}$, where $m(r)$ is the mass of the body below $r$.  So, $m(r)=\int_0^r\rho(r)\,4\pi r^2dr$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly you need an equation of state relating $P$ and $\rho$.  The simplest is for an ideal gas, derived from good old $PV=nRT$, which is $P=\rho{kT\over\mu}$.  $\mu$ is the molecular mass and $T$ is the temperature.  $k$ is the Boltzmann constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As noted in the question, state changes result in, as you'd expect, completely different equations of state.  Note that different species will undergo state changes at different temperatures and pressures, so the real equation of state can get quite complicated.  But the ideal gas law is good enough for getting a feel for the general behavior in a varying gravity field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you assume a well-mixed atmosphere (which is not necessarily consistent with hydrostatic equilibrium), then you can consider $\mu$ to be a constant.  You are left with the need for a temperature profile, $T(r)$ in order to do the integration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The temperature profile matters.  You can't assume a constant temperature except over small changes in altitude.  Below is a plot of temperature profiles from &lt;a href=&quot;http://www.astronomynotes.com/solarsys/s3c.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0JgLK.png&quot; alt=&quot;gas and ice giant temperature profiles&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-16T16:20:00.310" LastActivityDate="2014-02-16T16:20:00.310" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3715" PostTypeId="1" AcceptedAnswerId="3716" CreationDate="2014-02-15T19:42:15.087" Score="20" ViewCount="4512" Body="&lt;p&gt;What software language was used to program the martian rovers Spirit, Opportunity and Curiosity? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, how many engineers were involved in writing the software for the rovers?&lt;/p&gt;&#xA;" OwnerUserId="1465" LastActivityDate="2014-02-15T20:45:16.860" Title="What software language was used to program the martian rovers Spirit, Opportunity and Curiosity?" Tags="|curiosity|software|spirit|opportunity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3716" PostTypeId="2" ParentId="3715" CreationDate="2014-02-15T20:19:05.040" Score="23" Body="&lt;p&gt;Almost all in C on the rover computer.  On MSL, some of the C was generated automatically using UML state charts.  I don't know the exact team sizes.  On the order of 20 on MER and 40 on MSL for the flight software on the rover computer.  You would also need to consider the software on the instruments.  And the radio.  And the motor controllers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a &lt;a href=&quot;https://www.youtube.com/watch?v=9jVt5vb68xA&quot; rel=&quot;noreferrer&quot;&gt;good video on the MSL software development&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lots of good stuff in the answers to &lt;a href=&quot;https://softwareengineering.stackexchange.com/questions/159637/what-is-the-mars-curiosity-rovers-software-built-in/&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many different languages are used in the ground software.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-12T07:31:59.047" LastActivityDate="2014-02-15T20:45:16.860" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3717" PostTypeId="2" ParentId="3708" CreationDate="2014-02-15T21:39:42.930" Score="1" Body="&lt;p&gt;Something else to keep in mind:  When you are dealing with the gas giants you no longer have anything remotely resembling a perfect gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long ago I did a brute force calculation of the pressure &amp;amp; density of the atmosphere in a hole through the Earth.  The answers are insane--in reality the gas will compress to liquid density and cease to follow the gas law.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-02-15T21:39:42.930" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3718" PostTypeId="5" CreationDate="2014-02-16T00:30:32.603" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T00:30:32.603" LastActivityDate="2014-02-16T00:30:32.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3719" PostTypeId="4" CreationDate="2014-02-16T00:30:32.603" Score="0" Body="Questions about Near-Earth Objects or NEO for short, a term describing meteoroids, asteroids, comets, man-made spacecraft, space observatories, and space debris whose orbit brings them into proximity with Earth. See also: [near-earth-asteroid]" OwnerUserId="49" LastEditorUserId="58" LastEditDate="2020-02-20T15:29:16.560" LastActivityDate="2020-02-20T15:29:16.560" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3720" PostTypeId="5" CreationDate="2014-02-16T00:35:02.213" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T00:35:02.213" LastActivityDate="2014-02-16T00:35:02.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3721" PostTypeId="4" CreationDate="2014-02-16T00:35:02.213" Score="0" Body="Questions about past and future space missions that either will or have required multiple launches for a single space exploration campaign, or questions about multiple launches on individual missions that are jointly managed." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T00:35:02.213" LastActivityDate="2014-02-16T00:35:02.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3722" PostTypeId="5" CreationDate="2014-02-16T00:37:01.597" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T00:37:01.597" LastActivityDate="2014-02-16T00:37:01.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3723" PostTypeId="4" CreationDate="2014-02-16T00:37:01.597" Score="0" Body="Questions about dual-launch campaigns, which are space exploration missions requiring two separate launch vehicles to achieve a common goal." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T00:37:01.597" LastActivityDate="2014-02-16T00:37:01.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3724" PostTypeId="5" CreationDate="2014-02-16T00:40:22.797" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T00:40:22.797" LastActivityDate="2014-02-16T00:40:22.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3725" PostTypeId="4" CreationDate="2014-02-16T00:40:22.797" Score="0" Body="Questions about co-developed or otherwise jointly managed space exploration campaigns, such as  two or more spacecraft complementing each other in achieving a common goal." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T00:40:22.797" LastActivityDate="2014-02-16T00:40:22.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3726" PostTypeId="2" ParentId="3502" CreationDate="2014-02-16T02:14:51.413" Score="2" Body="&lt;h3&gt;Multiple reasons exist.&lt;/h3&gt;&#xA;&lt;h4&gt;Weapon of Mass Destruction&lt;/h4&gt;&#xA;&lt;p&gt;Any large body in orbit is a potential weapon of mass destruction. Drop a 20-ton rock from orbit, and it may not survive, but if you do it right, it creates a crater some 100m across.&lt;/p&gt;&#xA;&lt;p&gt;While NASA is not planning on using deadfall artillery, the possibility is a political issue.&lt;/p&gt;&#xA;&lt;h4&gt;Ownership and access&lt;/h4&gt;&#xA;&lt;p&gt;Legally, all natural bodies in space are common property of all Earth's peoples.&lt;/p&gt;&#xA;&lt;p&gt;Putting it in LEO would make it accessible to a variety of states that are seen as politically untrustworthy. The key example is North Korea - they have demonstrated the ability to launch SRBM's, which are precursors to a space program as well as to ICBMs. (The initial US space launchers were in fact developed as part of ICBM programs.) The concern that these nations may reach and deorbit a body as a form of terrorism is a real and present (but low probability) threat.&lt;/p&gt;&#xA;&lt;p&gt;A Lunar Orbit location has far less access to everyone, however, it's a near total bar to most nations. The US, Russia, China, India, and the EU have the capability to get there, as evidenced by the ability to land lunar rovers and install lunar orbiters. It's still close enough for most real-time experimental controls.&lt;/p&gt;&#xA;&lt;h4&gt;More margin for error&lt;/h4&gt;&#xA;&lt;p&gt;The lack of atmosphere&lt;sup&gt;1&lt;/sup&gt; allows for a miss to not result in slowing and impact, nor to result in surface melting of the body.&lt;/p&gt;&#xA;&lt;h4&gt;Lower ∆V&lt;/h4&gt;&#xA;&lt;p&gt;Less overall energy is needed to put items into a stable Lunar orbit than a stable Earth orbit. The speed change needed is less, and thus the duration of acceleration and total thrust can be less.&lt;/p&gt;&#xA;&lt;h4&gt;Less to impact&lt;/h4&gt;&#xA;&lt;p&gt;Fundamentally, inserting anything into Earth orbit involves hitting the mark at the right time and speed to not hit anything else in Earth orbit. There are huge numbers (thousands) of tracked items, and hundreds of useful items, including a manned facility, that might be affected.&lt;/p&gt;&#xA;&lt;p&gt;Lunar orbit is far less crowded, and no lunar orbiting equipment is essential infrastructure.&lt;/p&gt;&#xA;&lt;h4&gt;Much less impact on satellite orbits.&lt;/h4&gt;&#xA;&lt;p&gt;If I recall correctly, the intent is to move an asteroid of sub-kilometer diameter - but preferably over 100 m diameter. These are objects large enough to have a noticeable effect upon other orbiting bodies. The cumulative effect could be catastrophic. The impact is tiny, but would be persistent and affect all other orbiting structures notably.&lt;/p&gt;&#xA;&lt;h4&gt;Less tidal stress&lt;/h4&gt;&#xA;&lt;p&gt;Tidal stress being the difference in pull between the ends of an object in orbit around a more massive object.  Lunar orbit results in far less tidal stress than Earth orbit, due to the much lower lunar mass. (Note that Earth tidal stress will still exist in lunar orbit, but will be considerably less than even geosynchronous.&lt;/p&gt;&#xA;&lt;h4&gt;Proving the tech doesn't require a Earth orbit.&lt;/h4&gt;&#xA;&lt;p&gt;The technology in question is the ability to put asteroids into desired orbits. The principles are exactly the same whether that's Earth, Lunar, or Martian orbit. Success will result in proving the technology.&lt;/p&gt;&#xA;&lt;h4&gt;A base for future science&lt;/h4&gt;&#xA;&lt;p&gt;A lunar orbiting asteroid is a good base for further lunar and deep space oriented science. Being closer to the moon, it provides relay access for far-side rovers, as well as a location outside the van allen belts to test various practical radiation shielding approaches without having to land on the moon itself.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;Lunar atmosphere is present, but is at so low a fraction of a pascal at surface as to be safely ignored for orbital purposes.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="12102" LastEditDate="2021-08-12T23:01:51.003" LastActivityDate="2021-08-12T23:01:51.003" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="3728" PostTypeId="2" ParentId="1908" CreationDate="2014-02-16T07:42:59.133" Score="2" Body="&lt;h1&gt;No. It's too far.&lt;/h1&gt;&#xA;&lt;p&gt;Even if the Alcubierre-White Warp Drive does work as hoped for (10C), the nearest black holes would remain centuries away. The nearest is more than 1500 LY away. Our best long-term power sources are good for under a half a century, and won't come close to powering the Alcubierre-White Warp Drive.&lt;/p&gt;&#xA;&lt;p&gt;And the only scientifically reasonable drive that could make it feasible is not even yet to practical lab testing. Dr. White hasn't released even preliminary test data yet. We don't know if it will even generate a practical sublight warp, let alone Dr. White's predicted 10C.&lt;/p&gt;&#xA;&lt;h1&gt;Black Holes don't exist?&lt;/h1&gt;&#xA;&lt;p&gt;According to the leading mind on Black Holes, Dr. Stephen Hawking, as of the start of 2014, black holes don't exist. (Not that supermassive objects don't. It's just that it's that they don't meet the definition, and, per Dr. Hawking, nothing probably does.)&lt;/p&gt;&#xA;&lt;p&gt;So, from that point of view, there are no black holes to send a probe to.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-16T07:54:36.333" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3729" PostTypeId="2" ParentId="2466" CreationDate="2014-02-16T09:00:23.553" Score="-1" Body="&lt;p&gt;The 'T' is the point in which the spacecraft receives the 'Go for Launch' command. At the point where 'T-0' touched, the rocket takes off from the pad. Also, as 'Hash' has mentioned above, you can use the 'T Plus' measurement after the mission has begun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This convention has been used throughout NASA's history, and even when the Saturn V was still around, launching astronauts to the moon in the Cold War. Most of the progression NASA has made has been driven by the Space Race, which was like a competition against the USA and the Soviet Union (which is today Russia).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this helps you!&lt;/p&gt;&#xA;" OwnerUserId="2663" LastActivityDate="2014-02-16T09:00:23.553" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3730" PostTypeId="5" CreationDate="2014-02-16T13:00:01.240" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T13:00:01.240" LastActivityDate="2014-02-16T13:00:01.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3731" PostTypeId="4" CreationDate="2014-02-16T13:00:01.240" Score="0" Body="Questions about Global Navigation Satellite Systems (GNSS), such as existing Global Positioning System (GPS) and GLObal NAvigation Satellite System (GLONASS), but also future such systems in development, like European Galileo, China's Compass or India's IRNSS (Indian Regional Navigational Satellite System)." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-16T13:00:01.240" LastActivityDate="2014-02-16T13:00:01.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3732" PostTypeId="5" CreationDate="2014-02-16T16:50:35.170" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-02-16T16:50:35.170" LastActivityDate="2014-02-16T16:50:35.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3733" PostTypeId="4" CreationDate="2014-02-16T16:50:35.170" Score="0" Body="Mars One is an organization that proposes to establish a permanent human colony on Mars by sending people on a one-way journey there, making it possible to launch with smaller rockets and have more space for payload." OwnerUserId="288" LastEditorUserId="4660" LastEditDate="2015-07-04T14:51:54.173" LastActivityDate="2015-07-04T14:51:54.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3734" PostTypeId="2" ParentId="359" CreationDate="2014-02-16T17:48:07.370" Score="3" Body="&lt;p&gt;You can determine the microgravity by observing two or more free-floating objects and measuring their motion towards or away from each other. This is the bowling ball method. How can you tell if you are in free space, or falling down a long elevator shaft? Watch a pair of &quot;floating&quot; bowling balls. If they are moving towards each other faster than their own gravitation should produce, then you are falling into a gravity well. There is always a pseudo microgravity in orbits because parts of the craft closer to the planet are trying to follow a faster orbit than the parts that are further away. It is often called a tidal effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The late Dr. Robert Forward of Hughes Research designed &quot;space-time flatteners&quot; for shuttle experiments. These are arrangements of tungsten disks and spheres and toruses that counteract the tidal effects and gave very good approximations of being in free space far from any significant mass, albeit in a small areas in the middle of the apparatus. He once lamented that he could not patent them because he had used them extensively (on a much larger scale) in his science fiction.&lt;/p&gt;&#xA;" OwnerUserId="1432" LastEditorUserId="195" LastEditDate="2019-07-02T01:46:46.723" LastActivityDate="2019-07-02T01:46:46.723" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3735" PostTypeId="1" AcceptedAnswerId="3737" CreationDate="2014-02-16T18:14:50.283" Score="13" ViewCount="1842" Body="&lt;p&gt;An &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/16028570&quot;&gt;NIH&lt;/a&gt; abstract writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A daily water intake of 3.7 L for adult men and 2.7 L for adult women meets the needs of the vast majority of persons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is water on-board space-craft (such as ISS, Tiangong) rationed?&lt;/li&gt;&#xA;&lt;li&gt;Do the astronauts aboard ISS, and other human space programmes (historic, and contemporary) receive the quantum of water necessary for their normal bodily functions?&lt;/li&gt;&#xA;&lt;li&gt;Are astronauts at the end of a mission dehydrated as compared to the start?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-03-14T19:16:04.157" Title="Is water on-board space-craft (such as ISS) rationed?" Tags="|iss|water|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3736" PostTypeId="1" AcceptedAnswerId="3746" CreationDate="2014-02-16T18:17:00.213" Score="10" ViewCount="735" Body="&lt;p&gt;An &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/16028570&quot;&gt;NIH&lt;/a&gt; abstract writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A daily water intake of 3.7 L for adult men and 2.7 L for adult women meets the needs of the vast majority of persons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.mars-one.com/mission/roadmap&quot;&gt;Mars One&lt;/a&gt; touts a crew-size of four with a proposed transit duration of 210 Earth days. Assuming four males, the minimum figure works out to 3108 litres with the caveat nobody consume more than their 3.7 litres daily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The brief on MTV (&lt;a href=&quot;http://www.mars-one.com/technology/mars-transit-vehicle&quot;&gt;Mars Transit Vehicle&lt;/a&gt;) writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Transit habitat has a mass of about 20,000 kg. It will carry close to 800 kg of dry food, 3000 liters of water and 700 kg of oxygen on board. No water or Oxygen will be recycled, because the trip lasts only 210 days.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Even assuming 3000 litres is merely for the purpose of consumption, the short-fall is 108 litres. In other words, one crew member may have to forgo water for a month (a little in excess of 29 days). The short-fall increases still further if this 3000 litres also provides for cooking, and other mundane activities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will the Mars Transit Vehicle need to include a water recycling system?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-03-05T09:36:34.083" LastActivityDate="2014-03-05T09:36:34.083" Title="Will the Mars Transit Vehicle need to include a water recycling system?" Tags="|mars|crewed-spaceflight|water|interplanetary|mars-one|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3737" PostTypeId="2" ParentId="3735" CreationDate="2014-02-16T21:50:59.290" Score="15" Body="&lt;p&gt;Depends how you look at it. Onboard the International Space Station (ISS), water is definitely a precious resource and several measures were taken to consume less of it, such as recycling as much of it as possible, not washing clothes, reducing tap pressure, conserving it under shower and using wet cloths, using liquid waste vacuum tube on toilets, and so on. But it is also a consumable, since it's a source of oxygen through electrolysis and inefficiencies of the ISS' life support recycling used water further reduces its quantity, so it is supplied to the ISS regularly with resupply vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS astronauts aren't deprived of it though, and it's part of their daily regime to hydrate frequently to reduce effects of &lt;a href=&quot;https://space.stackexchange.com/q/2611/49&quot;&gt;blood volume loss&lt;/a&gt; due to microgravity. I can't say that was the case with all manned missions to space though, for example, if Chinese taikonauts had sufficient water supply during their stay at Tiangong 2. And some earlier US manned missions only took really limited, one day's worth of water supplies with them, since they were short-duration flights only, such as the early &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Mercury&quot; rel=&quot;nofollow noreferrer&quot;&gt;project Mercury&lt;/a&gt; flights. And for &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo&lt;/a&gt; missions, water was used to rehydrate the food and as onboard equipment coolant, on top of using it for drinking, so that was a bit of a problem during &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 13&lt;/a&gt; mission, but they managed to make supplies they had left after the cryogenic tank stir explosion last by restricting its use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, no astronaut yet suffered long term water shortage in space, with a slight exception of Apollo 13 crew but they made due. Recycling and conserving use is however essential to keeping supplies last even in Low-Earth Orbit (LEO) with resupply vehicles standing by and relatively easy escape route in case something goes badly wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources and suggested reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reference guide to the International Space Station, Assembly complete edition&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2000/ast02nov_1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Science News: Water on the Space Station&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2000/ast13nov_1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Science News: Breathing Easy on the Space Station&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.lpi.usra.edu/lunar/missions/apollo/apollo_13/return/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar And Planetary Institute: Apollo 13 Mission - Emergency Return&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/2611/49&quot;&gt;How do astronauts battle loss in blood volume in microgravity?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_toilet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Space toilet&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-02-16T21:50:59.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3738" PostTypeId="1" AcceptedAnswerId="12025" CreationDate="2014-02-17T11:21:23.853" Score="16" ViewCount="1624" Body="&lt;p&gt;The NASA &lt;a href=&quot;http://petecrownasa.wordpress.com/2011/12/03/the-ksc-firing-rooms-in-the-launch-control-center/&quot; rel=&quot;noreferrer&quot;&gt;Launch Control Center&lt;/a&gt; at Cape Canaveral has 4 Firing Rooms. Each can support the launch of a spacecraft. Similarly, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Vehicle_Assembly_Building&quot; rel=&quot;noreferrer&quot;&gt;VAB&lt;/a&gt; was divided into four bays, each large enough to support assembly of a &lt;a href=&quot;http://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;noreferrer&quot;&gt;Saturn V&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was operational tempo ever high enough that more than one Firing Room was in use at the same time? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vFyEA.jpg&quot; alt=&quot;LCC&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="168" LastEditDate="2014-02-18T05:43:15.327" LastActivityDate="2015-09-17T12:37:05.757" Title="NASA Launch Control: was all of its capacity ever used?" Tags="|nasa|apollo-program|launch-site|multi-launch|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3739" PostTypeId="2" ParentId="3738" CreationDate="2014-02-17T12:28:47.127" Score="9" Body="&lt;p&gt;First of all, it's extremely important to have redundancy for something like this, in the event of there being an issue with one of the rooms. Furthermore, the various rooms can be used for training by different teams at the same time, although I doubt one is doing a live fire in one and having a training in another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been a few instances of multiple NASA launches in a narrow time span, where I would expect multiple rooms to be in use. Specifically, the Gemini/ Agena launches, where Agena was a practice docking target, was launched within hours of Gemini (Usually 100 minutes before)&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-02-17T12:28:47.127" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3740" PostTypeId="2" ParentId="1479" CreationDate="2014-02-17T14:05:45.467" Score="13" Body="&lt;p&gt;We can use the &lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot;&gt;rocket equation&lt;/a&gt; to get an estimate of the amount of fuel we need to slow down from orbital speed to something survivable by a Cessna.&lt;br&gt;&#xA;Orbital speed is 7890 m/s. We also need to dissipate the potential energy that was stored as altitude: if we fire the rocket at orbital altitude until the Cessna's speed is zero, it'll start dropping in freefall and &lt;a href=&quot;http://en.wikipedia.org/wiki/Felix_Baumgartner&quot;&gt;gaining speed&lt;/a&gt; again. Here's the &lt;a href=&quot;http://en.wikipedia.org/wiki/Equations_for_a_falling_body&quot;&gt;speed&lt;/a&gt; gained in a drop:&lt;br&gt;&#xA;$\sqrt(2 * g * h)$&lt;br&gt;&#xA;Starting from 200 km and ignoring atmospheric influences, I get 1980 m/s. If the rocket has to brake away that speed as well, the total $\Delta V$ is 9870 m/s. Let's use an Isp of 300 seconds (e.g. the SpaceX Merlin rocket engine) and an empty mass of 1200 kg. Solving the rocket equation to give us the initial mass:&lt;br&gt;&#xA;$M_0 = M_1 * e^{\frac{9870}{300*9.81}}$&lt;br&gt;&#xA;So we end up with an initial mass of 34.3 tons.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We'll have to work out a flight profile where we save some of the rocket fuel until the Cessna hits the atmosphere so we can prevent overspeeding and overheating during the descent.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also need a structure to carry 34 tons of fuel in, and the rocket engine is going to have lots more thrust than the original Cessna engine so we may have to beef up the Cessna's structure to take the loads. It starts piling up quickly, and it gets worse when you consider that those 34 tons have to be launched as well. To get the original 1200 kg up, a Soyuz launcher would suffice. At 34 tons, you're beyond the heaviest launch vehicles we currently have.&lt;br&gt;&#xA;Compare this with a Soyuz descent module which weighs about 3000 kg. Clearly aerobraking and parachutes carries much less of a weight penalty than removing all that delta-V via thrust alone. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-02-17T14:30:48.900" LastActivityDate="2014-02-17T14:30:48.900" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3741" PostTypeId="1" AcceptedAnswerId="3996" CreationDate="2014-02-17T15:18:51.117" Score="20" ViewCount="1212" Body="&lt;p&gt;This &lt;a href=&quot;http://suzymchale.com/ruspace/isssozh.html#water&quot;&gt;article&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An interesting fact is that the Russian and American segment water supplies can’t be mixed. The U.S. water uses iodine for bacteria control, while the Russian water uses silver. If these substances mix, a silver-iodine precipitate forms in the water, which may clog the sublimator in the NASA EMU spacesuit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I assume this last mention about the precipitate means the water from both segments must be stored separately. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is potable water in the American &amp;amp; Russian segments still stored separately?&lt;/li&gt;&#xA;&lt;li&gt;Does it carry the caveat a Russian may not drink water in the American section, and v.v?&lt;/li&gt;&#xA;&lt;li&gt;If such caveat is meaningless, does it follow the precipitate ( is it silver iodide? ) absorbed into the body of the drinker? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-03-16T18:32:44.927" Title="Is potable water in the American &amp; Russian segments still stored separately?" Tags="|iss|nasa|water|russia|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3742" PostTypeId="2" ParentId="3708" CreationDate="2014-02-17T15:28:34.863" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;This formula assumes a constant gravitational acceleration over the whole height of the gas column - a reasonable assumption for Earth, as the atmosphere is thin compared to to the size of the planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For a &lt;a href=&quot;https://physics.stackexchange.com/questions/51908/why-arent-gas-planets-and-stars-fuzzy&quot;&gt;simple argument&lt;/a&gt;, if you assumed composition is the same, it's clear that the altitude change needed for pressure to change by a certain factor is &lt;em&gt;less&lt;/em&gt; on Jupiter than on Earth. Gravity is stronger, and the radius of Jupiter is larger. Both of these factors result in a smaller &lt;em&gt;characteristic height&lt;/em&gt; for the atmosphere. That gives some indication that the above quote may be &lt;em&gt;less correct&lt;/em&gt; than the reverse proposition. But there are many more complications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, Jupiter has a lot more light gas in its atmosphere than Earth. This is partly because Earth's gravity isn't perfect at holding in Hydrogen against the radiation in space. So this pushes in the other direction - making Jupiter's atmosphere have a longer characteristic length.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the characteristic length isn't the full story. Obviously Earth has a nice (agreed upon) ground level. Jupiter might have a phase change that vaguely resembles this, but even if there is an abrupt density change, it won't be anything like Earth. So this raises the question of how we should define the region of Jupiter's atmosphere in the first place.  Could we accept internal metallic Hydrogen in the center as &quot;atmosphere&quot;? Probably not, but that's just the most extreme case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Differential equations correctly describing the system can be found in other answers. For completeness, they are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{d}{dr} P(r) = g(r) \rho(r) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let's go into the special case solutions, because I think this is what the OP is asking for. In the case of constant gravity, ideal uniform gas, and constant temperature, the atmospheric pressure follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P(h) = P(0)\, e^{ - \frac{ h }{ H } } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $H$ here is the characteristic length.  For Earth, this is about 7.4 km.  It depends on the temperature, so it's always in flux.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if we add in the &lt;em&gt;tidal&lt;/em&gt; component of gravity, we can obtain a new expression. This is assuming a $\frac{1}{r^2}$ form of gravity, instead of a constant. In this case, the solution to the differential equation &lt;a href=&quot;https://physics.stackexchange.com/questions/89013/expression-for-atmospheric-pressure-with-altitude-including-tidal-forces&quot;&gt;yields the following pressure profile&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P(r) = P(r_0)\, e^{ \frac{r_0}{H}  \left( \frac{r_0}{r} - 1 \right) } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $H$ is the same characteristic length. $r_0$ is the radius of the planet itself, and $r$ (radius) replaces $h$ (altitude). This equation is accurate for the specific case of:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;constant temperature&lt;/li&gt;&#xA;&lt;li&gt;constant composition (homogenous)&lt;/li&gt;&#xA;&lt;li&gt;contribution to gravitational field by air itself can be neglected&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;1 and 2 are terrible assumptions, obviously.  For all planets, there tends to be a local minimum of temperature in the atmosphere, and deeper into it, the temperature will be many many times higher, which gives a magnitude of just how bad the constant temperature assumption is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The homogeneous assumption is also very bad, because the gases stratify subtly. It's not like oil-water separation, but the higher you go, the higher the concentration of the lighter elements will be. In fact, this is a major justification for why the gas giant atmospheres are such pure Hydrogen and Helium to begin with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nonetheless, this equation for pressure is still technically better than the constant gravity assumption. But that's not the most relevant on Jupiter. In fact, it's probably more relevant of a correction on Earth.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2014-02-18T12:15:17.750" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3744" PostTypeId="2" ParentId="3709" CreationDate="2014-02-17T17:09:32.030" Score="4" Body="&lt;p&gt;The Non-Atmospheric Universal Transport Intended for Lengthy United States Exploration &lt;a href=&quot;http://en.wikipedia.org/wiki/Nautilus-X&quot; rel=&quot;nofollow noreferrer&quot;&gt;(Nautilus-X)&lt;/a&gt;, a concept developed by the Technology Applications Assessment Team of NASA, is intended to initially take up residence at EML-1, after being assembled at the ISS.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The spacecraft was designed for long duration (one to twenty-four months) exo-atmospheric space journeys for a six-person crew. In order to limit the effects of microgravity on human health, the spacecraft would be equipped with a centrifuge.&lt;/p&gt;&#xA;&lt;p&gt;The spacecraft itself is proposed to be relatively cheap by manned spaceflight standards&lt;a href=&quot;http://hobbyspace.com/nucleus/?itemid=26786&quot; rel=&quot;nofollow noreferrer&quot;&gt;(2)&lt;/a&gt; as it is projected to only cost US$3.7 billion. In addition, it may only need 64 months of work.&lt;a href=&quot;http://www.geekosystem.com/nautilus-x/&quot; rel=&quot;nofollow noreferrer&quot;&gt;(3)&lt;/a&gt;&lt;a href=&quot;http://www.popsci.com/science/article/2011-02/new-nasa-reusable-spacecraft-concept-could-serve-multiple-missions-future&quot; rel=&quot;nofollow noreferrer&quot;&gt;(4)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GMRdH.jpg&quot; alt=&quot;Initial Operations&quot; /&gt;&#xA;&lt;a href=&quot;http://www.scribd.com/doc/51592987/Nautilus-X-Holderman-1-26-11&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.scribd.com/doc/51592987/Nautilus-X-Holderman-1-26-11&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There was supposed to be a demonstrator of the centrifuge section on the ISS, but I'm not sure if it's going to make it to a flight manifest.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Onf8k.jpg&quot; alt=&quot;ISS Demo&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.spaceanswers.com/futuretech/nautilus-x-the-multi-purpose-nasa-spacecraft-that-could-take-humans-to-the-moon-and-beyond/&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; from January 2014 includes an interactive graphic of the extended duration version by one of my favorite space artists, Adrian Mann.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-12-31T15:21:09.313" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3745" PostTypeId="2" ParentId="3663" CreationDate="2014-02-18T00:42:18.800" Score="13" Body="&lt;p&gt;The software used for this at JPL is all homegrown.  A great deal of effort goes into the optimization algorithms for complex, multi-body and/or low-thrust trajectories.  You can try to request tools &lt;a href=&quot;https://download.jpl.nasa.gov&quot;&gt;here&lt;/a&gt;.  Examples are &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/30355/1/95-0944.pdf&quot;&gt;CATO&lt;/a&gt;, &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40782/1/06-2356.pdf&quot;&gt;Mystic&lt;/a&gt;, and &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40263/1/06-2513.pdf&quot;&gt;MALTO&lt;/a&gt;.  However these tools are not really intended to be &quot;user friendly&quot;, and require a great deal of domain expertise to operate.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-02-18T00:42:18.800" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3746" PostTypeId="2" ParentId="3736" CreationDate="2014-02-18T02:04:20.087" Score="6" Body="&lt;h1&gt;They need one, but they don't admit it...&lt;/h1&gt;&#xA;&lt;p&gt;In general, most long term space habitats are expected to engage in water recycling. NASA has always done so.&lt;/p&gt;&#xA;&lt;h1&gt;They Claim&lt;/h1&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.mars-one.com/technology/mars-transit-vehicle&quot; rel=&quot;noreferrer&quot;&gt;Mars One site&lt;/a&gt;, however, indicates that they will &lt;strong&gt;not&lt;/strong&gt; be recylcing.&lt;/p&gt;&#xA;&lt;p&gt;In their own words:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It will carry close to 800 kg of dry food, 3000 liters of water and 700 kg of oxygen on board. No water or Oxygen will be recycled, because the trip lasts only 210 days. Not recycling these components eliminates the need for recycling systems, backups, spare components and reduces power and cooling requirements. The 3000 liters of water is also used for radiation shielding. - See more at: &lt;a href=&quot;http://www.mars-one.com/technology/mars-transit-vehicle#sthash.grUA54kZ.dpuf&quot; rel=&quot;noreferrer&quot;&gt;http://www.mars-one.com/technology/mars-transit-vehicle#sthash.grUA54kZ.dpuf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h1&gt;Discussion&lt;/h1&gt;&#xA;&lt;p&gt;I expect that this is an unrealistic mission plan on their part, and, since they're based in the US, the Feds will require them to include some recycling gear, as 3.57L is not much. (I drink half that much on a typical day of reading. Sometimes, I drink more than that much in a day, especially if working.)&lt;/p&gt;&#xA;&lt;p&gt;Further, since they plan on using that water as a radiation shield, they must either replenish it, or they will be exposing the crew to ever increasing levels of radiation.&lt;/p&gt;&#xA;&lt;p&gt;Further, given the stored food, they're going to need to purge water from the atmosphere aboard, so concentration of water vapor in a dehumidification process can be a ready source for some additional water.&lt;/p&gt;&#xA;&lt;p&gt;Also note: bathing in 0G can be done in under 2L, but typical on the ISS is close to 4L to bathe - but that bathing isn't a daily need, either, in 0G.&lt;/p&gt;&#xA;&lt;p&gt;NASA notes in an &lt;a href=&quot;http://www.nasa.gov/audience/foreducators/stseducation/materials/Sustaining_Life.html&quot; rel=&quot;noreferrer&quot;&gt;educational page&lt;/a&gt; that up to about 10.6 Tons of water per year is needed per astronaut. That's 10,600L per year, or 30L per day.&lt;/p&gt;&#xA;&lt;p&gt;So, the MarsOne 3000L for 4 people is unreasonable. It would be rationing the whole time, and require that the astronauts not be sweating. Humans can lose up to 2L per day just by respiration if the air is too dry, and need at least a liter per day for metabolic purposes, and up to another 2L can be sweat out if engaged in high activity or exposed to high heat.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.mars-one.com/technology/mars-transit-vehicle&quot; rel=&quot;noreferrer&quot;&gt;http://www.mars-one.com/technology/mars-transit-vehicle&lt;/a&gt;&#xA;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.mars-one.com/faq/health-and-ethics/how-much-living-space-will-the-astronauts-have&quot; rel=&quot;noreferrer&quot;&gt;http://www.mars-one.com/faq/health-and-ethics/how-much-living-space-will-the-astronauts-have&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2000/ast02nov_1/&quot; rel=&quot;noreferrer&quot;&gt;http://science1.nasa.gov/science-news/science-at-nasa/2000/ast02nov_1/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/audience/foreducators/stseducation/materials/Sustaining_Life.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/audience/foreducators/stseducation/materials/Sustaining_Life.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-18T02:04:20.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3747" PostTypeId="2" ParentId="1479" CreationDate="2014-02-18T02:41:39.230" Score="7" Body="&lt;p&gt;While a Cessna is extremely implausible, due to a variety of stress and temperature factors, &lt;a href=&quot;https://en.wikipedia.org/wiki/Burt_Rutan&quot; rel=&quot;nofollow&quot;&gt;Burt Rutan&lt;/a&gt; developed a means for non-heat-shielded descent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mode is called &quot;shuttlecock mode&quot; and involves a high bypass configuration on a transsonic airframe; &lt;a href=&quot;http://www.scaled.com/projects/tierone/&quot; rel=&quot;nofollow&quot;&gt;SpaceShip One&lt;/a&gt; wasn't shedding orbital velocity. SpaceShip One was at very low orbital speed, insufficient to even qualify as an orbit in all but the most technical terms. The reason for shuttlecock mode is that it generates a huge amount of drag on a very low amount of material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The proposed Cessna can't do it, but a SpaceShipOne style shuttlecock mode could, in theory, make use of the principles used for removing vertical velocity in order to remove orbital velocity via near-vacuum drag; put it into shuttlecock mode, maintain the correct angle, and have the angle continuously change as it decays into an Earth-impacting ellipse. (Rutan has spoken of this in interviews.) Just as a badminton shuttlecock slammed due forward at 80 miles per hour rapidly drops to under 1 MPH, and as it does so, angles down, so also could an orbital craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trick will be maintaining a drag coefficient that doesn't result in heating too quickly - and a proper shuttlecock mode theoretically can do just that... provided the angles are correct on initial entry. Further, since the primary drag is behind the center of mass, it should, as with a badminton shuttlecock, maintain the correct angle by dynamic stability.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2014-02-19T07:15:47.033" LastActivityDate="2014-02-19T07:15:47.033" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3748" PostTypeId="1" AcceptedAnswerId="3749" CreationDate="2014-02-18T06:18:05.093" Score="7" ViewCount="1533" Body="&lt;p&gt;A few days ago, I was reading the wikipedia entry for the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Cometary_Explorer#End_of_mission&quot;&gt;International Cometary Explorer (ICE) / International Sun-Earth Explorer 3 (ISEE-3)&lt;/a&gt; satellite, that was launched August 12, 1978.  Down near the bottom of the page it states: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On February 4, 2014, the Goddard Space Flight Center announced that the Deep Space Network equipment necessary to transmit signals to the spacecraft had been decommissioned in 1999, and that replacing it was not economically feasible&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Does anyone know what equipment it was that was decommissioned leaving us unable to contact the spacecraft?  Also on that note what would it take to contact ICE/ISEE-3 or any spacecraft at a distance of ~1AU?  Would it be possible to contact ICE/ISEE-3 on a 10-meter dish?  5-meter dish?  &lt;/p&gt;&#xA;" OwnerUserId="901" LastActivityDate="2014-06-17T18:02:54.157" Title="Equipment needed to contact ICE/ISEE3 Spaceraft" Tags="|probe|communication|deep-space-network|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3749" PostTypeId="2" ParentId="3748" CreationDate="2014-02-18T08:31:13.133" Score="5" Body="&lt;p&gt;The obstacle is sending commands to the spacecraft. We can receive its signals and interpret them, but sending commands to adjust &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftTelemetry.do?id=1978-079A&quot; rel=&quot;nofollow&quot;&gt;ISEE&lt;/a&gt;'s orbit would be &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2014/02070836-isee-3.html&quot; rel=&quot;nofollow&quot;&gt;difficult&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's some information on the &lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?showtopic=3800&amp;amp;pid=207327&amp;amp;mode=threaded&amp;amp;show=&amp;amp;st=#entry207327&quot; rel=&quot;nofollow&quot;&gt;unmannedspaceflight forum:&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...&lt;a href=&quot;http://ipnpr.jpl.nasa.gov/progress_report/42-76/76K.PDF&quot; rel=&quot;nofollow&quot;&gt;This report&lt;/a&gt; ... indicates that ICEE-3 was never designed to&#xA;  use the DSN for communications and a special filter had to be added to&#xA;  the Block V masers at DSS-14 and DSS-63 in the mid-80s to communicate&#xA;  with it. According to &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsndocs/810-005/101/101E.pdf&quot; rel=&quot;nofollow&quot;&gt;this report&lt;/a&gt; the maser was replaced at&#xA;  DSS-14 (Goldstone) in 2010 but the one at DSS-63 (Madrid) isn't going&#xA;  to be until this November, but S-band uplink isn't supported at Madrid&#xA;  because of frequency conflicts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To summarize, ICE commands need to be sent on a frequency not normally used by the DSN. In order to send on this frequency, a filter needs to be added to the antenna to prevent the powerful broadcast from frying the incredibly sensitive receivers that are connected to the same antenna. This is not a matter of soldering a few components onto a breadboard. These are waveguides tuned to the filter frequency and able to withstand the power of the transmitter (20 kW?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is another factor that would have influenced NASA's decision not to pursue communication with ISEE: &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsn/&quot; rel=&quot;nofollow&quot;&gt;DSN&lt;/a&gt; availability. ISEE does not have any recording equipment; any measurements it takes must be transmitted immediately or they're lost. That means NASA would have to allocate a DSN antenna to the ISEE mission full-time, displacing several more recent missions. Normally DSN time is allocated in blocks of ~8 hours, so one antenna can serve several missions. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-03-07T07:21:36.907" LastActivityDate="2014-03-07T07:21:36.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3750" PostTypeId="2" ParentId="3703" CreationDate="2014-02-18T16:48:12.570" Score="3" Body="&lt;p&gt;Balloons can be quite useful for carrying scientific payloads, but only on Earth, Mars or something else &quot;benign&quot;. Not Uranus though. In that context, it's more science fiction (of the &lt;em&gt;remote&lt;/em&gt; future, that is) than a serious design consideration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are a poor design choice because:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;They are completely useless for the duration of the rest of the mission&lt;/li&gt;&#xA;&lt;li&gt;They involve tremendous risk if the mission's success is indeed to depend on them&lt;/li&gt;&#xA;&lt;li&gt;Using balloons would require very low injection velocities&lt;/li&gt;&#xA;&lt;li&gt;The &lt;em&gt;must&lt;/em&gt; deploy at a certain altitude, because otherwise the pressure would be too low and the balloons would likely pop &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That means you need a controlled and very slow decent into a volatile atmosphere, which costs &lt;em&gt;tremendous&lt;/em&gt; amounts of propellant, reducing the possible payload mass. Any real space mission to Uranus would therefore likely choose something like controlled high-speed descent of the main probe, until total destruction (which will unfortunately not be very deep).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, more to the point: Some work has been done no this subject, but not in much greater detail than a basic &lt;a href=&quot;https://space.stackexchange.com/questions/3708/what-is-the-barometric-formula-for-a-gas-giant&quot;&gt;exponential atmosphere&lt;/a&gt;. Existing Uranian atmospheric models differ quite significantly from one another. As you may know, they are all based on Voyager 2 (1989) spectral data for the composition (aided by telescopic data). A lot of the current knowledge is inferred from analyzing the formation mechanisms and dynamics of the clouds in the Uranian atmosphere. This, together with pretty basic physics (like the exponential atmosphere and insolation models to get the temperature profiles) composes all of mankind's knowledge about the Uranian atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Analyzing balloon flights on Uranus, given its &lt;em&gt;inherent&lt;/em&gt; uncertainties, certainly does not require anything better than an exponential atmosphere. It also does not require you to use a full-fledged temperature profile for the integration of the barometric equation; just use the planet's average temperature (&lt;a href=&quot;https://en.wikipedia.org/wiki/Barometric_formula#Estimating_the_temperature&quot; rel=&quot;nofollow noreferrer&quot;&gt;see the bottom of this wiki&lt;/a&gt;), and keep it constant; that significantly simplifies the integration. Then it's just a matter of plotting $P(r)$ vs. $g(r) = μ/r^2$ for a range of $r$ that describes the ~20% of the Uranian radius. &lt;/p&gt;&#xA;" OwnerUserId="97" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-02-18T16:54:06.777" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3751" PostTypeId="1" CreationDate="2014-02-18T15:28:30.403" Score="5" ViewCount="197" Body="&lt;p&gt;Would the impact result in sufficient gas expelled from the vaporization of the meteorite to carry sound waves a distance of a kilometre from the impact? Would the blast wave be sufficient to injure an unprotected astronaut? &lt;/p&gt;&#xA;" OwnerDisplayName="Steve d'Apollonia" LastActivityDate="2014-02-18T21:03:55.567" Title="What would an astronaut on the moon experience if a 1 meter diameter meteorite struck the surface one km away?" Tags="|asteroid|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3752" PostTypeId="2" ParentId="3751" CreationDate="2014-02-18T15:46:53.610" Score="2" Body="&lt;p&gt;It depends largely on the speed at impact, which could vary widely. If it's in a similar orbit to the Earth/moon system around the sun, catching up with the Earth/moon around the sun and the moon around the Earth, that would be pretty bad a kilometer away. The ejecta could certainly turn an astronaut into an astronaught.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, if the meteorite were coming in from a different direction, say it was orbiting the sun in a similar orbit to the Earth/moon but in the opposite direction, the speed at impact could be as high as 135,000 miles per hour, or 37.5 miles per second. It would be a spectacular show, launching fragments of the moon into orbit around the Earth and likely beyond. Not a lot, but some. Your astronaut 1 kilometer away could never survive.&lt;/p&gt;&#xA;" OwnerUserId="2681" OwnerDisplayName="Marc" LastActivityDate="2014-02-18T15:46:53.610" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3753" PostTypeId="2" ParentId="3639" CreationDate="2014-02-19T00:11:03.040" Score="4" Body="&lt;p&gt;This infographic should be a good representative of how many launches can be considered classified, so I'm reposting it here from the &lt;a href=&quot;https://space.meta.stackexchange.com/q/472/49&quot;&gt;Collection of space exploration related infographics&lt;/a&gt; thread on our &lt;a href=&quot;https://space.meta.stackexchange.com/&quot;&gt;Space Exploration meta&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h2&gt;T. McCall, M. Orcutt: Space over Time / History of Space Launches&lt;/h2&gt;&#xA;&lt;p&gt;&lt;em&gt;Infographic date: August 23, 2011&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;   &lt;a href=&quot;http://www.loveinfographics.com/wp-content/uploads/science/2011/11/history-of-space-launches-science-infographic.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tuo67.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Of the 7,000 spacecraft that have been launched into orbit or beyond,&#xA;more than half were defense satellites used for such purposes as&#xA;communication, ­navigation, and imaging. (The Soviet Union sent up a&#xA;huge number, partly because its satellites tended to be much&#xA;shorter-lived than those from the United States.) In the 1970s,&#xA;private companies began increasingly adding to the mix, ­launching&#xA;satellites for telecommunications and broadcasting.&lt;/p&gt;&#xA;&lt;p&gt;This graphic groups payloads by the nationality of the owner. A&#xA;satellite, a capsule of cosmonauts, or a deep-space probe would each&#xA;count as one payload. The data, which run through July 2011, were&#xA;drawn from hundreds of sources, including space agency documents,&#xA;academic journals, and interviews. They were compiled by Jonathan&#xA;­McDowell, an ­astrophysicist at the Harvard-­Smithsonian Center for&#xA;Astrophysics and author of Jonathan’s Space Report, a newsletter that&#xA;tracks launches.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Links:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.technologyreview.com/graphiti/425120/space-over-time/?p1=Mag_story1&quot; rel=&quot;nofollow noreferrer&quot;&gt;MIT Technology Review: Space over Time (description and links)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.technologyreview.com/sites/default/files/legacy/graphiti_charts.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Printable poster size (PDF file)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.loveinfographics.com/wp-content/uploads/science/2011/11/history-of-space-launches-science-infographic.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;Poster size (JPeG file)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Infographic collates data on space launches from 1959 through mid 2011 and was compiled by Jonathan ­McDowell, a Harvard-­Smithsonian Center ­astrophysicist, and it was designed by Mike Orcutt, a MIT Research Engineer and Tommy McCall, founder of &lt;a href=&quot;http://infographics.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Infographics.com&lt;/a&gt;, a data visualisation agency.&lt;/p&gt;&#xA;&lt;p&gt;Why I think it's a good representative of how many space launches are classified? Because there's always going to be a debate over what constitutes a &lt;em&gt;classified launch&lt;/em&gt;, so splitting these between military and nonmilitary government launches while including non-government launches in the numbers should provide a close enough overview, however we want to look at it. Some payloads might be only partially classified, but the point is, that with military launches, we simply don't know to what extent that is, while we can be fairly certain that with other government launches too many civilians are involved in projects to even be worth keeping any parts of them &lt;em&gt;under wraps&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Please, feel welcome to adding your own favorite space exploration related infographics to &lt;a href=&quot;https://space.meta.stackexchange.com/q/472/49&quot;&gt;our meta post where we're collecting them&lt;/a&gt; for easy reference. Thanks!&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-19T00:11:03.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3754" PostTypeId="2" ParentId="3708" CreationDate="2014-02-19T08:02:57.903" Score="0" Body="&lt;p&gt;For what it's worth:&lt;br&gt;&#xA;Using numbers that approximate Uranus (atmosphere just 350km deep, relativly low local g, cold, 100bar at lower limit of atmosphere) and ignoring the gravity from the atmosphere itself -&#xA;plugging the same numbers into the barometric formula and the one given by Alan (assuming constant temp), the differene would be about 6% at the top of the atmosphere.&#xA;The difference becomes bigger with higher g, colder Temp, and a smaller radius at the bottom of the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="102" LastActivityDate="2014-02-19T08:02:57.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3755" PostTypeId="2" ParentId="2638" CreationDate="2014-02-19T10:21:45.810" Score="8" Body="&lt;p&gt;This depends on your definition of &quot;proven&quot;. What we &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;know about Helium-3&lt;/a&gt; is based on lunar soil samples collected at 9 different locations (6x Apollo, 3x Luna sample return missions). Extrapolation from that yields a figure of 2.47 million tons of Helium-3 stored in the surface layer of the Moon.&lt;br&gt;&#xA;However, we also know that the He-3 is at a very low and variable concentration (1-15 ppb in samples). Traditionally, mining operations depend on a much more detailed survey of the area to be mined. Unlike mining on Earth, we expect He-3 to be basically everywhere on the Moon (it is supplied by the solar wind). The big question is, at which concentration does mining become profitable? This in turn depends on how much we need to spend to mine the He-3. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To gather 1 kg of He-3, we need to process 1-15 billion kg of rock. This requires thousands of tons of equipment (excavators, rock crushers, ovens) and lots of energy. On the other hand, one kg of He-3 might yield on the order of 3 GWh of energy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1843/practical-lunar-he-3-mining-approach?rq=1&quot;&gt;This question&lt;/a&gt; addresses the issues with mining the Helium and generating energy from it in far more detail.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://fti.neep.wisc.edu/pdf/wcsar9311-2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;One theoretical design&lt;/a&gt; for a mining system is estimated to produce 33 tons of He-3 while consuming ~800 MWh per year, which would be profitable. A &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890005477_1989005477.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA report&lt;/a&gt; estimates that we can extract ~80 times more energy from He-3 that we'd spend mining it. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-02-19T11:00:14.477" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3756" PostTypeId="1" AcceptedAnswerId="3757" CreationDate="2014-02-19T10:23:05.870" Score="30" ViewCount="4364" Body="&lt;p&gt;Will Voyager 1 or Voyager 2 be able to take a photo of their current view, using their cameras and send the photo to Earth? Do they have the power to do this?&lt;/p&gt;&#xA;" OwnerUserId="1427" LastEditorUserId="-1" LastEditDate="2014-02-19T13:53:23.043" LastActivityDate="2015-01-24T06:31:06.480" Title="Will Voyager 1 or 2 be able to take photos?" Tags="|probe|voyager|camera|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3757" PostTypeId="2" ParentId="3756" CreationDate="2014-02-19T11:13:19.960" Score="38" Body="&lt;p&gt;From the &lt;a href=&quot;http://voyager.jpl.nasa.gov/faq.html&quot;&gt;Voyager FAQ&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Question: Can the Voyager imaging cameras be turned back on?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Answer: It is possible for the cameras to be turned on, but it is not&#xA;  a priority for Voyager's Interstellar Mission. After Voyager 1 took&#xA;  its last image (the &quot;Solar System Family Portrait&quot; in 1990), the&#xA;  cameras were turned off to save power and memory for the instruments&#xA;  expected to detect the new charged particle environment of&#xA;  interstellar space. Mission managers removed the software from both&#xA;  spacecraft that controls the camera. The computers on the ground that&#xA;  understand the software and analyze the images do not exist anymore.&#xA;  The cameras and their heaters have also been exposed for years to the&#xA;  very cold conditions at the deep reaches of our solar system. Even if&#xA;  mission managers recreated the computers on the ground, reloaded the&#xA;  software onto the spacecraft and were able to turn the cameras back&#xA;  on, it is not clear that they would work.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In addition, it is very dark where the Voyagers are now. While you&#xA;  could still see some brighter stars and some of the planets with the&#xA;  cameras, you can actually see these stars and planets better with&#xA;  amateur telescopes on Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it might be able to happen, but not really. The cameras are probably damaged due to exposure to the cold of deep space, and the cameras wouldn't do much good anyways.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-02-19T11:13:19.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3758" PostTypeId="2" ParentId="3756" CreationDate="2014-02-19T12:51:27.460" Score="23" Body="&lt;p&gt;These &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2010-048&amp;amp;cid=release_2010-048&amp;amp;msource=f20100212&amp;amp;tr=y&amp;amp;auid=5935835&quot;&gt;were the last photos Voyager 1 took&lt;/a&gt;, on February 14, 1990:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/JKqMM.jpg&quot; alt=&quot;Pale Blue Dot&quot;&gt;&lt;br&gt;&#xA;This became known as the &quot;&lt;a href=&quot;http://petapixel.com/2013/03/25/the-pale-blue-dot-a-portrait-of-earth-shot-from-more-than-4-billion-miles-away/&quot;&gt;Pale Blue Dot&lt;/a&gt;&quot; photo. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the Solar System Family Portrait:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/kcCiu.jpg&quot; alt=&quot;Planets&quot;&gt;&lt;br&gt;&#xA;Voyager 1 had traveled 6 billion km at that time. After these images were taken, the camera was shut down as @Pearson says. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager took 60 photos. This shows all of them:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/mPN0Q.jpg&quot; alt=&quot;Voyager series&quot;&gt;&lt;br&gt;&#xA;On some of the photos, you can see rays of scattered light from the Sun. Because Earth is relatively close to the sun, these rays are visible in the Earth image.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://photojournal.jpl.nasa.gov/jpeg/PIA00451.jpg&quot;&gt;High-resolution version of the mosaic&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this list of &lt;a href=&quot;http://pds-rings.seti.org/voyager/iss/calib_images.html&quot;&gt;Voyager 2 photos&lt;/a&gt;, Voyager 2 took its last photo of Neptune on 1989-09-29:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/rsIRG.jpg&quot; alt=&quot;Neptune&quot;&gt;&lt;br&gt;&#xA;In 1998, the scan platform (which contains the cameras) was &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2&quot;&gt;shut down&lt;/a&gt;, as not enough power was available to operate it any more. This means that even if the cameras could be powered up again, there's no way to aim them.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-02-21T09:14:15.593" LastActivityDate="2014-02-21T09:14:15.593" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3759" PostTypeId="1" CreationDate="2014-02-19T13:02:40.877" Score="14" ViewCount="1635" Body="&lt;p&gt;The L1 and L2 points are thought to be unstable &quot;saddle&quot; points, meaning that there is stability in two directions of movement, but not in the other.  That raises an obvious question - when a spacecraft stationed there diverges toward either side of the saddle, and station-keeping is no longer maintained, where will it go?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several projects planned for these points.  The JWST won't be at the Sun-Earth L2 point &lt;em&gt;forever&lt;/em&gt;.  Likewise, we're looking at a plan for moving an asteroid to the Earth-Moon L2 point, and a lot of people raise questions about what would happen if it goes astray (although I'm not worried).  Unless we keep sending supply missions in perpetuity, it will drift away &lt;em&gt;eventually&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm most interested in the Earth-Moon system.  We have 2 points, and 2 directions it can fly off into.  Does that result in 4 distinct, deterministic, trajectories?  Which of these would crash into the moon and the Earth?  Would it happen with one orbit?  Or would any of them be chaotic?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2015-12-05T15:07:05.293" Title="If something &quot;falls off&quot; the L2 or L1 point, where will it go?" Tags="|orbital-mechanics|trajectory|lagrangian-points|planning|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3760" PostTypeId="2" ParentId="3759" CreationDate="2014-02-19T16:08:55.567" Score="8" Body="&lt;p&gt;Thanks to a very helpful suggestion in the comment, I was able to do an initial test with an online orbital simulator.  It can be found here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://astro.u-strasbg.fr/~koppen/body/ThreeBody.html&quot; rel=&quot;noreferrer&quot;&gt;http://astro.u-strasbg.fr/~koppen/body/ThreeBody.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This starts off (helpfully) with the Earth moon parameters, for the mass ratio and the spacing.  All we need to do is tell it where to put the initial particle.  Here is the screen where you can do that:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iPVul.png&quot; alt=&quot;parameters&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is &quot;details of initial situation&quot;.  As a side note, I believe they marked both the system CM and Earth, but they partially obscure each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that I've changed the parameters to what we need for simulation starting at L1.  It is around 84% of the way from Earth to the moon.  Make sure the angle is set correctly too.  Then we have two parameters for initial velocity - set both to zero because we are in the co-rotating reference frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's what I get doing the simulation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9GZpK.png&quot; alt=&quot;plot&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that it initially moves tangentially to the Earth-moon line, and then crashes into Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't tell if this is the answer I'm looking for.  Simplistically, I don't see how it can move in that direction.  Given the shape of the saddle point, I believe it should move either to the right or the left in the above plot.  But the mechanics in the co-rotating frame are notoriously spooky.  It still seems possible that this is right, but I can't tell.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The problem was that I misunderstood what the input parameters were.  The initial tangential velocity is still given in reference to a non-rotating frame.  In order to be &quot;stationary&quot; relative to the Earth-moon system, this parameter needs to be &lt;em&gt;equal&lt;/em&gt; to the radius parameter.  See help:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://astro.u-strasbg.fr/~koppen/body/ThreeBodyHelp.html#BUTTON&quot; rel=&quot;noreferrer&quot;&gt;http://astro.u-strasbg.fr/~koppen/body/ThreeBodyHelp.html#BUTTON&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;init.radial velocity&#xA;      with respect to the Centre of Mass, given in units of the oribital speed of the Moon&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;init.tangent.velocity&#xA;      ditto, positive velocities point &lt;strong&gt;in the same sense as the lunar motion&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So in the above L1 example, it the initial tangential velocity needs to be 0.84.  If you do that, you can get this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hjf4G.png&quot; alt=&quot;L1 moon&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is much more reasonable, and more likely correct.  In this example, it does crash into the moon, but only after about a month.  The scenarios appear to fit:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;L1 moon-side drop: orbits for 1 month, crashes into moon&lt;/li&gt;&#xA;&lt;li&gt;L1 earth-side drop: goes into high Earth orbit, looks stable&lt;/li&gt;&#xA;&lt;li&gt;L2 moon side drop: goes into moon orbit, can transition into high Earth orbit&lt;/li&gt;&#xA;&lt;li&gt;L2 far side drop: orbits both Earth and moon at very far distance&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2014-02-19T16:50:41.007" LastActivityDate="2014-02-19T16:50:41.007" CommentCount="0" CommunityOwnedDate="2014-02-19T16:08:55.567" ContentLicense="CC BY-SA 3.0" />
  <row Id="3761" PostTypeId="2" ParentId="1858" CreationDate="2014-02-20T10:03:12.453" Score="5" Body="&lt;p&gt;In addition to previous points above, I offer the following.  Firstly, the dV required to reach the outer planets, or leave the SS, can be partly picked up by repeated gravity assists from the planets (see Rosetta's flight plan with which it can catch a comet).  To do this you have to stay in the planes of the fly-by planets. Second, the dV to get a 'polar' or perpendicular orbit can be reduced if either the launch trajectory is north/south or if your path has a fly-by by a pole of another planet.  The Voyagers did this after their last planet encounter. (V2 is at about 55deg declination).  It can take very little dV to bend an outbound trajectory to meet a planet on a polar orbit approach.  By the way, launching into a terrestrial polar orbit uses more energy than an equatorial one because you can't take advantage of the angular velocity the rocket has sitting on the pad.&lt;/p&gt;&#xA;" OwnerUserId="1569" LastActivityDate="2014-02-20T10:03:12.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3762" PostTypeId="2" ParentId="1858" CreationDate="2014-02-20T12:26:50.567" Score="1" Body="&lt;p&gt;Space telescopes like Gaia and the James Webb will not study planets, but they are still placed in the ecliptic plane of the planets (and Earth's moon) in order to take advantage of their gravity. Another potential &quot;resource&quot; in the ecliptic is shadow from the Sun, and for a radio telescope on the far side of the Moon, a radio shadow from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reasons &lt;em&gt;not&lt;/em&gt; to stay in the ecliptic would be to study the Sun's poles, like the Ulussey probe did. Oort cloud and Kuiper belt objects can be very inclined and could require missions way outside of the ecliptic. Even such missions would take advantage of gravity assist by planets before they leave the ecliptic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A space telescope which would not be placed in the ecliptic would be a radio telescope placed so far out that it can use the Sun as a gravitational lens. That's at least 550 AU away, about 14 times the distance to Neptune. And maybe twice that because of the Sun's disturbing corona. It would get great magnification only in the direction of the Sun, so it would be sent on a course away from the object one would like to study. Maybe a specific star or the supermassive black hole at the center of the Milky Way. On an interstellar scale, the plane of the planets in the Solar system becomes irrelevant.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-02-20T12:34:13.523" LastActivityDate="2014-02-20T12:34:13.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3763" PostTypeId="1" AcceptedAnswerId="3764" CreationDate="2014-02-20T12:59:32.500" Score="7" ViewCount="504" Body="&lt;p&gt;JPL together with Goddard and a private company called Busek Company Inc. developed a new type of propulsion system known as &lt;a href=&quot;http://nmp.jpl.nasa.gov/st7/QUICKFACTS/QF_index.html&quot;&gt;micronewton thrusters&lt;/a&gt; for the LISA mission (which has since had a name change and a rescoping).  The goal of these small thrusters is to maintain the position of a spacecraft from small external disturbances such as solar winds and radiation pressure.  They were to be used on the LISA mission to have the spacecraft precisely follow the freely floating, mirrored, test masses inside of the spacecraft, the positions of which were to be measured by interferometry.  My questions are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What is the expected lifetime of the micronewton thrusters?&lt;/li&gt;&#xA;&lt;li&gt;What is the propulsion method employed; i.e. chemical, electric, etc.?&lt;/li&gt;&#xA;&lt;li&gt;Have they been flight tested yet?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="2699" LastEditorUserId="2699" LastEditDate="2014-02-20T15:10:58.387" LastActivityDate="2014-02-21T01:32:01.293" Title="Have the micronewton thrusters developed for the LISA mission been flight tested yet?" Tags="|propulsion|spacecraft|engine-design|engines|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3764" PostTypeId="2" ParentId="3763" CreationDate="2014-02-20T15:06:07.617" Score="3" Body="&lt;p&gt;The test masses, along with the thrusters, are part of the Disturbance Reduction System (DRS). They are electrostatic &lt;a href=&quot;http://en.wikipedia.org/wiki/Colloid_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;colloid thrusters&lt;/a&gt;. From &lt;a href=&quot;https://archive.org/stream/nasa_techdoc_20040171611/20040171611#page/n2/mode/1up&quot; rel=&quot;nofollow noreferrer&quot;&gt;&amp;quot;The Space Technology-7 Disturbance Reduction System&amp;quot;&lt;/a&gt; (James R. O'Donnell Jr., Oscar C. Hsu, John Hanson, Vlad Hruby; 2004):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Each DRS colloidal thruster consists of a set of emitters and electrodes that generate thrust by ejecting and electrostatically accelerating nanometer-sized, positively charged propellant droplets (colloids) (Fig. 6). The emitters are held at a positive variable voltage by a beam voltage source typically operating at 6kV. The extractor electrode located near the emitters is supplied by a variable voltage, nominally ranging between 1.5kV to 2.5kV below the emitter voltage. The extractor electrode asserts an electrostatic force on the fluid to form the so-called Taylor cone at the tip of each emitter. The cone tip transits into a cylindrical jet, some tens of nanometers in diameter, which ultimately breaks into a fine spray.&lt;/p&gt;&#xA;&lt;p&gt;After passing through the extractor, the spray is further accelerated as it flows through an accelerator electrode, which is held at a lower potential than the extractor electrode and negative with respect to the neutralizer cathode to prevent neutralizer electrons from flowing back into the thruster.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Idu7g.jpg&quot; alt=&quot;Figure 6 Colloidal Thruster Schematic&quot; /&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Colloid MicroNewton Thrusters, along with new Gravitational Reference Sensors, are being flight tested on this mission. I have not run across a mention of their expected lifetime.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-21T01:32:01.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3765" PostTypeId="1" AcceptedAnswerId="3773" CreationDate="2014-02-20T15:34:52.920" Score="20" ViewCount="3533" Body="&lt;p&gt;The fiery re-entry of spacecraft has been a staple of spaceflight since the beginning, making ablative heat shielding a necessary component of any craft wishing to return to Earth intact. This is the result of the shockwave caused by the high-speed encounter with the upper atmosphere, in turn dictated by the angle of re-entry (~40 deg for the Space Shuttle). The literature on this topic always says that too shallow of an angle would result in the craft &quot;skipping&quot; off of the atmosphere. This seems to imply that the atmosphere has a &quot;surface&quot; similar to the ocean, which is certainly not intuitive for a gaseous fluid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is, why is it impossible to gradually, gently deorbit using a shallow glidepath? Is there no combination of aerodynamic surface &amp;amp; thrust profile that could make this happen?&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2015-02-22T12:18:20.973" Title="Why is it not possible to deorbit in a shallow glidepath?" Tags="|crewed-spaceflight|reentry|spacecraft|atmosphere|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3766" PostTypeId="2" ParentId="3765" CreationDate="2014-02-20T15:57:09.070" Score="9" Body="&lt;p&gt;You actually can deorbit using a shallow glidepath that &quot;skips&quot; off the atmosphere, slowing down each time, called a &quot;skip trajectory.&quot; This helps to dissipate the heat the craft experiences over a longer time frame, and allows for a greater range of landing sites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But why does a craft skip when there is no well defined surface to the atmosphere? It happens because of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lift-to-drag_ratio&quot; rel=&quot;noreferrer&quot;&gt;lift-to-drag ratio&lt;/a&gt; of the craft. Most craft have angled surfaces, and as it deorbits into thicker parts of the atmosphere, they will create more lift. If the object reentering the atmosphere is a sphere, there will be almost no lift, but it will decelerate too quickly (for humans, anyway). Having flat surfaces is important for a softer deceleration, which creates a higher lift-to-drag ratio, and the potential to &quot;fly back out&quot; of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check out this website for more information:&#xA;&lt;a href=&quot;http://www.aerospaceweb.org/question/spacecraft/q0218.shtml&quot; rel=&quot;noreferrer&quot;&gt;http://www.aerospaceweb.org/question/spacecraft/q0218.shtml&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(sorry I don't have specific examples, my physics background is not very strong)&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-02-20T19:15:52.850" LastActivityDate="2014-02-20T19:15:52.850" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3767" PostTypeId="2" ParentId="2670" CreationDate="2014-02-20T17:22:16.677" Score="2" Body="&lt;p&gt;Interestingly, a fuel depot is one of the objectives of the Lunar Space Elevator project that a company called &lt;a href=&quot;http://liftport.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;LiftPort&lt;/a&gt; is pursuing.&lt;/p&gt;&#xA;&lt;p&gt;The idea is that an elevator could be constructed from the lunar surface to EML1 using COTS technology available today. Using a solar-powered lifter to transit payloads from EML1 to the lunar surface and back solves the problem of the large ΔV required to soft land a payload on the lunar surface or return it to lunar orbit.&lt;/p&gt;&#xA;&lt;p&gt;According to Michael Laine, the LiftPort CEO, lunar regolith could be processed into oxygen and hydrogen. He plans to extract that oxygen and hydrogen, then store it either at EML1, or at the Lunar Elevator counterweight, which would be 250,000 km from the lunar surface. At that point it could be used as fuel for any planetary or deep space mission.&lt;/p&gt;&#xA;&lt;p&gt;I don't have the technical background to know how feasible this plan is and would enjoy hearing others' insights into this intriguing project.&lt;/p&gt;&#xA;" OwnerUserId="2704" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-20T17:22:16.677" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3768" PostTypeId="2" ParentId="3709" CreationDate="2014-02-20T17:34:19.857" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;http://liftport.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;LiftPort Group&lt;/a&gt; is planning to station the &quot;top&quot; of their Lunar Space Elevator at EML1. From that point, they would drop an anchor station to the lunar surface while extending a counterbalance to 250,000 km above the lunar surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is a graphic illustrating the proposed Lunar Space Elevator.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/pFf8H.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Michael Laine, the LiftPort CEO, the Lunar Space Elevator will be in place and operational by the end of 2019. Their biggest remaining hurdle is funding, as they will need nearly $1B US to build the elevator.&lt;/p&gt;&#xA;" OwnerUserId="2704" LastActivityDate="2014-02-20T17:34:19.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3769" PostTypeId="2" ParentId="3765" CreationDate="2014-02-20T18:50:06.497" Score="17" Body="&lt;p&gt;When you reenter at a shallow angle your velocity is almost horizontal and you have orbital velocity.  A small amount of lift will lift your velocity above the vertical and send you back into orbit.  This is what is meant by skipping off the atmosphere.  As long as you have enough oxygen, you will survive this.  After a few skips (losing velocity each time due to drag) you will be under orbital velocity, so if you are going to reenter shallow you need to generate lift.  At hypersonic velocity the lift to drag ratio tends not to be very good, so generating that lift will generate a lot of drag, slowing you even more.  Then you drop lower into the atmosphere because you aren't generating enough lift, generate more heat, and burn up.  Theoretically if you had thrust at your disposal you could stay high while you lose the orbital velocity.  The amount of fuel required would be enormous, though.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-02-20T18:50:06.497" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="3770" PostTypeId="1" CreationDate="2014-02-20T21:21:20.440" Score="6" ViewCount="1383" Body="&lt;p&gt;Space guns have a lot of confounding factors for getting to LEO.  The projectile must be a rocket capable of large delta v in order to circularize the orbit.  Then, giving more sideways velocity requires a more shallow angle of launch, which makes the (already troublesome) drag even worse.  Both of these problems would be mitigated if the destination is switched from LEO to somewhere else, and the gun shoots almost exactly vertical.  If you could do this, sending regular supplies to somewhere in Earth-Moon space would have advantages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a qualifier, this requires larger velocities than LEO, which couldn't be delivered with a light gas gun.  These unprecedented speeds would be very difficult to achieve, but it's still possible in theory with a rail gun or Gauss gun, for instance.  Actually, I have &lt;a href=&quot;http://nextbigfuture.com/2009/02/nuclear-orion-home-run-shot-all-fallout.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;something vastly more radical&lt;/a&gt; in mind.  Agnostic to the gun type, the problem of a low Delta V path to somewhere that won't crash into the Earth is an interesting question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say, for instance, that we shoot vertically upward from Earth aiming for EML-1.  I realized from &lt;a href=&quot;https://space.stackexchange.com/questions/3759/if-something-falls-off-the-l2-or-l1-point-where-will-it-go&quot;&gt;an orbital simulator&lt;/a&gt; that this is the time-reverse of just falling from that point.  Here, a test particle starts at what &lt;em&gt;would&lt;/em&gt; be EML-1, but is stationary in the Earth-moon CM reference frame.  So the test particle is moving $0.86 \text{ km}/\text{s}$ relative to EML-1 reference frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9GZpK.png&quot; alt=&quot;simple fall&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I've neglected the rotation of the Earth itself.  The equator itself has a velocity of $0.46 \text{ km}/\text{s}$, which will affect the mechanics of approaching EML-1, if launched straight up from the ground.  Even a perfectly vertical shot would be following a highly elliptical orbit (although could crash into Earth).  I think this would reduce the Delta V to EML-1, but the math isn't obvious.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;With a single shot, would it be possible to achieve the EML-1 or 2 position &amp;amp; velocity without using any on-board propellant?&lt;/li&gt;&#xA;&lt;li&gt;If not, what would be the minimum Delta V needed to reach L1,2,3,4,5, or any staging area that is not a chaotic orbit, and will not crash into the Earth or Moon?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In a childish sense of how hills work, I can roll a ball up to the top, and it could balance there, given a perfect shot.  This might not be quite as simple, but I'm not sure.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-03-11T19:47:46.347" Title="Minimum Delta V to a staging area in cislunar space for a vertical space gun" Tags="|delta-v|lagrangian-points|space-gun|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3771" PostTypeId="2" ParentId="3765" CreationDate="2014-02-20T21:26:44.853" Score="3" Body="&lt;p&gt;It actually &lt;em&gt;is&lt;/em&gt; possible (in a sense).  In such cases, the trajectory is a slow spiral downward, but since the majority of the velocity will be lateral, the craft will tend to slow down faster than falling, therefore making the trajectory progressively steeper.  When this happens, the lifting effects of the other answers apply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This actually happens frequently, just usually not on purpose.  The ISS, for example, is very slowly deorbiting (due to air resistance).  This is why it has had to be boosted several times to a higher orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that this needs to happen very gradually.  If you're in a circular orbit and then burn retrograde, you'll be in an elliptical orbit, which will result in far and close passes to the planet (which could be mistaken for skipping off its atmosphere).&lt;/p&gt;&#xA;" OwnerUserId="2707" LastActivityDate="2014-02-20T21:26:44.853" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3772" PostTypeId="1" AcceptedAnswerId="3789" CreationDate="2014-02-21T03:40:36.870" Score="35" ViewCount="38787" Body="&lt;p&gt;I've read over the decades that a magnetic shield might protect a spacecraft from cosmic radiation. Its a fascinating idea that might only be theory or science fiction at the moment. In regards to that &lt;a href=&quot;http://physicsworld.com/cws/article/news/2008/nov/06/magnetic-shield-could-protect-spacecraft&quot;&gt;here is an article on that topic&lt;/a&gt; in case someone isn't sure what I mean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article even mentions simulations about midway into the article. Does anyone know if research has continued?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My big question though, is how much electrical power would be required to protect a space vessel in this way? How large would the field be in order to be useful?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Better yet, please use some practical objects that anyone can wrap their head around: What if you were using similar technology to protect the Apollo Command Module? Or the Discovery One from 2001 (140 meters long).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In laymans terms, how much electrical power would it take to create a magnetic shield?&lt;/p&gt;&#xA;" OwnerUserId="2709" LastEditorUserId="49" LastEditDate="2014-02-21T04:42:46.763" LastActivityDate="2017-11-25T17:25:41.270" Title="How much power would a spacecraft's magnetic shield require?" Tags="|magnetic-field|power|research|shielding|cosmic-radiation|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3773" PostTypeId="2" ParentId="3765" CreationDate="2014-02-21T08:29:25.230" Score="9" Body="&lt;p&gt;There was a great little space simulator originally designed for simulating Orbital Tethers (the &lt;a href=&quot;http://www.spacetethers.com/&quot;&gt;space tethers applet&lt;/a&gt;) that was expanded to do a variety of space based simulations, with readouts of various numbers relevant to the simulation:- distance, speed, temperature etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Running that software using a variety of the inbuilt scenarios, it was clear that a shallow re-entry, while lowering the maximum momentary heat the craft experiences, prolongs the &lt;em&gt;duration of the heating phase significantly.&lt;/em&gt;  As a result of that, a shallow re-entry will cause a craft to experience a &lt;em&gt;greater overall heat load&lt;/em&gt; than a steep one.&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="238" LastEditDate="2014-02-21T16:25:04.640" LastActivityDate="2014-02-21T16:25:04.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3774" PostTypeId="1" AcceptedAnswerId="3777" CreationDate="2014-02-21T09:20:17.110" Score="18" ViewCount="2260" Body="&lt;p&gt;If &lt;strong&gt;Deep Space Hibernation&lt;/strong&gt; of a probe saves energy and power, then why isn't it implemented on long planetary missions and Interstellar missions like,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;New Horizons, Voyagers, Pioneers, Cassini&lt;/strong&gt;, etc....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any plan to implement it on future missions ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(If the voyagers &amp;amp; pioneers had this facility, they could use that saved power to run science instruments, transmitters even now).&lt;/p&gt;&#xA;" OwnerUserId="1427" LastEditorUserId="897" LastEditDate="2014-02-21T09:41:23.893" LastActivityDate="2014-02-21T15:16:32.600" Title="Why not use deep space hibernation in all missions?" Tags="|probe|future-missions|deep-space|deep-space-hibernation|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3775" PostTypeId="5" CreationDate="2014-02-21T09:36:58.067" Score="0" Body="&lt;p&gt;Questions about deep space hibernation that are related to the hibernation technique in computers, which saves powers and energy, Long life probes uses hibernation to achieve more energy and power by hibernating the systems in deep space....&lt;/p&gt;&#xA;" OwnerUserId="1427" LastEditorUserId="1427" LastEditDate="2014-02-21T11:19:30.837" LastActivityDate="2014-02-21T11:19:30.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3776" PostTypeId="4" CreationDate="2014-02-21T09:36:58.067" Score="0" Body="Deep Space Hibernation" OwnerUserId="1427" LastEditorUserId="1427" LastEditDate="2014-02-21T11:19:42.307" LastActivityDate="2014-02-21T11:19:42.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3777" PostTypeId="2" ParentId="3774" CreationDate="2014-02-21T09:46:17.030" Score="23" Body="&lt;p&gt;Sometimes you don't want to hibernate the craft, because it has scientific operations to perform &lt;em&gt;during&lt;/em&gt; the interplanetary transfer phases. But even when you don't: &lt;strong&gt;Deep space hibernation is risky&lt;/strong&gt;. There is always the risk that the wake-up procedure - whether triggered by the vessel or by the ground station - won't work and the vessel will not recover. It also leaves a long period of time where you have absolutely no way to monitor the state of the vessel and its equipment and have no way to intervene when something goes wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it should only be done when there is a benefit to it. But often there isn't really a benefit. Temporarily saving energy is often not really an issue. Most vessels are powered by solar cells or radio-isotope generators. Both technologies have some wear over time and lose efficiency, but for neither does it matter how much of their energy is actually used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Voyager probes, for example, need to conserve energy not because they used up so much during the first part of their mission but because their radio-isotope generators are reaching the end of their (strictly chronological) lifetime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For solar-powered crafts, hibernation can make sense for two reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The mission requires the craft to spend long times eclipsed by celestial bodies or leads it on a trajectory far away from the sun, so there are mission phases where solar power is unavailable or severely limited.&lt;/li&gt;&#xA;&lt;li&gt;Depending on how the craft is constructed, it might be necessary to keep the craft oriented to the sun so the solar panels reach full efficiency. That might require the use of active thrusters which only have a limited amount of propellant. Ignoring orientation and going into power-saving mode during non-critical mission phases can conserve propellant.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-02-21T15:16:32.600" LastActivityDate="2014-02-21T15:16:32.600" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3778" PostTypeId="2" ParentId="3774" CreationDate="2014-02-21T09:49:36.630" Score="4" Body="&lt;p&gt;The probe &lt;strong&gt;New Horizons&lt;/strong&gt; have the hibernation feature implemented, for its long journey to pluto.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After passing Jupiter, New Horizons will spend most of its journey towards Pluto in hibernation mode...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Refer &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons#Hibernation_towards_Pluto&quot; rel=&quot;nofollow&quot;&gt;Wiki&lt;/a&gt; for more..&lt;/p&gt;&#xA;" OwnerUserId="2589" LastActivityDate="2014-02-21T09:49:36.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3779" PostTypeId="1" CreationDate="2014-02-21T10:54:32.163" Score="1" ViewCount="384" Body="&lt;p&gt;I've built a model of the Solar System by calculating Keplerian orbital elements. The model takes a time $t$ which is incremented with each frame of the simulation. I then calculate the new positions of each body. $t$ is really the time since perihelion for each orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that at the beginning of the simulation each object starts at its perihelion (since $t=0$). So, now I'd like to place a new body (a comet, for example) with a position $P$ and velocity $V$. From $P$ and $V$, I calculate the orbital elements such as eccentricity $e$, semi-major axis $M$, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I can calculate the next position of this new body (since $t$ has been incremented) using these orbital elements. However, since $t$ is the time since this new orbits perihelion, the calculated position will not be the next incremental position. One solution to this could be that I calculate the time ($t\text'$) at which this new orbit would position the body at $P$ and increment $t\text'$. This means that instead of having a global $t$, I have a $t$ for each body which is incremented at the same rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this sound sensible?&lt;/p&gt;&#xA;" OwnerUserId="2690" LastEditorUserId="49" LastEditDate="2014-02-24T14:08:59.190" LastActivityDate="2014-02-26T21:46:41.767" Title="Specifying a position and velocity of body in a Kepler simulation" Tags="|orbital-mechanics|mathematics|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3780" PostTypeId="2" ParentId="2493" CreationDate="2014-02-21T12:54:48.590" Score="13" Body="&lt;p&gt;Apollo 11 landed with &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_18-27_LM_Lunar_Landing.htm&quot; rel=&quot;noreferrer&quot;&gt;45 seconds&lt;/a&gt; of hover time remaining, but there is some leeway in that number. Apollo 17 landed with 117 seconds of hover time remaining. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's an article by Paul Fjeld in the &lt;a href=&quot;http://www.aiaahouston.org/Horizons/Horizons_2013_05_and_06.pdf&quot; rel=&quot;noreferrer&quot;&gt;May/June 2013 issue of AIAA Horizons&lt;/a&gt; (PDF) newsletter which addresses the question of Apollo 11 reserve fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the landing, you can hear the fuel level being called out (&quot;30 seconds&quot;), this was the time remaining until a &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/001499.html&quot; rel=&quot;noreferrer&quot;&gt;decision to abort&lt;/a&gt; had to be made. In an emergency, the crew could ditch the descent stage and start the ascent engine (yes, in flight!). This abort procedure took some time (to prepare the ascent engine for ignition), so the descent engine needed to run for a few seconds after the abort decision.&lt;br&gt;&#xA;The availability of the abort option meant NASA could limit the amount of reserve fuel without jeopardizing the crew. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a11/A11MissRules.pdf&quot; rel=&quot;noreferrer&quot;&gt;Flight Mission Rules&lt;/a&gt; (large PDF) contains some rules on when to abort the mission: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;5-91 There are no trajectory or guidance constraints which are cause for abort &#xA;  after crew take over of powered descent.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;25-12 The total continuous burn time of the descent engine shall not&#xA;  exceed 910 seconds of continuous operation independent of thrust level&#xA;  (based upon a lunar mission duty cycle).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;25-18 If possible the DPS will not be burned to propellent depletion,&#xA;  where ever possible the abort stage sequence will be initiated at low&#xA;  level plus 20 seconds during an abort from powered descent, however,&#xA;  at crew option abort stage sequence may be initiated at low level if a&#xA;  safe abort stage capability exists.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The crew had significant leeway, as long as an abort decision was made in time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The AIAA article also notes that fuel reserves were severely constrained by the weight budget (page 9). As early as 1963, &lt;a href=&quot;http://www.astronautix.com/l/lmdescentpropulsion.html&quot; rel=&quot;noreferrer&quot;&gt;discussions&lt;/a&gt; were going on regarding the amount of reserve to be carried. At one time, Grumman ran &quot;&lt;a href=&quot;http://www.astronautix.com/l/lmweight.html&quot; rel=&quot;noreferrer&quot;&gt;Operation Scrape&lt;/a&gt;&quot; in an effort to reduce weight of the LM. Remember that thanks to the rocket equation, every kg of fuel you land with carries a penalty of several kg of weight at the start of the descent, and maybe dozens of kg at liftoff from Earth. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="25067" LastEditDate="2018-08-21T08:10:57.953" LastActivityDate="2018-08-21T08:10:57.953" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3781" PostTypeId="1" CreationDate="2014-02-21T13:51:32.313" Score="4" ViewCount="4263" Body="&lt;p&gt;Given ...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A known position along a planets orbit &lt;/li&gt;&#xA;&lt;li&gt;The semi major axis &lt;/li&gt;&#xA;&lt;li&gt;The eccentricity of the orbit &lt;/li&gt;&#xA;&lt;li&gt;The period of orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;is it possible to calculate the time since periapse of this orbital position?&lt;/p&gt;&#xA;" OwnerUserId="2690" LastEditorUserId="49" LastEditDate="2014-02-24T13:40:34.157" LastActivityDate="2014-02-24T13:40:34.157" Title="Calculating the time since periapse of an orbital position" Tags="|orbital-mechanics|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3783" PostTypeId="2" ParentId="3774" CreationDate="2014-02-21T14:13:31.670" Score="7" Body="&lt;p&gt;Most of the time you don't ever want to use a deep hibernation mode on a spacecraft. If at all possible, you want it active and chatty (with mission control) the entire time, since it means you can do science, monitor its systems, and keep more diagnostic and recovery options available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Rosetta probe used deep-sleep mode because for a big part of its journey it was out past Jupiter, and did not receive enough solar power to remain fully functional. It's rendevouz now means it's closer, and can wake up again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is another more interesting reason that has to do with the machine lifetime of the onboard computers: space agencies have models of their computers which basically estimate the number of instruction-operations they can perform before they're likely to experience a hard failure due to wear or uncorrectable errors from radiation bit-flipping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of very long range spaceprobes, the probability of a failure starts to climb to 1 - that is, leave the ship fully operational and something will go wrong with the onboard computer (which you may or may not be able to recover from) before you reach your target. So deep-sleeping the spacecraft is a way to conserve the computers lifetime - you switch over to very robust real-time clock circuits, and power down the computer entirely.&lt;/p&gt;&#xA;" OwnerUserId="2713" LastActivityDate="2014-02-21T14:13:31.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3784" PostTypeId="1" AcceptedAnswerId="3785" CreationDate="2014-02-21T16:57:03.607" Score="5" ViewCount="240" Body="&lt;p&gt;I usually see the approach leg before an orbital injection trajectory around a body lying outside Earth's orbit drawn as catching up to it (moving in the same general direction/speed as the body's orbital vector) and approaching the far side of the planet/moon/etc. Does it ever make sense to approach the body from the near side, ahead of it, and still end up in orbit around the body? Can such a trajectory be accomplished without totally blowing your delta-v budget? If feasible and practicable, has any probe taken this approach vector?&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2014-02-22T03:51:21.210" Title="What is the best approach vector for orbital capture by an outer solar system body?" Tags="|orbital-mechanics|trajectory|deep-space|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3785" PostTypeId="2" ParentId="3784" CreationDate="2014-02-21T18:28:37.697" Score="7" Body="&lt;p&gt;In the frame of reference of the Sun, it is the outer solar system body (Mars, Jupiter, etc.) that catches up with the vehicle from Earth, not the other way around.  The energy of the vehicle from Earth is lower than the destination body, so that body is going faster around the Sun than the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the frame of reference of the destination body, the approach velocity of the vehicle is a vector fixed by the trajectory from Earth, and defines a plane, called the B-plane, through the body that is perpendicular to that approach velocity vector.  The vehicle can choose any point in the B plane to target, which will determine the closest approach distance to the body and a clock angle somewhere around the body.  Doing an orbit insertion burn at the closest approach point will result in an orbit that is prograde, retrograde, polar one way or the other, or somewhere in between depending on that clock angle, as you like.  Though the orbit is limited to an inclination no less than the declination of the approach vector.  So you can't go into an equatorial orbit with one maneuver.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-22T03:51:21.210" LastActivityDate="2014-02-22T03:51:21.210" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3786" PostTypeId="1" AcceptedAnswerId="3787" CreationDate="2014-02-22T04:18:15.773" Score="2" ViewCount="261" Body="&lt;p&gt;While going through links mentioned in &lt;a href=&quot;https://space.stackexchange.com/questions/3663/what-software-can-be-used-to-determine-deep-space-trajectories&quot;&gt;What software can be used to determine deep-space trajectories?&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/646/what-are-the-choices-today-for-orbital-mechanics-simulation-software&quot;&gt;What are the choices today for orbital mechanics simulation software?&lt;/a&gt;, I find that a great many (if not most) high-end trajectory analysis software depend on &lt;a href=&quot;http://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL DE files&lt;/a&gt; for their ephemeris data. Any chance that Sedna or other TNOs are covered already in the latest versions, or does our potential Sedna mission design team have to resort to the &lt;a href=&quot;http://www.americaspace.com/?p=44591&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mark One Cranium Computer&lt;/a&gt;? &lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="37597" LastEditDate="2022-09-27T16:43:20.827" LastActivityDate="2022-09-27T16:43:20.827" Title="Do the proper ephemerides yet exist to calculate trajectories to Sedna?" Tags="|trajectory|software|mission-design|ephemeris|sedna|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3787" PostTypeId="2" ParentId="3786" CreationDate="2014-02-22T04:57:23.183" Score="6" Body="&lt;p&gt;Yes, we have &lt;a href=&quot;http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=sedna&quot; rel=&quot;noreferrer&quot;&gt;good orbital elements for Sedna&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-02-22T04:57:23.183" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3788" PostTypeId="2" ParentId="3781" CreationDate="2014-02-23T01:01:28.087" Score="3" Body="&lt;p&gt;Well yes, you probably could. What you'd have to do is utilize &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion#Second_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler's second law&lt;/a&gt; that basically says that area swept by the focus-to-body line remains constant during equal intervals of time:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/jyN3U.gif&quot; alt=&quot;Animation of the Kepler&amp;#39;s second law for a planetary orbit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Animation of the Kepler's second law&lt;/strong&gt;: The same blue area is swept out in a given time (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Kepler-second-law.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your semi-major axis $a$ and eccentricity $e$ then gives you the total area $A_o$ that the one-orbit ellipse prescribes, and given your known position you could calculate the area prescribed since the periapsis, let's call it $A_p$. The fraction of $A_p$ in relation to $A_o$ is then directly proportional to the time slice since the periapsis $t$ of the total orbital period $T$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$A_o$ equals $a\cdot b\cdot \pi$ , where $a$ is already mentioned semi-major axis, and $b$ is semi-minor axis. To calculate $b$ of an ellipse, we can use the argument of eccentricity $e$ using $b=a{\sqrt  {1-e^{2}}}$, so our formula is then:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$A_o = a^2\pi{\sqrt  {1-e^{2}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How you could calculate the $A_p$, or the fraction of the $A_o$ since the periapsis comes down to what you meant with &lt;em&gt;a known position along a planet's orbit&lt;/em&gt;, so I'll leave out this part as an exercise for the reader, but your time since periapsis $t$ would then be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$t = T \cdot \frac{A_p}{A_o}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would of course be a lot simpler, if you calculated &lt;a href=&quot;https://en.wikipedia.org/wiki/Mean_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;mean anomaly&lt;/a&gt; $M$ as part of your orbital elements, and then simply do $t = \frac{M}{n}$, where $n$ is mean motion that equals $\frac{2\pi}{T}$.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-23T01:20:09.817" LastActivityDate="2014-02-23T01:20:09.817" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3789" PostTypeId="2" ParentId="3772" CreationDate="2014-02-23T07:02:06.267" Score="27" Body="&lt;p&gt;Yes, the research on shielding from energetic particles of solar wind plasma using dipole magnetic field continues, and perhaps the best indication of that is the filing of the &lt;em&gt;Spacecraft shield&lt;/em&gt; patent &lt;sup&gt;&lt;a href=&quot;http://www.google.com/patents/US20110049303&quot;&gt;(1)&lt;/a&gt;&lt;/sup&gt; in 2010, roughly 2 years after the publication of the Plasma Physics and Controlled Fusion journal &lt;sup&gt;&lt;a href=&quot;http://www.minimagnetosphere.org/downloads/pdfs/1Bamford.pdf&quot;&gt;(2)&lt;/a&gt;&lt;/sup&gt; that was noticed by the author of that Physics World article &lt;sup&gt;&lt;a href=&quot;http://physicsworld.com/cws/article/news/2008/nov/06/magnetic-shield-could-protect-spacecraft&quot;&gt;(3)&lt;/a&gt;&lt;/sup&gt; that you link to, as well as several other similar ones, e.g. in Universe Today &lt;sup&gt;&lt;a href=&quot;http://www.universetoday.com/20671/ion-shield-for-interplanetary-spaceships-now-a-reality/&quot;&gt;(4)&lt;/a&gt;&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One look at Ruth Bamford's recent publication &lt;sup&gt;&lt;a href=&quot;http://scholar.google.com/citations?user=Y3SKtQMAAAAJ&amp;amp;hl=en&amp;amp;sortby=pubdate&quot;&gt;(5)&lt;/a&gt;&lt;/sup&gt;, the lead researcher and the first quoted author of the said publication that prompted all those news articles, shows this scientist's involvement with several space exploration concept projects, so I'd wager that research seems very much alive. For example, this article titled &lt;em&gt;Conceptual Space Vehicle Architecture for Human Exploration of Mars&lt;/em&gt; &lt;sup&gt;&lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2012-5114&quot;&gt;(6)&lt;/a&gt;&lt;/sup&gt; from 2012 already displays involvement by Boeing and ULA, among others, so that's research that's already at least partially sponsored by the big space industry players.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the required size and power, most indicative source is mentioned patent (application number US 12/990,420)&lt;sup&gt;&lt;a href=&quot;http://www.google.com/patents/US20110049303&quot;&gt;(1)&lt;/a&gt;&lt;/sup&gt;, quoting:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In order to provide an effective shield, the strength of the shield&#xA;  magnetic field at the source is preferably at least 1×10&lt;sup&gt;−4&lt;/sup&gt;&#xA;  Tesla. To obtain a boundary between the shield magnetic field and a&#xA;  typical solar wind background magnetic field of around&#xA;  1×10&lt;sup&gt;−7&lt;/sup&gt; Tesla (perhaps 5×10&lt;sup&gt;−8&lt;/sup&gt; to&#xA;  5×10&lt;sup&gt;−6&lt;/sup&gt; Tesla depending on the conditions of the solar wind)&#xA;  at a distance of up to a few hundred metres from the spacecraft a&#xA;  field strength of less than 0.1 Tesla at the magnetic field source&#xA;  will generally be sufficient. Allowing for effects of field&#xA;  persistence in the plasma environment, average electrical power from&#xA;  about 100 W to 10 kW, and more preferably from about 500 W to 5 kW may&#xA;  be provided by the power supply to drive the magnetic field source&#xA;  to generate the shield magnetic field.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And the size of functional elements is described in the Universe Today article &lt;sup&gt;&lt;a href=&quot;http://www.universetoday.com/20671/ion-shield-for-interplanetary-spaceships-now-a-reality/&quot;&gt;(4)&lt;/a&gt;&lt;/sup&gt; as &lt;em&gt;&quot;no bigger than a large desk&quot;&lt;/em&gt;. Of course, these quoted power requirements do seem high, but the shield wouldn't have to be active all the time at its maximum power, and its output could be variable to match observational data by remote observatories feeding data on solar flux to the spacecraft that such dipole magnetic field would be shielding beforehand. If I remember correctly, larger Solar flares reach Earth on average in about a day and a half, so that's roughly ⅔ AU per day. And of course, at distances to the Sun where there is much danger from solar weather, there's also plenty of solar power that can be tapped into by using photovoltaics. For comparison, International Space Station's solar arrays, while large and heavy, are capable of 120 kW of generating capacity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it does seem feasible that future long-duration manned missions outside the Earth's magnetic field could use their own, portable and active magnetic field, and protect the crew from exposure to charged high-energy particles of Solar winds with it. Possibly on top of using &lt;em&gt;biological shielding&lt;/em&gt; by surrounding crew compartments with layers of water and propellants, like e.g. in this Conceptual Design of Crew Exploration Lander for Asteroid Ceres and Saturn Moons Rhea and Iapetus &lt;sup&gt;&lt;a href=&quot;http://enu.kz/repository/2010/AIAA-2010-795.pdf&quot;&gt;(7)&lt;/a&gt;&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Quoted sources and suggested reading&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.google.com/patents/US20110049303&quot;&gt;Google Patents: Spacecraft shield, US 20110049303 A1&lt;/a&gt;, Ruth Bamford, Robert Bingham, 2010&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.minimagnetosphere.org/downloads/pdfs/1Bamford.pdf&quot;&gt;The interaction of a flowing plasma with a dipole magnetic field: measurements and modelling of a diamagnetic cavity relevant to spacecraft protection&lt;/a&gt;, R. Bamford et al., Plasma Physics and Controlled Fusion, 2008 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://physicsworld.com/cws/article/news/2008/nov/06/magnetic-shield-could-protect-spacecraft&quot;&gt;Physics World: Magnetic shield could protect spacecraft&lt;/a&gt;, Nov. 6, 2008&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.universetoday.com/20671/ion-shield-for-interplanetary-spaceships-now-a-reality/&quot;&gt;Universe Today: Ion Shield for Interplanetary Spaceships Now a Reality&lt;/a&gt;, Nov. 4, 2008&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://scholar.google.com/citations?user=Y3SKtQMAAAAJ&amp;amp;hl=en&amp;amp;sortby=pubdate&quot;&gt;Google Scholar: Ruth Bamford&lt;/a&gt;, STFC, RAL Space, Rutherford Appleton Laboratory, U.K&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2012-5114&quot;&gt;Conceptual Space Vehicle Architecture for Human Exploration of Mars, with Artificial Gravity and Mini-Magnetosphere Crew Radiation Shield&lt;/a&gt;, M. Benton et al., AIAA Meeting Papers, 2012&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://enu.kz/repository/2010/AIAA-2010-795.pdf&quot;&gt;Conceptual Design of Crew Exploration Lander for Asteroid Ceres and Saturn Moons Rhea and Iapetus&lt;/a&gt;, M. Benton, American Institute of Aeronautics and Astronautics, 2010 (PDF)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-20T01:57:46.687" LastActivityDate="2015-10-20T01:57:46.687" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3790" PostTypeId="2" ParentId="29" CreationDate="2014-02-23T13:18:29.640" Score="8" Body="&lt;p&gt;NASA actually have looked into mining the gas giants, &lt;a href=&quot;http://mdcampbell.com/TM-2006-214122AtmosphericMining.pdf&quot;&gt;http://mdcampbell.com/TM-2006-214122AtmosphericMining.pdf&lt;/a&gt;, this paper outlines some the methods they might use, whether they will or not is a different question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were considering mining methods, of Uranus due to its relatively low wind speeds (compared to Jupiter, Saturn and Neptune, such as aerostats, &quot;scoopers&quot; and huge atmospheric cruisers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The aerostats would just float in the atmosphere, absorbing gas and extracting the useful isotopes. If I recall, the aerostat would be powered by a nuclear reactor that uses the gases taken in as fuel. Then a collection ship would come and collect the rare and valuable gases for return to orbit, where they would be returned to Earth (or elsewhere in the solar system) using NEP (Nuclear electric propulsion) or NTP (Nuclear termal propulsion). The problem here was that regardless of how low Uranus' wind speed is relative to other planets, it's still got some seriously high winds (although these may decrease in speed the higher in the atmosphere you are, however the higher you get, the lower the pressure, so the less gas you can take in and the less 3-He you can produce).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scoopers would do just what their name suggests, they would dip into the atmosphere and &quot;scoop&quot; up the gas, extracting the useful parts and discarding the rest, they would then return to orbit and send the gas off in NTP or NEP powered cargo ships. Here they realised that building many ships capable of &lt;em&gt;very&lt;/em&gt; high speed atmospheric re-entrys, that would have to be endured countless time, would be extremely hard if not impossible (with current technology) to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmospheric cruisers are perhaps the most sci-fi way of harvesting material from a planet I have heard of in a long time. These vehicles would be gigantic cruisers that fly forever throught the atmosphere of Uranus taking in huge amounts of gas and storing the valuable isotopes for transport up to an orbital station by smaller cruiser, that as far as I know would actually land &lt;em&gt;on&lt;/em&gt; the cruiser whilst it was flying. The problem here they realised was that building a ship this big and sending it to another planet (or even constructing it in-situ) would be an almost impossible task.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So all in all they came up with some pretty interesting ideas but I fear we are still quite a bit off from developing the actual technology. Nice to know that NASA has an actual &quot;dream-team&quot; so to speak. It seems to be their job to dream up the craziest, yet also most interesting, of things.&lt;/p&gt;&#xA;" OwnerUserId="2721" LastEditorUserId="2721" LastEditDate="2014-05-26T19:38:10.400" LastActivityDate="2014-05-26T19:38:10.400" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3792" PostTypeId="2" ParentId="3781" CreationDate="2014-02-23T17:22:33.443" Score="1" Body="&lt;p&gt;@iasksillyquestions  A standard algorithm you will find typically involves a conversion from position and velocity to an orbital element set, one of which could by your true anomaly.  You can then use a variety of methods to solve for your mean anomaly, which will in turn give you time since periapsis.  If you are evaluating against orbits with low eccentricity, it could tend to be confusing when discussing periapsis.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As another sidenote, for the two-body problem, specifying the period is essentially the same as specifying the semi-major axis.&lt;/p&gt;&#xA;" OwnerUserId="1478" LastActivityDate="2014-02-23T17:22:33.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3793" PostTypeId="2" ParentId="3781" CreationDate="2014-02-23T22:33:05.723" Score="1" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, you also need the location of the barycenter of the system about which the body is rotating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming you have that, you are left with an ambiguity.  The distance between the position and the barycenter is the current radius.  Unless that radius is exactly the periapsis or exactly the apoapsis, then that radius occurs twice in the orbit.  There are two possible times since periapsis for a given radius that is not exactly at the extremes.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-02-23T22:33:05.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3794" PostTypeId="2" ParentId="2423" CreationDate="2014-02-23T22:36:10.737" Score="25" Body="&lt;p&gt;There are some very good ideas.  This requires a multiple answer approach.  This can not be resolved by one method only. I don't believe mass is the issue (if) the iron core is large enough in comparison to the over all mass of the planet.  If that is the case then re-starting the mantel is within our current technology to do.  We may be able to reach and initiate colonizing the planet within a decade but this would be no more then a .001% colony till we transform the planet.  Hence it will take decades to do and if we don't start now then we are only delaying the inevitable.  I propose the following, a combination of the proposed ideas.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Go to the asteroid belt and grab a few good sized rocks.  Toss them at Mars so that they impact at a tangent.  Not only does this increase the green house gasses and increase the atmospheric temperature, but it also gently starts to increase the rotation of the planet.  Several of these impact placed and timed correctly can do the above.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A series of properly placed nuclear shape charges planted into the mantel to assist in the melting process. Not enough to cause any long lasting side effects. Just enough for part 3 to work.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;While grabbing rocks we build a new moon for Mars.  Placing this in orbit to increase and stabilize the tidal forces on Mars.  this will help heat the core and keep it molten.  It will take some artificial stabilization to keep the new moon in orbit till gravity and tidal forces straighten enough to hold it in place.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;All of this still may not be enough to hold back the solar winds.  so an artificial shield to strengthen the now occurring magnetic shielding maybe needed.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="2725" LastEditorUserId="49" LastEditDate="2014-09-05T00:47:41.670" LastActivityDate="2014-09-05T00:47:41.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3797" PostTypeId="1" AcceptedAnswerId="3812" CreationDate="2014-02-24T11:18:56.633" Score="4" ViewCount="170" Body="&lt;p&gt;I'm working on a tool to simulate comet trajectories. Just the usual method where you place the sun and the planets in their orbits and then solve for the comets' trajectories using the gravitation law (perhaps taking into account sublimation etc. for better precision). There are &lt;a href=&quot;http://jcometobs.web.fc2.com/pnum.htm&quot; rel=&quot;nofollow&quot;&gt;websites&lt;/a&gt; that will give you the orbital elements corresponding to a comet, with some epoch. However it is not specified where the comet is at the epoch. Question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to put a known comet on its observed ephemerides a fair distance away from the sun on its way in, so I can observe it coming in, turning, and going out. How can I find such a position using data in the available databases?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf&quot; rel=&quot;nofollow&quot;&gt;This&lt;/a&gt; is the algorithm I use to determine the position of the planets, I think it might be helpful in this case as well.)&lt;/p&gt;&#xA;" OwnerDisplayName="user1561" LastEditorDisplayName="user1561" LastEditDate="2014-02-24T11:27:00.253" LastActivityDate="2014-02-26T15:55:44.633" Title="How to specify a point along the ephemerides of a comet" Tags="|orbital-mechanics|ephemeris|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3798" PostTypeId="1" CreationDate="2014-02-24T18:22:08.420" Score="2" ViewCount="691" Body="&lt;p&gt;If I have a body $\text{A}$, which is orbiting a more massive body $\text{B}$, and I increase $\text{A}$'s velocity, I'd like to be able to calculate $\text{A}$'s new position after some time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have all the orbital elements of the previous orbit, and I can calculate orbital period $T$, semi-major axis $a$, eccentricity $e$ of the new orbit from the new initial position and increased velocity vectors $V$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, given ...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a known position along a planet's orbit $P$,&lt;/li&gt;&#xA;&lt;li&gt;a velocity vector $V$,&lt;/li&gt;&#xA;&lt;li&gt;the semi-major axis $a$,&lt;/li&gt;&#xA;&lt;li&gt;the eccentricity of the orbit $e$, and&lt;/li&gt;&#xA;&lt;li&gt;the period of orbit $T$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;... is it possible to calculate the new position of $\text{A}$ given its old position in the orbit and after some time $t$?&lt;/p&gt;&#xA;" OwnerUserId="2690" LastEditorUserId="49" LastEditDate="2014-02-24T18:57:58.040" LastActivityDate="2014-02-25T01:55:45.763" Title="Calculating position of a body after a velocity change" Tags="|orbital-mechanics|orbital-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3799" PostTypeId="1" CreationDate="2014-02-24T18:42:28.270" Score="9" ViewCount="1829" Body="&lt;p&gt;With the recent bootstrapping of so many space centric companies, it's starting to look like our economy will extend beyond the bounds of Earth sooner rather than later. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming that mining companies such as &lt;a href=&quot;http://www.planetaryresources.com/&quot;&gt;Planetary Resources&lt;/a&gt; succeed and other companies force competition, how important will it be for methods of transporting materials to Earth to become cheap?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, would cheaper &amp;amp; longer  paths such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot;&gt;Hohmann transfers&lt;/a&gt; be favored, or will more direct (and therefore more expensive) trajectories be preferred?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this question we narrow the source of material to the main asteroid belt and assume that resource needs to be brought within Earths SOI.&lt;/p&gt;&#xA;" OwnerUserId="2690" LastEditorUserId="1142" LastEditDate="2014-02-24T22:52:05.223" LastActivityDate="2015-11-26T04:37:48.467" Title="Mining and the transportation of resources" Tags="|mining|natural-resources|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3800" PostTypeId="2" ParentId="3798" CreationDate="2014-02-24T20:46:50.007" Score="7" Body="&lt;p&gt;Yes.  You have the position and the $\Delta V$.  From the orbital elements, calculate the velocity at that position.  Add the $\Delta V$.  You now have a new position and velocity.  Throw out the old orbital elements -- they are no longer useful.  Using the position and velocity, derive new orbital elements.  The velocity and radius can give you the energy and angular momentum, and from those you can get the new $a$ and $e$.  You will need to solve for the orientation of the ellipse (in 2D or 3D), and the true or eccentric anomaly of the position.  From that, you can compute the new position and velocity at any time.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-02-25T01:55:45.763" LastActivityDate="2014-02-25T01:55:45.763" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3803" PostTypeId="2" ParentId="3799" CreationDate="2014-02-24T23:52:21.467" Score="7" Body="&lt;p&gt;I can think of three generalized trajectories, actually:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;“More direct” routes, by which I mean quicker than a plain transfer. More Δv is required, and thus more expensive. Unless you have live cargo or other radiation-sensitive materials, this will be rarely used.&lt;/li&gt;&#xA;&lt;li&gt;Hohmann transfers, which will be the route of only the most highly valued resources.&lt;/li&gt;&#xA;&lt;li&gt;Finally, what has become known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot;&gt;Interplanetary Transport Network&lt;/a&gt;, a series of low- Δv pathways. This will no doubt be the route for most bulk mining such as water, carbonaceous materials, metal ore, etc. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Locally, a need will develop for “traffic enforcement” at both the Sun-Earth and Earth-Moon &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot;&gt;Lagrangian points&lt;/a&gt; with lots of barge traffic passing through.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2014-02-24T23:52:21.467" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3805" PostTypeId="1" AcceptedAnswerId="3809" CreationDate="2014-02-25T15:14:02.473" Score="19" ViewCount="3012" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;noreferrer&quot;&gt;RP-1&lt;/a&gt; is highly refined kerosene. If I bought a supply of commercial-grade kerosene, by what process could I convert it to RP-1, And, while I'm at it, do I have any good choices for non-cryogenic oxidizer to use with it, other than the highly-unstable &lt;a href=&quot;https://en.wikipedia.org/wiki/High-test_peroxide&quot; rel=&quot;noreferrer&quot;&gt;High Test Peroxide (HTP)&lt;/a&gt;?&lt;/p&gt;&#xA;&lt;p&gt;Oh, and in case anyone is seriously considering doing such things, I add the following caveat, from Asimov's foreword to the &lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;noreferrer&quot;&gt;book&lt;/a&gt; Mark Adler cites:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Now it is clear that anyone working with rocket fuels is outstandingly mad. I don’t mean garden-variety crazy or a merely raving lunatic. I mean a record-shattering exponent of far-out insanity.&lt;/p&gt;&#xA;&lt;p&gt;There are, after all, some chemicals that explode shatteringly, some that flame ravenously, some that corrode hellishly, some that poison sneakily, and some that stink stenchily. As far as I know, though, only liquid rocket fuels have all these delightful properties combined into one delectable whole.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-04-30T16:13:46.717" Title="Could I home-brew my own rocket fuel?" Tags="|fuel|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="3806" PostTypeId="1" CreationDate="2014-02-25T15:47:37.907" Score="-6" ViewCount="305" Body="&lt;p&gt;Would it be viable to send a wind turbine which will be propelled by wind during a lift to space? Will it be able to produce significant amount of electricity while in space? &lt;/p&gt;&#xA;" OwnerUserId="2736" LastActivityDate="2014-02-25T16:12:05.530" Title="Send a wind turbine to space" Tags="|energy|power|" AnswerCount="1" CommentCount="3" ClosedDate="2014-02-26T03:01:36.027" ContentLicense="CC BY-SA 3.0" />
  <row Id="3807" PostTypeId="2" ParentId="3806" CreationDate="2014-02-25T16:02:53.190" Score="4" Body="&lt;p&gt;If you attached a wind turbine to a rocket, it would create drag while in the atmosphere and slow the rocket down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in space, the wind turbine might have energy to use as it will continue to spin, but once you harness the energy it will slow down and eventually stop. Even if you don't directly harness the energy, friction would slow it to a stop. Better to use solar panels. &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-02-25T16:12:05.530" LastActivityDate="2014-02-25T16:12:05.530" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3808" PostTypeId="2" ParentId="3805" CreationDate="2014-02-25T21:21:44.240" Score="6" Body="&lt;p&gt;What is your objective?  For many purposes, any hydrocarbon will suffice.  Gasoline and diesel are quite available.  RP-1 has longer chains, which means more carbon per hydrogen and a  higher boiling point.  For an oxidizer, nitric acid might be the storable oxidizer of choice.  If you need a certain level of performance, you have a larger problrem.&lt;/p&gt;&#xA;" OwnerUserId="176" LastEditorUserId="176" LastEditDate="2014-04-28T12:38:43.867" LastActivityDate="2014-04-28T12:38:43.867" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3809" PostTypeId="2" ParentId="3805" CreationDate="2014-02-25T22:32:41.677" Score="18" Body="&lt;p&gt;You're not going to significantly improve on your commercial kerosene. The process of fractional distillation used means that they've already cooked off and catalyzed away most of the undesired materials. &quot;Low grade&quot; now generally means &quot;devoid of higher energy chains&quot; not &quot;mixed low and high energy chains,&quot; as it's the leftovers when they distilled out the higher grade already, rather than being undifferentiated kerosene. The higher grades command significant prices, and so it's worth it to the refinery to extract them, then sell the lower grade leftovers. Even then, they've highly processed them already.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A look at how Kerosene is refined: &#xA;&lt;a href=&quot;http://www.madehow.com/Volume-7/Kerosene.html&quot;&gt;http://www.madehow.com/Volume-7/Kerosene.html&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And a discussion of what is in oil: &#xA;&lt;a href=&quot;http://www.setlaboratories.com/overview/tabid/81/Default.aspx&quot;&gt;http://www.setlaboratories.com/overview/tabid/81/Default.aspx&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-02-25T22:32:41.677" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3810" PostTypeId="2" ParentId="1858" CreationDate="2014-02-25T22:53:57.870" Score="4" Body="&lt;h1&gt;Why is inclination low?&lt;/h1&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Energy cost to incline the orbit&lt;/li&gt;&#xA;&lt;li&gt;The lack of anything of interest outside the ecliptic&lt;/li&gt;&#xA;&lt;li&gt;Any inclined orbit still crosses the ecliptic&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;It costs a lot of energy to change the orbital inclination significantly. That said, NASA has done so with at least two probes, by use of gravity slingshots.  Plans for use of a Venus encounter to put a probe into solar polar orbit were public discussion in the 80's, but I have no idea if they are still on the drawing board; the early 90's Ulysses probe was slingshot off of Jupiter.&lt;/p&gt;&#xA;&lt;p&gt;Most of our probes are being sent to other planets in the system - deviation from the ecliptic plane is thus not in our interest; Voyager 1 being sent off was more to do with getting the desired encounters at Saturn. The joint NASA/ESA Ulysses probe used a jupiter assist to enter a fairly large 80° inclined orbit, specifically to get views of the solar poles. In theory, an encounter with Mars or Venus could do the same, and this was proposed for Pioneer H, but NASA instead put that craft on display in the Smithsonian.&lt;/p&gt;&#xA;&lt;p&gt;Any orbit intending to avoid the &amp;quot;clutter&amp;quot; of the ecliptic plane still crosses it twice an orbit, and still has risks; further the higher the inclination, the greater the crossing velocity and more severe the potential impact energy. Thus, while it reduces the overall chances of collisions slightly, it increases the risk from each encounter greatly. Further still, there are a lot of small bodies in highly inclined orbits already, and the coronal mass ejection risk isn't reduced either, so the only reasons to do so are for research that requires a highly inclined orbit anyway.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-02-25T22:53:57.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3812" PostTypeId="2" ParentId="3797" CreationDate="2014-02-26T09:47:55.407" Score="1" Body="&lt;p&gt;JPL's Solar system dynamics group has an online tool called &lt;a href=&quot;http://ssd.jpl.nasa.gov/?ephemerides&quot; rel=&quot;nofollow&quot;&gt;HORIZONS&lt;/a&gt; which has become the de-facto tool to look up  or compute ephemerides of the planets, natural satellites, and several thousand minor planets. They also provide the &lt;a href=&quot;http://naif.jpl.nasa.gov/naif/toolkit.html&quot; rel=&quot;nofollow&quot;&gt;SPICE toolkit&lt;/a&gt;, which you can use in your own software. This tool needs some pre-generated data files (like DE405/DE406 for planets), and allows you to generate accurate ephemerides for any and all bodies in those data files.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there is the &lt;a href=&quot;http://www.minorplanetcenter.net/iau/mpc.html&quot; rel=&quot;nofollow&quot;&gt;Minor Planet Center&lt;/a&gt;, the IAU body responsible for all minor planets (classification, naming, etc.). They offer a tool called &lt;a href=&quot;http://www.minorplanetcenter.net/iau/MPEph/MPEph.html&quot; rel=&quot;nofollow&quot;&gt;Minor Planet Ephemerides Service (MPES)&lt;/a&gt;, which allows you to generate ephemerides for a list of up to 100 minor planets at a time. Then they offer the &lt;a href=&quot;http://www.minorplanetcenter.net/iau/MPCORB.html&quot; rel=&quot;nofollow&quot;&gt;MPC Orbit database&lt;/a&gt;, which provides MPCORB.dat files (or dailies), which contain the current orbital elements of all known minor planets (and/or newly discovered ones). Note that unlike the site you linked to, the MPCORB.dat file includes all 6 orbital elements and epoch, meaning that you can uniquely determine its position and velocity in space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I recall correctly, either SPICE or MPC also provides drift values, that is, the first and second order rates of change of the orbital elements at the epoch. Meaning: more accuracy coming already from the ephemerides tool, less computation required on your side. In general: use the work that has already been done, instead of doing it yourself :) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you really have to or want to propagate the asteroids: I suggest you use &lt;a href=&quot;http://en.wikipedia.org/wiki/Perturbation_%28astronomy%29#Encke.27s_method&quot; rel=&quot;nofollow&quot;&gt;Encke's methods&lt;/a&gt; in combination with either a &lt;a href=&quot;http://en.wikipedia.org/wiki/Symplectic_integrator&quot; rel=&quot;nofollow&quot;&gt;symplectic integrator&lt;/a&gt; or a &lt;a href=&quot;http://www.mathworks.com/matlabcentral/fileexchange/25291-rkn1210-a-12th10th-order-runge-kutta-nystr%C3%B6m-integrator&quot; rel=&quot;nofollow&quot;&gt;high-order Runge-Kutta-Nyström&lt;/a&gt; integrator. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that these integrators cannot be used if you want to take into account things that depend on the instantaneous velocity, like radiation pressure. But hey, if you're going into &lt;em&gt;that&lt;/em&gt; kind of detail, well...perhaps better ask a new question on which books to read first :)  &lt;/p&gt;&#xA;" OwnerUserId="97" LastEditorUserId="97" LastEditDate="2014-02-26T15:55:44.633" LastActivityDate="2014-02-26T15:55:44.633" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3813" PostTypeId="2" ParentId="1206" CreationDate="2014-02-26T11:50:35.823" Score="4" Body="&lt;p&gt;Astrium is working on a &lt;em&gt;space harpoon&lt;/em&gt; capture system. They have been awarded a study contract by the French space agency CNES to develop space vehicles that can remove large items of debris from Earth's orbit. Their idea is to develop a pneumatic space harpoon that a spacecraft would fire into larger space debris targets and use tether to drag them out of orbit and into the atmosphere:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/CTSY7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The harpoon design is trying to minimize secondary debris from puncturing the target's body. It has a set of barbs, a shock absorber section that is the wide part in the photograph above and releases the barbs into open, hook-like position after impact, and a shaft that is inserted into the pneumatic gun that holds the tether with which the target is later towed by into the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information and references:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astrium.eads.net/en/press_centre/astrium-to-study-removal-of-large-space-debris.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astrium press release: Astrium to study removal of large space debris&lt;/a&gt;, April 17, 2013&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.theengineer.co.uk/aerospace/news/space-harpoon-could-tackle-satellite-debris-problem/1016107.article&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Engineer: Space harpoon could tackle satellite debris problem&lt;/a&gt;, April 18, 2013&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://edition.cnn.com/2013/04/18/world/space-junk-harpoon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CNN: Space harpoon plan to nail orbital garbage&lt;/a&gt;, April 21, 2013&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://edition.cnn.com/video/?/video/international/2013/04/18/space-harpoon-debris-astium-animation.cnn&amp;amp;video_referrer=&quot; rel=&quot;nofollow noreferrer&quot;&gt;CNN Video: Astrium Space Hook - How to capture space debris&lt;/a&gt;, April 18, 2013&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=51iviu4HP0I&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube: The Engineer report on Astrium Space Hook&lt;/a&gt; (with demonstration), April 18, 2013&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.aljazeera.com/video/europe/2013/04/20134227349673986.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Al Jazeera Video Report: UK floats harpoon solution to 'space junk'&lt;/a&gt;, April 22, 2013&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-02-26T11:50:35.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3815" PostTypeId="2" ParentId="2444" CreationDate="2014-02-26T14:18:12.047" Score="3" Body="&lt;p&gt;Not exactly &lt;em&gt;medicine&lt;/em&gt; per se, but baby formula enriched with docosahexaenoic (DHA) and arachidonic (ARA) acids that are present in human breast milk were discovered by chance in certain types of algae when they were researched for possible ways of producing oxygen:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The formula being fed to the baby contains an algae-based additive&#xA;  highly enriched in nutrients believed to be beneficial to infant&#xA;  mental and visual development; called Formulaid&lt;sup&gt; ®&lt;/sup&gt;, the additive is an&#xA;  offshoot of space research. &lt;sup&gt;&lt;a href=&quot;http://spinoff.nasa.gov/spinoff1996/42.html&quot; rel=&quot;nofollow&quot;&gt;(1)&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Research in growing superpotent crystalline structures is also promising, and they've already managed to develop techniques to grow super-fine insulin crystals so far back as in 1998:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;From this, scientists expect to be able to design new forms of insulin&#xA;  that the body can absorb more effectively than the forms now on the&#xA;  market. With certain chemicals bound to insulin, diabetics could&#xA;  inject themselves once every three days or so, instead of one to three&#xA;  times a day. &lt;sup&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/1998/notebook/msad22jul98_1/&quot; rel=&quot;nofollow&quot;&gt;(2)&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Research on protein crystallisation using microgravity now continues under the banner of CASIS ISS National Lab &lt;sup&gt;&lt;a href=&quot;http://www.iss-casis.org/Opportunities/Solicitations/RFPProteinCrystallization.aspx&quot; rel=&quot;nofollow&quot;&gt;(3)&lt;/a&gt;&lt;/sup&gt;. And there were all kinds of other medical technology spinoffs and medical advancements in general that were made possible through research in or for microgravity environment and long duration spaceflight. See &lt;em&gt;recommended reading&lt;/em&gt; links below for more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spinoff.nasa.gov/spinoff1996/42.html&quot; rel=&quot;nofollow&quot;&gt;NASA Spinoff: Nutritional Products from Space Research&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/1998/notebook/msad22jul98_1/&quot; rel=&quot;nofollow&quot;&gt;NASA Science News: Weightless Research has Heavy Implications - Crystal-clear view of insulin should lead to improved therapies for diabetics&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.iss-casis.org/Opportunities/Solicitations/RFPProteinCrystallization.aspx&quot; rel=&quot;nofollow&quot;&gt;CASIS Request for Proposals: Advancing Protein Crystallization Using Microgravity&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Recommended reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/363454main_medical_flyer.pdf&quot; rel=&quot;nofollow&quot;&gt;NASA Health and Medicine Spinoffs&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_spin-off_technologies#Health_and_medicine&quot; rel=&quot;nofollow&quot;&gt;Wikipedia on NASA spin-off technologies: Health and medicine&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/centers/kennedy/news/facts/nasaspinoff.html&quot; rel=&quot;nofollow&quot;&gt;NASA Kennedy Space Center: NASA Spinoffs Fact Sheet&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spinoff.nasa.gov/index.html&quot; rel=&quot;nofollow&quot;&gt;NASA Spinoff&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/content/top-space-station-research-awards-of-the-year-medical-advancements/stationresearch/&quot; rel=&quot;nofollow&quot;&gt;Top Space Station Research Awards of the Year – Medical Advancements&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/benefits/&quot; rel=&quot;nofollow&quot;&gt;International Space Station - ISS Benefits for Humanity&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-02-26T14:18:12.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3816" PostTypeId="2" ParentId="2370" CreationDate="2014-02-26T19:08:16.150" Score="18" Body="&lt;p&gt;The biggest problems with a space gun are inherent the simple nature of ballistics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can use railguns to accelerate projectiles very fast; much faster than explosive propellants can do over the same distance. The current record is 33 MJ; that's a one-kilo projectile propelled at approximately escape velocity (8127 m/s). So, theoretically, the most powerful railgun we have (currently in the hands of the Navy Labs, who are developing the technology for a new class of warship) could put a 1-kilo object into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are those problems I mentioned. First, in the Navy railgun, this amount of energy is currently imparted to the projectile over a distance of approximately 12 meters, in a timespan of 10 milliseconds. An 8000m/s velocity change in a 1kg projectile over .01 seconds equals an acceleration force of 800,000 Newtons, which is about 81,632.65 G-forces. Humans can survive about 14 Gs when lying flat on their back or stomach, and 4 Gs is the maximum we like submitting our astronauts to in a seated position, since they're not all fighter pilots in prime condition able to handle 6-8 Gs for long maneuvers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get the same delta-V with acceleration limited to 40m/s&lt;sup&gt;2&lt;/sup&gt; (just over 4G) would require 200 seconds, during which time you would have traveled 800,000 meters (the required length of the barrel of the railgun). 800,000 meters is just shy of 500 miles; you're talking about engineering a track from Dallas to El Paso, TX, with millimeter tolerances for deviation per kilometer of track from being perfectly straight. It would be the biggest and most expensive single civil engineering project the human race has ever undertaken; bigger by far than the current bullet train systems, bigger than the CERN supercollider, bigger than the U.S. interstate project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, at the instant you leave the barrel, you're travelling as fast as you ever will. Drag from air resistance will slow you down, and the air is thickest right at the surface of the Earth, where it's cheapest to build this 500-mile track. So as soon as you leave the barrel, you're blasted with Mach-25 winds that will instantly begin slowing you down below escape velocity. And if it happens to be raining in Dallas (remember you want to travel west to east, in the direction of the Earth's rotation, to take advantage of the extra 465 m/s of angular velocity), &lt;a href=&quot;http://whatif.xkcd.com/93/&quot;&gt;the raindrops will shatter the windshield&lt;/a&gt;. We mitigate this with modern rockets by limiting our acceleration and velocity until we exit the troposphere, at which point we throttle up the rocket to maximum power as the air thins. Our railgun, on the other hand, would have to be built on about a 7* incline, with the muzzle about 100,000 feet in the air, in order for the projectile to avoid the worst of the air drag as it exits the barrel. So now, you're taking what's already the most expensive project in history and adding to it the task of building a ramp 36 times taller than the Burj Khalifa, and that's the shorter dimension, by far. We're now also working against gravity as we accelerate, requiring us to add an additional 1.225m/s&lt;sup&gt;2&lt;/sup&gt; to our railgun's acceleration just to account for gravity (we've already been assuming that air drag and other friction within the barrel is negligible).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, 33MJ is our current world record for imparting kinetic energy into a projectile via magrail without completely destroying it, and that's only enough to get one kilo up to escape velocity. The Space Shuttle orbiter, empty, weighs 130,000 kg. The lightest vehicle we've ever put into orbit that is capable of supporting human life was the Mercury capsule at 1,400kg; a railgun capable of putting this capsule in orbit would have to be able to impart, in the ideal, not one millijoule less than 46.2GJ of kinetic energy to the capsule. In the grand scheme of things that's not much; it's about the yearly electricity consumption of the average refrigerator. However, that's after all inefficiencies and losses, which when you're dealing with electromagnetic inductors can be several orders of magnitude more than the energy delivered.&lt;/p&gt;&#xA;" OwnerUserId="411" LastEditorUserId="411" LastEditDate="2014-05-23T06:06:33.480" LastActivityDate="2014-05-23T06:06:33.480" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3817" PostTypeId="2" ParentId="3483" CreationDate="2014-02-26T20:16:51.820" Score="3" Body="&lt;p&gt;The existing answers pretty much show why it's not done but I'll add a couple more points also:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1)  Remember that a rocket going up these days pulls several Gs.  Note how far it goes before the engines shut down--hundreds of miles.  If you want the same G load you need as much distance--for humans to survive the trip your launcher needs to be hundreds of miles long.  Obviously it must shoot on a very flat trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2)  A rocket obtains most of it's velocity while pretty much outside the atmosphere.  A linear motor launcher obtains it's velocity on the ground and thus has a major problem with drag--even more so because it's going nearly horizontally.  You'll have to have an awfully big vehicle to avoid it losing &lt;strong&gt;ALL&lt;/strong&gt; it's speed in the atmosphere.  (You can't overcome this by adding to the launch velocity as the drag goes up as fast as the velocity does.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-02-26T20:16:51.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3818" PostTypeId="2" ParentId="3779" CreationDate="2014-02-26T21:46:41.767" Score="2" Body="&lt;p&gt;You can do both.  For each object, store the time of perihelion.  You have a global time for observation.  A subtraction gets the time since perihelion.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-02-26T21:46:41.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3820" PostTypeId="1" CreationDate="2014-02-27T06:41:27.673" Score="5" ViewCount="1748" Body="&lt;p&gt;I'm interested in downloading all of the Curiosity rover photos. Is there some sort of automated way to do this?&lt;/p&gt;&#xA;" OwnerUserId="2747" LastActivityDate="2021-01-06T09:31:01.127" Title="Download all Curiosity photos?" Tags="|rovers|curiosity|photography|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3821" PostTypeId="2" ParentId="2370" CreationDate="2014-02-27T13:20:58.063" Score="9" Body="&lt;p&gt;I think there is another, more fundamental problem which hasn't been mentioned by the other answers.  The drag on a body due to friction with the air increases quadratically with velocity.  The drag force is given by (&lt;a href=&quot;http://en.wikipedia.org/wiki/Drag_%28physics%29&quot;&gt;taken from this Wikipedia article&lt;/a&gt;)&#xA;$$&#xA;F_D=\frac12\rho C_D A v^2,&#xA;$$&#xA;where &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\rho$ is the density of the fluid.&lt;/li&gt;&#xA;&lt;li&gt;$C_D$ is the drag coefficient which admittedly can decrease with velocity.&lt;/li&gt;&#xA;&lt;li&gt;$A$ is the cross-sectional area.&lt;/li&gt;&#xA;&lt;li&gt;$v$ is the velocity of the object relative to the fluid.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is a problem because a gun would need to impart all of the energy necessary to get the object into orbit right at the beginning, meaning that all of the energy would need to be stored in the objects velocity rather than the chemical energy of its boosters.  This would significantly increase the total amount of energy necessary due to the quadratic dependence of the drag force.&lt;/p&gt;&#xA;" OwnerUserId="2699" LastActivityDate="2014-02-27T13:20:58.063" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3822" PostTypeId="2" ParentId="3820" CreationDate="2014-02-27T14:11:31.973" Score="9" Body="&lt;p&gt;Source: &lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?s=&amp;amp;showtopic=7375&amp;amp;view=findpost&amp;amp;p=188520&quot;&gt;http://www.unmannedspaceflight.com/index.php?s=&amp;amp;showtopic=7375&amp;amp;view=findpost&amp;amp;p=188520&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;MSL IMAGE RETRIEVAL TOOLS&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Links from the &lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?showtopic=7408&quot;&gt;Curiosity Image Retrieval Tools&lt;/a&gt; Thread.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Please note that we do not guarantee any of the programs listed.&#xA;They are links provided by or programs/website developed by members.&#xA;Please use them at your caution.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A &lt;a href=&quot;http://paranoid.dechengst.nl/files/MSLget.sh&quot;&gt;shell script&lt;/a&gt; that allows you to download all images from a given sol&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?act=attach&amp;amp;type=post&amp;amp;id=27458&quot;&gt;Curiosity RAW Get&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.stardownloader.com/&quot;&gt;Star Downloader&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mars-ogler.com/&quot;&gt;Mars Ogler&lt;/a&gt; Check out the &lt;em&gt;grid&lt;/em&gt; view in the menu options.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.downthemall.net/&quot;&gt;DownloadThemAll plugin for Firefox&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Automatically updated RAW images online - &lt;a href=&quot;http://msl-raw-images.com/&quot;&gt;link to external website&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;A basic web interface listing images for a particular Sol - &lt;a href=&quot;http://curiositymsl.com/rawlist&quot;&gt;link to external website&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Web interface to the raw images - &lt;a href=&quot;http://curiositymsl.com/table/view&quot;&gt;link to external website&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-02-27T14:11:31.973" CommentCount="0" CommunityOwnedDate="2014-02-27T14:11:31.973" ContentLicense="CC BY-SA 3.0" />
  <row Id="3823" PostTypeId="1" AcceptedAnswerId="3826" CreationDate="2014-02-27T15:04:27.870" Score="8" ViewCount="491" Body="&lt;p&gt;I have been trying to find information on how far away &lt;a href=&quot;http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/&quot;&gt;these new exoplanets&lt;/a&gt; are from Sol. Does anybody have a map or a table that shows their distance to us in light years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There only seem to be 100 of them that are Earth-sized, do we know how many of these are in the habitable zone?&lt;/p&gt;&#xA;" OwnerUserId="2709" LastEditorUserId="49" LastEditDate="2014-02-27T15:36:35.080" LastActivityDate="2014-02-27T16:25:19.253" Title="NASA just announced 715 new exoplanets were discovered, what do we know about them?" Tags="|planet|astronomy|exoplanet|kepler-telescope|goldilocks-zone|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3824" PostTypeId="2" ParentId="3823" CreationDate="2014-02-27T16:09:29.033" Score="2" Body="&lt;p&gt;There is a list of &lt;a href=&quot;http://exoplanetarchive.ipac.caltech.edu/cgi-bin/ExoTables/nph-exotbls?dataset=planets&quot; rel=&quot;nofollow&quot;&gt;confirmed planets&lt;/a&gt;, but this does not identify which planets were confirmed by the Lissauer and Rowe teams. Their papers (&lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/arXivValidationMultisII.pdf&quot; rel=&quot;nofollow&quot;&gt;Lissauer&lt;/a&gt;, &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/arXivValidationMultisIII.pdf&quot; rel=&quot;nofollow&quot;&gt;Rowe&lt;/a&gt; (large PDFs)) don't identify all of the planets they've confirmed.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/Multis-telecon-26Feb_FINAL-web.pdf&quot; rel=&quot;nofollow&quot;&gt;This presentation&lt;/a&gt; (pdf) says that 4 planets are in the habitable zone. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-02-27T16:09:29.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3825" PostTypeId="2" ParentId="2370" CreationDate="2014-02-27T16:10:04.910" Score="12" Body="&lt;p&gt;Since we've been fielding a lot of questions on this subject lately, I just want to chime in with some of the rules-of-thumb that seem very natural to me.  Unlike rockets, a gun sees the maximum density of the atmosphere at its fastest point.  So provided you understand the necessity to circularize an orbit, and you understand that high accelerations need hardened equipment, the viability of many proposals can be easily ruled out envelope calculations for the drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a helpful metric to look at the atmosphere with: the mass-thickness of the atmosphere is the mass per unit area, looking straight up into the sky at sea-level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{ \text{Mass of Atmosphere} }{ \text{Area} } := \mu_{atm} \approx 10 \frac{ \text{tons} }{ m^2 } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a naive sense, look at a bullet straight-on.  Divide its mass by the area (same area it presents to the atmosphere).  A more accurate approach would be to do some manipulation of the drag equation.  We're not interested strictly on the &lt;em&gt;force&lt;/em&gt; on the projectile.  Alternatively, I'll consider the velocity lost due to its trip through the atmosphere, $\Delta v$ here.  I obtained the following from the drag equation, under the assumption that the total velocity is notably greater than the loss in the atmosphere (if not, it's nonviable anyway).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{ \Delta v }{ v } = \frac{1}{2} \frac{C_d \epsilon \mu_{atm}}{ \rho D f \sin{( \theta)} }  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to practical considerations, if this ratio is &lt;em&gt;about&lt;/em&gt; 1, you don't have a chance.  It will burn up in the atmosphere, and even if not, you can't produce those speeds.  Speeds &lt;em&gt;near&lt;/em&gt; orbital have &lt;em&gt;never&lt;/em&gt; been demonstrated.  So if you can't push this ratio well below 1, the idea is out the window.  I'll go over all the terms below, dividing them into more-or-less into things that are completely impossible to push beyond a certain window, and things that have not inherent limitation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hard limits:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;C_d the drag coefficient for bullets are in the neighborhood of 0.2 for &lt;em&gt;well engineered&lt;/em&gt; projectiles.  You can easily push it a little bit higher or a little lower, but there is no where &lt;em&gt;close&lt;/em&gt; to an order of magnitude of wiggle room.  The high mach numbers make it an even tighter range for different shapes.&lt;/li&gt;&#xA;&lt;li&gt;epsilon, is the correction for a thinner atmosphere at higher altitudes.  For Mt. Everest the factor is something like 0.3.  I would believe a factor of 0.5 for placing it on a high mountain.  Obviously sea launches face the full factor of 1.&lt;/li&gt;&#xA;&lt;li&gt;rho, the average density of the projectile is constrained by your propellant if you're going to circularize its orbit.  This is very low, probably around 1.0 specific gravity.  If you're doing some other scheme (like a Rotovator), you could pack it full of steel payload, getting to 7.0 or 8.0 at theoretical maximum.  Unless you're sending Uranium or something like that.  I'll give it 2.0 for a practical payload.&lt;/li&gt;&#xA;&lt;li&gt;f, the length to diameter ratio of the projectile is limited by aerodynamic considerations.  Rockets tend to be very slender, but they have active control systems.  Fins can help push the envelope here.  But even with that, I'm calling a &quot;hard&quot; limit of about 10.&lt;/li&gt;&#xA;&lt;li&gt;sin(theta), this factor works &lt;em&gt;against&lt;/em&gt; you, and the best case scenario is 1.  This would apply for suborbital vertical launches.  For orbital launches, you risk defeating the point of a gun in the first place if this angle isn't low enough.  You could shoot something straight up and then burn the necessary 7.8 km/s, but this would make a very poor mass ratio, and probably can't be engineered to withstand the gun acceleration.  For Quichlaunch, I would give this a value of 0.5.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Soft limits:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;D, is the diameter, which serves as a metric for the overall size of the payload.  How large can it be?  How large of a gun can you construct?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So we have to use our (only) soft limit in order to engineer around the other hard limits.  I'll take a velocity loss ratio of 0.2 for now.  You could stretch this, but not by much.  You actually come out with a set of parameters like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ D \approx 4.5 cm \\&#xA; M \approx 11.7 kg $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This doesn't sound shockingly huge.  But if you change it to a sea-level launch, the required mass goes up by &lt;strong&gt;a factor of 8&lt;/strong&gt;, due to the fact that mass scales with D^3.  I was also extremely generous with the diameter to length factor.  A bullet shape that has a factor of 5 would be, again, 8 times the mass.  So if we're looking t something like a practical sea launch to ultimately reach orbit, the minimum mass is closer to the scale of 750 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see here how the minimum size depends strongly on the assumptions of the type of system you're using.  Nonetheless, 10-750 kg is still a starting range for the minimum bullet mass needed for the system to ever work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the bullet size and speed also set a limit on the minimum size of the gun itself.  If you can get pressures on the order of 50,000 psi, then the 750 kg case (Quicklaunch) at 8 km/s would need a volume of about 110 m^3.  That's big.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare, a nuclear reactor pressure vessel is probably about 2-3 times the volume of that.  It's also about 1/20th the pressure.  And it costs upwards of $100 million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Straightforward material requirements for a pressure vessel are proportional to the (pressure)x(volume) product.  So things are not looking good for our space gun, although the economics of heavy forgings are far from simple.  On the other hand, a minimalist space gun mounted on the slope of a tall mountain &lt;em&gt;might&lt;/em&gt; have a &lt;em&gt;hard&lt;/em&gt; cost minimum below the $10 million scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or you could just buy a Falcon 9 flight, for somewhere around $50 million.  Of course you could get better economics from the space gun if you used it enough times to amortize the cost sufficiently.  However, the total yearly demand for payload into LEO is only about &lt;a href=&quot;https://space.stackexchange.com/questions/88/total-mass-sent-into-orbit-over-all-history&quot;&gt;240 tons&lt;/a&gt;.  The Falcon 9 can carry 10 tons in one shot.  Only a small fraction of the payloads could be substituted by the space gun (hardened equipment).  So &lt;em&gt;maybe&lt;/em&gt; there would be enough demand for the space gun to replace 1 or 2 rocket launch equivalents.  On top of that, the risks are huge.  Global flight frequency would have to be much larger for people to seriously put the necessary capital into this.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-02-27T16:10:04.910" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3826" PostTypeId="2" ParentId="3823" CreationDate="2014-02-27T16:10:26.710" Score="5" Body="&lt;p&gt;Here's yesterday's &lt;a href=&quot;http://www.ustream.tv/channel/nasa-arc&quot;&gt;media briefing (audio stream)&lt;/a&gt; that explains what this announcement is all about, on top of already linked to &lt;a href=&quot;http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/&quot;&gt;news article&lt;/a&gt;. Many other links are available on &lt;a href=&quot;http://www.nasa.gov/ames/kepler/digital-press-kit-kepler-planet-bonanza/&quot;&gt;Kepler Planet Bonanza press kit&lt;/a&gt;, including &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/Multis-telecon-26Feb_FINAL-web.pdf&quot;&gt;presentation slides (PDF, 27MB warning!)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://kepler.nasa.gov/Mission/discoveries/&quot;&gt;table of confirmed planets&lt;/a&gt; is published on Kepler pages, including planet characteristics, their orbits and caracteristics of their stars. It also includes further links:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://kepler.nasa.gov/Mission/discoveries/candidates/&quot;&gt;Planet candidates&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://keplerebs.villanova.edu/&quot;&gt;Eclipsing Binaries&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://kepler.nasa.gov/Science/ForScientists/papersAndDocumentation/&quot;&gt;Papers&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://kepler.nasa.gov/Science/ForScientists/dataarchive/&quot;&gt;Data Archives&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://kepler.nasa.gov/Mission/discoveries/discoverynews/&quot;&gt;News Stories&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://kepler.nasa.gov/Mission/discoveries/discoveryanimations/&quot;&gt;Animations&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One thing to add, in case it isn't obvious to some, is that most of these exoplanets are far too distant to us to attempt any direct imaging, so they're detected as they transit in front of the star they orbit and the intensity of detected light emitted from it temporarily drops. Knowing duration, intensity and periodicity of these dips, we can deduct the candidate planet's orbital period (distance to star and velocity), inclination, eccentricity, and other orbital elements, and through known characteristics of the star they orbit also their size, mass and surface temperature estimates. This is relatively easy and reliable with &lt;em&gt;quiet&lt;/em&gt; stars, but a fair bit trickier with stars with irregular activity patterns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other links of interest:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://exoplanetarchive.ipac.caltech.edu/&quot;&gt;NASA Exoplanet Archive&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://exoplanets.org/&quot;&gt;Exoplanet Orbit Database - Exoplanet Data Explorer&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.planethunters.org/&quot;&gt;Planethunters.org Citizen Science Project&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-27T16:25:19.253" LastActivityDate="2014-02-27T16:25:19.253" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3827" PostTypeId="1" CreationDate="2014-02-27T16:59:04.240" Score="7" ViewCount="867" Body="&lt;p&gt;So, I've been reading a bit about thermal rocketry, from NTRs to beamed propulsion ideas. Now the basic concept is to squeeze more Isp out of traditional propellants such as liquid hydrogen. Specifically, if you heat H2 to around 2700K (depending on pressure), it breaks up into quantities of H1, which gives a big boost in exhaust velocity and thus Isp.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure I'm not the first person to think of this, but instead of elaborate schemes to beam energy to a moving craft, why not just use an energy source while on the ground to create heat, perhaps a dense plasma  like in fusion experiments, and then use that to heat the propellant that will lift the craft into orbit? Basically plug the rocket in. You only need around ~10 minutes to get into orbit, so this system would only have to be viable for a short time. You could use reflective barriers to minimize radiative heat loses and channel them to the heat exchanger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess this is more of a thought experiment than a serious proposal. What are the drawbacks of this sort of design?&lt;/p&gt;&#xA;" OwnerUserId="2749" LastActivityDate="2018-10-18T11:43:33.440" Title="Why not plug it into the wall?" Tags="|launch|rockets|propulsion|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3828" PostTypeId="2" ParentId="3827" CreationDate="2014-02-27T17:00:43.100" Score="2" Body="&lt;p&gt;Generically this is known as &lt;a href=&quot;http://en.wikipedia.org/wiki/Beam-powered_propulsion&quot; rel=&quot;nofollow&quot;&gt;Beamed Power Propulsion.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Beam-powered propulsion is a class of aircraft or spacecraft propulsion mechanisms that uses energy beamed to the spacecraft from a remote power plant to provide energy. Most designs are thermal rockets where the energy is provided by the beam, and is used to superheat propellant that then provides propulsion, although some obtain propulsion directly from light pressure acting on a light sail structure, and at low altitude heating air gives extra thrust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-02-27T17:00:43.100" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3829" PostTypeId="2" ParentId="3827" CreationDate="2014-02-27T17:09:59.057" Score="7" Body="&lt;p&gt;The problem would be enormous expansion in volume and/or increase in pressure on the tank walls, so you can't simply &lt;em&gt;preheat&lt;/em&gt; all the volume of the cryogenic propellants, you want to store them at high density but at still controllable pressure, then preheat them as they enter the expansion chamber to improve their exhaust velocity. Another way to think of this is also that the stored propellant density in relation to its density once expelled (&lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_expansion&quot; rel=&quot;noreferrer&quot;&gt;thermal expansion&lt;/a&gt;) is stored energy that the rocket uses to propel itself. This is what we refer to as &lt;a href=&quot;https://en.wikipedia.org/wiki/Working_mass&quot; rel=&quot;noreferrer&quot;&gt;reaction mass&lt;/a&gt;. If you preheated all of it, even if you managed to somehow stably store it without adding much to rocket's weight, you'd be throwing a lot of this potential energy away. Either that, or the pressure on the tank walls would be too high and we're again back at the tank weight and volume problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you only superheated some fraction of these propellants, to use that later on as your source of thermal energy to preheat the rest of it as it's fed into the engine, the problem becomes one of insulating this high temperature mass from the rest of your propellants to keep them stable. Which would again require more insulation, stronger structure to contain it, and needlessly add weight and complexity to the rocket. It would also have limited thermal potential, a bit like mixing hot water from the boiler with cold water, and you'd never be able to make use of all of the potential of thermal expansion. So it's simply easier and more economical to preheat propellants &lt;em&gt;on demand&lt;/em&gt; and reuse as much of the stored thermochemical energy they provide for it as they burn in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_engine#Introducing_propellant_into_a_combustion_chamber&quot; rel=&quot;noreferrer&quot;&gt;combustion chamber&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what it's worth, rockets actually are &lt;em&gt;plugged into the wall&lt;/em&gt; with umbilicals until minutes before launch when they are switched to internal power. So they can use all the &lt;em&gt;juice&lt;/em&gt; they require from the grid for their electrical power needs right up until the very last few minutes (usually around T-3 minutes) before flight, and so conserve their onboard power sources.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-02-27T17:35:36.750" LastActivityDate="2014-02-27T17:35:36.750" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3830" PostTypeId="1" AcceptedAnswerId="4071" CreationDate="2014-02-27T20:37:17.877" Score="19" ViewCount="8255" Body="&lt;p&gt;Presumably the various windows on the different modules of the ISS are consistent on the different segments of the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the windows on the US Segment made out of?  Do they have special coatings to block specific frequencies of light?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6944" LastEditDate="2020-09-09T12:15:56.543" LastActivityDate="2020-09-09T12:15:56.543" Title="What materials are the windows on the US Segment of the ISS made out of?" Tags="|iss|design|materials|spacecraft-window|us-segment|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3831" PostTypeId="1" AcceptedAnswerId="4073" CreationDate="2014-02-27T20:38:41.460" Score="13" ViewCount="766" Body="&lt;p&gt;Presumably the various windows on the different modules of the ISS are consistent on the different segments of the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the windows on the Russian Segment made out of?  Do they have special coatings to block specific frequencies of light? Are they different than on the US segment?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are they different from the Mir station in any significant way?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6944" LastEditDate="2020-09-09T12:13:34.413" LastActivityDate="2020-09-09T12:13:34.413" Title="What are the windows on the Russian segement of the ISS made of?" Tags="|iss|design|materials|spacecraft-window|russian-segment|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3832" PostTypeId="1" AcceptedAnswerId="3834" CreationDate="2014-02-28T00:15:39.210" Score="8" ViewCount="542" Body="&lt;p&gt;We are &lt;a href=&quot;https://space.stackexchange.com/questions/3823&quot;&gt;finding new planets&lt;/a&gt; in other solar systems all the time.  Some of them are in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Circumstellar_habitable_zone&quot; rel=&quot;nofollow noreferrer&quot;&gt;habitable zone&lt;/a&gt; of their sun.  Are we able to detect the atmosphere of any exoplanet?  Is there any atmospheric element that would be indicative of known life forms? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-02-28T01:02:43.397" Title="Can we detect atmosphere on exoplanets?" Tags="|atmosphere|exoplanet|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3833" PostTypeId="2" ParentId="3631" CreationDate="2014-02-28T00:44:09.210" Score="7" Body="&lt;p&gt;The mathematical names for those directions are tangent (the red arrows), normal (the green arrows, and binormal (the blue arrows). Geometers have made extensive use of these, so much so that these directions are a key part of the Fundamental Theorem of Curves. For example, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Frenet-Serret_formulas&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Frenet-Serret_formulas&lt;/a&gt;, &lt;a href=&quot;http://mathworld.wolfram.com/FundamentalTheoremofSpaceCurves.html&quot; rel=&quot;noreferrer&quot;&gt;http://mathworld.wolfram.com/FundamentalTheoremofSpaceCurves.html&lt;/a&gt;, and &lt;a href=&quot;http://math.rice.edu/~hardt/401F03/ftc.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://math.rice.edu/~hardt/401F03/ftc.pdf&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This theorem isn't of much use in orbital mechanics because torsion involves a third derivative of position with respect to time. Orbital mechanics is a study of second derivatives: F=ma.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The directions along the red arrows (v-bar) are useful in orbital mechanics because these are the directions along which you want to thrust to minimize gravity losses. The blue arrows are useful because angular velocity points in this direction. The green arrows? They're useful for geometers and for describing vehicles flying through an atmosphere. They're not so useful in orbital mechanics, which is perhaps why there isn't a standard orbital mechanics name for this direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Addendum&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When looking at the uncertainties in where a spacecraft is, those directions are oftentimes called along track (the red arrows), cross track (the blue arrows), and radial (the green arrows). One can look at &quot;radial&quot; as being either a bit of a misnomer or as being spot on correct. It's a misnomer in the sense that &quot;radial&quot; only points &quot;radially&quot; (toward / away from the planet in the case of a circular orbit. It's spot on in the sense that &quot;radial&quot; always points toward / away from the instantaneous center of curvature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A related set of directions is the local vertical / local horizontal system, or LVLH for short. In this system, +Z points to the center of the Earth, +Y points opposite the orbital angular velocity, and +X completes the right hand coordinate system (i.e., $\hat x = \hat y \times \hat z$). This means that $\hat x$ points along the velocity vector in the case of a circular orbit. This labeling a bit arbitrary. The Clohessy-Wilshire equations use +X as pointing away from the Earth, +Z as pointing along the angular momentum vector, and +Y completing the right hand coordinate system. Either the LVLH frame or CW frame used to describe the orbital mechanics of a spacecraft rendezvousing with the ISS.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-02-28T07:56:08.807" LastActivityDate="2014-02-28T07:56:08.807" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3834" PostTypeId="2" ParentId="3832" CreationDate="2014-02-28T01:02:43.397" Score="8" Body="&lt;p&gt;Yes, they are starting to examine exoplanet atmospheres. A spectroscopic analysis is made of the light from a exoplanet transiting its star. The spectra of the star is subtracted from the combined star/planet data, and the remainder is assumed to be the absorption spectra from light passing through the planet's atmosphere. Here's an article from a recent Centauri Dreams column that goes into some detail: &lt;a href=&quot;http://www.centauri-dreams.org/?p=30039&quot;&gt;Tau Boötis b: A ’3-D’ Look at Star and Planet.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as elements indicative of life forms, one would expect to see water, carbon dioxide and methane on a life-bearing planet, but, of course, these are common substances that can readily be explained by natural processes.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2014-02-28T01:02:43.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3835" PostTypeId="1" AcceptedAnswerId="3843" CreationDate="2014-02-28T02:58:00.537" Score="14" ViewCount="1035" Body="&lt;p&gt;Clearly there is some method of power transfer between the US Segment with the honking big arrays, to the Russian segment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But since the Russian segment is basically Mir 2 (Zvezda, and an FGB tug which is even older as Zarya), which means it core design was set ages ago, and the US Segment is fundamentally a newer design, how compatible are their power systems? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though as I think about this, it is likely that the US segment is probably based on the Shuttle experience, which means it is 1970's closer to the Salyut heritage of Mir. So who the heck knows!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there transformers/inverters required to transfer power? Or does it all run on a common system?  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-02-28T03:27:22.150" LastActivityDate="2015-02-01T03:15:20.820" Title="Are the power systems on the US and Russian segments of the ISS similar/compatible?" Tags="|iss|power|design|us-segment|russian-segment|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3836" PostTypeId="1" AcceptedAnswerId="3837" CreationDate="2014-02-28T05:23:05.917" Score="15" ViewCount="1941" Body="&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://www.space.com/24800-spacex-falcon9-rocket-landing-legs-photo.html&quot;&gt;SpaceX Adds Landing Legs to Falcon 9 Rocket for Next Launch&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still thinking why they added legs to the rocket's first stage?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the advantages of attaching the legs to the rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does that increase the structural weight thereby decreasing the payload mass? &lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2014-02-28T05:58:25.137" LastActivityDate="2015-01-28T22:06:20.237" Title="What is the intent of attaching legs to the rocket?" Tags="|launch|rockets|spacex|engineering|design|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3837" PostTypeId="2" ParentId="3836" CreationDate="2014-02-28T07:47:25.593" Score="17" Body="&lt;p&gt;They're part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_reusable_launch_system_development_program&quot;&gt;SpaceX reusable launch system development program&lt;/a&gt;. The intent is to make both first and second stages of the Falcon 9 and the Falcon Heavy reusable by returning them both to the launch pad, where they are to make vertical landings. Their technology demonstrator, &lt;a href=&quot;https://en.wikipedia.org/wiki/Grasshopper_%28rocket%29&quot;&gt;Grasshopper&lt;/a&gt;, made eight successful test flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it reduces the payload fraction, but if they can make the technology work, the cost per pound to orbit can be substantially reduced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Elon Musk can pull this off, then maybe the State of California should take another look at his &lt;a href=&quot;https://en.wikipedia.org/wiki/Hyperloop&quot;&gt;Hyperloop&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2014-02-28T07:47:25.593" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3838" PostTypeId="2" ParentId="3836" CreationDate="2014-02-28T11:19:16.697" Score="8" Body="&lt;p&gt;As Jerard mentioned, the goal is to land the first stage, and eventually the second stage, to improve re-useability. The legs provide an important piece to the puzzle to making this happen. And yes, it does reduce the lifting capacity, but there are many cases where the rocket is not completely used. It seems evident that SpaceX was avoiding doing re-usability testing for satellites which used a significant portion of the carrying capacity (Geo birds), while they seem to do more testing with smaller satellites, which are often LEO (CRS, Cassiope). I would expect this trend will continue, seeing more re-usability test on missions with surplus, which I would expect to be the next Orbcomm and CRS missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted that they will not attempt to land on land, but rather have a controlled landing on water. They would like to land on land, but they haven't been given the appropriate clearance to attempt the landing, and won't be until the system can be proven reliable in a low risk situation. Still, if successful, landing on water will provide valuable information on the post thrust lifetime of the rockets, as well as performance in air. Maybe SpaceX will even just set up some sort of a barge to land on to keep the rockets from landing in populated areas.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-02-28T14:21:33.097" LastActivityDate="2014-02-28T14:21:33.097" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3839" PostTypeId="2" ParentId="3836" CreationDate="2014-02-28T11:24:51.447" Score="12" Body="&lt;p&gt;As noted, the SpaceX plan is to build a rapidly reusable rocket.  That is the only real way to get costs down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the moment, no one has ever done what they are trying in several ways.  What I find most interesting is the regular testing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Grasshopper and Grasshopper 2 (officially named F9R-Dev1 and it was tested into a lovely explosion as they hit an edge of the test envelope, the purpose of testing) are testing the control software at the low end of the landing event.  (GH completed testing up to 1/2 mile, F9R-Dev1 was lost during testing, and F9R-Dev2 will go much higher in New Mexico).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every flight they can, SpaceX will be testing the landing system, as a 'bonus' use of the first stage until they succeed. Once they do, they will have the evidence in hand to demonstrate to the FAA that they can safely land a stage on land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus first they will test it over water to demonstrate control and the entire system end to end. Since their original testing, they added a landing barge, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow&quot;&gt;ASDS (Autonomous Spaceport Drone Ship)&lt;/a&gt;, which provides a 170X300 foot target to try and land on. The first attempt on the CRS-5 mission hit the corner of the barge, but ran out of hydraulic fluid for the grid fins. But boy they got close! Following missions aim to land on the drone ship. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going forward they are building a West Coast drone ship for Vandenberg launch recoveries. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives them additional flexibility.  If the ASDS is far downrange and they launch a heavy payload, the fuel required to return to launch site (RTLS) is less, if they actually land on the barge down range. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Testing on the barge makes the FAA certification for landing at KSC much easier, if they have proven repeatadly that they can land on target.  (They are building a landing site at KSC at LC-13). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon Heavy center core will be flying much higher and faster than a normal F9 core, or the two side cores, so using the ASDS for recovery will help payload as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their plans for reuse do involve a 15-30% payload hit but as part of the move from F9 to F9 v1.1 was a big boost in available payload. Thus they will have some flexibility in the future.  If your mission requires 100% of the payload, they could launch expendable and charge you more.  If it fits in the reuable mode payload range, charge much less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they can get Falcon Heavy reusable, then instead of expending a F9 it might be more effective to launch bigger things on the F-Heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The legs on a water landing mission seem sort of silly, but are really needed for several reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Test out the leg mechanism on a full up flight, end to end.&#xA;2) They need the angular momentum control to handle stage spin that they encountered on flight 6, with the Cassioppe mission.&lt;br&gt;&#xA;3) Land on the ASDS and prove their ability to hit a small target to the FAA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Sept 2013, they tested the recovery model by firing three engines after MECO to slow down and aim vaguely back to base. Then once it reentered the atmosphere, restarted the center engine (third firing of the mission for that engine!) above the water for the final deceleration, but the vehicle had been spinning and the reaction control system was overwhelmed and could not control it so the fuel centrifuged away from the intakes and the engine flamed out.  By extending the legs, like a figure skater, they expect to better be able to control the spin with the existing RCS system and land it on the water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if it fails on the Mar 2014 CRS-3 mission, they will try again on the next mission until they succeed.   (See above, what no one else seems to be doing, serious experimentation!)&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-01-28T22:06:20.237" LastActivityDate="2015-01-28T22:06:20.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3840" PostTypeId="2" ParentId="3836" CreationDate="2014-02-28T12:08:49.533" Score="7" Body="&lt;p&gt;SpaceX is intending to make the Falcon series a reusable rocket system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing legs are to allow the 1st stage to perform a vertical landing at or near the launch site, for refurbishment and reuse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The financials are based upon the theory that the fuel is not the most expensive component (Elon Musk, in a television interview). &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to SpaceX’s Elon Musk, \$200,000 in fuel and oxygen make up just 0.35% of his firm’s launch costs. The rest is in their \$56.5 million Falcon 9 rocket.  Musk likens this to throwing away a \$300 million 747 after a one way flight. If you give this a minute of thought, you’ll realize that Musk’s completely right and more effort into reusability should have been given a long time ago.&lt;br&gt;&#xA;  —&lt;a href=&quot;http://www.zmescience.com/space/spacex-reusable-rocket-100-times-cheaper-0432423/&quot;&gt;ZMEScience.com Article&lt;/a&gt; dated 20 Aug 2013.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;M\$0.2 of M\$56 is just a hair under 0.36% If operation and refurbishment costs are in the 5% range, that means launching 10 missions on one rocket body is 15.3% of the cost of 10 rockets... 1/6th the cost. At 20 launches, it would be 1/10 the cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the lost payload capacity isn't needed this flight, it's a suitable flight for the experiment. And if it can soft land, it will benefit SpaceX immediately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacelaunchreport.com/falcon9.html&quot;&gt;http://www.spacelaunchreport.com/falcon9.html&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;https://www.fbo.gov/index?s=opportunity&amp;amp;mode=form&amp;amp;id=d776bfdf8afabea49f5a2c2847c88519&amp;amp;tab=core&amp;amp;_cview=1&quot;&gt;https://www.fbo.gov/index?s=opportunity&amp;amp;mode=form&amp;amp;id=d776bfdf8afabea49f5a2c2847c88519&amp;amp;tab=core&amp;amp;_cview=1&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.space.com/13139-space-fully-reusable-rockets-works.html&quot;&gt;http://www.space.com/13139-space-fully-reusable-rockets-works.html&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.nasaspaceflight.com/2013/10/musk-plans-reusability-falcon-9-rocket/&quot;&gt;http://www.nasaspaceflight.com/2013/10/musk-plans-reusability-falcon-9-rocket/&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;http://www.zmescience.com/space/spacex-reusable-rocket-100-times-cheaper-0432423/&quot;&gt;http://www.zmescience.com/space/spacex-reusable-rocket-100-times-cheaper-0432423/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-02-28T12:08:49.533" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3841" PostTypeId="2" ParentId="3827" CreationDate="2014-02-28T13:51:26.877" Score="5" Body="&lt;p&gt;I'm not a physicist, and certainly can't do the exact maths. But let's have a look at the numbers involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very (!!!) roughly, a first-stage rocket engine consumes over 1000 L / second of fuel for about five minutes. That's 300 tons of fuel, roundabout, that would need to be heated by about 2700 K.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can get a plasma as hot as 10^7 K in the lab... but those are &lt;em&gt;minute&lt;/em&gt; amounts, which means that mixing them with even as little as 1 L of LH2 won't give you more than a puff of H2. Realistically, you will have to settle for much lower heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you allow for ten tons of &quot;heat storage&quot;, even assuming 100% efficiency would require the &quot;storage&quot; to be heated to 81,000 K. (Picture it in your mind. Ten tons of something 25 times as hot as the &lt;em&gt;boiling&lt;/em&gt; point of iron.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now imagine the engineering problems involved in getting it that hot in the first place, then keeping it insulated, and then transferring the heat to the fuel in the required amounts in the required time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then remember you don't get 100% efficiency, not with heat as an energy form. You either need yet more weight, or yet more K in the storage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's using the most optimistic numbers, which I am sure any true scientist will wince at and point out that I overdid it by several orders of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat as energy storage? No chance.&lt;/p&gt;&#xA;" OwnerUserId="2756" LastActivityDate="2014-02-28T13:51:26.877" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3842" PostTypeId="2" ParentId="3105" CreationDate="2014-02-28T14:28:03.560" Score="2" Body="&lt;p&gt;As MPV answers, the Falcon 9 is a launch vehicle.  While the second stage could potentially reach orbit, it has no real inherent payload capacity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, for the future vehicle and engines that SpaceX is planning post Falcon 9, is a methane powered engine named Raptor.  The speculation is that it is meant for a Mars departure stage, where a second or third stage on a future launcher (Falcon Heavy, or something bigger) will be used as the main engine/stage for missions to Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case, the upper stage would potentially be considered a space craft in that context.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-02-28T14:28:03.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3843" PostTypeId="2" ParentId="3835" CreationDate="2014-02-28T16:05:33.470" Score="11" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Iss#Power_and_thermal_control&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Russian segment of the station, like the Space Shuttle and most spacecraft, uses 28 volt DC from four rotating solar arrays mounted on Zarya and Zvezda. The USOS uses 130–180 V DC from the USOS PV array, power is stabilised and distributed at 160 V DC and converted to the user-required 124 V DC. The higher distribution voltage allows smaller, lighter conductors, at the expense of crew safety. The ROS uses low voltage. The two station segments share power with converters.(&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/behindscenes/truss_segment.html&quot; rel=&quot;noreferrer&quot;&gt;118&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;The USOS solar arrays are arranged as four wing pairs, with each wing producing nearly 32.8 kW.(118) These arrays normally track the sun to maximise power generation. Each array is about 375 m2 (450 yd2) in area and 58 metres (63 yd) long. In the complete configuration, the solar arrays track the sun by rotating the alpha gimbal once per orbit while the beta gimbal follows slower changes in the angle of the sun to the orbital plane. The Night Glider mode aligns the solar arrays parallel to the ground at night to reduce the significant aerodynamic drag at the station's relatively low orbital altitude.(&lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/3.23302&quot; rel=&quot;noreferrer&quot;&gt;147&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;The station uses rechargeable nickel-hydrogen batteries (NiH2) for continuous power during the 35 minutes of every 90-minute orbit that it is eclipsed by the Earth. The batteries are recharged on the day side of the Earth. They have a 6.5-year lifetime (over 37,000 charge/discharge cycles) and will be regularly replaced over the anticipated 20-year life of the station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The photovoltaic cells were state-of-the-art for circa 2005, &lt;a href=&quot;https://en.wikipedia.org/wiki/Photovoltaic_cell#Further_improvements&quot; rel=&quot;noreferrer&quot;&gt;but no longer.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;ROSCOSMOS &lt;a href=&quot;https://en.wikipedia.org/wiki/Russian_Orbital_Segment#Proposed_modules&quot; rel=&quot;noreferrer&quot;&gt;had plans&lt;/a&gt; to upgrade the power system on the Russian Orbital Segment (ROS):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On June 17, 2009, the Russian Federal Space Agency (Roskosmos) presented to NASA and the other ISS partners a proposal to add additional modules to the Russian segment to ensure its viability past 2016 or even 2020. To this end, a Nodal Module that would be attached to the nadir docking port of the MLM Nauka would facilitate the attachment of two additional, larger modules that would be capable of providing an independent power source to the Russian segment should current plans to deorbit the US segment of the ISS after 2016 move forward. As proposed, the Nodal Module would be launched during 2013 by a Soyuz launcher in a similar fashion to how the Pirs and Poisk MRM-2 modules were lifted to orbit. The two larger modules, nominally referred to as Scientific and Power Producing Modules 1 and 2, would be lifted to orbit via Proton launchers in 2014 and 2015, respectively. These two modules would be attached to the port and starboard sides of the Nodal Module, leaving its aft docking port accessible for possible future expandability and its nadir port accessible for docking by Soyuz or Progress spacecraft. Because of the proximity of the Nodal Module to the planned attachment point of MRM-1 on the nadir docking port of Zarya FGB to facilitate docking of Soyuz and Progress spacecraft, the module's forward-facing port will be unusable. As of January 2010, neither Roskosmos nor NASA have provided further details of these modules or verification that they have been officially funded by the Russian government or added to the ISS launch manifest schedule.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From NASA's &lt;a href=&quot;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&quot; rel=&quot;noreferrer&quot;&gt;Reference Guide to the International Space Station&lt;/a&gt;, pg. 90:&lt;br /&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/IBkVT.jpg&quot; alt=&quot;ISS Electrical Power System (EPS)&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Further reading:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Electrical_system_of_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;Electrical system of the International Space Station&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-02-01T03:15:20.820" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3844" PostTypeId="1" CreationDate="2014-02-28T16:37:58.433" Score="12" ViewCount="691" Body="&lt;p&gt;The phrase &quot;Life as we know it&quot; is often spoken with a knowing wink, and implies the speaker is in on the big secret. Life on Earth is &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbon-based_life&quot;&gt;carbon based&lt;/a&gt; and every form of life we know of is on Earth. Which leads to &lt;em&gt;Life as we know it&lt;/em&gt;, being carbon based.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Science Fiction occasionally offers &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbon-based_life#Fiction&quot;&gt;Silicon based life&lt;/a&gt; as an option.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Has there been an reliable studies, or factual based speculation on what a non-carbon based (&lt;em&gt;or possibly radically different carbon approach&lt;/em&gt;) life might be?  &lt;/li&gt;&#xA;&lt;li&gt;Would non-carbon based life leave different markers to search for? &lt;/li&gt;&#xA;&lt;li&gt;Are there any programs searching for these markers?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2015-04-21T15:01:49.477" LastActivityDate="2015-04-21T15:01:49.477" Title="Are we searching for life &quot;as we don't know it&quot;?" Tags="|remote-sensing|seti|astrobiology|life|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3845" PostTypeId="2" ParentId="3844" CreationDate="2014-02-28T17:44:16.420" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-carbon-based_biochemistries&quot; rel=&quot;nofollow&quot;&gt;&lt;strong&gt;Wikipeida&lt;/strong&gt;&lt;/a&gt; has a decent article about hypothetical forms of life. Silicon shares many chemical properties with Carbon, making it a good candidate. However, carbon is much more versatile than other similar elements in terms of what they can bond to. Silicon has limitations and interacts with much fewer elements than Carbon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One characteristic of &quot;life as we know it&quot; is that it requires some sort of solvent for biochemical reactions take place. All life we have studied is based on water, but there are many liquids that have the potential to provide life with a solvent. Currently when scientists study the makeup of different planets and moons, some markers of interest are non-water solvents, and this includes ammonia, hydrocarbons (methane), and many others. The presence of these liquids can be detected by using remote sensing and &lt;a href=&quot;http://en.wikipedia.org/wiki/Spectroscopy&quot; rel=&quot;nofollow&quot;&gt;&lt;strong&gt;light analysis&lt;/strong&gt;&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, &quot;life as we know it&quot; took on a very different meaning once scientists began discovering and classifying &lt;a href=&quot;http://en.wikipedia.org/wiki/Extremophile&quot; rel=&quot;nofollow&quot;&gt;&lt;strong&gt;Extremophiles&lt;/strong&gt;&lt;/a&gt;. The &quot;range&quot; of known life increased dramatically. We now know that life is extremely resilient and robust, and these findings are very exciting. However, this doesn't tell us much about non-carbon based life, it really only describes the variability in environments that can support life.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-03-06T16:52:14.007" LastActivityDate="2014-03-06T16:52:14.007" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3846" PostTypeId="2" ParentId="3844" CreationDate="2014-02-28T18:21:42.010" Score="5" Body="&lt;p&gt;Some rarely discussed option also include naturally occuring self-organising and self-replicating helical plasma structures, at least in theory: &lt;a href=&quot;http://doi.org/10.1088/1367-2630/9/8/263&quot; rel=&quot;nofollow&quot; title=&quot;title&quot;&gt;DOI:10.1088/1367-2630/9/8/263&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This article studies the theoretical possibility that some plasma structures with properties usually used to describe living organism. Given the right environment, these structure could duplicate themselves (reproduction), preserve information for a given time (memory), and most importantly, they exibit the same thermodinamic properties as life: they are able to reduce their local enthropy (by incrasing their environment's one).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interresting feature of these plasma structures is that they are shaped as helical strings, much like the DNA in our own cells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would difinately not be classified as &quot;life as we know it&quot;!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;: Here is a more direct answer to the original question, based on the info above and my opinion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sentence &quot;life as we know it&quot; is said in a cautionary way, meaning &quot;everything we know and is defined as life&quot;. It implies that there may be weird lifeforms we don't know about which we are not searching for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for these plasma structures, there is no space exploration program dedicated to searching for them, but results from dust experiments in microgravity (onboard the ISS) could be used to test the plasma structure hypothesis.&lt;/p&gt;&#xA;" OwnerUserId="2758" LastEditorUserId="2758" LastEditDate="2014-03-03T20:11:15.473" LastActivityDate="2014-03-03T20:11:15.473" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3847" PostTypeId="1" AcceptedAnswerId="3850" CreationDate="2014-02-28T19:34:00.670" Score="14" ViewCount="3040" Body="&lt;p&gt;Of course, I am asking about the EVA incident where the Luca's spacesuit malfunctioned and his helmet filled up with water nearly drowning him.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm really sorry, I am not assuming the mission centre did not evaluate all possibilities and this brilliant idea just didn't appear in their minds. But I am puzzled why the astronaut couldn't slurp in that water and swallow it?&lt;/p&gt;&#xA;" OwnerUserId="2759" LastActivityDate="2014-03-03T19:38:23.383" Title="Why couldn't Luca Parmitano just drink that water?" Tags="|iss|spacesuits|eva|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3848" PostTypeId="2" ParentId="3847" CreationDate="2014-02-28T19:51:48.393" Score="7" Body="&lt;p&gt;As a matter of fact, he did. However, the leak came from the cooling system, where there is 4 liters of water, far too much to expect one to drink. Plus, the cooling water isn't really meant to be drunk, and there are other issues as well. &lt;a href=&quot;http://www.theaustralian.com.au/news/world/flooded-helmet-of-astronaut-luca-parmitano-causes-space-emergency/story-e6frg6so-1226680548997&quot;&gt;Quote where he did:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Parmitano reported the leaking water tasted odd.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;His last words before becoming mum were: &quot;It's a lot of water.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-02-28T19:51:48.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3849" PostTypeId="2" ParentId="3847" CreationDate="2014-02-28T20:19:41.650" Score="5" Body="&lt;p&gt;He could indeed taste it (and apparently it didn't taste like drinking water). The water wasn't filling up from the bottom of his helmet. You are ignoring that this was a spacewalk in a zero G environment. There was no pool of water he could drink. Liquids mixed with gas in zero G behave unlike anything you see on the Earth. The water was not a single pool. It was instead suspended droplets of water spread throughout the breathing atmosphere in his helmet. Every breath he took included many, many droplets of water.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-02-28T20:19:41.650" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3850" PostTypeId="2" ParentId="3847" CreationDate="2014-02-28T20:50:29.147" Score="21" Body="&lt;p&gt;The biggest issue was that the source of the liquid wasn't established and that it might have not been safe to drink, potentially causing bigger problems than Luca immediately faced. He did try to drink a bit of the fluid, and reported that it has an odd taste, so it was not his drinking water. They opted for immediate retreat back to the Quest airlock and proceed with emergency  repressurisation of the airlock, so he could take the helmet off as soon as possible with the help of other of his ISS crewmates. It still took long and agonizing minutes to complete all these procedures, as Luca described in his &lt;a href=&quot;http://blogs.esa.int/luca-parmitano/2013/08/20/eva-23-exploring-the-frontier/&quot; rel=&quot;nofollow noreferrer&quot;&gt;EVA 23 blog post&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I move for what seems like an eternity (but I know it’s just a few&#xA;  minutes). Finally, with a huge sense of relief, I peer through the&#xA;  curtain of water before my eyes and make out the thermal cover of the&#xA;  airlock: just a little further, and I’ll be safe.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;I try to move as little as possible to avoid moving the water inside&#xA;  my helmet. I keep giving information on my health, saying that I’m ok&#xA;  and that repressurization can continue. Now that we are&#xA;  repressurizing, I know that if the water does overwhelm me I can&#xA;  always open the helmet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This rushed ingress turned out to have been the better option, and Luca could still have drunk some of the liquid if he couldn't breathe any longer due to it filling his EVA helmet. The exact cause of the leak of what was later determined to have been the Portable Life Support Unit (PLSS) cooling liquid has not yet been determined, except that it was reaching the helmet through the T2 helmet port so it originated from PLSS, but the general consensus is that it's better that he didn't drink the liquid up, unless he couldn't help it any longer and was simply left with no other option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Luca figured he could alternatively try and perform a controlled depressurisation of his helmet, but that would be a risky procedure:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The only idea I can think of is to open the safety valve by my left&#xA;  ear: if I create controlled depressurisation, I should manage to let&#xA;  out some of the water, at least until it freezes through sublimation,&#xA;  which would stop the flow. But making a ‘hole’ in my spacesuit really&#xA;  would be a last resort.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For now, since the root cause for liquids filling up Luca's helmet wasn't yet established precisely, they're settling with temporary fixes to prevent further such incidents by adding absorbent pads to the Extravehicular Mobility Units (EMU) and equip the helmets with a snorkel device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some relevant links:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/1625/49&quot;&gt;What was the source of the fluid in Luca Parmitano&amp;#39;s suit?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2013/08/root-cause-breakthrough-during-emu-investigation/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Spaceflight: Root cause breakthrough during Luca’s leaky EMU investigation&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://blogs.esa.int/luca-parmitano/2013/08/20/eva-23-exploring-the-frontier/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Luca Parmitano's ESA Blog: EVA 23: exploring the frontier&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/Hansen_PressConf.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS EVA 23 Mishap Investigation Board Media Briefing&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: NASA made the full report on the Mishap Investigation Board investigating US EVA 23 mishap and its root causes available to the public in the &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/Suit_Water_Intrusion_Mishap_Investigation_Report.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;International Space Station (ISS) EVA Suit Water Intrusion, IRIS Case Number S–2013–199–00005&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In summary, the causes for this mishap evolved from (1) inorganic&#xA;  materials causing blockage of the drum holes in the EMU water&#xA;  separator resulting in water spilling into the vent loop; (2) the NASA&#xA;  team’s lack of knowledge regarding this particular failure mode; and&#xA;  (3) misdiagnosis of this suit failure when it initially occurred on&#xA;  EVA 22.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The source of the inorganic materials blocking the water separator&#xA;  drum holes had not been experienced  during an EVA before and is still&#xA;  undergoing a concurrent investigation. The results of this&#xA;  investigation  will ultimately lead to resolution of this issue;&#xA;  however, since the concurrent investigation into the source  of the&#xA;  debris is expected to continue for many months, the MIB does not yet&#xA;  have the required data to  determine the root causes of the&#xA;  contamination source, which must ultimately be determined to prevent &#xA;  future mishaps.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Related article on Space.com: &lt;a href=&quot;http://www.space.com/24835-spacesuit-water-leak-nasa-investigation.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacesuit Leak That Nearly Drowned Astronaut Could Have Been Avoided&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-03-03T19:38:23.383" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3851" PostTypeId="1" AcceptedAnswerId="4144" CreationDate="2014-02-28T21:23:42.733" Score="8" ViewCount="546" Body="&lt;p&gt;If an L1 space station was used as a staging area for a mission to Mars, it looks like we would use the Oberth effect in a close flyby to Earth to make the burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I just realized - the idea, as stated, has a free degree of symmetry.  I imagine that we need to leave (v_infinity) with a velocity vector in the direction of Earth's orbit (as per the Hohmann transfer orbit).  We have to stay in the elliptic plane.  That means there are 2 well-timed passes that could accomplish this in a month (neglecting the position of Mars).  Standing on the moon's north pole, looking at the Earth, the trajectory could either fly on the left of right of Earth.  Put another way, the flyby could be prograde or retrograde.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One trajectory would obviously be better than the other, but which one?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously one factor would be the velocity of the moon itself.  But maybe the gravitational potential relative to the sun would factor in as well?  Initially thinking about it, the retrograde option would seem to start at a higher &lt;em&gt;solar&lt;/em&gt; gravitational potential.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-03-30T00:26:15.563" Title="For an EML-1 to Mars transfer orbit, would flying to the right or left of Earth be preferable?" Tags="|mars|trajectory|lagrangian-points|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3852" PostTypeId="1" CreationDate="2014-02-28T22:02:18.763" Score="-2" ViewCount="674" Body="&lt;p&gt;If there had been no lunar tides on Earth, would life have been less likely or evolved less?  If tides benefited life, how so?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Anthropic_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;anthropic principle&lt;/a&gt; resolves the rarity of Earth's features that are conducive to intelligent life, including position in the solar system, position in the galaxy, abundance of water, active geology, etc.  Might tides be another unusual feature with strong benefits for life?  Certainly it seems a rare feature; within the solar system, Earth's moon has 1% of its mass, compared with 0.02% for Saturn and Jupiter, or 0.000002% for Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there are significant benefits, it might be useful to measure tides on &lt;a href=&quot;https://space.stackexchange.com/q/1766/213&quot;&gt;exoplanets&lt;/a&gt;, or implement tides as part of &lt;a href=&quot;https://space.stackexchange.com/q/176/213&quot;&gt;terraforming&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="213" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-03-01T23:14:08.017" Title="What are the benefits of tides? Should we look for them on exoplanets?" Tags="|terraforming|exoplanet|tides|" AnswerCount="1" CommentCount="6" ClosedDate="2014-03-05T15:36:08.933" ContentLicense="CC BY-SA 3.0" />
  <row Id="3853" PostTypeId="2" ParentId="3844" CreationDate="2014-02-28T22:21:54.393" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Are we searching for life “as we don't know it”?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Of course not. How could we?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are (barely) searching for life by looking for electromagnetic signals from some distant civilization, but since we use electromagnetic signals for communication, that is life &quot;as we know it&quot;. That parenthetical &quot;barely&quot;: The organization that is doing this is SETI. NASA along with other space agencies have pretty much divorced themselves from the goings on of SETI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are (barely) searching for life by looking for signatures of biological life &lt;em&gt;as we know it&lt;/em&gt; on Mars, and there are plans to look for signatures of biological life &lt;em&gt;as we know it&lt;/em&gt; on the slew of recently discovered exoplanets. That parenthetical &quot;barely&quot;: NASA and other space agencies until recently have paid rather short shrift to looking for signs of life on Mars, past or present. With regard to exoplanets, right now we're still in the process of finding planets that &lt;em&gt;might&lt;/em&gt; support life as we know it. Looking in detail at whether some of those planets do support life as we know it: that is future work. Hopefully, it's near future work rather than the work of some future generation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With regard to life as &quot;we don't know it&quot; -- how do we look for something with regard to which we have no clue what it looks like?&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-02-28T22:21:54.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3854" PostTypeId="1" AcceptedAnswerId="3858" CreationDate="2014-02-28T22:22:07.263" Score="2" ViewCount="17556" Body="&lt;p&gt;(See the more focused version of this question: &lt;a href=&quot;https://space.stackexchange.com/q/3872/213&quot;&gt;How long would it take to move Venus and Mars into Earth's orbital zone using gravity assist?&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What methods and time scales might be involved in moving Venus or Mars into &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;stable orbits&lt;/a&gt; at Earth-like distances from the sun?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rearranging the lifeless rocky planets might make &lt;a href=&quot;https://space.stackexchange.com/q/1334/213&quot;&gt;terraforming&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/3223/213&quot;&gt;transportation&lt;/a&gt; easier. But the answer need not address &lt;strong&gt;usefulness or feasibility&lt;/strong&gt;.  Though I suspect that something like this can be done with less energy and time than &lt;a href=&quot;https://space.stackexchange.com/q/445/213&quot;&gt;interstellar travel&lt;/a&gt;, and so will be achieved earlier, that's outside the scope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Existing laws of physics only. Plausible guesses at future technology are welcome, but much more science than fiction. Try to think beyond the palpable constraints on energies and budgets of the present era. Not yet a Type II civilization on the &lt;a href=&quot;http://en.wikipedia.org/wiki/Kardashev_scale&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kardashev Scale&lt;/a&gt;, but well beyond Type I.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hint: &lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravity Assist.&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="213" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-12-21T08:01:17.660" Title="How would we move Venus or Mars into Earth's orbital zone?" Tags="|orbital-mechanics|solar-system|interplanetary|terraforming|celestial-mechanics|" AnswerCount="6" CommentCount="10" ClosedDate="2014-03-06T21:02:53.530" ContentLicense="CC BY-SA 3.0" />
  <row Id="3856" PostTypeId="2" ParentId="3854" CreationDate="2014-03-01T11:24:26.363" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Rearranging the lifeless rocky planets might make terraforming and transportation easier.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, it mightn't, because the amounts of energy it would involve are so ridiculously gigantic that terraforming a planet is a very easy job in comparison. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.pha.jhu.edu/~broholm/l24/node1.html&quot;&gt;kinetic energy of an orbiting body&lt;/a&gt; is $\epsilon_k = G\cdot \frac{m\cdot M}{2\cdot r}$ where $G$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot;&gt;gravitational constant&lt;/a&gt;, $m$ the mass of the orbiting planet, $M$ the mass of the body it orbits around, and $r$ the &lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-major_axis&quot;&gt;semi-major axis&lt;/a&gt; or the average distance between the primary focus of the ellipse and the orbiting body. The minimum energy required to move between two orbits is the difference of the two orbital energies. For Mars moving from its current orbit to Earth orbit, this comes out (if I calculated correctly) at $-9.98 \cdot 10^{31}\rm J$ (&lt;a href=&quot;https://en.wikipedia.org/wiki/Joule&quot;&gt;Joule&lt;/a&gt;). That is the total kinetic energy that would have to be removed from Mars' orbit for it to start orbiting at the average Earth's distance to the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is roughly the &lt;strong&gt;entire energy output of the Sun for three days&lt;/strong&gt;!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you could harness that much energy, terraforming a planet would be something you'd give to kids as homework (well, maybe a term project). And, of course, transportation would be something you don't even think about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, if you really want to concoct a hard SF scenario where planets are moved between orbits, the above estimation may also give you an idea about methods and timescales. Say, find a way to siphon off 0.01% of the sun's energy output and use it to move the planets over a timeframe of 80 years.&lt;/p&gt;&#xA;" OwnerUserId="537" LastEditorUserId="537" LastEditDate="2014-03-01T15:13:54.877" LastActivityDate="2014-03-01T15:13:54.877" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3857" PostTypeId="2" ParentId="3854" CreationDate="2014-03-01T19:01:33.587" Score="6" Body="&lt;p&gt;Let's see what it would take to move Mars to an Earth orbit. It's not a pretty picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most efficient way to move from one orbit to another is via a Hohmann transfer. We'll apply a delta-V to Mars to slow it down and put the planet into an elliptical transfer orbit that just intersects Earth's orbit, then another delta-V once Mars reaches perihelion. Assuming Mars is orbiting circularly at 1.524 AU, a retrograde delta-V of 2.65 km/s will put Mars on that transfer ellipse. Half an orbit later, another retrograde delta-V, this time 2.94 km/s, will put Mars in a 1 AU circular orbit. No problem! All we have to do is change Mar's velocity by 2.65 km/s and then later by 2.94 km/s, or a total delta-V of 5.59 km/s, and voila! we have Mars orbiting at 1 AU.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a huge problem. Those two instantaneous changes in velocity represent an enormous amount of energy and an even more enormous amount of momentum. Note that this is considerably less energy than Michael Borgwardt's answer. Mars' mass * ((2.65 km/s)&lt;sup&gt;2&lt;/sup&gt;/2 + (2.94 km/s)&lt;sup&gt;2&lt;/sup&gt;/2) is about 5*10&lt;sup&gt;30&lt;/sup&gt; joules, not 10&lt;sup&gt;32&lt;/sup&gt; joules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this is only part of the total energy needed. The only way we know how to make an object in space move in some direction is to eject some mass from the object in the opposite direction. Only a tiny fraction of the energy that goes into accelerating the ejected mass accelerates the rocket (or the planet in this case).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose we could eject a (small) mountain's worth of material (10&lt;sup&gt;12&lt;/sup&gt; kg) at 99% of the speed of light from Mars every single day. This is not the way to get Mars to follow a Hohmann transfer to Earth's orbit. It would takes centuries to build up that 2.65 km/s delta-V needed for the first leg of the transfer. A Hohmann transfer is not possible when moving planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ejecting 10&lt;sup&gt;12&lt;/sup&gt; kg per day from Mars at 0.99&lt;i&gt;c&lt;/i&gt; makes for a very slow transfer. It would take more than 47 centuries to move Mars from it's current orbit to Earth's orbit. The amount of energy consumed in the process of just ejecting the mass is 10&lt;sup&gt;36&lt;/sup&gt; joules. Almost all of the energy goes into that exhaust stream. Only a tiny fraction goes into moving Mars. In the process, we'll have expelled over 10&lt;sup&gt;18&lt;/sup&gt; kilograms of mass from Mars. Note that 10&lt;sup&gt;12&lt;/sup&gt; kg per day is about 1.5 orders of magnitude greater than rate at which coal and iron are extracted from the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mining a mountain's worth of material a day and sending it off into space at 99% of the speed of light: We can't do that. It's not even close. Maybe in 47 centuries we'll be able to do that. It's better to wait.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-01T19:01:33.587" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3858" PostTypeId="2" ParentId="3854" CreationDate="2014-03-01T21:12:57.770" Score="9" Body="&lt;p&gt;Setting aside the fact that this would be a very bad idea and that there are far less energy intensive ways to terraform the surface of a planet, the most energy efficient way to do this I can think of is by using Oort Cloud objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes very little $\Delta V$ to make an Oort cloud object dive towards the Sun.  In fact, it is the source of most comets.  The total mass out there is at least a few Earths.  Each object would be sent on a trajectory to swing close to Mars on the trailing side.  The object would be flung out of the Solar System, and Mars would slow down a little.  Repeat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might be possible to reuse an object.  A Jupiter flyby could pull an escaping object back into a solar orbit to return to Mars for another tug.  If you could cycle such an object, you would make a pump that transfers energy and angular momentum from Mars to Jupiter.  Don't worry too much about changing Jupiter's orbit -- it won't change much.  1/3000th as much as Mars' orbit changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more useful application of the approach would be to raise Earth's orbit to keep it habitable as the energy output of the Sun increases over the next few billion years.  In other words, &lt;em&gt;keeping&lt;/em&gt; Earth terraformed.  So do the above on the leading side of Earth to speed it up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the very long time scale of the increase of the Sun's energy, this might be a viable approach.  Something like self-replicating vehicles using materials in the Oort cloud, powered by fusion reactors, might make this somewhat affordable.  Again over long time scales.  I'd mainly be worried about running out of Oort cloud material if you couldn't reuse it.  Though you could then start raiding the Oort clouds of other nearby stars.  Then we'll have high-potential-energy-mass wars with the civilizations in other star systems who want to do the same thing ...&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-03-02T01:03:50.193" LastActivityDate="2014-03-02T01:03:50.193" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="3859" PostTypeId="2" ParentId="3852" CreationDate="2014-03-01T22:09:04.337" Score="2" Body="&lt;p&gt;100% of all planets on which evolution of intelligent life has been observed have unusually strong tides. But considering the sample size (one), it is quite a stretch to assume a causal link.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the theory of evolution, a trait evolves when this trait increases the chance of having off-springs which in turn also have off-springs which pass this trait on. That means an environment in which intelligence evolves needs to provide some kind of challenge which can be overcome through applying intelligence and which, when overcome, provides a better chance of having off-springs. Tides result in an ecosystem along the coastlines which is constantly changing and might provide such a challenging environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it would be quite a stretch to assume that it is the only possible way how the intelligence of any lifeforms could be challenged in a way which makes it beneficial for them to be more intelligent. Some &lt;a href=&quot;http://en.wikipedia.org/wiki/Cephalopod_intelligence&quot; rel=&quot;nofollow&quot;&gt;cephalopods&lt;/a&gt;, for example, are considered to be quite intelligent even though they evolved in the sea where tidal effects don't have much effect on their environment (octopuses seem to be capable of communication, learning, simple tool use and... &lt;a href=&quot;http://en.wikipedia.org/wiki/Paul_the_Octopus&quot; rel=&quot;nofollow&quot;&gt;sport betting :)&lt;/a&gt; ).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that is all just theoretical navel-gazing. To find out if there is a correlation between tides and intelligent life or not, we need more examples of planets with intelligent or at least non-intelligent life.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-03-01T23:14:08.017" LastActivityDate="2014-03-01T23:14:08.017" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3860" PostTypeId="2" ParentId="3854" CreationDate="2014-03-01T22:22:30.800" Score="4" Body="&lt;h1&gt;Within Known Physics&lt;/h1&gt;&#xA;&lt;p&gt;There are three vaguely plausible methods for changing a planet's orbit that come to mind without breaking the accepted laws of physics:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;direct application of thrust&lt;/li&gt;&#xA;&lt;li&gt;Consistent close passes of a series of massive objects&lt;/li&gt;&#xA;&lt;li&gt;targeted solar flares&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Direct application of thrust is going to be incredibly disruptive - it's either by impact or by insolation changes.&lt;/p&gt;&#xA;&lt;p&gt;Impact as a direct application of force is practical on a societal time scale, but not on an individual time scale. It would require minor alterations to many cometary/asteroidal bodies in crossing orbits, with lead times of one to two decades per each. Remember, a small perturbation can result in a significant change in position over a long time. The important elements of the project require ensuring that the final impact is on the correct position - the trailing (retrograde) face of venus to speed it up, or the spinward (prograde) face of mars. (Remembering the WWIVnet reduction of Niven's ditty: Spin out, out back, back in, in spin. Accelerate to spinward, you move out. To back, move in.) The issue is that this would take thousands of impacts, all of which would need to change the orbital velocity in the same direction, over a scale of a couple centuries, in order to have any appreciable effect. We are just now getting to the technological point where this is feasible - but it will likely never be practical.&lt;/p&gt;&#xA;&lt;p&gt;The use of altered insolation could produce a change in atmospheric loss in such a manner as to create a very slow, but steady, acceleration. Light pressure is already a known factor in orbital mechanics by itself; the mirror array would be accelerated away, so would need some form of acceleration of its own. Perhaps, a larger sail array. Timescale is, again, centuries. Mass is tremendous - comparable to entire navies. In the case of Venus, the desired thinning effect might be useful, as well.&lt;/p&gt;&#xA;&lt;p&gt;Moving massive objects by at the correct path is doable, but without a non-N-space drive, requires insane amounts of power to alter them. To accelerate, you need to route them close enough to the correct side. For venus, this means parading a bunch of bodies past the spinward side of Venus, and preferably, most of them doing so on the outbound leg of their own path. For mars, you want them on the trailing side and inbound leg. Ideally, for venus, they also should be moving prograde, while for mars, retrograde. Minor errors? Accumulate new moons. The drawback is that, to get sufficient mass, you're looking at moving large asteroids.&lt;/p&gt;&#xA;&lt;p&gt;It is theoretically possible to trigger solar flares; doing so could be used to apply recurrent pressure over several centuries. The side effects, however, are that doing so is likely to be seen as a threat of wiping out all life in the system.&lt;/p&gt;&#xA;&lt;p&gt;Edit:&#xA;&lt;em&gt;According to Wikipedia, solar wind generates between 1e-9 and 6e-9 nPa. Venus has a cross sectional area of about 1.15e14 m^2, a mass of around 4.87e24 kg, for a total net of about (3.5x1.15/4.8)e(-9+14-24) = 0.8e-19 nm/s^2 or 8e-26 m/s^2. Pressure at venus should be about 2.7x as strong... It would take 1,000,000,000,000,000,000 years to hit 8m/s. A CME can be 1000x more energetic and can add additional from atmospheric heating... It's theoretically useful, but in practice, worthless. If one's engineering them, one can possibly also get deeper upshot, more energy, and more particle density.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;h1&gt;Just outside current physics&lt;/h1&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A giant Alcubierre-White Warp Drive&lt;/li&gt;&#xA;&lt;li&gt;Gravity Manipulation - if the math is to be believed, gravity is a manipulable force.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The Alcubierre-White Warp Drive, in theory, could be used to place a body into a &amp;quot;fall into orbit&amp;quot; position. It doesn't matter if it generates superluminal pseudovelocities - only that it moves the object by changing the topology of space. Without Dr. White's transformations, the math would indicate insanely high energy requirements; even with, moving a bubble that large is likely to require a stupidly high amount of energy.&lt;/p&gt;&#xA;&lt;p&gt;Gravity manipulation is believed theoretically possible by a number of physicists. If one can either enhance or impede gravity's pull artificially, one can then make use of that to alter orbits. But the process may be surprisingly convoluted. You move out with it reducing, then turn it to enhancing to accelerate inbound, then turn it to reduce again as one passes perihelion, to increase overall orbital speed. Unfortunately, this also increases eccentricity. Careful manipulation of the process can reround the orbit, but during the transfer process, it's actually going to get closer. In practice, however, almost no one is willing to admit to thinking it within easy reach. Many physicists think it impossible, citing that gravity is a fundamental property of matter, not a force generated by matter; if it's a fundamental principle, it isn't alterable.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-12-21T08:01:17.660" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3861" PostTypeId="2" ParentId="3851" CreationDate="2014-03-01T23:33:02.863" Score="5" Body="&lt;p&gt;The Oberth effect isn't the only reason for flying past another celestial body during an interplanetary flight. There is also the &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational slingshot effect&lt;/a&gt; which increases the speed of a vessel without using any fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To perform a gravitational slingshot around a celestial body which results in an increase in relative velocity, one needs to pass behind that body on its orbital path. Passing in front of it reduces the relative velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ejz3F.gif&quot; alt=&quot;Gravitational slingshot with speed-gain&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/HNsEA.gif&quot; alt=&quot;Gravitational slingshot with speed loss&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such a fly-by will most certainly be on the &quot;right&quot; side as seen from moon's north pole. The reason is that the moon and thus also the EML-1 orbit the earth in a counter-clockwise direction (seen from north). Passing earth &quot;left&quot; would require to change from a prograde to a retrograde orbit, which requires more fuel than just going from a circular to a highly eccentric orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image source: &lt;a href=&quot;http://commons.wikimedia.org/wiki/Category:Gravitational_slingshots&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;&#xA;&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-03-02T12:18:45.147" LastActivityDate="2014-03-02T12:18:45.147" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3862" PostTypeId="2" ParentId="3844" CreationDate="2014-03-02T05:33:29.120" Score="0" Body="&lt;p&gt;To conduct a successful search for extraterrestrial life, one would have to keep an open mind to the existence of life based on alternative chemistries, but focus a search effort on that which is most likely to occur, and therefore be easiest/most likely to find. The point of this &lt;a href=&quot;http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-carbon-based_biochemistries&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article&lt;/a&gt; is that alternative chemistries are theoretically possible, but each of the theoretical alternatives is ultimately less viable or probable than the one we know. As stated in the article, it comes down to abundancies of the elements involved, the properties of liquid water as a solvent, carbon's rich chemical potential, and the other elements being most compatible with a carbon and water based chemistry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It isn't that an alternative couldn't exist, just that there is one and only one chemistry which seems far more likely to occur than any other.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-03-02T05:33:29.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3863" PostTypeId="1" AcceptedAnswerId="3864" CreationDate="2014-03-02T12:37:24.720" Score="22" ViewCount="12288" Body="&lt;p&gt;All videos of rocket launches I ever saw were performed in clear weather conditions. I also read many reports about launches which were delayed because of bad weather conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the complications which would occur during a rocket launch when it rains? What modifications to a rocket could be made to make it possible to launch it anyway?&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="49" LastEditDate="2015-04-27T22:55:24.223" LastActivityDate="2016-03-20T22:33:28.060" Title="Can a rocket be launched while it rains?" Tags="|launch|rockets|weather|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3864" PostTypeId="2" ParentId="3863" CreationDate="2014-03-02T12:53:54.667" Score="30" Body="&lt;p&gt;There is a huge difference between the US and Russian launchers, in terms of tolerance for weather.  Additionally, different launchers have different tolerances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians launch from Baikanour, which is kind crazy weather wise, and cold, and snow, and other weather is much more common, so they designed from the get go for weather tolerance.  It probably also helped that the R-7 ICBM became the Soyuz booster, and you don't get to delay nuclear war for bad weather.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US side is much more delicate, in general, and much more careful about weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle specifically had issues with launching and landing in rain. The tiles on the underside of the shuttle are very good at handling heat, but at the same time very delicate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rocket launching quickly goes supersonic, usually within the atmosphere, and usually within a minute or three of launch. Hitting individual rain, snow, or hail pellets at supersonic speeds is not a good thing. The tiles on the shuttle would have a real problem if that happened, alas as demonstrated by Columbia's final mission. Though that was a large strike, during launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to rain, a bigger concern for long thin launchers is upper atmospheric winds. The amount of control a rocket has, depends on a number of things, but the vast majority of control is by gimballing the engine. If it is really windy as it ascends, it is possible that the engine cannot control the force of the wind, which could potentially push the vehicle into the air flow and destroy it.  The Challenger orbiter itself was sort of destroyed more by the fact after the main tank ruptured, the orbiter tumbled in the airflow at multiple of Mach and was destroyed by the airflow than by the SRB having an issue or the external tank rupture (it did not explode, it burned) per se.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issue however varies with altitude. The Shuttles in the end were suspectible to cold weather at launch ground level, as Challenger demonstrated. The Russians are much more cold tolerant since they knew what normal conditions were in Baikanour (vs Florida).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once launched, your vehicle needs to be strong enough to handle supersonic impact of rain particles.  Once high enough, you need enough gimbal control to handle winds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much of each you tolerate in your design depends on how much you care about robustness vs cost. It is a huge trade off.  If you can build a much cheaper booster, but have to be more sensitive to weather, which way do you chose?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the R-7 booster case, it was an ICBM that became a launcher, so it was more important to be able to launch on demand than cost, per se. (Of course, when you launch 1700 of them, the cost does tend to go down a bit). With the Shuttle, it was a pampered princess in this regard.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-03-02T12:53:54.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3865" PostTypeId="2" ParentId="3854" CreationDate="2014-03-02T16:22:46.397" Score="5" Body="&lt;p&gt;It's not possible! At least not if you want to keep Earth safe in its orbit. And while I like some suggestions previously mentioned here because playing celestial game of pool sure is fun, they're inherently not possible because of the timescale at which all these orbital changes would have to be applied. So forget about Hohmann transfer on a planetary scale, or slow decrease of planet's orbital velocity by shooting other massive bodies past it or even into it. By the time you'd sufficiently decrease its orbital velocity for it to come close to orbital altitude of Earth, well, it's already in its own sphere of influence and all hell breaks loose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also forget about quantum hocus pocus and FTL. Whatever you did to temporarily move a whole planet out of realm of spacetime and general relativity to later beam / teleport / warp it to another place, you've also forfeit ability to manipulate its momentum with it, so once (might be instantaneous?) you'd &lt;em&gt;summon&lt;/em&gt; this planet to another orbital position and back to physical reality as we know it, it would have exactly the same momentum imparted on it as it always had, so in case of moving Mars into position of Earth's L&lt;sub&gt;3&lt;/sub&gt;, you'd only end up with Mars having a highly elliptical orbit with exactly the same semi-major axis as it always had, only much larger eccentricity that would cause it to intersect Earth's own orbital path. Given sufficient time, say a few orbits, this wouldn't end up well either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Celestial mechanics is hard! And it has very little to do with how we think of trajectories and orbits of small spacecraft with largely negligible mass in relation to celestial bodies they move about. Managing orbits of planets is nearly impossible, and what makes it so is that all the massive bodies of a star system are in semi-stable equilibrium because they evolved into ordered state from a primordial chaos of collapsing nebulae. Sufficiently disturb this equilibrium, and you've set off a chain reaction with unforeseen consequences. And moving a whole planet into lower orbit does qualify. To somewhat demonstrate this, let's consider these two graphs:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://blogs.scientificamerican.com/observations/2011/06/09/what-would-happen-if-earth-and-mars-switched-places/&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JQtyXm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LxuqQm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Graphs courtesy of: Renu Malhotra; Source: &lt;a href=&quot;http://blogs.scientificamerican.com/observations/2011/06/09/what-would-happen-if-earth-and-mars-switched-places/&quot; rel=&quot;noreferrer&quot;&gt;Scientific American: What Would Happen If Earth and Mars Switched Places?&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their captions are wrong, and they show orbital altitude of Mercury, Venus, Earth and Mars and these planet's perihelion, aphelion and semi-major axis plots, not all official planets of our Solar system from Mercury to Neptune. But nevermind that, what's important is that inner Solar system's current arrangement seems stable, but if we just swapped orbits of Mars and Earth (not even moving Mars into the same orbit with Earth), the whole system destabilizes. in roughly 10 million years, Mercury intersects Venus and Venus Mars, and there's no way of telling what would happen. Would they swap orbits &lt;a href=&quot;http://www.space.com/4755-trading-cosmic-places-neptune-uranus-swapped-spots.html&quot; rel=&quot;noreferrer&quot;&gt;like it's suggested that Uranus and Neptune did four billion years ago?&lt;/a&gt; Collide? Nobody knows, because &lt;a href=&quot;https://en.wikipedia.org/wiki/Perturbation_theory&quot; rel=&quot;noreferrer&quot;&gt;perturbation theory&lt;/a&gt; methods that enable us to solve these problems are inherently imprecise once other factors than those accounted for come into play.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what's wrong with all the current suggestions? Well, they all oversimplify the whole system and don't deal with all the intermediate states between start state A and final state B, merely suggest what's the required change in kinetic energy and how one could achieve that. And with FTL suggestions, the other way around. And that's fine, because in reality, there isn't any solution to your question anyway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In theory, I can think of a few end states for Mars to orbit at same or similar distance to the Sun as Earth does, for example putting Mars in Earth's opposition so they're co-orbital with Mars roughly where L&lt;sub&gt;3&lt;/sub&gt; would have been if Mars wasn't there, or putting Mars into a 90° or 270° inclination orbit that intersects Earth's in opposition, or even inclining Mars' orbit 180° to orbit retrograde and lowering its orbit parallel to Earth's but not at exactly at the same altitude, and let them swap altitude each time they approach each other (a horseshoe orbit). And these orbits could even be stable for a relatively long time. If... If other celestials weren't there, and all of a sudden having two celestials where it used to be only one wouldn't perturb their orbits and destabilize the whole inner Solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even assuming we could solve the issue of &lt;em&gt;which orbit to put another celestial on so it's at the same distance to the Sun as the Earth is&lt;/em&gt;, we'd end up destabilizing the whole inner Solar system, and set it back into chaos with each celestial fighting for its place &lt;em&gt;around the Sun&lt;/em&gt;. Mercury and Venus would violently change eccentricity, with end possibilities being either plummeting into the Sun, hitting one of the other three inner Solar system celestials, swapping orbits, or be ejected out of the inner Solar system completely. Main belt objects would destabilize orbits too, and the whole Solar system would be back in the Late Heavy Bombardment years. What would be the end outcome, and how would that affect outer Solar system? Nobody knows. But one thing is for sure. There isn't any comfortable way of doing it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what it's worth, swapping Earth's and Mars' orbit would require a lot less change in orbital energy, if you could increase Mars' eccentricity to the point of intersecting Earth's at exactly the right time for them to exchange orbits and not collide in the process. I didn't do calculations, but I'd say it should be around 20-30% of the required energy previously suggested. Oh, and according to my calculations, Michael Borgwardt's maths on how much change in kinetic energy would be required appears closer to $\approx -1.6\cdot 10^{32} \text{J}$ that I came out with, but it's really easy to mess with unit conversions required in the process, so I can't claim that's correct either. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-02T16:29:30.360" LastActivityDate="2014-03-02T16:29:30.360" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3866" PostTypeId="2" ParentId="3854" CreationDate="2014-03-02T17:39:38.833" Score="0" Body="&lt;p&gt;There's another problem here:  None of the rocket approaches are safe even if you could do them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thing is the acceleration is going to be small.  That means the body you are moving is going to pass very close to the Earth while it's orbit is being circularized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can you say &quot;tides&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(And that's assuming it's even possible to circularize the orbit in the first place.  I wouldn't be one bit surprised if this is impossible due to deflection from the flybys of the Earth.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-03-02T17:39:38.833" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3867" PostTypeId="2" ParentId="2515" CreationDate="2014-03-02T17:55:18.903" Score="2" Body="&lt;p&gt;Some satellite operators intentionally locate more than one satellite in a single orbital slot.  They employ a stationkeeping strategy to keep them physically separated.  Minimum distances of a few kilometers are usual.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-03-02T17:55:18.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3868" PostTypeId="1" AcceptedAnswerId="3871" CreationDate="2014-03-02T18:29:48.757" Score="36" ViewCount="13379" Body="&lt;p&gt;When I first saw Google Maps satellite view many years ago, I remember being greatly impressed (and somewhat surprised) with the image quality, considering the photographs came from up there in space.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I later learned that only some of the texture is from satellite imagery, and the higher resolution content is usually sourced from aircraft flyover in metropolitan areas.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I'm curious about just how good the state of the art in satellite imagery is today. For example, if a person of interest was looking up into the sky on a clear day, would the photography be good enough for facial recognition? How many centimetres on the ground per pixel in the image at maximum zoom? Is this technology still improving, or is there some sort of physical limit which now prevents increases in resolution?&lt;/p&gt;&#xA;" OwnerUserId="762" LastEditorUserId="49" LastEditDate="2014-03-02T18:47:02.720" LastActivityDate="2018-01-08T03:05:00.487" Title="Is photography from a satellite good enough to make out a person on the ground?" Tags="|artificial-satellite|low-earth-orbit|optical|remote-sensing|photography|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3869" PostTypeId="2" ParentId="3676" CreationDate="2014-03-02T18:42:35.293" Score="9" Body="&lt;p&gt;Using gravity to remotely look at the hidden interior of a planet or moon has a remarkably long history. The idea goes back to Newton. He suggested measuring the divergence of a plumb bob near a mountain from the surveyed normal as a means of assessing the mass of the Earth. He dismissed this idea as impractical given the low quality of surveying measurements at that time. It didn't take long for surveying techniques to improve. While surveying the Mason-Dixon Line, Charles Mason and Jeremiah Dixon noted measurable deviations of a plumb bob from surveyed normal. Mason later helped select a mountain that could be used to &quot;weigh the Earth&quot;. The end result was the 1774 Schiehallion experiment (&lt;a href=&quot;http://en.wikipedia.org/wiki/Schiehallion_experiment&quot;&gt;http://en.wikipedia.org/wiki/Schiehallion_experiment&lt;/a&gt;), which gave a result within 20% of the current value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The modern day successors to the Schiehallion experiment includes a number of satellites used to measure the gravity fields of the Earth, the Moon, and several of the planets. These satellites continue the work of Schiehallion experiment in giving insights into the internal makeup of these bodies. Some of these satellites (e.g., GRACE and GRAIL) had gravity observations as their sole job. Many others had gravity science as one of many experiments. For yet others such as the PFS-2 launched by Apollo 16 (&lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&quot;&gt;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&lt;/a&gt;), discovering weirdnesses of gravity was rather unintentional.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno's gravity science experiment has a predecessor in what are arguably the simplest of all satellites ever launched, the LAGEOS satellites. The LAGEOS satellites have no instruments, no sensors, no effectors. They are just solid brass balls covered with retroreflectors. Scientists instead ping those retroreflectors with ground-based lasers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity experiments give a somewhat muddled view of the interior. It helps (a lot!) to have alternate sources of data. Juno scientists will use both the gravity results and the magnetic field measurements to get an idea of what Jupiter's interior looks like. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-02T18:42:35.293" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3870" PostTypeId="2" ParentId="3868" CreationDate="2014-03-02T19:22:43.257" Score="26" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;would the photography be good enough for facial recognition?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Not yet. It's not even close. Facial recognition requires 50 to 100 pixels between the eyes, or on the order of 1 millimeter resolution. To see that kind of detail from a distance of 250 kilometers using blue-green light (500 nm) would require a lens or mirror that is 125 meters in diameter. Note: I'm being very generous with that distance of 250 km and with the use of blue-green light. For the physics involved, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Airy_disk&quot;&gt;http://en.wikipedia.org/wiki/Airy_disk&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The KH-12 satellites supposedly have a resolution of 6 inches or so. Given the sampling theorem, that means a one foot resolution in practice. You can't even see that it's a face that you're looking at with that kind of resolution, let alone whose face it is.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;is there some sort of physical limit which now prevents increases in resolution?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes. That light is a wave phenomenon places limits on the resolution of imaging equipment. High resolution requires big lenses or mirrors. See the article to which I linked early. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-02T19:29:30.703" LastActivityDate="2014-03-02T19:29:30.703" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3871" PostTypeId="2" ParentId="3868" CreationDate="2014-03-02T19:36:12.050" Score="30" Body="&lt;p&gt;There are limits. For one, there's atmospheric effects that scatter light in visible wavelength spectrum. You might be able to penetrate clouds and haze easier in the lower end of the spectrum and towards the infrared wavelengths, and those might still be usable for facial recognition though. Another limit is aperture of optical equipment used to take photographs, in case of telescopes, that's given by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawes%27_limit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dawes' limit&lt;/a&gt; and is derived from &lt;a href=&quot;https://en.wikipedia.org/wiki/Rayleigh_criterion#Explanation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rayleigh criterion&lt;/a&gt; to compute angular resolution of imaging devices. So the end resolution depends on weather, imaging wavelength, atmospheric scattering, distance, and diameter of primary mirror (aperture). Build quality obviously also has a major role.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let's first see what's the &lt;strong&gt;absolute minimum resolution&lt;/strong&gt; we'd require to recognize facial features and successfully identify individuals. According to &lt;a href=&quot;http://www.axis.com/academy/identification/resolution.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, to do that, we'd need to resolve a human face to a resolution of at least 40x40 pixels. So if we say that an average adult human face is 20 cm wide, we should resolve to within 5 mm. At also &lt;strong&gt;extremely low LEO orbits&lt;/strong&gt; that might be used by some surveying satellites, say 200 km above the surface of the Earth during perigee of highly elliptical &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Molniya orbits&lt;/a&gt;, this gives us a required angular resolution of $3.0\cdot 10^{-8}\text{ rad}$, or $0.00618794419\text { arc}$&amp;nbsp; (in &lt;a href=&quot;https://en.wikipedia.org/wiki/Radian&quot; rel=&quot;nofollow noreferrer&quot;&gt;radians&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Minute_of_arc#Symbols.2C_abbreviations_and_subdivisions&quot; rel=&quot;nofollow noreferrer&quot;&gt;arcseconds&lt;/a&gt;, respectively).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using Dawes' limit, we can then calculate theoretical minimum telescope's aperture (in centimeters):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$D = \frac{11.6}{R} = \frac{11.6}{0.00618794419} = 1,874.6\text{ cm}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or roughly 18.7 m (738 inch, 61.5 ft, 20.5 yard) diameter telescope. Not impossible, but such huge telescope would certainly be visible even with a naked eye (remember, that's minimum diameter, it would be by far taller than that) &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;nofollow noreferrer&quot;&gt;if it caught reflection&lt;/a&gt;, and that's usually not an option with military spy satellites we might not be aware of, while there certainly aren't any such commercial and/or scientific telescopes in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering I took the most extreme and ideal case examples into consideration for the sake of argument, and the numbers are an order of magnitude smaller than what David Hammen came out with in &lt;a href=&quot;https://space.stackexchange.com/a/3870/49&quot;&gt;his answer&lt;/a&gt; because of that, the answer is still:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;No, there is no such optical telescope in Earth's orbit with required resolution to identify individuals by their facial features. If it was, we'd know about it.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing that shouldn't be forgotten is that satellites can't simply change orbits and be at any place and at any time they'd need to be, to track and identify individuals even if they could do that by their facial features alone. If someone is going to put so capable optical equipment in the skies, it would more likely be on inconspicuously painted blimps / aerostats that could silently hover at a lot smaller altitudes, image targets from a shallow angle, and follow their movement easier. And all of that with a lot cheaper, smaller and harder to notice equipment. Or use helicopters, &lt;a href=&quot;https://en.wikipedia.org/wiki/Unmanned_aerial_vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;drones&lt;/a&gt;, CCTV,... or good old &lt;em&gt;boots on the ground&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-03-03T00:27:22.167" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3873" PostTypeId="2" ParentId="3863" CreationDate="2014-03-02T23:27:46.170" Score="13" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What are the complications which would occur during a rocket launch when it rains?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In addition to the issues raised by geoffc, another is that rain (or rather the rain clouds) obscure visibility of the rocket. Two key goals of launching from a US-based launch site are that the launch should not present a hazard to the US public at large, and perhaps even more importantly, that a US-based launch should not cause an international incident. A rocket veering out of control from Cape Canaveral has plenty of targets of opportunity for havoc: The communities around Cape Canaveral, Jacksonville, lots of cities on the coast. Internationally, a misbehaving rocket launched from Canaveral could theoretically hit Bermuda, the Bahamas, or Cuba.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The range safety rules for US launches are very strict with regard to weather conditions and the observability of the launching rocket. The rocket has to be visible throughout the launch, up to the point where it is high enough and moving fast enough that an onboard catastrophe would mean wreckage falling into the Atlantic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of those old videos of rockets blowing up shortly after lift-off were a result of the Range Safety Officer pushing the &quot;big red button&quot; that sent an electronic self-destruct signal to a misbehaving rocket. (Actually, it was two small switches, both of which red to ensure they weren't triggered accidentally and both which had to be armed prior to moding to fire. Details, details.)&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-02T23:27:46.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3874" PostTypeId="1" AcceptedAnswerId="3878" CreationDate="2014-03-03T05:19:56.880" Score="5" ViewCount="365" Body="&lt;p&gt;NASA recently released &lt;a href=&quot;http://futureplanets.blogspot.com/2014/02/boundaries-for-next-discovery-mission.html&quot; rel=&quot;noreferrer&quot;&gt;preliminary guidelines&lt;/a&gt; for proposals for the next &lt;a href=&quot;http://discovery.nasa.gov/index.cfml&quot; rel=&quot;noreferrer&quot;&gt;Discovery Program&lt;/a&gt; mission. The Principal Investigator (PI) budget, which covers the cost of the spacecraft, its science payload, and data analysis, is capped at \$450 million.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Outside of the PI’s budget, NASA will pay for the cost of the mission launch.  For the first time, NASA will pay for the cost of the mission’s operation outside of the PI budget.  In past competitions, missions with long flight times, and hence high operations costs, were at a disadvantage to missions with low operations costs.  This appears to be an effort to equalize operations costs.  Missions that would benefit are any with flight times of several years compared to the weeks or months to reach Venus, the moon, or Mars.  With operations costs not counted against the PI’s budget, the new competition probably keeps up with inflation compared to the previous competition.  (NASA’s announcement does emphasize that the operations costs projected for a mission must be ‘reasonable’.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If I were heading a team putting together a Discovery Program mission proposal, I would want some hard data to determine what NASA might consider 'reasonable'. Several points to consider:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has already announced that no radioisotope power systems will be available for your mission, so you're stuck with solar power. Also, previous Discovery missions could get 1/3 of their budget from a foreign space agency, but this did not apply to science instruments paid for by foreign governments; now, the 1/3 cap has been extended to science payload as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a savvy project manager, my first thought would be to try to see how previous mission proposals were budgeted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Are there any online sources for budgets from previous Discovery Program missions?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="49" LastEditDate="2014-03-03T14:51:23.340" LastActivityDate="2014-03-03T16:11:37.710" Title="What should a Discovery Program mission budget look like?" Tags="|nasa|future-missions|mission-design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3875" PostTypeId="2" ParentId="3868" CreationDate="2014-03-03T08:43:02.317" Score="-1" Body="&lt;p&gt;Considering 2008 satellite imaging can resolve surface features down to 0.41 meters in size, I think you can be pretty sure that by now (2014) if you are sitting in your backyard reading a newspaper, the govt can know which paper you are reading.&lt;/p&gt;&#xA;" OwnerUserId="2780" LastActivityDate="2014-03-03T08:43:02.317" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3876" PostTypeId="2" ParentId="3093" CreationDate="2014-03-03T12:57:22.223" Score="13" Body="&lt;p&gt;The Voyager &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810001583_1981001583.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Attitude and Articulation Control Subsystem&lt;/a&gt; uses a sun sensor to point the high-gain antenna at the sun. This is simple: rotate the spacecraft until you find the brightest spot in the sky.&lt;br&gt;&#xA;When this is done, the spacecraft's attitude is known in two of the three axes (X and Y). To fix the third axis (Z), the spacecraft rotates until its star tracker finds a star of a specified brightness. This &lt;a href=&quot;http://pds-rings.seti.org/voyager/uvs/vg1host.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;third axis fix is needed&lt;/a&gt; because you want to be able to point the camera platform in the correct direction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Voyager 1, one of the star trackers failed. The &lt;a href=&quot;https://oce.jpl.nasa.gov/mib/VOY-1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;failure analysis&lt;/a&gt; showed that components in the star tracker failed though a combination of manufacturing error and radiation. As a result, the star tracker reported an incorrect position, and the antenna wasn't pointed at Earth even though the spacecraft thought it was. Despite the link mentioned in the comments, the tracker did not lock onto the wrong star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was a pretty good solution given the limitations the Voyager team had to work with. They reduced their problem to finding a star of a specific brightness within a very narrow band of the sky, so they needed to store the brightness of just one star. The Voyager star tracking system used an analog computer, keeping this as simple as possible meant saving weight. Storing the position data of thousands of stars would have required carrying another &lt;a href=&quot;https://space.stackexchange.com/questions/2053/how-was-magnetic-tape-decay-prevented-in-voyager-1&quot;&gt;tape recorder&lt;/a&gt; plus a digital computer to analyze the data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern star trackers (e.g. &lt;a href=&quot;http://www.ballaerospace.com/page.jsp?page=104&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ball Aerospace&lt;/a&gt;, &lt;a href=&quot;http://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/Alphasat/Advanced_Star_Tracker&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;) use a catalog that contains position data for several thousand stars. They check the relative position of a number of stars to eliminate errors. Here's a screenshot of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Attitude_control#Star_tracker&quot; rel=&quot;nofollow noreferrer&quot;&gt;star tracking system on a high-altitude balloon&lt;/a&gt;:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/eT9FO.png&quot; alt=&quot;Star tracking system&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nmp.jpl.nasa.gov/st6/TECHNOLOGY/star_camera.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;From NASA's Space technology website:&lt;/a&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/7ISz3.jpg&quot; alt=&quot;star tracker cartoon&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-03-05T09:41:26.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3877" PostTypeId="2" ParentId="1551" CreationDate="2014-03-03T13:30:59.927" Score="24" Body="&lt;p&gt;Most US Launchers use a similar water suppression system for the same reasons as the Space shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some level, if you intend to launch often you do not wish your launcher to destroy the launch pad. A rocket launching usually has between 600,000 lbs thrust (Delta 4's single RS-68) to 7 million lbs thrust (Space Shuttle, or Saturn V range) and that is an amazing amount of power released in a small space and it really does not matter what you build your launch pad out of, it will take damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus minimizing the damage caused is just a good idea, across the board.  Water is cheap, easy to manage, and absorbs energy by converting steam, absorbs sound instead of the physical structure.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russians for the Soyuz boosters have an interesting approach that does not use water suppression, since Baikanour is often experiencing winter when they wish to launch, when flowing water won't work so well (and snow is not as effective as water for this task). They basically suspend the booster over the edge of a cliff, so that the exhaust goes down, and then out a vent, such that there is not much infrastructure directly below it to be damaged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Amusingly, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;noreferrer&quot;&gt;ASDS Barge&lt;/a&gt; that SpaceX is using for landing first stages has a deluge gun, to cool the deck during landing operations as well.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-15T01:35:21.653" LastActivityDate="2015-04-15T01:35:21.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3878" PostTypeId="2" ParentId="3874" CreationDate="2014-03-03T16:11:37.710" Score="3" Body="&lt;p&gt;Probably not.  You should team with someone who has previously proposed to and won Discovery proposals.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-03-03T16:11:37.710" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3879" PostTypeId="2" ParentId="203" CreationDate="2014-03-03T18:38:56.333" Score="4" Body="&lt;p&gt;The key issue is, how do I accelerate this 'thing' to orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To date, only rockets have succeeded. Rockets are pretty efficient at generating high thrust with high impulse but are complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other ideas touted in the part are thing like Orion, which used a giant vehicle, with a large plate, which on the downward facing side would detonate a nuclear device.  Then the plate would absorb the impact and connected via springs (huge as your would imagine) would transfer that momentum to the vehicle.  Clearly this is never going to happen. (Of course, the plot of the novell &lt;a href=&quot;http://en.wikipedia.org/wiki/Footfall&quot; rel=&quot;nofollow&quot;&gt;Footfall&lt;/a&gt; is that to repel an alien invasion, even Orion is acceptable). A rocket engine with no combustion, just riding the crest of nuclear explosions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other ideas are to use a high powered laser to heat a working fluid in the engine, so there is no rocket engine, combusting anything, rather the heated fluid exits the engine at high speed, transferring momentum.  A rocket engine, with no combustion really. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital beanstalks, a cable from a base in GeoSync, dropping down to the surface of the Earth is an idea, and it is still science fiction for the most part.  (Cable needs to be 22,300 miles long.  Enough to wrap several times around the Earth.  It need a counterbalance weight or cable the same length sticking out the other way).  But this would be an approach to reaching orbit without a rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Railguns, or high energy chemical guns are similar in approach, accelerate the vehicle in a gun.  Railguns work either by magnets (linear accelerator) or by vaporizing something with the current that generates the thrust.  Chemical guns light the fire behind the vehicle, but it is pushed by the edge of the explosion (Sort of confined, chemical Orion). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, no anti-gravity or neat trick to get to orbit.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-03-03T18:38:56.333" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3880" PostTypeId="1" AcceptedAnswerId="3881" CreationDate="2014-03-03T22:21:24.997" Score="40" ViewCount="7016" Body="&lt;p&gt;An Aldrin-Cycler is a vessel on an orbit on which it passes both Earth and Mars every few years without expending much fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vq6Yz.png&quot; alt=&quot;Aldrin cycler orbit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first glance this seems like a great thing for a permanent transportation system between Earth and a future Mars colony. But when you look closer at it you notice that it zooms past Earth and Mars with a quite high relative speed. Any payload which is supposed to travel with it would first need to accelerate a lot to rendezvous with the cycler and then, at the destination, decelerate a lot to rendezvous with the planet. The energy required for that would likely be higher than for a usual hohmann-transfer from one planet to the other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what practical applications would an aldrin-cycler have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(image source: &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2008/shupe_report/shupe_report.html&quot; rel=&quot;noreferrer&quot;&gt;http://ccar.colorado.edu/asen5050/projects/projects_2008/shupe_report/shupe_report.html&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-03-03T22:50:25.787" LastActivityDate="2022-06-07T01:13:16.273" Title="What uses would the Aldrin-Cycler have?" Tags="|mars|orbital-mechanics|interplanetary|low-energy-transfer|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3881" PostTypeId="2" ParentId="3880" CreationDate="2014-03-03T22:51:12.050" Score="35" Body="&lt;p&gt;Imagine a large vehicle that can recycle water and air, has shielding, and contains everything for a trip that would take months. This would be the cycler. The vehicle is heavy and is needed for a long trip; crew cannot survive in a small rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The benefit is that when you launch your crewed rocket to catch up with the cycler, you only need to accelerate the crew and food. So the rocket is very small and cost would be much smaller that accelerating the whole cycler every time.&lt;/p&gt;&#xA;" OwnerUserId="1126" LastEditorUserId="-1" LastEditDate="2014-10-25T17:36:01.260" LastActivityDate="2014-10-25T17:36:01.260" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3882" PostTypeId="2" ParentId="3880" CreationDate="2014-03-03T22:55:13.433" Score="29" Body="&lt;p&gt;One good thing about these periodic trajectories / interplanetary orbits, be it &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_cycler&quot;&gt;Mars Cyclers&lt;/a&gt;, &lt;a href=&quot;http://fdcg.eng.uci.edu/past-projects/earth-moon-cyclers/&quot;&gt;Earth-Moon Cyclers&lt;/a&gt; (gravity assist &lt;em&gt;UP/DOWN Escalator Orbits&lt;/em&gt;), or &lt;a href=&quot;http://starsdestination.blogspot.com/2012/05/resonant-cyclers.html&quot;&gt;Resonant Cyclers&lt;/a&gt; (fixed &lt;em&gt;VISIT Orbits&lt;/em&gt;) is that they can be maintained with a relatively small penalty for the total mass of the spacecraft, so they could be whole industry complexes processing raw goods for later delivery to the planets or moons they pass along their route, while using processing byproducts and remains for reaction mass, shielding, construction materials, or life support.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since mass comes at a small expense, spacecraft in circulating orbits in inner Solar system where sunlight is sufficient might as well maintain whole greenhouses and gardens for oxygen and food requirements of their inhabitants. This advantage in maintaining stable orbits of large mass bodies cheaply is also briefly described in &lt;a href=&quot;http://www.researchgate.net/profile/James_Longuski/publication/4665174_Circulating_transportation_orbits_between_earth_and_Mars/file/9c96052b470bf8d4fe.pdf&quot;&gt;Circulating transportation orbits between earth and Mars&lt;/a&gt;, Friedlander et al., SAIC / JPL (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Circulating orbits share one potential advantage for manned&#xA;  transportation between Earth and  Mars. They allow a large orbiting&#xA;  facility (herein called a &quot;CASTLE&quot;) providing all power, living and&#xA;  work space, life support, gravity environment, and solar storm shelter&#xA;  to be &quot;once-launched&quot;, thereby obviating the need to carry these&#xA;  massive elements repeatedly through large planetocentric ΔV&#xA;  maneuvers. Transportation to and from these CASTLES is carried out by&#xA;  smaller Space Taxis using hyperbolic rendezvous techniques.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Additionally, many of these trajectories also extend far beyond the visiting planets, several AU past Mars and into the Asteroid belt, where raw materials they would process could come from. Granted, these would still have to be delivered to cyclers while maintaining their momentum, but they would then have a long time for any needed trajectory corrections, and still deliver processed goods in time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, since there's small to no mass penalty assuming no loss of momentum during boarding, they could simply serve as a taxi service for interplanetary travel, providing temporary habitat with all the infrastructure required for it. Trajectories could also be maintained with large solar sails that don't use any consumables, so they could be viable means of propulsion for a very long time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Together with &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot;&gt;Interplanetary Transport Network&lt;/a&gt;, all of these could serve as a low-maintenance and &lt;em&gt;cheap&lt;/em&gt; connecting transport routes for, say, space mining industry, processing plants, space laboratories, personnel transport, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suggested further reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.popularmechanics.com/science/space/moon-mars/2076326&quot;&gt;Popular Mechanics: Buzz Aldrin's Roadmap To Mars - A PM Exclusive&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://courses.ae.utexas.edu/ase333t/past_projects/04spring/Cycler%20Website/index.html&quot;&gt;The University of Texas at Austin - Mars Cyclers&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://starsdestination.blogspot.com/2012/05/resonant-cyclers.html&quot;&gt;A Thought Experiment - Resonant Cyclers&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.damninteresting.com/the-martian-express/&quot;&gt;Damn Interesting - The Martian Express&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-04T00:45:03.140" LastActivityDate="2014-03-04T00:45:03.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3883" PostTypeId="1" CreationDate="2014-03-04T13:16:30.717" Score="7" ViewCount="682" Body="&lt;p&gt;Today while studying about nuclear reactions, I found that we can produce a large amount of energy. So, why not introduce this energy in our space exploration rockets?&#xA;It may decrease the time of exploration, or it may also increase the life of space travel as well.&lt;/p&gt;&#xA;" OwnerUserId="2795" OwnerDisplayName="Suyog" LastEditorUserId="58" LastEditDate="2014-03-04T14:46:44.693" LastActivityDate="2014-03-04T15:34:51.863" Title="Is nuclear fusion a viable means of powering spacecraft?" Tags="|propulsion|interstellar-travel|nuclear|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3884" PostTypeId="1" AcceptedAnswerId="3886" CreationDate="2014-03-04T13:35:39.177" Score="20" ViewCount="717" Body="&lt;p&gt;One way to create a heavy launch vehicle is by clustering large numbers of small engines together in a stage. An example of this approach is the SpaceX Falcon 9:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/mRCwH.jpg&quot; alt=&quot;Falcon 9 first stage engines&quot;&gt;&lt;br&gt;&#xA;Now I've heard people arguing against this approach, saying that the engines may interfere with each other and that you're better off using fewer, large engines. This argument often refers to the N-1 which used 30 engines on the first stage:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Brjft.jpg&quot; alt=&quot;N-1 first stage&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I realize that when you get more engines, you get more parts and as a result, more possible problems. But does the engine arrangement itself pose a problem when you have more than a few in close proximity? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="51" LastEditDate="2015-07-09T21:08:55.077" LastActivityDate="2015-07-09T21:08:55.077" Title="Does the proximity of engines in engine clusters cause interference?" Tags="|launch|falcon-9|engines|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3886" PostTypeId="2" ParentId="3884" CreationDate="2014-03-04T14:21:32.483" Score="10" Body="&lt;p&gt;The N-1 suffered from a lack of testing, at a test stand level, and instead had to do all up tests that exposed the failures, which their leadership could not tolerate.&lt;/p&gt;&#xA;&lt;p&gt;There are probably lots of different issues that can happen in multiple engine configurations.&lt;/p&gt;&#xA;&lt;p&gt;One upstream one that can happen is that the fuel/oxidizer flow, through the piping, which clearly gets more complex as the number increases, can generate resonance vibrations that cause fuel flow issues.&lt;/p&gt;&#xA;&lt;p&gt;Here is the SpaceX LOXtpus manifold, for just 9 engines!&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RxijW.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Imagine the plumbing for 32 engines.  These are not small volume pipes where variations in flow do not matter.&lt;/p&gt;&#xA;&lt;p&gt;Then you get exhaust impinging on each others exhaust.  I recall reading somewhere that the RS-68's under an SLS model would get too hot, from the exhaust of the SRB's or would trap some of the heat, making it an issue.  That was just a 2 SRB + 3 or 5 RS-68 model.  Again, extrapolate to 32.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-03-04T14:21:32.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3887" PostTypeId="2" ParentId="3883" CreationDate="2014-03-04T15:18:56.083" Score="5" Body="&lt;p&gt;Current experimental &lt;strong&gt;fusion reactors&lt;/strong&gt; are still decades away from generating a net energy output. The test reactor at the &lt;a href=&quot;http://en.wikipedia.org/wiki/National_Ignition_Facility&quot;&gt;National Ignition Facility&lt;/a&gt; made a crucial breakthrough last year when they &lt;a href=&quot;http://www.bbc.com/news/science-environment-24429621&quot;&gt;first managed to get more energy out of their fusion chamber than they put into it&lt;/a&gt;. But keep in mind that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The energy output was only positive when you measure the input energy as the energy of the laser beams entering the fusion chamber. But the process to create these laser-beams is only 1% effective. Also, there is no method yet to extract the energy from the fusion chamber and turn it into electricity. You can't assume that such a method would be 100% effective either. So the energy efficiency of the whole facility is still magnitudes away from being positive.&lt;/li&gt;&#xA;&lt;li&gt;The positive output was merely for 150 PICOseconds&lt;/li&gt;&#xA;&lt;li&gt;That test reactor currently has the size of a factory complex.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It will still take decades until we have a fusion facility capable of completely(!) powering itself. It will take even longer until the technology is feasible for economic use and even longer until the technology is miniaturized enough to be considered for use in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will try to update my answer when this has happened. But I am not sure I will still be alive then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is, however, another method with which we successfully managed to create a fusion reaction with positive output: The &lt;strong&gt;fusion bomb&lt;/strong&gt;. Unfortunately the only way to use their energy is in form of a single, huge explosion, larger than a common nuclear fission bomb (because a fission bomb is used to ignite the fusion reaction). Propelling a spacecraft through shockwaves produced by &lt;em&gt;nuclear bombs&lt;/em&gt; is obviously crazy. But not so crazy that nobody ever though about it. There is a theoretical concept called &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29&quot;&gt;Project Orion&lt;/a&gt; which is exactly that. In theory it doesn't even look bad. It might in fact currently be the only viable method for manned interstellar travel within our technical capabilities. But it never got above theoretical planning, because:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It would need to be huge to be efficient and thus insanely expensive.&lt;/li&gt;&#xA;&lt;li&gt;Putting nuclear weapons into orbit would be a violation of &lt;a href=&quot;http://www.atomicarchive.com/Treaties/Treaty4.shtml&quot;&gt;the Outer Space Treaty of 1967&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-03-04T15:34:51.863" LastActivityDate="2014-03-04T15:34:51.863" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3888" PostTypeId="2" ParentId="1840" CreationDate="2014-03-04T20:07:58.387" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281970%29027%3C1186:TDOAIC%3E2.0.CO;2&quot; rel=&quot;nofollow noreferrer&quot;&gt;This paper from 1970&lt;/a&gt; makes some progress to answering the question.  It gives the following graph:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/g1MWE.png&quot; alt=&quot;K values&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives the ion mobilities for different altitudes, and for both positive and negative charges.  Considering that the units match as well, I'm going to identify this as $k$ in the equation with fairly good confidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The correlation is from applying physical principles to the atmosphere, so there is no expectation that this would be correct in real life.  Actually, there are several confounding factors in real life, including a significant difference between day and night.  &lt;a href=&quot;http://www.vlf.it/parmigiani-frozen/frlight.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Attempts at a real real measured curve&lt;/a&gt; are much more messy, and even has a global maximum.  However, this global max is far above the 100 km mark, and up to that point, this exponential relationship looks like it could hold fairly well.  For a straightforward order-of-magnitude estimation (which is what this question wants), this seems perfectly sufficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, to compare values, the above graph's value for sea-level can be roughly estimated by observation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ k|_{h=0} = 1.5 - 2.0 \frac{ cm^2 }{ V s}  = 1.5 - 2.0 \times 10^{-4} \frac{ m^2 }{ V s}  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's compare to the Wikipedia article on ionocraft:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;k is the ion mobility coefficient of air, measured in dimension M−1 T2 I (Nominal value 2·10−4 m2 V−1 s−1).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;$$ k|_{h=0}   = 2.0 \times 10^{-4} \frac{ m^2 }{ V s}  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These numbers look to be in fairly good correspondence.  This gives a little more credibility to the graph's numbers.  &lt;em&gt;If&lt;/em&gt; they are correct, we're looking at a change in the value from sea-level to higher altitudes of around:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{ k|_{h=70 km} }{k|_{h=0}} \approx 2 \times 10^4 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is clear enough that the ionocraft equation would benefit from the difference in the k value.  The real question is if a detriment to the other independent variables would also exist?  Perhaps electrical arcing would happen at a lower voltage, which would further constrain the I and d tradeoffs.  That's still unclear to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also unclear if a factor of 10,000 would be sufficient to move from non-viable to viable.  Or if this would be sufficient to compensate for the cost of running wires from the ground to a high-altitude craft.  You could still go higher.  The global minimum looks closer to maybe 200 or 300 km in altitude.  By pushing the 70 km to maybe 100 or 150 km you could certainly buy yourself several more orders of magnitude in k.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-03-04T20:07:58.387" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3889" PostTypeId="1" AcceptedAnswerId="7908" CreationDate="2014-03-05T04:03:59.427" Score="13" ViewCount="1944" Body="&lt;p&gt;&lt;a href=&quot;http://cbboff.org/UCBoulderCourse/documents/LunarCyclerPaper.pdf&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; describes an Earth-moon cycler orbit.  In this concept, a 180 degree &quot;BackFlip&quot; is used to modify a more simple cycler orbit.  However, I can't figure out what cycler orbit it modifies, and correspondingly, what this thing is doing on the Earth-return leg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VXVSz.png&quot; alt=&quot;backflip&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think I understand the backflip itself, with the patched conic approximations.  The highly elliptical orbit is aimed at the moon such that the hyperbolic pass redirects the velocity to that angle above the plane.  So in terms of order, let's assume we start from &quot;Translunar Trajectory&quot; in the image:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Do a precisely timed burn from LEO to hit the moon with the right angle and extra velocity&lt;/li&gt;&#xA;&lt;li&gt;The (un-powered) pass by the moon redirects it into the 14-day backflip half-orbit&lt;/li&gt;&#xA;&lt;li&gt;The previous pass is repeated in reverse (or very close to reverse), putting it on the Earth Return Trajectory&lt;/li&gt;&#xA;&lt;li&gt;Then what?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;By the time you reach apogee again, the moon will have moved on.  And that's a really high apogee.  It seems to defeat the point.  This orbit should be repeatable indefinitely, and I don't see how it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The source of the idea is credited to &lt;a href=&quot;http://cbboff.org/UCBoulderCourse/documents/ASPapercosIOK.pdf&quot; rel=&quot;noreferrer&quot;&gt;Uphoff 1989&lt;/a&gt;.  I tried to figure it out from that paper, but it doesn't really add much information which isn't in the other paper.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="49" LastEditDate="2014-03-05T09:19:21.993" LastActivityDate="2021-10-10T02:32:40.070" Title="What does the BackFlip lunar cycler do in its pass by Earth?" Tags="|orbital-mechanics|the-moon|trajectory|gravity-assist|low-energy-transfer|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3890" PostTypeId="2" ParentId="182" CreationDate="2014-03-05T10:02:24.783" Score="15" Body="&lt;p&gt;Italian inventor and Monolite founder Enrico Dini believes it is and is together with UK based Foster + Partners working on a prototype massive &lt;a href=&quot;http://www.d-shape.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;D-Shape&lt;/a&gt; 3D printer that would use Lunar regolith as source material. They are testing 3D printing techniques using simulated regolith to which they're adding magnesium oxide. End results are fascinating:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/6IBIw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Example 3D printed structure using simulated Lunar regolith. Source: &lt;a href=&quot;https://web.archive.org/web/20140702003910/http://www.dvice.com/archives/2010/04/massive-3d-prin.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;DVICE&lt;/a&gt;, Credit: &lt;a href=&quot;http://www.fosterandpartners.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Foster + Partners&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently, &lt;a href=&quot;http://www.esa.int/Our_Activities/Technology/Building_a_lunar_base_with_3D_printing&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dini and Foster + Partners are in talks with ESA&lt;/a&gt; (European Space Agency) to create one of these D-Shape printers that would use Moon dust to fabricate an entire lunar base. So yes, seems there is a way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suggested further reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.esa.int/Our_Activities/Technology/Building_a_lunar_base_with_3D_printing&quot; rel=&quot;nofollow noreferrer&quot;&gt;European Space Agency: Building a Lunar base with 3D printing&lt;/a&gt;, Jan. 31, 2013&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://web.archive.org/web/20130601012018/http://www.dvice.com/2013-1-31/architects-unveil-plan-3d-printed-moon-base&quot; rel=&quot;nofollow noreferrer&quot;&gt;DVICE: Architects unveil plan for a 3D-printed Moon base&lt;/a&gt;, Jan. 31, 2013&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://web.archive.org/web/20140702003910/http://www.dvice.com/archives/2010/04/massive-3d-prin.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;DVICE: Massive 3D printer could someday crank out an entire Moon base&lt;/a&gt;, Apr. 20, 2010&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://i.stack.imgur.com/cDYWk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;imgur gallery: European Space Agency's lunar base concept&lt;/a&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-09-28T00:21:40.387" LastActivityDate="2015-09-28T00:21:40.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3891" PostTypeId="2" ParentId="3702" CreationDate="2014-03-05T13:11:50.653" Score="4" Body="&lt;p&gt;In general, spacecraft have two issues with temperature:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Getting rid of heat&lt;/li&gt;&#xA;&lt;li&gt;Thermal cycling&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In space, no one can hear you cry?  Nor can they see you sweat. The equipment onboard the station has serious design constraints on cooling. As you can imagine, without gravity to let hotter air (which is lighter) float higher, and cooler air to settle, internal heat control is hard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus they do a lot of work with fans, and heat sink systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting rid of the heat is the hard part, and as you can see from the size of the various radiator systems it takes a lot of work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is worth noting that the solar panels on Zarya (FGB Control Block) module on the Russian segment had to be folded up to allow room for the ETCS radiators to extend. That is a fairly large power loss for the Russian segment, almost half their power generation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have wondered, if they considered moving those panels to say the Nauka or Rassvet module later. (Of course they may not be designed for that, who knows).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermal cycling is a problem as well, since the temperature swings a lot, every 90 minutes or so in orbit. Materials issues are a concern from all the expanding/contracting. The radiators do not help much with that, since they focus on moving heat internal to the station out of the station.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-03-05T13:26:38.977" LastActivityDate="2014-03-05T13:26:38.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3892" PostTypeId="5" CreationDate="2014-03-05T15:16:44.627" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-03-05T15:16:44.627" LastActivityDate="2014-03-05T15:16:44.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3893" PostTypeId="4" CreationDate="2014-03-05T15:16:44.627" Score="0" Body="Questions on the Interplanetary Transport Network" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-03-05T15:22:09.420" LastActivityDate="2014-03-05T15:22:09.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3894" PostTypeId="1" AcceptedAnswerId="3896" CreationDate="2014-03-05T15:17:15.273" Score="8" ViewCount="479" Body="&lt;p&gt;In his excellent &lt;a href=&quot;https://space.stackexchange.com/a/2287/2699&quot;&gt;answer to the question of how we make a round trip journey to Mars&lt;/a&gt;, @MarkAdler mentions that &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerocapture&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerocapture&lt;/a&gt; would be used to get into orbit around Mars, saving fuel and therefore weight.  This sounds like a nice idea in principal.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if the spacecraft comes in too near the planet there is a risk of slowing too much and crashing into the surface.  On the flip side, coming in too far away from the planet risks not being captured and flying off into deep space.  My question is: Given a normal amount of fuel for such a mission; how narrow is this altitude window in which the spacecraft is captured but doesn't crash or get lost in space?&lt;/p&gt;&#xA;" OwnerUserId="2699" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-03-05T16:10:14.197" Title="What precision is needed for aerocapture?" Tags="|mars|orbital-mechanics|orbit|orbital-maneuver|aerobraking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3895" PostTypeId="1" AcceptedAnswerId="3964" CreationDate="2014-03-05T16:01:41.280" Score="13" ViewCount="1818" Body="&lt;p&gt;Cartographers have always been valued members of any exploration team. Even the teenage George Washington had experience with the &quot;compass and chain.&quot; Every Terra Incognito is in need of a good map.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much has been made, rightly so, of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot; rel=&quot;noreferrer&quot;&gt;Interplanetary Transport Network&lt;/a&gt;. This is usually described as a series of pathways through a stellar system that requires very little energy for a spacecraft to follow, rather like a bit of flotsam floating down a lazy river.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some linguistic problems with our definitions, however, which &lt;a href=&quot;https://space.stackexchange.com/questions/72/how-much-of-the-interplanetary-transport-network-is-currently-known&quot;&gt;this question and its answer&lt;/a&gt; illustrate. Both &quot;pathways&quot; and &quot;network&quot; seem to imply that there are fixed routes that can be drawn on a 3D map, but we're actually talking about time-dependent trajectories rather than fixed pathways, and the network is more like virtual packet-switching than your cable &amp;amp; router LAN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question: &lt;strong&gt;is it feasible to create a usable 4D &quot;map&quot; of the Interplanetary Transport Network?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &quot;usable 4D map&quot; I mean a program that a mission designer could input destination, launch window &amp;amp; maximum delta-V, and get usable flight plans back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &quot;feasible&quot; I mean, are the data sets sufficiently constrainable (given that the 'n' in the n-body problems &amp;amp; the number of trajectory legs can go large) that the software can be run on something less than a supercomputer.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-09-20T22:30:59.607" Title="Is it possible to &quot;map&quot; the Interplanetary Transport Network?" Tags="|trajectory|software|mission-design|low-energy-transfer|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3896" PostTypeId="2" ParentId="3894" CreationDate="2014-03-05T16:10:14.197" Score="6" Body="&lt;p&gt;A typical metric is that you want to exit with no more than 100 m/s of correction $\Delta V$ to get to your targeted conditions.  (That does not include the deterministic $\Delta V$ required to raise periapsis assuming a perfect exit state.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That generally requires that the flight through the atmosphere be guided or drag-modulated to account for both uncertainty in the entry flight path angle and uncertainty in the atmospheric density.  With such guidance, the entry flight path angle accuracy required isn't any tighter than what is needed for targeting a landing site, around a quarter of a degree.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-03-05T16:10:14.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3897" PostTypeId="1" CreationDate="2014-03-05T18:47:09.023" Score="16" ViewCount="4084" Body="&lt;p&gt;Long story short: I'm writing sci-fi and taking my protagonist to Europa. He's got 2 weeks to one month to get there from Earth, give or take a few days. That sounds, of course, preposterous in this day and age. However, it's the future, he's stolen an alien ship, and I need to know, using the ship's constant acceleration drive, how fast the spaceship would need to travel to get there in my literary time constraints (2 weeks-month). Any flight path between here and Jupiter is perfectly okay by me, but the ship will need to accelerate most of the way there and then decelerate before looping around Jupiter to land on Europa. Hoping I don't have to pull out the warp drive to make it happen. Any thoughts? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_travel_using_constant_acceleration&quot;&gt;http://en.wikipedia.org/wiki/Space_travel_using_constant_acceleration&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2802" LastEditorUserId="25" LastEditDate="2015-07-16T13:10:35.620" LastActivityDate="2015-07-16T13:10:35.620" Title="Space travel using constant acceleration drive: Earth to Europa" Tags="|orbital-mechanics|propulsion|trajectory|interplanetary|europa|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3898" PostTypeId="2" ParentId="3897" CreationDate="2014-03-05T20:08:17.027" Score="13" Body="&lt;p&gt;Jupiter is 778,500,000 km away from the sun, on average. Earth is 149,600,000 km. Thus, the distance to Jupiter is always between 630-930 million km. So, let's take that range, and figure out what the time would be, given 1 g of acceleration, and ignoring for the moment relativity. Let's also ignore starting/ending velocity, as I'm feeling lazy... Okay, so 1 g of acceleration, go half way, 1 g of deceleration. What does that give?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$d=\frac{1}{2}a\cdot t^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the min case:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t=2\cdot \sqrt{\frac{\frac{2}{2}630\cdot10^9}{9.8}}=253546s$, or about 71 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the max case:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t=2\cdot \sqrt{\frac{\frac{2}{2}930\cdot10^9}{9.8}}=308055$, or about 86 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, 2 weeks should be no problem, presuming you could accelerate that fast. Okay, so you want to stretch it 2 full weeks? Let's take the long path distance, and figure out what the acceleration would need to be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$a=\frac{d}{(\frac{t}{2})^2}=\frac{930\cdot10^9}{(\frac{1209600}{2})^2}=2.54 m/2$, or about half of the gravitational force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The max speed in each case:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Short- 2484750.8 m/s, or 0.83% of the speed of light&lt;/li&gt;&#xA;&lt;li&gt;Long- 3018939 m/s, or 1.01% of the speed of light&lt;/li&gt;&#xA;&lt;li&gt;2 week- 1536192 m/s, or 0.51% of the speed of light&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="2813" LastEditDate="2014-03-06T12:11:03.130" LastActivityDate="2014-03-06T12:11:03.130" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3899" PostTypeId="2" ParentId="3897" CreationDate="2014-03-05T22:08:14.743" Score="10" Body="&lt;p&gt;@PearsonArtPhoto's (excellent) answer doesn't consider &lt;em&gt;efficiency&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Direct travel – by accelerating directly toward the spot Jupiter will be at when you arrive, and then decelerating for arrival – is not the most efficient way to reach Jupiter. To date, no human-made spacecraft has used such a method. So taking the distance to Jupiter and calculating travel time based on simple &quot;point A to point B&quot; classical mechanics &lt;em&gt;may&lt;/em&gt; not be the best answer, depending on the needs of your story.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With present-day technology, our spacecraft achieve interplanetary travel using specially calculated &lt;a href=&quot;http://en.wikipedia.org/wiki/Category%3aOrbital_maneuvers&quot;&gt;orbital transfer maneuvers&lt;/a&gt;. These require &lt;em&gt;far&lt;/em&gt; lower quantities of energy, but also take far longer (several years would not be uncommon for a voyage to Jupiter). These methods work by using a transfer orbit which intersects both Earth's orbit and Jupiter's orbit. A small orbital change is made at the beginning of the journey to move from Earth's orbit to the transfer orbit, and then (perhaps years later), at the point where the transfer orbit intersects Jupiter's orbit, another small change is made to join Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Physically, there is absolutely nothing wrong with your craft going directly there in the allotted 2-week time span (per the other answers) using a direct thrust method. But I wanted to point out that &lt;strong&gt;doing so in your story will imply, to a knowledgeable reader, that your characters/technology possess effectively &quot;unrestricted&quot; amounts of energy, and have reached a point where they simply don't care about energy efficiency the way modern humans do&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know anything about your story so I have no idea if this is a problem at all, but it may be something to consider. You did mention this was a stolen &lt;em&gt;alien&lt;/em&gt; craft, so if the craft happened to have a &quot;full tank of gas&quot; when it was stolen, and sufficiently advanced alien propulsion/energy technology, then there should be no problem.&lt;/p&gt;&#xA;" OwnerUserId="2804" LastEditorUserId="2804" LastEditDate="2014-03-06T05:35:24.407" LastActivityDate="2014-03-06T05:35:24.407" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3902" PostTypeId="1" AcceptedAnswerId="3906" CreationDate="2014-03-06T09:52:09.347" Score="34" ViewCount="4542" Body="&lt;p&gt;NASA and others seem careful to talk about microgravity instead of zero gravity. Why is for example the ISS not in zero gravity? Is it because of movements onboard? Because of variations in center of mass caused by the inhomogeneity of Earth and the direction to the Moon? Because of the eccentricity of the orbit? Or is microgravity a more fundamental intrinsic property of orbiting?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen the term &quot;picogravity&quot;. Are there some established limits for micro, pico and zero gravity? Are there terms for the surface gravity of the large moons (say 0.10 to 0.17 g) and for larger asteroids (such as 0.001 g to 0.028g)?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2019-02-23T08:26:17.730" Title="What causes microgravity (i.e. non-zero gravity) in orbit?" Tags="|microgravity|gravity|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3904" PostTypeId="1" AcceptedAnswerId="3905" CreationDate="2014-03-06T06:02:22.043" Score="8" ViewCount="864" Body="&lt;p&gt;So, a gravity assist in very simple terms is when an object uses a planet's gravity and rotating momentum to increase the objects speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is true, why doesn't the ISS get further and further away from us as it gains angular momentum from Earth? As I understand every once in a while, the ISS has to do a burn to get back into stable orbit around Earth because of the very thin atmospheric drag on the ISS. That would mean the ISS isn't increasing it's speed, it's slowing down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a separate but related question, if gravity assists do indeed work that way, isn't our moon stealing angular momentum from Earth every second it's in orbit around us? As I understand, the moon is gradually being tugged away from Earth, not by stealing Earth's angular momentum, but because the tidal effects of the moon on Earth bump the moon into a higher orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please, no math, keep it to worded answers. I just want to understand the concept behind the explanations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="2816" OwnerDisplayName="Err" LastEditorUserId="2816" LastEditDate="2014-03-16T09:57:00.590" LastActivityDate="2014-03-16T09:57:00.590" Title="Gravity Assist - Orbiting Satellites &amp; Speed Increases" Tags="|the-moon|gravity|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3905" PostTypeId="2" ParentId="3904" CreationDate="2014-03-06T11:04:31.017" Score="7" Body="&lt;p&gt;Gravity assist doesn't use relevant amounts of &lt;a href=&quot;http://en.wikipedia.org/wiki/Angular_momentum&quot;&gt;angular momentum&lt;/a&gt; of Earth's rotation, but almost exclusively Earth's &lt;a href=&quot;http://en.wikipedia.org/wiki/Momentum&quot;&gt;orbital momentum&lt;/a&gt;, which may be interpreted as angular momentum of the Earth-Sun system. Hence the ISS doesn't convert relevant amounts of angular momentum from Earth's rotation to momentum of ISS (may be interpreted as angular momentum of the Earth-ISS system). There exists a couple of &lt;a href=&quot;http://virgo.lal.in2p3.fr/NPAC/relativite_fichiers/anderson_2.pdf&quot;&gt;minor rotation-dependent effects&lt;/a&gt;, dependent on Earth's mass distribution, magnetic fields, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity assist can accelerate, or deaccelerate relative to Sun's (almost) &lt;a href=&quot;http://en.wikipedia.org/wiki/Inertial_frame_of_reference&quot;&gt;inertial frame&lt;/a&gt;, depending on the velocity vectors of the planet and the probe/asteroid. Gravity assist applies mainly to &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist&quot;&gt;flyby&lt;/a&gt;, or &lt;a href=&quot;http://en.wikipedia.org/wiki/Oberth_effect&quot;&gt;momentum-based propulsion&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS orbits Earth (no flyby), hence gravity assist isn't applicable, with the exception of propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since gravity assist doesn't work the way you presumed, it doesn't apply to the Moon.&#xA;Nevertheless our moon is &quot;stealing&quot; angular momentum from Earth by &lt;a href=&quot;http://en.wikipedia.org/wiki/Tidal_acceleration&quot;&gt;tidal acceleration&lt;/a&gt;. &lt;a href=&quot;http://en.wikipedia.org/wiki/%CE%94T&quot;&gt;Earth's rotation is slowing down&lt;/a&gt; by this effect; a day gets longer about 2.3 milliseconds per century by tidal friction due to Moon and Sun (but there are other effects, as well, changing the length of a day).&lt;/p&gt;&#xA;" OwnerUserId="1454" OwnerDisplayName="Gerald" LastEditorUserId="1142" LastEditDate="2014-03-06T14:46:02.263" LastActivityDate="2014-03-06T14:46:02.263" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3906" PostTypeId="2" ParentId="3902" CreationDate="2014-03-06T13:38:57.923" Score="56" Body="&lt;p&gt;The reason the Space Station is called a micro-g environment rather than a zero g environment is because the Space Station is rotating, because it's in low Earth orbit, and because it's big (for a spacecraft).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Station nominally rotates at the orbital rate so as to keep the nadir-pointing windows pointing downward. This alone means an accelerometer attached to the Space Station at a distance of 7.7 meters from the ISS center of mass will register an acceleration of one micro-g.  Every additional 7.7 meters from the ISS center of mass adds an additional micro-g to this sensed acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On top of that, the Space Station has a rather high drag coefficient compared to other spacecraft thanks to those huge solar arrays. That drag becomes measurable (i.e., micro-g level), particularly during periods of high solar activity (e.g., now, and presumably 11 years from now) and/or when the Space Station altitude is low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One last factor is gravity gradient. The ISS is big. The variation in gravitational acceleration over a span of 100 meters is as much as 14 micro-g at ISS altitude. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-06T14:30:06.283" LastActivityDate="2014-03-06T14:30:06.283" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3908" PostTypeId="2" ParentId="3904" CreationDate="2014-03-06T16:49:47.740" Score="4" Body="&lt;p&gt;There are two types of gravity-assists. When an object passes behind another, it accelerates. When it passes in front of another, it decelerates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When looking from the sun watching the movement of the ISS while it orbits the earth, you would notice that its relative speed to yourself doesn't appear to be constant. It seems to change based on its position on its orbit around earth. It would appear to move faster while traveling in the direction the earth is heading and slower while traveling in the oppsite direction. You could say that a satellite orbiting a planet gets &lt;em&gt;two&lt;/em&gt; gravity-assists per orbit, one accelerating and one decelerating. In the end these two assists cancel each other out.&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2014-03-06T16:49:47.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3909" PostTypeId="1" CreationDate="2014-03-06T17:09:44.553" Score="3" ViewCount="54" Body="&lt;p&gt;The ICE/ISEE-3 spacecraft is in space since 1978. It'll return near the Earth in august 2014. can we contact it to put it in L1 Lagrange point ? Is it too old ? The decision is in the hands of Nasa.&lt;/p&gt;&#xA;" OwnerUserId="2817" LastActivityDate="2014-03-06T17:09:44.553" Title="What's about Nasa agreement for waking up ICE/ISEE-3?" Tags="|nasa|probe|" AnswerCount="0" CommentCount="2" ClosedDate="2014-03-06T21:19:26.293" ContentLicense="CC BY-SA 3.0" />
  <row Id="3910" PostTypeId="2" ParentId="3902" CreationDate="2014-03-06T20:24:15.413" Score="0" Body="&lt;p&gt;Gravity is the force of attraction between objects. When you are in orbit, there is still that force of attraction (gravity) pulling you and the earth together.  The reason you don't crash into the earth is not because you have &quot;zero gravity&quot;, but because you are moving.  In fact, you are falling.  Gravity is pulling you straight down towards the center of the earth.  The thing is, you're moving so fast that you are always missing the earth.  If it weren't for the earth's gravitational pull, you would continue off into space in a straight line.  Gravity keeps you in orbit, thus you are not in &quot;zero&quot; gravity. And if it weren't for your forward motion, you'd fall directly towards the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The effect of gravity is always there.  Think of it this way - the moon feels the earth's gravity, right?  That's why it orbits the earth.  It, too, is just continually missing the earth because of its motion. And the earth feels the moon's gravity.  That is why we have tides.  The water on the earth, and in fact everything on earth, feels the pull of the moon's gravity.  It's so small (microgravity) that you don't notice it on your body, but it's strong enough to cause the tides.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And similarly, when you are in orbit in the space station, everything in your body (your blood, skin, hair, etc.) are still pulled towards the earth - sort of a tidal force affecting your bodily fluids.  Thus you are not in &quot;zero&quot; gravity - your blood is not truly floating wherever it wants to go - it is constantly being pulled towards the earth.  So being oriented with your head nearer the earth than your toes is like standing on your head on the surface of the earth, it's just that the forces are much weaker.  In true zero gravity - out in deep space so far away from any galaxies that gravitational pull is at a minimum (it's never truly zero), your blood would act differently than it does in low earth orbit where it feels the earth's gravitational pull.&lt;/p&gt;&#xA;" OwnerUserId="2821" LastActivityDate="2014-03-06T20:24:15.413" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3911" PostTypeId="1" AcceptedAnswerId="3916" CreationDate="2014-03-06T23:29:02.957" Score="9" ViewCount="1638" Body="&lt;p&gt;&lt;a href=&quot;http://www.spacedaily.com/reports/The_COTS_Enigma_999.html&quot;&gt;An article from 2007&lt;/a&gt; made the following statement:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX is in many ways the exact opposite of Kistler. It's run by Internet wunderkind Elon Musk, does almost all of its work in-house with a staff of young engineers not tainted by experience. It envisions a simple float-back booster (similar to Von Braun's classic 1952 Ferry Rocket) not burdened with weighty flyback hardware.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If my understanding is correct, &quot;weighty flyback hardware&quot; is exactly what SpaceX is working on.  So it seems that the approach changed since the publishing of that article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More specifically, what &lt;em&gt;had&lt;/em&gt; the company been planning before they went with the flyback route?  Parachutes?  Airbags?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-03-07T03:18:53.283" Title="What was SpaceX's original vision for booster reusability?" Tags="|spacex|reuse|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3912" PostTypeId="2" ParentId="3911" CreationDate="2014-03-07T00:07:53.823" Score="7" Body="&lt;p&gt;In that context, the phrase &lt;em&gt;weighty flyback hardware&lt;/em&gt; is implying a Space Shuttle or the RocketPlane mentioned in the article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That sort of wehicle will be launched to a complete orbit, do its job and return to earth. SpaceX is not doing that. Falcon 9 rocket only launches a small portion of the rocket to the orbit, compared to previously mentioned vehicles( at least in size and somewhat in mass ). The Falcon first stage doesn't reach orbital velocity an as such doesn't fit in the same category as the in vehicles I mentioned in the first paragraph. It also doesn't fit by category as it is a basically just a booster, while the other two are the operating vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course you can also call the Falcon first stage a &lt;em&gt;weighty flyback hardware&lt;/em&gt;, but I doubt the author was implying that.&lt;/p&gt;&#xA;" OwnerUserId="1126" LastActivityDate="2014-03-07T00:07:53.823" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3913" PostTypeId="1" CreationDate="2014-03-07T00:14:15.330" Score="5" ViewCount="135" Body="&lt;p&gt;Could using the gravity of another celestial body allow for something like a &lt;a href=&quot;http://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot; rel=&quot;nofollow&quot;&gt;bi-elliptic transfer&lt;/a&gt;&#xA;&lt;br&gt;&#xA;that uses less delta-v than the theoretical &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Bi-elliptic_transfer#Example&quot; rel=&quot;nofollow&quot;&gt;infinite apogee&lt;/a&gt;&quot; delta-v requirement?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If yes, has such a transfer ever actually been performed?&lt;/p&gt;&#xA;" OwnerDisplayName="user2822" LastEditorUserId="49" LastEditDate="2014-03-08T09:37:02.063" LastActivityDate="2014-03-08T09:37:02.063" Title="Could another celestial body make a bi-elliptic transfer significantly more efficient?" Tags="|orbital-mechanics|orbital-maneuver|delta-v|gravity-assist|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3914" PostTypeId="2" ParentId="3911" CreationDate="2014-03-07T00:39:35.187" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;An article from 2007 …&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That was a ridiculously biased and rather ignorant article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SpaceX reusability goal has always been to bring the launch vehicles back to Earth by relighting after separation and then using controlled descent to bring them back to Earth. Whether that qualifies as &quot;not burdened with weighty flyback hardware&quot; -- that's up to you. SpaceX has recently come close to attaining that first goal of bringing the first stage back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether SpaceX does achieve it's goal of reusability is a bit secondary in this regard. They have achieved NASA's primary goal with regard to the COTS program. Contrary to the predictions of that article, both SpaceX and Orbital Sciences have been successful in launching vehicles that berthed with the ISS. Even more importantly, these two companies are placing significant downward pressures on that other Borg-like, cost-plus-fixed-fee launch contractor to reduce its costs. If those old-style companies want to play the same game as SpaceX and Orbital, great. The more the merrier. I don't care if launch providers use big dumb boosters that aren't reusable boosters so long as they are &lt;em&gt;cheap&lt;/em&gt; big dumb boosters that aren't reusable.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-07T00:46:49.433" LastActivityDate="2014-03-07T00:46:49.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3915" PostTypeId="2" ParentId="3913" CreationDate="2014-03-07T03:02:52.393" Score="4" Body="&lt;p&gt;Do you mean &lt;a href=&quot;http://en.wikipedia.org/wiki/PAS-22&quot; rel=&quot;nofollow&quot;&gt;like this&lt;/a&gt;?  In that case, a geosynchronous satellite stranded in a 51° inclination orbit used a very high apogee transfer with two lunar flybys at the high apogee to remove 40° of that inclination.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-03-07T04:00:23.187" LastActivityDate="2014-03-07T04:00:23.187" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3916" PostTypeId="2" ParentId="3911" CreationDate="2014-03-07T03:18:53.283" Score="10" Body="&lt;p&gt;SpaceX's initial plan for re usability was that it would be a lot easier than it turned out to be.  They initially had parachute equipment on the first couple of Falcon 9 launches, and they found that the first stage never survived the reentry to the atmosphere. There is a shot I remember from the first or second Falcon 9 launch of a recovery team in SCUBA gear, holding a structure built to cradle and float the first stage in the water. But I cannot find the image, darn you Google!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After which they gave up on that approach and morphed into the current one, which is to use three engines above the upper atmosphere to slow down enough to survive reentry based on the data they learned from the first few flights. Then a single engine for the final approach to the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interesting question is how much the landing legs, which appear to have aero surfaces on the bottom, will contribute to 'flying' the stage to a landing.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-03-07T03:18:53.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3917" PostTypeId="1" AcceptedAnswerId="3920" CreationDate="2014-03-07T14:36:38.833" Score="9" ViewCount="742" Body="&lt;p&gt;I've read &lt;a href=&quot;http://www.spaceflightnow.com/news/n1403/05spacexula/&quot; rel=&quot;noreferrer&quot;&gt;an interesting article&lt;/a&gt; that mentions Michael Gass, president and CEO of United Launch Alliance (ULA), alluding to ULA innovations during the latest Senate Appropriations Subcommittee hearing at Capitol Hill this Wednesday, March 5, 2014:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Not surprisingly, Michael Gass, president and CEO of United Launch&#xA;  Alliance, the company that up to now has been the sole provider of&#xA;  military Evolved Expendable Launch Vehicle -- EELV -- rockets for&#xA;  national security payloads, disagreed, &lt;strong&gt;saying his company constantly&#xA;  innovates&lt;/strong&gt; and that its Atlas 5 and Delta 4 rockets were the most&#xA;  reliable in the world.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The whole context is described in the linked to article, so I won't repeat what the purpose of that hearing was. But this statement does present an opportunity to discuss this here without being seen as argumentative. If Michael Gass believes that statement is good enough for the U.S. Senate hearing, and it shouldn't present an opportunity for Elon Musk to contest it in his comeback, surely there's ample proof supporting this assertion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, for the better of me, I can't think of a single substantial innovation on the part of ULA that isn't proprietary to Boeing and Lockheed Martin from the times before the merger / partnership. Unless Gass was alluding to some other than technological innovations? Thus my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What innovations was Michael Gass alluding to during his latest address to the U.S. Senate Appropriations Subcommittee? Is there, from technological standpoint, any evidence that ULA is an innovative player in the space launch business?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="25" LastEditDate="2014-03-07T15:21:19.753" LastActivityDate="2014-03-07T15:21:19.753" Title="What ULA innovations was Michael Gass alluding to during latest U.S. Senate address?" Tags="|rockets|cost|ula|eelv|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3918" PostTypeId="5" CreationDate="2014-03-07T14:50:26.740" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-07T14:50:26.740" LastActivityDate="2014-03-07T14:50:26.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3919" PostTypeId="4" CreationDate="2014-03-07T14:50:26.740" Score="0" Body="Questions about Evolved Expendable Launch Vehicle (EELV), a US Air Force expendable launch system program assuring access to space for national security, military and other US government payloads, using Delta IV and Atlas V launch systems." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-07T14:50:26.740" LastActivityDate="2014-03-07T14:50:26.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3920" PostTypeId="2" ParentId="3917" CreationDate="2014-03-07T14:58:47.417" Score="8" Body="&lt;p&gt;One possibility is actually building two disparate launch vehicles in the same shared facility.  The Delta 4 and Atlas 5 cores are pretty distinctive and different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond that, he is blowing smoke. Atlas 5/Delta 4 are basically identical to before the ULA merger.  They have managed to get launch cycle times down somewhat. but not enough to be considered an innovation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch success is not innovation. That is the normal expectation. If you fail to launch successfully, you have failed.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-03-07T14:58:47.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3921" PostTypeId="2" ParentId="3917" CreationDate="2014-03-07T15:12:41.110" Score="5" Body="&lt;p&gt;Here's a list from Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Atlas_V&quot;&gt;&lt;strong&gt;Atlas V&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;In 2006, ULA offered an Atlas V HLV (Heavy Lift Vehicle) option that would use three Common Core Booster (CCB) stages strapped together to lift a 29,400 kg (64,816 lb) payload to low Earth orbit&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceflight101.com/atlas-v-541.html&quot;&gt;541 configuration&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Some consideration of using the Delta tooling system to allow for a different engine.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Delta_IV&quot;&gt;&lt;strong&gt;Delta IV&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*There's been a few new configurations flown (Medium+(5,4), Medium+(5,2)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, not much that's public, perhaps some stuff that hasn't hit the floor yet.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-03-07T15:12:41.110" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3922" PostTypeId="1" AcceptedAnswerId="4919" CreationDate="2014-03-07T20:12:59.993" Score="10" ViewCount="8614" Body="&lt;p&gt;One of the issues the Air Force has with using SpaceX as a launch provider is that their payloads are designed for vertical integration.  That is, the booster stands on the pad upright, and the payload is attached atop it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 and Falcon Heavy are integrated in the hangar, horizontally, that is, lying on its side. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the consequences to changing this approach?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-08-07T23:15:50.873" LastActivityDate="2019-01-16T11:08:18.637" Title="What are the consequences of Vertical vs Horizontal integration for SpaceX?" Tags="|launch|spacex|falcon-9|falcon-heavy|ula|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3923" PostTypeId="2" ParentId="3922" CreationDate="2014-03-07T20:23:09.113" Score="3" Body="&lt;p&gt;One thing I thought of would be the need for an enclosure at the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle used the RSS (Rotating Servicing System) that was a monstrosity of a thing that covered the entire cargo bay, while on the pad, provided environmental control.  As you can imagine, not a cheap structure, that had to survive being fairly close to 7 million lbs of thrust as the shuttle took off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ariane 5 and Delta have a structure like the upside down letter U that rolls over top the has doors that close to enclose the vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus building one of these (which actually SpaceX destroyed at LC40 at CCAFS when they took it over) would be a cost.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2019-01-16T11:08:18.637" LastActivityDate="2019-01-16T11:08:18.637" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="3924" PostTypeId="1" AcceptedAnswerId="8893" CreationDate="2014-03-07T20:40:00.200" Score="10" ViewCount="2127" Body="&lt;p&gt;With the CRS-3 (SpaceX-3) launch of a Falcon 9 v1.1 with a Dragon capsule to the ISS the F-9 will launch with landing legs for the first time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk &lt;a href=&quot;https://twitter.com/elonmusk/status/438378373027360768&quot; rel=&quot;noreferrer&quot;&gt;Tweeted&lt;/a&gt; an image of the legs mounted on the first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/D1Dms.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see a bit better detail from an earlier tweet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mmtMg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottoms look sculpted, and the question is, do they have an aerodynamic function for controlling the flight?  We already know that like a figure skater, they are expected to help control the amount of spin so the ACS system can counteract the roll.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will they allow the stage control to 'fly' to a more accurate landing?  Do they have sufficient control in the legs to do that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been noted that there are actually two versions of the legs built.  The original image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/A2WZl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then Steve Jurvetson posted one from the SpaceX lobby.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/qPirl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are supposed to be the model used since the CRS-3 flight. Note the arrow pointing out the major obvious difference.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-12-22T06:06:56.503" LastActivityDate="2016-01-25T18:33:01.540" Title="Are the Falcon 9 landing legs aerodynamic surfaces as well?" Tags="|spacex|reuse|falcon-9|booster-flyback|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3925" PostTypeId="2" ParentId="1300" CreationDate="2014-03-08T00:22:37.753" Score="4" Body="&lt;p&gt;It depends on your definition of artificial intelligence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is a Kalman filter AI? By most definitions, no. Yet a Kalman filter's job is to generate an ever improving estimate of state. It &quot;learns&quot; by comparing what it think it knows (imperfectly) versus the imperfect results from sensors. Some Kalman filters are extremely sophisticated. They handle sensors that report at different rates, they estimate biases in the sensors as well as where the vehicle is, and they help detect sensor failures. The mathematical hearts of a Kalman filter and a Bayesian belief network (which is AI) are one and the same. The key reason a Kalman filter isn't deemed to be AI is that it wasn't developed by AI researchers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is a star tracker AI? A modern star tracker looks for multiple bright stars at once and matches the detected stars against a database of stars. It is doing simple pattern matching. There's not much room in a star tracker's microprocessor for any advanced AI techniques. There's not much room for what most would call AI, period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the Autonomous Landing Hazard Avoidance Technology (ALHAT) being developed by NASA AI? This is a lot closer to what many would call AI. However, is edge detection AI? Is detecting circles and ellipses (aka craters) AI? Does it matter whether the algorithm was developed by an AI practitioner or a signal processing expert? Does matching the detected craters against a database AI? Does it matter that the point of all this is to feed the information to a Kalman filter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's the long to midrange part of ALHAT. As the vehicle get's close to the landing site, the technology has to switch from &quot;where am I&quot; mode to &quot;where can I land&quot; mode. It must autonomously find and guide the vehicle to a good landing site. A good landing site is relatively flat and free of obstructions. Does it matter if the technique used to detect a good landing site uses what most would call AI to categorize obstructions as rocks, versus purely mathematical techniques such as wavelet or Fourier transformations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It shouldn't matter whether the techniques came out of the signal processing community or the AI community, but it does. (Better: AI communities; there's a noted lack of communications amongst different branches of AI, and even a lot of bickering of what AI is.) That I could rewrite a Kalman filter as a Bayesian network and suddenly it's AI? That doesn't make sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another issue that gets in the way of &quot;true&quot; AI from being used in space operations is the languages preferred by AI researchers. LISP won't fly, nor will Prolog (Buran excluded, and it didn't fly). The last thing one wants to happen is to have the computer go into garbage collection mode just as the vehicle is about to land. The algorithms have to be translated to the stodgy languages used by flight software developers, and then toned down further because dynamic memory allocation is taboo in flight software.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One last thing that gets in the way of employing AI in space exploration: A state of the art flight computer is a state of the practice desktop computer from the previous millennium. We won't be able to say &lt;em&gt;Siri, open the pod bay doors&lt;/em&gt; on a flight computer anytime soon. Your smart phone has a much faster CPU and a whole lot more memory than does the flight computer on a spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-08T00:22:37.753" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3926" PostTypeId="5" CreationDate="2014-03-08T04:32:40.657" Score="0" Body="&lt;p&gt;An ephemeris provides a means for quickly estimating the positions, and sometimes orientations, of solar system bodies as a function of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ephemerides used to be in the form of printed lists of positions of heavenly bodies tabulated at fixed intervals of time. High accuracy ephemerides are now more likely to be in the form of tables of Chebyshev polynomial coefficients stored in a binary data file. Software is needed to find the right set of coefficients based on the time interval and body of interest, and then to apply those to obtain the position and orientation of the body at the specific point of time. Additional software may be needed to transform those data into the desired form (e.g., relative position, right ascension and declination, azimuth and elevation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The producers of the highest precision ephemerides include the Jet Propulsion Laboratory, which produces the JPL Development Ephemeris and Small Body Ephemerides series of ephemeris models; the Institute of Applied Astronomy at the Russian Academy of Sciences, who produce the Ephemerides of the Planets and Moon (EPM) series of ephemerides; and the IMCCE at the Observatory of Paris, which produces the INPOP series of ephemerides.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-08T07:59:59.567" LastActivityDate="2014-03-08T07:59:59.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3927" PostTypeId="4" CreationDate="2014-03-08T04:32:40.657" Score="0" Body="An ephemeris provides a means for quickly estimating the positions, and sometimes orientations, of solar system bodies as a function of time." OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-08T07:59:14.627" LastActivityDate="2014-03-08T07:59:14.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3928" PostTypeId="5" CreationDate="2014-03-08T09:42:42.820" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-08T09:42:42.820" LastActivityDate="2014-03-08T09:42:42.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3929" PostTypeId="4" CreationDate="2014-03-08T09:42:42.820" Score="0" Body="Questions about Heavy Lift Launch Vehicles (HLLV) that are capable of lifting payloads of between 20 to 50 metric tons to Low-Earth Orbit (LEO)." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-08T10:24:14.233" LastActivityDate="2014-03-08T10:24:14.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3930" PostTypeId="2" ParentId="39" CreationDate="2014-03-08T13:15:07.593" Score="8" Body="&lt;p&gt;In addition to NASA's in-house SLS (see aramis's answer), NASA is sinking a good deal of cash into its Commercial Crew Development (CCDev) program, \$1.5 billion between 2010 and 2014.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is CCDev?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CCDev is essentially &quot;son of COTS&quot; (Commercial Orbital Transportation Services). COTS was a remarkably successful program. For a mere \$800 million, NASA has two new vehicles at its disposal for delivering cargo to the International Space Station. The goal of CCDev is to continue the spirit of the COTS program, but this time with a goal of putting humans into space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first phase of the project, CCDev1, involved \$49.8 million spread amongst five companies: Blue Origin, Boeing, Paragon, Sierra Nevada, and ULA. The second phase, CCDev2, involved \$315.5 million spread amongst four companies, three of which were in CCDev1. Those three were Blue Origin, Boeing, Paragon, and Sierra Nevada. SpaceX was also selected for CCDev2. The third phase, Commercial Crew integrated Capability (CCiCap) involved \$1167.5 million spread amongst only three companies: Boeing, Sierra Nevada, and SpaceX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What's next?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final part, Commercial Crew Transportation Capability (CCtCap) is expected to be even bigger. The FY14 budget for this final part is \$696 million (cut from a request of \$821 million). The first phase of this final part, CPC1, involved \$29.6 million, spread amongst the CCiCap contractors. How many of those three finalists will receive CCtCap is undetermined.  (Note: Any company could submit a CCtCap proposal. However, the three companies that had collectively received over a billion have a huge leg up on any newcomers.) Rumor has it that CCtCap will involve two companies to foster competition, but budget concerns might make it one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Who's building what?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boeing is working with Bigelow Aerospace on a new capsule, the &lt;em&gt;CST-100&lt;/em&gt;. The Boeing capsule is designed to be compatible with the Atlas V, Delta IV, and Falcon 9 launch vehicles. The initial work is targeting launch with the Atlas V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sierra Nevada is working on the &lt;em&gt;Dream Chaser&lt;/em&gt; that Sierra Nevada acquired with it's purchase of SpaceDev. The &lt;em&gt;Dream Chaser&lt;/em&gt; is the only lifting body left in the CCDev competition. Plans are to have it be launched on an Atlas V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has been working on outfitting its &lt;em&gt;Dragon&lt;/em&gt; capsule with life support and making its &lt;em&gt;Falcon 9&lt;/em&gt; launch vehicle human rated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;Addendum&lt;/strong&gt; (to address comments)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The end goal of CCDev has always been an end-to-end Crew Transportation System. The initial work focused on various parts of that ultimate goal, with most of the funding going toward the vehicles that will dock with the ISS and return to Earth. However, a small portion of the very first chunk CCDev money did go to United Launch Alliance to work toward making their launch vehicles human rated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RFP for the final phase of the CCDev program, CCtCap, was released in its final form in November 2013 with a submission deadline of January 22, 2014. All proposals had to be for an end-to-end Crew Transportation System with a manned demo mission in late 2017 (budget pending). The capsule or lifting body is but a part of that end-to-end system. The launch vehicle is another rather important part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goals of CCDev and SLS are largely orthogonal to one another. The primary goal of CCDev is to restore the ability of the US to send humans to the International Space Station (ISS) &quot;and other low Earth orbit destinations&quot;. The primary goal of the SLS is to restore the ability of the US to send humans beyond low Earth orbit and eventually, outside of Earth orbit. CCDev is not intended to send humans beyond LEO; SLS is not intended to go to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch vehicle proposed by a CCtCap bidder will not be the Space Launch System (SLS) vehicle. There is a backup plan to use SLS for human flights to the ISS, but only if CCDev fails.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;General discussion on CCDev:&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Commercial_Crew_Development&quot;&gt;http://en.wikipedia.org/wiki/Commercial_Crew_Development&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.thespacereview.com/article/2370/1&quot;&gt;http://www.thespacereview.com/article/2370/1&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.thespacereview.com/article/2406/1&quot;&gt;http://www.thespacereview.com/article/2406/1&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.thespacereview.com/article/2443/1&quot;&gt;http://www.thespacereview.com/article/2443/1&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vehicles:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boeing's CST-100: &lt;a href=&quot;http://en.wikipedia.org/wiki/CST-100&quot;&gt;http://en.wikipedia.org/wiki/CST-100&lt;/a&gt;&lt;br&gt;&#xA;Sierra Nevada's Dream Chaser: &lt;a href=&quot;http://en.wikipedia.org/wiki/Dream_Chaser_(spacecraft)&quot;&gt;http://en.wikipedia.org/wiki/Dream_Chaser_(spacecraft)&lt;/a&gt;&lt;br&gt;&#xA;Human-rated Atlas V: &lt;a href=&quot;http://en.wikipedia.org/wiki/Atlas_V#Future_developments&quot;&gt;http://en.wikipedia.org/wiki/Atlas_V#Future_developments&lt;/a&gt;&lt;br&gt;&#xA;SpaceX's Dragon and Falcon 9: &lt;a href=&quot;http://en.wikipedia.org/wiki/Dragon_(spacecraft)&quot;&gt;http://en.wikipedia.org/wiki/Dragon_(spacecraft)&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Falcon_9&quot;&gt;http://en.wikipedia.org/wiki/Falcon_9&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CCtCap:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GSA solicitation page: &lt;a href=&quot;https://www.fbo.gov/?s=opportunity&amp;amp;mode=form&amp;amp;id=e016cb01d032ec7468ca85035072a43c&amp;amp;tab=core&amp;amp;_cview=0&quot;&gt;https://www.fbo.gov/?s=opportunity&amp;amp;mode=form&amp;amp;id=e016cb01d032ec7468ca85035072a43c&amp;amp;tab=core&amp;amp;_cview=0&lt;/a&gt;&#xA;NAIS solicitation page: &lt;a href=&quot;https://prod.nais.nasa.gov/cgibin/eps/sol.cgi?acqid=158768&quot;&gt;https://prod.nais.nasa.gov/cgibin/eps/sol.cgi?acqid=158768&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS/MPCV:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS/MPCV as a backup to CCDev: &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/Dumbacher_MPCVSLS_508.pdf&quot;&gt;http://www.nasa.gov/sites/default/files/files/Dumbacher_MPCVSLS_508.pdf&lt;/a&gt;, slide #10.&lt;br&gt;&#xA;SLS as a backup to CCDev: &lt;a href=&quot;http://www.nasa.gov/pdf/510449main_SLS_MPCV_90-day_Report.pdf&quot;&gt;http://www.nasa.gov/pdf/510449main_SLS_MPCV_90-day_Report.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From page 7 of the latter,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The vehicle must be capable of serving as a backup system for supplying and supporting cargo and crew delivery requirements for the International Space Station (ISS) in the event such requirements are not met by available commercial or partner-supplied vehicles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-10T14:31:23.580" LastActivityDate="2014-03-10T14:31:23.580" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3931" PostTypeId="2" ParentId="3799" CreationDate="2014-03-08T15:17:22.740" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Assuming that mining companies such as Planetary Resources succeed and other companies force competition, how important will it be for methods of transporting materials to Earth to become cheap?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Very important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider the claim by Deep Space Industries (DSI) that asteroid 2012 DA14 is worth \$195 billion. (&lt;a href=&quot;http://www.space.com/19758-asteroid-worth-billions-2012-da14-flyby.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.space.com/19758-asteroid-worth-billions-2012-da14-flyby.html&lt;/a&gt;) That is nonsense. It is currently worth \$0, and that will remain its value for a long time to come.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By way of analogy, there is a readily accessible gold deposit right here on Earth that is worth \$1 to \$100 trillion (calculated by multiplying the size of the deposit by the price per ounce). Yet nobody goes after that deposit. Why? Because that reservoir of gold is the world's oceans. The cost of extracting gold dissolved in ocean water vastly overwhelms the value of the extracted gold. The value of that vast deposit is \$0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Extracting minerals from asteroids might be valuable in the distant future, but not for the foreseeable future. The near-term goals of companies such as DSI and Planetary Resources are much more modest. The immediate goals are to go after volatiles. Beyond low Earth orbit, a pound of water (or a pound of methane), is currently far more valuable than is a pound of gold on the surface of the Earth. Extracting water or fuel from an asteroid and using that to refuel vehicles in geosynchronous orbit or provide water to the ISS could make for a going venture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last thing you would want to do is to waste those hard-extracted resources to bring them back toward Earth on a fast trajectory.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-08T15:17:22.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3932" PostTypeId="2" ParentId="3924" CreationDate="2014-03-09T00:31:29.310" Score="6" Body="&lt;p&gt;They are not used as aerodynamic surfaces during launch, but they are used to help balance the spacecraft and provide some control during the landing sequence. As to what level they will be used, only someone who works for SpaceX could tell you and they won't, but the Falcon 9 has full thrust vector control, not requiring external fins or other such items for control.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-03-09T00:31:29.310" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3933" PostTypeId="2" ParentId="3015" CreationDate="2014-03-09T00:53:41.397" Score="4" Body="&lt;p&gt;This is what I use in class, these are standard formulas from any text book. Define 2 functions&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;sphereOfInfluence[dominantMass_, minorMass_, distanceBetween_] := &#xA;   (minorMass/dominantMass)^(2/5)*distanceBetween&#xA;&#xA;sphereOfGravitation[dominantMass_, minorMass_, distanceBetween_] := &#xA;   (minorMass/dominantMass)^(1/2)*distanceBetween&#xA;   (*valid only for  minorMass&amp;lt;&amp;lt;dominantMass*)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;For example, to find earth SOI w.r.t sun&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;sunMass = 1.989*10^30;&#xA;earthMass = 5.944*10^24;&#xA;earthSunDistance = 1.495978*10^8;&#xA;earthSOI = sphereOfInfluence[sunMass, earthMass, earthSunDistance]&#xA;&#xA;(* 922790.  in km *)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;To find moon SOI w.r.t. earth &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;earthMass = 5.944*10^24;&#xA;moonMass = 7.3483*10^22;&#xA;moonEarthDistance = 384400;&#xA;moonSOI = sphereOfInfluence[earthMass, moonMass, moonEarthDistance]&#xA;&#xA;(* 66317.3  in km *)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;To find earth gravitational sphere of influence w.r.t. sun&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;earthgSOI = sphereOfGravitation[sunMass, earthMass, earthSunDistance]&#xA;(* 258611. km *)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;As expected, the  gravitational sphere of influence is much smaller than the SOI. &lt;/p&gt;&#xA;" OwnerUserId="2829" LastActivityDate="2014-03-09T00:53:41.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3934" PostTypeId="1" CreationDate="2014-03-09T03:06:05.627" Score="13" ViewCount="757" Body="&lt;p&gt;I was reading an &lt;a href=&quot;http://www.spaceflightinsider.com/space-flight-news/orion/exploration-flight-test-1/two-delta-iv-heavy-boosters-arrive-ccafs-leadup-nasas-eft-1-mission/&quot; rel=&quot;nofollow&quot;&gt;article about the 2 of the 3 cores arriving&lt;/a&gt; by barge from Decatur at CCAFS and it noted that the two would be the Starboard and center cores.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This made me wonder what differences in the core stages might exist. That is, do they build the three cores differently for a Delta 4 Heavy launch? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know one of the critiques for the Delta 4 performance wise is that the mount points for the solid rocket boosters apparently are heavy, and are there on every launch, even those not using it.  Somehow Atlas V differs in its mounting of solid boosters that does not carry a similar penalty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So does the center core require additional structure to attach a core to either side, or is it already built into all the common core boosters?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My ultimate question that I am looking to get answered is: Are the CBC modules in a three core Delta 4 Heavy interchangeable?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="8561" LastEditDate="2015-03-26T17:06:46.673" LastActivityDate="2015-03-26T18:27:59.687" Title="Are the three core boosters in a Delta 4-Heavy interchangeable?" Tags="|ula|delta-iv|delta-iv-heavy|cbc|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3935" PostTypeId="5" CreationDate="2014-03-09T03:06:54.317" Score="0" Body="" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-03-09T03:06:54.317" LastActivityDate="2014-03-09T03:06:54.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3936" PostTypeId="4" CreationDate="2014-03-09T03:06:54.317" Score="0" Body="United Launch Alliance (ULA) is an American joint venture between Lockheed Martin and Boeing that provides launch vehicle services." OwnerUserId="51" LastEditorUserId="40257" LastEditDate="2021-05-18T18:11:00.413" LastActivityDate="2021-05-18T18:11:00.413" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="3937" PostTypeId="5" CreationDate="2014-03-09T03:07:29.447" Score="0" Body="" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-03-09T03:07:29.447" LastActivityDate="2014-03-09T03:07:29.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3938" PostTypeId="4" CreationDate="2014-03-09T03:07:29.447" Score="0" Body="Common Booster Core is the name given to each first stage booster in a Delta-4 Heavy booster." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-03-09T03:07:29.447" LastActivityDate="2014-03-09T03:07:29.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3939" PostTypeId="1" CreationDate="2014-03-09T05:20:05.767" Score="5" ViewCount="757" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IL2OL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Source: Lockheed Martin press release: &lt;a href=&quot;http://www.lockheedmartin.com/us/news/press-releases/2014/march/0306-ss-orion.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Boosters for Orion’s Launch Vehicle Arrive to Cape Canaveral&lt;/a&gt;, Image credit: NASA&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the above image you can see that the nozzles are not open; instead they are capped. My question is: why are they capped, and is the same applicable to all rocket stages?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="49" LastEditDate="2014-03-09T11:32:51.717" LastActivityDate="2014-03-09T11:32:51.717" Title="Why is the nozzle of Delta IV capped?" Tags="|rockets|transport|security|delta-iv|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3940" PostTypeId="2" ParentId="3015" CreationDate="2014-03-09T05:59:57.623" Score="23" Body="&lt;p&gt;&lt;strong&gt;Sphere of influence&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravitational sphere of influence asks which of two gravitating bodies should be used as the origin for purposes of modeling the behavior of some third body such as a spacecraft. This comes into play in at least two key places:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;In a patched conic approximation, what's the right place to switch from one conic to another?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;When a spacecraft is moving away from a large body and toward a smaller body, when should the spacecraft navigation switch from a large body centric to a small body centric point of view?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;When looking at things from the perspective of a frame of reference with its origin at the center of the smaller body, the gravitational acceleration toward the larger body is calculated as a &quot;third body effect&quot; (sorry for the confusing nomenclature; it's not mine).  The answer to the above questions (where should I switch patched conics / where should I switch my flight software) is the surface on which this perturbing third body acceleration is equal in magnitude to the gravitational acceleration toward the smaller body. This surface has a rather complex shape, approximately that of an oblate spheroid, but it cannot be expressed in terms of the elementary functions. However, the &quot;right&quot; place along the line connecting the two bodies can be expressed simply. Relative to the smaller body, this distance is $R\left(\frac m M\right)^{2/5}$, where $R$ is the distance between the two bodies, $m$ is the mass of the smaller body, and $M$ is the mass of the larger body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who developed this concept? That's a good question. Some aerospace textbooks call this the Lagrange sphere of influence after Joseph-Louis Lagrange, others call it the Tisserand sphere of influence after Felix Tisserand, and yet others just call it the sphere of influence, period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;Hill sphere&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hill sphere asks a rather different question: Given a smaller body orbiting a larger body, can an even smaller body orbit the small body? The Hill sphere (aka the Roche sphere) looks at things from the perspective of energy rather than force. One of the key developments initiated by Lagrange was to switch from the Newtonian focus on force to instead focus on energy. Lagrangian physics, and later, Hamiltonian physics, were complete re-writes of classical mechanics. In many cases, particularly where energy is conserved, it makes a lot more sense to look at things from the perspective of energy rather than force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what determines whether an orbit is stable? The answer is very complex. In the three body problem, if that third object stays within an extremely complex boundary called the Roche lobe, the orbit of that third object about the smaller body will be stable for at least some amount of time. The Roche lobe just touches the L1 and L2 points and fans out from there. George Hill used the L1 point to define a sphere that approximated the Roche lobe. This is still intractable; the L1 point is defined by a fifth order polynomial that cannot be solved in terms of the elementary functions. Hill further simplified things by realizing that a simple cubic equation yields a very good approximation of that intractable fifth order equation. The result is $R\left(\frac m {3M}\right)^{1/3}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;So which is &quot;right&quot;?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So which is &quot;right&quot;, the Lagrange/Tisserand sphere of influence or the Hill sphere? First off,  it's important to note that both are approximations. This muddies the waters as to which is &quot;right&quot;. More importantly, the two concepts attack very different questions. If you are doing mission planning to send a spacecraft to the Moon or some other planet, you should be using the sphere of influence. Similarly, you should probably be using the sphere of influence if you are building the guidance and navigation subsystem for that spacecraft. On the other hand, if you are bringing an asteroid back to Earth for mining, you probably want to place it in a distant selenocentric retrograde orbit. Now the Hill sphere is the correct choice with regard to the orbit in which you place your asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the question is concerned, the sphere of influence is the correct answer simply because the questioner asked about the sphere of influence, not the Hill sphere. Whether it was Lagrange or Tisserand, both have &quot;sphere of influence&quot; naming precedence over Hill simply because both predate Hill.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will admit that that is being ridiculously nit-picky. The &quot;right&quot; answer is that the two concepts address two different questions. Both are &quot;right.&quot;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sphere of influence: &lt;a href=&quot;http://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)&quot;&gt;http://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)&lt;/a&gt;, or practically any textbook on the astrodynamics field within aerospace engineering. For example, both Vallado (&lt;em&gt;Fundamentals of Astrodynamics and Applications&lt;/em&gt;) and Bate, Mueller, and White (&lt;em&gt;Fundamentals of Astrodynamics&lt;/em&gt;) write about the sphere of influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hill sphere: &lt;a href=&quot;http://en.wikipedia.org/wiki/Hill_sphere&quot;&gt;http://en.wikipedia.org/wiki/Hill_sphere&lt;/a&gt;, or practically any textbook or journal article that addresses invariant manifolds as applied to space exploration. For example, Koon, Lo, Marsden, and Ross (&lt;em&gt;Dynamical Systems, the Three-Body Problem and Space Mission Design&lt;/em&gt;) write about the Hill sphere.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-09T06:14:43.297" LastActivityDate="2014-03-09T06:14:43.297" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3941" PostTypeId="2" ParentId="3939" CreationDate="2014-03-09T06:30:52.710" Score="3" Body="&lt;p&gt;A link to the relevant article: &lt;a href=&quot;http://www.spaceflightinsider.com/space-flight-news/orion/exploration-flight-test-1/two-delta-iv-heavy-boosters-arrive-ccafs-leadup-nasas-eft-1-mission/&quot; rel=&quot;nofollow&quot;&gt;http://www.spaceflightinsider.com/space-flight-news/orion/exploration-flight-test-1/two-delta-iv-heavy-boosters-arrive-ccafs-leadup-nasas-eft-1-mission/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From that article,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On Tuesday, March 4, two Delta IV Common Booster Cores (CBC’s) arrived  at Port Canaveral aboard United Launch Alliance’s (ULA’s) Delta Mariner barge. The barge previously departed ULA’s Decatur, Alabama, manufacturing facility on  Sunday, Feb. 23, and traveled along the Tennessee, Ohio and Mississippi Rivers prior to entering the Gulf of Mexico, rounding the Florida peninsula, and making  its way northward along the Atlantic coast. … Upon arrival at the Port, the two CBC’s were transported to ULA’s Horizontal Integration Facility (HIF) at Space Launch Complex 37 (SLC-37) at Cape Canaveral Air Force Station (CCAFS).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's a &lt;strong&gt;long&lt;/strong&gt; journey. As the crow flies it's only 1000 km or so. As the barge moves, it's at least twice that. Storms can fling debris, and so can traffic on the short segments where the boosters were on the road as opposed to on a barge. Those are protective shrouds to prevent damage along the route from Decatur, Alabama to Cape Canaveral, Florida.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-09T06:30:52.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3942" PostTypeId="2" ParentId="3939" CreationDate="2014-03-09T10:16:00.280" Score="4" Body="&lt;p&gt;Airplane engines, and rocket engines are usually transported with a cover on them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The journey of a thousand miles begins with a single piece of unexpected debris. The covers try to protect them.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-03-09T10:16:00.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3943" PostTypeId="1" CreationDate="2014-03-09T14:48:35.253" Score="4" ViewCount="2101" Body="&lt;p&gt;Years ago, J. Michael Straczynski of Babylon 5 reported that NASA had shown keen interest in using the show's &lt;a href=&quot;https://en.wikipedia.org/wiki/Starfury&quot; rel=&quot;nofollow&quot;&gt;Starfury&lt;/a&gt; design as a forklift for work in zero-g. I think he told them it was a great idea but to please keep the name Starfury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the plans for the Starfury forklift die out? Is it still a long term viable option?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems a neat idea to me, since a design could either be robotic or manned.&lt;/p&gt;&#xA;" OwnerUserId="2709" LastEditorUserId="49" LastEditDate="2014-03-09T15:52:02.880" LastActivityDate="2014-03-09T15:52:02.880" Title="Did the StarFury die on the drawing board?" Tags="|technology|spacesuits|robotic-missions|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3944" PostTypeId="1" AcceptedAnswerId="3946" CreationDate="2014-03-09T14:58:26.067" Score="6" ViewCount="212" Body="&lt;p&gt;Would the length of a space craft be a limitation when considering aerobreaking or gravity-assist?  I have to assume there would be some crazy stress tension on a longer body craft in one of those circumstances. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would most certainly affect breaking if &quot;air bags&quot; were deployed.  Unless those are just science fiction.&lt;/p&gt;&#xA;" OwnerUserId="2709" LastActivityDate="2014-03-09T16:55:28.553" Title="Could the length of a craft affect it adversely during aero-breaking or gravity assist?" Tags="|gravity-assist|aerobraking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3945" PostTypeId="2" ParentId="3943" CreationDate="2014-03-09T15:40:34.640" Score="2" Body="&lt;p&gt;According to J. Michael Straczynski himself, when asked about it in an &lt;a href=&quot;http://www.republibot.com/content/interview-joe-straczynski&quot; rel=&quot;nofollow&quot;&gt;interview for Republibot 3.0's blog&lt;/a&gt; in May 2009:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I haven't heard anything new about this in several years, so I have to&#xA;  assume it's dormant for now.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One thing to consider here is that NASA &lt;em&gt;shows interest&lt;/em&gt; for many things. I've recently watched &lt;em&gt;ISS On-Orbit&lt;/em&gt; updates where current ISS crew were interviewed by some lively bunch of space exploration fans, and one question was asking them what they thought of Gravity (the movie). They were courteous, of course, but it was interesting how they masterfully avoided actually answering it. If you get my meaning. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-03-09T15:40:34.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3946" PostTypeId="2" ParentId="3944" CreationDate="2014-03-09T16:12:32.183" Score="4" Body="&lt;p&gt;Yes, with gravity-assist slingshots, parts of a very long craf could be a subject to torque due to gravity gradient along the craft's truss, and if it's not constructed to withstand these forces, it could tear it apart or deform by tidal forces. Due to this torque, the craft could also enter into a violent spin. We can actually observe similar happening to larger comets and asteroids as they shoot past massive celestial bodies, perhaps most famous of those is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;comet Shoemaker–Levy 9&lt;/a&gt; that  that broke apart and collided with Jupiter in July 1994:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/GN0mn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image of Comet Shoemaker–Levy 9 fragments (total: 21), taken on May 17, 1994 (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Comet Shoemaker–Levy 9&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same could of course happen also during aerobraking, but mostly due to difference in decelerating forces along the craft's body due to atmospheric drag. At velocities involved, atmosphere actually behaves as a fluid, and the flow shapes along the ram-facing side and bends around its corners. As it does that, it has to increase its velocity relative to the rest of the medium the craft is surrounded with, and increases its kinetic force with it. As these deflected jets catch up with the slower flow around the craft, rapid deceleration and change in vector cause shockwaves and form local eddies along the edge of the craft's ram-facing side. This is actually easier to demonstrate than using some comets that impacted Jupiter, just take a sheet of paper, hold it above your head and release. You'll notice it'll slightly bend at its edges due to these air currents forming along its bottom side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oh, and using airbags as Inflatable Aerodynamic Decelerators (IAD) isn't science fiction. For example, &lt;a href=&quot;http://www.nasa.gov/directorates/spacetech/game_changing_development/HIAD/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA is developing HIAD&lt;/a&gt; (Hypersonic Inflatable Aerodynamic Decelerator), and there are other companies taking similar approach to aerobraking, for example &lt;a href=&quot;http://www.ilcdover.com/Ballutes-Decelerators/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ILC Dover&lt;/a&gt; developing many IAD variants. Here's &lt;a href=&quot;http://www.nasa.gov/directorates/spacetech/game_changing_development/HIAD/irve3-success.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA video&lt;/a&gt; of Inflatable Re-entry Vehicle Experiment 3 (IRVE-3) flight demonstration of inflatable technology, and this is how IRVE-3 looked like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/kCRzl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;NASA's Inflatable Re-entry Vehicle Experiment 3 (IRVE-3) completely  inflated under vacuum conditions in Transonic Dynamics&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Tunnel at Langley Research Center. Image credit: NASA Langley / Kathy Barnstorff&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-09T16:55:28.553" LastActivityDate="2014-03-09T16:55:28.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3947" PostTypeId="2" ParentId="1415" CreationDate="2014-03-09T17:53:18.597" Score="29" Body="&lt;p&gt;In addition to all the issues raised by TidalWave, you also need to consider the exhaust products.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space shuttle main tank carried about 730 tons of hydrogen and oxygen. Burning that produces about 730 tons of water, the happy chemical we all love. Burning about 730 tons of hydrogen and fluorine produces about 730 tons of very hot hydrogen fluoride, which is highly toxic and extremely corrosive. It attacks glass, metals and pretty much everything else, most likely including rocket nozzles and launch pads. It causes horrific burns to human flesh. You do not want this.&lt;/p&gt;&#xA;" OwnerUserId="2784" LastActivityDate="2014-03-09T17:53:18.597" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3948" PostTypeId="1" AcceptedAnswerId="3949" CreationDate="2014-03-09T19:28:02.457" Score="20" ViewCount="14674" Body="&lt;p&gt;What should be the escape velocity for our galaxy and can we calculate it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, if we assume that we don't know the mass of our galaxy, you may consider it as small 'm'.&lt;/p&gt;&#xA;" OwnerUserId="2795" LastEditorUserId="-1" LastEditDate="2019-04-24T23:03:18.813" LastActivityDate="2019-07-24T11:53:26.857" Title="What is the escape-velocity for our galaxy?" Tags="|orbital-mechanics|rockets|energy|escape-velocity|celestial-mechanics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="3949" PostTypeId="2" ParentId="3948" CreationDate="2014-03-09T20:11:17.837" Score="18" Body="&lt;p&gt;This is actually pretty difficult to do, because it depends on where from due to uneven distribution of matter (local parameters), how far from the galactic center due to radial velocity, the direction in which you want to reach escape velocity (how much of the radial velocity can be used), and that it's hard to estimate mass of the Milky Way (global parameters) and there will always be significant uncertainty with methods that are currently available to us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can probably expect a much more reliable overview of mass distribution, stellar velocities and total mass estimates once the &lt;a href=&quot;http://sci.esa.int/gaia/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gaia observatory&lt;/a&gt; completes its billion stars stellar kinematics survey, but so far the best we have is probably the results of the &lt;a href=&quot;http://www.rave-survey.aip.de/rave/pages/project/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radial Velocity Experiment (RAVE) survey&lt;/a&gt;;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RAVE took a sample of 90 high-velocity Milky Way stars (some moving at over 300 km/s) for which their position and velocity was determined sufficiently precise, and then compared their movement to models of other, similar spiral galaxies, to reach an estimate of Milky Way's total mass at about 1.6 trillion &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_mass&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar masses&lt;/a&gt;. Once they had that, they could calculate estimated escape velocities for our stellar neighbourhood, for which mass distribution and radial velocity with respect to the galactic barycenter are of course most well understood.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar system's orbital velocity is estimated at roughly 220 km/s, and galactic escape velocity for our vicinity at about 537 km/s. &lt;strong&gt;So in the direction of Solar system's velocity vector, velocity required to escape Milky Way is ~ 317 km/s&lt;/strong&gt;. And much more, if this Solar system's own orbital momentum cannot be used to full extent and a launch in other directions is required. This is of course assuming you can launch on a trajectory that avoids getting too close to gravitational influence of other solar systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exact methods used to calculate this are a fair bit too complex to even attempt describing them here, so I'll refer you to some interesting sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://arxiv.org/abs/1309.4293v1&quot; rel=&quot;nofollow noreferrer&quot;&gt;The RAVE survey: the Galactic escape speed and the mass of the Milky Way&lt;/a&gt;, Piffl et al., 17 Sep 2013 (&lt;a href=&quot;http://arxiv.org/pdf/1309.4293v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Complete PDF&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://arxiv.org/abs/astro-ph/0611671&quot; rel=&quot;nofollow noreferrer&quot;&gt;The RAVE Survey: Constraining the Local Galactic Escape Speed&lt;/a&gt;, Smith et al., 9 May 2007 (&lt;a href=&quot;http://arxiv.org/pdf/astro-ph/0611671v2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Complete PDF&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And a bit lighter reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.newscientist.com/article/dn24249-stars-escape-velocity-shows-how-to-exit-the-milky-way.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Scientist: Stars' escape velocity shows how to exit the Milky Way&lt;/a&gt;, 23 Sep 2013&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.rave-survey.aip.de/rave/pages/project/Intro.jsp&quot; rel=&quot;nofollow noreferrer&quot;&gt;RAVE: the RAdial Velocity Experiment - Introduction to RAVE&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="21562" LastEditDate="2019-07-24T11:53:26.857" LastActivityDate="2019-07-24T11:53:26.857" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="3950" PostTypeId="1" CreationDate="2014-03-10T12:05:47.593" Score="3" ViewCount="551" Body="&lt;p&gt;Launch happens when trajectory plane contains launch site, launch asymptote and center of Earth. On a launch day there will be two launch opportunities, one at morning and other at the night. Which launch parameter makes these two launch time separate? &lt;/p&gt;&#xA;" OwnerUserId="2832" LastEditorDisplayName="user8858" LastEditDate="2017-07-06T19:41:22.833" LastActivityDate="2017-07-06T19:41:22.833" Title="Which parameter causes two launch times in a launch day?" Tags="|launch|interplanetary|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3951" PostTypeId="1" AcceptedAnswerId="3952" CreationDate="2014-03-10T14:01:20.567" Score="13" ViewCount="1565" Body="&lt;p&gt;Before the Space Shuttle retirement, there was a huge pressure on NASA, U.S. Administration, and everyone else directly involved in U.S. human space exploration using Heavy-Lift Launch Vehicle (HLLV) to come with suggestions to keep the U.S. access to space and International Space Station (ISS) gap-free and all the existing workforce employed in the industry, doing what they do best, while trying to reuse existing technology to achieve these goals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Advisory committee was formed by the President G. W. Bush, Vice President J. D. Quayle and the Space Council, chaired by Norman R. Augustine, more commonly know as the Augustine Commission. Commission's end report included several proposed variants for the continuation for U.S. manned flight, among which Option 4B that called for a direct Space Shuttle derived HLLV reusing many parts of Space Shuttle, like it's two boosters, external tank, Space Shuttle Main Engines (SSME), and sitting Orion crew module on top of it, dealing away with the Space Shuttle Orbiter (SSO). The whole idea enjoyed large support of the space community, as can be seen for example in this &lt;a href=&quot;http://vimeo.com/7209149&quot; rel=&quot;noreferrer&quot;&gt;video compilation supporting Augustine Commission Report Option 4B&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/17X79.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Exploded diagram of the DIRECT v3 Jupiter-130 Launch Vehicle. Artist: Philip Metschan, DIRECT Team. Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/DIRECT&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on DIRECT&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Review of U.S. Human Spaceflight Plans Committee document titled &lt;a href=&quot;http://www.nasa.gov/pdf/396093main_HSF_Cmte_FinalReport.pdf&quot; rel=&quot;noreferrer&quot;&gt;SEEKING A HUMAN SPACEFLIGHT PROGRAM WORTHY OF A GREAT NATION&lt;/a&gt; (PDF), Option 4B, perhaps better known as &lt;a href=&quot;http://www.directlauncher.org/&quot; rel=&quot;noreferrer&quot;&gt;DIRECT v3&lt;/a&gt;, is described as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Variant 4B is the Shuttle extension variant. It offers the only&#xA;  foreseeable way to eliminate the gap in U.S. human-launch capability:&#xA;  by extending the Shuttle to 2015 at a minimum safe-flight rate. It&#xA;  also takes advantage of synergy with the Shuttle by developing a&#xA;  heavy-lift vehicle that is more directly Shuttle-derived than the Ares&#xA;  family of vehicles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I can't state how much this option makes sense to me, but as much as I'd like it to, the whole thing isn't happening, is it? And the then so much discussed U.S. human access to space and workforce gap is now a reality, isn't it? So, I guess this naturally begs the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What eventually killed the Augustine Commission Report Option 4B, Directly Shuttle-Derived Heavy-Lift Launch Vehicle? Are there any good objective reasons that made decision makers believe it was worth scrubbing much of the existing Space Shuttle technology for a new U.S. HLLV in the form of first Ares, then SLS, that even now seems more as a gap-filler project that won't deliver for many years to come, even if all goes according to plans?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do apologize if this seems as a bit of a rant, I guess it is, but I'm looking for objective explanation of what went wrong, and if possible, please substantiate your claims with references. Which decisions made U.S. lose its status as the world leader in human space exploration and caused this almost intolerable gap in U.S. based human spaceflight and  access to space for manned missions?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-06-30T18:09:43.400" LastActivityDate="2014-06-30T19:36:52.257" Title="What eventually killed the Augustine Commission Report Option 4B, Directly Shuttle-Derived Heavy-Lift Launch Vehicle, a.k.a. DIRECT?" Tags="|rockets|nasa|crewed-spaceflight|hllv|shuttle-derived|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3952" PostTypeId="2" ParentId="3951" CreationDate="2014-03-10T15:17:23.827" Score="8" Body="&lt;p&gt;In one sense, Option 4B was rejected because that option was to send humans back to the Moon. NASA and Congress have opted to instead send humans beyond Earth orbit, either an asteroid or to Mars.&lt;/p&gt;&#xA;&lt;p&gt;In a larger sense, Option 4B lives on in part in the Space Launch System. Shuttle-derived components are an important part of SLS.&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.directlauncher.org&quot; rel=&quot;noreferrer&quot;&gt;http://www.directlauncher.org&lt;/a&gt;,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;After several years of ongoing relationship with NASA, the DIRECT Team is pleased to see that many of the principles that defined our Jupiter HLV design have found application in NASA's new flagship launch vehicle, the SLS.&lt;/p&gt;&#xA;&lt;p&gt;While we are disappointed that the agency has chosen to introduce costly additions to the heavy-lift design, rather than make maximum utilization of existing components, technology, and infrastructure, we are pleased that the Administration, Congress and NASA have found common ground and charted a course forward for our nation as we move beyond the era of the Space Shuttle.&lt;/p&gt;&#xA;&lt;p&gt;As we look forward to that future, the entire team offers our very best wishes to NASA as the agency embarks on a new path of once again leading mankind beyond the confines of Low Earth Orbit; back to the Moon, on to Mars, and then beyond, into the deepest reaches of our vast solar system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;On October 13, 2010, the DIRECT Team declared success.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;DIRECT Team Declares Success and Looks to the Future&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The DIRECT Team congratulates and applauds the President, Senate, and the House of Representatives for passing legislation enabling NASA to begin work on the Space Launch System Heavy Lift Launch Vehicle, based on existing Space Shuttle components.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;For more, &lt;a href=&quot;http://www.directlauncher.org/documents/Direct-Team-Declares-Success-PR-101310.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.directlauncher.org/documents/Direct-Team-Declares-Success-PR-101310.pdf&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;In particular, SLS is directly reusing the Shuttle Main Engine, just as the DIRECT team proposed. It is also reusing the Shuttle External Tank (with modifications) and the Shuttle solid rocket boosters (with modifications).&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-03-10T15:17:23.827" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="3955" PostTypeId="2" ParentId="3950" CreationDate="2014-03-10T18:21:04.817" Score="5" Body="&lt;p&gt;There will be roughly two opportunities per day if&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The launch site's latitude is less than the orbital inclination,&lt;/li&gt;&#xA;&lt;li&gt;The launch is directly into the desired orbit, with orbit insertion happening when the launch vehicle reaches the desired altitude (i.e., no transfer orbit), and&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;All you care about is being on the right orbital plane at the right altitude but not where the vehicle is in the orbit.&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The orbital plane is roughly fixed in inertial space. (Note: This ignores precession due to the non-spherical nature of the Earth and due to external bodies. That's why I said &quot;roughly.&quot;) The Earth's slightly less than 24 hour rotation rate (the sidereal day) means that the launch site will switch from being on one side of the orbital plane to the other side twice every sidereal day. Add in precession and this will change somewhat, but it's still roughly twice per day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That last condition, all you care about is being on the right orbital plane at the right altitude but not where the vehicle is in the orbit, usually does not apply. Space vehicle operators tend to care a lot about where the vehicle will be on the orbital plane. For example, it's rather important if the vehicle is to rendezvous with some other object, if it is to be in a particular sun-synchronous orbit with a particular local sub-satellite point time, or if it is to be in just the right spot at geosynchronous altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;TL;DR version: &lt;em&gt;Which parameter causes two launch times in a launch day?&lt;/em&gt; The answer is the Earth's ~24 hour rotation rate.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-10T18:21:04.817" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3957" PostTypeId="1" AcceptedAnswerId="3959" CreationDate="2014-03-10T23:08:50.613" Score="15" ViewCount="2412" Body="&lt;p&gt;What are the differences between a sounding rocket and a suborbital flight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sounding rockets have long been used for science purposes. Some of them have trajectories elliptic enough to do microgravity research during a few minutes. Sub-orbital is a term which mostly seems to be used when talking about crewed flights. Is the difference simply that sub-orbital flights are optimized for carrying a crew? Are sounding rockets too rough or small for carrying humans?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2014-03-10T23:42:03.673" LastActivityDate="2023-03-26T00:12:09.800" Title="What distinguishes a sounding rocket from a suborbital flight?" Tags="|rockets|terminology|sub-orbital|sounding-rocket|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="3958" PostTypeId="2" ParentId="3957" CreationDate="2014-03-10T23:37:38.997" Score="8" Body="&lt;p&gt;Some sounding rockets are sub-orbital. Sub-orbital merely means going into space with less than orbital velocity. Going into space: That's a bit arbitrary, but a person who goes 50 miles or higher (~80km) gets astronaut wings; another definition is the nice round figure of 100 kilometers (~62 miles) in altitude; that's the Kármán line. Yet another is the nice round figure of 400,000 feet (~122 km; ~76 miles); that's &quot;entry interface&quot; for NASA vehicles that reenter the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By whatever definition one uses, some sounding rocket flights are less than sub-orbital. Sounding rockets sent into the stratosphere may top out at much less than 50 miles altitude. A balloon is cheaper than a rocket, but balloons can only go to an altitude of 31 miles / 50 kilometers or so. If your experiment needs to go higher than that you need to use a sounding rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as acceleration is concerned, sounding rockets go whoosh! and they're gone. However, their payload capacities are rather small. They aren't intended for carrying massive people and their even more massive environmental control and life support systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the &lt;a href=&quot;http://sites.wff.nasa.gov/code810/files/SRHB.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Sounding Rocket Handbook&lt;/a&gt; for more info on sounding rockets and their capabilities.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="195" LastEditDate="2018-02-05T23:37:55.453" LastActivityDate="2018-02-05T23:37:55.453" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3959" PostTypeId="2" ParentId="3957" CreationDate="2014-03-10T23:38:16.753" Score="11" Body="&lt;p&gt;If the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sounding_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;sounding rockets&lt;/a&gt; go past the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kármán line&lt;/a&gt; at 100 kilometres (62 mi) above the mean sea-level that's now officially &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/a/2077/49&quot;&gt;the edge of space&lt;/a&gt;&lt;/em&gt; since the &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;nofollow noreferrer&quot;&gt;Outer Space Treaty&lt;/a&gt; sort of ratified that, there is no difference, and they're all &lt;a href=&quot;https://en.wikipedia.org/wiki/Sub-orbital_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;sub-orbital spaceflight&lt;/a&gt;. Meaning, they don't reach required &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital velocity&lt;/a&gt; (around 8 km/s) to stay in orbit and their flight is on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Trajectory_of_a_projectile&quot; rel=&quot;nofollow noreferrer&quot;&gt;ballistic trajectory&lt;/a&gt;. If they don't even reach the boundary of outer space, then they don't really qualify as spaceflight and are just &lt;a href=&quot;https://en.wikipedia.org/wiki/Missile&quot; rel=&quot;nofollow noreferrer&quot;&gt;missiles&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while &lt;em&gt;suborbital spaceflight&lt;/em&gt; and &lt;em&gt;sounding rockets&lt;/em&gt; aren't really directly comparable, and the former defines the speed and achievable trajectory and the latter identifies its purpose and often also size with usually small payloads for short-duration microgravity and/or upper atmosphere / ionosphere environment experiments, most sounding rockets are indeed suborbital. But it's not necessarily the other way around and not all suborbital spaceflights are sounding rockets, they could for example also be manned suborbital spaceflights, like say certain &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_tourism&quot; rel=&quot;nofollow noreferrer&quot;&gt;space tourism&lt;/a&gt; vehicles or &lt;a href=&quot;https://en.wikipedia.org/wiki/Point-to-point_suborbital_spaceflight#Point-to-point_sub-orbital_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;point-to-point suborbital transportation routes&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-03-11T00:55:54.580" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3960" PostTypeId="1" CreationDate="2014-03-11T19:34:40.020" Score="21" ViewCount="1764" Body="&lt;p&gt;The concept of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hybrid_rocket&quot;&gt;hybrid rocketry&lt;/a&gt; is very interesting, and seems like it has many advantages over its solid and liquid forebears. But having only a superficial idea of the challenges preventing it from mainstream use (namely combustion instability and adverse grain regression rates) and not wanting to fall for a case of &quot;what you see is all there is&quot;, I must ask what else there is that is preventing hybrid rocketry from being used for more large scale applications, instead of just suborbital joyrides and small scale amateur endeavours.&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="1142" LastEditDate="2015-07-13T03:08:38.080" LastActivityDate="2018-07-18T13:22:52.773" Title="Challenges Facing Hybrid Rocketry" Tags="|rockets|hybrid-rocket|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3961" PostTypeId="2" ParentId="3770" CreationDate="2014-03-11T19:47:46.347" Score="3" Body="&lt;p&gt;I think there's a simplistic way to constrain this problem.  Just apply simple conservation of angular momentum.  Take the moon out of the picture entirely and just focus on the rise from the surface of Earth to the L1 distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the center of the Earth as the origin, we easily identify a tangential and radial component of the velocity.  The tangential component is the only one that counts toward angular velocity.  The angular momentum balance is simply:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ r_1 v_{t1} = r_2 v_{t2} \\&#xA;v_{t2} = \frac{ r_1 v_{t1} } { r_2}  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These values are all specified by the nature of the problem.  Roughly, we find:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ v_{t2} \approx \frac{ (6,300 \text{ km}) ( 0.46 \frac{ \text{ km} }{s}) } {326,000 \text{ km} } \approx 5 \frac{m}{s} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, any angular momentum you start with will be virtually irrelevant for the purposes for circularization of extremely high orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Out of the orbits that rotate along with the Earth-moon system, it seems that a fancy shot to &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/04/cartoon-delta-v-map.html&quot; rel=&quot;nofollow&quot;&gt;EML-2 would win with 0.4 km/s&lt;/a&gt;, which is better the 0.86 km/s for EML-1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could go beyond lunar orbit, but you would need to be over 3 lunar distances in order to beat EML-2, and the stability of such an orbit would get extremely iffy.  The Earth-sun L2 point would probably beat that.  The Earth-sun L1 and L2 points are in the neighborhood of 0.3 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe you could just do a very highly elliptical orbit below lunar distance.  I went through the calculations for that (system of energy and angular momentum with 2 unknowns).  If you do a burn at apogee at 1/2 lunar distance then you need a burn of about 0.36 km/s in order to reduce the eccentricity sufficiently.  On the bright side, Earth's rotation is now giving you more nearly 0.01 km/s for an apogee boost as opposed to the 0.005 km/s at L1 distance.  Whoo hoo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I doubt that orbit is very stable, but you could push the tradeoff to either side.  So it looks like the Lagrange points are the winners.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-03-11T19:47:46.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3962" PostTypeId="1" CreationDate="2014-03-12T12:14:48.693" Score="5" ViewCount="168" Body="&lt;p&gt;Could humanity have entered the space age without relativity theory or quantum physics?&#xA;(Other non-Newtonian physics would be allowed, it is actually the importance, or lack thereof, of relativity and quantum mechanics for space exploration which I want to highlight here)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a &quot;space age&quot; I mean things like sending crewed Sputniks to LEO, landing men on the Moon, sending probes to outer planets, putting communication satellites in geostationary orbit, space telescopes in Lagrange points and such stuff which has been done today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK a GPS system would have had some (manageble?) errors, and interplanetary trajectories would've been a bit off. But could such little mysteries not have been practically handled with some engineering rule of thumb adjustments as you go?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the greatest problems have been for a purely Newtonian space age humanity?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2014-03-12T17:07:57.203" LastActivityDate="2014-03-12T17:07:57.203" Title="Could humanity have traveled in space with knowledge of only Newtonian physics?" Tags="|rockets|" AnswerCount="0" CommentCount="20" ClosedDate="2014-03-12T14:33:25.613" ContentLicense="CC BY-SA 3.0" />
  <row Id="3963" PostTypeId="2" ParentId="1173" CreationDate="2014-03-12T12:46:35.043" Score="34" Body="&lt;p&gt;This is an old question, but nobody has properly answered it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ultimate answer is the Columbia disaster. This disaster demonstrated that the growing expense of, and inherent risks in, the Shuttle program precluded long term use of the Shuttle. From chapter 9, page 210 of the &lt;a href=&quot;http://www.nasa.gov/columbia/home/CAIB_Vol1.html&quot;&gt;&lt;em&gt;Report of Columbia Accident Investigation Board&lt;/em&gt;&lt;/a&gt; (emphasis theirs):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Even so, based on its in-depth examination of the Space Shuttle Program, the Board has reached an inescapable conclusion: &lt;em&gt;Because of the risks inherent in the original design of the Space Shuttle, because that design was based in many aspects on now-obsolete technologies, and because the Shuttle is now an aging system but still developmental in character, it is in the nationʼs interest to replace the Shuttle as soon as possible as the primary means for transporting humans to and from Earth orbit.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;The decision to retire the Shuttle came shortly after the CAIB made their report. In early 2004 when President George W. Bush announced of his &lt;a href=&quot;http://www.spaceref.com/news/viewpr.html?pid=13404&quot;&gt;&lt;em&gt;Vision for Space Exploration&lt;/em&gt;&lt;/a&gt; where he said (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To meet this goal, we will return the Space Shuttle to flight as soon as possible, consistent with safety concerns and the recommendations of the Columbia Accident Investigation Board. The Shuttle's chief purpose over the next several years will be to help finish assembly of the International Space Station. &lt;em&gt;In 2010, the Space Shuttle -- after nearly 30 years of duty -- will be retired from service&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;NASA immediately began following this mandate. Starting in 2004, NASA began the long process of continuing Shuttle program operations (after returning to flight) to complete the construction of the International Space Station, and then retire the Shuttle in 2010. (Ultimately that retirement would occur 2011 rather than 2010. President Obama added two additional flights to the original manifest.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Part of the process begun in 2004 was a decision to make various &lt;em&gt;lifetime buys&lt;/em&gt; of parts that needed to be replaced on every Shuttle flight. They knew exactly how many more flights there would be needed. Add parts for a couple of contingency flights, and they knew exactly how much to buy. Many of those parts were one of a kind items. There were specialty bolts and connectors of non-standard dimensions and made of exotic alloys. There were vintage 1970s era pieces of electronics. Many of these were made by mom and pop fabricators. They stayed in business primarily because they were doing something good for the country. When they fulfilled those lifetime buy purchases, many of those mom and pop fabricators simply went out of business. They retired with the Shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This process was largely complete in 2008. By 2009, the decision to terminate the Shuttle program was irrevocable. The logistics chain was gone. For more on this, see &lt;a href=&quot;http://www.spaceref.com/news/viewsr.html?pid=28974&quot;&gt;&lt;em&gt;Wayne Hale's NASA Blog: Shutting Down the Shuttle&lt;/em&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-12T12:46:35.043" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3964" PostTypeId="2" ParentId="3895" CreationDate="2014-03-12T18:13:44.940" Score="5" Body="&lt;p&gt;There is a tool &quot;LTool&quot; by &lt;a href=&quot;http://en.wikipedia.org/wiki/Martin_Lo&quot; rel=&quot;nofollow&quot;&gt;Martin Lo&lt;/a&gt; from JPL to compute low-energy transfers via chaotic trajectories near Lagrange Points.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Martin W. Lo and Roby S. Wilson The LTool Package &lt;a href=&quot;http://www.ieec.fcr.es/libpoint/abstracts/lo2.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.ieec.fcr.es/libpoint/abstracts/lo2.pdf&lt;/a&gt; or copy &lt;a href=&quot;http://www.ieec.cat/hosted/web-libpoint/abstracts/lo2.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.ieec.cat/hosted/web-libpoint/abstracts/lo2.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Martin Lo, LTool Version 1.0G delivery memorandum // JPL TRS 1992+, 29-Sep-2000 &lt;a href=&quot;http://hdl.handle.net/2014/16203&quot; rel=&quot;nofollow&quot;&gt;http://hdl.handle.net/2014/16203&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As I know, tools are not public, and used internally at the JPL.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Martin Lo published several maps in his articles, for example in &quot;The InterPlanetary Superhighway and the Origins Program&quot; (2002)&#xA;&lt;a href=&quot;http://www.gg.caltech.edu/~mwl/publications/papers/IPSAndOrigins.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.gg.caltech.edu/~mwl/publications/papers/IPSAndOrigins.pdf&lt;/a&gt; - check Figure 11-12 for Poincare maps. They shows only principal capabilities of ITN (IPS - interplanetary superhighway). Searching of exact paths is hard due to chaotic nature of ITN and manifolds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get exact trajectory (Figure 16) Martin Lo used &quot;MYSTIC/SDC low thrust trajectory optimization program by Gregory Whiffen at JPL&quot;.&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2014-03-12T18:24:04.433" LastActivityDate="2014-03-12T18:24:04.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3965" PostTypeId="1" AcceptedAnswerId="3966" CreationDate="2014-03-13T02:20:46.117" Score="9" ViewCount="483" Body="&lt;p&gt;Charon, a moon of Pluto, was discovered in 1978. Today we know that Pluto has three additional moons, but they were discovered 3 decades later? Also, Hubble Space Telescope was in operation since 1990 - how could it miss for 15 years that a planet has so many other moons?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="49" LastEditDate="2014-03-13T03:00:59.763" LastActivityDate="2019-02-18T16:18:43.213" Title="Why were Nix, Cerberos and Styx discovered later than Charon?" Tags="|moons|astronomy|pluto|hubble|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3966" PostTypeId="2" ParentId="3965" CreationDate="2014-03-13T04:05:38.737" Score="16" Body="&lt;p&gt;This is the 1978 image of the Pluto system that led to the discovery of Charon. This is a negative, so the big black blob in the middle is Pluto and Charon. Charon? It's the little bump on the upper right of that blob. You can barely make out Charon. Additional satellites? No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://web.archive.org/web/20190218161646if_/http%3A//www.redchairblogs.com/starstruck/files/2012/07/charon-discovery-281x300.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;This is a 1990 image of Pluto and Charon taken by the Hubble:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/f/f1/Pluto-picture.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was before multiple upgrades to Hubble's optics, and multiple upgrades of the ground software that processes Hubble imagery. The light gathering quality and the resolution just weren't there in 1990 to resolve those small moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;This is a sequence of Hubble photographs from 2002 and 2005.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://cbsnews1.cbsistatic.com/hub/i/r/2011/04/20/0fa45d0b-a643-11e2-a3f0-029118418759/resize/620x465/d650b488c6ab30ffad8c001649c156ba/plut6.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice the marked improvement in the quality of the images compared to 1990. However, it wasn't until overexposed images were taken in 2005 that enough light was gathered to suggest that Pluto-Charon system had additional moons.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="58" LastEditDate="2019-02-18T16:18:43.213" LastActivityDate="2019-02-18T16:18:43.213" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="3967" PostTypeId="2" ParentId="2734" CreationDate="2014-03-13T05:08:26.407" Score="7" Body="&lt;p&gt;Streamlining is not the problem: those fancy-looking outer shells will burn up early on in the re-entry process.  The pieces of a satellite that hit the ground are the high-density ones: things like pressurized fuel tanks, thick structural members, and radiothermal generators.  If a satellite uses unpressurized maneuvering fuel, has lightweight structural members, and is solar powered, it is unlikely that any dangerously large pieces will hit the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &quot;potential planetary penetrator&quot; looks more like a &lt;a href=&quot;https://en.wikipedia.org/wiki/Kinetic_bombardment#Project_Thor&quot;&gt;telephone pole&lt;/a&gt; than a Ferrari.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-03-13T05:08:26.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3969" PostTypeId="2" ParentId="2803" CreationDate="2014-03-13T12:44:57.093" Score="4" Body="&lt;p&gt;If departure and destination points are 180 degrees apart (as in a Hohmann transfer), Lambert iterations will give a polar heliocentric transfer orbit. That is an orbit with inclination 90º. Obviously such a transfer orbit would take huge plane change expense at departure and destination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can be mitigated by using a broken plane transfer orbit. See &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/01/deboning-porkchop-plot.html&quot; rel=&quot;nofollow&quot;&gt;Deboning The Porkchop Plot&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you stay in the ecliptic plane, you can miss Mars by a good margin. The only places where Mars comes close to the ecliptic plane is around the ascending and descending nodes.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-03-15T21:13:55.917" LastActivityDate="2014-03-15T21:13:55.917" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3970" PostTypeId="1" AcceptedAnswerId="3971" CreationDate="2014-03-13T12:45:24.413" Score="17" ViewCount="19430" Body="&lt;p&gt;I like looking at photos of stuff in earths orbit. But when I google satellites I often get a lot of nice but not as exciting CGI. I understand it might be hard (but not impossible) for a satellite to take a photo of itself. Surely it can't be that difficult to put a camera on one though?&lt;/p&gt;&#xA;" OwnerUserId="2849" LastActivityDate="2016-03-16T00:17:13.393" Title="Why are photos of satellites most often computer generated?" Tags="|artificial-satellite|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3971" PostTypeId="2" ParentId="3970" CreationDate="2014-03-13T14:11:40.067" Score="21" Body="&lt;p&gt;Satellites are expected to keep their distance from each other to avoid collisions. That means when a satellite would ever get close enough to another satellite to take a picture of it, something went really wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually the only object which would ever be close enough to a satellite in orbit to take a good picture would be the upper stage of its launch vehicle right after separation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Equipping the upper stage with a camera would add additional mass which has questionable use for the mission goal. You might have a chance to spot some external damage caused to the satellite during launch or separation. But the chances are low because the upper stage would see the satellite only from one direction. Also, light conditions in space aren't that well - you only have a single light source and almost no ambient light. Any damage to the parts in the shadow would be hardly visible. Well, you could perform some orbital maneuvers with both upper stage and satellite to get some good shots from all angles, but really, why bother? Either the satellite works or it doesn't, and spotting some scratches somewhere is hardly going to tell you what exactly went wrong during launch. Also, the risk to &lt;em&gt;cause&lt;/em&gt; damage &lt;em&gt;with&lt;/em&gt; these maneuvers isn't non-negligible either (a malfunction or operator error might cause upper stage and satellite to collide).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what reason remains to take pictures?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Public Relations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the public will be much more excited by a shiny, polished CGI image from an exciting angle than a grainy, badly-lit (hard shadows everywhere) photograph from a boring perspective. Look at &lt;a href=&quot;https://pbs.twimg.com/media/BYeYohbCAAA8MFZ.jpg&quot;&gt;this press image from ESAs SWARM mission&lt;/a&gt;, for example. Yes, pure CGI. But you couldn't take a photo nearly as awesome-looking under real space conditions. So why even bother trying?&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-03-13T14:30:11.673" LastActivityDate="2014-03-13T14:30:11.673" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3972" PostTypeId="1" AcceptedAnswerId="3975" CreationDate="2014-03-13T14:45:18.810" Score="17" ViewCount="28177" Body="&lt;p&gt;If we launch a Saturn V rocket from space (far away from Earth so that Earth's gravity has no effect on it) then what would be the maximum velocity achievable? How close could it get us to the speed of light?&lt;/p&gt;&#xA;" OwnerUserId="2722" LastEditorUserId="49" LastEditDate="2014-03-13T15:54:32.000" LastActivityDate="2015-08-17T20:34:38.617" Title="Maximum speed reachable by Saturn V" Tags="|delta-v|saturn-v|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="3974" PostTypeId="2" ParentId="3970" CreationDate="2014-03-13T15:35:28.247" Score="11" Body="&lt;p&gt;One other thing to note is also that most of these computer generated graphics are prepared before the satellite even launches and might be used to get an impression of the whole mission, simulate satellites operation, assess mission feasibility, even to serve as a visualisation aid during design, selection and funding stages of the project. When the satellite is designed, engineers would build a computer generated 3D model of its parts and as assembled anyway, so taking those 3D models and running them through some eye-pleasing ray tracing software comes at little to no expense to mission planners, and agency's PR officers. And they tend to look a lot better and are available long before any &lt;em&gt;selfies&lt;/em&gt; would be, so they are used in public media earlier. Once that's set in motion, certain images of the satellite design might have already imprinted into people's memories and by now a recognizable public image of the whole mission will of course be used more often than some low-quality &lt;em&gt;selfies&lt;/em&gt;. Pre-existing CGI (often including mission demonstration videos) then remove that little potential value for vanity photography, unless they're already needed to conduct satellites mission anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if these cameras and the arms needed to distance them away from the satellite so the satellite can &lt;em&gt;self-reflect&lt;/em&gt; would be strictly for vanity purposes, such as &lt;em&gt;selfies&lt;/em&gt;, and public media already has press kit materials they need, then there isn't any technical and scientific justification to add such devices to them, and would only make them heavier and thus more expensive to launch. For self-inspection purposes, other types of sensors are more effective given the environmental constraints (lighting, solar radiation introducing sensor noise, atomic oxygen oxidizing exposed equipment, micrometeorites and dust disabling it altogether, and whatnot), they can be built into the hardware itself, are lighter, consume less power, and with no or fewer moving parts they're not just another point of failure with little to no benefit to the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If satellites would have sets of arms for &lt;em&gt;selfies&lt;/em&gt;, it would mean you're wasting precious weight and volume constraints, and the arms would likely have to be retracted during the launch into the satellite's body so it's weight-balanced, spin stable and fits into its payload slot, further reducing its usable volume and surface area that might otherwise be used for scientific equipment, propulsion and avionics systems, solar panels, heat radiators, heat insulation, and so on. Any such protruding arms would also open up and expose parts of the satellite to the environment once extended, and needlessly complicate satellite's design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in a nutshell, it might make sense to include such hardware if that's required for satellite's self-inspection (say, International Space Station has cameras on its truss and robotic arms, and is technically still an artificial satellite), but with most smaller satellites that rarely has any real value. But for better or worse, it's not unheard of that something like this has been done solely for vanity and PR purposes. From description of the &lt;a href=&quot;http://artemis-space.com/2013/12/19/artemis-open-space-programme-paves-the-way-for-civil-society-to-enter-space/&quot;&gt;ARTEMIS SmallSats Constellation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Users can even take “Selfies” and upload their images to the&#xA;  micro-satellites. The onboard cameras will capture the image of users&#xA;  Selfies against the background of Space and the Earth and will send&#xA;  the Selfie back to it’s user, creating a truly unique, once in a&#xA;  lifetime message.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-03-13T15:35:28.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3975" PostTypeId="2" ParentId="3972" CreationDate="2014-03-13T15:40:05.060" Score="25" Body="&lt;p&gt;The amount by which a spacecraft is able to change its velocity is called it's Δv (delta-velocity) budget. You can calculate the Δv-budget of each stage of a rocket using the &lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket_equation&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt; which reads:&#xA;$$&#xA;\Delta v = I_{sp} * 9.81  * \ln \frac {Mass_{full}} {Mass_{dry}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where I&lt;sub&gt;sp&lt;/sub&gt; is the specific impulse (&quot;fuel-efficiency&quot;) of the engine. The factor of 9.81 (the gravity of earth) in this context  is the factor used to convert specific impulse to exhaust velocity. You could also express the formula above using exhaust velocity, but I decided to use the specific impulse, because the numbers I found for the efficiency of the Saturn V were in I&lt;sub&gt;sp&lt;/sub&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The masses and I&lt;sub&gt;sp&lt;/sub&gt; of each stage can be looked up on &lt;a href=&quot;http://en.wikipedia.org/wiki/Saturn_V&quot;&gt;wikipedia&lt;/a&gt;. I made an overview as a table and calculated the delta-v using &lt;a href=&quot;http://www.strout.net/info/science/delta-v/intro.html&quot;&gt;this handy online calculator&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;      | Individual stage    | Total vessel         |&#xA;Stage | Full mass | Dry mass| Full mass | Dry mass | Isp   | Δv&#xA;------+-----------+---------+-----------+----------+-------+--------------&#xA;I     | 2,300,000 | 131,000 | 2,900,000 | 731,000  | 263s  | 3554.2 m/s&#xA;II    |   480,000 | 36,000  |   600,000 | 156,000  | 421s  | 5561.5 m/s&#xA;III   |   120,800 | 10,000  |   120,800 |  10,000  | 421s  | 8796.2 m/s&#xA;                                                            ===========&#xA;                                                            17911.9 m/s   &#xA;* masses are in kg&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;That means a Saturn V started in space could reach a speed of about &lt;strong&gt;18 km per second&lt;/strong&gt; or &lt;strong&gt;64,500 km/h&lt;/strong&gt; relative to its initial frame of reference. The speed of light is about 300,000 km per second, so this is still just about 0.006% of the speed of light. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This speed is just the rocket without any payload. When you add a payload, that mass needs to be added to the dry- and full-mass of the total vessel in each stage and the delta-v of each stage decreases accordingly. I didn't do the math, but when you would add the Apollo 11 payload, the 12 km/s stated in the answer by Mark Adler appears to be plausible (but note that the Apollo 11 payload would add three additional stages with additional Δv budget - the service module, the landing module descent stage and the landing module ascent stage).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the first two stages of the Saturn V were optimized for operating in the atmosphere of Earth, not for the vacuum of space. When you would build a rocket only for operating in vacuum, you could expect each stage to be as efficient as the 3rd of the Saturn V. Without the acceleration requirement during launch, you wouldn't need such large and heavy engines like those on the first stages of the Saturn V. You would reach a much higher Δv by having only a single small engine and dropping only the empty fuel tanks themself. But that still would not nearly be enough to get close to the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another interesting trivia fact: Incidentally, 18 km per second is quite close to the current speed of &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1&quot;&gt;Voyager I&lt;/a&gt;. Note that Voyager I was launched with a rocket much smaller than a Saturn V (a &lt;a href=&quot;http://en.wikipedia.org/wiki/Titan_IIIE&quot;&gt;Titan IIIE&lt;/a&gt;). How could it still reach such a high speed? By performing multiple &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist&quot;&gt;gravity slingshots&lt;/a&gt; around the various planets it flew by. What conclusion can we take from that? When you want speed, don't just add more power. Use the power you have in a smarter way.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-03-14T10:00:32.557" LastActivityDate="2014-03-14T10:00:32.557" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="3976" PostTypeId="2" ParentId="2071" CreationDate="2014-03-13T19:12:16.240" Score="2" Body="&lt;p&gt;What exactly is being measured?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's one's own timeline while travelling, then a wristwatch is fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's mis-synchronization of clocks that are distant and possibly moving, then one clock must be chosen as a standard, either distant and unchangeable (pulsars) or local and convenient (an accurate clock in Greenwich).&lt;/p&gt;&#xA;" OwnerUserId="1235" LastActivityDate="2014-03-13T19:12:16.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3977" PostTypeId="1" AcceptedAnswerId="3978" CreationDate="2014-03-13T19:54:32.060" Score="12" ViewCount="13351" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Phobos_%28moon%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phobos (one of Mars' moons)&lt;/a&gt; has the following dimensions: 27 × 22 × 18 km. It has low mass, and little or no gravity/atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;Let's just assume that it's possible to walk around on Phobos without dying or getting hurt. With it being so small, you should be able to just walk to one of the edges of Phobos.&lt;/p&gt;&#xA;&lt;p&gt;This brings me to my first questions:&#xA;Since there is so little gravity/atmosphere and it is so small, &lt;strong&gt;would you be able to &amp;quot;fall off&amp;quot; Phobos?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;This concept is hard for me to grasp because on Earth, it's hard to imagine to &amp;quot;fall off&amp;quot; the Earth, because you're never physically on an edge (since the Earth is round).&#xA;Phobos however doesn't seem to have a smooth round shape (and is &lt;em&gt;a lot&lt;/em&gt; smaller).&lt;/p&gt;&#xA;&lt;p&gt;Question number two:&#xA;&lt;strong&gt;When you're standing on of the edges of Phobos, would you be able to &amp;quot;stare down&amp;quot; into the empty vacuum of space?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;I compare this in my head with standing on the edge of a chair or table, looking down to the floor. Only in this case, you're staring straight down into space. Does this comparison make sense, or am I looking at this the wrong way?&lt;/p&gt;&#xA;" OwnerUserId="2855" LastEditorUserId="17520" LastEditDate="2022-10-07T08:49:02.153" LastActivityDate="2022-10-07T08:49:02.153" Title="Would it be possible to &quot;fall off&quot; Phobos?" Tags="|mars|moons|" AnswerCount="2" CommentCount="7" ClosedDate="2014-03-14T12:09:26.807" ContentLicense="CC BY-SA 4.0" />
  <row Id="3978" PostTypeId="2" ParentId="3977" CreationDate="2014-03-13T20:03:46.267" Score="11" Body="&lt;p&gt;In order to leave a gravity well, you need to exceed the &lt;a href=&quot;http://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;noreferrer&quot;&gt;escape velocity&lt;/a&gt;. The escape velocity depends on the mass of the body and your current distance from its center. The escape velocity on the surface of Phobos is approximately 11.4 m/s or 41 km/h. It likely varies depending on where you are on Phobos because the moon is quite irregular-shaped. You would expect it to be less when you are on top of a mountain than inside a crater, but it's hard to tell because we don't know much about Phobos' internal structure. When its internal density is as unsymmetrical as its external shape, its gravity field might have considerable local variations. But you can expect the escape velocity to be roughly about 40 km/h anywhere on Phobos' surface. When you manage to run or jump so fast that you exceed that speed, you would escape from Phobos and then end up in an orbit around Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reaching such a high speed using just your own body strength, especially while you are wearing a heavy space suit, is unlikely. But with a bit of technical help it might be possible. Riding a bicycle on Phobos might be enough (remember that bicycle-across-the-moon scene from E.T.? It would look a bit like that).&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-03-13T20:18:41.250" LastActivityDate="2014-03-13T20:18:41.250" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="3979" PostTypeId="2" ParentId="2071" CreationDate="2014-03-13T20:36:36.850" Score="4" Body="&lt;p&gt;For the kind of space travel we are performing in this day and age, time dilation effects are too small to be of much concern for mission planning. The time coordination of most space missions is synchronized to the time at the mission control. The International Space Station does its scheduling and daily routine according to Coordinated Universal Time (UTC).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you &lt;em&gt;would&lt;/em&gt; perform space travel with relativistic speeds (note that we are entering the realm of science fiction here), you have to consider what goal you want to reach with your time-keeping. When you need to check on your fusion reactor temperature every hour, you would do so according to a conventional clock in your own reference frame, because your reactor wouldn't care about how much time has passed on Earth. But when mission control back on Earth expects hourly status reports from you, you would have to separately track the time according to Earth's frame of reference to ensure that they arrive on time. Your earth-clock would have to be aware of your current speed in earth's frame of reference so it can adjust its speed accordingly. Also keep in mind that information can only travel with the speed of light, so the earth-clock would also need to be aware of your current distance from Earth (in Earths reference frame), and add that number of light-seconds to the time it displays.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2019-06-06T07:59:21.640" LastActivityDate="2019-06-06T07:59:21.640" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="3980" PostTypeId="2" ParentId="3977" CreationDate="2014-03-13T20:51:48.623" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;would you be able to &quot;fall off&quot; Phobos ?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Right now, no. In a few million years, yes. Phobos is slowly spiraling in towards Mars. In a few million years it will be close enough to Mars that tidal forces from Mars will tear Phobos apart. The very rocks that loosely comprise Phobos will fall off of Phobos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Phobos is 27 × 22 × 18 km. Your &quot;edge of Phobos&quot; is that point on Phobos that is the furthest from the center of Phobos, or about 13.5 km. I'll use this figure of 13.5 km as the basis for my answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, Phobos's orbit about Mars brings the center of Phobos to with 9234 km of the center of Mars. At this distance, the tidal acceleration toward Mars at a distance of 13.5 km from the center of Phobos is &lt;a href=&quot;http://www.wolframalpha.com/input/?i=%2842828+km%5E3%2Fs%5E2%29%2a%281%2F%289234+km+-+%2827+km%29%2F2%29%5E2+-+1%2F%289234+km%29%5E2%29&quot; rel=&quot;noreferrer&quot;&gt;1.47 mm/s&lt;sup&gt;2&lt;/sup&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's very small, but then again so is the gravitational acceleration toward Phobos. An overly simplistic calculation using $GM/r^2$ gives a value of &lt;a href=&quot;http://www.wolframalpha.com/input/?i=G%2a%281.0659e16+kg%29%2F%2813.5+km%29%5E2&quot; rel=&quot;noreferrer&quot;&gt;3.9 mm/s&lt;sup&gt;2&lt;/sup&gt;&lt;/a&gt; at the furthest most point of Phobos from its center of mass. Since Phobos isn't spherical, the gravitational acceleration toward Phobos will be a bit more than this at that distance. But it's not going to be a whole lot more than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tidal forces currently aren't enough to make you &quot;fall off&quot; Phobos. At least not yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They will be as Phobos spirals ever inward. The tidal forces from Mars will rip Phobos apart when Phobos falls to within less than about 7000 km from Mars. That will happen in a few million years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way to look at it: Phobos's Hill sphere is currently &lt;a href=&quot;http://www.wolframalpha.com/input/?i=%289234+km%29%2a%28%28phobos+mass%29%2F%283%2a%28mars+mass%29%29%29%5E%281%2F3%29&quot; rel=&quot;noreferrer&quot;&gt;16.37 km&lt;/a&gt;. That's just a tiny bit more than the 13.5 km figure used above. Phobos's Hill sphere will shrink as Phobos spirals inward. It will become less than 13.5 km when Phobos periapsis distance drops to about 7600 km. At that point Phobos will cease to exist since Phobos apparently is a rubble pile.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="921" LastEditDate="2014-03-14T11:20:48.063" LastActivityDate="2014-03-14T11:20:48.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3981" PostTypeId="2" ParentId="2071" CreationDate="2014-03-13T22:41:26.500" Score="8" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;With the time dilatation (and probably some other factors) one cannot simply rely on ones watch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Ultimately, a &quot;perfect watch&quot; is &lt;strong&gt;all&lt;/strong&gt; one can rely upon. A good approximation of a &quot;perfect watch&quot; is an atomic clock that is located at the center of mass of your (relativistic) spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic problem is that &lt;a href=&quot;http://en.wikipedia.org/wiki/Relativity_of_simultaneity&quot;&gt;simultaneity is relative&lt;/a&gt; even in special relativity. In general relativity, the simultaneity of relativity is a manyfold aggravated issue. Ben Franklin's statement that &quot;Three can keep a secret, if two of them are dead&quot; is very applicable to relativistic mechanics. How do you keep time, synchronously? As long as more than one are trying to do it, you can't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can (kind of) pretend that relativistic effects are linear, and in that case you end up with something like JPL's ephemeris time T&lt;sub&gt;eph&lt;/sub&gt; or the IERS's/IAU's TCB (barycentric coordinate time) or TDB (barycentric dynamic time). None of them are perfect; they can't be.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-13T22:41:26.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3982" PostTypeId="1" AcceptedAnswerId="3983" CreationDate="2014-03-14T09:13:41.317" Score="6" ViewCount="2507" Body="&lt;p&gt;After reading &lt;a href=&quot;https://space.stackexchange.com/questions/3970/why-are-photos-of-satellites-most-often-computer-generated&quot;&gt;this question&lt;/a&gt; and seeing &lt;a href=&quot;http://apod.nasa.gov/apod/ap090116.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;an image of ISS&lt;/a&gt; shot from earth, a doubt came in mind that if shooting moon from earth with high details is possible, why is it difficult to shoot artificial satellites from earth?&lt;/p&gt;&#xA;" OwnerUserId="2860" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2018-09-04T06:26:52.920" Title="Why is taking a picture of the moon surface from earth possible but taking one of an artificial satellite is difficult?" Tags="|the-moon|artificial-satellite|telescope|photography|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3983" PostTypeId="2" ParentId="3982" CreationDate="2014-03-14T11:22:21.163" Score="24" Body="&lt;p&gt;There are two parts to this question. One is around speed (or apparent angular velocity) and one is around size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon's apparent velocity is small, so it is easy to set up a telescope or camera to take a photograph at an appropriate exposure. Many satellites, and the ISS are much closer, so the apparent velocity from the observer's perspective is a problem - you need to track at speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon is big. The fact that you can take a good picture of it doesn't mean you are taking a picture at high detail. In fact you are getting pictures of the moon at probably lower detail than pictures of closer satellites. But the size of the moon means this probably isn't an issue. Being able to resolve a rock 50m across on the moon is overkill for many purposes, but even 20m resolution when trying to photograph a satellite probably means you can't see it at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also an issue around brightness - satellites have limited periods of visibility as they move in and out of the Earth's shadow, and typically are not visible at all during the day (obvious exceptions include Iridium flashes - but you need to know where and when to look). The moon reflects a lot of light, which makes finding, focusing and tracking it very simple.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2014-03-14T11:33:51.353" LastActivityDate="2014-03-14T11:33:51.353" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3986" PostTypeId="2" ParentId="3982" CreationDate="2014-03-14T12:27:14.623" Score="8" Body="&lt;p&gt;Really simplified answer: Anyone can see the moon. It's huge. You normally can't see an artificial satellite, it's tiny. When you can see it, it's a tiny point, like a distant star. Same goes for a camera / telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you call &quot;High detail&quot; of the moon is probably tens or hundreds of metres across per pixel on the moons surface. A satellite is just not that big.&lt;/p&gt;&#xA;" OwnerUserId="2863" LastEditorUserId="2863" LastEditDate="2018-09-03T13:27:57.337" LastActivityDate="2018-09-03T13:27:57.337" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="3987" PostTypeId="1" AcceptedAnswerId="18057" CreationDate="2014-03-14T12:43:56.937" Score="26" ViewCount="8750" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=wuNGQPBZv0E&amp;amp;t=39&quot; rel=&quot;noreferrer&quot;&gt;This video&lt;/a&gt; shows the stage separation of the 1st stage of Apollo 4 (incorrectly identified as Apollo 11) followed by the jettisoning of the ring-shaped fairing between the two stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The moment the fairing is jettisoned (at 0:40), it appears to catch fire and burns in a very spectacular-looking way. What exactly caused this effect?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Lspii.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Lspii.jpg&quot; alt=&quot;Still image of burning interstage ring from Apollo 4&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="195" LastEditDate="2020-04-13T16:05:34.883" LastActivityDate="2020-06-13T22:46:38.710" Title="Why did the Saturn V stage fairing appear to burn after separation?" Tags="|launch|stages|upper-stage|saturn-v|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="3988" PostTypeId="2" ParentId="3987" CreationDate="2014-03-14T13:21:01.480" Score="17" Body="&lt;p&gt;The burning seems to be after the 5 J-2 engines on the second stage fire. Each J-2 has about 230KLbs of thrust for well over a million lbs of thrust aggregate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ANYTHING exposed to 1 million lbs of LOX/LH thrust is very likely to burn. No doubt they ran some oxygen and hydrogen through the engines to pre-cool them before igniting, (like the SSME does, like most engines do) so there was likely a local non-vacuum environment for a short period of time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sort of like how a Delta-4 Heavy's hydrogen venting for engine preconditioning hangs around enough to light the outside on fire.  Just with less oxygen in the Saturn V case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, I think you are seeing little ullage style engines firing to force a tumble onto the interstage for a more destructive reentry. &lt;a href=&quot;http://en.wikipedia.org/wiki/Saturn_V&quot;&gt;Wikipedia&lt;/a&gt; says they were solid rockets:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Eight small solid fuel separation motors backed the S-IC from the interstage at an altitude of about 67 kilometers (42 mi).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-03-14T13:21:01.480" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3989" PostTypeId="2" ParentId="3987" CreationDate="2014-03-14T13:21:08.210" Score="7" Body="&lt;p&gt;My guess would be it's predominantly due to cryo-pumping, cryogenic propellants in the interstage liquefying and solidifying atmospheric gases, among which oxygen, because of liquid hydrogen propellant temperatures being low enough to liquefy atmospheric air on contact when Saturn V was still on the launchpad (but fuelled), so the whole interstage ring held onto it and provided oxidizer for the excess hydrogen (low oxidizer to fuel mixture) in the exhaust of the upper stage. Notice chunks of ice falling off the interstage (and the upper stage) on separation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's first see what we're talking of here. From &lt;a href=&quot;http://history.msfc.nasa.gov/saturn_apollo/documents/Second_Stage.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V Second Stage Fact Sheet&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lFWE6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Interstage&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The interstage, fabricated at NAA's Tulsa plant, is a semimonocoque&#xA;  structure. Semimonocoque means that the skin has a minimum of internal&#xA;  framework. The interstage is slightly over 18 feet in height and 33&#xA;  feet in diameter. The structure has internal circumferential&#xA;  supporting frames and external hat sections positioned vertically to&#xA;  provide structural rigidity.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;After first stage burnout and initial separation, eight rocket motors&#xA;  attached equidistantly around the interstage are fired for&#xA;  approximately 4 seconds. These motors, called ullage motors (an old&#xA;  brewer's term referring to the gaseous zone in a tank above the&#xA;  liquid), provide positive acceleration and therefore pressure to force&#xA;  the stage's propellants into the feed lines to the J-2 engines. This&#xA;  is called the ullage maneuver. The interstage is separated from the&#xA;  second stage approximately 30 seconds after it separates from the&#xA;  first stage. The two-step separation of the interstage is called&#xA;  dual-plane separation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This means that the burn you see after the interstage separation can't be from the ullage motors, because they're fired to provide acceleration for the second stage propellants before the interstage separation and the video you link to shows the second stage engine has already been ignited. Plus, the ullage motors fire externally to the interstage, and the video clearly shows burning also within the interstage ring, perhaps more so than the burn external to it. The second stage interstage also separated the Saturn V &lt;a href=&quot;https://en.wikipedia.org/wiki/File:SaturnV_S-IC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;first stage&lt;/a&gt; and the second stage LOX tanks, so the interstage provides a leak path for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_oxygen&quot; rel=&quot;nofollow noreferrer&quot;&gt;LOX&lt;/a&gt; cryogenic oxidizer, and some of it would slush on contact with the interstage and form chunks of ice, mixing leaked cryogenic oxidizer, atmospheric air and nitrogen gas used for purging the excess gases in the interstage. Due to the nitrogen purging, there wouldn't be much of it, and indeed there isn't as seen in the video, but some there clearly is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a nutshell, the interstage brought along with it ice chunks of Earth's atmosphere that facilitated the burn with its oxygen where you wouldn't really expect it, once they started to outgas due to sublimation in hard vacuum and heat from the upper stage exhaust. Part of the required oxygen might also have been provided by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Titanium_dioxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;titanium dioxide&lt;/a&gt; white paint on the interstage itself, with oxygen releasing from them at the temperatures of the upper stage exhaust, or heat insulation materials used in the inner side and soaked in cryogenic propellants (cryo-ingestion of leaked propellants mixed with nitrogen used for purging them from the interstage) catching fire.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I discuss cryo-pumping and cryo-ingestion in more detail in my answer to &lt;a href=&quot;https://space.stackexchange.com/a/3404/49&quot;&gt;Why was the Shuttle's LOX tank on top of the LH2 tank, since that makes it more top-heavy?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-03-17T12:40:09.010" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3990" PostTypeId="2" ParentId="3970" CreationDate="2014-03-14T14:18:32.530" Score="3" Body="&lt;p&gt;Additional to the other answers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;if you would use your upper stage to take photos of your satellite, and even add lighting equipment you still would not get a nice shot, because most satellites take some time to reach their final configuration - they have to unfold solar panels, etc.&lt;/p&gt;&#xA;" OwnerUserId="2866" LastEditorUserId="49" LastEditDate="2014-03-14T14:23:09.137" LastActivityDate="2014-03-14T14:23:09.137" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="3991" PostTypeId="1" AcceptedAnswerId="3993" CreationDate="2014-03-14T14:32:06.313" Score="4" ViewCount="652" Body="&lt;p&gt;Let's say you are standing on the Moon, with the Earth directly above your head (the moon being &lt;a href=&quot;http://en.wikipedia.org/wiki/Tidal_locking&quot; rel=&quot;nofollow&quot;&gt;tidally locked&lt;/a&gt; with the earth. At this point you and the Moon are sharing an orbit, both experiencing the same orbital elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You climb into a cannon that has just enough force for you to reach escape velocity (or, maybe more appropriately, leave the Moon's sphere of influence). Pull the trigger, and force is applied orthogonal to your velocity vector. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does your new orbit differ from the Moon's? What orbital elements changed in this process? Would your orbital period relative to the moon decrease?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These answers are all really great, but I made things too complicated by involving the moon. Instead, more simply, how do orbital elements change when force is applied orthogonal to the velocity vector of an orbiting mass? Think about a satellite in GSO. &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-03-17T19:39:43.427" LastActivityDate="2014-03-17T19:39:43.427" Title="How do orbital elements change when force is applied orthogonal to the velocity vector?" Tags="|orbital-mechanics|the-moon|soi|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="3992" PostTypeId="2" ParentId="3970" CreationDate="2014-03-14T15:08:40.757" Score="3" Body="&lt;p&gt;There is a matter of information security as well.  Satellites are expensive, both for governments and businesses, and providing your competitors with information to reverse engineer your intellectual property doesn't make any sense.  Any photograph may very well contain sensitive information about the construction of that satellite that an intelligent engineer can utilize to aid in reverse engineering.  The great thing about the computer generated models is that you can control what the public sees.   &lt;/p&gt;&#xA;" OwnerUserId="2868" LastActivityDate="2014-03-14T15:08:40.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3993" PostTypeId="2" ParentId="3991" CreationDate="2014-03-14T16:43:03.833" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Would your orbital period relative to the moon decrease?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Somewhat surprisingly, yes. You will end up in a slightly smaller orbit about the Earth with a slightly smaller period. That's assuming that (1) a patched conic is a valid approach (it probably isn't in this case), and (2) I've done my math right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll assume the Moon is in a circular orbit at 384500 km, moving at 1.022 km/s relative to the Earth. I'll assume you fire the canon at the sub-Earth point at a velocity of 2.375 km/s, aiming straight towards the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using a patched conic approximation, you are on a radial parabolic trajectory. It will take &lt;a href=&quot;http://www.wolframalpha.com/input/?i=%28sqrt%282%29%2F3%29%2a%28sqrt%28%2866000+km%29%5E3%2F%284902.8000+km%5E3%2Fs%5E2%29%29+-+sqrt%28%281784+km%29%5E3%2F%284902.8000+km%5E3%2Fs%5E2%29%29%29&quot; rel=&quot;nofollow&quot;&gt;1.315 days&lt;/a&gt; or &lt;a href=&quot;http://www.wolframalpha.com/input/?i=%281.3153445+days%29%2F%2827.321661+days%29&quot; rel=&quot;nofollow&quot;&gt;0.04814 sidereal months&lt;/a&gt; to reach a distance of 66000 km from the center of the Moon (that's the radius of Moon's sphere of influence; c.f. the 60000 km radius of the Moon's Hill sphere), at which point that initial 2.375 km/s velocity will have dropped to &lt;a href=&quot;http://www.wolframalpha.com/input/?i=sqrt%282%2a%284902.8000+km%5E3%2Fs%5E2%29%2F%2866000+km%29%29&quot; rel=&quot;nofollow&quot;&gt;0.3854 km/s&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Converting to geocentric coordinates, you will be &lt;a href=&quot;http://www.wolframalpha.com/input/?i=sqrt%28%28384500+km%29%5E2+-+2%2a%28384500+km%29%2a%2866000+km%29%2acos%282%2api%2a0.04814292%29+%2B+%2866000+km%29%5E2%29&quot; rel=&quot;nofollow&quot;&gt;322100 km&lt;/a&gt; from the center of the Earth and moving at a speed of &lt;a href=&quot;http://www.wolframalpha.com/input/?i=sqrt%28%28%281.022+km%2Fs%29%2acos%282%2api%2a0.04814292%29%29%5E2+%2B+%28%281.022+km%2Fs%29%2asin%282%2api%2a0.04814292%29+%2B+%280.3854474+km%2Fs%29%29%5E2%29&quot; rel=&quot;nofollow&quot;&gt;1.195 km/s&lt;/a&gt; relative to the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the vis-viva equation, this means you will be in an elliptical orbit about the Earth with a semi-major axis of &lt;a href=&quot;http://www.wolframalpha.com/input/?i=1%2F%282%2F%28322097+km%29+-+%281.195+km%2Fs%29%5E2%2F%28398600.4418+km%5E3%2Fs%5E2%29%29&quot; rel=&quot;nofollow&quot;&gt;380700 km&lt;/a&gt;. That orbit has a period of &lt;a href=&quot;http://www.wolframalpha.com/input/?i=2%2api%2asqrt%28%28380700+km%29%5E3%2F%28398600.4418+km%5E3%2Fs%5E2%29%29&quot; rel=&quot;nofollow&quot;&gt;27.056 days&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You won't stay in that orbit for long. The Moon is going to perturb that orbit, big time.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-14T17:39:09.567" LastActivityDate="2014-03-14T17:39:09.567" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="3995" PostTypeId="1" AcceptedAnswerId="4041" CreationDate="2014-03-14T21:20:06.537" Score="30" ViewCount="8207" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Transposition,_docking,_and_extraction&quot; rel=&quot;noreferrer&quot;&gt;Transposition, Docking and Extraction&lt;/a&gt; (TD&amp;amp;E) maneuver was done during the Apollo Moon missions right after the translunar injection. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot; rel=&quot;noreferrer&quot;&gt;Command/Service Module&lt;/a&gt; (CSM) separated from the &lt;a href=&quot;https://en.wikipedia.org/wiki/S-IVB&quot; rel=&quot;noreferrer&quot;&gt;S-IVB&lt;/a&gt;, pitched 180 degrees, docked with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;Lunar Module&lt;/a&gt; (LM) and extracted it from the upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/d/dc/Apollo11-03.png/320px-Apollo11-03.png&quot; alt=&quot;TD&amp;amp;E, part 1&quot;&gt;&#xA;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Apollo11-04.png/320px-Apollo11-04.png&quot; alt=&quot;TD&amp;amp;E, part 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess the reason for this delicate and potentially risky maneuver was that the 2 modules had to be stacked like that (CSM on top of the LM) on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;noreferrer&quot;&gt;Saturn V&lt;/a&gt; rocket due to aerodynamics, but during the mission the order had to be switched, so the service module engine could perform the &lt;a href=&quot;http://history.nasa.gov/afj/loiessay.htm&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbit Insertion&lt;/a&gt;. Is that correct?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were there some other options considered? Was the engineering team trying to find a way how to avoid this maneuver somehow and simplify the mission profile? What led them to pursue this setup in the end?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm speculating that the chosen design meant the least possible load for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Launch_escape_system&quot; rel=&quot;noreferrer&quot;&gt;Launch Escape System&lt;/a&gt; (LES), because anything else I can imagine means the LES would have to carry more mass: CSM installed upside down (docked to the LM) would mean the LES must carry not only CM, but also SM. More mass for LES requires more fuel in the LES which is more mass to carry for the first stage with all the rocket equation problems. Unfortunately I cannot find a relevant resource to confirm this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a plenty of online resources that describe the details of the procedure as such (which I already understand), but not the reasons why it must have been done in the first place. Even though I feel I probably understand at least part of the reason, a reliable reference would be nice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Checked resources:&lt;br&gt;&#xA;&lt;a href=&quot;http://www.ehartwell.com/InfoDabble/Apollo_lunar_mission_tutorial:_Transposition,_Docking_and_Extraction&quot; rel=&quot;noreferrer&quot;&gt;Apollo mission tutorial&lt;/a&gt; only describes the procedure.&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Transposition,_docking,_and_extraction&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; only describes the procedure.&lt;br&gt;&#xA;&lt;a href=&quot;http://history.nasa.gov/ap11fj/03tde.htm&quot; rel=&quot;noreferrer&quot;&gt;The Apollo 11 Flight Journal&lt;/a&gt; only describe the procedure.&lt;br&gt;&#xA;&lt;a href=&quot;http://books.google.cz/books?id=x-taL4N0sjIC&amp;amp;pg=PA133&amp;amp;lpg=PA133&amp;amp;dq=transposition,%20docking%20and%20extraction&amp;amp;source=bl&amp;amp;ots=v7awJXtXXO&amp;amp;sig=nV3_TVDjUo118rRaamfHhfmLpUc&amp;amp;hl=cs&amp;amp;sa=X&amp;amp;ei=CD0oU46tE4nxhQfepIH4Bg&amp;amp;ved=0CGoQ6AEwBQ#v=onepage&amp;amp;q=transposition,%20docking%20and%20extraction&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;How Apollo flew to the Moon book&lt;/a&gt; - I only have this preview, not the full book. The available sections again deal with the procedure, not the reasons behind.&lt;br&gt;&#xA;Etc.&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="20089" LastEditDate="2017-06-17T04:00:04.027" LastActivityDate="2017-06-24T19:32:28.327" Title="Reasons behind the &quot;Transposition, docking and extraction&quot; maneuver" Tags="|apollo-program|docking|saturn-v|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3996" PostTypeId="2" ParentId="3741" CreationDate="2014-03-14T21:23:01.147" Score="8" Body="&lt;p&gt;&lt;strong&gt;Not only it is stored separately, it is also supplied to the International Space Station (ISS) as two different waters. But if water from both sources was consumed, silver iodide precipitate wouldn't form, since the U.S. segment water is deiodinated before it's used for human consumption.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Silver iodide is actually a &lt;a href=&quot;https://en.wikipedia.org/wiki/Cloud_seeding&quot; rel=&quot;noreferrer&quot;&gt;cloud seeding&lt;/a&gt; agent and while I personally doubt it has any serious consequences to our health, the scientific discussion on that front is &lt;a href=&quot;http://toxnet.nlm.nih.gov/cgi-bin/sis/search/a?dbs%20hsdb%3a@term%20@DOCNO%202930&quot; rel=&quot;noreferrer&quot;&gt;still ongoing&lt;/a&gt;. But, luckily for us, we don't even have to consider that as a possibility, or what happens when you'd drink both Russian and U.S. segment waters onboard the ISS and the silver-iodide precipitate formed. Here's a nice excerpt from an &lt;a href=&quot;http://www.spacesafetymagazine.com/2011/10/27/americans-russians-drink-waters-iss/&quot; rel=&quot;noreferrer&quot;&gt;article in Space Safety Magazine&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Since 2008 the SMAT (Metropolitan Water Company Turin) is  supplying&#xA;  the space station with drinking water, truly two different kind of&#xA;  water for the US and Russian segment. Both segments require a very&#xA;  stable water, able to maintain its properties for months in zero&#xA;  gravity, but while the Americans prefer little mineralized water, &#xA;  treated with iodine salts, the Russians prefer heavier waters, with&#xA;  more minerals and treated with silver salts.  For the Americans, the&#xA;  SMAT selected the very light water from Pian della Mussa, coming down&#xA;  from the valleys of Lanzo. For the Russians, the water comes from&#xA;  wells located in the region of Collegno because of the mineral&#xA;  content. A sampling of these waters was sent to NASA, which certified&#xA;  it after thorough checks.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More is explained in the linked to article, but as you see, astronauts and cosmonauts drink different water onboard the ISS, each from their own stored supplies. How strictly this &lt;em&gt;no water mixing&lt;/em&gt; rule is enforced, I wouldn't know, but as mentioned, I wouldn't think it would make much of a problem in quantities a person consumes. Or a few of them. Besides, U.S. potable water is deiodinated before consumption, as the article later explains, so that's even less of an issue then, if it's unlikely to happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you can't mix the two separately stored water supplies, complete with their different disinfecting agents, because water is used also for other purposes (coolant, source of oxygen via electrolysis,...) and the silver-iodide precipitate would find its way directly into systems that aren't designed to handle it well. And I believe that's what the article that you're quoting is suggesting.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-16T18:32:44.927" LastActivityDate="2014-03-16T18:32:44.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="3997" PostTypeId="2" ParentId="3995" CreationDate="2014-03-14T21:23:51.667" Score="12" Body="&lt;p&gt;Probably the biggest reason is they did not want to cut a hole through the heat shield, same reason the &lt;a href=&quot;https://space.stackexchange.com/questions/3220/why-is-the-descent-module-of-the-soyuz-in-the-middle-of-its-stack&quot;&gt;Soyuz has the ascent/descent element in the middle of the stack&lt;/a&gt;, instead of at the top. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the Lunar module is not very robust, and probably needed the structural bracing for launch. So probably would not have worked well on top of the Command Module.  It would have made the Launch Escape System much harder or more expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus Soyuz is your counter example on a much smaller scale.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-03-14T21:23:51.667" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="3998" PostTypeId="2" ParentId="3995" CreationDate="2014-03-14T23:44:27.967" Score="2" Body="&lt;p&gt;The Saturn V was equipped with a launch escape system. In case of a dangerous malfunction during launch, there was a smaller rocket on top of the Saturn V which had the job to drag the crew capsule away from the rest of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This could only work when the crew was in the topmost part of the rocket. The LM could only house two crewmembers, so all crewmembers had to be in the command module during launch, so the CM had to be the topmost part of the payload.&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2014-03-14T23:44:27.967" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="3999" PostTypeId="2" ParentId="3991" CreationDate="2014-03-15T07:14:34.937" Score="1" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/kPQbh.png&quot; alt=&quot;3 body sim&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a screen capture from an orbital sim for a variety of pellets launched straight up from the moon's near point. The red #1 pellet is just under escape velocity. The blue #11 is a hair over. Tidalwave, The blue dot is the earth. The white orbit is the moon. I placed darker colored pellets at EML1, EML2 as well as EML4 and EML5.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rainbow pellets are from what I call orbital shotgun. The pellets velocity and direction vary from one extreme (pellet one) to another (Pellet 11). Here is the page I used to do this sim: &lt;a href=&quot;http://clowder.net/hop/railroad/emlShotGun.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Shotgun&lt;/a&gt; Making smaller time increments makes the model more accurate but slower. I had to make time increments pretty small for this sim. It can be seen that pellet 1 doesn't quite make it past EML1 and falls back to the moon. The rest of the pellets escape the moon's sphere of influence and fall into earth centric orbits&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general pellets the pellets will hit the earth, moon, or be completely ejected. Just a tiny variation in velocity can make a huge difference in outcome. In this diagram the fastest pellet varies from the slowest by .6 meters/second:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/6A57R.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-03-15T20:23:06.830" LastActivityDate="2014-03-15T20:23:06.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4000" PostTypeId="1" AcceptedAnswerId="4033" CreationDate="2014-03-15T13:31:44.197" Score="13" ViewCount="2182" Body="&lt;p&gt;Repeated Google searches over the past year, as well as recent searches of NTRS, AIAA, and arXiv, show no signs that anyone has seriously considered a mission to Sedna. I would like to propose a thought problem to further explore how one can design this mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the elements of my Lambert’s problem:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Launch date: January 1, 2026&lt;/li&gt;&#xA;&lt;li&gt;Arrival date: July 1, 2076&lt;/li&gt;&#xA;&lt;li&gt;Position 1: circular Earth orbit of 2000 km&lt;/li&gt;&#xA;&lt;li&gt;Position 2: Sedna at perihelion&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For my vehicle, let’s say I have a 17U cubesat into which I’ve managed somehow to shoehorn an &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576511001834&quot;&gt;indium needle FEEP engine&lt;/a&gt; capable of a maximum thrust of 100μN, and say I have magically reduced the power/science package/structural elements so that my mass ratio is 50, and I’ve revved my Isp up to 30,000. My orbital pre-maneuver mass is 500 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Any chance I could get to Sedna for a ballistic orbital capture well before perihelion, with a little gas left in the tank?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: besides a gestimate of feasibility, I am interested in getting an idea about the work flow of the trajectory design process, i.e., what other assumptions need to be made, how long would it take to calculate, etc.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2015-08-31T03:54:10.720" LastActivityDate="2016-08-15T00:35:46.660" Title="What's the best flightpath to get to Sedna by 2076?" Tags="|trajectory|mission-design|gravity-assist|sedna|" AnswerCount="5" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="4001" PostTypeId="1" AcceptedAnswerId="4005" CreationDate="2014-03-15T14:11:12.447" Score="7" ViewCount="2020" Body="&lt;p&gt;One recurring idea is to use continuous thrust in a spacecraft to create artificial gravity. Of course this would need a whole lot of fuel and is thus very impractical. But how impractical exactly? I tried to solve this; my question is whether &lt;strong&gt;this is correct:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, we need the travel time if we accelerate constantly, for the first half of the trip, and break for the second half:&#xA;$$ t_t = \sqrt{\frac{x}{a{}}}$$&#xA;with acceleration a and distance x&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the travel itself we have the following relationship:&#xA;$$\dot{m} v_e = m(t)a$$&#xA;with $\dot{m}$ being the mass stream of the exhaust and $v_e$ its velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I solve this - but I'm not good at differential equations - to:&#xA;$$m(t)=m_0 e^{-\frac{a}{v_e}t}$$&#xA;and&#xA;$$\dot m(t)=-m_0 \frac{a}{v_e} e^{-\frac{a}{v_e}t}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a certain payload, $m_t$ I want to get to the target, so knowing my time to target $t_t$ I get&#xA;$$m_0=m_t e^{2\sqrt{\frac{x}{a}}\frac{a}{v_e}}$$&#xA;or, simpler,&#xA;$$m_0=m_t e^{t_t\frac{a}{v_e}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My fuel need, then, is the difference between $m_t$ and $m_0$.&lt;/p&gt;&#xA;" OwnerUserId="102" LastEditorUserId="102" LastEditDate="2014-03-15T16:29:29.330" LastActivityDate="2017-01-12T16:06:59.887" Title="Constant acceleration Rockets" Tags="|rockets|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4002" PostTypeId="2" ParentId="3991" CreationDate="2014-03-15T17:34:04.950" Score="4" Body="&lt;p&gt;&quot;Your new orbit&quot; isn't simply a two-body orbit about the Earth that you might be thinking of, since the Moon is still there after you (barely) escape.  In the long term it will be a complicated trajectory that will eventually result in you impacting the Moon, escaping the Earth-Moon system, or possibly even impacting the Earth.  So the concept of orbital elements is not terribly useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, you can imagine an effective orbital period when you are orbiting the Earth but are far from the Moon.  That is related to your total specific energy, where if that is less than the Moon's, your orbital period is less than the Moon's, and vice-versa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some examples of your trajectory where you are launched at or a little above the lunar escape velocity.  (The gray circle is the Moon's orbit.  The blue disk in the middle is the Earth to-scale.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shown under each trajectory is your specific energy in the Earth-Moon system.  You can see that initially it is always less than the Moon's (the gray line), and so your &quot;orbital period&quot; is shorter than the Moon's.  But it can go above the Moon's after a few lunar flybys, and usually does.  Where the plot doesn't go all the way across, it impacted the Moon where it ended.  In one case, you can see that you escape the Earth-Moon system completely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/kCBHw.png&quot; alt=&quot;plots of trajectories and specific energy&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some interesting borderline cases.  Below 0.978 they all do the same thing -- immediately reimpact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9BTXL.png&quot; alt=&quot;plots of borderline cases&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-03-15T22:57:12.633" LastActivityDate="2014-03-15T22:57:12.633" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="4003" PostTypeId="1" AcceptedAnswerId="12882" CreationDate="2014-03-15T21:23:13.893" Score="21" ViewCount="7391" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_1&quot;&gt;Vanguard 1&lt;/a&gt; holds the record for being in space longer than any other man-made object, along with the upper stage of its booster. It was launched on March 17, 1958, but was last heard from in May of 1964.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What still-orbiting satellite that is still transmitting a signal holds the record as oldest?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any older-still satellites from which we could potentially get a signal if they were reactivated (such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Prospero_%28satellite%29#Current_status&quot;&gt;Prospero&lt;/a&gt;)?&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-03-15T22:03:09.873" LastActivityDate="2017-04-19T10:08:03.823" Title="What is the oldest orbital satellite we can contact?" Tags="|artificial-satellite|history|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4005" PostTypeId="2" ParentId="4001" CreationDate="2014-03-15T21:57:32.090" Score="7" Body="&lt;p&gt;You are missing a factor of two in your exponent. The half-way time is $t_h = \sqrt{x/a}$, so the total time is twice that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, you apparently incorporated that factor of two here:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$m_0 = m_t \exp\left(2\sqrt{\frac x a}\frac a {v_e}\right)$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note that this simplifies to $m_0 = m_t \exp\left(\frac {2\sqrt{a x}} {v_e}\right)$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Suppose you want to go a mere 384,500 km (the mean distance between the Earth and the Moon) at 1g and are using a chemical rocket with an effective exhaust velocity of 4400 m/s. This means your initial mass is 1.32×10&lt;sup&gt;12&lt;/sup&gt; times the final mass. Another way to look at it: Over 99.9999999999% of the initial mass of the vehicle is fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We don't know how to build such a vehicle. We will &lt;em&gt;never&lt;/em&gt; know how to build such a vehicle. A realistic rocket has the fuel comprising at most 90 to 95 percent of the initial mass. Your $m_t$ isn't just payload. It also includes the rocket, structure, and empty fuel tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way around the nastiness of the ideal rocket equation is to use multistage rockets. The Apollo vehicles, for example, were essentially a six stage rocket. It launched with the three stage Saturn rocket, and the payload itself was another three stage rocket. This helps, but not near enough so that you can accelerate / decelerate the whole way. The Apollo vehicles coasted most of the way to the Moon and then coasted most of the way back to Earth. The vehicle that is en route to Pluto is coasting and has been for years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way around this is to use a much lower acceleration. Why do you need to go at 1g? Yet another way around this is to use a better rocket. 4400 m/s is barely enough to get us off the surface of the Earth. Ion thrusters have a much higher specific impulse. However, ion thrusters have no &lt;em&gt;oomph&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Back to your original question,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One recurring idea is to use continuous thrust in a spacecraft to create artificial gravity. Of course this would need a whole lot of fuel and is thus very impractical. But how impractical exactly?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The answer is that it is extremely impractical with the rockets we now know how to make. Maybe it would be possible if we had nuclear rockets, or even better, antimatter rockets. We don't. For now, those are the stuff of science fiction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;High specific impulse engines do fire all the time, but not to create artificial gravity.  Ion thrusters have to fire almost all the time because that's the only way to make the rocket go anywhere. For example, it took over a year for SMART-1 to reach the Moon.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-01-12T16:06:59.887" LastActivityDate="2017-01-12T16:06:59.887" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4006" PostTypeId="2" ParentId="1564" CreationDate="2014-03-15T22:17:43.667" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What is the oldest that's still in use?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Depending on what &quot;still in use&quot; means, I would venture that it is LAGEOS-1, launched on 4 May 1976. That's two years before GOES-3, and over a year before the Voyager spacecraft. The LAGEOS satellites are simple to the extreme. They are balls of solid brass covered with retroreflectors, and that's it. No sensors, no effectors, no attitude control, no comm, no avionics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Researchers ping those retroreflectors with ground-based lasers. That was and remains the intended use of these satellites. The returns from those pings tells researchers range, range rate, and bearing. From that, the orbits of the LAGEOS satellites can be determined with very high precision. From those very precise orbit determinations, LAGEOS provides insight on diverse topics such as the validity of general relativity, the shape of the Earth and the distribution of mass within the Earth, and plate tectonics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LAGEOS satellites will cease to serve their initial function when they deorbit (that's million of years from now), when their retroreflectors degrade to such an extent that they can't be seen (that's also a long time), or when they are smashed by another satellite or by orbital debris.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-15T22:22:59.087" LastActivityDate="2014-03-15T22:22:59.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4007" PostTypeId="2" ParentId="233" CreationDate="2014-03-16T01:12:02.310" Score="8" Body="&lt;p&gt;L4 and L5 are stable in an ideal circular 3 body scenario where the central body is 26 times more massive than orbiting body. But that isn't a very accurate model for the real world. While the sun is a lot more massive than 26 times earth's mass, there are more than 3 bodies exerting an influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I included the Sun's influence in my orbit sims, the Earth-Moon L4 and L5 were destabilized by the Sun's influence. I haven't looked at Sun-Earth L4 and L5, but I would expect Venus' influence to destabilize Earth trojans. Venus would come within 0.28 AU of an earth trojan each 1.6 years (Venus Earth synodic period). Jupiter also exerts a substantial tug. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter has a bunch of trojans. But, outside of the Sun, Jupiter is by far the biggest frog in the pond. Jupiter's trojans are less vulnerable to perturbations by the weaker planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I would venture to guess the escape velocity from Sun-Earth trojan regions is 0 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Did some Googling, there's an Earth trojan &lt;a href=&quot;http://en.wikipedia.org/wiki/2010_TK7&quot; rel=&quot;noreferrer&quot;&gt;2010 TK7&lt;/a&gt;. But it's not very stable. Wikipedia says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;2010 TK7's orbit has a chaotic character, making long-range&#xA;  predictions difficult. Prior to A.D. 500, it may have been oscillating&#xA;  about the L5 Lagrangian point (60 degrees behind Earth), before&#xA;  jumping to L4 via L3. Short-term unstable libration about L3, and&#xA;  transitions to horseshoe orbits are also possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-02-13T16:30:15.460" LastActivityDate="2015-02-13T16:30:15.460" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4008" PostTypeId="2" ParentId="3904" CreationDate="2014-03-16T01:34:23.190" Score="3" Body="&lt;p&gt;The moon's orbital is increasing due to &lt;a href=&quot;http://en.wikipedia.org/wiki/Tidal_acceleration&quot; rel=&quot;nofollow&quot;&gt;tidal acceleration&lt;/a&gt;. This will happens to satellites whose orbital radius is above geosynchronous orbit (about 36,000 km above earth's surface).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But ISS is well below geosynchronous orbit. So it experience tidal &lt;em&gt;deceleration&lt;/em&gt;. It's being pulled closer to the earth like Phobos is slowly getting closer to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would expect ISS' tidal deceleration to be tiny in comparison to deceleration from atmospheric friction though.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-03-16T01:34:23.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4009" PostTypeId="2" ParentId="1564" CreationDate="2014-03-16T01:55:18.020" Score="23" Body="&lt;p&gt;The first satellites listed by &lt;a href=&quot;https://celestrak.org/satcat/search.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;SATCAT&lt;/a&gt; with a status of + (operational) are &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/calsphere-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;CALSPHERE 1 &amp;amp; 2&lt;/a&gt;, passive spherical surveillance calibration targets built by the Naval Research Laboratory, first one launched in December of 1962, but apparently no longer in use. &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/lcs-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;LCS 1&lt;/a&gt;, a hollow metal sphere with a precisely defined radar cross-section launched on the 6th of May 1965, is still used (along with LCS 4, 7th Aug 1971) to calibrate ground-based radars.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Lincoln_Calibration_Sphere_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lincoln Calibration Sphere 1&lt;/a&gt; says (though not sure if it's up to date:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Lincoln Calibration Sphere 1, or LCS-1, is a large aluminium sphere in Earth orbit since 6 May 1965. **It is still in use, having lasted for over 50 years. (2, 3)&lt;/p&gt;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://etda.libraries.psu.edu/files/final_submissions/10290&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low Earth Orbit Target Design for the Optical Calibration of the Falcon Telescope Network&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.naic.edu/%7Ephil/hardware/radarcal/radarcal.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radar calibration (via satellites)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;See also&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://apps.dtic.mil/dtic/tr/fulltext/u2/638442.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Quality of the Lincoln Calibration Sphere, KSD-TR-66-417, Lincoln Laboratory Technical Note 1966-41&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2020-09-04T11:20:36.377" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="4011" PostTypeId="1" CreationDate="2014-03-16T15:42:43.300" Score="8" ViewCount="3138" Body="&lt;p&gt;It seems possible to produce methane and oxygen from the atmospheric CO&lt;sub&gt;2&lt;/sub&gt; and icy soil on Mars, and from icy regolith in the polar craters of the Moon. It has been suggested that CH&lt;sub&gt;4&lt;/sub&gt;+O&lt;sub&gt;2&lt;/sub&gt; could be used not only as rocket fuel, but also for internal combustion engines (ICE) in rovers and other industrial machinery on the Moon and Mars. The advantage over solar- or RTG-powered electric engines would be that an ICE can deliver great effect relative to the mass needed for a system. Electric rovers are slow without pre-charged batteries or nuclear reactor respectively, and heavy with them onboard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But wouldn't an ICE overheat in the vacuum of the Moon? Would the thin Martian atmosphere be enough to cool a powerful car-sized ICE? Or would a methane and oxygen combustion engine have a completely different design than a conventional gasoline ICE, maybe with a hot gas exhaust like a rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Non heat related comments on the feasibility of locally supplied &quot;methane rovers&quot; on the Moon and Mars would be welcomed too.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2014-03-16T16:16:16.887" LastActivityDate="2023-11-30T15:16:24.047" Title="Methane internal combustion engines for rovers on Mars and Moon. Feasibility?" Tags="|mars|the-moon|rovers|fuel|isru|" AnswerCount="6" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4012" PostTypeId="2" ParentId="4011" CreationDate="2014-03-16T16:43:37.430" Score="3" Body="&lt;p&gt;Who cares about going fast? The primary concern is just &lt;em&gt;going&lt;/em&gt;. Going fast? Those Mars rovers, for example, are anything but fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary advantage of a methane-powered rover over a solar-powered rover is that the methane-powered rover can run at night. This advantage pretty much disappears for a solar-powered rover operating in the nearly perpetually-lit polar regions of the Moon. Burning methane and oxygen might be a handy backup to solar power in case the rover accidentally drives itself into one of the nearly perpetually-shadowed areas in the same locale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary disadvantage of a methane-powered rover is that this is &quot;eating your seed corn.&quot; The methane and oxygen used to power that rover would be extremely valuable if only they weren't used to move the rovers around. Initially (and probably for a long time to come), it will make more sense to apply those precious resources to other uses than to use it to make a fast go-cart.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-16T16:43:37.430" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4013" PostTypeId="2" ParentId="4011" CreationDate="2014-03-16T16:54:25.140" Score="7" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Convective_heat_transfer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Convective heat transfer&lt;/a&gt; within tenuous atmospheres wouldn't work, lunar &lt;a href=&quot;https://en.wikipedia.org/wiki/Exosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;exosphere&lt;/a&gt; is near vacuum, and it would be fairly limited on Mars with its average of ~ 0.6% mean sea-level Earth's atmospheric pressure, so yes, ICE blocks would have to be redesigned to either facilitate fuel and oxidizer also as a closed-loop liquid coolants (and also preheat them in the process to improve combustion), or use separate coolants (e.g. there plenty of &lt;a href=&quot;https://en.wikipedia.org/wiki/Dry_ice&quot; rel=&quot;nofollow noreferrer&quot;&gt;dry ice&lt;/a&gt; precipitate in and above Martian regolith, so that's a lot of solid form CO&lt;sub&gt;2&lt;/sub&gt;). Whichever option would be used, these coolants would be transporting excess heat into likely large and heavy radiators, losing heat predominantly via &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_radiation&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermal radiation&lt;/a&gt; alone. But since you need to pump oxidizer to the ICE on top of fuel, they would have to be redesigned anyway.&lt;/p&gt;&#xA;&lt;p&gt;So designs might be substantially different to what we're used to here on Earth, possibly depending on how you store your oxidizer (cryogenic &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_oxygen&quot; rel=&quot;nofollow noreferrer&quot;&gt;LOX&lt;/a&gt;?), how much heat you're producing, and where else in the system you might want to use it. Mars obviously has an advantage in cooling ICE blocks with still some non-negligible atmospheric pressure and an average temperature at ~ -55°C, but that might also mean that you'd have to first heat the system up before even starting it. Excess heat could as well be redirected to heat up the cabin space, or otherwise made useful. But designs would be completely different for each celestial, even just local environment on any of them. For example, on Titan, you'd really only require the oxidizer, since there's &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Titan#Composition&quot; rel=&quot;nofollow noreferrer&quot;&gt;plenty of methane in its lower atmosphere&lt;/a&gt;. See &lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/181254/feasibility-of-sourcing-fuel-from-air&quot;&gt;this answer for more details on scavenging design of such an engine&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Our ICE designs would adapt and evolve, like they have here on Earth for all kinds of environments, from dry and hot deserts, to arctic conditions. How? Well, I expect lots of new innovations, and some of them will be written in big letters in the history of any of the new worlds we'd colonize. And these innovation processes have already started, for example, Wickman Spacecraft &amp;amp; Propulsion Co. (WSPC) developed a way to directly burn Martian atmospheric CO&lt;sub&gt;2&lt;/sub&gt; with their &lt;a href=&quot;http://www.wickmanspacecraft.com/marsjet.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Jet Engine&lt;/a&gt;. Not an ICE, but there might be others with goals of developing an &lt;em&gt;air-breather&lt;/em&gt; ICE suitable for Mars. The race to win the best design has barely started. Would they use CH&lt;sub&gt;4&lt;/sub&gt; + O&lt;sub&gt;2&lt;/sub&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Trisilane&quot; rel=&quot;nofollow noreferrer&quot;&gt;Trisilane&lt;/a&gt; Si&lt;sub&gt;3&lt;/sub&gt;H&lt;sub&gt;8&lt;/sub&gt; + CO&lt;sub&gt;2&lt;/sub&gt;, something else entirely? Who knows ...&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="6834" LastEditDate="2023-11-27T05:35:14.087" LastActivityDate="2023-11-27T05:35:14.087" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="4014" PostTypeId="2" ParentId="4000" CreationDate="2014-03-16T17:29:54.683" Score="6" Body="&lt;p&gt;It's possible for an object with Jupiter's L1 Jacobi constant to have a widely varying semi major axis. And something with Jupiter's L2 Jacobi constant might find it's way to Saturn's L1. Etc. But changing the semi-major axis via repeated perturbations typically takes many passages. Sedna's perihelion is about 76 AU. A Hohmann trip from 1 AU to 76 AU would take 120 years. And a trip that relies on repeated planetary perturbations for its delta V would take much longer. A 56 year trip to Sedna via ITN doesn't sound plausible to me. A mass ratio of 50 with a healthy ISP might do it (since I don't know how to model constant low thrust, I can't tell you yes or no). But I'm not sure that's what Lo et al have in mind when they talk about the ITN. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-03-16T17:29:54.683" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4017" PostTypeId="1" AcceptedAnswerId="4018" CreationDate="2014-03-17T00:45:38.957" Score="24" ViewCount="5337" Body="&lt;p&gt;I was just wondering, as a Fortran user. As far as I know, the simulations taken in order to carry the Apollo 11 mission was written in Fortran 2. I would like to confirm this from a reliable source. Also, I would be interested in the papers, if any, published around this topic (the numerical simulations research done at that time).&lt;/p&gt;&#xA;" OwnerUserId="2998" OwnerDisplayName="user2820579" LastEditorUserId="49" LastEditDate="2014-03-17T11:21:59.100" LastActivityDate="2022-12-23T08:43:25.693" Title="Fortran and the Apollo 11 mission" Tags="|nasa|apollo-program|rockets|guidance|computing|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4018" PostTypeId="2" ParentId="4017" CreationDate="2014-03-17T01:01:02.607" Score="33" Body="&lt;p&gt;I worked at the MIT Instrumentation Lab during the Apollo program.&#xA;Although I did not work on that program, I knew many of the people who did.&lt;/p&gt;&#xA;&lt;p&gt;The guidance computers for the command module and lunar landing module were each programmed in their own assembly language.&#xA;The missions were exhaustively simulated on a large IBM 360 Model 75 computer, using a language called &lt;a href=&quot;https://encyclopedia2.thefreedictionary.com/MAC-360&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;MAC-360&lt;/em&gt;&lt;/a&gt; created by &lt;a href=&quot;http://en.wikipedia.org/wiki/J._Halcombe_Laning&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Hal Laning&lt;/em&gt;&lt;/a&gt;.&#xA;It resembled Fortran in being mostly about mathematical calculations.&#xA;It had a 3-line format for representing vector and matrix calculations.&lt;/p&gt;&#xA;&lt;p&gt;At the time, Fortran was the primary programming language, especially for math.&#xA;There were many others - Cobol, Algol, B (a predecessor to C), Lisp, and of course numerous assembly languages.&lt;/p&gt;&#xA;&lt;p&gt;There is more information &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Guidance_Computer&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;here on the Apollo Guidance Computer&lt;/em&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="4564" OwnerDisplayName="Mike Dunlavey" LastEditorUserId="7062" LastEditDate="2022-12-23T08:43:25.693" LastActivityDate="2022-12-23T08:43:25.693" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="4019" PostTypeId="1" AcceptedAnswerId="4020" CreationDate="2014-03-17T13:37:26.477" Score="44" ViewCount="11177" Body="&lt;p&gt;We can break this down into two categories:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;For internal security purposes, does the ISS have any type of weapon on board? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;For global security purposes, are there any orbiting weapons (that we know of)?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Obviously governments launch lots of top-secret payloads, so ignoring those for now.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="51" LastEditDate="2014-04-20T11:19:15.127" LastActivityDate="2023-12-16T23:01:57.050" Title="Are there any weapons in space?" Tags="|iss|security|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4020" PostTypeId="2" ParentId="4019" CreationDate="2014-03-17T13:47:19.763" Score="45" Body="&lt;p&gt;There's a &lt;a href=&quot;http://www.nbcnews.com/id/23131359/#.Uyb7hFyWXTQ&quot;&gt;handgun on the Soyuz attached to the Space Station&lt;/a&gt;. Does that count?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Outer_Space_Treaty&quot;&gt;Outer Space Treaty&lt;/a&gt; forbids placing weapons of mass destruction in space. Other than that handgun on the Soyuz, that's about it. That handgun is a weapon of bear or wolf destruction, not a weapon of mass destruction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question is a bit tougher to answer when you consider the fact that &lt;strong&gt;anything&lt;/strong&gt; moving at 17,000 mph can be a rather potent kinetic weapon. China and the US each blew up one of their own satellites as a demonstration of their prowess. If they could destroy one of their own, they can certainly destroy satellites belong to some other country. The Outer Space Treaty doesn't appear to address anti-satellite weapons.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-17T13:58:20.420" LastActivityDate="2014-03-17T13:58:20.420" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4022" PostTypeId="2" ParentId="4019" CreationDate="2014-03-17T21:46:45.590" Score="16" Body="&lt;p&gt;The Soviet space station &lt;a href=&quot;http://en.wikipedia.org/wiki/Salyut_3#On-board_gun&quot; rel=&quot;nofollow noreferrer&quot;&gt;Salyut 3&lt;/a&gt; used to have a gun (an aircraft autocannon) equipped on it (&lt;a href=&quot;http://www.russianspaceweb.com/almaz_ops2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;The cannon was a modification of &lt;a href=&quot;https://en.wikipedia.org/wiki/Nudelman-Rikhter_NR-23&quot; rel=&quot;nofollow noreferrer&quot;&gt;NR-23&lt;/a&gt; autocannon used in Tu-22 aircraft.&#xA;Russian version of &lt;a href=&quot;https://ru.wikipedia.org/wiki/%D0%A1%D0%B0%D0%BB%D1%8E%D1%82-3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Salyut-3 Wikipedia article&lt;/a&gt; claims that this weapon system, named &amp;quot;Shchit-1&amp;quot; (in Russian &amp;quot;Щит-1&amp;quot;, translated as &amp;quot;Shield-1&amp;quot;), was implemented as a self-defense measure against potential capture of the station by Space Shuttle orbiter:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The leaders of the Soviet military-industrial complex had concerns that one of the &amp;quot;military&amp;quot; applications of the Shuttles would be inspection and removal of Soviet spacecrafts from orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The following quotes (translated from Russian) are from &lt;a href=&quot;http://www.popmech.ru/science/8406-artilleriya-na-orbite-boevye-orbitalnye-stantsii/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this webpage&lt;/a&gt;, based on an article in &amp;quot;Popular Mechanics&amp;quot; (&amp;quot;Популярная механика&amp;quot;) magazine, &lt;a href=&quot;https://www.popmech.ru/magazine/2003/12-issue/&quot; rel=&quot;nofollow noreferrer&quot;&gt;№10 (12), October 2003&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The OPS [OPS-2 (Orbital Piloted Station-2), aka Almaz-2, aka Salyut-3] was equipped with an aircraft rapid-fire NR-23 cannon (modification of Tu-22 jet bomber tail gun) designed by Nudelman - Rikhter.&lt;/p&gt;&#xA;&lt;p&gt;Estimated firing range for shooting orbital targets was more than three thousand meters. The gun fired 950 shots per minute. The shell weighing 200 g flew at a speed of 690 m/s.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Below, allegedly, is the photo of the cannon (&lt;a href=&quot;https://judgesuhov.livejournal.com/118410.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/q1oph.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/q1oph.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;When shooting in space, the recoil of the cannon was equivalent to a thrust of 218.5 kgf [2.143 kN], therefore the station had to be stabilized, which was easily handled by two main engines with a thrust of 400 kgf [3.9 kN] each and engines of rigid stabilization [sic] [attitude control thrusters, perhaps] with a thrust of 40 kgf [0.39 kN] each.&lt;/p&gt;&#xA;&lt;p&gt;The cannon was mounted rigidly under the belly [sic] of the OPS [-2]. It could be aimed at desired point with help of the sight by the means of turning the entire station manually or by using remote control in order to [automatically?] follow the target. The firing of the gun was controlled by a &amp;quot;programmatic control equipment&amp;quot; (&amp;quot;PKA&amp;quot; [in Russian &amp;quot;ПКА&amp;quot;]), which calculated the salvo [sic] required to destroy the target with the shell flight duration of one to five seconds.&lt;/p&gt;&#xA;&lt;p&gt;On 24th of January 1975, when the station ... was being deorbited, the gun fired its first (and ... last) salvo [sic]: the developers needed to know how firing the cannon affects dynamics and vibrational stability of the Almaz [-2 station]. The tests were successful ... and shells were fired [in direction] against the orbital velocity vector [firing was controlled from the ground], and burned in atmosphere ... before the station did.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2886" LastEditorUserId="52956" LastEditDate="2023-12-16T23:01:57.050" LastActivityDate="2023-12-16T23:01:57.050" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="4023" PostTypeId="2" ParentId="4019" CreationDate="2014-03-17T21:56:51.033" Score="4" Body="&lt;p&gt;Anything big enough in orbit  can be used as a  weapon due to its immense gravitational potential and kinetic energy. You can smash a satellite into another one as sabotage,the brilliant pebble project demonstrated here (&lt;a href=&quot;http://en.wikipedia.org/wiki/Brilliant_Pebbles#Brilliant_Pebbles&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Brilliant_Pebbles#Brilliant_Pebbles&lt;/a&gt;) or just deorbit it for ground bombardment if you can calculate the trajectory well enough (&lt;a href=&quot;http://en.wikipedia.org/wiki/Rods_from_god#Project_Thor&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Rods_from_god#Project_Thor&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="2887" LastEditorUserId="2887" LastEditDate="2014-03-19T18:58:27.843" LastActivityDate="2014-03-19T18:58:27.843" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4025" PostTypeId="2" ParentId="2841" CreationDate="2014-03-18T01:58:35.897" Score="5" Body="&lt;p&gt;While the rover is an automated vehicle, it is likely it can be used in a personnel transport role.&lt;/p&gt;&#xA;&lt;p&gt;Any vehicle that can carry a person's suited mass, and has a platform big enough to sit upon, and doesn't exceed about 3G's of thrust is a suitable personnel transport. It may not be ideal, it may not be comfortable, but it doesn't need to be.&lt;/p&gt;&#xA;&lt;p&gt;The Mars One Project's &lt;a href=&quot;http://www.mars-one.com/technology/the-rover&quot; rel=&quot;noreferrer&quot;&gt;page for the Rover&lt;/a&gt; notes that it can move the landers and has a trailer. This clearly exceeds the mass of a couple suited people, provides a platform large enough to sit upon, and the acceleration must needs be able to be reduced at need to accomplish other goals on the page.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.mars-one.com/technology/living-unit&quot; rel=&quot;noreferrer&quot;&gt;Living Unit page&lt;/a&gt; notes that the unit is large enough for a shower and kitchen. This means it's big enough to mass at least several tons, probably several dozens, plus the inflatable habitat (also many tons). It also means a large enough size that the trailer to haul it will be large enough to have people sit upon.&lt;/p&gt;&#xA;&lt;p&gt;If needed, a shirt-sleeve environment trailer could be developed and shipped, as well. Portable computing technology will allow instructing the autonomous rover where to take the crew using it as a vehicle.&lt;/p&gt;&#xA;&lt;p&gt;Searching for images, in the image at &lt;a href=&quot;http://tech.co/mars-one-startup-humans-to-mars-2023-2012-08&quot; rel=&quot;noreferrer&quot;&gt;tech.co&lt;/a&gt; the rover looks to be about 2m tall overall, so it might be large enough for an astronaut to ride on directly, between the panels. It wouldn't be comfortable, but it may be serviceable.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.mars-one.com/technology/the-rover&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Mars One - Rover - The Technology&lt;/em&gt;  (www.mars-one.com/technology/the-rover)&lt;/a&gt; accessed 17 Mar 2014&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.mars-one.com/technology/living-unit&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Mars One - Living Unit - The Technology&lt;/em&gt;  (www.mars-one.com/technology/living-unit)&lt;/a&gt; accessed 17 Mar 2014&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://tech.co/mars-one-startup-humans-to-mars-2023-2012-08&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Tech Cocktail - Mars One: A Startup To Send Humans To Mars By 2023&lt;/em&gt; (tech.co/mars-one-startup-humans-to-mars-2023-2012-08)&lt;/a&gt; accessed 17 Mar 2014&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-03-18T01:58:35.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4026" PostTypeId="1" CreationDate="2014-03-18T11:59:43.867" Score="13" ViewCount="16007" Body="&lt;p&gt;&lt;strong&gt;&lt;em&gt;Note: This has become a frequently asked question.&lt;/em&gt;&lt;/strong&gt;  Some related situations which have been regarded as duplicates of this question include these:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Leaving vehicle / going outside without spacesuit or helmet&lt;/li&gt;&#xA;&lt;li&gt;Wearing regular clothes instead of a spacesuit&lt;/li&gt;&#xA;&lt;li&gt;Damage to spacesuit or helmet&lt;/li&gt;&#xA;&lt;li&gt;Depressurization or loss of spacesuit oxygen&lt;/li&gt;&#xA;&lt;li&gt;Taking off / removing spacesuit or helmet&lt;/li&gt;&#xA;&lt;li&gt;doing any of these on Mars instead of in space (the air pressure on Mars is pretty close to a vacuum, so the answers here apply for Mars too)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;See also &quot;&lt;a href=&quot;https://space.stackexchange.com/q/864/26446&quot;&gt;Puncturing space suit during EVA. What would happen?&lt;/a&gt;&quot; for smaller tears or punctures, and &quot;&lt;a href=&quot;https://space.stackexchange.com/q/122/26446&quot;&gt;How would an astronaut&amp;#39;s hand react to a hard vacuum?&lt;/a&gt;&quot; for effects specific to the hands.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Original question:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I really want to know the consequences of helmet damage during EVA. Recently I watched a movie called &quot;Mission to Mars&quot; in which one of the astronauts sacrifices his life by taking his mask off. His face was badly affected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the image for reference.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/4A6gO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4A6gO.jpg&quot; alt=&quot;Mission to Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="Praveen Kadambari" LastEditorUserId="897" LastEditDate="2018-11-21T14:00:14.890" LastActivityDate="2019-01-20T05:45:51.847" Title="What are the consequences if an astronaut's helmet gets damaged during a spacewalk?" Tags="|astronauts|health|eva|failure|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="4027" PostTypeId="1" AcceptedAnswerId="4028" CreationDate="2014-03-18T14:34:26.097" Score="21" ViewCount="5793" Body="&lt;p&gt;The Earth's natural satellite has a great deal of angular momentum that seems to me could be used to derive a considerable boost for leaving the local SOI. Has any probe used a Lunar flyby as part of its flightpath? Are there reasons why this maneuver is not used regularly?&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2018-10-14T07:18:02.603" Title="Has Luna been used for gravity assist for interplanetary probes?" Tags="|the-moon|trajectory|gravity-assist|mission-design|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4028" PostTypeId="2" ParentId="4027" CreationDate="2014-03-18T14:46:56.207" Score="21" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot;&gt;STEREO&lt;/a&gt; satellites used multiple gravitational assists from the Moon to significantly decrease the amount of fuel needed to put those two satellites into heliocentric orbits. The first flyby resulted in STEREO ahead (STEREO-A) being ejected from the Earth-Moon system with a semi-major axis slightly less than that of the Earth-Moon system. STEREO-A has a period of 346 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STEREO behind (STEREO-B) didn't pass as close to the Moon as did STEREO-A. It didn't quite escape. It instead went into a large orbit and later re-encountered the Moon. This second flyby did eject STEREO-B from the Earth-Moon system, but with a slightly larger semi-major axis than that of the Earth-Moon system. STEREO-B has a period of 388 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following nasa.gov page contains videos and graphics that depict how the two STEREO satellites used the Moon for gravitational assists:&lt;br&gt;&#xA;&lt;a href=&quot;http://svs.gsfc.nasa.gov/vis/a000000/a003300/a003364/&quot;&gt;STEREO's Routes to Solar Orbits&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;The Japanese &lt;a href=&quot;http://en.wikipedia.org/wiki/Nozomi_%28spacecraft%29&quot;&gt;Nozomi spacecraft&lt;/a&gt; also made use of a lunar flyby. Unfortunately, things went awry on the subsequent Earth flyby and then went awry again at Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lunar flybys can be useful if the intent is to stay in the vicinity of the Earth-Moon system. A number of near Earth vehicles have used the Moon's gravity to reduce propellant needs. The &lt;a href=&quot;http://en.wikipedia.org/wiki/WIND_%28spacecraft%29&quot;&gt;WIND&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Geotail&quot;&gt;GEOTAIL&lt;/a&gt; spacecraft took advantage of the &quot;&lt;a href=&quot;http://cbboff.org/UCBoulderCourse/documents/ASPapercosIOK.pdf&quot;&gt;double lunar swingby&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another proposed use is to reduce fuel needed to place a vehicle in geostationary orbit. Putting a vehicle into a geostationary orbit is a surprisingly expensive operation, particularly if the launch isn't from an equatorial site. The delta-V using a traditional geostationary transfer orbit followed by a circularization and plane change burn at geosynchronous altitude is greater than escape delta-V. Instead transferring into a highly eccentric orbit that reaches the Moon's altitude at apogee enable using the Moon to affect the plane change and to raise perigee to geostationary. See &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/1.17469&quot;&gt;Ramanan and Adimurthy, &lt;i&gt;Precise Lunar Gravity Assist Transfers to Geostationary Orbits&lt;/i&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Excluding Nozomi, there's a common thread in these uses: The vehicles aren't going to another planet. One reason the Moon isn't used much for interplanetary missions is because things don't line up properly, and in the rare cases when they do, the opportunities are rather short-lived.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-18T16:00:41.490" LastActivityDate="2014-03-18T16:00:41.490" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4029" PostTypeId="2" ParentId="4027" CreationDate="2014-03-18T15:01:12.853" Score="7" Body="&lt;p&gt;As for why it is not used regularly, it's because usually time is of the essence.  If you're trying to hit a particular outgoing trajectory to, say, Mars, which is good for maybe two or three weeks once every two years, the Moon would be potentially in a useful position for a gravity assist for only a few days during that time, if at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need about three weeks of launch days to have a sufficient probability of launching, and your spacecraft and launch system must accommodate that entire period.  So you could not mine the benefit of a lunar assist that might be available over only a few days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's all if you launch directly to escape.  If you plan to hang around in Earth orbit and use longer-lived propulsion systems to escape, then you could plan to use a lunar assist, so long as you assure that you get into orbit in time to take advantage of it.  &lt;a href=&quot;http://issfd.org/ISSFD_1999/pdf/IFL_3.pdf&quot; rel=&quot;noreferrer&quot;&gt;Nozomi did that&lt;/a&gt;, using lunar flybys to reduce the propellant needed to escape and inject.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Towards the end of the MER-A 21-day launch period there was a little dip in the C3 curve.  It was due to distant lunar flybys on those days.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-03-18T20:00:20.380" LastActivityDate="2014-03-18T20:00:20.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4030" PostTypeId="2" ParentId="4027" CreationDate="2014-03-18T15:33:51.860" Score="5" Body="&lt;p&gt;Of some interest as well is the &lt;a href=&quot;http://en.wikipedia.org/wiki/PAS-22&quot; rel=&quot;noreferrer&quot;&gt;PAS-22&lt;/a&gt; spacecraft, which was intended for Geostationary orbit, had a failed rocket that put it into a very poor orbit, and recovered by flying by the moon to assist in inclination changes, allowing it to achieve orbit around the moon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-03-18T15:33:51.860" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4032" PostTypeId="2" ParentId="4026" CreationDate="2014-03-18T17:09:00.333" Score="18" Body="&lt;p&gt;Don't ever take what's portrayed in a scifi movie as fact. You can use the fingers on one hand to count the number of scifi movies that go out of their way to be faithful to science. &lt;em&gt;&quot;Mission to Mars&lt;/em&gt; was not one of them. (Nor was &lt;em&gt;Total Recall&lt;/em&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will you die if you take your helmet off in space? Of course. Your brain needs oxygen, as does the rest of your body. Without oxygen, you will go unconscious rather quickly. Then you'll die shortly later. You won't blow up. You won't instantly freeze. You might however get an ugly postmortem sunburn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this &quot;Ask an Astrophysicist&quot; page at NASA, &lt;a href=&quot;http://web.archive.org/web/20140313144730/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html&quot; rel=&quot;noreferrer&quot;&gt;Human Body in a Vacuum&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You do not explode and your blood does not boil because of the containing effect of your skin and circulatory system. You do not instantly freeze because, although the space environment is typically very cold, heat does not transfer away from a body quickly. Loss of consciousness occurs only after the body has depleted the supply of oxygen in the blood. If your skin is exposed to direct sunlight without any protection from its intense ultraviolet radiation, you can get a very bad sunburn.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-07-02T22:57:08.633" LastActivityDate="2016-07-02T22:57:08.633" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4033" PostTypeId="2" ParentId="4000" CreationDate="2014-03-18T19:28:45.037" Score="10" Body="&lt;p&gt;An Ion thrust probe on a direct course is a viable option for a flyby. You need a total delta-V of around 70,000kph for a direct course accelerate-coast-decelerate. A slightly better path is a single flyby non-hohman. This is because a current extant probe has sufficient delta-V to make the trip in the specified timeframe with current technology and with current fuel loads. (Noting that the probe in question would be out of fuel by the time it was out of power.)&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Data&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The Dawn spacecraft has a total expected vector accumulation of 38,000 kph, or around 10.4 kps (space.com), while the voyager 2 ephemeris data (theskylive.com) shows 15.4kps solar relative velocity. A 2020-launched probe could, for a flyby, use a direct launch to get to Sedna - note that this would put perihelion at about 35 years travel time without a more efficient engine nor more relative fuel than the Dawn spacecraft. The Dawn probe, by replacing one or two instruments with additional fuel, could likely arrive direct course with fuel to decelerate, but would lack the power to make use of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first order approximation is sufficient for initial mission development purposes - I'm not an engineer, and the actual calculus is not needed, since most of the trip would be coasting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Topaz Nuclear Reactor was theoretically capable of 5 years operation on its 12kg of fuel, in a roughly 350kg total installed mass, producing 5 kWe. This is adequate to operate the Dawn Thrusters. More recent &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Discussion&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The cubesat is impractically small. It's a good size for a minimal science payload, however. Dawn has 425 kg fuel, about 90 kg of drives on a 1200 kg spacecraft; it would need at least 4 kWe to drive the ion drives at the far end. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives us a reasonably launchable mission profile - by using solar for the outbound, and remote starting the fission plant for the deceleration phase, we can get increased launch performance. By reducing the mission instruments, we can increase the fuel load as well. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;some really vague estimates due to lack of concrete data&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;We need about 450 kg of power plant, about 100 kg of drives, about 50 kg of navigational support, about 100 kg of structure. This means we can get about 600 kg of fuel, and note that I'm assuming spare fuel for a slightly more efficient nuclear reactor, and about a 6% improvement over the NStar thruster efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars' orbital velocity is around 23 kp/s; a martian flyby should be able to provide 5 kps or more - and redirecting for the outbound direct flight, with only a 6 month lag or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;600 / 425 ≅ 1.41 &lt;br&gt;&#xA;38,000 * 1.41 ≅ 53640 kmh ∆V total fuel&lt;br&gt;&#xA;53640 / 2 ≅ 26820 kmh transit velocity (halving for deceleration at end)&lt;br&gt;&#xA;This is about 5577 hours per AU, or 232 days per AU, or 0.637 years per AU&lt;/p&gt;&#xA;&#xA;&lt;p&gt;we need a boost - we can get an addition 50% from a mars flyby/redirect. Which gives us about 155 days per AU, or about 0.42 years per AU.&lt;br&gt;&#xA;76 * 0.42 ≅ 31.9 years. We need to double the acceleration and deceleration times&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - modified hohman to mars for redirect, and then straight out to sedna, braking on the last few years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dawn has 2100 days acceleration...&lt;br&gt;&#xA;2100 * 1.41   ≅ 2960 days or about 8.1 years at each end &lt;br&gt;&#xA;roughly 40 years travel time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;this first order approximation says we might be able to pull it off, and is close enough to make it practical for someone more skilled in calculus than I to work out the gravitational, flyby, and accelerational effects. I've also been extremely conservative by ignoring the initial earth velocity of 6.28 AU/year. Further, an additional small boost can be obtained from a lunar flyby at launch. So, being able to approximate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original question's probe isn't big enough for the needed &lt;em&gt;current&lt;/em&gt; hardware; I'm working from current &quot;off the shelf&quot; tech, only extrapolating for a 40 year fuel supply for the nuclear reactor (at 350kg for the reactor for 5 years, and 12 kg per additional 5 years, noting that the half life for the reactor's fuel is in excess of 10^7 years... The probe, once on station, could operate with reduced fuel efficiency for several years further.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Bottom Line&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;A suitable probe the size of the Dawn probe could be put on station with off the shelf components and careful engineering, plus a variation of an extant space rated power plant. The original question's probe is implausible.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.space.com/8579-nasa-spacecraft-breaks-speed-boost-record.html&quot;&gt;http://www.space.com/8579-nasa-spacecraft-breaks-speed-boost-record.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://theskylive.com/voyager2-tracker&quot;&gt;http://theskylive.com/voyager2-tracker&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor&quot;&gt;https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;www.esto.nasa.gov/conferences/nstc2007/papers/Patterson_Michael_D10P3_NSTC-07-0014.pdf&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&quot;&gt;http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-03-18T19:28:45.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4034" PostTypeId="1" AcceptedAnswerId="4036" CreationDate="2014-03-18T20:19:26.613" Score="44" ViewCount="56304" Body="&lt;p&gt;Can I see the International Space Station (ISS) from the surface with the naked eye? If yes, how can I determine that it really is the ISS?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="49" LastEditDate="2014-03-19T03:51:20.883" LastActivityDate="2020-09-14T18:00:44.500" Title="Can I see the ISS from the surface with the naked eye?" Tags="|iss|observation|" AnswerCount="6" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4035" PostTypeId="2" ParentId="4034" CreationDate="2014-03-18T20:36:25.870" Score="27" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Cj0L2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Cj0L2.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://www.flickr.com/photos/perry-pics/5482376910/in/album-72157629890298141/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This image taken by Kam75 on Flickr.&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;Yes, you can. When it is illuminated by the sun, it is brighter than any star and moves with a quite high speed across the night sky which makes it quite easy to spot. Because of its low orbit it can only be seen from a small area at any given time, but thanks to its strongly inclined orbit it regularly passes over most inhabited areas of earth. You can check its current position and when it will have its next overpass near your location on &lt;a href=&quot;http://www.heavens-above.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heavens Above&lt;/a&gt;. First &lt;a href=&quot;http://www.heavens-above.com/SelectLocation.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;set your location and time zone&lt;/a&gt;, then go to &lt;a href=&quot;http://www.heavens-above.com/PassSummary.aspx?satid=25544&quot; rel=&quot;nofollow noreferrer&quot;&gt;the subpage of the ISS&lt;/a&gt; to check when it will do its next overpass near you.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2020-09-14T18:00:44.500" LastActivityDate="2020-09-14T18:00:44.500" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="4036" PostTypeId="2" ParentId="4034" CreationDate="2014-03-18T20:42:19.227" Score="34" Body="&lt;p&gt;Yes, it's perfectly possible to observe the International Space Station with a naked eye from most latitudes, unless you're observing from one of the two polar regions, in which case the angle at which it would be visible would likely be too low on the horizon and blocked by local topography. From one of my answers on &lt;a href=&quot;https://astronomy.stackexchange.com/a/1260/13&quot;&gt;Astronomy.SE&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Two basic conditions need to be met for anyone on the globe to observe the International Space Station (ISS) from any location:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ISS needs to pass roughly overhead of your location, and&lt;/li&gt;&#xA;&lt;li&gt;it needs to do that during night so it's visible to the naked eye&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now, obviously there are other requirements, like e.g. weather (if it's overcast we won't be able to see it through the clouds). And it won't be visible when it's in the Earth's shadow and doesn't reflect any light during the wee hours of the night.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's discuss the two basic conditions mentioned a bit more. ISS has an orbital inclination of 51.65°. Why is that is explained in &lt;a href=&quot;https://space.stackexchange.com/a/2536/49&quot;&gt;this answer&lt;/a&gt;, but to quickly recap, this orbital inclination was chosen because it's near ideal for launches from the Baikonur Cosmodrome, where most launches towards the ISS are made from, it is still achievable from the US launch facilities and most US built launchers, and it allows the ISS to go over majority of the populated areas on Earth. So it was decided both for economic reasons, as well as to allow ISS to do as much science as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what this inclination means is that the ISS will make a ground track (path along the surface of the Earth directly below it) that moves towards the West with each new orbit, even though the ISS itself moves West to East (or saying it otherwise, it's in a prograde Low-Earth orbit). Its ground track could look something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/tkXR0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Ground track of the International Space Station with an area where it is observable from the Earth's surface&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;shown in the white circle (Image source: &lt;a href=&quot;http://www.isstracker.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Tracker&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS orbital period, the time it takes it to circumnavigate the globe and complete one orbit, is roughly 93 minutes. As you can see in the screen grab from the &lt;a href=&quot;http://www.isstracker.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Tracker&lt;/a&gt; above, its ground track moves westward on each new orbit, which is due to Earth's rotation on its axis, so this shift towards the West will be equal to how much the Earth rotates in the meantime, which in those 93 minutes comes out at roughly 22,5° or a good 2,505 km (1,556 mi) of the Earth's total equatorial circumference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But ISS's orbit isn't Sun-synchronous, meaning it won't be over any given Earth latitude at the same local mean solar time, so when it will be observable depends also on its orbital position relative to the position of the Sun. We require that it is dark, remember? ISS isn't bright enough (doesn't reflect enough light with its truss and solar panels) during the day to be brighter than the day skies are. So we have another parameter, which is local time. As you probably anticipated by now, the length of the day isn't really equal at any day of the year, and moving further away from the equator only increases this difference of the length of the day in winter, or the length of it during summer.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;And so on, and I'm omitting part of that answer that isn't relevant to your question to save you the trouble. But it might help you understand the station's movement over our night skies better. Since that question is about the ISS timetables for given coordinates, you might have guessed by now that there are various services on the internet and software that will help you determine when and where to look for it. NASA hosts one of such tools too, on their &lt;a href=&quot;http://spotthestation.nasa.gov/sightings/index.cfm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spot The Station&lt;/a&gt; website. Or perhaps &lt;a href=&quot;http://iss.astroviewer.net/observation.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS observation tool on Astroviewer.net&lt;/a&gt; that I find fairly straightforward to use. And there are &lt;a href=&quot;https://encrypted.google.com/search?q=observe%20iss%20timetable&quot; rel=&quot;nofollow noreferrer&quot;&gt;many others&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for identifying it from among other visible objects, the easiest is simply knowing that it should be about as bright as some of the brightest stars and planets on the night's sky (depends on exact angle of their solar panels to the Sun and your observation point, it's distance and atmospheric conditions, but it doesn't get much brighter than, say, planet Venus), with major difference that it won't follow the sidereal time (its apparent motion won't follow the rotation of the Earth), so it will move much faster than the stars, it won't have location lights flashing on it like planes do, won't produce any noise, no airplane contrails behind it, and it won't change course and/or speed. Average observable time from a single location during one of its orbits will be roughly 3 minutes, before it again disappears over the horizon. And if you first observed it rising and setting close towards southeast or it shot straight over you, chances are you'll be able to observe it once more some 90 minutes later rising from the southwestern direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also some softwares that could help you confirm it's indeed the ISS, I personally use &lt;a href=&quot;https://play.google.com/store/apps/details?id=com.isitbroken.android.stardroid&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sky Map +&lt;/a&gt; for my Android smartphone. It is free and uses your phone's internal gyroscope, compass and/or GPS to enable you to follow and identify visible features of the night's sky (can be filtered by type) by holding the phone in front of you and towards the observed object. Simple.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2014-11-07T20:10:39.983" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4037" PostTypeId="1" AcceptedAnswerId="4038" CreationDate="2014-03-18T20:46:53.887" Score="5" ViewCount="476" Body="&lt;p&gt;The tensions between the US and Russia are currently growing.  What effects (if any) can these tensions have on day-to-day operations on the International Space Station, or any other joint undertakings?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, to what extent do the individual space agencies depend on good relations between the nations?  To what extent are the agencies self-sufficient?&lt;/p&gt;&#xA;" OwnerUserId="2831" LastActivityDate="2014-03-18T21:04:12.083" Title="What effect may the tensions between the US and Russia have on the ISS?" Tags="|iss|law|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4038" PostTypeId="2" ParentId="4037" CreationDate="2014-03-18T20:51:18.387" Score="5" Body="&lt;p&gt;There has been a lot of speculation, but it is all that, speculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the Russian control of manned access to the station (Only Soyuz carries humans today in 2014) and the majority of control of cargo (Progress, and Kurs docking computer systems used on ATV) they have a pretty strong position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, only Progress and ATV can deliver fuel to the station, Dragon, Cygnus, and HTV which berth on the US segment cannot deliver fuel to the Zvezvda module for reboost uses. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US would be foolish to push on this issue.  Conversely the Russians would be foolish to try and use this for leverage, since they cannot afford to run the station on their own either. Sort of mutually assured destruction if either tries to use the ISS as leverage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Come 2015/2016 if SpaceX can deliver on Dragon manned (DragonRider), Boeing with CST100, SNC with Dream Chaser, then the story can change. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For now, all sides seem to be keeping their heads down on this issue, so no good comes of it, in any direction.  But that being said, politicians have done stupider things in the past.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said that, assuming things went horribly south, an interesting question would be whether the US would be willing to launch humans on a Dragon, without the escape system in place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right now, Elon Musk has strongly suggested that should a Soyuz develop a problem that it could not reenter, a Dragon could be launched in Cargo config with seats, to safely return a crew.  Whether the US would be willing to launch on it is a different story.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, it would be a great idea to get Dragon/CST-100/DreamChaser ready as soon as possible.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-03-18T21:04:12.083" LastActivityDate="2014-03-18T21:04:12.083" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4039" PostTypeId="2" ParentId="4034" CreationDate="2014-03-18T22:08:33.643" Score="14" Body="&lt;p&gt;Yes, you can see the ISS with the naked eye under certain conditions. As for identifying it, it will be quite bright compared to stars and it will be visibly moving. It will not have flashing anti-collision lights like a plane. It will be a more-or-less constant intensity bright light and will appear to be moving across the sky faster than a cruising jet airliner would appear to be moving.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To know when it's going to be visible at your location, you can sign up for e-mail or text message notifications directly from NASA at &lt;a href=&quot;http://spotthestation.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;Spot the Station&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The conditions for seeing it are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) It must be night (otherwise the sky will be too bright to see it.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) It must be flying relatively close to directly over your location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) As with viewing anything else in outer space, the sky at your location must be relatively clear during the flyover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) It must be either shortly before dawn or shortly after dusk (well, for most locations, at least.) The reason for this is that, by definition, night occurs when your part of Earth is facing away from the sun, yet the sun must be hitting the space station for it to be visible with the naked eye. Once it passes into the shadow of the Earth, it will disappear. The closer the flyover is to dawn or dusk at your location, the longer you'll be able to see the ISS for this reason. It will usually pass into the shadow of Earth (and, thus, disappear) before it actually sets on the horizon. The longest visibilities tend to be about 6 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flyovers of the space station will occur roughly 90 minutes apart, though usually only one flyover will be visible to the naked eye per morning or night, due to the aforementioned reason of it needing to be close to dawn or dusk to see it.&lt;/p&gt;&#xA;" OwnerUserId="2900" LastEditorUserId="2900" LastEditDate="2015-02-11T14:33:27.090" LastActivityDate="2015-02-11T14:33:27.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4041" PostTypeId="2" ParentId="3995" CreationDate="2014-03-19T08:27:50.567" Score="34" Body="&lt;p&gt;Three factors come to mind from my reading over the years: Heat Shield integrity, Escape Tower limitations, and avoiding fuel over the CM on the pad.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Throughout this answer, keep in mind that the stacking order (from the top) is &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_escape_system&quot; rel=&quot;noreferrer&quot;&gt;Escape Tower (ET)&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Command_Module_.28CM.29&quot; rel=&quot;noreferrer&quot;&gt;Command Module (CM)&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Service_Module_.28SM.29&quot; rel=&quot;noreferrer&quot;&gt;Service Module (SM)&lt;/a&gt;, Lunar Module Shroud, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;Lunar Module (LM)&lt;/a&gt;. See &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/b/b2/Saturn_v_schematic.jpg&quot; rel=&quot;noreferrer&quot;&gt;the full Saturn V diagram&lt;/a&gt; or &lt;a href=&quot;http://www.popsci.com/two-become-one-how-apollo-spacecraft-stuck-together#page-3&quot; rel=&quot;noreferrer&quot;&gt;this simplified diagram&lt;/a&gt;.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;The ET, in order to keep its own mass minimal, only lifts the CM and itself. When it is triggered, the CM/SM joins are broken, and the ET's rockets fire.&lt;/p&gt;&#xA;&lt;p&gt;The CM/SM connections all routed around the heat shield on the CM, none of them go through it. NASA has stated in several documents that this was to preserve the heat shield integrity.&lt;/p&gt;&#xA;&lt;p&gt;The CM and SM do need to be connected — the capsule is only good for a few hours comfortable operation without the SM's support systems, and NASA's safety culture precluded any in-flight connections being made that were not mission essential. (A failure to dock to the LM would not be a mission failure, per se. It would abort the landing, but the LM was essentially just pushed along with the CM/SM.)&lt;/p&gt;&#xA;&lt;p&gt;The shroud serves several purposes — one of which is transferring the load of the ET/CM/SM stack down to the S-IVB. The shroud also prevents damage to the LM before and during launch, and keeps the LM separated from the SM, as well as streamlining the stack, and preventing animals being accidentally carried. Note that the LM is not actually connected to the SM while inside the shroud — the CM/SM stack separates from the shroud, which then itself opens, revealing the LM.&lt;/p&gt;&#xA;&lt;p&gt;The LM itself is a dual stage design — the Lunar Descent Module, and the Lunar Ascent Module.&lt;/p&gt;&#xA;&lt;h1&gt;Alternate Stacking&lt;/h1&gt;&#xA;&lt;p&gt;NASA did consider some alternate stacking schemes, but these were not for the Apollo CM/SM/LM as launched — these final designs were concepts approved well before production designs were completed. Two of the three other stackings considered (see link 1) are worth discussion.&lt;/p&gt;&#xA;&lt;p&gt;One alternate stack was a CM/SM/LunarLaunch/LunarLanding stack — no on-orbit docking maneuvers. It would have, however, required a larger launcher than the Saturn V, and would have had the potential for losing all three astronauts at once at the moon, and would not have had the orbiting CM for comm relay and visual confirmation. The costs of designing an even larger launcher were considered prohibitive.&lt;/p&gt;&#xA;&lt;p&gt;The second alternate stack given serious consideration was to launch the LM separately on a second Saturn V. This would have allowed a considerably more powerful LEM and a larger CM as well. It was not, however, considered cost effective, and so was rejected.&lt;/p&gt;&#xA;&lt;h1&gt;Discussion and Elaboration&lt;/h1&gt;&#xA;&lt;p&gt;The CM design could not have had the LM above it — it would have made the escape tower too massive, and the Apollo project was run almost to the safety limit already. Further, that would have put several tons of highly explosive fuel above the CM at load, which would have been rejected even by the (now relatively lax) safety standards of 1960's NASA.&lt;/p&gt;&#xA;&lt;p&gt;The SM cannot be stacked above the CM for the same reasons, plus even more mass.&lt;/p&gt;&#xA;&lt;p&gt;That the capsule need not be 2-axis balanced was apparently not considered until later in the program, in the 1970's.&lt;/p&gt;&#xA;&lt;p&gt;The SM has to be able to use its engines; they're used for mid-course corrections, lunar orbit insertion and trans-Earth injection. Therefore, the LM cannot remain stacked underneath the SM if the SM is to have a single engine. The single engine is a matter of simplicity.&lt;/p&gt;&#xA;&lt;p&gt;The LM cannot be used as the TLI/LOI engine, either — it would require more structural mass, and the LM engine is left on the surface of the moon.&lt;/p&gt;&#xA;&lt;p&gt;The LM in the middle requires disconnecting and reconnecting the CM/SM, or having the CM be self-sufficient aside from the rocket motor, or having the connections run around the LM; for a variety of reasons, this was not practical.&lt;/p&gt;&#xA;&lt;p&gt;So, for reasons of simplicity, the historic stacking mode was the best combination of habitability, safety, and price for the available technology set.&lt;/p&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://smartech.gatech.edu/bitstream/1853/8042/3/SSEC_SE2_doc.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://smartech.gatech.edu/bitstream/1853/8042/3/SSEC_SE2_doc.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://news.bbc.co.uk/dna/place-lancashire/plain/A3770174&quot; rel=&quot;noreferrer&quot;&gt;http://news.bbc.co.uk/dna/place-lancashire/plain/A3770174&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceflight.nasa.gov/history/apollo/apollo_mission.swf&quot; rel=&quot;noreferrer&quot;&gt;http://spaceflight.nasa.gov/history/apollo/apollo_mission.swf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-24T19:32:28.327" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4042" PostTypeId="1" AcceptedAnswerId="51082" CreationDate="2014-03-19T12:30:42.273" Score="8" ViewCount="426" Body="&lt;p&gt;In the fictional 2003 book &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/142153441X&quot;&gt;The Next Continent&lt;/a&gt; (p. 241-252), set in 2030, the Moon Treaty is used in the UN International Court of Justice to put a commercial project at the south pole of the moon on hold.  The project was being conducted by a Japanese company, and while Japan never signed the Moon Treaty, it is argued (in the fictional plot line) that &lt;a href=&quot;http://en.wikipedia.org/wiki/Customary_international_law&quot;&gt;customary international law&lt;/a&gt; applies to the situation.  Supposedly, the Moon Treaty bans use of the moon's resources for commercial purposes.  The majority of their tourist-oriented moon palace is made of lunar ice, so in-situ resource use is unavoidable.  In this case, the plaintiff is the US government, whose competing NASA moon base is scientific and not for profit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Legally, how realistic is this?  The author seems to have done his research.  I want to know if these arguments are legally sound, legally weak, or laughable.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2021-03-27T18:40:39.577" Title="Could customary law realistically be used to block commercial use of the moon's resources?" Tags="|the-moon|law|natural-resources|" AnswerCount="2" CommentCount="20" ContentLicense="CC BY-SA 3.0" />
  <row Id="4043" PostTypeId="1" CreationDate="2014-03-19T12:49:30.047" Score="5" ViewCount="664" Body="&lt;p&gt;Is the shuttle ahead of the ISS in the orbit before docking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the international space station catchup with the shuttle in the same orbit?&lt;/p&gt;&#xA;" OwnerUserId="2907" LastEditorUserId="58" LastEditDate="2016-06-07T17:14:08.877" LastActivityDate="2016-06-07T17:14:08.877" Title="Position of shuttle viz iss" Tags="|orbital-mechanics|iss|space-shuttle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4044" PostTypeId="2" ParentId="4043" CreationDate="2014-03-19T15:10:22.920" Score="11" Body="&lt;p&gt;Directions: The ISS is in a nearly circular orbit. The &quot;v-bar&quot; axis is oriented along the ISS's velocity vector, with +vbar being along the velocity vector and -vbar against it. The r-bar axis is oriented radially, with +rbar pointing toward the center of the Earth and -rbar pointing out into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle docked with a specialized component on the ISS, Pressurized Mating Adapter 2 (PMA-2). The location of PMA-2 on the ISS bass changed a number of times. On the first five missions, PMA-2 was oriented upward, which meant docking on the -rbar. On the next two, PMA-2 was oriented downward, which meant docking on the +rbar. After that, PMA-2 has been oriented forward, which meant docking on the +vbar. In all cases, final approach was very slow. It took forty minutes for the Shuttle to go the last 320 feet With those vbar rendezvous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting to that final approach was tricky. It's not like the movies. Rendezvous is a long process. The Shuttle approached the Station from behind, in a slightly lower orbit than the ISS. The rendezvous timeline started almost six hours prior to docking. The action started about two and half hours prior to the Ti burn. At this point, the Shuttle performed a height adjustment burn that made the Shuttle's apogee equal to the ISS' orbital altitude. Half an orbit later, at apogee, the Shuttle performed it's last phasing burn. This made the Shuttle's perigee about 40,000 feet lower than apogee. An orbit later (three hour prior to docking), the Shuttle performed the Ti (transition initiation) burn. This burn raised perigee to about 10,000 feet below apogee. The Ti burn started the final hop to the Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About 4/5 the way into this hop is when the crew took over. At this point, the Shuttle was about 600 feet below the ISS. The Shuttle began flying the Twice Orbital Rate V-Bar Approach (TORVA) to slowly bring the Shuttle up to the vbar and in front of the ISS. Once on the vbar, the Shuttle slowly worked its way toward the docking port, docking about forty minutes after arriving on the +vbar.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/567076main_RNDZ_135_F.pdf&quot;&gt;STS 135 Flight Data File / Rendezvous&lt;/a&gt;.&lt;br&gt;&#xA;Pages 1-5 to 1-8 have nice diagrams of the rendezvous and docking. All Shuttle flights to the ISS that docked on the vbar followed very similar profiles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023479.pdf&quot;&gt;History of Space Shuttle Rendezvous&lt;/a&gt;.&lt;br&gt;&#xA;In addition to covering all Shuttle missions that involved rendezvous, this document contain a 50 page summary of pre-Shuttle rendezvous operations.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-19T15:26:28.600" LastActivityDate="2014-03-19T15:26:28.600" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4045" PostTypeId="2" ParentId="4026" CreationDate="2014-03-19T17:59:08.727" Score="-2" Body="&lt;p&gt;There was the Russian astronaut who had to empty his space suit to re-enter his ship on the very first space walk:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As they had with the first satellite and first man in space, the Soviets again stunned the world on March 18, 1965 with the first spacewalk (and the first EVA) performed by Alexey Leonov from the Voskhod 2 spacecraft, for 12 minutes outside the spacecraft. Leonov had no means to control his motion other than pulling on his 50.7-foot (15.5 m) tether. After the flight, he claimed this was easy, but his space suit ballooned from its internal pressure against the vacuum of space, stiffening so much that he could not activate the shutter on his chest-mounted camera&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-&lt;a href=&quot;http://en.wikipedia.org/wiki/Extra-vehicular_activity&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Extra-vehicular_activity&lt;/a&gt;  (in &quot;Development History&quot;)&lt;/p&gt;&#xA;" OwnerUserId="2912" LastActivityDate="2014-03-19T17:59:08.727" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4046" PostTypeId="2" ParentId="4034" CreationDate="2014-03-19T18:23:16.640" Score="6" Body="&lt;p&gt;I signed up for alerts at &lt;a href=&quot;http://spotthestation.nasa.gov/index.cfm&quot; rel=&quot;nofollow&quot;&gt;spotthestation.nasa.gov&lt;/a&gt;.  There you can register your location and have alerts sent to your email when the space station will be passing over you.  It also tells you what part of the sky it will first appear and will it will disappear along with the angle above the horizon and the duration it is visible.  Here is a sample of what the email will contain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time: Fri Mar 14 5:53 AM, Visible: 2 min, Max Height: 73 degrees, Appears: SSE, Disappears: NE&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this helps.  Happy viewing&lt;/p&gt;&#xA;" OwnerUserId="2913" LastEditorUserId="4660" LastEditDate="2014-10-09T20:12:16.013" LastActivityDate="2014-10-09T20:12:16.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4047" PostTypeId="1" AcceptedAnswerId="4048" CreationDate="2014-03-19T19:48:03.810" Score="4" ViewCount="387" Body="&lt;p&gt;It's improbable to get a payload into a perfect orbit, but obviously there is room for error, otherwise payloads in GSO would not work as well as they do. If the target eccentricity for a payload in GSO is 0, what is the margin of error? Have there been any egregious errors when maneuvering a payload to GSO?&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2014-03-21T15:05:29.137" Title="How much do eccentricities of different satellites in GSO vary?" Tags="|artificial-satellite|orbit|geosynchronous|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4048" PostTypeId="2" ParentId="4047" CreationDate="2014-03-19T20:46:41.120" Score="8" Body="&lt;p&gt;Once in GSO, the eccentricity is perturbed by a few effects, particularly the pressure of sunlight (not the solar wind), on the satellite.  It accelerates the satellite at 6PM local and decelerates the satellite at 6AM local, forcing the orbit to become elliptical.  The eccentricity causes the satellite to oscillate east-west on a 24 hour cycle.  Its control is part of the normal east-west stationkeeping.  Generally the box is $\pm 0.05^\circ$ in longitude, but this can vary.  A reasonable target eccentricity is $0.00028$ in this range.&lt;/p&gt;&#xA;" OwnerUserId="176" LastEditorUserId="176" LastEditDate="2014-03-20T16:39:53.267" LastActivityDate="2014-03-20T16:39:53.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4049" PostTypeId="1" CreationDate="2014-03-20T13:14:53.470" Score="4" ViewCount="247" Body="&lt;p&gt;After seeing &lt;a href=&quot;http://www.bbc.co.uk/news/health-26458868&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; article about a skin-tight space suit, I was wondering what were the different ways of combating the negative effect of being in space, e.g. artificial gravity? How far away are these technologies? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see this as the biggest barrier for long-duration space travel, as the technology exists for actual travel (albeit slow and expensive). Is this premise accurate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; I know the scope of this question is broad. I don't need the answer to be too in depth. &lt;/p&gt;&#xA;" OwnerUserId="65" LastEditorUserId="65" LastEditDate="2014-03-20T14:49:19.113" LastActivityDate="2014-08-11T21:20:30.417" Title="Combating negative effects of space" Tags="|microgravity|radiation|humans|environment|travel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4050" PostTypeId="1" AcceptedAnswerId="4053" CreationDate="2014-03-20T15:24:57.207" Score="33" ViewCount="17584" Body="&lt;p&gt;Lagrange points are the points in a multi-body gravitational system in which the gravitational force and centrifugal force sum to zero.  The image below from &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;this Wikipedia article&lt;/a&gt; shows the 5 Lagrange points in a two body system.  The L1, L2, and L3 Lagrange points are stable in one direction but unstable in the others while the L4 and L5 Lagrange points are unstable in both directions.  My question is:  Is there a buildup of space trash at these stable (in one direction) Lagrange points?  Or does their instability in the other dimension prevent such a buildup?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; The L4 and L5 Lagrange points are stable in both directions &quot;provided that the ratio of M1/M2 is greater than 24.96,&quot; to quote the Wikipedia article linked above, which is the case for the Earth-Moon system.  In light of this fact, my updated question is: Is there a buildup of space trash at &lt;em&gt;any&lt;/em&gt; of the Earth-Moon Lagrange points?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/aM9L5.jpg&quot; alt=&quot;Lagrange points in a two-body system&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2699" LastEditorUserId="2699" LastEditDate="2014-03-23T17:44:20.573" LastActivityDate="2014-03-23T17:44:20.573" Title="Is there a lot of space trash at the Earth-Moon Lagrange points?" Tags="|orbital-mechanics|debris|lagrangian-points|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4051" PostTypeId="2" ParentId="4050" CreationDate="2014-03-20T16:49:37.987" Score="12" Body="&lt;p&gt;There's a number of missions that have headed to various of the Lagrange points, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrange_point#List_of_missions_to_Lagrangian_points&quot;&gt;Wikipedia&lt;/a&gt; for a good list. Let's look at what it takes to stay there:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://jwst.nasa.gov/faq_scientists.html#lifetime&quot;&gt;James Webb&lt;/a&gt;- Proposed for L1 point, can only take enough fuel for a 10 year mission.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.universetoday.com/103212/astronomers-capture-images-of-herschel-telescope-heading-toward-its-graveyard-orbit/&quot;&gt;Hershel Space Telescope&lt;/a&gt;: Was deliberately moved from L2 to a heliocentric because &quot; the spacecraft would be in a slow tumble, receding from its stable L2 orbit, subjected to solar radiation pressure. And as ESA’s ground stations were no longer communicating with it, so we wanted to basically check the orbits and make sure that for future science, it was not mistakenly detected as an asteroid.&quot;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There has never been an L3 mission, but the same sort of logic follows, fuel usage is required to maintain that orbit, and as a result, there shouldn't be a large amount of debris left behind there.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="2843" LastEditDate="2014-03-21T00:15:49.307" LastActivityDate="2014-03-21T00:15:49.307" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4052" PostTypeId="2" ParentId="4050" CreationDate="2014-03-20T17:46:58.877" Score="23" Body="&lt;p&gt;The Lagrange points L1, L2 and L3 are stable in prograde- and retrograde direction, but unstable on the radial axis. That means any object on these points will drift away in radial direction unless it uses small amounts of thrust to balance on these points, so any concentration of natural mass or debris at these points is impossible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only the points L4 and L5 are stable, and objects, so-called &lt;a href=&quot;http://en.wikipedia.org/wiki/Trojan_%28astronomy%29&quot;&gt;&quot;Trojans&quot;&lt;/a&gt;, tend to orbit these points. There is also one which orbits the Earth/Sun L4: &lt;a href=&quot;http://neo.jpl.nasa.gov/news/news173.html&quot;&gt;2010 TK&lt;sub&gt;7&lt;/sub&gt;&lt;/a&gt;. It's rare for trojans to orbit closely to the lagrange points. The reason is that these points would only be 100% stable when the Sun and Earth would be the only objects in the solar system. But due to the gravitational influence of the other planets, an object parked exactly at L4 or L5 would be slowly but steadily dragged away from its point and end up in an orbit around it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The L4 and L5 points of Moon/Earth appear to be clean, except for &lt;a href=&quot;http://en.wikipedia.org/wiki/Kordylewski_cloud&quot;&gt;some faint clouds of dust&lt;/a&gt;, and even the existence of those is disputed.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-03-20T18:36:58.657" LastActivityDate="2014-03-20T18:36:58.657" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4053" PostTypeId="2" ParentId="4050" CreationDate="2014-03-20T18:28:23.147" Score="21" Body="&lt;p&gt;To add to other answers, the L1-L2 Lagrange points are unstable because they need to follow radial velocity of the two parent bodies as they orbit each other, in our case the Earth in a heliocentric orbit around the Sun, but none of them are really at the orbital altitude matching their radial velocity (too slow in L1 and too fast in L2). Since the Earth's orbit isn't exactly circular (orbital eccentricity of ~ 0.017), their altitude will also slightly change during one orbital period. This is less of a case with L4-L5, and arguably also L3, depending on orbital eccentricity of parent bodies, and is where we might find &lt;a href=&quot;https://en.wikipedia.org/wiki/Trojan_asteroids&quot; rel=&quot;nofollow noreferrer&quot;&gt;Trojan&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Hilda_family&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hilda&lt;/a&gt; family of asteroids, respectively. All these Lagrange points can also be perturbed by gravitational influence of other celestials in the system, for example Jupiter's and even Moon's orbit in case of L1-L2 Sun-Earth points. And this is of course about instabilities in the velocity vector along the M1 parent body. Orthogonal to it and towards the M1 and M2 bodies, it's just the tipping point in direction towards M1 or M2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slightly simplifying, what I'm talking about is that the heliocentric velocity (using $v_o \approx {2 \pi a \over T}$) will be $\text{SEL1} \approx 29.49\ \text{km/s}\ \ $ and $\text{SEL2} \approx 30.08\ \text{km/s}\ \ $, where Earth's orbital velocity is $v_o \approx 29.78\ \text{km/s}\ \ $. This difference will be maintained by the L1-L3 saddle points, not too dissimilar to surfing at the tip of a wave. Any lateral movement will tip your balance towards one of the two parent bodies (M1 or M2).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Lagrange point satellite orbits need to be managed, what's usually referred to as &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_station-keeping&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital station-keeping&lt;/a&gt;. Effects of these perturbations and instabilities can be somewhat offset by placing Lagrange point satellites into &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Halo&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lissajous orbits&lt;/a&gt; and using precision orbital insertion, but not even these will be stable without using onboard propellants and corrections to their orbits. Any orbital debris or whole defunct satellites will eventually spiral towards the Earth (&lt;a href=&quot;https://space.stackexchange.com/a/3760/49&quot;&gt;see update to this answer&lt;/a&gt;, but they hold too much orbital momentum to really fall towards the Sun, as their orbital period actually matches that of the Earth's, but their semi-major axis towards the Sun doesn't for about ± 1.5 million kilometers or roughly ± 1%).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt;: All this means that the L1 and L2 points would essentially be free from long-term orbital debris. And with L3-L5, unless the bodies there formed from the same protoplanetary disk and share same radial velocity that's required to stay at that altitude, there is nearly no chance that any other body with significantly different orbital energy would be captured at those points. But if we deliberately placed satellites there, any debris from them would stay there for a lot longer than with L1 and L2 points (and perhaps L3, as mentioned before).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: Apparently, I've misread the question initially and was answering for the Sun-Earth lagrange points instead of the Earth-Moon ones. OK, no problem, most of the problems remain the same in theory, only the L1-L3 saddle points are even more unstable. Moon's orbital eccentricity is ~ 0.055, so L1-L3 points move even more along the Earth-Moon axis. On average, EML1 is 326,380 km away from Earth and 58,019 km away from the Moon, EML2 448,914 km and 64,515 km respectively, and their velocities would be (again, average) ~ 0.87 and 1.2 times average orbital velocity of the Moon (1.022 km/s). They are even more perturbed, especially by the Sun itself and the complexity of the Moon's orbit relative to the Sun (it never curves on itself in loops though, contrary to popular beliefs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a nice image depicting Earth-moon lagrange points:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/eXWhr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The Lagrange points for the Earth-moon system. Credit: David A. Kring, &lt;a href=&quot;http://www.lpi.usra.edu/exploration/&quot; rel=&quot;nofollow noreferrer&quot;&gt;LPI-JSC Center for Lunar Science and Exploration&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is how Lissajous orbits of the &lt;a href=&quot;http://www.nasa.gov/mission_pages/artemis/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARTEMIS&lt;/a&gt; (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun) mission's P1 spacecraft's EML1 and EML2 orbits looked like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/l1Gn8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The view from above of the ARTEMIS orbits as they make the transition from the kidney-shaped Lissajous orbits on either side of&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;the moon to orbiting around the moon. Credit: &lt;a href=&quot;http://www.nasa.gov/mission_pages/artemis/news/lunar-orbit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA/Goddard Space Flight Center&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/3pSrl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Illustration of Artemis-P1 liberations orbits, side or ecliptic view. Credit: &lt;a href=&quot;http://www.nasa.gov/mission_pages/themis/news/artemis-orbit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA/Goddard&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lagrange points calculator: &lt;a href=&quot;http://www.orbitsimulator.com/formulas/LagrangePointFinder.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbitsimulator.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Bad Astronomy Blog: &lt;a href=&quot;http://blogs.discovermagazine.com/badastronomy/2008/09/29/the-moon-that-went-up-a-hill-but-came-down-a-planet/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Moon that went up a Hill but came down a planet&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;NASA: &lt;a href=&quot;http://www.nasa.gov/mission_pages/artemis/news/lunar-orbit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARTEMIS Spacecraft Prepare for Lunar Orbit&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;NASA: &lt;a href=&quot;http://www.nasa.gov/mission_pages/themis/news/artemis-orbit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARTEMIS - The First Earth-Moon Libration Orbiter&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-03-21T01:19:44.953" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4054" PostTypeId="1" AcceptedAnswerId="4055" CreationDate="2014-03-20T19:00:43.467" Score="10" ViewCount="6049" Body="&lt;p&gt;What are typical delta-v for deep space maneuvers, used by deep space missions to outer planets or for &quot;&lt;a href=&quot;http://voyager.jpl.nasa.gov/&quot;&gt;interstellar missions&lt;/a&gt;&quot; ? For example, Voyager 1 now has 17 km/s speed relative to the Sun. What part of the speed was created by the rocket, what was delta-v of its hydrazine thruster, and what was delta-v from gravitational slingshots (gravity assists)?&lt;/p&gt;&#xA;" OwnerUserId="2843" LastActivityDate="2016-11-08T18:28:19.097" Title="Delta-v of deep space maneuvers in deep space missions like Voyager or Pioneer" Tags="|voyager|deep-space|delta-v|pioneer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4055" PostTypeId="2" ParentId="4054" CreationDate="2014-03-20T22:50:32.720" Score="14" Body="&lt;p&gt;&lt;strong&gt;Voyager 1&lt;/strong&gt;: separated from Centaur stage with velocity 18.3 km/s (relative to Earth, &lt;a href=&quot;http://books.google.com/books?id=Gp-2B7AUPVUC&amp;amp;pg=PA120#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Dave Doody, &lt;em&gt;Deep Space Craft: An Overview of Interplanetary Flight&lt;/em&gt;, 2010, page 120&lt;/a&gt;). Then 76.5N Injection Propulsion Unit of V1 did burn its solid fuel in 43 seconds giving additional 1,7 km/s (?), injecting the spacecraft into Hohmann transfer orbit. IPU was separated, and Mission Module itself has only sixteen thrusters (0.9N each) used only to fine-tuning (statistical maneuvers?). So Voyager 1 (and its brother Voyager 2) had no Deep Space Maneuvers, flying on free-fall cruise with minor corrections, and doing unpowered flybys (gravity assists)..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Voyager 2&lt;/strong&gt; should be similar to Voyager 1, and there is &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:Voyager_2_velocity_vs_distance_from_sun.svg&quot; rel=&quot;noreferrer&quot;&gt;plot of its velocity relative to Sun&lt;/a&gt; with 4 passive gravity assists: Jupiter in 1979, Saturn in 1981, Uranus in 1986 and Neptune in 1989.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Messenger&lt;/strong&gt; did Deep Space Maneuver (DSM) at 2005-12-12 with delta-v of 316 m/s (in 524 second burn by using 18% of propellants - same book, page 71), preparing for Venus gravity assist 10 months later. Total DSM count is 5, and there was 830 m/s injection maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Cassini&lt;/strong&gt; did one huge DSM of -450.2 m/s with 440 N engine to prepare Venus gravity-assist. (There were several small TCM - Trajectory correction maneuvers). Also it used 626 m/s delta-v to insert itself into Saturn orbit. (page 72 same book or &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19490/1/98-0900.pdf&quot; rel=&quot;noreferrer&quot;&gt;1998 plans, page 3&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Galileo&lt;/strong&gt;  interplanetary mission (VEEGA=Venus-Earth-Earth gravity assists), was started by IUS solid stage engines, with 3.1 km/s of heliocentric velocity (&lt;a href=&quot;http://articles.adsabs.harvard.edu/full/gif/1992SSRv...60...23D/0000027.000.html&quot; rel=&quot;noreferrer&quot;&gt;page 27&lt;/a&gt; of &lt;a href=&quot;http://adsabs.harvard.edu/full/1992SSRv...60...23D&quot; rel=&quot;noreferrer&quot;&gt;1992SSRv...60...23D&lt;/a&gt;). &lt;a href=&quot;http://articles.adsabs.harvard.edu/full/gif/1992SSRv...60...23D/0000028.000.html&quot; rel=&quot;noreferrer&quot;&gt;TCM were&lt;/a&gt;: 15.7 m/s TCM-1, 0.7 m/s TCM-2, zero TCM-3 (cancelled), &lt;a href=&quot;http://articles.adsabs.harvard.edu/full/gif/1992SSRv...60...23D/0000030.000.html&quot; rel=&quot;noreferrer&quot;&gt;24.2 m/s&lt;/a&gt; TCM-4A, 11 m/s TCM-4B, ~1-m/s TCM-5, 3-5 m/s for TCM-9 and 10, 21 m/s TCM-14. Venus gravity assists added 2.3 km/s (heliocentric). First Earth flyby &lt;a href=&quot;http://articles.adsabs.harvard.edu/full/gif/1992SSRv...60...23D/0000031.000.html&quot; rel=&quot;noreferrer&quot;&gt;raised&lt;/a&gt; velocity of spacecraft by 5.2 km/s, from 30.1 km/s to 35.3 km/s (heliocentric) and orbit period to 2 years. Second Earth flyby added 3.7 km/s, raising total to 39 km/s (5,6 years period). Jupiter insertion maneuver (JOI) was planned to be &lt;a href=&quot;http://articles.adsabs.harvard.edu/full/gif/1992SSRv...60...23D/0000038.000.html&quot; rel=&quot;noreferrer&quot;&gt;-630 m/s&lt;/a&gt;. Galileo's &quot;satellite tour&quot; was planned to use total delta-v of 6000-8000 m/s from many gravity assists, which is described as 4-5 times larger than total delta-v of spacecraft thrusters (&lt;a href=&quot;http://articles.adsabs.harvard.edu/full/gif/1992SSRv...60...23D/0000024.000.html&quot; rel=&quot;noreferrer&quot;&gt;page 24&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Saturn Atmospheric probe&lt;/em&gt; was planned to have &lt;a href=&quot;http://www.nap.edu/reports/13117/App%20G%2018a_Saturn-Probe.pdf#page=24&quot; rel=&quot;noreferrer&quot;&gt;450 m/s&lt;/a&gt; delta-v DSM to set up Earth flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;New Horizons&lt;/em&gt; was planned to have &lt;a href=&quot;http://www.boulder.swri.edu/~tcase/Kouch_Acta57_2005.pdf#page=8&quot; rel=&quot;noreferrer&quot;&gt;397 m/s&lt;/a&gt; delta-v budget &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;NEAR&lt;/em&gt; &lt;a href=&quot;http://techdigest.jhuapl.edu/TD/td2301/dunham.pdf#page=5&quot; rel=&quot;noreferrer&quot;&gt;used&lt;/a&gt; 268 m/s DSM-1 and 932 m/s DSM-2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, 300-500 m/s delta-v are typical for deep space maneuvers in classic interplanetary missions. Much greater delta-v, of 2-5 km/s was gained by unpowered gravity assists.&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2016-11-08T18:28:19.097" LastActivityDate="2016-11-08T18:28:19.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4056" PostTypeId="1" AcceptedAnswerId="4057" CreationDate="2014-03-20T23:17:57.723" Score="11" ViewCount="7855" Body="&lt;p&gt;In some papers there is &quot;launch energy&quot; characteristic of spacecraft launched into space from Earth. Sometimes it is written as &quot;C₃&quot; (C&lt;sub&gt;3&lt;/sub&gt;). For example, &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40727/1/JPl%20Pub%2008-03_A.pdf&quot;&gt;Ares V document&lt;/a&gt; lists &quot;C₃&quot; as equal or less than 35 km²/s².&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the meaning of  &quot;C₃&quot; (&lt;a href=&quot;http://en.wikipedia.org/wiki/Characteristic_energy&quot;&gt;Characteristic energy&lt;/a&gt;?), and what is difference between 35 km²/s² (start from Earth), 10 km²/s² (&quot;delivering 8 tonn to mars at a C₃ of 10 km²/s²&quot;), 80  km²/s² (required for sample return from Jovian moons), and 110 km²/s² (needed to reach Saturn without gravity assists - in direct mission)&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2014-03-21T00:00:29.780" LastActivityDate="2014-03-21T00:13:37.287" Title="What is the &quot;launch energy&quot; or C₃ measured in km² / s²" Tags="|launch|energy|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4057" PostTypeId="2" ParentId="4056" CreationDate="2014-03-21T00:13:37.287" Score="16" Body="&lt;p&gt;It is what your velocity would be at a sufficient distance from Earth that its gravity doesn't matter, squared.  That velocity is $v_\infty$, so $C_3=v_\infty^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It can be calculated at any distance from Earth as your specific energy (energy per unit mass), times two:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$C_3 = v_\infty^2 = v^2-{2\mu\over r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So wherever you are, use the magnitude of your velocity, $v$, at that point, your distance from the center of the Earth or the barycenter of the Earth-Moon system, $r$, and the GM of Earth or the Earth-Moon system, $\mu$.  Note that as $r$ goes to $\infty$, the expression goes to your velocity squared.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The term has an ancient derivation as the third integration constant when integrating the equation of motion.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-03-21T00:13:37.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4058" PostTypeId="1" AcceptedAnswerId="4059" CreationDate="2014-03-21T13:51:20.120" Score="11" ViewCount="2915" Body="&lt;p&gt;&lt;a href=&quot;https://solarsystem.nasa.gov/missions/profile.cfm?Target=Beyond&amp;amp;MCode=Voyager_1&quot; rel=&quot;noreferrer&quot;&gt;NASA site mentions&lt;/a&gt; following facts about Voyager.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Did you know that the record is a 30-cm gold-plated copper disc? Together with a needle and playing instructions, it is mounted on the body casing and on it are recorded natural Earth sounds, 90 min of music, 115 pictures, and greetings in 60 languages.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Voyager 1 will remain within the confines of the solar system until it emerges from the Oort cloud in another 14,000 to 28,000 years&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Thinking about the time span 14,000+ years to go just to get out from Oort cloud, it would take millions of years to reach another solar system.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Aren't we endangering our future generations by sending out such a record to unknown aliens?&lt;/li&gt;&#xA;&lt;li&gt;Knowing that these crafts will take thousands of years just to get out of solar system these golden records concept seems like a publicity stunt by someone at NASA. What was the real need for this?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="2722" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2021-08-13T21:39:32.690" Title="Why did we send those golden records on Voyager crafts?" Tags="|nasa|voyager|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4059" PostTypeId="2" ParentId="4058" CreationDate="2014-03-21T14:13:49.293" Score="19" Body="&lt;p&gt;The thought was that long, long after we're gone as a species and perhaps even as a life-bearing planet, some record of our species, our culture, and of life on Earth would be preserved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it provides directions to our solar system, but so does the velocity and position of the spacecraft itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, it is very difficult to see how it would be found by anyone, unless they knew exactly where to look.  Running across it accidentally would be like winning the lottery many times over.  Space is big.  Really big.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-03-21T14:13:49.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4060" PostTypeId="2" ParentId="4047" CreationDate="2014-03-21T15:05:29.137" Score="2" Body="&lt;p&gt;@Stu, here is an example of a satellite in GSO, and the impact of slight eccentricity coupled with high inclination with the &lt;a href=&quot;http://www.jaxa.jp/countdown/f18/overview/orbit_e.html&quot; rel=&quot;nofollow&quot;&gt;JAXA description of QZSS &lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With that example in mind, you can almost visualize the space of trajectories with varying eccentricity in GSO.  If an egregious error were made, you could expect to see variations in both, but will depend on the magnitude the error creates in the initial conditions of inclination and eccentricity.  Other perturbations could make the situation only go downhill (pun?) from here, based upon the type of error made and ability to control north-south / east-west station keeping.  An older (1982) UT Austin conference paper on inclined/eccentric GSO is &lt;a href=&quot;http://adsabs.harvard.edu/abs/1982asdy.conf.....N&quot; rel=&quot;nofollow&quot;&gt;cited here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1478" LastActivityDate="2014-03-21T15:05:29.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4061" PostTypeId="2" ParentId="4058" CreationDate="2014-03-21T16:47:00.160" Score="12" Body="&lt;p&gt;The Voyager probes, or any other spacecraft leaving the Solar system, do not pose a risk to future generations. As you mention in your question, they move relatively slowly. As Mark mentions in his answer: &lt;em&gt;&quot;Space is big. Really big&quot;&lt;/em&gt;. So the chances any starfaring non-Earth based life will find one of these craft are remote at best.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As mentioned in the question about &lt;a href=&quot;https://space.stackexchange.com/questions/2835/&quot;&gt;Interstellar eavesdropping&lt;/a&gt;, we have been broadcasting to the Universe for a long time. There is a sphere currently about 90 light-years in size, where every solar system has had the opportunity to become aware of the Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that gold plated record on Voyager is comparatively less noticeable than a single grain of gold sand on an ocean beach.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-03-21T22:20:28.987" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4062" PostTypeId="1" AcceptedAnswerId="4063" CreationDate="2014-03-21T17:07:18.637" Score="13" ViewCount="1554" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/questions/2835&quot;&gt;Interstellar eavesdropping&lt;/a&gt; points out that we have been broadcasting to space for a long time, currently we are approaching a century.  This means every star system in within 100 light years has had the opportunity to become aware of Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are of course technical issues in receiving and translating our broadcasts, particularity across light years, I am allowing those issues to be the subject of different questions.  This question focuses on the size and membership of the broadcast sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many star systems have our broadcasts been exposed to? What will the exposure be as time goes on? (i.e. another 100 or 1,000 years).  Of the &lt;a href=&quot;https://space.stackexchange.com/questions/tagged/exoplanet&quot;&gt;exoplanets&lt;/a&gt; we have discovered so far how many are in our broadcast sphere?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-03-21T19:10:05.340" Title="How big is Earth's sphere of broadcast influence?" Tags="|communication|astrobiology|exoplanet|extra-solar-object|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4063" PostTypeId="2" ParentId="4062" CreationDate="2014-03-21T18:24:38.750" Score="12" Body="&lt;p&gt;Within 100 light-years, there are approximately 14,600 stars. Many of these are not even known. It is difficult to determine exactly the number of star systems as opposed to stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;113 star systems within 100 light-years are confirmed to have exoplanets in them (listed from nearest to furthest):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Alpha Centauri B     → cont'd ↓             → cont'd ↓           → cont'd ↓&#xA;Epsilon Eridani      Upsilon Andromedae     Beta Pictoris        HD 142&#xA;Gliese 674           47 Ursae Majoris       GJ 3483              HD 16417&#xA;Gliese 876           Gamma Cephei           HD 189733            HD 39194&#xA;Gliese 832           HIP 79431              HD 192263            HD 96700&#xA;82 G. Eridani        HD 136352              7 Canis Majoris      Gliese 439&#xA;Gliese 581           HD 176051 B            GJ 3634              23 Librae&#xA;Gliese 667 C         HD 38858               HD 217107            HD 30562&#xA;Fomalhaut            Gliese 317             Alpha Arietis        HD 134606&#xA;61 Virginis          Tau Boötis             HD 24496 A           HD 20367&#xA;Gliese 433           Mu Arae                HD 114783            HD 181433&#xA;Gliese 785           51 Pegasi              HD 33564             Rho Indi&#xA;Gliese 176           HD 113538              HD 97658             HD 204941&#xA;HD 102365            Gliese 777             HD 104067            HD 82943&#xA;Gliese 849           HD 128311              CFBDS 1448           HD 179949&#xA;Gliese 436           Gliese 676 A           16 Cygni B           HD 93083&#xA;Pollux               HD 7924                HD 210277            HD 125595&#xA;Gliese 649           Iota Horologii         HD 215152            HD 70642&#xA;Gliese 86            GJ 3021                HD 45184             HD 13808&#xA;54 Piscium           HD 10647               HD 4308              HD 218566&#xA;Gliese 370           14 Herculis            HD 164922            HD 52265&#xA;HIP 57050            HD 154088              HD 90156             HD 114386&#xA;Ross 458             HD 189567              94 Ceti              HD 75289&#xA;Gliese 179           Epsilon Reticuli       HIP 12961            HD 101930&#xA;55 Cancri            HD 87883               HD 1461              HD 111232&#xA;HD 69830             83 Leonis B            HIP 70849            HD 150433&#xA;HD 40307             70 Virginis            HD 134060            HD 102195&#xA;HD 147513            Pi Mensae              HD 156668            HD 50554&#xA;GJ 1214              HD 154345              HD 60532             Kappa Coronae Borealis&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Within just 50 light years, 93 exoplanets have been discovered (listed from nearest to furthest):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Alpha Centauri Bb    → cont'd ↓            → cont'd ↓           → cont'd ↓ &#xA;Luhman 16 Ab         Gliese 581 c           Gliese 649 c         HD 40307 f&#xA;Epsilon Eridani b    Gliese 581 d           Gliese 649 b         HD 40307 g&#xA;Epsilon Eridani d    Gliese 667 Cb          Pollux b             Upsilon Andromedae b&#xA;Epsilon Eridani c    Gliese 667 Cc          Gliese 86 b          Upsilon Andromedae c&#xA;Epsilon Indi b       Gliese 667 Cf          HIP 57050 b          Upsilon Andromedae d&#xA;Tau Ceti b           Gliese 667 Ce          54 Piscium Ac        Upsilon Andromedae e&#xA;Tau Ceti c           Gliese 667 Cd          54 Piscium Ab        47 Ursae Majoris b&#xA;Tau Ceti d           Gliese 667 Cg          Gliese 370 b         47 Ursae Majoris c&#xA;Tau Ceti e           Fomalhaut b            Gliese 179 b         47 Ursae Majoris d&#xA;Tau Ceti f           61 Virginis b          55 Cancri e          Gamma Cephei Ab&#xA;Teegarden's star b   61 Virginis c          55 Cancri b          HIP 79431 b&#xA;Gliese 674 b         61 Virginis d          55 Cancri c          HD 136352 b&#xA;Gliese 876 d         Gliese 785 b           55 Cancri f          HD 136352 c&#xA;Gliese 876 c         Gliese 785 c           55 Cancri d          HD 136352 d&#xA;Gliese 876 b         Gliese 433 b           HD 69830 b           HD 176051 Bb&#xA;Gliese 876 e         Gliese 433 c           HD 69830 c           Gliese 163 b&#xA;Gliese 832 b         Gliese 849 b           HD 69830 d           Gliese 163 e&#xA;VB 10 b              Gliese 849 c           HD 147513 b          Gliese 163 c&#xA;82 G. Eridani b      HD 102365 b            GJ 1214 b            Gliese 163 f&#xA;82 G. Eridani c      Gliese 176 b           HD 40307 b           Gliese 163 d&#xA;82 G. Eridani c      Gliese 436 c           HD 40307 c           Gliese 317 b&#xA;Gliese 581 e         Gliese 436 b           HD 40307 d           Gliese 317 c&#xA;Gliese 581 b         Gliese 436 d           HD 40307 e           HD 38858 b&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Based on the average number of confirmed exoplanets per star system, there are about 140 exoplanets total within 100 light-years in confirmed planetary systems. I won't list the rest of them here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we use the same average across our estimated number of stars, we arrive at about 17,500 exoplanets in 100 light-years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also some rogue planets within 100 light-years which aren't even considered in any of these numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, in another 100-1000 years, there will be practically countless planets in our sphere of broadcast influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even more interesting, eight out of the 93 discovered exoplanets within 50 light-years are potentially habitable. If this same ratio extended out to our 17,500 estimate, that would mean about 1,500 potentially habitable planets within 100 light-years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_exoplanetary_host_stars&quot;&gt;List of exoplanetary host stars - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_nearest_exoplanets&quot;&gt;List of nearest exoplanets - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980123d.html&quot;&gt;Number of stars within 100 light years of Earth - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.dailygalaxy.com/my_weblog/2012/11/lost-rogue-planet-found-floating-in-space-100-light-years-distant.html&quot;&gt;New Lost Planet Found Floating in Space 100 Light Years Away - The Daily Galaxy&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_potential_habitable_exoplanets&quot;&gt;List of potentially habitable exoplanets - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="49" LastEditDate="2014-03-21T19:10:05.340" LastActivityDate="2014-03-21T19:10:05.340" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4064" PostTypeId="1" AcceptedAnswerId="4066" CreationDate="2014-03-22T14:15:41.270" Score="19" ViewCount="15899" Body="&lt;p&gt;There's a &lt;a href=&quot;http://www.snopes.com/history/american/gauge.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;kind of urban legend&lt;/a&gt; that the Space Shuttle boosters diameter could have been larger if it was not restricted by the limitations imposed by the railroads, specifically the railroad gauge. I can't find any convincing detailed analysis of how fair the claim was about the boosters diameter being dependent on the railroad gauge.&lt;/p&gt;&#xA;&lt;p&gt;Did the boosters have exactly that diameter because the larger the better and the current one is the largest one that could be afforded with transporting the boosters by railroad?&lt;/p&gt;&#xA;" OwnerUserId="2948" LastEditorUserId="7062" LastEditDate="2021-07-22T16:57:18.580" LastActivityDate="2021-07-22T16:59:57.653" Title="How fair is the claim that Shuttle boosters diameter was dictated by the railroad gauge?" Tags="|rockets|space-shuttle|construction|standards|booster|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="4065" PostTypeId="1" AcceptedAnswerId="4068" CreationDate="2014-03-22T14:56:48.660" Score="5" ViewCount="1039" Body="&lt;p&gt;There have been several hard science fiction stories where sheets or a ball of ice is used on a spacecraft. The function is usually as forward shielding against dust and small debris. Is this practical at all?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fuel costs aside, it seems the ice shield would wear away after a while and there would be no way to rebuild it. Would it make more sense to use something more easily rebuilt like ceramic plating?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other stories about using the same ice as reaction mass. I assume the atoms would be split for such a purpose as an &quot;atomic rocket&quot; per se. However you would also run out of ice after a while, assembly of ice seems more daunting than using some type of high atmosphere scoop on a gas giant. Am I off base with my assumptions?&lt;/p&gt;&#xA;" OwnerUserId="2709" LastEditorUserId="49" LastEditDate="2015-10-20T16:21:17.013" LastActivityDate="2022-08-11T08:54:59.760" Title="Is a ball of ice practical as reaction mass and forward shielding?" Tags="|propulsion|crewed-spaceflight|interstellar-travel|water|shielding|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4066" PostTypeId="2" ParentId="4064" CreationDate="2014-03-22T15:44:51.490" Score="21" Body="&lt;p&gt;The claim may be based on a misunderstanding. The solid rocket boosters were &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;made in Utah and transported to the launch site in 4 segments&lt;/a&gt; by rail, which did limit their size.&#xA;But it was not the &lt;a href=&quot;http://en.wikipedia.org/wiki/Track_gauge&quot; rel=&quot;nofollow noreferrer&quot;&gt;track gauge&lt;/a&gt; that influenced this limit, but the &lt;a href=&quot;http://en.wikipedia.org/wiki/Loading_gauge&quot; rel=&quot;nofollow noreferrer&quot;&gt;loading gauge&lt;/a&gt;, which is only very indirectly related to the track gauge.&lt;/p&gt;&#xA;&lt;p&gt;However, the diameter of the shuttle boosters (12 feet) exceeds the width of all loading gauge standards in the USA (which differ mostly in height, the width is generally 10 feet 8 inches). Shipments that exceed the standard loading gauge in width are not all that uncommon, more mundane industrial parts often exceed them, but it requires careful planning and is very expensive as it often involves temporarily dismantling blocking structures, and of course at some point (such as with a tunnel) you run into hard limits.&lt;/p&gt;&#xA;&lt;p&gt;So it doesn't look like transport by railroad &lt;em&gt;dictated&lt;/em&gt; the booster designs, but it probably &lt;em&gt;influenced&lt;/em&gt; them. I'm pretty sure that if making them wider would have yielded some large advantage and been otherwise feasible, then a solution to the transportation problem would have been found.&lt;/p&gt;&#xA;" OwnerUserId="537" LastEditorUserId="7062" LastEditDate="2021-07-22T16:59:57.653" LastActivityDate="2021-07-22T16:59:57.653" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="4067" PostTypeId="2" ParentId="4065" CreationDate="2014-03-22T19:34:43.633" Score="6" Body="&lt;p&gt;Arthur C. Clarke's &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Songs_of_Distant_Earth&quot; rel=&quot;nofollow&quot;&gt;Songs of Distant Earth&lt;/a&gt; featured a generation star ship which traveled from star system to star system. They would rebuild their water ice forward shield each time they stopped at a star system. The forward shield was needed because impact from even a small speck of dust at small fractions of c could do a lot of damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrogen and oxygen are abundant. See &lt;a href=&quot;http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Abundance_of_elements_in_the_Universe&quot; rel=&quot;nofollow&quot;&gt;Abundance of Elements in the Universe&lt;/a&gt;. So water should be available in most star systems where a generation ship would visit. Water ice starts sublimating at a good rate at 90 Kelvin, but temperatures in the interstellar void should usually be well below that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ceramics might be a better choice for an ablation shield diving into a thick planetary atmosphere. But for a star ship traveling at .1 c through the interstellar medium, water ice might be the good choice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Highly speculative as we're not doing interstellar trips in the near future (if ever).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's travel about our solar system you're talking about, I don't think a dust shield is needed.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-10-20T19:06:38.110" LastActivityDate="2015-10-20T19:06:38.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4068" PostTypeId="2" ParentId="4065" CreationDate="2014-03-22T20:43:01.677" Score="7" Body="&lt;p&gt;The so-called &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;tyranny of the rocket equation&lt;/a&gt;, which becomes clear by examining &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt;, defining  maximum change in velocity $\Delta v$ as: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v = v_\text{e} \ln {\left(\frac{m_0}{m_1}\right)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $m_0$ is the wet mass, $m_1$ dry mass, and $v_\text{e}$ effective exhaust velocity of reaction mass, shows that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rockets are momentum machines. They spew gas out of a nozzle at high&#xA;  velocity causing the nozzle and the rocket attached to it to move in&#xA;  the opposite direction. Isaac Newton correctly defined the mathematics&#xA;  for this exchange of momentum in 1687. Conservation of momentum&#xA;  applied to a rocket was first done by Russian visionary and scientist&#xA;  Konstantin Tsiolkovsky in 1903. All our rockets are governed by&#xA;  Tsiolkovsky’s rocket equation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This means that reaction mass that &lt;em&gt;conventional rockets&lt;/em&gt; (chemical rockets, thermal rockets,...) need to carry with them, while necessary to achieve &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust&quot; rel=&quot;nofollow noreferrer&quot;&gt;thrust&lt;/a&gt; ($\mathbf{T} = \frac{dm}{dt} {v}$) and with it movement in the opposite direction to their exhaust, directly following principles described in &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's laws of motion&lt;/a&gt;, also increases their initial mass and with it impede total achievable $\Delta v$. And as you approach relativistic speeds, all that &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_in_special_relativity&quot; rel=&quot;nofollow noreferrer&quot;&gt;mass &lt;em&gt;becomes heavier&lt;/em&gt;&lt;/a&gt; and requires even more energy to accelerate faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this means that increasing rocket's mass by adding additional required systems or shielding mass to it directly lowers its performance. It will accelerate slower, and reach its destination later. As far as exposure to &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_ray&quot; rel=&quot;nofollow noreferrer&quot;&gt;cosmic rays&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_proton_event&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar high energy proton flux&lt;/a&gt; is concerned, radiation dose is governed by the duration of exposure, its intensity (proximity to its source), and a few other, more-or-less invariable factors we can't control with rocket systems alone, like our body density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is not much we can do to block the most energetic chargeless particles in cosmic rays (such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Neutrino&quot; rel=&quot;nofollow noreferrer&quot;&gt;neutrinos&lt;/a&gt;, and if I remember correctly, we'd need many light-years thick block of led to block half of them). As for smaller eV radiation, short of reducing exposure time, there are ways to deflect most charged particles with a &lt;a href=&quot;https://space.stackexchange.com/a/3789/49&quot;&gt;dipole magnetic field&lt;/a&gt;, and/or blocking lower energy mass particles by shielding mass, where water (or water ice) is &lt;a href=&quot;https://space.stackexchange.com/a/393/49&quot;&gt;fairly good at that&lt;/a&gt; (see another, &lt;a href=&quot;https://space.stackexchange.com/a/1826/49&quot;&gt;also relevant answer&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it makes perfect sense to decrease exposure time, accelerate fast away while in the inner Solar system (proximity to the Sun), decelerate even faster at your destination since the rocket is by then lighter, and shorten total trip time. So yes, reusing any rocket's mass for multiple purposes makes perfect sense, and even if that mass is a consumable, it would be required as reaction mass anyway, so why not use it as a shielding mass for &lt;em&gt;biological &#xA;shield&lt;/em&gt; as well? Since it's water, you'd likely use it for other purposes as well, such as a source of oxygen, potable water, coolant,... and you can &lt;a href=&quot;https://space.stackexchange.com/a/3737/49&quot;&gt;recycle most of that&lt;/a&gt;, so there would always be at least some available and not all of it consumed as reaction mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, unless that's required by the rocket's design, forward shield only makes sense if the source of the most of radiation you'd like to block is coming from in front of the rocket, meaning you're likely travelling at a significant fraction of the speed of light already (at which point a collision with a grain of dust might be catastrophic, due to $E_\text{k} =(m c^2)/(\sqrt{1 - v^2/c^2}) - mc^2$ with $E_\text{k}\approx\tfrac{1}{2} mv^2$ when $v$ is much smaller than $c$, see explanation &lt;a href=&quot;https://en.wikipedia.org/wiki/Kinetic_energy#Relativistic_kinetic_energy_of_rigid_bodies&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;), are faster than harmful matter from behind you, and you're for some reason not concerned by the impacts orthogonal to your vector of travel, otherwise you'd likely want to shield the crew module all around, not merely from in front of the ship. Like for example in this &lt;a href=&quot;http://enu.kz/repository/2010/AIAA-2010-795.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Conceptual Design of Crew Exploration Lander for Asteroid Ceres and Saturn Moons Rhea and Iapetus&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/dpdZ2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So &lt;strong&gt;TL;DR&lt;/strong&gt; - It makes perfect sense to reuse as much of the vehicle's reaction mass for multiple purposes, including increasing protection from harmful radiation and impacting with smaller debris while in areas and at relative velocities where it's more likely to happen and total kinetic energy discharged on impact can be shielded against. It will also decrease total travel time, further lowering exposure time to harmful radiation. But solely forward shielding likely doesn't make much sense, if this bioshield mass could instead be distributed all around the crew module. As the vehicle increases velocity and if that on itself changes direction of harmful radiation and likelihood of impact at ram-facing side of the craft, then part of the threat mitigation could also be gradual use of bioshield mass from where it's less needed first.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-10-23T16:08:03.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4069" PostTypeId="1" AcceptedAnswerId="4348" CreationDate="2014-03-23T02:55:20.350" Score="9" ViewCount="1193" Body="&lt;p&gt;When reading &lt;a href=&quot;http://www.spaceflight101.com/soyuz-2-1b-glonass-754-launch-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz 2-1B - Glonass-M Launch Updates on Spaceflight 101&lt;/a&gt;, I noticed an interesting photo showing these two red &quot;horns&quot; on the payload fairing (all photos courtesy of the Russian Ministry of Defence, here is an &lt;a href=&quot;http://function.mil.ru/news_page/country/more.htm?id=11501279@egNews&quot; rel=&quot;nofollow noreferrer&quot;&gt;article in Russian&lt;/a&gt;, and &lt;a href=&quot;http://www.google.com/translate?hl=en&amp;amp;ie=UTF8&amp;amp;sl=ru&amp;amp;tl=en&amp;amp;u=http://function.mil.ru/news_page/country/more.htm?id=11501279@egNews&quot; rel=&quot;nofollow noreferrer&quot;&gt;English translation via Google Translate&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/qjhKK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their red color and that they don't really look strong enough to withstand acceleration during launch seems to suggest these are to be removed before launch and must be umbilical ports to access &lt;a href=&quot;http://www.astronautix.com/stages/fregat.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fregat upper stage&lt;/a&gt; and the sole passenger - &lt;a href=&quot;http://gpsworld.com/glonass-m-satellite-shipped-to-launch-site/&quot; rel=&quot;nofollow noreferrer&quot;&gt;GLONASS-M satellite&lt;/a&gt;. This photograph seems to confirm the umbilical port part, as it looks like Fregat and GLONASS-M are being fueled / charged before the rocket is moved into its upright position:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/AgFRe.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, I've noticed they're still in place after moving Soyuz rocket vertically:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Jb121.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this made me really curious;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What purpose do these red &quot;horns&quot; on the Soyuz 2-1B payload fairing serve? Is there more to them than merely being service / umbilical ports, or are they indeed removed before launch? If latter, why would they be still in place as the rocket is moved vertically?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my understanding, the Fregat upper stage is loaded with 5,350 kg of UDMH + N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; (&lt;a href=&quot;https://en.wikipedia.org/wiki/Unsymmetrical_dimethylhydrazine&quot; rel=&quot;nofollow noreferrer&quot;&gt;Unsymmetrical Dimethylhydrazine&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrogen_Tetroxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nitrogen Tetroxide&lt;/a&gt; fuel and oxidizer) long before that, during payload integration, but I'm not sure about GLONASS-M consumables (perhaps cold gas for ACS thrusters?), or if they both require grid power before switching to internal power minutes before launch? Is there any page available where their purpose is explained in more detail?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="13208" LastEditDate="2017-02-27T21:34:05.660" LastActivityDate="2017-02-27T21:34:05.660" Title="What purpose do these red &quot;horns&quot; on the Soyuz 2-1B payload fairing serve?" Tags="|launch|russia|payload|upper-stage|soyuz-launcher|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4070" PostTypeId="1" AcceptedAnswerId="4078" CreationDate="2014-03-23T11:46:02.100" Score="4" ViewCount="1605" Body="&lt;p&gt;Orbital Sciences, which uses 2 Russian NK-33 engines on its Antares booster, has been suing Lockheed Martin/ULA over anti-trust violations, since they were not allowing Orbital to buy RD-180 engines, which power the Atlas V booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NK-33 is the engine built in the 1960's for the N-1 moon rocket, with the Soyuz-2-1v and Orbital Antares booster using up the stock that remains from the 1960's.  (Which is pretty interesting in itself.).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the NK-33/AJ-26 is about a 330Klb (Sea level) thrust engine, and with two that is around 660Klbs thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RD-180 is a 860Klb thrust engine, which is quite a growth from 660Klbs, and I can imagine this will require a substantial redesign.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is NOT a drop in replacement, and seems like it would result in a functionally new booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the flip side, Yuzhnoye Design Bureau is building the first stage for Orbital, and they also build the Zenit first stage, which uses the RD-170 engine, of which the RD-180 is a half-size derivative. (RD-170 was also used on the strap-on boosters for the Energia rocket)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how is one a replacement for the other?  Sure, same fuels, RP1/LOX, but beyond that a fairly big change in design.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="1255" LastEditDate="2014-03-24T10:24:04.023" LastActivityDate="2014-03-24T10:24:04.023" Title="How is an RD-180 engine a reasonable replacement for 2 NK-33/AJ-26 engines on Antares?" Tags="|launch|rd-180|nk-33|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4071" PostTypeId="2" ParentId="3830" CreationDate="2014-03-23T13:04:42.690" Score="15" Body="&lt;p&gt;ESA-built &lt;a href=&quot;http://en.wikipedia.org/wiki/Cupola_(ISS_module)&quot; rel=&quot;noreferrer&quot;&gt;Cupola&lt;/a&gt; uses &quot;Fused silica and borosilicate glass&quot; for all its windows including biggest 80 cm one (reference is &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/International_Space_Station/Cupola&quot; rel=&quot;noreferrer&quot;&gt;http://www.esa.int/Our_Activities/Human_Spaceflight/International_Space_Station/Cupola&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;http://www.esa.int/esapub/bulletin/bulletin137/bul137h_deloo.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.esa.int/esapub/bulletin/bulletin137/bul137h_deloo.pdf&lt;/a&gt; on page 7 (labelled 66) there is text about previous 50 cm window on ISS &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The high-tech 12 cm thick glass is actually a composite of four laminated panes consisting of a thin exterior 'debris' pane that protects it from micrometiorites, two internal pressure panes, and an interior 'scratch' pant to absorb accidental marking from inside&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The reference guide from NASA &lt;a href=&quot;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/pdf/508318main_ISS_ref_guide_nov2010.pdf&lt;/a&gt; shows typical window in &quot;&lt;a href=&quot;http://www.nasa.gov/externalflash/ISSRG/pdfs/mmod.pdf&quot; rel=&quot;noreferrer&quot;&gt;Micrometeoroid and Orbital Debris (MMOD) Protection&lt;/a&gt;&quot; (page 98 of guide, figure &quot;Typical Glass Windowpane design&quot;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/URUGw.jpg&quot; alt=&quot;International space station windowpane design&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Outer removable shutter, Outer meteoroid/debris pane 0.93 cm, Redundant pressure pane 2.54 cm, Primary pressure pane 2.54 cm, Inner scratch pane 1.14 cm&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All panes have 14.3 cm width total.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This paper &lt;a href=&quot;http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1120&amp;amp;context=nasapub&amp;amp;sei-redir=1&quot; rel=&quot;noreferrer&quot;&gt;Transparent Armor Ceramics as Spacecraft&#xA;Windows (2012)&lt;/a&gt; says &quot;&lt;em&gt;Only one material has been researched and used extensively: fused silica. Fused silica has performed well in the space shuttle orbiter and the international space station (ISS) with no catastrophic failures.&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110013394.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110013394.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The window in the U.S. Laboratory was made from fused silica glass and the hatch windows were made of Chemcor, both provided by &lt;a href=&quot;http://en.wikipedia.org/wiki/Corning_Inc.&quot; rel=&quot;noreferrer&quot;&gt;Corning, Inc&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA-STD-5018.pdf (&lt;a href=&quot;https://standards.nasa.gov/documents/viewdoc/3315793/3315793&quot; rel=&quot;noreferrer&quot;&gt;https://standards.nasa.gov/documents/viewdoc/3315793/3315793&lt;/a&gt;) and SSP 30560 (offline) have some requirements for glass usage in space: &lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2014-03-23T13:50:06.553" LastActivityDate="2014-03-23T13:50:06.553" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4073" PostTypeId="2" ParentId="3831" CreationDate="2014-03-23T13:42:25.360" Score="11" Body="&lt;p&gt;According to &quot;Final Report of the International Space Station Independent Safety&quot; (2007)&#xA;&lt;a href=&quot;http://www.nasa.gov/pdf/170368main_IIST_%20Final%20Report.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/pdf/170368main_IIST_%20Final%20Report.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russian windows have less meteoroid protection (less layers). Also there is no internal scratch to protect it from internal vandals, unlike the &lt;a href=&quot;https://space.stackexchange.com/a/4071/2843&quot;&gt;NASA/ESA design&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;5.11 Service Module Windows. Observations&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The 13 SM windows are Russian-heritage hardware for which designers did not consider currently available data regarding MMOD in their design. The design consists of a two-windowpane (primary and secondary) configuration with the volume between the panes pressurized. This results in the external pane being the primary pressure pane. Unlike other windows on the ISS, most Russian windows do not have an external debris pane to protect against MMOD or an internal scratch pane to protect against damage caused by inadvertent crew activities. The probability of a critical failure (i.e., loss of a primary pane) is estimated by NASA to be one chance in six for all SM windows combined and one chance in nine for SM windows 1 and 2 over a six-year period. Loss of the primary pane would result in loss of redundancy in the window with respect to maintaining ISS atmospheric pressure. Early in the NASA assessment of the SM windows there were concerns that failure of the primary pane might cause failure of the secondary pane as well due to near instantaneous change in differential pressure since the volume between the two panes is pressurized. However, tests conducted by NASA using SM window hardware have demonstrated that the loss of the primary pane does not result in the loss of secondary pane.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.gpo.gov/fdsys/pkg/GAOREPORTS-T-NSIAD-00-128/html/GAOREPORTS-T-NSIAD-00-128.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.gpo.gov/fdsys/pkg/GAOREPORTS-T-NSIAD-00-128/html/GAOREPORTS-T-NSIAD-00-128.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Space station windows must also be certified to last at least 15 years.&#xA;  However, the Service Module windows are based on the same design used for&#xA;  the existing Mir space station and are designed to last 5 years. The Russian&#xA;  space agency had not provided NASA with sufficient test data to verify that&#xA;  the windows would last for 15 years, citing instead the fact that no windows&#xA;  failed during the Mir's 14 years in orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The glass used for windowpane is silica/quartz glass (&lt;a href=&quot;http://www.sunhome.ru/journal/125924/p2&quot; rel=&quot;noreferrer&quot;&gt;http://www.sunhome.ru/journal/125924/p2&lt;/a&gt; &quot;«Для космоса» используется не обычное стекло, а кварцевое...  марки СК и КВ (последняя — не что иное, как плавленый кварц) ... (КВ10С, К-108)&quot;). There are several organisations involved in space glass development: &quot;НИИ авиационного стекла Минавиапрома  (теперь это ОАО «&lt;a href=&quot;http://web.archive.org/web/20080710031632/http://www.intg.org/&quot; rel=&quot;noreferrer&quot;&gt;НИИ технического стекла&lt;/a&gt;») ...  &lt;a href=&quot;http://en.wikipedia.org/wiki/Vavilov_State_Optical_Institute&quot; rel=&quot;noreferrer&quot;&gt;Государственный оптический институт им. С. И. Вавилова&lt;/a&gt;, НИИ резинотехнической промышленности, Красногорский механический завод ... &lt;a href=&quot;http://ru.wikipedia.org/wiki/%D0%9B%D1%8B%D1%82%D0%BA%D0%B0%D1%80%D0%B8%D0%BD%D1%81%D0%BA%D0%B8%D0%B9_%D0%B7%D0%B0%D0%B2%D0%BE%D0%B4_%D0%BE%D0%BF%D1%82%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%BE%D0%B3%D0%BE_%D1%81%D1%82%D0%B5%D0%BA%D0%BB%D0%B0&quot; rel=&quot;noreferrer&quot;&gt;Лыткаринский завод оптического стекла&lt;/a&gt;&quot; (&lt;a href=&quot;http://www.sunhome.ru/journal/125924/p1&quot; rel=&quot;noreferrer&quot;&gt;http://www.sunhome.ru/journal/125924/p1&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-03-23T13:42:25.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4074" PostTypeId="2" ParentId="3234" CreationDate="2014-03-23T19:22:56.413" Score="7" Body="&lt;p&gt;The book &quot;Soyuz: A Universal Spacecraft&quot; by Rex Hall, David Shayler (2003) has the text on &lt;a href=&quot;http://books.google.com/books?id=dbGchpi1HP8C&amp;amp;pg=PA41&quot;&gt;page 41&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Propulsion Module, ... Descent module, ... Orbital Module... materials used in the fabrication of these modules .... an aluminium alloy was finally chosen. The thermal insulation covering the modules consists of two layers - the upper layer consisting of an &lt;em&gt;asbestos-cloth laminate&lt;/em&gt;, and the lower layer of 'a light, heat-insulating material'.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&quot;Leaving Earth: Space Stations, Rival Superpowers, and the Quest for Interplanetary Travel&quot; by Robert Zimmerman (2003) says at &lt;a href=&quot;http://books.google.com/books?id=j053AgAAQBAJ&amp;amp;pg=PA280&amp;amp;&quot;&gt;page 280&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Protecting Soyuz-TM's bell-shaped descent module were eight blankets of insulation material, each made of many layers of golden foil and an outer micrometeoroid layer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(note, &quot;golden foil&quot; may describe polymer film, covered by thin layer of any metal)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were several EVAs to fix some equipment under &quot;vacuum-screen thermal insulation&quot; (EVTI), like by Titov and Monarov (30.06.1988, 20.10.1988, page 254 of Leaving Earth) fixing X-ray telescope TTM &quot;Roentgen&quot; on &lt;a href=&quot;http://en.wikipedia.org/wiki/Kvant-1&quot;&gt;Kvant-1&lt;/a&gt; (Mir Space station). Or EVA on ISS by &lt;a href=&quot;http://en.wikipedia.org/wiki/Expedition_17&quot;&gt;Expedition 17&lt;/a&gt; (10-11.07.2008; Orlan spacewalk &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition17/eva20a.html&quot;&gt;EVA-20A&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="2843" LastActivityDate="2014-03-23T19:22:56.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4075" PostTypeId="1" AcceptedAnswerId="4076" CreationDate="2014-03-23T19:28:47.897" Score="9" ViewCount="909" Body="&lt;p&gt;As I remember, Gaia (&lt;a href=&quot;http://en.wikipedia.org/wiki/Gaia_%28spacecraft%29&quot;&gt;wiki&lt;/a&gt;, &lt;a href=&quot;http://sci.esa.int/gaia/&quot;&gt;ESA&lt;/a&gt;) was planned to flight away from Earth, so it orbit is not LEO, and not GEO. It is located near 1.5 millions kilometers away from Earth, orbiting thousands kilometers away from L2 Lagrangian point of Sun-Earth system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was main reasons to put it so far?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How far is it from the L2 point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is sun shadowed there?&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2014-03-23T19:58:11.327" LastActivityDate="2014-03-25T18:29:15.483" Title="What are reasons to put Gaia space telescope into L2 Lagrangian point of Sun-Earth system?" Tags="|lagrangian-points|gaia|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4076" PostTypeId="2" ParentId="4075" CreationDate="2014-03-23T20:01:14.093" Score="4" Body="&lt;p&gt;According to ESA, it's target Lissajous orbit around SEL2 is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;263,000 × 707,000 × 370,000 km, 180 day-long orbit around L2&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's the only place that suits Gaia's design constraints, to keep its solar panels pointed towards the Sun at all times, prevent thermal cycling, and keep its antenna pointed towards the parts of the Earth where it can be communicated from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KBGl7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;A cutaway of the Gaia space observatory showing its instruments Credit: EADS Astrium&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sun-synchronous -1° retrograde polar orbit (frozen orbit) wouldn't do, since the majority of its ground track is over the middle of nowhere. L1 wouldn't do, since the antenna can't be a part of the Sun shield then, L3 is behind the Sun, L4 &amp;amp; L5 still don't enable its antenna to point towards the Earth (maximum permissible tilt for its Sun shield relative to the Sun is at 15°), and higher or lower heliocentric orbits would put Earth in opposition during its 5 year mission, complicate communications (1 billion pixels creates a large data stream) and reduce total insolation needed to power the space observatory, if in higher altitude heliocentric orbit.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-03-25T13:08:34.060" LastActivityDate="2014-03-25T13:08:34.060" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4077" PostTypeId="1" AcceptedAnswerId="4957" CreationDate="2014-03-23T20:39:18.790" Score="9" ViewCount="450" Body="&lt;p&gt;Earth has strong magnetic field, which changes concentration of charged particles from Sun near the planet. There is rather safe torus near the Earth (LEO non-polar orbits), and also there is long tail, in which activity of Sun emitted particles is reduced (in this picture Sun is at left side and Earth is in center) &lt;a href=&quot;http://en.wikipedia.org/wiki/File:Magnetosphere_Levels.svg&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/File:Magnetosphere_Levels.svg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DBVpj.png&quot; alt=&quot;Artist&amp;#39;s concept of the Earth&amp;#39;s magnetosphere. en.wikipedia.org/wiki/File:Magnetosphere_Levels.svg&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question - what is geometry of magnetotail, if we check it 1.5 million kilometers away from Earth (at the line Sun-Earth-L2). What is estimated diameter of &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetopause&quot; rel=&quot;noreferrer&quot;&gt;magnetopause&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetotail&quot; rel=&quot;noreferrer&quot;&gt;magnetotail&lt;/a&gt; here?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will Earth provide extra protection from charged particles for the spacecraft hung exactly in L2 Lagrange point of the System Sun-Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will there substantial protection for the spacecraft orbiting around L2 at distance of 200,000 to 300,000 kilometers away from L2 (in the plane, perpendicular to the line Sun-Earth)&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorDisplayName="user12" LastEditDate="2014-05-20T15:06:35.987" LastActivityDate="2014-07-10T23:12:17.657" Title="What is the diameter of magnetopause 1.5 millions kilometers behind the Earth?" Tags="|radiation|lagrangian-points|the-sun|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4078" PostTypeId="2" ParentId="4070" CreationDate="2014-03-24T04:09:38.387" Score="5" Body="&lt;p&gt;The ultimate answer depends on the structural margins used in the construction of the first stage. It's possible that it was overdesigned with this switch in mind, in which case you could simply replace the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, as osgx points out, the RD-180 is throttleable, so it could simply be throttled down to 76% to reach 660 klbf. At it's lowest range, however, (47%) the thrust is 404 klbf, but the NK-33 is only throttleable down to 55% [1], which translates to 363 klbf from two of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason that's important is because it says here [2] that the Antares does throttle down towards the end of the flight to maintain structural limits on the rocket. However, it's unclear how far it throttles down. If it only throttled down to 61%, that matches the lower end of the RD-180, so a structural redesign of the first stage would not be necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if it required a structural redesign of the rocket, it might still be a reasonable replacement because, as you note, the engine is no longer in production. If they kept using it, they would have to pay to restart the production line, which could be a lot more costly than a structural redesign of the rocket. The RD-180 is in production for the Atlas V, as you also mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another question is gimbaling, which is the process of moving the nozzles slightly in order to correct the course of the rocket. This isn't really a problem, since the NK-33 can gimbal up to 6 degrees [3], and the each of the RD-180's nozzles can gimbal up to 8 degrees [4]. As osgx noted, a single RD-180 has two nozzles, and each NK-33 has one, so the nozzle count is fine. If the RD-180 had more limits on its gimbaling, Orbital would need a new solution for steering the rocket on its way up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, Orbital has time to come up with a solution. They purchased enough engines to complete their obligations to NASA under their Commercial Resupply Services agreement [5], and to date no one else has signed up to use the Antares for their payload [6]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) &lt;a href=&quot;http://www.spaceandtech.com/spacedata/engines/nk33_specs.shtml&quot; rel=&quot;noreferrer&quot;&gt;http://www.spaceandtech.com/spacedata/engines/nk33_specs.shtml&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) &lt;a href=&quot;http://www.spaceflight101.com/antares-a-one-mission-updates.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.spaceflight101.com/antares-a-one-mission-updates.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) &lt;a href=&quot;http://spaceflightnow.com/antares/cots1/countdowntimeline.html&quot; rel=&quot;noreferrer&quot;&gt;http://spaceflightnow.com/antares/cots1/countdowntimeline.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) &lt;a href=&quot;http://www.spaceflightnow.com/atlas/ac201/000514rd180.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.spaceflightnow.com/atlas/ac201/000514rd180.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5) &lt;a href=&quot;http://www.spaceflightnow.com/news/n1403/20ulaorbital/#.Uy-ue_ldV8E&quot; rel=&quot;noreferrer&quot;&gt;http://www.spaceflightnow.com/news/n1403/20ulaorbital/#.Uy-ue_ldV8E&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;6) &lt;a href=&quot;http://en.wikipedia.org/wiki/Antares_(rocket)#Missions&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Antares_(rocket)#Missions&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2014-03-24T04:09:38.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4079" PostTypeId="2" ParentId="1667" CreationDate="2014-03-24T08:53:45.910" Score="16" Body="&lt;p&gt;I think you already know the answer, it is not really a technical issue, it is (mainly) political. The Germans have experience with liquid engines (they make major parts for the liquid main engine of Ariane 5) and the Italians have experience with solid fuel (they make part of the solid boosters for Ariane 5 and the majority of the solid fueled Vega-rocket). There are others involved in the different engines but that is the basics. The French (although also involved in the liquid main engine) probably have a preference for solid fuel because there is a direct relationship between the solid boosters of the Ariane 5 and the French submarine-based nuclear missiles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quick comments on your recap:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;A rocket stage with liquid engines can be tested before launch to make sure it works. Just fuel it up again and launch it. SpaceX fires up the engines on the launch pad for a couple of seconds as a final test a couple of days before launch. Can&amp;rsquo;t do that with solids...&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Yes solid rockets vibrate more but that&amp;rsquo;s not really a problem unless you are launching people. Plenty of mostly solid fueled rockets launching satellites in the world. The suggested NASA-rocket for launching people using a solid booster from the shuttle as first stage apparently had serious problems with vibrations.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Hydrochloric acid and aluminum compounds are the pollution from solid fuel. So on the grounds of environmental concerns maybe one should stick to liquid oxygen + liquid hydrogen, only water vapor as exhaust. And it only takes water and electricity to make the stuff.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To finish. The real problem for Ariane 6 (and 5) is that none of them can compete with SpaceX. Europe should really try to bypass the political inefficiency the same way as NASA is trying to do, with real free market competition.&lt;/p&gt;&#xA;" OwnerUserId="2954" LastEditorUserId="6816" LastEditDate="2018-11-05T15:38:17.873" LastActivityDate="2018-11-05T15:38:17.873" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="4080" PostTypeId="1" AcceptedAnswerId="4135" CreationDate="2014-03-24T09:46:31.430" Score="4" ViewCount="995" Body="&lt;p&gt;Mercury's orbital speed is 48 km/s or about 10 AU/year. Wouldn't probes accelerate towards the Sun if we give them a little push from LEO, and only 0.6-1.6 AU from here take a big gravity assist from Mercury? Its orbital period is only 3 months, so we could frequently choose any direction of its graviational assist we need to get to any other planet (compared to Jupiters 11 year orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How large fraction of Mercury's orbital speed could a probe get through gravity assist, if it passed really nearby? And could a gas giant capture into its orbit a probe with such high speed? Would the heat at Mercury be a challenge for a flyby probe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think Mercury has been used for gravity assists inspite of its vicinity and high speed, so what's the catch?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-03-27T20:02:22.523" Title="How could Mercury be used as a gravity sling to the outer planets?" Tags="|gravity-assist|mercury|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4081" PostTypeId="2" ParentId="441" CreationDate="2014-03-24T10:16:07.653" Score="10" Body="&lt;p&gt;Centrifuges with long tethers (or airbeams) could be used also on the ground. Below is an illustration where the crewed part of the carousel is below ground for protection against radiation. According to the source the centrifugal radius would be 33 meter long to achieve 0.5 g (with 0.17 g given naturally on the Moon).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/w5Ais.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/wn8pA.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From www.cislunarone.com by Dr. Doug Plata.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firm ground with useful gravity is indeed a rare resource in the Solar system. Mars and Mercury with 0.38 g might be enough for human health. If Lunar gravity is enough then we have half a dozen other bodies. But then there's Ceres with 0.03 g which certainly is useless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more phantastic idea is to build a tunnel inside an asteroid and spin it up so that you'd walk with your feet outwards on the inside of the surface. Future space exploration could be a funny thing!&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-03-24T10:16:07.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4082" PostTypeId="2" ParentId="768" CreationDate="2014-03-24T10:33:07.840" Score="3" Body="&lt;p&gt;I sincerely hope that my answer contributes in a meaningful way.  I found a very nice visualisation of the manouvers that the Rosetta will go through after catching up to 67P (see below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.youtube.com/watch?v=fNBUep7mPdI&quot; rel=&quot;nofollow&quot;&gt;http://www.youtube.com/watch?v=fNBUep7mPdI&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems as if the short answer to your question may indeed be 'yes' but I guess anything may happen when the spacecraft reaches its target and the 'we don't know yet' may be equally relevant.  From the animation it seems as if ESA is planning to approach 67P from the sunny side to avoid the coma and tail as much as possible.&lt;/p&gt;&#xA;" OwnerUserId="2955" LastActivityDate="2014-03-24T10:33:07.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4083" PostTypeId="2" ParentId="4064" CreationDate="2014-03-24T10:59:30.993" Score="9" Body="&lt;p&gt;There's a &lt;a href=&quot;http://www.oldrocketforum.com/printthread.php?t=5656&amp;amp;pp=40&quot; rel=&quot;nofollow noreferrer&quot;&gt;forum post here&lt;/a&gt; that has some interesting information:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Well, there was SOME justification for the decision to cast the&#xA;propellant in Utah-- the cool dry climate there, especially the low&#xA;humidity, helped considerably in getting a CONSISTENT propellant&#xA;casting, especially in casting multiple segments for a solid rocket&#xA;motor.&lt;/p&gt;&#xA;&lt;p&gt;Aerojet had created the world's largest solid rocket motor, a 260 inch&#xA;behemoth which was static tested as a possible (cheap? or so the&#xA;thinking went) replacement for the Saturn IB first stage (cluster's&#xA;last stand) instead of the nine clustered tanks, thrust structure, and&#xA;eight clustered liquid engines used on that stage. They cast it in&#xA;Florida (IIRC) because the 22 foot diameter and massive weight&#xA;precluded moving it very far if at all. They test fired it in a 'silo'&#xA;test stand (which is there to this day, but abandoned) and there were&#xA;some difficulties encountered with the casting of the grains and the&#xA;motor itself which were basically 'traced back' to variable&#xA;temperature and humidity messing up the pours. The engine didn't&#xA;perform as expected, and basically broke the test stand from the&#xA;vibration, etc. so they abandoned the idea.&lt;/p&gt;&#xA;&lt;p&gt;A few years later when shuttle decided to go with solid rocket&#xA;boosters, Morton Thiokol, who had extensive experience with solid&#xA;rockets from their ballistic missile work for Air Force, won the&#xA;contract. Their facilities were in Utah, and really sold the idea that&#xA;their products (large ICBM solid rocket motors for missiles like&#xA;Minuteman) had given them considerable experience in this area, and&#xA;they wouldn't have the problems casting the much-larger shuttle SRB&#xA;segments because the dry climate in Utah made casting pours more&#xA;consistent and easier. BUT, because of the weight of the fully-fuelled&#xA;segments, the only realistic way to transport them was by rail-- they&#xA;would be too heavy and too big for road transport, and MUCH too heavy&#xA;for air transport, and Utah doesn't have any water shipping ports&#xA;capable of shipping them by barge. This meant that the SRB's were&#xA;ultimately limited in diameter, which of course in a core-burning&#xA;solid rocket, determines the burn duration (more or less).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The Aerojet 260&amp;quot; booster was &lt;a href=&quot;https://web.archive.org/web/20121002061554/http://www.astronautix.com/engines/aj2602.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;an attempt to get around size limits&lt;/a&gt; of solid rockets. According to the forum post, this attempt failed.&lt;/p&gt;&#xA;&lt;p&gt;Note that ESA decided to use a &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Launch_vehicles/Boosters_EAP&quot; rel=&quot;nofollow noreferrer&quot;&gt;segmented booster design&lt;/a&gt; for Ariane 5, as well. The top segment is fueled in Italy, the two lower segments are fueled at the Kourou spaceport. They are joined by a construction that is &lt;a href=&quot;http://www.esa.int/esapub/bulletin/bullet93/b93carr.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;similar&lt;/a&gt; to the Shuttle SRBs.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="42448" LastEditDate="2021-07-21T11:53:06.457" LastActivityDate="2021-07-21T11:53:06.457" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="4084" PostTypeId="1" AcceptedAnswerId="4088" CreationDate="2014-03-24T11:12:55.517" Score="13" ViewCount="22364" Body="&lt;p&gt;Is ISS orbiting with or against the rotation of Earth? Is it important in any aspect for the space station?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="25" LastEditDate="2014-03-24T13:37:45.693" LastActivityDate="2019-10-25T06:35:57.920" Title="Does ISS follow Earth's rotation?" Tags="|iss|orbit|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4085" PostTypeId="1" AcceptedAnswerId="4087" CreationDate="2014-03-24T12:18:05.340" Score="26" ViewCount="70178" Body="&lt;p&gt;I was watching &lt;a href=&quot;http://youtu.be/OnoNITE-CLc?t=2m53s&quot; rel=&quot;nofollow noreferrer&quot;&gt;space shuttle launch&lt;/a&gt; recently, and at 2:53 you can see the space shuttle from an angle, even though it's high above the earth and you should see only the bottom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the space shuttle fly straight up at its launch or did it have a curve in its path?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IpPJ7.png&quot; alt=&quot;Launching space shuttle path&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2956" LastEditorUserId="7062" LastEditDate="2020-06-07T09:50:10.887" LastActivityDate="2020-06-07T09:50:10.887" Title="Did the space shuttle fly straight &quot;up&quot; when leaving Earth?" Tags="|launch|space-shuttle|" AnswerCount="7" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="4086" PostTypeId="2" ParentId="4085" CreationDate="2014-03-24T12:47:04.177" Score="9" Body="&lt;p&gt;Any launch to orbit follows a common path. First they have to get above the atmosphere, to keep slowing from it down to a minimum. Then they need to re-orient to accelerate to maintain orbit. If you simply launched straight up, you'd fall straight down, it's the speed that makes the difference. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a pretty good wikipedia article about how this is managed called &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_turn&quot;&gt;Gravity Turn&lt;/a&gt;. Basically, the rocket only goes straight up for a short period of time, then turns slightly off to allow Gravity to actually work for the rocket.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-03-24T12:47:04.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4087" PostTypeId="2" ParentId="4085" CreationDate="2014-03-24T12:53:26.297" Score="40" Body="&lt;p&gt;Everything (not only space shuttles) that goes into the Earth orbit must curve its path on the way up. If a vehicle went straight up and did not achieve escape velocity, it would fall back to Earth after the fuel runs out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main objective of the rocket engine is not only to get the cargo above the atmosphere, but more importantly to accelerate it in horizontal direction to the orbital speed (7.5 km/s for the orbital altitudes of the Shuttle and International Space Station). That is why all rockets / shuttles curve their path gradually to horizontal direction and then burn a lot of fuel in horizontal direction. If the cargo did not reach enough horizontal speed, it would fall back to Earth.&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="921" LastEditDate="2014-03-26T18:22:12.360" LastActivityDate="2014-03-26T18:22:12.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4088" PostTypeId="2" ParentId="4084" CreationDate="2014-03-24T13:03:13.097" Score="23" Body="&lt;p&gt;The ISS orbit is prograde - in the direction of the Earth rotation. Prograde orbits are orbits with inclination less than 90 degrees. ISS inclination is 51.6 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These orbits are slightly easier to reach, because they don't require as much fuel, as you get additional &quot;kick&quot; from the Earth during launch. Retrograde (opposite direction) orbits are rare, because they require more fuel to reach. It would be inefficient to build the ISS in retrograde orbit, because all the craft would have to spend additional fuel to get there. The ISS orbit was chosen to make it &lt;a href=&quot;http://spaceflight.nasa.gov/feedback/expert/answer/mcc/sts-112/09_04_12_54_17.html&quot;&gt;accessible from the key launch sites&lt;/a&gt; in United States (Florida) and Kazakhstan (Baikonur).&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="1255" LastEditDate="2014-03-24T14:29:59.170" LastActivityDate="2014-03-24T14:29:59.170" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4089" PostTypeId="1" AcceptedAnswerId="4090" CreationDate="2014-03-21T15:38:46.133" Score="22" ViewCount="9727" Body="&lt;p&gt;If we provide adequate water supply will it be possible to grow plants/seeds/moss/algae (from Earth) on Mars? How different are Mars soil from Earth's soil? &lt;/p&gt;&#xA;" OwnerUserId="2722" OwnerDisplayName="jorel" LastEditorUserId="25" LastEditDate="2014-04-25T10:02:25.030" LastActivityDate="2014-07-16T12:58:14.767" Title="Is it possible to grow plants on Mars?" Tags="|mars|colonization|plants|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4090" PostTypeId="2" ParentId="4089" CreationDate="2014-03-21T23:22:23.827" Score="22" Body="&lt;p&gt;&lt;a href=&quot;http://mars.jpl.nasa.gov/msl/news/whatsnew/index.cfm?FuseAction=ShowNews&amp;amp;NewsID=1438&quot;&gt;Some of the rocks on Mars&lt;/a&gt; aren't too bad for growth of plants or bacteria. They contain a considerable amount of clay minerals (about 20%), and several other minerals which are considered as essential and sufficient for bacteria to survive, at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probable chlorinated salts, e.g. &lt;a href=&quot;http://spacescience.arc.nasa.gov/story/curiosity-finds-%E2%80%9Csurprising%E2%80%9D-amounts-water-perchlorate-mars&quot;&gt;perchlorates&lt;/a&gt;, which are likely and wide-spread in Martian soil, don't favour plant growth, but there are also some &lt;a href=&quot;http://toxics.usgs.gov/highlights/perchlorate_sw_soils.html&quot;&gt;soils on Earth containing perchlorates&lt;/a&gt;. &lt;a href=&quot;http://www.atsdr.cdc.gov/toxprofiles/tp162-c6.pdf&quot;&gt;Some plants, and some bacteria can cope with a certain concentration of perchlorates&lt;/a&gt;. But it would certainly be better to clean Martian soil from perchlorates before trying to grow plants on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basaltic rocks are wide-spread on Mars. They need some weathering to be suitable for growth of plants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://msl-scicorner.jpl.nasa.gov/Instruments/RAD/&quot;&gt;Solar energetic particles, and galactic cosmic rays&lt;/a&gt; are more abundant on Mars than on Earth. That's not optimal for plants, but probably just acceptable. A severe problem is high &lt;a href=&quot;https://cab.inta-csic.es/rems/rems-science/&quot;&gt;ultra violet radiation&lt;/a&gt; due to a missing ozone layer on Mars. Plants wouldn't survive this uv level without shielding. Hence at least a protection by glass or something similar would be needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/msl/multimedia/pia16460.html&quot;&gt;Carbon dioxide&lt;/a&gt; for the respiration of plants is present on Mars, but the &lt;a href=&quot;http://www.nasa.gov/mission_pages/msl/news/msl20121102.html&quot;&gt;atmospheric pressure&lt;/a&gt; is too low for &lt;a href=&quot;http://cft.fis.uc.pt/eef/FisicaI01/fluids/thermo20.htm&quot;&gt;water to stay liquid&lt;/a&gt; over a sufficiently long period. Hence a pressurized containment would be needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Btw.: &lt;a href=&quot;http://www.nasa.gov/mission_pages/phoenix/news/phoenix-20080620.html&quot;&gt;There is permafrost in some regions on Mars&lt;/a&gt;; you just need to warm it, and keep it confined, to get water.&lt;/p&gt;&#xA;" OwnerUserId="1454" OwnerDisplayName="Gerald" LastActivityDate="2014-03-21T23:22:23.827" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4091" PostTypeId="1" CreationDate="2014-03-24T14:50:43.470" Score="17" ViewCount="1845" Body="&lt;p&gt;Say I'm designing a probe or satellite. To control its attitude, I can add some gyros, or I can put thrusters on each corner, or I can add both and pick one in flight according to the situation. How do I decide which to use? How does it depend on the design lifetime, mass, expected frequency of attitude changes, and orbit of the probe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It'd be great if you could support your explanation with an example or two.&lt;/p&gt;&#xA;" OwnerUserId="2807" LastEditorUserId="25" LastEditDate="2016-07-19T17:27:18.780" LastActivityDate="2016-07-19T17:27:18.780" Title="What factors determine whether a spacecraft/probe/satellite uses gyroscopes or propellant thrusters to rotate?" Tags="|artificial-satellite|propulsion|spacecraft|attitude|reaction-wheel|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4092" PostTypeId="2" ParentId="4091" CreationDate="2014-03-24T15:14:49.277" Score="17" Body="&lt;p&gt;For the record, there's actually a third possibility, Magnetorquers. Here's why you would use one vs the other. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Magnetorquers&lt;/strong&gt; - Inexpensive, low maintenance, but don't work in all situations. Used by LEO spacecraft typically, and small in size. They work by pushing off of a magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Reaction Wheels&lt;/strong&gt; (Or Gyros) - The trick here is that you can't take out a momentum change using only reaction wheels. You either need to stop the spacecraft from moving, or else offload the momentum with another form of propulsion. These work great if you can reasonable expect to have the spacecraft with the same angular momentum eventually. They tend to break down over time, but can last for many years. The reason they can't is because basically they exchange momentum from the spacecraft with momentum in the wheel. You can only make a permanent change if you are pushing against something external, or pushing something towards or away from you, Reaction Wheels just change the momentum until the wheel stops spinning. They can change the orientation, just not the momentum. See &lt;a href=&quot;https://www.youtube.com/watch?v=FidvWCDA_8w&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; for a good demonstration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Thrusters&lt;/strong&gt; - These work all the time, but use fuel. Thus, you are limited to how much you can use them based on the fuel load vs. the lifetime of the mission. Also their exhaust can be dangerous to your satellite, if it carries something sensitive - that is the case of the Hubble Space Telescope, where &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf11-2.php&quot; rel=&quot;noreferrer&quot;&gt;thrusters could contaminate the optics&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, you use what you can, and will cause the minimal impact to your spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="921" LastEditDate="2014-03-29T07:23:34.977" LastActivityDate="2014-03-29T07:23:34.977" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="4093" PostTypeId="1" AcceptedAnswerId="4098" CreationDate="2014-03-24T15:36:54.007" Score="9" ViewCount="3187" Body="&lt;p&gt;I have a position and velocity vector for an orbiting object and can calculate the orbital elements. From a simple two-body situation, how can I predict where in the orbit the object will be at some future time? What are the mathematical steps involved?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-03-24T20:58:27.400" LastActivityDate="2014-03-25T23:07:50.743" Title="How can I predict what an object's orbital state vectors will be in the future?" Tags="|orbital-mechanics|trajectory|mathematics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4094" PostTypeId="2" ParentId="4089" CreationDate="2014-03-24T17:12:44.977" Score="12" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How different are Mars soil from Earth's soil?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Mars has no soil.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soil has an organic component, which (so far as we know) is lacking in Mars regolith. Actually, the Viking probes may have found signs of organic compounds in the Martian soil, but even until today the results of the experiment are in dispute. Interestingly all Martian landers since, with the notable exception of Phoenix and Curiosity, have been designed to &lt;em&gt;not&lt;/em&gt; search for organics.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="3179" LastEditDate="2014-04-25T08:55:44.793" LastActivityDate="2014-04-25T08:55:44.793" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4095" PostTypeId="1" CreationDate="2014-03-24T17:57:25.367" Score="25" ViewCount="6708" Body="&lt;p&gt;Why are petrol and diesel not used as rocket fuel like kerosene ?&#xA;Which properties make them unfit for use in rocket fuel ?&lt;/p&gt;&#xA;" OwnerUserId="2962" LastEditorUserId="12102" LastEditDate="2017-06-23T08:36:32.743" LastActivityDate="2020-04-19T19:18:45.467" Title="Petrol as rocket fuel" Tags="|rockets|fuel|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4096" PostTypeId="2" ParentId="4095" CreationDate="2014-03-24T18:09:00.380" Score="22" Body="&lt;p&gt;I'll try to find a source for this, but it is my understanding that in the early days of rocketry (read: 1950's, in this context), engineers tried to use jet fuel (and probably other fuels), but the way the fuel was produced led to large variations in thrust and specific impulse from test to test.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, they'd fuel up the rocket engine (on a test stand), light it up and measure the thrust. Then they'd do it again, and they would get a substantially different thrust. This led to a specification called RP-1 (Rocket Propellant 1) which dictates that any fuel carrying the designation of RP-1 must have a certain level of various impurities (e.g. sulfur) and no more. This specification is tighter than specifications for jet fuel and other hydrocarbon fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: The wikipedia page on RP-1 [1] gives a more detailed description of what I said, although they attribute the problems more directly to problems with cooling the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) &lt;a href=&quot;http://en.wikipedia.org/wiki/RP-1#Usage_and_history&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/RP-1#Usage_and_history&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="-1" LastEditDate="2014-10-14T17:06:56.517" LastActivityDate="2014-10-14T17:06:56.517" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4097" PostTypeId="2" ParentId="4085" CreationDate="2014-03-24T19:57:22.350" Score="16" Body="&lt;p&gt;It only goes straight up for a few seconds as it's clearing the pad. It turns to an angled path almost immediately after clearing the launch platform and begins traveling more horizontally than upwards very soon. If you listen to the audio from NASA TV during a shuttle launch, you'll hear them call out altitude, speed, and distance downrange at semi-regular intervals. By about 50 seconds, it is twice as far horizontally downrange as it is above the surface in altitude. You can listen to the audio in &lt;a href=&quot;http://www.youtube.com/watch?v=r4l14VC9bGM&quot;&gt;this shuttle launch from NASA TV&lt;/a&gt;. As others have said, this is because anything in low Earth orbit must accelerate to around 18,000 mph horizontally in order to stay in orbit. Basically, the speed tangent to the surface of the Earth must be such that the acceleration towards the Earth from gravity causes the object to fall around the Earth in a closed loop rather than falling into the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="2900" LastActivityDate="2014-03-24T19:57:22.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4098" PostTypeId="2" ParentId="4093" CreationDate="2014-03-24T20:41:13.533" Score="10" Body="&lt;p&gt;There are two ways.  One is to numerically integrate from the current time to the future time.  That is conceptually the easiest, as well as the simplest to implement (given a good integration routine).  On the other hand, it is the most compute intensive, most prone to accumulated numerical error depending on how far you have to go, and provides no insight into the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For situations that are not your two-body problem, that is really the only option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second is to derive the six orbital elements from the six components of position and velocity (assuming some convention for $t=0$).  That tells you a great deal about the future of the object, such as the size, shape, and orientation of the orbit, and the orbit period.  From the elements you can compute the position and velocity at any future time with a few simple formulas, without having to integrate through the times in between.  Again, assuming your two-body problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer the question in the comments, the position and velocity can be computed using the orbital elements thusly.  In the plane of the orbit, where $\mu$ is the $GM$, $a$ is the semi-major axis, $e$ is the eccentricity, and $\tau$ is the eccentric anomaly (more on that one in a bit):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$x=a\left(\cos\tau-e\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$y=a\sqrt{1-e^2}\sin\tau$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$z=0$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_x=-\sqrt{\mu\over a}{\sin\tau\over 1-e\cos\tau}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_y=\sqrt{\mu\over a}{\sqrt{1-e^2}\cos\tau\over 1-e\cos\tau}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_z=0$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can then rotate those coordinates to the actual plane of the orbit by applying an Euler rotation with the angles $\Omega$, $i$, and $\omega$, where those are the longitude of the ascending node, the inclination, and the argument of periapsis respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The eccentric anomaly goes from $0$ to $2\pi$ over one orbit, and is a convenient substitute for time.  It is related to time by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t=\sqrt{a^3\over\mu}\left(\tau-e\sin\tau\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no closed form solution to get $\tau$ from $t$, so you need to solve that numerically.  Or if you are, for example, plotting, you can calculate the coordinates and $t$ all from $\tau$ and plot parametrically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This all assumes the convention that $t=0$ and $\tau=0$ is at periapsis.  You will need to offset $t$ for your solution, since I expect that you will want $t=0$ to be at your initial condition, which is probably not at periapsis.  That is where the sixth orbital element for the anomaly comes into play.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will give you the answer in a fake coordinate system for the two-body problem that is pretty darned close to the real coordinate system if the body being orbited is much, much heavier than the object orbiting.  However if that's not the case, then you need to convert the coordinates back to the real situation for both bodies, since they are both orbiting their combined center of mass.  An example of that case is the Moon orbiting the Earth.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-03-24T21:26:15.693" LastActivityDate="2014-03-24T21:26:15.693" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4100" PostTypeId="2" ParentId="4095" CreationDate="2014-03-25T00:16:54.573" Score="5" Body="&lt;p&gt;Rockets work differently to internal combustion engines. It is easier to cool an engine than it is to cool a rocket, due to the scale and usage of the exothermic reactions. rockets use specific propellants that are tested for their purity. Car engines can have fuel that have some impurities because it is ignited intermittently and is used to transform chemical energy into kinetic energy. Most Rockets ignite their fuel constantly and the resulting release of force from the chemical energy pushes the rocket away from the aperture of the rocket, even in vacuums.      &lt;/p&gt;&#xA;" OwnerUserId="2971" LastEditorUserId="49" LastEditDate="2014-03-25T10:20:16.263" LastActivityDate="2014-03-25T10:20:16.263" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4101" PostTypeId="2" ParentId="4085" CreationDate="2014-03-25T08:06:52.837" Score="-4" Body="&lt;p&gt;The Earth moves, and if the shuttle is travelling in a straight path it wouldn't make sense, because the Earth is moving, and we would perceive for it to be moving in a curved way. Not sure, but I think this is the coriolis effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Found a cool video on what happens if we dig a hole to the middle of the Earth. Somewhere in this video, it suggests that people wouldn't be able to dig a massive hole into the Earth and jump into it because of the coriolis effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cheers, hope this helps.&lt;/p&gt;&#xA;" OwnerUserId="2976" LastActivityDate="2014-03-25T08:06:52.837" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4102" PostTypeId="2" ParentId="2240" CreationDate="2014-03-25T08:11:57.287" Score="9" Body="&lt;p&gt;Actually, Philae does have a propulsion system. As explained in &lt;a href=&quot;https://space.stackexchange.com/questions/2771/are-any-precautions-in-place-to-prevent-loss-of-the-rosetta-lander-the-way-hayab&quot;&gt;this related question&lt;/a&gt;, its &lt;a href=&quot;https://solarsystem.nasa.gov/docs/p395.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Active Descent System&lt;/a&gt; uses a &lt;a href=&quot;http://adsabs.harvard.edu/full/2004ESASP.548..449B&quot; rel=&quot;nofollow noreferrer&quot;&gt;cold-gas thruster&lt;/a&gt; to propel the lander towards the comet if needed.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-03-25T08:11:57.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4103" PostTypeId="2" ParentId="2500" CreationDate="2014-03-25T08:51:08.143" Score="34" Body="&lt;p&gt;According to Aviation Week &amp;amp; Space Technology, 1967 August 28:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA has retroactively assigned the labels of Apollo 1, 2 and 3 to the&#xA;  first three unmanned missions flown.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The magazine also refers to AS-501 as Apollo 4.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By my calculation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apollo 1 = suborbital 1966 Feb 26 as AS-201&lt;/li&gt;&#xA;&lt;li&gt;Apollo 2 = suborbital 1966 Aug 25 as AS-202&lt;/li&gt;&#xA;&lt;li&gt;Apollo 3 = orbital 1966 059A as AS-203&lt;/li&gt;&#xA;&lt;li&gt;Apollo 4 = orbital 1967 113A as AS-501&lt;/li&gt;&#xA;&lt;li&gt;Apollo 5 = orbital 1968 007A and 1968 007B as AS-204&lt;/li&gt;&#xA;&lt;li&gt;Apollo 6 = orbital 1968 025A as AS-502&lt;/li&gt;&#xA;&lt;li&gt;Apollo 7 = manned orbital 1968 089A as AS-205&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, &lt;a href=&quot;http://history.nasa.gov/SP-4009/contents.htm&quot;&gt;NASA History SP-4009&lt;/a&gt;, for the period &lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4009/v4p1h.htm&quot;&gt;1967 March 25 to April 24&lt;/a&gt;, tells a different story. After some discussion (in which George Low offered the above as an option), it was eventually decided to keep the burnt mission as Apollo 1, to not number AS 201, 202 and 203, and the next would be Apollo 4 (AS-501) and thereafter number the missions in flight order:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;March 25 - April 24&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA Hq. Office of Manned Space Flight informed KSC, MSFC, and MSC of&#xA;  approved designations for Apollo and Apollo Applications missions: &lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;all Apollo missions would be numbered sequentially in the order flown, with the next mission to be designated Apollo 4, the following&#xA;  one Apollo 5, etc., and &lt;/li&gt;&#xA;  &lt;li&gt;the Apollo Applications missions would be designated sequentially as AAP-1, AAP-2, etc. The number designations would not differentiate&#xA;  between manned and unmanned or uprated Saturn I and Saturn V missions.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;  &#xA;  &lt;p&gt;In a letter to George E. Mueller, OMSF, on March 30, MSC Deputy&#xA;  Director George M. Low offered two suggestions, in keeping with the&#xA;  intent of the NASA instruction yet keeping the designation Apollo 1&#xA;  for spacecraft 012. NASA Hq. had approved that designation before the&#xA;  January 27 fire claimed the lives of Astronauts Virgil I. Grissom,&#xA;  Edward H. White II, and Roger B. Chaffee; and their widows requested&#xA;  that the designation be retained. The suggestions were:&lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;Consider the AS-201, 202, and 203 missions part of the Saturn I (as opposed to uprated Saturn I) series; reserve the designation&#xA;  Apollo 1 for spacecraft 012; and number the following flights Apollo&#xA;  2, etc., or &lt;/li&gt;&#xA;  &lt;li&gt;Designate the next flight Apollo 4, as indicated by Headquarters, but apply the scheme somewhat differently for missions already flown.&#xA;  Specifically, put the Apollo 1 designation on spacecraft 012 and then,&#xA;  for historic purposes, designate 201 as mission 1-a, 202 as mission 2&#xA;  and 203 as mission 3.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;  &#xA;  &lt;p&gt;A memorandum to the NASA space flight Centers, North American&#xA;  Aviation, and certain Headquarters personnel from the NASA Assistant&#xA;  Administrator for Public Affairs on April 3 stated that the Project&#xA;  Designation Committee had approved the Office of Manned Space Flight's&#xA;  recommendations and that Mueller had begun implementation of the&#xA;  designations.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;On April 24, OMSF further instructed the Centers that AS-204 would be&#xA;  officially recorded as Apollo 1, &quot;first manned Apollo Saturn flight -&#xA;  failed on ground test.&quot; AS-201, AS-202, and AS-203 would not be&#xA;  renumbered in the &quot;Apollo&quot; series, and the next mission would be&#xA;  Apollo 4.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;TWX, Mueller, NASA OMSF, to KSC, MSFC, MSC, &quot;Apollo and AAP Mission&#xA;  Designation,&quot; March 25 and April 24, 1967; ltr., Low to Mueller, March&#xA;  30, 1967; memo, Julian Scheer, NASA Assistant Administrator for Public&#xA;  Affairs, to distr., April 3, 1967.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there is no official Apollo 2 or Apollo 3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this helps,&#xA;Andrew Rennie&#xA;President&#xA;Space Association of Australia, Inc.&lt;/p&gt;&#xA;" OwnerUserId="2977" LastEditorUserId="49" LastEditDate="2014-03-25T10:32:56.027" LastActivityDate="2014-03-25T10:32:56.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4104" PostTypeId="1" CreationDate="2014-03-25T13:12:47.887" Score="12" ViewCount="3569" Body="&lt;p&gt;Is there any reason why we can't launch from other primary locations other than Cape Canaveral? I know we have begun launching some rockets from Virginia, but is the Space Coast really the ideal location for launches in the US? Florida is a lot closer to the equator than say New Jersey and the launch facilities are more established at Cape Canaveral. The reason I ask is with the new commercial space launch initiatives, many in Congress are pushing for their States to get a piece of the space action because it creates and nurtures jobs, with Space Florida aggressively trying to build on past successes to encourage more users of existing launch facilities.&lt;/p&gt;&#xA;" OwnerUserId="2757" LastEditorUserId="25" LastEditDate="2014-03-25T22:21:53.643" LastActivityDate="2024-02-09T18:30:23.713" Title="Is Cape Canaveral the ideal launch location in the US?" Tags="|rockets|launch-site|commercial|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4106" PostTypeId="2" ParentId="4104" CreationDate="2014-03-25T13:39:54.127" Score="10" Body="&lt;p&gt;The space coast is one of the best locations for launches to equatorial orbit in the US:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It's as far south as you can get and still be in the US, so it can still take advantage of the US' infrastructure. And as you point out the closer to the equator the better&lt;/li&gt;&#xA;&lt;li&gt;It's right next to an ocean, which is good for safety as downrange is unpopulated&lt;/li&gt;&#xA;&lt;li&gt;You can launch to the east without endangering life - launching to orbit to the east is preferable due to the rotation of the earth&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Launching to the west or polar orbits is done from the west coast, again for safety reasons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course there's much more to the space industry than assembly and launch facilities. Rockets must be designed and tested, modules must be built - things which can happen just about anywhere. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-03-25T13:39:54.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4107" PostTypeId="2" ParentId="4104" CreationDate="2014-03-25T13:40:21.037" Score="14" Body="&lt;p&gt;This is a bit odd question. Cape Canaveral (I imagine you mean both the &lt;a href=&quot;https://en.wikipedia.org/wiki/Patrick_Air_Force_Base&quot; rel=&quot;nofollow noreferrer&quot;&gt;Patrick Air Force Base&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kennedy_Space_Center&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kennedy Space Center&lt;/a&gt; with it?) is hardly the only launch site in the US, but it makes sense to launch prograde from it, since the groundtrack then puts launches over the Atlantic. In case of retrograde or highly inclined launches, it makes more sense to launch from US West Coast (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Vandenberg_Air_Force_Base&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vandenberg Air Force Base&lt;/a&gt;) where the ground track puts them over the Pacific.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideal? For most launches I'd say yes, since prograde launches are far more frequent than retrograde (or hushed purpose ones, where launching over mainland US could keep its early flight away from prying eyes, but those are even rarer). Cape Canaveral launch sites are also closer to the equator and with it launch vehicles receive a slight additional boost by Earth's rotation, again, if launched prograde. I discuss this a bit more in my answer to &lt;a href=&quot;https://space.stackexchange.com/a/3511/49&quot;&gt;Are there any benefits to launching from Vandenberg AFB instead of Cape Canaveral?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But even brushing aside with politics or pressure from the space launch industry in Florida, I'd have to concur, it makes sense. You're soon after launch over the ocean, there's not any heavy traffic shipping lanes close by that can't be rerouted for the duration of the launch window, and it avoids the heavy traffic &lt;a href=&quot;https://en.wikipedia.org/wiki/North_Atlantic_Tracks&quot; rel=&quot;nofollow noreferrer&quot;&gt;North Atlantic Tracks&lt;/a&gt; air routes. It is also one of the major US space flight centers, with lots of the space flight industry and their facilities at disposal.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-03-25T13:40:21.037" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4108" PostTypeId="1" CreationDate="2014-03-25T14:00:27.557" Score="10" ViewCount="4025" Body="&lt;p&gt;It looks like a liquid fuel rocket engine is made of rather thin metal. Here's &lt;a href=&quot;http://en.wikipedia.org/wiki/File:RD-107_Vostok.jpg&quot; rel=&quot;noreferrer&quot;&gt;photo of RD-107&lt;/a&gt; from Wikipedia&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RGzCwl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nothing personal, but it looks like it's made of several shiny thin metal cans (connected with gazillion of pipes for feeding fuel and oxidizer). When the rocket is flying with the engine working, a jet of burning fuel is going down and pushing the rocket up which means that all the rocket weight (dozens of tons and btw the engine accelerates the rocket so that weight should be multiplied by some number larger than one) is resting on the engine (and the engine is resting on the jet being produced) so all the weight is in fact resting on those shiny thin metal cans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why isn't the engine crushed by the rocket weight?&lt;/p&gt;&#xA;" OwnerUserId="2948" LastEditorUserId="897" LastEditDate="2014-03-26T16:01:18.503" LastActivityDate="2014-11-13T01:13:36.320" Title="How does a thin metal rocket engine not get crushed by the weight of the flying rocket?" Tags="|rockets|engine-design|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4109" PostTypeId="2" ParentId="4108" CreationDate="2014-03-25T14:17:59.337" Score="5" Body="&lt;p&gt;The weight is supported quite evenly by the pressure of the exhaust gases in the nozzle.  Spread the weight over a wide enough area, and the resulting stresses are quite manageable.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-03-25T14:17:59.337" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4111" PostTypeId="2" ParentId="4075" CreationDate="2014-03-25T17:07:38.587" Score="4" Body="&lt;p&gt;The Earth Sun Lagrange point 2 (SEL2) has the following advantages for the Gaia telescope:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Gaia is NOT shadowed in (orbit around) the SEL2. The Earth is way too far away to cast any shadow there. This is good for Gaia because it gets its electric power from its solar panels.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In SEL2 Gaia has constant solar exposure. In LEO it would, like the Hubble Space Telescope or the ISS, go into and out of Earth shadow every few hours. Gaia has very sensitive instruments which need predictable and constant temperatures. Not necesserily minimal temperature, but rather constant.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;LEO would also have the Earth cover a large fraction of the sky. The Hubble Space Telescope is designed to focus on a tiny part of the sky at a time and its operation is carefully planed with respect to where it is relative to Earth. But the purpose of Gaia is to systematically over time scan the entire sky. It wouldn't be practical for it to be near any large object.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Gaia uses the parallax, the angular differences from which it observes stars as it orbits the Sun. In a larger orbit it would get larger angles and better precision in its measurements. So it would basically be good. But it would have two major drawbacks:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, it would take longer to complete the orbits, and it needs in principle to complete at least two orbits to get both distance and movement data (this might be more complex depending on how it operates). If it were out at Jupiter, it would would have about 5 times as large a parallax, but it would take more than 10 years to complete one single orbit around the Sun. It might not have that long a life and scientists might not want to wait that long. (Maybe a follow up will???)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, SEL2 is at a constant distance and direction from Earth. If in any other orbit, it would drift away and eventually be hundreds of millions kilometers away, on the other side of the Sun. This would make telecommunications a challenge and likely require more investments in both onboard equipment and ground control.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-03-25T18:29:15.483" LastActivityDate="2014-03-25T18:29:15.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4112" PostTypeId="2" ParentId="4108" CreationDate="2014-03-25T17:31:38.533" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Nothing personal, but it looks like it's made of several thin metal cans connected with gazillion of pipes. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Those &quot;gazillion of pipes&quot; are the mechanism via which force is transferred from the rocket engines to the structure of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a simple experiment: Get a couple of empty aluminum soda cans. Stand them upright on a flat surface. Now stand on them, one can per foot. Do it carefully and those two soda cans will support &lt;em&gt;all&lt;/em&gt; of your weight. Think about that. All of your weight is supported on two very thin cylindrical structures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, notice how those &quot;gazillion of pipes&quot; form lots of triangles. Triangles make for very sound structures. Look at a bridge. Look in your attic. You'll see lots and lots of triangles.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-03-25T17:31:38.533" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4113" PostTypeId="2" ParentId="4104" CreationDate="2014-03-25T19:52:56.320" Score="6" Body="&lt;p&gt;Another way to think about this question might be &amp;quot;What US territory is closest to the equator.&amp;quot;  There are definately better options that Florida, but they are outside the continental US.  Obviously using these sites is a bit of a trade-off since it's not as convenient to the rest of the industrial base.&lt;/p&gt;&#xA;&lt;p&gt;Here are the latitudes of some possible alternate sites:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Cape Canaveral: 28 degrees N.&#xA;Hawaii: 21 degrees N.&#xA;Marshall Islands: 9 degrees N.&#xA;Baker Island: 0.11 degrees N.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Baker island looks best on paper, but it is just a tiny nature preserve, not really practical for rocket operations.&lt;/p&gt;&#xA;" OwnerUserId="1165" LastEditorUserId="1165" LastEditDate="2024-02-09T18:30:23.713" LastActivityDate="2024-02-09T18:30:23.713" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="4114" PostTypeId="1" AcceptedAnswerId="4116" CreationDate="2014-03-25T21:27:04.320" Score="20" ViewCount="6587" Body="&lt;p&gt;In this screenshot from the recent Soyuz TMA-12M launch, a crewmember is seen using a stick to enter commands into the computer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/XYubL.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this stick standard, taken on all Soyuz flights? Also, why aren't the input devices moved such that they can be used without a pointer?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2014-05-28T20:20:39.207" Title="Why is a stick required to operate Soyuz's computer?" Tags="|launch|soyuz-spacecraft|flight-computer|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4115" PostTypeId="2" ParentId="4104" CreationDate="2014-03-25T22:21:04.597" Score="6" Body="&lt;p&gt;In addition to the optimal latitude for equatorial prograde launches, let me explain a few other reasons why Flordia is a good location.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It is easily accessible by land, allowing for the transportation of rocket parts. Islands are much more difficult to access.&lt;/li&gt;&#xA;&lt;li&gt;You can't go much further to the south in Florida without being over major islands, which would cause problems with potential loss of life if something happened.&lt;/li&gt;&#xA;&lt;li&gt;As mentioned, there aren't any major shipping lanes/ aicraft flight lanes overhead, that can't be diverted. The numbers are reasonable.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Wallops Island in Virginia is an increasingly attractive option, especially for getting to the International Space Station, as it's slightly easier to achieve that orbit from Wallops than the Cape. But due to the large amount of infrastructure at the Cape, I'm confident it will remain the primary launch site for many missions based out of the United States.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-03-25T22:21:04.597" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4116" PostTypeId="2" ParentId="4114" CreationDate="2014-03-25T23:01:40.360" Score="22" Body="&lt;p&gt;It's really hard to answer this without complicating about how Soyuz spacecraft designers obviously didn't. Or crack a joke or two. It's a cramped little space vehicle and that space between the three crew, the seats they occupy and the control panels is pretty much all the space they have to crawl into it and later out of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving the panels closer would simply leave too little space with all the gear surrounding them, the hatch wouldn't open all the way down, and a stick to push buttons happens to be good enough for the job. Here's how the Soyuz capsule looks from inside:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/hDuv6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, that's all the space there is. Actually, even less, as I don't see the backup parachutes and a few other pieces of equipment. Because it's from a training vehicle / simulator. OK, let me try once more:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/QNxyV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;ISS crew members, U.S. astronaut Daniel Burbank and Russian cosmonauts Anton Shkaplerov and Anatoly Ivanishin, crowded&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;inside their Soyuz capsule shortly after landing in Kazakhstan, on April 27, 2012. (Reuters/Sergei Remezov)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comfy! And this is how &quot;big&quot; it looks from the outside on landing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/uNECc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Russian support personnel work to help get Expedition 29 crew members out of the Soyuz TMA-02M spacecraft shortly after the&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;capsule landed on November 22, 2011. (AP Photo/NASA/Bill Ingalls)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That wide open compartment on the capsule on the last photograph is where the drogues and main parachutes are stored, and is not where the crew comes out of. The orange thing with white stripes inside is an inflated &quot;airbag&quot; that pushes the chutes out. And there are three astronauts / cosmonauts inside, and ...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The reentry capsule also contains life-support systems, batteries for&#xA;  the descent phase of the mission, primary and backup parachutes and&#xA;  solid-propellant engines for landing. The reentry capsule is also&#xA;  equipped with eight small attitude control thrusters, which are used&#xA;  exclusively during the descent phase of the flight.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Source: &lt;a href=&quot;http://www.russianspaceweb.com/soyuz_sa.html&quot; rel=&quot;noreferrer&quot;&gt;RussianSpaceWeb.com&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the &quot;stick&quot; it is. And a small furry toy mission mascot that the spacecraft commander usually receives from his / her daughter / son / other family member, hangs it in that little free space left and in front of the onboard crew camera to double as a &quot;weightlessness indicator&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not wanting to leave you completely empty-handed and dismissing your question with &quot;that's how it is&quot;, here's a nice blog post that I found searching for these photographs above: &lt;a href=&quot;http://jalopnik.com/5883458/how-to-fly-a-soyuz-space-capsule&quot; rel=&quot;noreferrer&quot;&gt;How To Fly A Soyuz Space Capsule&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a nice video, showing where the crew sit, and why a stick is a useful extension tool.  &lt;a href=&quot;https://www.youtube.com/watch?v=YfCws6c6_Tw&quot; rel=&quot;noreferrer&quot;&gt;Inside a Soyuz capsule&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="51" LastEditDate="2014-03-26T19:04:36.493" LastActivityDate="2014-03-26T19:04:36.493" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4117" PostTypeId="2" ParentId="4093" CreationDate="2014-03-25T23:07:50.743" Score="3" Body="&lt;p&gt;@Stu, I'll add some information from the equations of motion perspective.  These comments are more geared towards artificial satellite orbiting Earth problem--additional bodies and perturbations throws a lot of this out the window, but some fundamentals remain.  Apologies in advance if you already know this.&lt;/p&gt;&#xA;&lt;p&gt;We make some assumptions on the relative masses of the two-bodies involved, and assume the central body has a far greater mass.  We also assume an inertial reference frame, and that the bodies are point masses.  Using these we can develop the 2BP equation of motion where &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; is the position vector from the central body center of mass, to the second body&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;\ddot{\vec{r}}=-\frac{\mu}{r^2} \frac{\vec{r}}{r}&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Now, the fact that we have an &lt;span class=&quot;math-container&quot;&gt;$r^3$&lt;/span&gt; in the denominator shouldn't worry you - This is the inverse square gravity law, and the additional &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; scales &lt;span class=&quot;math-container&quot;&gt;$\vec{r}$&lt;/span&gt; to produce a unit vector.  This looks a little nasty to solve in closed form, since we have a non-linear, ordinary differential equation (which is also coupled due to the &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; term).  There are 3 second-order ODEs, which can be expressed as 6 first-order ODEs.  If you have access to a numerical integrator, all you need now are initial conditions on your position and velocity vectors (the 6 states), set your desired time, and away you go.  You can propagate this problem backward in time as easily as forward in time.&lt;/p&gt;&#xA;&lt;p&gt;With respect to orbital element and position velocity conversions, the unperturbed two-body problem essentially has one degree of freedom, that being your choice of anomaly in the plane.  With one anomaly specified, you can calculate any of the other two.  Given the mean anomaly &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt;, you can iterate to find the eccentric anomaly &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt;, and from there determine the true anomaly &lt;span class=&quot;math-container&quot;&gt;$\nu$&lt;/span&gt; (with care to avoid quadrant ambiguity with the arctangent function).&#xA;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;M=E-e\sin{E}&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;\tan{\frac{\nu}{2}} = \sqrt{\frac{1+e}{1-e}}\tan{\frac{E}{2}}&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Propagating the anomalies allows one to more easily visualize the orbit, versus observing position/velocity vectors.  There exist numerous open-source routines to easily convert (typically named rv2oe or rv2coe) between position/velocity and orbital elements.  This is a good website for &lt;a href=&quot;https://celestrak.org/software/vallado-sw.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vallado's collection of software&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1478" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2014-03-25T23:07:50.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4118" PostTypeId="1" AcceptedAnswerId="4120" CreationDate="2014-03-26T01:14:16.257" Score="22" ViewCount="10161" Body="&lt;p&gt;While a launch vehicle is being fueled on the launch pad, what is holding it up? Surely not all that weight is being held up by the engine nozzles. What parts of the launch vehicle are supporting its entire weight prior to ignition? Is there a term for this? I am sure it's different for each vehicle, but any example would be great. &lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2017-01-13T06:05:18.610" Title="What is a launch vehicle resting on prior to launch?" Tags="|launchpad|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4119" PostTypeId="2" ParentId="4118" CreationDate="2014-03-26T06:09:52.933" Score="7" Body="&lt;p&gt;Most rockets are supported at the base of the fuselage. This is generally the location where the engine is connected to the framework of the rocket. EG The base of the STS SRBs are connected to the pad with 4 explosive bolts. They blow a few ms after the ignition of the boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An exception is the Soyuz (R-7 and its descendants) which are actually suspended over the pad by 4 large 'arms'. These lock into place after erection and are released during launch.&lt;/p&gt;&#xA;" OwnerUserId="1076" LastEditorUserId="415" LastEditDate="2015-06-11T14:45:51.367" LastActivityDate="2015-06-11T14:45:51.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4120" PostTypeId="2" ParentId="4118" CreationDate="2014-03-26T08:48:27.137" Score="27" Body="&lt;p&gt;The platform holding launch vehicle is called &quot;launch pedestal&quot; or &quot;пусковой стол&quot; or &quot;стартовый стол&quot; (in russian). Usually there are special &quot;Hold-Down Arms&quot; or &quot;support arms&quot; (&quot;опоры пускового стола&quot;) to hold rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Examples:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1.. Proton is supported at bottom. There is launch pedestal model: &lt;a href=&quot;http://www.cardmodels-r.narod.ru/html/Proton-LP1.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.cardmodels-r.narod.ru/html/Proton-LP1.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3NJfg.jpg&quot; alt=&quot;Proton&amp;#39;s  launch pedestal - model by cardmodels-r.narod.ru&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the middle there is round interface plane &quot;блок разъемов пускового стола&quot; (with electrical ports). Six white elements are top view of support arms. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side view, arrow pointing to interface plane (actually, to position of &quot;lift-off switch&quot; - &quot;контакт подъема&quot;) :&lt;img src=&quot;https://i.stack.imgur.com/4Blgj.jpg&quot; alt=&quot;servicing the proton launch pedestal, support arms are visible&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When rocket goes up, support arms will rotate and hide in the green protection cones, and interface plane will go down and protected with special doors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is cross-section of proton pedestal. In the middle we have interface plane, and support arms with rotation mechanism is shown in green:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/3O97V.jpg&quot; alt=&quot;cross section of Proton&amp;#39;s launch pedestal&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last stages of installing Proton from transport vehicle (&quot;Установщик&quot;, 8У260) to launch pedestal: &lt;img src=&quot;https://i.stack.imgur.com/Qj1IT.jpg&quot; alt=&quot;installing Proton from transport vehicle to launch pad&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.. Saturn's &quot;Hold-Down Arms and Tail Service Masts&quot; - &lt;a href=&quot;http://www.hq.nasa.gov/pao/History/SP-4204/ch13-4.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.hq.nasa.gov/pao/History/SP-4204/ch13-4.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hold-down arm:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/BA2AT.jpg&quot; alt=&quot; hold-down arm for saturn V&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Four hold-down arms had to secure the Saturn V firmly on the mobile launcher during assembly, transportation to the launch site, and its stay on the launch pad in all kinds of weather. These devices also had to have the strength to hold down the launch vehicle after ignition, until all engines registered full thrust. Then they automatically and simultaneously released the Apollo-Saturn for liftoff.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In cross-section of arm - &lt;a href=&quot;http://www.hq.nasa.gov/pao/History/SP-4204/images/m287a.gif&quot; rel=&quot;noreferrer&quot;&gt;http://www.hq.nasa.gov/pao/History/SP-4204/images/m287a.gif&lt;/a&gt; we can see bottom part - the &quot;support arm&quot; and the top &quot;upper link&quot; which is used as &quot;hold-down arm&quot; to hold down the Saturn before reaching full thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;3.. Saturn 1 support arm's photo: &lt;a href=&quot;http://heroicrelics.org/ussrc/s-i-bl-i-support-arm/index.html&quot; rel=&quot;noreferrer&quot;&gt;http://heroicrelics.org/ussrc/s-i-bl-i-support-arm/index.html&lt;/a&gt; and hold down arms: &lt;a href=&quot;http://heroicrelics.org/msfc/s-i-bl-i-holddown-arm/index.html&quot; rel=&quot;noreferrer&quot;&gt;http://heroicrelics.org/msfc/s-i-bl-i-holddown-arm/index.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Schematics of LC-34 Launch pad &lt;a href=&quot;http://www.hq.nasa.gov/pao/History/SP-4204/ch2-4.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.hq.nasa.gov/pao/History/SP-4204/ch2-4.html&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;4.. NASA’s Space Launch System (SLS) - &lt;a href=&quot;http://www.nasaspaceflight.com/2012/11/nasa-sls-mobile-launcher-umbilical-plans/&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasaspaceflight.com/2012/11/nasa-sls-mobile-launcher-umbilical-plans/&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here we can see at right 3 Vehicle Support Posts of nearest booster. There are 4 support posts for each booster, and there are two boosters.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/rU3zb.jpg&quot; alt=&quot;SLS’s support umbilicals and Vehicle Support Posts&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“Provide 8 Vehicle Support Posts for SLS Boosters using as-designed Ares 1 stationary VSPs, utilizing Shuttle Hold-down Post blast shields, bushing, spherical bearings, and shoe retainers,” the VSPs will be 68 inches tall, 43 inches wide, and 49 inches deep.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The VSPs will provide structural load support of the SLS rocket through the Solid Rocket Boosters.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Currently, it is expected that the VSPs will be static, providing a weight-of-vehicle-only support pedestal for the SRBs&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2014-03-26T08:58:30.400" LastActivityDate="2014-03-26T08:58:30.400" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4121" PostTypeId="1" AcceptedAnswerId="4126" CreationDate="2014-03-26T11:04:16.850" Score="13" ViewCount="1923" Body="&lt;p&gt;The images taken by Pioneer 10 and 11 were of much lower quality than those of later planetary probes. I wonder if this is representative also for their other instruments. For example, they don't seem to have discovered volcanism on Io or any rings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should the Pioneer 10 &amp;amp; 11 be considered as disappointments or poorly designed missions (maybe prematurely rushed for prestige in the space race)? Or were they the necessary groundbreakers which made the Voyagers so successful? Was there really a big gap in science results (and interplanetary probe technology) between Pioneer and Voyager in the mid-70s and if so, was the reasons for it just general technological maturity, or some change in mission design philosophy at NASA?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I rarely see the Pioneers mentioned, while Voyager is still often admired even after the Galileo and Cassini orbiters. The first two (out of only seven to date) probes to the outer planets seem to have missed the spotlight in space exploration history.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-03-26T18:39:25.307" Title="What significance did Pioneer 10 &amp; 11 have?" Tags="|nasa|probe|interplanetary|pioneer|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4122" PostTypeId="2" ParentId="4085" CreationDate="2014-03-26T12:44:26.660" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;at 2:53 you can see the space shuttle from an angle, even though it's high above the earth and you should see only the bottom.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Shuttle time-lapse image showing the curved path it takes after takeoff:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/gDi6z.jpg&quot; alt=&quot;National Geographic image of Shuttle launch&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, just after liftoff the Shuttle does a &lt;a href=&quot;http://stason.org/TULARC/science-engineering/space/53-Why-does-the-shuttle-roll-just-after-liftoff.html&quot; rel=&quot;noreferrer&quot;&gt;roll maneuver&lt;/a&gt; so it will fly with the top of the orbiter facing down. This is done to minimize the loads on the Shuttle components. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/afmwy.jpg&quot; alt=&quot;Shuttle on its back&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-03-26T12:44:26.660" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4123" PostTypeId="2" ParentId="4017" CreationDate="2014-03-26T13:10:08.273" Score="12" Body="&lt;p&gt;In my youth, I am now retired, I worked on the apollo program at north american space and information systems in downey calif. chief contractor for the Apollo Command/Service Modules.  I also worked at Rocketdyne that was a part of north american.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the code I used and developed at that time was in fortran.  The fortran programs performed analysis of test firing data, analysis of rocket processes (combustion performance, chemical kinetics, heat transfer, etc) and programs performing spacecraft thermal analysis. I am not sure of the version but I do remember fortran 4 coming out during our efforts and there were some software changes needed to accommodate that newer version.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't speak regarding command and control software.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Truly exciting times and an experience I'll never duplicate.  I still occasionally use this &quot;ancient&quot; software on hobby projects.  It's still the state-of-the-art.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tom Kosvic&lt;/p&gt;&#xA;" OwnerUserId="2994" LastActivityDate="2014-03-26T13:10:08.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4124" PostTypeId="2" ParentId="4121" CreationDate="2014-03-26T15:04:52.873" Score="18" Body="&lt;p&gt;Yes, Pioneer 10 and 11 each carried a 0.000001 megapixel camera.  The single pixel was scanned over the body by the constant spin of the spacecraft in one direction and a slew of the spacecraft in the other.  Building up one image took a long time, over which the relative position of the spacecraft and the body changed quite a bit, distorting the image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That does not mean that the missions were &quot;poorly designed&quot;.  They were launched in the early 70's, so all of the planning and payload selection was in the 60's.  Each had &lt;em&gt;one-third&lt;/em&gt; the mass to work with as Voyager did, and much less power.  The science objectives were focused on fields and particle experiments, which were considered more scientific at the time than taking &quot;pretty pictures&quot;.  Later the science value of really good images was better appreciated.  (The science definition was led by James Van Allen, who was a fields and particles guy.)  Even if they wanted to make it an imaging mission, Pioneer had nowhere near the resources that Voyager did to accomplish that.  Nor did they have the technology.  With the mass they had, they did what they could to maximize the science return.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Pioneers really did pioneer, and provided important data on radiation and micrometeoroids that was used in the design of the Voyagers.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-03-26T15:04:52.873" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4125" PostTypeId="2" ParentId="4085" CreationDate="2014-03-26T16:14:13.387" Score="4" Body="&lt;p&gt;I had some old optimal control software lying around.  It's a really good match for this question, so I decided I would use that to attack the theoretical underpinnings of this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The central question I see is, does the shuttle &lt;em&gt;want&lt;/em&gt; to roll to some non-vertical angle?  Of course, by &quot;the shuttle&quot; I really mean the engineers who design it.  And by the engineers, I really mean the underpinning mathematical reality.  In short, will you burn less fuel if your trajectory is tilted somewhat?  We would think the answer is &quot;yes&quot;, but it's not easy to justify that.  It's also not clear what the motivation is.  If there were no atmosphere, we would prefer to launch horizontally, actually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to answer this is to use numerical methods to brute-force the problem to a solution.  Develop a figure-of-merit &quot;score&quot; function, and then vary the trajectory so that you get the best value.  Numerically, you can probably accomplish this by guessing some initial trajectory and then using the calculated Jacobian and Hessian to approximately locate the nearest critical point, where the figure-of-merit is (hopefully) at a local minimum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to simulate, here are some of the things I take into account:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The thrust from the engines comes out to 3gs, and this is constant&lt;/li&gt;&#xA;&lt;li&gt;Earth's gravity pulls it down&lt;/li&gt;&#xA;&lt;li&gt;Atmospheric drag follows the drag equation, and I used Falcon9 mass and diameter parameters&lt;/li&gt;&#xA;&lt;li&gt;The state vector gives the angles that the thrust is applied at, relative to vertical&lt;/li&gt;&#xA;&lt;li&gt;A simple rk4 method is used to integrate this model given a certain control&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;At the end of the simulation, a figure of merit function tallies its score.  These are chosen arbitrarily in order to get the outcome we want.  In particular, I:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Penalize high eccentricities&lt;/li&gt;&#xA;&lt;li&gt;Penalize a low semi-major axis&lt;/li&gt;&#xA;&lt;li&gt;Penalize long burn times&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The idea is that an optimizer program will find you the best combination of all these three for the lowest price.  This is what I obtained in terms of the angle at which the thrust is applied.  This is relative to the vertical, in the sense that 0 degrees would be applying thrust straight up.  Note that thrust and velocity aren't always in the same direction.  This represents the &quot;best&quot; way to angle the rocket to get the best performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/arWLP.png&quot; alt=&quot;Thrust angles&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This type of method does often have problems with it.  A lot of the &quot;harry&quot; behavior there is likely due to model errors as problems with the derivative calculations and other types of artifacts.  The simulation runs over 500 intervals... and there are 17 independent variables for which the &lt;em&gt;second&lt;/em&gt; derivative determines the system.  Since it's near the critical point by definition, the figure-of-merit is highly insensitive to these variables and that can cause problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nevertheless, it shows what I wanted it to show - the optimal trajectory clearly follows a non-vertical path near the surface.  FYI, the initial angle here is about 27 degrees relative to the vertical.  I would expect that real-world scenarios would be using numbers that are somewhat close to this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mathematically, that is a fairly rigorous justification for the angle at which rockets fly once they're clear of the pad.  It would, indeed, be &lt;em&gt;more optimal&lt;/em&gt; to tilt the launchpad itself.  Logistically, that sounds like a really bad idea.  So the engineers compromise by turning to the optimal angle after the launch pad has been cleared.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-03-26T16:14:13.387" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4126" PostTypeId="2" ParentId="4121" CreationDate="2014-03-26T18:39:25.307" Score="11" Body="&lt;p&gt;Pioneer 10 and 11 were built to &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_10&quot;&gt;gain experience in sending probes&lt;/a&gt; to the outer solar system. NASA had no experience at all of space outside Mars' orbit. The opportunity for a Grand Tour (one probe could cover all 4 of the outer planets) was recognized early enough to do some experiments. There &lt;a href=&quot;http://books.google.nl/books?id=evUP4glYmvEC&amp;amp;pg=PA36&amp;amp;redir_esc=y#v=onepage&amp;amp;q&amp;amp;f=false&quot;&gt;wasn't much of a budget&lt;/a&gt; though, so the scientific payload was limited. Remember that the Pioneers were built in the middle of the eyewateringly-expensive Apollo program.&lt;br&gt;&#xA;The Pioneer 5-9 series of spacecraft were all focused on measuring fields and particles. It made sense to do environmental measurements so they could prepare later space probes for the circumstances they'd be likely to encounter. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-03-26T18:39:25.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4127" PostTypeId="1" AcceptedAnswerId="4128" CreationDate="2014-03-26T20:15:14.603" Score="11" ViewCount="4189" Body="&lt;p&gt;According to Wikipedia and SpaceX, this demo flight is scheduled for 2014.  A group of us from San Diego want to come and watch the launch from a distance.  Is there any scheduled date for the launch?  If there's no specific date planned yet, what's the best resource/website to check to keep up with any news?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="3000" LastEditorUserId="49" LastEditDate="2014-03-26T20:26:00.840" LastActivityDate="2018-01-17T22:33:31.647" Title="What is the planned date for the Falcon Heavy Demo launch from Vandenberg AFB?" Tags="|launch|rockets|spacex|observation|falcon-heavy|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4128" PostTypeId="2" ParentId="4127" CreationDate="2014-03-26T20:19:35.323" Score="12" Body="&lt;p&gt;Rocket launches are notorious for not making deadlines. Especially the first launch of a new type (ISS launches, interestingly enough, have a very narrow window, usually a minute or three, so they eventually get to the point where they can launch on time). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore any date given today, for a date in the future is utterly meaningless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX will undoubtedly make a BIG deal about a Falcon Heavy initial launch. Of course, you will be able to check the &lt;a href=&quot;http://www.spacex.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt; home page. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, the most reliable source for launch info that I have found is at SpaceFlight Now's &lt;a href=&quot;http://spaceflightnow.com/launch-schedule/&quot; rel=&quot;nofollow noreferrer&quot;&gt;launch tracking page&lt;/a&gt;. Which as of this writing, does not even have a date listed for a Falcon Heavy launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt; The SpaceFlight Now page, as of January 4th 2017, lists a &quot;2nd Quarter 2017&quot; launch for the Falcon Heavy Demo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update2:&lt;/strong&gt; As of Jan 2018, the launch is expected any day now, as a Wet Dress Rehersal has completed, but a Static Fire is still pending. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally first launch will be from Florida at pad LC-39A no longer at Vandenberg.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-01-17T22:33:31.647" LastActivityDate="2018-01-17T22:33:31.647" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4129" PostTypeId="2" ParentId="4127" CreationDate="2014-03-26T21:43:41.743" Score="9" Body="&lt;p&gt;SpaceX has in particular been notorious about delaying their recent flights. The advent of the Falcon 9 v1.1 was about a year late, as you can see from looking at some of the mission dates in the history on SpaceFlight Now. For instance, the upcoming CRS mission shows that the following have been listed as it's launch date:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;April 6, Sept. 30, Nov. 11, Dec. 9, Jan. 15, Feb. 11, Feb. 22, March 1 and March 16. [March 14]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;SpaceFlightNow is a fantastic resource, even many in the industry use it to keep track of things. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the launch of the Falcon 9 heavy, I wouldn't put any serious money towards watching it happening until it's a month away. Even then, don't be surprised if it doesn't launch for a few weeks after the scheduled date. Stuff happens in the space industry.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-03-26T21:43:41.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4130" PostTypeId="2" ParentId="4091" CreationDate="2014-03-27T12:47:08.767" Score="1" Body="&lt;p&gt;Further to the answer of @PearsonArtPhoto, it &lt;strong&gt;&lt;em&gt;is&lt;/em&gt;&lt;/strong&gt; possible to use a reaction wheel to make a permanent change in attitude.  Consider a non-rotating satellite carrying a non-rotating gyro.  If a motor is used to spin up the gyro, Newton's Third Law requires a reverse torque be exerted on the satellite, and it will start to rotate in the opposite direction.  After a suitable period of time, the gyro is braked to a stop, at which point conservation of angular momentum dictates that the satellite and gyro are both non-rotating again, with the satellite pointing in the desired, new, direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the satellite experiences any &lt;strong&gt;&lt;em&gt;outside&lt;/em&gt;&lt;/strong&gt; torque, then the reaction wheel can be used to store the new angular momentum, keeping the attitude of the satellite fixed. In this case, the gyro must be kept rotating constantly, until the angular momentum can be &quot;dumped&quot;, or some opposite outside torque comes along...&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2014-03-27T12:47:08.767" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4132" PostTypeId="1" CreationDate="2014-03-27T13:26:39.683" Score="10" ViewCount="10028" Body="&lt;p&gt;A micrometeorite (including space debris in LEO) hitting the metal wall of the ISS would likely cause a shower of secondary high speed fragments. But there are other &quot;softer&quot; materials proposed which would just let the object through without much interaction. For example by Bigalow Aerospace.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would happen to a human who is hit by a micrometeorite directly? Take a 1 mm diameter metal object at 30 km/s impact speed as an example. Much smaller but much faster than a bullet from a gun. Would it just make a thin hole through the body, or would it like most military bullets tumble and tear up a much larger cavity in the tissue? Does it matter much how fast the projectile is spinning?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, micrometeorites have never caused any actual injury or even any important damage to equipment, although it is a much talked about potential threat.&lt;/p&gt;&#xA;" OwnerUserId="1457" OwnerDisplayName="LocalFluff" LastActivityDate="2014-03-27T19:22:13.127" Title="What happens if an astronaut is hit by a tiny micrometeorite?" Tags="|debris|impact|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4133" PostTypeId="2" ParentId="441" CreationDate="2014-03-27T18:58:44.473" Score="4" Body="&lt;p&gt;I talk about this at &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/12/whats-minimum-spin-hab.html&quot; rel=&quot;nofollow&quot;&gt;What's the minimum spin hab?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have two data points: 0 g and 1 g. We don't know the effects of partial g. Maybe Mars or Lunar gravity can keep us healthy. Or maybe not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Valeri Polyakov spent 438 days in orbit yet suffered only minor bone loss. However he had a lot of self discipline and maintained a challenging exercise regimen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides muscle and bone atrophy, drainage is also a problem. We rely on gravity to drain our sinuses, for example. I don't think a full g is needed for liquid to flow down hill. Besides helping with the body's housekeeping chores, partial g could also enable flush toilets, conventional showers, etc. Less troublesome hygiene would be a huge morale booster and improve health (in my opinion).&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="58" LastEditDate="2014-03-27T19:14:39.550" LastActivityDate="2014-03-27T19:14:39.550" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4134" PostTypeId="2" ParentId="4132" CreationDate="2014-03-27T19:22:13.127" Score="13" Body="&lt;p&gt;The &lt;a href=&quot;http://www.nasa.gov/externalflash/ISSRG/pdfs/mmod.pdf&quot; rel=&quot;noreferrer&quot;&gt;modules of the ISS&lt;/a&gt; are protected by &lt;a href=&quot;http://ares.jsc.nasa.gov/ares/hvit/basic.cfm&quot; rel=&quot;noreferrer&quot;&gt;Whipple shields&lt;/a&gt;: basically an extra wall outside the main pressure hull. A micrometeorite would puncture this shield and disintegrate, leaving nothing big enough to penetrate the main hull. In this image, the shield is on the right:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/WL9wl.png&quot; alt=&quot;Whipple shield&quot;&gt;&lt;br&gt;&#xA;In a space suit, you don't have as much protection (it'd weigh too much, and be very bulky if it had a big air gap), but the outer layer of a spacesuit is designed to offer &lt;a href=&quot;http://www.jaxa.jp/article/special/eva/aoki_e.html&quot; rel=&quot;noreferrer&quot;&gt;some protection from micrometeorites&lt;/a&gt;. As with a Whipple  shield, the micrometeorite would disintegrate when it hits the outer layer. You'd be left with lots of tiny puncture wounds. &#xA;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_Micrometeoroid_Garment&quot; rel=&quot;noreferrer&quot;&gt;outer layer of a spacesuit&lt;/a&gt; (NASA calls this the Thermal Micrometeoroid Garment) is made of layers of (for the Space Shuttle suits) kevlar and dacron interspersed with thermal foil layers. The &lt;a href=&quot;http://history.nasa.gov/SP-368/s6ch6.htm&quot; rel=&quot;noreferrer&quot;&gt;Apollo suits were slightly different&lt;/a&gt;.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.bigelowaerospace.com/ba330.php&quot; rel=&quot;noreferrer&quot;&gt;Bigelow claims&lt;/a&gt; that their design offers better protection than NASA's aluminium Whipple shield.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A study on rats showed &lt;a href=&quot;http://spacemedicineassociation.org/timeline/1962/33020156-1.pdf&quot; rel=&quot;noreferrer&quot;&gt;damage similar to a shotgun blast&lt;/a&gt; (PDF with graphic images), plus burns (and explosions if the spaceship used a pure-oxygen atmosphere). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-03-27T19:22:13.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4135" PostTypeId="2" ParentId="4080" CreationDate="2014-03-27T20:02:22.523" Score="8" Body="&lt;p&gt;It takes more than a little push to drop a probe to Mercury. A 1 AU x .387 AU heliocentric orbit would be moving 22.25 km/s at aphelion. Earth moves about 30 km/s. So Vinfinity for Trans Mercury Insertion would be 7.78 km/s. From LEO the burn would be about be about 5.5 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At perihelion the ship would be traveling 57.5 km/s wrt sun.  It'd be traveling a hyperbolic path wrt to mercury.  Vinfinity wrt to Mercury is about 9.2 km/s. If Mercury periapsis were 0 km from Mercury's surface, turning angle of hyperbola would be about 10.2 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the Mercury gravity assist would be good for 1.7 km/s at best. Which wouldn't be sufficient to boost aphelion to Mars, much less Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From LEO, Trans Jupiter Insertion is about 6.3 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding a gas giant's gravity capturing the probe: An earth to Jupiter Hohmann would be a hyperbola wrt to Jupiter. Vinfinity wrt to Jupiter would be about 6 km/s. From Jupiter's view point, it'd see the distant probe approaching at 6 km/s. It'd accelerate as it fell into Jupiter's well, then after Jupiter periapsis as it climbs up the well. Eventually it'd slow to 6 km/s wrt to Jupiter. From Jupiter's point of view, the probe would have the same velocity outgoing as it does incoming. It is with regard to the sun that Jupiter's gravity changes the velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter capture might be possible through one of Jupiter's moons changing the probes path, or maybe if the probe's periapsis passes through Jupiter's atmosphere. But Jupiter's gravity alone would not change a hyperbola to an ellipse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a pic showing how a hyperbola's velocity approaches Vinf as it grows more distance from the central body: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xsV7i.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-03-27T20:02:22.523" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="4136" PostTypeId="2" ParentId="3465" CreationDate="2014-03-28T15:35:19.503" Score="1" Body="&lt;p&gt;Your best bet is to start with &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_determination#Gauss.27_method&quot; rel=&quot;nofollow&quot;&gt;Gauss's method of orbit determination&lt;/a&gt;, which allows you to determine a two body orbit from three angular observations.  Unfortunately, I don't have a good online reference handy, but it is discussed in &lt;a href=&quot;http://books.google.com/books/about/Fundamentals_of_Astrodynamics.html?id=UtJK8cetqGkC&quot; rel=&quot;nofollow&quot;&gt;Fundamentals of Astrodynamics&lt;/a&gt; by Bate, Mueller, and White.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="49" LastEditDate="2014-03-28T19:14:10.413" LastActivityDate="2014-03-28T19:14:10.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4137" PostTypeId="1" CreationDate="2014-03-28T16:12:46.407" Score="12" ViewCount="1840" Body="&lt;p&gt;I'm running a numerical simulation that calculates an interplanetary trajectory from Earth to Mars, and I'm trying to get a spacecraft into an orbit about Mars upon intercept, but am having a little trouble. I've been reading Orbital Mechanics for Engineering Students (by H. Curtis), which defines an &quot;aiming radius&quot; which will be able to help me with this orbit problem. The aiming radius is defined as&#xA;$$&#xA;\Delta= r_p\sqrt{1+\frac{2\mu_2}{r_pv_\infty^{2} }},&#xA;$$&#xA;where $v_\infty$ is just the spacecraft's hyperbolic excess speed relative to Mars when it enters Mars' SOI, $r_p$ is the periapse radius of your chosen orbit about Mars, and $\mu_2$ is Mars' gravitational parameter (unfortunately not the Pokemon). As far as I can tell, $\Delta$ essentially defines the lateral offset distance between the spacecraft's velocity vector and Mars' velocity vector, set when the spacecraft enters Mars' SOI, so that the spacecraft will have a periapse radius (or closest approach radius) of $r_p$ when passing Mars. And from there you can apply the required $\Delta v$ to get into a Mars orbit once you reach the $r_p$ radius point. I'm horrible at explaining things, so this image from the above-mentioned book (p. 370) will hopefully clear things up:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/05IyX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, this is all good and well, but in my case, with a chosen $r_p$ of $4.13418 \times 10^6$ meters, I get a lateral offset value for $\Delta = 8.26836 \times 10^6$ meters. If I was only concentrating on what happens in Mars' SOI (and disregarding the spacecraft's long and arduous heliocentric transfer orbit), I'd set the initial position of the spacecraft to be $-8.26836 \times 10^6$ along the &lt;code&gt;x-axis&lt;/code&gt; and a little less than $5.77 \times 10^8$ meters along the &lt;code&gt;y-axis&lt;/code&gt;, with Mars centred at $(0, 0)$ and then start the simulation to see what happens. But since I have to take the heliocentric part of the flight into consideration this makes things a bit trickier as I need to patch the heliocentric conditions to the mars-centric (is that the correct word?) conditions in order to get an accurate trajectory. So my question is this: How do I design the heliocentric transfer in such a way that when the spacecraft enters Mars SOI, it has the required $\Delta$ (or has a value close to it) for me to get the required value for $r_p$ when the spacecraft reaches its closest approach to Mars? My numerical sim already does a mid-course correction halfway through the heliocentric transfer (using a Lambert solver), so I was thinking I could do something there to get the required $\Delta$ at SOI entrance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any help would be great!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I just realised that the spacecraft will only approach Mars with its velocity vector parallel to that of Mars at the entry into Mars' SOI if this is a perfect Hohmann transfer. With my Lambert trajectory I see that angle between their velocity vectors is just over 5 degrees upon entry into Mars' SOI so this method would not work. Is there an analytical solution that exists for velocity vectors that are not parallel?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="1064" LastEditDate="2014-03-28T19:36:41.357" LastActivityDate="2014-03-28T22:13:06.123" Title="Mars Orbit Upon Intercept" Tags="|mars|orbital-mechanics|orbital-maneuver|interplanetary|soi|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4138" PostTypeId="1" AcceptedAnswerId="4145" CreationDate="2014-03-28T18:49:04.637" Score="9" ViewCount="392" Body="&lt;p&gt;Every extra bit of &lt;a href=&quot;https://space.stackexchange.com/questions/1989&quot;&gt;weight cost extra to send to orbit&lt;/a&gt;.  Presumably it also impacts the possible payload.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a generally healthy build, and otherwise physically able and fit person. In astronaut selection in general or for specific missions does a prospective astronaut's weight (&lt;em&gt;mass for the purist&lt;/em&gt;) impact selection?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="47149" LastEditDate="2022-05-27T16:17:40.200" LastActivityDate="2022-05-27T16:17:40.200" Title="Does a persons weight affect their potential selection to be an astronaut?" Tags="|launch|astronauts|mass|economics|astronaut-recruitment|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="4139" PostTypeId="2" ParentId="4137" CreationDate="2014-03-28T20:04:25.937" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I see that angle between their velocity vectors is just over 5 degrees&#xA;  upon entry into Mars' SOI so this method would not work. Is there an&#xA;  analytical solution that exists for velocity vectors that are not&#xA;  parallel?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Law_of_cosines&quot; rel=&quot;noreferrer&quot;&gt;Law of cosines&lt;/a&gt;. I think of it as a more generalized version of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pythagorean_theorem&quot; rel=&quot;noreferrer&quot;&gt;Pythagorean theorem&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$c^2 = a^2 + b^2 - 2ab \cdot \cos\alpha$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see if $\alpha$ is 90° that the 3rd term is 0 and it goes to the Pythagorean theorem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for example, if Mars is moving 24 km/s with respect to the Sun, your ship 27 km/s wrt the Sun, and the 27 km/s vector is 5 degrees from the 24 km/s vector, then the difference between the two is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\sqrt{(27^2 + 24^2 - 2 \cdot 24 \cdot 27 \cdot cos(5°))}\ \approx 3.73$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your $v_{\infty}$ would be about 3.73 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zHzHt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-03-28T22:13:06.123" LastActivityDate="2014-03-28T22:13:06.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4140" PostTypeId="1" AcceptedAnswerId="4143" CreationDate="2014-03-29T12:48:50.893" Score="14" ViewCount="4688" Body="&lt;p&gt;If one spacecraft wants to overtake another (e.g. for a rendezvous), it has to (counterintuitively) brake, go into a lower orbit and use the higher angular velocity of that orbit/shorter orbital period.&lt;br&gt;&#xA;Now I expected that when I add thrust in the prograde direction, my linear speed would increase, even though this wouldn't lead to my overtaking the target because my higher orbit has a lower angular speed.&lt;br&gt;&#xA;How does the prograde thrust not lead to a linear speed increase but to a decrease? I guess all of the energy in the thrust (and more) is converted to potential energy (altitude), but I still don't see how this would work.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edited in light of the first two answers)&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-02T12:19:56.740" LastActivityDate="2019-02-20T00:07:13.720" Title="Orbital mechanics: trying to understand counterintuitive behavior (braking to overtake)" Tags="|orbital-mechanics|" AnswerCount="7" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4141" PostTypeId="2" ParentId="4140" CreationDate="2014-03-29T13:31:33.497" Score="1" Body="&lt;p&gt;As you near the central gravitating body, pull of gravity grows stronger and objects fall faster. For circular orbits speed scales with r&lt;sup&gt;-(1/2)&lt;/sup&gt;. For example, earth is moving at about 30 km/s about the sun. A circular orbit 4 times as distant would move 15 km/s. 9 times as distant, 10 km/s. 16 times as distant, 7.5 km/s, etc. Gravity wells can be fairly well modeled by those spiral funnels you've probably seen at shopping malls or science museums: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sLBR5.jpg&quot; alt=&quot;spiral funnel&quot;&gt;&#xA;&lt;sup&gt;(Michael Hanna of Online Vending gave me permission to use his photo).&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides distance/time, another sort of velocity is angular velocity measured in radians/time. Where 2π radians is a full circuit. Angular velocity is often denoted with the Greek letter ω (omega). For example ω for a second hand is 2π radians/60 seconds. ω for an minute hand is 2π radians per hour. For orbiting bodies, ω slows even more than distance/time. It scales with r&lt;sup&gt;-(3/2)&lt;/sup&gt;. For example a body four times as distance from earth will take 8 years to make a circuit. A body nine times as distant will take 27 years to make a circuit.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="709" LastEditDate="2014-03-31T01:12:49.517" LastActivityDate="2014-03-31T01:12:49.517" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4142" PostTypeId="2" ParentId="4140" CreationDate="2014-03-29T15:00:14.400" Score="1" Body="&lt;p&gt;For perfectly circular orbits, the further out you are, the slower your angular and linear velocity, which is why Mercury is moving the fastest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where it gets counter-intuitive is the overtake scenario. Consider two objects sharing the same circular orbit. For one to overtake the other, it first decreases its orbital velocity. That transforms its orbit into an ellipse where it is at its maximum orbital altitude (apoapsis). As it proceeds along that orbit its linear and angular velocity increases as its orbital altitude decreases. At its lowest point (periapsis), it could then decrease its velocity further to establish a circular orbit at a lower altitude and higher velocity, or simply continue on its elliptical path until the next apoapsis, at which point it would accelerate to restore its original velocity on the original orbit, but further ahead than it would have been if it had not made any changes.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-03-29T15:00:14.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4143" PostTypeId="2" ParentId="4140" CreationDate="2014-03-29T18:39:53.550" Score="14" Body="&lt;p&gt;I'm considering &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbit_phasing&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbit phasing&lt;/a&gt; between two spacecrafts on the same circular orbit, but not in the same phase.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Now I expected that when I add thrust in the prograde direction, my linear speed would increase&#xA;  How does the prograde thrust not lead to a linear speed increase but to a decrease? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;When you do prograde impulse, your speed (just after the burn) will increase. But at the same time you will change your orbit, because the impulse was the first part of Hohmann transfer. Now your orbit is not circular, but more elliptic. The burn was at periapsis (closer to the planet) of new elliptic orbit, and after flying half of new ellipsis, you will be at apoapsis (greatest distance to planet). At apoapsis your speed will be lower than it was on circular orbit, and it is the reason to say that &quot;prograde thrust ..&quot; will decrease speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we are doing classic &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann Transfer&lt;/a&gt;, we will do one more prograde impulse at apoapsis, to move to higher circular orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in phasing we will fly another half or ellipsis and do retrograde burn in periapsis to return to original circular orbit. Our phase will be changed comparing to phase of target. By selecting right ellipsis we can tune phases to do rendezvous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on phase difference, we may different elliptic orbits, sometimes higher, and sometimes lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NGWyA.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NGWyA.png&quot; alt=&quot;orbit phasing&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://ccar.colorado.edu/asen5050/projects/projects_2012/bartkowicz/website/orbital_mechanics_4_files/image004.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;colorado.edu&lt;/a&gt;)&lt;/sub&gt;  &lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="10273" LastEditDate="2019-02-20T00:07:13.720" LastActivityDate="2019-02-20T00:07:13.720" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="4144" PostTypeId="2" ParentId="3851" CreationDate="2014-03-30T00:20:56.730" Score="2" Body="&lt;p&gt;EML1 is moving in a prograde orbit at about .86 km/s. A .7 km/s braking burn will drop it into a prograde orbit with a perigee deep in earth's gravity well (about 300 km altitude).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To go to a retrograde orbit, you would need to completely kill your .86 km/s velocity. But at 0 km/s, you'd fall straight to earth for an impact. To raise perigee to above earth's atmosphere, you'd need another .17 km/s. So the retrograde burn would require a little more than 1 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the burn dropping you into a prograde orbit is preferable as it takes less delta V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is only one time during the moon's orbit when a braking burn would drop a ship to a perigee at the correct longitude. But during the launch window it's possible EML1 could be as much as two weeks away from the place where you want to do the burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can be mitigated by letting your payload orbit more than once. For example, if EML1 reaches the place where you need to do the burn 10 days early, go ahead and do the burn. It will take 5 days to reach perigee. Then another 10 days it will complete another orbit and it's back at the perigee location you want, but now it's the right time as well as the right place. And you can make the orbit more or less than 10 days with small perigee or apogee burns. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another option is to drop from EML2, do a small burn at perilune to get to deep perigee. It takes a little more than 8 days to reach perigee. And it takes less delta v than dropping from EML1 -- about .4 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bVLmo.jpg&quot; alt=&quot;Farquhar diagram&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the perilune burn drops the ship to a deep perigee, it too is in about a 10 day orbit.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-03-30T00:26:15.563" LastActivityDate="2014-03-30T00:26:15.563" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4145" PostTypeId="2" ParentId="4138" CreationDate="2014-03-30T12:05:35.190" Score="4" Body="&lt;p&gt;The Space Shuttle was not specifically payload limited on most missions, such that the difference of even 100 lbs on a crew member would have mattered.  Soyuz is much the same. Perhaps earlier Mercury and Gemini missions were tighter on performance, and it might have been vaguely considered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Height however is a bigger issue. Soyuz is quite cramped, and they needed to make some modifications to allow US astronauts, who tended to be taller than their Russian/Soviet partners, and for the female astronauts who tended to be shorter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, US space suits for EVA have limited ranges in terms of how large and how small they can be sized, which can constrain astronaut selection a bit. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-03-30T12:05:35.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4146" PostTypeId="1" AcceptedAnswerId="4152" CreationDate="2014-03-30T13:15:17.033" Score="11" ViewCount="1905" Body="&lt;p&gt;It seems to me that the April 12, 1981 launch of &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Columbia&quot;&gt;Columbia&lt;/a&gt; (STS-1) was the manned spaceflight with the most untested hardware of any manned space launch. Even the &lt;a href=&quot;http://en.wikipedia.org/wiki/Vostok_%28spacecraft%29&quot;&gt;Vostok&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Mercury_spacecraft&quot;&gt;Mercury&lt;/a&gt; launch systems had unmanned launches before the manned flights. However the Space Shuttle never had an unmanned test flight, and I could find no information about an unmanned SRB launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am well aware of the ground-based testing of the SRBs and the SSMEs. However, were these systems ever flight-tested before the manned flights?&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-03-31T09:48:32.773" Title="Have SRBs or SSMEs ever launched without a space shuttle?" Tags="|crewed-spaceflight|space-shuttle|testing|ssme|srb|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4147" PostTypeId="2" ParentId="4146" CreationDate="2014-03-30T13:27:15.183" Score="9" Body="&lt;p&gt;The SRB's were ground tested, as were the SSME's.  Consider for a moment the scale/scope of a test stand that can hold an SRB full fire test.  (The SRB's put out 2.8 million lbs of thrust.  To compare, even an F-1 engine was only 1.5 Mlbs). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ironically the only real standalone launch of an SRB was the Ares-1X launch, well after the Shuttle was ready to be retired.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-03-30T13:27:15.183" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4148" PostTypeId="2" ParentId="4140" CreationDate="2014-03-30T15:18:35.890" Score="12" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How does the prograde thrust not lead to a linear speed increase but to a decrease?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A prograde thrust does lead to a linear speed increase. Suppose the lagging spacecraft performs a prograde delta-V. The spacecraft that performed the burn will be approaching the other spacecraft immediately after applying the delta-V and for the next half minute or so (assuming low Earth orbit conditions). Orbital mechanics effects will start to make themselves present shortly after applying the delta-V, but these effects will be negligible at first. A About half of a minute after applying that delta-V, the closing speed will have noticeably started to decrease and the active spacecraft will have started to move above the target vehicle's orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These orbital mechanics effects will be in full swing ten minutes or so later. Now the forward progress of the active vehicle will have stopped. The active vehicle will continue to rise above the target vehicle's orbit for another 35 minutes or so until it reaches apogee. The active vehicle will reach perigee 45 minutes or so after that. At this point it will be considerably behind the target vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The opposite happens with retrograde burns. Immediately after the burn, the active vehicle will fall even further behind the target vehicle. It will also fall below the target vehicle and eventually pick up speed to overtake the target vehicle after making a full orbit.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-03-31T11:22:35.937" LastActivityDate="2014-03-31T11:22:35.937" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4151" PostTypeId="2" ParentId="2456" CreationDate="2014-03-31T00:22:52.350" Score="1" Body="&lt;p&gt;I'm not sure the premise of the question -- that all rocket engines provide more thrust in vacuum -- is correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A design feature of &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerospike_engine&quot; rel=&quot;nofollow&quot;&gt;aerospike engines&lt;/a&gt; is that they overcome this very problem, to provide nearly uniform levels of thrust both in and out of the atmosphere.  AFAIK they have never flown for orbital missions, but they have been built, and fired extensively on test stands, and smaller models have flown on tests.&lt;/p&gt;&#xA;" OwnerUserId="1165" LastActivityDate="2014-03-31T00:22:52.350" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4152" PostTypeId="2" ParentId="4146" CreationDate="2014-03-31T09:48:32.773" Score="7" Body="&lt;p&gt;The obvious way to test the SRBs and SSME would be to fly an unmanned Shuttle mission. Any alternative meant developing a one-off rocket just for the tests.&lt;br&gt;&#xA;NASA considered doing unmanned Shuttle flights from the beginning. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;STS-1 pilot John Young, discussing the topic, &lt;a href=&quot;http://www.collectspace.com/ubb/Forum30/HTML/001038.html&quot; rel=&quot;noreferrer&quot;&gt;in his 2006 interview with collectSPACE&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;They wanted to fly the thing unmanned. I went to many, many meetings&#xA;  where they wanted to fly the thing unmanned, but finally the program&#xA;  manager up at Headquarters, John Yardley, he said he wasn't going to&#xA;  come across California with nobody in the spacecraft.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So, we got to fly it manned. It's probably the safe way to do it. We&#xA;  looked at California and there were all kinds of places you can land&#xA;  out there...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/11350-space-shuttle-pilot-bob-crippen-30th-anniversary.html&quot; rel=&quot;noreferrer&quot;&gt;Robert Crippen, his copilot, agrees&lt;/a&gt; in this Space.com interview:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But John and I were both proponents of us being onboard, because we&#xA;  thought the flight had a bigger chance of success, and it would have&#xA;  been very difficult to make it unmanned on the first one.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On the first four flights, the two-man crew had &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_abort_modes&quot; rel=&quot;noreferrer&quot;&gt;ejector seats&lt;/a&gt; to increase survivability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back then, there were rumors that there was only one real obstacle to making an unmanned Shuttle flight: there was no way to extend the landing gear remotely.&lt;br&gt;&#xA;The same thread had another post that ties in with that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In speaking with a number of the 1970s shuttle program guys (Engineering and Ops), there was at least an impression that there was a contingent of astronauts and managers that fought hard to keep an overtly unmanned flight option off the shuttle, despite how simple it would be to build in. This was supposedly done to ensure that pilot astronauts would not be regarded as superfluous later on - if we don't need them to fly it now, why have them fly it later? This would therefore reduce the importance of the pilots relative to the mission specialists. I've seen some documentation in the JSC archives (it may have even been among the &quot;Young-grams&quot; lending some credence to at least the existence of such a faction, if not their reasoning.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the end, NASA did develop the means to do unmanned missions. The &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-3xx&quot; rel=&quot;noreferrer&quot;&gt;Remote Control Orbiter&lt;/a&gt; cabling first flew on STS-121. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-03-31T09:48:32.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4153" PostTypeId="1" AcceptedAnswerId="5147" CreationDate="2014-03-31T10:49:18.993" Score="21" ViewCount="7666" Body="&lt;p&gt;Solid cores, either for solid-fueled of hybrid rocket motors, use various propellant &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid-fuel_rocket#Grain_geometry&quot; rel=&quot;noreferrer&quot;&gt;grain geometries&lt;/a&gt; to achieve thrust curve needed. For example, some of these could look like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/PzDnY.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Common solid propellant core cross-sections of grain geometries, including circular, finocyl and non-circular bores.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are other profiles in use, such as C-slot (wedge cut out at the side of the core), Moon burner (off-center circular bore), and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But all of these are actually 2-dimensional bores, and it seems reasonable to me that such solid-fueled stages could benefit from a 3-dimensional grain geometry. Depending on the fuel and propellant mixture used (and with hybrid rocket stages that use solid fuels), some of the common ones seem adhesive enough to enable additive manufacturing techniques (a.k.a. 3D printing) to simply print a perfect grain geometry needed for the trust/burn profile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, Ammonium Perchlorate (NH&lt;sub&gt;4&lt;/sub&gt;ClO&lt;sub&gt;4&lt;/sub&gt; or AP) or Nitrous Oxide (NO) are powerful oxidizers that are often used together with Polybutadiene ((C&lt;sub&gt;4&lt;/sub&gt;H&lt;sub&gt;6&lt;/sub&gt;)&lt;sub&gt;n&lt;/sub&gt;, or PB) groups in &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydroxyl-terminated_polybutadiene&quot; rel=&quot;noreferrer&quot;&gt;HTPB&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Polybutadiene_acrylonitrile&quot; rel=&quot;noreferrer&quot;&gt;PBAN&lt;/a&gt; mixtures, together with aluminum (Al) powder and curing agents to form fairly homogenous and adhesive solid propellant grain, and with hybrid rockets, the solid cores that seem to be considered the most lately use paraffin wax. All these substances should be fairly straightforward to 3D print with, and could possibly even enable fine-grain control over fuel, oxidizer, binder  / curative and catalyst components mixture.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Just to expand a bit on what I had in mind, additive manufacturing with paraffin wax is perfectly feasible since the material allows for thermal manipulation from its liquid to solid phase without chemically destabilizing the fuel's chemistry and degrading its performance. Here's one example complex part made with paraffin additive printing by &lt;a href=&quot;http://reprap.org/wiki/SLS_wax_printer&quot; rel=&quot;noreferrer&quot;&gt;SLS wax printer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/OKjmg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Solid wax sprocket gear (source: &lt;a href=&quot;http://reprap.org/wiki/SLS_wax_printer&quot; rel=&quot;noreferrer&quot;&gt;RepRap SLS wax printer Wiki&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might not be the most detailed print ever, but it would enable advanced grain geometry and potentially increase performance of hybrid rocket motors once the fuel is being depleted and the geometry of the oxidizer path changes, as is demonstrated to be a problem in e.g. &lt;a href=&quot;http://www.spg-corp.com/docs/JPC47_Paper.pdf&quot; rel=&quot;noreferrer&quot;&gt;High Performance Hybrid Upper Stage Motor&lt;/a&gt; (PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not too sure that thermal manipulation could be used to such extent for non-hybrid solid motor grain where oxidizer is already a component of the mixture, but judging by &lt;a href=&quot;http://nvlpubs.nist.gov/nistpubs/jres/61/jresv61n2p77_A1b.pdf&quot; rel=&quot;noreferrer&quot;&gt;Thermal Degradation of Polyacrylonitrile, Polybutadiene, and Copolymers of Butadiene With Acrylonitrile and Styrene&lt;/a&gt; (PDF), PB seems to be thermally stable without degradation beyond its evaporation point at roughly 407 °C. AP's melting point is at 240 °C and Aluminum powder melting point is at 660.4 °C (&lt;a href=&quot;http://www.braeunig.us/space/propel.htm&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, thermal manipulation required for additive manufacturing could be limited to  temperatures that don't degrade the grain and simply (re)enable their adhesive properties before curing agents are applied and the layer is cooled down to a solid. And I doubt that high precision additive printing at high temperatures would be required to still benefit from a more complex 3D grain geometry than simple 2D and/or 3D geometries by stacking sections one atop the other permit. This might be especially useful for solid fuel grain geometry in hybrid motors, though.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So here's my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Has anyone already considered using state-of-the-art additive manufacturing techniques to 3D-print solid core propellants instead of casting them into molds or boring simple 2-dimensional profiles into them?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If yes&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;what performance benefits could be expected from 3D-printed solid / hybrid cores, &lt;/li&gt;&#xA;&lt;li&gt;how far have such techniques progressed, and &lt;/li&gt;&#xA;&lt;li&gt;who's developing them? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If no&lt;/strong&gt;: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;what challenges need to be solved to enable manufacturing of 3D-printed solid cores&lt;/li&gt;&#xA;&lt;li&gt;which grain geometries could be considered first (honeycomb, bubble, helix, petal, non-homogeneous propellant mixtures,...) that can't be achieved using simple 2D grain geometry techniques (casting, boring,...)?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-02T09:34:43.180" LastActivityDate="2017-10-10T14:44:31.190" Title="Could 3D printing be used to achieve perfect grain geometry of solid and hybrid rocket motors?" Tags="|rockets|technology|fuel|design|solid-fuel|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4154" PostTypeId="1" AcceptedAnswerId="4157" CreationDate="2014-03-31T10:52:35.487" Score="13" ViewCount="10506" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Iron&quot;&gt;Iron&lt;/a&gt; and steel are made primarily of the same substance and they tend to &lt;a href=&quot;https://en.wikipedia.org/wiki/Rust&quot;&gt;rust&lt;/a&gt;.  I believe other materials like &lt;a href=&quot;https://en.wikipedia.org/wiki/Non-ferrous_metal&quot;&gt;non-ferrous metal&lt;/a&gt; are generally preferred for their lighter weight and strength, in most space applications.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably it will be found to be as common in asteroids as it is on earth (&lt;a href=&quot;https://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust&quot;&gt;&lt;em&gt;4th most common&lt;/em&gt;&lt;/a&gt;).  In the not to distant future, we should be harvesting minerals from asteroids and building things in space. So if we build say a frame to support solar panels out of steel, will we need to try and figure out how to apply paint in zero gravity and zero atmosphere, or would rust not be a concern?&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2019-03-04T05:06:56.663" Title="Would unpainted iron or steel rust in space?" Tags="|construction|vacuum|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4155" PostTypeId="2" ParentId="4154" CreationDate="2014-03-31T11:15:17.197" Score="5" Body="&lt;p&gt;It could be a concern, if it's anywhere near some oxidizer, say on the inside and having contact with atmospheric oxygen, propellant oxidizer, from your own propellants leaking forward of your path, freezing solid on the frame and slowly outgassing due to lack of pressure (baking when exposed to the Sun), splashing onto the frame from high oxidizer burn mixtures, or ionospheric atomic oxygen (e.g. &lt;a href=&quot;https://space.stackexchange.com/q/3200/49&quot;&gt;see here&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But unless required oxygen is already present in the environment, or you somehow add it there (unwittingly or otherwise), there wouldn't be any oxidation, thus no rust. And with up to a few proton particles of matter per cubic centimeter in outer space, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements&quot; rel=&quot;nofollow noreferrer&quot;&gt;only ~ 0.05% of that oxygen atoms&lt;/a&gt;, it would take a long, long time for any rust to form. One thing to add is that while your question is explicitly about iron, with some other metals like Aluminum, a few atoms thick oxidized layer can even be intentional and can protect material from further decay.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-03-31T11:15:17.197" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4157" PostTypeId="2" ParentId="4154" CreationDate="2014-03-31T13:43:51.587" Score="19" Body="&lt;p&gt;&quot;Rusting&quot;, or more chemically correct &quot;oxidation&quot;, is a reaction with oxygen in the atmosphere. Iron reacts with oxygen and turns into iron oxide, the reddish-brown substance commonly referred to as &quot;rust&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space there is no atmosphere with any oxygen to react with, so any iron in space would not rust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This, however, assumes that there is really no oxygen in space. When a spacecraft is in a low orbit around Earth, there are still small traces of atmosphere it flies through. These small traces could bond to any iron and cause it to rust, although far, far slower than on ground level. Another source of oxygen are any rocket engines. Most rocket engines run on some liquid fuel and liquid oxygen. A leak of the latter could cause corrosion to any iron which gets exposed to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;By the way:&lt;/strong&gt; The lack of oxidation in vacuum also has another interesting effect which could be either a blessing or a curse for space construction: It allows you to &lt;a href=&quot;http://en.wikipedia.org/wiki/Cold_welding&quot; rel=&quot;noreferrer&quot;&gt;cold-weld&lt;/a&gt;. Pure, non-oxidized metals have an interesting property: When they touch, they stick and form a single piece. That means you could break a piece of metal into two, put the parts together again, and they would fuse without a trace. This effect is hard to reproduce on earth, because the moment you break a piece of metal, the exposed area comes into contact with oxygen and a nano-scale corrosion layer forms immediately which prevents cold-welding. But it works in an artificial pure vacuum environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For space construction, this could be a blessing because it makes it much easier to put large structures together. Just move two girders together and they fuse the moment they touch. No welding, screws or bolts required. But it could also be a curse, because it is easy for accidental welding to occur. Any surfaces which are designed to touch and separate again (like mechanical gears or joints) need to be coated to avoid accidental cold-welding from happening. Exposing such parts to an oxygen-atmosphere for a short duration could be enough, though.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="15906" LastEditDate="2019-03-03T10:29:37.933" LastActivityDate="2019-03-03T10:29:37.933" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="4158" PostTypeId="2" ParentId="4153" CreationDate="2014-03-31T14:26:42.977" Score="5" Body="&lt;p&gt;A quick search does not turn up any projects in this direction. That's not to say nobody has thought about it, of course.&lt;br&gt;&#xA;A few possible problems occur to me: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The process has to be safe. Most of the current 3D printing processes apply heat to fuse the printed material together. Firing a laser at a bunch of propellant may not be an option. If you don't use heat, you'll have to wait for the propellant to solidify.  &lt;/li&gt;&#xA;&lt;li&gt;Printing would take long. A single Shuttle SRB segment weighs 150 tons. When you're casting, you can pretty much dump the entire load in at once. May not be too much of a problem in the space industry, though. &lt;/li&gt;&#xA;&lt;li&gt;You'll have to prevent cracks forming in the propellant due to e.g. being too late to print the next layer. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There have been efforts toward printing concrete. This has similar problems (curing time, size of concrete structures).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are printers that can print wax, these prints are often used for lost-wax casting of e.g. &lt;a href=&quot;http://www.shapeways.com/materials/silver?li=nav&quot; rel=&quot;nofollow&quot;&gt;silver&lt;/a&gt;. I haven't been able to find material specifications for the wax they use, though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-04-01T19:46:00.573" LastActivityDate="2014-04-01T19:46:00.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4159" PostTypeId="2" ParentId="4140" CreationDate="2014-03-31T15:31:50.123" Score="10" Body="&lt;p&gt;There is a faster way to overtake a leading spacecraft than a retrograde burn. Don't do a purely horizontal burn. Do a prograde burn but with a downward component. This will lower perigee. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/Dh3i1.png&quot; alt=&quot;prograde and down&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the red portion of the new orbit, your spacecraft will be deeoer in the gravity well and moving faster. The red arc is also shorter than the circular arc of the original orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise if you're in the lead and you want the ship following you to catch up, you can do a retrograde burn with an up component.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-03-31T16:08:39.667" LastActivityDate="2014-03-31T16:08:39.667" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4160" PostTypeId="1" CreationDate="2014-03-31T15:38:10.563" Score="11" ViewCount="1668" Body="&lt;p&gt;I understand that more power is required to launch a satellite into retrograde orbit, but once it's up there how much does the orbit differ from the more usual West to East? Would you still use $v\approx\sqrt{\frac{GM}{r}}$ to find the speed or are there more things to consider, such as the drag in LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm assuming a simplified circular orbit. &lt;/p&gt;&#xA;" OwnerUserId="3020" LastEditorUserId="2784" LastEditDate="2014-04-01T00:01:55.623" LastActivityDate="2014-04-01T04:08:40.580" Title="Would a satellite in retrograde orbit have a different speed than it would in prograde?" Tags="|orbital-mechanics|artificial-satellite|orbit|low-earth-orbit|retrograde-orbit|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4161" PostTypeId="1" CreationDate="2014-03-31T15:59:58.163" Score="7" ViewCount="771" Body="&lt;p&gt;&quot;Inspiration Mars&quot; has the idea to use a free return trajectory for a crew of two rounding Mars. While attractive in some ways, this free return trajectory has the disadvantage of ending up with a very high re-entry speed as it comes back to Earth's atmosphere. Heavy heat shields and/or lots of fuel for breaking rockets counterbalances some of the advantages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is often (always?) suggested to use adaptations of capsules like Orion (NASA) or Dragon (SpaceX) as re-entry vehicles. But they are greatly overdimensioned for a crew of two using it for a few hours as they approach Earth after having discarded the rest of their spaceship. I'm thinking of a claustrophobic cage like Gemini. They wouldn't need to operate anything. They would need none or minimal life support systems except their space suites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't these be some great advantages of a tiny REV?:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Smaller surface area of the capsule needs a smaller heat shield. But maybe the physics involved is not that simple?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Lower mass of capsule and heat shield makes the braking rockets and also the parachutes more efficient in slowing down the speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Tiny crewed re-entry vehicles were used about 50 years ago. Making them smaller than those used and planned today should make the challenge of high speed re-entry easier, even if a capsule design has to be made from scratch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And finally, would a fast re-entry vehicle have any more generally useful application?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;I've looked through some of the student contributions in the Mars Society / Inspiration Mars contest, so I feel like a rocket scientist now! :p&#xA;Link (posted on marssociety.org Mar 28, 2014, 3:48 PM by M Stoltz):&#xA;&lt;a href=&quot;http://www.marssociety.org/home/press/announcements/marssocietypostsinspirationmarsstudentdesignreportsonline&quot; rel=&quot;noreferrer&quot;&gt;http://www.marssociety.org/home/press/announcements/marssocietypostsinspirationmarsstudentdesignreportsonline&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1255" LastEditDate="2014-03-31T16:36:27.290" LastActivityDate="2014-04-10T07:31:38.653" Title="Would a smaller re-entry vehicle better handle very high re-entry speeds?" Tags="|mars|crewed-spaceflight|reentry|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4162" PostTypeId="2" ParentId="4160" CreationDate="2014-03-31T16:35:56.997" Score="16" Body="&lt;p&gt;To first order, no.  Considering only Newtonian gravitational forces and a spherical gravitational field, the satellite has no way to tell if the body beneath it is spinning or not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exceptions would be a) if you are close enough to a body with an atmosphere to measure its drag.  Since the atmosphere rotates with the body, the drag will be higher retrograde than prograde.  b) If the gravitational field is not spherical (and they never are), bumps in the gravity field will go by faster in the retrograde than the prograde orbits.  And c) if you can measure accurately enough to see &lt;a href=&quot;https://en.wikipedia.org/wiki/Frame-dragging&quot; rel=&quot;noreferrer&quot;&gt;General Relativity frame dragging&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-04-01T04:08:40.580" LastActivityDate="2014-04-01T04:08:40.580" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4163" PostTypeId="2" ParentId="4161" CreationDate="2014-03-31T16:44:27.827" Score="7" Body="&lt;p&gt;Regardless of the size, unless someone is being foolish, the vehicle will be designed to handle the reentry speeds.  So a small one won't handle them &quot;better&quot; than a large one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However you are correct that it is easier, and will take much less mass for a small one to handle the speeds and other landing responsibilities than a large one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it has already been accepted that there will be a separate habitation space from the entry vehicle, and for Inspiration Mars it has, then it makes sense to make the entry vehicle as small as possible for the crew to be able to fit inside and to support the crew independently for an hour, if that.  That is basically the philosophy of the Soyuz spacecraft, which unlike Apollo had a separate habitation space.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-03-31T16:44:27.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4164" PostTypeId="2" ParentId="4161" CreationDate="2014-03-31T17:25:32.207" Score="2" Body="&lt;p&gt;In regards to point two, if a vehicle is smaller it means it will be a lot lighter. Lighter vehicles mean smaller re-entry parachutes, which also means that you have a reduction in carry weight when taking off. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bigger the vehicle the bigger the canopy will have to be. This also means the bigger the deployment drogue will have to be as well as the thickness of the used suspension materials which will connect the vehicle to the chute. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lighter the weight of the capsule, the smaller the parachute will have to be. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1385/use-of-steerable-parachutes-for-re-entry/1414#1414&quot;&gt;See my other answer here for more information on parachutes&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="521" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-03-31T17:46:57.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4165" PostTypeId="2" ParentId="4161" CreationDate="2014-03-31T22:38:38.247" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Ballistic_coefficient&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ballistic Coefficient&lt;/a&gt; is an important quantity for looking at re-entry. Ballistic coefficient scales with mass/cross sectional area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Increase a shape's dimensions by a factor of r and it's cross sectional surface area will increase by r^2 and its volume by r^3. So ballistic coefficient becomes less as size increases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a pic of a range of objects all having the same ballistic co-efficient:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/x7w7U.png&quot; alt=&quot;Ballistic Coefficient&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, it's easier to have a more favorable ballistic coefficient for small payloads.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-03-31T22:38:38.247" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4168" PostTypeId="1" AcceptedAnswerId="4172" CreationDate="2014-04-01T15:25:10.873" Score="5" ViewCount="657" Body="&lt;p&gt;The Russian &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Piloted_Orbital_Station&quot; rel=&quot;nofollow&quot;&gt;MARPOST&lt;/a&gt; plan in its simplest form, to me, sounds like a great idea: Establish a manned orbital presence around Mars before deploying humans to the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia article says that the plan was to have the design ready by 2012 and the ship ready by 2021, but it has no current information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this mission still in the works or has it been canned? If it is still going, is it on schedule?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why hasn't this sort of plan been considered by others? If it has, why hasn't it caught on?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-04-01T16:57:59.403" LastActivityDate="2014-04-01T21:26:44.210" Title="Is the MARPOST (Mars Piloted Orbital Station) mission still in the works?" Tags="|mars|russia|mars-orbiters|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4169" PostTypeId="1" AcceptedAnswerId="4175" CreationDate="2014-04-01T18:32:40.423" Score="37" ViewCount="35878" Body="&lt;p&gt;Metallic surfaces touching in the vacuum can stick to each other and fuse. This is known as the &amp;quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Cold_welding&quot; rel=&quot;nofollow noreferrer&quot;&gt;cold welding&lt;/a&gt;&amp;quot; and it seems to be a possible issue in space if it happens where it should not. It was discussed for example in the 2009 paper &lt;a href=&quot;http://esmat.esa.int/Publications/Published_papers/STM-279.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Assessment of Cold Welding&#xA;between Separable ContactSurfaces due to Impact and&#xA;Fretting under Vacuum STM-279&lt;/a&gt; from ESA which summarizes the tests of the cold welding phenomena under various conditions and concludes that it can be a problem in space.&lt;/p&gt;&#xA;&lt;p&gt;I would like to know whether there are some specific examples how this is prevented in spacecraft design. For example the &lt;a href=&quot;http://www.aerospace-technology.com/projects/space_station/space_station7.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Solar Array Rotary Joints&lt;/a&gt; (SARJ) have fairly large rotating metallic areas exposed to the vacuum. Are there any countermeasures implemented in the SARJ against the cold welding?&lt;/p&gt;&#xA;&lt;p&gt;There are also other moving devices on the ISS: &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/structure/elements/mss.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Canadarm2&lt;/a&gt;, &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/iss/dextre/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dextre&lt;/a&gt;, &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/iss/mobile-base/default.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mobile Base System&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/External_Active_Thermal_Control_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heat Rejection System Radiators&lt;/a&gt;, etc. Was it necessary to take the cold welding into account when some (or all) of these were manufactured?&lt;/p&gt;&#xA;&lt;p&gt;Other spacecraft also have moving parts: the Space Shuttle had the payload bay doors, space probes carry solar arrays and deployable high-gain antennas, etc.&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="12102" LastEditDate="2021-07-26T23:05:43.237" LastActivityDate="2021-07-26T23:05:43.237" Title="How is unwanted cold welding prevented in space?" Tags="|iss|materials|manufacturing|structural-design|cold-welding|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="4170" PostTypeId="5" CreationDate="2014-04-01T20:13:43.973" Score="0" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Cold_welding&quot; rel=&quot;nofollow&quot;&gt;Cold welding&lt;/a&gt; in general is a process of joining 2 parts without heating them. It is being used in industry, but it also has a significance in space exploration where cold welding of metallic surfaces can happen due to the absence of atmosphere that would exidize the surfaces and thus the atoms of the metal come in direct contact, allowing the merge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cold welding in space applications was explored by &lt;a href=&quot;http://esmat.esa.int/Publications/Published_papers/STM-279.pdf&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; paper from ESA.&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="921" LastEditDate="2014-04-01T20:15:32.547" LastActivityDate="2014-04-01T20:15:32.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4171" PostTypeId="4" CreationDate="2014-04-01T20:13:43.973" Score="0" Body="Questions about the phenomenon of cold welding that happens with metals in the vacuum of space." OwnerUserId="921" LastEditorUserId="921" LastEditDate="2014-04-01T20:15:38.783" LastActivityDate="2014-04-01T20:15:38.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4172" PostTypeId="2" ParentId="4168" CreationDate="2014-04-01T21:20:35.277" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Wikipedia article says&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;but the wikipedia itself is not &quot;reliable source&quot; of information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will try to list several sources about MARPOST (russian МАРПОСТ) here. 4 sources from wiki article:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;RIA 03/2005: &lt;a href=&quot;http://en.ria.ru/onlinenews/20050330/39700840.html&quot; rel=&quot;nofollow&quot;&gt;RUSSIA SUGGESTS MANNED MARTIAN-MISSION PLAN&lt;/a&gt;: &quot;&lt;em&gt;It will take 10 - 12 years to implement the MARPOST project with annual expenses totaling nearly USD 1.5 billion&lt;/em&gt;&quot;; no exact dates or the prjoect&lt;/li&gt;&#xA;&lt;li&gt;Huff.Post 10/2009 &lt;a href=&quot;http://www.huffingtonpost.com/2009/10/29/russia-hopes-to-fly-human_n_338297.html&quot; rel=&quot;nofollow&quot;&gt;Russia Hopes To Fly Humans To Mars With Nuclear Spaceship&lt;/a&gt;: &quot;&lt;em&gt;Perminov said the preliminary design could be ready by 2012, and then it would take nine more years and cost 17 billion rubles (about $600 million, or euro400 million) to build the ship.&lt;/em&gt;&quot; (Article doesn't name the project &quot;MARPOST&quot;, only saying about &quot;...the nuclear spaceship should be used for human flights to Mars...&quot;)&lt;/li&gt;&#xA;&lt;li&gt;(ru) &quot;Независимая газета&quot; (&lt;a href=&quot;http://en.wikipedia.org/wiki/Nezavisimaya_Gazeta&quot; rel=&quot;nofollow&quot;&gt;lit &quot;Independent Newspaper&quot;&lt;/a&gt;), 01/2005 &lt;a href=&quot;http://www.ng.ru/science/2005-01-12/15_heart.html&quot; rel=&quot;nofollow&quot;&gt;«Сердце» марсианского корабля&lt;/a&gt; (Heart of the martian spacecraft) - discussing &lt;a href=&quot;http://en.wikipedia.org/wiki/Electrically_powered_spacecraft_propulsion&quot; rel=&quot;nofollow&quot;&gt;Electric&lt;/a&gt; (powered by Solar panels) and nuclear engines, again the project doesn't named MARPOST.&lt;/li&gt;&#xA;&lt;li&gt;(ru book from 07/2006) &lt;a href=&quot;http://epizodsspace.no-ip.org/bibl/pil-ex-na-mars/obl.html&quot; rel=&quot;nofollow&quot;&gt;Пилотируемая экспедиция на Марс/ Под ред. А.С. Коротеева. — М.: Российская академия космонавтики им. К.Э. Циолковского, 2006, 320 с, илл., 1000 экземпляров&lt;/a&gt; /somebody should update link on Wikipedia/ - &quot;Излагается современный Российский проект пилотируемой экспедиции на Марс — «МЭК».&quot; (The modern Russian project of manned expedition to Mars - &quot;MEK&quot; is presented)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, next proposed manned martian mission is &quot;MEK&quot; (&quot;Межпланетный Экспедиционный Комплекс&quot; = &quot;Mezhplanetniy Ekspeditsioniy Komplex&quot;, Interplanetary Expeditionary Complex). There were no news about MARPOST in 2010-2013 in russian segment of internet (The Runet), but there are several news about &quot;Межпланетный Экспедиционный Комплекс&quot;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;RIA, 08/2010 &lt;a href=&quot;http://ria.ru/science/20100802/261018841.html&quot; rel=&quot;nofollow&quot;&gt;На околоземной орбите построят межпланетный экспедиционный комплекс&lt;/a&gt; (Interplanetary Expeditionary Complex will be built at near-earth orbit): &quot;Разрабатываемая РКК &quot;Энергия&quot; концепция развития космической деятельности на Марсе в период до 2040 года предполагает создание пилотируемого межпланетного экспедиционного комплекса (МЭК) непосредственно на околоземной орбите&quot; (The concept of development space activity on Mars /and Moon/ in period up to 2040, which is developing in &lt;a href=&quot;http://en.wikipedia.org/wiki/S.P._Korolev_Rocket_and_Space_Corporation_Energia&quot; rel=&quot;nofollow&quot;&gt;RKK Energiya&lt;/a&gt;, suggests building manned Interplanetary Expeditionary Complex on the orbit of Earth). The newspost cites next article:&lt;/li&gt;&#xA;&lt;li&gt;07/2010, &lt;a href=&quot;http://www.energia.ru/ru/news/news-2010/public_07.doc&quot; rel=&quot;nofollow&quot;&gt;&quot;Космическая миссия поколений XXI века&quot;&lt;/a&gt; (V.A. Lopota. Space Mission Of 21st Century Generations) from magazine &quot;Полёт&quot; pages 3-12. Launch and flight testing of nuclear reactor for various missions, including IEC, was planned to start in 2020.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Martian mission concept on the website of RKK Energiya (&lt;a href=&quot;http://www.energia.ru/english/energia/mars/concept.html&quot; rel=&quot;nofollow&quot;&gt;http://www.energia.ru/english/energia/mars/concept.html&lt;/a&gt; - FEATURES OF RSC ENERGIA'S MARTIAN MISSION CONCEPT) lists no any dates.&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2014-04-01T21:26:44.210" LastActivityDate="2014-04-01T21:26:44.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4173" PostTypeId="1" AcceptedAnswerId="4174" CreationDate="2014-04-01T21:27:51.963" Score="13" ViewCount="1171" Body="&lt;p&gt;What would be the positive and negative affects of using other planets and/or space itself for storing trash - instead of wasting place for that on Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, what are the possible obstacles of doing that? I have a guess that for example, shooting nuclear waste out would prevent several environmental damage that nuclear radiation can cause.&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="2752" LastEditDate="2014-04-01T23:14:48.400" LastActivityDate="2017-09-25T18:06:26.187" Title="Sending trash into space" Tags="|contamination|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4174" PostTypeId="2" ParentId="4173" CreationDate="2014-04-02T00:05:18.883" Score="26" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Also, what are the possible obstacles of doing that?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At a bare minimum, there are at least four obstacles:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;We can't do it.&lt;/li&gt;&#xA;&lt;li&gt;It would cost too much even if we could do it technically.&lt;/li&gt;&#xA;&lt;li&gt;It would be too risky even if we could do it technically and financially.&lt;/li&gt;&#xA;&lt;li&gt;We just don't want to do it.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;We can't do it.&lt;/strong&gt;&lt;br&gt;&#xA;The nuclear industry generates 2000 to 2300 metric tons of waste in the form of spend fuel per year (&lt;a href=&quot;http://www.nei.org/Knowledge-Center/Nuclear-Statistics/On-Site-Storage-of-Nuclear-Waste&quot;&gt;Source: Nuclear Energy Institute&lt;/a&gt;). We certainly don't want to put that waste into low Earth orbit. At a minimum, we need to trash the Moon with that waste. (Anything else will cost even more.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll take the Falcon Heavy as a baseline. It will be able to put 13,200 pounds of payload into a translunar injection orbit. At a minimum, that means 384 Falcon Heavy launches per year. Note very well: That bare minimum means stacking the waste, unprotected, on top of the Falcon Heavy launch vehicle. That is unrealistic for a number of reasons. Being completely unrealistic and assuming that there's only one pound of encapsulating material per pound of waste, that means over two launches per day, every day of the year, just to keep up with the current production rate of nuclear waste.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make matters worse, that 2000-2300 metric tons per year is the tip of the iceberg. Per that same source, we have accumulated almost 70,000 metric tons of used fuel. There's also contaminated water, contaminated housing equipment, contaminated control rods; etc. There's a whole lot of stockpiled nuclear waste. There's no way to get rid of it in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;It would cost too much if we could do it.&lt;/strong&gt;&lt;br&gt;&#xA;Using my ridiculously optimistic factor of one pound of encapsulating material per pound of waste, getting rid of that 70,000 metric tons of used fuel would mean a cost of almost 2 trillion dollars. That number is ridiculously optimistic. Plug in any realistic number and the resulting cost will make the war on terror look like small potatoes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;It would be too risky even if we could do it technically and financially.&lt;/strong&gt;&lt;br&gt;&#xA;My factor of factor of one pound of encapsulating material per pound of waste is ridiculously optimistic. Look what NASA has to do for the radioisotope thermoelectric generator it uses for deep space probes. Those RTGs are built like tanks, to the n&lt;sup&gt;th&lt;/sup&gt; degree. Rather than my one to one factor, a more realistic factor would be tens to one, or more. That would make the war on terror look like seed potatoes rather than small potatoes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the bright side (pun intended), if there was a mishap that caused one of those multiple launches per day to crash on the banks in Bermuda, it would give a brand new meaning to money burning a hole in ones wallet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;We just don't want to do it.&lt;/strong&gt;&lt;br&gt;&#xA;The ability to have multiple launches per day is every space fanatic's dream. Wasting that fantastic capability on launching waste into space? That's every space fanatic's nightmare.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-04-02T00:05:18.883" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4175" PostTypeId="2" ParentId="4169" CreationDate="2014-04-02T12:34:32.647" Score="33" Body="&lt;p&gt;I'll mostly focus on prevention of accidental cold-welding in vacuum of space, but for reference, one of the possible causes for the Galileo high-gain antenna deployment anomaly that was considered &lt;sup&gt;&lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/32404/1/94-0141.pdf&quot; rel=&quot;noreferrer&quot;&gt;(1)&lt;/a&gt;&lt;/sup&gt; was also &lt;em&gt;&amp;quot;Retention of the ribs at the mid-point restraint due to friction, cold welding, or adhesion&amp;quot;.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Preventing accidental cold-welding:&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Materials selection&lt;/strong&gt;: Contact surfaces could either be made of different materials that do not cold-weld, and/or of materials with low contact adhesion (dissimilar metals). Aerospace &amp;amp; Advanced Composites (AAC) publishes &lt;a href=&quot;http://www.aac-research.at/products/products_AAC_SpaceTesthouse_ColdWelddatabase_en.html&quot; rel=&quot;noreferrer&quot;&gt;Cold Weld Database&lt;/a&gt; listing materials combinations and their adhesion in four categories also for fretting and impact contacts.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Coated surfaces&lt;/strong&gt;: Selected materials can be coated (painted, anodised, oxidized, chemical film coated,...) to reduce contact adhesion. Coating effectiveness can be reduced through fretting and impact damage.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Reduce actuator dependency&lt;/strong&gt;: By reducing number of moving parts, surfaces prone to fretting damage, triboelectric wear and exposure of unprotected materials to cold-welding also reduce in size.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Secure passive structures&lt;/strong&gt;: Secure latches, locks, fasteners, and so on to prevent wear and abrasive particle contaminants finding way between other moving parts.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Cleanliness&lt;/strong&gt;: Reduce particle and molecular contamination of the assembly to reduce abrasion, wear and corrosion of surface coated materials. Environmental sealing, flushing or purging with inert gases or liquids can be used before deployment, and in some cases also during use.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Lubrication&lt;/strong&gt;: Use of specific material surface properties or an applied material between two contacting or moving surfaces in order to reduce friction, wear or adhesion.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Reduce contact surface&lt;/strong&gt;: Either by increasing margin between them, reducing surface size altogether, and/or using non-polished finish low contact area surfaces where possible.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Reduce environmental exposure&lt;/strong&gt;: Thermally insulate parts exposed to thermal cycling, shield against micrometeorite, radiation damage, stray light induced emissions, electrostatic charge and chemical interaction (e.g. atomic oxygen) that can degrade or damage coating and/or introduce contaminants and expose bare metals prone to cold-welding.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;And so on and these are just some of the mitigation techniques that I thought were specific to preventing accidental cold-welding, the list is by no means complete. More specifically, ESA adheres to ECSS Space engineering Mechanisms (ECSS-E-ST-33-01C) that covers this in the section &lt;em&gt;4.7.5.4.5 Separable contact surfaces&lt;/em&gt; &lt;sup&gt;&lt;a href=&quot;http://www.everyspec.com/ESA/download.php?spec=ECSS-E-ST-33-01C.048198.pdf&quot; rel=&quot;noreferrer&quot;&gt;(2)&lt;/a&gt;&lt;/sup&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;For more general overview of cold-welding in space applications, I suggest reading ESA's &lt;em&gt;Assessment of Cold Welding between Separable Contact Surfaces due to Impact and Fretting under Vacuum&lt;/em&gt; &lt;sup&gt;&lt;a href=&quot;http://esmat.esa.int/Publications/Published_papers/STM-279.pdf&quot; rel=&quot;noreferrer&quot;&gt;(3)&lt;/a&gt;&lt;/sup&gt;. From its abstract:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A common failure mode seen during the testing and operation of&#xA;spacecraft  is termed ‘cold welding’. European laboratories refer to&#xA;this as ‘adhesion’,  ‘sticking’ or ‘stiction’. This publication is&#xA;intended to provide the space  community with the most recent&#xA;understanding of the phenomenon of  ‘cold welding’ in relation to&#xA;spacecraft mechanisms with separable contact  surfaces. It presents&#xA;some basic theory and describes a test method and the  required&#xA;equipment. Cold welding between two contacting surfaces can occur&#xA;under conditions of impact or fretting. These surfaces may be bare&#xA;metals,  or inorganically or organically coated metals and their&#xA;alloys. Standard  procedures for quantifying the propensity of&#xA;material surface pairs to cold  weld to each other are proposed. Of&#xA;particular interest will be the contact data  of different materials,&#xA;which are presented in numerical form and as tables  summarising&#xA;contacts between materials that can be either recommended or&#xA;considered unsuitable for use under vacuum. The data have been&#xA;compiled in  a database that can be accessed online.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/32404/1/94-0141.pdf&quot; rel=&quot;noreferrer&quot;&gt;Galileo High Gain Antenna Deployment Anomaly&lt;/a&gt;, Michael R. Johnson, JPL (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.everyspec.com/ESA/download.php?spec=ECSS-E-ST-33-01C.048198.pdf&quot; rel=&quot;noreferrer&quot;&gt;Space engineering Mechanisms&lt;/a&gt;, ECSS Secretariat, ECSS-E-ST-33-01C, 6 March 2009 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://esmat.esa.int/Publications/Published_papers/STM-279.pdf&quot; rel=&quot;noreferrer&quot;&gt;Assessment of Cold Welding between Separable Contact Surfaces due to Impact and Fretting under Vacuum&lt;/a&gt;, B. D. Dunn, ESA STM-279, November 2009 (PDF)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-04-02T12:34:32.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4176" PostTypeId="1" CreationDate="2014-04-02T14:19:56.107" Score="9" ViewCount="1415" Body="&lt;p&gt;Being a fan of &lt;a href=&quot;http://www.spaceflightnow.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spaceflightnow.com&lt;/a&gt; and it's launch schedule page, I thought it would be great if it were possible for &quot;launch enthusiast&quot; to track live launch trajectories using Google Earth, or a similar technology, in addition to watching the online stream. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know if such a capability exists?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="6537" LastEditDate="2018-01-05T21:01:57.790" LastActivityDate="2018-01-05T21:01:57.790" Title="Are any tools available for realtime visual tracking of live launches?" Tags="|launch|rockets|software|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4177" PostTypeId="1" AcceptedAnswerId="4182" CreationDate="2014-04-02T14:51:39.930" Score="5" ViewCount="1594" Body="&lt;p&gt;this is my first question here; and a lot of what I've learnt so far has been pretty much self taught so I thought I best hear from professionals. :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First let me set the scene as to what I am trying to achieve...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am trying to calculate the fuel cost for a single stage to orbit spaceplane travelling from London Heathrow to Sydney Australia as part of my dissertation. For the purpose of avoiding supersonic overflight of populated areas (i.e. Europe), it is necessary to depart to the south-west in the climb to orbit. Since I am already facing that direction once I reach 7.84km/s at 100km above sea level, I thought I may as well just orbit in that direction (i.e between retrograde and polar) and follow a great circle that would take me over Sydney.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I've calculated so far: is the change in net thrust, acceleration, mass, velocity, ect throughout the climb. What I've ended up with is a mass for my propellant and oxidiser used throughout the ascent under rocket thrust. Things like drag have been accounted for. I have assumed the atmosphere is under ISA conditions, with still and dry air. And I'm assuming a simplified circular orbit with a spherical earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the point...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I've NOT done, is take into account the direction I'm launching in. My calculations currently assume there's no spin at all. But I've read that a retrograde orbit is more costly to launch to than a prograde one. Is it a case of nothing more than increased drag? If so, are there altitudes at which this comes into effect? One would think that close to the surface (i.e. on a conventional airliner), the direction of the earths rotation is entirely negligible, so when is it not for spaceflight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I am assuming (just off the top of my head), that as acceleration due to gravity decreases in the ascent, the actual velocity of the aircraft in space and the velocity relative to the earths surface begins to differ, with the latter being the higher (if retrograde) and therefore with greater drag, if this is true than I suppose what I am asking is how to work it all out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that was a lot to read but I really appreciate your help!&#xA;(Also, if my orbit passed over Sydney, is my inclination just the Latitude of Sydney? Thanks again!) &lt;/p&gt;&#xA;" OwnerUserId="3025" LastEditorUserId="1255" LastEditDate="2014-04-02T19:20:22.490" LastActivityDate="2014-04-02T22:16:25.767" Title="How exactly does the inclination and direction (specifically retrograde) of orbit affect the velocity I need to attain orbit?" Tags="|orbital-mechanics|low-earth-orbit|delta-v|ssto|retrograde-orbit|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4178" PostTypeId="2" ParentId="4177" CreationDate="2014-04-02T15:01:54.657" Score="2" Body="&lt;p&gt;The speed you need to obtain is identical (Or close enough for most purposes) for all orbits. The velocity depends on the inclination of the orbit, basically your velocity vector needs to line up with the inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you are talking about, however, doesn't have to do with the orbital velocity at all, but rather with the amount of fuel required. The short version of this is, the Earth's rotation is a start velocity for the orbital velocity. Furthermore, the position has some influence, as without special effort, you can't have a lower inclination than your latitude (Actually, there is a few degrees difference by nature, but it isn't a lot...)&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-04-02T15:01:54.657" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4180" PostTypeId="2" ParentId="4173" CreationDate="2014-04-02T20:46:59.563" Score="11" Body="&lt;p&gt;One more reason on top of David Hammen's &lt;a href=&quot;https://space.stackexchange.com/a/4174/574&quot;&gt;excellent answer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today's trash may well be tomorrow's raw materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;100 years from now, today's nuclear waste might be useful for &lt;em&gt;something&lt;/em&gt;. We just don't know what, and removing it from the planet could turn out to be a very bad idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another point: Most of the mass of a launched rocket never leaves the atmosphere; it's fuel burned on the way up, producing that huge plume of smoke. (I don't have the numbers on this; does anyone else?) At least with current technology, launching waste into space doesn't even begin to make sense unless the benefit of getting rid of the waste outweighs the added pollution from the launch itself. A laser or electromagnetic launch system might change the tradeoffs, but I'm still skeptical that it would pay off in the foreseeable future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, all of this could change in the more distant future. If launch costs drop low enough it just might make sense to, for example, put hazardous waste on the Moon &lt;em&gt;where we can get it back if it turns out to be valuable&lt;/em&gt;. (The latter would be a good reason &lt;em&gt;not&lt;/em&gt; to drop it into the Sun.)&lt;/p&gt;&#xA;" OwnerUserId="574" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-04-02T20:46:59.563" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4181" PostTypeId="2" ParentId="4154" CreationDate="2014-04-02T21:13:26.560" Score="6" Body="&lt;p&gt;Iron/steel in low earth orbit most likely would eventually rust, due to the presence of highly reactive atomic oxygen.  I don't have data on steel specifically (most steel launched is stainless), but atomic oxygen does cause erosion of hydrocarbon polymers, silicone polymers, aluminum, and silver.  I would direct your attention to &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040087142.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt;, which shows micrographs of surfaces exposed to AO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Summary&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atomic oxygen is formed in the low Earth orbital environment (LEO) by photo dissociation of diatomic oxygen by short wavelength (&amp;lt; 243 nm) solar radiation which has sufficient energy to break the 5.12 eV O2 diatomic bond in an environment where the mean free path is sufficiently long (~ 108 meters) that the probability of reassociation or the formation of ozone (O3) is small. As a consequence, between the altitudes of 180 and 650 km, atomic oxygen is the most abundant species. Spacecraft impact the atomic oxygen resident in LEO with sufficient energy to break hydrocarbon polymer bonds, causing oxidation and thinning of the polymers due to loss of volatile oxidation products. Mitigation techniques, such as the development of materials with improved durability to atomic oxygen attack, as well as atomic oxygen protective coatings, have been employed with varying degrees of success to improve durability of polymers in the LEO environment. Atomic oxygen can also oxidize silicones and silicone contamination to produce non-volatile silica deposits. Such contaminants are present on most LEO missions and can be a threat to performance of optical surfaces. The LEO atomic oxygen environment, its interactions with materials, results of space testing, computational modeling, mitigation techniques, and ground laboratory simulation procedures and issues are presented. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Illustration&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see it very clearly on this picture as discussed &lt;a href=&quot;https://space.stackexchange.com/questions/3200/what-is-the-cause-of-discoloration-along-the-edges-of-the-quest-joint-airlock-th&quot;&gt;here&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0lcPB.jpg&quot; alt=&quot;oxydized edges picture&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Rick Mastracchio as EV1 (red stripes) and Mike Hopkins as EV2 (no stripes) leaving the airlock during US EVA-24 (Credit: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="6642" LastEditDate="2019-03-04T05:06:56.663" LastActivityDate="2019-03-04T05:06:56.663" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="4182" PostTypeId="2" ParentId="4177" CreationDate="2014-04-02T21:51:35.007" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Also, if my orbit passed over Sydney, is my inclination just the Latitude of Sydney?&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, the inclination would be the inclination of the great circle passing through Sidney and London. A low-tech method of finding this great circle is to wrap a band of cardboard around a globe with one edge on the equator. Tape the band closed and you have a band of earth's circumference. Then tilt this band so the great circle edge passes through both Sidney and London.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The northern most or southern most latitude of this great circle is the inclination. When I do this, London is pretty close to the northern most part of the orbit. It's a fair approximation to call this orbit's inclination 52º. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;But I've read that a retrograde orbit is more costly to launch to than a prograde one. Is it a case of nothing more than increased drag?&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No. No matter where you are, you'd need to achieve an ~7.8 km/s wrt earth's center. If you're at the equator, you're already moving close to .5 km/s eastward. So you would only need another 7.3 km/s eastward velocity to achieve orbit (this is assuming you're already above earth's atmosphere). To achieve a 7.8 km/s westward velocity, you'd need to kill your .5 km/s velocity so the total westward burn would be 8.3 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speed of a point on earth's surface is cos(latitude) * 2π * earth's radius / (24 hours). At 52º latitude, this comes to about .3 km/s. At London the earth's surface is moving .3 km/s eastward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To enter a southwestward orbit that passes over Sidney You will need to achieve a ~7.8 km/s westward velocity vector from London. Since you're already moving .3 km/s eastward, your westward burn would have to be 8.1 km/s.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-04-02T22:16:25.767" LastActivityDate="2014-04-02T22:16:25.767" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4183" PostTypeId="1" AcceptedAnswerId="4184" CreationDate="2014-04-03T18:15:28.990" Score="12" ViewCount="1489" Body="&lt;p&gt;A couple of recent questions; &lt;a href=&quot;https://space.stackexchange.com/questions/4169&quot;&gt;How is the unwanted cold welding prevented in space?&lt;/a&gt; &amp;amp; &lt;a href=&quot;https://space.stackexchange.com/questions/4154&quot;&gt;Would unpainted iron or steel rust in space?&lt;/a&gt; indicate that some pure metals like iron that have been forged in the vacuum of space (beyond LEO), will &lt;a href=&quot;https://en.wikipedia.org/wiki/Cold_welding&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;cold weld&quot;&lt;/a&gt; together on contact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The couple of existing questions and answer we have &lt;a href=&quot;https://space.stackexchange.com/questions/182&quot;&gt;here&lt;/a&gt; &amp;amp; &lt;a href=&quot;https://space.stackexchange.com/questions/4153&quot;&gt;here&lt;/a&gt; on 3D printing do not seem to consider cold welding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being able to print metals in 3D without welding or casting could have a huge impact to humanity in general. I tried to imagine how it might work... A container of powered iron would immediately cold weld its self into a lump. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to print 3D with cold weldable metals? If so are there any projects working on the process? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-04-03T18:46:36.830" Title="Can you print 3D with iron in space?" Tags="|technology|materials|cold-welding|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4184" PostTypeId="2" ParentId="4183" CreationDate="2014-04-03T18:46:36.830" Score="10" Body="&lt;p&gt;I think that first thing we need to address is that cold-welding isn't really as easy and practical as means of permanently welding pieces together as it might seem immediately apparent from its name (it's also know by others, e.g. &lt;em&gt;stiction&lt;/em&gt;). What happens is that materials that can join into a cold-weld (similar metals) only do that on immediately adjoining surfaces, where atoms of one piece quite literally fill in the gaps between atoms of another piece so they're at that point essentially a single piece. So if you want to achieve a strong weld, the two surfaces you're joining would have to match perfectly. This obviously limits the use of printing with predominantly random surface geometry particles, since the structures wouldn't be as strong as it might be anticipated. Second thing is, that in order to reduce the unwelded space between two surfaces, either a lot of pressure is required, or a lot of heat, at which point you're already welding in more conventional sense and it most certainly can't be considered a &quot;cold&quot; weld. Assuming the former requirement (we sort of disqualified the latter) is met, that means that you'd be either firing small (probably nano-scale) particles at great speed to the surface you'd want them to cold-weld (impact cold-welding), or apply a great deal of pressure once the particles are more or less in the place intended. With additive printing, the latter is again not practical (we'd be back to now ancient days of needle-printing, not state-of-the-art additive manufacturing one).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it would have serious limitations. But there are some fields that I can think of where the phenomenon of cold-welding could be of some use, for example with self-assembling nanostructures or self-healing metallic surfaces. Nanostructures are interesting for multiple applications, from medicine to create artificial manifolds on which crystalline and molecular structures could grow (say artificially grown tissues for transplantation, more potent medicine,...), to computing (data storage, central processing units, you name it), even batteries with greatly increased charge surfaces. And with self-healing surfaces, these could again be used in all kinds of fields, from astronomy to again computing, wherever nano-scale precision is required and parts of equipment used can be damaged on scale that the surfaces used could be repaired by self-healing to resume their function in mint condition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in short, yes I imagine it could be useful, on nano-scale, but has serious limitations for large industrial-scale applications and it won't replace traditional welding techniques, such as laser and electron beam welding that could also be simplified if they were used in the vacuum of space (they are done in vacuum anyway).&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-04-03T18:46:36.830" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4185" PostTypeId="2" ParentId="308" CreationDate="2014-04-04T00:20:35.420" Score="2" Body="&lt;p&gt;Adding to the other answers, the body may decay very fast when colliding with space debris - depending on it's size. Probably more like 'exploding', as it would be brittle after freezing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This assumes that the body is in an orbit where other debris exists - put that seems probable to me in general.&lt;/p&gt;&#xA;" OwnerUserId="3029" LastActivityDate="2014-04-04T00:20:35.420" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4187" PostTypeId="1" AcceptedAnswerId="4188" CreationDate="2014-04-04T11:55:28.810" Score="35" ViewCount="10157" Body="&lt;p&gt;During the PSLV (India's &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;Polar Satellite Launch Vehicle&lt;/a&gt;) C24 launch on 4 April 2014, once stage 3 (PS3) had burned out, it wasn't immediately jettisoned, but instead there was a coasting phase for over a minute before it was jettisoned and stage 4 (PS4) ignited:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7NGXR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;PSLV C24 Flight Profile&lt;/strong&gt;. Image: Indian Space Research Organization, Image source: &lt;a href=&quot;http://www.spaceflight101.com/pslv-c24-launch-updates-irnss-1b.html&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101 PSLV C24 Launch Updates&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Event                       Time      Alt. (km)  Vel. (m/s) &#xA;-----------------------------------------------------------&#xA;PS1 Ignition                00.00.0        0.02       452.0 &#xA;Booster 1,2 Ignition        00:00.5        0.02       452.0 &#xA;Booster 3,4 Ignition        00:00.7        0.02       452.0 &#xA;Booster 5,6 Ignition        00:25.0        2.60       606.5 &#xA;Booster 1,2 Separation      01:10.0        23.3      1424.6 &#xA;Booster 3,4 Separation      01:10.1        23.4      1429.3 &#xA;Booster 5,6 Separation      01:32.0        39.4      2026.7 &#xA;PS1 Separation              01:51.5        56.4      2389.9 &#xA;PS2 Ignition                01:51.7        56.4      2389.9 &#xA;Payload Fairing Separation  03:24.5        112.8     3709.6 &#xA;PS2 Separation              04:23.5        129.9     5376.7 &#xA;PS3 Ignition                04:24.7        130.1     5376.3 &#xA;PS3 Separation              10:08.7        184.3     7734.1 &#xA;PS4 Ignition                10:18.7        186.1     7732.0 &#xA;PS4 Cutoff                  18:48.8        454.2     9638.4 &#xA;IRNSS-1 Separation          19:25.0        506.3     9598.9 &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The PS3 burn time is &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;quoted&lt;/a&gt; at only 83 seconds (1.38 min), and the flight profile shows PS3 ignition to separation time of 344 seconds (5.73 min). This doesn't make sense to me, especially after hearing the quote on a previous launch that launches always avoid carrying any mass for longer than they have to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the benefit to carrying this 3rd stage for some time?&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="12102" LastEditDate="2019-12-19T00:31:20.973" LastActivityDate="2019-12-19T00:31:20.973" Title="Why is there a &quot;coasting&quot; phase in some space launches?" Tags="|launch|isro|booster|stage-separation|pslv|" AnswerCount="6" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4188" PostTypeId="2" ParentId="4187" CreationDate="2014-04-04T13:02:03.113" Score="39" Body="&lt;p&gt;Keeping a burnt-out stage attached doesn't hurt until it's time to start the next stage. In fact, keeping it attached until shortly before it's time to ignite the next stage can improve the total launch delta V. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stage three separation occurred at an altitude of 184 km. There's still air up there. It's not very dense air, but for a vehicle going at close to orbital velocity, there's going to be a decent amount of drag on the vehicle. Drag acceleration is proportional to drag cross section and inversely proportional to mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since rockets fly a zero angle of attack profile, that burnt-out stage won't increase the drag cross section by much. Keeping it attached obviously increases the mass compared to that of the vehicle without that stage attached. Keeping the stage attached reduces deceleration due to drag.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="49" LastEditDate="2014-04-04T13:13:56.327" LastActivityDate="2014-04-04T13:13:56.327" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4189" PostTypeId="2" ParentId="3145" CreationDate="2014-04-04T15:50:25.407" Score="-1" Body="&lt;p&gt;Again I can't comment, so I post my 5 cents as an answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one-way direction of flight comes from an expensive budget, but also technology is not advanced enough. Under lack of technology I mean:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Solar sail - no working samples at the moment. Definitely it would be noticable if our probes fly there-around in the solar system.&lt;/li&gt;&#xA;&lt;li&gt;Space elevator, no proper carbon nanofibers at the moment. Or some other materials which help to build a space elevator.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But technology is rolling forward, so that &quot;one way&quot; may (or may not) be converted to &quot;Welcome back&quot;. Or if they (the cosmonauts) will find 1 cubic km of gold on Mars ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some assumed (and argued in answers) probability of biological infestation, like very minor threat - but in a situation where we have near zero information about the Mars environmnent, we can only guessing and preparing ourselves for different scenarios (because any possible scenario may happen). &lt;a href=&quot;https://space.stackexchange.com/a/3278/3039&quot;&gt;Robert Walker answered&lt;/a&gt; and covered much of that question.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, first, it's interesting how our (in human body) microbiological friends (human microflora) will behave in those new circumstances (environment). Second, we have to be over-cautious about XNA. And an XNA may not necessarily  consider the human body like a host or target to infect - the human body may just be a package of food for it (for some pretty self-sufficient stuff, like bacteria, etc.). We can be pretty sure - chemistry is the same both on Mars and Earth - &quot;nobody will eat us&quot; - I would not rely on that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Earth: Deep ocean microbiology isn't covered well, also like deep under-earth microbiological stuff, and there can be interesting symbiotic systems, which parts can be not a welcome for us. About Mars ... nobody may be sure what wait us there - be ready.&lt;/p&gt;&#xA;" OwnerUserId="3039" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-04-05T08:32:59.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4190" PostTypeId="2" ParentId="3531" CreationDate="2014-04-04T18:06:22.857" Score="7" Body="&lt;p&gt;A minimum energy ellipse between departure and destination corners of a Lambert space triangle is described on page 65 of the 1993 edition of Prussing and Conway's Orbital Mechanics textbook. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this particular Lambert Space triangle, both r1 and r2 would be the radius of the earth, 6378 km. The 3 points of the triangle would be earth's center, Sidney and London. θ would be the angle between Sidney and London, about 152 degrees. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ydtO5.png&quot; alt=&quot;minimum energy ellipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second focus of this minimum energy ellipse would lie on the center of the chord connecting Sidney and London.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Distance between foci, (2e*a), is r cos(θ/2). The major axis of this ellipse (2a) is r(1 + sin(θ/2)). So it can be seen that eccentricity e is cos(θ/2)/(1+sin(θ/2)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case altitude of perigee would be about -847 km. Altitude of apogee would be 665 km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speed at earth's surface would be 7.84 km/s with a flight path angle of 6.86º, nearly horizontal. Following a nearly horizontal path through earth's troposphere at 7.84 km/s is very impractical. Typically trajectory includes a vertical ascent to get above the thick atmosphere before the major horizontal burn is made. If a 100 km vertical ascent is made, why not just go for an orbital flight from London to Sidney? The delta V would be about the same. This suborbital flight makes little sense to me.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-04-04T18:19:34.447" LastActivityDate="2014-04-04T18:19:34.447" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4191" PostTypeId="2" ParentId="4187" CreationDate="2014-04-04T19:49:34.580" Score="17" Body="&lt;p&gt;The coasting period is certainly to perform a &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_turn&quot;&gt;Gravity Turn&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is a trajectory optimization that uses gravity to steer the vehicle&#xA;  onto its desired trajectory. ... the thrust is not used to change the&#xA;  ship's direction, so more of it is used to accelerate the vehicle into&#xA;  orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Once the vehicle has coasted into the right angle, P4 ignition takes place completing the journey to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More relevant information from Wikipedia regarding the coasting period (emphasis added):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If the rocket were not producing thrust, the flight path would&#xA;  be a simple ellipse like a thrown ball (it's a common mistake to think&#xA;  it is a parabola: it is only true if you consider Earth is flat, and&#xA;  gravity always points in the same direction, which is a good&#xA;  approximation for short distances), leveling off and then falling back&#xA;  to the ground. The rocket is producing thrust though, and rather than&#xA;  leveling off and then descending again, by the time the rocket levels&#xA;  off, it has gained sufficient altitude and velocity to place it in a&#xA;  stable orbit.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;If the rocket is a multi-stage system where stages fire sequentially&lt;/strong&gt;,&#xA;  &lt;strong&gt;the rocket's ascent burn may not be continuous&lt;/strong&gt;. Obviously, some time&#xA;  must be allowed for stage separation and engine ignition between each&#xA;  successive stage, but some rocket designs call for extra free-flight&#xA;  time between stages. &lt;strong&gt;This is particularly useful in very high thrust&#xA;  rockets, where if the engines were fired continuously, the rocket&#xA;  would run out of fuel before leveling off and reaching a stable orbit&#xA;  above the atmosphere.&lt;/strong&gt; The technique is also useful when launching&#xA;  from a planet with a thick atmosphere, such as the Earth. Because&#xA;  gravity turns the flight path during free flight, the rocket can use a&#xA;  smaller initial pitchover angle, giving it higher vertical velocity,&#xA;  and taking it out of the atmosphere more quickly. This reduces both&#xA;  aerodynamic drag as well as aerodynamic stress during launch. Then&#xA;  &lt;strong&gt;later during the flight the rocket coasts between stage firings,&#xA;  allowing it to level off above the atmosphere, so when the engine&#xA;  fires again, at zero angle of attack, the thrust accelerates the ship&#xA;  horizontally, inserting it into orbit.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="3045" LastEditorUserId="3045" LastEditDate="2014-04-04T20:15:45.017" LastActivityDate="2014-04-04T20:15:45.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4192" PostTypeId="2" ParentId="4187" CreationDate="2014-04-04T23:27:14.453" Score="10" Body="&lt;p&gt;Kerbal Space Program, the video game, teaches us that it is most effective to thrust at apoapsis (highest point of orbit) to increase our periapsis (lowest point of orbit). Waiting to fire the next stage there will make better use of fuel. Note that you want all the force applied at apoapsis, so you want to burn a little before and after it, since it is a moving target as you thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Coasting to get further out of atmosphere can also increase the efficiency of some engines as they will have a higher specific impulse in vacuum compared to an atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other answers also point out that it helps minimizing deceleration due to drag to leave empty stages and their mass attached during ascent.&lt;/p&gt;&#xA;" OwnerUserId="3047" LastEditorUserId="49" LastEditDate="2014-04-05T18:31:19.457" LastActivityDate="2014-04-05T18:31:19.457" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4193" PostTypeId="2" ParentId="4187" CreationDate="2014-04-05T00:19:21.697" Score="12" Body="&lt;p&gt;Any rocket flight will involve a burn in it's final orbital position.  If you want to enter a 500km orbit your engine will shut down at 500km up.  The profile shows it needs nearly 19 minutes to reach altitude but the rocket burn times only add up to 15 minutes.  You can't magically make it get there in 15 minutes because you don't have extra fuel.  Thus the engines must be quiet for 4 minutes.  You can either accomplish this by restarting an engine or by delaying the ignition of an engine in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is more typically done by a small engine used for the final burn (for example, the Shuttle jettisoned the main tank and used it's orbital maneuvering engines for the final burn) but in this case it obviously made more sense to use the 4th stage.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-04-05T00:19:21.697" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4194" PostTypeId="2" ParentId="19" CreationDate="2014-04-05T14:55:57.283" Score="7" Body="&lt;p&gt;I've been looking into this recently, so I'll leave an answer even though @Chandough's answer will have some of the same information.  &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How much of a problem is it really to orbiting satellites? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For the satellites in low earth orbit (LEO) it represents a fairly significant risk.  The image below shows some of the space junk in LEO that is tracked, taken from &lt;a href=&quot;http://en.wikipedia.org/wiki/Kessler_syndrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Wikipedia article&lt;/a&gt;.  The problem is that it is possible that the amount of trash in this region reaches a critical density at which each collision creates many more pieces of debris which are capable of destroying other satellites and the process runs away like a downhill chemical reaction, pulverizing everything in LEO.  This scenario is known as the Kessler syndrome, and you can read more about it &lt;a href=&quot;http://en.wikipedia.org/wiki/Kessler_syndrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TLCQK.png&quot; alt=&quot;Space debris in LEO&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Are there many cases of spacecraft being damaged by it, or is it a bit overhyped?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are numerous cases which you can read about in &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_debris#Threat_to_manned_spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;this section of the Wikipedia article on space debris&lt;/a&gt;.  I'll tell here about the most significant one which occurred in 2009 and is documented &lt;a href=&quot;http://en.wikipedia.org/wiki/2009_satellite_collision&quot; rel=&quot;nofollow noreferrer&quot;&gt;in this Wikipedia article&lt;/a&gt; and &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/newsletter/pdfs/ODQNv13i2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA report&lt;/a&gt;.  That collision was between two satellites, one an active communications satellite operated by Iridum Communications Inc., and the other a Russian Cosmos satellite which had been out of use since the mid 1990s.  They collided at almost right angles to each other, and the initial debris field and the expected debris field 6 months after the collision can be seen below.  All of the pieces tracked in these images are capable of destroying another spacecraft if they were to collide with it!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sf7HQ.png&quot; alt=&quot;Debris field of the Iridum Cosmos collision&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What measures could be taken to &quot;clean up&quot; the junk we have orbiting the earth? Is the technology available to do an effective job of cleaning it up?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I think @Chandough did a pretty good job on this one, but there is one idea which I quite like that he didn't mention so I'll tell you about it.  First, the difficulty with sending a satellite to grab the junk is the fuel cost of constantly readjusting the orbit to get from one piece of junk to the next.  The &lt;a href=&quot;http://aero.tamu.edu/news/removing-space-debris-tamu-sweeper-sling-sat&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sling-Sat&lt;/a&gt; (pictured conceptually below) will try to avoid this by using the space junk to propel itself from one piece of junk to the next.  It will do so by capturing a piece of junk, then ejecting it with the right trajectory to simultaneously adjust its orbit to get to the next piece and adjust the orbit of the trash to reenter Earth's atmosphere (lower the perigee).  It is still in the feasibility study stages, but you can see a video about it &lt;a href=&quot;https://www.youtube.com/watch?v=Aoow-t7qu7k&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yzapx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2699" LastActivityDate="2014-04-05T14:55:57.283" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4197" PostTypeId="2" ParentId="1865" CreationDate="2014-04-05T22:37:12.673" Score="3" Body="&lt;p&gt;A problem with asteroids is revisit times. The more earth like an asteroid's orbit, the greater the synodic period. Let T1 be period of one body and T2 period of 2nd body. The synodic period is |(T1*T2)/(T1-T2)|.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example if the period of a near earth asteroid were 1.1 years, synodic period with earth would be (1.1*1) / (1.1-1) which would be 11 years. Moreover the lowest delta V rendezvous assumes a tangent transfer orbit that just touches earth's orbit as well as just touches the asteroid's orbit near aphelion. This makes launch windows even more infrequent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have no experience mining asteroids. Learning how will be a process of trial and error. For a conventional earthly mine, establishing necessary infrastructure would take multiple trips. This would be especially true of an asteroid mine where we're learning as we go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say it will take 10 missions to the asteroid before we have a working mine. If launch windows are 11 or more years apart, that'd take more than a century.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To eliminate the problem of revisit times, it would be a good idea to park the rock in high lunar orbit. For many rocks, this can be done with a tiny amount of delta V. Once parked in lunar orbit, launch windows open every two weeks and trip times are less than a week. Revisit times are no longer a show stopper. It becomes much more plausible to establish working infrastructure within a short time frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We could possibly park small rocks in lunar orbits. But even if delta V is quite small, it's way beyond our present abilities to park larger rocks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Another consideration is safety. Should a retrieval go awry and a rock impact the earth, it should be small enough that it would burn up harmlessly in the upper atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-04-06T02:00:44.943" LastActivityDate="2014-04-06T02:00:44.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4199" PostTypeId="1" CreationDate="2014-04-06T12:02:06.660" Score="8" ViewCount="1173" Body="&lt;p&gt;Are there any known materials which could only (or a lot easier) be manufactured in microgravity and which would have properties which would make them useful for industrial applications back on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any companies which seriously consider to commercially manufacture these materials in factories in low-earth orbit?&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2014-07-30T08:50:03.740" Title="Are there useful materials which can only be manufactured in microgravity?" Tags="|microgravity|materials|commercial|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4200" PostTypeId="2" ParentId="4187" CreationDate="2014-04-06T18:44:27.243" Score="5" Body="&lt;p&gt;Some &quot;coasting&quot; phases are designed to limit the aerodynamic stresses on the spacecraft. The US space shuttle is an example of this. After around 26 seconds the main engines are throttled back for the next 34 seconds. In actuality this is called a &quot;Thrust Bucket&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this isn't a true &quot;coast phase&quot; because the solid rocket boosters are still burning, it is in some sense a &quot;coast&quot; because you are trying to limit acceleration to reduce heat and aerodynamic stress on the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While other answers here accurately describe your particular use-case for coasting, this answer notes that there are other reasons for the general question of &quot;Why is there a 'coasting' phase in some space launches&quot; for the benefit of future visitors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Credit to TildalWave for &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/events/1stage/&quot; rel=&quot;nofollow&quot;&gt;this reference&lt;/a&gt; and naming/clarifying what I was going on about.&lt;/p&gt;&#xA;" OwnerUserId="3061" LastEditorUserId="3061" LastEditDate="2014-04-07T18:51:20.103" LastActivityDate="2014-04-07T18:51:20.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4201" PostTypeId="1" CreationDate="2014-04-06T21:32:10.570" Score="5" ViewCount="947" Body="&lt;p&gt;Are there any known Trojan objects residing in the Sun-Earth L&lt;sub&gt;4&lt;/sub&gt; (SEL4) and L&lt;sub&gt;5&lt;/sub&gt; (SEL5) Lagrange points, also named &quot;Greek&quot; and &quot;Trojan&quot; groups, respectively?&lt;/p&gt;&#xA;" OwnerUserId="3058" LastEditorUserId="49" LastEditDate="2014-04-07T09:43:52.420" LastActivityDate="2014-04-07T20:49:00.557" Title="Does the Earth have any Trojan asteroids?" Tags="|asteroid|solar-system|lagrangian-points|astronomy|near-earth-object|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4202" PostTypeId="2" ParentId="4201" CreationDate="2014-04-06T22:52:51.787" Score="2" Body="&lt;p&gt;There is a large amount of dust, and one known asteroid. From &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point#L4_and_L5&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Sun–Earth L4 and L5 points lie 60° ahead of and 60° behind the&#xA;  Earth as it orbits the Sun. The regions around these points contain&#xA;  interplanetary dust and at least one asteroid, 2010 TK7, detected&#xA;  October 2010 by WISE and announced July 2011.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Earth–Moon L4 and L5 points lie 60° ahead of and 60° behind the&#xA;  Moon as it orbits the Earth. They may contain interplanetary dust in&#xA;  what is called Kordylewski clouds; however, the Hiten spacecraft's&#xA;  Munich Dust Counter (MDC) detected no increase in dust during its&#xA;  passes through these points.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-04-06T22:52:51.787" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4204" PostTypeId="2" ParentId="4201" CreationDate="2014-04-07T05:48:35.877" Score="8" Body="&lt;p&gt;Asteroid &lt;a href=&quot;http://en.wikipedia.org/wiki/2010_TK7&quot; rel=&quot;noreferrer&quot;&gt;2010 TK7&lt;/a&gt; is called an Earth Trojan. But it's orbit isn't as long lived as the Jupiter Trojans. According to Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;2010 TK7's orbit has a chaotic character, making long-range predictions difficult. Prior to A.D. 500, it may have been oscillating about the L&lt;sub&gt;5&lt;/sub&gt; Lagrangian point (60 degrees behind Earth), before jumping to L&lt;sub&gt;4&lt;/sub&gt; via L&lt;sub&gt;3&lt;/sub&gt;. Short-term unstable libration about L&lt;sub&gt;3&lt;/sub&gt;, and transitions to horseshoe orbits are also possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;L&lt;sub&gt;4&lt;/sub&gt; and L&lt;sub&gt;5&lt;/sub&gt; are quite stable in circular 3 body orbit system. But a 3 body system is a poor model for Sun-Earth L&lt;sub&gt;5&lt;/sub&gt; (SEL5) asteroid. Jupiter's tug on an Earth Trojan can exceed Earth's tug by a factor of 18. Venus' tug can exceed Earth's pull by a factor of 11.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, Jupiter and its Trojans can be fairly well modeled by a 3 body system. Outside of the Sun, Jupiter is by far the biggest frog in our pond. The perturbations of other planets don't disrupt Jupiter's Trojans that much.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="49" LastEditDate="2014-04-07T09:49:17.747" LastActivityDate="2014-04-07T09:49:17.747" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4205" PostTypeId="2" ParentId="4201" CreationDate="2014-04-07T10:30:25.677" Score="6" Body="&lt;p&gt;To add to the other answers, Wikipedia has another page listing &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_objects_at_Lagrangian_points&quot; rel=&quot;noreferrer&quot;&gt;objects at Lagrangian points&lt;/a&gt;, and for Sun–Earth L&lt;sub&gt;4&lt;/sub&gt; (SEL4) it currently lists:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Asteroid &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_TK7&quot; rel=&quot;noreferrer&quot;&gt;2010 TK7&lt;/a&gt; is the first discovered &quot;tadpole&quot; orbit companion to Earth, orbiting L&lt;sub&gt;4&lt;/sub&gt; with a mean distance of about one&#xA;  astronomical unit.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;noreferrer&quot;&gt;STEREO&lt;/a&gt; A (Solar TErrestrial RElations Observatory – Ahead) made its closest pass to L&lt;sub&gt;4&lt;/sub&gt; in September 2009, on its orbit around&#xA;  the Sun, slightly faster than the Earth.&lt;/li&gt;&#xA;  &lt;li&gt;Dust clouds&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And for Sun–Earth L&lt;sub&gt;5&lt;/sub&gt; (SEL5):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Asteroid &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_SO16&quot; rel=&quot;noreferrer&quot;&gt;2010 SO&lt;sub&gt;16&lt;/sub&gt;&lt;/a&gt;, in a horseshoe companion orbit with Earth, is currently proximal to L&lt;sub&gt;5&lt;/sub&gt; but at a high inclination.&lt;/li&gt;&#xA;  &lt;li&gt;STEREO B (Solar TErrestrial RElations Observatory – Behind) makes its closest pass to L&lt;sub&gt;5&lt;/sub&gt; in October 2009, on its orbit around the Sun,&#xA;  slightly slower than the Earth.&lt;/li&gt;&#xA;  &lt;li&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Spitzer_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Spitzer Space Telescope&lt;/a&gt; is in an Earth-trailing heliocentric orbit drifting away c. 0.1 AU per year. In c. 2013–15 it will pass L&lt;sub&gt;5&lt;/sub&gt;&#xA;  in its orbit.&lt;/li&gt;&#xA;  &lt;li&gt;Dust clouds&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For those mentioned dust clouds (&quot;zodiacal cloud&quot;, or interplanetary dust cloud, or IPD for short) that Wikipedia lacks citation and reference, I suggest &lt;a href=&quot;http://arxiv.org/abs/astro-ph/9806250&quot; rel=&quot;noreferrer&quot;&gt;The COBE diffuse infrared background experiment search for the cosmic infrared background. II. Model of the interplanetary dust cloud&lt;/a&gt;, T. Kelsall et al., The Astrophysical Journal 508 (1998), pages 44-73 (&lt;a href=&quot;http://arxiv.org/pdf/astro-ph/9806250v1.pdf&quot; rel=&quot;noreferrer&quot;&gt;PDF here&lt;/a&gt;, &lt;a href=&quot;http://arxiv.org/ps/astro-ph/9806250v1&quot; rel=&quot;noreferrer&quot;&gt;accompanying images here&lt;/a&gt;) which provides density, positional and volumetric measurement data. The Earth's dust cloud is estimated to have a radius of about a tenth that of the Earth's orbit (~ 0.1 AU).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On top of &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_SO16&quot; rel=&quot;noreferrer&quot;&gt;2010 SO&lt;sub&gt;16&lt;/sub&gt;&lt;/a&gt;, orbits of other &lt;a href=&quot;https://en.wikipedia.org/wiki/Aten_asteroid&quot; rel=&quot;noreferrer&quot;&gt;Aten&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_asteroid&quot; rel=&quot;noreferrer&quot;&gt;Apollo&lt;/a&gt; Near-Earth Objects (NEO) with similar orbital period to Earth's own around the Sun can be periodically perturbed by gravitational influence of other Solar system celestials (like mentioned by David), to occasionally intersect with Sun-Earth L&lt;sub&gt;4&lt;/sub&gt; and L&lt;sub&gt;5&lt;/sub&gt; in their &lt;a href=&quot;https://en.wikipedia.org/wiki/Horseshoe_orbit&quot; rel=&quot;noreferrer&quot;&gt;horseshoe&lt;/a&gt; or tadpole resonant orbits between the Sun and the Earth, or between the Earth and other planets in the inner Solar system. These would however typically be named as &lt;a href=&quot;https://en.wikipedia.org/wiki/Hilda_family&quot; rel=&quot;noreferrer&quot;&gt;Hilda family&lt;/a&gt; of asteroids, and not Trojans. But their orbit might at times still put them at Sun-Earth L&lt;sub&gt;4&lt;/sub&gt; and L&lt;sub&gt;5&lt;/sub&gt; points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To somewhat illustrate what I mean with resonant tadpole and horseshoe orbits and how they can intersect Earth-Sun L&lt;sub&gt;4&lt;/sub&gt; and L&lt;sub&gt;5&lt;/sub&gt; Lagrange points, here's an image from Nature article on &lt;a href=&quot;http://www.nature.com/nature/journal/v387/n6634/full/387651a0.html&quot; rel=&quot;noreferrer&quot;&gt;The Earth's secret companion&lt;/a&gt; (sadly paywalled), that's discussing asteroid &lt;a href=&quot;https://en.wikipedia.org/wiki/3753_Cruithne&quot; rel=&quot;noreferrer&quot;&gt;3753 Cruithne&lt;/a&gt;, which is another example of an Aten asteroid that moves in an &quot;overlapping horseshoe&quot; path w.r.t. the Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/fN4BD.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Tadpole and horseshoe resonant orbits in a Sun-planet system, shown in a frame rotating with the planet's orbit (source: &lt;a href=&quot;http://www.nature.com/nature/journal/v387/n6634/fig_tab/387651a0_F1.html&quot; rel=&quot;noreferrer&quot;&gt;Nature&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for Earth-Sun L&lt;sub&gt;4&lt;/sub&gt; and L&lt;sub&gt;5&lt;/sub&gt; we have following natural satellites:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Trojan asteroid &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_TK7&quot; rel=&quot;noreferrer&quot;&gt;2010 TK7&lt;/a&gt; oscillating between SEL4 and SEL3&lt;/li&gt;&#xA;&lt;li&gt;Asteroid &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_SO16&quot; rel=&quot;noreferrer&quot;&gt;2010 SO&lt;sub&gt;16&lt;/sub&gt;&lt;/a&gt; in a horseshoe orbit extending beyond SEL4 and SEL5 points via SEL3&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/3753_Cruithne&quot; rel=&quot;noreferrer&quot;&gt;3753 Cruithne&lt;/a&gt; that is in 1:1 resonant orbit with Earth and crosses SEL4&lt;/li&gt;&#xA;&lt;li&gt;A plethora of NEO Aten and Apollo asteroids that can cross any of the three SEL3 to SEL5 points:&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;More resonant near-Earth objects (NEOs) have since been discovered.&#xA;  These include &lt;a href=&quot;https://en.wikipedia.org/wiki/54509_YORP&quot; rel=&quot;noreferrer&quot;&gt;54509 YORP&lt;/a&gt;, (85770) &lt;a href=&quot;https://en.wikipedia.org/wiki/%2885770%29_1998_UP1&quot; rel=&quot;noreferrer&quot;&gt;1998 UP&lt;sub&gt;1&lt;/sub&gt;&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/2002_AA29&quot; rel=&quot;noreferrer&quot;&gt;2002 AA&lt;sub&gt;29&lt;/sub&gt;&lt;/a&gt;, and 2009 BD&#xA;  which exist in resonant orbits similar to Cruithne's.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And some dust in blobs, stripes, rings, and whatnot shapes that's of disputable density, and a few man-made satellites that on periods enter these Lagrange regions in Earth's orbit. But so far, &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_TK7&quot; rel=&quot;noreferrer&quot;&gt;2010 TK7&lt;/a&gt; is the first and only identified Earth Trojan.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-07T20:49:00.557" LastActivityDate="2014-04-07T20:49:00.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4206" PostTypeId="1" AcceptedAnswerId="4207" CreationDate="2014-04-07T16:25:29.817" Score="16" ViewCount="3687" Body="&lt;p&gt;The majority of &lt;a href=&quot;http://kepler.nasa.gov/Mission/discoveries/&quot;&gt;exoplanets found so far&lt;/a&gt; are comparable in size to Jupiter or Saturn. Which, as we know, are problematic for human habitation. But their moons have a lot of potential.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of these large exoplanets are in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Circumstellar_habitable_zone&quot;&gt;Goldilocks zone&lt;/a&gt;. Given what we know about the moons of Jupiter &amp;amp; Saturn, if we should find duplicates of them circling in the Goldilocks zone of another star, would the moons be habitable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By habitable, I mean more likely to support independent/aboriginal life than Mars, or requiring significantly less supportive technology than Mars.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2014-04-07T16:35:21.357" LastActivityDate="2020-08-16T04:34:05.570" Title="If Saturn or Jupiter were in Earth's orbit, would any of their moons be habitable for humans?" Tags="|solar-system|jupiter|exoplanet|saturn|goldilocks-zone|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4207" PostTypeId="2" ParentId="4206" CreationDate="2014-04-07T17:21:23.533" Score="16" Body="&lt;p&gt;For Jovian and Saturnine moons, the simplest answer is no, at least not much more habitable than our own Moon is, because none of these moons have their own magnetic field or sufficient mass and their atmosphere would eventually thin out via the ionospheric hydrogen loss due to the solar wind. A bit more difficult answer is how many of these moons would be well within the parent planet's own magnetic field and still sufficiently protected from solar winds and cosmic radiation to sustain a breathable atmosphere, liquid water, and &lt;em&gt;life as we know it&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being within the goldilocks zone also means that these celestials would orbit well within the star's &lt;a href=&quot;https://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29&quot;&gt;snow line&lt;/a&gt;, so their surfaces would form completely differently than they indeed have in the outer Solar system, past the snow line. They wouldn't acrete as much of water ice as they did, and the liquid water would be more exposed to escaping hydrogen (hydrogen atoms aren't really strongly bound to oxygen atoms in water molecules, and UV radiation in upper Earth's atmosphere is sufficient to produce extremely light hydrogen ions that move to ionosphere).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it's difficult to say how such moons would even look like. Would they be barren rocks? Or could they hold onto enough water and sustain a breathable atmosphere? Would they even form at all from the protoplanetary disk, or would we mostly see captured planetesimals orbiting goldilocks Jupiters? I'm not sure that anyone can even answer these questions, there's enough of discussion even over our own Solar system's formation, but I'm fairly confident that there's not much point in discussing if Titan, Enceladus, Europa and alike moons of Jupiter and Saturn could support own, water-based lifeforms or be suitable for human colonisation, unless we can establish with a fair degree of certainty that they could even exist. For all we know, it might not be possible for them to hold onto so much water as some of the Jovian and Saturnine moons do. So it's quite possible we'd see much smaller, rocky moons and moonlets, with even smaller mass and surface gravity than we currently do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I'm not going to say it's impossible. Jupiter has incredibly strong magnetic field, likely sustained by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Metallic_hydrogen&quot;&gt;metallic hydrogen&lt;/a&gt; within its core, and is large enough to stretch nearly to other neighbouring planets. But how all this hydrogen actually moves in protoplanetary disks as their stars evolve, increase their temperature and push the snow line and blow lighter particles away, while at the same time consume a great deal of hydrogen to sustain their nuclear fusion, is still a bit of a mystery, as is the source of all the Earth's water. Perhaps we gained most of it via Late Heavy Bombardment of icy asteroids as the Earth's surface started to solidify? So what I'm saying is, that if such Jupiters form closer to their parent star, and within the goldilocks zone, we don't really know if they'd even be able to sustain a strong enough magnetic field. Composition of their inner cores might be entirely different than of our Jupiter. Even other gas giants (which leaves Saturn really, because Uranus and Neptune are ice giants) in our own Solar system don't support such strong magnetic field as Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, these are just some thoughts on the subject, and why I think it's too speculative and we lack sufficient understanding of even our own Solar system's formation, to transpose this knowledge onto exoplanets we know even less about. I guess the proof of the pudding will have to be in the eating. I.e. we'll know, when we detect some.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-07T23:47:52.473" LastActivityDate="2014-04-07T23:47:52.473" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="4209" PostTypeId="1" CreationDate="2014-04-07T21:55:38.177" Score="9" ViewCount="3308" Body="&lt;p&gt;Does any open source software exist that can compute GPS ground coverage in real time?  At work I used STK which would do the job perfectly, but I'm at school now so I only have the free version.  For those who don't know STK can simulate the GPS constellation and determine how much coverage any particular Earth coordinates are getting.  You can also place transmitters or jammers and see how that affects the coverage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the free version of STK doesn't come with coverage definitions or the ability to place new transmitters, which is what I need.  Does anyone know of an open source software package that can simulate the GPS constellation, compute ground coverage, and allows placement of transmitters/jammers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I read this question: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/646/what-are-the-choices-today-for-orbital-mechanics-simulation-software/647#647&quot;&gt;What are the choices today for orbital mechanics simulation software?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which had suggestions for orbital mechanics software.  I was hoping that either the GMAT or ORSA software packages mentioned there might be able to do what I want, but it looks like they're more for space exploration stuff rather than GPS.  If anyone has used these packages and knows if they can load the GPS constellation I'd love to hear about it.&lt;/p&gt;&#xA;" OwnerUserId="3069" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2022-02-17T19:36:48.157" Title="Is there any open source software that can compute GPS ground coverage? I need a free alternative to AGI's System Tool Kit (STK)" Tags="|navigation|software|gps|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4211" PostTypeId="1" AcceptedAnswerId="20629" CreationDate="2014-04-07T23:07:40.320" Score="14" ViewCount="33080" Body="&lt;p&gt;I know it is not that hard for those familiar with the equations for it, but I am having trouble with the math. In order to fully understand the orbit elements of TLEs I read &lt;a href=&quot;http://satelliteorbitdetermination.com/orbit_elements_wiki.htm&quot;&gt;this&lt;/a&gt; which I found very helpful, but I am struggling with turning that into latitude and longitude coordinates. I am coming into this with no knowledge of the necessary algorithms which is what I am hoping someone can point me to. &lt;a href=&quot;https://space.stackexchange.com/questions/2512/leo-metric-to-determine-the-ground-track-of-a-satellite&quot;&gt;This&lt;/a&gt; post provides some relevant information but if I understand correctly is only explaining how to get the longitude at a specific time while I would like the full coordinates.&lt;/p&gt;&#xA;" OwnerUserId="3072" LastEditorUserId="3072" LastEditDate="2014-04-08T00:40:01.217" LastActivityDate="2023-02-09T17:38:00.993" Title="Calculate Satellite Coordinates From TLE Data" Tags="|artificial-satellite|low-earth-orbit|ground-track|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4212" PostTypeId="2" ParentId="4206" CreationDate="2014-04-08T00:45:23.397" Score="2" Body="&lt;p&gt;On the first reading I thought you were asking about possible earth like moons orbiting gas giants in habitable zones of other star systems. This is fairly plausible in my opinion. I can't think of a reason why moons of about earth's mass couldn't be orbiting some of the gas giant exo-planets &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But on rereading it seems like you're asking about &lt;em&gt;duplicates&lt;/em&gt; of Jupiter and Saturn's moons. This is somewhat less plausible. Most of these moons don't have much air or magnetic field (as TildaWave points out).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Titan has a thick nitrogen atmosphere, with some liquid ethane and methane that evidently evaporates and rains back down on occasion. But Titan gets about 1/100 of the sunlight we do. Move it to a warmer place and I'm not sure Titan would hang on to those volatiles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which isn't to say Titan or Ganymede sized moons couldn't have an atmosphere in a habitable zone. Each moon has a different history that made it what it is today. I believe it's possible there are Titan sized moons in habitable zones with a history that's resulted in lots of volatiles. But Titan or Ganymede sized moons with a different histories wouldn't be &quot;duplicates&quot;. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-04-08T00:45:23.397" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4213" PostTypeId="2" ParentId="4211" CreationDate="2014-04-08T00:45:44.180" Score="3" Body="&lt;p&gt;Depending on which algorithm/set of equations you are using to convert, you may need to convert the TLE parameters into ECEF coordinates, then convert that into latitude, longitude, and altitude.  Here is a page that explains the ECEF-to-LLA conversion: &#xA;&lt;a href=&quot;http://www.gmat.unsw.edu.au/snap/gps/clynch_pdfs/coordcvt.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.gmat.unsw.edu.au/snap/gps/clynch_pdfs/coordcvt.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A common math difficulty is that the true anomaly and the mean anomaly are related by Kepler's equation, which is a transcendental equation as your first link mentioned.  An iterative method like Newton's Method is usually used for this part of the conversion.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't seem to find a webpage that has the set of algorithms for converting TLE to ECEF, but this page gives the algorithm for converting GPS ephemerides (orbital parameters) into ECEF coordinates: &lt;a href=&quot;http://web.ics.purdue.edu/~ecalais/teaching/geodesy/EAS_591T_2003_lab_4.htm&quot; rel=&quot;nofollow&quot;&gt;http://web.ics.purdue.edu/~ecalais/teaching/geodesy/EAS_591T_2003_lab_4.htm&lt;/a&gt;&#xA;If I remember correctly the TLE conversion is pretty similar, so that might get you on the right track.  If you don't have a textbook with the algorithm, it might be online in a paper or something.&lt;/p&gt;&#xA;" OwnerUserId="3069" LastActivityDate="2014-04-08T00:45:44.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4214" PostTypeId="2" ParentId="829" CreationDate="2014-04-08T05:29:55.347" Score="17" Body="&lt;p&gt;Hefty gravity wells can give a healthy Oberth benefit. Doing a burn deep in Neptune's well makes sense. Suggesting an Oberth maneuver near a Pluto sized object is pretty silly. Don't know if Randall Munroe knows this. Maybe that's part of his joke.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heading all the way back from the Kuiper belt to the inner solar system take 30 years. Then back out another few decades. Seeing as a mission to the outer system already takes decades, I don't think mission planners would go for this. They like to see their probe return data within their lifetimes. And it's doable to send a probe out of the system with existing rockets and assists from Jupiter. This is already demonstrated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the sun offers potentially huge Oberth benefits should we want to leave the solar system at a good clip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At .1 A.U. from the sun, solar escape velocity is about 133.2 km/s. Falling from Neptune, an object would be moving 133 km/s by the time it reaches a .1 A.U. perihelion. From there it would only need .2 km/s to achieve solar escape. Doing a 4.2 km/s burn at this perihelion would give a solar V infinity of 33 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To go from earth to Neptune would take a 8.3 km/s LEO burn. Once at a 30 A.U. aphelion, a gravity swing by from Neptune could be more than sufficient to throw the ship back to a .1 AU perihelion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get a solar Vinfinity of 33 km/s, an 18 km/s LEO burn would be needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;A 33 km/s solar Vinfnity might be good for doing an inner Oort Cloud object fly by within a reasonable time. But 33 km/s is only a little more than .0001 c. It'd take nearly 40,000 years to reach Alpha Centauri.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-04-08T05:35:03.817" LastActivityDate="2014-04-08T05:35:03.817" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4215" PostTypeId="1" CreationDate="2014-04-08T15:18:05.043" Score="5" ViewCount="881" Body="&lt;p&gt;I have been trying to work out how much ΔV would be required to deorbit a spacecraft after it had achieved escape velocity from the moon but I don't know how to work out what the radius of the orbit would be after it had escaped the moons gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say you were in a lunar orbit at 100km, and you made you burn at your lowest altitude (relative to earth) e.g. moons orbit minus 100km, when you got out of the moons gravity well and into the earths, how big would your orbit around the earth be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would I just have to rearrange the vis-viva equation to calculate my radius with the speed that I had after the burn?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know thats not very well put but I hope you understand.&lt;/p&gt;&#xA;" OwnerUserId="3073" LastEditorUserId="3073" LastEditDate="2014-04-08T17:21:59.027" LastActivityDate="2019-02-06T12:14:40.740" Title="What would your altitude be after you had achieved escape velocity from the moon?" Tags="|orbital-mechanics|orbit|orbital-maneuver|delta-v|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4216" PostTypeId="2" ParentId="4215" CreationDate="2014-04-08T17:36:38.710" Score="6" Body="&lt;p&gt;There isn't a single altitude associated with escape velocity. You could use a Jules Verne-style gun and achieve escape velocity at altitude 0. From the orbit you describe, it depends on how much thrust you use.&lt;br&gt;&#xA;You may be able to escape from the moon without reaching escape velocity, if you can get to a point where Earth's gravity is stronger than the Moon's.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-04-08T17:36:38.710" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4217" PostTypeId="2" ParentId="4215" CreationDate="2014-04-08T19:21:01.597" Score="6" Body="&lt;p&gt;A parabolic departure from the moon isn't needed. A ellipse with an apolune near EML1 suffices.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When first messing around with 3 body scenarios I tried using the vis-viva equation and other tools from two-body mechanics. It bit me, especially if I had trajectories passing through EML1 or EML2. For such a trajectory, a tiny change in initial velocity can make a big change later on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get an apolune near EML1, you need a perilune near the far side. Here is an orbital sim firing from the moon's far point:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y8Tgy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pale blue dot in the middle is earth. White circular arc is moon. The slowest red pellet was fired at 2.315 km/s and fell back towards the moon. The fastest pellet (blue) was fired at 2.355 km/s. Most these pellets sailed past EML1 and into approximately 100,000 km x 300,000 km earth orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in a 100,000x300,000 km orbit, a .6 km/s apogee burn can drop you to an atmosphere grazing perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However there is a direct route that takes less delta V:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/XUfjf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All these pellets depart from the moon's surface at the far point. Launches are westward. The earth grazing blue pellet was fired off at 2.6 km/s&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-04-11T14:32:22.707" LastActivityDate="2014-04-11T14:32:22.707" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4219" PostTypeId="2" ParentId="4161" CreationDate="2014-04-08T21:55:02.423" Score="0" Body="&lt;p&gt;I think your point 1 isn't phrased as well as it could be. What I think you mean is &quot;&lt;strong&gt;cross-sectional area&lt;/strong&gt;&quot;, not &quot;&lt;strong&gt;surface area&lt;/strong&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are using a material with reasonable heat transmission (i.e. not ceramic blocks, see below) having a larger surface area allows you to spread the thermal load more effectively. This is contrasted by having a larger cross-sectional area which will increase the thermal loading in the form of drag/friction/shockwave heating/etc. Thus larger surface area is beneficial, and a larger cross-sectional area is not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that what I'm referring to is the &quot;heat sink&quot; method of dealing with reentry heat. This technique is not suited for all craft. The space shuttle, for instance, did not use it, instead opting to protect against convective heat transfer. The shuttle can be thought of as a giant thermos, making heat transfer to the skin difficult, whereas the heatsink method can be thought of as absorbing all of the heat to radiate it back over time (once the heat source is removed).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interesting thermodynamics point: conductive transfer is how friction is applied, but with re-entry shockwaves you can design it so that ones heat load is nearly all convective... which means an air gap is effectively acting as your first layer of insulation.&lt;/p&gt;&#xA;" OwnerUserId="3061" LastActivityDate="2014-04-08T21:55:02.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4221" PostTypeId="1" AcceptedAnswerId="4320" CreationDate="2014-04-09T00:32:00.373" Score="9" ViewCount="4727" Body="&lt;p&gt;Various questions have been asked on this StackExchange about free versions of software that can do various things, such as &lt;a href=&quot;https://space.stackexchange.com/questions/646/what-are-the-choices-today-for-orbital-mechanics-simulation-software&quot;&gt;&lt;strong&gt;simulations&lt;/strong&gt;&lt;/a&gt; or &lt;a href=&quot;https://space.stackexchange.com/questions/2355/satellite-tracking-software&quot;&gt;&lt;strong&gt;tracking&lt;/strong&gt;&lt;/a&gt;. Many of the answers point to free software available around the internet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am more curious about the software used by governmental space agencies and corporations. Is most of the software used for simulation and trajectory planning built in-house, or are there some industry standards?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-04-20T20:00:15.843" Title="What software is often used to plan, launch, and track orbits?" Tags="|software|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4222" PostTypeId="2" ParentId="741" CreationDate="2014-04-09T01:13:57.527" Score="3" Body="&lt;p&gt;The delta-V a space vehicle can achieve depends on the &lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;rocket equation&lt;/a&gt;. It comes down to the proportion of total mass available to be expelled as propellant, and the velocity of that expelled mass. The velocity of the expelled mass depends on the amount of stored energy which can be converted into kinetic energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chemical rockets have only so much chemical energy stored in the reactants (which are also the propellant mass), which determines the limiting velocity of expelled propellant, which imposes an upper limit on the achievable delta-V of the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Switching to a different type of propulsion in which the propellant mass is expelled at much higher velocity can theoretically permit a vehicle to achieve much higher velocities. Of course, it presupposes the availability of an energy source which can store much more useable energy per unit of fuel mass - such as nuclear, and raises the question of just how to accelerate the propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Vasimir&quot; rel=&quot;nofollow&quot;&gt;VASIMIR&lt;/a&gt; is one example of a thrust technology which expells its exhaust gas at far higher velocity than achievable with chemical propellants. It may not be the technology to propel an interstellar probe (several factors appear to limit the delta-V it could apply to a spacecraft), but some other technology aiming to do the same thing (very high velocity exhaust) just might.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To some extent it does come down to money - to fund research into and development of new propulsion technologies. But it also comes down to basic physics. &lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-04-09T01:13:57.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4223" PostTypeId="2" ParentId="2912" CreationDate="2014-04-09T01:16:58.073" Score="4" Body="&lt;p&gt;I have a copy of that book -- autographed by J. S. Lewis! Most of the book is factual. But occasionally Lewis will indulge in little science fiction vignettes. He describes an imaginary asteroid hab on page 116. This cycler has a 2 year year period. It passes by the earth each two years. At its aphelion it goes by the main belt.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Cycler 3 is 800 meters long and averages 400 meters in diameter, except for the earth gravity wheel at the waist, which is 650 meters in diameter. Before reconstruction, the parent asteroid (2002 AK) was a carbonaceous asteroid of about half these dimensions and much less regular shape. The interior, originally mined to extract water for use as propellant to time the orbit into resonance, has been further excavated and restructured to permit accommodations…&lt;/p&gt;&#xA;&lt;p&gt;Meteoroid, cosmic ray, and solar flare proton protection provided by the outer walls are all well in excess of Earth-surface standards. ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Lewis list some rules for this cycler:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;There must be no further extraction of water from the native (asteroidal) regolith on the surface of the cycler. The water content of the surface layer is an integral part of the shielding of the interior against light cosmic-ray primaries and energetic solar protons…&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;All cycles of the biological elements hydrogen, carbon and nitrogen MUST be closed. Certified manifests of all these elements must be uploaded by all arriving vehicles. Departure manifest must also be filed and reconciled …&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I don't want to quote too much. But yes, you have the general idea. I think anyone interested in use of space resources as well as settlement of space should have a copy of &lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/com/0201328194&quot; rel=&quot;nofollow noreferrer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lewis' Mining the Sky&lt;/a&gt; Another excellent book by Lewis is &lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/com/0201489503&quot; rel=&quot;nofollow noreferrer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rain of Iron and Ice&lt;/a&gt;, also about the asteroids and asteroid impacts.&lt;/p&gt;&#xA;&lt;p&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-04-09T01:16:58.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4224" PostTypeId="2" ParentId="741" CreationDate="2014-04-09T06:40:23.800" Score="4" Body="&lt;p&gt;It is limited by how powerful and dense we can make lasers.  A &lt;a href=&quot;http://www.lunarsail.com/LightSail/rit-1.pdf&quot; rel=&quot;nofollow&quot;&gt;laser sail concept&lt;/a&gt; skirts the issues of propellant and the rocket equation, and so offer the promise of the highest speeds attainable.  Of course, &lt;a href=&quot;http://www.geoffreylandis.com/lightsail/lightsail89.html&quot; rel=&quot;nofollow&quot;&gt;it wouldn't be easy&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-04-09T06:40:23.800" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4225" PostTypeId="2" ParentId="4161" CreationDate="2014-04-09T13:55:44.520" Score="3" Body="&lt;p&gt;There is a weird project currently being studied to &quot;land&quot; probes on Mars without parachute or retro-rockets or whatelse: probes will be so thin and large that they will just slowly fall down like paper sheets without getting damaged.&#xA;&lt;a href=&quot;http://www.space.com/25000-planetary-exploration-flat-landers-tech-nasa.html&quot; rel=&quot;nofollow&quot;&gt;http://www.space.com/25000-planetary-exploration-flat-landers-tech-nasa.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is possible because what is needed for a &quot;soft landing&quot; is a small vertical speed.&#xA;An object left in free-fall in atmosphere accelerates up to &quot;terminal velocity&quot;, when air drag force becomes as high as gravitational force, so no more force results applied to the body, hence no acceleration, hence no speed variation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air drag force is $F= \frac 1 2 \rho C_d A v^2$, where $\rho=$ air density, $C_d$=Air Drag coefficient and A=cross-section area . The larger is the cross section area, the higher will be air drag force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravitational force (accelerating object): $F=mg$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air drag force (braking object): $F=-\frac 1 2 \rho C_d A v^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence maximum achievable speed for a free-falling object is:&#xA;$v_t=\sqrt \frac {mg}{\frac 1 2 \rho C_d A}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can tune m, $C_d$ and A to achieve a low terminal velocity (5-10 m/s), you get a soft landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, rather than &quot;smaller&quot; vehicle, you need a &quot;larger&quot; vehicle&lt;/p&gt;&#xA;" OwnerUserId="3075" LastEditorUserId="3075" LastEditDate="2014-04-10T07:31:38.653" LastActivityDate="2014-04-10T07:31:38.653" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4227" PostTypeId="1" CreationDate="2014-04-05T13:50:15.447" Score="10" ViewCount="223" Body="&lt;p&gt;Europa is one of the most interesting moons in our solar system. It seems possible that some kind of living beings are able to live beneath its frozen surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question.&lt;/strong&gt; Does NASA (or any other international organization) have a plan to search for the life in Europa in the near future? What is the date?&lt;/p&gt;&#xA;" OwnerDisplayName="Saint Georg" LastActivityDate="2014-04-09T14:56:10.110" Title="Does NASA have a plan to search for the life in Europa?" Tags="|life|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4228" PostTypeId="2" ParentId="4227" CreationDate="2014-04-05T14:36:53.657" Score="6" Body="&lt;p&gt;ESA's &lt;a href=&quot;http://sci.esa.int/juice/50068-science-objectives/&quot;&gt;JUICE&lt;/a&gt; mission is scheduled for launching in 2022.&#xA;One of the science objectives is the search for organic molecules near Europa.&#xA;Originally it was planned as NASA's JEO, as part of the joint mission &lt;a href=&quot;http://en.wikipedia.org/wiki/Europa_Jupiter_System_Mission_%E2%80%93_Laplace&quot;&gt;Laplace&lt;/a&gt; with NASA, JAXA, Rocosmos; for details of the NASA plans see the &lt;a href=&quot;http://www.lpi.usra.edu/decadal/2013_2022/decadalReport.pdf&quot;&gt;Decadal Survey&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA's &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29&quot;&gt;Juno&lt;/a&gt; mission is focused onto Jupiter itself, rather than on the moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has been a &lt;a href=&quot;http://solarsystem.nasa.gov/europa/blogs_icy_worlds_workshop_2014.cfm&quot;&gt;workshop on the habitability of icy worlds&lt;/a&gt; in February 2014. Europa has been discussed, see e.g. &lt;a href=&quot;http://www.hou.usra.edu/meetings/icyworlds2014/pdf/4060.pdf&quot;&gt;this abstract&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1454" OwnerDisplayName="Gerald" LastActivityDate="2014-04-05T14:44:12.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4229" PostTypeId="1" CreationDate="2014-04-09T15:39:05.400" Score="9" ViewCount="3950" Body="&lt;p&gt;I don't understand this and must ask a probably very stoopid question here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Oberth effect says that a rocket is much more efficient when (and in the orbital direction of) a payload when it already has the highest speed. This AFAIK is because force, or &quot;work&quot;, is a function of the distance moved and not of the time spent, while instead the force from a rocket engine is a function of time. So the faster a rocket is, the larger is the distance over which a rocket engine produce force during its fuel=time window.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is obviously anti-intuitive to everyday human experiences. Something which is already fast, such as a fast car or airplane, is less, not more, easy to accelerate further for the same amount of fuel consumed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What causes the Oberth effect? Is it directly given from Newtonian mechanics? If so, why did Oberth &quot;discover&quot; it 300 years afterwards? Has it got something to do with microgravity or rocket reaction propulsion? If we imagine a maglev train in a vacuum tube on Earth (i.e. no friction, no air resistance, no diminishing engine efficiency), would it get more and more energy efficient the more it is accelerated, because of the Oberth effect?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1142" LastEditDate="2014-12-04T20:31:23.263" LastActivityDate="2019-06-17T15:54:45.100" Title="Oberth effect for Earth vehicles" Tags="|orbital-maneuver|physics|rockets|oberth-maneuver|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4231" PostTypeId="2" ParentId="2749" CreationDate="2014-04-09T16:09:26.573" Score="2" Body="&lt;p&gt;There's a little funding to search for hazardous asteroids. &lt;a href=&quot;http://neo.jpl.nasa.gov/cgi-bin/neo_ca&quot; rel=&quot;nofollow&quot;&gt;JPL has a growing data base&lt;/a&gt; of known near earth asteroids. We're doing a good job of finding the Chicxulub sized rocks. Rocks big enough for extinction level events are easy to see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While we may have a good inventory of dino killers, potential city killers are another matter. Tunguska sized rocks are much harder to see. The vast majority of these rocks are probably still undiscovered. And the Tunguska or Chelyabinsk sized rocks are probably a million times more common than the Chicxulub sized rocks. To find these smaller rocks we may need orbital telescopes devoted to the search.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope to see the Asteroid Redirect Mission ARM funded. This mission calls for parking a small asteroid (5 to 7 meters) in lunar orbit. The proposed retrieval vehicle would be SEP (solar election propulsion) as described in the &lt;a href=&quot;http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf&quot; rel=&quot;nofollow&quot;&gt;Keck Report&lt;/a&gt;. Such a vehicle would be too small to deflect a Chicxulub sized rock. But it might be able to deflect a city killer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The co-authors of the Keck report include &lt;a href=&quot;http://en.wikipedia.org/wiki/John_S._Lewis&quot; rel=&quot;nofollow&quot;&gt;John S. Lewis&lt;/a&gt;, &lt;a href=&quot;http://www.planetaryresources.com/team/&quot; rel=&quot;nofollow&quot;&gt;Chris Lewicki&lt;/a&gt;, &lt;a href=&quot;https://solarsystem.nasa.gov/deepimpact/science/dyeomans.cfm&quot; rel=&quot;nofollow&quot;&gt;Don Yeomans&lt;/a&gt;, as well as other well known scientists and engineers who study asteroids. Many of the co-authors are now part of Planetary Resources or Deep Space Industries, two companies that hope to mine asteroids for profit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Deflecting larger asteroids would require space infrastructure. Our elected representatives are too ignorant and short sighted to fund it. But if mining space resources could return a profit, infrastructure would naturally grow. That is why I see Planetary Resources or Deep Space Industries as the best bets for planetary defense.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-04-09T16:09:26.573" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4232" PostTypeId="2" ParentId="4229" CreationDate="2014-04-09T18:37:37.420" Score="7" Body="&lt;p&gt;Assume a vehicle in orbit about some gravitational body. The Oberth effect says that the way to get the biggest increase in energy from a maneuver that changes the vehicle's velocity by some &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; is to perform that maneuver at periapsis (i.e., when the vehicle is going fastest with respect to the gravitating body). The total mechanical energy before the maneuver is &lt;span class=&quot;math-container&quot;&gt;$E_0 = \frac{v^2} 2 - \frac {\mu} r$&lt;/span&gt;, where &lt;span class=&quot;math-container&quot;&gt;$\mu=GM$&lt;/span&gt; is the gravitational parameter for the planet/star. Assuming an impulsive maneuver, the total mechanical energy after the maneuver is &lt;span class=&quot;math-container&quot;&gt;$\frac{(v+\Delta v)^2} 2 - \frac {\mu} r$&lt;/span&gt;, or &lt;span class=&quot;math-container&quot;&gt;$E_0 + \frac{\Delta v^2} 2 + v \cdot \Delta v$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Mechanical energy is a constant over the orbit, and &lt;span class=&quot;math-container&quot;&gt;$\Delta v^2$&lt;/span&gt; will be the same wherever the maneuver is performed. The only thing that does change is that last term, &lt;span class=&quot;math-container&quot;&gt;$v \cdot \Delta v$&lt;/span&gt;. To maximize the change in energy we want two things to be true: Velocity &lt;span class=&quot;math-container&quot;&gt;$v$&lt;/span&gt; needs to be at its maximum (i.e., periapsis) and the &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; to be applied along the velocity vector.&lt;/p&gt;&#xA;&lt;br&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Something which is already fast, such as a fast car or airplane, is less, not more, easy to accelerate further for the same amount of fuel consumed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;A car that's going fast is already in a high gear where you naturally get less torque. It's also using a good deal of power just to keep going the same speed. Let off the gas and the car will slow down. There's not much power left for acceleration. A plane that is already going fast (with respect to the airflow) similarly is using power just to maintain speed. Spacecraft operate in vacuum, and it doesn't matter how fast a vehicle is already going when a vehicle performs a maneuver. For a rocket, delta V is delta V. It is independent of the current velocity.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;One needs to be careful here. Imagine a rocket located in deep space, far removed from any gravitating bodies, about to test fire its thrusters. Suppose a number observers will watch this test, with different observers moving at different velocities relative to the rocket. Ignoring relativistic effects, all observers will agree on the rocket's change in velocity. The observers will not agree on how much the rocket's energy has changed. Some may even see the rocket as losing energy.&lt;/p&gt;&#xA;&lt;p&gt;The observers will agree on the total change in energy of the rocket and its exhaust stream. Comparing &lt;span class=&quot;math-container&quot;&gt;$m\Delta v^2$&lt;/span&gt; to this total change in energy yields a much more meaningful measure of efficiency than does comparing &lt;span class=&quot;math-container&quot;&gt;$m\Delta v^2$&lt;/span&gt; to the change in the rocket's energy.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;If we imagine a maglev train in a vacuum tube on Earth (i.e. no friction, no air resistance, no diminishing engine efficiency), would it get more and more energy efficient the more it is accelerated, because of the Oberth effect?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The Oberth effect isn't about efficiency so much as it is about effectiveness. An impulsive maneuver any along the rocket's orbit will instantaneously change velocity by the same amount, regardless of where the maneuver occurs. (For a non-impulsive burn, acceleration will be the same regardless of where the maneuver occurs.) The effectiveness of that change is the issue.&lt;/p&gt;&#xA;&lt;p&gt;There are bound to be inefficiencies in a maglev system, even with a train running in vacuum. These inefficiencies will inevitably vary with velocity. In other words, acceleration from a given amount of power will be a function of velocity.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-04-11T15:08:40.273" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4233" PostTypeId="2" ParentId="4229" CreationDate="2014-04-09T19:42:59.220" Score="8" Body="&lt;p&gt;The basis of the Oberth effect is that kinetic energy goes up at the square of velocity but a rocket burn provides the same delta-v no matter what speed the rocket is moving at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you borrow some velocity from a gravity well and then burn your rocket you're going faster as you exit the gravity well and thus lose less velocity on the way back out.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-04-09T19:42:59.220" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4234" PostTypeId="2" ParentId="2738" CreationDate="2014-04-09T20:42:12.270" Score="10" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29#Internal_structure.&quot; rel=&quot;noreferrer&quot;&gt;oblateness&lt;/a&gt; of Ceres seems to indicate water. Moreover &lt;a href=&quot;http://www.theverge.com/2014/1/26/5347762/dwarf-planet-ceres-is-spewing-water-into-space&quot; rel=&quot;noreferrer&quot;&gt;water geysers were recently detected by Herschel Space telescope&lt;/a&gt;. Hopefully we'll learn more when the Dawn spacecraft arrives February of 2015. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planetary chauvinists like to point out mass of the Main Belt is small compared to Earth or Mars. But most of a planet's mass is inaccessible. As we burrow down, pressure and temperature increases. Only a thin shell of a planet's surface mass can be reached. A planet's potential real estate and resources are better gauged by surface area than volume.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the total surface area of the asteroids far exceeds surface area of large moons and rocky planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, the entire &lt;strong&gt;volume&lt;/strong&gt; of an asteroid is accessible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://cseligman.com/text/planets/internalpressure.htm&quot; rel=&quot;noreferrer&quot;&gt;According to Seligman&lt;/a&gt; a body's internal pressure can be approximated by 3/8 π G g&lt;sub&gt;R&lt;/sub&gt;&lt;sup&gt;2&lt;/sup&gt;(1-(r/R)&lt;sup&gt;2&lt;/sup&gt;). Where R is distance from surface to center and r is distance from center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If my arithmetic's correct, the pressure of Ceres' center is about the same as the rock 2.7 km below earth's surface, about 1400 atmospheres. And the South African &lt;a href=&quot;http://en.wikipedia.org/wiki/TauTona_Mine&quot; rel=&quot;noreferrer&quot;&gt;TauTona mine&lt;/a&gt; goes as deep as 3.9 km underground. Thus it's possible to tunnel clear through Ceres. Of course the volume of smaller asteroids would be even more accessible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planetary chauvinists like to say planets can be terraformed. They like to imagine Martians walking in shirt sleeves under the Martian sky, breathing Martian air. In &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2014/02/terraforming-mars-vs-orbital-habs.html&quot; rel=&quot;noreferrer&quot;&gt;my opinion&lt;/a&gt; terraforming is impractical and a waste of resources. I believe living places beyond earth will be wholly enclosed, artificial environments. This will be true regardless whether the habs are asteroidal, lunar or martian.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another argument for planets is the need for gravity to stay healthy. In spinning habs, inertia in a rotating frame (aka centrifugal force) can provide acceleration to prevent bone and muscle atrophy as well as drain sinuses, enable flush toilets, showers etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The asteroids' very shallow gravity wells make beanstalks (aka Clarke Towers) more doable. If the asteroid has a healthy angular velocity $\omega$, even more so. Ceres' sidereal day is about 9 hours, an $\omega$ more than double, almost triple that of Mars and Earth. A &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2012/09/beanstalks-elevators-clarke-towers.html&quot; rel=&quot;noreferrer&quot;&gt;Ceres beanstalk&lt;/a&gt; could be as short as 706 kilometers and taper ratio using an ordinary substance like Kevlar would only be 1.02. A far, far less challenging project than an earth or even a Mars elevator. A Vesta elevator could be as short as 265 kilometers with a Kevlar taper ratio of 1.01. I believe asteroidal elevators could greatly reduce reaction mass needed for delta V. Thus the mass fractions mandated by Tsiolkovky's rocket equation become less of a challenge. Travel throughout the main belt could be less of a problem than travel between planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my answer is yes. Yes, colonizing Ceres is feasible. As well as colonizing much of the Main Belt.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="25" LastEditDate="2015-11-05T18:55:29.403" LastActivityDate="2015-11-05T18:55:29.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4235" PostTypeId="2" ParentId="4229" CreationDate="2014-04-09T21:01:35.233" Score="15" Body="&lt;p&gt;Kinetic energy is 1/2 mv&lt;sup&gt;2&lt;/sup&gt;. Here's my visualization:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0FnVU.jpg&quot; alt=&quot;1/2 mv2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you add to that velocity v, you get a bigger square:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1TRFj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here V&lt;sub&gt;b&lt;/sub&gt; is velocity imparted by your burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for example, if you accelerate a kilogram at rest to 1 meter/sec, you get .5 joules of kinetic energy. If you accelerate a kilogram moving 10 meters/sec to 11 meters/second, you get 10.5 joules of kinetic energy. An Oberth benefit of 10 joules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It &lt;em&gt;is&lt;/em&gt; harder to accelerate stuff moving fast. If you have a mass driver on the moon, Accelerating something to 1 km/s would take 100 times as much energy as accelerating it to .1 km/s. Or accelerating something to 4 km/s would take 16 times as much energy as 1 km/s. But on earth the difficulty is even greater as you have atmospheric drag. Atmospheric drag scales roughly with velocity cubed.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-04-09T21:01:35.233" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4238" PostTypeId="2" ParentId="3799" CreationDate="2014-04-10T01:17:08.167" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;For example, would cheaper &amp;amp; longer paths such as Hohmann transfers be favored, or will more direct (and therefore more expensive) trajectories be preferred?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Neither. Hohmann orbits are tangent transfer orbits between one circular orbit and another.  Asteroid orbits can be decidedly elliptical. The transfer orbit would probably still be more or less tangent to departure and destination orbits. But it's path might be a lot different than the 180 degree path we think of as Hohmann.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To illustrate here's a pic of possible tangent transfer orbits between a circular earth orbit and an elliptical asteroid orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/AEu4a.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most the possible tangent transfer orbits are pastel but I've darkened the red one. You can see the angle between departure and arrival is 100ª or 260º -- quite different from a Hohmann's 180 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also Hohmann orbits assume impulsive burns at periapsis and apoapsis of transfer orbit. Such impulsive burns are best done with high thrust chemical propellents. However chemical propellents don't have an exhaust velocity much over 4 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Keck Report&lt;/a&gt; suggests a solar electric propulsion vehicle that uses xenon propellant. Low thrust but an exhaust velocity around 30 km/s. With a high exhaust velocity, we can impart some delta V to an asteroid without a huge mass of propellent. But these low thrust burns can take a long time to accelerate an object. A slow steady burn can result in spiral trajectories rather than elliptical orbits.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For this question we narrow the source of material to the main asteroid belt and assume that resource needs to be brought within Earths SOI.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A bad assumption. To lower a main belt asteroid to a 1 A.U. perihelion would take around 4 km/s. Delta V budgets would be way too high to realize a profit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Keck report talks about near earth asteroids that only require about .2 km/s to be parked in lunar orbit. This is doable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's interesting that Chris Lewicki is one of the co-authors of the Keck report. Lewicki is the chief engineer for Planetary Resources.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;how important will it be for methods of transporting materials to Earth to become cheap?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Very.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the things that makes spaceflight so expensive is the &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2012/08/mf-is-mofo-tyranny-of-rocket-equation_21.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;tyranny of the rocket equation&lt;/a&gt;. With all propellent at the bottom of a deep gravity well, every spacecraft has a minimum delta V budget of around 8 or 9 km/s. But with a propellent source high on the slopes of earth's gravity well, the exponent in the rocket equation would be broken. This could make space travel much cheaper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water rich asteroids are potential sources of propellent. Water can be broken into hydrogen and oxygen, one of the better chemical bipropellents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, in fact, importing water rich asteroids is &lt;a href=&quot;http://www.planetaryresources.com/company/roadmap/#harvesting-water&quot; rel=&quot;nofollow noreferrer&quot;&gt;one of the first things Planetary Resources hopes to do&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-11-26T04:37:48.467" LastActivityDate="2015-11-26T04:37:48.467" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4239" PostTypeId="2" ParentId="2597" CreationDate="2014-04-10T08:01:30.217" Score="5" Body="&lt;p&gt;I found at SpaceMath ( &lt;a href=&quot;http://spacemath.gsfc.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spacemath.gsfc.nasa.gov&lt;/a&gt;  ) a &lt;a href=&quot;http://spacemath.gsfc.nasa.gov/Calculus/3Page24.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;document&lt;/a&gt; stating that the density of a planetary atmosphere is defined by the exponential function:&lt;br /&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$N(z) = N(0) e^{-z/H}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where H is the &lt;a href=&quot;http://astro.unl.edu/naap/scaleheight/sh_bg1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;scale-height&lt;/a&gt; of the gas.&lt;/p&gt;&#xA;&lt;p&gt;The amount of surface shielding for radiation arriving from 90° direction (vertical) is given by evaluating the integral of N(z), which results in:&lt;/p&gt;&#xA;&lt;p&gt;Equivalent Shielding = &lt;span class=&quot;math-container&quot;&gt;$N(0)H$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Values listed in that document are:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Mars: D = 0.020 kg/m3 x 11.1 km = 22 gm/cm2&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Earth: D = 1.2 kg/m3 x 8.5 km = 1,020 gm/cm2&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;We can extract:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Mars:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;N(0) = 0.020 kg/m3&lt;/li&gt;&#xA;&lt;li&gt;H = 11.1 km&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Earth:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;N(0) = 1.2 kg/m3&lt;/li&gt;&#xA;&lt;li&gt;H = 8.5 km&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Doing again the math with all steps:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;ES = &lt;span class=&quot;math-container&quot;&gt;$0.020 \frac{kg}{m^3} * 11.1 km$&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$0.020 \frac{kg}{m^3} * 11100 m$&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$0.020 \frac{kg}{m^2} * 11100$&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$222\frac{kg}{m^2}$&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$222000\frac{g}{m^2}$&lt;/span&gt; =  &lt;span class=&quot;math-container&quot;&gt;$\frac{222000}{10000}\frac{g}{cm^2}$&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$22.2\frac{g}{cm^2}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Earth&lt;/strong&gt;:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;ES =  &lt;span class=&quot;math-container&quot;&gt;$1.2 \frac{kg}{m^3}*  8.5 km$&lt;/span&gt;  = &lt;span class=&quot;math-container&quot;&gt;$1.2 \frac{kg}{m^3}*  8500 m$&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$1.2 \frac{kg}{m^2}*  8500 $&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$10200 \frac{kg}{m^2} $&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$10200000 \frac{g}{m^2} $&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$\frac{10200000}{10000} \frac{g}{cm^2} $&lt;/span&gt; = &lt;span class=&quot;math-container&quot;&gt;$1020 \frac{g}{cm^2} $&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Other values for comparison:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;onboard ISS: &lt;span class=&quot;math-container&quot;&gt;$ 10 \frac{g}{cm^2}$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo space suite: &lt;span class=&quot;math-container&quot;&gt;$ 0.1 \frac{g}{cm^2}$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;It's worth noting that a bare body on Mars' surface receives half the radiation of an astronaut onboard ISS.&lt;/p&gt;&#xA;&lt;p&gt;It's also worth noting that to get equivalent shielding of the whole Earth atmosphere by using a water shielding, only 10 meters thickness is required (given that water density is 1000 kg m3, compared to 1.2 kg/m3 for Earth atmosphere).&lt;/p&gt;&#xA;&lt;p&gt;(&lt;span class=&quot;math-container&quot;&gt;$R_{Earth}$&lt;/span&gt;  =  6378 km, &lt;span class=&quot;math-container&quot;&gt;$R_{Mars}$&lt;/span&gt; = 3374 km)&lt;/p&gt;&#xA;&lt;p&gt;In tabular format:&lt;/p&gt;&#xA;&lt;div class=&quot;s-table-container&quot;&gt;&#xA;&lt;table class=&quot;s-table&quot;&gt;&#xA;&lt;thead&gt;&#xA;&lt;tr&gt;&#xA;&lt;th&gt;&lt;/th&gt;&#xA;&lt;th&gt;N0 (kg/m3)&lt;/th&gt;&#xA;&lt;th&gt;H (km)&lt;/th&gt;&#xA;&lt;th&gt;ES (g/cm2)&lt;/th&gt;&#xA;&lt;th&gt;Received radiation (normalized to  Earth)&lt;/th&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/thead&gt;&#xA;&lt;tbody&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Mars&lt;/td&gt;&#xA;&lt;td&gt;0.020&lt;/td&gt;&#xA;&lt;td&gt;11.1&lt;/td&gt;&#xA;&lt;td&gt;22.2&lt;/td&gt;&#xA;&lt;td&gt;x45&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Earth&lt;/td&gt;&#xA;&lt;td&gt;1.2&lt;/td&gt;&#xA;&lt;td&gt;8.5&lt;/td&gt;&#xA;&lt;td&gt;1020&lt;/td&gt;&#xA;&lt;td&gt;1&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Moon (in spacesuite)&lt;/td&gt;&#xA;&lt;td&gt;-&lt;/td&gt;&#xA;&lt;td&gt;-&lt;/td&gt;&#xA;&lt;td&gt;0.1&lt;/td&gt;&#xA;&lt;td&gt;x10200&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;ISS&lt;/td&gt;&#xA;&lt;td&gt;-&lt;/td&gt;&#xA;&lt;td&gt;-&lt;/td&gt;&#xA;&lt;td&gt;10&lt;/td&gt;&#xA;&lt;td&gt;x102&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/tbody&gt;&#xA;&lt;/table&gt;&#xA;&lt;/div&gt;" OwnerUserId="3075" LastEditorUserId="3075" LastEditDate="2022-05-18T09:13:30.173" LastActivityDate="2022-05-18T09:13:30.173" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="4240" PostTypeId="2" ParentId="1067" CreationDate="2014-04-10T08:12:26.607" Score="1" Body="&lt;p&gt;It depends on amount of time astronauts spend in space, as DNA changes are as more probable as more radiation the body receives, and spacesuits, and habitat themselves, offer much less radiation shieldeing than earth atmosphere: ISS radiation shielding is 1/100 of Earth atmosphere shielding (10g/cm2 vs 1000 g/cm2); Apollo space suits offered just 1/10000 of Earth shielding (don't know about current spacesuits)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2597/what-if-any-reduction-in-radiation-exposure-does-the-atmosphere-of-mars-provid/4239#4239&quot;&gt;What, if any, reduction in radiation exposure does the atmosphere of Mars provide?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3075" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-04-10T08:12:26.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4242" PostTypeId="1" AcceptedAnswerId="4243" CreationDate="2014-04-10T18:54:23.077" Score="4" ViewCount="251" Body="&lt;p&gt;December 2014 is the scheduled date for the Exploration Flight Test-1 (EFT-1) mission of the Orion space capsule. It will be launched on a Delta IV Heavy rocket as a surrogate for the unavailable SLS booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will it carry a Launch Escape System/Launch Abort System (LES/LAS) on this mission? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="26446" LastEditDate="2018-11-06T03:54:37.453" LastActivityDate="2018-11-06T03:54:37.453" Title="Will the EFT-1 mission for Orion carry an escape tower?" Tags="|sls|orion-spacecraft|launch-escape-system|delta-iv-heavy|tractor-les|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4243" PostTypeId="2" ParentId="4242" CreationDate="2014-04-10T19:15:12.550" Score="4" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.nasaspaceflight.com/2013/12/heat-shield-eft-1-orion-ksc/&quot; rel=&quot;nofollow&quot;&gt;NASASpaceflight.com&lt;/a&gt; article from December 4, 2013, yes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Another element that will heading uphill on EFT-1 will be an inert&#xA;  Launch Abort System (LAS).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Most of the LAS won’t be active for the flight – not least because&#xA;  there will be no humans on board the Orion.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;However, the jettison motor will be active, allowing for a separation&#xA;  testing during the ascent phase. As such, this will be a nominal&#xA;  detachment of the LAS when it is no longer required during the flight&#xA;  into space, as opposed to an actual abort test.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;“The LAS will be the first actively controlled launch escape system&#xA;  ever flown, and it incorporates a throttleable solid-rocket motor for&#xA;  attitude control, a novel reverse-thrust abort motor, advanced solid&#xA;  propellants, and lightweight composite materials in the abort motor&#xA;  case and fairing,” noted LAS project manager for NASA, Kevin Rivers.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The first ascent abort test of the LAS is scheduled for around 2019,&#xA;  after the uncrewed Exploration Mission -1 (EM-1), but in plenty of&#xA;  time ahead of the first crewed mission where it will be required,&#xA;  namely Exploration Mission -2 (EM-2).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Other sources confirm this, for example the &lt;a href=&quot;http://www.universetoday.com/107564/nasa-pressing-towards-fall-2014-orion-test-flight-service-module-complete/&quot; rel=&quot;nofollow&quot;&gt;Universe Today article on NASA Pressing Towards Fall 2014 Orion Test Flight – Service Module Complete&lt;/a&gt; from January 22, 2014.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More about the EFT-1 LAS can be read in the &lt;a href=&quot;http://www.nasaspaceflight.com/2013/02/atk-launch-abort-system-arrives-ksc-eft-1/&quot; rel=&quot;nofollow&quot;&gt;NASASpaceflight.com article on ATK Launch Abort hardware arrives at KSC ahead of EFT-1&lt;/a&gt; article on the same website.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-10T19:20:59.113" LastActivityDate="2014-04-10T19:20:59.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4244" PostTypeId="2" ParentId="1747" CreationDate="2014-04-11T00:27:07.653" Score="0" Body="&lt;p&gt;From the article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It's been derisively referred to as the &quot;Senate Launch System&quot; in a lot of places, because it's being designed and built in a very distributed manner, with work being spread across many different contractors in many different states.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That says it for me. The SLS is a make work program that provides employment in areas the senators represent. Also known as pork.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-04-11T00:27:07.653" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4245" PostTypeId="2" ParentId="1747" CreationDate="2014-04-11T12:59:33.183" Score="2" Body="&lt;p&gt;Currently there are plenty of launch system available in the world for the 10t - 20t range, but none which can get more than 23 tons to low earth orbit. The SLS, however, will have a LEO capacity of 70t - 130t. That would open up a whole new market segment on which neither ESA nor Roskosmos can offer anything. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is, however, another player who emerges on the market for launch systems and who is currently developing a rocket with LEO capacity in that scale: China with its Long March 9. Leaving that market exclusively to the Chinese could be a dangerous move from a geopolitical point of view. The Chinese having a world-wide monopoly on launching 100t chunks into orbit would be quite a problem for US international interests.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-04-11T13:29:04.760" LastActivityDate="2014-04-11T13:29:04.760" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4246" PostTypeId="2" ParentId="2638" CreationDate="2014-04-11T15:41:03.580" Score="2" Body="&lt;p&gt;I've heard it said that IF we get fusion going, lunar regolith would have a specific energy less than low grade coal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hobbe's answer gives some good cites so I'll use his numbers. 3GWh - that's 3 * 10&lt;sup&gt;9&lt;/sup&gt; watts * 3600 seconds which is 1.08e13 joules. 1 to 15 billion kg of rock gives a specific energy of 10800 to 720 joules per kilogram. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Energy_density#Energy_densities_of_common_energy_storage_materials&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; gives the specific energy of coal as 24MJ/kg. 10800J/kg is about .00045 the specific energy of coal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure these He3 deposits woud be worth mining if they were on earth's surface.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-04-11T15:41:03.580" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4247" PostTypeId="2" ParentId="2813" CreationDate="2014-04-11T19:01:24.397" Score="9" Body="&lt;p&gt;I believe the use of 'former' is representative of both their employment status and their licensing/qualification status.  For instance a internet search for &quot;former pilot&quot; (including quotes) finds several references people who are either former employees and/or who have lost their license. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;John Glenn is referenced as a &quot;former astronaut&quot; until the &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-95&quot; rel=&quot;noreferrer&quot;&gt;STS-95 mission&lt;/a&gt;, where he re-qualifies and is considered a current or active astronaut.  Because his qualification for the 1998 flight is now expired he is again a &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/htmlbios/glenn-j.html&quot; rel=&quot;noreferrer&quot;&gt;&quot;former&quot;&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2014-04-11T19:01:24.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4250" PostTypeId="1" AcceptedAnswerId="4252" CreationDate="2014-04-12T18:22:42.937" Score="16" ViewCount="16208" Body="&lt;p&gt;The International Space Station is constantly losing orbital energy due to atmospheric drag. How does station-keeping work for the ISS? What sort of propulsion system is used to keep it in the desired orbit, and where are these thrusters located? I imagine it could be as simple as pressurized nitrogen.&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2018-09-28T13:33:07.873" Title="What propulsion methods does the ISS use for station-keeping?" Tags="|iss|propulsion|station-keeping|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4251" PostTypeId="1" CreationDate="2014-04-12T14:36:13.860" Score="3" ViewCount="92" Body="&lt;p&gt;When an astronaut goes to outer space, what can you measure? I'm thinking about the person's safety, so this is what I got so far:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Oxygen level&lt;/li&gt;&#xA;&lt;li&gt;Heart beat&lt;/li&gt;&#xA;&lt;li&gt;Suit pressure&lt;/li&gt;&#xA;&lt;li&gt;Blood pressure&lt;/li&gt;&#xA;&lt;li&gt;Solar radiation&lt;/li&gt;&#xA;&lt;li&gt;Body temperature&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.universetoday.com/77070/how-cold-is-space/&quot; rel=&quot;nofollow&quot;&gt;Suit temperature&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Is there any other relevant constant that I could measure for safety?&lt;/p&gt;&#xA;" OwnerDisplayName="Francisco Presencia" LastEditorUserId="168" LastEditDate="2014-04-13T05:48:33.390" LastActivityDate="2014-04-13T14:08:51.010" Title="What parameters should be monitored during spacewalks?" Tags="|radiation|life-support|eva|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4252" PostTypeId="2" ParentId="4250" CreationDate="2014-04-12T18:33:20.720" Score="20" Body="&lt;p&gt;Biprop attitude control thrusters using UDMH and N2O4 in a docked &lt;a href=&quot;http://www.spaceref.com/iss/spacecraft/progress.html&quot;&gt;Progress vehicle&lt;/a&gt; are used to reboost ISS using about &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_unmanned_spaceflights_to_the_International_Space_Station&quot;&gt;four Progress vehicles a year&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More recently, ESA's ATVs have also been doing reboosts using &lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/bipropellant-thrusters/220n-atv-thrusters.html&quot;&gt;MMH/N2O4 thrusters&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the order of &lt;a href=&quot;http://www.quora.com/International-Space-Station/How-does-the-International-Space-Station-maintain-its-orbit-and-what-propellant-does-it-use&quot;&gt;2 m/s per month is required&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-04-12T21:06:35.893" LastActivityDate="2014-04-12T21:06:35.893" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4254" PostTypeId="2" ParentId="4250" CreationDate="2014-04-13T10:58:19.677" Score="12" Body="&lt;p&gt;In addition to Progress and ATV vehicles, docked at the aft end of &lt;a href=&quot;http://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot;&gt;Zvezda&lt;/a&gt;, using their main engines to boost the orbit, the Zvezda module also has engines that can be used. In fact, the Progress cargo vehicles can refuel the Zvezda module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The two main engines on Zvezda can be used to raise the station's altitude. This was done on April 25, 2007. This was the first time the engines had been fired since Zvezda arrived in 2000.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The US plans to send a &lt;a href=&quot;http://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket&quot;&gt;VASIMR&lt;/a&gt; engine by Ad Astra, to the station to test its ability to provide thrust to the station for orbital maneuvers.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-04-13T10:58:19.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4256" PostTypeId="2" ParentId="4251" CreationDate="2014-04-13T14:08:51.010" Score="2" Body="&lt;p&gt;Just kidding: You should be monitoring the amount of water in the astronaut's helmet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not just kidding: Modern spacesuits are complex devices. You should be monitoring for signs of problems with the suit. The manufacturer of the spacesuit will have all kinds of fault detection, isolation, and recovery (FDIR) mechanisms built into the suit. You should be monitoring each and every one of these. You should also be paranoid and assume that the FDIR isn't perfect. That means you should be monitoring things such as the CO&lt;sub&gt;2&lt;/sub&gt; level. You should monitor anything and everything you can think of that might be a sign of a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consumables are another obvious set of things you are missing. What if the astronaut's metabolism is high? If you aren't monitoring the amount of stored breathing atmosphere you won't know the astronaut will die six hours into a planned eight hour EVA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, you need to monitor the voice loop. Despite your best efforts to monitor &lt;em&gt;everything&lt;/em&gt;, there will inevitably be some things you miss. When the astronaut says &quot;my helmet is filling with water&quot; you need to work feverishly to get that astronaut back inside ASAP.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-04-13T14:08:51.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4257" PostTypeId="1" AcceptedAnswerId="4259" CreationDate="2014-04-13T15:50:24.517" Score="11" ViewCount="1148" Body="&lt;p&gt;And how tenable is this distinction now that a third space agency carries out manned flights (China), and others are poised to follow?&lt;br&gt;&#xA;I can think of no other profession/job title where this distinction is made. Also, with the increased cooperation between spacefaring nations, the lines are getting blurred: what do we call an American who's employed by NASA, but is trained in Russia for a flight to the ISS aboard a Soyuz? Is it logical to have some ESA personnel be called astronauts, and others cosmonauts? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="49" LastEditDate="2014-04-14T12:26:09.103" LastActivityDate="2014-04-14T12:26:09.103" Title="Why do we differentiate between astronauts and cosmonauts?" Tags="|terminology|astronauts|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4258" PostTypeId="1" CreationDate="2014-04-13T19:30:59.793" Score="7" ViewCount="1940" Body="&lt;p&gt;Given the life expectancies of all its components (mechanical, electrical, electronic, etc.), how long could the Curiosity rover reasonably be expected to continue performing useful science in an extended mission, and what are the most likely candidates for mission-terminating failures?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the Spirit/Opportunity experiences, one might expect to see mechanical failures of various sorts high on the list. Would that be a fair statement or are there other factors at play?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its on board power plant apparently won't exhaust its fuel for a very long time, although output would gradually decrease. Presumably, the output will be sufficient to perform useful science for many years. However, at some point, things will eventually start to wear out (e.g. wheel bearings), fail (e.g. stuck bits in computer memory) or run out (carrier gas for the chromatograph?). Some of these things might only impact specific experiments or degrade overall mission capability. What could bring the mission to a definite end and what would be the potential time frame of its occurrence?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-04-14T14:39:05.910" Title="How long might the Curiosity Mars rover last, and what would be the most likely cause to end its mission capability?" Tags="|mars|rovers|curiosity|" AnswerCount="2" CommentCount="3" ClosedDate="2014-04-14T15:05:54.947" ContentLicense="CC BY-SA 3.0" />
  <row Id="4259" PostTypeId="2" ParentId="4257" CreationDate="2014-04-14T07:59:34.670" Score="5" Body="&lt;p&gt;As David pointed out, no language has a long established word for a space explorer, because space exploration is incredibly new.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over time it may be that we end up using one word (which may be from the Americans, Russians, Chinese, Indians or whoever) but the simple answer is that it really doesn't matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the moment it &lt;strong&gt;slightly&lt;/strong&gt; helps to differentiate between countries, giving us an expectation of type of craft, suit, language even., but it doesn't matter either way with collaborations such as the ISS.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2014-04-14T07:59:34.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4260" PostTypeId="2" ParentId="4258" CreationDate="2014-04-14T10:15:37.200" Score="3" Body="&lt;p&gt;Curiosity was designed to last for a &lt;a href=&quot;http://www.space.com/17963-mars-curiosity.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;two-year primary mission&lt;/a&gt;. &lt;a href=&quot;http://www.nasa.gov/jpl/msl/mars-rover-curiosity-20131220/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Experience&lt;/a&gt; has shown that the wheels are a &lt;a href=&quot;https://space.stackexchange.com/questions/3187/curiosity-why-did-they-reinvent-the-wheel-instead-of-using-pressure-tires?rq=1&quot;&gt;critical component&lt;/a&gt;. They are accumulating damage more quickly than expected. This has prompted some &lt;a href=&quot;http://www.nasa.gov/jpl/msl/update-20140129/&quot; rel=&quot;nofollow noreferrer&quot;&gt;route changes&lt;/a&gt;, driving over sand instead of rocks. They're also &lt;a href=&quot;http://www.nasa.gov/jpl/msl/traverse-map-20140219/&quot; rel=&quot;nofollow noreferrer&quot;&gt;trying driving in reverse&lt;/a&gt; to limit wheel damage.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-04-14T10:15:37.200" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4261" PostTypeId="2" ParentId="4258" CreationDate="2014-04-14T12:41:57.720" Score="3" Body="&lt;p&gt;As Hobbes explained, the driving system of Curiosity already shows some wear, so it is likely that the MSL rover will one day be unable to move. That, however, does not necessarily mean the end of the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Spirit rover became unable to move in 2009, but that did not mean that its scientific mission ended. It was reclassified as a &quot;stationary science platform&quot; and still returned scientific data until 2010.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on where Curiosity will eventually break down and which instruments will still be functional then, it could make sense to keep in contact with it to collect environmental data from the surrounding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Contact to Spirit was lost about a year after its breakdown. The exact reason why Spirit stopped responding isn't certain, but the generally accepted theory is that the orientation in which the rover became immobile wasn't ideal for the solar collectors to collect enough energy to survive the Martian winter, resulting in damage to the rover due to insufficient heating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can not really happen to Curiosity, because it does not require solar power. It is powered by radioisotope thermoelectric generators which have a minimum lifetime of 14 years. During various unmanned probe missions, RTGs have proven to be extremely reliable even beyond their warranty time. The Voyager probes were launched in the 70s and their RTGs still produce enough energy to communicate with Earth to this day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless Curiosity loses contact to Earth due to a defect in the computer- or communication system (which both have redundancies), it could stay responsive for a very long time. But the question is whether or not its scientific value as a stationary research platform will be considered high enough to justify the funding for the ground equipment and personnel back on Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The end of Curiosity will likely not be the failure of any mechanical part but rather the failure of a JPL manager to persuade a committee to grant further funds for the program.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-04-14T14:39:05.910" LastActivityDate="2014-04-14T14:39:05.910" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4262" PostTypeId="1" AcceptedAnswerId="65385" CreationDate="2014-04-14T17:27:14.167" Score="6" ViewCount="2038" Body="&lt;p&gt;In the prelude for SpaceX CRS-3 mission, the failure of a Multiplexer-Demultiplexer (MDM) on the International Space Station (ISS) seems to be a real concern for launching.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the pre-launch press conference, they discussed that there are several types of MDM's that they have on station. Some seem to be internal, some external. Mike Suffredini seemed to indicate that they could use different parts of different MDM's to make up a needed type.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the different types of MDM's, and how compatible/pluggable are they?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-04-21T20:34:21.020" LastActivityDate="2024-02-07T19:10:13.723" Title="What are the MDM's on the ISS, and how many different kinds are there?" Tags="|iss|flight-computer|us-segment|mdm|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4264" PostTypeId="1" AcceptedAnswerId="4265" CreationDate="2014-04-14T18:34:54.900" Score="4" ViewCount="300" Body="&lt;p&gt;What was the first commercial space vehicle, that is not the property of a government entity; to make a delivery of goods to a community (&lt;em&gt;one or more&lt;/em&gt;) in Earth orbit that was at the time occupied by human(s) who arrived by a separate means of transportation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking for the first time, &quot;Bobs Express Space Delivery&quot; was hired to and succeeded in making a delivery. Bob loads the goods into a vehicle he owns and operates, and delivers to someone in space.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will give 50 bonus points for an answer that includes a copy of the delivery receipt, signed by a member of the space community that received the delivery. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="12102" LastEditDate="2021-09-05T23:10:51.930" LastActivityDate="2021-09-05T23:10:51.930" Title="When was the first commercial delivery to an orbiting space station?" Tags="|launch|commercial|spaceflight-firsts|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4265" PostTypeId="2" ParentId="4264" CreationDate="2014-04-14T18:46:45.113" Score="7" Body="&lt;p&gt;I think the only two such examples would be Dragon and Cygnus vehicles. In those cases, NASA paid for the services, but does not own the launcher or vehicle (as it did in the case of Saturn 1B/V (Skylab) or Space Shuttle).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Dragon's first mission to the ISS was May 25, 2012, as a COTS 2/3 Demo flight, and delivered cargo.  SpaceX owns the launcher, the Dragon vehicle, and received payment for the flight. In fact, the various used Dragons, are displayed at SpaceX headquarters in Hawthorne, CA, since they belong to SpaceX after the mission, not NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cygnus's first such flight, also a demo flight, was Sept 18, 2013.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To disqualify other possibilities:&#xA;Progress/Soyuz - Launcher and space craft owned by Russian/Soviet government.&#xA;HTV - Launcher and space craft owned and provided by JAXA, Japanese space agency.&#xA;ATV - Launcher (owned by Arianespace) and vehicle owned by ESA, European space agency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo capsule, on a Saturn 1B resupplied the Skylab missions, but they too were government owned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not sure the Chinese system has progressed far enough yet, in terms of a real station, but Tiadong would be the same as Soyuz in this context anyway.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-04-14T18:46:45.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4266" PostTypeId="2" ParentId="4262" CreationDate="2014-04-14T19:00:46.963" Score="5" Body="&lt;p&gt;Note that this applies only to the U.S. section of the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 3 different types of MDMs used on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tier 1: Top level control. These are &lt;em&gt;inside&lt;/em&gt; the ISS and they have 3 on board, but only one is active.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tier 2: There are 5 different types of these, only one is external to the ISS... it is called the EXT MDM. There are 2 of every tier 2 MDM, one primary and a backup.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tier 3: I think there are 5 of these as well, these are sensor/component level control, there are over 30 of these in the ISS if I'm not mistaken, some are inside, some outside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a compatibility standpoint the MDMs come in 3 different sizes, which hold 4, 10, or 16 cards. Cards and the entire assembly is easy to move around and essentially slide in and out. My understanding is that they are generally specialized cards in each MDM and cannot be swapped for other good ones, hence the backups for each system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one that failed recently is EXT-2 MDM (Tier 2) with a size of 16 (not all are used).  A spare EXT MDM is &lt;em&gt;inside&lt;/em&gt; the ISS, but will have to be mounted outside before it would be useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2014/04/imminent-spacewalks-external-mdm-failure-iss/&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; is an excellent read regarding the failure of the EXT-2 MDM and discusses the options available to the flight crew.&lt;/p&gt;&#xA;" OwnerUserId="3061" LastActivityDate="2014-04-14T19:00:46.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4267" PostTypeId="2" ParentId="4262" CreationDate="2014-04-14T19:13:05.490" Score="4" Body="&lt;p&gt;At least two from the perspective of CPU. The &quot;Enhanced MDMs&quot; have a better CPU and better memory than do the &quot;Standard MDMs&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are at least four from the perspective of structure. The MDMs have cards that talk to the 1553 bus and to various &quot;dumb&quot; devices that don't use the 1553 bus for communication.  How many cards? The standard MDMs come in three different sizes. Some have room for only four cards (an MDM-4), some for ten cards (an MDM-10), and others, sixteen cards(an MDM-16).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another variation on the structure is the chassis, and these differ between MDMs mounted externally versus those mounted inside the ISS. The external MDMs are designed for replacement by a suited astronaut while the internal ones are designed for replacement by a shirt-sleeved astronaut. That means different attachment mechanisms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What goes into those MDMs varies a lot. The cards on one MDM-16 will differ from those on another. (Stupid analog devices!) An MDM supposedly is an Orbital Replaceable Unit, the lowest level of object an astronaut is expected to be able to replace. That however would require keeping too many spares, one for each specific MDM. Just as hobbyists can &quot;build&quot; their own computers at home, astronauts can &quot;build&quot; an MDM in the work area. Find the right chassis, add the right cards, and presto! there's the replacement MDM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2014/04/imminent-spacewalks-external-mdm-failure-iss/&quot; rel=&quot;nofollow&quot;&gt;Spacewalk set to fix external MDM failure on ISS – CRS-3 to launch Monday&lt;/a&gt; at nasaspaceflight.com&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.militaryaerospace.com/articles/print/volume-9/issue-4/news/honeywell-engineers-enhance-space-station-computers.html&quot; rel=&quot;nofollow&quot;&gt;Honeywell engineers enhance Space Station computers&lt;/a&gt; at militaryaerospace.com (old!)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www51.honeywell.com/aero/common/documents/MDMs.pdf&quot; rel=&quot;nofollow&quot;&gt;MDMs&lt;/a&gt; at honeywell.com (Honeywell manufactures the Space Station MDMs).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-04-14T19:18:34.300" LastActivityDate="2014-04-14T19:18:34.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4269" PostTypeId="1" AcceptedAnswerId="4270" CreationDate="2014-04-14T22:17:32.223" Score="7" ViewCount="539" Body="&lt;p&gt;Does building a vessel such that it can maintain a breathable atmosphere in a vacuum (like all our manned space vessels) also makes this vessel watertight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, why?&lt;/p&gt;&#xA;" OwnerUserId="122" LastActivityDate="2014-04-15T22:12:08.600" Title="Does building a vessel to work in vacuum also make it watertight?" Tags="|life-support|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4270" PostTypeId="2" ParentId="4269" CreationDate="2014-04-14T23:10:17.980" Score="11" Body="&lt;p&gt;At a first glance yes. If the inner crew compartments with breathable atmosphere are supposed to be air-tight to the pressure difference of at least one 1 atmosphere, and the hull is a rigid structure that's not meant to expand with change of pressure to within this 1 atmosphere, then it's fairly reasonable to assume the same pressure difference could be sustained by it in the opposite direction (where outside pressure is larger to the inner pressure for up to the same 1 atmosphere, or ~ 100 kPa, or ~ 14,7 psi, or the depth of ~ 10 m of water at sea-level Earth, where the Earth's atmosphere itself would equalize the pressure and the 10 m of water would increase external pressure to plus one atmosphere).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this is not necessarily the case, and vessel could be constructed to withstand a positive pressure (where inside pressure is bigger) better than the negative pressure (where the outside pressure is bigger), or even intentionally designed to expand with increased internal pressure, like some inflatable / expandable modules, e.g. Bigelow Aerospace's &lt;a href=&quot;http://www.bigelowaerospace.com/genesis-1.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Genesis 1&lt;/a&gt; and &lt;a href=&quot;http://www.bigelowaerospace.com/genesis-2.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Genesis 2&lt;/a&gt; expandable space habitat technology. These would clearly collapse under larger external than internal pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another concern with diving space vessels into the waters, if they're not designed for it, could be exposed electrical wiring, water soluble or water reactive materials used on its exterior, or pressure of water flow (waves, currents, vortices,...) damaging and/or bending equipment. And if their buoyancy wouldn't stabilize at the depth that it could still withstand external pressure, it could sink beyond its crash depth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your space faring vessel could experience problems due to the change of direction of the positive to negative pressure, and the properties of the water body, such as density, electrical conductivity, being a solvent to some materials, even facilitate corrosion and lose heat faster due to convective heat transfer. And the mass of this water your vessel would displace (buoyancy) would have to be roughly equal to the mass of your vessel, to prevent it sinking too deep and into external pressure it couldn't withstand any longer and implode. All of which are problems that are largely absent (or altogether different) than in the vacuum of outer space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in reality, no. Not unless they're specifically designed to land on water, like e.g. &lt;a href=&quot;https://space.stackexchange.com/q/2757/49&quot;&gt;the crew capsule of the Soyuz spacecraft&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-04-15T13:57:33.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4271" PostTypeId="1" CreationDate="2014-04-15T04:44:32.353" Score="4" ViewCount="130" Body="&lt;p&gt;How are sleeping and cognition in microgravity, with rapid changes in magnetic field, different from the normal human life on Earth?&lt;/p&gt;&#xA;" OwnerUserId="980" LastEditorUserId="49" LastEditDate="2014-04-15T09:58:13.810" LastActivityDate="2014-04-15T09:58:13.810" Title="What is the impact of micro-G environment and magnetic field in LEO on blood flow and brain function?" Tags="|microgravity|health|medical|magnetic-field|pressure-gradient-loss|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4272" PostTypeId="5" CreationDate="2014-04-15T11:41:35.980" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-15T11:41:35.980" LastActivityDate="2014-04-15T11:41:35.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4273" PostTypeId="4" CreationDate="2014-04-15T11:41:35.980" Score="0" Body="Questions about body and intracranial blood pressure gradient loss as a consequence to human and animal cardiovascular health, their cognitive function and well-being due to staying in microgravity environment and loss of constant acceleration, and measures taken to combat adverse effects and possible long-term consequences to their cardiovascular system due to prolonged stay and adjustment to microgravity conditions." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-15T11:48:40.077" LastActivityDate="2014-04-15T11:48:40.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4274" PostTypeId="2" ParentId="4269" CreationDate="2014-04-15T22:12:08.600" Score="6" Body="&lt;p&gt;Axiomatically, yes, an airtight craft is also watertight, since the molecules of water are less able to flow than those of air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current spacecraft are all rated for at least 1 bar (100 kPa) internal over external pressure difference. All are generally considered airtight (though all leak slightly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A terminology clarification: by overpressure, I mean an internal pressure higher than the external pressure; by underpressure, I mean the internal pressure is lower than the external. In the differential I use + to mean greater internal and - to mean greater external.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are, however, additional considerations. Direction of water resistance, direction of pressure, and duration of exposure all are relevant considerations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since spacecraft at surface generally are at +0 Bar relative (i.e.: 1 bar external and 1 bar internal) pressure, and at altitude are at +1 Bar, the bracing is all set to keep the pressure in. Certain fittings have in fact failed with less than -1 bar underpressure on apollo capsules. Note that water pressure rapidly climbs with depth in 1G - each 10m depth generates roughly 1 bar external pressure. Any Apollo, Gemini, or Mercury capsule was supposed to be able to withstand 0.5 bar under pressure for a short period during landing, and the capsule was held afloat by large balloons. And, with a singular exception, this worked. And the Apollo capsule flown was capable of ±0.8 Bar relative during pressure tests.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, many systems on the spacecraft are external to the pressure vessel, and while shielded from ram air pressures on launch, are not shielded from ambient static air pressure. Some of these are potentially damaged by water in quantity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pressure vessels designed for keeping water in have to deal with a mass thousands of times greater than those for air. So, an internal water environment capsule (say, for dolphins) would need to be braced differently for acceleration, and thus an air-design capsule cannot safely be considered watertight during launch, even if it would hold the full mass internally on the pad. Likewise, for movement through water, the forces are considerably higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One further consideration is that some of the seals are not viable for water environments (internal or external) except for very short durations due to chemical considerations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, practically, a spacecraft is not a viable submersible watercraft as an axiomatic condition, but is capable of preventing water intrusions within a reasonable range of exposures.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-04-15T22:12:08.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4275" PostTypeId="1" AcceptedAnswerId="4280" CreationDate="2014-04-16T07:07:15.493" Score="5" ViewCount="545" Body="&lt;p&gt;I am a learner in this field. I have a doubt after watching movie &quot;Gravity&quot;. In that movie, an explosion occurs in a spacecraft and the particles of that vehicle move so faster than a bullet, (&lt;em&gt;as mentioned by George in that movie&lt;/em&gt;) from a very long distance to the spot where the astronauts (&lt;em&gt;actors&lt;/em&gt;) crew members are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is will those objects (&lt;em&gt;or parts of spacecraft&lt;/em&gt;) stops/slow down ever?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I think right now is they don't stop because when Sandra (&lt;em&gt;actress&lt;/em&gt;) goes into rotations, she just doesn't stop until George (&lt;em&gt;actor&lt;/em&gt;) helps her out. &lt;/p&gt;&#xA;" OwnerUserId="82" LastEditorUserId="415" LastEditDate="2014-09-26T12:14:51.860" LastActivityDate="2014-09-26T12:14:51.860" Title="Moving object in space" Tags="|orbit|atmospheric-drag|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4276" PostTypeId="2" ParentId="2443" CreationDate="2014-04-16T09:21:07.243" Score="6" Body="&lt;p&gt;From the technical definition of drugs, and your definition of abuse I can say I've been told of alcohol 'abuse' on the ISS. Abuse is a strong word, though; a celebratory drink was had when there was a crew change a while back; not what I would call abuse but it was 'taken when not needed'. There's even a picture out there on the internet somewhere of the crews at the time of the changes with the bottle! (I believe it was cognac.)&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="3291" LastEditDate="2014-05-17T21:44:11.597" LastActivityDate="2014-05-17T21:44:11.597" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4277" PostTypeId="2" ParentId="3647" CreationDate="2014-04-16T09:36:03.973" Score="2" Body="&lt;p&gt;There are a number of terrestrial uses for plasma, which could be adapted for space purposes; these could theoretically harvest the required plasma from the Van Allen belts. There are even a few terrestrial uses for plasma that have been suggested as future space technologies if some further development is made. Have a look into plasma windows (used in electron beam welding), they have the potential to be used as airlocks that hold the pressure differential but allow the passage of higher momentum objects - such as humans in space suits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second thought that springs to mind is fusion. Magnetic fields can be used to contain the plasma used in fusion, I would be hesitant to suggest that the plasma in the Van Allen belts has the right properties, but there may be a cross of with the technologies that can allow some form of energy exchange from the plasma to a spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your spacecraft were to pass through the south Atlantic anomaly (where the highest concentration of anti-protons has been found I believe), I would imagine you could mine the antimatter. Of greater concern would be storage and eventual return to Earth (if that was the intention of your question). Having antimatter held in a magnetic bottle brought back to Earth by reentry would not only be a scientific challenge, but the political implications of what could happen if something went wrong would be a massive barrier to any sort of mission like this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few links you might be interested in:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Plasma_window&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Plasma_window&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetic_confinement_fusion&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Magnetic_confinement_fusion&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1142" LastEditDate="2015-12-30T01:29:55.477" LastActivityDate="2015-12-30T01:29:55.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4278" PostTypeId="1" AcceptedAnswerId="4285" CreationDate="2014-04-16T09:40:36.257" Score="9" ViewCount="337" Body="&lt;p&gt;I read on the &lt;a href=&quot;http://www.planetary.org/blogs/casey-dreier/2014/0415-nasas-burden-of-success-and-the-future-of-opportunity.html&quot;&gt;Planetery Society blog&lt;/a&gt; that it costs USD 25 million this year to maintain the Opportunity rover on Mars and the LRO Lunar orbiter. I would like to see a break down of those budgets. I imagine that the only material thing involved is communication, and I'd guess that the LRO wouldn't even have to use the Deep Space Network, and some skilled planning of the movements. USD 25 million p.a. seems ridiculously much. Where is the hog that bloats this budget?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think it costs USD 25 million/year to maintain two communication satellites in geosynchronous orbit (or does it?) so what is the big difference?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2014-04-16T17:05:10.840" LastActivityDate="2014-10-01T16:40:24.143" Title="What causes the costs of operating an existing planetary mission?" Tags="|mars|nasa|planetary-science|economics|maintenance|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4279" PostTypeId="2" ParentId="3456" CreationDate="2014-04-16T10:33:36.610" Score="3" Body="&lt;p&gt;So a quick bit of research has told me the highest lift chopper is the Mil Mi-26. This chopper has the highest lift of any chopper to go into production so it's probably going to be a likely candidate for capturing your as-heavy-as-possible reentry craft. This chopper can lift 20 metric tons - for comparison you'd need 24 of them to lift the ISS! (Although the chopped holds the record for being able to lift 62 tons to 2000m - that'll mean you only need 8 to lift the ISS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When talking about mass savings it's very, very dependent on the reentry vehicle in question. If it's a manned mission it would certainly have redundancies which multiply the mass required. A parachute and spashdown would probably be the closest approximation in mass, in that both require a parachute to reduce the speed and the rest is on the recovery side. All other system, retro rockets etc are much more massive, they often require bespoke propulsion systems that aren't used at any other time of the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reentry accuracy and recovery method both feed into one another. If your reentry footprint size is too large you wont be able to use this sort of method. It may be that the mass savings you get from not requiring retro rockets are spent on hardware to allow a more accurate reentry making this method not worthwhile.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-04-16T10:33:36.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4280" PostTypeId="2" ParentId="4275" CreationDate="2014-04-16T10:59:08.910" Score="7" Body="&lt;p&gt;Each of the exploded debris will gain a given $\Delta$V based on the force from the explosion. This $\Delta$V will be comparatively small to the orbital velocity, typically you'd be looking at 100s of meters per second against 1000s. Objects in orbit stay in orbit ALMOST indefinitely. Drag is the best friend of gravity. Gravity stops these object escaping the Earth, they have to be moving a lot faster to get away from the gravity pull. But it keeps them in orbit going around and around the Earth. Each time a object orbits the Earth it might hit a few very sparsely spread atoms. These atoms collide with the debris and slow it down. Over a period of months, years, decades, etc all objects in orbit will eventually reenter the atmosphere. Low Earth orbit is a couple of months, or years. Geosynchronous orbit is more like millions and billions of years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short answer: Yes, eventually they will slow down and reenter the Earth's atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="353" LastEditDate="2014-05-14T16:38:31.493" LastActivityDate="2014-05-14T16:38:31.493" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="4281" PostTypeId="1" CreationDate="2014-04-16T14:37:41.697" Score="15" ViewCount="8517" Body="&lt;p&gt;Assuming we ever manage to solve the energy generation problem (and whatever other problems may face it) and build a functioning Alcubierre drive, how &quot;fast&quot; would we be able to travel? That is, from the perspective of an external observer, how much distance could an Alcubierre-driven spacecraft cross in a given amount of time? Or, phrased yet another way, from the perspective of the stellar bodies we're leaving from and arriving at, how quickly can we cross that distance?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it simply (not that this is a &quot;simple&quot; matter...) a function of how much energy we can put into it, or does it have its own theoretical limits?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand (or at least think I do) that the craft does not exceed c within its local space-time &quot;bubble&quot;, but I also understand that the contraction/expansion of the surrounding space-time results in the craft &quot;breaking&quot; that speed limit relative to other bodies around it, and I'm interested in knowing what that speed could be -- hence the multiple phrasings of the question, because I'm not entirely sure what the &quot;correct&quot; way to phrase this question is.&lt;/p&gt;&#xA;" OwnerUserId="3106" LastEditorUserId="44505" LastEditDate="2023-02-28T14:13:00.650" LastActivityDate="2023-02-28T14:13:00.650" Title="If we can make an Alcubierre drive, how fast can it move a ship?" Tags="|interstellar-travel|velocity|ftl|alcubierre-drive|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4282" PostTypeId="2" ParentId="4281" CreationDate="2014-04-16T15:18:05.203" Score="11" Body="&lt;p&gt;My understanding is that you're asking what the theoretical maximum speed would be if you could build and use a Alcubierre drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've done a bit of research and it seems the current thinking is that even though the physics of an Alcubierre drive don't break relativity they do not work in thermodynamics. In a paper by [Stefano Finazzi, Stefano Liberati, and Carlos Barceló] it is suggested that the superluminal speed of the bubble would cause massive heating due to hawking radiation. This heating would be so intense it would not only destroy anything inside the bubble (you and your spacecraft) it would also destabilize the bubble itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ref: Stefano Finazzi, Stefano Liberati, and Carlos Barceló, &quot;Semiclassical instability of dynamical warp drives, Physical Review. Link: &lt;a href=&quot;http://journals.aps.org/prd/abstract/10.1103/PhysRevD.79.124017&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://journals.aps.org/prd/abstract/10.1103/PhysRevD.79.124017&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorDisplayName="user20636" LastEditDate="2018-06-05T19:23:48.863" LastActivityDate="2018-06-05T19:23:48.863" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="4283" PostTypeId="2" ParentId="4278" CreationDate="2014-04-16T16:09:27.863" Score="-1" Body="&lt;p&gt;According to &lt;a href=&quot;http://solarsystem.nasa.gov/missions/profile.cfm?InFlight=1&amp;amp;MCode=MarsSciLab&amp;amp;Display=ReadMore&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt;, 1.8 billion out of the 2.5 billion dollar had been spent on sience alone to make this project possible. I didn't find further information on what is causing the current cost, but concluding from the above the sience for this is pretty expensive, just imagine the computation power required to simulate where it's going and the antennas at the ground to control it. Also there are many people hired as ground crew (Source: TV-Documentary) and they're probably quite busy evaluating all the data with their expensive supercomputers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course most of this is speculation, but it's a fact that supercomputers and &#xA;good scientist are expensive and I don't see another source of costs.&lt;/p&gt;&#xA;" OwnerUserId="3109" LastActivityDate="2014-04-16T16:09:27.863" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4284" PostTypeId="2" ParentId="4278" CreationDate="2014-04-16T17:00:44.813" Score="5" Body="&lt;p&gt;First of all, the quoted number you provided is actually for both missions, LRO and MER combined, not individually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The material costs include:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Time on the Deep Space Network.&lt;/li&gt;&#xA;&lt;li&gt;Bandwidth to serve the data to the public&lt;/li&gt;&#xA;&lt;li&gt;Some amount of money spent on Public Relations meetings&lt;/li&gt;&#xA;&lt;li&gt;Occasional team reviews, which are not all located at one location.&lt;/li&gt;&#xA;&lt;li&gt;Servers for processing the data&lt;/li&gt;&#xA;&lt;li&gt;Writing new software tools&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So before I go in to the costs associated with the program, let me first explain why this is more complex than a communications satellite. Rovers have to do different things every day. Planning is required to figure out how to make it do what is desired, and what overall path the rover should take. Communication Satellites are usually relatively calm, not having significant change from day to day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From my experience, it takes about 30 full time people to run such a system, and another 5 of them being IT support, at a bare minimum. Why so many? Several will be writing tools to make the work of the team easier, let's call that number 5. Some will be working on planning the next several days operations, let's call that number 5. Another 5 analysing the previous days operations, 5 looking at the health of the spacecraft, 5 taking some sort of leadership/administrative position, and the last 5 doing the first batch of science. These numbers might seem high, but remember that there are multiple instruments that need specific planning, plus the movements and other items. The number of people required to make this happen goes down with time, but it still requires plenty of operations. Plus there is an external scientific team, which all contribute part time to the effort, of which there is around 20 at a minimum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's just say 40 full time equivalent heads, at a standard rate of around \$200K/ person (This factors administrative costs, and is actually a low estimate for skilled personnel). That would give a cost of \$10 million just for the key people, let's say another \$1 million for travel, PR, computing power, and a few million to use NASA's facilities. They also run a engineering unit on the ground, and pay some maintenance fees for the use of the various instruments (To troubleshoot anomalies that might arise). This seems to adequately fill the entire budget &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/508_2015_Budget_Estimates.pdf&quot; rel=&quot;nofollow&quot;&gt;budget&lt;/a&gt; that NASA provides, \$13.2 million. This seems to match reasonably close to my estimate. Just to give you an idea, even Voyager runs \$5.3 million per year, and they have a team of only 10 people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that LRO is similarly priced. While you don't have to drive LRO, you do have to plan for a lot of images, and that requires substantial work. Each instrument will be planning their possible images up to 3 weeks in advance, coordinating with each other team to ensure they can point the spacecraft in the right direction. The team size and cost are thus similar for the two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, these two spacecraft do an enormous amount of work, for a small amount of money. But I don't think one could reasonably reduce the cost of the missions.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-10-01T16:40:24.143" LastActivityDate="2014-10-01T16:40:24.143" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4285" PostTypeId="2" ParentId="4278" CreationDate="2014-04-16T17:20:34.653" Score="11" Body="&lt;p&gt;It's mostly people.  Engineers to operate the vehicle and a science team to direct the activities, interpret the results, and write papers.  There are also support personnel to keep the ground systems running, testbed personnel to keep the testbeds running and run tests on them.  And some management and business people.  And a small portion of the budget for outreach activities.  After overhead, you need on the order of \$250K to \$300K per full-time person per year.  So it's not that many people across two spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For MER, new command sequences are developed, validated, and uploaded to the vehicle every day or two, and results from the previous sequences are evaluated every day or two to feed the planning for the next upload.  The planning includes detailed designs of drives and arm motions, rock brushing/grinding, operations of cameras and spectrometers, determination of available energy for the day and what activities will fit, determination of the available relay data for the data and prioritization of the data products on that downlink, etc.  I could go on, but it's quite the effort.  The more I think about it, the more amazed I am that they were able to do all that with only $13.2M in fiscal year 2013.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't believe that there is a publicly available budget breakdown.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-04-16T21:07:28.117" LastActivityDate="2014-04-16T21:07:28.117" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4287" PostTypeId="5" CreationDate="2014-04-17T15:31:20.020" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-04-17T15:31:20.020" LastActivityDate="2014-04-17T15:31:20.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4288" PostTypeId="4" CreationDate="2014-04-17T15:31:20.020" Score="0" Body="The main airlock on the International Space Station (ISS), known as the Quest Joint Airlock, formerly the Joint Airlock Module" OwnerUserId="-1" LastEditorUserId="58" LastEditDate="2014-04-17T16:04:26.083" LastActivityDate="2014-04-17T16:04:26.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4289" PostTypeId="1" CreationDate="2014-04-17T15:51:01.873" Score="7" ViewCount="1243" Body="&lt;p&gt;Let's say I build a ship, get in it and take off into the dark. When I leave Earth, I measure my acceleration at 1g and my ship is capable of producing that level of power indefinitely. There will be a point when the amount of energy I am using will not be enough to continue accelerating me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I reach the point of diminishing returns, do I still feel 1g acceleration? At some point I will stop accelerating - how fast will I be going? What would be the time dilation? What would I, as a passenger on that ship, experience during my travel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm just thinking if we could find a way to accelerate indefinitely (I'm looking, it's around here somewhere), we wouldn't actually have to achieve c in order to make the trip to wherever seem short to the people on the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="3117" LastActivityDate="2014-04-17T23:32:27.013" Title="Maybe I can't really reach the speed of light, but how close could I get?" Tags="|interstellar-travel|travel|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4290" PostTypeId="2" ParentId="4289" CreationDate="2014-04-17T16:29:05.370" Score="6" Body="&lt;p&gt;Right I'm going to assume the constant power your ship is capable of producing is actually a constant force your ship is under. (i.e. your ship is using some purely electrical form of acceleration - no loss of mass due to propellant).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic law of physics behind this is that as you approach the speed of light you mass increases. Therefore to maintain a constant &lt;em&gt;acceleration&lt;/em&gt; you need an increasing &lt;em&gt;force&lt;/em&gt;. However, if your force is not increasing you acceleration decreases. The rate of decrease is such that you will approach the speed of light but never reach it, or to put it another way you would need an infinite length of time to reach the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This being said, you never actually stop accelerating. Your acceleration may get so low that you can't measure it. As for the observer on the spacecraft, they will still experience the 1g of acceleration - as in their frame of reference they aren't going at/near the speed of light. The time dilation experienced will be dependent on the length of the journey. As a passenger on the ship you will experience the universe around you accelerating in the opposite direction to your travel. You will also experience the blue/red shift of object outside the spacecraft infront/behind you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also to answer your title question you could get infinitely close.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-04-17T16:29:05.370" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4291" PostTypeId="1" CreationDate="2014-04-17T16:33:05.043" Score="9" ViewCount="1006" Body="&lt;p&gt;Would anyone have any insights or articles on the best location to land on Mars for a manned mission where the crew would establish their base and live on Mars for a couple months? (But I know we are not ready to live long-term on Mars)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Sorry for the broadness. I meant more in terms of the best location to land in terms of &lt;strong&gt;habitability&lt;/strong&gt;, since the crew will be establishing base and living on it for several months. Or could it be said that the location would not matter, since technology could negate the differences in location (e.g. technology could keep the temeperature ideal)? I would appreciate any insights or any new information that I could learn.&lt;/p&gt;&#xA;" OwnerUserId="3118" LastEditorUserId="58" LastEditDate="2014-04-17T16:54:40.757" LastActivityDate="2014-04-18T15:57:52.180" Title="What is the ideal location for a human habitat on Mars?" Tags="|mars|crewed-spaceflight|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4292" PostTypeId="2" ParentId="4289" CreationDate="2014-04-17T18:15:32.603" Score="2" Body="&lt;p&gt;Assuming constant acceleration you'd be sailing across a Poincare disk. See &lt;a href=&quot;http://euler.slu.edu/escher/index.php/Circle_Limit_Exploration&quot; rel=&quot;nofollow&quot;&gt;M. C. Escher's Circle Limits&lt;/a&gt; for nice illustrations of Poincare Disks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the point of view of each angel or devil in Escher's circle limit, he/she is in the middle of the disk. Near neighbors are somewhat scrunched. Distant neighbors closer to the edge are a lot scrunched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each angel or devil could be thought of as an inertial frame. With constant acceleration you'd be sailing across the disk. After you sailed over a devil you could watch him receding. He'd get closer and closer to the edge but never reaching it. From the devil's point of view he'd see you receding. He'd watch you getting closer and closer to the speed of light but never reaching it.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-04-17T18:23:30.043" LastActivityDate="2014-04-17T18:23:30.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4293" PostTypeId="1" CreationDate="2014-04-17T18:18:24.213" Score="6" ViewCount="249" Body="&lt;p&gt;I asked a question about reaching space using non-rocket aircrafts on aviation.se and I found out that it may be possible to reach the space for a short moment and then a normal aircraft designed to fly in atmosphere would start to fall. Still, that would allow to reach a certain altitude. I wonder what use could that be. Maybe transporting some supplies to orbital stations? But then with the velocity differences, it would result in a crash.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://what-if.xkcd.com/58/&quot; rel=&quot;noreferrer&quot;&gt;xkcd illustration&lt;/a&gt; of such ascend:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/q8Sks.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any use currently or theoretically in reaching out to space shortly without the ability to stay in orbit?&lt;/p&gt;&#xA;" OwnerUserId="990" LastEditorUserId="49" LastEditDate="2014-04-17T18:46:53.893" LastActivityDate="2014-04-17T20:34:56.600" Title="What use could be getting to space just for a bit?" Tags="|sub-orbital|ballistics|sounding-rocket|spaceplane|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4294" PostTypeId="2" ParentId="4293" CreationDate="2014-04-17T19:33:19.403" Score="7" Body="&lt;p&gt;There can be many reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Test of technology&lt;/li&gt;&#xA;&lt;li&gt;Doing &lt;em&gt;in-situ&lt;/em&gt; measurements in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Mesosphere&quot; rel=&quot;noreferrer&quot;&gt;mesosphere&lt;/a&gt;, which is too high for balloons, too low for satellites&lt;/li&gt;&#xA;&lt;li&gt;Performing &lt;a href=&quot;http://en.wikipedia.org/wiki/Microgravity&quot; rel=&quot;noreferrer&quot;&gt;microgravity&lt;/a&gt; experiments where parabolic flights have too short segments of microgravity but where launching a spacecraft is too expensive&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This list is almost certainly incomplete.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2014-04-17T20:34:56.600" LastActivityDate="2014-04-17T20:34:56.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4295" PostTypeId="1" CreationDate="2014-04-17T21:33:20.690" Score="21" ViewCount="24775" Body="&lt;p&gt;I'm looking for a serious, realistic looking scale model of the International Space Station to build at home. Even better, if it would include some electronics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm open to free or commercial products.&lt;/p&gt;&#xA;" OwnerUserId="3120" LastEditorUserId="3120" LastEditDate="2014-04-18T11:50:27.073" LastActivityDate="2020-06-07T14:12:18.533" Title="Is there a realistic looking ISS scale model available somewhere?" Tags="|iss|" AnswerCount="6" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4296" PostTypeId="2" ParentId="3205" CreationDate="2014-04-17T22:51:57.580" Score="4" Body="&lt;p&gt;Would you count the Transfrer Compartment of Zvezda &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/oWjTA.gif&quot; alt=&quot;Transfrer Compartment of Zvezda&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and Pressurized Adaptor of Zarya (each looking like a spherical part of the module)? &#xA;There even was &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_International_Space_Station_spacewalks&quot; rel=&quot;nofollow noreferrer&quot;&gt;an official EVA from TC of Zvezda.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MLM Nauka is going to have a a similar spherical compartment. In addition to that a separate airlock for experiments will be provided. See &lt;a href=&quot;http://www.youtube.com/watch?v=-M7gCWzJSx4#t=4m00&quot; rel=&quot;nofollow noreferrer&quot;&gt;this short 2012 Roscosmos video about MLM&lt;/a&gt;. At 4:00 the airlock is demonstrated. &#xA;Sorry this is all in russian.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2014-04-18T09:09:10.513" LastActivityDate="2014-04-18T09:09:10.513" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4297" PostTypeId="2" ParentId="4289" CreationDate="2014-04-17T23:32:27.013" Score="2" Body="&lt;p&gt;You can get as close as you want. The usual E=1/2 mv&lt;sup&gt;2&lt;/sup&gt; breaks down at relativistic speeds. Instead, you get the famous formula E=mc&lt;sup&gt;2&lt;/sup&gt;. And as you might know, your mass increases as you approach light speed, by a factor (1-v&lt;sup&gt;2&lt;/sup&gt;/c&lt;sup&gt;2&lt;/sup&gt;)&lt;sup&gt;-1/2&lt;/sup&gt;. In other words, as you approach light speed your mass goes to infinity. That in turn means with a constant force your acceleration decreases to zero, but it never becomes zero.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2014-04-17T23:32:27.013" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4298" PostTypeId="2" ParentId="4295" CreationDate="2014-04-18T00:17:43.043" Score="12" Body="&lt;p&gt;There is a pretty cool paper model available at &lt;a href=&quot;http://axmpaperspacescalemodels.com/index.php/international-space-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;AXM Models&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another cool paper model is available at &lt;a href=&quot;https://csiropedia.csiro.au/build-your-own-space-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CSIRO&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think paper models are pretty cool.  For example this paper &lt;a href=&quot;http://www.ksl.com/?sid=27371770&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ironman model&lt;/a&gt;!&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2020-06-07T14:12:18.533" LastActivityDate="2020-06-07T14:12:18.533" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="4299" PostTypeId="2" ParentId="4295" CreationDate="2014-04-18T07:54:20.130" Score="19" Body="&lt;p&gt;There are some plastic model kits as well: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://web.archive.org/web/20141227125959/http://www.realspacemodels.com/html/isspg.htm&quot; rel=&quot;noreferrer&quot;&gt;Intermountain Railway&lt;/a&gt; had a very nice (and very expensive) ISS model at 1/144 scale, generally regarded to be the best available. It's out of production so it's getting hard to find. &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/B0000BX89P&quot; rel=&quot;noreferrer&quot;&gt;Revell&lt;/a&gt; has an 1/144 scale ISS as well. When I wrote this in April of 2014, this kit was out of production and being offered for increasingly silly prices (\$160, when the original price was closer to \$50). &#xA;As of November 2014, Revell have done a limited-edition reissue of this kit (kit number 04841). &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.amazon.de/Heller-52909-Jules-Verne-Ariane/dp/B0017J2QEQ&quot; rel=&quot;noreferrer&quot;&gt;Heller&lt;/a&gt; has the ISS in 1/125. You can find kits of just the ISS, or the ISS plus an ESA ATV and an Ariane 5. These are readily available. This kit has one large drawback: it doesn't include the main truss and the 8 large solar arrays. &lt;/li&gt;&#xA;&lt;li&gt;There are more at smaller scales (Takara 1/700, iirc Herpa does one at a similar scale)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=4382.0&quot; rel=&quot;noreferrer&quot;&gt;Discussion of ISS models&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of these include electronics, by the way. Are you looking for anything specific? Lighting? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorDisplayName="user10509" LastEditDate="2016-02-08T08:55:15.727" LastActivityDate="2016-02-08T08:55:15.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4301" PostTypeId="1" AcceptedAnswerId="4302" CreationDate="2014-04-18T10:43:50.647" Score="17" ViewCount="5816" Body="&lt;p&gt;Iron and aluminum are the two most common metals in the Earth's crust.  While we can't know for sure what we'll find for raw resources when we start building factories in space, if we assume that iron and aluminum are both readily available, would one of these be easier to work then the other?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know from &lt;a href=&quot;https://space.stackexchange.com/questions/4154/would-unpainted-iron-or-steel-rust-in-space/4155#4155&quot;&gt;this answer&lt;/a&gt; that both iron and aluminum have similar longevity and oxidization concerns in the vacuum of space.  Despite aluminum being much &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth#Chemical_composition&quot; rel=&quot;noreferrer&quot;&gt;more common in the Earth's crust&lt;/a&gt;, it was the second of the two to be widely used due to the comparative difficulty in smelting. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like a &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_furnace&quot; rel=&quot;noreferrer&quot;&gt;solar furnace&lt;/a&gt; could be used, but I am not sure where to begin looking for how, or for what raw materials would be best suited as our first space-sourced building material.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="5965" LastEditDate="2018-03-07T16:22:29.690" LastActivityDate="2023-09-21T01:08:23.060" Title="What are the considerations for smelting iron and aluminum in space?" Tags="|materials|vacuum|" AnswerCount="6" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4302" PostTypeId="2" ParentId="4301" CreationDate="2014-04-18T14:16:58.880" Score="11" Body="&lt;p&gt;Iron is generally used for steel and I will address iron in the question as referring to steel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need a phenomenal amount of electricity to smelt aluminum in &lt;a href=&quot;http://en.wikipedia.org/wiki/Hall-H%C3%A9roult_process&quot;&gt;the most commonly used method&lt;/a&gt; here on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Steel on the other hand requires careful management of additives to produce the correct strength and other characteristics. Steel with too much carbon is brittle and too little is weak. This is most commonly managed by using a special kind of coal called &lt;a href=&quot;http://en.wikipedia.org/wiki/Coke_%28fuel%29&quot;&gt;coking coal&lt;/a&gt;. Steel production requires vast amounts of Oxygen, as what you are doing is burning the coke to heat up the ore and infuse oxygen, then burning the oxygen out of the steel to burn out impurities which produces an immense amount of heat that needs to be rejected to the environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither of these would be easy to work with in a spacecraft due to heat and oxygen concerns. On the surface of a planet/moon/something it would be easier to work with aluminum, due to steels reliance on other minerals and massive oxygen/fuel need. Aluminum is most commonly smelted with the only major inputs being Bauxite and electricity. Steel requires coal, oxygen, iron, and other trace elements like Chromium for stainless steel.&lt;/p&gt;&#xA;" OwnerUserId="3061" LastActivityDate="2014-04-18T14:16:58.880" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4303" PostTypeId="2" ParentId="4291" CreationDate="2014-04-18T15:57:52.180" Score="7" Body="&lt;p&gt;If you're only talking several months, or a year and a half which is a typical surface duration for a human expedition, then habitability is generally not a factor in site selection.  The expedition brings their own resources for a mission of that duration, including water, which is partially recycled, and nuclear power.  The exception is a reliance on Mars atmospheric CO&lt;sub&gt;2&lt;/sub&gt; from which to extract oxygen for breathing and return rocket propellants (using the C as well).  Since the CO&lt;sub&gt;2&lt;/sub&gt; is everywhere, it has no bearing on site selection.  You can see an example in &lt;a href=&quot;http://ww.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf&quot;&gt;NASA's Design Reference Architecture&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main landing site selection criteria would be landing safety, which includes a limit on altitude, and, presuming that it is a scientific expedition, how well the landing site is expected to meet the science objectives.  There will likely be an upper limit on latitude, based on the inclination of the return orbiter and capability of the rocket that will take them from the base to Mars orbit.  A latitude limit is also useful for bounding the requirements on habitat thermal control.  (Habitats include roving vehicles and spacesuits.)  The science objectives quoted are usually related to the search for evidence of life that had originated on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other constraints may derive from planetary protection concerns, in particular protecting Earth from putative Martian organisms that the crew might bring back outside of sealed sample containers.  From the report:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In order for humans to explore Mars and return to Earth safely, it&#xA;  will be necessary to identify sites on Mars that are free of hazards&#xA;  to the Earth’s biosphere. This is because astronauts on the martian&#xA;  surface inevitably would be exposed to local martian materials such as&#xA;  dust, and the plan is to return the astronauts to Earth at the end of&#xA;  the mission. The astronauts are therefore a potential vector for the&#xA;  transport of martian dust, which must be shown in advance to be&#xA;  sufficiently safe. The Space Studies Board has recommended the&#xA;  designation of Zones of Minimum Biological Risk (ZBRs) that are&#xA;  regions demonstrated to be safe for humans. That is, astronauts would&#xA;  only be allowed in areas that are demonstrated to be safe. For the&#xA;  initial landing site, such testing would probably have been performed&#xA;  as a part of the precursor mission activities, which may include&#xA;  analysis on Earth of returned martian samples, particularly wind-blown&#xA;  dust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same planetary protection concerns require the protection of Mars habitats from human-borne microorganisms:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The strategy that was adopted for the current DRA envisions targeting&#xA;  the human landing site that would be located within an area that is&#xA;  already known to be safe to humans (a zone of minimum biological risk)&#xA;  and in which microbial contamination would be permitted.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If humans have to avoid the very thing we might be looking for, it would put into question why we would send them.  If such constraints cannot be avoided, a human expedition might be limited to the search for evidence of life in the fossil record.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-04-18T15:57:52.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4304" PostTypeId="1" AcceptedAnswerId="4305" CreationDate="2014-04-18T16:36:42.173" Score="6" ViewCount="1010" Body="&lt;p&gt;We have seen in these questions that one of the primary concerns in sending humans (or anything for that matter, but especially humans) to Mars is contamination of the Martian environment:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3109/how-would-the-search-for-life-on-mars-be-affected-by-having-boots-on-the-ground&quot;&gt;How would the search for life on Mars be affected by having boots on the ground?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4291/what-is-the-ideal-location-for-a-human-habitat-on-mars&quot;&gt;What is the ideal location for a human habitat on Mars?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If any Earth biota were to get muddled into the Martian environment it might adversely affect our search for life on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Dutch Mars One mission is not primarily intended to search for life, but regardless it is still a manned mission to Mars. Do they plan to take any precautions to prevent contamination? I ask because they are not affiliated with existing space agencies, so the current restrictions do not apply to them. If they do have a process  planned already to prevent contamination, what does this process entail (in brief)?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-04-18T19:01:30.357" Title="Does Mars One plan on taking precautions to ensure the Martian environment is not contaminated with Earth biota?" Tags="|mars|life|mars-one|forward-contamination|sterilization|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4305" PostTypeId="2" ParentId="4304" CreationDate="2014-04-18T16:44:13.073" Score="6" Body="&lt;p&gt;There is an official Mars One FAQ page &lt;a href=&quot;http://www.mars-one.com/faq/mission-to-mars/will-the-mission-be-harmful-to-mars-environment&quot; rel=&quot;nofollow&quot;&gt;answering exactly this question&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mars One will take specific steps to ensure that the Mars environment&#xA;  (which we will study, and on which we will depend) will not be harmed.&#xA;  The Mars base will be forced to recycle just about everything, and pay&#xA;  close attention to its energy use and minimize the leakage of&#xA;  materials and energy. Nutrients are scarce on Mars. They either need&#xA;  to be imported from Earth, or extracted from the ground or atmosphere.&#xA;  Solar panels, which will also be launched from Earth, will generate&#xA;  the settlement's electricity. All of this means that a Mars resident&#xA;  will have a much smaller ecological footprint than that of the average&#xA;  person on Earth.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In addition, the development and operation of the settlement itself&#xA;  can greatly improve our sustainability efforts on Earth. The necessity&#xA;  to recycle everything on Mars will provide a high-profile boost to our&#xA;  recycling industry, as will the demand for lightweight solar panel&#xA;  technology. New methods of cultivating crops and growing plants on&#xA;  Mars can also teach us on Earth a great deal about how to improve our&#xA;  environment from experiences on another planet.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Prevention of environmental contamination&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Mars One will take the required actions to prevent environmental&#xA;  contamination caused by the importation of Earth life (humans and&#xA;  their companion organisms). Mars One has begun discussions with the&#xA;  ICSU Committee on Space Research’s (COSPAR’s) panel on planetary&#xA;  protection and the COSPAR panel on exploration to identify the&#xA;  measures that need to be taken with respect to prevent this&#xA;  contamination. &lt;a href=&quot;http://www.mars-one.com/about-mars-one/advisers/christopher-p.-mckay-phd-usa&quot; rel=&quot;nofollow&quot;&gt;Dr. Chris McKay&lt;/a&gt; of the COSPAR panel on exploration and&#xA;  &lt;a href=&quot;http://www.mars-one.com/about-mars-one/advisers/prof.-john-d.-rummel-phd-usa&quot; rel=&quot;nofollow&quot;&gt;Prof. Dr. John D. Rummel&lt;/a&gt; of the COSPAR panel of planetary protection&#xA;  are two of our advisers. Based on discussions with these two panels,&#xA;  Mars One will acquire the necessary systems and take the required and&#xA;  necessary actions to protect Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If I remember correctly, this FAQ question is a reply to one of the questions to &lt;a href=&quot;http://www.reddit.com/r/IAmA/comments/1tw2fy/i_am_bas_lansdorp_cofounder_of_marsone_mankind&quot; rel=&quot;nofollow&quot;&gt;Bas Lansdorp's AMA&lt;/a&gt; on Reddit. Or perhaps &lt;a href=&quot;https://www.youtube.com/watch?v=TePLtbTzzZ0&quot; rel=&quot;nofollow&quot;&gt;Mars One press conference&lt;/a&gt; held in Washington DC in December 10, 2013. I'll update, if I find some more info on it. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-18T17:10:36.810" LastActivityDate="2014-04-18T17:10:36.810" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4306" PostTypeId="1" AcceptedAnswerId="4314" CreationDate="2014-04-18T17:35:07.290" Score="15" ViewCount="6771" Body="&lt;p&gt;I'm sure we're all familiar with the go/no go standard phrasing for reporting whether something is OK or not. Having a standard phrasing clearly makes comprehension easier and more reliable, especially in an international field where not everyone speaks the same languages, and especially when mistakes can be very costly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it seems like these two phrases in particular are the worst possible choice. If it's hard to hear the speaker and you don't hear the first syllable of &quot;no go&quot;, it just sounds like &quot;go&quot;. And even if you hear the first syllable but not the second, it still sounds enough like &quot;go&quot; to be easily misinterpreted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like the only advantage of go/no go is that the two phrases are different lengths, so that if you can hear the stress but the sounds themselves are indistinct, you can still tell them apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whose idea was go/no go, and why did those phrases catch on, instead of two more distinguishable ones, such as &quot;go&quot; and &quot;not good&quot;?&lt;/p&gt;&#xA;" OwnerUserId="2807" LastEditorUserId="49" LastEditDate="2015-02-10T16:42:09.437" LastActivityDate="2019-12-08T00:10:52.173" Title="Why do they say &quot;go&quot; and &quot;no go&quot;" Tags="|history|terminology|procedure|launch-sequence|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4307" PostTypeId="1" CreationDate="2014-04-18T18:07:43.640" Score="2" ViewCount="25" Body="&lt;p&gt;If we were to ever send a probe to check for life then it's of paramount importance that we do not contaminate the target site with Earth organisms. And if there's anything we've learned from &lt;a href=&quot;http://en.wikipedia.org/wiki/Extremophile&quot; rel=&quot;nofollow&quot;&gt;extremophiles&lt;/a&gt; is that life is very persistent. How would we go about sterilizing a probe so that we would be sure without a shadow of doubt that no trace of life is clinging to it until it reaches its destination?&lt;/p&gt;&#xA;" OwnerUserId="122" LastEditorUserId="49" LastEditDate="2014-04-18T18:19:21.310" LastActivityDate="2014-04-18T18:19:21.310" Title="How would we sterilize a probe to look for life?" Tags="|probe|astrobiology|forward-contamination|sterilization|integration|" AnswerCount="0" CommentCount="0" ClosedDate="2014-04-18T20:10:58.080" ContentLicense="CC BY-SA 3.0" />
  <row Id="4308" PostTypeId="2" ParentId="4304" CreationDate="2014-04-18T19:01:30.357" Score="4" Body="&lt;p&gt;Once you open the hatch, Mars is contaminated. No matter how hard you try to sterilize everything before opening up, there will be microbes on the outside of suits, airlock surfaces, etc. Maybe you could flood the airlock with hot acid or some other biocide and then drain it, but that will be considered too much useless payload, and is not 100% guaranteed anyway. There's a good chance that some stuff has already made it to Mars on landers (that we attempted to sterilize) and crashed orbiters, so we'll never be absolutely certain that any bug we find there is truly native (unless its biochemistry is radically different from anything on Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All we can do is hope that the chemical environment of Mars's surface is so harsh that no Earth life could survive there. That doesn't bode well for the survival of astronauts or settlers, who will track some surface materials into their living quarters and breathe it in. I just wish people weren't in such a rush to get to Mars, and would do some more robotic looking for life on and below the surface (and sample returns), before people get there and definitely contaminate it.&lt;/p&gt;&#xA;" OwnerUserId="3127" LastActivityDate="2014-04-18T19:01:30.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4309" PostTypeId="2" ParentId="4306" CreationDate="2014-04-18T20:50:41.260" Score="13" Body="&lt;p&gt;Having worked mission communications in CAP for live Search and Rescue operations, on the radio, go/no-go is VERY clear. It's one syllable versus two. (Three is the interrogative.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The longer &quot;affirmative&quot; &quot;negative&quot; are 4 and 3, and not as clear, and further, subject to more interference due to length.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this usage parallels the morse use of C (-.-.) and N (-.) in mode, tho' 1 repeat being negative in morse, and in modern use, the question mark (..--..) being used for interrogative mode.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-04-18T20:50:41.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4310" PostTypeId="2" ParentId="2855" CreationDate="2014-04-18T21:00:22.633" Score="2" Body="&lt;p&gt;Because of near light speed, and the nearest &quot;intergalactic&quot; space being over 1000 light years, no one on Earth would still be alive by the time the ship reached it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those on the ship, however, could survive it if the velocity was in excess of 0.9999 C. If I do the math correctly, at that point, the Lorentz factor exceeds 100, putting the perceived time to between 10 and 50 years to the edge of the galactic disk.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-04-18T21:00:22.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4311" PostTypeId="1" AcceptedAnswerId="4318" CreationDate="2014-04-18T23:36:00.527" Score="6" ViewCount="6057" Body="&lt;p&gt;If someone is a programmer, but without any kind of knowledge or know-how about engineering and would want to work in space industry, what would be his chances to find a job anywhere? (NASA or other space agencies)&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="25" LastEditDate="2014-04-20T02:19:31.023" LastActivityDate="2014-04-20T02:20:24.320" Title="Which are the chances of a programmer without engineering-related degrees to work in space industry?" Tags="|nasa|software|career|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="4312" PostTypeId="1" AcceptedAnswerId="4313" CreationDate="2014-04-19T00:00:54.783" Score="18" ViewCount="4444" Body="&lt;p&gt;Someone claimed to me that they could see the actual shape of the ISS as it passed overhead without use of any optical device. I would think this isn't possible since it is only about 100 meters wide but over 370 km in altitude. I read through &lt;a href=&quot;https://space.stackexchange.com/questions/4034/can-i-see-the-iss-from-the-surface-with-the-naked-eye&quot;&gt;this question&lt;/a&gt; already but I wanted to know if it was possible to discern the solar arrays or general shape of the station (assuming you're in a decently remote area).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: This was claimed to have been observed recently, so it wasn't a case of a shuttle rendezvous with the station.&lt;/p&gt;&#xA;" OwnerUserId="3131" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2023-07-28T21:31:57.207" Title="Can you make out the shape of the ISS with the unaided eye as it passes overhead?" Tags="|iss|artificial-satellite|observation|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4313" PostTypeId="2" ParentId="4312" CreationDate="2014-04-19T01:32:19.527" Score="27" Body="&lt;p&gt;Having observed the International Space Station numerous times passing over the early night skies, on a clear night and when my eyes are rested (I stare a lot at monitors like I'd guess most of us here) having normal visual acuity (20/20 vision), I can assure you that your friend's claims are quite impossible even for an experienced amateur astronomer with many tricks up his sleeves, such as knowing how to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Averted_vision&quot; rel=&quot;noreferrer&quot;&gt;averted vision&lt;/a&gt; and observing the station when it barely reflects any light (usually farthest East during its early night sky when it would most likely start entering the Earth's shadow), to reduce &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_diffraction&quot; rel=&quot;noreferrer&quot;&gt;atmospheric diffraction&lt;/a&gt; of high-contrast close bright sources on a dark background reducing into a single point of light:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/tzzXi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;strong&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Airy diffraction patterns generated by light from two points passing through a circular aperture, such as&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;the pupil of the eye. Points far apart (top) or meeting the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rayleigh_criterion#Explanation&quot; rel=&quot;noreferrer&quot;&gt;Rayleigh criterion&lt;/a&gt; (middle) can be distinguished.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Points closer than the Rayleigh criterion (bottom) are difficult to distinguish. Image and quote: &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_resolution&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But before we even consider what other limitations to discerning features of a distant object might there be, let's first see what's still reasonably plausible for a human eye. So the question is, what's the human eye's resolution and the minimum spacing between two bright objects at the ISS orbital altitude (370 km or 230 mi)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.cis.uab.edu/sloan/PRtopo/Curcio_JCompNeurol1990_PRtopo_searchable.pdf&quot; rel=&quot;noreferrer&quot;&gt;Human Photoreceptor Topography&lt;/a&gt;, Curcio et al., 1990 (PDF) that lists several sources as well as own measurements of the spatial density of cones and rods in whole-mounted human retinas, the greatest cone density recorded was 324,100 cones/mm&lt;sup&gt;2&lt;/sup&gt;. That gives us acuity (or row-to-row spacing we need to discern at least two individual features) of 86.3 cycles/°. So for our best case, with a &lt;em&gt;great eye&lt;/em&gt;, neglecting any atmospheric effects, the ISS right above us when it's closest, and optimal contrast with the background sky, we get minimum separation of objects of 74.83 m. If there was no air between the observer and the station!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while 74.83 m seems just about right with the ISS truss length at 109 m (and solar panel arrays stretch a bit further than its truss, so their two sides' center spots would indeed be separated about that much), we shouldn't forget that there is about as much matter in between the observer and the subject that the observer is looking through, as if he was looking through roughly 10 meters of water. So indeed, the two sides of the ISS would be quite impossible to discern, even from a high altitude observation point, no air or light pollution, and extremely clear night.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is however still possible that your friend isn't making anything up, saw an &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;noreferrer&quot;&gt;iridium flare&lt;/a&gt; of two closely following satellites flying in formation, and confused them for the ISS. Those are not that rare, I've seen two such double iridium flares just last month, and I can assure you that I was not that bored to constantly look at the night skies. Those double (or sometimes triple, quadruple,...) flares of &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_formation_flying&quot; rel=&quot;noreferrer&quot;&gt;satellites flying in formation&lt;/a&gt; can look something like this (long exposure taken with a zoom camera):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Zji5v.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inexperienced observer could easily confuse these for a single object, since they appear to move as if they were connected to each other, like the ISS solar panels are via its truss. They move exactly like you'd expect of Low Earth Orbit satellites (like the ISS) to move, which is of course because they are. And they can be as bright as the ISS. But they do keep a bit larger distance to each other than the ISS's roughly 100 yards. Actually, there's a lot less space between the ISS solar panel arrays, since that's approximately its total length.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-20T02:56:46.440" LastActivityDate="2014-04-20T02:56:46.440" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4314" PostTypeId="2" ParentId="4306" CreationDate="2014-04-19T02:39:00.820" Score="16" Body="&lt;p&gt;To add to &lt;a href=&quot;https://space.stackexchange.com/a/4309/49&quot;&gt;@aramis' excellent explanation&lt;/a&gt; on clarity and ability to discern the go/no-go &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_status_check&quot; rel=&quot;noreferrer&quot;&gt;launch status check&lt;/a&gt; polling, these seem to have been introduced to NASA's (and U.S. in general) launch terminology during the first manned spaceflights of &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Mercury&quot; rel=&quot;noreferrer&quot;&gt;Project Mercury&lt;/a&gt;. I wasn't able to find a good example for Project Mercury launches, but I did find &lt;a href=&quot;https://www.youtube.com/watch?v=E4vz4xM_4Ok&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_5&quot; rel=&quot;noreferrer&quot;&gt;Gemini 5&lt;/a&gt; launch, where the CBS commentator already adopted this terminology (from Gemini launch command) and says several times that the &lt;em&gt;launch status is &quot;go&quot;&lt;/em&gt;, &lt;em&gt;spacecraft status is &quot;go&quot;&lt;/em&gt;, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before manned spaceflight, this &quot;go/no-go&quot; polling was used in military aviation as &quot;go/no-go for liftoff&quot;, especially in the U.S. navy with aircraft carrier pilots and likely also test pilots, where status of several complex systems has to be coordinated between stations before they can be cleared for takeoff. Since first astronauts came from those aviators, it is no wonder that this military aviation terminology found its way into the spaceflight terminology, where station polling makes just as much sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One other thing to note is that these statuses of such complex systems are not communicated verbally alone, but also signalised visually. First manned spaceflights coincided with the advent of computing with first computer networks soon to follow (like &lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-Automatic_Ground_Environment&quot; rel=&quot;noreferrer&quot;&gt;SAGE&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-automatic_business_research_environment#History&quot; rel=&quot;noreferrer&quot;&gt;SABRE&lt;/a&gt;), and mission controllers could have other centralized means of checking the status of individual stations before computer networks, for example with light switchboards in their mission command / launch control center. The status of individual stations would thus be called out via radio, as well as entered into the centralized system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So not everything depends on verbal commands alone, and several layers of control are used with station polling to avoid any miscommunication errors.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-02-10T16:45:15.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4315" PostTypeId="2" ParentId="4312" CreationDate="2014-04-19T03:28:20.073" Score="11" Body="&lt;p&gt;Human vision has an unaided resolution (typically) of around 0° 4'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS orbits at 370 km. It is approximately 73 m wide, 109m m long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3.7e5 * sin(4') ≅ 430.5&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each &quot;pixel&quot; is roughly 430 m. That the station is visible at all is a simple matter of brightness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get a visible shape reliably would require a roughly 10x scope, at which point the image would be between 1 x 2 pixels and 3x3 pixels, depending upon where exactly it falls - sufficient, over time, to infer shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even at the fovea, where peak individuals have been noted to have as narrow a discrimination as low as 0° 0' 21&quot; of arc resolution, the peak resolution would be about 21 m per pixel. Still only about 4x5 pixels worth, at the very peak. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also: human shape discrimination isn't a continuous field of uniform resolution, but a variable field ranging from about (typically) a ≤1° zone of 1' resolution through an outer band of ≥1° per cone at the very edges.  To make out the station at peak is barely doable, but only if looking right at it and then only while it remains within the narrow view of the central fovea.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html&quot; rel=&quot;nofollow&quot;&gt;NASA Facts &amp;amp; Figures ISS (http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2016-02-15T04:30:31.847" LastActivityDate="2016-02-15T04:30:31.847" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4316" PostTypeId="2" ParentId="4301" CreationDate="2014-04-19T08:08:01.580" Score="11" Body="&lt;p&gt;Current iron and aluminium processes rely on gravity:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Impurities (which are lighter than the metal being processed) are skimmed off the top of the molten metal.&lt;/li&gt;&#xA;&lt;li&gt;Molten metal can be relied on to stay at the bottom of the crucible, and not e.g. cling to the lid.&lt;/li&gt;&#xA;&lt;li&gt;Pouring under gravity is much easier than having to rely on pressure or somesuch to get the molten metal out.&lt;/li&gt;&#xA;&lt;li&gt;I suspect that for e.g. steel production, convection currents play a role in making a homogenous mass of steel (spreading the oxygen and other additives evenly through the metal)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-04-19T08:08:01.580" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4317" PostTypeId="1" CreationDate="2014-04-19T13:01:35.317" Score="2" ViewCount="290" Body="&lt;p&gt;On the SpaceX launch site (slc-40 launch pad) there are lot of cables suspended around top of the rocket (between four white rods). What is the function of them? Why no such cables is used on other launch sites?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are cables damaged by starting rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ers6m.jpg&quot; alt=&quot;Falcon 9 2014 april start - cables around top of the rocket&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/WLa4R.jpg&quot; alt=&quot;Falcon december 2013 launch, cables around rocket&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2843" LastActivityDate="2014-04-19T13:01:35.317" Title="SpaceX launch site - why all the cables are needed around top of the rocket" Tags="|spacex|launch-site|falcon-9|launchpad|" AnswerCount="0" CommentCount="1" ClosedDate="2014-04-19T13:09:39.357" ContentLicense="CC BY-SA 3.0" />
  <row Id="4318" PostTypeId="2" ParentId="4311" CreationDate="2014-04-19T14:36:55.410" Score="9" Body="&lt;p&gt;If you were trained as a computer scientist with a reasonable amount of math and science classes, yes. If you are a self taught programmer, or if you majored in computer science so that you only had to take a token number of pre-calculus math and Physics for Poets type classes, not so likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are plenty of places in the aerospace industry where computer science skills dominate over engineering. Some examples:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Flight software is oftentimes tested in a simulation environment. Somebody has to write the simulation engine. There's a physics engine part that you won't write, but there's a huge computer science part as well.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The flight software itself has to receive data from sensors and other computers, send data to effectors and other computers. How those inputs are converted to outputs, that's going to be directed by engineers. But those input / output processes themselves: That's a computer science problem.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The flight software runs on computers, sometimes multiple computers for redundancy purposes. How do you know if one of those computers has failed, has a rogue I/O card, has had it's memory messed up by a single event upset? That once again is a computer science problem.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Modern flight software is multithreaded. Do aerospace engineers know about threads? Maybe, maybe enough to be dangerous. Making that right, getting the scheduling algorithm right, that's yet another computer science problem.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Getting stuff into space is a huge logistics problem. That means databases galore. Once again, this is a problem for computer science majors to tackle rather than aerospace engineers.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-04-19T14:36:55.410" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4319" PostTypeId="2" ParentId="4311" CreationDate="2014-04-20T02:15:07.997" Score="8" Body="&lt;p&gt;In addition to David's answer, there's a few other key fields where programmers are often needed. The biggest need is usually in managing the data from the spacecraft. For instance, at my company, there's around 8 programmers that I'm aware of working on stuff related to the spacecraft. Of those:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Two work on the spacecraft themselves.&lt;/li&gt;&#xA;&lt;li&gt;1.5 work on building tools to simplify the process of command processing and data analysis.&lt;/li&gt;&#xA;&lt;li&gt;The rest work on programming the ground network to talk with the satellites, and optimize the comm schedule, etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;From my work with the HiRISE camera, here's the approximate breakdown of programmers at that time:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;An unknown number programmed the flight software. I didn't have many dealings with them.&lt;/li&gt;&#xA;&lt;li&gt;One programmed a bunch of web tools used to assist with planning upcoming passes, and a lot of other misc. tasks.&lt;/li&gt;&#xA;&lt;li&gt;One programmed a PR tool to look at images.&lt;/li&gt;&#xA;&lt;li&gt;One programmed a Java application to do the short term planning and commanding.&lt;/li&gt;&#xA;&lt;li&gt;One managed the databases.&lt;/li&gt;&#xA;&lt;li&gt;One did the image process programming.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Of these, all of them were computer science types. I'm not as familiar with those writing the Flight Software, however.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a non-engineering oriented programmer, you are less likely to be writing the spacecraft code itself, but there's plenty of work outside of that realm. I as a self taught programmer ended up programming a significant part of a simulation used by a aerospace company. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-04-20T02:20:24.320" LastActivityDate="2014-04-20T02:20:24.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4320" PostTypeId="2" ParentId="4221" CreationDate="2014-04-20T16:57:58.707" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is most of the software used for simulation and trajectory planning built in-house, or are there some industry standards?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's not an either/or question. The answer is &quot;Yes&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, there is no single tool that can answer all questions with regard to spaceflight. Drastically different tools were needed to plan the trajectory of Cassini–Huygens to Saturn, to model the detailed behavior of Huygens on its descent to Titan's surface, to perform hardware-in-the-loop simulations of the vehicle, and to develop best estimates of trajectory for the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Organizations that do this professionally have been doing so for a long time. The tools used were in-house, custom-developed tools. Some commercial tools do exist such as STK and Matlab/Simulink, but these are fairly new compared to the 50+ year history of spaceflight. Some organizations are turning from their custom tools to commercial tools, but typically only on well-contained problems where the commercial tool does the job right out of the box. For a complex simulation project, the costs of maintaining those custom tools and modifying them to exactly suit the needs of a new simulation project are small compared to the costs of number of licenses for those rather expensive commercial tools.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give an idea of the number of those in-house tools, The Jet Propulsion Laboratory uses &lt;a href=&quot;http://naif.jpl.nasa.gov/naif/toolkit.html&quot; rel=&quot;noreferrer&quot;&gt;SPICE&lt;/a&gt; and &lt;a href=&quot;https://gipsy-oasis.jpl.nasa.gov/index.php?page=home&quot; rel=&quot;noreferrer&quot;&gt;GIPSY-OASIS&lt;/a&gt; for some of their work in this area. JPL uses yet another set of software to generate its Development Ephmerides. The NASA Johnson Space Center uses &lt;a href=&quot;http://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-24209-1-copernicus.html#.U1Oadl5tLLQ&quot; rel=&quot;noreferrer&quot;&gt;Copernicus&lt;/a&gt;, &lt;a href=&quot;http://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-24492-1-sim-toolkit.html#.U1Oae15tLLQ&quot; rel=&quot;noreferrer&quot;&gt;Trick&lt;/a&gt;, and &lt;a href=&quot;http://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-24556-1-jeod2.html#.U1OaoF5tLLQ&quot; rel=&quot;noreferrer&quot;&gt;JEOD&lt;/a&gt; for some of their work in this area. Every NASA center appears to have their own sets of homebrew tools. The Department of Defense has multiple sets of tools. So do the European Space Agency and Roscosmos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One reason for the number of tools is that the people who use those tools have deep knowledge of how to use them. Switching to something else would be extremely costly. Another is a bit of a &quot;not invented here&quot; attitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet another reason is secrecy. If you look at STK, you'll see that large chunks of it are off limits to non-US organizations. The same goes for many of those tools developed at NASA. The reason is that the technology that enables accurately planning a vehicle mission to the Moon (or beyond) also enables enemies to accurately plan a missile mission to some US city. Exporting those technologies is subject to Export Administration Regulations (EAR) and International Traffic in Arms Regulations (ITAR). If it's Department of Defense, it's almost certainly classified in some way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the DoD had their druthers, $\vec F=m\vec a$ would be classified top secret NOFORN.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-04-20T20:00:15.843" LastActivityDate="2014-04-20T20:00:15.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4321" PostTypeId="2" ParentId="4221" CreationDate="2014-04-20T17:57:44.300" Score="4" Body="&lt;p&gt;You might like this website of the guy who plans trajectories for living: &lt;a href=&quot;http://astrogatorsguild.com&quot; rel=&quot;nofollow&quot;&gt;http://astrogatorsguild.com&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He uses STK.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some missions in NASA use this software &lt;a href=&quot;http://gmatcentral.org/display/GW/GMAT+Wiki+Home&quot; rel=&quot;nofollow&quot;&gt;http://gmatcentral.org/display/GW/GMAT+Wiki+Home&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but from what I know, they usually use some additional tools beside GMAT.&lt;/p&gt;&#xA;" OwnerUserId="1469" LastActivityDate="2014-04-20T17:57:44.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4322" PostTypeId="2" ParentId="3161" CreationDate="2014-04-21T05:19:08.573" Score="4" Body="&lt;p&gt;This article:&#xA;&lt;a href=&quot;http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/&quot; rel=&quot;nofollow&quot;&gt;http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;also notes that methane/LOX engines do not suffer coking buildup as LOX/RP1 engines do, and can be run less oxygen-rich, which is easier on the pumps.&lt;/p&gt;&#xA;" OwnerUserId="3139" LastActivityDate="2014-04-21T05:19:08.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4325" PostTypeId="1" AcceptedAnswerId="4327" CreationDate="2014-04-21T15:34:25.630" Score="26" ViewCount="22708" Body="&lt;p&gt;Buzz Aldrin is credited (or was tasked) with being the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_11&quot;&gt;Apollo 11 Lunar Module pilot&lt;/a&gt;. However in fact it was Neil Armstrong who piloted the craft down when the LM guidance computer overflowed, with Aldrin reading out to navigation data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why didn't Aldrin, the LM pilot, &quot;take the stick&quot; when human intervention became necessary?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; Even though the question has been answered, I just found this mention that also on Apollo 12 was &lt;a href=&quot;http://www.dickgordon.com/ApolloXII-4.shtml&quot;&gt;the LM piloted not by the pilot, but rather by the commander&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="4558" LastEditDate="2016-01-20T12:57:54.683" LastActivityDate="2021-01-14T21:17:52.343" Title="Why did Armstrong pilot the LM, when Aldrin was tasked as Lunar Module Pilot?" Tags="|the-moon|crewed-spaceflight|nasa|apollo-program|lunar-landing|" AnswerCount="6" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4326" PostTypeId="2" ParentId="434" CreationDate="2014-04-21T15:49:13.897" Score="0" Body="&lt;p&gt;Remove government funding from most of our defense related &quot;private&quot; concerns and you'd have nothing left.  So, I opine that SpaceX is probably more a commercial enterprise than most of our defense industry.&lt;/p&gt;&#xA;" OwnerUserId="3143" LastActivityDate="2014-04-21T15:49:13.897" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4327" PostTypeId="2" ParentId="4325" CreationDate="2014-04-21T16:21:19.273" Score="25" Body="&lt;p&gt;The question should really be, why did Armstrong pilot the Lunar Module (LM) manually, and the autopilot was switched off. According to &lt;a href=&quot;http://www.doneyles.com/LM/Tales.html&quot; rel=&quot;noreferrer&quot;&gt;Tales From The Lunar Module Guidance Computer&lt;/a&gt; by Don Eyles, Armstrong and Aldrin never switched places during the LM powered descent and landing phase. Armstrong was on the left side of the LM cockpit where the manual controls were, and Aldrin on the right and responsible for working the DSKY (Lunar Module Display and Keyboard Unit), essentially instructing the flight computer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, here's a photo of the Apollo Lunar Module's interior (source and credit: &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a12/a12.orient.html&quot; rel=&quot;noreferrer&quot;&gt;NASA HQ&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/yORjA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yORjA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that the &lt;em&gt;1202 Program Alarm&lt;/em&gt; that the LM was experiencing prior to the landing prompted the switch from &lt;code&gt;AUTO&lt;/code&gt; to &lt;code&gt;ATT HOLD&lt;/code&gt; flight mode. From mentioned source:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;ATT HOLD meant the digital autopilot's Rate-Command Attitude-Hold&#xA;  mode, in which the astronaut could command an attitude rate by&#xA;  deflecting a joystick. After the stick was released the autopilot&#xA;  nulled rates to maintain the present attitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Armstrong, as the mission commander, had to make a decision whether to proceed with the landing or not, while dealing with it with the mission control in Houston. It turned out that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Due to an error in the checklist manual, the rendezvous radar switch&#xA;  was placed in the wrong position. This caused it to send erroneous&#xA;  signals to the computer. The result was that the computer was being&#xA;  asked to perform all of its normal functions for landing while&#xA;  receiving an extra load of spurious data which used up 15% of its&#xA;  time. The computer (or rather the software in it) was smart enough to&#xA;  recognize that it was being asked to perform more tasks than it should&#xA;  be performing. It then sent out an alarm, which meant to the&#xA;  astronaut, &lt;em&gt;I'm overloaded with more tasks than I should be doing at&#xA;  this time and I'm going to keep only the more important tasks&lt;/em&gt;; i.e.,&#xA;  the ones needed for landing...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;— Margaret Hamilton, lead Apollo flight software designer&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And the computer was prompting Armstrong and Aldrin with other alarms, for all of which they received a go to proceed from Houston. According to &lt;a href=&quot;https://mitpress.mit.edu/sites/default/files/titles/content/9780262134972_sch_0001.pdf&quot; rel=&quot;noreferrer&quot;&gt;Human and Machine in Spaceﬂight&lt;/a&gt;, David A. Mindell, 2008 Massachusetts Institute of Technology (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With the push of a button, the astronaut could abort the landing, an&#xA;  action practiced countless times in simulation. Yet he held off.&#xA;  Armstrong later explained himself as a mechanism:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;‘‘In simulations we have a large number of failures and we are&#xA;  usually spring-loaded to the abort position. And in this case in the&#xA;  real ﬂight, we are spring-loaded to the land position.’’&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Houston checked out the problem. Young engineers recognized it from a&#xA;  recent simulation, and conferred with their support teams in the back&#xA;  room. They quickly found the cause. The computer was overloading and&#xA;  restarting but not shutting down. It was ignoring low-priority tasks,&#xA;  but these were not critical for the mission. ‘&lt;em&gt;‘We're go on that&#xA;  alarm,’’&lt;/em&gt; the ground controller replied, meaning the LM could proceed.&#xA;  For his role in clearing the landing, engineer Steven Bales later&#xA;  accepted a presidential award on behalf of the ﬂight control team.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Armstrong surveyed the computer display; it had frozen. He checked the&#xA;  LM’s systems. The vehicle seemed to be responding to his commands,&#xA;  meaning the computer was still running. So he continued.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Emphasis mine. So it seems that the switch to manual was due to a set of circumstances prior to the landing. Armstrong, as the mission commander, continued piloting the LM in &lt;code&gt;ATT HOLD&lt;/code&gt; flight mode after the &lt;em&gt;1202 Program Alarm&lt;/em&gt;, and all subsequent alarms they were in contact with Houston about. After mission control gave a go on the alarms, and having the control over the flight computer, he made a command decision and took &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11#Insignia&quot; rel=&quot;noreferrer&quot;&gt;The Eagle&lt;/a&gt; down to its landing spot — &lt;a href=&quot;https://en.wikipedia.org/wiki/Tranquility_Base&quot; rel=&quot;noreferrer&quot;&gt;Tranquility Base&lt;/a&gt; at &lt;a href=&quot;https://en.wikipedia.org/wiki/Sea_of_Tranquility&quot; rel=&quot;noreferrer&quot;&gt;the Sea of Tranquility&lt;/a&gt; (Mare Tranquillitatis), the Moon.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-21T22:36:38.423" LastActivityDate="2014-04-21T22:36:38.423" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="4328" PostTypeId="5" CreationDate="2014-04-21T20:40:19.933" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-21T20:40:19.933" LastActivityDate="2014-04-21T20:40:19.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4329" PostTypeId="4" CreationDate="2014-04-21T20:40:19.933" Score="0" Body="A Multiplexer/Demultiplexer (MDM) is an electronics unit that sends and receives multiple streams of data, and routes all incoming and outgoing data to its correct destination. They're in essence flight computers responsible for such tasks as controlling power, attitude control, environmental control, and communications systems, as well as monitoring of scientific payloads." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-21T20:40:19.933" LastActivityDate="2014-04-21T20:40:19.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4330" PostTypeId="1" AcceptedAnswerId="4331" CreationDate="2014-04-22T07:18:17.673" Score="17" ViewCount="29000" Body="&lt;p&gt;I was wondering, what is the minimum amount of energy required to put 1 kg of payload into LEO?&lt;/p&gt;&#xA;" OwnerUserId="3144" LastEditorUserId="49" LastEditDate="2014-04-22T14:31:08.397" LastActivityDate="2019-03-14T13:17:50.853" Title="How much Energy is required to put 1 kg in LEO?" Tags="|rockets|low-earth-orbit|energy|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4331" PostTypeId="2" ParentId="4330" CreationDate="2014-04-22T11:49:10.333" Score="9" Body="&lt;p&gt;Here's a simplistic and optimistic approach.  First find the mass ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \frac{ m_0 }{ m_p } = e^{ \frac{ 10,000 }{ 4462 } } = 9.4 $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This varies a great deal in practice, in both of those two numbers above.  The delta_v to orbit fluctuates widely between about 9 km/s to 11 km/s, and the propellant velocity by a much larger margin.  I also haven't taken into account take masses and staging.  Basically, this is optimistically low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To convert this information into energy, there are two approaches I want to follow.  For one, you could apply the reaction energy of &lt;span class=&quot;math-container&quot;&gt;$232 kJ/mol$&lt;/span&gt; for the reaction of liquid Hydrogen and Oxygen.  The other, you could just calculate the kinetic energy of the propellant.  The latter first, since it's simple (and more wrong):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ E_k = \frac{ 1}{2}   \frac{ m_0 }{ m_p } v^2 \\&#xA;= \frac{ 1}{2}  \frac{ 9.4 kg }{ 1 kg} \left( 4,462 \frac{m}{s} \right)^2 = 94 \frac{ MJ }{ 1 kg }  $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope the units help describe what the context is.  This is the energy required per mass of payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, let's use the more chemistry-based approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ E = 232 \frac{ kJ }{ mol } \frac{ mol }{ 20 g } 9.4 \frac{ kg }{ 1 kg} = 109 \frac{ MJ }{ 1 kg} $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well this shows surprising parity.  I thought that rocket engines would be less efficient, but I guess not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to this, manufacture of the propellant isn't thermodynamically trivial.  A lot of chemical production processes will require a significant multiple of the stored enthalpy.  I can't easily find the number for Hydrogen production.  So suffice it to say that the energy required will probably be a good deal higher than the above number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the above number of 100 MJ is about 28 kW-hours, which is about $3 of electricity.  But that depends on where you live.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="21562" LastEditDate="2019-03-14T13:17:50.853" LastActivityDate="2019-03-14T13:17:50.853" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="4332" PostTypeId="1" AcceptedAnswerId="4338" CreationDate="2014-04-22T16:52:07.590" Score="24" ViewCount="6960" Body="&lt;p&gt;Why does NASA send rovers such as Curiosity instead of remotely building a small building on the surface of Mars?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="49" LastEditDate="2015-07-09T12:19:16.247" LastActivityDate="2015-07-09T12:19:16.247" Title="Why hasn't NASA planted a stationary lab on Mars yet?" Tags="|mars|nasa|rovers|lander|experiment|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4333" PostTypeId="2" ParentId="4332" CreationDate="2014-04-22T18:51:15.133" Score="20" Body="&lt;p&gt;In the 1970s, NASA sent two &lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_1&quot;&gt;Viking&lt;/a&gt; probes to Mars. Is this the 'unmovable lab' you're looking for?&lt;br&gt;&#xA;The drawback of a stationary lander is that it can only gather data on one location. When you have finished analyzing that location, the mission is over. &#xA;A rover, on the other hand, can gather data in different locations. Its mission can last as long as the rover can move.&lt;br&gt;&#xA;Remotely building a small building seems like a lot of effort for little value. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-04-22T18:51:15.133" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4334" PostTypeId="2" ParentId="4332" CreationDate="2014-04-22T20:36:16.720" Score="15" Body="&lt;p&gt;The short answer is that it's easier to build a rover with all the lab-stuff on it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reuse of things in-space is harder than one might think. If there was one re-usable lab, it would need to be re-stocked, repaired, etc. Different rovers would need to be 'compatible' with the existing lab, hampering new development. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not only that, but the rovers would need to travel back and forth to the lab-unit, and the rovers are slow as it is!&lt;/p&gt;&#xA;" OwnerUserId="692" LastActivityDate="2014-04-22T20:36:16.720" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4335" PostTypeId="2" ParentId="4332" CreationDate="2014-04-22T21:22:24.040" Score="3" Body="&lt;p&gt;Planetary scientists have yet to identify one single location on Mars where a stationary lab or other structure makes sense. I suppose if you could make a case for many Martian years' worth of data at the same location you might sell a mission, a weather-station concept, perhaps. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as other permanent structures go, the only plans thus far I've seen have been to prepare for or be part of manned missions, the most interesting of which to me are &lt;a href=&quot;http://en.wikipedia.org/wiki/In-situ_resource_utilization#Mars&quot; rel=&quot;nofollow&quot;&gt;in-situ resource utilization&lt;/a&gt; schemes sent ahead to produce consumables to support colonization.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="49" LastEditDate="2014-04-22T23:31:31.967" LastActivityDate="2014-04-22T23:31:31.967" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4336" PostTypeId="2" ParentId="4332" CreationDate="2014-04-23T01:26:17.573" Score="5" Body="&lt;p&gt;Re Quora Fean's comment&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I imagine the OP means to have a fixed lab at a convenient location, and a smaller rover collecting samples and bringing them to this lab.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's similar to how the &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Pathfinder&quot;&gt;1997 Mars Pathfinder mission&lt;/a&gt; worked. The Sojourner rover didn't have a robotic arm. It also didn't have the ability to communicate with the Earth. That was the base station's job. The Pathfinder base station also carried a number of stationary experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have more lander missions to Mars than there have been rover missions. NASA has had the most rover missions, and even they have had more landers than rovers. NASA has had four lander missions (one of which failed), three rover missions, and one lander/rover mission (Pathfinder/Sojourner).&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-04-23T01:26:17.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4337" PostTypeId="1" CreationDate="2014-04-23T03:20:45.683" Score="11" ViewCount="1769" Body="&lt;p&gt;I'm creating a super sonic ping pong ball cannon like the one recently shown on Mythbusters. I plan on using pressure rated steel pipe so it's as safe as realistically possible and showing it off at science and technology exhibits for kids. I think it will be a big hit and a great way to get kids interested in physics and science. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The device is pretty simple and well explained in this video here:&#xA;&lt;a href=&quot;https://www.youtube.com/watch?v=YYNCGZCul1Q&quot; rel=&quot;noreferrer&quot;&gt;https://www.youtube.com/watch?v=YYNCGZCul1Q&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key to the whole thing is ~100 psi on one side of a burst disk with a de Laval nozzle attached to a barrel on the other side. The barrel and nozzle is all in a pretty strong vacuum. The pressure chamber is using 4&quot; pipe and the barrel is 1.5&quot; diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the most common usage for de Laval nozzles is in rocket motors, I ended up here. Reading and research has led me to believe that a convergent angle of 30 degrees is common in rockets. I've also read that the divergent angle of 15 degrees works but less works as well. What I can't find anywhere is the throat size. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the goal to get the highest velocity possible using 100 PSI and 4&quot; diameter on the convergent side and 1.5&quot; diameter on the divergent side which is the best convergent angle, divergent angle, and most importantly, the throat diameter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've found equations that I think would provide this information but they are well beyond my ability to solve so any help would be greatly appreciated!&lt;/p&gt;&#xA;" OwnerUserId="3148" LastActivityDate="2020-04-26T17:45:54.303" Title="Convergent-Divergent / De Laval Nozzle Dimensions" Tags="|nozzle|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4338" PostTypeId="2" ParentId="4332" CreationDate="2014-04-23T03:46:33.430" Score="49" Body="&lt;p&gt;To first answer the question &lt;em&gt;ad litteram&lt;/em&gt;, NASA has so far successfully landed four &lt;em&gt;stationary labs&lt;/em&gt; on Mars (Mars landing dates in parentheses):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_1#Lander&quot; rel=&quot;nofollow noreferrer&quot;&gt;Viking Lander 1&lt;/a&gt; (July 20, 1976), &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_2#Lander&quot; rel=&quot;nofollow noreferrer&quot;&gt;Viking Lander 2&lt;/a&gt; (September 3, 1976), &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/mars-pathfinder/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Pathfinder&lt;/a&gt; (July 4, 1997) and &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/phoenix/main/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phoenix Mars Lander&lt;/a&gt; (May 25, 2008). &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Here are their landing sites on a map of Mars generated with &lt;a href=&quot;http://ode.rsl.wustl.edu/mars/indexMapSearch.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Orbital Data Explorer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/WLGgm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WLGgm.jpg&quot; alt=&quot;NASA Mars mission landing sites&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And in 2016, the NASA's &lt;a href=&quot;http://www.jpl.nasa.gov/missions/insight/&quot; rel=&quot;nofollow noreferrer&quot;&gt;InSight&lt;/a&gt; (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander, is planned to land on Mars on 20 September 2016. These are all &lt;em&gt;stationary, non-mobile&lt;/em&gt; in-situ robotic laboratories.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why use immobile landers? I touch one major point later on in my answer with troubles with rover wheels, but not all science we want to do on Mars (or anywhere else) actually requires a mobile science laboratory. So if you can avoid actuator dependency and having to use other moving parts, you could obviously build a more robust lander that would be cheaper and lighter, and also require simplified AI and power constraints to run it. Some long-term uses could be measuring environmental conditions and changes, i.e. a weather station, communications relay station, seismology, radiation and insolation experiments, deep ice / regolith drilling and sampling, prefabricated greenhouse / biodome experiments laboratory, astronomy, even to serve as a base laboratory for other mobile units, manned missions, or as a component unit to &lt;a href=&quot;https://en.wikipedia.org/wiki/In-situ_resource_utilization&quot; rel=&quot;nofollow noreferrer&quot;&gt;in-situ resource utilization&lt;/a&gt; (ISRU) processes, production and fabrication.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for why there aren't more of them. Perhaps this infographic can paint the clearest picture (click on the image for description and links to poster-size versions):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://space.meta.stackexchange.com/a/474/49&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mSbT0.jpg&quot; alt=&quot;Bryan Christie, Harold Clenet: Missions to Mars infographic&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, landing on Mars is hard, all those lines that cut at the outermost ring around Mars on the infographic depict failed missions to the red planet. Among many failures is also one of NASA's landers - &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_polar_lander&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Polar Lander&lt;/a&gt;. So there's no wonder that we can see adult men shedding tears of joy in &lt;a href=&quot;https://www.youtube.com/watch?v=34voepoD_8Y&quot; rel=&quot;nofollow noreferrer&quot;&gt;videos&lt;/a&gt; of the &lt;a href=&quot;http://mars.jpl.nasa.gov/msl/&quot; rel=&quot;nofollow noreferrer&quot;&gt;MSL Curiosity&lt;/a&gt; landing as it successfully touched down on August 6, 2012. Sky Crane landing of MSL was dubbed by NASA as &lt;em&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/video/index.php?id=1090&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity's Seven Minutes of Terror&lt;/a&gt;&lt;/em&gt;, and for a good reason;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nobody ever attempted anything quite like it before. MSL was too heavy to be safely delivered to Martian surface like its lighter predecessors were, decelerated to acceptable drop-off speed using a set of drogue and main chutes and encased in a &lt;em&gt;grape of airbags&lt;/em&gt; serving as impact attenuators and cushioning their final landing by bouncing on surface to a complete stop. For parcels heavier than, say, one metric ton, landing on Mars involves depending on far more complicated Entry, Descent and Landing (EDL) systems. One ingenious design that is currently being developed by NASA's Jet propulsion Laboratory (JPL) is the &lt;a href=&quot;http://www.nasa.gov/mission_pages/tdm/ldsd/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low-Density Supersonic Decelerator&lt;/a&gt; (LDSD). If the &lt;a href=&quot;http://www.nasaspaceflight.com/2014/06/ldsd-rocket-powered-saucer-landing-technology/&quot; rel=&quot;nofollow noreferrer&quot;&gt;quirks&lt;/a&gt; of this new approach to entry and descent parts of EDL can be successfully dealt with and the tech proves reliable, it would enable delivery of heavier payloads to the landing phase of EDL on Mars, but it doesn't address the landing itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But difficulties don't end with successful landing. For robotic missions (and there have been no other so far), Martian environment is harsh. Extremely fine grain and sharp dust due to lack of atmospheric moisture that can find its way into the narrowest of openings, corrosive &lt;a href=&quot;http://www.space.com/21554-mars-toxic-perchlorate-chemicals.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;perchlorate salts in its top regolith&lt;/a&gt;, dust storms and dust devils that can randomly cover or &lt;a href=&quot;http://www.space.com/25577-mars-rover-opportunity-solar-panels-clean.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;clean&lt;/a&gt; solar panels, low atmospheric pressure (~ half percent of Earth's mean sea-level pressure) that limits convective heat transfer and complicates aerobraking and parachute descent, low average temperatures and extreme diurnal temperature cycles, with temperature difference due to atmospheric pressure within tens of degrees head-to-toe, rough and unweathered, untraversed terrain that hides all kinds of nasty surprises like bursting fans of evaporating dry ice from under the regolith during Martian springs, deep pockets of sand, sand filled lava chimneys, and the list could go on and on, really.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then there's the communication delay of a minimum of around 4 minutes in one direction, depending on proximity of Mars to Earth, but more commonly in the order of over 20 minutes for the communications round trip, that further complicates design of robotic missions and substantially lowers what we can expect from them. If I can reuse Douglas Adam's analogy with human fascination with digital wrist watches, as far as robotic autonomy is concerned, we're now roughly at the stage of being fascinated with automated house cleaners that can find their way around obstacles and plug themselves to the grid when they're running out of juice. We're not yet far enough for a base laboratory station with robotic explorers returning samples to it for analysis on another planet. These units would require autonomy (artificial intelligence) that we can't yet provide for them and each of the units would be more complicated in design than any of the currently still operational rovers on Mars (MSL and Opportunity), including future ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/ExoMars_rover&quot; rel=&quot;nofollow noreferrer&quot;&gt;ExoMars rover&lt;/a&gt; and NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020_rover_mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars 2020 rover&lt;/a&gt; missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MSL is also &lt;a href=&quot;https://space.stackexchange.com/q/3187/49&quot;&gt;experiencing problems with its wheels&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_%28rover%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Opportunity&lt;/a&gt; (MER-B) rover is &lt;a href=&quot;http://www.newscientist.com/article/dn16803-mars-rover-opportunity-experiences-wheel-trouble.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;driving backwards for years now&lt;/a&gt;, dragging the right front wheel along as it started drawing more electrical current than the rest of them since mid 2011. &lt;a href=&quot;http://www.nasa.gov/mission_pages/mars/missions/mer/mer-20060404_prt.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Same mitigation&lt;/a&gt; or driving in reverse to reduce stress, wear and tear on damaged wheels was used with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Spirit_%28rover%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spirit&lt;/a&gt; (MER-A) rover, and is &lt;a href=&quot;http://www.space.com/24764-mars-rover-curiosity-driving-reverse.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;now also used with MSL Curiosity rover&lt;/a&gt;. That's all due to the distance they managed to make through the rough terrain since they became operational. If these rovers had to double back to return samples to a stationary laboratory, the distance they'd have to traverse could double for about the same amount of science done, and their range from the base laboratory they could explore would be rather limited. Which would be a shame, since &lt;a href=&quot;http://mars.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars still doesn't fail to surprise us on daily basis&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is also quite large, with huge diversity in features, and lots of intraversible barriers surrounding them. So it would be a shame to invest in exploring a single area so much, when you can cover for the same money larger area in many different areas, and have some redundancy in place too, in case some of the units fail or something else goes wrong before even safely landing on Mars. If you have one &lt;em&gt;stationary laboratory&lt;/em&gt; and one &lt;em&gt;explorer/worker unit&lt;/em&gt; that depend on each other, you've just increased your chance of failure 100%, since a failure of any of the two would render the other unit obsolete. With the current Mars mission success rate statistics, nobody is willing to lower their chances from the mission planning stage onwards.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-09-27T11:17:55.303" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4339" PostTypeId="1" AcceptedAnswerId="4347" CreationDate="2014-04-23T04:04:30.890" Score="14" ViewCount="6982" Body="&lt;p&gt;Using the Python library pyephem I am calculating passes for the ISS based on TLE data and my lat/long, but how can I tell which of the many passes it returns per day are visible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that satellites are only visible around sunrise or sunset, but is there any systematic way to calculate wether or not a pass is in the range of time when it could be visible?&lt;/p&gt;&#xA;" OwnerUserId="3072" LastEditorUserId="3072" LastEditDate="2014-04-23T04:15:48.533" LastActivityDate="2018-10-24T20:32:40.637" Title="Calculating Which Satellite Passes are Visible" Tags="|artificial-satellite|orbit|low-earth-orbit|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4340" PostTypeId="1" AcceptedAnswerId="4341" CreationDate="2014-04-23T08:11:08.933" Score="4" ViewCount="844" Body="&lt;p&gt;Are there any planets or moons in the Outer Solar System, where a man-made object has landed and made photographs of its surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can remember an article somewhere that said there's only one moon where anything has ever landed and sent pictures of the surface, but I can't remember which moon that was, and I'm also not sure that it was actually the only case.&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="897" LastEditDate="2014-04-25T06:37:30.567" LastActivityDate="2014-04-25T06:37:30.567" Title="Has there been any mission to land on a planet or moon in the Outer Solar System?" Tags="|solar-system|landing|photography|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="4341" PostTypeId="2" ParentId="4340" CreationDate="2014-04-23T09:38:33.277" Score="10" Body="&lt;p&gt;In 2005 the &lt;a href=&quot;http://sci.esa.int/cassini-huygens/47052-huygens/&quot;&gt;Huygens probe&lt;/a&gt; successfully landed on Titan, satellite of Saturn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that any body with a solid/liquid surface could be a suitable target. Many jovian or saturnine moons like Enceladus, Phoebe or Hyperion feature solid surfaces.&lt;/p&gt;&#xA;" OwnerUserId="298" LastActivityDate="2014-04-23T09:38:33.277" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4342" PostTypeId="1" AcceptedAnswerId="4345" CreationDate="2014-04-23T10:35:07.097" Score="8" ViewCount="481" Body="&lt;p&gt;A couple of comments on the question &lt;a href=&quot;https://space.stackexchange.com/questions/4340&quot;&gt;Has there been any mission to land on a planet or moon in the Outer Solar System?&lt;/a&gt; suggesting that there is no scientific benefit to a crash on a moon, reminded me of the recent LADEE intentional crash. It seems like there may have been at least half a dozen, intentionally crashed for scientific rationale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many space craft have been intentionally crashed through 2014 (&lt;em&gt;include pending/planned&lt;/em&gt;)? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-04-23T18:17:24.707" Title="How many space craft have been intentionally crashed through 2014?" Tags="|impact|uncrewed-spaceflight|end-of-life|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4344" PostTypeId="1" CreationDate="2014-04-23T12:41:48.010" Score="9" ViewCount="753" Body="&lt;p&gt;What is the difference between re-entry from the Moon and Re-entry from low Earth orbit? &lt;/p&gt;&#xA;" OwnerUserId="3155" LastEditorUserId="2699" LastEditDate="2014-04-24T00:26:15.137" LastActivityDate="2014-04-24T00:26:15.137" Title="Re-entry from Moon vs. re-entry from low Earth orbit" Tags="|the-moon|low-earth-orbit|reentry|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4345" PostTypeId="2" ParentId="4342" CreationDate="2014-04-23T13:56:33.330" Score="7" Body="&lt;p&gt;The vast majority of these have been around the moon. Here's a list of what I can find (&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_artificial_objects_on_the_Moon&quot; rel=&quot;nofollow&quot;&gt;Using Wikipedia&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Luna- 2 (Spacecraft and rocket)&lt;/li&gt;&#xA;&lt;li&gt;Ranger- 4, 6, 7, 8, 9&lt;/li&gt;&#xA;&lt;li&gt;Apollo LEMs- Apollo 12, 14, 15, 17.&lt;/li&gt;&#xA;&lt;li&gt;Apollo S-IVB- Apollo 13, 14, 15, 16, 17&lt;/li&gt;&#xA;&lt;li&gt;1990's missions- Hiten, Lunar Prospector&lt;/li&gt;&#xA;&lt;li&gt;2000's missions- SMART-1, Moon Impacting Probe, Chang'e 1, Chandrayaan-1, SELENE, LCROSS shepherding spacecraft, LCROSS Centaur, &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, on to the planets/comets/ etc!&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Scott and Amundsen- (Mars Impactors for the Deep Space 2 mission)&lt;/li&gt;&#xA;&lt;li&gt;Deep Impact- impacted a comet&lt;/li&gt;&#xA;&lt;li&gt;Galileo Probe - &quot;Impacted&quot; Jupiter, although there isn't really a Jupiter surface&lt;/li&gt;&#xA;&lt;li&gt;Galileo- After the end of it's mission, &quot;impacted&quot; Jupiter.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;By my count, that's 25 Lunar Impacting missions, and 5 others, for a total of 30.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what reasons do they do intentional crashes? Here's some of the most common reasons.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Most of the Apollo crashes were to assist in determining the composition of the moon. The Astronauts left seismometers on the moon, which the exact landing spot and time of an intentional crash allowed them to figure out the composition of the moon.&lt;/li&gt;&#xA;&lt;li&gt;Some crashed to create plumes, and figure out what was in them. Most of the later lunar probes (LCROSS, SMART-1, etc) have done this, plus the Deep Impact space probe. Some of these were an afterthought, as a mission was ending they wanted to get some value out of it, others were done as a primary mission.&lt;/li&gt;&#xA;&lt;li&gt;Some use the impact of the crash to try and get scientific instruments on the ground. These include MIP, and the Deep Space 2 missions.&lt;/li&gt;&#xA;&lt;li&gt;Some are done to prevent contamination. This includes the Galileo crash, as Cassini will also follow.&lt;/li&gt;&#xA;&lt;li&gt;Luna-2 was actually crashed as a propaganda strike, littering the surface with Soviet ribbons.&lt;/li&gt;&#xA;&lt;li&gt;The Ranger missions were designed to see the moon up close, for a very short period of time. They ended with impact of the Moon.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Some lunar missions (Quite a few) ended in less controlled manners. There is a finite lifetime for a Lunar mission, of only a few weeks to months without fuel. Missions weren't listed that didn't crash at a pre-defined time and place. LADEE for one is included in this list.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-04-23T18:17:24.707" LastActivityDate="2014-04-23T18:17:24.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4346" PostTypeId="2" ParentId="4344" CreationDate="2014-04-23T16:05:51.197" Score="14" Body="&lt;p&gt;I suspect you're asking about low earth orbit versus a lunar return trajectory. (Strictly speaking, that lunar return trajectory was an orbit). The difference between the two is velocity. The Shuttle reentered about about 7.7 kilometers per second. The Apollo spacecraft, at about 11 kilometers/second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That extra 3.3 kilometers per second doesn't sound like much, but it is. Since kinetic energy is proportional to the square of velocity, that means that energy per unit mass for a lunar return is more than double that of a reentry from low earth orbit. That doubling in energy in turn mandates drastically different protection against the temperatures that build up due to reentry.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-04-23T16:05:51.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4347" PostTypeId="2" ParentId="4339" CreationDate="2014-04-23T21:57:40.227" Score="14" Body="&lt;p&gt;After doing some research I have found the answer to my problem on Celestrak. They have a great article about exactly this that I will summarize here, though if your interested in doing this you should read it &lt;a href=&quot;https://celestrak.org/columns/v03n01/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;There are 3 main requirements for a satellite to be visible:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;• The satellite must be above the observer's horizon.&#xA;• The sun must be below the observer's horizon enough to darken the sky.&#xA;• The satellite must be illuminated by the sun.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;The first point we can ignore because by definition a pass calculated by any library or website has to be over the horizon for it to be a pass over the specified location.&lt;/p&gt;&#xA;&lt;p&gt;This second point is crucial for what I am interested in. The sun must be at least -6˚ below the horizon so that it is dark enough that the light from the sun reflected on the satellite is bright enough relative to the sky to stand out, this is called the nautical twilight. So the sun must have set, but it also must not be below -18˚ otherwise not enough sunlight can get to the satellite for it to be visible. This is why you can only see satellites for a few hours around dawn or dusk.&lt;/p&gt;&#xA;&lt;p&gt;The third point is that the object must not be eclipsed by the earth. If you are not using pyephem or another library that calculates this then I highly suggest you read their article which details how to calculate it. Since pyephem does calculate wether the object is eclipsed I will not get into the math here and simply say that this can be found using iss.eclipsed for example.&lt;/p&gt;&#xA;&lt;p&gt;To tie this all together, using pyephem one can find the next pass and wether it's visible like this:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;def seconds_between(d1, d2):&#xA;    return abs((d2 - d1).seconds)&#xA;&#xA;def datetime_from_time(tr):&#xA;    year, month, day, hour, minute, second = tr.tuple()&#xA;    dt = datetime.datetime(year, month, day, hour, minute, int(second))&#xA;    return dt&#xA;&#xA; def get_next_pass(lon, lat, alt, tle):&#xA;&#xA;    sat = ephem.readtle(str(tle[0]), str(tle[1]), str(tle[2]))&#xA;&#xA;    observer = ephem.Observer()&#xA;    observer.lat = str(lat)&#xA;    observer.long = str(lon)&#xA;    observer.elevation = alt&#xA;    observer.pressure = 0&#xA;    observer.horizon = '-0:34'&#xA;&#xA;    now = datetime.datetime.utcnow()&#xA;    observer.date = now&#xA;&#xA;    tr, azr, tt, altt, ts, azs = observer.next_pass(sat)&#xA;&#xA;    duration = int((ts - tr) *60*60*24)&#xA;    rise_time = datetime_from_time(tr)&#xA;    max_time = datetime_from_time(tt)&#xA;    set_time = datetime_from_time(ts)&#xA;&#xA;    observer.date = max_time&#xA;&#xA;    sun = ephem.Sun()&#xA;    sun.compute(observer)&#xA;    sat.compute(observer)&#xA;&#xA;    sun_alt = degrees(sun.alt)&#xA;&#xA;    visible = False&#xA;    if sat.eclipsed is False and -18 &amp;lt; degrees(sun_alt) &amp;lt; -6 :&#xA;        visible = True&#xA;&#xA;    return {&#xA;             &amp;quot;rise_time&amp;quot;: timegm(rise_time.timetuple()),&#xA;             &amp;quot;rise_azimuth&amp;quot;: degrees(azr),&#xA;             &amp;quot;max_time&amp;quot;: timegm(max_time.timetuple()),&#xA;             &amp;quot;max_alt&amp;quot;: degrees(altt),&#xA;             &amp;quot;set_time&amp;quot;: timegm(set_time.timetuple()),&#xA;             &amp;quot;set_azimuth&amp;quot;: degrees(azs),&#xA;             &amp;quot;elevation&amp;quot;: sat.elevation,&#xA;             &amp;quot;sun_alt&amp;quot;: sun_alt,&#xA;             &amp;quot;duration&amp;quot;: duration,&#xA;             &amp;quot;visible&amp;quot;: visible&#xA;           }&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="3072" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2015-11-06T07:56:24.020" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4348" PostTypeId="2" ParentId="4069" CreationDate="2014-04-24T00:55:11.630" Score="6" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.arianespace.com/launch-services-soyuz/Soyuz_Users_Manual_CSG_June06.pdf#page=85&quot; rel=&quot;noreferrer&quot;&gt;http://www.arianespace.com/launch-services-soyuz/Soyuz_Users_Manual_CSG_June06.pdf#page=85&lt;/a&gt; pages 85, 59-61 the red horns are placed exactly over some Air-conditioning system hatches, which are used to stabilize temperature in the fairing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When  Soyuz rocket is transferred, the horns are used to plug air conditioning unit from the transport platform to the fairing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the rocked is moved to vertical state, the conditioning may be temporary unplugged (for example, Proton rocket have 4-hours with unplugged air conditioning: &lt;a href=&quot;http://www.federalspace.ru/464/&quot; rel=&quot;noreferrer&quot;&gt;http://www.federalspace.ru/464/&lt;/a&gt; &quot;&lt;em&gt;Во время установки РН на пусковое устройство в течение 4-х часов система кондиционирования должна быть отключена от КГЧ. После подвода к РН башни обслуживания работа системы кондиционирования возобновляется.&lt;/em&gt;&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The system is finally unplugged several hours before start (again, in case of Proton-K, 2 hours before start - &quot;&lt;em&gt;Система кондиционирования воздуха должна быть отключена за 2 часа до пуска.&lt;/em&gt;&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Soyuz there are some numbers in the linked pdf, page 60 &quot;Table 3.6 Air Conditioning under the Fairing&quot;. Several conditioning systems are listed: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;CARAVAN  - 3 hours for moving the rocket from UCIF to Launch pad&lt;/li&gt;&#xA;&lt;li&gt;&quot;None&quot; for 1 hour (mating on LV)&lt;/li&gt;&#xA;&lt;li&gt;&quot;High mode venting&quot; (up to 4 days, when rocket is installed on the LP)&lt;/li&gt;&#xA;&lt;li&gt;&quot;Low mode venting&quot; &lt;/li&gt;&#xA;&lt;li&gt;&lt;ul&gt;&#xA;&lt;li&gt;up to 45 minutes - from H0 -45minutes to liftoff&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;ul&gt;&#xA;&lt;li&gt;and some more time for aborted launch... seems up to 4 hours, before High mode is connected back.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The red horns are installed at two top intakes of &quot;Low pressure air-conditioning system&quot;. There are two more inlets at bottom part of fairing, near the yellow pipes on your second image.&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2014-04-24T01:04:32.667" LastActivityDate="2014-04-24T01:04:32.667" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4349" PostTypeId="1" AcceptedAnswerId="4358" CreationDate="2014-04-24T01:28:41.150" Score="15" ViewCount="1048" Body="&lt;p&gt;I've been watching some recorded videos from the April 22 - 24, 2014 &lt;a href=&quot;http://h2m2014.exploremars.org/&quot; rel=&quot;noreferrer&quot;&gt;Humans2Mars conference&lt;/a&gt; (videos and live streams, when available, are on the &lt;a href=&quot;http://new.livestream.com/viewnow/exploremars&quot; rel=&quot;noreferrer&quot;&gt;National Institute of Aerospace channel on Livestream&lt;/a&gt;), in particular the &lt;a href=&quot;http://h2m.exploremars.org/participant/mike-raftery/&quot; rel=&quot;noreferrer&quot;&gt;Mike Raftery&lt;/a&gt;'s (Boeing; Director, ISS Utilization and Exploration) presentation on a concept manned mission to Mars that was prepared by Boeing and Energia, starting roughly 15 minutes into &lt;a href=&quot;http://new.livestream.com/viewnow/exploremars/videos/48845903&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt;. This slide caught my attention:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/kq3YS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, two mission stages call for an orbital assembly at Earth-Moon L&lt;sub&gt;2&lt;/sub&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Lagrange point&lt;/a&gt; (EML2), but all the component parts are supposed to &lt;em&gt;spiral to EML2&lt;/em&gt; beforehand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, we know (&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_rendezvous#History&quot; rel=&quot;noreferrer&quot;&gt;with some trial and error&lt;/a&gt;, failed rendezvous attempt of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_4&quot; rel=&quot;noreferrer&quot;&gt;Gemini 4&lt;/a&gt; with spent &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_%28rocket_family%29#Titan_II&quot; rel=&quot;noreferrer&quot;&gt;Titan II&lt;/a&gt; launch vehicle's upper stage, and then first successful rendezvous of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_6A&quot; rel=&quot;noreferrer&quot;&gt;Gemini 6&lt;/a&gt; with &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_7&quot; rel=&quot;noreferrer&quot;&gt;Gemini 7&lt;/a&gt;, and later first docking of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_8&quot; rel=&quot;noreferrer&quot;&gt;Gemini 8&lt;/a&gt; with unmanned &lt;a href=&quot;https://en.wikipedia.org/wiki/Agena_target_vehicle&quot; rel=&quot;noreferrer&quot;&gt;Agena Target Vehicle&lt;/a&gt;) that the dynamics of the in-orbit rendezvous and docking aren't exactly intuitive. You can't simply apply thrust in the velocity vector to catch up with your target vehicle, because that will add energy to your orbit and you'll end up increasing your altitude and with it distance to the target. And we also know that stationkeeping in Lagrange points isn't anything like orbiting around some massive body and &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot; rel=&quot;noreferrer&quot;&gt;halo&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;noreferrer&quot;&gt;Lissajous&lt;/a&gt; orbits are the usual approach to maintaining one's position relative to L-points. So here's my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;With Lagrange point stationkeeping requiring entirely different approach to maintaining attitude than when in orbits around parent bodies, do we sufficiently understand flight mechanics of EML2 rendezvous and assembly / docking? Have we already done anything like it, run extensive physics simulations, or will proof of concept missions be required before risking billions of dollars of hardware on a manned mission to Mars and do that first as soon as it's required, if it indeed called for EML2  assembly?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-04-24T19:04:42.860" LastActivityDate="2014-04-25T02:05:02.207" Title="Do we sufficiently understand mechanics of Lagrange point stationkeeping for EML2 rendezvous and assembly?" Tags="|orbital-mechanics|lagrangian-points|orbital-assembly|station-keeping|rendezvous|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4352" PostTypeId="1" AcceptedAnswerId="4355" CreationDate="2014-04-23T13:56:09.663" Score="11" ViewCount="1296" Body="&lt;p&gt;The Lunar Atmosphere and Dust Environment Explorer (LADEE) was intentionally crashed into the moon by NASA &lt;strong&gt;but why did they crash the craft instead of leaving it in orbit&lt;/strong&gt;, or was it a part of some mission to study how the impact takes place?&lt;/p&gt;&#xA;" OwnerUserId="3173" OwnerDisplayName="Randomizer" LastEditorUserId="49" LastEditDate="2014-04-24T19:00:14.460" LastActivityDate="2014-04-26T06:20:58.753" Title="Why did NASA intentionally crash the Lunar Atmosphere and Dust Environment Explorer (LADEE) on the moon?" Tags="|the-moon|nasa|probe|ladee|scientific-data|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4353" PostTypeId="2" ParentId="4352" CreationDate="2014-04-23T18:08:54.767" Score="8" Body="&lt;p&gt;LADEE was going to run out of fuel, and they wanted - and succeeded - to collect data of Moon's atmosphere and levitated dust.&#xA;(LADEE means 'Lunar Atmosphere and Dust Environment Explorer'.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fuel is needed to compensate orbital degradation due to Moon's non-symmetric gravitational field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Online communication was possible due to superfast laser data transmission (Lunar Laser Communication Demonstration, or LLCD).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More details &lt;a href=&quot;http://www.csmonitor.com/Science/2014/0418/Moon-orbiter-LADEE-crashes-triumphantly-after-amazing-mission-video&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://www.americaspace.com/?p=55743&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1454" OwnerDisplayName="Gerald" LastActivityDate="2014-04-24T13:46:58.147" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4354" PostTypeId="1" AcceptedAnswerId="6444" CreationDate="2014-04-24T16:56:24.803" Score="2" ViewCount="1246" Body="&lt;p&gt;Lots of articles out about how Orbital Sciences is talking with three vendors for a replacement for the &lt;a href=&quot;https://en.wikipedia.org/wiki/NK-33&quot; rel=&quot;nofollow noreferrer&quot;&gt;NK-33/AJ26&lt;/a&gt; engine pair currently used on the Antares booster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An &lt;a href=&quot;http://www.spacenews.com/article/financial-report/40319orbital-hopes-to-parlay-stratolaunch-work-into-more-capable-antares&quot; rel=&quot;nofollow noreferrer&quot;&gt;article on SpaceNews&lt;/a&gt; notes three vendors and says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thompson told investors on April 17 that Orbital is weighing offers from three companies for the Antares first-stage engine. Two are from Russia, including the current supplier, and one is from the United States.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and also: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Orbital Sciences Corp.’s contract with startup Stratolaunch Systems on a rocket to be launched from beneath a large aircraft will produce an enhanced version of Orbital’s Antares rocket capable of handling 15-25 percent more payload, depending on the orbit, Orbital Chief Executive David W. Thompson said April 23.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, current vendor seems to be &lt;a href=&quot;http://en.wikipedia.org/wiki/Kuznetsov_Design_Bureau&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kuznetsov&lt;/a&gt; and it looks like they are considering reopening NK-33 production.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume the other Russian company would have to be whoever makes the &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-180&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-180&lt;/a&gt;.  Asked in a question &quot;&lt;a href=&quot;https://space.stackexchange.com/questions/4070/how-is-an-rd-180-engine-a-reasonable-replacement-for-2-nk-33-aj-26-engines-on-an&quot;&gt; How is an RD-180 engine a reasonable replacement for 2 NK-33/AJ-26 engines on Antares?&lt;/a&gt; it seems that the RD-180, with slightly higher thrust would likely be used to increase the payload, per my second quote snippet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who could be the US company?  What RP1-LOX engine is out there, from a US manufacturer, since Orbital does not have time for a new engine development project, if they intend to bid on the second Cargo Resupply contract for NASA.  They need engines sooner than that. Knowing Orbital, maybe it is ATK offering a solid booster (Minotaur writ large?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what could the three engines be?  I guess new build NK-33's made in Russia, RD-180, and some unknown US engine. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2022-05-30T11:18:38.430" Title="What replacement engine options are there for Antares?" Tags="|engines|rd-180|nk-33|orbital|antares|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4355" PostTypeId="2" ParentId="4352" CreationDate="2014-04-24T17:22:19.787" Score="9" Body="&lt;p&gt;Once in orbit, always in orbit doesn't apply to the moon--the gravity is too uneven.  Stuff left in lunar orbit generally crashes into the moon after a while.  (All the Apollo landers that were left in lunar orbit are gone by now.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to observing the crash they also wanted to control the crash--make sure it didn't happen on top of anything sitting on the moon.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-04-24T17:22:19.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4356" PostTypeId="1" CreationDate="2014-04-24T20:10:42.623" Score="10" ViewCount="948" Body="&lt;p&gt;Google maps has a new (to me) feature, and it is pretty awesome!  If you start at maps.google.com, switch to Earth view, and zoom all the way out, you get a view something like the one shown below.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has a lot of features, namely&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Sun&lt;/li&gt;&#xA;&lt;li&gt;The Moon&lt;/li&gt;&#xA;&lt;li&gt;The bright and dark sides of the Earth&lt;/li&gt;&#xA;&lt;li&gt;The bright and dark sides of the Moon&lt;/li&gt;&#xA;&lt;li&gt;The Milky Way&lt;/li&gt;&#xA;&lt;li&gt;The dark side of the Earth shows anthropogenic light&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Are these features geometrically correct?  Is the inclination of the galactic plane wrt to the plane of the ecliptic correct? Is the angular size of the Moon wrt to that of the Earth correct? Is it possible to find the other planets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Zhg0e.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2699" LastActivityDate="2015-03-19T16:44:55.710" Title="Is Google Earth's view from space geometrically correct?" Tags="|earth-observation|ecliptic-plane|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4358" PostTypeId="2" ParentId="4349" CreationDate="2014-04-24T21:14:26.307" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I'm interested in any considerations to EML2 docking that are different to orbital docking.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Some aspects of EML2 docking will be much easier. Some will be a bit harder. Some, we'll have to completely rethink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the plus side, the EML2 region is much closer to flat space than is low Earth orbit. Docking will be much more intuitive. That pesky orbital mechanics that makes docking in LEO non-intuitive is pretty much a non-issue. On the downside, this gravitational flatness means NASA won't be able to take advantage of orbital mechanics. New far-field rendezvous techniques will have to be  invented.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that EML2 rendezvous and docking will be easier than LEO rendezvous and docking &lt;em&gt;if everything goes according to plan&lt;/em&gt;. But what if things go wrong? NASA and others take advantage of orbital mechanics for LEO rendezvous and docking when things go wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, suppose the HTV, Dragon, or Cygnus is approaching the ISS on the r-bar, and suppose the astronauts or the mission control don't like what they're seeing. The simplest solution is tell the vehicle to play 'possum. This is a passive abort. Orbital mechanics naturally takes the vehicle down and in front of the ISS. The ISS has a 24 hour period (mandated by the &lt;a href=&quot;http://spacecraft.ssl.umd.edu/design_lib/SSP50235.ISSvehicleIDD.pdf&quot; rel=&quot;noreferrer&quot;&gt;ISS Visiting Vehicles IDD&lt;/a&gt;) before the ISS has to worry about that misbehaving vehicle colliding with the ISS. Passive abort won't work for EML2 rendezvous and docking. Active abort will always be needed, and that will impact the redundancy design of the active vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;Update&lt;/strong&gt;&lt;br&gt;&#xA;Suppose you're in the same orbit as a target vehicle in low Earth orbit but are trailing it by a bit. You fire the thrusters to move towards the target. For a short time, low Earth orbit acts like flat space; you do move towards the target. Then, oops, orbital mechanics starts to rear its ugly head and you start rising above it and falling behind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravity gradient is much smaller in the vicinity of EML2 than it is in low Earth orbit. This diminished gravity gradient is one of the ways in which the EML2 looks &quot;flat&quot;. Another way in which it looks flat is from the perspective of gravitational acceleration. It's small. Finally, from the perspective of an Earth-Moon rotating frame, the EML2 point is flat. The region about it is close to flat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What this means is that a spacecraft has a long time between performing an impulsive burn and seeing gravity perturb the flat space response to that impulsive burn. Near-field rendezvous is &quot;simpler&quot;, more intuitive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The downside is that rendezvous is &quot;simpler&quot;. NASA's astronauts were indeed confused by those non-intuitive behaviors during the first Gemini missions, but that was fifty years ago. People have learned how to deal with those behaviors since then. In fact, they've learned how to take advantage of them. There are lots of ways to take advantage of a gravity field. Phasing orbits rather than Lambert-based approaches. Passive aborts rather than active ones. Twice Orbital Rate V-bar Approach (TORVA). We'll have to make far-field rendezvous much slower and we'll probably have to burn more fuel to do rendezvous in the EML2 region.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-04-25T02:05:02.207" LastActivityDate="2014-04-25T02:05:02.207" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4359" PostTypeId="2" ParentId="4354" CreationDate="2014-04-24T21:59:54.863" Score="2" Body="&lt;p&gt;The Falcon 9 uses an RP1-LOX engine developed by SpaceX called the &lt;a href=&quot;http://en.wikipedia.org/wiki/Merlin_%28rocket_engine%29&quot; rel=&quot;nofollow&quot;&gt;Merlin&lt;/a&gt;. The thrust is significantly lower (using a Merlin-1D) than a RD-180 (155,000 lb*f vac. vs 933,400 lb*f vac.), but they are smaller (2.92m high x 1.67m diameter x 630Kg vs 3.56m high x 3.15m diameter x 5,480Kg) and the Falcon 9 uses nine of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eye balling the diameters you could fit about 4 Merlins per RD-180, still significantly less thrust, but 10,000Kg less.&lt;/p&gt;&#xA;" OwnerUserId="3061" LastActivityDate="2014-04-24T21:59:54.863" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4360" PostTypeId="1" AcceptedAnswerId="4364" CreationDate="2014-04-24T23:17:24.287" Score="8" ViewCount="283" Body="&lt;p&gt;When a spacecraft approaches its target (whatever is it a moon or a planet), how do they take photographs about it? Do they take as much as possible, or just in certain situations? What does it depends on?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, is it the same during a stay in the atmosphere/on the surface?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="1142" LastEditDate="2014-04-25T14:19:57.790" LastActivityDate="2014-05-09T15:05:36.340" Title="When approaching target planets/moons, how frequently do spacecrafts take photographs?" Tags="|spacecraft|photography|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4361" PostTypeId="1" AcceptedAnswerId="4362" CreationDate="2014-04-24T23:19:37.873" Score="13" ViewCount="2915" Body="&lt;p&gt;Does NASA have projects with spacecraft which are &lt;strong&gt;specifically&lt;/strong&gt; made for orbiting somewhere around the Sun? If yes, what can they be used for?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="3180" LastEditDate="2014-04-25T08:23:25.700" LastActivityDate="2023-07-18T11:36:09.837" Title="Does NASA have spacecraft that are specifically made for orbiting around the Sun?" Tags="|orbit|spacecraft|the-sun|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4362" PostTypeId="2" ParentId="4361" CreationDate="2014-04-25T01:19:21.850" Score="27" Body="&lt;p&gt;I suspect you mean objects that haven't left the solar system (and what is the boundary?) and that aren't orbiting another planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every object en route to another planet that has left the Earth's sphere of influence and has not yet entered the target planet's sphere of influence is orbiting the Sun. Whether the Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 missions count is a matter of debate. Dawn is en route to Ceres. Rosetta is en route to 67P/Churyumov–Gerasimenko; the encounter will be later this month. New Horizons, while on an escape trajectory, hasn't reached Pluto's orbit yet. India's Mars Orbiter Mission and the US's MAVEN satellite are both en route to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also a number of vehicles in pseudo orbits about the linear Sun-Earth Lagrange points (SEL1 and SEL2). These Lagrange point orbits are becoming the go-to place to put orbiting observatories.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to these, a number of satellites have intentionally been placed into heliocentric orbits with the intent to study either interplanetary space or the Sun itself. These include&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pioneer 5, to study interplanetary space between the Earth and Venus;&lt;/li&gt;&#xA;&lt;li&gt;Pioneer 6, 7, 8, and 9, to study the solar wind, solar magnetic field, and solar and cosmic rays;&lt;/li&gt;&#xA;&lt;li&gt;Helios 1 and 2, to study various solar processes;&lt;/li&gt;&#xA;&lt;li&gt;Ulysses, to study the Sun from a very high inclination orbit;&lt;/li&gt;&#xA;&lt;li&gt;Genesis, a sample return mission of solar wind particles;&lt;/li&gt;&#xA;&lt;li&gt;STEREO A and B, which monitor the Sun.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;STEREO A and B are the only two that are currently active.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-04-25T03:16:39.313" LastActivityDate="2014-04-25T03:16:39.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4363" PostTypeId="2" ParentId="4361" CreationDate="2014-04-25T02:14:34.887" Score="21" Body="&lt;p&gt;In addition to the definition given by David H, I would add &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/b/b9/Ulysses_2_orbit.jpg&quot; rel=&quot;noreferrer&quot;&gt;Ulysses&lt;/a&gt;, which is specifically observing the sun from out of the plane of the ecliptic and might mean what the questioner is asking for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ulysses made two passes around the sun, one over each pole, and its mission is officially over after the second pass.  In order to get out of the plane of the ecliptic, it did a gravity assist at Jupiter, so it could come back towards the sun, over the top/bottom.  Pretty amazing orbital mechanics management to do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has a &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Ulysses_2_orbit.jpg&quot; rel=&quot;noreferrer&quot;&gt;nice graphic of how that orbit was managed&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/b/b9/Ulysses_2_orbit.jpg&quot; alt=&quot;2nd orbit of Ulysses around the Sun&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="18788" LastEditDate="2017-03-03T15:25:50.690" LastActivityDate="2017-03-03T15:25:50.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4364" PostTypeId="2" ParentId="4360" CreationDate="2014-04-25T12:01:57.527" Score="10" Body="&lt;p&gt;When a probe approaches a small body like a comet or asteroid, they only get a chance to take a few high-res pictures, because what they make are mosaics. That means that the pictures are actually stitched together from tens of smaller pictures. After a few of these, the perspective and distance will have changed so much, that they can no longer stitch the images together seamlessly. If they inaccurately estimate the relative position of the object to the spacecraft, they may not get the frame they wanted. So they have to either do the math right, or they have to have instruments on board so the probe can decide what the perfect frame is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speed of taking a single images varies wildly. It all depends on how dim the object is and how big the imaging sensor is. If the objects is bright, it takes only a fraction of a second to gather enough photons for a usable image. On the other hand, the Hubble space telescope can take weeks to picture distant galaxies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next limit is the transmission speed. The probe has some internal storage, but eventually it will have to phone home to unload the scientific paydirt. If it is close to earth, that can happen as fast as it can take pictures. If it is in outer space, it can take a very long. The Voyager probes transmit signals at only 160 bits per second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if a probe flies by an object quickly, or is very far from earth, it needs to be picky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbiting probes have more time for transmission, but here also, pictures are often carefully planned in order to get the most out of the probe's lifetime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought that the HRSC instrument on the Mars Express orbiter scanned the Martian surface continually in a line underneath the orbiter, but I was unable to find a source to back this up. Maybe I misremembered. In any case, it pictured more than 60% of the Martian surface, you can't say it's very selective about what it wants to see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rovers like Curiosity have an array of cameras on board. Some take a continuous video stream for navigation, others take mosaic pictures as described above. The picturing time can be anywhere from instantaneous to seconds. Curiosity once took 12 seconds to take a picture of nearby Ceres and Vesta. Pictures like this are &quot;made to order&quot;, but Curiosity also carries artificial intelligence to decide if anything in its surroundings is &quot;out of place&quot; and needs investigating.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2014-05-09T15:05:36.340" LastActivityDate="2014-05-09T15:05:36.340" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4365" PostTypeId="1" AcceptedAnswerId="4382" CreationDate="2014-04-25T15:58:47.473" Score="7" ViewCount="344" Body="&lt;p&gt;I'm only just beginning to scratch the surface of the possibilities in interstellar travel. I know that pulsars can be independently identified by means of measuring their pulses, and that once enough have been identified they can be used to &lt;a href=&quot;https://space.stackexchange.com/questions/942/how-can-spacecraft-navigate-without-contact-from-earth&quot;&gt;identify one's position in space&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: How long (on average) would a pulsar need to be observed in order to identify it from its pulses alone? More to the point, how long would a craft need to watch the pulsars it finds to identify them and get a &quot;fix&quot; on its location based on that? (That is, is the distribution uniform, or is it skewed one way or the other and, if so, how does that affect the average time required to identify enough pulsars?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the purposes of this question, we're assuming highly accurate observational and measuring tools and equipment, rather than a bearded old man with a telescope in his backyard making all his calculations by hand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose another way to ask the same thing would be: What's the average cycle duration of pulsars, and how much skew is there to the distribution?&lt;/p&gt;&#xA;" OwnerUserId="3106" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-04-29T16:19:21.253" Title="How quickly could a craft find its location using only pulsars?" Tags="|interstellar-travel|navigation|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="4367" PostTypeId="2" ParentId="4352" CreationDate="2014-04-26T06:20:58.753" Score="3" Body="&lt;p&gt;Several reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The space probes are orbiting around the moon, below a certain altitude, and definitely at altitudes &lt;a href=&quot;http://www.nasa.gov/content/frequently-asked-questions-ladee-planned-impact/&quot; rel=&quot;nofollow&quot;&gt;as low as 1 - 2 miles&lt;/a&gt;, fuel must be used to keep it in orbit. After it has completed its mission, there will be no/little more fuel left for it to escape the moon's gravity pull again. (Quote from previous reference &lt;code&gt;At the time of impact, the spacecraft will have used all of its fuel.&lt;/code&gt;) So the probes will crash anyways.&lt;/li&gt;&#xA;&lt;li&gt;From &lt;a href=&quot;http://www.nytimes.com/2012/12/14/science/space/two-space-probes-to-crash-intentionally-on-moon.html&quot; rel=&quot;nofollow&quot;&gt;this New York Times article&lt;/a&gt; about a previous planned (of &lt;a href=&quot;http://www.nasa.gov/mission_pages/grail/main/index.html&quot; rel=&quot;nofollow&quot;&gt;Grail&lt;/a&gt;), the purpose of a &lt;em&gt;planned&lt;/em&gt; crash is so that the crash site does not destroy a historic landmark, such as Neil Armstrong's footprints. A less sentimental reason is that the crash site can also be strategically planned to help further missions locate a specific place on the moon, using the indentation the impact will create.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="3195" LastActivityDate="2014-04-26T06:20:58.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4368" PostTypeId="1" AcceptedAnswerId="4370" CreationDate="2014-04-26T10:31:53.737" Score="16" ViewCount="9449" Body="&lt;p&gt;Often I see small crosses on space pictures like this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2kqM1.jpg&quot; alt=&quot;Armstrong on the Moon&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are regularly spaced, thin and black. What are them? What's their purpose?&lt;/p&gt;&#xA;" OwnerUserId="105" LastActivityDate="2014-04-27T06:00:24.380" Title="What are the small crosses on space pictures, and what's their purpose?" Tags="|apollo-program|photography|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4369" PostTypeId="1" CreationDate="2014-04-26T11:25:50.230" Score="10" ViewCount="690" Body="&lt;p&gt;By &quot;lead designer&quot; I mean a person who oversaw the preliminary design phase for a manned spacecraft, being intimately involved in and ultimately responsible for crucial design choices and compromises. It is a shame that we know the names of space entrepreneurs and CEOs making financial decisions but often fail to honor the memory of the engineers who did the actual work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that I intend to make full use of recent changes in the &quot;Community Wiki&quot; rules. In case you haven't noted, you can edit your answers an unlimited number of times, all the votes accruing to your reputation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please also note that unless there's a verifiable tie, I will not upvote later answers copying names from other posts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For your convenience, here is the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_manned_spacecraft&quot;&gt;list of manned spacecraft&lt;/a&gt; (different generations of the same spacecraft count, but I'm not listing all the Soyuz versions here, for instance):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Vostok&lt;/li&gt;&#xA;&lt;li&gt;Mercury&lt;/li&gt;&#xA;&lt;li&gt;Gemini&lt;/li&gt;&#xA;&lt;li&gt;Voskhod&lt;/li&gt;&#xA;&lt;li&gt;Apollo CSM&lt;/li&gt;&#xA;&lt;li&gt;Apollo LM&lt;/li&gt;&#xA;&lt;li&gt;Soyuz&lt;/li&gt;&#xA;&lt;li&gt;STS (the Space Shuttle)&lt;/li&gt;&#xA;&lt;li&gt;TKS&lt;/li&gt;&#xA;&lt;li&gt;Buran&lt;/li&gt;&#xA;&lt;li&gt;Shenzhou (921)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Stations:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;DOS/Salyut&lt;/li&gt;&#xA;&lt;li&gt;Skylab&lt;/li&gt;&#xA;&lt;li&gt;Almaz/Salyut&lt;/li&gt;&#xA;&lt;li&gt;Mir&lt;/li&gt;&#xA;&lt;li&gt;ISS&#xA;(as well as its nodes)&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Zarya&lt;/li&gt;&#xA;&lt;li&gt;Zvezda&lt;/li&gt;&#xA;&lt;li&gt;Unity&lt;/li&gt;&#xA;&lt;li&gt;Destiny&lt;/li&gt;&#xA;&lt;li&gt;Harmony&lt;/li&gt;&#xA;&lt;li&gt;Tranquility&lt;/li&gt;&#xA;&lt;li&gt;Kibo&lt;/li&gt;&#xA;&lt;li&gt;Columbus&lt;/li&gt;&#xA;&lt;li&gt;Rassvet&lt;/li&gt;&#xA;&lt;li&gt;Leonardo&lt;/li&gt;&#xA;&lt;li&gt;Pirs&lt;/li&gt;&#xA;&lt;li&gt;Poisk&lt;/li&gt;&#xA;&lt;li&gt;Cupola &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Tiangong-1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Designer names for proposed / developmental manned spacecraft are welcome.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="12102" LastEditDate="2021-06-09T03:22:09.253" LastActivityDate="2021-06-09T03:22:09.253" Title="Who are the actual lead designers of manned spacecraft?" Tags="|crewed-spaceflight|history|design|mission-design|engineering|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4370" PostTypeId="2" ParentId="4368" CreationDate="2014-04-26T14:57:35.210" Score="20" Body="&lt;p&gt;These crosshair marks are called &lt;em&gt;registration marks&lt;/em&gt; and are produced by the camera optics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These marks ensure that the exact geometry of the image (and succesive angle/distance measurements) are preserved despite deformation of the negative, or deformations during image processing such as scanning or printing to paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are taking images &lt;em&gt;on the moon&lt;/em&gt;, you'd better preserve the most information intact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same techniques are used in aerial images for cartography.&lt;/p&gt;&#xA;" OwnerUserId="1348" LastActivityDate="2014-04-26T14:57:35.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4371" PostTypeId="1" AcceptedAnswerId="4379" CreationDate="2014-04-27T00:01:17.927" Score="6" ViewCount="679" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Thermocouple#Power_production&quot; rel=&quot;noreferrer&quot;&gt;It seems&lt;/a&gt; that a nuclear thermoelectric generator was used to power spacecrafts for some exploration missions. Can a thermopile that harvest the light/shade temperature difference on Earth's orbit be used to power a small satellite? What about a Mercury satellite, would it be more reliable and efficient than solar panels which could melt on those conditions?&lt;/p&gt;&#xA;" OwnerUserId="1263" LastActivityDate="2014-04-27T21:32:41.477" Title="Could a solar thermopile be enough to power a small satellite?" Tags="|artificial-satellite|power|solar-power|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4372" PostTypeId="2" ParentId="4368" CreationDate="2014-04-27T06:00:24.380" Score="17" Body="&lt;p&gt;Expanding on the answer of @Juancho, the marks are also called reticles or fiducials.  They are engraved into a thin, optically perfect piece of glass that is placed just in front of the film in the astronauts camera. (You remember film, that long piece of cellophane coated with Jello that was ... But I digress). When a picture is taken, the image of the fiducial is recorded on the film along with the image of the lunar landscape.  If the film is distorted during subsequent handling or processing, the distortion can be detected and eliminated by applying a transformation to the image that moves each fiducial back to its the correct position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that these crosses &lt;strong&gt;appear&lt;/strong&gt; to go behind some bright objects on the lunar surface has been cited by Apollo deniers as evidence of the great conspiracy...&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2014-04-27T06:00:24.380" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4375" PostTypeId="1" CreationDate="2014-04-27T16:03:28.933" Score="11" ViewCount="1203" Body="&lt;p&gt;Have any spacecraft been either launched or proposed which involve a survivable close encounter with the Sun? This would include solar observation missions as well as any missions designed to make a close pass as part of their trajectories en-route elsewhere, and by &quot;close&quot; I mean at least within the orbit of Mercury. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How close have spacecraft approached or have been proposed to approach, and how long were they / will they be within the most extreme thermal/radiation/etc. conditions that would be encountered? What features would be required / have been incorporated into any such spacecraft in order to survive those conditions?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="12102" LastEditDate="2016-09-20T03:44:05.943" LastActivityDate="2016-09-20T03:44:05.943" Title="Have any spacecraft been launched or proposed to make a survivable close encounter with the Sun?" Tags="|spacecraft|the-sun|parker-solar-probe|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4377" PostTypeId="2" ParentId="4375" CreationDate="2014-04-27T19:50:50.550" Score="16" Body="&lt;p&gt;&lt;a href=&quot;http://solarprobe.jhuapl.edu/&quot;&gt;Solar Probe&lt;/a&gt;, planned to launch in 2018, will get to within 8.5 solar radii of the Sun's surface.  For comparison, Mercury gets no closer than 65 solar radii.  Solar Probe will use a &lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/&quot;&gt;thick carbon-carbon, carbon foam shield&lt;/a&gt; when in close proximity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The record so far is held by &lt;a href=&quot;http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Helios_02&quot;&gt;Helios-2&lt;/a&gt; in 1976, which got just inside Mercury's orbit at 0.29 AU, about 61 solar radii above the surface of the Sun.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-04-27T19:59:01.737" LastActivityDate="2014-04-27T19:59:01.737" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4378" PostTypeId="2" ParentId="4375" CreationDate="2014-04-27T19:51:46.347" Score="8" Body="&lt;p&gt;The neatest, and the most prominent example of a &quot;close encounter&quot; with the Sun is the &lt;a href=&quot;https://en.wikipedia.org/wiki/MESSENGER&quot;&gt;MESSENGER&lt;/a&gt;. Additional information may be gleaned from this &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000012856.pdf&quot;&gt;proposal&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main problem for unmanned craft is, of course, the Sun itself. Protection from &lt;strong&gt;overheating&lt;/strong&gt; relies on a combination of design decisions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a heat-resistant sunshade (on MESSENGER, made from ceramic cloth)&lt;/li&gt;&#xA;&lt;li&gt;3-axis stabilization to keep the instruments and the craft from pointing at the Sun outside of the sunshade's protection&lt;/li&gt;&#xA;&lt;li&gt;solar panels (kept outside the sunshade) are rotatable, made from photovoltaic cells on the sun-facing side and from PV cells and optical reflectors (radiating excess heat into space) on the reverse side&lt;/li&gt;&#xA;&lt;li&gt;providing for radiators' unfettered view of the cold space background&lt;/li&gt;&#xA;&lt;li&gt;using state-of-the-art materials&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If you consider the requirement to pass by Mercury on the hot side, you have to be prepared to tackle two extra problems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;a lot of extra heating &quot;from below&quot; (infrared) that's not caught by the sunshade -&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;using side radiators to pick up heat, transfer it via heat pipes and dump through radiators viewing cooler space&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;fast heating/cooling of the spacecraft (&lt;strong&gt;thermal cycling&lt;/strong&gt;) -&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Thermal cycling induces mechanical stress, so the main idea is to allow for flexible expansion and contraction of the structure.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Keeping the most cycling-sensitive items (electronics, batteries, thrusters etc.) physically de-coupled from the radiators and the shade.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Careful periapsis height selection to limit thermal change rate.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To achieve all this, one has to do a lot of modelling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For &lt;strong&gt;manned spacecraft&lt;/strong&gt; at the current technology level, going inside the orbit of Venus for any prolonged period looks like a suicide mission - instead of thermal constraints you're bound by radiation dose limits.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-04-27T19:51:46.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4379" PostTypeId="2" ParentId="4371" CreationDate="2014-04-27T21:32:41.477" Score="9" Body="&lt;p&gt;There's no theoretical reason why not.  The main reason why they aren't used is practical: the best solar panels convert about 22% of available energy to electricity, while the best thermoelectric generators operate at about 8% of Carnot efficiency, which for a reasonable temperature gradient works out to converting about 4% of available energy.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-04-27T21:32:41.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4381" PostTypeId="2" ParentId="4369" CreationDate="2014-04-28T02:32:40.723" Score="15" Body="&lt;p&gt;This answer is about OKB-1 (later known as TsKBEM, then NPO Energia, today known as RSC Energia) spacecrafts.&#xA;I plan to edit, to make it more exact and detailed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Vostok and Voskhod.&lt;/strong&gt; (3K)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The chief conceptual designer was &lt;a href=&quot;http://en.wikipedia.org/wiki/Konstantin_Feoktistov&quot; rel=&quot;noreferrer&quot;&gt;Konstantin Feoktistov&lt;/a&gt;.&#xA;The administrative chain of all the people &quot;responsible for the design&quot; looked like that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Korolev (OKB-1. Chief designer)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ru.wikipedia.org/wiki/%D0%91%D1%83%D1%88%D1%83%D0%B5%D0%B2,_%D0%9A%D0%BE%D0%BD%D1%81%D1%82%D0%B0%D0%BD%D1%82%D0%B8%D0%BD_%D0%94%D0%B0%D0%B2%D1%8B%D0%B4%D0%BE%D0%B2%D0%B8%D1%87&quot; rel=&quot;noreferrer&quot;&gt;K. Bushuev&lt;/a&gt; (Deputy Chief Designer)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Mikhail_Tikhonravov&quot; rel=&quot;noreferrer&quot;&gt;Tikhonravov&lt;/a&gt; (Head of &quot;Department 9 of spacecrafts&quot;) &#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Konstantin_Feoktistov&quot; rel=&quot;noreferrer&quot;&gt;Feoktistov&lt;/a&gt; (Head of Conceptual Design Sector)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Source, for example: &lt;a href=&quot;http://history.nasa.gov/SP-4110/vol3.pdf%E2%80%8E&quot; rel=&quot;noreferrer&quot;&gt;Chertok &quot;Rockets and People&quot;, volume 3&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Soyuz&lt;/strong&gt; (7K)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One needs to understand, that the initial designs were the ones of the soviet &#xA;lunar programs. Including such complexes as &lt;a href=&quot;http://www.friends-partners.org/partners/mwade/craft/soyuza.htm&quot; rel=&quot;noreferrer&quot;&gt;Soyuz A-B-V&lt;/a&gt;, for example.&#xA;And &lt;a href=&quot;http://en.wikipedia.org/wiki/N1_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;N1-L3&lt;/a&gt;, where L3 = &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_7K-LOK&quot; rel=&quot;noreferrer&quot;&gt;7K-LOK&lt;/a&gt; + &lt;a href=&quot;http://en.wikipedia.org/wiki/LK_Lander&quot; rel=&quot;noreferrer&quot;&gt;LK&lt;/a&gt;.&#xA;7K-LOK is a kind of Soyuz with the familiar Landing and Orbital modules,&#xA;but a larger Service Module. &lt;img src=&quot;https://i.stack.imgur.com/24gUk.png&quot; alt=&quot;LOK&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The family included such spacecrafts as &lt;a href=&quot;http://en.wikipedia.org/wiki/Zond_5&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 7K-L1&lt;/a&gt; which were sent to fly around the moon under the names of Zond-5,6,7,8. &lt;img src=&quot;https://i.stack.imgur.com/YZSHR.png&quot; alt=&quot;Zond-5&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only structural part all those Soyuz variants have in common is, probably, the landing module. The design works, including the search for the optimal shape, landing schemes, of the Landing Module were headed by &lt;a href=&quot;http://www.astronaut.ru/as_rusia/energia/text/timchenko.htm&quot; rel=&quot;noreferrer&quot;&gt;Vladimir Timchenko&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Korolev (OKB-1. Chief designer)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ru.wikipedia.org/wiki/%D0%91%D1%83%D1%88%D1%83%D0%B5%D0%B2,_%D0%9A%D0%BE%D0%BD%D1%81%D1%82%D0%B0%D0%BD%D1%82%D0%B8%D0%BD_%D0%94%D0%B0%D0%B2%D1%8B%D0%B4%D0%BE%D0%B2%D0%B8%D1%87&quot; rel=&quot;noreferrer&quot;&gt;K. Bushuev&lt;/a&gt; (Deputy Chief Designer)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Vladimir Roschin (Head of &quot;Department 11 of Gas Dynamics&quot;) &#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://www.astronaut.ru/as_rusia/energia/text/timchenko.htm&quot; rel=&quot;noreferrer&quot;&gt;Timchenko&lt;/a&gt; (Head of Design Sector)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;After the concept of the landing module was ready, there was a joint team of Departments 9 and 11 headed by &lt;a href=&quot;http://en.wikipedia.org/wiki/Konstantin_Feoktistov&quot; rel=&quot;noreferrer&quot;&gt;Feoktistov&lt;/a&gt; to work out the designs of 7K (i.e. Soyuz).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good source is the following writeup: &lt;a href=&quot;http://m1.sm.bmstu.ru/wiki/%D0%92%D0%BE%D1%81%D0%BF%D0%BE%D0%BC%D0%B8%D0%BD%D0%B0%D0%BD%D0%B8%D1%8F_%D0%BE%D0%B1_%D0%B8%D1%81%D1%82%D0%BE%D1%80%D0%B8%D0%B8_%D1%81%D0%BE%D0%B7%D0%B4%D0%B0%D0%BD%D0%B8%D1%8F_%D1%81%D0%BF%D1%83%D1%81%D0%BA%D0%B0%D0%B5%D0%BC%D0%BE%D0%B3%D0%BE_%D0%B0%D0%BF%D0%BF%D0%B0%D1%80%D0%B0%D1%82%D0%B0_%D1%82%D1%80%D0%B0%D0%BD%D1%81%D0%BF%D0%BE%D1%80%D1%82%D0%BD%D0%BE%D0%B3%D0%BE_%D0%BA%D0%BE%D1%81%D0%BC%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%BE%D0%B3%D0%BE_%D0%BA%D0%BE%D1%80%D0%B0%D0%B1%D0%BB%D1%8F_%C2%AB%D0%A1%D0%BE%D1%8E%D0%B7%C2%BB&quot; rel=&quot;noreferrer&quot;&gt;Memories of the Soyuz lander module creation&lt;/a&gt; (in Russian).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Salyut&lt;/strong&gt; and &lt;strong&gt;Soyuz-T&lt;/strong&gt; (DOS-7K)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Salyut re-used Almaz hull design, fitted with Soyuz guidance and control systems.&#xA;Soyuz-T is a member of Soyuz family adapted to be a ferry serving the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The proposal came from a team of TsKBEM engineers (Bushuev, Ohapkin, Chertok, Kryukov, Feoktistov, Raushenbach).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Vasily_Mishin&quot; rel=&quot;noreferrer&quot;&gt;V. Mishin&lt;/a&gt; (Designer General, TsKBEM Head of Enterprise)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ru.wikipedia.org/wiki/%D0%91%D1%83%D1%88%D1%83%D0%B5%D0%B2,_%D0%9A%D0%BE%D0%BD%D1%81%D1%82%D0%B0%D0%BD%D1%82%D0%B8%D0%BD_%D0%94%D0%B0%D0%B2%D1%8B%D0%B4%D0%BE%D0%B2%D0%B8%D1%87&quot; rel=&quot;noreferrer&quot;&gt;K. Bushuev&lt;/a&gt; (Chief Designer, general manager of DOS-7K)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://ru.wikipedia.org/wiki/%D0%A1%D0%B5%D0%BC%D1%91%D0%BD%D0%BE%D0%B2,_%D0%AE%D1%80%D0%B8%D0%B9_%D0%9F%D0%B0%D0%B2%D0%BB%D0%BE%D0%B2%D0%B8%D1%87&quot; rel=&quot;noreferrer&quot;&gt;Yuri Semenov&lt;/a&gt; (Lead Designer of DOS-7K complex)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Konstantin_Feoktistov&quot; rel=&quot;noreferrer&quot;&gt;K. Feoktistov&lt;/a&gt; (Deputy Chief Designer, deputy general manager of &#xA;DOS-7K) &#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Leonid Gorshkov (designer of DOS orbital block)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ru.wikipedia.org/wiki/%D0%A6%D1%8B%D0%B1%D0%B8%D0%BD,_%D0%9F%D0%B0%D0%B2%D0%B5%D0%BB_%D0%92%D0%BB%D0%B0%D0%B4%D0%B8%D0%BC%D0%B8%D1%80%D0%BE%D0%B2%D0%B8%D1%87&quot; rel=&quot;noreferrer&quot;&gt;P.Tsybin&lt;/a&gt; (manager of 7K-T)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Dmitry Slesarev (Deputy Lead Designer responsible for 7K-T)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Sources of information: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://epizodsspace.no-ip.org/bibl/energia46-96/08.html&quot; rel=&quot;noreferrer&quot;&gt;RSC Energia '46-'96&lt;/a&gt; (in Russian)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://books.google.ru/books?id=EbDGMiXvdG0C&amp;amp;lpg=PA24&amp;amp;ots=jxEgggEou7&amp;amp;dq=dmitry%20slesarev%20soyuz-t&amp;amp;pg=PA25#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Salyut - The First Space Station: Triumph and Tragedy&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mir&lt;/strong&gt; core module (DOS-7) and &lt;strong&gt;Zvezda&lt;/strong&gt; ISS Service Module (DOS-8)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same &lt;a href=&quot;http://epizodsspace.no-ip.org/bibl/energia46-96/08.html&quot; rel=&quot;noreferrer&quot;&gt;RSC Energia '46-49&lt;/a&gt; gives the list of conceptual designers of Mir station, it doesn't go into finer details.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;V. Glushko (Designer General, NPO Energia)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Konstantin_Feoktistov&quot; rel=&quot;noreferrer&quot;&gt;K.Feoktistov&lt;/a&gt; (Deputy Designer General)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;L. Gorshkov (head of department)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;E. Demchenko, A. Nesterenko, M.M.Lemelev, V. Tokarev, F.Arkov, N. Beresnev, V. Bobkov, N. Brukhanov, A. Kochkin and others. &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The role of Konstantin Feoktistov&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One could note that the only name really belonging  to all of the above lists is Konstantin Feoktistov. He has participated in the Sputnik designs. And retired from RSC Energia in 1990, 4 years after Mir launch. In different roles he was creating them all. I am surprised his memoir book is not present on Amazon in English translation.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2014-05-03T01:35:21.810" LastActivityDate="2014-05-03T01:35:21.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4382" PostTypeId="2" ParentId="4365" CreationDate="2014-04-28T10:44:45.943" Score="2" Body="&lt;p&gt;Here's a &lt;em&gt;correct&lt;/em&gt; answer: A craft couldn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We haven't mapped the whole of space yet so... &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a craft with no other means of identifying its position&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;... can't know if it's near known or unknown pulsars. If you really have no idea of where you are or what your attitude is then looking around you isn't actually that useful. For example, you wake up in a forest, and from the flora and fauna you surmise that you must be in the Amazon rain forest. But you could easily be on another planet, in another galaxy, far far away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if you have knowledge of your approximate position, say somewhere in the milky way, then you can start using pulsars and other things you see in space to calculate your position. You would need three known pulsars to triangulate your position (with 2 pulsars you can only know you're in 1 of 2 possible locations). If you know enough about the pulsars you could do some interesting red shift calculations to find out how far away from them you are. When it comes to the actual identification process, the duration of observation required is dependant on how many pulsars it might be, based firstly on your expected position and then on initial observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example if you see a pulsar and you think you're near 1 of 10 pulsars then you can assume the pulsar you're seeing is one of the 10. Then you observe the pulsing of the pulsar and see at what rate it pulses. The longer you observer the pulsar the more accurate you calculation of it's spin can be, and hence the easier it is to rule out the other 9 pulsars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for a simple answer not being given to this question (by me at least), is that it is highly dependant on which pulsars you are looking at. For example take the following collection of pulsars and pulse frequencies:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;A  0.98&#xA;B  0.12&#xA;C  0.51&#xA;D  0.78&#xA;E  0.15&#xA;F  12.01&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;It's clear that if you are trying to identify pulsar F it is going to be an easy task. If you see more than one pulse per second you can be confident that the other pulsars are ruled out. Now consider the following collection:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;A  13.52&#xA;B  12.08&#xA;C  10.54&#xA;D  15.23&#xA;E  11.98&#xA;F  12.01&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;To identify pulsar F in this collection you Need to be able to observe the pulsar for around 13 seconds to rule out pulsar B as a potential candidate. This assumes that you can only count the number of pulses in a given number of seconds. This is unlikely, but there will be a limit to how accurately you can time pulses, dependent on the design of your hardware.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To surmise, the duration required to identify pulsar F in collection 2 is 13 times that of collection 1; this is why it's dependent on the number and characteristics of pulsars in range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT 2:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following is based on values for 1,861 pulsars.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;median: 1.88 Hz&#xA;highest: 716.35556 Hz  (PSR J1748-2446ad)&#xA;lowest: 0.084897259 Hz  (PSR J1841-0456)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BRCcl.gif&quot; alt=&quot;Distribution plots&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This looks to me like a large number in the 1 - 4Hz range say around 700. So if the values are evenly distributed in this range there is approximately 0.004hz between each pulsar's equatorial frequency.  The longest frequency involved in this large subset is 1Hz, which suggests that if you can measure to the nearest 0.004Hz then you can identify the majority of pulsars in 1 second or less. If your measuring accuracy is lower this will increase the time require.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: The data is a composite from 52 published works. A list can be found at the bottom of the page &lt;a href=&quot;https://www.wolframalpha.com/input/?i=pulsars%20frequency&amp;amp;a=*DPClash.AstronomicalP.frequency-_*EquatorialFrequency-&quot; rel=&quot;nofollow noreferrer&quot;&gt;HERE&lt;/a&gt;, under &lt;em&gt;external sources&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-04-29T16:19:21.253" LastActivityDate="2014-04-29T16:19:21.253" CommentCount="19" ContentLicense="CC BY-SA 3.0" />
  <row Id="4383" PostTypeId="2" ParentId="4360" CreationDate="2014-04-28T13:49:32.310" Score="4" Body="&lt;p&gt;There is no single answer to this question because there are many variables involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Limiting factors.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several factors come into play when designing a mission.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The orbit of the vehicle.&lt;br&gt;&#xA;A flyby mission offers very limited opportunities to observe the target object. For a vehicle in a bound orbit, the opportunities are limited by the vehicle's lifetime. The very different natures of flyby versus bound orbits dictate very different strategies in how a mission is designed and performed.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Communications and onboard storage.&lt;br&gt;&#xA;How fast data can be downlinked while communication is enabled, how often and for how long communication is enabled, and storage capacity to save data between downlinks all limit the kind of pictures and number of pictures that can be taken. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How the &quot;camera&quot; works.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note the quotes. Don't think of a camera as only something that takes a picture comprising N*M pixels in one swell foop. There are a number of other ways to take a &quot;picture&quot;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Some &quot;cameras&quot; take but a single pixel at a time. The scanning radiometers fall into this category. A number of the Earth-observing weather satellites use this technique. A rotating mirror focuses light from a specific angle onto a very narrow field of view sensor, which captures the data as one pixel (or one set of pixels, one per wavelength). The mirror rotates a bit, typically cross-track, and the sensor captures that data. In short order, the sensor will have generated individual pixels that represent a swath of data. It starts over, capturing another swath of data. In the meanwhile, the satellite has moved. The combination of mirror rotating and satellite motion lets the &quot;camera&quot; form a continuous image.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Other cameras collect a row of pixels at one time, typically across-track. This is how the HiRISE camera on the Mars Reconnaissance Orbiter works. It takes an N pixels × 1 pixel snapshot of what's immediately below, waits a bit, and takes another N×1 picture. As is the case with the scanning radiometer approach, this continuous scanning process eventually builds up a nice big image.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Yet other cameras work more or less like a digital camera you can buy off the shelf. They take an N pixels by M pixels snapshot of what's in the field of view, wait some time, and take another. The upside of this approach is that all of the pixels in a single image represent the same point in time. A downside is that it's much harder to stitch images together to form a composite.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-04-30T14:11:12.897" LastActivityDate="2014-04-30T14:11:12.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4384" PostTypeId="1" AcceptedAnswerId="4386" CreationDate="2014-04-28T17:53:00.697" Score="11" ViewCount="950" Body="&lt;p&gt;Imagine that Mars was orbiting around the sun in the opposite direction that it is now.  Assuming both Mars and Earth are now travelling in opposite directions, how might we land on Mars?  More specifically, what would our launch trajectory and path look like?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Translating this to more simple concepts, we can look at Earth and Mars as 2 cars travelling down a straight highway.  As they exist now, Earth and Mars would be travelling on the same side of the highway at relatively similar speeds.  Jumping from one moving car (while unadvised), would be relatively simple.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this theoretical question, Earth and Mars would be travelling on opposite sides of the highway and would have greatly different relative speeds.  Jumping from one moving car to the other would be extremely difficult.  Is the only way to make this jump, to reduce the relative speed, that is to bring the car to a stop and then start reversing until it is reversing at the same speed?&lt;/p&gt;&#xA;" OwnerUserId="690" LastEditorUserId="690" LastEditDate="2014-04-30T13:12:01.527" LastActivityDate="2014-05-02T16:04:57.600" Title="Theoretical: How might we land on a planet/moon that has an orbit going in the reverse direction of Earth?" Tags="|orbit|probe|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4385" PostTypeId="1" AcceptedAnswerId="4391" CreationDate="2014-04-28T18:28:35.677" Score="7" ViewCount="513" Body="&lt;p&gt;The United States shot down a satellite (&lt;a href=&quot;https://en.wikipedia.org/wiki/USA_193&quot;&gt;USA-193&lt;/a&gt;) in low Earth orbit, with Hydrazine on board, to prevent a toxic disaster. They did it at low altitude to prevent long lived debris. Was such a technique effective?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-19T17:25:07.203" LastActivityDate="2017-09-15T17:04:56.647" Title="What was the lifetime of the debris from the US Anti-satellite exercise for USA-193?" Tags="|artificial-satellite|debris|military|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4386" PostTypeId="2" ParentId="4384" CreationDate="2014-04-28T18:49:21.780" Score="5" Body="&lt;p&gt;A good heat shield.  Simply aim for the planet and use aerobraking.  You'll hit the atmosphere at a velocity similar to the probe Galileo dropped into Jupiter--we did it then, we can do it again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can get home the same way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the request for numbers:  The Galileo probe hit Jupiter at 47km/sec.  If you hit Mars at it's farthest from the sun it's orbital velocity is 22 km/sec, doubled to 44 km/sec as it's going the wrong way.  It's actually a little bit less than that as it won't be going quite the 22 km/sec of a body in Mars' orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The aerobrake will be tricky given how thin the Martian atmosphere is but that's not the same thing as saying it's impossible.  You can pass through the atmosphere twice--make your approach on a line that will be tangent to the surface after considering the aerobraking effects.  You also don't need to shed all your velocity--so long as you exit with less than 5 km/sec gravity will bring you back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't believe this is possible for a manned mission but the original didn't specify that.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2014-04-29T04:22:16.250" LastActivityDate="2014-04-29T04:22:16.250" CommentCount="24" ContentLicense="CC BY-SA 3.0" />
  <row Id="4387" PostTypeId="2" ParentId="4384" CreationDate="2014-04-28T19:05:17.067" Score="12" Body="&lt;p&gt;The only practical way I can think of to maybe slow down enough to land on a retrograde Mars would be to do a Jupiter flyby to mostly reverse your solar orbit direction.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-04-28T19:10:47.523" LastActivityDate="2014-04-28T19:10:47.523" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4388" PostTypeId="1" CreationDate="2014-04-28T21:15:39.833" Score="2" ViewCount="1777" Body="&lt;p&gt;This may be impossible to answer, since the data is often not disclosed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in an article, &lt;a href=&quot;http://thehill.com/policy/defense/204316-contractor-speeds-up-deliveries-of-russian-engines#ixzz2zthK4wBf&quot; rel=&quot;nofollow&quot;&gt;about ULA getting RD-180's from Russia&lt;/a&gt; it claims: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each engine reportedly costs between \$11 to \$15 million.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have never seen prices quoted for a Merlin 1C or 1D, or say an RS-68 (Or RS-68A), an SSME or an NK-33/AJ-26. Comparing different engines in terms of cost could be very interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, an RD-180 has the thrust of 10 Merlins or so, or almost 2 RS-68s or SSME's so probably worth noting common number of units of each used in a typical launcher. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-04-28T21:25:43.660" LastActivityDate="2014-04-29T16:49:18.343" Title="What are the costs of the various different engines in current use?" Tags="|launch|engines|ssme|rd-180|merlin-1d|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4389" PostTypeId="2" ParentId="4384" CreationDate="2014-04-28T23:04:29.643" Score="7" Body="&lt;p&gt;The basic process of getting from Earth to normal-Mars, without any slingshot manoeuvers, is a Hohmann transfer orbit:  give the probe enough velocity to put it in an elliptical orbit with perihelion at Earth's orbit, and aphelion at Mars' orbit, timing the start  so that Mars is there when the probe arrives.  A second burn at aphelion to circularize the orbit, matching speed with Mars, and you're done. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing different to reach retrograde-Mars is that &lt;strong&gt;first&lt;/strong&gt;, you need to kill the Earth's $30$ km/sec velocity, then add $30$ km/sec back in in the opposite direction, and then do all the Hohmann burns.  So, you've just added a Delta-V of $60$ km/sec to your job...&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2014-04-28T23:04:29.643" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4390" PostTypeId="2" ParentId="4385" CreationDate="2014-04-29T07:56:23.260" Score="2" Body="&lt;p&gt;I suspect there's no 100 answer to your question about how long debris lasts. Low earth orbit satellites have a small amount of drag from the upper atmosphere so must regularly thrust to keep altitude. Once the satellite was destroyed the pieces that remained in the same orbit would have been subjected to different amounts of drag without any balancing thrust. These would have fallen to earth in days, weeks or months depending on the shape of the pieces, how much drag they incur, and the vectors imparted by the impact force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some pieces, likely very few, may have been put into a significantly different orbit by the force of the impact, so it is possible that they may be in orbit significantly longer. These pieces are a threat to other satellites. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for whether it was effective the answer is no. The satellite's orbit was decaying but its path was predictable enough for other satellites to avoid it. Having one big chunk of satellite reenter as a piece is much better than several million small pieces of ex-satellite in unpredictable orbits. Shooting the satellite turned it into a cloud of debris which is harder to avoid, the opposite effect to the stated goal. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-04-29T07:56:23.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4391" PostTypeId="2" ParentId="4385" CreationDate="2014-04-29T09:49:57.727" Score="11" Body="&lt;p&gt;There's two things to look at in answering this question. First the large, track-able objects, and second the small un-track-able objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The large objects (I'm defining large as big enough to track) consisted of 174 objects according to &lt;a href=&quot;https://www.space-track.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Track&lt;/a&gt; the US public TLE database. These objects were tracked until re-entry, the last of which was 28/Oct/2009. Simple answer here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The small objects are a little more complicated. We can reliably say that smaller objects typically have a lower ballistic coefficient since if we assume (big assumption) that the objects are spherical, mass is proportional to r^3 whereas wetted surface area is proportional to r^2 (r = radius). So from this definition, and knowing that the large objects all de-orbited due to drag by 28/Oct/2009 we can be pretty confident that the small pieces mostly de-orbited by that time also. A secondary concern however is that since small objects have a lower mass they are more freely accelerated by the explosion. So if any objects are expected to be in orbits with higher semi-major axes the smaller objects would be more likely. However this isn't too much of a problem since the perigee of the small object orbits will stay close to the original satellite orbit and the perigee can effectively re-circularise and orbit due to drag alone; at which point the small objects would de-orbit relatively quickly as discussed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for whether the technique was effective, it's a mixed bag really. As @GdD states in their answer it created a greater orbital hazard whilst the spacecraft was in orbit. But it was a good proof of concept for the technology, and the re-entering objects would have been much smaller and hence much more likely to burn up in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to provide more references but you can find all the data on the large object debris from &lt;a href=&quot;https://www.space-track.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Track&lt;/a&gt;, but you do have to sign up (free).&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="6816" LastEditDate="2017-09-15T17:04:56.647" LastActivityDate="2017-09-15T17:04:56.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4392" PostTypeId="2" ParentId="3805" CreationDate="2014-04-29T13:17:05.350" Score="3" Body="&lt;p&gt;From &lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;nofollow&quot;&gt;John D. Clark's &lt;em&gt;Ignition!&lt;/em&gt;&lt;/a&gt; (from Mark Adler's link in second comment under question), the section on the specifications for RP-1 (pp 104-105), you could improve your kerosene by reducing the olefin content to less than 1% and the aromatics to less than 5%. From &lt;a href=&quot;http://www.astronautix.com/props/loxosene.htm&quot; rel=&quot;nofollow&quot;&gt;called2voyage's second cite&lt;/a&gt;, treat it by acid washing and sulphur dioxide extraction. You've not improved the hydrocarbon ratio, but at least you'll have far less tar &amp;amp; soot gumming up the works.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="415" LastEditDate="2014-04-30T16:13:46.717" LastActivityDate="2014-04-30T16:13:46.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4393" PostTypeId="2" ParentId="4388" CreationDate="2014-04-29T16:49:18.343" Score="1" Body="&lt;p&gt;We can put an upper bound on the cost of a Merlin; SpaceX quotes a cost of &lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;nofollow&quot;&gt;\$135M for a Falcon Heavy&lt;/a&gt; launch. That includes 28 Merlins. If you assume the engines are half the total cost of the rocket, that's 2.4M per engine. Of course it could be considerably less...  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-04-29T16:49:18.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4394" PostTypeId="2" ParentId="3161" CreationDate="2014-04-29T22:26:03.723" Score="18" Body="&lt;p&gt;Another disadvantage to hydrogen is that it requires advanced metallurgy to prevent hydrogen embrittlement, where more common alloys tend to become prone to fracture and fatigue in high hydrogen environments. &lt;/p&gt;&#xA;" OwnerUserId="3225" LastActivityDate="2014-04-29T22:26:03.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4395" PostTypeId="1" AcceptedAnswerId="4396" CreationDate="2014-04-30T10:48:11.597" Score="15" ViewCount="3082" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_spectroscopy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astronomical spectroscopy&lt;/a&gt; can be used to determine the chemical composition of distant bodies. In the related question &lt;a href=&quot;https://space.stackexchange.com/questions/3832&quot;&gt;Can we detect atmosphere on exoplanets?&lt;/a&gt; we learned that, even now, we are studying exoplanet atmospheres.&lt;/p&gt;&#xA;&lt;p&gt;We know that currently the atmosphere of Earth is about 21% oxygen and about 0.035% carbon dioxide.  As I understand it, Earth's early atmosphere had a much higher concentration of carbon dioxide, created by volcanic activity.&lt;/p&gt;&#xA;&lt;p&gt;What atmospheric concentration of oxygen (O&lt;sub&gt;2&lt;/sub&gt; or O&lt;sub&gt;3&lt;/sub&gt;) can be explained by tectonic or chemical actions, and what concentration could only be achieved by some kind of life?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="47149" LastEditDate="2022-05-29T16:02:36.217" LastActivityDate="2022-05-29T16:02:36.217" Title="What concentration of oxygen in a planetary atmosphere would be indicative of life?" Tags="|atmosphere|life|oxygen|geology|exoplanet|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="4396" PostTypeId="2" ParentId="4395" CreationDate="2014-04-30T13:09:02.553" Score="22" Body="&lt;p&gt;I would argue that no specific level of molecular or atomic oxygen in atmosphere is indicative of &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbon-based_life&quot;&gt;carbon-based life&lt;/a&gt; (i.e. life as we know it on Earth). A planet could have oxygen rich atmosphere which could be due to naturally occurring thermo-chemical reactions (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Sabatier_reaction&quot;&gt;Sabatier&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/Bosch_reaction&quot;&gt;Bosch&lt;/a&gt; reaction), loss of hydrogen in water vapor through &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_escape&quot;&gt;atmospheric escape&lt;/a&gt; and managing to hold onto the heavier oxygen, even by thermal / magmatic release of oxygen in &lt;a href=&quot;https://en.wikipedia.org/wiki/Oxide_minerals&quot;&gt;oxide minerals&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, we also know of &lt;a href=&quot;https://en.wikipedia.org/wiki/Anaerobic_organism&quot;&gt;anaerobic organisms&lt;/a&gt;, some of which (&lt;a href=&quot;https://en.wikipedia.org/wiki/Obligate_anaerobe&quot;&gt;obligate anaerobes&lt;/a&gt;) don't tolerate oxygen rich environment and are essentially killed by its presence in notable concentrations. So we have both control ends with a non-indicative correlation; carbon-based life could exist even without molecular or atomic oxygen present in the environment and life forms develop that don't utilize it directly in their metabolism, and don't release it as a metabolic byproduct, and at the same time high concentrations of atmospheric oxygen could be due to other, inorganic causes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More interesting than measuring atmospheric composition and amount of oxygen in it on average might however be measuring changes in atmospheric composition during planet's diurnal cycles, seasonal changes, and otherwise finding that it potentially has a role in metabolism of carbon-based life. Other metabolic byproducts might potentially be more indicative of activity of life, such as trace amounts of methane that could originate via &lt;a href=&quot;https://en.wikipedia.org/wiki/Methanogenesis&quot;&gt;methanogenesis&lt;/a&gt; that change in concentration as the environment changes daily or seasonally. Periodically changing ratio of other atmospheric gases, even in trace amounts and the changes can't be explained by inorganic processes, could also be more indicative of life than the presence of oxygen itself.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-04-30T13:09:02.553" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4397" PostTypeId="1" AcceptedAnswerId="4398" CreationDate="2014-04-30T14:05:13.027" Score="3" ViewCount="439" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OdSOI.jpg&quot; alt=&quot;This image of the NASA mission control&quot; /&gt; shows the world map screen at the centre. I'm probably wrong about this (feel free to shoot me down) but the map up there appears to render what look like 3 different orbit tracks.&lt;/p&gt;&#xA;&lt;p&gt;Why are there 3 orbit tracks?&#xA;Does Mission control track multiple craft in adjacent orbit? Are these something like an nominal, sub-nominal, and supra-nominal orbit? What is the typical margin between these orbits?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="37597" LastEditDate="2021-08-26T02:25:11.963" LastActivityDate="2021-08-26T02:25:11.963" Title="Does mission control track craft in an adjacent orbit on the world map?" Tags="|orbit|tracking|mission-control|ground-track|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="4398" PostTypeId="2" ParentId="4397" CreationDate="2014-04-30T14:24:37.813" Score="6" Body="&lt;p&gt;As @David said, those are the current and the two next orbits of the same spacecraft. On this photo you can see the ISS superimposed on one of the orbits, and the orbit numbers (4, 5, 6) to the left.&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/6lCkN.jpg&quot; alt=&quot;Mission control screen&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @Gerrit said, if the spacecraft isn't doing any maneuvering, the next orbit can be predicted with 100% accuracy. The only information you need is the orbit's period. As the Earth rotates beneath the spacecraft, the orbit's period determines the distance between the ground tracks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edited because my initial answer was incorrect)&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-04-30T17:05:51.653" LastActivityDate="2014-04-30T17:05:51.653" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4399" PostTypeId="2" ParentId="4395" CreationDate="2014-04-30T15:34:30.850" Score="2" Body="&lt;p&gt;Oxygen is about 1% of the universe's mass. So that suggests that looking at any random sample of mass throughout the universe you would expect to see 1% oxygen. Now of course you wouldn't be surprised to see higher concentrations and lower concentrations in any given sample, but the larger your sample size the more likely it is that the value will approach 1%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmosphere of a rocky planet like Earth is a tiny fraction of the planets mass (8.6134×10^-5% for Earth). However if we assume that oxygen content in the core and on the surface of a rocky planet without life is similar we can easily assume that any given lifeless planet should have around 1% oxygen in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now finding out the variance or standard deviation of oxygen in planetary atmosphere is a problem. I've found &lt;a href=&quot;http://seagerexoplanets.mit.edu/ftp/Papers/EMR2008.pdf&quot; rel=&quot;nofollow&quot;&gt;this paper&lt;/a&gt; that seems to be based on data for a large number of exoplanets, the problem is the data isn't shared, just the results. In Table 1 of that paper data is shown for three different types of exoplanet. We could calculate the variance based on this but variance from 3 data points isn't very accurate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It comes down to two aspects in my eyes. First can you attribute the oxygen content to another source? Second is the oxygen content high enough to suggest life. It will never be a guarantee from this type of observation, so you really need to know how confident you want to be. Once you know you're confidence level you could calculate your confidence interval based on standard deviation of exoplanets oxygen content. However even this is dependent on how likely you think exoplanets are to support life, if it's very unlikely then you can be more confident if you have just a few similar oxygen content observations; if it's very likely then you would expect the majority of observations to have a high oxygen content.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-04-30T15:34:30.850" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4400" PostTypeId="2" ParentId="4384" CreationDate="2014-04-30T15:58:27.493" Score="4" Body="&lt;p&gt;Getting to a contra-orbiting 'Mars' can be done using a solar sail that uses the Sun to 'crank' its heliocentric orbit into a reverse direction.  As mentioned, Jupiter can also be used this way so just take your pick.  In either case your approach trajectory relative to the planet would be in the same direction as the planet is orbiting.  The dynamics of going into orbit and landing are the same as for the real Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a solar sail mission, the time of flight would be a matter of payload mass related to sail mass and sail area with characteristic accelerations based on how close you wanted to get to the Sun during the cranking phase of the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such missions are no more challenging to a solar sail than for the real case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is similar to the never-flown Halley’s Comet rendezvous.  The mission plan there involved using a close-in heliocentric orbit (.25 AU) to crank the orbit into a 145-degree inclination.  Another 35 degrees and it would have been completely reversed.  This cranking would have taken about 440 days.  Had it been done at I AU, with only about 1/6th the acceleration available, it would have taken many years.   For the present problem, the inclination would have to be 180 degrees and the sail would then have to fly a more or less ordinary, but reversed, trajectory out to the counter-Mars.  Its a good thing the problem did not include a return to Earth.  &lt;/p&gt;&#xA;" OwnerUserId="3227" LastEditorUserId="3227" LastEditDate="2014-04-30T19:34:59.023" LastActivityDate="2014-04-30T19:34:59.023" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4402" PostTypeId="2" ParentId="2558" CreationDate="2014-04-30T16:48:04.703" Score="10" Body="&lt;p&gt;The MESSENGER mission is giving us a very tantalizing picture of Mercury's resources.  No surprise is that much of the surface is basaltic minerals containing about 42% Oxygen on average by weight.  Iron is notably diminished at about 1.5% - odd considering the planet's overall density is 5.4 gm/cm3 - while Sulfur is at around 2.3%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Colonizing any planet has to include an idea of the returns versus the expense.  Is it worth the cost to develop the technology to cope with Mercury's heat and cold, radiation, toxic regolith and resource issues?  My answer is an emphatic YES!!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mercury's orbital position is its greatest asset.  Being able to launch spacecraft to any other planet three or four times a year suggest a very aggressive program of exploration can be based there.  If solar sails are used to fly probes to other planets, the 'chemical cost' of launching from Mercury is limited to 4.2 Km/sec.  This being the velocity needed to get off Mercury's surface to a near-escape orbit for rendezvous with a sail.  In contrast, launching from Earth dooms us to at least 11.2 km/sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an astronomical platform, Mercury can support all types of instruments in all wavelengths.  So can the Moon.  The difference is that, on Mercury, a given instrument has 88 days of ultimate darkness to do full-sky surveys and 44 days to basically stare at any specific object.  The Moon, at best, offers only 14 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For those interested in colonizing Mars, Mercury can produce and transport mega-tonnages to Mars in much less time than any Earth/Moon-based program as Mercury has seven times the launch opportunity.  This is due to Mercury's synodic period with Mars being about 101 days, as compared to the 780-day Earth-Mars period.  The fact that Mercury can bring 20 times as much solar power to bear on production than facilities on Mars doesn't hurt Mercury's potential either.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could go on. . .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The environmental challenge is more about learning the details of what is there.  We are doing that with MESSENGER now, and hopefully with a lander mission of some kind as a follow-up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One last thing:  If I'm asked what is the single greatest challenge to colonizing Mercury, it will be the kinetic energy effect on manned vehicle's going to Mercury.  The only answer available right now is a very large, high-impulse propulsion with a very light payload.       &lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2014-04-30T16:48:04.703" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4403" PostTypeId="2" ParentId="4395" CreationDate="2014-04-30T19:50:20.867" Score="6" Body="&lt;p&gt;Oxygen is a highly reactive element. Gaseous oxygen rapidly combines with other elements. If there's any large amount of free oxygen in the atmosphere, after any significant amount of geological time, it's reasonable to assume that there is some process continuously releasing it to make up for what's being lost in oxidation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On our planet, that process is photosynthesis done by living plants. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That doesn't mean no other mechanism is possible, but it does suggest that SOMETHING unusual and interesting is going on which is driving a reaction backward against its natural flow.&lt;/p&gt;&#xA;" OwnerUserId="3231" LastActivityDate="2014-04-30T19:50:20.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4404" PostTypeId="2" ParentId="2558" CreationDate="2014-04-30T21:04:17.487" Score="2" Body="&lt;p&gt;This is an old question, but since it recently received a new answer, I'll add my own.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given today's technology, the answer is &quot;no way.&quot; Given our knowledge of what might be possible, the current answer is &quot;who knows?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right now, humans haven't the foggiest idea regarding how to make a sustainable human colony in space. Sending people into space is not anything like the 16th century age of discovery where a sailing ship could be sent around the world and expect to find air, water, and food wherever the ship went.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one thing we do know is that we don't know how to do it. We can't make a sustainable human colony in Arizona, let alone on the Moon, Mars, or Mercury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Too many scifi fans get hung up on the physics of space exploration and completely ignore the biological aspects of the problem. The physics part is comparatively easy. Biology? We just don't know how to do it. Making a self-sustaining space colony for now is a perpetually TRL 1 concept. In other words, it's science fiction for now, and I suspect it will be for a &lt;strong&gt;long&lt;/strong&gt; time.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-04-30T21:04:17.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4405" PostTypeId="2" ParentId="4395" CreationDate="2014-04-30T23:12:23.897" Score="9" Body="&lt;p&gt;The first &lt;a href=&quot;http://en.wikipedia.org/wiki/Great_Oxygenation_Event&quot;&gt;third&lt;/a&gt; of Earth's &lt;em&gt;own&lt;/em&gt; existence had neglible atmospheric oxygen.  (Simple) life existed during a few hundred million years of this period.  Thus, atmospheric oxygen does not correlate with life on a planet.&lt;/p&gt;&#xA;" OwnerUserId="1235" LastActivityDate="2014-04-30T23:12:23.897" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4406" PostTypeId="1" AcceptedAnswerId="4408" CreationDate="2014-04-30T23:22:22.450" Score="7" ViewCount="1414" Body="&lt;p&gt;I'm working on a school project on calculating the Keplerian orbital motion of objects and then plotting/animating the trajectories. One feature I want to include is plotting a Hohmann transfer from one orbiting height to another one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is do any of you know where I can find an equation of motion that would represent a Hohmann transfer &quot;orbit&quot;. I've looked around online and the best I've found is the equation to calculate the change in velocity needed, which doesn't really help me plot the motion of this orbit. Thanks in advance for any help.&lt;/p&gt;&#xA;" OwnerUserId="3236" LastActivityDate="2014-05-01T00:54:04.960" Title="Hohmann Transfer Equation of Motion" Tags="|orbital-mechanics|propulsion|moons|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4407" PostTypeId="2" ParentId="4406" CreationDate="2014-05-01T00:30:11.793" Score="0" Body="&lt;p&gt;With the equation of motion I assume you mean the position as a function of time. However this doesn't exist for eccentric Keplerian orbits, at least not explicitly. Because you would have to solve &lt;a href=&quot;http://en.wikipedia.org/wiki/Kepler&amp;#39;s_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler's equation&lt;/a&gt;. This can be approximated numerically. But it would be possible to calculate it the &lt;a href=&quot;https://physics.stackexchange.com/questions/69380/what-are-common-methods-for-calculating-the-time-dependency-of-elliptical-orbit&quot;&gt;other way around&lt;/a&gt;, so the time as a function of the position.&lt;/p&gt;&#xA;" OwnerUserId="1086" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2014-05-01T00:30:11.793" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4408" PostTypeId="2" ParentId="4406" CreationDate="2014-05-01T00:54:04.960" Score="5" Body="&lt;p&gt;You can use:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$x=a\left(\cos\tau-e\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$y=a\sqrt{1-e^2}\sin\tau$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;to plot.  $a$ is the semi-major axis and $e$ is the eccentricity.  The central body being orbited (e.g. the Sun) is at $\left(0,0\right)$.  For a Hohmann transfer, you are going from periapsis to apoapsis, or vice-versa, so run $\tau$ from $0$ to $\pi$, or $\pi$ to $2\pi$.  You may need to rotate the $\left(x,y\right)$ coordinates to line up with your departure and arrival points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\tau$ is not time.  It is the eccentric anomaly.  If you want to put time ticks on the plot, you can use:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t=\sqrt{a^3\over\mu}\left(\tau-e\sin\tau\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;to compute the time, where $\mu$ is the $GM$ of the central body.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-05-01T00:54:04.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4409" PostTypeId="2" ParentId="1134" CreationDate="2014-05-01T08:36:45.870" Score="5" Body="&lt;p&gt;Satellites are not sealed. A satellite is exposed to rapidly changing pressure during lift-off and has to be designed to release air trapped within the main frame (the bus) in a controlled manner without damage. An example, due to changing temperatures in space, they are wrapped in a thermal blanket, which has to have vent holes as it could rip during lift-off and potentially obstruct the payload (eg the antenna).&lt;/p&gt;&#xA;" OwnerUserId="3241" LastActivityDate="2014-05-01T08:36:45.870" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4410" PostTypeId="1" CreationDate="2014-05-01T12:34:36.200" Score="2" ViewCount="132" Body="&lt;p&gt;There happen to be several questions out here on DIY spacecraft/launch-systems ... so I figure this question should be the next that comes to mind&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless one happens to be on the craft, and well-trained in handling spacecraft at the extremely large velocities a launch vehicle may attain - there may be a ramp-up curve, or un-manned launch. Regardless such probably require specialized education, and processing power (firm-ware &amp;amp; wet-ware, both).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming it becomes possible for a person to put together a spacecraft on their own,&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Can a person &lt;strong&gt;control&lt;/strong&gt; an unmanned spacecraft on their own? &lt;/li&gt;&#xA;&lt;li&gt;Is a single person on ground in communication with a manned spacecraft enough?&lt;/li&gt;&#xA;&lt;li&gt;What is the minimum team size required to control an unmanned spacecraft from launch through orbit? What about a manned spacecraft?&lt;/li&gt;&#xA;&lt;li&gt;Are there any regulations either from &lt;a href=&quot;http://www.nar.org/&quot; rel=&quot;nofollow&quot;&gt;NAR&lt;/a&gt;, or &lt;a href=&quot;http://www.nasa.gov&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt; that define a lower-limit &amp;amp; profiles for mission control of any rocket/space launch?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="49" LastEditDate="2014-05-01T13:31:09.517" LastActivityDate="2014-05-01T13:31:09.517" Title="What is the minimum team-size for mission control?" Tags="|law|mission-control|" AnswerCount="0" CommentCount="3" ClosedDate="2014-05-01T20:01:33.187" ContentLicense="CC BY-SA 3.0" />
  <row Id="4411" PostTypeId="1" CreationDate="2014-05-01T14:30:40.810" Score="6" ViewCount="778" Body="&lt;p&gt;Is it possible to establish a practical 'cycler' transportation system between Earth and Mercury?  The concept involves a small, manned payload (7.5 mt or less) doing the Earth departure and Mercury Orbit Insertion burns.  The crew inhabits the pre-deployed Cycler and, prior to Mercury encounter, converts water stored on the cycler into the propellants needed for the MOI burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Principle numbers for the concept are: Mercury's orbital period: 87.9 days; the Mercury-Earth synodic period: 115.9 days; the Earth's orbital period: 365.25 days and the cycler's orbital period.  Several periods are possible, but I arbitrarily chose a 351.6 day orbit.  This gives a transfer time (either way) of 175.8 days.  Evidently, the match-up is not exact enough to prevent a larger delta-V than desired.     &lt;/p&gt;&#xA;" OwnerUserId="3227" LastEditorUserId="49" LastEditDate="2014-05-01T17:41:25.680" LastActivityDate="2019-03-13T09:31:12.263" Title="Cyclers To Mercury?" Tags="|orbital-mechanics|trajectory|interplanetary|mercury|low-energy-transfer|" AnswerCount="1" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="4412" PostTypeId="2" ParentId="4395" CreationDate="2014-05-01T19:17:38.333" Score="10" Body="&lt;p&gt;As @TidalWave said, no specific level of oxygen is &lt;em&gt;by itself&lt;/em&gt; sufficient to potentially indicate life.  However, based on &lt;a href=&quot;http://arstechnica.com/science/2014/04/search-for-extraterrestrial-life-clouded-by-exomoon-atmospheres/&quot;&gt;this article&lt;/a&gt;, what astronomers are looking for is the combination of both oxygen and methane in a single atmosphere.   &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Both oxygen and methane can be created independently by nonliving processes, so their individual presence is of little interest. What scientists are looking for is both of them in the atmosphere of a single body. If these reactive gases are not constantly replenished by living things, they will react with each other, creating carbon dioxide and water. As a result, we should not observe them in the same atmosphere without a large, living source.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The article itself is about a study which suggests that we can be fooled by detecting both signatures from an exoplanet with an exomoon with opposite atmospheres (i.e. one is O2, the other is CH4), and is quite interesting.&lt;/p&gt;&#xA;" OwnerUserId="898" LastActivityDate="2014-05-01T19:17:38.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4413" PostTypeId="1" CreationDate="2014-05-01T22:25:29.287" Score="5" ViewCount="681" Body="&lt;p&gt;How do you determine (what is the formula?) the delta-V for a sub-orbital 'hop' from one location on the Moon to another?  Is it different from a typical orbital calculation? &lt;/p&gt;&#xA;" OwnerUserId="3227" LastEditorUserId="49" LastEditDate="2014-05-01T22:51:56.757" LastActivityDate="2016-02-01T17:48:52.853" Title="Lunar sub-orbital trajectories" Tags="|the-moon|trajectory|gravity|sub-orbital|ballistics|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4414" PostTypeId="2" ParentId="4384" CreationDate="2014-05-01T23:17:20.977" Score="0" Body="&lt;p&gt;Firstly current tech and widely accepted theories:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately the key point in this question is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is the only way to make this jump, to reduce the relative speed&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The answer is yes. It really comes down to &lt;a href=&quot;http://en.wikipedia.org/wiki/Momentum#Conservation&quot; rel=&quot;nofollow noreferrer&quot;&gt;conservation of momentum&lt;/a&gt;, or more specifically &lt;a href=&quot;http://en.wikipedia.org/wiki/Angular_momentum#Conservation_of_angular_momentum&quot; rel=&quot;nofollow noreferrer&quot;&gt;conservation of angular momentum&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your spacecraft at Earth has a fixed angular momentum around the sun. A few points people often think can get around this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) It doesn't matter how you change the attitude of your spacecraft the angular moment stays the same. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) If you give the spacecraft angular momentum along a different axis so that it goes around the sun in a polar orbit it still won't have the right angular momentum when it gets to 180deg from it's starting position because it still has its original angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In essence if you wish to match velocity with retrograde Mars then you will need to reverse your angular momentum vector (and increase the magnitude).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the more interesting concepts that may be able to short circuit the problem:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first unusual method that springs to mind is an &lt;a href=&quot;https://space.stackexchange.com/questions/4281/if-we-can-make-an-alcubierre-drive-how-fast-can-it-move-a-ship/4282#4282&quot;&gt;Alcubierre drive&lt;/a&gt;, mainly due to that question being asked recently. It seems to me that you would be able to shift your spacecraft to the opposite side of the sun using such a drive. At which point the spacecraft would be moving in the same direction as retrograde Mars &lt;em&gt;IF&lt;/em&gt; the Alcubierre drive can transport an object in one dimension (eg X direction) whilst the object has a velocity perpendicular. I don't seem any immediate reason why this type of drive wouldn't work here (or at least no more reason than it working anywhere else) so this is a possibility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second unusual method that springs to mind is the very sci-fi idea of wormholes. In theory a wormhole could be exist connecting opposite sides of the sub where the relative velocity of the two objects would be much closer. Similar to the Alcubierre drive in a way. A problem here is that some current thinking on wormholes is that they are very small. We're talking the same scale as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Planck_length&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planck length&lt;/a&gt; for the neck of the wormhole. Also good luck trying to create one; &lt;a href=&quot;http://www.google.com/patents/US20030197093&quot; rel=&quot;nofollow noreferrer&quot;&gt;this patent&lt;/a&gt; should get you started!&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-05-02T16:04:57.600" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4415" PostTypeId="2" ParentId="4413" CreationDate="2014-05-01T23:25:24.907" Score="1" Body="&lt;p&gt;In its simplest form it isn't different from any other type of orbital calculation. The only difference you may notice is that the perigee of the orbit may be within the surface of the moon. For a quick thought experiment, imagine drawing an ellipse onto a 2D picture of the moon.  If you make this ellipse pretty small (and pretty circular) you can connect two close by points by drawing about half of the ellipse, starting at your take off and stopping at your landing point. As you go for a larger hop you draw more and more of the ellipse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things get a little more complicated when you take into account how you want to take off and land, but in essence it's just like any other type of orbit.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1142" LastEditDate="2014-05-02T16:30:11.903" LastActivityDate="2014-05-02T16:30:11.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4416" PostTypeId="1" CreationDate="2014-05-02T02:30:21.460" Score="6" ViewCount="288" Body="&lt;p&gt;To get orbit details of a large number of objects &lt;a href=&quot;http://en.wikipedia.org/wiki/Two-line_element_set&quot;&gt;Two line elements&lt;/a&gt; (TLE) datasets are often used. However since the only value that's related to mass in the TLEs is the BSTAR drag term how could I find the mass of each orbital object?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume I could look it up from launch details (although if this is the solution shouldn't this value be included in the TLEs?) but this seems cumbersome also what about objects that are created from collisions between satellites. Is there a database somewhere that includes the mass of each object?&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-10-29T17:36:53.273" Title="How can someone find the mass of debris objects in orbit" Tags="|debris|reference-request|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4417" PostTypeId="2" ParentId="4413" CreationDate="2014-05-02T05:46:30.070" Score="5" Body="&lt;p&gt;There was &lt;a href=&quot;https://space.stackexchange.com/questions/3531/would-a-point-to-point-suborbital-spaceflight-have-a-negative-perigee/4190#4190&quot;&gt;an earlier question about suborbital hops&lt;/a&gt;. I will reuse some of the diagrams and explanation from that answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A minimum energy ellipse between departure and destination corners of a Lambert space triangle is described on page 65 of the 1993 edition of Prussing and Conway's Orbital Mechanics textbook. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this particular Lambert Space triangle, both $r_1$ and $r_2$ would be the radius of the moon, 1738 km. The 3 points of the triangle would be moon's center and the departure and destination points on the lunar surface. $\theta$ would be the angle between the two points. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ydtO5.png&quot; alt=&quot;minimum energy ellipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second focus of this minimum energy ellipse would lie on the center of the chord connecting the points on the lunar surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Distance between foci, $2e\cdot a$, is $r \cos(\frac\theta2)$. The major axis of this ellipse ($2a$) is $r(1 + \sin(\frac\theta2))$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Knowing $r$ (1738 km) and $a = r(1 + \sin(\frac\theta2))$, the vis viva equation can be used to get $\Delta v$ for take off as well as soft landing at other end of suborbital hop. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vis viva equation is&#xA;$$v = \sqrt{GM\left(\frac2a - \frac1r\right)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another useful piece of info is what angle you should depart from the moon's surface. If the destination is near, the angle will be close to $45^\circ$. As the angle between departure and destination approaches $180^\circ$, the flight path angle will approach $0^\circ$, that is, horizontal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I made &lt;a href=&quot;http://clowder.net/hop/TMI/LunarSuborbital.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;a spreadsheet&lt;/a&gt; for this. User can input data into the colored cells. I set it for Luna, but a user could also input mass and radius of other bodies, Ceres and Mercury for example.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="13262" LastEditDate="2016-02-01T17:48:52.853" LastActivityDate="2016-02-01T17:48:52.853" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4418" PostTypeId="2" ParentId="4369" CreationDate="2014-05-02T05:47:25.803" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Maxime_Faget&quot; rel=&quot;noreferrer&quot;&gt;Max Faget&lt;/a&gt; was involved in the design of every US manned spacecraft flown to date.  Mercury, Gemini, Apollo, and Shuttle. It looks like Elon Musk will end Max's monopoly.&lt;/p&gt;&#xA;" OwnerUserId="3250" LastActivityDate="2014-05-02T05:47:25.803" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4419" PostTypeId="1" AcceptedAnswerId="4421" CreationDate="2014-05-02T09:07:42.600" Score="28" ViewCount="4517" Body="&lt;p&gt;In the film 'Gravity', astronauts can be seen having casual chat with mission control. Do astronauts do that in real life? Do they have any specific policy that states what can be communicated over radio?&lt;/p&gt;&#xA;" OwnerUserId="3253" LastActivityDate="2021-05-27T09:05:51.607" Title="Do astronauts go on a random banter with mission control like in the film 'Gravity'?" Tags="|communication|astronauts|mission-control|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4420" PostTypeId="2" ParentId="4413" CreationDate="2014-05-02T10:08:57.653" Score="4" Body="&lt;p&gt;There is one difference with typical orbital calculations: the Moon's gravity field is uneven, due to &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2013-184&quot; rel=&quot;nofollow&quot;&gt;mass concentrations&lt;/a&gt; in various places. If your trajectory crosses one of these mass concentrations, your trajectory will be changed a bit. The gravity value &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&quot; rel=&quot;nofollow&quot;&gt;differs by about 0.3%&lt;/a&gt; across the Moon's surface. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-05-02T10:08:57.653" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4421" PostTypeId="2" ParentId="4419" CreationDate="2014-05-02T10:36:58.620" Score="28" Body="&lt;p&gt;If you listen to live (or recorded) ISS to ground conversations you will find that the astronauts are human, and have perfectly normal conversations when not specifically running through mission checklists etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These conversations include banter, birthday wishes, conversations about the weather (yep - seriously) and any manner of normal topics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I couldn't find any examples of swearing or offensive language, so there is probably a directive to avoid cursing on air (that is just a guess of mine though)&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2014-05-02T10:36:58.620" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4422" PostTypeId="2" ParentId="4416" CreationDate="2014-05-02T14:12:31.700" Score="3" Body="&lt;p&gt;I don't know of any way to accurately determine the mass of a piece of debris on orbit without knowing the material beforehand. Their mass is much too low to use gravitational influence as a measure and I don't think the &quot;usual&quot; method for small asteroids applies. This would be assuming an albedo to determine the size by the amount of light it reflects, and then determining the mass by assuming (again) a density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This fails with debris. Debris tracking is mostly done by Lidar. While the albedo could be determined this way, this would only give you information on the surface, which may be coated for maximum reflectivity or emissivity. For large enough objects they may be able to determine a size by direct measurement. However, the object is likely to be fragile and made of many different materials, rather than solid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My best guess is that they use an average density of a typical satellite fragment if they have no pre-launch information. I wouldn't assume that is is very accurate.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-05-02T14:12:31.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4423" PostTypeId="1" AcceptedAnswerId="6527" CreationDate="2014-05-02T14:41:20.010" Score="11" ViewCount="625" Body="&lt;p&gt;I was reading about this new &lt;a href=&quot;http://www.bbc.com/news/science-environment-27245720&quot;&gt;space meal contest&lt;/a&gt; it talks about many of the issues creating appetizing meals for Astronauts.  It mentions the loss of taste...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Astronauts often lose their sense of smell, which diminishes the sense of taste. This is because blood flows to their heads owing to the lack of gravity, and this causes their faces to swell and blocks their noses&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Which got me thinking what the impact would be on &lt;a href=&quot;https://en.wikipedia.org/wiki/Snore&quot;&gt;snoring&lt;/a&gt;, which in my experience is at least partly based on positioning and gravity effects on soft parts. Does no gravity mean no snoring? Or does the lack of gravity and related soft tissue swelling increase snoring? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2014-12-22T13:12:23.647" Title="Do Astronauts snore when sleeping in Zero G?" Tags="|microgravity|sleep|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4425" PostTypeId="2" ParentId="2433" CreationDate="2014-05-02T16:12:15.550" Score="13" Body="&lt;p&gt;When the amount of evidence, circumstantial or direct, for a given proposition vastly outweighs evidence against that proposition, most reasonable people are willing to accept the proposition as true. This does occasionally lead to having to change your mind about a proposition when new evidence comes along; this is how the scientific method works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the specific case of space travel, consider this: &lt;a href=&quot;https://en.wikipedia.org/wiki/V-2_rocket&quot; rel=&quot;noreferrer&quot;&gt;Germany launched some big rockets during WWII&lt;/a&gt;, and some big rockets landed in England, and we have good circumstantial evidence that those were the same rockets, and some back-of-the-envelope calculations regarding energy from chemical reactions, volume of fuel, mass of rocket, models of aerodynamic drag, etc. all agree that, yes, a rocket using &lt;em&gt;that&lt;/em&gt; design and &lt;em&gt;that&lt;/em&gt; fuel can go &lt;em&gt;that&lt;/em&gt; far.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After WWII, other countries (like the USA and USSR) then built bigger and better rockets, that could go further, and all of those seemed to perform more or less according to our models as well, and we improved our models to better align with our observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eventually the USSR built &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_1&quot; rel=&quot;noreferrer&quot;&gt;Vostok 1&lt;/a&gt;: a pod big enough to hold Yuri Gagarin, on top of a rocket big enough to, according to our calculations, throw that pod high enough that the atmosphere is thin enough that we call it &quot;outer space&quot;. Lots of people saw the rocket take off, and later, far away, a pod fell out of the sky and &lt;a href=&quot;https://en.wikipedia.org/wiki/Yuri_Gagarin&quot; rel=&quot;noreferrer&quot;&gt;Yuri Gagarin&lt;/a&gt; landed, nearby, by parachute. One interesting thing here is that the USSR lied about that at the time and said he landed with the pod, so already this story is kind of fishy and you wouldn't be completely crazy to have doubts about this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But since then, there have been &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_human_spaceflights&quot; rel=&quot;noreferrer&quot;&gt;over 300 crewed spaceflights&lt;/a&gt;, on rockets that we know quite a bit about, that clearly have taken off at point A and had parts reappear later at point B, with photographs and video of people getting into them at one end, photos and video of the beginning of the flight, and photos and video of very similar-looking people getting out of them at the other end, across more than half a century. &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_astronauts_by_first_flight&quot; rel=&quot;noreferrer&quot;&gt;More than 550 individuals&lt;/a&gt; have claimed to have flown in space; as far as I know, none have ever recanted the claim and said &quot;it was a hoax&quot;—and I'd ignore one or two voices out of 550 who claimed hoax anyway, given the evidence in favor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's a &lt;em&gt;lot&lt;/em&gt; of circumstantial evidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point, even the most skeptical humans generally believe that the burden of proof is on those who doubt that human spaceflight happened. It seems, at this point, vastly easier to actually put people into space than to orchestrate a conspiracy of this scale. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want me to doubt the fact of crewed spaceflight, you have to explain to me where those rockets went, where the crew boarding them went, who the people were who exited similar-looking capsules hundreds and thousands of miles away, and most importantly, why we'd bother with such a scheme.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of this applies to anyone doubting the moon landings, as well. The ratio of evidence may be slightly less extreme, but there's still &lt;a href=&quot;https://en.wikipedia.org/wiki/Third-party_evidence_for_Apollo_Moon_landings&quot; rel=&quot;noreferrer&quot;&gt;a wealth of evidence&lt;/a&gt; in favor of them. A Saturn V rocket produces about 18km/s of delta-V; where was it going if not to the moon?&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-06-28T20:44:33.223" LastActivityDate="2018-06-28T20:44:33.223" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="4426" PostTypeId="2" ParentId="2890" CreationDate="2014-05-02T16:33:00.317" Score="11" Body="&lt;p&gt;Consider that, in order for the escaping crew to survive outside the re-entering capsule, they'd need a heat-shielded, thick-walled escape pod; in order not to hit the ground at fatal speed, the escape pod would need to be broad and blunt in order to bleed off speed via aerodynamic drag. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd basically be talking about another re-entry capsule inside your re-entry capsule. From an engineering standpoint, it makes far more sense to build redundancy (multiple parachutes) and reliability into the single capsule. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-05-02T16:33:00.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4427" PostTypeId="2" ParentId="4369" CreationDate="2014-05-02T21:16:16.333" Score="4" Body="&lt;p&gt;&lt;strong&gt;Buran&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following is the relevant administrative chain of Buran proper, not the whole Energia-Buran complex. The whole complex as well as Energia rocket herself were designed mostly in NPO Energia.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Gleb_Lozino-Lozinskiy&quot; rel=&quot;nofollow&quot;&gt;Gleb Lozino-Lozinsky&lt;/a&gt; (Designer General, Director General (~CEO) of &lt;a href=&quot;http://en.wikipedia.org/wiki/NPO_Molniya&quot; rel=&quot;nofollow&quot;&gt;NPO Molnia&lt;/a&gt;)&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://wiki.airforce.ru/index.php?title=%D0%94%D0%B5%D0%BC%D0%B5%D0%BD%D1%82%D1%8C%D0%B5%D0%B2_%D0%93%D0%B5%D0%BD%D0%BD%D0%B0%D0%B4%D0%B8%D0%B9_%D0%9F%D0%B5%D1%82%D1%80%D0%BE%D0%B2%D0%B8%D1%87&quot; rel=&quot;nofollow&quot;&gt;Gennady Dementyev&lt;/a&gt; (Chief Designer)&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.buran.ru/images/jpg/selet.jpg&quot; rel=&quot;nofollow&quot;&gt;Yakov Seletsky&lt;/a&gt; (Head of &quot;Department 400 of configuration(?)&quot;)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.buran.ru/images/jpg/samson.jpg&quot; rel=&quot;nofollow&quot;&gt;Evgeny Samsonov&lt;/a&gt; (Head of &quot;Department 500 of aerodynamics&quot;)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some sources claim, that the essential part of the conceptual design works were done by Lozino-Lozinsky himself. However strange that may sound.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The information is rather sparse, though, so probably the &lt;em&gt;real&lt;/em&gt; key people are somewhere down the administrative chain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are lists of &lt;em&gt;all&lt;/em&gt; the people responsible for Buran airframe though, such as &lt;a href=&quot;http://www.buran.ru/htm/kontruk.htm&quot; rel=&quot;nofollow&quot;&gt;at the bottom of this page&lt;/a&gt;. So the right person is definitely there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources: &lt;a href=&quot;http://buran.ru&quot; rel=&quot;nofollow&quot;&gt;buran.ru&lt;/a&gt; mostly.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2014-05-03T07:15:13.020" LastActivityDate="2014-05-03T07:15:13.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4428" PostTypeId="2" ParentId="4369" CreationDate="2014-05-03T01:09:06.367" Score="2" Body="&lt;p&gt;This answer is about spacecrafts designed in OKB-52, later known as TsKBM, then as &lt;a href=&quot;http://en.wikipedia.org/wiki/NPO_Mashinostroyeniya&quot; rel=&quot;nofollow&quot;&gt;NPO Mashinostroeniya&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Almaz&lt;/strong&gt; (OPS), &lt;strong&gt;TKS&lt;/strong&gt; and &lt;strong&gt;VA&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Vladimir_Chelomey&quot; rel=&quot;nofollow&quot;&gt;Chelomey&lt;/a&gt; (OKB-52, Designer General) &#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.npomash.ru/history/en/efremov.htm&quot; rel=&quot;nofollow&quot;&gt;Efremov&lt;/a&gt; (Deputy Designer General)&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://www.npomash.ru/press/images/tribuna2009/histwri.JPG&quot; rel=&quot;nofollow&quot;&gt;Vladimir Polyachenko&lt;/a&gt; (Chief Lead Designer of &quot;Almaz&quot; station)&lt;/strong&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;http://www.findpatent.ru/byauthors/417465/&quot; rel=&quot;nofollow&quot;&gt;Anatoly Blagov&lt;/a&gt; (Lead Designer of &lt;a href=&quot;http://en.wikipedia.org/wiki/VA_spacecraft&quot; rel=&quot;nofollow&quot;&gt;VA&lt;/a&gt;)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Blagov designed re-entry capsules (VAs) initially for &lt;a href=&quot;http://www.astronautix.com/craft/lk1.htm&quot; rel=&quot;nofollow&quot;&gt;LK-1 project&lt;/a&gt;, then for &lt;a href=&quot;http://en.wikipedia.org/wiki/VA_spacecraft#Almaz_APOS&quot; rel=&quot;nofollow&quot;&gt;Almaz&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/VA_spacecraft#TKS_missions&quot; rel=&quot;nofollow&quot;&gt;TKS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The design of TKS should have been conducted by someone in Fili branch of TsKBM (later known as KB Salyut, today part of Khrunichev).&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2014-05-03T01:32:21.400" LastActivityDate="2014-05-03T01:32:21.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4429" PostTypeId="1" AcceptedAnswerId="4444" CreationDate="2014-05-03T09:19:09.293" Score="6" ViewCount="855" Body="&lt;p&gt;A body on Earth's surface can be positioned easily knowing the GPS coordinates, assuming the body is in direct path or 4 GPS satellites. But how is a satellite (say Maven or India's Mangalyaan) which is travelling to Mars positioned, or its position calculated to steer it on the right path? I guess the GPS satellites are facing the Earth not to waste power and resources by facing away from the Earth. But how is a satellite out of reach of GPS satellites positioned, and its position calculated with required precision?&lt;/p&gt;&#xA;" OwnerUserId="3266" LastEditorUserId="6214" LastEditDate="2015-01-30T04:28:19.837" LastActivityDate="2015-01-30T04:28:19.837" Title="How are artificial satellites of non-Earth celestial bodies positioned?" Tags="|artificial-satellite|orbit|solar-system|navigation|gps|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4430" PostTypeId="2" ParentId="4429" CreationDate="2014-05-03T09:47:26.310" Score="0" Body="&lt;p&gt;For the relatively rare missions to other planets I would assume the satellites are almost constantly tracked. Either way we can quite easily predict the future position of any object in orbit around a central body with a very high accuracy, for a short period of time. So assuming regular checks are made to negate the non conservative perturbations of the orbit, we can just use astrodynamics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A point of interest is that some of the non conservative perturbations will be practically non excitant in interplanetary (drag for example), others may be increased (solar pressure may be greater outside of the Earth's magnetic field, but smaller further from the sun).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT 2:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well star trackers take pictures of space and try and match that to a star chart; essentially saying if I've taken a picture of that area of the sky I must be facing this way. Sun sensors are a lot simpler, we're pretty confident of the solar flux at any given distance from the sun, so if we are at mars we know which part of the spacecraft is pointing at the sun based on the brightness. You also have horizon trackers, these look toward the planet and find the outline of the planet based on the difference between the dark background of space and the planet's surface. There are also gyroscopes that can be used to measure your attitude.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-05-04T13:54:10.323" LastActivityDate="2014-05-04T13:54:10.323" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4431" PostTypeId="2" ParentId="1134" CreationDate="2014-05-03T09:48:47.397" Score="8" Body="&lt;p&gt;Early Russian probes were pressurized. The reason was to give the equipment similar conditions which could be tested on Earth, and to simplify the thermal environment (making conduction/convection possible, not just radiation as in vakuum). They used nitrogen at 113 hectopascal, this source says: Andrew J. LePage at the space review&#xA;&lt;a href=&quot;http://thespacereview.com/article/2477/1&quot;&gt;http://thespacereview.com/article/2477/1&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-05-03T09:48:47.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4432" PostTypeId="1" CreationDate="2014-05-03T17:36:48.850" Score="3" ViewCount="501" Body="&lt;p&gt;I was wondering if it is possible to send a small payload to LEO or somewhere above 200 km (130 miles) if you are not a rocket scientist? By small payload I mean under 1 kg (Solid State Drive, GPS, etc.). I have read that there are companies which will take your payload and put it into the LEO or GSO/GEO, but could not find any information about pricing.&lt;/p&gt;&#xA;" OwnerUserId="3271" LastEditorUserId="49" LastEditDate="2014-05-03T18:11:34.420" LastActivityDate="2019-08-12T01:22:46.970" Title="Small payload to Low Earth Orbit" Tags="|orbit|low-earth-orbit|geostationary|payload|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4433" PostTypeId="2" ParentId="4432" CreationDate="2014-05-03T18:32:21.583" Score="3" Body="&lt;p&gt;You can get a &lt;a href=&quot;http://www.nasa.gov/directorates/heo/home/CubeSats_initiative.html&quot; rel=&quot;nofollow&quot;&gt;ride for free&lt;/a&gt; if it is educational:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To participate in the CSLI program, CubeSat investigations should be&#xA;  consistent with NASA's Strategic Plan and the Education Strategic&#xA;  Coordination Framework. The research should address aspects of&#xA;  science, exploration, technology development, education or operations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Or &lt;a href=&quot;http://spaceflightservices.com/pricing-plans/&quot; rel=&quot;nofollow&quot;&gt;you can pay&lt;/a&gt; $125K.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-05-03T18:32:21.583" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4434" PostTypeId="1" CreationDate="2014-05-03T19:32:29.260" Score="9" ViewCount="2277" Body="&lt;p&gt;Of the potential volume of a liquid fueled rocket's propellant tank, how much of that volume is actually dedicated to propellants (since &lt;a href=&quot;http://en.wikipedia.org/wiki/Ullage&quot; rel=&quot;nofollow noreferrer&quot;&gt;ullage&lt;/a&gt; is needed because of pressurization, boiloff, etc.)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Particularly, I'm looking for this measure with respect to the following fuels &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;RP-1, &lt;/li&gt;&#xA;&lt;li&gt;LOX, and &lt;/li&gt;&#xA;&lt;li&gt;LH2 fuels.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/khS7e.jpg&quot; alt=&quot;Note the gaseous hydrogen in the Ariane 5 EPC: that&amp;#39;s the ullage space I&amp;#39;m asking about.&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="3272" LastEditorUserId="288" LastEditDate="2014-05-04T07:30:13.220" LastActivityDate="2014-07-16T09:34:21.913" Title="How much potential volume in a rocket's tank is actually used for fuel?" Tags="|rockets|fuel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4435" PostTypeId="1" CreationDate="2014-05-03T20:03:43.510" Score="10" ViewCount="3766" Body="&lt;p&gt;There are such things as &lt;a href=&quot;http://en.wikipedia.org/wiki/Earth-Moon-Earth_communication&quot;&gt;moon-bounce communication&lt;/a&gt; whereby radio signals are 'bounced' off the lunar surface to a distant location on Earth. This mechanism may be likened to the way a mirror reflects light. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.df5ai.net/ArticlesDL/EMEPhysics2/EMEscatter.html&quot;&gt;This article&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;it has been discovered that radiowaves, unlike light and infrared radiation, are reflected back to the Earth principally from a small region at the center of the visible disk &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Radio communication on Earth is strongly dependent upon the ionosphere - a layer of the atmosphere comprised of charged particles sympathetic to radio energy. Luna does not have an ionosphere but lunar dust levitates. The dust is ionized by the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I am curious about&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is long range radio communication possible on the lunar surface?&lt;/li&gt;&#xA;&lt;li&gt;Was/Is radio propagation on the lunar surface part of any mission/programme to-date (or proposed)?&lt;/li&gt;&#xA;&lt;li&gt;Could analysis of the signal quality on CSM/LM communication tapes of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_program&quot;&gt;Apollo program&lt;/a&gt; yield anything useful in this context?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2022-06-17T21:43:18.830" Title="Is long range radio communication possible on the lunar surface?" Tags="|the-moon|apollo-program|communication|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4436" PostTypeId="2" ParentId="4435" CreationDate="2014-05-03T22:22:28.943" Score="-2" Body="&lt;p&gt;This is from logic and opinion, no references:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is long range radio communication possible on the lunar surface?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No. The dust around the moon will be much more discrete than the ionosphere which acts more like a continuum. It would be a bit like trying to see your reflection in ground up mirrors dust. If you have the appropriate wavelength then it might be possible, but it would be dependent on a high enough number of particles levitating.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Was/Is radio propagation on the lunar surface part of any&#xA;  mission/programme to-date (or proposed)?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I assume this question is about long range propagation. I've had a look over everything I could think to look over and I haven't found anything substantial, I assume because it wouldn't work.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Could analysis of the signal quality on CSM/LM communication tapes of&#xA;  the Apollo program yield anything useful in this context?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes. This could be used to test if the frequencies used have the wavelength. It may also be used to predict to what which frequency would be required, since there would be a degree of Doppler shift in the frequency at times and you might cross into a frequency that produces some measurable 'bounce', after that you could extrapolate the best frequency. &lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-05-03T22:22:28.943" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4437" PostTypeId="2" ParentId="4435" CreationDate="2014-05-04T00:31:24.773" Score="16" Body="&lt;p&gt;The lack of an ionosphere seems, intuitively, to mean great difficulty in propagating over-the-horizon radio waves, but according to &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20080046911&quot;&gt;this paper&lt;/a&gt; by Robert M. Manning, it should, in theory, be possible to radio to another point beyond your line-of-sight even without the use of a lunar communications satellite. From the abstract:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Two potential low-frequency propagation mechanisms characteristic of the lunar landscape are the lunar regolith and the photoelectron induced plasma exosphere enveloping the Moon. Although it was hoped that the regolith would provide for a spherical waveguide which could support a trapped surface wave phenomena, it is found that, in most cases, the regolith is deleterious to long range radio wave propagation. However, the presence of the plasma of the lunar exosphere supports wave propagation and, in fact, surpasses the attenuation of the regolith. Given the models of the regolith and exosphere adopted here, it is recommended that a frequency of 1 MHz be considered for low rate data transmission along the lunar surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-05-04T00:36:25.380" LastActivityDate="2014-05-04T00:36:25.380" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4438" PostTypeId="1" AcceptedAnswerId="4439" CreationDate="2014-05-04T04:17:03.457" Score="10" ViewCount="2137" Body="&lt;p&gt;The Mars One website lists SpaceX as a supplier for their launch vehicle and that they did receive a &quot;letter of interest&quot; from them along other suppliers. Mars One cofounder Bas Lansdorp has visited SpaceX, but they have been very quiet about their relationship with Mars One. With Elon Musk's goal of Mars colonization it appears that the two organizations do have similar interests, but is the relationship possibly competitive? Will SpaceX publicly support Mars One? If so, what milestones, if any are they waiting for from Mars One to make such a statement?&lt;/p&gt;&#xA;" OwnerUserId="3161" LastActivityDate="2014-11-25T12:08:50.167" Title="What is SpaceX's stance on Mars One?" Tags="|mars|colonization|spacex|mars-one|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4439" PostTypeId="2" ParentId="4438" CreationDate="2014-05-04T07:08:44.283" Score="11" Body="&lt;p&gt;The cooperation between Mars One and SpaceX will become serious after they sign a binding contract of some sort. Until this is the case, their relationship will be very loose and will not force any party to do anything by any date. So far there is &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_One#Launcher&quot;&gt;no such contract&lt;/a&gt; signed between the 2 parties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, SpaceX won't be doing anything, until Mars One makes an obligatory order for a launcher or some other hardware.&lt;/p&gt;&#xA;" OwnerUserId="921" LastActivityDate="2014-05-04T07:08:44.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4440" PostTypeId="1" AcceptedAnswerId="4452" CreationDate="2014-05-04T11:16:07.477" Score="8" ViewCount="1227" Body="&lt;p&gt;I wonder what the most important potentials and problems are with folding out structures in space. The basic idea is to achieve large structures in space while reducing launch mass and launch volume, and also avoiding human space walking construction work in microgravity. What stress are different kinds of structures in space exposed to? Such as astronauts moving around in them, engines accelerating them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can think of three purposes with having big structures in space:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1) Parabolic dish for radio telescopes or solar collectors.&lt;/li&gt;&#xA;&lt;li&gt;2) Living&#xA;space for astronauts.&lt;/li&gt;&#xA;&lt;li&gt;3) Radial distance for an &quot;artificial gravity&quot;&#xA;centrifuge.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Concerning #1 above, does the geometry of a parabola provide some easy way to &quot;throw out&quot; a foil to get the desired shape, or must it be painstakenly formed inch by inch? And would a thin metal foil work well as a radio telescope dish? Could even a useful mirror for optical telescope be unfolded? Russia has a 10 meter radio telescope in space, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Spektr-R&quot;&gt;Spektr-R&lt;/a&gt; interferometer. It must've been folded out in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On #2 &lt;a href=&quot;http://www.bigelowaerospace.com/genesis-2-tracking.php&quot;&gt;Bigelow Aerospace&lt;/a&gt; already have two (uncrewed) expandable space stations in LEO and will attach one to the ISS next year. How are they unfolded? They use the term &quot;expandable&quot; instead of &quot;inflatable&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On #3, rotation to create radial acceleration that simulates gravity, large distances are prefered in order to avoid unnatural shifts in weight as crew moves radially. At least a distance long enough that two meters, i.e. standing up, is a practically negligable fraction. I've seen a proposal to use &quot;air beams&quot; instead of wires. An &quot;air beam&quot; is like a hose inflated with air or other gasses so that it gets stiff. It would allow for (human) movements inside of it and doesn't get slack like a wire could. The only actual application of air beams I find today is for tents(!) Would it be a practical technology in space? &lt;a href=&quot;http://spaceshowclassroom.files.wordpress.com/2011/05/artificial-gravity-iac2010-presentation-revised-2011april30.pdf&quot;&gt;Here's &quot;some guy's&quot; ideas about air beams for artificial gravity.&lt;/a&gt; Slide 10 has an image that demonstrates the strength an air beam can have.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-05-05T21:55:19.457" Title="Folding structures in space - What are the potential benefits and problems?" Tags="|space-station|construction|artificial-gravity|space-telescope|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4441" PostTypeId="1" CreationDate="2014-05-04T13:05:49.680" Score="41" ViewCount="21191" Body="&lt;p&gt;Assume you want a satellite to constantly point its radio dish towards Earth while orbiting it, or its solar panels towards the Sun if it is instead orbiting it. Is any of the following true about what is needed to achieve this?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;continuous change in its orientation,&lt;/li&gt;&#xA;&lt;li&gt;once and for all giving it the right spin to begin with,&lt;/li&gt;&#xA;&lt;li&gt;it happens naturally.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The last point is true for a toy car on a sloping road curve, it keeps the same side facing the center of a circular track.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6816" LastEditDate="2016-07-13T22:18:24.687" LastActivityDate="2018-10-30T06:16:08.823" Title="Does a satellite naturally turn in phase with its orbit, always facing Earth?" Tags="|orbital-mechanics|artificial-satellite|attitude|rotation|" AnswerCount="6" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4442" PostTypeId="2" ParentId="4441" CreationDate="2014-05-04T13:22:21.353" Score="37" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is any of the following true about what is needed to achieve this: &lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;continuous change in its rotation, &lt;/li&gt;&#xA;  &lt;li&gt;once and for all giving it the right spin to begin with, &lt;/li&gt;&#xA;  &lt;li&gt;it happens naturally.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The answer is &quot;yes&quot; to all three questions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a vehicle is shaped right and is given the right rotation to start with, torques that naturally occur such as gravity gradient torque and torque from atmospheric drag from can help keep the vehicle rotating in the desired orientation. &lt;strong&gt;However, this is never perfect and there are always residual undesired torques.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vehicles need to have some kind of active attitude control system so they can keep themselves properly oriented. If that attitude control relies on fuel, the depletion of the fuel tanks marks the end of the vehicle's useful life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;Update: Approaches to attitude control&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Use thrusters.&lt;/strong&gt;&lt;br&gt;&#xA;The vehicle can only do this so often before it runs out of fuel. For most vehicles, that's the end of the mission. Approaches that reduce the need to use thrusters will extend the vehicle's useful life or enable a bigger payload. In some cases thee alternate approaches entirely eliminate the need for thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Take advantage of torques from the environment.&lt;/strong&gt;&lt;br&gt;&#xA;Vehicles from Landsat to the Space Station take advantage of rather than fight the external torques exerted on the vehicle by the environment. Environmental torques include gravity gradient torque, atmospheric torque, and magnetic torque. (There's also solar radiation pressure torque, but this is a tiny disturbance.) Some small vehicles in low Earth orbit equipped with magnetic torquers don't use any fuel. They remain functional until they reenter the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Take advantage of rotation.&lt;/strong&gt;&lt;br&gt;&#xA;A rotating object has angular momentum, which makes it harder to turn than if the object wasn't rotating. This adds stability to the vehicle (but also instabilities in some cases). Some of the earliest satellites were spin stabilized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next step up in complexity is to construct the vehicle so that it has comprises two parts that rotate about a common axis but at different speeds. Most communications satellites are dual spin satellites. The rotor (plastered with solar arrays) rotates rather quickly for stability while the communications platform rotates but once per day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another approach is to place the rotating parts inside the vehicle. These internal rotating devices include momentum wheels, reaction wheels, and control moment gyros. A momentum wheel, like the rotor in a communications satellite, is intended to rotate at a constant angular velocity. A motor with a simple controller is needed to bring the wheel up to speed and then keep it at that speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding the ability to change the commanded rotation rate to that momentum wheel controller turns the momentum wheel into a reaction wheel. With this ability, angular momentum can be transferred between the main body of the spacecraft to the reaction wheel. A vehicle with three reaction wheels, one per rotation axis, provides an active means of controlling vehicle rotation. Reaction wheels have a basic problem in that rotation speed must be between a minimum value (lest the stabilizing influence be lost) and a maximum value (lest the wheel lose structural integrity). A vehicle that uses reaction wheels needs some alternate control mechanism to help keep the vehicle stable while reaction wheels at their limits are brought back to the nominal rotation rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An alternative approach is a control moment gyro (CMG). These are essentially momentum wheels with another motor that pushes against the rotating wheel. (Think of the apocryphal stories of physicists who put airplane gyros in suitcases and then spun them up as a practical joke.) The amount of torque generated by CMGs per unit of power applied can be quite impressive. Just as reaction wheels have operational issues, so do CMGs. In the case of CMGs the problem is gimbal lock. Rotations about one or more axes eventually become uncontrollable. A vehicle that uses CMGs needs some alternate control mechanism to help keep the vehicle stable while CMGs are restored to their nominal rotation axes.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-05-06T14:07:37.313" LastActivityDate="2014-05-06T14:07:37.313" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="4443" PostTypeId="2" ParentId="4441" CreationDate="2014-05-04T13:22:29.403" Score="16" Body="&lt;p&gt;The best way to keep an antenna always pointed at Earth, if you can manage it, is to stick a large weight at the tip of your antenna. The weight will receive more pull, and naturally keep the antenna pointed at that direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short of having something like that to help passively, the next best solution is to spin stabilize. By spinning around an axis, you can guarantee that the axis always maintains it's direction, like spinning a top. Of course, there can be some wobble, which might become an issue, but this can be managed if worked carefully enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can't do one of those two, then you will most likely have an unstable system. Density fluctuations, turning to maintain solar power, solar wind and light pressure, thermal gradients, all can cause a very small perturbation. These will be magnified with time. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-05-04T13:22:29.403" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4444" PostTypeId="2" ParentId="4429" CreationDate="2014-05-04T14:42:11.567" Score="8" Body="&lt;p&gt;Knowledge and maintenance of a spacecraft's state (its position, velocity, attitude, and attitude rate) is a collaboration between ground controllers and the spacecraft itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Vehicle guidance, navigation, and control systems&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some vehicles keep track of both translational state (where they are) and rotational state (where they are pointing). Others just keep track of rotational state; determining translational state is done by people back on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The software that does this is a Kalman filter. A Kalman filter maintains a notion of the spacecraft's state and the uncertainty in that state. The filter uses two basic operations to advance the state and uncertainty over time, propagate and update. The propagate step advances state and its uncertainty per a set of differential equations (e.g., F=ma). The update step refines the state per sensor readings that reflect on the state.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some sensors such as accelerometers provide information used in the propagate step rather than in the update step. The position error grows without bound when position is propagated from sensed accelerations only. Another name for this is &quot;dead reckoning&quot;. Measuring position is not easy. Solving the dead reckoning problem was the primary motivation for developing GPS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Others sensors such as star trackers provide information that can be used in the update step. This means that attitude is far easier to keep track of in space than is position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another thing that makes propagating position difficult is that accelerometers only sense due to non-gravitational forces. They cannot sense acceleration due to gravity. This means that a spacecraft inertial navigation system that does estimate position (not all do) needs to model gravitation in the flight software. This is not easy for a vehicle in orbit about a rocky body. The Earth, the Moon, and Mars have non-spherical gravitational fields. In addition to having a model of the non-spherical nature of the gravity field, the flight software must also have a model of the rotation of the body about which they are orbiting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While landers do have to know where they are to a high degree of precision, orbiting vehicles oftentimes don't. Many orbiters do not use their inertial navigation system to propagate position. Translational state is propagated only to the extent needed to perform the trajectory correction maneuvers commanded by the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the earliest missions to Mars, these correction burns were in the form of timed burns. The vehicle pointed itself in a commanded orientation and then fired its thrusters for a commanded duration with the burn starting at a commanded time. There was a big problem with the timed burn approach: They weren't very accurate. Having a five to ten percent variance in commanded versus actual $\Delta v$ was very common, even with a well-calibrated thruster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nowadays those maneuvers are instead commanded in terms of a commanded $\Delta v$ to be performed at a commanded burn start time. The vehicle uses it's accelerometers to measure the accumulated $\Delta v$ and stops the burn when this reaches the desired value. This is a much simpler task for the flight software than is a full-blown positional inertial navigation. The flight software doesn't need to know where the vehicle is, or even how fast it's going. It just needs to know how much its velocity has changed, as sensed by the accelerometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ground processing&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the cruise phase of the mission, the onboard system typically does not keep track of position. Estimating and maintaining the vehicle's translational state is the job of the people on the ground who remotely monitor and control the vehicle. Both NASA and India have deep space networks that communicate with their interplanetary vehicles. One of the jobs of these deep space networks is estimating vehicles' orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time lag between transmission of a signal to a vehicle and the reception of the vehicle's response gives a good estimate of the distance (range). The doppler shift in the received signal gives a good estimate of the rate at which the distance is changing (range rate).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might think that where those ground-based antennas have to point to communicate with the spacecraft would also give a clue as to where the spacecraft is. That's not the case, at least not with a single ground station. The spacecraft would be so far off course as to be unusable if the antenna had to be aimed in a measurably different direction than expected. The only useful measurements from a single ground station are range and range rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Angular measurements are possible during the short periods of time when the spacecraft is simultaneously visible to two ground stations. During those intervals, NASA uses very long baseline interferometry (VLBI) techniques to create what is equivalent to a single antenna that is thousands of kilometers across. Those VLBI measurements provide angular measurements in addition to the range / range rate measurements provided by a single antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point of taking these measurements is that the vehicle is never in exactly the orbit it should be in. The spacecraft's orbit can be re-estimated after collecting a number of these ground-based measurements of the spacecraft's state. The process is somewhat similar to the Kalman filter used in the spacecraft's flight software. A Kalman filter processes measurements once and then discards them. The batch least squares filters used by ground controllers process all of the data at once, and then can reuse them to even further refine the estimate of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the spacecraft is never in exactly the right orbit, ground controllers will occasionally command the spacecraft to perform the previously mentioned trajectory correction maneuvers to put the spacecraft back on course toward the target.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-05-04T14:42:11.567" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4445" PostTypeId="2" ParentId="4440" CreationDate="2014-05-04T16:15:52.957" Score="8" Body="&lt;p&gt;This is done by necessity, all the time, for solar panels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/imFIZ.jpg&quot; alt=&quot;ISS panels&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also &lt;a href=&quot;http://www.kiss.caltech.edu/workshops/apertures2008/talks/santiagoprowald.pdf&quot; rel=&quot;noreferrer&quot;&gt;large deployable antennas&lt;/a&gt; used for communication satellites &lt;a href=&quot;http://www.northropgrumman.com/BusinessVentures/AstroAerospace/Products/Pages/AstroMesh.aspx&quot; rel=&quot;noreferrer&quot;&gt;and soon for&lt;/a&gt; the &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/smap&quot; rel=&quot;noreferrer&quot;&gt;Soil Moisture Active Passive mission&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZVx3Y.jpg&quot; alt=&quot;SMAP antenna deployed on ground&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA plans to &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/news/beam_feature.html&quot; rel=&quot;noreferrer&quot;&gt;test an inflatable habitat on ISS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vojqu.jpg&quot; alt=&quot;Bigelow inflatable ISS module&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The separation of rotating structures for artificial gravity is likely more efficient using a tether than a deployed rigid structure.  You don't need crew quarters on both ends.  There's plenty of other non-crewed mass to put at the other end.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-05-04T16:22:09.580" LastActivityDate="2014-05-04T16:22:09.580" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4446" PostTypeId="1" AcceptedAnswerId="4447" CreationDate="2014-05-05T02:00:32.893" Score="17" ViewCount="4729" Body="&lt;p&gt;Almost on every mission the astronauts did some digging, but how deep did they reach? According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Regolith#Moon&quot;&gt;this article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The regolith is generally from 4 to 5 metres thick in &lt;em&gt;mare&lt;/em&gt; areas and&#xA;  from 10 to 15 metres in the older highland regions.[3] Below this true&#xA;  regolith is a region of blocky and fractured bedrock created by larger&#xA;  impacts, which is often referred to as the &quot;megaregolith&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So did they ever see what was beneath the regolith?&lt;/p&gt;&#xA;" OwnerUserId="1263" LastEditorUserId="168" LastEditDate="2014-05-05T07:58:46.767" LastActivityDate="2022-07-13T20:41:14.990" Title="The maximum depth Apollo astronauts dug into lunar regolith?" Tags="|the-moon|apollo-program|regolith|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4447" PostTypeId="2" ParentId="4446" CreationDate="2014-05-05T02:23:26.587" Score="22" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;So did they ever see what was beneath the regolith?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No. The Apollo program sent a deep core drill (&quot;deep&quot;: 3 meters) on Apollo 15, 16, and 17. The deepest sample was 292 centimeters during Apollo 17.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://curator.jsc.nasa.gov/lunar/lsc/A17drill.pdf&quot;&gt;Apollo 17 Samples 70001 to 70006 Deep Drill Core&lt;/a&gt; for details on the sample.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-05-05T02:23:26.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4448" PostTypeId="1" AcceptedAnswerId="4457" CreationDate="2014-05-05T10:56:10.723" Score="3" ViewCount="2008" Body="&lt;p&gt;If there was a satellite that had a periapsis at 250km, approximately how much delta-v would it take during each orbit to offset the atmospheric drag? Would it be different depending on apoapsis?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea is for a satellite to go through and pick up debris and then release it at periapsis &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting it at 250km ensures that any debris will be removed and if the satellite brakes apart its debris will be removed as well.&lt;/p&gt;&#xA;" OwnerUserId="3277" LastEditorUserId="3277" LastEditDate="2014-05-05T21:51:49.113" LastActivityDate="2014-05-06T07:06:17.570" Title="Delta-v from atmospheric drag" Tags="|low-earth-orbit|atmospheric-drag|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4449" PostTypeId="1" CreationDate="2014-05-05T11:08:50.947" Score="8" ViewCount="606" Body="&lt;p&gt;I'm seeking a software which helps in the visualization of solar sail trajectories.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I'm not mistaken, the European Space Agency uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Celestia&quot; rel=&quot;nofollow&quot;&gt;Celestia&lt;/a&gt; to render trajectories for the general public.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that Celestia allows one to define Keplerian orbits. Is it also possible to define non-Keplerian orbits and let the software compute and display the trajectory? My current idea is to use Matlab in order to generate a list of points of the orbit, and plot those in Celestia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a software which helps generate and visualize arbitrary non-Keplerian orbits? Specifically, I would like to compute several Earth Mars trajectories and make figures out of them to include them in a report.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My objective is to play around with several interplanetary cycling orbits which could be used by solar sails. Is there any software you would recommend for that? I do not mind using several different programs, e.g. one for generating a discrete list of points of an orbit, and another one for visualization.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="3185" LastEditDate="2016-02-16T16:24:01.327" LastActivityDate="2016-06-13T01:54:08.563" Title="Visualization of non-Keplerian orbits for solar sails" Tags="|orbit|trajectory|software|solar-sail|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4450" PostTypeId="2" ParentId="4441" CreationDate="2014-05-05T17:31:54.860" Score="-3" Body="&lt;p&gt;Picture this:  Take a toy airplane and tie a string to one wing.  Now spin in place and let the plane fly at the end of the string.  You are the Earth and the plane is a satellite.  Does the plane really &quot;rotate&quot;?  Or is it flying straight all the time but its course is being changed because of the string?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its the exact same thing with a satellite only the string is gravity.  In reality the satellite is flying straight because it was launched forward, and it is constantly falling towards the Earth, but its forward speed exactly offsets the pull of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So don't think of it so much as turning as it flying forward with continuous automatic course changes into a circular path.&lt;/p&gt;&#xA;" OwnerUserId="3281" LastActivityDate="2014-05-05T17:31:54.860" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4451" PostTypeId="2" ParentId="4441" CreationDate="2014-05-05T18:22:04.143" Score="16" Body="&lt;h1&gt;Keeping the Same Face &amp;quot;Down&amp;quot;&lt;/h1&gt;&#xA;&lt;p&gt;There's a term for this when it naturally occurs: Tidelocking.&lt;/p&gt;&#xA;&lt;h1&gt;Natural orientation&lt;/h1&gt;&#xA;&lt;p&gt;One can make use of tidal stress to keep an orientation naturally.&lt;/p&gt;&#xA;&lt;p&gt;When an object of significant length is placed into orbit, the side closer to the center of gravity receives somewhat more &amp;quot;pull&amp;quot; than the far end, and it rotates around its own center of mass. This eventually damps rotation to match the orbital duration. This can, however, take years to accomplish.&lt;/p&gt;&#xA;&lt;p&gt;It also can take a lot of material, and has other effects.  It's a tiny force, but it's constant and profound. It's fractions of a centimeter per second per second at geosynchronous orbit. Just enough to have a stable effect.&lt;/p&gt;&#xA;&lt;p&gt;Short bodies, and especially ones that are round, blob-like, or blocky, will eventually tide-lock as well,but much more slowly.&lt;/p&gt;&#xA;&lt;p&gt;Further, even large objects have orbital decay issues. Orbital decay comes from several sources: atmospheric drag, solar wind drag, solar wind force, and tidal stresses. Atmospheric drag at most low-earth orbits results in falling before tidal force matters much.  Solar wind drag is similar, but several orders less. Solar wind acceleration is always &amp;quot;attempting&amp;quot; to force the periapsis to be on the sunward side, but is a tiny force. Tidal stresses attempt to drag the orbit to the same duration as the rotation of the body orbited.&lt;/p&gt;&#xA;&lt;p&gt;Most objects people are considering are too small to self-orient naturally before decay.&lt;/p&gt;&#xA;&lt;h1&gt;Unnatural orientation&lt;/h1&gt;&#xA;&lt;p&gt;If one places an object in orbit, and sets its rotation length to the same as its orbit length, then one has essentially replicated the effects of tidelocking... as long as the long axis is also down.&lt;/p&gt;&#xA;&lt;p&gt;Keep in mind that the object rotates on its center of mass. The center of gravitic force, however, may not be on the center of mass, and so tidal stress will slowly alter the orientation of the object. In earth orbit, this is complicated by the tidal stress of the moon, as well. Mind you, the moon's tidal stress is very tiny - nanometers per second per second - dwarfed by the millimeters per second per second of the earth, but sufficient to induce orbital deformations.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-05-05T18:22:04.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4452" PostTypeId="2" ParentId="4440" CreationDate="2014-05-05T21:55:19.457" Score="6" Body="&lt;p&gt;There have been folding structures used in space since the early days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many spacecraft have folded their solar arrays for launch; many further have used rolled up arrays on folding arms, so as to minimize launch volume (and thus launch fairing mass).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A folding parabolic antenna was used on a few missions; the most notable is notable for its failure to open: Galileo. The actual cause of the failure isn't truly &lt;em&gt;known&lt;/em&gt;, but is thought to be vacuum welding. A prabola can be created by panel shapes, and then held open with springing; it's not quite as smooth as a casting, but is still doable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Large multi-segment telescopes are used on planet; they are a practical way to get a large (≥5m) telescope into orbit. Any such telescope would likely also have additional components deployed to shadow and to protect the objective mirror.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Folding arms and even wheel-carriages have been used on rovers, including the manned rover for the Apollo landings. (&lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/000731.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.collectspace.com/ubb/Forum29/HTML/000731.html&lt;/a&gt; shows the stowed rover being loaded into the LM. &lt;a href=&quot;http://www.armaghplanet.com/blog/nasas-lunar-rover-everything-you-need-to-know.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.armaghplanet.com/blog/nasas-lunar-rover-everything-you-need-to-know.html&lt;/a&gt; shows part of the deployment process in one of the included videos.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary considerations for habitat space include internal pressure; a folded unit needs very little structure when it can be held in deployed mode by its own internal pressure. Several current designs (as yet unflown) use a central structural core, with an inflated habitat surrounding it. The Central core includes the docking port for mating to existing craft/structures, as well as the power and data connections. Bigelow  Aerospace is involved in the design of the ones to be tested in 2015. (&lt;a href=&quot;http://www.nasa.gov/exploration/technology/deep_space_habitat/xhab/&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/exploration/technology/deep_space_habitat/xhab/&lt;/a&gt;, &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/news/beam_feature.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/mission_pages/station/news/beam_feature.html&lt;/a&gt;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key benefits of all the expandables are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;reduced launch dimensions, which reduce fairing dimensions and thus fairing mass&lt;/li&gt;&#xA;&lt;li&gt;reduced launch mass due to less need to support stress during launch.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The key drawbacks include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mechanical failure of the extension motors&lt;/li&gt;&#xA;&lt;li&gt;mechanical failure of the attachments during launch (eg: Skylab)&lt;/li&gt;&#xA;&lt;li&gt;corrosion or vacuum welding rendering it fixed into the stowed position (eg: Galileo)&lt;/li&gt;&#xA;&lt;li&gt;failure to deploy in correct sequence (eg: most parachute failures; major concern on the Apollo Lunar Rover)&lt;/li&gt;&#xA;&lt;li&gt;packaging failing to release (common in halo equipment airdrops; a major concern with the Pathfinder, Spirit, and Opportunity rovers)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some additional drawbacks noted for inflatables:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;lack of hard shell for impact resistance&lt;/li&gt;&#xA;&lt;li&gt;lack of metal shell for radiation reduction.&lt;/li&gt;&#xA;&lt;li&gt;potential for excessive force upon inflation to damage the module&lt;/li&gt;&#xA;&lt;li&gt;potential for damage in packing (a concern for the Pathfinder mission)&lt;/li&gt;&#xA;&lt;li&gt;potential for damage in removal of packaging&lt;/li&gt;&#xA;&lt;li&gt;potential for damage if deployed near sharp objects (also a concern mentioned in the Pathfinder data, albeit more as a failure post-deployment).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some of those have potential solutions - a concrete &amp;amp; fabric inflatable has been developed that is airtight once deployed - &lt;a href=&quot;http://www.youtube.com/watch?v=vv3SII568v0&quot; rel=&quot;noreferrer&quot;&gt;http://www.youtube.com/watch?v=vv3SII568v0&lt;/a&gt; is a brag video by the manufacturer. Likewise, foaming polymer compounds could be used to create rigid walls.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-05-05T21:55:19.457" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4456" PostTypeId="2" ParentId="4438" CreationDate="2014-05-06T06:45:13.580" Score="0" Body="&lt;p&gt;Mars One has said multiple times that they have no intent of building their own launch hardware; as such, there's no way they could be &quot;competitors&quot; to SpaceX... at least, not directly.&lt;/p&gt;&#xA;" OwnerUserId="3291" LastActivityDate="2014-05-06T06:45:13.580" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4457" PostTypeId="2" ParentId="4448" CreationDate="2014-05-06T07:06:17.570" Score="4" Body="&lt;p&gt;The retrograde delta-v that the spacecraft will receive from atmospheric drag depends on two things: the mass of the spacecraft and the force of drag. (And the drag force will be integrated over the all of the time that the spacecraft is in the atmosphere.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drag force, in turn, depends on four things:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The velocity of the spacecraft&lt;/li&gt;&#xA;&lt;li&gt;The cross-sectional area of the spacecraft (i.e., how much of the spacecraft the air is &quot;dragging against&quot;)&lt;/li&gt;&#xA;&lt;li&gt;The density of the air&lt;/li&gt;&#xA;&lt;li&gt;The drag coefficient of the spacecraft&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Spacecraft velocity will depend entirely on orbital characteristics, and air density will depend on altitude as well as time of day and climate/Sun conditions. And, in turn, the drag coefficient will add additional dependencies on the object's shape and the viscosity of the air.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would it be different depending on apoapsis?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As you go through the atmosphere, many of the properties above will change, and based on the orbit, your velocity at different parts of the orbit, as well as what parts of the atmosphere you're passing through, will change as well. With a fixed perigee, the position of your apogee will determine the overall shape of the orbit, which will change all of these factors. So yes, the acceleration due to drag that the spacecraft experiences will vary drastically depending on the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, you'll need to specify more characteristics of the orbit in order for your question to be answered properly. However, it's possible to approximate an answer to the question if you assume a circular orbit (i.e., apogee distance = perigee distance) and make lots of assumptions about the shape of your craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we go by the tables at the end of &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1881883108&quot; rel=&quot;nofollow&quot;&gt;Space Mission Analysis and Design&lt;/a&gt;, if you have a spacecraft that has 50kg of mass per square meter of cross-sectional area, and you're in a 250km circular orbit, then you must impart a delta-v of between 636 m/s and 2,002 m/s each year (depending on solar activity) in order to maintain altitude; i.e., you can lose that much delta-v during the course of one year. If your spacecraft is more massive, then it will lose velocity more slowly (as it has greater momentum).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I agree with the other comments, however, that your approach seems to be somewhat flawed, as you'll need to impart a delta-v to the debris lest it simply continue to follow the same orbit you're in. (It will eventually deorbit on its own, sure, but very slowly.)&lt;/p&gt;&#xA;" OwnerUserId="3291" LastActivityDate="2014-05-06T07:06:17.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4458" PostTypeId="1" AcceptedAnswerId="4459" CreationDate="2014-05-06T13:30:33.967" Score="9" ViewCount="508" Body="&lt;p&gt;Although many people have been disheartened by the lack of large scale space exploration endeavours since the Apollo program, I was wondering if there has been an appreciable decrease in the cost-per-kg to space since then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone have data regarding cost-per-kg to space and how this value has changed through the decades?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="49" LastEditDate="2014-05-06T14:37:41.203" LastActivityDate="2014-05-06T14:37:41.203" Title="Is access to space becoming more affordable?" Tags="|cost|cost-to-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4459" PostTypeId="2" ParentId="4458" CreationDate="2014-05-06T13:48:10.053" Score="6" Body="&lt;p&gt;Numbers for cost of kilos of cargo to orbit are so hard to get it is likely impossible to answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, what is the cost of a Shuttle launch?  Well, what do you include? Numbers have ranged between \$500 million to \$2 billion.  That is a fairly large range of numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any time a national program is involved, there are many hidden costs that are very hard to attribute properly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian program is a great example where it is not clear they need to actually make money and cover costs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very few commercial launchers have real price lists, there is always a deal, or some detail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ULA example is interesting.  While the US military just did a bulk buy of 36 cores for a theoretically fixed cost, it should be easy, right?  But what about the ongoing billion dollars a year they get as ongoing funds for the program?  Does that count into the cost? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the cost of the range get added?  Air Force runs the range in Kennedy/Canaveral, should their costs be included in costs? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even SpaceX which is theoretically posting prices, \$56 million for 28,990lbs to orbit, suggests \$1931/lb, or \$4,248.2/kilo which is pretty low.  But then consider the contract with NASA for cargo to the ISS at $1.6 billion for 20,000 kilos of up mass. Which comes out to closer to \$80,000/kilo which seems ridiculous in comparison to the raw cost.  But of course some of that money was designed to spur development and let the next contract be more affordable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course you need to normalize orbits for payload capacity to get a comparable cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are overall trends.  Government run, national prestige systems (Space Shuttle, JAXA H-2A/B, ESA Vega) do not really need to be cost effective.  They exist for political reasons, darn the costs.  These have no incentive to get cheaper, so have not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians were desperate for foreign currency, and were willing to sell anything at any price they could manage.  China took a similar approach initially.  Whether they make or lose money on these deals is very opaque. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best news is that Antares from Orbital, and Falcon 9 from SpaceX have a goal of actually making money so they are incentivized to actually reduce costs and run an actual business.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time will tell, but the new entrants give us hope. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-05-06T14:13:38.773" LastActivityDate="2014-05-06T14:13:38.773" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="4460" PostTypeId="1" AcceptedAnswerId="4462" CreationDate="2014-05-06T14:40:14.127" Score="5" ViewCount="238" Body="&lt;p&gt;I'm curious what treaties, if any, are in place as far as restricting military, engineering, or scientific applications on the lunar surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;e.g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming someone definitively discovers there's Helium-3 in worthwhile quantities on the moon, is it first-come, first-serve? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can any nation place a permanent structure on the lunar surface?  Can they engage in terraforming activities?&lt;/p&gt;&#xA;" OwnerUserId="3296" LastEditorUserId="49" LastEditDate="2014-05-06T15:21:39.187" LastActivityDate="2014-05-06T15:21:39.187" Title="What lunar legalities are currently in place?" Tags="|the-moon|law|mining|terraforming|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4461" PostTypeId="2" ParentId="4449" CreationDate="2014-05-06T14:45:17.967" Score="1" Body="&lt;p&gt;If the plan is to keep the solar sail perpendicular to the photon flus, and simply furl and unfurl it (either partially or fully), then I think the problem is relatively simple: the orbits remain Keplerian.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The photons will exert a purely radial outward force, that varies with the photon intensity;  in other words, it varies inversely with the square of the distance from the sun.  It reduces the &lt;strong&gt;effective&lt;/strong&gt; force of gravity without changing its behavior as a function of distance.  The &quot;sailing ship&quot; would still be moving under a reduced net centripetal inverse square force.  It could be mathematically modelled by simply reducing the mass of the Sun in Newton's Law of Universal Gravitation &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example. consider a ship in a circular orbit around the Sun at Earth's distance.  Open the sail partially, and suddenly the ship's velocity is too high for its circular orbit under the reduced net centripetal force; instead it finds itself in an elliptical orbit, at perihelion.  Keep the sail open at the correct fixed size, and the ship could follow that elliptical orbit until it reaches say, Mars' orbit, at aphelion.  Open the sail further just the right amount, and the orbit becomes circular, but at a different velocity than Mars has...&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2014-05-06T14:45:17.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4462" PostTypeId="2" ParentId="4460" CreationDate="2014-05-06T15:12:53.847" Score="6" Body="&lt;p&gt;This is tough to answer because it's of course politically convoluted. Currently, we have the &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot;&gt;Outer Space Treaty&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon_Treaty&quot;&gt;Moon Treaty&lt;/a&gt;, but while OST seems more formal, the Moon Treaty is really more of a gentlemen's agreements and not ratified by any major space-faring nation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in reality, there is currently no globally agreed on legislation regarding exploitation of lunar resources. &lt;a href=&quot;http://www.oosa.unvienna.org/&quot;&gt;United Nations Office for Outer Space Affairs&lt;/a&gt; (UNOOSA) seems a fair bit more active in the last few years and there's all kinds of other public forums and organisations dealing with this subject, all trying to eventually draft proposals that would be acceptable to the international community and ratified before we really start industrializing the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for the time being and according to OST, as long as you're using the Moon for peaceful purposes, don't place weapons of mass destruction on it, and don't interfere with peaceful exploration of it by other nations, there is no international legislation standing in your way to do so. But since other nations might have different views than yours, make sure you establish a dialog regarding your intentions first, because you'll meet with opposition here on Earth, if they for some reason don't like what you're doing up there on the Moon.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-05-06T15:12:53.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4464" PostTypeId="1" CreationDate="2014-05-06T18:22:01.257" Score="6" ViewCount="491" Body="&lt;p&gt;Are there any private space probes presently active? &#xA;I'm asking about instruments which have left / are leaving the Earth-Moon system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know there is a plan for a telescope in one of the Earth - Sun Lagrange points, but I believe it's a government project. &lt;/p&gt;&#xA;" OwnerUserId="3058" LastEditorUserId="6537" LastEditDate="2015-04-23T02:28:48.633" LastActivityDate="2015-04-23T02:28:48.633" Title="Are there any private space probes presently active?" Tags="|probe|commercial|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4465" PostTypeId="1" CreationDate="2014-05-06T18:57:56.877" Score="0" ViewCount="324" Body="&lt;p&gt;command: &quot;get satellite in correct position&quot;. Intension: satellite adjusts to a certain angle and move through space. Is the command sufficient?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Position := which{t,s,d,x,y,z}?&lt;/p&gt;&#xA;" OwnerUserId="3298" LastActivityDate="2016-02-05T07:14:22.147" Title="Which coordinates does &quot;Satellite is in correct position&quot; imply? time and direction or only x,y,z?" Tags="|artificial-satellite|terminology|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4466" PostTypeId="1" AcceptedAnswerId="4467" CreationDate="2014-05-06T22:31:05.420" Score="13" ViewCount="6989" Body="&lt;p&gt;SpaceX's attempt at a reusable first stage rocket sounds amazing, but how they're going to get it to land back on the launchpad is beyond me. Assuming that the first stage will separate with a speed of about 3 km/s at a downrange distance of around 150 km, they'll essentially need to provide more than 3 km/s retrograde delta-v and somehow cover the 150 km back to the pad. Or am I being stupid and missing something obvious? I would have thought that a mobile water-based pad would be ideal, as it could save a ton of the required delta-v, but I haven't heard anything about such plans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, does anyone know how they're going to do it? Does the Falcon 9 v1.1 really have enough fuel to provide enough delta-v to get back to the pad and slow down sufficiently before doing a controlled landing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I'm trying to create a rough simulation using some differential equations, but I cannot find a reliable enough source for the downrange distance, altitude and velocity (i.e. x and y-components or at least speed and flight path angle above horizon) of the first stage at MECO and separation. I know these values will be different depending on the mission, but are there any stats for, say, a resupply mission to the ISS?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="552" LastEditDate="2016-01-06T02:38:42.200" LastActivityDate="2016-04-12T20:36:29.367" Title="How is SpaceX going to land their Falcon first stage?" Tags="|spacex|reuse|falcon-9|booster-flyback|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4467" PostTypeId="2" ParentId="4466" CreationDate="2014-05-06T22:35:15.273" Score="13" Body="&lt;p&gt;They have said that they will reserve around 15% of the fuel capacity of a first stage for reusability operations. At the point they need to impart the Delta-V to return to base, they will thus be 85% empty. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the need for only 3 engines instead of the 9 main engines for the retro propulsion burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I won't try to do the math, but they claim they have, and think it will work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was some discussion I did not entirely follow about whether 2 or 3 burns are required to return.  They need to cancel the velocity in the forward direction, and start heading back.  Then need to control entry back into the atmosphere in a survivable fashion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember they are lofted upwards and forwards when MECO-1 and stage separation occur.  They can ride the upward component and focus on the forward vector to cancel it out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Falcon Heavy it is trickier, since the middle core stage burns till much later and is quite a bit higher and faster at it's MECO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some have speculated that launching out of Texas, and recovering in Florida might buy some margin on reusability operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the CRS-5 flight (Dec 2014 scheduled date) Musk has said they will use a barge being built in Louisiana (50mX70m) to try to land on at first, until they can prove to the FAA they can control the landing accurately enough. Musk predicts only 50% chance of success on first attempt. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the CRS-8 flight, Apr 8, 2016, they successfully landed a first stage on the ASDS barge &quot;Of Course I Still Love You&quot;. They made several attempts in which they literally hit the barge, demonstrating the ability to get back to the barge, and finally the ability to land.  Elon Musk in the press conference after the launch suggested that fully 1/3 of their future flights would use the barge for landing. Usually heavy GEO missions that needed the extra performance.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-04-12T20:36:29.367" LastActivityDate="2016-04-12T20:36:29.367" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="4468" PostTypeId="1" AcceptedAnswerId="4470" CreationDate="2014-05-07T01:22:35.507" Score="8" ViewCount="329" Body="&lt;p&gt;I looked over &lt;a href=&quot;http://curator.jsc.nasa.gov/lunar/lsc/A17drill.pdf&quot;&gt;this NASA report&lt;/a&gt; and it seems to me they didn't analyze for traces of organics any of the deep core samples. Is my assumption correct and if yes why wasn't this test considered?&lt;/p&gt;&#xA;" OwnerUserId="1263" LastActivityDate="2014-05-07T02:17:24.637" Title="Any regolith lunar samples tested for organics?" Tags="|the-moon|nasa|apollo-program|regolith|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4469" PostTypeId="2" ParentId="4466" CreationDate="2014-05-07T01:45:49.093" Score="7" Body="&lt;p&gt;Based upon the statement of 15% of fuel being reserved, and three engines used for the landing phase... and doing a simple &quot;back of the envelope&quot; kind of calculation...&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;**Stock Data**&#xA;  28.0 T   Falcon 9 v1.1 dry mass (est)&#xA;   5.0 T   this author's wing-it for the landing legs on the v1.2&#xA; 411.0 T   Fuel Mass (est)&#xA; 598.7 TT  Full thrust (at MSL) 9 engine cluster&#xA;  66.5 TT  Thrust per engine&#xA; 170.0 sec Burn Duration&#xA;1530.0 e•s engine-seconds burn duration&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;This gives us enough to work from.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;  28.0 T   Falcon 9 v1.1 dry mass (est)&#xA;   5.0 T   this author's wing-it for the landing legs on the v1.2&#xA;  61.6 T   Fuel Mass (est) at separation&#xA;  94.6 T   est. mass at separation&#xA; 199.5 TT  Thrust on 3 engines at full throttle.&#xA;  66.5 TT  Thrust per engine&#xA;  46.5 TT  Thrust per engine minimum (70%)&#xA;  76.5 sec Burn Duration 3 engines full throttle&#xA; 229.5 e•s engine-seconds burn duration&#xA; 327.9 e•s burn duration engine seconds at 70% throttle (minimum)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;It looks to me like they separate at about 144 seconds, fall for a bit, then begin powered flight on a single throttled back engine. Elon has said the goal is 40% throttle minimum, and so I would expect the Merlin 1E to go lower still - but the thing's still got ≥1G at landing. with the 1D @ 70%. At separation, it's got about 0.6G per engine. More than adequate to cancel forward velocity, and begin the landing sequence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is throttle-back, not adequate thrust!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacelaunchreport.com/falcon9.html#components&quot;&gt;http://www.spacelaunchreport.com/falcon9.html#components&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacex.com/falcon9&quot;&gt;http://www.spacex.com/falcon9&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2012/06/spacex-merlin-1d-orbital-fire-aj-26-engine/&quot;&gt;http://www.nasaspaceflight.com/2012/06/spacex-merlin-1d-orbital-fire-aj-26-engine/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-05-07T01:45:49.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4470" PostTypeId="2" ParentId="4468" CreationDate="2014-05-07T02:17:24.637" Score="8" Body="&lt;p&gt;They did, although apparently not on that particular sample and for the analysis that you link to, but the low carbon content was discouraging. No carbon, no organic compounds. Later studies of Apollo Lunar samples have provided some interesting, though perhaps expected results. It seems most of Lunar regolith organics that were found could have been &lt;em&gt;imported&lt;/em&gt; to it via accretion of zodiacal (interplanetary) dust particles and meteor impacts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.hou.usra.edu/meetings/metsoc2013/pdf/5105.pdf&quot;&gt;ALLOCHTHONOUS ADDITION OF METEORITIC ORGANICS TO THE LUNAR REGOLITH&lt;/a&gt;, Thomas-Keprta et al., 76th Annual Meteoritical Society Meeting, 2013 (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Introduction:&lt;/strong&gt; Papers presented at the first Lunar Science  Conference&#xA;  and those published in the subsequent Science Moon Issue &#xA;  reported the carbon (C) content of Apollo 11 soils, breccias, and&#xA;  igneous rocks as ranging from ~50 to 250  parts per million. These&#xA;  values are unexpectedly low given that multiple processes should have&#xA;  contributed to the lunar C inventory. We estimate the contribution of&#xA;  C from exogenous sources (e.g., cometary and asteroidal dust) alone&#xA;  is ~500 ppm, which is  ~4x greater than the reported average. So while&#xA;  the assessment of  indigenous organic matter in returned lunar samples&#xA;  was one of  the primary scientific goals of the Apollo program,&#xA;  extensive  analysis of Apollo samples yielded no evidence of any&#xA;  significant  indigenous organic species. We have identified for the&#xA;  first time  arguably indigenous complex organic matter on the surfaces&#xA;  of  the 74220 pyroclastic beads, collected on the rim of Shorty crater&#xA;  during the Apollo 17 mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'll let you read the rest of the paper from its source, but the analysis was done on same Apollo 17 samples as the ones in the paper that you're linking to in the question. I suspect there were many other tests for organics conducted on them, but this report should answer your question well enough. Apollo Lunar regolith samples were tested for presence of organics.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-05-07T02:17:24.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4471" PostTypeId="2" ParentId="122" CreationDate="2014-05-07T06:28:11.613" Score="15" Body="&lt;p&gt;It depends how suit is built. And in fact pretty similar thing happend to Joe Kittinger during his (&gt;50 years long standing before it was recently beaten by Baumgartner during Redbull Stratos flight) record jump flight from the stratosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kittinger lost pressurization in one of his gloves while still rising to altitude. He could abort the mission but he pressed on even as he lost feeling of his hand. The pressure outside was pretty close to vacuum (from PoV of human body everything above ~20km (or 12.5 miles) is like vacuum since boiling point of water falls below body temperature). After the jump he could not feel nor control his hand for a short period of time, the hand was considerably swollen, yet after a few hours it returned to mostly normal condition.&lt;/p&gt;&#xA;" OwnerUserId="3300" LastEditorUserId="15906" LastEditDate="2019-06-11T13:58:38.870" LastActivityDate="2019-06-11T13:58:38.870" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="4472" PostTypeId="2" ParentId="4464" CreationDate="2014-05-07T06:29:43.433" Score="3" Body="&lt;p&gt;The only project I'm aware of is the &lt;a href=&quot;http://www.amsat-dl.org/p5a/p5a-to-mars.pdf&quot; rel=&quot;nofollow&quot;&gt;Amsat P5a Mars&lt;/a&gt; (PDF) project for an amateur radio relay satellite, but that's still in the &lt;a href=&quot;http://www.ticket-to-mars.org/en_EN/mission.html&quot; rel=&quot;nofollow&quot;&gt;planning stage&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-05-07T06:29:43.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4473" PostTypeId="2" ParentId="864" CreationDate="2014-05-07T06:38:08.683" Score="7" Body="&lt;p&gt;In fact there was a puncture incident during one of the earlier Space Shuttle EVAs. Glove got punctured due to it's fabrication/assembly defect. The thing was that astronaut didn't even notice until after the EVA he found strange red spot on his hand. Hole was not big and his skin sealed it, so air leak was below any alarm levels. And astronaut was excited enough (it was his first EVA) that he didn't notice.&lt;/p&gt;&#xA;" OwnerUserId="3300" LastActivityDate="2014-05-07T06:38:08.683" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4474" PostTypeId="2" ParentId="4465" CreationDate="2014-05-07T07:39:54.580" Score="4" Body="&lt;p&gt;A satellite's position in its target (or current) orbit is characterized by seven variables:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Time (specifying one of the reference frames to account for relativistic effects).&lt;/li&gt;&#xA;&lt;li&gt;Position vector (cartesian or polar coordinates) in a known reference frame (Earth-centric fixed, or Earth-centric rotating, for instance).&lt;/li&gt;&#xA;&lt;li&gt;For the same reference frame, instantaneous velocity vector.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If you are responsible for launching it, you have to make sure the satellite can be controlled. To achieve this, the satellite's attitude and rotation rates should be within pre-determined limits. This gives you 6 more numbers to watch at separation and orbit insertion.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-05-07T07:39:54.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4475" PostTypeId="2" ParentId="4466" CreationDate="2014-05-07T14:07:10.243" Score="4" Body="&lt;p&gt;You're missing something obvious: Air friction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The terminal velocity, that is the speed, the rocket would have when air friction and gravity are in balance, is a small fraction of orbital velocities in a few kilometers altitude. So just by reentering the atmosphere, they kill almost all of their momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After reentry, they may also use aerodynamic forces to help get back to the launch pad, but I don't know if that will be a significant factor. In any case, while they need to speed the rocket up quickly during launch, they don't need (or want) a lot of speed to return to base.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-05-07T14:07:10.243" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4476" PostTypeId="1" AcceptedAnswerId="4478" CreationDate="2014-05-07T14:10:09.417" Score="8" ViewCount="1296" Body="&lt;p&gt;I am reading &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40318/1/07-0390.pdf&quot;&gt;this paper&lt;/a&gt; about cyclers between Saturn and some of it's moons. In the beginning is discusses the Earth-Mars cycler as &quot;suffering&quot; from the &quot;risky requirement to perform hyperbolic rendezvous.&quot; So, what is a hyperbolic rendezvous? Does it mean that the orbiting vehicle must enter a body's SOI at a high-speed hyperbolic trajectory? Is it the speed which makes it risky?&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2015-05-22T09:28:31.327" Title="What is a hyperbolic rendezvous?" Tags="|low-energy-transfer|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4477" PostTypeId="2" ParentId="4476" CreationDate="2014-05-07T14:46:05.287" Score="7" Body="&lt;p&gt;Coming from &lt;a href=&quot;https://engineering.purdue.edu/people/james.m.longuski.1/ConferencePapersPresentations/2006GuidanceStrategyforHyperbolicRendezvous.pdf&quot;&gt;one of the sources&lt;/a&gt;, a hyperbolic trajectory rendezvous is dangerous because if the rendezvous doesn't do as planned, the cycler taxi will escape the body's gravitational pull and venture out into space.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the paper:&#xA;&quot;Apollo missions included a similar risk when the lunar module docked with the command/service module in lunar orbit. If this rendezvous failed, two of the three &#xA;astronauts would not make it home.&quot;&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2014-05-07T14:46:05.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4478" PostTypeId="2" ParentId="4476" CreationDate="2014-05-07T18:24:36.883" Score="10" Body="&lt;p&gt;The sentence reads &quot;Earth-Mars cycler applications generally suffer from &lt;strong&gt;long repeat periods&lt;/strong&gt;, &lt;strong&gt;infrequent launch opportunities&lt;/strong&gt;, and the risky requirement to perform hyperbolic rendezvous&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bolded portions are important parts of that sentence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a rendezvous is botched during planetary fly by, there won't be another opportunity until next fly by. The taxi is now on a heliocentric orbit that won't fly by a cycler or planet for &lt;strong&gt;years&lt;/strong&gt;. Passengers aboard are as good as dead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturnian moon cyclers also fly by the moons at hyperbolic trajectories with regard to the moons. But trip times and launch opportunities are on the order of days or weeks instead of years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a perhaps an over-simplified pic of two possible Mars cyclers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/63zDd.png&quot; alt=&quot;Aldrin vs Visit 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see fly bys are infrequent for both VISIT 2 and Aldrin Cyclers, though Aldrin flies by both earth and Mars more often. But the Aldrin Cycler has much bigger &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vinfinity&lt;/a&gt; (also known as hyperbolic excess velocity). The Vinfinity vectors are colored red in the illustration above. You can see the Aldrin Cycler Vinfinity at Mars fly by is quite large, around 10 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Aldrin cycler also needs its line of apsides rotated about 50 degrees each synodic period. Buzz Aldrin hopes to accomplish that with earth's gravity instead of propellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://clowder.net/hop/railroad/HohmannGasGiantMoons.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here's&lt;/a&gt; a spreadsheet that shows synodic period and trip times between moons of Saturn as well as moons of Jupiter. If you play with it you can see the tempo for travel between gas giant moons is a lot quicker than travel between planets.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-05-07T18:35:15.703" LastActivityDate="2014-05-07T18:35:15.703" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4480" PostTypeId="2" ParentId="4464" CreationDate="2014-05-07T18:55:36.397" Score="3" Body="&lt;p&gt;Planetary Resources hopes to launch &lt;a href=&quot;http://www.planetaryresources.com/technology/&quot; rel=&quot;nofollow&quot;&gt;Arkyd prospector probes&lt;/a&gt;. The Arkyd 100 and 200 would be in the earth-moon neighborhood. But the Arkyd 300 would leave the earth-moon neighborhood and orbit an asteroid on a heliocentric orbit.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-05-07T18:55:36.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4481" PostTypeId="2" ParentId="4465" CreationDate="2014-05-07T18:59:23.273" Score="4" Body="&lt;p&gt;I can't tell what context you're referring to, but IMHO the word &quot;position&quot; refers only to the satellite's current location in space, specified for example by (x, y, z) coordinates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that interpretation, being in the correct &quot;position&quot; is useless if the satellite is moving in the wrong direction and/or at the wrong speed. If a communications satellite is directly above the desired spot on the equator, at exactly the right altitude for a geosynchronous orbit, but it happens to be plummeting toward the Earth at 1000 kilometers per second, then it's in the right &quot;position&quot; but it's going to have a very bad day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The word &quot;position&quot;, at least as it's usually used in English, does not include information about the velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there's a context in which the word &quot;position&quot; does include both location and velocity (which both need to be correct), then please update your question to clarify just what that context is. It's common, in technical fields, for English words to be used in a way that differs from their common English meanings.&lt;/p&gt;&#xA;" OwnerUserId="574" LastActivityDate="2014-05-07T18:59:23.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4482" PostTypeId="1" CreationDate="2014-05-08T04:04:39.813" Score="5" ViewCount="681" Body="&lt;p&gt;I'm working on a project that is dealing with calculating the orbital motion of satellites around some given body and one thing I was wanting to try and study was the path that a satellite would take to land on the body it was orbiting. I'm wondering if it's possible to use the concept of Hohmann transfers to calculate the trajectory of the satellite as it lands. Do you all think that this would be an acceptable way to approach this problem or is there a more efficient way to do this? Thanks for any help.&lt;/p&gt;&#xA;" OwnerUserId="3236" LastActivityDate="2015-01-10T19:33:21.363" Title="Landing Satellite via Hohmann Transfer" Tags="|artificial-satellite|low-earth-orbit|landing|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4483" PostTypeId="2" ParentId="4211" CreationDate="2014-05-08T05:12:59.207" Score="21" Body="&lt;p&gt;There are a number of software packages, many of them free, that deal with those two line elements. &lt;strong&gt;Use one of them.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those two line elements are not Keplerian elements. They are instead Brouwer-Lyddane mean orbital elements. Keplerian elements assume a spherical central body and no forces other than gravitation. The Brouwer-Lyddane mean orbital elements address the first six spherical harmonics and attempt to account for atmospheric drag. The mathematics of Keplerian elements is a bit messy. The mathematics of those two line elements is beyond messy. It's a &quot;math-out&quot;. (Think of a blizzard where all you see is whiteness. Blizzards are white-out conditions. The paper describing the two line elements is a math-out. All you see is mathematics.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mathematics is described in &lt;a href=&quot;http://adsabs.harvard.edu/full/1981CeMec..24..367H&quot;&gt;F.R. Hoots, &quot;Reformulation of the Brouwer geopotential theory for improved computational efficiency&quot;, &lt;em&gt;Celestial Mechanics&lt;/em&gt; 24 (1981)&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-05-08T05:12:59.207" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4484" PostTypeId="2" ParentId="4482" CreationDate="2014-05-08T05:28:54.180" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I'm wondering if it's possible to use the concept of Hohmann transfers to calculate the trajectory of the satellite as it lands. Do you all think that this would be an acceptable way to approach this problem or is there a more efficient way to do this?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The answer is &quot;no&quot;, and it has nothing to do with efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For simplicity, let's assume the target body is spherical. A Hohmann transfer to the surface of that body would result in an orbit just above the surface of the body. That orbital rate is inevitably going to differ markedly from the velocity at which a point on the surface of the target body is moving. You don't want to be orbiting the body centimeters above the surface. You want to be landing on the body, and that means making your velocity equal to the body's rotational velocity rather than that orbital velocity.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-05-08T05:28:54.180" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4485" PostTypeId="1" AcceptedAnswerId="4486" CreationDate="2014-05-08T13:24:42.657" Score="12" ViewCount="2743" Body="&lt;p&gt;Some NASA satellites such as &lt;a href=&quot;http://sohowww.nascom.nasa.gov/data/realtime/c3/512/&quot; rel=&quot;noreferrer&quot;&gt;LASCO&lt;/a&gt; transmit images which are viewable by the public in real time. Are there any high-resolution Earth-facing satellites that also are viewable by the public in real time? Either government-funded or private satellites would be of interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &lt;strong&gt;real-time&lt;/strong&gt; I mean a public feed of reasonably recent data in a format supported by common consumer equipment, available over a network accessible by common consumer equipment. The web interface to &lt;a href=&quot;http://sohowww.nascom.nasa.gov/data/realtime/c3/512/&quot; rel=&quot;noreferrer&quot;&gt;LASCO&lt;/a&gt; would be the canonical example of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &lt;strong&gt;high resolution&lt;/strong&gt; I mean the ability to discern objects that can be seen in their entirety by a human standing on the ground. Thus an image of the panhandle of Florida would not be high resolution, but the ability to resolve pad 39 A would be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit: The information reported here is exactly the type of satellite information that I am looking for:&lt;/strong&gt;&#xA;&lt;a href=&quot;http://time.com/3042640/satellite-russian-ukraine-shelling/&quot; rel=&quot;noreferrer&quot;&gt;http://time.com/3042640/satellite-russian-ukraine-shelling/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="472" LastEditDate="2014-07-28T12:50:22.807" LastActivityDate="2014-07-28T12:50:22.807" Title="Which high-resolution Earth-facing satellites transmit data to the public in realtime?" Tags="|earth-observation|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4486" PostTypeId="2" ParentId="4485" CreationDate="2014-05-08T13:42:15.097" Score="11" Body="&lt;p&gt;International Space Station (ISS) now hosts the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/917.html&quot; rel=&quot;noreferrer&quot;&gt;High Definition Earth Viewing (HDEV) experiment&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The High Definition Earth Viewing (HDEV) experiment places four&#xA;  commercially available HD cameras on the exterior of the space station&#xA;  and uses them to stream live video of Earth for viewing online.  The&#xA;  cameras are enclosed in a temperature specific housing and are exposed&#xA;  to the harsh radiation of space.  Analysis of the effect of space on&#xA;  the video quality, over the time HDEV is operational, may help&#xA;  engineers decide which cameras are the best types to use on future&#xA;  missions. High school students helped design some of the cameras'&#xA;  components, through the High Schools United with NASA to Create&#xA;  Hardware (HUNCH) program, and student teams operate the experiment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Live streams are available via the &lt;a href=&quot;http://www.ustream.tv/channel/iss-hdev-payload/theater&quot; rel=&quot;noreferrer&quot;&gt;HDEV experiment channel on USTREAM&lt;/a&gt; to everyone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another project is &lt;a href=&quot;http://www.urthecast.com/&quot; rel=&quot;noreferrer&quot;&gt;UrtheCast&lt;/a&gt; that is commercial in nature (currently requires registration) and also aims to stream live feeds from two high-definition cameras onboard the ISS in the near future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what exactly you mean with &quot;real-time&quot; though, since the &lt;a href=&quot;http://sohowww.nascom.nasa.gov/data/realtime-images.html&quot; rel=&quot;noreferrer&quot;&gt;SOHO&lt;/a&gt;'s (and also e.g. &lt;a href=&quot;http://sdo.gsfc.nasa.gov/data/&quot; rel=&quot;noreferrer&quot;&gt;SDO&lt;/a&gt;, &lt;a href=&quot;http://stereo.gsfc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;STEREO&lt;/a&gt;,...) images of the Sun aren't exactly real-time due to the time it takes to complete observations for each frame and transmit their data back to Earth. They are frequently updated though, did you mean any satellite imagery of the Earth that is publicly available as soon as it's available to their operators?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If latter, then yes, there are many Earth-observing satellites with their data streams publicly available, for example from &lt;a href=&quot;http://earthnow.usgs.gov/earthnow_app.html&quot; rel=&quot;noreferrer&quot;&gt;LANDSAT&lt;/a&gt;, &lt;a href=&quot;http://www.eumetsat.int/website/home/Images/RealTimeImages/index.html&quot; rel=&quot;noreferrer&quot;&gt;EUMETSAT&lt;/a&gt;, and so on. There are also websites that aggregate data from multiple sources and publish them as they become available, e.g. the University of Wisconsin-Madison's &lt;a href=&quot;https://www.ssec.wisc.edu/sose/real_time.html&quot; rel=&quot;noreferrer&quot;&gt;SOSE&lt;/a&gt; (Satellite Observations in Science Education) program.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-05-08T15:24:52.400" LastActivityDate="2014-05-08T15:24:52.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4487" PostTypeId="2" ParentId="4485" CreationDate="2014-05-08T15:01:53.707" Score="5" Body="&lt;p&gt;If you have the necessary antennae and ground segment, you can receive data from many satellites.  Many of those transmit in near-real-time.  For example, the &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/n/noaa-poes-series-5th-generation&quot; rel=&quot;noreferrer&quot;&gt;5th generation of POES satellites&lt;/a&gt; (polar orbiting weather satellites) provide direct readout to all users across the globe.  To use the data, you will need:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a directed antenna pointing at the satellite&lt;/li&gt;&#xA;&lt;li&gt;accurate knowledge of the location of the satellite&lt;/li&gt;&#xA;&lt;li&gt;a ground station with the necessary software to process the measurements&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Many national weather offices have this equipment.  I don't know if any amateurs do.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2014-05-08T15:01:53.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4488" PostTypeId="1" AcceptedAnswerId="4491" CreationDate="2014-05-08T15:08:29.523" Score="8" ViewCount="971" Body="&lt;p&gt;In the context of the Russian space system, I asked the question &lt;a href=&quot;https://space.stackexchange.com/questions/3652/what-is-different-from-the-r-7-that-launched-sputnik-to-the-soyuz-launcher-used&quot;&gt;What is different from the R-7 that launched Sputnik, to the Soyuz launcher used for Soyuz/Progress launches?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus we see that much of the current Soyuz booster has strong legacy ties back to the R-7 booster from the 1950's. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the US side, what is the oldest legacy launcher in service?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delta-4 uses a new engine and looses pretty much all of its Delta heritage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atlas-V uses almost nothing in common with the earlier Atlas versions.  (Unless you count the Atlas 3, which also loses much of its commonality with the earlier Atlases, but is a stepping stone to Atlas-V). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle would have been 30+ years old.  We might say SLS using Shuttle components would continue the Space Shuttle longer if we stretch the definition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To stretch the idea even further (to its own detriment) you could consider that the Merlin engine design is based on the Lunar Module Ascent engine design.  But that seems like a two level stretch, a bridge too far in this context.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Capsule design, which of course has Apollo heritage, since how could SpaceX/Boeing ignore the tons of research papers NASA produced, but are not legacy in the sense meant here. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-05-08T15:43:26.057" Title="What US Legacy launcher is oldest in service?" Tags="|launch|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4489" PostTypeId="2" ParentId="4488" CreationDate="2014-05-08T15:12:08.933" Score="6" Body="&lt;p&gt;The RL-10 engine, used in the upper stages of the Delta-4 and Atlas-V might count as a large heritage component. It dates back to 1962 for a first flight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see on the launch calendar that a Delta-2 is about to launch but that is a step away from the orginal Delta design as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Castor solid rocket engines have been in use since 1960's and the upper stage of an Antares is using one even today.  (Of course Antares is a hodge podge of technology.  Engines built in the 1970's and kept in storage till now. First stage structure from the people who build Zenits, Castor upper stages, etc).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-05-08T15:12:08.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4490" PostTypeId="1" CreationDate="2014-05-08T15:28:36.357" Score="2" ViewCount="72" Body="&lt;p&gt;What are the oldest US space launch vehicles, or vehicle stages still in use?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will definitely give you upvotes for answers suggesting any rocket or stage first launched in 1957-1965 and still in use today (actually launched after 2005). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If nothing is found in that period explain and provide younger candidates.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2014-05-08T15:28:36.357" Title="The oldest US space launch vehicles and stages still in use" Tags="|rockets|history|stages|" AnswerCount="0" CommentCount="4" ClosedDate="2014-05-08T15:41:16.533" ContentLicense="CC BY-SA 3.0" />
  <row Id="4491" PostTypeId="2" ParentId="4488" CreationDate="2014-05-08T15:43:26.057" Score="10" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Centaur_%28rocket_stage%29&quot;&gt;Centaur&lt;/a&gt; upper stage is the one that springs instantly to mind. Its first successful flight was in 1965, and it is still being used by ULA as an upper stage for its &lt;a href=&quot;http://en.wikipedia.org/wiki/Atlas_V&quot;&gt;Atlas V&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2014-05-08T15:43:26.057" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4492" PostTypeId="1" CreationDate="2014-05-08T22:11:11.453" Score="6" ViewCount="329" Body="&lt;p&gt;SpaceX has a practice of a hot fire test before the real launch.  This gives them a chance to test any number of things, similar to launch, to identify possible issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They test:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fueling&lt;/li&gt;&#xA;&lt;li&gt;Launch procedure&lt;/li&gt;&#xA;&lt;li&gt;Actually ignite the engines&lt;/li&gt;&#xA;&lt;li&gt;and more&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But what other boosters test this way? I do not recall Saturn 1B or V's doing this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anything with a Solid rocket booster (Space Shuttle, Araine 5, H-2A/H-2B, Titan 4, Atlas 5 with strapons, Delta 4 with strapons, Long March, Vega) obviously cannot do a hot fire, since you only get one burn out of a solid booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thinking about that list, I realized I could not think of a Russian solid rocket booster.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-05-08T22:43:11.960" Title="What boosters/launchers conduct hot fire tests before launch?" Tags="|launch|spacex|testing|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4493" PostTypeId="2" ParentId="3652" CreationDate="2014-05-08T22:19:45.320" Score="20" Body="&lt;p&gt;I've tried to write it down, but then I've changed it into the following scheme.&#xA;That way it is much cleaner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the scheme doesn't include the recent Soyuz-2.1v vehicle.&#xA;It seems like it is not &quot;based&quot; on R7 anymore. The conic boosters and core stage of R7 are no longer present.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FSSeT.png&quot; alt=&quot;R7 rocket family&quot;&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2014-05-08T22:19:45.320" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4494" PostTypeId="2" ParentId="4492" CreationDate="2014-05-08T22:43:11.960" Score="5" Body="&lt;p&gt;NK-33 engines can be started several times. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(It was one of the measures to counter problems the designers of N1 were struggling with. The large set of multiple start NK-33 has been produced in parallel with the very last N1 test flight. After that the program has been closed without giving multiple start NK-33 a chance).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That gives Antares and Soyuz-2.1v. &lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2014-05-08T22:43:11.960" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4495" PostTypeId="1" CreationDate="2014-05-09T01:44:30.457" Score="2" ViewCount="117" Body="&lt;p&gt;The Cassini mission has made several flybys of Saturn's second largest moon, Rhea.  Do we have any data about that moon's surface geochemistry?  I do know there is a thin Oxygen/Carbon Dioxide atmosphere and that the planet is 75/25 ice/rock.  &lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2014-05-09T01:44:30.457" Title="Surface material on Saturn's moon Rhea" Tags="|planetary-science|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4496" PostTypeId="1" AcceptedAnswerId="4498" CreationDate="2014-05-09T07:08:02.310" Score="22" ViewCount="3195" Body="&lt;p&gt;The first American moon launches were in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Ranger_program&quot;&gt;ranger program&lt;/a&gt;.  Ranger 3 and 5 both missed the moon in their 1962 attempts.  Where are they now? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="335" LastEditDate="2014-05-09T09:23:32.507" LastActivityDate="2014-05-11T06:53:55.347" Title="When you shoot for the moon and miss, where do you end up?" Tags="|orbital-mechanics|the-moon|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4497" PostTypeId="2" ParentId="4496" CreationDate="2014-05-09T08:51:14.363" Score="22" Body="&lt;p&gt;It depends on how fast you go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you exceed Earth escape velocity, you will leave Earth orbit and end up in an orbit around the Sun, similar to the orbit of Earth. That's what happened to Ranger 3 and 5.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you do not exceed Earth escape velocity, you will end up on a highly eccentric Earth orbit (very high apogee, very low perigee). When the craft does a direct-ascent (in contrast to entering a parking orbit around Earth first) the perigee will likely be below the atmosphere or even below the Earth's surface - in other words, the craft falls back to Earth and crashes (if it survives atmospheric reentry).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But keep in mind that the Moon's gravity acts on the craft during a fly-by. Depending on how closely the craft passes the Moon and on what side, the Moon's gravity will alter its trajectory. This can give the craft an additional gravity slingshot sending it onto an Earth escape trajectory, change a crashing orbit into a surviving Earth orbit or change a surviving orbit into a crashing orbit. But a surviving orbit will not be stable. In any case, the orbit will cross the Moon's orbit again, so at some points in the future it will make additional Moon fly-bys which will again alter its orbit. So at some point in the future the craft will crash either into the Moon or into the Earth or get sent onto an escape trajectory.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="3291" LastEditDate="2014-05-11T06:53:55.347" LastActivityDate="2014-05-11T06:53:55.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4498" PostTypeId="2" ParentId="4496" CreationDate="2014-05-09T12:11:16.503" Score="16" Body="&lt;p&gt;To add some numbers to Ranger 3 and 5 orbits, from &lt;a href=&quot;http://satlist.nl/RAE/RAE1962.doc&quot; rel=&quot;noreferrer&quot;&gt;RAE Table of Earth Satellites&lt;/a&gt; (DOC) for artificial objects launched in 1962 (Source: &lt;a href=&quot;http://www.satlist.nl/index.html&quot; rel=&quot;noreferrer&quot;&gt;SATLIST, Ruimtevaart Filatelie Club Nederland&lt;/a&gt;), last revised in 2001:&lt;/p&gt;&#xA;&lt;h3&gt;Ranger 3&lt;/h3&gt;&#xA;&lt;p&gt;Atlas 121 D - Agena B 6003 primary payload (Cospar ID: 1962 01 A, NORAD No. 00221), a hexagonal cylinder 2.51 m long, 1.52 m in diameter and 390 kg in weight (wet mass), passed 36,800 km ahead of the Moon on Jan. 28, 1962 23:45 UTC and is now in a 0.985 x 1.164 AU heliocentric orbit with orbital eccentricity of 0.083 and 0.40° inclination to the ecliptic. Its orbital period around the Sun is roughly 406 days.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Heliocentric orbit (Sun orbit):&#xA;----------------------------------------------&#xA;Perihelion x Aphelion       0.985 x 1.164  AU.&#xA;Eccentricity                0.083&#xA;Incl. to ecliptic           0,40°&#xA;Period (around the Sun)     406 days&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Ranger 3 Agena B stage (Cospar ID: 1962 01 B, NORAD No. 00222), a 6.7 m long and 1.52 m diameter cylinder with dry mass of roughly 770 kg is in unknown heliocentric orbit.&lt;/p&gt;&#xA;&lt;h3&gt;Ranger 5&lt;/h3&gt;&#xA;&lt;p&gt;Atlas 215 D - Agena B 6005 primary payload (Cospar ID 1962 55 A, NORAD No. 00439), a hexagonal cylinder 2.51 m long, 1.52 m in diameter and 342 kg in weight (wet mass), passed 735 km behind the Moon on Oct. 21, 1962 15:50 UTC and is now in a 0.951 x 1.064 AU heliocentric orbit with orbital eccentricity of 0.056 and 0.41° inclination to the ecliptic. Its orbital period around the Sun is roughly 369 days.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Heliocentric orbit (Sun orbit):&#xA;----------------------------------------------&#xA;Perihelion x Aphelion       0.951 x 1.064 A.U.&#xA;Eccentricity                0.056&#xA;Incl. to ecliptic           0.41°&#xA;Period (around the Sun)     369 days&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Ranger 5 Agena B stage (Cospar ID 1962 55 B, NORAD No. 00440) is in unknown heliocentric orbit.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-05-09T12:11:16.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4499" PostTypeId="1" AcceptedAnswerId="4502" CreationDate="2014-05-09T13:24:24.073" Score="8" ViewCount="1048" Body="&lt;p&gt;The Mars Reconnaissance Orbiter has taken direct images of the MSL Curiosity rover, its parachute, heat shield and sky crane. Could it image, or has it even already, the failed Mars Express lander with its Beagle 2 rover? If it can't, then for what reason? Would such an image be likely to give clues to why it failed?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2018-02-11T23:03:30.467" LastActivityDate="2018-02-11T23:03:30.467" Title="Could the crashed Mars Express lander be imaged by the MRO orbiter?" Tags="|mars|probe|imaging|lander|mars-reconnaissance-orbiter|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4500" PostTypeId="2" ParentId="4499" CreationDate="2014-05-09T14:08:35.557" Score="8" Body="&lt;p&gt;The reason why HiRISE (The camera on MRO that took the images mentioned) was able to take those pictures is that it knew exactly where the object landed. It has in fact attempted to find the Beagle 2 rover, which is a difficult task. There's an excellent article on the attempt on the &lt;a href=&quot;http://www.uahirise.org/ESP_024618_1920&quot;&gt;HiRISE website&lt;/a&gt;. Depending on exactly what happened, different things might be seen, letting one know exactly where in the EDL process the lander failed. However, finding it is quite difficult, without knowing where to look for it.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-05-09T14:08:35.557" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4501" PostTypeId="1" AcceptedAnswerId="4504" CreationDate="2014-05-09T14:35:36.950" Score="11" ViewCount="619" Body="&lt;p&gt;I know that many artificial satellites are tide-locked with Earth, which allows them to always face a directional antenna towards their ground station. But some vehicles, such as space telescopes, and orbiters observing other bodies are required to rotate to point at other objects in space. What are the common ways used by such satellites to maintain communication links with Earth?&lt;/p&gt;&#xA;" OwnerUserId="2807" LastActivityDate="2014-05-09T23:11:08.227" Title="How do non-Earth-facing vehicles communicate with Earth?" Tags="|communication|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4502" PostTypeId="2" ParentId="4499" CreationDate="2014-05-09T15:20:43.813" Score="9" Body="&lt;p&gt;Maybe. A search of University of Arizona's &lt;a href=&quot;http://www.uahirise.org/results.php?keyword=beagle&amp;amp;order=release_date&amp;amp;submit=Search&quot; rel=&quot;nofollow noreferrer&quot;&gt;HiRISE catalog&lt;/a&gt; reveals 22 images, most captioned 'Search for Beagle 2.' Page 2 of the search leads us to the following image: &lt;a href=&quot;http://www.uahirise.org/PSP_002347_1915_cut_b.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Portion of Beagle 2 Landing Ellipse in Isidis Planitia cut b&lt;/a&gt;:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/cP32w.jpg&quot; alt=&quot;color image&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Or maybe not: see David's comment below. From the &lt;a href=&quot;https://en.wikipedia.org/wiki/Beagle_2#Mission_progress&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article:&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On 20 December 2005, Professor Pillinger released specially processed images from the Mars Global Surveyor which suggested that Beagle 2 came down in a crater at the landing site on Isidis Planitia.&lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Mars_Express/Possible_evidence_found_for_Beagle_2_location&quot; rel=&quot;nofollow noreferrer&quot;&gt;(4)&lt;/a&gt; It was claimed that the blurry images show the primary impact site as a dark patch and, a short distance away, Beagle 2 surrounded by the deflated airbags and with its solar panels extended.&lt;a href=&quot;http://news.bbc.co.uk/2/hi/science/nature/4542174.stm&quot; rel=&quot;nofollow noreferrer&quot;&gt;(5)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Ua0m.png&quot; alt=&quot;annotated first image&quot; /&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;However, Mars Reconnaissance Orbiter's HiRISE camera subsequently observed the area, in February 2007, and revealed that the crater was empty.&lt;a href=&quot;http://hiroc.lpl.arizona.edu/images/PSP/diafotizo.php?ID=PSP_002347_1915&quot; rel=&quot;nofollow noreferrer&quot;&gt;(6)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;See also: &lt;a href=&quot;http://www.planetary.org/multimedia/space-images/charts/mars_landing_site_map_lakdawalla.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Map of all Mars landing sites, failed and successful&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; According to this January 16th &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=4446&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL article&lt;/a&gt;, MRO has finally located the Beagle2.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A set of three observations with the orbiter's High Resolution Imaging Science Experiment (HiRISE) camera shows Beagle 2 partially deployed on the surface of the planet, ending the mystery of what happened to the mission more than a decade ago. They show that the lander survived its Dec. 25, 2003, touchdown enough to at least partially deploy its solar arrays.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5uH2Q.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;br /&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/R4s3P.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Subsequent re-imaging and analysis by the Beagle 2 team, the HiRISE team and NASA's Jet Propulsion Laboratory, Pasadena, California, have confirmed that the targets discovered are of the correct size, shape, color and dispersion to be Beagle 2. JPL planetary geologist Tim Parker, who has assisted in the search and processed some of the images said, &amp;quot;I've been looking over the objects in the images carefully, and I'm convinced that these are Beagle 2 hardware.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Analysis of the images indicates what appears to be a partially deployed configuration, with what is thought to be the rear cover with its pilot/drogue chute (still attached) and main parachute close by. Due to the small size of Beagle 2 (less than 7 feet, or 2 meters across for the deployed lander) it is right at the limit of detection of HiRISE, the highest-resolution camera orbiting Mars. The targets are within the expected landing area at a distance of about three miles (five kilometers) from its center.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-01-16T15:02:59.380" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4503" PostTypeId="1" CreationDate="2014-05-09T19:18:38.607" Score="22" ViewCount="3497" Body="&lt;p&gt;I am doing a science project for school and i was wondering if anybody could help me out. I need to know how many and what kinds of animals have been on the iss.&lt;/p&gt;&#xA;" OwnerUserId="3317" LastEditorUserId="12102" LastEditDate="2021-07-23T05:33:04.157" LastActivityDate="2021-07-23T05:33:04.157" Title="How many and/or what kind of animals have ever been on the ISS?" Tags="|iss|experiment|animals|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4504" PostTypeId="2" ParentId="4501" CreationDate="2014-05-09T20:16:44.330" Score="16" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What are the common ways used by such satellites to maintain communication links with Earth?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In general, the strategy is to&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Have an antenna that can be pointed toward the Earth.&lt;/li&gt;&#xA;&lt;li&gt;Have multiple antennae that can be pointed toward the Earth as a backup plan in case something goes wrong.&lt;/li&gt;&#xA;&lt;li&gt;Have a backup backup plan in case something goes very, very wrong.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The need to send data to and receive commands from the Earth can strongly influence overall vehicle design. The mechanisms by which a vehicle will achieve these goals is made very early on in the planning and design of the vehicle and of the mission. While these are the overarching design goals, there is no one single answer as to how to achieve these goals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll look at three spacecraft to illustrate, The &lt;em&gt;New Horizons&lt;/em&gt; mission on its way to Pluto, and the &lt;em&gt;Wilkinson Microwave Anisotropy Probe&lt;/em&gt; that mapped the cosmic microwave background radiation from a pseudo-orbit about the Sun-Earth L2 point, and the &lt;em&gt;Mars Reconnaissance Orbiter&lt;/em&gt; that is currently orbiting Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;New Horizons&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;New Horizons&lt;/em&gt; is currently en route to Pluto. It's cruise phase operations are fairly simple. It is currently operating as a spin-stabilized vehicle in hibernation mode. The vehicle rotates about the axis of the communications system with the axis pointed more or less earthward. This is the stable axis of rotation, by design. The vehicle gets once a year checkups from the Earth. The communications system comprises a stack of three antennae, plus one other antenna elsewhere. One is a high gain but low beam angle antenna. This will be the primary antenna for sending data to the Earth. The second is a medium gain antenna with a 4 degree beam angle. The third is a low gain antenna with a hemispherical beam width. (The fourth antenna is identical to the third, but aimed in the opposite direction.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The low gain antennae are no longer of any use. The vehicle is too far from the Earth. They were the backup to the backup plan during the very first part of the cruise phase. If something had gone very, very wrong early on, controllers on the Earth could have sent low bandwidth signals to the vehicle to try to urge it back to health. There's only one backup now, the medium gain antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometime in the next year, the vehicle will switch from it's current spin-stabilized mode to 3-axis mode to perform its primary mission. The vehicle will use some of its precious hydrazine to de-spin itself and then use more of that precious hydrazine for active attitude control. To keep things simple, the antenna on &lt;em&gt;New Horizons&lt;/em&gt; are not steerable. The vehicle will temporarily lose comm with the Earth during its closest approach with Pluto. The main job there is to record as much data as possible. Once the approach is over, the vehicle will reacquire comm by turning the vehicle as a whole so antenna axis once again points Earthward. It will then transmit that recorded data toward the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if that doesn't work? The vehicle has two emergency modes, Earth acquisition and Sun acquisition. The first resort on failing to reacquire comm is to enter Earth acquisition mode. If that first backup plan doesn't work, the vehicle will switch to Sun acquisition mode and aim the comm axis toward the Sun. Since the medium gain antenna has a 4 degree beam width, the Earth should be in view at distance of 40 AU. If that backup to the backup doesn't work, it's goodbye mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Wilkinson Microwave Anisotropy Probe&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The WMAP was also a spin-stabilized vehicle, but with a twist. It's rotation axis was not around one of the principal axes of the vehicle. It intentionally precessed by a bit. It was also slowly reoriented throughout its pseudo orbit so the science package pointed away from the Sun and Earth, shielded from radio noise from the sun and Earth by a large sun shield. The communications equipment and solar panels were on the sunward side of the sun shield. This orientation obviated the need for gimbalable communications antennae.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The WMAP communications system comprised dual redundant medium gain antennae and two low gain omnidirectional antennae. The backup plan was to switch from one medium gain antenna to the other. The backup plan to the backup plan was to use the omni antennae to find the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mars Reconnaissance Orbiter&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MRO also has multiple levels of redundancy. It has but a single large antenna dish. That the MRO dish needs to be gimbal able means that the dish is a single point of failure. However, failures are much more likely to happen in the electronics rather than in the dish and its controls. The vehicle has two X-band transponders that use this single dish. A third Ka band transponder uses the same dish (but this is a demo project rather than a redundancy). In addition to these transponders, the vehicle also has two omni antenna as independent backups to the backup. Mars is close enough to Earth so that those omnis provide an alternate means of contacting the vehicle (at a low bit rate) even if the vehicle is lost in space / lost in time or if some failure has occurred in the main communications system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I omitted something in the above: How does the spacecraft know where to point the antenna? Once again there is no single answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the above spacecraft, along with almost all other modern spacecraft, autonomously keep track of their orientation in space. Now all the spacecraft needs to know is how to point toward the Earth. One way for a spacecraft to know this pointing information is via command from ground controllers, typically in the form of a function of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another approach is for the spacecraft to keep track of where it is in space. This can be done autonomously, via command from the ground, or a combination of autonomy and ground update. Now it's a simple matter of calculating where the Earth and Sun are as a function of time. That's the job of an ephemeris model. The ephemerides are calculated on the ground. The ephemeris model is just a bunch of Chebychev polynomial coefficients, where the polynomial is a function of time.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-05-09T23:11:08.227" LastActivityDate="2014-05-09T23:11:08.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4505" PostTypeId="1" AcceptedAnswerId="4506" CreationDate="2014-05-09T21:23:58.453" Score="45" ViewCount="43975" Body="&lt;p&gt;Kerbal Space Program is an independent spaceflight simulation game, which has become quickly popular due to being (kind of) precise at simulating actual spaceflights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how precisely? How close is Kerbal Space Program to reality in the aspect of difficulty of flights?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="3291" LastEditDate="2014-05-11T06:05:05.400" LastActivityDate="2022-04-02T03:20:50.413" Title="How realistic is Kerbal Space Program?" Tags="|orbital-mechanics|rockets|kerbal-space-program|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4506" PostTypeId="2" ParentId="4505" CreationDate="2014-05-09T22:50:25.980" Score="48" Body="&lt;p&gt;Kerbal Space Program is somewhat of a medium fidelity simulation. It manages a few things quite well, and a few things not as well. Let me try and give a list (which might be a bit out of date):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Good:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The orbit simulation is quite accurate, including how to change inclination, raise/lower orbits, leave a planet, and approach a new planet.&lt;/li&gt;&#xA;&lt;li&gt;The staging is somewhat accurate.&lt;/li&gt;&#xA;&lt;li&gt;The use of fuel, acceleration, mass, etc. are pretty accurate.&lt;/li&gt;&#xA;&lt;li&gt;Structural stability is pretty accurate, at least modeling the key components.&lt;/li&gt;&#xA;&lt;li&gt;The new Aerodynamic's model is pretty good. It doesn't model everything perfectly, but is pretty good overall.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Bad:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Only one body affects an orbiting object, the object of most influence. It might even do something like a Hill sphere. However, there are plenty of multi-object systems which aren't managed at all correctly; Lagrange points aren't taken into account, for instance.&lt;/li&gt;&#xA;&lt;li&gt;The aerodynamics model doesn't model well things like breaking the sound barrier.&lt;/li&gt;&#xA;&lt;li&gt;Reaction wheels aren't modeled properly (Can do with a mod, but...)&lt;/li&gt;&#xA;&lt;li&gt;A number of effects aren't really modeled, but can be included with mods (Communication delay, rocket fuel settling, etc)&lt;/li&gt;&#xA;&lt;li&gt;Re-entry heating is now modeled, although it isn't great.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Ugly:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;No life support requirements.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For more, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Kerbal_Space_Program#Inaccuracies&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-12-21T13:02:41.727" LastActivityDate="2016-12-21T13:02:41.727" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="4507" PostTypeId="1" AcceptedAnswerId="4509" CreationDate="2014-05-10T15:33:44.100" Score="16" ViewCount="5564" Body="&lt;p&gt;In Buzz Aldrin's book &lt;a href=&quot;https://www.amazon.com/Encounter-Tiber-John-Barnes-ebook/dp/B00COWLXUS/ref=as_li_ss_til?tag=httpwwwpearso-20&amp;amp;linkCode=w01&amp;amp;linkId=ZNIGS4AGSHWPODM3&amp;amp;creativeASIN=B00COWLXUS&quot;&gt;Encounter with Tiber&lt;/a&gt;, he mentions that some of the Space Shuttle's fuel tanks actually made it to orbit. Did this actually happen, and if so, for how long did they stay in orbit, and what altitude did they achieve?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2023-01-24T09:34:49.813" Title="Did some of the Shuttle fuel tanks actually orbit?" Tags="|space-shuttle|external-tank|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4509" PostTypeId="2" ParentId="4507" CreationDate="2014-05-10T18:09:04.750" Score="18" Body="&lt;p&gt;Based on this description of the &lt;a href=&quot;http://www.space-shuttle.com/abortmain.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle flight profile&lt;/a&gt;, no external tank would ever have completed so much as a single orbit. An external tank would achieve essentially the same orbital apogee as the orbiter itself, but that is all. The shuttle fired its OMS engines to achieve an actual orbit AFTER tank separation. This means that the tank remained on a trajectory which would intersect (re-enter) the atmosphere with no chance of ever completing an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only way an external tank could remain in orbit would be if it was left attached until after the orbiter had completed its OMS burns to establish an orbital trajectory. To do so would increase the mass to which the OMS engines must apply the required delta-V... by more than the vehicle's payload capacity. This would require reducing, or more likely eliminating the internal payload. The upshot being that a shuttle flight in which a tank is inserted into orbit (if it were even possible) would be a mission to insert a tank into orbit and almost certainly accomplish nothing else. See &lt;a href=&quot;https://space.stackexchange.com/questions/264/energy-cost-of-bringing-the-space-shuttle-external-tank-to-orbit?rq=1&quot;&gt;this question&lt;/a&gt; for a more comprehensive analysis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aldrin has apparently exercised a bit of creative license; the title you mention is after all, a work of fiction. &lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-05-11T19:29:53.273" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4510" PostTypeId="2" ParentId="4505" CreationDate="2014-05-10T19:56:25.577" Score="14" Body="&lt;p&gt;One thing that is different is the lack of &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_decay&quot; rel=&quot;noreferrer&quot;&gt;orbital decay.&lt;/a&gt; In kerbal, if you get above 68km, you will stay there forever. In real life, even extremely high orbits are subject to decay, since our atmosphere is much more complicated than KP can model.&lt;/p&gt;&#xA;&lt;p&gt;Things like the ISS have to constantly make adjustments to stay in orbit. Things like solar flares have a tendency to spew atmospheric particles up into higher orbits and make orbiting objects slow down.&lt;/p&gt;&#xA;&lt;p&gt;I think the game is definitely much easier with its simplified atmospheric model, and I for one am glad to not have to worry about my orbit changing once I achieve one.&lt;/p&gt;&#xA;" OwnerUserId="3322" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-05-10T19:56:25.577" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4512" PostTypeId="2" ParentId="4507" CreationDate="2014-05-11T15:23:05.853" Score="6" Body="&lt;p&gt;There are generally speaking 2 conops to get into orbit with the Space Shuttle. The first one, and most commonly used, requires two thrusts post Main Engine CutOff (MECO) to achieve orbit. This one drops the ET in the Indian Ocean. The &quot;Direct Insert&quot; requires only one OMS maneuver, and the tank landed in the Pacific Ocean. So the External tank might technically have been in orbit briefly, but in a reentry orbit.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-05-11T15:23:05.853" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4513" PostTypeId="1" CreationDate="2014-05-12T01:41:07.193" Score="10" ViewCount="3756" Body="&lt;p&gt;Have any living animals successfully reproduced entirely in space? If none have, what animals have had the most complete reproductive cycles in space?&lt;/p&gt;&#xA;" OwnerUserId="3161" LastEditorUserId="49" LastEditDate="2014-05-12T15:00:42.077" LastActivityDate="2018-11-28T17:06:12.773" Title="What animals, if any, have reproduced in space?" Tags="|life-support|research|animals|life|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4514" PostTypeId="2" ParentId="4513" CreationDate="2014-05-12T13:42:36.330" Score="6" Body="&lt;p&gt;Till now no animals have been successfully reproduced in space.In 1979 Russians carried out an experiment in space to find out whether rats can reproduce in space , in that space mission the rats showed no signs of copulation because of weightlessness in space.&#xA;Then Japanese &#xA;researchers artificially fertilized&#xA;mouse eggs with sperm that had&#xA;been stored inside a three-&#xA;dimensional clinostat, a machine that&#xA;mimics weightlessness by rotating&#xA;objects in such a way that the effects&#xA;of gravity are spread in every&#xA;direction. Some embryos were&#xA;ultimately implanted in female mice&#xA;and survived to a healthy birth, but at&#xA;lower numbers than a regular-gravity&#xA;control group because of the microgravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In answer to your second question three space experiments&#xA;were carried out by the Russian scientists by using aquatic invertebrate animals such as&#xA;Amphipods, Gastropods (pond&#xA;snails), Ostracods and Daphnia&#xA;(water flea) .&#xA;The first experiment&#xA;used a Space shuttle only and it was&#xA;a 10-day flight. The other two space&#xA;experiments were carried out in the&#xA;Space station Mir (Shuttle/Mir&#xA;mission), and the flight units had&#xA;been kept in microgravity for 4&#xA;months. Daphnia produced their&#xA;offspring during a 10-day Shuttle&#xA;flight. In the first Mir experiment, no&#xA;Daphnia were detected when&#xA;recovered to the ground. However,&#xA;they were alive in the second Mir&#xA;experiment. Daphnia were the most&#xA;fragile species among the&#xA;invertebrate animals employed in the&#xA;present experiments. All the animals,&#xA;i.e., Amphipods, pond snails,&#xA;Ostracods and Daphnia had survived&#xA;for 4 months in space, i.e., they had&#xA;produced their offspring or repeated&#xA;their life-cycles under microgravity&lt;/p&gt;&#xA;" OwnerUserId="536" LastActivityDate="2014-05-12T13:42:36.330" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4515" PostTypeId="1" AcceptedAnswerId="4517" CreationDate="2014-05-12T20:10:24.917" Score="15" ViewCount="3068" Body="&lt;p&gt;Beside solar winds and cosmic rays, would they need some form of shielding from whatever deadly comes from these planets ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And about this potentially deadly stuff coming from the planets, what would it be ? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiations only or something else as well ?&lt;/p&gt;&#xA;" OwnerUserId="1368" LastActivityDate="2014-05-13T01:42:46.447" Title="Can humans live in something like the ISS around gas giants?" Tags="|orbit|life|humans|gas-giant|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4517" PostTypeId="2" ParentId="4515" CreationDate="2014-05-12T21:44:50.793" Score="23" Body="&lt;p&gt;This is a pretty broad questions, as it would depend on which gas giant you have in mind. Excluding Uranus and Neptune as &lt;a href=&quot;https://en.wikipedia.org/wiki/Ice_giant&quot; rel=&quot;noreferrer&quot;&gt;ice giants&lt;/a&gt;, this leaves us with Jupiter and Saturn in our own Solar system, and they're still hard to directly compare in terms of how hazardous environment they'd represent to an orbiting space station. But they have one deadly thing in common:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To reach them from the Earth requires years long travel through deep space. As far as manned missions to space are concerned, we haven't done anything quite like it yet. Those Apollo visits of the Moon in the late 60's and early 70's were days long short-duration excursions in comparison, so first thing we'd have to figure out is how to protect astronauts from deadly exposure to &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind&quot; rel=&quot;noreferrer&quot;&gt;Solar wind&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_ray&quot; rel=&quot;noreferrer&quot;&gt;cosmic rays&lt;/a&gt;, and avoid Solar charged particles flux of &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot; rel=&quot;noreferrer&quot;&gt;Coronal Mass Ejections&lt;/a&gt; (CMEs) for the duration of the round trip and their stay at one of the gas giant orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://missionjuno.swri.edu/jupiter/magnetosphere&quot; rel=&quot;noreferrer&quot;&gt;Jupiter's magnetosphere&lt;/a&gt; is also the second largest and strongest structure in the Solar system. Its magnetic field can accelerate electrically charged particles to immense speeds. Their effect can be observed at Jupiter's poles as polar auroras, where they interact with its upper atmosphere and create brilliantly blue circle areas of ionized gases, many times the Earth in size. These magnetically accelerated particles mostly come off one of it's moons - &lt;a href=&quot;https://en.wikipedia.org/wiki/Io_%28moon%29&quot; rel=&quot;noreferrer&quot;&gt;Io&lt;/a&gt; and its volcanic activity. They can be the size of dust particles and cause significant damage on impact with the craft's shielding, especially as the time goes by and the damage accumulates, to barely noticeable but in time deadly nonetheless nano-scale particles and ions (charged particle radiation) that can rip through living tissues and cause tissue scarring, genetic defects, and worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/fPkAR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Synchrotron Emission observations of Jupiter by the radio telescopes of the Very Large Array (source: &lt;a href=&quot;http://missionjuno.swri.edu/media-gallery/magnetosphere&quot; rel=&quot;noreferrer&quot;&gt;SwRI on mission JUNO&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Polar regions are also where the Jupiter &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_reconnection&quot; rel=&quot;noreferrer&quot;&gt;magnetically reconnects&lt;/a&gt; with the Sun's magnetosphere, so they present a double danger of both charged particles from the Solar winds as well as those accelerated by the Jupiter's own magnetosphere. So there really isn't any orbit around Jupiter that could be considered nearly as safe as Low Earth Orbit (LEO) of the International Space Station (ISS), that is still well within the Earth's magnetosphere and &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;noreferrer&quot;&gt;Van Allen radiation belts&lt;/a&gt;, with an exception of the &lt;a href=&quot;https://en.wikipedia.org/wiki/South_Atlantic_Anomaly&quot; rel=&quot;noreferrer&quot;&gt;South Atlantic Anomaly&lt;/a&gt; that the station still passes through during some of its Earth orbits and for perhaps up to 10 minutes or less at the time (its orbit is ~ 92 minutes long).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are other potential problems, such as Jupiter's emissions in the radio frequencies interfering with communications equipment, electrically charged plasma damaging other sensitive electronic parts, and otherwise putting your Jupiter orbiting mission in jeopardy. The list of ways the universe can kill us is infinite, the list of ways to protect ourselves from those dangers, sadly, isn't.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-05-12T22:11:21.410" LastActivityDate="2014-05-12T22:11:21.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4518" PostTypeId="2" ParentId="4515" CreationDate="2014-05-13T01:42:46.447" Score="9" Body="&lt;p&gt;Live, yes. Thrive? No. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS and stations like it are not &quot;benign environments&quot; - that is, they are life sustaining, but not safe for long term habitation on the order of years. They lose air, they admit both EM and particulate radiation, and are only minimally protected from magnetic induction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NASA lifetime space exposure limit is under 5 Seiverts (Sv), with a per year limit of 0.5 Sv. Apollo 14 hit 0.014 Sv in 9 days - a rate above the annual threshold. Certain space events should be able to generate 0.01 Sv per day, even through the ISS type station's walls. (Radiation bunkerage has been proposed for such situations; the launch costs are prohibitive.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter and Saturn both are, themselves, radiation sources. Their radiation belts are also in the orbital regimes - a factor that's had to be designed for in NASA's probes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radiation that needs to be shielded against includes some gamma, some X, Cosmic and Solar Wind, the extensive IR emissions, some radio waves, the relatively dense microbody environments, the incredibly intense magnetic fields (and their potential induced current).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/centers/johnson/slsd/about/divisions/hacd/hrp/space-radiation.html&quot;&gt;http://www.nasa.gov/centers/johnson/slsd/about/divisions/hacd/hrp/space-radiation.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/284273main_Radiation_HS_Mod1.pdf&quot;&gt;http://www.nasa.gov/pdf/284273main_Radiation_HS_Mod1.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-05-13T01:42:46.447" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4520" PostTypeId="1" AcceptedAnswerId="4521" CreationDate="2014-05-13T13:01:46.943" Score="8" ViewCount="261" Body="&lt;p&gt;Apollo XI mission carried &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_plaque&quot; rel=&quot;noreferrer&quot;&gt;a plaque&lt;/a&gt; to the Moon featuring a map of the Earth. So do the Pioneers with &lt;a href=&quot;http://en.wikipedia.org/wiki/Pioneer_plaque&quot; rel=&quot;noreferrer&quot;&gt;a plaque&lt;/a&gt; with symbolic indications that may help to find its origin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, here comes the question: Did the Huygens probe carry any hint that may be helpful to find the Earth? If not, why? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question may extend to any spacecraft that is expected to survive a landing in other places than Earth, like Mars rovers, or spacecraft whose final location is not a known one, e.g. New Horizons.&lt;/p&gt;&#xA;" OwnerUserId="298" LastEditorUserId="298" LastEditDate="2014-05-13T13:22:38.160" LastActivityDate="2014-05-13T15:29:14.387" Title="Did Huygens probe carry any hint to help find its home planet?" Tags="|probe|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4521" PostTypeId="2" ParentId="4520" CreationDate="2014-05-13T15:04:27.520" Score="7" Body="&lt;p&gt;No plaques are needed to identify the origins of any of our deep space probes, landers, rovers and so forth. Any civilisation technologically at least as advanced as we are from the 20th century onward shouldn't have problems identifying that they could only have came from the Earth. In fact, I would dare go as far to claim you'd only really require a small part of one probe, perhaps an electrolytic or a ceramic capacitor, to infer that the only place where it could have been manufactured was on the Earth, and you could likely establish also sufficiently precisely when, by the level of decay of its materials or even &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiocarbon_dating&quot;&gt;carbon dating&lt;/a&gt; them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A whole spacecraft would hold other clues, such as the strength / resolution of its communications subsystem and where its antennae are pointing, limiting possible source within our Solar system or closer, and give you a hint of where to look for other &lt;a href=&quot;https://en.wikipedia.org/wiki/Technosignature&quot;&gt;technosignatures&lt;/a&gt;. Some of the materials used by it could only have been synthesized and they don't exist naturally anywhere in our neighborhood. Any industry producing them would leave behind markers, such as, say, atmospheric pollutants that could be detected from another celestial in our Solar system, perhaps by &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_spectroscopy&quot;&gt;astronomical spectroscopy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even with a few clues, technologically advanced civilisation could establish when they were manufactured, when they were last used even (exposure to the environment of different parts could give a clue about that), what direction in the skies they were communicating with and a good approximation of at most how far away they were from where their signals were supposed to be picked up. They could then investigate possible candidate celestials and establish that the Earth is the only one that contains traces of industrial pollutants needed for fabrication of some of the probe's materials in its upper atmosphere and that they must be there from around the time when the probe was built. As soon as they'd realize no other celestials in our vicinity match all these requirements, they would've solved the problem of establishing some probe's origins. Simple really.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plaques and alike would likely be meaningless to anyone not understanding our languages or having required equipment to play records, analogue or digital. If analogy is required, just think of all the data carriers from as little as a few decades ago that we don't even have any readers for anymore. Remember floppy disks? &lt;a href=&quot;https://en.wikipedia.org/wiki/Punched_card&quot;&gt;Punched cards&lt;/a&gt;? Do you still keep around all the required hardware to read them? Probably not, and even if you did, they might not be working any more, and/or you wouldn't be able to make sense of the data they hold. And even the launching programs don't for some of the spacecraft that are still functional, like e.g. &lt;a href=&quot;http://www.centauri-dreams.org/?p=30419&quot;&gt;NASA misplacing communications equipment needed to control the ICE/ISEE-3&lt;/a&gt;, that is &lt;a href=&quot;http://www.rockethub.com/42228&quot;&gt;now a crowdfunding effort&lt;/a&gt; to redirect it to its original orbit and do science again by means of &lt;a href=&quot;https://en.wikipedia.org/wiki/Software-defined_radio&quot;&gt;software-defined radio&lt;/a&gt; equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So those plaques, gold records, DVDs (&lt;a href=&quot;http://saturn.jpl.nasa.gov/kids/amazing-dvd.cfm&quot;&gt;like on the Cassini spacecraft&lt;/a&gt;), and whatnot that we're attaching to our probes and landers are really more for ourselves than any yet unknown civilisation (distant future us, or anyone else). And they're a subject to decay, so they won't last or be readable for eternity anyway.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-05-13T15:29:14.387" LastActivityDate="2014-05-13T15:29:14.387" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4522" PostTypeId="2" ParentId="4513" CreationDate="2014-05-13T15:25:45.810" Score="7" Body="&lt;p&gt;The nematode, or roundworm, known as &lt;em&gt;Caenorhabditis elegans&lt;/em&gt;, is able to reproduce fully in space from mating through development.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though simple, the nematode &lt;em&gt;is&lt;/em&gt; an animal and it is one of the most studied creatures on the International Space Station. I am not aware of any other animals that have been observed to successfully reproduce in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the relevant portion of my source material (emphasis added):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;ICE-First-Aging is one of several experiments that investigates the effects of space flight on a model organism in the nematode worm family (Caenorhabditis elegans) and aims to develop links to human physiology in space. &lt;strong&gt;The organism chosen for this study is known to be able to mate, reproduce and develop apparently normally during space flight.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Fruit flies have also successfully reproduced in space, and there was an attempt to study the reproduction of geckos in space--unfortunately, the geckos froze at some point during the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/720.html&quot; rel=&quot;nofollow&quot;&gt;International Caenorhabditis elegans Experiment First Flight-Aging (ICE-First-Aging) - 02.15.14 - NASA&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.theguardian.com/science/2014/sep/01/russia-death-five-geckos-space-sex-mission&quot; rel=&quot;nofollow&quot;&gt;Russia confirms death of five geckos on space sex mission - The Guardian&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-09-02T15:37:54.373" LastActivityDate="2014-09-02T15:37:54.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4523" PostTypeId="2" ParentId="604" CreationDate="2014-05-13T15:47:29.427" Score="8" Body="&lt;p&gt;Strictly speaking, the answer to your question is &lt;strong&gt;yes&lt;/strong&gt;. As I have explained in answer to a related question (&lt;a href=&quot;https://space.stackexchange.com/questions/4513/what-animals-if-any-have-reproduced-in-space&quot;&gt;What animals, if any, have reproduced in space?&lt;/a&gt;), nematodes are able to experience a full reproduction cycle in space. Nematodes &lt;em&gt;are&lt;/em&gt; considered pregnant at a point in this cycle, and their pregnancy does meet your definition:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Egg and Sperm join, Cells divide and attach to the uterus&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, obviously nematodes are not mammals, and though some are ovoviviparous, the species observed in space is not. I am not aware of any mammals that have been observed to get pregnant in space.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-05-13T15:47:29.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4524" PostTypeId="2" ParentId="4503" CreationDate="2014-05-13T16:16:43.687" Score="26" Body="&lt;p&gt;No answer to this question is likely to be complete, but I have tried to gather what sources are available to give a good representation of the animal research on the International Space Station (ISS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a list of the types of animals that have been on the ISS with as much detail as I could find about them. The final portion is an attempt at getting a lower limit on their numbers:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Drosophila melanogaster&lt;/em&gt;, a species of fruit fly, more than 450&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/72XzXm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/72XzXm.jpg&quot; alt=&quot;Fruit fly&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: NASA&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Caenorhabditis elegans&lt;/em&gt;, a species of nematode or roundworm, more than 8 culture bags containing an unspecified amount of nematodes&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rduqS.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rduqS.jpg&quot; alt=&quot;C. elegans, roundworm&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: Cell Image Library, Bob Goldstein&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Oryzias latipes&lt;/em&gt;, a species of rice fish, known as the medaka, the Japanese rice fish, or the Japanese killifish, more than 5&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ib2a9m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ib2a9m.jpg&quot; alt=&quot;Medaka&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: NASA&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Euprymna scolopes&lt;/em&gt;, a species of squid, known as the Hawaiian Bobtail squid, at least 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3VN7Cm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3VN7Cm.jpg&quot; alt=&quot;Hawaiian Bobtail squid&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: University of Florida, Jamie Foster (used with permission)&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Bombyx mori&lt;/em&gt;, a species of silkworm, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AbPTCm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AbPTCm.jpg&quot; alt=&quot;Domestic silkworm&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Larvae pictured. Credits: Wikipedia user Fastily under the &lt;a href=&quot;https://creativecommons.org/licenses/by-sa/3.0/deed.en&quot; rel=&quot;noreferrer&quot;&gt;Creative Commons Attribution-Share Alike 3.0 Unported license&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Girardia tigrina&lt;/em&gt;, a species of planarian or flatworm, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wsj0Zm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wsj0Zm.jpg&quot; alt=&quot;Girardia tigrina, flatworm&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: Continenticola under the &lt;a href=&quot;http://creativecommons.org/licences/by/3.0/&quot; rel=&quot;noreferrer&quot;&gt;Creative Commons Attribution CC BY Licence&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Helix lucorum&lt;/em&gt;, a species of snail, unspecified amount greater than 7&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8Of5fm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8Of5fm.jpg&quot; alt=&quot;Helix lucorum, snail&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &lt;a href=&quot;http://imagesfrombulgaria.com/v/Animals/Mollusca/Unsorted/DSC05960.JPG.html?g2_imageViewsIndex=1&quot; rel=&quot;noreferrer&quot;&gt;&quot;Helix lucorum 2&quot;&lt;/a&gt; by Petar Iankov, &lt;a href=&quot;http://www.ImagesFromBulgaria.com&quot; rel=&quot;noreferrer&quot;&gt;http://www.ImagesFromBulgaria.com&lt;/a&gt; - by Petar Iankov, (caption cropped from original source image by User:Snek01). Licensed under &lt;a href=&quot;http://creativecommons.org/licenses/by/2.5&quot; rel=&quot;noreferrer&quot;&gt;CC BY 2.5&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Helix_lucorum_2.jpg#/media/File:Helix_lucorum_2.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Coturnix japonica&lt;/em&gt;, a species of quail, about 36 or more&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oABZgm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oABZgm.jpg&quot; alt=&quot;Japanese quail&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Japanese Quail&quot; by Ingrid Taylar - Flickr: Mikiko the Quail. Licensed under &lt;a href=&quot;http://creativecommons.org/licenses/by/2.0&quot; rel=&quot;noreferrer&quot;&gt;CC BY 2.0&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Japanese_Quail.jpg#/media/File:Japanese_Quail.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Mus musculus&lt;/em&gt;, a species of mouse, much more than 40&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bcKGL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bcKGL.jpg&quot; alt=&quot;Mouse&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: NASA&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Danio rerio&lt;/em&gt;, a species of danio fish, known as the zebrafish, at least 18&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BSDgum.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BSDgum.jpg&quot; alt=&quot;Zebrafish&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: JAXA&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Nephila clavipes&lt;/em&gt;, a species of golden orb spider, at least 2&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Aa1qRm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Aa1qRm.jpg&quot; alt=&quot;Nephila clavipes, golden orb spider&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: Danielle Anthony&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Vanessa cardui&lt;/em&gt;, a species of painted lady butterfly, at least 4&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5O1Q5m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5O1Q5m.jpg&quot; alt=&quot;Vanessa cardui, painted lady butterfly&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: Jeffery Pippen (used with permission)&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Danaus plexippus&lt;/em&gt;, a species of milkweed butterfly, known as the monarch butterfly, unspecified amount likely greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6MZWhm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6MZWhm.jpg&quot; alt=&quot;Monarch butterfly&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: NASA&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Tetramorium caespitum&lt;/em&gt;, a species of ant, known as the pavement ant, about 800&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MM1Vsm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MM1Vsm.jpg&quot; alt=&quot;Pavement ant&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Tetramorium caespitum casent0005827 profile 1&quot; by The photographer and www.antweb.org. Licensed under &lt;a href=&quot;http://creativecommons.org/licences/by/3.0/&quot; rel=&quot;noreferrer&quot;&gt;CC BY-SA 3.0&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Tetramorium_caespitum_casent0005827_profile_1.jpg#/media/File:Tetramorium_caespitum_casent0005827_profile_1.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Bees, unspecified taxon, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hwtXlm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hwtXlm.jpg&quot; alt=&quot;Bee&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: NASA&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Tardigrada&lt;/em&gt;, the phylum of tardigrades or waterbears, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PtVrg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PtVrg.jpg&quot; alt=&quot;Tardigrade&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: Bob Goldstein and Vicky Madden, UNC Chapel Hill - &lt;a href=&quot;http://tardigrades.bio.unc.edu/pictures/&quot; rel=&quot;noreferrer&quot;&gt;http://tardigrades.bio.unc.edu/pictures/&lt;/a&gt; Licensed under &lt;a href=&quot;https://creativecommons.org/licenses/by-sa/3.0&quot; rel=&quot;noreferrer&quot;&gt;CC BY-SA 3.0&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/w/index.php?search=tardigrade&amp;amp;title=Special%3ASearch&amp;amp;go=Go&amp;amp;ns0=1&amp;amp;ns6=1&amp;amp;ns12=1&amp;amp;ns14=1&amp;amp;ns100=1&amp;amp;ns106=1#/media/File:Waterbear.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Daphnia magna&lt;/em&gt;, a species of water flea, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pjs4ym.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pjs4ym.jpg&quot; alt=&quot;Daphnia magna, water flea&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Daphnia magna-female adult&quot; by Hajime Watanabe - PLoS Genetics, March 2011. Licensed under &lt;a href=&quot;http://creativecommons.org/licenses/by/2.5&quot; rel=&quot;noreferrer&quot;&gt;CC BY 2.5&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Daphnia_magna-female_adult.jpg#/media/File:Daphnia_magna-female_adult.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Artemia&lt;/em&gt;, a genus of fairy shrimp, known as the brine shrimp, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hDLYXm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hDLYXm.jpg&quot; alt=&quot;Artemia, fairy shrimp&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Artemia salina 4&quot; by © Hans Hillewaert. Licensed under &lt;a href=&quot;http://creativecommons.org/licenses/by-sa/4.0&quot; rel=&quot;noreferrer&quot;&gt;CC BY-SA 4.0&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Artemia_salina_4.jpg#/media/File:Artemia_salina_4.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Tenebrio molitor&lt;/em&gt;, a species of darkling beetle, known as the mealworm beetle, at least 6 to 8&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sJXi5m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sJXi5m.jpg&quot; alt=&quot;Mealworm beetle&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Tenebrio molitor MHNT&quot; by Didier Descouens - Own work. Licensed under &lt;a href=&quot;http://creativecommons.org/licenses/by-sa/4.0&quot; rel=&quot;noreferrer&quot;&gt;CC BY-SA 4.0&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Tenebrio_molitor_MHNT.jpg#/media/File:Tenebrio_molitor_MHNT.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Pogonomyrmex barbatus&lt;/em&gt;, a species of ant, known as the red harvester ant, at least 6&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/a1JRdm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/a1JRdm.jpg&quot; alt=&quot;Red harvester ant&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Pogonomyrmex barbatus casent0006306 profile 1&quot; by The photographer and www.antweb.org. Licensed under &lt;a href=&quot;http://creativecommons.org/licenses/by-sa/3.0&quot; rel=&quot;noreferrer&quot;&gt;CC BY-SA 3.0&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Pogonomyrmex_barbatus_casent0006306_profile_1.jpg#/media/File:Pogonomyrmex_barbatus_casent0006306_profile_1.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Musca domestica&lt;/em&gt;, a species of fly, known as the housefly, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NxOQQm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NxOQQm.jpg&quot; alt=&quot;Housefly&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Musca domestica housefly&quot; by Muhammad Mahdi Karim (www.micro2macro.net) - Own work. Licensed under &lt;a href=&quot;http://www.gnu.org/licenses/old-licenses/fdl-1.2.html&quot; rel=&quot;noreferrer&quot;&gt;GFDL 1.2&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Musca_domestica_housefly.jpg#/media/File:Musca_domestica_housefly.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Streptocephalus&lt;/em&gt;, a genus of fairy shrimp, variety used is known as the Dry Lake fairy shrimp, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mkzeW.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mkzeW.jpg&quot; alt=&quot;Streptocephalus, fairy shrimp&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Streptocephalus woottoni&quot; by &lt;a href=&quot;http://www.werc.usgs.gov/Project.aspx?ProjectID=211&quot; rel=&quot;noreferrer&quot;&gt;USGS&lt;/a&gt;. Licensed under Public Domain via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Streptocephalus_woottoni.jpg#/media/File:Streptocephalus_woottoni.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Aedes albopictus&lt;/em&gt;, a species of mosquito, known as the tiger mosquito or the forest mosquito, unspecified amount greater than 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TX2UEm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TX2UEm.jpg&quot; alt=&quot;Tiger mosquito&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Aedes Albopictus&quot; by James Gathany/CDC - This media comes from the Centers for Disease Control and Prevention's Public Health Image Library (PHIL), with identification number #4487. Note: Not all PHIL images are public domain; be sure to check copyright status and credit authors and content providers. Licensed under Public Domain via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Aedes_Albopictus.jpg#/media/File:Aedes_Albopictus.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Eisenia fetida&lt;/em&gt;, a species of earthworm, known as the red wiggler worm, at least 20 to 30&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FR96m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FR96m.jpg&quot; alt=&quot;Red wiggler worm&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;Redwiggler1&quot; by Mihai Duguleana; - Transferred from en.wikipedia to Commons. Licensed under Public Domain via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Redwiggler1.jpg#/media/File:Redwiggler1.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The study of animals on the space station has been very popular with the public to the extent that the Onion, the satirical &quot;news&quot; source, saw fit to make up their own animal for the ISS: &lt;a href=&quot;http://www.theonion.com/articles/mischievous-raccoon-wreaks-havoc-on-international,1180/&quot; rel=&quot;noreferrer&quot;&gt;Mischievous Raccoon Wreaks Havoc On International Space Station&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also possible that some animals, such as dust mites, have gotten on board outside of study purposes. Dust mites were known to inhabit the Mir space station and other space missions. Current allergen studies on the ISS have not involved the detection of dust mites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/unuyDm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/unuyDm.jpg&quot; alt=&quot;Dust mite&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: &quot;CSIRO ScienceImage 11085 A scanning electron micrograph of a female dust mite&quot; by CSIRO. Licensed under &lt;a href=&quot;http://creativecommons.org/licenses/by/3.0&quot; rel=&quot;noreferrer&quot;&gt;CC BY 3.0&lt;/a&gt; via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:CSIRO_ScienceImage_11085_A_scanning_electron_micrograph_of_a_female_dust_mite.jpg#/media/File:CSIRO_ScienceImage_11085_A_scanning_electron_micrograph_of_a_female_dust_mite.jpg&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, of course, there is one I have left off:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Homo sapiens&lt;/em&gt;, a species of hominin, known as the human, 220 and counting&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oPB01.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oPB01.jpg&quot; alt=&quot;Human astronauts aboard the ISS&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Credits: NASA&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/experiments_hardware.html#Biology-and-Biotechnology&quot; rel=&quot;noreferrer&quot;&gt;International Space Station Experiments by Hardware - 05.06.14 - Biology and Biotechnology - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/space_spiders_live.html&quot; rel=&quot;noreferrer&quot;&gt;Spiders in Space -- Live! - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/286.html&quot; rel=&quot;noreferrer&quot;&gt;Commercial Generic Bioprocessing Apparatus Science Insert - 03: Spiders and Butterflies (CSI-03) - 01.15.14 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.tudelft.nl/en/current/university-magazines/delft-outlook/former-editions/delft-outlook-2007-2/kort-delfts/space-bees/&quot; rel=&quot;noreferrer&quot;&gt;Space bees - Delft University of Technology&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.techtimes.com/articles/2824/20140121/ants-invade-international-space-station-and-they-are-on-a-mission.htm&quot; rel=&quot;noreferrer&quot;&gt;Ants invade International Space Station...and they're on a mission - Tech Times&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/800.html&quot; rel=&quot;noreferrer&quot;&gt;BIOKon In Space (BIOKIS) - 02.12.14 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1033.html&quot; rel=&quot;noreferrer&quot;&gt;Surface, Water and Air Biocharacterization - A Comprehensive Characterization of Microorganisms and Allergens in Spacecraft Environment (SWAB) - 04.30.14 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576510001839&quot; rel=&quot;noreferrer&quot;&gt;Survival of dormant organisms after long-term exposure to the space environment - Novikova, et al.&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/2005.html&quot; rel=&quot;noreferrer&quot;&gt;NanoRacks-Darca Bat Yam High School-Emergence of Artemia Eggs (NanoRacks-DBYHS-Artemia) - 08.27.15 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1944.html&quot; rel=&quot;noreferrer&quot;&gt;NanoRacks-McMinnville High School-Exoskeleton Density Analysis of Mealworms in a Microgravity Environment (NanoRacks-MHS-Exoskeleton Density ) - 08.27.15 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1937.html&quot; rel=&quot;noreferrer&quot;&gt;NanoRacks-Valley Christian High School-Ant Colony Behavior in a Microgravity Environment (NanoRacks-VCHS-Ant Colony Behavior) - 08.27.15 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1954.html&quot; rel=&quot;noreferrer&quot;&gt;NanoRacks-National Center for Earth and Space Science-Yankee Clipper (SSEP Mission 6) (NanoRacks-NCESSE-Yankee Clipper) - 08.27.15&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/2125.html&quot; rel=&quot;noreferrer&quot;&gt;NanoRacks-NDC-Bell Middle School-Efficiency of Vermicomposting in a Closed System (NanoRacks-NDC-BMS-Vermicomposting) - 08.27.15 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2020-03-26T18:59:26.593" LastActivityDate="2020-03-26T18:59:26.593" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="4525" PostTypeId="1" AcceptedAnswerId="4527" CreationDate="2014-05-13T20:31:15.630" Score="33" ViewCount="7806" Body="&lt;p&gt;A &lt;a href=&quot;http://in.reuters.com/article/2014/05/13/uk-ukraine-crisis-russia-usa-idINKBN0DT1QX20140513&quot;&gt;new story came&lt;/a&gt; out today about Russia potentially preventing the US from using the ISS. One of the claims made was: &lt;em&gt;&quot;The Russian segment [of ISS] can exist independently from the American one. The U.S. one cannot.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Leaving aside all the political considerations of whether any of this is a good idea, is this claim true? Can the US side not survive on it's own? Why was such a design made so that it couldn't survive but the Russian side could?&lt;/p&gt;&#xA;" OwnerUserId="1482" LastEditorUserId="49" LastEditDate="2014-05-14T01:27:54.810" LastActivityDate="2023-07-02T12:07:26.317" Title="Can the US Part of ISS survive independent of the Russian?" Tags="|iss|us-segment|russian-segment|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4526" PostTypeId="2" ParentId="1766" CreationDate="2014-05-13T20:35:11.153" Score="0" Body="&lt;p&gt;This is a well known exercise, codified as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Drake_equation&quot; rel=&quot;nofollow&quot;&gt;Drake equation&lt;/a&gt;. Excerpt from this wikipedia link:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Drake equation is:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$$N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L$$&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;where:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$N$ = the number of civilizations in our galaxy with which&#xA;  radio-communication might be possible (i.e. which are on our current&#xA;  past light cone); &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;and&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$R_{\ast}$ = the average rate of star formation in our galaxy&lt;br /&gt;&#xA;  $f_p$ = the fraction of those stars that have planets&lt;br /&gt;&#xA;  $n_e$ = the average number of planets that can potentially support life per star that has planets&lt;br /&gt;&#xA;  $f_l$ = the fraction of planets that could support life that actually develop life at some point&lt;br /&gt;&#xA;  $f_i$ = the fraction of planets with life that actually go on to develop intelligent life (civilizations)&lt;br /&gt;&#xA;  $f_c$ = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space&lt;br /&gt;&#xA;  $L$ = the length of time&#xA;  for which such civilizations release detectable signals into space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2639" LastEditorUserId="49" LastEditDate="2014-05-13T21:41:18.860" LastActivityDate="2014-05-13T21:41:18.860" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4527" PostTypeId="2" ParentId="4525" CreationDate="2014-05-13T21:00:27.370" Score="33" Body="&lt;p&gt;What is being referred to here is the fact that the guidance, navigation, control, and propulsion capabilities of the ISS are entirely contained in the Russian &lt;em&gt;Zvezda&lt;/em&gt; module.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SYxff.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SYxff.jpg&quot; alt=&quot;ISS Diagram&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Image Credit: ESA&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;NASA does have a module on the ground that is capable of replacing the &lt;em&gt;Zvezda&lt;/em&gt; module temporarily for three years called the Interim Control Module, but the US currently has no way of delivering this to orbit.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4bQho.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4bQho.jpg&quot; alt=&quot;Interim Control Module&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;We have designs for a long term module to replace Zvezda, but the design was never approved and is now outdated and would need to be revised.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Zvezda&lt;/em&gt; was originally designed to be part of a successor to the Russian Mir space station, but was gifted to the International Space Station as part of a good faith effort between Russia and the US.&lt;/p&gt;&#xA;&lt;p&gt;This is why there is a strong reliance on &lt;em&gt;Zvezda&lt;/em&gt; today. The US had plans to backup the &lt;em&gt;Zvezda&lt;/em&gt; module, but they were not needed at the time, since &lt;em&gt;Zvezda&lt;/em&gt; was successful.&lt;/p&gt;&#xA;&lt;p&gt;As far as the political situation is concerned, if the Russian government really did break faith with the US in this way, it might be enough impetus to reignite the space race. With the proper budget (which a reignited space race might provide), the US has ample capability to strike out on its own. There would be political ramifications from this, obviously, and at this time we cannot know how the US, Russia, and the world would be affected by a new space race. It could be good or bad, and likely a mix of both. It may cause an acceleration of technological development, but it could also have nasty side effects.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;For more information:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_Propulsion_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Propulsion Module - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Interim_Control_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;Interim Control Module - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Zvezda_(ISS_module)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zvezda (ISS module) - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="50091" LastEditDate="2023-07-02T12:07:26.317" LastActivityDate="2023-07-02T12:07:26.317" CommentCount="20" ContentLicense="CC BY-SA 4.0" />
  <row Id="4528" PostTypeId="2" ParentId="4525" CreationDate="2014-05-13T21:01:21.687" Score="4" Body="&lt;p&gt;It depends.  The US side technically owns (as in paid for) the Zarya FGB tug which is critical to the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Zvezda service module is a major module as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian plan is basically to take the next two modules they are launching, (one day, maybe, in the future, someday) to form the base of a new station.  The MLM and UM. The question becomes, will they take Zvezda with them or not. There are alternate plans for that both ways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see more about these modules in the answer to the question I asked about &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;Will the ISS need more docking ports&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they leave Zvezda, then the station reboost facility is still there, you just need to get fuel to it.  Which normally Progress does, but ATV can do it as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, the ATV gets its Kurs docking computer from Russia aftermarket.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on how badly Russia wants to piss off the US and vica versa will determine the outcome.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-05-13T21:16:46.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4529" PostTypeId="1" AcceptedAnswerId="4530" CreationDate="2014-05-14T01:25:19.040" Score="0" ViewCount="6755" Body="&lt;p&gt;I am just curious what record is fastest orbit around the Earth. Although I suppose there are three different metrics one could use to measure what does one mean by fast so here are the three I thought of.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Shortest Orbital Period&lt;/li&gt;&#xA;&lt;li&gt;Fastest Ground Speed&lt;/li&gt;&#xA;&lt;li&gt;Fastest Orbital Velocity&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If someone could give me a quick answer to any or all three of those categories that would be great. Would also be awesome to see the fastest orbit for manned and unmanned spacecraft (separately). If anyone could point me to the information that would be awesome!&lt;/p&gt;&#xA;" OwnerUserId="3344" LastActivityDate="2014-05-14T02:07:32.147" Title="What's the record for the fastest orbit around Earth?" Tags="|artificial-satellite|crewed-spaceflight|orbit|spacecraft|uncrewed-spaceflight|" AnswerCount="1" CommentCount="6" ClosedDate="2014-05-14T04:15:04.963" ContentLicense="CC BY-SA 3.0" />
  <row Id="4530" PostTypeId="2" ParentId="4529" CreationDate="2014-05-14T01:53:42.207" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Stardust_%28spacecraft%29#Sample_return&quot; rel=&quot;nofollow&quot;&gt;Stardust sample return entered earth's atmosphere at about 12.9 km/s&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow&quot;&gt;New Horizons&lt;/a&gt; left the earth at about 16.26 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither of these have a defined orbital period as both are hyperbolic. Anything leaving Low Earth Orbit at a speed above 11 km/s has exceeded escape velocity and isn't coming back.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-05-14T02:07:32.147" LastActivityDate="2014-05-14T02:07:32.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4531" PostTypeId="1" CreationDate="2014-05-14T07:20:44.803" Score="40" ViewCount="10540" Body="&lt;p&gt;I'm reading various articles on Tesla S electric car and many of them &lt;a href=&quot;http://www.hybridcars.com/tesla-model-s-scores-highest-ever-in-federal-crash-tests/&quot;&gt;(one example)&lt;/a&gt; say that the car body is very strong because of using &lt;em&gt;aerospace grade bolts&lt;/em&gt;. What are those I wonder? I found &lt;a href=&quot;http://www.thomasnet.com/articles/hardware/aerospace-fastener-types&quot;&gt;this review&lt;/a&gt; on aerospace fasteners that says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The key difference is quality. Aerospace products need to be more durable in order to withstand a lot of high pressure and temperature environments, such as leaving the earth’s atmosphere or exposure to burning rocket fuel. Additionally, aerospace products must be lightweight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So what are those &lt;em&gt;aerospace grade bolts&lt;/em&gt;? Are they just made of stronger (and perhaps lighter) alloys or do they have any additional properties?&lt;/p&gt;&#xA;" OwnerUserId="2948" LastActivityDate="2024-01-18T14:23:00.953" Title="What exactly makes a bolt &quot;aerospace grade&quot;?" Tags="|materials|construction|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4532" PostTypeId="2" ParentId="4531" CreationDate="2014-05-14T11:13:09.907" Score="20" Body="&lt;p&gt;Parts for use in aircraft have to be certified. This means they need to be produced from &lt;a href=&quot;http://www.faa.gov/aircraft/safety/programs/sups/standard_parts/media/standard_parts.pdf&quot;&gt;standardized materials, using a standardized and traceable process&lt;/a&gt;. The part must be checked thoroughly during production, and the entire history of the part must be recorded. The tests and paperwork often account for most of the cost of the part. \$100 bolts are not uncommon.&lt;br&gt;&#xA;&quot;Aerospace grade&quot; means that the materials and process used to create the part are suitable for use in aircraft parts. I suspect aerospace suppliers will sell you two versions of the same part: one with the entire papertrail for use in aircraft, and the other without the papertrail for use elsewhere. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-05-14T11:13:09.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4534" PostTypeId="2" ParentId="4525" CreationDate="2014-05-14T15:31:54.967" Score="8" Body="&lt;p&gt;Others have already answered the first part of the question (&quot;is it true?&quot;). I would like to address the second part (&quot;Why was such a design made?&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS was launched in parts over the course of several years. That means it was necessary to first launch those modules which were vital to the survival of the station and then launch any of the more science-oriented modules. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first module was the Functional Cargo Block module, which provided electrical power, propulsion and guidance. This module made sure that the station itself could maintain its orbit (at that point it was still autonomous). The second (major) module was the service module which provided life support and made the station habitable for a crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The project planners handed the responsibility for these modules to Russia who named them Zarya and Zvezda. At that time Russia had the most experience with building and maintaining a habitable space station due to the Mir project. That meant the responsibility for the two most survival-critical modules was well-placed in their hands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After these modules were in orbit, adding other modules providing the same functionality would have been redundant and a waste of the already tight budget of the project. &lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-05-14T17:13:48.193" LastActivityDate="2014-05-14T17:13:48.193" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4535" PostTypeId="1" AcceptedAnswerId="6481" CreationDate="2014-05-14T16:26:38.993" Score="7" ViewCount="605" Body="&lt;p&gt;For whatever reasons, it seems like the standard upper stage engine in the U.S. inventory is the RL-10. A high performance engine with nice specific impulse (I&lt;sub&gt;sp&lt;/sub&gt; = 450-465 s), but very low thrust (25 Klbs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ares-1&lt;/a&gt;, the initial notion was to try a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Main Engine&lt;/a&gt; (SSME) on the second stage, and air start it, which ended up being a non-starter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SSME is a great performer in I&lt;sub&gt;sp&lt;/sub&gt; (452 seconds in vacuum and 366 seconds at sea-level) and much higher thrust (512 KLbs thrust), but that is literally 20 times the thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the difficulty in air starting (really starting period) an SSME, they changed to the notion of using a &lt;a href=&quot;https://en.wikipedia.org/wiki/J-2X&quot; rel=&quot;nofollow noreferrer&quot;&gt;J-2X&lt;/a&gt; with good I&lt;sub&gt;sp&lt;/sub&gt; (448 s in vacuum) and about half the thrust (294 Klbs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ares-1's payload to LEO was not huge (56 KLbs), compared to, say, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV_Heavy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta-4 Heavy&lt;/a&gt; that uses the &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10&quot; rel=&quot;nofollow noreferrer&quot;&gt;RL10&lt;/a&gt; as an upper stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a 20 fold increase in thrust seems like it would be more of a hindrance than help.  J-2X with only a 10-fold increase in thrust seems like it would be problematic as well.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can throttle all you want, but a 10-fold or a 20-fold thrust increase seems like a big deal to accommodate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that I&lt;sub&gt;sp&lt;/sub&gt; is king in upper stages, which is why the fairly low thrust RL10 is so effective (similar to the discussion in the question &lt;a href=&quot;https://space.stackexchange.com/questions/4070/how-is-an-rd-180-engine-a-reasonable-replacement-for-2-nk-33-aj-26-engines-on-an&quot;&gt;How is an RD-180 engine a reasonable replacement for 2 NK-33/AJ-26 engines on Antares?&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-12-18T22:38:31.963" Title="Was the notion of using an air-started SSME on Ares-1 a reasonable idea?" Tags="|engines|upper-stage|ssme|rl-10|j-2|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4537" PostTypeId="2" ParentId="4531" CreationDate="2014-05-14T17:03:37.250" Score="54" Body="&lt;p&gt;Disclaimer: I worked as an aerospace engineer for 15 years for the USAF. Our organization managed the 53 Federal Stock Group &lt;sup&gt;(&lt;a href=&quot;https://mn.gov/admin/assets/DISP_h2book%5B1%5D_tcm36-281917.pdf&quot; rel=&quot;noreferrer&quot;&gt;1&lt;/a&gt;)&lt;/sup&gt; (among others), which includes Bolts, Screws, etc. By this I mean to suggest I have some (dusty) knowledge of this subject.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;While the quality control is very much a part of the process, as suggested, that alone is not the only difference. How the bolt is produced (the manufacturing process used) is also key.&lt;/p&gt;&#xA;&lt;p&gt;So, for example, it is common to have bolts with &amp;quot;rolled threads&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;Quoting from some industry literature &lt;sup&gt;(&lt;a href=&quot;https://web.archive.org/web/20160617194212/http://horstengineering.com/threadrolling/process/thread-rolling/&quot; rel=&quot;noreferrer&quot;&gt;2&lt;/a&gt;)&lt;/sup&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Often, rolled threads are required by design because of their superior&#xA;tensile, shear, and fatigue strength. Other processes remove material&#xA;to produce the thread form, but thread rolling displaces the material&#xA;with hardened steel dies...&lt;/p&gt;&#xA;&lt;p&gt;The result of moving the material grains (molecules) into the shape of&#xA;the thread rather than weakening it by removing material, is that the&#xA;grains become denser at the critical parts of the thread, especially&#xA;in the root and on the flank below the pitch diameter...&lt;/p&gt;&#xA;&lt;p&gt;                       &lt;img src=&quot;https://i.stack.imgur.com/hJ4z3.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;                                                          &lt;sup&gt;&lt;strong&gt;Figure 1&lt;/strong&gt;: non-rolled thread&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So, looking at the &amp;quot;not rolled&amp;quot; thread (fig. 1), one can see that threads which are cut (for example on a lathe) have exposed grain boundaries. Such a condition is undesirable for certain applications, for example under high impact loads. These areas would be prone to fracture/failure.&lt;/p&gt;&#xA;&lt;p&gt;Compare the above illustration to the &amp;quot;rolled thread&amp;quot; illustration:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;                       &lt;img src=&quot;https://i.stack.imgur.com/te0k4.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;                                                              &lt;sup&gt;&lt;strong&gt;Figure 2&lt;/strong&gt;: rolled thread&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Here (Fig.2), as explained above, one can see that the grain boundaries &lt;em&gt;flow&lt;/em&gt; around the contours of the thread, alleviating the exposed grain boundary condition and thereby producing a superior thread.&lt;/p&gt;&#xA;&lt;p&gt;So, in conclusion, it is both the quality control requirements &lt;em&gt;and&lt;/em&gt; the manufacturing process used to produce the bolt which make it aircraft quality.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://mn.gov/admin/assets/DISP_h2book%5B1%5D_tcm36-281917.pdf&quot; rel=&quot;noreferrer&quot;&gt;FEDERAL SUPPLY CLASSIFICATION Groups and Classes&lt;/a&gt; (PDF)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://web.archive.org/web/20160617194212/http://horstengineering.com/threadrolling/process/thread-rolling/&quot; rel=&quot;noreferrer&quot;&gt;Horst Engineering: Thread Rolling&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="3348" LastEditorUserId="18850" LastEditDate="2022-02-13T12:57:06.387" LastActivityDate="2022-02-13T12:57:06.387" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="4539" PostTypeId="2" ParentId="4531" CreationDate="2014-05-14T23:04:34.847" Score="5" Body="&lt;p&gt;I have little knowledge of the aerospace industry other than that gleaned from growing up in Derby (UK, East Midlands) which is the home of Rolls Royce. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of materials used there are a number of specialised alloys used in aero engines and in particular &lt;a href=&quot;http://en.wikipedia.org/wiki/Nimonic&quot; rel=&quot;noreferrer&quot;&gt;Nimonic&lt;/a&gt; which is used for making bolts for aero engines. Nimonic was developed by &lt;a href=&quot;http://en.wikipedia.org/wiki/Henry_Wiggin&quot; rel=&quot;noreferrer&quot;&gt;Henry Wiggin and Company&lt;/a&gt;, Birmingham UK.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My brother worked for a company in Derby and operated a CNC lathe cutting the bolts (I assume threads, milling etc). The alloy is very expensive so running up a bill on a bad day is too easy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Links&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.rolls-royce.com/civil/index.jsp&quot; rel=&quot;noreferrer&quot;&gt;Rolls Royce Civil Aerospace&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.derbytelegraph.co.uk/Rolls-Royce-reveals-2020-vision/story-20710131-detail/story.html&quot; rel=&quot;noreferrer&quot;&gt;Article from Derby Evening Telegraph&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.flightglobal.com/pdfarchive/view/1951/1951%20-%200148.html&quot; rel=&quot;noreferrer&quot;&gt;1951 Advert for Nimonic&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this is of some interest.&lt;/p&gt;&#xA;" OwnerUserId="2593" LastActivityDate="2014-05-14T23:04:34.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4541" PostTypeId="2" ParentId="4531" CreationDate="2014-05-15T00:33:45.890" Score="21" Body="&lt;p&gt;While not a matter of materials science, my A&amp;amp;P gave me his explanation of the subject when my plane was in for annual:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Mechanic:&lt;/strong&gt; These two screws are identical. This one comes from Home Depot and costs 25 cents. This one is comes from an aviation supplier and is certified for use in your plane; it costs \$2.00 per screw. If I use the Home Depot one and it fails in flight, I'm liable. If I use the \$2.00 one, the manufacturer is.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Me:&lt;/strong&gt; Well, what's different between the two?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Mechanic:&lt;/strong&gt; About $1.75 of liability insurance. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's an exaggeration for sure, but the principle is important: If a screw breaks on your patio deck, there's no presumption of liability or fitness. It's a screw; it broke. It happens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if the screw fails on a plane and you lose an aileron, that's a different story. The lawyers are coming for somebody's head. So while a huge portion of the cost of materials covers the liability of the manufacturer, the insurance company isn't going to grant a liability policy unless the manufacturer can demonstrate that the bolt &lt;em&gt;won't&lt;/em&gt; fail in flight, that literally every reasonable precaution, testing procedure, manufacturing technique, and quality control mechanism known to the industry is being employed to ensure that these bolts are safe. Because if the manufacturer misses even one reasonable precaution, then the insurance company will have to pay out. &lt;/p&gt;&#xA;" OwnerUserId="1257" LastActivityDate="2014-05-15T00:33:45.890" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4542" PostTypeId="2" ParentId="1132" CreationDate="2014-05-15T13:35:20.400" Score="5" Body="&lt;p&gt;Let's set aside, for the moment, field-warping, etc., and consider a hypothetical ultra-powerful propulsion system which has no significant effect on space-time beyond the immediate region of the craft itself. Let's also say that your craft can (somehow) negate the effects of extremely high acceleration. Even with this ultimate magic bullet, your trajectories will still be tracing conic sections. Newton and Kepler still apply, merely corrected by relativistic equations at higher speeds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No matter how fast you go, if you're subluminal, your path is still a curve traced on a gravity well. Kepler and Newton are just providing you with a way to map your journey. Subluminal ultra-high speed will not break the mathematics, but once you get to a substantial percentage of the speed of light, relativistic effects cannot be ignored.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2016-06-12T18:02:14.210" LastActivityDate="2016-06-12T18:02:14.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4543" PostTypeId="1" AcceptedAnswerId="4544" CreationDate="2014-05-15T15:13:35.377" Score="6" ViewCount="1041" Body="&lt;p&gt;Including the suborbital flight of Alan Shepard, what altitudes did the first men in space reach? I am looking for a list of maybe the first 5-10 people who reached orbit, but also the first suborbital flights around that period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An infographic gets bonus points!&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="12102" LastEditDate="2021-09-05T23:17:00.083" LastActivityDate="2021-09-05T23:17:00.083" Title="What altitudes did the first astronauts reach?" Tags="|history|spaceflight-firsts|altitude|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4544" PostTypeId="2" ParentId="4543" CreationDate="2014-05-15T15:31:58.250" Score="9" Body="&lt;p&gt;This really depends on your definition of astronaut. The Google definition is: &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;a person who is trained to travel in a spacecraft.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I'd suggest that the first person ever &lt;em&gt;trained&lt;/em&gt; to travel in a spacecraft never actually flew. Again this depends on your definition of training though I suppose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However I fear this isn't the answer you're looking for. If you're definition is the first man into space then we're talking about Yuri Gagarin, at 324.9 km apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly if you type &quot;yuri gagarin altitude&quot; into Google it tells you his height - either that or he flew a very low spacecraft at some point. Put together the two facts from this answer and what do you have? &lt;a href=&quot;https://www.wolframalpha.com/input/?i=vostok%201%20apogee/%20yuri%20gagarin%20height&quot; rel=&quot;nofollow&quot;&gt;Nonsense!&lt;/a&gt;. :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;The following apogee altitudes are in kilometres. They are ordered by launch date.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Vostok 1    324.9       (Gagarin)&#xA;Mercury 3   187.5       (Shepard, suborbital)&#xA;Mercury 4   190.31      (Grissom, suborbital)&#xA;Vostok  2   244         (Titov)&#xA;Mercury 6   265         (Glenn)&#xA;Mercury 7   260         (Carpenter)&#xA;Vostok  3   218         (Nikolayev)&#xA;Vostok  4   211         (Popovich)&#xA;Mercury 8   283         (Schirra)&#xA;Mercury 9   267         (Cooper)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Enjoy!&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="195" LastEditDate="2016-01-28T17:14:00.327" LastActivityDate="2016-01-28T17:14:00.327" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="4546" PostTypeId="1" AcceptedAnswerId="4548" CreationDate="2014-05-15T18:44:19.777" Score="13" ViewCount="973" Body="&lt;p&gt;How does water spray behave in Low Earth Orbit (LEO), from the side exposed to the Sun to the side in Earth's shadow? Could it be used as means of augmenting atmospheric drag and deorbiting defunct spacecraft and debris from LEO faster than their orbits would naturally decay?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only mention of drag augmentation as means of active debris removal that seem to be under consideration for the moment are the use of gossamer drag augmentation structures and deployable sails, or expanding foams, e.g. as mentioned in ESA's &lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/ARI/ARI%20Study%20Report/ACT-RPT-MAD-ARI-10-6411-Pisa-Active_Removal_of_Space_Debris-Foam.pdf&quot;&gt;Active Removal of Space Debris: Expanding foam application for active debris removal&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The core idea of this method is to increase the area-to-mass ratio of&#xA;  these objects such that  the atmospheric drag can cause their natural&#xA;  re-entry, thus “cleaning up” different regions in the  near-Earth&#xA;  space. The drag augmentation system proposed does not require any&#xA;  docking system and  just an uncontrolled re-entry can follow, thus it&#xA;  seems a short-term application free from the usual  technological&#xA;  issues of these debris removal systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Venerable idea, but what about the other way around? Instead of increasing the area-to-mass ratio of space junk, would it be feasible to increase local atmospheric density by means of spraying water ahead of their orbits, while perhaps at the same time using such spraying as means of maintaining orbit (propulsion for orbital reboost) of the &quot;drag augmentation device&quot;?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-05-15T19:05:11.730" LastActivityDate="2018-04-23T18:57:51.603" Title="Does it snow in LEO?" Tags="|low-earth-orbit|debris|water|atmospheric-drag|drag-augmentation|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4547" PostTypeId="2" ParentId="4546" CreationDate="2014-05-15T23:03:07.933" Score="9" Body="&lt;p&gt;Based on your comment stating you're question is irrespective of if the water is liquid/snow etc. I'd suggest reading about tungsten dust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tungsten dust has been &lt;a href=&quot;http://www.popsci.com/technology/article/2011-04/fighting-orbital-space-debris-cloud-tungsten-dust&quot;&gt;proposed&lt;/a&gt; as a solution to this problem, in fact you can even read the &lt;a href=&quot;http://www.google.co.uk/patents/US8469314&quot;&gt;patent for it&lt;/a&gt;. The general concept is to release a collection of tungsten dust into a specific orbit to more quickly de-orbit craft in that region of space. The downsides are pretty major though, since the dust itself will de-orbit it will potentially pass through other regions of space; it will also cause a significant increase in drag in ALL objects that fly through it so you might end up de-orbiting a spacecraft that shouldn't be de-orbited. On the plus side though, since the particles are very small they have a reasonably low ballistic coefficient so they will only stay in orbit for a short period of time. I remember discussing this with a colleague and for &lt;a href=&quot;http://en.wikipedia.org/wiki/Envisat&quot;&gt;Envisats&lt;/a&gt; orbit the lifetime of the dust was only about 8 years.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-05-15T23:03:07.933" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4548" PostTypeId="2" ParentId="4546" CreationDate="2014-05-16T05:12:11.833" Score="10" Body="&lt;p&gt;&lt;a href=&quot;http://www.nwra.com/resumes/andreas/publications/Icarus_Moon.pdf&quot; rel=&quot;noreferrer&quot;&gt;Edgar Andreas&lt;/a&gt; made a nice chart for water sublimation in a vacuum at various temperatures:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0tbSG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 270 K it looks like a square centimeter of water ice surface sublimates 100 grams per hour. &lt;a href=&quot;http://hypertextbook.com/facts/2001/JudyMoy.shtml&quot; rel=&quot;noreferrer&quot;&gt;A typical snow flake masses 3 milligrams&lt;/a&gt;. Unless the snowflakes were cryogenic, they'd quickly sublimate&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Root-mean-square_speed&quot; rel=&quot;noreferrer&quot;&gt;Root square mean speed&lt;/a&gt; of water molecules at 270 K would be ~.618 km/s (if my arithmetic's right). The water vapor cloud would quickly dissipate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;O2 has a higher molar weight than H20. At 270 K it's root square mean speed would be about .46 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Better yet would be CO2. Root square mean speed at 270 K would be .4 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it seems to me all these would dissipate too quickly to intercept and bring down much orbital debris.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-05-16T05:12:11.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4549" PostTypeId="2" ParentId="2370" CreationDate="2014-05-16T20:12:47.670" Score="10" Body="&lt;p&gt;Several posts have mentioned atmospheric drag. Drag slows the spacecraft. Traveling 8 km/s through the troposphere also induces heat and stress. The spacecraft would need a sturdy structure and thermal protection system or it'd burn up and/or crumple like  beer can.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An important factor would be the flight path angle as the space craft exits the muzzle. If the spacecraft departs at a 0 degree flight path angle (in other words, horizontal), it must travel a long ways through the atmosphere. A huge column of air would be displaced:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/r6cAr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That part of the path traveling through the atmosphere is colored red.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over a trip of around 8000 kilometers the ship endures slowing resistance, heat and pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the flight path angle is close to 90 degrees (in other words, near vertical), the trip through the atmosphere is a lot shorter. The ship endures extreme atmospheric friction for only about 100 kilometers. But then the orbit would be an elongated ellipse that  would quickly return and crash into earth's surface:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Nm0in.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A typical rocket starts with a nearly vertical ascent. As the air grows thinner, it leans more towards the horizontal. It doesn't do the major horizontal burn until the ship is well above most the atmosphere. This isn't an option for space guns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The notion is much more viable for airless worlds like Luna or Ceres.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-05-16T20:12:47.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4550" PostTypeId="2" ParentId="2370" CreationDate="2014-05-17T05:36:39.807" Score="6" Body="&lt;p&gt;Fundamentally, the issues are that a functional spacecraft is only engineered for between 3 G's and 30 G's; typical rockets only to about 100 Gs. (3.5 for the shuttle, 30-60 for ICBMs.) A reasonable launch tube would require an exit velocity in excess of 7.8km/s (the LEO speed). Adding 1 km/s for atmospheric losses, that gives a need for about 8.8km/s; for simplicity, let's round up to 9km/s. Note that a 180 km high suborbital flight was attained by the US Navy HARP project, with a 3.6 km/s launch velocity and near vertical launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;9 km/s, at a reasonably robust 30 G's, is a hair over 30.6 seconds. That requires (using d=0.5AT^2) 137644 m... almost 138 km. (The HARP space gun was 41m long, and launched at 3600m/s, near vertical, and around 18500 G.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Peak electronics safe launch is around 15500 G's (used on artillery shells), but designing spacecraft for that is highly unlikely. At that acceleration, and using the same 9m/s target, it's 274 meters long, and in the barrel for 0.06 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more reasonable benchmark is the 100 G's that is routinely survived in momentary impact events - it's a good benchmark for frame survival of relatively thin-skinned craft. (After all, most cars survive this level relatively intact. It's also the acceleration of the Sprint Interceptor Missile.) At 100 G's, that's 9.2 seconds in the tube, and about 41.5 km long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A human-safe launch is 12 G's... for a target of 9 km/s, it needs to be 345 km long, and is 70.6 seconds in the tube.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As can be seen, the length of the tube is a huge limit. Essentially, one has to launch a small missile - it doesn't need a whole lot of delta-V, just enuough to raise the perigee - and thus the reasonable level is a high acceleration missile. At that it's impractical, but doable. (Designs for this have been worked up by various engineering schools as sample problems.) The problem being that the gas can only accelerate so fast, and to maintain a consistent thrust requires adding additional combustion along the tube.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible now, but impractical. It would be monumental architecture, unusable for humans, but a practical method of delivering certain categories of payload to orbit - bulk fluids, such as fuel, water, and air, and raw materials for construction, such as girders. It would require on-orbit tugs. Any failure has no abort mode, as well, since the payload is unpowered in flight.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_gun&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Space_gun&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/G-force&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/G-force&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-05-17T05:36:39.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4551" PostTypeId="1" AcceptedAnswerId="4552" CreationDate="2014-05-17T10:07:00.053" Score="3" ViewCount="298" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=tlDC_3PF4PQ&quot; rel=&quot;nofollow&quot;&gt;In situ resource utilization of water ice in eternally shadowed craters&lt;/a&gt; on the Moon's poles might provide water for rocket fuel and astronaut survivability cheaper compared to launching water from Earth. Today we have great lunar maps with &lt;a href=&quot;http://lroc.sese.asu.edu/images/gigapan&quot; rel=&quot;nofollow&quot;&gt;2 meter resolution of the entire north pole&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Can we today specifically pinpoint one or a few ideal locations for the first polar ISRU landings?&lt;/em&gt; Specific as in a location with a name and an arrow to it on a map. Or is even more orbital reconnaissance needed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine a landing of stationary solar panels where insolation is constant, to have not only constant power but also (near) constant thermal conditions for convenience. And then a rover which rolls out a power cable from that station as it travels into a nearby eternal (billion years old) shadow where water could've been &quot;trapped&quot;. I think that a &quot;lunar hopper&quot; would have a power supply problem in the eternal darkness which might be &lt;a href=&quot;http://www.newscientist.com/article/dn17810-moon-is-coldest-known-place-in-the-solar-system.html#.U3cwvvnHnAY&quot; rel=&quot;nofollow&quot;&gt;the coldest place in the Solar system&lt;/a&gt;. Nuclear RTGs are expensive and would kind of symbolically defeat the concept of cheap lunar solar-water-ISRU.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I gather that precision landing on the Moon is not much of a problem nowadays. Steep slopes should be avoided though, and since a sharp contrast between eternal sun and eternal shadow is sought after in order to shorten rover travel distance, this might be the dominating geographic restriction. (Or maybe the &quot;rover&quot; could be designed as a cat yarn ball and be kicked down off the brink). Use your imagination and reasoning to come up with attractive geographical conditions which could be pinpointed!&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-05-17T10:15:37.110" LastActivityDate="2015-12-29T01:31:34.797" Title="Where should a lunar polar ISRU mission land?" Tags="|the-moon|lunar-landing|isru|pole|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4552" PostTypeId="2" ParentId="4551" CreationDate="2014-05-17T15:00:52.843" Score="3" Body="&lt;p&gt;Kamaz of &lt;a href=&quot;http://moonmapper.wordpress.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Future Moon&lt;/a&gt; did this graphic of Whipple Crater:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/R09Ey.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The deepest red near the top is thought to be a peak of nearly constant illumination. The deepest blue is a cold trap that might have rich volatile deposits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As can be seen by the spacing of the isolines, the crater floor's flat. Unfortunately the crater rim that enjoys near constant illumination is steep.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rover traveling from the illuminated rim to the shaded floor would need to cross about 8 km of terrain, some of it pretty rough.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-05-17T15:00:52.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4553" PostTypeId="2" ParentId="604" CreationDate="2014-05-18T13:50:45.220" Score="3" Body="&lt;p&gt;Another attempt to figure out for vertebrates is to be conducted on the &lt;a href=&quot;http://en.samspace.ru/products/satellites_of_scientific_purpose/ka_foton_m_4/&quot; rel=&quot;nofollow&quot;&gt;Foton-M4 spacrecraft&lt;/a&gt; this summer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check out the google translation of the flight experiment program:&#xA;&lt;a href=&quot;http://biosputnik.imbp.ru/science_f.html&quot; rel=&quot;nofollow&quot;&gt;http://biosputnik.imbp.ru/science_f.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;Gecko-F4&lt;/em&gt; experiment states as its first objectives:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;to create the conditions for sexual behavior, copulation and&#xA;  breeding of geckos in the orbital experiment;    &lt;/li&gt;&#xA;  &lt;li&gt;to record on camera the sexual behavior of Phelsuma ornata and possible oviposition, and&#xA;  to maximize the probability of survival for eggs that can be laid&#xA;  during the experiment;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Let's wait just a couple of months.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2014-05-19T09:07:54.603" LastActivityDate="2014-05-19T09:07:54.603" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4557" PostTypeId="1" AcceptedAnswerId="4601" CreationDate="2014-05-19T02:26:18.027" Score="4" ViewCount="967" Body="&lt;p&gt;This is a somewhat similar question to &lt;a href=&quot;https://space.stackexchange.com/q/3111/49&quot;&gt;What risk management options are available for Mars orbiters and rovers to mitigate threat of impacting with Siding Spring cometary debris?&lt;/a&gt;, only this time much closer both in time as well as distance, and potentially affecting safety of many more satellites and more importantly, one manned space station - the ISS with its current &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_40&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expedition 39/40&lt;/a&gt; crew of three.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/k8tCw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Map of projected peak viewability for 2014 May Camelopardalids meteor shower. (Credit and source: &lt;a href=&quot;http://www.nasa.gov/topics/solarsystem/features/watchtheskies/may-camelopardalids.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA/MSFC/Danielle Moser&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you probably know, on the night of 23rd to 24th of May 2014 (06:00 - 08:00 a.m. UTC on May 24 2014), the Earth will be in the path of the predicted cometary debris from comet &lt;a href=&quot;https://en.wikipedia.org/wiki/209P/LINEAR&quot; rel=&quot;nofollow noreferrer&quot;&gt;209P/LINEAR&lt;/a&gt;, called Camelopardalids because they'll radiate from the direction of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Camelopardalis&quot; rel=&quot;nofollow noreferrer&quot;&gt;Camelopardalis&lt;/a&gt; constellation (camel + leopard = a giraffe, or at least that's what ancient Greeks used to believe). Actually, that time is it predicted peak, we don't really know what exactly to expect, how wide the debris field is, how large some of its chunks, how numerous, and &lt;a href=&quot;http://www.universetoday.com/111474/may-meteor-storm-alert-all-eyes-on-the-sky/&quot; rel=&quot;nofollow noreferrer&quot;&gt;some have previously speculated it might be a proper meteor storm&lt;/a&gt;, not shower. So potentially something fascinating to see on your northern hemisphere night skies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Ib48c.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The speckled area shows where on the Moon impacts from meteoroids from the new Camelopardalid meteor shower could occur.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;(Source: &lt;a href=&quot;http://www.universetoday.com/112057/potential-weekend-meteor-shower-will-pelt-the-moon-too/&quot; rel=&quot;nofollow noreferrer&quot;&gt;UniverseToday.com: Potential Weekend Meteor Shower Will Pelt the Moon Too!&lt;/a&gt;, Credit: Bill Cooke)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But my question isn't about aesthetics, how visually pleasing it might be for us on the Earth's surface and protected by the complete thickness of its atmosphere. It is about safety of astronauts and cosmonauts aboard the International Space Station (ISS) and all the hardware in Earth's or &lt;a href=&quot;http://www.universetoday.com/112057/potential-weekend-meteor-shower-will-pelt-the-moon-too/&quot; rel=&quot;nofollow noreferrer&quot;&gt;even Moon's orbit&lt;/a&gt;, not being offered any protection from our atmospheric blanket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;More specifically, what procedures will be followed onboard the ISS during the meteor shower peak as their orbit puts them in the path of this debris field? How are NASA and Roscosmos (ESA and JAXA won't have any astronaut onboard ISS during that time) preparing to mitigate this threat, how significant do they assess it might be, i.e. what risk management options do they have in place? Will the ISS crew &lt;em&gt;weather the storm&lt;/em&gt; in Soyuz spacecrafts ready to abandon the ship? Will the ISS rotate its solar panel arrays to reduce exposed surface area? Or is the threat not considered significant enough and they'll go about their business as per usual?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, which Earth orbits might be at risk the most, and have any satellite operators already announced activation of any risk management procedures, like perhaps folding or rotating their solar panels and antennae to reduce the surface area potentially exposed to increased risk of collision? The question is about this specific 2014 May Camelopardalids meteor shower event.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-05-22T16:14:33.623" Title="Do meteor showers from 209P/LINEAR cometary debris pose a threat to the ISS and other Earth-orbiting satellites?" Tags="|iss|artificial-satellite|safety|impact|risk-management|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4560" PostTypeId="1" AcceptedAnswerId="4569" CreationDate="2014-05-19T18:38:33.253" Score="10" ViewCount="1597" Body="&lt;p&gt;Nuclear thermal rockets may have great benefits for Solar system exploration. But one consideration is that the launch of a nuclear engine from Earth's surface, as the payload on a chemical rocket, might fail. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How should the danger of such a scenario be assessed? Would the heavy metal radiactive engine piece fall almost intact on one place? Or would it create a radiactive cloud which dangerously contaminates a wide area? Could a failure for example be compared with one of many hundreds nuclear bomb tests in the atmosphere?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What saftey measures could lower this risk? Maybe an abort system like that used on the crewed Apollo missions, which could separate the payload and land it in a controlled way.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Should they maybe be launched from Western Sahara where their trajectories cover a uninhabited area (&lt;a href=&quot;http://looklex.com/e.o/sahara.htm&quot;&gt;4 million people live in Sahara&lt;/a&gt;, most of which in the far eastern parts along the Nile river and the rest mostly in a few concentrated oasis) and where the polluted sites could relatively easily be cleaned by scooping up contaminated sand.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For example, assume a 20 ton nuclear thermal rocket engine component was the payload of one of these two recently failed Proton launches, what would the environmental impact be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;9 minutes after launch:&#xA;&lt;a href=&quot;http://www.spaceflight101.com/ekspress-am4r---proton-launch-updates.html&quot;&gt;http://www.spaceflight101.com/ekspress-am4r---proton-launch-updates.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few seconds after launch:&#xA;&lt;a href=&quot;http://www.spaceflight101.com/proton-m-block-dm-03-glonass-launch-2013.html&quot;&gt;http://www.spaceflight101.com/proton-m-block-dm-03-glonass-launch-2013.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="4660" LastEditDate="2015-01-09T05:15:23.920" LastActivityDate="2017-02-08T23:50:28.433" Title="Dangers of launching a nuclear thermal rocket" Tags="|engine-design|environment|nuclear|risk-management|ntr|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4561" PostTypeId="1" AcceptedAnswerId="4563" CreationDate="2014-05-19T20:08:08.213" Score="27" ViewCount="10077" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Buran_%28spacecraft%29&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Buran was the first space shuttle to perform an unmanned flight, including landing in fully automatic mode. ... &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same article writes further &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The automated landing took place on a runway at Baikonur Cosmodrome where, despite a lateral wind speed of 61.2 kilometres per hour (38.0 mph), it landed only 3 metres (9.8 ft) laterally and 10 metres (33 ft) longitudinally from the target mark. Specifically, as Buran approached Baikonur Cosmodrome and started landing, spacecraft sensors detected the strong crosswind and &quot;the robotic system sent the huge machine for another rectangular traffic pattern approach, successfully landing the spacecraft on a second try.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle&quot;&gt;NASA Space Shuttle&lt;/a&gt; (I use the term to refer to the assembly family rather than a tail) headed either to the Shuttle Landing Facility at Kennedy Space Center (TTS), or multiple runways at Edwards Air Force Base (EDW). In the event it undershot it could splash into the Atlantic. If it overshot, it could splash into the Pacific. A similar argument may be applied to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Buran_%28spacecraft%29&quot;&gt;Buran&lt;/a&gt; landing at Baikonur - except that Baikonur is land-locked first...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently the Shuttle landing strip at both places was specially &lt;strike&gt;reinforced&lt;/strike&gt; designed for a high-speed landing. The wikipedia article on the &lt;a href=&quot;http://en.wikipedia.org/wiki/Shuttle_Landing_Facility&quot;&gt;Shuttle Landing Facility&lt;/a&gt; (TTS) writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The runway surface consists of an extremely high-friction concrete strip designed to maximize the braking ability of the Space Shuttle at its high landing speed, with a paving thickness of 40.6 cm (16.0 in) at the center. It uses a grooved design to provide drainage, and further increase the coefficient of friction. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm unsure about a similar provision being made for the Buran. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite all the high-technology precision involved - having an extremely limited set of runways to head for seems contrary. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could the Space Shuttle have landed on any long runway other than those specially prepared at TTS, and EDW?&lt;/li&gt;&#xA;&lt;li&gt;Was/Were any such long runways along it's flight-path similarly prepared for that purpose, and/or designated port-of-refuge for the Space Shuttle?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-05-20T03:48:08.930" LastActivityDate="2017-04-05T22:36:35.223" Title="Could the Space Shuttle have landed on any long runway other than those specially reinforced at Kennedy (TTS), and Edwards (EDW)?" Tags="|space-shuttle|landing|kennedy-space-center|" AnswerCount="11" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4563" PostTypeId="2" ParentId="4561" CreationDate="2014-05-19T21:16:17.130" Score="35" Body="&lt;p&gt;STS-3 landed at White Sands runway 17.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to Kennedy, Edwards, and White Sands, several sites were selected as targets for a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Transoceanic_abort_landing&quot; rel=&quot;noreferrer&quot;&gt;transoceanic abort landing (TAL)&lt;/a&gt;, but no launch ever had to perform such an abort. If something went bad during launch before the vehicle had enough energy to get to a TAL site, a mission might have had to do something even more drastic such as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Return_to_launch_site_.28RTLS.29&quot; rel=&quot;noreferrer&quot;&gt;return to launch site (RTLS) abort&lt;/a&gt; or an &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Post-Challenger_abort_enhancements&quot; rel=&quot;noreferrer&quot;&gt;east coast abort landing (ECAL)&lt;/a&gt;. Any airport with a long enough runway would do. There were several airports designated as east coast abort landing sites, ranging from Myrtle Beach to airports in Newfoundland and Labrador.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="415" LastEditDate="2016-11-16T12:45:59.880" LastActivityDate="2016-11-16T12:45:59.880" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4564" PostTypeId="1" AcceptedAnswerId="4565" CreationDate="2014-05-19T21:54:16.157" Score="8" ViewCount="579" Body="&lt;p&gt;A recent discussion on the ISS has raised some questions in my mind about &lt;em&gt;Zvezda&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4525/can-the-us-part-of-iss-survive-independent-of-the-russian&quot;&gt;Can the US Part of ISS survive independent of the Russian?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does &lt;em&gt;Zvezda&lt;/em&gt; alone lack from being a capable space station? Are there any existing Russian modules or modules in development that could be attached to enable it to be a capable space station if it is not already? If Russia detached &lt;em&gt;Zvezda&lt;/em&gt; from the ISS today, approximately how quickly could they have a functioning independent space station?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-05-20T13:28:02.400" Title="What does Zvezda alone lack from being a capable space station?" Tags="|iss|russia|zvezda|russian-segment|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4565" PostTypeId="2" ParentId="4564" CreationDate="2014-05-19T22:05:45.913" Score="9" Body="&lt;p&gt;Zvezda has crew accommodations, orbital control, docking ports to expand (forward end) and accept refueling (aft end). The docking compartment can be used as an airlock for spacewalks. It is far more complete on its own than any other module in the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fundamentally, Zvezda is Mir-2. It is the actual Mir-2 module.  Had ISS not occurred, the plan was to use this module to start a new station complex. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like Mir, Zvezda doesn't have Control Moment Gyroscopes (they arrived on Kvant-1). Some work needs to be done to improve communication abilities: Luch-5 satellite relay infrastructure could be utilized more heavily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where Zvezda lacks compared to Mir, is in the forward docking compartment, it only has 3 (or 4 if you count the one attached to Zvezda itself).  Forward, zenith and nadir.  No side ports. This is only an issue as a standalone, as the UM for the future Russian segment is a docking module with 6 ports.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is not a lot of room in just the one module, nor a lot of power available, which is why Mir had the various add on modules.  But by itself it could probably do a good job as a space station (as many Salyut stations and Mir did before it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As it happens the Russian plan is to use the Nauka MLM module, due to be launched and docked to the earth facing (nadir) port on Zvezda, along with the UM docking ball (6 ports, so lots of options), and the Solar Power platform to form the core of a new station.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-05-20T13:28:02.400" LastActivityDate="2014-05-20T13:28:02.400" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4566" PostTypeId="1" AcceptedAnswerId="4568" CreationDate="2014-05-19T23:51:57.233" Score="4" ViewCount="298" Body="&lt;p&gt;Since the retirement of the Space Shuttles, all crew transfers from and to the ISS are performed using the Russian Soyuz crafts launched with the launch system of the same name.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should Russia withdraw from the ISS program, which other launch systems and crafts exist or are in development which could fulfill this role?&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2020-08-02T02:58:15.420" Title="What options exist for crew transfers to the ISS?" Tags="|iss|crewed-spaceflight|soyuz-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4567" PostTypeId="2" ParentId="4561" CreationDate="2014-05-20T00:11:20.870" Score="27" Body="&lt;p&gt;The Space Shuttle didn't need a &quot;specially reinforced&quot; runway. If a runway could accept a 747 it could probably handle the shuttle. The shuttle's maximum landing weight is in the range of a large airliner (less than half that of a 747), so load is not a problem. The major issue would probably be length... the shuttle's high landing speed meant it needed a lot of room to stop, so it would have been limited to the longer runways of major airports - which would almost certainly be strong enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably the biggest issue with landing a shuttle at an alternate airport would be traffic. A high-speed glider on an unscheduled approach, committed to landing on its first try would present some serious air traffic control problems at any major airport. Not to mention the hazardous (flammable/explosive/toxic) chemicals on board. If an airport was specifically &quot;prepared&quot; to be a shuttle alternate, it wouldn't be about physical/structural preparations, it would be about closing it and clearing it of all air traffic and people for a suitable window of time in case the shuttle needed to land there.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2014-05-20T00:21:33.230" LastActivityDate="2014-05-20T00:21:33.230" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="4568" PostTypeId="2" ParentId="4566" CreationDate="2014-05-20T00:16:33.893" Score="4" Body="&lt;p&gt;NASA has had a series of development projects over the decades.&lt;/p&gt;&#xA;&lt;p&gt;Most recently the &lt;a href=&quot;http://www.nasa.gov/offices/c3po/home/cots_project.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;COTS&lt;/a&gt; (Commercial Orbital Transportation Service) mainly for developing cargo capacity had a D variant that was never really excersized.&lt;/p&gt;&#xA;&lt;p&gt;The manned component morphed into CCDev, &lt;a href=&quot;http://en.wikipedia.org/wiki/Commercial_Crew_Development&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCiCAP&lt;/a&gt;, and CCtCAP which has been working with SpaceX (Dragon/DragonRider), Sierra Nevada Corp (SNC Dream Chaser) and Boeing (CST-100).&lt;/p&gt;&#xA;&lt;p&gt;Dragon and CST-100 are capsules, and Dream Chaser is a lifting body designed for landing on a runway.  Dragon intends to land under power on land (using the unused launch abort SuperDraco thrusters).  CST-100 plans to land on airbags on land.&lt;/p&gt;&#xA;&lt;p&gt;All three are making progress along their milestones, but all are several years out from regular manned launches.  (2020 update: 2020 was the correct year for SpaceX, CST-100 maybe).&lt;/p&gt;&#xA;&lt;p&gt;In some ways SpaceX may be closest as they have launched and re-entered Dragon at least 5 times unmanned.  (2020 update: 20+ times now)&lt;/p&gt;&#xA;&lt;p&gt;SNC has done an unmanned drop test to practice landing.  (Had an issue with one of the landing gear not deploying so it flipped, but met rest of milestones, and the landing gear was not flight equipment on the test).&lt;/p&gt;&#xA;&lt;p&gt;CST-100 has done a couple of parachute drop tests to test the airbag landing system they intend to use for landing on land.&lt;/p&gt;&#xA;&lt;p&gt;SNC and Boeing plan to launch on an Atlas-V with unspecified upgrades for man rating. which is interestingly in question now, with the Ukraine/Russia/RD-180 issue.&lt;/p&gt;&#xA;&lt;p&gt;Dragon of course is flying on a Falcon 9 booster, 4 flights of the F9 1.1 variant at time of this writing, (Update 2020: almost 90 flights) and the base capsule has proven its chops on reentry in 5 missions so far (Update 2020: 20+ missions). They have re-tested their parachute deploy system to validate performance during an abort case (Needs much faster deployment in abort case). They have two launch abort tests due in 2014 (pad abort, and Max-Q abort).&lt;/p&gt;&#xA;&lt;p&gt;Other than Soyuz, only manned spacecraft in active use would then be Shenzou but that seems even less likely.  It is unclear if they have a compatible docking system.&lt;/p&gt;&#xA;&lt;p&gt;The actual docking system for the proposed new vehicles is due to be launched on a Cargo Dragon in 2015 for installation on the forward facing PMA-2 to convert it to a NDS (NASA Docking System) format. Current cargo craft (JAXA HTV, SpaceX Dragon, and Orbital Cygnus) berth to a CBM, which requires on station intervention to berth and unberth. Docking to a PMA (via NDS/LIDS system) is meant to be possible without people on station.&lt;/p&gt;&#xA;&lt;p&gt;Updated 2020:&#xA;Since this answer was originally written, many things have changed.  Dream Chaser has dropped out of the race.&lt;/p&gt;&#xA;&lt;p&gt;The Dragon V2/Crew Dragon flew a demo mission unmanned, a full abort, and a 2 person crew to the ISS.&lt;/p&gt;&#xA;&lt;p&gt;The IDA's were delivered and installed and the PMA modules moved as needed.&lt;/p&gt;&#xA;&lt;p&gt;NASA has agreed to allow the use of previously flown boosters and capsules on future crew missions.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2020-08-02T02:58:15.420" LastActivityDate="2020-08-02T02:58:15.420" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="4569" PostTypeId="2" ParentId="4560" CreationDate="2014-05-20T03:03:58.200" Score="14" Body="&lt;p&gt;It's&amp;nbsp;not dangerous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core would never be operated on Earth, and so would not become radioactive like you're thinking.  A nuclear reactor on Earth that has been in operation is extremely radioactive due to the fission products, but the original fuel was not.  The unburned U-235 fuel has a half-life of 700 million years, which means that its radioactivity is extremely low.  If the launch vehicle blew up and somehow dispersed the material, the effect would be minimal.  Even that is unlikely, since the reactor would be designed to contain the material in the event of a launch failure.  The force of a catastrophic launch failure explosion on the payload is not that great.  As an example, the entire shuttle cabin survived the Challenger explosion, and some of the astronauts remained conscious immediately after the explosion (three emergency air packs were manually activated, though one that was found was not).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main issue would probably be one of proliferation.  You'd want to make sure that you picked up the bigger pieces or the intact reactor so that no one could make a bomb out of it.  Though making a bomb from the material would be hard, since reactor fuel is not enriched nearly as much as bomb fuel needs to be.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="6471" LastEditDate="2017-02-08T23:50:28.433" LastActivityDate="2017-02-08T23:50:28.433" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4570" PostTypeId="2" ParentId="4561" CreationDate="2014-05-20T05:46:20.600" Score="6" Body="&lt;p&gt;Columbia actually landed at White Sands Space Harbor in 1982.  See &lt;a href=&quot;http://www.airfields-freeman.com/NM/WhiteSands_NM_82_Shuttle.jpg&quot; rel=&quot;noreferrer&quot;&gt;http://www.airfields-freeman.com/NM/WhiteSands_NM_82_Shuttle.jpg&lt;/a&gt; for a picture of the landing.&lt;/p&gt;&#xA;" OwnerUserId="3380" LastActivityDate="2014-05-20T05:46:20.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4571" PostTypeId="1" AcceptedAnswerId="4572" CreationDate="2014-05-20T08:23:23.843" Score="4" ViewCount="813" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.youtube.com/watch?v=P9M__yYbsZ4&quot; rel=&quot;nofollow&quot;&gt;this YouTube video which starts with a 5 minutes summary&lt;/a&gt; the Liquid Fluoride Thorium Reactor is an extremely safe alternative to the current ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not only Thorium is abundant on Earth, but this technology cannot be used to build weapons, and they automatically shut down in an emergency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would this be useful to power rocket engines or at least to generate power to them?&lt;/p&gt;&#xA;" OwnerUserId="3336" LastActivityDate="2017-02-21T19:19:13.473" Title="Nuclear powered engines with LFTR technology?" Tags="|energy|nuclear|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4572" PostTypeId="2" ParentId="4571" CreationDate="2014-05-20T08:50:41.210" Score="4" Body="&lt;p&gt;Any form of power generation &lt;em&gt;could&lt;/em&gt; be used in space. It's a question of whether it's financially and practically feasible. From a spacecraft perspective mass is a huge contributor to cost - typical estimates are $10,000/kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now from what I've read on Wikipedia about &lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reactor&quot; rel=&quot;nofollow&quot;&gt;LFTR&lt;/a&gt;, they can produce a whole lot of power in &lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reactor#Hybrid_.22one_and_a_half_fluid.22_reactor&quot; rel=&quot;nofollow&quot;&gt;this table&lt;/a&gt; it seems the most efficient system produces 1000-MW(e) from 700kg of fuel. That lasts approximately 12.5 years according to the &lt;a href=&quot;http://www.energyfromthorium.com/pdf/ORNL-4541.pdf&quot; rel=&quot;nofollow&quot;&gt;reference&lt;/a&gt;. Lets assume that that time span is acceptable for our mission. However we probably don't need 1000MW - or anything close to that; depending on the payload you're designing for you power requirements might be tiny or huge but 1GW is larger than I recollect hearing (maybe look into &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunokhod_programme#Rover_description&quot; rel=&quot;nofollow&quot;&gt;Lunokhod&lt;/a&gt; since the rovers are RTG powered).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question then becomes scale. Can you effectively scale down your 1GW design to 1MW? That might be more in the range that would be useful for spacecraft. You probably can't assume it's simply 1/1000th of the mass since smaller engines typically have larger dry:wet mass ratios. We haven't actually covered dry system mass yet either. I can't find any estimates for the dry system mass (dry mass is total mass less fissile material mass) but typically with anything nuclear related it can be many thousands of time the fissile mass. So a 700kg fissile mass may require a system mass of hundreds of tonnes, or more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However that all being said, if there is the need for a huge, high power spacecraft like those seen in sci-fi films it may be a reasonable alternative to solar power especially for interstellar travel - at pluto you're spacecraft would require &lt;a href=&quot;http://www.wolframalpha.com/input/?i=1,000,000,000W%20/%20solar%20flux%20at%20pluto&quot; rel=&quot;nofollow&quot;&gt;truly gigantic solar arrays&lt;/a&gt; to match power with this system.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-05-20T08:50:41.210" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4573" PostTypeId="2" ParentId="4561" CreationDate="2014-05-20T09:37:59.173" Score="13" Body="&lt;p&gt;In France, a French Air Force base with a 5 km long runaway is approved for Transoceanic Abort Landing (TAL) but was never used.&#xA;The runaway is the longuest one in europe and also used by French Army contractor as a test facility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source : &lt;a href=&quot;http://en.wikipedia.org/wiki/Istres-Le_Tub%C3%A9_Air_Base&quot;&gt;Wikipedia&lt;/a&gt; and being French&lt;/p&gt;&#xA;" OwnerUserId="3381" LastActivityDate="2014-05-20T09:37:59.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4574" PostTypeId="2" ParentId="4561" CreationDate="2014-05-20T10:27:02.713" Score="9" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/RAF_Fairford&quot;&gt;RAF Fairford&lt;/a&gt; was the only TransOceanic Abort Landing site in the UK. It was never used to land a shuttle, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I'm aware the American portion of the base is being shutdown, so this runway won't be available in the future.&lt;/p&gt;&#xA;" OwnerUserId="3382" LastActivityDate="2014-05-20T10:27:02.713" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4575" PostTypeId="2" ParentId="4513" CreationDate="2014-05-20T10:32:46.103" Score="4" Body="&lt;p&gt;Japanese rice fish, Oryzias latipes, spent 15 days in 1994 abord the Colombia Shuttle where they reproduced: &lt;a href=&quot;http://cosmo.ric.u-tokyo.ac.jp/SPACEMEDAKA/IML2/e/text/textcontents_E.html&quot; rel=&quot;nofollow&quot;&gt;http://cosmo.ric.u-tokyo.ac.jp/SPACEMEDAKA/IML2/e/text/textcontents_E.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same species where brought aboard the ISS in 2012, &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/aquatic.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/mission_pages/station/research/news/aquatic.html&lt;/a&gt;, with facilities for breeding - though I havn't found the results of that experiment.&lt;/p&gt;&#xA;" OwnerUserId="52" LastActivityDate="2014-05-20T10:32:46.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4576" PostTypeId="1" CreationDate="2014-05-20T14:27:19.053" Score="8" ViewCount="992" Body="&lt;p&gt;There was a lot of talk about it back in '06 when the news came out, and how it could reach a specific impulse approaching 20,000, but I haven't heard any updates on it and the ESA link doesn't work.  Anybody know if it's still being worked on and if it's something viable for in-use production in the near-future?&lt;/p&gt;&#xA;" OwnerUserId="3296" LastEditorUserId="8157" LastEditDate="2015-02-11T11:27:57.953" LastActivityDate="2015-09-04T17:38:44.517" Title="Whats the status of the Dual Stage 4 Grid Ionic Thruster?" Tags="|propulsion|advanced-propulsion|ion-thruster|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4577" PostTypeId="1" CreationDate="2014-05-20T14:51:51.573" Score="8" ViewCount="147" Body="&lt;p&gt;I have been doing a bit of light reading on space telescopes and have been amazed by the reported sensitivity of some of the instruments. In particular, the Laser Interferometer Space Antenna (LISA) which is said to be able to detect a relative change in displacements of 20 picometres over a distance of 1 million km. I'm keen to understand what is the technology/underlying physics behind detectors such as this? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that this is a very broad question as detectors are chasing many different things but a general overview will suffice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If possible, specifically, what gives rise to the phenomenal resolution of LISA?&lt;/p&gt;&#xA;" OwnerUserId="3384" LastEditorUserId="3384" LastEditDate="2014-12-22T15:30:29.360" LastActivityDate="2014-12-22T15:30:29.360" Title="Sensitivity of Space Telescopes and Instruments" Tags="|gravity|instrument|measurement|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4578" PostTypeId="2" ParentId="4561" CreationDate="2014-05-20T16:13:09.147" Score="1" Body="&lt;p&gt;I know that there were multiple alternate landing sites approved, even relatively minor locations. In particular, I grew up in the small town which hosted Sewart AFB (Smyrna, TN), and a big deal was made about the fact that we were one of the 'emergency landing sites' for the shuttle.&lt;/p&gt;&#xA;" OwnerUserId="3385" LastActivityDate="2014-05-20T16:13:09.147" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4579" PostTypeId="2" ParentId="4561" CreationDate="2014-05-20T18:06:56.117" Score="9" Body="&lt;p&gt;Yes, there were several designated &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_space_shuttle_landing_sites&quot;&gt;landing sites and abort landing sites&lt;/a&gt; both inside and outside the USA. Of these, only Kennedy, Edwards and White Sands were ever used (White Sands only once).&lt;/p&gt;&#xA;" OwnerUserId="2784" LastActivityDate="2014-05-20T18:06:56.117" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4580" PostTypeId="1" AcceptedAnswerId="4582" CreationDate="2014-05-20T19:21:15.807" Score="5" ViewCount="304" Body="&lt;p&gt;There is &lt;a href=&quot;https://space.stackexchange.com/questions/4561/could-the-space-shuttle-have-landed-on-any-long-runway-other-than-those-speciall&quot;&gt;this question&lt;/a&gt; that is very interesting. But it made me wonder about upcoming systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming it gets finished, and &lt;a href=&quot;http://nasajsc.force.com/track/apex/sbm__sbx_emailviewonline?id=701d0000000yUI2AAM&amp;amp;mt=4200533936655&amp;amp;cmid=00vd000000IGfIKAA1&quot; rel=&quot;nofollow noreferrer&quot;&gt;this blog&lt;/a&gt; makes it seem more real.. Where will it take off from and land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[ &lt;/p&gt;&#xA;" OwnerUserId="3387" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-05-20T21:37:00.907" Title="What will be the launch and landing sites for Orion?" Tags="|launch-site|landing|orion-spacecraft|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4581" PostTypeId="2" ParentId="4560" CreationDate="2014-05-20T19:52:47.300" Score="-4" Body="&lt;p&gt;I believe that any spacecraft that will be beyond the use of solar power, will, for now, use nuclear technology. They use a form of &lt;a href=&quot;http://en.wikipedia.org/wiki/Plutonium-238&quot; rel=&quot;nofollow&quot;&gt;plutonium (238)&lt;/a&gt;, that can actually be hand held. This isotope has a span of 80+ years, before it decays. Also, it decays to &lt;a href=&quot;http://en.wikipedia.org/wiki/Uranium-234&quot; rel=&quot;nofollow&quot;&gt;Uranium-234&lt;/a&gt;, not 235, and then to Lead-206. According to the wiki articles,there is only enough Pu238 to last for missions planned to 2022. We'll either have to find more, or, we won't be sending spacecraft beyond Mars,for a while.&lt;/p&gt;&#xA;" OwnerUserId="3387" LastActivityDate="2014-05-20T19:52:47.300" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4582" PostTypeId="2" ParentId="4580" CreationDate="2014-05-20T19:54:58.830" Score="4" Body="&lt;p&gt;Launch will only be from LC-39B at Cape Caneveral, (except the very first demo flight on a Delta 4 Heavy, which will launch from LC-37 at the Cape) one of the two former Shuttle pads.  LC-39A will be turned over to SpaceX to use for their launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing will be in the ocean and only the Orion capsule itself will be recovered.  Not sure if Pacific or Atlantic, probably will depend on the mission and what is easiest to access from the incoming path.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-05-20T21:37:00.907" LastActivityDate="2014-05-20T21:37:00.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4583" PostTypeId="1" AcceptedAnswerId="4584" CreationDate="2014-05-20T23:42:05.490" Score="4" ViewCount="2987" Body="&lt;p&gt;I think the ultimate souvenir would be not a moon rock or a martian, but space &quot;nothing&quot;, but I wonder if this is physically possible... Can an astronaut go out to space with an open container, close and seal it and bring it back to Earth? Is there a container that could withstand the pressure of Earth's atmosphere upon reentry? If this is possible, what does my container of space &quot;nothing&quot; truly consist of?&lt;/p&gt;&#xA;" OwnerUserId="3388" LastActivityDate="2022-04-16T17:59:05.640" Title="Is it possible to bring &quot;nothing&quot; back from space?" Tags="|astronauts|atmosphere|vacuum|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4584" PostTypeId="2" ParentId="4583" CreationDate="2014-05-21T03:25:00.170" Score="14" Body="&lt;p&gt;Space is not quite as empty as you might think, and your average jam jar is easily strong enough to hold atmospheric pressure against an internal vacuum. More to the point, there wouldn't be any need to go into space to create a jar full of nothing. Current technology can easily create a vacuum that would compare with that of &quot;outer space&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for what your container will contain, it depends on your point of view. In everyday terms, there is nothing, except perhaps a few stray particles bouncing around. If you agree with the relevant underlying theory, your container is full of quantum foam. But then, the quantum foam is everywhere, and the container just happens to be empty of anything else.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-05-21T03:25:00.170" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4585" PostTypeId="2" ParentId="4577" CreationDate="2014-05-21T07:46:49.823" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://lisa.nasa.gov/Documentation/LISA-PRJ-RP-0001.pdf&quot; rel=&quot;nofollow&quot;&gt;LISA&lt;/a&gt; (long PDF) uses &lt;a href=&quot;http://en.wikipedia.org/wiki/Interferometry&quot; rel=&quot;nofollow&quot;&gt;interferometry&lt;/a&gt;. This is a method that allows very accurate measurement of the difference between two lengths.&lt;br&gt;&#xA;Basically, a laser beam is split. Each half of the beam travels a different path. A small difference in path lengths causes a phase difference between the beams. Both beams are combined in a heterodyne detector, which produces an output signal proportional to the difference in either frequency or phase of the two beams (I'm not sure which is used in LISA yet).&lt;br&gt;&#xA;The laser produces a frequency on the order of $10^{14} Hz$, a frequency difference of 0.1 Hz is easily measured and yields an accuracy of $10^{-15}$.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://list.caltech.edu/lib/exe/fetch.php?media=documents%3atechnology%3alisa_whitepaper.pdf&quot; rel=&quot;nofollow&quot;&gt;Paper that calculates the accuracy of this method.&lt;/a&gt; (long PDF)&lt;br&gt;&#xA;&lt;a href=&quot;http://lisa.gsfc.nasa.gov/Documentation/sts_1.04.pdf&quot; rel=&quot;nofollow&quot;&gt;Report on the underlying technology of LISA&lt;/a&gt; (even longer PDF)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: my initial answer was incorrect. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-05-22T18:15:34.763" LastActivityDate="2014-05-22T18:15:34.763" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4586" PostTypeId="1" AcceptedAnswerId="4594" CreationDate="2014-05-21T08:10:26.143" Score="16" ViewCount="508" Body="&lt;p&gt;What is the purpose of the 4 vents at the aft section of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt;? The spacecraft is operating in vacuum and the interior of the telescope is devoid of any gas, so venting seems pointless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venting might have played some role during ascent to orbit (when the telescope had to get rid of the air inside the instrument section), but whether this is the case I'm not sure. Can there be other reasons behind that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DmWya.jpg&quot; alt=&quot;Hubble Telescope aft section&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="921" LastEditorUserId="415" LastEditDate="2014-05-21T15:50:50.020" LastActivityDate="2014-05-21T15:50:50.020" Title="What is the purpose of Hubble's aft vents?" Tags="|spacecraft|hubble|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4587" PostTypeId="1" AcceptedAnswerId="4596" CreationDate="2014-05-21T08:23:00.987" Score="12" ViewCount="7320" Body="&lt;p&gt;In &lt;em&gt;Space Mission Engineering: The new SMAD&lt;/em&gt;, page 555, section 18.7.2, the following thrust formula is given for a solar sail:&#xA;$$F=\frac{2RSA}{c}\sin^2\theta=9.113\times10^{-6}\frac{RA}{D^2}\sin^2\theta$$&#xA;Where, $F$ is the thrust; $R$ is the fraction of incident light; $D$ is the distance from the Sun in astronomical units; $S$ the solar flux in $W/m^2$; $c$ the speed of light; $A$ the sail area in $m^2$ and $\theta$ the sail tilt angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this formula calculate instantaneous thrust? There is no time factor in this equation, so how does one go to calculate the thrust of a given solar sail over a period of time? Is there an &lt;em&gt;integration friendly&lt;/em&gt; formula instead?&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="1391" LastEditDate="2014-05-21T16:25:20.500" LastActivityDate="2021-06-10T17:06:08.607" Title="Solar sail thrust calculation" Tags="|propulsion|solar-sail|mathematics|advanced-propulsion|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4588" PostTypeId="2" ParentId="2698" CreationDate="2014-05-21T08:49:47.570" Score="5" Body="&lt;p&gt;Most probably english. For technical communication, Hindi is avoided because it lacks technical words and instead borrows them from Sanskrit. Relaying highly technical information in chaste Sanskrit-derived technical language would unwieldly, so for all practical purposes - English.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hindi may be used for colloquial conversation. Again language in India depends on the mutual comfort of persons involved, if the astronaut is from a state in the Deep South, where Hindi is usually nor spoken nor understood; conversation is probably going to be conducted in english.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;India has 22+ national languages which enjoy wide patronage.&lt;/p&gt;&#xA;" OwnerUserId="3393" LastActivityDate="2014-05-21T08:49:47.570" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4590" PostTypeId="1" AcceptedAnswerId="10116" CreationDate="2014-05-21T11:23:10.480" Score="12" ViewCount="441" Body="&lt;p&gt;The utility of using liquid metals or ionic salts to automate casting of a telescope mirror could offer a cheap way to build a massive telescope on the moon or in space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/A616P.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image above, along with details of the idea are from &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2008/09oct_liquidmirror/&quot; rel=&quot;noreferrer&quot;&gt;http://science1.nasa.gov/science-news/science-at-nasa/2008/09oct_liquidmirror/&lt;/a&gt;. There is also information on the International Liquid Mirror Telescope Project currently building one India ( &lt;a href=&quot;http://www.aeos.ulg.ac.be/LMT/&quot; rel=&quot;noreferrer&quot;&gt;http://www.aeos.ulg.ac.be/LMT/&lt;/a&gt;), but there is no mention of proposals or plans to build one in space or on the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there plans or detailed conceptual studies to build and deploy a liquid space telescope in space or on the moon by NASA or any other organization?&lt;/p&gt;&#xA;" OwnerUserId="3161" LastEditorUserId="3161" LastEditDate="2014-05-21T12:50:14.023" LastActivityDate="2015-07-21T17:25:29.327" Title="What are the current state in plans to put a liquid telescope on the moon or in space?" Tags="|future-missions|telescope|mission-design|space-telescope|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4592" PostTypeId="1" AcceptedAnswerId="4595" CreationDate="2014-05-21T11:39:50.577" Score="7" ViewCount="723" Body="&lt;p&gt;Highlights from NASA's webpage lists studies done during the Space Shuttle Columbia mission STS-107 including student experiments, one of them including fish. What did the student fish experiment investigate and did any results survive the disaster? I was told by a NASA employee that the experiment was on goldfish and that the results did not survive the crash.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-107.html&quot;&gt;http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/archives/sts-107.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3161" LastEditorUserId="29286" LastEditDate="2020-08-31T18:06:26.247" LastActivityDate="2020-08-31T18:06:26.247" Title="What was the design and/or results of the fish experiement onboard Space Shuttle Columbia disaster mission STS-107?" Tags="|space-shuttle|life|experiment|animals|shuttle-columbia|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4594" PostTypeId="2" ParentId="4586" CreationDate="2014-05-21T15:41:40.780" Score="14" Body="&lt;p&gt;From the &lt;a href=&quot;http://asd.gsfc.nasa.gov/archive/hubble/a_pdf/news/SM2-MediaGuide.pdf&quot;&gt;HST Service mission 2 Media Guide&lt;/a&gt; (large PDF)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The shroud and bulkhead support a gas purge system used to prevent contamination of the science instruments before launch. All vents used to expel gases are light-tight; that is, no stray light can enter the OTA focal plane.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From this description, I suspect that the interior of the telescope was filled with e.g. nitrogen under a slight overpressure before launch. During the launch, as the ambient pressure decreased, this nitrogen was expelled via these vents.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-05-21T15:41:40.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4595" PostTypeId="2" ParentId="4592" CreationDate="2014-05-21T16:02:28.050" Score="7" Body="&lt;p&gt;A scholarly article documented the student experiment involving fish was published in the journal Zoological Science by the Zoological Society of Japan and summarizes the set-up and results of the experiment. Apparently there were no goldfish on-board from a student experiment; these were &lt;a href=&quot;https://en.wikipedia.org/wiki/Japanese_rice_fish&quot;&gt;Medaka&lt;/a&gt; fish fry. There is no mention in the abstract if the results were transmitted or survived from the crash:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Development and Swimming Behavior of Medaka Fry in a Spaceflight aboard the Space Shuttle Columbia (STS-107) by Maki Niihori, Yoshihiro Mogami 2, Kiyoshi Naruse, and Shoji A. Baba &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the abstract from &lt;a href=&quot;http://www.bioone.org/doi/abs/10.2108/zsj.21.923&quot;&gt;http://www.bioone.org/doi/abs/10.2108/zsj.21.923&lt;/a&gt;, emphasis added:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A space experiment aimed at closely observing the development and swimming activity of &lt;strong&gt;medaka fry&lt;/strong&gt; under microgravity was carried out as a part of the S*T*A*R*S Program, a space shuttle mission, in STS-107 in January 2003. Four eggs laid on earth in an artificially controlled environment were put in a container with a functionally closed ecological system and launched on the Space Shuttle Columbia. Each egg was held in place by a strip of Velcro in the container to be individually monitored by closeup CCD cameras. In the control experiment, four eggs prepared using the same experimental set-up remained on the ground. There was &lt;em&gt;no appreciable difference in the time course of development between space- and ground-based embryos. In the ground experiment, embryos were observed to rotate in place enclosed with the egg membrane, whereas those in the flight unit did not rotate. One of the four eggs hatched on the 8th day after being launched into space. All four eggs hatched in the ground unit. The fry hatched in space was mostly motionless, but with occasional control of its posture with respect to references in the experimental chamber. The fry hatched on ground were observed to move actively, controlling their posture with respect to the gravity vector.&lt;/em&gt; These findings suggest that &lt;strong&gt;the absence of gravity affects the initiation process of motility of embryos and hatched fry&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="3161" LastEditorUserId="49" LastEditDate="2014-05-21T23:23:05.027" LastActivityDate="2014-05-21T23:23:05.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4596" PostTypeId="2" ParentId="4587" CreationDate="2014-05-21T20:07:07.160" Score="7" Body="&lt;p&gt;Thrust is a &lt;a href=&quot;http://en.wikipedia.org/wiki/Thrust&quot; rel=&quot;noreferrer&quot;&gt;reaction force&lt;/a&gt;, so yes this formula provides instantaneous thrust. I think what you're asking is how does thrust (force) translate into velocity. Force is a rate of change of momentum (i.e. F=ma). To get velocity you need to integrate that over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think about it, you're given F and m, you solve for acceleration and then integrate over time with an initial condition to get velocity. I think by asking for thrust over time you are meaning how much work is performed. Use &lt;a href=&quot;http://en.wikipedia.org/wiki/Work_%28physics%29#Mathematical_calculation&quot; rel=&quot;noreferrer&quot;&gt;work formulas&lt;/a&gt;, such as this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FGVjb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for the amount of work over a dt of time or the full amount over a span of time here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/v8LYx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="3061" LastEditorUserId="3061" LastEditDate="2014-05-21T20:20:20.923" LastActivityDate="2014-05-21T20:20:20.923" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4597" PostTypeId="1" AcceptedAnswerId="4598" CreationDate="2014-05-22T03:33:49.917" Score="22" ViewCount="1775" Body="&lt;p&gt;I have read that the New Horizons probe plans on visiting some other Kuiper-belt objects once it passes Pluto, but not sure which ones. This brings up a few questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are there any leading KBO candidates?      &lt;/li&gt;&#xA;&lt;li&gt;Are they just looking for objects in its current trajectory, or does the probe have enough fuel to significantly alter its orbit on closest approach with Pluto?  &lt;/li&gt;&#xA;&lt;li&gt;Will New Horizons follow a hyperbolic trajectory past Pluto?    &lt;/li&gt;&#xA;&lt;li&gt;How close will it get to the surface?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="603" LastEditorUserId="49" LastEditDate="2014-05-22T12:14:55.840" LastActivityDate="2017-01-02T00:43:52.207" Title="What's after Pluto for New Horizons?" Tags="|future-missions|new-horizons|pluto|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4598" PostTypeId="2" ParentId="4597" CreationDate="2014-05-22T04:14:35.950" Score="22" Body="&lt;p&gt;Edit as of August 31, 2015: Looks like 2014 MU69 will be the post Pluto target. Paul Gilster has a &lt;a href=&quot;http://www.centauri-dreams.org/?p=33931&quot; rel=&quot;noreferrer&quot;&gt;nice article&lt;/a&gt; on Centauri Dreams.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit as of Jan. 3, 2015: &lt;a href=&quot;http://www.centauri-dreams.org/?p=31765&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; says 3 potential candidates were found in December of 2014.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.huffingtonpost.com/2014/05/21/pluto-probe-new-horizons-crisis_n_5364138.html&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; says the New Horizons team is running out of time to find KBOs to check out after it passes Pluto. No post Pluto targets at this time. Hopefully they'll find candidate(s) soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it's orbit is hyperbolic. It'll be flying by Pluto at a pretty good clip, &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons#Pluto_approach&quot; rel=&quot;noreferrer&quot;&gt;nearly 14 km/s at closest approach&lt;/a&gt;. According to the linked Wikipedia article it's expect to come within 10,000 km of Pluto and 27,000 km of Charon.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-08-31T13:56:03.770" LastActivityDate="2015-08-31T13:56:03.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4599" PostTypeId="2" ParentId="4587" CreationDate="2014-05-22T07:02:07.913" Score="4" Body="&lt;p&gt;Solar sails are unique when compared to other propulsion methods because in a solar sail, the acceleration is &lt;em&gt;continuously available&lt;/em&gt; throughout the flight. Steady solar flux on the sail produces a steady force, albeit reducing with distance to the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course it depends on angle, and the occasional eclipse if passing through a body's shadow will cause a dip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equation you've provided calculates force at a given moment in the craft's flight. However as earlier mentioned, thrust in solar sails is not temporary, it is a steady process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to get the solution you want, you can integrate the equation using calculus and magic, but if you're like me, you want a simpler method for research purposes. The method I used was to simply solve the equation multiple times for differing values of distance from the Sun in AU, according to a hypothetical flight plan. Now I had a dataset of thrust values from Earth till Neptune, in steps of 1 AU. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is simple now to calculate a rough estimate of the sail's final velocity using these 'steps of thrust'. Try this yourself using steps of any size, though I recommend writing a small computer program to automate the task like I did.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="12102" LastEditDate="2017-12-08T03:50:23.747" LastActivityDate="2017-12-08T03:50:23.747" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4600" PostTypeId="1" CreationDate="2014-05-22T11:50:31.670" Score="3" ViewCount="400" Body="&lt;p&gt;I guess it might seems like a spam or a troll one, but actually I'm serious. I'm firt year Mechanical engineer (3 and half to go) and I'd love to work there, Since I was young kid I found that space and space exploration are very intersting thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sorry about my spelling mistakes, My origin is not from th US&lt;/p&gt;&#xA;" OwnerUserId="3397" LastActivityDate="2014-05-22T11:50:31.670" Title="How to work at NASA" Tags="|nasa|" AnswerCount="0" CommentCount="4" ClosedDate="2014-05-22T13:17:06.660" ContentLicense="CC BY-SA 3.0" />
  <row Id="4601" PostTypeId="2" ParentId="4557" CreationDate="2014-05-22T15:14:22.570" Score="2" Body="&lt;p&gt;Not (yet) exactly an answer I was hoping for, since I'm the most interested in procedures onboard the ISS, but since I wasn't able to find any info on that and the questions seems to be unreasonable to some, here's &lt;a href=&quot;http://chandra.harvard.edu/blog/node/500&quot; rel=&quot;nofollow noreferrer&quot;&gt;an update from the NASA's Chandra X-ray observatory&lt;/a&gt; (select and to the question most relevant quotes for brevity) somewhat demonstrating that NASA is taking this meteor shower as a potential threat to their missions in orbits around the Earth:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Even though much of the debris in meteor showers is very tiny, it can&#xA;  pose serious potential risks for spacecraft like Chandra. There are&#xA;  two main concerns: the first is that particles moving at very high&#xA;  speeds will generate electric fields &lt;em&gt;[ed. &lt;a href=&quot;https://en.wikipedia.org/wiki/Triboelectric_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;triboelectric charge&lt;/a&gt;]&lt;/em&gt; that could disrupt the&#xA;  electronics aboard the spacecraft. The second big worry is that a&#xA;  particle would impact Chandra, causing damage to a key instrument or&#xA;  other important piece of the telescope.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Several hours before the meteor shower is expected to begin, the team&#xA;  will make sure that the spacecraft is pointed in the opposite&#xA;  direction of where the meteors are coming from (that is known as the&#xA;  &quot;radiant&quot; of the meteor shower.) They will also feather, or turn,&#xA;  Chandra's solar arrays in a direction to minimize the amount of their&#xA;  surface area that will be exposed to the oncoming meteors.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/NRZDv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;While the team at the OCC has handled many meteor showers in Chandra's&#xA;  nearly 15 years of operations, the Camelopardalids are different&#xA;  because there is relatively little known about the stream of material&#xA;  that Earth (and Chandra) will pass through. Because of this&#xA;  uncertainty, the Mission Planning Team is taking extra precautions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Quotes, image source and more detailed info &lt;a href=&quot;http://chandra.harvard.edu/blog/node/500&quot; rel=&quot;nofollow noreferrer&quot;&gt;in the article&lt;/a&gt;. They don't seem to be taking this event much differently than any other meteor shower events, with a bit of extra caution and simple to implement procedures, but the threat mitigation procedures don't synchronize with the space observatory's own entry into the Earth’s radiation belts where it would experience down time to protect itself from damaging radiation within the &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Van Allen radiation belts&lt;/a&gt;, so the combination of pointing it away from the meteor shower radiant, rotating its solar panels to reduce area surface exposed to potential impacting debris and its own scheduled down time will result in a much longer calibration observation (self-inspecting time when the observatory doesn't conduct its normal observations) than usual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, apparently, at least some Earth-orbited hardware will follow &lt;a href=&quot;https://en.wikipedia.org/wiki/Risk_management&quot; rel=&quot;nofollow noreferrer&quot;&gt;risk management&lt;/a&gt; procedures (interestingly, the Wikipedia article on risk management uses ISS as an example) to help reduce risk of impacting with 209P/LINEAR cometary debris and prevent damage to its hardware, in particular:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Point the most sensitive parts of the spacecraft away from the meteor shower radiant&lt;/li&gt;&#xA;&lt;li&gt;Rotate the solar panel arrays to reduce their surface area exposure to incoming debris&lt;/li&gt;&#xA;&lt;li&gt;Secure shutters and covers to protect most sensitive equipment from debris and el. discharge&lt;/li&gt;&#xA;&lt;li&gt;Command the craft to enter &quot;safe mode&quot; and continue with self-inspection&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So while this does partially answer my question, it's merely regarding a single craft in Earth's orbit (64.2 hours, 133,000 x 16,000 km), so I'm still hoping to find more answers. In particular, regarding risk management procedures onboard the ISS that will be activated during the 209P/LINEAR meteor shower peak. I'll keep an eye out for further updates from NASA and Roscosmos officials on this matter myself, too.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-05-22T16:14:33.623" LastActivityDate="2014-05-22T16:14:33.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4604" PostTypeId="1" CreationDate="2014-05-24T12:17:29.787" Score="12" ViewCount="599" Body="&lt;p&gt;To throw away an investment in space infra-structure the size of the ISS  seems an incredible waste. Even if there is only a limited chance of ever using it again, the probability of this happening should be multiplied with the gigantic investment itself in order to reach an assesment of the potential value of the long term storage investment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, if there is 10% chance of salvaging 30% of the investment, it is still worth billions. Another aspect of the value of a determined salvage operation is the learning experience. By using the existing NEXT ion engines and the station's existing solar power array, NASA or whoever does it might learn something  about cost effective industrial space operations, which hopefully will come in handy one day .  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If The ISS were boosted to a height of 5000 km , it would be stable for a very long time . Without airpressure in the modules, many of the processes of degeneration of materials would efectively end , except maybe the ageing of gaskets and pressure seals. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While NASA has a 2015 plan for using the Vasimir ion engine for &lt;em&gt;experimenting&lt;/em&gt; with ISS orbit boost, this has never been a real practical plan for boosting the ISS to a stable orbit, but rather a way to get more funding for a PET project...The total drag on ISS is about 0.9 N , a single NEXT engine can push 0.25  ,  6  ion engines together with ''normal'' chemical boosts for 10 more years could go along way towards a stable orbit. The NEXT program has build atleast 4 prototype engines , probably sitting on a shelf somewhere .. WHY not use them  ?&lt;/p&gt;&#xA;" OwnerUserId="3407" LastEditorUserId="288" LastEditDate="2014-05-24T12:47:39.697" LastActivityDate="2014-05-25T07:32:30.443" Title="Can the ISS be brought to a long term storage orbit using existing NEXT ion engines?" Tags="|iss|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4605" PostTypeId="2" ParentId="4466" CreationDate="2014-05-24T15:27:47.873" Score="6" Body="&lt;p&gt;Horizontal component of first stage velocity will be under Mach 6 or under 2,000 meters/sec as Elon Musk stated previously (in a Popular Mechanics magazine article from 2012) for recovery of his 1st stage. Remember that SpaceX announced F9R wind tunnel tests at NASA-Marshall up through Mach 5. Remember that overcoming gravity losses is ~ 1,500 meters/sec of 1st stage delta V so total 1st stage burn out delta V is ~ 3,500 meters/sec = 2,000 meters/sec horizontal velocity + 1,500 meters/sec gravity losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That 15% number is a reduction of orbital payload by 15% (or 30%) and probably not a massive 15% fuel reserve on 1st stage of 60+ tons propellant. A 32 ton propellant reserve is more likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air Force RLV booster studies and Kistler had estimated rocket-back maneuver to launch pad, including bleeding off Mach 4 to 7 in horizontal velocity, to require ~ 3,000 meters/sec delta V (according to the cited AF RLV booster return study that states 11,000 fps - 1,000 fps = 10,000 fps = 3,000 m/s as delta V needed for return to launch pad). This is closer to 32 tons propellant reserved for the 3,000 m/s delta V needed for the 1st stage to rocket back to the original launch pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk has already said that F9R landing legs weight are less than Tesla Model S weight of 2.1 tons and that mass fraction of F9R 1st stage is under 4%. You should assume 18-tons including legs in empty weight and ~ 32-tons fuel to bring 1st stage back to pad. They need to be able to throttle Merlin 1D back to 30% thrust (i.e. 20-tons thrust) for a soft landing of an empty 18-ton booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air friction alone can slow booster to subsonic velocities, but it can also tear booster apart. SpaceX has to control reentry and position vehicle using multiple smaller firings so it does not &quot;belly flop&quot; (Elon Musk's words) into the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Use these numbers as your estimates:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;418-ton fueled 1st stage with 18-ton empty weight (i.e. 16-tons for 4% mass fraction and 2 tons for the new landing legs) -- &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.spaceflight101.com/falcon-9-v11.html&lt;/a&gt;;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;50-ton burn out weight of 1st stage with 32-tons fuel left for rocket-back maneuver;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2nd stage fueled weight of 75-tons, empty weight of 5 tons, and payload weight of 13-tons, for 88-tons total payload + 2nd stage weight carried by 1st stage; and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;total F9R rocket weight of 506 tons to place 13-ton payload into LEO and to rocket back 1st stage to launch pad. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also assume 300-sec average Isp on 1st stage and 340-sec Isp on 2nd stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above estimates fit with the F9R v1.1 weight and performance estimates made by this web site -- &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.spaceflight101.com/falcon-9-v11.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above estimates should work in your calculations for F9R taking a 13-ton payload to LEO and then the F9R 1st stage having the delta V of 3,000 meters/second (obtained from 32-tons reserved propellant) to be able to perform a rocket-back maneuver to the launch pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I estimate that it will be less than 3-tons of added weight penalty on the F9R 2nd stage for the propellant, heat shield, (4) Super Draco thrusters, and landing legs needed to recover and reuse the F9R 2nd stage similar to recovery and reuse of the Dragon-2 spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that a fully reusable F9R that reuses both its 1st and 2nd stages could carry 10-tons to LEO and less than 1-ton to GTO.&lt;/p&gt;&#xA;" OwnerUserId="3409" LastEditorUserId="3409" LastEditDate="2014-05-25T18:51:47.557" LastActivityDate="2014-05-25T18:51:47.557" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4606" PostTypeId="1" AcceptedAnswerId="4611" CreationDate="2014-05-24T18:51:16.223" Score="8" ViewCount="618" Body="&lt;p&gt;The &lt;em&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Rosetta&lt;/a&gt;&lt;/em&gt; probe had to fly-by several bodies to acquire the necessary velocity to match &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow&quot;&gt;67P/Churyumov–Gerasimenko&lt;/a&gt; (Chury). Towards this end it was launched almost a decade in advance. &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Philae_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Philae&lt;/a&gt;&lt;/em&gt; will fire a pair of harpoons to anchor itself to the comet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thinking aloud - between them, &lt;em&gt;Rosetta&lt;/em&gt; and &lt;em&gt;Philae&lt;/em&gt; mass around 192&amp;nbsp;kg. Chury's mass is 3.14×10^12± 0.21×10^12&amp;nbsp;kg&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Call this a thought-experiment ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say now &lt;em&gt;Rosetta&lt;/em&gt; were to be launched shortly before Chury were at a suitable rendezvous. (As contrasted to 10 years in advance.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say further &lt;em&gt;Rosetta&lt;/em&gt; were to cold-fire a pair of harpoons to intercept Chury. The harpoons could be attached to a winch aboard &lt;em&gt;Rosetta&lt;/em&gt; to reel the craft in towards the comet. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could &lt;em&gt;Rosetta&lt;/em&gt; have lassoed Chury, and hitched a ride instead of spending 10 years to match velocity, and orbit?&lt;/li&gt;&#xA;&lt;li&gt;How much force would such a tether/lasso experience? &lt;/li&gt;&#xA;&lt;li&gt;Is any material produced by our contemporary technology capable of withstanding this force whilst being light enough to be launched into deep-space?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;PS: The draw-back of the lasso method is it's touch-and-go nature. On the other hand, it may have the potential to afford a relatively &lt;code&gt;quick&lt;/code&gt; study for any target-of-opportunity.&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="286" LastEditDate="2014-11-17T22:05:54.957" LastActivityDate="2014-11-17T22:05:54.957" Title="Could Rosetta have lassoed Chury to hitch a ride instead of chasing after it for 10 years?" Tags="|rosetta|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4607" PostTypeId="2" ParentId="4604" CreationDate="2014-05-24T20:33:28.993" Score="3" Body="&lt;p&gt;I don't know the characteristics of NEXT, this is just a prototype, right?&#xA;But assuming&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;specific impulse around 3000 seconds, &lt;/li&gt;&#xA;&lt;li&gt;the ISS mass to be 450 000 kg, &lt;/li&gt;&#xA;&lt;li&gt;the delta-V required to boost from 400 km to 5000 km is around 1700 m/s.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;we need around 27 tons of Xenon fuel to be delivered to 400 km orbit first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's more than a single Proton or a single Space Shuttle payload.&#xA;The whole enterprise gets as expensive as to deliver a couple of new modules to the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While deorbiting the station only requires a single Progress, they say.&#xA;A many times cheaper solution.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2014-05-25T07:32:30.443" LastActivityDate="2014-05-25T07:32:30.443" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4608" PostTypeId="1" AcceptedAnswerId="4657" CreationDate="2014-05-25T01:03:32.080" Score="22" ViewCount="7463" Body="&lt;p&gt;Something goes bad on Earth, ISS is cut from any kind of supply and the escape pods can't be used. If there are 2 people on board at that time, how long can they survive there? Did someone write a protocol for this type of situation?&lt;/p&gt;&#xA;" OwnerUserId="1263" LastActivityDate="2023-03-19T12:20:14.153" Title="How long can a 2 person crew survive on ISS totally cut from Earth?" Tags="|iss|life-support|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4609" PostTypeId="2" ParentId="4608" CreationDate="2014-05-25T11:17:45.513" Score="7" Body="&lt;p&gt;The state of supplies on the ISS changes over time. Right at the end of the Shuttle era, they stockpiled lots of hardware, clothes, food and water supplies. But most specifically they made sure that large hardware components, that were replaceable, were left on station, since the Shuttle was uniquely qualified to carry them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over time, they have drawn down that reserve, and then replenish parts of it via Dragon, Progress, Cygnus, ATV, and HTV launches.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current state of supplies thus fluctuates over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water is an interesting consumable. The Shuttle which used fuel cells for power, would produce excess water while on station, and they would spend much of the flight transferring the water.  With the absence of the shuttle visiting the station, more water needs to be brought on cargo flights to cover that lack.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-05-25T18:19:39.790" LastActivityDate="2014-05-25T18:19:39.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4610" PostTypeId="2" ParentId="3241" CreationDate="2014-05-26T13:14:54.420" Score="13" Body="&lt;p&gt;They do use sparklers: the Radially Outward Firing Initiators (ROFI), derived from the Shuttle system, are started at &lt;a href=&quot;http://www.nasa.gov/mission_pages/launch/delta_IV_count_101_prt.htm&quot;&gt;T-15 seconds&lt;/a&gt;. Note that a hydrogen fire is the expected result of this system; they just didn't expect it to be this extensive.&lt;br&gt;&#xA;On the &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=23774.195&quot;&gt;NasaSpaceflight forum&lt;/a&gt;, someone suggested that the scorching of the insulation is caused by the hydrogen not mixing with air immediately, so the ROFI doesn't burn all of it until the oxygen turbopumps are spooled up just prior to ignition. The &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=9350.315&quot;&gt;flame duct&lt;/a&gt; is enclosed which allows hydrogen to pool, that doesn't help either.&lt;br&gt;&#xA;On the Space Shuttle pad, the water deluge system helps keep down any fires. The Delta IV is launched dry. &#xA;The launch photo you show, with the insulation extensively blackened, &lt;a href=&quot;https://groups.yahoo.com/neo/groups/space-modelers/conversations/topics/48749?o=1&amp;amp;d=-1&quot;&gt;may have been an anomaly&lt;/a&gt; due to some part of the timing being off (i.e. more hydrogen present than expected when the ROFI started). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-05-26T13:14:54.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4611" PostTypeId="2" ParentId="4606" CreationDate="2014-05-26T13:22:20.297" Score="7" Body="&lt;p&gt;Interesting question. There's a few problems with using a harpoon for capturing something of this nature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Tether strength - the tether material is going to be under a reasonable amount of tensile stress when Rosetta is pulling itself in, but nothing compared to the tensile shock it will experience if you have any serious delta V between Rosetta and the target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Targeting/Accuracy - if the two orbits are out of plane with one another and they are going some reasonable speed then you'll only have a very small window in which to fire your harpoon. A cross speed of 1000m/s means that the 4km target will pass in 4 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Harpoon speed - Assuming you have targeted your harpoon correctly, you still need it to reach the target. If there is a few km/s between your speed and that of your target (assuming your target is going faster) then your harpoon has to be fired at least as fast as that. On top of that it's going to be travelling the distance between your spacecraft and the target, so you had better be close or have a LONG tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) Barb strength - so lets assume you've managed to hit your target and your tether isn't going to give way. The barbs that you have planted deep into the target are now carrying the shock. This will typically be compressive so that's not as bad as it could be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a lot that can go wrong, mainly because you didn't match the speed of the target, but if you could do that then why would you need a harpoon!&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-05-26T13:22:20.297" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4612" PostTypeId="1" AcceptedAnswerId="4613" CreationDate="2014-05-26T14:14:11.240" Score="5" ViewCount="194" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; writes to say Low Earth Orbit spans the altitude from 160KM above Earth through 2000KM above Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch vehicles appear to be highly stream-lined. Yet only the first few score KM (as far as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Karman_line&quot; rel=&quot;noreferrer&quot;&gt;Karman Line&lt;/a&gt;) are in atmosphere. To the best of my comprehension, dead-weight is a no-no on launch vehicles/systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The additional mass of aerodynamic sheets may be necessary when the payload for a launch must be protected from the elements. Yet in all other cases it probably constitutes dead-mass. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Is the streamlining of a launch vehicle/stages/components worth the additional fuel required to carry it beyond the Karman line?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What is the &lt;strong&gt;earliest&lt;/strong&gt; launch vehicles may/should jettison the streamlining dead-weight? (This would probably have to be on a case-by-case basis, but any recommended guidelines/parameters ... ?)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-05-26T15:24:32.533" Title="Is the streamlining of a launch vehicle worth the additional fuel required to carry it beyond the Karman line?" Tags="|design|atmospheric-drag|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4613" PostTypeId="2" ParentId="4612" CreationDate="2014-05-26T15:09:44.727" Score="5" Body="&lt;p&gt;This dead weight is called &quot;fairing&quot;. The time to do payload fairing separation is determined by heat flux from the air (&lt;em&gt;maximum free molecular heat flux&lt;/em&gt;) that can be tolerated by the payload (and its more sensitive parts), and by the mass margins provided by the launch vehicle, as well as impact point constraints downrange (you don't want lawsuits from angry relatives). Nobody wants to carry fairing any longer than absolutely necessary. The exact timings may vary from 150 seconds to 350 seconds after lift-off, speaking from memory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To achieve separation, a carefully orchestrated and tested sequence of explosive bolts, detonation cord and separation springs is used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact method of &quot;streamlining&quot; depends on the launch profile. For instance, drag reduction may include a shock wave &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag-resistant_aerospike&quot; rel=&quot;noreferrer&quot;&gt;aerospike&lt;/a&gt; for ballistic missiles that are programmed to get out of Dodge and 'clear the datum' A.S.A.P.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-05-26T15:24:32.533" LastActivityDate="2014-05-26T15:24:32.533" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4614" PostTypeId="1" CreationDate="2014-05-26T16:19:41.033" Score="5" ViewCount="1091" Body="&lt;p&gt;For long term crewed space travel, gravity might be simulated by rotating a habitat in a long tether to a counter weight. Conventional chemical rocket engines give a spacecraft a brief initial impulse. Artificial gravity mode could be initiated afterwards by unfolding the tethers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But some rocket engines are designed to give continuous propulsion over long periods of time. I think of for example nuclear thermal, ion electric and solar sails. How could such rocket engines be fitted on a spacecraft which has a long rotating tether? If placed at the center of mass, wouldn't such an engine bend the tethers and would that be a sound way to engineer it? Would rigid structures be required instead of a flexible unfoldable wire?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-05-26T19:41:37.113" Title="How could a spacecraft be continuously propelled if it rotates to simulate gravity (by tether)?" Tags="|propulsion|spacecraft|engines|artificial-gravity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4615" PostTypeId="2" ParentId="4614" CreationDate="2014-05-26T17:00:46.833" Score="6" Body="&lt;p&gt;Some of the required rigidity could be provided by the rotation alone, by what's the simplest to describe as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifugal_force&quot;&gt;centrifugal force&lt;/a&gt; (it really doesn't exist, it's merely a product of other forces), and it would be simplest to limit lateral wobble, if the thrust is applied perpendicular to the rotation vector from all its extremes simultaneously, i.e. your non-rigid wheel rotates 90° to your velocity vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My first edit was wrong. Rotating in the axis along your velocity vector would not only complicate engine design, but also eventually collapse the structure as one part of the wheel decelerates while the other accelerates during one single rotation. So unless you could compensate for the propulsive force with magnetic fields to keep the extreme parts of the wheel at a stable distance, this wouldn't work. But rotating the wheel perpendicular to the velocity vector could. If the two forces (centrifugal from our rotation and propulsive from our engines) are kept at exactly the 90° angle, they're non-interactive and the extreme ends of the wheel would keep exactly the same distance to each other as if the engines were off. That's true because the centrifugal force vector is equal in all directions along the wheel's plane of rotation, and the force on each of its parts is canceled out by its opposing end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem is, that it would be imperative to keep the thrust vector (your normal) perpendicular to the wheel's rotational plane, the two (or more, but that's even harder to do then) engines would have to be perfectly synchronised, and run in a mutually compensating mode, where all the other engines would have to adjust to the worst of them. So while the engines perform exactly as expected, this could be relatively simple. Simply ignore that the whole structure rotates too and consider all its extreme parts with engines as individual spaceships flying in precision formation. If your velocity vector is exactly perpendicular to the plane of rotation, these forces don't have any influence on each other As one engine starts acting up though, all the rest of them have to follow suit and compensate, otherwise you end up with and out-of-plane rotation, precession of its extreme parts and eventual collision between them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in my opinion; doable, but complicated. A rigid structure with a central engine would be simpler...&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-05-26T19:41:37.113" LastActivityDate="2014-05-26T19:41:37.113" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="4616" PostTypeId="2" ParentId="29" CreationDate="2014-05-26T22:36:35.177" Score="3" Body="&lt;p&gt;Earth to Jupiter Hohmann:&#xA;$V_{\infty}$ departing Earth 8.8 $\frac{km}{s}$,&#xA;$V_{\infty}$ approaching Jupiter 5.6 $\frac{km}{s}$,&#xA;Hyperbolic periapse velocity at Jupiter (0 km altitude) &lt;strong&gt;60 $\frac{km}{s}$&lt;/strong&gt;,&#xA;Trip time 2.7 years&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth to Saturn Hohmann:&#xA;$V_{\infty}$ departing Earth 10.3 $\frac{km}{s}$,&#xA;$V_{\infty}$ approaching Saturn 5.4 $\frac{km}{s}$,&#xA;Hyperbolic periapse velocity at Saturn (0 km altitude) &lt;strong&gt;36 $\frac{km}{s}$&lt;/strong&gt;,&#xA;Trip time 6 years&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth to Uranus Hohmann:&#xA;$V_{\infty}$ departing Earth 11.3 $\frac{km}{s}$,&#xA;$V_{\infty}$ approaching Uranus 4.7 $\frac{km}{s}$,&#xA;Hyperbolic periapse velocity at Saturn (0 km altitude) &lt;strong&gt;22 $\frac{km}{s}$&lt;/strong&gt;,&#xA;Trip time 16 years&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth to Neptune Hohmann:&#xA;$V_{\infty}$ departing Earth 11.6 $\frac{km}{s}$,&#xA;$V_{\infty}$ approaching Neptune 4 $\frac{km}{s}$,&#xA;Hyperbolic periapse velocity at Saturn (0 km altitude) &lt;strong&gt;24 $\frac{km}{s}$&lt;/strong&gt;,&#xA;Trip time 31 years&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Numbers come from &lt;a href=&quot;http://clowder.net/hop/railroad/Hohmann.xls&quot; rel=&quot;nofollow&quot;&gt;Hohmann.xls&lt;/a&gt; which assumes circular, coplanar orbits. Since the orbits are inclined, the actual $V_{\infty}$'s would be higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trip times and $V_{\infty}$s alone make these resources hard to reach. But the velocities needed to scoop gas out of these deep gravity wells are the real show stoppers. As others have mentioned, these massive planets are hard to leave.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of $\Delta V$, there are closer possible sources of hydrogen. Our moon's polar cold traps may have rich deposits of $NH_3$ and $H_2O$ -- or maybe not -- we still don't know much about these regions. Another possible source of hydrogen are carbonaceous asteroids with Earth-like orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for helium, I don't know of any accessible deposits besides what we have on Earth. There's thought to be some helium lodged in lunar regolith, but you'd have to process a lot of regolith for a small amount of helium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would you want to use the helium for?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="168" LastEditDate="2014-05-27T07:27:07.653" LastActivityDate="2014-05-27T07:27:07.653" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4617" PostTypeId="1" AcceptedAnswerId="4622" CreationDate="2014-05-27T10:57:10.313" Score="10" ViewCount="4198" Body="&lt;p&gt;We know there is at least &lt;a href=&quot;https://space.stackexchange.com/questions/4019/are-there-any-weapons-in-space/4020#4020&quot;&gt;one gun in the ISS&lt;/a&gt; the caliber seems to a state secret.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Whipple_shield&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; and &lt;a href=&quot;http://ston.jsc.nasa.gov/collections/TRS/_techrep/TP-2003-210788.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA large pdf&lt;/a&gt; there are over 100 shield configurations on the ISS.  These would be optimized for external impact protection. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the best and worse case scenario for a gun fired inside the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Best case; Lowest energy gun fired at the most protected area of the hull&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Worst case; Highest energy gun fired at the least protected area of the hull&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-05-27T21:34:10.113" Title="Could a gun fired inside the ISS rupture it?" Tags="|iss|impact|security|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4618" PostTypeId="2" ParentId="1081" CreationDate="2014-05-27T15:31:32.017" Score="5" Body="&lt;p&gt;What Thomas Pornin said. But I'll draw a picture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jJtBN.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you plug in $R = 6378$, $h = 780$ and $a = 5^\circ$ you get &#xA;$\cos(b) - 0.0875 \sin(b) = 0.8920$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From there numeric methods can be used to find $b \approx 22.42^\circ$.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="6816" LastEditDate="2016-01-31T14:49:50.010" LastActivityDate="2016-01-31T14:49:50.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4619" PostTypeId="1" CreationDate="2014-05-27T15:33:57.073" Score="6" ViewCount="154" Body="&lt;p&gt;I'm a master student working on networks analysis in general. A network is something that has nodes and there are links between the nodes. Nodes and links could have attributes. An evolving network is one that changes overtime (new nodes and links are added..etc). An example of that is Facebook. Nodes are users and links represent the friendship relationship. Users have attributes (gender, age ..etc). A Facebook network as you know is an example of a social network. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issue is that so many people studied traditional evolving networks like social networks, the web, or transportation networks. Currently I'm looking for novel examples of evolving networks to study them. So I thought there might be some examples or applications in &lt;em&gt;Space Exploration&lt;/em&gt; that could represent some kind of an evolving network. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question: Can you give me examples in &lt;em&gt;Space Exploration&lt;/em&gt; for evolving networks?&lt;/p&gt;&#xA;" OwnerUserId="3416" LastActivityDate="2014-05-27T22:54:34.443" Title="Examples of evolving networks in Space Exploration" Tags="|deep-space-network|" AnswerCount="2" CommentCount="6" ClosedDate="2014-05-27T23:36:29.850" ContentLicense="CC BY-SA 3.0" />
  <row Id="4621" PostTypeId="2" ParentId="4619" CreationDate="2014-05-27T17:12:34.883" Score="4" Body="&lt;p&gt;There are a few examples.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best example is at Mars. New orbiting spacecraft arrive on a regular basis, which usually have a relay instrument to relay back information from landing probes to Earth. As new probes land, or new orbiting spacecraft arrive, then the number nodes changes. There are plans to more formalize the protocol. The current plan is to call it the &lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_Internet&quot; rel=&quot;nofollow&quot;&gt;Interplanetary Internet&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Closer to home, new satellites for constellations such as GPS, Irridium, GlobalStar, and Orbcomm are added. Irridium in particular will talk with one satellite to another, the others will just add functionality when new satellites are launched. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-05-27T17:12:34.883" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4622" PostTypeId="2" ParentId="4617" CreationDate="2014-05-27T17:40:00.533" Score="14" Body="&lt;p&gt;There are lots of variables here. Gun type, wall thickness, type of shielding used.&lt;br&gt;&#xA;According to &lt;a href=&quot;https://answers.yahoo.com/question/index?qid=20100513145026AAxJPAe&quot;&gt;this email exchange&lt;/a&gt;, handgun bullets can penetrate 1-3 cm of aluminium. It also states that:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(in) a M113A2 APC (armored personnel carrier) the aluminum (hull) is about 3/4 inches&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The first data I've found for the wall thickness of a space station module is for &lt;a href=&quot;http://esabase2.net/wp-content/uploads/2009/09/R099-516pub_01_02_5ECSD-ESABASE2.pdf&quot;&gt;ESA's proposed Columbus module&lt;/a&gt;: they used a Whipple shield with two walls of 2 mm and 3 mm thick. At first glance, not enough to stop a bullet. And the bullet will probably be going too slow to disintegrate on impact (as a micrometeoroid would).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I know, in a Whipple shield, only the inner wall is airtight, so if you shoot from the inside, you're going to have a leak. At ~1 cm diameter, it's not big enough to cause immediate catastrophic decompression, but it'd be time for emergency procedures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Worst case is that your bullet trajectory goes through a fuel (or oxidizer) tank. This will most likely result in contamination and exposure to highly toxic substances.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="168" LastEditDate="2014-05-27T21:34:10.113" LastActivityDate="2014-05-27T21:34:10.113" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="4623" PostTypeId="2" ParentId="4619" CreationDate="2014-05-27T22:47:14.740" Score="4" Body="&lt;p&gt;Here's an explanation of space debris from a networks perspective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A collection of nodes with properties who are connected (in proximity) with other nodes whose properties are similar. If two nodes coincide in a specific collection of time dependent properties (say position in x, y and z but not attitude) then the result is the removal of these nodes and the introduction of a new collection of nodes that have similar properties to the parent nodes. Also you have the arbitrary addition of nodes (launching new objects) and the eventual subtraction of nodes based on a time that is a function of the nodes properties (atmospheric drag de-orbiting and re-entry).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quite close to evolutionary networks I think.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact I remembered a paper I read a few months back about this very topic:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576509003191&quot; rel=&quot;nofollow&quot;&gt;A new analysis of debris mitigation and removal using networks&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This might get you well on your way to your lit review, or even the outlines of your model! :D&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-05-27T22:54:34.443" LastActivityDate="2014-05-27T22:54:34.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4624" PostTypeId="1" AcceptedAnswerId="4625" CreationDate="2014-05-28T04:45:42.420" Score="4" ViewCount="463" Body="&lt;p&gt;When discussing orbital mechanics, you learn that all orbits roughly follow an ellipse which is obtained as the intersection of a cone with an inclined plane, creating conic sections.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/v4rml.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is a plot of different mathematical variables of a cone. I am trying to figure out if the ellipses formed by orbits follow a special rule where the opening angle (at the top of the diagram) of the cone is 90 degrees, or if it is irrelevant what that angle is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TL4rA.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-05-28T04:51:09.977" LastActivityDate="2014-05-28T05:32:12.260" Title="Does the opening angle of the cone matter?" Tags="|orbital-mechanics|mathematics|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4625" PostTypeId="2" ParentId="4624" CreationDate="2014-05-28T05:32:12.260" Score="3" Body="&lt;p&gt;You can think of the cone as a flashflight beam and the cutting plane as the floor:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wmZqb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shine the beam straight down and you get a circle. Tilt the beam and the circle stretches into an ellipse. Tilt it more and the ellipse gets longer and longer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the top edge of the light beam becomes parallel with the floor, the ellipse stretches out to infinity and you have a parabola of eccentricity one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus by varying the tilts you can get any ellipse from eccentricity 0 to 1. Doesn't matter how thin the cone is, so long as it's angle is more than zero.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-05-28T05:32:12.260" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4626" PostTypeId="1" AcceptedAnswerId="4627" CreationDate="2014-05-28T16:47:22.960" Score="6" ViewCount="1251" Body="&lt;p&gt;If a satellite covered with a thick layer of copper, were to go straight through a solar storm in the opposing direction, would the satellite produce electricity, provided the copper(or other suitable material) doesn't melt? Could the satellite harvest this energy? Can this be used to study solar storms further?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;in my opinion it should, by laws of electromagnetic induction, but I am asking this to know whether it would be practically possible?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="3419" LastEditorUserId="109" LastEditDate="2014-05-28T18:36:28.627" LastActivityDate="2018-02-05T18:53:01.033" Title="Can electricity be generated from solar storms" Tags="|probe|power|research|solar-wind|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4627" PostTypeId="2" ParentId="4626" CreationDate="2014-05-28T17:55:37.930" Score="4" Body="&lt;p&gt;You need both a source and a sink to produce electric current by electromagnetic induction, otherwise you're really just a target at the mercy of the incoming energy discharge. So if you're going to have large static collectors in the way of high-energy electron flux of &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_storm&quot; rel=&quot;nofollow&quot;&gt;solar storms&lt;/a&gt;&lt;/em&gt; (pretty broad definition), you better have ways to use, store or convert all of that energy to some useful purpose, otherwise it's going to build-up on static charge and can damage your equipment by destructive electrostatic discharge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But yes, the energy in &lt;em&gt;solar storms&lt;/em&gt;, as you put it, could be converted into something useful, like electricity. Problem is, they're rather unpredictable in both time and intensity, and we can already convert solar radiation energy into useful forms with simpler, proven technology (e.g. photovoltaics), measure proton flux with various &lt;a href=&quot;https://en.wikipedia.org/wiki/Particle_detector&quot; rel=&quot;nofollow&quot;&gt;particle detectors&lt;/a&gt;, magnetic fields with &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetometer&quot; rel=&quot;nofollow&quot;&gt;magnetometers&lt;/a&gt;, heat with thermocouples, infrared sensors, ad infinitum. And then there's the use of solar radiation pressure for propulsion with &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;nofollow&quot;&gt;solar sails&lt;/a&gt;, attitude control by magnetic fields with &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetorquer&quot; rel=&quot;nofollow&quot;&gt;magnetorquers&lt;/a&gt;, even &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19980236665&quot; rel=&quot;nofollow&quot;&gt;electrodynamic tether propulsion&lt;/a&gt; (PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But all these systems have one critical thing in common: They can either be deactivated, protected, or the whole energy they collect is used, converted, stored or dumped so the electric fields don't build-up as static charge. The latter is a bit of a problem in the vacuum of space, and that's why such charged particle flux is really more of a hindrance to space exploration missions with precious and sensitive equipment onboard, so they rather try minimizing their potentially mission-ending effects, than consider them something worth waiting for. Unless you're actually measuring it, usually from a relatively safe distance and well protected, like &lt;a href=&quot;http://sdo.gsfc.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;SDO&lt;/a&gt; and the two &lt;a href=&quot;http://stereo.gsfc.nasa.gov/spacecraft.shtml&quot; rel=&quot;nofollow&quot;&gt;STEREO&lt;/a&gt; spacecraft do while performing their heliophysics observations of our Sun.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-05-28T17:55:37.930" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4629" PostTypeId="2" ParentId="3211" CreationDate="2014-05-28T20:09:09.357" Score="14" Body="&lt;p&gt;&lt;strong&gt;Earth&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Carbon nanotubes might endure the enormous stress of an earth elevator but only short lengths have been manufactured so far.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be a mega engineering project that would dwarf earlier human endeavors. An earth elevator would need to extend at least to geosynchronous orbit at about 36,000 km altitude. And unless there were a truly enormous counterweight just beyond geosynch, it would have to extend well beyond geosynchronous orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A problem not often considered is &lt;em&gt;throughput&lt;/em&gt;. An elevator car's mass adds to stress but not tensile strength, so we can't have too many elevator cars. And how fast will the elevator cars travel? They will most likely need their own power source.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We don't don't know the number of elevator cars or their speeds, but it's possible the &lt;a href=&quot;http://www.spaceward.org/elevator-feasibility&quot;&gt;small throughput wouldn't be sufficient to maintain the elevator&lt;/a&gt;, much less send humanity spaceward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another problem is orbital debris. At an altitude of 300 kilometers and above, the elevator would be passing through a debris cloud moving about 8 km/s w.r.t. the elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Moon&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two quasi-stable regions that hover over fixed regions on the moon's surface. Earth Moon Lagrange 1 and Earth Moon Lagrange 2 (EML1 and 2). These hover over the moon's near and far points, respectively. Elevators from EML1 or EML2 would endure much less stress than an earthly elevator -- it's possible to make this from Kevlar rather than exotic materials like carbon nanotubes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EML1 and 2 are about 60,000 km from the moon's surface. It would have to be even longer than an earth elevator. So throughput is also an issue with a lunar elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Debris density in a lunar elevator's neighborhood is much lower than in low earth orbit. So it is much less vulnerable than an earth elevator to random collisions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Phobos&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Phobos has a minute gravity well and high angular velocity (a revolution each 7 hours, if memory serves). Like our moon, Phobos is tide-locked. The Mars Phobos L1 and L2 points hover about 3 kilometers over Phobos' surface. An elevator extending 10 or 20 kilometers beyond this points is sufficient to maintain tension.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A tether from Stickney Crater to Mars' upper atmosphere would be about 5600 km in length. Throughput is much less of an issue. The tether foot would be moving about 0.6 km/s w.r.t. Mars' surface. Small suborbital hops from Mars would suffice for foot rendezvous. A lander released into Mars atmosphere at 0.6 km/s would suffer much less entry, descent and landing issues than the typical Mars lander entering at 6 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going outward from Phobos: 2500 kilometers out the tether is moving at Mars' escape velocity. 6200 kilometers out this tether is moving fast enough for trans-Earth insertion. 8000 kilometers out, a Phobos elevator is moving fast enough for trans-Ceres insertion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ceres and Vesta&lt;/strong&gt;&#xA;Both these asteroids have shallow gravity wells and spin a lot faster than earth. So elevators could be much shorter and endure less stress.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a chart from my &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2012/09/beanstalks-elevators-clarke-towers.html&quot;&gt;space elevator page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FLZzK.png&quot; alt=&quot;Elevator Chart&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The top altitude column is how tall the elevator would need to be if there were no counterweight to provide tension. Taper ratio is ratio of tether thickness at synchronous orbit to tether thickness at surface.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="-1" LastEditDate="2014-08-12T13:39:21.447" LastActivityDate="2014-08-12T13:39:21.447" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4631" PostTypeId="1" AcceptedAnswerId="4632" CreationDate="2014-05-29T00:19:56.420" Score="-1" ViewCount="148" Body="&lt;p&gt;I've seen in several places that there seems to be an extinction cycle on earth of approximately 27 million years. I've read this here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-1294372/Life-Earth-wiped-27-million-years.html&quot; rel=&quot;nofollow&quot;&gt;http://www.dailymail.co.uk/sciencetech/article-1294372/Life-Earth-wiped-27-million-years.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And also saw a similar explanation on Cosmos last week. Since the Earth is about 4.54 billion years old, that means there have been around 168 different extinction cycles. From what I understand, the Earth will either heat or freeze at extreme temperatures and then either cool or warm until water and life is a possibility again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that this is correct (and definitely correct if I'm wrong, I'm eager to learn!), what are the chances there was a full civilization like ours that got completely wiped out in a different extinction cycle than ours? Is it possible or are there other factors in the history of our planet that would have prevented life from flourished to the point that we are in now?&lt;/p&gt;&#xA;" OwnerUserId="3423" LastActivityDate="2014-05-29T01:42:44.817" Title="Life on Earth previous to us" Tags="|life|" AnswerCount="1" CommentCount="2" ClosedDate="2014-05-29T10:53:38.757" ContentLicense="CC BY-SA 3.0" />
  <row Id="4632" PostTypeId="2" ParentId="4631" CreationDate="2014-05-29T01:42:44.817" Score="4" Body="&lt;p&gt;It's a mesmerizing thought to consider that there may have been an advanced, perhaps spacefaring civilization of &quot;first people&quot; on Earth many millions of years ago, which was wiped out by some global catastrophe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, the reality is not that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any advanced civilization would be composed of beings with certain properties we find on Earth only in humans. To build a city you need a society, so you need large brains capable of functioning in a complex social setting. You need the ability to formulate and communicate abstract concepts, so you need language, and again, a big brain. And, you need appendages capable of a strong and precise grasp, capable of manipulating tools and other objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a clear fossil record which shows the evolutionary history of our planet, laid down in sedimentary rock. The same rock shows the extinction events you referred to. Those &quot;pre-extinction&quot; records show no evidence of any creature exhibiting the sort of properties needed to build a city. And, there are no artifacts or fossilized impressions of artifacts buried deep down amongst the dinosaurs or other ancient creatures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fossil record shows that big brains are a relatively recent innovation - much more recent than the last extinction event. So, we humans are the first people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps some future catastrophe will wipe out our civilization, only to be reborn a million years further on. If that were to happen, even if no human survives the event, no city remains standing, there will still be evidence of our existence preserved in rock for many millions of years. It may not be obvious to the untrained eye, but to a person (or future-being) who knows what they are looking at, it will be incontrovertible proof that we were once here.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-05-29T01:42:44.817" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4633" PostTypeId="2" ParentId="1081" CreationDate="2014-05-29T03:39:59.110" Score="1" Body="&lt;p&gt;Without defining an objective function (i.e. what you are trying to optimize) the question is ill-formed. What defines optimal? If I say 6 satellites and someone else says 5, without an objective function you have no objective means of determining who is correct. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two trivial examples:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;If the objective is the cheapest option and there are no other technical requirements being met, then one satellite is the answer. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If the objective is to maximize revisit time over a certain target then infinity satellites is the answer. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Both examples are degenerate and serve to illustrate that in practice objective functions tend to be weighted considerations; for example, a balance of performance and cost.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2014-05-29T03:39:59.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4634" PostTypeId="1" AcceptedAnswerId="4636" CreationDate="2014-05-29T03:40:59.190" Score="8" ViewCount="1022" Body="&lt;p&gt;If the New Horizons team &lt;a href=&quot;http://www.huffingtonpost.com/2014/05/21/pluto-probe-new-horizons-crisis_n_5364138.html&quot;&gt;doesn't find&lt;/a&gt; a KBO target for the probe, can they turn it into an orbiter for the Pluto system? I know entering orbit right now  would be impossible, but would it be possible to achieve this by making a first fly by, turning back and getting some help from &lt;a href=&quot;http://www.solarsystemscope.com/&quot;&gt;Saturn or Jupiter&lt;/a&gt; in the future years?&lt;/p&gt;&#xA;" OwnerUserId="1263" LastActivityDate="2015-07-21T16:22:44.423" Title="Can New Horizons be turned into an orbiter for the Pluto system?" Tags="|gravity-assist|new-horizons|pluto|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4635" PostTypeId="1" AcceptedAnswerId="4638" CreationDate="2014-05-29T03:44:23.763" Score="7" ViewCount="1294" Body="&lt;p&gt;In today's Soyuz TMA-13M launch, the vehicle seemed to take a while to lift off after the boosters fired up. &lt;a href=&quot;http://youtu.be/MBL29YEgIOQ?t=33s&quot; rel=&quot;nofollow&quot;&gt;Here is the video.&lt;/a&gt; A huge flame and cloud of exhaust build up around the Soyuz, and it seems dangerous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was this an abnormal amount of time for the launch tower arms to hold onto the launch vehicle after booster ignition? If so, what went wrong? &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="13208" LastEditDate="2017-02-27T21:34:24.690" LastActivityDate="2017-02-27T21:34:24.690" Title="Did something go wrong during the Soyuz TMA-13M launch?" Tags="|launch|soyuz-launcher|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4636" PostTypeId="2" ParentId="4634" CreationDate="2014-05-29T03:51:38.997" Score="10" Body="&lt;p&gt;See &lt;a href=&quot;https://www.youtube.com/watch?v=78U0_XcFP_I&quot;&gt;this video&lt;/a&gt; of New Horizon's trajectory. After it flies by Pluto, it's not coming back. It's leaving the solar system on a hyperbolic trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;If the hyperbolic periapsis was moved to within 10 km of Pluto's surface, the turning angle would be increased to about .4 degrees. Not much of a path change, not sufficient to send it back towards one of the gas giants. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-05-29T11:53:22.803" LastActivityDate="2014-05-29T11:53:22.803" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4637" PostTypeId="2" ParentId="3161" CreationDate="2014-05-29T06:19:19.197" Score="7" Body="&lt;p&gt;A good question.  In pre-&lt;a href=&quot;https://en.wikipedia.org/wiki/Evolved_Expendable_Launch_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;EELV&lt;/a&gt; studies, NASA and the U.S. Air Force looked at LOX/methane.  EELV resulted in the LOX/kerosene &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;noreferrer&quot;&gt;Atlas V&lt;/a&gt; and the LOX/hydrogen &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV&quot; rel=&quot;noreferrer&quot;&gt;Delta IV&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;At the 4th International Conference on Launcher Technology in 2002, &lt;a href=&quot;http://thehuwaldtfamily.org/jtrl/research/Propulsion/Rocket%20Propulsion/Propellants/DLR,%20Comparative%20Study%20of%20Kerosene%20and%20Methane%20Engines.pdf&quot; rel=&quot;noreferrer&quot;&gt;Burkhardt et al.&lt;/a&gt; compared a reusable LOX/kerosene launch vehicle using the RD-180 type engine from the Atlas V with a LOX/methane vehicle using a possible engine of the same efficient staged combustion cycle:&lt;/p&gt;&#xA;&lt;p&gt;The LOX/methane engine had &lt;strong&gt;about 3% higher specific impulse&lt;/strong&gt; but that advantage was outweighed by the &lt;strong&gt;lower density&lt;/strong&gt; of the liquid methane compared to kerosene.&lt;/p&gt;&#xA;&lt;p&gt;LOX/kerosene was slightly better performing overall in terms of payload and expected to be lower cost to build and operate, the same result as the pre-EELV studies.&lt;/p&gt;&#xA;&lt;p&gt;The reason LOX/hydrogen is comparable to or better than LOX/kerosene is that the specific impulse is much higher overcoming the even lower density problem.  For the Space Shuttle the main engines operated from ground to orbit so the higher specific impulse of hydrogen at higher altitude was the reason for its use.&lt;/p&gt;&#xA;&lt;p&gt;For a first stage that only operates to low altitud followed by a LOX/hydrogen second stage as in the Atlas and Delta, kerosene has comparable payload performance and may be lower cost because of &lt;strong&gt;vehicle size&lt;/strong&gt;. For the Delta IV another advantage is &lt;strong&gt;commonality with the upper stage propellants&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Methane is not currently supplied at launch sites so a &lt;strong&gt;major facility investment would be needed&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Lack of long experience with operation&lt;/strong&gt; is another negative for methane.&lt;/p&gt;&#xA;&lt;p&gt;If the Raptor were to be used in space as &lt;strong&gt;in a Mars mission&lt;/strong&gt; then the fact that &lt;strong&gt;both LOX and liquid methane are relatively easy to store in space&lt;/strong&gt; compared to hydrogen or kerosene would be an advantage.&lt;/p&gt;&#xA;&lt;h2&gt;References:&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://thehuwaldtfamily.org/jtrl/research/Propulsion/Rocket%20Propulsion/Propellants/DLR,%20Comparative%20Study%20of%20Kerosene%20and%20Methane%20Engines.pdf&quot; rel=&quot;noreferrer&quot;&gt;Comparative Study of Kerosene and Methane Propellant Engines for Reusable Liquid Booster Stages&lt;/a&gt;. Holger Burkhardt, Martin Sippel, Armin Herbertz, Josef Klevanski. 4th International Conference on Launcher Technology &amp;quot;Space Launcher Liquid Propulsion&amp;quot; 3-6 December 2002 – Liège (Belgium).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="3424" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-05-29T08:00:28.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4638" PostTypeId="2" ParentId="4635" CreationDate="2014-05-29T08:38:49.300" Score="11" Body="&lt;p&gt;It seems to be normal. The Soyus (like Falcon 9 and Saturn V) use a hold-down, or hold before launch, feature. Engines are fired up while the rocket is physically held down. This is a safety measure, if the engines don't fire to full thrust without any detected anomaly, &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/spacex_orbcomm_presskit_final.pdf&quot; rel=&quot;nofollow&quot;&gt;the launch is automatically canceled&lt;/a&gt; (page 8) by shutting down the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The firing time on ground seems to fit with &lt;a href=&quot;http://spaceflightnow.com/soyuz/vs01/countdowntimeline.html&quot; rel=&quot;nofollow&quot;&gt;this description&lt;/a&gt; of the Soyus launch sequence.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-05-29T18:39:39.357" LastActivityDate="2014-05-29T18:39:39.357" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4639" PostTypeId="2" ParentId="4635" CreationDate="2014-05-29T10:49:26.517" Score="11" Body="&lt;p&gt;Nothing went wrong and the TMA-13M crew (Alexander Gerst (ESA), Maxim Suraev (Roscosmos) and Reid Wiseman (NASA)) are by now onboard the ISS. The way it looked to me (watched it live) was that the camera's aperture ratio wasn't adjusted fast enough with the fast changing incident light, so the frames were overexposed as the Soyuz-FG rocket engines were ignited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A combination of rocket's own exhaust, reflected light from the deflected exhaust plume (Soyuz rockets launched from Baikonur Cosmodrome are placed over a quarry-like &quot;hole in the ground&quot;, for lack of a better word) additionally illuminating the site, and camera's &lt;a href=&quot;https://en.wikipedia.org/wiki/F-number&quot;&gt;F-Stop&lt;/a&gt; in the same or similar position as prior to the engine ignition with dark night's background and the whole site only illuminated by a set of reflector lights, meant that the live transmission camera produced an overexposed image during liftoff. I could swear that the camera's position was also slightly more towards the &quot;quarry&quot; this time, more in the way of the deflected exhaust plume, but I can't confirm it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'd have to say it just appeared that there was more flame than it should have been, and as @LocalFluff mentioned in his answer, no anomalies were reported.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-05-29T10:49:26.517" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4640" PostTypeId="1" AcceptedAnswerId="4642" CreationDate="2014-05-29T14:31:48.553" Score="13" ViewCount="2161" Body="&lt;p&gt;Europa orbits Jupiter at an average speed of &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/galileanfact_table.html&quot;&gt;13.7 km/s&lt;/a&gt;. At the same time Jupiter orbits the Sun at a speed of &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html&quot;&gt;13.1 km/s&lt;/a&gt;. This means that Europa has an orbital speed relative to the Sun of &lt;strong&gt;26.8&lt;/strong&gt; km/s when it is in the shadow behind Jupiter once every 85 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's orbital speed is &lt;strong&gt;29.8&lt;/strong&gt; km/s which is only 3.0 km/s different from Europa's max solar orbital speed. Compared with Mars which has an orbital speed of &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/galileanfact_table.html&quot;&gt;24.0 km/s&lt;/a&gt; which is 5.8 km/s different form Earth's.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does this mean that it is &lt;em&gt;easier&lt;/em&gt; to land a spacecraft on Europa, than it is to land on Mars?&lt;/li&gt;&#xA;&lt;li&gt;Io with 17.3 km/s around Jupiter gives it a max of 13.1 + 17.3 = 30.4 km/s around the Sun when in shadow once every 43 hours. Just a 0.6 km/s difference, our own Luna orbits us at 1.0 km/s. Does any sizable object have a more similar speed to Earth than Io does?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And considering that Europa has no atmosphere and just about 1/3 of Mars' surface gravity, it looks like an easy target to go to! According to a table on &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/appmissiontable.php&quot;&gt;this page&lt;/a&gt; (which refers to a calculator by Erik Max Francis) a Hohmann transfer from Earth to Jupiter takes 2 years and 9 months. That's pretty quick compared to many of the current missions which travel even 10 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shouldn't also radiation problems be smaller for a lander than for an orbiter (either of Europa itself or an orbiter around Jupiter which makes repeated close Europa fly bys) because the moon covers half of the radiating source. And even more in a deep crack. And sensitive electronics might perhaps be drilled/melted down a few meters into the ice to get complete shielding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a monster factor in the Jupiter system which makes it much harder to land on Europa (or Io) than I've made it look here? Harder than to land on Mars.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-05-29T21:12:16.957" Title="Is it easier to land on Europa/Io than on Mars? What are the main considerations?" Tags="|orbital-mechanics|lander|europa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4641" PostTypeId="2" ParentId="2076" CreationDate="2014-05-29T14:33:41.130" Score="4" Body="&lt;p&gt;Outer space begins at the Kármán line (correctly stated by @TildalWave). This has been accepted by Fédération Aéronautique Internationale (FIA). Quoting &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Kármán line, or Karman line, lies at an altitude of 100 kilometres (62 mi) above the Earth's sea level, and commonly represents the boundary between the Earth's atmosphere and outer space. This definition is accepted by the Fédération Aéronautique Internationale (FAI), which is an international standard setting and record-keeping body for aeronautics and astronautics.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But it is &lt;strong&gt;not&lt;/strong&gt; where atmospheric weather phenomena ceases. Weather phenomena typically ceases in the thermosphere which reaches 8-15 km above the Earth (18 km above equator), but stratospheric weather phenomena does exist, e.g. nacreous or &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_stratospheric_cloud&quot; rel=&quot;nofollow&quot;&gt;polar stratospheric clouds&lt;/a&gt; which can form as high as 25 km above the mean sea-level. Still, that's far below the Kármán line at 100 km above the Earth's surface.&lt;/p&gt;&#xA;" OwnerUserId="3419" LastEditorUserId="49" LastEditDate="2015-05-28T23:19:59.177" LastActivityDate="2015-05-28T23:19:59.177" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4642" PostTypeId="2" ParentId="4640" CreationDate="2014-05-29T17:03:59.067" Score="9" Body="&lt;p&gt;Francis' Python BOTEC looks at &lt;strong&gt;Hohmann&lt;/strong&gt; orbits. To simplify he assumes circular coplanar orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a pic comparing an earth-to-Mars Hohmann vs an earth-to-Jupiter Hohmann:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mBO1v.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At perihelion the Mars transfer orbit is moving 33 km/s vs earth's 30 km/s. Departure Vinf is 3 km/s. At aphelion the transfer is 21.5 km/s vs Mars 24 km/s. Arrival Vinf is 2.5 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At perihelion the Jupiter transfer orbit is 39 km/s. Departure Vinf is 9 km/s. At aphelion the transfer is 7.5 km/s vs Jupiter's 13 km/s. Arrival Vinf is 5.5 km/s. You wouldn't want to rendezvous with the moon when it's on the far side of the moon when it's moving fastest wrt sun. Rather on the near side of Jupiter when the moon's moving slowest wrt to the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To depart for Trans Mars insertion you would need to inject into a hyperbolic orbit wrt earth. Speed of a hyperbolic orbit is sqrt(vinf^2 + Vesc^2). To remember speed of a hyperbolic orbit I use this device:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/s0mKh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At LEO, Vesc is about 11 km/s. Vinf is for Mars insertion is 3 km/s. Sqrt(11^2 + 3^2) = 11.4 km/s. LEO velocity is about 7.8 km/s. So from LEO you'd need 3.6 km/s for Trans Mars Insertion (TMI).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From LEO Trans Jupiter Insertion (TJI) is about 6.3 km/s. The departure delta V is much higher for a Jupiter trip than a Mars trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Francis' table shows delta Vs from a low circular orbit about 1 planet to a low circular orbit about another planet. It takes a lot less delta V to park in a capture orbit. I talk about this at &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2012/06/inflated-delta-vs.html&quot; rel=&quot;noreferrer&quot;&gt;Inflated Delta Vs&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can make Mars' atmosphere work for us. It suffices to park in a  Mars capture orbit with periapsis passing through Mars upper atmosphere. At each periapsis atmospheric friction will slow the orbit, thus saving reaction mass. .7 km/s is needed to park in a Mars capture orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exiting an earth-to-Jupiter orbit and parking in a 599408 km altitude orbit (Europa) takes 6.5 km/s. Parking in Io altitude orbit takes about 7.8 km/s. That doesn't take into account descending the moons' gravity wells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, takes a lot less delta V to get to Mars than Jupiter's moons  &lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-05-29T21:12:16.957" LastActivityDate="2014-05-29T21:12:16.957" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4643" PostTypeId="1" AcceptedAnswerId="4644" CreationDate="2014-05-30T10:52:02.880" Score="19" ViewCount="2377" Body="&lt;p&gt;In the 1985 work &lt;em&gt;'The Cat Who Walks Through Walls'&lt;/em&gt; by Robert Heinlein, Richard Ames makes the following internal observation; The character is in an underground abode. The accommodations are in a side room off of a main corridor. The atmosphere is appropriate for shirt sleeves, think of it like being in a large mall with heat and air, low gravity being the only noticeable indicator you are not on Earth.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At one-sixth gee a rock shelf is softer than a foam mattress in Iowa. I went to sleep quickly.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://scifi.stackexchange.com/questions/57193/soft-as-a-rock-bed-on-the-moon-seeking-source-quote/57202#57202&quot;&gt;(&lt;em&gt;Source&lt;/em&gt;)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a lunar colonist find a rock shelf softer than a foam mattress? Would you only need a blanket and flat &amp;amp; level surface of solid rock to sleep comfortably in Moon's gravity?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:43:38.653" LastActivityDate="2014-06-04T15:17:15.067" Title="Do you need a mattress on the Moon?" Tags="|colonization|lunar-landing|habitat|myth-busting|sleep|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4644" PostTypeId="2" ParentId="4643" CreationDate="2014-05-30T12:26:57.123" Score="17" Body="&lt;p&gt;&lt;strong&gt;Probably not.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good measure of comfort is how much external body pressure is induced from laying on a hard surface. Earth gravity is enough to cause an adult considerable discomfort when lying on a hard surface. &lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Paresthesia&quot;&gt;Paresthesia&lt;/a&gt;, or the sensation of having arms and limbs falling asleep, would be a good standard to measure against to answer this question.&lt;/strong&gt; The sensation can be &lt;strong&gt;felt with as little as 20 mmHg (2.9 kPa) or just 1/2 pound per square inch&lt;/strong&gt; according to this detailed description from fact finding website &lt;a href=&quot;http://www.todayifoundout.com/index.php/2012/01/what-causes-arms-legs-and-feet-to-fall-asleep/&quot;&gt;TodayIFoundOut&lt;/a&gt;. &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/2419514&quot;&gt;One study&lt;/a&gt; from the Journal of Neurological Science &quot;Effects of graded experimental compression on slow and fast axonal transport in rabbit vagus nerve&quot; gives some details on pressure points including discussion of human implications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;For the sake of argument:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Any &lt;strong&gt;external pressure on the body in excess of 20 mmHg (2.7 kPa), or 1/2 lb per&#xA;square inch, would be enough to make a mattress a necessity.&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;The &lt;strong&gt;sleeping surface is flat&lt;/strong&gt;, even, and hard. If you were sleeping on lava&#xA;rocks, there would be some jagged surface points poking into the skin&#xA;that might give isolated pressure points.&lt;/li&gt;&#xA;&lt;li&gt;The person sleeping is &lt;strong&gt;average weight and size&lt;/strong&gt; at &lt;a href=&quot;http://en.wikipedia.org/wiki/Body_weight#Average_weight_around_the_world&quot;&gt;62.0 kg or 136.7 lbs&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Calculating the surface area of the body in contact with the sleeping surface will be a bit trickier to estimate. Again for the sake of argument let's assume that less than half of the body will be in contact with the surface, say 25% of the body. Ratios of body fat will give some ranges here too and there many calculations to choose from. &lt;a href=&quot;http://en.wikipedia.org/wiki/Body_surface_area#Calculation&quot;&gt;Wikipedia shows average Body Surface Areas of men and women at 1.6 and 1.9 m&lt;sup&gt;2&lt;/sup&gt;, respectively&lt;/a&gt;. Let's take the difference between men and women at an average of 1.75 m&lt;sup&gt;2&lt;/sup&gt; and 25% of that gives us 0.4375 m&lt;sup&gt;2&lt;/sup&gt; or roughly 4.71 ft&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So given the average weight at 136.7 lbs (62.0 kg) on Earth gravity, and assuming 25% body surface contact, laying on a flat surface will distribute the weight to 29 lbs per square foot (142 kg/m²) or 2.42 lbs/inch (43.2 kg/m). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;On the moon, gravity is 1/6th that of earth, so we can reduce weight to 23.8lbs. Our mass however does not change, it's still at 62.0kg so applying moon gravity (1.6249 m/s2) gives us a force of 0.101 kPa or just 0.756 mmHg or a cozy 0.015 lbs per square inch, not enough to induce Paresthesia. While it may not be &quot;comfortable&quot;, it would not be uncomfortable.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A purely hypothetical answer of course--one would have to actually simulate 1/6th Earth gravity to test and in &lt;a href=&quot;http://www.space.com/6984-apollo-astronauts-sleep-lunar-liftoff.html&quot;&gt;an interview with Space.com&lt;/a&gt;, the crew of Apollo 12 recalled what little sleep they had with little mention of physical comfort albeit &lt;a href=&quot;http://www.popsci.com/blog-network/vintage-space/seeing-inside-apollo-lunar-module&quot;&gt;they had hammocks to sleep on&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something else to keep in mind: &lt;strong&gt;People can comfortably lay on a bed of nails given enough surface area.&lt;/strong&gt; Comfort, of course, is subjective, but we know it is possible in Earth gravity. (&lt;a href=&quot;http://entertainment.howstuffworks.com/arts/circus-arts/bed-of-nails.htm&quot;&gt;http://entertainment.howstuffworks.com/arts/circus-arts/bed-of-nails.htm&lt;/a&gt;) Given this observation, it is not a stretch of imagination to assume this would be painless enough to sleep on given 1/6th Earth gravity on the moon.&lt;/p&gt;&#xA;" OwnerUserId="3161" LastEditorUserId="-1" LastEditDate="2014-05-30T16:07:57.350" LastActivityDate="2014-05-30T16:07:57.350" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4646" PostTypeId="1" AcceptedAnswerId="4647" CreationDate="2014-05-29T22:59:24.590" Score="63" ViewCount="23354" Body="&lt;p&gt;Living in space is pretty dangerous, and the wonderful people of the ISS float around the Earth at risk of radiation and micrometeoroids. I'm curious if they're any more safe than if they simply were on a base on the Moon. It seems to me the same dangers apply, but far more can be done with the microgravity and potential resources on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is more dangerous, how much more and in what ways? And if the danger isn't that much different, why aren't NASA/ESA/Russia doing it? &lt;/p&gt;&#xA;" OwnerUserId="3443" OwnerDisplayName="simontemplar" LastEditorUserId="6816" LastEditDate="2019-03-26T20:31:01.733" LastActivityDate="2019-03-26T20:31:01.733" Title="Is a Moon-base inherently more dangerous than a Space Station?" Tags="|iss|the-moon|radiation|meteorites|lunar-base|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4647" PostTypeId="2" ParentId="4646" CreationDate="2014-05-30T07:35:14.653" Score="64" Body="&lt;p&gt;Here are a few reasons, probably not the only ones:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Radiation. The Moon isn't protected by Earth's magnetic field, so it gets more radiation. This isn't good in the long term. Also, radiation shielding is heavy. Aluminium just won't do it. The ISS gets protection from the magnetosphere but not from the atmosphere, so the astronauts still get 1 millisievert per day - 5x as much radiation as on jet aircraft and 365x as much as on the ground.&lt;/li&gt;&#xA;&lt;li&gt;If something goes wrong, home is quite a long way away. The Apollo missions took a few days to reach the moon and a few days to come back. If something goes wrong on the ISS, the astronauts can get into a Soyuz capsule and be on the ground in a few hours (it's only ~300km away from Earth).&lt;/li&gt;&#xA;&lt;li&gt;Added difficulty of having to land on the Moon. A few probes have crashed attempting to do so (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_15&quot;&gt;Luna 15&lt;/a&gt;). It may be that the techique is perfected by now.&lt;/li&gt;&#xA;&lt;li&gt;Two weeks of complete darkness, every month. The ISS orbits the Earth in just over 90 minutes, so it switches between day and night very quickly. This is not the case for any object on the surface of the Moon. As a result, solar power is probably unusable (this was a problem for the Jade Rabbit rover), you would have to use some form of nuclear power such as that used by the Curiosity MSL rover.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://xxx.lanl.gov/abs/1206.6328&quot;&gt;Toxic&lt;/a&gt; moon dust. It was found in the lungs of Apollo astronauts (because the way they would get into their lander after an EVA was to go in, close the door, pressurise, and take off their dust-covered suits). A possible solution would be &lt;a href=&quot;https://en.wikipedia.org/wiki/Suitport&quot;&gt;a combination of docking port and spacesuit&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user3405" LastActivityDate="2014-05-30T13:25:05.047" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4649" PostTypeId="2" ParentId="4646" CreationDate="2014-05-30T14:59:32.563" Score="15" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;If it is more dangerous, how much more and in what ways?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Proffesorfish and Eli Skolas have both given thoughtful answers comparing the hazards of I.S.S. vs Moonbase. If humans were preceded by robots to establish infra-structure, I believe a moonbase could be &lt;strong&gt;less&lt;/strong&gt; hazardous. Radiation shielding from local resources could be added to Bigelow habs. At the poles there may be volatiles that could be harvested for life support as well as propellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for the second part of the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And if the danger isn't that much different, why aren't NASA/ESA/Russia doing it&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We don't have a moon base because it's a lot harder. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest difference between I.S.S. and a lunar base is delta V. It takes 9 km/s to reach the I.S.S. and 15 km/s to reach the lunar surface. At this time there's no infra-structure and thus no propellent available at the lunar surface. So another 3 km/s must be added for the return trip. An 18 km/s delta v budget is vastly different from a 9 km/s delta V budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A capsule from the I.S.S. re-enters at 8 km/s and one from the moon would re-enter earth's atmosphere at about 11 km/s. More robust structure and thermal protection would be needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also a lunar soft landing is a lot harder than rendezvous with the I.S.S.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="49" LastEditDate="2015-01-23T14:43:15.937" LastActivityDate="2015-01-23T14:43:15.937" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4650" PostTypeId="1" AcceptedAnswerId="4659" CreationDate="2014-05-30T15:47:32.407" Score="16" ViewCount="51790" Body="&lt;p&gt;The ISS is in orbit about 250 miles high, the students in &lt;a href=&quot;https://space.stackexchange.com/questions/2076&quot;&gt;this related question&lt;/a&gt; get 62 miles in less than a minute.  This &lt;a href=&quot;http://www.nasa.gov/press/2014/may/new-crew-launches-to-space-station-to-continue-scientific-research/#.U4ilzSjtN8E&quot; rel=&quot;noreferrer&quot;&gt;NASA Press release&lt;/a&gt; indicates the The Soyuz TMA-13M launch on May 2014 has a 6 hour trip duration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 65 miles per hour I can drive 250 miles in less than 4 hours.  Is it really a 6 hour trip by rocket?   If so why? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2018-12-15T23:33:53.067" Title="How long does it take to get to ISS?" Tags="|launch|iss|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4652" PostTypeId="2" ParentId="203" CreationDate="2014-05-30T17:08:24.640" Score="0" Body="&lt;p&gt;The critical issue isn't height but Speed. Could a combination of precooled jets, ion drives and rockets enable us to journey into space cheaper, safer and more efficiently than the current rocket based method? Using a precooled jet engine to gain altitude then using an ion drive to give extra thrust you could achieve the required velocity. Reversing the process you could land the craft conventionally removing the need for heat shields and other hefty re-entry devices. &lt;a href=&quot;http://youtu.be/KFL623O9CXQ&quot; rel=&quot;nofollow&quot;&gt;http://youtu.be/KFL623O9CXQ&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3437" LastActivityDate="2014-05-30T17:08:24.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4653" PostTypeId="2" ParentId="203" CreationDate="2014-05-30T17:45:41.350" Score="8" Body="&lt;p&gt;Multiple posters have mentioned but dismissed beanstalks because materials science isn't up to producing a sufficiently strong cable.  So far nobody has mentioned the other ladder into space approach--launch loops.  They don't require super materials because they are supported by objects moving faster than orbital velocity.  The big downside is that they are supported by moving objects--if they quit moving they fall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic idea is you build two stations that will pitch iron bars back and forth.  You use extremely strong magnets to turn the bars around, once the bars are moving the energy input is minimal.  They originally fly back and forth on a ballistic trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you build a track on top of the flying bars.  The track deflects the bars downward, getting lift in the process.  The speed of the bars is upped to compensate--they continue to follow the same course.  Now you have what amounts to a train track that extends into space.  There's no upper limit on the speed of a maglev train in vacuum.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Technically possible:  Yes.&#xA;Cost:  Tremendous&#xA;Safety:  Not for me!&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;There's also the combination of the two ideas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Build a set of towers in a ring around the Earth.  They support a tube, inside the tube you have an iron ring.  Spin the ring above orbital velocity, generate enough upward force to counteract the weight of the towers.  Since we have countered the load of the towers we are free to build higher.  Then build another ring to take the load.  Repeat until you are as high as you want to go.  If you're going past geosync the rings turn below orbital velocity in order to generate downward force rather than upward force.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Technically possible:  Yes.  You can build the rings and towers as close as needed to cope with material limits.&#xA;Cost:  Tremendous^2.&#xA;Safety:  While there are moving parts it's not like the launch loop that requires a turning station.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-05-30T17:45:41.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4655" PostTypeId="1" CreationDate="2014-05-30T21:10:39.240" Score="9" ViewCount="6164" Body="&lt;p&gt;If Orion is being designed for interplanetary space travel, where will the crew live for the long duration?  The Orion Crew Module is not much bigger than Apollo and I would not think a crew of 4-6 could spend months in such a small space unless they were in some sort of hibernation.&lt;/p&gt;&#xA;" OwnerUserId="3439" LastActivityDate="2016-08-26T18:12:06.073" Title="Is the Orion Crew Module large enough for long duration flight?" Tags="|interplanetary|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4656" PostTypeId="2" ParentId="4655" CreationDate="2014-05-30T21:35:24.527" Score="7" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_Service_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion Service Module&lt;/a&gt; does not extend the habitat area, but it does extend the configuration's life support capabilities by, &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Highlights/Orion&quot; rel=&quot;nofollow noreferrer&quot;&gt;according to ESA&lt;/a&gt;, &lt;em&gt;providing propulsion, power, thermal control, as well as supplying water and gas to the astronauts in the habitable module.&lt;/em&gt; The &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion MPCV&lt;/a&gt; (Multi-Purpose Crew Vehicle) however will come with &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Docking_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Docking System&lt;/a&gt;, allowing it to mate in orbit with other modules, such as e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Habitat&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Habitat&lt;/a&gt;. Wikipedia on Orion MPCV puts it like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The MPCV resembles its Apollo-era predecessors, but its technology and&#xA;  capability are more advanced. It is designed to support long-duration&#xA;  deep space missions of up to 21 days maximum active crew time plus 6&#xA;  months quiescent. During the quiescent period crew life support would&#xA;  be provided by another module such as a Deep Space Habitat. The&#xA;  spacecraft's life support, propulsion, thermal protection and avionics&#xA;  systems are designed to be upgradeable as new technologies become&#xA;  available.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And here's an image of a concept full-length Orion configuration, again from &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Highlights/Orion&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA pages&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/1Ss7h.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, the Orion Crew Module itself, or together with the Orion Service Module aren't enough to support long duration manned missions beyond cislunar space or missions of over &lt;em&gt;21 days maximum active crew&lt;/em&gt;, and the Crew Module and Service Module that will be launched atop the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;SLS&lt;/a&gt; (Space Launch System) Crew Block will have to dock in-orbit with previously launched Deep Space Habitat or other habitat modules atop the SLS Cargo Block launchers.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-05-30T23:05:29.257" LastActivityDate="2014-05-30T23:05:29.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4657" PostTypeId="2" ParentId="4608" CreationDate="2014-05-30T21:55:45.403" Score="14" Body="&lt;p&gt;The longevity of the crew in terms of water and air recycling, as far as I know, is the easy part. The true test is the caloric intake of the two inhabitants and the longevity of the ISS itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The largest issues are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Food &amp;amp; water - there will be a limited supply for the inhabitants and once the food and water are used up (water is also used to &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2000/ast13nov_1/&quot;&gt;generate oxygen via electrolysis&lt;/a&gt;) there is no way to get more, assuming your dire situation continues with the Earth. This aspect depends on how much food and water is/was stocked at the time of the crisis and how well the two ration these resources.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Orbit - perhaps the largest issue hindering long term survival in the ISS would be that the space station is constantly losing altitude. The orbital path of the space station is roughly 370 km (average between min and max altitude ranges) and loses roughly 100 m of altitude per day. Currently the space station is &quot;boosted&quot; back up to a higher orbit periodically by the Soyuz and the Zvezda module's 2 main engines (formerly by the Space Shuttle when it was still in service). Without those constant visits and the ability of the Zvezda module, it is only a matter of time before the ISS will de-orbit itself.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Possible solution to the Orbit issue: &lt;a href=&quot;http://www.parabolicarc.com/2013/12/19/ad-astra-nasa-agree-continue-work-vasimr-engine/&quot;&gt;NASA signed a deal with Ad Astra to test out the VASIMR&lt;/a&gt; (basically a electromagnetic thruster) which could make boosting the space station easier and cheaper, but that is not slated to join the ISS until 2015.&lt;/p&gt;&#xA;" OwnerUserId="3440" LastEditorUserId="49" LastEditDate="2014-05-30T23:43:27.377" LastActivityDate="2014-05-30T23:43:27.377" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4658" PostTypeId="1" AcceptedAnswerId="4670" CreationDate="2014-05-30T23:02:16.727" Score="7" ViewCount="1403" Body="&lt;p&gt;Trying to ask a series of questions but I'm breaking up the overall point into easier &quot;chunks&quot; to not make the overall question too broad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question 1: Designing a craft for transfer from Earth to Lunar Orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Situation: Move three people, and/or supplies to lunar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;For ease of calculations, say the mass of the vehicle is 31,000 kg (similar mass of the SM and CM for Apollo 11) including the mass of the solar sail.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Starting Earth orbit is LEO, say 600 km (to greatly reduce drag rather than being at ISS levels), and the ending orbit around the Moon will be at 300 km (also similar to Apollo 11's orbit).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;According to wikipedia: 800 x 800 m sail would generate 5 newtons (1.1 lbf) of force.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;OR hoping someone can use the thrust calculations that apply: $$F=\frac{2RSA}{c}\sin^2\theta=9.113\times10^{-6}\frac{RA}{D^2}\sin^2\theta$$&#xA;Where, $F$ is the thrust; $R$ is the fraction of incident light; $D$ is the distance from the Sun in astronomical units; $S$ the solar flux in $W/m^2$; $c$ the speed of light; $A$ the sail area in $m^2$ and $\theta$ the sail tilt angle.(via Chris R.'s post: &lt;a href=&quot;https://space.stackexchange.com/questions/4587/solar-sail-thrust-calculation&quot;&gt;Solar sail thrust calculation&lt;/a&gt;)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How large would the sails need to be to propel the ship from Earth to Lunar Orbit with a turnaround of 5 days (Apollo 11 took 4 days 6 hours 45 mins)? (relevant sub-question: what would be the mass of the sail itself, subtract that from the total assumed mass of 31,000 kg)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Would this method be feasible to perform the transfer orbit?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Could this trip be repeatable? As in, could they return from Lunar orbit to that of the Earth? - POSSIBLY ANSWERED by TidalWave (see comments)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I know there are issues like this trip like radiation protection, can only be made at certain times, etc. What I'm hoping is to basically prove or disprove a personal hypothesis about the feasibility and sustainability of a solar sail &quot;powered&quot; craft for purposes of shipping. More on what is shipped in another question :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I tried to make this as concise and fill as many variables as I could so that those who are more in the know with this stuff and the math (that's giving me a headache LOL) could have an easier go. If I didn't explain something clearly enough or you need me to fill in more variables I can try my best to do that. Thanks in advance to those who answer :)&lt;/p&gt;&#xA;" OwnerUserId="3440" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-11-07T21:06:55.773" Title="Is a manned craft powered by Solar Sail feasible from Earth Orbit to Lunar Orbit" Tags="|orbital-mechanics|propulsion|orbital-maneuver|solar-sail|advanced-propulsion|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4659" PostTypeId="2" ParentId="4650" CreationDate="2014-05-31T00:28:58.867" Score="21" Body="&lt;p&gt;Docking is a slow process. The Space Shuttle rendezvous timeline took 6 hours from start to finish. With 4½ hours to go, the Shuttle was 250,000 feet (76 km) behind the Space Station. With 3 hours to go, the Shuttle was about 50,000 feet (15 km) behind. Relative velocities became slower as the Shuttle closed in on the Station. The last 400 feet were incredibly slow, taking about 40 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's typical. Docking is slow. The Soyuz expedited docking is rather bold. Approach and rendezvous is typically measured in days, not hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;Update&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is docking such a slow process? The answer is safety, errors (faults), uncertainties, and reaching the target (phasing). I'll deal with these in reverse order.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Achieving the Space Station's orbit is easy. Just do a Hohmann transfer and be done with it.  That's not nearly good enough when it comes to rendezvous. The target vehicle has to reach a specific point on the Space Station's orbit at a specific time to dock or berth with it. Having spare time helps. A vehicle could make the Hohmann transfer from the insertion orbit to the Station orbit immediately after launch, but that means zero tolerance for uncertainties, errors, and safety. An easy solution for dealing with reaching the target: Take your time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are lots of uncertainties in launching a vehicle into space. The vehicle might well be placed in orbit, but not the planned one. Having a 20 km uncertainty in altitude an even bigger uncertainty in along-track is typical. That launch uncertainty was one of the key reasons TMA-12M had to use the standard slow approach rather than the planned fast track approach. Uncertainties pop up all over the place. An easy solution to dealing with these uncertainties: Take your time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a big difference between uncertainties and errors. Uncertainties are what confound the vehicle when everything is behaving according to spec. Errors are what confound the vehicle when things don't behave according to spec. A thruster might provide 97% of nominal thrust one time, 102% the next. That's uncertainty. When the thruster is commanded off and a valve breaks so it stays off, permanently: That's a fault. That failed valve doesn't manifest itself as an error until the next time the control system commands the thruster on. The thruster remains off, just as expected, up until that &quot;thruster on&quot; command.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rendezvous has to be fault tolerant. In fact, it has to be two failure safe for unmanned vehicles coming to the Station, and two failure tolerant for manned vehicles. The difference between the two: An unmanned SpaceX that goes reeling off into space because it suffered two failures is a loss, but a manageable loss, so long as it doesn't crash into the Station. A manned vehicle that goes reeling off into space because it suffered two failures is not a manageable loss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dealing with those failures takes time. Ensuring that a vehicle that has sustained two failures will not collide mandates a gradual phasing (which takes time) and a very, very slow final approach (which takes a lot of time). The Station safety rules dictate a rather cautious approach to rendezvous.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-06-02T02:58:40.410" LastActivityDate="2014-06-02T02:58:40.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4660" PostTypeId="1" AcceptedAnswerId="4664" CreationDate="2014-05-31T03:24:22.430" Score="7" ViewCount="481" Body="&lt;p&gt;Pioneer 10 was about &lt;a href=&quot;http://en.wikipedia.org/wiki/Pioneer_10#Timeline&quot;&gt;80 AU&lt;/a&gt; when contact was lost in 2003 and both Voyager 1 and 2 traveled more than &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_1#Interstellar_medium&quot;&gt;100 AU&lt;/a&gt; and they are still operating. This put those 3 probes for years in the range of &lt;a href=&quot;http://en.wikipedia.org/wiki/Kuiper_belt&quot;&gt;Kuiper belt&lt;/a&gt; and even close to orbital aphelion on an &lt;a href=&quot;http://en.wikipedia.org/wiki/Scattered_disc&quot;&gt;SDO&lt;/a&gt;. I know that those 2 zones are pretty rarefied, the possibility of a close encounter is minimal and no gravitational deviations beside the &lt;a href=&quot;http://en.wikipedia.org/wiki/Pioneer_anomaly&quot;&gt;Pioneer anomaly&lt;/a&gt; were observed. My question: one of &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_2#Scientific_instruments&quot;&gt;the instruments&lt;/a&gt; on board of these probes could have detect an object, in case of a distant encounter?&lt;/p&gt;&#xA;" OwnerUserId="1263" LastActivityDate="2014-05-31T11:37:54.463" Title="Any evidence for a KBO fly by from one of the Voyagers or Pioneers?" Tags="|voyager|pioneer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4661" PostTypeId="2" ParentId="2076" CreationDate="2014-05-31T04:41:04.130" Score="5" Body="&lt;p&gt;To contradict everyone else, the US Air Force used an altitude of 50 miles when awarding the &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronaut_Badge&quot; rel=&quot;noreferrer&quot;&gt;Astronaut Badge&lt;/a&gt; to pilots of the X-15.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-05-31T04:41:04.130" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4662" PostTypeId="2" ParentId="203" CreationDate="2014-05-31T08:49:12.390" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.jpaerospace.com/&quot; rel=&quot;nofollow&quot;&gt;JP Aerospace&lt;/a&gt;, which currently has the &lt;a href=&quot;http://www.theregister.co.uk/2011/11/02/space_airships/&quot; rel=&quot;nofollow&quot;&gt;altitude record for airships&lt;/a&gt; at 95,085 feet, plans to fly &lt;strong&gt;&lt;a href=&quot;http://www.jpaerospace.com/atohandout.pdf&quot; rel=&quot;nofollow&quot;&gt;airships to orbit&lt;/a&gt;&lt;/strong&gt;. Once boyancy has taken it above almost all of the atmosphere, it will however use a rocket engine of sorts, an electrochemical propulsion which during several hours accelerates it to orbital velocity. &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_airship&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; has an overview of orbital airships and here's a lengthy &lt;a href=&quot;http://thespaceshow.wordpress.com/?s=airship&amp;amp;searchbutton=go%21&quot; rel=&quot;nofollow&quot;&gt;radio interview&lt;/a&gt; with John Powell of JP Aerospace.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-05-31T08:49:12.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4663" PostTypeId="2" ParentId="4606" CreationDate="2014-05-31T10:13:02.340" Score="1" Body="&lt;p&gt;The lander had already been modified to adapt to the new comet since C46 was lost in terms of the launch window opportunities. Technology might have become better in the nine or so years but in 2002/2003 there would have been little advance that would allow the mission director to guarantee a significant better chance of success by postponing for say another eight years, and have approval from the funders, knowing the revised trajectory detail is far more risky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My original point however was with the difference in velocities, a high(er) powered harpoon would be required to launch the grappling claw, and with that comes all sorts of new calculations, not only the movement of the platform once the harpoon has been fired, but also the trajectory of the craft on the way to C47, and ultimately the mass of equipment and fuel at lauch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would guess there is also a reduced risk of Rosetta sustaining damage through smaller particles travelling with Chury by coming up relatively slowly alongside.&lt;/p&gt;&#xA;" OwnerUserId="3438" LastActivityDate="2014-05-31T10:13:02.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4664" PostTypeId="2" ParentId="4660" CreationDate="2014-05-31T10:35:49.980" Score="7" Body="&lt;p&gt;The list you linked to, shows that the instruments that are still active, are meant for the detection of solar wind and cosmic rays. The magnetometer could be useful, but a planet needs a liquid core to generate a magnetic field. It is unlikely that any KBO is warm enough to have a liquid core (or we'd be able to detect it using IR telescopes).&lt;br&gt;&#xA;What's left is optical detection using the camera system. This has been switched off for years. It's unlikely the probe has enough power available to use it. Even if it did, the camera probably broke when its heaters were switched off.&lt;br&gt;&#xA;Also, the camera system was built for imaging nearby planets. It's 1970s technology that probably doesn't have enough resolution and sensitivity to search for dark, far-away objects. Another problem would be data transmission: to do a thorough search, you need lots of images. To transmit these at high speed (115 kbit/s), NASA needs to allocate a &lt;a href=&quot;http://voyager.jpl.nasa.gov/science/&quot; rel=&quot;nofollow&quot;&gt;70 m DSN antenna&lt;/a&gt;.&lt;br&gt;&#xA;There is one way these spacecraft could contribute to KBO detection: a big enough course deviation due to gravity of a nearby KBO would be detectable from Earth.&lt;br&gt;&#xA;&lt;a href=&quot;http://news.bbc.co.uk/2/hi/science/nature/460095.stm&quot; rel=&quot;nofollow&quot;&gt;There is some evidence this already happened once&lt;/a&gt;. With only the course deviation to go on, they haven't been able to define the size or orbit of this KBO, though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-05-31T11:37:54.463" LastActivityDate="2014-05-31T11:37:54.463" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4665" PostTypeId="2" ParentId="4606" CreationDate="2014-05-31T11:00:22.323" Score="9" Body="&lt;p&gt;Comet Chury moves at a &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions&quot;&gt;speed of 37 km/s&lt;/a&gt;. If Rosetta were launched just in time for a rendezvous near Earth, it'd be traveling at maybe 12 km/s (relative to Earth) at a large angle relative to Chury.&lt;br&gt;&#xA;Let's just focus on Chury's speed for the moment and ignore the rest. Rosetta flies by Chury at an angle of let's say 90° and fires its harpoon. Now it has to build up 37 km/s in the direction that Chury travels. If we allow Rosetta one second to do this, it will be subjected to $37 km/s^2$ of acceleration, or 3800 G. Rosetta would need to be built like an artillery shell to withstand this force.&lt;br&gt;&#xA;Rosetta would also need to reel out on the order of 37 km of bungee cord (in one second), with a tensile strength of 3800 x 200 = 760 tons.&lt;br&gt;&#xA;So not only is this an impossible task, it's 4 orders of magnitude away from being possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could use a longer cable and/or reel it out beforehand, but that's just rearranging deckchairs. The big problem is the G-force. To lower the G-force by a factor of 1000, you need 1000 times the length of cable: the two are inversely proportional.&lt;br&gt;&#xA;The only way around the problem is by getting Rosetta to match Chury's speed and direction, and then you're back to either ESA's original approach using gravitational slingshots, or you could use a Saturn V-class vehicle and launch Rosetta at the required speed. The choice between those is a matter of budget. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="1567" LastEditDate="2014-06-06T16:44:24.130" LastActivityDate="2014-06-06T16:44:24.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4667" PostTypeId="2" ParentId="203" CreationDate="2014-06-01T15:46:10.260" Score="4" Body="&lt;p&gt;To get out of the atmosphere in any useful way, you have to accelerate to at least the velocity associated with a low Earth orbit, and do so without falling back to the ground before that velocity is achieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ion drives will almost certainly never do this. They are much more efficient with propellant (much less propellant mass expended to achieve a given delta-V) than a chemical rocket, but cannot be built to deliver the necessary vehicle acceleration. You need at least 1g to balance surface gravity, and then some to get you moving up and away. Ion engines deliver very small fractions of a g; they work well for spacecraft already in space on long-term missions, but are not considered to ever be a space &lt;em&gt;launch&lt;/em&gt; technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a couple of interesting technologies which could be applied to space launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One is the &lt;a href=&quot;http://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29&quot; rel=&quot;nofollow&quot;&gt;SABRE&lt;/a&gt; engine. If successful, it could replace vertical pad-launch rocket with runway take-off almost like a conventional airliner for access to orbit. Although effectively a rocket engine at high altitudes where there is insuffucient air to operate a jet engine, SABRE would operate as an air-breathing jet at lower altitudes, substantially reducing the mass of propellant the vehicle would require at lift of for flight to orbit, as compared to conventional rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another is the &lt;a href=&quot;http://en.wikipedia.org/wiki/NERVA&quot; rel=&quot;nofollow&quot;&gt;nuclear rocket&lt;/a&gt;. This was pioneerd in the 50s and 60s, pursued to the point where a feasible launch system could be built, although non ever was. The nuclear rocket replaces chemical reactions with nuclear reactions as the power source to produce a stream of hot gas for propulsion. The exhaust velocity would be much higher than that of any chemical rocket, significantly reducing the mass fraction of the vehicle's propellant.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-06-01T15:46:10.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4668" PostTypeId="1" AcceptedAnswerId="4669" CreationDate="2014-06-01T20:14:38.410" Score="13" ViewCount="780" Body="&lt;p&gt;What would be the major hurdles of setting up an autonomous mining operation on the Moon to shoot lunar rock to something like the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would think it would be a much easier way to get radiation shielding and fuel which can be prohibitively expensive to launch from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the most difficult part? Is it really cheaper to keep building our space things on Earth, and launching them through our considerable atmosphere and gravity?&lt;/p&gt;&#xA;" OwnerUserId="3452" LastEditorUserId="49" LastEditDate="2014-06-01T20:42:36.557" LastActivityDate="2014-10-27T20:55:56.427" Title="Could lunar mining be used to service Low Earth Orbit?" Tags="|the-moon|low-earth-orbit|mining|natural-resources|transport|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4669" PostTypeId="2" ParentId="4668" CreationDate="2014-06-01T22:15:18.400" Score="10" Body="&lt;p&gt;In terms of delta-v budget there could be large savings in transporting required materials from the Moon to LEO (∆V ~ 2.74 km/s) instead of Earth to LEO (∆V ~ 9.3 - 10 km/s)  &lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space_.E2.80.94_high_thrust&quot; rel=&quot;noreferrer&quot;&gt;[1]&lt;/a&gt;&lt;/sup&gt;, assuming you can reduce excess &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_insertion&quot; rel=&quot;noreferrer&quot;&gt;orbit insertion&lt;/a&gt; velocity (~ 3.3 km/s from about 11 km/s reentry speed to 7.7 km/s for ISS) that builds up due to Earth's larger &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;noreferrer&quot;&gt;Hill sphere&lt;/a&gt; with &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerocapture&quot; rel=&quot;noreferrer&quot;&gt;aerocapture&lt;/a&gt; in Earth's atmosphere and match apogee to target orbital altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would require an additional orbit circularization burn at apogee (about half the delta-v of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;noreferrer&quot;&gt;Hohmann transfer&lt;/a&gt; orbit raising maneuvers, and below 0.78 km/s &lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Example&quot; rel=&quot;noreferrer&quot;&gt;[2]&lt;/a&gt;&lt;/sup&gt;) to match target LEO orbit such as the one of the ISS, but the total delta-v budget would still stay significantly lower than that of Earth to LEO. This wouldn't be easy, but it is possible:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/zm2EF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;(Image source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Aerocapture.svg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, establish gravity assisted &quot;visit&quot; &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_cycler&quot; rel=&quot;noreferrer&quot;&gt;Earth-Moon cyclers&lt;/a&gt; first, and the delta-v expenditure for the &quot;taxi&quot; parts might then be even lower, if higher altitude equatorial orbits are targeted (the &quot;castle&quot; part of the cycler has to stay in the Earth-Moon orbital plane and avoid Earth's atmospheric drag in LEO). The required delta-v from the Moon to Low Lunar Orbit is then even lower (∆V ~ 1.87 km/s), but the final orbit circularization and inclination change (in case of inclined target orbits like the one of the ISS) still remain. There might still be savings in required delta-v though, even compared to the aerocapture method, once &quot;castles&quot; are in their cycler orbits, if the cycler's Earth flyby velocity doesn't greatly exceed that of the &quot;taxi's&quot; target orbit. The two required rendezvouses complicate this a bit, but you don't require thermal shield for aerocapture and a large part of the cycler (the &quot;castle&quot; part) is fully reusable, in contrast to a fully expendable cargo delivery vehicle with the aerocapture method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem is, that we currently have no mining operations and required support facilities on the Moon, no raw materials processing and manufacturing facilities in LEO or the Moon itself, and no required transport infrastructure or consumables, like e.g. needed chemical propellants, that would also have to be produced on the Moon for such an operation to make economic sense and, of course, be possible. So in the long run, yes, lunar mining could serve as a source of supplies needed in LEO. But we're a long way from even getting close to that, so the most difficult part would have to be the one of funding and logistics, assuming the Moon can provide all the resources needed to run this, and in some future point becomes more profitable to do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt; - It's more about economics (how much), politics (if, who and when), legality (who objects), and ethics (with what arguments) than about the technical and scientific feasibility. The Moon certainly has resources that we could use in LEO, among many others e.g. water locked in its polar permanently shadowed regions, oxygen locked in many oxide minerals, metals like titanium,... and no living soul on it to object large scale industrial exploitation of them. But it would be an ambitious marathon to get to that point, and the currently prevailing obstacles described before are managed by 4-5 year long mandated positions. Good luck!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space_.E2.80.94_high_thrust&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Delta-v budget: Earth–Moon space — high thrust&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Example&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Hohmann transfer orbit: Example&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-06-02T03:42:08.800" LastActivityDate="2014-06-02T03:42:08.800" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4670" PostTypeId="2" ParentId="4658" CreationDate="2014-06-02T00:13:06.100" Score="7" Body="&lt;p&gt;How large would the sail need to be for a five day transfer? Too large. Solar sails are typically not very useful to increase your semimajor axis when orbiting a body other than the Sun (eg Earth). This is because when you're travelling away from the sun, the solar pressure will accelerate you, and when you're travelling towards the sun, it will slow you down, so the net result is 0. That's not strictly true, but you can make a sail much more useful if you have active control over its attitude. For example, have the sail perpendicular to the solar pressure when moving away, but parallel when moving towards. Still, solar radiation pressure is small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trans-Lunar Injection requires about &lt;a href=&quot;http://en.wikipedia.org/wiki/Trans-lunar_injection#History&quot; rel=&quot;nofollow&quot;&gt;3 km/s delta V&lt;/a&gt;. Solar radiation pressure is about &lt;a href=&quot;http://en.wikipedia.org/wiki/Radiation_pressure#Pressures_of_absorption_and_reflection&quot; rel=&quot;nofollow&quot;&gt;10 micro newtons/meter squared&lt;/a&gt;. Assuming you have a turn around of 5 days, you'll need to get all the required force in that time. Your craft weighs 31,000 kg, so we have the mass. From F=ma we can see that since a=dV/dt, F=31,000 x 3000/(5 x 24 x 3600) which is 215N of constant force. Not too high, considering. The sail area however is Force/Pressure = 215/10E-6 = 215E5 or 21,500,000 meters squared. That's big, &lt;a href=&quot;http://www.wolframalpha.com/input/?i=21500000m2/vatrican%20area&quot; rel=&quot;nofollow&quot;&gt;49 times the size of Vatican city&lt;/a&gt;. That also has a bunch of assumptions that make this estimate lower than the true value (such as 100% pressure in right direction, zero shade, minimum delta V).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for the mass. If we make it out of &lt;a href=&quot;http://www2.dupont.com/Kapton/en_US/assets/downloads/pdf/Gen_Specs.pdf&quot; rel=&quot;nofollow&quot;&gt;pretty thin kapton&lt;/a&gt;, lets say 0.1 micrometer thickness, then we're looking at a reasonable material for the job. The sail weight would be 0.0000001 x 21500000 x 1420 (&lt;a href=&quot;http://www2.dupont.com/Kapton/en_US/products/HN/&quot; rel=&quot;nofollow&quot;&gt;Density of kapton&lt;/a&gt;) then we're only talking a mass of about 3,000kg. Reasonable. Except when you consider the real mass driver in sail systems is the structure. Up to 99% of the mass is structure. If that's the case here, then you're talking about 300,000kg. And that's bigger than your system mass. Woops!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it feasible? No. Could you return using this method, yes. If you have attitude control.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="6214" LastEditDate="2014-11-07T21:06:55.773" LastActivityDate="2014-11-07T21:06:55.773" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4671" PostTypeId="2" ParentId="4668" CreationDate="2014-06-02T07:41:41.500" Score="6" Body="&lt;p&gt;High initial costs compared to current demand rate is the basic economic problem. The infrastructure would need more spacecrafts to serve in order for it to become worthwhile. Prolonging the life of GEO satellites might become the crucial stepping stone to make this concept profitable. But they are today not built be refueled. I think that tugging is much easier than transfering fuel. The satellite manufacturers could probably make this kind of lunar industry happen, if they agree on a standard.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-06-02T07:41:41.500" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4672" PostTypeId="1" CreationDate="2014-06-02T12:20:47.503" Score="4" ViewCount="248" Body="&lt;p&gt;A friend and I travelled to Kazakhstan in May and booked a trip to see the 29 May launch.  Everything was fine, we had permits for everything except the launch itself (all the museums, Gagarin's cottage etc), and that was just a matter of time, and then on the Monday before the launch we suddenly had everything cancelled. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tour company said that all 70 tourists were cancelled (because of an earlier test flight crash), but in the photos of the pre-launch briefings etc., it looks like there are a lot of tourists taking photos, and a child wandering around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anyone heard any info on any cancellations? Someone in the UK said it could be down politics and that some unhelpful comments were made by some of our political/royal 'elite' towards the Russians.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was frustrating to miss the launch (and more the museums, etc.), but more frustrating to not know the reason. If anyone has any insight, that would be great.&lt;/p&gt;&#xA;" OwnerUserId="3453" LastEditorUserId="49" LastEditDate="2015-07-20T18:38:38.987" LastActivityDate="2015-07-20T18:38:38.987" Title="Baikonur trip cancelled at the 11th hour (29 May 2014)" Tags="|soyuz-spacecraft|launch-site|tourism|baikonur-cosmodrome|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4674" PostTypeId="1" CreationDate="2014-06-02T18:27:44.637" Score="4" ViewCount="4956" Body="&lt;p&gt;I am wondering if anybody can help me with calculating Delta-V in Kerbal Space program?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a ship in circularized 100 km orbit around planet Kerbin. Planet's radius is 600 km (standard gravitational parameter &lt;em&gt;μ&lt;/em&gt; = 3.5316000×10&lt;sup&gt;12&lt;/sup&gt; m&lt;sup&gt;3&lt;/sup&gt;s&lt;sup&gt;-2&lt;/sup&gt;). I want to perform a Hohmann transfer to a target orbit of 400 km above surface, i.e. 1000 km radius to planet's center of gravity (orbit's focus).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anybody please explain how can I calculate Delta-V cost of a Hohmann transfer orbit raising maneuver?&lt;/p&gt;&#xA;" OwnerUserId="3455" LastEditorUserId="49" LastEditDate="2014-06-02T19:35:53.160" LastActivityDate="2014-06-02T20:05:11.743" Title="Calculate Delta-V for a Hohmann transfer to a higher orbit" Tags="|orbital-mechanics|orbital-maneuver|delta-v|kerbal-space-program|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4675" PostTypeId="2" ParentId="4674" CreationDate="2014-06-02T19:09:36.300" Score="4" Body="&lt;p&gt;KSP also offers a nav computer that lets you plot a course and then gives you the relevant markers (and burn w/time estimate) to achieve what you want. If you want to do this to change your orbit I recommend the in game tool, but if you are trying to familiarize yourself with the math then that's a different issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To use the game nav tool: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Switch to map view and locate your periapsis (the point on your orbit closest to the orbiting body). Click that mark on your orbit and add a prograde burn until your apoapsis is raised to the desired altitude. When your ship arrives at your periapsis start the designated prograde burn and stop thrusting when your real apoapsis matches that of your projected apoapsis. Do the same process at your apoapsis increasing your periapsis to the desired altitude. Then perform the required prograde burn and you've just increased your orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember to burn with SAS on and if you over burn you can quickly retrograde burn to decrease your apoapsis/periapsis in case you increase either value too much.&lt;/p&gt;&#xA;" OwnerUserId="3440" LastEditorUserId="3440" LastEditDate="2014-06-02T20:05:11.743" LastActivityDate="2014-06-02T20:05:11.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4676" PostTypeId="1" AcceptedAnswerId="4677" CreationDate="2014-06-02T19:31:38.353" Score="10" ViewCount="1216" Body="&lt;p&gt;Soft ball question here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know if SpaceX will be open to taking non-space agency/military personnel into space? And if so, in what capacity will they allow? And any estimates on the price?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know the Russians offer a space flight service including a stay on the ISS for somewhere in the neighborhood of $45-50 million which includes a 2 week stay in the ISS. I was just wondering if SpaceX has anything like that planned or at least has shown interest in unlocking space for the general public.&lt;/p&gt;&#xA;" OwnerUserId="3440" LastActivityDate="2014-06-03T04:00:45.583" Title="Will SpaceX open up space to the public?" Tags="|spacex|commercial|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4677" PostTypeId="2" ParentId="4676" CreationDate="2014-06-02T19:38:32.437" Score="15" Body="&lt;p&gt;&lt;a href=&quot;http://www.bigelowaerospace.com&quot;&gt;Bigelow Aerospace&lt;/a&gt; has been talking with anyone close to being able to launch people, about using their services for manned launch.  Bigelow plans on launching inflatable habitats for rent to national governments who want a cheap space program.  (South Korea is often touted as a customer).  Bigelow at one point offered prices for crew launched by Dragon and for crew launched by CST-100 (Boeing). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus Bigelow is looking for anyone who can launch manned.  They have held off launching their &lt;a href=&quot;http://en.wikipedia.org/wiki/Bigelow_Aerospace#BA_330&quot;&gt;BA-330&lt;/a&gt; module, since with Genesis 1 and 2 having proven most of their needs, but no way to get there, no point in launching the habitat for people, with no people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA does not 'own' Dragon after cargo deliveries, nor will it 'own' the Dragon V2 (DragonRider?) capsule on missions.  Rather they purchase a service, and SpaceX provides it. Thus SpaceX can sell launches and manned flights to whomever is willing to pay.  You can bet they will sell launches that people want to buy. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-06-02T20:06:44.030" LastActivityDate="2014-06-02T20:06:44.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4678" PostTypeId="1" AcceptedAnswerId="4679" CreationDate="2014-06-03T02:11:21.990" Score="33" ViewCount="46220" Body="&lt;p&gt;In the book &quot;&lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/0553288105&quot; rel=&quot;noreferrer&quot;&gt;The Gods Themselves&lt;/a&gt;&quot;, Isaac Asimov imagines humans living on the Moon would enjoy much greater freedom of movement on the Moon. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each pair went up in unison; each pair rose and fell in a more&#xA;  complicated pattern. One pair kicked off simultaneously to cross the&#xA;  tube in a low parabola, convex upward, each reaching the handhold the&#xA;  other had abandoned, and somehow skimming past each other in mid-air&#xA;  without touching. That evoked louder applause.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Earthman said, &quot;I suspect I lack the experience to appreciate the&#xA;  finer points of skill. Are these all native Lunarites?&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;They have to be,&quot; said Selene. &quot;The gymnasium is open to all Lunar&#xA;  citizens and some immigrants are fairly good, considering. For this&#xA;  kind of virtuosity, however, you must depend on babies that are&#xA;  conceived and born here. They have the proper physical adaptation, at&#xA;  least more than native Earthmen have, and they get the proper&#xA;  childhood training. Most of these performers are under eighteen.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;I imagine it's dangerous, even at Moon-gravity levels.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;Broken bones aren't very uncommon. I don't think there's been an&#xA;  actual death, but there's been at least one case of broken spine and&#xA;  paralysis. That was a terrible accident; I was actually watching— Oh,&#xA;  wait now; we're going to have the ad libs now.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This raises the question: How far could one fall on the moon before sustaining injuries? In an &lt;a href=&quot;http://www.livescience.com/6040-brute-force-humans-punch.html&quot; rel=&quot;noreferrer&quot;&gt;interview with a biomedical engineer&lt;/a&gt;, it is said that 4,000 Newtons of force would break a femur. Could a 170 lb (77 kg) person jump high enough on the Moon to sustain that much force on landing? How high would that person have to fall to experience that much force?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Terminal velocity during free fall on Earth is &lt;a href=&quot;http://www.greenharbor.com/fffolder/math.html&quot; rel=&quot;noreferrer&quot;&gt;reported to be about 110 mph&lt;/a&gt; (49 m/s). That would most probably result in a fatal fall. How high would a person on the Moon have to fall to reach that on the Moon?&lt;/p&gt;&#xA;" OwnerUserId="3161" LastEditorUserId="6392" LastEditDate="2018-07-06T18:26:22.893" LastActivityDate="2021-12-31T23:29:00.967" Title="How far can you fall on the Moon without injury?" Tags="|the-moon|colonization|lunar-landing|physical-activity|moonwalking|" AnswerCount="10" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="4679" PostTypeId="2" ParentId="4678" CreationDate="2014-06-03T03:19:22.187" Score="30" Body="&lt;p&gt;The simplest would be defining some arbitrary impact velocity that is at the limit of being fatal, and we then consider everything else (surface properties, subject's physique,...) except gravitational acceleration constant. We can also neglect air resistance to make it simpler, since we're more interested in a safe height to jump off on the Moon, than that on the Earth. Plus air resistance at such small heights wouldn't change our results much anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's, for the sake of argument, assume we have a crash test dummy that won't break into pieces, if it impacts the ground no faster than 60 km/h. At Earth's mean sea-level surface gravitational acceleration (&lt;a href=&quot;https://encrypted.google.com/search?q=earth%27s%20gravity&quot; rel=&quot;noreferrer&quot;&gt;9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;&lt;/a&gt;) that comes out as a jump at a height of 14.16 m, and the free fall duration of 1.7 s before impact. I'm cheating by using an &lt;a href=&quot;http://www.ambrsoft.com/CalcPhysics/acceleration/acceleration.htm&quot; rel=&quot;noreferrer&quot;&gt;online free fall and constant acceleration calculator&lt;/a&gt;, but the maths for a &lt;a href=&quot;https://en.wikipedia.org/wiki/Free_fall#Free_fall_in_Newtonian_mechanics&quot; rel=&quot;noreferrer&quot;&gt;free fall without air resistance&lt;/a&gt; go as follows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$$v(t)=-gt+v_{0}$$ $$y(t)=-\frac{1}{2}gt^2+v_{0}t+y_0$$&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;where&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;$v_{0}$ is the initial velocity (m/s).&lt;/li&gt;&#xA;  &lt;li&gt;$v(t)$ is the vertical velocity with respect to time (m/s).&lt;/li&gt;&#xA;  &lt;li&gt;$y_0$ is the initial altitude (m).&lt;/li&gt;&#xA;  &lt;li&gt;$y(t)$ is the altitude with respect to time (m).&lt;/li&gt;&#xA;  &lt;li&gt;$t$ is time elapsed (s).&lt;/li&gt;&#xA;  &lt;li&gt;$g$ is the acceleration due to gravity (9.81 m/s&lt;sup&gt;2&lt;/sup&gt; near the surface of the earth).&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We can derive everything else we need from these two equations, let me know if you require a more detailed answer in this part. Moving on, now let's see at which height of a jump we reach 60 km/h impact on the surface of the Moon. Moon's average surface gravity acceleration is &lt;a href=&quot;https://encrypted.google.com/search?q=moon%27s%20gravity&quot; rel=&quot;noreferrer&quot;&gt;1.622 m/s&lt;sup&gt;2&lt;/sup&gt;&lt;/a&gt;. Plugging that into our online constant acceleration calculator (inserting acceleration, start velocity of 0 km/h and end velocity of 60 km/h), we get:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Jump from a height of 85.63 m&lt;/li&gt;&#xA;&lt;li&gt;Free fall time of 10.28 s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Recalculation for 110 mph (177 km/h) impact velocity post edit of the question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Jump from a height of 745.41 m (2,445.58 ft) on the Moon, 123.29 m (404.49 ft) on Earth&lt;/li&gt;&#xA;&lt;li&gt;Free fall time of 30.32 seconds on the Moon, 5.01 seconds on Earth&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And for 116 mph (187 km/h), &lt;a href=&quot;https://encrypted.google.com/search?q=4000%20newton%20per%20170%20lbs%20per%20second%20in%20mph&quot; rel=&quot;noreferrer&quot;&gt;equaling 4,000 N force&lt;/a&gt; on impact of a 170 lbs (77.11 kg) object:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Jump from a height of 829.49 m (2,721.42 ft) on the Moon, 137.2 m (450.12 ft) on Earth&lt;/li&gt;&#xA;&lt;li&gt;Free fall time of 31.98 seconds on the Moon, 5.29 seconds on Earth&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: Both these last impact velocities (110 and 116 mph) are over 80.85 mph (130.12 km/h) that I calculate (see comments for details, thanks @LorenPechtel!) is the terminal velocity on the Moon for a freeflying skydiver, if all other conditions remain constant (mass of the skydiver, drag coefficient, fluid density, projected area of the falling object) both on the Earth and on the Moon. I.e. this would mean equal atmospheric conditions, no spacesuit, and for a skydiver whose terminal velocity on Earth is 320 km/h (200 mph or 90 m/s), as given in &lt;a href=&quot;https://en.wikipedia.org/wiki/Terminal_velocity#Examples&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia examples of terminal velocity&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if we accept that a human can survive an impact at given velocities (110 and 116 mph), Moon's gravity isn't sufficient to counteract atmospheric drag of 1 atmospheric pressure to such speeds to kill you on impact. I.e. the maximum survivable altitude to jump off in such conditions would be, theoretically, infinite. In reality, you'd then have to deal with the heat released during atmospheric reentry beyond terminal velocity, which becomes a different question altogether. And it would of course be unfeasible, the Moon doesn't have sufficient gravity and magnetic field to sustain Earth-like atmospheric pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solution to calculate lunar terminal velocity, with a known terminal velocity of the same object and same atmospheric density on Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{(M)} = 0.4067 \cdot v_{(E)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $v_{(E)}$ is terminal velocity for Earth's gravity. Derived from:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v= \sqrt{\frac{2mg}{\rho A C_d }}, \ x = \frac{2m}{\rho A C_d }$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $x$ denotes our constant mass of the object, drag coefficient, fluid density, and projected area of the falling object. Thus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${v_{(M)}}^2 = \frac{g_{(M)}{v_{(E)}}^2}{g_{(E)}} \Rightarrow {v_{(M)}}^2 = \frac{1.622}{9.80665} \cdot {v_{(E)}}^2 \Rightarrow v_{(M)} = \sqrt{0.1654 \cdot {v_{(E)}}^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a belly-to-earth (i.e., face down) free-fall position, again using &lt;a href=&quot;https://en.wikipedia.org/wiki/Terminal_velocity#Examples&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia quoted values&lt;/a&gt;, in Earth's gravity achievable terminal velocity is only 195 km/h (122 mph or 54 m/s). On the Moon, using our conversion value of 0.4067, this amounts to 79.3 km/h (49.6 mph or 22 m/s). I would still argue that this is not survivable due to lack of vegetation and unweathered (sharp) terrain on the Moon, but it is a food for thought. For a 170 lbs, this &quot;only&quot; amounts to an impact force of &lt;a href=&quot;https://encrypted.google.com/search?q=%2849.6%20mph%20*%20170%20lbs%29%20per%20second&quot; rel=&quot;noreferrer&quot;&gt;1,710 Newtons&lt;/a&gt;. With some luck and landing into a deep pocket of lunar dust and no larger boulders in the way, it &lt;em&gt;might&lt;/em&gt; be survivable. Still, that's in a face down position, so most likely not.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Another thought though, that our subjects are acrobats that would probably achieve jumps of such heights with their own power. So they would first have to work against the same gravitational acceleration that will later try to kill them on impact. Point being, that if you can't jump on Earth to heights that would be fatal to land from, you won't be able to do that on the Moon either. So the frequency of injuries and fatalities should be much the same, assuming all other conditions are equal, and no severe bone density loss due to staying in roughly 1/6-th the Earth's gravity. Of course, bone density loss usually comes with muscle atrophy, so the ability to jump as high might decrease, too.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-19T07:22:38.960" LastActivityDate="2015-12-19T07:22:38.960" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="4680" PostTypeId="2" ParentId="4676" CreationDate="2014-06-03T04:00:45.583" Score="5" Body="&lt;p&gt;The question is, how do you define &quot;public&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Passenger-safe rockets are expensive things to build and operate. That seems unlikely to change any time soon, even as commercial space flight becomes better established. Given current technology, I expect that, at best, we might see a 10-minutes-in-space suborbital flight reach the point where it's as affordable as a high-end cruise or safari or the like, meaning that many folks could afford it as a once-in-a-lifetime event if it's really that high on their personal priorities. In other words, I expect that SpaceX &quot;tourist&quot; tickets will probably always be closer to the price of a house than to the price of a car.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll be delighted if someone proves me wrong.&lt;/p&gt;&#xA;" OwnerUserId="3231" LastActivityDate="2014-06-03T04:00:45.583" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4682" PostTypeId="2" ParentId="2473" CreationDate="2014-06-03T04:56:00.947" Score="8" Body="&lt;p&gt;Some engines will be used on the grass hopper platforms. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some engines will presumably fail during qualification/smoke (pun not intended*) testing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some engines will end up in the stock pile for future launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I am guessing most of the remainder will be used for R&amp;amp;D testing, while improving the design [Merlin C =&gt; Merlin D &amp;amp; Merlin D =&gt; Merlin E(?)] &lt;/p&gt;&#xA;&#xA;&lt;p&gt;i.e., tweak design, build engine, test performance etc, compare with simulations, rinse and repeat. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Pun kinda intended a little bit.&lt;/p&gt;&#xA;" OwnerUserId="3460" LastEditorUserId="3460" LastEditDate="2016-04-12T22:53:06.507" LastActivityDate="2016-04-12T22:53:06.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4683" PostTypeId="1" AcceptedAnswerId="4685" CreationDate="2014-06-03T08:58:31.760" Score="9" ViewCount="434" Body="&lt;p&gt;Some of what I've read theorises that Mars' oceans evaporated and blew away, but it seems to me that (potential) &lt;a href=&quot;http://en.wikipedia.org/wiki/Seasonal_flows_on_warm_Martian_slopes&quot;&gt;observed water flows&lt;/a&gt; and exposure of (potential) &lt;a href=&quot;http://en.wikipedia.org/wiki/File%3aPIA10741_Possible_Ice_Below_Phoenix.jpg&quot;&gt;water ice under the surface dust&lt;/a&gt; by the Phoenix lander suggests the liklihood that a lot of that ocean is still there, aside from the polar ice caps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is, ignoring the innumerous hazards of going to and settling Mars, how could we harvest that ice and turn it into water to be used for drinking and fuel, with the least amount of cost and effort? Could there be an automated way of doing this so that robots could do it whilst humans stay on Earth for a while initially?&lt;/p&gt;&#xA;" OwnerUserId="3443" LastActivityDate="2014-06-03T14:20:31.470" Title="If Mars has a frozen ocean, how can it be used by settlers?" Tags="|mars|terraforming|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4684" PostTypeId="2" ParentId="4646" CreationDate="2014-06-03T09:34:44.140" Score="2" Body="&lt;p&gt;Since micro-meteor won't come from the lunar ground but can come from all around the space station it is at least 2 times safer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But since there is gravity it attract a little more meteor, it is not easy to quantify those but if they are getting fast and have a lot energy (the scary one) they won't be impacted by gravitation.&lt;/p&gt;&#xA;" OwnerUserId="3461" LastActivityDate="2014-06-03T09:34:44.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4685" PostTypeId="2" ParentId="4683" CreationDate="2014-06-03T09:44:28.927" Score="11" Body="&lt;p&gt;The Martian atmospheric pressure is around &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Mars&quot;&gt;600 Pascals&lt;/a&gt;. At that pressure the &lt;a href=&quot;http://www.jbind.com/pdf/Cross-Reference-of-Boiling-Temps.pdf&quot;&gt;boiling point of water is around 0&amp;deg;C&lt;/a&gt;. The average temperature on the surface of &lt;a href=&quot;http://en.wikipedia.org/wiki/Climate_of_Mars#Temperature&quot;&gt;Mars is -55&amp;deg;C&lt;/a&gt;. The melting point of water is also lower, but this &lt;a href=&quot;http://en.wikipedia.org/wiki/Pressure_melting_point&quot;&gt;usually changes by a much smaller fraction&lt;/a&gt; so we'll assume you want to get it to just below boiling. So you only need to increase the temperature by 55&amp;deg;C to be able to store it in liquid form (and then slowly raise its temperature and pressure thereafter). The specific heat capacity of water is around &lt;a href=&quot;http://en.wikipedia.org/wiki/Heat_capacity#Specific_heat_capacity_of_building_materials&quot;&gt;2,000J/Kg.K&lt;/a&gt;. That means you need 2,000 Joules of energy to raise 1kg of water by 1&amp;deg;C. So to raise 1kg by 55&amp;deg;C you need: 2,000x55 = 110,000 Joules. On top of that you need to provide the energy for the enthalpy of fusion. for water this is 334,000J/kg, bringing your total value up to 444,000J/kg. Now you just need to know how much water you need per day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from that though there's one more useful calculation; the size of solar array required for melting 1kg. The solar flux at &lt;a href=&quot;http://www.wolframalpha.com/input/?i=solar%20flux%20at%20mars&quot;&gt;Mars is around 557W/m&lt;sup&gt;2&lt;/sup&gt;&lt;/a&gt;. Assuming you get that for around half the day (if you have tracking panels) then you're looking at 557 * 12 * 3600 (Martian day is pretty much the same as Earth) = 24,062,400 Joules/m&lt;sup&gt;2&lt;/sup&gt;/day. Solar panels efficiencies are pretty bad, &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_cell_efficiency&quot;&gt;around 14% is typical&lt;/a&gt;: 24,062,400 *0.14 = 3,368,736. That means you need 444,000/3,368,736 = 0.1318 m&lt;sup&gt;2&lt;/sup&gt;/kg/day of solar array to melt ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is achievable by robots, but giving an example is a little to broad to answer here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope my thesis has this density of references.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-06-03T14:20:31.470" LastActivityDate="2014-06-03T14:20:31.470" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4686" PostTypeId="1" AcceptedAnswerId="4687" CreationDate="2014-06-03T15:07:37.753" Score="20" ViewCount="5671" Body="&lt;p&gt;In the following image (coming from &lt;a href=&quot;http://www.satflare.com/track.asp?q=25544#TOP&quot; rel=&quot;noreferrer&quot;&gt;SAT-Flare&lt;/a&gt;) you can see the orbit of the ISS (in red) and the day/night terminator (in blue). The terminator separates night from day along the surface of the Earth. How often will the ISS orbit align with the terminator? When it does so, how long will such an alignment last? And lastly, can it align any closer than it does in this image?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jUmXZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-06-03T15:53:06.713" LastActivityDate="2018-11-09T13:40:08.493" Title="How often does the ISS orbit align with the day/night terminator?" Tags="|iss|orbital-mechanics|low-earth-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4687" PostTypeId="2" ParentId="4686" CreationDate="2014-06-03T15:36:11.337" Score="21" Body="&lt;p&gt;The short answer: 2-4 times per year the ISS enters a &quot;full sun&quot; period. Right before this happens, the ISS will line up near the terminator at the beginning and the end of this period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Full answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll try and cover this without explaining too much orbital mechanics. The key thing is what is known as the beta angle, which is the angle between the sun and the orbital plane. A 0 degree beta will have the satellite in maximum eclipse, while a high enough beta angle will allow the satellite to actually be in the sun the entire time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so how does this beta angle come about, and what value of it is required to see this effect? The first one is easier, so I'll answer it first. The angle has to be such that the beta angle is greater than the size of the Earth as appearing from that orbit. I have an offline table that has the value (Called Maximum NADIR angle), the Maximum NADIR angle for a satellite of the ISS's altitude is between 350 and 400 km, so the value is about 70 degrees. The higher up, the lower this angle becomes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, full sun happens when the beta angle is above 70, and it lines up with the terminator when the satellite is at 70 exactly. How often does this actually happen? There are two factors which determine the beta angle. The first is the Earth's contribution, which can go up to 23 degrees, depending on the season (Northern Summer +23, Winter -23). This corresponds to the Earth's axial tilt. The second corresponds to the orbital inclination, and this also varies. The math on the last part is somewhat complex, it rotates at the rate of &lt;a href=&quot;http://en.wikipedia.org/wiki/Nodal_precession&quot; rel=&quot;noreferrer&quot;&gt;nodal procession&lt;/a&gt;. The ISS's nodal precession period is about every 2 months, as I can tell from plotting it's beta angle in STK. So, the bottom line is that this can only happen when the inclination, of 51 degrees, adds to the relative Earth sun angle, at most 23 degrees, equals 70 degrees. Note that -70 is fine as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5V8TM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see from this plot, it isn't very common. It seems to happen only once per peak season, although it might be possible to have it happen twice per half year, but no more than that. Thus, it happens between 2-4 times per year, always in near one of the solstices. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also worth noting that it's a pretty unique thing, this only tends to happen to satellites in LEO with inclinations between 40-70 degrees. There isn't a whole lot of satellites in that range.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-06-03T17:15:28.113" LastActivityDate="2014-06-03T17:15:28.113" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4688" PostTypeId="1" AcceptedAnswerId="4698" CreationDate="2014-06-03T17:57:29.757" Score="146" ViewCount="184593" Body="&lt;p&gt;It is said that in order for an object or a projectile to leave Earth's gravitational pull, it must reach Earth's escape velocity, meaning reach a speed of 7 miles per second (~11 km per second). Well, as far as I understand, you could easily escape Earth's gravity even at 1 mph (0.45 m/s) - directed away from the surface and you will eventually reach space. So why is the escape velocity 7 miles/s (11 km/s)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it because the object has to gain a certain speed once it reaches orbit in order to maintain that altitude? Or is it because practically an object can't carry infinite amount of fuel, and so it has to reach a certain speed to maintain its orbit before all fuel is gone?&lt;/p&gt;&#xA;" OwnerUserId="3456" LastEditorUserId="33" LastEditDate="2017-05-12T14:00:03.603" LastActivityDate="2021-02-02T13:22:21.640" Title="Couldn't I escape Earth's gravity traveling only 1 mph (0.45 m/s)?" Tags="|gravity|escape-velocity|" AnswerCount="12" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4689" PostTypeId="2" ParentId="4688" CreationDate="2014-06-03T18:06:39.750" Score="35" Body="&lt;p&gt;You are confusing velocity and acceleration. If you were to jump standing on the surface of the Earth you might experience &lt;a href=&quot;https://uk.answers.yahoo.com/question/index?qid=20110521093548AAaKBbd&quot;&gt;8&amp;nbsp;m/s&lt;/a&gt; which is &lt;a href=&quot;https://www.google.co.uk/search?q=8m/s%20in%20mph&amp;amp;rlz=1C1CHFX_en-GBGB543GB543&amp;amp;oq=8m/s%20in%20mph&amp;amp;aqs=chrome..69i57j0j69i58.4475j0j7&amp;amp;sourceid=chrome&amp;amp;es_sm=93&amp;amp;ie=UTF-8&quot;&gt;17&amp;nbsp;mph&lt;/a&gt; velocity upward, but the acceleration of gravity would act to retard your motion, slowing your velocity down. If you have a high enough velocity, the effect of (de) acceleration can not slow you down before you get far enough away from the gravitational source.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you could keep a constant velocity of 1&amp;nbsp;mph, you would defiantly be able to escape the earth. The problem is that would require constant thrust. If you're going 11&amp;nbsp;km/s then you can just relax and watch the world shrink in your rear view mirror.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="286" LastEditDate="2014-06-06T11:38:29.977" LastActivityDate="2014-06-06T11:38:29.977" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="4690" PostTypeId="2" ParentId="4688" CreationDate="2014-06-03T18:32:08.677" Score="8" Body="&lt;p&gt;Unless you are &lt;strong&gt;very far&lt;/strong&gt; away from Earth, if you are only moving away at 1 mph the gravity of Earth will pull you back to Earth (&lt;em&gt;assuming you do not have an infinite fuel supply to maintain a 1mph thrust&lt;/em&gt;). So you are correct when you say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is it because the object has to gain a certain speed once it reaches orbit in order to maintain that altitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Think of a ball tossed in the air, it starts by moving quickly, but as it rises higher it goes slower, than stops and falls back down.  At some point it is moving away from Earth at 1mph, but gravity overcomes that momentum.  Air Resistance has some impact on the ball, but you can throw horizontally much farther than you can up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity works pretty much them same on the surface of the Earth as it does a 1000 miles up.  When you throw something horizontally it falls towards the earth in an arc, attracted by the gravity of the Earth.  If it is moving fast enough the curvature of the Earth will match the arc of the falling object, this is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital speed&lt;/a&gt; and the object will not hit the earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit 4 years later to consider a solar sail&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you had a nearly infinite fuel supply, and you kept moving away from Earth at 1 mph, yes you could escape.  You could do this with a &lt;a href=&quot;https://space.stackexchange.com/questions/tagged/solar-sail&quot;&gt;solar sail&lt;/a&gt; there are a couple of &lt;a href=&quot;https://space.stackexchange.com/questions/1594/can-solar-sail-prevent-orbital-decay&quot;&gt;issues using the sail near Earth&lt;/a&gt; but assuming you start in a high stable orbit, you could easily expand until your escape.  Of note, using a solar sail, as you move farther from Earth your speed would increase unless you lowered the efficiency of the sail.  In other words, if you started with a solar sail to get 1 mph thrust, you would need to work to maintain that speed, otherwise you would soon be going faster. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2018-11-16T17:28:52.333" LastActivityDate="2018-11-16T17:28:52.333" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="4691" PostTypeId="2" ParentId="4688" CreationDate="2014-06-03T18:52:01.213" Score="103" Body="&lt;p&gt;Escape velocity reduces as you get further away from the Earth.  If you proceed upwards at a constant speed of 1 mph (which as noted will require continuous thrust to counteract gravity), &lt;strong&gt;you will eventually reach a distance where the escape velocity is equal to 1 mph&lt;/strong&gt;.  Then, you will have reached escape velocity and are no longer gravitationally bound to the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This distance is extremely great; around 4&amp;times;10&lt;sup&gt;12&lt;/sup&gt; km or 26000 AU.  In practice, third-body effects (moon, sun, other planets) will dominate when you get beyond 10&lt;sup&gt;5&lt;/sup&gt; km away from the Earth.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="288" LastEditDate="2014-06-06T11:36:49.677" LastActivityDate="2014-06-06T11:36:49.677" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4692" PostTypeId="2" ParentId="4688" CreationDate="2014-06-03T19:55:20.440" Score="62" Body="&lt;p&gt;To sum up the answers: the escape velocity is the velocity that, at a given distance, is sufficient to escape the gravitational field so that &lt;em&gt;no additional energy (= acceleration) is needed&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, if you are 26000 AU from Earth, you don't need any more fuel to counteract Earth's gravity, you just float away. However, when at Earth's surface, you will need additional acceleration to sustain the 1mph velocity - otherwise you just fall back down like the tossed ball.&lt;/p&gt;&#xA;" OwnerUserId="3464" LastActivityDate="2014-06-03T19:55:20.440" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4693" PostTypeId="1" AcceptedAnswerId="4766" CreationDate="2014-06-03T22:07:43.207" Score="10" ViewCount="985" Body="&lt;p&gt;The three CCtCAP competitors, Dream Chaser, CST-100, and Dragon are all designed to carry a crew of 7.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Operationally, NASA plans on launching crews of 4, since that allows the station to support a crew of 7 (3 by Soyuz, 4 by US side) and the station has 'room' (4 sleeping berths in Unity, 3 in Zvezda, though that might only be 2) and life support for a long term crew of 7 total.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Direct handoff of crews does not seem to be likely, where, say a Dragon is docked to PMA-2 on the forward end of Node 2, and a second vehicle say a CST-100 docks to the zenith (space facing) PMA-3 and there is a Soyuz on the Russian segment.  That would imply 7 on station + 7 more, but that might stress the life support system too much.  (Shuttle brought its own fairly robust life support system).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More likely is indirect handoff, where a Dream Chaser, docked to PMA-2, undocks with a complement of 4, reenters, and a week later a CST-100 launches to deliver a crew of 4.  Which is how the Soyuz handoff has been happening for most of the stations lifetime.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Why indirect?  For one thing NASA requires a spare docking port be available, so if they arrive, and PMA-2 is busted, they need to have PMA-3 available, and at the moment, there are only going to be 2 PMA's available for docking.  For berthing (HTV, Dragon V1, and Cygnus) the nadir (earth facing) CBM port on Node 2, and on Node 1 will be left available for that, once some modules get minorly shuffled around the station.  Right now, the backup CBM port is the zenith (space facing) CBM port, that is due to be replaced with PMA-3). &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-06-10T18:38:50.770" Title="If NASA's plan is to launch 4 astronauts to the ISS on CCtCAP vehicles, why design for 7?" Tags="|iss|crew-size|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4695" PostTypeId="2" ParentId="4688" CreationDate="2014-06-03T22:10:13.613" Score="31" Body="&lt;p&gt;I think if you are powered (rocket/motor ) you can go at any speed and escape the gravity. The escape velocity is only for objects thrown (projected into space), with the initial velocity and they are not powered.&lt;/p&gt;&#xA;" OwnerUserId="3468" LastActivityDate="2014-06-03T22:10:13.613" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4696" PostTypeId="2" ParentId="4643" CreationDate="2014-06-03T23:56:21.987" Score="8" Body="&lt;p&gt;A rock surface might or might not be comfortable, but certainly there are lighter and more compact alternatives to mattresses that would be comfortable in lunar gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Apollo Lunar Module they used lightweight hammocks, and astronauts made a point of reporting how much more pleasant they were to sleep in under lunar versus on the Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;We ran a simulated night in the simulator before we went - and got a lousy night's sleep. When you get to one-sixth g, it's just terrific; but you try to sleep in those hammocks in one g: not terrific.&quot; - Dave Scott, Apollo 15 Commander (&lt;a href=&quot;http://www.hq.nasa.gov/alsj/a15/a15.eva2wake.html#1382037&quot; rel=&quot;noreferrer&quot;&gt;http://www.hq.nasa.gov/alsj/a15/a15.eva2wake.html#1382037&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/NLnHB.jpg&quot; alt=&quot;Apollo LM ascent stage aft view showing hammock&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were other issues with sleep comfort, mainly related to poor spacesuit fits.  From Apollo 15 onwards they would remove their spacesuits to sleep, which helped a lot.  Much more information available here: &lt;a href=&quot;http://www.workingonthemoon.com/WOTM-Sleep.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.workingonthemoon.com/WOTM-Sleep.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-06-03T23:56:21.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4697" PostTypeId="1" AcceptedAnswerId="4699" CreationDate="2014-06-04T00:06:49.150" Score="13" ViewCount="6519" Body="&lt;p&gt;It is rumored that NASA uses the movie &lt;a href=&quot;http://www.todayifoundout.com/index.php/2010/01/nasa-uses-the-movie-armageddon-in-their-management-training-program/&quot;&gt;Armageddon in management training&lt;/a&gt;, to count the number of impossibilities. Specifically, they claim there are 168 scientific impossibilities in the movie. Two questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Does NASA really use this in their training?&lt;/li&gt;&#xA;&lt;li&gt;Does this list of 168 impossibilities really exist?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-06-04T13:23:06.847" LastActivityDate="2014-06-04T13:23:06.847" Title="Armageddon in NASA training" Tags="|nasa|training|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4698" PostTypeId="2" ParentId="4688" CreationDate="2014-06-04T02:40:44.847" Score="141" Body="&lt;p&gt;The force of gravity decreases with distance. It follows an inverse-square relationship... essential to know when you're grinding out the math, but not essential to a conceptual understanding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that gravity decreases with distance means that at some distance, it will be negligible; an object sufficiently distant from Earth may be considered to have &quot;escaped&quot; Earth's gravity. In reality, the force of gravity has no distance limit; two objects would have to be at infinite distance from each other to have no gravitational interaction, but for practical purposes, one can think of finite distances where gravitational forces become small enough to ignore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider an object some large distance from Earth... right at the edge of what we would consider the Earth's gravitational &quot;sphere of influence&quot;. Some tiny movement toward Earth will increase the gravitational attraction, accelerating the object toward Earth. The process will escalate with the object's velocity and acceleration increasing. If we ignore the effects of Earth's atmosphere, the object will continue its acceleration until it strikes the Earth's surface at some velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, let's reverse everything. The object magically launches up from Earth's surface at exactly the same speed as our falling object had at the instant of impact. As it rises up, gravity tugs on it and it slows down. As it gets further away, gravity diminishes so it decelerates more slowly. Eventually, it gets to some distance where it has come to a stop, but Earth's gravity no longer has any effect on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity our object had at Earth's surface is Earth's escape velocity. In precise terms, a body's escape velocity is the velocity an object in &quot;free fall&quot; must have in order to escape the gravitational influence of that body - no more and no less. Technically, escape velocity can be specified for any distance from the center of a body, and the value will decrease with distance, but when a planet's escape velocity is stated, it is usually for the planet's surface. Mathematically, it is calculated as an integral of the body's gravitational acceleration from some specified distance to infinity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An object does not have to travel at escape velocity to escape a planet's gravity, but the same amount of energy needed to accelerate an object to escape velocity must be applied to an object (giving it potential energy) to lift it out of the planet's gravitational sphere of influence. The difference is that at escape velocity, the object needs no external influence to escape; at anything less than escape velocity, some external force must be applied.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-06-04T02:40:44.847" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4699" PostTypeId="2" ParentId="4697" CreationDate="2014-06-04T03:35:50.737" Score="13" Body="&lt;p&gt;Try an exact phrase web search for &amp;quot;NASA shows the film as part of its management training program. Prospective managers are asked to find as many inaccuracies in the movie as they can.&amp;quot; You'll get lots of hits. Lots and lots of them. That smacks of hand-me-down news, which makes the story ring a bit false. Where a source is attributed it's the &lt;a href=&quot;http://www.newscientist.com/article/mg19526191.500-feedback.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Feedback&lt;/em&gt; section of the September 1, 2007 edition of &lt;em&gt;New Scientist&lt;/em&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Armageddon games&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;WHY does NASA show the movie Armageddon as part of its management training programmes? ... In reality, the screenings are just a game for NASA's space geeks: who can find the highest number of impossible things in the movie? The record, Feedback is told, stands at 168.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The ellipsis is a few fluff sentences I edited out to keep my quote within the realm of fair use. I didn't cut that much. Note that there's no attribution, no context. It's a fluff piece. Who knows whether it's true?&lt;/p&gt;&#xA;&lt;p&gt;That said, I can think of one context where this might well have happened. NASA does send their management trainees to fairly extensive and sometimes intensive classes. Occasionally they need to do something fun to lower the intensity. Some companies hold a paintball session to accomplish that end. The &amp;quot;fun&amp;quot; session at a NASA retreat is reportedly a bit more nerdish. Tearing a &amp;quot;so bad it's good&amp;quot; sci-fi movie to shreds would fit right in.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;em&gt;New Scientist&lt;/em&gt; article was printed 6½ years ago. If the article was true, there's no knowing if it was something NASA still did in 2007. It might have been something NASA did years before that, but the news came to &lt;em&gt;New Scientist&lt;/em&gt; by word of mouth. I highly doubt it that this is used now because (a) the movie is now old and (b) management everywhere regularly changes how they do training. Now they might rip &lt;em&gt;Gravity&lt;/em&gt; to shreds.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-06-04T03:42:11.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4700" PostTypeId="2" ParentId="4678" CreationDate="2014-06-04T08:41:59.593" Score="7" Body="&lt;p&gt;I'm starting to sound like a broken record, but I agree with Tildal's post (yet again). There is one more thing I would add that concerns an Earth native who recently travels to the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jumping ability comes from strength in muscles and tendons, while bones support weight. It is possible to jump on Earth and sprain an ankle and such if one jumps too high and/or lands incorrectly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good landings come from apropriate training, coordination, etc. As a counter point, one who just came from Earth's higher grav might be able to jump very high on natural strength alone and not be able to land as easily as they jumped. The more dense bones might be able to take the force, but I am concerned with the ability of the tissues to absorb the shock and take the damage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you combine the speeds that Tildal writes about with a solid ground and number of factors could contribute to injury. It could be possible that an untrained individual may not be able to react quickly enough to the increase in speed or not be able to accurately estimate the distance to the ground (much like when jumping out of a plane on the Earth, the trees come up rather quickly). Any number of issues could arrise that could cause serious damage. I don't see people dying per se, but with speed and a solid ground, one could hurt themselves much more easily than they would on Earth with a similar jump.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In relation to the book (great book btw), these acrobats are trained and Lunar natives or at least they have been on the Moon long enough to become skilled with movement with the lower gravity. It is possible (with the aid of some suspension of disbelief) that they are able to react more quickly, and perform movements that help mitigate or disperse any force applied, much like a parachute landing fall (plf) can save someone from falling a great height by rolling the force in a different direction.&lt;/p&gt;&#xA;" OwnerUserId="3440" LastActivityDate="2014-06-04T08:41:59.593" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4701" PostTypeId="1" CreationDate="2014-06-04T13:04:08.470" Score="23" ViewCount="5720" Body="&lt;p&gt;&lt;a href=&quot;http://www.astronomy.org/programs/moon/moon.html&quot; rel=&quot;noreferrer&quot;&gt;According to this site&lt;/a&gt;, NASA produced a public domain painting representing what the Olympics could look like on a lunar colony. In the upper right of the picture, there are people flying by flapping wearable wings. Given Moon's gravity with Earth's atmospheric pressure in a lunar colony dome, like the one shown here, could this be possible? How much force would you have to push down on your arms to take off? Would taking off be feasible, or is just gliding?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rnzHQ.jpg&quot; alt=&quot;Lunar Olmpics&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="3161" LastEditorUserId="49" LastEditDate="2014-06-04T14:56:39.300" LastActivityDate="2019-03-17T09:11:10.437" Title="Could you fly on the Moon, in Earth's atmospheric pressure, by flapping wearable wings?" Tags="|the-moon|crewed-spaceflight|colonization|humans|myth-busting|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4702" PostTypeId="2" ParentId="4701" CreationDate="2014-06-04T14:03:16.037" Score="22" Body="&lt;p&gt;The lift force you would need to produce would have to be equal (stable flight) or greater (take off) than the force pulling you back towards the moon. If your mass is average for a &lt;a href=&quot;http://en.wikipedia.org/wiki/Body_weight#By_region&quot;&gt;male at 62kg&lt;/a&gt;, the force from the lunar gravity would be F=ma, = 62kg x &lt;a href=&quot;https://www.google.co.uk/webhp?sourceid=chrome-instant&amp;amp;rlz=1C1SVEF_enGB442GB442&amp;amp;ion=1&amp;amp;espv=2&amp;amp;ie=UTF-8#q=gravity%20of%20moon&quot;&gt;1.622m/s2&lt;/a&gt; = 100N (almost exactly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Lift_%28force%29#Lift_coefficient&quot;&gt;lift force = 0.5 x density x lift coefficient x area x velocity^2&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is as far as you can really calculate without specifying shape/size of the wings etc. However &lt;a href=&quot;http://www.roymech.co.uk/Useful_Tables/Human/Human_strength.html&quot;&gt;this site&lt;/a&gt; shows that the human arm can exert around 50N -60N (that's per arm). So with the correct design it seems like you have 1.1 times the required force available. This all ignores the mass of the wings themselves, the percentage of force that is applied in the appropriate direction etc, it at least implies you would be able to hover on the spot until you get tired. I suppose doing so would become comparable to the world record for &lt;a href=&quot;http://uk.ask.com/question/what-is-the-world-record-for-the-longest-period-of-time-treading-water&quot;&gt;treading water&lt;/a&gt; - after 85 hours you'd be pretty bored.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-06-04T14:09:25.433" LastActivityDate="2014-06-04T14:09:25.433" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4703" PostTypeId="1" CreationDate="2014-06-04T23:19:21.617" Score="15" ViewCount="705" Body="&lt;p&gt;I'm interested in understanding what effect does long-term radiation and microdust exposure would do to the surface roughness of a high-precision mirror like the one on the Hubble. Since it has been on orbit for 24 years, it provides an unique opportunity to estimate accumulated environmental damage to high-quality mirror surfaces in general.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since surface roughness is directly related to scattering losses and Strehl ratio, I figure that a measurable decay of image quality and total luminosity would enable an estimate of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do astronomers working on the Hubble track for optical degradation of the optics? if yes, what techniques do they use? what fraction of that degradation is actually related to mirror degradation?&lt;/p&gt;&#xA;" OwnerUserId="638" LastEditorUserId="638" LastEditDate="2014-09-18T20:37:17.133" LastActivityDate="2018-02-25T04:24:35.987" Title="Accumulated environmental damage to Hubble main mirror" Tags="|radiation|optical|hubble|space-telescope|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4704" PostTypeId="1" AcceptedAnswerId="5702" CreationDate="2014-06-05T00:23:23.000" Score="11" ViewCount="2509" Body="&lt;p&gt;In &lt;a href=&quot;http://www.newscientist.com/article/dn25677-us-must-play-nice-with-china-to-put-astronauts-on-mars.html&quot;&gt;this article&lt;/a&gt; is stated that in order to land on Mars NASA might need the help of the ESA or even the Chinese, not because it doesn't have the capability, but because it needs more financial help. Of course for the moment this collaboration might get little traction in the US government because of security reasons. Although little publicized, it appears that NASA &lt;a href=&quot;http://spaceindustrynews.com/how-to-donate-to-nasa/&quot;&gt;accepts donations&lt;/a&gt;, but you can't suggest/know what &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PD_1210_001G_&amp;amp;page_name=main&quot;&gt;they&lt;/a&gt; do with your money. Probably this type of approach doesn't create a sense of accomplishment within the people that manage to find out they can actually donate to NASA. My question is what legal biases prevents NASA from launching targeted, well advertised campaigns (kickstarter &lt;a href=&quot;http://www.rockethub.com/projects/42228-isee-3-reboot-project-by-space-college-skycorp-and-spaceref&quot;&gt;like&lt;/a&gt;), that could raise a part of the money needed for specific missions?&lt;/p&gt;&#xA;" OwnerUserId="1263" LastActivityDate="2020-06-17T07:12:32.280" Title="Why doesn't NASA create targeted campaigns for donations?" Tags="|nasa|future-missions|cost|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="4705" PostTypeId="2" ParentId="2741" CreationDate="2014-06-05T00:34:20.980" Score="4" Body="&lt;p&gt;If you could power a &quot;massless&quot; propulsion method (like a bicycle that climbed out of the gravity field), you could easily escape Ceres, but this highlights the need for huge power thrust at the early stage, because propulsion needs mass that it can &quot;throw backwards&quot;, and if you accelerate slowly, that means you have to carry all that mass with you for longer.&lt;/p&gt;&#xA;" OwnerUserId="3495" LastActivityDate="2014-06-05T00:34:20.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4706" PostTypeId="1" CreationDate="2014-06-05T12:07:26.583" Score="5" ViewCount="17080" Body="&lt;p&gt;How are astronauts protected from radiation (alpha, beta, gamma rays, uv rays) by their spacecraft?&#xA;Is the spacecraft built in a special material? &lt;/p&gt;&#xA;" OwnerUserId="3502" LastEditorUserId="897" LastEditDate="2014-06-06T09:25:30.607" LastActivityDate="2019-11-29T13:18:11.280" Title="How are astronauts protected from radiation in a spacecraft?" Tags="|astronauts|radiation|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4707" PostTypeId="1" AcceptedAnswerId="4708" CreationDate="2014-06-05T16:39:21.383" Score="6" ViewCount="1172" Body="&lt;p&gt;Does regolith mean any threat for people visting the Moon or the Mars? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If yes, what is that?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="25" LastEditDate="2014-06-05T17:34:40.693" LastActivityDate="2014-06-18T09:33:24.630" Title="What are the dangers of regolith?" Tags="|mars|the-moon|regolith|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4708" PostTypeId="2" ParentId="4707" CreationDate="2014-06-05T17:02:15.260" Score="6" Body="&lt;p&gt;Firstly, the Lunar and Martian regolith will be quite different in composition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_regolith#Harmful_effects_of_lunar_dust&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; has a short list of the harmful effects of Lunar dust, these have been added to, to include potential harmful effects of Martian dust.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Darkening of surfaces, leading to a considerable increase in radiative heat transfer &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Abrasive nature of the dust particles may rub and wear down surfaces through friction&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Negative effect on coatings used on gaskets to seal equipment from space, optical lenses that include solar panels and windows as well as wiring&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Possible damage to astronaut's lungs, nervous, and cardiovascular systems&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;it can short circuit equipment since some of it is metallic dust&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;wedge into small openings or spacesuit fabric or stick to it magnetically and cause triboelectric (rubbing) and/or surface photoelectric discharge&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;contain dangerous chemicals like e.g. calcium perchlorate salts on Mars&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;it can cover &quot;traps&quot; on untraversed lands, such as dry ice fans, other volatiles, ravines, cracked lava tubes&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Cause substantial surface temperature differential&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;airborne dust can cause electric storms&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;reduce visibility&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;As for the Martian soil there is a whole host of articles discussing this topic, for example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0304388601001528&quot; rel=&quot;nofollow noreferrer&quot;&gt;potential hazard of electrostatic adhesion and discharge&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Or for how  I found this paper: &lt;a href=&quot;http://scholar.google.co.uk/scholar?q=potential%20hazards%20of%20martian%20regolith&amp;amp;hl=en&amp;amp;as_sdt=0&amp;amp;as_vis=1&amp;amp;oi=scholart&amp;amp;sa=X&amp;amp;ei=zJ-QU-viDILR7AbZ5YDoAg&amp;amp;ved=0CCYQgQMwAA&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google Scholar&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One difference between the Martian and Lunar regolith is the degree to which is it moved. Mars has an &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars#Atmosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;atmospheric pressure&lt;/a&gt; of around 600 Pa, where as the moon is much lower at &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon#Atmosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.3 nPa&lt;/a&gt; (0.3*10&lt;sup&gt;-9&lt;/sup&gt; Pa) - that's about &lt;a href=&quot;http://www.wolframalpha.com/input/?i=600/0.3x10%5E%28-9%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;2 thousand billion&lt;/a&gt; times lower. So you could logically expect the Martian atmosphere to carry a much larger amount of regolith in &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/0019103582901270&quot; rel=&quot;nofollow noreferrer&quot;&gt;dust storms&lt;/a&gt; - creating a large issue when looking at regolith covering sensors etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDITED to add notes from &lt;a href=&quot;https://space.stackexchange.com/users/49/tildalwave&quot;&gt;TidalWave&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-06-05T17:54:25.530" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4709" PostTypeId="2" ParentId="4688" CreationDate="2014-06-05T18:00:17.910" Score="12" Body="&lt;p&gt;The key difference is that &quot;escape velocity&quot; is how fast you would have to throw a stone straight up from the Earth's surface (ignoring air drag), for it to escape from Earth's gravitational influence. It would be &lt;em&gt;coasting&lt;/em&gt; the whole way, always losing speed due to Earth's gravitational pull. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, on the other hand, you have a rocket engine with sufficient fuel, you can just keep rising slowly (1 mph), which is almost a hover, until you've gotten way out into space and Earth's gravity is overwhelmed by the Sun, Jupiter, etc. You could keep throttling back to maintain the same upward speed (gravity decreases with distance, and the rocket carries less fuel) if you wanted to, or let the rocket speed up.&lt;/p&gt;&#xA;" OwnerUserId="3127" LastEditorUserId="6816" LastEditDate="2015-08-17T21:06:02.913" LastActivityDate="2015-08-17T21:06:02.913" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4710" PostTypeId="2" ParentId="4688" CreationDate="2014-06-05T20:10:01.007" Score="5" Body="&lt;p&gt;Looking at this in another way, &lt;strong&gt;consider the concept of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_well&quot; rel=&quot;noreferrer&quot;&gt;gravity wells&lt;/a&gt;.&lt;/strong&gt; The gravity well of course is not a &quot;real&quot;, physical well, but it is a commonly used metaphor to describe how much energy is required to escape from the gravitational effect of a body, and it provides a reasonably straight-forward way of answering your question. (Space buffs, bear with me below; this is meant as an explanation, not a university-level physics and astronomy lecture.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are at or near the bottom of a gravity well (say, at the surface of the Earth) and want to climb out of that well, you basically have two options. Either climb very fast for a short distance (this is the approach taken for getting off the surface of the Earth, for reasons stated in other answers), or climb slowly for a much longer distance (this works once you are far enough away from the body forming the gravity well that the predominant gravitational forces acting on you are small or negligible). Each way of looking at it represents the same thing: you provide some sort of energy input, usually in terms of fuel of some kind, which is used to climb the &quot;side&quot; of the gravity well. The energy provided as input becomes potential energy as you climb farther from the surface, and at some point, your potential energy exceeds the gravitational pull at that point of the body that forms the gravity well; you &quot;continue on a tangent&quot; and move straight on from that point forward rather than following the curve of the gravity well. Once that happens, you have reached escape velocity from that body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you don't climb far enough for your rate of climb at the time you stop actively climbing, then when you stop climbing (let's assume you cannot grab hold of anything, because in space there is nothing to hold on to) you will fall back toward the body that forms the gravity well you are trying to climb out of; you did not attain escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, there are usually multiple gravitational forces to contend with at any one point. However, one of them will project a stronger force on you than the others; that's the concept behind the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sphere_of_influence_%28astronomy%29&quot; rel=&quot;noreferrer&quot;&gt;sphere of influence&lt;/a&gt;. Near Earth (yes, that most definitely includes low Earth orbit), it's Earth's gravity that dominates; take a trip to Luna and its gravity will exert the greater force once you pass &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#L1&quot; rel=&quot;noreferrer&quot;&gt;the Earth-Moon system L1 Lagrangian point&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://xkcd.com/681/&quot; rel=&quot;noreferrer&quot;&gt;The &quot;depth&quot; of a gravity well&lt;/a&gt; is often given as its escape velocity, in km/s or some other convenient measurement of velocity, taken at the bottom of the well. Hence, the depth of Earth's gravity well is approximately 11.2 km/s, which is the escape velocity at the Earth's surface. &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity#Overview&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia gives the escape velocity at 9,000 km above the Earth's surface as 7.1 km/s&lt;/a&gt;, but as we have seen in other answers, getting to 9,000 km above the surface itself takes a lot of energy, negating the gain from the lower &quot;absolute&quot; speed necessary to break free of the Earth's gravity.&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2014-06-05T20:10:01.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4712" PostTypeId="2" ParentId="4706" CreationDate="2014-06-06T09:10:34.017" Score="12" Body="&lt;p&gt;In current spacecraft, the materials for e.g. the hull are chosen primarily because they are light and strong. Aluminium is common. A few mm of aluminium &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2005/27jan_solarflares/&quot; rel=&quot;nofollow noreferrer&quot;&gt;blocks most of the radiation&lt;/a&gt; you would encounter in low Earth orbit. &lt;a href=&quot;https://space.stackexchange.com/questions/1034/how-are-the-astronauts-in-the-iss-protected-from-solar-flares&quot;&gt;In the ISS&lt;/a&gt;, 95% of the radiation is blocked.&lt;br&gt;&#xA;This is enough for low Earth orbit: these orbits are inside the Van Allen Belts, so they are protected from the worst radiation. If we want to go beyond LEO for longer periods, more protection is needed. You could make the hull thicker, but this makes the launch more expensive. One way around this would be to use a small asteroid and build the spaceship inside it, but that leaves the problem of capturing an asteroid and bringing it to Earth. We've never done that.&lt;br&gt;&#xA;Another way is a magnetic shield. This requires a lot of power. I seem to remember a figure of 500 kW for a reasonable shield for a small spacecraft (I can't find the reference at the moment though). The ISS (with its massive solar arrays) can produce about 200 kW.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some interesting related questions and answers:&lt;br&gt;&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/797/radiation-shielding-magnetic-or-mass-which-is-more-efficient&quot;&gt;Efficiency of radiation shielding&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/388/what-materials-provide-the-best-protection-from-cosmic-radiation?rq=1&quot;&gt;Materials&lt;/a&gt;  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-06-06T09:10:34.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4713" PostTypeId="1" CreationDate="2014-06-06T09:47:47.127" Score="5" ViewCount="203" Body="&lt;p&gt;What would an extra-vehicular activity (EVA) be like on a spacecraft which consists of modules along a (100s of meters long) rotating tether that causes simulated gravity? See for example these &lt;a href=&quot;http://thespaceshow.files.wordpress.com/2014/05/partial-gravity-iac2010-presentation-revised-2014may22.pdf&quot; rel=&quot;noreferrer&quot;&gt;slides&lt;/a&gt; and &lt;a href=&quot;http://thespaceshow.files.wordpress.com/2014/05/designconceptsforamannedartificialgravityresearchfacilityforiac2010sept14-rev-2014may22.pdf&quot; rel=&quot;noreferrer&quot;&gt;pdf&lt;/a&gt; for Joe Carroll's ideas about this kind of spacecraft/spacestation.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could the astronaut stand and walk on the &quot;roof&quot; of a module?&lt;/li&gt;&#xA;&lt;li&gt;Where does the astronaut go if he/she falls off the side of a module?&lt;/li&gt;&#xA;&lt;li&gt;What would be easier and more difficult respectively, with rotation than in microgravity?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'd guess that manual handling of tools would be much more Earth like and thus construction work much more efficient. Maybe large construction projects in LEO and beyond should be performed in rotating artificial gravity?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-06-07T15:03:05.863" Title="EVA on rotating spacecraft - differences from microgravity" Tags="|eva|artificial-gravity|rotation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4714" PostTypeId="2" ParentId="4713" CreationDate="2014-06-06T10:11:06.727" Score="2" Body="&lt;p&gt;Assuming the EVA is at the module end, rather than the center of rotation, several interesting effects happen.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;things &quot;fall&quot;.&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Things fall in an apparent arc, rather than down.&lt;/li&gt;&#xA;&lt;li&gt;This arc does appear to have  a significant bend.&lt;/li&gt;&#xA;&lt;li&gt;This arc always is to trailing/back/antispin.&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;li&gt;Any item has weight varying by distance from center of rotation. This means that the items gain weight when lowered; the mass never changes, but the forces required to move/hold it vary.&lt;/li&gt;&#xA;&lt;li&gt;Any surface where the spin is to one's advantage is an area where one is likely to be working in a &quot;downward&quot; motion - specifically, down as &quot;away from center of rotation.&quot;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-06-06T10:11:06.727" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4715" PostTypeId="1" CreationDate="2014-06-06T10:45:04.457" Score="14" ViewCount="1113" Body="&lt;p&gt;Quoting from &lt;em&gt;&lt;a href=&quot;https://news.yahoo.com/nasa-warned-plan-send-humans-mars-may-fail-022806799.html&quot;&gt;NASA warned plan to send humans to Mars may fail&lt;/a&gt;&lt;/em&gt; news article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Obama administration is opposed to another moon landing, saying&#xA;  such a mission would be too costly. It wants instead to focus on&#xA;  capturing an asteroid and placing it into the Moon's orbit for future&#xA;  exploration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How useful is &quot;placing an asteroid in lunar orbit&quot;? What are implications as a function of research?&lt;/p&gt;&#xA;" OwnerUserId="3522" LastEditorUserId="25" LastEditDate="2016-03-25T14:43:59.883" LastActivityDate="2016-03-25T14:43:59.883" Title="How useful is placing an asteroid in Lunar orbit?" Tags="|the-moon|nasa|asteroid|future-missions|research|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4716" PostTypeId="2" ParentId="4715" CreationDate="2014-06-06T14:28:22.073" Score="10" Body="&lt;p&gt;There are entities that hope to mine asteroids, &lt;a href=&quot;http://www.planetaryresources.com/&quot;&gt;Planetary Resources&lt;/a&gt; and &lt;a href=&quot;http://deepspaceindustries.com/&quot;&gt;Deep Space Industries&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The proposed Asteroid Retrieval Mission is based upon the &lt;a href=&quot;http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf&quot;&gt;Keck Report&lt;/a&gt;, a paper outlining how a small Near Earth Asteroid (NEA) could be parked in lunar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Co-authors of the Keck Report include&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Chris Lewicki (Chief engineer of Planetary Resources),&lt;/li&gt;&#xA;&lt;li&gt;J. S. Lewis (Author of &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0201328194&quot;&gt;Mining the Sky&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Don Yeomans (of &lt;a href=&quot;http://neo.jpl.nasa.gov/neo/&quot;&gt;JPL's Near Earth Object Program&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And a long list of other prominent scientists and engineers whose focus is near earth asteroids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many of the co-authors of the Keck Report are also on the staff of Planetary Resources or Deep Space Industries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Developing a retrieval vehicle with powerful Solar Electric Propulsion (SEP) engines would be a big step towards making asteroid mining plausible. Robust SEP might also be helpful for ferrying massive payloads from earth orbit to lunar orbit, so the technology would also make establishing a lunar base more doable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why park an asteroid in lunar orbit? Why not mine it in its heliocentric orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Early asteroid mining will be a process of trial and error. So establishing infra structure will require multiple trips. Trip times to an NEA would be months to a better part of a year. The most accessible asteroids with earth like orbits also have very rare launch windows, on the order of years and decades apart.  Light lag latency to a NEA in heliocentric orbit can be 10s of minutes. Since signal strength falls with inverse square of distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's compare that to an asteroid in lunar orbit. Trips times are less than a week. Launch windows from a given low earth orbit occur each two weeks. Light lag latency is about 3 seconds. &lt;a href=&quot;http://www.softwarelicensingblog.com/2009/08/articles/telecom-and-other-interesting-1/that-is-one-small-step-for-bandwidth-one-giant-leap-for-isps/&quot;&gt;Telerobots at ~1 lunar distance can enjoy high bandwidth&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For these reasons lunar orbit is a much better place to develop the fledgling asteroid mining technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Placing an asteroid in lunar orbit would be very useful.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-06-06T14:59:24.270" LastActivityDate="2014-06-06T14:59:24.270" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4717" PostTypeId="2" ParentId="4678" CreationDate="2014-06-06T15:02:45.853" Score="6" Body="&lt;p&gt;First of all, let's ask how far does a fall from Earth have to be to have injury? For simplicity, I'm going to assume a health middle aged adult, the typical range for astronauts. Looking around, the &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/8584940&quot;&gt;height of serious injury&lt;/a&gt; on the Earth appears to be 7 m, or something relatively close to that. In fact, the height might be even lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm also going to assume that the person was well conditioned. We know that bone mass is lost in zero gravity, and I'm going to assume that the person falling still have an Earth bone mass. As bone mass is lost, the chance of injury increases dramatically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, a 7 m fall from Earth will end up with a velocity of about 11.7 m/s. What distance is required to have that speed on the moon? The answer is about 43 meters. Of course, that's the serious injury level, and in fact the fall distance might well be less than that, as you lose bone mass from being in low gravity for so long. But this should be a good starting point at least.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-06-06T15:02:45.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4718" PostTypeId="2" ParentId="4606" CreationDate="2014-06-06T16:09:17.663" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/content/comet-hitchhiker-harvesting-kinetic-energy-from-small-bodies-to-enable-fast-and-low-cost/#.U5HgyPnHnAY&quot; rel=&quot;nofollow&quot;&gt;Hitchhiking a comet by tether&lt;/a&gt; is a concept which actually today has been &lt;a href=&quot;http://www.parabolicarc.com/2014/06/06/nasa-selects-12-niac-phase-projects-funding/&quot; rel=&quot;nofollow&quot;&gt;awarded $100,000&lt;/a&gt; to &lt;em&gt;&quot;conduct a nine-month initial definition and analysis study of their concepts&quot;&lt;/em&gt;, by NASA. This is part of the NIAC program (NASA Innovative Advanced Concepts). So at least a PhD in Aeronautics and Astronautics, MIT 2012, will spend some time to find out what questions one needs to ask about this.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-06-06T16:09:17.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4719" PostTypeId="2" ParentId="4678" CreationDate="2014-06-06T16:19:20.350" Score="10" Body="&lt;p&gt;Without going into detailed calculations, I recall that the gravitational acceleration on the Moon's surface is about $\frac16$ of what it is on the Earth's surface. Now by the simple formula that work is force times distance, for an object to acquire the same kinetic energy (and therefore the same velocity) as falling from a height $h$ on Earth, it should fall from a height about $6h$ on the Moon. (This is ignoring friction, which is certainly valid on the Moon, and valid for moderate (yet fatal) heights on Earth.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The damage done on hitting the ground is mostly a function of the velocity of impact, so whatever height makes you uncomfortable on the Earth, six times that heigh should make you uncomfortable on the Moon. You will have longer to prepare for impact on the Moon; in fact six times as long, since with the final velocities being equal, so are the average velocities (half the final velocity when starting stationary), and falling from height $6h$ on the Moon will take $6$ times as long as falling from height $h$ on Earth.&lt;/p&gt;&#xA;" OwnerUserId="3492" LastActivityDate="2014-06-06T16:19:20.350" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4720" PostTypeId="1" AcceptedAnswerId="4724" CreationDate="2014-06-06T17:11:23.920" Score="22" ViewCount="4899" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/4715/415&quot;&gt;How useful is placing an asteroid in the Moon's orbit?&lt;/a&gt; quotes &lt;a href=&quot;https://news.yahoo.com/nasa-warned-plan-send-humans-mars-may-fail-022806799.html&quot; rel=&quot;noreferrer&quot;&gt;a news article&lt;/a&gt; saying the Obama administration is opposed to a lunar landing mission because of the cost of such a mission:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Obama administration is opposed to another moon landing, saying such a mission would be too costly. It wants instead to focus on capturing an asteroid and placing it into the Moon's orbit for future exploration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But placing an asteroid in lunar orbit and going there to pay a visit to it isn't exactly cheap either (even if &lt;a href=&quot;https://space.stackexchange.com/questions/4720/what-is-the-marginal-cost-of-landing-on-the-moon?noredirect=1#comment9879_4720&quot;&gt;the delta-v to bring the asteroid into a lunar orbit isn't that large&lt;/a&gt; and everything goes perfectly, there's still the cost of first getting to the asteroid in its original orbit, then getting to lunar orbit to do anything with it). Sending robots to do the exploration and possibly mining of the asteroid would almost certainly cut down on the cost, but that same argument also holds for a lunar landing mission: technology has advanced a lot since the 1960s, so we can do a lot with robots today that we couldn't reasonably or even at all do back then. The difference in capability between the early planetary landing probe missions and the recent Mars missions, for example, shows this well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this got me wondering. If we assume for a second that we would want to go to the Moon in some manner, then roughly &lt;strong&gt;how much higher would the monetary cost be just to &lt;em&gt;land&lt;/em&gt; on the Moon and getting back into lunar orbit,&lt;/strong&gt; compared to only &lt;em&gt;establishing a (reasonably) stable lunar orbit?&lt;/em&gt; It's probably safe to say that it would add a fair bit to the cost of the mission, but how much? Let's ignore the matter of human EVA capability; people aren't going to be walking around on the surface of the moon from a lunar orbit either (even though landing people on the Moon without human EVA capability seems a bit silly, to put it mildly; there isn't a lot that can be done by Moon-local remote control that can't be done over an Earth-Moon link, if only you have enough power and receiver sensitivity for a good quality downlink from the Moon), and a lot can be done without sending humans either to lunar orbit or to the lunar surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good answer would explore this both from the angle of manned and unmanned missions. Order of magnitude figures are quite good enough.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-12-07T03:10:55.683" Title="What is the marginal cost of *landing* on the Moon?" Tags="|the-moon|lunar-landing|cost|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4721" PostTypeId="2" ParentId="4678" CreationDate="2014-06-06T17:40:30.700" Score="17" Body="&lt;p&gt;Going away from the scientific answers, and more towards the psychological answers, I'd like to approach how one makes a mistake bad enough to break a bone.  The first step involves getting enough energy to break the bone with a fall in the first place.  The body should naturally do a reasonable job of balancing the muscle strength needed to do tasks against the bone strength needed to oppose the muscles.  This is why you see little fear of broken bones on the ISS -- their muscles and bones atrophy in harmony.  However, when they come back to earth, their muscles are forced to push themselves to the limit to fight gravity, and the bones haven't been given an opportunity to catch up.  Think similar to forgetting to stretch before physical activity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to get the energy to break your bones, you'd probably have to rely on something other than jumping.  A tall fall could do it, just like on earth (only taller).  This is where the psychology comes in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have not had enough time to get used to low gravity rules, your brain may not realize it is in trouble until too late.  You may get cocky, and think you can take the 1km fall because the 900m one wasn't so bad.  The brain has a lot of safeguards to prevent us from hurting ourselves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most likely culprit would be mass vs weight.  If someone had not gotten used to low gravity (such as an Earthlander visiting the Moon), they may drastically underestimate the inertia of an object because it &quot;doesn't weigh much.&quot;  This may cause them to carelessly put themselves in a position where they must stop a high-mass object with their body, and fail to realize how much trouble they are in until it is too late.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has happened at least once in our spacewalks.  An engineer designed a procedure that involved powering down a spinning dish and then stopping it to do maintenance.  The earthbound engineer accidentally assumed that the dish would have little inertia, because they were weightless.  The spacewalker found it was virtually impossible to stop the disk.  They ended up finding a smooth ring on the dish, and appyling friction for several minutes to get the rotational rate down to where the spacewalker had the muscle (and bone) to actually stop the rotational inertia of the disk.&lt;/p&gt;&#xA;" OwnerUserId="3532" LastActivityDate="2014-06-06T17:40:30.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4722" PostTypeId="2" ParentId="4706" CreationDate="2014-06-06T19:49:33.467" Score="5" Body="&lt;p&gt;Most space-based radiation, primarily charged particles, is very effectively blocked with hydrogen-rich materials.  Scientists for the Lunar Reconnaissance Orbiter discovered that, pound for pound, polyethylene was a more effective radiation shield than aluminum. &lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-06-06T19:49:33.467" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4723" PostTypeId="2" ParentId="2840" CreationDate="2014-06-06T20:04:41.810" Score="13" Body="&lt;p&gt;The Tindallgrams have some notes on the matter.  See pages 307 to 354 of &lt;a href=&quot;http://www.collectspace.com/resources/tindallgrams/tindallgrams02.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.collectspace.com/resources/tindallgrams/tindallgrams02.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Briefly, improvements were made in a couple of areas:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The model of the moon's gravity field (based on tracking data from earlier Apollo and Lunar Orbiter flights)&lt;/li&gt;&#xA;&lt;li&gt;Real-time estimation of the spacecraft's trajectory - a new &quot;Lear Processor&quot; was brought online at Misson Control's Real-Time Computer Complex, which was running a specialized Kalman Filter to provide optimal use of all the tracking information.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It was too late in the design cycle to incorporate any changes to the gravity model into the LM guidance computer program, so instead the RTCC computed offsets to the landing site position based on the latest information during powered descent.  This offset was verbally relayed to the astronauts who entered it into the PGNCS, adjusting the target landing site that the autopilot was aiming for so that the &lt;em&gt;actual&lt;/em&gt; landing site would be closer to Surveyor.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2019-07-08T06:02:35.160" LastActivityDate="2019-07-08T06:02:35.160" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="4724" PostTypeId="2" ParentId="4720" CreationDate="2014-06-07T00:23:32.033" Score="22" Body="&lt;p&gt;1.87 km/sec Δv between low lunar orbit and the surface.  Thus you need 3.74 km/sec plus some reserve because suicide burns are for KSP, not real life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make a trip from low Earth orbit to low lunar orbit is 4.04 km/sec Δv and I believe the return is 1.40 km/sec Δv.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus your total Δv is 5.44 for the trip to low lunar orbit (and less if you go to a higher orbit, I don't know how high you can go and yet remain in orbit) vs 9.18+ km/sec Δv for the landing.  The tyranny of the rocket equation says the latter trip is &lt;strong&gt;MUCH&lt;/strong&gt; harder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:  FraserOfSmeg provided a link giving \$1.2M/kg to the lunar surface.  There's an even more interesting piece of data there--the same company is charging \$198K/kg to lunar orbit.  (Edit: They no longer list the price to orbit, only to the surface.)  Thus the cost of landing from orbit increases the price sixfold.  Furthermore, looking at the ascent stages for the Apollo lunar landers they were more than 50% fuel.  Even if we can reuse the descent engine that means a lot more weight involved.  Using the Apollo numbers combined with the Astrobotic ones I get \$2.2M/kg returned to lunar orbit.  Thus the price of a landing/return mission is elevenfold higher than an orbital mission.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2016-12-07T03:10:55.683" LastActivityDate="2016-12-07T03:10:55.683" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4725" PostTypeId="1" CreationDate="2014-06-07T07:23:25.013" Score="6" ViewCount="5083" Body="&lt;p&gt;According to &lt;em&gt;&lt;a href=&quot;https://www.google.com.ar/patents/US8616496&quot; rel=&quot;noreferrer&quot;&gt;Systems and methods for a self-deploying vehicle drag device&lt;/a&gt;&lt;/em&gt; (US 8616496 B2) patent's background of the invention:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The United States Federal Communications Commission (FCC) requires&#xA;  CubeSats and other picosatellites to be designed to re-enter the&#xA;  atmosphere within 25 years of the end of their useful lifetimes.&#xA;  Without an assistance, it is estimated that a CubeSat may take over&#xA;  150 years to de-orbit from an 800 km altitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The purpose of deorbiting devices is to cause the satellite to deorbit within the 25-year post-mission lifetime limit, as also recommended by the Inter-Agency Space Debris Coordination Committee (IADC) and ISO 24113 &lt;em&gt;Debris mitigation requirements and compliance&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(e) The orbital lifetime of objects passing through LEO (lower than  2,000&#xA;  km) shall be shorter than 25 years after the end of operation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why does it take 150 years for a CubeSat satellite in 800 km Low Earth Orbit (LEO) to de-orbit naturally, without the use of deorbiting devices?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="3542" LastEditorUserId="49" LastEditDate="2014-06-16T16:52:15.277" LastActivityDate="2018-04-01T20:11:42.760" Title="Deorbit time for satellites in LEO" Tags="|artificial-satellite|low-earth-orbit|debris|atmospheric-drag|drag-augmentation|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4726" PostTypeId="1" CreationDate="2014-06-07T10:23:11.937" Score="4" ViewCount="378" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Does the Dragon reusability concept of SpaceX include the ability to reactivate the engines after landing to make a &quot;jump&quot; to a new soft landing location some kilometers or so away on the surface?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is it (will it be) capable of hoovering at low altitude over the landscape for tens of minutes to for example get feedback from human control on Earth to Mars?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Or is the Dragon concept designed just to make one soft landing, not able to take off and land again without refurbishing?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-06-07T12:10:18.457" Title="Will the SuperDraco powered Dragon be able to &quot;jump&quot; on the Moon or Mars?" Tags="|mars|the-moon|spacex|lander|dragon|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4727" PostTypeId="1" CreationDate="2014-06-07T10:23:39.220" Score="9" ViewCount="4245" Body="&lt;p&gt;I have made a &lt;a href=&quot;https://gist.github.com/grom358/367295eeb2c8efa10e69&quot; rel=&quot;noreferrer&quot;&gt;RK4 simulator&lt;/a&gt; for launching a cannonball from a tower. Which has the state variables:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Position vector r&lt;/li&gt;&#xA;&lt;li&gt;Velocity vector v&lt;/li&gt;&#xA;&lt;li&gt;Mass of planet M&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To simplify it, working in only 2 dimensions (orbits on equatorial plane) and ignoring the cannonball mass since M is much greater then mass of cannonball. The cannonball also moves ballistically (no air drag etc).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The output from the simulator firing 100km above surface (Orange is the planet, Blue is 60 degree launch angle, Yellow is 30 degree launch angle):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VNOrt.png&quot; alt=&quot;Suborbital Trajectory&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A closer look at some trajectories (Green: 60, Yellow: 45, Orange: 30, Blue: 0) &#xA;&lt;img src=&quot;https://i.stack.imgur.com/NhsZC.png&quot; alt=&quot;Suborbital Trajectory Zoomed&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to be able to calculate the apoapsis of the orbit, what is the formula given v, r and M?&lt;/p&gt;&#xA;" OwnerUserId="3543" LastActivityDate="2017-03-17T16:13:18.327" Title="How to calculate apoapsis of sub-orbital trajectory?" Tags="|orbital-mechanics|orbit|sub-orbital|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4728" PostTypeId="2" ParentId="4725" CreationDate="2014-06-07T10:29:27.823" Score="3" Body="&lt;p&gt;If you're asking why the time limit is set to 25 years then it's more for legal reasons than anything astrodynamic related. The choice of 25 years was at this point the owners of the satellite are not culpable should it re-enter, crash into another satellite etc. So the rational was give people as long as possible, without it being so long that they are not responsible for their actions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately I can't give a linked reference for this, since it was an answer to a question at the ESA Space Debris conference in 2013.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're asking why a satellite might take 25 years to de-orbit, it's because of the very high velocity of orbiting object (~7 km/s) and hence the objects having a high kinetic energy. For low Earth orbits, de-orbiting only passively occurs due to drag. To reduce your very high kinetic energy you need to collide and lose that kinetic energy with the rarified gas atmosphere. It take such a long time because the atmosphere is extremely thin. If you are interested in knowing how long an object will take to de-orbit due to drag, the key variables are altitude and ballistic coefficient.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-06-08T15:08:47.173" LastActivityDate="2014-06-08T15:08:47.173" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4729" PostTypeId="2" ParentId="4726" CreationDate="2014-06-07T12:10:18.457" Score="4" Body="&lt;p&gt;Yes, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Draco_%28rocket_engine_family%29#SuperDraco&quot; rel=&quot;nofollow&quot;&gt;SuperDraco thrusters&lt;/a&gt; are restartable. The propellants used are MMH (&lt;a href=&quot;https://en.wikipedia.org/wiki/Monomethyl_hydrazine&quot; rel=&quot;nofollow&quot;&gt;Monomethyl Hydrazine&lt;/a&gt;) as fuel and &lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrogen_tetroxide&quot; rel=&quot;nofollow&quot;&gt;Dinitrogen Tetroxide&lt;/a&gt; as oxidizer which are &lt;a href=&quot;https://en.wikipedia.org/wiki/Hypergolic_propellant&quot; rel=&quot;nofollow&quot;&gt;hypergolic&lt;/a&gt;, meaning they ignite on contact with each other and that greatly simplifies engine's ability to restart, throttle and achieve full thrust in &lt;a href=&quot;http://www.gizmag.com/superdraco-test/32292/&quot; rel=&quot;nofollow&quot;&gt;quoted&lt;/a&gt; 100 ms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ability to hop from one landing site to another and hover will of course depend on amount of propellants available for such maneuvers relative to vehicle's payload mass, gravity and atmospheric density, i.e. achievable engine efficiency given specific conditions and thrust to weight ratio for a specific mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since they'll be used on DragonRider, the crewed version of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Dragon&lt;/a&gt; spacecraft, their target performance will have to match requirements for &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_escape_system&quot; rel=&quot;nofollow&quot;&gt;LES/LAS&lt;/a&gt; (Launch Escape System / Launch Abort System) for human-rated vehicles, and SpaceX has plans to soft-land them on Earth as a part of their fully re-usable launch vehicle (Falcon 9 and later Falcon Heavy) once returning from LEO orbital speed to a soft landing in Earth's gravity, so I would imagine hovering and hopping could also be achievable on the Moon or Mars in reduced gravity w.r.t. Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One note though, that there would be little point in waiting for Earth based control station's communications to Mars during critical maneuvers like this. Depending on Mars' position relative to the Earth (closest at ~ &lt;a href=&quot;http://www.universetoday.com/14824/distance-from-earth-to-mars/&quot; rel=&quot;nofollow&quot;&gt;54.6 million kilometers&lt;/a&gt;), one-way communications delay is at minimum &lt;a href=&quot;https://encrypted.google.com/search?q=54.6%20million%20kilometers%20in%20light%20minutes&quot; rel=&quot;nofollow&quot;&gt;roughly 3 minutes&lt;/a&gt;. Mars landing craft will require far greater autonomy than having to hover over some surface to wait for input from Earth.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-06-07T12:10:18.457" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4730" PostTypeId="2" ParentId="4727" CreationDate="2014-06-07T12:38:07.697" Score="9" Body="&lt;p&gt;First calculate the semi-major axis $a$ where $\mu$ is standard gravitational parameter of the planet that in orbit around:&#xA;$$E=\frac{v^2}{2}-\frac{\mu}{r}$$&#xA;$$a = -\frac{\mu}{2E}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then with the eccentricity vector:&#xA;$$ e = {\left |v \right |^2 r \over {\mu}} - {(r \cdot v ) v \over{\mu}} - {r\over{\left|r\right|}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can now calculate: &#xA;$$r_p=a(1-\left|e\right|)$$ $$r_a=a(1+\left|e\right|)$$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$r_a\,\!$ is the radius at apoapsis (i.e., the farthest distance of the orbit to the center of mass of the system, which is a focus of the ellipse).&lt;/li&gt;&#xA;&lt;li&gt;$r_p\,\!$ is the radius at periapsis (the closest distance).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="3543" LastActivityDate="2014-06-07T12:38:07.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4732" PostTypeId="2" ParentId="4713" CreationDate="2014-06-07T15:03:05.863" Score="2" Body="&lt;p&gt;Things that get easier:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Applying force (linear or rotational). Gravity allows you to have sufficient friction to counteract forces. &lt;/li&gt;&#xA;&lt;li&gt;Storing objects. If you store tools or parts in a basket, they stay there. No more need to tether everything all the time. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Things that get harder:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Moving heavy objects.&lt;/li&gt;&#xA;&lt;li&gt;Working in a space suit: EVA suits are heavy.&lt;/li&gt;&#xA;&lt;li&gt;Getting rescued if you fall off the spacecraft.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-06-07T15:03:05.863" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4734" PostTypeId="2" ParentId="4720" CreationDate="2014-06-07T18:36:02.033" Score="6" Body="&lt;p&gt;Finding a nice equation for this is next to impossible. The cost per kilogram might steadily decrease from 1-10kg, but then increase thereafter since you need a different launcher/modular system to get the mass to the surface (speculative reasoning).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This all being said, &lt;a href=&quot;http://www.astrobotic.com/&quot; rel=&quot;noreferrer&quot;&gt;Astrobotic&lt;/a&gt; give you a cost per kilogram to the surface of the moon at \$1.2m. That's not specifically for a manned payload, but the majority of the cost difference would be design/build side (and the fact the you need to launch many kilos for a manned mission). Typical estimates for the cost/kilogram to LEO are in the range of \$10,000. As for the cost in the interim of lunar orbit but not landing I would imagine it would be something very similar to the extra amount of propellant needed to get you to the orbit. If that ends up being 20 kilograms of propellant, then you're talking an extra \$200,000.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't give exact figures because the mass of propellant required is based on the dry mass of the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is how I would suggest costing a mission like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Calculate mass required on for operations in orbit around the moon&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Add mass required for surface operations (if applicable)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Add mass for earth re-entry (if applicable)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) Add propellant mass for decent and landing onto the moon (if applicable)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5) Add propellant mass for take off from moon and orbit manoeuvres leading to Earth re-entry (if applicable)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;6) Add propellant mass for lunar orbit insertion&lt;/p&gt;&#xA;&#xA;&lt;p&gt;7) Add Calculate mass required for launch&lt;/p&gt;&#xA;&#xA;&lt;p&gt;8) Total launch cost x \$10,000 is flight cost&lt;/p&gt;&#xA;&#xA;&lt;p&gt;9) Add design\ build\ test costs&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-06-08T09:26:42.310" LastActivityDate="2014-06-08T09:26:42.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4735" PostTypeId="1" CreationDate="2014-06-08T00:07:24.897" Score="8" ViewCount="1532" Body="&lt;p&gt;If a hypothetical space colony were on the other side of the sun from earth, would it be possible for them to communicate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so how would this work?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing I can think of would be bouncing waves off other bodies orbiting the sun that share a line of sight. Is this possible? Has it been done?&lt;/p&gt;&#xA;" OwnerUserId="3452" LastActivityDate="2017-11-10T14:39:17.410" Title="Is it possible to communicate in space while the sun is between parties?" Tags="|communication|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4736" PostTypeId="2" ParentId="4735" CreationDate="2014-06-08T00:13:37.397" Score="6" Body="&lt;p&gt;A constellation of 3 satellites in polar orbit around the sun should give sufficient coverage as repeaters to the other side of the solar system. This would probably be the most reliable method of maintaining communication.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT to include comment from TidalWave:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could instead position one (or more) satellites at the Sun-Earth Lagrangian points SEL4 or SEL5 for the same benefit.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-06-08T09:26:02.727" LastActivityDate="2014-06-08T09:26:02.727" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4737" PostTypeId="2" ParentId="4735" CreationDate="2014-06-08T00:49:06.240" Score="12" Body="&lt;p&gt;First of all, the sun is almost never directly in the path of transmission. What actually happens is that the sun is in the beam path of the antennas. As the Sun puts out a lot of radio noise, it interferes with the signals. So, how can one combat this? There's 3 ways that I can think of:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Reduce the beam width. Use a laser instead of a radio waves, or a larger dish, or a higher frequency. All will reduce the effective beam width. This reduces the amount of interference from the Sun.&lt;/li&gt;&#xA;&lt;li&gt;Reduce the bandwidth. This is commonly done with Mars missions near opposition. Just don't send as much data. This works surprisingly well, although you usually don't get science back.&lt;/li&gt;&#xA;&lt;li&gt;Set up a relay satellite of some kind. The best would be to set one up at L4 or L5 for the Earth Sun point. Any point will either be in view of this point, or the Earth. This also helps in ranging information to find the exact location of satellites, it could be a handy thing.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-06-08T00:49:06.240" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4738" PostTypeId="2" ParentId="4706" CreationDate="2014-06-08T03:24:30.997" Score="16" Body="&lt;p&gt;This may sound strange, but part of the protection is a &lt;em&gt;lack&lt;/em&gt; of shielding.  The high-energy particles that make up a large portion of the radiation environment mostly just pass through the human body.  They do, however, interact with traditional shielding materials (eg. lead) to produce secondary radiation that does affect humans.  Because of this, light shielding is &lt;em&gt;worse&lt;/em&gt; than no shielding: it's thick enough to turn high-energy, low-danger radiation into medium-energy, high-danger radiation, but not thick enough to absorb that radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence the use of hydrogen-rich shielding (water, plastics): it's not as effective as traditional materials, but it doesn't produce secondary radiation.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-06-08T03:24:30.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4739" PostTypeId="2" ParentId="4715" CreationDate="2014-06-08T12:50:51.100" Score="2" Body="&lt;p&gt;The Asteroid Retrieval Mission (ARM) is most fundamentally critizied out of disappointment because that's what's left of the envisioned great crewed space mission of our lifetime. Instead of building a permanent lunar base or going to Mars or to a real asteroid, we'll just make a trip back to lunar orbit. That does not inspire. Russian, Chinese and even private entities might do that before ARM completes. This priority will make NASA lose its outstanding place in the imagination of human kind and turn it into an outcompeted second rate space flight organization with recruitment problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even as an asteroid mining technology demonstration it is flawed. Such tiny meteoroids are out of the question when it comes to mining. And Solar Electric Propulsion (SEP) cannot be used to move real asteroids with thousands times the mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The science of it is also questionable. Asteroids are likely very diverse. They may have formed in different parts of the solar system at different times in different ways. Some primordial, others more recent impact fragments. A close look at a sample of one will not say much at all about the typical asteroid. Especially since meteoroid sized objects very well may be systematically different from real asteroids. Maybe they are mostly rubble piles which separate easily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The meteoroid size and propulsion technology is also irrelevant for planetary defence. The Chelyabinsk meteor was larger than what ARM plans to go for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The political explanation of why the ARM mission exists, seems more logical than any rationale based on science or space economy development. ARM seems to be a political way to motivate the continuation of already ongoing projects like the SLS, Orion and SEP. They all have good potential uses, but the combination is strained and the usefulness limited to keeping those projects financed. Many critics are convinced that ARM will actually never happen, that it will soon be canceled.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-06-08T12:57:48.697" LastActivityDate="2014-06-08T12:57:48.697" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4740" PostTypeId="1" CreationDate="2014-06-08T19:21:33.743" Score="11" ViewCount="436" Body="&lt;p&gt;Expanding a bit on this question: &lt;a href=&quot;https://space.stackexchange.com/questions/4735/is-it-possible-to-communicate-in-space-while-the-sun-is-between-parties&quot;&gt;Is it possible to communicate in space while the sun is between parties?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any wavelength usable by our presently available technologies at which the Sun is both quiet and transparent? If so, could a signal on such a frequency propagate through without distortion?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is known that the Sun produces electromagnetic energy of various wavelengths, but is it actually quiet in some bands?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is quiet in a given band, is it transparent? Or is it opaque to any EM energy because it is a giant ball of plasma?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it does happen to be transparent in some wavelength, does it refract, scatter, or otherwise disorganize EM energy to the point that no external signal could propagate through it intact?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2019-09-12T16:19:09.480" Title="Is there any wavelength at which the Sun is both transparent to and quiet of electromagnetic radiation?" Tags="|the-sun|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4741" PostTypeId="1" AcceptedAnswerId="4745" CreationDate="2014-06-08T19:27:11.037" Score="25" ViewCount="4420" Body="&lt;p&gt;They had a crew round the Moon before going for landing, but they didn't test any of the new landing and ascent operations uncrewed on the lunar surface. Why? Couldn't it be done by remote control? Were they for some reason very confident that the risk with landing and ascent was comparatively low? Was it perceived as too costly? Were uncrewed launches politically difficult to motivate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideas about crewed space missions today often include the preplacemen of a return vehicle and other payloads before the crew arrives. For example the Mars Direct and derivatives, and I think also the orbiting realtime telerobotic Herro mission ideas to Mars and Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the Apollo program have preplaced an uncrewed lander, or other equipment, on a site within range of where later a crewed landing took place? Or didn't they have that precision in landing area (they did land near a Surveyor)?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2021-07-12T12:34:43.820" Title="Why didn't the Apollo program do an uncrewed landing/ascent rehearsal?" Tags="|apollo-program|uncrewed-spaceflight|crewed-spaceflight|" AnswerCount="7" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4742" PostTypeId="2" ParentId="4741" CreationDate="2014-06-08T19:52:36.060" Score="15" Body="&lt;p&gt;Time.&lt;/p&gt;&#xA;&lt;p&gt;They were coming down to the wire on Kennedy's challenge.  There might also have been technical reasons. Look how close to the end of the timeline Apollo 12 was.  It easily could have slipped.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="716" LastEditDate="2021-07-12T12:34:43.820" LastActivityDate="2021-07-12T12:34:43.820" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="4743" PostTypeId="1" AcceptedAnswerId="4807" CreationDate="2014-06-08T19:54:02.547" Score="32" ViewCount="8895" Body="&lt;p&gt;The records attached to the Voyager spacecraft are apparently just like an old-school vinyl disk, except made of gold (sort of like a metal master), in that the information is encoded in mechanically cut grooves. Based on depictions of the spacecraft, it appears that while the record is attached to the outside of the vehicle body, the playing surface is facing in (presumably for both protection and to display some engraved illustrations). In the series &quot;Cosmos: A SpaceTime Odyssey&quot;, Neil deGrasse Tyson states rather simply that the record should still be playable in a billion years, but says nothing about the duration as either a minimum or a maximum, or how the recording may be emperiled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the Voyager craft don't crash into anything, and encounter nothing larger or more substantial than cosmic ray particles, what would be the limit at which the record would still be in a playable condition? And by what mechanisms might it be damaged to the point its recording could no longer be played (or its content no longer recoverable by any known means)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would cosmic radiation eventually erode the entire spacecraft into dust? If it remained in cold insterstellar space, never to encounter anything but cosmic radiation, could it remain intact for the projected life of the universe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point of my question is this: Is there (or are there) any process(es) which is/are entirely unavoidable by which the recorded content is doomed to eventually become lost, perhaps even while some semblance of the original spacecraft still remains? Is the entire vehicle utimately doomed to certain destruction within the projected life of the universe? Conversely, if not vaporized, smashed, or otherwise damaged in some sort of encounter(s), would the recordings be more or less guaranteed to remain intact and playable if the craft is found by some intelligence, no matter how far into the future that might be?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2014-06-12T01:06:47.423" LastActivityDate="2020-05-23T02:35:21.307" Title="For how long will the Voyager records remain playable?" Tags="|voyager|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4744" PostTypeId="1" AcceptedAnswerId="4746" CreationDate="2014-06-08T20:21:50.510" Score="30" ViewCount="4187" Body="&lt;p&gt;Voice communication between ground and spacecraft during missions such as Apollo seemed to be always punctuated by beeps. What was the origin of these beeps? Were they some sort of unavoidable artifact of the communications system in use? Did they have a purpose as a sort of automatic &quot;Over!&quot; as is customary in voice communication on simplex radio? Was it to help the communications equipment switch over between transmit and receive modes? Was it some sort of feedback occuring when both parties had their microphones keyed simultaneously?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="12102" LastEditDate="2017-02-26T11:13:09.293" LastActivityDate="2017-02-26T11:13:09.293" Title="Why were there beeps punctuating the voice communications during missions such as Apollo?" Tags="|nasa|apollo-program|radio-communication|audio|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4745" PostTypeId="2" ParentId="4741" CreationDate="2014-06-08T20:25:34.137" Score="26" Body="&lt;p&gt;As Loren Pechtel mentions, time pressure was a big part of it; also, Apollo 10 did actually test a large part of the descent, ascent and rendezvous operations even through they didn't slow all the way to 0 and touch down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was by no means certain that Apollo 11 would successfully make a landing.  A variety of abort options were available at every stage of the operation, so if something had gone unrecoverably wrong it's still likely that the crew would have survived, and lessons could have been learned to allow a successful landing on Apollo 12.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-06-08T20:25:34.137" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="4746" PostTypeId="2" ParentId="4744" CreationDate="2014-06-08T20:55:32.707" Score="30" Body="&lt;p&gt;Those were called &lt;a href=&quot;http://www.hq.nasa.gov/alsj/quindar.html&quot; rel=&quot;noreferrer&quot;&gt;Quindar tones&lt;/a&gt; and they were used for in-band signaling in the ground communications network to key the transmitters at the ground stations.  An &quot;intro&quot; tone is generated when CapCom in Houston presses his/her push-to-talk (PTT) button.  This triggers the high power amplifiers at the MSFN ground station (often on the other side of the world) to start transmitting.  When CapCom is done talking, the PTT is released and the equipment at Houston generates a slightly different &quot;outro&quot; tone.  The equipment at the ground station recognizes that and stops transmitting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Michael Kjörling mentioned, comms were full-duplex.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="6816" LastEditDate="2016-06-15T21:57:51.467" LastActivityDate="2016-06-15T21:57:51.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4747" PostTypeId="2" ParentId="4740" CreationDate="2014-06-08T21:00:16.873" Score="8" Body="&lt;p&gt;It would only be transparent to radiation of wavelength longer than the characteristic scale of the sun.  That would be in the 0.1 Hz range, even lower frequency than &lt;a href=&quot;http://en.wikipedia.org/wiki/Extremely_low_frequency&quot; rel=&quot;noreferrer&quot; title=&quot;ELF&quot;&gt;ELF&lt;/a&gt;.  It's not practical to transmit or receive on such frequencies (you would need an antenna larger than the Earth), and even if you could, there's so little bandwidth that the data rates would be incredibly slow (perhaps one byte per minute).  A relay station is a better option.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-06-08T21:00:16.873" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4748" PostTypeId="1" AcceptedAnswerId="4775" CreationDate="2014-06-08T22:38:41.947" Score="17" ViewCount="4198" Body="&lt;p&gt;What’s more feasible: to build a self-sustaining colony on Mars or as a self-sustaining space station? The 1976-book &lt;a href=&quot;https://en.wikipedia.org/wiki/The_High_Frontier%3a_Human_Colonies_in_Space&quot;&gt;The High Frontier&lt;/a&gt; advocates the latter while Elon Musk seems &lt;a href=&quot;http://www.forbes.com/sites/ericmack/2013/12/12/elon-musk-on-the-details-of-colonizing-mars/&quot;&gt;pretty confident&lt;/a&gt; that Mars is the better option. Why? Are they taking other factors into account? Me, I’m not asking where it would be nicer to have a station but where it’s easier to get it done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things in favour of a station on Mars:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You already have some gravity.&lt;/li&gt;&#xA;&lt;li&gt;Less radiation (&lt;a href=&quot;http://www.sci-news.com/space/science-mars-radiation-measurements-surface-01629.html&quot;&gt;0.7 vs 1.8 mSv/day&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Some sort of raw materials on your front door&lt;/li&gt;&#xA;&lt;li&gt;Lots of space to put stuffs without it floating away.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Things in favour of a space station, say a design similar to an updated O'Neil cylinder like the &lt;a href=&quot;http://www.nss.org/settlement/space/kalpana.htm&quot;&gt;Kalpana One&lt;/a&gt; (or is there something more up-to-date?) in GEO or near-earth space:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Closer to Earth. I guess if you want to reach Mars within closer to three than the full six months, you’d need a lot more propellant than to get to the space station. (I can find many different Delta-V estimates, probably depending on lots of parameters like how much you aerobrake and what constellation the planets are in. And yes, I'm aware that Delta-V to dollars is exponential.)&lt;/li&gt;&#xA;&lt;li&gt;Little Delta-V needed to get to and fro asteroids, which should make it easier to mine them. You can then cherry-pick (to a certain extent) raw materials from asteroids.&lt;/li&gt;&#xA;&lt;li&gt;A lot more solar power (no atmosphere, no sandstorms, no/less night)&lt;/li&gt;&#xA;&lt;li&gt;Some processes are probably actually easier in 0g.&lt;/li&gt;&#xA;&lt;li&gt;Seems like no heating needed (or is that a minus because it’s hard to get rid of the excess heat?)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Any more factors? How would you weigh them in terms of feasibility?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: More objectively, what do experts think and how do they reach that conclusion? Has our knowledge/assumptions changed between O'Neill's and Musk's approaches or do they just have different opinions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit2&lt;/strong&gt;: With &quot;self-sustaining&quot; I mean able to sustain itself without major imports of materials from earth which would make the endeavour at current launching costs very expensive. Importing small masses of specialised equipment (like computers and instruments) from earth is allowed, as well as obviously whatever you need to get it all going. More importantly, regularly importing raw materials from asteroids (and/or the Martian surface for the Martian colony) is allowed—for the purpose of this question, I consider those facilities part of the colony.&lt;/p&gt;&#xA;" OwnerUserId="4558" LastEditorUserId="4558" LastEditDate="2014-06-15T18:29:29.377" LastActivityDate="2023-04-13T21:15:31.063" Title="Feasibility: self-sustaining colony on Mars vs. Space Station" Tags="|mars|colonization|space-station|" AnswerCount="3" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="4749" PostTypeId="2" ParentId="4741" CreationDate="2014-06-09T00:16:33.720" Score="14" Body="&lt;p&gt;Remember the state of computers at the time. It is very unclear that an automated landing could have been executed -- indeed, the first moon landing was under manual control, and came uncomfortably close to running out of descent fuel before landing,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remote control from Earth would not be practical due to speed-of-light issues. The moon is about 1.28 light-seconds from Earth. By the time you could see what's happening, react to it, and send a parameter change to the lander, over 2.6 seconds have passed. It is VERY hard to operate something remotely with that much lag; it's probably effectively impossible to fly with that delay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remote control from the orbiter might have been possible... I don't remember how long the orbiter was in communications range of the lander on each pass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in the end, the answer is that the space race was seen as just that, a race, and called for making judgements about the degree of risk (as best it could be calculated) versus the time that would be required to reduce that risk. The astronauts knew that what they were doing wasn't &quot;safe&quot;, but they too estimated the risks and judged them worth taking. The fact that they were generally military test pilots (and that, despite being a &quot;civilian&quot; agency, NASA was also functioning as a military research program) also meant the level of risk considered acceptable was higher than it would be for civilians. The landing was made at the earliest date when they thought it stood acceptable odds of succeeding; a test landing, even if it could have been conducted, might not have told them much about how a human in direct control of the vehicle would affect those odds.&lt;/p&gt;&#xA;" OwnerUserId="3231" LastEditorUserId="3231" LastEditDate="2014-06-09T14:00:21.417" LastActivityDate="2014-06-09T14:00:21.417" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="4751" PostTypeId="1" AcceptedAnswerId="4752" CreationDate="2014-06-09T01:09:58.867" Score="11" ViewCount="26594" Body="&lt;p&gt;So I have briefly gone over answers to how things are stored as from this answer: &lt;a href=&quot;https://space.stackexchange.com/a/2273/1247&quot;&gt;https://space.stackexchange.com/a/2273/1247&lt;/a&gt; but how is the information sent back? I'm thinking of probes like the Voyagers, the Pioneers, the Mars probes and Hubble.&lt;/p&gt;&#xA;" OwnerUserId="1247" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-01-15T12:29:48.030" Title="How does NASA get information from space probes?" Tags="|artificial-satellite|nasa|probe|communication|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4752" PostTypeId="2" ParentId="4751" CreationDate="2014-06-09T03:06:10.407" Score="10" Body="&lt;p&gt;Mostly it's by radio waves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some &quot;information&quot; is physically transported back to Earth. The Apollo missions brought 382 kg of lunar samples back to Earth and the Soviet Union Luna program brought 0.326 kg of lunar samples back to Earth. That kind of return is highly valuable information. Even with the success of the Mars rover programs, a Mars sample return mission would still provide very useful insight into the geology of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recently NASA has experimented with using lasers instead of radio communications. The much higher frequencies associated with laser communication have the potential of a much higher communications rate. The Lunar Laser Communication Demonstration on the Lunar Atmosphere and Dust Environment Explorer achieved a 622 megabits per second downlink rate. That's orders of magnitude higher than the rate achievable with radio waves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update regarding latencies:&lt;/strong&gt;&lt;br&gt;&#xA;The speed of light dictates the latency for vehicles well beyond the Earth such as the rovers at Mars and the New Horizons vehicle en route to Pluto. The one-way travel time between New Horizons and Earth will be around 4.5 hours when the vehicle flies by Pluto. Commands need to be sent at least 4.5 hours in advance, making for a 9 hour delay between sending a command and receiving the vehicle's response to the command.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For vehicles in low Earth orbit such as the ISS and the Hubble, speed of light is but a small part of the communications delay. Watch a NASA video where mission controllers talk with the astronauts aboard the ISS. There's about a good four or five second delay, even for simple questions such as &quot;ISS crew, are you go?&quot; The one way travel time between low Earth orbit (or Earth surface) and geosynchronous orbit is about a quarter of a second. Double that and you get half a second for round trip light delay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So where does that extra four seconds or so come from? The answer is bits bouncing around inside and between computers. The mission controller's &quot;are you go?&quot; query is digitized, mixed with other signals to be sent to the ISS, and transmitted via ground links to White Sands. The signal enters White Sands at the NASA Ground Terminal, where it is mixed with other signals that will eventually be sent to the appropriate Tracking and Data Relay Satellite. The signal constructed by the NGT next makes a short hop to the White Sands Ground Terminal, which does even more data manipulation. The signal is next send to an antenna and transmitted to the TDRS. The signal arriving at the TDRS has to be split into component parts, another computer at work. That signal is sent to the ISS, where it goes through yet another computer before the crew hear it. The entire process is reversed when the crew reply &quot;Go!&quot; to the query.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-06-09T15:27:35.987" LastActivityDate="2014-06-09T15:27:35.987" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4753" PostTypeId="1" AcceptedAnswerId="18827" CreationDate="2014-06-09T06:45:53.957" Score="8" ViewCount="1980" Body="&lt;p&gt;No body is immune to impact by other bodies. Earth too is no stranger to body-play; Micro-micro meteorites accumulate by the giga-ton annually in the form of dust. Larger impact bodies such as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Chicxulub_impact&quot;&gt;Chicxulub&lt;/a&gt; capable of delivering as much as &lt;strong&gt;100 Teratons&lt;/strong&gt; equivalent of TNT are also out there - but less frequent. As many as 132 meteorites found on Earth are identified as of &lt;a href=&quot;http://en.wikipedia.org/wiki/Martian_meteorite&quot;&gt;Martian origin&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On October 17, 2013, NASA reported, based on analysis of argon in the&#xA;  Martian atmosphere by the Mars Curiosity rover, that certain&#xA;  meteorites found on Earth thought to be from Mars were actually from&#xA;  Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Back in the 1960s/70s, &lt;a href=&quot;http://en.wikipedia.org/wiki/project_Orion_%28nuclear_propulsion%29&quot;&gt;Project Orion&lt;/a&gt; was a theoretical study on a nuclear pulse propelled space-ship. Several devices being flung out opposite to the desired direction of thrust. Each device imparting an absurdly humongous specific impulse on detonation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I therefore find myself wondering &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How large would the impact have been on Mars to impart escape velocity to the rocks that eventually turned up on Earth?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Even given the turbulent environment on Mars, is there a possibility one of the impact craters - the source of an eventual martian meteorite, may be discovered by one of the missions presently on/under-way to Mars?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is there a probability of discovering a Venerean meteorite here on Earth? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Could Chicxulub, or a comparable impact have imparted escape velocity, or larger to a rock such that it would eventually wind up on Mars, or Venus?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2020-05-17T22:13:07.137" Title="Is there a meteorite from Earth on Mars?" Tags="|mars|impact|meteorites|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4754" PostTypeId="2" ParentId="4753" CreationDate="2014-06-09T07:54:15.143" Score="4" Body="&lt;blockquote&gt;- Is there a probability of discovering a Venerean meteorite here on Earth?&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Possible but unlikely. No (known) venerean meteorite has been found on Earth. Several factors make it much more unlikely than a meteorite from Mars (I should write &quot;much less common&quot;, certainly some are around, but so few that we might never find them). Venus' deeper gravity well and shorter distance to the Sun turn meteoroids away from Earth orbit. And the thick atmosphere of Venus slows down ejecta.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-06-09T07:54:15.143" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4756" PostTypeId="2" ParentId="4751" CreationDate="2014-06-09T08:52:18.660" Score="7" Body="&lt;p&gt;As David said, probes generally communicate via radio. Long-range space probes often carry a dish antenna.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;https://en.wikipedia.org/wiki/Parabolic_antenna&quot; rel=&quot;nofollow noreferrer&quot;&gt;parabolic dish antenna&lt;/a&gt; focuses the signal into a narrow cone. Compared to a standard wire antenna that transmits evenly in all directions, a dish makes sure as much of the signal as possible is sent in the direction of the receiver.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives the strongest radio signal possible within the limited power budget of the probe.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For space probes that aren't in Earth orbit, NASA operates the Deep Space Network (DSN): a series of large dish antennas designed to receive the very weak radio signals from far-away space probes. They can still communicate with the Voyager probes on the other end of the solar system.&lt;br&gt;&#xA;For Mars ground missions, the rover or ground station often communicates with an orbiter. The orbiter relays the signal to Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you use a dish antenna on Earth, you can make it very large, in order to gather more of the signal. The larger the dish, the weaker the signals it can pick up.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the last step, the receiver on Earth is very sensitive. The receiver is often cooled to very low temperatures to reduce noise. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a result, this chain (transmitter with dish antenna, sensitive receiver with large dish antenna) can pick up a 10 W transmitter from 100 AU away. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find more details in these related questions:&lt;br&gt;&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/958/how-does-voyager-1-send-signals-to-earth&quot;&gt;Communication with Voyager&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/3167/how-many-space-craft-can-a-station-on-the-dsn-communicate-with-concurrently&quot;&gt;DSN operations&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble is close enough that it doesn't need the DSN. It's in low Earth orbit, so latency isn't really an issue. For Mars, latency is on the order of 15 minutes. For Voyager, it's around 15 hours (one way).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-01-15T12:29:48.030" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4757" PostTypeId="2" ParentId="1102" CreationDate="2014-06-09T14:15:15.720" Score="7" Body="&lt;p&gt;One of the issues that entered into the decision to have Armstrong out the hatch first was that he was a civilian, and Aldrin was still in the Air Force. Someone high up thought it would look better to have a civilian plant the first boot-print on the Moon, rather than a military man. Given the US's unpopularity in the world due to the Vietnam War, and the desire to make the Space Program look like a peaceful civilian endeavor, this was probably a wise choice in the end.&lt;/p&gt;&#xA;" OwnerUserId="3127" LastActivityDate="2014-06-09T14:15:15.720" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4759" PostTypeId="2" ParentId="4753" CreationDate="2014-06-09T19:42:17.643" Score="8" Body="&lt;p&gt;The majority of the rocks from Mars appear to have come from a single impact on Mars, which &lt;a href=&quot;http://www.space.com/24962-mars-meteorites-single-impact.html&quot; rel=&quot;noreferrer&quot;&gt;formed Mojave Crater&lt;/a&gt;. I'm finding it difficult to get an exact power for that impact, but the crater is 58 km in diameter. It is a very large crater, one of the type that rarely impacts anywhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Discovering a Veneran meteorite would be quite rate, as LocalFluff mentioned. It's not impossible, but it would have to be the result of a very large impact, which seems unlikely. It is likely there is rocks from Venus around, just that they are likely buried after a long time in the past.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It certainly is possible that a rock from Earth could have made it to  Mars, but that isn't all that likely. Most likely, any such rock would be from the &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2002/18oct_earthrocks/&quot; rel=&quot;noreferrer&quot;&gt;Late Bombardment&lt;/a&gt; period, as that's the only time in Earth's history with such power, at least, with Earth as we know it. I have little doubt that the impact that resulted in the Moon showed the solar system, however, it is unlikely that many of those fragments have continued to survive, except perhaps in deep space.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-06-10T10:17:50.520" LastActivityDate="2014-06-10T10:17:50.520" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4761" PostTypeId="2" ParentId="4715" CreationDate="2014-06-10T04:59:53.340" Score="2" Body="&lt;p&gt;Long term, an asteroid-to-orbit mission has several benefits:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;geopolitical value&lt;/li&gt;&#xA;&lt;li&gt;asteroid defense value&lt;/li&gt;&#xA;&lt;li&gt;economic value&lt;/li&gt;&#xA;&lt;li&gt;scientific value&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Capture of an asteroid has several geopolitical considerations that give it value, ones which don't require manned missions by anyone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The proven ability to move asteroids into orbit, especially lunar-coorbit (LaGrange points) or Lunar orbit, would establish definitively a level of fine control over asteroid movement. That level of fine control is a requisite for use of asteroids as weapons of mass destruction, and is directly on par with orbital insertion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is undoubtedly a part of the considerations - such a practical capability is bidirectional - if you can put one into stable orbit, you can also crash it, and also send it away from Earth. In other words, by proving you can make a successful insertion, you prove you can prevent one, as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The commercial applications, much as with the Apollo program, are a justification, but are probably not the root for the program. (Apollo established that the US could in fact deliver a missile payload anywhere desired within cislunar space.) The mining value of a nickel-iron asteroid as base metal is not worth the effort - it would cost more to recover the metal than the metal is worth. One that has significant other metals would be financially valuable, but the programmatic costs and landing costs may not be viable long term. Planetary Resources thinks they can manage it and make money - but if they had secure numbers, we'd see backers lining up to privately fund it. The risk is too high. (Further, the geopolitical aspects make it impractical to pull it off without it looking like a threat.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scientific value of an asteroid to orbit is immense - on par with putting men on the moon, but only slightly so. In addition to the mission itself and its science value as a near-system trans-lunar mission, there is the benefit of it being a platform for a much more radiation-proofed space platform, which would enable truly long-term science missions in microgravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The combination makes the asteroid to orbit overall a higher priority for politicians than the pure science of further lunar missions or the science adventure of Mars.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-06-10T04:59:53.340" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4762" PostTypeId="1" CreationDate="2014-06-10T05:05:59.213" Score="6" ViewCount="198" Body="&lt;p&gt;A planet in an elliptical orbit around a star conserves angular momentum.  (This is tantamount to saying that it sweeps out equal areas of the ellipse in equal times). If conditions are such that there are objects gathered at the $L_4$ and $L_5$ points of that orbit, do those objects also conserve angular momentum?  They do of course in the case of the circular orbit, but that is rather trivial.  What about elliptical orbits with considerable eccentricity?&lt;/p&gt;&#xA;" OwnerUserId="1101" LastActivityDate="2019-11-14T22:13:09.503" Title="Do objects at the $L_4$ and $L_5$ points conserve angular momentum?" Tags="|orbital-mechanics|orbit|lagrangian-points|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4763" PostTypeId="2" ParentId="4762" CreationDate="2014-06-10T08:31:57.797" Score="4" Body="&lt;p&gt;At $L_4$ and $L_5$ you are orbiting the larger body in the system (so at the Sun-Earth $L_4$/$L_5$ you're orbiting the sun). You orbit the larger body with a slightly greater semi major axis than that of the orbit of the smaller body, so typically in a two body system you would expect your object to have a greater orbital period than it does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you have stated from Kepler's second law, the area swept out in a given time is constant for a elliptical orbit. This still holds true for an object at $L_4$/$L_5$ since they will be moving constantly around the larger body (sun). A good way of looking at this is that if the smaller body (Earth) does it then the object does it, just with a true anomaly 60 degrees greater or lower than the smaller body (Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also by way of observation you can see that the object at $L_4$/$L_5$ will (by definition) maintain it's position relative to the smaller body (Earth) and hence will have a constant angular momentum since the smaller body (Earth) will have a constant angular momentum.  &lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-06-10T08:31:57.797" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4764" PostTypeId="1" CreationDate="2014-06-10T10:06:45.337" Score="8" ViewCount="421" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How rapid do the attractive properties of GEO and lagrange points deteriorate with the distance from optimum? How &quot;large&quot; are such locations?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;At what distance would station keeping require, say, twice the delta-v per year?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is there any reason for spacecrafts to try to stay put in a lagrangian point, or can they as well orbit them (like Gaia does) and if so, how large could such orbits be and does this mean that the L-points are practically non-scarce locations?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-04-19T13:05:45.980" Title="How does gravitational stability change with the distance from special places such as geostationary orbit and lagrange points?" Tags="|artificial-satellite|gravity|lagrangian-points|geostationary|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4765" PostTypeId="1" AcceptedAnswerId="4782" CreationDate="2014-06-10T17:43:50.107" Score="21" ViewCount="1246" Body="&lt;p&gt;I'm obviously not a rocket scientist, so this might rank among the stoopidest questions around here. I wonder if it is feasible to design a rocket engine which could be reused in space by refilling it with different kinds of rocket fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some imaginary future of a developed space economy, could the same rocket engine sometimes use LOX+H2 extracted from an asteroid, and then sometimes replace that fuel with LOX+CH4 produced in Mars' atmosphere? Or any other combination of more than one relatively easily available fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I don't refer only to the &quot;engine&quot;, but to the rocket as a whole. This includes the tanks for fuel or other propellant and the plumbing which goes with it. I'm not even sure about the difference between an &lt;em&gt;engine&lt;/em&gt; and the rest of a &lt;em&gt;rocket&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-06-10T18:14:48.610" LastActivityDate="2018-05-30T08:51:09.787" Title="Could a rocket engine be designed to use different fuels? (Like LOX+H2, LOX+CH4)" Tags="|engine-design|fuel|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4766" PostTypeId="2" ParentId="4693" CreationDate="2014-06-10T18:38:50.770" Score="10" Body="&lt;p&gt;These spacecraft are not designed solely for use on the ISS. They could be used with a &lt;a href=&quot;http://www.bigelowaerospace.com/&quot;&gt;Bigelow&lt;/a&gt; space station that would utilize the 3 extra seats. The private firms are probably thinking forward to the post-ISS era.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is reasonable to infer that NASA will use the extra volume from the 3 empty seats for cargo.&lt;/p&gt;&#xA;" OwnerUserId="4579" LastActivityDate="2014-06-10T18:38:50.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4767" PostTypeId="1" CreationDate="2014-06-10T22:55:43.417" Score="2" ViewCount="140" Body="&lt;p&gt;I'm working on a project that involves magnetorquers, and as someone with only a basic understanding of physics (and even more basic understanding of electromagnetism), I have absolutely no idea how these things do what they do. As far as I've been able to suss out, it has something to do with an induced current causing an electromagnetic field and that somehow translates to momentum? Can someone help please?&lt;/p&gt;&#xA;" OwnerUserId="4580" LastActivityDate="2014-06-10T22:55:43.417" Title="How do magnetorquers work?" Tags="|artificial-satellite|physics|" AnswerCount="0" CommentCount="1" ClosedDate="2014-06-11T10:27:35.687" ContentLicense="CC BY-SA 3.0" />
  <row Id="4768" PostTypeId="1" CreationDate="2014-06-11T00:47:04.510" Score="12" ViewCount="1369" Body="&lt;p&gt;If we assume that humans will eventually visit (if not colonize) Mars, and we consider the benefits to terrestrial navigation and communication of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Global_Positioning_System&quot;&gt;GPS&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&quot;&gt;Iridium&lt;/a&gt; systems...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a precursor to human travel to Mars logically be the establishment of a GPS-style constellation of navigation satellites and/or an Iridium-style constellation of communication satellites in Martian orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that something Mars mission planners have written into their plans, have considered, or have dismissed as unnecessary?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="286" LastEditDate="2014-06-11T21:30:40.157" LastActivityDate="2022-08-26T23:40:31.977" Title="GPS / Iridium for human presence on Mars?" Tags="|mars|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4769" PostTypeId="2" ParentId="4768" CreationDate="2014-06-11T00:51:07.897" Score="12" Body="&lt;p&gt;Each orbiter at Mars has a secondary mission as a data relay satellite for other Mars missions. Mars Reconnaissance Orbiter (NASA), Mars Express (ESA), Mars Odyssey (I think) are all used to relay data from the smaller landers back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a small network of communication satellites is already in orbit with a few more coming.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of them have clocks of the accuracy needed for GPS style navigation, but some similar system seems likely, even if just for search and rescue, once serious manned exploration/habitation of Mars occurs.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that is probably several decades away still, Elon Musk notwithstanding.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-06-11T00:51:07.897" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4771" PostTypeId="1" AcceptedAnswerId="4774" CreationDate="2014-06-11T04:16:28.737" Score="14" ViewCount="13980" Body="&lt;p&gt;When watching the &lt;a href=&quot;http://www.ustream.tv/channel/iss-hdev-payload&quot;&gt;ISS HD Earth Viewing Experiment&lt;/a&gt;, I realized that they turn the cameras off when the ISS enters night. How often are they turned off when its dark? How come they aren't left on?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="58" LastEditDate="2020-02-24T20:46:57.343" LastActivityDate="2020-05-28T08:45:41.887" Title="Why aren't the ISS HDEV cameras left on at night?" Tags="|iss|camera|hdev|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4772" PostTypeId="2" ParentId="948" CreationDate="2014-06-11T07:48:01.583" Score="11" Body="&lt;p&gt;In addition to Geoff's answer, the comment by Garan contains a major reason why things stop working eventually. Any machine with moving parts is subject to wear and tear. You can keep a machine going for very long if you maintain it; look at the steam engines in various industrial heritage museums for instance. These machines are/were monitored constantly by a man with an oil can, and parts were swapped out or repaired when needed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A planetary rover is a whole different ballgame: we can't maintain or repair it. Once a part malfunctions, it's gone for good. With careful design, you can add some redundancy. Every rover we've built has at least two main computers: if one fails, the other can take over.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For mechanical parts, this is more difficult. Wheels for instance, are a major point of failure: the wheel itself gets damaged, and there are lots of sensitive moving components (see Geoff's comments on bearings). If you want to be able to jettison a damaged wheel assembly, you're adding more moving parts and more chances of failure. Adding the mechanics to replace damaged parts makes things even more complicated, making the rover larger and heavier, which quickly makes the project more expensive.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The power supply has a similar problem. Solar panels, radiothermal generators, fission plants all decay eventually. Solar panels are down to ~70% of their initial output in 25 years. In an RTG, the thermocouples become less efficient over time. Even on Earth, we have to decommission fission plants eventually due to corrosion and decay of critical components. RTGs and fission plants also run out of fuel eventually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the technological barriers, there are two major reasons not to build rovers for a very long lifespan: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Cost. Adding redundancy adds weight and complexity, and increases the launch cost. A Rolls-Royce can be counted on to remain operational for far longer than a Ford, but the Rolls costs enough that you can afford to buy ten Fords vs. one Rolls.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;New science. We've only been doing this for a few decades. Every planetary mission has new and unexpected findings. It makes sense to run a rover for a few years, and then sit down and examine what it has found that warrants further study with new, improved instruments. One long-lived rover limits you to one set of instruments. Science advances quickly enough that each rover we've built is vastly more capable than the previous one. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-06-11T07:59:27.297" LastActivityDate="2014-06-11T07:59:27.297" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4773" PostTypeId="2" ParentId="4768" CreationDate="2014-06-11T07:52:55.180" Score="13" Body="&lt;p&gt;A Mars GPS system cannot be a high priority considering the challenges of life support. GPS is a hit on Earth because it has billions of users. If you divide the multi billion dollar cost on say 12 astronauts on Mars, you need extremely productive use to motivate the investment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without global magnetic field, a compass is useless on Mars, so there is a need for navigational aid. But radio beacons on the ground at landing sites would be much more cost effective.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-06-11T07:52:55.180" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4774" PostTypeId="2" ParentId="4771" CreationDate="2014-06-11T08:45:40.143" Score="32" Body="&lt;p&gt;I believe they are and my network analysis tools show nearly constant transmission (data bandwidth remains the same), even when the image is all black or all gray. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/High_Definition_Earth_Viewing_cameras&quot;&gt;High Definition Earth Viewing&lt;/a&gt; (HDEV) Experiment quick notes on its &lt;a href=&quot;http://www.ustream.tv/channel/iss-hdev-payload/theater&quot;&gt;USTREAM channel&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Black Image = International Space Station (ISS) is on the night side&#xA;  of the Earth. &lt;/li&gt;&#xA;  &lt;li&gt;Gray Image = Switching between cameras, or&#xA;  communications with the ISS is not available.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Same is repeated in German on the &lt;a href=&quot;http://www.columbuseye.uni-bonn.de/&quot;&gt;Columbus Eye pages&lt;/a&gt; by the University of Bonn. But it doesn't say anywhere that the cameras are switched off during that time, and I get stream bandwidth in the average of 700 - 800 kB which cannot be down to buffering, since it remains constant for the whole duration of ISS's orbit in Earth's shadow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/917.html&quot;&gt;NASA on High Definition Earth Viewing&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;HDEV Design for Operations&lt;/strong&gt;:   The HDEV operates one camera at a time. &#xA;  The HDEV is designed so that when the system is initially powered on,&#xA;  after a 1-2 minute warm up period, the Cameras are turned on one at a&#xA;  time in a repeating cycle.  The Forward looking camera is powered&#xA;  first, followed by the Nadir and each aft looking camera, such that&#xA;  the HDEV video “follows” a location on the earth as the ISS passes&#xA;  overhead.  This auto-cycle mode of the HDEV does not require any input&#xA;  from ground operators, so the HDEV can be operated any time that the&#xA;  ISS power and data resources are available, without requiring a ground&#xA;  controller present to operate the payload.  The only command required,&#xA;  is the initial “power on” command, which is performed by ESA’s&#xA;  Columbus Control Center as schedule by ISS Payload Operations.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Alternately as desired by ground controllers, the HDEV video can be&#xA;  commanded.  Ground operators have the choice to change the cycle of&#xA;  the images noted in the auto-cycle mode (either changing which cameras&#xA;  that are powered on, or changing the length of time they are powered&#xA;  on), or, if desired, ground controllers can command a single camera to&#xA;  remain powered on and no auto-cycle to take place.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So here's what I'm thinking is happening;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Before entering the daylight part of a new ISS orbit, the aft facing camera is activated because the front facing ones would produce too much camera glare (reflections from the station, front glass elements,...) when the &lt;a href=&quot;https://en.wikipedia.org/wiki/Beta_angle&quot;&gt;beta angle&lt;/a&gt; of the Sun w.r.t. the station at dawn would be too direct.&lt;/li&gt;&#xA;&lt;li&gt;Aft facing cam doesn't produce any glare most of the times (I've seen it on occasion though, you can see the front lens element, black lens rings and protective glass defects in the reflection of the enclosure's front glass), since the only sources that the sunlight could reflect directly towards the camera are the Soyuz TMA solar panels (currently, two Soyuz spacecraft are docked to the station).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So it appears to me that during a lot of the time in one orbit, there simply isn't anything to see and that's why the screen remains black. For example, here's a screen capture from the aft facing camera when the station was entering the Earth's shadow, with a barely visible arc of Earth's atmospheric glow at dusk on the left hand side of the image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/4mctq.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4mctq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, the rest of the image remains uniformly black, even though the camera is apparently on. The remaining question is, why don't we see any lightnings, meteors and light pollution on the surface of the Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For lightnings and meteors, I doubt that the frame rate of these transmissions is fast enough to catch most of them. Remember, there is limited bandwidth available to them, so the streams would be highly compressed, and the cameras are COTS (Commercial Off-The-Shelf) quality. And a sporadic bright artefact that can be seen would more likely be produced by on-sensor noise, especially when the station is exposed to more radiation when over the &lt;a href=&quot;https://en.wikipedia.org/wiki/South_Atlantic_Anomaly&quot;&gt;South Atlantic Anomaly&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for light pollution, the cameras don't seem to employ any lens filters, like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Polarizing_filter_%28photography%29&quot;&gt;polarizing filters&lt;/a&gt; that could help reduce &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_diffraction&quot;&gt;atmospheric light diffraction&lt;/a&gt; and suppress glare, and their angle w.r.t. the atmosphere is simply too shallow, so all the relatively small point sources of light are blurred-out with the rest of the background, and camera sensors are not sensitive enough to pick up such small differences in illumination, or stream compression is set too high and ignores such small difference between the individual frames.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To somewhat easier appreciate image quality of these HDEV cameras, and how much the atmosphere blurs out surface features, here's my own work in trying to improve image quality of a few HDEV snaps, using only simple image post-processing filters (output levels, color normalisation, unsharp mask,...), &quot;before and after&quot; style:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Central Australia (Simpson Desert National Park)&lt;/strong&gt;:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/m2pw7.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/m2pw7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/Hkrny.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hkrny.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Andes at dawn (Peru, Chile, Bolivia, Argentina)&lt;/strong&gt;:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/hFHNv.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hFHNv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/nooV9.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nooV9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;And this is one such &quot;black frame&quot; from one of HDEV cams a few minutes ago, somewhere over South Pacific Ocean, west of Ecuador:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/KnGNo.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KnGNo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's probably hard to believe looking at this captured frame on most monitors, but the image uses no less than 69 unique colors and the live stream data rate was in excess of 700 kB/s during the capture. This is the same frame, with highly exaggerated difference between pixels (using color palette stretching):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/3hT3U.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3hT3U.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;(Click on any image for a higher resolution, 978 x 550 pixel lossless compression PNG image)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-06-11T11:12:31.963" LastActivityDate="2014-06-11T11:12:31.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4775" PostTypeId="2" ParentId="4748" CreationDate="2014-06-11T08:58:33.847" Score="7" Body="&lt;p&gt;There is one central question with creating a colony somewhere uninhabited: resources. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Material resources&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;With a ship-based colony, the material resource is just whatever you put up there, and then whatever solar power you can collect. In order to be self-sustaining, you therefore need to be able to create a closed system which requires no further input. At present, we are very close to that goal technologically, and I think with the addition of some small amount of support from Earth, a colony is within our capacity now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Mars (or the Moon), there is an available resource under your feet and above the dome roof, so the limitation is really our technological capacity to reprocess the atmosphere and rock. Thankfully, the atmosphere does contain a small amount of oxygen and the rocks are similar to Earth rocks, so we are quite capable of extracting raw materials from Mars itself. Even if we weren't, we could fall back on a hermetic environment like that of the ship-based colony, so it is at least no worse than the ship-based colony.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Costs of the initial mission&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;If it is technologically possible, what is the cost to Earth-inhabitants of either path?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Mars mission to establish an entire colony (not just to send 1 or 2 astronauts) would be more expensive by quite a margin. The equipment and resources for a Mars colony would be no less massive than for an orbital one, and then we are subjected to the tyranny of the rocket equation for the fuel to make the journey. There is a difficult balance here; the slower and more efficient the journey (more gravitational slingshots, using the &quot;interplanetary highways&quot; etc), the greater the material required to support life during the flight. However, the Mars mission could be done much more slowly, by establishing a small base and slowly expanding it using largely local resources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By comparison, having only succeeded in getting a few small lumps of metal to Mars, we have put a whole space station in orbit. The costs are far lower because the delta-V requirement is lower. It is conceivable that a station could be constructed and populated near Earth and then shifted to a Lagrange point via ion drives and so on, which are largely solar powered, further reducing the mass to be lifted. &lt;a href=&quot;http://en.wikipedia.org/wiki/BA_330&quot;&gt;Inflatable habitations&lt;/a&gt; seem a particularly appropriate option, permitting a large structure to require minimal volume and mass on the rocket. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Time to independence from Earth resources&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The space habitat wins here. A Mars mission would require a lot more preparation, and a lot more time to build up from the initial payload to a working environment. It would be capable of expansion, unlike the habitat, but it would take much longer. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Potential inhabitants (and thus sponsors)&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The space habitat wins here again. An Earth-orbit habitat or a Lagrange point habitat would both be reachable from Earth in days, so a rich family could take a holiday there and older people could retire there (consider the benefits of lower gravity environment for the elderly). Mars is more of an undiscovered country; America was populated largely by the poor because the journey was so long and promise was more of opportunity than quality of life. Yet voyagers to Mars would need to be well-funded by governments, so it would not be within the means of ordinary people's life savings.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Summary&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;I would illustrate the difference as being a choice between creating a holiday resort on an island in the sea versus creating a colony at the South pole. Despite the limits of the island, the tradeoff of proximity, cost and timing would make it both a better initial proposition and a better long-term bet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One day we will populate Mars, but I suspect it will not be an expanding colony until there are several habitats near Earth.&lt;/p&gt;&#xA;" OwnerUserId="519" LastActivityDate="2014-06-11T08:58:33.847" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="4776" PostTypeId="2" ParentId="948" CreationDate="2014-06-11T11:22:14.627" Score="5" Body="&lt;p&gt;Yet another reason is how much data you can squeeze. A rover is a very limited lab, and after some time, it has gathered all data it can from a specific area. They are slow, so travelling to far away lands is very impractical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity, the most advanced rover, has a &lt;a href=&quot;http://marsrover.nasa.gov/mission/spacecraft_rover_wheels.html&quot; rel=&quot;noreferrer&quot;&gt;mean speed of 1 cm/s&lt;/a&gt; on flat land. It takes more than a full day to travel one kilometre on ideal conditions. On the actual surface, you have to avoid rocks and climb up and down hills; that will probably reduce the speed by a rough factor of 2. My hometown is about 200 km away from the next geological unit, so it would take Curiosity more than a year to just get there, without stopping to do science. So far, including scientific experiments, it has travelled about &lt;a href=&quot;http://curiosityrover.com/rovermap1.html&quot; rel=&quot;noreferrer&quot;&gt;5 km&lt;/a&gt; in almost two years, so the actual mean speed is less than a millimetre per second..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course in the same area there are different kinds of rocks, and it takes a long time to fully sample an area, and you never know when you are going to find a new thing. But even then, the investment necessary to get the next big new thing is huge (because it is far away). And, by the time the rover has gotten there, we could have sent a new one, with better more sensitive equipment, to another part of Mars looking for exciting more new things.&lt;/p&gt;&#xA;" OwnerUserId="3273" LastActivityDate="2014-06-11T11:22:14.627" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4777" PostTypeId="2" ParentId="4741" CreationDate="2014-06-11T14:37:02.217" Score="4" Body="&lt;p&gt;Without Neil Armstrong taking over manual control, Apollo 11 would have crashed into a rock filled crater. Possibly something like this happened to the unmanned Mars probe Beagle2? A failure of that would have halted the programme completely preventing the first manned mission from starting. You think even back then NASA was taking risks? Later the risks t hey took proved fatal but back then they pulled it off. &#xA;Perhaps the astronauts were expendible? Behind the scenes maybe the experts considered that it was just so risky that they might as well send the men and take the chance to get to the moon before the Kennedy deadline.  As far as I know, most things hade backups except the heat shield and the ascent engine of the LM, so it was maybe a change worth taking.&lt;/p&gt;&#xA;" OwnerUserId="4585" LastActivityDate="2014-06-11T14:37:02.217" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4778" PostTypeId="1" AcceptedAnswerId="4779" CreationDate="2014-06-11T16:21:14.037" Score="11" ViewCount="2603" Body="&lt;p&gt;The &lt;s&gt;Centrifugal Battery&lt;/s&gt; (&lt;em&gt;flywheel as an energy storage device&lt;/em&gt;) has been around [a while]&lt;s&gt;&lt;a href=&quot;http://strangengines.wordpress.com/2008/09/03/albert-pott%E2%80%99s-centrifugal-battery-1857/&quot; rel=&quot;nofollow noreferrer&quot;&gt;since at least 1857&lt;/a&gt;.&lt;/s&gt;  This &lt;a href=&quot;https://space.stackexchange.com/questions/948/what-happens-to-defunct-lunar-and-mars-rovers-what-technological-barriers-preve/4776?noredirect=1#comment11026_4776&quot;&gt;comment&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/948/what-happens-to-defunct-lunar-and-mars-rovers-what-technological-barriers-preve/949#949&quot;&gt;geoffc's answer (batteries and Bearings)&lt;/a&gt; got me wondering what would the advantages and disadvantages of what in theory (&lt;em&gt;with ceramic/magnetic bearings&lt;/em&gt;) could be a battery without end to its rechargeable life would be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or said another way; Assuming a Centrifugal Battery built with ceramic/magnetic bearings which would essentially be capable of endless (or 10,000 year) recharging capability.  Would the low (&lt;em&gt;Mars, Moon, etc&lt;/em&gt;) or zero gravity, make the Centrifugal Battery a better or worse option then it is on Earth.  As &lt;a href=&quot;https://en.wikipedia.org/wiki/Kinetic_energy_recovery_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kinetic energy recovery systems&lt;/a&gt; these have been in use on Earth since the &lt;a href=&quot;http://www.mpoweruk.com/alternatives.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;1950's&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorDisplayName="user20636" LastEditDate="2018-04-18T03:11:34.143" LastActivityDate="2018-04-18T05:15:15.907" Title="Would a flywheel energy storage have a distinct advantage or disadvantage in zero or low gravity?" Tags="|energy|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="4779" PostTypeId="2" ParentId="4778" CreationDate="2014-06-11T18:21:16.283" Score="10" Body="&lt;p&gt;There are two issues I see with this idea:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The overall mass of the system necessary to be able to store meaningful amounts of energy.&lt;/li&gt;&#xA;&lt;li&gt;The effect of such a device on the attitude control capabilities of a spacecraft.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If those can be solved, or at least mitigated, this might be a viable idea.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-06-11T18:21:16.283" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4780" PostTypeId="2" ParentId="4778" CreationDate="2014-06-11T20:05:47.617" Score="10" Body="&lt;p&gt;You are correct that flywheels can be used for energy storage like batteries.  As with all engineering trades, this comes down to many factors:  mass, cost, efficiency, lifetime, reliability ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being in 0g means you don't have to support the weight of the flywheel on the bearings.  That is an advantage, but not a huge one, compared to 1g.  Having electromagnetic bearings will eliminate bearing wear as a failure mode, but there will be a complex electronic unit to handle suspension and power transfer.  It will have some failure rate.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that we see so many batteries in use indicates which way the trade seems to come out.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-06-11T20:05:47.617" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4781" PostTypeId="2" ParentId="4748" CreationDate="2014-06-11T22:12:37.467" Score="1" Body="&lt;p&gt;Sustainability on Mars does not require 100% closure of all loops in a life-support system.  It only requires economically sustainable levels of closure - 'Good is good enough.'  NASA is far too hung up on total system closure, particularly for Mars where there is no question that essential resources are there. &lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2014-06-11T22:12:37.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4782" PostTypeId="2" ParentId="4765" CreationDate="2014-06-11T22:35:49.840" Score="13" Body="&lt;p&gt;Typically the difference between an engine of a rocket and a rocket is the engine only encompasses the injectors and control of those ejectors. Arguably, any control over the nozzle direction would also come under &quot;engine&quot; but it's a fuzzy line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rough breakdown of a rocket propulsion system is: fuel storage, piping, injectors, combustion chamber, nozzle. So lets go through them 1 by 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fuel storage: this is your tanks. Now different fuels have different densities, and for a given pressure rated tank you will be able to hold a different amount of liquid from one fuel to the next. Also different fuels can react with the tanks inner walls in different ways. Further to this, helium (I know this isn't hydrogen but it's for example purposes) can actually leak through aluminium. So you can see there are a lot of issues with this, but none of them rule out using different fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Piping: very similar to your fuel tank, we'll bundle valves into this section as well. Aside from the solid/fluid interactions that we discussed with the tank there's a couple of other issues with the piping. It needs to be big enough to let the right amount of fuel through, which means making it big enough for the fuel that requires the largest diameter piping, but then you are potentially over flowing the other fuels. Still, using the engine at some reduced efficiency would still qualify as using it! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Injectors: as I understand it the injectors are to mix and ignite the fuel. I would imagine (could be wrong here) that these things are designed bespoke for the fuel. The fluid mechanics of it are likely to be a nice complicated task, but again if we don't mix well but still ignite then we'll just have a reduced efficiency!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combustion chamber: this is very similar to the issues related to the injectors, just to a lesser extent I would imagine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nozzle: this is a tricky one. The purpose of the nozzle is to turn the heat into speed. Now an ideal nozzle converges up to the point where the the exhaust reaches mach 1, and then diverges (subsonic fluid flow increases in speed through a convergent nozzle, supersonic is the opposite). I see no reason why you couldn't make the convergent part of your nozzle long enough for the exhaust that requires the maximum length, since you would just get a reduced efficiency from the other exhausts!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in summary, I see no reason why it wouldn't work. You might just get a large reduction in efficiency, meaning more propellant being required for a specific delta V.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="921" LastEditDate="2018-05-30T08:51:09.787" LastActivityDate="2018-05-30T08:51:09.787" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="4783" PostTypeId="1" AcceptedAnswerId="4787" CreationDate="2014-06-12T07:59:35.127" Score="14" ViewCount="3509" Body="&lt;p&gt;The moon whizzes around the Earth, dragging water after it and producing tides. In a comment on &lt;a href=&quot;https://space.stackexchange.com/questions/4778/would-a-centrifugal-battery-have-a-distinct-advantage-or-disadvantage-in-zero-or/4780#4780&quot;&gt;this question&lt;/a&gt;, I asserted this would sap the momentum of the moon causing it to fall to Earth. This was promptly rebutted by people with references, but I don't see why this should be so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I admit I forgot about planetary rotation and its effect on the water but nevertheless I would have expected tides to dissipate energy from both as friction induced heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, an assertion that momentum is being transferred to the moon is trivially falsifiable for any astronomical observatory equipped with modern clocks, so I'm going to simply assume it's true. The question then becomes what is the mechanism by which tides transfer momentum to the moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it have to do with the balance point in a very assymetric two body system transferring energy from a large mass to a small one, akin to a slingshot orbit? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or does the energy transfer from the body with the water sloshing about to the one without? (And if so, why?)&lt;/p&gt;&#xA;" OwnerUserId="4586" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2019-02-22T22:01:14.553" Title="Why is momentum transferred to the moon?" Tags="|orbital-mechanics|the-moon|gravity|celestial-mechanics|tides|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4784" PostTypeId="1" CreationDate="2014-06-12T08:02:07.653" Score="22" ViewCount="6293" Body="&lt;p&gt;During the last ten years all 100 or so launches by &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V#Atlas_V_launches&quot;&gt;Atlas V&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5#Ariane_5_flights&quot;&gt;Ariane V&lt;/a&gt; together have been successful. (One Atlas V payload entered too low orbit, but that would hardly have risked the life of a crew). Why is that not enough for them to be man-rated and carry crewed capsules?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there requirements on technical details which overrule actually demonstrated performance? Or is there something else than reliability which plays a role, like higher G-forces than a human can take?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="897" LastEditDate="2014-06-12T12:44:09.720" LastActivityDate="2023-07-25T20:49:54.663" Title="Why are the very reliable rockets Atlas V and Ariane V not rated for human flight?" Tags="|rockets|crewed-spaceflight|reliability|human-rating|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4786" PostTypeId="2" ParentId="4784" CreationDate="2014-06-12T11:32:29.423" Score="21" Body="&lt;p&gt;Man rating seems like an obvious thing.  Safe enough for manned flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in reality, there really was/is no standard for it.  Some things are generic. Sufficient (Usually triple) redundancy in flight computers/controls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has cited building to 1.4X structural margins expected, instead of 1.25. (This is a hard one to retrofit in after the fact).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA actually came up with an attempt at a standard.  Thing is, Soyuz does not meet it, yet has flown how many hundreds of manned flights (And 1700 total flights for the booster).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per the Wikipedia entry on &lt;a href=&quot;https://en.wikipedia.org/wiki/Human-rating_certification&quot;&gt;Human Rating Certification&lt;/a&gt; NASA in 2008 came out with a standard for the new entrants. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(NASA) has published &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8705_002B_&quot;&gt;NASA Procedural Requirement NPR 8705.2B&lt;/a&gt; - Human Rating Requirements for Space Systems, defining the certification process and a set of technical requirements to be applied to its crewed space systems in addition to the standards and requirements that are mandatory for all of NASA's space flight programs&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Shuttle would not meet the standards either.  Not sure about Apollo/Saturn V (and NASA would probably very much like people NOT to ask that question, I suspect.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So asking why were Atlas V/Ariane V are not human rated is hard to answer. Mostly because until recently, human rated did not have a very hard target to hit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously no one plans to build an unreliable rocket. But obviously as well, this is still rocket science and it is hard. Additionally, if you believe in a man rated standard, a large number of successful flights alone does not prove it is man rated. But if you are NASA you get to ignore the rules (Shuttle/Soyuz) at will, if it is expedient.  Which leads to the conclusion that man-rating is sort of tenuous at best.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-06-12T13:32:36.067" LastActivityDate="2014-06-12T13:32:36.067" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4787" PostTypeId="2" ParentId="4783" CreationDate="2014-06-12T13:01:53.320" Score="11" Body="&lt;p&gt;The Earth's rotation rate is very gradually slowing down. Over the short term, the Earth's rotation rate changes a bit chaotically, sometimes speeding up, sometimes slowing down. These short term changes are due to transfers of angular momentum amongst various components of the Earth. The long term secular variations in the Earth's rotation rate are due to tidal interactions between the Earth, Sun, and Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The slowing of the Earth's rotation rate means a reduction in angular momentum. Because angular momentum is a conserved quantity, that lost angular momentum doesn't just disappear. It has to be transferred to something else. That something else are the two bodies responsible for the slowing of the Earth's rotation rate, and that is the Moon and the Sun, mostly the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;The textbook explanation for this is the Earth's tidal bulge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Newton's explained the tides by noting that the tidal force is away from the center of the Earth along the axis connecting the Earth's and Moon's center of mass and is directed inward toward the center of the Earth along the great circle normal to this axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.mercer.edu/hpage/tidal%20asymmetry/fig%205.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://physics.mercer.edu/hpage/tidal%20asymmetry/fig%205.gif&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Newton reasoned that this force should make Earth's oceans bulge outward along that axial line and draw inward along that circle where the tidal force is inward. This equilibrium theory of the tides gives a nice, simple explanation for transfer of angular momentum from the Earth's rotation to the Moon's orbit. The Moon pulls the tidal bulge slightly forward. This off-axis tidal bulge on the Earth in turn results in a tangential component of the gravitational Earth's gravitational force on the Moon, in the direction of the Moon's velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A similar thing happens when a rocket in circular orbit fires it's thrusters against the velocity vector. The rocket accelerates forward, but in doing so moves to a higher orbit where it slows down. The same thing is happening to the Moon. That continuous forward acceleration very gradually raises the Moon to an every higher orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a very slow process. Currently the Moon is receding from the Earth at about 3.8 centimeters/year, and that slow creep is anomalously high. The long term rate based on various fossil records is about 1.7 cm/yr over the last 2 to 3 billion years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;There's one thing wrong with this model: Newton's tidal theory is false. While Newton did develop the correct model for the tidal force, his static model of the tides is just wrong. There is no Newtonian tidal bulge. There can't be, for a number of reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, Newton's equilibrium theory of the tides is falsified by observations of the tides. Consider the North Sea. Per Newton's equilibrium theory, there would be two high tides per day in the North Sea, one when the Moon is at zenith and the other when it is at nadir. Since the North Sea is rather small, those high tides should occur more or less at the same time all across the North Sea. That isn't what happens. At any time of day, it's always high tide somewhere in the North Sea, low tide somewhere else. The same goes for Patagonia, New Zealand, and a few other spots on the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Laplace developed the correct theory of the tides. Laplace's dynamic theory of the tides takes into account the very things that say why Newton's tidal bulge cannot exist. Instead of a tidal bulge, the oceans comprise a number of amphidromic systems. There are several amphidromic points in the oceans where tides are nearly non-existent. The tides rotate about these amphidromic points. There are three such points in the North Sea, which explains why tides in the North Sea are so ridiculously complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if Newton's equilibrium theory of the tides and his tidal bulge are false, what explains the transfer of momentum? The interactions at the coasts results in asymmetries in the rotations about these amphidromic points. The net effect of these asymmetries are to induce a tangential component in the Moon's gravitational acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;This is not a &quot;net tidal bulge&quot;. There is no tidal bulge.&lt;/strong&gt; Those amphidromic systems result from the geometry of the oceans and the impossibility of a tidal bulge. The geometry of the oceans has changed drastically over the ages. Currently there are two huge north-south land barriers that impede the tides, the Americas in the western hemisphere and Afro-Eurasia in the eastern hemisphere. This configuration is the reason for the currently anomalously recession rate. The tides have had much freer flow during other times in the Earth's history such as when the Earth's continents were organized as a single supercontinent. The recession rate was anomalously low during those periods.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lambeck, &quot;&lt;a href=&quot;http://rsta.royalsocietypublishing.org/content/287/1347/545&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tidal Dissipation in the Oceans: Astronomical, Geophysical and Oceanographic Consequences&lt;/a&gt;&quot;, &lt;em&gt;Phil. Trans. R. Soc. Lond. A&lt;/em&gt; 287:1347 (1977)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Proudman and Doodson, &quot;&lt;a href=&quot;http://rsta.royalsocietypublishing.org/content/224/616-625/185.full.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Principal Constituent of the Tides of the North Sea&lt;/a&gt;&quot;, &lt;em&gt;Phil. Trans. R. Soc. Lond. A&lt;/em&gt;, 224 (1924).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thompson, &quot;&lt;a href=&quot;http://faculty.washington.edu/luanne/pages/ocean420/notes/tidedynamics.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tide Dynamics: Dynamic Theory of Tides&lt;/a&gt;&quot;, Lecture notes for Ocean 420 at Univ. of Washington.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wotal, &quot;&lt;a href=&quot;http://www.oberlin.edu/faculty/swojtal/SFWpage/161Stuff/161Lect17/index.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tides and their origin&lt;/a&gt;,&quot;&#xA;Lecture notes for Geology 161 at Oberlin College.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="10273" LastEditDate="2019-02-22T22:01:14.553" LastActivityDate="2019-02-22T22:01:14.553" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="4789" PostTypeId="1" CreationDate="2014-06-12T14:07:47.473" Score="8" ViewCount="801" Body="&lt;p&gt;A few days ago, &lt;a href=&quot;http://aviationweek.com/blog/smoke-space-station-russian-module-clears-quickly&quot;&gt;a Russian water reclamation system took up smoking&lt;/a&gt;. The smoke that was produced traveled through the air ducts and was thus propagated throughout the station. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the article, ground controllers 'isolated' the US ventilation from the Russian system:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ventilation system for the station's Russian segment was isolated from the U.S. Operating Segment with commands issued by NASA flight controllers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How is this accomplished? Is a tube connecting the Russian and American side literally blocked?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="7062" LastEditDate="2021-09-13T17:22:58.943" LastActivityDate="2021-09-13T17:22:58.943" Title="How are the US and Russian ventilation systems separated on the ISS?" Tags="|iss|fire|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4790" PostTypeId="2" ParentId="4789" CreationDate="2014-06-12T15:25:34.710" Score="2" Body="&lt;p&gt;I don't know of any specific explanations of that particular issue, but in general it sounds like a fairly standard ventilation fire suppression system.  &lt;a href=&quot;http://www.systemair.com/Documents/Downloads/UK/Fire%20Safety.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; is one example of fire prevention in ductwork; in large buildings in the US and other countries being able to isolate sections of the buildings automatically would be required by building code.  In the answer to your direct question, it is likely that the ductwork connecting the Russian and the American side is indeed literally blocked (with a damper).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.boeing.com/boeing/defense-space/space/spacestation/systems/eclss.page&quot; rel=&quot;nofollow&quot;&gt;This Boeing article&lt;/a&gt; discusses shutoff systems and other fire prevention systems, and as well discusses the separate ways the Russian and US segments add oxygen to the air mix in the Atmospheric Control and Supply section.  This answers the obvious followup question &quot;If it is blocked, how do they breathe?&quot;; as with many things on the ISS, there are backups for the backups - in this case, independent atmospheric systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100017014.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; shows some interesting cross sections of the ventilation system, and while it is focused on acoustic abatement, it is pretty easy to see how you might implement fire abatement blocks in the system.&lt;/p&gt;&#xA;" OwnerUserId="3432" LastActivityDate="2014-06-12T15:25:34.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4791" PostTypeId="2" ParentId="4784" CreationDate="2014-06-12T15:49:58.800" Score="9" Body="&lt;p&gt;&lt;a href=&quot;http://www.space.com/15170-private-space-taxis-atlas5-rocket.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space.com&lt;/a&gt; answered this very question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The emergency detection system needs to be finished, for example. And a human-rated Atlas 5 will use a two-engine Centaur upper stage rather than the single-engine version currently in use, Patton said, so some more tweaks will be needed to accommodate the change.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's a good list of other things, but the bottom line is, it's reliable enough, it just needs a few additional tweaks that a man rated spacecraft requires.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the reason why these types of things are required is a flight profile has to be used that is fairly flat, so that if there is an emergency there will be enough time to slow down before impact. Any reentry vehicle if launched straight up will burn up on its way down, they aren't designed to do that. So the upper stage has to have enough thrust to allow for a relatively flat trajectory, which takes Delta and Arianne out. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-05-22T09:43:19.593" LastActivityDate="2019-05-22T09:43:19.593" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="4792" PostTypeId="1" CreationDate="2014-06-12T17:00:45.087" Score="9" ViewCount="1099" Body="&lt;p&gt;My understanding is that the parts of a typical &lt;a href=&quot;http://en.wikipedia.org/wiki/flywheel_energy_storage&quot; rel=&quot;nofollow noreferrer&quot;&gt;flywheel energy storage&lt;/a&gt; are pretty much the same as the parts of a typical &lt;a href=&quot;http://en.wikipedia.org/wiki/reaction_wheel&quot; rel=&quot;nofollow noreferrer&quot;&gt;reaction wheel&lt;/a&gt; subsystem --&#xA;both have a flywheel, electric motor/generator, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible for a spacecraft to use a flywheel to store and later supply energy, and also (perhaps at an even later time) re-use the same subsystem as a reaction wheel for &lt;a href=&quot;http://en.wikipedia.org/wiki/attitude_control&quot; rel=&quot;nofollow noreferrer&quot;&gt;attitude control&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible for a spacecraft to use a sufficient(*) number of flywheels to simultaneously control attitude and store energy, and later simultaneously control attitude and supply energy?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(*) 3 flywheels are not sufficient, but my understanding is that many spacecraft already have 4 or more reaction wheels&#xA;( &lt;a href=&quot;https://space.stackexchange.com/questions/8/optimal-placement-of-4-reaction-wheels&quot;&gt;Optimal placement of 4 reaction wheels?&lt;/a&gt; ).&lt;/p&gt;&#xA;" OwnerUserId="397" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-11-11T09:38:48.720" Title="Can the reaction wheels also be used to store energy?" Tags="|energy|attitude|power|reuse|reaction-wheel|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4793" PostTypeId="1" AcceptedAnswerId="4805" CreationDate="2014-06-12T18:24:57.297" Score="9" ViewCount="1148" Body="&lt;p&gt;Mass to LEO is somewhat of a standard in superficially comparing the payload capacity of different rocket, at least for the googling general public. There are many LEOs and it depends on where the launch takes place and whatnot. But especially when it accounts for what has actually been lifted to LEO, it is a rough guideline and useful for relative comparision (I hope).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder if such a mass to LEO could be easily and roughly converted to a mass on the lunar surface? And if so, in a couple of distinct scenaria such as to the equator or to a lunar pole, if that matters much. And for simplicity lets not consider long term low thrust propulsion, just chemical rocketry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With heavy rockets I mean the &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_launchers_families&quot;&gt;12-23 tonnes&lt;/a&gt; to LEO types (Atlas, Delta, Ariane, Proton, Zenit, Mitsubishi, Long March, Falcon). How much can they &lt;em&gt;land softly&lt;/em&gt; on the surface of the Moon today?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-06-12T18:45:38.100" LastActivityDate="2019-02-21T10:53:55.380" Title="What tonnage can current heavy rockets land on the Moon?" Tags="|rockets|lunar-landing|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4794" PostTypeId="1" CreationDate="2014-06-13T13:41:58.573" Score="6" ViewCount="978" Body="&lt;p&gt;How can I find a estimation of the delta-V needed to lift-off from a planet and go into a low orbit around it?&lt;/p&gt;&#xA;" OwnerUserId="4600" LastEditorUserId="8693" LastEditDate="2019-09-26T18:54:26.730" LastActivityDate="2021-01-08T07:11:41.033" Title="Delta-V required for lift-off from a planet/ asteroid" Tags="|launch|delta-v|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4796" PostTypeId="1" CreationDate="2014-06-13T14:49:56.893" Score="3" ViewCount="473" Body="&lt;p&gt;I want to calculate the Elevation angle between Earth and Mars .I have the Mars and Earth exact position in space  ,but need the elevation Angle from a specific Earth Station antenna ( e.g. canberra  station ) .may help me ?&#xA;thank alot&lt;/p&gt;&#xA;" OwnerUserId="3394" LastActivityDate="2014-06-13T22:50:53.077" Title="Elevation Angle from Earth Station antenna to Mars" Tags="|artificial-satellite|orbital-elements|earth-observation|communication-satellite|" AnswerCount="2" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4797" PostTypeId="1" AcceptedAnswerId="4800" CreationDate="2014-06-13T15:05:29.060" Score="9" ViewCount="7699" Body="&lt;p&gt;I'm wondering just how many burns the Apollo 11 mission required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did they first have to reach a circular orbit around Earth by a single burn at apogee or did they target the Moon immediately?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, when they've reached the Moon did they have to retroburn at perilune to circularize, or was the trajectory calculated so that it happened automatically?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On their way back was a single burn enough to leave the Moon or did they have to execute some corrections along the way?&lt;/p&gt;&#xA;" OwnerUserId="3336" LastEditorUserId="6642" LastEditDate="2019-12-17T15:43:05.877" LastActivityDate="2019-12-17T15:43:05.877" Title="How many burns did the Apollo 11 mission need?" Tags="|apollo-program|trajectory|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4798" PostTypeId="2" ParentId="4796" CreationDate="2014-06-13T15:37:13.147" Score="4" Body="&lt;p&gt;You can use the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow&quot;&gt;JPL HORIZONS system&lt;/a&gt;.  Select the target body (Mars), the observer location (lat/long, or city name), and table settings (check Apparent AZ &amp;amp; EL).  Select the time span you want if you don't like the default (every day for the next month).  Then &quot;Generate Ephemeris&quot;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-06-13T15:37:13.147" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4800" PostTypeId="2" ParentId="4797" CreationDate="2014-06-13T16:59:40.680" Score="19" Body="&lt;p&gt;According to the plan, an Apollo mission to the moon uses &lt;strong&gt;eleven-ish&lt;/strong&gt; burns.  Here are the expected maneuvers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JKwqQ.png&quot; alt=&quot;Apollo mission profile&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Launch from earth.  This you might count as three burns, since it requires all three stages of the Saturn V rocket.  They're not only launching from earth at this point, but they're also burning to get into a circular orbit around the earth.&lt;/li&gt;&#xA;&lt;li&gt;Leave earth orbit for the moon (translunar injection).  Here they relight the third stage of the Saturn V, using up the rest of its fuel.  At this point, they should be on a &lt;em&gt;free-return&lt;/em&gt; trajectory, meaning that if they do nothing else, they'll slingshot around the moon and come back to the vicinity of the earth.&lt;/li&gt;&#xA;&lt;li&gt;Make a midcourse correction.  These corrections are made using the engine built onto the Service Module (SM).&lt;/li&gt;&#xA;&lt;li&gt;Make another midcourse correction, as needed.&lt;/li&gt;&#xA;&lt;li&gt;Get into an elongated orbit around the moon.  This uses the SM engine.&lt;/li&gt;&#xA;&lt;li&gt;Circularize the orbit around the moon.  This also uses the SM engine.&lt;/li&gt;&#xA;&lt;li&gt;Get into descent orbit.  This uses the descent engine on the Lunar Module (LM).&lt;/li&gt;&#xA;&lt;li&gt;Descend to the moon.  This also uses the descent engine on the LM.&lt;/li&gt;&#xA;&lt;li&gt;Ascend from the moon.  This uses the ascent engine on the LM, leaving the descent portion behind.&lt;/li&gt;&#xA;&lt;li&gt;Leave lunar orbit for the earth (transearth injection).  This uses the SM engine.&lt;/li&gt;&#xA;&lt;li&gt;Make a midcourse correction.  This also uses the SM engine.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="552" LastEditorUserId="552" LastEditDate="2014-06-13T20:32:28.257" LastActivityDate="2014-06-13T20:32:28.257" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4801" PostTypeId="2" ParentId="4794" CreationDate="2014-06-13T18:44:25.157" Score="3" Body="&lt;p&gt;As a first approximation, you can calculate the &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;nofollow&quot;&gt;orbital speed&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_o \approx \sqrt{\frac{GM}{r}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where Vo is the orbital speed, G is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow&quot;&gt;gravitational constant&lt;/a&gt;, M is the mass of the planet, and r is the radius of the orbit.&lt;br&gt;&#xA;That gives you the speed you need to achieve. You can't say $v_o = \Delta V$, because $\Delta V$ contains several other factors:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;potential energy due to the height of the orbit  &lt;/li&gt;&#xA;&lt;li&gt;speed gained or lost by launching along or against the planet's rotation (you need to calculate the rotation speed of your launch site). On Earth's equator, this is some 1600 km/h.  &lt;/li&gt;&#xA;&lt;li&gt;energy losses due to e.g. drag  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-06-14T07:37:56.070" LastActivityDate="2014-06-14T07:37:56.070" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4802" PostTypeId="2" ParentId="4796" CreationDate="2014-06-13T21:24:53.417" Score="4" Body="&lt;p&gt;Do you really want to calculate it entirely from first principles?  If not, you might find the PyEphem library useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or e.g. this code:  &lt;a href=&quot;https://github.com/swift-nav/libswiftnav/blob/master/src/coord_system.c&quot; rel=&quot;nofollow&quot;&gt;https://github.com/swift-nav/libswiftnav/blob/master/src/coord_system.c&lt;/a&gt; (you'll have to convert to the ECEF coordinate system first)&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2014-06-13T22:50:53.077" LastActivityDate="2014-06-13T22:50:53.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4803" PostTypeId="2" ParentId="4655" CreationDate="2014-06-14T06:57:36.650" Score="2" Body="&lt;p&gt;One proposal is for a Mars Transfer Vehicle (MTV) for the astronaut crew.  The MTV has been given the name Copernicus,  for the 16th century mathematician.  This vehicle will be constructed on earth, and lifted into Low Earth Orbit (LEO) by several heavy lift SLS vehicles.   The MTV will then need to be assembled; either in LEO or at the Mars-Earth Lagrange Point.  Copernicus (~93.7m long) will consist of 3 basic components:  1) Crewed payload element (~33.8m long), 2) a propulsion module, and 3) an integrated saddle truss &amp;amp; LH2 propellant drop tank assembly that connects the payload and propulsion elements.  The payload consists of an inflatable Crew Transfer Habitation (CTH) (also called a Transhab), a long life CEV/SM for vehicle to vehicle transfer as well as end-of-mission Earth entry, a secondary T-shaped docking module, a contingency consumables container, and a connecting structure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two cargo vehicles would make the trip to Mars 1 to 2 years ahead of the crew, and will be used to pre-deploy a cargo lander to the landing site,  and a habitat lander that is placed in Mars orbit.  Once these vehicles check out, the MTV is readied and departs.  Following the trans-Mars insertion maneuver,  the drained LH2 drop tank is jettisoned.  Once in Mars orbit, the MTV rendezvous with the awaiting orbiting hab lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source:  S.K. Borowski, D.R. McCurdy &amp;amp; T.R. Packard.  &quot;Nuclear Thermal Propulsion (NTP):  A Proven Growth Technology for Human NEO/Mars Exploration Mission.&quot;  Aerospace Conference. 01/2012.&lt;/p&gt;&#xA;" OwnerUserId="4604" LastActivityDate="2014-06-14T06:57:36.650" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4804" PostTypeId="1" AcceptedAnswerId="4812" CreationDate="2014-06-15T06:54:24.430" Score="8" ViewCount="1770" Body="&lt;p&gt;Space Adventures would send up two space tourists for about $150 million with a professional Russian cosmonaut for a mission around the Moon lasting 16-17 days: &lt;a href=&quot;http://news.discovery.com/space/private-spaceflight/fly-me-to-the-moon-for-150-million-140613.htm&quot;&gt;http://news.discovery.com/space/private-spaceflight/fly-me-to-the-moon-for-150-million-140613.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the potential issues, and do they have workarounds for them?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How can a Soyuz reach the Moon? &lt;/li&gt;&#xA;&lt;li&gt;How can a Soyuz manage life support for three people for 17 days?&lt;/li&gt;&#xA;&lt;li&gt;Can the heat shield take the load?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="4606" LastEditorUserId="51" LastEditDate="2014-06-16T23:15:54.193" LastActivityDate="2014-06-17T01:29:28.427" Title="Is it possible to fly with Soyuz spacecrafts around the Moon?" Tags="|the-moon|soyuz-spacecraft|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4805" PostTypeId="2" ParentId="4793" CreationDate="2014-06-15T12:51:02.640" Score="3" Body="&lt;p&gt;Here's a table that shows the &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space_.E2.80.94_high_thrust&quot; rel=&quot;nofollow noreferrer&quot;&gt;delta-v requirements in the Earth-Moon system&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/ig1l2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ig1l2.jpg&quot; alt=&quot;Delta-V Budget. Earth-Moon space.&quot;&gt;&lt;/a&gt;&#xA;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By this it takes 5,930 m/s to go from LEO to the lunar surface. The &lt;a href=&quot;http://www.astronautix.com/stages/cenaurv2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centaur V2 upper stage&lt;/a&gt; could do this as a single stage one-way. It has a loaded mass of 23 metric tons (mT) and empty mass of 2.2 mT, with an Isp of 451s. Then it could carry 5 mT from LEO to the lunar surface:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ 451 \cdot 9.81 \ln{\frac{23 + 5}{2.2 + 5}} = 6,010 \text{ m/s} $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would need landing legs but these are typically a fraction of the landed weight and would need even less mass for the 1/6th gravity of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total gross mass, loaded stage plus payload, of 28 mT could be carried to LEO by the Delta IV heavy which has been upgraded to carry 28 mT to orbit.&lt;/p&gt;&#xA;" OwnerDisplayName="user3544" LastEditorUserId="10273" LastEditDate="2019-02-21T10:53:55.380" LastActivityDate="2019-02-21T10:53:55.380" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="4806" PostTypeId="2" ParentId="4743" CreationDate="2014-06-16T02:57:17.677" Score="1" Body="&lt;p&gt;Unless as you state something large impacts with the disc, which is certainly unlikely until they enter into another solar system. The disc should remain readable, there are some processes which may distort the disc but they wouldn't damage the possibility of recovering the data stored on the disc if this distortion is taken into account when trying to recover the data.&lt;/p&gt;&#xA;" OwnerUserId="3437" LastActivityDate="2014-06-16T02:57:17.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4807" PostTypeId="2" ParentId="4743" CreationDate="2014-06-16T03:55:28.500" Score="13" Body="&lt;p&gt;There are, in principle, three ways the record might be destroyed: impact with another body; heating/melting; or atomic deterioration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no idea as to what the ultimate trajectory of Voyager might be, namely whether it might eventually either A. run into an icy body in an Oort Cloud equivalent of another solar system; or B. make a close enough pass to another star to cause the thermal load to become high enough to melt the record. Someone may have run a rough calculation on Voyager's far-distant trajectory, but I'm not aware of it if so. But the galaxy is very much mostly empty space so this is unlikely. But since either of these seem unknowable for now let's just address the possible impacts of cosmic rays or other particles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The record is constructed of gold-plated copper. Stable isotopes of both metals do not decay atomically. Since the record is analog, not digital, the information on the disc is macro-scale (bumps on a groove) and thus not subject to the kinds of losses from cosmic ray bombardment that data encoded in digital bits would be. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So given that - and again, assuming that there are no direct impacts or close heat encounters with another body, I think the answer as to how long the disc will remain playable (or at least recoverable by a civilization sophisticated to recover it in the first place) is: indefinitely.&lt;/p&gt;&#xA;" OwnerUserId="4607" LastEditorUserId="4607" LastEditDate="2014-06-16T19:52:16.043" LastActivityDate="2014-06-16T19:52:16.043" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4809" PostTypeId="1" AcceptedAnswerId="9991" CreationDate="2014-06-16T14:52:33.663" Score="9" ViewCount="981" Body="&lt;p&gt;Space Adventures is selling a seat for a flight around the Moon in a Soyuz for \$150 million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reading a &lt;a href=&quot;http://spaceflightnow.com/news/n1406/15lunarsoyuz/&quot;&gt;SpaceFlight Now&lt;/a&gt; article, it says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The probable flight plan calls for the moon-bound crew to fly to the space station on a Soyuz rocket and spacecraft for a few days, then undock and rendezvous with a habitation module and Block DM propulsion stage launched separately atop a Proton booster. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is the Earth Orbit Rendezvous model which was considered for Apollo. I suppose turning around and getting the Lunar Module out of the Saturn V 3rd stage sort of counts, but that was post-TLI. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has any large scale EOR launch ever been done before? I suppose Gemini-Agena, but they just used Agena to boost and lower orbits to practice for the Apollo missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I mean by EOR, is not just the act of the rendezvous.  It is the act of launch multiple times, collect the parts together, and go off to do something useful.  (Which thinking about it, is NOT what was meant for Apollo by EOR really).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, has anyone launched a crewed vehicle and then transport vehicle (Hab + Block DM) to dock together to go off and do something useful.  (That is what Agena/Gemini was. But never very far, or beyond Earth orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is actually a really good thing, for several reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It tests out an EOR approach, and if it works, demonstrates it can work. (The proof is in the doing).&lt;/li&gt;&#xA;&lt;li&gt;Fuel depots are only a small step further along than this EOR approach.&lt;/li&gt;&#xA;&lt;li&gt;Making hab modules is a good thing, since the first is usually expensive, more would be cheaper once everything is ready to go. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So a question: Is there any other example of EOR of a reasonable scale?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-06-16T20:07:38.973" LastActivityDate="2015-07-17T05:54:28.467" Title="If Space Adventures actually does a Soyuz around the Moon, will this be the first real Earth Orbit Rendezvous launch?" Tags="|launch|rendezvous|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4811" PostTypeId="2" ParentId="3880" CreationDate="2014-06-16T22:27:28.763" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Any payload which is supposed to travel with it would first need to&#xA;  accelerate a lot to rendezvous with the cycler and then, at the&#xA;  destination, decelerate a lot to rendezvous with the planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is some work that has been done to find optimal cycler orbits, check &lt;a href=&quot;https://engineering.purdue.edu/people/james.m.longuski.1/ConferencePapersPresentations/2002ALow-ThrustVersionoftheAldrinCycler.pdf&quot; rel=&quot;noreferrer&quot;&gt;this work&lt;/a&gt;. Some of the computed orbits reduced the delta-V for inbound taxis almost to 5.9 Km/sec. Not enough to be comparable to a normal orbital transfer, but the report doesn't suggest this is a lower bound, so it could be improved in principle. &lt;/p&gt;&#xA;" OwnerUserId="638" LastEditorUserId="638" LastEditDate="2014-06-17T19:41:57.707" LastActivityDate="2014-06-17T19:41:57.707" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4812" PostTypeId="2" ParentId="4804" CreationDate="2014-06-17T01:16:54.087" Score="9" Body="&lt;h1&gt;Soyuz Spacecraft&lt;/h1&gt;&#xA;&lt;p&gt;The Soyuz spacecraft was originally developed for a moonshot attempt. This goal was abandoned when the US beat them to the moon, but the capsule was still developed with lunar capability in mind.&lt;/p&gt;&#xA;&lt;p&gt;It's important to understand, however, that Soyuz isn't a single capsule design - it's a family of over 9 distinct spacecraft types:&lt;br /&gt;&#xA;— Soyuz A&lt;br /&gt;&#xA;— Soyuz 7K-OK&lt;br /&gt;&#xA;— Soyuz 7K-L3&lt;br /&gt;&#xA;— Soyuz 7K-T&lt;br /&gt;&#xA;— Soyuz 7K-TM&lt;br /&gt;&#xA;— Soyuz T&lt;br /&gt;&#xA;— Soyuz TM&lt;br /&gt;&#xA;— Soyuz TMA&lt;br /&gt;&#xA;— Soyuz TMA-M&lt;/p&gt;&#xA;&lt;p&gt;The proposed Soyuz-K is one of the family, but one which has not been flown yet.&lt;/p&gt;&#xA;&lt;p&gt;The name Soyuz is also used for a series of rockets used to launch Soyuz spacecraft.&lt;/p&gt;&#xA;&lt;h1&gt;Current Capsules&lt;/h1&gt;&#xA;&lt;p&gt;The current capsules are intended for under 4 days of operation; that said, it's mostly a matter of additional life support (food, water, oxygen, and CO2 scrubber) for a lunar shot. 3.5 kL is pretty tight for a 3 person craft, but it's not unreasonable.&lt;/p&gt;&#xA;&lt;p&gt;The Soyuz TMA-01 mission operated from roughly 23:11 Thursday to 00:01 sunday in autonomous mode - just shy of 49 hours of operational flight - prior to docking, and another 2:36 of operation for landing. The TMA-02M and TMA-03M missions are of similar flight time. Apollo 13, on it's lunar flyby, had a total elapsed mission time of 5 days, showing that the capsule would need to be carrying some extra supplies.&lt;/p&gt;&#xA;&lt;h1&gt;Soyuz-K&lt;/h1&gt;&#xA;&lt;p&gt;This is why the plans were not for a TMA-M, per se, but the &amp;quot;Soyuz-K&amp;quot; configuration.  This is a TMA-M with a modified Orbital Habitation Module, and one or more orbital tug modules added. An off-the-shelf orbital tug, the Fregat, has enough delta-V to put a TMA-M with an endurance oriented habitation module into circumlunar flight, similar to the overall flown Apollo 13 mission of around 6 days. The Fregat module would be launched separately on a Proton rocket flight, and docked in orbit.&lt;/p&gt;&#xA;&lt;p&gt;Other variations could include additional habitat, larger thrust units, or even a lander, launched aboard one or more additional flights, and docked in orbit.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/project/soyuz.htm#more&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/project/soyuz.htm#more&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.spaceflight101.com/soyuz-spacecraft-information.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.spaceflight101.com/soyuz-spacecraft-information.html&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-01M&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Soyuz_TMA-01M&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-02M&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Soyuz_TMA-02M&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-03M&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Soyuz_TMA-03M&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://en.ria.ru/russia/20061018/54922208.html&quot; rel=&quot;noreferrer&quot;&gt;http://en.ria.ru/russia/20061018/54922208.html&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.russianspaceweb.com/soyuz_acts.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.russianspaceweb.com/soyuz_acts.html&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Soyuz_(spacecraft)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-06-17T01:23:15.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4813" PostTypeId="2" ParentId="4804" CreationDate="2014-06-17T01:29:28.427" Score="5" Body="&lt;p&gt;The plan as described in a &lt;a href=&quot;http://spaceflightnow.com/news/n1406/15lunarsoyuz/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Now article&lt;/a&gt; is to use a Soyuz-K as Aramis describes, launched on a standard Soyuz launcher, dock at the ISS, and wait for a Proton launch of a Habitation Module, powered by a &lt;a href=&quot;http://en.wikipedia.org/wiki/Blok_DM-03&quot; rel=&quot;noreferrer&quot;&gt;Blok DM&lt;/a&gt; upper stage for Trans Lunar Injection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They intend to use a more robust heat shield for faster reentry support, and are contemplating a skip re-entry to reduce demands on the heat shield. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the Blok DM will provide the impulse to get to the Moon, the Hab module will provide the extra space and life support for the longer mission, and a new heat shield to handle the higher re-entry speeds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz was originally designed to compete with Apollo, in the race to the moon, but it has been modified since that mission as can be seen from its very extensive flight history to orbit, Salyut, Mir, and ISS missions.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-06-17T01:29:28.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4814" PostTypeId="1" CreationDate="2014-06-17T15:03:19.293" Score="4" ViewCount="423" Body="&lt;p&gt;Does any mission have a propulsion system such that thrusters are aligned automatically, in order to only give delta-v, but without changing angular momentum? If so, how is this done? That is to say, how is the support of such thrusters designed, in order to ensure that the reaction force vector passes through the CoG of the spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="4611" LastEditorUserId="6214" LastEditDate="2015-01-15T09:20:00.880" LastActivityDate="2015-01-16T00:53:49.440" Title="Is there an existing method of automatic thruster alignment?" Tags="|propulsion|spacecraft|attitude|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4815" PostTypeId="1" CreationDate="2014-06-17T15:36:21.597" Score="6" ViewCount="180" Body="&lt;p&gt;Would it be feasible and would it make sense to minimalistically design a sample-return-only mission (or class of missions) along these lines:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Light weight plume collecting aerogel (like the Stardust mission) as the only science instrument payload.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;No payload to communicate with Earth. Trajectory correction (if necessary) done autonomously based on minimal sensors and propulsion system.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Battery as the only energy resource. Deep sleep mode almost all of the time.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Return trajectory formed by gravity assist at Jupiter or Saturn.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Earth based radar locates it when it returns, an Earth orbiting spacecraft docks with it and lands it on Earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The main ideas being that other kinds of science are best done by an orbiter or a fast flyby. A sample return needs a different kind of trajectory than all the other instruments. Also, since it returns physically, radio communication is not necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;&lt;strong&gt;Actually, why not send ONLY a sheet of aerogel? Nothing else, no propulsion, no electronics, no energy source, no heat source. And a whole series of them to improve chances of success.&lt;/strong&gt; Are such trajectories possible without any correction by the probe?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-06-18T08:12:16.970" LastActivityDate="2014-06-26T13:28:31.050" Title="How could a minimal sample return only from the plumes of an ice moon, Ceres or Io be designed?" Tags="|probe|mission-design|sample-return|plume|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4816" PostTypeId="1" AcceptedAnswerId="4818" CreationDate="2014-06-17T16:05:05.030" Score="10" ViewCount="328" Body="&lt;p&gt;The obvious drawback with a flyby mission compared with an orbiter is of course that an orbiter could make several flyby's. I wonder if the following ideas about mission design could help make flyby missions attractive enough to outcompete orbiters for exploration of outer Solar System objects (Uranus, Neptune, dwarf planets, asteroids)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a flyby probe splits up into several sub-probes with instruments, they could each fly by the object of study in different trajectories. Images could be taken from several angles and a better 3D measurment of magnetic and gravity fields would be obtained. The main probe would be the only one with Earth communication capability, a long term energy source and apropulsion system. For outer planets, each sub-probe could also pass nearby one moon each. Sub-probes would only be awake on batteries during the flyby and communicate only with the nearby main probe. With 3 sub-probes + the main probe one would have the equivalent of 4 well positioned orbits by an orbiter (near a moon, such as Cassini at Enceladus).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that fast flyby missions have several advantages compared to the same investment in an orbiter, for distance objects. Voyager being a triumph and hopefully soon New Horizons too. On the plus side:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Earlier arrival by many years, which means that the instruments will the more sophisticated and/or cheaper, giving more science per dollar invested.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Earlier science results helps design efficient follow up science, ground based or new flyby's, to target the discoveries and new questions raised.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The time value of similar high risk business investments is not rarely 20% per year or so, several years earlier delivery of results could double the present value of the investment.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Every flyby mission would fly by Jupiter for gravity assist, and near one of its moons. Then the object of interest, for example Neptune and one of its moons (before which it would split into sub-probes). Then a KBO. It would also test the limits of the heliosphere, like Voyager. So its not exactly a mayfly.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-06-17T16:11:54.503" LastActivityDate="2014-06-17T21:28:18.077" Title="Are multiple sub-probes feasible for fast flyby missions to outer Solar System objects?" Tags="|probe|planet|mission-design|flyby|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4817" PostTypeId="2" ParentId="3748" CreationDate="2014-06-17T18:02:54.157" Score="4" Body="&lt;p&gt;A private group, with some cooperation from NASA is making progress on recovering the spacecraft.  Using a slightly re-tuned cellphone transmitter and the Arecibo observatory dish, they have been able to command the spacecraft to do things and get the correct responses using only 400W.  A major challenge has been finding and interpreting documentation on the spacecraft and its supporting ground systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On 18-Jun-2014, they are paying NASA's DSN to provide an updated, precise trajectory for the spacecraft.  On 21-Jun, they plan to fire thrusters to increase it's spin rate to specification.  Around the end of the month, they expect to adjust the trajectory to put it back in an earth orbit (probably at the earth-sun L1 point).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For long-term retrieval of information, they have enlisted support from Morhead State University, Radio Amateurs in Germany, and others.  Additional radio and radio-astronomy amateurs have picked up the signal with small dishes.  So, no, DSN is NOT required to listen to the spacecraft at this time.  Nor will it be when its placed in a new orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Follow along at &lt;a href=&quot;http://spacecollege.org/isee3/&quot; rel=&quot;nofollow&quot;&gt;http://spacecollege.org/isee3/&lt;/a&gt;, where you can find a lot of the gory details, and additional references.&lt;/p&gt;&#xA;" OwnerUserId="4612" LastActivityDate="2014-06-17T18:02:54.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4818" PostTypeId="2" ParentId="4816" CreationDate="2014-06-17T21:28:18.077" Score="6" Body="&lt;p&gt;The sad reality is that by splitting a probe into a few subprobes you decrease mass, thermal control, and energy budgets available for scientific payloads.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;mass: you still have to build the frame sturdy enough to withstand high g's at launch; command and data handling equipment is the same for a large and a small probe, etc.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;thermal control - heaters can serve adjacent instruments in a large probe;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;power, downlink bandwidth etc. - you can stagger observations/transmissions to get the best value for your money.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Reliability of several probes will also likely be correlated, with an extra twist of managers having no time to control quality of each unit. &lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-06-17T21:28:18.077" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4819" PostTypeId="2" ParentId="4814" CreationDate="2014-06-18T02:51:32.943" Score="2" Body="&lt;p&gt;Any propulsion system that uses propellant is going to be hard pressed to meet this goal. As propellant is used up the centre of mass changes (the propellant contributes to the centre of mass). You could design a propulsion system with a spherical propellant tank, which is aligned with the centre of the spacecraft, thereby negating the effect of loss of fuel on the CoM, however that would still require you to design your spacecraft with zero products of inertia. If you've ever attempted a complete satellite configuration, right down to the nuts and bolts, then you will know just how hard it is to get zero's for the products of inertia (I'd suggest that it's borderline impossible to be completely zero - material inconsistencies etc.).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two of methods do spring to mind however. First you have things like solar sails, these need no propellant from the spacecraft, and in theory as long as they are aligned perpendicular to the force vector from the sun they should experience a constant force over the surface (solar particle flux in directions of the sail length and width should be symmetrical about the centre of the sail). All this then requires is that you design your spacecraft to have a centre of mass directly aligned with the sun force vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other thought that sprung to mind was a spin stabilised spacecraft. If you are performing a long duration, continuous burn you mitigate any part of the burn that is not along the preburn angular momentum vector. So if you want to go directly forward and not change angular momentum, spin around. Change your angular momentum to negate any accidental changes in your angular momentum. &lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-06-18T02:51:32.943" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4820" PostTypeId="2" ParentId="4814" CreationDate="2014-06-18T15:09:52.423" Score="1" Body="&lt;p&gt;You can mount the thruster on a gimbal and use disturbance torques to align it.  Launch vehicles often gimbal the nozzle for this purpose.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-06-18T15:09:52.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4822" PostTypeId="1" CreationDate="2014-06-18T21:03:26.137" Score="15" ViewCount="1085" Body="&lt;p&gt;Can anyone breakdown the cost of developing and implementing an upper stage nuclear thermal engine? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After development, what could the price be of each engine produced?&lt;/p&gt;&#xA;" OwnerUserId="4579" LastEditorUserId="4660" LastEditDate="2015-01-09T05:06:15.730" LastActivityDate="2015-07-15T06:36:11.857" Title="What are the economics of Nuclear Thermal Propulsion?" Tags="|propulsion|nuclear|upper-stage|ntr|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4823" PostTypeId="2" ParentId="4822" CreationDate="2014-06-18T22:13:13.287" Score="12" Body="&lt;p&gt;Any sensible answer to your question should make an implicit assumption on the number of engines to be ordered overall. That is, we need to know the number of planned manned flights to Mars and other important destinations, since fixed (sunk) costs of an NTR-based program are significant. The following ideas are based on a few NASA documents discussing NTRs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's see what has to be researched and done today (you can guesstimate person-hours and equipment expenses required, and multiply it by 2.5 to allow for your optimism):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fuel elements together with cladding resistant to the hellish combination of high neutron flux and hot hydrogen&lt;/li&gt;&#xA;&lt;li&gt;A new test rig has to be built at the Nevada test site without making the US look like a violator of test ban treaties&lt;/li&gt;&#xA;&lt;li&gt;Hydrothermoneutronic simulation codes must be re-validated&lt;/li&gt;&#xA;&lt;li&gt;A fueling, test and integration facility should be built at Cape Canaveral&lt;/li&gt;&#xA;&lt;li&gt;A dedicated fuel production facility&lt;/li&gt;&#xA;&lt;li&gt;An engine assembly line&lt;/li&gt;&#xA;&lt;li&gt;NTR control software and hardware&lt;/li&gt;&#xA;&lt;li&gt;The whole suite of vibration, radiation damage, electromagnetic interference, drop, abort etc. etc. tests should be conducted&lt;/li&gt;&#xA;&lt;li&gt;Environmental impact statements should be prepared and filed for all of the above activities and facilities&lt;/li&gt;&#xA;&lt;li&gt;A stringent quality control program must be established bridging the efforts of NASA and DoE.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are a few concrete sums scattered across the NTRS, but I'd be wary of trusting them blindly. There are also many unknowns, from the number of firings required to the technology to manufacture fuel elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yTNF3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Resources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;NUCLEAR THERMAL ROCKET (NTR) PROPULSION: &quot;BEFORE THE DECADE IS OUT&quot;. S. K. Borowski, M. R. Lapointe, M. G. Houts, J. W. Warren. LPI NETS-2012. &lt;a href=&quot;http://lpi.usra.edu/meetings/nets2012/pdf/3088.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://lpi.usra.edu/meetings/nets2012/pdf/3088.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Videos:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;(alleged testing of a cermet fuel element under load) &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ue9VLdqngLw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-07-15T06:36:11.857" LastActivityDate="2015-07-15T06:36:11.857" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4824" PostTypeId="2" ParentId="4792" CreationDate="2014-06-19T06:00:24.463" Score="11" Body="&lt;p&gt;It would be highly problematic for reaction wheels to serve dual purposes as reaction control devices and energy storage mechanisms. It might be possible, but the implementation would be extremely complicated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starting with a simpler case of a three-wheel design, a desired spacecraft attitude and/or slew rate uniquely determines the wheel speed. That is, given a set of initial conditions that include spacecraft attitude and current wheel speed, the wheel speeds at the desired attitude and rate is completely determined. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means reaction wheels would make crummy energy storage devices, because if you want to control attitude you wouldn't be able control how much energy was stored in the wheels, and conversely if you needed to store or use energy, it would change the spacecraft orientation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps if the spacecraft was not 3-axis controlled that would be acceptable. Likewise, with enough reaction wheel redundancy you might have enough degrees of freedom to control energy storage and attitude simultaneously, but the math involved sounds like Ph. D work to me. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, batteries aren't really that expensive, supply very clean power, have high energy density, are reliable, and can be recharged with solar arrays very efficiently. In contrast, wheels tend to be expensive because they have to be precisely balanced and have moving parts; this also causes wheels to have generally lower reliability than solid-state subsystems. I'm not inclined to replace a battery with a wheel. &lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="4660" LastEditDate="2014-11-11T09:38:48.720" LastActivityDate="2014-11-11T09:38:48.720" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4825" PostTypeId="2" ParentId="4792" CreationDate="2014-06-19T08:28:19.413" Score="5" Body="&lt;p&gt;When you have two reaction-wheels on the same axis, you can accelerate one in one direction and the other in the opposite direction. The net torque would be 0, but you would have energy stored in both. When you then want to still use them as reaction-wheels, you just have to transfer momentum from one of the wheels to the other to get a net torque.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, keep in mind that it is mechanically impossible to create a flywheel which is completely free of friction, even in microgravity. That means that this contraption is only suitable for short-term energy storage.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="195" LastEditDate="2014-11-10T23:55:40.340" LastActivityDate="2014-11-10T23:55:40.340" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4826" PostTypeId="1" AcceptedAnswerId="4828" CreationDate="2014-06-19T08:39:01.963" Score="13" ViewCount="3821" Body="&lt;p&gt;I hear a lot of talk about SSTO, and it got me to wonder, what's so good about them?&#xA;Doesn't it make sense to get rid of parts that you no longer need for a mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way I see it, if it emptied a tank of fuel, I don't want to be carrying it with me, so I dump it (hopefully in such a way that it can be reused in the future).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I ask, what are the advantages of SSTOs?&lt;/p&gt;&#xA;" OwnerUserId="4619" LastActivityDate="2016-05-28T16:57:55.713" Title="What are the advantages of SSTO craft over Multi-stage crafts?" Tags="|ssto|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4827" PostTypeId="1" AcceptedAnswerId="4853" CreationDate="2014-06-19T09:09:17.987" Score="10" ViewCount="3665" Body="&lt;p&gt;36 years after launch we still communicate with &lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html&quot;&gt;Voyager&lt;/a&gt; and &lt;a href=&quot;http://www.planetary.org/blogs/jason-davis/2014/20140606-decades-silence-isee.html&quot;&gt;ISEE&lt;/a&gt;. And contact with the Voyagers could continue for another 10 years or so. I doubt that it was designed with this in mind (but who knows about those NASA nerds). Has the New Horizons probe past Pluto been designed for an extraordinarily long lifetime?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could we, and would it be rational to, design future probes for very long lifetimes? For example a century. What instruments would be of interest on long timescales, and what functionality is easiest to preserve? Consider this also for probes like the ISEE which remain in the inner Solar system with the benefits of solar energy and Earth communication.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-09-07T12:43:53.953" Title="How long can probes function, e.g. New Horizons? Can and should we build &quot;century probes&quot;?" Tags="|probe|long-duration|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4828" PostTypeId="2" ParentId="4826" CreationDate="2014-06-19T09:25:11.350" Score="12" Body="&lt;p&gt;The idea behind a SSTO craft is that it can not just get to orbit in one piece, but also back to the ground, refuel and start again. That means after the initial investment of building the SSTO launch system, a launch only costs you the fuel (theoretically - practically you will always have some components which will need to be replaced more or less regularly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a multi-stage craft, the spent stages usually sink into the ocean, disintegrate in the atmosphere or end up in a graveyard orbit which means that they can only be used once and need to be rebuild from scratch. Each launch has the price-tag of building the whole launch system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A middle-ground between both methods is a multi-stage launch system where the spent stages can be recovered, refueled and reused. This seems to combine the advantages of both methods - no wasted hardware and no dead weight into orbit. But it is technically a lot more complex to do. The Space Shuttle was the first launch-system which tried this. Its solid rocket boosters were recovered and refueled. Unfortunately the execution was flawed: Refilling the boosters with solid fuel was almost as expensive as rebuilding them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A newer system which tries this is the reusable version of the Falcon 9. &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX_reusable_launch_system_development_program&quot;&gt;SpaceX is currently developing a lower stage which returns to the launch-site autonomously&lt;/a&gt;. The system uses liquid-fuel, which is a lot cheaper and a lot easier to get into the booster than solid-fuel. Also, because the stage returns to the launch-site on its own power, an expensive recovery-operation becomes unnecessary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The downside compared to an expendable launch system is that the lower stage needs to conserve some of its fuel for returning and landing, which means less fuel for the payload. The downside compared to an SSTO is that each stage must be a full-fledged craft capable of autonomous navigation, maneuvering and landing which inevitably means that lots of systems will be redundant.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-06-19T12:29:29.637" LastActivityDate="2014-06-19T12:29:29.637" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4830" PostTypeId="2" ParentId="4826" CreationDate="2014-06-19T13:53:08.333" Score="3" Body="&lt;p&gt;A distinction should be made between 'Single-Stage Orbiters' and REUSEABLE single stage orbiters.  The Space Systems/Loral company investigated a design about 15 years ago called 'Aquarius', that would have put a metric ton &lt;em&gt;and itself&lt;/em&gt; into low orbit.  It was to be ocean launched and so would have had zero launch facility costs.  Each launch would have cost $3 Million. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Think about it: a rocket launching a ton of payload that arrives in orbit fully intact with power and RCS systems still operational.  Shuttle cost a billion dollars to launch.  That would have paid for 330 Aquarius launches, delivering a cumulative payload of 300 tons - and 660 potentially habitable propellant tanks. I leave it to the reader to imagine the possibilities. . .      &lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2014-06-19T13:53:08.333" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4832" PostTypeId="1" CreationDate="2014-06-19T20:37:40.563" Score="4" ViewCount="769" Body="&lt;p&gt;It is said that a launch to orbit requires a Delta V of 9.3–10 km/s, and this encompasses orbital speed, air drag, gravity drag, and the energy to raise altitude to low Earth orbit.  But I'm troubled by vastly different estimates of the amount of Delta V that goes to the altitude term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To demonstrate, consider the two obvious physical models and the numbers they produce.  First, we have a perfectly vertical &quot;toss&quot; of a rocket, like a sounding rocket or a suborbital flight.  Second, we can ignore Earth's atmosphere and imagine two burns to reach LEO. Assuming 200 km orbit:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;$v\approx \sqrt{2 h a} $, for about 1.98 km/s, or use the more accurate $GM/r$ potential for &lt;strong&gt;1.95 km/s&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;With the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Hohmann Transfer formula&lt;/a&gt;,  get $\Delta v_1$ and $\Delta v_2$ to be 61 m/s and 60 m/s, for a grand total of 121 m/s, or &lt;strong&gt;0.121 km/s&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;That's a big difference there.  For an actual rocket launch, what is the appropriate mental model?  On the one hand, the early parts of the launch aren't very close to the horizontal.  But on the other hand, the characteristic height of our atmosphere is like 8 km, which is much lower than the altitude of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one were to give a ballpark figure for how much of the delta v budget goes into raising the altitude, what would be a good estimate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, does the burn even last for half an orbit?  How Hohmann-ish is the climb to orbit?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="49" LastEditDate="2014-06-20T02:16:11.910" LastActivityDate="2014-06-20T11:44:23.553" Title="Is an orbital launch more like a vertical climb or a Hohmann Transfer?" Tags="|launch|delta-v|altitude|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4833" PostTypeId="2" ParentId="4832" CreationDate="2014-06-20T02:40:37.723" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is an orbital launch more like a vertical climb or a Hohmann Transfer?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is a false dilemma. Neither is correct. Both of your calculations are wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your vertical climb interpretation ignores that the velocity at the end of the launch needs to be horizontal and about 7.8 km/s. Your Hohmann transfer interpretation ignores that the velocity at the start of the launch is not a horizontal velocity of about 7.8 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a non-rotating, atmosphere-free Earth and impulsive delta Vs, your vertical climb can be fixed by adding a 7.784 km/s delta V on reaching the maximum height. This gives a total delta V of &lt;a href=&quot;http://www.wolframalpha.com/input/?i=sqrt%28%28earth+geopotential%29%2F%286378+km%29%29%2a%28sqrt%282%2a200%2F6578%29%2Bsqrt%286378%2F6578%29%29&quot; rel=&quot;noreferrer&quot;&gt;9.733 km/s&lt;/a&gt; and an elapsed time of a bit over 4 minutes. Your Hohmann transfer can be fixed by adding a 7.905 km/s delta V to your initial impulsive burn. This gives a total delta V of &lt;a href=&quot;http://www.wolframalpha.com/input/?i=sqrt%28%28earth+geopotential%29%2F%286378+km%29%29%2a%28sqrt%28%281%2B200%2F6378%29%2F%281%2B100%2F6378%29%29+%2B+%28sqrt%28%281%2F%281%2B200%2F6378%29%29%29+-+sqrt%281%2F%28%281%2B200%2F6378%29%2a%281%2B100%2F6378%29%29%29%29%29&quot; rel=&quot;noreferrer&quot;&gt;8.027 km/s&lt;/a&gt; and an elapsed time of a bit less than 90 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with both of the above calculations are the assumptions of a non-rotating, atmosphere-free Earth and impulsive delta Vs. A real launch takes about 8 to 11 minutes to complete. Thrusting is finite and more or less continuous, and the trajectory gradually turns from purely vertical at launch to purely horizontal at orbit insertion.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-06-20T11:44:23.553" LastActivityDate="2014-06-20T11:44:23.553" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4836" PostTypeId="2" ParentId="2738" CreationDate="2014-06-20T20:30:53.770" Score="0" Body="&lt;p&gt;From a strictly physical point of view, yes, we could colonize Ceres.  Why would we want to?  Colonizing any object in space is more of an economics and political issue than a material or physical issue.  Human missions to Ceres will cost billions of dollars and there needs to be a justification for it that is acceptable to the prevailing culture.  And cultures change.  What would motivate the general public or private investors to colonize any place, on Earth or in space? Answer that question and you will see the future of Ceres - or not.&lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2014-06-20T20:30:53.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4837" PostTypeId="1" AcceptedAnswerId="4838" CreationDate="2014-06-20T21:04:53.580" Score="5" ViewCount="12473" Body="&lt;p&gt;What are the equations used to calculate rocket thrust, acceleration, and weight? Obviously this depends on the type of fuel and engine and a lot of other factors, but I'm interested in a simplified ideal case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, a solid rocket booster would provide a constant force thrust, but it would get lighter as it burns up its fuel, so it's thrust-to-weight-ratio would change mid flight, and acceleration would increase.&lt;/p&gt;&#xA;" OwnerUserId="23" LastActivityDate="2016-10-24T15:35:33.023" Title="How to calculate rocket acceleration?" Tags="|rockets|propulsion|thrust|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4838" PostTypeId="2" ParentId="4837" CreationDate="2014-06-20T21:46:46.773" Score="7" Body="&lt;p&gt;This is for idealized case (see below), but &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt; should have you covered:&lt;/p&gt;&#xA;&lt;h1&gt;&lt;span class=&quot;math-container&quot;&gt;$$\Delta v = v_\text{e} \ln \frac {m_0} {m_1}$$&lt;/span&gt;&lt;/h1&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;where:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$m_0$&lt;/span&gt; is the initial total mass, including propellant,&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$m_1$&lt;/span&gt; is the final total mass,&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$v_\text{e}$&lt;/span&gt; is the effective exhaust velocity,&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; is delta-v - the maximum change of velocity of the vehicle (with no external forces acting),&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$\ln$&lt;/span&gt; refers to the natural logarithm function.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There's many online &lt;em&gt;rocket equation&lt;/em&gt; calculators to simplify this for you, for example this &lt;a href=&quot;http://www.strout.net/info/science/delta-v/intro.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta-V Calculator&lt;/a&gt;, and you can find many more on &lt;a href=&quot;http://www.projectrho.com/rocket/calculators.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atomic Rockets Online Calculators&lt;/a&gt; page.&lt;/p&gt;&#xA;&lt;p&gt;And average &lt;a href=&quot;https://en.wikipedia.org/wiki/Acceleration#Definition_and_properties&quot; rel=&quot;nofollow noreferrer&quot;&gt;acceleration&lt;/a&gt; over a period of time is then:&lt;/p&gt;&#xA;&lt;h1&gt;&lt;span class=&quot;math-container&quot;&gt;$$\boldsymbol{\bar{a}} = \frac{\Delta \mathbf{v}}{\Delta t}$$&lt;/span&gt;&lt;/h1&gt;&#xA;&lt;p&gt;Effective exhaust velocity is mostly given in &lt;a href=&quot;https://en.wikipedia.org/wiki/Exhaust_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;Specific Impulse&lt;/a&gt; in:&lt;/p&gt;&#xA;&lt;h1&gt;&lt;span class=&quot;math-container&quot;&gt;$$I_{\rm sp}=\frac{v_{\rm e}}{g_{\rm 0}}$$&lt;/span&gt;&lt;/h1&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;where:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$Isp$&lt;/span&gt; is the specific impulse measured in seconds&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$v_{\rm e}$&lt;/span&gt; is the average exhaust speed along the axis of the engine (in ft/s or m/s)&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$g_0$&lt;/span&gt; is the acceleration at the Earth's surface (in ft/s2 or m/s2).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;And &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust&quot; rel=&quot;nofollow noreferrer&quot;&gt;thrust&lt;/a&gt; is:&lt;/p&gt;&#xA;&lt;h1&gt;&lt;span class=&quot;math-container&quot;&gt;$$T=v_{\rm e}\frac{\Delta m}{\Delta t}$$&lt;/span&gt;&lt;/h1&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;where:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$T$&lt;/span&gt; is the thrust generated (force),&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$\frac{\Delta m}{\Delta t}$&lt;/span&gt; is the rate of change of mass with respect to time (mass flow rate of exhaust), and&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$v_{\rm e}$&lt;/span&gt; is the speed of the exhaust gases measured.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;You can derive your own equations out of these ones, based on the data that you have available.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Do note that, as mentioned previously, this is idealized case and in reality none of your input data will remain constant during entire flight, not merely T/W ratio. If, for example, you have a thrust profile as a function of time available, these same equations apply, but you'll have to recalculate for each change in thrust, which will give you other graphs, e.g. mentioned T/W ratio as a function of time, and so on. And there's variables that will change with altitude, velocity (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_coefficient&quot; rel=&quot;nofollow noreferrer&quot;&gt;drag coefficient&lt;/a&gt;), attitude and even time since some specific event (e.g. propellants boil-off rate, grain geometry of solid motors as mentioned by Adam in the comments, fuel-to-oxidizer mixture ratio, thermal expansion, ad nauseum,...). The list of variables is in reality infinite, and it's due to these that rocket science isn't as easy as ABC, as these few listed basic equations might suggest. The list of all equations that you might need is in reality unmanageably long.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-06-21T20:02:59.190" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4839" PostTypeId="2" ParentId="4672" CreationDate="2014-06-20T22:35:52.207" Score="3" Body="&lt;p&gt;It was probably your tour company that cancelled the trip. I was there, along with 16 other people, on a trip through Smithsonian Journeys and the Harvard Alumni Association, although the trip was actually conducted by Mir in Seattle. We toured Star City, then flew down to Baikonur on a chartered Roskosmos flight and saw the launch. Our group was at the Cosmodrome when the cosmonauts came out in their Sokol launch suits, and boarded the bus to the launch pad. I have video I shot of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/pRyBZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The three-strong crew prepare for takeoff ahead of their successful mission to the International Space Station (source: &lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2642038/Spaceship-3-man-crew-docks-space-station.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;MailOnline&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the above photo, I was on the left side of the cosmonauts in a large group of visitors and tourists. We were also at the observation area, 1.2 km from the launch pad when the rocket went up at 1:52 AM. Sorry to hear that you missed the trip. Hopefully you had trip insurance! Mir is running a trip again in December.&lt;/p&gt;&#xA;" OwnerUserId="4630" LastEditorUserId="49" LastEditDate="2014-06-20T23:23:26.587" LastActivityDate="2014-06-20T23:23:26.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4840" PostTypeId="1" CreationDate="2014-06-21T07:24:15.250" Score="2" ViewCount="733" Body="&lt;p&gt;I was just watching a story on TV about a man who tells a story of abduction (your basic run-of-the-mill abduction story) and was thinking about how every time someone explains these stories, the aliens have legs and arms and are generally upright beings similar to humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, the odds of a planet being similar to Earth and possible to sustain life in a human form are pretty rare (I know that it's not impossible by any means, particularly with the billions and billions of planets out there, but still slim nonetheless). Second, on top of that, the added odds of life evolving to the point where it develops a similar intelligence and body structure have to be even more astronomically rare. And on top of that, them making contact with our planet as opposed to any other planet adds less odds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now obviously you can tell I don't believe any of these abduction/alien encounter stories but I've never heard anyone attempt to debunk them in this way (or at least present a logical probability of it happening). Has anyone heard of this kind of explanation of why the odds are so slim of this ever happening to anyone? If not, anyone have a ballpark figure? I understand the possibility is there (even if it is in the 1 in 1 trillion+) but come on now...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps this is better suited for the skeptics forum but I figure you all have more of a scientific background that would help rationally explain this. I'm sorry if I'm posting in the wrong place, just let me know and I can move it. Thanks everyone!&lt;/p&gt;&#xA;" OwnerUserId="3423" LastActivityDate="2014-06-22T02:08:47.433" Title="Logical Debunking of Alien Abduction/Encounters" Tags="|life|astrobiology|" AnswerCount="1" CommentCount="9" ClosedDate="2014-06-22T02:08:57.203" ContentLicense="CC BY-SA 3.0" />
  <row Id="4841" PostTypeId="2" ParentId="4840" CreationDate="2014-06-21T11:15:16.917" Score="3" Body="&lt;p&gt;Before I answer this I want to say I don't believe any of the alien abduction stories either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that being said, the reason no one debunks them using statistics of likelihood evolution of life etc., is &lt;a href=&quot;http://en.wikipedia.org/wiki/Drake_equation&quot; rel=&quot;nofollow&quot;&gt;Drake's equation&lt;/a&gt;. Put simply Drakes equation calculates the probability of life on other planets, and shows that irrespective of what coefficients you use for probability that: stars have planets, planets are in the goldilocks zone, life evolves etc., you're pretty much guaranteed that there is other life out there. Mainly because of the ridiculously high number of stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the second logical debunking method: so there's life out there but why would it look like us? It almost certainly wouldn't look exactly like us, but there is a theory that due the the process of evolution the species that survives to evolve into the most intelligent (e.g. humans) will likely have similar appearance, this is called &lt;a href=&quot;http://en.wikipedia.org/wiki/Parallel_evolution&quot; rel=&quot;nofollow&quot;&gt;parallel evolution&lt;/a&gt;. For example if we make the assumption that the alien brain will be physically similar to ours, then it'll likely need to be around the same size. It also logical that the brain will have any 'eyes', 'ears' etc. close to it to allow for faster data transfer to the brain so it can make quicker decisions about the environment. There's &lt;a href=&quot;http://en.wikipedia.org/wiki/Parallel_evolution#Examples&quot; rel=&quot;nofollow&quot;&gt;evidence&lt;/a&gt; of this type of parallel evolution on our planet so this is a reasonably solid argument.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for why they would pick out planet to visit, we've been &lt;a href=&quot;http://en.wikipedia.org/wiki/Arecibo_message&quot; rel=&quot;nofollow&quot;&gt;sending out radio waves&lt;/a&gt; for some time now. We also have a lot of objects in orbit around the planet. Lets not delve into the sci-fi of what hardware and science the aliens have, but it's easy to say they're advanced enough to 'find' us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that's the reason why these theories aren't debunked off the cuff. However there is some recent discussion about the &lt;a href=&quot;http://www.technologyreview.com/view/424795/probability-of-et-life-arbitrarily-small-say-astrobiologists/&quot; rel=&quot;nofollow&quot;&gt;utility of drakes equation&lt;/a&gt;, and applying Bayesian maths shows the likelihood to be much lower than expected especially this early on in the universe. There's also some &lt;a href=&quot;http://en.wikipedia.org/wiki/Speed_of_light&quot; rel=&quot;nofollow&quot;&gt;fundamental principals of physics&lt;/a&gt; which we haven't yet managed to break, that would massively increase the travel time for these aliens or make it impossible for them to have heard our radio waves unless they were already quite close. Other reasons against it become almost as sci-fi as the alien abductions themselves, so I wont go any further.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-06-21T11:36:46.803" LastActivityDate="2014-06-21T11:36:46.803" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4842" PostTypeId="1" AcceptedAnswerId="5489" CreationDate="2014-06-21T14:15:14.243" Score="4" ViewCount="317" Body="&lt;ul&gt;&#xA;&lt;li&gt;How far has the MOM (Mars Orbiter Mission) travelled, and &lt;/li&gt;&#xA;&lt;li&gt;Where is it now? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It is scheduled to reach Mars orbit on September 1 this year.&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="288" LastEditDate="2014-06-21T19:49:30.117" LastActivityDate="2014-10-01T19:41:26.657" Title="Is it possible to know the position of MOM?" Tags="|mars|isro|mars-orbiter-mission|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4843" PostTypeId="1" AcceptedAnswerId="4852" CreationDate="2014-06-21T15:41:51.560" Score="17" ViewCount="1360" Body="&lt;h2&gt;Lunar Lithobraking&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;If your goal is simply to deliver a science package somewhere on the moon, might it be easier and cheaper to design the package to survive lunar impact? As an example, consider the Lunar X Prize, with the goal to land a device that can transmit HD video back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question has two parts.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Is it possible to engineer a package to survive lunar impact intact?&lt;/li&gt;&#xA;&lt;li&gt;Would it be cheaper (in mass, Δv, and/or $) to use this method of landing over retrorockets?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ClCBf.jpg&quot; alt=&quot;Lunar Impact&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Thoughts&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Advantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Much lower mission Δv (-3km/s?).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Disadvantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;High peak force on package during lithobraking. &lt;/li&gt;&#xA;&lt;li&gt;Lower precision on final destination.&lt;/li&gt;&#xA;&lt;li&gt;Dust thrown by impact may cover package.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Could use spent rocket and fuel tank from trans-lunar injection as a crumple zone to reduce peak deceleration. Springs, airbags, foam, etc could further lengthen impact time, reducing shock.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Research&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pages.citebite.com/s3e3l8g4j0poy&quot; rel=&quot;noreferrer&quot;&gt;Ranger Program – NASA's balsa wood moon crasher&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://news.google.com/newspapers?nid=1893&amp;amp;dat=19611023&amp;amp;id=5K8fAAAAIBAJ&amp;amp;sjid=LdcEAAAAIBAJ&amp;amp;pg=667,1724883&quot; rel=&quot;noreferrer&quot;&gt;Ranger Program – newspaper images&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pages.citebite.com/k3u3m8r6d5naa&quot; rel=&quot;noreferrer&quot;&gt;Airbag landers scale as v^2 while rockets scale as v.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4720/what-is-the-marginal-cost-of-landing-on-the-moon&quot;&gt;SSE: Marginal Cost of landing on Moon&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4602" LastEditorUserId="4602" LastEditDate="2014-06-23T13:33:26.603" LastActivityDate="2014-06-23T13:33:26.603" Title="Is lunar lithobraking 1. survivable and 2. cheaper than a rocket landing?" Tags="|lunar-landing|lander|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4845" PostTypeId="1" CreationDate="2014-06-21T16:32:57.967" Score="3" ViewCount="400" Body="&lt;p&gt;Years ago space settlement advocates were working out ambitious schemes for orbiting colonies housing 10,000 people.  They were to be built of lunar-derived materials and would have been occupied primarily with building dozens of huge power satellites.  Given the known resources on the Moon, and its advantageous location for interplanetary flights, has there ever been a design for a LUNAR-BUILT spacecraft to go to Mars?   &lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2014-06-23T07:32:27.750" Title="Lunar-built Spacecraft" Tags="|spacecraft|orbital|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4846" PostTypeId="1" CreationDate="2014-06-21T19:03:11.447" Score="8" ViewCount="384" Body="&lt;p&gt;Yesterday (June 20, 2014), SpaceX intended to launch a Falcon 9 to bring 6 satellites of the Orbcomm OG2 satellite constellation into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the launch was first postponed for an hour and then, 10 minutes before the new liftoff time, aborted. This wasn't the first time there were problems with this particular mission. It was already postponed twice, once by SpaceX and once by the customer Orbcomm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why exactly did the team decide to abort the launch this time, and was it related to the issues they had before?&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="49" LastEditDate="2014-06-26T22:55:38.330" LastActivityDate="2014-07-01T02:41:02.803" Title="What went wrong during the OG2 Mission 1 launch attempt on June 20th 2014?" Tags="|spacex|falcon-9|abort|launch-sequence|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4847" PostTypeId="2" ParentId="4846" CreationDate="2014-06-21T21:15:55.733" Score="7" Body="&lt;p&gt;From &lt;a href=&quot;http://www.orbcomm.com/networks/og2-launch&quot; rel=&quot;nofollow&quot;&gt;Orbcomm OG2 Mission 1 Update&lt;/a&gt; (June 20, 2014): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During today’s countdown, engineers noticed fluctuations in pressure&#xA;  on the Falcon 9 vehicle’s second stage and are taking additional time&#xA;  to evaluate. The next available launch opportunity is tomorrow&#xA;  evening, Saturday, June 21st. Additional updates will be posted as&#xA;  schedules are confirmed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;http://www.spaceflight101.com/orbcomm-g2-launch-1---falcon-9-launch-updates.html&quot; rel=&quot;nofollow&quot;&gt;Spaceflight 101&lt;/a&gt; put it like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Late in the countdown, a pressure drop on the rocket's second stage&#xA;  was noticed during regular pressure checks leading to a scrub of the&#xA;  launch when the team ran out of launch window time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As also described in &lt;a href=&quot;http://www.spaceflightnow.com/falcon9/010/140620scrub/&quot; rel=&quot;nofollow&quot;&gt;Spaceflight Now article: Delay-stricken SpaceX launch scrubbed by technical issue&lt;/a&gt;, this has however already happened earlier in the countdown, shortly before beginning the terminal countdown for liftoff at the beginning of the launch window and the countdown clock was recycled with the next launch attempt at the end of the roughly one hour long launch window. Problems were not resolved before the last station polling for the day, and the launch was scrubbed with the next launch opportunity one day later, June 21.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Weekend launch opportunities on June 21 - June 22 were also scrubbed. Saturday one for the weather, and Sunday one &lt;a href=&quot;http://www.spacenews.com/article/launch-report/41050spacex%E2%80%99s-shotwell-orbcomm-launch-not-likely-before-july-14&quot; rel=&quot;nofollow&quot;&gt;due to problems with thrust vector control actuator on the first stage&lt;/a&gt;, according to Gwynne Shotwell, SpaceX president and COO, mentioning this during &lt;a href=&quot;http://thespaceshow.com/detail.asp?q=2268&quot; rel=&quot;nofollow&quot;&gt;John Batchelor Show / The Space Show Special&lt;/a&gt; (roughly 3 minutes into the recording).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.orbcomm.com/networks/og2-launch&quot; rel=&quot;nofollow&quot;&gt;Orbcomm OG2 Mission 1 Update&lt;/a&gt; (June 23, 2014):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX is taking a closer look at a potential issue identified while&#xA;  conducting pre-flight checkouts during yesterday's [June 22] countdown. SpaceX&#xA;  will stand down Tuesday, June 24 while our engineering teams evaluate&#xA;  further, which will also allow the Range to move forward with&#xA;  previously scheduled maintenance. We are currently targeting the first&#xA;  week of July and will work with the Range to confirm the next&#xA;  available launch opportunities.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-07-01T02:41:02.803" LastActivityDate="2014-07-01T02:41:02.803" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4848" PostTypeId="2" ParentId="4842" CreationDate="2014-06-22T02:33:24.967" Score="5" Body="&lt;p&gt;Download &lt;a href=&quot;http://eyes.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;Eyes on the Solar System&lt;/a&gt;.  Mars Orbiter Mission is one of the spacecraft tracked therein.  It looks to be a little behind MAVEN, also tracked.  The measuring tools can tell you how far away it is from anything else that's tracked, like the Sun, Earth, Mars, and MAVEN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clicking on additional information button in its toolbar, it displays an external link to &lt;a href=&quot;http://www.isro.org/mars/updates.aspx&quot; rel=&quot;nofollow&quot;&gt;ISRO's Mars Orbiter Mission page&lt;/a&gt;, with regular mission updates. Currently latest is from ten days ago as of writing this answer (12-06-2014):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At present, the radio distance between the Spacecraft and the Earth is&#xA;  102 million km. A radio signal from the Earth to the Spacecraft now&#xA;  takes about 340 seconds. The spacecraft so far has traveled a distance&#xA;  of 466 million km as part of its total Journey of 680 million km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For current distance to Earth, you can also use &lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow&quot;&gt;DSN Now&lt;/a&gt; online tool and click on the DSN (Deep Space Network) station that tracks MOM at that time. When available, the range is displayed in the right column.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-06-22T03:22:44.337" LastActivityDate="2014-06-22T03:22:44.337" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4849" PostTypeId="2" ParentId="4843" CreationDate="2014-06-22T03:04:10.773" Score="8" Body="&lt;ol&gt;&#xA;&lt;li&gt;No&lt;/li&gt;&#xA;&lt;li&gt;Yes&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Though 1. depends on your &quot;science package&quot;.  If your &quot;science package&quot; is simply a large chunk of mass whose purpose is to vaporize and eject water from the lunar regolith (see &lt;a href=&quot;http://www.nasa.gov/mission_pages/LCROSS/main/index.html&quot; rel=&quot;noreferrer&quot;&gt;LCROSS&lt;/a&gt;), then yes, it will &quot;survive&quot; in the sense of serving its purpose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not aware of any penetrator designs with actual electronic science instruments and telecommunications equipment that have been shown to survive 2 to 3 km/s impacts, which would be required for an unassisted impact on the Moon.  Penetrators are designed for hundreds of m/s, so you would need a rocket to remove about 99% of the energy before you could expect even a penetrator designed specially for that purpose to survive.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-06-22T03:04:10.773" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="4850" PostTypeId="2" ParentId="4845" CreationDate="2014-06-22T16:59:54.457" Score="1" Body="&lt;p&gt;The advantage of building on the moon is that it is a shallower gravity well than earth, so requires less propulsion to get to Mars (or elsewhere).  The major disadvantage is that (for the foreseeable future) it is very hard to operate there-there is not infrastructure.  Remote operations are extremely difficult under these conditions.  If we get to the point of an infrastructure on the moon appropriate to build a spacecraft, technology will have advanced so much that any design done now would be useless.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-06-22T16:59:54.457" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4851" PostTypeId="2" ParentId="4845" CreationDate="2014-06-23T07:32:27.750" Score="3" Body="&lt;p&gt;There have been lots of plans for Lunar built spacecraft. There have been few that have been credible, and all are in the realm of speculation.&lt;/p&gt;&#xA;&lt;p&gt;At present, any such plans are relegated to either distant conceptual plans (NASA, ESA, and ФКА/Roscosmos), fuel processing only (NASA and ESA), or science fiction literature.&lt;/p&gt;&#xA;&lt;h1&gt;Distant Conceptual Plans&lt;/h1&gt;&#xA;&lt;p&gt;NASA, ESA, and Roscosmos all have had conceptual plans for lunar construction and lunar bases since inception. None of these are even to the practical testing stage. NASA is funding some specific technological testing by college students in a variety of fields; I don't know about ESA nor Roscosmos testing towards it.&lt;/p&gt;&#xA;&lt;p&gt;NASA has recently had some funding that lead to testing of the ability to seal up natural lava tubes into pressure habitat structures. Likewise, they've made chemically comparable ground regolith and tested it for cement and concrete usages. These are extremely early steps towards the &amp;quot;eventual&amp;quot; lunar construction base.&lt;/p&gt;&#xA;&lt;p&gt;The actual plans themselves are little better than science-based whimsy - without major changes to the funding, none of them are likely to see any major progress.&lt;/p&gt;&#xA;&lt;h1&gt;Fuel Only plans&lt;/h1&gt;&#xA;&lt;p&gt;A number of NASA plans involve automated harvesting of lunar water to make O2 and H2, then shipping that fuel to an orbit assembled spacecraft for orbital fueling. By launching with tanks dry, the mass of the booster stage can be much reduced, allowing for a larger tank size per unit, and fewer earth to LEO missions to assemble the final martian mission; similar units would refuel the lander at mars, and possibly also the transfer vehicle.&lt;/p&gt;&#xA;&lt;p&gt;Of the various plans, these, which don't actually meet the question's narrowest scope, are the only ones likely to be used. The technology exists, the water exists, the robotics can be made to perform the needed tasks. The Mars missions on the drawing board are looking at various methods of getting fuel into orbit, and lunar water is one that is getting serious consideration.&lt;/p&gt;&#xA;&lt;h1&gt;Science Fiction&lt;/h1&gt;&#xA;&lt;p&gt;Pretty much all other variations are speculative fiction. Many science fiction stories include plausible human habitation on Luna, and manufacturing located at the lunar habitats. A few of these rise to credible plans. Most make assumptions about some form of fusion power and high efficiency electric thrusters, both of which are plausible but not yet implemented technologies.&lt;/p&gt;&#xA;&lt;p&gt;The best rise to or exceed the quality of what NASA has released, but unlike NASA, there is little chance of these being implemented.&lt;/p&gt;&#xA;&lt;h1&gt;Select references&lt;/h1&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/centers/marshall/news/news/releases/2009/09-083.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nasa.gov/centers/marshall/news/news/releases/2009/09-083.html&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.universetoday.com/106808/this-moon-rover-could-hunt-for-lunar-water-in-2018/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.universetoday.com/106808/this-moon-rover-could-hunt-for-lunar-water-in-2018/&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.esa.int/Our_Activities/Preparing_for_the_Future/Space_for_Earth/Energy/Helium-3_mining_on_the_lunar_surface&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.esa.int/Our_Activities/Preparing_for_the_Future/Space_for_Earth/Energy/Helium-3_mining_on_the_lunar_surface&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.dailygalaxy.com/my_weblog/2009/11/moon-water-will-lunarbase-humans-be-able-to-drink-it-a-weekend-special.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.dailygalaxy.com/my_weblog/2009/11/moon-water-will-lunarbase-humans-be-able-to-drink-it-a-weekend-special.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-06-23T07:32:27.750" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4852" PostTypeId="2" ParentId="4843" CreationDate="2014-06-23T08:31:41.803" Score="10" Body="&lt;p&gt;Fundamentally, all landers eventually use lithobraking - it's called touchdown, and the last few m/s are shed that way. Apollo lunar lander landing gear were design rated for a maximum of 5 feet per second vertical velocity at touchdown.&lt;/p&gt;&#xA;&lt;h1&gt;High Velocity Impact&lt;/h1&gt;&#xA;&lt;p&gt;Currently, artillery electronics can be hardened for short duration 30000 Gee accelerations according to public data; it's almost a certainty that the actual numbers are higher. Similar decelerations are acceptable provided the electronics are designed for that.&lt;/p&gt;&#xA;&lt;p&gt;Typical science packages are not going to be able to survive that. Certain science packages might be able to do so, and plowing into a 100m long skid (or penetration) from 300m/s is shedding 45kj/kg in 0.6 sec or so, and about 6900 G's.&lt;/p&gt;&#xA;&lt;p&gt;In order to survive this, special construction techniques are needed, and the types of experiments are limited severely.&lt;/p&gt;&#xA;&lt;h1&gt;Lithobreaking as Sole Method&lt;/h1&gt;&#xA;&lt;p&gt;The speeds needed for orbit are high enough that lithobraking is implausible for a a sole method even for the best hardened projectile electronics.&lt;/p&gt;&#xA;&lt;p&gt;For translunar or more distant missions, the speeds and energies are higher still, and a shallow graze would result in a skip into orbit or past the target.&lt;/p&gt;&#xA;&lt;h1&gt;Lithbreaking as final process&lt;/h1&gt;&#xA;&lt;p&gt;The Pathfinder Rover is considered to have used Lithobraking via it's airbag bounce landing. This was used to shed a 14 meters per second at 18 G peak impact - too high for human safety, but well within human survivability. And, since it performed multiple science activities, easily within the realm of delivery for science payloads.&lt;/p&gt;&#xA;&lt;p&gt;Similar systems could be used on the moon, albeit with much longer runs and higher bounces.&lt;/p&gt;&#xA;&lt;h1&gt;Price and Mass&lt;/h1&gt;&#xA;&lt;p&gt;The two competing issues are price and mass. For the Pathfinder rover, it was competetive; I've read (but cannot cite) that it was more expensive than rocket, but thought to be more likely to succeed. It was not more mass-efficient, but wasn't severely higher, and offered a number of other failure mode advantages.&lt;/p&gt;&#xA;&lt;p&gt;The system was not practical when scaled up for larger rovers - both mass and price resulted in a return to thrust based.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-06-23T08:31:41.803" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4853" PostTypeId="2" ParentId="4827" CreationDate="2014-06-23T11:57:15.060" Score="10" Body="&lt;p&gt;New Horizons has been designed for a lifetime of &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;20-25 years&lt;/a&gt;. New Horizons is powered by an RTG. Over time, this will lose power as the plutonium decays. &lt;a href=&quot;http://www.boulder.swri.edu/pkb/ssr/ssr-fountain.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Around 2030&lt;/a&gt; (PDF on New Horizons design), the RTG will no longer supply sufficient power to cover the spacecraft's needs even in its lowest-power cruise mode. For probes to the outer solar system, the power supply will be the limiting factor. You can design the power supply to last longer, you just have to add more plutonium to the RTG. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For probes that operate in the inner solar system where they can use solar panels, things get a bit easier. Solar panels degrade too, but not as quickly as an RTG. ISEE shows it's possible to keep a spacecraft running without batteries - but you'll have to plan the spacecraft's orbit to keep it out of shadow. A spacecraft that goes in and out of the shadow of a planet is subject to a deep thermal cycle, which tends to break things. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, spacecraft are subject to the slow destructive force of entropy. Systems will fail eventually. &lt;a href=&quot;https://space.stackexchange.com/questions/948/what-happens-to-defunct-lunar-and-mars-rovers-what-technological-barriers-preve&quot;&gt;This related question&lt;/a&gt; gives a detailed overview of the problems you encounter when designing a system for a very long lifetime. There comes a point where it's more cost-effective to launch a new satellite than to keep the old one operational. Advances in science also mean you'll eventually want to launch new, state-of-the-art instruments instead of having to make do with 100-year-old technology. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-06-23T11:57:15.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4855" PostTypeId="2" ParentId="1664" CreationDate="2014-06-24T02:23:48.117" Score="12" Body="&lt;p&gt;Spacecraft are designed to contain internal pressure of not more than one atmosphere; submarines are designed to withstand dozens of atmospheres of  external pressure. Furthermore, aerospace systems have very severe weight budgets - additional weight means additional fuel to power the engines to lift it, which itself adds weight... too much and the system will be unable to achieve its mission objective, or perhaps even get off the ground. Submersibles also have weight budgets, but they are nowhere as severe. The structural members of a spacecraft hull are predominantly operating in tension, at a significant fraction of the material's yield limits, and the most likely failure mode would be tensile fracture. The structural members of a submarine hull are predominantly operating in compression, and the most likely failure mode would be buckling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, the space and undersea environments are entirely different and present entirely different challenges to vehicle designers and structural engineers. So, no, the requirements are not at all similar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given all of the above, one could reasonably deduce that if one were to (hypothetically) seal up and submerge a space capsule, its structure would buckle (it would be crushed) at relatively shallow depth. Capsules such as Mercury, Gemini, and Apollo had sufficient bouyancy to float on the ocean surface. As long as the hatches and vents remained closed to prevent ingres of water, they could remain afloat indefinitely. They could only be submerged if there was opportunity for water to enter (and displaced air to escape). In such case there would be no pressure differential on the hull to crush it no matter how deep it sunk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to the flotation collars, they were needed to stabilize the capsule (keep them upright in the water) and ensure bouyancy after the hatch was opened.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2014-06-25T00:21:36.870" LastActivityDate="2014-06-25T00:21:36.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4856" PostTypeId="1" CreationDate="2014-06-24T04:20:22.283" Score="8" ViewCount="821" Body="&lt;p&gt;I have a question about Laplace angle-only method for orbit determination where the line of sight vectors are being interpolated. I read somewhere that the method fails (due to a matrix being singular) when the observation site is in the motion plane of the observed body, but failed to see proof for that. Could anyone explain to me why is that?&lt;/p&gt;&#xA;&lt;p&gt;The Laplace method could be found &lt;a href=&quot;http://repository.tamu.edu/bitstream/handle/1969.1/ETD-TAMU-2011-12-10242/SCHAEPERKOETTER-THESIS.pdf?sequence=2&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. In pages 8-12 it explains the method, and in page 12 it references to Escobal's book that should explain that, however I do not have access to that book.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Schaeperkoetter, Andrew Vernon (2011) Masters Thesis &lt;a href=&quot;https://oaktrust.library.tamu.edu/handle/1969.1/ETD-TAMU-2011-12-10242&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Comprehensive Comparison Between Angles-Only Initial Orbit Determination Techniques&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;From page 12:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Escobal (1) determines that this will only occur when the observation&#xA;site lies on the great circle of the satellite’s orbit.&lt;/p&gt;&#xA;&lt;p&gt;(1) ESCOBAL, P. R., Methods of Orbit Determination, John Wiley and Sons, Inc.,&#xA;New York, 1965.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4636" LastEditorUserId="12102" LastEditDate="2020-12-16T01:34:05.080" LastActivityDate="2020-12-24T15:29:33.210" Title="Laplace Method for Orbit Estimation" Tags="|orbital-mechanics|artificial-satellite|mathematics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="4858" PostTypeId="1" AcceptedAnswerId="4861" CreationDate="2014-06-25T14:10:50.120" Score="41" ViewCount="15119" Body="&lt;p&gt;It is touted by the media that space is becoming more dangerous and risky due to an increase in space debris. Why haven't more craft been hit by the debris? I presume that it is not at a critical level yet. Do the space agencies have avoidance techniques or are the strikes not reported/not critical?&lt;/p&gt;&#xA;" OwnerUserId="65" LastEditorUserId="709" LastEditDate="2014-06-26T14:33:09.250" LastActivityDate="2019-01-20T16:35:11.093" Title="Why haven't more spacecraft/satellites been hit by debris?" Tags="|spacecraft|debris|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4859" PostTypeId="1" CreationDate="2014-06-25T14:17:08.143" Score="10" ViewCount="387" Body="&lt;p&gt;I know there will always be a risk of a strike in space but what is being done to combat the man made risk of debris from finished satellites? I read about a &lt;a href=&quot;http://space.epfl.ch/page-87472-en.html&quot; rel=&quot;noreferrer&quot;&gt;Swiss Organisation&lt;/a&gt; that is launching a device to clean up one of their satellites that was finishing its life. Will there be law mandating countries cleanup after themselves (countries that launch can't come off the excuse that they are too poor to cleanup)? Do things like building in decay in the orbit of a satellite so that it will burn up when it is finished exist? Are there other ways of making space safer again?&lt;/p&gt;&#xA;" OwnerUserId="65" LastEditorUserId="65" LastEditDate="2014-06-25T22:44:59.563" LastActivityDate="2015-10-06T11:14:50.950" Title="What is being done to cleanup space of junk, particulary laws and litigation?" Tags="|debris|law|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4860" PostTypeId="2" ParentId="4858" CreationDate="2014-06-25T14:23:12.053" Score="17" Body="&lt;p&gt;The size of the various things that can collide are 'small' relative to the amount of 'space' available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Increase the surface area, increase the risk of collisions.  A &lt;a href=&quot;http://articles.latimes.com/1996-02-26/news/mn-40319_1_space-shuttle-columbia&quot;&gt;tether experiment on the shuttle&lt;/a&gt;, that was 12 miles long, tore, possibly due to a collision back in 1996.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar wings of the space station have recorded minor debris hits I think.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle windows have shown debris hits over the year, but very small ones that did not break through.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As gerrit comments, space is really big. Satellites, in general are very small in comparison.  The odds of two tiny things hitting each other in huge spaces is fairly low but not impossible.  Thus the low number of hits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, increase the number of objects, size of objects, and the probability goes up.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-06-25T14:23:12.053" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4861" PostTypeId="2" ParentId="4858" CreationDate="2014-06-25T16:33:08.703" Score="60" Body="&lt;p&gt;They are hit all the time. It just hasn't been critical.  So far. Looking at the outside portion of the Hubble WFPC2 camera after it was returned, I saw a huge number of large and small pits in the paint and underlying aluminum from debris hits. &lt;a href=&quot;http://airandspace.si.edu/explore-and-learn/multimedia/detail.cfm?id=6572&quot; rel=&quot;noreferrer&quot;&gt;Below is an image of that surface&lt;/a&gt; after all of those sites were cored out for analysis. Note that the cores are much larger than the size of the damaged areas, but it at least gives you an idea of the number of hits.  It was on orbit for 15 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mxXCI.jpg&quot; alt=&quot;WFPC2 after coring&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-06-25T16:33:08.703" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4862" PostTypeId="2" ParentId="4859" CreationDate="2014-06-25T17:26:04.850" Score="8" Body="&lt;p&gt;There is no law per se, since there is no way to enforce it.  There are general guidelines and ideas in place that pretty much everyone follows.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Geosync satellites have a reserve of fuel allocated to boost higher or lower in orbit at end of life, to preserve the slots safety.&lt;/li&gt;&#xA;&lt;li&gt;Second stages are not supposed to be pressurized after use, and efforts made to prevent them from exploding (A common occurrence in the past, and source of much debris).&lt;/li&gt;&#xA;&lt;li&gt;LEO satellites should be able to either a) demonstrate that they will de-orbit naturally within a certain time frame (Much easier than in GEO) b) de-orbit themselves over an ocean if there is a risk of landfall in-tact c) re-orbit to a disposal zone (a higher orbit).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is a rough distillation of the IADC guidelines, not everyone abides by them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, it is much too late for some of the longer lived debris, and the Chinese ASAT test did not help. Nor did the collision of the Iridium and Russian satellite.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many thoughts on how to clean up, beyond waiting for stuff to deorbit.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ElectroDynamic Tethers that can attach to deorbit, either &lt;a href=&quot;http://www.tethers.com/tt.html&quot; rel=&quot;nofollow&quot;&gt;built in&lt;/a&gt; or &lt;a href=&quot;http://www.amostech.com/TechnicalPapers/2010/Posters/Levin.pdf&quot; rel=&quot;nofollow&quot;&gt;after the fact.&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Ground based lasers that vaporize the leading edge of the object, to generate thrust to slow it down, leading to deorbit  (Or if small enough, vaporize the entire object).&lt;/li&gt;&#xA;&lt;li&gt;Some satellite that can collect debris somehow.  (Insert much hand waving).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The goal is to get rid of the biggest stuff with an EDDE like approach, and maybe a laser based approach to focus on the little stuff in dangerous orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither is cheap nor fast.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2015-10-05T20:48:07.197" LastActivityDate="2015-10-05T20:48:07.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4864" PostTypeId="1" AcceptedAnswerId="4879" CreationDate="2014-06-25T19:43:51.170" Score="16" ViewCount="1889" Body="&lt;p&gt;Providing a theoretical journey to some far place, could the void of space be used as a good refrigerator for food since it is very cold ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming that radiations in the void space are somehow avoided for such idea, is this viable ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, what about the vacuum of space, would food be affected by such condition and require a human-friendly pressure level ?&lt;/p&gt;&#xA;" OwnerUserId="1368" LastActivityDate="2016-04-25T05:50:18.810" Title="Could the void of space act as a good refrigerator for food in long journeys?" Tags="|food|vacuum|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4869" PostTypeId="2" ParentId="4858" CreationDate="2014-06-26T13:51:27.500" Score="5" Body="&lt;p&gt;According to this article -- &lt;a href=&quot;http://www.space.com/5542-satellite-destroyed-space-collision.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.space.com/5542-satellite-destroyed-space-collision.html&lt;/a&gt; -- there have been only 4 collisions with space debris as of 2009, only one resulting in a satellite being destroyed. I suspect there's a question of definitions there: collisions with tiny enough objects travelling in similar orbits (and thus at similar speeds, resulting in very low delta-v) could be barely noticeable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which, I guess, leaves the question of whether the low &quot;actual collision rate&quot; is because the danger isn't really all that big, or because people are actively working to avoid collisions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reports that the U.S. government is monitoring 18,000 pieces of space debris (a number I just got from that same article) sound at first hearing like there is a huge amount of debris out there. But space is a big place. There are a lot more than 18,000 things you could run into on US highways, between other cars, pedestrians, animals, objects that fall off the back of trucks, etc. These have certainly caused damage and fatalities, but they have not made U.S. highways so dangerous as to be unusable. (And yes, I'm well aware that there are many ways in which &quot;dead skunk in the middle of the road&quot; and &quot;10 cm object flying about in space&quot; are not the same, the analogy is far from perfect, but I think it's valid enough to make my point.)&lt;/p&gt;&#xA;" OwnerUserId="4656" LastActivityDate="2014-06-26T13:51:27.500" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4870" PostTypeId="1" AcceptedAnswerId="4872" CreationDate="2014-06-26T14:44:12.687" Score="8" ViewCount="2355" Body="&lt;p&gt;This question was inspired by this &lt;a href=&quot;https://space.stackexchange.com/questions/4858/why-havent-more-spacecraft-satellites-been-hit-by-debris&quot;&gt;other question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/4861/4657&quot;&gt;one of its answers&lt;/a&gt;.  Essentially we have a lot of debris flying around in close proximity to Earth, and it is often going quite fast - fast enough to tear into or through somebody.  Satellites and other such bodies have a lot of physical shielding, but even given any sort of armoring in a spacesuit, it just wouldn't be the same.  So when astronauts go outside a station or a shuttle in a spacesuit, what keeps random debris from flying through their suits and into their bodies?  How are they safe?&lt;/p&gt;&#xA;" OwnerUserId="4657" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-06-26T23:16:58.290" Title="How are astronauts able to spacewalk in safety, despite all the debris?" Tags="|debris|spacesuits|safety|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4871" PostTypeId="2" ParentId="4870" CreationDate="2014-06-26T14:58:43.187" Score="1" Body="&lt;p&gt;Space is very big and even though there is a lot of junk, astronauts are small targets.&#xA;Nothing keeps it from happening really.&lt;/p&gt;&#xA;" OwnerUserId="4594" LastActivityDate="2014-06-26T14:58:43.187" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4872" PostTypeId="2" ParentId="4870" CreationDate="2014-06-26T15:09:39.150" Score="13" Body="&lt;p&gt;Safe is a big word, so can we first settle for &lt;em&gt;safe enough that their EVA are worth the risk&lt;/em&gt;? OK, another big word in threat mitigation is &lt;a href=&quot;https://en.wikipedia.org/wiki/Risk_management&quot;&gt;risk management&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What this means is, that on top of larger debris being tracked by ground radars, their orbits predicted days in advance and assessing risk of these know debris entering the &lt;em&gt;pizza box&lt;/em&gt; (imaginary square protected area around the station), EVA are scheduled to take place when and where the risk of collision with smaller (untracked) debris is predicted to be the smallest, and every step taken that they decrease exposure time. Obviously, this goes the same for &lt;em&gt;space weather&lt;/em&gt;, for example scheduling during low Solar activity and during station's orbits that don't cross the &lt;a href=&quot;https://en.wikipedia.org/wiki/South_Atlantic_Anomaly&quot;&gt;South Atlantic Anomaly&lt;/a&gt; regions. And having backup EVA dates, if they have to be scrubbed for some unexpected reason, e.g. detected Solar flare pointing towards the Earth a day or so before the planned excursion, the station having to do orbital maneuvers to avoid larger debris,...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This risk management might mean taking fewer EVA and doing more tasks during each excursion, keeping the number of EVA astronauts / cosmonauts to the minimum that can still deal with unexpected situations (that would be two in egress and one in the station as support crew, and ground stations tracking their progress and monitoring other sensor readings), determining and prioritizing tasks necessity, placing ingress / egress and work locations in station's &lt;em&gt;wake&lt;/em&gt; or otherwise protected by station's own hardware to minimize exposure to highest relative velocity debris, including changing station's own configuration, like rotating solar arrays, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while &lt;em&gt;safe&lt;/em&gt; is a lot to demand, various risk management procedures work, and make EVAs &lt;em&gt;safer&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-06-26T15:32:36.163" LastActivityDate="2014-06-26T15:32:36.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4873" PostTypeId="2" ParentId="4870" CreationDate="2014-06-26T23:16:58.290" Score="1" Body="&lt;p&gt;IIRC, like the Apollo Lunar EVA suits, the ISS suits are multi-layered to give the best chance of mitigating impact of sub-radar sized flecks etc. Think of it as 'spaced armour' on a vehicle. Outer layer takes the big hit, spalls a bunch of lesser splashes towards next layer. Recurse until non-lethal...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, this is why so much emphasis has gone into the Canadarm and tele-operated 'robots'.&lt;/p&gt;&#xA;" OwnerUserId="4664" LastActivityDate="2014-06-26T23:16:58.290" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4874" PostTypeId="1" AcceptedAnswerId="4875" CreationDate="2014-06-26T23:30:03.087" Score="10" ViewCount="7712" Body="&lt;p&gt;What would happen if I throw a grenade in space? Would it explode? Or will it just keep floating forever?&lt;/p&gt;&#xA;" OwnerUserId="4665" LastEditorUserId="49" LastEditDate="2014-06-27T00:07:21.853" LastActivityDate="2014-06-29T00:46:06.543" Title="What would happen if I throw a grenade in space?" Tags="|orbital-mechanics|vacuum|physics|ballistics|explosives|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4875" PostTypeId="2" ParentId="4874" CreationDate="2014-06-27T00:01:00.330" Score="17" Body="&lt;p&gt;Yes, it would explode. Most &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_hand_grenades&quot;&gt;hand grenades&lt;/a&gt; are nowadays triggered chemically, electrically or contain a fuze enclosed within the assembly, so they don't require atmospheric oxygen to ignite, are watertight and otherwise more reliably &lt;em&gt;go off&lt;/em&gt; at a preset time since activation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would however create a large number of dangerous debris that would &lt;em&gt;float forever&lt;/em&gt;, if exploded in higher orbits or deep space, or at least until they impact something, i.e. transfer their momentum onto another body. In lower orbits, the smaller the fragments, the lesser their kinetic potential and would eventually decay their orbit due to still some atmospheric pressure and thus atmospheric drag slowing them down. Depending on the materials these fragments are made of, and their surface area, they might evaporate on reentry, further disintegrate due to surface ablation into smaller fragments, or make it to ground at these fragments' terminal velocity more or less intact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do note though that if you're in Earth's orbit, you're orbiting it at many times the speed of firearm projectiles (roughly 7.7 km/s in LEO while firearm bullets usually achieve a bit over the speed of sound at sea level or even less, see e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Orders_of_magnitude_%28speed%29&quot;&gt;this page&lt;/a&gt; for comparison), so while hand grenade fragments would travel away in all directions from the point of explosion, they would retain more-or-less same orbit you're in. They might &lt;em&gt;boomerang&lt;/em&gt; (can it be used as a verb?) and hit you many orbits around the Earth later at same relative velocity they were at first moving away from you. Not recommended!&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="58" LastEditDate="2014-06-27T13:02:44.110" LastActivityDate="2014-06-27T13:02:44.110" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4876" PostTypeId="1" AcceptedAnswerId="4891" CreationDate="2014-06-27T01:59:14.377" Score="12" ViewCount="945" Body="&lt;p&gt;Assigning slots to different satellites in various polar orbits seems very difficult on account of them passing over the North Pole. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cJ0I6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This loose intersection seems like the place where mission errors would become apparent. Is this one of the most congested regions of space? What additional measures and tools are taken into consideration for collision avoidance? How difficult is it?&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2014-06-29T21:22:16.430" Title="How is space assigned for satellites that follow intersecting orbits?" Tags="|orbit|orbital-maneuver|collision-avoidance|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4878" PostTypeId="2" ParentId="4864" CreationDate="2014-06-27T05:37:31.167" Score="11" Body="&lt;p&gt;The effect of the void of space is not so much cold, rather it is an insulator. Though heat radiates away, there is no convection nor conduction. Many spacecraft take precautions not to &lt;em&gt;overheat&lt;/em&gt; in space, very few spacecraft have a too-cold condition. Even the &lt;a href=&quot;http://www.spitzer.caltech.edu/&quot;&gt;Spitzer space telescope&lt;/a&gt; has since warmed up since the coolant ran out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, the void of space will likely remove most of the moisture from the food. Though much food is &quot;dried&quot;, even dried food contains a significant portion of water. The void of space will destroy this, and other potential volatiles in the food as well.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-06-27T05:37:31.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4879" PostTypeId="2" ParentId="4864" CreationDate="2014-06-27T07:44:19.090" Score="22" Body="&lt;p&gt;On planet earth, food spoils for several reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Oxidation&lt;/li&gt;&#xA;&lt;li&gt;Dehydration&lt;/li&gt;&#xA;&lt;li&gt;Microorganisms&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Oxidation is a reaction with the oxygen in the atmosphere. You can witness this when you cut an apple in half and leave it. It will only take minutes until the cuts become brown. Oxidation is usually harmless, but often gives the food an unpleasant texture, look and/or taste. In the vacuum of space there is no oxygen, so by storing food in space, oxidation can be prevented.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dehydration is when food loses moisture to the atmosphere. It becomes dry and stale. This often does not affect the nutritious value, but gives most kinds of food an unpleasant texture. Food dehydrates because its own moisture is higher than the moisture of the atmosphere. The moisture of space is 0, so being exposed to vacuum greatly increases the dehydration process. However, some kinds of food can be rehydrated by adding water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of all the kinds of spoiling mentioned here, microorganism growth is by far the most harmful one. It is practically impossible to produce food in a completely sterile way. Any food product will have smaller or larger traces of bacteria or fungi (&quot;mold&quot;) on it or will get contaminated with these during transport and storage. In small quantities, the human body can deal with ingesting them. In fact, most of these organisms themself aren't even toxic. But when the environment the food is stored in is beneficial, these organisms will procreate. They will consume parts of the food and turn it into chemicals which are often toxic to humans (see &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Mycotoxin&quot;&gt;Mycotoxins&lt;/a&gt;&quot;). However, almost all of such microorganisms require a pressurized atmosphere. In vacuum, they either die or become dormant. That means storing food in vacuum is a great way to preserve food from mold and bacteria.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;tl;dr:&lt;/strong&gt; Food in vacuum of space will dry up immediately, but when you don't care about that, it will be preserved really well.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-06-27T12:58:51.047" LastActivityDate="2014-06-27T12:58:51.047" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4880" PostTypeId="2" ParentId="3161" CreationDate="2014-06-27T08:23:47.073" Score="47" Body="&lt;p&gt;Methane (CH&lt;sub&gt;4&lt;/sub&gt;) and RP-1 are roughly equivalent in realizable performance. As previously mentioned by other posters, CH&lt;sub&gt;4&lt;/sub&gt; has slightly higher impulse – about 370 s in vacuum vs the 360 s – at the same chamber pressure of 7 MPa. But, this is counterbalanced by its lower bulk density of about 830 kg/m&lt;sup&gt;3&lt;/sup&gt; vs about 1030 kg/m&lt;sup&gt;3&lt;/sup&gt;. Bulk Density is the density of the combined Fuel and Oxidizer load in their appropriate ratios. Even though Methane is &quot;only&quot; 430 kg/m&lt;sup&gt;3&lt;/sup&gt; it is burned with 3.5 parts oxygen compared to 2.1 parts for RP-1, hence a CH4 rocket will be carrying more oxygen and less fuel by weight. Oxygen is pretty dense at a little over 1140 kg/m&lt;sup&gt;3&lt;/sup&gt; which is denser in fact than RP-1 (about 810 kg/m&lt;sup&gt;3&lt;/sup&gt;). If we assume that chamber pressures and engine cycle efficiency will be equal, RP-1 outperforms CH&lt;sub&gt;4&lt;/sub&gt; simply because a 20% larger tank will impose weigh penalties that slightly outweigh the 3% increase in specific impulse. However, the RP-1 advantage is contingent upon operating at an equal chamber pressure which may not be the case. And, Methane (CH&lt;sub&gt;4&lt;/sub&gt;) has additional advantages that are applicable in specific scenarios.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reasons CH&lt;sub&gt;4&lt;/sub&gt; is a front runner for SpaceX's Raptor can probably be attributed to four factors:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Methane does not coke (polymerize) at the operating temperatures of a rocket engine – it's coking point is roughly twice as high. This makes it easier to make an engine reusable and re-usability is a key SpaceX objective.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Because Methane does not coke, it is also easier to implement a full-flow stage combustion (FFSC) cycle where all the fuel and oxidizer flow goes through the pre-burner. Compared to contemporary Russian partial flow stage combustion engines higher chamber pressures are attainable resulting in a total impulse advantage of about 30 seconds, or 9%. This eliminates the performance deficiency of CH&lt;sub&gt;4&lt;/sub&gt; compared to RP-1.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If SpaceX intends to use the same fuel in all the stages, CH&lt;sub&gt;4&lt;/sub&gt; can be considered a better upper stage fuel and a worse lift-off fuel, even without enabling higher working pressures. This is because upper stages are typically 1/8th to 1/10th the size of the 1st stage, and here impulse is more important than density. Using Methane with the aforementioned FFSC cycle means that SpaceX can potentially get equivalent 1st stage performance and better upper stage performance.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Even though it is, IMHO, somewhat dubious that early Mars mission will use in-situ fuel production. If that ever becomes an applicable practice, Methane can be produced from water and CO&lt;sub&gt;2&lt;/sub&gt; while RP-1 cannot.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Other than that, there is the non-factor that somewhat favor Methane, such as regular grade Natural Gas being good enough and not having to highly refine the fuel from regular kerosene to RP-1 to achieve low coking characteristics and consistent densities. I say it is a non-factor, because fuel cost is such a negligible part of launch costs that it really doesn't matter if fuel cost a few times more or less. Fuel is typically only about 0.3% of the cost of flying a rocket to orbit, so fuel cost really doesn't matter – Not even when you compare highly expensive propellant combos like Hydrazine/Tetroxide to the relatively cheap Kerosene/Oxygen.&lt;/p&gt;&#xA;" OwnerUserId="4669" LastEditorUserId="4669" LastEditDate="2014-06-30T02:25:42.497" LastActivityDate="2014-06-30T02:25:42.497" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="4881" PostTypeId="1" AcceptedAnswerId="4884" CreationDate="2014-06-27T12:07:39.023" Score="3" ViewCount="930" Body="&lt;p&gt;How many human made satellites/probes have been directed outside of our solar system (or are currently within the solar system which in time will travel outside)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a source that shows information about these satellites, e.g. trajectory, speed and other historical info etc?&lt;/p&gt;&#xA;" OwnerUserId="3433" LastEditorUserId="49" LastEditDate="2014-06-30T15:50:19.333" LastActivityDate="2016-08-04T19:37:56.893" Title="How many satellites/probes will or have travelled outside the solar system?" Tags="|artificial-satellite|extra-solar-object|escape-velocity|reference-request|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4882" PostTypeId="1" AcceptedAnswerId="4883" CreationDate="2014-06-27T12:55:23.480" Score="12" ViewCount="1348" Body="&lt;p&gt;What requirements and restrictions are there for a civilian organisation sending a satellite into LEO? (Apart from cost. I'm talking about any laws, agreements between nations etc)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is in relation to a launch in the US, but with the payload coming from the UK or Europe.&lt;/p&gt;&#xA;" OwnerUserId="3433" LastEditorUserId="3433" LastEditDate="2014-06-27T16:10:04.083" LastActivityDate="2014-06-27T16:10:04.083" Title="Civilian requirements/restrictions for sending satellite into LEO" Tags="|low-earth-orbit|law|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4883" PostTypeId="2" ParentId="4882" CreationDate="2014-06-27T13:46:44.130" Score="6" Body="&lt;p&gt;There is a general assumption that country of launch is responsible to enforce liability in case your launch fails and does damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the US that is the FAA/AST where you need to get a launch license.  So being in orbit may or may not be directly regulated, the getting there part is. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-06-27T13:46:44.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4884" PostTypeId="2" ParentId="4881" CreationDate="2014-06-27T14:06:58.080" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_artificial_objects_escaping_from_the_Solar_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page on List of artificial objects escaping from the Solar System&lt;/a&gt; is a direct answer to your question. Please see further links therein for &lt;em&gt;trajectory, speed and other historical info&lt;/em&gt; of each of these objects:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h2&gt;Space probes&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_10&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pioneer 10&lt;/a&gt; – Launched in 1972, flew past Jupiter in 1973. Contact lost in January 2003 and is heading in the direction of Aldebaran in Taurus.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_11&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pioneer 11&lt;/a&gt; – Launched in 1973, flew past Jupiter in 1974 and Saturn in 1979. Contact lost in November 1995. The spacecraft is headed toward the constellation of Aquila (The Eagle),  Northwest of the constellation of Sagittarius. Barring incident, Pioneer 11 will pass near one of the stars in the constellation in about 4 million years.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 2&lt;/a&gt; – Launched in August 1977, flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. Probe has (depending on the source) not passed the heliosheath, but is still active.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1&lt;/a&gt; – Launched in September 1977, flew past Jupiter in 1979 and Saturn in 1980, making a special close approach to Saturn's moon Titan. Probe passed the heliopause on 25 August 2012 to enter interstellar space, and is still active.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons&lt;/a&gt; – Launched in 2006, probe made flyby of Jupiter in 2007, will make a flyby of Pluto in 2015. Flyby of Pluto may be followed by a flyby of a Trans-Neptunian object in the Kuiper belt.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h2&gt;Other objects&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;New Horizons third stage, a &lt;a href=&quot;https://en.wikipedia.org/wiki/STAR-48&quot; rel=&quot;nofollow noreferrer&quot;&gt;STAR-48&lt;/a&gt; booster, is on a similar escape trajectory out of the Solar System as New Horizons, but will pass millions of miles from Pluto. It will probably cross Pluto's orbit in late 2015.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The only objects to date to be launched directly into&#xA;a solar escape trajectory were the New Horizons spacecraft, its third&#xA;stage and its two small de-spin masses. The New Horizons &lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_(rocket_stage)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centaur&lt;/a&gt; stage&#xA;is not escaping; it is in a 2.83-year orbit.&lt;/p&gt;&#xA;&lt;p&gt;Many of the upper stages of all these spacecraft except Pioneer 11 are&#xA;also escaping the Solar System as they initially followed a trajectory&#xA;very close to that of their respective spacecraft and their subsequent&#xA;encounters with Jupiter boosted them all into hyperbolic solar escape&#xA;trajectories. The upper stage of Pioneer 11 is believed to be in solar&#xA;orbit because its encounter with Jupiter would not have resulted in&#xA;escape. Pioneer 11 gained the required velocity to escape the Solar&#xA;System in its subsequent encounter with Saturn.&lt;/p&gt;&#xA;&lt;p&gt;Since the Pioneers were launched first, they had a head start on the&#xA;Voyagers, but because they were traveling slower they were eventually&#xA;overtaken for most distance from the Sun: Voyager 1 passed Pioneer 10&#xA;on February 17, 1998.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-06-30T15:45:29.207" CommentCount="0" CommunityOwnedDate="2014-06-30T15:37:47.320" ContentLicense="CC BY-SA 3.0" />
  <row Id="4887" PostTypeId="1" CreationDate="2014-06-28T03:31:58.743" Score="0" ViewCount="684" Body="&lt;p&gt;How much does time slow down with gravitational time dilation, on a black hole with these graph trajectories.?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you watch a journey into a black hole on Youtube before reading this question, it only takes four minutes to watch the whole video, you will understand better what I mean by graph trajectory zones. &#xA;Orbiting a black hole is dangerous. There are four graph trajectory zones when orbiting a black hole. The green zone is a safe zone, the yellow zone is a risky zone, the orange zone is a danger zone where there are no orbits, neither stable nor unstable.&#xA;Then there is the red zone, the Event Horizon. &#xA;So if you had a spacecraft and wanted to slow yourself down with gravity from the black hole, so that you could visit Earth in the future in your lifetime. &#xA;I heard from this show you can orbit a black hole for a year and four years will have passed on Earth from your perspective. &#xA;How much would time slow down if you orbited in the clour coded regions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Question 1: If you orbited in the green zone for a year, how much time would pass on Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question 2: If you orbited in the yellow risky zone for a year, how much time would pass on the Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question 3: If you orbited in the orange zone, where there are no orbits stable, or unstable, this area is called the photon shere where light can just stay in orbit. &#xA;So the spacecraft would be orbiting in the orange zone, as close to the Event Horizen as possible. Is this orange zone the best place to be in terms of slowing down a space shuttle in time, to kind of travel in time relative to outside observers watching?&#xA;Also will the spacecraft get sphagettified in this orange region?&lt;/p&gt;&#xA;" OwnerUserId="4674" LastEditorUserId="8693" LastEditDate="2016-05-02T18:58:31.020" LastActivityDate="2016-05-02T18:58:31.020" Title="how much does time slow down with gravitational time dilation, on a black hole with these graph trajectories.?" Tags="|travel|time|" AnswerCount="1" CommentCount="2" ClosedDate="2014-06-28T12:32:41.917" ContentLicense="CC BY-SA 3.0" />
  <row Id="4889" PostTypeId="2" ParentId="4887" CreationDate="2014-06-28T04:31:18.380" Score="4" Body="&lt;p&gt;I don't get your colors, but the time dilation (how much more slowly your time passes relative to the outside observer, so greater than one) is $1/\sqrt{1-{r_S\over r}}$, where $r$ is your distance from the center of the mass, and $r_S$ is the Schwarzschild radius for that mass.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-06-28T04:31:18.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4890" PostTypeId="2" ParentId="2092" CreationDate="2014-06-28T12:19:33.333" Score="2" Body="&lt;p&gt;I haven't done an exhaustive search, but I came across one example: &lt;a href=&quot;http://www.astronautix.com/details/zond6810.htm&quot; rel=&quot;nofollow&quot;&gt;Zond 6&lt;/a&gt;. This was an unmanned test of the Soyuz 7K-L1, a stripped-down version of the 7K-L3 design for a manned capsule for moon missions. Zond 6 carried turtles, fruit flies and various other biological samples to test the capsule's life support systems. It was sent on a free-return trajectory around the Moon.&lt;br&gt;&#xA;During the return flight, a hatch seal failed, depressurizing the cabin and killing all life on board. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-06-28T12:19:33.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4891" PostTypeId="2" ParentId="4876" CreationDate="2014-06-28T16:22:08.640" Score="11" Body="&lt;p&gt;As far as I know, other than for the GEO ring satellites are not assigned slots prior to launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the United States various regulatory bodies approve the launch of Earth-orbiting satellites depending on their mission (e.g. the FCC approves frequencies and communications, NOAA approves Earth-imaging satellites, etc.). Part of this licensing process typically includes an analysis that shows the &lt;a href=&quot;https://space.stackexchange.com/a/2768/127&quot;&gt;25-year on-orbit requirement&lt;/a&gt; will be met. As of 2008, this includes what's known as a &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/protect/collision_avoidance.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;conjunction analysis&lt;/a&gt;, which looks at the probability of the new spacecraft intersecting old ones. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the short answer is that slots are not assigned, but (at least in the US) analysis is required to show the spacecraft has a low probability of hitting something already in orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately in practice conjunction analysis is very computationally expensive and requires government assistance (as the orbits of many national security assets are not published). Also, because of the uncertainty in ephemerides and other perturbations that build-up on orbit, it's not a good prediction of the probability of collision on a timescale anywhere close to 25 years. That's why despite this type of analysis things still collide.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-06-28T16:22:08.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4892" PostTypeId="2" ParentId="4874" CreationDate="2014-06-29T00:46:06.543" Score="2" Body="&lt;p&gt;It depends on how long the grenade has been exposed to space.  You might find the vacuum got to the mechanism if it's been out there long enough.  Assuming the trigger doesn't fail due to vacuum welding it's going to go boom.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-06-29T00:46:06.543" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4893" PostTypeId="1" AcceptedAnswerId="4894" CreationDate="2014-06-29T07:03:28.173" Score="12" ViewCount="4233" Body="&lt;p&gt;I wonder why the &lt;a href=&quot;http://www.nasa.gov/exploration/systems/mpcv/&quot;&gt;Orion Multi-Purpose Crew Vehicle&lt;/a&gt; will only allow ocean landings while the Soyuz spacecraft is capable of landing on land?&lt;br&gt;&#xA;Also, what mode of landings does the Chinese space program envision for its spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="3387" LastEditorUserId="198" LastEditDate="2014-12-05T17:23:18.437" LastActivityDate="2019-01-20T17:06:11.730" Title="Why will the Orion system land on water, while the Soyuz lands on land?" Tags="|reentry|soyuz-spacecraft|orion-spacecraft|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4894" PostTypeId="2" ParentId="4893" CreationDate="2014-06-29T10:37:35.960" Score="14" Body="&lt;p&gt;Back in the early days of space flight, the Soviets did not have large expanses of warm water available to them, where there is no fear of 'enemies'. Unlike the US with large coastlines on the Atlantic and Pacific Oceans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus necessity was the mother of invention, and the Soviets needed to land on land.  They use parachutes, and solid rockets that fire in the last second or three of flight to slow down the final touchdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US on the other hand, used water to cushion the landing as it is much simpler. Hard for a water surface to fail. (I suppose if it froze, that might count).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, as noted by several commenters, the Russians, launching mostly over land, needed to handle an abort that ended on land, and the Americans launching mostly over water needed to handle an abort that ended on water. On the gripping hand, eventually in both those cases, an abort could end on the other type of surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is easier to NOT rely on something always working, with no time for backups to take over.  So the US took the easy way out historically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion is not very 'revolutionary' and is not really doing anything new.  So it is no surprise they did not try to change the landing mode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course the Commercial manned space providers are not planning on water landings, since they must do something revolutionary to make an affordable business case.  (NASA does not have to deal with petty things like that).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So SNC with Dream Chaser is a runway lander.  Boeing with CST-100 is a land lander, but with the final blow cushioned by air bags. (Which means throwing away the heat shield every landing).  SpaceX planned initially on powered landings for Dragon V2 but that plan has changed over time back to water landings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am pretty sure the Chinese using a Soyuz clone, land on solids cushioning the last bit, on land.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2019-01-20T17:06:11.730" LastActivityDate="2019-01-20T17:06:11.730" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="4895" PostTypeId="1" CreationDate="2014-06-29T11:16:17.690" Score="16" ViewCount="819" Body="&lt;p&gt;When New Horizons arrives at Pluto next year, how much time will we get to image and explore (measurements, spectra-analysis) the Pluto system? Any ideas if we will see (or be able to count) Pluto's moons?&lt;/p&gt;&#xA;" OwnerUserId="3387" LastEditorUserId="25" LastEditDate="2016-12-19T21:32:07.440" LastActivityDate="2016-12-19T21:32:07.440" Title="New Horizons - How much time will we get to image the Pluto system?" Tags="|new-horizons|pluto|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4896" PostTypeId="1" CreationDate="2014-06-29T17:48:52.120" Score="4" ViewCount="47" Body="&lt;p&gt;Soon, the Dawn spacecraft will reach and orbit Ceres. Any ideas about what will happen after this work?&lt;/p&gt;&#xA;" OwnerUserId="3387" LastEditorUserId="49" LastEditDate="2014-06-30T13:07:14.523" LastActivityDate="2014-06-30T13:07:14.523" Title="Dawn - What happens after Ceres?" Tags="|nasa|asteroid|mission-design|asteroid-belt|dawn-mission|" AnswerCount="0" CommentCount="2" ClosedDate="2014-06-30T13:00:24.920" ContentLicense="CC BY-SA 3.0" />
  <row Id="4897" PostTypeId="2" ParentId="4876" CreationDate="2014-06-29T19:05:28.237" Score="5" Body="&lt;p&gt;The polar region is congested, but there are a couple of mitigating factors that make it not as bad as it may initially appear to be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Altitude separation&lt;/li&gt;&#xA;&lt;li&gt;The periods of polar orbits at similar altitudes are similar, so the timing of conjunctions varies slowly, facilitating their prediction.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;No special slot assignments are made, and conjunctions are dealt with operationally, primarily based on &lt;a href=&quot;http://www.oosa.unvienna.org/pdf/pres/stsc2011/tech-33.pdf&quot; rel=&quot;noreferrer&quot;&gt;analysis&lt;/a&gt; and &lt;a href=&quot;https://www.space-track.org/documents/CSM_Guide.pdf&quot; rel=&quot;noreferrer&quot;&gt;notifications&lt;/a&gt; by &lt;a href=&quot;http://www.vandenberg.af.mil/library/factsheets/factsheet.asp?id=12579&quot; rel=&quot;noreferrer&quot;&gt;JSpOC&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-06-29T19:05:28.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4898" PostTypeId="2" ParentId="4876" CreationDate="2014-06-29T21:22:16.430" Score="4" Body="&lt;p&gt;I dispute the premise of the question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two satellites pose a danger of mutual collision if they are in orbit at the &lt;strong&gt;same&lt;/strong&gt; altitude, but in &lt;strong&gt;different&lt;/strong&gt; orbital planes.  In this case, they could possibly collide at the two points where the line of intersection of the two planes crosses the orbit.  &lt;strong&gt;Those two points exist for any pair of satellites&lt;/strong&gt;.  The point on the earth directly below the possible collision point is, IMO, irrelevant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question for mission control is not, &quot;Do we make it through this insane traffic jam safely?&quot;.  Rather, the question is, &quot;Do we miss Satellite A?  Satellite B?  Satellite C?...&quot;&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2014-06-29T21:22:16.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4899" PostTypeId="1" CreationDate="2014-06-29T23:14:04.397" Score="3" ViewCount="284" Body="&lt;p&gt;The US government created NASA to focus American efforts toward a single goal: put a man on the moon in a non-military context (albeit with mostly military flight crew and inherently military overtones in the goal itself). Having accomplished that mission, NASA has remained at the center of almost all US space efforts, except for those with military objectives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With all the various interests in space from commercial, institutional, military, and private entities towards a variety of objectives including exploration, research, resource development, and even tourism, does the US government still want to keep NASA central to it all, or is there an alternative vision? I once read something which declared an aim to get NASA &quot;out of the space launch business&quot;, but it still appears to be heavily involved in the development and construction of space hardware and the ongoing business of launching payloads into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider aviation... there is the FAA and the American air traffic control system which exists to promote aviation and to promote increased safety in aviation. It doesn't transport people or goods, build aircraft, or do aeronautical research, generally doesn't operate aircraft at all, leaving that to private, institutional, and commercial operators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a similar model in the works for America in space?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2014-06-30T03:29:01.990" LastActivityDate="2014-06-30T17:15:29.003" Title="What is the US government's long term vision for space development and exploration?" Tags="|nasa|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4900" PostTypeId="1" AcceptedAnswerId="4903" CreationDate="2014-06-30T02:25:08.790" Score="3" ViewCount="1614" Body="&lt;p&gt;I just had this thought today:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I wonder what the shattering would look like if you &quot;dropped&quot; a glass or a ceramic cup in zero gravity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Obviously you can't drop it, you'd have to like throw it. But then what would it look like when breaking?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was surprised not to find any YouTube videos showing it... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what would happen? Will the pieces be shattered in all directions? Is it somehow harder to break a glass in space? Does it look like an explosion? Why has no one tried and filmed it yet?&lt;/p&gt;&#xA;" OwnerUserId="3481" LastActivityDate="2014-06-30T10:37:21.930" Title="What would it look like to shatter a glass or break similar material in Zero Gravity?" Tags="|gravity|physics|experiment|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4901" PostTypeId="2" ParentId="4899" CreationDate="2014-06-30T06:30:44.447" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What is the US government's long term vision for space development and exploration?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;None.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Call me a troll, but the US government is composed of &lt;strong&gt;short-term-elected officials&lt;/strong&gt; who use NASA to &lt;a href=&quot;http://www.bloomberg.com/news/2014-01-08/congress-makes-nasa-finish-useless-350-million-structure.html&quot;&gt;provide jobs for their constituency&lt;/a&gt;. This is often called 'pork' when discussed in a derogatory fashion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.nasa.gov/content/charles-f-bolden-jr-nasa-administrator-july-17-2009-present/&quot;&gt;NASA Administrator&lt;/a&gt; and especially the &lt;a href=&quot;http://www.jpl.nasa.gov/&quot;&gt;scientists and engineers&lt;/a&gt; employed by NASA may look long-term, but &lt;strong&gt;they are not the US government referenced in the question&lt;/strong&gt;. NASA has from almost the beginning been a political tool. That in itself is not a bad thing (look what the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Race&quot;&gt;space race&lt;/a&gt; has &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_spin-off_technologies&quot;&gt;brought us&lt;/a&gt;) but today as there is no &lt;a href=&quot;https://en.wikipedia.org/wiki/Soviet_space_program&quot;&gt;adversary&lt;/a&gt; there is &lt;a href=&quot;http://www.universetoday.com/53232/nasa-budget-details-constellation-cancelled-but-where-to-next/&quot;&gt;no long term vision&lt;/a&gt; for space development and exploration.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-06-30T06:30:44.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4902" PostTypeId="1" CreationDate="2014-06-30T08:31:20.537" Score="21" ViewCount="4287" Body="&lt;p&gt;The &lt;a href=&quot;http://jwst.nasa.gov&quot; rel=&quot;noreferrer&quot;&gt;James Webb Space Telescope&lt;/a&gt; (JWST) project could be fantastic, It has the promise of seeing farther than &lt;a href=&quot;http://hubblesite.org/gallery/&quot; rel=&quot;noreferrer&quot;&gt;Hubble&lt;/a&gt;, almost to the beginning of our space/time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, &lt;a href=&quot;https://space.stackexchange.com/questions/284/why-should-the-james-webb-space-telescope-stay-in-the-unstable-l2&quot;&gt;orbiting at L2&lt;/a&gt; - while closer by far than other LaGrange points - is still 1.5m KM away from Earth. That is four times more distant than humans have ever travelled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is this: This project has had a &lt;a href=&quot;http://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Program_status&quot; rel=&quot;noreferrer&quot;&gt;difficult construction history&lt;/a&gt;. (Most do but this has been more challenging). Are there ideas or plans for rescue/repair/restore missions if/when something goes wrong?&lt;/p&gt;&#xA;" OwnerUserId="3387" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-07-03T09:40:00.080" Title="The JWST - What happens if/when it breaks?" Tags="|hubble|space-telescope|james-webb-telescope|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4903" PostTypeId="2" ParentId="4900" CreationDate="2014-06-30T10:37:21.930" Score="3" Body="&lt;p&gt;When you break glass in zero gravity, the shards will keep flying in a straight line instead of falling to the ground. As @Deer Hunter said, this makes it incredibly dangerous to try out in an open space.&lt;br&gt;&#xA;Doing it safely means doing the experiment inside a sealed container, so this moves from being something that can be done just for fun, to an expensive planned experiment. The question then becomes: what can we learn from this experiment? At first glance, I'd say not much. There's no reason to expect glass breaks differently in zero gravity. So you'd have an expensive experiment with no apparent value. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-06-30T10:37:21.930" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4904" PostTypeId="2" ParentId="4902" CreationDate="2014-06-30T11:31:12.300" Score="10" Body="&lt;p&gt;None whatsoever. If JWST has issues, it is basically out of luck.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe one day, Orion might be able to go visit, but Orion is a lousy repair platform compared to the Shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle had more crew (7 vs 4), more room for equipment, a place to dock the Hubble as a work platform, an RMS to move heavy equipment around (in and out of the Hubble).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-06-30T11:31:12.300" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="4905" PostTypeId="5" CreationDate="2014-06-30T11:33:40.943" Score="0" Body="&lt;p&gt;The &lt;a href=&quot;http://jwst.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;James Webb Space Telescope&lt;/a&gt; is a Great Observatory, planned for launch to the L2 Lagrange point.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-06-30T11:33:40.943" LastActivityDate="2014-06-30T11:33:40.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4906" PostTypeId="4" CreationDate="2014-06-30T11:33:40.943" Score="0" Body="The James Webb Space Telescope is a Great Observatory, planned for launch to the L2 Lagrange point." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-06-30T11:33:40.943" LastActivityDate="2014-06-30T11:33:40.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4907" PostTypeId="2" ParentId="4902" CreationDate="2014-06-30T12:49:29.823" Score="17" Body="&lt;p&gt;There is &lt;a href=&quot;http://www.space.com/3833-nasa-adds-docking-capability-space-observatory.html&quot;&gt;a docking ring&lt;/a&gt; on the JWST, so in theory astronauts could visit it. It would be easier to get to JWST than to the Moon, but more difficult than LEO like we have been doing. Edward Weiler, director of NASA Goddard Space Flight Center, had this to say on the subject:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We cannot make the James Webb Space Telescope fully serviceable like&#xA;  the Hubble because that would cost so much money that I don't think&#xA;  this country could afford it. However, what if you have a bad day when&#xA;  you put this thing a million miles out and everything folds out except&#xA;  for an antenna ... it gets stuck? Or a solar panel doesn't fold out&#xA;  completely, and you say, 'gee, I wish we could send an astronaut just&#xA;  to give it a kick'?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-06-30T12:49:29.823" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4908" PostTypeId="1" CreationDate="2014-06-30T14:29:35.777" Score="3" ViewCount="3695" Body="&lt;p&gt;I'm working on a way of simulating orbits for a video game I'm making. It works by taking the apoapsis, periapsis and current distance and calculating from there the velocity, radial velocity, period etc. There are no vectors involved, as no 2D space is being simulated - only the numbers themselves. The simulation works well with the orbit put on rails, but I run into problems as soon as I change the orbital velocity, because I then need to re-calculate the apoapsis and periapsis based on this change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've tried a few methods, but they don't seem to be working due to the lack of vectors, which they all use. What's the correct way of doing this? If we say that I'm orbiting an object where mu is 10,000, my current distance from the object is 500 metres, the apoapsis is 700 metres, the periapsis is 300 metres and I accelerate by 20 m/s, what would be new apoapsis and periapsis be? Bearing in mind that this is intended for programming use so any pseudocode would also be appreciated.&lt;/p&gt;&#xA;" OwnerUserId="4677" LastEditorUserId="25" LastEditDate="2015-08-27T12:19:49.437" LastActivityDate="2015-12-25T14:10:55.383" Title="How to calculate new periapsis and apoapsis after orbital velocity change" Tags="|orbital-mechanics|orbit|orbital-elements|computing|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="4909" PostTypeId="2" ParentId="4899" CreationDate="2014-06-30T16:06:59.473" Score="1" Body="&lt;p&gt;I suspect that most government agencies (around the world), have elements that can be manipulated for local goals - it's probably the best and worst sides of representative government.. However, NASA is really two agencies, that of human spaceflight, and that of robotic spaceflight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the former, I think that this administration has finally recognized that human exploration cannot advance without private sector leadership. And to me, that is very exciting, although I wish I was 20 years younger and could see more of what is about to unfold.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as robotic exploration, I couldn't have lived in a more exciting time.. I've seen images from just about every planet and moon in our solar system, we have orbiters around Mercury, Venus, Mars, Saturn and even some asteroids and comets. We have two landers on Mars.. We have craft on their way to Ceres, Jupiter, and Pluto. We have amazing space and terrestrial telescopes, and we are getting better at making these projects - at least (knock on wood), the success rate is improving.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than the engineering feat of getting places, (&lt;a href=&quot;http://www.jpl.nasa.gov&quot; rel=&quot;nofollow&quot;&gt;think Jet Propulsion Lab&lt;/a&gt;, Orbital Sciences, and others), each NASA center is linked to a set of Universities. For example, JPL has CalTech and Arizona State University while Goddard is linked to Johns Hopkins, Maryland, and others. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this structure is brilliant, as the academics are rightly getting to do their research, while engineering is working with the academics to build on for the next steps. This synergy is what NASA brings to the table.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, NASA has invested a ton in Earth Resource Science, so that we can learn about what has happened on Earth in the past, and our possible impact at present. &lt;/p&gt;&#xA;" OwnerUserId="3387" LastActivityDate="2014-06-30T16:06:59.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4910" PostTypeId="2" ParentId="4899" CreationDate="2014-06-30T17:15:29.003" Score="3" Body="&lt;p&gt;'The U.S. government's long-term vision' is actually a contradiction in terms.  There is no 'national goal' for space in the U.S.  What we have are a series of short-term goals strung out to look like a long-term goal. . .build a spacecraft. . .land on the Moon. . .build a reusable spacecraft. . .build a space station. . .build a cheaper spacecraft. . .grab an asteroid. . .go to Mars. . .  None of it is integrated into one, over-arching vision accept when politicians want to look visionary.  Their vision - when they have one - only lasts as long as their terms in office.  Sometimes not even that long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are wondering about colonizing space or the Moon or Mars or. . .  Forget it.  NASA does not (indeed, CANNOT) do 'colonization'.  All NASA can ever do is put a few very few semi-lucky souls out there and bring them back alive.  they have not always succeeded.   &lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2014-06-30T17:15:29.003" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4911" PostTypeId="1" CreationDate="2014-06-30T17:45:14.197" Score="8" ViewCount="1308" Body="&lt;p&gt;Shuttle, and its cousin &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle-C&quot; rel=&quot;nofollow&quot;&gt;Shuttle-C&lt;/a&gt;, are now history as the launch pads have been converted for use by other systems.  Claims made for the Shuttle-C were that it would have taken 4 years and cost $6 billion to develop.  Given the time/costs associated with the first generation of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow&quot;&gt;SLS&lt;/a&gt; units, exactly how are they an improvement on Shuttle-C?   &lt;/p&gt;&#xA;" OwnerUserId="3227" LastEditorUserId="168" LastEditDate="2014-07-08T06:22:57.993" LastActivityDate="2014-07-08T06:22:57.993" Title="Shuttle-C: Did we make the right move?" Tags="|nasa|space-shuttle|shuttle-derived|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4912" PostTypeId="2" ParentId="4911" CreationDate="2014-06-30T18:14:27.733" Score="10" Body="&lt;p&gt;The reality is that any estimate out of NASA on costs needs to be taken with a salt lick the size of LC-39B's former tower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Had they done Shuttle-C, odds are likely it would cost as much, be as delayed as, SLS.  They can screw up anything it would seem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In theory, Shuttle-C said, replace JUST the orbiter with a non-winged payload fairing.  Still needs Space Shuttle main engines (SSME), still needs Orbital Maneuvering System (OMS) pods. No heat shield or wings though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This in theory allows for a much simpler design, fundamentally a fairing (which is well understood) on top of a boattail that holds the engine 'stuff'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle is on the order of &lt;a href=&quot;http://coolcosmos.ipac.caltech.edu/ask/268-How-much-did-the-Space-Shuttle-weigh-&quot;&gt;165,000 lbs&lt;/a&gt;, by itself empty. Then 65,000 lbs payload or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means if you reduce the 'overhead' of the orbiter to only 40,000lbs (40%) potentially you could get a mighty fine payload to orbit. &lt;a href=&quot;http://www.globalsecurity.org/space/systems/sts-c.htm&quot;&gt;This article&lt;/a&gt; mentions &lt;em&gt;&quot;... anywhere from 100,000 to 170,000 pounds (45,360 to 77,112 kilograms)&quot;&lt;/em&gt; to LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seems like the general shape for launch could be preserved, since it is just the deletion of the wings that affects aerodynamics.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This minimal change model did not seem to have any attraction, when the purpose of SLS is a jobs program. Without wings and heat shield you could probably reduce the people working on the Shuttle a fair bit as well.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS in theory, in later models should be able to launch more payload than Shuttle-C, but that is still many many years out, and I will believe it when I see it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-06-30T18:21:30.287" LastActivityDate="2014-06-30T18:21:30.287" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4913" PostTypeId="2" ParentId="3951" CreationDate="2014-06-30T19:36:52.257" Score="2" Body="&lt;p&gt;As pointed out, NASA wants to go beyond Earth orbit.  THAT is the problem.  They are losing taxpayer support with every discussion about Mars because no one has ever given a reason for going to Mars that means anything to people not trying to protect their aerospace/NASA job.  People are scared the economy will collapse next Tuesday, if not sooner, and NASA wants to go to Mars.  This comes across as out of touch on so many levels.  What they (taxpayers) need is to hear someone talk about BENEFITS in a way that allows them to feel confident about the expense.  Sort of like trying to talk the entire country into taking an expensive camping trip.  Technically neat concepts like DIRECT will not, by themselves, generate that confidence.  &lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2014-06-30T19:36:52.257" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4915" PostTypeId="1" AcceptedAnswerId="4916" CreationDate="2014-07-02T02:08:40.267" Score="4" ViewCount="717" Body="&lt;p&gt;Could the current technology used in adaptive optics to see far into the sky, be used to improve satellite or even aerial photographs?&lt;/p&gt;&#xA;" OwnerUserId="962" LastEditorUserId="857" LastEditDate="2014-07-02T11:46:15.013" LastActivityDate="2014-07-04T11:34:30.190" Title="Do adaptive optics work the other way around?" Tags="|artificial-satellite|photography|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4916" PostTypeId="2" ParentId="4915" CreationDate="2014-07-02T02:49:34.300" Score="7" Body="&lt;p&gt;Well yes, degradation due to blurring and scintillation of atmospheric turbulence goes both ways, and the most apparent benefits could be not for optical imaging systems, but for data communications and reducing signal degradation. See e.g. &lt;a href=&quot;http://opticalengineering.spiedigitallibrary.org/article.aspx?articleid=1088685&amp;amp;journalid=92&quot;&gt;Improved optical communications performance combining adaptive optics and pulse position modulation&lt;/a&gt; or &lt;a href=&quot;https://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-22-13-16000&quot;&gt;Effectiveness of adaptive optics system in satellite-to-ground coherent optical communication&lt;/a&gt;. Optical imaging from orbit can benefit also from other, less invasive image enhancement techniques (see below), but AO could be one of them. If we used them to image some other worlds than the Earth;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a few problems with using adaptive optics in orbit, though. First is that AO use powerful lasers and those drain a lot of power. To my knowledge far more of it than laser communications systems, even from such a distance as the Moon, like e.g. &lt;a href=&quot;http://esc.gsfc.nasa.gov/267/271/335.html&quot;&gt;LADEE's LLCD&lt;/a&gt; (Lunar Laser Communications Demonstration). But perhaps more importantly, that I can't imagine pointing such powerful lasers from orbit to ground without protest from targeted nations. As I hear, &lt;a href=&quot;http://www.aps.org/publications/apsnews/200910/optics.cfm&quot;&gt;Air Force doesn't exactly approve the use of ground based AO&lt;/a&gt; either:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Air Force Space Command regulates their use to protect passing&#xA;  satellites. All uses of the lasers must be approved days ahead of time&#xA;  by the Laser Clearing House at Vandenberg Air Force base to prevent&#xA;  the beam from crossing paths with an approaching spacecraft.  The&#xA;  lasers cannot damage a craft’s hull, but they could potentially burn&#xA;  out sensitive optical equipment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, for &lt;em&gt;the other way around&lt;/em&gt;, you can imagine FAA and FCC, possibly DOE, having something to say about their use. Not to mention what all problems would ensue, if AO rated lasers were used to improve optical imaging of territories of other nations. Imaging other worlds with thick atmospheres though (but with refractive index below critical angle, which rules out, say, Venus), sure. Power budget permitting.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-07-02T02:49:34.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4917" PostTypeId="2" ParentId="3922" CreationDate="2014-07-02T16:09:00.390" Score="4" Body="&lt;p&gt;With vertical integration, the payload force is absorbed in the same direction as the launch forces, so you only need to make the rocket strong enough to hold the payload in the vertical direction.&lt;br&gt;&#xA;With horizontal integration, that the rocket needs to be built to withstand the bending load of the payload when the rocket is horizontal. This generally means a heavier construction. Rockets are built as lightly as possible. Some rockets can only support their own weight when the fuel/oxidizer tanks are pressurized. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vertical integration in an assembly building also means you need to transport the stack vertically to the launch pad. This is doable in mild climates with plenty of warning for bad weather (Cape Canaveral). Baikonur, for instance, has high winds so often that transporting the stack vertically is not feasible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &quot;N-1 for the Moon and Mars, a reference guide to the Soviet superbooster&quot;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-07-02T16:09:00.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4919" PostTypeId="2" ParentId="3922" CreationDate="2014-07-03T09:07:09.443" Score="12" Body="&lt;p&gt;The primary issue with SpaceX's horizontal integration approach is that the payload has to be able to be rotated.&lt;/p&gt;&#xA;&lt;h1&gt;Horizontal Mating&lt;/h1&gt;&#xA;&lt;p&gt;The primary costs of horizontal integration are the transitional equipment - essentially, a tilt-table the length of the assembled rocket. The rocket itself isn't designed for carrying the horizontal load itself, but is designed with some minimal extra structure to enable the rotation and the use of the horizontal support. Since the rocket has to be shipped in this orientation anyway, it's not a major cost issue, and the rotation has to happen at some point.&lt;/p&gt;&#xA;&lt;p&gt;The benefits of the horizontal integration, especially for the payload attach, are that the clean environment is maintained near the floor, and it is possible to use entirely non-crane based materials movement. This reduces structural overhead, and risk to staff. A 3 m fall is a far different beast from a 50 m fall. This means dropped tools, falling staff, and other incidental mishaps have far less risk. Likewise, a crane is many potential points of failure more than a modified forklift. (SpaceX does use cranes for some activities; they don't take full advantage of the potential, because certain activities are easier by crane.)&lt;/p&gt;&#xA;&lt;p&gt;The drawbacks, however, are that the payload must be able to maintain it's integrity both in the mating position and in the launch position, as well as all the points between. This is mostly risks to sensors, antennae, and solar panels; most of these can be designed for the transition at minor cost in mass, and in the case of sensors, adding covers for during the mating process.&lt;/p&gt;&#xA;&lt;h1&gt;Vertical Mating&lt;/h1&gt;&#xA;&lt;p&gt;The benefits of vertical integration are that the payload can be very slightly reduced in mass, and slightly reduced in complexity, but at the cost of working 50+ meters above the ground, and everything needing to be hoisted by cranes or special elevators. Further, the work environment creates a high velocity impact with dropped tools, and there is additional structural mass for the clean work area at altitude.&lt;/p&gt;&#xA;&lt;h1&gt;Movement to the pad.&lt;/h1&gt;&#xA;&lt;p&gt;Horizontal mating also allows for horizontal movement - SpaceX can move the assembled rocket horizontally, and erect it onto the pad, rather than moving it in vertical position. Horizontal transfer is more stable and easier to manage safely.&lt;/p&gt;&#xA;&lt;h1&gt;Some notes&lt;/h1&gt;&#xA;&lt;p&gt;It is well worth noting that the rockets are generally not fueled until after payload mating. This is true for many rockets, not just dragon. &lt;a href=&quot;http://www.spacex.com/news/2013/02/09/f9dragon-preparing-iss&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Notes&lt;/a&gt; that they only fuel the vehicle for the Wet Dress Rehearsal and Launch day, and detank the fuel between them; it's already on the pad vertically.&lt;/p&gt;&#xA;&lt;p&gt;Note that the dragon capsule is loaded in vertical orientation; in theory, this could be done after mating, but that would require craning of the aeroshell into place.&#xA;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/1/13/KSC-loading_of_Dragon-with_workers.jpg&quot; alt=&quot;Loading of Dragon&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.spaceflight101.com/dragon-c2-falcon-9-assembly.html&quot; rel=&quot;noreferrer&quot;&gt;Dragon C2&lt;/a&gt; was mated to the trunk vertically prior to horizontal integration with the Falcon 9 launch vehicle. The &lt;a href=&quot;http://www.universetoday.com/93832/just-in-from-spacex-dragon-and-falcon-9-assembly-now-complete/&quot; rel=&quot;noreferrer&quot;&gt;assembled mission&lt;/a&gt; is in a low-slung hangar. This hangar is a lower cost structure due to reduced load on the members than a vertical one. Further, note that the structure is wide enough for the Falcon 9 heavy - it's about 5x the width of the assembled falcon 9 vehicle, and the Falcon 9 heavy is about 3.4x as wide.&lt;/p&gt;&#xA;&lt;p&gt;Further note: the &lt;a href=&quot;http://www.spacex.com/news/2014/05/30/dragon-v2-spacexs-next-generation-manned-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;Dragon 2 manned capsule&lt;/a&gt; is designed for passenger loading in vertical mode on the pad. Loading it prior would be difficult, given the seating.&#xA;&lt;a href=&quot;http://www.spacex.com/sites/spacex/files/v2_seats.jpg&quot; rel=&quot;noreferrer&quot;&gt;Dragon 2 interior http://www.spacex.com/sites/spacex/files/v2_seats.jpg&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-01-15T23:40:48.520" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="4920" PostTypeId="2" ParentId="4902" CreationDate="2014-07-03T09:40:00.080" Score="12" Body="&lt;p&gt;As of 2013, NASA still had a docking ring for the JWST. ① While they have no plans to service JWST, they left the docking capability just in case.&lt;/p&gt;&#xA;&lt;p&gt;The most likely service vehicles are either an Orion capsule (4 man) or Dragon 2 Capsule (7 man); a Dragon 2 atop a Falcon Heavy could easily reach and have delta-V sufficient for return. ②&lt;/p&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;p&gt;① &lt;a href=&quot;http://www.space.com/21232-nasa-james-webb-space-telescope-infographic.html&quot; rel=&quot;noreferrer&quot;&gt;Space.com JWST Infographic (http://www.space.com/21232-nasa-james-webb-space-telescope-infographic.html)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;② &lt;a href=&quot;http://www.spacex.com/news/2014/05/30/dragon-v2-spacexs-next-generation-manned-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Dragon 2 (http://www.spacex.com/news/2014/05/30/dragon-v2-spacexs-next-generation-manned-spacecraft)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-07-03T09:40:00.080" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4922" PostTypeId="1" AcceptedAnswerId="4924" CreationDate="2014-07-03T19:25:08.850" Score="16" ViewCount="1342" Body="&lt;p&gt;What type of computers are used (generally) in robotic ( i.e. space probes and rovers) vs human craft (such as the ISS, soyuz)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Subquestion:&#xA;What kind of specs are important on computers to operate in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I ask because I was curious as to the differences between say the laptops that they sometimes use on the ISS and space probes and also what kind of electronics principles do and don't work (or just are and are not efficient) in space.&lt;/p&gt;&#xA;" OwnerUserId="3440" LastActivityDate="2014-07-05T17:01:53.080" Title="Space computing, general question" Tags="|electronics|computing|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4923" PostTypeId="1" CreationDate="2014-06-22T08:44:45.077" Score="17" ViewCount="5665" Body="&lt;p&gt;I've looked at the design of the James Webb Space Telescope and I got curious about something, some years ago, it seems that the international space station was hit by micro-meteorites. I'm wondering if the same couldn't happen in the James Webb Space Telescope. &lt;/p&gt;&#xA;" OwnerDisplayName="Igäria Mnagarka" LastEditorUserId="12102" LastEditDate="2021-08-23T00:42:22.873" LastActivityDate="2021-08-23T11:52:23.783" Title="Aren't the mirrors of the James Webb Space Telescope too unprotected?" Tags="|lagrangian-points|space-telescope|impact|james-webb-telescope|meteoroid|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4924" PostTypeId="2" ParentId="4922" CreationDate="2014-07-03T20:27:34.990" Score="15" Body="&lt;p&gt;The laptops on the ISS can only be used for non-time critical uses. Those laptops are used for some mission critical tasks, but only if the task can handle the time needed to replace a toasted laptop with another. Those laptops are commercial off-the-shelf, and they do not fare well against cosmic rays. Most cosmic ray hits result in a &quot;single event upset&quot;, where bits in memory get flipped. Reboot the computer and it's usually fine. Usually. Some cosmic ray hits do more damage and the computer truly is toast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most flight computers used to control safety or mission critical aspects of a spacecraft or a robot are extremely expensive, radiation-hardened computers whose ground counterparts were state of the practice (not state of the art) in some previous millennium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one exception is SpaceX (see &lt;a href=&quot;http://aviationweek.com/blog/dragons-radiation-tolerant-design&quot;&gt;http://aviationweek.com/blog/dragons-radiation-tolerant-design&lt;/a&gt;), which achieved the required redundancy by using lots of computers. The tradeoff is a bit more expensive software to keep all those computers in sync and find the ones that have gone off the ranch versus much less expensive and much, much, much more capable computing hardware. SpaceX chose the off-the-shelf flight computer route. Flying with computers that were state of the practice fifteen to twenty years ago puts severe limitations on what can be done onboard.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-07-03T20:45:42.527" LastActivityDate="2014-07-03T20:45:42.527" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4925" PostTypeId="2" ParentId="4923" CreationDate="2014-07-04T00:14:45.853" Score="30" Body="&lt;p&gt;No. Not too unprotected, as you put it. There are several misconceptions that I find common about the JWST, that need to be addressed:&lt;/p&gt;&#xA;&lt;h3&gt;JWST primary mirror elements are not made of glass and do not shatter on impact&lt;/h3&gt;&#xA;&lt;p&gt;It's primary hexagon mirror elements are made out of &lt;a href=&quot;https://en.wikipedia.org/wiki/Beryllium&quot; rel=&quot;noreferrer&quot;&gt;Beryllium&lt;/a&gt; powder pressed into blocks, that were later cut in half to create two mirror blanks and had most of their back side cut away leaving reinforcing rib structures and thin and light front mirror surfaces. Those  front surfaces were later precisely shaped to their specification at JWST operational temperature (-400°F, -240°C) and polished to a mirror finish. Beryllium was specifically chosen due to its lightness, strength and ability to maintain shape at mentioned cryogenic temperatures. Read more about this process on &lt;a href=&quot;http://www.webb.nasa.gov/mirrors.html&quot; rel=&quot;noreferrer&quot;&gt;JWST - The Primary Mirror&lt;/a&gt; (includes images and videos).&lt;/p&gt;&#xA;&lt;p&gt;Unlikely impacts with micrometeorites (see below) would result in &lt;em&gt;bullet holes&lt;/em&gt;, not fractures and shattered mirror surfaces, and those punctures are something that can be established during mirror calibration mode and corrected for with image post-processing. No large mirror is free from slight defects throughout their lifetime, and this is something astronomers are well used to dealing with. The important thing is that the primary mirrors still maintain their shape, together with secondary mirror maintain focus, and most of their joint surface doesn't distort end image too much with any such imperfections. Smaller &lt;em&gt;problem areas&lt;/em&gt; can be corrected for in software, or otherwise adjusted to in hardware.&lt;/p&gt;&#xA;&lt;h3&gt;JWST target halo orbit around Sun-Earth Lagrange point 2 is not littered with debris&lt;/h3&gt;&#xA;&lt;p&gt;Sun-Earth &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Lagrange point&lt;/a&gt; 2, or SEL2 for short, the point around which JWST will be stationkeeping in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot; rel=&quot;noreferrer&quot;&gt;halo orbital regime&lt;/a&gt; (its own orbital plane around SEL2 is perpendicular to the Sun-Earth plane), is one of the least gravitationally attractive points in the near-Earth space, roughly 1.5 million km more distant to the Sun than the Earth, and with the same orbital period as Earth itself. Think of this point either as the top of a hill, or perhaps better yet (since it's not really a &amp;quot;hill&amp;quot; per se, that would require negative gravitational attraction, or anti-gravitational point), as a nearly flat surface with increasingly steep slope towards its parent massive bodies, in our case the Sun and the Earth. In another analogy, where is the safest place to stand if you're at risk of being swept by an avalanche? At the flat top of otherwise a steep hill, of course.&lt;/p&gt;&#xA;&lt;p&gt;This is a lot different to orbiting around the Earth in LEO (Low Earth Orbit), where the International Space Station (ISS) is;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;First, close Earth orbits attract far more interplanetary dust that can be temporarily gravitationally captured in its orbit or at least intersect close Earth orbits, depending on their trajectory and relative velocity. I explain a bit more on this in &lt;a href=&quot;https://astronomy.stackexchange.com/q/373/13&quot;&gt;Parking a telescope at a Lagrange point: is this a good idea from a debris point of view?&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Second, SEL2 is unstable and requires constant &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_station-keeping&quot; rel=&quot;noreferrer&quot;&gt;stationkeeping&lt;/a&gt; (albeit due to &amp;quot;flatness&amp;quot; of space there, this is relatively cheap in terms of required delta-v). So any orbital debris from man-made objects wouldn't stay there for long and would fast gravitate (fall) away from the region. They would also have slow relative velocity, since there isn't any such thing as &amp;quot;retrograde orbit&amp;quot; around Lagrange points.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;And finally, JWST will not be the first man-made object that navigated the region. Other space observatories were stationkeeping at SEL2 before, such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Wilkinson_Microwave_Anisotropy_Probe&quot; rel=&quot;noreferrer&quot;&gt;Wilkinson Microwave Anisotropy Probe&lt;/a&gt; (WMAP), &lt;a href=&quot;https://en.wikipedia.org/wiki/Herschel_Space_Observatory&quot; rel=&quot;noreferrer&quot;&gt;Herschel Space Observatory&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Planck_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Planck Space Observatory&lt;/a&gt;, it's currently occupied by ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Gaia_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Gaia&lt;/a&gt; in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;noreferrer&quot;&gt;Lissajous orbital regime&lt;/a&gt;, and future missions include (on top of JWST), ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Euclid_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Euclid&lt;/a&gt;, and NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Wide_Field_Infrared_Survey_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Wide Field Infrared Survey Telescope&lt;/a&gt; (WFIRST).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h3&gt;JWST is not defenseless against collisions with debris and micrometeorites&lt;/h3&gt;&#xA;&lt;p&gt;Still, even with all said, collision with objects in transit through the SEL2 region are not excluded, so there are a few &lt;em&gt;defense mechanisms&lt;/em&gt; that JWST will have available to it and its mirrors;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The most obvious one is that it will deploy a large, multi-layered sunshield that will be kept pointing towards the Sun, as the name suggests. The &lt;em&gt;aft&lt;/em&gt; of the craft will not merely be these deployable sail-like sunshields though, but will also host more robust structures, such as observatory's communications subsystem, navigation subsystem and engines, and so on. All of them should protect the mirrors from roughly half of the impact vectors, while they point in the other direction.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Halo orbit is, as mentioned, inclined roughly 90° to the Sun-Earth plane, which means that the JWST would only pass the plane at which most transient heliocentric dust would orbit at that orbital altitude two times during each of its large 800,000 km (500,000 mi) radius orbit, and only for relatively short duration. Any of these heliocentric interplanetary dust transiting SEL2 would also have small relatively velocity to the observatory. SEL2's radial velocity is roughly 30.08 km/s (&lt;span class=&quot;math-container&quot;&gt;$v_o \approx {2 \pi a \over T}$&lt;/span&gt;), while orbital speed at Earth + 1.5 million km heliocentric altitude is roughly 29.64 km/s (&lt;span class=&quot;math-container&quot;&gt;$v_o \approx \sqrt{\frac{GM}{r}}$&lt;/span&gt;). So we're in theory talking of impact velocity in the range of 440-445 m/s, or about 1.5 times the speed of sound at sea-level Earth. In terms of possible relative velocities of man-made debris in LEO, this is roughly &lt;a href=&quot;https://encrypted.google.com/#q=(1-((443+m%2Fs)+%2F+((7.7+km%2Fs+*+2)))%5E2)*100&quot; rel=&quot;noreferrer&quot;&gt;99.9%&lt;/a&gt; smaller kinetic potential (&lt;span class=&quot;math-container&quot;&gt;$E_\text{k} =\tfrac{1}{2} mv^2$&lt;/span&gt;) per same debris mass than in LEO with retrograde debris hitting prograde satellites at up to 15.4 km/s (e.g. ISS orbits at roughly 7.7 km/s, or 4.8 mi/s).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Larger Near Earth Objects (NEO) will of course continue to be tracked by Earth-based and in-orbit observatories, and needless to say JWST can detect even the darkest surface NEO on its own that are hard to detect in visual spectrum (albeit that won't be its mission, most new NEO detections are still by chance), since it'll be doing observations in the infrared range, and can detect these objects' radiated and reflected heat signature. In case a collision with a larger object would be predicted, and this object's trajectory well established, JWST could complete collision avoidance maneuvers with its own thrusters, if needs be.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;And there are other &lt;a href=&quot;https://en.wikipedia.org/wiki/Risk_management&quot; rel=&quot;noreferrer&quot;&gt;risk management&lt;/a&gt; options available to JWST, including preparing conjunction analysis for potential collisions with other satellites stationkeeping in SEL2 or intersecting it (won't be many, but it'll still be done, no question about it), planning its stationkeeping and attitude maneuvers to avoid hazards and protect its most sensitive parts against debris, interplanetary dust, micrometeorites, solar and cosmic events, and other detected threats, when, where, and as needed. But in a general sense, SEL2 is a relatively safe place to be compared to near-Earth orbits, as far as orbital debris and micrometeorite impacts go, as per your question.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="40257" LastEditDate="2021-08-23T11:52:23.783" LastActivityDate="2021-08-23T11:52:23.783" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="4926" PostTypeId="1" AcceptedAnswerId="4927" CreationDate="2014-07-04T03:02:20.400" Score="7" ViewCount="776" Body="&lt;p&gt;How long (weeks, month, years) does it normally take engineers to find good interplanetary trajectories, such as Cassini, Messenger or BepiColombo?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would be particularly interested in the time required for the MESSENGER trajectory. I found some reference on &lt;a href=&quot;https://solarsystem.nasa.gov/news/display.cfm?News_ID=27035&quot;&gt;NASA's Solar System Exploration: News &amp;amp; Events&lt;/a&gt;, which states that it was a &lt;em&gt;&quot;challenge&quot;&lt;/em&gt;, but unfortunately no specific time.&lt;/p&gt;&#xA;" OwnerUserId="4684" LastEditorUserId="897" LastEditDate="2014-07-04T08:03:53.320" LastActivityDate="2016-02-04T02:09:39.837" Title="How much time is needed for interplanetary trajectory design?" Tags="|trajectory|interplanetary|mission-design|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4927" PostTypeId="2" ParentId="4926" CreationDate="2014-07-04T04:18:17.500" Score="9" Body="&lt;p&gt;The big question is, when do you start timing it and when do you stop? For MESSENGER (you mention you're most interested in James V. McAdams' work on its trajectory analysis, for which he was named the 2008 Engineer of the Year by the AIAA Baltimore Section), McAdams himself starts one of the related publications (&lt;a href=&quot;http://messenger.jhuapl.edu/the_mission/publications/McAdams.2003a.pdf&quot;&gt;MESSENGER MERCURY ORBIT TRAJECTORY DESIGN&lt;/a&gt;, AAS 03-209, PDF) as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After nearly three decades of Mercury orbiter spacecraft mission&#xA;  studies, recent  improvements in ballistic trajectory design and&#xA;  spacecraft technology opened the door to low-cost mission options.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And later in the same document:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During the last 15 years, space agencies around the world have&#xA;  invested heavily in  studies of “comprehensive” Mercury orbiter&#xA;  missions that often require advanced propulsion, two orbiting&#xA;  spacecraft, and sometimes a lander or surface penetrator.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So as you can see, it's tough to precisely pinpoint when MESSENGER trajectory analysis started. So I would suggest looking for when mission specific information was made available, to enable further refinement of its final trajectory. I would suggest the date of publishing the 1998 Discovery Program Announcement of Opportunity (AO), which (quoting &lt;a href=&quot;http://messenger.jhuapl.edu/the_mission/publications/McAdams.et.al.2001.pdf&quot;&gt;IMPROVEMENTS IN TRAJECTORY OPTIMIZATION FOR MESSENGER: THE FIRST MERCURY ORBITER MISSION&lt;/a&gt;, AAS 01-458, PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... imposed restrictions on launch dates, available launch vehicles,&#xA;  and total mission cost. Thermal protection, propellant mass fraction,&#xA;  and a comprehensive science payload left one choice – the Delta&#xA;  2925H-9.5 (formerly 7925H-9.5), which is the highest-performance&#xA;  expendable launch vehicle allowed within Discovery. The Delta&#xA;  2925H-9.5  launches from Cape Canaveral Air Force Station, thereby&#xA;  limiting declination of launch  asymptote (DLA) to ± 28.5˚ without&#xA;  performance degradation. The AO guideline that “a  mission launch&#xA;  every 12 to 24 months,” the AO requirement “that launch can take place&#xA;  by September 30, 2004,” and a Discovery mission (CONTOUR) launch set&#xA;  for July 1,  2002, all constrain any MESSENGER baseline and backup&#xA;  launch opportunities to begin  and end between July 1, 2003, and&#xA;  September 30, 2004.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Announcement of Opportunity: Discovery Program, NASA AO 98-OSS-04, was published on March 31, 1998.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now all you have to do is decide when to stop considering any work being done on MESSENGER's initial trajectory analysis. Perhaps its launch date on August 3, 2004? Or maybe its first Mercury flyby in January 2008? Depends on how much of its trajectory would you like to include in this timing of yours, but MESSENGER is currently still on its extended mission since March 17, 2012. And its trajectory is still being designed and periodically corrected. Your call.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For also a pretty interesting read, here's Takuto Ishimatsu's thesis for Master of Science in Aeronautics and Astronautics at Massachusetts Institute of Technology: &lt;a href=&quot;http://dspace.mit.edu/handle/1721.1/45216&quot;&gt;Interplanetary trajectory analysis for 2020-2040 Mars missions including Venus flyby opportunities&lt;/a&gt; (35.61 Mb PDF). And if you're interested in more &lt;em&gt;convoluted&lt;/em&gt; trajectories, I'd suggest looking at the work on the &lt;a href=&quot;https://en.wikipedia.org/wiki/File%3aISEE3-ICE-trajectory.gif&quot;&gt;ICE/ISEE-3&lt;/a&gt; one. It's simply fantastic, but it's &lt;a href=&quot;http://www.rockethub.com/42228&quot;&gt;still being worked on&lt;/a&gt; too, 36 years after its launch. I currently don't have a good link for it's trajectory design available, but it shouldn't be too difficult to find with a web search, it's been all over the news recently.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-07-04T04:18:17.500" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4928" PostTypeId="2" ParentId="4915" CreationDate="2014-07-04T11:24:07.477" Score="3" Body="&lt;p&gt;Adaptative Optics systems correct the wavefront error by deforming the mirror, and they do so several times per second,ie at a specific frequency $f$. This frequency depends on the speed $v$ of the turbulence in the atmosphere in relation to the observer, and the characteristic size of the turbulence $r_{0}$ (actually it is the coherence length).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rule of thumb is $f&amp;gt;\frac{v}{r_{0}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For ground system, $v$ is roughly the wind velocity in altitude. If you want to put an adaptative optics sytem in a satellite, $v$ will be on the order of the orbital velocity, ie ~8km/s. You will have to increase f a lot to compensate for that, meaning your mirror actuators and your wavefront sensor will have to work a lot faster, which is very challenging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other aspect of the problem is that AO is not needed for satellite imaging: the blur due to turbulence is in genral negligible compared to the blur due to diffraction, because satellite have much smaller mirrors than ground telescopes, and their distance to the turbulent layers in the atmosphere is much larger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: a good read on adaptative optics:&lt;a href=&quot;http://www.google.co.uk/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0CCUQFjAA&amp;amp;url=http%3A%2F%2Fcfao.ucolick.org%2Faosummer%2Fbook%2Fpdf%2F3.1_quirrenbach.pdf&amp;amp;ei=3pC2U6r3HYmr7AbeyYG4BA&amp;amp;usg=AFQjCNG2cCClXT1zlXtDbEcRT_haQbH_uQ&amp;amp;bvm=bv.70138588,d.ZGU&quot; rel=&quot;nofollow&quot;&gt;The Effects of Atmospheric Turbulence on Astronomical Observations A. Quirrenbac&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="4685" LastEditDate="2014-07-04T11:34:30.190" LastActivityDate="2014-07-04T11:34:30.190" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4929" PostTypeId="2" ParentId="4922" CreationDate="2014-07-05T06:16:46.583" Score="12" Body="&lt;h1&gt;NASA Built-in Hardware&lt;/h1&gt;&#xA;&lt;p&gt;The dedicated mission computers are tested extensively for vibration resistance, resistance to cosmic and solar radiation (both particulate and EM), magnetic fields, and are also prioritized for low energy consumption and high reliability. Generally, this results in processors a generation or three older than current desktops, and often in larger architecture versions and slower speeds.&lt;/p&gt;&#xA;&lt;p&gt;Further, the motherboards and enclosures are often custom made designs, like the Shuttle's upgraded AP-101B, using a 400 kHz single core main processor, and 24 dedicated IO processors, in a radiation resistant case with a vibration resistant motherboard, and a very small amount of memory - a few hundred kilobytes.&lt;/p&gt;&#xA;&lt;p&gt;Many satellites use off-the shelf processors; for years, the Zilog Z-80, Intel 8080, and Motorolla 68000 were staple processors, because they were both inexpensive (having outlived the patents), and had already passed the NASA testing requirements. Newer processors have since replaced them in common use, but missions into the late 1990's made use of NASA-specific production 8080 chips. (A top-end Z80 is 20 MHz, 8-bit clean, single core, and $12. This has resulted in a couple of students looking to use it on microsat projects - it's cheap and space rated.)&lt;/p&gt;&#xA;&lt;h1&gt;Off the Shelf Hardware&lt;/h1&gt;&#xA;&lt;p&gt;NASA has, since about 1992, allowed the use of off the shelf IBM laptops for some mission critical applications. They've found that the devices are unreliable in orbit, due to magnetic field fluctuations. Still, they are reliable enough to be used  in general purpose non-flight-essential uses.&lt;/p&gt;&#xA;&lt;p&gt;Hand calculators, including HP-41's, have been used by several astronauts over the years; these devices have larger circuits, slower processors, less memory, and have always been used only for non-mission-critical applications. (The HP-41 also was used to simulate the AP-101. It was also the backup plan in case of failure of the AP-101 cluster.)&lt;/p&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/flyout/flyfeature_shuttlecomputers.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/mission_pages/shuttle/flyout/flyfeature_shuttlecomputers.html&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/avionics/dps/gpc.html&quot; rel=&quot;noreferrer&quot;&gt;http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/avionics/dps/gpc.html&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://hpinspace.wordpress.com/category/hp-41/&quot; rel=&quot;noreferrer&quot;&gt;http://hpinspace.wordpress.com/category/hp-41/&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.hq.nasa.gov/pao/History/computers/Ch4-6.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.hq.nasa.gov/pao/History/computers/Ch4-6.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-07-05T06:16:46.583" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4930" PostTypeId="2" ParentId="4922" CreationDate="2014-07-05T17:01:53.080" Score="2" Body="&lt;p&gt;For mission-critical computers (flight control, etc.) ESA uses the &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Engineering/LEON_the_space_chip_that_Europe_built&quot; rel=&quot;nofollow noreferrer&quot;&gt;LEON processor&lt;/a&gt;, based on &lt;a href=&quot;https://space.stackexchange.com/questions/729/why-did-the-esa-choose-sparc-for-leon&quot;&gt;Sun's SPARC&lt;/a&gt; architecture. They used &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Engineering/Leading_up_to_LEON_ESA_s_first_microprocessors&quot; rel=&quot;nofollow noreferrer&quot;&gt;other custom designs&lt;/a&gt; before LEON.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-07-05T17:01:53.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4931" PostTypeId="1" AcceptedAnswerId="4932" CreationDate="2014-07-06T08:54:10.403" Score="2" ViewCount="427" Body="&lt;p&gt;Within what degree of accuracy can a satellite be placed into orbit around earth, or any object for that matter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it placed to within a few meters or rather a few kilometers?&lt;/p&gt;&#xA;" OwnerUserId="4693" LastEditorUserId="25" LastEditDate="2014-07-06T12:42:50.273" LastActivityDate="2014-07-06T12:47:22.077" Title="Placement of satellites" Tags="|launch|artificial-satellite|rockets|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4932" PostTypeId="2" ParentId="4931" CreationDate="2014-07-06T12:47:22.077" Score="4" Body="&lt;p&gt;Just to give you an idea, here's the accuracy numbers from the &lt;a href=&quot;http://decadal.gsfc.nasa.gov/pace-201206mdl/Launch%20Vehicle%20Information/Falcon9UsersGuide_2009.pdf&quot; rel=&quot;nofollow&quot;&gt;Falcon 9&lt;/a&gt;. Each rocket has it's own uncertainty, but the F9 is pretty representative.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low Earth Orbit&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Perigee ± 10 km&lt;/li&gt;&#xA;&lt;li&gt;Apogee ± 10 km&lt;/li&gt;&#xA;&lt;li&gt;Inclination ± 0.1 deg&lt;/li&gt;&#xA;&lt;li&gt;Right Ascension of Ascending Node ± 0.15 deg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Geosynchronous Transfer Orbit&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Perigee ± 7.4 km&lt;/li&gt;&#xA;&lt;li&gt;Apogee ± 130 km&lt;/li&gt;&#xA;&lt;li&gt;Inclination ± 0.1 deg&lt;/li&gt;&#xA;&lt;li&gt;Right Ascension of Ascending Node ± 0.75 deg&lt;/li&gt;&#xA;&lt;li&gt;Argument of Perigee ± 0.3 deg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are the parameters that really count for orbits. They could represent a fairly large uncertainty in the predicted path.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-07-06T12:47:22.077" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4934" PostTypeId="1" AcceptedAnswerId="5000" CreationDate="2014-07-07T11:51:19.867" Score="8" ViewCount="351" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Bigelow_Aerospace&quot; rel=&quot;nofollow&quot;&gt;Bigelow Aerospace&lt;/a&gt; has two inflatables on orbit (&lt;a href=&quot;http://en.wikipedia.org/wiki/Genesis_I&quot; rel=&quot;nofollow&quot;&gt;Genesis I&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Genesis_II&quot; rel=&quot;nofollow&quot;&gt;Genesis II&lt;/a&gt;) with plans for attaching the &lt;a href=&quot;http://en.wikipedia.org/wiki/Bigelow_Expandable_Activity_Module&quot; rel=&quot;nofollow&quot;&gt;BEAM&lt;/a&gt; module to the ISS. These are precursors to a full scale station, based on multiple (3?) BA-330 full size modules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan is for solar panels at the ends of the BA-330 modules.  They seem like they will have lower power output than the ISS, based simply on size.  (Assuming efficiency is vaguely similar, even though the panels will be 20 years newer). Ignoring the power issue for a moment, I am curious about the heat rejection problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, paradoxically, it would seem, getting rid of heat is one of the harder problems. Between the 90 min day/night cycle, the temperature cycling, and the amount of heat humans and equipment put out, getting heat OUT of the station is a major design issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS has some very large radiators, the ammonia cooling loop system, that have made the news in past years for a variety of problems. (MMOD on one of the radiatiors, ammonia leaks, failed pumps, etc). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does Bigelow plan to handle the heat rejection issue?  None of the images I have seen, show obvious large radiators.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="12102" LastEditDate="2021-08-09T01:47:49.287" LastActivityDate="2021-08-09T01:47:49.287" Title="How does Bigelow plan to handle heat rejection on the BA-330?" Tags="|heat|ammonia|bigelow|radiators|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4935" PostTypeId="1" CreationDate="2014-07-07T15:08:13.457" Score="8" ViewCount="592" Body="&lt;p&gt;My daughter and I are debating whether it is technically feasible to bring a nickel iron asteroid to the surface or the earth non-destructively.  I felt it was impossible.  Her thought was that with a propulsion system like M2P2 (already on the drawing boards) that would act on the entire ferro-magnetic body at once so as to avoid breaking it up, and a long enough time to fiddle with its orbit, one could put it in an elliptical solar orbit whose aphelion precisely matched the position, rotational velocity, and orbital velocity of some spot on the surface of the earth.  With a big enough chunk, atmospheric effects would be tiny.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would not exactly be a Hohmann transfer orbit to earth from the vicinity of Mercury, but would be similar in concept.  After some thought, I granted that it might be possible but calculating the orbital mechanics would be tough slogging and I am pretty sure the required precision is not currently available in any propulsion system.  The earth is also not perfectly round and the thing would probably have to bash through a couple of mountain ranges before coming to rest unless the whole last bit of its orbital spiral was over an ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question remains though.  If we had the time, computing power, and precision thrust, could this be done?  Imagine a few kilometer high piles of nickel iron (with very interesting impurities) in isolated spots around the equator.  The impact on industry, science, the economy and human life in general would be profound.&lt;/p&gt;&#xA;" OwnerUserId="4699" LastActivityDate="2014-07-09T16:17:24.780" Title="Can one put a large nickel-iron asteroid into an elliptical solar orbit that results in a soft(ish) earth landing?" Tags="|orbital-mechanics|asteroid|mining|advanced-propulsion|computing|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="4936" PostTypeId="1" AcceptedAnswerId="4937" CreationDate="2014-07-07T15:28:49.930" Score="11" ViewCount="2382" Body="&lt;p&gt;I'm sure it cannot because I've never heard it suggested even on the pretty imaginative space flight communities on line. But the question is why, what makes it a bad idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some orbital data I find on Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Altitude&lt;/strong&gt;:&#xA;ISS 220-435 km,&#xA;HST 569 km,&#xA;&lt;em&gt;diff 134-349 km&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Inclination&lt;/strong&gt;:&#xA;ISS 51.6 degrees,&#xA;HST 28.5 degrees,&#xA;&lt;em&gt;diff 23.1 degrees&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the energy of the higher altitude of HST together with its remaining rocket fuel be enough to dock with the ISS? Could the ISS be refueled enough to reach an orbit to meet HST half way or further?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK the HST has no conventional docking ability, it needs to be grabbed by a robotic arm. And ISS is the only thing which has that capability (the &quot;Canadarm&quot;), if even that is suitable for this purpose. Implementing such a contraption on Dragon or Cygnus or ATV or Soyuz would take longer than the HST can survive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be a meaningful investment to save the Hubble Space telescope any further? Or is it too aged to be used as an optical platform for upgraded instruments? And would its operations be hurt by joining orbits with the ISS?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="4703" LastEditDate="2014-07-07T19:48:55.377" LastActivityDate="2015-03-03T23:11:18.647" Title="Could the Hubble HST be saved by docking with the ISS?" Tags="|orbital-mechanics|iss|low-earth-orbit|hubble|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4937" PostTypeId="2" ParentId="4936" CreationDate="2014-07-07T16:46:55.540" Score="24" Body="&lt;p&gt;The altitude difference is minor compared to the plane change.  Even 23 degrees of plane change would require a prohibitively large change in velocity&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2 * 7.5 km/s * sin(23 degrees / 2) = 3.0 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality the plane change required is greater than 23 degrees, because the two orbits don't have the same right ascension of ascending node.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble actually has no propulsive capability at all, but for comparison a typical earth orbiting satellite might have a 200 m/s delta-V capacity; Shuttle with full OMS tanks had approx 300 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be even less practical to move the ISS - far heavier, so far more impulse required for the same delta V.  In a lower inclination orbit, the ISS could not be reached by crew and cargo vehicles launched from Baikonur.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even disregarding those factors, if Hubble were docked to the ISS, it would be useless because &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Pointing control and stability of the station are insufficient for long astronomical exposures.&lt;/li&gt;&#xA;&lt;li&gt;Outgassing from the various activities on the ISS would contaminate the optics.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-07-07T16:46:55.540" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4938" PostTypeId="2" ParentId="4935" CreationDate="2014-07-07T17:39:20.840" Score="10" Body="&lt;p&gt;If gravity was repulsive between the Earth and the asteroid, this could at least make sense in principle.  In that case, getting close is like pushing on a spring, and with a carefully managed trajectory, you might be able to finish the trajectory on the surface, with zero gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But gravity isn't like a pushing spring, it's like a pulling spring.  That means that you can't cancel your incoming velocity by some carefully managed trajectory.  This argument can be made from many different perspectives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Energetically, you have a problem getting rid of the potential energy &lt;em&gt;relative to Earth&lt;/em&gt;, not relative to the sun.  You need to both get within the Earth's sphere of influence (SOI), and reduce your velocity to a small amount in order to have a soft landing.  Short of quantum tunneling, this just doesn't make any sense.  In order to &lt;em&gt;exist at all&lt;/em&gt; on the edge of Earth's SOI you must have tremendous gravitational potential energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see somewhat what the &lt;em&gt;thought&lt;/em&gt; is.  If you had a lower energy orbit relative to the sun, you could match the total kinetic-potential energy to a point on the Earth's surface.  But the problem is that there's no way to transition between the two.  That would also not be the case for a Mercury-Earth elliptical orbit.  The kinetic energy at perihelion is lower than an Earth orbit relative to the sun, but that's irrelevant.  Kinetic energy is huge relative to the Earth itself.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-07-07T17:39:20.840" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4940" PostTypeId="1" CreationDate="2014-07-07T23:03:12.563" Score="6" ViewCount="844" Body="&lt;p&gt;Could a trajectory exist that gave an object kinetic energy relative to the Earth/Moon that somehow passes zero as the Earth moved across the other's path, tugging on it?  And having it happen at a potential energy minimum (e.g., a flat place on the surface of the Earth or Moon)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had been wondering about this as a way to soft-land on the moon with minimal $\Delta\ v$.  A recent question mentioned something similar as a way to soft-land on Earth, but didn't appear set up to cancel the relative kinetic energy.  My question also doesn't specify a keplerian orbit for the &quot;lander&quot;, just whether a trajectory could exist and what features might describe a trajectory satisfying this scenario.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A compelling answer would show quantitatively the potential energies being considered and the range of relative kinetic energies for which a solution, if any, exists.  I presume the lander's relevant kinetic energies come from the E/M system moving in orbit around the sun, the moon around the Earth, and their rotations ranging from effectively zero at the poles to an absolute maximum at their equators.  I'm also presuming that inclinations, tilts, topography, and (Earth's) atmosphere don't drastically effect the answer's validity unless of course &quot;making it&quot; actually relies on any of those.  Since some responses so far suggest it might be possible only elsewhere in the solar system, an answer that lets other planet and maybe moon masses, radii and velocities be substituted would be extra useful, though not necessary.&lt;/p&gt;&#xA;" OwnerUserId="3230" LastEditorUserId="3230" LastEditDate="2014-07-08T17:49:01.453" LastActivityDate="2014-07-10T15:16:07.070" Title="Soft landing by gravity assist?" Tags="|orbital-mechanics|retrograde-orbit|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4941" PostTypeId="2" ParentId="4935" CreationDate="2014-07-08T02:06:15.403" Score="7" Body="&lt;p&gt;While AlanSE did a fine job of addressing it I think I can do a better job of addressing where she's going wrong:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, I think an orbit as she is envisioning exists.  The problem is that Earth has mass and will pull it out of solar orbit.  It won't be in that nice orbit when it hits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest you could theoretically come to a soft landing would be to put it in Earth orbit and then use whatever engine you have to lower the perigee to zero.  It's going to hit at about 8km/sec but you could make that a rolling impact rather than a great splat.  If the Earth were a billiard ball with nothing on it that matters you could land stuff that way.  The real Earth has things like mountains and cities, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see only two even remotely possible landing sites:  Antarctica and Siberia.  Whether it would stop rolling before leaving those areas is a matter for those far more skilled than I.  I wouldn't say either touchdown is ecologically benign even if it stops in time, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides, we don't need it.  If you can move the rock move it into Earth orbit.  Put a mining facility on it, extract &amp;amp; purify the metals and foam them.  Take some of the slag, foam it and use it as a heat shield.  Put it in a mass driver and give it enough of a kick to deorbit.  The dropzone must be water and huge because there's no drive or guidance on it.  The slag burns in the upper atmosphere, make sure there's enough that it doesn't burn through.  Once it's been slowed it's just a falling object, it's terminal velocity is low enough that it won't melt or be vaporized on impact.  While it might not survive the impact intact the pieces float.  Go pick them up.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-07-08T02:06:15.403" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="4942" PostTypeId="2" ParentId="4940" CreationDate="2014-07-08T12:54:15.667" Score="5" Body="&lt;p&gt;I don't think so, at least for the Earth/Moon system.  It's tempting to imagine, say, an Earth-centered elliptical orbit that just touches the Moons surface, but that won't work out well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take a look at the Wikipedia pages for &quot;Sphere of Influence  (astrodynamics)&quot; and &quot;Hill sphere.&quot;  Once you get within about 60,000km of the Moon, the Moon's gravity dominates. From there on, to a first approximation, you're falling towards the Moon from whatever initial trajectory you entered on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, think about it as a single body problem: is there any trajectory you could fall from space on that has you hitting the ground at an acceptable speed?  Start with your spaceship at the edge of the moon's Hill sphere.  At the very least, it has to be approaching the moon with velocity of zero or greater--otherwise, it would move away from the moon back into an Earth orbit.  So, the lowest speed it can reach the Moon's surface with is what it would have falling from 60,000km.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As TildalWave pointed out, you might be able to pull something off if you're landing on a fast spinning object.  Not sure if that's true, the thing might have to be spinning faster than its orbital velocity for that to work, but there's probably some narrow window of barely stable fast spinning and lithobraking that would work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think your idea will only work for bodies that are &lt;strike&gt;smaller&lt;/strike&gt; larger than their own Hill sphere--in that case, you can treat the spaceship as in orbit around something else until it reaches the surface.&lt;/p&gt;&#xA;" OwnerUserId="4707" LastEditorUserId="4707" LastEditDate="2014-07-08T13:09:35.490" LastActivityDate="2014-07-08T13:09:35.490" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4943" PostTypeId="2" ParentId="4940" CreationDate="2014-07-08T15:16:50.317" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;A recent question mentioned something similar as a way to soft-land on Earth, but didn't appear set up to cancel the relative kinetic energy. My question also doesn't specify a keplerian orbit for the &quot;lander&quot;, just whether a trajectory could exist and what features might describe a trajectory satisfying this scenario.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The specific gravitational potential follows a simple $1/r$ function. But of course, neighboring bodies exist with their own gravity well. So you can be at the same potential in two different places. This is my best thinking of what these questions are trying to communicate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me use the &lt;a href=&quot;http://xkcd.com/681/&quot; rel=&quot;nofollow noreferrer&quot;&gt;xkcd gravity wells infographic&lt;/a&gt; to illustrate those thoughts. (license is non-commercial CC-SA-2.5, so this is actually legal use)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/fEXux.png&quot; alt=&quot;weird idea&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't say that you can't do something like this, but just that there's no obvious way with &lt;em&gt;rocket&lt;/em&gt; technology. In quantum physics, things jump straight through potential barriers like this all the time. But our ships are (extremely) sufficiently non-quantum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there's a much more simple manifestation of such a &quot;jump&quot;, through space elevators/tethers. Consider a space elevator from the moon's surface to EML-2. You could make the space elevator fully self-powered, if delivering material from the moon's surface to high Earth orbit. Just have material on the &quot;up&quot; side of the elevator get pulled by material going down the &quot;down&quot; side of the elevator. Since the &quot;down&quot; side isn't limited by the moon's surface, you can get energy out of the process. You can also deliver things from a high Earth orbit (really a pseudo-orbit, I'm glossing over some details) to the surface of the moon in a zero-energy process. Main drawback is that you need a space elevator in the first place.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-07-08T15:16:50.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4944" PostTypeId="1" CreationDate="2014-07-08T16:23:45.300" Score="9" ViewCount="188" Body="&lt;p&gt;Let's say that we're going to seriously shield a manned space station (to the tune of 1-2 m thickness) in LEO so that radiation doses are a reasonable multiple of background on Earth. Let's also assume that we have freedom of attitude selection, so we can have one direction constantly pointing toward the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could you omit the Earth-facing wall because cosmic rays won't come from below? Or maybe the radiation will scatter back up? What would an optimized shielding envelope look like?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2015-07-02T17:56:19.040" Title="Shape of shielding for a tidally-stabilized LEO space station" Tags="|low-earth-orbit|space-station|shielding|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4946" PostTypeId="2" ParentId="4940" CreationDate="2014-07-08T18:26:32.610" Score="17" Body="&lt;p&gt;Proof By Contradiction:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume such a &quot;Soft&quot; Landing Trajectory (SLT) exists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then anything that happens to be sitting at the SLT landing spot could be &quot;launched&quot; into the &lt;strong&gt;reverse&lt;/strong&gt; SLT with the same tiny residual energy input. Just run the equations backwards in time...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we see chunks of the moon just drifting off?&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2014-07-08T18:26:32.610" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4947" PostTypeId="2" ParentId="4935" CreationDate="2014-07-09T16:17:24.780" Score="3" Body="&lt;p&gt;Alan SE gave a good answer. But I'd like to add some stuff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Alan mentioned we need to consider earth's gravity. Once an asteroid comes within earth's sphere of influence, it's path is no longer well modeled as an ellipse about the sun. At that point it becomes better modeled as a hyperbola with a focus at earth's center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speed of a hyperbola is $\sqrt{V_{escape}^2+V_{infinity}^2}$ I remember this by thinking of $V_{escape}$ and $V_{infinity}$ as legs of a right triangle. Then the hypotenuse is the hyperbola's speed. This memory device relies on the good old Pythagorean theorem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what's &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow&quot;&gt;$V_{infinity}$&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For practical use, $V_{infinity}$ is the asteroid's speed with regard to earth when it enters earth's Sphere Of Influence (SOI). Earth moves about 30 km/s about the sun. If the asteroid's moving a little slower than earth, say 29.5 km/s, then $V_{infinity}$ is .5 km/s. If the asteroid were moving a little faster, say 30.5 km/s, we'd still have a .5 km/s $V_{infinity}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best we can do is match earth's 30 km/s which would make $V_{infinity}$ zero. In which case $\sqrt{V_{escape}^2+V_{infinity}^2}$ becomes $\sqrt{V_{escape}^2+0^2}$ which is simply $V_{escape}$. Then the orbit becomes a parabola rather than a hyperbola.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{escape}$ is about 11 km/s near earth's surface. A parabolic orbit is the slowest you can get.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;However...&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible to use the moon's gravity to slow the rock's speed relative to earth. Using the moon we can drop the speed from a parabolic orbit to an elliptical capture orbit about the earth. The rock can then be parked in a lunar orbit for surprisingly little delta V. I would suggest you and your daughter check out the &lt;a href=&quot;http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf&quot; rel=&quot;nofollow&quot;&gt;Keck Report&lt;/a&gt;. Co-authors of this asteroid retrieval study include Chris Lewicki (chief engineer for &lt;a href=&quot;http://www.planetaryresources.com/asteroids/usage/&quot; rel=&quot;nofollow&quot;&gt;Planetary Resources&lt;/a&gt;) and J. S. Lewis (author of &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0201328194&quot; rel=&quot;nofollow&quot;&gt;Mining the Sky&lt;/a&gt;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I give Planetary Resources better than even odds for retrieving asteroids and making a profit. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-07-09T16:17:24.780" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4948" PostTypeId="1" CreationDate="2014-07-09T16:19:50.603" Score="0" ViewCount="2154" Body="&lt;p&gt;I was reading this news paper &lt;a href=&quot;http://timesofindia.indiatimes.com/india/With-eye-on-space-debris-Isro-starts-countdown-for-PSLV-launch-3-minutes-late/articleshow/37371744.cms&quot; rel=&quot;nofollow&quot;&gt;article&lt;/a&gt; which says&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Keeping a close watch on some space debris that threatens to get in the way of the satellites to be launched by PSLV-C23 on Monday morning, the Indian Space Research Organisation (Isro) started the 49-hour countdown for the launch three minutes late, at 8.52am on Saturday.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so my questions is why the space debris does not fall on to earth or why they dont roam in universe.The space debris might be from a satellite launch long before so why do they still orbit around earth.If you say this is due to the orbital velocity with which the satelite is launched and so the debris also get the speed and orbits the earth then why they dont get attracted towards as the faulty satellites do&lt;/p&gt;&#xA;" OwnerUserId="1006" LastActivityDate="2014-07-09T21:44:55.817" Title="why dont space debris fall on earth or roam in universe" Tags="|artificial-satellite|debris|" AnswerCount="1" CommentCount="5" ClosedDate="2014-07-10T11:09:56.343" ContentLicense="CC BY-SA 3.0" />
  <row Id="4949" PostTypeId="1" AcceptedAnswerId="4950" CreationDate="2014-07-09T16:44:21.183" Score="36" ViewCount="8226" Body="&lt;p&gt;I was recently rereading this previous question of mine:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2372/what-would-happen-if-a-mutiny-occured-on-the-international-space-station&quot;&gt;What would happen if a mutiny occurred on the International Space Station?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I was considering different contingencies, the thought occurred to me that Mission Control locking out the crew members from the systems might not be effective if the ISS is &quot;hackable&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider the unlikely scenario in which the crew of the International Space Station (ISS) decides to mutiny and ally with China. Mission Control locks out the crew from the controls, but then the crew hacks the ISS to regain control. China, as their partner, promises to send resources in lieu of the regular deliveries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe it is impossible for the crew to regain control from aboard the station, but there is another possible scenario: someone hacks remote control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider in the above unlikely scenario if instead of the crew hacking the ISS, China hacked remote control of the ISS in cooperation with the mutinied crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is either type of &quot;hacking&quot; the ISS possible? If so, how difficult would each be to pull off?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-07-23T12:51:12.133" Title="Is the International Space Station &quot;hackable&quot;?" Tags="|iss|security|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4950" PostTypeId="2" ParentId="4949" CreationDate="2014-07-09T17:53:25.023" Score="46" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station#Communications_and_computers&quot;&gt;Wikipedia&lt;/a&gt; contains some information about the computer systems which control the ISS. Short version is: it is a mess.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently, some modules have their own systems and communication frameworks. Internally, WiFi is used. The &quot;key systems&quot; are documented to run Debian Linux, while they previously used Windows -- which means that these systems are basically PC, and run common operating systems. As such, they &lt;em&gt;must&lt;/em&gt; also have the same vulnerabilities. I also expect that they don't get patched for security fixes on a regular basis. So the bottom-line is that the ISS should be highly &quot;hackable&quot; -- provided that the attacker gains some access to the communication network of the ISS. Hacking into the ISS from the ground would imply penetrating one of the two communication networks which link the ISS to the external world (the American &lt;a href=&quot;http://en.wikipedia.org/wiki/TDRSS&quot;&gt;TDRSS&lt;/a&gt; and the Russian &lt;a href=&quot;http://en.wikipedia.org/wiki/Lira_%28ISS%29&quot;&gt;Lira&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On-board astronauts should find it easy to take full control of the ISS systems, because it is highly improbable that these PC are physically shielded (shielding is weight, and weight is &lt;em&gt;very&lt;/em&gt; expensive when talking about orbital things). Also there is a big Tradition in human space exploration (and human exploration in general) to keep the local human in control of things -- that's the reason why we send humans and not chimpanzees in orbit. In that sense, the ISS crew cannot &quot;hack into&quot; the ISS computers because they already have full control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gaining control of the ISS system would not buy much to any attacker. At best, they could spread some local mayhem, probably kill the crew or threaten to do so if some ransom is not paid (e.g. by swinging the robotic arm into the hull, or sabotaging the oxygen inflow, or using the the ISS orbital control engines to precipitate it down to the atmosphere, or playing some Britney Spears at full volume in the audio system). Though an atmospheric reentry may fail to burn the complete station, targeting a precise area with the debris seems overly difficult, and the impact energy would not be high, so a subverted ISS would not account for a plausible orbital bombardment weapon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To summarize: hack into the ISS, yeah, but what for ? It is not a boat; it cannot be made to land anywhere. Its strategic military value is nil. The ISS is good for Science and Public Relations, but very useless otherwise.&lt;/p&gt;&#xA;" OwnerUserId="311" LastActivityDate="2014-07-09T17:53:25.023" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="4952" PostTypeId="2" ParentId="4948" CreationDate="2014-07-09T21:44:55.817" Score="4" Body="&lt;p&gt;In general terms, once an object is in orbit, it will stay in orbit.  Doesn't matter if it's debris, a satellite, rocket body, etc.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing that can bring it back down to earth or break it out of earth orbit to &quot;roam the universe&quot; is a significant energy change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only natural energy change that can bring the satellite down is atmospheric drag, which will only impact the very lowest orbits - those with a perigee (closest point of the orbit to earth) &amp;lt; 500-600 km altitude.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing that can break it out of earth orbit to &quot;roam the universe&quot; is a lot more energy to boost the object up to escape velocity.  Something that doesn't happen naturally.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastActivityDate="2014-07-09T21:44:55.817" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4953" PostTypeId="2" ParentId="4940" CreationDate="2014-07-10T15:16:07.070" Score="3" Body="&lt;p&gt;The answer is yes if you only land part of the craft. You use a &quot;wheel&quot; of long lines or whips (one is enough) that rotate so that as they pass the landing point or orbit, the cable end is like a huge wheel running on the surface and a payload can be set down with zero velocity. The complete dynamics of inter-solar system travel with whips was described by the late Robert Forward. His company after he left Hughes Research, Tethers Unlimited, has made some devices for NASA and works on various tether ideas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has potential for a Moon landing x-prize - a no-motor no-fuel soft landing. &lt;/p&gt;&#xA;" OwnerUserId="1432" LastActivityDate="2014-07-10T15:16:07.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4956" PostTypeId="1" CreationDate="2014-07-10T19:56:15.923" Score="7" ViewCount="173" Body="&lt;p&gt;Given the interest in using lunar water for propellant manufacture, how do you calculate the production rate for electrolytic breakdown and liquefaction of water into cryogens?  A corollary question is how do you calculate power needed?&lt;/p&gt;&#xA;" OwnerUserId="3227" LastEditorUserId="168" LastEditDate="2014-07-11T03:59:12.377" LastActivityDate="2014-07-11T15:17:13.400" Title="Electrolysis Production Rate" Tags="|isru|chemistry|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4957" PostTypeId="2" ParentId="4077" CreationDate="2014-07-10T23:12:17.657" Score="1" Body="&lt;p&gt;Some interesting info from ISEE-3 via eoPortal: &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/i/isee&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://directory.eoportal.org/web/eoportal/satellite-missions/i/isee&lt;/a&gt; from the figure 12 &quot;ISEE-3 spacecraft trajectory overview from halo orbit to geomagnetic tail&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0HxVU.jpg&quot; alt=&quot;ISEE-3 spacecraft trajectory overview from halo orbit to geomagnetic tail&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the tail is depicted correctly, its diameter near L2 is ~1.5 times less then Moon's orbit diameter (0.77 mln km), or near 0.5 million kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.physics.usyd.edu.au/~cairns/teaching/lecture14/node4.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Global Magnetosphere&lt;/a&gt; page from &quot;Earth's Magnetosheath, Magnetopause and Magnetosphere&quot; lecture by Iver Cairns, 1999 has interesting picture &quot;Figure 14.7: Noon-midnight cross section of the magnetosphere and geomagnetic tail [Hughes, 1995]. Magnetic field lines are shown using solid lines while the dashed lines shows particles moving along the field subject to the ...&quot;, and I think that sizes are in R_e units (6400km).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/LR1p6.gif&quot; alt=&quot;Figure 14.7: Noon-midnight cross section of the magnetosphere and geomagnetic tail  - Hughes, 1995&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original source of this figure is Hughes95, &lt;a href=&quot;http://www.ss.ncu.edu.tw/~lyu/lecture_files_en/IntroSpacePhysics_KivelsonRussell/SPIntro/SPIntro_9_new.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ss.ncu.edu.tw/~lyu/lecture_files_en/IntroSpacePhysics_KivelsonRussell/SPIntro/SPIntro_9_new.pdf&lt;/a&gt; page 232 (he taked the picture from &quot;Pilipp and Morfill 1978&quot;). The &quot;X line&quot; point is located near 115 R_e. Moon orbit semi-axis is at 60 R_e (60 Earth radii). Sun-Earth L2 is 240 R_e, so we can expect something around 60 R_e diameter of Magnetopause at L2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;200,000 - 300,000 km halo orbit around L2 is equal to 30-40 R_e. So I can conclude that Earth magnetosphere may cover the exact L2  point. But at the same time, it is unlikely to give much protection or constant protection to the satellite orbiting so far from L2 (actually, it is periapsis of halo orbit parameters of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Gaia_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gaia&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="2843" LastActivityDate="2014-07-10T23:12:17.657" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4958" PostTypeId="2" ParentId="2339" CreationDate="2014-07-11T10:04:30.173" Score="14" Body="&lt;p&gt;NASA did research on &lt;code&gt;Feasibility of liquid-metal vacuum seals&lt;/code&gt;, the article was published in 1963 and is accessible &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640018287.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; (please look up section B, page 29).&lt;/p&gt;&#xA;&lt;p&gt;Here is quote of the articles conclusion sections.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A procedure was found for using a liquid-metal seal in an ultrahigh-vacuum system.&lt;/p&gt;&#xA;&lt;p&gt;Leak reates through a &lt;span class=&quot;math-container&quot;&gt;$5.5$&lt;/span&gt; inch-diameter seal were so small that, with a chamber pressure of &lt;span class=&quot;math-container&quot;&gt;$1$&lt;/span&gt; micron, there was no indication on a leak detector with a sensitivity of &lt;span class=&quot;math-container&quot;&gt;$3*10^{-10}$&lt;/span&gt; standard cc of helium per second. Also, leakage was small enough to give no appreciable in-bleed to a system at a chamber pressure of &lt;span class=&quot;math-container&quot;&gt;$7*10^{-9}$&lt;/span&gt; torr.&lt;/p&gt;&#xA;&lt;p&gt;The liquid-metal seal, essentially eliminating gas permeation and requiring a very small force per unit length to achieve a seal, has several advantages over the conventional O-ring and metallic shear seals. However, because of the extensive preliminary procedure required to prepare the flanges and liquid metal for the seal, the liquid-metal seal would be difficult to make use of on a routine basis.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This technology is considered to be used in &lt;a href=&quot;http://en.wikipedia.org/wiki/Nautilus-X&quot; rel=&quot;noreferrer&quot;&gt;Nautilus-X&lt;/a&gt;, which as you can see, has a spinning wheel that should provide about a half of Earths gravity experience.&lt;/p&gt;&#xA;&lt;h2&gt;Edit:&lt;/h2&gt;&#xA;&lt;p&gt;If the portion of the aircraft that is spinning is small enough, the only problems are ones that you listed, power and air pressure, but imagine we are speaking about very big spacecraft with a very big spinning ring (or even more than one big spinning ring), then you would have much more problems, for example liquid exchange (which is usefull for heat exchange or simply water supply for the crew).&lt;/p&gt;&#xA;" OwnerUserId="4627" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-07-11T12:23:56.367" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4959" PostTypeId="1" CreationDate="2014-07-11T12:05:39.727" Score="8" ViewCount="4303" Body="&lt;p&gt;The basic idea of staged rocketry is to get rid of the mass of empty tanks et cetera as soon as possible during launch, isn't it? So you would ideally want to drop them one at a time as soon as they are emptied.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK, Delta IV Heavy does, and Falcon Heavy and Angara are planned to, drop their common booster side cores both at once (with Angara planning up to 6 strap on common cores, I don't know if they will drop 2 at a time or all 6 at once). But why not drop just one at a time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One argument for simultaneous separation might be to maintain symmetry and stability. But at least naively, if the rocket rotates 90 degrees so that the three cores are on top of each other, then removing one of them should not create any asymmetry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, while SpaceX plans to reuse its boosters, Delta IV Heavy does not. SpaceX needs boosters on their side cores in order to soft land them. But why does Delta IV Heavy have rocket engines on its side cores? Engines which are dropped when there's fuel left in the central core. Why not fix all rocket engines to the central core and just use drop tanks on the side?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Atlas V, Ariane 5, Proton, SLS the empty weight of the smallish side boosters might be too little to care too much about. But it must be very different with rockets using 3 or more similarly sized cores in a bundle.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-07-11T13:22:26.810" LastActivityDate="2018-02-01T12:24:56.273" Title="Why do rockets separate from both its strap-on booster cores simultaneously instead of one at a time? (Delta IV Heavy, Falcon Heavy, Angara)" Tags="|launch|rockets|stages|delta-iv-heavy|booster|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4960" PostTypeId="2" ParentId="4959" CreationDate="2014-07-11T14:20:41.023" Score="10" Body="&lt;p&gt;Consider the amount of thrust the various cited side boosters provide:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 Heavy: 9 Merlin 1Ds at 155Klbs each = 1.4 Mlbs thrust&lt;br&gt;&#xA;Delta 4 Heavy: 1 RS-68 ~ 600Klbs thrust&lt;br&gt;&#xA;Araine 5:  P230 solid 1.45 Mlbs thrust&lt;br&gt;&#xA;Angara: RD-191 on a URM ~432 KLbs thrust  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are not minor numbers (conceding my point is less valid on Angara and D4-H)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you generate a thrust instability of 1.4 million lbs on anything, it is not going to end well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However since the paired side boosters typically ignite at the same time, burn at the same rate, using identical components, they burn out at the same time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core may be throttled down (D-4H and probably Angara in the future, only a single URM model has launched sub orbital so far) or use cross feed (F9-H maybe) in the case of identical core modules.  It may be a different setup entirely (Ariane 5, Vulcain main engine vs solids) when the center core and strapons are totally different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the Atlas models with lots of smaller GEM solid strapons would air ignite some of the strapons once they reached a certain speed/altitude and had dropped others to get performance benefits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second question you asked was, why engines in the side cores. That is sort of the question of why is everything not like Proton.  Proton looks like it has a center core, and a bunch of smaller strapons, but the strapons have engines using fuel from the center core. And they do not separate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of F-9H and D-4H carrying all those extra engines to orbit would be a big weight penalty, defeating the porpoise of staging. The extra side cores are not so much as a bigger stage 1, rather they are a stage 1 (side cores) and stage 1+ or 2 (center core). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original Atlas did the opposite. It had three engines, and would drop the two side engines as staging, but keep the shared tankage (Ballon tanks, very lightweight). &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="19627" LastEditDate="2018-01-03T17:09:15.647" LastActivityDate="2018-01-03T17:09:15.647" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4961" PostTypeId="2" ParentId="4956" CreationDate="2014-07-11T14:37:46.963" Score="6" Body="&lt;p&gt;I believe electrolyzing would take at least 286,000 joules per mole of water. See this &lt;a href=&quot;http://en.wikipedia.org/wiki/Electrolysis_of_water#Thermodynamics_of_the_process&quot;&gt;Wikipedia article&lt;/a&gt;. A mole of water would give ~16 grams of oxygen and ~2 grams of hydrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to make a million grams of propellent we'd need at least (1,000,000/18)*286,000 joules. That's about ~15,888,888,889 joules for a tonne of propellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://nmp.jpl.nasa.gov/st8/tech/solar_array2.html&quot;&gt;this NASA article&lt;/a&gt;, the ISS solar array wings have a specific power of 32 watts per kilogram. The Dawn space craft's solar array has a specific power of 80 watts per kilogram. They believe in the near future we'll have arrays with a specific power of 175 watts per kilogram. Being an optimist, I'll use 175 watts/kilogram as the specific power of our power source.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make arithmetic easier let's consider a 1000 kg power source. At 175 watts per kilogram, that would be a 175,000 watt power source. It cranks out 175,000 joules per second. Recall we need about 16 billion joules to make a tonne of propellent. 15,888,888,889 joules/175,000 watts = 90,793 seconds which is about 1 day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So it'd take a 1 tonne power source at least 1 day to make a tonne of propellent&lt;/strong&gt; (please check my arithmetic, I sometimes make mistakes).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there are minable lunar volatiles, they would likely be in the permanently shadowed regions on the floors of the polar craters. &lt;a href=&quot;http://www.newscientist.com/article/dn17810-moon-is-coldest-known-place-in-the-solar-system.html&quot;&gt;These regions are already very cold&lt;/a&gt; -- around 30 to 40 Kelvin. I believe that'd be cold enough to store oxygen with little boil off. But &lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_hydrogen&quot;&gt;hydrogen&lt;/a&gt; needs to be cooled to about 20 K.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once at a Earth Moon Lagrange 1 or 2 (EML1 or 2), the moon and earth are a good distance from a depot. The sun is the major heat source we need to worry about. A conical Multi Layer Insulation (MLI) shield can block the sun and leave a depot open to space which is about 3 Kelvin. Hydrogen boil off can be used for station keeping. &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/APracticalAffordableCryogenicPropellantDepotBasedonULAsFlightExperience20087644.pdf&quot;&gt;This ULA paper&lt;/a&gt; argues that we can have cryogenic propellent depots with acceptable boil off rates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, we'd need a massive power source to produce lunar propellent at a good rate. In my opinion this is one of the biggest obstacles to lunar ISRU propellent. But I don't think it's a show stopper.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-07-11T15:17:13.400" LastActivityDate="2014-07-11T15:17:13.400" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4962" PostTypeId="2" ParentId="4959" CreationDate="2014-07-11T15:09:08.287" Score="15" Body="&lt;p&gt;Your assumption that rotating the rocket removes asymmetry is incorrect.&lt;br&gt;&#xA;The total thrust of the rocket can be visualized as a vector. This vector should point at the center of gravity of the rocket. If it doesn't, the thrust will change the direction of the rocket. It's easy to see that a rocket with 1 booster will not have its thrust vector pointing at its CoG.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: If you rotate the rocket as you describe, the direction change will be &quot;up&quot; or &quot;down&quot;. Either way the rocket will try to fly in a circle. Throttling won't help keep the rocket on course, unless you throttle the booster engines to 0, and then why bother keeping the booster around?&lt;br&gt;&#xA;As @TildalWave says in the comments, you might be able to compensate by gimballing the engines but then you lose some performance.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket body must be aligned with its velocity vector at all times. If it gets out of alignment (i.e. it slews or sideslips), drag rises enormously and the rocket is at risk of breaking up. If you drop one booster, you get asymmetric thrust and drag, both of which will try to slew the rocket around. You'd have to time your nozzle gimballing perfectly to survive. Dropping both boosters is a much simpler and less risky maneuver: you just have to make sure the separation charges fire simultaneously.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-07-12T08:09:59.747" LastActivityDate="2014-07-12T08:09:59.747" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4963" PostTypeId="2" ParentId="4959" CreationDate="2014-07-11T21:23:27.640" Score="7" Body="&lt;p&gt;All rockets need to keep the center of thrust (Ct) beneath the Center of Mass (Cm).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume for the moment a 2-booster rocket stack; the rocket and the boosters all produce thrust of 500 Tons-thrust. the left booster is 3m from centerline, and the right also 3m from centerline. The center of thrust can be figured multiplying the thrust by the distance from centerline for each thruster (getting the thrust moment), and then dividing by the total thrust of the system&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, the center of mass can be figured by taking the mass times the distance from reference centerline, then dividing by total mass. Let's assume each booster is mass 50T, as is the 1st stage, and the upper stage is 25T, and the payload 25T.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Every aircraft pilot is supposed to do similar calculations for loadout prior to takeoff...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Case 1: all three going.&lt;br&gt;&#xA;Left Booster 500TT @ -3m, moment -1500&lt;br&gt;&#xA;Right Booster 500TT @ +3m, moment +1500&lt;br&gt;&#xA;stage 1: 500TT @ 0m, moment 0&lt;br&gt;&#xA;Total Moment 0&lt;br&gt;&#xA;center of thrust 0 (= Moment/ThrustTotal = 0/1500)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Center of mass, case 1&lt;br&gt;&#xA;LB 50T x -3m  moment=-150&lt;br&gt;&#xA;RB 50T x +3m  moment=+150&lt;br&gt;&#xA;S1 50T x 0m  moment=0&lt;br&gt;&#xA;S2 25T x 0m  moment=0&lt;br&gt;&#xA;PL 25T x 0m  moment=0&lt;br&gt;&#xA;Total Moment = 0 (=150-150+0+0+0)&#xA;Center of mass = 0 (=  Moment/TotalMass = 0/200)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Case 2: Only the left going, right dropped&lt;br&gt;&#xA;Left Booster 500TT @ -3m, moment -1500&lt;br&gt;&#xA;stage 1: 500TT @ 0m, moment 0&lt;br&gt;&#xA;Total Moment -1500&lt;br&gt;&#xA;center of thrust -1.5m (= Moment/ThrustTotal = -1500/1000)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Center of mass, case 1&lt;br&gt;&#xA;LB 50T x -3m  moment=-150&lt;br&gt;&#xA;S1 50T x 0m  moment=0&lt;br&gt;&#xA;S2 25T x 0m  moment=0&lt;br&gt;&#xA;PL 25T x 0m  moment=0&lt;br&gt;&#xA;Total Moment = -150 (=-150+0+0+0)&#xA;Center of mass = -1 (=  Moment/TotalMass = -150/150)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that in case 2, the Ct and Cm do not match; this will force a spin to the right, since the center of thrust is left of the center of mass. (The actual rate of spin will be determined by how far UP the stack the center of mass is. These calcs can be done in 3D.)&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-07-11T21:23:27.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4964" PostTypeId="2" ParentId="4895" CreationDate="2014-07-12T01:57:20.290" Score="13" Body="&lt;p&gt;Observations will be of two different kinds. Distant, effectively telescopic observations starting months in advance of the flyby and then a torrent of activity during the actual flyby. There will definitely be a chance to observe the known moons of Pluto, which is part of the plan. You can see a &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons#Key_mission_dates&quot;&gt;timeline of Pluto encounter events on wikipedia&lt;/a&gt;. There will be a period of over two months where New Horizons will still be approaching the Pluto/Charon system but the resolution of the images returned will be better than what Hubble is able to achieve (which, admittedly isn't great), and will thus be able to make valuable observations over that period. The actual flyby will occur over a fairly short period of time. The probe will be traveling so fast that it travels over a million kilometers per day. By the time we receive telemetry from the probe indicating it is 100,000 km away from Pluto (nearly 10x the minimum flyby distance) the probe will have already completed its flyby and be 100,000 km on the other side of Pluto. So the vast majority of the most valuable scientific observations will be entirely pre-programmed and we won't see the data until many hours or days later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the smaller moons, New Horizons should be able to image them to a sub-km resolution.&lt;/p&gt;&#xA;" OwnerUserId="3371" LastEditorUserId="3371" LastEditDate="2014-07-19T08:13:57.143" LastActivityDate="2014-07-19T08:13:57.143" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4965" PostTypeId="2" ParentId="4895" CreationDate="2014-07-12T04:10:44.100" Score="7" Body="&lt;p&gt;Wikipedia has a &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons#Key_mission_dates.&quot; rel=&quot;noreferrer&quot;&gt;time table&lt;/a&gt; of the mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;July 14, 2015 it will pass within 13695 kilometers traveling at abour 14 km/s. Based on that info I did this drawing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rAhqF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia article also has a nice description of what they'll be doing during the &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons#Pluto_flyby&quot; rel=&quot;noreferrer&quot;&gt;Pluto Fly By&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-07-12T04:10:44.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4966" PostTypeId="2" ParentId="4017" CreationDate="2014-07-13T01:50:47.790" Score="10" Body="&lt;p&gt;I too worked on the Apollo Space Program, in Downey, California.  I was the operator/maintenance/repair &quot;computer man&quot;, in charge of the Downey computer that was one of the three interfaced with each other.  The three computers in Downey, Johnson in Houston, and Kennedy Space had control rooms operated by the computer that was &quot;my baby&quot;.  While I can no longer remember how to do the work Fortran and Cobol were the languages (programs) in use when I ran the computer in Downey.  Not only did my computer control all of the displaying and monitoring equipment that made up the Control Room, but it was used to test the Apollo Command Vehicle.  I used to eat my lunch in the Command Capsule, while I flipped switches, read displays and conducted tests.   It was a great honor to have been a part of sending men to the Moon, and I am still very proud to have been an active participant.  I was hired away from The Apollo by IBM who asked me to help work the kinks out of System 360...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Luke G. Conley III&lt;/p&gt;&#xA;" OwnerUserId="5747" LastActivityDate="2014-07-13T01:50:47.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4967" PostTypeId="1" AcceptedAnswerId="5007" CreationDate="2014-07-13T11:47:21.657" Score="21" ViewCount="10505" Body="&lt;p&gt;I've been following the upcoming launch of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Antares_%28rocket%29&quot;&gt;Antares rocket&lt;/a&gt; to resupply the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot;&gt;ISS&lt;/a&gt; (scheduled for today, July 13, 2014). This launch was originally scheduled for May, and has been rescheduled multiple times before this, due to poor weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, airliners regularly take off and land even during snowstorms and thunderstorms. What is it about rockets that makes them so sensitive to bad weather? Is it simply a question of wanting to avoid any risk at all, with such an expensive piece of equipment?&lt;/p&gt;&#xA;" OwnerUserId="5748" LastEditorUserId="49" LastEditDate="2015-04-27T22:54:22.567" LastActivityDate="2024-02-11T16:45:12.653" Title="Why are rocket launches so sensitive to weather?" Tags="|launch|rockets|weather|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="4968" PostTypeId="1" AcceptedAnswerId="4988" CreationDate="2014-07-13T12:07:00.193" Score="13" ViewCount="8471" Body="&lt;p&gt;From what I understand, a rocket changes its direction during launch by &lt;a href=&quot;https://space.stackexchange.com/questions/2830/how-does-a-launch-vehicle-control-its-trajectory-during-the-first-stage&quot;&gt;gimbaling its engines&lt;/a&gt; (i.e. swiveling the engine nozzles so the thrust of the engine points in the correct direction).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what controls the gimbaling of the engines? Do engineers simply pre-program the sequence of gimbaling changes? Does the electronic logic on the rocket adjust the thrust direction based on altitude readings, etc? Or is ground control responsible for this?&lt;/p&gt;&#xA;" OwnerUserId="5748" LastEditorUserId="9092" LastEditDate="2021-02-19T14:31:59.667" LastActivityDate="2022-06-25T05:15:36.853" Title="How does a rocket navigate during launch?" Tags="|rockets|launch|navigation|thrust-vectoring|gimbaled-engines|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4969" PostTypeId="1" CreationDate="2014-07-13T12:17:11.807" Score="16" ViewCount="4176" Body="&lt;p&gt;Given two spacecraft (let's say, the ISS and an unmanned resupply capsule), how do the two spacecraft rendezvous with each other in space? If they're moving at the same speed in space, they'll never catch up; if one tries to move faster, it will be boosted into a higher orbit as a consequence of its speedup. Do they simply create a very slight speed differential (so the difference in altitude will be negligible) and then wait for the two to sync up?&lt;/p&gt;&#xA;" OwnerUserId="5748" LastActivityDate="2015-01-24T20:19:18.393" Title="How do spacecraft rendezvous in orbit?" Tags="|orbital-mechanics|orbit|rendezvous|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4970" PostTypeId="2" ParentId="4967" CreationDate="2014-07-13T14:47:47.853" Score="11" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;Why are rocket launches so sensitive to weather?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It could make the difference between an O-ring failing, or not.&lt;/p&gt;&#xA;&lt;p&gt;In the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster&quot; rel=&quot;noreferrer&quot;&gt;Challenger disaster&lt;/a&gt;..&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Disintegration of the vehicle began after an O-ring seal in its right solid rocket booster (SRB) failed at liftoff. The O-ring failure caused a breach in the SRB joint it sealed, allowing pressurized hot gas from within the solid rocket motor to reach the outside and impinge upon the adjacent SRB attachment hardware and external fuel tank. This led to the separation of the right-hand SRB's aft attachment and the structural failure of the external tank. Aerodynamic forces broke up the orbiter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This was partly attributed to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster#Ice&quot; rel=&quot;noreferrer&quot;&gt;low temperature weather&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This was believed to be the result of supercooled air blowing on the joint from the liquid oxygen tank vent. It was much lower than the air temperature and far below the design specifications for the O-rings. ..&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;The temperature on the day of the launch was far lower than had been the case with previous launches: below freezing at 28 to 29 °F (−2.2 to −1.7 °C); previously, the coldest launch had been at 53 °F (12 °C).&lt;/strong&gt; Although the Ice Team had worked through the night removing ice, engineers at Rockwell still expressed concern. Rockwell engineers watching the pad from their headquarters in Downey, California, were horrified when they saw the amount of ice. They feared that during launch, ice might be shaken loose and strike the shuttle's thermal protection tiles, possibly due to the aspiration induced by the jet of exhaust gas from the SRBs. Rocco Petrone, the head of Rockwell's space transportation division, and his colleagues viewed this situation as a launch constraint, and told Rockwell's managers at the Cape that Rockwell could not support a launch. However, Rockwell's managers at the Cape voiced their concerns in a manner that led Houston-based mission manager Arnold Aldrich to go ahead with the launch. Aldrich decided to postpone the shuttle launch by an hour to give the Ice Team time to perform another inspection. After that last inspection, during which the ice appeared to be melting, &lt;em&gt;Challenger&lt;/em&gt; was finally cleared to launch at 11:38 am EST.[12]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Aircraft need to trim weight and get by with the minimum strength with which they can provide a high level of safety for machines that might travel thousands of kilometers in a journey at speeds of 100s of kilometers per hour.&lt;/p&gt;&#xA;&lt;p&gt;Spacecraft operate faster &amp;amp; higher and must be closer still to the limits of the materials technology.  As the high-tech increases, so do the risks and the fragility (to weather, among other things) of the vehicle. At the same time, the margin for error and safety decreases.&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-07-13T14:47:47.853" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4971" PostTypeId="2" ParentId="4969" CreationDate="2014-07-13T15:55:31.253" Score="13" Body="&lt;p&gt;They use orbital mechanics, never fight them. First of all let's be clear about directions in Space:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/AXOrA.jpg&quot; alt=&quot;Radial in-track cross-track reference frame, generated using free version of AGI&amp;#39;s STK&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The X-vector of the RIC (radial, in-track, cross-track) frame is called radial as it points along the radial position of the object w.r.t. central body. The Y-vector is called in-track and points, for circular orbits, along the orbital velocity direction. The Z-vector is orbit normal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First approach method is V-bar that relies on changing the relative altitude. Essentially one of the S/C goes lower/higher than the other and hence it has a slightly different period than the other one. This causes relative motion between the two. They often make &quot;hops&quot; using this technique. So change the altitude a little, drift closer together, return to the same altitude, hold for a while to make sure everything is fine, and then keep hopping. Read about the &lt;a href=&quot;http://www.spaceops2012.org/proceedings/documents/id1314619-Paper-001.pdf&quot; rel=&quot;noreferrer&quot;&gt;ATV approach technique&lt;/a&gt; to learn more about this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes they also do a R-bar rendezvous. In this case the secondary spacecraft approaches the primary along the local radial direction (X-direction in the RIC frame), but this requires many engine firings in the radial direction rather than only several altitude (and orbital velocity of the secondary) change manoeuvres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But both techniques have been used many times and are generally well-understood.&lt;/p&gt;&#xA;" OwnerUserId="5749" LastEditorUserId="5749" LastEditDate="2014-08-26T11:15:20.307" LastActivityDate="2014-08-26T11:15:20.307" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4973" PostTypeId="1" AcceptedAnswerId="4976" CreationDate="2014-07-13T17:34:14.640" Score="9" ViewCount="1336" Body="&lt;p&gt;&lt;a href=&quot;https://www.orbital.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Sciences&lt;/a&gt; just launched (July 13, 2014 at 12:52 EST) their &lt;a href=&quot;https://www.orbital.com/NewsInfo/MissionUpdates/Orb-2/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orb-2 resupply mission to the International Space Station&lt;/a&gt; with Cygnus resupply vehicle onboard the Antares rocket (named SS Janice Voss in honor of Orbital's past employee and NASA astronaut) from NASA's Wallops Flight Facility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the initial liftoff phase though, I've noticed rather strong lateral movement of the launcher that I don't recall being that pronounced during previous Orbital Sciences Antares launches:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/gmSmA.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Orbital Sciences Antares rocket lifting off from Wallops Flight Facility on June 13, 2014. Notice its angle relative to lightning poles.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: Here's a &lt;a href=&quot;http://www.youtube.com/watch?v=F1qdhFPB3DI&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video of the launch&lt;/a&gt; (&lt;a href=&quot;http://www.youtube.com/watch?v=vYYNc2_EqQY&amp;amp;t=39&quot; rel=&quot;nofollow noreferrer&quot;&gt;high definition here&lt;/a&gt;) for those that missed it, or would like to see the liftoff sequence again. The part I'm asking about is in the first 10-15 seconds of the video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do recall &quot;some&quot; such movement from previous launches, but not as profound. If I had to guess, I'd assume some angle is &quot;nominal&quot; and intended for the rocket to clear launch site installations like the umbilical grapple and the rocket stand. But here's the thing, that I believe those lightning towers surrounding the rocket are connected up top with a set of cables, with their inner square being quite a bit tighter than the distance between the four towers. So my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is it &quot;normal&quot; for the Antares to &quot;tilt&quot; at such angle and drift from the stand immediately following ignition and before clearing the launch site, and what is the maximum lateral movement it could tolerate to successfully clear the launchpad?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-07-13T22:01:32.147" LastActivityDate="2014-07-14T15:56:03.653" Title="Quite some lateral wobble immediately following Antares Orb-2 liftoff. Is that normal?" Tags="|launch|nasa|launch-site|antares|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4974" PostTypeId="2" ParentId="4973" CreationDate="2014-07-13T18:02:07.617" Score="2" Body="&lt;p&gt;This is speculation - not a complete answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The smoke blowing off the pad made me take a gander at &lt;a href=&quot;http://www.wunderground.com/weather-forecast/US/VA/Wallops_Island.html&quot; rel=&quot;nofollow&quot;&gt;http://www.wunderground.com/weather-forecast/US/VA/Wallops_Island.html&lt;/a&gt; for today July 13th, 2014 AD&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graph shows the barometer falling between 1213 through 1320 - the time-frame corresponding to the launch. There is a corresponding rise in the wind-speed ; 1km/h from E at 1219 to 6km/h from SE. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would speculate there may have been a sharper spike than predicted/observed/reported on wunderground.com - enough to cause the observed wobble&lt;/p&gt;&#xA;&#xA;&lt;p&gt;p.s. Feel free to down-vote (+:&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-07-13T18:02:07.617" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4975" PostTypeId="1" CreationDate="2014-07-13T18:39:37.107" Score="5" ViewCount="1397" Body="&lt;p&gt;I heard about evacuated tube transportation technology and it's application to launch cargo or even shuttles to space. Example of such project would be &lt;a href=&quot;http://en.wikipedia.org/wiki/StarTram&quot; rel=&quot;nofollow&quot;&gt;StarTram&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that acceleration phase to gain &lt;a href=&quot;http://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow&quot;&gt;escape velocity&lt;/a&gt; happens in evacuated tube let's us cut the air resistance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, would it be possible to accelerate a space shuttle (or any object) using maglev technology without vacuum tube? What would be the impact of such object on it's surroundings when it would gain such high velocity so close to the ground?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is anyone familiar with vehicles that gain high speeds (not necessarily escape velocity, but at least supersonic) close to the ground?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I imagined was a shuttle that would take off as ordinary airplane, then navigate into the series of concrete blocks strongly attached to the ground. Each of those blocks would emit strong magnetic field, creating a maglev-like linear acceleration. The pilot (or maybe autopilot) would have to maintain the flight trajectory above blocks. After the last block, it would reach high enough velocity and then the pilot could alter the flight level to exit the Earth's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, such launch facility would require to be build in unpopulated areas, near the equator, and it would be hundreds of kilometers long...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the list of engineering problems I would like you to comment:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The impact of very high speed (supersonic or even escape velocity) of a shuttle close to ground on it's surroundings.&lt;/li&gt;&#xA;&lt;li&gt;Would this be possible to maintain stable flight with this velocity on such low altitude? Could this be safe?&lt;/li&gt;&#xA;&lt;li&gt;How much higher than escape velocity should be the speed of that shuttle so that it could reach low Earth orbit without loosing escape velocity because of all air resistance. We can assume the shuttle would have aerodynamics similar to recently operated by the NASA.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="4627" LastEditorUserId="49" LastEditDate="2015-08-16T05:31:04.090" LastActivityDate="2016-11-02T19:34:58.457" Title="Is non-evacuated tube maglev launch possible?" Tags="|launch|escape-velocity|launch-assist|" AnswerCount="3" CommentCount="1" ClosedDate="2016-11-03T21:55:30.487" ContentLicense="CC BY-SA 3.0" />
  <row Id="4976" PostTypeId="2" ParentId="4973" CreationDate="2014-07-13T22:38:38.883" Score="13" Body="&lt;p&gt;&lt;a href=&quot;https://twitter.com/TildalWave/status/488391531330797568&quot;&gt;I asked NASA via Twitter&lt;/a&gt; about it hoping they answer it during the Orbital Sciences/Cygnus-2 Post-Launch News Conference, but Stephen Clark from &lt;a href=&quot;http://spaceflightnow.com/&quot;&gt;SpaceflightNow.com&lt;/a&gt; beat me to it and asked &lt;a href=&quot;https://www.orbital.com/CorporateInformation/ExecutiveProfiles/FCulbertson/&quot;&gt;Frank L. Culbertson, Jr.&lt;/a&gt;, Vice President and General Manager of Orbital's Advanced Programs Group, this very same question over the phone. It can be seen in &lt;a href=&quot;http://www.ustream.tv/recorded/50015926&quot;&gt;this video&lt;/a&gt; roughly 1 hour 19 minutes into it. Here's a transcript:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Stephen Clark, SpaceflightNow.com&lt;/strong&gt;: At the launch video, it looks like there's a little bit of a tilt of the vehicle coming off the pad.&#xA;  Is that a normal maneuver or something unexpected?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Frank Culbertson, Orbital Sciences Corp.&lt;/strong&gt;: You asked about what some of us call &lt;em&gt;&quot;the wiggle of the pad&quot;&lt;/em&gt;, others call it &lt;em&gt;The&#xA;  Baumgartner Maneuver&lt;/em&gt;, but it was a little bit more pronounced today,&#xA;  or maybe we could just see it a bit better.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;As the rocket lifts off, it actually moves the tail of the rocket&#xA;  closer to the structure, so that it then flies away from the&#xA;  structure. That's to protect the tail and the surrounding hydraulic&#xA;  equipment and the electrical cables from the blast of being too close&#xA;  to the rocket. It's only a degree or so, but it is sometimes visible,&#xA;  depending on the viewing angle.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Yes, it is a heart-stopper when you see it and you don't expect it,&#xA;  but it was all as it should have been.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This so called Baumgartner Maneuver, named after Paul Baumgartner, Master Engineer at Orbital Sciences, is additionally explained by &lt;em&gt;antonioe&lt;/em&gt; in &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=31662.135&quot;&gt;one of the NASASpaceflight.com Forum posts&lt;/a&gt;, also regarding Antares:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What you saw MIGHT be the &quot;Baumgartner Maneuver&quot; - an initial pitch&#xA;  AWAY from the TEL makes the TAIL of the rocket actually get CLOSER to&#xA;  the TEL ... but then the resulting (small) lateral acceleration&#xA;  carries the rocket AWAY from it ... this maneuver had to be very&#xA;  carefully designed, a trade between the initial tail movement towards&#xA;  the obstacle which is the inevitable result of the pitch, and the&#xA;  desired effect...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;I was told (by Paul B. himself) that the pitch-away and the resulting&#xA;  tail-towards-TEL motion would be so small that they would not be&#xA;  perceptible ... maybe he was not counting on telephoto lenses and HD&#xA;  video (and sharp NASASpaceflight subscribers...)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And therein is described what I believe was the reason for the somewhat more pronounced effect of this maneuver this time around, as captured by the broadcast cameras - their position relative to the launchpad must have been slightly different than during previous Antares launches from Wallops and they captured this Baumgartner Maneuver better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the second part of my question, what is the maximum lateral drift that would still permit the launch vehicle to navigate through the cabling between the four lightning towers, it seems I assumed wrong and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mid-Atlantic_Regional_Spaceport_Launch_Pad_0&quot;&gt;Mid-Atlantic Regional Spaceport Launch Pad 0-A&lt;/a&gt; doesn't use any cables between them at all, like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Space_Launch_Complex_40#mediaviewer/File:CCAFS_LC40_Falcon_9_C2plus.jpg&quot;&gt;those at the Cape Canaveral Air Force Station Space Launch Complex 40 do&lt;/a&gt;. This becomes more apparent by looking closer to some of the photographs of the Launch Pad 0-A, for example these posted on &lt;a href=&quot;http://www.spaceflight101.com/cygnus-orb-2-mission-updates.html&quot;&gt;Spaceflight101.com page to Cygnus Orb-2 Mission Updates&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, judging by the photographs, the maximum tilt of the launch vehicle during liftoff could be several degrees more, and it would still be able to clear all the launchpad installations (mainly TEL or Transporter/Erector/Launcher and the umbilical cabling).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-07-14T15:56:03.653" LastActivityDate="2014-07-14T15:56:03.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4977" PostTypeId="1" CreationDate="2014-07-14T05:06:55.150" Score="5" ViewCount="94" Body="&lt;p&gt;Just what the title states.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia writes to say about the &lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_ECLSS&quot; rel=&quot;noreferrer&quot;&gt;ECLSS&lt;/a&gt; (Environment Control Life Support System) &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The highest priority for the ECLSS is the ISS atmosphere, but the system also collects, processes, and stores waste and water produced and used by the crew—a process that recycles fluid from the sink, shower, toilet, and condensation from the air&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Atmosphere is controlled by the following components&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_ECLSS#Air_revitalisation_system&quot; rel=&quot;noreferrer&quot;&gt;Air Revitalisation System&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_ECLSS#Oxygen_generating_system&quot; rel=&quot;noreferrer&quot;&gt;Oxygen Generation System&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_ECLSS#Elektron&quot; rel=&quot;noreferrer&quot;&gt;Elektron&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_ECLSS#Vika&quot; rel=&quot;noreferrer&quot;&gt;Vika&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_ECLSS#Vozdukh&quot; rel=&quot;noreferrer&quot;&gt;Vozdukh&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Do the various components of the ECLSS suffer failure at the same rate aboard ISS as they would here on Earth? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;Was a comparative evaluation ever performed against a control unit on Terra Firma?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-07-14T05:06:55.150" Title="Do the ECLSS components aboard ISS deteriorate at the same rate as they would on Earth?" Tags="|iss|eclss|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4978" PostTypeId="5" CreationDate="2014-07-14T05:20:09.093" Score="0" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_ECLSS&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;is a life support system that provides or controls atmospheric pressure, fire detection and suppression, oxygen levels, waste management and water supply. The highest priority for the ECLSS is the ISS atmosphere, but the system also collects, processes, and stores waste and water produced and used by the crew—a process that recycles fluid from the sink, shower, toilet, and condensation from the air&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More detail may be obtained from the linked article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider applying this tag when your question is on the subject of the&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ECLSS in general &lt;/li&gt;&#xA;&lt;li&gt;Atmosphere aboard ISS&lt;/li&gt;&#xA;&lt;li&gt;Conflagration control&lt;/li&gt;&#xA;&lt;li&gt;Waste management&lt;/li&gt;&#xA;&lt;li&gt;Water management&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-07-14T05:20:09.093" LastActivityDate="2014-07-14T05:20:09.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4979" PostTypeId="4" CreationDate="2014-07-14T05:20:09.093" Score="0" Body="Acronym for Environmental Control and Life Support System used on the International Space Station" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-07-14T05:20:09.093" LastActivityDate="2014-07-14T05:20:09.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4980" PostTypeId="1" AcceptedAnswerId="4981" CreationDate="2014-07-14T07:55:33.980" Score="9" ViewCount="819" Body="&lt;p&gt;I know that there are (at least) two space stations in operation: The ISS and the Chinese one (which I don’t remember the name now)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine, that China, EU, Russia and USA agree on cooperating in the space and they decide to join the Chinese space station and ISS together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would this be possible? I mean, would airlocks &quot;match&quot; together and could they be joined together?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am basically asking, if there is any recognised &quot;standard&quot; in how to build spacecraft. So, if tomorrow this happens could it be possible?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;India delivery ship successfully landing on ISS or Chinese Space station&lt;/li&gt;&#xA;&lt;li&gt;The same the opposite way: Russian Progress ship landing on China space station&lt;/li&gt;&#xA;&lt;li&gt;Any thinkable cross-joining of space ships&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="49" LastEditDate="2014-07-14T13:37:25.307" LastActivityDate="2014-07-14T13:37:25.307" Title="Are space stations compatible?" Tags="|space-station|docking|standards|docking-port|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4981" PostTypeId="2" ParentId="4980" CreationDate="2014-07-14T09:29:35.790" Score="11" Body="&lt;p&gt;The International Space Station's PMA docking adapters and &lt;a href=&quot;https://en.wikipedia.org/wiki/Tiangong-1&quot;&gt;Tiangong-1&lt;/a&gt; both use passive &lt;a href=&quot;https://en.wikipedia.org/wiki/Androgynous_Peripheral_Attach_System&quot;&gt;APAS-95 docking adapters&lt;/a&gt; for spaceship operations; the ISS additionally uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Common_Berthing_Mechanism&quot;&gt;CBM connectors&lt;/a&gt; for connecting modules (including most supply spacecraft) and Soyuz probe-and-drogue connectors for Russian modules and craft.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To cover your scenarios:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Connecting two APAS adapters requires at least one of them to be an active adapter, so the two stations can't dock, but a vehicle with an active APAS adapter can dock with either.&lt;/li&gt;&#xA;&lt;li&gt;If India makes a delivery ship with an APAS or CBM connector, yes.&lt;/li&gt;&#xA;&lt;li&gt;Progress uses the Soyuz probe-and-drogue system rather than APAS, so no.&lt;/li&gt;&#xA;&lt;li&gt;Mostly not without an adapter.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-07-14T09:29:35.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4982" PostTypeId="2" ParentId="4975" CreationDate="2014-07-14T12:09:55.263" Score="7" Body="&lt;p&gt;Supersonic at ground level has beeen done once, by the &lt;a href=&quot;http://www.thrustssc.com/thrustssc.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thrust SSC&lt;/a&gt; land speed record car. At supersonic speeds, a shockwave forms on the nose of the car or air/spacecraft. If you're close to the ground, this shockwave will reflect off the ground and potentially cause trouble. You'd also need to consider the noise level for the neighbours. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of the technological challenges are similar to those of a space gun, see &lt;a href=&quot;https://space.stackexchange.com/questions/2370/what-technological-hurdles-prevent-the-development-of-a-space-gun&quot;&gt;this question&lt;/a&gt;. The major limitation will be air resistance: if you accellerate to a significant fraction of orbital speed at ground level, you're losing enormous amounts of energy through drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using an aircraft as you describe, taking off under its own power and then descending onto the magnetic track, is even more difficult. Maglev trains hover a few cm above the track. The higher they go, the smaller the magnetic force. Imagine trying to fly an aircraft at a constant altitude of 5 cm over a concrete track.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-07-14T12:09:55.263" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4983" PostTypeId="1" CreationDate="2014-07-14T12:51:38.290" Score="6" ViewCount="1609" Body="&lt;p&gt;Most satellites are launched to a mere fraction of the altitude at which geosynchronous satellites orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How are geosynchronous satellites carried so high? Do they store a lot of propellant on board? Or do they keep their final stage longer than low-altitude satellites?&lt;/p&gt;&#xA;" OwnerUserId="5748" LastActivityDate="2014-07-19T00:39:15.343" Title="How are geosynchronous satellites launched to such an incredible height?" Tags="|artificial-satellite|geosynchronous|communication-satellite|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4984" PostTypeId="2" ParentId="4983" CreationDate="2014-07-14T13:20:11.973" Score="5" Body="&lt;p&gt;Satellites are launched to a range of heights from about 160-1200 km (LEO) out to 33800 km (geosynchronous).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Altitude is not the challenge here. Orbital speed is the hard bit. From &lt;a href=&quot;http://www.sciencelearn.org.nz/Contexts/Satellites/Looking-Closer/Launching-satellites&quot; rel=&quot;nofollow&quot;&gt;sciencelearn:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;even though it takes nearly a million joules of energy to lift a 1 kg mass to an altitude of 100 km, it takes over 30 million joules of extra energy to give it enough speed to stay in orbit around the Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Your last two sentences are correct though - the further out you want to go, the more propellant you need to carry, and you'll keep your stages burning longer. I know this sounds very simplistic, but it really all comes down this in the end: delta-V. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have a read of the other questions tagged &lt;a href=&quot;/questions/tagged/launch&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;launch&amp;#39;&quot; rel=&quot;tag&quot;&gt;launch&lt;/a&gt; for useful information on speeds, launch types etc.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2014-07-19T00:39:15.343" LastActivityDate="2014-07-19T00:39:15.343" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="4985" PostTypeId="2" ParentId="4975" CreationDate="2014-07-14T19:28:08.023" Score="6" Body="&lt;p&gt;While it is theoretically possible, it is most likely impractical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specific concerns include the bowshock, and the resultant sonic boom. If using also a tube, the air itself will also generate a considerable force profile and tube-exit turbulent flow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To use a non-evacuated tube, you're needing to generate supersonic airflow - this can be done, but requires a massive amount of moving air from high pressure reservoirs. (Functionally, any rifle does exactly this - generating the supersonic gas wave by detonation of gunpowder.) The effects of the rush of supersonic, or for launch to orbit, hypersonic, air out of the barrel will create a massive exit turbulence, and by venturi effect, drag surface air into the flow. It will also be the loudest cannon yet. To generate this supersonic flow, air will have to be added to the tube to prevent back pressure; at that point, the maglev may as well overpressure and make use of the pressurization, by massive pressure additions behind the vehicle timed to hit just as it passes. If the air in the tube is already moving at high speed (certain wind tunnels can produce over 400 kph), the in-tube drag can be reduced, but there will be a sudden deceleration when the non-moving air is encountered upon exit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that supersonic flight, especially in the hypersonic region, is a high drag configuration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pressure wave from the barrel exit will, as previously noted, transfer a lot of kinetic energy to the exterior air, resulting in an upward flow, which creates a surface low, which draws air in from around. This will pull surface materials towards the exit point. This is a multiple hazard situation - it's a hazard to anyone or anything near the exit point, and it's a hazard for repeat use because the exit port must be kept clear of debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bowshock only exists when the projectile is faster than the medium. All objects moving through fluids generate bowshock - but it's only a significant noise situation when the speed is a high fraction of the speed of sound in that fluid, relative to the fluid itself. (A bullet doing 400 m/s down a 200 m/s airflow isn't generating a 400 m/s bowshock, but a 200 m/s one.) Since the bowshock is a major portion of aerodynamic and hydrodynamic drag, the bowshock is going to vary widely based upon the nature of the air being moved through. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bowshock will sound like an explosion (and is, mechanically, nearly identical to one at range), as the transition to supersonic speed happens. The air, since it cannot accelerate quickly enough through normal kinetic transfer, converts the movement energy from directional vector to vibration. We hear vibrations, and this generates the large sound.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In an enclosed tube, the boom becomes relevant upon exit (the tube itself must be constructed to survive it. In an open system, the entire path of the rail must be capable of withstanding the noise and outward air rush. This makes the unenclosed supersonic system a major environmental quality issue for noise pollution, in addition to the previously mentioned venturi effect and ground drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, the post-vehicle shockwave can generate a low pressure zone that itself causes a second boom - few aircraft hit velocities high enough for the human ear to notice this second, weaker, cavitation, but in water, cavitation causes bubble formation and demonstrates the principle - the noise of a high speed submarine is both the bow shock and the collapse of the partial vacuum behind it. A launch system will have speeds that generate a much longer cavitation event than self-propelled aircraft do. This is going to be perceived as a longer boom, or even a second boom (certain high powered rifles generate a separate cavitation sonic event).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in the end, there is the turbulent airflow, the disruption of surrounding landscape, and the noise pollution, plus much increased drag.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-07-14T19:28:08.023" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4986" PostTypeId="2" ParentId="4688" CreationDate="2014-07-14T19:33:34.660" Score="2" Body="&lt;p&gt;to maintain a speed of 1 mph long enough to escape, one is accelerating about 34 feet per second per second (1.46 feet per second above gravity) straight up. To get outside the hill sphere (and into &quot;solar space&quot; rather than being in &quot;Earth Space&quot;), you're looking at 107 years of continuous 1.05G acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in theory, yes, but in practice, the delta-V makes it insanely expensive.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-07-14T19:33:34.660" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="4987" PostTypeId="2" ParentId="4983" CreationDate="2014-07-14T19:37:48.030" Score="13" Body="&lt;p&gt;Altitude is part of the problem. Assuming you wanted to get to GEO from a circular, 700 km altitude orbit you'd first need to do a burn of about 2.24 km/sec along the orbital velocity direction to raise the apogee of the transfer orbit (the half-ellipse):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/vuVKe.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft wouldn't normally do it itself, the launcher upper stage would typically inject you into a &lt;a href=&quot;http://en.wikipedia.org/wiki/Geostationary_transfer_orbit&quot; rel=&quot;noreferrer&quot;&gt;Geostationary Transfer Orbit&lt;/a&gt; (GTO) directly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What every spacecraft has to do itself, however, is circularisation of the orbit once it reaches the geostationary altitude. This is done by doing yet another burn along the orbital velocity direction, but this time of 1.2 km/sec. You can think of this burn as trying to raise perigee altitude and circularising the orbit or matching the spacecraft's velocity to match the geostationary orbit's velocity. Any inclination changes are also done there to reduce the mass of the fuel necessary (the slower you go the less fuel you need to change inclination by some amount).&#xA;&lt;img src=&quot;https://i.stack.imgur.com/DD46W.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's how you get to GEO. So the problem is, by the measure of delta-V, mainly due to altitude, less so regarding the velocity. The only problem is every spacecraft has to do the circularisation burn itself, which adds some complexity.&lt;/p&gt;&#xA;" OwnerUserId="5749" LastEditorUserId="5749" LastEditDate="2014-07-14T19:52:35.523" LastActivityDate="2014-07-14T19:52:35.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4988" PostTypeId="2" ParentId="4968" CreationDate="2014-07-15T03:43:10.427" Score="15" Body="&lt;p&gt;First of all, it's worth to mention that engine gimbaling is not the only way to control a rocket. You can use differential thrust, or vernier thrusters, or even aerodynamic flight control surfaces. But you are right on that, that all of these systems require some sort of control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gimbaling angles can't be pre-programed, because they have to dynamically adapt to minute changes in the vehicle's attitude. (That is the word used to describe where the nose is pointing.) If, during ascent, a sudden gust of wind tips the rocket to the left, it has to counter-act with something which creates a rotating moment in the other direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore the rocket guidance system works in a loop. It senses the current attitude. Compares that to a target value. Actuates in such a way that the error decreases. Then repeats. This is called closed-loop control. (As opposed to the open-loop control, what would be what you described, with the pre-programed gimbal angles.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are interested in this topic, there is quite a bit detail about the Saturn V Instrument Unit on wikipedia. That's the part of the Saturn V rocket which among other things, provided the guidance signals to the engines. &lt;a href=&quot;http://en.wikipedia.org/wiki/Saturn_V_Instrument_Unit&quot;&gt;http://en.wikipedia.org/wiki/Saturn_V_Instrument_Unit&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5750" LastEditorUserId="897" LastEditDate="2014-07-17T18:05:50.927" LastActivityDate="2014-07-17T18:05:50.927" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="4989" PostTypeId="1" CreationDate="2014-07-15T06:25:24.830" Score="3" ViewCount="2839" Body="&lt;p&gt;Looking at &lt;a href=&quot;http://youtu.be/lbHnSu-DLR4?t=15m3s&quot; rel=&quot;nofollow&quot;&gt;recent SpaceX launches&lt;/a&gt; there is some, probably water, sprayed to the flames. Why?&lt;/p&gt;&#xA;" OwnerUserId="4617" LastEditorUserId="5754" LastEditDate="2014-07-15T09:56:56.087" LastActivityDate="2014-07-15T09:56:56.087" Title="What is sprayed at a rocket launch? And why?" Tags="|launch|launchpad|" AnswerCount="1" CommentCount="0" ClosedDate="2014-07-15T14:01:35.643" ContentLicense="CC BY-SA 3.0" />
  <row Id="4990" PostTypeId="2" ParentId="4989" CreationDate="2014-07-15T07:06:17.983" Score="3" Body="&lt;p&gt;According to NASA, they spray water for two purposes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thousands of gallons of water flood the launch area at the crucial moments surrounding ignition, serving two purposes. Water keeps flames from spreading and prevents damage caused by sound waves.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.nasa.gov/audience/forstudents/9-12/features/F_Preventing_Fires_on_the_Launchpad_prt.htm&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/audience/forstudents/9-12/features/F_Preventing_Fires_on_the_Launchpad_prt.htm&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5760" LastActivityDate="2014-07-15T07:06:17.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4991" PostTypeId="1" AcceptedAnswerId="4995" CreationDate="2014-07-15T14:19:37.767" Score="18" ViewCount="2786" Body="&lt;p&gt;Computers and telemetry all need electrical power, is this just stored in batteries? If so what type of batteries are used? How much electrical power does a modern rocket consume on normal LEO or GTO flight? &lt;/p&gt;&#xA;" OwnerUserId="3372" LastEditorUserId="168" LastEditDate="2014-07-16T02:59:45.313" LastActivityDate="2014-07-19T08:42:05.317" Title="Where does electrical power on a rocket come from?" Tags="|rockets|electronics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4992" PostTypeId="1" AcceptedAnswerId="4994" CreationDate="2014-07-15T17:11:56.717" Score="15" ViewCount="3734" Body="&lt;p&gt;Finding water in some craters on the Lunar poles is a recent discovery. Was this expected during the Apollo era?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even without any thoughts about future in situ resource utilization, landing at a Lunar pole would have the advantage AFAIK of more even insolation, a more reasonable day temperature. So why didn't any Apollo mission land at a pole? Are there orbital mechanical reasons? Were they afraid to land in the cold shadow (and/or Earth radio shadow) of some mountain?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2019-09-05T06:32:59.283" Title="Why didn't Apollo land on the Lunar poles?" Tags="|the-moon|apollo-program|lunar-landing|isru|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4993" PostTypeId="1" AcceptedAnswerId="4997" CreationDate="2014-07-15T17:39:15.227" Score="13" ViewCount="1198" Body="&lt;p&gt;How were the Soviet Lunochod rovers during the 1970s controlled from Earth? Did they take images on the fly and a guy with a joystick steared it around to reach targets of opportunity? Well, probably not, but did they take advantage of the near real time radio communication?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rovers on Mars are given a single batch of commands once a day. So while the roundtrip travel of light delay is no more than 8 to 48 minutes, a rover would be remotely operated at exactly the same pace even if it were on a moon of Neptune 4 light hours away. But the Moon is so close that the 2-3 second delay would be at most irritating until gotten used to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a lecture by Dr. Jeff Norris at JPL from 12 sep 2013. I really recommend it, a potentially easy solution on a seemingly unfixable problem:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=EeSGuGw4aJU&quot;&gt;https://www.youtube.com/watch?v=EeSGuGw4aJU&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He describes how time delays can be handled by human psychology by simulating and visualizing in real time what would happen if the given commands give the expected results. While he focuses speculative on using this on Mars, eliminating the 2-3 seconds delay on the Moon in a virtual way certainly seems very doable.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="9092" LastEditDate="2021-03-15T10:02:26.707" LastActivityDate="2021-03-15T10:02:26.707" Title="Were the Lunokhod Lunar rovers remote controlled in real time?" Tags="|the-moon|rovers|teleoperations|lunokhod|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4994" PostTypeId="2" ParentId="4992" CreationDate="2014-07-15T17:52:52.627" Score="23" Body="&lt;p&gt;Apollo missions were on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Free_return_trajectory&quot;&gt;free-return trajectory&lt;/a&gt; which limits your initial Lunar orbital insertion inclination close to the Earth-Moon plane. Any orbital inclination change is at that stage rather prohibitive in terms of required delta-v for the Lunar Module both on descent as well as later ascent phase to match Command Module's orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/lwl4k.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Sketch of a circumlunar free return trajectory (not to scale). Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Free_return_trajectory&quot;&gt;Wikipedia on Free return trajectory&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Farthest landing from the Lunar equator was during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15&quot;&gt;Apollo 15&lt;/a&gt; mission with its landing site at the Hadley Rille/Apennine Mountains, a bit more than 26° Northern latitude, with Apollo 17 closely following, landing at roughly 20° North in the Taurus-Littrow region. Other Apollo mission all landed a lot closer to the Lunar equator, some at slightly Southern, some at Northern latitudes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/qy2me.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Apollo landing sites.&lt;/strong&gt; Image credit: NASA, Source: &lt;a href=&quot;http://airandspace.si.edu/explore-and-learn/multimedia/detail.cfm?id=5480&quot;&gt;Smithsonian National Air &amp;amp; Space Museum&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be other problems that you mention with landing at the Lunar poles, including surface / contact temperature delta that's a lot more challenging to handle due to surface convection than gaining or losing heat due to radiation alone in vacuum, but the missions were simply not designed for that. One thing that you gain with near-equator landing sites is ability to reuse slight surface rotation in your favor both on landing and liftoff, but more importantly, you also don't have to crank your orbital module's orbit to near polar inclination, which is both time and delta-v consuming.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-07-15T18:00:16.480" LastActivityDate="2014-07-15T18:00:16.480" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="4995" PostTypeId="2" ParentId="4991" CreationDate="2014-07-15T18:01:42.803" Score="17" Body="&lt;p&gt;For rockets it's normally batteries since they only need to operate for minutes, maybe hours at most when it comes to upper stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An example power budget and description of the hardware for one of the largest launch vehicles out there, the Ariane 5, can be found &lt;a href=&quot;http://adsabs.harvard.edu/full/2002ESASP.502..659B&quot;&gt;here&lt;/a&gt;. It's a bit outdated (2002) but you can get a feel for the numbers. Ariane 5 used to deliver 4 kWh over 6 hours with peak power consumption of 1100 W. It used Ag-Zn batteries, but lithium-ion are now getting increasingly more popular in Space so the newer launchers probably use those.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This'll, of course, scale with the size of the launcher.&lt;/p&gt;&#xA;" OwnerUserId="5749" LastEditorUserId="5749" LastEditDate="2014-07-15T18:18:16.947" LastActivityDate="2014-07-15T18:18:16.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4997" PostTypeId="2" ParentId="4993" CreationDate="2014-07-15T18:09:47.677" Score="19" Body="&lt;p&gt;The &lt;a href=&quot;http://books.google.nl/books?id=4sIiQdFo0RMC&amp;amp;pg=PA255&amp;amp;lpg=PA255&amp;amp;dq=lunokhod%20control&amp;amp;source=bl&amp;amp;ots=tzSf07rWCV&amp;amp;sig=1eMYiKMfpwApDLXTvZ4jbDLUrbM&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=zGvFU5fkBcG00QX1zYHQDg&amp;amp;ved=0CIYBEOgBMBI#v=onepage&amp;amp;q=lunokhod%20control&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Lunokhods&lt;/a&gt; were &lt;a href=&quot;http://www.airspacemag.com/space/the-other-moon-landings-6457729/?all&quot; rel=&quot;noreferrer&quot;&gt;controlled in real-time from Earth&lt;/a&gt; (long article well worth reading), exactly as you say. A camera relayed images to Earth (one image every 7-20 seconds) and a five-man team (driver, commander, navigator, radio antenna operator, and the flight engineer) would control the rover.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can imagine this was difficult: the driver has to anticipate massively due to the communications delay and the image delay. The cameras had a dead zone right in front of the rover, so the driver would also have to remember previous images (and with 1970s technology, no easy way to recall the previous image on screen).&lt;br&gt;&#xA;Each team worked a two-hour stretch before being relieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The control room looked like this:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/RFeKR.jpg&quot; alt=&quot;Lunokhod control room&quot;&gt;&lt;br&gt;&#xA;Yup, a guy with a joystick. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-07-15T19:26:11.037" LastActivityDate="2014-07-15T19:26:11.037" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="4998" PostTypeId="2" ParentId="4983" CreationDate="2014-07-15T19:00:07.410" Score="3" Body="&lt;p&gt;As an example, &lt;a href=&quot;http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf&quot; rel=&quot;nofollow&quot;&gt;Ariane 5&lt;/a&gt; (large PDF: Ariane 5 user manual) generally delivers its payload in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Geostationary Transfer Orbit&lt;/a&gt; (an elliptical orbit with its highest point at geostationary altitude). From there, the satellite's own engines circularize the orbit.&lt;br&gt;&#xA;It depends a bit on the rocket: the Ariane 5 ECA has a second stage that cannot be restarted, so it cannot do the circularization burn. With a restartable second stage, it's possible to push the satellite into its final orbit, but then you have to do another maneuver to remove the rocket stage from geostationary orbit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-07-15T19:00:07.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="4999" PostTypeId="2" ParentId="4975" CreationDate="2014-07-15T19:04:39.893" Score="3" Body="&lt;p&gt;Maglev launch without tubes &lt;strong&gt;is&lt;/strong&gt; possible, although not in the fashion you are envisioning.  Look at &lt;a href=&quot;http://en.wikipedia.org/wiki/Launch_loop&quot; rel=&quot;nofollow&quot;&gt;Launch loops&lt;/a&gt;--not that I would want one anywhere nearby--the whole thing falls apart if the power goes off.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-07-15T19:04:39.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5000" PostTypeId="2" ParentId="4934" CreationDate="2014-07-15T23:52:23.973" Score="8" Body="&lt;p&gt;Relevant information can be found in this patent of the company: &lt;a href=&quot;https://www.google.com/patents/US6481670&quot;&gt;https://www.google.com/patents/US6481670&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan seems to be to circulate coolant in tubes positioned between the inflatable layers.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The tube segment substantially enclose the spacecraft, and in particular, the critical layers that are desired to be protected. This protective barrier provides a thermal heat sink, temperature equalization, and a &lt;strong&gt;waste heat rejection system&lt;/strong&gt;, that can moderate the temperature extremes encountered by spacecraft. In addition, dependent on the fluid selected, substantial radiation shielding can be provided that can protect the entire spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Quote from the patent abstract, emphasis mine.&lt;/p&gt;&#xA;" OwnerUserId="5750" LastActivityDate="2014-07-15T23:52:23.973" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5001" PostTypeId="2" ParentId="4434" CreationDate="2014-07-16T09:34:21.913" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990009082.pdf&quot; rel=&quot;nofollow&quot;&gt;This report&lt;/a&gt; has the ullage volume of the LOX tank on the X-34 at 6%, and RP-1 at 0.5 to 1.5%.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-07-16T09:34:21.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5004" PostTypeId="1" CreationDate="2014-07-18T00:08:27.400" Score="5" ViewCount="1347" Body="&lt;p&gt;What would the effects of pregnancy in space environment be on a human fetus? I understand that astronauts on long-duration missions in microgravity environment experience bone density loss. Would that be a concern for human embryonic development, and if so, what precautions could be taken to avert bone growth / bone density issues?&lt;/p&gt;&#xA;" OwnerUserId="5773" LastEditorUserId="49" LastEditDate="2014-07-18T05:59:04.397" LastActivityDate="2014-07-18T05:59:04.397" Title="Does space environment affect human embryonic development?" Tags="|microgravity|humans|research|life|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5005" PostTypeId="2" ParentId="5004" CreationDate="2014-07-18T01:36:27.267" Score="9" Body="&lt;p&gt;We don't know. There was some research done on embryonic development of mice and rats in microgravity, both impregnated females sent to orbit and later recovered (mostly onboard Space Shuttle, but Russians also sent in orbit similar experiments), and even more frequently by using 3D clinostat to simulate microgravity via continuous three-dimensional rotation. Reduced number of successful full-term pregnancies was observed for females exposed to microgravity during early pregnancy, while fewer abnormal pregnancies for those exposed later. Findings suggest that embryonic stem cells have problems developing normally under microgravity conditions during early pregnancy and result in abnormal fetal development and terminated pregnancy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even if impregnation alone is not a problem, &lt;a href=&quot;https://space.stackexchange.com/questions/604/is-it-possible-to-get-pregnant-through-natural-means-in-space&quot;&gt;and it might be&lt;/a&gt;, due to increased radiation adversely affecting male fertility and sperm count, I dread to think what a full-term human pregnancy would result in, without intervention of biotechnology and biomedical engineering that can hopefully one day develop solutions to abnormal cell division and protein growth in microgravity. Increased frequency of cell mutations due to increased radiation is also not excluded, considering full-term human pregnancy lasts on average about 38 weeks, and for mice, gestation period is between 19 and 21 weeks, and rats a week or two more. With those sent in orbit already impregnated (or fertilized in-vitro) 9-20 days before launch, this means we only have experimental data on exposure duration of roughly half full-term human pregnancy duration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So regarding bone density, it's really hard to judge without any observational data. All we have are observations done on adult humans and smaller mammals, mostly rodents. Common sense suggests that human embryos, since they develop in uterus and placenta filled with water providing some unidirectional pressure on the developing embryo even in microgravity, shouldn't have problems developing sufficient bone density, assuming normal (for gravity) protein growth can be achieved during early pregnancy. Of course, once we have that capability, we have also solved bone density problems of adult astronauts on long duration microgravity missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before that though, I'd personally rather no one even tries. And it might not be down to my personal preference either, there are all kinds of &lt;em&gt;should we&lt;/em&gt; ethical questions to consider for planned human pregnancy experiments in microgravity, before we answer the ultimate one: Do we really have to?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further reading and references:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://online.liebertpub.com/doi/abs/10.1089/ast.2009.0415&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bioastronautics: The Influence of Microgravity on Astronaut Health&lt;/a&gt;, E. Blaber et al.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0006753;jsessionid=5ECB7D8C6C832748C58514B037F5FAB0&quot; rel=&quot;nofollow noreferrer&quot;&gt;Detrimental Effects of Microgravity on Mouse Preimplantation Development In Vitro&lt;/a&gt;, S. Wakayama et al.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.riken.jp/en/research/rikenresearch/highlights/6065/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Riken Research: Embryonic development—lost in space?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.wired.com/2009/08/spacebabies/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wired: Making Babies in Space May Be Harder Than It Sounds&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.space.com/9327-embryonic-idea-human-procreation-space-perilous.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space.com: Embryonic Idea: Human Procreation in Space Would Be Perilous&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Documents in Russian (Abstracts in English):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/9035797&quot; rel=&quot;nofollow noreferrer&quot;&gt;Effect of weightlessness on the mother-fetus system (results of embryological experiment NIH-R1 aboard the Space Shuttle&lt;/a&gt;, Serova L. V. et al.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/11496419&quot; rel=&quot;nofollow noreferrer&quot;&gt;Microgravity and development of the mammals: problems results prospects&lt;/a&gt;, Serova L. V.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Related posts on Space Exploration:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/4513/49&quot;&gt;What animals, if any, have reproduced in space?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/604/49&quot;&gt;Is it possible to get pregnant through natural means in space?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-07-18T02:22:20.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5006" PostTypeId="1" CreationDate="2014-07-19T04:06:47.767" Score="10" ViewCount="1380" Body="&lt;p&gt;Broadly speaking, most rocket systems appear to have at least 3 or 4 fold radial symmetry about their long axis (if you don't consider internals or smaller features not obvious from a distance). A notable exception is the Space Shuttle, which (broadly) displays only bilateral symmetry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Descriptions of launch profiles often include roll and pitch maneuvers. For the shuttle, it's quite obvious what is going on. Not so much for a vehicle like Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably, for vehicles such as Apollo, Gemini, and Mercury, pitch, roll, and yaw axes were consistent with the crew's seated positions (or vice versa) - &quot;pitch down&quot; equated to the crew experiencing a &quot;forward&quot; rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When sitting on the pad, in which directions were Apollo's pitch and yaw axes oriented relative to the launch tower? What visual cues did the vehicle have purposely or incidentally that one could use once the vehicle was in flight to discern its orientation about the roll axis relative to the viewer?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2014-07-22T00:38:37.610" LastActivityDate="2014-07-22T00:38:37.610" Title="Pitch and yaw axes of rocket systems such as Apollo" Tags="|apollo-program|spacecraft|terminology|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5007" PostTypeId="2" ParentId="4967" CreationDate="2014-07-19T04:26:04.677" Score="16" Body="&lt;p&gt;Lightning and wind are the primary weather concerns that preclude rocket launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rocket's control system needs to compensate for lots of un-modeled or difficult to model factors. For example, uncertainty in mass properties, engine performance, mechanical alignments, errors in inertial measurement units (IMU), gyroscopes, GPS, etc. If any of these errors become too large the system will not longer be able to compensate, the system will become uncontrollable, and the mission will be lost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wind, unsurprisingly, is one of these uncertain factors, and especially &lt;a href=&quot;http://en.wikipedia.org/wiki/Wind_shear&quot; rel=&quot;noreferrer&quot;&gt;wind shears&lt;/a&gt;, which are rapid changes in wind direction as a function of location, are difficult for control systems to handle because they can put enormous aerodynamic loads on a vehicle very quickly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lightning is dangerous for entirely different reasons. Rockets are full of sensitive electronics, &lt;a href=&quot;http://en.wikipedia.org/wiki/Lightning_rocket&quot; rel=&quot;noreferrer&quot;&gt;attract lightning nicely (even when not grounded)&lt;/a&gt;, and are vulnerable to direct lightning strike. Even something as typically benign as a reset or temporarily glitch that could potentially be tolerated in a satellite would be disastrous to a rocket under powered flight. Airplanes avoid lightning storms for the same reason, although they have more margin available to incorporate lightning mitigation into their designs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For these reasons (and to a lesser extent others, such as the extreme temperature example provided in another answer), rockets launches are frequently delayed for ideal conditions. There's no sense in risking a rocket and payload worth hundreds of millions of dollars for a week. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In contrast, rockets that must be able to be launched in at a moments notice (think &lt;a href=&quot;https://en.wikipedia.org/wiki/Intercontinental_ballistic_missile&quot; rel=&quot;noreferrer&quot;&gt;ICBMs&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Submarine-launched_ballistic_missile&quot; rel=&quot;noreferrer&quot;&gt;SLBMs&lt;/a&gt;) are designed with very different requirements. Here, the performance hit taken to design robustness to weather is accepted because a weapon isn't very useful if you have to wait for the storm to pass before it can be fired.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="12038" LastEditDate="2015-11-18T04:53:26.100" LastActivityDate="2015-11-18T04:53:26.100" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5008" PostTypeId="2" ParentId="4967" CreationDate="2014-07-19T08:31:00.173" Score="17" Body="&lt;p&gt;It's mainly just bad luck.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is really only one relatively common weather condition that you don't want to launch a rocket into and that's a thunderstorm. Granted, high winds can also be problematic. And it should go without saying that you don't want to launch during a hurricane or tornado.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets are perfectly capable of launching directly into a thunderstorm, just as a civil airliner might do, but there's a wee bit of a problem with doing so. Rockets are made of metal, and their exhaust is partially ionized, so during liftoff the entire rocket+exhaust trail is like a giant conductive wire tying the rocket to the ground. If the rocket flies into a thunderstorm then it will attract lightning strikes. Rockets can be made to withstand such strikes, and most of the current flows over the skin of the vehicle (just as with an airliner) but that adds a lot of difficulty. This actually happened during the Apollo 12 launch, and the lightning strike caused a huge number of problems with the onboard computers and with the telemetry link until folks were able to fix the telemetry and then slowly get everything else in order (fortunately the flight computers continued running normally).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As with all inherently dangerous activities the basic principle is generally to &quot;stack the odds in your favor&quot;, so if you can avoid launching a hypersonic rocket packed with literally kilotons of explosive fuel into a scenario where it is almost certain to draw a lightning strike then you do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why does weather cause so many launch delays? Well, that's mostly just an issue for the US. For orbital dynamics reasons it is most beneficial to launch in an Easterly direction from lower latitudes. In the continental US the lowest latitudes are in Florida, which are conveniently on the coast facing the entire Atlantic Ocean to the East, so that's where the bulk of America's space launches occur from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Coincidentally, South Florida also has the highest level of thunderstorm activity in the entire US. The area around Cape Canaveral typically sees a thunderstorm on one out of every five days of the year (for a total of 70 or more annually).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a high risk of thunderstorms right on top of the hub of US space launch activity translates to a high volume of weather related scrubs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spaceport up in Virginia has it slightly better than South Florida but the high incidence of inclement weather there still makes for an unfortunate combination.&lt;/p&gt;&#xA;" OwnerUserId="3371" LastEditorUserId="3371" LastEditDate="2014-07-20T05:27:45.060" LastActivityDate="2014-07-20T05:27:45.060" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5009" PostTypeId="2" ParentId="4991" CreationDate="2014-07-19T08:42:05.317" Score="12" Body="&lt;p&gt;Batteries usually. They're charged on the ground and discharged during flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes fuel cells are also used, although that's more typical for manned spacecraft such as Apollo and the Shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And in the case of the Shuttle the orbiters also required hydraulic power to actuate their flight surfaces, provided by hydrazine fueled auxiliary power units (APUs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, many military rockets (such as shoulder launched guided missiles) use &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermal_batteries&quot;&gt;thermal batteries&lt;/a&gt; which are capable of remaining &quot;charged&quot; for decades while not in use. Such batteries use a solid electrolyte which renders the battery inert at ordinary temperatures. A small pyro charge melts the electrolyte, activating the battery and enabling it to provide power.&lt;/p&gt;&#xA;" OwnerUserId="3371" LastActivityDate="2014-07-19T08:42:05.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5010" PostTypeId="2" ParentId="3236" CreationDate="2014-07-19T19:57:15.767" Score="4" Body="&lt;p&gt;I think the &quot;apart from checklists&quot; caveat in your question makes it difficult to give you the answer you want. Astronauts aboard the ISS have a checklist that they have to complete for basically every mission activity (&lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/123834main_iss_eva_sys_checklist.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; is the NASA checklist for EVA), &lt;em&gt;especially&lt;/em&gt; if its related to the outside of the bulkhead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Page 54 of that document makes it seem as though crewlock repressurization is designed to take approximately 15 minutes- 5 for prep, 10 for the actual repress process, and then I'm sure there are plenty of time consuming activities during the Post-EVA process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you feel like skimming through the rest of the EVA manual, you might note that some airlock pressurization criteria are things like &quot;Wait until |dP/dT| &amp;lt; 0.04 mmHg/min&quot; or &quot;Wait 25 seconds.&quot; This leads me to believe that there are some opportunities for conversational chatter/dirty jokes with the comm officer, but it probably depends on who's in the airlock (and on the other end). Besides, there is a lot to be focused on when you're running through these mission procedures, so I doubt ISS astronauts are bored in there. They do have to record instrument readings and do maintenance checks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know if it were me, I'd be far too transfixed by the upcoming (or recently completed) &lt;strong&gt;&lt;em&gt;space walk&lt;/em&gt;&lt;/strong&gt; to talk a lot about the weather down below, meal plans, or family, but hey...people can get used to anything. I'm sure to some of the more experienced spacewalkers, it becomes routine.&lt;/p&gt;&#xA;" OwnerUserId="5777" LastActivityDate="2014-07-19T19:57:15.767" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5011" PostTypeId="2" ParentId="4968" CreationDate="2014-07-20T08:39:27.143" Score="12" Body="&lt;p&gt;Rockets generally use an &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_navigation_system&quot; rel=&quot;noreferrer&quot;&gt;inertial navigation system&lt;/a&gt; (INS). This system uses the input from accelerometers and gyroscopes to calculate the rocket's position (relative to the launch pad) and attitude. This is a form of dead reckoning (&quot;I have traveled x km at n degrees, so my position must be y&quot;).&lt;br&gt;&#xA;The &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/718625.pdf&quot; rel=&quot;noreferrer&quot;&gt;INS&lt;/a&gt; takes care of both navigation and error correction (compensating for e.g. wind gusts that change the rocket's attitude). INS can be used autonomously, but if you do that, tiny measurement errors build up over time so your end position is not entirely accurate. So there's generally an error correction mechanism: a system that can provide outside confirmation of the rocket's position. This can be a &lt;a href=&quot;https://space.stackexchange.com/questions/3093/how-does-the-star-tracker-in-voyager-1-lock-onto-a-specific-star&quot;&gt;star tracker&lt;/a&gt; or a ground station that tracks the launch via radar.&lt;br&gt;&#xA;The INS controls the steering system of the rocket (be it a gimbaling system, vernier thrusters or other methods).&lt;br&gt;&#xA;In modern INS, the gyroscope is often in the form of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Ring_laser_gyroscope&quot; rel=&quot;noreferrer&quot;&gt;ring laser gyro&lt;/a&gt;, instead of the traditional mechanical gimbaled rotor.&lt;br&gt;&#xA;In addition to the INS, rockets contain a sequencer, this controls aspects of the launch that are time-sensitive. The sequencer decides when to stop the engines of the first stage, jettison the stage and fire the engines of the second stage, for example. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-07-20T08:39:27.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5012" PostTypeId="2" ParentId="3152" CreationDate="2014-07-20T13:06:30.923" Score="3" Body="&lt;p&gt;As yet I am unable to reply to the former two queries raised i.e. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Whether communication frequencies are allocated on a per-agency, or a per-mission basis.&lt;/li&gt;&#xA;&lt;li&gt;Whether allocated frequencies are re-cycled after the mission&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To the last question on the rationale behind allocation of micro-wave bands, here is what I found&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space communications are effected mostly through the radio part (It extends from frequencies below 1 Hz up to around 3000 GHz) of the Electromagnetic spectrum. More recently NASA demonstrated LASER communication on the LADEE. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Part of the rationale behind the allocation of frequencies for various services are the following&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Propagation changes with frequency      &lt;/li&gt;&#xA;&lt;li&gt;Signal generation techniques differ from one frequency to another&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The article &lt;a href=&quot;http://www.spaceacademy.net.au/spacelink/radiospace.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;referenced&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Not all of the electromagnetic spectrum can pass through the Earth's atmosphere... There are in fact only two main windows of the EM spectrum that are open to space ... &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jSy1j.gif&quot; alt=&quot;Windows in the E/M spectrum open to space&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Briefly, the use of higher frequencies allows &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Larger bandwidths, &lt;/li&gt;&#xA;&lt;li&gt;Better tracking capability and &lt;/li&gt;&#xA;&lt;li&gt;Minimal ionospheric effects &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It also requires greater pointing accuracy. A higher frequency means a smaller antenna on board the craft that may be pointed with a limited amount of motive power.&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-07-20T13:06:30.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5013" PostTypeId="2" ParentId="840" CreationDate="2014-07-20T16:21:46.140" Score="15" Body="&lt;p&gt;According to wikipedia, interstellar travel at 1G would take approximately 1 year + the distance in lightyears.  Proxima Centauri (4.2 light years) for example would take 5.2 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that time is from the viewpoint of stationary observers at the departure point.  The trip's duration from the traveler's viewpoint would be less due to the time dilation effect predicted by Einstein's Theory of Relativity.  The greater the distance, the greater the speed from the stationary observer's viewpoint.  From the stationary observer's viewpoint the traveler's rate of acceleration would slow as they approached the speed of light.  The traveler would see no change between their speed and the speed of light.  Instead they would experience time at an increasingly slower rate which would effectively cause the distance to the destination to become shorter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the time dilation effect, 1G acceleration should be sufficient to travel anywhere in our galaxy in less than a lifetime from the viewpoint of the traveler, but not the stationary observer.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information on the time dilation effect read Stephen Hawking's &quot;Brief History of Time&quot;&lt;/p&gt;&#xA;" OwnerUserId="5780" LastEditorUserId="5780" LastEditDate="2014-07-20T16:43:49.070" LastActivityDate="2014-07-20T16:43:49.070" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5016" PostTypeId="2" ParentId="840" CreationDate="2014-07-21T04:27:06.740" Score="53" Body="&lt;p&gt;Assuming acceleration is constant, $d=(1/2) a t^2$. So plotted over time, distance traveled is a nice parabola.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want the time it'd take for a specific distance, it's easy to manipulate  $d=(1/2) a t^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t=\sqrt{2d/a}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're using meters and seconds as your units, $a=9.8 meters/sec^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To travel half the distance to the moon would take about 1.75 hours. The other half distance spent decelerating would take the same amount of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yCUki.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using Days and AU (astronomical units) we can see 3 days will get about 2.5 AU (halfway to Jupiter). 4.5 days will get you 5 AU (halfway to Saturn). 9 days will get you 20 AU (more than halfway to the Kuiper belt)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/un7oC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It gets trickier for interstellar distances. In Newtonian mechanics v = at, so it'd take a little less than a year to reach c at 1 g acceleration. But relativity won't allow that, we can only get close to c.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our Newtonian model is okay for nearly a year of acceleration and after that relativity wrecks this nice parabola:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jSZDK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After 1 year at 1 g we will have traveled .5 lightyears and our velocity will be close to maxed out. There after we're moving at close to c, so add a little more than a year for each lightyear distance.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-07-21T06:22:49.177" LastActivityDate="2014-07-21T06:22:49.177" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5017" PostTypeId="1" CreationDate="2014-07-21T07:15:48.517" Score="3" ViewCount="156" Body="&lt;p&gt;Although I do enjoy counting down the last ten seconds before rocket launch, I am still puzzled: Why do we still use countdown? What is real purpose behind the countdowm?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do we need to launch rocket exactly at (example) 10:15:00 ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it just a thing we are used to do, because &quot;thats how we do it from beginning&quot; or is there any practical purpose of countdowns to exact time of launch?&lt;/p&gt;&#xA;" OwnerUserId="5754" LastActivityDate="2014-07-21T07:15:48.517" Title="Why is still countdown necessary?" Tags="|launch|rockets|" AnswerCount="0" CommentCount="1" ClosedDate="2014-07-21T14:09:20.043" ContentLicense="CC BY-SA 3.0" />
  <row Id="5018" PostTypeId="1" CreationDate="2014-07-21T13:54:29.347" Score="13" ViewCount="402" Body="&lt;p&gt;Is there any coordination between governmental space agencies in order to achieve complementary science goals with interplanetary and other space missions? If so, is this done somewhat informally through the academic world, or is there some formal institution for this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My impression is that missions from different governments sometimes overlap suboptimally. For example, are ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Express&quot;&gt;Mars Express&lt;/a&gt; orbiter and the ISRO's &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot;&gt;Mars Orbiter Mission&lt;/a&gt; really complementary with two NASA orbiters already there? Is the ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/ExoMars_Trace_Gas_Orbiter&quot;&gt;ExoMars Trace Gas Orbiter&lt;/a&gt; to be launched in 2016 really an added value to the NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/MAVEN&quot;&gt;MAVEN&lt;/a&gt; (Mars Atmosphere and Volatile EvolutioN Mission) launched in 2013? Would NASA have considered doing those missions on its own if no one else had? Do they fit rationally into the grand plan of Mars exploration? ESA has also followed NASA to Venus and Mercury. And now NASA considers to follow ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter_Icy_Moon_Explorer&quot;&gt;JUICE&lt;/a&gt; mission to Jupiter's moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA and ESA both have fairly diversified mission portfolios. But taken together, they seem to overlap a lot. JAXA's asteroid and comet missions, however, I think can fit with those of NASA and ESA because of the variety of the targets and big differences in mission profiles (&lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot;&gt;Hayabusa&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_%28spacecraft%29&quot;&gt;Dawn&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot;&gt;Rosetta&lt;/a&gt;). And Russia at least aims for the moons of Mars, rather than yet another Mars orbiter or rover.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2015-07-12T01:55:38.333" LastActivityDate="2017-02-13T19:28:42.297" Title="How are space agencies coordinating their science missions internationally?" Tags="|interplanetary|mission-design|planning|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5019" PostTypeId="2" ParentId="5006" CreationDate="2014-07-21T18:55:31.217" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Broadly speaking, most rocket systems appear to have at least 3 or 4 fold radial symmetry about their long axis.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's not quite true. There are little things that make every vehicle not quite symmetric. Many of those little things were important. Plumbing, electrical connections, communication equipment, navigation sensors, computers, and the crew compartments were not symmetrically laid out in Saturn V / Apollo rocket. The umbilical tower was a rather important part of the Saturn V / Apollo rocket system, and that umbilical tower gave a marked asymmetry to the system. The rocket proper had its x-axis (roll axis) pointing upward when the rocket was on the pad, the +y axis toward the umbilical tower, and the +z axis completing a right hand coordinate system.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When sitting on the pad, in which directions were Apollo's pitch and yaw axes oriented relative to the launch tower?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://books.google.com/books?id=x-taL4N0sjIC&amp;amp;pg=PA73&amp;amp;lpg=PA73#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;How Apollo Flew to the Moon&lt;/em&gt; by W. David Woods, page 73&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At this point, it is worth outlining the vehicle's coordinate system. The Saturn V's plus-x axis ran along the length of the rocket and out through the top of the escape tower. Its plus-y axis ran through the vehicle towards the umbilical tower and therefore was pointed north. The plus-z axis ran through the vehicle to the west. As the crew lay on the couches, their heads aimed east and therefore towards the minus-z axis.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-07-21T18:55:31.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5020" PostTypeId="1" AcceptedAnswerId="5022" CreationDate="2014-07-21T21:11:38.763" Score="5" ViewCount="258" Body="&lt;p&gt;It seems this would be an economical way to explore anything beyond the heliopause, given that the probe would carry limited fuel.&lt;/p&gt;&#xA;" OwnerUserId="5783" LastActivityDate="2014-07-21T23:58:01.640" Title="Can a single-apparition comet be used to propel a deep space probe outside of our solar system?" Tags="|probe|deep-space|comet|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5021" PostTypeId="2" ParentId="5020" CreationDate="2014-07-21T21:25:35.960" Score="3" Body="&lt;p&gt;In order to connect with the comet, you need to get yourself the same orbit as the comet has. And you could do that without any comet. That's the basic flaw in your idea there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want the comet to radically change the speed of your probe, then you need a technology to absorb that &quot;collision&quot; between your probe and the comet. If a high speed train passes you by at 360 km/h that is just 0.1 km/s. Imagine jumping abord it on the fly! That's a challenge. But in order to get any speed of relevance for interplanetary space travel, you need to 100 fold that speed difference. What structure could survive such a speed change?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, NASA has very recently funded a one man one year investigation into exactly this scenario (if only there were long tethers made out of nanotubes) :-D&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/content/comet-hitchhiker-harvesting-kinetic-energy-from-small-bodies-to-enable-fast-and-low-cost/#.U82FL9-7z0o&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/content/comet-hitchhiker-harvesting-kinetic-energy-from-small-bodies-to-enable-fast-and-low-cost/#.U82FL9-7z0o&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-07-21T21:25:35.960" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5022" PostTypeId="2" ParentId="5020" CreationDate="2014-07-21T23:58:01.640" Score="5" Body="&lt;p&gt;As Local Fluff mentioned, you'd have to match velocities with the comet for a non destructive rendezvous. And when you match velocities you're already on the same trajectory as a comet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But a comet might give radiation shielding, water, and other consumables -- a lot of mass that would be hard to launch from earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/098491546X&quot; rel=&quot;noreferrer&quot;&gt;Heart of the Comet&lt;/a&gt; by David Brin and Gregory Benford looks at landing humans on Halley's Comet. This was one of Brin's early novels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before he became a science fiction writer, Brin was a competent scientist (in my opinion). His doctoral dissertation was on cometary evolution. He argued that if perihelion outgassing wasn't too violent, a comet might accumulate an insulating mantle that would preserve an icey core. He speculated that many of the dead comets that no longer outgas might still have icey cores. So I believe Brin was well qualified to write a story set on a comet. I found the yarn entertaining.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Manned rendezvous with a long period comet would be very difficult. When in earth's neighborhood comets are moving at a pretty good clip so a soft landing would take a lot of delta V. As a long period comet heads back out into the outer solar system, passengers would have to endure many decades or even centuries of extreme cold and very little sunlight. So they'd have to have a robust and reliable nuclear power source. This sort of scenario is pretty much science fiction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However a probe rendezvous with a short period comet is not science fiction. &lt;a href=&quot;http://sci.esa.int/rosetta/&quot; rel=&quot;noreferrer&quot;&gt;Rosetta&lt;/a&gt; is due to rendezvous with comet 67P/Churyumov-Gerasimenko this August. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-07-21T23:58:01.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5023" PostTypeId="1" AcceptedAnswerId="5024" CreationDate="2014-07-22T08:06:03.117" Score="33" ViewCount="7689" Body="&lt;p&gt;I've seen a lot of documentaries, articles, papers, etc which show people living in the desert or other deserted places to simulate living on Mars. However, it seems to me that a better way to really test the feasibility, isolation, and environmental hostility would be to try to create some sort of permanent, self-sustaining colony on the continent of Antarctica.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, there are already permanent facilities there, but as far as I know they are all dependent on the outside world for everything. I'm wondering if it would be a good test run for equipment, people, policies, food production, etc as a sort of alpha test for colonising Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similar problems exist in both environments, but aside from the fact people can breathe outside and radio signals are essentially instantaneous, is there anything else really different? If it's not a better test run than a desert in North America or Australia, why not? Are there any similar ideas to approaching colonising Mars with test runs like this?&lt;/p&gt;&#xA;" OwnerUserId="3443" LastActivityDate="2018-02-04T17:12:10.573" Title="Would colonising Antarctica be a good test for colonising Mars?" Tags="|mars|colonization|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5024" PostTypeId="2" ParentId="5023" CreationDate="2014-07-22T08:42:25.853" Score="25" Body="&lt;p&gt;There are a number of problems with this idea. The top two I can see are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Antarctic is an incredibly difficult place to work and live. While that might sound like an excellent test for Mars colonisation, most groups really do not want their test subjects to die because we couldn't get to them in an emergency. Deserts are actually much more forgiving in this respect.&lt;/li&gt;&#xA;&lt;li&gt;The Antarctic is protected under international treaty. The amount of impact man has already had there is considered by many to be too much - so to get agreement from all the relevant parties would be very difficult.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That said, there is extensive data from the various bases in the Antarctic on isolation and hostility, as well as scientific tests so this certainly feeds in to the planning for Mars.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="5845" LastEditDate="2014-07-30T10:07:50.133" LastActivityDate="2014-07-30T10:07:50.133" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5025" PostTypeId="1" AcceptedAnswerId="5049" CreationDate="2014-07-22T10:24:26.150" Score="9" ViewCount="1641" Body="&lt;p&gt;The Dragon capsule of SpaceX is seriously considered by NASA for a sample return mission from Mars in 2022, according to &lt;a href=&quot;http://www.seti.org/weeky-lecture/red-dragon-low-cost-access-surface-mars-using-commercial-capabilities&quot;&gt;Larry Lemke&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a Dragon be used also for hopping around on the Moon? Hoover over its surface? Land through skylights of lunar caves? Will the Dragon become a general purpose spaceship with low costs because of serial production and reusability?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since it wouldn't return to Earth and the Moon has no atmosphere, I suppose its heat shield would be replaced by a lower mass structure or instruments. But otherwise, would any other modifications be necessary for repurposing a used LEO-Dragon for interplanetary missions, except the payload inside it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the frequent poster here &lt;a href=&quot;http://hopsblog-hop.blogspot.se/2014/06/travel-on-airless-worlds.html&quot;&gt;HopDavid&lt;/a&gt; (if I'm not mistaken), a delta-v of 1.68 km/s is enough to jump from any point of the Moon to any other point, for example from pole to pole. What delta-v could a landed Dragon feasibly achieve, given a certain fraction of its payload being fuel tanks?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-06-02T02:42:02.163" Title="Is the Dragon useful as a Lunar hopper?" Tags="|lunar-landing|dragon|hopper|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5026" PostTypeId="1" CreationDate="2014-07-22T10:24:50.220" Score="3" ViewCount="409" Body="&lt;p&gt;It is not an unknown scenario in science fiction; a sun that stays stable in the sky of a planet.  Humans (or other sapient species) without space suits battling hot deserts, cold dark night sides, or the lush land at the never moving border of night and day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_locking&quot; rel=&quot;nofollow&quot;&gt;Tidal locking (or captured rotation) occurs when the gravitational gradient makes one side of an astronomical body always face another, an effect known as synchronous rotation. For example, the same side of the Moon always faces the Earth.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is life for humans and our supporting species possible on a planet where the sun does not move across the sky? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking for answers with credible science, best case scenario, with an oxygen generating biosphere evolving naturally. Is it possible or does some science rule out the possibility? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="6471" LastEditDate="2016-10-09T15:04:09.290" LastActivityDate="2016-10-09T15:04:09.290" Title="Could a planet that is tidal locked to its sun be habitable to naked humans?" Tags="|planetary-science|tidal-locking|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5027" PostTypeId="1" AcceptedAnswerId="5029" CreationDate="2014-07-22T12:12:34.883" Score="6" ViewCount="1939" Body="&lt;p&gt;What technology or techniques are used to test rovers (that are designed to work on the moon, for example) on earth ? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can the environment (e.g. gravity) of the moon be simulated on earth?&lt;/p&gt;&#xA;" OwnerUserId="414" LastEditorUserId="12102" LastEditDate="2024-02-12T04:02:14.357" LastActivityDate="2024-02-12T15:09:24.510" Title="How to simulate the environment of the moon for rovers on earth?" Tags="|the-moon|gravity|simulation|testing|simulator|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5028" PostTypeId="2" ParentId="5023" CreationDate="2014-07-22T12:23:34.380" Score="16" Body="&lt;p&gt;When I think of (non-political) challenges when settling the Antarctic, two things come to my mind:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Long winter nights&lt;/li&gt;&#xA;&lt;li&gt;Snow&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Any self-sustained facility will probably be solar powered&lt;sup&gt;1&lt;/sup&gt;. The long winter nights make this much harder than it would be on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You probably want to go outside from time to time. Snow fall and blizzards create problems very different from the ones sand storms would cause on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You don't need to go far away to test self sustained facilities, you can just put it in your backyard and employ a no-cheating rule.&#xA;The only exception is if you want to test for specific environmental conditions, but I think deserts are a better approximation of Mars than the Antarctic in that regard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;) although I wouldn't be surprised if some people out there think about searching for oil on Mars&lt;/p&gt;&#xA;" OwnerUserId="2927" LastActivityDate="2014-07-22T12:23:34.380" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5029" PostTypeId="2" ParentId="5027" CreationDate="2014-07-22T13:20:28.173" Score="3" Body="&lt;p&gt;One cannot simulate microgravity on Earth, except for a few seconds at a time in a &lt;a href=&quot;http://en.wikipedia.org/wiki/Reduced_gravity_aircraft&quot; rel=&quot;nofollow&quot;&gt;diving airplane&lt;/a&gt;. One should be able to simulate low gravity such as the 0.17g lunar gravity for a few more seconds. The simulated (low and?) microgravity business seems to be expanding with suborbital ambitions and &lt;a href=&quot;http://www.citizensinspace.org/2014/03/swiss-space-systems-to-offer-microgravity-aircraft-flights/&quot; rel=&quot;nofollow&quot;&gt;Swiss Space Systems&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I don't think it would be practical for much rover testing. And I think that the engineering is based on calculations without any actual tests aboard diving airplanes. (If real lunar gravity testing was done for the Apollo program I'd love to hear about it).&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-07-22T13:20:28.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5030" PostTypeId="2" ParentId="5023" CreationDate="2014-07-22T13:39:37.920" Score="7" Body="&lt;p&gt;I think such tests are pretty useless. However, the Mars Society successfully use them as public relations events to promote space travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since gravity, temperature swings, dust, atmosphere, radiation, ground materials doesn't match at all between Mars or any place on Earth, the only potential thing to test is social psychology. But even there I see partly huge differences, partly stuff which has already been demonstrated many times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You arrive at Mars after 6-9 months in a capsule. Submarines and space stations have already done that. We know that it isn't a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Real Mars travelers would not be able to take off their helmet to breath air, as they can on the Antarctic. They can't go home if they get sick. If anything psychological is important, I think that those differences are among the most important. The very difference between a simulation and the real thing. Human psychology understands that difference and reacts differently. If at any single moment you get desperate and panic knowing there is no way out, you could go berserk and cause accidents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the general discussions of this I've seen, I think that there's far too much focus on people getting angry with each others. That's not the psychological problem. People realize that they don't have that luxury on a real mission. Mutinies on ships have been very rare inspite of year long journeys with unfair conditions between officers and other crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, the Antarctic seems to be the &lt;strong&gt;worst&lt;/strong&gt; place on Earth to simulate Mars environment. Because one of the very few things common between Mars and Earth is the length of day. Only on the poles is even that very different with months long days and nights. Unless you want to simulate a Mars polar landing, its axis tilt is the same as that of Earth.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-07-22T13:45:02.990" LastActivityDate="2014-07-22T13:45:02.990" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5031" PostTypeId="1" AcceptedAnswerId="5032" CreationDate="2014-07-22T18:02:02.010" Score="27" ViewCount="111023" Body="&lt;p&gt;I have heard, that upon standing on the moon, Neil Armstrong at some point said &lt;em&gt;&quot;Good luck Mr. Gorsky&quot;&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The story being that while a child, playing baseball, through an open window he heard Mrs. Gorsky yell at her husband: &lt;em&gt;&quot;Oral sex, you want oral sex? When the kid next door walks on the Moon!&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that just made up, or real?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-06-20T15:38:41.783" LastActivityDate="2015-06-20T15:38:41.783" Title="Did Neil Armstrong really say &quot;Good Luck Mr. Gorsky&quot; on the Moon?" Tags="|the-moon|nasa|apollo-program|lunar-landing|myth-busting|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5032" PostTypeId="2" ParentId="5031" CreationDate="2014-07-22T18:16:47.183" Score="43" Body="&lt;p&gt;It's a cute story, an urban legend, but that's all it is. According to &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a11/a11.step.html&quot;&gt;NASA HQ Apollo 11 Lunar Surface Journal&lt;/a&gt; that includes full transcript and &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a11/a11a1091545-1101226.mp3&quot;&gt;MP3 audio clip&lt;/a&gt; (52 MB) of all the conversations between Apollo 11 astronauts and mission control, as a note near the beginning of the page, there it stands:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During November 1995, a clever (and slightly risqué) story was widely&#xA;  circulated on the Internet concerning a statement Neil is supposed to&#xA;  have made during the Apollo 11 EVA. At the suggestion of several&#xA;  readers, let me state that Neil never said &quot;Good luck, Mr. Gorsky&quot; at&#xA;  any time during the mission. Indeed, on November 28, 1995, Neil wrote&#xA;  for the ALSJ, &quot;I understand that the joke is a year old. I first heard&#xA;  it in California delivered by (comedian) Buddy Hackett&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there we have it. It's fiction.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-07-22T18:16:47.183" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5033" PostTypeId="1" AcceptedAnswerId="5034" CreationDate="2014-07-22T19:54:03.910" Score="3" ViewCount="258" Body="&lt;p&gt;This question is inspired by the following comment on &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/5023/would-colonising-antarctica-be-a-good-test-for-colonising-mars#comment13675_5023&quot;&gt;Would colonising Antarctica be a good test for colonising Mars?&lt;/a&gt;&#xA;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This isn't a direct answer, but I think colonising underwater would be a better practice for colonizing Mars. Some problems are reversed in direction (you're trying to keep water out rather than air in), but the challenges are similar: you can't breathe outside and have to take precautions going in and out, reduced sunlight of similar length, isolation both physical and communication-wise (assuming there isn't an undersea cable), and unusual hazards.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Suppose we had a manned capsule that could dive into the depths of the ocean at a controllable rate. What acceleration would it need to maintain to simulate Martian gravity within the capsule?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the capsule were diving to the deepest point in the ocean (roughly 7 miles or 11 km), how long would the inhabitants have in the simulated Martian gravity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose the capsule could also ascend at a controllable rate. If it turned around at the deepest point and ascended, how much additional time would this give in simulated Martian gravity before reaching the surface? I would think it would double the time, but I'm not sure.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-07-22T20:43:36.523" Title="How rapidly would a manned capsule have to dive to simulate Martian gravity?" Tags="|mars|artificial-gravity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5034" PostTypeId="2" ParentId="5033" CreationDate="2014-07-22T20:38:32.163" Score="4" Body="&lt;p&gt;Fluid resistance and with pressure and temperature changing viscosity of water would limit such tests to a rather short distance, depending on your acceptable margin of error. Unless you're able to constantly adjust buoyancy / rate of descent, and somehow overcome challenges of eventually hitting terminal velocity of your test apparatus in water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without it, and assuming ability to dynamically adjust buoyancy on descent, I get nearly exactly 60 seconds till the depth of 11 km at 6.096 m/s&lt;sup&gt;2&lt;/sup&gt; (Earth sea-level gravity of 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt;, minus Mars standard datum gravity of 3.711 m/s&lt;sup&gt;2&lt;/sup&gt;), which is roughly how much of Earth's gravitational acceleration you need to remove to simulate that of Mars. Remember, we're still on Earth (well, slowly descending deeper into it, but 11 km isn't all that much compared to Earth's equatorial radius of ~ 6,378 km), so those 9.80665 m/s&lt;sup&gt;2&lt;/sup&gt; didn't disappear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, your capsule would reach in 11 kilometers of free fall at Martian gravity over 366 m/s, which is above Mach 1 at mean sea-level Earth's standard atmosphere. So you'd have to employ some heavy drag reducing techniques, say a series of implosions in front of your capsule (on a parabolic trajectory) that would help if fall through it easier. I have no idea how to achieve that and keep capsule's acceleration more or less constant. Perhaps by installing a 11 km deep tube that had water evacuated and you replace it with some less dense (and inert, we don't want implosions at such depth!) gas. At such immense pressure (up to 1,100 atm), it would have to be quite a tube though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the ascent part though, you can't really do it. Your capsule's proper acceleration vector is negative to Earth's gravity, so all the forces combined within it don't partially cancel acceleration effect of Earth's gravity out, but add to it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-07-22T20:43:36.523" LastActivityDate="2014-07-22T20:43:36.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5035" PostTypeId="2" ParentId="5023" CreationDate="2014-07-22T22:20:45.393" Score="6" Body="&lt;p&gt;Some parts of Antarctica apparently experience Mars-like temperatures occasionally; however, the environment there is quite different from Mars in many other respect, in particular the floor. An Antarctica base building must cope with being established on packed snow, which has its own engineering challenges that do not echo what would be experienced by Mars colonists. For instance, it is expected that dust will be a significant issue on Mars, and Antarctica is mostly dust-less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simulation of Mars colony has been done and &lt;a href=&quot;http://en.wikipedia.org/wiki/Flashline_Mars_Arctic_Research_Station&quot; rel=&quot;noreferrer&quot;&gt;is ongoing&lt;/a&gt;, but at the other pole, on &lt;a href=&quot;http://en.wikipedia.org/wiki/Devon_Island&quot; rel=&quot;noreferrer&quot;&gt;Devon Island&lt;/a&gt;, which is part of the Canadian Arctic. The rocky ground makes it a closer match (not close, but closer nonetheless, and about as good as you can get on Earth). Also, as @Rory mentioned, human presence in Antarctica is regulated by international treaties which can make some operations a bit more complex. I may add that Devon Island can be reached for much less money, because it is quite closer to North American countries from which most of the equipment and people come, and also because it can be reached by boat (not year round, but often enough), whereas an inland Antarctica base requires air-lifting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Among Martian conditions which are very hard to simulate anywhere on Earth, one may cite:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Low gravity (about 1/3rd of Earth).&lt;/li&gt;&#xA;&lt;li&gt;Very low atmospheric pressure, requiring pressurized buildings.&lt;/li&gt;&#xA;&lt;li&gt;That pesky oxidized dust that is prone to infiltrate everywhere.&lt;/li&gt;&#xA;&lt;li&gt;Occasional dust storms which may last for weeks.&lt;/li&gt;&#xA;&lt;li&gt;Occasional very low temperatures.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="311" LastActivityDate="2014-07-22T22:20:45.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5036" PostTypeId="1" AcceptedAnswerId="5037" CreationDate="2014-07-22T23:24:08.007" Score="7" ViewCount="468" Body="&lt;p&gt;&lt;a href=&quot;http://www.spacex.com/news/2014/07/22/spacex-soft-lands-falcon-9-rocket-first-stage&quot;&gt;SpaceX has stated&lt;/a&gt; &quot;Flights 14 and 15 will attempt to land on a solid surface with an improved probability of success.&quot; Are they going to land at the Cape, or some other solid surface?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-12-15T13:43:43.170" Title="Where is SpaceX planning to land their booster rockets for Mission 14 and 15?" Tags="|launch|rockets|spacex|landing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5037" PostTypeId="2" ParentId="5036" CreationDate="2014-07-23T00:12:47.830" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;http://www.spacex.com/news/2014/07/22/spacex-soft-lands-falcon-9-rocket-first-stage&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX website&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At this point, we are highly confident of being able to land successfully on a floating launch pad or back at the launch site and refly the rocket with no required refurbishment&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In a Tweet, Elon Musk revealed the landing barge, now known as an ASDS (Autonomous spaceport drone ship) will be towed to the landing area. Beyond that, much is still unknown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/536262624653365248&quot; rel=&quot;nofollow noreferrer&quot;&gt;ASDS Tweet&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JUyot.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interesting that they claim they have enough evidence they can refly with no required refurbishment.  That seems a claim too far from the current known evidence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing on a floating launch pad with proper cameras would be awesome to see I suspect! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect, having run qualification testing on MANY Merlin 1 engines now (10 F9 flights, 5 F1 flights, that is at least 105 engines), through multiple generations (Merlin 1A, 1B, 1C, and 1D now), having run a Merlin 1D through many flight cycles on Grasshopper, and having run a trio of Merlin 1D's on F9R-Dev1 for two flights they have WAY more flight data than almost any other engine program that considered reuse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One imagines a full launch/landing will still have surprises, but it is possible they know most of what they were looking for.  The unknown stuff is what will bite them. Which is what the F9R-Dev2 at Mojave will be aiming at covering.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-12-15T13:43:43.170" LastActivityDate="2014-12-15T13:43:43.170" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5039" PostTypeId="1" CreationDate="2014-07-23T03:07:52.953" Score="8" ViewCount="762" Body="&lt;p&gt;With few exceptions, orbit-reaching rocket systems have a service life limited to a single launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle was intended at concept to be highly (and economically) reusable. Although much of the vehicle was reusable, it took a lot of work and money to prepare the vehicle for its next flight. Ultimately, its reusability was not economically advantageous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current SpaceX Falcon 9 is claimed to require no refurbishment from soft landing to next launch. Even if the claim is inflated, it is presumably less expensive per unit of payload to re-fly than the Shuttle ever was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: what is the current state-of the art for reusable surface-to-orbit vehicle systems where reuse actually confers an economic advantage over a non-reusable vehicle of comparable capacity? How many flights until either end-of-life or major refurbishment for the highest-wear components (rocket nozzles, re-entry thermal protection, etc.)? Does it number in the tens, hundreds, or perhaps thousands?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if the main structure and systems of a given vehicle are fully reusable, would it be more economic to manufacture high-wear items like rocket nozzles, heat shields, etc. to be &quot;single-use&quot;/disposable than to engineer them for maximum lifetime?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="25" LastEditDate="2017-12-19T13:28:54.107" LastActivityDate="2017-12-19T13:28:54.107" Title="What is the practical limit on reusable chemical rockets?" Tags="|rockets|reuse|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5040" PostTypeId="1" AcceptedAnswerId="5041" CreationDate="2014-07-23T04:20:37.560" Score="77" ViewCount="44606" Body="&lt;p&gt;As we all know Jupiter is a gaseous gas giant and it has a large mass, almost twice the sum of all other planets in the Solar system. So, if it happens that we go to Jupiter, and, as we know it does not have a hard surface, we could not stand on it. So what would happen if we dived into it? Would we float on the &lt;em&gt;surface&lt;/em&gt; (I don't think so), or would we be crushed into pieces due to gravity dragging us to the center of the planet with its whole mass above us?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="29286" LastEditDate="2020-07-27T16:12:57.507" LastActivityDate="2020-07-27T16:12:57.507" Title="What will be the effect if we stand on Jupiter?" Tags="|planetary-science|atmosphere|jupiter|physics|gas-giant|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="5041" PostTypeId="2" ParentId="5040" CreationDate="2014-07-23T06:27:30.297" Score="123" Body="&lt;p&gt;(*) Jupiter, for all intents and purposes, doesn't have a solid surface to stand on. Not any more than you could say that Earth's atmosphere has it, before you hit Terra Firma. It's an enormous ball composed of &lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter#Composition&quot; rel=&quot;noreferrer&quot;&gt;mostly Hydrogen and Helium&lt;/a&gt;, but also other heavier elements in smaller parts, and it's so massive that its own gravity compresses these gases into liquid the further into its interior we go. Lighter elements dominate in its upper atmosphere in gas state; these gradually compress due to its own pressure into liquids, deeper still metallic Hydrogen, and eventually mesh of metallic Hydrogen, rock, and other heavier elements that sunk deeper into its core. Nobody would be able to &quot;stand&quot; on any of these layers. In fact the temperature and pressure become so great, it's been calculated that even diamonds (it's speculated they could form as precipitates in certain layers of Jupiter's interior from black clouds of soot, where, if proven true, would mean it quite literally &lt;a href=&quot;http://www.bbc.com/news/science-environment-24477667&quot; rel=&quot;noreferrer&quot;&gt;&quot;rains diamonds&quot;&lt;/a&gt;) eventually melt into, again speculated, gooey form of liquid carbon that's not so much unlike tar, except that it isn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ouTvX.jpg&quot; alt=&quot;Jupiter vertical section&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;A pie slice of Jupiter's pressure, temperature and density layers. Source:&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://lasp.colorado.edu/education/outerplanets/giantplanets_interiors.php&quot; rel=&quot;noreferrer&quot;&gt;Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what would happen if you dived into Jupiter? Well, how long you'd last would depend on what gear you're in that protects you from its hostile environment. Pressure would at first gradually increase in its upper atmosphere to the point that it's sufficient for its violent storms to throw you around. That's the Jupiter's cloud layer. You might be &quot;lucky&quot; though, and fall into it at its poles where giant atmospheric depressions keep all of it somewhat lower, slightly prolonging the inevitable outcome. As the pressure increases, so does thermal convection. You'd start losing heat increasingly more rapidly, and it's not a nice Mediterranean spring temperature either. At one atmosphere (pressure equal to mean sea-level on Earth) the temperature goes as low as &amp;minus;108 °C. That's below the coldest temperatures ever recorded on Earth's surface (~ &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2013/09dec_coldspot/&quot; rel=&quot;noreferrer&quot;&gt;&amp;minus;93 °C in the East Antarctic Plateau&lt;/a&gt;), even for its polar regions during winters. All the while, you'd also be bombarded by &lt;a href=&quot;http://www.nasa.gov/mission_pages/juno/multimedia/pia03478.html&quot; rel=&quot;noreferrer&quot;&gt;Jupiter's radiation&lt;/a&gt;. And if you're falling into it from its poles and you thought you were lucky for a few hundred kilometers more, think again because those are the regions where Jupiter magnetically reconnects with Sun's own magnetic field, increasing speed of charged particles to the point that we can observe fantastic &quot;electric blue&quot; polar aurorae the size of many Earths where this Solar proton flux ionizes Jupiter's upper atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you have three main adversaries to fight against with your environmental protective gear you're in: radiation, pressure, and temperature. And if you enter its upper atmosphere too fast, also contact ionisation, triboelectric charge, surface ablation&amp;hellip; nothing too charming and all of it quite terminating on its own. When would any of these be too much to stand and your equipment fails is anyone's guess, but it wouldn't take very long at Jupiter's gravity (24.79 m/s²), regardless of your initial descent rate, until you dive too deep for comfort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eventually, once long dead from gas giant inhospitability, your remains would submerge deeper into Jupiter's liquid Hydrogen layer. First freeze solid, then thaw as the temperature and pressure increase to nearly 5,000 °C and about 2 million times Earth's sea-level atmospheric pressure. You'd nearly implode, if your body wasn't mostly water, which doesn't compress easily. You would still compress greatly as all your body's once functioning cavities collapse. Not the best time for a selfie. Your journey isn't yet over though, because you and your gear you're in are still denser than that particular Jovian layer and would sink deeper still towards its metallic Hydrogen layer that starts at a density of roughly 1 g/cm&lt;sup&gt;3&lt;/sup&gt; and continues to nearly 25 g/cm&lt;sup&gt;3&lt;/sup&gt; (with average density of ~ 4 g/cm&lt;sup&gt;3&lt;/sup&gt;, or slightly more than 4 times your own body's density, if we excluded an EVA suit you'd have to be in, adding to your overall density. At that point, you're being zapped by tremendous electric currents that give Jupiter such an enormous &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter&quot; rel=&quot;noreferrer&quot;&gt;magnetosphere&lt;/a&gt;, the second largest structure in our Solar system besides the Sun's own heliosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These currents would tear your remains apart into indistinguishably small fragments, and induce chemical decomposition by free atomic Hydrogen radicals randomly exchanging electrons. It would look a bit like submerging a body into hydrofluoric acid while deep frying it at the same time, if perhaps not more violent. I don't know, I can only imagine, I've never actually done it. Honest! Anyway, fragments of what was once you would decompose into its constituent chemical elements, lose valence and bound with surrounding free hydrogen protons. Heavier compounds would sink even deeper, where the pressure and current would eventually cause them to lose hydrogen protons and recombine with themselves or other heavier elements and electron hungry molecules present into so compressed and hot state not even current science is able to tell their exact nature and behavior.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Either case, you'd be spread all over Jupiter's interior in various states, and become a part of it for near to eternity. Pretty epic, but please don't do it.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(*) Not all of it might be necessarily &lt;em&gt;exactly&lt;/em&gt; true, since some parts I describe are a subject to still ongoing research, but this was kinda fun so I went for it. I'll revise to add some references at a later time for the parts that there are any available.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="6816" LastEditDate="2016-06-15T03:02:40.153" LastActivityDate="2016-06-15T03:02:40.153" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5042" PostTypeId="2" ParentId="5039" CreationDate="2014-07-23T08:30:40.290" Score="8" Body="&lt;p&gt;The current state of the art is changing rapidly. SpaceX is the best-known proponent of reusable launch systems, and they're in the middle of developing their first reusable rocket. SpaceX have not yet reused a single rocket, so it's difficult to say with any certainty how economic their approach will turn out to be.&lt;br&gt;&#xA;We have a few data points:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Heat shields. SpaceX claim their &lt;a href=&quot;http://www.spacex.com/news/2013/04/04/pica-heat-shield&quot; rel=&quot;nofollow noreferrer&quot;&gt;PICA-X heat shield&lt;/a&gt; can be reused &quot;hundreds of times&quot; without refurbishment. It remains to be seen how much time and money needs to be spent on inspecting the heat shield to make sure it's safe to reuse. PICA-X looks a lot more resilient than the Shuttle's ceramic tile system. The Dragon's heat shield contains a few dozen tiles, compared to thousands on the Shuttle, so inspection time should be a lot shorter. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/EMYhk.jpg&quot; alt=&quot;Dragon heat shield&quot;&gt;  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Engines. The critical, high-wear item in a rocket engine is also the most expensive to manufacture: the turbopump. Compared to this, the nozzle is trivial. SpaceX is developing a new rocket engine that uses methane instead of RP-1; a major reason to do this is the fact that methane will leave no residue inside the engine unlike RP-1. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Until SpaceX publish some real numbers on the cost of reuse vs. manufacturing a new rocket, all we can do is speculate. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-07-23T08:30:40.290" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5043" PostTypeId="2" ParentId="5027" CreationDate="2014-07-23T12:20:53.510" Score="8" Body="&lt;p&gt;NASA built the Reduced Gravity Simulator, which allowed astronauts to train for walking on the Moon:&lt;br /&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/5muPu.gif&quot; alt=&quot;RGS&quot; /&gt;&lt;br /&gt;&#xA;I haven't been able to find a similar system for a rover.&lt;/p&gt;&#xA;&lt;p&gt;For earth-bound tests where gravity is important (e.g. for checking the suspension), NASA often builds a replica vehicle at a lower weight (for Mars with a gravity of ~0.3G the replica will weigh 0.3 times as much as the original rover). Here's a replica of Curiosity being tested on Earth:&lt;br /&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/UAcB4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UAcB4.jpg&quot; alt=&quot;Curiosity low-weight replica&quot; /&gt;&lt;/a&gt;&#xA;As you can see, the suspension looks like Curiosity's while the body is much smaller to get the weight down.&lt;br /&gt;&#xA;Other test vehicles are built to test other aspects (thermal and vacuum tests, for instance), each test vehicle is built to simulate one aspect of the rover.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="46357" LastEditDate="2024-02-12T15:09:24.510" LastActivityDate="2024-02-12T15:09:24.510" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5044" PostTypeId="2" ParentId="4949" CreationDate="2014-07-23T12:51:12.133" Score="6" Body="&lt;p&gt;Probably. The ISS has been infected with computer viruses multiple times. You can read about it &lt;a href=&quot;http://www.tripwire.com/state-of-security/top-security-stories/kaspersky-russian-astronaut-infected-international-space-station-malware/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This was not the first instance of malware being transported to the&#xA;  space station, as other reports note that as early as 2008, another&#xA;  Russian astronaut had brought a laptop running Windows XP that was&#xA;  infected with the W32.Gammima.AG worm which quickly spread to other&#xA;  systems running the same operating system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If an attacker manages to infect an astronaut's PC with a malware, it can easily be transported to the space station and since the space station uses known operating systems, infection is plausible. The challenging part might be to cause physical damage without knowing the details about the hardware and the control systems but that won't be a problem if it's a state sponsored espionage but like Thomas Pornin said, there is no point in doing it.&lt;/p&gt;&#xA;" OwnerUserId="5799" LastActivityDate="2014-07-23T12:51:12.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5045" PostTypeId="2" ParentId="5025" CreationDate="2014-07-23T13:57:40.067" Score="2" Body="&lt;p&gt;Dragon V2 has more than enough delta-v to land on Earth. Only problem on Moon it would be too much. SuperDraco engines are throttleable (~7%) and redundant so fewer engines could be used, but still - too much engine. Probably best solution is to run off some scaled versions of SuperDraco on their 3D printer and push through test program. SuperDraco's incompatibility is not a deal-breaker due to SpaceX's CAM flexibility.&lt;/p&gt;&#xA;" OwnerUserId="5761" LastEditorUserId="5761" LastEditDate="2014-07-31T09:24:15.300" LastActivityDate="2014-07-31T09:24:15.300" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5048" PostTypeId="1" CreationDate="2014-07-23T19:48:53.250" Score="13" ViewCount="1024" Body="&lt;p&gt;I've been playing a lot of &lt;a href=&quot;http://kerbalspaceprogram.com/&quot;&gt;Kerbal Space Program&lt;/a&gt; lately. I've launched quite a few  &quot;half-failed&quot; missions where my craft will get to where it's going, or very close to it, only for me to realize that it doesn't have enough power to get back. Most of these missions are now roughly analogous to the real-world problem of dysfunctional or retired satellites which remain in orbit as space junk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I've found that the majority of my space junk problem (not really a problem just yet, but I'm sure it will be eventually) isn't with these missions. Most of my space junk is from spent rocket stages that were ejected, and now remain, in orbit or on exit trajectories from Kerbin (the Earth analogue in KSP).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a shot of Kerbin, looking at the south pole, which shows many of the 65 trackable items currently categorized as &quot;debris&quot;. All of these are upper stages that were discarded from previous missions - actual flyable (albeit perhaps un-fueled) spacecraft and probes are categorized separately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xtQCG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In considering how I'd clean up this mess, now that I've obtained the &quot;Advanced Grabbing Unit&quot; (effectively a big claw I can put on a spacecraft to grab stuff, e.g. so I can bring stuff down from orbit or re-direct it when it lacks its own power) I realized that any missions I'd launch to clean up this junk would probably just end up creating more in the process. After all, I'd need a lot of fuel and power to get back up there in the first place, and to get around once I am, and eventually I'd end up wanting to drop off empty components in orbit again. This got me to thinking a little less about cleanup, and more about prevention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I'm wondering if this is a significant consideration in the design of real-world spacecraft. Do real-world space agencies and commercial providers make any special efforts, when designing their rockets, to make sure that most expendable stages (not just reusable ones, like the Space Shuttle SRBs) are returned to Earth instead of being left in orbit? Is any consideration given at all to stages which would be discarded on an exit trajectory from Earth? (e.g.: Try to get them captured by the sun or another Sol-based celestial object, instead of leaving them to fly out where they might eventually impact some distant civilization.) Are there any common design methods or technologies that are specifically intended to address this issue?&lt;/p&gt;&#xA;" OwnerUserId="126" LastEditorUserId="12102" LastEditDate="2021-08-16T15:44:16.267" LastActivityDate="2021-08-16T15:44:16.267" Title="Do space agencies take measures to prevent spent upper stages from becoming space junk?" Tags="|design|mission-design|debris|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5049" PostTypeId="2" ParentId="5025" CreationDate="2014-07-24T00:25:07.063" Score="9" Body="&lt;p&gt;The Manned Dragon capsule, the Dragon 2, is designed to be able to do a powered escape from a launch stack. That requires at least 1.1 Gees thrust (and ISTR a requirement of 3 Gees). Further, it's designed to perform a powered landing after escaping the launch stack. But it's also required to be able to detach in flight at any altitude, including LEO, and perform a controlled descent to soft landing on a hard surface..&lt;/p&gt;&#xA;&lt;p&gt;The Dragon 2 capsule has &amp;quot;120,000 pounds of axial thrust&amp;quot;... 60 tons, for an easier measure. (Note that each of the 8 superdraco motors is capable of 8 tons, but due to angles, 3.5 of the 64 tons of total thrust is non-axial (a fancy way of saying sideways), and the rest is throttled back for safety reasons.&lt;/p&gt;&#xA;&lt;p&gt;At liftoff, the Dragon 1 masses some 26,000 pounds (including 6000 pounds payload capacity); the Dragon 2 probably masses around 30,000 pounds (15 Tons). This means that, on 2 engines, it is probably capable of 1G of thrust.&lt;/p&gt;&#xA;&lt;p&gt;Two superdraco engines are required as a bare minimum for flight (due to load balancing), and preferably 4, due to the configuration of the thruster nacelles. Each engine has a 25 second duration on internal fuel (according to the wikipedia entry). Assuming that, as with other SpaceX engines, they can be throttled back to about 65%, this puts the minimum thrust to about 19.5 tons, or roughly 0.66 Gee, and possibly up to 38 seconds thrust.&lt;/p&gt;&#xA;&lt;p&gt;If we assume even deeper throttleback (Elon Musk has stated a goal of 30%), then we could get this down to 10 tons of thrust, and up to about 80 seconds. And there are 4 such pairs.&lt;/p&gt;&#xA;&lt;p&gt;The notes on the Dragon 1 capsule give an endurance on orbit of 2 weeks. The Dragon 1 is a parachutes-to-water design.&lt;/p&gt;&#xA;&lt;p&gt;Also note: Earth landing requires 6 of the 8 superdraco engines working. So, if aiming for earth return, either refuelling or limited use are required.&lt;/p&gt;&#xA;&lt;h1&gt;On the Moon&lt;/h1&gt;&#xA;&lt;p&gt;The moon's roughly 0.16 Gee gravity makes the Dragon 2 thrust an excess. In the case of hopping, this means it will do exactly that. Assuming the exhibited 65%, it's going to have to use bursts.&lt;/p&gt;&#xA;&lt;p&gt;Landing from Earth orbit requires at least 2 pairs. That leaves room for landing and a few hops on the moon. dry-landing on earth is likely to need well more than that.&lt;/p&gt;&#xA;&lt;p&gt;The capsule itself seems, from the very limited data available, to be suitable for this role.&lt;/p&gt;&#xA;&lt;p&gt;The capsule, however, relies upon the trunk for the solar panel connections. Those are not a suitable fit for the landing, unless the trunk is fitted with landing gear.&lt;/p&gt;&#xA;&lt;p&gt;Further, the capsule door is some 6' off the ground when landed. Use as a taxi would require a ladder.&lt;/p&gt;&#xA;&lt;p&gt;In such a taxi use, the total of about two hundred seconds of thrust and lack of solar panels would necessitate a support infrastructure.&lt;/p&gt;&#xA;&lt;h1&gt;Delta-V estimates&lt;/h1&gt;&#xA;&lt;p&gt;Noting that NASA gave about a 25% above minimum delta-V, as a safety margin. The total delta-V on the Apollo LM was about 15,380 feet per second, just a hair over 4.7 km/s total delta V. Low Lunar Orbit (LLO) to Lunar Surface is about 1.8 km/s delta V.&lt;/p&gt;&#xA;&lt;p&gt;The best estimates of current Dragon V2 delta V put it around 1.6 km/s delta V. On the other hand, the capsule itself is rigged for external fuel connections, in addition to its internal fuel, so a modified &amp;quot;landing trunk&amp;quot; is a very real possibility. Adding a trunk with another 3-4 km/s worth of fuel, and some landing legs and a ladder, and you have your down-and-back, with the majority of mass beneath the center of thrust (which is a fairly stable condition).&lt;/p&gt;&#xA;&lt;p&gt;Note that LEO to Earth surface needs about 4.3 km/s of delta-V... most of which Dragon is slated to achieve by drag from Aerobraking. (Versus the Apollo CM, which did it all by drag - first by aerobraking, then by parachutes.)&lt;/p&gt;&#xA;&lt;h1&gt;Bottom Line&lt;/h1&gt;&#xA;&lt;p&gt;It could be pressed into such a use. It would not be optimal. It's not actually designed for this role.&lt;/p&gt;&#xA;&lt;h1&gt;References:&lt;/h1&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacex.com/news/2014/05/30/dragon-v2-spacexs-next-generation-manned-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;http://www.spacex.com/news/2014/05/30/dragon-v2-spacexs-next-generation-manned-spacecraft&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.spacex.com/dragon&quot; rel=&quot;noreferrer&quot;&gt;http://www.spacex.com/dragon&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/SuperDraco&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/SuperDraco&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.space.com/26063-spacex-unveils-dragon-v2-manned-spaceship.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.space.com/26063-spacex-unveils-dragon-v2-manned-spaceship.html&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Delta-v_budget&lt;/a&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&lt;/a&gt;&#xA;&lt;a href=&quot;https://www.reddit.com/r/spacex/comments/33f75a/just_how_much_deltav_will_the_dragon_2_capsule/&quot; rel=&quot;noreferrer&quot;&gt;https://www.reddit.com/r/spacex/comments/33f75a/just_how_much_deltav_will_the_dragon_2_capsule/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-06-02T02:42:02.163" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5050" PostTypeId="1" AcceptedAnswerId="5051" CreationDate="2014-07-24T03:02:39.783" Score="8" ViewCount="742" Body="&lt;p&gt;How much propellant is required to return the Falcon 9 Reusable's first stage after separation?&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="49" LastEditDate="2014-07-24T03:24:05.147" LastActivityDate="2015-01-24T23:35:19.580" Title="SpaceX Falcon 9 Reusable first stage propellant for return" Tags="|spacex|fuel|stages|falcon-9|reuse|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5051" PostTypeId="2" ParentId="5050" CreationDate="2014-07-24T11:30:56.780" Score="7" Body="&lt;p&gt;SpaceX has stated on numerous occasions that it requires about 30% fuel reserve to return the first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One example from Trent Waddingtons &lt;a href=&quot;http://shitelonsays.com/transcript/spacex-press-conference-september-29-2013-2013-09-29&quot; rel=&quot;nofollow&quot;&gt;excellent Musk-Stalking site:&lt;/a&gt; (Regardless of the URL)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Question on performance hit for attempting landing the first stage] We effectively lose, in terms of performance... It really depends on what we want to do with the stage if we were to do an ocean landing or a return to launch site landing. If we do an ocean landing, the performance hit is actually quite small at maybe in the order of 15%. If we do a return to launch site landing, it's probably double that, it's more like a 30% hit (i.e., 30% of payload lost).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Further that they planned that into the payload considerations of the booster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even further that if that means a payload is too big for a Falcon 9, then the Falcon Heavy is the way to go. They believe it to be worthwhile to take this approach to reduce overall costs, by reusability rather than expending stages.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-01-24T23:35:19.580" LastActivityDate="2015-01-24T23:35:19.580" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5052" PostTypeId="2" ParentId="5048" CreationDate="2014-07-24T11:33:39.657" Score="7" Body="&lt;p&gt;Some early upper stages would maintain residual fuel/oxidizer in the tanks after they were done.  Over time, they would explode, causing large debris fields. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most nations who can launch agree to vent propellants from upper stages that remain in orbit or try to deorbit them when practicable.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An upper stage used to launch to GeoSync is much harder to deorbit than a LEO upper stage as you might imagine.  But upper stages to GEO often have a very high apogee, with a low perigee, so the perigee can often take them back through the atmosphere for reentry.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-07-24T11:33:39.657" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5053" PostTypeId="1" AcceptedAnswerId="5055" CreationDate="2014-07-24T12:54:37.917" Score="8" ViewCount="291" Body="&lt;p&gt;I know that because of the Earth's magnetosphere, we are largely isolated from the influences of the solar winds. But what happens when a probe leaves the Earth's atmosphere and travels inward through the Solar System (like the Venera probes)? How does the solar wind affect the probe? What if it was a vehicle was carrying people? What measures should been taken into account?&lt;/p&gt;&#xA;" OwnerUserId="5798" LastEditorDisplayName="user12" LastEditDate="2014-07-27T03:29:41.447" LastActivityDate="2014-07-27T03:29:41.447" Title="Does solar wind have any influence on probes?" Tags="|probe|solar-wind|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5055" PostTypeId="2" ParentId="5053" CreationDate="2014-07-24T19:26:33.507" Score="5" Body="&lt;p&gt;Of course it affects the probes, as the number and energy of the elementary particles increase as we get higher from the earth in general (even airplanes get higher dose of radiation than cars) and when we pass the outer layers of the earth's atmosphere in specific.  Major Engineering conferences take place to discuss this issue, that's why your question is a good one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Technological challenges appear in the engineering of the material that we may use to protect our equipment of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can have further technical information in this &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0CBwQFjAA&amp;amp;url=http%3A%2F%2Fwww.uio.no%2Fstudier%2Femner%2Fmatnat%2Ffys%2FFYS4220%2Fh11%2Fundervisningsmateriale%2Fforelesninger-vhdl%2FRadiation%2520effects%2520on%2520space%2520electronics.pdf&amp;amp;ei=BlzRU_XyE8fT7AbSxoDACA&amp;amp;usg=AFQjCNGPUstd3-WBOws-dhVaN0xuM9NLbw&amp;amp;sig2=64FzPznMYXIZlA4newznGw&amp;amp;bvm=bv.71667212,d.ZGU&amp;amp;cad=rja&quot; rel=&quot;noreferrer&quot;&gt;presentation&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="5814" LastActivityDate="2014-07-24T19:26:33.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5057" PostTypeId="2" ParentId="5053" CreationDate="2014-07-25T02:05:50.143" Score="3" Body="&lt;p&gt;Yes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar wind was calculated for as far back as the last couple Mariner probes, and the Pioneer and Voyager probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, NASA and ESA also account for light pressure, including reflected IR from on-board thermal sources. The Pioneer Anomaly is the best example of this - a few hundred watts of thermal energy bouncing off the side produced a sufficient thrust to put the Pioneer probe off predicted location by a measurable amount.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-07-25T02:05:50.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5059" PostTypeId="1" CreationDate="2014-07-25T15:02:52.020" Score="0" ViewCount="731" Body="&lt;p&gt;I am very interested in outer space and the advancement of technology. I would like to get your opinion on what time-frame we can expect humans to start to colonize on the moon? I find this topic really interesting.&lt;/p&gt;&#xA;" OwnerUserId="5819" LastActivityDate="2014-07-25T17:41:21.697" Title="When will humans start to inhabit the moon?" Tags="|the-moon|colonization|" AnswerCount="2" CommentCount="6" ClosedDate="2014-07-26T06:52:30.470" ContentLicense="CC BY-SA 3.0" />
  <row Id="5060" PostTypeId="2" ParentId="5059" CreationDate="2014-07-25T15:50:18.943" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2330220/Astronauts-LIVE-moon-2020-Nasa-report-claims.html&quot; rel=&quot;noreferrer&quot;&gt;Bigelow Airspace&lt;/a&gt; did a study in which they claim various companies were interested in moon bases. They claim that inflatable moon bases could be in use by the late 2020s or early 2030s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0316J.jpg&quot; alt=&quot;Inflatable Space Base&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nydailynews.com/news/world/russia-plans-colonize-moon-permanent-lunar-base-report-article-1.1785673&quot; rel=&quot;noreferrer&quot;&gt;Russia&lt;/a&gt; plans on establishing a permanent presence on the moon in 2030.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iCOXe.jpg&quot; alt=&quot;Moon Base Concept&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;http://www.tuvie.com/futuristic-instant-moon-base-for-astronauts/&quot; rel=&quot;noreferrer&quot;&gt;[Image Source]&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these too optimistic? It is too early to tell. Technologically speaking, they are certainly possible, but whether or not they will be economically feasible is still a grey area.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2014-07-25T15:50:18.943" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5061" PostTypeId="2" ParentId="5059" CreationDate="2014-07-25T17:41:21.697" Score="4" Body="&lt;p&gt;There are not too many good reasons (I mean, economically justified, as opposed to mere awesomeness) to establish permanent bases on the Moon, let alone colonies. &quot;Good&quot; reasons include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Science:&lt;/strong&gt; the Moon is a good place for astronomy. The lack of atmosphere allows for a clear view, and there are craters at the lunar poles where one can benefit from permanent shadow (direct sunlight implies heat, which is a big issue for observations in the infrared band). Any place on the far side of the Moon will be good for radio-astronomy (a nice shield against emissions from Earth-based mankind).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Helium-3&quot; rel=&quot;nofollow&quot;&gt;Helium-3&lt;/a&gt;:&lt;/strong&gt; this isotope of Helium is exceedingly rare on Earth, but relatively more abundant on the Moon (exact mechanism for this abundance is not really known, but there it is). &lt;sup&gt;3&lt;/sup&gt;He can theoretically be used for nice aneutronic nuclear fusion reactions (reactions which yield no free neutron as byproduct, thus much less resultant radioactive hazard). When fusion plants begin to work, and then are improved to also work with &lt;sup&gt;3&lt;/sup&gt;He fusion (which is harder to achieve than Deuterium-Tritium fusion), then (and only then) it may become economically sound to go mining on the Moon.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Transportation:&lt;/strong&gt; the Moon can be a good step in the colonization of other bodies in the Solar system: it is much easier to send bulk material to Mars if you start from the Moon surface than the Earth surface, since lunar gravity is much lower than Earth's. This makes sense if there &lt;em&gt;is&lt;/em&gt; some bulk material that is needed for a Martian colony, and is available both on Earth and the Moon.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Agriculture:&lt;/strong&gt; on the Moon you can have all the sunlight you want, a lot of free space to grow crops, and controlled meteorology. You would need, of course, some sort of pressurized dome, but the gas can be extracted from the rocks, and plants can actually help in recycling the CO&lt;sub&gt;2&lt;/sub&gt; from the human colonists. The Moon as the basis for global human nutrition is a long term goal, unlikely to be relevant for a long time. It may help a lot, though, if some food must be produced and sent to some other base on a less propitious emplacement. (An agricultural Moon society is described in Heinlein's sci-fi novel &quot;The Moon is a harsh mistress&quot;; but sci-fi is &lt;em&gt;fiction&lt;/em&gt;.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It is unlikely that Moon colonization would begin for mere lack of space on Earth. It is &lt;em&gt;way&lt;/em&gt; easier to establish a permanent colony in the Sahara, Gobi desert or Antarctica than the Moon. As long as there is space in Earth deserts (and space is not lacking there), then the Moon does not make much sense as a destination for pioneers.&lt;/p&gt;&#xA;&#xA;&lt;hr /&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Once the decision to go to the Moon is taken&lt;/strong&gt;, it can be done in a relatively short time. At the time of the Apollo missions, the time frame from &quot;political decision&quot; to &quot;Armstrong and Aldrin set foot on the Moon&quot; was less than a decade. Technology from the 1960s was sufficient. In fact, the promise of Moon base &quot;within ten years&quot; has been made again and again, repeatedly, and it always failed to become reality, because of lack of money and political push, not because of any insurmountable technical issue. Shortly put: a Moon base, yeah, but what for ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, USA went to the Moon because they needed to beat the Soviets. Once they won the race, it became meaningless; they did a few extra performances (Apollo 11 to 17) then cancelled the show because of its price and falling audiences (Apollo 18 to 20 were never launched). What &lt;em&gt;might&lt;/em&gt; motivate the USA to go again to the Moon would be a Chinese mission. If China sets foot on the Moon and begins to build a permanent base, then you can be sure that within 8 years (at most) some American base will be started too.&lt;/p&gt;&#xA;" OwnerUserId="311" LastActivityDate="2014-07-25T17:41:21.697" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5062" PostTypeId="1" CreationDate="2014-07-27T05:01:30.607" Score="12" ViewCount="1079" Body="&lt;p&gt;What are some common design guidelines/practices to combat the electrical effects of spacecraft charging(e.g. ESD, ground plane shift).&#xA;Is it to focus on more resilient parts, and reduce resistance/impedence in the ground path, or are there other design guidelines/strategies?&lt;/p&gt;&#xA;" OwnerUserId="5826" LastEditorUserId="5826" LastEditDate="2014-08-01T21:30:55.913" LastActivityDate="2020-08-11T20:54:32.777" Title="Strategies for combating ESD and ground plane potential shifts on spacecraft charging?" Tags="|spacecraft|atmosphere|engineering|design|electronics|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5064" PostTypeId="2" ParentId="4741" CreationDate="2014-07-27T16:04:50.420" Score="1" Body="&lt;p&gt;While Apollo 10 flew all the way to the Moon and could theoretically have executed a landing, its ascent stage was deliberately short-fueled to discourage the flight crew from making any such attempt (doing so would have left them stranded). According to this Wikipedia article:  &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_10&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Apollo_10&lt;/a&gt; an important objective was to perform system calibrations necessary for the actual landing mission, presumably requiring a human operator present in the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apart from time pressure, (and perhaps that there was no functionally adequate substitute for a human operator to perform the required tasks) this was also very much about cold war geopolitics; Americans in space, preparing to land on the Moon, sending back live video in the process, had immeasurable PR value in the context of the cold war. Flying the system unmanned, especially after other manned missions, would not have had the same impact. After all, the whole exercise was about sending humans to the Moon.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-07-27T16:04:50.420" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5065" PostTypeId="1" AcceptedAnswerId="5068" CreationDate="2014-07-27T16:09:07.197" Score="17" ViewCount="7082" Body="&lt;p&gt;When the Space Shuttle launched, it would perform a roll and then pitch to establish its ascent trajectory. There is fairly straightforward rationale for this. The vehicle's aerodynamic shape, the benefit to keep the horizon visible through the flight deck's upper windows being the most obvious.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flying long before the Shuttle, the Apollo launch profile has been described in a very similar way - a roll to orient the vehicle so that a pitch maneuver puts it onto trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is this: With no large-scale external features to break the vehicle's apparent radial symmetry, why was it necessary/preferred for Apollo to roll first then pitch instead of forgoing the roll and simply yaw or yaw/pitch? Since the net thrust and drag forces are aligned with the vehicle's long axis, would it make any difference which way the vehicle is oriented in roll? Were the considerations similar to Space Shuttle (despite the capsule windows being covered until escape tower jettison)? Did Mercury and/or Gemini do the same?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-12-08T22:36:39.297" Title="Why did Apollo use roll and pitch maneuvers similar to Space Shuttle to establish ascent trajectory?" Tags="|launch|rockets|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5067" PostTypeId="2" ParentId="5026" CreationDate="2014-07-27T18:50:38.063" Score="4" Body="&lt;p&gt;LocalFluff's comment is the key to how such a planet could be habitable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A one-face planet will be uninhabitable as the atmosphere will freeze out on the dark side unless there's something major that prevents this (and that something major probably is pretty dangerous.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, a tidally locked but eccentric planet isn't inherently uninhabitable.  You can get enough wobble to keep it from freezing out.  The thermal swings will be major but in a sheltered location and with a sufficient atmosphere it might be possible, especially if they retreat to caves at the point of close approach.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-07-27T18:50:38.063" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5068" PostTypeId="2" ParentId="5065" CreationDate="2014-07-27T20:45:14.243" Score="22" Body="&lt;p&gt;You essentially asked the same question a week ago, Anthony X. The initial orientation of the Saturn V / Apollo stack on the launchpad was fixed. The launchpad had a north-south / east-west orientation, as did the launch vehicle + umbilical tower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That launchpad orientation was close to but not quite the optimal orientation of the stack for going to the Moon. The optimal orientation would have placed the vehicle with its axes oriented such that all that was needed post-launch was a gradual pitch down to place the vehicle on a gravity turn. That was not possible. The easiest way to achieve that was to have the stack oriented reasonable close to that optimal orientation at launch and then roll shortly after to place the vehicle in that launch-time-specific optimal orientation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your problem is your view that the vehicle was apparently axially symmetric. That's just not true, both from an automated guidance perspective and from a human factors perspective. The latter was very important. NASA didn't quite trust their automation in the 1960s. The human crew were an integral part of the failure detection process. The commander had ultimate authority to trigger an abort. He needed to see the horizon. Orienting the vehicle so that the horizon was visible and nominally flat was essential to making he commander's job easier.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-07-27T20:45:14.243" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5069" PostTypeId="1" AcceptedAnswerId="5114" CreationDate="2014-07-28T11:43:06.983" Score="11" ViewCount="4734" Body="&lt;p&gt;It gives me great pleasure to hear about the feat that Voyager 1 achieved. I was wondering if Voyager 1/2 could collide with Asteroids (those,if present, outside our solar system) or any other matter present in its way. Did NASA know exactly what path was Voyager going to follow and the possible collisions it could encounter in its journey till date? Could there be any unforeseen object in its path? &lt;/p&gt;&#xA;" OwnerUserId="5833" LastEditorUserId="897" LastEditDate="2014-07-28T19:35:21.900" LastActivityDate="2014-12-17T05:33:49.073" Title="What is the possibility of Voyager 1/2 colliding with matter (Asteroids or planetoids) present in space?" Tags="|asteroid|voyager|collision-avoidance|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5070" PostTypeId="2" ParentId="5069" CreationDate="2014-07-28T14:57:01.583" Score="6" Body="&lt;p&gt;The probability of Voyager colliding with any matter any time soon is &lt;strong&gt;unknown&lt;/strong&gt;, but likely small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have no way of detecting small outer solar system objects, because they are small and far away.  Therefore, we don't know how many of those bodies there are, and thus we cannot begin to estimate the probability quantitatively.  But, space is big, so in all likelihood we can say that the probability is very small.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2014-07-29T14:09:38.193" LastActivityDate="2014-07-29T14:09:38.193" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5071" PostTypeId="1" CreationDate="2014-07-28T17:40:45.583" Score="10" ViewCount="3395" Body="&lt;p&gt;I tried to work this out. My thinking is as follows. If the same amount of energy is given to a low mass particle and a high mass particle, the low mass particle is faster. A particle with a mass of 1 will be twice as fast as a particle with a mass of 4 but the momentum of the particle with a mass of 1 will be twice that of the particle with mass 2, which seems to go against what is seen with regards to molar mass and specific impulse.&lt;/p&gt;&#xA;" OwnerUserId="5836" LastEditorUserId="414" LastEditDate="2014-07-29T06:10:41.433" LastActivityDate="2014-08-05T15:24:29.183" Title="Why do lower molar mass rocket exhaust products give higher specific impulse?" Tags="|propulsion|specific-impulse|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5072" PostTypeId="2" ParentId="5071" CreationDate="2014-07-29T06:09:37.627" Score="7" Body="&lt;p&gt;For a chamber where the pressure is $p_c$ and contains a gas of molar mass $\mu$ and specific heat $\gamma$ and the ambient pressure condition be $p_e$. Then, the equation of the  for the exhaust velocity is given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_j=\sqrt{\frac{2*\gamma*R_0*T_c}{(\gamma-1)*\mu}*(1-\frac{p_e}{p_c})^\frac{\gamma-1}{\gamma}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $V_j$= the velocity of the exhaust &lt;/p&gt;&#xA;&#xA;&lt;p&gt;and $\mu$ is the molar mass &lt;/p&gt;&#xA;&#xA;&lt;p&gt;the exhaust jet velocity is inversely proportional to the square root of the molar mass .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are many ways to define specific impulse in two ways (both of them are equivalent)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;1) $I_{sp}=\frac{I}{m_p}$ where I=$m_p*V_j$ which leads you to $I_{sp}=V_j$&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;2)$I_{sp}=\frac{thrust}{mass..flow..rate}$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;you can see that the specific impulse is directly depends upon the exhaust velocity which is inversely  molar mass &lt;/p&gt;&#xA;&#xA;&lt;p&gt;hence lower molar mass rocket exhaust products give higher specific impulse :)&lt;/p&gt;&#xA;" OwnerUserId="414" LastActivityDate="2014-07-29T06:09:37.627" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5073" PostTypeId="1" AcceptedAnswerId="5074" CreationDate="2014-07-29T10:35:40.320" Score="5" ViewCount="1012" Body="&lt;p&gt;If I rode a space elevator to the top, assuming only natural forces apply, would the Earth be over my head or under my feet?  If it was over my head how is that possible?  &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2014-11-22T00:56:09.113" Title="Standing at the top of a space elevator would 'up' be towards the Earth?" Tags="|gravity|space-elevator|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5074" PostTypeId="2" ParentId="5073" CreationDate="2014-07-29T13:00:27.210" Score="7" Body="&lt;p&gt;Above geosynchronous orbit, so called centrifugal force exceeds force of gravity. So earth would be above you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Centrifugal acceleration (actually just inertia in a rotating frame) is $\omega^2r$ where ω is angular velocity in radians.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's sidereal day is about 23.93 hours, so ω is about 2 pi radians/23.93 hours. Or 7.3e-5 radians/second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration from gravity is $GM_e/r^2$&#xA;where $M_e$ is mass of earth and r is distance from earth's center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will find that  $GM_e/r^2$ and $\omega^2r$ exactly cancel at geosynchronous altitude. Above geosynchronous $\omega^2r$ exceeds $GM_e/r^2$. You will weigh more as you move further beyond geosynchronous height. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If my arithmetic is right, you wouldn't feel a full g until you're about 1.8 million kilometers from earth's center.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-07-29T13:06:04.963" LastActivityDate="2014-07-29T13:06:04.963" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5075" PostTypeId="1" AcceptedAnswerId="5076" CreationDate="2014-07-29T16:39:30.713" Score="25" ViewCount="7706" Body="&lt;p&gt;If I remember correctly,  The construction and launch of the Voyager spacecrafts in the early 1970s had an urgency as there was to be an unusual lineup of the outer planets (Jupiter, Saturn, Uranus and Neptune). At that time, we had only been &quot;space-faring&quot; for about 10 years!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this planetary lineup really occur infrequently? And if so, when is the next time that a Voyager-like mission (using today's technology), could be mounted?&lt;/p&gt;&#xA;" OwnerUserId="3387" LastEditorUserId="58" LastEditDate="2020-02-24T20:42:22.477" LastActivityDate="2022-08-19T23:10:13.517" Title="When is the next Outer Planet lineup (Voyager)" Tags="|planetary-science|voyager|outer-planets|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5076" PostTypeId="2" ParentId="5075" CreationDate="2014-07-29T18:42:57.097" Score="23" Body="&lt;p&gt;The lineup occurs once every 175 years. The launch window the last time this alignment occurred was from 1976 to 1980 [1], so the next time it would open would be around 2151-2154.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another Voyager-like mission could be mounted at any time really. We have the technology to do it, but the cost of mounting such a mission outside of the Grand tour launch window would be much higher, because the spacecraft would need more propellant to be able to alter its trajectory to get from place to place. Such a mission would also take longer, citing from the same source as above, the Grand tour would take &quot;...eight to thirteen years, depending on the trajectory, compared to thirty years for a direct flight to Neptune alone-by employing a maneuver called gravity assist&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The beauty of the gravity assist is that you use the gravity field of a large body to change course. A common misconception is that the gravity assist increases speed, but it actually leaves speed unchanged. It's more accurate to say that the gravity assist changes direction, since velocity is both a magnitude (speed) AND a direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I recall correctly from a study I made of these trajectories a couple years ago, the Voyagers saved something like 30 km/s of delta-v by employing gravity assists. I can't find a source for this at the moment, but I will dig up my notes and see if I can confirm. But 30 km/s is a huge savings, as most outer solar system probes only launch with a few hundred m/s of delta-v that they can spend performing maneuvers [2]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1] &lt;a href=&quot;http://history.nasa.gov/SP-4219/Chapter11.html&quot; rel=&quot;noreferrer&quot;&gt;http://history.nasa.gov/SP-4219/Chapter11.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[2] &lt;a href=&quot;https://space.stackexchange.com/questions/4054/delta-v-of-deep-space-maneuvers-in-deep-space-missions-like-voyager-or-pioneer&quot;&gt;Delta-v of deep space maneuvers in deep space missions like Voyager or Pioneer&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-07-30T16:40:26.577" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5077" PostTypeId="2" ParentId="4199" CreationDate="2014-07-30T01:51:20.273" Score="2" Body="&lt;p&gt;Ultra high quality &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetostriction&quot; rel=&quot;nofollow&quot;&gt;magnetostrictive&lt;/a&gt; materials can be made in microgravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perfect &lt;a href=&quot;https://en.wikipedia.org/wiki/Monocrystal&quot; rel=&quot;nofollow&quot;&gt;single crystal&lt;/a&gt; structures can be made in microgravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are extremely useful here on earth.&lt;/p&gt;&#xA;" OwnerUserId="5842" LastEditorUserId="168" LastEditDate="2014-07-30T08:50:03.740" LastActivityDate="2014-07-30T08:50:03.740" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5078" PostTypeId="2" ParentId="5027" CreationDate="2014-07-30T03:11:11.550" Score="5" Body="&lt;p&gt;For a robotics competition I attended, the whole robotics challenge (with mobile robots) was on a slick surface, and the robots had to have slick wheels. The idea was that the friction could be lowered to that of something on the lunar surface, but by altering the coefficient of friction, not the normal force. &lt;/p&gt;&#xA;" OwnerUserId="5843" LastActivityDate="2014-07-30T03:11:11.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5081" PostTypeId="1" CreationDate="2014-07-30T09:27:36.213" Score="9" ViewCount="1033" Body="&lt;p&gt;I have been flying rocketbelts for 21yrs. I have a new rocketbelt(some call jetpacks); a copy of the Bell rocketbelt. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe that the angles are incorrect and the throat opening is too large. My thrust tubes have an inner diameter of 1.75&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the the &lt;strong&gt;math&lt;/strong&gt; to determine the dimensions for 95% efficient nozzles? Alternatively, if the specific dimensions may be provided that shall suffice too.&lt;/p&gt;&#xA;" OwnerUserId="5846" LastEditorUserId="288" LastEditDate="2014-07-30T10:44:36.733" LastActivityDate="2019-02-09T21:36:11.823" Title="How are rocket belt nozzle dimensions calculated for a specific thrust tube diameter?" Tags="|design|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5084" PostTypeId="2" ParentId="5071" CreationDate="2014-07-30T14:14:31.940" Score="8" Body="&lt;p&gt;Suppose we have our choices of particles, we can convert all applied energy to exhaust velocity, and the energy is divided amongst the same number of particles whether they have an atom mass of 1 or 131. (Note well: Those are some big assumptions.) The velocity of an exhaust particle is $v=\sqrt{2E/m}$ where $E$ is the energy applied to an individual particle and $m$ is the mass of the particle. Multiplying by mass yields the momentum: $p=mv = \sqrt{2Em}$. So, yes you get higher velocity but less momentum with smaller particles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that it's the exhaust velocity that counts when it comes to specific impulse, not the exhaust momentum. So why do ion thrusters inevitably use xenon, which has a very high atom mass, as exhaust?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One reason is the thrust to power ratio, which is ideally $2/v_e$. Attaining a high specific impulse engine needs more power than does a lower specific impulse engine. Ion thrusters are power hungry beasts; they would function best with Mr. Fusion. (Sadly, it's almost 2015 but Mr. Fusion is nowhere in sight.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason is that specific impulse isn't the be all and get all in rocket science. Even with chemical propulsion, where the energy source and the propellant are one and the same thing, a high specific impulse might be not be beneficial. Given a desired total $\Delta v$, there's an optimal exhaust velocity (specific impulse) that minimizes energy consumption. An exhaust velocity higher than that limit is just as detrimental as is an exhaust velocity under that limit. This becomes even more important with ion propulsion because the energy source and the propellant are very different things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A third reason are those big assumptions made at the start. There are always inefficiencies in any system. In the case of ion thrusters, the energy needed to ionize the particles does not contribute to exhaust velocity. That energy ultimately goes to increasing the entropy of the universe. Big particles are a big win here because big atoms tend to hold their electrons less tightly than do smaller ones and because fewer atoms need to be ionized to attain a desired thrust. This makes ion thrusters considerably more efficient when they use larger particles, which in turn makes xenon the goto propellant for ion thrusters.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-07-30T15:18:47.067" LastActivityDate="2014-07-30T15:18:47.067" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5088" PostTypeId="1" CreationDate="2014-07-31T21:13:55.253" Score="7" ViewCount="199" Body="&lt;p&gt;A teacher recently told me that in order to have hopes of detecting Kaluza-Klein type spin 2 particles we would have to have an accelerator as big as the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is (nowadays) of course out of reach for humans, but for the sake of argument let's think that some more technologically advanced alien civilization can. Let's think that they are performing high energy experiments somewhere in this hypothetical huge accelerator. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This accelerator would emit synchrotron radiation which prompts my question, could we, by detecting this  synchrotron radiation, be able to guess that this is a signature of a high energy experiment being performed by someone or is this totally hopeless?&lt;/p&gt;&#xA;" OwnerUserId="5857" LastEditorUserId="5857" LastEditDate="2016-01-10T23:20:28.543" LastActivityDate="2016-01-10T23:40:53.533" Title="Could we detect synchrotron radiation of alien high energy experiments?" Tags="|radiation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5089" PostTypeId="2" ParentId="5088" CreationDate="2014-07-31T22:01:45.807" Score="5" Body="&lt;p&gt;I don't think so. There's all kinds of polarized radiation sources all over the spectrum in the universe and &lt;a href=&quot;https://en.wikipedia.org/wiki/Polarization_in_astronomy&quot; rel=&quot;nofollow&quot;&gt;known to astronomers&lt;/a&gt;, from masers (&lt;a href=&quot;https://en.wikipedia.org/wiki/Astrophysical_maser&quot; rel=&quot;nofollow&quot;&gt;the astronomy ones&lt;/a&gt;, but a similar question could be if we could detect high energy emissions of &lt;a href=&quot;https://en.wikipedia.org/wiki/Maser&quot; rel=&quot;nofollow&quot;&gt;masers of the other kind&lt;/a&gt;, that could be used as source of microwave &lt;a href=&quot;https://en.wikipedia.org/wiki/Beam-powered_propulsion&quot; rel=&quot;nofollow&quot;&gt;beam-powered propulsion&lt;/a&gt;), &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetar&quot; rel=&quot;nofollow&quot;&gt;magnetars&lt;/a&gt; splitting and merging x-rays and polarizing its surroundings in the process, and so on. And there's plenty of other naturally occurring &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_radio_source&quot; rel=&quot;nofollow&quot;&gt;astronomical radio sources&lt;/a&gt; too, that could easily mask any polarized sources and convert it all into unstructured &lt;a href=&quot;https://en.wikipedia.org/wiki/White_noise&quot; rel=&quot;nofollow&quot;&gt;white noise&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So unless these &lt;a href=&quot;https://en.wikipedia.org/wiki/Synchrotron_radiation&quot; rel=&quot;nofollow&quot;&gt;synchrotron radiation&lt;/a&gt; sources exhibit some other distinct and indisputably intelligent design characteristics (i.e. they would be intentionally used to transmit their location, as a sort of a beacon, perhaps repeating a sequence of prime numbers or alike), they could be easily lost to background noise or dismissed as a natural source. To my knowledge, nobody is (currently) explicitly looking at polarized radiation sources with intention of discovering high energy &lt;a href=&quot;https://en.wikipedia.org/wiki/Technosignature&quot; rel=&quot;nofollow&quot;&gt;technosignatures&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But since such source radiation would be neither narrow beam nor capable of high bandwidth communications due to its sheer required size stretching several light hours, it wouldn't make much sense to use it as anything else but a beacon anyway. And, as far as we know, &lt;a href=&quot;http://news.sciencemag.org/physics/2014/07/physicists-spot-potential-source-oh-my-god-particles&quot; rel=&quot;nofollow&quot;&gt;we might have already detected&lt;/a&gt; just that in the so-called &lt;a href=&quot;https://en.wikipedia.org/wiki/Oh-My-God_particle&quot; rel=&quot;nofollow&quot;&gt;Oh-My-God particles&lt;/a&gt;, it would just be a bit of a stretch drawing such &lt;em&gt;&lt;a href=&quot;http://www.bigear.org/wow20th.htm#vast&quot; rel=&quot;nofollow&quot;&gt;vast conclusions from half-vast data&lt;/a&gt;&lt;/em&gt;, to quote Jerry R. Ehman, the person that discovered the &lt;a href=&quot;https://en.wikipedia.org/wiki/Wow!_signal&quot; rel=&quot;nofollow&quot;&gt;Wow! signal&lt;/a&gt; and famously circled the data print-out and wrote &lt;em&gt;Wow!&lt;/em&gt; next to it. They might just as well be emissions of a perfectly natural process that we don't yet fully understand.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-01-10T23:40:53.533" LastActivityDate="2016-01-10T23:40:53.533" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5090" PostTypeId="1" AcceptedAnswerId="5091" CreationDate="2014-08-01T07:49:14.233" Score="14" ViewCount="2202" Body="&lt;p&gt;A British inventor invented an engine that can provide thrust in outer space without a propellant. &lt;a href=&quot;http://www.wired.co.uk/news/archive/2014-07/31/nasa-validates-impossible-space-drive&quot;&gt;This engine&lt;/a&gt; has been tested by a Chinese team, and more recently by NASA:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engine itself is &lt;a href=&quot;http://emdrive.com/&quot;&gt;The EmDrive:&lt;/a&gt; and is FAR too complicated for me to understand.&#xA;If this engine actually is legitimate, what would be the consequences for Science and Space exploration?&lt;/p&gt;&#xA;" OwnerUserId="5860" LastEditorDisplayName="user12" LastEditDate="2014-08-01T16:33:08.420" LastActivityDate="2016-04-25T07:10:51.823" Title="What are the consequences of NASA validating this &quot;impossible&quot; space drive?" Tags="|nasa|engine-design|emdrive|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5091" PostTypeId="2" ParentId="5090" CreationDate="2014-08-01T09:27:58.667" Score="12" Body="&lt;p&gt;Looking through the linked article, several forces are mentioned, including 720 mN and 30-50 uN. These are extremely low levels, but still can have some use. At first glance, these seem similar to the numbers provided by Ion Drives, and Ion Drives also use a large amount of power, so let's compare the two of them, and see how they compare. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster#Comparisons&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, I find that the thrusters typically operate around 2-3 kW of power, with a thrust around 70 mN for that given power level. The numbers according to the website provided for the Chinese test are 720 mN with a power level of 2.5kW. So according to that article, they are potentially more energy efficient than ion drives, at least according to the Chinese. I say potentially because the NASA levels reported are significantly different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would assume this would replace the Ion Drive as the long duration maintenance thruster for maintenance, when it has been fully vetted, and be used for some deep space missions. This will increase the time and capabilities of these missions, but not provide a radical difference than what we have now. It might make more possible long durations missions, such as Dawn, or even interplanetary missions, but the power is far too little to do something like lift off of a planet, for instance. I imagine there will be other uses found as well, ones that we can't easily think of at the moment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will save some on the fuel. &lt;a href=&quot;http://www.nasa.gov/mission_pages/dawn/news/dawn-20071009.html&quot; rel=&quot;noreferrer&quot;&gt;Dawn&lt;/a&gt;, for instance, had 425 kg of Xenon for use as fuel. That is a significant portion of the spacecraft, which can be used for other things, or just make the mission cheaper. &lt;a href=&quot;http://www.chemicool.com/elements/xenon.html&quot; rel=&quot;noreferrer&quot;&gt;Xenon&lt;/a&gt; has a cost of about \$1200 per kg, and thus just the fuel alone costs around \$600,000.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-08-01T21:47:13.017" LastActivityDate="2014-08-01T21:47:13.017" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5092" PostTypeId="1" AcceptedAnswerId="5093" CreationDate="2014-08-01T11:05:32.363" Score="1" ViewCount="171" Body="&lt;p&gt;Would it be possible (not necessarily plausible) for an astronaut to bring a paperback novel along on a mission and accidentally leave it in space somehow?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would an astronaut even be allowed to bring such a thing on a mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would happen to the book over time (e.g. would it stay preserved, orbit as space trash)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you.&lt;/p&gt;&#xA;" OwnerUserId="5861" LastActivityDate="2014-08-01T13:54:23.377" Title="Paperback Books in Space?" Tags="|debris|feasibility|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5093" PostTypeId="2" ParentId="5092" CreationDate="2014-08-01T13:54:23.377" Score="8" Body="&lt;p&gt;Astronauts on most missions are allowed to bring personal items with them. Depending on what kind of mission, there are of course limits on mass and volume for their personal luggage. However, a paperback book would fit into the parameters of most missions. So an astronaut bringing a novel into space as a personal item is plausible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the question is how this item would get into open space. Most missions avoid putting anything into orbit to avoid unnecessary space debris. Trash is usually collected and sent back to earth. The ISS uses the expendable Progress transport capsules for this purpose, which then burn up in the atmosphere. So the only way for a book to end up in orbit would be by accident. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could happen that the book ends up in the airlock during an EVA and leaves the vessel this way. However, this would be quite a sign of negligence to have unexpected objects in an airlock. Using an airlock for an EVA is a very dangerous activity, so the airlock should be checked beforehand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could also happen due to a catastrophic accident on the vessel. A loss of cabin pressure through a large enough hole could cause some loose items to get blown into space. A collision with debris could cause this kind of damage. Such an event would also be likely to cause the destruction of the vessel and the demise of the crewmembers on board.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding the book itself: In either case it would end up in an orbit very similar to that of the vessel which lost it. When that orbit is high enough to avoid atmospheric friction (the ISS orbit would not), it could stay there for a very long time as another piece of space debris. However, the strong sun radiation would likely cause the paper and ink to bleach, so it might not be readable anymore after a few decades.&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2014-08-01T13:54:23.377" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5094" PostTypeId="1" AcceptedAnswerId="5096" CreationDate="2014-08-01T14:02:59.877" Score="18" ViewCount="9815" Body="&lt;p&gt;This morning's BBC News says that a 2020 rover mission to Mars will &lt;a href=&quot;http://www.bbc.com/news/science-environment-28582903&quot; rel=&quot;noreferrer&quot;&gt;&quot;make oxygen&quot;&lt;/a&gt;. There is a brief line indication that it will extract it from atmospheric CO₂, but no other details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What process will the rover use? Is this a plausible method for obtaining oxygen on Mars for human life support?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2015-04-20T21:44:05.593" LastActivityDate="2015-04-20T21:46:03.800" Title="How do you 'make oxygen' on Mars?" Tags="|mars|rovers|atmosphere|experiment|isru|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5095" PostTypeId="2" ParentId="5094" CreationDate="2014-08-01T14:21:14.137" Score="6" Body="&lt;p&gt;There are many proposed chemical processes that can lead to taking elements found commmonly on Mars, and using them to make more useful things for humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The entire process is vaguely know as &lt;a href=&quot;http://en.wikipedia.org/wiki/In_situ_resource_utilization&quot;&gt;In Situ Resource Utilization (ISRU)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That Wikipedia link nicely summarizes some of the likely approaches to be taken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a great Sci Fi book that looked at  the problem as a series of successive chemical processes, where the output of the first is used by the second, then third, so that they get a complete cycle going.  Cannot find the reference.  (There is an accident on earth, starts a fire, primary character cannot go due to lung damage).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-08-01T14:21:14.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5096" PostTypeId="2" ParentId="5094" CreationDate="2014-08-01T15:16:48.077" Score="11" Body="&lt;p&gt;To be somewhat more specific than @geoff that gave an overview, regarding MOXIE (Mars OXygen In situ resource utilization Experiment), according to brief notice in the &lt;a href=&quot;http://www.nasa.gov/press/2014/july/nasa-announces-mars-2020-rover-payload-to-explore-the-red-planet-as-never-before/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA press release&lt;/a&gt; that accompanied the Mars 2020 Rover scientific payload announcement:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Mars Oxygen ISRU Experiment (MOXIE), an exploration technology&#xA;  investigation that will produce oxygen from Martian atmospheric carbon&#xA;  dioxide.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And a mock-up of the presumed layout of these announced scientific experiments and equipment:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xvnnE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;An artist concept image of where seven carefully-selected instruments will be located on NASA’s Mars 2020 rover. The instruments will conduct unprecedented science and exploration technology investigations on the Red Planet as never before. Image credit: &lt;a href=&quot;http://www.nasa.gov/press/2014/july/nasa-announces-mars-2020-rover-payload-to-explore-the-red-planet-as-never-before/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More detailed information will slowly trickle into the press as specifics are decided upon during the course of next few years, and indeed &lt;a href=&quot;http://spaceref.com/news/viewpr.html?pid=43670&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceRef just published a story on it&lt;/a&gt; (I'd assume following media teleconference that was scheduled a few hours after yesterday's public announcement of the Mars 2020 Rover science package on NASA TV):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;MOXIE -- short for Mars OXygen In situ resource utilization Experiment&#xA;  -- was selected from 58 instrument proposals submitted by research teams around the world. The experiment, currently scheduled to launch&#xA;  in the summer of 2020, is a specialized reverse fuel cell whose&#xA;  primary function is to consume electricity in order to produce oxygen&#xA;  on Mars, where the atmosphere is 96 percent carbon dioxide. If proven&#xA;  to work on the Mars 2020 mission, a MOXIE-like system could later be&#xA;  used to produce oxygen on a larger scale, both for life-sustaining&#xA;  activities for human travelers and to provide liquid oxygen needed to&#xA;  burn the rocket fuel for a return trip to Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More on MOXIE &lt;a href=&quot;http://spaceref.com/news/viewpr.html?pid=43670&quot; rel=&quot;nofollow noreferrer&quot;&gt;in the article&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this experiment is going to extract oxygen from the atmospheric carbon dioxide that constitutes 96% of Martian atmosphere. And the modus operandi for this particular experiment seems will be what &lt;a href=&quot;https://en.wikipedia.org/wiki/In_situ_resource_utilization#Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia describes&lt;/a&gt; as &lt;em&gt;electrolysis of the atmosphere&lt;/em&gt;, possibly using &lt;a href=&quot;https://en.wikipedia.org/wiki/Zirconia&quot; rel=&quot;nofollow noreferrer&quot;&gt;zirconia&lt;/a&gt; as a catalyst:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\require{mhchem}\ce{2CO2} + \text{energy} → \ce{2CO + O2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See e.g. &lt;a href=&quot;http://papers.sae.org/981655/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zirconia Electrolysis Cells for Oxygen Generation from Carbon Dioxide for Mars In-Situ Resource Utilization Applications&lt;/a&gt;. Additional note that carbon monoxide is merely an immediate reaction product and, while it &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbon_monoxide#Niche_uses&quot; rel=&quot;nofollow noreferrer&quot;&gt;could be used as fuel on Mars&lt;/a&gt;, it can also be further reduced by use of catalysts and additional energy into elemental carbon and oxygen, or by adding hydrogen by secondary &lt;a href=&quot;https://en.wikipedia.org/wiki/Bosch_reaction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bosch reaction&lt;/a&gt; to graphite and water, and so on. Since it's not yet known exactly how MOXIE will do that, and what use its reaction products will be put to, I'd reserve further judgment on it for the time being.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are of course also other possibilities to extract oxygen on Mars, for example from possible water ice pockets embedded deep into its regolith and using electrolysis to split water into oxygen and hydrogen, from dry ice that we know exists in abundance in Martian regolith and as a precipitate above it in polar regions and then using same or similar oxygen extraction methods as MOXIE will employ, to superheating and melting oxidized minerals, rust and metallic ore to release locked oxygen in it, and so on. But for the first steps, extracting it directly from atmospheric CO&lt;sub&gt;2&lt;/sub&gt; seems to be the easiest way to do it without needing to employ a whole truckload of machinery that we can't afford to land there, or don't really know how to yet.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-04-20T21:46:03.800" LastActivityDate="2015-04-20T21:46:03.800" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5097" PostTypeId="1" CreationDate="2014-08-01T16:32:57.363" Score="17" ViewCount="1973" Body="&lt;p&gt;So apparently NASA just built an &lt;a href=&quot;http://sploid.gizmodo.com/nasa-reveals-new-impossible-engine-can-change-space-t-1614549987&quot;&gt;impossible propulsion device&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sawyer's engine is extremely light and simple. It provides a thrust by &quot;bouncing microwaves around in a closed container.&quot; The microwaves are generated using electricity that can be provided by solar energy. No propellant is necessary, which means that this thrusters can work forever unless a hardware failure occurs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why is this device considered impossible?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastActivityDate="2014-10-21T19:16:37.563" Title="Why is the &quot;impossible&quot; space drive impossible?" Tags="|propulsion|advanced-propulsion|emdrive|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5098" PostTypeId="2" ParentId="4822" CreationDate="2014-08-01T16:58:07.343" Score="10" Body="&lt;p&gt;In the past 6 years NASA and the DOE have put a fair amount of effort into estimating the cost to recapture NTP technology assuming either graphite based or tungsten CERMET fuel.  The estimates to include fuel development, reactor development, reactor testing and infrastructure ranged from $4 to 6 billion over 10 to 15 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, where there has been little thought thus far is in how many engines would be required per decade and how reusable each engine is. The lack of thought is partially due to the fact that the answers would be mission and architecture dependent and as of now there is no mission or architecture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the Moon, Mars and beyond program (Constellation) there were some VERY preliminary studies that indicated a decade of lunar exploration could be 40 to 50% cheaper with NTP than chemical engines, but I am not sure at what point you made up the development cost.&lt;/p&gt;&#xA;" OwnerUserId="5856" LastEditorUserId="49" LastEditDate="2014-08-01T17:21:50.983" LastActivityDate="2014-08-01T17:21:50.983" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5099" PostTypeId="2" ParentId="5097" CreationDate="2014-08-01T18:29:49.490" Score="15" Body="&lt;p&gt;It's a violation of Newton's 3rd law of motion? This is a fairly complete answer to the question in the absence of any deeper argument by the proponents of the device, but I'll get to those in a minute. Firstly, it's important to recognize that our universe follows a set of predictable and consistent laws. This might have been controversial in the 1700s. One of those laws is that forces must be accompanied by an &lt;strong&gt;opposite and equal&lt;/strong&gt; force an another object. Now there are way that modern physicists can revise this in creative ways. For instance, there could be many bodies acted upon by a force that can't be divided up into 1-to-1 interactions, but these are all objections that don't change the bottom line. You can't push against nothing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another revision might be massless particles. Radio waves are photons, and they have momentum like &lt;strong&gt;photons&lt;/strong&gt;. You can propel yourself by emitting any kind of light - but the forces are miniscule compared to what is being claimed here. It also requires tremendous amounts of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's move on to the &quot;modern physics&quot; adjustments to Newton's 3rd law, and possible violations of it. It is sometimes claimed that &lt;strong&gt;general relativity doesn't preserve Newton's 3rd law&lt;/strong&gt;. Maybe this is semantics. Practically, it's obvious that gravitational waves are scientifically verified and predicted by general relativity. Much like photons, these can carry momentum along with them. Since this is a &quot;ripple in spacetime&quot; and not momentum carried by some matter, there's certainly room for argument. However, momentum is still conserved. Also like photons, this is mostly useless in terms of the actual thrust involved and the difficulty of building a practical machine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, most of what I'm reading about this subject makes reference to &quot;new&quot; physics, mostly quantum gravity type stuff. There are many theories that would predict that some sort of reactionless drive is possible. However, proving such a principle &lt;em&gt;through&lt;/em&gt; a reactionless drive prototype is contextually bizarre. If you could show even the slightest variation from classical physical theory, then that's interesting by its own merit. What I'm saying is that any such machine should &lt;em&gt;first&lt;/em&gt; be interesting to physicists, &lt;em&gt;then&lt;/em&gt; be interesting to spaceflight. It's entirely possible that we will discover some novel way to break Newton's 3rd law, but then we'll have no practical way to use it in spaceflight. After all, this was the exact case with my above 2 historical examples.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More than likely, they're not on to anything. Dr. Harold White uses real physics and makes claims backed up by genuine science, in addition to having access to a large body of peer scientists to back things up. However, his statements are difficult to parse correctly, and this combines with irresponsible reporting to produce headlines which are often blatant falsities. In his paper abstract, he refers to force produced by virtual particles. Great, that's fine. This is not tremendously surprising, because this is within established physics. It might just be that they achieve &quot;propulsive momentum transfer via the quantum vacuum virtual plasma&quot; without diverging from current theory. But that doesn't make it a reactionless drive! Current theory still preserves momentum. Maybe no one has done a detailed study to see how this particular case balances momentum, but we have every reason to believe that it does. Even if &lt;em&gt;they&lt;/em&gt; are onto something, that lends no credibility to the bogus experiments that are searching for some true reactionless drive.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2014-08-01T20:09:26.370" LastActivityDate="2014-08-01T20:09:26.370" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5100" PostTypeId="2" ParentId="5090" CreationDate="2014-08-02T07:06:16.903" Score="14" Body="&lt;h1&gt;The Drive's Impacts&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;If the drive works as described, it could reduce the costs to launch to those in the range needed to commercialize manned flight. They claim to be able to design a 2G shuttle using it for primary thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that a consistent 0.01G (0.1 m/s² vehicle would be capable of making it to mars in a matter of days rather than months. It would be about 15 days, instead of 6 months, at closest approach. A 1G drive reduces closest approach to about 35 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drive as described uses energy conversion from frequency to kinetic force. Assuming the papers are correct, the measurements weren't fudged, and the apparatus actually operates, it's a potential revolution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drive isn't without consumables, however. The drive requires cryogenic cooling. This is as surely a limit as the fuel for an ion thruster. In theory, a sufficiently potent heat pump and radiator system could be used to provide both closed cooling and (very very) miniscule additional thrust. In practice, cryogenic coolants generally are open cycle; using them involves venting them.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;The Impact of NASA Confirmation&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The impact of NASA replicating the chinese and inventor's results indicates that the drive may have actual validity, and that the physics may be incomplete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Primarily, it is a credible agency giving a credible result of &quot;apparently works&quot; to a drive that's revolutionary in concept - both concept of design and concept of mode of operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondarily, it's a clear &quot;Conservation of Momentum needs refinement&quot; result. Until the specific method is evaluated better, it's just a &quot;Useful anomaly&quot; rather than a deal breaker.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Cascade Effects&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;If NASA finds it cheap and credible, then it will get tested further (tho' not, of need, by NASA). If that testing bears out the initial findings, then it will lead to commercial use of the drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the drive bears out in orbital flight the tested ratings, then the cascade through physics will include many new theory tweaks trying to explain it, and a lot of tests, until eventually a consensus explanation arises.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Ongoing Testing Notes&lt;/h2&gt;&#xA;&#xA;&lt;h3&gt;Nov 2015&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Just a note that NASA is apparently still testing the device. &lt;a href=&quot;http://www.iflscience.com/space/nasa-allegedly-conducts-more-successfully-emdrive-tests0&quot; rel=&quot;nofollow&quot;&gt;source: IFLScience, 3 Nov 2015&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;24 Apr 2016&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;on 13 Apr 2016, M.E. McCulloch released a paper, &lt;em&gt;Testing quantised inertia on the emdrive&lt;/em&gt;, explaining how the drive works without violating conservation of energy, based upon. &lt;a href=&quot;http://arxiv.org/abs/1604.03449&quot; rel=&quot;nofollow&quot;&gt;Source: arXiv.org &gt; physics &gt; arXiv:1604.03449&lt;/a&gt; Paper is up for peer review. The basis is &lt;em&gt;Unruh Radiation Effect&lt;/em&gt;, a not yet widely accepted bit of theoretical physics, which can be used to predict thrust of an EM Drive based upon the chamber, the power, and the wavelengths, and changes in the effective inertial mass of the photon due to frequency shifts as it bounces around. McCulloch does not, however, actually define the maximum theoretically ideal Q value...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the Conclusion of McCulloch:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;More than eight tests in four independent labs have shown that when microwaves&#xA;  resonate within an asymmetric cavity an anomalous thrust is generated pushing&#xA;  the cavity towards its narrow end.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This force can be predicted fairly well by using a new model for inertia (MiHsC)&#xA;  which assumes that the inertial mass of the photons is caused by Unruh radiation&#xA;  whose wavelengths have to fit exactly inside the cavity so that the photons’&#xA;  inertial mass is greater at the wide end. To conserve momentum a new force&#xA;  appears to to push the cavity towards its narrow end, and the predicted force&#xA;  is similar to the thrust observed.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;MiHsC suggests that the thrust can be increased by increasing the input power,&#xA;  the Q factor, or using a dielectric. As a direct test MiHsC predicts that the&#xA;  thrust can be reversed by making the length L equal to the width of the narrow&#xA;  end.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2016-04-25T07:10:51.823" LastActivityDate="2016-04-25T07:10:51.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5101" PostTypeId="2" ParentId="5094" CreationDate="2014-08-02T07:39:13.027" Score="6" Body="&lt;p&gt;In simplest terms: You find a chemical that is made of a mixture of oxygen and anything else, and then cause the molecules of that chemical to give up their oxygen (and thus becoming something else).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2 H₂O can be cracked into 2 H₂ + O₂ by electrolysis  &lt;/li&gt;&#xA;&lt;li&gt;CO₂ can be cracked into C + O₂ by highly inefficient electrolysis, or via photosynthesis.  &lt;/li&gt;&#xA;&lt;li&gt;2 CO₂ can be cracked and recombined to 2 CO + O₂&lt;/li&gt;&#xA;&lt;li&gt;Hematite and Iron Oxide (both of which are common) can be (given enough energy) cracked to release the oxygen, and result in iron and oxygen.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastEditorUserId="49" LastEditDate="2015-04-20T21:41:43.513" LastActivityDate="2015-04-20T21:41:43.513" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5102" PostTypeId="2" ParentId="5090" CreationDate="2014-08-02T15:27:58.620" Score="18" Body="&lt;h2&gt;Call me unimpressed.&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;NASA did not &quot;validate&quot; this &quot;impossible&quot; space drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, this was a &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140006052&quot;&gt;conference paper&lt;/a&gt;, not a peer reviewed scientific journal paper. Even if the results had been published in a peer reviewed scientific journal, I would still not call it &quot;validation.&quot; The peer reviewed literature is where science starts, not ends. To be &quot;validated&quot;, the results would have to be widely replicated elsewhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let's dig deeper. From the conference paper,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thrust was observed on both test articles, even though one of the test articles was designed with the expectation that it would not produce thrust. Specifically, one test article contained internal physical modifications that were designed to produce thrust, while the other did not (with the latter being referred to as the “null” test article).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This supposed &lt;strike&gt;EmDrive&lt;/strike&gt; Cannae Drive&lt;sup&gt;a&lt;/sup&gt; is a highly extraordinary claim; it violates conservation of momentum. Extraordinary claims require extraordinary evidence. The authors can't even reject the null hypothesis. That's not extraordinary evidence. It's more along the lines of &quot;Move along, move along, there's nothing to see&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Footnote&lt;/strong&gt;:&lt;br&gt;&#xA;&lt;sup&gt;a&lt;/sup&gt; The article being tested apparently is not Shawyer's EmDrive. It is a different device, Guido Fetta's Cannae Drive, which uses the &quot;quantum vacuum virtual plasma&quot; (whatever that is) as the means for obtaining thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's the media that made the connection to the EmDrive. That connection is nowhere to be found in the conference papers by White et al. or by Fetta. In particular, &lt;a href=&quot;http://www.google.com/patents/US20140013724&quot;&gt;Fetta's patent&lt;/a&gt; claims no prior art, which must mean it is somehow distinctly different from Shawyer's EmDrive, at least in the eyes of the patent office.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-08-06T18:35:40.143" LastActivityDate="2014-08-06T18:35:40.143" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5103" PostTypeId="1" AcceptedAnswerId="5104" CreationDate="2014-08-02T23:46:08.783" Score="15" ViewCount="3936" Body="&lt;p&gt;SpaceX lands their booster rocket vertically, this seems complicated. The space shuttle landed with wings which seems more simple. Why is it preferable for the rocket to land vertically as opposed to with wings?&lt;/p&gt;&#xA;" OwnerUserId="5879" LastActivityDate="2017-04-10T01:16:57.627" Title="Why is it preferable for SpaceX to land their booster vertically rather than fly it down with wings?" Tags="|rockets|spacex|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5104" PostTypeId="2" ParentId="5103" CreationDate="2014-08-03T01:16:03.343" Score="35" Body="&lt;p&gt;Wings are heavy.  Control surfaces are hard to move at reentry speeds, so need powerful hydraulics. These must be triply redundant. More weight to carry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wings are not useful in orbit and are dead mass carried up, and then back down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle could not go around for a second pass, so any goof up during landing and the crew was all dead.  Thankfully that never happened, but I seem to remember one Shuttle did land short and was close to a problem. &lt;a href=&quot;http://www.avweb.com/news/safety/183035-1.html?redirected=1&quot; rel=&quot;noreferrer&quot;&gt;A story of short landing shuttles&lt;/a&gt; is interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whereas vertical landing seems complex, the reality is, the engines and fuel are already there. They are needed on the way up. If you wish to recover them, you need to bring them back, so might as well use them on the way down. The fuel is already in the booster, just use some for return instead of all to orbit. (Reduces payload, yes, but may be worth it, time will tell if it pays off).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is nicely lowering risk with incremental testing via Grasshopper, F9R-Dev1, and then real tests on otherwise paid for commercial flights. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the point is, yes vertical landing requires extra fuel, and has payload costs.  But so does a set of wings. Wings look simple, but as you get deeper into it, you add more and more subsystems, until it eats up much more payload than you would have expected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Revisiting this answer almost three years later, they have successfully landed 8 or more stages either on the ASDS barge fleet or on land. Thus it is clear, the proof is in the pudding. (And the pudding is delicious and fun to watch).  Mission after mission lands. Reuse has happened. Whether reuse is quick and affordable remains to be seen at this time.  I guess I shall have to revisit this in another two years, based on what comes next.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-04-10T01:16:57.627" LastActivityDate="2017-04-10T01:16:57.627" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5105" PostTypeId="2" ParentId="5090" CreationDate="2014-08-03T02:59:42.107" Score="3" Body="&lt;p&gt;&lt;em&gt;If&lt;/em&gt; it works as advertised, it can have quite an impact on satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They need to make small corrections to their orbits every now and then. Over years, it adds up to quite a bit of fuel, which had to be ferried to orbit at launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This new system's only input seems to be energy, which is plentiful at space. If the power comes from an increase in the size of solar panels, you'd need to do the math to see if it's worth it. But it could come from off-peak power in some satellites which are not used full-on all the time. These adjustments have a fairly flexible schedule.&lt;/p&gt;&#xA;" OwnerUserId="2765" LastActivityDate="2014-08-03T02:59:42.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5107" PostTypeId="2" ParentId="5103" CreationDate="2014-08-04T02:14:48.717" Score="21" Body="&lt;p&gt;A vertical landing system, versus a winged one, does several things:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;increases available landing sites (see note 1)&lt;/li&gt;&#xA;&lt;li&gt;encourages more agile boosters (which enables better course corrections)&lt;/li&gt;&#xA;&lt;li&gt;requires very little additional hardware&lt;/li&gt;&#xA;&lt;li&gt;preserves a larger fraction of disposable launch payload (see note 2)&lt;/li&gt;&#xA;&lt;li&gt;doesn't preclude a disposable use if payload maximization is essential (See note 3)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note 1:&lt;/em&gt;&lt;br&gt;&#xA;The vertical landing system needs about a 100m radius circle to land in. (Due to engine blast, mostly.) It needs that to be a relatively firm surface. Any flat spot with a concrete pad&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The winged landing requires at least 20m wide, by at least a kilometer long. It needs to be nearly flat, fairly smooth, and clear of approach obstacles. Further, there is the consideration of approach directions - NASA had a network of some 30 approved runways for different landing approaches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note 2:&lt;/em&gt;&lt;br&gt;&#xA;THe only additional equipment needed for the vertical launch are the landing gear and the additional gimbals for the thrust. All the additional equipment can be radially symmetrical for both mass and atmospheric drag purposes. The primary mass penalty is the combination of the fuel mass for landing fuel and the loss of that same fuel for payload launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For winged landing, the additional equipment needed includes the wings, the landing gear, and the control surfaces. The wings inherently are not balanced - the produce a sideways force during a vertical takeoff, and are not readily made multi-axis symmetrical. The fuel penalty is primarily due to the additional mass of the wings - but it's a significant penalty because the the mass of the wings is so profound - counting control surfaces, the wings are often most of the dry mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note 3:&lt;/em&gt;&lt;br&gt;&#xA;The mass penalty for disposable use in a vertical landing craft is only the landing gear and gimbals. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass penalty for disposable use in a winged design still includes the massive wings, and the extra fuel needed to lift them; disposable use gains no benefit.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2016-04-11T20:46:23.223" LastActivityDate="2016-04-11T20:46:23.223" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5108" PostTypeId="1" CreationDate="2014-08-04T11:24:08.823" Score="1" ViewCount="102" Body="&lt;p&gt;&lt;a href=&quot;http://www.asterank.com&quot; rel=&quot;nofollow&quot;&gt;Here is a comparison of nearby asteroids&lt;/a&gt; with their worth in US Dollars. Does our technology have the capability to reach any of these asteroids and bring back enough of the materials to be considered profitable?&lt;/p&gt;&#xA;" OwnerUserId="5883" LastEditorUserId="5883" LastEditDate="2014-08-05T01:11:57.727" LastActivityDate="2014-08-05T16:34:42.623" Title="Is our technology advanced enough to start asteroid mining?" Tags="|asteroid|solar-system|" AnswerCount="0" CommentCount="3" ClosedDate="2014-08-06T12:30:43.053" ContentLicense="CC BY-SA 3.0" />
  <row Id="5109" PostTypeId="2" ParentId="3288" CreationDate="2014-08-04T17:06:06.533" Score="10" Body="&lt;p&gt;I found it!  The SPIF (Spacecraft Processing and Integration Facility) was used for the Shuttle program, and renamed to SPIF after the Challenger incident.  Used to be the SMAB (Solid Motor Assembly Building).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The IUS (Intertial Upper Stage) were assembled there for Shuttle payloads.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The location is shown on this map:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1nQIk.png&quot; alt=&quot;KSC/CCAFS Locations&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The internal layout looks like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BN6FH.jpg&quot; alt=&quot;SPIF Interior layout&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The outside looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qs9Sn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several bays, with room for 2 payloads to be processed, a third in the transfer aisle, and airlocks to get the payloads in and out, without contamintating anything.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the last image, bottom left there are three doors side by side.  Two are access ways to two bays, and the third is the airlock for incoming payloads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is leasing space in the SPIF, apparently as they need it (Used for Asiasat 8 and 6 launches, may be used for Dragons as well to make space in LC40 hangar and allow for two or more payload processing flows at once. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a picture of a DSP being mounted on a IUS for a Shuttle launch:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/IQyKS.jpg&quot; alt=&quot;DSP mounted on IUS&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a picture of the outside of the building.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/U1Cap.jpg&quot; alt=&quot;Outside of SPIF-SMAB&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-08-04T17:11:47.783" LastActivityDate="2014-08-04T17:11:47.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5110" PostTypeId="1" CreationDate="2014-08-05T01:49:53.150" Score="12" ViewCount="1105" Body="&lt;p&gt;If we assume that the claims for the &lt;a href=&quot;http://en.wikipedia.org/wiki/EmDrive&quot;&gt;EmDrive&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Woodward_effect&quot;&gt;Woodward effect&lt;/a&gt; and/or &lt;a href=&quot;http://en.wikipedia.org/wiki/Quantum_vacuum_plasma_thruster&quot;&gt;quantum vacuum plasma thruster&lt;/a&gt; are born out (notice I did NOT mention the &lt;a href=&quot;http://en.wikipedia.org/wiki/Dean_drive&quot;&gt;Dean Drive&lt;/a&gt;), what would be the upper limit of potential applications based on predicted if not measured force per unit of input power and present-day power generation technologies (including nuclear)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the stated results seem to suggest that for practical power input, only modest forces yielding tiny fractions of a g could be applied to a spacecraft, making reactionless drive (if it actually works) suitable for nothing more than orbital stationkeeping or long duration robotic spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other numbers seem to suggest far greater capability: enough force per unit power to yield more than 1g vehicle acceleration for current power generation technologies. One could imagine flying cars, easy flight into space, and interplanetary vehicles completing trips in hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the source of the disparity in claims? Are there errors in the physics/math? Are the differences due to different mechanisms of operation (e.g. Woodward effect vs quantum vacuum plasma)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course this is all based on claims which have yet to be rigorously proven out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the claimed yields in force per unit power and what is the potential scalability for each of the proposed reactionless drive concepts/principles? Are we talking space probe microthrusters, flying cars, or flying battleships? How close is each principle to a peer reviewed science based answer as to whether it is a reality with a reference design or merely some experimental error, anomaly, or hoax? Are we talking months, years, or decades?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way... where does the input power go in these drives? If a test article is simply pushing against a stationary test instrument, is all input the power converted to heat in the device, or does some of it disappear into the quantum vacuum foam or whatever (as with the putative momentum transfer)?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="1142" LastEditDate="2014-10-21T18:48:50.410" LastActivityDate="2015-09-06T20:24:41.813" Title="Hypothesized parameters of reactionless drives" Tags="|propulsion|emdrive|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5111" PostTypeId="2" ParentId="5110" CreationDate="2014-08-05T10:19:52.957" Score="2" Body="&lt;p&gt;from the report of yang Juan and NASA I computed that&#xA;EmDrive was reported as 200-1000mN/kW and Cannae Drive as 1.4-5mN/kW&lt;/p&gt;&#xA;&#xA;&lt;p&gt;since the null reactor of Fetta worked despite Fetta's theory, and since it is about 0.5% of EmDrive measured performance, until new evidence one can consider the possibility that Cannae Drive is a parasitic EmDrive (it works because of a tiny asymmetry)...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The impact of EmDrive, and even Cannae Drive is huge for satellites and proves, provided you consider it is easy to install solar panel providing few kW, or nuclear reactor of few kW/MW.&#xA;continuous acceleration of few Newton, and even milli-Newton are much higher than what ion-drive propose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for satellite the main cause of satellite expiration is when they have no fuel... if they can correct their trajectory with an EmDrive, their life will be the one of their solar panel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for heavier application, flying cars, rocket launching, one have to base prediction on Shawyer (see his IAC2013 paper). He estimate from his theory (probably broken, no problem if test succeed) that using superconducting cavities one could sustentate tons per kW... This is only projection of his theory, so unlike his reproduced test, this is to take with more caution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;note that EmDrive theory explain that momentum is conserved, that if the thrust is used to accelerate the reactor then the energy of the microwave is consumed (through Doppler effect). Conservation of energy and momentum is enforced according to EmDrive theory... This is only theory, so...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Be careful it can be an artifact, but a smart artifact linked to microwave resonance (see Emdrive FAQ), not a dumb one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;hope this helps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;sources: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yang Juan EmDrive test &lt;a href=&quot;http://www.emdrive.com/yang-juan-paper-2012.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.emdrive.com/yang-juan-paper-2012.pdf&lt;/a&gt;&#xA;NASA Cannae Drive test: &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2014-4029&quot; rel=&quot;nofollow&quot;&gt;http://arc.aiaa.org/doi/abs/10.2514/6.2014-4029&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;much data is missing to judge of the quality of Cannae Drive test.&#xA;For EmDrive test, the FAQ on EmDrive site gives details.&lt;/p&gt;&#xA;" OwnerUserId="5888" LastActivityDate="2014-08-05T10:19:52.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5113" PostTypeId="2" ParentId="5071" CreationDate="2014-08-05T15:24:29.183" Score="-2" Body="&lt;p&gt;To give a hands-on explanation: Imagine you have a kilogram of a &quot;light&quot; gas like hydrogen and the same weight of a &quot;heavier&quot; gas like air in the same size containers of 1m^3. Now punch a hole into container and see how far you fly the other way in both scenarios.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-08-05T15:24:29.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5114" PostTypeId="2" ParentId="5069" CreationDate="2014-08-05T16:15:27.237" Score="23" Body="&lt;p&gt;The Voyager probes are outside the Kuiper belt now, and have a very long way to go before entering the Oort cloud. They are now in a place that is almost completely devoid of matter. Or at least I couldn't find any estimates as to how dense the solar system is there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about when they where still in the Kuiper belt?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Wikipedia is to be believed, the Kuiper belt has a mass of about 4.59*10^23kg and is mostly made of ice. Let's say the average object is a sphere with a diameter of 1m (put in a better estimate, if you want to). An ice sphere of 1m diameter has a mass of 489kg, meaning there are 9.385*10^20 such objects. Each has a cross section of 0.785m^2.&#xA;Let's say for simplicity, the Kuiper belt is a torus with a major radius of 40AU and a minor radius of 10 AU. (I haven't found a good number for the north-south dimension of the Kuiper belt, sorry). That means that it's volume is 2.64*10^37m^3. That means there is an average free volume of 2.81*10^16m^3 per object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can borrow from gas kinetics to get an average free path length, that the Voyagers can fly before colliding with something. (I know this doesn't directly answer your question, but it gives us an idea)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\lambda = \frac{1}{n\sigma}&#xA;$$&#xA;with $\frac{1}{n}$ the mean free Volume and $\sigma$ the average cross section.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We get: &lt;em&gt;drum whirl&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;239 282.67 AU&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meaning that the voyager probes could almost fly through the Kuiper belt all the way to alpha centauri before smashing into anything. Now keep in mind that material density is much lower where they are right now...&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-08-05T16:15:27.237" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5116" PostTypeId="1" AcceptedAnswerId="10663" CreationDate="2014-08-05T16:57:03.247" Score="11" ViewCount="1427" Body="&lt;p&gt;The oxygen turbopump in the &lt;a href=&quot;http://en.wikipedia.org/wiki/RL10&quot;&gt;RL10&lt;/a&gt; upper stage engine has been openly discussed by NASA for some decades now, in publications like &lt;a href=&quot;http://www.zerognews.com/special/sp8000/archive/00000158/01/sp8107.pdf&quot;&gt;SP-8107&lt;/a&gt;. On page 8 of this document is a drawing of this turbopump. The lower end of the oxygen pump shaft seems to be splined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the back of my head, I seem to remember reading somewhere that it has something to do with a possible usage of the engine in an ICBM, but that doesn't make much sense, considering it uses liquid hydrogen as fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone tell me if it is indeed splined, and what is supposed to be connected there, if anything?&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="49" LastEditDate="2014-08-05T18:01:40.090" LastActivityDate="2015-08-15T17:19:30.967" Title="Function of the RL10 Oxygen Turbopump's Splined Shaft End" Tags="|engineering|oxygen|rl-10|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5117" PostTypeId="1" AcceptedAnswerId="5118" CreationDate="2014-08-06T12:58:52.993" Score="34" ViewCount="3742" Body="&lt;p&gt;From &lt;a href=&quot;http://www.newscientist.com/article/dn25995-cometmapping-rosetta-spacecraft-prepares-for-duty.html&quot; rel=&quot;noreferrer&quot;&gt;this link&lt;/a&gt; I fount the following representation of Rosetta's orbit relative to 67P:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/c6dMf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;https://www.youtube.com/watch?v=Mf1zsACcXc4&quot; rel=&quot;noreferrer&quot;&gt;video from ESA&lt;/a&gt; depicts a similar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;is that really the relative orbit?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If yes: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;why is it triangular initially? does this bring some specific benefits?&lt;/li&gt;&#xA;&lt;li&gt;doesn't that shape costs a ludicrous amount of propellant? or in the Sun-centered fixed frame of reference the actual orbit undergoes much smaller changes of direction than it would appear from the images shown? (and thus the total $\Delta V$ required is not as big)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="5862" LastEditorUserId="49" LastEditDate="2015-07-25T14:39:36.500" LastActivityDate="2015-07-27T12:47:15.857" Title="Is this really Rosetta's orbit around 67P?" Tags="|orbit|propulsion|orbital-maneuver|rosetta|mission-design|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5118" PostTypeId="2" ParentId="5117" CreationDate="2014-08-06T13:20:50.043" Score="31" Body="&lt;p&gt;This was one of the questions just now during the &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Call_for_Media_Rosetta_s_comet_rendezvous&quot;&gt;Rosetta press briefing&lt;/a&gt;. &lt;a href=&quot;https://www.youtube.com/watch?v=Mf1zsACcXc4&quot;&gt;This video&lt;/a&gt; was shown during the presentation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Mf1zsACcXc4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The triangular trajectory are hyperbolic orbits with respect to the comet and they'll (also, among other tasks also mentioned in the image you're attaching) serve to establish its mass. In essence scientists will be looking at how the comet's gravity changes these &quot;straight&quot; legs of the triangular Rosetta's quasi-orbit around comet 67P and estimate its density / mass more precisely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trajectory changing maneuvers aren't all that costly to the Rosetta spacecraft, and it was mentioned (during the briefing) that we're talking of delta-v of only a few meters per second during each of these maneuvers. So in a sense, since the comet's own gravity isn't all that great and the probe was already successfully injected into comet's own heliocentric orbit, they're not expensive to Rosetta's own propellants and would be more alike to satellite station-keeping maneuvers, like say the ones that satellites in halo or Lissajous orbits at Lagrange points would be performing, also on a fairly regular basis.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-25T14:39:59.140" LastActivityDate="2015-07-25T14:39:59.140" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5119" PostTypeId="2" ParentId="5110" CreationDate="2014-08-06T15:51:34.763" Score="5" Body="&lt;p&gt;For the EM-Drive, the disparity comes from the test results of the different test apparati. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;at least 5 test run versions have been made:&lt;br&gt;&#xA;— The 2 test rigs by the developer&lt;br&gt;&#xA;— The Chinese replication&lt;br&gt;&#xA;— The NASA test (tested in the same program as two cannae thrusters - see Brady, et al)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each version produces a different thrust. This may be due to non-optimal configuration, sub-optimal materials, or improper frequency setting for the chamber. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that each replication was a ground-up build from specifications, rather than borrow the developer's version and test it, as far as I can tell.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note: the claimed peak efficiencies require a superconducting chamber; which said chamber is in development, and was not tested by NASA nor the Chinese, both of whom appear to be using room temperature devices.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can say that the &lt;a href=&quot;http://emdrive.com/demonstratorengine.html&quot; rel=&quot;noreferrer&quot;&gt;SPR model&lt;/a&gt; and another &lt;a href=&quot;http://emdrive.com/feasibilitystudy.html&quot; rel=&quot;noreferrer&quot;&gt;SPR Model&lt;/a&gt; don't look the same as the &lt;a href=&quot;http://www.theregister.co.uk/2014/08/04/nasa_tests_impossible_microwave_drive_that_could_get_us_to_mars_in_weeks/&quot; rel=&quot;noreferrer&quot;&gt;NASA one (photo at TheRegister.co.uk)&lt;/a&gt;. They are similar in material, and show a generally conical profile, but there are visible differences in construction. The chinese replication looks different still (&lt;a href=&quot;http://nextbigfuture.com/2014/07/renewed-hope-for-emdrive-with-nasa.html&quot; rel=&quot;noreferrer&quot;&gt;photo at next big future blog&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;While I have seen and heard references to a European lab confirmation, I cannot find who that is at the moment.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For the Cannae drive... NASA replications in Brady, et al, show a difference between the slotted and unslotted drive - but again, the NASA test articles were manufactured in-house, not the same articles tested by others.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;References:&lt;/em&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://nextbigfuture.com/2014/07/renewed-hope-for-emdrive-with-nasa.html&quot; rel=&quot;noreferrer&quot;&gt;http://nextbigfuture.com/2014/07/renewed-hope-for-emdrive-with-nasa.html&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://nextbigfuture.com/2009/10/emdrive-progress.html&quot; rel=&quot;noreferrer&quot;&gt;http://nextbigfuture.com/2009/10/emdrive-progress.html&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.theregister.co.uk/2014/08/04/nasa_tests_impossible_microwave_drive_that_could_get_us_to_mars_in_weeks/&quot; rel=&quot;noreferrer&quot;&gt;http://www.theregister.co.uk/2014/08/04/nasa_tests_impossible_microwave_drive_that_could_get_us_to_mars_in_weeks/&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://rt.com/usa/177204-nasa-space-drive-emdrive/&quot; rel=&quot;noreferrer&quot;&gt;http://rt.com/usa/177204-nasa-space-drive-emdrive/&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140006052&quot; rel=&quot;noreferrer&quot;&gt;http://ntrs.nasa.gov/search.jsp?R=20140006052&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/EmDrive&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/EmDrive&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://emdrive.com/feasibilitystudy.html&quot; rel=&quot;noreferrer&quot;&gt;http://emdrive.com/feasibilitystudy.html&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://emdrive.com/demonstratorengine.html&quot; rel=&quot;noreferrer&quot;&gt;http://emdrive.com/demonstratorengine.html&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://emdrive.com/dynamictests.html&quot; rel=&quot;noreferrer&quot;&gt;http://emdrive.com/dynamictests.html&lt;/a&gt;&lt;br&gt;&#xA;&lt;strong&gt;&lt;em&gt;Anomalous Thrust Production from an RF Test Device Measured on a Low-Thrust Torsion Pendulum&lt;/em&gt;&lt;/strong&gt; &lt;em&gt;David A. Brady, Harold G. White, Paul March, James T. Lawrence, and Frank J. Davies&lt;/em&gt; retrieved 14 aug 2014 from &lt;a href=&quot;http://www.libertariannews.org/wp-content/uploads/2014/07/AnomalousThrustProductionFromanRFTestDevice-BradyEtAl.pdf&quot; rel=&quot;noreferrer&quot;&gt;libertariannews.org&lt;/a&gt;; abstract at &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140006052&quot; rel=&quot;noreferrer&quot;&gt;NASA.gov&lt;/a&gt;. Note: the paper is clear that the cannae and the trucated conic chamber were tested.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2014-08-14T18:08:45.057" LastActivityDate="2014-08-14T18:08:45.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5120" PostTypeId="1" AcceptedAnswerId="5121" CreationDate="2014-08-06T19:29:16.737" Score="4" ViewCount="693" Body="&lt;p&gt;When taking photos in space with consumer grade cameras modules, what factors need to be taken into account? For example, is auto-focus needed? Will the photos end up washed out because of the additional sunlight? Are pictures of the moon feasible (we have +/- 5 degree pointing)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The camera is a secondary payload, just meant to take opportunistic photos. But it would be nice if they where of a high quality. Here is an example of a camera we are considering: &lt;a href=&quot;https://www.sparkfun.com/products/12804&quot; rel=&quot;nofollow&quot;&gt;https://www.sparkfun.com/products/12804&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4686" LastActivityDate="2014-08-07T09:52:37.730" Title="Considerations for Taking Photos from CubeSat in LEO" Tags="|photography|cubesat|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5121" PostTypeId="2" ParentId="5120" CreationDate="2014-08-06T20:05:30.500" Score="8" Body="&lt;p&gt;Auto focus is not required, and would be a hindrance. You should simply focus at infinity, that will be sufficient for any images without a large telescope and very high density. I believe HiRISE has to worry about the difference in focusing at 300 km vs stars/Earth/etc, but this usually isn't a concern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photos could end up washed out if you have the sun in your field of view, or near your field of view, this can best be prevented by putting a shield of some kind around your lens, such as a lens hood. They won't be perfect, but they will help you to improve your protection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest concern, however, is one you haven't mentioned. The platform stability is critical. Take a camera without flash and take a picture from a car, at night, etc, and you'll see that the platform must remain stationary, or else the pictures will turn out poorly. There are a few ways to manage this, the key ones being:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Use a short exposure.&lt;/li&gt;&#xA;&lt;li&gt;Keep the platform stable, using gyroscopes or other similar.&lt;/li&gt;&#xA;&lt;li&gt;Using an adaptive technique, ground post processing.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The first two are preferred, but the third can be used in some cases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to do this, I suggest you first figure out your requirements. What kind of exposure will you need to get the picture you want? How much camera shake are you willing to have? What pixel density do you want? What is your weight and power constraints? All of these must be considered to find the system that can best work with yours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-08-07T09:52:37.730" LastActivityDate="2014-08-07T09:52:37.730" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5122" PostTypeId="2" ParentId="5097" CreationDate="2014-08-06T21:48:40.300" Score="4" Body="&lt;p&gt;Newton's 3rd law is empirical, and this observation could in theory be the first black swan. That by itself is unlikely, given the amount of empirical data gathered so far (the historically more likely hypothesis is measurement error; wouldn't be the first case).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I consider more damning is that per &lt;a href=&quot;http://en.wikipedia.org/wiki/Noether%27s_theorem&quot; rel=&quot;nofollow&quot;&gt;Emmy Noether's first theorem&lt;/a&gt;, spatial symmetry is associated with the conservation of impulse. And that symmetry hasn't been disproven yet. This is a stronger statement, because we &lt;em&gt;know&lt;/em&gt; Newtonian physics are wrong (see General Relativity) but Noether's theorem still holds in GR.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastEditorUserId="1142" LastEditDate="2014-10-21T19:16:37.563" LastActivityDate="2014-10-21T19:16:37.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5123" PostTypeId="1" AcceptedAnswerId="5124" CreationDate="2014-08-07T09:58:32.800" Score="0" ViewCount="312" Body="&lt;p&gt;Now that it is a really special comet, and every one is talking about it, why doesn't it get a name, instead of just a number?&lt;/p&gt;&#xA;" OwnerUserId="5895" LastActivityDate="2014-08-07T18:21:29.457" Title="Why doesn't 67P get a name?" Tags="|rosetta|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5124" PostTypeId="2" ParentId="5123" CreationDate="2014-08-07T10:24:47.770" Score="7" Body="&lt;p&gt;In fact there is a better name for &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot;&gt;67P&lt;/a&gt;, which is 67P/Churyumov–Gerasimenko. It was named after the two people who helped discover it in 1969.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Churyumov &lt;a href=&quot;http://www.space.com/24292-rosetta-spacecraft.html?cmpid=514630_20140807_29243276&quot;&gt;identified 67P&lt;/a&gt; while studying photographs of Comet 32P/Comas Solá, taken by Gerasimenko.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FYI, the official comet naming convention can be found at the &lt;a href=&quot;http://www.cbat.eps.harvard.edu/cometnameg.html&quot;&gt;IAU website&lt;/a&gt;. They give the comet the name based on the information they have on the discoverers. &lt;a href=&quot;https://en.wikipedia.org/wiki/Naming_of_comets&quot;&gt;Wikipedia&lt;/a&gt; also has great information on naming comets.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="1103" LastEditDate="2014-08-07T18:21:29.457" LastActivityDate="2014-08-07T18:21:29.457" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5125" PostTypeId="1" CreationDate="2014-08-07T10:28:23.627" Score="14" ViewCount="2392" Body="&lt;p&gt;Astronauts in microgravity don't use their legs much. This has negative health effects in the long run. But I wonder if it doesn't have the good side of lowering human need for food and water and thus the mass of supplies? Do people eat less in space than on Earth?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="58" LastEditDate="2016-06-15T19:47:50.337" LastActivityDate="2016-06-15T19:47:50.337" Title="Do humans need less calories and water in microgravity?" Tags="|microgravity|life-support|humans|astronaut-lifestyle|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5126" PostTypeId="1" AcceptedAnswerId="5128" CreationDate="2014-08-07T11:07:12.163" Score="9" ViewCount="517" Body="&lt;p&gt;The pictures of Rosettas orbit show it doing some straight legs past 67P for the next while.-&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean that at the current distance, the force of 67P's gravity is not really significant, and Rosetta is not really (yet) &quot;in orbit&quot; but rather just flying in the same general direction at the same time?&lt;/p&gt;&#xA;" OwnerUserId="5895" LastEditorUserId="49" LastEditDate="2014-08-07T15:15:26.463" LastActivityDate="2014-08-07T15:15:26.463" Title="Is Rosetta really &quot;in orbit around&quot; 67P, or just conveniently co-located?" Tags="|rosetta|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5127" PostTypeId="2" ParentId="5125" CreationDate="2014-08-07T12:15:14.330" Score="8" Body="&lt;p&gt;Apparently the sense of taste is dulled in space.  Thus favorite food stuffs are things like Tabasco sauce, and other spicy things. Thus they may eat less, due to reduced taste and desire to eat. (Partial answer at best, I know).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is confirmation from Chris Hadfield (&lt;a href=&quot;https://www.youtube.com/watch?v=xFgS9alvV1w&quot; rel=&quot;nofollow&quot;&gt;video&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="4686" LastEditDate="2014-08-08T18:43:06.550" LastActivityDate="2014-08-08T18:43:06.550" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5128" PostTypeId="2" ParentId="5126" CreationDate="2014-08-07T12:40:17.460" Score="7" Body="&lt;p&gt;Rosetta will shift between different &lt;a href=&quot;https://en.wikipedia.org/wiki/Hyperbolic_trajectory&quot; rel=&quot;noreferrer&quot;&gt;hyperbolic trajectories&lt;/a&gt;. This means that it won't move in an ellipse around the comet, but escape it if not repeatedly powered to another hyperbolic trajectory in a series of flybys. Trajectory changes are planned every Wednesday and Sunday. It is not what most ordinary people would think of as an &amp;quot;orbit&amp;quot;, hence the confusion.&lt;/p&gt;&#xA;&lt;p&gt;See from 16:45 in &lt;a href=&quot;http://www.esa.int/spaceinvideos/Videos/2014/08/Rosetta_at_comet_First_images_science_results&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; of the press conference August 6th.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-08-07T13:00:55.560" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5129" PostTypeId="1" AcceptedAnswerId="5140" CreationDate="2014-08-07T12:41:01.817" Score="10" ViewCount="365" Body="&lt;p&gt;Title says it all really: how accurately do they know where the Rosetta space vehicle is at any given time, and how do they know that?&lt;/p&gt;&#xA;" OwnerUserId="5895" LastActivityDate="2019-11-10T19:49:13.583" Title="How accurately do they know where Rosetta is, and how do they know?" Tags="|rosetta|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5130" PostTypeId="2" ParentId="5125" CreationDate="2014-08-07T13:43:12.903" Score="11" Body="&lt;p&gt;Almost certainly not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, astronauts work out for hours every day, specifically to prevent the deterioration of bones and muscles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, the human energy budget is dominated not by the energy cost of moving around, but by a) maintaining body temperature and b) the brain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, just removing gravity from the equation isn't necessarily going to save you any energy when it comes to moving around: While you no longer have to fight against gravity in some situations; you &lt;em&gt;do&lt;/em&gt; have to replace it in others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(This answer only addresses calories: Water requirements are invariant wrt. gravity.)&lt;/p&gt;&#xA;" OwnerUserId="5898" LastActivityDate="2014-08-07T13:43:12.903" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5131" PostTypeId="2" ParentId="5125" CreationDate="2014-08-07T13:57:09.537" Score="10" Body="&lt;p&gt;From one &lt;a href=&quot;http://lsda.jsc.nasa.gov/scripts/experiment/exper.aspx?exp_index=611&quot;&gt;NASA study&lt;/a&gt; which examined several space shuttle crews: &quot;Energy expenditures were similar before and during flight.&quot; However, there was a decrease in energy intake (also noted in this &lt;a href=&quot;http://ajcn.nutrition.org/content/60/5/801S.short&quot;&gt;journal article&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the energy requirements seem to be the same, but for whatever reasons the astronauts seem to take in less food and water. So yes, people eat less in space, but they shouldn't!&lt;/p&gt;&#xA;" OwnerUserId="4686" LastActivityDate="2014-08-07T13:57:09.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5132" PostTypeId="1" AcceptedAnswerId="5133" CreationDate="2014-08-07T15:14:12.777" Score="8" ViewCount="437" Body="&lt;p&gt;Rosetta's lander, Philae is scheduled to land on Comet 67P in November 2014. This will involve use of &quot;harpoon&quot; to anchor itself to make sure Philae remains attached to the surface despite weak gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the time lag, I understand we cannot have control over this in real time. Considering this, I have following questions about this scheduled landing:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Did we have sufficient information about the composition of this Comet to determine the thrust required for anchoring the harpoon? Or is it yet to be determined?  &lt;/li&gt;&#xA;&lt;li&gt;Will there be any auto-checks to verify successful anchoring? &lt;/li&gt;&#xA;&lt;li&gt;What degree of control will we have about the exact location of the landing site?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1103" LastEditorUserId="1103" LastEditDate="2014-08-09T05:59:08.083" LastActivityDate="2014-08-09T05:59:08.083" Title="Rosetta's lander, Philae will &quot;anchor&quot; itself on Comet 67P. What level of control can we have over this?" Tags="|landing|comet|rosetta|lander|philae|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5133" PostTypeId="2" ParentId="5132" CreationDate="2014-08-07T16:53:04.627" Score="8" Body="&lt;p&gt;Rosetta's approach and odd orbit (described well in the question and answer here: &lt;a href=&quot;https://space.stackexchange.com/questions/5117/is-this-really-rosettas-orbit-around-67p&quot;&gt;Is this really Rosetta&amp;#39;s orbit around 67P?&lt;/a&gt;) are designed to gather the necessary information needed to achieve a safe orbit and eventually land Philae.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing site is being selected now: &quot;As many as five possible landing sites will be identified by late August, before the primary site is identified in mid-September. The final timeline for the sequence of events for deploying Philae – currently expected for 11 November – will be confirmed by the middle of October.&quot; (&lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_arrives_at_comet_destination&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Comet_rendezvous&quot; rel=&quot;nofollow noreferrer&quot;&gt;press release&lt;/a&gt; goes into a little bit more detail about the approach and landing: planned released from about one km, velocity at touchdown of about 1 m/s. Since the comet's mass is so small, the worry is that the lander may just bounce off the surface (according to &lt;a href=&quot;http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; the escape velocity is around a measly .46 m/s), thus the tether. So the landing site will need to be suitable for the harpoon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, satellites control systems are able to make adjustments to maintain and transition to specified orbits / attitudes. Ground operators can send a desired configuration, and the control system will move to that configuration while controlling for any number of perturbations. Search &quot;control theory&quot; for more information about how this can be done.&lt;/p&gt;&#xA;" OwnerUserId="4686" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-08-08T18:35:39.357" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="5134" PostTypeId="2" ParentId="886" CreationDate="2014-08-07T20:18:03.347" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How feasible was Project Orion and how feasible would it be if started&#xA;  today?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In general, the only limitation on space flight has been political capital.  Project Orion would thrive in an environment in which there was substantial interest (~500B USD range) in interstellar but not enough resources (~3-5T USD) or time (requiring space based solar) for development of antimatter propulsion.  If you gave NASA half a trillion dollars and told them to build an interstellar Project Orion, I'm sure that all the technical problems would be worked out relatively quickly.  Specifically addressing some common concerns, I'd expect it to be assembled in orbit from predominately asteroids pulled in by gravity tractors.  There isn't any particular reason to assembly such a large craft in a gravity well.  While the proposed pure-fusion detonators have not been developed, there hasn't been reason to.  Regardless, launching large amounts of fissile material to seed detonators, while costly, is still exclusively a question of political capital.  Space elevator is also always an option, especially for multiple vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the main difference would be that, given this implementation rather than a from surface launch, there's little to no reason not to employ existing crew to orbit methods and then use nuclear reactors powering ion thrusters, as it scales considerably better.  However, if nuclear pulse detonation was, for any reason (including surface launch) desired, it certainly could be done.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Specifically, what would be the real world impact of nuclear fallout?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't think a surface launch would be pursued today under almost any circumstances, including systemic failure of the planet to sustain life, due to the increased efficiency of orbital assembly.  However, polar launches would largely reduce fallout, as would using conventional detonations from the launch pad for the initial pulse.  &lt;em&gt;Overall, however, this in an unresolved problem&lt;/em&gt;.  However, Freeman Dyson estimated on the order of a single death (from cancer through radiation) from each launch, and this is likely fairly reasonable, especially given the astounding number of nuclear detonations that have already occured worldwide.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And, would there be methods to mitigate or eliminate the fallout, so Project Orion could became a realistic method of space travel?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While I've outlined a few things, I'll summarize here:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Polar launch&lt;/li&gt;&#xA;&lt;li&gt;Conventional initial pulse&lt;/li&gt;&#xA;&lt;li&gt;Orbital assembly&lt;/li&gt;&#xA;&lt;li&gt;Antimatter propulsion&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The most important thing to stress though is that &lt;em&gt;Project Orion is a realistic method of space travel&lt;/em&gt;.  The consequences of launch would likely be negligible, the investment would likely generate enough new knowledge to pay for itself, and the value of having life off-planet as an insurance policy is incalculably valuable.  Personally I favor holding off for antimatter for humans and nuclear powered ion for cargo, but if we had to launch in 2020 (possibly sooner?), Project Orion could do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Happy travels!&lt;/p&gt;&#xA;" OwnerUserId="5904" LastActivityDate="2014-08-07T20:18:03.347" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5135" PostTypeId="2" ParentId="5125" CreationDate="2014-08-07T23:45:00.943" Score="4" Body="&lt;p&gt;In the short term on long micro g missions, because blood no longer pools in the legs due to gravity, too much blood returns to the heart.  This will make the kidneys actually reduce blood volume by increasing urine production for a while.  I suppose you can say that during this time, we need less water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FWIW, this loss of blood volume causes all sort of issues when returning to normal g after prolonged missions&lt;/p&gt;&#xA;" OwnerUserId="5906" LastActivityDate="2014-08-07T23:45:00.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5136" PostTypeId="2" ParentId="2659" CreationDate="2014-08-08T18:16:37.360" Score="2" Body="&lt;p&gt;Every time I hear of utilizing the Moon for anything, I generally assume that highest degree of reasonable applicablity is Helium-3 extraction.  Fortunately for people interested in this topic, the media tends to find it fairly interesting:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2716417/Could-moon-fuel-Earth-10-000-years-China-says-mining-helium-satellite-help-solve-worlds-energy-crisis.html&quot; rel=&quot;nofollow&quot;&gt;http://www.dailymail.co.uk/sciencetech/article-2716417/Could-moon-fuel-Earth-10-000-years-China-says-mining-helium-satellite-help-solve-worlds-energy-crisis.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sciences360.com/index.php/jaxa-nasa-lunar-base-moon-helium-3-technology-fusion-power-7166/&quot; rel=&quot;nofollow&quot;&gt;http://www.sciences360.com/index.php/jaxa-nasa-lunar-base-moon-helium-3-technology-fusion-power-7166/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the current state of affairs on Earth, energy policy is becoming increasingly important and open to innovative and non-conventional solutions.  He-3 fusion is certainly an excellent medium- to long-term alternative to &lt;code&gt;&amp;lt;environmentalist&amp;gt;&lt;/code&gt; completely and irreparably destroying the environment for no reason by ignoring obvious externalities &lt;code&gt;&amp;lt;/environmentalist&amp;gt;&lt;/code&gt;.  Japan has traditionally been fairly open to the nuclear option and, recent difficulties aside, will probably continue to look toward nuclear in the future.  Fusion is substantial more desirable than fission, of course, and many would argue that He-3 will trivialize some subset of the problems facing fusion development.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of &lt;em&gt;how&lt;/em&gt;, we aren't really actually sure how much He-3 there is, how to extract it, how to return it (or even if the reactors should be on lunar surface or in orbit and energy otherwise returned).  And of course, there's always the possibility that there is other, unexpected mineral or otherwise wealth to be harvested.  I'd expect some InSight like drilling operation with a goal of sample return rather seismic modeling.  At least, if I were JAXA, that's what I'd do.&lt;/p&gt;&#xA;" OwnerUserId="5904" LastEditorUserId="5904" LastEditDate="2014-08-19T22:00:28.997" LastActivityDate="2014-08-19T22:00:28.997" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5137" PostTypeId="1" AcceptedAnswerId="5139" CreationDate="2014-08-08T19:43:28.903" Score="6" ViewCount="2941" Body="&lt;p&gt;Propulsion exhaust speed is lower than speed of light, I think and if it is so, than how could spaceship go faster that exhaust speed? Right? So exhaust speed is a maximum speed for spaceship? (If there is no gravity, etc.. just empty space and propulsion)&lt;/p&gt;&#xA;" OwnerUserId="5913" LastActivityDate="2014-08-09T15:54:46.313" Title="Space travel: Is there an speed limit lower than speed of light?" Tags="|rockets|propulsion|interstellar-travel|travel|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5138" PostTypeId="2" ParentId="5137" CreationDate="2014-08-08T20:35:56.977" Score="-3" Body="&lt;p&gt;Actually u r right for every action there is an equal and opposite reaction.  So when there is no gravity the max speed will be equal to it. But there is no fixed limit lesser then the speed of light.  Only speed of light is constant and unbreakable . according to theory of relativity and that's because time become slow to preserve that constant value.&lt;/p&gt;&#xA;" OwnerUserId="5914" LastActivityDate="2014-08-08T20:35:56.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5139" PostTypeId="2" ParentId="5137" CreationDate="2014-08-08T20:57:10.830" Score="13" Body="&lt;p&gt;It's not exactly how you put it, &lt;em&gt;exhaust speed&lt;/em&gt; is not a &lt;em&gt;maximum speed for a spaceship&lt;/em&gt;. While &lt;a href=&quot;https://en.wikipedia.org/wiki/Exhaust_velocity&quot;&gt;exhaust velocity&lt;/a&gt; does define &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust&quot;&gt;thrust&lt;/a&gt; together with the rate of expelled mass and is given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T=v\frac{\Delta m}{\Delta t}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;where thrust $T $ is a reaction force that is a product of exhaust velocity $v$ and mass change $\Delta m$ in time $\Delta t$, you're neglecting &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertia&quot;&gt;inertia&lt;/a&gt;. In a sense, if you invest some of the propellants to achieve some thrust and you're on your way at a given speed, the remaining rocket's propellants now carry this same rocket's &lt;a href=&quot;https://en.wikipedia.org/wiki/Momentum&quot;&gt;momentum force&lt;/a&gt;. This is more usually referred to as &lt;a href=&quot;https://en.wikipedia.org/wiki/Conservation_of_momentum#Conservation&quot;&gt;conservation of momentum&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while the exhaust velocity normally doesn't change (assuming same performance engines and propellants) and is directly related to rocket's performance (what we usually refer to as &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot;&gt;specific impulse&lt;/a&gt;), all of the remaining mass of the rocket, including remaining propellants, now carry this same speed. Since thrust is given by exhaust velocity with respect to the rocket itself, you'd constantly add rocket's own speed to new thrust in time, to potentially go beyond the speed of the exhaust velocity. Does this make sense?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v = v_\text{e} \ln \frac {m_0} {m_1}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to read more about how to calculate maximum achievable speed (mostly referred to as $\Delta v$, &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot;&gt;delta-v&lt;/a&gt;, or change in velocity), I'd suggest reading the Wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt; or other similar questions on our own site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if we assume wet-to-dry mass ratio of 10:1 (payload and the rocket weigh together 1/10th the weight of propellants), the rocket will reach $\ln(10) \approx 2.3 \cdot v_e$, or delta-v of roughly 2.3 times the propellant exhaust velocity. For more practical example, say our rocket's total mass (wet mass; propellants + rocket's dry mass) is 100 metric tons, and the rocket's dry mass (when it used up all its propellants) is 10 metric tons. Assuming  specific impulse $I_{sp}$ of 460 seconds (exhaust velocity of ~ 4,511 m/s, or $I_{sp}$ in seconds multiplied by $g \approx 9.80665\ m/s^2$) that's achievable with LOX/LH (liquid oxygen / liquid hydrogen) cryogenic propellant engines such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot;&gt;Space Shuttle Main Engines&lt;/a&gt; in vacuum, the total delta-v using Tsiolkovsky rocket equation above comes out at 10,387 m/s. So again those ~ 2.3 times the exhaust velocity, i.e. &lt;a href=&quot;https://en.wikipedia.org/wiki/Natural_logarithm&quot;&gt;natural logarithm&lt;/a&gt; $\ln(x)$, where $x$ is $\frac {m_0} {m_1}$, or our wet-to-dry &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_ratio&quot;&gt;mass ratio&lt;/a&gt;. If you want to play with other parameters, you can use this &lt;a href=&quot;http://www.strout.net/info/science/delta-v/intro.html&quot;&gt;Delta-V Calculator&lt;/a&gt; for convenience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, what you said in the questions would be true for some other methods of propulsion where the source of your thrust is stationary to the point you use to define your speed against, such as, say, &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot;&gt;solar sails&lt;/a&gt; or some forms of &lt;a href=&quot;https://en.wikipedia.org/wiki/Beamed_energy_propulsion&quot;&gt;beam-powered propulsion&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for &lt;em&gt;limits&lt;/em&gt;, well traditional rocket propulsion methods come with their own limit often referred to as &lt;em&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot;&gt;the tyranny of the rocket equation&lt;/a&gt;&lt;/em&gt; since, simplifying - your total delta-v depends on how much of propellants you carry, but the more of them you carry the heavier you get and it becomes more expensive to accelerate them to any speed (not to be confused with &lt;em&gt;exhaust velocity&lt;/em&gt;, that stays the same).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-08-08T22:57:16.483" LastActivityDate="2014-08-08T22:57:16.483" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5140" PostTypeId="2" ParentId="5129" CreationDate="2014-08-08T23:53:52.440" Score="14" Body="&lt;p&gt;Rosetta is mainly tracked through radiometric means. They use 3 different methods: range, Doppler and ΔDOR. [2]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Range measurements are conducted by sending a &quot;ping&quot; radio transmission to the spacecraft, which then responds with a &quot;pong&quot;. The time it takes for the spacecraft to respond, that is the reaction time of the onboard electronics, is precisely calibrated. The ground observer then can measure the time elapsed between ping and pong. That's the time it takes for the radio waves to propagate back and forth in space plus the calibrated reaction time. Radio waves travel with the speed of light which is precisely known. From this it can be calculated how far the spacecraft is in the line-of-sight direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doppler measurement goes as follows: The spacecraft is transmitting on a known frequency. If it's velocity has a non-zero component along the line-of-sight direction, then the observer will measure in the received transmission in a slightly different frequency. This is called the &lt;a href=&quot;http://en.wikipedia.org/wiki/Doppler_effect&quot; rel=&quot;noreferrer&quot;&gt;Doppler effect&lt;/a&gt;, and can be used to determine speed with which the spacecraft is approaching or leaving.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;∆DOR is best described by this quote from &lt;a href=&quot;http://en.wikipedia.org/wiki/Doppler_effect&quot; rel=&quot;noreferrer&quot;&gt;1&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;∆DOR requires two widely separated ground stations to measure the difference in the arrival time of the signal from both Rosetta and a directionally close-by extragalactic radio source (quasar) whose inertial direction is precisely known. It provides a direct measurement of one coordinate of Rosetta's position in the plane-of-sky which is a perfect complement to the line-of-sight Doppler and range measurements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;During the comet flybys and the 67P/Churyumov-Gerasimenko phase Rosetta utilizes its onboard cameras to better track its position relative to the comet nucleus. This is needed because even tough radiometric tracking provides a very accurate position estimate of the spacecraft relative to the solar system, the comets trajectories are not known with sufficient precision. [2][3]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The accuracy of the position estimation varies. When they make more measurements they gain a more precise picture. For example the Mars swing-by paper&lt;a href=&quot;http://en.wikipedia.org/wiki/Doppler_effect&quot; rel=&quot;noreferrer&quot;&gt;1&lt;/a&gt; mentions that the altitude at closest approach was known with 3sigma uncertainty of 1.1 kilometer. In that paper Figure 8th provides an illuminating illustration on how the position estimation's accuracy evolved during the days around the swing-by due to frequent measurements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Rosetta Navigation at Its Mars Swing-By: &lt;a href=&quot;http://issfd.org/ISSFD_2007/8-4.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://issfd.org/ISSFD_2007/8-4.pdf&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Preparations and Strategy for Navigation During Rosetta Comet Phase: &lt;a href=&quot;http://issfd.org/ISSFD_2012/ISSFD23_IN2_2.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://issfd.org/ISSFD_2012/ISSFD23_IN2_2.pdf&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Optical Measurements for the Flyby Navigation of Rosetta at Asteroid Steins: &lt;a href=&quot;http://issfd.org/ISSFD_2009/AOCSII/Lauer.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://issfd.org/ISSFD_2009/AOCSII/Lauer.pdf&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="5750" LastEditorUserId="-1" LastEditDate="2019-11-10T19:49:13.583" LastActivityDate="2019-11-10T19:49:13.583" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5141" PostTypeId="2" ParentId="5137" CreationDate="2014-08-09T15:54:46.313" Score="6" Body="&lt;p&gt;TildalWave gave a correct and detailed answer.  For a very simple proof, though:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even hydrogen/oxygen engines (the most powerful fuel that has actually been used) have an exhaust velocity far below orbital velocity.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-08-09T15:54:46.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5142" PostTypeId="1" CreationDate="2014-08-09T23:42:01.277" Score="4" ViewCount="1524" Body="&lt;p&gt;I'm looking for a formula can be used to calculate the magnitude of a passing satellite (e.g. the ISS) based on it's TLE&lt;/p&gt;&#xA;" OwnerUserId="5916" LastActivityDate="2014-08-12T20:12:39.143" Title="how to calculate the brightness of a passing satellite?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5143" PostTypeId="1" AcceptedAnswerId="5152" CreationDate="2014-08-10T13:19:54.650" Score="17" ViewCount="3244" Body="&lt;p&gt;With the example of SpaceX and Orbital Sciences building brand new launchers in a small number of years, it seems that the time has come technologically where it is not quite as much 'rocket science' as in the past.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of countries in the world who have necessary resources and technical know-how to do this as well.  If so, why are there so few countries with Satellite launching Vehicles of their own?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the US sufficiently advanced that other countries cannot compete?  (Using the SpaceX example).  Is outsourcing most of the components sufficient to count as your 'own' launcher? Take the Orbital Sciences Antares for example, where the first stage is made in the Ukraine by the makers of the Zenit rocket, the engines are from the 1960's, and the upper stage is by ATK, formerly a separate company (they just recently merged with OSC).&lt;/p&gt;&#xA;" OwnerUserId="5918" LastEditorUserId="897" LastEditDate="2014-08-13T14:22:22.227" LastActivityDate="2015-04-13T18:24:53.370" Title="Why have so few countries developed satellite launching vehicles of their own?" Tags="|launch|artificial-satellite|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5145" PostTypeId="5" CreationDate="2014-08-10T13:51:39.227" Score="0" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Philae_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Wikipedia writes to say about Philae&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The lander will achieve the first controlled touchdown on a comet nucleus. The lander’s instruments will obtain the first images from a comet’s surface and make the first in situ analysis to find out what it is made of&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-08-10T13:51:39.227" LastActivityDate="2014-08-10T13:51:39.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5146" PostTypeId="4" CreationDate="2014-08-10T13:51:39.227" Score="0" Body="If your question is pertinent to &#xD;&#xA; - Rosetta Lander, and &#xD;&#xA; - Comet 67P/Churyumov–Gerasimenko&#xD;&#xA;please apply this tag" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-08-10T13:51:39.227" LastActivityDate="2014-08-10T13:51:39.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5147" PostTypeId="2" ParentId="4153" CreationDate="2014-08-10T14:36:27.000" Score="16" Body="&lt;p&gt;Try looking at research done at The Aerospace Corporation, Penn State and Utah State. It turns out that there are very good reasons to print the fuel for hybrids. Adding a third dimension, beyond the images, above, of standard grain shapes can cause increased mixing of oxidizer and fuel, raising Isp. Regression rate was significantly increased in tests at both universities. Imagine image #3 above, where the star is swept through a helix, rather than made as a straight extrusion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some links...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First description of printed grains... &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2011-5821&quot;&gt;http://arc.aiaa.org/doi/abs/10.2514/6.2011-5821&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;USU's comparison of ABS to HTPB... &lt;a href=&quot;http://enu.kz/repository/2011/AIAA-2011-5909.pdf&quot;&gt;http://enu.kz/repository/2011/AIAA-2011-5909.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;USU shows increased regression rate... &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2014-3751&quot;&gt;http://arc.aiaa.org/doi/abs/10.2514/6.2014-3751&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PSU shows increased regression rate in acrylic &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2013-4141&quot;&gt;http://arc.aiaa.org/doi/abs/10.2514/6.2013-4141&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5920" LastEditorUserId="5920" LastEditDate="2014-08-11T07:38:18.640" LastActivityDate="2014-08-11T07:38:18.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5148" PostTypeId="2" ParentId="3519" CreationDate="2014-08-10T22:24:14.063" Score="10" Body="&lt;p&gt;Q: Why is planning and executing the maneuver so difficult it had not been done before 2007?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A: The 1st ZPM was performed in Nov 2006 when the ISS was rotated 90 degrees. The paper describing this maneuver can be obtained at &lt;a href=&quot;https://www.academia.edu/4543547/First_Ever_Flight_Demonstration_of_Zero_Propellant_Maneuver_Attitude_Control_Concept&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.academia.edu/4543547/First_Ever_Flight_Demonstration_of_Zero_Propellant_Maneuver_Attitude_Control_Concept&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also downloadable (without requiring a login) at: &lt;a href=&quot;https://www.researchgate.net/publication/268554560_First_Ever_Flight_Demonstration_of_Zero_Propellant_ManeuverTM_Attitute_Control_Concept&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.researchgate.net/publication/268554560_First_Ever_Flight_Demonstration_of_Zero_Propellant_ManeuverTM_Attitute_Control_Concept&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Re difficulty in planning and executing maneuver:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;As ZPM's take longer than an equivalent thruster maneuver (it can take up to 5-10 X longer time), the ISS operational timeline must be altered and that impacts many other subsystems which have to be coordinated. Anything that disrupts an existing operational timeline increases risk and additional work has to be performed which costs more and since ZPM's were not included in ISS budget that means the money has to come from somewhere else.&lt;/li&gt;&#xA;&lt;li&gt;Because ZPM's are slower than thruster maneuvers, separate thermal analysis has to be performed to evaluate impact of a &quot;stale sun&quot;, ie the sun shinning too long on a particular spot on the ISS. Note that maneuvers that take less than an orbit (about 90min) do not need any additional analysis. Since the ZPM's take longer than 90min additional work has to be performed which costs money.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Finally, about the timing, it is a complex combination of availability of solution and its maturity, opportune timing where management and operations line up with new technology. Just having a good idea and an implementation was not enough as you also need to change perception and status quo that this is actually possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Q: What are the sources of errors that can occur during the maneuver?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A: Some of the error sources that directly affect how much momentum is used during the ZPM:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Uncertainty in knowledge of ISS inertia. Other than knowing the inertia of each ISS component there is also the inertia variation due to rotating solar arrays.&lt;/li&gt;&#xA;&lt;li&gt;Uncertainty in initial conditions (attitude &amp;amp; rate).&lt;/li&gt;&#xA;&lt;li&gt;Uncertainty in aerodynamic drag that results in a disturbance torque on the ISS&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Q: How does the fact that the ISS is not a rigid body complicate the attitude control problem for large maneuvers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A: The general answer is none. The ZPM in general does not excite the ISS flexible structure unlike thruster firings because it uses smooth CMG actuators. The design of the ZPM is based on rigid body dynamics. There is a feedback CMG control system wrapped around the ZPM commands that includes flex filters that suppress the impact of structural vibrations on attitude and rate measurements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Q: Assuming loss of propulsive attitude control (i.e. failure of Russian segment's thrusters), can ZPMs result in off-nominal situations (e.g. the Station spinning beyond control authority of the CMGs)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A: The general answer is no. Remember that there is a stable feedback attitude controller wrapped around the ZPM and the design always includes substantial momentum margin (at least 50%). The feedback controller's job is to get the ISS to track the ZPM trajectory. The momentum margin is designed to deal with mismatches between modeled and actual ISS dynamics that can result in using more momentum than anticipated. So as long as the CMG momentum is not saturated we would have control authority. And how do we know that? Because before flight, there is extensive analysis on the impact of ISS uncertainties on the maximum momentum that would be required during a ZPM. In the end we are taking a calculated risk that an out of control situation will not occur. Theoretically it is possible that momentum could saturate but the likelihood of that happening is very small. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly the opposite happened. In 2007 the Russian segment computers all failed at the same time resulting in loss of automatic thruster control while the Shuttle was docked to the ISS. If thruster control was not recovered, Shuttle undocking could have put the ISS in such a spin. And a ZPM was one of the two options considered to recover a spinning ISS. Analysis showed that an ISS spinning up to 0.1deg/sec could be recovered and returned to stable attitude using a ZPM. Thankfully the computers were recovered before the Shuttle had to undock. A reference for this is the paper &lt;a href=&quot;https://www.academia.edu/5497196/ISS_Contingency_Attitude_Control_Recovery_Method_For_Loss_Of_Automatic_Thruster_Control&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.academia.edu/5497196/ISS_Contingency_Attitude_Control_Recovery_Method_For_Loss_Of_Automatic_Thruster_Control&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also downloadable (without requiring a login) at: &lt;a href=&quot;https://www.researchgate.net/publication/273441779_ISS_Contingency_Attitude_Control_Recovery_Method_For_Loss_Of_Automatic_Thruster_Control&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.researchgate.net/publication/273441779_ISS_Contingency_Attitude_Control_Recovery_Method_For_Loss_Of_Automatic_Thruster_Control&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and also directly from NASA at: &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20080009592&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/search.jsp?R=20080009592&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5921" LastEditorUserId="12102" LastEditDate="2018-03-28T02:48:58.140" LastActivityDate="2018-03-28T02:48:58.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5150" PostTypeId="1" CreationDate="2014-08-11T12:28:54.690" Score="13" ViewCount="3134" Body="&lt;p&gt;I am about to program space exploration simulation game in the future, but I need to consider many factors. I would like to be as much realistic as possible, but keeping the game playable as well and also attractive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think that space battles are not very probable, but I also think that without them, game will loose attractiveness for many potential players. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I need to have a realistic point of view, to decide what to change and what ignore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have created this scenario:&#xA;As space explorers people have found Earth-like planet in habitable zone and started to colonize it. There where no evidence of advanced civilization until one day they come to our settlement and say: this planet was our for many decades and you have attacked our territory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All diplomatic ways failed (they wanted to set up their own colony there) and they started war against people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How this war/battles/strategy/weapons/battleships/fighters should look like? But in realistic way? With technologies we know/have or are reachable in close future lets say in 10-20 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am really looking forward to your answers/comments/links, and I hope my question doesn't break any community rules. :) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;Thank you all for answers, you give me lot of inspiration and ideas for brainstorming. I really feel that it helped me to get better understood of complexity of space conflicts. Despite of the question was marked as opinion-based, it helped me and may help other as well.&lt;/p&gt;&#xA;" OwnerUserId="5913" LastEditorUserId="5913" LastEditDate="2014-08-13T20:01:42.190" LastActivityDate="2014-08-13T20:01:42.190" Title="Realistic space battle, how it could looke like? No hollywood version or videogames like" Tags="|colonization|interstellar-travel|technology|construction|design|" AnswerCount="4" CommentCount="6" ClosedDate="2014-08-12T13:44:07.823" ContentLicense="CC BY-SA 3.0" />
  <row Id="5151" PostTypeId="2" ParentId="5150" CreationDate="2014-08-11T13:11:33.070" Score="1" Body="&lt;p&gt;Fast projectiles would be effective space weapons today. For sure, some such weapons are already prepared for use against military satellites if a new world war broke out today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I must say that your combination of colonizing a planet, and &quot;10-20 years&quot;, do not work together. Even sending four humans for a short visit to Mars within 20 years is optimistic. It takes tens of thousands of years to travel even to the nearest star, and even that optimistically assume that several problems would be solved within centuries (like an energy source which lasts for tens of thousands of years, communication system and not least a completely new kind of propulsion system). Today we cannot start sending even a tiny probe to another star. Also aliens claiming the same planet &quot;since decades&quot; is a bit silly. They should've been around many millions of years, some percentage of the age of the galaxy which is ten billion years.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-08-11T13:11:33.070" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5152" PostTypeId="2" ParentId="5143" CreationDate="2014-08-11T13:40:53.867" Score="19" Body="&lt;p&gt;In attempting to answer this, I read two different questions here. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Why is it so hard to develop a space vehicle/rocket?&lt;/li&gt;&#xA;&lt;li&gt;In contrast, why did SpaceX and Oribtal look so easy? &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Thus for #1, because it is rocket science.  For #2 There is one word to answer it.  Heritage. The best example in modern history of how that heritage manifests is NASA. The second example of heritage that is less specific is the military.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;US corporations can publicly leverage all the research NASA did in the 50s till now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When SpaceX designed the capsule shape for Dragon, they could look at all the work NASA did in the Mercury/Gemini/Apollo days on different shapes, see the results and make a better decision.  Much less 'testing' on their own that they needed to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For parachute design for Dragon, same thing. Apollo did a TON of parachute testing, and SpaceX leveraged all of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their engine designer, Tom Mueller, came from TRW where he worked on rocket engines there. He was able to reuse the basic design approach of the Lunar Lander ascent engine for a simple engine design, then cluster it for performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be clear, SpaceX has said this loud and clear on many occasions that they stand on the shoulders of giants and would not be possible without the research NASA had done over the decades.  (I.e. The actual purpose of NASA!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital cheated, and used the overseas heritage equivalents of NASA. They outsourced it. They hired the &lt;a href=&quot;https://en.wikipedia.org/wiki/Yuzhnoye_Design_Bureau&quot;&gt;guys who build Zenit (Yuzhnoye)&lt;/a&gt; to build their first stage.  (Zenit is also LOX/Kerosene powered, and uses an RD-170 (an RD-180 is 'half' of an RD-170, and the RD-191, used in Angara, is 1/4 of an RD-170)).  They cheated further by reusing engines from the Soviet era N-1 program, that were already built, and in storage. Then, they used existing Castor engines from ATK. (I use cheat in a non-perjorative fashion here. Buying off the shelf is just smart business. I question the use of the NK-33/AJ-26 engine, but still).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delta 4 uses a newly developed core, newly developed engine, and that was hard and not cheap.  But that was Boeing, who had acquired companies with all the expertise and IP in house already.  (That may be a LH2 issue, liquid Hydrogen is just plain hard to work with).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look how much trouble India and China had developing indigenous launchers, when they could not piggy back on NASA or Russian experience.   Conversely, look at Shenzou which is a major clone of Soyuz. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Israel with the Shavit did what the US/Soviets did in the 1950/1960s (Atlas, Delta, Thor) and re-purposed an ICBM as a space launcher.  (The Russian Soyuz booster (as opposed to Soyuz spacecraft) is basically the R-7 ICBM from the 1950's with upgrades). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ESA did not have any real ICBMs to tag off, and you see that in Ariane, being more of a fresh design.  Which was not easy either.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-13T18:24:53.370" LastActivityDate="2015-04-13T18:24:53.370" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="5153" PostTypeId="2" ParentId="5150" CreationDate="2014-08-11T15:11:34.750" Score="13" Body="&lt;p&gt;I would like to firstly echo the suggestion to read Project Rho. Throughout the entire site, there is a tremendous amount of hard science and it is all deeply entertaining.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.projectrho.com/public_html/rocket/spacegunexotic.php&quot;&gt;http://www.projectrho.com/public_html/rocket/spacegunexotic.php&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, a space war would look more like the Cold War than any other war we've known. With nuclear ICBMs, we effectively established that our capacity for destruction vastly exceeds our ability to evade or shield against attacks. Because of that, we wind up in a stalemate, where both sides are perpetually armed but refuse to attack. The role for a &quot;conventional&quot; military has remained strong more-or-less due to developed nations taking action in developing nations. In space, this seems much less likely. Could there even be a developing nation in space? We're looking a concept of warfare that is much colder than anything we've known.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some predictions and extrapolations are certainly still obvious. Today, we have nuclear subs as a &quot;net&quot; which keep weapons on alert. For a society that spans the solar system, you could realistically maintain a similar net. Such a net could consist of probes in free orbit with either people or computers capable of making a decision to fire, and where to fire.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To break down the overall strategies of the weapons, you could go several routes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Something that is too fast to detect&lt;/li&gt;&#xA;&lt;li&gt;Something that is too large to divert&lt;/li&gt;&#xA;&lt;li&gt;&quot;Peaceful&quot; space technologies which are inherently dual-use&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Planetary colonies are uniquely unmovable. Their future location can be calculated exactly as far as we care, so if an asteroid is diverted toward one, it's lights out for that colony unless they have similar technology. This presents a logical decision matrix, where if one society has vastly larger rockets, then they can win at war by imparting more momentum to a rock than the other society can (as a means of diversion). However, asteroids are far away, and even a small course correction could avoid impact, so the advantage would only be present if one side of the conflict had &lt;em&gt;much&lt;/em&gt; better technology. So this is only relevant for a highly asymmetrical war.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something too fast to detect looks much more deadly. Consider something in-between a linear particle accelerator and a Gauss gun. With a small enough payload, it's conceivable to launch it at a significant fraction of the speed of light. This becomes a simple kinetic energy weapon, and depending on how fast it's traveling, it could be virtually impossible to detect in advance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given this thinking, war would look more like a chain reaction - completely preplanned. Similar to the historical Cold War, it comes down to a probability matrix where you have x probability of winning if you fire first, and y probability of winning if the other side fires first. In some particularly intense situations, it's possible that firing first gives your best chance of survival. But this situation has a kind of super-rational game theory, where the logic of your opponent feeds back onto your own logic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In all situations, pre-planning basically wins. If one side implements a plan to annihilate the other side, there is no hope. The only other option is if the defenders build infrastructure in lockstep with the aggressors, which seems unlikely. The probability of annihilation is most likely to be either 100% / 0% or 100% / 100%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most likely space war story would be bogged down in heavy political logic, and possibly a great deal of espionage. For interstellar warfare, the most effective deterrent might be sending a message back to your home system. Throw in some game theory as well. For instance, conceal the time that the first message was sent, and use this to force their hand into a peace negotiation.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-08-11T15:11:34.750" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5154" PostTypeId="1" CreationDate="2014-08-11T15:36:53.420" Score="2" ViewCount="99" Body="&lt;p&gt;Yutu was ferried to Luna by way of &lt;a href=&quot;http://en.wikipedia.org/wiki/Chang%27e_3&quot; rel=&quot;nofollow&quot;&gt;Chang`e&lt;/a&gt; in December 2013&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia article on the mission writes to say the mission duration for the lander, and rover were as follows.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lander: 1 Year&lt;/li&gt;&#xA;&lt;li&gt;Rover: 3 months&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;After Lunar land-fall on December 14th 2013 ADG, Yutu purportedly suffered a mechanical failure in January leaving it immobile within sight of the Lander. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacedaily.com/reports/Chinese_lunar_rover_alive_but_weak_999.html&quot; rel=&quot;nofollow&quot;&gt;This report&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Its functionality has been weakened considerably by the bitterly cold lunar nights after the rover experienced a &quot;mechanical control abnormality&quot; in January&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same report writes further to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;With each lunar night, the functionality of Yutu is yet again weakened&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia article for Yutu lists the following systems/instruments&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Ground Penetrating Radar&lt;/li&gt;&#xA;&lt;li&gt;Spectrometer&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Alpha &lt;/li&gt;&#xA;&lt;li&gt;Infra-red&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Camera Pair&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Navigation&lt;/li&gt;&#xA;&lt;li&gt;Hazard&lt;/li&gt;&#xA;&lt;li&gt;Panoramic&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Besides the twin mechanical failures leaving Yutu immobile and it's Solar Panels unable to lock-down - what other instruments/systems aboard the rover have suffered degradation/demise over the last 7-odd months?&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-08-11T15:36:53.420" Title="Besides mechanical failure, what other systems have failed/deteriorated on Yutu?" Tags="|the-moon|lunar-landing|yutu|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5155" PostTypeId="2" ParentId="5150" CreationDate="2014-08-11T17:10:48.327" Score="6" Body="&lt;p&gt;There are two answers, and they depend upon available ΔV (delta-V).&lt;/p&gt;&#xA;&lt;p&gt;Realistically, there are only 3 weapons worth mention: Lasers, Missiles, and Particle Accelerator Weaponss (PAW).&lt;/p&gt;&#xA;&lt;p&gt;Weapon ranges are going to be measured in megameters (1e6 meters) for missiles, and maybe a dozen kilometers (around 1e4 meters) for lasers and PAWs.&lt;/p&gt;&#xA;&lt;h1&gt;Low ΔV&lt;/h1&gt;&#xA;&lt;p&gt;If available ΔV is low, then it's simply a slow drift together, and he whose fire control software and hardware works best wins. You'll never see the beams of the lasers or PAWs, only the effects of a hit. The battle will be a single pass. There may be some adjustment of formation, but its not going to be a lot. Missiles will be delivering nukes and will probably be used for generating detonation laser events instead of direct impact. (The available delta-V isn't enough to prevent the missile from being lasered before impact.)&lt;/p&gt;&#xA;&lt;h1&gt;High ΔV&lt;/h1&gt;&#xA;&lt;p&gt;If available ΔV is high (several G-hours), and the time is present to use them, everything has to be automated. A single pass, using kinetic kill missiles at high relative velocity. You send the missiles ahead, and use your lasers and PAWs for automated point defense. The goal of the point defense fire is to prevent the missiles from terminal guidance, not to detonate them, and secondarily, to disable the backup warheads. The missiles are armed with nukes, but that's only a backup plan; the primary goal is to actually impact, for the kinetic impact is huge. Maneuver is to avoid missile impact. The initial commit to battle is a sudden burst of missiles towards the enemy. And then, the missiles start to die. Next, the missiles or fragments impact, and massive crunches happen.&lt;/p&gt;&#xA;&lt;p&gt;So, in either case, two formations approach each other, fly a single pass, and hope it's decisive. The next &amp;quot;pass&amp;quot; is going to be a way off in time... more like a second whole battle than a continuation of the first.&lt;/p&gt;&#xA;&lt;p&gt;Silent, and sudden flowers of death. And a significant wait until battle ranges.&lt;/p&gt;&#xA;&lt;h1&gt;Onboard&lt;/h1&gt;&#xA;&lt;p&gt;Onboard, in either case... program the computers, put on your vacuum suits, hit &amp;quot;commit&amp;quot;, and then hide in the solar storm shelter, and wait. If there's still a ship after the battle pass, emerge, and assess the damage. Repair what you can. Find out which other ships survived, and match to grab survivors to the limits of your own life support systems.&lt;/p&gt;&#xA;&lt;h1&gt;Casualties&lt;/h1&gt;&#xA;&lt;p&gt;The lucky ones die from near instant vaporizations.&lt;/p&gt;&#xA;&lt;p&gt;The slightly less lucky die from shrapnel.&lt;/p&gt;&#xA;&lt;p&gt;The least lucky are the guys who can't signal another ship, and fell off-ship during an impact event, and have no ship air, and realize that they are going to run out of suit air... and suffocate, slowly; floating there, waiting for death, praying for rescue, all alone in the dark...&lt;/p&gt;&#xA;&lt;p&gt;Most people aboard ship will either live or die in groups - either the shelter took the hit, or it didn't. Some minor bangs and bruises in those that survived. The partials happen when the shelter is lasered open but not squarely hit... anyone in the wrong area is cooked alive near instantly; the rest see the hit. Fragments of molten metal floating in the remainder of the compartment cause minor holes and burns in the others. And, unless their radios are on, you hear nothing... despite their screams and sobs.&lt;/p&gt;&#xA;&lt;p&gt;If detonation lasers happen, you can add radiation burns to those whose shelters were compromised. If the ship's point defense was overwhelmed, you're at ground zero of a nuclear flare... no cloud, just a bright light and vaporizing metal, plastic, and flesh. Chunks of ship disappear, and the survivors pick up lots of rads.&lt;/p&gt;&#xA;&lt;h1&gt;Maneuver&lt;/h1&gt;&#xA;&lt;p&gt;This isn't to say the ships are not maneuvering... they are. They are rotating to keep the weapons in arc, and spinning on the long axis to minimize laser and PA loiter times on any one spot. Changing facing is a series of pulses from a variety of RCS thrusters, timed to fire only when in the right position ... lots of small sudden jolts, not one smooth acceleration... Kind of a &amp;quot;throw you sideways&amp;quot; alarm clock effect. And when RCS engines get killed, it becomes an uneven tattoo.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-08-11T17:10:48.327" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5156" PostTypeId="1" CreationDate="2014-08-11T18:59:15.460" Score="6" ViewCount="572" Body="&lt;p&gt;I get the impression that NASA has done alot to make a crewed landing on Mars possible. Not as a focused mission like Apollo, but piece by piece in spite of the lack of dedicated and lasting political commitment. I think that maybe most people who are lucky enough to work with human spaceflight probably prefer to help a mission to Mars when they can, and maybe manage to do so partly in the shadow of other pretexts. Now, my claims above may be &quot;opinionated&quot;, and may be edited out, but as a matter of fact, the question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How much of what is needed to land humans on Mars has been investigated and prepared today?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below are my own impressions to begin with, I hope for better versions of these hypothetical answers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;+) What has been, or is being, done:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Low-Density_Supersonic_Decelerator&quot; rel=&quot;noreferrer&quot;&gt;LDSD&lt;/a&gt;, a way to land heavy payloads on Mars. For what if not humans?&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Suitport&quot; rel=&quot;noreferrer&quot;&gt;Suitports&lt;/a&gt;, spacesuits which avoid toxic dust and bio contamination.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/ATHLETE&quot; rel=&quot;noreferrer&quot;&gt;Athlete&lt;/a&gt;, the mobile Mars rover/habitat concept.&lt;/li&gt;&#xA;&lt;li&gt;Mars 2020 rover &lt;a href=&quot;http://mars.jpl.nasa.gov/mars2020/news/whatsnew/index.cfm?FuseAction=ShowNews&amp;amp;NewsID=1683&quot; rel=&quot;noreferrer&quot;&gt;oxygen ISRU demonstration&lt;/a&gt;. Obviously just for human purposes.&lt;/li&gt;&#xA;&lt;li&gt;Orion, together with a habitat module it'd be quite useful for a trip to Mars.&lt;/li&gt;&#xA;&lt;li&gt;SLS, huge and expensive rocket, but if financed would be great for a Mars mission.&lt;/li&gt;&#xA;&lt;li&gt;Rovers and orbiters to Mars, quite a set of robotic precursors.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Telecommunications_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt; has at least investigated a communication satellite at Mars.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;-) What remains to be done:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Artificial centrifugal gravity, in order for astronauts to arrive healthy.&lt;/li&gt;&#xA;&lt;li&gt;Fully (water) recycling life support systems. The ISS is only partly there.&lt;/li&gt;&#xA;&lt;li&gt;The travel habitat on route to Mars. But, they do invite Bigalow to dock with the ISS next year!&lt;/li&gt;&#xA;&lt;li&gt;The forward bio contamination issue. Would the exobiologists destroy the data they look for upon arrival?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(My linked sources are mostly Wikipedia, because I'm anyway not quite competent to qualify the usefulness of sources on this topic beyond what most who read and comment here can do themselves)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-08-19T19:31:00.057" Title="What has been done and what remains to be done to land humans on Mars?" Tags="|mars|nasa|humans|" AnswerCount="1" CommentCount="6" ClosedDate="2014-08-22T08:23:51.163" ContentLicense="CC BY-SA 3.0" />
  <row Id="5157" PostTypeId="2" ParentId="4049" CreationDate="2014-08-11T21:20:30.417" Score="3" Body="&lt;p&gt;I think one of the important things to consider about artificial gravity, especially in deep space, is the impact of thrust.  Since, especially on interstellar, rockets could be expected to be accelerating or decelerating for the entire duration of the flight, simply doing so at 1 g would completely alleviate the need for any form of artificial gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what other negative effects your thinking of, but radiation comes to mind for me.  Of course, you can always bury your human cargo behind some form of radiation shielding and can reasonably expect much of the radiation to come from the nearest star.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an example of the previous two points, in the case of a Project Orion nuclear propulsion model, it would be reasonable to use 1 g acceleration half way to the destination, micro-thrusters to change orientation, and and then 1 g deceleration to destination with the propulsion plate being used to deflect radiation from stars.  While far from a comprehensive solution, I hope this address some of your concerns.&lt;/p&gt;&#xA;" OwnerUserId="5904" LastActivityDate="2014-08-11T21:20:30.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5158" PostTypeId="2" ParentId="5156" CreationDate="2014-08-11T21:39:08.023" Score="1" Body="&lt;p&gt;To address some concerns and add my own:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Artificial gravity, full water recycling, and a lovely travel habitat (I don't imagine needing more than two docked Orion MPCV capsules) for a Mars mission are probably not totally necessary.  While there would certainly be non-zero health consequences to launching without a comprehensive astronaut/marstronaut health solution in place, I believe existing fitness routines, reasonable water reuse, and a moderate degree of living room would probably be enough to keep human cargo within reasonable health range for recovery.  Considering historical naval conditions and pharmaceutical advances, I think we can overcome a lot of problems derived from humans being organic.  I, for one, am in favor of extensive drug use on a single crew member to allow toleration of sub-optimal conditions, and possible reduce cabin fatigue and metabolism.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bio contamination doesn't seem particularly solvable, so I have no real recommendations on that front, other than some personal confusion over why we take so much care to preserve when Earth is regularly destroyed, often without any reason.  Something something break eggs to make omelets something something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know one of the biggest concerns with using Orion for Mars landing specifically is that we have absolutely no mechanism in place getting back out of the gravity well (or even land in it but that's a much easier task).  At least, there's nothing that I've heard of anywhere ever that represents a practical solution (that is, passes scientific and political muster).  For now, at least, there's no provision for a return trip and while that doesn't bother me personally, it is part of the current program charter for the Orion mars mission (&quot;safe return&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars mission that'd I'd expect to be possible by 2020 range would be using two docked Orion capsules for one or two crew members to drift through a transit orbit to and back from Mars, mainly using discipline and tolerance to endure lack of artificial gravity and living space.  Metabolism would be the main concern here, but even just using two capsules provides a reasonable amount of hybrid living space/storage area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, we could always drop 500 billion dollars on nuclear rockets and do the whole trip in a month (with a landing) but then we get to do fun things like find ways to launch nuclear material with international law and get Congress to actual do something, so in the present political environment I'd say finding a way to land and relaunch Orion is the easiest 'hard' problem.&lt;/p&gt;&#xA;" OwnerUserId="5904" LastEditorUserId="5904" LastEditDate="2014-08-19T19:31:00.057" LastActivityDate="2014-08-19T19:31:00.057" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5159" PostTypeId="1" AcceptedAnswerId="5177" CreationDate="2014-08-12T07:19:45.367" Score="5" ViewCount="2965" Body="&lt;p&gt;&lt;a href=&quot;http://thespacereview.com/article/2576/1&quot;&gt;This guest blog text&lt;/a&gt; by &lt;a href=&quot;http://hea-www.harvard.edu/~elvis/&quot;&gt;Martin Elvis&lt;/a&gt; mentions a kind of orbit which I've never heard of before. How does the orbital mechanics work so that a DRO could benefit an interplanetary mission?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... transfer in highly elliptical orbit (HEO) or a distant retrograde orbit (DRO) around the Moon on both the outbound and return legs. This saves taking about 20 metric tons of mass to and from Mars, and allows for some hardware reuse on later missions. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;He refers to &lt;a href=&quot;http://www.lpi.usra.edu/sbag/meetings/jul2014/presentations/0900_Wed_Gates_ARM_activities.pdf&quot;&gt;these slides&lt;/a&gt; by Michele Gates from a Small Bodies Assesment Group &lt;a href=&quot;http://www.lpi.usra.edu/sbag/&quot;&gt;presentation&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-08-12T07:39:08.697" LastActivityDate="2014-08-13T07:35:02.383" Title="How could a trip to Mars benefit from a &quot;Distant (Lunar) Retrograde Orbit&quot; DRO?" Tags="|mars|orbital-mechanics|the-moon|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5160" PostTypeId="2" ParentId="5150" CreationDate="2014-08-12T08:52:42.117" Score="3" Body="&lt;p&gt;Over Solar System distances and high velocities, more than cometary speeds, smart kinetics and &quot;smart mines&quot; are interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You obviously have to have a way to look for smart mines and munitions deployed in massive numbers and ad-hoc networked as well as silent, all the way down to nano-bots and destructive replicators. In other words, new systems are approached very slowly with years of close examination and test probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Among kinetic weapons perhaps the most interesting as the triple gun (or more). This is a fairly massive projectile launched by rail gun intended to intercept a distant target, light minutes or light hours away. The launch package is itself a smaller rail gun that can fire a smart missile as it nears the target zone and has updated information on the target. The smart missile can have high acceleration sub-munitions or be a third smaller rail gun that fires a smart high acceleration missile into an &quot;expectation area&quot; that brings the missile with range and speed of the target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this is to manage the time lag to the target and the target's high acceleration and unpredictability. We think of these as very expensive expendables, but consider the current state of 3D printers and maybe they are not unreasonable. The time and information lag dictates new strategies and an examination of old strategies from the days of sailing ships.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many pre-planned and compiled scenarios are crucial to avoid any delays in decision making.&lt;/p&gt;&#xA;" OwnerUserId="1432" LastActivityDate="2014-08-12T08:52:42.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5161" PostTypeId="2" ParentId="5159" CreationDate="2014-08-12T09:17:39.293" Score="6" Body="&lt;p&gt;You never heard of Distant Retrograde Orbits because they were never before utilized. Quoting Kirstyn Johnson's (Colorado Center for Astrodynamics Research, University of Colorado at Boulder) work titled &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2013/Johnson_Kirstyn/finalorbit.html&quot;&gt;Understanding NASA's Asteroid Redirect Mission, article on Distant Retrograde Orbits&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Currently, research has not revealed a spacecraft that has utilized a&#xA;  Distant Retrograde Orbit in its operation, despite 40-50 years of&#xA;  research going into this topic. The proposed JIMO mission [&lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Orbiter&quot;&gt;Jupiter Icy Moons Orbiter&lt;/a&gt;], before it&#xA;  was cancelled in 2005, was looking into using a DRO for its stability&#xA;  and low-energy transfer qualities. DROs have also been of interest to&#xA;  potential spacecraft in creation of an initial warning system for&#xA;  large coronal mass ejections heading towards Earth. However, neither&#xA;  of these missions came to fruition. Therefore, there is not mission&#xA;  experience in flying a spacecraft to and into such an orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Some of the benefits of such orbits are avoiding instabilities of closer orbits due to third body gravitational perturbations (a term “quarantine orbit” is mentioned therein and referring to &lt;a href=&quot;https://en.wikipedia.org/wiki/Lyapunov_stability&quot;&gt;Lyapunov stability&lt;/a&gt;), and lower required delta-v for orbital transfers, but come with a caveat that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Due to the retrograde then prograde motion of the orbit in the&#xA;  inertial frame, a satellite's motion cannot be described by Keplerian&#xA;  orbital elements, making the problem more difficult to analyze.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Refer to the &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2013/Johnson_Kirstyn/finalorbit.html&quot;&gt;mentioned article&lt;/a&gt; for more detailed explanations and images helping visualize benefits and difficulties of using DRO.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-08-12T09:17:39.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5162" PostTypeId="2" ParentId="5143" CreationDate="2014-08-12T10:38:11.340" Score="13" Body="&lt;p&gt;Shorter answer: Rocket Science is really, really (really) expensive. Unless you have a) boatloads of money to (literally) burn or b) are launching something you &lt;em&gt;really&lt;/em&gt; don't want anyone else to know about (spy satellite, weapons) it's just easier and cheaper to pick up the phone and get competitive quotes.&lt;/p&gt;&#xA;" OwnerUserId="5933" LastActivityDate="2014-08-12T10:38:11.340" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5163" PostTypeId="1" AcceptedAnswerId="5165" CreationDate="2014-08-12T11:10:06.103" Score="32" ViewCount="6161" Body="&lt;p&gt;Ignore for a moment the question of whether building such a &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_elevator&quot;&gt;space elevator&lt;/a&gt; is practical. Let's also ignore the minute &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta-v&quot;&gt;delta-v&lt;/a&gt; involved in &quot;stepping off&quot; something in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we were to build a space elevator that extends from the surface of the Earth (on the equator) up to &lt;a href=&quot;http://en.wikipedia.org/wiki/Geostationary_orbit&quot;&gt;geostationary Earth orbit (GEO)&lt;/a&gt;, then &lt;strong&gt;would it be possible&lt;/strong&gt; to use that space elevator to get to GEO, then simply &quot;step off&quot; the elevator &lt;em&gt;and remain in GEO&lt;/em&gt; without any additional acceleration component? Why or why not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found myself in a discussion about this over lunch and can't quite make up my mind whether it seems plausible or not, so I'm turning to the collective expertise here to hopefully get an answer.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="5946" LastEditDate="2014-08-12T23:00:03.920" LastActivityDate="2022-11-29T19:35:20.730" Title="If we build a space elevator from Earth surface to GEO, could I step off it at GEO and remain in GEO?" Tags="|geostationary|space-elevator|" AnswerCount="6" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5164" PostTypeId="2" ParentId="5163" CreationDate="2014-08-12T11:24:51.703" Score="9" Body="&lt;p&gt;Yes. The elevator stop at GEO will itself be in orbit at GEO. Thus with a minimal delta-v away from it, you will also be in GEO orbit, at GEO orbital velocity.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-08-12T11:24:51.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5165" PostTypeId="2" ParentId="5163" CreationDate="2014-08-12T11:37:59.237" Score="27" Body="&lt;p&gt;Yes, GEO is a balance point for anchored to Earth space elevator. It has to be, to keep its rotation rate synchronized with Earth's own and keep tether stable. This necessarily means that at GEO altitude, the elevator rotates at GEO orbital speed. Stepping off it at that altitude then means you're in a stable equatorial geosynchronous orbit, or GEO (geostationary, with nadir pointing towards more or less same spot on Earth's surface).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your exact resultant orbit would however depend on the force with which you stepped off it and its vector, so you might end up in slightly higher, slightly lower, or in orbit around the elevator's balance point, depending also on it's balance point mass (and yours, no pun implied).&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-08-12T11:37:59.237" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5166" PostTypeId="1" AcceptedAnswerId="5172" CreationDate="2014-08-12T14:35:40.290" Score="5" ViewCount="522" Body="&lt;p&gt;Per &lt;a href=&quot;http://www.spacenews.com/article/launch-report/41515spacex-sets-november-january-dates-for-launch-abort-tests-of-crew-capable&quot;&gt;SpaceNews article&lt;/a&gt; SpaceX has set November 2014 for a Pad Abort test from LC-40 in Florida, and January 2015 for a Max-Q abort test from Vandenberg Airforce Base in California.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have only done one launch from Vandenberg so far, so why test there, instead of in Florida, where manned flights will be performed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is to clear space/time in the launch schedule at Florida, why do the Pad Abort in Florida at all?  Do it in California and not interfere with regular launch operations.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-02T23:05:03.883" Title="Why is SpaceX testing Pad Abort at LC-40 (CCAFS) but Max-Q abort at Vandenberg?" Tags="|spacex|testing|abort|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5167" PostTypeId="1" AcceptedAnswerId="5168" CreationDate="2014-08-12T14:42:05.467" Score="5" ViewCount="1525" Body="&lt;p&gt;How much is it possible to measure from Earth and how accurately without sending probes to the planets? I am curious about how atmospheric boundary (altitude), atmospheric pressure, scale height were measured.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, when were the probes sent to do more precise atmosphere measures? Was it known where the atmospheric boundary is, or would the probe slowly lower it's orbit constantly detecting for presence of an atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="5937" LastEditorUserId="897" LastEditDate="2014-08-13T14:20:26.957" LastActivityDate="2014-08-13T14:20:26.957" Title="How do we measure the atmospheres of Solar system planets?" Tags="|probe|planetary-science|atmosphere|solar-system|planet|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5168" PostTypeId="2" ParentId="5167" CreationDate="2014-08-12T15:04:11.517" Score="6" Body="&lt;p&gt;The only alternative to sending probes is by measuring through &lt;a href=&quot;https://en.wikipedia.org/wiki/Remote_sensing&quot; rel=&quot;noreferrer&quot;&gt;remote sensing&lt;/a&gt;.  Remote sensing is any measurement without actually &quot;being there&quot; (which includes what your eyes are doing!).  For remote sensing of atmospheric properties, pretty much the only way¹ is by measuring &lt;a href=&quot;https://en.wikipedia.org/wiki/Electromagnetic_radiation&quot; rel=&quot;noreferrer&quot;&gt;electromagnetic radiation&lt;/a&gt;: either radiation emitted by the atmosphere, or radiation emitted by another source and then absorbed by the atmosphere.  Detailed information can be obtained through &lt;a href=&quot;https://en.wikipedia.org/wiki/Spectroscopy&quot; rel=&quot;noreferrer&quot;&gt;spectroscopy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everything emits and absorbs electromagnetic radiation.  Different molecules have very specific spectral signatures for their emission as a function of wavelength.  This emission further depends on temperature, pressure, and some other factors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/44z5w.gif&quot; alt=&quot;Jupiter spectrum&quot;&gt;&#xA;&lt;br /&gt;&lt;sup&gt;Spectrum of Jupiter, from &lt;a href=&quot;http://lasp.colorado.edu/~bagenal/3720/CLASS5/5Spectroscopy.html&quot; rel=&quot;noreferrer&quot;&gt;LASP&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, we compare the observed spectrum with lab measurements for known species: H₂, H₂O, CH₄, CO₂, etc.  We try to add up the different species contributions, until they match the observed spectrum.  That is how we can determine the composition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information, see &lt;a href=&quot;http://lasp.colorado.edu/~bagenal/3720/CLASS5/5Spectroscopy.html&quot; rel=&quot;noreferrer&quot;&gt;this page at LASP&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this is very commonly used not only for planetary sciences, but also in remote sensing of the Earth atmosphere, such as for weather and climate monitoring.  Without atmospheric remote sensing, the daily weather forecast would not exist.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;¹&lt;sup&gt;Theoretically, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravimetry&quot; rel=&quot;noreferrer&quot;&gt;gravimetry&lt;/a&gt; can be used to get mass distributions, but this requires being very close to the object and then still, atmospheres are not massive enough.  But we can measure Earth ocean currents!&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2014-08-12T15:22:47.277" LastActivityDate="2014-08-12T15:22:47.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5169" PostTypeId="2" ParentId="3211" CreationDate="2014-08-12T16:38:43.160" Score="3" Body="&lt;p&gt;There's also the rotovator concept.  It's a lot trickier to handle because it's actually in orbit rather than tethered so you have to be very careful with the energy balance to avoid pulling it out of orbit--if you launch 10 tons you also need to land 10 tons (even if that 10 tons is nothing but slag from some refinery in space.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the strength requirements are lower and can also do ones that only do part of the job.  (The rocket equation still makes this very valuable, though.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take a cable like you would for an elevator but instead of anchoring it you put it in orbit and spin it opposite it's orbital direction--the end closest to the planet is moving a lot slower than orbital velocity, the far end is moving a lot faster.  On a full size one the tip will be moving at orbital velocity--in other words, stationary with respect to the planet.  It could dip into the upper atmosphere (or come near the surface on an airless body) and a craft could come up to meet it.  (It might actually work better if it was moving some hundreds of miles per hour so an aircraft could grab it.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-08-12T16:38:43.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5172" PostTypeId="2" ParentId="5166" CreationDate="2014-08-12T17:20:52.060" Score="9" Body="&lt;p&gt;I suspect these two abort situations test two different things, and as such have different requirements. As you mentioned, the Florida SpaceX launch schedule is quite busy, and I suspect they want to minimize the downtime there as much as possible. I suspect the pad abort test has to be done at Flordia, however, due to the unique communication lines and such at the Florida site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Max Q abort can happen at Vandenburg, because nothing will be plugged in, after the initial launch. Thus there is nothing that really needs to be configured to have the abort happen correctly.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-08-12T17:20:52.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5175" PostTypeId="2" ParentId="5142" CreationDate="2014-08-12T20:12:39.143" Score="3" Body="&lt;p&gt;In general, this is a very hard problem.  You might take a look at this &lt;a href=&quot;http://www.amostech.com/TechnicalPapers/2011/NROC/HEJDUK.pdf&quot; rel=&quot;nofollow&quot;&gt;paper&lt;/a&gt;, which looks at heuristic models for estimating satellite visual magnitude as a function of phase angle, range, object size and object type.  Though IIRC, it's only looking at geosynchronous objects.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastActivityDate="2014-08-12T20:12:39.143" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5176" PostTypeId="2" ParentId="5167" CreationDate="2014-08-12T20:17:54.167" Score="3" Body="&lt;p&gt;Wikipedia has a series of articles about the &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Mars&quot; rel=&quot;nofollow&quot;&gt;atmosphere of each of the major bodies in the Solar system&lt;/a&gt;; you can learn quite a bit about the history of investigation of each.  For the moon, we observed no visual distortion of the surface with telescopes, so we expected no atmosphere, and confirmed it with probes (Ranger flyby in 1961, Surveyor landing in 1966).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Observation suggested Mercury had essentially no atmosphere, and Mariner 10 confirmed this (something like a hundred-trillionth of Earth's atmospheric pressure) in 1974. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The optically fuzzy edge of Venus hinted at a thick atmosphere as early as 1761; Mariner 2 came close to Venus in 1962 with a pair of radiometers, one measuring microwave radiation and the other infrared, and determined that the upper cloud cover was relatively cool while the surface was hot. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-08-12T20:17:54.167" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5177" PostTypeId="2" ParentId="5159" CreationDate="2014-08-13T07:35:02.383" Score="7" Body="&lt;p&gt;The basic idea is to stage components of a large manned mission in a high Earth orbit, where you have already imparted the majority of the energy needed to get to destinations in the solar system.  The $\Delta V$ to get from LEO to a high-Earth orbit with an apoapsis at the distance of the Moon is 80% to 85% of the $\Delta V$ to get from LEO to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can use highly efficient electric propulsion systems to get to HEO, since you don't care how long it takes for uncrewed components to get there, in order to minimize the launch mass, and thus the cost.  These can be fueled chemical propulsion stages, habitats, and other pieces.  Our Moon then provides a convenient and nearly free means to both raise and drop components' periapsis for storage in a circular HEO, rendezvous with a crew, and to use a low periapsis for a near-Earth chemical propulsive maneuver for efficient injection to an interplanetary trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://nicadd.niu.edu/~mwiesner/Landau_Strange.pdf&quot;&gt;this article&lt;/a&gt; for an overview of the concept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A stable way to store such pieces is in a DRO around the Moon, which is energetically cheap to get into and out of from HEO.  You can also put some of your returned hardware, such as your deep-space habitat, in DRO for resupply and reuse.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-08-13T07:35:02.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5178" PostTypeId="1" AcceptedAnswerId="5181" CreationDate="2014-08-13T13:45:18.277" Score="9" ViewCount="1418" Body="&lt;p&gt;So I was chasing something else when &lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_orbital_rocket_engines&quot;&gt;this page&lt;/a&gt; popped into my field&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm quite sure the page isn't exhaustive. What little content &lt;em&gt;is&lt;/em&gt; there on the page though seems to indicate the below listed propellant are what have been used majorly over the last 6 decades, or so of space exploration.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;RP1/LOX&lt;/li&gt;&#xA;&lt;li&gt;&lt;strike&gt;Xenon&lt;/strike&gt;&lt;/li&gt;&#xA;&lt;li&gt;LH2/LOX&lt;/li&gt;&#xA;&lt;li&gt;&lt;strike&gt;Argon&lt;/strike&gt;&lt;/li&gt;&#xA;&lt;li&gt;N2O4/MMH&lt;/li&gt;&#xA;&lt;li&gt;Methane/LOX&lt;/li&gt;&#xA;&lt;li&gt;LH2/LOX&lt;/li&gt;&#xA;&lt;li&gt;APCP&lt;/li&gt;&#xA;&lt;li&gt;HTPB&lt;/li&gt;&#xA;&lt;li&gt;N2O4/UDMH&lt;/li&gt;&#xA;&lt;li&gt;H2O2/Kerosene&lt;/li&gt;&#xA;&lt;li&gt;Solid&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The listed propellants are both launch systems, as well as ion/deep-space systems so it looks a little odd to read.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The propellants of choice, as per this page, are (in descending order)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;LH2/LOX&lt;/li&gt;&#xA;&lt;li&gt;RP1/LOX&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The list above seems to indicate space-craft launch vehicle propellants have not evolved much over this period with. Is this impression correct?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-09-29T16:20:19.087" LastActivityDate="2015-01-16T20:56:36.723" Title="Have launch system propellants evolved since RP1/LH2?" Tags="|fuel|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5179" PostTypeId="2" ParentId="5178" CreationDate="2014-08-13T13:59:26.913" Score="2" Body="&lt;p&gt;For the most part, yes that impression is correct.  In terms of chemical rockets, LH2 is still king in ISP.  Non-Chemical rockets have improved to get better/different performance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid fuels have progressed over time, with all sorts of different compounds being used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chemical propulsion, in terms of raw performance, and in terms of things that can be used, has mostly plateaued. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-08-13T13:59:26.913" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5181" PostTypeId="2" ParentId="5178" CreationDate="2014-08-13T14:17:22.580" Score="9" Body="&lt;p&gt;The performance of a rocket engine is limited mainly by chemistry: a given rocket fuel always has the same performance, as long as the engine is strong enough to contain the burning fuel. The stronger the engine, the higher the chamber pressure which increases performance, but that also runs into chemical limits (the strength of the chamber materials).&lt;br&gt;&#xA;So to really improve performance, you'd need to switch to a different fuel. But there are only so many fuels that can be used. Your list contains most of them. There have been some experiments with a &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_rocket_propellants&quot;&gt;fluorine/lithium/LH2 mix&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The impracticality of this chemistry highlights why exotic propellants are not actually used: to make all three components liquids, the hydrogen must be kept below -252°C (just 21 K) and the lithium must be kept above 180°C (453 K). Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, including hydrogen. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which makes working around the launch pad difficult, damages the environment, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket. Finally, both lithium and fluorine are expensive and rare, enough to actually matter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This shows the difficulties you can run into when researching better propellants: your propellant may have side effects that make it undesirable to use in practice.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The chemistry of combustion was pretty well known when the first rockets were developed, so the best chemicals available have been used from the start, and the scope for evolution was/is limited.&lt;br&gt;&#xA;Instead, there has been a lot of research into other forms of propulsion. Nuclear thermal drives and ion drives have been developed, for example. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-08-13T14:17:22.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5182" PostTypeId="2" ParentId="5163" CreationDate="2014-08-13T14:46:50.563" Score="0" Body="&lt;p&gt;Note that the answer is &quot;yes&quot; for any orbit.  Step out of the spaceship/elevator/phonebooth with minimal energy and you'll be in the exactly same orbit that the original one was (modulo whatever force you used to step out).&lt;/p&gt;&#xA;" OwnerUserId="5950" LastActivityDate="2014-08-13T14:46:50.563" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5183" PostTypeId="2" ParentId="5163" CreationDate="2014-08-13T18:19:45.483" Score="3" Body="&lt;p&gt;It depends upon exactly where you step off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The elevator, in order to remain in place, has to actually extend well past GEO. An object orbits based upon the speed at its center of mass. As you go down, orbital velocity required increases, and as you go up, it decreases... and the elevator also has an issue of as the elevator goes down from the center of mass, its speed decreases, and as you go up from the center of mass, speed increases. (Keeping in mind, down is towards the planet.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, remember the laws of motion:&lt;br&gt;&#xA;— An object in motion remains in motion until acted upon by an outside force.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity is constantly acting, but the speed means that the pull isn't fast enough to bring one down, since gravity works over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if you step off 10m below the center of mass, you're in a decaying orbit already - the orbital speed needed is higher, but your retained orbital speed is lower. That difference is VERY small. Small enough to be ignored in the short term. But if you &quot;hang&quot; your tools nearby, and wander away, a few days later they've moved downward and trailing of the station, and will continue to do so, accelerating downward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you instead step off above the center of mass, you are above the orbital speed; you have attained escape velocity and will appear to arc away. It won't be a nice clean straight line, as you're close enough to orbital velocity that gravity keeps you curved. It will be a long slow spiral outward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This effect, when maximized by a few kilometers, can be used, in theory, to launch interplanetary spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-08-13T18:19:45.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5184" PostTypeId="1" CreationDate="2014-08-14T12:49:27.087" Score="20" ViewCount="726" Body="&lt;p&gt;Given that the ISS will eventually come to the end of its mission (currently extended to 2020 (&lt;a href=&quot;http://en.wikipedia.org/wiki/International_Space_Station_program#End_of_Mission&quot;&gt;source&lt;/a&gt;)), would it be possible to re-purpose it for non-human space flight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To clarify: does the equipment in orbit already provide enough utility to conduct useful science missions that would justify the operational costs and the costs of retrofitting the station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that risk and cost would be greatly reduced without a human crew, and we would be remiss to let all of that infrastructure de-orbit. But I don't know what kind off science experiments could be performed on the station remotely and if the benefits outweigh the costs.  &lt;/p&gt;&#xA;" OwnerUserId="4686" LastEditorUserId="63" LastEditDate="2015-04-15T21:16:18.443" LastActivityDate="2016-01-11T11:28:57.823" Title="Extending Use of ISS Beyond End of Mission" Tags="|iss|robotic-missions|uncrewed-spaceflight|science|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="5185" PostTypeId="2" ParentId="902" CreationDate="2014-08-15T00:21:37.840" Score="14" Body="&lt;p&gt;Milton is not, in fact, the first English author to use 'space' in this sense. He is antedated by Lucy Hutchinson and John Evelyn, both of whom use 'space' to translate 'spatium' in their English versions of Lucretius's &lt;em&gt;De Rerum Natura&lt;/em&gt;. Milton probably did help the English sense to catch on, and he too was alluding to Lucretius. By the way, Paradise Lost, book 1, line 650 (which has the singular, 'space may produce new worlds') is a better example to use than the lines in book 8.&lt;/p&gt;&#xA;" OwnerUserId="5956" LastEditorUserId="58" LastEditDate="2017-06-14T21:16:50.483" LastActivityDate="2017-06-14T21:16:50.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5186" PostTypeId="1" CreationDate="2014-08-15T02:42:58.010" Score="10" ViewCount="686" Body="&lt;p&gt;I hoping to look at learn more about Main Engine Cut Off (MECO) for various scenarios.&#xA;5 quantities I want are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Booster mass at lift off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Booster mass at MECO&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Booster velocity at MECO (including direction, horizontal and vertical components)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) Booster altitude at MECO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5) Booster distance from launch pad at MECO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scenarios I want to compare are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ordinary expendable launch&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reusable booster flying a vertical path and Return To Luanch Site (RTLS)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reusable booster flying eastward and then horizontal boost back for RTLS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reusable booster landing on a sea platform downrange.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me the quantities I'm looking for would vary for each scenario.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's my belief reuse would have a hit on payload mass. I've heard vertical lofting RTLS would have a 15% hit on payload mass and horizontal boost back RTLS would have a 30% hit on payload mass. I don't know if that's accurate as I don't know what math and models are used to arrive at those figures.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="1142" LastEditDate="2014-10-26T04:20:36.347" LastActivityDate="2015-04-17T22:00:36.480" Title="Numbers for Falcon 9 booster MECO scenarios?" Tags="|spacex|reuse|falcon-9|launch-sequence|booster|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5187" PostTypeId="1" CreationDate="2014-08-15T13:51:12.700" Score="27" ViewCount="3770" Body="&lt;p&gt;Also, I believe that the second stage continues to burn for quite some time even after seperation of stage 3.&lt;/p&gt;&#xA;" OwnerUserId="5958" LastEditorUserId="198" LastEditDate="2020-02-03T23:09:13.353" LastActivityDate="2023-05-01T21:07:02.480" Title="Why does the Soyuz Rocket system fire the third stage while stage two is still attached?" Tags="|soyuz-launcher|stage-separation|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5188" PostTypeId="2" ParentId="5163" CreationDate="2014-08-15T18:26:43.647" Score="15" Body="&lt;p&gt;A Clarke style space elevator is a (very large) gravity gradient stabilized vertical tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When in a rotating frame (as on a merry-go-round) you feel a tug. It's just inertia but feels like an acceleration. This so called acceleration is &lt;strong&gt;$\omega^2r$&lt;/strong&gt; where $\omega$ is angular velocity expressed in radians/time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity's acceleration is &lt;strong&gt;$GM_{earth}/r^2$&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a vertical tether moving in a circular orbit, there's a point where &lt;strong&gt;$\omega^2r$&lt;/strong&gt; and &lt;strong&gt;$GM/r^2$&lt;/strong&gt; exactly cancel and someone on the tether at this point would feel zero net acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a space elevator this point would be at geosynchronous altitude about 36,000 kilometers above earth's surface or about 42,000 kilometers from earth's center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below geosynchronous altitude, &lt;strong&gt;$GM_{earth}/r^2$&lt;/strong&gt; overwhelms &lt;strong&gt;$\omega^2r$&lt;/strong&gt;. Someone on an elevator platform below geosynch would feel a tug towards the earth. This earthward tug grows stronger for platforms at lower altitudes. Someone jumping off the leading eastern edge of such a platform would fall towards the earth along the elliptical paths shown. The jumper would also move further and further east of elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eloJM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above geosynch altitude, &lt;strong&gt;$\omega^2r$&lt;/strong&gt; overwhelms &lt;strong&gt;$GM_{earth}/r^2$&lt;/strong&gt;. Someone on a elevator platform above geosynch would feel a tug away from the earth. This tug away from earth grows stronger at higher altitudes. Someone jumping off the trailing western edge of such a platform would fall away from the earth along the elliptical paths shown. This jumper would move further and further west of elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Call the ~42,000 km geosynch radius $r_g$. At  $2^{1/3}r_g$, the elevator is moving escape velocity. Someone jumping off a platform at this altitude would fall into a parabolic orbit away from the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Someone stepping off the platform at geosynch would feel no acceleration up or down. He would step into the blue orbit of the diagram above. He wouldn't be moving with regard to the elevator.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-08-15T18:35:41.720" LastActivityDate="2014-08-15T18:35:41.720" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5190" PostTypeId="1" AcceptedAnswerId="21152" CreationDate="2014-08-16T10:08:43.230" Score="18" ViewCount="2821" Body="&lt;p&gt;Orion is being developed in tandem with the SLS. But Falcon Heavy is scheduled to fly earlier, early 2015, and be cheaper. So I wonder what kind of missions Orion could do with FH. Could it for example do a crewed flight around the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Exploration_Flight_Test_1&quot; rel=&quot;noreferrer&quot;&gt;first test&lt;/a&gt; launch with Orion is currently planned for December 2014, atop a Delta IV Heavy. According to Wikipedia articles about &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV_Heavy&quot; rel=&quot;noreferrer&quot;&gt;Delta IV&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_Heavy&quot; rel=&quot;noreferrer&quot;&gt;Falcon Heavy&lt;/a&gt;, the latter will be able to lift twice as much to LEO and 50% more to GEO. &#xA;Falcon 9 is underway to be human rated, and I would think that the technical similarities should mean that the FH is human rated soon after that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there some show stopper for FH+Orion?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any reason to launch Orion instead of Dragon?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="195" LastEditDate="2018-01-25T22:19:05.577" LastActivityDate="2019-07-20T12:51:30.720" Title="What missions could be done with Orion on Falcon Heavy?" Tags="|nasa|spacex|crewed-spaceflight|orion-spacecraft|falcon-heavy|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5191" PostTypeId="2" ParentId="5190" CreationDate="2014-08-17T10:04:52.763" Score="13" Body="&lt;p&gt;The problem is that Orion + Service Module is very heavy. On the order of 21,000 Kilos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is too heavy for anything but a Delta 4-Heavy or Falcon Heavy.  (SLS probably won't ever really exist).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the Falcon Heavy with cross feed offers 53,000 kilos to LEO. (Cannot find the non-cross feed numbers, since that is a better representation of initial performance).  However with the coming updates in summer of 2015 to the core Falcon 9 first stage (slight stretch, Merlin 1D's at 100% not the current 85% thrust level, propellant chilling for densificiation) the expectation is to get to 50,000 kg range without cross feed. As an update in 2017, SpaceX announced that in full expendable mode, no crossfeed, Falcon Heavy after all the Falcon 9 single stick, and Merlin 1D improvements was on track for 64,000lb to LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper stage, being a Merlin 1D is probably an issue as the ISP is not wonderful. (Great ISP of 340 for LOX/RP1 but not a great ISP for an upperstage. RL-10 has much lower thrust but better ISP by a long shot, in the mid 400 range).  Thus a third stage (or else a larger Service Module) would be wanted for further out missions. With SpaceX developing the Raptor LOX/CH4 engine, it would not be inconceivable for them to construct a higher energy upper stage one day. There are many costs involved in doing that which would make it unlikely, but still a possibility if the need really arose. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-04-20T00:47:15.433" LastActivityDate="2017-04-20T00:47:15.433" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5192" PostTypeId="1" CreationDate="2014-08-17T15:10:12.177" Score="4" ViewCount="844" Body="&lt;p&gt;It is believed that at any point in time, a handful of small asteroids (TCO's) are temporarily orbiting the Sun-Earth Lagrange points 1 &amp;amp; 2. They are random samples from the Asteroid Belt and therefor represent that size distribution, mostly up to a couple of meters in diameter. They have irregular orbits which last about 9 months on average until they escape. (Btw, what are such orbits called, Lissajous although they are so short lived?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that it is really cheap to travel from LEO to Lagrange points. When I add up what I easily find online, going from LEO via an Earth-Moon L-point to a Sun-Earth L-point could require only 1 km/s. Is that reasonable? How long time would such a trip take? How much of an extra challenge would it be to rendezvous with an asteroid in a particular orbit around an L-point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would a long-term mission look like to redirect a handful such TCO's per year to LEO for studies by astronauts? I imagine one would like to have a small observatory in Lagrange orbit to detect the small candidates, and a mobile Solar electric tow vehicle to push them to LEO one by one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=9JeIUTz6qho#t=2309&quot; rel=&quot;nofollow&quot;&gt;this very recent Keck talk&lt;/a&gt; the leading asteroid astronomer Dr. William Bottke mentions a mission to TCO's as an alternative to the controversial ARM. Here's a &lt;a href=&quot;http://www.space.com/23016-nasa-asteroid-capture-mission-minimoons.html&quot; rel=&quot;nofollow&quot;&gt;short news article&lt;/a&gt; and a &lt;a href=&quot;http://arxiv.org/abs/1112.3781&quot; rel=&quot;nofollow&quot;&gt;paper&lt;/a&gt; about such asteroids/moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/2006_RH120&quot; rel=&quot;nofollow&quot;&gt;2006 RH120&lt;/a&gt; is the only TCO yet observed, thanks to its large size of about 5 meters. That is about the size that the NASA Asteroid Redirect Mission aims to deal with. One of the advantages of going to a SEL point instead of to a NEO, is that one can go to SEL any day while a NEO has specific launch windows.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-08-19T05:23:42.580" LastActivityDate="2014-08-19T06:29:46.630" Title="What would be the delta-v of rendezvousing with temporarily captured asteroids in Sun-Earth L-points?" Tags="|asteroid|lagrangian-points|future-missions|delta-v|" AnswerCount="1" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="5193" PostTypeId="1" CreationDate="2014-08-18T01:24:30.220" Score="13" ViewCount="1907" Body="&lt;p&gt;Space elevators may be a reality only in the future, but I know we have the basic principles down pat. Something has been bugging me, though. If we had the proper materials, we could build such an elevator high into the sky. But is there a limit to how high? This leads me to consider a special case:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take two doubly-tidally locked bodies (i.e. they are always facing the same way relative to each other - as if half of the Earth was always facing the Moon). For simplicity, say they are more like Pluto and Charon: they are essentially the same shape, size, and composition, and have approximately the same mass. Would it ever be possible to build a space elevator (now more like a space &lt;em&gt;bridge&lt;/em&gt;) from one to the other? Could it be built in a similar fashion to the normal hypothesized space elevators?&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-08-18T01:30:38.630" LastActivityDate="2015-09-27T23:18:49.387" Title="Space Elevator Between Doubly-Tidally Locked Bodies" Tags="|space-elevator|tidal-locking|" AnswerCount="3" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5195" PostTypeId="2" ParentId="5193" CreationDate="2014-08-18T05:54:27.230" Score="14" Body="&lt;p&gt;One scheme for building elevators is to start from an anchor mass in synchronous orbit and to extend tethers down and up. The tether above will pull the anchor mass up and the tether below would pull the mass down. Care is taken to balance the two and keep building until the tether reaches the planet's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Synchronous orbit for earth is about 36,000 kilometers above earth's surface. Mars synchronous orbit is about 17,000 kilometers above Mars' surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A tide locked moon doesn't have a synchronous orbit as earth and Mars do. However there are two stable regions that hover in the same spot as seen from the tide locked moon's surface: &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point#L1&quot; rel=&quot;noreferrer&quot;&gt;Lagrange 1&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point#L2&quot; rel=&quot;noreferrer&quot;&gt;Lagrange 2&lt;/a&gt;. And in fact there are &lt;a href=&quot;http://www.lpi.usra.edu/meetings/leag2011/pdf/2043.pdf&quot; rel=&quot;noreferrer&quot;&gt;proposals to build lunar elevators&lt;/a&gt; from earth-moon lagrange 1 or earth-moon lagrange 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0qBv9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Pluto-Charon L1 is a point where 3 accelerations cancel out: Pluto's gravity, so-called centrifugal acceleration, and Charon's gravity. As the tether extends both directions it would be a balancing act to keep one side from overpowering the other. Some station keeping would be needed to keep the elevator balanced as the tethers move out from L1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8XKVz.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tether reaches Charon after unfurling 4569 km from L1, To balance this, an 8612 km length is needed on the Pluto side of L1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can the balancing act be maintained while the tether is being unfurled ~8600 km on one side and ~4600 km on the other? I believe it can. Once the elevator is anchored to Charon, the pull of Pluto's tug can exceed the other accelerations. In fact this is a good idea to maintain tension.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An elliptical orbit would cause the elevator length to vary. According to a 2012 Tholen and Buie paper &lt;a href=&quot;http://adsabs.harvard.edu/abs/2012AJ....144...15B&quot; rel=&quot;noreferrer&quot;&gt;Charon's orbit is circular&lt;/a&gt;. In 1997 Tholen and Buie had estimated the eccentricity to be .003 to .008. In 2002 Stern Bottke and Levison &lt;a href=&quot;http://iopscience.iop.org/1538-3881/125/2/902/fulltext/&quot; rel=&quot;noreferrer&quot;&gt;expressed skepticism&lt;/a&gt; for that result noting that tidal evolution of the Pluto-Charon would drive eccentricity to values negligibly close to zero. Tholen and Buie's 2012 paper are from longer and more careful observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An orbit inclined to the Pluto's or Charon's equators would also cause stretching and flexing. Again this system has orbital elements favoring an elevator. &lt;a href=&quot;http://en.wikipedia.org/wiki/Charon_(moon)&quot; rel=&quot;noreferrer&quot;&gt;i = .001º&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Pluto Charon elevator would be much shorter than an earthly elevator and in a much shallower gravity well. With far less stress, no exotic bucky tubes needed. Ordinary materials like Kevlar could be used. Taper ratio for a Kevlar Pluto Charon elevator would be less than 1.3. But the extreme cold of these bodies could make Kevlar brittle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: corrected arithmetic errors in calculating L1 distance from surfaces of Pluto and Charon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2nd Edit: Put in cites to support my claim that Charon's orbit is very nearly circular. Also explained the needed balancing act while tethers are extended from L1. This balancing act isn't the show stopper some think it is. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-08-23T21:30:39.173" LastActivityDate="2014-08-23T21:30:39.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5196" PostTypeId="2" ParentId="5193" CreationDate="2014-08-18T08:52:28.923" Score="12" Body="&lt;p&gt;In theory, yes, one could build a bridge.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several potential problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first problem is that no known natural orbit is circular; all of them are ellipses. If, for example, the semi-major axis of the co-orbital elipse is 11000km, and the semi-minor axis is 10000 km, you need 1000km of &quot;flex&quot; in your bridge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, the &quot;stable&quot; point in the middle is not all that stable, If the two are not the same mass, the L1 point will be closer to the smaller body while the barycenter is closer to the larger. And L1 points are not actually all that stable, anyway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the stable orbital balance point is harder to establish, one cannot &quot;hang&quot; the bridge. If one cannot hang the bridge, then one must push up the bridge. Compressive strength is a major flaw; at present, all known materials require hanging the elevator from orbit, rather than building it up, and even then, only carbon nanotube is viable for Earth orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the issue of putting the thing into orbit, the geosynchronous point will be the L1. &quot;stable&quot; L1 orbits are not points, but complex wobbles around the L1 point, due to N-body solution issues. This instability of the L1 makes the initial station unable to be actually all that geosynchronus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further still, even tho tidally locked, all know tidally locked bodies &quot;wobble&quot; - the lock isn't absolute, and their orbits are also inclined versus the equator. This means that you have to allow for the non-match - the anchor point must move, or the system must flex from the L1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This makes, for this author, the bridge to be unlikely, at least barring some form of orbital alterations to eliminate the inclination and eccentricity.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2014-08-18T08:52:28.923" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="5197" PostTypeId="2" ParentId="5192" CreationDate="2014-08-19T06:29:46.630" Score="9" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cMNdP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above are 4 ellipses. They all have a 400 km altitude perigee, but widely varying apogees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The blue ellipse has an ~1.5 million km apogee, about the distance to SEL1 or SEL2.&#xA;The larger grey ellipse has an apogee at EML2 altitude, the smaller grey ellipse has an apogee at lunar altitude.&#xA;The red ellipse has an apogee at EML1 altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A striking thing about all these ellipses is they have very nearly the same perigee speed. The blue one has a perigee speed of 10.82 km/sec. The red one has a perigee velocity of 10.73 km/sec. They are all just a hair under escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 400 km LEO is about 7.7 km/s. So no matter which of these Lagrange points you're heading for, you need a LEO burn of about 3 or 3.1 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the EML1, EML2, SEL1, SEL2 are all quite close to one another in terms of delta V. EML2, SEL1 and SEL2 all have C3 close to zero. Which makes them much closer to Mars, asteroids, or other destinations in our solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you wanted to park an extraterrestrial propellent source in the earth moon neighborhood, high lunar orbit is a more strategic location than LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It can take take less than 1 km/s delta V to go &lt;strong&gt;from&lt;/strong&gt; these places to LEO. This is because &lt;strong&gt;aerobraking&lt;/strong&gt; can be used to shed that 3 to 3.1 km/s delta V at perigee. But aerobraking can only help you slow orbits. For speeding up from LEO to a big transfer ellipse, you need around 3 km/s worth of propellent plus some to park when you reach your apogee. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-08-19T06:29:46.630" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5199" PostTypeId="1" AcceptedAnswerId="5249" CreationDate="2014-08-19T12:26:00.317" Score="8" ViewCount="346" Body="&lt;p&gt;I see that escape velocity on 67P/Churyumov–Gerasimenko is &lt;a href=&quot;http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot;&gt;0.64 m/s&lt;/a&gt;.  (Contrast Earth's &lt;a href=&quot;http://en.wikipedia.org/wiki/Earth&quot;&gt;11,186 m/s&lt;/a&gt;.)  With such light gravity I imagine any material kicked up would take hours to land, or orbit indefinitely.  The thing &lt;a href=&quot;http://apod.nasa.gov/apod/ap140819.html&quot;&gt;seems covered in sand&lt;/a&gt; and fine particles, in line with the &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19278/1/98-0577.pdf&quot;&gt;rubble pile&lt;/a&gt; model of cometary nuclei.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have precautions been taken with the landing of Philae and harpooning its anchor to lessen kicking up dust?  Are we likely to see landing dust in photographs?&lt;/p&gt;&#xA;" OwnerUserId="213" LastActivityDate="2023-12-14T17:56:23.387" Title="Will kicked-up &quot;dust&quot; be a problem for Rosetta and lander Philae?" Tags="|landing|rosetta|comet|dust|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5200" PostTypeId="1" AcceptedAnswerId="5216" CreationDate="2014-08-19T16:27:21.583" Score="9" ViewCount="2021" Body="&lt;p&gt;For the past few days, since I found out about this, I've been trying to figure out what was the reasoning behind taking dinosaur bones into space. Everywhere I looked I couldn't find any explanation to why, and this place doesn't seem to be the best to ask this question, but I don't know where else I could.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that in 1985 NASA's Spacelab 2 took a piece of bone from a Maiasaura Peeblesorum dinosaur. But why? What did they do with it? What was that they tried to find out?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was also a second dinosaur bone that went into space 3 years later, Coelophysis, again, with unknown reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this question is way off the topic, please guide me to the right place. This has baffled me for awhile now.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;EDIT&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Here are the links I found with the information about dinosaurs in space:&lt;br&gt;&#xA;&lt;a href=&quot;http://www.enchantedlearning.com/subjects/astronomy/glossary/Firstdinospace.shtml&quot;&gt;The First Dinosaurs in Space&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.smithsonianmag.com/science-nature/dinosaurs-in-space-3945429/?no-ist&quot;&gt;Dinosaurs In Space!&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="353" LastEditorUserId="58" LastEditDate="2014-08-21T17:36:21.927" LastActivityDate="2014-08-21T17:36:21.927" Title="Why did we take dinosaur bones into space?" Tags="|life|experiment|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5205" PostTypeId="1" AcceptedAnswerId="5207" CreationDate="2014-08-20T04:30:58.587" Score="11" ViewCount="2505" Body="&lt;p&gt;A tough exterior of heat-dissipating material is generally used for reentry modules. I am wondering if there have been any attempts or experiments at using liquid cooling systems as thermal protection during reentry. And if not, why? Too costly and complex?&lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2014-08-20T11:50:08.883" Title="Have liquid cooling systems been tested for reentry surfaces?" Tags="|reentry|protection|crew-module|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5206" PostTypeId="1" AcceptedAnswerId="5209" CreationDate="2014-08-20T04:39:56.280" Score="7" ViewCount="783" Body="&lt;p&gt;Specifically a location in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Celestial_sphere&quot;&gt;celestial sphere&lt;/a&gt; that is fixed, most likely a star, which creates a set of still coordinates with respect to the Sun. There are many ways to do this, but I am interested in frames used for interplanetary missions, specifically for New Horizons and Cassini, and how the frames are used for telemetry and navigation.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="68" LastEditDate="2014-08-20T14:38:29.413" LastActivityDate="2014-10-20T12:19:53.653" Title="What conventional reference frame is used for interplanetary telemetry and navigation?" Tags="|navigation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5207" PostTypeId="2" ParentId="5205" CreationDate="2014-08-20T11:50:08.883" Score="12" Body="&lt;h1&gt;Main problems&lt;/h1&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Would be cost-effective only for reusable craft, mostly hypersonic spaceplanes (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Rockwell_X-30&quot; rel=&quot;noreferrer&quot;&gt;NASP&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;li&gt;Active cooling may fail spectacularly &lt;strong&gt;=&amp;gt;&lt;/strong&gt; Mission loss.&lt;/li&gt;&#xA;&lt;li&gt;It is hard to achieve required number of cycles before refurbishment due to thermal stress.&lt;/li&gt;&#xA;&lt;li&gt;Too heavy! &lt;strong&gt;-&amp;gt;&lt;/strong&gt; Cannot compete with ablative/heat sink TPS for non-reusable craft.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jumRw.png&quot; alt=&quot;TPS Concepts&quot; /&gt;&lt;/p&gt;&#xA;&lt;h1&gt;Timeline for Active TPS&lt;/h1&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MunRE.png&quot; alt=&quot;Active TPS development&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://hdl.handle.net/2060/19930003259&quot; rel=&quot;noreferrer&quot;&gt;Current Technology for Thermal Protection Systems, 1993&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-08-20T11:50:08.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5208" PostTypeId="1" AcceptedAnswerId="5213" CreationDate="2014-08-20T14:20:44.223" Score="6" ViewCount="4515" Body="&lt;p&gt;While building model rockets with my son, I figured I would ask a question that had been bothering me for a while. When you look at the packaging of the Estes rockets, regardless of complexity level (I tried a level 3, and that was WAY more effort than I wanted to put in), it recommends a different engine for first launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually B class. Then, for later launches, you can do higher powered engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why the lower power for the first flight?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-08-21T01:09:58.973" LastActivityDate="2014-08-21T01:09:58.973" Title="Why do Estes model rockets recommend a smaller engine (B class) for first launches, but support bigger engines later?" Tags="|safety|range-safety|model-rocketry|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5209" PostTypeId="2" ParentId="5206" CreationDate="2014-08-20T15:22:14.050" Score="10" Body="&lt;p&gt;That would be the &lt;a href=&quot;http://www.iers.org/IERS/EN/DataProducts/ICRF/icrf.html&quot; rel=&quot;nofollow&quot;&gt;International Celestial Reference Frame&lt;/a&gt;. The &lt;a href=&quot;http://en.wikipedia.org/wiki/IERS&quot; rel=&quot;nofollow&quot;&gt;International Earth Rotation and Reference Systems Service (IERS)&lt;/a&gt; is the group responsible for maintaining the reference frames used by astronomers and others, as well as maintaining time. (They're the ones who decide when the next leap second will occur.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the predecessors to the ICRF were based on stars (e.g., the frame widely known as J2000 was based on the Fifth Fundamental Catalogue (FK5)), the ICRF is not based on stars. It is instead based on quasars. There are two advantages to using quasars instead of stars:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Quasars are very, very, very far away, drastically reducing the problem of proper motion.&lt;/li&gt;&#xA;&lt;li&gt;Astronomers detect those quasars with radio telescopes. This means they can use Very Long Baseline Interferometry to get an extremely precise reading of where those quasars are in the sky, much more precise than visual frequency telescopes can obtain.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The IERS released revision 2.0 of the ICRF in 2009. I don't know if JPL has upgraded to this newer standard in the more recent releases of its Development Ephemeris.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update: How the frames are used for telemetry and navigation.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll get to telemetry and navigation, but before that, they are first used for mission planning. You need to know the orbits of the Earth, the target planet, and any intermediate planets used for gravity assist to find the launch opportunities for a mission. You need a set of planetary ephemerides to do this. Supporting those interplanetary missions is the primary reason JPL originally developed its &lt;a href=&quot;http://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris&quot; rel=&quot;nofollow&quot;&gt;Development Ephemeris&lt;/a&gt; back in the 1960s, and has continued to refine them ever since.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ephemeris plays an important role not only in mission planning but also in mission operations. The vehicle is never exactly on the trajectory on which it is supposed to be. The results from orbit determination tell the mission controllers what trajectory the vehicle is on (to within some error), and that estimated trajectory coupled with the ephemeris in turn informs the controllers about midcourse corrections the vehicle needs so that it will reach the target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reference frame comes into play in multiple places. The ephemeris is necessarily  expressed in some specific frame of reference. Which one? The scientists involved in these operations use a plethora of reference frames, more so in the past than now. There are different fundamental planes (e.g., Earth equator versus the ecliptic). Since the Earth precesses and nutates, which equatorial plane? Since the orbits are not quite Keplerian, which ecliptic? This non-constancy of the equatorial and ecliptic planes leads to even more choices with regard to frames of reference. The developers of an ephemeris need to be very aware of this plethora of frames so as to correctly interpret all the observations that lead to the ephemeris. The users of the ephemeris need to know the frame in which that ephemeris is expressed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The developers of an ephemeris explicitly assume that the frame they are using is an inertial frame. What if it isn't? What if that frame is rotating with respect to some true inertial frame? That the frame is rotating results in a mismatch in the equations of motion the planets actually follow in that frame versus the equations of motion used in the development of the ephemeris. Having a very good estimate of what constitutes an inertial (non-rotating) frame of reference helps a lot in improving the accuracy of those ephemerides. The same applies to a lesser extent to the problem of vehicle orbit determination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This frame rotation problem motivated JPL to be one of the first organizations to switch from the FK4-based Mean of 1950 frame to the FK5-based J2000 frame, and from that frame to the ICRF. That old FK4 frame wasn't bad; that's what was used to plan and operate the Pioneer and Voyager missions. It wasn't great, either. A better frame means better ephemerides and better orbit determination. Less fuel needs to be allocated for midcourse corrections with a better estimate of what constitutes an inertial frame, and reduced fuel mass means more mass can be added as scientific payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbit determination is one key part of navigation. Another part is vehicle pointing. The vehicle's flight software may not need to know where the vehicle is in space, but it certainly does need to know its attitude. The star trackers and other attitude determination equipment are expressed in terms of some frame of reference, and the flight software needs to be aware of this. It also of course needs a frame of reference in which to express that computed attitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, there's telemetry. The Earth-based communications equipment need to point toward the vehicle and the vehicle needs to point it's communications equipment toward the Earth in order for communications to occur. The vehicle side of this problem can be made simple by telling the vehicle where to point as a function of time. The vehicle doesn't need to know where it is or where the Earth is. It just needs to where to point its antennae. Earth-based side of the problem is much trickier. This requires knowledge of where the vehicle is in inertial space (orbit determination) and also requires knowledge of the Earth's rotation. This leads to the incredibly complex Earth rotation/nutation/precession model. The current best estimate involves the &lt;a href=&quot;http://www.iausofa.org/sofa_pn.pdf&quot; rel=&quot;nofollow&quot;&gt;IAU 2000/2006 precession nutation model&lt;/a&gt;, which has thousands of terms, plus &lt;a href=&quot;http://www.iers.org/nn_10398/IERS/EN/Science/EarthRotation/EOP.html&quot; rel=&quot;nofollow&quot;&gt;Earth Orientation Parameters&lt;/a&gt; supplied by the IERS.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="1142" LastEditDate="2014-10-20T12:19:53.653" LastActivityDate="2014-10-20T12:19:53.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5210" PostTypeId="1" AcceptedAnswerId="5212" CreationDate="2014-08-20T17:53:25.640" Score="8" ViewCount="492" Body="&lt;p&gt;&lt;a href=&quot;http://gizmodo.com/5992441/how-nasas-nuclear-rockets-will-take-us-way-beyond-mars&quot;&gt;This article&lt;/a&gt; dated. 2013 writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After a multi-decade hiatus, both NASA and the Russian Federal Space Agency (which developed many of its own NTRs during the Cold War but never physically tested their designs) announced in April 2012 that they would be revival of nuclear-engine powered rocket technology and coordinating a new $600 million joint engine project along with potential involvement from France, Britain, Germany, China, and Japan&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The then powers-that-be brought the Outer Space Treaty (henceforth, OST) into existence. The development reported above brings to mind the questions &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Whether the reported article is accurate?&lt;/li&gt;&#xA;&lt;li&gt;Whether consultations are under-way at present to amend OST?&lt;/li&gt;&#xA;&lt;li&gt;Whether Roscosmos, ESA, NASA et.al may revive nuclear-engine space technology without violating the OST?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="4660" LastEditDate="2015-01-09T05:11:10.050" LastActivityDate="2015-01-09T05:11:10.050" Title="Will Roscosmos et.al attempt to revive nuclear-engine space technology violate the OST?" Tags="|law|nuclear|ntr|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5211" PostTypeId="2" ParentId="5210" CreationDate="2014-08-20T18:24:17.103" Score="7" Body="&lt;p&gt;I can't find any sources that corroborate, confirm, or falsify the article, so I'm going to treat it as if it was accurate, in order to answer the question. That said, given some of the articles (complete with an obscenity or two) that popped up when I first saw the site, I wouldn't exactly cite this thing in a publication, report, essay, paper, . . . You get the idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Would this violate the OST? (Bullet #3)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's good news here, and then there's bad news. The good news is that the project would not violate the OST (see &lt;a href=&quot;http://history.nasa.gov/1967treaty.html&quot;&gt;http://history.nasa.gov/1967treaty.html&lt;/a&gt; for a detailed transcript). The treaty does not prohibit &lt;em&gt;peaceful&lt;/em&gt; detonations; however, it does prohibit detonations of weapons (see Article IV). Great, right? In fact, the program wouldn't even detonate anything (at least, not on purpose) - it would use a nuclear reactor. So the OST would be non-applicable because no detonations would be taking place. The program should be fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've been putting nuclear-related power sources (well, primarily RTGs - radioisotope thermoelectric generators) in space for quite some time. They are an excellent source of power for satellites and unmanned spacecraft - such as the Pioneer probes. There shouldn't be any opposition, given the success of these devices. The big differences are that most previous nuclear devices were RTGs, not nuclear rectors, and only provided electrical power to the spacecraft - the nuclear reactor one proposed would power the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice that I said unmanned. There are quite a few concerns about humans and nuclear-powered craft - after all, how many people do you know that would enjoy spending months 50 feet from a nuclear reactor? Health problems are, or course, on the top of the list, and an emergency - think a mini-meltdown - could be un-fixable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think that the reasons outlined above would make any modifications to the treaty unnecessary &lt;strong&gt;(Bullet #2)&lt;/strong&gt;. That is, as long as the rockets kept on using nuclear &lt;em&gt;reactors&lt;/em&gt;. You may have heard of Project Orion - the crazy idea to power a spacecraft via &quot;nuclear pulses&quot; (dropping mini nuclear bombs out the back). Well, aside from the obvious health risks, there is another issue with this type of propulsion. The OST? Hardly. It allows for peaceful detonations. Such a rocket would, however, violate a little something called the Partial Nuclear Test Ban Treaty of 1963 (also called the Limited Test Ban Treaty, as well as a dozen or so other names), which makes any detonations aboveground (and thus in the atmosphere or outer space) illegal. So should an international consortium decide to, for some reason, go in that direction, then the PNTBT would need to be modified. Fortunately, it doesn't seem that the project is going, or ever will go, in that direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like I said before, the only part of your question I can't answer is the first part &lt;strong&gt;(Bullet #1)&lt;/strong&gt;. There is a brief mention of something on the Wikipedia page for Nuclear Thermal Rockets, but I can't find anything else (its source is here: &lt;a href=&quot;http://www.space-travel.com/reports/NASA_Researchers_Studying_Advanced_Nuclear_Rocket_Technologies_999.html&quot;&gt;http://www.space-travel.com/reports/NASA_Researchers_Studying_Advanced_Nuclear_Rocket_Technologies_999.html&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this helps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find a transcript of the PNTBT at &lt;a href=&quot;http://www.state.gov/t/isn/4797.htm&quot;&gt;http://www.state.gov/t/isn/4797.htm&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-08-20T18:36:19.880" LastActivityDate="2014-08-20T18:36:19.880" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5212" PostTypeId="2" ParentId="5210" CreationDate="2014-08-20T20:26:02.080" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Whether the reported article is accurate?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's rather bad in my opinion. The news is outdated and incorrect. This part was completely incorrect:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;After a multi-decade hiatus, both NASA and the Russian Federal Space Agency (which developed many of its own NTRs during the Cold War but never physically tested their designs) announced in April 2012 that they would be revival of nuclear-engine powered rocket technology and coordinating a new $600 million joint engine project along with potential involvement from France, Britain, Germany, China, and Japan.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is wrong with that?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It was April 2011, not 2012.&lt;br&gt;&#xA;Getting the year wrong is a perhaps minor quibble. Getting completely wrong what NASA and Roscosmos agreed to is anything but a minor quibble. What happened is that Russia and NASA agreed to &lt;strong&gt;&quot;talk&quot;&lt;/strong&gt;. A meeting scheduled for April 15, 2011 was announced on April 5, 2011.&lt;/li&gt;&#xA;&lt;li&gt;As far as I can tell, those talks went nowhere.&lt;br&gt;&#xA;There were no announcements from NASA or Russia after this meeting. If something had come out of that meeting, you would have heard about it three years ago.&lt;/li&gt;&#xA;&lt;li&gt;The $600 million figure should be 17 billion rubles.&lt;br&gt;&#xA;That's how much Russia alone had planned to spend in this area, with or without support from other governments. Whether that spending is actually happening, that's a different story.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There's a lot of other not-quite-right stuff in that article. Yes, NASA is funding spending on nuclear thermal propulsion. NASA funds all kinds of futuristic projects, but at very low levels. This is one of many. Whether any of these projects come to fruition is a different question. Some of them do.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Whether consultations are under-way at present to amend OST?&lt;br&gt;&#xA;  Whether Roscosmos, ESA, NASA et.al may revive nuclear-engine space technology without violating the OST?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's no need to do that. The Outer Space Treaty forbids putting weapons of mass destruction in space. A nuclear power plant and a nuclear bomb are very different things. The materials used in a nuclear power plant are not fissile. Nuclear power plants not only do not explode but they can not explode. They are not weapons.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.ria.ru/science/20091028/156623290.html&quot;&gt;This October 2009 RIA Novosti article&lt;/a&gt; covered Roscosmos's announcement that they would begin working on a nuclear-powered rocket, at an estimated cost of 17 billion rubles. You can also read about this announcement in &lt;a href=&quot;http://www.csmonitor.com/World/Global-News/2009/1029/russians-to-ride-a-nuclear-powered-spacecraft-to-mars&quot;&gt;this Christian Science Monitor article&lt;/a&gt; and &lt;a href=&quot;http://www.wired.com/2009/11/nuclear-propulsion-in-space/&quot;&gt;this article from wired.com&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.ria.ru/science/20100111/157509227.html&quot;&gt;This January 2010 RIA Novosti article&lt;/a&gt; announced the allocation of 500 million rubles (16.7 million dollars) on the project. You can also read about this in &lt;a href=&quot;http://www.theregister.co.uk/2010/01/14/russian_nuke_spaceship_initial_funds/&quot;&gt;this article from the Register&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.ria.ru/russia/20110406/163403884.html&quot;&gt;This early April 2011 RIA Novosti article&lt;/a&gt; briefly alludes to a then upcoming meeting between the heads of Roscosmos and NASA. This &lt;a href=&quot;http://en.ria.ru/science/20110413/163514275.html&quot;&gt;mid-April 2011 RIA Novosti article&lt;/a&gt; gives slightly more detail on those talks. You can also read about this April 2011 meeting in this &lt;a href=&quot;http://www.space-travel.com/reports/Russia_And_US_To_Discuss_Nuke_Powered_Spaceship_Project_999.html&quot;&gt;April 2011 space-travel.com article&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find any reliable sources for what the outcome of that meeting was.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="288" LastEditDate="2014-08-21T17:05:07.503" LastActivityDate="2014-08-21T17:05:07.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5213" PostTypeId="2" ParentId="5208" CreationDate="2014-08-20T21:06:20.520" Score="8" Body="&lt;p&gt;A smaller engine means a lower peak altitude and a smaller recovery area.  You wouldn't want your first flight to involve losing sight of it as it passes 1000 ft AGL and letting it drift a mile away, never to be seen again, would you?&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-08-20T21:06:20.520" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5214" PostTypeId="2" ParentId="5208" CreationDate="2014-08-21T00:46:48.290" Score="6" Body="&lt;p&gt;The issue is if something's wrong with your rocket.  The smaller the engine the slower it will be going if things go badly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen one case where the motor mounting broke free on second stage ignition.  I don't know exactly what went wrong but the rocket hesitated in flight and pretty much fell over before the second stage was producing any substantial thrust.  (I suspect the engine might have been defective.)  Fortunately it was still heading slightly into the sky at that point so there was no danger to any of us on the ground.  (The rocket headed far from the launch site and did a belly landing.  The ejection charged fired after it had come to a stop--the nose had been blown forward and the still-furled chute was laying there.  Reglue the mount, touch up the paint and it was good to fly.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've also seen a fin come off on liftoff.  The rocket headed up on an arcing trajectory and was never found--the smoke trail suggested it was spinning rapidly, the chute probably tied itself up and it was heading far from the vertical, way beyond our outer trackers.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-08-21T00:46:48.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5215" PostTypeId="1" CreationDate="2014-08-21T14:18:58.220" Score="15" ViewCount="966" Body="&lt;p&gt;I'm working on a simple rocket flight simulation program, and am trying to reproduce aspects of Goddard's first successful rocket flight. My sources put the empty weight at 5.75-6lbs, loaded weight 10.25-10.4 lbs, and thrust 9 lbs (40N), using a fuel-rich gasoline/LOX propellant with Isp of about 150s. The rocket is supposed to have reached an altitude of 41' (12.5m) and turned to cover 184' (56m) horizontally in a 2.5 sec flight after burning 20 seconds without leaving the launch stand. Fully fueled, the rocket didn't reach 1:1 thrust-to-weight, so couldn't lift off; my calculations agree that 20-25 seconds of fuel consumption is about where the weight would have dropped to 9 lbs.; so far so good.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in order to reach 12.5m altitude in no more than 2.5 seconds, the rocket would have to accelerate at about 4m/s² on average (d = 1/2 at²), implying thrust-to-weight ratio of around 1.4; this thrust could be achieved only very briefly right before fuel-out, and that would leave no time at all for the fall (actually a powered dive!) from that peak altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we take the widely reported 41' altitude as counting the rocket's 10' height, that still leaves us with a 9.5m apogee to account for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My simulation doesn't yet include rotation off the vertical, so my final apogee is much higher (Goddard's rocket had another 40 or 50 seconds of propellant available after liftoff; it flipped over and crashed quite quickly), but 2.5 seconds after liftoff I see only 0.04m of vertical travel, nowhere near the reported figure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I overlooking something obvious? My model includes drag, but at the low speeds in the first seconds of travel, it's negligible.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Update: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My best guess at this point is that the nozzle burnthrough that caused the rocket to go off-course also greatly increased the thrust while decreasing the Isp (i.e. it let a lot of unburned propellant out of the combustion chamber in a hurry); if I change thrust to 100N and Isp to 50s right at takeoff, my simulation can reach the reported altitude at the required time. Does that seem plausible?&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="5963" LastEditDate="2014-08-21T17:57:23.660" LastActivityDate="2014-08-22T16:17:39.910" Title="How did Goddard's rocket reach 41 feet altitude in under 2.5 seconds?" Tags="|rockets|simulation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5216" PostTypeId="2" ParentId="5200" CreationDate="2014-08-21T15:57:32.717" Score="6" Body="&lt;p&gt;Neither mission made use of the dinosaur bone fragments in any of their recorded experiments. The most reasonable explanation is that the astronauts who carried them aboard did so for the opportunity to say &quot;Dinosaurs in Space!&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/experimentSearch.do?spacecraft=STS%20%2051F/Spacelab%202&quot;&gt;STS-51F Experiments - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/missions/sts-89/mission-sts-89.html&quot;&gt;STS-89 Mission Details - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2014-08-21T15:57:32.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5217" PostTypeId="2" ParentId="2476" CreationDate="2014-08-21T19:15:39.450" Score="9" Body="&lt;p&gt;The simplest answer is that McDivitt thrusted the Gemini Spacecraft towards the Titan second stage in the same direction that both two objects were travelling. The two were separated far enough that it would take several minutes for Gemini to reach the Titan second stage with amount of thrust generated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they were in a zero-g field with nothing else imparting a force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But they were in orbit. Because thrusting towards the Titan second stage was also  the same as thrusting in the direction of their orbit around earth this had the effect of raising the orbit of the Gemini spacecraft. Because the Gemini was now in a higher orbit it was causing the spacecraft to move a slower relative to the Titan's second stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see this effect by the fact a spacecraft orbiting the earth at a 200 miles altitude takes roughly 90 minutes to orbit the earth and a spacecraft at 26,200 miles takes roughly 24 hours to orbit the earth (otherwise known as geosynchronous orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The distance between the two spacecraft was enough that the effect of the Gemini shifting orbit dominated how the two objects interacted. McDivitt various attempts at approaching the Titan kept shifting the Gemini into higher orbits causing the counter intuitive effect of moving away from the Titan second stage even though he was thrusting straight at it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same effect does happens when the range is short. However your orbital altitude doesn't raise or lower fast enough to make a difference. Thus it can be largely ignored.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately NASA solved this by correctly timing the final approach to a target object. The target was allowed to rise to a certain number of degrees above the horizon relative to the spacecraft. The spacecraft was in a lower orbit and starting out moving faster than the target. Then the commander pointed the Gemini straight at the target. The radar locked on giving the rate of range closure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The commander then applied thrust toward the target. There was an instrument that showed distance to target and rate of range closure on a scale. The commander job was to keep the rate of range closure at the right spot for the current distance. Generally by thrusting forward at periodic intervals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were looking from the outside what you would see is the Gemini spacecraft rising up and towards the target, passing underneath, and then curling upwards to come to a stop a few hundred meters forward of the target's orbital position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the entire maneuver the Gemini spacecraft would be pointed at the target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the radar failed on Gemini 12, it killed the crucial rate of range closure. Pilot Buzz Aldrin then used handheld instruments (sextant, etc) and charts to tell Command Jim Lovell when to apply forward thrust. They ultimately successfully completed the rendezvous. &lt;/p&gt;&#xA;" OwnerUserId="151" LastEditorUserId="151" LastEditDate="2018-12-14T13:24:54.777" LastActivityDate="2018-12-14T13:24:54.777" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5218" PostTypeId="2" ParentId="5178" CreationDate="2014-08-21T20:40:53.883" Score="8" Body="&lt;p&gt;LH2 has several nice properties that make it the propellant of choice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One, it releases a lot of energy when combusted with oxygen; this is obviously one of the most important factors in a rocket fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two, the molecular weight of both the fuel and its combustion product is low, which yields high exhaust velocity, which is directly proportional to specific impulse, the fuel-efficiency of a rocket. Obviously hydrogen is going to be a winner here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three, the combustion product of LH2/LOX is mostly friendly, non-toxic water vapor; any unburned fuel or oxidizer is also relatively harmless. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the down side, the low density of LH2 means that your fuel tanks are much bigger than they would be for kerosene/LOX, and you have to keep LH2 very cold; it's impractical to store it in the rocket's tanks for any substantial length of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can't afford the volume of LH2 (like in the already-enormous first stage of Saturn V) you go with RP-1 (kerosene)/LOX instead, which is almost as powerful and convenient; the combustion products include a lot of relatively-harmless CO &amp;amp; CO2 in addition to water vapor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you need to keep the rocket fueled for long periods of time - like in an ICBM that has to launch in a hurry - then LOX isn't practical, and you start having to get into room-temperature fuels like the hydrazine family of fuels with N2O4 oxidizer, which are toxic in both raw and combusted forms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everything else tends to fall short of LH2 or kerosene in safety, stability, convenience, performance, or all of the above. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So those are the factors for the majority of conventional launchers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's growing interest in methane/LOX for Mars return missions, since it's &lt;a href=&quot;http://en.wikipedia.org/wiki/In_situ_resource_utilization&quot; rel=&quot;nofollow&quot;&gt;theoretically possible to produce methane for the return trip&lt;/a&gt; on the surface of Mars. The specific impulse is in the same ballpark as kerosene or a bit better. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If overall thrust isn't a factor, as for upper stages, ion engines using Xenon as reaction mass have vastly higher ISP but terribly low thrust; it takes them a long time to leave Earth's orbit to go anywhere.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-11T15:38:19.060" LastActivityDate="2014-12-11T15:38:19.060" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5219" PostTypeId="2" ParentId="5215" CreationDate="2014-08-22T16:17:39.910" Score="6" Body="&lt;p&gt;It could be that what happened is a slight &quot;Ground Effect&quot;. Essentially, the exhaust will push off the ground, which pushes off the rocket slightly as well. This normally isn't a big deal, but it does have an effect when a rocket stays near the ground for an extended period of time. I think what this would do in effect is to increase the acceleration for a period of time. This would also increase the time, as it would take slightly less fuel to push the rocket forward. The effect would be something like a 50% increase in thrust which falls off based on the height, although I don't know the specifics. Take a look at &lt;a href=&quot;https://en.wikipedia.org/wiki/Ground_effect_%28aerodynamics%29&quot;&gt;Wikipedia&lt;/a&gt; for more details.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-08-22T16:17:39.910" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5221" PostTypeId="1" CreationDate="2014-08-24T17:43:27.093" Score="7" ViewCount="261" Body="&lt;p&gt;&lt;a href=&quot;http://www.bbc.com/news/world-europe-28910662&quot; rel=&quot;nofollow&quot;&gt;The British Broadcasting Corporation&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The European Space Agency (Esa) says the latest two satellites for Europe's version of the American GPS satellite navigation system have not gone into the correct orbit. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same article elaborates as follows&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Observations taken after the separation of the satellites from the Soyuz VS09 (rocket) for the Galileo Mission show a gap between the orbit achieved and that which was planned,&quot; said launch service provider Arianespace, in a statement. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The article is open on in-flight corrective measures&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Arianespace declined to comment on whether their trajectories could be corrected, the AFP news agency reports.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This remark may qualify as hear-say (because it is!). A space-craft is tracked from launch through orbit by highly qualified personnel using high-end instruments &amp;amp; techniques. In the present instance the satellites were launched into the wrong orbit. The team at launch &lt;strong&gt;may&lt;/strong&gt; have determined the discrepancy, and outcome thereof, from the flight-profile of the launch vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can a launch-provider determine from the flight-profile whether the payload will enter the wrong orbit?&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="1142" LastEditDate="2016-02-07T15:03:44.623" LastActivityDate="2016-02-07T15:03:44.623" Title="Can a launch-provider determine from the flight-profile whether the payload will be in the wrong orbit?" Tags="|launch-vehicle|flight-profile|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5222" PostTypeId="2" ParentId="5221" CreationDate="2014-08-25T01:58:38.500" Score="12" Body="&lt;p&gt;It's important to keep in mind two very important concepts:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Launch and orbit insertion are typically two distinct phases of flight.&lt;/li&gt;&#xA;&lt;li&gt;Ground controllers have very limited options during both phases.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Once the launch starts, about the only thing the mission control team can do is make the vehicle blow up. Most of those fantastic explosions you see of launch vehicles blowing up are because the mission control team made that happen. It's better to have a blown-up ex-rocket than a fully function but out of control rocket that hits some city. That didn't happen in this case. The launch appears to have been nominal. The problem instead apparently occurred during orbit insertion.&lt;/p&gt;&#xA;&lt;p&gt;ESA issued a &lt;a href=&quot;http://www.arianespace.com/news-press-release/2014/8-22-2014-orbital-injection.asp&quot; rel=&quot;noreferrer&quot;&gt;very short press release on August 22&lt;/a&gt; that merely stated the vehicles were in the wrong orbit. They issued a &lt;a href=&quot;http://www.arianespace.com/news-press-release/2014/8-23-2014.asp&quot; rel=&quot;noreferrer&quot;&gt;much longer press release the following day&lt;/a&gt; that stated what that meant.&lt;/p&gt;&#xA;&lt;p&gt;From that latter press release, the satellites were supposed to have been placed in a circular orbit with a semi-major axis of 29,900 kilometers and an inclination of 55 degrees. They are instead in an elliptical orbit with an eccentricity of 0.23, a semi-major axis of 26,200 km, and an inclination of 49.8 degrees. There's not much that can be done to fix the problem. What little fuel was left in the upper stage has been purged.&lt;/p&gt;&#xA;&lt;p&gt;My guess is that everything went more or less okay during the launch and during the first transfer burn. The problem arose during the final orbit insertion. Those numbers suggest a significant underburn at orbit insertion. Minor underburns (this was not a minor underburn) can be caused by a number of things such as navigation errors and erroneous sensors. Significant underburns such as this are more often a result of bigger problems such as engine failure or running out of fuel.&lt;/p&gt;&#xA;&lt;p&gt;So why didn't they see this on the ground? They did see this on the ground, but it took the mission control team a couple of hours to realize that the vehicles were in the wrong orbit. GPS doesn't work at that altitude. The only way to determine what orbit a high altitude vehicle is in is to make multiple range/range-rate measurements from the ground, spread out over time. Orbit determination isn't something that can be done by one single observation.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Update:&lt;/h3&gt;&#xA;&lt;p&gt;I didn't keep track of this answer. Today is October 27th, but the root cause was discovered over two weeks ago. The root cause was loss of attitude control due to frozen fuel lines on the Fregat upper stage. Hydrazine fuel lines that fed attitude control thrusters were routed too close to liquid helium lines that supplied helium used to pressurize the fuel tanks. Per the article &lt;a href=&quot;http://spectrum.ieee.org/tech-talk/aerospace/satellites/a-simple-plumbing-problem-sent-galileo-satellites-into-wrong-orbits&quot; rel=&quot;noreferrer&quot;&gt;A Simple Plumbing Problem Sent Galileo Satellites Into Wrong Orbits&lt;/a&gt; at IEEE Spectrum,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The fuel — hydrazine — didn’t reach the thrusters because it froze in its feed line, which was too close to another feed line carrying liquid helium. Both lines were mounted on a same support structure that, unfortunately, functioned as a thermal bridge cooling the hydrazine. The fault has also been discovered in other Fregat stages that are now under construction in Russia, and was apparently caused by ambiguities in the design documents of Fregat.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-10-27T22:28:19.747" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5223" PostTypeId="2" ParentId="5221" CreationDate="2014-08-25T17:00:38.903" Score="4" Body="&lt;p&gt;The simple answer is yes. Arianespace was asked to deliver the satellites to a given separation orbit. They track the rocket, so have data that tells what orbit was achieved.  They provide the separation orbit elements to the customer about 30 minutes after separation, then a more accurate set later. In this case the discrepancy is large enough to be easy to see in the first set of elements. Once the satellites are separated, Arianespace cannot do anything to correct the orbit. Arianespace does not officially know whether the satellites can fix the problem themselves, hence the last statement. Arianespace may in fact know, but it is not their place to make that announcement.&lt;/p&gt;&#xA;" OwnerUserId="176" LastEditorUserId="49" LastEditDate="2014-08-26T01:33:32.007" LastActivityDate="2014-08-26T01:33:32.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5224" PostTypeId="1" CreationDate="2014-08-25T18:43:00.373" Score="8" ViewCount="196" Body="&lt;p&gt;&quot;Lander&quot; is maybe not a good job description for the Rosetta sub probe Philae. I understand that there is no practically useful gravity there. Philae will try to hook itself onto the comet surface in order to get stationary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it has completed its primary mission on one surface site, could it unhook itself and float (fall? hop?) to another location and do new science there? Within its battery life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;em&gt;Could Philae just detach itself and tumble but still make use of its instruments on another part of the comet?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-08-25T20:55:57.000" LastActivityDate="2014-11-19T05:03:52.350" Title="Could the Philae comet lander &quot;hop&quot;?" Tags="|microgravity|rosetta|comet|robotic-missions|hopper|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5225" PostTypeId="1" AcceptedAnswerId="5230" CreationDate="2014-08-26T09:45:57.950" Score="21" ViewCount="5757" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_2&quot;&gt;Voyager 2&lt;/a&gt; started its journey some 37 years back so all of the gases must have been exhausted, but it's still moving at a speed of 15km/s. I can say that this might be due to no opposing force to slow down the speed. But if some space debris gets in its way, or any of the extra-planetary objects get in its way or comes too close, then how will that change its path?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the only the way to change the direction is by gases; are there any gases still present?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="58" LastEditDate="2020-03-11T18:02:00.310" LastActivityDate="2020-03-11T18:02:00.310" Title="Can Voyager still use its thrusters to avoid hazards?" Tags="|voyager|collision-avoidance|" AnswerCount="5" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5226" PostTypeId="1" AcceptedAnswerId="5233" CreationDate="2014-08-26T10:19:49.767" Score="8" ViewCount="2847" Body="&lt;p&gt;For example, if it takes a delta-v of 4 km/s to fly from LEO to Mars in 8 months, according to one mission plan, would it take half the time, 4 months, if the delta-v was doubled to 8 km/s? Is there any rule of thumb relationship, or must a completely new trajectory be calculated for any change of delta-v?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2019-02-25T12:50:09.217" Title="Is there a simple relation between delta-v and travel time?" Tags="|orbital-mechanics|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5227" PostTypeId="2" ParentId="5226" CreationDate="2014-08-26T11:06:35.960" Score="2" Body="&lt;p&gt;It's not necessary to take an entirely different route every time. There is the minimum energy transfer, that you mentioned, but you can use more fuel(or better yet, use a more efficient engine) to take a similar, but different route that takes you there faster. The raise in energy requirement is moderate at first, but becomes uncomfortable if you want to cut your travel time by more than a month or so. I think there is a table in a book I have at home (&amp;quot;Astronautics&amp;quot; by Ulrich Walter). I will let you know.&lt;/p&gt;&#xA;&lt;p&gt;Also, there are entirely different courses you can take, but I don't think any of them are faster. For example, in some constellations a Venus flyby gives you the chance to return after only a short stay at Mars, rather than waiting out a full cycle.&lt;/p&gt;&#xA;&lt;p&gt;Edit:&lt;/p&gt;&#xA;&lt;p&gt;I misremembered. In Astronautics I found the following two figures (which are even better; read below):&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WlDtB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WlDtB.png&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The image above makes three examples of orbits we could fly. You could fly even more extreme courses than the 70 day one, but you can see how the required earth escape speed &lt;span class=&quot;math-container&quot;&gt;$v_\infty$&lt;/span&gt; becomes uncomfortable.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dlNHT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dlNHT.png&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This graph gives us the the same in exact numerical terms. You can see that shortening the time a bit is quite cheap, but then it quickly becomes expensive. &lt;span class=&quot;math-container&quot;&gt;$t_x$&lt;/span&gt; is the transfer time. &lt;span class=&quot;math-container&quot;&gt;$t_H$&lt;/span&gt; is the time for the Hohmann-transfer. I won't explain all variables involved, I don't think anyone has the patience to read that.&lt;/p&gt;&#xA;&lt;p&gt;It is true that you also need an burn to turn into Mars' orbit, but I disagree with HopDavid about this being the major factor:&lt;/p&gt;&#xA;&lt;p&gt;The &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; for the for the injection is &lt;span class=&quot;math-container&quot;&gt;$v_\infty\sqrt{\frac{1-e}{2}}$&lt;/span&gt; where &lt;span class=&quot;math-container&quot;&gt;$e$&lt;/span&gt; is the eccentricity of the orbit around mars and &lt;span class=&quot;math-container&quot;&gt;$v_\infty$&lt;/span&gt; is the excess velocity with regard to mars now. This can be very high, because you can slowly circularize the orbit for free by aerobraking. For &lt;span class=&quot;math-container&quot;&gt;$e=0.95$&lt;/span&gt; the Mars injection takes only 16% of &lt;span class=&quot;math-container&quot;&gt;$v_\infty$&lt;/span&gt;. Of course, there is a limit to how eccentric your orbit can be, because you may escape Mars' sphere of influence if you come in too fast.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-02-25T12:50:09.217" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="5228" PostTypeId="2" ParentId="5225" CreationDate="2014-08-26T11:15:59.263" Score="26" Body="&lt;p&gt;Space is almost completely empty. The voyager probes are exceedingly unlikely to collide with anything (as demonstrated here: &lt;a href=&quot;https://space.stackexchange.com/questions/5069/what-is-the-possibility-of-voyager-1-2-colliding-with-matter-asteroids-or-plane/5114#5114&quot;&gt;What is the possibility of Voyager 1/2 colliding with matter (Asteroids or planetoids) present in space?&lt;/a&gt;) larger than a mote of dust. If they did collide with anything at 15 km/s, then it slowing them down would be the least of their worries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They do have some hydrazine fuel left, which they use to control their orientation (attitude). &lt;a href=&quot;http://www.space.com/11527-nasa-voyager-spacecraft-leave-solar-system.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.space.com/11527-nasa-voyager-spacecraft-leave-solar-system.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrazine, by the way, is not a gas, but a liquid. The probe thrusters catalytically split the hydrazine to produce gases for thrust.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-08-26T11:15:59.263" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5229" PostTypeId="2" ParentId="5226" CreationDate="2014-08-26T11:37:34.063" Score="4" Body="&lt;p&gt;A particular delta-v (relative to a given celestial body, such as the Moon) actually &lt;em&gt;implies&lt;/em&gt; a particular transfer orbit toward the target body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, delta-v is the &lt;em&gt;difference&lt;/em&gt; in velocity (produced by a particular maneuver), it is not an absolute velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's say you start out at 11 km/s relative to the Earth (in an approximate Earth-Moon system transfer orbit) and then near the Moon apply a further delta-v of 4 km/s along your direction of travel. Ignoring orbital effects, that will push your orbital speed to 15 km/s. If you instead double the delta-v (which means you need to bring along a &lt;em&gt;lot&lt;/em&gt; more fuel, which increases the fuel requirement for achieving the initial E-M transfer orbit, which means you need even more fuel just to get off the ground, and round and round we go in the rocket equation...) to 8 km/s, not only do you end up in a different orbit (so have to redo all the orbital calculations) but also your final velocity is only 19 km/s at most. You gain about 26% in terms of absolute velocity by doubling the delta-v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An astronaut on a spacewalk in Earth orbit might start out with an orbital velocity of on the order of 7-8 km/s (the orbital velocity of their spacecraft) and apply a delta-v of a fraction of a m/s in order to get to where they want to go. And so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a perhaps somewhat extreme but very real example, consider that &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13#Crew_survival_and_return_journey&quot; rel=&quot;nofollow&quot;&gt;Apollo could abort the 11 km/s transfer orbit to the moon by applying an approximate 1.8 km/s delta-v&lt;/a&gt; about a third to half of the way from the Earth to the Moon. (True &quot;abort&quot; would require a delta-v of twice the current velocity, to reverse the direction of travel while in the end resulting in the same orbital velocity in a reversed orbit. Even ignoring whether it's practical to &quot;reverse&quot; the orbit like that in the first place, applying an unplanned delta-v of well over 20 km/s to something as massive as the Apollo CSM, let alone the CSM/LM pair, is not an easy feat.) That abort wasn't an option in the case of Apollo 13, but it (and &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Direct_Abort_Trajectory_-_Lunar_Landing_Symposium,_MSC_Jun66.jpg&quot; rel=&quot;nofollow&quot;&gt;the illustrative graph&lt;/a&gt; shown on the Wikipedia page linked) goes to show some of how it's not quite as simple as &quot;double the delta-v, halve the time&quot; or even &quot;double the velocity, halve the time&quot;.&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2014-08-26T11:37:34.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5230" PostTypeId="2" ParentId="5225" CreationDate="2014-08-26T12:21:34.850" Score="29" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;But if some space debris comes on its way or any of the extra planetary objects comes on its way then how it changes its path?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Not only won't Voyager 2 change it's path, it can't change it's path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose you drop your cellphone from the top of a tall building. Your cellphone is going to fall and hit the ground, hard. Your cellphone has no sensors to detect the hazard, no effectors to avoid the hazard, and no hazard avoidance software to develop a plan that uses those non-existent effectors to avoid the hazard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now suppose one of those Voyager spacecraft is on a collision trajectory with some Oort cloud object. Just as your cellphone is going to hit the ground, hard, so will that Voyager spacecraft hit that object. The Voyager spacecraft have no way to detect that they are on a collision course because the cameras that took all of those beautiful images were powered off a long time ago. Even if they spacecraft could see the imminent collision, the spacecraft have so little fuel left in them that nothing could be done to avoid that collision. Finally, even if the spacecraft still had powered sensors to detect the collision and powered effectors to avoid the collision, it still couldn't avoid that collision (without help from the Earth) because the computers on those spacecraft and the software on those computers are extremely primitive.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-08-26T12:21:34.850" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="5231" PostTypeId="1" AcceptedAnswerId="5239" CreationDate="2014-08-26T13:03:20.397" Score="6" ViewCount="906" Body="&lt;p&gt;As a launch vehicle begins to veer slightly off course during a launch, what sort of algorithms are used to determine how far off the vehicle's trajectory is pointing from the intended trajectory and what thrust vector adjustments should be made? In my head I imagine you would use data from sensors to determine the thrust vector, which ideally would have a 0° angle from the intended or planned trajectory. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Great answers so far. Does anyone have specific examples of which control systems are used in modern day launch systems?&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-08-27T13:01:29.740" LastActivityDate="2021-09-24T12:47:30.403" Title="What algorithms are used to correct the flight-path?" Tags="|navigation|software|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5232" PostTypeId="1" AcceptedAnswerId="5234" CreationDate="2014-08-26T16:54:06.140" Score="14" ViewCount="1526" Body="&lt;p&gt;I was reading about the various TransHab proposals (at &lt;a href=&quot;http://www.astronautix.com/craft/traodule.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/craft/traodule.htm&lt;/a&gt;, among others), and I started to think about pushing the idea to its limits. Assuming you had a large enough payload area on a rocket (or space shuttle - once upon a time), you could launch some pretty big modules. Most of the plans called for multi-&quot;story&quot; modules, with different levels. But a much bigger, simpler version would, of course, simply have no levels, and be just one giant cavity. Reminiscent of Star Wars landing bays, right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there's a catch. Most (okay, &lt;em&gt;all&lt;/em&gt;) of the modules for space stations over the years are tight, cramped, and not for the overly claustrophobic. One upside is that you can pull yourself around using bars on the &quot;walls&quot;. In a large, cavernous module, that would be hard (unless you plan on using an MMU all the time!). Artificial gravity could make this even worse, by trapping any astronauts on a single side!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is this: Is there an upper limit to the interior of space stations because it becomes impossible to move around in a controlled fashion? Are space station modules doomed to be thin and cramped forever?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I don't think that this is a duplicate of &lt;a href=&quot;https://space.stackexchange.com/questions/819/are-there-any-theoretical-size-limits-of-man-made-space-stations-structures&quot;&gt;Are there any theoretical size limits of man-made space stations/structures?&lt;/a&gt;, simply because I am interested in the interior of such a space station - my hypothetical module could simply be the size of a gymnasium and still be of a problematic scale, as opposed to the miles-long structures discussed in the other question.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2020-01-24T03:49:48.340" Title="Is there an upper limit for the internal size of space stations?" Tags="|space-station|crew-module|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5233" PostTypeId="2" ParentId="5226" CreationDate="2014-08-26T17:10:11.217" Score="9" Body="&lt;p&gt;You seem to be thinking in terms of speed, a scalar quantity. Of course if you double speed, you cover a given distance in half the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But velocity is a vector quantity. It has direction as well as magnitude. Here is a diagram of adding vector quantities:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FYY27.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the above illustration, the red vector would be the delta V (change in velocity) in going from the blue vector to the black vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So changing speed isn't the only delta V. Changing direction also boosts delta V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example imagine two cars with different speeds 30 mph and 40 mph. If they're going the same direction in parallel lanes, the difference is 10 mph. If they T-Bone at an intersection, the difference is 50 mph. If they meet head on, the velocity difference has a 70 mph magnitude. Direction can make a huge difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an illustration of an earth to Mars Hohmann:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TIF2z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at the transfer orbit's velocity vectors at earth and at Mars. Notice the velocity vectors are pointing the same direction, so only a change in speed is needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now here's an illustration of a non Hohmann earth to Mars transfer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/biZep.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice that the mars velocity vector is nearly the same size as the transfer orbit's velocity vector when it crosses Mars' orbit. We don't need to speed up or slow down much. But changing direction requires a lot of delta V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Deviating from Hohmann for a 4 month trip would boost your delta V by a lot more than double. This is due to direction change non Hohmann transfers require.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are three more examples, transfer orbits depicted by Rikki-Tikki-Tavi in another answer to this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vu9Jg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Rikki's illustration 3 &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vinfinity&lt;/a&gt; quantities are given, those with regard to earth at departure. His illustration lacks the Vinfinity numbers with regard to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My illustration shows Vinfinity vectors as red arrows. It has Vinfinity quantities for both earth and Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside of a planet's sphere of influence, the transfer orbits can be modeled as ellipses or a parabola about the sun. But within a planet's sphere of influence, the path is better modeled as an ellipse about the planet. Vinfinity is a hyperbola's velocity at an infinite distance from the gravitating body. A &lt;a href=&quot;http://en.wikipedia.org/wiki/Hyperbolic_trajectory#Velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;hyperbola's speed&lt;/a&gt; can be found by $\sqrt{V_{esc}^2+V_{inf}^2}$. Escape velocity is $\sqrt{2GM/r}$. As you can see escape velocity grows larger as the planet gets closer. At the edge of a planet's sphere of influence, escape velocity is close to zero and the hyperbolic velocity is very close to Vinfinity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Escape velocity is around 5 km/s near Mars' surface. So a hyperbolic orbit grazing Mars' atmosphere would have a speed of $\sqrt{5^2+V_{inf}^2}$ km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Rikki's 3 examples this becomes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\sqrt{5^2+2.65^2}$ km/s which = ~5.7 km/s (the Hohmann orbit)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\sqrt{5^2+6.23^2}$ km/s which = ~8 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\sqrt{5^2+20.31^2}$ km/s which = ~21 km/s (the parabolic transfer orbit)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 5.7 km/s is what needs to be shed for Mars landing coming from a Hohmann orbit. Aerobraking can accomplish this for smaller pay loads but for more massive payloads this is hard. Only .7 km/s is needed to brake into a capture orbit about Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the 8 km/s hyperbola periapsis velocity, you will need to shed twice as much kinetic for a soft landing. 3 km/s delta V would be needed to brake into a capture orbit about Mars. Rikki seems to believe this is trivial, but it's not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a 21 km/s hyperbola periapsis velocity I don't think it's practical to use aerobraking for a soft landing. 16 km/s would need to be shed to brake into a capture orbit. 16 km/s is about what it takes to get from earth's surface to the moon's surface. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-08-28T01:48:16.767" LastActivityDate="2014-08-28T01:48:16.767" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5234" PostTypeId="2" ParentId="5232" CreationDate="2014-08-26T18:12:48.927" Score="15" Body="&lt;p&gt;Have you seen tours of the space station? The modules are not particularly cramped. Some parts are, especially where they connect with other modules, but most seem quite spacious actually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kibo, the Japanese module and also the largest module on the station, has seemed somewhat too spacious for the astronauts, as they could get stuck in the middle. [1]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From wikipedia, these are Kibo's dimensions, and thus a version of an answer to your question:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Length: 11.19 m (36.7 ft)&#xA;Diameter: 4.39 m (14.4 ft)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Since they could to get stuck in Kibo, I'd bet that it would be tough to build a module much larger without &quot;levels&quot; or some kind of infrastructure to get around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1] &lt;a href=&quot;http://www.universetoday.com/14967/space-station-astronauts-could-get-stranded-in-kibo/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.universetoday.com/14967/space-station-astronauts-could-get-stranded-in-kibo/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="27346" LastEditDate="2020-01-24T03:49:48.340" LastActivityDate="2020-01-24T03:49:48.340" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="5235" PostTypeId="1" AcceptedAnswerId="5237" CreationDate="2014-08-26T20:01:33.040" Score="11" ViewCount="1200" Body="&lt;p&gt;Let's say you have magical unlimited delta-v, but very low thrust-to-weight ratio. You could travel by accelerating constantly for half the trip, then braking for the other half, until your arrival.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember someone describing this maneuver and using a name, but I can't remember the name or find any references. What is it called?&lt;/p&gt;&#xA;" OwnerUserId="5999" LastEditorUserId="6006" LastEditDate="2014-09-17T17:56:21.993" LastActivityDate="2014-09-17T17:56:21.993" Title="What's the name of this maneuver for unlimited delta-v?" Tags="|orbital-mechanics|delta-v|terminology|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5236" PostTypeId="2" ParentId="4969" CreationDate="2014-08-26T20:34:41.200" Score="5" Body="&lt;p&gt;Here's a fairly abstract way to see how it can be done: consider two ships fairly close together, on similar but not identical orbits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's one vector running from the current position of ship A to ship B. If you thrust along that vector, the ships will tend to get closer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's another vector that represents the difference in velocities between the two ships. If you thrust along that vector, you can reduce the difference in velocities, i.e. make the two ships more stationary relative to one another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be clear that if those two vectors aren't directly opposed, there's some vector in between those two, which if you thrust along, will both close the range and reduce the relative speed. If you repeatedly compute and use those vectors, eventually you can get both range and relative speed arbitrarily close to zero, at which point you're docked.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-08-26T20:34:41.200" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5237" PostTypeId="2" ParentId="5235" CreationDate="2014-08-26T20:36:53.810" Score="14" Body="&lt;p&gt;That's a &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/torchships.php&quot;&gt;brachistochrone&lt;/a&gt; or constant-acceleration trajectory; Heinlein was fond of it and called the turnover the &quot;skew-flip maneuver&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-08-26T20:36:53.810" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5238" PostTypeId="2" ParentId="5231" CreationDate="2014-08-26T21:28:36.260" Score="5" Body="&lt;p&gt;There's a whole bunch of &lt;a href=&quot;http://en.wikipedia.org/wiki/Control_theory&quot; rel=&quot;noreferrer&quot;&gt;theoretical and practical study&lt;/a&gt; of this kind of problem. The most important algorithm is called the &lt;a href=&quot;http://en.wikipedia.org/wiki/PID_controller&quot; rel=&quot;noreferrer&quot;&gt;PID controller&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-08-26T21:28:36.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5239" PostTypeId="2" ParentId="5231" CreationDate="2014-08-27T00:34:32.160" Score="6" Body="&lt;p&gt;Another common technique is what is called a &lt;a href=&quot;http://en.wikipedia.org/wiki/Kalman_filter&quot;&gt;Kalman Filter&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simple PID controller has a controllable output, a sensor, and a feedback loop. The standard example is a car's cruise control: the output is the amount of fuel sent to the engine. The sensor is the speedometer. The speedometer's measurement is compared to the desired speed, and the output is adjusted. This allows the car to adjust the amount of gas as the car goes up and down hills.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this works well for many devices, more complicated systems have a number of controllable outputs, and must also deal with noisy sensor inputs. This is where the Kalman filter (and it's many derivatives) comes into play. It is often used for guidance systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic idea is that the Kalman filter keeps track of the state of the system, has a model of how the system works, and makes predictions based on this model. The predicted output can then be compared to new sensor readings, and the state knowledge is updated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if our car was actually a self driving car, we could use a Kalman filter to help it navigate. The filter starts with a known state of the car: stopped, wheel straight, pointing north, and at lat X and long Y. To reach a new destination, the filter can generate inputs (throttle and wheel angle), make predictions based on the physics model of the car (if I turn the wheel 90 degrees at highway speeds bad things happen), take sensor inputs (compass, speedometer, mileage gauge), and finally see if those predictions are reasonably close to the sensed environment. Repeat.&lt;/p&gt;&#xA;" OwnerUserId="4686" LastActivityDate="2014-08-27T00:34:32.160" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5240" PostTypeId="2" ParentId="5225" CreationDate="2014-08-27T07:59:04.880" Score="16" Body="&lt;p&gt;When Voyager was a shiny new probe just heading out back in 1977 (or Pioneer, Apollo, or really anything else we've sent beyond low earth orbit) it didn't have the ability to avoid hazards. It's trajectory was more-or-less fixed the moment the Titan booster cut it loose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel it carried was for course corrections - gentle nudges to go past Jupiter at just the right angle to get to Saturn in a few years. Yes, we could consider Jupiter to be a hazard it avoided but moon-size and larger hazards are very predictable - the Titan profile considered all of that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;True hazards, the ones that pop up without planning, come in &quot;dust speck&quot; and &quot;rocks and larger&quot; categories. We have nothing, even today, to detect dust specks coming toward us at spacecraft speeds and rocks would need to be pretty big to notice 24+ hours before collision. You need that much time to collect sensor data, return it to earth, analyze it, predict a collision, program a maneuver to avoid the collision, send it to the spacecraft and fire the thrusters to nudge it out of the way. You can't do this just-in-time without pretty big fuel reserves, but if you see it far enough out you only need 1 Newton in the right direction to avoid it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every spacecraft gets hit by dust. If it takes a big particle (say, 1mm) in something critical that component is dead. 1 gram at LEO speeds produces 22,000 joules on impact - this is more than a .50BMG round point-blank, and that round can split a truck engine in half. Rises to 144,000 Joules at Voyager's speed. That's a 30mm cannon round. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;if it gets hit by an impossible-to-see-coming apple-sized object, it's space junk. &lt;/p&gt;&#xA;" OwnerUserId="5933" LastEditorUserId="5933" LastEditDate="2014-08-27T11:07:13.913" LastActivityDate="2014-08-27T11:07:13.913" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5242" PostTypeId="2" ParentId="5232" CreationDate="2014-08-27T12:41:48.640" Score="9" Body="&lt;p&gt;As Nickolai suggested, this is more of a technical problem, and so there are several imaginable technical solutions. Reading the article he linked, it seems to be a temporary annoyance in Kibo, rather than a serious problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Swimming&quot; might work for relatively small distances. If modules get bigger, you would of course need a more sophisticated solution, such as a small hand-held cold gas thruster. Although it would take practice to operate in a way that doesn't send the astronaut spinning wildly. (This solution appears in the anime &quot;Planetes&quot;, which is excellent, btw)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simpler solution would be to create an air draft in the station that pushes the astronauts towards one side. This approach is of course limited in how large the module can be, because the required energy to create such a draft would become unreasonable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously some draft always has to exist in a space station, or the CO2 that the Astronauts produce wouldn't get transported away from them. So you could rescue yourself by sending the air conditioning into overdrive.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="23891" LastEditDate="2020-01-24T01:03:28.417" LastActivityDate="2020-01-24T01:03:28.417" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="5243" PostTypeId="2" ParentId="5231" CreationDate="2014-08-27T12:58:41.077" Score="6" Body="&lt;p&gt;I'd like to add a little to &lt;a href=&quot;https://space.stackexchange.com/a/5239&quot;&gt;Superdesk's answer about the Kalman filter&lt;/a&gt;. Superdesk gives a great practical description; I'd like to add a more theoretical characterisation: the way I like to think about the Kalman filter. It also leads to an interesting bit of historical trivia about the Kalman filter that I don't think many people know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The classical Kalman filter assumes that the observations of the parameters of the system model described in Superdesk's answer belong to a &lt;a href=&quot;http://en.wikipedia.org/wiki/Gaussian_process&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gaussian random process&lt;/a&gt;. The aim of the Kalman filter is to estimate the parameters of these hidden Markov processes and to update the parameter estimates with their &lt;em&gt;maximum likelihood values&lt;/em&gt; (i.e. the values that maximize the a prior probability of observing what is actually observed) with each new measurement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now imagine you have a model of a process - a driving car as in Superdesk's answer. You could simply take the car's whole sensor output history at each sensor as well as your model and brute force compute the parameters that give the maximum likelihood. You'd be doing the same thing as the Kalman filter, but this is NOT the Kalman filter. You'd also be doing a huge amount of number crunching, with an increasing workload at each step as your histories get longer and longer. It would be an impractical approach, especially in the early Apollo missions!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No, the point of the Kalman filter as opposed to general, brute force maximum likelihood estimation is that, as each new measurement comes in, the parameters of the (assumed) Gaussian &lt;a href=&quot;http://en.wikipedia.org/wiki/Probability_density_function&quot; rel=&quot;nofollow noreferrer&quot;&gt;probability distribution functions&lt;/a&gt; are updated by simple recurrence relationships that make use of former parameter estimates of these parameters and the new observation ALONE. You don't have to go back and get all the old observational data and do a maximum likelihood estimate from the newly augmented full dataset from scratch. So the Kalman filter is a maximum likelihood estimator, but it achieves this with vastly less computation than brute force maximum likelihood estimation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Indeed, the Kalman filter was invented to drastically reduce HAND statistical calculations. For, even though we credit Rudolf Kalman with its invention, it was in fact invented by the great mathematician Carl Friedrich Gauss and first published by him in 1809, 150 years before Kalman. Gauss used the Kalman filter to simplify hand calculations needed to find optimal estimates of planetary orbits from astronomical observations. You can see that a simple recursive algorithm involving only the parameter estimates and most recent data would make hand calculation practicable: brute force maximum likelihood estimation would have been impossible. Kalman was unaware of Gauss's work, and indeed his method of proof that the simple algorithm is indeed maximum likelihood was quite different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kalman actually only found respect and credit for his work when he proposed it to Stanley_F._Schmidt and the latter led to the Kalman filter's adoption as a key path estimator during the Apollo missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://www.le.ac.uk/users/dsgp1/COURSES/MESOMET/ECMETXT/recurse.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this exposition, &quot;Recursive Estimation and the Kalman Filter&quot; in D.G.S. Pollock's &quot;Kalman Filters&quot;&lt;/a&gt; on the history.&lt;/p&gt;&#xA;" OwnerUserId="6005" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-08-27T13:37:51.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5244" PostTypeId="2" ParentId="5187" CreationDate="2014-08-27T21:31:59.730" Score="17" Body="&lt;p&gt;&lt;strong&gt;From the &lt;a href=&quot;http://www.roscosmos.ru/457/&quot; rel=&quot;noreferrer&quot;&gt;description of Soyuz-U on the Roscosmos site&lt;/a&gt;:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Разделение второй и третьей ступеней происходит по «горячей схеме».&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Третья ступень (блок «И»), состоящая из переходного отсека, бака горючего, бака окис­лителя, хвостового отсека и двигателя,&#xA;  уста­новлена на центральном блоке и соединена с ним с помощью&#xA;  ферменной конструкции.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Блок «И» снабжен двигательной уста­новкой, состоящей из&#xA;  четырехкамерного двигателя однократного включения и четы­рех&#xA;  поворотных рулевых сопел, используе­мых для управления полетом по трем&#xA;  осям. Маршевый двигатель третьей ступени вклю­чается примерно за две&#xA;  секунды до отклю­чения центрального блока.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Газы, истекающие из сопел двигателя третьей ступени, непосредственно отде­ляют ступень от центрального блока.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;I have slightly edited the google translation, to make it a little more intelligible:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The second and third stage separation is done &quot;the hot way&quot;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The third stage (block &quot;I&quot;), consisting of the transfer compartment, &#xA;  fuel tank, oxidizer tank, tail section and the engine is&#xA;  mounted on the central unit and connected to it by a truss.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Block &quot;I&quot; is equipped with a propulsion system consisting of a&#xA;  four-chambers single start engine, and four rotary steering nozzles used for&#xA;  flight control in three axes. The main engine of the third stage is&#xA;  started about two seconds before the center unit shuts off.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The gases flowing out of the third stage engine nozzles directly &#xA;  separate the stage from the central unit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The last sentence seems to be the answer to your question.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2014-08-28T12:53:49.733" LastActivityDate="2014-08-28T12:53:49.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5245" PostTypeId="2" ParentId="5187" CreationDate="2014-08-28T08:18:11.963" Score="30" Body="&lt;p&gt;When you shut down a rocket stage, its acceleration drops to zero and the rocket becomes weightless. As a result, the propellants may start sloshing around their tanks. This could lead to gas entering the pipes leading to the engines (since the propellant will have 'sloshed' away from the pipe entrance). If a gas pocket were to hit the engine's turbopump, even a small one would probably lead to destruction of the turbopump, and ultimately the destruction of the rocket.&#xA;There are &lt;a href=&quot;https://space.stackexchange.com/questions/2446/starting-a-liquid-fuel-engine-in-zero-gravity&quot;&gt;several ways to avoid this&lt;/a&gt;. Many rockets use small ullage motors to provide some acceleration and settle the propellants. So you fire the ullage motors, and then you can start the main engines.&lt;br&gt;&#xA;The Russians went for a less complex design: by starting the third stage while the second stage is still attached, they can avoid ullage motors and the precise timing they require. The stage separation mechanism also becomes less complex (as Horsh pointed out).&lt;br&gt;&#xA;The disadvantage is they'll have to cover the top of the second stage in insulation, to prevent the hot gases from burning a hole in the second stage. So, less complexity but more weight, and probably more drag due to the open lattice of the interstage. This method was also used on the N-1. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-08-28T20:19:13.020" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5246" PostTypeId="1" AcceptedAnswerId="5248" CreationDate="2014-08-28T15:57:50.637" Score="53" ViewCount="20012" Body="&lt;p&gt;AFAIK, NASA and others are using mainly gold to protect surfaces from heat radiation.&lt;/p&gt;&#xA;&lt;p&gt;But, as we can see with our own eyes, chrome or silver had probably a much better albedo, because they are white, with very good reflectivity - eligible to make mirrors. Gold is yellow, thus it doesn't reflect very well, at least not in the whole visible spectrum.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/w53F7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/w53F7.png&quot; alt=&quot;Satellite multi-layer insulation&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Thus, why is gold used?&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="286" LastEditDate="2021-09-07T20:22:54.827" LastActivityDate="2022-05-07T01:24:28.090" Title="Why is gold used in space technology to protect from heat radiation?" Tags="|protection|heat-shield|heat|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="5247" PostTypeId="2" ParentId="5232" CreationDate="2014-08-28T20:44:29.043" Score="13" Body="&lt;p&gt;With respect to &quot;getting stuck&quot; in a large open space in an orbiting space station,  I believe the dangers have been exaggerated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First:  A &quot;stuck&quot; astronaut would need to work pretty hard to &lt;strong&gt;get&lt;/strong&gt; stuck.  If he/she is moving from the wall of the compartment to the middle, it's tricky to stop in mid-air.  There &lt;em&gt;could&lt;/em&gt; be a line run from one side to the other, then removed. Or fellow astronauts could do it to a unsuspecting victim.  Sort of the orbital equivalent to being stuffed in a high-school locker...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, say you are stuck.  This means that you are in your own orbit around the earth, unaffected by anything the space station does.  The center of mass of the space station is following &lt;strong&gt;its&lt;/strong&gt; orbit, and all the parts of the station (except you) are pulled along in the same orbit by the structural integrity of the station.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the center of mass of the space station is located inside the large space, and that's where you're stuck, well, then you are stuck (except see below).  As a bonus, you'll still be stuck if someone changes the station's orientation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Picture the line traced by the center of mass of the station as it orbits and how this line passes through the structure of the space station. You can only be stuck, motionless, if you are located on this line.  You can only be stuck, with oscillations, if you are located close to this line. The oscillations get bigger, around the line, as you get farther from the line. If the oscillations are too big, you'll get close enough to something to grab it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, if you are stuck, you are no longer affected by any decay in the space station orbit.  The ISS changes speed, and thus altitude, because of the slight amount of air friction, and must be periodically boosted back up.  Eventually the space station will come to meet you.  The time scale for rescue would depend on the state of the earth's atmosphere and the shape and orientation of the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, you could always sneeze, or ...But I digress...&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2014-08-28T20:44:29.043" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5248" PostTypeId="2" ParentId="5246" CreationDate="2014-08-28T20:45:00.347" Score="90" Body="&lt;p&gt;It's not actually gold, but I think this is a common misconception, so allow me to elaborate for a bit.&lt;/p&gt;&#xA;&lt;p&gt;The stuff that you see satellites covered in is not normal foil, its just the outer layer of so-called &amp;quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Multi-layer_insulation&quot; rel=&quot;noreferrer&quot;&gt;Multi Layer Insulation&lt;/a&gt;&amp;quot; or MLI. That means that there are several layers of foil, each separated by a spacer, so that transfer between the layers happens by radiation, not heat conduction. This severely limits how much heat is transfered between the two sides of the insulation.&lt;/p&gt;&#xA;&lt;p&gt;That means that the satellite's sensitive parts (the batteries are usually most critical) are protected from fluctuations of the temperature on the surface, such as when the satellite passes through the shadow of the earth.&lt;/p&gt;&#xA;&lt;p&gt;However, if the satellite stays out there for years, no thermal resistance in the world will keep the satellite from eventually reaching an equilibrium temperature, where the amount of energy that goes into the satellite matches what goes out. But you can influence at which temperature this happens, by using special surfaces.&lt;/p&gt;&#xA;&lt;p&gt;To illustrate this, let's look at some examples from more-or-less daily life:&lt;/p&gt;&#xA;&lt;p&gt;When you put something into a convector oven (which works by circulating hot air), let's say at 200°C (400°F), it will eventually also be 200°C hot. The same thing happens if you heat something with infrared light from all directions. If your light source is 400°C (750°F) hot, eventually the object that the light falls on, will also be 400°C hot.&lt;/p&gt;&#xA;&lt;p&gt;However, have you ever noticed how on a hot summer day, a gray concrete wall in the sunshine is only warm to the touch, while stainless steel, for example of a slide, can be scorching hot? Doesn't the polished metal reflect more light than the gray wall? The special space foil of satellites would barely get warm at all.&lt;/p&gt;&#xA;&lt;p&gt;The answer to why this is lies in the difference of wavelength of the emitted and the absorbed light.&lt;/p&gt;&#xA;&lt;p&gt;Things, in general, emit radiation with a wavelength inversely proportional to their temperature, this is &lt;a href=&quot;http://en.wikipedia.org/wiki/Black-body_radiation&quot; rel=&quot;noreferrer&quot;&gt;Black Body Radiation&lt;/a&gt;. So the Sun radiates visible light (because it is very hot), while you radiate infrared light, which has a much higher wavelength (because you are way cooler).&lt;/p&gt;&#xA;&lt;p&gt;How much of that energy do things radiate? It depends on the emittance &lt;span class=&quot;math-container&quot;&gt;$\epsilon$&lt;/span&gt; of that object's surface. High emittance means that the object will cool faster by radiation, if the surroundings are cool. Low emittance means that the object will stay warm longer, even if the environment is cool.&lt;/p&gt;&#xA;&lt;p&gt;Another quantity is &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt;, the absorptivity. High &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt; means that the object will heat faster in our infrared oven. The factors &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\epsilon$&lt;/span&gt; are the difference between black body radiation, and &amp;quot;gray body radiation&amp;quot; which is basically the same. These factors just account for the fact that not all surfaces are black, meaning that some surfaces reflect light.&lt;/p&gt;&#xA;&lt;p&gt;Funny thing: For any given wavelength &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt; is always the same as &lt;span class=&quot;math-container&quot;&gt;$\epsilon$&lt;/span&gt;. That is why in our radiation oven (where the IR light comes from all directions), the object will assume the temperature of the oven, even if the oven is in a vacuum.&lt;/p&gt;&#xA;&lt;p&gt;But the sunlight comes just from one direction, so the satellite (or the wall, or the slide) can radiate to the other directions and will never be as hot as the surface of the sun, so the blackbody light it emits has a different wavelength than the radiation it receives. And that's important, because while &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\epsilon$&lt;/span&gt; are always equal to each other, they are different for different wavelengths.&lt;/p&gt;&#xA;&lt;p&gt;(I advise you take this in for a second before reading on)&lt;/p&gt;&#xA;&lt;p&gt;Usually when we talk about the absorptivity &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt;, we mean the absorptivity in the sun's light, &lt;span class=&quot;math-container&quot;&gt;$\alpha_s$&lt;/span&gt; (s for solar), because that is the absorptivity for the light, that the satellite mostly receives. Since the satellite's temperature causes it to emit infrared light, the &lt;span class=&quot;math-container&quot;&gt;$\epsilon$&lt;/span&gt; we usually talk about is &lt;span class=&quot;math-container&quot;&gt;$\epsilon_{IR}$&lt;/span&gt; (IR for... you know).&lt;/p&gt;&#xA;&lt;p&gt;A body with high a high ratio of &lt;span class=&quot;math-container&quot;&gt;$\frac{\alpha_s}{\epsilon_{IR}}$&lt;/span&gt; will get very hot in the sun, even if both values are very low. Both values are quite low for the slide's polished steel, but &lt;span class=&quot;math-container&quot;&gt;$\epsilon_{IR,Steel}=0.05$&lt;/span&gt; is lower still than &lt;span class=&quot;math-container&quot;&gt;$\alpha_{s,Steel}=0.37$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;As a sidenote, absorptivity+reflectivity+transmittance=1, this means that all light gets either absorbed, or reflected or let through a surface.&lt;/p&gt;&#xA;&lt;p&gt;Because the sun is very bright in space, we usually therefore want to create a surface that emits a lot in the infrared and absorbs little at the sun's most powerful wavelengths.&lt;/p&gt;&#xA;&lt;p&gt;To that end, manufacturers of Multi Layer Insulation use the second surface mirror effect:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/VcksD.png&quot; alt=&quot;SSM&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;(image from Multek/Sheldahl catalog, presumed in fair use)&lt;/p&gt;&#xA;&lt;p&gt;They use a material that is transparent to the sun's radiation and highly emitting in the infrared as a cover (substrate). This is often brownish Kapton. Below that they have a material that reflects as much of the sun's light as possible, which usually silver or aluminum. Gold would be a bad choice, because, as you correctly pointed out, it has quite high absorptivity.&lt;/p&gt;&#xA;&lt;p&gt;That way they have the lowest possible ratio of &lt;span class=&quot;math-container&quot;&gt;$\frac{\alpha_s}{\epsilon_{IR}}$&lt;/span&gt;, which means that the equilibrium temperature of the satellite stays the lowest possible.&lt;/p&gt;&#xA;&lt;p&gt;So what you see is not gold, but a thin layer of brown plastic above a surface of silver or aluminum.&lt;/p&gt;&#xA;&lt;p&gt;See also:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.thermalfluidscentral.org/encyclopedia/index.php/Absorptivity_and_emissivity_of_metal_and_deposited_coating_on_metals&quot; rel=&quot;noreferrer&quot;&gt;Some more values of &lt;span class=&quot;math-container&quot;&gt;$\epsilon$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="47149" LastEditorDisplayName="user20636" LastEditDate="2022-05-07T01:24:28.090" LastActivityDate="2022-05-07T01:24:28.090" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="5249" PostTypeId="2" ParentId="5199" CreationDate="2014-08-29T15:48:41.223" Score="7" Body="&lt;p&gt;I did some research, and according to &lt;a href=&quot;http://www.rmki.kfki.hu/~szalai/Publications/Bibring-Philae-2006-fulltext.pdf&quot;&gt;this&lt;/a&gt; source, Philae has only one, upward-pointing, thruster, the &quot;Active Descent System&quot;. It has no downward-facing thrusters that could kick up dust. This thruster will likely only be used when Philae is already on the ground to keep it there while it anchors itself, not for course correction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rosetta contains a mechanism to eject Philae with an adjustable force, in order to send in on the right path. However, if that mechanism fails, there is also a backup mechanism to eject Philae with a fixed pre-defined force. If that method is used, they may need the cold gas thruster to adjust the course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect they will be careful not to point it at the comet.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-08-29T15:48:41.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5250" PostTypeId="1" CreationDate="2014-08-29T17:13:55.587" Score="13" ViewCount="4122" Body="&lt;p&gt;Let's imagine a rocket like the Falcon 9 recovers its first stage via a soft landing, but doesn't attempt a return to the launchpad. For the sake of argument, let's imagine that there are no geographic constraints on the landing location, as would be the case in an ocean barge recovery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roughly where does that put us? In such a scheme, what is a reasonable number for the distance between the launchpad and the recovery location? There's no doubt we have plenty of historical precedent, but I'm sure there's fairly broad interest in the subject and I can't manage to easily look this up myself.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="51" LastEditDate="2015-07-12T14:00:27.510" LastActivityDate="2021-08-11T09:58:21.407" Title="How far downrange would a reusable Falcon 9 first stage land (without fly-back)?" Tags="|spacex|reuse|falcon-9|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5251" PostTypeId="2" ParentId="5250" CreationDate="2014-08-29T19:29:33.477" Score="7" Body="&lt;p&gt;The difference between landing downrange, and fly-back is only a matter of degrees. That is, to land downrange you still expend fuel slowing your forward motion.  With fly-back, you expend more fuel to do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon has said that 15-30% of payload mass is the penalty for reuse, depending on where they recover the stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good possible option is to launch from Brownsville, TX (Boca Chica Beach) and recover in Florida.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exact numbers would be interesting to see. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-08-29T19:29:33.477" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5252" PostTypeId="1" CreationDate="2014-09-01T03:24:10.660" Score="17" ViewCount="1502" Body="&lt;p&gt;Has anyone admitted to having sex in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What policies/protocols are there in place for this kind of activity?&lt;/p&gt;&#xA;" OwnerUserId="6027" LastEditorDisplayName="user20636" LastEditDate="2019-12-11T08:31:09.710" LastActivityDate="2019-12-11T08:31:09.710" Title="Has anyone had sex in space?" Tags="|health|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="5253" PostTypeId="1" CreationDate="2014-09-01T07:18:18.297" Score="24" ViewCount="1573" Body="&lt;p&gt;What sort of paths would payloads follow after being released from a space elevator such as the one described in Clarke's &lt;em&gt;Fountains of Paradise&lt;/em&gt;?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-09-01T07:28:33.607" Title="What's the path of something dropped from a space elevator" Tags="|space-elevator|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5254" PostTypeId="2" ParentId="5253" CreationDate="2014-09-01T07:28:33.607" Score="26" Body="&lt;p&gt;After being released from an elevator, the payload and earth are a 2-body system. The paths in 2 body systems are conic sections: ellipses, parabola, and hyperbolas. What sort of conic section depends on where it is released from the elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The elevator makes a revolution each &lt;a href=&quot;http://scienceworld.wolfram.com/astronomy/SiderealDay.html&quot; rel=&quot;noreferrer&quot;&gt;sidereal day&lt;/a&gt;.   The angular velocity (aka ω) is 2 π radians/sidereal day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speed of an object in &lt;a href=&quot;http://www.yorku.ca/marko/PHYS1010/Class09.pdf&quot; rel=&quot;noreferrer&quot;&gt;uniform circular motion&lt;/a&gt; is ωr. In this case r is the point’s distance from earth’s center. So called centrifugal acceleration is $ω^2r$. This is a push away from the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides $ω^2r$, there is another acceleration in the opposite direction: gravity. This acceleration is $GM/r^2$. When r=42,164 kilometers,  $ω^2r$ and $GM/r^2$ exactly cancel. This is the orbit radius of geosynchronous satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For convenience let’s choose sidereal days as our time unit and geosynch radius as our unit of length. Then orbital period is $r^{3/2}$ sidereal days. For example if the semi-major axis of an orbit is 4 geosynch lengths, then it’s period is 8 sidereal days. If a = 9, the period is 27 sidereal days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using these units  our speed ωr becomes 2πr/sidereal day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An orbit’s &lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_relative_angular_momentum#Definition&quot; rel=&quot;noreferrer&quot;&gt;specific angular momentum&lt;/a&gt;  is &lt;strong&gt;r&lt;/strong&gt; X &lt;strong&gt;v&lt;/strong&gt; where &lt;strong&gt;r&lt;/strong&gt; is position vector and &lt;strong&gt;v&lt;/strong&gt; is velocity vector. The magnitude of the specific angular momentum is rv sinφ where φ is the angle between position and velocity vectors. For objects traveling a circular path, the angle between  position and velocity vectors is 90º. Sin 90º is 1, so the magnitude of angular momentum is simply rv.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recall the speed is  ωr. So $rv= ωr*r = ωr^2$ which is $2πr^2$/sidereal day&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_relative_angular_momentum#Elliptical_orbit&quot; rel=&quot;noreferrer&quot;&gt;Angular momentum is also equal to twice the elliptical orbit’s area divided by it’s period&lt;/a&gt;. An ellipse’s area is πab where a is semi major axis and b is semi minor axis. $b=(1-e^2)^{1/2}$ where e is eccentricity. So twice the ellipse’s area would be $2πa^2(1-e^2)^{1/2}$ With the units we’re using the orbital period would be $a^{3/2}$ sidereal days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the magnitude of the specific angular momentum is&#xA;$$\frac{2πa^2(1-e^2)^{1/2}}{a^{3/2}}$$ square geosynch lengths per sidereal day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{2πa^2(1-e^2)^{1/2}}{a^{3/2}}$$ reduces to $$2π(a(1-e^2))^{1/2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now we have&#xA;$2πr^2= 2π(a(1-e^2))^{1/2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which reduces to&#xA;$r^2= (a(1-e^2))^{1/2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the velocity vector is perpendicular to the position vector, r is either the perigee or apogee of the orbit. If released below geosynch, it’s apogee. If released above geosynch r is the perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If r is the perigee, r = (1-e)a. If r is the apogee, r=(1+e)a.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These equations can be rewritten  a = r/(1-e) or a = r/(1+e)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting these values of a into $r^2= (a(1-e^2))^{1/2}$ we get&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$r^2= (r(1-e^2)/(1-e))^{1/2}$ or $r^2= (r(1-e^2)/(1+e))^{1/2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These reduce to&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$e=1-r^3$ or $e=r^3-1$ depending on whether r is more or less than 1 geosynch length.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Knowing the eccentricities that correspond to a specific r, we can draw orbits of payloads released from various locations on the elevator:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IQX1v.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When r=1, the orbit is circular. I colored this orbit blue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may notice I cut the illustration off at $r=2^{1/3}$ geosynch lengths. As r approaches $2^{1/3}$, e approaches 2-1. When e reaches 1, the orbit is a parabola. When e exceeds 1, the orbit is a hyperbola. My argument rests on regarding specific angular momentum as twice an elliptical orbit’s area over the orbital period. For parabolas and hyperbolas those quantities aren’t defined. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does $e = r^3-1$ when $r &amp;gt;2^{1/3}$? I suspect it does. But this is only conjecture on my part.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-09-01T07:28:33.607" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5256" PostTypeId="1" CreationDate="2014-09-01T15:55:05.223" Score="6" ViewCount="417" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/exploration/systems/mpcv/#.VASWDN-7z0o&quot;&gt;Orion&lt;/a&gt; is designed to land in the ocean and is therefor not reusable after landing (only some internal systems will be reused). But can Orion be reused while in space? What would the main issues be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could it for example be used as a shuttle between ISS and the captured asteroid in the ARM mission? The crew would land with Soyuz or &lt;a href=&quot;http://www.spacex.com/dragon&quot;&gt;DragonRider&lt;/a&gt;. The propulsion stage of Orion would be refueled and its crew will be supplied as two or more out of the regular ISS crew. Or as a shuttle between ISS and a (&lt;a href=&quot;http://www.bigelowaerospace.com/ba330.php&quot;&gt;BA330&lt;/a&gt;?) station orbiting the Moon, from which the crew lands on the Moon with a modified DragonRider?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-09-02T02:50:55.637" Title="Can the Orion be reused in space?" Tags="|reentry|crewed-spaceflight|orion-spacecraft|reuse|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5257" PostTypeId="1" CreationDate="2014-09-01T16:34:17.340" Score="7" ViewCount="910" Body="&lt;p&gt;Dragon will land on Earth using only the propulsion from its rocket engines. Has any operational or test spacecraft done that before? If not, what is the highest altitude from which propulsion-only landing has been demonstrated?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-09-16T10:00:12.750" Title="Has any spacecraft landed on Earth with propulsion only? Or else, from what highest altitude?" Tags="|propulsion|spacecraft|landing|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5258" PostTypeId="2" ParentId="5257" CreationDate="2014-09-01T19:30:37.830" Score="8" Body="&lt;p&gt;I think &lt;a href=&quot;http://en.wikipedia.org/wiki/McDonnell_Douglas_DC-X&quot;&gt;DC-XA&lt;/a&gt; in 1996 has the record, a landing from 3km altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More recently, SpaceX's &lt;a href=&quot;http://en.wikipedia.org/wiki/Grasshopper_(rocket)&quot;&gt;Grasshopper and Falcon&lt;/a&gt; development vehicles have done powered landings from altitudes on the order of 1km, and a number of other companies such as Armadillo Aerospace and Masten Space Systems have demonstrated powered landings with small test craft from low altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More info &lt;a href=&quot;http://en.wikipedia.org/wiki/VTVL&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-09-01T19:30:37.830" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5259" PostTypeId="2" ParentId="5256" CreationDate="2014-09-02T02:50:55.637" Score="7" Body="&lt;p&gt;The big question is if the Service Module can be refueled in orbit. It is based on the ESA's ATV, which can transfer fuel from itself to the Zvezda module of the ISS. So could it flow fuel in reverse? Seems like a smaller step, than being able to transfer fuel at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However it seems that the main engine on the ATV uses different fuel (same oxidizer though) than is used by Zvezda.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quoting from &lt;a href=&quot;http://en.wikipedia.org/wiki/Automated_Transfer_Vehicle&quot;&gt;Wikipedia on the ATV&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Up to 4,700 kilograms (10,400 lb) of propellant for the re-boost maneuver and refueling the station. The ATV propellant used for re-boost (monomethylhydrazine fuel and N2O4 oxidizer) is of a different type from the payload refueling propellant (UDMH fuel and N2O4 oxidizer).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Then the question becomes life support consumables. Can they be refilled on orbit? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Uses solar arrays for power, so no fuel cells to worry about refueling.  What is the battery life, since the sun is not always available.  So that might limit life span of the vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another issue is the seals around the fuel supplies. The propellants used are UMDH or MMH are amazing corrosive, and there are life spans for the seals. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-09-02T02:50:55.637" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5260" PostTypeId="1" AcceptedAnswerId="5262" CreationDate="2014-09-02T03:45:27.520" Score="1" ViewCount="8465" Body="&lt;p&gt;I am wondering to know if it's possible to stand still (no movement at all with respect to the Sun) in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can it be possible? I am thinking that when we move upwards from Earth's surface an upward thrust is needed. When we reach space, if we apply an equivalent opposite force, then can velocity go to zero and hence stand still in space?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="1142" LastEditDate="2017-01-23T22:58:03.500" LastActivityDate="2021-11-28T17:49:01.760" Title="Is it possible to stand still in space?" Tags="|spacecraft|geostationary|the-sun|thrust|" AnswerCount="6" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="5261" PostTypeId="2" ParentId="5260" CreationDate="2014-09-02T18:06:09.657" Score="12" Body="&lt;p&gt;In the way you defined it, no motion relative to the sun, it is not possible to stand still, because the sun itself is not a solid object. Instead different latitudes rotate with different velocities. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_rotation&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Solar_rotation&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could stand still with regard to the sun's equator by being in a tight orbit around the sun with an orbital period of 24.47 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I understand you correctly, however, you don't want to be in an orbit around the sun. If this is the case, you need to keep firing your engine, so you are not pulled into the sun. As HopDavid correctly pointed out to you, the most plausible concept for this is a Statelite. However, the sun will still be spinning below you, so you are still not fixed in place with regard to most of the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From your question and subsequent replies, I think maybe you may enjoy reading a basic textbook on orbital mechanics. I don't think stackexchange can give you the structured introduction that you need in order to move on in this complicated subject.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2017-01-23T10:27:04.117" LastActivityDate="2017-01-23T10:27:04.117" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5262" PostTypeId="2" ParentId="5260" CreationDate="2014-09-02T19:25:14.483" Score="0" Body="&lt;p&gt;Yes, it is possible to be essentially standing still in relationship to the Sun.  There are many comments and an answer telling you why you can't and, as far as they go, they are correct.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you move far enough away from our sun, it become just another star.  If you move to about half way between Earth and &lt;a href=&quot;https://en.wikipedia.org/wiki/Andromeda_Galaxy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Andromeda&lt;/a&gt;, each galaxy will be so far away from you that no individual sun will have any reasonable impact on your position in space.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are, of course, many things to consider, the spinning of the Sun, the Sun's orbit around the &lt;a href=&quot;https://en.wikipedia.org/wiki/Milky_Way&quot; rel=&quot;nofollow noreferrer&quot;&gt;Milky Way&lt;/a&gt;, the movement of all the galaxies in the Universe.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you travel about 1.25 million light years towards Andromeda, and kill your relative speed, you could stand essentially still in regards to our sun.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="1142" LastEditDate="2017-01-23T22:55:15.507" LastActivityDate="2017-01-23T22:55:15.507" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5263" PostTypeId="1" CreationDate="2014-09-03T06:12:52.560" Score="0" ViewCount="200" Body="&lt;p&gt;Space Colonization has become a popular trend in Sci-Fi movies. It would be a thrilling experience if we could travel from planet to planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have we found another earth-like planet close enough for us to travel there for a Honeymoon and back to earth whenever we want?&lt;/p&gt;&#xA;" OwnerUserId="6036" LastEditorUserId="109" LastEditDate="2014-09-03T13:12:06.883" LastActivityDate="2014-09-03T13:12:06.883" Title="Have we found another earthlike planet?" Tags="|colonization|planet|exoplanet|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5264" PostTypeId="2" ParentId="5257" CreationDate="2014-09-03T08:03:03.610" Score="4" Body="&lt;p&gt;The Apollo Lunar Module did propulsive-only landings on the Moon from an &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_11g_Lunar_Orbit_Phase.htm&quot; rel=&quot;nofollow&quot;&gt;altitude of 110 km and a speed of 1625 m/s&lt;/a&gt;.&lt;br&gt;&#xA;For Earth, propulsive-only landing from orbit is not popular because the weight of the fuel would be much higher than the weight of aerodynamic landing equipment (shielding, parachutes).&lt;br&gt;&#xA;Also, even in a propulsive landing, atmospheric drag is going to contribute something to the deceleration. Pure propulsive landings on Earth would be very inefficient (decelerate to 0 above the atmosphere, then keep running the engines to slowly descend through the atmosphere). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-09-03T08:03:03.610" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5265" PostTypeId="2" ParentId="5263" CreationDate="2014-09-03T12:38:03.083" Score="1" Body="&lt;p&gt;No and there will never be such a thing. Basic relativity theory predicts that any space ship traveling faster than the speed of light could also be used as a time machine, and of course with a time machine come many paradoxes. One of them is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Grandfather_paradox&quot; rel=&quot;nofollow&quot;&gt;grandfather paradox&lt;/a&gt;, but the more your study the topic, the more paradoxes pop up. From my experience with physicists, the more they study relativity, the more certain they are that light speed cannot be broken. For an introduction I suggest you read &quot;The Elegant Universe&quot; by Brian Greene.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are finding more and more promising candidates for possibly habitable worlds, however, even though we have only studied a &lt;a href=&quot;http://en.wikipedia.org/wiki/Kepler_%28spacecraft%29#Field_of_view&quot; rel=&quot;nofollow&quot;&gt;tiny portion&lt;/a&gt; of the sky in detail. This may lead one to conclude that a habitable world is within a few light-years of the earth in some direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A word of caution, though: Until now, we have only detected earth-sized planets &lt;a href=&quot;http://en.wikipedia.org/wiki/Exoplanet#Detection_methods&quot; rel=&quot;nofollow&quot;&gt;indirectly&lt;/a&gt;, so for all we know, they might be tiny gas giants, or as uninhabitable as Venus. And to travel to even the nearest of them is &lt;a href=&quot;http://en.wikipedia.org/wiki/Interstellar_travel#Challenges&quot; rel=&quot;nofollow&quot;&gt;far beyond our current capabilities&lt;/a&gt;. Even if we allow for several lifetimes of travel.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-09-03T12:38:03.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5268" PostTypeId="1" AcceptedAnswerId="5271" CreationDate="2014-09-04T07:12:24.917" Score="5" ViewCount="254" Body="&lt;p&gt;I understand that there is thinking to &lt;a href=&quot;http://www.nasa.gov/mission_pages/asteroids/initiative/#.VAgQR8GUdWg&quot;&gt;grab an asteroid&lt;/a&gt; to be able to use it for study and resource. When/where is the best place to do this? From its own orbit? From an Earth/Moon close encounter? (Context: I thought of this question as I read of a close encounter upcoming this weekend).&lt;/p&gt;&#xA;" OwnerUserId="3387" LastActivityDate="2014-09-05T03:37:46.400" Title="What would be the best moment to grab an asteroid?" Tags="|asteroid|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5269" PostTypeId="2" ParentId="5257" CreationDate="2014-09-04T11:51:15.210" Score="2" Body="&lt;p&gt;&quot;can&quot; and &quot;should&quot; need to be applied here. Rocket-only landings on a large planet with a substantial atmosphere are a very poor choice. Parachutes are MUCH lighter (although somewhat less accurate), wings are very accurate assuming you have a place to roll out your forward speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Skimming the Draco page is a bit confusing - are they using rockets all the way down, or just at the end? A Soyuz-type landing is basically a parachute descent - the 2 seconds of rocket thrust at the end are solely to make the impact less ainful for the passengers.&lt;/p&gt;&#xA;" OwnerUserId="6039" LastActivityDate="2014-09-04T11:51:15.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5270" PostTypeId="2" ParentId="5268" CreationDate="2014-09-04T13:27:23.950" Score="0" Body="&lt;p&gt;Grabbing it is easy - approach in a gently intersecting orbit and attach yourself with any convenient method. When and where is a question of convenience. We have done this many times with satellites, the moon, Mars, and Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving it somewhere else is a different (but also simple) question. Attach a rocket of suitable size, point, and fire.&lt;/p&gt;&#xA;" OwnerUserId="5933" LastActivityDate="2014-09-04T13:27:23.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5271" PostTypeId="2" ParentId="5268" CreationDate="2014-09-04T14:30:32.277" Score="3" Body="&lt;p&gt;One option might be to rendezvous with a temporarily captured orbiter (TCO) which makes a close encounter by passing by the Sun-Earth Lagrange point 1 or 2 (the Moon is not involved!) It is about as easy to reach in terms of delta-v as the easiest to reach known Near Earth Asteroid in stable orbit. But the Lagrange point has launch window everyday, not just once per asteroid orbit, and I think also some benefits for maneuvering because of the gravitational instability of the Lagrange point. And it is only 1% of an AU away, NEA's have much longer perigeo maximum distance from Earth. Since (small) asteroids are believed to pass by there regularly, maybe one mission could &quot;grab&quot; several of them to get a more representative sample, with huge benefits to science about them?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've recently asked related questions about this here:&#xA;&lt;a href=&quot;https://astronomy.stackexchange.com/questions/6184/can-the-gaia-telescope-detect-small-temporarily-captured-asteroids-near-its-lagr&quot;&gt;https://astronomy.stackexchange.com/questions/6184/can-the-gaia-telescope-detect-small-temporarily-captured-asteroids-near-its-lagr&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5192/what-would-be-the-delta-v-of-rendezvousing-with-temporarily-captured-asteroids-i&quot;&gt;What would be the delta-v of rendezvousing with temporarily captured asteroids in Sun-Earth L-points?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A challenge for this is that one probably won't be able to observe the target asteroid from Earth before the mission launches. They are too small, only a 5 meter TCO has ever been discovered, the rest are obviously smaller. But one could turn that challenge into a reason to develop the ability to react flexibly to previously unknown information when on site in space. Especially the military should be interested.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-09-04T14:38:26.877" LastActivityDate="2014-09-04T14:38:26.877" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5272" PostTypeId="1" AcceptedAnswerId="5273" CreationDate="2014-09-04T15:00:42.443" Score="20" ViewCount="2062" Body="&lt;p&gt;How do astronauts/cosmonauts manage the atmosphere at the International Space Station as their missions are years long, versus shorter flights?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they recycle? Import via cargo vessels? Electrolyze water? Chemical processes?&lt;/p&gt;&#xA;" OwnerUserId="6040" LastEditorUserId="49" LastEditDate="2015-09-20T21:11:45.930" LastActivityDate="2015-09-20T21:11:45.930" Title="Air in International Space Station" Tags="|iss|atmosphere|life-support|eclss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5273" PostTypeId="2" ParentId="5272" CreationDate="2014-09-04T15:24:03.663" Score="22" Body="&lt;p&gt;Several approaches are taken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cargo vehicles bring up Oxygen and other atmospheric components (Nitrogen, etc).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian segments life support system works different and independant of the US side, which is a nice redundancy model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian side has a system that recycles water from urine and moisture that needs to be removed to electrolyze to oxygen and hydrogen (and throws hydrogen overboard).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have a backup system of 'candles' that 'burn' and release oxygen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US side I am not as familiar with specifics. But there is a great article on &lt;a href=&quot;http://en.wikipedia.org/wiki/ISS_ECLSS&quot;&gt;ECLSS at the ISS&lt;/a&gt;.  ECLSS is Environmental Control and Life Support System.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many interaction points between different systems, using by products of others.  So it is not just about 'air' or atmosphere, it is also about water, carbon dioxide, nitrogen, and more.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-09-04T16:48:37.497" LastActivityDate="2014-09-04T16:48:37.497" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5276" PostTypeId="2" ParentId="5268" CreationDate="2014-09-05T03:37:46.400" Score="6" Body="&lt;p&gt;I've played with this. It seems to me there are two major delta Vs:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Delta V to nudge an asteroid's heliocentric orbit so the rock passes through earth's sphere of influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Once in earth's sphere of influence, delta V to shed enough velocity so that the hyperbolic orbit with regard to earth becomes an elliptical capture orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a matter of course we already have many rocks that pass through our sphere of influence. And there are far more that pass near the earth's sphere influence and need only a small nudge to alter its pass so it passes through.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping a rock within our sphere of influence is tougher. It helps if the rock has an earth like orbit: low inclination, semi major axis close to 1 A.U. and a low eccentricty (nearly circular). However a rock with an earth like orbit would have a large synodic period with regard to earth. It would only rarely come near earth's neighborhood. But there since there's a lot of rocks with earth like orbits, earth approaches from these more accessible rocks aren't exceedingly rare.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the rock flies by the moon's path while passing through our neighborhood, the moon can change the rock's velocity with regard to earth. Sometimes the moon can change a hyperbolic path to a capture orbit. &lt;a href=&quot;http://www.newscientist.com/article/mg21328464.600-hundreds-of-tiny-moons-may-be-orbiting-earth.html&quot;&gt;It's thought this occurs naturally&lt;/a&gt;. If humans were to give a rock a strategic nudge here and there, it could occur much more often.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf&quot;&gt;Keck Report&lt;/a&gt; outlines how a robotic craft with robust solar electric propulsion might retrieve an asteroid like 2008 HU4 and park it in lunar orbit. It is interesting the many of the Keck Report authors are prominent asteroid guys like J. S. Lewis (Author of Mining the Sky) or Chris Lewicki (of Planetary Resources).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/04/catching-asteroid.html&quot;&gt;I also took a crack at speculating how 2008 HU4 might be captured when it nears the earth in 2015.&lt;/a&gt; Of course we don't have time to build a retrieval vehicle in time for 2008 HU4's 2015 fly by, but more chances will comes along.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-09-05T03:37:46.400" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5277" PostTypeId="1" AcceptedAnswerId="5279" CreationDate="2014-09-05T10:55:46.977" Score="7" ViewCount="485" Body="&lt;p&gt;As we know, something released from around 36000 km height from a space elevator will remain in geostationary orbit around the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things released from lower will go into an elliptical orbit around the center of the earth. If the pericenter of its orbit is at least so far from the center of the Earth, as the radius of the Earth, plus around 500 km, I think, its orbit will be highly elliptic, but relative stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How high do we need to release something from a space elevator to reach that?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To clarify,&lt;/strong&gt; what is the height at which you could release an object from a space elevator and achieve an orbit with perigee &gt;= Re + 500 km (where Re is the earth radius)?&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="58" LastEditDate="2014-09-05T15:45:07.980" LastActivityDate="2017-11-11T03:42:05.900" Title="Needed height from the release of a space elevator to get a stable elliptical orbit?" Tags="|orbital-mechanics|geostationary|space-elevator|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5278" PostTypeId="1" AcceptedAnswerId="5283" CreationDate="2014-09-05T14:54:45.513" Score="16" ViewCount="1941" Body="&lt;p&gt;After WWII, both the US and USSR began developing their early rockets based on the German V-2 work. USSR flew their R-1 copy of the V2 in 1948; the US flew Corporal in 1947.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It wasn't until 1957 that the first artificial satellite, Sputnik 1, was launched into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, tension between military, political, and scientific goals had a lot to do with the priorities of both nations' rocket programs, but is there any particular &lt;em&gt;technical&lt;/em&gt; reason a Sputnik-type satellite couldn't have been launched sooner? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Sputnik_1&quot;&gt;Sputnik&lt;/a&gt; orbiter itself wasn't fancy: a tough shell around a big pack of batteries and a radio transmitter, trailing whip antennas. Contemporarily with the 84kg Sputnik, the US was building the functionally similar Vanguard satellite in a much smaller package massing 1.5kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sputnik's launcher was an &lt;a href=&quot;http://en.wikipedia.org/wiki/Sputnik_(rocket)&quot;&gt;R-7&lt;/a&gt;, a fairly complex, 20-combustion-chamber, one-and-a-half-stage ICBM design (amazingly, the same basic design is still in use today!). The two US satellite programs under development when Sputnik launched, &lt;a href=&quot;http://en.wikipedia.org/wiki/Vanguard_(rocket)&quot;&gt;Vanguard&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_I&quot;&gt;Explorer&lt;/a&gt;, were using 3- and 4-stage designs with simpler, single-chamber, liquid-fueled lower stages and solid uppers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume that building a reliable, staged launcher of sufficient power was the major technical hurdle, but the US had flown the two-stage &lt;a href=&quot;http://en.wikipedia.org/wiki/WAC_Corporal&quot;&gt;WAC Corporal&lt;/a&gt; to 50 miles altitude as early as 1946.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was closed-loop guidance used/required for Sputnik, Vanguard, or Explorer? For my purposes, precise guidance isn't required; any orbit that could be sustained for at least a day or two would be acceptable for &quot;Sputnik-style success&quot;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="758" LastEditDate="2015-07-28T22:48:27.227" LastActivityDate="2016-04-07T08:52:04.913" Title="Sputnik: What took so long?" Tags="|history|sputnik|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5279" PostTypeId="2" ParentId="5277" CreationDate="2014-09-05T16:21:37.457" Score="11" Body="&lt;p&gt;I think the question is asking what the release altitude from a space elevator would result in an orbit with a perigee of $R_E + 500 km$ ($R_E$ is the earth radius &amp;mdash; 6378.135 km)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity of the space elevator varies linearly along its length, with the velocity at GEO altitude ($R_{GEO}$) equal to GEO orbital velocity ($V_{GEO} = \sqrt{\mu/R_g}$).  So for a position $r$ on the elevator:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_{elev} = {r\over R}\sqrt{\mu/R_{GEO}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This velocity vector is perpendicular to the position vector, as is the orbital velocity vector at apogee (and perigee).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital velocity as a function of radius $r$ is given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_r = \sqrt{\mu(1/r-1/a)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $a$ is the semi-major axis&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a = {R_{apogee} + R_{perigee}\over 2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The release point will be the apogee of the achieved orbit, so we can re-write the equations as such:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_{elev} = {R_{apogee}\over R_{GEO}}\sqrt{\mu/R_{GEO}}$$&#xA;$$V_{apogee} = \sqrt{\mu(1/R_{apogee} - 2/(R_{apogee} + R_{perigee}))}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can now set $V_{elev} = V_{apogee}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${R_{apogee}\over R_{GEO}}\sqrt{\mu/R_{GEO}} = \sqrt{\mu(1/R_{apogee} - 2/(R_{apogee} + R_{perigee}))}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After some algebra, you can solve for $R_{perigee}$ as a function of $R_{apogee}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R_{perigee} = (R_{apogee}^3/(2 R_{GEO}^3))[1/(1/R_{apogee} - R_{apogee}^2/(2 R_{GEO}^3))]$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solving for $R_{apogee}$ would be a bit messier I think, so I plugged this into Excel and used solver to numerically find the solution for $R_{perigee} = R_E + 500 = 6878.135$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R_{apogee} = 30276.54 km$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this is the radius from the earth center, not the height, as was used in the original question.  Other quantities used:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mu = 398601$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$R_{GEO} = 42164.2 km$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wCXBu.png&quot; alt=&quot;Graph showing rising curve of release perigee crossing the fixed 500km threshold at an apogee of about 30000km&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="6816" LastEditDate="2017-11-11T03:42:05.900" LastActivityDate="2017-11-11T03:42:05.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5281" PostTypeId="1" AcceptedAnswerId="5284" CreationDate="2014-09-06T13:52:51.553" Score="9" ViewCount="2863" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_launchers_families&quot; rel=&quot;noreferrer&quot;&gt;this Wikipedia list&lt;/a&gt;, the rockets Delta IV Heavy and Proton can launch the same mass payload to LEO, 23 tons. But the capacity, according to Wikipedia, is dramatically different when it comes to what mass payload those same rockets can launch to GEO geostationary orbit, or to TLI trans lunar injection. (13 and 7, or 9 and 5.7 tons for DIVH and Proton).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this? Are public tables like this practically useless for comparison of different launch systems? Or does the launch capacity to different altitudes depend on what upper stage is compatible with the launch system? Or what else?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="897" LastEditDate="2014-09-07T13:00:35.750" LastActivityDate="2014-09-07T13:00:35.750" Title="Why are there such large differences in launcher payloads to higher orbits (GEO, Lunar) compared to LEO?" Tags="|rockets|orbit|low-earth-orbit|payload|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5282" PostTypeId="2" ParentId="5278" CreationDate="2014-09-06T21:17:24.380" Score="8" Body="&lt;p&gt;Sputnik-1 was political statement. Its science value was close to zero. Some see the entire Space Race as a series of political statements: &amp;ldquo;If we can do this, think of what we can do to your cities.&amp;rdquo; A rocket capable of putting a satellite into orbit, even a small one such as Sputnik-1, is more than capable enough to deliver a nuclear bomb to some city a third of the way around the Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's one caveat to that last sentence: The bomb has to reasonably lightweight. There was no impetus at first to develop ballistic missiles  because the first nuclear weapons were extremely massive devices with relatively low yields. Both sides of the Cold War made significant improvements to those initial Los Alamos devices. It was these improvements that provided the impetus to build long range ballistic missiles.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="8693" LastEditDate="2016-04-07T08:52:04.913" LastActivityDate="2016-04-07T08:52:04.913" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="5283" PostTypeId="2" ParentId="5278" CreationDate="2014-09-07T02:03:47.523" Score="19" Body="&lt;p&gt;I'm sure it could have been done somewhat earlier, but there was still quite a big technical step between the suborbital sounding rockets / short-range ballistic missiles derived from the V-2, and a multi-stage, high performance vehicle capable of orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Advances in guidance systems were definitely necessary, but I think raw performance in terms of delta V was the real differentiator.  The V-2 provided less than 2 km/s of delta V to its payload; an orbital launch needs about 10 km/s (including gravity and drag loss).  This requires a combination of staging, improved mass fraction, and better engine performance.  That takes time to develop, no matter how you look at it.  Much of the same is needed for true ICBM-like ranges (typically 7-8 km/s), which was the real driver for development of those technologies.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-09-07T02:03:47.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5284" PostTypeId="2" ParentId="5281" CreationDate="2014-09-07T04:07:08.993" Score="17" Body="&lt;p&gt;The numbers on the Wikipedia page are a little messed up, so you can't do apples-to-apples comparisons.  You'd need to go to the actual payload planner's guides to see what the performance is to what orbits.  Here are the guides:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Launch_Vehicles/Delta_IV_Users_Guide_June_2013.pdf&quot; rel=&quot;noreferrer&quot;&gt;Delta IV Launch Services User's Guide&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ilslaunch.com/sites/default/files/pdf/Proton%20Mission%20Planner%27s%20Guide%20Revision%207%20%28LKEB-9812-1990%29.pdf&quot; rel=&quot;noreferrer&quot;&gt;Proton Launch System Mission Planner's Guide&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Proton launch to a LEO with the launch site inclination (51.5°) is 23 t.  A Delta IVH launch to a LEO with the launch site inclination (28.7°) is 28.79 t.  Quite a bit more.  The 23 t on the Wikipedia page is for a Delta IVH launch to a &lt;em&gt;polar&lt;/em&gt; orbit (90°), at 23.56 t.  So you would expect the Delta IVH to also deliver quite a bit more to GTO or TLI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(You said GEO, but you must mean GTO, since launch vehicles rarely if ever deliver a spacecraft all the way to GEO -- they drop off the spacecraft in GTO, Geosynchronous Transfer Orbit, which has an apogee around GEO and a low perigee).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's more, the Proton is handicapped with a launch site so far to the North.  Large plane change maneuvers are required to reduce the inclination of the orbit to drop off the spacecraft closer to (but still not at) the GEO inclination of 0°.  This figure from the guide shows how large the plane change is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KXpWe.png&quot; alt=&quot;Proton Mission Profile from SC injection into GTO from Parking Orbit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 6.92 t GTO figure for the Proton is a drop off to a 31.1° inclination orbit, with a perigee at 2175 km.  That leaves 1800 m/s to the spacecraft to get to GSO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Delta IVH delivers 14.22 t to a 27° orbit, requiring essentially no plane changes to get there, with a 185 km perigee leaving 1804 m/s to the spacecraft to get to GSO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Proton is even further handicapped by a lower Isp upper stage (the Бриз-М or Breeze-M) using storable propellants, as compared to the Delta IVH upper stage using LH2/LOX.  So there will be lower performance for the Proton as compared to the Delta IVH when boosting past LEO, even if they were at the same starting point in orbit and mass.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-09-07T04:37:55.260" LastActivityDate="2014-09-07T04:37:55.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5285" PostTypeId="1" CreationDate="2014-09-07T12:32:15.127" Score="10" ViewCount="1201" Body="&lt;p&gt;I read the previous question on this site about astrodynamics &lt;a href=&quot;/questions/tagged/software&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;software&amp;#39;&quot; rel=&quot;tag&quot;&gt;software&lt;/a&gt; and I have tried STK (free version) and GMAT, but I could not find software to simulate and optimize a launch trajectory. Is there a free (and ITAR-free) solution for this kind of problems?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally, I would like to be able to simulate trajectories of a reusable first stage returning to the launch site.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="49" LastEditDate="2014-09-07T16:44:52.640" LastActivityDate="2014-09-07T16:44:52.640" Title="Software to simulate / optimize ascent trajectory" Tags="|launch|trajectory|software|reuse|simulation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5286" PostTypeId="2" ParentId="5285" CreationDate="2014-09-07T13:56:30.610" Score="8" Body="&lt;p&gt;Software written in the US? If your launch is anything but a toy rocket that goes up a few hundred feet and if you expect any decent level of accuracy/fidelity from the simulation, probably not. Software written elsewhere, perhaps. ITAR is a US-specific concept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that the very same software that will let you simulate a high energy launch with reasonably high accuracy and fidelity is exactly the same software that is needed by nefarious groups hellbent on sending a missile to a US city. This is one of the central areas that results in a piece of space-related software being deemed as ITAR-restricted.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-09-07T14:06:34.163" LastActivityDate="2014-09-07T14:06:34.163" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5287" PostTypeId="1" CreationDate="2014-09-07T14:14:17.497" Score="17" ViewCount="952" Body="&lt;p&gt;On Earth the magnetic poles are &lt;em&gt;near&lt;/em&gt; the geographic poles.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Does Earth have this situation by pure luck? &lt;/li&gt;&#xA;&lt;li&gt;What is the situation on other planets?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1108" LastEditorUserId="603" LastEditDate="2014-09-08T03:10:23.867" LastActivityDate="2016-12-06T18:39:15.863" Title="Is it a pure coincindence that the magnetic poles of Earth are near the geographic poles?" Tags="|planetary-science|magnetic-field|planet|pole|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5288" PostTypeId="2" ParentId="5257" CreationDate="2014-09-07T16:51:25.523" Score="1" Body="&lt;p&gt;The Dragon V2 is designed to use areobrakeing first to slow down then rockets to land at the end without parachutes, however, they are retaining the parachutes as a backup. SpaceX is going for rapid reuse-ability not total efficiency since it is much cheaper to do 2 200 thousand dollar flights then 1 100 million flight.&lt;/p&gt;&#xA;" OwnerUserId="6049" LastActivityDate="2014-09-07T16:51:25.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5289" PostTypeId="2" ParentId="5287" CreationDate="2014-09-08T02:55:32.813" Score="16" Body="&lt;p&gt;It is not coincidence and it does not apply to just the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sun, Earth, Jupiter, Saturn, Uranus, and Neptune all have sizable magnetic fields. Mercury and Ganymede have smaller but still noticeable magnetic fields. All of these bodies have one thing in common: They have a sizable amount of rotating, electrically conductive fluid somewhere beneath the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The underlying theory of why those objects that do have a magnetic field do have them is magnetic dynamo theory. This theory suggests that the object's rotational and magnetic axes need to be a bit offset from one another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some enigmas that dynamo theory still needs to explain. Saturn's magnetic field is almost perfectly aligned with it's rotation axis. Uranus and Neptune have magnetic fields that are tilted by a bit too much, and the center of their fields are offset from the center of the planet by a considerable amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, Venus, the Earth's Moon, Mars have negligible magnetic fields, despite all three apparently having partially liquid cores. What they most likely don't have is a convective liquid core. These three bodies have a stagnant lid: no active vulcanism, no plate tectonics. This keeps heat bottled up inside and keeps the liquid core from roiling.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-12-06T18:39:15.863" LastActivityDate="2016-12-06T18:39:15.863" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5290" PostTypeId="1" CreationDate="2014-09-08T03:34:10.457" Score="8" ViewCount="2041" Body="&lt;p&gt;How does the Falcon 9 v1.1 form this exhaust pattern with its nine Merlin 1D engines? Is it a bigger, rougher &lt;a href=&quot;http://www.aerospaceweb.org/question/propulsion/q0224.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;shock diamond&lt;/strong&gt;&lt;/a&gt;? I am trying to learn more about how one could form with a cluster of engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sgvpT.png&quot; alt=&quot;Falcon 9 exhaust pattern&quot;&gt;&#xA;&lt;sub&gt;&lt;br&gt;Aft end of the Falcon 9 v1.1 a minute or two into the launch.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-09-10T03:39:43.627" LastActivityDate="2015-06-10T19:28:49.457" Title="Does the Falcon 9 v1.1 form a shock diamond when in flight?" Tags="|propulsion|spacex|engine-design|falcon-9|physics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5291" PostTypeId="2" ParentId="5278" CreationDate="2014-09-08T08:42:11.300" Score="10" Body="&lt;p&gt;The RAND Corporation issued far-seeing reports on artificial satellites as early as 1946 (&lt;a href=&quot;http://www.rand.org/pubs/special_memoranda/SM11827.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.rand.org/pubs/special_memoranda/SM11827.html&lt;/a&gt;) - a must read! There was no immediate military need to develop satellites right then, so the whole agenda lingered on. Von Braun was directed by the Army to focus on intermediate-range missiles, and the Air Force was bomber-heavy, and eager to defend its turf (even though Curtis LeMay commissioned the above-mentioned study).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Advent of the &quot;super&quot; (thermonuclear weapons) changed the situation drastically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was &lt;a href=&quot;https://en.wikipedia.org/wiki/John_von_Neumann&quot; rel=&quot;noreferrer&quot;&gt;John von Neumann&lt;/a&gt; who was instrumental in advocating for ICBMs in the States, having the privileged position inside the nuclear establishment and keen insight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The United States had a positional advantage over the Russkies, with bases around Russian borders, so there was no real pressure to develop at the very limits of technical possibilities. The USSR, prodded by Andrey Sakharov's extraordinary weight requirements (1953) for a bomb-carrying ICBM, and its own lack of reliable bomber force, made Korolyov's designers choose the now familiar &quot;packet&quot; design for the R-7 missile, together with radio-controlled guidance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest is history: in 1955 President Eisenhower's press secretary promised to launch a small satellite during the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Geophysical_Year&quot; rel=&quot;noreferrer&quot;&gt;International Geophysical Year&lt;/a&gt; - unfortunately, led by the NRL in &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Vanguard&quot; rel=&quot;noreferrer&quot;&gt;Project Vanguard&lt;/a&gt; - and not von Braun's ABMA team. The US duplicated effort, choosing to hide the Corona/WS-117 development.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soviets retaliated in kind, making the decision to re-use R-7 and jury-rig the Sputnik, and were the first past the post.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To re-cap, here are the factors that delayed satellites till 1957:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Troubles with developing thermonuclear weapons.&lt;/li&gt;&#xA;&lt;li&gt;International Geophysical Year slated for 1957.&lt;/li&gt;&#xA;&lt;li&gt;Initial reliance on balloons/spy planes for aerial reconnaissance.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some books to skim through:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Neil Sheehan. A Fiery Peace in a Cold War: Bernard Schriever and the Ultimate Weapon. 2010, Vintage books.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-09-12T07:45:28.753" LastActivityDate="2014-09-12T07:45:28.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5292" PostTypeId="1" CreationDate="2014-09-08T09:02:28.120" Score="9" ViewCount="1510" Body="&lt;p&gt;A quick follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/2423/how-would-it-be-possible-to-kick-start-marss-magnetic-field&quot;&gt;How would it be possible to kick start Mars&amp;#39;s magnetic field?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the Venerean atmosphere is contained by merely an induced magnetic field. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How powerful would magnetic field need to be to contain sea-level Earth equivalent atmospheric pressure and composition as it exists today at Martian standard datum? &lt;/li&gt;&#xA;&lt;li&gt;If future terraforming initiatives get some greenhouse effect going on Mars, would the magnetic field need to be gradually strengthened / reinforced as the Martian atmosphere also gradually thickened? Would the atmospheric escape rate to outer space increase as it thickens, or would interaction between the ionosphere and the solar wind sufficiently strengthen induced magnetic field on its own?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-06-02T11:19:15.697" Title="How strong a magnetic field does Mars need to contain Earth-like atmosphere?" Tags="|mars|planetary-science|magnetic-field|terraforming|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5293" PostTypeId="2" ParentId="5292" CreationDate="2014-09-08T13:59:16.223" Score="8" Body="&lt;p&gt;Earth's atmosphere is also slowly &quot;leaking&quot; into space, but very slowly. This is because there are &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmospheric_escape&quot;&gt;multiple processes&lt;/a&gt; involved in escaping atmospheres. One of those processes is Jeans escape, where due to &lt;a href=&quot;http://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_distribution&quot;&gt;Maxwell speed distribution&lt;/a&gt; and long enough &lt;a href=&quot;http://en.wikipedia.org/wiki/Mean_free_path&quot;&gt;mean free path&lt;/a&gt; molecules are able reach &lt;a href=&quot;http://en.wikipedia.org/wiki/Escape_velocity&quot;&gt;escape velocity&lt;/a&gt; and escape the atmosphere before &quot;bumping&quot; into another molecule. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dominant process for an escaping atmosphere differ from planet to planet. For example Venus has a very weak magnetic field and orbits closer to the sun, but has an atmosphere two orders of magnitude denser than that of Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So giving Mars a strong(er) magnetic field and a &quot;new&quot; atmosphere does not mean its will keep it. It probably will lower the rate at which its atmosphere is escaping, however it will not stop it. I am not an expert on this, but I suspect that Mars is just too light to maintain a thick atmosphere, since more mass would increase its escape velocity. &lt;/p&gt;&#xA;" OwnerUserId="1086" LastEditorUserId="1086" LastEditDate="2014-12-30T01:58:30.643" LastActivityDate="2014-12-30T01:58:30.643" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5294" PostTypeId="1" CreationDate="2014-09-08T17:55:10.587" Score="5" ViewCount="2490" Body="&lt;p&gt;I am looking for the effects (mainly in the short term) that you would experience if you were to go on an EVA wearing an IVA suit. For example, I don't think that the IVA suits have any form of protection from radiation. Unlike most EVA suits that have reflective coating made from Mylar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am mainly looking for things you would experience on that EVA, not a couple of days-years after(if you survived), but anything is useful :)&lt;/p&gt;&#xA;" OwnerUserId="6053" LastActivityDate="2014-09-08T23:26:29.173" Title="What would happen if you wore 'soft suit'(IVA suit) in the vacuum of space?" Tags="|spacesuits|eva|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5296" PostTypeId="2" ParentId="5294" CreationDate="2014-09-08T23:26:29.173" Score="4" Body="&lt;p&gt;An astronaut entering an EVA in an ACES would probably suffocate before they are boiled to death (lucky them).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The life support system is normally plugged into the spacecraft. There is only a 10 minute backup oxygen supply in the suit itself. Meanwhile the unfavorable  &lt;a href=&quot;https://space.stackexchange.com/questions/5246/why-is-gold-used-in-space-technology-to-protection-from-heat-radiation/5248#5248&quot;&gt;ratio of absorptivity to emittance&lt;/a&gt; of the pumpkin orange suit would cause it to heat up quickly. This is not helped by the fact that the astronauts body heat has nowhere to go. And it is conceivable they might be &quot;agitated&quot; and therefore give off a fair bit of body heat in this situation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume heat capacity the stagnant water in the water cooling system of the suit together with its many cloth layers would keep the suit at survivable temperatures for that time, though. Long enough for the oxygen to run out at least.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-09-08T23:26:29.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5297" PostTypeId="1" CreationDate="2014-09-10T06:57:45.183" Score="40" ViewCount="28811" Body="&lt;p&gt;The ISS orbits West to East covering almost every part of the land on Earth, but excludes the polar regions.  Why was this orbit chosen?&lt;/p&gt;&#xA;" OwnerUserId="6057" LastEditorUserId="2707" LastEditDate="2014-12-11T09:18:44.257" LastActivityDate="2020-04-18T18:11:13.780" Title="Why doesn't ISS pass over the polar regions?" Tags="|iss|orbit|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5298" PostTypeId="2" ParentId="5297" CreationDate="2014-09-10T07:32:42.863" Score="65" Body="&lt;p&gt;The decision to orbit (and assemble) the International Space Station (ISS) in 51.65° prograde Low Earth Orbit (LEO) was driven by its accessibility and utility. You see, inclination changes are terribly expensive for visiting spacecraft delivering astronauts / cosmonauts, consumables, science experiments and other cargo, and ideally, you'd always launch rockets in orbit with its inclination matching that of the launch site's azimuth to maximize rocket's carrying capability. I.e. you'd launch directly due East to take as much advantage of the Earth's rotation as possible to reach required orbital velocity and the resulting orbital inclination is then a direct result of the Earth's own rotation on its axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since most launches to the station are done from Baikonur Cosmodrome, Kazakhstan (Progress and Soyuz rockets), with the launch site roughly 46° North of the equator, ISS' orbit nearly matches that of the launch site. Nearly, but not quite, since directly due East and downrange of Baikonur the land relatively soon (for a rocket flight) enters the territory of China, so to avoid that, the target orbital inclination was increased to 51.65° and the rockets don't overfly China at all. This is also important during crew landings in case they under / overshot the targeted landing site:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/eEWBl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Rockets coming from the Baikonur Cosmodrome, Kazakhstan are sent into orbits inclined 51.65 degrees to avoid overflying China.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This decision was made jointly by NASA and Roscosmos and is also convenient for Earth observations and covers 75 percent of the Earth and 95 percent of its population under its direct line of sight. I explain this a bit more in &lt;a href=&quot;https://space.stackexchange.com/questions/2535/why-did-the-recent-sirius-fm6-launch-use-such-a-high-inclination&quot;&gt;Why did the recent Sirius FM6 launch use such a high inclination?&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/4034/can-i-see-the-iss-from-the-surface-with-the-naked-eye&quot;&gt;Can I see the ISS from the surface with the naked eye?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if the station instead orbited in a prograde polar orbit (overpassing polar regions, but not in marginally retrograde and permanently sunlit &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;Sun-synchronous Orbit&lt;/a&gt;, e.g. the so-called dawn-to-dusk SSO where satellites circumnavigate the Earth &lt;em&gt;riding&lt;/em&gt; the &lt;a href=&quot;https://en.wikipedia.org/wiki/Terminator_(solar)&quot; rel=&quot;noreferrer&quot;&gt;solar terminator&lt;/a&gt;, which would be a thermal management nightmare for the station), on top of requiring more launches and keeping the station more expensive with it, it would also further increase the time interval at which it would orbit over the launch sites. In its current orbit, this time interval is a bit more than a day, or about 16 of its circa 93 minutes long orbits. If this took significantly longer, it would also make launch windows less frequent and impact traffic to and fro the station. It would also limit crew escape options in an emergency, since you wouldn't want to do a deorbit burn over the polar regions, and it would expose the crew onboard the station to &lt;a href=&quot;http://www.nasa.gov/centers/langley/science/polar-radiation.html&quot; rel=&quot;noreferrer&quot;&gt;more radiation over those regions&lt;/a&gt; too - which would mean it would have to orbit lower still to decrease overall radiation exposure, which would in turn increase required propellant mass to periodically re-boost its orbit due to higher atmospheric drag, and make it even more expensive to maintain / service.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-09-11T17:33:30.843" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5299" PostTypeId="1" AcceptedAnswerId="9701" CreationDate="2014-09-10T13:35:18.693" Score="22" ViewCount="1119" Body="&lt;p&gt;A number of times now, the cargo vehicles (Dragon, Cygnus, or HTV) seem to have arrived while the station was between crew switchovers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With indirect crew handovers, three crew depart in a Soyuz, reducing ISS Crew size from 6 to 3, and a week or two later another Soyuz launches with a crew of 3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I have noticed is that a couple of times now, cargo missions have docked, while running a crew of 3 only. Since the cargo vehicles implies additional work. (They only stay docked about a month, on the US side, since with only one primary CBM port available (Harmony nadir. Harmony zenith is secondary)  See &lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;Will the ISS need more docking ports&lt;/a&gt; for details.)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With only a month on station, there is a fair bit of cargo to unload, store somewhere (Tracked of course, else no one will be able to find anything), then refill the vehicle with downmass (Dragon only) or trash (Cygnus, HTV, Progress, ATV).  There are requirements to balance the mass on reloading, so it is non-trivial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would think, targeting 6 person crew compliments would make more sense. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But is there a reason? Perhaps the schedule is so tight on the Visiting Vehicle Schedule they cannot 'waste' the two weeks when the crew is reduced?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Harmony, where the CBM for cargo docking resides, is really central to the station, (you must go through Harmony to get to Columbus (ESA module), Kibo (JAXA module) and rest of the station.  So maybe a crew of three is easier, since there are fewer people in the way?  Or moving cargo disturbs fewer people or experiments?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-06-29T12:01:19.040" Title="Does the ISS team plan cargo flights when at half crew on purpose?" Tags="|iss|spacex|docking|jaxa|cygnus|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5300" PostTypeId="1" CreationDate="2014-09-10T07:56:15.150" Score="4" ViewCount="168" Body="&lt;p&gt;I have seen many questions answered about the roll program with relation to the shuttle launch and reducing stresses on the launch vehicle and various other reasons, but why was the roll program needed for the Saturn five rocket being a circular vehicle. Having watched films of other launch's in different countries using circular rockets, they do not appear to go through a roll program or is it carried out at a different part of the launch procedure.&lt;/p&gt;&#xA;" OwnerDisplayName="Philip48" LastEditorUserId="49" LastEditDate="2014-09-10T18:00:15.090" LastActivityDate="2014-09-10T18:00:15.090" Title="Launch roll program for the Saturn five rocket" Tags="|launch|nasa|apollo-program|rockets|saturn-v|" AnswerCount="0" CommentCount="6" ClosedDate="2014-09-10T17:57:38.730" ContentLicense="CC BY-SA 3.0" />
  <row Id="5301" PostTypeId="1" CreationDate="2014-09-10T17:46:47.517" Score="6" ViewCount="1950" Body="&lt;p&gt;Pushinka (&lt;em&gt;Fluffy&lt;/em&gt;) was the name of one of the Strelka's (&lt;em&gt;Arrow's&lt;/em&gt;) six pups that she had after being safely landed to Earth from orbit to which it was launched aboard Vostok-1K rocket No. 2 on 19 August 1960. Wikipedia describes this in a bit more detail in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Soviet_space_dogs#Belka_and_Strelka&quot; rel=&quot;noreferrer&quot;&gt;Soviet space dogs article, section on Belka and Strelka&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/u4zdw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Kennedy family dog Pushinka on the White House North Lawn.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is to my understanding that Chairman Nikita Khrushchev and US President John F. Kennedy maintained a rather warm relationship despite the cold war era already being at full swing, and they communicated regularly. BBC News magazine covers this &lt;a href=&quot;http://www.bbc.com/news/magazine-24837199&quot; rel=&quot;noreferrer&quot;&gt;Pushinka: A Cold War puppy the Kennedys loved&lt;/a&gt; story in even more detail and also republishing Kennedy's letter to Khrushchev upon receiving the pup:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/wX5P9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Both above photographs courtesy of &lt;a href=&quot;http://www.jfklibrary.org/&quot; rel=&quot;noreferrer&quot;&gt;John F. Kennedy Presidential Library &amp;amp; Museum&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a heartwarming story for sure, but what exactly were Khrushchev's intentions with it, besides, you know, &lt;em&gt;rubbing it in&lt;/em&gt; that the Soviet communist space program was at that time more advanced than the &lt;em&gt;free world&lt;/em&gt; capitalist one?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Was this information ever disclosed to the public, perhaps in some memoirs / biography or analyzed in more detail by some cold war historian like Martin Sandler that the BBC article mentions?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was quite common in those times for such presents to have a hidden message to it, and I find the whole thing a bit curious to say the least. Was there a more obscure agenda behind this &lt;em&gt;present&lt;/em&gt; to JFK's daughter Caroline? I guess it could be seen as a good will / public relations gesture of sorts, and that's I guess how it was received by the general public. But it could as well serve as a proof that the outer space isn't hostile to life on Earth? Or perhaps serve as any number of other messages that I can't think of.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorDisplayName="user54" LastEditDate="2014-09-10T20:22:47.600" LastActivityDate="2014-09-10T20:22:47.600" Title="Why exactly did Khrushchev present Pushinka to JFK's daughter Caroline?" Tags="|history|russia|animals|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5302" PostTypeId="2" ParentId="5297" CreationDate="2014-09-10T21:12:10.860" Score="13" Body="&lt;p&gt;If you are asking the mechanics of the orbit, then only a polar orbit will ever pass directly over the poles.  Think of the orbit as a circle like a hula hoop around a basketball (the ISS's orbit would be very close to the ball).  You can tilt the hoop any way you want, but it will only pass over the poles if you stand it on end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the Earth did not rotate, the ISS would appear to travel the same path over the surface of the Earth each orbit, but as the Earth rotates the ISS's orbit falls over different parts of the Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Ghuqd.gif&quot; alt=&quot;orbits&quot;&gt;&lt;br&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sub&gt;The lighter blue area is where the Space Station can be seen from the ground. Image: &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2000/ast01dec_1/&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's orbit were at &lt;a href=&quot;http://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;noreferrer&quot;&gt;geostationary&lt;/a&gt; distance (might be a little different if not over the equator) but inclined to the equator at the same angle, it would appear to follow a longitudinal line on the Earth's surface from 51.65 degrees north to 51.65 degrees south and back each day.&lt;/p&gt;&#xA;" OwnerUserId="6061" LastEditorUserId="415" LastEditDate="2014-10-01T09:09:42.943" LastActivityDate="2014-10-01T09:09:42.943" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5303" PostTypeId="1" AcceptedAnswerId="5304" CreationDate="2014-09-10T21:21:05.013" Score="4" ViewCount="2449" Body="&lt;p&gt;I've tried to get my head around this and have decided that, for the moment, I don't want to learn to solve logarithmic equations. Too much else going on. &#xA;I was reading &lt;a href=&quot;http://www.marssociety.org/home/about/mars-direct&quot; rel=&quot;nofollow&quot;&gt;Mars Direct&lt;/a&gt; and got the impression from the very abbreviated info there that about 80% would be fuel. That surprised me because the gravity is so much weaker and the air is so thin on Mars. Is that right?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2014-09-10T21:38:48.007" LastActivityDate="2014-09-16T16:54:46.090" Title="What fraction of a liquid rocket launching from Mars to return to Earth would be fuel?" Tags="|mars|rockets|fuel|payload|mass|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5304" PostTypeId="2" ParentId="5303" CreationDate="2014-09-10T22:41:35.580" Score="16" Body="&lt;p&gt;Here's a Map to the Solar System. It details, roughly how much $\Delta v$ you need to get from one place to the next.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/piNqI.png&quot; alt=&quot;Subway Map&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can take the rocket equation to quickly calculate your Fuel fraction for any given $\Delta v$. We take the basic form ($m_0$ starting mass; $m_1$ final mass; $v_\text{e}$ effective propellant exit velocity)&#xA;$$&#xA;\Delta v = v_\text{e} \ln \frac {m_0} {m_1}&#xA;$$&#xA;And introduce the fuel fraction $M_f=\frac{m_0-m_1}{m_0}$&#xA;$$&#xA;M_f = 1-\frac {m_1} {m_0}=1-e^{-\frac{\Delta V}{ v_\text{e}}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I was too tired to derive that by myself, so I took it out of Wikipedia. Not that it's hard to do...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars Direct wants to use in-situ-produced methane and oxygen for the return. The theoretical ISP of this fuel combination is 368.9s. That equals an effective exhaust velocity of $368.9\text{s}\cdot \text{g}=3618\frac{\text{m}}{\text{s}}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To return to earth we need at least an intercept. Dennis Tito wants to return from mars and plunge right into the earth's atmosphere (yes, I know it's more complicated), so if you fly that way, you only need to intercept earth, not go into an orbit before you land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That way your $\Delta v$ would be $(3800+1400+1060)\frac{\text{m}}{\text{s}}=6260\frac{\text{m}}{\text{s}}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluck that into the equation above and you get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;M_f = 1-e^{-\frac{6260\frac{\text{m}}{\text{s}}}{3618\frac{\text{m}}{\text{s}}}}=0.82&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this a very ideal calculation, with a flatrate model for Mars air drag and gravity losses, perfect engine, etc, 80% is a bit optimistic. But then again, maybe the maker of that solar system map took a worse constellation for reference, than Mars Direct did. I don't know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source for the ISP: Modern Engineering for Design of Liquid-Propellant Rocket Engines (AKA The Huzel)&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2014-09-16T16:54:46.090" LastActivityDate="2014-09-16T16:54:46.090" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="5307" PostTypeId="1" AcceptedAnswerId="5308" CreationDate="2014-09-11T19:51:50.670" Score="13" ViewCount="688" Body="&lt;p&gt;I know the station's solar panels rotate to track the sun (and also rotate to be in-line with the station for lower drag on night passes), but how does the station itself rotate? I imagine it has to rotate for thermal management issues, or maybe not? I also believe it rotates for docking/supply ship approach, but I'm mostly interested in how it rotates for thermal management. Which axis? What's the rotation rate?&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2018-04-11T17:44:20.830" Title="How does the space station rotate, if at all?" Tags="|thermal-control|attitude|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5308" PostTypeId="2" ParentId="5307" CreationDate="2014-09-12T01:06:40.483" Score="12" Body="&lt;p&gt;Most of the time (when it's not maneuvering for experiments and/or visiting vehicles) the ISS maintains a &quot;fixed&quot; attitude in the noninertial Local Vertical, Local Horizontal &lt;a href=&quot;http://www.asi.org/adb/04/02/00/iss-coordinate-systems.pdf&quot; rel=&quot;noreferrer&quot;&gt;reference frame&lt;/a&gt;.  This means that it rotates about the Y body axis (the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/multimedia/iss_labs_guide.html&quot; rel=&quot;noreferrer&quot;&gt;&quot;port / starboard&quot; axis&lt;/a&gt;) at a rate of one revolution per orbit, about 0.067 degrees per second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For easier reference, here's the International Space Station’s coordinate system (image credit: &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/multimedia/iss_labs_guide.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/c5qqK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="49" LastEditDate="2014-09-12T03:08:09.093" LastActivityDate="2014-09-12T03:08:09.093" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="5309" PostTypeId="1" AcceptedAnswerId="5318" CreationDate="2014-09-12T06:08:04.370" Score="21" ViewCount="7793" Body="&lt;p&gt;From what I understand, pogo oscillation was already a known problem for Saturn V rocket's first stage (and likely other stages, not sure) long before, say, Apollo 11, especially on its central out of 5 main F-1 engines due to, I guess, not sturdy enough support structure (cruciform) and the central engine's force moving it upwards, shortening the fuel line during high thrust and vice-versa when the engine was off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might have been a known problem even before the first Saturn V flight. They (von Braun et al.) were dealing with combustion instabilities that they were trying to solve (or at least establish where the problem is) by adding a small explosive charge to the engines assembly and detonating it during hot fire testing of the first stage engines. That didn't go too well, and this pogo oscillation problem persisted at least also for the first Saturn V flight of Apollo 4.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, did they make structural adjustments to the Saturn V main stage cruciform during the Apollo program, or somehow otherwise dampen the pogo oscillation and sloshing problems, or perhaps make adjustments to its ascent (thrust) profile, or they just flew them &lt;em&gt;as is&lt;/em&gt; (which would be rather risky, wouldn't it - the term &lt;em&gt;rapid unplanned disassembly&lt;/em&gt; comes to mind)?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-09-12T06:49:40.150" LastActivityDate="2018-01-09T02:01:59.730" Title="How did they solve Saturn V pogo oscillation problems?" Tags="|rockets|nasa|apollo-program|engineering|saturn-v|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5310" PostTypeId="1" AcceptedAnswerId="5316" CreationDate="2014-09-12T06:44:06.470" Score="9" ViewCount="2217" Body="&lt;p&gt;How far is it from the Venus?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does Mercury gives too big perturbations for a stable Lissajous orbit?&lt;/p&gt;&#xA;" OwnerUserId="6026" LastActivityDate="2019-10-07T00:56:50.783" Title="Stability of Lissajous orbits around Sun-Venus L1" Tags="|lagrangian-points|venus|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5311" PostTypeId="2" ParentId="5310" CreationDate="2014-09-12T08:31:54.483" Score="5" Body="&lt;p&gt;Lagrange points move with the distance between primary and secondary body's barycenter, so the exact distance between Venus and &lt;span class=&quot;math-container&quot;&gt;$\text{SVL}_1$&lt;/span&gt; (Sun-Venus Lagrange point 1) would depend on Cytherean distance to the Sun in its orbit. For mean distance though, we can simplify this to Cytherean semi-major axis (&lt;span class=&quot;math-container&quot;&gt;$108,208,000\ \text{km}$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$0.723327\ \text{AU}$&lt;/span&gt;). Using following formula (refer to &lt;a href=&quot;http://www.unc.edu/%7Emarzuola/Math547_S13/Math547_S13_Projects/H_Kerner_Section003_Lagrangian_Points.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Lagrangian Points, An Application of Linear Algebra, Hannah Rae Kerner, 2013&lt;/a&gt; for how it is derived):&lt;/p&gt;&#xA;&lt;h2&gt;&lt;span class=&quot;math-container&quot;&gt;$$L_1 = R \left(1 -\left(\sqrt[3\ \ \ ]{\frac{\alpha}{3}}\right)\right)$$&lt;/span&gt;&lt;/h2&gt;&#xA;&lt;p&gt;Where &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt; is the ratio of &lt;span class=&quot;math-container&quot;&gt;$M_2$&lt;/span&gt; (secondary body's mass) to the combined mass of the system (&lt;span class=&quot;math-container&quot;&gt;$M_1 + M_2$&lt;/span&gt;), or &lt;span class=&quot;math-container&quot;&gt;$\frac{M_2}{M_1 + M_2}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;So in our case, since Cytherean mass is &lt;span class=&quot;math-container&quot;&gt;$0.815\ M_E$&lt;/span&gt; and the Sun's mass &lt;span class=&quot;math-container&quot;&gt;$333,000\ M_E$&lt;/span&gt; (&lt;span class=&quot;math-container&quot;&gt;$M_E$&lt;/span&gt; is Earth's mass), this mass ratio &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt; is &lt;span class=&quot;math-container&quot;&gt;$2.4474 \cdot 10^{-6}$&lt;/span&gt; and the mean &lt;span class=&quot;math-container&quot;&gt;$\text{SVL}_1$&lt;/span&gt; distance to Venus is &lt;span class=&quot;math-container&quot;&gt;$-0.009344\ R$&lt;/span&gt; or, to make it fit our formula, &lt;span class=&quot;math-container&quot;&gt;$0.99057\ R$&lt;/span&gt; from the Sun (primary body).&lt;/p&gt;&#xA;&lt;p&gt;So, in kilometers and astronomical units (AU), mean distance of &lt;span class=&quot;math-container&quot;&gt;$\text{SVL}_1$&lt;/span&gt; is then:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$-1,011,090\ \text{km}$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$-0.00676\ \text{AU}$&lt;/span&gt; from Venus, or&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$107,196,910\ \text{km}$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$0.71657\ \text{AU}$&lt;/span&gt; from the Sun&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;I calculated this using more precise values without cutting decimals off, but to confirm it, let's &lt;em&gt;cheat&lt;/em&gt; a bit and use &lt;a href=&quot;http://orbitsimulator.com/formulas/LagrangePointFinder.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;one of the online Lagrange point calculators&lt;/a&gt;. It assumes circular orbit (so only mean distance will be somewhat precise, like the one I calculate), and the figures for &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; of &lt;span class=&quot;math-container&quot;&gt;$108,208,000\ \text{km}$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$M_1 = 1\cdot M_\odot$&lt;/span&gt; (one &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_mass&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar mass&lt;/a&gt;) and &lt;span class=&quot;math-container&quot;&gt;$M_2 = 1\cdot M♀$&lt;/span&gt; (mass of Venus) come out as:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$-1,007,985\ \text{km}$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$-0.00674\ \text{AU}$&lt;/span&gt; from Venus, or&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$107,200,015\ \text{km}$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$0.71659\ \text{AU}$&lt;/span&gt; from the Sun&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;So fairly close, but I'm unsure what values that calculator uses as input, or at which point it cuts off decimal places so some small-ish discrepancy was expected.&lt;/p&gt;&#xA;&lt;p&gt;If we also apply Cytherean orbital eccentricity to our calculations (&lt;span class=&quot;math-container&quot;&gt;$e = 0.0067$&lt;/span&gt;), we get a movement of the &lt;span class=&quot;math-container&quot;&gt;$\text{SVL}_1$&lt;/span&gt; distance to Venus from &lt;span class=&quot;math-container&quot;&gt;$-1,004,259\ \text{km}$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$-0.006713\ \text{AU}$&lt;/span&gt; at perihelion to &lt;span class=&quot;math-container&quot;&gt;$-1,017,920\ \text{km}$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$-0.006804\ \text{AU}$&lt;/span&gt; at aphelion.&lt;/p&gt;&#xA;&lt;p&gt;Or simply &lt;span class=&quot;math-container&quot;&gt;$-1,011,090\ \text{km} \pm 0.67\%$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;I'll let others describe &lt;span class=&quot;math-container&quot;&gt;$\text{SVL}_1$&lt;/span&gt; stability and how much it would be perturbed by Mercury, but I suspect gravitational perturbations by Earth and Jupiter to be more severe. Of course, just that the distance of Lagrange point 1 from the primary and secondary body varies with their distance between each other during one orbit of the secondary body (Venus) around the primary one (Sun) due to being non-circular makes this &lt;span class=&quot;math-container&quot;&gt;$\text{SVL}_1$&lt;/span&gt; saddle point rather unstable. &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lissajous&lt;/a&gt; or indeed &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Halo orbits&lt;/a&gt; around these gravitationally flat points in space only make stationkeeping somewhat easier and these instabilities (and satellite's own due to precision burn requirements) easier to control. They don't cancel them out in any way, regardless how feeble they might be.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-09-12T16:21:59.727" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5312" PostTypeId="1" AcceptedAnswerId="5314" CreationDate="2014-09-12T11:25:59.530" Score="16" ViewCount="4467" Body="&lt;p&gt;This is a special case follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/916/are-sun-synchronous-orbits-possible-around-any-body&quot;&gt;Are sun-synchronous orbits possible around any body?&lt;/a&gt; Reading &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Sun-synchronous orbit&lt;/a&gt; a few days ago when trying to refresh my memory on orbital mechanics of SSO, I noticed this as a rather vague mention:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sun-synchronous orbits are possible around other &lt;a href=&quot;https://en.wikipedia.org/wiki/Oblate_spheroid&quot; rel=&quot;nofollow noreferrer&quot;&gt;oblate&lt;/a&gt; planets, such&#xA;  as &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars&lt;/a&gt;. But &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus&lt;/a&gt;, for example, is too spherical to have a satellite&#xA;  in sun-synchronous orbit. See the article &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus&lt;/a&gt; where a flattening&#xA;  coefficient of zero for this planet is cited.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have a problem imagining why this would be so. I would have thought (without doing actual calculations) that its long day that's longer than its year would be more problematic, e.g. for permanently sunlit dawn-to-dusk SSO but not all SSO, than it being too spherical. So what in orbital dynamics of SSO prohibits them around perfectly spherical bodies? Or is that Wikipedia quote simply inaccurate?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-09-13T01:03:12.627" Title="Why would Sun-synchronous orbits be impossible around perfectly spherical bodies?" Tags="|orbital-mechanics|venus|sun-synchronous|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5313" PostTypeId="2" ParentId="5312" CreationDate="2014-09-12T13:52:09.233" Score="7" Body="&lt;p&gt;Wikipedia is correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To have a sun synchronous orbit the satellite's orbital plane needs to precess by at a rate equal to the planet's mean orbital rate about the sun. The Keplerian orbital plane doesn't change. Some perturbing force is needed to make the plane precess. IN the case of sun synchronous satellites, the source of the perturbation is the planet's oblateness.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-09-12T13:52:09.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5314" PostTypeId="2" ParentId="5312" CreationDate="2014-09-12T14:06:30.103" Score="25" Body="&lt;p&gt;An orbit has an angular momentum that is conserved.  That angular momentum vector keeps the orbit at a fixed orientation in inertial space.  As a planet orbits the Sun, the local time of the ascending node moves around the clock over that planet's year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only way for an orbit to be Sun-synchronous is for there to be a torque applied to the orbit to rotate the angular momentum vector of the orbit in the correct direction once per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It turns out rather conveniently that the gravity field of an oblate planet provides just such a torque.  But only at the right orbit altitude and inclination.  The first coefficient of the spherical harmonic expansion of a non-spherical gravity field is called $J_2$, and results just from the oblateness.  The rotation rate of the ascending node is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\dot{\Omega}=-{3\over 2}\ J_2 \left(r_e\over a\left(1-e^2\right) \right)^2\ \ \sqrt{\mu\over a^3}\ \cos{i}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $r_e$ is the equatorial radius of the planet (e.g. Earth).  If $J_2$ is large enough, you can select $a$, $e$ (usually chosen to be zero), and $i$ to get a rotation rate that matches the inertial rotation of a Sun-centered frame, about a degree a day.  For Earth, the inclinations are around 97° to 101° (mostly polar and retrograde) for altitudes of 400 km to 1400 km respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If $J_2$ is too small, you can't get sufficient torque to rotate the orbit fast enough.  That is the case for Venus, whose $J_2$ is about 0.4% that of Earth's.  As was pointed out by the mongoose, this lack of oblateness is due to Venus' extremely low rotation rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can get an intuitive feel for where this torque comes from by considering what happens when a retrograde, mostly polar orbit approaches the equator.  An equatorial bulge will accelerate the orbit a little causing it to cross the equator sooner than it would if there were no equatorial bulge.  If the orbit is retrograde, then the local time below the node crossing will move a little later each orbit.  If that amount is the same as the amount by which the inertial frame, e.g. the constellations, moves earlier each orbit, then the orbit is Sun-synchronous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the eccentricity of the orbit of the planet, as well as the tilt of its rotation axis to the ecliptic, a real &quot;Sun-synchronous&quot; orbit isn't perfect, but it's still pretty good.  For Earth, the local time of the ascending node will vary about $\pm 15$ minutes, following the analemma.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-09-13T01:03:12.627" LastActivityDate="2014-09-13T01:03:12.627" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5315" PostTypeId="1" CreationDate="2014-09-12T15:15:17.963" Score="0" ViewCount="64" Body="&lt;p&gt;What is the man-made object which is currently the farthest from our planet?&#xA;Where is it in this very moment?&lt;/p&gt;&#xA;" OwnerUserId="6073" LastActivityDate="2014-09-12T15:15:17.963" Title="The farthest man-made object" Tags="|solar-system|extra-solar-object|" AnswerCount="0" CommentCount="3" ClosedDate="2014-09-12T15:22:12.177" ContentLicense="CC BY-SA 3.0" />
  <row Id="5316" PostTypeId="2" ParentId="5310" CreationDate="2014-09-12T18:36:47.713" Score="6" Body="&lt;p&gt;I get SVL1 as 1,007,927 kilometers from Venus' center. But that's too much precision. Since Venus' orbit isn't perfectly circular, you don't have that many significant digits. I usually say about a million kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've made a &lt;a href=&quot;http://clowder.net/hop/railroad/L1-2_distances.xls&quot; rel=&quot;noreferrer&quot;&gt;spreadsheet&lt;/a&gt; calculating L1 and L2 distances from various bodies orbiting the sun:&#xA;Sun plus Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, and Pluto.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spreadsheet also gives various planet moon L1 and L2 distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spreadsheet is based on equations found on pages 133 to 138 of Szebehely's &lt;em&gt;Theory of Orbits - The Restricted 3 Body Problem&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think Mercury is that much of an influence. I would guess pressure from sunlight would have a greater effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But even if the sun and venus and probe were a perfect circular 3-body system, L1 would not be stable.  You would need station keeping propellent in any event.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-09-12T18:36:47.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5317" PostTypeId="2" ParentId="1635" CreationDate="2014-09-12T18:42:17.190" Score="2" Body="&lt;p&gt;Ballon talk has me thinking about the alternative to rockets that carbon nanotubes suggest may be available in the hopefully near future. The space elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a practical standpoint I have thought there is no way they will be able to directly launch a 66,000 mile tow cable into orbit, but assuming we come up with a way to weave together long lengths of nanotubes into a cable or strap and we can fly out, and move a sizeable near earth asteroid to a geosynchronous earth orbit, without precipitating a dinosaurs having a bad day kind of celestial collision, it seems to me that the slow steady climb of a tethered balloon and a crew equipped with space suits will be involved in the final knitting together of the tether.  I assume the cable will start being woven together from the asteroid geosynchonously stationed somewhere over Jarvis, Baker, or Howland Islands along the equator in the middle of the Pacific.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I picked those because they are US territories and uninhabited and would seem to make pretty good potential space ports. If private enterprise were to build it, I assume they would want their airspace protected by the world's strongest military to avoid calamities and repairs.  Hence those islands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is would the tether being lowered into the atmosphere burn up as gravity tugged it towards the ground?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also assume a balloon would be needed because a rocket would be moving to fast to work on capturing the descending tether and you would need a relatively stable platform  for the crew to work on knitting the final pieces together in a similar way to the joining of the intercontinental railroad in Utah.&lt;/p&gt;&#xA;" OwnerUserId="6075" LastActivityDate="2014-09-12T18:42:17.190" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5318" PostTypeId="2" ParentId="5309" CreationDate="2014-09-12T19:24:56.850" Score="20" Body="&lt;p&gt;Apparently the issue could not be addressed in time for the Apollo 13 mission as is evidenced by this note in the &lt;a href=&quot;http://www.hq.nasa.gov/pao/History/SP-4204/ch22-7.html&quot;&gt;NASA archives&lt;/a&gt; available online. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During the Apollo 13 flight the pogo effect had reappeared, this time on the second stage. Severe oscillations had forced an early shutdown (two minutes ahead of schedule) of the inboard engine.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.hq.nasa.gov/pao/History/SP-4204/ch22-7.html&quot;&gt;NASA archives&lt;/a&gt; write to say about the Apollo 14 pogo oscillation fix&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Marshall and North American engineers devised three changes to the second stage. They installed a helium gas accumulator in the LOX line of the center engine. This reservoir served to dampen fluid pressure oscillations, keeping them out of phase with the vibrations of the thrust structure and engines. North American added a cutoff device to shut down the center engine in case the accumulator failed to control the oscillations. Finally, simplified propellant valves were installed on all five J-2 engines. The valves controlled the propellant mixture to the engines, providing a rich mixture for high thrust during the early portion of the burn and a leaner mixture later.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More reading in this context are &lt;a href=&quot;http://www.clavius.org/techsvpogo.html&quot;&gt;posted on this web-page&lt;/a&gt;. The author there writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Second, there was no one single mission before which pogo was a serious problem, and after which it wasn't. Pogo occurred in the Saturn V all the way up to Apollo 17. But during the program engineers took a series of steps to make pogo less of a problem for each successive mission. But there was often a delay because engineers needed time to analyze the results from a Saturn V flight, and in most cases the next Saturn V was already &quot;stacked&quot; in the assembly building and couldn't be easily modified. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2014-09-12T19:30:32.953" LastActivityDate="2014-09-12T19:30:32.953" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5320" PostTypeId="1" AcceptedAnswerId="5376" CreationDate="2014-09-13T01:28:30.573" Score="12" ViewCount="5137" Body="&lt;p&gt;To try and make parameters clear, can we say we are talking about 50% 'more Earth'? As in, Earth, but 1.5 times as big and heavy? And let us include the atmosphere. If there is 50% more of it by volume, then that would also make it thicker at sea level, right? And then it is also affected by higher gravity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I considered using a known Super-Earth as a reference, but the reputation of many is in question, and none seem at all promising as possible homes of intelligent aliens. Yet, the perennial question is &lt;a href=&quot;http://en.wikipedia.org/wiki/Fermi_paradox&quot; rel=&quot;noreferrer&quot;&gt;where is everybody&lt;/a&gt;? Perhaps we have an unusually easy time getting to orbit from our nice liquid-water rocky planet. Maybe that is a partial answer to that question. Just a thought.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-09-18T15:00:06.823" LastActivityDate="2014-11-04T11:43:35.247" Title="On a Super-Earth 1.5x the volume and mass of Earth, would our rocket technology allow us to reach orbit?" Tags="|launch|rockets|gravity|technology|" AnswerCount="3" CommentCount="8" ClosedDate="2018-04-05T20:24:58.587" ContentLicense="CC BY-SA 3.0" />
  <row Id="5321" PostTypeId="1" CreationDate="2014-09-13T09:33:31.487" Score="11" ViewCount="558" Body="&lt;p&gt;There's been a news article around claiming that cosmonauts have found sea plankton on the outside of the international space station, for example &lt;a href=&quot;http://www.space.com/26888-sea-plankton-space-station-russian-claim.html&quot;&gt;here&lt;/a&gt;. Is there any substance to this claim? Have any papers been published about it? Is there any public documentation of the EVA when the sampling was taken and the whole project which must have preceded and followed it?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A Russian official claims that samples collected by cosmonauts show&#xA;  evidence of sea plankton on the outside of the International Space&#xA;  Station, news agencies are reporting.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-09-15T12:59:03.973" Title="Plankton found outside the ISS, what is the substance in that claim?" Tags="|iss|astrobiology|astronauts|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5322" PostTypeId="1" AcceptedAnswerId="5324" CreationDate="2014-09-13T19:14:52.363" Score="22" ViewCount="2353" Body="&lt;p&gt;Those formerly on either side of the Iron Curtain can now admit that the Cold War had its plusses and minuses. The key minus, of course, is that the United States and the Soviet Union constructed enough nuclear devices to destroy the world many times over. After relations began to improve between the two nations (i.e. the Soviet Union collapsed, the Cold War ended, and a lot of people started sleeping better), nuclear disarmament became a hot-button issue. I can't find reliable figures regarding the number of missiles and warheads disarmed during and after the Cold War, but I imagine they number(ed) in the thousands. Once they were no longer suited for their original purpose, however, something had to have been done with the missiles. My question, in essence, boils down to this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were any former missiles repurposed for peaceful space transportation?&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="856" LastEditDate="2014-09-20T07:13:23.267" LastActivityDate="2014-12-15T23:05:47.800" Title="Were decommissioned Cold War missiles repurposed for peaceful spaceflight?" Tags="|rockets|technology|reuse|repurpose|icbm|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5323" PostTypeId="2" ParentId="2825" CreationDate="2014-09-13T20:21:38.417" Score="8" Body="&lt;p&gt;Directly from the Mars One site - &lt;a href=&quot;http://www.mars-one.com/faq/finance-and-feasibility/how-much-does-the-mission-cost&quot;&gt;Finance and Feasibility, Mission Cost&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After discussions with potential suppliers for each component and&#xA;  close examination, Mars One estimates the cost of putting the first&#xA;  four people on Mars at six billion US Dollar. The six billion figure&#xA;  is the cost of all the hardware combined, plus the operational&#xA;  expenditures, plus margins. For every next manned mission, Mars One&#xA;  estimates the costs at four billion US$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That is excluding the cost of maintaining 4 astronauts on Mars until they die.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, some have criticized the budget as being too low, instead suggesting it to be at the 100 billion Dollar mark. A manned mission to Mars was proposed by NASA in 2009 (which would return to Earth) which had a projected cost of $100 billion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having a look at some other proposed missions, such as Mars Direct, I would estimate that the cost to send 4 astronauts to Mars, without a return trip, would cost somewhere between 4 to 10 billion Dollars. Including the cost to maintain the mission.&lt;/p&gt;&#xA;" OwnerUserId="6053" LastEditorUserId="49" LastEditDate="2014-09-13T21:50:30.057" LastActivityDate="2014-09-13T21:50:30.057" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5324" PostTypeId="2" ParentId="5322" CreationDate="2014-09-13T21:14:43.863" Score="27" Body="&lt;p&gt;Yes, definitely. For example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Taurus, now &lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur-C&quot; rel=&quot;noreferrer&quot;&gt;Minotaur-C&lt;/a&gt; first stage (US, Orbital Sciences Corporation), based on &lt;a href=&quot;https://en.wikipedia.org/wiki/LGM-118_Peacekeeper&quot; rel=&quot;noreferrer&quot;&gt;Peacekeeper&lt;/a&gt; ICBM first stage. 9 launches since 1994, out of which 3 failures.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_I&quot; rel=&quot;noreferrer&quot;&gt;Minotaur I&lt;/a&gt; (US, Orbital Sciences Corporation), repurposed Minuteman missile with  M55A1 first stage and SR19 second stage. 11 launches since 2000, all successful.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_II&quot; rel=&quot;noreferrer&quot;&gt;Minotaur II&lt;/a&gt; a.k.a. Chimera (US, Orbital Sciences Corporation), consist of the M55A1 first stage and SR19AJ1 second stage of a decommissioned Minuteman missile, now serves as long range suborbital launch vehicle. 8 launches since 2000, all successful.&lt;/li&gt;&#xA;&lt;li&gt;Minotaur III (US, Orbital Sciences Corporation), based on SR-118 first stage, SR-119 second stage and SR-120 third stage of the &lt;a href=&quot;https://en.wikipedia.org/wiki/LGM-118_Peacekeeper&quot; rel=&quot;noreferrer&quot;&gt;LGM-118 Peacekeeper&lt;/a&gt; ICBM. No launches yet.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_IV&quot; rel=&quot;noreferrer&quot;&gt;Minotaur IV&lt;/a&gt; (US, Orbital Sciences Corporation) is also based on the first three stages of LGM-118 Peacekeeper ICBM. 5 successful launches since 2010.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_V&quot; rel=&quot;noreferrer&quot;&gt;Minotaur V&lt;/a&gt; (US, Orbital Sciences Corporation), also based on the first three stages of LGM-118 Peacekeeper ICBM. Launched &lt;a href=&quot;https://en.wikipedia.org/wiki/LADEE&quot; rel=&quot;noreferrer&quot;&gt;LADEE&lt;/a&gt; on her maiden flight.&lt;/li&gt;&#xA;&lt;li&gt;Conceptual Minotaur VI is based on the Minotaur IV+, using an additional SR-118 first stage.&lt;/li&gt;&#xA;&lt;li&gt;Start-1 (Russia, Moscow Institute of Thermal Technology) is a launch vehicle derived from first three stages of &lt;a href=&quot;https://en.wikipedia.org/wiki/RT-2PM_Topol&quot; rel=&quot;noreferrer&quot;&gt;RT-2PM Topol&lt;/a&gt; ICBM. 6 launches since 1993.&lt;/li&gt;&#xA;&lt;li&gt;Start (Russia, Moscow Institute of Thermal Technology) is a five stage variant of Start-1 and uses another RT-2PM Topol second stage. Its so far single launch in 1995 was a failure.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Strela_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;Strela&lt;/a&gt; (Russia, NPO Mashinostroyeniya, JSC &quot;Khartron&quot;) is orbital launch vehicle derived from the Soviet/Russian &lt;a href=&quot;https://en.wikipedia.org/wiki/UR-100&quot; rel=&quot;noreferrer&quot;&gt;UR-100NU&lt;/a&gt; ICBM missile. It launched Kondor radar imaging satellite for the Russian military in June 2013 and is scheduled to launch another one this December 18, 2014.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Rokot&quot; rel=&quot;noreferrer&quot;&gt;Rockot&lt;/a&gt; (Russia / Europe, &lt;a href=&quot;https://en.wikipedia.org/wiki/Eurockot_Launch_Services&quot; rel=&quot;noreferrer&quot;&gt;Eurockot Launch Services&lt;/a&gt; owned by EADS Astrium and Khrunichev State Research and Production Space Center), like Strela, is also a derivative of the UR-100N  ICBM missile. It made 25 launches, 2 of them were failures.&lt;/li&gt;&#xA;&lt;li&gt;Not based on ICBM but a ballistic missile nonetheless, &lt;a href=&quot;https://en.wikipedia.org/wiki/Shavit&quot; rel=&quot;noreferrer&quot;&gt;Shavit&lt;/a&gt; (Israel, Republic of South Africa) some variants in this family of launch vehicles are based on first two stages of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jericho_(missile)&quot; rel=&quot;noreferrer&quot;&gt;Jericho II&lt;/a&gt; ballistic missile.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And I'm sure this list isn't complete and many other families of launch vehicles were either designed to serve dual purpose - as ICBM as well as orbital or suborbital launch vehicles, or smaller elements of them were later repurposed for peaceful use, such as e.g. stage engines, avionics, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Missile silos can also be &lt;a href=&quot;http://1800recycling.com/2010/11/repurposed-nuclear-missile-recycle-silos&quot; rel=&quot;noreferrer&quot;&gt;repurposed for peaceful purposes&lt;/a&gt;, and &lt;a href=&quot;http://www.space.com/23188-ballistic-missiles-disaster-relief-launches.html&quot; rel=&quot;noreferrer&quot;&gt;it's been contemplated&lt;/a&gt; to even repurpose some ballistic missile interceptors as humanitarian &lt;em&gt;relief missiles&lt;/em&gt; delivering urgent supplies fast to disaster zones. Though basic idea behind it &lt;a href=&quot;https://en.wikipedia.org/wiki/Berlin_Blockade&quot; rel=&quot;noreferrer&quot;&gt;isn't anything new&lt;/a&gt; in terms of repurposing military &lt;em&gt;air and space&lt;/em&gt; vehicles for philanthropic causes, using ballistic missiles to reach distant zones in distress to help instead of cause additional havoc is a rather novel concept.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-12-15T23:05:47.800" LastActivityDate="2014-12-15T23:05:47.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5325" PostTypeId="2" ParentId="5322" CreationDate="2014-09-13T22:41:30.010" Score="14" Body="&lt;p&gt;There are also the &lt;a href=&quot;http://en.wikipedia.org/wiki/Dnepr-1&quot;&gt;Dnepr-1&lt;/a&gt; rockets, made from converted R-36 missiles. One that launched in &lt;a href=&quot;http://www.nasaspaceflight.com/2014/06/russian-dnepr-rocket-record-launch-37-satellites/&quot;&gt;June 2014&lt;/a&gt; delivered a payload of 37 satellites, making it the current record-holder for most satellites delivered in one launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk likes to tell the story of going to Russia to buy ICBMs when he was first thinking of getting into a space project - that's how he puts it, he never mentions that they were already converted, and has at least once joked that he felt like the sellers might have been willing to sell the warheads too if he'd tried. That was the beginnings of SpaceX.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-09-13T22:41:30.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5327" PostTypeId="2" ParentId="5322" CreationDate="2014-09-14T07:25:06.473" Score="11" Body="&lt;p&gt;Converting old ICBMs for space exploration is hardly a new thing.  Many of the retired &lt;a href=&quot;https://en.wikipedia.org/wiki/SM-65_Atlas&quot;&gt;SM-65 &lt;em&gt;Atlas&lt;/em&gt;&lt;/a&gt; missiles were re-purposed as the first stages of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Template:Atlas_rockets&quot;&gt;wide variety of orbital launch vehicles&lt;/a&gt;.  The &lt;a href=&quot;https://en.wikipedia.org/wiki/SM-68_Titan&quot;&gt;SM-68 &lt;em&gt;Titan&lt;/em&gt;&lt;/a&gt; was similarly converted, but in smaller numbers.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-09-14T07:25:06.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5329" PostTypeId="1" CreationDate="2014-09-14T15:24:52.660" Score="17" ViewCount="2941" Body="&lt;p&gt;As a follow-up to  &lt;a href=&quot;https://space.stackexchange.com/questions/1703/does-cosmic-dust-pose-a-problem-for-long-term-satellites-telescopes-and-probes&quot;&gt;Does cosmic dust pose a problem for long-term satellites, telescopes and probes?&lt;/a&gt;, assuming satellite's long duration stay in Earth's orbit – let's for the sake of argument assume a run-of-the-mill communications satellite in Geostationary Orbit (GEO) – would its solar panels gather dust and require cleaning?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the rate at which this dust would gather, or if that is unknown, how fast do solar panels of GEO satellites deteriorate due to external causes that can't be attributed to material degradation due to exposure to solar radiation, solar wind, micrometeorites and other causes unrelated to persistent dust covering solar cells?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, assuming this can be identified as a genuine concern, what techniques can they employ to clean their solar panels or help mitigate persisting dust problem?&lt;/p&gt;&#xA;" OwnerUserId="6081" LastEditorUserId="58" LastEditDate="2019-02-18T18:23:53.623" LastActivityDate="2023-04-19T12:19:40.907" Title="Do solar panels on satellites gather dust and need cleaning?" Tags="|artificial-satellite|solar-power|space-dust|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5331" PostTypeId="5" CreationDate="2014-09-14T17:51:46.120" Score="0" Body="&lt;p&gt;Intercontinental Ballistic Missiles, often known by their abbreviation &lt;strong&gt;ICBM&lt;/strong&gt;, are a class of rockets developed during the Cold War for the purpose of delivering warheads (primarily nuclear) to nearly anywhere on Earth. ICBM creation during the Cold War also contributed to the development of rocketry for space exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Questions should focus on the rocket portion of the vehicle. Discussions about the warheads and their use (beyond trajectories set by the rocket) are off-topic.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="6816" LastEditDate="2019-01-15T08:21:28.593" LastActivityDate="2019-01-15T08:21:28.593" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5332" PostTypeId="4" CreationDate="2014-09-14T17:51:46.120" Score="0" Body="Intercontinental Ballistic Missiles (ICBM) are a class of rockets developed for the purpose of delivering warheads to targets almost anywhere on Earth. Use for discussing the rocket portion of the vehicle. Discussing warheads themselves are off-topic. " OwnerUserId="5963" LastEditorUserId="6816" LastEditDate="2019-01-15T07:06:03.137" LastActivityDate="2019-01-15T07:06:03.137" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5333" PostTypeId="2" ParentId="1635" CreationDate="2014-09-15T08:56:34.533" Score="2" Body="&lt;p&gt;It may be possible, but &lt;strong&gt;won't help much&lt;/strong&gt; if you want to reach an orbit. This is easily seen considering the &lt;strong&gt;energy budget&lt;/strong&gt;. The specific orbital energy for a satellite orbiting Earth at mean altidude (orbital semimajor axis minus Earth radius to be precise) $H$ above Earth's surface is&#xA;$$&#xA;E_{sat} = -\frac{GM}{2(R+H)} \approx -\frac{GM}{2R} + \frac{gH}{2} &#xA;$$&#xA;with $M$ and $R$ the Earth mass and radius, respectively, and $g=GM/R^2$ as usual. The specific energy reached by a ballon at altitude $h$ is&#xA;$$&#xA;E_{bal} = -\frac{GM}{R+h} \approx -\frac{GM}{R} + gh&#xA;$$&#xA;(ignoring the small effect from Earth's rotation, i.e. assuming launch at a pole).&#xA;Thus, when launching a rocket from the altitude $h$, it must still provide the difference&#xA;$$&#xA;\Delta E =E_{sat}-E_{bal} \approx \frac{GM}{2R} + g(H/2-h).&#xA;$$&#xA;Thus, the main contribution $GM/2R$ is not helped by increasing $h$ from $h=0$ (launch at zero altitude).&lt;/p&gt;&#xA;" OwnerUserId="5976" LastActivityDate="2014-09-15T08:56:34.533" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5334" PostTypeId="2" ParentId="5321" CreationDate="2014-09-15T12:59:03.973" Score="5" Body="&lt;p&gt;Lynn Rothschild, evolutionary biologist and astrobiologist at NASA Ames Research Center, is &lt;a href=&quot;http://radio.seti.org/blog/2014/09/big-picture-science-is-it-true-lynn-rothschild-space-plankton/&quot; rel=&quot;noreferrer&quot;&gt;skeptical&lt;/a&gt; and says that there is very little information about the claim.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-09-15T12:59:03.973" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5335" PostTypeId="1" CreationDate="2014-09-15T16:40:47.667" Score="12" ViewCount="828" Body="&lt;p&gt;In my answer to &lt;a href=&quot;https://space.stackexchange.com/a/2624/49&quot;&gt;What would it feel like to be in a Martian dust storm?&lt;/a&gt; I assert that Martian dust storms would be best avoided, and as one of the reasons (besides carrying corrosive chemicals like &lt;a href=&quot;https://en.wikipedia.org/wiki/Perchlorate#Perchlorate_on_Mars&quot; rel=&quot;noreferrer&quot;&gt;perchlorate salts&lt;/a&gt;, hydrogen peroxide, tiny and sharp dust particles being strongly ablative, decreased visibility and so on) I also mention triboelectric charge that such dust storms could cause on the surface of the suit and potentially lead to catastrophic discharge tearing the fabric of the suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there's another possible concern which I also mention, and that is &lt;a href=&quot;http://ns.umich.edu/new/releases/7199&quot; rel=&quot;noreferrer&quot;&gt;lightnings on Mars&lt;/a&gt;. To me, that we directly detected them in 2009 isn't really surprising, since the Martian regolith is covered with rust, iron oxides which gives it its reddish surface tint that the planet is famous for. This got me thinking however, that if Martian dust storms carry sufficient electric potential that its &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0032063311001061&quot; rel=&quot;noreferrer&quot;&gt;triboelectricity has been suggested as the leading candidate for generating hydrogen peroxide on Mars&lt;/a&gt; that was first detected in 2003, such lightnings cannot be rare and far between, and the overall electric charge capacity of these dust storms must be pretty impressive despite their relatively low kinetic strength.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which brings me to my question. Assuming asking for maximum strength electric discharge events (read: lightnings) would be rather frivolous if we barely detected any (not too surprising considering reduced visibility), perhaps more fruitful would be asking about their average charge capacity that they carry;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Has any orbiter been measuring Martian dust storms capacitance, or can their strength perhaps be established indirectly by measuring its effect on atmospheric chemistry, e.g. local changes in the amount of atmospheric hydrogen peroxide before, during and after the storm?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other possible ways of establishing this, including laboratory tests on simulated Martian regolith in Mars-analog atmospheric pressure environment, or even computed models, so I don't want to limit answers to a handful of options that I mentioned to establish this. Any additional insight is welcome, but please provide references in your answers.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2020-01-15T22:39:22.843" Title="How strong is the electric charge capacity of Martian dust storms?" Tags="|mars|planetary-science|dust|regolith|lightning|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5336" PostTypeId="1" CreationDate="2014-09-15T18:51:52.770" Score="5" ViewCount="359" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Pb40.png&quot; alt=&quot;enter image description here&quot;&gt;There's something that's visible in the SE (roughly) from Denver now ~12:40PM. It seems to be tracking north. I tried taking a photo but my camera doesn't have the resolution. I'm really curious. I understand the ISS can be visible during the day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/a9yxU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6092" LastEditorUserId="6092" LastEditDate="2014-09-15T20:10:33.587" LastActivityDate="2021-05-09T07:57:34.030" Title="Any idea what's in the SE sky from Denver around 12:40PM EDT?" Tags="|iss|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5337" PostTypeId="1" AcceptedAnswerId="5339" CreationDate="2014-09-15T20:53:48.990" Score="7" ViewCount="873" Body="&lt;p&gt;I'm just curious why this is important to SpaceX.  The reusable launch system that SpaceX is developing was supposedly not funded by any of NASA's contract.  What benefits would this give to SpaceX?  Does this just give SpaceX more freedom with using this technology since there is no contract involved?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See this quote from &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX_reusable_launch_system_development_program&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on SpaceX reusable launch system development program&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In June 2014, COO Gwynne Shotwell clarified that all funding for&#xA;  development and testing of the reusable launch system technology&#xA;  development program is private funding from SpaceX, with no&#xA;  contribution by the US government. SpaceX has not publicly&#xA;  disclosed the cost of the development program.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6093" LastEditorUserId="49" LastEditDate="2014-09-15T23:45:36.763" LastActivityDate="2014-09-16T00:38:28.013" Title="What are the benefits of saying a commercial space development program is funded privately?" Tags="|nasa|spacex|economics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5338" PostTypeId="2" ParentId="176" CreationDate="2014-09-15T21:06:15.393" Score="2" Body="&lt;p&gt;I'm afraid there aren't really any good candidates for terraforming in Earth's solar system.&#xA;Mars and Venus were once quite similar to Earth naturally.  The fact that they aren't now demonstrates that they would have to be artificially maintained over geologic time scales.&#xA;Worse, their substantially different physical parameters would keep them from ever being truly Earth-like. Romantic fantasies aside, what would be the point of taking centuries if not millennia to build EPCOT Earths when artificial environments that better reproduce the conditions that we prefer could be built in years or decades.  Built from carbon nanotubes, such structures could be hundreds of miles wide and thousands of miles long. Space enough for anyone I should think, and the people who build them would actually be able to live in them.  If you want a project that stretches over generations, build such a space colony, attach a space drive and head for the stars. &lt;/p&gt;&#xA;" OwnerUserId="6094" LastActivityDate="2014-09-15T21:06:15.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5339" PostTypeId="2" ParentId="5337" CreationDate="2014-09-15T23:35:13.930" Score="8" Body="&lt;p&gt;One of the most important things for any company looking to achieve something new in the field of aerospace is to have a good budget. NASA is funded by the United States government, which has its plusses and minuses. The big plus is that its budget is rather large: Close to or over 18 billion dollars, for the last part of the 2000s. Is that good? Yes. There are a lot of things you can do with 18 billion dollars. But there's a catch. NASA competes for a share in the federal budget with many other agencies (okay, &lt;em&gt;all&lt;/em&gt; other federal agencies), and if some of them were to need more funding (e.g. in times of war, or if there was a natural disaster requiring a lot of money), NASA's budget could be slashed dramatically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a private company, on the other hand, the money that goes in is dedicated completely to that company's goals. If SpaceX receives, say, 8 billion dollars (I haven't been able to find an accurate figure) in private funding, all of that 8 billion will go to its projects. This isn't the case with a government's budget: Taxpayers pay a sum of money each year, an officials divvy up the money where it is needed. You may want a certain percent of your money to go to NASA, but you have no control. If you give the same amount of money to SpaceX, you know it will go to its projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, the advantages of having a privately-funded budget are a) Relatively constant funding, and b) All money the company takes in goes to that company's projects, where the investors want it, as opposed to a government taking in money from taxes and not necessarily distributing it where taxpayers want it to be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I don't agree with Gwynne Shotwell for the reason that a private company is dependent on its investors and may do what the investors want, while a government (in theory) doesn't always have to do what the people want, but that is a personal opinion, and (I don't think) relevant to your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source for NASA's budget:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.gpo.gov/fdsys/pkg/BUDGET-2010-BUD/pdf/BUDGET-2010-BUD-23.pdf&quot;&gt;http://www.gpo.gov/fdsys/pkg/BUDGET-2010-BUD/pdf/BUDGET-2010-BUD-23.pdf&lt;/a&gt;. Wikipedia's page on NASA's budget references this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TildalWave recently brought up a good point regarding NASA-private company relations, and suggested I include it here, so I'll add in some more information:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A portion of SpaceX's budget comes from government contracts. This is the case with many private companies: the government needs someone to manufacture tanks, aircraft, or, in this case, rockets, and pays a private company to do so. A company such as SpaceX can earn a lot of money this way. The trouble, though, is that competing for a government contract is high-risk. Chances are, there are many other competitors with similar proposals, and the government can pick and choose. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This case was exemplified in the early conceptual stages of the space shuttle program - the pictures in the book &quot;Space Shuttle: The History of the National Space Transportation System: The First 100 Missions&quot; by Dennis R. Jenkins gives a great look at how dramatically the design changed from time to time, depending on the contractor and new requirements. Tens of contractors, sub-contractors, and sub-sub-contractors competed for a piece of the pie, and many failed in their efforts. The take-away message is that even if a private company competes for government contracts, it can't assume in its budget that it will get them. A private company operating entirely under its own budget doesn't have those issues.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-09-16T00:38:28.013" LastActivityDate="2014-09-16T00:38:28.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5340" PostTypeId="2" ParentId="176" CreationDate="2014-09-16T00:39:46.467" Score="6" Body="&lt;p&gt;Terraform, shmerraform. The planet that would take the &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;least resources to make fully habitable &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;is the Moon. Even on Mars it would take &lt;a href=&quot;http://science.howstuffworks.com/terraforming2.htm&quot;&gt;centuries&lt;/a&gt; of extremely intensive and expensive work before you could walk outside without a spacesuit on. Functionally, if you are looking for a new place to live in the solar system, think in terms of enclosed spaces, unless you are thinking of your great-great-great-great-great-great-grandkids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have a way of building suitably resistant inflatable domes over large areas, the line between terraforming and artificial environments can really blur. That seems like the most plausible place where a human will first stand unprotected, surrounded by vegetation, look up, and see stars. The one-sixth gravity and the proximity to Earth give you a huge advantage in achieving that on the moon. Compared to the cost of terraforming, we're talking peanuts, even if you did it over hundreds of square kilometers. I can't find a reference so far concerning domes, but this &lt;a href=&quot;http://www.space.com/21588-how-moon-base-lunar-colony-works-infographic.html&quot;&gt;infographic&lt;/a&gt; at Space.com points out several of the things the moon has going for it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Found a reference for a &lt;a href=&quot;http://www.nss.org/settlement/moon/shackleton.html&quot;&gt;domed lunar city&lt;/a&gt;! The proposal is to put a dome over &lt;a href=&quot;http://en.wikipedia.org/wiki/Shackleton_(crater)&quot;&gt;Shackleton Crater&lt;/a&gt; at the lunar south pole, 25 miles in diameter and 5000 feet high. Sounds good to me.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-09-20T01:14:10.293" LastActivityDate="2014-09-20T01:14:10.293" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5341" PostTypeId="2" ParentId="5336" CreationDate="2014-09-16T01:55:47.973" Score="9" Body="&lt;p&gt;&lt;a href=&quot;http://www.flightradar24.com/HBAL374/455c412&quot; rel=&quot;noreferrer&quot;&gt;Flightradar24&lt;/a&gt; says that a couple of Google's &lt;a href=&quot;http://www.google.com/loon/&quot; rel=&quot;noreferrer&quot;&gt;Project Loon&lt;/a&gt; high-altitude balloons have been meandering around Denver airspace for the past day or so.  It's quite possible that what you've been seeing was one of them.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-09-16T01:55:47.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5344" PostTypeId="2" ParentId="5329" CreationDate="2014-09-16T02:51:31.287" Score="14" Body="&lt;p&gt;Satellites do not have any capacity to clean the solar arrays.  The solar arrays degrade over life due to many factors:  radiation damage, UV darkening, micrometeroids, etc.  The manufacturers have models of degradation and design for it.  Adhering dust is the least of it.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-09-16T02:51:31.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5345" PostTypeId="2" ParentId="5252" CreationDate="2014-09-16T07:45:37.587" Score="15" Body="&lt;p&gt;Wikipedia has an article on the topic of &lt;a href=&quot;http://en.wikipedia.org/wiki/Sex_in_space&quot; rel=&quot;nofollow noreferrer&quot;&gt;sex in space&lt;/a&gt;. The notable conclusion from that document is that no human spaceflight agency has any documentation regarding flight experiments conducted on human sexuality in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, rats and I think other non-mammals have been conceived, undergone embryonic development, been born, and been raised in space. Some information on the rats can be found &lt;a href=&quot;https://web.archive.org/web/20130109085357/http://www.aticourses.com/blog/index.php/tag/document-12-571-3570/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that when we finally do get around to doing it, some enterprising designers / engineers have already developed &lt;a href=&quot;http://en.wikipedia.org/wiki/2suit&quot; rel=&quot;nofollow noreferrer&quot;&gt;garments designed specifically for copulation in zero-G&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorDisplayName="user20636" LastEditDate="2019-12-10T15:05:05.023" LastActivityDate="2019-12-10T15:05:05.023" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="5346" PostTypeId="2" ParentId="5257" CreationDate="2014-09-16T10:00:12.750" Score="2" Body="&lt;p&gt;Note also that the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_program&quot; rel=&quot;nofollow&quot;&gt;Space Transportation System&lt;/a&gt; meets the requirements set by the OP:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;no use of parachutes (and no bouncing airbags). Heat shield and chock absorbing landing legs, and maybe before that aerobreaking, might be practically complementary.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Orbiter would use the OMS to propulsively manoeuvre out of LEO into the atmosphere, and then aerobrake until touchdown. Note that Endeavour was in fact fitted with a drag chute that would deploy after touchdown, and drag chutes were subsequently retrofitted to most (all?) other orbiters. However, the orbiters were capable of, and performed many, landings without the drag chute.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-09-16T10:00:12.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5347" PostTypeId="1" CreationDate="2014-09-16T10:31:41.497" Score="18" ViewCount="2861" Body="&lt;p&gt;Although I have seen many perfectly symmetric nozzles, I have also seen designs such as this (Virgin Galactic) which is only symmetrical about one axis:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SDsYK.jpg&quot; alt=&quot;Virgin Galactic SpaceShipTwo&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this?&lt;/p&gt;&#xA;" OwnerUserId="4693" LastEditorUserId="709" LastEditDate="2014-09-16T23:49:33.227" LastActivityDate="2018-10-24T22:08:31.180" Title="Why the non-symmetric design of rocket nozzles?" Tags="|rockets|propulsion|engine-design|engines|nozzle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5348" PostTypeId="1" CreationDate="2014-09-16T15:01:02.960" Score="3" ViewCount="386" Body="&lt;p&gt;ISRO &lt;a href=&quot;http://isp.justthe80.com/planetary-exploration/mars-obiter#TOC-Restarting-Liquid-Engine&quot; rel=&quot;nofollow&quot;&gt;expressed concerns&lt;/a&gt; on restarting the Liquid Apogee Motor (LAM) for Mars Orbit Insertion (MOI) after a long coasting period of its &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;nofollow&quot;&gt;Mars Orbiter Mission&lt;/a&gt; (MOM). ISRO has redesigned the LAM plumbing to isolate the fuel / oxidizer lines used for Trans-Mars Injection (TMI) and instead use separate (and fresh) fuel / oxidizer lines for the MOI manoeuvre.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISRO will test fire the LAM ahead of MOI for a short duration to check its health. Plan B, if it does not pass the test, is to use the attitude thrusters for MOI to enter a mission salvaging orbit around Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both the LAM and the attitude thrusters use the same fuel/oxidizer from shared tanks:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.isro.org/mars/pdf/press%20briefing%20on%20MOI.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.isro.org/mars/pdf/press%20briefing%20on%20MOI.pdf&lt;/a&gt; (page:8)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is it then, that the thrusters are considered more reliable than the LAM?&lt;/p&gt;&#xA;" OwnerUserId="1507" LastEditorUserId="1507" LastEditDate="2014-09-17T05:20:01.773" LastActivityDate="2014-09-17T05:20:01.773" Title="ISRO's Mars Orbiter Mission - LAM vs Attitude thrusters reliability" Tags="|mars|isro|engine-design|engines|mars-orbiter-mission|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5349" PostTypeId="2" ParentId="5347" CreationDate="2014-09-16T15:18:35.383" Score="15" Body="&lt;p&gt;According to comments on &lt;a href=&quot;http://www.parabolicarc.com/2014/01/12/spaceshiptwo-powered-flight-3-photo-essay/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this thread&lt;/a&gt;, the shape of the nozzle on SpaceShip Two is to provide &quot;free pitch-up&quot; during the short powered flight period. That nozzle shape would direct thrust force downward slightly, under the craft's center of gravity, causing it to slowly rotate nose-upward. Presumably modifying the nozzle shape was easier than tilting the whole engine, and the nozzle can be customized for different flight profiles (i.e. more or less fuel carried on a particular flight). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=6RwHN50s1yw&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt;, you can see the pitch-up maneuver during the powered burn from roughly 0:30 to 1:00. Ordinary aircraft achieve pitch-up by using control surfaces (elevators/elevons), but using these creates drag. Directing the rocket motor thrust instead gives a pitching force, with less drag. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-10-24T22:08:31.180" LastActivityDate="2018-10-24T22:08:31.180" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="5352" PostTypeId="1" AcceptedAnswerId="5353" CreationDate="2014-09-16T21:46:22.840" Score="4" ViewCount="1960" Body="&lt;p&gt;As I understand it, our current breathable atmosphere on Earth is the result of early lifeforms transforming huge amounts of CO2 into O2. Before early life, even Earth's atmosphere wasn't livable for modern humans, let alone that of current Mars, Venus, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But is there any reason why a planet couldn't develop naturally, without the aid of life, to have an atmosphere that humans could survive breathing? Obviously it wouldn't be common, but it is possible, or is there some chemical process I'm not aware of that would prevent the formation of a breathable atmosphere through non-biological means?&lt;/p&gt;&#xA;" OwnerUserId="1216" LastActivityDate="2022-05-24T01:15:55.680" Title="What are the odds of finding a lifeless planet with breathable air?" Tags="|atmosphere|life|oxygen|chemistry|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5353" PostTypeId="2" ParentId="5352" CreationDate="2014-09-16T22:48:44.097" Score="8" Body="&lt;p&gt;It's actually not that far-fetched, but non-biological processes involved with creation of such breathable atmospheres likely wouldn't be sustainable for a very long time. Let's imagine a largely water ice world that was first formed beyond its host star's &lt;a href=&quot;https://en.wikipedia.org/wiki/Frost_line_(astrophysics)&quot; rel=&quot;nofollow noreferrer&quot;&gt;snow line&lt;/a&gt; but later swapped orbit with another body that orbited within the star's &lt;a href=&quot;https://en.wikipedia.org/wiki/Circumstellar_habitable_zone&quot; rel=&quot;nofollow noreferrer&quot;&gt;Goldilocks zone&lt;/a&gt;. Surface ice would start melting because it was exposed to more heat from its sun, water vapor would increase atmospheric pressure, and stronger UV radiation would start splitting weak hydrogen bond to &lt;span class=&quot;math-container&quot;&gt;$\require{mhchem}\ce{H_2O -&amp;gt; H+ + OH-}$&lt;/span&gt; and then &lt;span class=&quot;math-container&quot;&gt;$\ce{2OH- -&amp;gt; O^2- + H_2O}$&lt;/span&gt; in its ionosphere.&lt;/p&gt;&#xA;&lt;p&gt;Now, if the planet moved at just about right distance to its parent star and the star isn't too violent (our Sun for example isn't), the atmosphere would start losing hydrogen to space with solar wind while the remaining oxygen would be too heavy and sink towards the surface. Planet would still also be losing oxygen to space, but at a slower rate than it's losing hydrogen. And nearly &lt;sup&gt;8&lt;/sup&gt;/&lt;sub&gt;9&lt;/sub&gt; the mass of water is oxygen. Some of this oxygen would recombine into its molecular state &lt;span class=&quot;math-container&quot;&gt;$\ce{O_2}$&lt;/span&gt; and some would form hydrogen peroxide &lt;span class=&quot;math-container&quot;&gt;$\ce{H_2O_2}$&lt;/span&gt; when recombining with the rest of liquid water on planet's surface, leaving mostly oxygen rich atmosphere above it which would be technically breathable.&lt;/p&gt;&#xA;&lt;p&gt;This couldn't be considered a self-sustainable process though, due to hydrogen loss to outer space. There are also other non-biological processes that could enrich planet's atmosphere with oxygen, like e.g. naturally occurring &lt;a href=&quot;https://en.wikipedia.org/wiki/Sabatier_reaction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sabatier&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Bosch_reaction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bosch&lt;/a&gt; reactions, but these reactions require high temperature and large quantities of hydrogen so such atmosphere would be less likely and too hot and quite possibly too dense (pressure, violent weather, ...) for comfort.&lt;/p&gt;&#xA;&lt;p&gt;Another problem with no life present is that such atmospheres could contain high concentrations of toxic elements and molecules in them, such as too high concentrations of carbon oxides (&lt;span class=&quot;math-container&quot;&gt;$\ce{CO}$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\ce{CO_2}$&lt;/span&gt;) which are both toxic to the human body, cyanides (containing &lt;span class=&quot;math-container&quot;&gt;$\ce{CN-}$&lt;/span&gt;), even too high concentrations of gaseous state hydrogen peroxide above the planet's surface. So called &lt;em&gt;filler gases&lt;/em&gt; like all the non-reactive, inert gases, e.g. nitrogen, argon, even helium would be fine, though. So it gets complicated and it's a lottery. But it's not out of the realm of possibility.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="47149" LastEditDate="2022-05-24T01:15:55.680" LastActivityDate="2022-05-24T01:15:55.680" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5354" PostTypeId="2" ParentId="5348" CreationDate="2014-09-17T00:24:04.990" Score="2" Body="&lt;p&gt;It's not clear that the thrusters are considered more reliable. It may be as simple as it being unlikely that both the LAM and the thrusters would fail, and the thrusters have quite a bit of redundancy if a subset fail. Depending on how they're arranged, four working thrusters might be enough to orient the craft for the MOI burn, and two could do the &quot;Plan B&quot; burn. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-09-17T00:24:04.990" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5356" PostTypeId="1" CreationDate="2014-09-17T12:52:51.053" Score="10" ViewCount="1006" Body="&lt;p&gt;If NASA were to apply the safety standards it is using on the CCtCAP program (Dragon V2 and CST-100 now chosen) would Soyuz meet those standards? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose it helps if we are clear on what the safety standards are?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much of it is paperwork, and for Soyuz, at this point in time, test flights are obviously not relevant (hundreds of manned flights so far?) and has an abort system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But would Soyuz 'pass'?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-09-17T14:50:29.907" LastActivityDate="2014-10-14T20:54:40.457" Title="Would Soyuz meet NASA current rating standards for manned spaceflight?" Tags="|crewed-spaceflight|soyuz-spacecraft|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5357" PostTypeId="1" CreationDate="2014-09-17T12:54:55.420" Score="12" ViewCount="1096" Body="&lt;p&gt;If NASA were to apply the safety standards it is using on the CCtCAP program (Dragon V2 and CST-100 now chosen) would the Space Shuttle have met those standards?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose it helps if we are clear on what the safety standards are?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle is now in the past, but would it have been able to pass the standards?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No abort system to speak of. Lots of manned flights already, so demonstrated it could work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lots of paperwork required to be filled out though. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-09-17T19:00:27.803" LastActivityDate="2014-09-17T19:46:34.680" Title="Would the Space Shuttle have met the current NASA manned flight standards?" Tags="|crewed-spaceflight|space-shuttle|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5358" PostTypeId="1" AcceptedAnswerId="5361" CreationDate="2014-09-17T13:33:02.313" Score="3" ViewCount="2826" Body="&lt;p&gt;I am doing a report on TLE's and I was looking up what the numbers mean. I found a very helpful site. See here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/tle_def.html&quot; rel=&quot;nofollow&quot;&gt;http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/tle_def.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However I do not understand what the checksum is. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anybody help me?&lt;/p&gt;&#xA;" OwnerUserId="6099" LastEditorUserId="25" LastEditDate="2014-09-17T14:13:38.223" LastActivityDate="2023-01-05T08:09:43.180" Title="what does Check sum TLE mean?" Tags="|debris|orbital-elements|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5359" PostTypeId="2" ParentId="5356" CreationDate="2014-09-17T13:39:27.137" Score="3" Body="&lt;p&gt;No. For example, one of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Commercial_Crew_Development&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCDev&lt;/a&gt; (Commercial Crew Development) key requirements includes: &lt;strong&gt;&lt;em&gt;Deliver and return four crew and their equipment&lt;/em&gt;&lt;/strong&gt;. &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz capsule&lt;/a&gt; only has the &lt;a href=&quot;https://space.stackexchange.com/a/4116/49&quot;&gt;capacity to &lt;em&gt;cram&lt;/em&gt; three astronauts / cosmonauts&lt;/a&gt; at a time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Refer to &lt;a href=&quot;http://commercialcrew.nasa.gov/document_file_get.cfm?docid=107&quot; rel=&quot;nofollow noreferrer&quot;&gt;Commercial Crew Program: Key Driving Requirements Walkthrough&lt;/a&gt; (Powerpoint presentation), which starts off with:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Three Primary Functions of the spacecraft in support of this:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Deliver and return four crew and their equipment&lt;/li&gt;&#xA;  &lt;li&gt;Provide Assured Crew Return in the event of an emergency&lt;/li&gt;&#xA;  &lt;li&gt;Serve as a 24-hour Safe Haven in the event of an emergency&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And continues:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The CTS shall be capable of exchanging up to four NASA ISS crewmembers&#xA;  every 150 to 210 days. (3.1.2.4; R.CTS.13)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Of course, Soyuz spacecraft also isn't American (US domestic) vehicle and is not a part of a NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Commercial_Orbital_Transportation_Services&quot; rel=&quot;nofollow noreferrer&quot;&gt;COTS&lt;/a&gt; (Commercial Orbital Transportation Services) program.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-09-17T13:39:27.137" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5360" PostTypeId="2" ParentId="5357" CreationDate="2014-09-17T13:58:06.410" Score="8" Body="&lt;p&gt;No. Like in your &lt;a href=&quot;https://space.stackexchange.com/q/5356/49&quot;&gt;similar question about Soyuz spacecraft meeting NASA's CTS requirements&lt;/a&gt;, I'll quote from Commercial Crew Program: Key Driving Requirements Walkthrough (Powerpoint presentation):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The CTS shall be capable of being docked to the ISS for 210 days to&#xA;  provide an assured crew return capability for four NASA crew.&#xA;  (3.1.2.3; R.CTS.12)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;CTS stands for Crew Transportation Systems, but what this means is that Space Shuttle wouldn't meet it, since it was capable of a maximum of 12 days of being docked to the station due to its electrical power constraints.  Quoting a Boeing Frontiers article &lt;a href=&quot;http://www.boeing.com/news/frontiers/archive/2005/july/i_ids4.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle upgrade lets astronauts at ISS stay in space longer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Station-Shuttle Power Transfer System (SSPTS) allows the ISS to&#xA;  supplement the shuttle's electrical power using electricity generated&#xA;  by ISS solar arrays. This results in a lower consumption of liquid&#xA;  hydrogen and oxygen, components used by the shuttle's fuel cells for&#xA;  making electricity. The power transfer system upgrade also will result&#xA;  in a 50 percent increase in the amount of time the orbiter can dock to&#xA;  the station: from about seven days to between nine and 12 days,&#xA;  depending on the mission configuration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So 12 day and a far cry off one of CCDev (Commercial Crew Development) key requirements of being &lt;em&gt;capable of being docked to the ISS for 210 days&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-09-17T13:58:06.410" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5361" PostTypeId="2" ParentId="5358" CreationDate="2014-09-17T14:12:21.197" Score="5" Body="&lt;p&gt;A checksum is a way to verify the contents of a message.  Basically, a checksum value is transmitted with the message, and you can calculate the checksum yourself when you receive the message.  If the transmitted checksum and the calculated checksum don't match, there is an error in the message you received.&lt;/p&gt;&#xA;&lt;p&gt;TLE data uses a very simple checksum algorithm - basically the sum of all the digits in the line mod 10, with minus signs counting as 1, letters and whitespace as 0,  Here's the python code I use for calculating the TLE checksum:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;def checksum(line):&#xA;    L = string.strip(line)&#xA;    cksum = 0&#xA;    for i in range(68):&#xA;        c = L[i]&#xA;        if c == ' ' or c == '.' or c == '+' or c in string.letters:&#xA;            continue&#xA;        elif c == '-':&#xA;            cksum = cksum + 1&#xA;        else:&#xA;            cksum = cksum + string.atoi(c)&#xA;&#xA;    cksum %= 10&#xA;    &#xA;    return cksum&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Here's some more info on checksums in general:&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Checksum&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Checksum&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="50249" LastEditDate="2023-01-05T08:09:43.180" LastActivityDate="2023-01-05T08:09:43.180" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5362" PostTypeId="2" ParentId="5358" CreationDate="2014-09-17T14:13:09.063" Score="2" Body="&lt;p&gt;A Checksum in general is a way of summarizing the data such that one can figure out if the data was valid. An easy example is the even/odd parity, by which an 8 bit character was passed, with the first 7 bits containing the substance of the character, and the last declaring an even/odd number of bits.&lt;/p&gt;&#xA;&lt;p&gt;As far as the specific example you provided, the definition can be found at &lt;a href=&quot;https://celestrak.org/columns/v04n03/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The last column on each line (fields 1.14 and 2.10) represents a modulo-10 checksum of the data on that line. To calculate the checksum, simply add the values of all the numbers on each line—ignoring all letters, spaces, periods, and plus signs—and assigning a value of 1 to all minus signs. The checksum is the last digit of that sum.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2014-09-17T14:13:09.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5363" PostTypeId="2" ParentId="5358" CreationDate="2014-09-17T14:24:42.197" Score="2" Body="&lt;p&gt;The simple rule for a Two-Line Element Set (TLE) &lt;a href=&quot;https://en.wikipedia.org/wiki/Checksum&quot; rel=&quot;nofollow noreferrer&quot;&gt;checksum&lt;/a&gt; is:&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Modulo 10 of a sum of all the numerals in a line (except the checksum itself, of course, but including the line number), where all the non-numerals (letters, signs and spaces) are substituted with a 0 except the minus sign (-) that is substituted with 1.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;So for example, if you have a TLE:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;ISS (ZARYA)&lt;br&gt;  1 25544U 98067A   08264.51782528 -.00002182  00000-0&#xA;-11606-4 0  2927&lt;br&gt;  2 25544  51.6416 247.4627 0006703 130.5360 325.0288 15.72125391563537&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Then the checksum of the last line is:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://encrypted.google.com/search?q=(2%2B0%2B2%2B5%2B5%2B4%2B4%2B0%2B5%2B1%2B0%2B6%2B4%2B1%2B6%2B0%2B2%2B4%2B7%2B0%2B4%2B6%2B2%2B7%2B0%2B0%2B0%2B0%2B6%2B7%2B0%2B3%2B0%2B1%2B3%2B0%2B0%2B5%2B3%2B6%2B0%2B0%2B3%2B2%2B5%2B0%2B0%2B2%2B8%2B8%2B0%2B1%2B5%2B0%2B7%2B2%2B1%2B2%2B5%2B3%2B9%2B1%2B5%2B6%2B3%2B5%2B3)%20mod%2010&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;code&gt;(2+0+2+5+5+4+4+0+5+1+0+6+4+1+6+0+2+4+7+0+4+6+2+7+0+0+0+0+6+7+0+3+0+1+3+0+0+5+3+6+0+0+3+2+5+0+0+2+8+8+0+1+5+0+7+2+1+2+5+3+9+1+5+6+3+5+3) modulo 10&lt;/code&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Which is in our case 7. Refer to e.g. &lt;a href=&quot;https://celestrak.org/columns/v04n03/#FAQ01&quot; rel=&quot;nofollow noreferrer&quot;&gt;CelesTrak-s Frequently Asked Questions: Two-Line Element Set Format&lt;/a&gt; for more detailed information and even an interesting read on the &lt;a href=&quot;https://celestrak.org/NORAD/documentation/checksum.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Two-Line Element Set Checksum Controversy&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2014-09-17T14:24:42.197" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5364" PostTypeId="1" AcceptedAnswerId="5365" CreationDate="2014-09-17T14:25:22.133" Score="10" ViewCount="751" Body="&lt;p&gt;Since the last shuttle retired in 2011, US astronauts have been paying to ride to space on Russian rockets.  According to &lt;a href=&quot;http://www.bbc.com/news/science-environment-29228900&quot;&gt;recent news: BBC 16 September 2014 &lt;/a&gt; the earliest US launch is late 2017.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently politics of these two nations are having some differences of opinion.  If the US suddenly found it impossible/impractical to use Russian transport to get to the ISS, could a US based astronaut launch occur sooner? Alternately could a ride be arranged with another nation like China?&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2020-05-01T04:19:15.230" Title="Is there any chance of a US based astronaut launch prior to 2017?" Tags="|iss|future-missions|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5365" PostTypeId="2" ParentId="5364" CreationDate="2014-09-17T14:50:45.583" Score="12" Body="&lt;p&gt;Currently the US has a policy against participating with China in any space endeavors (&lt;em&gt;&quot;Due to U.S. obstruction, China now can only have limited international exchanges and cooperation on space technologies and activities.&quot;&lt;/em&gt; &lt;a href=&quot;http://www.chinausfocus.com/foreign-policy/sino-us-competition-and-cooperation-in-outer-space/&quot;&gt;[1]&lt;/a&gt;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk says that SpaceX could provide manned transport in 2016, a year early of NASA's plans. &lt;a href=&quot;http://www.ibtimes.com/musk-says-spacex-manned-craft-can-be-ready-fly-2016-1592434&quot;&gt;[2]&lt;/a&gt;  This does leave 2015 as a gap year if things went south with Russia, but there are some launch vehicles (such as Ariane 5) that are technically capable of launching manned capsules if the situation became dire enough. Such a situation is improbable, however.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2014-09-17T14:50:45.583" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5366" PostTypeId="1" CreationDate="2014-09-17T15:14:26.867" Score="6" ViewCount="232" Body="&lt;p&gt;The CCtCAP Competitors, Dragon V2 (SpaceX) and CST-100 (Boeing) are being subjected to the NASA human rating standards.  Although I asked the questions:&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/5357/would-the-space-shuttle-have-met-the-nasa-manned-flight-standards&quot;&gt;Would the Shuttle meet Human rating standards?&lt;/a&gt;&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/5356/would-soyuz-meet-nasa-current-rating-standards-for-manned-spaceflight&quot;&gt;Would Soyuz have met Human Rating standards?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious next question is, will Orion be held to the same standards? I mean this in a safety perspective, not in terms of crew size/time on station standards.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-09-17T18:00:08.120" Title="Will Orion be subjected to the same Human Rating (Safety, demo flight) standards as the CCtCap competitors?" Tags="|crewed-spaceflight|orion-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5367" PostTypeId="2" ParentId="5366" CreationDate="2014-09-17T18:00:08.120" Score="4" Body="&lt;p&gt;A brief preview to the other two question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would the Shuttle meet Human rating standards?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes and no. The Shuttle was human rated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the Columbia review board initially was leaning toward recommending an immediate shutdown of the Shuttle program, and if not that, no further use beyond Assembly Complete. The first recommendation didn't make it into their final report. The second did, with an additional recommendation that should NASA should go through a complete recertification of the entire Shuttle program should NASA continue to use the Shuttle beyond Assembly Complete. That would not have been possible with NASA's limited budget. That recommendation was a way to kill the program without saying so explicitly. NASA took that recommendation very seriously. The Shuttle program ended with Assembly Complete.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would Soyuz have met Human Rating standards?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Soyuz is a Russian vehicle that docks with the Russian half of the Space Station. They're immune to US internal regulations. Human rating concerns explicitly exclude Soyuz. They're immune.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The obvious next question is, will Orion be held to the same standards?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes. Those human rating concerns are a driving issue for the Orion project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Orion project has already undergone a pad abort test and has worked toward getting the parachute system human rated. The upcoming Exploration Flight Test-1 will form yet another step toward that human rating. Critical Design Review will be in April, 2015, and that is when human rating concerns become paramount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least traditionally, human rating isn't just about the vehicle. It's about the entire end-to-end system. That includes how the vehicle is manufactured, every little piece, every little fastener. If a bolt is bought from a commercial supplier, NASA wanted to see how that supplier kept up to standards. Orion is being held to those old-time standards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your question is phrased in a way that is suspicious of those safety ratings. I suspect that if anything, the rules will be relaxed a bit with CCtCap.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-09-17T18:00:08.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5368" PostTypeId="2" ParentId="5356" CreationDate="2014-09-17T19:12:32.440" Score="7" Body="&lt;p&gt;As I noted in &lt;a href=&quot;https://space.stackexchange.com/a/5367/2752&quot;&gt;my answer to a similar question on Orion&lt;/a&gt;, Soyuz is immune. The US human ratings standards apply to vehicles developed on behalf of NASA, and to non-US vehicles that would dock with the US side of the International Space Station. (Note that there are no such vehicles, and none are planned.)&lt;/p&gt;&#xA;&lt;p&gt;The Soyuz is built by Russia, launched by Russia using Russian-built launch vehicles, and docks with the Russian side of the ISS. The NASA human safety ratings rules don't apply to Soyuz. NASA has to abide by Russia's rules when NASA contracts with Russia to use Russian vehicles to fly US astronauts to the ISS.&lt;/p&gt;&#xA;&lt;h3&gt;Update&lt;/h3&gt;&#xA;&lt;p&gt;To make my answer a bit more on-target, there is no way that Soyuz could pass US safety requirements, for at least two reasons.&lt;/p&gt;&#xA;&lt;p&gt;One is that Russia has its own engineering concepts of how to address safety and reliability, and those are rather different from those used in the US. I'm not saying Soyuz isn't safe, and I'm not passing judgement on the approaches used in the US versus those used in Russia. They're just different. Meeting the US safety standards would necessarily entail Russia switching to a US-style of documentation, data capture, traceability, testing, verification, and validation. For one thing, that would be ridiculously expensive. For another, Russia is a rather proud nation. It's not going to happen.&lt;/p&gt;&#xA;&lt;p&gt;Another issue is that for Russia to comply with those US safety concepts, Russia would necessarily have to divulge a lot of Russian military secrets to NASA. That's even less likely to happen than Russia switching to a US-style of documentation, data capture, traceability, testing, verification, and validation.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-10-14T20:54:40.457" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5369" PostTypeId="2" ParentId="5357" CreationDate="2014-09-17T19:46:34.680" Score="6" Body="&lt;p&gt;The Shuttle did pass human rating standards, and post-Challenger, it did have a (limited) crew escape capability. Pre- and post-Challenger, it had abort modes galore that were designed around human safety concerns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every bolt, every wire, every &lt;em&gt;everything&lt;/em&gt; used on the Shuttle was controlled from the prior to pouring molten metal into a form the bolt or wire, and on up the manufacturing chain. This is part of what made the Shuttle so expensive to operate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This also applies to lesser extent to commercial aircraft. Fasteners certified for use in aircraft cost a bunch of money. Oftentimes you can buy exactly the same bolt at your local hardware store, and all you will be missing is a hole in your wallet and a piece of paper that traces the pedigree of the bolt. MIL specs and NASA human rating specs make those FAA specs look like child's play.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle did have a few CRIT-1A exceptions, the lack of a reliable launch escape system being one of them. Rules, even human safety rules, always have an out. Sometimes you might have to wait until the next ice age to get a waiver to a rule. Waivers are amazingly easy to get at other times, as was the case with regard to the a reliable launch escape system. The waiver rules are built into the human safety rating standards. Since the Shuttle had the requisite paperwork for those exceptions (with signoffs all the way to the top), it was human rated.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-09-17T19:46:34.680" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5370" PostTypeId="1" AcceptedAnswerId="5377" CreationDate="2014-09-17T20:57:23.127" Score="10" ViewCount="1928" Body="&lt;p&gt;I have been considering if it would be possible to place a satellite in a Sun-synchronous orbit in one of the lunar frozen orbits at 86°. According to NASA scientists there are four inclinations for orbits wherein a satellite can stay in low lunar orbit indefinitely:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;There are actually a number of 'frozen orbits' where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27º, 50º, 76º, and 86º&quot;—the last one being nearly over the lunar poles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-&lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&quot; rel=&quot;noreferrer&quot;&gt;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is whether precession would still be possible in a frozen orbit. An orbit over the lunar poles could potentially allow a continuous supply of solar energy. I believe this orbit might be possible with zero eccentricity at a bit above 200 km altitude. This is provided that the satellite can remain in the frozen orbit and would still precess due to the oblateness of the moon. However I am unsure if there might be some reason that this kind of orbital precession might not work as well since I am not very familiar with frozen orbits. I'm also unaware how long it might take for the Earth to perturb this orbit. Is a lunar Sun-synchronous orbit possible when using a frozen orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also did find an interesting way of reaching a lunar polar orbit on Hop's blog (who had directed me to this site):&#xA;&lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/08/lunar-ice-vs-neo-ice.html&quot; rel=&quot;noreferrer&quot;&gt;http://hopsblog-hop.blogspot.com/2013/08/lunar-ice-vs-neo-ice.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kind regards,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Peter&lt;/p&gt;&#xA;" OwnerUserId="6102" LastEditorUserId="10132" LastEditDate="2015-07-16T22:52:46.717" LastActivityDate="2015-07-16T22:52:46.717" Title="Is a lunar Sun-synchronous orbit possible at the &quot;frozen&quot; inclination of 86°?" Tags="|orbital-mechanics|the-moon|orbit|orbital-maneuver|trajectory|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5372" PostTypeId="1" AcceptedAnswerId="5375" CreationDate="2014-09-17T21:25:44.523" Score="10" ViewCount="9445" Body="&lt;p&gt;So ULA and Blue Origin have announced they are designing a LOX/LNG rocket engine (&lt;a href=&quot;http://www.ulalaunch.com/ula-and-blue-origin-announce-partnership.aspx?title=United+Launch+Alliance+and+Blue+Origin+Announce+Partnership+to+Develop+New+American+Rocket+Engine&quot;&gt;link&lt;/a&gt;).  From what I understand, LNG is just a less pure form of methane (CH4).  Is this correct?  Why are they announcing a LNG engine, not a methane engine?&lt;/p&gt;&#xA;" OwnerUserId="6103" LastEditorUserId="51" LastEditDate="2014-09-17T22:13:44.740" LastActivityDate="2018-01-10T17:31:02.287" Title="How does liquified natural gas compare to liquid methane?" Tags="|rockets|engine-design|engines|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5373" PostTypeId="1" AcceptedAnswerId="5381" CreationDate="2014-09-17T22:06:26.747" Score="5" ViewCount="728" Body="&lt;p&gt;Does the material that hits the moon all vaporize and disperse so much it doesn't lead to enriched deposit areas, or could it sometimes be useful?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm thinking in particular of a permanent moon base, the beginnings of a moon economy.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-09-18T14:37:40.893" LastActivityDate="2014-09-18T15:45:01.740" Title="How much meteorite material hits the moon and could it be useful?" Tags="|the-moon|mining|meteoroid|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5374" PostTypeId="2" ParentId="5372" CreationDate="2014-09-17T22:13:10.847" Score="3" Body="&lt;p&gt;Liquid Natural Gas is primarily CH4 (Methane).  However since it is derived from natural sources, not synthesized, it has other components in low quantities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low enough that a power plant, stove, dryer, or even car engine would probably not care.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But a rocket engine is much more sensitive, using TONS of fuel a second and anything less than purity can be a real issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RP1 is similarly basically kerosene.  But RP1 is really just a refined form of kerosene, minimizing other non-desired components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Technically CH4 or Methane is different than LNG, but in the common parlance, LNG is becoming cheap and more available due to fracking and horizontal drilling, people are more commonly aware of LNG.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus it is likely a marketing decision.  LOX/LNG people will comprehend as cheap fuel, where as Methane of CH4 looks expensive and exotic. But by designing for LNG, they save costs on fuel, which in a reusable world, might become a major factor. Interestingly SpaceX, with its Raptor engine is still sticking with Methane instead of LNG.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-01-10T17:31:02.287" LastActivityDate="2018-01-10T17:31:02.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5375" PostTypeId="2" ParentId="5372" CreationDate="2014-09-17T22:13:19.157" Score="10" Body="&lt;p&gt;In theory, Liquid Natural Gas (LNG) is less pure and may contain smaller amounts of higher saturated Hydrocarbons (Alkanes) like Ethane, Propane, Butane, and also Nitrogen, Carbon Oxides, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, even Liquid Methane propellants may be mixed with other compounds intentionally for combustion stability, performance or to control exhaust temperature, and ullage gases (Nitrogen or Helium) providing propellant pressure and preventing sloshing may mix with it in smaller amounts &lt;em&gt;by design&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From performance standpoint (specific impulse), there ought not be much advantage in favor of Liquid Methane. LNG with its higher Hydrocarbons introduces slightly more Carbon to the combustion so the exhaust temperature might also slightly increase with more Carbon Dioxide combustion products, demanding throttling the engine back a bit. But since rockets usually fly with a &lt;em&gt;fuel rich&lt;/em&gt; mixture, and fuel rich burning of LNG produces more free Hydrogen (that's how most of it is actually produced), you'd technically also get slightly higher exhaust products velocity as the secondary burn, as the exhaust products mix with atmospheric Oxygen. This should even things out, as long as you design for somewhat more pronounced combustion instabilities of LNG.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So not much of a difference there, and perhaps the most notable one is that LNG uses &lt;em&gt;natural&lt;/em&gt; in its name, which might go a mile or two farther on fumes alone in the increasingly more important realm of public relations.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: This is related so I wanted to add it to this thread:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;17 Sep 2014, &lt;a href=&quot;https://www.youtube.com/watch&quot;&gt;United Launch Alliance and Blue Origin Announce Partnership To Develop New American Rocket Engine&lt;/a&gt; (YouTube video)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;During the press conference announcing United Launch Alliance (ULA) and Blue Origin partnership in developing BE-4 (Blue Engine 4), a US built successor (but not a 1:1 replacement) to Russian built &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-180&quot;&gt;RD-180&lt;/a&gt; engine, &lt;a href=&quot;https://en.wikipedia.org/wiki/Jeff_Bezos&quot;&gt;Jeff Bezos&lt;/a&gt; (entrepreneur and founder of Blue Origin) and Tory Bruno (ULA President) discuss a few details on the announced new engine. A few takeaways are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;BE-4 will use Oxygen enriched stage combustion cycle&lt;/li&gt;&#xA;&lt;li&gt;Using Liquified Natural Gas (LNG) as fuel and Liquid Oxygen (LOX) as oxidizer&lt;/li&gt;&#xA;&lt;li&gt;Booster engine capable of ~ 550,000 lbf of thrust&lt;/li&gt;&#xA;&lt;li&gt;Single turbopump design&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It is yet unclear which vehicles it is planned to be used on, pending engine's further validation. It is however probably not too bold of an assertion that the first family of launch vehicles it will be used on will be the same one that currently depends on supply and availability of RD-180 engines and the vehicles that ULA uses to provide &lt;em&gt;assured access to space&lt;/em&gt; to United States Air Force (USAF) and its Evolved Expendable Launch Vehicle (EELV) program - so &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot;&gt;Atlas V&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering RD-180's thrust of 860,568 lbf (3.83 MN) with its dual nozzle design, it's also probably fair to assume that BE-4 powered booster stages will initially use two of these engines for a total thrust of roughly 1,100,000 lbf (sea-level) on new EELV workhorses delivering heavier payloads to higher orbits and a single engine for lighter payload applications, possibly as a successor to &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_II&quot;&gt;Delta II&lt;/a&gt; (more than double the thrust of Rocketdyne RS-27A could simplify its dependence on solid rocket boosters).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BE-4 engines will not be used on &lt;a href=&quot;http://www.nbcnews.com/science/space/bezos-vs-musk-blue-origin-ula-turn-heat-rocket-battle-n205581&quot;&gt;also newly announced&lt;/a&gt; Blue Origin's &lt;a href=&quot;https://en.wikipedia.org/wiki/Blue_Origin_New_Shepard&quot;&gt;New Shepard&lt;/a&gt; suborbital spaceship. It will use a Liquid Hydrogen / Liquid Oxygen (LH2/LOX) fueled &lt;a href=&quot;https://en.wikipedia.org/wiki/BE-3&quot;&gt;BE-3&lt;/a&gt; engines.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-09-18T13:09:20.497" LastActivityDate="2014-09-18T13:09:20.497" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5376" PostTypeId="2" ParentId="5320" CreationDate="2014-09-18T04:48:13.670" Score="11" Body="&lt;p&gt;I'm going to go with &quot;1.5x the size&quot; as referring to volume, so combined with 1.5 times the mass results in the same density as Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The alternative of 1.5 times the radius or circumference combined with 1.5 times the mass would result in a Super-Earth with a sub-Earth density of 2.4 g/cc.  That would be like an icy moon, not a rocky planet, and would result in a &lt;em&gt;lower&lt;/em&gt; surface gravity than Earth (2/3 g), and the &lt;em&gt;same&lt;/em&gt; orbital velocity as Earth, making it overall a little easier to get into orbit.  That does not sound like the intention of the questioner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So with 1.5 times the volume and mass of Earth, we would have about a 14.5% higher surface gravity, $1.5\over {1.5}^{2\over 3}$, and a 14.5% higher orbital velocity, $\sqrt{1.5\over {1.5}^{1\over 3}}$.  There should be no problem at all with our current rocket technology to get into orbit from the planet, so long as its atmosphere isn't any thicker than ours.  In fact, there should be no problem to stick with two stages to orbit with that modest increase in orbital velocity.  The increase in surface gravity could be accommodated with a modest 14.5% increase in thrust to maintain the same thrust to weight ratio.  Perhaps just somewhat larger engines or one or two more engines added to the first stage cluster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However this Super-Earth isn't very super as Super-Earths go.  Most of the discoveries are two to ten times Earth mass, as opposed to the 1.5 times Earth mass posed here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for an explanation for where everybody is, no, this isn't it.  First off, our discoveries of predominantly Super-Earths is observational bias.  It is easier to find larger planets than small ones.  I'm sure that there are plenty of Earths and sub-Earths out there.  Second, you can just add stages to your chemical rocket to get off of a larger planet.  It will simply cost more, so you will need to be more determined.  Third, you don't need to use chemical rockets.  You could use electromagnetic rail launchers, nuclear fission high Isp engines, or thermonuclear bomb propulsion (see Orion).  All within our current technological capability.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-09-18T14:01:36.077" LastActivityDate="2014-09-18T14:01:36.077" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5377" PostTypeId="2" ParentId="5370" CreationDate="2014-09-18T05:20:59.473" Score="8" Body="&lt;p&gt;The Moon's $J_2$ of $2.0330530\times 10^{-4}$ gives only very low, not very inclined retrograde orbits.  The inclinations start at 130° at a zero altitude orbit, and go up to 180° at about 200 km altitude.  See &lt;a href=&quot;https://space.stackexchange.com/a/5314/265&quot;&gt;this answer&lt;/a&gt; for more details.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-09-18T20:28:50.070" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5378" PostTypeId="2" ParentId="5320" CreationDate="2014-09-18T12:15:41.903" Score="7" Body="&lt;p&gt;There is a great blog post on the NASA homepage about &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;noreferrer&quot;&gt;&quot;the tyranny of the rocket equation&quot;&lt;/a&gt;. It is going into quite some detail, and it gives in one of the last paragraphs:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, you don't want to venture in space, but reach orbit, and your Earth has 1.5 times the mass and not the diameter, so there's still hope.&#xA;Just in case that the link goes dead, I am listing some of the interesting details of the blog post, as well as some additional information. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation#Derivation&quot; rel=&quot;noreferrer&quot;&gt;rocket equation gives&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$M_f = 1-\frac {m_1} {m_0}=1-e^{-\Delta v\ / v_\text{e}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $M_f$ is the fraction of propellant by total mass of the spacecraft, $\Delta V$ is the necessary escape velocity and $v_e$ is the exhaust velocity. This can be rewritten to&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v=- \ln(1-M_f) v_e$$&#xA;The orbital velocity of a body (without adjusting for aerodynamic drag) is given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v=\sqrt{\frac{G M}{R}} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming that you're interested in an Earth with $M'=1.5M$, this gives $R'=(1.5)^{\frac{1}{3}} R$, so $\Delta v'= \sqrt{\frac{1.5}{1.5^{1/3}}} \Delta v=1.145 \Delta v$, just like Mark Adler wrote in his answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we need some maximal possible values for exhaust speed and the ratio propellant/mass, which are handily given in the NASA post:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hydrogen-oxygen is the most energetic chemical reaction known for use in a human rated  rocket. Chemistry is unable to give us any more. In the 1970’s, an experimental nuclear thermal rocket engine gave an energy equivalent of 8.3 km/s. This engine used a nuclear reactor as the source of energy and hydrogen as the propellant.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The common soda can, a marvel of mass production, is 94% soda and 6% can by mass. Compare that to the external tank for the Space Shuttle at 96% propellant and thus, 4% structure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, if we use $M_f=0.96$ and $v_e=8\;\mathrm{km/s}$, we get a maximum possible $\Delta v = 25.8\;\mathrm{km/s}$.&#xA;The orbital velocity of Earth is about $7.9\;\mathrm{km/s}$, so the orbital velocity of your bigger Earth would be about $9.0\;\mathrm{km/s}$. So it would still be possible to reach orbit, also with a less efficient propellant.&lt;/p&gt;&#xA;" OwnerUserId="6105" LastEditorUserId="6213" LastEditDate="2014-11-04T11:43:35.247" LastActivityDate="2014-11-04T11:43:35.247" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5379" PostTypeId="2" ParentId="5372" CreationDate="2014-09-18T12:22:33.487" Score="0" Body="&lt;p&gt;It's purely economics. Price is key for the space launch business, and one of the most important components of launch price is fuel. The price of kerosene is about 20USD per million btu, and LNG can be bought in the US for around 6USD per million mBtu. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back before fracking became mainstream it was thought that the future was in liquidizing natural gas from fields where pipelines were not possible and transporting it cryogenically. There was a big market in the US and Japan, with China looking to become bigger then both in the future. The economics seemed to be there so hugely expensive liquification plants started to be built all over the world. Then fracking came on the scene and the US gas price went through the floor and LNG lost its biggest market. This has caused the LNG pricing to drop. A substantial portion of LNG receiving terminals in the US are in the gulf coast, convenient for the proposed launch sites of both companies. So what you have is loads of LNG to be bought at competitive prices with a delivery infrastructure which is already in place. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something that kerosene does have going for it is energy density, kerosene has somewhere around 50%  more energy per unit of volume than LNG, so you need bigger fuel tanks with corresponding weight penalties. This means you need more fuel to lift more weight. However at more than 3 times the cost LNG must be still worth it or these companies wouldn't be doing it.  &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-09-18T12:22:33.487" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5380" PostTypeId="1" AcceptedAnswerId="7977" CreationDate="2014-09-18T12:58:51.383" Score="5" ViewCount="445" Body="&lt;p&gt;This &lt;a href=&quot;http://russell.ae.utexas.edu/FinalPublications/ConferencePapers/2010May_BornSymposium_AAS_HaloToHalo.pdf&quot; rel=&quot;nofollow&quot;&gt;paper&lt;/a&gt; studies transfers from halo orbit to halo orbit in the Jovian system. I wonder if there are any locally stable orbits connecting Earth-Moon L1 and L2 points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: One should also consider trajectories based on low-thrust engines (ion, solar sails, magsails).&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="1142" LastEditDate="2015-01-27T15:41:37.360" LastActivityDate="2015-01-29T16:45:38.487" Title="Are L1 to L2 and back &quot;cheap&quot; transfers/cyclers possible?" Tags="|the-moon|lagrangian-points|ballistics|earth|low-energy-transfer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5381" PostTypeId="2" ParentId="5373" CreationDate="2014-09-18T15:45:01.740" Score="4" Body="&lt;p&gt;Dennis Wingo and I had an argument over this. Wingo argues asteroid impacts have left rich ore deposits on the moon. My response (which he did not like) was citing &lt;a href=&quot;http://www.barringercrater.com/about/history_5.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Barringer's disastrous effort&lt;/a&gt; to mine Barringer Crater in Arizona. Barringer was correct that the impactor was metallic. But the impact scattered the ore far and wide leaving it too diluted to be profitably mined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dennis pointed to intact metallic meteorites found on earth's surface as well as Mars surface. He argued meteorites hit the moon slower than Mars and Earth due to the moon's shallower gravity well. I replied that the planet's atmosphere slowed the impactors so they only hit at &lt;a href=&quot;http://en.wikipedia.org/wiki/Terminal_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;terminal velocity&lt;/a&gt;. Of course the moon doesn't have enough atmosphere to slow down meteorites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I made a chart of speeds at which asteroids would strike the moon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ux0l3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An explanation of how I built the above histogram can be found at my blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/09/one-legged-stools.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;One Legged Stools&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very large meteorites have a higher mass/(cross section surface area ratio) and thus a faster terminal speed. So it might be possible big impactors like the Sudbury meteorite hit the earth faster than the slowest speeds in the above histogram. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sciencedaily.com/releases/2012/03/120308143213.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here&lt;/a&gt; is an article speculating the lunar magnetic anomalies are metallic deposits left by impacts. If so, we have a pretty good idea of locations of Sudbury like deposits on the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I am coming around to Wingo's opinion that there are rich lunar metal deposits left by impactors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean they can be profitably mined? No. The delta V from earth's surface to the moon and back is around 20 km/s. Transportation expense prevents profitable platinum or gold mining. Even deposits of pure lunar crack cocaine would be a money pit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To profitably mine extra-terrestrial metals, we need to break the tyranny of the rocket equation. Extra terrestrial propellent might do that. So I would argue the most valuable lunar commodity won't be gold or platinum. It will be water. &lt;a href=&quot;http://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Water might be found in the cold traps at the lunar poles&lt;/a&gt;. And it is thought that the lunar water at the cold traps is exogenic in origin. Some of the volatile gasses from cometary or asteroidal impacts will find their way to the 40 Kelvin lunar cold traps, freeze and stay. Thus it's possible rich water and volatile deposits have accumulated over the eons in spite of being widely dispersed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my answer to your question is a tentative yes. Yes, it's possible meteorites have left useful deposits on the moon's surface.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-09-18T15:45:01.740" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5382" PostTypeId="2" ParentId="1330" CreationDate="2014-09-18T16:50:38.013" Score="8" Body="&lt;p&gt;While Dream Chaser is, indeed, based on HL-20, one of the few significant changes was removing the launch abort adapter and replacing it with an &lt;a href=&quot;http://www.parabolicarc.com/2014/07/08/sierra-nevada-completes-dream-chaser-propulsion-reaction-control-milestone/&quot;&gt;integrated abort system&lt;/a&gt;. This was made possible by using a pair of liquid bipropellant engines (though originally &lt;a href=&quot;http://www.parabolicarc.com/2014/08/19/snc-abandons-hybrid-motors-dream-chaser/&quot;&gt;the plan was to use hybrid motors&lt;/a&gt;) as the spacecraft's main propulsion system. The same engines could therefore be used for both orbital adjustments and aborts.&lt;/p&gt;&#xA;" OwnerUserId="989" LastEditorUserId="989" LastEditDate="2014-09-18T20:26:46.547" LastActivityDate="2014-09-18T20:26:46.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5383" PostTypeId="2" ParentId="5320" CreationDate="2014-09-18T17:35:50.020" Score="7" Body="&lt;p&gt;There's a very large gap between what's theoretically possible in rocketry and what's practical and what an entity has the resources and political will to accomplish. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;a href=&quot;http://en.wikipedia.org/wiki/Multistage_rocket#Delta-v&quot; rel=&quot;noreferrer&quot;&gt;staged version of the rocket equation&lt;/a&gt;, the possible achievable delta-V is proportional to the number of stages when mass ratio and engine efficiency is held constant. In short, you can get any delta-V you want by adding additional stages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you needed twice the delta-V to reach orbit (because of a larger planet and denser atmosphere, say), then instead of a 1.5 stage launcher for a one-man orbital mission (like the R-7/Vostok or Atlas-Mercury missions), a 3-stage launcher like a Saturn 1B (counting the Apollo service module as a third stage, here) might be required. (I'm handwaving a bit here; R-7 and Atlas aren't literally 1.5 stages for purposes of the rocket equation.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typically, each stage is 4-5 times as large as the one above it in the stack, so linear increases in delta-V requirements lead to exponential increases in overall rocket size. Complexity and cost probably increase faster than linearly with stage size, considering the assembly facilities, transport, etc. required to support them. Eventually, as you turn up the gravity, you might hit a point where a nation wouldn't be willing to take on a Saturn-Apollo sized development program to achieve the goal of a single person in orbit. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-09-18T17:35:50.020" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5384" PostTypeId="1" CreationDate="2014-09-19T01:17:30.627" Score="5" ViewCount="365" Body="&lt;p&gt;Many people would argue that the International Space Station is one of the most extraordinary human accomplishments in space exploration. Considering the short-lived space stations of the pre-ISS era, it really is an astonishing achievement. The thing is, it needed a lot of people to get it off the ground (pun definitely intended).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ISS programme is a joint project among five participating space agencies: NASA, Roscosmos, JAXA, ESA, and CSA.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, as in the past, we are looking far beyond Earth orbit, primarily to Mars (although President Obama has the Moon back on the table). However, whenever I hear the phrase &quot;men/women on Mars&quot;, it is always in the context of an American mission by NASA. Such a mission seems pretty hard for one nation alone. Doable? Yes. After all, we did put men on the Moon (but that was during the Cold War). But it would still be very difficult. So my question boils down to this: Are future large-scale manned missions (e.g. Mars and beyond) likely to be international efforts, or just from one country? Why?&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2015-01-10T22:42:52.133" LastActivityDate="2015-01-10T22:42:52.133" Title="International Collaboration on Large-Scale Manned Space Exploration Projects" Tags="|mars|iss|crewed-spaceflight|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5385" PostTypeId="1" AcceptedAnswerId="5506" CreationDate="2014-09-19T01:30:19.067" Score="6" ViewCount="287" Body="&lt;p&gt;The area in front of a vehicle re-entering the atmosphere is elevated mostly because of the high pressure of the shock wave leading it.  Given the space immediately behind the vehicle is &lt;em&gt;not&lt;/em&gt; being pressurized, it stands to reason it should be cooler.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much cooler?  Have any measurements been made?  Failing that, do the software models of re-entry show a temperature for that region?&lt;/p&gt;&#xA;" OwnerUserId="238" LastActivityDate="2014-10-03T03:51:20.450" Title="Temperature of the area behind a reenty vehicle" Tags="|reentry|temperature|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5386" PostTypeId="2" ParentId="5384" CreationDate="2014-09-19T04:20:52.253" Score="0" Body="&lt;p&gt;I hold a theory, and I'm not sure that this theory is anything similar to something well accepted in the scientific community, but here it goes: My theory states that as technology progresses the differences in communities decreases. The critical point at which technological progression becomes so great and the community differences decreases to near nothing we come to a new state of human civilization known as a 'Unity'. In this new state of civilization no longer are we viewed as Japanese, European, American, and so-on; But rather we are known as Humans, or Tellurians. This theory holds true relative to the apparent area and number of the people forming the community. For instance, with our current technology say we inhabited two other planets with equal amounts of people on each summing to a total of 21-billion people. Assuming that 21-billion people remained a constant, it would take a progression of technology three times longer to reach the critical point of unification because of the sheer number of people as opposed to the 7-billion people we currently have.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does this relate to your question?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe that as our technology improves and our desire for greater accomplishments increases, not only does the unified community become necessary but according to my theory it's unavoidable. For now, I believe that most countries are trying to accomplish as much as they can alone, however NASA can only get so far without a way of transportation first to get to the ISS (which I assume would be the first stop).  As someone from a time before us might say, &quot;It's written in the stars.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(This is my first post. I know I'm not using a lot of facts but I hope that I can get some positive-critical feedback on this concept.)&lt;/p&gt;&#xA;" OwnerUserId="6107" LastActivityDate="2014-09-19T04:20:52.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5387" PostTypeId="1" AcceptedAnswerId="5389" CreationDate="2014-09-19T04:41:58.540" Score="20" ViewCount="12182" Body="&lt;p&gt;From my understanding, space is not entirely empty. Infact, between the voids of space are molecules or maybe individual atoms of hydrogen just floating around. I understand that gathering massive quantities of this from space would be a difficult endeavor; however, if we could refine technology to use hydrogen thrusters and create a collection device that would collect the sporadic rogue hydrogen atoms floating in space to refuel our reserves could we travel space on an interstellar level indefinitely?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second part to the question if for some reason the first is completely implausible. If the intake of hydrogen isn't sufficient enough to replenish the reserves we have, would it be possible to collect what we could from interstellar space and then fill up on hydrogen from a nearby comet, asteroid, star, or planet(oid) when we run low?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very farsighted questions, I know, but a real question nonetheless.&lt;/p&gt;&#xA;" OwnerUserId="6107" LastEditorUserId="1142" LastEditDate="2014-10-13T13:03:25.150" LastActivityDate="2020-02-06T08:26:30.890" Title="Can we use interstellar hydrogen as a fuel for interstellar travel?" Tags="|interstellar-travel|interstellar-medium|hydrogen|fuel-system|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5388" PostTypeId="1" CreationDate="2014-09-19T08:51:13.760" Score="4" ViewCount="452" Body="&lt;p&gt;Does anyone know what would be the exact or approximate altitude for the stage separation and spacecraft separation of LEO insertion? Moreover, does anyone has a picture including all step of LEO insertion? Thank you very much :)&lt;/p&gt;&#xA;" OwnerUserId="6108" LastEditorUserId="4660" LastEditDate="2014-09-20T15:34:48.737" LastActivityDate="2014-09-20T15:34:48.737" Title="LEO insertion altitude" Tags="|low-earth-orbit|altitude|launch-sequence|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5389" PostTypeId="2" ParentId="5387" CreationDate="2014-09-19T11:10:55.073" Score="23" Body="&lt;p&gt;It's theoretically possible to collect fuel from near empty space, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;nofollow noreferrer&quot;&gt;bussard ramjet&lt;/a&gt; is an example of an engine designed to do just that. The principle is that you use magnetic fields to collect and concentrate hydrogen atoms from the near vacuum of space, and then a &lt;a href=&quot;http://en.wikipedia.org/wiki/Fusion_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;fusion rocket&lt;/a&gt; would turn some of this into propulsion for speed and a fusion reactor would convert the some into power to maintain the fields. The rest would get stored in tanks. In order to make this self-sustaining you would need to get going to a significant proportion of the speed of light so that the amount of hydrogen collected from space would be greater than the fuel needed to maintain the collection fields. So you'd have to have some fuel to begin with to get you going fast enough. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's where the second part of your question comes in: are there sources of hydrogen fuel to be had in space? Yes, hydrogen is the most abundant element, there's loads out there. All you need is to find some water and split it by electrolysis, although you could also free hydrogen from other compounds. Once you have hydrogen fusion power these actions would be reasonably economical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if the technology actually works you could go from one star to another using a bussard ramjet, use your fuel reserves to slow down, explore (trade, lay waste, etc), then find a hydrogen source to replenish your reserves and go to the next star. Just make sure you don't end up in a solar system without enough matter or you'll never get going again. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;:&#xA;This is all very theoretical, whether it would work or not depends on how efficient you could make hydrogen collection, how much power you could get from the rocket, etc. It's far beyond current technical capabilities at any rate. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2020-02-06T08:26:30.890" LastActivityDate="2020-02-06T08:26:30.890" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="5390" PostTypeId="1" AcceptedAnswerId="5391" CreationDate="2014-09-19T11:40:16.963" Score="13" ViewCount="4028" Body="&lt;p&gt;Is it possible to detonate a nuclear bomb in space? If possible, then what will happen? In the Armageddon movie we watched that, after discovering that an asteroid the size of Texas is going to impact Earth in less than a month, NASA recruits a team of deep core drillers to drill and blast a nuclear war-head. Is it possible?&lt;/p&gt;&#xA;" OwnerUserId="6109" LastEditorUserId="23462" LastEditDate="2022-04-21T02:06:16.117" LastActivityDate="2022-04-21T02:06:16.117" Title="Nuclear Explosion in Space" Tags="|nuclear|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="5391" PostTypeId="2" ParentId="5390" CreationDate="2014-09-19T15:58:46.267" Score="29" Body="&lt;p&gt;You can detonate nuclear bombs in space, it's been done several times. There are technical challenges to it but nothing too complex.&lt;/p&gt;&#xA;&lt;p&gt;In an atmosphere much of the damage from a nuclear weapon is from the blast wave which is caused by the heat and pressure of the explosion. This wave goes out from center of the explosion, and then back again as the pressure drops. This is why most nuclear weapons are designed for an air burst rather than a surface burst in order to maximize the destruction caused.&lt;/p&gt;&#xA;&lt;p&gt;A nuclear weapon in space has a different effect than one detonated in the atmosphere or underground as there's no matter for the explosion to push against. What will happen is a burst of radiation and plasma which disperses evenly outward from the detonated device.&lt;/p&gt;&#xA;&lt;p&gt;What would happen if you detonated a nuclear device inside an asteroid? That depends on the composition and size:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If it's small enough it could be completely vaporized, however if it's that small it's probably not a danger&lt;/li&gt;&#xA;&lt;li&gt;It could blast it into many pieces, which would be a bit better than one big asteroid as it gives the smaller pieces a better chance of burning up in the atmosphere, although the earth would get pelted by many smaller impacts rather than one big one, spreading damage over a wider area&lt;/li&gt;&#xA;&lt;li&gt;The asteroid could contain the blast and remain intact, or the force of the explosion could be lost through fissures in the body of the asteroid and also leave it intact. It is &lt;strong&gt;extremely&lt;/strong&gt; unlikely that it would break into 2 clean pieces predictably as in that (awful, awful) movie.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;A nuclear blast theoretically could be used to deflect an asteroid by being detonated close to it, the heat and radiation could ablate the side of the asteroid facing the explosion causing it to change course. The effect is likely to be small and would need to be achieved while the asteroid is a considerable distance away. We would need many years' warning to achieve this, which is why it is pretty important that we start tracking asteroids and comets far better than we do now.&lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2022-04-19T14:21:54.513" LastActivityDate="2022-04-19T14:21:54.513" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="5392" PostTypeId="2" ParentId="5388" CreationDate="2014-09-19T16:58:36.327" Score="5" Body="&lt;p&gt;What you want to search for to get this information is 'launch sequence'. It depends on the rocket and the payload. What happens up to first stage separation is pretty standard, what happens with second stages is more variable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the rocket has boosters (the thin cylinders that are strapped onto the sides), they will separate between 30 to 60 kilometers altitude depending on what rocket it is and its configuration. With or without boosters, the first stage will separate at 100 to 130 km, again depending on what rocket it is. For the most part, what the first stage does is get the vehicle out of the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second stage gets the vehicle up to orbital speed. Therefore what it does exactly depends a lot on what the payload is. On some rockets and with some payloads, once it has done that it's job is over and it separates. In other cases it burns twice, once to get up to orbital speed, and again on the opposite side of its orbit to raise orbital height and make it circular. What height that is depends on the payload. Then it is discarded.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-09-19T20:17:22.247" LastActivityDate="2014-09-19T20:17:22.247" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5393" PostTypeId="2" ParentId="5390" CreationDate="2014-09-19T21:29:05.930" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is it possible?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As depicted in that awful movie? No. The concept in general? Yes. A nuclear standoff explosion is widely regarded as the best, most realistic approach to diverting an incoming asteroid or comet, assuming some minimal amount of lead time. Weeks or months do not qualify as &quot;minimal&quot;. We need a couple of years, at least, even with nukes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A nuclear standoff explosion involves exploding a nuclear bomb at some distance away from the target object. (Note well: This is exactly how our every country that has nuclear weapons plans to use them, and has tested using them. This is a Technology Readiness Level 9 concept.) More than half of the radiation and neutrons created by the explosion go off into space, but a good portion will bathe the asteroid/comet with gamma rays, X-rays, and high energy neutrons. This will cause the outer layer of the part of the asteroid/comet that faces the bomb to vaporize. It is this secondary explosion that provides the needed impetus to divert the object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We cannot divert all incoming objects. The asteroid that exploded over Chelyabinsk in early 2013, or a dinosaur killer comet that we don't see until months prior to impact: Those are problems for some future generation to solve. Those Chelyabinsk-level objects are far too hard to see. Diversion is not possible if we can't see the incoming object ahead of time. Suppose a dinosaur killer by some fluke of its trajectory manages to sneak up on us until just months before impact. We're already dead. At some point there is nothing we can do except say goodbye to one another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What we can solve using current technology, or reasonable extensions to current technology, are potential impactors that we first detect years prior to the impact. The nuclear option is the only viable approach if we detect the impactor less than a decade prior to impact. With that short of a lead time, a non-nuclear approach would require hundreds to millions of launches and hundreds to millions of cooperating vehicles. A nuclear approach would require very few launches, typically, just one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Non-nuclear approaches &lt;em&gt;might&lt;/em&gt; work if we detect the impactor several decades prior to impact and if we take action soon after detection. That action includes spending a rather sizable chunk of money on technology that is not quite ready for prime time, and that spending needs to start soon after detection. The problem here is that that multiple decades of lead time is more than enough time for the uglier side of humanity to raise its ugly head. &quot;Why should we spend such huge amounts of money to solve a problem that won't be a problem for half a century when we have so many pressing problems right now?&quot; So we won't spend the requisite amounts of money. We'll instead twiddle our thumbs. We'll wait and wait until the problem is a decade away or less, making the nuclear option the only viable option.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-09-19T21:29:05.930" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5394" PostTypeId="1" AcceptedAnswerId="10847" CreationDate="2014-09-20T18:47:57.897" Score="10" ViewCount="253" Body="&lt;p&gt;Is there enough information to conclude whether or not the water ice in the permanently shaded craters around the poles likely also contains carbon and nitrogen chemicals? (Or any other potentially useful substances not present or hard to access on the moon?)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="6816" LastEditDate="2017-01-03T23:23:18.443" LastActivityDate="2017-01-03T23:23:18.443" Title="Is the water ice in craters near the moon's poles likely to be accompanied by other volatiles?" Tags="|the-moon|planetary-science|colonization|water|crater|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5395" PostTypeId="1" CreationDate="2014-09-20T21:35:46.720" Score="15" ViewCount="1088" Body="&lt;p&gt;For the SpaceX Falcon 9 launch on its Dragon/CRS-4 cargo delivery mission to the International Space Station, &lt;a href=&quot;http://www.spaceflight101.com/dragon-spx-4-mission-updates.html&quot; rel=&quot;noreferrer&quot;&gt;according to Spaceflight 101&lt;/a&gt;, two downrange safety zones have been identified:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The SpX-4 soft splashdown attempt will demonstrate a cross-track&#xA;  maneuver to bring the first stage down in a zone located off-track to&#xA;  assess the maneuverability during the propulsive boost back. Two&#xA;  safety zones have been identified for this mission, a ballistic&#xA;  splashdown zone and a target for the soft splashdown.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;My best guess as to the importance of this so-called &lt;em&gt;cross-track maneuver&lt;/em&gt; (with first stage descending off-track with an angle relative to the ballistic descent) is that SpaceX will need this capability on Falcon 9 (and later Falcon 9 Heavy) in case they proceed with the Brownsville, TX (Boca Chica Beach) launch site, so they can perform in-flight dogleg maneuver to avoid overflying oil rigs in the Gulf of Mexico that are considered to be populated areas and FAA wouldn't allow them to fly over them or attempt first stage landings in the area for range safety. Combined with recovering first stage off-track, this ability could enable SpaceX a large selection of future launch and first stage downrange recovery / landing sites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/FWM97.jpg&quot; alt=&quot;Google Earth view of US offshore oil and gas platforms in the Gulf of Mexico&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Google Earth view of US offshore oil and gas platforms in the Gulf of Mexico. Data: &lt;a href=&quot;http://www.boem.gov/&quot; rel=&quot;noreferrer&quot;&gt;BOEM&lt;/a&gt;. Credit: &lt;a href=&quot;https://www.flickr.com/photos/robshell/8435708729/in/photostream/&quot; rel=&quot;noreferrer&quot;&gt;Rob Shell&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the achievable angle between the two tracks (ballistic trajectory and the off-track trajectory, what I referred to as &lt;em&gt;cross-track angle&lt;/em&gt; in the title) does have potential significance. If there are other uses for what I describe, do please mention it in your answer, but the main question that I have is rather simple;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Has it been published anywhere what the value of this &lt;em&gt;cross-track angle&lt;/em&gt; between the two CRS-4 mission safety zones for the Falcon 9 first stage landing is, or alternatively where exactly have they been designated to be, so I can calculate the angle myself?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely, since these would be safety exclusion zones, maritime and aviation traffic advisories were published by USCG and FAA, respectively?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="12102" LastEditDate="2021-08-09T03:13:38.017" LastActivityDate="2021-08-09T03:13:38.017" Title="What is the value of the cross-track angle between the two designated CRS-4 first stage recovery zones?" Tags="|falcon-9|landing|reuse|booster-flyback|dogleg-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5396" PostTypeId="1" AcceptedAnswerId="5397" CreationDate="2014-09-21T07:20:52.360" Score="9" ViewCount="1235" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/c52Vt.png&quot; alt=&quot;two satellites connected by rope&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could I tie together two satellites at different altitudes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an existing name for this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can this be stable, or will it spin out of control? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How strong of a rope will I need?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sake of argument, let's say:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;They're orbiting Earth&lt;/li&gt;&#xA;&lt;li&gt;They both mass 1000 kg. (Not sure if it matters.)&lt;/li&gt;&#xA;&lt;li&gt;The bottom one is at the edge of low earth orbit (160 km)&lt;/li&gt;&#xA;&lt;li&gt;The top one is at the other edge of low earth orbit (2000 km)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="5941" LastEditorUserId="5941" LastEditDate="2014-09-21T07:45:03.270" LastActivityDate="2019-06-24T06:31:25.607" Title="Could I tie together two satellites at different altitudes?" Tags="|space-elevator|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5397" PostTypeId="2" ParentId="5396" CreationDate="2014-09-21T13:43:47.703" Score="11" Body="&lt;p&gt;Technically, what you're describing is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Tethered_formation_flying&quot;&gt;tethered formation flying&lt;/a&gt;. Dynamics of such systems are rather complex, but it's simple to say the way you proposed it in your example wouldn't work without constant propulsion to account for different orbital decay rate and attitude control to maintain their rotation rate along the velocity vector constantly aligned so it's oriented towards each other along the tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would have to be done on both ends and constantly, so you don't gain anything from maintaining such a system unless you really need the two satellites physically connected. Say to remove the need for them to communicate wirelessly with each other, which could be tapped into or the non-tethered constellation detected via compromising electromagnetic emanations of its members. Or the lower altitude end serves as an &lt;em&gt;anchor mass&lt;/em&gt; for &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrodynamic_tether&quot;&gt;electrodynamic tether&lt;/a&gt;, and generates sufficient current that their attitude can be maintained via, say, &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetorquer&quot;&gt;magnetorquers&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most striking obstacle from your own example however is that the atmospheric drag on the lower altitude satellite will be significantly larger than that on the higher altitude satellite. For the whole tethered system that means its orbit would decay significantly faster, with the lower altitude satellite dragging both of them into even lower orbit, removing the whole system's &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_orbital_energy&quot;&gt;specific orbital energy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In simpler terms, pull of the lower altitude satellite on the higher altitude one via tether and along the velocity vector axis, or v-bar, and making the lower one lag behind if they weren't tethered, would decay the system's orbit faster. Constant tug on the tether would inevitably pull the higher altitude end lower than what used to be our lower end of the system by simply increasing its orbital eccentricity to the point that its periapsis is below the Earth's surface half an orbit around the Earth later. In a sense, it would run aground, assuming it didn't burn up in the lower atmosphere first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Smaller scale tethered formation flying is possible though, and if they're close enough to one another, the distance between such formation's members can also be maintained by electromagnetic force alone. See e.g. Department of Defense (DoD) SPHERES-Resonant Inductive Near-field Generation System, a.k.a. SPHERES-RINGS experiment, also demonstrated during one &lt;a href=&quot;http://www.youtube.com/watch?v=w9G0p4H3bcc&quot;&gt;Space Station Live&lt;/a&gt; (YouTube video). The experiment is meant to test wireless power transfer and formation flight using electromagnetic fields. Before the link disappoints you though, we're talking of &lt;em&gt;stretch of an arm&lt;/em&gt; distances here, not nearly two thousand kilometers between them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also see other types of &lt;a href=&quot;https://en.wikipedia.org/wiki/Momentum_exchange_tether&quot;&gt;momentum exchange tethers&lt;/a&gt;. Perhaps the most similar to what you're after could be considered the &lt;a href=&quot;https://en.wikipedia.org/wiki/Skyhook_(structure)&quot;&gt;skyhook&lt;/a&gt; concept, and what I describe would happen  to it if it was following orbital altitudes from your example, for a moment at least, would be similar to the rotovator concept, with the exception that its final periapsis would be too low and you'd end up with 1,840 kilometers of extremely strong and thin cable crashing at its terminal velocity towards the densely populated equatorial regions.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-09-21T14:49:30.257" LastActivityDate="2014-09-21T14:49:30.257" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5398" PostTypeId="2" ParentId="5396" CreationDate="2014-09-21T13:48:57.773" Score="7" Body="&lt;p&gt;Yes you can. It would be a &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity-gradient_stabilization&quot;&gt;gravity gradient stablized&lt;/a&gt; vertical tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two forces at play: force of gravity and what we used to call centrifugal force. Centrifugal force isn't actually a force, just inertia in a rotating frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration from gravity is $GM/r^2$ and centrifugal acceleration is $\omega^2r$ where $\omega$ is angular velocity given in radians/time. r is distance from central body's center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the lower object, $GM/r^2$ overwhelms $\omega^2r$. For the upper body $\omega^2r$ dominates. This keeps the tether aligned vertically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see this sort of stablization in our solar system. There are many &lt;a href=&quot;http://en.wikipedia.org/wiki/Tidal_locking&quot;&gt;tide locked&lt;/a&gt; moons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two masses you described wouldn't need exotic materials like bucky tubes. Mundane Kevlar would suffice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Tilda Wave mentioned, the lower end is deep enough in the atmosphere that atmespheric friction would likely bring it down. Orbital debris is relatively dense in low earth orbit. Even if the tether is thin, the great height gives a big cross sectional area -- there is a likelihood the tether would be cut.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-09-21T13:59:20.577" LastActivityDate="2014-09-21T13:59:20.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5399" PostTypeId="2" ParentId="5396" CreationDate="2014-09-21T13:49:07.223" Score="6" Body="&lt;p&gt;If we assume regular circular orbits around the earth for both satellites then they will start with quite different speeds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At an altitude of 6351 km a satellite in a circular orbit would move at about 7.874 km/sec.&lt;br&gt;&#xA;At an altitude of 8371 km a satellite in a circular orbit would move at about 6.900 km/sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Formula used:  $v = \sqrt{GM/r}$, and for the Earth G times M equals 398600.5 km&lt;sup&gt;3&lt;/sup&gt;/sec&lt;sup&gt;2&lt;/sup&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They would have a speed difference of almost a kilometer per second.  At these speeds it is safe to say that a rope would snap and that both satellite would get a jolt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do not assume that and carefully place the two in a non-circular orbit or one where one if trusting, then it gets more complex than a can quickly answer (and given two post which already appeared while I was writing that other probably understand those complexities better).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iIEyo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="686" LastEditorUserId="242" LastEditDate="2014-09-22T04:29:22.190" LastActivityDate="2014-09-22T04:29:22.190" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5400" PostTypeId="1" AcceptedAnswerId="5401" CreationDate="2014-09-21T19:57:08.793" Score="11" ViewCount="1847" Body="&lt;p&gt;I read a while ago about the famous Soviet N-1, called by some the most powerful rocket (in terms of thrust at liftoff) in history. It used an incredible 30 engines in its first stage. The American Saturn V, on the other hand, used only five F-1 engines in its first stage. The difference was that each F-1 was much more powerful than a single one of the (initially NK-15) engines used on the N-1. The advantage of the 30-engine system was that if one engine failed, there were backups, and more fuel could be shifted to or from others to compensate (although this led to some amusing mishaps with the KORD computer controlling the engines).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the other advantages/disadvantages for each of the systems (30-engines vs. 5-engines)? What made the Soviets choose a 30-engine system, while the Americans chose a 5-engine system?&lt;/p&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2017-12-19T13:20:55.713" Title="Quality vs. Quantity for Rocket Engines" Tags="|rockets|engine-design|engines|saturn-v|n-1|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5401" PostTypeId="2" ParentId="5400" CreationDate="2014-09-21T20:12:04.400" Score="15" Body="&lt;p&gt;Combustion instability is a very hard problem to crack.  It gets harder as the engine gets larger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Americans took on the challenge of a 1.5 million lb thrust engine and beat it (F-1, I suppose even the J-2 on the second (5) and third stages (1)).  The Soviets tried and failed and went with a small (NK-15) engine.  They needed the thrust so they clustered them.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel lines at some point combine, and back pressure can cause vibrations in other feed pipes.  Saturn V suffered from Pogo oscillations even with only 5 engines, you can imagine the N-1 would have had bigger issues with 30.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be interesting is to hear how this has affected Falcon 9 with 9 engines. I think it is the largest number of engines on a stage currently flying.  (Or ever?  Since the N-1? What had more engines?)&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-09-21T23:20:45.140" LastActivityDate="2014-09-21T23:20:45.140" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5402" PostTypeId="1" AcceptedAnswerId="5414" CreationDate="2014-09-21T23:23:51.050" Score="14" ViewCount="3445" Body="&lt;p&gt;The Saturn V had 5 engines (F-1) on the first stage, five engines on the second stage (J-2).  The N-1 had 30 engines on its first stage (NK-15).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 (1.0 and 1.1) have 9 Merlin 1 engines on the first stage.  (Merlin 1C on F9 1.0, and Merlin 1D on F9 1.1). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any stages on other launchers with more than 9, or even close to 9 engines? (Not in design phase, but metal bent, and possibly an attempt at a launch.  Can't ask for success since that kicks out the N-1, just an attempt will make me happy). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot think of any currently launching, but in the past? How would I count the Falcon Heavy with 27 engines firing at liftoff?  I think like I would count a Atlas V 551 with 5 GEM Solid rockets firing, as not counting as on a single stage. Same as Energia with 4 strapon Zenit boosters would not really count. Energia would thus have had 4 engines on the first stage.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="5963" LastEditDate="2015-01-10T19:36:05.267" LastActivityDate="2020-09-22T17:48:46.333" Title="Other than then N-1 (30 engines) has there been a single stage of a rocket with more engines than a Falcon 9?" Tags="|spacex|falcon-9|engines|n-1|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5403" PostTypeId="1" AcceptedAnswerId="6525" CreationDate="2014-09-22T00:11:53.050" Score="23" ViewCount="2132" Body="&lt;p&gt;As seen in Gravity, and also in this youtube video: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/-l7MM9yoxII?start=490&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;, the Soyuz main engine is covered by a metal plate that it mechanically moved out of the way before firing. What is the function of this covering?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ee1ST.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="168" LastEditDate="2015-02-27T18:31:31.470" LastActivityDate="2015-02-27T18:31:31.470" Title="Why does the Soyuz cover its main engine?" Tags="|soyuz-spacecraft|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5405" PostTypeId="1" AcceptedAnswerId="5406" CreationDate="2014-09-22T07:18:10.600" Score="19" ViewCount="6222" Body="&lt;p&gt;We all know that the Space Shuttle Solid Rocket Boosters (SRB) were the pair of large solid rockets used by the United States' NASA Space Shuttle during the first two minutes of its powered flight. Together, they provided about 83% of liftoff thrust for the Space Shuttle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would have happened if the Solid Rocket Boosters didn't separate after the separation point?&lt;/p&gt;&#xA;" OwnerUserId="6109" LastEditorUserId="6109" LastEditDate="2014-09-22T16:48:32.040" LastActivityDate="2017-06-14T18:03:31.563" Title="What would have happened if Space Shuttle Solid Rocket Boosters didn't separate?" Tags="|rockets|space-shuttle|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5406" PostTypeId="2" ParentId="5405" CreationDate="2014-09-22T07:39:03.600" Score="25" Body="&lt;p&gt;The empty SRBs would have added an additional mass to the vessel which would need to be carried into orbit until separation of the main tank. The additional fuel needed for this would mean that it would not have been possible to reach orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The standard procedure for an abort between SRB separation and main-engine cut-off was the Transoceanic Abort Landing (TAL). The orbiter would have been separated from the external tank, continue on its sub-orbital trajectory and perform a landing on the designated abort runway in Europe (for every space shuttle launch, two different airports in Europe were prepared especially for this eventuality. It never happened).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should the separation of the orbiter from the external tank also fail... well, the orbiter was unable to glide aerodynamically and land with the tank still attached, so this failure would have been catastrophic.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-09-22T12:43:06.403" LastActivityDate="2014-09-22T12:43:06.403" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="5407" PostTypeId="1" AcceptedAnswerId="5412" CreationDate="2014-09-22T09:31:15.080" Score="2" ViewCount="167" Body="&lt;p&gt;My lecture slides on spacecraft magnetometers for use in attitude sensing only mention the use in the context of Earth orbits, but leave it open to interpretation if they can be used for attitude sensing in orbits of other planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can a spacecraft magnetometer be used in non-terrestrial orbits, or other planets lack the necessary magnetic field?&lt;/p&gt;&#xA;" OwnerUserId="1552" LastEditorUserId="49" LastEditDate="2014-09-22T18:11:46.880" LastActivityDate="2014-09-22T18:11:46.880" Title="Can spacecraft magnetometers be used in orbits of planets other than Earth?" Tags="|orbit|planetary-science|attitude|magnetic-field|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5408" PostTypeId="2" ParentId="5396" CreationDate="2014-09-22T10:14:37.150" Score="0" Body="&lt;p&gt;Centrifugal force has nothing to do with satellite movement as it is too low to counteract the gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to stay together, at least one of the satellites should have a certain mean of propulsion, as his orbit would decay otherwise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can calculate here &lt;a href=&quot;http://www.csgnetwork.com/satorbdatacalc.html&quot; rel=&quot;nofollow&quot;&gt;http://www.csgnetwork.com/satorbdatacalc.html&lt;/a&gt;&#xA;To stay in orbit, the 160km satellite should have a 87 min orbit revolution time, and the 2000km one - 127 min. So no, they cannot stay at the same distance from one another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a space elevator, the satellites must be geostationary, and that is only possible ( without propulsion ) at 35,786 km and a speed of 3.07 km/s.&lt;/p&gt;&#xA;" OwnerUserId="6134" LastActivityDate="2014-09-22T10:14:37.150" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5409" PostTypeId="1" AcceptedAnswerId="5415" CreationDate="2014-09-22T11:44:03.917" Score="21" ViewCount="10053" Body="&lt;p&gt;As I understand it, the Space Shuttle was essentially a glider when it was coming back to land, and the engines were not there to facilitate powered flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what was the procedure, had the Space Shuttle had to do an aborted landing?&lt;/p&gt;&#xA;" OwnerUserId="2584" LastEditorUserId="49" LastEditDate="2014-09-22T16:24:15.323" LastActivityDate="2023-05-19T01:50:43.073" Title="If the Space Shuttle missed its landing approach, what could have been done?" Tags="|space-shuttle|landing|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5410" PostTypeId="1" CreationDate="2014-09-22T16:14:25.960" Score="9" ViewCount="676" Body="&lt;p&gt;After the SpaceX CRS-4 launch of Dragon on F9, Hans Koenigsmann, SpaceX head of launch operations is &lt;a href=&quot;http://www.spacenews.com/article/launch-report/41932spacex-launches-fourth-cargo-mission-to-space-station&quot; rel=&quot;noreferrer&quot;&gt;quoted as saying&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Koenigsmann said that shorter gaps between launches should become more routine for SpaceX. “In the future, I anticipate this will be the norm,” he said at a pre-launch briefing Sept. 20, crediting parallel processing at the launch site for enabling the fast turnaround. “We’re ramping up for that launch rate and even more than that.” &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Thus, when we look at the LC-40 hangar, it looks fairly narrow for two cores.  Especially since the primary core needs to be mounted on the rail transporter, which runs down the center of the building.  So does not seem likely to even be able to shuffle over and share the space.  Additionally it seems like that would need a second cradle to handle the booster stage.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Dragon capsule is managed at the back end (assuming the Falcon rolls out the front, reverse terms as needed) of the hangar, and they have the SPIF building for payload processing if needed as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how are they handling parallel processing of cores?  Leave it on the truck transporter in the hangar? Or is the payload the bottleneck, and thus cores are easier to prepare for launch.  (Which would actually be the best outcome, right?  Cores are easy, payloads are hard means pushing more cores through the process faster is easier).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-07-10T02:55:48.527" LastActivityDate="2017-11-06T18:26:20.947" Title="How does SpaceX manage parallel processing at the LC-40 Complex?" Tags="|launch|spacex|falcon-9|lc-40|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5411" PostTypeId="2" ParentId="5402" CreationDate="2014-09-22T16:22:10.093" Score="4" Body="&lt;p&gt;As far as I know, SpaceX holds the record. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_IB&quot; rel=&quot;nofollow&quot;&gt;Saturn 1B&lt;/a&gt; takes second place with 8 engines. Everything else I know off has smaller numbers. &#xA;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_%28rocket_family%29&quot; rel=&quot;nofollow&quot;&gt;Delta&lt;/a&gt; has up to 9 boosters, but they would not all fire simultaneously so you could argue those amount to 2 stages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(shows you what I know, @Russell has found the winner) &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-09-22T19:37:27.363" LastActivityDate="2014-09-22T19:37:27.363" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5412" PostTypeId="2" ParentId="5407" CreationDate="2014-09-22T17:06:49.100" Score="4" Body="&lt;p&gt;For attitude determination using magnetic field you need:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A measurable field&lt;/li&gt;&#xA;&lt;li&gt;A good model for the field&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;On Earth, the usual model is called &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Geomagnetic_Reference_Field&quot; rel=&quot;nofollow&quot;&gt;IGRF&lt;/a&gt;. You need the model to correlate the measured field with the estimated field at a certain position in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronomynotes.com/solarsys/plantblb.htm&quot; rel=&quot;nofollow&quot;&gt;Here&lt;/a&gt; is a list of magnetic field strengths on planets in the solar system. &lt;/p&gt;&#xA;" OwnerUserId="1348" LastActivityDate="2014-09-22T17:06:49.100" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5413" PostTypeId="2" ParentId="5409" CreationDate="2014-09-22T19:30:49.613" Score="12" Body="&lt;p&gt;The answer is that they made sure everything went smooth, understanding they had one chance only:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;They rather used an alternative landing strip than risk landing in bad weather (unlike commercial flights) &lt;/li&gt;&#xA;&lt;li&gt;They were able to control speed of descent to make sure to make it to the runway. &lt;/li&gt;&#xA;&lt;li&gt;The runway also was long enough that if they landed midway (too long) they had enough room to stop. This prevented &quot;normal&quot; runways to be used for the shuttle.&lt;/li&gt;&#xA;&lt;li&gt;Emergency teams were ready in case that 1,2,3 was not enough to save the crew.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="6139" LastEditorUserId="6139" LastEditDate="2014-09-22T20:20:21.590" LastActivityDate="2014-09-22T20:20:21.590" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5414" PostTypeId="2" ParentId="5402" CreationDate="2014-09-22T19:33:26.267" Score="17" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Jupiter-C&quot; rel=&quot;noreferrer&quot;&gt;Jupiter-C&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_I&quot; rel=&quot;noreferrer&quot;&gt;Juno&lt;/a&gt; second stage was a cluster of 11 (!) solid rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VxTyN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on what you're willing to count, the R-7 &amp;amp; Soyuz launcher family have 20 thrust chambers going at launch -- 16 on strap-on boosters, 4 in the sustainer core. (Each cluster of 4 chambers shares one set of fuel pumps, etc., so it comes down to whether you define that as 5 engines or 20.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RBIlz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-11-11T17:22:39.723" LastActivityDate="2014-11-11T17:22:39.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5415" PostTypeId="2" ParentId="5409" CreationDate="2014-09-22T20:13:12.340" Score="30" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;If the Space Shuttle missed its landing approach, what could have been done?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Nothing. Not one thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the reentry burn was complete, the Shuttle was either going to explode on the way down or land at or near the intended landing site. There was no such thing as an aborted/retried landing. If the Shuttle didn't land at the intended landing site, it was still going to land near that site. The reentry burn made the decision to land irrevocable. The only way to avoid landing was not to perform that reentry burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because landing was a one-shot event, the number one perceived threat to a safe landing was bad weather. There were quite a few Shuttle flights that spent an extra day or two on orbit because of forecasts of inclement weather at or near the intended landing site. Some of the Shuttle flights that were intended to land at the Cape ended up landing at Edwards Air Force Base because of continued threats of bad weather at the Cape. One flight, STS-3, ended up landing at White Sands because of continued threats of bad weather at Edwards. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="415" LastEditDate="2015-10-15T14:00:48.577" LastActivityDate="2015-10-15T14:00:48.577" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5416" PostTypeId="1" AcceptedAnswerId="5426" CreationDate="2014-09-22T21:07:46.800" Score="17" ViewCount="15804" Body="&lt;p&gt;Today I saw this picture:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nxIh2.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Is this true? I have no idea how many satellites there are orbiting earth, but I do know that we have sent quite a few of them.&lt;/p&gt;&#xA;&lt;p&gt;So, is it known how many satellites we have orbiting the earth right know? (I guess there must be some inactive ones)&lt;/p&gt;&#xA;" OwnerUserId="5881" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-11-19T17:22:28.400" Title="How many satellites are orbiting the earth?" Tags="|artificial-satellite|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5417" PostTypeId="2" ParentId="5416" CreationDate="2014-09-22T21:42:58.663" Score="9" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.ucsusa.org/nuclear_weapons_and_global_security/solutions/space-weapons/ucs-satellite-database.html&quot;&gt;this site&lt;/a&gt;, there are over 1100 operational satellites in Earth orbit. In addition to those and inactive satellites, there's a tremendous amount of &quot;space junk&quot; -- debris from launchers -- also in orbit around the Earth.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-09-22T21:42:58.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5419" PostTypeId="1" AcceptedAnswerId="5486" CreationDate="2014-09-23T03:33:20.123" Score="13" ViewCount="2243" Body="&lt;p&gt;A lucky discovery of &lt;a href=&quot;http://www.reddit.com/r/KerbalAcademy/comments/1jh29x/atmospheric_drag_and_orbital_v/&quot;&gt;a Kerbal engineer's writing&lt;/a&gt; gave this deep insight into the mechanics of climbing to orbit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the best way to minimize d/v losses through the atmosphere is to always go at exactly the terminal velocity at any given altitude (because this is the point of intersection between the graphs of drag losses vs gravity losses).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I had always found the mathematics of the gravity turn to be daunting. This would really toss a lot of that detail away, allowing many quick and useful back-of-the-envelope calculations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But why should I believe it? Maybe it's just a Kerbal old wives tale. Is there any real theoretical model that suggests something like this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, it's not clear &lt;em&gt;what&lt;/em&gt; terminal velocity should be used anyway. The mass of the rocket is changing throughout the climb, and with mass, the terminal velocity changes. Would this rule use the dynamic mass value, or the final mass, or next mass before staging, or something else?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-10-01T16:46:36.630" Title="Climbing at terminal velocity minimizes losses? But why? And &quot;of what&quot;?" Tags="|launch|trajectory|guidance|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5420" PostTypeId="1" CreationDate="2014-09-23T04:33:45.533" Score="4" ViewCount="450" Body="&lt;p&gt;In March, the ISS had to be &lt;a href=&quot;http://www.baynews9.com/content/news/baynews9/news/article.html/content/news/articles/cfn/2014/3/17/Space_station_moves_away_from_space_junk.html&quot; rel=&quot;nofollow&quot;&gt;maneuvered to avoid space junk&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA said Monday the space station had to dodge part of an old&#xA;  satellite. Sunday night's firing of on-board thrusters pushed the&#xA;  orbiting lab up a half-mile.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Then I remembered that arstechnica did a &lt;a href=&quot;http://arstechnica.com/science/2013/07/how-nasa-steers-the-international-space-station-around-space-junk/&quot; rel=&quot;nofollow&quot;&gt;great piece on how the ISS is moved&lt;/a&gt; and in that article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Additionally, the station has several sets of thrusters that allow it&#xA;  to rotate and translate. The Zvezda service module is equipped with&#xA;  thrusters, and there are thrusters on docked vehicles like the&#xA;  Progress resupply craft and the ESA ATV that can also be employed.&#xA;  Space shuttles could also be used when they were still operational.&#xA;  For a typical debris avoidance maneuver, the station will be subjected&#xA;  to delta V of between 0.5 and 1 meter per second.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I am wondering what is the power required to move the ISS? I am not a physics expert but in space as there is no friction, I don't think it would need massive amounts of propulsion to move the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more curious question is could a human &quot;push&quot; the ISS and make it move?&lt;/p&gt;&#xA;" OwnerUserId="2584" LastActivityDate="2023-10-26T19:10:08.887" Title="What is the power required to move the space station?" Tags="|iss|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5421" PostTypeId="1" CreationDate="2014-09-23T05:37:23.200" Score="19" ViewCount="1364" Body="&lt;p&gt;This question may be a bit silly, but &lt;a href=&quot;https://www.youtube.com/watch?v=lMtXfwk7PXg&quot;&gt;this video&lt;/a&gt; of Chris Hadfield wringing out a wet washcloth on the ISS got me thinking...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the video you can clearly see drops of water floating away from the cloth, and as I understand it, they'll be sucked into the ventilation system and recombined with the astronauts' water supply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So &lt;em&gt;how much water could be in the 'internal atmosphere' of the ISS (or any spacecraft) before problems start arising?&lt;/em&gt; Suppose some water tank burst and now X percent of the ISS volume is filled with water. What is a dangerous X and how would the water be dealt with?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely a lot of water floating around would get into instruments and damage important circuitry. Could the ventilation systems handle it? Would the &lt;a href=&quot;http://en.wikipedia.org/wiki/Surface_tension&quot;&gt;surface tension&lt;/a&gt; of water be a blessing or a curse? Is the thought of having astronauts 'swim' through the ISS in search of air pockets pure science fiction? &lt;/p&gt;&#xA;" OwnerUserId="6144" LastActivityDate="2014-09-24T12:39:29.437" Title="My space station is filling with water" Tags="|iss|water|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5422" PostTypeId="1" AcceptedAnswerId="5434" CreationDate="2014-09-23T07:55:32.690" Score="9" ViewCount="783" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_debris&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As of 2009, about 19,000 pieces of debris larger than 5 cm (2.0 in) are tracked,[1] with another 300,000 pieces smaller than 1 cm below 200 km altitude&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Control collision avoidance attempt to maintain a clear 2KM cube around the ISS. Whenever an object is expected the pass through this virtual cube, ISS has to manoeuvre. 2KM might sound a lot, but we're talking of a velocity of 7km/s - so it's not even a half-a-second away!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbit around Mars is probably not quite as crowded. On the other hand, there are 3 live spacecraft in orbit&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;MO&lt;/li&gt;&#xA;&lt;li&gt;ME&lt;/li&gt;&#xA;&lt;li&gt;MRO&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There may also be other craft from older missions in Martian orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the imminent arrival of NASA MAVEN, and ISRO MOM, the number will increase this week.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Given the signal lag&lt;/strong&gt;, is there any active collision avoidance system aboard any of these craft ? Is such a system needed at present?&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2015-06-02T18:13:50.490" Title="Is there any active collision avoidance aboard the spacecraft in Mars' orbit?" Tags="|mars|nasa|isro|collision-avoidance|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5423" PostTypeId="1" AcceptedAnswerId="5428" CreationDate="2014-09-23T13:17:20.830" Score="41" ViewCount="56149" Body="&lt;p&gt;GPS satellites transmit time values regularly, but if we put in consideration the time delay between the satellite and the receiver, the time value received wouldn't be accurate insofar as I know GPS receivers aren't able to determine their position until they've got accurate time. Our devices don't have any atomic clocks synchronized with GPS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do our GPS receivers calculate time from the signals of GPS?&lt;/p&gt;&#xA;" OwnerUserId="6148" LastEditorUserId="168" LastEditDate="2014-09-23T14:28:16.910" LastActivityDate="2024-03-27T22:50:44.637" Title="How does GPS receiver synchronize time with GPS satellites?" Tags="|artificial-satellite|time|gps|" AnswerCount="6" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5424" PostTypeId="1" CreationDate="2014-09-23T13:32:02.543" Score="3" ViewCount="236" Body="&lt;p&gt;This question comes from NASA and is the &lt;strong&gt;NASA's Balance Mass Challenge: Using &quot;Dead Weight&quot; on Mars Spacecraft to Advance Science and Technology&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While landing on Mars, the Mars Science Laboratory (MSL) system ejected approximately 300 kg of Balance mass to offset its center of gravity before atmospheric entry and then rebalance its center of gravity after atmospheric entry. This 300 kg might be used on future missions for Mars-related science and technology applications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question NASA hopes to answer is, &quot;If you had up to 150 kg of ejectable mass prior to entry and another 150 kg during the entry and landing phase of a Mars mission, what could you do with it that was useful and advances knowledge in a scientific or technological way?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.innocentive.com/ar/challenge/9933607&quot; rel=&quot;nofollow&quot;&gt;https://www.innocentive.com/ar/challenge/9933607&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="690" LastActivityDate="2014-09-23T13:32:02.543" Title="If you had 150 kg of ejectable mass prior to entry and another 150 kg during entry/landing of a Mars mission, what could you do with it?" Tags="|mars|landing|science|" AnswerCount="0" CommentCount="3" ClosedDate="2014-09-23T14:36:20.080" ContentLicense="CC BY-SA 3.0" />
  <row Id="5425" PostTypeId="2" ParentId="5423" CreationDate="2014-09-23T14:17:35.040" Score="18" Body="&lt;p&gt;To give a big picture view of how the GPS solution is determined, consider the following equation:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\rho_i = \sqrt{(x_i-x_u)^2+(y_i-y_u)^2+(z_i-z_u)^2} +c\Delta t$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where &lt;span class=&quot;math-container&quot;&gt;$\rho$&lt;/span&gt; is essentially a range/distance from the user to the GPS satellite, &lt;span class=&quot;math-container&quot;&gt;$x,y,z$&lt;/span&gt; are position coordinates, the subscript &lt;span class=&quot;math-container&quot;&gt;$i$&lt;/span&gt; indicates the particular satellite, &lt;span class=&quot;math-container&quot;&gt;$c$&lt;/span&gt; is the speed of light, and &lt;span class=&quot;math-container&quot;&gt;$\Delta t$&lt;/span&gt; is a time delay.&lt;/p&gt;&#xA;&lt;p&gt;Assuming that you have knowledge of the GPS space vehicle (SV), the &lt;span class=&quot;math-container&quot;&gt;$x_i,y_i,z_i$&lt;/span&gt; values are known from the satellite ephemeris (this can be obtained from publicly available data, and more accurate ephemeris can be obtained via more secure methods).  There are now 4 unknowns, implying that we need 4 GPS SVs to solve for the user location &lt;span class=&quot;math-container&quot;&gt;$(x_u,y_u,z_u)$&lt;/span&gt; and time delay.  More SVs can be observed, and an over-determined solution can be found from various numerical methods (e.g., a least squares solution), or a best-4 SV solution can be employed.&lt;/p&gt;&#xA;&lt;p&gt;The time delay is essentially in the &lt;span class=&quot;math-container&quot;&gt;$\Delta t$&lt;/span&gt; term.  Various errors can be accounted for by augmenting the system of equations to include, but in no way limited to, ionospheric &amp;amp; tropospheric delays, relativity effects, and clock errors present in the receiver.&lt;/p&gt;&#xA;&lt;p&gt;A multitude of simple and complex differential methods exist to essentially exploit similar delays between measurements and remove them without even solving for them (e.g., &lt;a href=&quot;http://en.wikipedia.org/wiki/Differential_GPS&quot; rel=&quot;nofollow noreferrer&quot;&gt;differential GPS&lt;/a&gt; and &lt;a href=&quot;http://www.navipedia.net/index.php/RTK_Fundamentals&quot; rel=&quot;nofollow noreferrer&quot;&gt;real time kinematics&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;http://www.nbmg.unr.edu/staff/pdfs/Blewitt%20Basics%20of%20gps.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;short paper&lt;/a&gt; that discusses the observation equations and, more specifically, the GPS signal and code-generation.&lt;/p&gt;&#xA;" OwnerUserId="1478" LastEditorUserId="-1" LastEditDate="2024-03-27T07:02:49.683" LastActivityDate="2024-03-27T07:02:49.683" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="5426" PostTypeId="2" ParentId="5416" CreationDate="2014-09-23T14:51:44.410" Score="17" Body="&lt;p&gt;Depends on how you count them.  Russell Borogove's answer of 1100 is probably in the right neighborhood for operational satellites.  The number of dead satellites, debris objects and rocket bodies is far higher.  If you have a Space-Track.org account, take a look under the &amp;quot;Boxscore&amp;quot; for the current count, broken down by country/organization of origin.&lt;/p&gt;&#xA;&lt;p&gt;The totals on Sept. 23, 2014:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Payloads:       3984&#xA;Rocket Bodies:  2005&#xA;Debris:        11193&#xA;Total:         17182&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;This is the total, as released by the US Air Force, and doesn't include a significant population which aren't publicly released for various reasons.  It also doesn't include large swaths of the debris population which is either to small to reliably track, or hasn't been catalogued for various reasons.  Depending on where you draw the line for the lower size bound, some estimates are as high as 100,000 objects.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="1235" LastEditDate="2020-08-10T20:34:12.150" LastActivityDate="2020-08-10T20:34:12.150" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5427" PostTypeId="2" ParentId="5422" CreationDate="2014-09-23T17:14:25.517" Score="7" Body="&lt;p&gt;A few notes...&#xA;The conjunction screening volume for ISS isn't a cube.  It's a rectangular box which is longer in-track and smallest in the radial direction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It kind of sounds like you're suggesting this volume is so that the spacecraft has time to maneuver.  This isn't the case - any maneuver to mitigate a predicted conjunction would have to occur long before the event - at least half a rev is good, more is better.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason you have a screening volume larger then the satellite is because the satellite orbits have a significant amount of uncertainty, commonly referred to as the covariance.  So even if you predict a 2 km miss between two objects, there is still a non-zero chance of collision because of the position uncertainty.  The iridium/cosmos collision had a predicted miss distance from TLE data of on the order of 0.5 to 1.5 km, depending on which data you use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is also the reason why conjunction predictions, even for ISS are only done a few days to a week out.  The position covariance grows with time so eventually it's to big to do useful analysis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So back to Mars.  Space is big - really really big.  5 objects in orbit around one planet should have a vanishingly small chance of colliding.  With over 17K tracked objects around Earth, collisions are still a very very rare occurrence - 4 events to date.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;space is big&quot; argument isn't very satisfying however, because these objects aren't placed there randomly and there are orbit altitudes and inclinations which are more useful then others, so you would expect that objects would be concentrated there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back to covariance - does anybody know the accuracy to which we know the orbital positions of these probes?  Sounds like a good follow-up question.  If the covariance of the objects becomes to large, you really can't do effective collision avoidance analysis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best strategy may be to simply allow a sufficient altitude difference between the missions - which shouldn't be difficult given the population now or in the foreseeable future.  But the risk really is quite low.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="4709" LastEditDate="2014-09-23T20:42:48.513" LastActivityDate="2014-09-23T20:42:48.513" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5428" PostTypeId="2" ParentId="5423" CreationDate="2014-09-23T19:13:03.660" Score="35" Body="&lt;p&gt;&quot;...the time value received wouldn't be accurate insofar as I know GPS receivers aren't able to determine their position until they've got accurate time.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time value isn't used to tell the receiver what time it is (at least not directly, although that is helpful later).  It's used so that the receiver can tell relatively what the distance is to each satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you hear Sat A say that the time is 0.00000 and Sat B says the time is 0.00010, then if they are in sync, you must be closer to B than to A.  You can tell exactly how much closer you are by the specific time difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Repeat the calculations with a few other satellites and you will find that there is only one place (and time) that the receiver can be located.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GPS receiver computes a solution that simultaneously provides Position, Velocity, and Time (PVT).  It's not that one is calculated first, then the other is.  They all fall out simultaneously.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2014-09-23T19:13:03.660" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="5429" PostTypeId="2" ParentId="5422" CreationDate="2014-09-23T19:21:20.710" Score="6" Body="&lt;p&gt;My initial thought was that, for a small number of orbiting craft, the orbits could be selected so as to not ever intersect, but some quick checking doesn't make it obvious that this is possible, since many of the orbiters have very eccentric, highly inclined orbits.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;MRO, 93° inclination, 250km-316km altitude&lt;/li&gt;&#xA;&lt;li&gt;MEx, 86.3°, 298km-10107km&lt;/li&gt;&#xA;&lt;li&gt;MAVEN, 75°, 150km-6200km&lt;/li&gt;&#xA;&lt;li&gt;MOM, 150°, 365km-80000km&lt;/li&gt;&#xA;&lt;li&gt;Mars Odyssey, 93°, ~400km&lt;/li&gt;&#xA;&lt;li&gt;Mars Global Surveyor, 93°, 378km&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what the significance of the popular 93° inclination is; the three orbiters in that inclination all have very different orbital periods. It's theoretically possible that the periods of the orbits could be selected to harmonize, but I don't think it's likely; probably all the involved agencies watch each other's orbits and on the very unlikely chance a close approach is predicted, whoever's got the most maneuverable orbiter will nudge it.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="265" LastEditDate="2014-09-25T00:35:39.550" LastActivityDate="2014-09-25T00:35:39.550" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5430" PostTypeId="1" AcceptedAnswerId="5431" CreationDate="2014-09-23T19:51:05.917" Score="6" ViewCount="3058" Body="&lt;p&gt;Why do gps satellites rotate over the Earth? Some says because of it there are at least 4 gps satellites at any location of Earth. But I can't understand it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would happen if gps satellites stays at where they are like tv broadcast satellites? Like this? There are still at least 4 gps satellites at any location of Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KnnNg.jpg&quot; alt=&quot;gps&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6148" LastActivityDate="2022-03-05T10:07:47.223" Title="Why do gps satellites rotate over the Earth" Tags="|artificial-satellite|orbit|gps|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5431" PostTypeId="2" ParentId="5430" CreationDate="2014-09-23T20:13:57.250" Score="22" Body="&lt;p&gt;The only class of orbit in which a satellite always remains over the same point on the earth's surface is a geostationary orbit, which can only be directly above the equator.  Putting the GPS satellites in such orbits would have several disadvantages:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Poor satellite visibility for users at high latitudes - the satellites would always be low on the southern horizon (for a northern hemisphere user) and often blocked by buildings or terrain.  At high enough latitudes they would not be visible at all.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Poor solution geometry - GPS navigation works best when the satellites are in &lt;a href=&quot;http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS)&quot; rel=&quot;nofollow noreferrer&quot;&gt;diverse directions&lt;/a&gt;.  In addition to poor DOP, if the satellites are all in one plane as they would be in the case of geostationary orbits, there's an ambiguity - the distances to the satellites would be the same for a user at the same longitude and either sign of latitude, so the receiver wouldn't be able to tell whether it was in the northern or southern hemisphere.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Greater launch cost - geostationary orbits are higher energy than MEO; not all launch vehicles can get there and those that can would have a lower payload capacity.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;For completeness, there would be some minor advantages to placing GNSS spacecraft in geostationary orbits, though these are outweighed by the disadvantages given above:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Simpler acquisition for receivers - because a satellite in GEO has ~zero velocity with respect to the Earth's surface, a stationary or slowly-moving receiver will see very little Doppler effect on the ranging signal. This can make the search phase of the signal acquisition process faster and simplify the receiver's circuitry. When GPS receivers were first developed this would have been a significant factor but these days the search is very fast anyway and silicon is cheap.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;More benign radiation environment for the spacecraft - the MEO orbits used by GPS and most other GNSS pass through some of the stronger portions of the Van Allen radiation belts, which requires expensive (and in some senses antiquated) radiation-hardened electronics. GEO is no picnic either but the radiation dose is significantly less than that in MEO.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2022-03-05T10:07:47.223" LastActivityDate="2022-03-05T10:07:47.223" CommentCount="15" ContentLicense="CC BY-SA 4.0" />
  <row Id="5432" PostTypeId="2" ParentId="5423" CreationDate="2014-09-23T20:22:24.790" Score="1" Body="&lt;p&gt;Let's say the time on your receiver was way off, let's say off by 10 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the delay for the signal from each of 4 satellites would appear to be 10 minutes plus some small fraction of a second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The signal from the satellite with the shortest delay would be the satellite that is closest to you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Subtracting that delay from the delays of the other 3 satellites will let you calculate the difference of the delay between each of the satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some other information you know as facts:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The absolute location of all 4 satellites relative to earth... latitude, longitude, altitude (orbit height). &lt;/li&gt;&#xA;&lt;li&gt;Given that the &quot;true&quot; distance from you to the closest satellite cannot be less than that satellites' altitude (if you were directly below it), and cannot be further from you than the next further satellite.&lt;/li&gt;&#xA;&lt;li&gt;And the &quot;true&quot; distance from you to the furthest satellite cannot be less than the distance of the next closer satellite, and cannot be further from you than a point on the horizon at that satellites' altitude.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;With just this information from these 4 satellites it should be possible to fairly accurately calculate your latitude and longitude. Adding data from additional satellites would allow improved accuracy including calculation of your altitude on earth.&lt;/p&gt;&#xA;" OwnerUserId="1496" LastActivityDate="2014-09-23T20:22:24.790" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5433" PostTypeId="2" ParentId="5421" CreationDate="2014-09-24T00:54:43.157" Score="3" Body="&lt;p&gt;There isn't enough water on the ISS to create the scenario you describe in your last sentence.  A rapid leak would still be a serious problem, for the reasons you mention of electrical shorts and associated galvanic corrosion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The designers probably keep tank pressures somewhat low and pipe diameters small to reduce the likelihood of a rapid leak (e.g. in the event of a failed valve).&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-09-24T00:54:43.157" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5434" PostTypeId="2" ParentId="5422" CreationDate="2014-09-24T01:34:12.917" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;blockquote&gt;&#xA;    &lt;p&gt;Given the signal lag, is there any active collision avoidance system aboard any of these craft?&lt;/p&gt;&#xA;  &lt;/blockquote&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No. For that matter, such an active collision avoidance system does not exist for the International Space Station, either. The Space Station moves when told to do so by the ground. The Space Station has no capability of detecting when a collision is imminent. At seven kilometers per second, detection of an imminent collision would mean an unavoidable collision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The maneuvers by the ISS are based on predictions by ground-based assets with lots of computing power, which in turn are based on lots and lots of measurements made by other ground-based assets. We don't have radar stations positioned across the surface of Mars to make those measurements, and we don't have banks of blades and supercomputers dedicated to processing those non-existent measurements and solving the orbit determination problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS is a very special asset. It is big, really really big, which makes the collision problem much more significant than anything else we have in space. It's cost is even bigger than it's size; it's loss would be catastrophic. That a collision could kill people makes the problem even bigger. The Three Letter Acronym organizations charged with keeping track of what's doing what in space are specially charged with protecting that singular asset.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-09-24T01:34:12.917" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5435" PostTypeId="2" ParentId="5421" CreationDate="2014-09-24T06:48:16.687" Score="7" Body="&lt;p&gt;The &lt;a href=&quot;http://www.spaceref.com/iss/ops/emergency.procedures.pdf&quot;&gt;ISS Emergency Procedures book&lt;/a&gt; (alas, only 2000 edition (configuration 5A) is publicly available) lists the following contingency:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.514 WATER CONDENSATE TANK HIGH WARNING SAFING - WARN (EMER/5A/FIN)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;If Contingency Water Container fill not in progress Then&#xA;    Perform H2O Vent&#xA;    If Condensate Qty not decreasing Then &#xA;       Perform routine {3.702 WATER CONDENSATE TANK BLOCKAGE/ HIGH} (SODF: ECLSS: MALFUNCTION: WRM)&#xA;    End If&#xA;Else&#xA;    If Contingency Water Container fill in progress, contact MCC-H&#xA;End If&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-09-24T06:48:16.687" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5436" PostTypeId="1" AcceptedAnswerId="5446" CreationDate="2014-09-24T07:36:16.647" Score="16" ViewCount="3667" Body="&lt;p&gt;It has been reported that India's Mars mission, Mangalyaan, cost only \$74 million compared to \$671 million cost of Maven. What is the reason for this difference in cost? Are there any significant differences in the operation of two missions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://firstbiz.firstpost.com/life/worlds-most-budget-friendly-mars-mission-how-mangalyaan-stacks-up-against-others-101153.html&quot;&gt;http://firstbiz.firstpost.com/life/worlds-most-budget-friendly-mars-mission-how-mangalyaan-stacks-up-against-others-101153.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6160" LastEditorUserId="168" LastEditDate="2014-09-24T08:20:59.663" LastActivityDate="2014-10-20T12:34:26.233" Title="Cheap cost of Mangalyaan" Tags="|nasa|isro|mars-orbiter-mission|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5437" PostTypeId="2" ParentId="5436" CreationDate="2014-09-24T09:44:06.450" Score="21" Body="&lt;p&gt;Comparing Mangalyaan (MOM) to Maven there's a significant difference in mission, scientific capabilities and operational lifetime. Maven's primary purpose is to study the martian atmosphere, whereas MOM is more of a general purpose probe with surface study instruments. Maven has over 4 times the payload (65kg vs 15kg) and nearly double the number of instruments, which translates to more capability, and more capability means higher cost. Then there is the operational lifetime, Maven is planned to last up to 10 years, MOM's is 6-10 months although it will likely be extended. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words Maven's higher cost comes at least in part from more capability and a longer operational lifetime. That does not take away from the fact that Mangalyaan is a real feat, especially on such a low cost. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-09-24T09:44:06.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5438" PostTypeId="2" ParentId="5421" CreationDate="2014-09-24T12:39:29.437" Score="4" Body="&lt;p&gt;Water doesn't compress, so in order to have the ISS fill up with water you would need a water tank with the same internal volume of the ISS to contain it. You would then need to spend an awful lot of energy to lift that much water (that's a lot of water, and a lot of weight) to orbit. This clearly hasn't happened. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say you did have that much water, you'd have significant technical challenges actually filling the ISS up with it as you'd have to work out how to move the displaced oxygen out of the way for the water to replace it. It wouldn't happen by accident. What would happen is you'd have a leak and the astronauts would shut down the water supply until they found and repaired it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it's just not going to happen. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-09-24T12:39:29.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5439" PostTypeId="1" AcceptedAnswerId="5444" CreationDate="2014-09-24T14:51:07.450" Score="28" ViewCount="51785" Body="&lt;p&gt;I just read in the BBC news that &lt;a href=&quot;http://www.bbc.com/news/science-environment-28268186&quot;&gt;four nations have placed satellites in Mars orbit&lt;/a&gt;. How many active or inactive artificial satellites are there in orbit around Mars (2014)?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="58" LastEditDate="2016-06-15T15:02:21.300" LastActivityDate="2022-10-25T20:14:40.430" Title="How many active or inactive artificial satellites are in Mars orbit?" Tags="|orbital-mechanics|mars|artificial-satellite|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5440" PostTypeId="1" CreationDate="2014-09-25T03:53:09.223" Score="4" ViewCount="453" Body="&lt;p&gt;I was looking for pictures of Mars taken by spacecrafts and came across this &lt;a href=&quot;http://timesofindia.indiatimes.com/india/After-scripting-space-history-Mangalyaan-starts-clicking-images-from-Mars/articleshow/43352300.cms&quot; rel=&quot;nofollow&quot;&gt;news article&lt;/a&gt; of MOM taking its first pictures, however there no images in the news article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anybody please tell me where can I see the real picture of Mars either taken from MAVEN or from MOM?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="603" LastEditDate="2014-09-25T18:39:42.580" LastActivityDate="2019-02-26T02:16:42.037" Title="Mars Pictures Taken By MOM" Tags="|mars|imaging|mars-orbiter-mission|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5441" PostTypeId="2" ParentId="5440" CreationDate="2014-09-25T05:43:01.810" Score="1" Body="&lt;p&gt;Here is the only image taken by MOM that has been publicly released so far (credit: ISRO):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/FKs7Z.jpg&quot; alt=&quot;MOM first light&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MAVEN doesn't have any visible light cameras.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many previous Mars flyby and orbiter missions have taken tens of thousands of pictures of Mars; &lt;a href=&quot;http://apod.nasa.gov/apod/index/Mars.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here are a few nice ones&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-09-25T05:43:01.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5443" PostTypeId="2" ParentId="5420" CreationDate="2014-09-25T08:06:44.813" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I don't think it would need massive amounts of propulsion to move the ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Depends. How fast to you want to move it? Atmospheric drag and radiation pressure from the sun move the ISS (and everything else in LEO) constantly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS weighs about 420,000 kg - multiply by the desired acceleration and press Equals. Anything over a couple of thousand Newtons should have a measureable effect.&lt;/p&gt;&#xA;" OwnerUserId="5933" LastActivityDate="2014-09-25T08:06:44.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5444" PostTypeId="2" ParentId="5439" CreationDate="2014-09-25T17:32:22.197" Score="32" Body="&lt;p&gt;There are currently 14-16 known artificial satellites in Mars orbit (8 active):&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars-2, launched 1971 May 19, USSR, inactive&lt;/li&gt;&#xA;&lt;li&gt;Mars-3, launched 1971 May 28, USSR, inactive&lt;/li&gt;&#xA;&lt;li&gt;Mariner 9, launched 1971 May 30, USA, inactive, &lt;strong&gt;&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1971-051A&quot; rel=&quot;nofollow noreferrer&quot;&gt;estimated to deorbit October 2022 or later&lt;/a&gt;&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Mars-5, launched 1973 July 25, USSR, inactive&lt;/li&gt;&#xA;&lt;li&gt;Viking 1, launched 1975 August 20, USA, inactive, &lt;strong&gt;&lt;a href=&quot;https://web.archive.org/web/20171107021706/https://www.enu.kz/repository/2009/AIAA-2009-6002.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;estimated to deorbit 2019 or later&lt;/a&gt;&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Viking 2, launched 1975 September 9, USA, inactive&lt;/li&gt;&#xA;&lt;li&gt;Phobos-2, launched 1988 July 12, USSR, inactive&lt;/li&gt;&#xA;&lt;li&gt;Mars Global Surveyor, launched 1996 November 7, USA, inactive, &lt;strong&gt;&lt;a href=&quot;https://www.latimes.com/archives/la-xpm-1996-10-27-mn-58293-story.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;estimated to deorbit 2047 or later&lt;/a&gt;&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;2001 Mars Odyssey, launched 2001 April 7, USA, active&lt;/li&gt;&#xA;&lt;li&gt;Mars Express, launched 2003 June 2, European Space Agency (launched by Russia), active&lt;/li&gt;&#xA;&lt;li&gt;Mars Reconnaissance Orbiter, launched 2005 August 12, USA, active&lt;/li&gt;&#xA;&lt;li&gt;Mars Orbiter Mission (MOM) or &lt;em&gt;Mangalyaan&lt;/em&gt;, launched 2013 November 5, India, active&lt;/li&gt;&#xA;&lt;li&gt;Mars Atmosphere and Volatile Evolution (MAVEN), launched 2013 November 18, USA, active&lt;/li&gt;&#xA;&lt;li&gt;ExoMars Trace Gas Orbiter, launched 2016 March 14, ESA and Russia, active&lt;/li&gt;&#xA;&lt;li&gt;Emirates Mars Mission aka Hope/Al-Amal, launched 2020 July 19, UAE and Japan, active&lt;/li&gt;&#xA;&lt;li&gt;Tianwen-1, launched 2020 July 23, China, active&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;One small clarification: Mark Adler has pointed out that since we do not actively track the inactive orbiters, they may not actually be in orbit. They are generally expected to be in orbit longer, however.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.russianspaceweb.com/spacecraft_planetary_mars.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;A complete list of the world's launch attempts in the direction of Mars - Russian Space Web&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2022-10-25T20:14:40.430" LastActivityDate="2022-10-25T20:14:40.430" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5445" PostTypeId="2" ParentId="5440" CreationDate="2014-09-25T18:03:19.080" Score="0" Body="&lt;p&gt;Beside the image posted by @pericynthion MOM had clicked 5 more images. They are going to be soon released by ISRO. Also they are preparing for investigation on Comet Siding Spring. You can have details in these articles:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;timesofindia.com/home/science/After-Mars-MOM-looks-to-sniff-methane-on-Comet-Siding-Spring/articleshow/43449086.cms&lt;/p&gt;&#xA;&#xA;&lt;p&gt;phys.org/news/2014-09-india-spacecraft-mars-photos.html&lt;/p&gt;&#xA;" OwnerUserId="1417" LastActivityDate="2014-09-25T18:03:19.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5446" PostTypeId="2" ParentId="5436" CreationDate="2014-09-25T18:33:26.140" Score="17" Body="&lt;p&gt;As per &lt;a href=&quot;http://www.hindustantimes.com/specials/coverage/marsorbitermission/marsorbitermission/from-lighter-rocket-to-slingshot-isro-innovates-its-way-to-mars/sp-article10-1267931.aspx&quot;&gt;&quot;From lighter rocket to slingshot, Isro innovates its way to Mars&quot;&lt;/a&gt; from Hindustan Times:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main reasons for the low cost are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Low labour cost. In India scientists are working with very low pay which indirectly drives cost down.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Scientists relied on technologies they had used before. India had their PSLV launches before. The hardware of Chandrayaan was similar to MOM. Also LAM (Liquid Apogee Motor) has been tested several times in space for positioning satellites.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Kept the size of the payload small, at 15 kg. Initially they planned to send 12 payloads but due to approaching 2013 launch window they sent 5 payloads.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Saved on fuel by using a smaller rocket (PSLV) that was adapted from a launch vehicle that first flew in 1993.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Only one physical model of the Mangalyaan was built.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Two-thirds of the craft's parts were made by Indian companies which drives cost down.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This is not a comparison with MAVEN and is solely based on the Hindustan Times article.&lt;/p&gt;&#xA;" OwnerUserId="1417" LastEditorDisplayName="user56" LastEditDate="2014-09-25T23:44:30.703" LastActivityDate="2014-09-25T23:44:30.703" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5447" PostTypeId="1" AcceptedAnswerId="5490" CreationDate="2014-09-25T19:18:54.350" Score="6" ViewCount="789" Body="&lt;p&gt;If you really, really, don't care about getting from one place in space to another quickly, a solar sail can be a good option. A solar sail is light (no pun intended) and requires no onboard fuel. At first glance, it sounds like an aerospace engineer's dream. The big issue, though, is that it takes a long time to accelerate to meaningful speeds. The problem can be tamed a little by making the solar sail bigger, but that means that it can become unwieldy (and thus harder to deploy), and a little hard to stuff into a rocket. So once again, here I go with a &quot;size&quot; question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How big can solar sails &lt;em&gt;reasonably&lt;/em&gt; become (assuming finite resources)? What are the biggest ones currently on the drawing board?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, as called2voyage has pointed out, you could make a very large solar sail with &quot;an asteroid and a 3-D printer.&quot; Is space-based manufacturing a &lt;em&gt;reasonable&lt;/em&gt; option?&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-09-25T22:40:08.823" LastActivityDate="2014-10-02T00:39:02.113" Title="How big can solar sails be?" Tags="|propulsion|solar-sail|design|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="5448" PostTypeId="1" CreationDate="2014-09-25T19:32:46.427" Score="3" ViewCount="199" Body="&lt;p&gt;The &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/hyperloop_alpha-20130812.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hyperloop&lt;/a&gt; is like a &lt;a href=&quot;http://en.wikipedia.org/wiki/Mass_driver&quot; rel=&quot;nofollow noreferrer&quot;&gt;mass driver&lt;/a&gt; - magnetically accelerate a capsule in an evacuated tube. But it uses air bearings and linear induction motors, unlike other designs. Considering SpaceX's CEO Elon Musk came up with the idea, I expected that somewhere someone would talk about whether the system could be modified for launching cargo at some point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concept is to use a tube which has had 99.9% of the air pumped out of it. Capsules are sent down it which use pumps on their front ends to suck in the build-up of air that is created as the capsule travels down the tube at high speed, and send that air out their back ends, and also to 'skis', where it is used to form an air cushion between the capsule and the walls of the tube. Linear induction motors are used to accelerate the capsule. In that scheme, the rotor on the capsules are just hollow aluminum blades, constituting only 5% of the capsule's weight. The air cushion suspension system forms 25% of its weight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would suspending the capsule with air provide a net benefit to the design of mass driver orbital launch systems? Does it lower power requirements, maybe, or allow a simpler design overall? Would air suspension continue to work at supersonic speeds? Are there other relevant differences between the two systems that i've missed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd appreciate any input on how to improve this question. It was suggested it is quite similar to the question '&lt;a href=&quot;https://space.stackexchange.com/questions/4975/is-non-evacuated-tube-maglev-launch-possible&quot;&gt;Is non-evacuated tube maglev launch possible?&lt;/a&gt;', but that question is about magnetic levitation, and the key thing here is that the capsule is not magnetically levitated. That question in fact proposes a very different system.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-09-27T18:07:34.100" Title="Would a mass driver that uses a cushion of air to suspend its capsule be more efficient?" Tags="|design|mass-driver|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5449" PostTypeId="1" CreationDate="2014-09-26T08:48:46.003" Score="8" ViewCount="4964" Body="&lt;p&gt;Is there a breakdown of the 2.5 Billion USD cost for Curiosity?&#xA;I am particularly interested in the cost of the software development of the rover.&lt;/p&gt;&#xA;" OwnerUserId="99" LastActivityDate="2015-04-20T16:43:43.990" Title="Cost Breakdown for Curiosity Development" Tags="|curiosity|cost|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5450" PostTypeId="1" AcceptedAnswerId="5452" CreationDate="2014-09-26T09:32:39.403" Score="21" ViewCount="2734" Body="&lt;p&gt;We know that the Apollo astronauts were subject to rigorous medical testing prior to launch, and we know that this caused &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13#Prime_and_backup_crew&quot; rel=&quot;noreferrer&quot;&gt;at least one Apollo crew member to be replaced very late in the process&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, &lt;strong&gt;what if something still happened during the actual spaceflight which prevented a crew member from fulfilling their mission role?&lt;/strong&gt; Were there contingency plans in place for such a scenario, or was it considered unlikely enough to not warrant specific planning beyond something like &quot;if that happens, we'll just grab the backup crew member, drop them in a chair next to CAPCOM and say 'sort this out'&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been claimed that the Apollo CM was designed such that the command module pilot alone could fly the CSM back to Earth in the event that the lunar-landing mission failed. Given that the CSM was completely separate during the lunar-landing phase of the mission, that makes very good sense; lots of things could go wrong during that phase of the mission, and losing two astronauts was less bad compared to losing three.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was the lunar module designed and the people involved trained similarly, such that &lt;strong&gt;in the event of a crew member problem among the lunar landing crew while on the lunar surface, either lunar-landing astronaut&lt;/strong&gt; (commander or lunar module pilot) could at the very least &lt;strong&gt;ascend from the lunar surface and execute a successful rendezvous with the manned command module?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;The inspiration for this question was &lt;a href=&quot;https://space.stackexchange.com/questions/1302/why-did-apollo-11-need-someone-to-stay-behind-in-lunar-orbit#comment11121_1303&quot;&gt;a comment&lt;/a&gt; by &lt;a href=&quot;https://space.stackexchange.com/users/429/anthony-x&quot;&gt;Anthony X&lt;/a&gt;, which doesn't appear to have received any followup.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-01-02T19:47:26.643" Title="Could a single crew member fly the Apollo LM?" Tags="|apollo-program|lunar-landing|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5452" PostTypeId="2" ParentId="5450" CreationDate="2014-09-26T17:48:50.163" Score="13" Body="&lt;p&gt;General answer: Yes, but the workload would have been high, and the ability to deal with problems would have been compromised.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lunar ascent and rendezvous was mostly automatic, and the main job of the crew (apart from some sequencing steps) was to carefully monitor the process, comparing progress of various parameters to paper charts and be ready to take action in the event of a fault.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-09-26T17:48:50.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5453" PostTypeId="1" CreationDate="2014-09-26T20:27:14.440" Score="8" ViewCount="2417" Body="&lt;p&gt;I have been watching From the Earth to the Moon, and just saw Part 4 which dramatised the Apollo 8 mission.&lt;/p&gt;&#xA;&lt;p&gt;During Apollo 8 the crew placed themselves in orbit around the moon, for one day, and then returned to earth, using a single engine (on the SPS). A big deal was made in this episode about the possibility, but unlikelihood, of something going wrong with the engine, stranding the crew in lunar orbit.&lt;/p&gt;&#xA;&lt;p&gt;I understand there was no planned contingency or backup system, but with all the people at NASA, someone would have been thinking what might be done in that case? If it didn't fire they surely wouldn't just sit there waiting for their oxygen to run out?&lt;/p&gt;&#xA;&lt;p&gt;For example, did crew have suits that they could EVA with? (In the episode it was said that the fuel is self igniting and one &amp;quot;only&amp;quot; needs to open the valves.)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;What would the procedures be, had the crew of Apollo 8 found themselves in lunar orbit with a non-responsive engine?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6173" LastEditorUserId="7062" LastEditDate="2021-09-13T11:59:48.253" LastActivityDate="2021-09-13T11:59:48.253" Title="What would have happened if the SPS did not ignite on Apollo 8?" Tags="|the-moon|rockets|apollo-program|failure|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="5454" PostTypeId="2" ParentId="3428" CreationDate="2014-09-27T01:04:08.977" Score="2" Body="&lt;p&gt;Answers so far have addressed India's launch capability, but it should be noted that quite aside from delivering a ton or two to $C_3 &amp;gt; 0$, the technical challenge of building a lander that will successfully touch down on Mars is substantial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Experience from the development of Chandrayaan-2 will be helpful, but that has yet to be built and flown, and Mars presents more difficulties than the moon:  Navigation and communication are much more challenging, and atmospheric entry and deceleration are wholly new.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this is not to say that India can't do it!  It just might take more time and money, and possibly more than one attempt, than one might imagine from looking only at launch vehicle availability.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-09-27T01:04:08.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5456" PostTypeId="2" ParentId="5453" CreationDate="2014-09-28T06:27:03.697" Score="6" Body="&lt;p&gt;This was one of the few scenarios where the Apollo program had no backup or abort available. If the SPS engine failed when &lt;em&gt;entering&lt;/em&gt; lunar orbit, the spacecraft would just loop around the moon and head home. If it failed while &lt;em&gt;exiting&lt;/em&gt; lunar orbit, they don't get to come home.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other no-options situation was liftoff from the moon's surface. And Apollo 11 broke the ascent motor's breaker switch! They were able to reset it with a pen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible that minor failures could be fixed from inside the spacecraft, but it's difficult even today to service things in space unless they are specifically designed for servicing. How will you undo a fastener? Turning a wrench on a bolt will move you more than it will move the bolt. In the 1960s this was simply not possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA had a very quiet procedure for return-engine failure, and the Nixon administration had a draft speech written which was promptly filed-and-forgotten when everything worked.&lt;/p&gt;&#xA;" OwnerUserId="5933" LastActivityDate="2014-09-28T06:27:03.697" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5457" PostTypeId="2" ParentId="5450" CreationDate="2014-09-28T06:36:10.600" Score="6" Body="&lt;p&gt;I expect it was quite possible. Any planned stage after One of Three suddenly drops dead would have been summarily cancelled and the remaining crew would do whatever mission-abort step was suitable at the time. Sometimes this involved doing absolutely nothing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CSM was certainly able to return with one crew aboard, and they must have considered Plan B lunar ascents - there was a real chance that one of them would have a fatal accident on the lunar surface. Punch a reasonable-size hole in your suit and there's no way you will get back into the LM, close the door and pump it back up before you are out of air.&lt;/p&gt;&#xA;" OwnerUserId="5933" LastActivityDate="2014-09-28T06:36:10.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5459" PostTypeId="1" AcceptedAnswerId="5460" CreationDate="2014-09-28T15:11:31.743" Score="8" ViewCount="1625" Body="&lt;p&gt;Do Russians have means of launching a manned mission to the Moon? Do they have an engine with sufficient thrust and stability needed (&lt;a href=&quot;https://en.wikipedia.org/wiki/RD-170&quot;&gt;RD-170&lt;/a&gt; has bigger thrust than the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot;&gt;F-1&lt;/a&gt; engine, if I read the data properly), and a suitable launch vehicle to pull off such a feat?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can perhaps &lt;a href=&quot;https://en.wikipedia.org/wiki/Proton_(rocket_family)&quot;&gt;Proton&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot;&gt;Energia&lt;/a&gt; rockets be somehow modified for the purpose of launching a manned spacecraft to &lt;a href=&quot;https://en.wikipedia.org/wiki/Trans-lunar_injection&quot;&gt;TLI&lt;/a&gt; and do they have a spacecraft capable of landing a manned mission on the Moon's surface and later lifting it off for a likely &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit_rendezvous&quot;&gt;LOR&lt;/a&gt; approach?&lt;/p&gt;&#xA;" OwnerUserId="6179" LastEditorUserId="49" LastEditDate="2014-09-28T16:24:52.400" LastActivityDate="2016-09-12T00:33:40.063" Title="Russian manned Moon landing capability today" Tags="|rockets|crewed-spaceflight|lunar-landing|engines|russia|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5460" PostTypeId="2" ParentId="5459" CreationDate="2014-09-28T15:49:56.087" Score="8" Body="&lt;p&gt;When it comes to rockets, engine power is important, but so is quantity. I asked a question to that effect (&lt;a href=&quot;https://space.stackexchange.com/questions/5400/quality-vs-quantity-for-rocket-engines&quot;&gt;Quality vs. Quantity for Rocket Engines&lt;/a&gt;) and got a really good answer. For a moon shot, having one super-powerful engine is all well and good, but so is having six or so smaller engines. There are the plusses and minuses of both configurations (again, see Geoffc's answer). So just because a single engine &lt;em&gt;X&lt;/em&gt; is more powerful than another single engine &lt;em&gt;Y&lt;/em&gt; doesn't mean that a rocket with &lt;em&gt;X&lt;/em&gt; is better than a rocket with &lt;em&gt;Y&lt;/em&gt;, if there are more of &lt;em&gt;Y&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if engines don't have a direct bearing, what does? Configuration of the rocket. Energia/Energiya/Энергия used strap-on boosters to supplement its main engines. The advantage of that is that, in theory, you could make modifications to add on more boosters. You also might be able to avoid the pogo problem if they were separated enough, and you have redundancy built in if the boosters are independent. Then you have Energia's second stage, which could be another smaller rocket (&lt;em&gt;Polyus&lt;/em&gt;) or the space shuttle &lt;em&gt;Buran&lt;/em&gt;. Polyus was really used as a one-time test, and would not have been used in later Energia launches if the system had proliferated, so there is plenty of room for modification there. In fact, there were more versions of Energia on the drawing board at the time the program ended, which were very powerful. In short: Energia would have required  a lot of modifications to reach the moon, but it could certainly be modified to be more powerful with more boosters and a better upper stage configuration (a necessity to reach the moon).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Proton family of rockets is slightly harder to analyze, partly because they are so varied, but mostly because they were originally designed as ICBMs. They use four stages to get their payloads to orbit. The first stage contains six engines (RD-275s), while the other stages use 3, 1, and 1 engines (different types). While I'm hesitant to say that Proton would be harder to modify simply because Energia's strap-ons are relatively convenient, but I will say that it would require partially (if not fully) redesigning the first stage. While the upper stages can vary, the lower stage does not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To summarize: Energia, in its prime, would have been easier to modify than the Proton; unfortunately, it was retired decades ago. It strap-on booster design would have been convenient because in an expanded system, boosters could have been added and removed as necessary. Both systems, however, would have required extensive modification to reach the moon, and Russia overall does not appear to have that capability. I hope this helps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, some comparisons between Energia, Proton, and the American Saturn V:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First stage: E (4 boosters, 4-nozzle RD-170); P (6 RD-275); S (5 F-1)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Payload to LEO: E (220,000 lb); P (46,000 lb); S (260,000 lb)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Total launches: E (2); P (397+); S (13, with more in the extended Saturn family)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Energia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Proton_(rocket_family)&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Proton_(rocket_family)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Saturn_V&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-10-04T14:46:59.317" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5463" PostTypeId="1" AcceptedAnswerId="5465" CreationDate="2014-09-29T07:33:57.337" Score="38" ViewCount="14841" Body="&lt;p&gt;Stupid question obviously. But did you ever had an idea which sounded so brilliant, but you know it is totally stupid? So, lets hear my idea:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do you know how we launch jet fighters from navy ships? If not, have look at &lt;a href=&quot;https://www.youtube.com/watch?v=S7f6GrBP1BA&quot;&gt;this video&lt;/a&gt; for an example. In nutshell:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;attach to catapult&lt;/li&gt;&#xA;&lt;li&gt;Jet fighter goes to full throttle&lt;/li&gt;&#xA;&lt;li&gt;launch the jet fighter&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The plus sides of this method obviously is, that we save amount of space needed for acceleration. But we must, we simply &lt;strong&gt;must&lt;/strong&gt; also save some fuel in jet fighter, don’t we?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The stupid idea:&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine same device attached to a space rocket horizontally. The launch sequence would be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;attach rocket to catapult&lt;/li&gt;&#xA;&lt;li&gt;rocket blasts off its engines&lt;/li&gt;&#xA;&lt;li&gt;catapult helps rocket gain initial speed&lt;/li&gt;&#xA;&lt;li&gt;To the space we go&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, I am not saying to build catapult good enough to &quot;fire&quot; rocket to the space. I am reasoning to &lt;strong&gt;save rocket's fuel&lt;/strong&gt;, which means we can put less fuel to the rocket and replace the fuel by (usable) cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because we are not using such approach, I:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Am total genius and deserve Nobel prize in year 2015&lt;/li&gt;&#xA;&lt;li&gt;Have something terribly wrong in my thought process&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;And I am so obviously stuck with this stupid idea (catapults, catapults &lt;em&gt;everywhere&lt;/em&gt;) that I cannot find anything wrong with the idea. Can someone of you please kindly explain me, where did I go wrong?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Legal disclaimer:&lt;/em&gt; If I am not wrong with the idea, I claim all the copyright for such glorious way to get to the space and would like to thank my parents. Getting Nobel prize for Space Exploration (introduced just because of me). It would not be possible without them.&lt;/p&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="49" LastEditDate="2015-08-16T05:20:07.197" LastActivityDate="2017-12-20T21:49:04.867" Title="Why don't we use catapults to get to space?" Tags="|launch|physics|launch-assist|" AnswerCount="10" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="5464" PostTypeId="2" ParentId="5463" CreationDate="2014-09-29T08:08:18.480" Score="11" Body="&lt;p&gt;Mass of carrier-based F-18: 40,000kg&#xA;Mass of Delta-II rocket: 230,000kg&#xA;Mass of the Space Shuttle stack: 2,000,000kg&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if we can knock off the fuel needed to get it moving we still have to have enough to accelerate the thing to 7km/s, and that's most of the fuel. You are going to need a stronger structure to handle the catapult forces (which adds weight which requires more force which needs a stronger structure.....) and a mighty big cable to apply that much force. I leave the math as an exercise for the reader.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets can simply apply smooth, steady thrust until they are going fast enough, and some of the earlier rockets like the Centaur had pressurized bodies - the frame was only about 1-2 mm thick and could not hold itself up without internal pressure. No way you could hook it up to a launcher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And to totally invalidate your idea, it's already been done in fiction. Part II of &lt;em&gt;2001: A Space Odyssey&lt;/em&gt; opens with a Pan-Am spaceplane being track-launched before the rockets lit up. Arthur C. Clarke beat you to it by 45 years.&lt;/p&gt;&#xA;" OwnerUserId="6039" LastActivityDate="2014-09-29T08:08:18.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5465" PostTypeId="2" ParentId="5463" CreationDate="2014-09-29T08:14:59.990" Score="23" Body="&lt;p&gt;I'm leaning toward option 2, that you &lt;em&gt;&quot;have something terribly wrong in [your] thought process&quot;&lt;/em&gt;. Here's at least some arguments why; I by no means claim this to be an exhaustive list:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mach 3 (which is pretty fast for a fighter aircraft, toward the upper end of the currently attainable range) is right about 1 km/s (Google calls it 1020.87 m/s), and you don't need to go that fast originally, just fast enough for sustained flight so probably closer to Mach 0.5 &lt;em&gt;at most&lt;/em&gt; as you leave the runway. Earth orbital speed is about 7 km/s (for comparison, translunar is 11 km/s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The weight of an aircraft is trivial compared to practically any spacecraft. Work to accelerate grows exponentially with target speed &lt;em&gt;and object mass&lt;/em&gt; -- either of which makes this a serious problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration kills; go much above 10G (which is what, 100 m/s^2?) and a human becomes a wet spot on the rear wall. The acceleration during a spacecraft launch and climb process is nontrivial, yet much easier to manage because it can be kept to a safe level while climbing out of the worst depths of Earth's gravity well. No need to jump to anywhere near even orbital velocity right off the launchpad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if after the vessel is catapulted to a considerable speed the main engine fails to light up? If a current-design spaceship main engine fails to light up on the launchpad, that's a serious annoyance and probably grounds for some grim looks toward a number of technicians, but nothing really worse than that. If you are already airborne and moving at a considerable speed, in a fully fueled spacecraft (keep in mind that &lt;a href=&quot;https://space.stackexchange.com/q/3805/415&quot;&gt;rocket fuel is some really nasty stuff&lt;/a&gt;), and the main engine doesn't kick in on schedule for any reason whatsoever, then what do you do? Depending on the (ballistic) flightpath and spacecraft maneuverability, even ignition a few seconds late could spell some very serious trouble downhill.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmosphere is thickest close to ground, exactly where with your scheme you would be seeing a very high rate of acceleration. With more common rocket designs, acceleration close to ground is relatively trivial, and they only really start picking up speed some distance above ground. (Close to the ground, you are more concerned with getting off the ground than you are with building up speed. Even if you climb slowly for the first 10 km, you still have &lt;em&gt;plenty&lt;/em&gt; of time to build up speed before you reach orbital altitudes.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not like the idea hasn't been proposed. Jules Verne did something quite close to what you are proposing in 1865 in &lt;a href=&quot;https://en.wikipedia.org/wiki/From_the_Earth_to_the_Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;De la terre à la lune&lt;/em&gt;&lt;/a&gt;, beating you to it by about 150 years. It was &lt;a href=&quot;https://en.wikipedia.org/wiki/From_the_Earth_to_the_Moon#Technical_feasibility_of_a_space_cannon&quot; rel=&quot;nofollow noreferrer&quot;&gt;seriously considered in 1903&lt;/a&gt; and concluded that it wouldn't work even from a technological standpoint, although perhaps with modern materials it could be slightly more practical. That still doesn't solve the issue of the acceleration, however, which was touched on even in &lt;a href=&quot;https://scifi.stackexchange.com/a/18399/3248&quot;&gt;fiction works in the 1950s&lt;/a&gt; at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line: since it has been proposed and found a non-viable alternative, I would suggest that you do the math and tell us why it &lt;em&gt;would&lt;/em&gt; work, addressing the concerns raised in this as well as the (currently only) other answer.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-10-02T07:24:56.373" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5466" PostTypeId="2" ParentId="5453" CreationDate="2014-09-29T18:46:10.660" Score="3" Body="&lt;p&gt;As noted in comments, the SPS ignition system was made extremely simple, with redundant valves and hypergolic fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Chaikin's &quot;A Man On The Moon&quot;, the astronauts asked for some sort of manually actuated valve system in addition, and didn't get it; in this case the additional weight and complexity was judged unnecessary, and the small risk of multiple valve systems failing was considered acceptable.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-09-29T18:46:10.660" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5467" PostTypeId="2" ParentId="5459" CreationDate="2014-09-29T18:49:04.280" Score="3" Body="&lt;p&gt;Remember also that the launcher is only one part of a complete manned moon landing and return program -- an important part, to be sure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a manned moon mission, Russia would still have to develop one or more stages for translunar injection, lunar orbit insertion, and transearth (return) injection, as well as a manned lander; while Soyuz is a proven design with the necessary time endurance, it has nowhere near the maneuvering delta-v or thrust of the Apollo service module.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-09-29T18:49:04.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5468" PostTypeId="1" CreationDate="2014-09-29T19:26:59.353" Score="15" ViewCount="1629" Body="&lt;p&gt;&lt;a href=&quot;http://athena.cornell.edu/mars_facts/sb_dead_reckoning.html&quot;&gt;This website&lt;/a&gt; conveys the impression a rover on the Martian surface necessarily navigates by dead-reckoning. Stereo images are used to determine how far a rover has travelled; an advancement over older technology which may have merely counted wheel revolution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth's surface, accurate navigation is made easier by the aid of satellite navigation systems such as GPS. A system such as GPS is apparently a bunch of satellites in orbit queried by a dedicated receiver to determine the position of the receiver itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pure speculation on my part - feel free to shoot the thought down. I guess each question in the list below could be a separate question in itself ... let me know if that is a better way to go about it &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How many satellites would be needed in orbit around Mars to be used as a navigation aid? My guess is the smaller sphere may translate to a lesser number than required for Earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Are the rovers on Mars capable of communicating with the satellites in orbit?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Can the 5 satellites currently in orbit around Mars be made into a &lt;em&gt;jury-rigged system to provide a satellite based navigation system&lt;/em&gt; for the rovers?&lt;/strong&gt; &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-09-29T23:16:36.230" Title="Can the satellites in Martian orbit be put to use providing a rudimentary GPS system on Mars?" Tags="|mars|rovers|navigation|gps|mars-orbiters|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5469" PostTypeId="2" ParentId="5463" CreationDate="2014-09-29T20:24:08.380" Score="7" Body="&lt;p&gt;I'll try a little bit of Fermi estimation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mass Limit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similar to rockets, there is an exponential equation for tethers. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_tether&quot;&gt;Wikipedia article on it&lt;/a&gt; is actually quite solid in terms of the gritty technical details. Easily enough, we can plug in some velocity at the point of release and use the material characteristic velocity for M5 fiber. As far as I can tell, this is the most reasonably optimistic out of the practically available materials. Then the ratio of the mass of the tether to the mass of the rocket will be given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$         \frac M m = \sqrt { \pi } V_r \mathrm{e}^{ {V_r}^2 } \mathrm{erf} ( {V_r} )   $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping with the spirit of Fermi estimation, let's image a Falcon 9 with a launchpad mass of 500,000 kg, which puts 10,000 kg into orbit. If we use an engineering factor of 3, then we can easily calculate the velocity at which the catapult assist system will weigh as much as the rocket on the launchpad. This is in the neighborhood of 3 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's important to not go much further than this mass limitation. No company does more than a few launches per year. If per-mass the tether costs a similar order of magnitude as the rocket, then the mass ratio is the number of launches you'll need to amortize the cost. If the tether material is 10% the cost of the entire launch assist system, and you need to pay off investors within one year, then we're clearly settling on (mass of rocket) = (mass of tether). It doesn't matter much if you change your financial risk tolerance here, because the mass equation is so insanely exponential.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Needless to say, this could still work. If a rocket reduced its Delta V budget by 3 km/s, then it could practically double or triple its payload delivered to orbit. You would be tempted to think this makes sense. Actually, in space, it does make sense. The problem is that we're still on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Length Limit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Strange enough, the above mass ratio equation says nothing about the &lt;em&gt;tether length&lt;/em&gt;. It could be long or it could be short. If the edge velocity is the same, it doesn't change the total tether mass. If you go faster, the centripetal acceleration is higher so the tether must be thicker - and this perfectly balances the effect of a shorter tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, g-forces actually limit the length of the tether, and these are constrained by biology. Let's say that our limit is 3g. The equation is simply $3g=\frac{v^2}{r}$. If you plug in 3 km/s, you get a tether radius of 300 km. Oh my. This will not do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most radical version of the tether launch assist system consists of either high altitude balloons or aircraft. These are limited in altitude due to atmosphere density, typically to about 10 km in the case of large jets. Combining the limitation of 3g and 10 km, you find that the launch assist system can only aid to the tune of 0.5 km/s. So with this system, you can only improve the rocket's mass fraction by about 20%, but only at the cost of reducing the starting mass by the requirement that a jet or balloon can pick it up in the first place. More than likely, it hurts more than it helps!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ways Around Length Limit?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your idea was to &lt;em&gt;reduce&lt;/em&gt; the propellant the rocket needs. You can see from my calculations that the limitation isn't the bulk mass of the tether. The problem is that the length need is so high that you don't have anywhere to attach it to. But your idea of attaching it is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Imagine same device attached to a space rocket horizontally.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sounds like the tether connects the rocket to an anchor (I assume a moving anchor) on the ground. But the problem is that the rocket must pull the tether up with it here. That increases the amount of propellant the rocket needs, which is already violating your objective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, this is why any serious proposals in this family involve launching hardened commodity payloads, not people, and not conventional rockets. By increasing the acceleration, you can keep the structure to within roughly a kilometer scale. But such a megastructure is a hard sell when it will only marginally increase the payload in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, you could try a scheme to &quot;attach&quot; it to something not bound by terrestrial constraints. For instance, something already in orbit. This is the approach of various partial space elevators. These can reasonably be &gt;300 km in radius, but then the mass constraint pops back up again since you must put it in orbit to begin with.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-09-29T20:24:08.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5470" PostTypeId="2" ParentId="5468" CreationDate="2014-09-29T20:40:19.517" Score="18" Body="&lt;p&gt;Not the way GPS works, no, since the Mars orbiters don't have atomic clocks.  (Though future Mars orbiters &lt;a href=&quot;http://www.jpl.nasa.gov/video/index.php?id=1239&quot;&gt;may someday&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The relay radios on the Mars orbiters provide a 2-way Doppler data type that can be used to locate surface assets to, as I recall, within about 100 meters.  Even a single orbiter with Doppler data collected over a few passes can get this accuracy.  (Yes, the surface assets are not only able to communicate with the orbiters -- they depend on the radio relays to return their required science data volume.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the cameras on the surface assets can be used to pinpoint their location on orbital maps to far greater accuracy by using topographic features.  So radio navigation from the orbiters is neither needed nor beneficial.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-09-29T20:40:19.517" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5471" PostTypeId="2" ParentId="5468" CreationDate="2014-09-29T22:48:47.517" Score="10" Body="&lt;p&gt;Mark Adler already explained that the satellites aren't equipped anyway to provide a satellite navigation system. But even when they were technically capable of working as a GPS satellite, 5 satellites would not be enough. You need a direct line of sight to at least 4 satellites, and with only 5 satellites this will happen very rarely, because even considering the largest possible orbit and the smallest possible planet you will only ever be able to observe up to 50% of the sky from a position on the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason you need four satellites is that a GPS satellite sends only two pieces of information:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Its current position&lt;/li&gt;&#xA;&lt;li&gt;Its exact current time according to its atomic clock&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Due to the signal only traveling with the speed of light, you can compare the time received by the satellite with your own time to learn your exact distance to the satellite. That way you know you are on the surface of an invisible sphere around the satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you also have a signal from a second satellite, you have two spheres. The intersection of two spheres is a circle. You now know you are on this circle which will be perpendicular to the surface of the planet. When you assume that you are on the ground (which isn't so sure in case you are in an aircraft or in a mountainous region) you have two possible locations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A distance measurement to a third satellite is needed to know your precise position in three-dimensional space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why the fourth satellite? Because until now we assumed that you know your current time as accurately as the satellites know. Unless you also have an atomic clock calibrated to GPS time this is unlikely to be the case. To obtain the necessary time reference to make an accurate distance measurement to the satellites, you also need a fourth satellite.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-09-29T23:16:36.230" LastActivityDate="2014-09-29T23:16:36.230" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5472" PostTypeId="1" AcceptedAnswerId="5519" CreationDate="2014-09-29T23:10:19.883" Score="12" ViewCount="985" Body="&lt;p&gt;For decades, the Baikonur Cosmodrome was the premier Soviet space launch site. Many historic launches took place there, and it earned its place in the history books. But then the Soviet Union fell apart in 1991, and Kazakhstan (where it is located) became Russia's new best friend. While I don't know the details of the agreement, I do know that Kazakhstan leases it to Russia, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikonur_Cosmodrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; says the lease will extend to about 2050, by which time Baikonur will be nearing 100 years old. But Russia is still in an awkward situation - for Americans, it would be like having Cape Canaveral in Quebec. I know the Russians are building the Vostochny Cosmodrome (courtesy of TildalWave &lt;a href=&quot;https://space.stackexchange.com/questions/5459/russian-manned-moon-landing-capability-today&quot;&gt;here&lt;/a&gt;), but it's not meant to replace Baikonur. So for those who didn't bother to read through this morass of text, my question is this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are the Russians planning to replace the Baikonur Cosmodrome?&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-01-10T22:39:51.727" Title="Are the Russians planning to replace the Baikonur Cosmodrome?" Tags="|launch-site|russia|baikonur-cosmodrome|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5473" PostTypeId="1" AcceptedAnswerId="5487" CreationDate="2014-09-30T05:22:50.853" Score="4" ViewCount="2572" Body="&lt;p&gt;How much fuel would be required to send a 300g satellite to space using a &lt;a href=&quot;https://en.wikipedia.org/wiki/Rockoon&quot; rel=&quot;nofollow noreferrer&quot;&gt;rockoon&lt;/a&gt; that can travel to a height of 32 km, and then send the satellite to a inclination of 28, Apogee: 350 km, Perigee: 280 km, using an &lt;a href=&quot;https://en.wikipedia.org/wiki/Ammonium_perchlorate_composite_propellant&quot; rel=&quot;nofollow noreferrer&quot;&gt;APCP&lt;/a&gt; propellant?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please provide the exact calculation if possible so I can follow.&lt;/p&gt;&#xA;" OwnerUserId="1267" LastEditorUserId="1267" LastEditDate="2018-10-03T10:56:16.247" LastActivityDate="2018-10-03T10:56:16.247" Title="How much fuel would be required to send a 300g satellite to space using Rockoon?" Tags="|rockets|low-earth-orbit|fuel|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="5475" PostTypeId="2" ParentId="5463" CreationDate="2014-09-30T09:59:18.203" Score="6" Body="&lt;p&gt;As others have said, the scale of both the construction and the forces involved place this project firmly in the realm of the impossible. However, that does not mean the idea hasn't been considered in a different form.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While not exactly a catapult, There has been a somewhat similar idea involving what's basically a particle accelerator for space cargo that can withstand the pressure. Imagine the Cern Large Hadron collider, but built into a mountain with an exit port aimed at a stable orbit entry point. The theory is that you get the craft up to speed on the ground floor and then launch it into orbit from there. While the G-forces are not viable for anything living, properly protected equipment that can withstand both the G-forces and the magnetic forces involved could be brought into outer space for a ludicrously low price compared to traditional rocket-aided launch methods (on the order of 10,000 times lower for the launch itself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Wikipedia article on Mass Drivers: &lt;a href=&quot;http://en.wikipedia.org/wiki/Mass_driver&quot;&gt;http://en.wikipedia.org/wiki/Mass_driver&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Wikipedia article on Space Gun: &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_gun&quot;&gt;http://en.wikipedia.org/wiki/Space_gun&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Article on New Scientist: &lt;a href=&quot;http://www.newscientist.com/article/dn10180-huge-launch-ring-to-fling-satellites-into-orbit.html&quot;&gt;http://www.newscientist.com/article/dn10180-huge-launch-ring-to-fling-satellites-into-orbit.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also see the different sources on the Wikipedia articles.&lt;/p&gt;&#xA;" OwnerUserId="5860" LastEditorUserId="5860" LastEditDate="2014-10-02T12:37:05.390" LastActivityDate="2014-10-02T12:37:05.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5476" PostTypeId="2" ParentId="5463" CreationDate="2014-09-30T10:30:18.450" Score="9" Body="&lt;p&gt;You are not the first to think of this, of course. The idea of an electromagnetic catapult to push a payload to orbital speeds has been floated before... a couple of plans which spring to mind are the Launch Loop and StarTram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Launch_loop&quot;&gt;http://en.wikipedia.org/wiki/Launch_loop&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/StarTram&quot;&gt;http://en.wikipedia.org/wiki/StarTram&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key issue is the sheer size of the whole assembly, which is generally measured in hundreds or thousands of kilometres with the launch end of the system needing to be many thousands of metres up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 'Gen 1.5' version of the StarTram sounds closest to your proposition, as it only provides 4km/s of accelleration to the payload, with the rest provided by onboard propulsion systems. The peak accelleration for a shorter version of the system (50-80km) could be under 15g, and so potentially surviveable for fit humans (if not particularly pleasant).&lt;/p&gt;&#xA;" OwnerDisplayName="user6190" LastActivityDate="2014-09-30T10:30:18.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5477" PostTypeId="1" AcceptedAnswerId="5480" CreationDate="2014-09-30T11:14:31.990" Score="18" ViewCount="6527" Body="&lt;p&gt;A recent picture of Curiosity rover &lt;a href=&quot;http://mars.jpl.nasa.gov/msl-raw-images/msss/00746/mcam/0746MR0032080400403454E01_DXXX.jpg&quot; rel=&quot;noreferrer&quot;&gt;depicts a surprisingly accurate sphere on the surface of Mars.&lt;/a&gt; What is it, and how is it possible that it is so perfecly spherical?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Gpn9N.jpg&quot; alt=&quot;NASA photo&quot;&gt;&#xA;&lt;sup&gt;&lt;br /&gt;Photo by NASA Mars Science Laboratory, Mastcam: Right, taken&#xA;2014-09-11 14:46:58 UTC. &lt;a href=&quot;http://mars.jpl.nasa.gov/msl/multimedia/raw/?s=746&amp;amp;camera=MAST_&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&#xA;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="49" LastEditDate="2014-10-04T10:33:10.840" LastActivityDate="2021-06-03T15:13:18.823" Title="What is the mysterious &quot;Ball of Mars&quot;?" Tags="|mars|planetary-science|curiosity|regolith|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5478" PostTypeId="2" ParentId="5178" CreationDate="2014-09-30T11:29:35.853" Score="1" Body="&lt;p&gt;It's unfair to say that propellants haven't developed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until last decade Europe still used storeable propellants for it's Ariane 4 main stage. India, China and Russia (Khrunichev) continue to do to so, although Russia with Angara and China with Long March 5 and onward are moving towards LOX/Kerosene.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would expect that the hypergolic storables will continue to be phased out because of their high toxicity and low performance. They will be replaced by &quot;greener&quot; propellants, as mechanisms to start the engine are developed further. Even for roll-control-thrusters I expect that in some years monoprop thrusters will be replaced by laser-ignited thrusters running on the same propellant as the main stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, there have been interesting developments into methane combustion, which offers a compromise between kerosene's high-density-low-performance and hydrogen's high-performance-low-density.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-09-30T11:29:35.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5479" PostTypeId="2" ParentId="5477" CreationDate="2014-09-30T11:42:14.390" Score="19" Body="&lt;p&gt;Curiosity photographs thousands of rocks every week. It does that for years. You will find rocks with almost any shape you can think of. This one happens to be somewhat round in one direction. From the shadows and the way it stays on what is apparently a slope, I would infer that it is not a sphere, but is actually ragged on the other side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm no geologist (do we have one?), but I don't think this requires further explanation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I did have to venture a guess I would say that maybe its porous and thus light. Together with Mars' low gravity it might have tumbled in the wind to dull the edges a little.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-09-30T11:42:14.390" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5480" PostTypeId="2" ParentId="5477" CreationDate="2014-09-30T15:49:11.710" Score="19" Body="&lt;p&gt;As called2voyage♦ pointed out in a &lt;a href=&quot;https://space.stackexchange.com/questions/5477/what-is-the-mysterious-ball-of-mars/5480#comment14888_5477&quot;&gt;comment&lt;/a&gt;, this looks like a &lt;a href=&quot;http://en.wikipedia.org/wiki/concretion&quot; rel=&quot;nofollow noreferrer&quot;&gt;concretion&lt;/a&gt;.  It is not the first concretion observed on Mars, and it was most likely formed back when Mars had liquid water.  More information from &lt;a href=&quot;http://news.discovery.com/space/curiosity-finds-a-weird-ball-on-mars-140924.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;an article on Discovery News, dated 24 September&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;According to MSL scientists based at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., the ball isn’t as big as it looks — it’s approximately one centimeter wide. Their explanation is that it is most likely something known as a “&lt;a href=&quot;http://en.wikipedia.org/wiki/Concretion&quot; rel=&quot;nofollow noreferrer&quot;&gt;concretion&lt;/a&gt;.” Other examples of concretions have been found on the Martian surface before — take, for example, the tiny haematite concretions, or “blueberries”, &lt;a href=&quot;http://www.nature.com/nature/journal/v429/n6993/full/nature02600.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;observed by Mars rover Opportunity in 2004&lt;/a&gt; — and they were created during sedimentary rock formation when Mars was abundant in liquid water many millions of years ago.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From a &lt;a href=&quot;https://space.stackexchange.com/questions/5477/what-is-the-mysterious-ball-of-mars/5480?noredirect=1#comment173376_5480&quot;&gt;comment&lt;/a&gt; by &lt;a href=&quot;https://space.stackexchange.com/users/27764/mgarey&quot;&gt;mgarey&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Fun fact - concretions are also found in many places in Utah in the United States, in particular near the Mars Desert Research Station near Hanksville, Utah. Here's an &lt;a href=&quot;https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/moqui-marbles/&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; about them. Feel free to incorporate this into your answer if you want.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2021-06-03T15:13:18.823" LastActivityDate="2021-06-03T15:13:18.823" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="5481" PostTypeId="2" ParentId="925" CreationDate="2014-09-30T16:27:32.043" Score="4" Body="&lt;p&gt;&lt;a href=&quot;https://www.kickstarter.com/projects/zacinaction/kicksat-your-personal-spacecraft-in-space/posts/843807&quot; rel=&quot;nofollow&quot;&gt;KickSat did not deploy&lt;/a&gt; unfortunately. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other candidates for the smallest satellite, one that was capable of two way communications, would have been the &lt;a href=&quot;http://amsat-uk.org/2013/10/24/wren-a-ham-radio-sstv-pocketqube/&quot; rel=&quot;nofollow&quot;&gt;WREN PocketQube&lt;/a&gt; or the &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/pocket-pucp.htm&quot; rel=&quot;nofollow&quot;&gt;POCKET-PUCP&lt;/a&gt; satellite launched out of PUCP-SAT-1. Neither of these have been reported as operational. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the current candidate for the world's smallest (communications) satellite would appear to be &lt;a href=&quot;http://www.50dollarsat.info/&quot; rel=&quot;nofollow&quot;&gt;$50SAT&lt;/a&gt;, launched on November 21st 2013 and still operational. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless anyone knows of another smaller satellite that actually went operational. &lt;/p&gt;&#xA;" OwnerUserId="6196" LastEditorUserId="49" LastEditDate="2014-09-30T18:19:12.943" LastActivityDate="2014-09-30T18:19:12.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5483" PostTypeId="2" ParentId="5459" CreationDate="2014-09-30T20:54:20.890" Score="11" Body="&lt;p&gt;While HDE 226868's answer was an excellent summation of the challenges in trying to up-rate the Energia or Proton launchers, I think the answer to the actual question implied in the title, &quot;Russian manned Moon landing capability today&quot; is:  No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No nation today has an operational launch vehicle (aka, rocket stack) to get humans even to lunar orbit, much less a landing on the Moon with return.  The largest rocket, in terms of payload to lunar injection, is the Delta IV Heavy, which has roughly 1/4th the payload capacity of the Saturn V, which was the rocket used for the Apollo lunar missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russia is currently developing the Angara family of rockets, but the initial heavy lift version - the Angara A5 - will have nearly the same capacity as the Delta IV, meaning in the range of 25 - 30 metric tons to LEO, and a fraction of that to TLI (trans-lunar injection).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometime next year, SpaceX plans to launch their first Falcon Heavy, which will (according to SpaceX) have the capability to take 53 metric tons to LEO, or 16 metric tons to TLI (per Wikipedia's page on Falcon Heavy).  But even that will be roughly 1/2 the payload of a Saturn V, and any lunar mission based on Falcon Heavy would require multiple launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the answer is, &quot;No&quot;.  Could Russia develop the capability given time and money?  Sure, probably.  But today?  Nobody has it.&lt;/p&gt;&#xA;" OwnerUserId="6199" LastActivityDate="2014-09-30T20:54:20.890" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5484" PostTypeId="1" AcceptedAnswerId="5485" CreationDate="2014-10-01T05:11:06.650" Score="12" ViewCount="7324" Body="&lt;p&gt;Earth's geostationary orbit has many human-made satellites which have helped vastly with communications and research. Aside from this artificial collection, are there any other objects orbiting a massive body at synchronous or stationary orbits? Generally, since masses are considered to be point masses, an orbit of any radius can be constructed, including ones that would require crossing through an atmosphere or the surface. Are there any such orbits that can't exist because of a combination of body radius, strong gravitational pull, and fast rotation? You can also consider cases were the rotation is so slow that the orbital radius would require the object to exit the massive body's gravitational sphere-of-influence (Venus).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My next question is how do you calculate the orbital parameters of geostationary orbits on other bodies (specifically orbital velocity and radius)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My thought is that we are very lucky on Earth to have such a geostationary orbit that resides inside our &lt;a href=&quot;http://en.wikipedia.org/wiki/Hill_sphere&quot;&gt;Hill Sphere&lt;/a&gt;, outside of our atmosphere, and at a distance that serves well for communication. These sorts of systems around other bodies could prove very useful in the future.&lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="603" LastEditDate="2014-10-01T13:53:48.500" LastActivityDate="2017-06-15T20:30:45.180" Title="Geosynchronous orbits around other Solar System objects" Tags="|orbital-mechanics|geostationary|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5485" PostTypeId="2" ParentId="5484" CreationDate="2014-10-01T14:57:20.513" Score="17" Body="&lt;p&gt;Geosynchronous orbit radius can be calculated imposing the orbital period to be equal to the Earth's rotational period, which results in:&#xA;$R_{GEO}=\sqrt[3]{\frac{GM_ET_{rot}^2}{4\pi^2}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $G = 6.673 \cdot 10^{-11} \frac{Nm^2}{kg^2}$ is the universal gravitational constant, $M_E = 5.97\cdot 10^{24} kg$ is the Earth Mass and $T_{rot} = 86164 s$ is the Earth rotation period.&#xA;A circular orbit having the resulting radius ($46164 km$ for Earth) is called Geosynchronous; if it also have 0 inclination it is a Geostationary orbit, since a spacecraft put in such an orbit will always be over the same point on the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbital speed on any circular orbit can be calculated with the following formula:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ V_c=\sqrt{\frac{GM}{R}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Geosynchronous case this results in about $3.07 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same computations can be performed for any celestial body using the appropriate values:&#xA;$R_{synch}=\sqrt[3]{\frac{GM_{planet}T_{rot,planet}^2}{4\pi^2}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the computation of the radius, you could compare it with the planet's radius and the radius of the Hill Sphere (Sphere of influence of the planet).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the following, I report the results of an approximate computation for each Solar System planet (plus the Moon), considering the Hill sphere relative to the Sun (to the Earth for the Moon):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mercury: $R_{synch}=242843 km$  ,  $R_H=220594 km$  ,  $R_{planet}=2440 km$  ,  $V_c=0.3 km/s$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus: $R_{synch}=1535681 km$  ,  $R_H=1010369 km$  ,  $R_{planet}=6052 km$  ,  $V_c=0.46 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moon: $R_{synch}=88463 km$  ,  $R_H=129417 km$  ,  $R_{planet}=1737 km$  ,  $V_c=0.24 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars: $R_{synch}=20429 km$  ,  $R_H=1083941 km$  ,  $R_{planet}=3390 km$  ,  $V_c=1.45 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter: $R_{synch}=160052 km$  ,  $R_H=53155071 km$  ,  $R_{planet}=69911 km$  ,  $V_c=28.14 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saturn: $R_{synch}=111606 km$  ,  $R_H=65439558 km$  ,  $R_{planet}=58232 km$  ,  $V_c=18.43 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Uranus: $R_{synch}=82674 km$  ,  $R_H=70064595 km$  ,  $R_{planet}=25362 km$  ,  $V_c=8.37 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neptune: $R_{synch}=83395 km$  ,  $R_H=115863626 km$  ,  $R_{planet}=24622 km$  ,  $V_c=9.03 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto: $R_{synch}=18892 km$  ,  $R_H=7633076 km$  ,  $R_{planet}=1184 km$  ,  $V_c=0.22 km/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see from these numbers, the orbits relative to Mercury and Venus would be outside the Hill sphere. Every orbit, instead, is well above the relative planet surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These computations only determine the radius of the orbits, then, their stability has to be considered, which is a much more complicated matter. Even the Geostationary orbit is not stable and satellites spend fuel to maintain this orbit. In general each orbit will be influenced by the actual shape of the planet (flatness, asymmetries...), by the gravitational attraction exerted by other nearby bodies like moons (and even by more distant but bigger ones like the Sun or Jupiter) and many other factors. A precise computation of these effects requires an accurate knowledge of the Solar System dynamics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once one of these orbit has been proved to exist, nothing would prevent a natural body to follow it. Charon and Pluto are mutually tidally locked which means that Charon is essentially in one of these orbits.   &lt;/p&gt;&#xA;" OwnerUserId="685" LastEditorUserId="20070" LastEditDate="2017-06-15T20:30:45.180" LastActivityDate="2017-06-15T20:30:45.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5486" PostTypeId="2" ParentId="5419" CreationDate="2014-10-01T16:46:36.630" Score="12" Body="&lt;p&gt;Climbing at terminal velocity is the most fuel efficient speed to gain (vertical) height while in steady state, such that the density, velocity, gravitational acceleration and other parameters stay constant during ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To proof this I will first have to define the required thrust, $F$, which has to overcome &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravitational_acceleration#For_point_masses&quot;&gt;gravity&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Drag_(physics)#Drag_at_high_velocity&quot;&gt;drag&lt;/a&gt; (such that the sum of all forces would add up to zero) and can be calculated as follows,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;F = mg + \frac{1}{2} \rho v^2 C_D A,&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $m$ is the mass of the rocket, $\rho$ the density of the atmosphere, $v$ the velocity, $C_D$ the drag coefficient and $A$ the cross section area of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This thrust can also be &lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_impulse#Specific_impulse_as_a_speed_.28effective_exhaust_velocity.29&quot;&gt;related&lt;/a&gt; to mass flow of the expelled fuel, like so&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;F = v_e \dot{m},&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $v_e$ is equal to the effective exhaust velocity of the rocket engine and $\dot{m}$ the mass flow ($\frac{dm}{dt}$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To optimize fuel efficiency you would have to minimize the amount of fuel needed to gain  height, thus minimize&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{d m}{d y} = \frac{dm/dt}{dy/dt} = \frac{\dot{m}}{v} = \frac{mg + \frac{1}{2} \rho v^2 C_D A}{v_e v}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can be done by taking the derivative of this expression with respect to $v$ and set this equal to zero,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{d}{dv} \frac{mg + \frac{1}{2} \rho v^2 C_D A}{v_e v} = \frac{v_e \rho v^2 C_D A - v_e (mg + \frac{1}{2} \rho v^2 C_D A)}{v_e^2 v^2}=0,&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\rho v^2 C_D A - mg - \frac{1}{2} \rho v^2 C_D A = \frac{1}{2} \rho v^2 C_D A - mg = 0,&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{1}{2} \rho v^2 C_D A = mg.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the best velocity indeed turns out to be the same as the magnitude on terminal velocity (drag equal to gravity), but in the opposite direction of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However during the ascent only a small initial portion of the trajectory will be vertical. Once the rocket would start to pitch sideways then terminal velocity will probably not be the most efficient anymore.&lt;/p&gt;&#xA;" OwnerUserId="1086" LastActivityDate="2014-10-01T16:46:36.630" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5487" PostTypeId="2" ParentId="5473" CreationDate="2014-10-01T18:02:12.820" Score="5" Body="&lt;p&gt;Randall Munroe in his &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0544456866&quot; rel=&quot;nofollow&quot;&gt;What If&lt;/a&gt; book says it well:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The reason it's hard to get to orbit &lt;strong&gt;isn't&lt;/strong&gt; that space is high up.&lt;br&gt;&#xA;  It is hard to get to orbit because you have to go &lt;em&gt;fast&lt;/em&gt;  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Being $32$ km higher doesn't save much at all.  Lifting a mass from $32$ km to $302$ km uses $270,000 \cdot 9.8 \approx 2650$ kjoules/kg.&lt;br /&gt;&#xA;Lifting a mass from sea level to $302$ km uses about $3,000$ kjoules/kg.&lt;br /&gt;&#xA;Accelerating to $7.73$ km/sec orbital velocity given &lt;a href=&quot;http://www.calctool.org/CALC/phys/astronomy/earth_orbit&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; uses $\frac 12 (7730000)^2 \approx 3\cdot 10^{10} $ kjoules/kg, over $10,000$ times more.&lt;/p&gt;&#xA;" OwnerUserId="176" LastEditorDisplayName="user10509" LastEditDate="2015-07-29T11:57:06.407" LastActivityDate="2015-07-29T11:57:06.407" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5489" PostTypeId="2" ParentId="4842" CreationDate="2014-10-01T19:41:26.657" Score="1" Body="&lt;p&gt;This answer is only to provide some supplemental information on the discussion in the comments section of the original question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a reference to an animation of the MOM and MAVEN orbits at &lt;a href=&quot;http://sankara.net/mom.html&quot; rel=&quot;nofollow&quot;&gt;http://sankara.net/mom.html&lt;/a&gt; . Since I am the developer of the animation, I would like to clarify a few things:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The animation uses orbit data fetched from NASA JPL's &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow&quot;&gt;HORIZONS&lt;/a&gt; web interface&lt;/li&gt;&#xA;&lt;li&gt;For the Mars Orbiter Mission, JPL publishes the information obtained from its Deep Space Network and/or the Indian Space Research Organization&lt;/li&gt;&#xA;&lt;li&gt;I update the animation whenever JPL publishes an update on HORIZONS. So the metrics reported by the animation are perhaps the best available publicly. &lt;/li&gt;&#xA;&lt;li&gt;I believe NASA Eyes must be using orbit data from a source equivalent to HORIZONS&lt;/li&gt;&#xA;&lt;li&gt;As of September 2014, the animation reports distances from Sun, Earth, and Mars. Not the distance traveled since launch. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6150" LastActivityDate="2014-10-01T19:41:26.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5490" PostTypeId="2" ParentId="5447" CreationDate="2014-10-02T00:39:02.113" Score="6" Body="&lt;p&gt;This is not a complete answer (there really isn't one, since what's considered &quot;reasonable&quot; will vary with the person answering), but:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With current technology, assuming that we didn't do fabrication in space (which would allow much thinner sails that don't require folding and packing in a rocket), and without relying on anything that hasn't been developed yet, solar sails can be made of 2 µm thick Mylar, which has &lt;a href=&quot;http://www.grafixplastics.com/mylar_prop.asp&quot; rel=&quot;noreferrer&quot;&gt;a density of 1.39 g/cc&lt;/a&gt;, not counting the extremely thin layer of aluminum deposited onto it.  This works out to approximately 360 m^2 per kilogram.  Assuming the heaviest-lift booster for 2015 will be a Falcon Heavy (53,000 kg to LEO), that works out to 19 km^2, or if you prefer, a circular solar sail about 4.9 kilometers in diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this doesn't include the weight of the rigging, control systems, and so on that would be required, I just wanted to give a rough guide. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best bet for a near-future solar sail really would be space-based manufacturing, though. Per &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail#Materials&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, an engineer named Eric Drexler has created 0.1 µm aluminum film using vapor deposition. Space-based manufacturing (presumably using an asteroid as feedstock) would allow arbitrarily large solar-sails to be constructed.&lt;/p&gt;&#xA;" OwnerUserId="6199" LastActivityDate="2014-10-02T00:39:02.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5492" PostTypeId="2" ParentId="5477" CreationDate="2014-10-02T11:33:01.977" Score="7" Body="&lt;p&gt;Recently I have found a similar ball-shaped stone (limestone or dolomite?), 4 cm in diameter, in a &quot;gully&quot; on a hillside in Budaörs, Hungary. I think it is Mother Nature furnishing curious things like this.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/QPTDN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6211" LastActivityDate="2014-10-02T11:33:01.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5493" PostTypeId="2" ParentId="4108" CreationDate="2014-10-02T13:20:52.510" Score="-1" Body="&lt;p&gt;pressure though out a generic rocket thrust chamber and nozzle while throttling through different ranges. &lt;a href=&quot;https://www.youtube.com/watch?v=SlPHYc0mXEw&quot; rel=&quot;nofollow&quot;&gt;https://www.youtube.com/watch?v=SlPHYc0mXEw&lt;/a&gt; this demonstrates that on a vast majority of rocket designs that are used in optimum power ranges often there is a significant pressure decrease around the throat and pressure is drasticly lower in the nozzle section. it does not have to be this way and you could design a rocket with near equal pressure distribution in the chamber and nozzle however this usually would result in a grossly under expanded nozzle and other drawbacks. i hope for the sake of credibility of this site you reconsider your answers. the primary section that transmits thrust to the rocket structure is the injector plate and machinary ahead of it this is commonly known as the power head and is the reason most heavy lifting engines are connected from here. further  in no case does any rocket transmit thrust from the nozzle alone. also if a significant portion of thrust was generated from the nozzle, this would imply that the forces on all of the nozzle before the throat would sum up and need to pass through the bottle neck of the throat, the throat being the cross section of least material area density and thus higher stress per unit. this would be poor design in most cases. &lt;/p&gt;&#xA;" OwnerUserId="6175" LastActivityDate="2014-10-02T13:20:52.510" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5494" PostTypeId="1" AcceptedAnswerId="19147" CreationDate="2014-10-02T16:00:03.523" Score="12" ViewCount="5250" Body="&lt;p&gt;The 2013 NASA's Office of Inspector General (OIG) report, &lt;a href=&quot;http://oig.nasa.gov/audits/reports/FY14/IG-14-031.pdf&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Extending the Operational Life of &#xA;the International Space Station &#xA;Until 2024&lt;/em&gt;&lt;/a&gt;, shows the ISS annual operating costs at $2.9 billion. How much of this cost can be linked to the fact that the ISS has a human crew? It takes a lot to support a human crew in space: more mass to launch in resupply, launches to ferry crew back and forth, life support systems, higher risk monitoring, etc. If it is possible to separate these costs out, I would be very interested in the results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Some users have mentioned that estimating this depends on what you count and what you don't. Let's imagine that Robonaut were completely capable of performing solo operations on the ISS, and that Robonaut has now been given full responsibility for performing all on-board activities on the ISS. How would this change the annual operating cost? (Don't try to estimate what it would take to maintain Robonaut at this level of activity--assume current Robonaut costs whether those are lower or higher than they would be in this imaginary scenario.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Please try not to get hung up on the plausibility of this scenario. I'm not asking for a fully automated ISS--I'm using it as the premise to demonstrate how I want the costs separated.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-09-28T14:15:44.400" LastActivityDate="2016-11-26T23:12:55.647" Title="How much of the International Space Station annual operating costs are due to human crew?" Tags="|iss|cost|crewed-spaceflight|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="5495" PostTypeId="1" CreationDate="2014-10-02T16:39:10.110" Score="4" ViewCount="379" Body="&lt;p&gt;Space Policy Online's report on &lt;a href=&quot;http://www.spacepolicyonline.com/news/stratolaunch-and-sierra-nevada-courting-on-dream-chaser&quot; rel=&quot;nofollow&quot;&gt;IAC2014 Day Three: Stratolaunch and Sierra Nevada Courting on Dream Chaser&lt;/a&gt; mentions that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... Stratolaunch would be used to launch a 75 percent version of Dream&#xA;  Chaser into low Earth orbit (LEO).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And goes on to somewhat elaborate:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A Stratolaunch-Dream Chaser system envisions using Stratolaunch to&#xA;  launch a 75 percent version of Dream Chaser into space with cargo or&#xA;  two-three crew.  It could launch and return to the launch site within&#xA;  24 hours in a “responsive space” mode.  It could take off from&#xA;  anywhere in the world and deliver cargo or people to any inclination&#xA;  orbit and, with its cross-range capability, land anywhere there is a&#xA;  runway that can handle a 747 or A320 aircraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But I didn't find any further information on what exactly is meant with these &quot;75%&quot;. 75 percent of / by what? Of its size? Carrying capacity? By volume? Time supporting orbital activities?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, failing to find further info on this so-called &lt;em&gt;75 percent version of Dream Chaser&lt;/em&gt;, short of what the linked article mentions about the differences with the full-scale Dream Chaser that Sierra Nevada competed with for NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Commercial_Crew_Development&quot; rel=&quot;nofollow&quot;&gt;CCDev / CCtCap&lt;/a&gt;, i.e. that the smaller scale version will support a two to three crew flights only, how else will it differ from current Dream Chaser design?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't that mean, for example, it's basically going back to the &lt;a href=&quot;https://en.wikipedia.org/wiki/HL-20_Personnel_Launch_System&quot; rel=&quot;nofollow&quot;&gt;HL-20 Personnel Launch System design&lt;/a&gt; and using Launch Escape System (LES) adapter instead of integrating LES into its body? 75% also can't refer to supported crew size since the HL-20 was meant to service up to ten astronauts, the version of Dream Chaser that SNC competed for NASA's CCDev / CCtCap would have serviced seven, and this new version is supposed to service up to three astronauts. That's more than half the reduction in crew size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So which new design features would this &lt;em&gt;&quot;75% version&quot;&lt;/em&gt; keep from its bigger brethren (Dream Chaser / HL-20), and which of its systems would go back to the drawing boards or have already undergone change?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-03T09:18:33.480" LastActivityDate="2014-10-03T09:18:33.480" Title="What is the &quot;75 percent version of Dream Chaser&quot;?" Tags="|crewed-spaceflight|spacecraft-development|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5496" PostTypeId="1" CreationDate="2014-10-02T16:54:38.247" Score="2" ViewCount="176" Body="&lt;p&gt;The &lt;a href=&quot;http://www.isro.org&quot; rel=&quot;nofollow&quot;&gt;ISRO&lt;/a&gt; &lt;a href=&quot;http://www.isro.org/mars/payload.aspx&quot; rel=&quot;nofollow&quot;&gt;Mars Orbiter Mission&lt;/a&gt; web-site writes to say&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mars Orbiter Mission carries five scientific payloads to observe Martian surface, atmosphere and exosphere extending up to 80,000 km for a detailed understanding of the evolution of that planet, especially the related geologic and the possible biogenic processes on that interesting planet. These payloads consist of a camera, two spectrometers, a radiometer and a photometer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same web-site goes on to provide a brief of each of these instruments. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question then that comes to mind is - &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the space-craft have a dedicated channel for each instrument to convey it's data? &lt;strong&gt;OR&lt;/strong&gt;&#xA;Is the data from each instrument queued before dispatch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm also curious to know what is the total bandwidth required/utilized by MOM for all 5 instruments.&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-10-02T16:54:38.247" Title="Is each instrument on ISRO MOM independently in 24x7 contact with Earth?" Tags="|mars|communication|isro|mars-orbiter-mission|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5498" PostTypeId="1" CreationDate="2014-10-02T17:24:50.703" Score="3" ViewCount="262" Body="&lt;p&gt;A quick follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/5496/is-each-instrument-on-isro-mom-independently-in-24x7-contact-with-earth&quot;&gt;Is each instrument on ISRO MOM independently in 24x7 contact with Earth?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every 26 months Sol interposes to occult Earth from the gaze of the Warlord. The last time this occurred was in June-July 2014. This occultation is not only visual, but applies to perhaps all frequencies in the E/M spectrum. Ergo, communication between Earth/Mars just can not happen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Colonization_of_Mars#Communication&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In reality most missions at Mars have had communications blackout periods of the order of a month.&lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1196/annals.1370.007/abstract&quot; rel=&quot;nofollow noreferrer&quot;&gt;[38]&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Although the MOM has a planned life far shorter than the 26-month blackout frequency, one can not help wondering&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What is the data storage capability for each instrument aboard MOM?&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;IF&lt;/strong&gt; MOM was to be extended into the next comm.blackout period, would the onboard instruments be put on standby for that duration?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-12-04T03:25:31.183" Title="What will MOM do when Earth isn't looking?" Tags="|mars|communication|isro|data-storage|mars-orbiter-mission|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5499" PostTypeId="1" CreationDate="2014-10-02T17:39:56.457" Score="8" ViewCount="590" Body="&lt;p&gt;A brief follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/5498/what-will-mom-do-when-earth-isnt-looking&quot;&gt;What will MOM do when Earth isn&amp;#39;t looking?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As already premised in the referenced question; terrestrial artifacts on/around Mars encounter a communications blackout every 26 months, or so. This &lt;a href=&quot;http://www.wral.com/communication-blackout-mars-rovers-work-on-out-of-contact-with-nasa/12309745/&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; conveys the blackout may be more of a precautionary note to avoid a corrupt command from throwing a spoke in the wheel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there, today, a relatively error-free means to communicate with orbiters/rovers on the Martian surface whilst Sol occults Earth/Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;p.s. The impression from the answer chain indicates it is possible to &lt;em&gt;bounce&lt;/em&gt; signals off other craft, and may have been done during the Apollo program. Perhaps other long-term missions too. As a follow-up question - &lt;strong&gt;Have there been any instances of a terrestrial robot on/around Mars actually communicating thus? What was/were the relay/s involved, and the circumstances under which it was done?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-10-03T08:08:00.540" Title="Is there a means to communicate with robots on/around Mars whilst the Sun is between Mars &amp; Earth?" Tags="|mars|communication|the-sun|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5500" PostTypeId="2" ParentId="5495" CreationDate="2014-10-02T17:43:13.633" Score="2" Body="&lt;p&gt;Your linked article says the 75% version would carry cargo or 2-3 crew; the Wikipedia article on Dream Chaser says it could carry 7 crew. From that, my guess would be that they're talking about a 75%-scale version of the ship. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This article seems to back that interpretation: &lt;a href=&quot;http://www.sncorp.com/press_more_info.php?id=636&quot; rel=&quot;nofollow&quot;&gt;http://www.sncorp.com/press_more_info.php?id=636&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-10-02T17:43:13.633" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5501" PostTypeId="2" ParentId="5499" CreationDate="2014-10-02T20:12:07.283" Score="7" Body="&lt;p&gt;For a direct opposition, no. You could use a relay probe, or, if you have enormous amounts of transmission power at your disposal, bounce a signal off another planetary body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it rarely happens that Mars is actually occluded by the sun, because the orbits are ever so slightly inclined to each other. And if it does, it only lasts a short while.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the Sun is surrounded by lots of charged particles that it emits, the heliosphere. It extends into interplanetary space, but it's most dense near the sun. This makes it more challenging to get signals through during any opposition. More power and heavy error correction helps, of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it's not a question of &quot;can you get a signal through?&quot; but &quot;how long is the blackout and how large is the antenna?&quot;, both on earth, and on the probe.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-10-02T20:12:07.283" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5502" PostTypeId="2" ParentId="5499" CreationDate="2014-10-02T21:00:50.640" Score="4" Body="&lt;p&gt;No.  Communications between Earth and assets at Mars are suspended for three weeks or so around solar conjunction, depending on the angles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no real danger of corrupt commands as they are checked for errors.  What would happen if you tried is no commanding.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-10-03T08:08:00.540" LastActivityDate="2014-10-03T08:08:00.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5504" PostTypeId="2" ParentId="5440" CreationDate="2014-10-02T22:56:59.753" Score="1" Body="&lt;p&gt;For all MOM images as they become available, see &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.isro.org/pslv-c25/Imagegallery/mom-images.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.isro.org/pslv-c25/Imagegallery/mom-images.aspx&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most recently posted image is &lt;a href=&quot;https://i.stack.imgur.com/bTCT7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bTCT7.jpg&quot; alt=&quot;An almost full image of Mars from MOM &quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;http://www.isro.org/pslv-c25/Imagegallery/satellitesimages/img-4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;isro.org&lt;/a&gt;)&lt;/sub&gt;  &lt;/p&gt;&#xA;" OwnerUserId="6214" LastEditorUserId="10273" LastEditDate="2019-02-26T02:16:42.037" LastActivityDate="2019-02-26T02:16:42.037" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5505" PostTypeId="2" ParentId="5499" CreationDate="2014-10-02T23:38:49.510" Score="1" Body="&lt;p&gt;I like the answers already posted, but I'll add an example (which you've probably heard of) to demonstrate that while it would definitely be impossible, it really wouldn't be a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the days of the Apollo program, early missions to the moon did not actually land on the moon, but merely orbited it (although many times). The flights, among other things, helped pave the way for the now more-famous landings on the moon. There was one important thing that affected each mission, though. In order for the craft to orbit, it obviously had to go around the back of the moon. This led to a notorious period of radio silence, which was dramatized in &lt;em&gt;Apollo 13&lt;/em&gt;. The astronauts (I'm no longer talking about the movie) were able to get along just fine so long as no problems occurred onboard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can see that it is impossible to communicate between objects on opposite sides of the Sun for the same reason that made Deke Slayton just a little more nervous during each mission. Clearly, autonomous landers, probes, or other robots on Mars (or anywhere else, for that matter) would be just fine without human intervention for a while.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-10-03T00:53:07.033" LastActivityDate="2014-10-03T00:53:07.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5506" PostTypeId="2" ParentId="5385" CreationDate="2014-10-03T03:51:20.450" Score="3" Body="&lt;p&gt;Of more useful physical and engineering meaning than temperature is the heat flux, which is the energy being deposited on a unit area in unit time.  Even a very high temperature with very little energy behind it may do no damage.  You can touch a hot pot for a short time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These papers on detailed thermodynamic models of the &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090024218.pdf&quot; rel=&quot;nofollow&quot;&gt;MSL heatshield&lt;/a&gt; and the &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090024230.pdf&quot; rel=&quot;nofollow&quot;&gt;MSL backshell&lt;/a&gt; provide a great deal of information on the predicted heat flux on the front and back.  You will see peak heat fluxes on the front on the order 100 W/cm^2, but only on the order of 10 W/cm^2 on the back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that that does not mean that the temperatures have the same proportions.  Temperature is complicated and depends on material characteristics, where exactly in the material (solid or fluid) it is measured, how heat is absorbed and dissipated, local geometries, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to heat flux, of significant engineering interest is the total heat load, the total energy deposited per unit area over the entire entry (the integral of the heat flux), measured in J/cm^2.  You will find those in the papers as well.  The thermal protection systems have to be designed to take both the maximum heat flux, experienced in the steepest expected trajectory, as well as the maximum heat load, experienced in the shallowest (and so longest) expected trajectory.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-10-03T03:51:20.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5507" PostTypeId="2" ParentId="5499" CreationDate="2014-10-03T06:19:22.720" Score="2" Body="&lt;p&gt;&lt;em&gt;Direct&lt;/em&gt; communication between two planets while on opposite sides of the sun is nearly impossible since the signal gets swamped out by noise from the sun.  In principle, of course, just as today we can bounce signals off communication satellites in orbit around Earth and communicate between opposite sides of the planet, someday we could set up satellites in orbit around the sun to relay signals.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;Terminology note:&lt;/em&gt;  The term for that, when Mars and Earth are on opposite sides of the sun, is &quot;conjunction&quot;, and the term for Mars and Earth being close together is &quot;opposition&quot;.  Personally this makes no sense to me intuitively, but it probably made sense to early astronomers who thought of Earth as being the center of the universe.  Since it was astronomers that proved otherwise, by definition astronomy started at a time when everyone thought in geocentric terms.  And in all fairness, almost all telescopes are still on Earth or in orbit around Earth, and the problem really is that the sun is too close to Mars from the point of view of the Deep Space Network.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6218" LastActivityDate="2014-10-03T06:19:22.720" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5508" PostTypeId="1" AcceptedAnswerId="5510" CreationDate="2014-10-03T14:45:53.823" Score="9" ViewCount="1724" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Health_threat_from_cosmic_rays&quot;&gt;Wikipedia&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Exposures on the ISS average 150 mSv per year, although frequent crew rotations minimize individual risk&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I assume here the crew aboard the space-station receive more radiation during their stint than they would on Terra Firma over a year. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Random synapses firing ... &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;As compared to UV exposure on Earth' surface, how much more UV does a crew-member aboard ISS receive during the typical course of an expedition?&lt;/li&gt;&#xA;&lt;li&gt;If the answer to the above is noticeably larger, would it translate to a visible tan on crew at debark as compared to the same crew at embarkation? &#xA;&lt;ul&gt;&#xA;&lt;li&gt;What about non-ISS expeditions?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Does the ISS medical inventory include a sun-tan/sunscreen lotion/cream? &#xA;&lt;ul&gt;&#xA;&lt;li&gt;How much of it is ferried in a typical resupply mission?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2020-02-13T02:27:01.380" Title="Does the ISS medical inventory include a sun-tan/sunscreen lotion/cream?" Tags="|iss|radiation|health|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5509" PostTypeId="2" ParentId="4108" CreationDate="2014-10-03T14:50:19.730" Score="7" Body="&lt;p&gt;The forces that act on the walls of a rocket engine look like this: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Efr3t.jpg&quot; alt=&quot;Rocket engine cross-section&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The propellant is ignited in the chamber, this causes the combustion products to expand, and the pressure in the chamber and nozzle rises above the external atmospheric pressure.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only force vectors in this diagram that contribute to pushing the rocket forward, are the ones that point in the direction of travel or less than 90° off that direction. I.e. the net thrust is created by the force on the top of the chamber (usually the injector plate) and to some degree by the vertical component of the forces on the nozzle.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, you should be able to cut off the chambers and nozzles, and place the fully-fueled rocket on the injector plates.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also see that the sides of the chamber are under the same amount of force as the injector, so the chamber walls need to be strong enough to withstand this pressure. &#xA;From the throat down, the pressure decreases, so a smaller force acts on the end of the nozzle, and this section doesn't have to be as strong. So the bottom end of the nozzle is usually thinner than the walls of the combustion chamber. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://what-when-how.com/space-science-and-technology/rocket-propulsion-theory/&quot;&gt;More details here&lt;/a&gt; (this is also where I found the schematic)&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-10-03T19:26:30.163" LastActivityDate="2014-10-03T19:26:30.163" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5510" PostTypeId="2" ParentId="5508" CreationDate="2014-10-03T15:08:48.037" Score="15" Body="&lt;p&gt;Ultraviolet radiation does not get through the aluminum hull of the space station, so no such protection is needed for the crew inside. The glass of &lt;a href=&quot;http://history.nasa.gov/spacesuits.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;EVA suits&lt;/a&gt; is made not to let UV light through, either. The same is true for the glass of the ISS' windows. So a random member of the ISS crew receives about as much UV radiation as you sitting inside writing this. Unless you use a CRT monitor, then they receive less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ionizing radiation the astronauts have to worry about is much shorter wave gamma radiation. And for that, there is no sunblock.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="11721" LastEditDate="2020-02-13T02:27:01.380" LastActivityDate="2020-02-13T02:27:01.380" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="5511" PostTypeId="1" AcceptedAnswerId="5513" CreationDate="2014-10-03T16:55:58.903" Score="14" ViewCount="3504" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rings_of_Saturn#Physical_characteristics&quot;&gt;Saturn's rings&lt;/a&gt; are almost pure water ice in nice manageable chunks from specks to boulders a few meters across. Assuming there is something truly useful to do with it - once again talking about orbital depots or human bases on the moon - is it feasible to collect that ice and drag it back to the neighborhood of Earth? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compared to snagging asteroids passing through the neighborhood or collected at the Trojan points like &lt;a href=&quot;http://www.planetaryresources.com/&quot;&gt;Planetary Resources&lt;/a&gt; plans, or shipping water from Earth, it seems like it could have advantages. We know exactly where it is, it seems easy to get to, there's plenty of it, and it's in small chunks. What would be the problems?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-10-03T19:42:21.623" LastActivityDate="2020-02-11T10:11:55.963" Title="What about gathering water ice from Saturn's rings?" Tags="|water|mining|saturn|rings|ice|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5512" PostTypeId="2" ParentId="5511" CreationDate="2014-10-03T21:01:14.317" Score="-1" Body="&lt;p&gt;Saturn is &lt;em&gt;not&lt;/em&gt; easy to get to. The Earth is already full of water, we don't need to transport ice from 10 AU away to home to have another whiskey on the rocks. Water fills the oceans and the clouds and the polar ices and the underground and even inside ourselves. It is actually a problem that we have too much water, flooding and tsunamis are common causes of catastrophic death. Saturn's water would be used where it is. We would build the X-factory over there instead of bringing home the raw materials.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-10-03T21:01:14.317" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5513" PostTypeId="2" ParentId="5511" CreationDate="2014-10-03T22:49:18.920" Score="13" Body="&lt;p&gt;For a Hohmann transfer to Saturn, I get 15.7 km/s for both burns. The transfer time is also a simple formula. I obtain roughly &lt;strong&gt;6 years&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare to the lunar ice. It is roughly &lt;strong&gt;2.8 km/s&lt;/strong&gt; to get to, and the trip time would be a &lt;strong&gt;few days&lt;/strong&gt;, even from Low Earth Orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As suggested by the other answer, you could compare to Earth's surface. If we're using some reference frame far from Earth's surface, then it's nearly &lt;strong&gt;12.8 km/s&lt;/strong&gt; going up (EDIT: corrected from newbie mistake, LEO gets Oberth). But this gets complicated. The first 9 km/s is constrained by atmospheric physics. Although, leaving the moon also has a similar thrust-to-weight requirement placed on the rockets by gravity drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These numbers aren't totally fair to Saturn. If I use the edge of the rings (nearly the &quot;F&quot; ring), I get an orbital velocity of 16.6 km/s. In order to mine these rings, you'll need to follow a hyperbolic trajectory, and then do your final Hohmann burn when you're close to your destination, inside Saturn's gravity well. This will reduce the Delta V you'll need for the final burn. That'll reduce the final burn by a little bit. Breaking up the 15.7 km/s, the initial insertion burn is 10.3 km/s and the heliocentric circularization burn is 5.4 km/s. So we would need to correct this all for the Oberth effect, which is a multiplier of $\sqrt{1 + \frac{2 V_\text{esc}} {\Delta v}} $, sphere of influence assumptions, yada yada. That reduces the previous numbers to (respectively) 5.8 km/s and 1.4 km/s. Boy that was an unpleasant calculation, but the total comes to approximately &lt;strong&gt;7.2 km/s&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Commentary:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's true that mining the lunar ice would involve challenges that Saturn's rings don't have. But if the destination is close to Earth, those challenges would be more workable than the nuclear, ion, or whatever kind of drives that a Saturn hauler would involve. A similar tradeoff might present itself for other sources closer than Saturn, for instance, Jupiter trojan asteroids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I won't dispute the quality of Saturn's ice. It is likely more pure than the alternative sources. It might also be in convenient size chunks. Importantly (in its favor) it can be delivered by drives with a low thrust-to-weight ratio. Any value would do for this, although there might be some Delta V penalty due to reduced Oberth effect. Solar electric would be penalized heavily due to the distance from the sun. Nuclear power sources are vastly superior at that distance. 9.5 AU --&gt; squared means 90 times less power per unit area than near Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real problem is justifying that 6 year time frame. Who is going to wait 12 years for their return on investment? Now I grant you, because of the absence of a &lt;em&gt;gravity drag&lt;/em&gt; penalty, it's possible that a ship of monstrous size could deliver huge quantities of ice. Because you're not dealing with any surface-to-orbit transition, your ship could be fragile, and your engine could have a low thrust. Given that, the only thing that scales directly with the payload is the propellant. If we can get something like 30 km/s reaction velocities, then it's believable that making the round trip would work... although only if you can cannibalize water for propellant, and at somewhat poor margins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, sure, there is a business case in a &lt;em&gt;very specific&lt;/em&gt; scenario where we have a mature market for water in space, someone could reduce costs of procurement by making a ridiculous-looking bloated spaceship that runs the Saturn to Earth orbit route. I'm still not convinced that it would be better than other alternatives beyond the frost line, but most of those have angular clustering. A business might want deliveries more than once every year, and for that, Saturn versus Jupiter trojans (for instance) provide some seasonal diversity. But the advantage is nil except for with extremely large quantities. That raises the question - how could the depreciation possibly compete with lunar ice? If you &lt;em&gt;can&lt;/em&gt; run regular ferries from lunar poles to orbit, that seems more economic unless your bloated Saturn ship could be much lower cost (because the lunar scheme has much higher trip frequency). Maybe it could. We can't really say. The biggest variable will be your rate of return. If you demand 6% rate of return, then you're effectively cutting the value of the ice in half over 12 years. &lt;em&gt;This&lt;/em&gt; is why it's a hard sell, but that's not really the rocket science part of it.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Addition: Found the example of a &quot;bloated spaceship&quot; that I had in mind. Saw it on Atomic Rockets. Here is &lt;a href=&quot;http://www.neofuel.com/waterships/watership.htm&quot; rel=&quot;noreferrer&quot;&gt;the original source&lt;/a&gt;. Apparently they had Phobos in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eMBET.jpg&quot; alt=&quot;bloated&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have some problems with their off-the-bat assumptions. If Phobos has water, why doesn't Eros? I don't know these things. I'm doubtful that anyone &lt;em&gt;else&lt;/em&gt; knows the bulk composition or the difficulty of refining the below-the-surface precious water of such inner system bodies. Myself, I would like to picture that &quot;spaceship&quot; with the bladder being 100x its above relatively size. These things will make a hot air balloon look lean.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2014-10-03T23:39:04.753" LastActivityDate="2014-10-03T23:39:04.753" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5515" PostTypeId="1" CreationDate="2014-10-04T15:58:00.817" Score="1" ViewCount="311" Body="&lt;p&gt;All the articles I have found show the VCCW engines using flat plate injectors. With the inherent combustion stability of a Pintle injector system, it would seem to be a better choice, yet I find no evidence that it was tried. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone have any references on this?&lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-10-04T15:58:00.817" Title="Pintle injector in a Vortex Cooled Chamber Wall (VCCW) Engine" Tags="|rockets|engine-design|engines|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5516" PostTypeId="2" ParentId="2843" CreationDate="2014-10-04T17:18:31.790" Score="5" Body="&lt;p&gt;One that comes to mind is Copenhagen Suborbitals, a Danish non-profit aerospace organization that has constructed and launched several privately built rockets.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their homepage is &lt;a href=&quot;http://copsub.com&quot; rel=&quot;noreferrer&quot;&gt;http://copsub.com&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has a good write-up at &lt;a href=&quot;http://en.wikipedia.org/wiki/Copenhagen_Suborbitals&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Copenhagen_Suborbitals&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also find them on YouTube - search for &quot;CphSuborbitals&quot; &lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-10-04T17:18:31.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5517" PostTypeId="2" ParentId="2843" CreationDate="2014-10-04T17:42:27.537" Score="2" Body="&lt;p&gt;I'll admit that the manned aspect of the following program is fairly tangential to its main goals, but it is something, and the company could become a major player in the SSTO market within the next decade or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm talking about &lt;a href=&quot;http://www.reactionengines.co.uk/index.html&quot; rel=&quot;nofollow&quot;&gt;Reaction Engines&lt;/a&gt;. They're an English group primarily concerned with the &lt;a href=&quot;http://www.reactionengines.co.uk/space_skylon.html&quot; rel=&quot;nofollow&quot;&gt;Skylon&lt;/a&gt; spaceplane. It's original design was unmanned, but there have been some side efforts to put a passenger module in the payload bay. It's unclear as to whether the craft would still be autonomous; I would think so because there would be a massive design challenge to make it pilotable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From their website:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Reaction Engines Ltd ('REL') is a privately held company located in the United Kingdom and was formed in 1989 to develop the technologies needed for an advanced combined cycle air-breathing rocket engine class called SABRE that will enable aircraft to operate easily at speeds of up to five times the speed of sound or fly directly into Earth orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylon_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, the group aims to have the first test flights in 2019.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the company's other projects include the &lt;a href=&quot;http://www.reactionengines.co.uk/sabre.html&quot; rel=&quot;nofollow&quot;&gt;SABRE&lt;/a&gt; engine and the group's &lt;a href=&quot;http://www.reactionengines.co.uk/mach5cruise.html&quot; rel=&quot;nofollow&quot;&gt;LAPCAT&lt;/a&gt; design. &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_Engines_A2&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; goes into some detail about their entrance in that (called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_Engines_A2&quot; rel=&quot;nofollow&quot;&gt;A2&lt;/a&gt;), but I can't verify that information from the company's official website. The A2 would definitely be manned, but it would not be capable of even sub-orbital flight, nor is it designed to do that.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2014-10-04T17:42:27.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5519" PostTypeId="2" ParentId="5472" CreationDate="2014-10-04T21:24:48.070" Score="16" Body="&lt;p&gt;Although it is quite difficult to give a definite and documented answer about Russian Space Program and Russian politics in general, I'm trying to express some considerations that could someway answer your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considerations against Baikonur's replacement:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;ISS access: With the end of the Space Shuttle Program, Baikonur is the only launch site which supports crewed missions to the ISS. This means that any country participating in the ISS program has to rely on Russians to reach the ISS and Russians are getting paid huge amounts of money for this (&lt;a href=&quot;http://www.space.com/20897-nasa-russia-astronaut-launches-2017.html&quot;&gt;about $70 million per astronaut from USA&lt;/a&gt;), without mentioning the political power they have gained. By the way, the ISS orbit was chosen to be accessible from the USA (Cape Canaveral), from European operated spaceport in South America (Kourou, French Guyana), Japan (Tanegashima Space Center) and from Russia (Baikonur, precisely).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Current agreements and development plans: As you pointed out, Russia already has an agreement with Kazakhstan for the lease of the cosmodrome until 2050 at $115 Million a year. The agreement was signed in Astana on January 09, 2004 and was &lt;a href=&quot;http://www.spacedaily.com/reports/Kazakhstan_Finally_Ratifies_Baikonur_Rental_Deal_With_Russia_999.html&quot;&gt;ratified by the Kazakh parliament in April 2010&lt;/a&gt;. In addition, Russia and Kazakhistan are planning to renew Site 250 at Baikonur to launch Angara rockets.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Possible alternatives: Ok, Russia is huge, but not all places are good to build a Spaceport. An ideal place would be as close to equator as possible (quite a problem in Russia), it would have usually clear skies and good weather (also a problem in the majority of Russia), not too close to population centers (this may be easier).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;History: Baikonur is the oldest (and largest) cosmodrome in the world and it is steeped in history. A lot of historical events took place here, including the Sputnik 1 flight, the Yury Gagarin's flight and the first flight with a woman onboard (Valentina Tereškova), so I guess the location is also significant for propaganda reasons.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Considerations in favour of Baikonur's replacement:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Independence: Although a long term agreement between Russia and Kazhakistan is in place, I believe that Russia would be glad to stop relying on foreign countries for its space program. Russia is indeed building a new cosmodrome, Vostochny, and planning to expand Plesetsk cosmodrome in this sense. So a sort of &quot;nationalization plan&quot; of space launches seems to be ongoing in Russia.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Age: Baikonur was founded in 1955. As you pointed out, it will be almost 100 years old at the end of the current agreement. In addition, a lot of launch pads are currently inactive, so they wouldn't need to be replaced in a new cosmodrome.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Conclusion:&#xA;Comparing the two groups of considerations I would conclude that Baikonur will continue to be an active cosmodrome for a long time and its replacement is anything but probable and imminent. At most the total number of launches will decrease in favour of Plesetsk and Vostochny. Again, this is more a personal opinion based on investigations on the web rather than a definite answer.&lt;/p&gt;&#xA;" OwnerUserId="685" LastEditorDisplayName="user56" LastEditDate="2014-10-07T00:50:00.353" LastActivityDate="2014-10-07T00:50:00.353" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5522" PostTypeId="1" CreationDate="2014-10-05T10:06:16.617" Score="10" ViewCount="756" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/q/5422/49&quot;&gt;recent question/discussion&lt;/a&gt; on this site brought out the interesting fact that most probes around Mars are in Sun synchronous near polar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://isro.org/pressrelease/scripts/pressreleasein.aspx?Sep24_2014&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISRO web-site for the Mars Orbiter Mission&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The inclination of orbit with respect to the equatorial plane of Mars is 150 degree, as intended. In this orbit, the spacecraft takes 72 hours 51 minutes 51 seconds to go round the Mars once. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Wikipedia on &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Inclination&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;an inclination greater than 90° and less than 180° is a retrograde orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How does MOM benefit from this particular retrograde orbit? &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-03-16T13:51:40.710" Title="Why is MOM in a very retrograde orbit?" Tags="|mars|orbit|isro|mars-orbiter-mission|retrograde-orbit|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5525" PostTypeId="1" CreationDate="2014-10-05T14:31:26.550" Score="16" ViewCount="2276" Body="&lt;p&gt;One of the roles that Progress (and ATV) fulfill at the ISS is station reboost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This works because they can dock at the back of the Zvezda module, and boost inline with the station.  I cannot find a value for the thrust level of the main engine, but it suffices to boost the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Dragon currently berths to the CBM ports on the bottom (or top as backup) of the Harmony (node 2) module on the US side, and is thus perpendicular to the main line of the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But a Dragon V2 will be docking to the PMA on the front end of Harmony (or the PMA which is due to be moved to the top of the Harmony node, and the backup CBM port will be the bottom of Unity, as the MPLM berthed there is due to be moved.  See the answer to &quot;&lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;Will the ISS need more docking ports&lt;/a&gt;&quot; question for more info and images of how the re-arrangement will look). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The front end of Harmony is basically the opposite end from where ATV and Soyuz currently do their reboost tasks. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So will Dragon or Dragon V2 be able provide station reboost if needed?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-10-03T10:38:37.603" Title="Can Dragon, or Dragon V2 be used to boost the ISS orbit?" Tags="|iss|spacex|dragon|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5526" PostTypeId="2" ParentId="5525" CreationDate="2014-10-05T14:37:36.927" Score="10" Body="&lt;p&gt;I shall answer myself, since in wondering and writing about it, I think I know the answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably not.  The issue is that a Dragon capsule (Cargo version) only has &lt;a href=&quot;http://en.wikipedia.org/wiki/Draco_%28rocket_engine_family%29&quot;&gt;Draco thrusters&lt;/a&gt; for maneuvering. These have a thrust of 90 lbs.  That is just too little to make much of a difference.  Now if they could thrust for hours, sure that would help, but of course there is a fairly limited supply of fuel for each Draco on the Dragon so it is very unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now Dragon V2 is a slightly different vehicle and will have 8 &lt;a href=&quot;http://en.wikipedia.org/wiki/Draco_(rocket_engine_family)#SuperDraco&quot;&gt;Super Draco engines&lt;/a&gt;, with a thrust closer to 16,400 lbs. That starts to be enough thrust to be useful, but again it is fuel constrained, since they are intended to be used for powered landing, and depleting that fuel supply before landing seems like a generically bad plan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But even more so, the Dragon and Dragon V2 will have a trunk section attached that has the solar panels.  (V1 has extendable panels, V2 looks to have them flat on the surface of the trunk itself, an interesting concept, which begs the question of why the change!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Draco engines can fire around the trunk, after all that is how it maneuvers in orbit and for docking/berthing.  But the Super Dracos are meant to thrust after the trunk is discarded.  So the trunk is likely 'in the way'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is interesting that while Dragon can fulfill upmass and downmass requirements, there is still a hole in the US capacity to reboost the station.  (Which begs the question, could Dream Chaser or CST-100 reboost the station.  Dream Chaser, no, since it docks tail end first to the station, which is where its engines are.  CST-100?  A good question).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-10-05T14:37:36.927" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5527" PostTypeId="1" AcceptedAnswerId="14608" CreationDate="2014-10-05T14:41:59.720" Score="15" ViewCount="1438" Body="&lt;p&gt;In the question: &lt;a href=&quot;https://space.stackexchange.com/questions/5525/can-dragon-or-dragon-v2-be-used-to-boost-the-iss-orbit&quot;&gt;&quot;Can Dragon, or Dragon V2 be used to boost the ISS orbit?&quot;&lt;/a&gt; I asked if the Dragon V1/V2 can provide station reboost functionality that is currently provided by the ATV or Progress cargo vehicles docked to the back end of the Zvezda module. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am pretty sure Dream Chaser would not be able to, since its two engines flank its airlock/docking port, and it back up to the station and docks tail first.  Thus its engines are blocked. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how about the Boeing CST-100?  Can it provide sufficient thrust, when docked to the front of Harmony on the PMA for station reboost?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps a necessary precursor question is, can the LIDS/NDS docking system attachment, connected to the PMA, connected to the CBM port on the Harmony module take the thrust level needed for station reboost? One assumes it should, since the Shuttle provided some level of station reboost while docked to that port.  Though because of the geometry of the connection, I wonder if the Shuttle did not apply a very large amount of thrust.  Perhaps there is a limit.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-11-01T17:50:31.830" Title="Can a Boeing CST-100 be used to provide station reboost?" Tags="|iss|docking|docking-port|cst-100|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="5528" PostTypeId="1" AcceptedAnswerId="5529" CreationDate="2014-10-05T17:07:45.907" Score="4" ViewCount="886" Body="&lt;p&gt;In the process of thinking about some hard science fiction, I need to substitute some mass fractions that will minimize the amount of energy used in order to be maximally generous to the author. So this produces the following specifiers in terms of an academic problem:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Infinite propellant is available for free&lt;/li&gt;&#xA;&lt;li&gt;The ejection velocity of the propellant can be selected by the pilot (and the engine is 100% efficient at all values, which is for the sake of argument)&lt;/li&gt;&#xA;&lt;li&gt;A hard Delta V requirement exists due to the mission profile&lt;/li&gt;&#xA;&lt;li&gt;What is the mass ratio that minimizes the energy used by the engine?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I'm thinking the mathematics might be sufficiently available here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket#Energy_efficiency&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Rocket#Energy_efficiency&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have a notion of &quot;overall efficiency&quot;, which I think directly maps to the concept of engine efficiency I have in mind. This metric has an absolute maximum, at a value of ve/v of about 1.6. But is it &lt;em&gt;really&lt;/em&gt; 1.6? I can't find any reference to it. So I come here to ask:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What is an algebraic expression for maximum overall efficiency as defined here? No decimals. Just with math functions, like exp() or ln().&lt;/li&gt;&#xA;&lt;li&gt;Does this necessarily correspond to an absolute most energy efficient mass ratio as well?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-10-05T19:17:34.393" Title="Most energy efficient mass ratio of a rocket, or exhaust velocity" Tags="|interplanetary|specific-impulse|mass-fraction|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5529" PostTypeId="2" ParentId="5528" CreationDate="2014-10-05T19:01:59.887" Score="5" Body="&lt;p&gt;Yes, the Wikipedia answer applies to your problem and is correct.  I get $v_e=0.6275\Delta V$.  (Your &quot;ve/v of about 1.6&quot; is backwards -- it should be v/Ve, as shown correctly on the Wikipedia plot.)  This gives a mass ratio of about $4.9$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no closed form for that number, $x\approx 0.6275$.  You need to iteratively solve ${1\over 2}=x\left(1-e^{-1/x}\right)$.  (Unless you use the &lt;a href=&quot;http://mathworld.wolfram.com/LambertW-Function.html&quot; rel=&quot;noreferrer&quot;&gt;Lambert W-function&lt;/a&gt;, but you probably don't have that function on your calculator.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The derivation is simple, combining the rocket equation and $E={m v^2\over 2}$, and is left as an exercise for the reader.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-10-05T19:17:34.393" LastActivityDate="2014-10-05T19:17:34.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5530" PostTypeId="2" ParentId="2240" CreationDate="2014-10-05T19:39:38.133" Score="4" Body="&lt;p&gt;The odds of any unexpected event altering Philae's trajectory is negligible. I suspect that the most likely cause for putting it off-target, ironically, would be a propulsion system malfunction, or a leak in a propellant tank! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not sure what you mean with the last question, &quot;Is it's mass of 100kg on Earth, and Chury's infinitesimal gravity adequate to ensure touch-down on the comet along the planned trajectory?&quot;, but Philae is equipped with harpoons to anchor it - otherwise, even if it had thrusters and guidance systems, it would be very difficult to keep it from bouncing off the comet. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-10-05T19:39:38.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5531" PostTypeId="1" AcceptedAnswerId="5539" CreationDate="2014-10-06T04:19:30.150" Score="39" ViewCount="12926" Body="&lt;p&gt;There are several significant advantages to &lt;a href=&quot;http://en.wikipedia.org/wiki/Air_launch_to_orbit&quot;&gt;air launching&lt;/a&gt; a satellite-carrying rocket:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;save fuel/propellant as a horizontal take-off is more energy efficient&lt;/li&gt;&#xA;&lt;li&gt;fully mobile platform allowing the launch to be conducted anywhere&lt;/li&gt;&#xA;&lt;li&gt;less affected by weather allowing it to be deployed almost any time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What are the reasons preventing all such rockets from being launched from the air?&lt;/p&gt;&#xA;" OwnerUserId="6229" LastEditorUserId="6816" LastEditDate="2017-06-29T18:25:10.087" LastActivityDate="2018-07-23T12:46:10.207" Title="Why aren't all satellite-carrying rockets launched from airplanes?" Tags="|launch|air-launch|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5532" PostTypeId="2" ParentId="5531" CreationDate="2014-10-06T05:37:32.297" Score="34" Body="&lt;p&gt;Here are a few disadvantages to air launch:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Most launch vehicles are too heavy to be carried by any extant aircraft, e.g. Atlas V 401 masses 335 tonnes, compare to the Airbus A380's maximum load of 89 tonnes.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;For launch vehicles with cryogenic propellants, loading and topoff would be extremely challenging.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;It is generally not safe for personnel to be closer than a couple of miles to a rocket launch; that would be a problem for the aircraft's crew.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That said, the principle isn't impossible for smaller vehicles, as demonstrated by the &lt;a href=&quot;http://en.wikipedia.org/wiki/Pegasus_(rocket)&quot;&gt;Orbital Sciences Pegasus&lt;/a&gt; and the proposed &lt;a href=&quot;http://en.wikipedia.org/wiki/Stratolaunch_Systems&quot;&gt;Stratolaunch&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-10-06T05:37:32.297" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5533" PostTypeId="2" ParentId="5531" CreationDate="2014-10-06T11:38:29.800" Score="21" Body="&lt;p&gt;Air launch does not actually provide that much benefit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The benefits are basically, that starting at 40 or 50,000 feet allows the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The nozzle can be closer to vacuum optimized on the first stage which is good.  &lt;/li&gt;&#xA;&lt;li&gt;The launcher can fly to the equator, for a 0 degree inclination launch which is useful.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But as noted, the mass limits on the booster are huge, and rockets are much bigger than any aircraft in existence can lift. Look at the size of Stratolaunch's proposed airplane. It will literally be the biggest airplane in existence and can only lift a smaller than Falcon 9 booster due to mass limits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reality is that launching from the ground gets the rocket up to the 50,000 foot mark within the first minute or two.  The issue with space flight is not getting high enough, rather it is going fast enough to get into orbit.  So rockets typically fly mostly straight up to get out of the 'optimum' amount of atmosphere before turning to accelerate into an orbital path. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus airlaunch seems like a good idea, but its limits and minor benefits generally do not pay.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pegasus was using a fairly powerful launch aircraft (L-1011) but only had a miniscule payload.  Stratolauncher will use a huge launch aircraft, possibly at the limits of how big one can be, and will still have a fairly small payload. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-10-06T11:38:29.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5534" PostTypeId="2" ParentId="5531" CreationDate="2014-10-06T12:55:45.590" Score="13" Body="&lt;p&gt;As noted on the wikipedia page on air launch to orbit, &lt;strong&gt;This article has multiple issues&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One issue is that it compares a vehicle launched from an aircraft versus a similar vehicle launched vertically from the ground. That's not a good comparison. A better comparison would be to look at the aircraft that carries the rocket as a substitute for the first stage. In this comparison, air launch falls far short. A decent first stage will bring the rocket close to the top of the sensible atmosphere and provide a reasonable fraction of the total delta V. A good target is 50 to 80 kilometers altitude and 2 to 3 km/second velocity. Most air launch proposals involve a large subsonic aircraft that might reach the top of the troposphere, or 9 to 15 kilometers altitude and a velocity of 200 to 300 meters/second. Air launch makes for a very lousy first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another issue is limitations that the air launch places on the rocket. For ground-based launches, various economies of scale and the cube-square law in general mean that bigger is better. Cost per kilogram of payload to orbit decreases as launch vehicle size increases. There is a point at which diseconomies of scale and nonlinearities (e.g., pogo) start to kick in, meaning that there is some point after which bigger is not better, but that point is well beyond the capability of any planned or envisioned air launch system.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-10-06T12:55:45.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5535" PostTypeId="1" CreationDate="2014-10-06T13:39:34.843" Score="1" ViewCount="1898" Body="&lt;p&gt;For a school project I have to find coordinates using the TLE information. Does anyone know how to do this or know a program that can do this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks in advance&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. I know there is a question just like this on here but it is not very clear to me yet.&lt;/p&gt;&#xA;" OwnerUserId="6099" LastEditorUserId="415" LastEditDate="2014-10-07T12:20:27.770" LastActivityDate="2014-10-07T12:20:27.770" Title="How to find coordinates based on TLE information?" Tags="|artificial-satellite|" AnswerCount="1" CommentCount="2" ClosedDate="2014-10-07T12:20:12.953" ContentLicense="CC BY-SA 3.0" />
  <row Id="5536" PostTypeId="2" ParentId="5535" CreationDate="2014-10-06T14:48:08.063" Score="2" Body="&lt;p&gt;You need to use a piece of software called SGP4 (Simple General Perturbations algorithm 4).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Simplified_perturbations_models&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Simplified_perturbations_models&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given a TLE (Two Line Element) this software will give you the ECI (Earth Centered Inertial) position and velocity coordinates of the object as a function of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are alot of different implementations out there but you need to have some programming expertise to use them directly.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastActivityDate="2014-10-06T14:48:08.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5537" PostTypeId="1" CreationDate="2014-10-06T15:04:24.420" Score="6" ViewCount="227" Body="&lt;p&gt;What is the intensity of radio noise around Jupiter or Saturn on various frequencies compared to Earth? Does it significantly affect bandwidth for communication of satellites around Jupiter/Saturn with Earth? If yes, how is it solved?&lt;/p&gt;&#xA;" OwnerUserId="6213" LastActivityDate="2015-07-22T14:22:32.827" Title="What is the intensity of radio noise around Jupiter and Saturn?" Tags="|communication|jupiter|saturn|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5538" PostTypeId="1" CreationDate="2014-10-06T15:24:09.717" Score="6" ViewCount="681" Body="&lt;p&gt;The CCtCAP customers have to do abort tests to satisfy the NASA Human rating standards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is Orion going to do a pad abort, and a Max-Q abort, like SpaceX is required to do?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is Orion being held to a different standard, since it is owned by NASA?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the cost of an SLS booster is on the order of a billion dollars or two (launching once every 2-4 years, it cannot cost any less realistically just in standing army salaries), you can imagine why they would bend any rule to avoid spending that much money on a test. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot imagine an Orion capsule is 'cheap' to 'waste' on an abort test either.  But that sounds like an excuse, or else a massive failure, if CCtCAP is cheap enough to be willing to waste boosters and vehicles on tests, vs SLS where it is to valuable to test failure cases.  Sort of like the Shuttle...&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6537" LastEditDate="2018-01-10T01:38:27.817" LastActivityDate="2018-01-10T01:38:27.817" Title="Did Orion do a pad abort test? A live fire abort test?" Tags="|orion-spacecraft|human-rating|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5539" PostTypeId="2" ParentId="5531" CreationDate="2014-10-06T15:29:27.573" Score="50" Body="&lt;p&gt;This post highlights some misconceptions, so let's do this with big letters.&lt;/p&gt;&#xA;&lt;h1&gt;Launch is about going fast, not high&lt;/h1&gt;&#xA;&lt;p&gt;A &lt;a href=&quot;http://en.wikipedia.org/wiki/Flight_altitude_record#Balloons&quot; rel=&quot;noreferrer&quot;&gt;helium balloon will get you to the edge of space&lt;/a&gt;. It takes a large and expensive rocket to get into orbit. Ok, one &lt;a href=&quot;http://en.wikipedia.org/wiki/Falcon_1&quot; rel=&quot;noreferrer&quot;&gt;at least 20 m long&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;h1&gt;Very very fast&lt;/h1&gt;&#xA;&lt;p&gt;&amp;quot;But the plane can give you 600 mph&amp;quot; you say. Well, that's nice, we have 268 m/s of the required 7 km/s. Only 6,700 m/s to go.&lt;/p&gt;&#xA;&lt;h1&gt;Actually, what I mean is attaining a high KE&lt;/h1&gt;&#xA;&lt;p&gt;1 kg going 268 m/s has &lt;span class=&quot;math-container&quot;&gt;$0.5*1*268^2=36$&lt;/span&gt; kJ of KE. That leaves &lt;span class=&quot;math-container&quot;&gt;$0.5*1*(7,000-268)^2 = 22.7$&lt;/span&gt; MJ to go; all that plane stuff has achieved about 7.5% of the needed energy transfer. [Edited with relative numbers, thanks @NPSF3000]&lt;/p&gt;&#xA;&lt;h1&gt;How about a nuclear explosion in a barrel, then?&lt;/h1&gt;&#xA;&lt;p&gt;Now you're talking. Like a &lt;a href=&quot;http://savvyparanoia.com/the-fastest-man-made-object-ever-a-nuclear-powered-manhole-cover-true/&quot; rel=&quot;noreferrer&quot;&gt;nuclear pop gun blowing a disc of steel up to 70,000 m/s&lt;/a&gt;. Kinda. There's a whole wikipedia article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_gun&quot; rel=&quot;noreferrer&quot;&gt;space guns&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;h1&gt;Save some money, use a rocket you have sitting around at home&lt;/h1&gt;&#xA;&lt;p&gt;The way to really cheap launches is to use &lt;a href=&quot;http://en.wikipedia.org/wiki/Intercontinental_ballistic_missile&quot; rel=&quot;noreferrer&quot;&gt;one of those spare enormous orbital vehicles you have lying about the place&lt;/a&gt;. Particularly &lt;a href=&quot;http://en.wikipedia.org/wiki/R-36_(missile)#Derivatives&quot; rel=&quot;noreferrer&quot;&gt;the really big ones&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="519" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-10-08T09:02:15.887" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="5540" PostTypeId="1" AcceptedAnswerId="5543" CreationDate="2014-10-06T17:19:16.593" Score="14" ViewCount="3519" Body="&lt;p&gt;Obviously, with no humans living permanently off-planet yet, Earth-style dates and times are the most common system is use, and reasonably so.  And it helps that Mars's day is almost the same length as Earth's, so even Martian colonists could use roughly a 24-hour day.  (I was always charmed by the idea in the Mars Trilogy of simply stopping the clocks at midnight for a 38-minute &quot;witching hour,&quot; a sort of half-hour Mardi Gras every night, which would become a bit of local cultural flavor for people visiting Mars).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But eventually, when substantial numbers of people are living on the Moon and on Mars and on Venus and on ships in between, etc, a 24-hour day and 365-day year will be an increasingly irrelevant timescale.  There will probably need to be a &quot;stardate&quot; or some other interplanetary date/time system that is standardized across all planetary bodies, and even later, all solar systems.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few systems exist today that could work, such as Unix timestamps, but often these measure such small increments (like seconds) that they are better for use by computers than by people.  But at some point, a person on Venus will need to say to a person on Titan, &quot;it happened back in February,&quot; and I can't imagine that learning the complete calendars of every planetary body will be the most practical option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any effort, by NASA or SpaceX or any organization with some kind of practical authority, to establish such a system for tracking long-term times and dates in space?  Some interplanetary equivalent of Greenwich Mean Time or Zulu time, for dates as well as hours?&lt;/p&gt;&#xA;" OwnerUserId="1216" LastActivityDate="2018-01-30T03:04:48.803" Title="Has an interplanetary date/time system been established yet?" Tags="|interplanetary|time|measurement|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5541" PostTypeId="1" CreationDate="2014-10-06T17:37:23.553" Score="2" ViewCount="57" Body="&lt;p&gt;Why did Curiosity require a skycrane? It couldnt use a bouncy ball like spirit since it was too big, but why couldnt it land like the viking?&lt;/p&gt;&#xA;" OwnerUserId="6233" LastActivityDate="2014-10-06T17:37:23.553" Title="What's the advantage of a skycrane system?" Tags="|skycrane|" AnswerCount="0" CommentCount="1" ClosedDate="2014-10-06T18:54:59.137" ContentLicense="CC BY-SA 3.0" />
  <row Id="5542" PostTypeId="2" ParentId="5540" CreationDate="2014-10-06T17:52:18.197" Score="8" Body="&lt;p&gt;In contexts in which there are virtually little relativistic effects, the current standard &lt;a href=&quot;http://en.wikipedia.org/wiki/Coordinated_Universal_Time&quot; rel=&quot;noreferrer&quot;&gt;Coordinated Universal Time (UTC)&lt;/a&gt; would apply. As long as communication could be established with Earth, times could certainly be synchronized. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In relativistic contexts, it doesn't make much sense to try to synchronize time. Instead, just give the time in that frame of reference. To someone in another frame of reference, they can compute what the time means to them based on the effects of relativity.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2014-10-08T13:27:44.787" LastActivityDate="2014-10-08T13:27:44.787" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5543" PostTypeId="2" ParentId="5540" CreationDate="2014-10-06T18:28:46.033" Score="6" Body="&lt;p&gt;For the vast billions of people living in a relatively natural environment, our &lt;a href=&quot;http://en.wikipedia.org/wiki/Daylight_saving_time_by_country&quot; rel=&quot;noreferrer&quot;&gt;already insanely convoluted&lt;/a&gt; time systems in use on Earth are reasonably appropriate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the foreseeable future, all our colonists on the moon and Mars will be living indoors almost all the time, in bodies adapted to 24-hour cycles, and for every one of them there will be many more people on Earth working with them -- dozens or hundreds. It makes no sense for them to use any time system other than that of their ground controllers -- either UTC or local time for the mission control site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rather than converting between time systems, colonists on other bodies merely need to give up their assumptions that the sun comes up at the same time every day (which it doesn't anyway on Earth, because seasons), that the seasons change the same way every year (which they don't anyway on Earth, because hemispheres), and the like. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anyone came to me with a suggestion for a new interplanetary time system, I would refer them to &lt;a href=&quot;http://xkcd.com/927/&quot; rel=&quot;noreferrer&quot;&gt;this comic&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-10-06T18:28:46.033" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5544" PostTypeId="2" ParentId="5538" CreationDate="2014-10-06T22:45:59.867" Score="5" Body="&lt;p&gt;Orion has already had a pad abort test back when it was part of the Constellation program, in 2010.  Pad Abort 1 has its own &lt;a href=&quot;http://en.wikipedia.org/wiki/Pad_Abort_1_%28Orion%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia entry&lt;/a&gt;, strangely enough.  Pad Abort 1 test was a test of the tractor-style launch abort system (which pulls the capsule with a small rocket mounted on top, a la Apollo).  There was a proposed alternate launch abort system, called &lt;a href=&quot;http://en.wikipedia.org/wiki/Max_Launch_Abort_System&quot; rel=&quot;nofollow&quot;&gt;Max Launch Abort System&lt;/a&gt; (MLAS) (see the Wikipedia entry), a prototype of which was tested in 2009, that was meant to perform better at Max Q, but the whole thing was later dropped.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, according to &lt;a href=&quot;http://www.parabolicarc.com/2014/02/11/atk-upgrade-space-floridas-launch-complex-46/&quot; rel=&quot;nofollow&quot;&gt;Parabolic Arc magazine&lt;/a&gt;, &quot;NASA intends to launch its Orion Multi Purpose Crew Vehicle Ascent Abort 2 test flight (AA‑2) from SLC-46 in 2018.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflightinsider.com/organizations/nasa/nasa-official-talks-ascent-abort-2/&quot; rel=&quot;nofollow&quot;&gt;Info on AA-2.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, the short answer is, Orion has already had a pad abort test, and NASA seems to be planning an actual (mid-flight) ascent abort test in 2018, prior to the first planned manned Orion flight sometime after 2020.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6199" LastEditorUserId="51" LastEditDate="2014-10-06T22:53:44.340" LastActivityDate="2014-10-06T22:53:44.340" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5545" PostTypeId="2" ParentId="5540" CreationDate="2014-10-06T23:14:08.143" Score="2" Body="&lt;p&gt;Several special-purpose time systems are in use that account for both general and special relativistic effects.  These were developed by necessity by the IAU following on from work done by the JPL and other operators of spacecraft in deep space, and astronomers working with planetary orbits to high precisions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See: &lt;a href=&quot;http://en.wikipedia.org/wiki/Barycentric_Coordinate_Time&quot; rel=&quot;nofollow&quot;&gt;Barycentric Coordinate Time&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Ephemeris_time&quot; rel=&quot;nofollow&quot;&gt;Ephemeris Time&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Geocentric_Coordinate_Time&quot; rel=&quot;nofollow&quot;&gt;Geocentric Coordinate Time&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Barycentric_Dynamical_Time&quot; rel=&quot;nofollow&quot;&gt;Barycentric Dynamical Time&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Terrestrial_Time&quot; rel=&quot;nofollow&quot;&gt;Terrestrial Time&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some briefer definitions and formulae relating the time systems here: &lt;a href=&quot;http://tycho.usno.navy.mil/systime.html&quot; rel=&quot;nofollow&quot;&gt;http://tycho.usno.navy.mil/systime.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-10-06T23:14:08.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5546" PostTypeId="1" AcceptedAnswerId="5548" CreationDate="2014-10-07T00:49:32.643" Score="1" ViewCount="556" Body="&lt;p&gt;We're fortunate to live in an era where there are so many advances being made in aerospace engineering. There are a lot of new companies creating new machines to get to space. However, a lot of them are of the SSTO/spaceplane variety: &lt;a href=&quot;https://en.wikipedia.org/wiki/Dream_Chaser&quot; rel=&quot;nofollow&quot;&gt;Dream Chaser&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylon_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Skylon&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipOne&quot; rel=&quot;nofollow&quot;&gt;SpaceShipOne and variants&lt;/a&gt;, the &lt;a href=&quot;https://en.wikipedia.org/wiki/XCOR_Lynx&quot; rel=&quot;nofollow&quot;&gt;Lynx&lt;/a&gt;, . . . You get the idea. Yet there aren't a lot of rockets being privately developed, with Elon Musk and SpaceX leading the charge with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_(rocket_family)&quot; rel=&quot;nofollow&quot;&gt;Falcon&lt;/a&gt; family. Does this pattern indicate a trend towards spaceplanes and away from rockets?&lt;/p&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2014-10-18T21:52:05.807" Title="Are today's rockets the last of a dying breed?" Tags="|rockets|spaceplane|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5548" PostTypeId="2" ParentId="5546" CreationDate="2014-10-07T04:45:24.247" Score="8" Body="&lt;p&gt;I don't think there's any sort of trend here; of the spaceplanes you mention, one exists only on paper, and none of the others are SSTO: they're all suborbital and mostly multistage. They're spaceships, not launchers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of them can do anything near what Atlas 5 or Falcon 9 are routinely doing today. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's not a lot of need for development of new conventional launchers, because we have a fair selection of proven reliable designs, and no obvious way to make dramatic performance improvements to them.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-10-07T04:45:24.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5549" PostTypeId="2" ParentId="5546" CreationDate="2014-10-07T10:59:40.233" Score="4" Body="&lt;p&gt;Consider bullets. Gunpowder has changed a little since it was invented, but the bullet being propelled has changed significantly, in shape and material and advances like explosive tips. So it is with spaceplanes and suborbital vehicles - the rockets are largely the huge explosive that gives you lots of kinetic energy. Making a rocket that doesn't explode in construction, on the launchpad, in the launch sequence, on takeoff or in motion is difficult, and then there's not really much improvement to make after that. Messing about with the payload has much more capacity for variation; spaceplanes, satellites, space stations, weapons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets are brutally constrained by the combination of chemistry and the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow&quot;&gt;Tyranny Of The Rocket Equation&lt;/a&gt;. The rest is just minor tweaks and optimisations. Multi-stage launch vehicles were perhaps the greatest advance in rocket science, and people are always trying to put that genie back in the bottle, asking for fully reusable vehicles, smaller vehicles, greener fuels, cheaper launches. But without new chemistry or an entirely new engine based on nuclear power, it's all doomed to repeat the lessons of the 20th century.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, give up on carrying the energy source on the launch vehicle, which is the source of tyranny for chemical rockets. Perhaps the best candidate is &lt;a href=&quot;http://en.wikipedia.org/wiki/Beam-powered_propulsion&quot; rel=&quot;nofollow&quot;&gt;beam-powered propulsion&lt;/a&gt;, which quotes a rough number of 1 MW per kg payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;TLDR: Rockets are big, expensive and there's not much to do to improve on what exists after decades of military research. The only option for significant improvement is to replace the entire technology with nuclear or beam-powered propulsion.&lt;/p&gt;&#xA;" OwnerUserId="519" LastActivityDate="2014-10-07T10:59:40.233" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5551" PostTypeId="1" AcceptedAnswerId="5553" CreationDate="2014-10-07T20:20:38.640" Score="19" ViewCount="1279" Body="&lt;p&gt;Can we use GPS in orbit at the moon? What about on Mars? &lt;/p&gt;&#xA;" OwnerUserId="6246" LastActivityDate="2020-03-02T13:32:21.057" Title="What is the deepest position in space we can get a GPS signal?" Tags="|mars|the-moon|navigation|gps|" AnswerCount="5" CommentCount="2" ClosedDate="2021-08-07T04:42:28.423" ContentLicense="CC BY-SA 3.0" />
  <row Id="5552" PostTypeId="2" ParentId="5551" CreationDate="2014-10-07T20:41:15.097" Score="2" Body="&lt;p&gt;While I can't fully address this I can give part of an answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're looking at 4 satellites there are actually two possible solutions to the equations.  Normally you can reject one because it's far out from the Earth is on or very near the Earth's surface.  If you try to use it out in deep space you can't make this assumption and thus a fix will require five satellites, not four.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, the farther from Earth you get the more closely grouped the satellites will become from your vantage point and thus the more inaccurate the fix.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll leave it to others to determine how far away you can get before the signal is too weak and if the satellites themselves have antennas that direct their energy earthward (which they very well might.  A directional antenna cuts the power needed for the transmitter and thus the area of the solar cells needed to power it.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that most civilian units do not function in space, period.  This is a design decision, not a technical limitation--it keeps some black hat from using one to guide their missile.  Some of them shut down at altitudes low enough to cause problems for people sending balloons to the edge of space.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-10-07T20:41:15.097" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5553" PostTypeId="2" ParentId="5551" CreationDate="2014-10-07T21:19:56.107" Score="22" Body="&lt;p&gt;GPS is regularly used in low earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible to use the signals from the sidelobes of the GPS transmit antennas to receive service in a high earth orbit up to and potentially beyond geosynchronous altitude:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7dtF4.png&quot; alt=&quot;sidelobes&quot;&gt; (Image credit: GPS World)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it is challenging because the signals are substantially weaker and coverage is intermittent.  &lt;a href=&quot;http://www.emergentspace.com/assets/1/7/SSVP-v1.07-2column.pdf&quot; rel=&quot;noreferrer&quot;&gt;These&lt;/a&gt; &lt;a href=&quot;http://spacejournal.ohio.edu/issue9/pdf/geosynchronous.pdf&quot; rel=&quot;noreferrer&quot;&gt;papers&lt;/a&gt; and &lt;a href=&quot;http://www.gps.gov/multimedia/presentations/2010/01/bangkok/miller.pdf&quot; rel=&quot;noreferrer&quot;&gt;this presentation&lt;/a&gt; discuss the concept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GPS navigation has been demonstrated from HEO during an extension of the &lt;a href=&quot;http://gpsworld.com/giove-a-uses-gps-side-lobe-signals-for-far-out-space-navigation/&quot; rel=&quot;noreferrer&quot;&gt;GIOVE-A&lt;/a&gt; mission.  I haven't been able to find any reports of attempted reception at geosynchronous altitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The moon would be a real stretch (though it has been &lt;a href=&quot;http://m.ion.org/abstract.cfm?paperID=1856&quot; rel=&quot;noreferrer&quot;&gt;proposed and simulated&lt;/a&gt;), and reception at Mars is completely out of the question.  However, it should be noted that navigation of deep space probes including those at Mars is made with similar fundamental techniques to GPS (i.e. autocorrelation of long pseudorandom bit sequences).&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-10-07T21:19:56.107" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5555" PostTypeId="1" AcceptedAnswerId="5571" CreationDate="2014-10-08T02:54:05.550" Score="4" ViewCount="1210" Body="&lt;p&gt;&lt;strong&gt;Background&lt;/strong&gt;: While watching &lt;a href=&quot;http://short.arc.nasa.gov/sservidss-glotch&quot; rel=&quot;nofollow&quot;&gt;SSERVI Director’s Seminar Series: RIS4E&lt;/a&gt; (Science and Exploration of the Moon, Near Earth Asteroids, the Moons of Mars enabled by the Remote In Situ and Synchrotron Studies for Science and Exploration), more info &lt;a href=&quot;http://sservi.nasa.gov/event/science-and-exploration-of-the-moon-near-earth-asteroids-the-moons-of-mars-enabled-by-the-remote-in-situ-and-synchrotron-studies-for-science-and-exploration-ris4e-sservi-team/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, speaker Tim Glotch mentioned (roughly 26 minutes into the presentation) that they used kites and might use drones in the future to perform &lt;a href=&quot;https://en.wikipedia.org/wiki/Lidar&quot; rel=&quot;nofollow&quot;&gt;LiDAR&lt;/a&gt; scans of pits and otherwise untraversable terrain during their simulated lunar analog environment tests at Hawaii.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, as also mentioned by Tim Glotch in presentation, kites and conventionally propelled drones using aerodynamic lift wouldn't work on the Moon since there's no atmospheric pressure to speak of, but this got me thinking that future &lt;em&gt;foot on the ground&lt;/em&gt; activities on the Moon might use some way of hovering communications and science equipment higher above ground level, say to support various in situ ground operations and communicate with a distant or obstructed from direct line of sight base. One idea I had could be a type of a suction tube that collects surface dust at its &lt;em&gt;head&lt;/em&gt; end touching the ground, sucking dust towards its &lt;em&gt;tail&lt;/em&gt; end, where the exhaust is redirected towards the ground to achieve thrust and enable it to hover at some distance to the ground. Wild? Perhaps, but similar works here on the Earth at six times the gravity too by using jets of water to propel &lt;a href=&quot;http://www.youtube.com/watch?v=RS6EVBr3azM&quot; rel=&quot;nofollow&quot;&gt;aquatic hoverboards&lt;/a&gt;. Add a spatially aware controller software, a few high precision accelerometers and a gimballed nozzle to the mix, and you get an autonomously hovering structure that could suspend communications antennae and/or remote sensing equipment above the lunar ground by the pressure of its dust, and be portable and easily erectable / collapsible wherever needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To the question&lt;/strong&gt;: Wild ideas aside (and yes, there are a few other problems with it I didn't yet manage to address like that it would stir a lot of dust unless it's a highly efficient and precise recycler / rebreather system), one thing I'm rather unsure of is how does one design the &lt;em&gt;suction head&lt;/em&gt; so it starts working in a nearly no atmospheric pressure environment and with no surface tension to speak of? And once such a system would start, would the lunar dust itself, now hopefully an aerosol inside the tube, provide sufficient surface tension, or would it clog the suction tube if no additional gas is continuously supplied to such a system and mixed with the dust intake?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what is worth, this is not only relevant to my imagination gone wild, but will surely have to be addressed to run, say, mass separation systems for lunar regolith &lt;a href=&quot;https://en.wikipedia.org/wiki/In_situ_resource_utilization&quot; rel=&quot;nofollow&quot;&gt;ISRU&lt;/a&gt; (In Situ Resource Utilisation), removal of sharp silicate dust from breathable air environments, and if this works out well enough, perhaps even hovercraft as means of (safe and durable) transport on the sharp and abrasive lunar regolith surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Has any research institute, groups or inventive individuals published papers or patents on designing industrial continuous suction systems for use in exospheric conditions, preferably with tests done on lunar regolith simulant, and what were their conclusions / results?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the lunar dust analog to additionally supplied gas mass ratio that could start and sustain suction and what is the expected durability and most frequent failure points of such equipment? Also, has anything like it, on a smaller scale, perhaps been done / tested by any of the Apollo missions?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt; - Yes, I'm basically asking how does one design a vacuum cleaner that would work in vacuum. Well, kinda. Read the question. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-08T03:11:20.350" LastActivityDate="2014-10-09T14:27:15.730" Title="&quot;Lunar vacuum cleaner&quot;: Design considerations for regolith dust suction system in exospheric conditions" Tags="|the-moon|dust|isru|exosphere|space-industry|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5557" PostTypeId="2" ParentId="5555" CreationDate="2014-10-08T11:11:30.660" Score="4" Body="&lt;p&gt;You cannot vacuum in a vacuum, if you want to use a pressure differential to move dust on the moon you will have to supply the gas yourself so it's no longer a vacuum. You could set up a system which releases gas which is then sucked up, however I doubt it's a viable proposition because you won't be able to match the vacuum power of space itself. Having the gas in the first place is a big problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One possible solution would be to build some sort of structure which when set down and anchored to a flat enough part of the lunar surface would be able to create a seal strong enough for partial atmospheric pressure to be established. Once atmosphere is pumped in you can then vacuum and collect the dust, then pump the atmosphere back out again. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems overly complicated and has many challenges compared to simply creating an efficient scooping mechanism. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-10-08T11:11:30.660" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5561" PostTypeId="1" CreationDate="2014-10-08T14:43:08.543" Score="20" ViewCount="3890" Body="&lt;p&gt;The Earth would have a thin red ring around it, where sunlight is scattered by the atmosphere, right? Would the corona of the sun be very visible? &lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2019-01-04T09:53:17.200" Title="What would a lunar eclipse look like from the surface of the moon?" Tags="|the-moon|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5562" PostTypeId="2" ParentId="5561" CreationDate="2014-10-08T15:20:15.000" Score="17" Body="&lt;p&gt;NASA have already produced their impression of what it is likely to look like. &lt;a href=&quot;http://sciencenotes.org/tag/lunar-eclipse/&quot; rel=&quot;noreferrer&quot;&gt;Sciencenotes.org&lt;/a&gt; has this picture from NASA's Scientific Visualization Studio&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6ZGYL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if you want to see a full animation, &lt;a href=&quot;http://svs.gsfc.nasa.gov/vis/a000000/a004100/a004157/index.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Scientific Visualization Studio also has this&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From their work it appears the corona &lt;strong&gt;would&lt;/strong&gt; be very visible as with the Earth blocking out all direct light, the eye will be able to make out the faint corona. It is, after all, the principle by which various solar observatories work. The red ring around the Earth also should be obvious - it is the sun's light being refracted through the Earth's atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2014-10-08T19:55:47.473" LastActivityDate="2014-10-08T19:55:47.473" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5563" PostTypeId="2" ParentId="5561" CreationDate="2014-10-08T15:40:00.627" Score="4" Body="&lt;p&gt;I did find this artist's interpretation and a discussion at &lt;a href=&quot;http://en.es-static.us/upl/2011/06/lunar_eclipse_from_moon_550.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earthsky&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rcDt4.jpg&quot; alt=&quot;Eclipse of the moon as seen from the lunar surface&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-10-08T15:40:00.627" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5566" PostTypeId="2" ParentId="5561" CreationDate="2014-10-08T19:33:10.560" Score="23" Body="&lt;p&gt;It would look a lot like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bt6nF.jpg&quot; alt=&quot;kayuga probe eclipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These images were taken by the &lt;a href=&quot;http://www.kaguya.jaxa.jp/index_e.htm&quot; rel=&quot;noreferrer&quot;&gt;SELENE (Kaguya) probe&lt;/a&gt; in Feb 2009 during a penumbral eclipse. They were not taken from the surface, but from a 50km orbit. Because of the negligible atmosphere on the moon, I assume there wouldn't be much difference. You can find other images in &lt;a href=&quot;http://wms1.wms.selene.darts.isas.jaxa.jp/selene_viewer/en/observation_mission/2009/02/&quot; rel=&quot;noreferrer&quot;&gt;their gallery&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also see this in video form, both on the &lt;a href=&quot;http://wms.selene.darts.isas.jaxa.jp/selene_viewer/en/observation_mission/hdtv/059/diamond_ring_earth.html&quot; rel=&quot;noreferrer&quot;&gt;JAXA site&lt;/a&gt; and &lt;a href=&quot;https://www.youtube.com/watch?v=Zq0ptByQrcg&quot; rel=&quot;noreferrer&quot;&gt;YouTube&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Zq0ptByQrcg?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6257" LastEditorUserId="10273" LastEditDate="2019-01-04T09:53:17.200" LastActivityDate="2019-01-04T09:53:17.200" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="5568" PostTypeId="1" AcceptedAnswerId="5584" CreationDate="2014-10-09T08:15:07.300" Score="26" ViewCount="6559" Body="&lt;p&gt;How can Yuri Gagarin's Vostok 1 flight be considered to have been orbital, since he did not go all the way around Earth? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the Wikipedia article about Vostok 1, the launch site was near Tyuratam at 45°55′12.72″N 63°20′32.32″E, and the landing site was at 51.270682°N 45.99727°E.  Other online sites are in general agreement with these locations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So he landed 17 degrees west of the launch site. Sure sounds like a (long) &lt;strong&gt;sub&lt;/strong&gt;-orbital flight.&lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2018-08-27T23:36:14.233" Title="How can Yuri Gagarin's Vostok 1 flight be considered to have been orbital, since he did not go all the way around Earth?" Tags="|crewed-spaceflight|history|sub-orbital|orbital|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5569" PostTypeId="2" ParentId="5568" CreationDate="2014-10-09T08:57:57.350" Score="42" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia's article on Vostok 1&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/1/13/Vostok_1_orbit.png&quot; alt=&quot;flightpath of Vostok 1&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Path of Gagarin's complete orbit; the landing point is west of the&#xA;  takeoff point because of the eastward rotation of the Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The flight was at least one orbit of the Earth (note the latitude of the landing is higher than the launch, &lt;a href=&quot;https://space.stackexchange.com/a/5574/519&quot;&gt;see @DavidHammen's answer&lt;/a&gt;), and beyond that the mission involved firing retrorockets to leave orbit, so not only was it an orbit, the craft did attain orbital speed. Without retrorockets firing to reduce its orbital speed, it would complete many orbits around the Earth before its orbit would naturally decay due to atmospheric drag. The accomplishment of getting a human into orbit, round the Earth and back to the ground safely was there.&lt;/p&gt;&#xA;" OwnerUserId="519" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-10-10T10:07:23.323" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="5570" PostTypeId="2" ParentId="5540" CreationDate="2014-10-09T13:26:09.140" Score="1" Body="&lt;p&gt;You need to distinguish between time and calendar. Establishing a universal time is pretty easy, you just need an epoch and count intervals from there. Establishing a universal calendar on the other hand is pretty unlikely and is quite likely to be unnecessary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Converting between time scales is fairly easy, just change the interval scale and the epoch (the formula is the same as the one you use for converting from Fahrenheit to Celsius just with different constants) then adjust for relativistic effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Converting between calendars, on the other hand, requires extensive knowledge of local historical politics. It is highly unlikely that the politics of earth would be of interest to a civilisation of regular space dwellers.&lt;/p&gt;&#xA;" OwnerUserId="227" LastActivityDate="2014-10-09T13:26:09.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5571" PostTypeId="2" ParentId="5555" CreationDate="2014-10-09T14:27:15.730" Score="3" Body="&lt;p&gt;Have a look at &lt;a href=&quot;http://sci.esa.int/science-e/www/object/doc.cfm?fobjectid=41723&quot; rel=&quot;nofollow&quot;&gt;this paper&lt;/a&gt;, which discusses using magnets to collect the regolith in tubes similar to a maglev system. It also claims that the very fine dust particles can remain electrostatically suspended above the surface, forming an abrasive pseudo-atmosphere. The lunar hoverboard would not be very popular with your fellow lunar residents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fortunately it seems to be easily meltable with microwaves. So roadbuilding might be as simple as a bulldozer followed by a downward-pointing magnetron.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, if I wanted a liquid-phase seal that I could feed incoming material through without having to cycle an airlock, I'd consider a column of mercury in a &quot;U&quot; shape. You will eventually lose mercury to vapor evaporation on the vacuum side, but very slowly.&lt;/p&gt;&#xA;" OwnerUserId="1521" LastActivityDate="2014-10-09T14:27:15.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5572" PostTypeId="2" ParentId="4095" CreationDate="2014-10-09T16:03:43.177" Score="5" Body="&lt;p&gt;Goddard's early liquid-fueled rockets starting in 1926 did use gasoline (petrol) as the fuel, because it was easy to obtain. RP-1 kerosene is just an optimization - cleaner and somewhat more efficient.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-10-09T16:03:43.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5573" PostTypeId="2" ParentId="4034" CreationDate="2014-10-09T19:48:39.323" Score="6" Body="&lt;p&gt;Here is an example photo of the ISS passing overhead near sunset in Tübingen, Germany:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IHWl9.jpg&quot; alt=&quot;super wide angle multiple exposure shot of ISS overhead&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The line is dashed because it is a long series of exposures that were then stitched together. &#xA;Here is a link to a &lt;a href=&quot;http://www.allthesky.de/movies/index.html?iss07feb08v.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; of the same event. Credit: &lt;a href=&quot;http://www.allthesky.com/various/iss07feb08.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;© T. Credner &amp;amp; S. Kohle, AlltheSky.com&lt;/a&gt; (use for private and non-profit educational purpose only).&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2014-10-09T20:04:49.277" LastActivityDate="2014-10-09T20:04:49.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5574" PostTypeId="2" ParentId="5568" CreationDate="2014-10-09T19:54:49.347" Score="24" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;So he landed 17 degrees west of the launch site. Sure sounds like a (long) sub-orbital flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You are looking at the wrong parameter. The Earth rotated underneath the orbiting spacecraft during the 108 minute flight. A better parameter to look at is latitude.  He launched to the northeast and landed 5.35 degrees north of the launch site. He passed the latitude of the launch site and then kept going for a bit. From launch to landing, his flight covered more than one orbit.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-10-09T19:54:49.347" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5575" PostTypeId="1" AcceptedAnswerId="5577" CreationDate="2014-10-09T22:08:20.613" Score="5" ViewCount="874" Body="&lt;p&gt;I saw &lt;a href=&quot;https://space.stackexchange.com/users/6214/david-ratti&quot;&gt;David Ratti&lt;/a&gt;'s question about &lt;a href=&quot;https://space.stackexchange.com/q/5568/2854&quot;&gt;Yuri Gagarin's voyage on Vostak 1 and how it was considered orbital, and that it was considered the first.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering what constitutes one full trip around the Earth, it was only considered &quot;orbital&quot; because he reached a distance and speed where he could have stayed in &quot;orbit&quot; around the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is one full Orbit of the Earth Measured?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Point of Reference from Earth or Point of Reference from a stationary orbit plotted around the Earth that is not affected by it's rotation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;I am probably using the wrong terminology, I apologize.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="2854" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-10-10T17:05:18.150" Title="What is one full orbit?" Tags="|orbit|terminology|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5576" PostTypeId="2" ParentId="5575" CreationDate="2014-10-09T23:03:43.927" Score="2" Body="&lt;p&gt;One approach is to consider the orbit in an inertial frame, disregarding the position with respect to the surface of the Earth (and the Earth's rotation).&#xA;Then the period is &#xA;$$&#xA;T = 2 \pi \sqrt{\frac{a^3}{\mu}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $T$ is the period, $a$ is the semimajor axis and $\mu$ is the gravitational parameter (the universal gravitational constant multiplied by the mass of the central body).&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-10-09T23:03:43.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5577" PostTypeId="2" ParentId="5575" CreationDate="2014-10-09T23:19:34.830" Score="13" Body="&lt;p&gt;In the case of a Keplerian orbit, an orbit is a 360 degree change in true anomaly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose a spacecraft in low Earth orbit (LEO) passes directly over some point on the Earth. Where is that point relative to the spacecraft one orbit later? I'll use 90 minutes, or 1/16 days, as the orbital period for our spacecraft. The Earth rotates to the east by 22.5 degrees during that 90 minute orbit. Thus the point over which the spacecraft passed one orbit ago is now 22.5 degrees to the east of the spacecraft's sub-satellite point. Alternatively, the spacecraft ground track shifts 22.5 degrees to the west per orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To illustrate this, the image below portrays the ground track for the International Space Station over two orbits. This clearly depicts that the ground track shifts by about 22.5 degrees to the west per orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/3/3f/Satellite_Orbital_Elements_ISS_Groundtrack.png&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.447" LastActivityDate="2014-10-10T14:10:50.480" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5578" PostTypeId="2" ParentId="5117" CreationDate="2014-10-10T05:30:38.823" Score="4" Body="&lt;p&gt;It also has to do with the period. At 100km if Rosetta was just in a stable gravitational orbit due it would have a period of about 90 days, much less than the two weeks required. At 30km the gravitational period is about 2 weeks so this may explain why it is now in a circular orbit.&lt;/p&gt;&#xA;" OwnerUserId="6276" LastActivityDate="2014-10-10T05:30:38.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5579" PostTypeId="1" CreationDate="2014-10-10T08:11:52.347" Score="1" ViewCount="899" Body="&lt;p&gt;In the context of possible colonization of Ceres, would not such colony be totally impossible because of high probability of collision of Ceres with other asteroids in the Asteroid Belt?&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="812" LastEditDate="2014-10-10T16:56:36.570" LastActivityDate="2014-10-12T12:29:23.863" Title="What is the danger for Ceres of collision with other asteroids?" Tags="|colonization|astronomy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5580" PostTypeId="2" ParentId="5579" CreationDate="2014-10-10T10:17:59.527" Score="6" Body="&lt;p&gt;There are a few common misconceptions about the main belt's density and relative velocities between its members that seem to cause quite many of such questions similar to yours even on our site. This seems to boil down to how asteroid belt (aka main belt, because it isn't really the only asteroid belt in the Solar system) is often visually represented in media, greatly exaggerating its density for visualisation purposes. I'd first like to invite you to read through a few answers to these related questions that discuss main belt density from various perspectives:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/231/whats-the-particle-density-of-the-asteroid-belt&quot;&gt;What&amp;#39;s the (particle) density of the asteroid belt?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3202/how-do-spacecraft-navigate-through-the-asteroid-belt-to-avoid-collision&quot;&gt;How do spacecraft navigate through the Asteroid belt to avoid collision?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1382/how-frequently-do-asteroids-collide-with-each-other&quot;&gt;How frequently do asteroids collide with each other?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You'll notice that, while the chance of a collision is never zero, it is indeed astronomically slim. But it gets better, at least as far as collisions between main belt members are concerned. You see, Ceres' mass, despite being the largest member of the main belt, is only about 1.3% that of our own Moon's mass. That means it's gravitationally a lot less attractive body, which further reduces the chance that it would gravitationally perturb orbits of other asteroids in the main belt sufficiently enough to cause them to destabilize and potentially intersect with Cerean own some orbits around the Sun later. With Cerean orbital period of 4.6 Julian years, even if that happened, it would take many, many years for such orbits to intersect and for both bodies to be at exactly the same time at exactly the same place in their orbital position for all of this to result in impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Better still, relative velocities between main belt objects are small, and all the rest of them are much less massive than Ceres, both facts substantially reducing kinetic potential of such collision events. So if we indeed colonized it, and we'd first have to have the technology to do so, we'd also have means to predict future collisions, and have sufficient time to redirect any threatening objects away from its intersecting orbit with Ceres. Do that many years in advance and it becomes a trivial matter even for today's technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: Actually, come to think of it, considering the rarity of some of the other main belt objects getting relatively close to Ceres, they could be considered a blessing rather than a curse. You get to mine them for likely much needed resources relatively cheaply in the region where there's nearly no other significant mass to be found for millions of kilometers in any direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For some numbers, refer to &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/001910359290060K&quot; rel=&quot;nofollow noreferrer&quot;&gt;Collision rates and impact velocities in the main asteroid belt&lt;/a&gt;, Paolo Farinella, Dipartimento di Matematica, Università di Pisa and Donald R. Davis, Planetary Science Institute, Tucson, Arizona paper that, while not exactly recent (came out in my college years) and is sadly paywalled, in its abstract does give some relevant numbers to our problem. Reproducing the abstract of the mentioned paper here for convenience:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We have computed, using G.W. Wetherill's (1967, J. Geophys. Res. 72,&#xA;  2429–2444) algorithm, mutual collision probabilities and impact&#xA;  velocities for a set of 682 asteroids of diameter &gt; 50 km, intended to&#xA;  represent a bias-free sample of asteroid orbits.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;For every asteroid,&#xA;  we have obtained the intrinsic collision probability, $P_i$, the average&#xA;  collision velocity, $V$, and the number of projectile orbits which can&#xA;  intersect the target asteroid's orbit, Ncross, using the proper&#xA;  orbital elements of A. Milani and Z. Knežević (1990, Celest. Mech. 49,&#xA;  247–411). The average values and the corresponding standard deviations&#xA;  for the whole asteroid sample are:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;$〈P_i〉 = 2.85 ± 0.66 \cdot 10^{−18} \text{ km}^{−2} \text{ year}^{−1}$,&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;$〈V〉 = 5.81 ± 1.88 \text{ km/sec}$, &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;$〈N_{cross}〉 = 601$ (88% of the existing orbit pairs). &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;No significant differences were found in the average&#xA;  values of $P_i$, $V$, or $N_{cross}$ using osculating elements instead of proper&#xA;  elements, although results for individual asteroids could change by&#xA;  $\approx 10\%$.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A running box mean of the intrinsic collision probability with&#xA;  semimajor axis shows a peak near $2.7 \text{ AU}$ with $〈Pi〉= 3.4 \cdot 10^{−18} \text{ km}^{−2} \text{ year}^{−1}$ &#xA;  followed by a nearly monotonic decrease to $〈Pi〉 = 1.8 \cdot 10^{−18}&#xA; \text{ km}^{−2} \text{ year}^{−1}$ at $3.3 \text{ AU}$.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Collision probabilities are nearly independent&#xA;  of eccentricities but show a significant decrease with larger&#xA;  inclinations. As expected, collisional velocities grow rapidly with&#xA;  increasing orbital eccentricities and inclinations, but they show&#xA;  surprisingly little variation across the asteroid belt.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Family asteroids undergo collisions with other members of the same family two&#xA;  to three times more frequently than with nonfamily projectiles, but&#xA;  the relative speed of these intrafamily impacts is comparatively low&#xA;  for the Koronis and Themis families.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, even generously overestimating the number of colony threatening mass asteroids in the main belt that intersect Cerean orbit, let's for the sake of argument say there's one million of them, and taking the whole Cerean surface area into account ($2,850,000 \text{ km}^2$), the average time period between two such civilisation ending events (assuming nothing would be done to thwart against it), comes out at one such event every roughly 125 thousand years. Remember, this is a rough, generous estimate and in reality should happen orders of magnitude less frequently than that (read: millions of years).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Non-civilisation ending collisions would happen a lot more frequently though. Not exactly &lt;em&gt;cloudy with a chance of meatballs&lt;/em&gt;, but in one of my calculations roughly two collisions with over one thousand metric ton (about 10 times less massive than the Chelyabinsk meteor) object per year at mentioned speeds (which are over five times smaller, so roughly 26.7 times less kinetic potential per same mass objects, than what the Earth and the Moon &lt;em&gt;scoop&lt;/em&gt; by their own orbital speeds around the Sun alone).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, a chance of a collision with other objects that are not members of the main belt is also non zero, and could potentially be much more threatening with higher relative velocities involved and some of the trans-Neptunian objects (such as Kuiper belt or Oort cloud objects) could be hurled by other collisions between them towards the inner Solar system. But, as stated, Ceres is a really small target to hit, and the area is also protected by the gas and icy giants (Jupiter, Saturn, Uranus and Neptune) between the main belt and TNO and that are much, much stronger attractors clearing the neighborhood through the process known as accretion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, TL;DR - however you look at it, the chance of a collision with an object of significant mass is orders of magnitude smaller than if we set a colony on our own Moon, for example. And it would be many times easier in terms of required energy, to protect parts or even whole of the Cerean surface from such events.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-10-12T12:29:23.863" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5582" PostTypeId="1" CreationDate="2014-10-10T14:39:40.480" Score="2" ViewCount="862" Body="&lt;p&gt;I wonder how difficult it would be to create an artificial planetary magnetic field with generators? What power would they need?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is inspired by thinking about possible colonization of Jupiter's moons Io and Europa, which are located inside the Jovian radiation belt. Is it possible to create with easy means an artificial magnetic field such that it would shield the surfaces of these moons from radiation? Or would it require an astronomical amount of power?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By easy means I mean a device that would not require power greater than a normal industrial power plant, best of all, solar-powered or based on once-charged superconductor coils.&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="6537" LastEditDate="2018-01-05T20:34:46.477" LastActivityDate="2023-09-10T06:28:05.273" Title="What sort of power requirements are necessary to create an artificial magnetic shield around planetary &amp; moon-sized bodies?" Tags="|colonization|terraforming|" AnswerCount="0" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="5583" PostTypeId="1" AcceptedAnswerId="5588" CreationDate="2014-10-10T23:11:30.273" Score="15" ViewCount="27863" Body="&lt;p&gt;If an artificial satellite is in orbit around the Earth, it collides with dust and gas and loses a very small amount of kinetic energy to these collisions, and eventually will spiral down towards Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be possible to use solar energy to keep the satellite in orbit forever*?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*as long as the sun is burning, so &quot;forever&quot;=&quot;about 5 billion years&quot;&lt;/p&gt;&#xA;" OwnerUserId="481" LastActivityDate="2022-03-11T18:41:12.527" Title="Can an artificial satellite stay in orbit forever?" Tags="|artificial-satellite|orbit|orbital-lifetime|" AnswerCount="6" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5584" PostTypeId="2" ParentId="5568" CreationDate="2014-10-11T02:14:25.927" Score="30" Body="&lt;p&gt;Using the times of injection and retrofire in this diagram:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yrsyg.jpg&quot; alt=&quot;map of Vostok-1 flight path over Earth, with injection at 0617 UTC, and retrofire at 0725 UTC&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftOrbit.do?id=1961-012A&quot; rel=&quot;noreferrer&quot;&gt;orbit information from NASA&lt;/a&gt;, I get that Yuri did about 272° to 273° of a 360° orbit.  So about 3/4ths of an orbit.  I am not including the ten minutes it took to get from the launch pad to orbit as being in orbit, nor am I including the 30 minutes it took from the deorbit burn to landing as being in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ok, he didn't complete one full orbit.  So what?  He was &lt;em&gt;in orbit&lt;/em&gt; every second of that 3/4ths of an orbit.  So, yes, Yuri did indeed go orbital.  He didn't need to go 360° to make that claim.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will posit a definition that you are in orbit, even if only for a portion of an orbit, if your calculated orbital lifetime is at least one orbit.  For typical ballistic coefficients, that would be about 150 km.  Yuri's orbit was 315 km x 169 km, with a much longer lifetime than one orbit.  I calculate about a 19-day lifetime for that orbit and the (relatively high) ballistic coefficient of a Vostok spacecraft pointed into the wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It certainly wasn't a sub-orbital flight, as were the flights of Alan Shepard and Gus Grissom which never got close to achieving the speeds required for orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-10-11T18:08:58.593" LastActivityDate="2014-10-11T18:08:58.593" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5586" PostTypeId="1" CreationDate="2014-10-11T15:24:48.567" Score="4" ViewCount="1951" Body="&lt;p&gt;Because the mission was launched on a free return trajectory, the crew of Apollo 13 survived. Has that abort option ever been foregone in any mission that flew or was seriously planned?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(A free return trajectory is one that will allow a ship to return to its origin because it picks up enough speed on a low pass around a second body to escape its gravity and head back home. No fuel is needed to return. The Apollo flights landed instead of heading back by retro-firing their engines to slow down, except for Apollo 13.)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2014-10-12T11:01:16.780" LastActivityDate="2014-10-12T11:01:16.780" Title="Does NASA ever plan crewed missions without a free return trajectory?" Tags="|trajectory|safety|abort|free-return|crewed-spaceflight|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5587" PostTypeId="2" ParentId="5586" CreationDate="2014-10-11T16:29:53.747" Score="6" Body="&lt;p&gt;Apollo 13 was &lt;em&gt;not&lt;/em&gt; on a free return trajectory when the explosion happened, in order to target their landing site.  They had to execute a 30-second burn of the Lunar Module engine to get on a free return trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, yes, it was planned to not be on a free return for much of the Apollo 13 trajectory.  So that must count as &quot;ever&quot;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-10-11T16:29:53.747" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5588" PostTypeId="2" ParentId="5583" CreationDate="2014-10-11T16:32:20.113" Score="3" Body="&lt;p&gt;The answer is a solid 'no'. There is no way a satellite could stay in orbit indefinitely. I'm still not sure what you mean by &amp;quot;solar energy,&amp;quot; but that will not work forever. Everything fails, in due course. Any mechanisms onboard will eventually break down, and, over time, the satellite will come crashing to Earth. The only way around this would be to give the satellite an orbital boost and/or repair it, but that is (in my opinion) a cheap loophole. The satellite would not be self-sufficient.&lt;/p&gt;&#xA;&lt;p&gt;To add in what David Hammen and TildalWave were teaching me: The satellite could be thrown out of orbit by perturbations from another body and sent somewhere completely different, then potentially recapture by another celestial body. Then the forces of orbital decay will go at it again, (and as TildalWave would say) ad nauseum.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="58" LastEditDate="2022-03-11T18:41:12.527" LastActivityDate="2022-03-11T18:41:12.527" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="5589" PostTypeId="2" ParentId="5586" CreationDate="2014-10-11T16:33:37.770" Score="2" Body="&lt;p&gt;The Apollo missions are the only manned missions thus far to leave Earth's orbit, and as such are the only ones for which a &quot;free return&quot; (to Earth) trajectory has meaning. (As noted by Mark Adler, the Apollos didn't normally fly free-return trajectories, but could easily get on such trajectories.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For near-Earth missions, a &quot;free return&quot; to Earth means a sub-orbital trajectory (like the first two Mercury missions). Once you're in orbit, you're not in free return (discounting eventual orbital decay from atmospheric drag), but you can return for a very small energy investment. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-10-11T16:33:37.770" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5590" PostTypeId="2" ParentId="3483" CreationDate="2014-10-11T23:46:03.780" Score="0" Body="&lt;p&gt;I like your idea, but I was thinking more along the lines of using an electromagnet amplified by a centralized magnet amplifier. Explanation: A small electromagnet inside a slightly larger one, and that slightly larger one inside of an even larger one, and so on until you get the proper strength. Each magnet amplifies the magnetic field of the smaller one inside of it. You could build a silo with this theory applied, insert your &quot;payload&quot; into the &quot;barrel&quot;, so to speak, turn the magnet(s) on and the payload will launch. If your trying to launch to, say, the moon, you should build your payload to be durable, because it will make a rather firm impact, obviously.&lt;/p&gt;&#xA;" OwnerUserId="6288" LastActivityDate="2014-10-11T23:46:03.780" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5591" PostTypeId="2" ParentId="5586" CreationDate="2014-10-12T04:03:00.743" Score="8" Body="&lt;p&gt;Apollo missions 8, 10, and 11 were free-return trajectories, but this greatly limits the choice of landing sites.  From Apollo 12 onward, the initial trajectory was a highly elliptical orbit that did not reach the Moon (&lt;em&gt;technically&lt;/em&gt; suborbital: the perigee was inside the Earth's atmosphere); once everything checked out, a mid-course correction was used to put the spacecraft on a non-free-return lunar trajectory.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-10-12T04:03:00.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5592" PostTypeId="1" AcceptedAnswerId="5596" CreationDate="2014-10-13T01:19:05.177" Score="22" ViewCount="26382" Body="&lt;p&gt;I really enjoy watching the International Space Station fly over head.  I subscribe to alerts so I know when it is in my area during the night time.  I wouldn't be able to see it if it weren't for the light.  Perhaps the light is only so we can see it more easily.  I'm not exactly sure why there is a light on it and I've searched for answers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is there a light on the International Space Station facing us on Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a very bright white light.  It's not a blinking light for collision avoidance.  It uses power, maybe not a lot but still a consideration.&lt;/p&gt;&#xA;" OwnerUserId="690" LastEditorUserId="49" LastEditDate="2014-10-13T02:15:23.793" LastActivityDate="2018-04-07T06:22:24.580" Title="Why does the International Space Station have a downward facing light?" Tags="|iss|observation|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5593" PostTypeId="2" ParentId="5592" CreationDate="2014-10-13T01:48:24.387" Score="40" Body="&lt;p&gt;It's not a powered light; what you're seeing is sunlight reflected from the solar panels.  That's why you can only see it during near overhead passes around dusk and dawn - the sun has not yet &quot;set&quot; from the space station's altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://nasa.gov/vision/space/travelinginspace/f_skywatch.html&quot;&gt;http://nasa.gov/vision/space/travelinginspace/f_skywatch.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Space Station is one of the most visible man made objects in the sky, because it reflects sunlight and often looks like a slow-moving star. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6295" LastEditorUserId="605" LastEditDate="2014-10-13T15:31:33.317" LastActivityDate="2014-10-13T15:31:33.317" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5594" PostTypeId="2" ParentId="5472" CreationDate="2014-10-13T04:20:09.273" Score="4" Body="&lt;p&gt;Astro Watch published an article &lt;a href=&quot;http://www.astrowatch.net/2014/10/russia-to-continue-baikonur-use-along.html?m=1&quot; rel=&quot;nofollow&quot;&gt;&quot;Russia to Continue Baikonur Use Along with New Vostochny Spaceport&quot;&lt;/a&gt; . It says:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;• Roscosmos have the contractual obligations, they pay the rent for the land allotted for the space infrastructure. The lease agreement expires in 2050.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;• Another spaceport - Vostochny - is now under construction and its first priority goal is to ease the burden on Baikonur that currently accounts for 80% of Russia’s space activity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;• Under the current Russian space program, Baikonur remains a busy spaceport, with numerous commercial, military and scientific missions being launched annually. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;• All manned Russian spaceflights are launched from Baikonur.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lorenzo already mentioned most of the points and I agree with his conclusion that with new spaceceports Russia will decrease burden on Baikonur.&lt;/p&gt;&#xA;" OwnerUserId="1417" LastEditorUserId="1417" LastEditDate="2014-10-13T04:26:31.053" LastActivityDate="2014-10-13T04:26:31.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5596" PostTypeId="2" ParentId="5592" CreationDate="2014-10-13T15:09:35.440" Score="42" Body="&lt;p&gt;There are a few positional lights on the visiting spacecraft to the International Space Station (ISS), also doubling as indicators that the visiting spacecraft docked to the station are powered and similar reasons. And the Canadarm2 has lights on it so it can be remotely / CCTV operated also when the station is in the Earth's shadow, and they do have lights inside the station so astronauts / cosmonauts can see, and some of this light would be reflected towards the Earth when someone is in the Cupola pointing towards nadir and its protective shutters are open. But none of these lights are nearly as powerful enough to see them from at least 410 km (254 miles) which is station's current orbital altitude and would be the minimum distance between you and the station when it orbits directly overhead from the observer, and most of them do blink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you can see from the ground is however this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/tvnD0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's perhaps not best known is that the station's solar panel arrays are double-sided to also collect some of the sunlight reflected off the Earth's albedo (what our planet reflects from the Sun). They generate roughly up to 120 kW of power (on average about 84 kW) needed by the station's large number of equipment, life support, experiments,... to function. It stores excess collected power in batteries for when the station doesn't generate electric power with its photovoltaics (that's where that average comes from, lower than its maximum output), but more importantly to your question, they're roughly the size of a U.S. football field:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/w14xZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Image source and credit: &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html&quot; rel=&quot;noreferrer&quot;&gt;International Space Station - Facts and Figures&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what you see is indeed as @GWP mentions in his/her answer, even if that started as a rather vague one-line answer. How can we be sure? Simple. The station simply doesn't have sufficient power to run any such lights powerful and large enough to be seen from the ground, as even at 100% efficiency they would consume roughly the same amount of power that the station generates at its own photovoltaics efficiency of well under 50% (yes, they're due for an upgrade if they want to run it till 2024 and run even more experiments). The rest of the incident light from the Sun is mostly reflected, and some absorbed as heat that needs to be radiated to space. But you'd want as little of this heat absorption, since convective heat transfer doesn't really work in the near vacuum in the Low Earth Orbit (LEO), so the solar arrays use coating that matches its efficiency and reflects the rest in wavelengths it's not as efficient in converting incident sunlight into electricity. And if it can utilize less than 50% of what light it has available to it and reflects as much as possible of the rest,... well, you can do the maths.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The light you see is sunlight reflected off the station's enormous solar power arrays, and can only be visible when the observational conditions are dark enough that the bright spot of the station isn't lost in the sunlit background, and the arrays reflect sunlight towards you, so when the station is not in Earth's shadow. When the station is barely entering the Earth's shadow, and if it was visible before it, you can also visually observe it slowly fading into the shadow with your own eyes. That's another proof that it doesn't keep lights strong enough to be seen from the surface on, if the fact that there's absolutely no good reason to do that isn't good enough of a proof on its own.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are four frames from the station showing two Soyuz spacecraft docked to it entering the Earth's shadow as seen from one of the &lt;a href=&quot;http://www.ustream.tv/channel/iss-hdev-payload&quot; rel=&quot;noreferrer&quot;&gt;ISS HD Earth Viewing Experiment&lt;/a&gt; aft facing cams (click on images for larger versions):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RC86r.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RC86rm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/XIUJF.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XIUJFm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/ZcVBS.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZcVBSm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/lH3Ti.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lH3Tim.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This happens about every 93 minutes on the station, or each time it completes one of its orbits. Except during the period of station's high beta angle when it might not enter Earth's shadow at all and its orbit remains more or less aligned with the Earth's day/night terminator for a few days or so. For more on all of this, read e.g.:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4034/can-i-see-the-iss-from-the-surface-with-the-naked-eye&quot;&gt;Can I see the ISS from the surface with the naked eye?&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4312/can-you-make-out-the-shape-of-the-iss-as-it-passes-overhead&quot;&gt;Can you make out the shape of the ISS with the unaided eye as it passes overhead?&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4686/how-often-does-the-iss-orbit-align-with-the-day-night-terminator&quot;&gt;How often does the ISS orbit align with the day/night terminator?&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;OK, so why white if the station's solar arrays reflect brownish-golden color on their own? Simply because of the intensity of reflected sunlight when compared to the darker background of dusk or dawn when the station can be seen with a naked eye. When the station is in the process of entering Earth's shadow and/or its beta angle (the angle between the Sun, the station, and the observer) is shallower, and the station's solar arrays are rotated to collect as much sunlight so towards the Sun, the amount of this reflected light will also be smaller, the light beam will also lose some of its intensity through the atmosphere through a process known as atmospheric diffraction (see the other linked question), and your eyes will be able to distinguish colors more precisely. It will also appear a bit more yellowish at those times. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To somewhat reaffirm my last point, consider this frame capture from the &lt;a href=&quot;http://www.ustream.tv/channel/iss-hdev-payload&quot; rel=&quot;noreferrer&quot;&gt;ISS HD Earth Viewing Experiment&lt;/a&gt; showing solar arrays on two Soyuz visiting spacecraft docked to the station and reflecting incident sunlight at different angles:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/0jNQt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, the solar panels are actually not white, but due to the intensity of reflected sunlight, the array of the foremost Soyuz spacecraft appears bright white. It's not exactly white as neither the average wavelength of the light emitted by our Sun by intensity is, and the reflected light does include color component of the materials used on the panels, but for all intents and purposes, it's close enough.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-07-12T14:35:33.937" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5598" PostTypeId="2" ParentId="5583" CreationDate="2014-10-13T19:17:06.157" Score="4" Body="&lt;p&gt;From the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would it be possible to use solar energy to keep the satellite in&#xA;  orbit forever*?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Followed by the comment:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How would you use solar energy?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Forever (or even 5 billion years) is a long time. As the other posters have pointed out, it's unlikely that any artificial satellite could survive that long in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, possibly of use is a propellantless means by which you can keep the satellite in orbit for an indefinite* amount of time using solar-power. This can be done by running an electric current through a conductive &lt;a href=&quot;http://en.wikipedia.org/wiki/Electrodynamic_tether#Tether_propulsion&quot; rel=&quot;nofollow&quot;&gt;tether&lt;/a&gt; to create an electromagnetic field that pushes against the Earth's electromagnetic field to reboost the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*The problem here is material science: The tether will be be relatively long and exposed, and susceptible to damage by micrometeorite (so will the rest of the satellite, for that matter). One way to engineer around it is to use a multiply-redundant network of cables, a design trademarked as a &quot;&lt;a href=&quot;http://www.tethers.com/hoytether.html&quot; rel=&quot;nofollow&quot;&gt;Hoytether&lt;/a&gt;&quot;. Unfortunately, this still only extends the expected lifetime to less than 200 years, which is a far cry from forever.&lt;/p&gt;&#xA;" OwnerUserId="6306" LastActivityDate="2014-10-13T19:17:06.157" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5599" PostTypeId="2" ParentId="5583" CreationDate="2014-10-13T19:28:15.323" Score="13" Body="&lt;p&gt;The answer to the actual question posted, &quot;Can an artificial satellite stay in orbit forever?&quot; (where &quot;forever&quot; is assumed to be a couple of billion of years) is &lt;strong&gt;maybe&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are obvious constraints:  the satellite could not be in low-Earth orbit (aka, LEO) due to decay from friction and gravitational variations of the Earth, or even in higher orbits up to and possibly beyond geosynchronous orbit, for the same reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the satellite would have to be so massive, that micro-meteoroid impacts would not significantly change its orbit.  What is the proof of this?  Our moon has been in Earth orbit for several billion years, and while it is drifting (very very slowly) away, it's still there.  It is in &lt;em&gt;theory&lt;/em&gt; possible to construct a massive artificial satellite that would be in a similar orbit (although not too similar, or the Moon might disturb it).  I'm thinking of something along the lines of a Death Star here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And even then, with a massive Death Star, there is always some tiny chance that something else (close-passing comet, or an asteroid impact) disturbs it.  But is it &lt;strong&gt;possible&lt;/strong&gt;?  I think that it is.  &lt;/p&gt;&#xA;" OwnerUserId="6199" LastEditorUserId="6199" LastEditDate="2014-10-22T23:56:40.660" LastActivityDate="2014-10-22T23:56:40.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5600" PostTypeId="1" AcceptedAnswerId="5605" CreationDate="2014-10-13T21:41:30.243" Score="9" ViewCount="483" Body="&lt;p&gt;According to the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/3207/how-is-the-moons-coordinate-system-defined&quot;&gt;this question&lt;/a&gt;, the 0° longitude of the Moon is that which points toward Earth.  On Earth it goes through Greenwich (although I was amused to hear that &lt;a href=&quot;http://en.wikipedia.org/wiki/Prime_meridian&quot; rel=&quot;nofollow noreferrer&quot;&gt;the French disagreed&lt;/a&gt;), which is a fairly arbitrary (albeit historical) decision, and on Mars it was chosen &lt;a href=&quot;http://en.wikipedia.org/wiki/Geography_of_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;apparently at random&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the exploration of space continues, and more and more planets, moons, planetoids and asteroids are being mapped, (and with the rapid discovery of swarms of exoplanets, which further discoveries will no doubt eventually allow us to map in detail as well), is there a system for setting up coordinate systems on each?  Some traditional or systematic structure for choosing a Prime Meridian on alien worlds?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or are such decisions made more haphazardly, perhaps left up to whoever first chooses to make the first map?  If so, is there any standard approval body, or official way of settling disputes between competing coordinate systems?&lt;/p&gt;&#xA;" OwnerUserId="1216" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-10-14T13:25:59.513" Title="Is there a system for choosing the Prime Meridian on a given body/world?" Tags="|planetary-science|exoplanet|mapping|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5601" PostTypeId="1" CreationDate="2014-10-13T21:54:26.773" Score="2" ViewCount="1902" Body="&lt;p&gt;I am confused with energy requirement equations...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I anywhere near with my following assumptions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v$ from surface to LEO is 9000 m/s&#xA;typical exhaust velocity $v_e$ = 4500 m/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and mass of payload $m_l = 40\mathrm{t}$ and &#xA;$m_p$ = mass of propellant&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$=&amp;gt; \Delta v = v_e \, \ln \frac{m_p+m_l}{m_l}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$=&amp;gt; m_p = m_l (\mathrm{e}^{\frac{\Delta v}{v_e}}-1)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$=&amp;gt; m_p = 40 000 (\mathrm{e}^\frac{9000}{4500}-1) = 256 \mathrm{t}$  of propellant&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy with 100% efficiency would then be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$E = 0.5 \, m_p \, v_e^2$    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;or (gets me the same result?)   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$E = 0.5 \, m_l (\mathrm{e}^\frac{\Delta v}{v_e}-1) \, v_e^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$=&amp;gt; E = 2588 \mathrm{GJ}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I in a right order of magnitude here...?&lt;/p&gt;&#xA;" OwnerUserId="6311" LastEditorUserId="3185" LastEditDate="2014-10-14T11:31:56.927" LastActivityDate="2014-10-20T01:39:08.673" Title="How much energy is required to send a payload of 40 t from Earth's surface to LEO?" Tags="|low-earth-orbit|energy|fuel|delta-v|payload|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5602" PostTypeId="2" ParentId="5583" CreationDate="2014-10-14T05:37:22.780" Score="8" Body="&lt;p&gt;You seem to think atmospheric drag is the only influence. Above a certain height there are factors stronger than atmospheric drag. As David Hammen says &quot;Perturbations from the Moon, the Sun, and the Earth's non-spherical gravity field. Tidal perturbations. Radiation pressure and its cousin, the Yarkovsky effect. The Kozai mechanism.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also as Hammen says, these can result in a number of outcomes: collision with the earth or moon, or ejection from the earth moon system. An indefinite earth orbit is unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Station keeping with solar power may be possible if the satellite has an electrodynamic tether. But a tether that could last indefinitely seems unlikely. Most man made devices break down over time.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-10-14T05:37:22.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5603" PostTypeId="1" AcceptedAnswerId="5606" CreationDate="2014-10-14T09:11:28.380" Score="13" ViewCount="10682" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/questions/2699&quot;&gt;What is a “space elevator”?&lt;/a&gt; describes two key points about the length of a space elevator: equatorial geosynchronous orbit at 22,245 miles (35,800 km) above mean sea level, and an additional 22,245 miles for counter balance.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/questions/1350&quot;&gt;Benefit of sling shot effect with a space elevator&lt;/a&gt; talks about launching from the far/upper end of the elevator. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space elevators are not uncommon in science fiction. In a story I was just reading, travel time to the launch point was greater then a week; that seems overly slow.  For the two main points, what would impact travel time from Earth and what would be reasonable travel times to expect?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Stop at Geosynchronous orbit&lt;/li&gt;&#xA;&lt;li&gt;Release/Launch point (not sure if you would need to stop here, or just keep going)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2018-12-07T13:04:29.820" Title="How long would it take to ride to the top of a space elevator?" Tags="|space-elevator|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5604" PostTypeId="2" ParentId="5603" CreationDate="2014-10-14T11:44:26.043" Score="2" Body="&lt;p&gt;That just depends on how much energy you put in. If the elevator is heavy, it may not be trivial to transport the energy to it (say, by microwaves). If you need a week to GEO, it means you're traveling at 60 m/s (216 km/h or 134 mph). That sounds reasonably fast to me, but who knows how efficient and light microwave antennae will become in the future, or what way future space people will have to transmit power.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="415" LastEditDate="2014-10-15T11:00:42.743" LastActivityDate="2014-10-15T11:00:42.743" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5605" PostTypeId="2" ParentId="5600" CreationDate="2014-10-14T13:25:59.513" Score="4" Body="&lt;p&gt;One need look no further than the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were (and to some extent still are) some rather severe internecine wars within the Jet Propulsion Laboratory on the &quot;right&quot; way to represent the Moon's orientation. One group insists that the right way to describe the Moon's orientation is via its principal axes (the MOON-PA point of view). Per this point of view, any other approach is not just dumb, it's dumber than dumb. Another group insists that the right way to describe the Moon's orientation is via its orientation with respect to the Earth (the MOON-ME point of view). Per this point of view, that other description is even dumberer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a disinterested third party observer of these internecine wars, that these wars exist to me is what is dumb. I'd much rather have one agreed-upon frame that in one swell foop describes the Moon's rotation, it's gravity field, and it's topology. That desire is apparently a &quot;good luck with that&quot; kind of proposition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As TildalWave mentioned in a comment, there is an interdivisional working group within the International Astronomy Union (IAU) that is dedicated to describing the rotational states of the planets, satellites, asteroids, and comets that comprise the solar system. This is the Cartographic Coordinates &amp;amp; Rotational Elements Working Group, described &lt;a href=&quot;http://www.iau.org/science/scientific_bodies/working_groups/100&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://astrogeology.usgs.gov/groups/IAU-WGCCRE&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. This group can only make recommendations. It has no binding powers that say &quot;this is how thou shalt do it.&quot; In fact, this group to some extent operates at the whims of other IAU committees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the IAU definition of the &quot;north pole&quot; is different for a planet than for a small solar system body. The &quot;north pole&quot; of a planet or the satellite of a planet always has a positive +z component with respect to the solar system invariant plane. Oh the other hand, the north pole of some non-planetary / non-satellite body is that direction from which the body appears to be rotating counterclockwise. There has been one case where a planet has been relegated to a non-planet status, and that one case was troublesome with regard to the direction in which the &quot;north pole&quot; points. Pluto appears to rotate clockwise when observed from above the invariant plane, counterclockwise when observed from below. This means that the 2006 decision to designate Pluto a non-planet status meant that the description of Pluto's rotation had to be totally revamped.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-10-14T13:25:59.513" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5606" PostTypeId="2" ParentId="5603" CreationDate="2014-10-14T14:44:04.760" Score="18" Body="&lt;p&gt;For a first order estimate, we can use Wikipedia's &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_vehicle_speed_records&quot;&gt;list of vehicle speed records&lt;/a&gt;. Let's look at ground vehicles, ignoring rocket-powered vehicles (which sort of defeats the point of using an elevator). For &lt;a href=&quot;http://en.wikipedia.org/wiki/Wheel-driven_land_speed_record&quot;&gt;wheel driven vehicles&lt;/a&gt;, the speed records seems to be around 750 km/h (I'm rounding a bit). For &lt;a href=&quot;http://en.wikipedia.org/wiki/Maglev#Records&quot;&gt;maglev rail&lt;/a&gt; vehicles, the record is close to 600 km/h.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To geosynchronous orbit (double time to release point):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\text{35,800 km / 750 km/hr = 47.7 hrs = 1.99 days}$$&#xA;$$\text{35,800 km / 600 km/hr = 59.67 hrs = 2.49 days}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, it's quite likely that your space elevator isn't going to try out for any speed records, so your travel times could realistically be greater than this. So using a slower car, a week or more to the release points seems plausible. On the other hand, if high-speed were a priority, and the power was available, there's no reason they couldn't be made to go faster with the proper engineering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For one thing, this simple estimate doesn't account for the fact that there will be no air resistance to fight against once you leave the atmosphere, which means that you won't be fighting against terminal velocity if you go too fast. Although, if any part of your car is in contact with the cable/rail, there will still be friction and heating limits in terms of how fast you can go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Wikipedia references &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_elevator#cite_note-LangGTOSS-39&quot;&gt;a technical paper&lt;/a&gt;, &lt;em&gt;Space Elevator Dynamic Response to In-Transit Climbers&lt;/em&gt; (&lt;a href=&quot;http://home.comcast.net/~GTOSS/Paper_10152148Lang_Climb.pdf&quot;&gt;direct link to pdf&lt;/a&gt;), by David Lang, which simulates the stresses and oscillations in the cable of a space elevator. For simulation purposes, it assumes two different values of climber speed: a &quot;nominal&quot; case being 200 km/hr, and a &quot;fast&quot; case of 400 km/hr. These are both slower than my estimates above, but are of a similar order of magnitude (and &lt;em&gt;considerably&lt;/em&gt; less than the values used in ForgeMonkey's answer).&lt;/p&gt;&#xA;" OwnerUserId="6306" LastEditorUserId="6306" LastEditDate="2014-10-14T23:06:39.470" LastActivityDate="2014-10-14T23:06:39.470" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5607" PostTypeId="2" ParentId="5603" CreationDate="2014-10-14T15:44:47.403" Score="9" Body="&lt;p&gt;Assuming energy transmission isn't an issue (and any society that has the technology to build a space elevator probably won't have problems with throwing insane amount of energy about) and ignoring other engineering problems the limiting factor becomes the acceleration that humans can tolerate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say we want to keep the acceleration around what it is for astronauts today: about 3-4 g. So to make the maths easy let’s say we're going to accelerate, relative to the cable of the elevator, at 3 g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At liftoff the passengers would experience 4 g, our 3 g + Earth's. So we accelerate up until we reach halfway (17,900 km), and then accelerate in the opposite direction until we arrive at GEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in the equations of motion gives us:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$s=ut+\frac{at^2}{2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$17,900,000 = 15\text{ t}^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t=1,092\text{ s}$ (approx) to get halfway so twice that, 2,184 seconds or about 36 minutes to go from the surface to GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's take a VERY rough stab at the energy requirements. Let's say our elevator masses 20 tonnes or 20,000 kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Completely ignoring the effects of Earth's gravity or atmospheric resistance (maybe someone with more time can add these), finding the force required to sustain the acceleration: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F=ma=20,000*30 = 600,000\text{ N}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over a distance to give work: 600,000 N over 36,000,000 m gives 21,600,000,000,000 Joules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that work is done in 36 minutes: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\text{21,600,000,000,000 Joules / 36 minutes = 10 Gigawatts}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or about the total power output of Scotland.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-02-15T11:56:23.997" LastActivityDate="2015-02-15T11:56:23.997" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5608" PostTypeId="2" ParentId="5592" CreationDate="2014-10-14T17:12:38.093" Score="11" Body="&lt;p&gt;The ISS is covered with highly reflective material to help regulate its temperature. It isn't so easy to radiate energy away in space, a vacuum is actually the best insulator known to science. So when sunlight hits the station the best thing is to prevent as much of its energy as possible from being absorbed, thus everything is bright white. There are also giant radiators on it for the same reason. They are mostly there to prevent the solar panels from overheating - because they are actually the only things on the ISS that aren't bright white, they are a dark coppery color. The radiators also help cool the rest of the station. Because the solar panels are angled for best capture of light, they rarely reflect much light towards they ground. For the most part, what you see from the ground is light reflected from the body of the space station and from the radiators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of photos around online taken with telescopes of the ISS passing overhead near dusk or dawn. The one below is in the public domain and was taken by Ralf Vandebergh. In it, the bright sections are on the main truss and the hab modules. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RQp5r.jpg&quot; alt=&quot;Ralf Vandebergh ISS photo&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here are links to several more copyrighted ones:&#xA;&lt;a href=&quot;http://blogs.esa.int/atv/2012/04/05/atv-3-seen-docked-to-iss-from-ground/&quot; rel=&quot;noreferrer&quot;&gt;One from the European Space Agency&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another from &lt;a href=&quot;http://www.wired.com/images_blogs/wiredscience/2010/05/ralf.jpg&quot; rel=&quot;noreferrer&quot;&gt;Vandebergh&lt;/a&gt; that appeared in Wired&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And several by &lt;a href=&quot;http://legault.perso.sfr.fr/STS-133.html&quot; rel=&quot;noreferrer&quot;&gt;Thierry Legault&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the ISS does reflect sunlight off the solar panels towards the ground, what is seen is a flare, and it can be very bright indeed. Here is what they say about it on the &lt;a href=&quot;http://www.amnh.org/our-research/hayden-planetarium/blog/how-to-spot-the-international-space-station-and-other-satellites&quot; rel=&quot;noreferrer&quot;&gt;Hayden Planetarium&lt;/a&gt; website:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly &quot;flare&quot; in brilliance to as bright as magnitude -8; more than 16 times brighter than Venus!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-10-14T17:12:38.093" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5609" PostTypeId="2" ParentId="5601" CreationDate="2014-10-15T01:47:40.927" Score="2" Body="&lt;p&gt;The 'sending' part of your question is what makes this a tangle. Because sending is a process, it isn't possible to speak in terms of perfect efficiency. You can't isolate the fuel it took to put the payload into orbit from the fuel it took to put the rocket carrying the payload into orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may be that what you seek is a &lt;em&gt;theoretical&lt;/em&gt; minimum energy required to put 40 tons in orbit. In that case maybe it means something to say what the &lt;a href=&quot;http://en.wikipedia.org/wiki/Kinetic_energy&quot; rel=&quot;nofollow&quot;&gt;kinetic energy&lt;/a&gt; of 40 tons of material moving at orbital velocity is, because if there was a way to get it there with absolutely no energy losses, that is all the energy you would need. Orbital speed at 200km above the Earth, which is where low earth orbit is considered to begin, is 6.9 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;0.5 x 40,000kg x 6900 (m/s) $^2$ = 952.2 GJ&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe if there was a &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_elevator&quot; rel=&quot;nofollow&quot;&gt;space elevator&lt;/a&gt; to get above the atmosphere, and you had a way to make the mass of the cargo pod hauling the payload negligible compared to the payload itself, and you were able to minimize losses due to friction, and then the payload could be accelerated sideways to orbital speed with maybe a high-efficiency magnetically levitating &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_driver&quot; rel=&quot;nofollow&quot;&gt;mass driver&lt;/a&gt;, which had a cargo pod whose mass was also negligible, you could start to approach that amount. Exactly how close we could ever get to that ideal figure is highly debatable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thinking about engineering problems in terms of the theoretical ideal has real application. It is good to know exactly how much room there is for improvement sometimes. For instance, it has often been noted that building a rocket and then only using it once is awfully inefficient. But it doesn't really serve a purpose unless you can identify a place where the process can be improved, and design a method to make it so. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Failing that, the amount of energy it currently takes to get 40 tons of material to orbit, stated in terms of the energy in the fuel it takes to launch it, requires that we choose a rocket to do the job and then identify what fuel it uses, and how much. There is no rocket currently in service that can carry a payload of 40 tons. So let's imagine a &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot; rel=&quot;nofollow&quot;&gt;Falcon 9&lt;/a&gt; that is 3 times as big, since we are only going for a rough estimate anyhow and I was able to find enough data to go on. That data didn't include the oxidizer to fuel ratio the Merlin engine uses, but we are already pretty abstract here, let's use the 2.56 ratio referenced on Wikipedia for &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;nofollow&quot;&gt;LOX/RP-1&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case, it would burn about 400,000 kg of RP-1 to get to LEO, which at a specific energy of roughly 46 MJ/kg is 18,400 GJ, or 19 times the theoretical minimum mentioned above. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-10-20T01:39:08.673" LastActivityDate="2014-10-20T01:39:08.673" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5610" PostTypeId="1" AcceptedAnswerId="6647" CreationDate="2014-10-15T13:23:44.207" Score="16" ViewCount="767" Body="&lt;p&gt;While U.S. Air Force doesn't disclose the tasks and roles for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_X-37&quot; rel=&quot;noreferrer&quot;&gt;X-37&lt;/a&gt; Orbital Test Vehicle class spacecrafts, there are many things known about the vessel, such as its delta-v budget, its cargo bay capacity, its launch vehicle and hence the available orbits, its electric power capacity and many more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what military or applied tasks could a spacecraft similar to X-37 be useful for?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KAzov.jpg&quot; alt=&quot;X-37B (on orbit) [Boeing]&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;X-37B (on orbit) [Boeing]&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="49" LastEditDate="2014-12-25T18:44:48.617" LastActivityDate="2015-04-30T09:47:04.750" Title="What could the X-37 be useful for?" Tags="|robotic-missions|uncrewed-spaceflight|military|spaceplane|x-37|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5611" PostTypeId="1" AcceptedAnswerId="5613" CreationDate="2014-10-15T16:15:03.880" Score="43" ViewCount="3923" Body="&lt;p&gt;Exploiting the moon's resources is covered by &quot;The Moon Treaty&quot;. Is there something similar or in the works for regulating mining asteroids and other non-lunar bodies?&lt;/p&gt;&#xA;" OwnerUserId="122" LastActivityDate="2018-03-16T11:21:58.820" Title="Legality of asteroid mining" Tags="|asteroid|mining|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5612" PostTypeId="1" CreationDate="2014-10-15T14:18:57.560" Score="3" ViewCount="404" Body="&lt;p&gt;Say we find an asteroid that is rich in ores and we would like to mine that asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How could this be done?  The gravity on an asteroid is so minimal that pushing a drill into the asteroid would basically just push you off the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose you could clamp onto the asteroid, but wouldn't you need to drill a spot for the clamps to take purchase first?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the current ideas for getting the drills started?  Or will they be using something other than drills?&lt;/p&gt;&#xA;" OwnerUserId="6106" OwnerDisplayName="Scottie" LastActivityDate="2014-10-15T18:30:57.960" Title="How will they mine asteroids?" Tags="|asteroid|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5613" PostTypeId="2" ParentId="5611" CreationDate="2014-10-15T18:03:22.567" Score="41" Body="&lt;p&gt;The United States (along with Russia, China, Japan, India, and most other space-faring nations) did not sign (or in some cases, ratify) the Moon Treaty, and so companies that are registered in any of these countries are also not bound by it.  So, that treaty is essentially pointless for anyone who actually has the ability to get to the moon in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The more relevant treaty is the Outer Space Treaty of 1967.  This treaty covers a few different things; much of it is focused on prohibiting putting nuclear weapons in space, or using the Moon or other celestial bodies as military bases, etc). Article II of the Outer Space Treaty also states that &quot;&lt;em&gt;Outer space, including the moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means&lt;/em&gt;.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, we can't go to a &quot;celestial body&quot; (more on that in a minute) and claim it as a part of the United States, say. On the other hand, Article I says, &quot;&lt;em&gt;Outer space, including the moon and other celestial bodies, &lt;strong&gt;shall be free for exploration and use&lt;/strong&gt; by all States without discrimination of any kind, on a basis of equality and in accordance with international law, and there shall be free access to all areas of celestial bodies&lt;/em&gt;.&quot; (my emphasis added).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Article I seems to say that anybody can go &quot;use&quot; the Moon and other celestial bodies, but that (per Article II), we can't claim sovereignty, or use force (or presumably coercion) to keep other nations away from a celestial body.  That seems to leave open the possibility of mining an asteroid, since that's &quot;use&quot;, as long as you don't &quot;claim&quot; the entire thing as a sovereign part of your country, or use force to keep other nations away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However - and this is key - all of that hinges on what, exactly, is a &quot;celestial body&quot;. At what point is something sufficiently large that it counts as a celestial body?  The Moon, other planets in the solar system, and dwarf planets like Pluto and Ceres certainly count, but what about a medium size asteroid?  What about a small one?  What about a little chunk of rock, 6 inches across?  Nobody has yet delineated what, exactly, is covered by the Outer Space Treaty, but it's very easy to imagine a court in the U.S. ruling that small asteroids either aren't covered, or if they are, that mining constitutes allowed &quot;use&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the facts on the ground (pun intended!) are that so few nations or companies have the means to get a spacecraft to an asteroid at all, that when someone &lt;em&gt;does&lt;/em&gt; manage to do so, there won't be any practical way to contest their use of it.  Treaties that the U.S. signs become law in the U.S., meaning that their interpretation can end up being decided by U.S. courts (at least, as the law applies to U.S. companies).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TLDR:  legally it's a slightly gray area, but commercial mining of asteroids is probably allowed, because the treaty allows &quot;use&quot; of celestial bodies, and because asteroids might not even count as &quot;celestial bodies&quot;.&lt;/p&gt;&#xA;" OwnerUserId="6199" LastActivityDate="2014-10-15T18:03:22.567" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5614" PostTypeId="2" ParentId="5612" CreationDate="2014-10-15T18:30:57.960" Score="8" Body="&lt;p&gt;Resource extraction from asteroids is something that has been studied for decades.  There is a great article and info-graphic at:  &lt;a href=&quot;http://www.space.com/15391-asteroid-mining-space-planetary-resources-infographic.html&quot;&gt;Space.com&lt;/a&gt; that shows different ideas, from both NASA and Planetary Resources (a company currently planning to mine a near-Earth asteroid). I very much encourage you to read it!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the easiest and most useful things to mine would be water, which can be &quot;mined&quot; from an asteroid that has chunks of ice on it. Someday, the water will be valuable for its own sake (for future inhabitants of space stations or lunar colonies, whatever, to drink!) but in the nearer term, the ice would be melted, then split into hydrogen and oxygen (using electrolysis).  Hydrogen and oxygen are used to fuel cryogenic rocket engines, and having a bunch of rocket fuel already out in Earth orbit somewhere would be a huge boon for sending large probes to the outer planets, fueling up manned missions to Mars, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More advanced mining ideas would be mining metals for use in automated construction/fabrication in space; I'm sure mining in microgravity is going to have its share of technical challenges, but your question about anchoring a probe or machine to the surface has a bunch of answers, some of which I can think of:  use powered darts that fire from the probe during approach and which embed themselves in the asteroid (much like a piton used in climbing).  Alternatively, throw a net around the entire asteroid (assuming a small one), and use the net to keep the probe tight against the asteroid to allow drilling the first holes for anchors.  Lots of ways to tackle that problem.&lt;/p&gt;&#xA;" OwnerUserId="6199" LastActivityDate="2014-10-15T18:30:57.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5615" PostTypeId="1" CreationDate="2014-10-15T23:43:56.693" Score="7" ViewCount="313" Body="&lt;p&gt;What altitude would the LES (launch escape subsystem) reach on Apollo launches before falling back to Earth?&lt;/p&gt;&#xA;" OwnerUserId="6329" LastEditorUserId="288" LastEditDate="2014-10-22T11:49:02.700" LastActivityDate="2014-10-22T11:49:02.700" Title="On Apollo launches, what altitude would the LES reach?" Tags="|apollo-program|trajectory|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5616" PostTypeId="2" ParentId="5615" CreationDate="2014-10-16T04:05:16.263" Score="4" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;nofollow&quot;&gt;this article about duplicating a Saturn V/Apollo trajectory in simulation&lt;/a&gt;, the LES is jettisoned at 198 seconds into the flight, at which time altitude is just under 100km and vertical speed is about 650m/s (very rough numbers obtained by eyeballing those graphs). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we disregard the (very brief) acceleration from the jettison motor, and ignore atmospheric drag (negligible at this altitude, as PearsonArtPhoto points out), a pure ballistic trajectory from that point would make it top out at about 120km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Okay, so my initial estimate was completely horrible.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-10-16T16:21:22.957" LastActivityDate="2014-10-16T16:21:22.957" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5617" PostTypeId="1" CreationDate="2014-10-16T06:46:53.007" Score="12" ViewCount="2015" Body="&lt;p&gt;It is quite common to find (pseudo?) scientific statements to the effect that blowing up asteroids which are heading our way is a very bad idea, and that the movie Armageddon got this point totally wrong. I would like to explore this a little bit more. I have particularly seen statements that breaking it up makes it &lt;em&gt;significantly worse&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shouldn't breaking up the asteroid into 2 drastically increase its surface area or cross-section when its going through the atmosphere? Wouldn't this then both increase the chance of burning up in the atmosphere itself, causing an air-burst (not too sure about this one) or at the very least slow down at a slightly higher rate? Wouldn't all of these reduce the damage done?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the extreme, if the asteroid is broken down to the size of sand or nearly so, wouldn't the damage to earth be minimal?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally I would figure this has a bit more finality to it rather than diverting the asteroid by rockets or gravity. This may not matter much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This method would be lot more effective than the diversion methods if the asteroid were discovered too late as well wouldn't it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am thinking more carpet bomb than a single giant bomb such as in the movie.&lt;/p&gt;&#xA;" OwnerUserId="6332" LastActivityDate="2015-11-19T14:39:36.950" Title="Is it (or why is it) worse to break up a asteroid on a collision course with Earth?" Tags="|asteroid|impact|" AnswerCount="7" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5618" PostTypeId="2" ParentId="429" CreationDate="2014-10-16T07:19:08.880" Score="2" Body="&lt;p&gt;The difference is that the phasing delay is eliminated with the help of ISS maneuvers and  a more precise path can be flown by updated flight computers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine a &quot;normal&quot; launch, such as the shuttle might have used from Florida.  When you get to orbit, small changes in altitude are fairly easy to accomplish, but inclination changes are very expensive.  In order to reach the station, the shuttle waits until the earth's rotation takes it just about underneath the orbit.  During a small window it then launches into the same inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the time of launch, the station may be at any point along the orbit path.  The difference between the station's location and the shuttle's location is the phase difference.  To conserve fuel, the shuttle isn't going to fly above the station and then descend back, so all of the difference has to be made up by flying lower than the station.  This gives only a relatively small altitude range between the two.  This difference is sufficient to allow the shuttle to reach the station within 3 days no matter the phase.  When the phase is correct, the shuttle can begin the approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can imagine that having the ISS at one particular point in the orbit would allow for the approach to the station to happen immediately.  The trick is how to launch when that is the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The big thing for the fast approach profile is that the launch is planned well in advance and the ISS is maneuvered to the correct point.  This allows the phase to be within limits at the time of launch.  Moving the ISS is expensive, so this is only done for crewed flights now.  Cargo can wait it out.  The article below points out that updated flight control systems were also necessary.  The profile requires that the craft perform burns outside the range of ground stations, and that wasn't possible on previous versions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2012/08/progress-m-16m-launch-test-new-fast-rendezvous-iss/&quot; rel=&quot;nofollow&quot;&gt;http://www.nasaspaceflight.com/2012/08/progress-m-16m-launch-test-new-fast-rendezvous-iss/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2014-10-16T07:19:08.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5620" PostTypeId="2" ParentId="5617" CreationDate="2014-10-16T15:04:23.897" Score="7" Body="&lt;p&gt;The main problem is that you will have no idea what the result of your attempt will be until you try it.  Is the asteroid a rubble pile that will completely blow apart, or is it a solid object that will remain largely unaffected?  Or some combination of the two with big solid pieces inside surrounded by small rubble?  How big will the resulting pieces be?  How many and which ones will remain on an Earth impact trajectory?  Will you now have many remaining objects large enough on an impact trajectory that you have to take further measures to mitigate, multiplying the effort required with less time remaining?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this really an experiment you want to be doing when many lives are at stake and time is of the essence?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is better to use a gravity tug approach, or maybe spray paint one pole side of the asteroid white to use the Yarkovsky effect to deflect it as a whole without disturbing its integrity.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-10-16T15:04:23.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5621" PostTypeId="1" CreationDate="2014-10-16T15:19:28.743" Score="8" ViewCount="1726" Body="&lt;p&gt;I've been working on rocket stabilization for my graduation project similar to this video: &lt;a href=&quot;http://www.youtube.com/watch?v=qnUfsLCa8h8&quot; rel=&quot;nofollow&quot;&gt;Roll gyro stabilized rocket with automatic control system.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used a low cost MEMS sensor (MPU-6050) and was able to find accurate roll, pitch &amp;amp; yaw of the rocket using a kalman filter and sensor calibration. Later I installed some servo motors on the body to control movable fins, similar to the video. I built an air tunnel to test &quot;making fast air flow directly to the rocket.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My goal is to set a specific pitch and yaw angle to the PID controller so that it moves the fins and makes the rocket stable within it (like the video). What I'm unable to determine is the transfer function that will translate the desired pitch to an action by the motor to make the rocket compensate for deviations and stay stable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, I want pitch = 30 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sensor reads 35, thus I have a 5 degree error, which is propagated to the PID controller . &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The PID will then output a value that will be translated to a motion by the servo connected to the fins that control the pitch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do I find this equation, taking into consideration speed and other dependencies.&lt;/p&gt;&#xA;" OwnerUserId="6336" LastEditorUserId="6214" LastEditDate="2014-10-27T19:06:56.283" LastActivityDate="2019-04-22T17:56:26.170" Title="Finding rocket transfer function for rocket stabilization" Tags="|rockets|mission-control|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5622" PostTypeId="2" ParentId="5617" CreationDate="2014-10-16T15:25:17.977" Score="12" Body="&lt;p&gt;The general idea behind this is as follows. After a certain size of impact, there isn't really much that will increase the devastation. So if you take two rocks, each half the size of the first, and toss it, you will quite likely have 2 large impacts, instead of 1 large impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you manage to make all of the pieces small enough, then there would be less overall devastation than a single large impact, because small bits will burn up. If every piece was, say, smaller than 10m in diameter, no pieces would survive the Earth's atmosphere. Of course, that much dust in the atmosphere would cause a &quot;nuclear winter&quot;, but that's another issue entirely. Likely that would happen no matter what occurred, although it might be somewhat worse for the large amounts of dust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the question then becomes, at what size is a chunk of rock harmless, and at what size does it not significantly matter what size it is? Using this &lt;a href=&quot;http://www.purdue.edu/impactearth&quot; rel=&quot;noreferrer&quot;&gt;handy calculator&lt;/a&gt;, I've played around with that. I'm going to assume you either have 1 200m rock, or 8 100m rocks widely spread. The 100m rocks produce an explosion of 22.7 Mtons, and a 1.2 km crater. The 200m rock provides a 182 MTon blast, with a crater of 3 km in diameter. The total energy is slightly larger for the 100m rocks, and the crater area is slightly larger as well. In addition, with 8 hits instead of just 1 you are more likely to be in a populated area. A 400m rock would produce an explosion of about 1.45 GTon, with a crater of about 5 km, which is also less damage than 8 200m rocks would cause. Overall, more damage comes from small rocks than larger rocks of the equivalent size, unless you can make the rocks small enough to not hit the Earth at all.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-11-19T14:39:36.950" LastActivityDate="2015-11-19T14:39:36.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5623" PostTypeId="2" ParentId="5617" CreationDate="2014-10-16T22:36:43.770" Score="2" Body="&lt;p&gt;For meteorites large enough, blowing them apart before impact might not help &quot;spread the damage&quot;, unless some of the chunks are diverted into missing the earth completely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a really large meteorite (like the dinosaur killer) hits, a lot of the kinetic energy of the incoming mass is transferred to high-velocity ejecta.  Some of this will escape the earth;  the rest will be in various elliptical orbits that &lt;strong&gt;must&lt;/strong&gt; intersect the earth at some point in the future.  When they do, they will deliver their kinetic energy to the atmosphere where they hit, a random point somewhere on earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the amounts of energy are so high that this random spread of part of the impact energy uniformly all over the world will still raise the surface temperature to destructive levels...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Blowing the meteor into gravel or dust might just speed the process up a little...&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2014-10-16T22:36:43.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5624" PostTypeId="1" AcceptedAnswerId="5626" CreationDate="2014-10-17T01:34:02.730" Score="8" ViewCount="1545" Body="&lt;p&gt;What would mankind use in order to adapt or merely cope with the effects of a large increase of gravity on an alien world? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming there are no other problems we would face.&lt;/p&gt;&#xA;" OwnerUserId="6345" LastEditorUserId="49" LastEditDate="2014-12-02T14:04:08.703" LastActivityDate="2014-12-02T14:04:08.703" Title="Adapting to Increased Gravity on Alien Planets" Tags="|gravity|technology|planet|hypergravity|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5625" PostTypeId="2" ParentId="5624" CreationDate="2014-10-17T02:15:10.553" Score="2" Body="&lt;p&gt;A stool.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There exists a technology known as &quot;sitting&quot; which is commonly applied in multi-G environments on and near Earth, for example during some airflight. So, the answer is that we'd use our bottoms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the same density, surface gravity increases only linearly with the radius of the planet. And already at a few times earth's radius, planets are thought to collect huge hydrogen atmospheres, like our gas giants, where we'd rather swim than sit. Bare rocks with more than double Earth surface gravity are likely very rare.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-10-17T02:15:10.553" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5626" PostTypeId="2" ParentId="5624" CreationDate="2014-10-17T02:39:34.493" Score="9" Body="&lt;p&gt;This is not only relevant to the off chance, distant future human colonisation or otherwise long term stay on celestial bodies with larger than Earth surface gravity, but also constant acceleration space travel at over 1 g. It's a fair question, and to be fair in return, as far as adaptation to hypergravity conditions goes, we haven't a clue to what degree it could still be considered safe for humans and at what point you'd start taking risks of increased heart failure rates, respiratory problems, dizziness and inability to perform basic functions as the inner ear fails and we'd become disoriented, or even experience hypoxia or equally deadly nitrogen narcosis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some studies of mammals adapting (or not as they have shown) have been done with centrifuges. For example, &lt;a href=&quot;http://www.euroestech.net/resources/Rat_HyperG_2001_holleyabstract.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA Ames et al. conducted a study on rats&lt;/a&gt; (PDF) that concluded:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Exposing rats to hypergravity via chronic centrifugation resulted in&#xA;  an acute gravity dose  dependent decrease in both core body&#xA;  temperature, and gross locomotor activity. Circadian rhythm phase and&#xA;  amplitude were altered by the exposure to the hyperdynamic conditions&#xA;  on the centrifuge.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But these were rather short-term (days or weeks) experiments on four-legged critters with small bodies. Not the best representatives for what problems humans would face in hypergravity at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My take is that natural adaptation of humans to hypergravity environment is likely impossible in environments with largely increased gravity because we're simply too tall and our cardiovascular and respiratory systems wouldn't cope for long periods of standing up and doing things as we normally would. I've given away one clue how to partially solve this now, though. Don't stand up. If this could be done for extended periods of time and still permit humans to do much anything to make it worth the trouble, that I don't know, but it's a start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way could be by wearing compression garments, not unlike those that fighter pilots wear while they're expected to perform high gee maneuvers and compress lower body to momentarily cut the blood supply to the legs and increase blood pressure in the rest of the fighter pilot's body, allowing the blood to still reach their brains and they don't fall into what's now known as a G-LOC (G-force induced Loss Of Consciousness). Problem is, compression garments in constant hypergravity environments couldn't deflate for long enough time to let its occupant breathe normally again and allow blood flow through lower body. So they would have to be redesigned to likely inflate (compress) and deflate (decompress) at intervals following one's heartbeat to support human cardiovascular and respiratory function.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the while, we'd also most likely have to wear rebreathers to support and control our respiratory functions even if the hypergravity planet we'd be inhabiting has perfectly acceptable and breathable atmosphere. I've already mentioned some possible unfavorable scenarios regarding our respiratory system at such conditions, but another one not excluded would be pulmonary edema resulting in asphyxiation simply because of the additional pressure on our lungs and the associated difficulty with breathing. Healthy individuals in their prime years would likely be fine in this regard for a short time, but that's not enough for colonisation, merely perhaps outposts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to recap, we don't know. There, I said it.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Commentary&lt;/strong&gt;: Perhaps more interesting than the fact that we don't have enough data on adversity of hypergravity on humans is the question why don't we. One immediately apparent problem is the risk involved with doing such experiments and that some of the adverse effects of it might take years to develop and with no way of knowing we'd be able to prevent or at least manage them once they do, since we wouldn't even know what exactly to expect. But there are strictly technical limitations too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, centrifuge systems don't really simulate hypergravity conditions precisely enough since there's always a downward facing component (Earth's own gravity) adding to the list of acceleration forces. So the combined acceleration vector in such horizontal systems is actually inclined to the plane of rotation somewhere between 0 and 90° but never one or the other as long as the centrifuge rotates. And if the centrifuge is vertical, combined acceleration forces oscillate on each rotation and result in something akin to a tumble  dryer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, the largest such horizontal centrifuge system ever built and actually put to use was by Soviets during the early human spaceflight endeavours and it was big enough that it could house cosmonauts for days at a time. It used a sloped floor and &lt;em&gt;living quarters&lt;/em&gt; area. As you can probably imagine, it was a lot more fun for those running the experiments as it was for their subjects. Acceleration gradient along the radius and rotation alone through Coriolis effect produced some interesting but expected side effects:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/MC9eg.gif&quot; alt=&quot;enter image description here&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/aIOkL.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Cosmonauts training in Star City centrifuge system demonstrate &lt;a href=&quot;https://en.wikipedia.org/wiki/Coriolis_effect&quot; rel=&quot;noreferrer&quot;&gt;Coriolis effect&lt;/a&gt; on the flight of darts as the centrifuge rotates.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But perhaps more importantly, cosmonauts experienced dizziness, nausea, lost motoric skills due to effects of acceleration / simulated hypergravity gradient affecting the inner ear's and hippocampus' ability to translate sensory information to sense of orientation and ill-affected spatial memory, they would regularly &lt;em&gt;display their lunch&lt;/em&gt; after they consumed it (or tried to), and needed days to weeks to recover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was by no stretch of imagination a system that could precisely simulate effects of prolonged stay in hypergravity environment. And building even larger such horizontal systems to somewhat remove acceleration gradient, and one with direct access through its focal point as it spins, is out of the question due to technical and material limitations. The last thing you'd want in a centrifuge system is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)&quot; rel=&quot;noreferrer&quot;&gt;Galloping Gertie&lt;/a&gt; effect. Unless you build a circular track, you're fine with not being able to simply enter and exit the centrifuge system while it operates, and can maintain large enough and constant speed of the carts. Perhaps a circular track maglev?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, all this could be solved by building such centrifuge systems in orbit, i.e. space wheels rotating at larger than Earth's surface gravity equivalent centrifugal acceleration, but there's another problem that they would have to be absolutely enormous and with radius large enough to also remove the strong gravity gradient along a human body and approach levels of it small enough that it could be claimed it's largely insignificant and not affecting your experiment's results to a great degree.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doing this in orbit is however a far cry from our current technical capabilities, because of all the support facilities needed to run such experiments on humans in a controlled environment. Getting test subjects willing to participate might not be easy either since you're by law required to operate under full disclosure (in US and EU at last) yet you wouldn't really know what to expect. So you'd have to be prepared for any eventuality, and that simply means also having too many facilities in orbit that we don't yet have. You'd also run high liability risks of such operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this means that no space agency or research institutions have come even close to setting anything like it in motion, let alone started doing such experiments. It's one of the great unknowns we'll most probably have to deal with by relying on robotic exploration until limitations of our own bodies can be addressed by our technology. We're not there yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're interested, these issues were also discussed from a bit different perspective (studying effects of reduced gravity and long term presence on Mars on humans) during the recent The Space Show &lt;em&gt;Classroom edition&lt;/em&gt; by Dan Adamo, Dr. John Jurist, Dr. Jim Logan and the host Dr. David Livingston. They were mostly commenting on NRC's &lt;em&gt;Pathways to Mars&lt;/em&gt; study but they did mention lack of such studies and why that is so. Recording and further info are available &lt;a href=&quot;http://www.thespaceshow.com/detail.asp?q=2334&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; (MP3).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-19T17:54:33.683" LastActivityDate="2014-10-19T17:54:33.683" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5627" PostTypeId="2" ParentId="3524" CreationDate="2014-10-17T06:20:02.137" Score="2" Body="&lt;p&gt;Dr. Kiran Kumar of ISRO, in his speech at IITG Ahmedabad  has confirmed that the magnetic fields of the coils cancel each other when energized together. This was (fortunately) found in earth orbit, during the fourth orbit raising manoeuvre.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.youtube.com/watch?v=l_e7L0nJTQw#t=392&quot; rel=&quot;nofollow&quot;&gt;http://www.youtube.com/watch?v=l_e7L0nJTQw#t=392&lt;/a&gt;   ( from 5:02 onwards )&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe this confirms my original guess on the cause of failure:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;one cause of this anomaly which I could think of is, that the coils are wound in opposite directions. Or the current to one of the coils is being fed in opposite direction, maybe because the terminals got connected in the wrong order, thus resulting in cancellation of magnetic flux / force.&quot;&lt;/p&gt;&#xA;" OwnerUserId="1507" LastEditorUserId="1507" LastEditDate="2014-10-17T07:33:05.230" LastActivityDate="2014-10-17T07:33:05.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5628" PostTypeId="1" CreationDate="2014-10-17T11:20:46.587" Score="12" ViewCount="2412" Body="&lt;p&gt;Technically speaking, given enough mission time and operator attention...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Could Curiosity dig itself a &lt;a href=&quot;https://www.google.com/search?q=burrow&amp;amp;tbm=isch&quot; rel=&quot;nofollow noreferrer&quot;&gt;burrow&lt;/a&gt;?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7st0O.jpg&quot; alt=&quot;A burrow&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How long would that take? &#xA;What would be the technique for that? Has it already seen a soil good for that?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="-1" LastEditDate="2014-10-18T12:27:47.997" LastActivityDate="2014-10-18T12:27:47.997" Title="Could Curiosity dig itself a burrow?" Tags="|mars|curiosity|construction|habitat|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5629" PostTypeId="2" ParentId="5628" CreationDate="2014-10-17T12:28:48.907" Score="6" Body="&lt;p&gt;No. It can scoop up only &lt;a href=&quot;http://msl-scicorner.jpl.nasa.gov/samplingsystem/&quot; rel=&quot;noreferrer&quot;&gt;1 to 30 cubic centimeters&lt;/a&gt; of soil at a time. It has done so only few times during its two years on Mars. By locking some of the wheels while driving it can make some marks on the ground. It's not really a caterpillar.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-10-17T12:28:48.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5630" PostTypeId="2" ParentId="5628" CreationDate="2014-10-17T15:19:54.663" Score="8" Body="&lt;p&gt;It probably could, but with its wheels, not with the drill on its robotic arm. But why would it do that? The Curiosity rover has independent drive on all six of its wheels so technically all it would have to do is find a soft enough ground, lock the rest of its wheels and dig a hole with one or two of its front or rear wheels that also have individual steering motors. Check e.g. &lt;em&gt;Five Things About NASA's Mars Curiosity Rover&lt;/em&gt; article in &lt;a href=&quot;http://www.nasa.gov/mission_pages/msl/building_curiosity.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Science Laboratory Mission News&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Big Wheels&lt;/strong&gt;: Each of Curiosity's six wheels has an independent drive&#xA;  motor. The two front and two rear wheels also have individual steering&#xA;  motors. This steering allows the rover to make 360-degree turns&#xA;  in-place on the Mars surface. The wheels' diameter is double the wheel&#xA;  diameter on Spirit and Opportunity, which will help Curiosity roll&#xA;  over obstacles up to 75 centimeters (30 inches) high.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;With its drill it would probably take ages though since we're talking of a few inches deep holes at a time, and you'd lose ability to drill precisely enough anyway since the MAHLI (Mars Hand Lens Imager) cam couldn't see what the rover is doing with its robotic arm until the hole drilled would be wide enough. So I'd rule that option out as infeasible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as digging with wheels is concerned though, similar has actually happened to the Spirit rover (aka MER-A, Mars Exploration Rover A) as it unwittingly locked its wheels into the sandy pit but &lt;a href=&quot;http://mars.jpl.nasa.gov/mer/mission/status_spiritAll_2007.html#sol1384&quot; rel=&quot;nofollow noreferrer&quot;&gt;managed to break free&lt;/a&gt; after many attempts during what was then nicknamed the Free Spirit campaign:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/RZ4wP.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;MER-A right-rear wheel stalled in sand. Image Source &amp;amp; Credit: &lt;a href=&quot;http://www.nasa.gov/mission_pages/mer/images/mer20091201.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA/JPL-Caltech&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, I think Curiosity could &lt;em&gt;dig itself a burrow&lt;/em&gt; if its mission planners really wanted it to. It would have to find some terrain with locked dry ice and/or water ice in it first so the soil it would dig into would have &lt;em&gt;some&lt;/em&gt; adhesive properties yet still be soft enough to allow some progress in digging without constant caving in and the walls around the burrow collapsing, and it would have to run over excavated materials back and forth to compact them with its weight, but it's not out of the realm of possibility, if a bit on the vivid imagination side of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would probably be a lot easier simply finding a naturally occurring burrow, like perhaps a collapsed lava tube, wind eroded part of a soft rock or a hole that used to be occupied by dry ice that became surface exposed and evaporated, sometimes explosively so in what are known as dry ice fans and become seasonally active during the Martian springs.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-17T17:52:12.913" LastActivityDate="2014-10-17T17:52:12.913" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5631" PostTypeId="2" ParentId="5628" CreationDate="2014-10-17T17:00:28.800" Score="18" Body="&lt;p&gt;No, it couldn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Curiosity rover is 2.9 meters by 2.7 meters by 2.2 meters. It's tiny little scoop is 7 centimeters by 4.5 centimeters by 2 centimeters (or so; I can't find a reference on the last dimension). Assuming that the burrow is at least four times the volume of the object, this means more the robotic arm needs to make a million scoops with that tiny little scooper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity doesn't do anything with speed. By my timing, it takes Curiosity one or two minutes just to scoop up one scoop of fine Martian regolith. After scooping this tiny bit of material, Curiosity would then have to move a bit, call it two meters, to dump the contents away from the burrow. At 30 meters per hour, the Curiosity rover is not a race horse. The process per scoop would take five to ten minutes. At a 365/247/7 pace, a million scoops would mean Curiosity would need at least a decade or two to accomplish this task. Since the Curiosity rover doesn't have lights, so that means doubling the time, call it two decades, minimum. Curiosity's generator will die in fourteen years or so. It can't be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above completely ignores the fact that Curiosity's robotic arm almost certainly is not designed to make a million scoops. It's designed to make on the order of a hundred scoops, maybe less. The robotic arm will fail long before the burrow can be dug.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-10-17T17:38:31.177" LastActivityDate="2014-10-17T17:38:31.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5632" PostTypeId="2" ParentId="5617" CreationDate="2014-10-18T00:41:51.270" Score="1" Body="&lt;p&gt;While the existing answers all address the issue I would like to add a different perspective on what's going on here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Blasts extend in three dimensions.  However, on this scale humanity exists in only two dimensions.  All the energy expended up or down does basically no damage.  Thus damage scales at the 2/3 root of the energy.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-10-18T00:41:51.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5633" PostTypeId="1" AcceptedAnswerId="5659" CreationDate="2014-10-18T02:10:14.600" Score="2" ViewCount="686" Body="&lt;p&gt;Well, more specifically, using a counterweight like you'd find in a non-space elevator to give the rocket a little acceleration before firing up and spending all that fuel-to-lift-the-fuel-to-lift-the-etc.&lt;br&gt;&#xA;I understand from &lt;a href=&quot;https://space.stackexchange.com/questions/5463/why-dont-we-use-catapults-to-get-to-space?lq=1&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/2370/what-technological-hurdles-prevent-the-development-of-a-space-gun&quot;&gt;here&lt;/a&gt; that doing &lt;em&gt;all&lt;/em&gt; the acceleration through non-rocket methods would be prohibitively difficult.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, would it be feasible to &lt;em&gt;reduce&lt;/em&gt; the loaded-fuel requirement of a launch significantly, by keeping the rocket engines &lt;em&gt;off&lt;/em&gt; until the craft's velocity reached some acceptably higher positive number?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, to put specific numbers on &quot;significantly&quot;: &lt;strong&gt;how much extra delta-v would we need to add through non-rocket methods to reduce the carried fuel by, say, 10%?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image I've got in mind here is the rocket being pulled up along a partial space elevator (supposing that building a complete space elevator in one go turns out to be just too much), by the action of a counterweight. - I have no idea whether there's anything strong enough to even be used as a cable (or set of cables, or pulley system) here, given how massive spacecraft apparently are.  &lt;/p&gt;&#xA;" OwnerUserId="6222" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-10-21T06:42:54.203" Title="Could pull from a space elevator be used to assist a rocket launch?" Tags="|launch|delta-v|space-elevator|fuel|" AnswerCount="3" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5634" PostTypeId="2" ParentId="5617" CreationDate="2014-10-18T13:03:38.037" Score="4" Body="&lt;p&gt;The idea in the movie was to split the asteroid into two. This is actually a sound idea - an explosion (unless it's absolutely massive compared to the recipient) won't change the center of mass of the object, but if you cut it in half AND give each half enough velocity so both halves miss the planet, you're good until the next one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, there will be some debris that will still hit you. Random chance as to wether you survive but that's better than the zero chance if Texas hits you at multi-km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where the movie failed was the timing - you're not going to catch up to an inbound rock, drill into it's core, insert a multi-megaton explosive and crack the thing nicely in two if you start inside the moon's orbit. If we do notice anything that big coming at us with 18 days left, we had a good run.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you notice the impending doom years out you don't need to go to much trouble at all - a satellite-sized booster stage will change it's orbit enough to miss the planet. 12,000 km is a &lt;strong&gt;really&lt;/strong&gt; small target, doesn't take much to miss it.&lt;/p&gt;&#xA;" OwnerUserId="5933" LastActivityDate="2014-10-18T13:03:38.037" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5635" PostTypeId="1" AcceptedAnswerId="5706" CreationDate="2014-10-18T13:26:31.437" Score="15" ViewCount="8353" Body="&lt;p&gt;With something like a coil gun mass driver in mind where the more acceleration the cheaper the design in mind:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were to suspend the astronauts in a shallow layer of water and horizontally compared to the direction of acceleration - how many g could they survive?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I already know from a wiki article that &lt;a href=&quot;http://www.ejectionsite.com/stapp.htm&quot;&gt;a guy survived 46 g in a rocket sled&lt;/a&gt; unsuspended and horizontally oriented.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe the effect is nothing, but wouldn't the water support their mass perfectly and also supply pressure preventing blood draining/vein damage?&lt;/p&gt;&#xA;" OwnerUserId="6350" LastEditorUserId="265" LastEditDate="2014-10-18T15:36:17.857" LastActivityDate="2023-03-12T23:32:15.710" Title="Max g survivable suspended in water?" Tags="|launch|gravity|life-support|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5636" PostTypeId="2" ParentId="5601" CreationDate="2014-10-18T18:28:45.243" Score="1" Body="&lt;p&gt;Not sure, if I should &quot;answer&quot; or &quot;comment&quot;, but let's put some more questions...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I have a two stage launch and I still want to stick up with the $9000 \text {km/s} \Delta V$, should I then first calculate the fractions of propellant I need for the two stages?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$M_f = 1 - (M_1/m_0) = 1 - e^{-\Delta V/V_e}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And If the first stage would be until $5000 \text {m/s}$ and second until $9000 \text {m/s}$ with $V_e = 4500 \text {m/s}$, I would get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$M_{f1} = 1 - e^{-5000/4500} = 67.1% \text {propellant}$$&#xA;$$M_{f2} =  1 - e^{-4000/4500} = 58.9% \text {propellant}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the second one, I need to use the deltaV from $5000$ to $9000 \text {m/s}$ ($=4000 \text {m/s}$), right? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With assumption of $8%$ from the mass being construction, I end up with&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First stage:&#xA; - construction $8%$&#xA; - propellant $67.1%$&#xA; - Second stage $24.9%$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second stage:&#xA; - construction $8%$ ($2.0%$ from initial mass)&#xA; - Propellant $58.9%$ ($14.7%$ from initial mass)&#xA; - Final payload $33.1%$ $(8.3%$ initial mass)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in TOTAL the shares are:&#xA;- $8.25%$ the final payload&#xA;- $81.76%$ propellant&#xA;- $9.99%$ construction&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the payload needs to be $40 \text t$, I end up with:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First stage:&#xA;- Construction $38.8 \text t$&#xA;- Propellant $325.2 \text t$&#xA;- Second stage $120.8 \text t$&#xA;TOTAL: $484.8 \text t$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Second stage:&lt;/li&gt;&#xA;&lt;li&gt;Construction $9.7 \text t$&lt;/li&gt;&#xA;&lt;li&gt;Propellant $71.1 \text t$&lt;/li&gt;&#xA;&lt;li&gt;Final payload $40 \text t$&#xA;TOTAL $120.8 \text t$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So far so good (even though I am skipping many details as this is not really my field of science at all, I am trying to keep the focus of only getting the order of magnitude of the energy use...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I am in trouble though... Which energy equation should I use:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E = 0.5 \times (m_0-m_1) \times Ve^2$$   ($m_0-m_1$ would be the mass of the propellant)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E = 0.5 \times m_1 \times (e^{\Delta V/V_e}-1) \times V_e^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the first equation I get $3293 \text {GJ}$ and $0.720 \text {GJ}$, so $4013 \text {GJ}$ in TOTAL.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the second I got confused with m1. In first stage the $m_1$ is the mass of the second stage ($120.8 \text t$)? But with the second stage it must be the mass of the payload and the construction, right...? Or just the payload ($40 \text t$)? Or is this matter of definition, how loose boundaries do I want to set to these calculations? (the payload is ONLY the useful payload ($= 40 \text t$) or also the whole construction of the second stage (= useful payload + engines, empty tanks etc)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks a lot, if there's still more people willing to help in this interdisciplinary research of mine...&lt;/p&gt;&#xA;" OwnerUserId="6311" LastEditorUserId="5963" LastEditDate="2014-10-18T21:52:54.507" LastActivityDate="2014-10-18T21:52:54.507" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5637" PostTypeId="2" ParentId="5511" CreationDate="2014-10-18T20:00:21.057" Score="2" Body="&lt;p&gt;It occurs to me that the answer to this question depends on what you want to get for your money. If you are only interested in the ice itself, in a source of water for use on Earth on in LEO, then other options are preferable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you wish to get greater returns on your investment in the form of ancillary benefits which themselves have value, both marketable and otherwise, then Saturn would seem to be a much better option. The ring system, including some of it's smaller moons, are a much easier target from an engineering perspective for manned or partially-manned operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This goes beyond the reduction in processing required to make use of the water in situ or for market, and includes such things as the radiation environment, available human-friendly day/night cycles/orbital periods and useable resources - one of which being the generation of electricity. If the interest is long-term, multi-purpose industrial infrastructure, Saturn is a much more favorable source than the moon or even the Trojan asteroids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With potentially serious examination being undertaken of this kind of resource use by entities such as &lt;a href=&quot;http://www.planetaryresources.com&quot; rel=&quot;nofollow&quot;&gt;Planetary Resources&lt;/a&gt; and even the &lt;a href=&quot;http://www.whitehouse.gov/blog/2014/10/14/bootstrapping-solar-system-civilization&quot; rel=&quot;nofollow&quot;&gt;United States Government&lt;/a&gt;, it is important not only to consider what is best or even feasible, but what has the greatest likelihood of being funded or capitalized and actually occurring. In this light, multi-purpose endeavors could be the way to go.&lt;/p&gt;&#xA;" OwnerUserId="6352" LastActivityDate="2014-10-18T20:00:21.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5639" PostTypeId="2" ParentId="5384" CreationDate="2014-10-19T07:21:39.267" Score="2" Body="&lt;p&gt;International governmental cooperation is a huge burden. Several incompatible legal and bureaucratic systems and a multiplication of corrupt politicians who only care about where the money budget is spent so that their special interests can profit from it. Politicians never care about the results, they are only out grabbing as much cash as they can get their hands on. They are only in office for a few years anyway. Just see today how the ISS has suddenly become a pawn in some diplomatic game between Russia and the US. The more international a mission gets, the more of that kind of irrelevant stuff will dominate it, because some politician sees a way to profit from sabotaging the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS is a disaster for space exploration. It costs everything and delivers near to nothing. It is a prime example of how devastating &quot;international cooperation&quot; can be. And have a look at ESA where different countries are tearing each other apart about the &quot;Ariane 6&quot; because solid rockets are manufactured in one country and liquid rockets in another country. There will either be no Ariane 6, or a bad expensive compromise. Thanks to &quot;international cooperation&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Productive exploration will be done by companies or single governments. A government can have dedicated skilled bureaucrats (for some time anyway), but two governments cannot have that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rational space station had been a prototype of a spaceship to Mars, using concepts like simulated gravity and recycling. The ISS today only has a diplomatic political purpose, it is not a scientific or exploratory mission.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-10-19T07:29:28.633" LastActivityDate="2014-10-19T07:29:28.633" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5640" PostTypeId="2" ParentId="5633" CreationDate="2014-10-19T11:21:01.173" Score="1" Body="&lt;p&gt;No. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several problems here: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;If you use a counterweight, the maximum speed it can reach in freefall is Mach 1 or 300 m/s. Orbital speed is around 7 km/s, so you've reduced the speed requirement of the rocket only by a bit. This means the rocket still weighs almost as much as a ground-based rocket. A Soyuz rocket weighs around 600 tons at liftoff. Let's say you can save 100 tons. That still means you have to lift 500 tons, plus several times that for the counterweight. See &lt;a href=&quot;https://space.stackexchange.com/questions/5531/why-arent-all-satellite-carrying-rockets-launched-from-airplanes&quot;&gt;this related question&lt;/a&gt;, and &lt;a href=&quot;https://space.stackexchange.com/questions/744/effect-of-atmospheric-drag-on-rocket-launches-and-benefits-of-high-altitude-laun&quot;&gt;this one&lt;/a&gt;.   &lt;/li&gt;&#xA;&lt;li&gt;The counterweight must be connected to the rocket by a cable. If you separate the rocket from the space elevator at an altitude of 10 km, the cable is 10 km long. That's several times longer than the highest elevators in use today. &lt;/li&gt;&#xA;&lt;li&gt;How do you separate the rocket from the space elevator? The elevator must be strong enough to withstand the rocket exhaust. For ground-based launches you have hundreds, if not thousands of tons of concrete and steel for the launchpad, and hundreds of tons of water to reduce the damage done to the pad. All of this would have to be accelerated along with the rocket, and then braked to a standstill after separation.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;All in all you'd have to construct a tower 10 times higher than the highest skyscraper in use today, and build it strong enough to carry several thousand tons at the top. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-10-19T11:21:01.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5642" PostTypeId="1" AcceptedAnswerId="5644" CreationDate="2014-10-19T21:21:48.590" Score="8" ViewCount="9894" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/RP-1&quot;&gt;Wikipedia&lt;/a&gt; references the ratio of liquid oxygen to RP-1 as 2.56. Is this based on mass or volume? Are they using some typical combustion chamber pressure? &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-10-19T21:35:53.060" LastActivityDate="2019-04-15T19:52:43.360" Title="Are oxidizer to fuel mixture ratios based on mass or volume?" Tags="|fuel|oxidizer|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5643" PostTypeId="2" ParentId="5642" CreationDate="2014-10-19T22:43:11.803" Score="3" Body="&lt;p&gt;I believe that's the mass ratio. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_rocket_propellants#Propellant_table&quot; rel=&quot;nofollow&quot;&gt;liquid rocket propellants page&lt;/a&gt; on Wikipedia gives 2.77:1 as the mass ratio; the optimum varies with the exact engine design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's some interesting discussion of fuel ratios in this &lt;a href=&quot;http://yarchive.net/space/rocket/fuels/fuel_ratio.html&quot; rel=&quot;nofollow&quot;&gt;old usenet article&lt;/a&gt;.  &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-10-20T02:14:45.887" LastActivityDate="2014-10-20T02:14:45.887" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5644" PostTypeId="2" ParentId="5642" CreationDate="2014-10-19T22:44:18.620" Score="8" Body="&lt;p&gt;It's always by mass but there's one additional problem that rockets pretty much never use  &lt;a href=&quot;https://en.wikipedia.org/wiki/Stoichiometry&quot; rel=&quot;noreferrer&quot;&gt;stoichiometric&lt;/a&gt; oxygen to fuel ratio (O/F) that you'd expect if you calculated molar mass of propellant components an their ideal exhaust product(s). So the source should mention that somewhere if they're using stoichiometric ratios, typical O/F or even quoting some specific system's ideal O/F (and for where, usually given for sea-level and vacuum).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a note, quoting volumetric ratio wouldn't make sense without also quoting storage pressure and temperature. And in case of mixtures and otherwise non-pure (e.g. non-refined) propellants also their molar composition. And ullage volume. And so on.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-19T22:52:30.290" LastActivityDate="2014-10-19T22:52:30.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5646" PostTypeId="2" ParentId="5436" CreationDate="2014-10-20T12:22:05.487" Score="1" Body="&lt;p&gt;I believe India's Mars mission was much more cost-efficient than that of NASA's MAVEN due to the following reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;ISRO made use of Indian parts for the construction of Mangalyaan which drastically cut down the prices.&lt;/li&gt;&#xA;&lt;li&gt;Use of inter-planetary slingshot played a major role.&lt;/li&gt;&#xA;&lt;li&gt;MAVEN's capability and operation lifetime is much more than that of Mangalyaan's.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="6360" LastEditorUserId="49" LastEditDate="2014-10-20T12:34:26.233" LastActivityDate="2014-10-20T12:34:26.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5647" PostTypeId="1" AcceptedAnswerId="5648" CreationDate="2014-10-20T16:10:34.417" Score="3" ViewCount="957" Body="&lt;p&gt;...or, &quot;What is the inclination of a satellite that travels over Earth's equator?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering Earth's axial tilt is 23.4°, it's a significant difference - is the inclination calculated relative to the plane of ecliptic, or relative to Earth's global coordinates?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the sun-synchronous satellites travel at inclination of &lt;a href=&quot;https://space.stackexchange.com/a/823/103&quot;&gt;98 degrees&lt;/a&gt; I'm inclined (pun unintended) to believe it's the former but I prefer to ask in case I'm guessing wrong.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-10-20T16:37:28.330" Title="What is satellite inclination calculated relative to?" Tags="|orbital-mechanics|orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5648" PostTypeId="2" ParentId="5647" CreationDate="2014-10-20T16:32:10.293" Score="3" Body="&lt;p&gt;This is another one of those data that you'd normally expect to be accompanied by &lt;em&gt;with respect to what&lt;/em&gt; (plane of reference) metadata and explaining what they're quoted for. In Earth orbit, if this &lt;em&gt;&quot;w.r.t.&quot;&lt;/em&gt; isn't given, it's safe to assume that the reference &lt;em&gt;zero angle&lt;/em&gt; is the equatorial plane in the direction of Earth's rotation on its axis (e.g. TLE, Two-Line Element sets also give values with the plane of reference being the equatorial plane and prograde as 0 angle of the primary body, i.e. ascending node), but you'd often also find sources quoting values relative to something else, say the Laplace's invariable plane, ecliptic, or alike. Sadly, this crucial metadata is often also missing and you're then required to find more reliable quotes somewhere else or dig for metadata to properly interpret quoted values, since they can be ambiguous and a subject to misinterpretation without it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-20T16:37:28.330" LastActivityDate="2014-10-20T16:37:28.330" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5649" PostTypeId="1" AcceptedAnswerId="5655" CreationDate="2014-10-20T17:58:14.840" Score="26" ViewCount="9255" Body="&lt;p&gt;Could a satellite, without too much fuel consumption for station keeping, orbit the Earth so that it always has a line of sight to the Moon? It would need a near polar orbit which precesses in tandem with the Moon's orbit around Earth. Wouldn't that be the perfect communication satellite orbit for constantly staying in touch with Lunar missions? Have any satellites used such an orbit and does that type of orbit have a three letter name?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;I'm looking for the possibility of a satellite orbiting Earth's poles so that it can always have contact with the Earth facing side of the Moon, and with satellites in lunar polar orbit which precess so that they remain in line of sight with this satellite (and the rotating Earth). A potential use for this would be that such a polar Earth satellite would have a relatively small altitude and be able to relay data to Earth more easily than having to use something like the Deep Space Network spread across the equator.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-10-20T19:01:10.053" LastActivityDate="2014-10-23T07:06:47.587" Title="Can a satellite orbit Earth so that it always has the Moon in line of sight?" Tags="|the-moon|orbit|communication-satellite|earth|" AnswerCount="9" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5650" PostTypeId="2" ParentId="5649" CreationDate="2014-10-20T18:20:05.867" Score="23" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point&quot;&gt;http://en.wikipedia.org/wiki/Lagrangian_point&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A satellite orbiting at the L1 point or L2 point would be in constant contact with the moon.  Unfortunately, neither of these are particularly useful for keeping in contact with a lunar mission.  The L1 point is between the earth and the moon, and would offer no advantages over just putting the reciever on earth.  The L2 point is on the far side of the moon, and would therefore not be able to communicate with earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680015886.pdf&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680015886.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This article seems to be exactly what you want.  They conclude that a satellite near L2 would work.&lt;/p&gt;&#xA;" OwnerUserId="6364" LastActivityDate="2014-10-20T18:20:05.867" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5652" PostTypeId="2" ParentId="5642" CreationDate="2014-10-20T20:03:52.347" Score="2" Body="&lt;p&gt;It is a mass ratio as the ratio itself is based on the molecular masses of the fuel and oxidizer. Check out &lt;a href=&quot;http://www.braeunig.us/space/propuls.htm&quot; rel=&quot;nofollow&quot;&gt;http://www.braeunig.us/space/propuls.htm&lt;/a&gt; and there &quot;Combustion and Exhaust velocity&quot; for details. &lt;/p&gt;&#xA;" OwnerUserId="6311" LastActivityDate="2014-10-20T20:03:52.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5654" PostTypeId="2" ParentId="5649" CreationDate="2014-10-20T21:34:20.520" Score="3" Body="&lt;p&gt;Fillan Grady already gave the examples of orbiting a Lagrangian point, but based on your response to that, and subsequent edit, it seems you want a more traditional Earth orbit, rather than a Lagrangian &quot;cheat&quot;. But I will point out that these points &lt;em&gt;are&lt;/em&gt; valid ways to orbit the earth, and there is a reason they are so useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom-line: If you discount the L-points, it's not very likely, and certainly not in a low orbit. Every satellite orbit must follow a great circle, and therefore, must cross the equator at two points. This is true, even for polar orbits. Furthermore, these crossing points will remain fixed, relative to the celestial sphere (unless you invoke some sort of gravitational precession), while the Moon revolves around the Earth. That means that at some point, the Moon will almost certainly be opposite of the Earth during this crossing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you have to do is find a polar orbit high enough (and with the correct period and phase, relative to the Moon's orbit) that the Earth is small enough to not occult the moon during any of these crossings, ever. Consider a satellite in geosynchronous orbit (24 hour period). A back of the envelope &lt;a href=&quot;https://www.google.com/search?q=2*atan(6378.1/35786)%20in%20degrees&quot; rel=&quot;nofollow&quot;&gt;calculation&lt;/a&gt; says that the earth will appear to have an angular diameter of about 20 degrees. The angular portion of the Moon's orbit which this hides, on the opposite side the Earth, will be even greater than this. However, assuming a 27 day circular orbit, the Moon only covers about &lt;a href=&quot;https://www.google.com/search?q=360%2F27&quot; rel=&quot;nofollow&quot;&gt;13 degrees&lt;/a&gt; of its orbit per day, so if the Moon is just about to go behind the Earth during one orbit, it will not have come out the other side by the time your satellite returns to the same point 24 hours later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, a geosynchronous polar orbit would already take quite a bit of energy to achieve, and it isn't even close to providing what you would need. You'd have to go quite a bit higher, and it would almost certainly be cheaper to launch to L1, or to use network of multiple satellites on lower-altitude, lower-inclination orbits. But you've said that you want a low-altitude orbit -- that will make the Earth appear bigger, which will increase the time the Moon is behind it, while simultaneously increasing the frequency with which the satellite is in this shadow. &lt;/p&gt;&#xA;" OwnerUserId="6306" LastActivityDate="2014-10-20T21:34:20.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5655" PostTypeId="2" ParentId="5649" CreationDate="2014-10-20T21:55:54.410" Score="20" Body="&lt;p&gt;If I understand your edited question, then no.  While the Earth's J2 (oblateness) produces enough torque to rotate a Sun-synchronous orbit once a year, it does not produce enough torque to rotate a &quot;Moon-synchronous orbit&quot; once a month.  So there is no such orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not clear on what the utility of such an orbit would be, even if it did exist.  If you're going to point antennas at the Moon from the near vicinity of Earth, you can have many more antennas and much larger antennas on the surface of the Earth than what you would be able to put into Earth orbit for an equal amount of money.  You would only need to divide your total aperture by three to account for the downside of not always being able to see the Moon.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-10-21T02:04:17.053" LastActivityDate="2014-10-21T02:04:17.053" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5656" PostTypeId="1" AcceptedAnswerId="5657" CreationDate="2014-10-20T22:17:18.107" Score="13" ViewCount="903" Body="&lt;p&gt;In particular, are there any established property rights?&lt;/p&gt;&#xA;&lt;p&gt;Questions have been asked about the legal status of asteroid mining &lt;a href=&quot;https://space.stackexchange.com/questions/5611/legality-of-asteroid-mining&quot;&gt;here&lt;/a&gt;. The answer comes from the Outer Space Treaty of 1967. However, that treaty only seems to have covered &amp;quot;celestial bodies.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Intuitively, it seems like asteroid resources are closer to unmined natural resources while space debris is like a car that someone has left on the side of the road. (Albeit a road over which there appears to be no clear jurisdiction.)&lt;/p&gt;&#xA;&lt;p&gt;Practically speaking, I could envision scenarios where governments and corporations view space debris scrapping either positively or negatively. On the positive side, it would decrease the chances that debris would cause damage when falling out of orbit or that it would hit anything in-orbit. On the negative side, certain entities may not like strangers digging through their trash (so to speak).&lt;/p&gt;&#xA;" OwnerUserId="6368" LastEditorUserId="6368" LastEditDate="2022-07-01T18:12:36.980" LastActivityDate="2022-07-01T18:12:36.980" Title="What is the legal status of in orbit space debris?" Tags="|debris|law|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="5657" PostTypeId="2" ParentId="5656" CreationDate="2014-10-20T23:02:26.863" Score="13" Body="&lt;p&gt;According to the &lt;a href=&quot;http://history.nasa.gov/1967treaty.html&quot; rel=&quot;noreferrer&quot;&gt;Article VIII of the Outer Space Treaty&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A State Party to the Treaty on whose registry an object launched into&#xA;  outer space is carried shall retain jurisdiction and control over such&#xA;  object, and over any personnel thereof, while in outer space or on a&#xA;  celestial body. Ownership of objects launched into outer space,&#xA;  including objects landed or constructed on a celestial body, and of&#xA;  their component parts, is not affected by their presence in outer&#xA;  space or on a celestial body or by their return to the Earth. Such&#xA;  objects or component parts found beyond the limits of the State Party&#xA;  of the Treaty on whose registry they are carried shall be returned to&#xA;  that State Party, which shall, upon request, furnish identifying data&#xA;  prior to their return.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the launching nation holds property rights and is responsible for space debris they generate. Five years later, the 1972 &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Liability_Convention&quot; rel=&quot;noreferrer&quot;&gt;&quot;Space Liability Convention&quot;&lt;/a&gt; (in it's full title the &lt;a href=&quot;http://www.spacelaw.olemiss.edu/library/space/International_Agreements/Mulilateral/1972%20Liability%20Convention.pdf&quot; rel=&quot;noreferrer&quot;&gt;Convention on International Liability for Damage Caused by Space Objects&lt;/a&gt;, PDF) was based on the Article VIII of the OST and further provisions it, but sadly isn't specific about space debris in any part. And establishing whose debris caused damage isn't exactly equal to establishing origins of some part that fell on someone's land and you can analyze in laboratories. It isn't even clear if orbital debris tracking facilities (say, &lt;a href=&quot;https://en.wikipedia.org/wiki/North_American_Aerospace_Defense_Command&quot; rel=&quot;noreferrer&quot;&gt;NORAD&lt;/a&gt;) are legally obliged to share data on their origins in cases where it could hold the country running these facilities liable for damages to another country. And so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This and the fact that this issue is grossly under-regulated since mentioned first efforts were signed and ratified, and no internationally ratified law sufficiently addresses liability specifics as they pertain to orbital debris, comes with many interesting legal implications. But that's now over 47 years long and really tiring story so I won't bore you with all the details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a nice overview I'd suggest &lt;a href=&quot;http://www.unidir.org/programmes/emerging-security-threats/outer-space-security-conference-series&quot; rel=&quot;noreferrer&quot;&gt;UNIDIR's Outer Space Security Conference Series&lt;/a&gt;, e.g. the &lt;a href=&quot;http://www.unidir.org/programmes/emerging-security-threats/space-security-2014-implementation-and-compliance&quot; rel=&quot;noreferrer&quot;&gt;Space Security 2014: Implementation and Compliance&lt;/a&gt; one has some really interesting talks on the subject. Also check for related activities under umbrella of the &lt;a href=&quot;http://www.oosa.unvienna.org/&quot; rel=&quot;noreferrer&quot;&gt;United Nations Office for Outer Space Affairs&lt;/a&gt; (UNOOSA).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-27T17:38:27.417" LastActivityDate="2014-10-27T17:38:27.417" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5658" PostTypeId="2" ParentId="5649" CreationDate="2014-10-21T00:46:36.093" Score="15" Body="&lt;p&gt;It is possible in several ways.  Let's get the dumb ones out the way first.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;As I commented, you can orbit the moon itself.  Technically correct, since anything orbiting the moon is also orbiting the Earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You can make your satellite big enough so that it always has a line-of-sight.  A giant pole somewhat longer than the Earth's diameter can be arranged to orbit so as have a point with in line-of-sight.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Similarly, you can make your satellite encircle the Earth (a Niven ring or Dyson sphere)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Not outside-the-box answers:&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;4&quot;&gt;&#xA;&lt;li&gt;You can orbit at a Lagrange Point.  As this diagram of exactly your situation shows, any except $L_3$ are fine:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lHq1z.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;5&quot;&gt;&#xA;&lt;li&gt;I haven't run the numbers on this, but it should be possible to make a synchronized polar orbit with the moon.  I thought up a simple example:&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Say the orbital inclination of the moon were $0^{\circ}$ and circular.  Set up your satellite in the same orbit but at inclination $90^{\circ}$ and behind by a quarter orbit.  It might be a little hard to visualize, but in this configuration the line of sight never passes through the Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="2707" LastActivityDate="2014-10-21T00:46:36.093" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5659" PostTypeId="2" ParentId="5633" CreationDate="2014-10-21T01:26:53.440" Score="1" Body="&lt;p&gt;&lt;strong&gt;It's theoretically possible, but ludicrously impractical and missing the point.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't like to say things are impossible, but as another answer mentions, the numbers involved are too extreme.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You won't be limited by the speed of sound with a big enough counterweight, but the size of the counterweight, the height to which it must be stably lifted, and just the general &lt;em&gt;engineering&lt;/em&gt; pretty much rule this out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, all this is missing the point.  The hardest part of getting a rocket to orbit is to accelerate it &lt;em&gt;horizontally&lt;/em&gt;, not &lt;em&gt;vertically&lt;/em&gt;.  If you start from a standstill in LEO, you're still going to need a rocket almost the same size to get enough speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to escape the Earth's sphere of influence, an elevator-catapult makes a little more sense--but still only as much sense as a monumental tower-of-Babel-type-nonsense slingshot hurling a billion dollars of spaceship onto an escape trajectory could sound like.&lt;/p&gt;&#xA;" OwnerUserId="2707" LastActivityDate="2014-10-21T01:26:53.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5660" PostTypeId="2" ParentId="5649" CreationDate="2014-10-21T05:24:56.897" Score="0" Body="&lt;p&gt;Yes. You could place it at any of the Lagrange points that have LOS to the Moon. The first solution I would explore would be to place it in orbit about the Moon itself on a route that is generally perpendicular to the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There should be a number of acceptable orbital solutions to this problem, and dealing with larger bodies farther out will cost the least fuel for maintenance (but a larger amount for launch).&lt;/p&gt;&#xA;" OwnerUserId="6370" LastActivityDate="2014-10-21T05:24:56.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5661" PostTypeId="2" ParentId="5633" CreationDate="2014-10-21T06:26:51.887" Score="3" Body="&lt;p&gt;Are you thinking of a &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity-gradient_stabilization&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity gradient vertically stabilized tether&lt;/a&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mmyEg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike a Clarke tower, the &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/02/golden-tethers.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;elevator pictured&lt;/a&gt; isn't moving one circuit per sidereal day but with an orbital period of 2.15 hours. The tether &quot;center&quot; is about 2070 km above the earth's surface. Not really the center of the tether as the length above 2070 altitude needs to be longer to balance the portion below. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tether foot is at an altitude of about 200 km and moving 5.4 km/s. The tether top is at an altitude of 4265 km and moving 8.6 km/s which is escape velocity at that altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To rendezvous with the tether foot would take less delta V than achieving orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Catching a sub-orbital craft at the tether foot subtracts momentum. Repeated sub-orbital catches would eventually bring this tether down. But there are ways to restore momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way would be to run a current through the tether as it passes through earth's magnetic field. This would be an &lt;a href=&quot;http://en.wikipedia.org/wiki/Electrodynamic_tether&quot; rel=&quot;nofollow noreferrer&quot;&gt;electrodynamic tether&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way to preserve tether's momentum is to make super-orbital as well as suborbital catches. Super-orbital catches from a higher orbit (like from the moon or an asteroid parked in lunar orbit) would boost momentum. If super-orbital catches are balanced with sub-orbital catches, momentum can remain the same over time. Other momentum changing maneuvers are releasing payloads from the tether foot or from the tether top. Balancing these can also preserve momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not only is this elevator far shorter than a Clarke Tower, but endures much less stress. No scrith or Bucky tubes needed, Kevlar with a taper ratio of 5.2 could do the job.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However rendezvous with the tether foot at low altitudes would be very hard. In this &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=28099.0&quot; rel=&quot;nofollow noreferrer&quot;&gt;NasaSpaceFlight thread&lt;/a&gt;, Jim explains some of the difficulties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also the tether is passing through space with a relatively high debris density. It would likely be severed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, it seems such elevators would impractical for mitigating the ~9 km/s needed for achieving LEO.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-10-21T06:42:54.203" LastActivityDate="2014-10-21T06:42:54.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5662" PostTypeId="2" ParentId="5649" CreationDate="2014-10-21T06:28:16.590" Score="0" Body="&lt;p&gt;Besides of orbiting Lagrangian points, orbiting around Earth or Moon of 90 degrees with respect to Earth-moon plane inclination can be a solution. There is a simple image I made.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First one (orbiting around Earth) can be the cheapest solution of all because it is low-orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rafgk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6371" LastEditorUserId="6371" LastEditDate="2014-10-21T06:38:29.740" LastActivityDate="2014-10-21T06:38:29.740" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5664" PostTypeId="2" ParentId="5649" CreationDate="2014-10-21T11:00:56.573" Score="8" Body="&lt;p&gt;If the purpose is satellite communication between Earth and the Moon, here's my ranking of the options, from simple and cheap to exotic and expensive:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;No satellites&lt;/strong&gt;, a set of &lt;strong&gt;laser ground stations&lt;/strong&gt; operates from the Earth surface to one or two sites on the Moon. The Moon's Earth-facing side always has line-of-sight with Earth, it's a lot cheaper to create a ground-based laser network, and they don't have to be put on the end of a huge firework either. &lt;a href=&quot;http://en.wikipedia.org/wiki/Earth-Moon-Earth_communication&quot; rel=&quot;noreferrer&quot;&gt;Moon bounce&lt;/a&gt; [wikipedia]&lt;/li&gt;&#xA;&lt;li&gt;Earth-Moon &lt;strong&gt;L4/L5&lt;/strong&gt; Lagrange point satellite. One satellite, always has visibility of Earth and Moon. Pros are that you only needed one thing on a huge firework, and these points are stable so it should stay there without needing fuel for course correction. The downsides are that you still need an Earth-based network of ground stations or satellites to link to as the Earth turns, and data will be completing 2 sides of a triangle, so the already bad latency (1300ms) will be made worse (~2300ms). &lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/lagpt.html&quot; rel=&quot;noreferrer&quot;&gt;E-M L-points&lt;/a&gt; [hyperphysics]&lt;/li&gt;&#xA;&lt;li&gt;1 Earth sat in a &lt;strong&gt;wide polar orbit&lt;/strong&gt;. By synchronizing the orbit with that of the Moon, you could ensure that the Earth never blocks line of sight. However, this would require both a wide orbit to minimise the angular size of the Earth and a long period to permit the Moon to transit the angular size of the Earth during a fraction of its orbit. Helpfully long periods and large orbital radii go hand in hand. I think an odd fraction of the Moon orbital period (1/3, 1/5, 1/7) would permit an arrangement which prevented direct alignment of the Earth between them. I don't know if the Sun would drag this out of line, so occasional propulsion might be needed.&lt;/li&gt;&#xA;&lt;li&gt;Earth-Moon &lt;strong&gt;L1&lt;/strong&gt;. Again one satellite, so one giant rocket, but this time it will need propulsion as L1 is not a stable point (when it drifts away from L1, it will need to propel itself back). This should be doable with ion thrusters though, so only a moderately enlarged firework. Being directly between Earth and the Moon would have 2 Pros - easy to find, and the lowest latency.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;2+ Earth sats&lt;/strong&gt;, orthogonal Polar orbits or same orbit 180 degrees apart. One or other satellite can always see the Moon, as when one is obscured by the Earth, the other is not. Same requirement for a ground-based network. Lather, rinse repeat for more satellites.&lt;/li&gt;&#xA;&lt;li&gt;1 Earth sat in polar orbit (as per the question), but with a matter-scoop fuelled ion thruster performing &lt;strong&gt;continuous course correction&lt;/strong&gt;. Making the satellite and putting it up there is straightforward, but the propulsion would be entirely experimental. It would be worth putting up a sister satellite with nuclear fuel so that you could spend a bit longer refining the technology when the first one seizes up.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="519" LastActivityDate="2014-10-21T11:00:56.573" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5665" PostTypeId="1" CreationDate="2014-10-21T15:56:54.243" Score="2" ViewCount="82" Body="&lt;p&gt;In the ARRM  &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2013index/6asx9cc6.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;feasibility Study&lt;/a&gt; Table 4, parameters of four different options (three  different launchers and different trajectories) are gathered. What would be the total energy use of a mission like this (if I would choose for example &quot;the Direct drive 1&quot; with Falcon Heavy)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to just use the specific energy of hydrazine and then also calculate the energy use during launch? But what about SEP? Where do I get a exhaust velocity ($v_e$) for a ion thruster? And could I then still use the Energy equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E = 0.5 \times m_1 \times (e^{\Delta v/v_e}-1) \times v_e^2 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_1$ would then be the ARRV and the whole asteroid (ARRV having negligible mass compared to the 400 000 kg chunk of an asteroid).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v$ would just be summed up? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm well aware of the fact that &quot;energy use&quot; is a bit vague and not very practical to look at in space exploration, but to tell you the truth, this is what my thesis supervisor expects from me... Luckily in &quot;order of magnitude&quot; level though as whole space exploration is far from my own studies. ]&#xA;This whole question was a spin off from (&lt;a href=&quot;https://space.stackexchange.com/questions/5601/how-much-energy-is-required-to-send-a-payload-of-40-t-from-earths-surface-to-le/5636#5636&quot;&gt;How much energy is required to send a payload of 40 t from Earth&amp;#39;s surface to LEO?&lt;/a&gt;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks if someone has some tips to throw. :)&lt;/p&gt;&#xA;" OwnerUserId="6311" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-10-21T18:48:07.037" Title="Energy use of Asteroid Redirect Robotic Mission (ARRM)?" Tags="|probe|asteroid|energy|falcon-heavy|sls|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5666" PostTypeId="1" CreationDate="2014-10-21T17:42:38.997" Score="5" ViewCount="1015" Body="&lt;p&gt;Given that there are no organisms on other planets/moons, if we bury a dead human (or other organisms), will it decay? I think the body will be intact as on earth there are several organisms/micro organisms that help in decaying. Is my understanding correct?&lt;/p&gt;&#xA;" OwnerUserId="6377" LastEditorUserId="12102" LastEditDate="2020-04-26T01:27:18.883" LastActivityDate="2020-04-26T01:27:18.883" Title="What happens if we bury a human on other planets or moons" Tags="|colonization|planet|death|" AnswerCount="1" CommentCount="5" ClosedDate="2014-10-22T18:49:04.237" ContentLicense="CC BY-SA 3.0" />
  <row Id="5667" PostTypeId="2" ParentId="5666" CreationDate="2014-10-21T18:07:18.303" Score="11" Body="&lt;p&gt;This could be off-topic here, but I have an answer, so I might as well post it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Yes and No&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Decomposition&quot;&gt;Wikipedia&lt;/a&gt;, decomposition occurs due to two factors: the body's own chemicals and the effects of bacteria.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Autolysis&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Autolysis_(biology)&quot;&gt;Autolysis&lt;/a&gt; is the process of a bit of organic matter breaking itself down. The lysosomes in a cell use digestive enzymes that normally would break down food to inadvertently eat away at the cell. These proteins slowly start to destroy the organelles inside each cell. Eventually, this effects the entire body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Putrefaction&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be more familiar with &lt;a href=&quot;http://en.wikipedia.org/wiki/Putrefaction&quot;&gt;putrefaction&lt;/a&gt;. This is the process bacteria use to break down proteins in the body's tissue and get energy from it. Fungi also contribute. together, these organisms decompose organic matter, absorbing the nutrients for themselves.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So,&lt;/strong&gt; would this happen on another planet? Well, autolysis might. After all, the body's chemicals are still there. True, the vacuum of space (if the planet lacks an atmosphere) could cause some damaging effects, but perhaps the body would be okay and decompose. Putrefaction, on the other hand, might not happen. Unless extraterrestrial bacteria or other organisms exist to bite away at the tissue, there will be nothing there to aid in putrefaction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's always the chance that the environment will act like a &lt;a href=&quot;http://en.wikipedia.org/wiki/Bog&quot;&gt;peat bog&lt;/a&gt; and preserve the body, but to date, no subjects have volunteered to test this hypothesis.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="415" LastEditDate="2014-10-22T11:33:27.870" LastActivityDate="2014-10-22T11:33:27.870" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5668" PostTypeId="2" ParentId="5649" CreationDate="2014-10-21T19:16:08.900" Score="1" Body="&lt;p&gt;One more option--not in orbit at all.  You can use solar sails to hover a satellite over the north or south pole or Earth.  The patent on this has expired, you're free to do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that such a satellite must be far out, the lag going through the satellite will be considerably greater than the Earth-Moon lag.  On the flip side, it will retain communication with a equatorial-orbit satellite when it goes &quot;behind&quot; the moon so long as it's not in too low an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Statite&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Statite&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-10-21T19:16:08.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5669" PostTypeId="2" ParentId="2853" CreationDate="2014-10-22T08:56:40.653" Score="10" Body="&lt;p&gt;It's not that wind is not viable, it's just that it's less viable than other sources of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With such a small part of Earth atmosphere pressure, even the high wind speeds it's insufficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Energy scales linearly with pressure but with square of wind speed; for wind turbines on Earth the optimal wind speed is 50km/h; 200km/h of Mars would increase the energy by a factor of 16.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, typical Mars pressure of 600Pa is 0.006 of Earth's average 101,300 Pa. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combining these, it sets &lt;strong&gt;wind turbine efficiency on Mars a little short of 10% of corresponding turbine on Earth&lt;/strong&gt;. Sure with lower gravity and lower expected wind pressure the construction could be significantly lighter, but still one shouldn't expect any very high output, and the construction would need to be BIG to provide any meaningful amount of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTOH, while Mars, at 1.523679 AU only receives about 0.43 the amount of sunlight reaching Earth, the thin atmosphere blocks and reflects much less of it, meaning solar panels on Mars are about as efficient as on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, similarly to employing &lt;a href=&quot;https://space.stackexchange.com/questions/765/would-geothermal-energy-be-practical-on-a-lunar-base&quot;&gt;geothermal energy on the Moon&lt;/a&gt;, - yes, &lt;strong&gt;it's possible&lt;/strong&gt;, and no, &lt;strong&gt;it's not practical&lt;/strong&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If solar panels with tracking systems are roughly the same cost and complexity as wind turbines of the same output on Earth, on Mars you would get about 10% the energy output of solar at roughly the same cost/effort, or the same energy at 10x the cost/effort comparing to solar power. Maybe a little better if the turbines were optimized specifically for Mars conditions - but still far from breaking even.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-10-22T08:56:40.653" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5670" PostTypeId="1" AcceptedAnswerId="5675" CreationDate="2014-10-22T12:35:09.320" Score="3" ViewCount="206" Body="&lt;p&gt;&lt;a href=&quot;http://www.thehindu.com/sci-tech/science/im-safe-and-sound-tweets-mom-after-comet-sighting/article6520803.ece&quot; rel=&quot;nofollow&quot;&gt;This article&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There was an hour’s communication blackout during the comet’s fly-by ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What happened here? Did the tail of Siding Spring serve to block/distort communication between Mars, and Earth? &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="12102" LastEditDate="2019-12-18T06:57:30.353" LastActivityDate="2019-12-18T06:57:30.353" Title="Did the tail of Siding Spring distort communication between Mars, and Earth?" Tags="|communication|comet|mars-orbiter-mission|risk-management|maven|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5672" PostTypeId="1" AcceptedAnswerId="5673" CreationDate="2014-10-22T13:02:27.930" Score="15" ViewCount="1490" Body="&lt;p&gt;If one calculates the black-body temperature of an object in space, it turns out that it would be at room temperature somewhere between the orbits of Earth and Venus. But due to the greenhouse effect, Earth is warmer than expected from the calculated black-body value, which is around the freezing point of water. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what heating would a hypothetical large space station get for free from the sun? The space-station-within an asteroid design or even one based on a dark metal hull would seem to bow to the black-body law. But how about e.g. the O'Neill cylinder pictured below? It must receive most of its heating from the sun, yet I somehow don't expect the math for &quot;greenhouse earth&quot; to work here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mzzQa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6388" LastActivityDate="2014-11-01T19:38:43.720" Title="What would determine the interior temperature of a large space station?" Tags="|space-station|life-support|temperature|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5673" PostTypeId="2" ParentId="5672" CreationDate="2014-10-22T13:28:38.483" Score="12" Body="&lt;p&gt;As with any satellite, the problem is more one of overheating than of freezing. A black body takes up a lot of heat, but it also radiates a lot. Other materials, like solar panels, radiate a lot less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This phenomenon is called gray body radiation, and I have explained it here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5246/why-is-gold-used-in-space-technology-to-protection-from-heat-radiation/5248&quot;&gt;Why is gold used in space technology to protection from heat radiation?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to keep the station at bearable temperatures, the exterior will have to use radiating surfaces like the ones of the ISS. These can be directed to the sun or perpendicular to its rays depending on the current need for cooling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As with all technology, the simpler, the better. If possible, one would stick to such passive methods. If not, there are also means like coolant circulations, that would give a designer much more control over the temperatures in different parts of the station. However, it also adds a lot of components that may fail, which is something you want to avoid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;In your comment, you added that the station would be 50/50 &quot;glass&quot; and &quot;metal&quot;, and asked for an equilibrium temperature &quot;somewhere outside the orbit of Venus&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of these are problematic definitions. But even if they weren't that is not how spacecraft design works. You don't pick some materials and see what temperature comes out, and declare the spacecraft impossible if it didn't match your expectations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You decide on a temperature and then select materials accordingly. Somewhere around earth's orbit you have 1370 W/m² of thermal energy at your disposal. With that you can generate any temperature from freezing cold to scorching hot, depending on your heat management.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-10-28T15:43:51.313" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5674" PostTypeId="1" AcceptedAnswerId="10596" CreationDate="2014-10-22T15:38:51.717" Score="7" ViewCount="641" Body="&lt;p&gt;It's been almost a month since the ISRO's Mars Orbiter Mission (MOM) was put into orbit. Though it doesn't carry much scientific payload, the one most talked about is the methane sensor, which even MAVEN doesn't have. So my questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is it capable of positively confirming or denying the presence of methane?&lt;/li&gt;&#xA;&lt;li&gt;What has the sensor found up till now? Has it confirmed any presence of methane on Mars?&lt;/li&gt;&#xA;&lt;li&gt;If not, when can we except to hear some news?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6363" LastEditorUserId="49" LastEditDate="2014-10-22T16:14:44.217" LastActivityDate="2019-07-20T21:07:28.970" Title="Methane detection on Mars by MOM?" Tags="|mars|isro|experiment|mars-orbiters|mars-orbiter-mission|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5675" PostTypeId="2" ParentId="5670" CreationDate="2014-10-22T16:43:38.157" Score="4" Body="&lt;p&gt;Communications blackout was because they phased orbits of Mars orbiters to be &quot;behind Mars&quot; when at greatest risk of cometary dust impacts. &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2014/1009-mars-orbiter-mission-shifts.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;MOM phased its orbit too&lt;/a&gt; and so did NASA's / ESA's Mars orbiters. Depending on their orbit, this communications blackout should last roughly 40 minutes. With MOM's highly elliptical and over three Earth days long (nearly 3 Martian days) orbit, I wouldn't be too surprised if its time &quot;behind Mars&quot; was a bit longer than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2014-244&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The comet's nucleus will miss Mars by about 82,000 miles (132,000&#xA;  kilometers), shedding material hurtling at about 35 miles (56&#xA;  kilometers) per second, relative to Mars and Mars-orbiting spacecraft.&#xA;  At that velocity, even the smallest particle -- estimated to be about&#xA;  one-fiftieth of an inch (half a millimeter) across -- could cause&#xA;  significant damage to a spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So that's over Solar system's escape velocity at Martian orbital altitude and adding particles' velocity to orbiters' own orbital speed even the tiniest particles would carry huge kinetic potential ($E_\text{k} =\tfrac{1}{2} mv^2$, notice the square of velocity $v$) and it was simply more prudent to employ a few simple and affordable measures than &lt;em&gt;risk it&lt;/em&gt;. Phasing orbits was one of threat mitigation options and they took it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/BZdOH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;A slide on spacecraft mitigation plans shared during the &lt;a href=&quot;http://www.cometcampaign.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CIOC&lt;/a&gt; (Coordinated Investigations Of Comets) &lt;a href=&quot;http://www.cometcampaign.org/workshop&quot; rel=&quot;nofollow noreferrer&quot;&gt;Comet Siding Spring and Its Close Approach to Mars: Observer's Workshop&lt;/a&gt;, August 15, 2014, during the &lt;a href=&quot;http://new.livestream.com/viewnow/comets/videos/58947411&quot; rel=&quot;nofollow noreferrer&quot;&gt;Session 1: Setting the Stage; Spacecraft Observations&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA dubbed this &lt;a href=&quot;http://www.nasa.gov/jpl/mars/pia18611/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Duck &amp;amp; Cover&quot;&lt;/a&gt; on social media, but it's just one of the risk management options really.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Kw5NC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read a bit more in a related thread &lt;a href=&quot;https://space.stackexchange.com/questions/3111/what-risk-management-options-are-available-for-mars-orbiters-and-rovers-to-mitig&quot;&gt;What risk management options are available for Mars orbiters and rovers to mitigate threat of impacting with Siding Spring cometary debris?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-05-17T00:13:57.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5676" PostTypeId="1" AcceptedAnswerId="5678" CreationDate="2014-10-22T16:49:30.877" Score="10" ViewCount="1079" Body="&lt;p&gt;What does a typical Attitude and Orbit Control System (AOCS) look like for a very small satellite? How does such a satellite control its attitude and orbit? Are there any means of propulsion at this scale - like chemical or electrical thrusters or other AOCS related devices like torquers, reaction wheels or sun, earth and star sensors?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By very small I mean satellites like micro (10–100 kg), nano (1–10 kg), pico (0.1–1 kg) or femto (&amp;lt; 0.1 kg) satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There exists a related older question about cube satellites and their propulsion systems. However, this question is different to the older one, since this question is related to AOCS (which is not identical to propulsion system) and explicitely asks for the specific cases of micro, nano, pico and femto satellites.&lt;/p&gt;&#xA;" OwnerDisplayName="user5973" LastEditorDisplayName="user5973" LastEditDate="2015-09-04T11:29:00.950" LastActivityDate="2015-09-04T11:29:00.950" Title="AOCS for very small satellites (micro, nano, pico, femto)" Tags="|artificial-satellite|propulsion|orbit|altitude|attitude|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5677" PostTypeId="1" CreationDate="2014-10-22T22:12:19.550" Score="8" ViewCount="1464" Body="&lt;p&gt;During the space race (let's say 1956 to 1969, US and USSR), were there any advances in space exploration technology that were stolen by the other side?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've read a lot about espionage in military and diplomatic areas, but I've never found anything about non-military space-related espionage.  It seems that, in such a prestige-awarding contest, both countries would be eager to spy on what the other was doing.&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2014-10-28T03:50:07.747" Title="Espionage in the Space Race" Tags="|space-race|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5678" PostTypeId="2" ParentId="5676" CreationDate="2014-10-22T22:48:39.340" Score="5" Body="&lt;p&gt;As TidalWave pointed out, no very small satellites have flown with an active propulsion system, so they cannot change their orbit actively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To control their attitude, many employ a Magnetorquer, which is a magnet, that creates an infinitesimal, but sufficient, torque in the earth's magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They may have small reaction wheel systems, which allow better control while active. However, the satellite will loose attitude control and start tumbling when they power down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look here to see what's available:&#xA;&lt;a href=&quot;http://www.cubesatshop.com/index.php?option=com_virtuemart&amp;amp;page=shop.browse&amp;amp;category_id=7&amp;amp;Itemid=69&quot;&gt;http://www.cubesatshop.com/index.php?option=com_virtuemart&amp;amp;page=shop.browse&amp;amp;category_id=7&amp;amp;Itemid=69&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: They even carry a cold gas thruster system. According to the description, it &quot;is already integrated on a number of CubeSats that are ready for flight.&quot; I take this to mean that none have been launched yet. Also note, that &quot;a number&quot; may be &quot;one&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cubesatshop.com/index.php?page=shop.product_details&amp;amp;flypage=flypage.tpl&amp;amp;product_id=74&amp;amp;category_id=7&amp;amp;option=com_virtuemart&amp;amp;Itemid=69%22a%22&quot;&gt;Got a cool 81000€&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2014-10-22T22:56:22.807" LastActivityDate="2014-10-22T22:56:22.807" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5681" PostTypeId="1" AcceptedAnswerId="5682" CreationDate="2014-10-23T00:12:51.647" Score="4" ViewCount="2038" Body="&lt;p&gt;I'm curious about the delta-v of model rockets. While searching the web I found NASA's &quot;For Educators&quot; page &lt;a href=&quot;http://www.nasa.gov/audience/foreducators/rocketry/home/rockets_stack_up_nf.html&quot; rel=&quot;nofollow&quot;&gt;How Do Rockets Stack Up&lt;/a&gt; that was going to cover it, however the stats haven't been filled in. &lt;/p&gt;&#xA;" OwnerUserId="6397" LastEditorUserId="49" LastEditDate="2014-10-23T00:41:41.160" LastActivityDate="2014-10-23T11:02:24.980" Title="What is the delta-v range for model rockets?" Tags="|delta-v|model-rocketry|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5682" PostTypeId="2" ParentId="5681" CreationDate="2014-10-23T01:47:53.173" Score="4" Body="&lt;p&gt;It's common for the sort of model rocket you link (e.g. Estes kit with a D-class motor) to go to approximately 1000 ft altitude.  If we assume an impulsive burn (reasonable, given that burn time &amp;lt;&amp;lt; flight time) we can calculate the corresponding delta V with high school kinematics:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ v^2 = u^2 + 2as $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;v = final velocity = $0$ (at apogee)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;u = initial velocity (to be solved)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;a = acceleration due to gravity = -9.8 m/s$^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;s = distance traveled = 1000 ft ~= 300 m&lt;/p&gt;&#xA;&#xA;&lt;p&gt;solving,&#xA;$$ u = \sqrt{2 \cdot 9.8 \cdot 300} = 77 \space m/s $$&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3185" LastEditDate="2014-10-23T11:02:24.980" LastActivityDate="2014-10-23T11:02:24.980" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5684" PostTypeId="2" ParentId="5649" CreationDate="2014-10-23T07:06:47.587" Score="0" Body="&lt;p&gt;Yes, it's possible .. A polar orbit that has a longitudinal transition of the exact amount of speed the moon circles the Earth would allow such a thing. &lt;/p&gt;&#xA;" OwnerUserId="6400" LastActivityDate="2014-10-23T07:06:47.587" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5685" PostTypeId="1" AcceptedAnswerId="5687" CreationDate="2014-10-23T10:44:14.020" Score="3" ViewCount="381" Body="&lt;p&gt;I don't understand how we can get an uninterrupted line of sight with geostationary satellites with the presence of a high number of polar satellites in Earth's orbit. When microwave signals are used between GEO satellites and any base station, we must have a line of sight to convey information across, so isn't there a chance that this signal will be blocked by polar satellites as they occult geostationary satellites?&lt;/p&gt;&#xA;" OwnerUserId="6402" LastEditorUserId="49" LastEditDate="2014-10-24T14:52:19.333" LastActivityDate="2014-10-24T14:52:19.333" Title="How do polar satellites disrupt communications between GEO satellites and ground control?" Tags="|communication-satellite|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5686" PostTypeId="1" CreationDate="2014-10-23T12:08:25.503" Score="5" ViewCount="387" Body="&lt;p&gt;Let's start out with an air breathing SSTO, like Skylon. These things have pretty bad mass ratios and pretty razor thin margins. Given that the engines react with the oxygen in the air during the first stage, you'll have an unequal ratio of fuel to oxidizer. The limit of the air breathing stage is dictated by limits of the mach number with the ramjet operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what if you had a crazy scheme to extend this air breathing mode? It's a shorter trip to move air from the upper atmosphere to space than it is from ground to space. Additionally, your fuel tank will be contracting during the air breathing mode. There's extra empty volume in your craft being freed up while at the same time you're swimming in Oxidizer. What if your engines siphoned some extra air into an expanding 3rd tank near the end of your air breathing stage, which is then reacted with the fuel during the first part of your airless flight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would you have to use a gaseous form? Or given the cryo temperatures that Skylon has in some parts, could it be possible to liquify some amount of air? Perhaps the heat exchangers can't handle this, so it's completely ridiculous. Nonetheless, if they could, Oxygen liquifies before Nitrogen if I understand correctly, so this could tip the ratio in your favor. If you did use a gaseous form, perhaps this would require hardening the fuel tank, which would kill whatever mass ratio benefit you were hoping for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can we manage to rule this out categorically (as in there's no realistic scenario where it helps), or would it only be a matter of vast additional complexity?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2015-03-13T02:45:02.353" Title="Could a SSTO bladder of air scooped from atmosphere improve performance?" Tags="|ssto|launch-vehicle|oxidizer|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5687" PostTypeId="2" ParentId="5685" CreationDate="2014-10-23T14:45:10.167" Score="14" Body="&lt;p&gt;Satellites in LEO travel at around 7000 m/s. This means that a 1 m sized LEO satellite will take around 150 us (microseconds) to cross a given line between a GEO satellite and a ground antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it is a very short event, and all satellite communications are protected against error bursts (especially from interference) by using FEC (forward error correction codes) and other diversity methods.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, a LEO satellite crossing a particular GEO-to-ground line is very improbable (many LEO sats, but an extremely large area around Earth to &quot;hold&quot; them).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my guess is that these events are less probable and less important than other sources of interference and attenuation that satellite communications are exposed to normally (like &lt;a href=&quot;http://en.wikipedia.org/wiki/Sun_outage&quot;&gt;sun radiation&lt;/a&gt;, clouds and rain, ground based interference, aeroplanes).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. For a finer calculation, note that to effectively block a radio link, you would have to cover much of the section of the first &lt;a href=&quot;http://en.wikipedia.org/wiki/Fresnel_zone&quot;&gt;Fresnel zone&lt;/a&gt;; it's not just crossing the imaginary line between satellite and receiver.&lt;/p&gt;&#xA;" OwnerUserId="1348" LastActivityDate="2014-10-23T14:45:10.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5688" PostTypeId="2" ParentId="5686" CreationDate="2014-10-23T15:30:05.713" Score="4" Body="&lt;p&gt;Compression will by it's nature heat things up. Storing air while in flight would tend to heat the system up. Keeping rockets cool is a major challenge, so an increase of heat would likely impact things even further. Yes, you could get some improvements in the air breathing portion of the flight. But you gain a significant amount by launching outside of the atmosphere for a SSTO, and you can get easily to 50% of orbital speed using a SCRAMJET system. It seems unlikely that the extra weight and heat that would result from compressing and storing air in flight would be of any value at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 50% orbital speed comes from the fastest air-breathing aircraft, which was the X-43A, which set the &lt;a href=&quot;http://www.guinnessworldrecords.com/records-3000/fastest-aircraft-air-breathing-engine/&quot; rel=&quot;nofollow&quot;&gt;current record&lt;/a&gt; of 7,000 mph. This is approximately half of the orbital speed, which is between 15,430 mph to 17,450 mph (&lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;). I'm assuming that we can improve the technology a bit to get the rest of the way to half-way there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reference to the &lt;a href=&quot;http://www.reactionengines.co.uk/sabre_howworks.html&quot; rel=&quot;nofollow&quot;&gt;SABRE engine of Skylon&lt;/a&gt;, there's this quote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;While this sounds simple, the problem is that in air-breathing mode, the air must be compressed to around 140 atmospheres before injection into the combustion chambers which raises its temperature so high that it would melt any known material. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They can get around that to an extent, but the heat is quite dramatic. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2014-10-24T02:11:52.393" LastActivityDate="2014-10-24T02:11:52.393" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5689" PostTypeId="1" AcceptedAnswerId="10426" CreationDate="2014-10-24T03:36:59.963" Score="10" ViewCount="2026" Body="&lt;p&gt;A &lt;a href=&quot;http://exploration.grc.nasa.gov/education/rocket/gimbaled.html&quot;&gt;&lt;strong&gt;gimbaled thrust&lt;/strong&gt;&lt;/a&gt; system was used on the Space Shuttle, which is easily recognized in the beginning of &lt;a href=&quot;https://www.youtube.com/watch?v=OnoNITE-CLc&quot;&gt;this &lt;strong&gt;launch video&lt;/strong&gt;&lt;/a&gt;. Was this technology first used by jet fighters, or was it first invented for rocket engines? What were the early successes and failures? &lt;/p&gt;&#xA;" OwnerUserId="603" LastEditorUserId="12102" LastEditDate="2021-09-05T23:14:54.187" LastActivityDate="2021-09-05T23:14:54.187" Title="What was the first rocket to use gimbaled thrust?" Tags="|history|engine-design|thrust|spaceflight-firsts|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5690" PostTypeId="1" CreationDate="2014-10-24T04:56:17.930" Score="64" ViewCount="51810" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot;&gt;Wikipedia page for the International Space Station&lt;/a&gt; says that it has a fairly Earth-like, sea-level atmosphere: 21% oxygen, balance nitrogen at 101.3 kPa. Supposedly it's because a pure-oxygen environment is dangerous as in the Apollo 1 disaster, but in that case &quot;pure-oxygen&quot; meant 1.15 atm of O&lt;sub&gt;2&lt;/sub&gt;. It seems like a pure, 0.21 atm O&lt;sub&gt;2&lt;/sub&gt; atmosphere (or even lower) with no inert balance gas should be fine for people and would be ~ 80% less structurally demanding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One strange aspect might be an effective drop in boiling point to ~ 60 °C, but I'm not sure if anyone would be boiling water for a tea up there. My presumption is that it's not for any safety or human reason but solely for the sake of making ISS experiments more similar (and directly comparable, save for the microgravity environment) to those on Earth. Am I not considering something?&lt;/p&gt;&#xA;" OwnerUserId="881" LastEditorUserId="58" LastEditDate="2015-06-25T16:04:30.607" LastActivityDate="2018-10-14T00:46:18.783" Title="Why is the breathing atmosphere of the ISS a standard atmosphere (at 1 atm containing nitrogen)?" Tags="|iss|atmosphere|life-support|" AnswerCount="7" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5691" PostTypeId="2" ParentId="5690" CreationDate="2014-10-24T10:07:28.450" Score="15" Body="&lt;p&gt;Running a pure oxygen atmosphere at 0.21 is not going to be very healthy. Humans require atmospheric pressure within reasonable bounds of 'normal' in order to function correctly (gaseous transport across membranes etc) and a pure Oxygen atmosphere, even at lower pressures, is still going to be explosive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using nitrogen allows for normal atmospheric conditions and reduces that risk of explosion/fire.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you pointed out, it does allow for experiments to be run in Earthlike conditions (if we exclude that whole gravity thing...) but that is almost certainly going to be less of an issue, after all, many experiments are run in deliberately non-Earthlike conditions, or in sealed micro-environments with their own atmosphere etc.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2014-10-24T10:07:28.450" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="5692" PostTypeId="2" ParentId="5690" CreationDate="2014-10-24T12:33:16.967" Score="30" Body="&lt;p&gt;Rory mentions &lt;a href=&quot;https://en.wikipedia.org/wiki/Oxygen_saturation_%28medicine%29&quot; rel=&quot;noreferrer&quot;&gt;oxygenation rate&lt;/a&gt; which is an excellent point but there's additional reasons why not keep ISS atmosphere at a lower pressure - thermal convection and air cycling. Pressure at roughly one atmosphere means that the ventilation system on the station works better and no pockets of carbon dioxide or even carbon monoxide build up, which would be dangerous to astronauts. The air is easier recycled / replenished and mixed with oxygen (electrolysis of water) and carbon oxides removed from it (Sabatier reaction). The ventilation system also works more reliably at higher pressure and its parts lasts longer between failures. Astronauts / cosmonauts also exercise quite a bit on the station to combat adverse effects of prolonged stay in microgravity on human body, so air pressure also helps them shed excess body heat. Overheating is stressful to the body, lowers performance and can be deadly. And they use all kinds of equipment that requires air cooling too, and it would definitely complicate biology experiments or even render their results useless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nitrogen is also relatively cheap to deliver to the station since it's not really a consumable within the life support system and is only lost at a low rate to its inefficiencies, and is also used for all kinds of other things both on the station as well as visiting spacecraft (purging atmosphere, non-toxic fire suppressant, ullage gas in storage tanks to provide fluid / gas pressure,...). So it would be delivered to the station anyway. But in theory, if it made sense from the logistics standpoint, it could be replaced with some other inert and non-toxic gases like, say, Argon. Especially if they would for some reason decide to keep the atmosphere at a much increased pressure, where nitrogen narcosis might become a problem. But they won't do that, &lt;a href=&quot;https://space.stackexchange.com/a/3508/49&quot;&gt;there's no good reason to&lt;/a&gt; and the station probably couldn't support it structurally without coming dangerously close its ability to maintain pressure and not lose it to space.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-10-24T12:51:28.130" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5693" PostTypeId="2" ParentId="5690" CreationDate="2014-10-24T14:08:17.317" Score="6" Body="&lt;p&gt;Low pressure can imply &lt;a href=&quot;http://en.wikipedia.org/wiki/Decompression_sickness&quot; rel=&quot;nofollow noreferrer&quot;&gt;decompression sickness&lt;/a&gt;. Maintaining enough O&lt;sub&gt;2&lt;/sub&gt; is not sufficient; when overall pressure drops, gases dissolved in the blood (especially nitrogen) regain their freedom, and bubbles form.&lt;br&gt;&#xA;Given the price involved in sending a live human being up in orbit, it would not be very rational to first keep him in a decompression chamber for a few days; maintaining such a chamber in the ISS would also use substantial space, which is a scarce resource up there. Alternatively, decompression would be done prior to the flight, which would be technologically challenging (the vessel would have to be kept at low pressure at all times in the pre-flight phase).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early vessels like Apollo, DCS was solved by &quot;pre-breathing&quot;, i.e. having astronauts breathe pure O&lt;sub&gt;2&lt;/sub&gt; for half an hour before launch; and, more generally, by assuming that the astronauts were tough guys who could suck it up. &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19700034252&quot; rel=&quot;nofollow noreferrer&quot;&gt;Some problems remained&lt;/a&gt; which can explain why &quot;normal&quot; air was used in the Space Shuttle and the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Less seriously, some experiments on the ISS involve live subjects (e.g. small animals) that would not necessarily accommodate a low pressure; results would be biased. Unless, again, compression chambers are used.&lt;/p&gt;&#xA;" OwnerUserId="311" LastEditorUserId="15906" LastEditDate="2017-08-30T20:39:54.100" LastActivityDate="2017-08-30T20:39:54.100" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5694" PostTypeId="2" ParentId="5690" CreationDate="2014-10-24T15:46:11.343" Score="79" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Am I not considering something?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes. You are not considering Mir, Soyuz, and the Space Shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The International Space Station is a multinational program, jointly led by the US and Russia. While the US and Russia had to compromise on many design decisions, the makeup of the breathing atmosphere was not one of them. The decision to pressurize the ISS to one atmosphere with a standard mix of nitrogen and oxygen was probably one of the easiest design decisions agreed upon by those two countries. The Mir space station, the Soyuz capsules, and the Space Shuttle were all pressurized to one atmosphere. Making the ISS breathing atmosphere be anything but one standard atmosphere would have required extensive redesigns of the Soyuz capsule and the Shuttle, and would have precluded reuse of the Mir environmental control systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real question then is why the breathing atmosphere in Mir, Soyuz, and the Space Shuttle is a standard atmosphere, both in terms of pressure and composition. There are significant advantages to a reduced pressure, pure oxygen environment. Such an environment reduces spacecraft mass, structural integrity issues, and complexity. A pure oxygen environment eliminates the need to carry nitrogen tanks, eliminates the need to carefully monitor the oxygen/nitrogen mix, and eliminates the possibility of the bends (decompression sickness). The reduced pressure means the spacecraft can be a bit less bulky as well. There are additional advantages, particularly with respect to EVAs. Both the Soviet Union and the US initially planned to use pure oxygen breathing atmospheres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mercury, Gemini, and Apollo breathing atmospheres were pure oxygen. The Apollo 1 fire modified how that pure oxygen atmosphere was attained, but it did not change that the breathing atmosphere was transitioned to pure oxygen shortly after launch. The issues associated with a pure oxygen breathing atmosphere made NASA shift to having some nitrogen in the Skylab breathing atmosphere, but not much. The Skylab breathing air was 75% oxygen, 25% nitrogen. The use of a pure breathing atmosphere in the Apollo spacecraft continued to the very end, which created challenges for the Apollo-Soyuz test mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soviet space program switched from a pure oxygen atmosphere to standard atmosphere very early on. Valentin Bondarenko died in a pure oxygen fire three weeks before Yuri Gagarin's historic flight. Having a standard atmosphere mix drastically reduces the likelihood and severity of fires, and also greatly simplifies the pre-launch process. A pure oxygen atmosphere requires extensive pre-breathing to purge nitrogen from the bloodstream. A standard atmosphere meant the cosmonauts could enter the capsule without wearing a helmet and they were physiologically ready to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA eventually learned these lessons, too. The Space Shuttle used a standard atmosphere. Having a standard atmosphere in the ISS was the only logical decision.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-12-22T15:07:53.863" LastActivityDate="2016-12-22T15:07:53.863" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="5695" PostTypeId="2" ParentId="5617" CreationDate="2014-10-24T17:19:47.883" Score="0" Body="&lt;p&gt;Another reason you don't want to blow up an asteriod in the manner of &lt;strong&gt;&lt;em&gt;Armageddon&lt;/em&gt;&lt;/strong&gt; is radiation (assuming nuclear explosives are used to blow up the in bound asteriod).  I remember that when the movie came out, one of the criticisms I heard was the bomb irradiated gigatons of material that is about to impact Earth's atmosphere.  The radiation dose would likely be enough to kill or sicken everyone on the planet.&lt;/p&gt;&#xA;" OwnerUserId="36" LastEditorUserId="36" LastEditDate="2014-10-25T02:12:18.610" LastActivityDate="2014-10-25T02:12:18.610" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5696" PostTypeId="1" CreationDate="2014-10-24T17:20:10.507" Score="6" ViewCount="448" Body="&lt;p&gt;Launching instruments into orbit is expensive.  When a spacecraft in orbit stops functioning, fixing it is normally not an option; nor will it be possible to retrieve the instrument for diagnosis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If an instrument is attached to the ISS, it may or may not be possible to fix problems.  But what about bringing it back to Earth?  For example, &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/638.html&quot;&gt;SMILES&lt;/a&gt; stopped working relatively soon after it started making measurements.  The design did not allow for reparation.  Do they take the non-functioning instrument back to Earth or do they simply &quot;push away&quot; the instrument?  What would be the cost of retrieving it?&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2014-11-02T18:21:37.287" Title="What is the cost of taking something down from the ISS?" Tags="|iss|instrument|return|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5697" PostTypeId="2" ParentId="5696" CreationDate="2014-10-24T17:29:34.460" Score="4" Body="&lt;p&gt;At the moment the only way to retrieve payload from the ISS is via the SpaceX Dragon capsule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Previously the Space Shuttle admirably served this role, with the ability to take large and heavy payloads back, up to 32,000lbs (center of gravity/balance limits greatly limit this in practice).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon can return 5,000 lbs of payload, so significantly less.  (Beyond &lt;a href=&quot;http://en.wikipedia.org/wiki/Long_Duration_Exposure_Facility&quot; rel=&quot;nofollow&quot;&gt;LDEF&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-51-A&quot; rel=&quot;nofollow&quot;&gt;STS-51A two satellites&lt;/a&gt;, SpaceLab, MPLMs, not that much big was ever returned).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz can return on the order of 100 kilos max, and depends on the mass of the astronauts returning, so functionally almost nothing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space Shuttle costs are next to impossible to reasonably calculate. Consider the range of costs assigned to flights that can range from \$300 million to \$2 billion. How you would break that down for return payload?  Likely not possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon's contract is only for upmass, per se. The downmass is icing on the cake.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if &lt;a href=&quot;http://en.wikipedia.org/wiki/Center_for_the_Advancement_of_Science_in_Space&quot; rel=&quot;nofollow&quot;&gt;CASIS&lt;/a&gt; is providing your transit to the station, the question is, would they offer you a return cost?    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RFI for the second &lt;a href=&quot;http://www.nasa.gov/content/nasa-seeks-us-industry-feedback-on-options-for-future-space-station-cargo-service&quot; rel=&quot;nofollow&quot;&gt;Commercial Resupply Services&lt;/a&gt; (CRS-2, not to be confused with the CRS-2 flight by SpaceX, nor the CRS-2 mission flown by Orbital).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is interesting is that CRS-2 expects vendors to provide almost equal return or disposal services as to launch services. Thus for CRS-2, the commercial providers are bidding craft that can launch 14,000K a year in 4-5 launches, but also dispose of or return 14,000K of cargo in return.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More interesting is that it acknowledges that either the CBM or PMA may be used for berthing or docking, and that plans need to explain how to handle large cargo. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-02T18:21:37.287" LastActivityDate="2014-11-02T18:21:37.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5698" PostTypeId="1" AcceptedAnswerId="5699" CreationDate="2014-10-24T17:32:47.113" Score="10" ViewCount="1384" Body="&lt;p&gt;It's been done a lot, but how? Is it an extensive modification or relatively simple? The flight information for ICBMs on &lt;a href=&quot;http://en.wikipedia.org/wiki/Intercontinental_ballistic_missile#Flight_phases&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; makes it seem like they got pretty close to orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I reviewed the question &lt;a href=&quot;https://space.stackexchange.com/questions/5322/were-decommissioned-cold-war-missiles-repurposed-for-peaceful-spaceflight&quot;&gt;Were decommissioned Cold War missiles repurposed for peaceful spaceflight?&lt;/a&gt; the other day and it brought the matter to mind.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="198" LastEditDate="2019-01-14T23:12:37.547" LastActivityDate="2020-09-01T12:08:41.613" Title="How are intercontinental ballistic missiles (ICBMs) modified to be orbital launch vehicles?" Tags="|launch-vehicle|icbm|repurpose|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5699" PostTypeId="2" ParentId="5698" CreationDate="2014-10-24T17:58:01.163" Score="7" Body="&lt;p&gt;ICBM's typically &lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_ICBMs&quot; rel=&quot;nofollow noreferrer&quot;&gt;carry payloads&lt;/a&gt; of 1,000 kg or more of mass. Most ICBMs used for peaceful purposes can launch into orbit only small payloads. The single best example of this is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dnepr-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dnepr rocket&lt;/a&gt;. The rocket itself can carry 4,500 kg to LEO, but the SS-18 &lt;em&gt;Satan&lt;/em&gt; (&lt;a href=&quot;https://en.wikipedia.org/wiki/R-36_%28missile%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;R-36&lt;/a&gt;), from which it is based carries a 20 MT bomb. It's hard to say exactly what that translates to, but one can assume a near linear capacity based off of it's mass, which would give around 6,000 kg when compared to the Agni-V missile.&lt;/p&gt;&#xA;&lt;p&gt;They also fly a different trajectory. Most ICBMs actually fly at or very near orbital speed, although not in the right trajectory for orbital flight. The Wikipedia article for the &lt;a href=&quot;http://en.wikipedia.org/wiki/R-36_%28missile%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;R-36&lt;/a&gt; states that it flies at 7.9 km/s, which is actually orbital velocity. Simply re-programming the computer to fly at a more level flight will allow it to achieve orbit.&lt;/p&gt;&#xA;&lt;p&gt;The other way that is done in the instance of the Minotaur Rocket that Northrop Grumman produces is using the stages to build a rocket that can carry a larger payload in to orbit. They may use two first stages, for instance. Most of the ICBMs use solid rockets, and are vastly overpowered, so carrying two of the stages is totally possible.&lt;/p&gt;&#xA;&lt;p&gt;The bottom line is, some major software changes are required, and interface with the payload, payload fairing, and possibly a slight hit in mass, but little else is really required to launch an ICBM to orbit. But if you want a higher performance one, then just assemble the stages together from multiple rockets.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2020-09-01T12:08:41.613" LastActivityDate="2020-09-01T12:08:41.613" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5701" PostTypeId="2" ParentId="5690" CreationDate="2014-10-24T19:36:05.473" Score="11" Body="&lt;p&gt;When an object is burning in an atmosphere which is 80% nitrogen and 20% oxygen, the nitrogen will absorb a lot of the generated heat while doing nothing to assist in combustion.  While it's entirely possible that some other gas would be better than nitrogen (e.g. have a greater higher thermal mass per mole for better fire-suppression characteristics, or have comparable thermal mass at lower density for lower payload weight), nitrogen has the advantage that human beings can breathe an 80% concentration of it (at standard atmospheric pressure) for extended periods of time without ill effect.&lt;/p&gt;&#xA;" OwnerUserId="6001" LastActivityDate="2014-10-24T19:36:05.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5702" PostTypeId="2" ParentId="4704" CreationDate="2014-10-25T02:27:54.587" Score="18" Body="&lt;p&gt;Per  &lt;a href=&quot;https://standards.nasa.gov/documents/viewdoc/3315318/3315318&quot;&gt;https://standards.nasa.gov/documents/viewdoc/3315318/3315318&lt;/a&gt; .  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA Policy Directive&lt;br&gt;&#xA;COMPLIANCE IS MANDATORY&lt;br&gt;&#xA;NPD 1210.1G&lt;br&gt;&#xA;Effective Date: April 28, 2010&lt;br&gt;&#xA;Subject: Acceptance and Use of Monetary Gifts and Donations&lt;br&gt;&#xA;Responsible Office: Office of the Chief Financial Officer  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;POLICY The National Aeronautics and Space Administration (NASA) may accept and utilize monetary gifts, donations, or bequests given as cash, check, or money order, &lt;strong&gt;&lt;em&gt;provided they are unsolicited and offered without conditions on their use&lt;/em&gt;&lt;/strong&gt;.&lt;br&gt;&#xA;(Emphasis added)   &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-10-25T02:27:54.587" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5704" PostTypeId="2" ParentId="4704" CreationDate="2014-10-25T15:51:52.383" Score="5" Body="&lt;p&gt;As noted, NASA cannot campaign for donations.  However, even if someone else does the campaigning for them (e.g. the Planetary Society), the amount of funds you might expect to get are small in comparison to what is needed for a mission.  The Planetary Society managed to raise \$4M at one point to build a solar sail spacecraft, which is quite impressive, but down about three orders of magnitude from NASA annual spending on space science.  Similarly, the &lt;a href=&quot;https://www.kickstarter.com/projects/ryangrepper/coolest-cooler-21st-century-cooler-thats-actually&quot; rel=&quot;noreferrer&quot;&gt;largest kickstarter campaign so far&lt;/a&gt; raised \$13M (for an advanced beverage cooler).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another common question along these lines is selling advertising on spacecraft.  The Coca Cola or Nike logo on Mars might be worth something to them.  It turns out that the amounts are quite similar, where such an opportunity would be worth a few \$M to at the very most a small integer times \$10M.  Note that a Super Bowl 60-second spot costs \$8M, and that will likely have many more viewers than a Mars mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, since you're keen on this, the Planetary Society's Light Sail 1 is scheduled to launch in 2016, and they are &lt;a href=&quot;https://secure.planetary.org/site/Donation2?3440.donation=form1&amp;amp;df_id=3440&quot; rel=&quot;noreferrer&quot;&gt;accepting donations here&lt;/a&gt;.  NASA just &lt;a href=&quot;http://www.spacenews.com/article/civil-space/42227nasa-nixes-sunjammer-mission-cites-integration-schedule-risk&quot; rel=&quot;noreferrer&quot;&gt;cancelled their project to launch a solar sail&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-10-25T15:57:58.863" LastActivityDate="2014-10-25T15:57:58.863" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5705" PostTypeId="2" ParentId="5690" CreationDate="2014-10-25T21:23:20.867" Score="4" Body="&lt;p&gt;It is simpler to design because things behave like on Earth, and less chance to have things go horribly wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To add to what was already mentioned - less problems with overheating, and fires are less dangerous...  also lower air pressure lowers the boiling point of water.  Also simpler to not have to transition from earth's surface to different atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Down sides - structural of several times the air pressure, no need for nitrogen and balancing nitrogen, harder and more time consuming to do space walks - in space suit they use low pressure pure oxygen type atmosphere, more risk of decompression sickness (nitrogen boiling in blood), it may actually be &lt;em&gt;easier&lt;/em&gt; to get rid of co2 if only oxygen and co2 in atmosphere, etc...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A permanent space colony might use atmosphere like you suggest for such reasons... there are ways to adapt, eg if overheating is a problem then you lower temp of habitat, eg 5 degrees Celsius rather than 20 degrees.  At times you want to lose less heat - means you need less food and burn less oxygen, metabolism can slow down.   A colony might have constant wind/airflow coming from roof and going into holes in floor as one way to help you stay on ground in zero gravity and take care of problems like spilling a liquid, would also help take care of you overheating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But people are not so serious about thinking long term &quot;colony&quot;... if they were they would have a system that uses plants or similar to recycle the co2 into o2 and food rather than expensively have to ship up tonnes of consumables to keep astronauts alive... current system works for a few astronauts but would be unsustainable/too expensive for a colony of 100 or 1000 people.  Typical people, especially in government do not take risks/do stuff different because you suffer much more for failure than you can hope to gain from success in trying to do things newer/better.&lt;/p&gt;&#xA;" OwnerUserId="6414" LastActivityDate="2014-10-25T21:23:20.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5706" PostTypeId="2" ParentId="5635" CreationDate="2014-10-26T00:35:55.893" Score="9" Body="&lt;p&gt;Ok I will try to answer my own question as best as I can with known information.&#xA;(We assume some extra mass is not a problem in our design because we have a very cheap to launch coil gun.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An intuitive way if simplified of understanding the issues would be a balloon filled with water:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;If it is stretched enough the walls of the balloon burst - either through support deformation or hydraulic/water weight pressure from within.&lt;/li&gt;&#xA;&lt;li&gt;If you hold it with little support the water pressure deforms it.&lt;/li&gt;&#xA;&lt;li&gt;Shock - if pressure, compression or g differences occur it could create heat and or shear forces.&lt;/li&gt;&#xA;&lt;li&gt;Weight differences between materials become more pronounced at high g - heavier plastic will sink down compressing the balloon to a flat plane at sufficient g even if the balloon is floating in water around it.&lt;/li&gt;&#xA;&lt;li&gt;Ambient pressure may affect some materials adversely. The plastic may melt, chemical reactions could take place or the water could change phase to solid ice.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;None of 3-5 were issues during John Paul Stapp's tests - which I have now read in detail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have ranked 1-5 above in order of what would kill us (or a balloon first). Nr. 5 would not occur unless you made the balloon of styrofoam and/or reached incredible g. We can disregard that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nr. 4. will probably become a problem only at 50-150+ g as the human chest would cave into the lungs under its own weight as the air in the lungs offer no structural support.&#xA;Liquid suspension inside the lungs would help against this because it would reduce the mass difference between the lung space and ribs, probably pushing nr. 4 as an issue well above 100 g.&#xA;&lt;a href=&quot;http://www.wolframalpha.com/input/?i=cortical%20bone&amp;amp;a=*C.cortical%20bone-_*Material.dflt-&amp;amp;a=*DPClash.MaterialE.cortical%20bone-_*CorticalBoneICRP-&quot; rel=&quot;noreferrer&quot;&gt;Bone density is 1.85g/cm^3&lt;/a&gt; so water suspension would push this limit - whatever it is - up by 117% (1.85/0.85 = 217%). We know Strapp had no issues with this whatsoever at near 50 g so knowing nothing about bone strength/other similar issues I estimate this limit at 200-300g with lung liquid suspension and 100-150g without.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nr. 3 I do not see as an issue. This is because you could build your coil gun to ramp acceleration slowly, cool the suspension fluid and so on. This and in general careful construction should prevent any shocks.&#xA;You might also construct the &quot;launch bath&quot; as a rotating structure so that it could always face the acceleration in the most optimal fashion.&#xA;Further about 20 cm of water would at the bottom at 100g only reach 2 bar additional pressure (20 meters of water gives 2 bar). This is quite negligible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This brings us to nr. 2: Strapp actually cracked his ribs at around 18 g with some bad harnesses when facing the acceleration/deceleration. This makes a lot of sense as what looks like simple belts in his images would at this g have supported 1.4 tonnes if he weighed 80 kilos normally!&#xA;Turning the seat, better harnesses or in our case liquid suspension gets us well beyond 50g. If the liquid has the exact density of the human mass only internal differences in body mass/inability to compress evenly/negligible/without damage has consequences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally the big killer: Nr. 1. Strapp had both whiteouts and redouts. The first being where blood left his eyes and the second where blood pressure burst every vein in his eyeballs.&#xA;Going back to the balloon example it would be like holding a heavy water filled balloon with nothing to support it - pressure would make the walls of it burst.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water suspension may help here to some extent, but I don't know the limits: The eyeballs would be pressurized, most of the body mass, but the skull may prevent parts/all of the brain from being fully pressurized, as body fluids cannot move unrestricted past it and it will not itself flex - especially at rapid acceleration onset.&#xA;This low pressure at the back of the skull could mean veins bursting inside the brain - and any other body part not capable of &quot;transmitting&quot; pressure without tearing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However we &lt;a href=&quot;http://en.wikipedia.org/wiki/G-suit#The_GPS_series_.27Berger.27_Gradient_Pressure_Suit&quot; rel=&quot;noreferrer&quot;&gt;KNOW that water g-suits exist and work&lt;/a&gt;. These suits do not use hydraulics it seems, but the water itself. Our water suspension should work even better.&#xA;Further they are used in sitting position - a MUCH more vulnerable position to g force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Strapp, as far as I can tell, used NO pressure suit and still took these amazing forces (25g for 1.1 seconds with a peak of 46.2g) and walked away unhurt (his eyes healed almost completely in few days time without surgery) I have to assume pressure assistance - whether from water suspension or a g-suit - would help greatly.&#xA;I think if you &lt;em&gt;slowly&lt;/em&gt; go to high g from low g and let the pressure gradient created by the basin &quot;settle&quot; into the body's organs and fluids you could deal with this problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Upon leaving the muzzle of the coil gun however high pressure areas inside your body would suddenly have nothing pressing back - again creating bursting potential though less and of shorter duration. I would counter this by artificially increasing ambient pressure in the bath when leaving the muzzle to lessen the shock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could counter this by stopping the acceleration slowly, but that is not really an option with a realistic coil gun design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this in mind the pressure shock of going from 100g to 0g ie. 3 bar in the lowest parts of your body to 1 bar in the top of your body with no time to equalize is probably the limit.&#xA;Strapp experienced something like this at only 25g, but for a much longer duration - ie. constant ~0.5 bar pressure difference in his eyeball veins for 1.1 seconds with NO dynamic counteracting pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So based on guesswork, these ramblings, the numbers above, the fact that Strapp was &lt;em&gt;completely&lt;/em&gt; fine and died at 89 years old and that humans have been said to survive brief spikes of 1000g. I would say that breathable liquid suspension could allow astronauts to launch at 100g from a coil gun and be 99.9% okay after resting in a hyperbaric chamber/seeing a doctor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly enough a 50 g acceleration to low earth orbit speed (7.8 km/s) would use a 62 km long launch tube, of which 1.9 km could be horizontal on the ground and the other 60 km would have to be in the air suspended with gas balloons. The launch tube would curve as the velocity vector changed and would exit at about 40-45 km height where atmospheric influence would be at 1% or less (exit heat shield still needed).&#xA;The launch would take 15.9 seconds.&#xA;If the accelerator is all/mostly on the ground and only maglev suspension inside the airtube the astronauts would face some g for longer to pay for cheaper construction costs. Not sure how much longer/at what g the in-air tube ride would be.&lt;/p&gt;&#xA;" OwnerUserId="6350" LastEditorUserId="6350" LastEditDate="2014-10-29T09:53:44.000" LastActivityDate="2014-10-29T09:53:44.000" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5707" PostTypeId="1" AcceptedAnswerId="5708" CreationDate="2014-10-26T00:51:31.423" Score="6" ViewCount="3926" Body="&lt;p&gt;In a recent news &lt;a href=&quot;http://aviationweek.com/space/nasa-spacex-share-data-supersonic-retropropulsion&quot; rel=&quot;nofollow&quot;&gt;NASA, SpaceX Share Data On Supersonic Retropropulsion&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is what is Retropropulsion?&lt;/p&gt;&#xA;" OwnerUserId="6415" LastEditorUserId="26446" LastEditDate="2019-06-05T03:50:49.333" LastActivityDate="2019-06-05T03:50:49.333" Title="What is Retropropulsion?" Tags="|spacex|propulsion|terminology|retrorockets|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5708" PostTypeId="2" ParentId="5707" CreationDate="2014-10-26T01:03:20.603" Score="9" Body="&lt;p&gt;Firing your rocket engines against your velocity vector in order to decelerate.  In this case, specifically firing your rockets into a supersonic wind.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-10-26T01:03:20.603" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5710" PostTypeId="1" AcceptedAnswerId="5711" CreationDate="2014-10-26T02:35:21.430" Score="17" ViewCount="1070" Body="&lt;p&gt;So I went to the &lt;a href=&quot;http://aviationweek.com/space/nasa-spacex-share-data-supersonic-retropropulsion&quot;&gt;link&lt;/a&gt; in a recent question and discovered SpaceX turns its first stage around while it is going, what, several km/s in the upper atmosphere, restarts 3 of the engines and fires retrograde to slow the stage down and get it heading back towards the launchpad. Aside from suddenly realizing I had absolutely failed to appreciate the distances involved in this maneuver, I had a real 'Wait, what?' moment when thinking about turning around a mostly hollow tube at those speeds in even a thin atmosphere, aiming it precisely retrograde, and keeping it aimed that way as it slows down without losing control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That sounds stupendously difficult. Is it? How do they manage it?&#xA;(The link is to a good article in Aviation Week about the data NASA collected from the September CRS-4 launch during the retrograde burn, which the hope to use for design of propulsive deceleration on Mars.) &lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2018-08-12T15:00:00.720" Title="What are the challenges of turning a rocket stage around and firing its engines into a supersonic wind?" Tags="|launch|spacex|reuse|propulsive-deceleration|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5711" PostTypeId="2" ParentId="5710" CreationDate="2014-10-26T03:32:31.077" Score="18" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Having the business end of the rocket survive the dynamic pressure and heating of facing into the flow, where in general that part of the rocket is not designed to be aerodynamic.  This is a challenge whether or not the engines are firing.  Having the engines running can actually help a little here, rejecting flow into those nozzles, but they are not running all of the engines.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Starting an engine with a supersonic flow impinging on the nozzle.  This may or may not be a challenge, but you don't really know until you try.  It is, to put it gently, problematic to simulate or test on the ground.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Pointing the stick into the wind before you enter is not too difficult, but keeping it pointed into the wind is critical.  Just a little bit off and the side forces on the fuselage can overwhelm the control authority of the gimbaled nozzles, flip the vehicle, and subject it to side forces that can break up the vehicle.  You try to make the launch vehicle structure as light as possible, so it isn't designed to take full-on dynamic pressure loads from the side.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Once in the supersonic flow, the aerodynamic effects of the messy business end of the rocket can be complicated, making control a bit of a challenge.  You can have counterintuitive effects that redirect flow in unexpected directions at different angles of attack.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Predicting the effect of the running engines on the drag is a challenge.  The thrust plumes tend to reduce the drag, countering in part the intent of firing the engines to increase deceleration.  With enough of a thrust to drag ratio, this is not a show stopper, but you need to be able to predict how large the effect is to know if you have enough fuel.  This impact on drag also complicates what happens when you gimbal the engine, which is part of the challenge in #4.  Again, high thrust to drag ratio can reduce the surprises here.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2018-08-12T15:00:00.720" LastActivityDate="2018-08-12T15:00:00.720" CommentCount="14" ContentLicense="CC BY-SA 4.0" />
  <row Id="5713" PostTypeId="1" CreationDate="2014-10-26T07:01:14.080" Score="2" ViewCount="289" Body="&lt;p&gt;I recently read about Google exec Alan Eustace's record skydiving jump from a 'near space' altitude of 135,890 feet and was surprised to learn that the balloon which lofted him up to that height was made by the Tata Institute of Fundamental Research, in Hyderabad, India. I'd like to then ask whether India has any plans of its own to conduct a high-altitude manned balloon flight, perhaps to validate its own spacesuit designs which are under development. I know that a previous record-setting high-altitude balloon jump by Felix Baumgartner was said to have benefited spacesuit research. Are existing methods of spacesuit-testing cheaper, or would a high-altitude manned balloon flight actually allow for a cost savings, unorthodox though it might be? Also, what would be the approximate cost of such high-altitude balloon flights in USD (millions? tens of millions?) and could such activities ever become a viable area of space tourism?&lt;/p&gt;&#xA;" OwnerUserId="6419" LastEditorUserId="6419" LastEditDate="2014-10-26T07:08:26.697" LastActivityDate="2015-04-09T16:20:00.080" Title="Indian Manned Flight to 'Near Space' by ISRO?" Tags="|isro|balloons|tourism|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5714" PostTypeId="2" ParentId="1635" CreationDate="2014-10-26T15:10:26.383" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;http://lunar.xprize.org/teams/arca&quot; rel=&quot;nofollow&quot;&gt;ARCA team&lt;/a&gt; competing in the Google Lunar Xprize has its mission based on precisely this idea: launching the Helen 2 rocket from a helium balloon raised at 14.000m. They made only one &lt;a href=&quot;http://www.arcaspace.com/en/mission_4b.htm&quot; rel=&quot;nofollow&quot;&gt;successful launch&lt;/a&gt; and the rocket reached 40.000 m altitude. I don't know if they'll participate in the race with the same idea, but I mentioned it as an example that this was tried before.&lt;/p&gt;&#xA;" OwnerUserId="1263" LastActivityDate="2014-10-26T15:10:26.383" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5715" PostTypeId="2" ParentId="5696" CreationDate="2014-10-26T16:15:51.510" Score="2" Body="&lt;p&gt;SpaceX, not sure about &lt;em&gt;cost&lt;/em&gt; but here I found the &lt;em&gt;price&lt;/em&gt;. &#xA;On December 23, 2008, SpaceX won a $1.6 billion Commercial Resupply Services (CRS) contract to haul NASA cargo to the International Space Station.   The contract covered 12 missions planned to fly between 2010 and 2016.  SpaceX would use its Dragon spacecraft to perform the missions. (updated Jan8,14) Source : &lt;a href=&quot;http://www.spacelaunchreport.com/falcon9.html&quot; rel=&quot;nofollow&quot;&gt;http://www.spacelaunchreport.com/falcon9.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6421" LastActivityDate="2014-10-26T16:15:51.510" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5716" PostTypeId="1" AcceptedAnswerId="9343" CreationDate="2014-10-26T18:36:36.643" Score="5" ViewCount="1424" Body="&lt;p&gt;I was looking at some SpaceX pictures and realized that with the last ten years, and the coming two years, SpaceX will have built a TEL for:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon 1 - Kwajelin Atol &lt;/li&gt;&#xA;&lt;li&gt;Falcon 1 - SLC-4 (before they got kicked out, possibly reused at Kwajelin) &lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 - LC-40 &lt;/li&gt;&#xA;&lt;li&gt;Falcon 9/Heavy - SLC-4&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Coming up:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon 9/Heavy - LC39A&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9/Heavy - Brownsville/Boca Chica&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Beyond the sheer number of launch sites, that is a fairly large number of TEL's to design and build.  The upside is one assumes they are getting better at it, each one learning from design issues with the previous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who else in recent times (Ignore the crazy build ups in the 1960's and 70's of the Cold War), has done anything close?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose Vostochny the new Russian spacesport in the Far East of Russia will likely be the newest pads.  (Orbital's facilities at MARS are only really a single, though maybe count the Minotaur pads as well?) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ULA's pads were basically all built long ago, though maybe the Atlas/Delta pads could count as new.  But those are at most one in Florida, one in California (I do not know the complex numbers they launch from).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For completeness, I want to try and add in pictures of the various TEL's SpaceX has built.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LC-40 - Falcon 9&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ujyU8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ujyU8.jpg&quot; alt=&quot;LC-40 - Falcon 9 (CRS-3 mission)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CYvyK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CYvyK.jpg&quot; alt=&quot;Vandenberg - SLC 4 - Falcon 9 / Falcon Heavy&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LC-39A - Falcon 9 / Falcon Heavy&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Xpl61.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xpl61.jpg&quot; alt=&quot;LC-39A - Falcon 9, Falcon Heavy&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-05-27T20:04:29.590" LastActivityDate="2016-05-27T20:04:29.590" Title="Who has built the largest number of Transport/Erector/Launch (TEL)?" Tags="|launch|spacex|launchpad|record|tel|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5717" PostTypeId="5" CreationDate="2014-10-26T18:49:21.173" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-26T18:49:21.173" LastActivityDate="2014-10-26T18:49:21.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5718" PostTypeId="4" CreationDate="2014-10-26T18:49:21.173" Score="0" Body="TEL is an acronym for Transporter/Erector/Launcher that is a mobile platform that can transport, erect and launch a rocket. In the context of Space Exploration, it refers to TEL facilitating suborbital and orbital rocket launches and less so TEL used for military ballistic missiles or missile interceptors that might be by design a lot smaller, more mobile, can support multiple rockets and would often also integrate radar (TLAR or TELAR)." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-10-26T18:49:21.173" LastActivityDate="2014-10-26T18:49:21.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5719" PostTypeId="2" ParentId="2631" CreationDate="2014-10-27T01:03:01.767" Score="-1" Body="&lt;p&gt;The near side of the has huge regions of ancient lava flows, called Maria. While the far side is almost covered in crater impacts. Planetary geologists aren’t sure, but it is possible that the Earth used to have two Moons.&#xA;People say that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Billions of years ago, the second, smaller moon crashed into the far side of the Moon, covering up the darker Maria regions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(Quote from &lt;a href=&quot;http://www.universetoday.com/108186/whats-on-the-far-side-of-the-moon/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://article.wn.com/view/2012/10/22/New_study_suggests_moon_spun_off_Earth/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="6426" LastEditorUserId="5963" LastEditDate="2014-12-09T23:03:47.343" LastActivityDate="2014-12-09T23:03:47.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5720" PostTypeId="2" ParentId="5621" CreationDate="2014-10-27T06:18:09.287" Score="4" Body="&lt;p&gt;This might be better suited to aviation.stackexchange.com since your question is actually about aerodynamics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, what self respecting rocket guy doesn't know a thing or two about aerodynamics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rate of rotation is really what's important here. So your (35-30) = 5 is all well and good, but do you want to correct that in a second, 5 seconds, etc. Lets assume for the numbers that you want to correct it in a second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rate of rotation (for correction) = 5 (degrees) / 1 (seconds) = 5 degrees/second&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now this is where you need to know some masses. the amount of force required to give your rocket a 5 degree/second rotation is dependent on the mass of the rocket, or more specifically the distribution of mass radially (assuming it's axially symmetrical)... wait a minute this is getting too complicated, surely there's an easier way?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Luckily there is. If you want to stop the rocket spinning you just have to make sure you turn the fins a little bit if it's spinning slowly and a lot if it's spinning quickly. What you need is a &lt;a href=&quot;http://en.wikipedia.org/wiki/Closed-loop_transfer_function&quot; rel=&quot;nofollow noreferrer&quot;&gt;Closed loop transfer function!&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The specific design of the closed loop transfer function can be worked out from estimations of the mass distributions - much, much easier. So step by step:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) find the coefficient of lift of the aerofoil used for the fin&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) estimate the mass of the rocket&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) pick some reasonable example values for rate of rotation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) calculate the forces required to stop these rotations - see &lt;a href=&quot;http://en.wikipedia.org/wiki/Angular_momentum#Definition&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Momentum#Relation_to_force&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5) calculate the angles required from the fins to match these force&lt;/p&gt;&#xA;&#xA;&lt;p&gt;6) build your close loop transfer function&lt;/p&gt;&#xA;&#xA;&lt;p&gt;7) test on some sort of computer program before flight!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Step 7 isn't strictly necessary, but if you've got your number wrong and haven't realised then you could actually make things worse. Somewhat unrelated, but I came across this video of a high spin rate rocket while producing this answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.youtube.com/watch?v=9HbU5aczSXw&quot; rel=&quot;nofollow noreferrer&quot;&gt;Estimate this spin rate!&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="716" LastEditDate="2019-04-22T17:56:26.170" LastActivityDate="2019-04-22T17:56:26.170" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="5721" PostTypeId="2" ParentId="1635" CreationDate="2014-10-27T12:49:42.497" Score="7" Body="&lt;p&gt;Yes, some people are trying to do it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bloostar.com/&quot; rel=&quot;noreferrer&quot;&gt;http://www.bloostar.com/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are advantages to this solution, such as less drag during ascent (which is especially important for small launchers due to the square/cube law), and a better Isp due to lower pressure at launch. You might also reduce payload fairing mass.&#xA;However, the mass of the launcher is severely constrained and you have much less control over your launch procedure. Aborting and getting the launcher and payload back will be very hard.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="4685" LastEditDate="2014-10-29T08:32:23.283" LastActivityDate="2014-10-29T08:32:23.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5722" PostTypeId="1" CreationDate="2014-10-27T14:22:25.767" Score="1" ViewCount="128" Body="&lt;p&gt;Why aren't moored helium balloons hoovering a few kilometers over many cities as a competing alternative to expensive satellite launches? They could provide services with &lt;em&gt;local&lt;/em&gt; advantage such as: high powered radio communication, precision navigation, live traffic observation, micro meteorology and more which takes advantage of the low altitude and local adaptions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some benefits I can think of:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;unlimited access to cheap electric power through the ground cable, which could constantly power payload and even rotors to achieve extra altitude and control (an electric chopper in a leash),&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;reusability and therefor much lower cost,&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;no space graded equipment needed and therefor much lower cost again,&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;serviceability of payload and therefor higher reliability,&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;high degree of adaptation to specific local and current demands.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What is it that makes moored balloons apparently incapably to exist as a competitor with commercial (and military) geostationary satellites, GPS, LEO-satellites? Where's the deadly flaw?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular:&#xA;&lt;a href=&quot;http://www.nasa.gov/pdf/380911main_SOFIA_QuickFacts.pdf&quot; rel=&quot;nofollow&quot;&gt;SOFIA&lt;/a&gt; is an IR telescope on a Bo747 which is used &lt;a href=&quot;https://en.wikipedia.org/wiki/Stratospheric_Observatory_for_Infrared_Astronomy&quot; rel=&quot;nofollow&quot;&gt;at 12 km altitude&lt;/a&gt; in order to get over the clouds and most of the IR distorting atmosphere. Could moored balloons with similar payload capacity reach as high? &lt;a href=&quot;http://www.nasa.gov/pdf/380911main_SOFIA_QuickFacts.pdf&quot; rel=&quot;nofollow&quot;&gt;&quot;Telescope installation weight: 17 tons&quot;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-10-27T14:27:55.490" LastActivityDate="2014-10-28T12:22:37.560" Title="What are the uses and limitations of moored (powered) balloons? (Compared to GEO, sounding rockets et c.)" Tags="|commercial|sub-orbital|balloons|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5723" PostTypeId="2" ParentId="1081" CreationDate="2014-10-27T18:48:30.373" Score="3" Body="&lt;p&gt;Ok this is an old question, but I think I can contribute a bit if anyone is interested!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For continuous  coverage (always at least one satellite in view) or better (multiple satellites in view) the optimal configuration is usually a Walker constellation (GPS, Galileo) or a &quot;Streets of Coverage&quot; constellation (the wikipedia article calls this a &quot;Walker Star&quot; constellation - I hadn't heard it called that before).  Iridium is an example of the latter.  This &lt;a href=&quot;http://en.wikipedia.org/wiki/Satellite_constellation#Walker_Constellation&quot; rel=&quot;nofollow&quot;&gt;wikipedia&lt;/a&gt; article gives a basic introduction, a google search on those terms will take you as deep as you like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of which Walker constellation is optimal (ie. choosing the inclination, number of planes, plane phasing), this is usually a numerical exercise of evaluating the various candidates, though some can be trivially eliminated by inspection.  The allowable inclination bounds will be set by the angle &lt;em&gt;b&lt;/em&gt; referenced above (sometimes called the Earth Central Angle) and the latitude range you intend to cover.  For circular orbits, the coverage will be symmetric about the equator and uniform in longitude over a sufficient time span.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For intermittent coverage constellations, my experience has been that for average revisit times &amp;lt; the period of the orbit a Walker constellation will often still be optimal.  Beyond that and you may be able to find something better. &lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="4709" LastEditDate="2014-10-27T19:08:47.320" LastActivityDate="2014-10-27T19:08:47.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5724" PostTypeId="2" ParentId="5677" CreationDate="2014-10-27T18:56:52.623" Score="2" Body="&lt;p&gt;There are some who question the origin of the design of the Soyuz capsule - See &quot;Was the Soyuz Design Stolen?&quot; &lt;a href=&quot;http://www.astronautix.com/articles/wastolen.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.astronautix.com/articles/wastolen.htm&lt;/a&gt; for more details.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per the above Astronautix.com article:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;The General Electric Apollo D-2 / Soyuz Design Concept&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The fundamental concept of both designs can easily be summarised as obtaining minimum overall vehicle mass for the mission. This is accomplished by minimising the mass of the re-entry vehicle. There were two major design elements to achieve this:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;Put all systems and space not necessary for re-entry and recovery outside of the re-entry vehicle, into a separate jettisonable 'mission module', joined to the re-entry vehicle by a hatch. Every gram saved in this way saves two or more grams in overall spacecraft mass - for it does not need to be protected by heat shields, supported by parachutes, or braked on landing.  &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Use a re-entry vehicle of the highest possible volumetric efficiency (internal volume divided by hull area). Theoretically this would be a sphere. But re-entry from lunar distances required that the capsule be able to bank a little, to generate lift and 'fly' a bit. This was needed to reduce the G forces on the crew to tolerable levels. Such a manoeuvre is impossible with a spherical capsule. After considerable study, the optimum shape was found to be the Soyuz 'headlight' shape - a hemispherical forward area joined by a barely angled cone (7 degrees) to a classic spherical section heat shield.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;This design concept meant splitting the living area into two modules - the re-entry vehicle, with just enough space, equipment, and supplies to sustain the crew during re-entry; and a mission module. As a bonus the mission module provided an airlock for exit into space and a mounting area for rendezvous electronics.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The end result of this design approach was remarkable. The Apollo capsule designed by NASA had a mass of 5,000 kg and provided the crew with six cubic meters of living space. A service module, providing propulsion, electricity, radio, and other equipment would add at least 1,800 kg to this mass for the circumlunar mission. The General Electric D-2 or Soyuz spacecraft provided the same crew with 9 cubic meters of living space, an airlock, and the service module for the mass of the Apollo capsule alone!  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The modular concept was also inherently adaptable. By changing the fuel load in the service module, and the type of equipment in the mission module, a wide variety of missions could be performed. The superiority of this approach is clear to see: the Soyuz remains in use 30 years later, while the Apollo was quickly abandoned.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dk7He.jpg.&quot; alt=&quot;General Electric Apollo D-2 design; Image from The  Unwanted Blog on up-ship.com&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;General Electric Apollo D-2 design; Image from &lt;em&gt;The Unwanted Blog&lt;/em&gt; on up-ship.com&lt;/p&gt;&#xA;" OwnerUserId="6214" LastEditorUserId="6214" LastEditDate="2014-10-28T03:50:07.747" LastActivityDate="2014-10-28T03:50:07.747" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5725" PostTypeId="2" ParentId="5722" CreationDate="2014-10-27T19:18:48.107" Score="0" Body="&lt;p&gt;I suspect the major objection to large numbers of moored balloons would be the mooring cables themselves, as they would interfere with other aircraft. This was the &lt;strong&gt;&lt;em&gt;intended&lt;/em&gt;&lt;/strong&gt; function of barrage ballons over London in World War II. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That having been said, free-flying high-altitude balloons have been and continue to be studied for communications (Google and others) and surveillance (DARPA) purposes.&lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-10-27T19:18:48.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5726" PostTypeId="2" ParentId="4668" CreationDate="2014-10-27T20:55:56.427" Score="1" Body="&lt;p&gt;Ultimately, the most difficult challenge would be one of competing economically against entities that mine asteroids rather than the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll have to recheck for exact numbers, but I generally understand that the energy cost of getting off the moon exceeds the energy cost of bringing in asteroid material by around an order of magnitude.&lt;/p&gt;&#xA;" OwnerUserId="3230" LastActivityDate="2014-10-27T20:55:56.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5727" PostTypeId="1" CreationDate="2014-10-27T21:08:35.877" Score="3" ViewCount="476" Body="&lt;p&gt;I remember hearing that asteroids and other objects can actually gain speed while passing a planet or large object to due the gravity, but still not enter orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this true? If so, how is this possible? I'm asking because the phenomenon could be used to generate energy from gravity, and obviously we have not been able to do that.&lt;/p&gt;&#xA;" OwnerUserId="1545" LastEditorUserId="12102" LastEditDate="2018-09-29T07:19:59.653" LastActivityDate="2018-09-29T17:12:07.677" Title="Objects speeding up due to planets gravity?" Tags="|gravity|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5728" PostTypeId="2" ParentId="5727" CreationDate="2014-10-27T21:19:15.160" Score="8" Body="&lt;p&gt;It is true, and it works by transferring momentum from the large object to the small one. The movement of the large object is slowed to a degree equivalent to the energy transferred to the small object, but because usually the things involved are planets vs small asteroids, the change is imperceptible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When used to increase the speed of space vessels, this is called a &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist&quot;&gt;gravity assist&lt;/a&gt; or a slingshot maneuver. For instance, in the case of the &lt;a href=&quot;https://www.youtube.com/watch?v=hRjaBORkhog&quot;&gt;Cassini&lt;/a&gt; probe, it has been used many times. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-10-27T21:19:15.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5729" PostTypeId="2" ParentId="5727" CreationDate="2014-10-28T00:28:35.117" Score="8" Body="&lt;p&gt;How you describe this interaction depends on your frame of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at things from the reference frame of the planet, then the object gains no energy or speed.  The object approaches (speeding up) and departs (slowing down).  The relative velocity between the object and the planet is the same at the same distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we may instead choose to look at things from the reference frame of the sun.  In that frame the momenta of the object and planet change.  Some of the orbital momentum and kinetic energy of the planet is transferred to the object.  There is no net increase or creation of energy in the interaction.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Velocity, momentum, and kinetic energy are all frame-dependent properties, and we sometimes choose different frames to examine things.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2014-10-28T00:28:35.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5730" PostTypeId="2" ParentId="5722" CreationDate="2014-10-28T09:43:25.523" Score="2" Body="&lt;p&gt;Balloons can  be interesting as comms relay, in fact the French satellite builder Thales is developing balloons for comms relay (note it is not moored in order to reach higher altitudes)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://translate.google.com/translate?hl=en&amp;amp;sl=fr&amp;amp;u=http://www.sciencesetavenir.fr/high-tech/20140307.OBS8892/stratobus-le-ballon-autonome-futuriste-de-thales.html&quot; rel=&quot;nofollow&quot;&gt;http://translate.google.com/translate?hl=en&amp;amp;sl=fr&amp;amp;u=http://www.sciencesetavenir.fr/high-tech/20140307.OBS8892/stratobus-le-ballon-autonome-futuriste-de-thales.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is the communications market is not asking for it: if you want comms around cities, you put a few antennas on hilltops. If you want comms in the middle of nowhere, you use a  satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Weather can also be a concern at low altitudes so you would not have coverage 100% of the time.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="1142" LastEditDate="2014-10-28T12:22:37.560" LastActivityDate="2014-10-28T12:22:37.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5731" PostTypeId="2" ParentId="5672" CreationDate="2014-10-28T19:35:04.460" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Black-body_radiation&quot; rel=&quot;nofollow&quot;&gt;Black body radiation&lt;/a&gt; cannot be used in modelling this because the station is not going to be in thermal equilibrium. Something that size is going to have complex heat dynamics within it. What applies is &lt;a href=&quot;http://en.wikipedia.org/wiki/Non-equilibrium_thermodynamics&quot; rel=&quot;nofollow&quot;&gt;non-equilibrium thermodynamics&lt;/a&gt;, which is so messy many natural systems are still beyond our capacity to analyze. Fortunately, space stations are floating in a vacuum far from other objects, and thus are receiving heat purely as radiation from one direction - the sun - and not losing it by conduction or convection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the sun's radiation on the surface of the station was immediately distributed evenly throughout its entire mass as heat, then black body calculations would apply. But that is not what will happen. The radiation that falls on it will partly be absorbed as heat, and that heat will slowly be conducted through the materials of the station, at different rates depending on the materials involved, and once it meets a gas or a fluid, also by convection. At the same time, heat is also being radiated by the station's surface. The key thing here is that conduction and convection happen at a snail's pace compared to radiation. The closest comparison that can be made between the speed of radiation (the speed of light) and the speed of conduction, is &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermal_diffusivity&quot; rel=&quot;nofollow&quot;&gt;thermal diffusivity&lt;/a&gt;, which is measured in m&lt;sup&gt;2&lt;/sup&gt;/s. The fastest rate for a natural material is that of graphite (or any of its carbon lattice cousins, such as diamond or graphene), at a rate of 1.22 x 10&lt;sup&gt;-3&lt;/sup&gt; m&lt;sup&gt;2&lt;/sup&gt;/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's leave aside convection and consider just conduction, since most mass will be solid and it is easier to think about. Let's imagine the cylindrical space station pictured above with its long axis at right angles to the sun. Conduction occurs in all directions throughout a material, and its rate increases with difference in temperature across a given distance. So, near the surface of the side of the station in the sun, heat is passing both inward and outward faster than it is a meter from the surface. But throughout the volume, half of all heat is passing in a net direction towards the sun-ward surface, and half is passing in a net direction inwards, away from it. Of the half that is moving towards the surface, the farther it is away from that surface, the longer it takes to get there. Thus heat is being delivered to the station surface by the sun much faster than it is being delivered by conduction from the station interior, and the net heat flow is emphatically inwards. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other side of the station, out of the sun, heat is arriving by conduction and being radiated away, but because the rate at which it arrives by conduction is slow, the radiation is slow. If it were to conduct heat away as quickly as the sun is delivering heat on the other side, it would need to be as bright over its whole radiative area in the infra-red spectrum as the sun is  over its area at that distance. It will not be. In fact, as it cools down because it is radiating heat away, it will radiate heat more slowly than it did when it first rotated out of the sun's rays. The heat gradient across the volume of the material there will decrease as it cools and heat will therefore pass across it more slowly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider an example to get a sense of how much difference this makes. The heat that is conducted and convected from the &lt;a href=&quot;http://en.wikipedia.org/wiki/Earth&amp;#39;s_internal_heat_budget&quot; rel=&quot;nofollow&quot;&gt;interior of the Earth&lt;/a&gt; to its surface is still partly composed of the heat generated during its formation, 4.5 billion years ago. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To simplify your calculation you can consider the entire opaque mass of your hypothetical space station to be one material, for instance material from a stony asteroid. It might take some doing, but you can probably find the figures for the thermal conductivity and specific heat capacity of such material. You will need these properties for glass too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will need to know how much of it, in mass and volume, will be on the side facing the sun, on average. Basically it is a tube with half of its surface taken up by glass, and half by the stony material, so on average half of the side facing the sun will be the one material, and half the other. So figure what the volume, surface area, and mass would be of a quarter-tube of each material with the length, arc, and thickness you envision. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you need to consider how much of the incident light on the station's sun-ward face will be absorbed as heat, so you'll need to know the albedo of the stony material, and the reflectivity of the glass. And with the configuration shown in the example image, some light is going to enter the station and heat the air and the opaque solid masses it falls on, and some is going to pass right through the glass on the other side, so there's another confounding factor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is why when complex engineering is done, scale models and prototypes are made and tested. It would take quite a bit of doing to properly test a small scale model of such a station in order to approximately determine its thermal characteristics, and the result would be of limited usefulness. My guess is if a large space station were under construction, the builders would simply put in more radiative cooling capacity than they thought they would ever need, and then deploy it however worked best to get the desired temperature. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the long axis of the station is perpendicular to the sun, take the area of its cross section and multiply this by the W/m&amp;#178; of solar energy at the distance of its orbit. Presumably this is near Earth orbit, so it would be about 1370 W/m&amp;#178;. Then apply half that energy to the stony material. The glass is problematic because of all the different places light hitting it ends up, you are going to need to estimate a percentage that is absorbed. It becomes a big fudge factor. But really, the result of all this calculation is something engineers might use to place some parameters on working designs, and can be taken in that spirit. Knowing the thermal properties of the materials you can &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermal_conduction#Fourier.27s_law&quot; rel=&quot;nofollow&quot;&gt;determine&lt;/a&gt; how much of the incident radiation will be absorbed as heat over a given unit of time, and how much this will change the temperature of the mass overall. You will need to know the temperature on the outside surface of the material &lt;a href=&quot;http://en.wikipedia.org/wiki/Heat_equation&quot; rel=&quot;nofollow&quot;&gt;when it enters shade&lt;/a&gt;, and what the gradient across the thickness of the material will be, being the difference between the temperature on the inside of the station (posit a comfortable living temperature) and the temperature on the outside after its time in the sun. That will let you calculate how quickly it will lose heat. But compared to the rate at which it will be absorbing heat, this will be slow enough that it is not a big factor - until the station gets hot enough the two radiative figures balance, at a temperature far too hot for life. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you can start to consider how much extra heat radiating capacity will be needed to keep the station at a comfortable temperature.  &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-11-01T19:38:43.720" LastActivityDate="2014-11-01T19:38:43.720" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5732" PostTypeId="1" AcceptedAnswerId="5736" CreationDate="2014-10-28T23:16:29.840" Score="7" ViewCount="827" Body="&lt;p&gt;In today's crash, seen below, there was a control operator who stated, right as the rocket blew up, &quot;Avionics Power nominal&quot;. What does that mean exactly? Was he referring to the rocket power, power to a computer, or something else entirely?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=TjrqN02gUdk&quot;&gt;https://www.youtube.com/watch?v=TjrqN02gUdk&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="160" LastEditDate="2014-10-29T17:34:26.577" LastActivityDate="2014-10-29T17:34:26.577" Title="What entails &quot;Avionics&quot; on the Antares Rocket?" Tags="|launch|rockets|failure|antares|avionics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5733" PostTypeId="1" CreationDate="2014-10-28T23:20:07.063" Score="21" ViewCount="2713" Body="&lt;p&gt;The Antares rocket carrying Cygnus just exploded. &lt;a href=&quot;https://youtu.be/BSr4hUcROwo?t=4m&quot; rel=&quot;nofollow noreferrer&quot;&gt;A video is available here&lt;/a&gt;, explosion at around 4:19.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the video, right as the rocket begins to slide down and explode, a flight controller says &quot;Avionics power nominal&quot;. This is the scene when he begins his sentence:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4LbT9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would seem that nothing would be nominal as the vehicle is exploding. What did he mean, exactly? Was his reaction time simply slow?&lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="16614" LastEditDate="2018-07-01T00:08:19.147" LastActivityDate="2018-07-01T00:17:56.830" Title="Why did a flight controller say &quot;Avionics power nominal&quot; as Antares exploded?" Tags="|antares|failure|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="5735" PostTypeId="1" CreationDate="2014-10-28T23:41:02.397" Score="9" ViewCount="583" Body="&lt;p&gt;From the raw video of the event (which just happened), a trained eye can narrow the possibilities, given good general knowledge of the rocket. What in the video indicates which systems were involved, what kind of failure occurred, and how the failure progressed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that proper analysis will follow, but what should one look at in the video as definite information as to the nature of this event?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Here is the video released by &lt;a href=&quot;https://www.youtube.com/watch?v=HQQoKTuVScM&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And a &lt;a href=&quot;https://www.youtube.com/watch?v=o4SH1B7tzJY&amp;amp;src_vid=A4sahTq42mk&amp;amp;feature=iv&amp;amp;annotation_id=annotation_748814267&quot; rel=&quot;nofollow&quot;&gt;second angle&lt;/a&gt; including a fair bit of close-up footage, although it misses some key seconds. (It is a large group of reporters filming from nearby, and members of the public, and includes a lot of personal reactions).&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="23891" LastEditDate="2019-03-31T04:21:41.607" LastActivityDate="2019-03-31T04:21:41.607" Title="How should one analyze the video of the Antares CRS-3 explosion to find the clues to the cause?" Tags="|launch|failure|antares|rocket-explosion|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="5736" PostTypeId="2" ParentId="5732" CreationDate="2014-10-28T23:53:50.803" Score="14" Body="&lt;p&gt;Avionics is short for &quot;aviation electronics&quot;; on an aircraft it can include computer equipment, communications, guidance and navigation, radar, etc. &quot;Avionics power nominal&quot; implies that the rocket's guidance computers were getting the right amount of electrical power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably the Antares rocket's avionics power went off-nominal a short time later, perhaps when it hit the ground.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-10-29T00:02:10.360" LastActivityDate="2014-10-29T00:02:10.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5737" PostTypeId="2" ParentId="5733" CreationDate="2014-10-29T00:03:07.740" Score="22" Body="&lt;p&gt;It's eight or ten seconds from the sudden flare at the base of the rocket 'til it hits the ground and actually explodes. I'm not an expert, but I don't see any particular reason why power to the guidance systems would have been off-nominal immediately after the initial flare, and if the controller were watching the power readouts instead of the visual, he might have no idea anything was out of the ordinary.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-10-29T00:11:42.763" LastActivityDate="2014-10-29T00:11:42.763" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5739" PostTypeId="1" AcceptedAnswerId="5741" CreationDate="2014-10-29T02:03:40.543" Score="31" ViewCount="19225" Body="&lt;p&gt;While watching the ORB3 Antares explosion press conference, I began to wonder about the design of the NASA logo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the white stars on the blue background represent some particular stellar pattern, or is it an abstract design?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, what does the red elongated chevron represent?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/e7G42.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/e7G42m.jpg&quot; alt=&quot;NASA logo&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="26446" LastEditDate="2020-09-23T04:07:53.480" LastActivityDate="2020-09-23T04:07:53.480" Title="What constellation is on the NASA logo?" Tags="|nasa|insignia|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5740" PostTypeId="2" ParentId="5739" CreationDate="2014-10-29T02:56:01.817" Score="4" Body="&lt;p&gt;This &lt;a href=&quot;http://history.nasa.gov/meatball.htm&quot; rel=&quot;nofollow&quot;&gt;article&lt;/a&gt; from the NASA History Program Office discusses the history of the NASA &quot;meatball&quot; logo.  While not addressing a specific constellation, the red chevron represents aeronautics via an airfoil that was &quot;the latest design in hypersonic wings at the time the logo was developed.&quot;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last portion of the article has a very interesting discussion on the difficulties in printing the logo in black/white and color scanning it.&lt;/p&gt;&#xA;" OwnerUserId="1478" LastActivityDate="2014-10-29T02:56:01.817" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5741" PostTypeId="2" ParentId="5739" CreationDate="2014-10-29T03:51:42.823" Score="27" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.nasa.gov/audience/forstudents/5-8/features/symbols-of-nasa.html&quot;&gt;Symbols of NASA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;From the wing of the space shuttle to the top of the NASA homepage,&#xA;  the agency's official insignia is probably its best-known symbol.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The round red, white and blue insignia, nicknamed the &quot;meatball,&quot; was&#xA;  designed by employee James Modarelli in 1959, NASA's second year. The&#xA;  design incorporates references to different aspects of the mission of&#xA;  the National Aeronautics and Space Administration. The round shape of&#xA;  the insignia represents a planet. The stars represent space. The red&#xA;  v-shaped vector represents aeronautics. The circular orbit around the&#xA;  agency's name represents space travel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_insignia&quot;&gt;Wikipedia on NASA insignia&lt;/a&gt; adds that the red chevron is in the alternate shape of the constellation Andromeda.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for constellations, they're highly stylized and not drawn to scale or apparent magnitude of stars and galaxies in constellations, but I'll go by what I recognize:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Top one is &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28constellation%29&quot;&gt;Orion&lt;/a&gt; (Hunter), left brightest star is &lt;a href=&quot;https://en.wikipedia.org/wiki/Rigel&quot;&gt;Rigel&lt;/a&gt;. Notice the Three Kings in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion%27s_Belt&quot;&gt;Orion's Belt&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_Nebula&quot;&gt;Orion Nebula&lt;/a&gt; (M42) in the Orion's Sword.&lt;/li&gt;&#xA;&lt;li&gt;Bottom left one is &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_%28constellation%29&quot;&gt;Cygnus&lt;/a&gt; (Swan), the bottom brightest star is &lt;a href=&quot;https://en.wikipedia.org/wiki/Deneb&quot;&gt;Deneb&lt;/a&gt;. Notice that it closely follows the shape of a swan in flight, albeit the apparent magnitude and position of other stars again seems rather random.&lt;/li&gt;&#xA;&lt;li&gt;Bottom right one is a tricky one but might be the part of &lt;a href=&quot;https://en.wikipedia.org/wiki/Andromeda_%28constellation%29&quot;&gt;Andromeda&lt;/a&gt; with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Andromeda_Galaxy&quot;&gt;Andromeda Galaxy&lt;/a&gt;, or the part of &lt;a href=&quot;https://en.wikipedia.org/wiki/Lyra&quot;&gt;Lyra&lt;/a&gt; with &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega&quot;&gt;Vega&lt;/a&gt;, or more likely - simply some randomly painted stars.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Otherwise, there is no exactly such constellations or a groups of stars in the night's sky that would follow the triangle that the three groups on the logo represent. They all appear to be constellations that would appear on the Northern hemisphere night skies at least during some parts of the year (Orion for example isn't always visible or is too low on horizon at times), but they certainly don't represent the &lt;a href=&quot;https://en.wikipedia.org/wiki/Summer_Triangle&quot;&gt;Summer Triangle&lt;/a&gt; between Deneb in Cygnus, Vega in Lyra and &lt;a href=&quot;https://en.wikipedia.org/wiki/Altair&quot;&gt;Altair&lt;/a&gt; in &lt;a href=&quot;https://en.wikipedia.org/wiki/Aquila_%28constellation%29&quot;&gt;Aquila&lt;/a&gt;. I'm fairly certain about the Orion and Cygnus, alas, I fail to clearly recognize the group of stars on the bottom right of the logo.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-10-29T03:51:42.823" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5742" PostTypeId="1" AcceptedAnswerId="7987" CreationDate="2014-10-29T04:28:37.573" Score="11" ViewCount="2377" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster&quot;&gt;Wikipedia's&lt;/a&gt; explanation of the sequence of events:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The O-ring failure caused a breach in the SRB joint it sealed, allowing pressurized hot gas from within the solid rocket motor to reach the outside and impinge upon the adjacent SRB attachment hardware and external fuel tank. This led to the separation of the right-hand SRB's aft attachment and the structural failure of the external tank. Aerodynamic forces broke up the orbiter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I understand that lots of debris was being created by the SRB destruction, and the destruction of the fuel tank intuitively seems fatal. But this text doesn't go in that direction. In spite of the various debris, it is stated that aerodynamic forces tore up the orbiter itself. Maybe it's a combination of the two, but it certainly implies that aerodynamic forces were &lt;em&gt;sufficient&lt;/em&gt; to break up the orbiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this Space Shuttle was climbing, not reentering. That should put it well below orbital velocity and (I would assume) well below the velocity that the reentry vehicle sees at that altitude. If it has a heat shield which can withstand the tremendous aerodynamics of reentry, then why was this point in flight obviously destructive when the attitude changed?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="6944" LastEditDate="2020-09-01T17:44:35.087" LastActivityDate="2020-09-01T17:44:35.087" Title="How could aerodynamic forces break up the Challenger orbiter?" Tags="|space-shuttle|attitude|failure|shuttle-challenger|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5743" PostTypeId="2" ParentId="5742" CreationDate="2014-10-29T07:46:21.240" Score="9" Body="&lt;p&gt;From the same Wikipedia article: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Challenger veered from its correct attitude with respect to the local airflow, resulting in a load factor of up to 20 (or 20 g)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The SRB pushed the stack sideways. With the nose no longer pointing into the wind, aerodynamic load grew much larger than it would be on a normal ascent or descent.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-10-29T14:06:29.637" LastActivityDate="2014-10-29T14:06:29.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5744" PostTypeId="1" AcceptedAnswerId="5745" CreationDate="2014-10-29T10:38:03.070" Score="36" ViewCount="12865" Body="&lt;p&gt;It was brought to the attention of the community that right before the crash of Antares the commentator noted:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://spaceflightnow.com/2014/10/29/orbital-sciences-rocket-station-cargo-ship-lost-in-spectacular-launch-mishap/&quot;&gt;&quot;Main engines at 108%&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It was noted that other engines were also reported to work at levels higher than 100% during launch, such as Space Shuttle operating at 104%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is that and why is it considered nominal?&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="1142" LastEditDate="2014-10-29T11:53:21.363" LastActivityDate="2017-09-20T20:17:36.033" Title="Why are rocket engines operating above 100% often considered nominal?" Tags="|launch|engine-design|engines|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5745" PostTypeId="2" ParentId="5744" CreationDate="2014-10-29T10:55:47.520" Score="48" Body="&lt;p&gt;The original design called for say 500,000lbs of thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After years of development, tweaks, changes in the real world (bonuses, like the pressure of the fuel in the line from the entire length of the tank boosts performance (SLS has a sort of issue with this)) means the production engine actually produces 540,000lbs of thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus full power is now 108%.  It is a bit silly, but easier to keep the specs than constantly redefine 100%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they said &quot;engines at 500,000lbs thrust&quot; instead of &quot;engines at 100%&quot; there would be no need to change.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-10-29T10:55:47.520" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5746" PostTypeId="2" ParentId="5742" CreationDate="2014-10-29T12:12:59.743" Score="8" Body="&lt;p&gt;Any high-performance aircraft will break up if abruptly turned from its direction of travel at speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In normal flight, the thin leading edges of the aerodynamic surfaces catch relatively little air resistance, so there's relatively little force on the wings and it's directed down the length of the craft. As the aircraft turns out of the direction of travel, the effective aerodynamic cross section of the wings and tail suddenly gets a lot bigger, and the load on the wings goes rapidly to 20g as noted in Hobbes' answer. The maximum expected load on the shuttle was about 3g (in reentry), and the airframe was rated for 5g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind also that the hottest, fastest part of reentry happens in very thin air at high altitude. In contrast, the Challenger explosion happened during ascent at 48,000 feet -- at 14% of sea level pressure, this is still relatively dense air compared to the beginning of reentry, and a bad place to be flying sideways at mach 2. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-10-29T17:14:32.237" LastActivityDate="2014-10-29T17:14:32.237" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5747" PostTypeId="1" CreationDate="2014-10-29T14:32:19.603" Score="10" ViewCount="784" Body="&lt;p&gt;The Huygens probe didn't last long. What are the challenges of operating a rover on the surface of Titan for at least a few months and what technologies would be used to overcome these?&lt;/p&gt;&#xA;" OwnerUserId="6447" LastActivityDate="2018-09-18T01:58:19.073" Title="Titan rover survival challenges" Tags="|rovers|titan|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5748" PostTypeId="1" AcceptedAnswerId="5749" CreationDate="2014-10-29T14:36:58.117" Score="4" ViewCount="874" Body="&lt;p&gt;Was &lt;a href=&quot;http://www.youtube.com/watch?v=SZixaHZS1RE&quot; rel=&quot;nofollow&quot;&gt;Progress M-25M&lt;/a&gt; launched in response to &lt;a href=&quot;http://en.wikipedia.org/wiki/Cygnus_CRS_Orb-3&quot; rel=&quot;nofollow&quot;&gt;Antares' catastrophic failure&lt;/a&gt; or were they 2 different launches, both planned to reach the ISS &quot;separately&quot;?&lt;/p&gt;&#xA;" OwnerUserId="6448" LastActivityDate="2014-10-29T18:28:18.670" Title="Progress M-25M launched because of Antares?" Tags="|launch|iss|soyuz-spacecraft|failure|antares|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5749" PostTypeId="2" ParentId="5748" CreationDate="2014-10-29T14:46:30.233" Score="9" Body="&lt;p&gt;They were planned separately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes a long time to plan and execute a launch, and both Antares ORB-3 and Progress M-25M had their launch dates set months ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To see how far ahead some of these launches are planned look at: &lt;a href=&quot;http://www.spaceflightnow.com/tracking/&quot; rel=&quot;nofollow&quot;&gt;http://www.spaceflightnow.com/tracking/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance in August next year Progress 60P and SpaceX CRS-8 will also launch within a week of each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[edit] As TildalWave points out below, these are only tentative launch dates. In fact ORB-3 was initially scheduled to launch the day before it did, but was scrubbed for range safety issues (a boat failed to clear the exclusion zone).&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2014-10-29T18:28:18.670" LastActivityDate="2014-10-29T18:28:18.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5750" PostTypeId="2" ParentId="5748" CreationDate="2014-10-29T14:47:47.220" Score="8" Body="&lt;p&gt;The ISS is currently supplied (or was) by the following vehicles:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Space Shuttle - retired&lt;/li&gt;&#xA;&lt;li&gt;ESA Automated Transfer Vehicle (ATV) - last one has flown - retired&lt;/li&gt;&#xA;&lt;li&gt;JAXA H2 Transfer Vehicle (HTV) - which has only a few flights left before retirement&lt;/li&gt;&#xA;&lt;li&gt;Russian Progress - cargo, fuel, air, water - active&lt;/li&gt;&#xA;&lt;li&gt;Russian Soyuz - crew, very minimal cargo&lt;/li&gt;&#xA;&lt;li&gt;US SpaceX Dragon - contract for 12 flights, second contract for future flights is being worked out.  Only remaining vehicle that return cargo to Earth.&lt;/li&gt;&#xA;&lt;li&gt;US Orbital Sciences Cygnus- Contract for 8 flights, failure on 4th mission. Possible competitor for second cargo contract.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The schedule for all these vehicles is planned by the ISS team and is very complicated and interlocking. Each is given a window of time at the station that allows for crew time to load/unload them after the berth/dock.  Some (HTV, Cygnus, Dragon) share ports and thus can conflict.  ATV, Soyuz, and Progress all share the same docking systems, but there are 4 such ports available so they conflict less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the short answer is that the Progress flight was completely independent of the Cygnus launch, except that the timing was planned so as not to have concurrent docking operations using up needed crew time to process them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/structure/resupply.html&quot;&gt;Past history of visiting vehicles&lt;/a&gt; from NASA.&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_unmanned_spaceflights_to_the_International_Space_Station&quot;&gt;Past history of visiting vehicles&lt;/a&gt; from WikiPedia.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-10-29T14:47:47.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5751" PostTypeId="2" ParentId="5744" CreationDate="2014-10-29T15:08:25.790" Score="8" Body="&lt;p&gt;In aeronautical terms, the word &lt;em&gt;nominal&lt;/em&gt; simply means the component is operating within acceptable tolerances/parameters. In other words, it means that the engine is operating within a permissible deviation from a specified value. More often than not, these are expressed as a percentage of the nominal value. Later on in the flight, you'll likely hear them &lt;em&gt;reduce&lt;/em&gt; the thrust to (for example) 50% of its nominal value to reduce acceleration loads as the propellant tanks empty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's nothing sacrosanct about operating at &quot;100%&quot; of that nominal value. It's just a reference point to calculate trajectories and other operational parameters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The throttling capability of an engine (Thrust Magnitude Control (TMC)) allows the mission controllers to change the thrust of an individual rocket engine as needed. When a rocket is sitting on the launch pad, there's a big change in momentum needed to get it going. If the engine is designed to operate beyond the stated nominal value (in this case to 108%), you can push the engine in such a manner to get it going where it needs to be. Such capabilities are vital to achieve and maintain the trajectories needed for that specific load and mission. &lt;/p&gt;&#xA;" OwnerUserId="68" LastEditorUserId="68" LastEditDate="2014-10-29T16:33:17.800" LastActivityDate="2014-10-29T16:33:17.800" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5752" PostTypeId="2" ParentId="5744" CreationDate="2014-10-29T16:11:16.100" Score="12" Body="&lt;p&gt;100% is usually the power level where the turbo-machines operate at their design flow and pressure (head), which means they operate at maximum efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At launch it is often favorable to go above this value to reduce gravity losses. Although the turbo-machinery does not work at peak efficiency, the specific impulse may actually increase as well because running at a higher chamber pressure lessens the over-expansion in the nozzle at sea level. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Small glossary:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Chamber pressure: The pressure in the combustion chamber.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;over-expansion: Because of the lessening atmosphere along the rocket's path, the nozzle is only perfect at one point of the mission. At launch it is too big, and at altitude it is too small.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Specific impulse: Kind of a fuel efficiency of the engine.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Turbo-machinery: The pumps that are used to feed the propellants into the combustion chamber.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-10-29T16:11:16.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5753" PostTypeId="2" ParentId="5635" CreationDate="2014-10-29T16:59:37.277" Score="5" Body="&lt;p&gt;One big issue with G-forces is the way it affects the bloodflow. At high accelerations, the heart is unable to pump the blood 'uphill'. In aircraft, this leads to blackouts around 9 G. Having the acceleration vector towards the back instead of the feet mitigates this, but you're still going to have a blood supply gradient over your body. Surviving this for one second is feasible, but having parts of the body without sufficient blood for one minute will give problems.&lt;/p&gt;&#xA;&lt;p&gt;A G-suit as worn by pilots shows that an external force on the body can help with this a bit: it constricts the blood vessels of the lower body, making more blood available in the higher regions. This works because pilots experience G-forces towards their feet. The legs have lots of surface area relative to their volume, and major arteries close to the surface.&lt;/p&gt;&#xA;&lt;p&gt;If the blood pools around the spine and in the back of the head, adding surface pressure is not going to help force the blood 'up'.&lt;/p&gt;&#xA;&lt;p&gt;(Having thought about it some more, I have to rewrite this part of my answer, my initial assumption was incorrect).&lt;/p&gt;&#xA;&lt;p&gt;A body suspended in water must have enough buoyancy to float. If you're not buoyant enough, you'll sink toward the 'bottom' of the container under acceleration, and eventually you'll be supported by a solid surface instead of by water. If you're buoyant enough to float, you'll stay afloat under acceleration.&lt;/p&gt;&#xA;&lt;p&gt;Any water on top of the body will add pressure: a 10-cm layer of water at 50g will feel like 50 kg/dm^2; that's not healthy.&lt;/p&gt;&#xA;&lt;p&gt;A body almost submerged in water is supported by the water, which helps mitigate some of the effects of the G-force.&lt;/p&gt;&#xA;&lt;p&gt;From the &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_breathing&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article on liquid breathing&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15 to 20 G. Extending acceleration protection beyond 20 G requires filling the lungs with fluid of density similar to water. An astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme G forces because the forces on a liquid are distributed equally, and in all directions simultaneously. However effects will be felt because of density differences between different body tissues, so an upper acceleration limit still exists.&lt;/p&gt;&#xA;&lt;p&gt;Liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. Perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application.&lt;/p&gt;&#xA;&lt;p&gt;Original source:&#xA;Guyton, Arthur C. (1986). Textbook of Medical Physiology, 7th Ed., Aviation, Space, and Deep Sea Diving Physiology. W. B. Saunders Company. p. 533.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This ignores the fact that the lungs aren't the only air-filled cavity in the human body. All cavities would have to be filled with liquid to prevent collapse.&lt;/p&gt;&#xA;&lt;p&gt;All of this only works if the acceleration is limited to one direction only. A curved launch tube would add a centripetal acceleration force, which the water would not help against. You'd crash into the side of the container.&lt;/p&gt;&#xA;&lt;p&gt;PS &lt;a href=&quot;http://history.nasa.gov/SP-4201/ch2-4.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; has lots of information on the effects of G-loading on the human body.&lt;/p&gt;&#xA;&lt;p&gt;1: bottom and top are relative to the direction of the acceleration force.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-10-30T18:56:59.197" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="5754" PostTypeId="2" ParentId="5747" CreationDate="2014-10-29T17:07:27.337" Score="11" Body="&lt;p&gt;Titan's atmosphere is not extremely thick. Its only 1.5 bars at ground level. Its also not corrosive, consisting mostly of nitrogen and, to a lesser degree, methane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only real challenge is the distance from the sun. Reaching Titan requires a considerable amount of delta-v (about 20km/s from the earth's surface). Once you are on Titan, solar panels are unusable because of the low amount of light that reaches the surface (0.1% of the level at the earth's surface). Also because of this low energy input, temperatures are frigid cold, at about -180°C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space is also cold out there, but the atmospheric temperature is worse, because in space there is no convection. Heat can only be exchanged by radiation. In Titan's atmosphere, there is lots of convection, quickly cooling the spacecraft below the allowable temperatures of sensitive equipment, such as the batteries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only way to keep a spacecraft operating and supplied with energy for a longer period of time on Titan would be a nuclear power source. A nuclear reactor is possible, but it makes more sense to use a large radioisotope thermoelectric generator (RTG). Therein the heat from the decay of a plutonium ingot is converted to electrical power. RTGs have been used other space missions, such as Voyager or Curiosity, but in order to keep a titan probe warm, you would need a big one, and that makes it expensive to build and launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The required size, of course, depends on the amount of insulation, so its an optimization problem: Save some weight on the RTG, and carry more insulation, or carry a larger RTG and save some weight on the insulation. &lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2018-09-18T01:58:19.073" LastActivityDate="2018-09-18T01:58:19.073" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="5757" PostTypeId="1" AcceptedAnswerId="5758" CreationDate="2014-10-29T18:57:31.477" Score="16" ViewCount="20808" Body="&lt;p&gt;Superficially the Soyuz and Progress spacecraft look very similar:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hdut0.jpg&quot; alt=&quot;Soyuz TMA-7&quot;&gt;&#xA;(Soyuz TMA-7)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xeAsP.jpg&quot; alt=&quot;Progress M-52&quot;&gt;&#xA;(Progress M-52)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it would be easy to imagine that they share a common service module and/or orbital module. But does anyone know if this is the case, or how many common systems they do in fact share?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-01-31T19:49:51.217" LastActivityDate="2021-04-15T14:11:00.053" Title="How similar are Soyuz and Progress?" Tags="|iss|soyuz-spacecraft|russia|progress|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5758" PostTypeId="2" ParentId="5757" CreationDate="2014-10-29T22:15:29.730" Score="18" Body="&lt;p&gt;They are essentially the same. The Progress resupply spacecraft is a direct derivative of the Soyuz, where the reentry module was replaced by a fuel tank. Hence the similar shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both are launched with the same Soyuz rockets.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-10-29T22:15:29.730" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5759" PostTypeId="2" ParentId="5735" CreationDate="2014-10-29T23:10:47.533" Score="5" Body="&lt;p&gt;I'll just start with some things that came to my mind. Not planning to provide a complete list here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can look at the craft itself prior to and shortly after launch. Was there any leakage or jets of propellant where there shouldn't be any? For example, smoke was visible during the final launch of the space shuttle Challenger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparing the video to a &lt;a href=&quot;https://www.youtube.com/watch?v=vYYNc2_EqQY&quot;&gt;previous launch&lt;/a&gt; we find no obvious leakage. However, the night launch makes it difficult to see much of the vehicle at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next thing that came to my mind was the guidance system failing and causing automatic self-destruction, a bit like on that recent Proton launch. The rocket does sway to the side a bit after launch, but that may be on purpose, in order not to recontact the launch tower. It does the same in the other video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I'm going out on limb and speculate a bit from the poor footage available. The flame in of the exhaust turns from yellow-blue to bright yellow a split second before the explosion. One explanation (not the only explanation) for this would be a failure in the oxygen supply, so the rocket quickly goes to extremely fuel-rich combustion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the oxygen pump of the &lt;a href=&quot;http://lpre.de/resources/articles/AIAA-1998-3361.pdf&quot;&gt;NK-33&lt;/a&gt; is on the same shaft as the fuel pump, and located between the turbine and the fuel pump. So the fuel pump can't keep working when the oxygen pump has stopped (or exploded).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now it would be interesting to have a high-res video from another perspective to see if both engines fail simultaneously. If they do, my oracle is this: There was loss of pressure on the oxygen tank or an obstruction in the ducts. This caused the combustion to go extremely fuel rich for a split second, until the dry-running turbo-pump bearings (which must be cooled with liquid oxygen) overheated. This ignited the metal of the pump housing, leading to the rest of all this unpleasantness.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-10-29T23:10:47.533" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5761" PostTypeId="2" ParentId="5747" CreationDate="2014-10-30T12:35:12.137" Score="6" Body="&lt;p&gt;Additional to @Rikki's answer in addition to heat and power you need to be able to move and explore, there are several challenges to this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Titan is far from the sun, so has very little light. On Mars there's enough light for conventional camera instruments to work. Making navigation decisions is possible based mostly on visual information. On titan you'd need to design a system that can &quot;see&quot; terrain in a very dark environment&lt;/li&gt;&#xA;&lt;li&gt;Titan is very far away from earth, so communications takes a great deal of time. It's unrealistic for a titan rover to be directed from earth as nothing would get done, so the rover would need to be able to navigate and use its science instruments with a great deal of autonomy, which means a high level of AI&lt;/li&gt;&#xA;&lt;li&gt;Parts of Titan are covered with liquid hydrocarbons in which a rover would easily get mired. It is theorized that it rains hydrocarbons as well, so a rover would need to be able to recognize and avoid seas, rivers, ponds, and puddles of liquid methane which would be a hazard&lt;/li&gt;&#xA;&lt;li&gt;We know comparatively little about the surface of Titan. Before committing a rover to the surface of a planetoid we would want to do extensive, accurate mapping of the surface, which would have to be incorporated as part of, or a separate mission to pave the way for a rover mission&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, for a rover to be successful it would need to independently move around largely unknown terrain which is pitch black, close to absolute zero, and covered with liquid hydrocarbon all the while being able to do meaningful science. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-10-30T12:35:12.137" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5762" PostTypeId="1" CreationDate="2014-10-30T18:19:27.900" Score="13" ViewCount="2156" Body="&lt;p&gt;I was reading about what is involved in making an ephemeris and wondered about this. Feeling I'd read pieces that extrapolated orbits over many thousands of years, I looked for an example and &lt;a href=&quot;http://phys.org/news/2011-07-minor-planets-ceres-vesta-earth.html&quot;&gt;found this&lt;/a&gt; rather interesting one at phys.org:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Astronomy &amp;amp; Astrophysics is publishing a new study of the orbital&#xA;  evolution of minor planets Ceres and Vesta, a few days before the&#xA;  flyby of Vesta by the Dawn spacecraft. A team of astronomers found&#xA;  that close encounters among these bodies lead to strong chaotic&#xA;  behavior of their orbits, as well as of the Earth's eccentricity. This&#xA;  means, in particular, that the Earth's past orbit cannot be&#xA;  reconstructed beyond 60 million years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris&quot;&gt;JPL's Development Ephemeris&lt;/a&gt; gives a sense of how tremendously complex these calculations are, and how many observations are used. This quote gives a bit of a sense:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;DE418[23] was released in 2007 for planning the New Horizons mission&#xA;  to Pluto. New observations of Pluto, which took advantage of the new&#xA;  astrometric accuracy of the Hipparcos star catalog, were included in&#xA;  the fit. Mars spacecraft ranging and VLBI observations were updated&#xA;  through 2007. Asteroid masses were estimated differently. Lunar laser&#xA;  ranging data for the Moon was added for the first time since DE403,&#xA;  significantly improving the lunar orbit and librations. Estimated&#xA;  position data from the Cassini spacecraft was included in the fit,&#xA;  improving the orbit of Saturn, but rigorous analysis of the data was&#xA;  deferred to a later date. DE418 covered the years 1899 to 2051, and&#xA;  JPL recommended not using it outside of that range due to minor&#xA;  inconsistencies which remained in the planets' masses due to time&#xA;  constraints&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How quickly does uncertainty creep into such calculations, and what sort of margin of error is involved? Does the computing ability of modern computers make a difference - that is, can all known objects and forces be entered into a program that crunches the numbers, or do the smaller things have to be dropped out of the formula? Does this affect mission planning to asteroids?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-10-31T16:47:36.877" LastActivityDate="2014-11-17T14:24:58.970" Title="How accurate are modern ephemerides, and how does their accuracy degrade over time?" Tags="|orbit|trajectory|ephemeris|calculation|prediction|" AnswerCount="1" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="5763" PostTypeId="1" CreationDate="2014-10-31T11:39:17.963" Score="5" ViewCount="668" Body="&lt;p&gt;If our regular huge power plants on the ground could be used to power part of a rocket launch, launch mass would be smaller since fuel would not have to be carried on board. I wonder what are the main weaknesses and problems with this? How could it be done, if at all?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One limit of electric ion engines seems to be that it requires quite a huge electric power plant to match the effect generated by the explosion in a chemical rocket engine. But with a reusable first stage ion electric engine which is physically connected to the electric grid on the ground, I imagine that one could turn up the power until the cable glows and electric effect is not the limit anymore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another limit might be how large an ion engine can be in terms of gas mass flow per second, or how many small ones that can be bunched together. Maybe lifting the cable is a problem, even if it is put on the ground/sea surface under the planned launch trajectory, because one would have to reach a substantial fraction of the escape speed in order for this to be worthwhile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there other electric rocket engine types which would work better than ion engines when very high electric effect is available?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6944" LastEditDate="2019-07-27T23:05:56.413" LastActivityDate="2019-07-28T00:11:24.687" Title="Could Earth's electric grid be used to power rocket launches via cable?" Tags="|launch|propulsion|power|ion-thruster|design-alternative|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5765" PostTypeId="2" ParentId="5763" CreationDate="2014-10-31T14:41:19.273" Score="2" Body="&lt;p&gt;Laser propulsion may be a better option if unlimited power and imaginary technology were available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, a land based, ground powered laser is aimed at the base of the rocket, and the heat from the laser provides the energy to heat the propellant that is exhausted to generate thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course this has not been demonstrated to any serious level but there are small projects considering it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are issues in building powerful enough lasers, that can aim accurately enough over the time duration of a rocket flight.  Upper stages will of course still be needed, since the first stage MIGHT stay in line of sight long enough to work, but the upper stages will be out of sight pretty quickly. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-10-31T14:41:19.273" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5766" PostTypeId="2" ParentId="5763" CreationDate="2014-10-31T22:21:37.573" Score="4" Body="&lt;p&gt;1)  Nobody has built an ion engine that can lift from Earth.  Your system might work to take off from a place like Phobos but nothing bigger.  (And it wouldn't work very well even there.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2)  The strongest wires out there will take you no farther than the stratosphere before they snap under their own weight.  If you the strongest cables we can build to support the wires we can get into space--but getting up there is easy compared to building up the speed to get into orbit.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-10-31T22:21:37.573" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5767" PostTypeId="1" AcceptedAnswerId="7736" CreationDate="2014-10-31T23:49:58.987" Score="6" ViewCount="1193" Body="&lt;p&gt;I just found out about &lt;a href=&quot;http://www.bbc.com/news/world-us-canada-29857182&quot; rel=&quot;nofollow&quot;&gt;today's horrific crash&lt;/a&gt; of one of Virgin Galactic SpaceShipTwo spaceplanes, the &lt;em&gt;VSS Enterprise&lt;/em&gt;. It apparently was due to an 'anomaly.' I realize that details are still unavailable, but perhaps someone knows just enough to answer this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that there exists one other SpaceShipTwo (&lt;em&gt;the VSS Voyager&lt;/em&gt;), with three more planned. Will whatever malfunction occurred impact these craft? Are any major design changes in order?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I expect that we won't know the full story for some time; the investigation is ongoing. However, perhaps details may come out within the next week or so that could shed light on the situation.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The investigation is going to take a &lt;em&gt;long&lt;/em&gt; time. As &lt;a href=&quot;http://www.bbc.com/news/world-us-canada-29869070&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; says,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The investigation into the Virgin Galactic spacecraft crash in California's Mojave Desert could take about a year, the head of the US transport safety agency has said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I expect that this question won't receive any answers until the final report is released.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-11-09T02:07:01.823" LastActivityDate="2015-01-10T09:29:51.080" Title="Could the 'anomaly' in the recent Virgin Galactic crash impact the other SpaceShipTwos?" Tags="|spaceplane|virgin-galactic|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5769" PostTypeId="1" CreationDate="2014-11-01T00:56:52.837" Score="5" ViewCount="375" Body="&lt;p&gt;Here is a photo of SpaceShip 2's exhaust shortly after ignition during the test flight in January&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6pmAg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do they have that segmented structure, and how does it affect thrust?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-11-01T01:06:48.363" Title="Why are the exhaust plumes exiting the nozzles of SpaceShip Two segmented along their length?" Tags="|nozzle|plume|development|exhaust|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5770" PostTypeId="2" ParentId="5769" CreationDate="2014-11-01T01:06:48.363" Score="11" Body="&lt;p&gt;Those are &lt;a href=&quot;http://en.wikipedia.org/wiki/Shock_diamond&quot;&gt;Mach diamonds&lt;/a&gt;.  They form due to the interaction of the exhaust flow with its own supersonic shockwaves.  All rockets and some jet engines produce them; their visibility varies with the propellant combination, mixture ratio and environmental conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flow pattern doesn't appreciably affect thrust since it takes place after the exhaust has left the nozzle (where thrust is produced).&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-11-01T01:06:48.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5771" PostTypeId="1" AcceptedAnswerId="6620" CreationDate="2014-11-01T07:07:49.230" Score="17" ViewCount="4537" Body="&lt;p&gt;An important advantage of having a space station in LEO, as opposed to a space ship orbiting another celestial object, is allegedly that the crew can be saved by a quick return to Earth. But what would actually happen on the ISS if for example a catastrophic fire breaks out? Does the crew rush into the life boat Soyuz capsule within a minute and push the big red &quot;return-to-Earth-button&quot;, or do they have to get dressed and prepare in other ways?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And does the entire crew fit into the Soyuz life boat they have today?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-29T05:09:59.360" Title="How fast can the crew evacuate the ISS? (ACRV)" Tags="|iss|soyuz-spacecraft|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5773" PostTypeId="1" CreationDate="2014-11-01T11:57:24.830" Score="6" ViewCount="208" Body="&lt;p&gt;How, if at all, does suborbital flight benefit real space flight outside of the atmosphere? I mean suborbital flights as a business on its own, e.g. for tourism, not just suborbital testing of space equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Historically, military applications like the V2 and ICBM's were very helpful for space flight, which I think can even be said became a spin off from such suborbital projects. But what about today's suborbital projects? Virgin Galactic, Lynx, Skylon, Copenhagen Suborbital and others all seem to be very aerodynamical with little in common with space flight. (With the exception of Blue origin which seems conceptually easier to scale into real space travel, and of course JPaerospace fantastic idea about &quot;airship to orbit&quot; which however remains to be proven.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could there for example be an orbital and then a lunar upgrade of the VG SpaceShip Two? Doesn't look so superficially. If a suborbital company of today decides to go into space instead, do they have competence and experience required thanks to their suborbital RnD? Or do suborbital flights have more in common with airplanes than with spaceships?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-11-01T13:16:49.077" LastActivityDate="2019-08-16T00:18:49.803" Title="What are the synergies between today's suborbital private businesses and space flight?" Tags="|sub-orbital|crewed-spaceflight|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5774" PostTypeId="1" CreationDate="2014-11-01T13:33:31.663" Score="11" ViewCount="1113" Body="&lt;p&gt;In what main ways would a human return to the Moon facilitate human travel to Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is often argued that the Moon could work as a stepping stone to Mars. I want to know what the merits are of that argument. The differences seem huge. Travel time is really the main problem with going to Mars, while a Moon trip is done in a week. People have to survive away from Earth during an entire Mars opposition period of about 2 years. Atmosphere and gravity differ so that the two missions do best in having separate lander/ascent vehicle designs, I would suppose. Space suites have very different challenges too with different kinds of dust and temperature variations. The day/night cycle is a tough problem on the Moon, while Mars on the other hand has less than half the insolation, requiring different energy supply and heating/cooling systems. The Apollo lunar rover would hardly hold up in the double Martian gravity. Communication to Earth from a rotating Mars 400 000 000 km away is quite different than from the front side of the Moon. The resources which could potentially be made use of are very different too, such as water ice in eternally shadowed craters at Lunar poles and CO2 in the Martian atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If humans returned to the Moon, what could they bring from that to also go to Mars? For example, what Apollo equipment would be useful on Mars?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-11-01T13:38:59.597" LastActivityDate="2018-06-11T19:36:54.467" Title="What are the synergies between human landing on the Moon and on Mars?" Tags="|mars|the-moon|crewed-spaceflight|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5775" PostTypeId="1" AcceptedAnswerId="5799" CreationDate="2014-11-01T15:05:43.293" Score="15" ViewCount="1497" Body="&lt;p&gt;Many rocket systems are equipped with self-destructs to prevent an out-of-control vehicle from wandering too far from its intended path and becoming a hazard. Presumably, the objective is to kill the rocket engine's thrust (and disable any of the upper stages) so that the vehicle or its debris falls within a designated zone. The question is: what is the intended action of the self-destruct devices? Is it intended only to terminate thrust, or is there also design intent to release and flare off propellant and/or fragment the vehicle (or are those just side-effect)? Given that there are size and weight constraints, any added piece of equipment aboard a rocket can only do so much... how is it ensured that the devices will do their job reliably, when and only when commanded? How does the type of rocket (solid fuel vs liquid fuel) affect the design and implementation of the destruct system?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="10791" LastEditDate="2016-02-12T18:41:06.490" LastActivityDate="2016-02-12T18:41:06.490" Title="How are self-destructs implemented in rocketry?" Tags="|rockets|design|range-safety|flight-termination-system|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5776" PostTypeId="2" ParentId="5767" CreationDate="2014-11-01T23:16:16.590" Score="0" Body="&lt;p&gt;The cause of the accident will have to be clearly investigated. For all we know, the choice of fuel could be completely irrelevant to the accident, and could also have happened with the old rubber fuel at this point in time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the evidence is compiled, the accident cause is properly investigated and understood, design recommendations are made: then we can modify the remaining vehicles.&lt;/p&gt;&#xA;" OwnerUserId="1552" LastActivityDate="2014-11-01T23:16:16.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5777" PostTypeId="2" ParentId="5774" CreationDate="2014-11-02T11:20:47.730" Score="8" Body="&lt;p&gt;Bob Zubrin, famed for &quot;&lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/145160811X&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Case for Mars&lt;/a&gt;&quot; argues that there is no direct synergy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;EDL (Entry, Descent, Landing) needs are totally different.  No atmosphere on Moon, so parachutes/heat shields/deceleration systems are completely different for Mars vs Moon.&lt;/li&gt;&#xA;&lt;li&gt;ISRU (In-Situ Resource Utilization) which makes Mars much more affordable, is totally different on Mars vs Moon.  (No CO2 in the atmosphere on the moon, but tons on Mars).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now there is a direct synergy, if going the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;SLS&lt;/a&gt; route as NASA seems determined. The current SLS plan is to fly maybe once every 4 years if lucky.  That just plain doesn't work. Even in best case, only a Mars launch window around every 2 years.  So what do you do with your large booster in between those launch windows? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well the Moon is a great destination. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="15888" LastEditDate="2018-06-11T19:36:54.467" LastActivityDate="2018-06-11T19:36:54.467" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="5779" PostTypeId="2" ParentId="88" CreationDate="2014-11-02T15:48:37.333" Score="35" Body="&lt;p&gt;Using the links in Erik's answer and comments, I threw this together.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;#!/usr/bin/python3&#xA;&#xA;&amp;quot;&amp;quot;&amp;quot;&#xA;(C) 2014 Dotan Cohen&#xA;&#xA;This program is free software: you can redistribute it and/or modify&#xA;it under the terms of the GNU General Public License version 3.&#xA;&#xA;http://www.gnu.org/licenses/gpl-3.0.html&#xA;&amp;quot;&amp;quot;&amp;quot;&#xA;&#xA;import re&#xA;import time&#xA;import urllib.request&#xA;from bs4 import BeautifulSoup&#xA;&#xA;&#xA;def main():&#xA;&#xA;    year_links = []&#xA;    year_launches = []&#xA;    total_mass = 0&#xA;&#xA;    start_url = 'http://space.skyrocket.de/directories/chronology.htm'&#xA;    year_links = get_year_links(start_url)&#xA;&#xA;    for link in year_links:&#xA;        launches = get_year_launches(link)&#xA;        for launch_url in launches:&#xA;            year_launches.append( launch_url )&#xA;        &#xA;    for launch in year_launches:&#xA;        time.sleep(0.1) # Be nice to server&#xA;        total_mass += get_launch_mass(launch)&#xA;&#xA;    print(&amp;quot;Total mass: %i KG&amp;quot; % (total_mass, ))&#xA;&#xA;    return True&#xA;&#xA;&#xA;def get_year_links(start_url):&#xA;&#xA;    year_links = []&#xA;&#xA;    html = urllib.request.urlopen(start_url).read()&#xA;    parsed_page = BeautifulSoup(html, 'lxml')&#xA;    links = parsed_page.find_all('a')&#xA;&#xA;    for link in links:&#xA;        link = str(link)&#xA;        if 'doc_chr' in link:&#xA;            tmp_link = link[link.find('&amp;quot;')+1:]&#xA;            tmp_link = tmp_link[:tmp_link.find('&amp;quot;')]&#xA;            year_links.append(tmp_link)&#xA;&#xA;    return year_links&#xA;&#xA;&#xA;def get_year_launches(year_link):&#xA;&#xA;    year_launches = []&#xA;&#xA;    url = 'http://space.skyrocket.de/' + year_link[3:]&#xA;    html = urllib.request.urlopen(url).read()&#xA;    parsed_page = BeautifulSoup(html, 'lxml')&#xA;    links = parsed_page.find_all('a')&#xA;&#xA;    for link in links:&#xA;        link = str(link)&#xA;        if 'doc_sdat' in link:&#xA;            tmp_link = link[link.find('&amp;quot;')+1:]&#xA;            tmp_link = tmp_link[:tmp_link.find('&amp;quot;')]&#xA;            year_launches.append(tmp_link)&#xA;&#xA;    return year_launches&#xA;&#xA;&#xA;def get_launch_mass(launch_url):&#xA;&#xA;    mass = 0&#xA;&#xA;    try:&#xA;        url = 'http://space.skyrocket.de/' + launch_url[3:]&#xA;        html = urllib.request.urlopen(url).read()&#xA;        parsed_page = BeautifulSoup(html, 'lxml')&#xA;&#xA;        for pre_mass_element in parsed_page.find_all(text='Mass:'):&#xA;            mass_element = pre_mass_element.find_next()&#xA;            mass = int(re.search(r'(\d+)', mass_element.text).group(0))&#xA;&#xA;    except Exception:&#xA;        pass&#xA;&#xA;    return mass&#xA;&#xA;&#xA;if __name__ == '__main__':&#xA;    main()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;The code took 54 minutes to run, here is the output:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Total mass: 10500435 KG&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;The code visits a website that &lt;a href=&quot;http://space.skyrocket.de/directories/chronology.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;lists launches by year&lt;/a&gt;, and from each year's page it visits each launch's dedicated webpage. From there the code finds the &amp;quot;Mass&amp;quot; table cell. It then extracts the text of the next cell, pulls out the first integer, and adds that to the running total mass. Python!&lt;/p&gt;&#xA;&lt;p&gt;Since the answer is in code, it can be periodically rerun as new launches occur and as details of old launches are revised.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update 25 December, 2017:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Total mass: 13367669 KG&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update 23 December, 2019:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;$ time ./mass.py&#xA;&#xA;Total mass: 14466896 KG&#xA;&#xA;real    199m30.311s&#xA;user    26m51.576s&#xA;sys     0m38.556s&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update 15 February, 2022:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;$ ./mass.py&#xA;&#xA;Total mass: 14933443 KG&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update 2 August, 2023:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;$ time ./mass.py&#xA;&#xA;Total mass: 16333981 KG&#xA;&#xA;real    363m1.754s&#xA;user    74m24.274s&#xA;sys     1m1.314s&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="472" LastEditorUserId="472" LastEditDate="2023-08-02T17:34:49.733" LastActivityDate="2023-08-02T17:34:49.733" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="5780" PostTypeId="1" CreationDate="2014-11-02T19:29:00.953" Score="16" ViewCount="661" Body="&lt;p&gt;The Soviets/Russian had a lot of station experience (Salyut, Almaz, Mir, and then ISS) that was used to build better and better stations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US did Skylab and Shuttle/Spacelab/SpaceHab, and learned some stuff, clearly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the major takeaways from the ISS, that would be useful on the 'next' space station?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-05T17:06:38.400" Title="What lessons in Space Station design have come out of the ISS?" Tags="|iss|mir|crewed-spaceflight|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5781" PostTypeId="1" AcceptedAnswerId="5782" CreationDate="2014-11-02T19:31:14.337" Score="15" ViewCount="3716" Body="&lt;p&gt;The famous &lt;a href=&quot;https://en.wikipedia.org/wiki/Fermi_paradox&quot; rel=&quot;noreferrer&quot;&gt;Fermi paradox&lt;/a&gt; asks an important question: if space-faring civilizations exist and are not extremely rare, why are they not already here? Von Neumann proposed a &lt;a href=&quot;https://en.wikipedia.org/wiki/Self-replicating_spacecraft#Von_Neumann_probes&quot; rel=&quot;noreferrer&quot;&gt;probe&lt;/a&gt;, that flies to nearest star system, there it utilizes local resources and creates copies of itself, which are sent in multiple copies to nearest stars. Even if the traveling speed of such probes would be quite small (~100 km/s), they should be able to colonize the whole galaxy in few hundreds of millions of years, time relatively short with respect of its history.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Self-replication seems to be quite a universal concept and if there is more than hundreds of civilizations in the galaxy, at least one should try it. We would very probably be able to recognize it, if it was the case. Since we do not live in the earliest part of the habitable galactic zone there probably was enough time for other civilizations to evolve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/yEoW7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The GHZ &lt;code&gt;[Galactic Habitable Zone]&lt;/code&gt; in the disk of the Milky Way based on the star formation rate, metallicity (blue),&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;sufficient time for evolution (gray), and freedom from life-extinguishing supernova explosions (red). The white contours&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;encompass 68% (inner) and 95% (outer) of the origins of stars with the highest potential to be harboring complex life today.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The green line on the right is the age distribution of complex life and is&#xA;obtained by integrating $P_{GHZ} (r, t)$ over $r$&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;code&gt;[relative number of potentially suitable planetary systems as a function of space and time]&lt;/code&gt;.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Image credit: &lt;a href=&quot;http://arxiv.org/ftp/astro-ph/papers/0401/0401024.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way&lt;/a&gt;, Lineweaver et al. (PDF).&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the absence of the Von Neumann probes, we can conclude that either the intelligent life is extremely rare, or that it is really difficult to construct the Von Neumann probe. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is exactly my question: &lt;em&gt;Is it possible, that construction of the Von Neumann probe is simply not technologically viable?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;Some challenges to overcome:&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The journey takes tens of thousands of years, everything aboard must either survive functional this long, or rebuild itself from scratch.&lt;/li&gt;&#xA;&lt;li&gt;Intelligence with required lifespan might not be possible to construct.&lt;/li&gt;&#xA;&lt;li&gt;If rebuilding of machinery is required, it must be done with quite small payload.&lt;/li&gt;&#xA;&lt;li&gt;Viable energy source with required lifespan must exist for the probe. (In other words: fusion might not be possible outside stars.)   &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I realize this is a bit speculative questions. The reason why I ask it here is that I would like a serious answer that accounts for real properties of technology and limits set by the physics, rather than general &quot;anything not strictly forbidden by the laws of physics is possible&quot;, which I would probably receive in many other places. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: A great reference to the Fermi paradox &lt;a href=&quot;http://waitbutwhy.com/2014/05/fermi-paradox.html?doing_wp_cron=1415264933.3859341144561767578125&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6213" LastEditorUserId="49" LastEditDate="2015-04-21T14:50:44.783" LastActivityDate="2021-06-15T06:13:04.837" Title="Could Von Neumann probes be forbidden by technological limitations?" Tags="|spacecraft|technology|interstellar-travel|fermi-paradox|self-replication|" AnswerCount="3" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="5782" PostTypeId="2" ParentId="5781" CreationDate="2014-11-02T20:18:02.813" Score="14" Body="&lt;p&gt;There's no reason a Von Neumann probe could not be built once a civilization reaches the appropriate technology level, it's a matter of desire. When a civilization becomes advanced enough all it needs to build anything is a supply of matter or energy, as the one can be used to make the other. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you say &quot;the absence of Von Neumann probes&quot; I suspect that you are making the fundamental error of &lt;a href=&quot;http://en.wikipedia.org/wiki/Anthropomorphism&quot;&gt;anthropomorphization&lt;/a&gt;, assuming other life would have the same motivations as humans. Other life may not want to colonize the universe, or have a desire to explore, so they may have the technology but not the desire. Or, if they discover another species they may not want contact, - species which have the technology to explore may not want to be discovered, and any intelligence able to build a Von Neumann probe wouldn't have any problems hiding it from us. So just because we don't see a hundred probes out there right now doesn't mean they do not exist, or cannot be made. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After all, nobody knows how life started on Earth? Maybe we are here because billions of years ago an ancient Von Neumann probe seeded our planet with the makings of life? Not a new idea. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, a Von Neumann probe only makes sense if a civilization is limited to sub light speeds and has short life spans, and I wouldn't make either assumption. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-11-02T20:18:02.813" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5784" PostTypeId="2" ParentId="5780" CreationDate="2014-11-03T04:18:23.360" Score="9" Body="&lt;p&gt;Possible lessons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Resupply is hard.&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It takes a lot of time to load/unload cargo.&lt;/li&gt;&#xA;&lt;li&gt;Automated docking is better than manual berthing.&lt;/li&gt;&#xA;&lt;li&gt;Big doors are good for moving big cargo.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-03T04:18:23.360" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5785" PostTypeId="1" AcceptedAnswerId="5788" CreationDate="2014-11-03T05:40:00.147" Score="14" ViewCount="3269" Body="&lt;p&gt;I am very new to space exploration by humans (was always much more interested in astronomy) and have just begun looking at rocket launches in the 1960s because my little four year old is interested in them, and I was 4 years old when Apollo 8 launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There seems always to be a very clear and emphatic statement &quot;&lt;em&gt;We have cleared the tower&lt;/em&gt;&quot; when this event happens in the Apollo missions and earlier ones. I am not even sure who makes this statement, but it is always very noticeable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is this: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Does the point of having cleared the tower signal a procedural shift: e.g. defining measures for various staff to take in an emergency and therefore something that everyone would need to know about?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding is that before clearing the tower, there was very little anyone could do but hope that the launch vehicle actually clears the tower, and after that time there may be some hope of a successful escape with the escape rockets (I'm specifically talking about Apollo here, although my recollection is that there were escape rockets even for Mercury). Or is this kind of obvious statement perhaps just for the general public as a kind of exuberant &quot;this really is IT!&quot;?&lt;/p&gt;&#xA;" OwnerUserId="6005" LastActivityDate="2023-04-22T06:26:51.280" Title="Is there a &quot;procedural shift&quot; meaning to the statement &quot;we have cleared the tower&quot;" Tags="|launch|failure|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5787" PostTypeId="1" AcceptedAnswerId="5790" CreationDate="2014-11-03T05:51:23.367" Score="4" ViewCount="556" Body="&lt;p&gt;Just wondering, does anyone know a good way to calculate latitude and longitude of the moon relative to the earth with PyEphem?&#xA;I've kinda hit a wall trying to figure it out...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks,&#xA;Pete&lt;/p&gt;&#xA;" OwnerDisplayName="user6357" LastEditorDisplayName="user6357" LastEditDate="2018-05-17T11:32:40.430" LastActivityDate="2018-05-17T11:32:40.430" Title="Pyephem sublat and sublong equivalent for the Moon?" Tags="|the-moon|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5788" PostTypeId="2" ParentId="5785" CreationDate="2014-11-03T08:31:16.880" Score="24" Body="&lt;p&gt;Yes, there is a procedural change, at least with NASA launches. In NASA, launches are controlled by the Launch Control Center (LCC) until the rocket clears the tower, and then are handed over to the Mission Control Center (MCC). I'm not sure how other organizations do it.  &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-11-03T08:31:16.880" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5789" PostTypeId="2" ParentId="5780" CreationDate="2014-11-03T09:24:09.113" Score="9" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;EVA's have turned out to be an impractical way to work in space. &#xA;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/robonaut.html#.VFdHAjSG_2Z&quot;&gt;Service robots&lt;/a&gt; are taking over even on the crewed ISS.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Many of the different health problems with &lt;a href=&quot;https://en.wikipedia.org/wiki/Effect_of_spaceflight_on_the_human_body#The_effects_of_weightlessness&quot;&gt;microgravity&lt;/a&gt; have&#xA;been discovered in much greater detail. It could be a profound takeaway if it means that the next space station will have to be designed with rotating so called artificial gravity.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;( - I've heard that microgravity experiments (for medicine, materials sciences) have been a disappointment so far and that sounding rockets and orbiting automates would be preferable over a manned laboratory/factory in space, it would be interesting to learn more about this.)&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-11-03T09:24:09.113" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5790" PostTypeId="2" ParentId="5787" CreationDate="2014-11-03T11:29:49.370" Score="2" Body="&lt;p&gt;Use equatorial coordinates given in &lt;a href=&quot;https://en.wikipedia.org/wiki/Right_ascension&quot; rel=&quot;nofollow noreferrer&quot;&gt;Right Ascension&lt;/a&gt; for longitude and &lt;a href=&quot;https://en.wikipedia.org/wiki/Declination&quot; rel=&quot;nofollow noreferrer&quot;&gt;Declination&lt;/a&gt; for latitude as projected on celestial sphere. So you would do something along the lines of:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import ephem&#xA;m = ephem.Moon('2003/08/27', epoch=ephem.J2000)&#xA;print('%s %s' % (m.a_ra, m.a_dec))&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;This is explained in more detail in &lt;a href=&quot;http://rhodesmill.org/pyephem/coordinates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;PyEphem Coordinate Transformations&lt;/a&gt;. Note that &lt;code&gt;J2000&lt;/code&gt; is already an assumed epoch and you can simply provide the time since epoch to replace &lt;code&gt;'2003/08/27'&lt;/code&gt; in the example and omit the second parameter. Equatorial coordinate system is also already assumed when printing out RA/Dec. And if you want to transform RA/Lon and Dec/Lat to XYZ coordinates for a location on Earth's celestial sphere using Python, you'll continue with something like:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import math&#xA;&#xA;x = math.cos(m.dec) * math.cos(m.ra)&#xA;y = math.cos(m.dec) * math.sin(m.ra)&#xA;z = math.sin(m.dec)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Refer to the second part of &lt;a href=&quot;https://stackoverflow.com/a/11963439/1953464&quot;&gt;Brandon Rhodes' answer&lt;/a&gt; to &lt;a href=&quot;https://stackoverflow.com/q/11957633/1953464&quot;&gt;Getting J2000 XYZ coordinates for a location on earth in Python&lt;/a&gt; for more info on the latter part.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-05-23T12:37:21.217" LastActivityDate="2014-11-03T11:29:49.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5791" PostTypeId="1" CreationDate="2014-11-03T13:02:29.183" Score="3" ViewCount="485" Body="&lt;p&gt;With all the talk of re-engining Antares, many possibilities arise. Solids, new liquid engine, existing Russian engines (RD-180, RD-190), etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Focussing on the RD-1xx family though, an RD-180 is a bit overpowered for Antares, but more thrust is rarely a problem, and you need a stage redesign anyway, so roll with it, and boost your performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there was some unsourced speculation that maybe two RD-190's were possible, if ULA managed to lock them out of the RD-180 source.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RD-170 was a four chamber, but one turbopump engine. Is it one or 4 engines?  1 because although it has four 'bells' it only has one set of turbo-machinery.  (Used on Zenit as Sea Launch, and as strap on boosters for Energia)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Split in half, is the RD-180 with 2 bells, and one set of turbomachinery. Used on Atlas V (and Atlas III (now retired)). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Split in half again, is the RD-190 family, with a single bell and single set of turbomachinery.  Used on Angara family in Russia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust scales mostly linerally with the RD-190 having about 1/4 the thrust of the RD-170.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all, an immenseley successful family of engines, well done to the designers!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So on a re-engined Antares, would there be a benefit or cost, to using two RD-190s vs a single RD-180.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-03T20:08:55.513" Title="Would two RD-190's be better or worse than a single RD-180?" Tags="|antares|rd-180|rd-19x|rd-170|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5792" PostTypeId="1" AcceptedAnswerId="5796" CreationDate="2014-11-03T18:41:33.780" Score="15" ViewCount="6630" Body="&lt;p&gt;What is the minimum altitude required for a &lt;a href=&quot;https://en.wikipedia.org/wiki/CubeSat&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cubesat&lt;/a&gt;*-like object to orbit around the Earth? Could you initiate an orbit &lt;em&gt;inside&lt;/em&gt; the Earth's atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*A CubeSat is a 10 cm (1 liter) cube with a mass of no more than 1.33 kilograms. &lt;/p&gt;&#xA;" OwnerUserId="5880" LastEditorUserId="38" LastEditDate="2017-10-15T14:26:20.873" LastActivityDate="2017-10-15T14:26:20.873" Title="Minimum Orbit Altitude" Tags="|orbital-mechanics|low-earth-orbit|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5793" PostTypeId="2" ParentId="5792" CreationDate="2014-11-03T19:16:36.340" Score="4" Body="&lt;p&gt;Theoretically - You could orbit at any height, as long as the lowest point of your orbit doesn't hit the Earth...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your problem is that an orbit within the atmosphere has quite a bit of drag (minor understatement) so it will decay really quickly, and with a lot of spectacular light and sound.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a Cubesat, and in fact any satellite you want to complete at least one orbit, you are really going to have to be outside the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2014-11-03T19:16:36.340" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5794" PostTypeId="1" CreationDate="2014-11-03T19:52:29.637" Score="8" ViewCount="304" Body="&lt;p&gt;There have been two accidents in the past week, the Antares launch of Orbital Science's CRS-3 mission, and the test flight of SpaceShip Two that went awry, resulting in the death of one of the two pilots. Rockets being what they are, both events were extreme, involving the complete destruction of the ships involved. Space being what it is, both got a lot of attention from the press - people have a lot of feelings about space exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are procedures directed at proper documentation and data collection in the event of an accident begun when they occur? What methodologies are typically used? Because of the way such events tend to unfold, is it generally the case that few conclusions can be drawn until considerable analysis has been done?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-11-04T15:23:57.200" Title="How are accidents involving rockets and space vessels investigated?" Tags="|launch|procedure|failure|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5795" PostTypeId="2" ParentId="5791" CreationDate="2014-11-03T20:08:55.513" Score="2" Body="&lt;p&gt;Generally speaking, you want as few moving parts as possible, unless the extra parts add a useful redundancy. More moving parts = more things that can go wrong.&lt;br&gt;&#xA;Some examples: intercontinental aircraft used to require 4 engines, because that meant they could lose one and still make it to the other side of the ocean. These days jet engines are reliable enough that this requirement has been abandoned. In rocketry, the Saturn V and Falcon 9 have engine-out capability: they can lose an engine and still get to orbit.&lt;br&gt;&#xA;So for Antares the big question would be: if one engine fails, would the remaining engine have enough thrust to get the rocket to orbit? If not, then there's no point in having two engines, and you might as well go with the less complex and lighter single-engine option.&lt;br&gt;&#xA;I suspect the answer to that question is 'yes, for the second part of the trajectory'. Initially the rocket will be too heavy, but after some fuel has been burned there comes a point where one engine is enough. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-11-03T20:08:55.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5796" PostTypeId="2" ParentId="5792" CreationDate="2014-11-03T20:17:50.363" Score="35" Body="&lt;p&gt;The answer varies with atmospheric density (due to varying solar activity), with satellite geometry and mass, and with attitude.  But for a typical 3U cubesat, the minimum altitude for a circular orbit to complete at least one revolution is &lt;em&gt;approximately&lt;/em&gt; 150 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My colleagues and I collected the following orbital data from a cubesat we were operating:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/f10FK.png&quot; alt=&quot;Altitude vs time plot for last hours of 3U cubesat&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vertical jumps on the graph are where the orbit model was updated with new ranging measurements.  We last made contact with it at 145 km altitude, about 40 minutes prior to its final destruction.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2014-11-03T23:09:43.827" LastActivityDate="2014-11-03T23:09:43.827" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5797" PostTypeId="2" ParentId="5792" CreationDate="2014-11-03T21:37:13.253" Score="7" Body="&lt;p&gt;Here's a quick database exercise based on TLE data for the FLOCK 3U Cubesat constellation.  I took the last published elset for each of the 28 objects and calculated the apogee &amp;amp; perigee:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perigee:  162.6 +- 13.2 km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apogee:   175.1 +- 14.8 km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lots of caveats here.  Tracking can get sketchy near decay, and sensor coverage can vary alot from rev to rev.  So it's not clear how many revs were left for these objects.  I'd look at this as a rough upper bound.  pericynthion's data is based on last operator contact, so will be more accurate.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An aside, but something that threw me for a bit when looking at this was the launch date.  Event though the FLOCK satellites went up on an Antares in Feb of 2014, they were deployed from ISS.  So they were assigned the international designator (1998-067*) and launch date (1998-11-20) for ISS.  Something to keep in mind when working with this data.  The catalog numbers are sequential, the rest of the stuff can surprise you.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="4709" LastEditDate="2014-11-03T21:51:13.350" LastActivityDate="2014-11-03T21:51:13.350" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5798" PostTypeId="2" ParentId="5780" CreationDate="2014-11-03T22:46:03.127" Score="15" Body="&lt;p&gt;A couple from personal experience:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;If the station has several identical or nearly-identical components externally, label them with labels big enough to be seen from windows.  The US segment of the space station contains eight massive solar arrays.  When looking at imagery of them, it is often quite difficult to discern exactly which piece of hardware I'm looking at, especially when the image does not contain any other parts of the station for context.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Cables and hoses are a pain.  They don't behave nicely in microgravity, so any reconfiguration of external cables is pretty much required to be EVA.  Cables are also quite vulnerable to debris impacts, and while they may seem like small targets, the sheer number of exposed cables can mean a not-insignificant risk.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-11-03T22:46:03.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5799" PostTypeId="2" ParentId="5775" CreationDate="2014-11-03T22:58:33.527" Score="15" Body="&lt;p&gt;Flight termination systems serve two purposes:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Termination of thrust&lt;/li&gt;&#xA;&lt;li&gt;Dispersal of propellant&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Termination of thrust can be accomplished any number of ways.  For liquid fueled engines, it may be sufficient to simply stop the engine.  For solid motors, once the fuel grain is started, it cannot be stopped, so it needs to be rendered nonpropulsive.  This can be accomplished by either unzipping the casing using a linear shaped charge or puncturing the forward dome of the motor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For propellant dispersal, the requirements depend on the type of propellant.  For hazardous propellants like hydrazine and nitrogen tetroxide, the FTS is usually required to actually burn off the propellant to the greatest extent possible.  For more chemically benign fuels like LH2 and LOX, simple dispersal is typically sufficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Often, the same mechanism is used for both thrust termination and propellant dispersal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flight termination systems are required to be redundant to ensure success.  To prevent inadvertent initiation, safe-and-arm devices that physically isolate the destruct package from its initiator are used.  To prevent interference or malicious third-party initiation of a destruct package, destruct signals are encrypted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current US standard for flight termination systems is defined in AFSPCMAN 91-710 (though some of the juicier bits are not publicly available).&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2014-11-04T15:53:59.787" LastActivityDate="2014-11-04T15:53:59.787" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5800" PostTypeId="2" ParentId="5794" CreationDate="2014-11-04T10:05:06.067" Score="3" Body="&lt;p&gt;SpaceShipTwo is (more or less) an airplane.  As such, the crash is going to be &lt;a href=&quot;http://www.ntsb.gov/investigations/2014/mojave_ca/mojave_ca.html&quot; rel=&quot;nofollow&quot;&gt;investigated&lt;/a&gt; like any other airplane crash by the National Transportation Safety Board, probably in cooperation with the FAA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Antares explosion, on the other hand, doesn't fall under the jurisdiction of the NTSB.  The investigation will probably be handled by Orbital Science (as the owner of the rocket); NASA may conduct an investigation of their own.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-11-04T10:05:06.067" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5801" PostTypeId="2" ParentId="5794" CreationDate="2014-11-04T15:23:57.200" Score="1" Body="&lt;p&gt;The process is like any other careful investigation. You gather all the data you can.  Here it includes documentation of the build of the equipment, test data along the way, telemetry during the event.  Sometimes it is quickly obvious what went wrong, sometimes not.  Even if the cause of the immediate failure is easy to find (say the combustion chamber overpressured-that might come directly from telemetry), finding the root cause can be much harder.  Sometimes it is not obvious what data you want.  It is only after you have looked at the first data you gathered that you note there are other questions raised.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2014-11-04T15:23:57.200" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5803" PostTypeId="5" CreationDate="2014-11-04T16:07:55.320" Score="0" Body="&lt;p&gt;Most powerful liquid engine in production, 1.7 million lbs thrust, manufactured by NPO Energomash. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Used by Sea Launch as main engine for first stage in Zenit booster.&#xA;Zenit came from the Energia launch system, where the side boosters were each a full Zenit first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interesting design has a single set of turbomachinery to pump fuel and oxidizer to four engine bells. They were unable to develop a single engine bell design that could provide that level of thrust (comparable to the US F-1 engine) so they used 4 bells instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cut in half, with two bells, it is an RD-180 (Used in Atlas V booster).  Cut in quarters, with one bell, it is an RD-190 (Used in Angara booster).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-04T16:07:55.320" LastActivityDate="2014-11-04T16:07:55.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5804" PostTypeId="4" CreationDate="2014-11-04T16:07:55.320" Score="0" Body="Russian engine, designed by NPO Energomash for Zenit booster as side booster for Energia, and as first stage of Sea Launch.  1.7 Millions pounds thrust. LOX/Kerosene." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-04T16:07:55.320" LastActivityDate="2014-11-04T16:07:55.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5805" PostTypeId="5" CreationDate="2014-11-04T16:10:40.337" Score="0" Body="" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-04T16:10:40.337" LastActivityDate="2014-11-04T16:10:40.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5806" PostTypeId="4" CreationDate="2014-11-04T16:10:40.337" Score="0" Body="One quarter of an RD-170 engine, used by Angara booster, built by NPO Energomash. Burns LOX/Kerosene." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-04T16:10:40.337" LastActivityDate="2014-11-04T16:10:40.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5807" PostTypeId="5" CreationDate="2014-11-04T16:11:05.520" Score="0" Body="" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-04T16:11:05.520" LastActivityDate="2014-11-04T16:11:05.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5808" PostTypeId="4" CreationDate="2014-11-04T16:11:05.520" Score="0" Body="One half of an RD-170 engine, used by Atlas V booster, built by NPO Energomash. Burns LOX/Kerosene." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-04T16:11:05.520" LastActivityDate="2014-11-04T16:11:05.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5809" PostTypeId="1" AcceptedAnswerId="5810" CreationDate="2014-11-04T19:31:19.377" Score="8" ViewCount="658" Body="&lt;p&gt;The &lt;a href=&quot;http://deepspace.jpl.nasa.gov/&quot;&gt;NASA Deep Space Network&lt;/a&gt; is a means to get a 360 view of the heavens from zenith to nadir. The NASA DSN complexes were functional as of 1965 providing support to the Apollo Program. More recently ISRO MOM appears to rely upon the NASA DSN too. Ergo, one receives the impression a DSN is essential to deep-space missions.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Did the Soviet deep-space missions (looks like there were more Soviet missions than NASA/others) piggy-back on the NASA DSN?&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                  *OR*&#xA;&lt;/code&gt;&lt;/pre&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What was the Soviet counterpart to the NASA DSN? &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Where were their complexes located?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-11-04T19:56:33.060" Title="Where were the complexes of the Soviet counterpart to the NASA DSN located?" Tags="|nasa|russia|deep-space-network|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5810" PostTypeId="2" ParentId="5809" CreationDate="2014-11-04T19:56:33.060" Score="11" Body="&lt;p&gt;Soviet and Russian deep space missions did not use NASA's DSN.  The major antennas were all located in Soviet territory: Crimea, Moscow and Ussuriysk, spanning longitudes from 33 to 132 degrees east and allowing access to almost 270 of the possible 360 degrees of right ascension at any time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rather than using monolithic 70-meter parabolic antennas similar to the US DSN, the first antennas of the Soviet DSN were built from eight 16-meter dishes placed on two hulls of diesel submarines, welded together and laid down on railway bridge trusses. These trusses were mounted on bearings from battleship gun turrets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/XxtDs.jpg&quot; alt=&quot;Pluton antenna&quot;&gt; &lt;em&gt;Image credit: Rumlin CC-BY-SA&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Later, more traditional 64-meter and 70-meter antennas were constructed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Soviet_Deep_Space_Network&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Soviet_Deep_Space_Network&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mentallandscape.com/V_Telemetry.htm&quot; rel=&quot;noreferrer&quot;&gt;http://mentallandscape.com/V_Telemetry.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-11-04T19:56:33.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5811" PostTypeId="2" ParentId="3409" CreationDate="2014-11-04T22:48:31.643" Score="7" Body="&lt;p&gt;While it's true that nobody can give a credibly &lt;strong&gt;&lt;em&gt;very-accurate&lt;/em&gt;&lt;/strong&gt; estimate, that doesn't mean such estimates are useless.  We can look at similar engineering projects (similar in scale, timeline, and ideally similar in the regime) and make comparisons.  For an international mission to Mars, two projects spring to mind:  &lt;a href=&quot;http://en.wikipedia.org/wiki/ITER&quot;&gt;ITER&lt;/a&gt;, and the ISS.  Both involved (or involve) a fair number of unknowns, both are decade+ international engineering efforts where lots of R&amp;amp;D was required up front, and both also include building large-scale structures and a consortium of partners.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The  International Thermonuclear Experimental Reactor is the big fusion tokamok being built by France, Russia, the United States, Japan, and others, and the total cost is estimated to be roughly $50 billion dollars, with a total timeline of approximately 20 years, give or take.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS (which needs no intro here!) cost roughly $150 billion in total, and was a roughly 12-year project, not counting ongoing use of it. It's a better analog for a Mars mission in some respects, because it involved designing and building a long-duration habitat for humans in space, in-orbit construction by docking together modules, multiple launches of boosters, and solving a lot of the same life-support problems that a manned Mars mission will have to solve.  The ISS has 13 pressurized modules, which is (very likely) more than needed for a Mars mission, although they're only in LEO so they're much closer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I didn't look up the &amp;#36;6 billion estimate you listed in the question, but I'm hazarding a guess that this figure is for a fly-by, and not a landing and ascent.  Most of the estimates done by either NASA or outside groups have been higher.  One of the most recent credible estimates is by an expert panel (see this &lt;a href=&quot;http://news.nationalgeographic.com/news/2014/04/140422-mars-mission-manned-cost-science-space/&quot;&gt;National Geographic article&lt;/a&gt; from April 2014) which estimates &amp;#36;80 - 100 billion for a roughly 20-year program.  Or, put another way, about 1/10th the total cost of the F-35 fighter program ;-)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you read these cost breakdowns, the actual rocket launches are generally not a huge portion - even if we assumed a manned mission would require 4 x BFR or 4 x SLS Block II, at something like 2 billion/launch, and a complete dress-rehearsal unmanned, that's &amp;#36;16 billion or so.  A lot of the money will be spent designing, building, and testing the craft that takes the astronauts there and back, the Mars ascent vehicle, and solving the &quot;Mars landing problem&quot; (slowing down heavy payloads enough to get them on the surface safely).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While every estimate should be taken with a big grain of salt - and prices vary wildly with different types of missions, number of people, length of stay on Mars, etc - there are entire disciplines in engineering around cost estimation, so it's not impossible to generate useful estimates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line - somewhere in the &lt;code&gt;$&lt;/code&gt;50 billion - $100 billion range for a mission to put humans on the surface of Mars and bring them back, including R&amp;amp;D, and dependent on scope and ambition. I suspect that Elon Musk thinks he can do it for a lot less of course, lol.&lt;/p&gt;&#xA;" OwnerUserId="6199" LastActivityDate="2014-11-04T22:48:31.643" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5812" PostTypeId="1" AcceptedAnswerId="5813" CreationDate="2014-11-04T23:17:03.033" Score="11" ViewCount="2317" Body="&lt;p&gt;Rockets are expensive. And for the most part, one-time-use-only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SSTO promises to change that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, SSTOs will have limited payload capacity (as will every launch vehicle), and eventually missions we will require more Delta-V and more payload mass than a single trip can handle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we're going to put people on Mars, it's going to require a lot of equipment, and a lot of Delta-V to get it there.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What are the specific difficulties of assembling (anything) in space?&lt;/li&gt;&#xA;&lt;li&gt;Has anything been &quot;built&quot; in space before?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="192" LastActivityDate="2016-02-01T18:27:33.730" Title="Are we capable (engineering-wise) of building a spacecraft in space?" Tags="|rockets|cost|ssto|cost-to-orbit|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5813" PostTypeId="2" ParentId="5812" CreationDate="2014-11-04T23:36:07.693" Score="24" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Has anything been &quot;built&quot; in space before?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Absolutely. Perhaps not &quot;built&quot; in the sense you're talking about, but certainly assembled. A sterling example is the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;International Space Station&lt;/a&gt;. There are about a dozen distinct modules in the ISS that are pressurized and fit for occupation. The first was &lt;a href=&quot;https://en.wikipedia.org/wiki/Zarya&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Zarya&lt;/em&gt;&lt;/a&gt;, and the latest is &lt;a href=&quot;https://en.wikipedia.org/wiki/Leonardo_(ISS_module)&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Leonardo&lt;/em&gt;&lt;/a&gt;. They've been constructed by many different countries, and have been added on individually. There are also many unpressurized components. Here's a handy-dandy map - a guide to the ISS (&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/6/6f/ISS_configuration_2011-05_en.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;from Wikipedia&lt;/a&gt;)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/8RkEf.png&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Click to view full resolution version&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8RkEf.png&quot; alt=&quot;The ULF6 configuration of the International Space Station following the addition of the Alpha Magnetic Spectrometer and ExPrESS Logistics Carrier 3, launched by the Space Shuttle Endeavour on STS-134.&quot; title=&quot;Click to view full resolution version&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some details regarding the assembly process can be found &lt;a href=&quot;https://en.wikipedia.org/wiki/Assembly_of_the_International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, but I feel that &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Assembly&quot; rel=&quot;nofollow noreferrer&quot;&gt;this section&lt;/a&gt; better explains things. Many components were brought up on launches of the Russian Proton rockets, as well as numerous Space Shuttle launches. Each new module connected smoothly with the others. They were pre-built, but were put together in orbit.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The other thing a project like this is going to take is money. And a &lt;em&gt;lot&lt;/em&gt; of it. The ISS may have cost &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Cost&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;150 billion dollars&lt;/em&gt;&lt;/a&gt;. That's a staggering amount of money. That does appear to take into account the cost of Space Shuttle flights. Now, &lt;a href=&quot;http://zidbits.com/2010/11/the_most_expensive_object/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site&lt;/a&gt; says that a mission to Mars would be cheaper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To put that number in perspective, just over 10 years ago NASA estimated the cost to send astronauts to Mars. Their rough estimates put the price of a human Mars mission in the ballpark of $40 billion US dollars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Small change, clearly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A mission to Mars would require a decently-sized rocket. &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The ISS&lt;/a&gt; is about 357 feet long. That's roughly the size of a Saturn V rocket. Now, if you built the rocket in orbit, it might not need to be so large, because it would already be in orbit.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-11-05T23:53:44.970" LastActivityDate="2014-11-05T23:53:44.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5814" PostTypeId="2" ParentId="5522" CreationDate="2014-11-05T03:53:52.603" Score="-1" Body="&lt;p&gt;From the wikipedia page, it mentions that there are ground imaging sensors, so I could see how a retrograde imaging satellite would allow you to cover more area quickly, but with the highly elliptical orbit that it has, I don't think that's the case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is another &lt;a href=&quot;http://www.quora.com/How-did-ISROs-Mars-orbiter-end-up-in-a-retrograde-orbit&quot; rel=&quot;nofollow&quot;&gt;answer on Quora&lt;/a&gt; here: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;you are  thinking about spacecraft velocity w.r.t Sun (Vs) being  pro-grade or retrograde with respect to mars rotation. what we must&#xA;  consider is the spacecraft velocity w.r.t Mars(Vs - Vm =Vsm).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In the case of MOM, the space craft is ahead of Mars and Mars caught up to it (when you see things in solar system reference frame).&#xA;  The relative velocity of spacecraft w.r.t mars (Vsm) is actually in&#xA;  the retrograde direction to the Mars rotation.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So if you analyse from solar system frame, all that happened during the insertion burn is that the Vs of the spacecraft is&#xA;  increased so that when Mars arrives near spacecraft the magnitude of&#xA;  relative velocity of spacecraft w.r.t Mars (|Vsm|) is reduced enough&#xA;  for mars to capture the space craft into its orbit. Note: In this case&#xA;  |Vm| &gt; |Vs|&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6493" LastActivityDate="2014-11-05T03:53:52.603" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5815" PostTypeId="1" CreationDate="2014-11-05T08:21:19.697" Score="4" ViewCount="461" Body="&lt;p&gt;Since the White Knight Two doesn't transport passengers or cargo, why does it even have a fuselage? Wouldn't a purely wing shaped aircraft be better? Is it simply more convenient to use an existing and tested aircraft design, even if it was created for a different purpose?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2014-11-05T19:21:01.387" LastActivityDate="2014-11-05T19:21:01.387" Title="Why does the WhiteKnight2 of Virgin Galactic have so large fuselages?" Tags="|design|sub-orbital|virgin-galactic|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5816" PostTypeId="1" AcceptedAnswerId="5817" CreationDate="2014-11-05T08:39:35.527" Score="10" ViewCount="1280" Body="&lt;p&gt;Under what conditions would fuel depots in earth orbit make sense?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As it costs $X/kg to launch something into space, wouldn't it cost the same to launch fuel by itself as it would to launch it as part of the vehicle that's going to end up using it?&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2014-11-05T19:00:03.887" Title="Why would orbital fuel depots make sense?" Tags="|fuel-depots|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5817" PostTypeId="2" ParentId="5816" CreationDate="2014-11-05T09:05:18.787" Score="11" Body="&lt;p&gt;The $X/kg is a simplification.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets are only available in certain sizes; the launch vehicle with the greatest capacity that is presently operating is the Delta IV-H, delivering a little less than 28 metric tons to LEO.  Even hypothetical rockets have limits due to the logistics of assembling and launching them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to launch a large and heavy mission - perhaps for the human exploration of Mars - you have to split up your mission into multiple launches.  One way to do that is to establish a propellant depot in LEO, launch multiple tankers to it to top it off, and finally launch your main mission with empty or partially full tanks, fill up and depart for Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are alternative strategies - you could instead launch a fully fueled Earth Departure Stage, and separately launch the habitat and lander components, have them rendezvous in LEO and leave.  That does put more time pressure on the launch and rendezvous since a full EDS can't hang around for too long before boiloff becomes a problem.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-11-05T09:05:18.787" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5818" PostTypeId="2" ParentId="5816" CreationDate="2014-11-05T09:05:42.947" Score="2" Body="&lt;p&gt;Launching fuel from Earth without a specific purpose, waiting for a need to turn up, can't be a good idea. I've heard of fuel depots in the context of producing fuel on the Moon. So that is in a vision of a much more developed space flight than what we have today. The benefit would come from the lesser energy needed to launch from the Moon's lower gravity. And once produced on the Moon, the fuel might as well be put in a waiting depot in space. At a Lagrange point or some lunar orbits, the large distance to Earth makes any conventional Earth orbit easy to reach such as polar satellites, geostationaries, GPS. From Earth orbit, it would require a lot of fuel to navigate to different orbital inclinations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The market for refueling satellites is limited by the economic lifetime of satellites. One might want to replace a 15 years old communication satellite with new hardware technology anyway. And it might be cheaper to fuel it for 15 years of station keeping already before launch, rather than hassling with docking and refueling systems.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-11-05T09:05:42.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5819" PostTypeId="2" ParentId="5812" CreationDate="2014-11-05T10:37:44.920" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Has anything been &quot;built&quot; in space before?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Depends on your definition of &lt;code&gt;built&lt;/code&gt; (see HDE's answer for a detailed explaination of the ISS' assembly), but for the purposes of this answer I will presume you mean where a team in space would receive an IKEA-esque flat pack of parts and then assembles it into a working vessel in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Can it be done?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, in fact the ISS shows us a working example of a similar process, the crew frequently receive repair kits or scientific modules that have to be attached to the outside of the station on EVA. This involves alot the regular engineering works my might expect a construction team to do:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Bolting things into other things&lt;/li&gt;&#xA;&lt;li&gt;Welding joints&lt;/li&gt;&#xA;&lt;li&gt;Soldering circuitry&lt;/li&gt;&#xA;&lt;li&gt;eta eta&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Has it been done?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not to my knowledge. This might seem strange, but the reasoning is actually quite simple if we examine the potential advantages and disadvantages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Current Advantages&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Designers no longer have to be concerned about the vessel fitting inside the aerodynamic fairing of the rocket.&lt;/li&gt;&#xA;&lt;li&gt;Can split the final vessel into smaller, more efficient rockets&lt;/li&gt;&#xA;&lt;li&gt;Low gravity environment makes moving components &quot;easier&quot;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Current Disadvantages&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Still have to transport the same mass to space, so fuel savings are limited to what you can save by using smaller rockets.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Smaller limit on the size of the construction crew&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Every specialist you need for the construction also needs to be a&#xA;trained astronaut&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Extra Launches to assemble and maintain shipyard/construction team&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Loss of components due to drifting off into space&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;No heavy construction equipment&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Until we have a way of gathering fuel or the raw materials in space there isn't much to gain from building in space.&lt;/p&gt;&#xA;" OwnerUserId="3160" LastActivityDate="2014-11-05T10:37:44.920" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5820" PostTypeId="2" ParentId="5816" CreationDate="2014-11-05T11:32:23.870" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;As it costs $X/kg to launch something into space, wouldn't it cost the&#xA;  same to launch fuel by itself as it would to launch it as part of the&#xA;  vehicle that's going to end up using it?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why assume that the launch cost is the same?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some hypothetical scenarios why storing fuel in space might be a feasible idea.  &lt;em&gt;(Assuming a need for fuel in space.)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Regularly scheduled, low priority rockets (in advance) might be cheaper than sending large quantities of fuel irregularly (as needed).&lt;/li&gt;&#xA;&lt;li&gt;The rockets required to send fuel, may be inherently cheaper than rockets needed to transport other cargo (e.g. delicate/expensive satellite, humans).&lt;/li&gt;&#xA;&lt;li&gt;We may be able to use non-rocket technologies to get fuel into orbit.  Getting to orbit is actually quite cheap (few $/kilo according to my napkin), the problem we face is our technology is inefficient (rocket equation).  Some alternative solutions (e.g. Mass driver) may not be suitable for all cargo (high G), but fuel could be a valid use.&lt;/li&gt;&#xA;&lt;li&gt;Secondary Cargo - why pay to send 1000 kilo's to space (your rocket's optimal payload) when you only need to send a 500 kilo satellite?  Take a load of fuel as well to get a subsidised price.&lt;/li&gt;&#xA;&lt;li&gt;Cheaper source.  If fuel can be made on the Moon for example, then placing it in earth orbit would be far cheaper than lifting it from earth, while conversely building complex electronics/shipping earthers may be hard to do from moon.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6250" LastEditorUserId="6250" LastEditDate="2014-11-05T14:23:43.817" LastActivityDate="2014-11-05T14:23:43.817" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5821" PostTypeId="1" CreationDate="2014-11-05T11:48:14.927" Score="3" ViewCount="1016" Body="&lt;p&gt;In people vitamin D is created through effects of UV light on the skin &lt;a href=&quot;https://en.wikipedia.org/wiki/Vitamin_D#Biosynthesis&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;(source)&lt;/em&gt;&lt;/a&gt; on Earth the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ozone_layer&quot; rel=&quot;nofollow&quot;&gt;Ozone layer&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth%27s_magnetic_field&quot; rel=&quot;nofollow&quot;&gt;Earth's magnetic field&lt;/a&gt; protect us from many harmful effects of the sun.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there anyplace in the solar system where people could get enough sun to make vitamin D, without other significant health risks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clarify:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Anyplace in the solar system = planet, moon, other natural or man made object&lt;/li&gt;&#xA;&lt;li&gt;Significant health risks = without exceeding &lt;a href=&quot;https://www.osha.gov/&quot; rel=&quot;nofollow&quot;&gt;OSHA&lt;/a&gt; or at the very least the &lt;a href=&quot;http://srag-nt.jsc.nasa.gov/Publications/TM104782/techmemo.htm&quot; rel=&quot;nofollow&quot;&gt;NASA version&lt;/a&gt;, for exposure &lt;/li&gt;&#xA;&lt;li&gt;Windows, translucent suits or other man made filters using current technology that allow exposure to sun light, that will create vitamin D, while allowing safe exposure would be acceptable solutions.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastActivityDate="2014-11-05T12:41:20.677" Title="Where in the solar system can a person safely get enough sun to make vitamin D?" Tags="|colonization|health|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5822" PostTypeId="2" ParentId="5821" CreationDate="2014-11-05T12:26:18.320" Score="6" Body="&lt;p&gt;On any planet, moon or other natural or man made object, other than Earth, vitamin D is definitely going to be the least of your worries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is nowhere you could go without either a protective suit or a protective capsule/building.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, wearing a protective suit or staying indoors will remove your ability to make sufficient vitamin D, so other locations are guaranteed to be worse - they all have significant health risks!&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Rickets is one of the commonly known symptoms of Vitamin D deficiency, and this is incredibly common here on Earth among those at high latitudes, and those with high melanin levels living in medium latitudes, indicating that the UV absorbed by the skin here on Earth is close to being insufficient.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'd suggest vitamin tablets: problem solved &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, use UV lights, as NPSF3000 suggests.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2014-11-05T12:32:50.360" LastActivityDate="2014-11-05T12:32:50.360" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5823" PostTypeId="2" ParentId="5821" CreationDate="2014-11-05T12:41:20.677" Score="1" Body="&lt;p&gt;In your question, you're approaching the &lt;a href=&quot;https://en.wikipedia.org/wiki/Effective_dose_%28radiation%29&quot; rel=&quot;nofollow&quot;&gt;effective dose&lt;/a&gt; from a single entity standpoint - location based intensity (power per unit area) of radiation. Problem is, that you're neglecting other points to &lt;a href=&quot;https://en.wikipedia.org/wiki/Absorbed_dose&quot; rel=&quot;nofollow&quot;&gt;absorbed dose&lt;/a&gt;, such as exposure time and effectiveness of shielding. These could be anything, from microseconds long exposures to high intensity radiation, longer but still short exposures of smaller parts of the total surface area at the time, to reducing incident intensity by shielding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer to your question is actually - anywhere. Depends. If you can thwart against other threatening effects of being in close proximity to the Sun, such as heat, &lt;a href=&quot;https://en.wikipedia.org/wiki/Particle_radiation&quot; rel=&quot;nofollow&quot;&gt;particle radiation&lt;/a&gt; and so on, chances are good that you'll be more than capable of dealing with UV radiation too. All you need is some areas that are partially or fully opaque to this wavelength and limit your exposure time to acceptable dose of radiation.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-11-05T12:41:20.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5824" PostTypeId="2" ParentId="5780" CreationDate="2014-11-05T17:06:38.400" Score="8" Body="&lt;h3&gt;Long term international cooperation is hard&lt;/h3&gt;&#xA;&lt;p&gt;And it might not come with only advantages, such as share the assembly and operational costs, share science, technology, infrastructure, services, increase IP portfolio and so on.&lt;/p&gt;&#xA;&lt;p&gt;Some disadvantages are apparent since the Crimea crisis and US/EU sanctions against Russia, when momentarily, even the immediate future of the station wasn't clear and politics and hiccups in diplomacy nearly got the better of it. Additional multilateral reassurances were needed to continue with operations as previously agreed on, and US didn't have to recall all of its personnel from Star City in Russia, Baikonur in Kazakhstan,...&lt;/p&gt;&#xA;&lt;p&gt;US side isn't exactly guilt-free when it comes to operational complications either. Remember the &lt;em&gt;Government shutdown of 2013&lt;/em&gt; when most of NASA's operations were cut to bare minimum and 97% of its employees on unpaid leave due to US Congress failing to reach an agreement on the FY2014 budget in time? Sure, we all read that ISS operations won't be affected, and to a large degree they probably weren't. But a lot of background work related to ISS activities, including international cooperation, also went on a forced hiatus, for lack of a better word.&lt;/p&gt;&#xA;&lt;p&gt;So politics often can get in the way of research, and in an international theater, even more frequently so. But there's other points. One is definitely legal, better said - no shared liability; every nation party to the joint international project is equally and individually liable for any and all damages to property or land of other nations. At least that's according to the Outer Space Treaty and the later signed and ratified Space Liability Convention. This additionally complicates such deals between nations, sometimes, it might render them unworkable - but we don't read all that much about projects that failed before they were even established in the press, so this is hard to quantify.&lt;/p&gt;&#xA;&lt;p&gt;Another point is of difficulties involved with bailouts once the international contracts are signed. Politics change, even their leaders and the whole bandwagon. But you might be stuck with a contract that you later don't like. Our ideas about space exploration change, our needs for their science products change, or we simply want to do the same more efficiently, reshuffle things around a bit. International contracts that hold a large part of your budget hostage might prevent all that, or delay its progress - again, possibly rendering it as a fruitless endeavor.&lt;/p&gt;&#xA;&lt;p&gt;And finally, security problems. Let's not even mention interdependency and possible obstructions in case one of the nation parties can't or won't fulfill contract obligations. That's another issue altogether, but there are more pressing security concerns such as access control to proprietary or classified technology, and so on in an internationally shared location.&lt;/p&gt;&#xA;&lt;p&gt;I'm not saying that all of these problems can't be worked with, or some bypassed altogether. There's absolutely also perks to international projects like ISS is, but long term international cooperation is hard. ISS taught us valuable lessons in this regard, still does, and will continue to do so while its fate isn't exactly set in stone and each member state continues to present different, sometimes incompatible ideas of its future.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-11-05T17:06:38.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5825" PostTypeId="2" ParentId="5816" CreationDate="2014-11-05T19:00:03.887" Score="9" Body="&lt;p&gt;Humans like fast transit (less supplies, less radiation), so they want to use chemical propellant which is much less mass efficient than electric propulsion.  One way to mitigate this and lower the overall mass required for human missions is to use electric propulsion to transport chemical propellant to high energy locations, such as high Earth orbit or at asteroid or Mars destinations.  Then after you wait a few years to get the propellant to where it needs to be, the humans can zoot around the solar system using the stored propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would go ahead and store the engines with the chemical propellant, so there's no need to transfer fluids.  Just grab a complete stage that has been stationed for you and go.  The engines are a small fraction of the mass of a stage, so you might as well attach engines to your fuel tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally LH2/LOX is the chemical propellant discussed for this sort of thing, but that will require technology development and demonstration of zero boil-off for the time spans required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other way &quot;depots&quot; makes sense is if the propellants didn't come from Earth.  Asteroids and Phobos can be sources of water to make LH2 and LOX.  High-Earth orbit is energetically much closer to those locations than Earth.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-11-05T19:00:03.887" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5826" PostTypeId="2" ParentId="5815" CreationDate="2014-11-05T19:18:19.777" Score="3" Body="&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Scaled_Composites_White_Knight_Two&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One fuselage is an exact replica of that of SpaceShipTwo (to allow tourist training), and the other will carry 'cut-rate' day-trippers into the stratosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why even have one? Well, there is a pilot of White Knight. Given that you want to have Space Ship two be in the middle, White Knight needs to have two fuselages, to maintain stability with and without the SpaceShip Two. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, having two fuselages makes sense for stability, and if you have them anyways, why not make them useful?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-11-05T19:18:19.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5827" PostTypeId="2" ParentId="5773" CreationDate="2014-11-06T00:10:40.270" Score="2" Body="&lt;p&gt;Skylon (&lt;a href=&quot;http://www.reactionengines.co.uk/space_skylon.html&quot; rel=&quot;nofollow&quot;&gt;http://www.reactionengines.co.uk/space_skylon.html&lt;/a&gt;) is &lt;strong&gt;&lt;em&gt;designed&lt;/em&gt;&lt;/strong&gt; for orbital flights.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Xcor Lynx Mark III (see &lt;a href=&quot;http://xcor.com/lynxdevelopment/index.html&quot; rel=&quot;nofollow&quot;&gt;http://xcor.com/lynxdevelopment/index.html&lt;/a&gt;) will feature a dorsal pod to carry an &quot;upper stage capable of launching a small satellite into low earth orbit.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The VG SpaceShip 2 appears to have no potential for orbital use, and neither does Copenhagen Suborbital, but as with everything else in &quot;New Space&quot;, this may be subject to change without notice. &lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-11-06T00:10:40.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5828" PostTypeId="2" ParentId="2664" CreationDate="2014-11-06T01:19:30.300" Score="2" Body="&lt;p&gt;According to Wikipedia, &quot;At the end of the orbit insertion, MOM was left with 40 kg (88 lb) of fuel as against the 20 kg (44 lb) that was thought necessary for the six-month life span.&quot; (see &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;nofollow&quot;&gt;https://en.m.wikipedia.org/wiki/Mars_Orbiter_Mission&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-11-06T01:19:30.300" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5830" PostTypeId="1" AcceptedAnswerId="5831" CreationDate="2014-11-06T06:16:47.763" Score="4" ViewCount="130" Body="&lt;p&gt;With Orbital looking at using a 3rd party launch service for their CRS contract, they have stated that Cygnus was designed with this in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone have information or ideas on how they accommodated this feature?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine the 3 key areas are Power, Data and Structural. P &amp;amp; D is probably covered by a standard, but I can't imagine structural is easy todo quickly.&lt;/p&gt;&#xA;" OwnerUserId="1076" LastActivityDate="2024-01-26T02:24:13.817" Title="Cygnus on a 3rd party launcher" Tags="|antares|cygnus|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5831" PostTypeId="2" ParentId="5830" CreationDate="2014-11-06T13:24:18.363" Score="6" Body="&lt;p&gt;Cygnus has two main components.&lt;/p&gt;&#xA;&lt;p&gt;The Pressurized cargo compartment is built by Alenia-Thales in Italy.  The same folk who built the pressure vehicles for most of the ISS modules and the ATV.&lt;/p&gt;&#xA;&lt;p&gt;The bus or service module is based on a satellite design Orbital had on the shelf. It is based on GEOStar Satellite Bus and the Dawn spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;Thus the interface between Cygnus and the launcher is through the bus, which is a commercial product designed to be launched on commercial launchers.&lt;/p&gt;&#xA;&lt;p&gt;A clever approach, which is common to the entire Cygnus/Antares stack.  Use existing vendors, technologies, and components, just assemble them in an interesting fashion.&lt;/p&gt;&#xA;&lt;p&gt;Jan 2024: For a fun update to this old question, Cygnus has now launched on:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Antares 130 (NK-33/AJ-26 engines)&lt;/li&gt;&#xA;&lt;li&gt;Atlas V (RD-180 engine)&lt;/li&gt;&#xA;&lt;li&gt;Antares 230 (RD-191 engines)&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 (Merlin 1D engines)&lt;/li&gt;&#xA;&lt;li&gt;Antares 330 (Rutherford engines) in the future&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Seems like their flexible design payed off!&lt;/p&gt;&#xA;&lt;p&gt;Wonder if any other major payload has launched on as many different boosters?&lt;/p&gt;&#xA;&lt;p&gt;GPS Launched on (Based on &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_GPS_satellites&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Atlas E/F&lt;/li&gt;&#xA;&lt;li&gt;Delta II 6925-9.5 (counting the 7925-9.5 variant as the same)&lt;/li&gt;&#xA;&lt;li&gt;Delta IV M+ (4,2)&lt;/li&gt;&#xA;&lt;li&gt;Atlas V 401&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 B5&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;So depending on how you count Delta II variants this could go either way.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2024-01-26T02:24:13.817" LastActivityDate="2024-01-26T02:24:13.817" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="5834" PostTypeId="1" CreationDate="2014-11-07T01:34:45.133" Score="2" ViewCount="280" Body="&lt;p&gt;There's plenty of talk out there about a certain type of propulsion system, but I don't know the proper name for it.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You gather up / buy / otherwise acquire some kind of non-rocket material, like water or metal or rock: this is the reaction mass for your spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;You propel it out the back of your spacecraft at high speed using some kind of propulsion system.&lt;/li&gt;&#xA;&lt;li&gt;When you're running low on reaction mass, you gather up / acquire some more.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Ion engines would be a subset of this category.&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2014-11-07T01:34:45.133" Title="What are non-rocket-based reaction mass drives called?" Tags="|propulsion|terminology|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5835" PostTypeId="1" AcceptedAnswerId="5836" CreationDate="2014-11-07T01:41:47.253" Score="5" ViewCount="1033" Body="&lt;p&gt;Earlier I asked about &lt;a href=&quot;https://space.stackexchange.com/questions/5677/espionage-in-the-space-race&quot;&gt;espionage during the Space Race&lt;/a&gt;.  Were there any significant instances of cooperation during the Space Race?  More specifically, I'm asking about cooperation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;between the non-military space programs of the US and the USSR&lt;/li&gt;&#xA;&lt;li&gt;after WWII and before the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo-Soyuz Test Project&lt;/a&gt; in 1975&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="552" LastEditorUserId="3065" LastEditDate="2017-09-25T02:21:57.387" LastActivityDate="2019-07-22T09:50:43.470" Title="Were there significant instances of cooperation during the Space Race?" Tags="|nasa|space-race|soviet-union|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5836" PostTypeId="2" ParentId="5835" CreationDate="2014-11-07T03:00:01.047" Score="8" Body="&lt;p&gt;Well, both the US and the USSR participated in the July 1, 1957, to December 31, 1958 &lt;strong&gt;International Geophysical Year (IGY)&lt;/strong&gt;, an international effort to coordinate the collection of geophysical data from around the world. The IGY organizing committee resolved that &quot;&lt;em&gt;all observational data shall be available to scientists and scientific institutions in all countries.&lt;/em&gt;&quot; Even that limited level of scientific interchange between East and West  became possible only after the death of Joseph Stalin in 1953.&lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-11-07T03:00:01.047" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5837" PostTypeId="2" ParentId="2446" CreationDate="2014-11-07T03:26:21.863" Score="2" Body="&lt;p&gt;If the fuel and oxidizer tanks use a positive-displacement system, such as a piston or bellows, there is no need for a settling or ullage burn. This is the case with many (most?) attitude control thrusters, and even some main propulsion systems.&lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-11-07T03:26:21.863" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5838" PostTypeId="1" CreationDate="2014-11-07T11:59:18.997" Score="6" ViewCount="152" Body="&lt;p&gt;I got the relationship between power, thrust and specific impulse for an ion electrospray thruster as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P/T= 1/2 \times g_o \times I_{\mathrm{sp}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where: $P$ = power required (in Watts), $T$ = thrust (in Newtons), $g_o$ = gravitational parameter (9.81 m/s²), $I_{\mathrm{sp}}$ = specific impulse (in seconds).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I want to know how the dimensions of the emitter, propellant tank, extractor thickness, extractor - emitter gap, etc. affect the thrust or specific impulse of an electrospray thruster which can be used for CubeSats.&lt;/p&gt;&#xA;" OwnerUserId="3542" LastEditorUserId="12102" LastEditDate="2021-04-22T04:21:15.563" LastActivityDate="2021-04-22T04:21:15.563" Title="Relationship between part dimensions and performance of ion electrospray thruster" Tags="|propulsion|ion-thruster|thrust|specific-impulse|electric-propulsion|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5839" PostTypeId="2" ParentId="4034" CreationDate="2014-11-07T12:49:50.510" Score="0" Body="&lt;p&gt;Yes it is even possible over your smartphone to locate the exact  direction, latitude, longitude if weather permits.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Download iOS application &lt;a href=&quot;https://itunes.apple.com/gb/app/iss-spotter/id523486350?mt=8&quot; rel=&quot;nofollow&quot;&gt;ISS Spotter&lt;/a&gt; , it will show the exact&#xA;date and time when it is passing over your loaction (weather permits).&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;Download &lt;a href=&quot;https://itunes.apple.com/in/app/skyview-free-explore-universe/id413936865?mt=8&quot; rel=&quot;nofollow&quot;&gt;sky viewer&lt;/a&gt;, where you can get the mere exact direction to look.&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="6057" LastActivityDate="2014-11-07T12:49:50.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5842" PostTypeId="2" ParentId="3960" CreationDate="2014-11-08T10:31:19.553" Score="10" Body="&lt;p&gt;According to &lt;a href=&quot;http://spectrum.ieee.org/aerospace/space-flight/wax-fuel-gives-hybrid-rockets-more-oomph&quot; rel=&quot;noreferrer&quot;&gt;this recent article&lt;/a&gt; by one of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Propulsion_Group&quot; rel=&quot;noreferrer&quot;&gt;Space Propulsion Group&lt;/a&gt; partners, paraffin-based fuels give hybrid rockets more oomph, because the fuel that is exposed to combustion melts, atomizes, and becomes entrained in the flowing oxidizer [inset]. This enlarges the surface area over which the fuel can vaporize and react.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/c3Ntq.jpg&quot; alt=&quot;Wax Fuel Delivers More&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More fuel in the mix, faster burn, higher thrust. This does seem to address the problem with slow grain regression seen with other hybrid fuels. Since this makes possible single channel combustion, the problematic multi-channel design can be avoided (thus more fuel in the cylinder &amp;amp; no chunks of fuel flying loose during burn). Working with NASA Ames, they've hot fired hybrids with LOX and nitrous oxide. Recently they have been testing a 56-cm motor capable of 100,000 newtons of thrust.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, they are still subject to low-frequency combustion instabilities, along with high-frequency instabilities common to all kinds of rockets. It really flickers like a candle, as you can see in this video: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/6q0Coz1eu8s?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SPG website is &lt;a href=&quot;http://www.spg-corp.com/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2015-01-23T03:57:17.310" LastActivityDate="2015-01-23T03:57:17.310" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5843" PostTypeId="1" CreationDate="2014-11-08T14:56:11.920" Score="12" ViewCount="384" Body="&lt;p&gt;ISS is in low Earth orbit, and as such, its orbital progress is gradually slowed by atmospheric drag and consequently requires occasional reboosts. It also has to be occasionally maneouvered to avoid collisions with orbital debris. This all contributes to an environment aboard ISS which is not truly zero-g. Does this affect or limit the science or specialized processing which can be done when zero-g is called for? Is the difference between ISS microgravity and true zero-g small enough that it can be disregarded for the purposes of either experimentation or specialized manufacturing? Or are there processes and/or experiments for which the ISS is not useful simply because its microgravity is not a good enough approximation of zero-g? How does reboosting and other maneouvering affect the onboard microgravity environment? Do experiments/processes have to be suspended or scheduled around these events due to the accelerations they introduce? Are there long duration experiments/processes for which &quot;normal&quot; ISS microgravity would be adequate but cannot be done there because of the rate of interruptions by station maneouvering?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="12102" LastEditDate="2021-08-11T23:57:08.090" LastActivityDate="2021-08-11T23:57:08.090" Title="Does atmospheric drag or reboosting maneouvers affect the integrity of microgravity experiments aboard ISS?" Tags="|iss|microgravity|experiment|reboost|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5844" PostTypeId="2" ParentId="5843" CreationDate="2014-11-08T15:55:31.990" Score="13" Body="&lt;p&gt;The ISS microgravity is generally considered to be of pretty poor quality, but the major cause of that is vibration from mechanical equipment and astronaut movement rather than atmospheric drag and reboosts (which are infrequent).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For many experiments it's adequate, for others it is not - alternatives include drop tower tests and drag-free satellites such as &lt;a href=&quot;http://www.nt.ntnu.no/users/skoge/prost/proceedings/ifac11-proceedings/data/html/papers/2966.pdf&quot;&gt;GOCE&lt;/a&gt;.  The latter is rather clever: You put a test mass inside a cavity in a free-flying spacecraft.  You measure the test mass' position relative to the cavity with lasers or some other means.  As it drifts toward the edge of the cavity, you fire (extremely low-thrust) thrusters on the spacecraft to move the cavity walls away from the test mass.  This ensures that the spacecraft follows the same trajectory that an ideal mass would, free from the effects of drag, solar radiation pressure and other disturbances.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-11-08T15:55:31.990" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5845" PostTypeId="1" AcceptedAnswerId="12138" CreationDate="2014-11-08T21:17:31.323" Score="15" ViewCount="1258" Body="&lt;p&gt;There's no way for any probe that is sent to orbit/land on a distant body to get there only on precalculated flight parameters alone - minuscule errors in the beginning of flight would convert to huge differences in speed and trajectory near its end. Of course these errors must be corrected during the flight as they become apparent (and before they escalate to uncorrectable levels), but first - they need to become apparent. And while in the initial phase the probes would be able to measure their speed and location quite precisely using the GPS satellites, this is not viable, say, halfway to Saturn, and good few AU away from any near bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what methods do the probes use to measure their own flight parameters respective to planned ones?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-09-30T07:50:09.473" Title="How do probes locate themselves?" Tags="|probe|navigation|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5846" PostTypeId="1" CreationDate="2014-11-08T23:12:29.837" Score="16" ViewCount="3479" Body="&lt;p&gt;The thing about solids is that they can have awe inspiring amounts of thrust compared to liquid engines, as you can see in this collection of boosters and thrust levels:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Shuttle SRB - 2.8 Mlbs&lt;/li&gt;&#xA;&lt;li&gt;Ariane 5 boosters - 1.5 Mlbs&lt;/li&gt;&#xA;&lt;li&gt;Titan IV booster - 3.2 Mlbs (wow!)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Compared to liquid engines:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SSME - 660 Klbs&lt;/li&gt;&#xA;&lt;li&gt;F-1 - 1.5 to 1.8 Mlbs&lt;/li&gt;&#xA;&lt;li&gt;RD-170 - 1.6 Mlbs&lt;/li&gt;&#xA;&lt;li&gt;RD-180 - 800 Klbs&lt;/li&gt;&#xA;&lt;li&gt;Merlin 1D - 160Klbs&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But their &lt;a href=&quot;https://www.grc.nasa.gov/WWW/k-12/airplane/specimp.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISP&lt;/a&gt; (explanation of ISP at bottom of link) usually sucks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So with ATK (maker of US solid boosters) pushing to replace Antares first stage with a solid, and maybe even Atlas V first stage with a solid, the question I have is, do the numbers work? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISP is less important than thrust (per se) during the first few seconds and minutes of flight. You have to get the darn stack off the ground before it matters about efficiency.  Upper stages are ISP dependent, but less so the first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question I have is, how much so?  I.e. Can a Solid first stage really make sense in competition with high-performance liquid engines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Contrary examples would be: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Ares-1x flight (SRB first stage)&lt;/li&gt;&#xA;&lt;li&gt;Ariane 6 - proposed solid first and second stage&lt;/li&gt;&#xA;&lt;li&gt;Vega light launcher&lt;/li&gt;&#xA;&lt;li&gt;Minotaur&lt;/li&gt;&#xA;&lt;li&gt;Taurus&lt;/li&gt;&#xA;&lt;li&gt;Pegasus&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Of course Ares-1 was cancelled.  And Ariane 6 is unlikely to end up as a solid based booster. Minotaur, Taurus, and Pegasus barely launch these days. Vega is very overpriced. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="27346" LastEditDate="2019-12-18T12:24:52.183" LastActivityDate="2019-12-18T12:24:52.183" Title="Can a solid first stage compete with a liquid fueled first stage?" Tags="|solid-fuel|liquid-fuel|first-stage|" AnswerCount="5" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="5848" PostTypeId="1" AcceptedAnswerId="5849" CreationDate="2014-11-09T01:07:35.827" Score="12" ViewCount="3821" Body="&lt;p&gt;This is a very stupid hypothetical - sorry!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the Space Shuttle Orbiter plus external tank have been able to leave the pad just using the SSMEs (i.e. without the solid rocket boosters ~80% of its lift-off thrust).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so (and I'm guess it couldn't?) how fast/far could this configuration (basically the second stage) go on it's own before the ET ran dry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this stupid question I'm really trying to get some idea of the relationship between the power needed on the first and second stages and the contribution of the SSMEs.&lt;/p&gt;&#xA;" OwnerUserId="6516" LastEditorUserId="51" LastEditDate="2014-11-13T13:54:54.797" LastActivityDate="2021-05-19T19:15:39.537" Title="How far would the STS get without the SRBs" Tags="|rockets|space-shuttle|thrust|first-stage|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5849" PostTypeId="2" ParentId="5848" CreationDate="2014-11-09T01:43:09.240" Score="25" Body="&lt;p&gt;The reason for the SRBs is that the stack weighs something 4.4 million lbs.&lt;/p&gt;&#xA;&lt;p&gt;The two SRBs provide about 2.8 million lbs of thrust each.  The three SSMEs provide about 600Klbs each.&lt;/p&gt;&#xA;&lt;p&gt;If you subtract 5.6 million lbs of thrust, and the SRBs weigh 1.3 million lbs each, then the remaining stack, a little over 1.8 million lbs only has 1.18 million lbs of thrust.&lt;/p&gt;&#xA;&lt;p&gt;So it will sit exactly where it is, burning off fuel, destroying the launch pad, until enough LOX/LH2 is burned off by the SSMEs to get the total mass of the stack down to about 1.18 million lbs, then it will start moving.  But then there is not enough impulse to make it to orbit.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="512" LastEditDate="2021-05-19T19:15:39.537" LastActivityDate="2021-05-19T19:15:39.537" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="5850" PostTypeId="1" CreationDate="2014-11-09T03:04:04.677" Score="7" ViewCount="260" Body="&lt;p&gt;The problems with Ares-1 are well known. They were enough to close down the program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why was it based on the SRB? At first glance, the SRB was never launched to orbit, was never launched manned, and needed to be modified to prevent vibrations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why didn't NASA just use a modified Titan or Atlas. They're fairly successful, Atlas is 49 out of 50, and could probably be tweaked for more reliability. It's also made by Old Space, so it's great for congress.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What pushed NASA to try making a brand new design?&lt;/p&gt;&#xA;" OwnerUserId="6517" LastActivityDate="2017-12-19T13:29:39.910" Title="Why was Ares-1 based on the SRB?" Tags="|constellation-program|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5851" PostTypeId="1" AcceptedAnswerId="5853" CreationDate="2014-11-09T06:06:32.457" Score="2" ViewCount="895" Body="&lt;p&gt;Somewhere I read that the nearest earth-like planet is 19 light years away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is true, and there are no closer earth-like planets, do we have the knowledge, technology and resources to colonize it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean if the world decides to spent trillions of dollars or even more, can we build huge spaceship that could bring people there?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess they will have to live decades in the ship and their children would reach it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The technological restraints I think of is the launching and landing of such construction and the speed, it must reach it in reasonable time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can we accelerate such ship at least in half of the speed of light, so it could bring the people in 40 years maximum?&lt;/p&gt;&#xA;" OwnerUserId="6518" LastEditorUserId="58" LastEditDate="2020-03-24T15:08:44.913" LastActivityDate="2020-03-24T15:08:44.913" Title="Do we have the technology to colonize the nearest habitable planet by sending hundreds of people with spacecraft?" Tags="|colonization|interstellar-travel|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5852" PostTypeId="2" ParentId="5851" CreationDate="2014-11-09T12:10:12.353" Score="0" Body="&lt;p&gt;Half the speed of light is out. Any speck of dust at that speed would be a disaster (Space is not completely empty. Just mostly empty. )&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That would mean going a lot slower. Which in its turn implies a much longer journey. This probably changes from &quot;their children would reach it&quot; to &quot;many generations later we would reach it&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that implies a longer lasting ship. Which is something we probably can not do yet. Just consider the food problem. We did try to create sustainable closed ecospheres here on earth, but so far we failed. We always needed outside resources. Now consider that in a closed ecology aboard a spaceship where there is no way to get outside resources. There is just what you brought along (which as little as needed to save mass).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, technically we may be able to do this in a few decades.&lt;/p&gt;&#xA;" OwnerUserId="686" LastEditorUserId="686" LastEditDate="2014-11-09T12:36:47.950" LastActivityDate="2014-11-09T12:36:47.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5853" PostTypeId="2" ParentId="5851" CreationDate="2014-11-09T13:55:31.030" Score="13" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Do we have the knowledge, technology and resources to colonize it?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Simply put, no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, you need to forget about knowledge, technology and resources and look to biology, sociology, and psychology. How many people would be needed to create a self-sufficient modern colony? The number is rather large. It's not 100 people, or even 1,000. It's at least 14,000 per &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576513004669&quot;&gt;Cameron Smith, &quot;Estimation of a genetically viable population for multigenerational interstellar voyaging: Review and data for project Hyperion.&quot; Acta Astronautica 97:16-29 (2014)&lt;/a&gt;, and that's risking extinction. Sending 40,000 people is a better bet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can't do that. Not even close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose we knew how to put people in what science fiction authors call &quot;cold sleep.&quot; We can't do that, not even close. In that case, we don't need 40,000 colonists. We just need 40,000 embryos, plus 4,000 or so young women in cold sleep who will need to get very busy getting busy birthing implanted embryos upon waking up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about the target planet? We would have to terraform it if the target planet doesn't contain a breathable atmosphere. We can't do that, not even close. The closest we can do with current technology is to do what we've done on Earth, which is to try to de-terraform it. On the other hand, if the target planet does contain a breathable atmosphere, that would most likely mean bacteria and viruses that would greet the colonists with a hearty &quot;Yum!&quot; The colonists would have no defense. Either way, this is a showstopper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring that, can we send a spaceship containing a few thousand young women in cold sleep plus 40,000 embryos in cold sleep to a habitable planet 20 light years away? Those colonists need to eat. They need to eat quite a bit since each colonist needs to eat for two for twenty years straight. We need to send a self-sustaining food source. We can't do that, not even close. Every attempt to make a completely closed, self-sustaining ecological system has failed -- and that's right here on Earth. If we could do that, we would need to send a lot, lot more than 4,000 women and 40,000 embryos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even with the reduction from 40,000 people to 4,000 women, this spaceship is still extremely large. We need to send animals, plants, farm equipment, materials to make dirt, mining equipment, manufacturing equipment. This thing is several orders of magnitude larger than the International Space Station. Scaling things by an order of magnitude is hard. Scaling by several orders of magnitude--we can't do that, not even close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose we can get the spacecraft to 1/1000th the speed of light after exiting the solar system. That's about twenty times faster than the current velocity of the Voyager spacecraft, and those are tiny. Getting something the size of hundreds of space stations to that speed is once again a we can't do that kind of proposition. Scaling up by a factor of 100 to 1/10th the speed of light is science fiction. Even at that speed, it will take 200 years to reach the target star. We need equipment that will remain reliable and viable for centuries. This is yet again a we can't do that kind of proposition.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-11-09T13:55:31.030" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5854" PostTypeId="1" AcceptedAnswerId="5855" CreationDate="2014-11-09T19:40:22.917" Score="33" ViewCount="28498" Body="&lt;p&gt;In this picture from the AsiaSat 6 launch by Falcon 9 on 8th September 2014, the part of the rocket between the fairing and the engine appears to be surrounded by dust, water droplets or debris:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.spacex.com/sites/spacex/files/asiasat6_outside_pad_close_v.jpg&quot; alt=&quot;Falcon9 launch&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And another launch:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.spacex.com/sites/spacex/files/falcon_9_thaicom_engines.jpg&quot; alt=&quot;Falcon9 launch&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is smoke but many fewer particles in launches of other rockets. Saturn V:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/f/f0/Saturn_V_launches%2Bdestinations.jpg/800px-Saturn_V_launches%2Bdestinations.jpg&quot; alt=&quot;SaturnV&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atlas:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/7/7e/Atlas_V%28401%29_launches_with_LRO_and_LCROSS_cropped.jpg&quot; alt=&quot;Atlas&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These particles are particularly prominent in Falcon 9. &lt;strong&gt;What are they?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user3405" LastEditorDisplayName="user20636" LastEditDate="2019-01-05T21:08:57.490" LastActivityDate="2022-12-11T01:01:53.250" Title="What are the droplets/particles falling off rockets at launch?" Tags="|launch|falcon-9|cryogenics|ice|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="5855" PostTypeId="2" ParentId="5854" CreationDate="2014-11-09T19:54:20.917" Score="42" Body="&lt;p&gt;Ice. All these rockets use oxygen as the oxidizer component of their propellant. The Saturn 5 also used hydrogen in some of its engines (upper stages). They are stored in liquid state, which requires very low temperatures (below -183c for oxygen, below -253c for hydrogen). Despite insulation, some of the outside surfaces can get cold enough to condense and freeze moisture out of the air, so there will be accumulations of ice. When the vehicle launches, acceleration and vibration shakes it loose so it falls away.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2014-11-10T00:46:38.833" LastActivityDate="2014-11-10T00:46:38.833" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5856" PostTypeId="2" ParentId="5850" CreationDate="2014-11-09T20:19:27.607" Score="3" Body="&lt;p&gt;The design wasn't brand new, it was an extension of old technology which was extensively used and understood. It was nothing groundbreaking, just an up-scaled shuttle SRB. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding of the motivation behind Ares was that there were people within NASA who felt that they should still be in the launch game, and Ares was their last gasp. They chose the SRB system because it was something they could design and contract out at a relatively low cost, and with the level of expertise they still had. The combination of inexpensive private launch companies like SpaceX, cost overruns, and political changes killed the program before they could make it successful. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ares had problems for sure, but every new rocket system has failures. They would have made it work eventually, it was a matter of investment. NASA takes its direction from the US government, who told them to give up building their own launcher and use one of the many commercially available ones and to spend their time on more ground-breaking technologies and exploration missions. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="25" LastEditDate="2017-12-19T13:29:39.910" LastActivityDate="2017-12-19T13:29:39.910" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5857" PostTypeId="1" AcceptedAnswerId="5858" CreationDate="2014-11-09T23:21:26.930" Score="18" ViewCount="17987" Body="&lt;p&gt;I wonder what is concentration of iron in Moon's soil and whether iron ore is available there. Is there enough iron for successful industry?&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="4567" LastEditDate="2014-11-10T05:06:20.067" LastActivityDate="2017-08-15T14:51:46.633" Title="Is there enough iron on the Moon to support industry?" Tags="|the-moon|natural-resources|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5858" PostTypeId="2" ParentId="5857" CreationDate="2014-11-10T00:28:40.490" Score="20" Body="&lt;p&gt;Yes, oxides of typical engineering metals, such as iron, aluminum, magnesium, titanium and silicon (as a semiconductor and important alloy material) are abundant on the lunar surface. It is unknown whether there are large deposits of heavier elements like lead, tungsten, chromium or uranium. Although the moon is mainly composed of light elements, radioactive thorium has been proven to exist in the Compton–Belkovich Anomaly on the far side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A greater challenge to a moon colony could be the scarcity of hydrogen and therefore water. Even the poles of the moon, which likely hold the most water (in the form of ice), it is not entirely clear that enough water is present for a larger scale colony.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_water&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Lunar_water&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nitrogen is also uncommon, which makes it hard to create a breathable and non-flamable atmosphere. Nitrogen is also used in metal surface treatments. Cabon is also scarce, but it has the advantage of being a dense solid at room temperature, so it can be more easily shipped from earth than nitrogen or hydrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both nitrogen and carbon are available in a few parts per million in Lunar regolith.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://articles.adsabs.harvard.edu//full/1975lspa.book.....T/0000064.000.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://articles.adsabs.harvard.edu//full/1975lspa.book.....T/0000064.000.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2160.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2160.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources15.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources15.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2017-08-15T14:51:46.633" LastActivityDate="2017-08-15T14:51:46.633" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5859" PostTypeId="2" ParentId="2021" CreationDate="2014-11-10T18:19:06.527" Score="13" Body="&lt;p&gt;Some of the hydrogen will be disassociated.&#xA;For the reaction mass that is not dissociated, and passes through the engine in the form of diatomic hydrogen, in addition to the three translational degrees of freedom, heat energy is also put into the vibrational and rotational energy of the hydrogen molecule.  So the energy stored is 6/2kT, not 3/2 kT as in the first formula you quote.&#xA;(The vibrational and rotational energy is converted into exhaust kinetic energy as the hydrogen cools as it expands through the nozzle.  Some of it, however, isn't converted into kinetic energy because there isn't time.  This is known as &quot;frozen flow loss&quot;.  Frozen flow losses depend on details of the engine.)&lt;/p&gt;&#xA;" OwnerUserId="6529" LastActivityDate="2014-11-10T18:19:06.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5860" PostTypeId="1" CreationDate="2014-11-11T10:29:28.417" Score="4" ViewCount="444" Body="&lt;p&gt;If you look at the challenge of building a factory in space, there are many challenges - some examples:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lathes and other industrial equipment is heavy, thus expensive to lift. Weight is often helpful for vibration dampening&lt;/li&gt;&#xA;&lt;li&gt;Pumps want some static pressure at the inlet to avoid cavitation, this is often simply gravity head&lt;/li&gt;&#xA;&lt;li&gt;The vast majority of liquid-gas phase reactors I'm aware of use gravity to seperate the liquid from the gaseaous phase&lt;/li&gt;&#xA;&lt;li&gt;Industrial processes generate inconvenient dirt, in  zero-g this can get anywhere even more so than on earth&lt;/li&gt;&#xA;&lt;li&gt;Temperature changes of the environment may be more extreme than on earth, a convenient heat sink for low-temp process heat is generally not available&lt;/li&gt;&#xA;&lt;li&gt;Movement of machines may impart a torque on the spacecraft&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I could probably list more. For most of them, I could easily think of a solution, most solutions would add mass. Some could be solved by adding gravity by spinning the craft, leading to new interesting problems (and probably adding mass)&lt;br&gt;&#xA;My question is: &#xA; Has anyone done extensive research into what it would mean to build and operate a factory in space?&lt;br&gt;&#xA;What are the key findings and what are the key open questions?&lt;/p&gt;&#xA;" OwnerUserId="102" LastEditorUserId="603" LastEditDate="2014-11-11T13:17:18.610" LastActivityDate="2014-11-11T13:17:18.610" Title="Can we build a factory in zero gravity space?" Tags="|microgravity|materials|construction|" AnswerCount="0" CommentCount="11" ClosedDate="2014-11-12T00:07:04.640" ContentLicense="CC BY-SA 3.0" />
  <row Id="5861" PostTypeId="1" AcceptedAnswerId="10065" CreationDate="2014-11-11T12:12:33.037" Score="14" ViewCount="3488" Body="&lt;p&gt;There still seems to be uncertainty about how long the ISS will remain flying, mainly due to political wrangling and budgetary concerns. Even the longest plans state that operations are not expected to last beyond 2024.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, leaving aside political and budgetary considerations, how long could the ISS, as it currently exists, remain operational and safe enough for habitation and use by crew?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2022-02-05T00:04:05.973" Title="What is the theoretical life of the ISS?" Tags="|iss|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5862" PostTypeId="1" AcceptedAnswerId="5863" CreationDate="2014-11-11T17:36:57.020" Score="5" ViewCount="398" Body="&lt;p&gt;At NasaSpaceFlight.com, Chris Gerbhart &lt;a href=&quot;http://www.nasaspaceflight.com/2014/11/atk-expand-alternative-atlasv-rd-180/&quot; rel=&quot;nofollow&quot;&gt;writes&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The RD-180 has provided exceptional performance to the Atlas V and was chosen to be the engine of the first stage in no small part because of its safety record and reliable performance track record.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Who flew the RD-180 before it was used on Atlas III? They flew 6 Atlas III flights, and then transitioned to the Atlas V. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ya, the bigger brother RD-170 was flown on Energia on the side boosters and became the standalone Zenit booster.  But who else flew the RD-180, to give it a track record before the Atlas V?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If as Wikipedia suggests that the Atlas III was the first use of the RD-170, then what was the Russian intent in developing it from the RD-170 core?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-11T19:00:48.730" LastActivityDate="2014-11-11T19:00:48.730" Title="What was RD-180 used in, before Atlas III/V?" Tags="|rockets|engines|rd-180|atlas-v|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5863" PostTypeId="2" ParentId="5862" CreationDate="2014-11-11T17:53:27.163" Score="6" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/RD-180&quot;&gt;Wikipedia&lt;/a&gt;, RD-180 has only been used on the Atlas line, so I would assume they're talking about the track record of the whole RD-170 family -- the Energia and Zenit applications in particular.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-11-11T17:53:27.163" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5864" PostTypeId="5" CreationDate="2014-11-11T22:25:18.727" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-11-11T22:25:18.727" LastActivityDate="2014-11-11T22:25:18.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5865" PostTypeId="4" CreationDate="2014-11-11T22:25:18.727" Score="0" Body="Questions on spacecraft propulsion techniques that are either theoretical or not yet at a high technology readiness level." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-11-11T22:34:20.333" LastActivityDate="2014-11-11T22:34:20.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5866" PostTypeId="5" CreationDate="2014-11-11T22:43:55.390" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-11-11T22:43:55.390" LastActivityDate="2014-11-11T22:43:55.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5867" PostTypeId="4" CreationDate="2014-11-11T22:43:55.390" Score="0" Body="Questions on launch complexes, spaceports or other locales for the launch of spacecraft." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-11-11T23:29:40.503" LastActivityDate="2014-11-11T23:29:40.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5868" PostTypeId="5" CreationDate="2014-11-11T22:54:36.477" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-11-11T22:54:36.477" LastActivityDate="2014-11-11T22:54:36.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5869" PostTypeId="4" CreationDate="2014-11-11T22:54:36.477" Score="0" Body="Questions about rocket oxidizer, propellant, reactive mass, or other fuels used in spacecraft or rockets." OwnerUserId="1142" LastEditorUserId="25" LastEditDate="2015-10-05T17:37:46.610" LastActivityDate="2015-10-05T17:37:46.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5870" PostTypeId="2" ParentId="5199" CreationDate="2014-11-11T23:50:16.133" Score="1" Body="&lt;p&gt;The two harpoons fire into the dust at 300 feet/s (100m/s) on touchdown. &lt;a href=&quot;http://www.designnews.com/author.asp?section_id=1365&amp;amp;doc_id=274752&quot; rel=&quot;nofollow&quot;&gt;http://www.designnews.com/author.asp?section_id=1365&amp;amp;doc_id=274752&lt;/a&gt;&#xA;That is bound to kick up some dust. I have not found more info about this specific issue. &lt;a href=&quot;http://www.dlr.de/dlr/presse/en/desktopdefault.aspx/tabid-10308/471_read-6556/year-all/#/gallery/9154&quot; rel=&quot;nofollow&quot;&gt;http://www.dlr.de/dlr/presse/en/desktopdefault.aspx/tabid-10308/471_read-6556/year-all/#/gallery/9154&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6535" LastActivityDate="2014-11-11T23:50:16.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5871" PostTypeId="1" AcceptedAnswerId="5873" CreationDate="2014-11-12T04:09:32.833" Score="5" ViewCount="749" Body="&lt;p&gt;Alternatively, does anyone know it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm asking in regards to the upcoming &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=30543.0&quot; rel=&quot;noreferrer&quot;&gt;DSCOVR&lt;/a&gt; (Deep Space Climate Observatory) mission scheduled to launch aboard a Falcon 9 v1.1 rocket in January 2015. DSCOVR is 507 kg in mass and will be headed to the Sun-Earth L&lt;sub&gt;1&lt;/sub&gt; (SEL1).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm trying to calculate the payload fraction of the rocket based off the C&lt;sub&gt;3&lt;/sub&gt; (characteristic energy) involved. I'm assuming it'll be greater than 0 km&lt;sup&gt;2&lt;/sup&gt;/s&lt;sup&gt;2&lt;/sup&gt; and less than 12.2 km&lt;sup&gt;2&lt;/sup&gt;/s&lt;sup&gt;2&lt;/sup&gt;, which is the C&lt;sub&gt;3&lt;/sub&gt; of MAVEN to Mars.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="49" LastEditDate="2014-11-12T11:06:08.983" LastActivityDate="2014-11-14T07:07:29.490" Title="How can I calculate the Characteristic Energy to Sun-Earth L1?" Tags="|orbital-mechanics|lagrangian-points|falcon-9|earth-observation|noaa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5872" PostTypeId="1" AcceptedAnswerId="10218" CreationDate="2014-11-12T05:29:01.583" Score="8" ViewCount="306" Body="&lt;p&gt;Is data produced by the European Space Agency covered by any such thing as the &lt;a href=&quot;http://www.foia.gov/&quot; rel=&quot;nofollow&quot;&gt;Freedom of Information Act&lt;/a&gt; like in the United States?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, are data produced by ESA programs freely available much like they are for NASA programs?&lt;/p&gt;&#xA;" OwnerUserId="84" LastEditorUserId="58" LastEditDate="2015-07-24T15:10:02.753" LastActivityDate="2015-07-24T15:10:02.753" Title="Is there such a thing as the Freedom of Information Act for the ESA?" Tags="|esa|regulatory|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5873" PostTypeId="2" ParentId="5871" CreationDate="2014-11-12T06:35:19.973" Score="6" Body="&lt;p&gt;Actually, it's less than zero.  The Lagrange points are still effectively captured by the Earth, so you have not escaped.  &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020081116.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; states the required $C_3$ as $-0.7\,\mathrm{km^2/s^2}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we ignore the Sun, the $C_3$ required to get to the altitude of the S-E L1 point is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$C_3=-{2\mu\over{r_a+r_p}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mu$ is the $G M$ of the Earth and the Moon, and $r_a$ and $r_p$ are the apoapsis (L1) and periapsis (LEO).  The result is $C_3=-0.54\,\mathrm{km^2/s^2}$.  However the Sun is there and helps, hence the $C_3$ of $-0.7\,\mathrm{km^2/s^2}$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-11-14T07:07:29.490" LastActivityDate="2014-11-14T07:07:29.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5874" PostTypeId="1" AcceptedAnswerId="5886" CreationDate="2014-11-12T12:57:37.223" Score="17" ViewCount="8037" Body="&lt;p&gt;A Falcon 9 costs 61.2M dollars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Falcon Heavy (1 Falcon 9 + 2 boosters) costs 85M dollars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means 1 booster of the Falcon Heavy costs only 11.9M dollars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why does a Falcon 9 (9 Merlin 1D + 1 Merlin Vacuum 1D + 1 fairing) cost 50M dollars more than a single Falcon Heavy booster (9 Merlin 1D) ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacex.com/about/capabilities&quot;&gt;http://www.spacex.com/about/capabilities&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6540" LastEditorUserId="51" LastEditDate="2015-07-09T02:30:57.100" LastActivityDate="2018-01-06T19:46:50.513" Title="Why is there so little price difference between the Falcon 9 and the Falcon Heavy?" Tags="|rockets|spacex|falcon-9|falcon-heavy|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5875" PostTypeId="1" AcceptedAnswerId="5884" CreationDate="2014-11-12T13:03:38.133" Score="6" ViewCount="200" Body="&lt;p&gt;Philae is to land on 67P/Churyumov-Gerasimenko today at 17:02 CET. The comet is currently 3.409AU away from Earth. That means signals from the lander take a little over 28 minutes to reach Earth. Does that mean we get the first confirmation of landing no sooner than at 16:30 UTC, 28 minutes past planned 16:02 landing, or do we get them at 16:02, while the landing took place earlier?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2014-11-13T08:01:26.370" LastActivityDate="2014-12-16T14:03:39.187" Title="What time of events is given for probes a long way away from Earth?" Tags="|probe|time|uncrewed-spaceflight|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5876" PostTypeId="5" CreationDate="2014-11-12T13:08:47.850" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-11-12T13:08:47.850" LastActivityDate="2014-11-12T13:08:47.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5877" PostTypeId="4" CreationDate="2014-11-12T13:08:47.850" Score="0" Body="Questions about orbits whose position in the sky remains the same for a stationary observer on the primary body. " OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-11-12T14:06:22.227" LastActivityDate="2014-11-12T14:06:22.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5878" PostTypeId="5" CreationDate="2014-11-12T13:11:04.557" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-11-12T13:11:04.557" LastActivityDate="2014-11-12T13:11:04.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5879" PostTypeId="4" CreationDate="2014-11-12T13:11:04.557" Score="0" Body="Questions involving trajectories that reach space but do not attain orbit." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-11-12T14:06:10.567" LastActivityDate="2014-11-12T14:06:10.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5880" PostTypeId="5" CreationDate="2014-11-12T13:12:51.490" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-11-12T13:12:51.490" LastActivityDate="2014-11-12T13:12:51.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5881" PostTypeId="4" CreationDate="2014-11-12T13:12:51.490" Score="0" Body="Questions explicitly involving numerical analysis." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-11-12T14:06:27.007" LastActivityDate="2014-11-12T14:06:27.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5882" PostTypeId="2" ParentId="5874" CreationDate="2014-11-12T14:17:49.560" Score="5" Body="&lt;p&gt;First off, we shall see if that price for the Falcon Heavy survives contact with the enemy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did you get to 11.9 Million dollars for one booster?  I cannot find the reference now, but assume generic launch costs are about 10% of total cost.  Range, permits, fuel etc.  So say $55 million for the raw booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then assume the first stage is 70% of booster cost. That is about 38.5M.  Times 3 would be 115.5. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Economies of scale?  I suspect they expect when they are launching 10 F-9 and 10 F-H a year, and making 40 cores a year, and thus 380 Merlin engines a year they expect things to get cheaper. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or else they plan for reusing stages in that pricing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The proof will be in pudding, when they actually start delivering Heavy launches and what they really charge for future flights after the pricing settles down.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-12T14:17:49.560" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5883" PostTypeId="1" AcceptedAnswerId="5888" CreationDate="2014-11-12T14:50:45.400" Score="9" ViewCount="307" Body="&lt;p&gt;In other words, are the photos transferred &quot;live&quot; to Rosetta or did they plan to transfer them after landing? (and would they have been lost had the landing failed)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(yes, I am aware that &quot;nice photos&quot; are less important for mission control than for regular public like me :))&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Clarification:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I meant &lt;strong&gt;hypothetical&lt;/strong&gt; failed landing, where Philae could not land and started drifting into space, or tipped over randomly. What would happen then?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Assuming Philae can't be contacted, did Rosetta receive the descent photos before landing? (so they are safe and can be transmitted to Earth)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Or maybe Philae can be contacted by Rosetta even if it does not maintain the correct attitude and can still transmit any data it records?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6522" LastEditorUserId="6522" LastEditDate="2014-11-12T21:54:17.420" LastActivityDate="2014-11-12T22:09:53.910" Title="If Philae lander had bounced off the comet and become uncontrollable, could we still have seen some of the descent photos?" Tags="|rosetta|comet|philae|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5884" PostTypeId="2" ParentId="5875" CreationDate="2014-11-12T15:13:05.020" Score="5" Body="&lt;p&gt;The &lt;a href=&quot;http://rosetta.esa.int/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta website&lt;/a&gt; and live commentary has confirmed that communication of the touchdown is expected around 16:00 UTC, which implies touchdown just after 15:30 UTC comet local time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I agree that initially the wording on the website was not clear, but now we have definite commentary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://astronomy.stackexchange.com/q/2464/43&quot;&gt;Abby's Post on Astronomy&lt;/a&gt; has more info.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2014-12-16T14:03:39.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5885" PostTypeId="2" ParentId="5874" CreationDate="2014-11-12T17:01:21.087" Score="11" Body="&lt;p&gt;Look at the pricing on sodas at a movie theater sometime. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're looking at the retail &lt;em&gt;price&lt;/em&gt; of a launch on that page -- what SpaceX charges customers -- not the &lt;em&gt;cost&lt;/em&gt; of the hardware involved in the launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The price is whatever SpaceX says it is. If they set it too low, they can't afford to continue R&amp;amp;D on their future plans; if they set it too high, customers will find another provider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost of the hardware is only part of SpaceX's costs. The &lt;em&gt;management and logistics portions&lt;/em&gt; of designing, producing, and delivering the hardware and overseeing the launch -- which are enormous -- don't vary much with the size of the rocket involved.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-11-12T17:01:21.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5886" PostTypeId="2" ParentId="5874" CreationDate="2014-11-12T18:33:11.287" Score="22" Body="&lt;p&gt;@Russell has the right idea.  Pricing is about economics, what the market will bear.  What it costs to produce and launch is only one factor in that equation.  Let's look at it economically: dollars per kilogram to orbit.  Here are the numbers for payload to &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;GTO&lt;/a&gt;. First &lt;a href=&quot;https://web.archive.org/web/20141101012430/http://www.spacex.com/about/capabilities&quot; rel=&quot;nofollow noreferrer&quot;&gt;in 2014&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon 9: 4,850 kg to GTO for \$61.2M or &lt;strong&gt;&lt;em&gt;\$12500/kg&lt;/em&gt;&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Falcon Heavy: &quot;Up to 6,400 kg&quot; to GTO fro \$85M or &lt;strong&gt;&lt;em&gt;\$13281/kg&lt;/em&gt;&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And here they are &lt;a href=&quot;https://web.archive.org/web/20171231080118/http://www.spacex.com/about/capabilities&quot; rel=&quot;nofollow noreferrer&quot;&gt;in 2018&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon 9: &quot;Up to&quot; 5,500 kg to GTO for \$62M or &lt;strong&gt;&lt;em&gt;\$11270/kg&lt;/em&gt;&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Falcon Heavy: &quot;Up to&quot; 8,000 kg to GTO for \$90M or &lt;strong&gt;&lt;em&gt;\$11250/kg&lt;/em&gt;&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note the cost per kilogram is about the same and have converged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note the 2014 pricing on the Falcon Heavy is $85M for &quot;up to 6.4mT&quot; but it is capable of putting 21mT into GTO.  How much would that cost?  They don't say.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key comes when you look into the &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_Heavy#Reusable_technology_development&quot; rel=&quot;nofollow noreferrer&quot;&gt;potential reusability of the Falcon&lt;/a&gt; which SpaceX is gambling will drive the cost of launching stuff to LEO down to the magic \$2,200/kg (\$1000/lb).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX has indicated that the Falcon Heavy payload performance to&#xA;  Geosynchronous transfer orbit (GTO) will be reduced by addition of the&#xA;  reusable technology, but would fly at much lower launch price. With&#xA;  full reusability on all three booster cores, GTO payload will be 7,000&#xA;  kg (15,000 lb).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;7,000 kg is awful close to the marketed &quot;up to 6,400kg&quot; price.  They don't have that reusable part working yet, &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_reusable_launch_system_development_program#Falcon_9_booster_post-mission.2C_controlled-descent_tests&quot; rel=&quot;nofollow noreferrer&quot;&gt;but they're using every Falcon 9 launch to test it&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other part is volume.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_Heavy#Pricing_and_development_funding&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX plans to build a lot of these rockets and build them cheaper each time&lt;/a&gt;.  By using the same basic technology for all their rockets, they further increase their economies of scale.  And since launches have a fixed cost, more payload per launch &lt;em&gt;might&lt;/em&gt; mean cheaper launches... though &lt;a href=&quot;https://en.wikipedia.org/wiki/Fuel_fraction&quot; rel=&quot;nofollow noreferrer&quot;&gt;fuel fraction&lt;/a&gt; remains a tyrant.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX has claimed the cost of reaching low Earth orbit can be as low&#xA;  as US$1,000/lb if an annual rate of four launches can be sustained,&#xA;  and as of 2011 planned to eventually launch 10 Falcon Heavy and 10&#xA;  Falcon 9 annually.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there you have it.  The Falcon Heavy's posted price is for significantly less than its maximum capability and banking on yet to be proven technology and a steady stream of contracts to drive down the cost of launching stuff into orbit.  It's the vehicle SpaceX is banking on to drive launch costs down to encourage the sustained space economy they need to get to Mars.  Wait, Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Running a successful business is nice and all, but &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX#Goals&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX is one very, very rich man's dream to colonize Mars&lt;/a&gt;.  The Falcon Heavy is not the vehicle to do that, &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Colonial_Transporter#Super-heavy_lift_launch_vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;that will require a whole new engine&lt;/a&gt;, but it is the vehicle to &lt;a href=&quot;https://en.wikipedia.org/wiki/Red_Dragon_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;get to Mars cheaper than anyone else&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="4704" LastEditorUserId="4704" LastEditDate="2018-01-06T19:46:50.513" LastActivityDate="2018-01-06T19:46:50.513" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5887" PostTypeId="1" AcceptedAnswerId="9880" CreationDate="2014-11-12T18:39:54.867" Score="10" ViewCount="146" Body="&lt;p&gt;I just saw the pictures &lt;a href=&quot;http://www.bbc.com/news/science-environment-30026398&quot; rel=&quot;nofollow noreferrer&quot;&gt;from Philae&lt;/a&gt; I was very surprised to see that the surface looks like it is covered in dust, not that much different than our moon.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I personally was expecting it to be big chunks of rock and/or ice as essentially a single solid chunk.  Was there an expectation that it would be dusty?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EnCOr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="10234" LastEditDate="2015-07-13T03:30:18.380" LastActivityDate="2015-07-20T18:16:18.590" Title="Did we expect Comet 67P/Churyumov-Gerasimenko to be dusty?" Tags="|rosetta|comet|philae|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5888" PostTypeId="2" ParentId="5883" CreationDate="2014-11-12T18:42:33.057" Score="3" Body="&lt;p&gt;Based on the fact that we don't know 100% the status of Philae right now, but we do have some descent photos, I'd say yes, it is possible, so long as contact can be maintained.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-11-12T18:42:33.057" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5889" PostTypeId="1" AcceptedAnswerId="5890" CreationDate="2014-11-12T18:57:49.060" Score="12" ViewCount="756" Body="&lt;p&gt;There is currently a question as to if the harpoons fired, that is undergoing review. But I have to ask, how important are they? From what I've heard, a single leg touching the right spot could be a sufficient hold, and two legs had to have connection to even fire the harpoons. If the lander can stick to the comet with just a leg or two, why does it need harpoons?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-11-13T18:50:23.040" Title="How important are Philae's harpoons?" Tags="|comet|lander|philae|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5890" PostTypeId="2" ParentId="5889" CreationDate="2014-11-12T19:10:31.000" Score="16" Body="&lt;p&gt;The harpoons are intended to hold the lander to the surface long enough to get the ice screws on the lander's legs in securely. The ice screws are intended as the more permanent hold-down solution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bear in mind that the comet offers essentially no gravity; imagine trying to drive a wood screw into the ceiling without pushing the screwdriver upward at all. That's the ice screws, without the harpoons or hold-down thruster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The harpoons might not be able to keep their hold very long; they're fired at high speed into an icy surface we don't know much about, that's likely to shatter, evaporate, etc. So the screws are needed as a longer-term solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the plan was a three-part solution: the hold-down thruster to cancel the recoil of the harpoons, the harpoons and thruster to hold the lander in place until the ice screws were in. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that, and that (apprently) neither the thruster nor the harpoons worked, it's unclear right now (to me, not necessarily to ESA) how well the ice screws are driven in. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;UPDATE: Aaaaand Philae apparently bounced twice and is now lying on its side. Poor fella.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-11-13T18:50:23.040" LastActivityDate="2014-11-13T18:50:23.040" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5891" PostTypeId="1" AcceptedAnswerId="5907" CreationDate="2014-11-12T19:51:04.710" Score="18" ViewCount="3823" Body="&lt;p&gt;I noticed that 67P completes an orbit in 6.4 years. Assuming everything goes to plan, how long can Philae stay operational and keep sending data throughout the comet's orbit? &lt;/p&gt;&#xA;" OwnerUserId="122" LastActivityDate="2023-12-14T18:12:09.647" Title="How long can Philae stay &quot;alive&quot;?" Tags="|rosetta|philae|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5892" PostTypeId="2" ParentId="5891" CreationDate="2014-11-12T19:55:28.680" Score="20" Body="&lt;p&gt;The Wikipedia page for &lt;a href=&quot;http://en.wikipedia.org/wiki/Philae_%28spacecraft%29&quot;&gt;Philae&lt;/a&gt; suggest a 1-6 week mission duration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the design goal is always conservative.  Consider the Mars rovers (Spirit and Opportunity) whose mission design goal was 92 days and Opportunity is at 3945 days now and going strong. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They design longer, but ensure the minimum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how long could it last? No one specifically knows, and possibly only time will tell.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;News on the landing site suggests that they only have 60 hours of power in the battery and that the solar arrays are shaded and only get sunlight for 1 to 1.5 hours a day, which may not be enough to charge the battery. This poor location will likely reduced the life of the probe severely. However, they also are contemplating extreme notions such as using an extensible sensor to try and flip away to a better location.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-11-13T17:56:15.717" LastActivityDate="2014-11-13T17:56:15.717" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5893" PostTypeId="2" ParentId="5891" CreationDate="2014-11-12T20:53:42.363" Score="9" Body="&lt;p&gt;&lt;em&gt;While nothing is certain, even if there are no equipment failures it's unlikely that Philae would survive the comet becoming highly active as it approaches perihelion in fall 2015.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Seems I was wrong.  &lt;a href=&quot;https://twitter.com/Marco_Langbroek/status/533253606125101056&quot; rel=&quot;nofollow&quot;&gt;Quote&lt;/a&gt; from Dr Ulamec, lander team manager: &quot;&lt;strong&gt;Risk of the lander being blown off the surface by strong cometary jets is low - it has too high a density&lt;/strong&gt;&quot;&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2014-11-14T13:53:28.717" LastActivityDate="2014-11-14T13:53:28.717" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5894" PostTypeId="2" ParentId="5762" CreationDate="2014-11-12T21:30:15.590" Score="9" Body="&lt;p&gt;The current accuracy of the JPL Developmental Ephemerides released in September of 2013 is given by its authors in their article &lt;a href=&quot;http://ilrs.gsfc.nasa.gov/docs/2014/196C.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Planetary and Lunar Ephemerides &#xA;DE430 and DE431&lt;/a&gt;, quoted below:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The present-day lunar orbit is known to submeter accuracy through&#xA;  fitting lunar laser ranging data with an updated lunar gravity field&#xA;  from the Gravity Recovery and Interior Laboratory (GRAIL) mission. The&#xA;  orbits of the inner planets are known to subkilometer accuracy through&#xA;  fitting radio tracking measurements of spacecraft in orbit about them.&#xA;  Very long baseline interferometry measurements of spacecraft at Mars&#xA;  allow the orientation of the ephemeris to be tied to the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Celestial_Reference_Frame&quot; rel=&quot;noreferrer&quot;&gt;International&#xA;  Celestial Reference Frame&lt;/a&gt; with an accuracy of 0′′.0002. This&#xA;  orientation is the limiting error source for the orbits of the&#xA;  terrestrial planets, and corresponds to orbit uncertainties of a few&#xA;  hundred meters.The orbits of Jupiter and Saturn are determined to&#xA;  accuracies of tens of kilometers as a result of fitting spacecraft&#xA;  tracking data.The orbits of Uranus, Neptune, and Pluto are determined&#xA;  primarily from astrometric observations, for which measurement&#xA;  uncertainties due to the Earth’s atmosphere, combined with star&#xA;  catalog uncertainties, limit position accuracies to several thousand&#xA;  kilometers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Because the gravity of each body in the solar system affects all the others continuously, there is no way to calculate the future motion of bodies accurately for long periods of time. This is known as the &lt;a href=&quot;http://en.wikipedia.org/wiki/N-body_problem&quot; rel=&quot;noreferrer&quot;&gt;n-body problem&lt;/a&gt;. &lt;a href=&quot;http://en.wikipedia.org/wiki/Perturbation_theory&quot; rel=&quot;noreferrer&quot;&gt;Perturbation theory&lt;/a&gt; is used to find approximate projections of future behaviour. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Lyapunov_time&quot; rel=&quot;noreferrer&quot;&gt;Lyapunov time&lt;/a&gt; is the time it would take for the trajectory projections of the bodies of the system to diverge by a factor of e. For the solar system as a whole, this time is approximately 50 million years. Functionally this means that no matter how precise and complete the initial measurements, nothing can really be said about where anything will be in 50 million years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the near term, error accumulates slowly. DE431 projects orbits for 17,000 years into the future. DE430 uses a more complex model - it included a calculation of how the movement of the moon's core with respect to its mantle would affect its orbit. For this reason its accuracy was only considered appropriate for projections over the years 1550 to 2650. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The authors of DE430 and DE431 did not attempt to quantify the probable error margins for their ephemeris projections. They did carefully note the ranges of residual errors in all the observations used in the calculations. It must not be possible to state the probable error margins in such predictions, for the following reasons in descending order of importance:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Such systems are inherently chaotic&lt;/li&gt;&#xA;&lt;li&gt;Measurements and models used have inaccuracies&lt;/li&gt;&#xA;&lt;li&gt;There are factors known to influence outcomes which have not been quantified enough to include in the modelling (such as the orbit of the solar system around the galaxy)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="1142" LastEditDate="2014-11-17T14:24:58.970" LastActivityDate="2014-11-17T14:24:58.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5895" PostTypeId="2" ParentId="5883" CreationDate="2014-11-12T22:09:53.910" Score="5" Body="&lt;p&gt;There are two specific things that we have to have to get pictures in this case:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Communication:&lt;/strong&gt; PearsonArtPhoto already talked about this. If we can't receive information from the probe we can't get pictures.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;The instruments cannot have been damaged:&lt;/strong&gt; Take a look at &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/8/84/Atterrisseur-Philae-15494442575_099c1ff083_o.png&quot;&gt;this picture&lt;/a&gt; of Philae:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/8/84/Atterrisseur-Philae-15494442575_099c1ff083_o.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note: instruments are described on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Philae_(spacecraft)&quot;&gt;Wikipedia&lt;/a&gt; page)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equipment isn't in a precarious spot on the craft (given a landing at the right angle), but if it this at a bad angle, we could see some damage (hypothetically; at the moment, it seems we're good).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As per your edit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems &lt;a href=&quot;http://www.nbcnews.com/news/photo/look-out-below-lander-sends-image-descent-comet-n247266&quot;&gt;we have some pictures&lt;/a&gt; from the descent. You can find more information about them &lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2831125/Rosetta-mission-broken-thruster-computer-glitch-scupper-today-s-historic-landing-comet-67P.html&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://uk.news.yahoo.com/philae-comet-lander-separates-rosetta-091454913.html?.tsrc=yahoo&quot;&gt;here&lt;/a&gt;. There's also a cheerful last sentence about the latest pictures &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Touchdown!_Rosetta_s_Philae_probe_lands_on_comet&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first images from the surface are being downlinked to Earth and should be available within a few hours of touchdown.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2014-11-12T22:09:53.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5896" PostTypeId="1" AcceptedAnswerId="5908" CreationDate="2014-11-13T00:20:40.453" Score="2" ViewCount="352" Body="&lt;p&gt;How feasible would it be to put a large comet, like the one that Philae landed on, into a Lagrangian point, or some other spot that would make it convenient &amp;amp; SAFE for mining, colonization, experiments, etc?&lt;/p&gt;&#xA;" OwnerUserId="2810" LastActivityDate="2014-11-13T16:38:56.950" Title="Is it feasible to put a comet into a Lagrangian point?" Tags="|colonization|lagrangian-points|mining|comet|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5898" PostTypeId="1" CreationDate="2014-11-13T01:16:56.833" Score="10" ViewCount="6983" Body="&lt;p&gt;I am going to be beginning a degree in chemical engineering next year and also have a significant interest in space travel. I was wondering how much of a role chemical engineers play in space exploration. I understand there is a strong need for fuels and heat resistant materials and so forth, but how significant is this? Is there any significant  need for chemical engineers in the space industry? And if so, in what section are they most likely to be working?&lt;/p&gt;&#xA;" OwnerUserId="6254" LastActivityDate="2021-06-15T16:18:18.733" Title="Do chemical engineers play a significant role in space exploration?" Tags="|materials|space-industry|career|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5899" PostTypeId="1" AcceptedAnswerId="5900" CreationDate="2014-11-13T01:18:26.940" Score="37" ViewCount="39784" Body="&lt;p&gt;Here's an image (AS11-40-5922) from Apollo 11 that &lt;a href=&quot;http://history.nasa.gov/alsj/a11/images11.html#Mag40&quot; rel=&quot;noreferrer&quot;&gt;NASA describes as&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;View of the ascent stage from the northeast. Note the wrinkled surface of the RCS plume deflector and the warping of the rear of the ascent stage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DCx8x.jpg&quot; alt=&quot;Apollo 11&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to (don't laugh) the &lt;a href=&quot;http://www.theflatearthsociety.org/tiki/tiki-index.php?page=A%20Close%20Look%20at%20the%20Lunar%20Lander&quot; rel=&quot;noreferrer&quot;&gt;Flat Earth Society&lt;/a&gt;¹, this picture shows a lander made of paper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Upon close inspection one might notice that the Lunar Lander, a supposed six billion dollar hallmark of American engineering, is in truth made out of cardboard paper, a few old curtain rods, a roll of roofing paper, some floodlight holders, gold foil, and lots and lots of scotch tape to hold it all together on the hostile environment of the moon's surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Forget the source of the quote -- this image definitely shows a very wrinkly-looking ascent stage with what appear to be misaligned panels on the upper left.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does it look like this?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;1: Okay, go ahead and laugh.&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="25" LastEditDate="2015-11-11T17:35:14.393" LastActivityDate="2018-06-25T16:00:14.277" Title="Why does the ascent stage of Apollo 11's lunar module look like it's made of paper?" Tags="|crewed-spaceflight|apollo-program|spacecraft|myth-busting|apollo-11|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5900" PostTypeId="2" ParentId="5899" CreationDate="2014-11-13T02:17:41.433" Score="61" Body="&lt;p&gt;Like everything else, the ascent and descent stages were built to be as light as possible. But because they knew they would operate only in a vacuum, many things really didn't need to be sturdy, nor did the shape of it matter. It would never have to deal with aerodynamic drag. In fact, the descent stage was designed to buckle in the right places upon landing, that was how it absorbed the impact. It was only going to be used once, this was the most weight-efficient method of handling the shock of landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the complex insulation blankets covering the module had many layers, and contact points between the layers needed to be minimized so that heat wouldn't be passed through them by conduction. The black material is where thin Inconel sheets formed the outer layer of the insulation blanket, and they were painted matte black with Pyromark paint to improve their heat emission properties, so they would cool off quickly. (Black material both &lt;a href=&quot;http://www.thenakedscientists.com/HTML/questions/question/3136/&quot; rel=&quot;noreferrer&quot;&gt;absorbs and emits&lt;/a&gt; heat better than material of other colors.) Beneath the black layer were reflective layers to prevent the heat of the black layer penetrating into the module. This treatment was done where the exhaust of the reaction control thrusters heated the lunar modules. It had a tendency to crinkle, and on this particular module, that may have been accentuated by the fact it &lt;em&gt;was in fact installed at the last minute&lt;/em&gt;, as were the chutes under the thrusters. From the &lt;a href=&quot;http://home.earthlink.net/~pfjeld/lmdata/&quot; rel=&quot;noreferrer&quot;&gt;Lunar Module Coatings Page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A few months before flight, shock tunnel tests using a new thruster&#xA;  duty cycle revealed that the Pyromark painted Inconel lay-ups on the&#xA;  upper sides of the Descent Stage quads would not be sufficient&#xA;  protection against the hot plumes. A crash program to design a fix&#xA;  resulted in &quot;coal chute&quot; plume deflectors mounted below the&#xA;  down-firing jets. These were installed on LM 5 while it was on the&#xA;  pad, just before launch.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Another last minute thermal fix added 39 pounds of Kapton and Pyromark painted Inconel to the landing gear, pads and probe. One&#xA;  of the reasons for this added weight was a crew request(!) that they&#xA;  be allowed to keep the engine on past probe contact to pad touchdown.&#xA;  This would result in greater heating from the engine plume as it&#xA;  reflected off the lunar surface past the gear.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Considering the vast ambition of going to the Moon for the first time, it isn't surprising some fixes were last-minute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The foil is &lt;a href=&quot;https://en.wikipedia.org/wiki/Kapton&quot; rel=&quot;noreferrer&quot;&gt;Kapton&lt;/a&gt; MLI (&lt;a href=&quot;https://space.stackexchange.com/a/5248/4660&quot;&gt;multi-layer insulation&lt;/a&gt;) blankets, and it is actually pretty complex. In the places on the lunar modules that only needed to be a heat barrier to sunlight, high reflectivity was the most efffective approach, and those places are the shiny amber color of the Kapton. As there is no air in space to pass heat by convection, if you lower absorption of heat radiation by making surfaces that are highly reflective or emissive, and there are few contact points to pass heat by conduction, insulation can be highly effective. With the Kapton foil blankets, the contact points were reduced by &lt;em&gt;hand-crinkling&lt;/em&gt; an inner layer of the blanket. From the Apollo News Reference:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To make an even more effective insulation, the polymide sheets are&#xA;  hand crinkled before blanket fabrication. This crinkling provides a&#xA;  path for venting, and minimizes contact conductance between the&#xA;  layers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, this is bound to make the outer layer rather uneven.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the other covering material you see is also just there to protect whatever is underneath from the effects of sunlight. Perhaps they were also thinking a bit about keeping dust out. That is all it has to do, and it was made merely sufficient for that job. &#xA;Weight savings were more important than looks. The fancy stuff is underneath all those bare-bones panels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a different photo of the lander that gives a better sense of the complexity of it. The photo shows the Ascent Stage in the process of assembly, before the heat shielding had been put on it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Icj4.jpg&quot; alt=&quot;Apollo 11 lunar lander&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This photo of an LM test article shows the sturdy underlying aluminum and titanium structure pretty clearly:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/o8hnx.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/o8hnx.jpg&quot; alt=&quot;&amp;quot;naked&amp;quot; LM test article from the Smithsonian Air &amp;amp; Space Museum&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And a quote from the book &lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4205/ch7-3.html&quot; rel=&quot;noreferrer&quot;&gt;Chariots of Apollo&lt;/a&gt; available on the NASA website's History section:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;By the end of 1965, Scrape and SWIP had pruned away 1,100 kilograms,&#xA;  providing a comfortable margin below the control weight limit. One of&#xA;  the more striking changes to come from this drive for a lighter&#xA;  spacecraft was the substitution of aluminum-mylar foil thermal&#xA;  blankets for rigid heatshields. The gold wrapping characteristic of&#xA;  the lander's exterior saved 50 kilograms.&#xA;  Many of these weight-reducing changes made the lander so difficult to&#xA;  fabricate, so fragile and vulnerable to damage, that it demanded great&#xA;  care and skill by assembly and checkout technicians. Structural&#xA;  components took on strange and complex shapes, requiring careful&#xA;  machining to remove any excess metal&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;'Scrape' and 'SWIP' were both programs Grumman, the company that fabricated the Lunar Module, instituted specifically to reduce the weight of the LM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found both things on a great thread on the topic at &lt;a href=&quot;http://cosmoquest.org/forum/showthread.php?65853-Amazing-lunar-module-picture&quot; rel=&quot;noreferrer&quot;&gt;CosmoQuest&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can pore over the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/LM10HandbookVol1.pdf&quot; rel=&quot;noreferrer&quot;&gt;LM Apollo Operations Handbook&lt;/a&gt; for a great deal of technical information on the spacecraft, for more evidence.  &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="195" LastEditDate="2018-06-25T16:00:14.277" LastActivityDate="2018-06-25T16:00:14.277" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="5902" PostTypeId="1" CreationDate="2014-11-13T03:40:27.077" Score="0" ViewCount="291" Body="&lt;p&gt;Couldn't we launch the orion capsule on a launch system that is already developed? It makes no sense tp me that we would spend another half a decade just to build that vehicle. It doesn't make sense , and it's a waste of taxpayer dollars. If we really wanted to go to the moon we could do so in under two years. We have the tech , and we have the ability to manufacture. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think nasa is foolishly wasting money. It's a half hearted attempt and it would've been best if they hadn't made an attempt at all.&lt;/p&gt;&#xA;" OwnerUserId="6554" LastActivityDate="2014-11-13T04:47:03.300" Title="Nasa's Orion program doesn't make sense" Tags="|nasa|orion-spacecraft|" AnswerCount="2" CommentCount="3" ClosedDate="2014-11-13T08:06:50.200" ContentLicense="CC BY-SA 3.0" />
  <row Id="5903" PostTypeId="2" ParentId="5902" CreationDate="2014-11-13T03:51:39.837" Score="1" Body="&lt;p&gt;At some levels, the simple answer is that SLS (Space or Senate Launch System) is a jobs program and not about a space program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does not really make a whole heck of a lot of sense. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the justification for SLS?  Well to launch Orion in Lunar, Mars, or BEO mode, we need a booster bigger than the biggest available (Delta 4-Heavy, until Falcon Heavy is in operation).   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ok, so where is Orion going?  Dunno.  What is the mission?  Not sure.  How often do you plan to launch? Every 2-4 years if we are lucky, since there are no real missions, nor money to fund them, who knows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is somewhat ludicrous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you care about space, follow SpaceX and hope they succeed in reusability in Falcon 9 and Falcon Heavy and watch SLS get cancelled. If they succeed with the BFR and MCT model (Big F-ing Rocket, and Mars Colonial Transport fleet) then watch NASA fade to irrelevance due to mostly political shenanigans.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-13T03:51:39.837" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5905" PostTypeId="2" ParentId="5902" CreationDate="2014-11-13T04:47:03.300" Score="4" Body="&lt;p&gt;Delta IV has the capability to put Orion into LEO, but not to take it to the moon, and that doesn't include the (as yet unbuilt) lunar lander module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not clear to me what you're thinking of when you say &quot;If we really wanted to go to the moon we could do so in under two years. We have the tech , and we have the ability to manufacture.&quot; If you mean Saturn V, then no; we may have the plans on paper but we don't have the tooling or the knowledge of the people who actually built them in the 1960s; a number of industrial procedures that were acceptable in the context of that time probably aren't so today. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX's stuff is definitely exciting and I wouldn't be surprised to see them dominate over the long term, but if your goal is manned landing on the moon, SLS can in principle do it with a single launch, rather than the two or three you'd need with Falcon Heavy.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-11-13T04:47:03.300" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5907" PostTypeId="2" ParentId="5891" CreationDate="2014-11-13T09:39:08.263" Score="21" Body="&lt;p&gt;Philae can last many years (it's meant to survive perihelion pass, it has solar batteries, and it can withstand the long deep-space hibernation (it did it once already). Theoretically it could last as long as Opportunity on Mars.&lt;/p&gt;&#xA;&lt;p&gt;Unfortunately it doesn't have antennas to reach Earth - only Rosetta. It depends on Rosetta to forward data back to Earth. As long as Rosetta is in orbit, and there's enough sunlight for the batteries, Philae can work. Unfortunately  P67/Churyumov–Gerasimenko is as irregular and far from sphere as they get, so Rosetta can't attain a fully stable orbit without periodic corrections using its engines - and its fuel. This is the exhaustible resource that puts an upper limit on duration of the mission.&lt;/p&gt;&#xA;&lt;p&gt;Since the arrival didn't entirely go without a hitch...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Reaction control system problems&lt;/p&gt;&#xA;&lt;p&gt;In 2006, Rosetta suffered a leak in its reaction control system (RCS).[71] The system, which consists of 24 bipropellant 10-newton thrusters,[7] is responsible for fine tuning the trajectory of Rosetta throughout its journey. The RCS will operate at a lower pressure than designed due to the leak. This may cause the propellants to mix incompletely and so burn 'dirtier' and less efficiently, though ESA engineers are confident that they have sufficient fuel reserves to allow successful completion of the mission.[72]&lt;/p&gt;&#xA;&lt;p&gt;-- Wikipedia&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;it's unlikely the reserves will last long beyond the planned end of the mission.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-11-13T09:39:08.263" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5908" PostTypeId="2" ParentId="5896" CreationDate="2014-11-13T10:05:40.743" Score="5" Body="&lt;p&gt;I'm going to interpret your question as &quot;is it feasible to change a large comet's orbit to one we can easily access?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The short answer is no, we cannot with current technology. In order to move something as massive as Chury (100 Billion metric tons) in a controlled way you need a rocket that can produce a great deal of power, and you'd need an enormous amount of fuel to power it. Fuel is extremely expensive to lift to orbit, for every unit of fuel you put into orbit you expend 20 getting it there. Then you need to move all this engine and fuel out of orbit to meet the comet. That would be challenging to say the least, even if it is possible at all. There's simply be no benefit from that approach, even if the entire comet was made of valuable materials. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Realistically it is possible to nudge a comet off its orbit a bit with current technology. It may be necessary to do this if there's a comet or asteroid which could collide with the earth. This could be done by using a nuclear explosion, or series of explosions to heat one side of the comet, causing some of it to ablate with enough force to alter its trajectory. Painting or coating the body is also an idea, as is attaching a solar sail. None of these would be particularly precise, and would certainly not produce enough force to move a comet into an orbit where it could be reached, unless it was almost in that orbit anyway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there's to be any moving of celestial bodies then technology needs to move forward. You need a better engine technology that produces much more power for the amount of fuel consumed, most likely nuclear rocketry. You would also need to develop the ability to make fuel from material found in space. Comets have a significant amount of water on them, if you could manufacture fuel from that water on the surface of the comet itself you would then only need to get your powerful engine and your fuel factory there. It would still take a long time to do it, probably several decades, but once you are in space you need to take the long view. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-11-13T10:05:40.743" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5909" PostTypeId="2" ParentId="5891" CreationDate="2014-11-13T12:17:47.477" Score="6" Body="&lt;p&gt;In addition to what has been stated in other answers, it seems &quot;if everything goes according to plan&quot; is quite a bold assumption as there is serious concern that the lander may be stuck in the shade and/or wedged in a hole. In that case, &lt;a href=&quot;http://www.theguardian.com/science/across-the-universe/live/2014/nov/13/rosetta-mission-philae-lander-live-coverage-comet-esa#block-54649ad5e4b0f33efafdaae5&quot;&gt;battery life is about 60 hours&lt;/a&gt; before the lander must rely on solar power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that with bad luck, it might all be over in a little over two days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As has been said, only time will tell for certain.&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2014-11-13T12:17:47.477" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5910" PostTypeId="1" CreationDate="2014-11-13T12:28:35.287" Score="5" ViewCount="239" Body="&lt;p&gt;How well was the gravitational field of Philae's landing spot known before the landing? I am referring to the absolute value of the pull, the direction of pull, and the tidal forces.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-11-13T19:08:25.590" Title="Was the gravitational pull at Philae's landing site known prior to landing?" Tags="|landing|comet|rosetta|philae|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5911" PostTypeId="1" AcceptedAnswerId="5924" CreationDate="2014-11-13T14:50:34.957" Score="10" ViewCount="813" Body="&lt;p&gt;ESA indicates that the first bounce lasted two hours and reached a height of 1 km.  With the extremely weak surface gravity and low escape velocity of the body (&amp;lt; 1 m/s), and other publicly available information regarding the comet's mass properties and the landing site location, can we estimate how close the lander came to drifting away into space?&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2014-11-13T15:43:57.500" LastActivityDate="2017-06-23T00:28:37.137" Title="How close was Philae to escape velocity during its first bounce?" Tags="|comet|escape-velocity|philae|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="5912" PostTypeId="1" AcceptedAnswerId="5915" CreationDate="2014-11-13T15:20:25.543" Score="15" ViewCount="2128" Body="&lt;p&gt;Specifically in the case of complex mechanical space vehicles (e.g. rovers, landers), do we just refuel and send the battle tested and carefully fine tuned prototype on the mission, or do we take the risk of trying to make a near perfect replica and send it after basic lab testing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the actual Curiosity rove Earth before Mars?&lt;/p&gt;&#xA;" OwnerUserId="6565" LastActivityDate="2014-11-14T15:58:06.030" Title="Do we send the battle tested prototype or the freshly made copy out there?" Tags="|spacecraft|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5913" PostTypeId="1" AcceptedAnswerId="5923" CreationDate="2014-11-13T15:48:11.267" Score="4" ViewCount="287" Body="&lt;p&gt;How does the Curiosity rover analyse Mars' rocks to find signs of life? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it only based on life-supporting chemicals, or is there more involved?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What specific tests are performed?  We recall the &lt;a href=&quot;http://en.wikipedia.org/wiki/Viking_lander_biological_experiments&quot; rel=&quot;nofollow&quot;&gt;Viking labeled release&lt;/a&gt; experiments that caused so much &lt;a href=&quot;http://en.wikipedia.org/wiki/Viking_lander_biological_experiments#Controversy&quot; rel=&quot;nofollow&quot;&gt;controversy&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="4625" LastEditorUserId="1142" LastEditDate="2014-11-13T23:01:21.360" LastActivityDate="2014-11-13T23:01:21.360" Title="How does Curiosity analyse Mars' rocks for finding signs of life?" Tags="|mars|curiosity|astrobiology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5914" PostTypeId="1" AcceptedAnswerId="5957" CreationDate="2014-11-13T15:56:50.797" Score="1" ViewCount="396" Body="&lt;p&gt;When radio signals leave earth, they propagate out in a wave form. But how far they can “spread out” over distance until they become indistinguishable from background noise?&lt;/p&gt;&#xA;" OwnerUserId="4625" LastActivityDate="2014-11-15T15:02:14.080" Title="How far do the omnidirectional Earth transmissions (TV, Radio, etc) travel before they become indistinguishable from background radiation?" Tags="|cosmic-radiation|data-transmission|" AnswerCount="1" CommentCount="6" ClosedDate="2014-11-15T16:43:34.160" ContentLicense="CC BY-SA 3.0" />
  <row Id="5915" PostTypeId="2" ParentId="5912" CreationDate="2014-11-13T16:00:09.853" Score="16" Body="&lt;p&gt;Normally you build 2 prototypes of a spacecraft; one is called the &lt;em&gt;engineering prototype&lt;/em&gt; (or model), and the other one is called the &lt;em&gt;protoflight&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both must be identical, but the engineering model is normally reassembled several times during development and testing, and will stay on earth as a faithful reference of the flight model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The protoflight must be handled with extreme care (to avoid any damaging of connectors, or ageing of mechanical components). It is the protoflight that must undergo all final qualification tests (thermal-vacuum, vibration, acoustic, outgassing).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that for beasts such as telecommunications satellites, there is no budget for building a complete engineering model. These are integrated based on already tested and reliable components (avionics, propulsion, solar panels, transponders, antennas, etc.).&lt;/p&gt;&#xA;" OwnerUserId="1348" LastActivityDate="2014-11-13T16:00:09.853" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5916" PostTypeId="2" ParentId="5896" CreationDate="2014-11-13T16:38:56.950" Score="3" Body="&lt;p&gt;Parking a nice juicy body full of volatile ices in a high lunar orbit is one of my favorite daydreams. Lots of CHON - Carbon, Hydrogen, Oxygen, and Nitrogen. Stuff necessary for life support. Cometary volatiles could also be used to make propellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Departing from earth's surface for any destination beyond earth orbit takes 13 km/s or more. But EML2 has a C3 much closer to zero than earth's surface. Departing for Mars or other destinations from EML2 would be more like 1 km/s. A body of life support consumables and propellent at such a location would be a game changer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But is this daydream plausible? Let's look at some numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth moves at about 30 km/s relative to the sun. Comets falling from the Oort or Kuiper Belt are moving about 42 km/s when they're in our neighborhood. If they happen to be moving the same direction as earth, the relative velocity would be 12 km/s. However it is more likely the comet's orbit would intersect earth's orbit at an angle. In which case the velocity relative to earth would be more than 12 km/s. These are out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's short period comets. A lot of these have aphelions of around 5 astronomical units. When in earth's neighborhood these are moving 38 km/s relative to the sun. Speed relative to earth would be 8 km/s or more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the braking burn were done deep in earth's gravity well, a healthy Oberth benefit could be enjoyed. At a 300 km altitude perigee it'd take a 3 or 4 km/s braking burn to slow the comet into an earth capture orbit. So long as we're indulging in pleasant daydreams, let's imagine we've already established infrastructure on the comet for mining and manufacturing propellent. Exhaust velocity of hydrogen/oxygen bipropellent is about 4.4 km. Propellent mass fraction is $1-e^{-delta V/exhaust velocity}$. Half to 3/5 of the comet's mass would need to be burned within the few minutes the comet spends near perigee. Burning such an enormous amount of reaction mass in a very short time would require implausibly large rocket engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a braking burn so close to earth there is significant risk of impact. A small error could result in a major catastrophe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short period comets are also out.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are bodies with more earth like orbits that could plausibly be parked in our neighborhood. See the &lt;a href=&quot;http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf&quot; rel=&quot;nofollow&quot;&gt;Keck paper on asteroid retrieval&lt;/a&gt;. But these are orbits closer to the sun. Warmer rocks would lose volatile ices by sublimation in which case they are no longer comets. It is possible an accessible asteroid could hold onto some water in the form of hydrated clays. But this form of water would be more difficult to extract than the water ice found in comets. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-11-13T16:38:56.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5918" PostTypeId="2" ParentId="5912" CreationDate="2014-11-13T17:32:23.487" Score="20" Body="&lt;p&gt;Curiosity drove in a clean room on Earth, but it never got dirty until it got to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/brV88.jpg&quot; alt=&quot;Curiosity driving on ramps in clean room&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Different engineering models were used to test those aspects of the rover in the dirty outdoor Mars Yard at JPL.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general the actual flight vehicles are treated somewhat schizophrenically, handled very gently and with great care in clean rooms, then beaten all to hell in vibration and shock testing and frozen and heated in thermal vacuum chambers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally the &quot;battle-tested&quot; engineering model is not &quot;fine tuned&quot;.  It is hay-wired, jury-rigged, and duct-taped as needed to get it to work.  The flight hardware implements changes learned from the engineering model as well as for other reasons, e.g. changes in interfaces, and also it has all flight components where there may have been compromises in the components used in the engineering model.  It is a very rare, perhaps mythical, engineering model that can be flown.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-11-13T19:46:35.693" LastActivityDate="2014-11-13T19:46:35.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5920" PostTypeId="1" AcceptedAnswerId="5926" CreationDate="2014-11-13T17:55:39.357" Score="7" ViewCount="963" Body="&lt;p&gt;If SpaceX starts reusing boosters, it has to be assumed there is an increased risk of catastrophic failure until proved otherwise. If my payload was going to be on top of one of those boosters, some extra safety measures would be very reassuring. How about equipping the final stage bearing the payload with the kind of &lt;a href=&quot;http://www.space.com/26768-spacex-dragon-launch-abort-tests.html&quot;&gt;abort system&lt;/a&gt; the new Dragon capsules will have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much would the payload fairing need to be re-engineered and how much of a hit would the payload capacity take? Since they would be saving on the booster, couldn't the expense be worth it?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-11-14T03:37:02.327" Title="Has SpaceX considered using its abort system for cargo flights too?" Tags="|spacex|safety|reuse|abort|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5921" PostTypeId="2" ParentId="5920" CreationDate="2014-11-13T18:13:42.027" Score="4" Body="&lt;p&gt;There has been little publically said by SpaceX on this topic, but it would make sense to eventually phase out Dragon V1 (Current Dragon) in favour of the basic Dragon V2 design. Musk has said that they will stay with current cargo version for the near future. But that really just seems like it finishes out the CRS-1 contract.  CRS-2 is being negotiated/bid now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After all, why maintain two different product lines, when you can consolidate down to one? The main difference that will need to remain (maybe) is the docking port.  Current cargo uses a CBM port which is bigger/wider than the NDS/LIDS/IDS standard that cargo will have to use.  Of course CRS-2 makes it clear that using the PMA/LIDS/IDS/NDS dock vs berth is an acceptable possible option. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-13T18:13:42.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5922" PostTypeId="2" ParentId="5910" CreationDate="2014-11-13T19:00:50.730" Score="3" Body="&lt;p&gt;Rosetta had earlier this year been doing a triangular pattern of flybys to see how much the comet's mass would deviate it from a straight-line path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/08/21/determining-the-mass-of-comet-67pc-g/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; they had a mass estimate with 10% uncertainty, which tells the overall gravitation to the same uncertainty if 67P is considered as a point mass. They may have refined the mass estimate since then, without Wikipedia keeping up. ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a reasonably good model of the 3-D geometry of the comet body, and assuming relatively homogenous makeup, they could certainly make an estimate of the gravitational force on Philae as it approached the landing site. Both the model and the homogeneity would be subject to some uncertainty as well, though, so -- and this is just me guessing here -- there might be more like 15%-20% uncertainty in the estimate.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-11-13T19:08:25.590" LastActivityDate="2014-11-13T19:08:25.590" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5923" PostTypeId="2" ParentId="5913" CreationDate="2014-11-13T19:23:33.280" Score="3" Body="&lt;p&gt;It doesn't, not directly at least (it doesn't have a single microscope or anything like it), unless something large enough waves back to us and one of its cams catches that memorable moment. But it could, in theory, infer that there is some biology present. For example, it has five spectrometers (Wikipedia says four, but I count five): &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Chemistry_and_Camera_complex&quot; rel=&quot;nofollow&quot;&gt;ChemCam&lt;/a&gt; that includes an optical emissions spectrometer (&lt;a href=&quot;https://en.wikipedia.org/wiki/Laser-induced_breakdown_spectroscopy&quot; rel=&quot;nofollow&quot;&gt;Laser-Induced Breakdown Spectroscopy&lt;/a&gt;, or LIBS) that basically blasts rocks and soil with a laser and a high-resolution telescope camera (Remote Micro Imager or RMI) then spectrally analyzes composition of vapors.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Alpha_particle_X-ray_spectrometer&quot; rel=&quot;nofollow&quot;&gt;APXS&lt;/a&gt; (Alpha Particle X-ray Spectrometer) analyzing chemical composition by irradiating samples and then measuring scattered alpha particles and X-rays. It's part of the science package on MSL's robotic arm.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/CheMin&quot; rel=&quot;nofollow&quot;&gt;CheMin&lt;/a&gt; (Chemistry and Mineralogy) analyzing microcrystalline (read: powdered) mineral samples for structural characterization of materials using X-rays.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Quadrupole_mass_analyzer&quot; rel=&quot;nofollow&quot;&gt;QMS&lt;/a&gt; (Quadrupole Mass Spectrometer), part of its &lt;a href=&quot;https://en.wikipedia.org/wiki/Sample_Analysis_at_Mars&quot; rel=&quot;nofollow&quot;&gt;Sample Analysis at Mars&lt;/a&gt; (SAM), that samples ions by filtering them based on their charge and mass.&lt;/li&gt;&#xA;&lt;li&gt;Tunable Laser Spectrometer (TLS), also part of SAM, analyzing atmosphere of Mars in an otherwise similar fashion to ChemCam's LIBS + RMI.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And it also has a gas chromatograph (also part of SAM) and a long list of other cameras, including one (MAHLI - Mars Hand Lens Imager ) on its robotic arm, on its mast (MastCam), and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it packs plenty of instruments to analyze elemental composition of pulverized or gaseous samples, with contact or remotely (ChemCam), but, for example, if you placed a vial filled with &lt;a href=&quot;https://en.wikipedia.org/wiki/Amoeba&quot; rel=&quot;nofollow&quot;&gt;amoeba&lt;/a&gt; next to it, it couldn't tell you that it has life in it unless you gave it enough time to detect chemical, electrical, or otherwise physical change in metabolites. Assuming the sample was alive, and kept so during the analysis, of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this brings me to the point I'm trying to make; MSL (Mars Science Laboratory, aka the Curiosity rover) isn't equipped to detect life. It's not its purpose, that will be tasks for &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020&quot; rel=&quot;nofollow&quot;&gt;Mars 2020&lt;/a&gt; (NASA) and &lt;a href=&quot;https://en.wikipedia.org/wiki/ExoMars&quot; rel=&quot;nofollow&quot;&gt;ExoMars&lt;/a&gt; (ESA) rovers that are scheduled to launch in 2020 and 2018, respectively. MSL's purpose is to search for conditions for life as we know it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could, theoretically, &lt;em&gt;infer&lt;/em&gt; presence of life by detecting metabolites, or life's past existence, say, by chemically analyzing fossilized sediments, perhaps in clay deposits, but we've had no such luck yet. And if we did, it would most likely not be considered an indisputable proof either, considering that MSL lacks quite a bit of instrumentation to disqualify all other possible rational explanations for its findings.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-11-13T19:28:49.107" LastActivityDate="2014-11-13T19:28:49.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5924" PostTypeId="2" ParentId="5911" CreationDate="2014-11-13T20:11:26.550" Score="3" Body="&lt;p&gt;We can do a very rough back-of-the-envelope approximation, at least. Say 67P is a sphere 1.7 km in radius with mass of 1x10&lt;sup&gt;13&lt;/sup&gt; kg; Wolfram Alpha says surface gravity is ~2.3x10&lt;sup&gt;-4&lt;/sup&gt; m/s&lt;sup&gt;2&lt;/sup&gt; and escape velocity = 0.886 m/s. Gravity 1km above that (at periapsis) would be 9x10&lt;sup&gt;-5&lt;/sup&gt; m/s&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm too lazy to do the calculus of a ballistic trajectory through that gravitational gradient so let's just use the average, 1.6x10&lt;sup&gt;-4&lt;/sup&gt; m/s&lt;sup&gt;2&lt;/sup&gt;. If the first bounce is a 2-hour parabola, then periapsis is at 1 hour (3600s), vertical velocity is 0, and by v = v0 + at, initial velocity was therefore 0.576 m/s; periapsis works out pretty close to 1 km with that average acceleration as well, so that seems sane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Running the same equations for just the small lobe of the comet produces similar results; the lower mass is somewhat cancelled by the lower starting altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this was a fairly close call - Philae may have taken off at about 2/3 of escape velocity!&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-11-14T19:07:10.557" LastActivityDate="2014-11-14T19:07:10.557" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5925" PostTypeId="1" CreationDate="2014-11-13T21:30:17.353" Score="8" ViewCount="317" Body="&lt;p&gt;Why is there no POV video of the Philae lander landing? All I can find are animations made beforehand mixed with commentary and one or two stills.&lt;/p&gt;&#xA;" OwnerUserId="6571" LastEditorUserId="415" LastEditDate="2014-11-15T17:36:54.263" LastActivityDate="2014-11-15T17:36:54.263" Title="Why is there no POV video of the Philae lander landing?" Tags="|technology|philae|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5926" PostTypeId="2" ParentId="5920" CreationDate="2014-11-13T21:47:27.427" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;it has to be assumed there is an increased risk of catastrophic failure until proved otherwise&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I contest this point. What SpaceX is attempting to do is tranform the rocketry model into something more equivalent to the airline model. Various components on aircraft are rated for so many hours of flying, and are inspected at regular intervals. Although rocketry definitely pushes the limits of material science further than aircraft do, I see no reason why it shouldn't be possible to rate certain componentry on a rocket for a number of flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might end up being that a slightly used rocket ends up being more reliable than a brand new, untested one. There's no indication your statement is true. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Onto your actual question. It's simply not worth it - both in terms of cost and performance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon fairing is made of Aluminium Honeycomb attached to a carbon fiber laminate with cork. It weighs a few hundred kilograms. The reason it is so light is to maximize the total payload that can be carried to orbit. It would take major structural modifications for the fairing to handle the g-forces associated with a SuperDraco abort scenario (up to 8G, I believe). This would add many tonnes of weight (not to mention complexity and propellant) to the vehicle, and would significantly reduce the marketable payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going even further, if you were to face a late inflight abort (at stage separation, the Falcon 9  first stage is already traveling at approximately 1.8-2.0km/s, and needs to conduct a reentry burn for it to survive), you'd need heatshielding and protection built in. Which adds further mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, it is unlikely that the satellite would be able to handle the stresses associated with a complicated abort scenario. Some satellites are already built such that they can only handle stresses vertically, and require vertical integration on the pad (this is one of the many reasons the USAF is requiring Falcon 9 be stacked vertically for military and reconnaissance flights). If a satellite can't even handle being horizontal, it's unlikely it could handle the G's pulled in an abort. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A far better model, and the one that SpaceX is pursuing, &lt;strong&gt;is to simply make the vehicle so reliable you need not worry about failure&lt;/strong&gt;. That's SpaceX in a nutshell, really: Keep It Simple Stupid. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2014-11-13T21:57:07.103" LastActivityDate="2014-11-13T21:57:07.103" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="5928" PostTypeId="5" CreationDate="2014-11-13T22:51:16.630" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-11-13T22:51:16.630" LastActivityDate="2014-11-13T22:51:16.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5929" PostTypeId="4" CreationDate="2014-11-13T22:51:16.630" Score="0" Body="Questions more about celestial bodies themselves than exploration of them." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-11-14T09:55:38.090" LastActivityDate="2014-11-14T09:55:38.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5932" PostTypeId="2" ParentId="5925" CreationDate="2014-11-14T09:09:26.940" Score="16" Body="&lt;p&gt;According to &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/11/13/rosetta-operations-update/&quot;&gt;this statement by the ESA&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rosetta is presently sending signals to the ground stations at about 28 Kbps; Ignacio says that the spacecraft's own telemetry downlink uses about 1 or 2 Kbps of this, so the rest is being used to download science data from Rosetta and lander science and telemetry from the surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Given that you have 28 kbit/s for the total spacecraft data downlink, and assuming that the lander sends approximately the same amount of telemetry data as Rosetta, we are talking about approximately 25 kbit/s for science data (which I use here to mean data not related directly to the spacecraft itself or its operations, which may or may not be the same meaning as that used by the ESA).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;25 kbit/s isn't a whole lot at all. If we assume that is the payload transmission rate, we are talking about roughly 3100 &lt;em&gt;bytes&lt;/em&gt; per second. The &lt;em&gt;text&lt;/em&gt; of this answer, not including the links or formatting, is about 3000 bytes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take for example &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/11/12/here-comes-the-lander/&quot;&gt;this photo, taken during descent&lt;/a&gt;. The &quot;high resolution&quot; version posted on the web is 817 x 795 pixels, and it is in grayscale. If we assume one byte per pixel (8 bits shades of grey) and 50% compression (which is high for high-quality photographs, although by no means unattainable) that required downlinking about 320 kilobytes, taking about &lt;em&gt;104 seconds&lt;/em&gt; of payload downlink data capacity to downlink. And that's assuming that the native resolution of the imager wasn't higher and the image was later, &lt;em&gt;on Earth,&lt;/em&gt; downsampled or cropped for posting on the web.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if the lander had sufficient buffer memory to be able to record even, say, one image per minute during the descent and landing, it would still have needed twice as much time to downlink the data to Earth, assuming no other measurements are transmitted during that time. Given that the landing and settling took several hours, and the downlink isn't usable all the time due to occlusion of the Rosetta spacecraft, that would have left us for several hours receiving much less scientific data than we could have, in order to have a video of the descent, most of which would probably have been rather uninteresting to everyone except the most devout followers of the project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably wouldn't have been worth that tradeoff. I certainly wouldn't have made such a tradeoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And increasing the downlink speed isn't necessarily trivial; if increasing the downlink data transmission rate was easy, I can see no real reason why the team would have settled for 28 kbit/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above assumes, too, that we are talking about 28 kbit/s usable &lt;em&gt;after&lt;/em&gt; error correction is accounted for. I have no references to back that up, but I would expect error correction data to correspond to a &lt;em&gt;significant&lt;/em&gt; fraction of the raw downlink data transmission rate. The last thing you want is to send a spacecraft several hundred million kilometers away and then not be able to receive the transmission because it's been garbled beyond the ability of the error correction to recover from.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2014-11-14T17:54:08.093" LastActivityDate="2014-11-14T17:54:08.093" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5934" PostTypeId="2" ParentId="5911" CreationDate="2014-11-14T13:00:01.340" Score="2" Body="&lt;p&gt;The new &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/11/14/three-touchdowns-for-rosettas_lander/&quot; rel=&quot;nofollow noreferrer&quot;&gt;report&lt;/a&gt; says Philae was moving at 38 cm/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using Russell's figure of 0.886 m/s as escape velocity, we have 43% of escape velocity.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="195" LastEditDate="2017-06-23T00:28:37.137" LastActivityDate="2017-06-23T00:28:37.137" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5935" PostTypeId="2" ParentId="3373" CreationDate="2014-11-14T13:30:19.213" Score="12" Body="&lt;p&gt;ESA has a &lt;a href=&quot;http://wsn.spaceflight.esa.int/docs/Factsheets/27%20IBDM.pdf&quot; rel=&quot;noreferrer&quot;&gt;doc&lt;/a&gt; about the design of the docking ports (IDS - Internation Docking Standard, sometimes also called NDS NASA Docking System, sometimes called LIDS - Low Impact Docking System.  Though technically LIDS became NDS which fulfills the IDS standard).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On page 3 it says for the IDS ports, design goal is 30 cycles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the CBM ports, a &lt;a href=&quot;http://archive.org/stream/nasa_techdoc_19920015843/19920015843_djvu.txt&quot; rel=&quot;noreferrer&quot;&gt;NASA Tech Doc&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The 5/8-inch-diameter bolt with .625-18 UNJF rolled thread is made of Inconel 718 (AMS &#xA;  5664) and coated with a dry film lubricant to help prevent galling &#xA;  during the nearly 400 engage/disengage cycles required.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So 30 for the IDS, which Dragon is bringing to the station on Spx-7, and again on Spx-8 (since they will be moving PMA-3 to the Node 2 Zenith, to allow 2 docking ports, and freeing up Node 1 Nadir for the second CBM port).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;400 for the CBM interfaces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This makes sense as the IDS seems to be replaceable, given that it is being 'delivered' so when it gets near end of life, deliver a new one to replace it.  Whereas the CBM ports are pretty much built into the modules, so 400 is a better number.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-14T13:30:19.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5936" PostTypeId="2" ParentId="3294" CreationDate="2014-11-14T13:38:44.177" Score="5" Body="&lt;p&gt;I read in a &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=34093.msg1266163#msg1266163&quot; rel=&quot;noreferrer&quot;&gt;forum post on NASASpaceflight.com&lt;/a&gt; that cargo and visiting vehicles are eating up 27% of overall crew time. That is a surprisingly large number. Infrastructure is hard. We forget all those people behind the scenes filling the shelves in the supermarket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The context where this came up was in relation to the CRS-2 contract, where NASA wants only 4 flights a year with 20,000 kilos of cargo.  I.e. Do not do it in 20 launches, takes too much time to manage all those berthings and unloading events. Alas, there is no source cited there for the 27% number, but this is the best I have been able to find. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-14T13:38:44.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5937" PostTypeId="2" ParentId="5912" CreationDate="2014-11-14T15:30:59.860" Score="3" Body="&lt;p&gt;In the context of &lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_contamination&quot; rel=&quot;nofollow&quot;&gt;Interplanetary contamination&lt;/a&gt;, any craft that leaves earth for an interplanetary mission from most countries falls under the &lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_protection&quot; rel=&quot;nofollow&quot;&gt;Planetary protection&lt;/a&gt; section of the Outer Space treaty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The COSPAR defines categories within mission targets and indicates the precautions to take.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is in category III/IV, and therefore requires the craft to be perfectly clean. The decontamination procedure is described on the planetary protection page. It includes clean room assembly and components sterilization before assembly. I am pretty sure this rules out the possibility for the protoflight to make a test run outside the clean room before takeoff.&lt;/p&gt;&#xA;" OwnerUserId="6378" LastEditorUserId="6378" LastEditDate="2014-11-14T15:58:06.030" LastActivityDate="2014-11-14T15:58:06.030" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5938" PostTypeId="1" CreationDate="2014-11-14T17:41:23.543" Score="8" ViewCount="242" Body="&lt;p&gt;So I'm going through Brian Harvey's &quot;Rebirth of the Russian Space Program&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At one point the book mentions the Russian Satellite control centre at Serphukov-15 was totalled by fire. The consequence being some satellites drifted too far from their orbit before control could be restored. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By default Houston Centre is mission control for the ISS. Yet control may be transferred to Moscow in case Houston centre is not available. How is control transferred? Is it merely a hand-off the way an aircraft is transferred from one tower to another? &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is there legislation making it mandatory for a satellite/launch vehicle owner/provider to have a fully manned back-up control centre? &lt;/li&gt;&#xA;&lt;li&gt;What are the mission control backup options on a satellite/space-craft?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2015-01-14T21:34:35.057" Title="What are the mission control backup options on a satellite/space-craft?" Tags="|law|mission-control|backup|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5939" PostTypeId="1" CreationDate="2014-11-14T18:11:24.243" Score="18" ViewCount="4288" Body="&lt;p&gt;I was looking into whether there would be any way to source propellants from the moon itself in support of a future base and space stations in cis-lunar space. I found two documents exploring how aluminum in particular might work in combination with liquid oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One is by &lt;a href=&quot;http://www.wickmanspacecraft.com/lsp.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wickman Spacecraft &amp;amp; Propulsion&lt;/a&gt;, who say:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An additional option available with aluminum is to suspend the&#xA;  aluminum powder in gelled LOX to form a monopropellant...  As part of&#xA;  our research, we made a small rocket engine fueled by the LOX-aluminum&#xA;  monopropellant... The propellant tank was surrounded by a liquid&#xA;  nitrogen bath to keep the LOX from boiling off. The propellant feed&#xA;  lines ran through a liquid nitrogen bath on their way to the&#xA;  combustion chamber. A piston pushed against the propellant to feed the&#xA;  propellant into the rocket engine chamber.  While the thrust was only&#xA;  about a pound, the engine was started and stopped several times&#xA;  without a flashback of the combustion flame front into the propellant&#xA;  tank.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The other is from a document called the &lt;a href=&quot;http://www.asi.org/adb/06/09/03/02/095/al-o-propellants.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moon Miner's Manifesto&lt;/a&gt;, hosted on the website of the &lt;a href=&quot;http://www.asi.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Artemis Society International&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Aluminum and oxygen alone will provide a specific impulse somewhat&#xA;  lower than most hydrocarbons. Brower et al. expect a value of 285&#xA;  seconds... One [engine design] would be to pump aluminum powder as we&#xA;  do fluids. In this case, it will probably be necessary to use a&#xA;  carrier gas along with the powder to keep the aluminum grains from&#xA;  vacuum welding or sticking together from electrostatic forces... Another&#xA;  technique is a hybrid rocket engine using solid aluminum and liquid&#xA;  oxygen. A conceptual design for such an engine was proposed by Brower&#xA;  et al. Their design calls for a hexagonal array of aluminum bars the&#xA;  length of the combustion chamber. Liquid oxygen would be fed down the&#xA;  bars for regenerative cooling before reaching the flame at the bar&#xA;  tips. The engine could use oxygen and aluminum only, or could use&#xA;  tripropellant operation with hydrogen.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Brower et al. paper was presented at the 26th Joint Propulsion Conference in 1990 and is in the catalogue of the AIAA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously this needs a great deal of development, but at first blush it seems like it could have advantages over the usual proposal at this point - acquiring water ice from &lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid_mining&quot; rel=&quot;nofollow noreferrer&quot;&gt;asteroids&lt;/a&gt;, or from permanently shaded craters at the &lt;a href=&quot;http://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;moon's poles&lt;/a&gt; (if present in suitable quantities and concentrations), and splitting it into hydrogen and oxygen for use in H&lt;sub&gt;2&lt;/sub&gt;/LOX engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lunar highlands are composed largely of &lt;a href=&quot;http://www.permanent.com/lunar-geology-minerals.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;anorthite&lt;/a&gt; (CaAl&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;8&lt;/sub&gt;), a mineral that is 15% aluminum and 62% oxygen. Regions that are 80% anorthite or more seem to be common. Extraction of oxygen has been discussed in a &lt;a href=&quot;http://www.uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources08.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;number&lt;/a&gt; of &lt;a href=&quot;http://www.lpi.usra.edu/meetings/leagilewg2008/pdf/4084.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;papers&lt;/a&gt;, it requires heating regolith above 1100 Celsius so that the mineral oxides dissociate and the oxygen can be collected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: I &lt;a href=&quot;https://chemistry.stackexchange.com/questions/19687/what-possibilities-are-there-for-refining-aluminium-on-the-moon&quot;&gt;asked&lt;/a&gt; about this on the SE Chemistry community and got an answer from Jon Custer:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Fine powder feedstock... into an arc discharge to vaporize and&#xA;  ionize... electrostatic acceleration, then electromagnets to mass&#xA;  separate. Power with a solar panel array. No moving parts (except for&#xA;  the powder feed). No vacuum systems needed. And you separate all of&#xA;  the elements at once.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It has been pointed out to me by TildalWave that this would require a great deal of electricity and coolant. It occurred to me that perhaps the coolant could be avoided by putting the whole separator mechanism on rails, on a long shaded track running beside your solar panel field. If you set up the separator to dump the metal and slag products directly onto the ground under the track as it moves along, perhaps it is just a matter of figuring out how to get the flow and the rate at which the separator moves right, and the cooling could happen passively. There would be linear trails of metals and slag left on the ground which could slowly radiate away their heat until they are cool enough to be collected. There are probably reasons that wouldn't work, but I need to be told what they are. The oxygen that off-gasses could be collected from within the open-bottom chamber where the vaporized streams are ejected by the separator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could some process along these lines be competitive with asteroid mining? What are the issues and hurdles?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:57:06.083" LastActivityDate="2015-07-24T21:38:51.780" Title="How feasible is it to use aluminum and liquid oxygen as future propellant sourced from the moon?" Tags="|the-moon|colonization|mining|fuel|isru|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5940" PostTypeId="1" AcceptedAnswerId="5941" CreationDate="2014-11-14T19:33:29.830" Score="8" ViewCount="176" Body="&lt;p&gt;The determination of the mass of a celestial object is usually between hard and impossible unless one can observe something that orbits it. In the case of comet 67P/Churyumov–Gerasimenko, &lt;a href=&quot;https://space.stackexchange.com/questions/5117/is-this-really-rosettas-orbit-around-67p&quot;&gt;one of Rosetta's first tasks was to determine the comet's mass and gravitational field&lt;/a&gt;. However, in planning the mission, presumably some knowledge of this mass would have been necessary to plan things like Philae's descent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much did we know about this prior to launch? How was this knowledge obtained? Did it come from brightness or size measurements from imagery? If so, how reliable are these known to be?&lt;/p&gt;&#xA;" OwnerUserId="653" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-11-14T20:31:23.787" Title="What was known about 67P/C-G's mass before the arrival of Rosetta?" Tags="|comet|rosetta|mass|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5941" PostTypeId="2" ParentId="5940" CreationDate="2014-11-14T20:31:23.787" Score="8" Body="&lt;p&gt;Indeed it's extremely difficult to accurately determine the mass of a comet without flying something past it at close range.  Even if you assume an average density (which is not particularly safe, as little is yet known about the internal makeup of comets and how that varies from body to body), no earthbound telescopes (including Hubble) were able to resolve the nucleus before the mission to give an accurate volume estimate.  Researchers had to go by the magnitude and an assumed albedo, which is similarly poorly-characterized across the comet population.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One pre-encounter estimate by &lt;a href=&quot;http://www.lpi.usra.edu/meetings/acm2012/pdf/6282.pdf&quot;&gt;Maquet et al&lt;/a&gt; in 2012 gave $3.14 \pm 0.21 \times 10^{12}$ kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/08/21/determining-the-mass-of-comet-67pc-g/&quot;&gt;most recent publicly-available estimate&lt;/a&gt; using Rosetta orbital data is roughly three times as great: $1 \pm 0.1 \times 10^{13}$ kg.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-11-14T20:31:23.787" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5942" PostTypeId="2" ParentId="5911" CreationDate="2014-11-15T02:06:06.557" Score="3" Body="&lt;p&gt;It was never close.  Rosetta did not have escape velocity when it ejected Philae.  So as long as Philae was ejected in the retrograde direction, Philae would not have escape velocity either.  After the bounce, Philae would have even less energy due to landing gear attenuation and would therefore be even farther from escaping than it was pre-bounce.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2014-11-15T02:06:06.557" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5943" PostTypeId="1" AcceptedAnswerId="5948" CreationDate="2014-11-15T05:08:26.897" Score="11" ViewCount="10485" Body="&lt;p&gt;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0387713549&quot;&gt;This book&lt;/a&gt; writes to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In 1981 the USSR requested and was allocated seven satellite slots in 24-hr orbit for Earth observations, calling the system Prognoz&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Whom was the request made to? &lt;/li&gt;&#xA;&lt;li&gt;Who allocates/allocated satellite slots in Earth orbit? &lt;/li&gt;&#xA;&lt;li&gt;Is there an international body to assign/refuse orbit parameters for a space-craft?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2023-05-30T11:33:39.170" Title="Who/What is the international authority to assign/refuse orbit to a satellite/space-craft?" Tags="|artificial-satellite|orbit|regulatory|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5944" PostTypeId="1" AcceptedAnswerId="5946" CreationDate="2014-11-15T06:01:20.743" Score="2" ViewCount="833" Body="&lt;p&gt;I am reading this &lt;a href=&quot;http://www.ndtv.com/article/world/comet-probe-philae-uploads-last-minute-data-from-alien-world-621193&quot; rel=&quot;nofollow&quot;&gt;news article&lt;/a&gt; about the Rosetta mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In one paragraph it says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Philae's data collection from a comet travelling at 18 kilometres (11 miles) per second, currently at a distance of 510 million kilometres (320 million miles) from Earth, crowns a 10-year mission to study the origins of Solar System some 4.6 billion years ago, and maybe even life on Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This means that the comet is about 510 million kilometers distant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in the another paragraph it says like this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Philae landed Wednesday after a nail-biting seven-hour, 20-km descent from Rosetta, which had travelled more than a decade and 6.5 billion kilometres (four billion miles) to meet up with comet 67P/Churyumov-Gerasimenko in August this year&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So my question is why did Rosetta traveled this long distance of 6.5 billion kilometers?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="415" LastEditDate="2016-10-05T14:27:52.850" LastActivityDate="2016-10-05T17:21:39.693" Title="Why did Rosetta travel 6.5 billion km to reach a comet that was only 510 million km from Earth?" Tags="|orbital-mechanics|rosetta|philae|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5945" PostTypeId="1" CreationDate="2014-11-15T06:25:51.923" Score="6" ViewCount="1708" Body="&lt;p&gt;Rocket Lab, a company from New Zealand is working on a small rocket (18 m tall, 10 tonnes), for which there are already over 30 launches lined up at US $5M each. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_Lab#Smallsat_launch_vehicle&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In July 2014 it was announced that the company is developing a rocket&#xA;  called Electron, a carbon composite rocket with a payload to orbit of&#xA;  110 kilograms (240 lb) and a projected cost of less than US$5 million&#xA;  per launch. As of July 2014, the first test launch is planned for&#xA;  2015.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The whole purpose behind this rocket is to be able to place satellites into orbit far more quickly and for a fraction of the cost of conventional rockets. However, is 110 kg to LEO really a beneficial capability?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would have thought that most satellites would have a far greater mass than that and that any satellite small enough would have very limited functionality.Or will it most likely be used for very small scale projects such as scientific experiments?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a side question, what is a rocket like this likely to be fuelled by?&lt;/p&gt;&#xA;" OwnerUserId="6254" LastEditorUserId="49" LastEditDate="2014-11-15T10:57:26.150" LastActivityDate="2019-07-02T01:48:51.070" Title="Will the Rocket Lab's Electron rocket really be an effective launch system?" Tags="|rockets|artificial-satellite|low-earth-orbit|fuel|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5946" PostTypeId="2" ParentId="5944" CreationDate="2014-11-15T06:46:49.480" Score="13" Body="&lt;p&gt;It did not travel in a straight line.  It passed 4 planets for gravity assist&lt;sup&gt;1&lt;/sup&gt; in order to gain the velocity and change to the trajectory&lt;sup&gt;2&lt;/sup&gt; required to meet the comet.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;See &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist#Rosetta_.E2.80.93_first_spacecraft_to_match_orbit_with_a_comet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravity Assist: Rosetta – Rosetta – first spacecraft to match orbit with a comet&lt;/a&gt; for details.&lt;/li&gt;&#xA;&lt;li&gt;The comet is currently approaching the Sun, but further from the Sun than Earth. So effectively the vehicle needed not only to gain speed but flip the direction of travel almost opposite to where it was originally heading.&lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;http://www.livecometdata.com/rosetta-spacecraft-mission/rosetta-3d-model/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta Interactive 3D model&lt;/a&gt; shows the route it took - very impressive considering all the gravity assists required incredibly accurate calculations&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Can you please add more information saying why it did not travel in a straight line,why it has to pass 4 planets etc?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Explanation courtesy of &lt;a href=&quot;https://space.stackexchange.com/users/6583/theblastone&quot;&gt;TheBlastOne&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If it travelled in a straight line to the comet, it would need to reach a speed (relative to earth) that no launch vehicle can deliver. (Or at least: no launch vehicle could deliver the required acceleration at the time of Rosetta's launch; Ariane was the most powerful launcher available at that time.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="238" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-10-05T17:21:39.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5947" PostTypeId="1" CreationDate="2014-11-15T07:04:09.010" Score="19" ViewCount="10048" Body="&lt;p&gt;&lt;a href=&quot;http://en.memory-alpha.org/wiki/The_Naked_Now_(episode)&quot; rel=&quot;noreferrer&quot;&gt;In a somewhat famous &lt;em&gt;Star Trek: The Next Generation&lt;/em&gt;&lt;/a&gt; quote Data corrects Riker on the 'correct term' for a scenario where a hatch on a space capsule breaks open and all the crew are killed.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Riker:&lt;/strong&gt; &amp;quot;You were right. Somebody blew out the hatch. They were all sucked out into space.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Data:&lt;/strong&gt; &amp;quot;Correction, sir, that's blown out.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Riker:&lt;/strong&gt; &amp;quot;Thank you, Data.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Data:&lt;/strong&gt; &amp;quot;A common mistake, sir.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Is there a technical or scientific and objective correct answer? Was Data, from a technical perspective, correct in his assertion or are both statements equally valid?&lt;/p&gt;&#xA;" OwnerDisplayName="user6587" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-05-08T02:30:45.447" Title="Is the air &quot;sucked out&quot; or &quot;blown out&quot; into space when the hatch opens?" Tags="|terminology|" AnswerCount="9" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="5948" PostTypeId="2" ParentId="5943" CreationDate="2014-11-15T07:25:20.673" Score="14" Body="&lt;p&gt;Geostationary slots are a rare resource and are assigned to countries by the &lt;a href=&quot;http://www.itu.int&quot;&gt;ITU&lt;/a&gt; (International Telecommunications Union) since they are mainly used for communications and broadcasting, and need deconfliction both in frequency allocation (to reduce interference - the main raison d'être of ITU-R as a division of the ITU) and in collision avoidance (tighter orbital boxes are costly in terms of lifetime xenon expenditure).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Almost everything else in orbit selection is based on the operator's discretion, provided the downlink and uplink frequencies do not conflict with other satellite, fixed and mobile services.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbits are checked through national space control centers, though, to avoid crossing paths with other satellites and space debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information on Geostationary services, please have a look at &lt;a href=&quot;http://www.itu.int/ITU-R/space/snl/&quot;&gt;http://www.itu.int/ITU-R/space/snl/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: On sanctions and reprisals: violating an international treaty is bad since the culprits will find themselves shunned. States do have the ability and legal authority to jam or disrupt non-authorized transmitters that broadcast into their territories, so it shouldn't be surprising to the perpetrator to find his downlinks or uplinks jammed. It should be noted that GSO is far away from the Earth, and the link budgets are quite slim, so any modest interference will degrade the link beyond usefulness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TL;DR - A non-state actor (a corporation) will be held liable by its national telecommunications (and space) authorities, while a non-compliant state will be unable to effectively use the slot in GEO due to easy counter-measures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The usual disclaimer: I am not a space lawyer...&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-11-15T12:21:10.837" LastActivityDate="2014-11-15T12:21:10.837" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5949" PostTypeId="2" ParentId="5947" CreationDate="2014-11-15T09:39:48.383" Score="3" Body="&lt;p&gt;Could it be the difference about on whom the vacuum acts?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the hatch is opened, the vacuum's zero pressure causes all the air to rush out. However, the people, the hatch or anything else are blown out by the air which is rushing out to fill the vacuum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because without the air inside, there wouldn't be a &quot;blow out&quot;... (like when astronauts go out for space walk, they open the hatch after decompressing the chamber)&lt;/p&gt;&#xA;" OwnerUserId="6588" LastActivityDate="2014-11-15T09:39:48.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5950" PostTypeId="2" ParentId="5947" CreationDate="2014-11-15T09:41:44.463" Score="18" Body="&lt;h2&gt;Edit:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Thinking about this some more, the answer is a result of physics: gases exert pressure (due to molecules bumping into each other). This is what causes a gas to expand to occupy all available volume. The only force at work is a repellent one, so the answer must be &quot;blown out&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In everyday language, there are some exceptions. &#xA;We say a vacuum cleaner sucks. Physically, it pumps air from the vacuum chamber to the outside, lowering the pressure in the vacuum chamber and enabling air to rush in through the nozzle. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-04-21T07:52:46.013" LastActivityDate="2018-04-21T07:52:46.013" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="5951" PostTypeId="1" AcceptedAnswerId="5956" CreationDate="2014-11-15T11:13:31.273" Score="5" ViewCount="229" Body="&lt;p&gt;While listening to the latest updates it seems like there is a limited 60 hour battery capacity left for Philae. There seems to be some information on the web on &lt;a href=&quot;http://en.wikipedia.org/wiki/Wireless_power#Laser_method&quot; rel=&quot;noreferrer&quot;&gt;wireless power transfer using lasers&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Wikipedia: ...thereby delivering almost all emitted power at long ranges.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Since Rosetta is still in sampling sight of solar power, could it possibly transfer this power using the wireless method to Philae, or would the loss in transfer be too severe for any kind of practical solution? Seems like this could be an option for future mission to explore locations while blind to sun light.&lt;/p&gt;&#xA;" OwnerUserId="6578" LastActivityDate="2023-12-14T17:42:41.200" Title="Is it feasible for a future Rosetta type mission to use wireless transfer of power to Philae?" Tags="|power|rosetta|solar-power|philae|laser|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5952" PostTypeId="2" ParentId="5945" CreationDate="2014-11-15T11:35:06.310" Score="9" Body="&lt;p&gt;It pays to follow up on references that the Wikipedia you quote quotes itself. For example, &lt;a href=&quot;http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&amp;amp;objectid=11300831&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; in The New Zealand Herald lists some of the characteristics of the Electron rocket:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;18 m in length&lt;/li&gt;&#xA;  &lt;li&gt;1 m in diameter&lt;/li&gt;&#xA;  &lt;li&gt;Weighs more than 10 tonnes&lt;/li&gt;&#xA;  &lt;li&gt;Lift off mass 10,500 kg&lt;/li&gt;&#xA;  &lt;li&gt;Liquid oxygen and kerosene propellants&lt;/li&gt;&#xA;  &lt;li&gt;Top speed of 27,500 km/h&lt;/li&gt;&#xA;  &lt;li&gt;Payload of 110 kg&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Similar article, perhaps even slightly more detailed, appeared in &lt;a href=&quot;http://www.parabolicarc.com/2014/07/29/rocket-lab-announces-small-satellite-launcher/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Parabolic Arc&lt;/a&gt;. Both include a &lt;a href=&quot;http://vimeo.com/101964464&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; in which Peter Beck, CEO and founder of Rocket Lab Ltd. describes its application.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, some of what he talks about is of course marketing. At quoted NZ\$5.84 million (currently US\$4.62 million) per launch, and payload of 110 kg to LEO, this puts its cost per kilogram to LEO at nearly exactly US\$42,000 / kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's &lt;a href=&quot;https://space.stackexchange.com/questions/1989/what-is-the-current-cost-per-pound-to-send-something-into-leo&quot;&gt;10 times as much&lt;/a&gt; as on a Falcon 9 v 1.1, but you get to select your target orbit independently of other, primary payloads on a higher capacity launcher where your satellite might be a second class citizen. So there ought to be market for it, and if they indeed already have over 30 orders lined up, then they obviously made a compelling case for their Electron rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What use could a 110 kg payload be in LEO? Well, that can launch you up to roughly 80 1U &lt;a href=&quot;https://en.wikipedia.org/wiki/CubeSat&quot; rel=&quot;nofollow noreferrer&quot;&gt;CubeSat satellites&lt;/a&gt; on a single launcher, provided they'll all be happy in an identical target orbit and don't require too accurate deployment (I guess a simple lightweight piston would do to push them out of payload fairing all at once or in steps / with a timed delay, or perhaps spinning the upper stage and then slowly &quot;unzipping&quot; its payload fairing, there's many possible engineering options here to still keep its weight down and deploy multiple passengers with some required separation). With selection of higher LEO orbits at different inclinations, and/or more elaborate and precise deployment system that can fast reduce effective payload mass to a dozen of such nano-satellites, provided the launcher even is capable of more complex flight profiles required to attain them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LOX/Kerosene propellants likely means its second stage engine could be restarted, so it can benefit from, say, coasting phase (attains altitude at some fraction of required orbital speed, cuts main engine off, aka MECO, and then &lt;em&gt;coasts&lt;/em&gt; to near orbital velocity with the help of good old Newton before restaring the engine for the final orbital insertion burn).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, Parabolic Arc includes more detail:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Propellant mass: 9,200 kg&lt;/li&gt;&#xA;  &lt;li&gt;Maximum engine thrust : 146,000 N (14.8 tonnes)&lt;/li&gt;&#xA;  &lt;li&gt;Engine equivalent power: 530,000 hp&lt;/li&gt;&#xA;  &lt;li&gt;Nominal orbit: 500 km circular sun synchronous&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That it quotes a circular target orbit confirms that the second stage engine is restartable, since you require an additional burn to circularize the orbit after reaching its target apogee (but the rocket might fly higher than that before MECO - Main Engine Cut-Off - and starting the coasting phase).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if it could launch in slightly higher LEO orbits where satellite orbits aren't expected to decay as fast due to still some atmospheric drag (that decreases with orbital altitude), I guess it could launch a dozen or so small satellites in a satellite constellation and they could be used for communications, or deliver scientific experiments that would usually be done in low orbits (remote sensing, aka Earth observation, ionosphere experiments, microgravity experiments,...), or technological demonstrators (say, various drag augmentation devices or soft intercepts to deorbit orbital debris, aka space junk, in LEO faster). Your pick. The sky, erm, the low Earth orbit is the limit. 110 kg at the time (and that probably doesn't include the weight of the deployment mechanism).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;A few comments. What I find interesting about Electron is that it's extremely light for a two stage system (video shows a short second, upper stage and quoted capability requires at a minimum of two stages). Its total dry mass is only 1,190 kg (without the payload mass of 110 kg) which is nothing short of amazing for an all liquid propellants rocket (and LOX is cryogenic) with all its turbo pumps feeding it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's probably not very likely that it would set a new record for the highest engine thrust-to-weight ratio, currently held by Falcon's Merlin 1D rocket engine at T/W of 159.9 (440 kg in mass and thrust of 690 kN). If Electron was to beat that, its quoted 146 kN first stage engine shouldn't weigh more than &lt;span class=&quot;math-container&quot;&gt;$146,000\ \mathrm{kg\ m/s^2} / (159.9 \times 9.807\ \mathrm{m/s^2}) = 93.1\ \mathrm{kg}$&lt;/span&gt;. But at such small dry mass of the complete launcher, it can't be many times over that weight either. We shall see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One other interesting aspect of the launcher is however that, since it's an all liquid propellants turbo fed engine, it doesn't suffer as severe vibration during launch as solid or hybrid engine designs might, where essentially the whole stage (or in case of hybrids the solid part of it) basically acts as a flute when its solid grain pathways expand. This might be useful to more vibration sensitive payloads, or simply improve reliability of the launcher itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And also, as Deer Hunter comments, the launcher might be interesting simply because of its origins and in which country its provider is registered. It's usually only possible to launch military or any other classified payloads atop domestically built and operated launch systems to control access to classified information, conceal the purpose of your payload and/or to abide to defense-related articles, cryptography,... export regulations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exact use will also depend on launcher's proven reliability and ability of its upper stage to complete precision orbital insertions. So, with no actual flights, this remains yet to be established.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="195" LastEditDate="2019-07-02T01:48:51.070" LastActivityDate="2019-07-02T01:48:51.070" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="5953" PostTypeId="2" ParentId="5945" CreationDate="2014-11-15T11:35:30.853" Score="3" Body="&lt;p&gt;There are a fair number of satellites under 110 kg, however, the cost of launching them at a value so high will likely dissuade potential buyers. From &lt;a href=&quot;http://www.ucsusa.org/nuclear_weapons_and_global_security/solutions/space-weapons/ucs-satellite-database.html#.VGc5d8ka9u4&quot; rel=&quot;nofollow&quot;&gt;this database&lt;/a&gt;, I count 200 satellites that have been launched with a launch mass of less than or equal to 110 kgs. I can't find any that spend that much cash to launch, although there are some who might have.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-11-15T11:35:30.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5954" PostTypeId="1" AcceptedAnswerId="5955" CreationDate="2014-11-15T11:45:54.380" Score="7" ViewCount="534" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Pkl90.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All I could think about while looking at the designs for Philae (besides how courageous and ambitious the mission was) was that Philae itself looks like it was designed for landing on a known surface. We know from the press hearings that they landing site and comet shape were not known before the mission, or better put one of the goals of the mission itself was determining this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess this begs the question, why was Philae's design in shape and positioning format so determined that it can not adapt to a variety of surface scenarios?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My mind is full of ideas but especially two robots I once observed on YouTube inspire some design thought.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=HuC6q9kbryw&quot; rel=&quot;nofollow noreferrer&quot;&gt;MorpHex&lt;/a&gt; - A robot that is omnidirectional  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/results?search_query=cubli&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cubli&lt;/a&gt; - A robot that can do delicate balancing movements&lt;/p&gt;&#xA;" OwnerUserId="6578" LastEditorUserId="415" LastEditDate="2014-11-15T17:33:00.233" LastActivityDate="2014-11-17T06:03:40.383" Title="Could an upcoming comet lander be designed to cope with a greater variety of terrain types, compared to Philae?" Tags="|rosetta|design|philae|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5955" PostTypeId="2" ParentId="5954" CreationDate="2014-11-15T12:02:04.547" Score="9" Body="&lt;p&gt;Remember that Philae was designed in the 1990's. MorphHex is at the edge of what technology of today can accomplish, it wasn't possible 20 years ago when Philae was designed. Most of all because adaptability would have to be done autonomously because of the 28 minutes time delay. The harpoons were supposed to fire once two legs had hit the ground. But it could've bounced off already when the first leg touched down. In retrospect that engineering detail is easy to criticize and a bit more AI reactivity on board could've helped. But then they would've sacrificed some of the instrument payload, and taking the chance but still not being able to drill, for example, is better than not even having a drill on board to begin with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next comet lander will be quite different because of technological developments, and the experiences from Philae, and new evolved science targets. Not because Philae was poorly designed in its time.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="415" LastEditDate="2014-11-15T17:31:20.563" LastActivityDate="2014-11-15T17:31:20.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5956" PostTypeId="2" ParentId="5951" CreationDate="2014-11-15T14:52:39.053" Score="4" Body="&lt;p&gt;This is indeed possible. The US spent a fair amount of money in the 60s and 70s on beamed power. In recent years it has gotten some more interest for lunar rovers that can survive the 14 day lunar night (without RTGs). The losses are not negligible, and the pointing requirements are rather extreme but in practice there's no reason why this couldn't happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Losses: Initially the losses are driven by the solar panels on the satellite, if we assume they are 25% efficient then you've already lost a big lump of your 1300W/m2 (at 1 AU). Then you have the internal storage and transfer loses, maybe a few more percent ~2%. Then the beaming losses, creating the beam might be as inefficient as 10% or 20%, lets be optimistic and say 20%. There are potential losses based on the footprint size of the beam, but we'll (very optimistically) assume we can point just right. The receiver at the lander might manage another 25% efficiency, plus a few extra % for inboard storage etc ~ 2%. Total power efficiency: 0.25*0.97*0.20*0.25*0.98 = 0.012 = 1.2%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So about 1% of the power you get in orbit will make it to the lander. Of course all of these efficiencies can be improved upon, but it gives you an idea of the sort of problems. For 100W on the surface you would need 100W/0.012/1300W/m2 = 6.4m2 of solar panels in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once I get on my laptop (and off my mobile) I'll pepper this with links and correct typos.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="4660" LastEditDate="2016-01-07T16:59:36.033" LastActivityDate="2016-01-07T16:59:36.033" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5957" PostTypeId="2" ParentId="5914" CreationDate="2014-11-15T15:02:14.080" Score="2" Body="&lt;p&gt;There's a lot of factors in this question, but it really comes down to a link budget. With a fixed power, gain (gain = 1 for omnidirectional antennas), wavelength etc. it really just comes down to running the numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real equation of note here is the free space path loss equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.m.wikipedia.org/wiki/Free-space_path_loss#Free-space_path_loss_formula&quot; rel=&quot;nofollow&quot;&gt;http://en.m.wikipedia.org/wiki/Free-space_path_loss#Free-space_path_loss_formula&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That equation will give the the loss at a given distance. You can use the temperature of the background radiation in you link budget, and quickly come up with a distance at which the signal strength is less than the noise. Although it's typical when designing communications system to require a S/N ration of greater than 3, so that might be a more practical target.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-11-15T15:02:14.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5958" PostTypeId="1" CreationDate="2014-11-15T16:13:15.583" Score="3" ViewCount="390" Body="&lt;p&gt;Spacecraft/Satellites constitute the pay-load of a launch vehicle.&#xA;Deployment/injection into orbit is a task that &lt;em&gt;probably&lt;/em&gt; needs to be performed jointly by the pay-load, and the launch vehicle. Off the top of my heads, say, stuff like any preliminary systems check/activation that may need to be performed outside atmosphere before the pay-load is separated from the launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is there any sort of marriage between pay-load systems (electronic and otherwise) &amp;amp; the systems aboard the launch vehicle?  &lt;/li&gt;&#xA;&lt;li&gt;Can all satellites/spacecraft be used interchangeably with all launch vehicles?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-11-18T17:12:40.077" Title="Can all satellites/spacecraft be used interchangeably with all launch vehicles?" Tags="|launch|artificial-satellite|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5959" PostTypeId="2" ParentId="5954" CreationDate="2014-11-15T17:31:10.183" Score="5" Body="&lt;p&gt;The next comet lander will definitely be better designed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before the Rosetta mission, our knowledge about comets was very limited. When Philae was designed, the engineering team had no idea what they would encounter. No probe ever got closer to the nucleus of a comet than a few 100 km (The &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Impact_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Deep Impact&lt;/a&gt; craft crashed its impactor into comet Temple-1 &lt;em&gt;after&lt;/em&gt; the launch of Rosetta), so any theories about the properties of the surface were just speculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The data from the Rosetta mission, especially those from Philae which were collected despite the problems, will likely tell us much about how the surface looks and thus allow the team to design a more reliable anchoring mechanism.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, Philae didn't work completely as designed. The probe had an upwards facing thruster which was supposed to press it against the comet during the anchoring. This thruster didn't work for some reason which still needs to be figured out. Now the engineering team needs to find out if that thruster would have made the landing successful and depending on the result either remove it from the next design or prevent it from malfunctioning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might have some ideas how a lander for unknown terrain could be designed, but keep in mind that space missions have some additional requirements for equipment which can make a lot of designs infeasible:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The lander must be light: Any additional gram of mass makes the mission more expensive&lt;/li&gt;&#xA;&lt;li&gt;The lander must still be sturdy enough to survive acceleration during the launch&lt;/li&gt;&#xA;&lt;li&gt;The lander must survive years in a vacuum and high-radiation environment. The latter is a problem for off-the-shelf computer components. As a result a space probe often has a lot less processing power than you would expect from an earth robot. &lt;a href=&quot;http://www.cpushack.com/2014/11/12/here-comes-philae-powered-by-an-rtx2010/&quot; rel=&quot;nofollow&quot;&gt;The CPU on Philae runs at just 8 MHz!&lt;/a&gt;. That's likely much too slow for the real-time calculations the Cubli robot has to perform to balance on a tip.&lt;/li&gt;&#xA;&lt;li&gt;The design must be as simple as possible to avoid potential malfunctions of single components to ruin the whole mission. Looking at the MorphHex robot, it must have dozens of servomotors. Just one of them failing could make the whole robot inoperable.&lt;/li&gt;&#xA;&lt;li&gt;It must not use much energy. Electricity is a very limited resource on space probes.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-11-15T18:13:58.863" LastActivityDate="2014-11-15T18:13:58.863" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5960" PostTypeId="2" ParentId="5954" CreationDate="2014-11-15T17:38:58.733" Score="4" Body="&lt;p&gt;Philae is most certainly designed to &lt;strong&gt;land&lt;/strong&gt; on an unknown surface. It is not designed to &lt;strong&gt;be mobile&lt;/strong&gt; on such surfaces, like the robots in the OP's video are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that Philae has three legs and an extremely low center of gravity. It's tipping angle is probably above 45 degrees, and certainly far above 30 degrees. Three legs is the most stable and self-adapting configuration known. This is important for several reasons, most of which are related to power and reliability. Having three long legs means that &lt;strong&gt;active adaptation is not necessary&lt;/strong&gt;, saving the weight and complexity of actuators and feedback mechanisms such as sensors and computers. In addition to needing power, such systems may be susceptible to failure. Even the simple nitrogen purge thruster (i.e. cold gas thruster) failed, and that was little more than a needle and a diaphragm.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-11-15T17:38:58.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5961" PostTypeId="1" CreationDate="2014-11-15T18:03:25.753" Score="4" ViewCount="335" Body="&lt;p&gt;How much of the actual launch pad survives the heat and pressure of launch and is capable of being reusd?&lt;/p&gt;&#xA;" OwnerUserId="6594" LastActivityDate="2015-03-04T13:15:36.920" Title="What survives the actual launch of a rocket" Tags="|launch|reuse|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5964" PostTypeId="1" CreationDate="2014-11-16T10:52:44.377" Score="19" ViewCount="5611" Body="&lt;p&gt;The different parts of the Soyuz rocket have different colors:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;White&lt;/li&gt;&#xA;&lt;li&gt;Dark gray&lt;/li&gt;&#xA;&lt;li&gt;Orange&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4G4fd.jpg&quot; alt=&quot;Soyuz rocket with different colors&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Progress rockets I saw have different &quot;liveries&quot;, multicolored like this&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xTQDE.jpg&quot; alt=&quot;Multicolored Progress rocket&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and white like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qX3Ss.jpg&quot; alt=&quot;White progress&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose they are painted like that (sometimes multicolored, sometimes white) for a reason. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is/are these reason/s?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have several hypotheses:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Visual observation.&lt;/strong&gt; Soyuz rockets were designed at a time, when there were no rocketcams and the flight controllers used specialized telescopes to see what happens with the rocket (in addition to telemetry). If different stages have different colors it may be easier to determine whether they have separated or not. But it does not explain, why some Progress rockets (which have the same &quot;ancestor&quot; as Soyuz) are white.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Danger sign.&lt;/strong&gt; Orange parts are colored that way because some of them fall down to ground and may contain remainders of a highly toxic propellant.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Ease of recovery.&lt;/strong&gt; Orange parts fall to ground and are then searched for. Therefore, these parts are colored to be as easily detectable visually as possible (similar to the black boxes of aircraft). Allegedly (not sure, if it's actually true), in the 1950es the parts of rockets, which fell on the ground were collected for at least two reasons. First, in case of failure, to analyze the cause of it. Second, to prevent people from collecting the remainders of the rockets and selling them to competing nations.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;No suitable paint.&lt;/strong&gt; Russian industry is incapable of producing a paint, which would sustain varying and harsh conditions of a spaceflight (like heat coming from the engines at the start and cold of the space in the later stages of the flight). As far as I know, you cannot paint a device the way you like - the color must be certified for the environment, in which the device will be operated. This hypothesis is refuted by the fact that the Angara rocket is white.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ouctT.jpg&quot; alt=&quot;Angara rocket&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update 1 (16.11.2014 15:08 MSK)&lt;/strong&gt;: ChrisR suggested another hypothesis:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Weight&lt;/strong&gt;. The orange parts may be bare (unpainted) to make the rocket weigh less.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update 2 (16.11.2014 15:45 MSK):&lt;/strong&gt; TidalWave proposed another hypothesis:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Ice&lt;/strong&gt;. Prior to launch, the rocket is covered with ice and therefore appears white. I. e. rockets painted with the same color may appear as white and non-white, depending on whether the ice has melted (and/or been shaken off during launch) or not.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Weight hypothesis seems to be incorrect (see comments).&lt;/p&gt;&#xA;" OwnerDisplayName="user3049" LastEditorUserId="13208" LastEditDate="2017-02-27T21:22:25.610" LastActivityDate="2021-10-19T19:34:33.360" Title="Why are Soyuz rockets painted with different colors?" Tags="|progress|soyuz-launcher|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="5965" PostTypeId="1" CreationDate="2014-11-16T12:37:08.670" Score="2" ViewCount="252" Body="&lt;p&gt;What did Philae find?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not asking for yet another summary of on-board instruments and a list which were activated. I'm asking for the actual results - either semi-raw data dumps with results of measurements, or some papers where these are presented in 'cooked' form composition, magnetic profile, whatever was discovered. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; in case these are unavailable at time of writing this question, let it sit open, waiting for them, until they are.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2014-11-16T13:36:53.503" Title="What data did Philae collect?" Tags="|probe|rosetta|" AnswerCount="1" CommentCount="7" ClosedDate="2014-11-16T16:17:09.263" ContentLicense="CC BY-SA 3.0" />
  <row Id="5966" PostTypeId="1" CreationDate="2014-11-16T12:41:40.927" Score="4" ViewCount="147" Body="&lt;p&gt;It appears that we underestimated comet 67P's mass significantly (see  &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/q/5940/1103&quot;&gt;What was known about 67P/C-G&amp;#39;s mass before the arrival of Rosetta?&lt;/a&gt;&lt;/em&gt;) during the initial planning.&#xA; I wonder if this contributed to:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Stronger-than-expected gravity;&lt;/li&gt;&#xA;&lt;li&gt;Helped Philae to finally settle on the surface despite failure of its thrusters and harpoon?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1103" LastEditorUserId="12102" LastEditDate="2023-12-14T21:57:02.247" LastActivityDate="2023-12-14T21:57:02.247" Title="Could underestimation of 67P's mass have helped Philae?" Tags="|rosetta|comet|philae|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="5967" PostTypeId="1" AcceptedAnswerId="5969" CreationDate="2014-11-16T12:48:54.580" Score="8" ViewCount="3327" Body="&lt;p&gt;Somewhere I read the shuttle fuel tank kept structural integrity only thanks to internal pressure of the fuel, a'la inflated balloon;  empty it would collapse under own weight; and the walls were quite thin, a BB pellet could easily thwart a mission punching a hole. How much truth is to that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A perfect answer would provide a full crossection of the wall, air on one side, liquid hydrogen on the other, materials and distances in between. But a good summary on 'material design' of the shuttle - materials used, their estimated thickness, will suffice.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2022-07-21T10:18:07.830" Title="Structural composition of shuttle liquid fuel tank wall" Tags="|space-shuttle|construction|liquid-fuel|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5968" PostTypeId="2" ParentId="5965" CreationDate="2014-11-16T13:36:53.503" Score="3" Body="&lt;p&gt;ESA implements a &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/07/16/access-to-rosetta-data/&quot; rel=&quot;nofollow noreferrer&quot;&gt;proprietary period&lt;/a&gt; of 6-12 months for data from Rosetta/Philae. For example, the images released are only from the navigation camera. The high resolution color images won't be released until next year. But they could of course make some exceptions if they want.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-11-16T13:36:53.503" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5969" PostTypeId="2" ParentId="5967" CreationDate="2014-11-16T13:51:05.913" Score="14" Body="&lt;p&gt;You might be confusing it with the &lt;a href=&quot;http://en.wikipedia.org/wiki/Balloon_tank&quot; rel=&quot;nofollow noreferrer&quot;&gt;balloon tanks&lt;/a&gt; used by the original Atlas and the Centaur upper stage.  The Shuttle external tank was not a balloon tank - it did not need to be pressurized to be structurally stable under its own weight - but the pressurization did contribute to its ability to withstand flight loads, as is the case for almost all liquid-propellant rocket tanks.  The mechanism for this strength-enhancement is that the pressure provides a local restoring force for small deformations in the skin, resisting the effect of &lt;a href=&quot;http://en.wikipedia.org/wiki/Buckling&quot; rel=&quot;nofollow noreferrer&quot;&gt;buckling&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Super Light-Weight Tank (SLWT) used in the majority of Shuttle missions has a fairly simple basic structure.  In the barrel sections, the innermost layer is approx 0.1&quot; thickness of Al-Li alloy.  The thickness varies along the length of the tank due to varying hydrostatic loads.  Outside this there is a thin (&amp;lt;0.02&quot;) layer of epoxy primer, then approx 0.5&quot; of mysterious grey substance and then ~2&quot; of orange foam insulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/LnHvB.jpg&quot; alt=&quot;ET cross section engineering sample&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2019-12-04T01:40:22.510" LastActivityDate="2019-12-04T01:40:22.510" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="5970" PostTypeId="1" AcceptedAnswerId="5971" CreationDate="2014-11-16T19:46:43.793" Score="6" ViewCount="235" Body="&lt;p&gt;I saw the question &lt;a href=&quot;https://space.stackexchange.com/questions/5925/why-is-there-no-pov-video-of-the-philae-lander-landing&quot;&gt;&quot;Why is there no POV video of the Philae lander landing?&quot;&lt;/a&gt;, which lead me to this question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The data transfer rate of Rosetta is only 28kb/s, which is not strange because it is very, very far away.&#xA;But could that be improved so that a POV video could've been possible if Rosetta was launched today? It was launched in 2004, not to mention it should've launched in 2003, so it was ready that time.&lt;/p&gt;&#xA;" OwnerUserId="648" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-11-17T16:56:38.103" Title="Would a POV camera on Philae lander be possible if the mission was launched today?" Tags="|technology|rosetta|philae|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5971" PostTypeId="2" ParentId="5970" CreationDate="2014-11-16T21:08:21.820" Score="6" Body="&lt;p&gt;Technically yes, definitely. It might have even been as Rosetta and Philae were designed, but was not considered a priority or was thought wasteful to already limited resources. For example, from technical standpoint, we frequently get downstream in the order of several megabits per second (Mb/s) from Mars orbiters (check for example the &lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSN Now&lt;/a&gt; tool, select a dish receiving data from some orbiter, and click in the right column &lt;em&gt;+ more details&lt;/em&gt; to see real time uplink and downlink info). Granted, Mars is currently 263 million kilometers away, not 510 million, but you also don't require that much of a bandwidth for some highly compressed standard quality of perhaps lower frame rate high definition videos transmitted in real time. We could certainly &lt;em&gt;stuff&lt;/em&gt; some real time videos into downstreams of today's space-faring hardware. Or, alternatively, record them and transmit later at a slower data rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there still wouldn't be much chance that it would be included, since it would hardly have any scientific or technical value beyond perhaps for public outreach purposes. And even if it was, it would have low priority among many other scientific instruments, all sharing same bandwidth. Scientifically speaking, there is no real justification for such  bandwidth-starved real time videos on a celestial body where you get many times the scientific return on investment by using still cameras in multiple wavelengths whose data can be transmitted at the same cost to the spacecraft's bandwidth and power budgets. You can always stitch individual still frames for panoramas or display them in a time-lapse sequence for the animated effect, if that's really required. It might be nice to see them, so the general public can grasp easier what their governments have spent their hard-earned tax money on and why, but that's usually something for Public Affairs Officers (PAO) to justify once the mission budget has been approved, and those rarely (if ever) select scientific packages on spacecraft. Frankly, I think that's for the better and if public was asked what they'd want their scientist to do with fancy and expensive gadgets, it would probably yield fairly little science and have quickly-forgotten entertainment value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is however true that many space agencies lately put more effort into Education &amp;amp; Public Outreach (EPO, or sometimes E/PO), e.g. NASA does that through its &lt;a href=&quot;http://smdepo.org/welcome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Science Mission Directorate Education and Public Outreach&lt;/a&gt; (SMDEPO) community, among others, and such hardware is nowadays more frequently included on spacecraft, budget and other mission constraints permitting. One such example is &lt;a href=&quot;https://space.stackexchange.com/q/2373/49&quot;&gt;JunoCam on JUNO spacecraft&lt;/a&gt; that will exclusively serve for POE purpose, and has otherwise little scientific significance to the mission.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-11-17T16:56:38.103" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5972" PostTypeId="1" CreationDate="2014-11-16T15:28:02.990" Score="8" ViewCount="2402" Body="&lt;p&gt;There are &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;5 lagrangian points&lt;/a&gt;. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/VQZ0D.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any man-made satellites at any of those points? Is there a reason for the presence or absence of satellites at these points? &lt;/p&gt;&#xA;" OwnerUserId="6606" OwnerDisplayName="JoeHobbit" LastEditorUserId="49" LastEditDate="2014-11-16T23:36:55.463" LastActivityDate="2014-11-18T20:03:06.440" Title="Are there any man-made satellites at Lagrangian points?" Tags="|orbital-mechanics|artificial-satellite|lagrangian-points|station-keeping|celestial-mechanics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5974" PostTypeId="2" ParentId="5972" CreationDate="2014-11-16T15:50:10.737" Score="3" Body="&lt;p&gt;First of all you need to specify the bodies you are considering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 5 Lagrangian points in the Sun-Earth system and there are other 5 Lagrangian points in the Earth-Moon system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically you can find 5 Lagrangian points (which are equilibrium points int the synodic reference frame) for each system made of 3 bodies where $m_3 &amp;lt;&amp;lt; m_1,m_2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Sun-Earth system, L1 is preferred for Sun observation missions, whereas L2 is good for deep-space observation since it is possible to have the Sun always behind the spacecraft (which could be a telescope for instance) therefore it is possible to observe the whole emisphere without having the Sun in the field of view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;L4 and L5 are stable, which means that once you get there you do not have to spend propellant to stay there, whereas the others are not stable therefore you need to adjust your orbit nearly every period not to be driven away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Missions in L1: Explorer 3, Genesis, Wind, LISA (coming soon).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Missions in L2: Herschel&amp;amp;Planck, James Webb Space Telescope (coming soon hopefully)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Earth-Moon system L2 is used for telecommunication satellites.&lt;/p&gt;&#xA;" OwnerUserId="6608" OwnerDisplayName="Rhei" LastEditorUserId="6608" LastEditDate="2014-11-18T20:03:06.440" LastActivityDate="2014-11-18T20:03:06.440" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="5976" PostTypeId="1" CreationDate="2014-11-17T12:42:02.573" Score="8" ViewCount="1049" Body="&lt;p&gt;Were drilling samples taken by SD2 and analysed by Ptolemy, COSAC, and ÇIVA?&lt;/p&gt;&#xA;" OwnerUserId="6610" LastActivityDate="2014-11-18T04:12:06.657" Title="Philae Lander - Were drilling samples collected and analysed?" Tags="|rosetta|comet|philae|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5977" PostTypeId="2" ParentId="5976" CreationDate="2014-11-17T12:54:30.230" Score="9" Body="&lt;p&gt;&lt;s&gt;Yes&lt;/s&gt; No (see update). From &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Pioneering_Philae_completes_main_mission_before_hibernation&quot; rel=&quot;nofollow&quot;&gt;ESA's 15 November 2014 update on the Rosetta mission&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Pioneering Philae completes main mission before hibernation&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;code&gt;15 November 2014&lt;/code&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Rosetta’s lander has completed its primary science mission after&#xA;  nearly 57 hours on Comet 67P/Churyumov–Gerasimenko.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;After being out of communication visibility with the lander since&#xA;  09:58 GMT / 10:58 CET on Friday, Rosetta regained contact with Philae&#xA;  at 22:19 GMT /23:19 CET last night. The signal was initially&#xA;  intermittent, but quickly stabilised and remained very good until&#xA;  00:36 GMT / 01:36 CET this morning. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In that time, the lander returned all of its housekeeping data, as&#xA;  well as science data from the targeted instruments, including ROLIS,&#xA;  COSAC, Ptolemy, SD2 and CONSERT. This completed the measurements&#xA;  planned for the final block of experiments on the surface.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The article later mentions that the science teams are already studying this data to establish what type of cometary materials have been sampled with Philae's drill. Refer to &lt;a href=&quot;http://blogs.esa.int/rosetta/&quot; rel=&quot;nofollow&quot;&gt;Rosetta blog&lt;/a&gt; for more up to date information as it is made available.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: Teams behind these mentioned Philae instruments also have their own Twitter feeds. Here's some (adding latest info):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://twitter.com/Philae_COSAC&quot; rel=&quot;nofollow&quot;&gt;Philae COSAC&lt;/a&gt; (Cometary Sampling and Composition Experiment); Latest tweets suggest &lt;strong&gt;loss of subsurface data&lt;/strong&gt; despite attempting to drill through the hard crust in &lt;em&gt;desperate mode&lt;/em&gt; (aka power mode 4 beyond design specs). The team concludes that &lt;em&gt;Surface must be &gt;2 MPa hard!&lt;/em&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://twitter.com/Philae_MUPUS&quot; rel=&quot;nofollow&quot;&gt;MUPUS on Philae&lt;/a&gt; (MUlti PUrpose Sensor); It links to just released &lt;a href=&quot;http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-12176/year-all/#/gallery/17219&quot; rel=&quot;nofollow&quot;&gt;official DLR press release&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://twitter.com/RosettaSD2&quot; rel=&quot;nofollow&quot;&gt;Rosetta SD2&lt;/a&gt; (Sample and Distribution Device)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://twitter.com/Philae_ROMAP&quot; rel=&quot;nofollow&quot;&gt;Philae_ROMAP&lt;/a&gt; (Rosetta Lander Magnetometer and Plasma Monitor)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://twitter.com/Philae_Ptolemy&quot; rel=&quot;nofollow&quot;&gt;Philae Ptolemy&lt;/a&gt; (gas chromatograph and medium resolution mass spectrometer)&lt;/li&gt;&#xA;&lt;li&gt;and of course &lt;a href=&quot;https://twitter.com/Philae2014&quot; rel=&quot;nofollow&quot;&gt;Philae Lander&lt;/a&gt; and &lt;a href=&quot;https://twitter.com/ESA_Rosetta&quot; rel=&quot;nofollow&quot;&gt;ESA Rosetta Mission&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Rosetta's instruments also have their own Twitter feeds, such as &lt;a href=&quot;https://twitter.com/RosettaMIDAS&quot; rel=&quot;nofollow&quot;&gt;Rosetta MIDAS&lt;/a&gt; or &lt;a href=&quot;https://twitter.com/ConsertRosetta&quot; rel=&quot;nofollow&quot;&gt;Consert Rosetta&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-11-18T04:12:06.657" LastActivityDate="2014-11-18T04:12:06.657" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5978" PostTypeId="1" CreationDate="2014-11-14T21:21:34.413" Score="4" ViewCount="114" Body="&lt;p&gt;The Aphelion of Comet &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow&quot;&gt;Churyumov–Gerasimenko&lt;/a&gt; is about 5.6829 AU. It has a orbital period of 6.44 years. If it stays on the comet for the entire period of its orbit, can it be usable when it is in the same position after 6.44 years?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am guessing the wear and tear to be mainly from radiation and temperature variations. Are there any other dangers?&lt;/p&gt;&#xA;" OwnerUserId="4708" OwnerDisplayName="san1646" LastEditorUserId="1142" LastEditDate="2014-11-17T15:58:33.620" LastActivityDate="2014-11-17T15:58:33.620" Title="Can the Philae lander survive one orbit of 67P?" Tags="|probe|radiation|philae|temperature|end-of-life|" AnswerCount="1" CommentCount="4" ClosedDate="2014-11-17T21:00:02.997" ContentLicense="CC BY-SA 3.0" />
  <row Id="5979" PostTypeId="2" ParentId="5978" CreationDate="2014-11-16T14:24:54.203" Score="2" Body="&lt;p&gt;As the lander does not appear to be firmly fixed on the surface (the harpoons didn't fire, the points on the lander gear did not cut into the surface) and may even be at an odd angle, the issue will be what happens when the comet gets closer to the Sun and begins to give off gases and other materials - put simply these might just blow the lander into space.&lt;/p&gt;&#xA;" OwnerUserId="1519" OwnerDisplayName="adrianmcmenamin" LastActivityDate="2014-11-16T14:24:54.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5980" PostTypeId="2" ParentId="4019" CreationDate="2014-11-17T14:01:22.637" Score="13" Body="&lt;p&gt;As David Hammen states in his answer, the Soyuz emergency landing survival kit included a gun, but, according to &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Oberg&quot; rel=&quot;noreferrer&quot;&gt;James Oberg&lt;/a&gt;, &lt;a href=&quot;https://spectrum.ieee.org/tech-talk/aerospace/space-flight/how-i-stop-cosmonauts-carrying-guns&quot; rel=&quot;noreferrer&quot;&gt;not any more&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For decades, the standard Soyuz survival pack included a deluxe all-in-one pistol called the TOZ 82 with three barrels and a folding stock that doubled as a shovel and contained a swing-out machete. There were a few dozen rounds of three types of ammunition—rifle bullets, shotgun shells and flares—in a belt attached to the gun.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JvPFO.jpg&quot; alt=&quot;TOZ 82&quot; /&gt;&#xA;&lt;img src=&quot;https://spectrum.ieee.org/img/652e7096-d2a6-4de0-8180-11e664a6da6b%20(1)-1415891692933.jpg&quot; alt=&quot;barrel&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;According to the article, the pistol is still on the official list of kit contents, &amp;quot;But before every mission we meet to review that list and vote to remove it for this specific flight.&amp;quot;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-01-08T20:17:18.693" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="5981" PostTypeId="2" ParentId="5958" CreationDate="2014-11-17T15:56:59.977" Score="1" Body="&lt;p&gt;Every spacecraft is designed to fit one particular launcher, in fact every launcher has its own adapters which have diferent sizes and can carry different weigths. Moreover, the spacecraft has to satisfy constraint on maximun height and diameters which come from the selected launcher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During testing the loads applied to the structure of the spacecraft are the ones specified in the launcher manual and they are different for each launcher. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You should be able to download the Ariane 5 manual from the Arianespace website, just in case you want to know more about the requitements the spacecraft needs to satisfy.&lt;/p&gt;&#xA;" OwnerUserId="6608" LastEditorUserId="6608" LastEditDate="2014-11-17T16:26:20.937" LastActivityDate="2014-11-17T16:26:20.937" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="5983" PostTypeId="1" AcceptedAnswerId="5985" CreationDate="2014-11-17T21:13:32.083" Score="5" ViewCount="321" Body="&lt;p&gt;Recently read that power was lost in the Philae probe. Did the team under estimate how the probe would recharge or is it in a spot where it doesn't get enough sun. And since this was supposed to transmit data back for months.. what is going to happen now?&lt;/p&gt;&#xA;" OwnerUserId="1247" LastActivityDate="2014-11-17T23:22:37.963" Title="Since Philae lost power, what would be the approximate time to recharge it" Tags="|probe|rosetta|philae|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5984" PostTypeId="1" AcceptedAnswerId="5992" CreationDate="2014-11-17T21:21:57.237" Score="3" ViewCount="233" Body="&lt;p&gt;How would a cryofreeze work in order to keep the pilot alive long enough to survive an automated travel to some destination?&lt;/p&gt;&#xA;" OwnerUserId="1247" LastEditorUserId="1247" LastEditDate="2014-11-18T00:05:32.677" LastActivityDate="2015-03-30T12:40:19.947" Title="How would cryogenics work to keep a pilot alive during long term travel?" Tags="|technology|humans|cryogenics|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5985" PostTypeId="2" ParentId="5983" CreationDate="2014-11-17T23:14:55.550" Score="3" Body="&lt;p&gt;Nothing will happen with the Philae lander now for some undetermined time. How much time is not exactly clear though, but ESA operations said not to expect the lander to wake up in a couple of days or so. From &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/11/15/our-landers-asleep/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta blog&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;From now on, no contact would be possible unless sufficient sunlight&#xA;  falls on the solar panels to generate enough power to wake it up.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Problem is, that its batteries are now depleted and Philae is receiving only roughly 1.5 hours of illumination during every 12-hour rotation period of the comet. The comet is still on its approach to perihelion, closest point in its orbit to the Sun, so while the period of sunlight won't change, at least not by much since 67P isn't a very active comet and outgassing shouldn't change its rotation rate by much, total insolation (sunlight intensity) will increase as the comet comes closer to the Sun and the lander might be able to partially charge its batteries in the meantime too. Increased sunlight intensity will also increase intensity of comet's albedo (reflected light) and that might also help it a bit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Philae operators have tried to reposition its solar panels to receive more incident sunlight before the lander run out of battery juice, so that might have help it a bit and is yet to be established for how much, but it will most likely be an on-and-off adventure. From the same linked blog article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, given the low recharge current available from the solar&#xA;  cells, it is considered unlikely that contact with Philae will be&#xA;  established in the coming days.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And here's this sad graph showing Philae's battery systems' charge levels (source: &lt;a href=&quot;https://twitter.com/Philae2014/status/533403430489178112&quot; rel=&quot;nofollow noreferrer&quot;&gt;Philae Lander on Twitter&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/cg9Sl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From another of its tweets:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;.@ESA_Rosetta I'm feeling a bit tired, did you get all my data? I&#xA;  might take a nap… #CometLanding&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Marking success of its main mission objectives and then falling into deep sleep (hibernation) as its batteries run out of free electrons.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-11-17T23:22:37.963" LastActivityDate="2014-11-17T23:22:37.963" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5986" PostTypeId="1" AcceptedAnswerId="5987" CreationDate="2014-11-18T00:44:59.480" Score="9" ViewCount="1710" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z60Eo.jpg&quot; alt=&quot;Valkyrie B-70&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's B-70 Valkyrie carrying the X-15. What private space is trying to accomplish, was done in the 60's. In fact, The X-15 remains the fastest and highest flying manned aircraft ever flown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happened? Both were cancelled, but data from tests made by the X-15 were used for the shuttle, while the ones from the B-70 helped to bridge the gap between supersonic and hypersonic travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was this approach capable of reaching orbital flight? was the project cancelled because of technical hurdles, or simply it was deemed to expensive, compared to rockets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More info about X-15: &lt;a href=&quot;http://www.aerospaceweb.org/aircraft/research/x15/&quot; rel=&quot;noreferrer&quot;&gt;http://www.aerospaceweb.org/aircraft/research/x15/&lt;/a&gt;&#xA;More info about B-70 Valkyrie: &lt;a href=&quot;http://www.aerospaceweb.org/aircraft/research/xb70/&quot; rel=&quot;noreferrer&quot;&gt;http://www.aerospaceweb.org/aircraft/research/xb70/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="638" LastEditorUserId="12102" LastEditDate="2017-08-22T13:35:16.177" LastActivityDate="2019-07-07T14:16:56.027" Title="Was the X-15 program a dead end for orbital insertion?" Tags="|ssto|spaceplane|x-15|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="5987" PostTypeId="2" ParentId="5986" CreationDate="2014-11-18T01:29:04.827" Score="7" Body="&lt;p&gt;Mach 6 buys you roughly 1.8 km/s at those altitudes. OK, let's say that X-15 reached 2 km/s for good measure. But at that speed, Newton still works against you and you're not going to perpetually miss the Earth because of carrying so much momentum in the velocity vector that you're effectively free-falling in a circle around it. To achieve orbital speed you need to go roughly 7.5 km/s (about Mach 25).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But at the altitude that you still gain any aerodynamic lift, and where such a lifting body design still comes useful, your orbit is also going to decay rather fast, and need I say your airframe is probably melting by now if you're still in denser, lower atmosphere below the Kármán line (100 km or 62 mi). So you also have to climb fast and then circularize, at which point aerodynamics don't do anything for it any more, but relatively large aerodynamic surfaces of your lifting body will increase the surface area exposed to atmospheric heating on reentry that you need to somehow protect with a heat shield, if you want to also return from orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's how these experimental airplanes helped the development of Space Shuttle Orbiter (SSO) - the reusable, lifting body part of the Space Transportation System (STS). They helped establish limitations of then on the cusp materials science and propulsion technologies,  dynamic pressure on the body at rapid acceleration through lower atmosphere, and so on, and some of those findings were used to design the SSO while also helped dismiss failed, infeasible ideas so it ended up being relatively safe and reliable.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-11-18T01:29:04.827" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="5988" PostTypeId="2" ParentId="5986" CreationDate="2014-11-18T01:30:26.797" Score="9" Body="&lt;p&gt;Was the X-15 capable of reaching orbit?  Not on its own (it only carried enough fuel to reach 4500 mph), but with extra fuel tanks or a booster rocket, it could.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was the X-15 capable of &lt;em&gt;returning from orbit&lt;/em&gt;?  Unambiguously not.  The X-15 required an ablative heat-shield coating to reach the aforementioned 4500 mph; given the materials science of the time, there was no way of shielding an aircraft of its shape against the heat of re-entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Incidentally, your picture is probably a CG render or other artist's conception: the carried aircraft is emphatically not an X-15.  The wings are entirely wrong.)&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-11-18T01:30:26.797" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5989" PostTypeId="1" AcceptedAnswerId="6002" CreationDate="2014-11-18T02:30:13.033" Score="12" ViewCount="1357" Body="&lt;p&gt;It is possible to find numerous studies of space elevators, for Earth, Luna, Mars and even Phobos. But would a 'space elevator' work on an asteroid? I use scare quotes because typically the point of a space elevator is to escape from the gravity well of the planet, but in this case the second function of a space elevator is relevant - the ability to launch (or catch) objects at high velocities, acting as a momentum exchanger between the planet and spaceships.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concept is simple enough, find a &lt;strong&gt;rapidly rotating asteroid&lt;/strong&gt;, one which rotates in hours or minutes would be ideal, it doesn't matter too much how big it is as long as it's not so large that it has significant gravity. It also doesn't matter how small it is, as long as it's big enough to make the venture worthwhile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A cable is anchored to the asteroid (ideally to a solid iron core) and a counterweight is flung or launched off into space, so that the rotation of the asteroid causes the cable to maintain it's extension.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cable could then be extended further, until it reaches off as far into space as is desirable. Crawlers would bring objects up the cable and release them from the top. If the cable is long enough then the objects could be released with sufficient velocity to reach Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this scheme worked, then packets of material mined from the asteroid could be launched without the use of rocket fuel, requiring only the electricity for climbing the cable (which would not be great, as there is no real gravity to climb against - in fact mainly it would be braking which would be required as the cable would try to fling the crawlers off prematurely). Essentially the cable would transfer angular momentum from the asteroid to packet launches. The asteroid would slow down over time, but the cable could be extended to maintain launch velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, higher up the cable there would be increasing apparent gravity due to centripetal force, this would permit the construction of a station with comfortable gravity for human habitation. The cable could then also be used to catch incoming spaceships. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there problems with space elevator physics that would make the system hopelessly unstable in the absence of gravity? If it did work, how would the construction difficulty/cost compare with a space elevator on Luna or Mars?&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2014-11-18T21:17:38.633" LastActivityDate="2014-11-20T20:34:51.913" Title="Would a 'space elevator'/sling on a rotating asteroid work?" Tags="|asteroid|mining|space-elevator|" AnswerCount="5" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="5990" PostTypeId="2" ParentId="5976" CreationDate="2014-11-18T03:49:53.997" Score="2" Body="&lt;p&gt;Alas, no.  Reported today:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://twitter.com/erichand/status/534413817040867328&quot; rel=&quot;nofollow&quot;&gt;COSAC PI: Drill tried to deliver sample. Ovens heated up. But data&#xA;  show no actual delivery. &quot;There’s nothing in it.&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-11-18T04:01:02.570" LastActivityDate="2014-11-18T04:01:02.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5991" PostTypeId="2" ParentId="5989" CreationDate="2014-11-18T05:42:00.163" Score="1" Body="&lt;p&gt;A space elevator can't be built without gravity--for the system to be stable you must have more tension on the cable than the greatest force exerted by all climbers on the cable at any one instant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the weight of the climbers is not a realistic problem inertia is another matter if you have a cable on something that's really small--you could end up pulling the cable down a bit as your mass accelerated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice there's no reason for an elevator per se on really small objects--you can just throw stuff off anyway.  The only reason one would be built is as a sling--if the object has enough rotation a long enough cable would be turning pretty fast at the outer end&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-11-18T05:42:00.163" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5992" PostTypeId="2" ParentId="5984" CreationDate="2014-11-18T10:56:08.697" Score="5" Body="&lt;p&gt;The objective of cryofreezing is to slow or stop the body’s metabolism. Essentially, put you to sleep for a very long time without aging and thaw you out when you get to where you’re going.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it sounds whacky it’s actually closer to reality than it sounds. Slowing of the metabolic rate happens naturally in many animals during hibernation, and there is some &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/18186703&quot; rel=&quot;nofollow&quot;&gt;speculation&lt;/a&gt; that it may be possible to induce hibernation in humans. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://news.discovery.com/space/nasa-eyes-crew-deep-sleep-option-for-mission-to-mars-141003.htm&quot; rel=&quot;nofollow&quot;&gt;Nasa is even looking at this technique for mars missions.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.newscientist.com/article/mg22129623.000-gunshot-victims-to-be-suspended-between-life-and-death.html#.VGsYlTSsVgg&quot; rel=&quot;nofollow&quot;&gt;And it’s being trialled for trauma patients.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However hibernation will only get you so far, it only slows, not stops the metabolism, and it hasn’t yet been attempted for more than a few days at a time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In principle entering cryofreeze (or cryostasis is another technical term) is easy, find a massive freezer, anesthetise the patient, and sling them in. The problems come when we try to thaw them out. Currently the biggest problem is that as the water in the body freezes it forms crystals that puncture cell membranes. Think: full body freezer burn.  While there are potential solutions, mostly involving completely replacing the water with an antifreeze solution, they are still very much works in progress. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another problem is bacteria; with the human immune system offline there’s nothing to stop bacteria from slowly munching through whatever they please. So on entering long term cryofreeze you’d need to be completely sterilised (both inside and out).&#xA;&lt;a href=&quot;http://link.springer.com/article/10.1007%2Fs12038-013-9306-y#page-1&quot; rel=&quot;nofollow&quot;&gt;Here&lt;/a&gt; is a good paper on low temperature bacteria.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there are many unsolved problems before we can bring someone back from the freezer, and it’ll probably take another few decades before cryofreeze or induced hibernation can be seriously considered for use in crewed missions.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-03-30T12:40:19.947" LastActivityDate="2015-03-30T12:40:19.947" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5993" PostTypeId="1" AcceptedAnswerId="5999" CreationDate="2014-11-18T11:58:20.163" Score="9" ViewCount="1654" Body="&lt;p&gt;In &lt;a href=&quot;https://www.google.com/maps/views/view/streetview/nasa/bottom-of-launch-pad-39a/ttE5-TYtRtcbjEwMgVsl3g?hl=en&amp;amp;gl=us&amp;amp;heading=356&amp;amp;pitch=68&amp;amp;fovy=75&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Google Street View&lt;/a&gt; you can see what appear to be rails leading down into the flame trench under launch pad 39A. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/9zx71.jpg&quot; alt=&quot;Pad 39A taken from Google Street View&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was/is the purpose of those rails? &lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="49" LastEditDate="2015-07-12T20:25:33.667" LastActivityDate="2015-07-12T20:25:33.667" Title="What are/were the rails leading into the flame trench of pad 39A used for?" Tags="|history|launch-site|engineering|launchpad|kennedy-space-center|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5995" PostTypeId="2" ParentId="5989" CreationDate="2014-11-18T12:30:00.657" Score="3" Body="&lt;p&gt;What you are describing is not a space elevator, the point of one of those is to get to an orbit where there is a significant gravity well. Your idea is more of an angular momentum sling. It's not a bad idea, however there are considerations:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Although the crawlers would not need much energy to go out on the sling, they'd need a great deal to get back as they are fighting the outward force developed by the spin&lt;/li&gt;&#xA;&lt;li&gt;You will lose the spin the more you launch from the sling. Nothing is for free - the energy you impart to the cargo you launch will come out of the asteroid's spin. This may not be much but for smaller asteroids it may be a factor over time. You could regain momentum by capturing spacecraft or other objects in order to balance things out&lt;/li&gt;&#xA;&lt;li&gt;The sling would have to be very strong to contain the forces involved, and it would need a strong anchor, so it's not a simple bit of engineering. The technology is way beyond what we have now&lt;/li&gt;&#xA;&lt;li&gt;If your asteroid is spinning fast enough simply remaining on the surface is going to be a challenge, your mining operations would have to deal with this&lt;/li&gt;&#xA;&lt;li&gt;Finding an asteroid that is the right size, with the right spin, and in a location which would make it suitable for mining may not actually be possible&lt;/li&gt;&#xA;&lt;li&gt;It may not actually be worth it. The sling will have to be strong, and probably cannot be built on the asteroid from materials sourced there, so will likely have to be sent. The cost involved in building the sling, transporting it, mounting it, maintaining it, providing energy to the crawlers, and dealing with the spin's adverse effects on mining operations may not be worth the fuel savings&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-11-18T12:30:00.657" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="5996" PostTypeId="2" ParentId="5989" CreationDate="2014-11-18T14:22:19.773" Score="1" Body="&lt;p&gt;It's a realistic sci-fi. Something that technically could be done but is unlikely to be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary problem is a split between economy and safety:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;If you're aiming at a lonely asteroid, finding one with enough resources to make such construction worthwhile, and rotating in the right plane at the right speed will be exceedingly hard - in particular the first part, resources worth mining, in amounts that make the whole concept economically viable. Doing this for several hundred tons of iron - I don't think we'd find that economically viable in half a millennium.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Now the matter is different if we're working in a swarm of meteorites. Finding ones that would contain rare, expensive resources would be viable, and delivering these resources to the &quot;launch station&quot;, the specific asteroid with the elevator (or calling it a &quot;sling&quot; would be more apt), slowly, only to sling them towards Earth at high speed would be quite nice... except, acting in a swarm of meteorites, the rapidly turning lift would be at constant risk of damage, vastly higher than in open space.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, these two are the the big roadblocks that make it unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for various obstacles along the way...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;the material for the tether. Currently, only buckytubes, which are insanely expensive in such amounts. Delivering them from Earth (you'd have a hard time finding enough carbon in space). It would have to be similar to currently planned space elevator tether, obviously much shorter, so the gain on length/durability could be exploited to fling heavier masses. Nevertheless, currently this is the absolute (if hopefully short-term) show-stopper. Until we can produce buckytube tethers, no space elevators for us.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;anchoring. Asteroids, due to lack of gravity, are far less packed than Earth's native rock. You'll have some mining equipment on site, but the anchor tunnel network would need to be quite extensive.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;mass, speed and plane of rotation. In this case the situation isn't quite as bad. There's a lot of asteroids. Finding ones to fit the bill should be perfectly possible. Finding ones that fit the bill and are located in a worthwhile place or have valuable resources - well, much trickier.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;unless you launch more than land (likely in the mining scenario; even with delivery from other asteroids, you'd crash the raw materials into the asteroid at fairly low speeds before lifting them) you'd have to provide energy. The crawlers could provide energy when lifting, but would consume when used as catch/dock for speedy arrivals. In a pinch, you could manufacture energy by flinging any native rock at random, e.g. crawlers never travel up empty.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mining operations and delivery (drops) of raw materials would need to happen in polar regions, where the gravity of the asteroid would be much higher than counteracting centripetal forces. Luckily, with gravity generally low, transport of even very heavy loads shouldn't be that hard.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;as for depleting the rotational energy - picking a bigger asteroid resolves most headaches. The stored kinetic energy scales with square of rotary speed, and fifth power of its radius. $E =  {{4 }\over{15}} \pi \rho  r^5  \omega ^2 $&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="103" LastActivityDate="2014-11-18T14:22:19.773" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="5997" PostTypeId="2" ParentId="5986" CreationDate="2014-11-18T14:31:34.067" Score="4" Body="&lt;p&gt;As @TildalWave notes, the X-15 had nothing close to orbital capability, and wasn't intended to. There was some early consideration of putting an X-15 on top of a SM-64 Navaho missile, but NASA went with Project Mercury instead. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;X-15's big successes were in developing the technology and techniques for transition between aerodynamic control and reaction control and back. In this, it was very successful, and lessons learned on the X-15 were applied most directly to the Space Shuttle.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-22T14:32:08.730" LastActivityDate="2017-08-22T14:32:08.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="5998" PostTypeId="2" ParentId="5958" CreationDate="2014-11-18T17:12:40.077" Score="2" Body="&lt;p&gt;The short answer is No, not all the spacecrafts (S/C) can be launched with any Launcher.&#xA;Once this have been told let me be more accurate in the answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Selecting the Launcher:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lets assume the overall dimensions and weight of the S/C are known and the final orbit is, more or less, defined. In such case not any Launch Vehicle (LV) can fit the S/C inside (fairings are already designed) or it is too big to accommodate it inside as primary payload. In the last case a shared launch could be a good solution.&#xA;The weight and the final orbit are also crucial in order to decide either a LV is capable or not to meet the needs. The closer the injection orbit is to the final orbit, less fuel will be needed onboard the S/C (or longer lifetime the S/C can have). So no any LV has capability to inject the S/C weight where required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the previous parameters and, of course, taking into account the cost of the launcher, a list of candidate LV is selected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Re-designing the S/C:&#xA;Now the S/C design has to be accommodated for the specific Launcher so here two additional aspects are important:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The mechanical loads the LV will induce in the S/C. Can the S/C withstand those loads?&lt;/li&gt;&#xA;&lt;li&gt;The interface between the LV and the S/C (launcher adaptor).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For the first point, if you select one launcher and perform the whole S/C acceptance campaign based in such LV and finally it is launched with another launcher, it could happen some additional test might be performed to ensure the S/C can withstand with the more severe loads imposed by the new launcher. If the loads between selected launchers does not differ too much, the S/C can be tested to an envelope so this problem is avoided.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the LV adapter... lets say the most critical part is the separation device. Basically there are two main separator device systems: the bolted one and the clamp band. Some LV can install both, but this is not usual. It could happen the selected LV for your S/C has one system and it is different from the other LV capable of injecting the S/C... so if the final launcher is other, there is a problem, in time and cost, since the S/C needs to be modified.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you see, the design and verification of the S/C is a loop, as it is the design of the LV (it can be tweaked a little). However, the S/C authority and the LV authority are, in general, different entities so there is no (I insist, in general, but not always) &lt;em&gt;electronic marriage&lt;/em&gt; between the S/C and the LV. Actually the S/C is usually in &quot;standby&quot; until it is separated from the LV, saving energy.&#xA;What is the point of having the S/C On if the S/C is not detached from the LV, attitude will not work properly. Usually a connector will inform the S/C to boot once detached.&lt;/p&gt;&#xA;" OwnerUserId="6622" LastActivityDate="2014-11-18T17:12:40.077" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="5999" PostTypeId="2" ParentId="5993" CreationDate="2014-11-18T17:57:04.360" Score="9" Body="&lt;p&gt;Those are remnants of the Apollo program era when the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kennedy_Space_Center_Launch_Complex_39&quot; rel=&quot;nofollow noreferrer&quot;&gt;Launch Complex 39&lt;/a&gt; was built. They were used to transport large removable blast deflectors to the pad when the Saturn V and Saturn 1B launch vehicles were already transported there on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Crawler-transporter&quot; rel=&quot;nofollow noreferrer&quot;&gt;crawler-transporters&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ten9M.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Blast deflectors at the Complex 34 launch pad. Prior to a launch, one of the deflectors was moved into place underneath the launch pedestal. The rails on which the deflectors traveled are visible on the ground. (Source: &lt;a href=&quot;http://www.robsv.com/cape/c34s3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Rob Zone&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, these rails weren't used during the Space Transportation System (STS, aka Space Shuttle) era and the blast (aka exhaust plume) deflectors were since then permanently installed on the launch pad 39A.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-12T20:24:12.513" LastActivityDate="2015-07-12T20:24:12.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6000" PostTypeId="1" AcceptedAnswerId="6001" CreationDate="2014-11-18T18:56:58.300" Score="17" ViewCount="5475" Body="&lt;p&gt;I was shocked when I recently found out just how long it takes to get to Mars. I've been conducting a very informal survey amongst peers of how just how long people think it takes us to travel to Mars. My previous feelings were echoed by their answers: people think it takes us 3-5 years to travel to Mars. I feel like this has been reinforced by ideas like cryofreezing. I've seen several numbers put to it, but it looks like a 7month trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was also shocked when I learned that people are ACTUALLY considering Mars to be a 1 way trip. This sounds asinine to me. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was ALSO shocked to learn that Mars has 1/3 our gravity, making launch after landing seem really plausible to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously I'm not working with the whole picture here. What is holding us back, besides funding?&lt;/p&gt;&#xA;" OwnerUserId="6613" LastActivityDate="2020-07-14T21:23:14.817" Title="What is the largest hurdle of the mission to Mars?" Tags="|launch|mars|landing|crewed-spaceflight|" AnswerCount="4" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="6001" PostTypeId="2" ParentId="6000" CreationDate="2014-11-18T19:21:45.933" Score="39" Body="&lt;p&gt;The minimum-fuel (Hohmann transfer) travel time to Mars is about 8 months each way. It's possible to shave some off that time by using more fuel, but fuel-to-payload ratio is among the biggest engineering factors in ambitious space missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the alignment of the planets has to be just right to get that fuel-efficient course, and after reaching Mars the spacecraft would have to &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&amp;amp;NECs=on&amp;amp;chk_maxMag=on&amp;amp;maxMag=25&amp;amp;chk_maxOCC=on&amp;amp;maxOCC=4&amp;amp;chk_target_list=on&amp;amp;target_list=Mars%0D%0A&amp;amp;mission_class=roundtrip&amp;amp;mission_type=rendezvous&amp;amp;LD1=2022&amp;amp;LD2=2026&amp;amp;maxDT=4&amp;amp;DTunit=yrs&amp;amp;maxDV=15&amp;amp;min=DV&amp;amp;wdw_width=-1&amp;amp;submit=Search#a_load_results&quot; rel=&quot;noreferrer&quot;&gt;wait another year or so&lt;/a&gt; before heading home -- or spend vastly more fuel to make the trip. That makes the whole mission take about 32 months*.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, our long-term space station missions have involved regular resupply from the ground; a round-trip Mars mission needs to go &lt;em&gt;more than two and a half years&lt;/em&gt; on its own. That's a lot of supply cargo you have to carry and a lot of things that can break along the way, which means carrying a lot of weight in spare parts and/or people to maintain the equipment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars's 1/3 &lt;em&gt;g&lt;/em&gt; is about twice that of the moon, so you can imagine that a landing craft would need to pack a lot more engine and a lot more fuel than a lunar lander. Each ton of payload on a rocket incurs many tons of engine and fuel on the lower stages -- looking back to the moon missions, it's something like 75 tons of Saturn V paying for 1 ton of Apollo spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our moon landers only stayed on the surface for a couple of days. We'll be staying at Mars for more than a year, so we'll send a bunch of unmanned cargo flights in advance to drop off supplies and building materials for the surface station. That's more launches, but the payload and reliability requirements for those are relatively modest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We could send unmanned fuel tankers to rendezvous with the mission in Mars orbit; the total fuel requirement for the mission wouldn't change, but the primary spacecraft could be a little smaller this way, at the cost of a more complex (i.e. failure-prone) mission profile. Another option is &lt;a href=&quot;http://en.wikipedia.org/wiki/In_situ_resource_utilization&quot; rel=&quot;noreferrer&quot;&gt;ISRU&lt;/a&gt;, refining fuel on Mars itself for the return trip, but that's another fairly risky proposition, probably only suitable for the lander's ascent back to Mars orbit, if that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmosphere of Mars is awkward; unlike Earth, it's not dense enough to give the lander free braking (once a returning spacecraft reaches Earth atmosphere, it doesn't need to use any more fuel to land safely, just drag and parachutes), but it is just dense enough that the landing craft has to be built with aerodynamic considerations (again a huge contrast to the lumpy Apollo LM). Which means, yep, more weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of these problems are themselves insurmountable, but building and flying such a thing would be many times larger and more complex task than the Apollo flights. No nation presently has the will and budget surplus to make that happen. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;*) If you can provide another 5km/s of ∆v -- requiring a launcher about 5 times as big -- you can cut the total mission duration in half, with a 30-day stay on Mars instead of a 336-day stay.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="415" LastEditDate="2015-09-17T09:07:32.393" LastActivityDate="2015-09-17T09:07:32.393" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="6002" PostTypeId="2" ParentId="5989" CreationDate="2014-11-18T19:38:08.683" Score="8" Body="&lt;p&gt;Shallow gravity wells with a healthy angular velocity are much more amenable to space elevators. The elevators can be be much shorter. The stress is much less so the tether could be an ordinary material like Kevlar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P. K. Aravind outlined the equations for an elevator's length and taper ratio: &lt;a href=&quot;http://users.wpi.edu/~paravind/Publications/PKASpace%20Elevators.pdf&quot; rel=&quot;noreferrer&quot;&gt;The physics of the space elevator&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I attempted to incorporate his equations into a spreadsheet. Plugging in body radius, mass and angular velocity for various bodies I got these numbers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hcpgr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stationary altitude is the distance above body surface at which a circular orbit would have the same angular velocity as the body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Top altitude is the tether length needed to hold the tether taut and vertical. To balance the length below synchronous orbit, a length above is needed. If a counterweight is employed, it doesn't need to be this tall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taper ratio is the ratio of tether thickness at synchronous orbit vs the tether thickness at body surface. When the tether endures a lot of stress, the taper ratio is high.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see Vesta and Ceres have lengths and taper ratios far less challenging than Mars or Earth elevators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we ever make it to the main belt, I believe Ceres will be an important resource body. There is some reason to believe it has lots of water beneath it's surface. Hopefully we will know more about Ceres when &lt;a href=&quot;http://en.wikipedia.org/wiki/Dawn_(spacecraft)#Ceres_approach&quot; rel=&quot;noreferrer&quot;&gt;Dawn arrives there April, 2015&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you say, an elevator can fling payloads and thus provide velocity for injection to a transfer orbit. The ~2000 km Ceres elevator mentioned above could only provide ~.5 km/s. Possibly enough to reach neighboring asteroids but not enough to fling a payload earthward. A Ceres elevator 26,000 kilometers tall could provide about 5 km/s, enough for Trans Earth Injection. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other advantages as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For larger asteroids, surface gravity precludes ion engines. While ion engines have great ISP, they have very meager thrust. Even on bodies like Ceres or Vesta, a spacecraft's weight would exceed the tiny thrust of an ion engine. When thrust to weight ratio is less than one, a spacecraft can't get off the ground. However an ion engine could dock with an asteroid elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have more info on space elevators at my blog post: &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2012/09/beanstalks-elevators-clarke-towers.html&quot; rel=&quot;noreferrer&quot;&gt;Beanstalks, Elevators, Clarke Towers&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-11-18T20:26:36.597" LastActivityDate="2014-11-18T20:26:36.597" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6003" PostTypeId="2" ParentId="6000" CreationDate="2014-11-18T19:38:38.650" Score="22" Body="&lt;p&gt;In many ways, the short answer is nothing is holding us back except funding. There are technological hurdles but nothing beyond our capabilities. The investment necessary to do this is very hard to estimate because it depends on many unknowns, but is definitely in the billions. For a government, there is no return on that other than the knowledge that you did it, so if the scientific merit of it and the national pride isn't enough for you, you aren't going to pay. The Apollo missions were ultimately justified by a military imperative. America wouldn't have done it if they didn't fear what Russian superiority in space might mean. For a private enterprise, there is no profit motive. Really none. SpaceX claims that eventually they can turn a profit by selling residency on Mars. That is a very shaky assertion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are no more Saturn V rockets. To make a trip to Mars, you'd need something at least &lt;a href=&quot;http://www.nasaspaceflight.com/2014/08/battle-heavyweight-rockets-sls-exploration-rival/&quot;&gt;that big&lt;/a&gt;. Bigger would be better. So first that rocket has to be designed and put into production, an undertaking of at least 10 years. SpaceX is currently in the process of beginning to &lt;a href=&quot;http://www.nasa.gov/press/2014/april/nasa-spacex-cut-ribbon-to-launch-testing-partnership/#.VGuhZvnF-2U&quot;&gt;test prototype engines&lt;/a&gt; for a rocket that is meant for this. The rest of the rocket exists only on drawing boards. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you have the rockets, you will need at least 3, maybe 4. The &lt;a href=&quot;http://www.nss.org/settlement/mars/zubrin-promise.html&quot;&gt;Mars Direct&lt;/a&gt; proposal explains one approach to this, or you can look at &lt;a href=&quot;http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf&quot;&gt;NASA's mission plan&lt;/a&gt;, which is considerably more infrastructure intensive. Both plans largely depend on using self-contained chemical plants to produce some of the fuel needed to return to Earth, and also to produce things needed for survival on Mars - water and oxygen. Without the weight-savings in what you have to launch from Earth this provides, a Mars mission would be several times more expensive. You launch the first few rockets as cargo missions that stockpile needed supplies on Mars. Once you have confirmed those missions have successfully landed the cargo on Mars, you can send a crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing on Mars is a big challenge. You need to do a &lt;a href=&quot;http://en.wikipedia.org/wiki/Red_Dragon_(spacecraft)&quot;&gt;powered landing&lt;/a&gt; because the atmosphere is too thin to use aerobraking like a spacecraft reentering the atmosphere of Earth does, but the atmosphere is still enough to cause your craft to heat up tremendously due to friction with the atmosphere, so you need a heat shield. Powered landings - meaning you fire your engines to brake - were done on the moon, but because it has no atmosphere, this was much simpler. Engineering a lander capable of taking several tons of payload to the surface is probably the biggest technical challenge of a Mars mission.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-11-18T19:48:43.030" LastActivityDate="2014-11-18T19:48:43.030" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6004" PostTypeId="1" CreationDate="2014-11-18T20:08:33.003" Score="-2" ViewCount="167" Body="&lt;p&gt;If you have two magnets and you brace them toghter but at a distance less the the force the magnets will push each other apart. Then by useing a switch to fire the back magnet which will push the front magnet foward could this be used to propel a spaceship through space.Also you would have to make the front magnet smaller then the back one so the force backwards wont counter the force foward and fire the back magnet 100 times per second.&lt;/p&gt;&#xA;" OwnerUserId="6626" LastActivityDate="2014-11-18T21:09:07.063" Title="Useing two magnets to push a ship foward that is already in outerspace" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="2" ClosedDate="2014-11-18T21:28:17.160" ContentLicense="CC BY-SA 3.0" />
  <row Id="6005" PostTypeId="2" ParentId="6004" CreationDate="2014-11-18T21:09:07.063" Score="2" Body="&lt;p&gt;If you just let the magnets push each other apart, then you are doing the same thing as a rocket engine, but with less speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are doing something to keep the rear magnet nearby (like putting an engine on it), then you would do better to just put that engine on the ship in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There doesn't seem to be a situation where mediating the interaction with magnets would make sense.  Magnets are kind of neat at the small scale, but to work on something like a ship, it would have to be enormous or draw huge amounts of power.  Can you imagine how big a magnet you'd need to just push a car?&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2014-11-18T21:09:07.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6007" PostTypeId="1" AcceptedAnswerId="6008" CreationDate="2014-11-18T23:21:00.693" Score="11" ViewCount="26652" Body="&lt;p&gt;Say there were a base on the Moon with people living/studying there. Would they age differently than those on earth due to a change in speed through space? Does the Moon move faster through space (relative to Earth), as it is orbiting the Earth? I feel like the answer would be yes, but very minor.&lt;/p&gt;&#xA;" OwnerUserId="6627" LastEditorUserId="6627" LastEditDate="2014-11-18T23:29:23.843" LastActivityDate="2020-01-26T14:37:40.540" Title="Would a Moon resident age differently than someone living on Earth?" Tags="|the-moon|colonization|time|orbital|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6008" PostTypeId="2" ParentId="6007" CreationDate="2014-11-19T00:13:52.417" Score="18" Body="&lt;p&gt;The time dilation of the Moon relative to Earth is dominated by not being as deep in the gravitational field of Earth.  Not so much the velocity of the Moon.  Though the velocity effect isn't negligible, being about two orders of magnitude down from the gravity effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time passes about 0.66 parts per billion &lt;em&gt;faster&lt;/em&gt; on the Moon than on Earth, due to not being in as strong a gravity field.  The dilation, including all the effects of being further from the Earth's gravity field, being in the Moon's gravity field, and the velocity of the Moon, could easily be measured in sum by comparing good atomic clocks on the Earth and the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time dilation relative to some location in space not in a gravitational potential is given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$1\over\sqrt{1-{2\mu\over r c^2}-{v^2\over c^2}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $v$ is the velocity of the object which, to keep it simple, is in a direction that does not change the gravitational potential (e.g. a circular orbit), $\mu$ is the $GM$ of a gravitational source, $r$ is the distance from the center of that source, and $c$ is the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For these kinds of examples, the ratios are very small and so the above can be approximated:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$1+{\mu\over r c^2}+{v^2\over 2c^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The small amount that that is above one is the fractional amount that time is passing slower in the gravitational field and moving at some velocity, as compared to the fixed point far away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${1\over c^2}\left({\mu\over r}+{v^2\over 2}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can put in the numbers for Earth gravity, radius, and surface rotational speed to get our time dilation here relative to a reference with zero velocity relative to Earth's center.  For the Moon, we can put in the Moon's gravity, radius, and orbital velocity around the Earth to get the dilation there relative to the same reference point.  Then dividing the dilation at the Moon by the dilation at the Earth, we get the dilation at the Moon relative to Earth.  For the small ratios, this is just subtracting the fractional variations from one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${1\over c^2}\left({\mu_M\over r_M}+{v_M^2\over 2}-{\mu_E\over r_E}-{v_E^2\over 2}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives a negative value, which is time contraction on the Moon relative to Earth.  Note that $v_M$ is the velocity of the Moon about the Earth, whereas $v_E$ is the velocity of an object on the surface of the Earth.  (Varies with latitude.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-11-20T06:29:59.947" LastActivityDate="2014-11-20T06:29:59.947" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6010" PostTypeId="2" ParentId="6000" CreationDate="2014-11-19T00:47:31.453" Score="17" Body="&lt;p&gt;Having discussed this with a couple of people who know it well, seems to me the limiting issue will be &lt;strong&gt;human psychology&lt;/strong&gt;, specifically &lt;strong&gt;confined environment psychology&lt;/strong&gt;, compounded by gerontology and serious communication timelag:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the &lt;strong&gt;psychotype of people who sign up for a one-way permacolony trip&lt;/strong&gt; to Mars (esp. one with no chance of return, under any circumstances) will be utterly different to the psychotype for volunteers for a one/two-year research visit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;For a few who didn't read through, the following is primarily about the specific never-returning permacolony scenario:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;how many is the minimum number needed for a colony, such that you don't go nuts? 8? 20? 100? 1000? How many of you could live in a glasshouse with the same set of people for the rest of your natural life? (Compare to &lt;a href=&quot;http://en.wikipedia.org/wiki/McMurdo_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;McMurdo, Antarctica&lt;/a&gt;, which even with its winter population of 250, is practically like Club Med, and it's not the same 250 permanently, some new people rotate in/out every summer.)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;at what point do you get to childbirth on the colony? How do you manage that? Birth complications? Death in childbirth? How seriously weird does the kid grow up with no contact with any other children? Unless you send a batch of prospective parents. (How do you even toddler-proof an entire biosphere, anyway?)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;how would you even get young, fertile couples to go there - don't forget the 7% per-person cancer risk per trip, due to gamma ray exposure.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;consider also how stable and dependable each of those people would need to be, and what happens if/when anything goes wrong? serious illness? depression? nervous breakdown? boredom? loneliness? decrepitude? disability? death? You can't send them home, do you euthanase them? How do you do elderly care on Mars? How many infirm people can the others support, before you have to euthanase people? [or, per PeterMasiar, actively encourage them to commit suicide]&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;what is the governing law jurisdiction on Mars anyway, and is euthanasia allowed? (Do we need to create a new jurisdiction just for this?)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I think this is all totally uncharted territory. It's boldly going where no gerontology study has gone before.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;consider also the very unnerving &lt;a href=&quot;http://www.spaceacademy.net.au/spacelink/commdly.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;3-21 minute communication lag&lt;/a&gt;. Ok let's assume AI technology advances so you can have a chatterbot and upload it daily (or hourly) updates, at least you could talk to it in real-time, like the &lt;a href=&quot;http://www.imdb.com/title/tt1182345/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kevin Spacey robot in &lt;em&gt;Moon&lt;/em&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;See &lt;em&gt;confined environment psychology&lt;/em&gt;, and the negative outcome of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Biosphere_2#The_ethnosphere:_psychology_and_conflict&quot; rel=&quot;nofollow noreferrer&quot;&gt;two New Mexico Biosphere 2 closure experiments, 1991-1994&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;so I think the &lt;strong&gt;one-way permanent-colony scenario with no chance of return&lt;/strong&gt; is further out than people want to believe. Till that day it will be one/two-year scientific visit type trips.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6186" LastEditorUserId="6186" LastEditDate="2020-07-14T21:23:14.817" LastActivityDate="2020-07-14T21:23:14.817" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="6011" PostTypeId="2" ParentId="5224" CreationDate="2014-11-19T05:03:52.350" Score="3" Body="&lt;p&gt;During development, the team discussed the possibility of hopping Philae if that became necessary or desirable.  It would be done by activating various mechanical devices.  This was explained briefly during the Philae descent video coverage.  They tried to do it a bit when the lander ended up in an unfavorable position, but were not successful.&lt;/p&gt;&#xA;" OwnerUserId="5775" LastActivityDate="2014-11-19T05:03:52.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6012" PostTypeId="2" ParentId="3670" CreationDate="2014-11-19T05:25:04.743" Score="7" Body="&lt;p&gt;TildalWave's answer is good when only considering the two bodies, but as Pepijn and mart note, for many purposes it is also important to consider the other forces at work, which brings me to one of my favorite learnings about orbital dynamics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In many practical scenarios, we need to consider the influence of the Sun, which exerts tidal forces that become significant for larger orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The maximum theoretical distance at which object A (e.g. Rosetta) can orbit around object B (67P), when considering the influence of a larger more distant object C (the Sun) is determined by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;noreferrer&quot;&gt;Hill sphere (Wikipedia)&lt;/a&gt; of object B.  For example, as I recall the Hill radius for comet 67P when it is 3 AU from the Sun is about 600 km, and at 1.5 AU it would be half that.  Beyond that distance, A would really be orbiting C which would be pulling harder on it than B would.  It would be perturbed by B, not orbiting B.  When other influences are considered, you really need to be within about 1/2 or 1/3 of the Hill radius to maintain a stable orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if a sufficiently close orbit work, you can orbit anything.  But when 67P is closer to the Sun and more active, Rosetta will need to stay further away from it, quite possibly outside the Hill sphere, and thus no longer in orbit around 67P.&lt;/p&gt;&#xA;" OwnerUserId="5775" LastActivityDate="2014-11-19T05:25:04.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6014" PostTypeId="1" CreationDate="2014-11-19T11:34:08.383" Score="2" ViewCount="214" Body="&lt;p&gt;What are the benefits of having a heavy lift launch vehicle, compared to using several smaller launches and assemble the spacecraft in orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There must've been more than a hundred successful dockings (and berthings) in space, including in Lunar orbit, and especially for building and supplying the ISS. The only docking failure I know of is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_M-34&quot; rel=&quot;nofollow&quot;&gt;Progress M-34 to MIR&lt;/a&gt; in 1997 (they seem to have &lt;a href=&quot;https://www.youtube.com/watch?v=Iu3oy6_v73M&quot; rel=&quot;nofollow&quot;&gt;shut down the radar&lt;/a&gt; and looked out of the window holding a laser range finder...) and even then they didn't lose the mission. So docking seems mature and doesn't add substantial risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How important would the costs of docking systems and fuel for maneuvering in orbit be? Is the logistics of timing multiple launches a problem? Could we use today's small launchers to spend several years to build a Mars spaceship in LEO, like we did with the ISS, and fuel it and send it on its way whenever it is ready? Do economies of scale benefit large launchers for large missions, or is it the other way around because small launchers can be mass manufactured? What are the key issues with multiple small launches versus a single big one?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-11-19T13:52:20.223" Title="What are the tradeoffs of docking multiple small launches in orbit vs a single heavy launch?" Tags="|iss|docking|mir|hllv|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6017" PostTypeId="2" ParentId="6014" CreationDate="2014-11-19T13:52:20.223" Score="4" Body="&lt;p&gt;Many more smaller launchers means smaller modules. That is a limit on absolute volume limits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inflatables are a way to handle this.  Launch them folded, expand in space, in a later launch deliver water that is stored in the walls as additional radiation shielding.  So smaller launch, additional launches for outfitting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Volume is not always a limit though. You can live with many small rooms, vs one or two big rooms. (Washrooms are a good example. Soyuz has two habitable modules in orbit, only one is landed. The second is most useful since someone can use the 'facilities' while the other two hang out in the other module allowing privacy.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Docking hardware, wiring, plumbing, and assorted overhead of connecting multiple modules can start to eat into your margin very quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your structure needs to withstand thrust (Say Mars Transit vehicle) your docking structure must be that much stronger (and concomitantly heavier).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need to provide a maneuvering system, or a tug, to deliver your modules together, more overhead. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do it all in one launch, and follow Zubrin's Shuttle-Z style model, the upper stage engine that put you in orbit can also do your TMI maneuver, so you could save on a set of engines mass. Even if you needed to be refueled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Refueling, while non-trivial is a good way to launch most of the mass in one launch, one piece and deliver the fuel/oxidizer/propellant in a second similarly sized launch, without only minor overhead. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-19T13:52:20.223" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6018" PostTypeId="1" AcceptedAnswerId="6028" CreationDate="2014-11-19T14:21:02.910" Score="8" ViewCount="354" Body="&lt;p&gt;We all have seen the photos indicating the exact planned landing site, and the (much bigger) candidate area where Philae might have finally touched down, also the pictures that show Philae &quot;in-jump&quot; after first two touchdowns. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've also learned that Rosetta's orbit propagator used is of a completely new kind, allowing for very slow, controlled and accurate Rosetta flight paths around the comet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given these circumstances, wouldn't it be rather easy to visually locate Philae? ESA must have enough images pre- and post-touchdown to compare, and if not, why don't they take highres pictures of the final touchdown candidate rectangle and look carefully for Philae? Maybe even in a crowd project where everybody can look at the pictures?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why is it so difficult, or at least unsuccessful so far, to figure out where Philae is located, and how it is oriented?&lt;/p&gt;&#xA;" OwnerUserId="6583" LastEditorUserId="33" LastEditDate="2014-11-19T16:11:06.487" LastActivityDate="2019-01-07T07:55:28.753" Title="Why does it take so long to find Philae's exact location?" Tags="|rosetta|philae|remote-sensing|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6019" PostTypeId="2" ParentId="5117" CreationDate="2014-11-19T15:07:49.690" Score="8" Body="&lt;p&gt;Adding to the existing answers: &lt;strong&gt;The triangular flight paths were needed to get enough images of the comet's surface, which are required for the low paths Rosetta flies today (and for dropping Philae close to the surface).&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rosetta's orbit propagator is an innovation insofar as it uses landmarks on the comet (among other things) to find the comet's and its own orientation, and to find a route to a given target landmark location. For this, the comet first had to be imaged in great detail, and that happened during the &quot;triangle phase.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has never been done before, and makes Rosetta a very exciting pilot project for ESA's future far-reaching space missions, which will all use this orbit propagator. It is already a breathtaking success if you consider the complexity the software has to manage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, except for planned crash &quot;landings,&quot; no spacecraft has ever flown by (or, as Rosetta does all the time now, has flown around and around) a comet in altitudes as low as 10 km. This requires a completely new auto-navigation system, and that one needs a map of the comet that cannot be acquired from earth (or from great distances in general).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(It is called &quot;orbit propagator&quot; despite the fact that Rosetta is &lt;em&gt;not&lt;/em&gt; in an orbit because it is built upon other components that were core parts of &quot;real&quot; orbit propagators.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://issfd.org/ISSFD_2012/ISSFD23_IN2_2.pdf&quot; rel=&quot;nofollow&quot;&gt;http://issfd.org/ISSFD_2012/ISSFD23_IN2_2.pdf&lt;/a&gt; sheds some light on the propagator. Recommended reading imho.&lt;/p&gt;&#xA;" OwnerUserId="6583" LastEditorUserId="1142" LastEditDate="2015-07-27T11:27:16.507" LastActivityDate="2015-07-27T11:27:16.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6020" PostTypeId="2" ParentId="4295" CreationDate="2014-11-19T15:08:14.037" Score="7" Body="&lt;p&gt;I've been looking at the AXM models. I haven't built one yet, but I have been looking carefully at the various modules in the photographs of the finished item, and it looks good to me (I write books about how spacecraft work, for kids).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously not electronic though! But perhaps you could rig up your own lighting?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've found some on Amazon. Most of them include a Space Shuttle which means they're not 100% accurate any more. EG this one at 1/450 scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/B00ABY8XQ8&quot; rel=&quot;noreferrer&quot;&gt;http://www.amazon.com/Daron-Vision-International-Station-60-Piece/dp/B00ABY8XQ8/ref=sr_1_1?s=toys-and-games&amp;amp;ie=UTF8&amp;amp;qid=1416409406&amp;amp;sr=1-1&amp;amp;keywords=International+Space+Station&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6637" LastActivityDate="2014-11-19T15:08:14.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6021" PostTypeId="1" AcceptedAnswerId="6022" CreationDate="2014-11-19T15:33:16.650" Score="8" ViewCount="1138" Body="&lt;p&gt;A recent news article &lt;a href=&quot;http://www.bbc.com/news/science-environment-30097648&quot; rel=&quot;nofollow noreferrer&quot;&gt;Comet landing: Organic molecules detected by Philae (BBC 18 November 2014)&lt;/a&gt; talks about 'Organic molecules' being found on a comet.  The question here &lt;a href=&quot;https://space.stackexchange.com/questions/4468/any-regolith-lunar-samples-tested-for-organics&quot;&gt;Any regolith lunar samples tested for organics?&lt;/a&gt; talks about looking for organics on the Moon. The Wikipedia entry for &lt;a href=&quot;https://en.wikipedia.org/wiki/Organic&quot; rel=&quot;nofollow noreferrer&quot;&gt;Organic&lt;/a&gt; has several options, the first being &lt;a href=&quot;https://en.wikipedia.org/wiki/Organism&quot; rel=&quot;nofollow noreferrer&quot;&gt;Organism is any contiguous living system&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To most people I think that Organic equates to life and supporting of life.  But even that is a huge range. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is meant by 'Organics' when discussing space exploration? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it just a sensational term without defined meaning, or does it really have meaning, that implies the existence of life not from Earth (&lt;em&gt;extraterrestrial life&lt;/em&gt;)?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-02-03T15:43:38.630" Title="What is meant by 'Organics' when discussing space exploration?" Tags="|life|chemistry|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6022" PostTypeId="2" ParentId="6021" CreationDate="2014-11-19T15:57:02.777" Score="15" Body="&lt;p&gt;'Organics' usually refer to organic compounds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Wikipedia: &lt;a href=&quot;http://en.wikipedia.org/wiki/Organic_compound&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Organic_compound&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is no single &quot;official&quot; definition of an organic compound. Some&#xA;  textbooks define an organic compound as one that contains one or more&#xA;  C-H bonds. Others include C-C bonds in the definition. Others state&#xA;  that if a molecule contains carbon―it is organic.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So if someone says they have detected 'organics' you first have to ask them which definition they're using. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, organic compounds ≠ life. &lt;em&gt;Organic compounds make carbon based life possible, but don't indicate that such life exists&lt;/em&gt;, just that chemically it could. Because of the name, many news pundits get very exited when someone mentions organics, but really &lt;strong&gt;it just means there are molecules containing carbon present&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-02-03T15:43:38.630" LastActivityDate="2015-02-03T15:43:38.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6023" PostTypeId="2" ParentId="6021" CreationDate="2014-11-19T16:43:04.307" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What is meant by 'Organics' when discussing space exploration?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It means compounds that contain carbon, except possibly for some simple carbon compounds such as CO&lt;sub&gt;2&lt;/sub&gt;, carbonates and cyanides that historically have been deemed to be inorganic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the 19th century, carbon-containing compounds were classified as organic or inorganic based upon 19th century assumptions regarding whether or not the compound could only be formed by life. Compounds such as ammonia are deemed inorganic simply because they don't contain any carbon. Carbon dioxide is inorganic but methane is organic based on those arbitrary 19th century concepts. Life plays a big role in how much inorganic carbon dioxide is present in the Earth's atmosphere, and methane can certainly form without biological processes. Except for the presence of carbon, the distinction between organic and inorganic compounds is a bit arbitrary and a bit archaic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nonetheless, the distinction between organic chemistry and inorganic chemistry remains a very major distinction in identifying branches of chemistry.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2784" LastEditDate="2014-11-19T23:54:35.607" LastActivityDate="2014-11-19T23:54:35.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6026" PostTypeId="1" AcceptedAnswerId="14853" CreationDate="2014-11-19T19:37:11.697" Score="2" ViewCount="134" Body="&lt;p&gt;Beyond Mars and 67P, what is the next most viable mission for probe landing? How far are we from sending one to the Jupiter or Saturn system?&lt;/p&gt;&#xA;" OwnerUserId="6179" LastEditorUserId="603" LastEditDate="2014-11-19T19:53:44.793" LastActivityDate="2016-04-15T17:36:21.730" Title="Next lander/rover mission" Tags="|probe|rovers|landing|lander|moons|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6028" PostTypeId="2" ParentId="6018" CreationDate="2014-11-19T20:33:06.373" Score="8" Body="&lt;p&gt;Below is pair of a high resolution images from the OSIRIS camera of where Philae's first touched down, overlaid upon one another. One was pre-touchdown and the other very shortly after touchdown (and bounce):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dSrm8.gif&quot; alt=&quot;&quot;&gt;&lt;br&gt;&#xA;&lt;sub&gt;Image source: &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2014/11/Philae_spotted_by_Rosetta_after_first_landing&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.esa.int/spaceinimages/Images/2014/11/Philae_spotted_by_Rosetta_after_first_landing&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Rosetta operators knew to look at exactly that location because that is where Philae was targeted to land. The circled dark spot is where Philae made its first bounce. It disturbed a nice-sized area on the comet's surface. The spots marked &quot;Philae&quot; and &quot;Philae's shadow&quot; are barely visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at all the sensor noise in that image. Philae is barely more than a pixel across in that image, making it indistinguishable from that sensor noise. The only thing that let the operators identify those objects as Philae and its shadow is that they knew Philae had to be in that image somewhere, and very close to the touch down location. This is a well-lit image taken at a time when Philae's location was fairly well-known. Now the operators have to look for Philae when they don't know where it is. They are looking for a pixel-sized object amongst a haystack-full of artificial pixel-sized objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make matters worse, Philae is in extremely poorly-lit conditions. It is completely in shadow most of the time, partially in shadow almost all of the time.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="10273" LastEditDate="2019-01-07T07:55:28.753" LastActivityDate="2019-01-07T07:55:28.753" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="6029" PostTypeId="1" AcceptedAnswerId="6601" CreationDate="2014-11-20T01:10:08.637" Score="7" ViewCount="1107" Body="&lt;p&gt;So, let's save some travel time and throw a spaceship, rather than an airplane, across the Atlantic or Pacific ocean on a ballistic trajectory. How high would it have to go? How high do the doomsday ICBM's really go? How far would the Virgin Galactic spaceship go?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've heard that the altitude required is ridiculous, and the numbers I get when I put some numbers in some equations I've found, are ridiculous too. Does ballistic ocean crossing sub-orbital flight really need to put the passengers through the van Allen Belt?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-28T16:27:25.040" Title="What altitude would an ocean crossing sub-orbital flight need to reach?" Tags="|sub-orbital|altitude|ballistics|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6031" PostTypeId="1" AcceptedAnswerId="12027" CreationDate="2014-11-20T04:07:32.873" Score="9" ViewCount="10325" Body="&lt;p&gt;On Kerbin, rockets have an optimal speed depending on altitude for maximum fuel efficiency, as &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You can save fuel by being close to your terminal velocity during ascent. Lower velocity wastes delta-V on gravity, higher is wasted on air resistance&#xA;   See &lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Basic_maneuvers&quot; rel=&quot;nofollow&quot;&gt;http://wiki.kerbalspaceprogram.com/wiki/Basic_maneuvers&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I searched for a similar chart on earth but couldn't find any. Can someone give me an approximation of ideal speed for real rockets on earth, or maybe a few other planets ?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorDisplayName="user20636" LastEditDate="2020-08-31T17:52:14.160" LastActivityDate="2020-08-31T17:52:14.160" Title="Optimal speed per altitude for orbit launch" Tags="|launch|rockets|aerodynamics|launch-trajectories|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6032" PostTypeId="1" AcceptedAnswerId="6041" CreationDate="2014-11-20T09:41:13.177" Score="8" ViewCount="366" Body="&lt;p&gt;Since we have yet to lose personnel onboard the ISS due to starvation, I assume that space agencies have got a good handle on the mechanics for a rendezvous in low earth orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the (added) difficulty in organising a rendezvous between spacecraft in orbit around other planets? Are distance or the knowledge about the planet in question factors in the complexity of these maneuvers?&lt;/p&gt;&#xA;" OwnerUserId="4589" LastEditorUserId="4589" LastEditDate="2014-11-20T13:22:23.597" LastActivityDate="2014-11-24T17:25:21.130" Title="Can spacecraft rendezvous in orbit on planetary missions" Tags="|orbital-mechanics|orbital-maneuver|interplanetary|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6033" PostTypeId="2" ParentId="6018" CreationDate="2014-11-20T09:45:21.223" Score="3" Body="&lt;p&gt;Since they managed to acquire &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-philae-drifting-across-the-comet/&quot; rel=&quot;nofollow&quot;&gt;some more data points&lt;/a&gt; they can now estimate at least the axis towards the second landing spot, and the smart Flight Dynamics team should be able to calculate the landing area. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTOH, ~&lt;a href=&quot;http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/11/osiris_spots_philae_drifting_across_the_comet/15058700-1-eng-GB/OSIRIS_spots_Philae_drifting_across_the_comet.jpg&quot; rel=&quot;nofollow&quot;&gt;200m&lt;/a&gt; (my calculation; 28cm/px, 730px distance)  traveled from the landing site at &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/11/14/three-touchdowns-for-rosettas_lander/&quot; rel=&quot;nofollow&quot;&gt;15:34&lt;/a&gt; until 15:43 (= 9 minutes) is about 0.37m/s in horizontal direction. The second touchdown was at 17:25, 111 minutes = 6660 seconds later. That would be almost 2.5 kilometers of travel if it was flying over flat terrain. 67P is approximately 4.1 kilometres by 4.3 kilometres&lt;a href=&quot;http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow&quot;&gt;*&lt;/a&gt; so obviously the trajectory is curved, but that puts the second landing spot roughly half the comet away from the original one, probably somewhere in the middle &quot;saddle&quot; area - the area with deepest gorges and cliffs, most shade, most irregular terrain, and most hidden from all directions. And even if the flight dynamics lets us calculate that second spot, there were another 7 minutes of flight after the second bounce, and we have no clue about its direction (though hopefully the speed was low.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding David's notes to that - Philae being hidden in deep shadow - 1.5 hours of sunlight a day means it's (partially) visible (possible to appear on photos) about 6% of the time. Since it's highly unlikely (...6% chance per photo) that any of prior photos of Chury caught that area when it was lit up by Sun, chance that the 'differential analysis' (like for the first landing spot) will be viable is proportionally diminished. even if we take the photo with Philae in sunlight - simply no photo with visible ground without Philae to compare to! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably the best approach to finding Philae would be by finding the moments when radio connectivity is cut off/re-established with Rosetta with it orbiting the comet at various angles, and modelling that data - Rosetta's radio &quot;field of view&quot; edges - onto the comet's 3D model, finding where the edges of &quot;visibility&quot; from moments of entering/exitting the &quot;view&quot; converge. But for that to happen, Philae must talk. As long as it's silent, it's seeking a needle in the haystack... at night.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2014-11-20T09:45:21.223" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="6034" PostTypeId="2" ParentId="6000" CreationDate="2014-11-20T11:09:07.100" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;What is the largest hurdle of [a manned] mission to Mars?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;h2&gt;&lt;a href=&quot;http://www.space.com/21353-space-radiation-mars-mission-threat.html&quot; rel=&quot;noreferrer&quot;&gt;Radiation&lt;/a&gt;&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The daily dosage received by a human on earth is 0.00001 sieverts.&lt;/li&gt;&#xA;&lt;li&gt;The shortest round-trip to Mars involves a 0.66 sievert dose.&lt;/li&gt;&#xA;&lt;li&gt;You get a 5.5% increased chance of cancer from 1 sievert.&lt;/li&gt;&#xA;&lt;li&gt;The Earths magnetic field and atmosphere offer considerable protection against radiation.&lt;/li&gt;&#xA;&lt;li&gt;Mars doesn't. You might have to live underground.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;It's a problem on the moon and would be a larger problem on Mars and in transit.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One of the largest recorded [Solar Particle Events] occurred in August 1972, between the Apollo 16 and Apollo 17 missions. The August 1972 event is one of the largest recorded SPEs in flux density and contained more high-energy (10-200 MeV) protons than most other historic events. For this event, astronauts who were thinly shielded on the Lunar surface (for example, astronauts conducting extra vehicular activities [EVAs] such as a spacewalk) could have received fatal radiation doses.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;em&gt;From &lt;a href=&quot;http://hps.org/publicinformation/ate/faqs/spaceradiation.html&quot; rel=&quot;noreferrer&quot;&gt;Health Physics Society&lt;/a&gt;&lt;/em&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/D0tiY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;sup&gt;&lt;em&gt;From &lt;a href=&quot;http://www.spaceflightnow.com/mars/msl/130530rad/#.VG3OpfnJOUk&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight now&lt;/a&gt;&lt;/em&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the scale is logarithmic. A 1-year round trip to Mars (if possible) would involve receiving maybe 14 x the maximum permitted radiation dose of a worker at a nuclear power plant and several thousand times the dose of people living at sea level.&lt;/p&gt;&#xA;" OwnerUserId="3101" LastEditorUserId="3101" LastEditDate="2014-11-20T11:27:41.533" LastActivityDate="2014-11-20T11:27:41.533" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6035" PostTypeId="2" ParentId="6032" CreationDate="2014-11-20T11:51:53.230" Score="0" Body="&lt;p&gt;The point of going to Mars is to use its resources on the ground. Otherwise one could just stay in empty space, building a space station. An automated fuel factory (extracting CO2 from the atmosphere) will have to land on Mars well before the crew. And with it a power plant. And most practically the ascent vehicle which its purpose is to fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there doesn't remain much to put in a waiting orbit. Maybe the habitat, but if it and the crewed spaceship doesn't land near the ascent vehicle, they have a serious problem. Orbiting Mars doesn't give you many options anyway, until a lot of infrastructure has been constructed there. It is safer to make sure that all preplaced assets are on site and working before even launching the crew from Earth. The landers and ascent vehicles should be minimized, like elevators. A return-to-Earth spaceship, or even a &lt;a href=&quot;http://buzzaldrin.com/space-vision/rocket_science/aldrin-mars-cycler/&quot; rel=&quot;nofollow&quot;&gt;Mars cycler&lt;/a&gt;, might be a very good idea for the ascent vehicle to dock with in near Martian space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure someone more knowledgable here will explain how it is a huge waste of fuel to enter Mars orbit before landing, instead of just crashing down through the atmosphere on arrival.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-11-20T11:51:53.230" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6036" PostTypeId="1" AcceptedAnswerId="6042" CreationDate="2014-11-20T13:18:45.667" Score="8" ViewCount="497" Body="&lt;p&gt;While reading through the comments &lt;a href=&quot;https://space.stackexchange.com/questions/6000/what-is-the-largest-hurdle-of-the-mission-to-mars?lq=1&quot;&gt;on a question&lt;/a&gt; on the difficulty of a manned mission to Mars, people brought up the issue that sending supplies in advance would lock in the landing site. It seems to me that one solution would be to bring the supplies into orbit around Mars instead, possibly even constructing a space station in orbit as a form of base camp.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What tangible benefits can an orbiting station offer for (manned) Mars exploration? What are the possible limitations or concerns?&lt;/p&gt;&#xA;" OwnerUserId="4589" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-11-21T08:35:21.517" Title="Orbital station for Mars missions" Tags="|interplanetary|space-station|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6037" PostTypeId="1" CreationDate="2014-11-20T15:53:39.677" Score="0" ViewCount="248" Body="&lt;p&gt;To my (admittedly limited) understanding a human being in outer space wouldn't last very long, mainly due to three factors:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the extreme cold&lt;/li&gt;&#xA;&lt;li&gt;the intense radiation&lt;/li&gt;&#xA;&lt;li&gt;and the vacuum of space&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So if my understanding is correct, humans would die within seconds (if anyone could put an approximate number on this that would be great!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I want to know is that if a human being was exposed to space for half that time, would they suffer lasting and life threatening after effects?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, is there any amount of exposure to space that wouldn't result in your imminent or eventual death?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would exposure of just a hand kill you, or would you just lose that hand?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would being exposed to space from inside a room with a single open door help much (I am assuming here the room would shield you from much of the radiation in space), or would the other two factors still kill you off?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please feel free to correct my assumptions wherever I am wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many thanks for any help offered to my slightly morbid musings on space.&lt;/p&gt;&#xA;" OwnerUserId="339" LastActivityDate="2014-11-21T16:03:31.553" Title="Humans surviving in space" Tags="|eva|" AnswerCount="2" CommentCount="3" ClosedDate="2014-11-21T20:11:06.907" ContentLicense="CC BY-SA 3.0" />
  <row Id="6039" PostTypeId="2" ParentId="5989" CreationDate="2014-11-20T17:23:44.650" Score="5" Body="&lt;p&gt;There is a big problem with the proposal as-stated in the OP, and I don't believe anyone else has sufficiently hit this point. I'm quoting, with emphasis my own here:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If this scheme worked, then packets of material mined from the asteroid could be launched without the use of rocket fuel, requiring only the electricity for climbing &lt;strong&gt;the cable&lt;/strong&gt; (which would not be great, as there is no real gravity to climb against - in fact mainly it would be braking which would be required as the cable would try to fling the crawlers off prematurely). Essentially &lt;strong&gt;the cable&lt;/strong&gt; would transfer angular momentum from the asteroid to packet launches.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A cable is not a rigid member. It has tensile strength but it can bend. It can often be coiled. Cables are structurally very useful, but they only have one specific role to play. In this case, the proposal is attempting to over-use cables.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I looked at such a scheme a while ago, and here is a really brief mockup of what we're looking at:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/s6mlK.png&quot; alt=&quot;asteroid slingshot&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'll note that the darker line along which the payload moves corresponds to the conventional notion of a space elevator. Others have already pointed out that the gravity will be overcome very easily. However, the &quot;swing&quot; in the tether is not so easy to deal with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An Earth space elevator would be aligned directly with Earth's center when there is no payloads traveling up or down. Start moving lots of material into space (after all, that's the point) and the tether will be dragged along the direction of Earth's rotation. The tether will then be off-center. You can entertain this concept for an asteroid by either drawing the main elevator off-center, or add additional cables as I have done for the &quot;Coriolis Supports&quot; above. I like the above scheme because the system will reliably not swing and move about. Mathematically it doesn't matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You easily find that the &lt;em&gt;time to climb the tether&lt;/em&gt; is constrained by the &lt;em&gt;radius of the asteroid&lt;/em&gt;. Also the rate of rotation. This follows directly from the simple triangle in the diagram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where would we find such a slingshot useful? Well, &lt;a href=&quot;http://en.wikipedia.org/wiki/433_Eros&quot; rel=&quot;nofollow noreferrer&quot;&gt;Eros&lt;/a&gt; is one of the largest asteroids in the inner solar system. Very few large asteroids have a semimajor axis within Martian territory and Eros is a bit of an exception to the rule. The minimum useful requirement we might expect from it is a Hohmann transfer to/from Earth orbit. I did the calculations before, but you could easily put them together yourself if you want. Even being maximally generous, the time to climb the tether is on the order of months. Since these transfers should only take something like 8 months in the first place this is a &lt;em&gt;huge&lt;/em&gt; logistical problem!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about larger asteroids? For Ceres, Vesta, etc. it could absolutely work. You might have very few constraints on the limit of climbing rate. For these few giants it might be practical to toss things around the asteroid belt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a more practical approach on an asteroid like Eros, you might need a wacky structure kind of like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QnjYZ.jpg&quot; alt=&quot;suspension bridge&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, we're adding additional members which are not cables. For an asteroid sling shot these would need to be fully rigid, or it would need to combine compressive strength with enough mass that it's weight on the asteroid is enough to hold it in place. Kind of like that suspension bridge on Earth. By doing this, you're increasing the radius at which you are pulling the climbing payload, which is one way of increasing the torque (force x radius).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, once you've added all these elements, you have almost everything you need for a space trebuchet anyway. It is true that a powered sling would need its own power source (unlike using the asteroid's spin), but doing this would massively reduce the total mass of the structure. For this reason, I think that using the rotation of asteroids for transport is unlikely except for a few very select cases.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2014-11-20T20:34:51.913" LastActivityDate="2014-11-20T20:34:51.913" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6041" PostTypeId="2" ParentId="6032" CreationDate="2014-11-20T20:54:22.560" Score="6" Body="&lt;p&gt;It is a bit trickier due to the reduced availability and/or accuracy of terrestrial navigation aids such as the Near Earth Network and GPS.  But certainly possible though it hasn't yet been demonstrated outside of Apollo.  Rendezvous around Mars would rely on a combination of orbit solutions from DSN ranging with optical and cooperative radar measurements on board each spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MRO observations of MSL and Phoenix under parachute and IIRC of Mars Express show that ~1 km position knowledge and extremely accurate pointing are possible even with noncooperative targets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If unmanned, the initial launch trajectory would be preprogrammed based on earth's best estimate of the target spacecraft's orbit. Autonomous navigation would take place for the latter part of the rendezvous using the seeker spacecraft's onboard sensors and computers. This has already been demonstrated in earth orbit.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2014-11-24T17:25:21.130" LastActivityDate="2014-11-24T17:25:21.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6042" PostTypeId="2" ParentId="6036" CreationDate="2014-11-21T03:15:00.140" Score="10" Body="&lt;p&gt;Actually, there is one very sensible place to build an orbiting station - the moon Phobos. For example in the fictional Mars series by Kim Stanley Robinson, the majority of colonists land on Mars, but a small number establish a base on Phobos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compared with Earth's moon, Phobos orbits much, much closer to Mars. In fact it completes an orbit in less than 8 hours, compared with 1 month for Earth's moon. This puts Phobos almost in 'Near Mars Orbit'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A colony on Phobos would be in many ways easier than one on Mars, the delta-V requirement for reaching Phobos from Earth (and vice-verca) is very low - in fact less than for the Moon. Putting aside the long travel time, and the restrictive launch windows, it would be cheaper to put a base on Phobos than the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Phobos is very dark and likely rich in volatiles, such as water and hydrocarbons. These could be a valuable resource, including for export to Earth Orbit (i.e. to help fuel rockets from low Earth orbit to Mars). Conceivably fuel mined on Phobos could be used for both legs of the journey from Earth to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A manned colony on Phobos would have some advantages - one is teleoperating robots on the Martian surface without the time delay for communications from Earth. It could thus act as a staging ground for establishing the surface colony. Fuel could also be refined on Phobos then launched to the surface colony, a refinery on Phobos would not lose solar power during global dust storms. Compared with a space station in open space, a station on Phobos also enjoys protection from solar flares as the bulk of the moon can be used as a shield - this might be a minor matter as the journey in open space is already 7 months so the spaceship would have to be reasonably well protected, but it certainly doesn't hurt to have additional protection from radiation. Unlike a Mars colony, a colony on Phobos could easily lift off and move to another location, as the gravity is very low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.csc.caltech.edu/references/Hopkins-Phobos-Deimos-Paper.pdf&quot;&gt;Here&lt;/a&gt; is a pdf on missions to the moons of Mars and their role in exploration of Mars. Also a pdf on a &lt;a href=&quot;http://www.nss.org/settlement/mars/2003-SpaceColonizationUsingSpaceElevatorsFromPhobos.pdf&quot;&gt;phobos space elevator&lt;/a&gt; which indicates the kind of neat things a Phobos station might lead to.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2014-11-21T08:35:21.517" LastActivityDate="2014-11-21T08:35:21.517" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="6043" PostTypeId="1" AcceptedAnswerId="6044" CreationDate="2014-11-21T11:58:09.423" Score="13" ViewCount="4990" Body="&lt;p&gt;I am reading a &lt;a href=&quot;https://en.wikipedia.org/wiki/When_Worlds_Collide&quot;&gt;book&lt;/a&gt; where a gas giant is coming into our solar system.  It was a rogue planet, traveling between stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the knowledge we have now, would we expect a gas giant to freeze solid if is far enough away from a sun, e.g. traveling between stars, and/or being in a distant orbit around the Sun?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="519" LastEditDate="2014-11-21T14:09:43.003" LastActivityDate="2015-10-13T03:16:17.440" Title="If a gas giant is far enough away from a sun will it freeze solid?" Tags="|planetary-science|interstellar-travel|gas-giant|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6044" PostTypeId="2" ParentId="6043" CreationDate="2014-11-21T13:28:05.010" Score="13" Body="&lt;p&gt;There is an answer on wikipedia: &lt;a href=&quot;http://en.wikipedia.org/wiki/Rogue_planet#Retention_of_heat_in_interstellar_space&quot;&gt;Rogue planet&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is calculated that, for an Earth-sized object at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures above the melting point of water.[13] Thus, it is proposed that interstellar planetary bodies with extensive liquid-water oceans may exist. It is further suggested that these planets are likely to remain geologically active for long periods, providing a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It also mentions that if the planet retained a moon, it would have a source of tidal heating. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that one such rogue planet, &lt;a href=&quot;http://en.wikipedia.org/wiki/WISE_0855%E2%88%920714&quot;&gt;WISE 0855-0714&lt;/a&gt; has a temperature of 225-260 K and is the coldest such object seen so far. It is still far warmer than the 55 K temperature of Neptune, which is mostly fluid mantle.&lt;/p&gt;&#xA;" OwnerUserId="519" LastActivityDate="2014-11-21T13:28:05.010" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6045" PostTypeId="2" ParentId="6037" CreationDate="2014-11-21T13:39:06.563" Score="1" Body="&lt;p&gt;I will go deeper into the three factors you gave:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;The extreme cold&lt;/strong&gt; Well, not only cold, also heat. There's nothing that resists the hot sunwaves, so it can get hot. Temperature isn't much of a very direct problem, because space is almost vacuum, meaning that there is no matter to transfer the heat. If you forget about all the other factors, if you would stick out a hand in space, it'll not feel cold immediatly like you would feel if you enter a freezer. That's because there's no matter transferring the heat. So, your body temperature would slowly drain in forms of radiation.&#xA;If your hand is in the sun in space however, it would warm up and you would burn your hand. The temperature in space varies from 2,7 K to 497 K.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;The intense radiation&lt;/strong&gt; It's not that intense. It damages your DNA over a long time, causing cancer. Not much of a direct problem, because vacuum or temperature would kill you instead.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;The vacuum&lt;/strong&gt; This is the biggest threat. You would die in several seconds because of it. Water starts boiling, you can't breathe air, all the gas molecules would be sucked out of the body... So does the oxygen. It takes about 10-15 seconds to reach the blood from your lungs to your brains, and if that happens you pass out. Several seconds more and you are dead.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=pm6df_SExVw&quot; rel=&quot;nofollow&quot;&gt;Here's a video&lt;/a&gt; that explains all these things very well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now your questions:&#xA;Exposing your hand would not kill you, nor would lose your hand, but if you are wearing a space suit the air would escape it's suit, resulting in vacuum in the suit and then you would die.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a room with a door open will filter a very little part of radiation, but that doesn't matter because that's only usefull over a long time. Since the &quot;door&quot; is open means that the air will escape in a few milliseconds, which means you're in vacuum again.&#xA;That would kill you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And your &quot;amount of exposure of space&quot; question: I can't really give an answer to that because I don't see how you could give yourself that &quot;amount of exposure&quot;.&#xA;What I can give you is that people in the ISS have space exposure in forms of radiation, and they don't die. Radiation would give them an extra chance of cancer and other symptoms thought.&#xA;There are some other effects, see other questions for more info on that.&lt;/p&gt;&#xA;" OwnerUserId="648" LastActivityDate="2014-11-21T13:39:06.563" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6046" PostTypeId="2" ParentId="6037" CreationDate="2014-11-21T16:03:31.553" Score="1" Body="&lt;p&gt;Regarding the &quot;intense radiation&quot;: The Earth's magnetic field does a good job of protecting low Earth orbit from radiation, so depending on where you are, that isn't a huge concern. Earth's atmosphere gives even more protection, of course, so e.g. cancer risk from cumulative exposure is a thing for ISS crew, but it's not an immediate or certain thing. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-11-21T16:03:31.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6047" PostTypeId="1" AcceptedAnswerId="6538" CreationDate="2014-11-21T16:05:54.350" Score="23" ViewCount="1037" Body="&lt;p&gt;Having read that &lt;a href=&quot;http://en.wikipedia.org/wiki/Phobos_%28moon%29&quot;&gt;Phobos&lt;/a&gt; at 27 km long is around 30% porous, I am going to assume that there are a number of voids large enough for human entrance. If it turned out there was a void large enough and close enough to the surface somewhere on Phobos, it could be turned into a cheap but spacious habitat during a Mars exploration.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How could a probe detect and map voids? Are there techniques for this? Ground-penetrating radar?&lt;/li&gt;&#xA;&lt;li&gt;Would the benefits of a pre-existing cave make it worthwhile to convert it into a habitat, or is that just more work than building something on the surface?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Concerns:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;100m thick &lt;a href=&quot;http://en.wikipedia.org/wiki/Regolith&quot;&gt;Regolith&lt;/a&gt; on Phobos&lt;/li&gt;&#xA;&lt;li&gt;Seismic instability inside a cave, being as Phobos is a pile of rubble and Mars exerts tidal forces&lt;/li&gt;&#xA;&lt;li&gt;There may be ice below the regolith - a void next to some ice would be great&lt;/li&gt;&#xA;&lt;li&gt;Cosmic radiation shielding inside the cave&lt;/li&gt;&#xA;&lt;li&gt;Retaining an atmosphere inside a cave&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="519" LastEditorUserId="519" LastEditDate="2014-11-21T16:28:33.983" LastActivityDate="2015-01-23T21:36:22.467" Title="Is it possible to map the caves of Phobos?" Tags="|mars|habitat|caves|phobos|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6049" PostTypeId="2" ParentId="5603" CreationDate="2014-11-22T00:26:34.007" Score="1" Body="&lt;p&gt;One of the possible configurations of the Space Elevator is a gigantic rotating loop, going around a (very large) pulley at the Earth's surface and hanging out into space well beyond GEO. A vehicle would grab onto the ribbon just past the pulley and be lifted up to wherever it wanted to go, with no need for power other than life support. I did a rough design in which I assumed the loop would be traveling at 300 m/s -- just under Mach 1 to avoid &quot;breaking the sound barrier&quot; problems in the atmosphere. At that speed the vehicle would arrive at &lt;strong&gt;GEO in about 33 hours&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this configuration requires very strong ribbon materials, as the ribbon cannot taper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an important question because the Space Elevator transits the Van Allen radiation belts. Passengers have to get through them very quickly (as the Apollo astronauts did) or be inside heavy shielding.&lt;/p&gt;&#xA;" OwnerUserId="6666" LastActivityDate="2014-11-22T00:26:34.007" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6050" PostTypeId="2" ParentId="5073" CreationDate="2014-11-22T00:56:09.113" Score="4" Body="&lt;p&gt;Everywhere on the Space Elevator, &lt;strong&gt;&lt;em&gt;&quot;up&quot; is the direction toward the geosynchronous point&lt;/em&gt;&lt;/strong&gt; (GEO). Below GEO, gravity is exerting a stronger force on your body than the centrifugal force caused by rotation -- &quot;down&quot; is toward Earth; beyond GEO, the centrifugal force is stronger than gravity, so the net force on your body is away from the Earth. Right at GEO there's no &quot;up&quot; or &quot;down,&quot; and indeed for some distance on either side of GEO the net force is too small for your senses to detect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the surface you climb &lt;strong&gt;up&lt;/strong&gt; to GEO and then, without changing direction, you climb &lt;strong&gt;down&lt;/strong&gt; to the counterweight at the outer end of the elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rule for &lt;em&gt;&quot;what holds the Space Elevator up&quot;&lt;/em&gt; is that the sum of the net force on all the mass of the elevator above GEO must be larger than the sum of the net force on all the mass below GEO.&lt;/p&gt;&#xA;" OwnerUserId="6666" LastActivityDate="2014-11-22T00:56:09.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6051" PostTypeId="2" ParentId="6043" CreationDate="2014-11-22T03:30:34.090" Score="6" Body="&lt;p&gt;Eventually, yes but I don't think that it has much to do with the sun. Jupiter's magnetosphere shunts all of the solar wind (plasma) from the sun and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth. -&lt;a href=&quot;http://nineplanets.org/jupiter.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;nineplanets.org&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Presumably, Jupiter will freeze some day regardless of sunlight (having assumed the sun is immutable). I would compare the amount of energy Jupiter is receiving from the sun to the heat you add to your coffee by stirring it with a spoon (friction). This is probably a gross understatement but it's one way to look at it.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The magnetosphere of Jupiter is the &lt;strong&gt;cavity created in the solar wind&lt;/strong&gt; by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosphere is the largest and most powerful of any planetary magnetosphere in the Solar System, and by volume the largest known continuous structure in the Solar System after the heliosphere.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Jupiter's internal magnetic field is generated by electrical currents&lt;/strong&gt; in the planet's outer core, which is composed of liquid metallic hydrogen. Volcanic eruptions on Jupiter's moon Io eject large amounts of sulfur dioxide gas into space, forming a large torus around the planet. Jupiter's magnetic field forces the torus to rotate with the same angular velocity and direction as the planet. The torus in turn loads the magnetic field with plasma, in the process stretching it into a pancake-like structure called a magnetodisk. In effect, Jupiter's magnetosphere is shaped by Io's plasma and its own rotation, rather than by the solar wind like Earth's magnetosphere. Strong currents in the magnetosphere generate permanent aurorae around the planet's poles and intense variable radio emissions, which means that Jupiter can be thought of as a very weak radio pulsar. Jupiter's aurorae have been observed in almost all parts of the electromagnetic spectrum, including infrared, visible, ultraviolet and soft X-rays.&lt;/p&gt;&#xA;&lt;p&gt;The action of the magnetosphere traps and accelerates particles, producing intense belts of radiation similar to Earth's Van Allen belts, but thousands of times stronger. &lt;strong&gt;The interaction of energetic particles with the surfaces of Jupiter's largest moons markedly affects their chemical and physical properties. Those same particles also affect and are affected by the motions of the particles within Jupiter's tenuous planetary ring system.&lt;/strong&gt; Radiation belts present a significant hazard for spacecraft and potentially to human space travelers. -&lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Magnetosphere of Jupiter, Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There are a lot of variables; Jupiter is its own mini solar system. Humans have neither the means nor the time to study the effects of it otherwise (I.e, take away its moons, negate the sun or put Jupiter alone in a box for billions of years) and I wouldn't trust any simulation for a definitive answer. The WISE &lt;em&gt;object&lt;/em&gt; that Phil mentions could be compared to a rogue planet, and it is surprisingly similar in size (however its &lt;em&gt;mass&lt;/em&gt; is 3~10 times larger). &lt;a href=&quot;http://en.wikipedia.org/wiki/Jupiter_mass#mediaviewer/File:BrownDwarfs_Comparison_01.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter Mass, Wiki&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/trmEwm.png&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;If the sun had the capability of keeping Jovian planets from freezing I'd expect Mercury to be completely molten. Given the proximity of Mercury and Jupiter to the sun I believe tidal forces and convection to be the much larger factors at play here than sunlight. The point is moot. You can't live on gas giants anyway and even if it was frozen solid, 2.5x normal gravity would not be pleasant (and a bit nippy). The chances of a rogue planet entering our solar system and achieving Goldilocks status are, wait for it... &lt;em&gt;astronomically&lt;/em&gt; [echo] slim.&lt;/p&gt;&#xA;" OwnerUserId="6667" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-12T18:00:15.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6052" PostTypeId="1" CreationDate="2014-11-22T06:55:35.847" Score="1" ViewCount="559" Body="&lt;p&gt;What is the difference between colloid thrusters and ion electro spray thrusters&lt;/p&gt;&#xA;" OwnerUserId="3542" LastActivityDate="2018-03-05T03:46:39.567" Title="Difference between colloid thrusters and electro spray thrusters" Tags="|propulsion|ion-thruster|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6053" PostTypeId="2" ParentId="6052" CreationDate="2014-11-22T12:19:44.360" Score="2" Body="&lt;p&gt;They appear to be two names for the same thing. See &lt;a href=&quot;http://lmts.epfl.ch/MEMS-ion-source&quot; rel=&quot;nofollow&quot;&gt;http://lmts.epfl.ch/MEMS-ion-source&lt;/a&gt;, for example.&lt;/p&gt;&#xA;" OwnerUserId="6214" LastActivityDate="2014-11-22T12:19:44.360" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6055" PostTypeId="2" ParentId="810" CreationDate="2014-11-22T17:30:30.873" Score="2" Body="&lt;p&gt;There are satellites in polar orbit. Such an orbit can be effectively retrograde, depending on whether the launch is toward the North or South Pole.&lt;/p&gt;&#xA;" OwnerUserId="6672" LastEditorUserId="168" LastEditDate="2015-01-12T18:04:00.063" LastActivityDate="2015-01-12T18:04:00.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6057" PostTypeId="1" AcceptedAnswerId="6058" CreationDate="2014-11-22T21:23:17.883" Score="1" ViewCount="180" Body="&lt;p&gt;I was watching &lt;a href=&quot;https://www.planet.com&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt; about how the mini-satelites of PlanetLabs are being launched from the International Space Station, and something immediately came to my mind:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How could they point camera in the right direction?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="962" LastActivityDate="2014-11-22T23:16:41.330" Title="Mini-satelite how to stabilize orientation?" Tags="|artificial-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6058" PostTypeId="2" ParentId="6057" CreationDate="2014-11-22T23:10:58.567" Score="3" Body="&lt;p&gt;Typically, in a small satellite, a &lt;a href=&quot;http://en.wikipedia.org/wiki/Reaction_wheel&quot; rel=&quot;nofollow&quot;&gt;reaction wheel system&lt;/a&gt; is used to turn the craft, and optical detection used to determine orientation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cubesatkit.com/docs/datasheet/DS_CSK_ADACS_634-00412-A.pdf&quot; rel=&quot;nofollow&quot;&gt;Here's a module&lt;/a&gt; intended for use in CubeSats like PlanetLabs' which uses reaction wheels for orientation and, apparently, magnetometers and sun sensors (optical?) to determine orientation.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-11-22T23:16:41.330" LastActivityDate="2014-11-22T23:16:41.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6059" PostTypeId="1" CreationDate="2014-11-22T23:16:43.163" Score="4" ViewCount="730" Body="&lt;p&gt;Reading an article &lt;a href=&quot;http://aviationweek.com/space/final-sls-engines-are-still-unknown&quot; rel=&quot;nofollow&quot;&gt;about new engines for the SLS&lt;/a&gt; (Summary: There is no plan what to do once the current SSME's are expended), they mention that the J-2X is too powerful for the upper stage of SLS. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ultimately‭, ‬the 294,000-lb‭.-‬thrust J-2X engine may be too powerful for the SLS upper stage‭, ‬at least in the 105,000-ton Block IB‭ ‬intermediate variant en route to the 130-ton version needed for Mars missions‭. ‬“Essentially‭, ‬that stage wants to be about‭ [‬120,000‭ ‬lb‭. ‬thrust‭] ‬total‭,‬”‭ ‬May says‭, ‬and the agency is considering four RL-10s or two‭ ‬“60k-class”‭ ‬engines instead‭.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The original J-2 was about 234 Klbs of thrust. The J-2X seems to be uprated to 294 Klbs thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless, the J-2 was the second stage engine on Saturn V. There was a cluster of 5 J-2's. The third stage used a single J-2. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this leaves me wondering, if the upper stage on Saturn V was appropriate at 234 Klbs thrust, why is SLS, which is meant vaguely to be in the same class, more likely to need only 120 Klbs thrust?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-11-23T00:04:19.317" LastActivityDate="2014-11-23T00:04:19.317" Title="If the J-2X is too powerful for SLS, why was it appropriate for Saturn V?" Tags="|engines|saturn-v|sls|j-2|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6060" PostTypeId="1" AcceptedAnswerId="6061" CreationDate="2014-11-22T23:21:20.140" Score="10" ViewCount="1242" Body="&lt;p&gt;Radiation is often mentioned as a serious hazard to the health of astronauts and colonists on worlds without magnetospheres, I presume that the majority of this harmful radiation is the ultraviolet component of sunlight and bombardment by the solar wind, and thus radiation exposure would be greatly reduced at night. Is this correct? Or between cosmic rays and induced radioactivity in the surface is the environment still rather too hot for comfort?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another aspect of this question is whether a permanently shaded crater would be significantly less radioactive than a crater exposed to day and night, does direct solar bombardment induce significant radioactivity in the surface?&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-10-19T18:40:19.720" Title="Would working at night on the surface of Mars/Luna protect from radiation?" Tags="|colonization|radiation|space-weather|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6061" PostTypeId="2" ParentId="6060" CreationDate="2014-11-23T01:27:36.553" Score="14" Body="&lt;p&gt;The most dangerous &lt;a href=&quot;http://www.nasa.gov/pdf/284273main_Radiation_HS_Mod1.pdf&quot;&gt;radiation events&lt;/a&gt; in space are due to solar flares and coronal mass ejections - &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_particle_event&quot;&gt;solar proton events&lt;/a&gt; (SPEs). If you are caught outside in just your spacesuit or a lightly shielded vehicle during a major SPE, the dose of ionizing radiation (high-energy protons) could be enough to kill you if you stayed outside, oblivious to the danger, for several hours. SPEs don't happen that often, and when they do, you will have a few minutes warning that they are coming that could allow you to get to shelter in time to avoid the really dangerous ionizing radiation. The x-ray and gamma rays from the event will arrive at light speed and give advance warning of the wave of high-energy protons on its way. Astronauts working outside on a planet would likely try to always remain within reach of a shelter that can shield them from the ionizing radiation when alerted by heightened x-ray and gamma radiation of a SPE on its way. The x-rays and gamma rays aren't good but aren't that dangerous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you went out at night you would avoid the risk of SPEs altogether. But the ionizing radiation in interplanetary space is mostly GCRs - galactic cosmic rays. When the sun is more active, during solar maximums or briefly during SPEs, the amount of cosmic radiation is reduced, but it is always a concern. The cosmic radiation dose in interplanetary space has &lt;a href=&quot;http://www.srl.caltech.edu/ACE/ASC/DATA/bibliography/ICRC2005/usa-mewaldt-RA-abs1-sh35-oral.pdf&quot;&gt;been assessed&lt;/a&gt; by the &lt;a href=&quot;http://www.srl.caltech.edu/ACE/&quot;&gt;ACE probe&lt;/a&gt; as being about 0.4 Sv/year during the solar minimum of the sun's 11 year solar cycle, and 0.9 Sv/year during solar maximum. On any planet surface, half of that will be blocked out by the mass of the planet under your feet, making the dose 0.2 Sv/year to 0.45 Sv/year - if you were outside all the time. Shall we say for the sake of argument astronauts could be outside up to a quarter of the time? That would be 0.05 to 0.1 Sv/year, roughly. Whether you are out during the day or night makes no real difference to this dose, night work would just prevent the risk of being out of reach of adequate shelter in the event of an SPE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0.1 Sv is associated with a &lt;a href=&quot;http://www.nrc.gov/about-nrc/radiation/health-effects/rad-exposure-cancer.html&quot;&gt;slightly heightened&lt;/a&gt; risk of cancer. Crew on the ISS receive doses of 0.08 Sv over the course of their 6 month stays during solar maximums. Ionizing radiation is considered the cause of an increased incidence of &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2004/22oct_cataracts/&quot;&gt;cataracts&lt;/a&gt; among astronauts. Other than that, it is &lt;a href=&quot;https://en.wikipedia.org/wiki/Health_threat_from_cosmic_rays&quot;&gt;not clear&lt;/a&gt; what health risk this level of cosmic radiation exposure represents. If you lived on an off-world base it would be something to keep in mind as the decades wore on, maybe, but your main defense would to always be close enough to adequate shelter to avoid SPEs, and ensure you would be notified of their approach. If you do that, whether you are out in the day or the night makes no real difference.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-11-23T01:41:06.777" LastActivityDate="2014-11-23T01:41:06.777" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6062" PostTypeId="1" CreationDate="2014-11-23T05:45:42.087" Score="5" ViewCount="1120" Body="&lt;p&gt;Could we use some kind of rotating sling or magnetic loop to accelerate a spaceship to (very) high speed and then release it and hurl it away?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today all spaceships are propelled by engines and some fuel/energy source on board themselves. An alternative concept which has been proposed is to put the engine and fuel at or near Earth, and beam the propulsive power to the spaceship. The basic advantage would be that literally all resources in our world would be available for it, instead of only the &quot;luggage&quot; which we can put on the spaceship itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But wouldn't it, in principle, be even better to accelerate the spaceship before we launch it? Speed doesn't weigh down the payload.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-11-24T00:52:19.273" Title="Could a spaceship be &quot;charged&quot; with kinetic energy instead of having to be propulsed?" Tags="|advanced-propulsion|" AnswerCount="2" CommentCount="6" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6063" PostTypeId="2" ParentId="6062" CreationDate="2014-11-23T12:14:38.210" Score="3" Body="&lt;p&gt;In theory, yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://en.wikipedia.org/wiki/Non-rocket_spacelaunch&quot; rel=&quot;nofollow&quot;&gt;Non-rocket spacelaunch on Wikipedia&lt;/a&gt; for examples. (These are ground-based, but some of the concepts could be adapted to an in-orbit system)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, due to the high speeds needed for a spaceship, the structures needed to support such a launch would be huge. For example, if you want to achieve a velocity of 10 km/s, and willing to bear an acceleration of 5 g, a linear accelerator would be 10 km long; an equivalent circular or rotating structure would have to have a radius of a least 20 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an in-orbit design, you would have to bring this structure into orbit first, which would be a huge investment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rotating structures have an additional cost as you need to provide the rotational energy for the structure as well. (And you won't want to do this via rockets at the ends of the structure, as then you would have to accelerate the fuel to rotational speed, which means you are still subject to the tyranny of the rocket equation.)&lt;/p&gt;&#xA;" OwnerUserId="548" LastEditorUserId="548" LastEditDate="2014-11-23T14:27:32.383" LastActivityDate="2014-11-23T14:27:32.383" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6065" PostTypeId="1" AcceptedAnswerId="6068" CreationDate="2014-11-23T16:01:08.717" Score="8" ViewCount="1881" Body="&lt;p&gt;I was exploring some queries about the moon and came across the question &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/q/221/1006&quot;&gt;Which artificial satellites in lunar orbit are currently active?&lt;/a&gt;&lt;/em&gt;. In this, the accepted answer says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The lunar gravity field varies by as much as 1%.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I want to know why lunar gravity is not constant as Earth's is.&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2014-11-24T15:25:29.707" Title="Why does lunar gravity decrease?" Tags="|the-moon|gravity|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6066" PostTypeId="2" ParentId="5939" CreationDate="2014-11-23T16:03:13.100" Score="8" Body="&lt;p&gt;There are several issues at play here.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Are the raw materials needed to be found on the moon?&lt;/li&gt;&#xA;&lt;li&gt;Can the raw materials be realistically harvested on the moon?&lt;/li&gt;&#xA;&lt;li&gt;Can the raw materials be processed into a rocket propellant on the moon?&lt;/li&gt;&#xA;&lt;li&gt;Are rocket engines which use this propellant feasible?&lt;/li&gt;&#xA;&lt;li&gt;Are there more suitable materials for this purpose to be found on the moon?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Let's look at them in turn:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Are the raw materials needed to be found on the moon?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As per the OP's &lt;a href=&quot;http://www.asi.org/adb/06/09/03/02/095/al-o-propellants.html&quot; rel=&quot;noreferrer&quot;&gt;cited paper&lt;/a&gt; the materials exist in sufficient quantity on the moon. This is well confirmed via many sources, as is shown at &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon_rock&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Can the raw materials be realistically harvested on the moon?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Yes they can, and in fact were &quot;harvested&quot; by Apollo. Much refining would be required, but the raw materials can easily be harvested.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Can the raw materials be processed into a rocket propellant on the moon?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As per the paper cited above, the rocket propellant is a suspension of aluminum in LOX. Realistically, I do not foresee this suspension being used in actual production however the researchers do mention that this is not the only possible concoction. Perhaps in the moon's low-gravity, low-pressure, low-temperature environment an easier to form and easier to handle concoction may be feasible. Additional research is needed in this particular area.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Are rocket engines which use this propellant feasible?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Again using &lt;a href=&quot;http://www.wickmanspacecraft.com/lsp.html&quot; rel=&quot;noreferrer&quot;&gt;a paper cited by the OP&lt;/a&gt; the answer seems to be true. At the current state of technology (admittedly very early) it seems that only small, low power engines are possible. However, the moon is a low-gravity world with no appreciable atmosphere. Such a low-power application may be be enough for moon-station to moon-station hopping.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Are there more suitable materials for this purpose to be found on the moon?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Almost certainly. The electrolysis of water (used to acquire the oxygen) will produce hydrogen as a byproduct, as a trivial example. Water is limited in it's locations on the Moon, however, but can be found in some locations.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Conclusion&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Based on the above, I conclude that given an established mining and manufacturing presence on the moon, and giving enough power, &lt;strong&gt;an aluminum-oxygen rocket engine might use locally-sourced fuel for jaunting around the moon&lt;/strong&gt;. However, I do not expect that this technology could be used to power ships from the moon to other destinations.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="25" LastEditDate="2015-07-24T14:28:49.417" LastActivityDate="2015-07-24T14:28:49.417" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6067" PostTypeId="2" ParentId="6065" CreationDate="2014-11-23T16:31:15.997" Score="10" Body="&lt;p&gt;The answer appears to be &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)&quot;&gt;mascons&lt;/a&gt;. &quot;Mascons&quot; is short for &quot;&lt;strong&gt;Mas&lt;/strong&gt;s &lt;strong&gt;con&lt;/strong&gt;centration&lt;strong&gt;s&lt;/strong&gt;&quot;, which are regions of a celestial body where there are anomalies in density, which lead to abrupt changes in the local gravitational field of the body. They can even impact lunar satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/1/1d/MareSmithiiTG.jpg&quot;&gt;Here's&lt;/a&gt; an example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/1/1d/MareSmithiiTG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mascons may be formed by subsurface processes, such as changes in the Moon's crust following an impact with another small body. Hypothetical sub-surface lava flows are another explanation. However, the impact theory has gained more support, in part because of the high frequency in impacts on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read more about mascons &lt;a href=&quot;http://www.nasa.gov/mission_pages/grail/multimedia/zuber4.html&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://science.time.com/2013/05/30/revealed-the-awesome-explanation-for-the-moons-extra-gravity/&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2014-11-23T16:31:15.997" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6068" PostTypeId="2" ParentId="6065" CreationDate="2014-11-23T16:39:45.367" Score="17" Body="&lt;p&gt;The Moon's gravity does not &quot;decrease&quot;. The Moon is not of uniform composition. It contains a variety of materials. The different materials have different densities. These materials are not uniformly distributed. So, the gravity field around the Moon is not uniform. If you imagine a sphere of some radius exactly centered on the Moon's center of mass, gravity will not be the same at all points on the sphere. At any given point, it is constant over time, just not necessarily the same as all other points on that sphere. So, any object orbiting the Moon is not going to travel in an ideal elliptical or circular path. It will be perturbed by the irregularities in the Moon's gravity field. These effects can add up quickly, so a Lunar orbiting satellite has to make frequent adjustments to maintain an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The farther away you get from the Moon, the less significant those irregularities become, but you don't have to go very far before another factor takes hold: the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth also has an irregular distribution of mass, so Earth orbiting satellites also have to contend with similar perturbations, but to a smaller degree. The GPS satellites have to account for these perturbations. Although they don't have to make adjustments to their orbits very often, the paths of the satellites are affected enough to impact system accuracy if not factored in to the descriptions of their trajectories.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2014-11-23T16:39:45.367" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6071" PostTypeId="1" CreationDate="2014-11-23T21:05:55.797" Score="22" ViewCount="3960" Body="&lt;p&gt;I am not affiliated with any space agency. I do not have the resources to do so, but am I allowed to send a rocket to LEO without talking to any federal agency? Obviously that could be a danger to satelites and to the ISS, so I strongly suspect this is not allowed. What procedure does an individual have to follow to launch a rocket into LEO?&lt;/p&gt;&#xA;" OwnerUserId="6681" LastEditorUserId="5963" LastEditDate="2014-11-23T21:49:56.953" LastActivityDate="2014-11-24T10:22:53.557" Title="Am I allowed to send a rocket to LEO?" Tags="|rockets|law|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6072" PostTypeId="2" ParentId="6071" CreationDate="2014-11-23T21:45:25.240" Score="19" Body="&lt;p&gt;Short answer: No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(if you live in the USA)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer:&#xA;Law is complex, but for a review of how things stood in 2005 check out: &lt;a href=&quot;http://www.colonyfund.com/Reading/papers/NH_FAA_2005.pdf&quot;&gt;http://www.colonyfund.com/Reading/papers/NH_FAA_2005.pdf&lt;/a&gt;&#xA;It's quite a good article on most of the relevant FAA regulations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more detailed reading see the Office of Commercial Space Transportation: &lt;a href=&quot;https://www.faa.gov/about/office_org/headquarters_offices/ast/regulations/&quot;&gt;https://www.faa.gov/about/office_org/headquarters_offices/ast/regulations/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And in particular this section about “Amateur Rocket and Commercial Space Operations”: &lt;a href=&quot;https://www.faa.gov/air_traffic/publications/atpubs/AIR/air3101.html&quot;&gt;https://www.faa.gov/air_traffic/publications/atpubs/AIR/air3101.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However all this only applies inside the US, take a short trip into (or over, a la Stratolaunch) international waters and, as far as I know, you can do whatever you want. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a good answer on Quora that addresses regulation once you reach orbit: &lt;a href=&quot;http://www.quora.com/Are-the-orbits-of-satellites-regulated-by-an-organisation&quot;&gt;http://www.quora.com/Are-the-orbits-of-satellites-regulated-by-an-organisation&lt;/a&gt;&#xA;In short: there isn't much apart from radio frequency that is regulated.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2014-11-23T21:45:25.240" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6073" PostTypeId="2" ParentId="6062" CreationDate="2014-11-23T23:59:08.553" Score="2" Body="&lt;p&gt;There are a lot of schemes for transferring angular momentum from one thing to another but it's hard to find a scheme to exploit the resources of Earth better than rocket fuel (the energy density of kinetic energy storage is very low compared with the energy density of rocket fuel - it's comparable to batteries). &#xA;You could 'drop' rocks from the moon (actually launch with a mass driver), and then transfer that momentum using a tether to a payload coming up from Earth. The hunk of moon rock slows down so it doesn't hit like a fusion bomb, and the payload from Earth gets enough speed to reach Luna.&#xA;The problem with using electrical energy is the 'place to stand' problem &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“Give me a place to stand and I will move the whole Earth”&#xA;  — Archimedes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A motor needs to be mounted to something in order to move something else. And whatever it is mounted to, will receive an equal and opposite force. This is true whether you use a rotating motor or a linear accelerator. A mass driver would launch itself towards Earth as surely as it launches payloads away from Earth. What are you going to push against to unleash the stored or beamed energy? This is the challenge of turning beamed energy from Earth into kinetic energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nevertheless it is possible to imagine a system which should work. In principle you could build a slingshot which uses a heavy flywheel, coupled to a spindle boom cable assembly. As the flywheel spins, the boom turns in the opposite direction and drags the cable and any attached payload. Spin it up at an appropriate speed, and it will work exactly like a slingshot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i1234.photobucket.com/albums/ff418/dfpicts/flywheelsling.png&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;i&gt;Boom and cable would be relatively longer in reality&lt;/i&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the payload whips around the flywheel, they will orbit their common center of mass thanks to being tied together by the cable. I'm not sure exactly what the dynamics would be as it's really hard to visualize dynamics in microgravity and vacuum, but I believe it should be possible to gradually increase the angular velocity of the system - the flywheel in one direction, the payload in the other direction. You might get some nasty oscillations but there are potential solutions to that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the system has come up to enough speed, the spaceship could be released and for a long enough cable most of that angular momentum would become linear momentum, flinging the payload off into space at a speed depending on the length of the cable and the angular momentum imparted to the wheel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are a few problems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all as noted the flywheel and ship were 'orbiting' each other bound by the cable, when the ship is released the sling assembly flies away in the opposite direction. How fast, depends on the relative mass of the ship and sling assembly. This doesn't always have to be a problem, as it would let the flywheel change orbits. Another way to resolve it is to spin two payloads at once, one on each side, that would allow the flywheel to remain still and significantly reduce mass requirements for stability. You just need to have two things which need to go in opposite directions (but one could use a gravity turn around Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second problem is discharging the flywheel, because you just can't keep dumping more angular momentum into it because it would explode. But what you can do is attach another payload to the hook, and then brake the flywheel, that would transfer the momentum from the rapidly spinning flywheel to the payload. This would mean the system would spin the payload first in the clockwise direction, then the anticlockwise direction, needing only electrical power to spin or brake the flywheel (in this case no regenerative braking is possible - it's the 'place to stand' problem - all the energy dumped into the system ends up as kinetic energy of the payload(s)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third problem is of course the 'place to stand' problem, where do you put the motor which spins up the flywheel? Basically the flywheel and spindle have to 'push' against each other. So you could put the electromagnets and driving circuitry in the flywheel, or the spindle, whichever is most convenient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is another problem with this idea though. That is getting power to the motor while the two parts of the system are both spinning at thousands of rpm - there is no stationary point to attach anything! If the flywheel is huge so it doesn't need to spin very fast, then this would be easy as you could just mount solar panels on it (or receivers for beamed energy). Or if the boom and cable is very long so it doesn't rotate very fast then you could beam power to the spindle, which won't be moving much at all. The boom is at heart a compression structure so it would be heavy and expensive to make it long (although there would be other advantages to a long boom). So ideally you would use a heavy flywheel, which in practise would probably mean a captured asteroid, preferably an iron-nickel one, but it would also be possible to wrap up a bunch of space rocks in a cylinder of high tensile strength.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the system is orbit-stabilized by having it launch two payloads at once in opposite directions, then a power satellite could be placed nearby and relay power beamed from Earth, the nearby satellite could easily track both the Earth beamer stations, and the small power receivers on the rotating sling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should also be noted that the whole analysis could be performed on a linear accelerator as well. As with a rotating sling in order to stabilize the orbit it would be required to launch an an equal payload in the opposite direction. This might sound wasteful, but it would be cheaper to launch a 1 tonne spaceship and 1 a tonne rock into LEO, than a 1 tonne spaceship and 2 tonnes of rocket fuel. Having to launch stuff in the opposite direction thus isn't a deal breaker, as long as the mass driver provides more speed than the equivalent mass of rocket fuel would have given. A linear accelerator would not have rotation to worry about which would be great, but would probably need to be more massive than the equivalent sling, it would need flywheels (or superconductors) for energy storage equivalent to the flywheels in the sling, AND, it would need all the mass driver magnets as well.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.480" LastActivityDate="2014-11-24T00:52:19.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6074" PostTypeId="1" AcceptedAnswerId="6078" CreationDate="2014-11-24T01:12:56.957" Score="3" ViewCount="631" Body="&lt;p&gt;I recently answered &lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/4444/whats-the-most-logical-shape-for-my-city-in-space-space-station&quot;&gt;a question&lt;/a&gt; on Worldbuilding about space stations. In my answer, I proposed a toroidal station which would be composed up many small segments, each rotating. As was pointed out, this would require at least one an airlock between each of the wedges bookending each segment and the segment itself. The airlocks would have to rotate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we have the current technology to build rotating airlocks? Have any designs been tested?&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:23.630" LastActivityDate="2014-12-24T11:24:08.280" Title="Do we have the technology to make large rotating airlocks?" Tags="|space-station|airlock|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6076" PostTypeId="1" CreationDate="2014-11-24T05:41:23.850" Score="1" ViewCount="200" Body="&lt;p&gt;For an Ion thruster which emits single charged ions, the charge to mass ratio is find out by considering the charge( whether it is positive or negative) and the mass of that ion.when the ion thruster is a bipolar type which emits positive and negative ions at a time, how can we calculate charge to mass ratio of that system&lt;/p&gt;&#xA;" OwnerUserId="3542" LastActivityDate="2014-11-24T05:41:23.850" Title="Charge to Mass ratio of Ion thruster" Tags="|propulsion|ion-thruster|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6077" PostTypeId="2" ParentId="6071" CreationDate="2014-11-24T10:22:53.557" Score="10" Body="&lt;p&gt;No. As ForgeMonkey notes, it's banned in the US. The same restrictions apply pretty much anywhere. The problem is that the necessary technology is essentially dual-use, with clear military applications. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now not all countries are signatories to the various arms control treaties, so e.g. North Korean citizens aren't bound by these treaties. But in such dictatorships pretty much anything happens with government approval, and this certainly would. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sea launches would be from a vessel, which still has to be registered somewhere. Trying to launch a rocket from an unflagged ship would very quickly draw all kinds of attention to you, including probably a few major navies.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2014-11-24T10:22:53.557" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6078" PostTypeId="2" ParentId="6074" CreationDate="2014-11-24T10:51:45.433" Score="3" Body="&lt;p&gt;I'd go with &quot;yes, but&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two tricky parts: You need a large, gliding seal, and you need to build conduits for cables, pipes, etc. I'm not a mechanical engineer, so maybe I misrepresent a few things. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google for largest axial face seal found &lt;a href=&quot;http://www.process.vogel.de/stroemungstechnik/dichtungen/articles/315384/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; about a seal at 390mm diameter .. that's not much. Admittedly, thiy is for a compressor where the pressure differential is far higher than what you'd find in a space station, so there may be hope. The challenges in building seals are in creating and aligning to very, very smooth surfaces. for a large seal hoop this means a large lathe and probably many headaches in transporting and installing. But I'd say it's doable if someone throws enough money at the problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second challenge are conduits. &lt;a href=&quot;http://hydraulicspneumatics.com/200/TechZone/FittingsCouplin/Article/False/6421/TechZone-FittingsCouplin&quot; rel=&quot;nofollow&quot;&gt;We know how to transport fluids between rotating elemtents&lt;/a&gt;. The conduit would always need to be in the middle of the airlock, where it 'costs the most space (imagine the smallest circle you can fit a 1x1m cupboard through. now imagine the smallest circle you can fit said cupboard through if the center is blocked), compounding the problems with seal described above. You will also build your conduits once, retrofitting for more connections once they are built will be hard.&lt;br&gt;&#xA;For electrical power, brush contacts are available, for signal transmission you'd probably use something wireless.&#xA;Why do I talk so much about conduits? because you will want to move air, water, sewage, fuel  and a host of of stuff from on segment to the other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, I think we can but we will have a hard time to interconnct the parts of the station in a meaningful way.&lt;/p&gt;&#xA;" OwnerUserId="102" LastActivityDate="2014-11-24T10:51:45.433" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6079" PostTypeId="2" ParentId="6074" CreationDate="2014-11-24T10:52:52.590" Score="1" Body="&lt;p&gt;We can have rotating airlocks with current tech, but their durability and performance is rather poor (and of course the speed difference can't be too big.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The secret is making the airlock as two (or more) interlocking (&quot;telescope&quot;) pipes, with grease in between; enough of greased overlay between the pipes so that its friction/viscosity makes air pressure insignificant. Of course the joint will leak, albeit very slowly, and so, must be refilled with grease, and of course the viscosity will put quite a bit of resistance against the rotation, so don't expect this to be a zero-loss joint between modules rotating at different speed.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2014-11-24T10:52:52.590" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6080" PostTypeId="1" AcceptedAnswerId="6081" CreationDate="2014-11-24T13:03:50.933" Score="6" ViewCount="3110" Body="&lt;p&gt;The Dragon is designed to be able to bring unpressurized cargo to the ISS. How can such cargo be brought on board the ISS? What uses can such unpressurized cargo have?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="51" LastEditDate="2014-11-24T13:55:34.800" LastActivityDate="2014-11-24T16:31:51.040" Title="How is unpressurized cargo useful at the ISS?" Tags="|iss|spacex|dragon|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6081" PostTypeId="2" ParentId="6080" CreationDate="2014-11-24T13:41:58.140" Score="8" Body="&lt;p&gt;Unpressurized volume is used to deliver payloads that will be used externally on the International Space Station (ISS), i.e. &lt;em&gt;external cargo&lt;/em&gt;. It also holds the two Dragon's solar panels and about 14 cubic meters is available for other unpressurized cargo, or 34 cubic meters with extended trunk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One such example unpressurized cargo was the RapidScat that was delivered to the station aboard the Dragon SpX-4 and installed on &lt;a href=&quot;https://en.wikipedia.org/wiki/Columbus_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Columbus module's&lt;/a&gt; External Payloads Facility (CEPF) by &lt;a href=&quot;https://en.wikipedia.org/wiki/Dextre&quot; rel=&quot;noreferrer&quot;&gt;Dextre&lt;/a&gt; (Special Purpose Dexterous Manipulator) on &lt;a href=&quot;https://en.wikipedia.org/wiki/Mobile_Servicing_System&quot; rel=&quot;noreferrer&quot;&gt;Canadarm2&lt;/a&gt;. Here's RapidScat in action as seen from &lt;a href=&quot;http://www.ustream.tv/channel/live-iss-stream/theater&quot; rel=&quot;noreferrer&quot;&gt;one of ISS external cams&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/cdkOW.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole installation of RapidScat, along with some additional info on Dragon's unpressurized volume, are described in detail in &lt;a href=&quot;http://www.spaceflight101.com/dragon-spx-4-mission-updates.html&quot; rel=&quot;noreferrer&quot;&gt;Dragon SpX-4 Mission Updates Spacefligh101 article&lt;/a&gt;. Same source also has an &lt;a href=&quot;http://www.spaceflight101.com/rapidscat.html&quot; rel=&quot;noreferrer&quot;&gt;extensive article on the RapidScat&lt;/a&gt;. &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Dragon spacecraft&lt;/a&gt; adds that the first Dragon to deliver unpressurized cargo to the ISS was the &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-2&quot; rel=&quot;noreferrer&quot;&gt;SpaceX CRS-2 mission&lt;/a&gt; that delivered two Heat Rejection Subsystem Grapple Fixtures (HRSGFs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2012/10/dragon-iss-stay-minor-issues-falcon-9-investigation/&quot; rel=&quot;noreferrer&quot;&gt;NASASpaceFlight&lt;/a&gt; elegantly and concisely describes this Dragon's capability with:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;CRS-2 will debut the use of Dragon’s Trunk section, capable of&#xA;  delivering unpressurized cargo, prior to the payload being removed by&#xA;  the ISS’ robotic assets after berthing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Only other cargo delivery vehicle to the ISS that is currently also capable of delivering unpressurized cargo is JAXA's &lt;a href=&quot;https://en.wikipedia.org/wiki/H-II_Transfer_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;H-II Transfer Vehicle&lt;/a&gt; (HTV) with unpressurized payload mass of 1,500 kg.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-11-24T13:41:58.140" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6082" PostTypeId="2" ParentId="6080" CreationDate="2014-11-24T13:57:49.323" Score="7" Body="&lt;p&gt;There are many payloads that are large, and need transport to the station.  Things the Shuttle used to provide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you carried them pressurized, they would need to be sized to fit through the CBM port to get out of the Dragon, then through one of the airlocks (Quest or Russian side) to get out of the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle left some large line replaceable units (LRU) on the truss as spares. Should that supply be exhausted, Dragon could launch more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CanadaArm can reach in and grab the payloads and move them as needed. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-24T13:57:49.323" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6083" PostTypeId="2" ParentId="6080" CreationDate="2014-11-24T14:46:58.677" Score="2" Body="&lt;p&gt;As an analogy, suppose you are taking an airplane flight from New York to Los Angeles. Knowing in advance that the airplane food, if any, will be garbage (said nicely), you buy some food to take with you on the flight. When you partake a couple of hours into the flight you notice that your bag of potato chips seems to be about to explode, but your nice sandwich remains about the same as you bought it at sea level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both your sandwich and your bag of potato chips are in this sense &quot;unpressurized cargo&quot; (but see below). With regard to your sandwich, it's still edible because the approximately 1/3 drop in pressure isn't near enough to make it inedible. With regard to your bag of chips, it's still edible because the bag was sealed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting back to the problem at hand, anything that can handle the drop from one atmosphere of pressure to vacuum is &quot;unpressurized cargo.&quot; Some items are sealed against vacuum, so it's fine to expose those items to vacuum. Other items are oblivious to pressure changes, so once again it's fine to expose those items to vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stuff that can't handle anything close to that drop in pressure from one atmosphere to zero is &quot;pressurized cargo.&quot; This stuff needs special handling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the passengers in a modern airliner need special handling. The pressure in the cabin of such an airplane is not ambient. The pressure inside an airplane that flies from New York to Los Angeles is instead an artificially maintained environment. The pressure at the altitude at which that airplane flies is extremely low. If exposed to that low pressure, you would die in short order. Even peoples who have had generations of acclimation would die if exposed to that pressure. The passenger section of a modern airliner is a pressurized cargo space.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-11-24T14:46:58.677" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6084" PostTypeId="2" ParentId="6065" CreationDate="2014-11-24T15:25:29.707" Score="1" Body="&lt;p&gt;First off, gravitation from the Earth is not quite as uniform as you think. It varies by about 1/2 of one percent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, and far more importantly, the bigger an object is, the more uniform is its gravitational field. Look to Jupiter and Saturn. By far the biggest contributor to Jupiter's and Saturn's non-spherical gravitational fields are the fact that Jupiter and Saturn rotate at rather high rates. After accounting for this rotation, Jupiter and Saturn are essentially &quot;spherical.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the Earth is not nearly as massive as Jupiter or Saturn, it is still more or less &quot;spherical.&quot; While the gravitation deviations from that predicted by an ellipsoidal Earth are important to finding oil or minerals, they are rather small. The deviations of gravitation on the Moon from an ellipsoidal Moon are huge in comparison. Comets such as 67P/Churyumov–Gerasimenko and 25143 Itokawa provide even more extreme examples. The gravitation from these objects only looks spherical from a large distance (any finite-shaped object looks like a point mass from a sufficiently large distance). Close-up, gravitation from a comet is anything but spherical because they are so lumpy.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-11-24T15:25:29.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6085" PostTypeId="2" ParentId="6080" CreationDate="2014-11-24T16:31:51.040" Score="4" Body="&lt;p&gt;I think you're looking at it backwards. It's not &quot;how is unpressurised cargo useful?&quot;; it's &quot;how is pressurising cargo, that doesn't need to be pressurised, useful?&quot; It's &quot;what can I ship up to the ISS that doesn't need to be kept at atmospheric pressure the whole time, with all the space and logistical requirements that this adds?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anything that doesn't &lt;em&gt;need&lt;/em&gt; to be squeezed into the station's habitable area can simply be kept unpressurised during launch, and removed from the supply ship externally using the CanadaArm, which is great!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is pretty much reduced to technology that will live outside of the station long-term, since you'd otherwise have to cram it into an airlock anyway in order to get it inside.&lt;/p&gt;&#xA;" OwnerUserId="4703" LastActivityDate="2014-11-24T16:31:51.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6089" PostTypeId="1" CreationDate="2014-11-25T08:47:39.157" Score="7" ViewCount="318" Body="&lt;p&gt;I found this powerful online tool from NASA:&#xA;&lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi#top&quot;&gt;http://ssd.jpl.nasa.gov/horizons.cgi#top&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It allows getting orbital data for the comet, but what I actually need is the orientation of the axis of the comet w.r.t to Sun, to build a 3d model which shows shadows changes during day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to retrieve such information using this tool? Or do I need another source like SPICE or others?&lt;/p&gt;&#xA;" OwnerUserId="3075" LastActivityDate="2015-12-26T00:02:56.723" Title="How to use &quot;HORIZONS&quot; tool by NASA to get orientation of comet 67P/Churyumov-Gerasimenko axis?" Tags="|rosetta|comet|ephemeris|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6090" PostTypeId="1" AcceptedAnswerId="6098" CreationDate="2014-11-25T09:42:41.953" Score="11" ViewCount="500" Body="&lt;p&gt;Wikipedia has an article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_tide&quot;&gt;atmospheric tides&lt;/a&gt; but it is mostly earth based considerations.  I was wondering if they have any impact on satellites or rocket launches?  We know that satellites in &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot;&gt;Low Earth Orbit&lt;/a&gt; are subject to drag from the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do atmospheric tides impact that drag? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do any considerations for atmospheric tides need to be made during a rocket launch?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="6312" LastEditDate="2014-11-25T12:01:07.967" LastActivityDate="2014-11-26T12:12:20.473" Title="Do atmospheric tides have any impact on orbiting satellites or rocket launches?" Tags="|launch|low-earth-orbit|atmosphere|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6091" PostTypeId="2" ParentId="4438" CreationDate="2014-11-25T12:08:50.167" Score="2" Body="&lt;p&gt;Mars One's schedule, budget, business idea (live TV reality show from Mars?) and some of their technical proposals are unworkable. But they have been extremely effective in marketing! They have indeed already created popular support for missions to Mars. If they focus on their (Earthbound) marketing skills, they might in that respect very well become a part of real Mars missions designed and financed by others. SpaceX (and even NASA) could gain by cooperating with them.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-11-25T12:08:50.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6092" PostTypeId="2" ParentId="2825" CreationDate="2014-11-25T12:18:38.363" Score="6" Body="&lt;p&gt;Yes, this was released in mid-October 2014 by a group of Ph.D. candidates at MIT: &#xA;&lt;a href=&quot;http://web.mit.edu/sydneydo/Public/Mars%20One%20Feasibility%20Analysis%20IAC14.pdf&quot; rel=&quot;nofollow&quot;&gt;http://web.mit.edu/sydneydo/Public/Mars%20One%20Feasibility%20Analysis%20IAC14.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And they will be interviewed and available for listeners' questions today November 25th at &lt;a href=&quot;http://www.thespaceshow.com/newsletterfinal.htm&quot; rel=&quot;nofollow&quot;&gt;thespaceshow.com&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.thespaceshow.com/live.htm&quot; rel=&quot;nofollow&quot;&gt;Listen live&lt;/a&gt; 10-11:30 PM EST&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally speaking, the report is quite pessimistic, including items like not believing the station could survive longer than 68 days on Mars, lack of budgeting, timeline, etc.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-02-23T20:41:27.820" LastActivityDate="2015-02-23T20:41:27.820" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6093" PostTypeId="2" ParentId="4295" CreationDate="2014-11-25T18:26:30.930" Score="6" Body="&lt;p&gt;As far as using an Arduino board to control your model I had also planned a similar project involving the data from the nasa website to not only control with servos the station orientation but as you know the solar panels move to face the sun or away based on power requirements. I was looking at the very excellent &quot;open source&quot; program &quot;celeste&quot; which is a standalone program that can use current data from NASA TLE's and correctly give inclination of the station and it's position. Moreover if your Arduino board was wi-fi equipped with a link to the TLE's that nasa publishes monthly you could create an accurate and very convincing station model as it would be in real time telemetry. Add that to a display base of a flat panel monitor of High Resolution running the celeste program showing earth position below and your model/diorama would be unbelievable. &lt;/p&gt;&#xA;" OwnerUserId="6706" LastActivityDate="2014-11-25T18:26:30.930" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6094" PostTypeId="1" AcceptedAnswerId="6095" CreationDate="2014-11-25T22:17:42.157" Score="4" ViewCount="1418" Body="&lt;p&gt;Probably the only reasonable way how to get rid of waste heat in space is radiating the heat away. Most of the current satellites and probes probably do not produce much waste heat. However, assuming we would like to deploy a spacecraft that produces a &lt;em&gt;lot&lt;/em&gt; of waste heat, what are technical parameters of the best radiators we could use?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, I am interested in:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;How heavy the radiator is per square meter, including the cooling liquid?&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;What is the temperature of the radiator, if the spacecraft is to be cooled to 300 K?&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6213" LastEditorUserId="6213" LastEditDate="2014-11-26T10:07:17.373" LastActivityDate="2014-11-26T10:07:17.373" Title="Mass and efficiency of heat radiators" Tags="|construction|heat|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6095" PostTypeId="2" ParentId="6094" CreationDate="2014-11-26T09:01:11.717" Score="4" Body="&lt;p&gt;You can infer the state of the art from the following quote from &lt;a href=&quot;http://www.aerospace.org/publications/books-by-aerospace/spacecraft-thermal-control-handbook/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gilmore's 2002 handbook&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Most spacecraft radiators reject between 100 and 350 W&lt;/strong&gt; of internally generated electronics waste heat &lt;strong&gt;per square meter&lt;/strong&gt;. The upper end of this range is typical of a radiator that runs at a fairly high temperature (say 40°C) and experiences a relatively modest heat backload from the environment or other spacecraft surfaces.&lt;/p&gt;&#xA;&lt;p&gt;The lower end of the range might represent a radiator running below room temperature in low Earth orbit, where environmental backloads can be substantial. The actual sizing is determined by a thermal analysis that considers the desired operating temperature, worst-case satellite waste heat, environmental heating, and radiative and conductive interactions with other spacecraft surfaces. &lt;strong&gt;Weights for radiators typically vary from almost nothing, if an existing structural panel is used as a radiator, to around 12 &lt;span class=&quot;math-container&quot;&gt;$kg/{m^2}$&lt;/span&gt; for a heavy deployable radiator and its support/deployment structure&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The thing to keep in mind, though is that heat rejection capability goes as the &lt;strong&gt;fourth&lt;/strong&gt; power of the radiator temperature. A VASIMR engine or an NTR nuclear engine have higher operating temps (limited by avionics, of course) than a cryo tank, and you'd generally have many more problems with the latter.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-11-26T09:06:42.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6097" PostTypeId="2" ParentId="4301" CreationDate="2014-11-26T10:22:22.530" Score="3" Body="&lt;p&gt;It's important to understand the special challenges imposed by Earth's atmosphere, which would not be present to such a degree on other bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the beginning, Earth's atmosphere had very little free oxygen. After a while organisms capable of photosynthesis arrived and started generating free oxygen. Dissolved iron reacted with this oxygen and precipitated out of early oceans, becoming today's veins of iron ore which are exploited by the steel industry. In other words, most of the iron ore we use today is a result of interactions with an oxygen rich atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first kind of iron used by men was actually &lt;a href=&quot;http://en.wikipedia.org/wiki/Meteoric_iron#Cultural_and_historical_usage&quot; rel=&quot;nofollow&quot;&gt;meteoric iron&lt;/a&gt;. Not having been around for long enough to be oxidized by the atmosphere, it could be fashioned directly into tools. It should be clear now, that one of the advantages of being in space is the possibility of finding iron and other metals that have not been exposed to oxidization at all, and it might be possible to fashion it into parts simply by melting it. The quality may not be as high, but if it is cheap enough it may not matter for some purposes such as space stations, where a thicker metal wall (to compensate for weaker metal) would provide additional shielding.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2014-11-26T10:22:22.530" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6098" PostTypeId="2" ParentId="6090" CreationDate="2014-11-26T12:12:20.473" Score="10" Body="&lt;p&gt;The diurnal bulge in the atmosphere plays a rather minor role during launch. Launch is short in duration and involves an aerodynamically shaped vehicle that punches through the thickest part of the atmosphere. Whether its locally 2 PM (bulge is thickest) or 4 AM (bulge is thinnest), the density at orbit insertion altitude is tiny compared to that of the troposphere, stratosphere, and mesosphere through which the vehicle has plowed before getting to that orbit insertion altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The diurnal bulge plays a significant role on a satellite in low Earth orbit. The very short duration launch interval (ten minute or less) becomes years of operations for a satellite in low Earth orbit. The effects of drag pile up over the years. On top of that, most satellites have anything but a nice aerodynamic shape. One of the first things a satellite does after separating from the launch vehicle is to deploy its solar arrays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another big factor is the Sun. One solar flare, and poof! the upper atmosphere swells. Drag on a satellite can easily more than double between solar minimum and solar maximum.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-11-26T12:12:20.473" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6099" PostTypeId="1" AcceptedAnswerId="6100" CreationDate="2014-11-26T15:46:01.123" Score="9" ViewCount="2449" Body="&lt;p&gt;If I had a CubeSat sitting on my desk ready to go today, what documentation and testing would I need to do before I could get it launched? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably the launch provider won’t just launch anything I give them, but if they did I’ve got an Arduino in a cardboard box that’s flight ready. &lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2014-11-27T16:32:07.700" LastActivityDate="2015-04-22T11:35:31.857" Title="Documentation and testing for a CubeSat launch?" Tags="|launch|cubesat|testing|regulatory|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6100" PostTypeId="2" ParentId="6099" CreationDate="2014-11-26T16:18:53.177" Score="5" Body="&lt;p&gt;I found &lt;a href=&quot;http://www.nasa.gov/pdf/627972main_LSP-REQ-317_01A.pdf&quot;&gt;this document&lt;/a&gt; which seems to be a checklist of requirements for the CubeSat launch systems.  Listed in section 2 seems to be all the documentation and compliance the CubeSat launch provider would have to supply/meet.  I would assume that they would need similar documentation on each Cube also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everyone mentions a NASA safety review, but details are sparse on any exact documentation that would be needed with that.  NanoRacks has a &lt;a href=&quot;http://nanoracks.com/resources/documents/&quot;&gt;documentation section&lt;/a&gt; which includes reference information on Acoustic Noise Control and Analysis Plan, Flammability; Offgassing and Compatability Requirements; and Standard Materials and Requirements for Spacecraft which are likely key points of the safety review.&lt;/p&gt;&#xA;" OwnerUserId="6707" LastActivityDate="2014-11-26T16:18:53.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6101" PostTypeId="1" CreationDate="2014-11-26T16:19:13.800" Score="5" ViewCount="186" Body="&lt;p&gt;Compared with the interplanetary probes of the last decades, how can the next generation in the next decades be miniaturized? Not really meaning small in size, but low in mass. The purpose would be to allow a larger part of the mass budget for science instruments. And what are the most important developments in this direction which have already happened between for example Voyager and New Horizons?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do different functions contribute to mass on an interplanetary probe today?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have some naive imaginations:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;A physical structure which can survive launch from Earth. But maybe the individually sturdy components of a probe could be assembled to a fragile structure in microgravity at the ISS? For example, solar panels and radio dish could be folded out manually, without the probe having to carry the unfolding mechanism onboard.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Power system and thermal control. I suppose an RTG is as about slim as it can be. But what about the heat distribution system?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Radiation protection. One necessarily needs mass to stop cosmic rays. But it is only electronics which is sensitive to radiation (right?) So if one has multiple redundancies and can reprogram damaged chips to work around their damages, maybe shielding could be replaced by several almost massless microchips?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Communication with Earth. How thin and light can you make a reflecting radio dish? The rest, I suppose, is electronics which is of negligible mass already.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Orientation. Maybe one could use solar pressure instead of rotating wheels, or fuel for rocket engines?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-11-27T00:41:35.670" Title="How can interplanetary probes be miniaturized (mass minimized)?" Tags="|probe|interplanetary|design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6102" PostTypeId="1" AcceptedAnswerId="6107" CreationDate="2014-11-26T16:27:11.160" Score="24" ViewCount="15537" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7nohX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm becoming interested in how Mars missions are planning their landings. One aspect I see brought up time and again is Mars' awkward atmosphere - 'just enough there that you have to care about it'. This picture claims (I cannot vouch for it's validity) that Olympus Mons essentially pokes out of Mars' atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could an unmanned ship have a slow descent onto such a structure? Is it viable to roll down the mountain from there? Wikipedia says it is a 5 degree descent - hardly noticeable (though very noticeable in the picture above...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure this has been ruled out for good reasons, I'm just curious what they are.&lt;/p&gt;&#xA;" OwnerUserId="6613" LastActivityDate="2023-11-20T22:55:08.457" Title="Olympus Mons - viable Mars landing site?" Tags="|mars|landing|uncrewed-spaceflight|olympus-mons|" AnswerCount="5" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="6104" PostTypeId="2" ParentId="6102" CreationDate="2014-11-26T17:02:56.537" Score="0" Body="&lt;p&gt;I believe a 5 degree slope would be easily within the limits of a landing system.  From what I am reading the Apollo Landing Module had a slope tolerance of about 12 degrees.  I found a chart in this &lt;a href=&quot;https://www.hq.nasa.gov/alsj/LM_Landing%20Gear1973010151.pdf&quot; rel=&quot;nofollow&quot;&gt;document(page 11)&lt;/a&gt; which graphs the slope at landing sites for several Apollo missions.  It seems to imply that many landing sites had rather high slopes, between 5-10 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course this was on The Moon with a gravity of 0.1654g, where as Mars has a gravity of 0.376g, which makes it a difficult direct comparison.  There is also no atmosphere to consider on the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think that the wind on mars would pose a much larger problem than a 5 degree slope.  Any landing system would have to be stable enough not to get blown over if it got caught in a storm, a requirement that would likely make any system have to be far more stable than concern of 5 degree slope at the landing site.&lt;/p&gt;&#xA;" OwnerUserId="6707" LastActivityDate="2014-11-26T17:02:56.537" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6105" PostTypeId="2" ParentId="6102" CreationDate="2014-11-26T17:07:23.410" Score="5" Body="&lt;p&gt;The rovers we have exploring Mars move very slow. Every movement is planned on Earth based on analysis of the terrain. Even curiosity, the most capable rover ever built, will never go all that far from its landing point. Until we have rovers capable of independent movement over long distances rovers will be landed where they need to be to perform science. If good science can be done on Olympus Mons maybe they'll land one there, but given the distances involved they won't land there to avoid having to deal with the atmosphere. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2014-11-26T17:07:23.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6106" PostTypeId="2" ParentId="6102" CreationDate="2014-11-26T17:40:49.280" Score="16" Body="&lt;p&gt;The real problem with Martian mountains is that by landing high you forego the ability to shed at least several hundred meters per second off your re-entry velocity available in the denser atmosphere below. Thus, you are forced to build in larger EDL (entry, descent and landing) propellant reserves and, all other variables equal, forget about putting more instruments into the lander.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-11-26T17:40:49.280" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6107" PostTypeId="2" ParentId="6102" CreationDate="2014-11-26T18:15:53.913" Score="22" Body="&lt;p&gt;Sure, with enough rockets and propellant you could land there.  However the mass required for all that propellant would vastly reduce the mass of your payload, for an equivalent Earth launch mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars Science Laboratory was still going about Mach 17 (in Martian terms 1 Mach ~ 240m/s) when it was at the altitude of the top of Olympus Mons.  So we're talking a &lt;em&gt;lot&lt;/em&gt; of propellant.  A very rough calculation assuming a storable biprop system would leave you with 1/4 to 1/5 of the payload you would have at &quot;normal&quot; landing altitudes, with the same entry mass.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-11-26T19:54:38.070" LastActivityDate="2014-11-26T19:54:38.070" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6108" PostTypeId="2" ParentId="6101" CreationDate="2014-11-26T22:23:38.123" Score="3" Body="&lt;p&gt;I'll just answer your proposals first:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Unfolding mechanisms make up a minuscule part of the structural weight. Docking at the ISS would take an enormous amount of fuel and effort. This is definitely not a workable path.&lt;/li&gt;&#xA;&lt;li&gt;There are engineers who do nothing but optimize the thermal systems in spacecraft for their whole career. I find this enormously boring, but these people know what they are doing.&lt;/li&gt;&#xA;&lt;li&gt;Electronics are not routinely protected by anything like a lead casing. It wouldn't help anyway, because it would have to be insanely thick to keep out gamma radiation. Redundancy mechanisms are highly developed and always applied.&lt;/li&gt;&#xA;&lt;li&gt;You can save weight with lighter spacecraft antennae, if you make the antenna on the receiving side more powerful. But there are physical limits to this. New light materials and special antenna shapes also come in handy on the spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;The current trend is away from using the solar panels for radiation pressure. The amount of fuel you save generally does not make up for the additional complexity and cost.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I don't have the time to answer the rest of your question right now. Maybe I will find time tomorrow.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2014-11-27T00:41:35.670" LastActivityDate="2014-11-27T00:41:35.670" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6109" PostTypeId="1" CreationDate="2014-11-27T12:53:10.813" Score="11" ViewCount="233" Body="&lt;p&gt;The ESA is using ground-based telescopes to precisely gauge the position of the Gaia satellite. But the satellite appears at about the 21st magnitude, three magnitudes dimmer (~16 times) than expected!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this? If we can accurately measure the reflectance of the sunshield (as I assume has been done!), wouldn't it be a reasonably simple matter to calculate its apparent magnitude?&lt;/p&gt;&#xA;" OwnerUserId="1172" LastEditorUserId="25" LastEditDate="2016-03-30T13:52:05.597" LastActivityDate="2016-03-30T20:15:44.363" Title="Why is Gaia so much dimmer than we thought?" Tags="|spacecraft|observation|gaia|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6110" PostTypeId="2" ParentId="6099" CreationDate="2014-11-27T14:38:32.347" Score="4" Body="&lt;p&gt;You can also check out the &lt;a href=&quot;https://www.qb50.eu/index.php/tech-docs/category/5-qb50-system-requirements-document&quot; rel=&quot;nofollow&quot;&gt;System Requirements documents for the QB50 project&lt;/a&gt;. You'll find system requirements and testing (qualification and acceptance) as well as requirements regarding quality control and reporting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The QB50 project will launch 50 double and triple cubesats to study the lower thermosphere.&lt;/p&gt;&#xA;" OwnerUserId="558" LastActivityDate="2014-11-27T14:38:32.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6111" PostTypeId="1" AcceptedAnswerId="6116" CreationDate="2014-11-27T21:33:46.993" Score="10" ViewCount="763" Body="&lt;p&gt;A significant minority of the users over at the &lt;a href=&quot;http://www.reddit.com/r/spacex/&quot;&gt;/r/SpaceX Reddit community&lt;/a&gt; seem to be of the opinion that SpaceX will attempt to &lt;em&gt;reuse&lt;/em&gt; the Falcon 9 CRS-5 first stage provided it lands on the downrange barge successfully. I disagree, and believe that it's likely SpaceX will thoroughly examine and analyze the returned stage, whether that be through destructive or nondestructive testing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, I am led to believe that they will put significant focus on the engines, the turbopumps, and the tankage structure. Elon Musk has mentioned on a previous launch abort where they didn't see expected thrust rampup that they took Falcon 9 down and &quot;boroscoped&quot; the engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, what mechanisms and tests will SpaceX likely employ to understand the stresses and fatigues that the first stage undergoes? Will these tests be destructive? Does anyone have examples of tests commonly employed in the aerospace industry to check for fatigue, etc?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the answer to the second question is a &quot;no&quot;, would it be a good idea to look at reuse of the CRS-5 stage?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="49" LastEditDate="2015-12-22T06:03:12.143" LastActivityDate="2015-12-22T06:03:12.143" Title="Through what mechanisms would SpaceX test and examine the returned CRS-5 first stage?" Tags="|spacex|falcon-9|reuse|testing|booster-flyback|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6112" PostTypeId="2" ParentId="6111" CreationDate="2014-11-28T00:06:32.570" Score="8" Body="&lt;p&gt;Elon Musk said during the recent &lt;a href=&quot;http://aeroastro.mit.edu/aeroastro100/centennial-symposium&quot;&gt;MIT AeroAstro Centennial Symposium&lt;/a&gt; (&lt;a href=&quot;https://www.youtube.com/watch?v=PULkWGHeIQQ#t=181&quot;&gt;link to the video&lt;/a&gt;) that for the upcoming launch (CRS 5) he gives it about 50% or less chance of landing the first stage successfully on also &lt;a href=&quot;https://twitter.com/elonmusk/status/536262624653365248/photo/1&quot;&gt;recently announced floating platform&lt;/a&gt;, but if they do that he figures they'd be able to refly it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer to your first question seems to be: no. They do &lt;em&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=EgGVkw9zjI8&quot;&gt;destructive testing&lt;/a&gt;&lt;/em&gt; on the Grasshopper, which is SpaceX' development vehicle. Gwynne Shotwell (SpaceX President &amp;amp; CEO) said in one past interview (I don't remember which one, it might have been &lt;a href=&quot;http://www.atlanticcouncil.org/events/webcasts/webcast-atlantic-council-captains-of-industry-with-gwynne-shotwell&quot;&gt;the one for Atlantic Council&lt;/a&gt;), that the production of booster engines (Merlin 1D) is quality tested to assure roughly 50-flight reuse (again, if I remember correctly, but the number was impressive, someone will correct me if I'm way off). They need to assure high reusability of their booster stage so they would &lt;em&gt;destructively test&lt;/em&gt; production samples, say hot fire test engines in simulated full ascent profile till they fail, and in-flight test new technologies with their Grasshopper platform, but they don't destructively test flight-ready hardware and I don't expect them to, unless absolutely necessary. Such practice would be too expensive and clearly clash with their aggressive pricing scheme.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for &lt;a href=&quot;https://en.wikipedia.org/wiki/Nondestructive_testing&quot;&gt;nondestructive testing&lt;/a&gt; (NDT), methods usually employed in aerospace industry are visual inspection with high resolution cameras or microscopy in various wavelengths (visual, near-IR, IR, sometimes UV if you're looking for residual patterns and similar), 2D X-ray and 3D X-ray CT (Computed Tomography) scanning for deep material fatigue, ultrasound, vibration analysis, and so on, and sometimes as simple as partially refueling the stage, or nitrogen purging, and checking for leaks with thermal imaging system. &lt;a href=&quot;https://en.wikipedia.org/wiki/Nondestructive_testing#Methods_and_techniques&quot;&gt;Different methods and techniques&lt;/a&gt; might be employed for different parts of the booster stage, the list of them is really long and SpaceX isn't really forthcoming about exact methodology employed for the purpose. I guess they'll reassure clients to the level needed, or simply offer cheaper flights with reused boosters (which is their long-term plan anyway).&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-11-28T00:06:32.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6116" PostTypeId="2" ParentId="6111" CreationDate="2014-11-28T15:53:29.907" Score="4" Body="&lt;p&gt;A suggestion (unsourced, floating around the NSF forums) is that they most likely will NOT refly the first recovered stage. But will rather take it apart to examine it in great detail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, they may take parts that are otherwise redundant (Flight computers, electrical components, single engines) and reuse them on other flights. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, use parts whose failure is not mission critical by themselves, and gain more flight experience with them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you consider the center Merlin 1D, it has been fired:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Acceptance test fire in McGregor as an engine. (Full duration? Not sure)&lt;/li&gt;&#xA;&lt;li&gt;Acceptance test fire in McGregor as a complete core. (Pretty sure full duration)&lt;/li&gt;&#xA;&lt;li&gt;Hot fire on the pad, pre-launch. (Just ignition, but that is the hard part)&lt;/li&gt;&#xA;&lt;li&gt;Full duration fire for launch&lt;/li&gt;&#xA;&lt;li&gt;Hypersonic retro-propulsion fire (Short duration, length unknown)&lt;/li&gt;&#xA;&lt;li&gt;Braking/Landing burn (Unclear if 1 or 2 burns, time will tell)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So that is at least 6 firings of that engine. Then if integrated into another vehicle as a non-center engine, will probably repeat steps 1-4, getting it up to 10 firings in the real world, which could be helpful in determining life span of the Merlin engine in the real world. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By reflying some of the other 9 engines, they can gain engine life data with minimal risk to payload since they can handle an engine out almost all the way to orbit. (Usually first few seconds no rocket can tolerate an engine out, but since they ignite and hold before launch, they usually can get past that initial launch time window).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same idea for other things like avionics, which are triple redundant, get some real world flight time on a couple of units to understand how long they can last in a non-risky fashion. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-28T15:53:29.907" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6119" PostTypeId="1" AcceptedAnswerId="6120" CreationDate="2014-11-29T10:54:17.637" Score="13" ViewCount="2661" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/6090&quot;&gt;Do atmospheric tides have any impact on orbiting satellites or rocket launches?&lt;/a&gt; we learned that the atmosphere does impact satellites in Low Earth Orbit (LEO), and that &lt;a href=&quot;https://space.stackexchange.com/questions/6090/do-atmospheric-tides-have-any-impact-on-orbiting-satellites-or-rocket-launches/6098#6098&quot;&gt;&quot;most satellites have anything but a nice aerodynamic shape&quot;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why are LEO satellites not aerodynamically shaped? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-11-29T20:32:47.853" Title="Why are LEO satellites not aerodynamically shaped?" Tags="|artificial-satellite|low-earth-orbit|atmospheric-drag|aerobraking|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6120" PostTypeId="2" ParentId="6119" CreationDate="2014-11-29T12:17:09.393" Score="18" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why are LEO satellites not aerodynamically shaped?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The need for electrical power overwhelms the need to reduce drag. That means a sizable cross sectional area to incoming solar radiation. Sometimes that cross section to solar radiation corresponds nicely (or not so nicely) with cross section to drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's worse, it's hard to claim that any shape is &quot;aerodynamic&quot; in low Earth orbit. In the troposphere, the coefficient of drag of an streamlined object can be less than a tenth that of a spherical body, which in turn is about a quarter that of a well-designed parachute. On orbit, the standard value for the coefficient of drag is 2.2, regardless of shape. Keep in mind that parachutes typically have a drag coefficient of 1.75. Per this standard view, shape doesn't matter, and whatever shape an object has, it is less aerodynamic than a parachute. All that matters is cross sectional area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A somewhat recent paper by Kenneth and Mildred Moe, &lt;a href=&quot;http://www.colorado.edu/ASEN/asen5519_dsmc/papers/moe05.pdf&quot;&gt;Moe &amp;amp; Moe (2005), &quot;Gas–surface interactions and satellite drag coefficients,&quot; &lt;em&gt;Planetary and Space Science&lt;/em&gt; 53.8:793-801&lt;/a&gt; puts some doubt to this standard drag coefficient of 2.2. Above 200 km altitude, most shapes have a drag coefficient that exceeds 2.2. Objects in space are not &quot;aerodynamic&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update: Why shape doesn't matter (at least not that much)&lt;/strong&gt;&lt;br&gt;&#xA;For objects moving through the troposphere, the shape of the object has a dramatic effect on drag. The coefficient of drag can vary by factor of forty depending on shape. Shape is much less important in the thermosphere. There, coefficient of drag varies by perhaps a factor of two instead of forty. Moreover, the very shapes that are considered to be aerodynamically-shaped in the troposphere can have a very high coefficient of drag in the thermosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, a flat plate oriented normal to the wind flow has close to the worst shape an object can have in the troposphere with regard to drag. (A parachute is of course even worse.) An arrow with a nicely shaped arrowhead has a significantly smaller coefficient of drag than does a flat plate. The situation is reversed in the thermosphere. It's the flat plate that has a lower coefficient of drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for this reversal is the way drag works in the lower atmosphere versus the upper atmosphere. The mean free path between collisions of atmospheric molecules is extremely short in the troposphere. On the other hand, mean free path ranges from about a quarter of a kilometer at 200 kilometers altitude to 2.6 kilometers at 300 kilometers to hundreds of kilometers at 600 kilometers altitude. This long mean free path means that drag operates very differently in the thermosphere than in the troposphere. Viscosity is the dominant force in the troposphere. Viscosity is essentially zero in the thermosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, drag in the thermosphere is described by free molecular flow. Atmospheric molecules in the thermosphere don't &quot;know&quot; about the existence of the object that is subject to drag unless they collide with it. The object is long gone by the time surrounding molecules do interact with the molecules that collided with the object.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-11-29T20:32:47.853" LastActivityDate="2014-11-29T20:32:47.853" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="6121" PostTypeId="1" CreationDate="2014-11-29T16:50:26.077" Score="4" ViewCount="1872" Body="&lt;p&gt;Presumably, it has something to do with the sun being rather hot. All the same, are there any potential cases with current technology where swinging close by the sun would be worth the shielding for the velocity imparted? How close could an uncrewed spacecraft get to the sun with shielding?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If ultralight shielding technology comes along, would such maneuvers have broader applications?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/829/4660&quot;&gt;'Could the sun be used as a gravity assist outside the solar system (with current tech)?'&lt;/a&gt; got me thinking about this.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-11-29T22:45:03.090" Title="What are the problems with using the sun for Oberth maneuvers?" Tags="|the-sun|heat-shield|oberth-maneuver|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6122" PostTypeId="2" ParentId="6121" CreationDate="2014-11-29T17:01:11.647" Score="4" Body="&lt;p&gt;Edit: This answer refers to an earlier version of the question when it was about a gravity assist maneuver instead of an Oberth maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No, the answer is very simple. The problem is physical, not technical. In order to use an object for a gravity assist, you need to have escape velocity with regard to that object to begin with. You can fly close to the sun (no reference intended), but it won't give you any net momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it was otherwise, we wouldn't need thrusters to reach GTO from LEO, we could just swing by the earth several times, but that would violate conservation of momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe you could swing by other stars on an interstellar voyage, but I gather that this is not the question you want to ask.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2014-11-29T17:18:20.090" LastActivityDate="2014-11-29T17:18:20.090" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6123" PostTypeId="1" CreationDate="2014-11-29T18:55:52.647" Score="14" ViewCount="4603" Body="&lt;p&gt;It sounds from the text of this &lt;a href=&quot;https://twitter.com/elonmusk/status/536263260056850432?s=17&quot; rel=&quot;nofollow&quot;&gt;tweet&lt;/a&gt; like Elon Musk plans to continue to do barge landings over the long term:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Base is 300 ft by 100 ft, with wings that extend width to 170 ft. &lt;strong&gt;Will allow refuel &amp;amp; rocket flyback in future.&lt;/strong&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, presumably that is because they can save on launch mass because that requires less fuel. That weight savings can then be used to reinforce the structures of the rocket, give a bit more delta-v to the following stage, have a bigger payload, or maybe just have more fuel margin for maneuvering. It also implies they are confident they can make the barge sufficiently stable for landing in all sea conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the long-range plan would seem to be to launch from Brownsville TX and perhaps land in the Atlantic, in order to have the best distance in which to use air drag to slow the stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can the fuel they would save by doing this be calculated?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="51" LastEditDate="2015-07-09T12:54:29.677" LastActivityDate="2019-01-28T20:16:00.367" Title="How much fuel can SpaceX save by landing the Falcon 9 booster on a barge?" Tags="|launch|spacex|falcon-9|atmospheric-drag|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6125" PostTypeId="2" ParentId="6121" CreationDate="2014-11-29T21:14:17.727" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What are the problems with using the sun for Oberth maneuvers?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A number of recent papers have explicitly advocated using the Oberth effect near the Sun to achieve very, very high velocities. The only problem is heat. Conceptually, all we need is &lt;em&gt;unobtanium&lt;/em&gt; to overcome the heating problem. The current lack of this fantasy material is of course a big problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of those papers advocate using conventional thrusters with a spacecraft passing very close to the Sun. An economical way to achieve this is to use a gravitational assist from Jupiter to cancel almost all tangential velocity. It's a lot cheaper from a delta-V perspective to go to Jupiter than it is to dive straight toward the Sun. The basic idea is to send a vehicle towards Jupiter, let Jupiter send the vehicle diving almost straight toward the Sun, and fire massive set of thrusters near perihelion. The resulting $V_{\infty}$ is huge compared to anything yet achieved by humankind. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other papers advocate using solar sails in combination with sun-diving to achieve very high escape velocities. A gravity assist from Jupiter is not needed here. In fact, it's counterproductive; solar sails don't work very well beyond Mars' orbit. On the other hand, solar sails work quite nicely (and that's putting it mildly) inside of Mercury's orbit. All we need are solar sails made of &lt;em&gt;unobtanium&lt;/em&gt; as opposed to a very thin layer metal on plastic so the solar sail can dive very close to the Sun, and voila! the solar sail can easily escape the solar system at velocities that make those obtained by the Pioneer and Voyager satellites look miniscule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll try to dig up some references to these papers later.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-11-29T21:14:17.727" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6126" PostTypeId="2" ParentId="6121" CreationDate="2014-11-29T21:38:49.387" Score="7" Body="&lt;p&gt;The main problem is thermal control: you need a heatshield on your spacecraft to survive through the close pass. It is a solved problem though, meaning that  NASA is planning to send a mission in 2018 to explore the close environment of the sun as low as 8 solar radii:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://solarprobe.jhuapl.edu/mission/index.php&quot;&gt;http://solarprobe.jhuapl.edu/mission/index.php&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_Probe_Plus&quot;&gt;https://en.wikipedia.org/wiki/Solar_Probe_Plus&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a project called the &lt;a href=&quot;http://www.niac.usra.edu/files/library/meetings/annual/jun00/393McNutt.pdf&quot;&gt;Realistic Interstellar Explorer&lt;/a&gt;, which would have used a probe to explore the interstellar medium. The mission would have started with a gravity assist from Jupiter to get the perihelion down to 3-4 solar radii, and then a perihelion boost using solids or solar thermal propulsion (which turns the heating constraints into an advantage):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.google.fr/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0CCMQFjAA&amp;amp;url=http%3A%2F%2Fwww.niac.usra.edu%2Ffiles%2Flibrary%2Fmeetings%2Fannual%2Fjun00%2F393McNutt.pdf&amp;amp;ei=iTt6VNO-OovZasSBgqgN&amp;amp;usg=AFQjCNFOgO3udq1PHzPPyG6i6FiFSfExkw&amp;amp;sig2=5V7QWpZeNXLkMZ07i6dZRA&amp;amp;bvm=bv.80642063,d.d2s&quot;&gt;presentation here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal was to cross 1000 AU in 50 years.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="1142" LastEditDate="2014-11-29T22:45:03.090" LastActivityDate="2014-11-29T22:45:03.090" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6128" PostTypeId="2" ParentId="6123" CreationDate="2014-11-30T01:27:39.263" Score="7" Body="&lt;p&gt;When a Falcon 9 first stage launches, it appears there will be two basic modes available.  Reusable and expendable.  We have seen mostly expendable launches so far.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reusable mode, there is a fuel cost to brake the forward thrust, slow down for reentry through the atmosphere, and then finally for landing. While it is not an attempt to save fuel, rather that fuel must be reserved for recovery cannot be used for thrust to orbit and therefore the functional payload is reduced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The burn to control reentry into the atmosphere and landing cannot be easily obviated, but the burn to slow down forward flight might possibly be minimized or removed if there is a target downrange to land on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing back at the launch site is known as RTLS - Return to Launch Site. That is most fuel intensive and thus most payload reducing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondarily, the center core of a Falcon Heavy will be firing longer and thus faster/higher at MECO. The question becomes, can that be recovered? Well RTLS in that case would likely mean a huge payload reduction so probably not worth it.  But if there is a downrange target to land on, (an island, a barge, a continent) that is appropriate the cost of recovering the stage could be largely reduced.  (Cost as always being seen from a payload capacity view).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the booster is on the barge, what do you do next? Well maybe tilt it horizontal, transfer to a faster ship and sail it back to base? Maybe refuel and fly it back? It is after all a reusable rocket stage. (Probably need a cap, since the open top end of a second stage-lacking first stage is probably very un-aerodynamic). &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-11-30T01:27:39.263" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6129" PostTypeId="1" AcceptedAnswerId="6164" CreationDate="2014-11-30T01:43:42.050" Score="9" ViewCount="3891" Body="&lt;p&gt;I'm curious how people plan an orbital rendezvous. &lt;a href=&quot;https://space.stackexchange.com/q/4969&quot;&gt;This question&lt;/a&gt; explains the basic principles, but not in enough detail for me to understand how it actually works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From playing Kerbal Space Program, it seems that this is very hard to get right:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If I try to match the orbit (by adjusting my periapsis/apoapsis and inclination), I invariably end up with a large phase angle from my target.&lt;/li&gt;&#xA;&lt;li&gt;If I try to time the launch so that I go straight into my target's orbit, even being a second or two late can mean I end up several kilometers away from the target, but it is very hard to predict how long a launch will take to that accuracy.&lt;/li&gt;&#xA;&lt;li&gt;The process is very counter-intuitive, because if I am near my target and I try to thrust towards it, what happens instead is that I warp my orbit on the other side of the planet.&lt;/li&gt;&#xA;&lt;li&gt;When I try to manipulate the phase angle by going into a slightly higher orbit, it becomes hard to make predictions because the phase angle doesn't stay constant when my orbit is more eccentric than the target.&lt;/li&gt;&#xA;&lt;li&gt;If the phase angle difference is large, I need to either spend a lot of fuel making big adjustments to my orbit, or I need to wait for dozens of orbits to catch up to my target.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All of this seems very error prone and time intensive. KSP doesn't model things like provisions, so the problem is a bit easier. So long as you can create even a tiny difference in phase, you can just crank up the time warp and wait for it to synchronize.&#xA;But I'm sure that in reality, it wouldn't make sense to send astronauts up and then have the sit in orbit for days or weeks, painstakingly adjusting their phase angle with the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, how do space agencies plan their rendezvous missions? Do they have a very precise estimate for the launch, and end up at the right phase when they match orbits? Or is there a good algorithm for optimally synchronizing orbits once you are already in some orbit?&lt;/p&gt;&#xA;" OwnerUserId="6733" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-12-02T19:12:09.717" Title="How is an orbital rendezvous planned?" Tags="|planning|rendezvous|kerbal-space-program|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6130" PostTypeId="2" ParentId="6129" CreationDate="2014-11-30T04:08:02.377" Score="2" Body="&lt;p&gt;According to &lt;a href=&quot;http://blogs.esa.int/atv/2013/06/11/why-does-it-take-so-long-to-rendezvous-with-the-iss/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt;, it can take days to rendezvous with the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, Gemini 6A and 7 rendezvous'd in only about 6 hours from the second launch, and after using a standard 2-day profile for manned flights to ISS for a long time, &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-08M&quot; rel=&quot;nofollow&quot;&gt;Soyuz flights have switched to a 6 hour profile&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern missions do have better estimates for launch times than the typical KSP player does, but there's always a bit of uncertainty in launcher performance and trajectory, so the initial conditions for the rendezvous are unpredictable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going to ISS at 420km altitude, if you start in a lower 320km orbit, you'll gain about a degree of phase every 10 minutes. So the worst case would be 2.5 days for the initial interception, but several more iterations of approach have to occur after that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In KSP, the usual technique I use is to match planes with craft A in a circular orbit and craft B in an elliptical orbit with apoapsis just touching A's orbit. B is then gaining on A in each orbit. On each apoapsis, examine the next upcoming closest approach in the map view; when the next closest approach is an overshoot, burn prograde at the apoapsis (raising periapsis, making a bigger slower orbit) to reduce the overshoot to zero.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-11-30T06:26:42.373" LastActivityDate="2014-11-30T06:26:42.373" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6131" PostTypeId="2" ParentId="6129" CreationDate="2014-11-30T08:01:23.743" Score="4" Body="&lt;p&gt;Going from Earth, the most important thing is getting the plane right: both the inclination and the longitude of the ascending node (LAN), since Earth is rotating. Thus, launch windows first and foremost depend on getting directly into the target object's plane (which means timing the launch to a second).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, you are correct in ascribing importance to phase angle. Weeding out the &quot;phase angle too large&quot; windows for the desired time-to-rendezvous leaves you with a much smaller set of launch times. You also want to place restrictions on the section of the orbit where the final phases of rendezvous and docking take place (illumination and, often, radio comms visibility from mission control). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, you have to understand that there are errors in the rendezvous process that have to be removed with the least expenditure of propellant:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;launch timing error&lt;/li&gt;&#xA;&lt;li&gt;orbit insertion error&lt;/li&gt;&#xA;&lt;li&gt;thrust and time-of-burn discrepancies for the active spacecraft&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Thus, there are pre-defined times when orbit correction maneuvers are scheduled based on the errors estimated with the help of ground tracking and ballistics facilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer to your question is, accordingly, both: a goodly amount of prior planning and preparation is involved to launch &quot;just right&quot;, plus an optimizing algorithm of orbit correction maneuvers is employed. &lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-11-30T08:01:23.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6133" PostTypeId="1" AcceptedAnswerId="8059" CreationDate="2014-11-30T17:47:13.623" Score="13" ViewCount="1005" Body="&lt;p&gt;While watching &lt;a href=&quot;http://vimeo.com/31799422&quot; rel=&quot;noreferrer&quot;&gt;this video animation&lt;/a&gt; of Orion EFT-1, a question occurred to me. Previous capsules (Apollo, Gemini, Mercury) were used once and then relegated to museums (apart from a few &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_2#MOL&quot; rel=&quot;noreferrer&quot;&gt;test flight articles&lt;/a&gt;). The Space Shuttle was, by design, reusable, even if it never lived up to its initial economic projections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Could individual Orion capsules be reconditioned and launched again?&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; The &lt;a href=&quot;http://www.space.com/21541-nasa-orion-spacecraft-reusable.html&quot; rel=&quot;noreferrer&quot;&gt;Space.com&lt;/a&gt; article referenced by TildalWave talks about Orion re-usability in a general way, but I'm hoping someone can dig up somewhat more official references and/or provide greater detail, such as what processes might be required before sending the capsule on its second mission, how many times it could practically be used, etc.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="195" LastEditDate="2015-08-19T23:34:43.740" LastActivityDate="2015-08-19T23:34:43.740" Title="Could Orion capsules be reused?" Tags="|design|crewed-spaceflight|orion-spacecraft|economics|capsule|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="6135" PostTypeId="1" CreationDate="2014-12-01T04:53:55.690" Score="16" ViewCount="3313" Body="&lt;p&gt;The Wikipedia article on Mir places its total cost at a little over four billion dollars. The ISS on the other hand, is at 150 billion and that figure will surely increase more over the coming years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Understandably, the ISS is a lot more complex, and heavier than Mir, but is it really so much more as to warrant the extra 145 billion dollars?&lt;/p&gt;&#xA;" OwnerUserId="6254" LastActivityDate="2019-05-15T15:18:57.587" Title="Why was the ISS so much more expensive than Mir?" Tags="|space-station|crewed-spaceflight|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6137" PostTypeId="2" ParentId="6135" CreationDate="2014-12-01T12:43:46.613" Score="24" Body="&lt;p&gt;The comparison rests on two pieces of evidence:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://web.archive.org/web/20141027231301/http://www.nytimes.com/2001/03/24/science/24MIR.html&quot;&gt;Yuri Koptev's statement&lt;/a&gt; (possibly based on reference budget material collected by the officials of RKA and its Soviet predecessors) placing the price tag at 4.2 billion US dollars&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://claudelafleur.qc.ca/Programcosts.html&quot;&gt;Claude LaFleur's estimates&lt;/a&gt; (also &lt;a href=&quot;http://www.thespacereview.com/article/1579/1&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'm going to make a glorified &quot;Apples and Oranges&quot; argument here. Basically, there are several more steps to make the comparison meaningful.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Koptev's estimate must be traced back to year-by-year budget numbers in the original Soviet and post-Soviet roubles, and converted painstakingly to then-current US dollars via purchasing power parity (PPP), and afterwards, to current (say, 2010, to keep in line with LaFleur's figure) prices.&lt;/li&gt;&#xA;&lt;li&gt;The price structure in the U.S.S.R. and elsewhere in the &quot;First&quot; (aka free) world was VERY different. It is notoriously difficult to find reliable price comparisons between the US and the Soviets.&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The budget has to be broken up into several items to be compared line-by-line:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Design and article production (we should divide the number by the internal volume, in cubic meters, of the respective stations - e.g. 837 vs 350)&lt;/li&gt;&#xA;&lt;li&gt;Launch vehicle flights (here alone, Space Shuttle costs are an enormous influence vs. cheap labor by Russian/Soviet officers and conscripts in launch teams)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Normal&lt;/strong&gt; space segment operation and maintenance costs (divide by the man-days of life aboard the station)&lt;/li&gt;&#xA;&lt;li&gt;Ground segment costs (training centers, mission control, recovery, whatever)&lt;/li&gt;&#xA;&lt;li&gt;The cost of &lt;strong&gt;extraordinary&lt;/strong&gt; operations (saving the Mir from a bunch of mishaps for sure deserves a separate line)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I don't have the time and, quite likely, the required documents (Russian budget notes) now to conduct the whole comparison, but I'd like to pinpoint several key differences that matter for design/production costs:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The NASA Freedom/Alpha/ISS program underwent several re-designs (each concept redesign costs money, and does not allow to build production experience).&lt;/li&gt;&#xA;&lt;li&gt;Soviets had two space station programs going on, with capacity and qualified workers/engineers very much in place.&lt;/li&gt;&#xA;&lt;li&gt;The ISS liberally borrowed design experience and whole &lt;em&gt;surplus&lt;/em&gt; modules from the Mir-2 station.&lt;/li&gt;&#xA;&lt;li&gt;USOS modules for the ISS were outsourced to the Europeans.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I would also direct interested visitors to a bunch of &lt;a href=&quot;http://www.gao.gov/search?q=%22International+Space+Station%22&amp;amp;facets=a%3A2%3A%7Bs%3A16%3A%22tx_agency_0_term%22%3Bs%3A52%3A%22National+Aeronautics+and+Space+Administration+%5BNASA%5D%22%3Bs%3A10%3A%22topic_term%22%3Bs%3A5%3A%22Space%22%3B%7D&amp;amp;search_type=Solr&quot;&gt;US GAO reports on the ISS&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-12-01T16:48:17.923" LastActivityDate="2014-12-01T16:48:17.923" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6138" PostTypeId="1" CreationDate="2014-12-01T13:17:27.703" Score="4" ViewCount="317" Body="&lt;p&gt;NASA's Asteroid Redirect Mission &lt;a href=&quot;http://www.lpi.usra.edu/sbag/meetings/jul2014/presentations/1030_Wed_Mazanek_ARM_OptionB.pdf&quot; rel=&quot;nofollow&quot;&gt;ARM Option B&lt;/a&gt; would have a robotic probe pick up a boulder from the surface of a much larger asteroid, and put that boulder in lunar orbit. Boulders considered could have a diameter of up to 4 meters and a mass of up to 50 tons (at maximum density).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A potentially hazardous asteroid has a diameter of at least 100 meters and a mass of at least 1,000,000 tons, according to the same source (11th SBAG July 2014).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a 100 meter asteroid be deflected by picking up several (many) 4 meter boulders from it? I suppose at least one boulder could be manipulated per day. Not to put the boulders in lunar orbit, but dropping them off one by one in slightly divergent trajectories. Or alternatively, putting them down on another part of the asteroid if that is more effective. Planetary defense is one of the motivations for ARM, I wonder if this motive is applicable to ARM Option B.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-03-31T09:14:46.193" Title="Could an ARM Option B type of mission be used to deflect a hazardous asteroid?" Tags="|asteroid|robotic-missions|planetary-defense|asteroid-redirect-mission|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6139" PostTypeId="2" ParentId="6138" CreationDate="2014-12-01T14:39:53.140" Score="6" Body="&lt;p&gt;ARM Option B hardware could be useful in deflecting an asteroid, but probably not by the methods you propose or the current Option B mission profile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all: Picking up 50 ton boulders is going to have a minute effect on an asteroid massing 1,000,000 tons. Placing the boulders somewhere else on the asteroid will move the centre of mass (by a tiny amount) but that probably won’t do much good. Shifting the centre of mass does mean that the asteroid has moved, but unless we can shift the asteroid hundreds or thousands of kilometres we still have an earth intercept trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other problem is that many asteroids don’t have enough easily detachable boulders to make up a significant proportion of their mass. Even if 20% of the asteroid is boulders, that’s still 800,000 tons of asteroid we can’t move. So flinging the boulders away isn’t going to solve the problem. (The other, bigger, problem with this approach is the horrible amount of propellant we’re going to waste slowing the spacecraft down and returning to the asteroid after we release each boulder)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a good discussion of using the &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_tractor&quot; rel=&quot;nofollow&quot;&gt;Gravity Traction deflection&lt;/a&gt; technique (which is similar to what we’re discussing) on the NasaSpaceFlight forums a few weeks ago. Although the thread has since been deleted, in summary it concluded that the only situation in which Gravity Traction deflection would be the best solution (and more efficient that just pushing the asteroid) is when the asteroid consist of multiple smaller bodies (boulders) that wouldn’t have the cohesion to be pushed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in that case, where the asteroid is nothing but boulders, it’s quite possible that picking and throwing each one individually might be a good solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the significant metric in asteroid deflection (and in fact all orbits) is velocity. So the best solution for solid asteroids would be to use the propellant we have on board to push the asteroid like the full redirect Option A. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We might not need the fuel load of Option A to deflect the asteroid, after all we aren’t looking for a stable lunar orbit like the full Option A; we just want a few millimeters/sec of Δv to push it out of the way of Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find any Δv numbers of the ARM Option B missions, but if you can find them it would be relatively easy to estimate whether Option B would have sufficient Δv to deflect your 1,000,000 ton asteroid enough to miss Earth.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-03-31T09:14:46.193" LastActivityDate="2015-03-31T09:14:46.193" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6140" PostTypeId="1" AcceptedAnswerId="6142" CreationDate="2014-12-01T17:36:40.090" Score="11" ViewCount="1411" Body="&lt;p&gt;The SLS is a very opinionated subject among bloggers. I'm looking for what substance there might be behind the following general claim (without specific source). Is it truthful and if so, what is the official plan concerning it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;The SLS will reuse Space Shuttle Main Engines (SSME), but they are currently not manufactured and there are only 16 of them in storage. Since each SLS launch consumes 4 of them, the SLS only has a lifetime of 16/4=4 launches altogether.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Is it rational to invest in re-starting the manufacture of an engine which was basically developed in the 1970's?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How expensive and how long time would it take to develop a brand new custom made engine for the SLS first stage?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Are there ready replacement alternatives to the SSME engine available, and would their implementation compromise the general SLS architecture which presumably is optimized for the SSME?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-03-29T11:51:37.160" Title="How will the SLS first stage be supplied with engines? Copy manufacturing the RS-25 or replacing it?" Tags="|engine-design|ssme|sls|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6141" PostTypeId="2" ParentId="6140" CreationDate="2014-12-01T18:06:51.257" Score="10" Body="&lt;p&gt;Engines at the size and performance of an SSME are very difficult to develop.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME has one of the highest ISP's ever attained in a production engine. (Not some crazy Fluorine involving demo). It is anything but a simple, or even 'old' engine. Its re-manufacture would likely benefit from modern techniques, like the people examining starting F-1 engine rebuilds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been estimated (I have no source, only what is usually quoted) to take 6-10 years to develop a large engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME is not that high in thrust, compared to the F-1 (660 Klbs SSME vs 1.5-1.8 MLbs thrust) but is much higher than something like a Merlin 1D (165 Klbs thrust). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest comparable (LOX/LH2) engine is the RS-68 used by the Delta 4 booster but was rejected because the heat load under the first stage, surrounded by the SRBs was unsurvivable by the RS-68. (Whereas the SSME was always designed to be 'near' the SRBs during launch. I say 'near' since of course, on the Shuttle they were offset being on the back of the orbiter, not directly between as they would be on SLS)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it rational to restart SSME production?  Maybe. It is a well understood, mature engine, that might benefit from moderm manufacturing technologies. It is a very advanced engine with excellent performance. It has been pointed out in the comments that some of the newest engines of that remaining set of 16 were manufactured as late as 2008. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have this graphic of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#mediaviewer/File:SSME_Flight_History.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;which SSME was used on which STS flight.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hzui9.png&quot; alt=&quot;SSME history of use on STS missions&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How expensive would a new engine be?  Very. It would require performance similar to the SSME, else the size of tanks or # of engines would cascade rippling changes throughout the entire SLS stack probably killing it.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME was quite expensive and something comparable to it would be similarly hard to develop. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any ready replacements? Nothing really. The RS-68 is the closest and already rejected. Would a Russian engine be accepted? Not sure there is an appropriatly sized LOX/LH2 Russian engine on the shelf.  The Russians have been great at BIG Kerosene engines. (RD-170, RD-180, RD-193) But LH2 mechanics at the large scale have usually caused them issues. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Ariane 5 Vulcain main engine might be an interesting idea, as it currently runs beneath the A5 main stage with an SRB on either side of it, but I would guess the sizing is wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said all that, it seems like more reason to consider SLS less of a great option.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-12-03T22:22:57.690" LastActivityDate="2014-12-03T22:22:57.690" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="6142" PostTypeId="2" ParentId="6140" CreationDate="2014-12-01T18:56:02.520" Score="10" Body="&lt;p&gt;Some changes appear to be under consideration for future production of SLS RS-25s:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#Space_Launch_System&quot;&gt;Wikipedia's SSME page&lt;/a&gt;, about the SLS plan: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once the remaining RS-25Ds are used up, they are to be replaced with a&#xA;  cheaper, expendable version, currently designated the RS-25E ('E' for&#xA;  expendable). This engine may be based on one or both of two&#xA;  single-use variants which were studied in 2005, the RS-25E (referred&#xA;  to as the 'Minimal Change Expendable SSME') and the even more&#xA;  simplified RS-25F (referred to as the 'Low Cost Manufacture Expendable&#xA;  SSME'), both of which were under consideration in 2011.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-01T21:39:28.167" LastActivityDate="2014-12-01T21:39:28.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6143" PostTypeId="1" AcceptedAnswerId="6149" CreationDate="2014-12-01T21:00:58.387" Score="5" ViewCount="641" Body="&lt;p&gt;Please watch 45 seconds of this demonstration performed by no one less than Wernher von Braun himself!&#xA;&lt;a href=&quot;https://www.youtube.com/watch?v=1ZImSTxbglI#t=1800&quot; rel=&quot;noreferrer&quot;&gt;https://www.youtube.com/watch?v=1ZImSTxbglI#t=1800&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The numbers along the illustrated ellipse between Earth and the Moon are obviously in units of hours travel time from/to LEO. So that makes no physical sense. According to the illustration in the movie, the slowest speed is in lunar periapsis. The fastest speed at the nodes (minor axis end-points) of the ellipse. And this about 350 years after Johannes Kepler had proven that the opposite is true.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it is a Disney movie. But an educational documentary one, in this part at least. And purportedly, Wernher und Walt were personally good friends. Why make this mistake in public? Or is it I who make a fool out of myself now?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1142" LastEditDate="2014-12-02T05:42:36.717" LastActivityDate="2014-12-02T09:54:46.297" Title="Didn't Wernher von Braun understand Kepler's laws in 1955?" Tags="|orbital-mechanics|history|n-body-problem|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6144" PostTypeId="2" ParentId="6143" CreationDate="2014-12-01T21:46:35.127" Score="3" Body="&lt;p&gt;Over most of the flight, Earth's gravity dominates, so what you're referring to as lunar periapsis is better thought of, for Keplerian purposes, as Earth apoapsis, and so is rightfully the slowest speed. The highest speeds &lt;em&gt;shown&lt;/em&gt; are at the minor axis endpoints, but in order to reach the first of those after ~15 hours, and the far point of the ellipse at 121 hours, you must be going quite a bit faster in the first leg of the journey.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-01T21:46:35.127" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6145" PostTypeId="2" ParentId="6143" CreationDate="2014-12-02T01:41:10.857" Score="-2" Body="&lt;p&gt;They didn't have computers readily available at the time. I think they used an earth orbit for this simplified illustration, because you can calculate it by hand. They could have used patched conics, but it really doesn't matter for this kind of presentation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This may be a going out on a limb, but it may point to von Braun's ability to use &quot;good enough&quot; solutions for a set of requirements instead of over-engineering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer the question, Wernher von Braun was very aware of the mathematics of the two body problem. You can read about this in &quot;First Men to the Moon&quot;, where he suggest that the moon mission may have to carry a large set of course correction maneuvers stored on magnetic tape, because he expects the computers to be too big to fit on a spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2014-12-02T01:41:10.857" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6147" PostTypeId="1" AcceptedAnswerId="6148" CreationDate="2014-12-02T03:03:54.293" Score="0" ViewCount="198" Body="&lt;p&gt;If you had a space structure in the form of a disk shape. Inside it would have rooms and hall ways connecting the rooms. Each hall is a continues ring so if you walk in one direction in a hall you will end up where you started. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this example we have 3 halls inner mid and outer. If the structure is spinning at a speed that the mid hallway would have 1G would the inner hallway and outer hallway also have 1G? Or will each point in the structure experience a different g-force.&lt;/p&gt;&#xA;" OwnerUserId="6496" LastEditorUserId="6496" LastEditDate="2016-02-24T15:26:43.043" LastActivityDate="2016-02-24T15:26:43.043" Title="Multi level Gravity Wheel" Tags="|gravity|microgravity|spacecraft|artificial-gravity|" AnswerCount="1" CommentCount="0" ClosedDate="2014-12-02T12:11:37.077" ContentLicense="CC BY-SA 3.0" />
  <row Id="6148" PostTypeId="2" ParentId="6147" CreationDate="2014-12-02T03:15:43.843" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;If the G-force of the middle hallway was 1G would the person on the inner and outer halls also be experiencing 1G?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If the levels are spinning at the same rate (e.g. 4 times per minute) no.  For a given rate of rotation, the centripetal force increases in a linear way as you move further from, or closer to, the center.  Twice is far from the center, leads to twice the force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they are spinning independently of each other, possibly.&lt;/p&gt;&#xA;" OwnerUserId="238" LastActivityDate="2014-12-02T03:15:43.843" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6149" PostTypeId="2" ParentId="6143" CreationDate="2014-12-02T03:52:37.950" Score="12" Body="&lt;p&gt;Old Wernher got it right.  Of course.  It all makes perfect sense, at least the transit to the Moon part and the relative velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I calculate 121 hours for half of an Earth orbit with a periapsis 400 km above the equator and an apoapsis 100 km (60 mi as he stated) above the far side of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At apoapsis, the vehicle will be going about 0.2 km/s in the Earth frame.  The Moon is going about 1.0 km/s around the Earth.  So for the vehicle to encounter the Moon, it will need to be ahead of the Moon, and the Moon will catch up to it from behind at 0.8 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore as the Moon approaches the vehicle (or as the vehicle approaches the Moon &amp;mdash; same thing), the Moon will &lt;em&gt;slow down&lt;/em&gt; the vehicle as it pulls on it from behind (and even reverse its motion), in the reference frame of the Earth.  As the Moon passes, it will then be in front and pull the vehicle forward, speeding it up again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only in the frame of reference of the Moon will it appear that the vehicle speeds up as it approaches the Moon.  You cannot think of this problem as the Moon and Earth being immobile with the vehicle flying around them.  The Moon is moving, in fact much faster than the vehicle as it approaches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity at Earth closest approach is close to 11 km/s, so yes, as Wernher describes, the speed is slower at the Moon and faster at Earth.  &lt;em&gt;Much&lt;/em&gt; faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As already noted, the orbit's shape and speed is determined almost entirely by the Earth, with only a very small portion of the trajectory dominated by the Moon's gravity.  So you can use a two-body (Earth and vehicle) Kepler orbit to approximate the trajectory to the Moon very well, ignoring the Moon's gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you like, you can then correct for the Moon as a perturbation using patched conics, considering the Lunar flyby as a hyperbolic pass with a $V_\infty$ of 0.8 km/s, the difference between the vehicle velocity and the Lunar velocity.  In the Lunar reference frame, the initial and final velocities at $\infty$ have the same magnitude, but different directions.  When that is converted to the Earth frame, it will result in a $\Delta V$ to the vehicle which will cause it to depart from the initial Kepler orbit about the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's what the trajectory actually ends up looking like in the Earth frame, where the trajectory is orange and the path of the Moon is blue (Earth is at 0, 0):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2jgaq.png&quot; alt=&quot;loop around Moon, then to much larger orbit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After that close lunar flyby on the far side, the vehicle is in a much larger orbit of Earth with an apoapsis almost ten times the distance of the Moon.  So Wernher didn't have the post-flyby state correct.  The orbit he showed is what would happen if the Moon weren't there when you got to apoapsis.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-12-02T09:54:46.297" LastActivityDate="2014-12-02T09:54:46.297" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6150" PostTypeId="5" CreationDate="2014-12-02T05:47:01.587" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-12-02T05:47:01.587" LastActivityDate="2014-12-02T05:47:01.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6151" PostTypeId="4" CreationDate="2014-12-02T05:47:01.587" Score="0" Body="Questions involving determining the orbital mechanics considering the gravitational interactions of n (number) of bodies." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-02T11:23:57.503" LastActivityDate="2014-12-02T11:23:57.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6152" PostTypeId="1" AcceptedAnswerId="8877" CreationDate="2014-12-02T12:22:17.493" Score="6" ViewCount="775" Body="&lt;p&gt;Are there any missions planned to take a better look at the dwarf planets discovered in recent years? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Eris_%28dwarf_planet%29&quot;&gt;Eris&lt;/a&gt;, bigger than Pluto and over twice as massive as Earth's Moon, &lt;a href=&quot;http://en.wikipedia.org/wiki/Makemake_%28dwarf_planet%29&quot;&gt;Makemake&lt;/a&gt;, a large Kuiper belt object with its highly inclined orbit, or &lt;a href=&quot;http://en.wikipedia.org/wiki/Haumea_%28dwarf_planet%29&quot;&gt;Haumea&lt;/a&gt; flattened into ellipsoid due to rapid rotation? Any of the number of &lt;a href=&quot;http://en.wikipedia.org/wiki/Plutoid&quot;&gt;Plutoids&lt;/a&gt;? I'm finding it disconcerting that we have a number of objects nearly as big as what until recently was qualified as a planet, and we hardly know anything about them.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="1142" LastEditDate="2014-12-02T13:32:14.273" LastActivityDate="2015-04-23T11:22:40.680" Title="Are there any missions planned to the &quot;new&quot; dwarf planets?" Tags="|probe|tno|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="6153" PostTypeId="1" AcceptedAnswerId="6163" CreationDate="2014-12-02T13:04:38.977" Score="3" ViewCount="227" Body="&lt;p&gt;How would one go about sending a probe on an orbit that goes around the Sun in the opposite direction as most planets? Specifically, a rather high orbit - something like Pluto's. What happens to possibility of gravity assists and Obereth maneuvers? Would using planetary fly-bys make turning harder? Would it necessitate excessive fuel budget?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rationale:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Plutino&quot; rel=&quot;nofollow&quot;&gt;Plutinos&lt;/a&gt; are dwarf planets in 2:3 &lt;a href=&quot;http://en.wikipedia.org/wiki/Resonant_trans-Neptunian_object&quot; rel=&quot;nofollow&quot;&gt;mean-motion orbital resonance with Neptune.&lt;/a&gt;  There's a large number of Plutinos, and to examine them better, a fly-by mission would be very helpful. Now to examine a number of objects roughly scattered around a single orbit, a craft in the same orbit would be best - except flying in the same direction as Pluto, chasing them all would take a very long time. One flying head-on, would pass all of them in half their orbital period (123 years) in unpowered flight, much shorter if using gravity assists for &quot;shortcuts&quot; to skip from one to another.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2014-12-02T18:26:36.963" Title="What difficulties would there be for a probe to attain a high retrograde solar orbit?" Tags="|orbit|probe|retrograde-orbit|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6154" PostTypeId="1" CreationDate="2014-12-02T13:22:47.487" Score="23" ViewCount="24785" Body="&lt;p&gt;I assume this hasn't got a precise answer, but I was wondering if anyone had an indication of the maximum survivable long term g-forces, if the persons positioning was optimal?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, could you accelerate a craft 3g for say, 16 hours a day with the traveller in a prone position, and 1g for 8 hours a day so they can get up and exercise?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or could you do even 6g in prone position for most of the day?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like I said, I accept the answers would be estimates, but any discussion based on current research would be good.&lt;/p&gt;&#xA;" OwnerUserId="6742" LastEditorUserId="49" LastEditDate="2014-12-02T14:02:48.113" LastActivityDate="2022-08-22T00:43:19.903" Title="Maximum survivable long term g-forces" Tags="|humans|crewed-spaceflight|hypergravity|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6155" PostTypeId="5" CreationDate="2014-12-02T13:34:46.617" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-12-02T13:34:46.617" LastActivityDate="2014-12-02T13:34:46.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6156" PostTypeId="4" CreationDate="2014-12-02T13:34:46.617" Score="0" Body="A Trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-02T18:36:33.730" LastActivityDate="2014-12-02T18:36:33.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6157" PostTypeId="2" ParentId="6154" CreationDate="2014-12-02T14:12:26.927" Score="16" Body="&lt;p&gt;It is hypothesized that &lt;strong&gt;the only acceleration that can be tolerated for looong time without side effects is the normal acceleration&lt;/strong&gt;. Human subjects have been exposed to continuous high-G environment at most for seven days at 1.5 G. Although no immediate ill effects were found, extrapolation of the data to longer periods may be dangerously risky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Relevant PubMed abstracts:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pubmed/23926661&quot; rel=&quot;noreferrer&quot;&gt;https://www.ncbi.nlm.nih.gov/pubmed/23926661&lt;/a&gt;&#xA;&lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pubmed/11542364&quot; rel=&quot;noreferrer&quot;&gt;https://www.ncbi.nlm.nih.gov/pubmed/11542364&lt;/a&gt;&#xA;&lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pubmed/591467&quot; rel=&quot;noreferrer&quot;&gt;https://www.ncbi.nlm.nih.gov/pubmed/591467&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have not found any studies of chronic high-G exposure (beyond 1 hour limit). However, our esteemed fellow &lt;a href=&quot;https://space.stackexchange.com/users/7017/mins&quot;&gt;user&lt;/a&gt; &lt;a href=&quot;https://space.stackexchange.com/questions/3424/have-there-been-any-studies-on-the-health-impacts-of-prolonged-high-g&quot;&gt;cites&lt;/a&gt; &lt;a href=&quot;http://www.worldcat.org/title/biology-of-human-survival-life-and-death-in-extreme-environments/oclc/70215878&quot; rel=&quot;noreferrer&quot;&gt;Claude Piantadosi&lt;/a&gt; from Duke U.'s Center for Hyperbaric Medicine and Environmental Physiology.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Human G tolerance, like other physiological strains, is limited by different physiological factors at different levels of G stress.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There may be astronauts who can tolerate higher accelerations (and indeed, there have been tests subjecting humans to a week-long 1.5G regimen), but you don't want to stress cardiovascular system without need. If you can provide standard 1-g environment, you should do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://standards.nasa.gov/documents/viewdoc/3315785/3315785&quot; rel=&quot;noreferrer&quot;&gt;NASA-STD-3001 VOL 2.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;Rationale: The limits in these figures represent safe levels of&#xA;  sustained translational  acceleration under nominal and off-nominal&#xA;  conditions. Exposure to acceleration above these limits could&#xA;  significantly affect human performance for maneuvering and interacting&#xA;  with a spacecraft. The limits for return to Earth are lower than&#xA;  launch limits because crewmembers could have degraded capabilities&#xA;  because of deconditioning from exposure to reduced gravity. For the&#xA;  extreme conditions of a launch abort or emergency entry, limits are&#xA;  higher because it may be necessary to expose the crew to accelerations&#xA;  more severe than those experienced nominally. Humans are never to be&#xA;  exposed to translational acceleration rates greater than these&#xA;  elevated limits, as this significantly increases the risk of&#xA;  incapacitation, thereby threatening crew survival. In using figures 2&#xA;  through 6, the acceleration vectors are relative to the “axis” of the&#xA;  upper body, particularly with a focus on a line running from the eye&#xA;  to the heart. However, the acceleration limit charts do not account&#xA;  for all body types or temporary off-axis accelerations or body&#xA;  positions.  This is why the limits are set conservatively. Therefore,&#xA;  brief excursions past the limits in one axis should be reviewed and&#xA;  may be found to be acceptable.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0EYwi.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0EYwi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;Data for Curves&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LEM5H.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LEM5H.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Figure 2 — +G&lt;sub&gt;x&lt;/sub&gt; Sustained Translational Acceleration Limits&lt;/h3&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZMQVT.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZMQVT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;Data for Curves&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y50Eb.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y50Eb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Figure 3 — -G&lt;sub&gt;x&lt;/sub&gt; Sustained Translational Acceleration Limits&lt;/h3&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ARv4W.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ARv4W.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;Data for Curves&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FnLoL.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FnLoL.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Figure 4 — +G&lt;sub&gt;z&lt;/sub&gt; Sustained Translational Acceleration Limits&lt;/h3&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xKIUd.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xKIUd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;Data for Curves&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zF5G3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zF5G3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Figure 5 — -G&lt;sub&gt;z&lt;/sub&gt; Sustained Translational Acceleration Limits&lt;/h3&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IKlYY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IKlYY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;Data for Curves&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/D1R4P.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D1R4P.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Figure 6 — &amp;pm;G&lt;sub&gt;y&lt;/sub&gt; Sustained Translational Acceleration Limits&lt;/h3&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;h3&gt;6.5.2  Rotation Limits&lt;/h3&gt;&#xA;  &#xA;  &lt;h3&gt;6.5.2.1  Rotational Velocity [V2 6065]&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;The system &lt;strong&gt;shall&lt;/strong&gt; limit crew exposure to rotational velocities in yaw, pitch, and roll&#xA;  by staying  below the limits specified in figure 7, Rotational&#xA;  Velocity Limits.   &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;Rationale: The limits in this figure represent safe levels of sustained rotational acceleration for  crewmembers under nominal and&#xA;  off-nominal conditions.  Exposure to rotational acceleration  above&#xA;  these limits could significantly affect human performance for&#xA;  maneuvering and interacting  with a spacecraft.  The limits for return&#xA;  to Earth are lower than launch limits because  crewmembers could have&#xA;  degraded capabilities because of deconditioning from exposure to &#xA;  reduced gravity.  For the extreme conditions of a launch abort or&#xA;  emergency entry, limits are  higher because it may be necessary to&#xA;  expose the crew to accelerations more severe than those  experienced&#xA;  nominally.  Humans are never to be exposed to rotational acceleration&#xA;  rates greater  than these elevated limits as this significantly&#xA;  increases the risk of incapacitation, thereby  threatening crew&#xA;  survival.&lt;/em&gt;     &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;APPROVED FOR PUBLIC RELEASE — DISTRIBUTION IS UNLIMITED&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-12-22T14:15:10.273" LastActivityDate="2014-12-22T14:15:10.273" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="6158" PostTypeId="1" AcceptedAnswerId="6160" CreationDate="2014-12-02T14:56:00.990" Score="1" ViewCount="67" Body="&lt;p&gt;'Diurnal signature of the earth related to the the rotation of the earth is utilized and 24 hours of amplitude and phase data is used along with Ranging and Doppler shift data to calculate Angular position and velocity components'&#xA;- in DSN Measurement data types&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What exactly, is meant by Diurnal signature of the earth and how does it aid line of sight range and range-rate data to give angular information of the spacecraft  state? &lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2014-12-02T15:58:27.163" Title="Diurnal Signature of Earth in Angular state tracking" Tags="|navigation|deep-space|ground-station|ground-track|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6159" PostTypeId="2" ParentId="6153" CreationDate="2014-12-02T15:26:15.317" Score="-1" Body="&lt;p&gt;It would be foolish to attempt a complete orbit reversal starting from earth as the delta V would be enormous. For a comparison, if the delta V to reverse orbit at earth's orbital speed is a 1, out there at pluto's speed it would be 0.16. Factor in the lighter craft after having presumably expanded fuel to get there in the first place, it is now much cheaper to reverse the orbital direction once at the outer orbitals, near your target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Forgot to mention, since you haven't completed the Hohmann to circularize once you yet at the right orbit, and if you calculated your transfer right, you should shave off a big part of the pro-grade speed you need to cancel. &lt;/p&gt;&#xA;" OwnerUserId="5938" LastEditorUserId="5938" LastEditDate="2014-12-02T15:34:56.590" LastActivityDate="2014-12-02T15:34:56.590" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6160" PostTypeId="2" ParentId="6158" CreationDate="2014-12-02T15:58:27.163" Score="3" Body="&lt;p&gt;This is necessarily a short answer, I don't doubt there will be other more extensive answers later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The location of a spacecraft in space is changing due to gravity forces and perturbations from drag and solar pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The location of DSN stations in inertial reference frame is not constant either. Earth's orbital motion is the first component to consider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over very long time intervals, DSN stations drift with the continents they are located on, thus their longitude and latitude may be changing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Attitude of the Earth relative to far away stars includes three more components:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Fm27m.png&quot; alt=&quot;enter image description here - from Wikipedia - I&amp;#39;m lazy!&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;rotation (length of the day and Universal Time - UT1) is essentially a random process, highly correlated with Atmospheric Angular Momentum.&lt;/li&gt;&#xA;&lt;li&gt;nutation (with main oscillatory periods of 18.61 years, 183 days and so on)&lt;/li&gt;&#xA;&lt;li&gt;precession (oscillation with a dominant period of 26 thousand years) &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;DSN maintains a highly accurate and up-to-date model of Earth's orientation in space (based on Very Large Base Interferometry - observations of differences in arrival of signals from distant quasars to different DSN stations).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Range and range rate (Doppler) data acquired from spacecraft are dominated by the rotation of the Earth and errors in UT1 are the most critical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Diurnal signature of the Earth is thus the mathematical model of our Earth's orientation/rotational speed relative to an inertial reference frame with all its wobbly components that should be subtracted from raw Doppler shift observations to get true spacecraft coordinates after a few more calculations.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2014-12-02T15:58:27.163" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6161" PostTypeId="1" AcceptedAnswerId="6162" CreationDate="2014-12-02T16:20:58.377" Score="14" ViewCount="7939" Body="&lt;p&gt;Most of the representations of the Earth from space that one sees are inventions, meant to clearly show the things people most want to see - the outlines of the continents, instead of mostly clouds as in photos. There has been discussion of this in a recent &lt;a href=&quot;https://astronomy.stackexchange.com/questions/8105/can-you-see-man-made-lights-on-the-dark-side-of-the-earth-from-the-surface-of-th?noredirect=1#8105&quot;&gt;Astronomy&lt;/a&gt; question, and thus I wonder how many real photographs of the entire Earth there really are. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are the ones from the Apollo missions, I don't know how many, and I think I remember SELENE took a few. Are there any others? How many are we talking about in total?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2020-11-12T03:06:48.050" Title="How many photographs are there of the whole Earth?" Tags="|photography|earth-observation|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6162" PostTypeId="2" ParentId="6161" CreationDate="2014-12-02T17:34:59.787" Score="23" Body="&lt;p&gt;Millions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meteorological satellites constantly take photos of Earth in a very wide spectrum, and the visible spectrum is a part of it, and many of these satellites travel on pretty high orbits with good overview of the whole Earth. There are many portals with these photos; finding specifically visible spectrum images may be tricky, but, say, pick any hour of any day of past four years in &lt;a href=&quot;http://www-pm.larc.nasa.gov/cgi-bin/site/showdoc?docid=22&amp;amp;domain=FY2D_fd&quot;&gt;this one&lt;/a&gt;, or &lt;a href=&quot;http://www-pm.larc.nasa.gov/&quot;&gt;find&lt;/a&gt; one that fits your needs better. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2014-12-02T17:34:59.787" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6163" PostTypeId="2" ParentId="6153" CreationDate="2014-12-02T18:26:36.963" Score="3" Body="&lt;p&gt;You can use one or more Jupiter flybys, as needed, to crank the orbit around.  Ulysses used a Jupiter flyby to get into an orbit over the poles of the Sun.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-12-02T18:26:36.963" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6164" PostTypeId="2" ParentId="6129" CreationDate="2014-12-02T18:36:13.133" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;So, how do space agencies plan their rendezvous missions?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Carefully, and with computers. Computer also plan and perform most rendezvous burns.  Rendezvous nowadays is likely to be fully automated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first step in planning a rendezvous is to determine the launch opportunities and launch windows. Cooperative rendezvous (e.g., the Soyuz, ATV, HTV, Dragon, or Cygnus rendezvousing with the Space Station) involves an active chaser vehicle and a passive target vehicle. If the chaser vehicle is launched, as are all the vehicles cited above, that cannot occur at any arbitrary time. Launching at any time would necessarily entail extremely expensive plane changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vehicle must instead launch when the launch will make the chaser be in more or less the same orbital plane as the target vehicle. This happens roughly twice a day, near the time when the orbital plane crosses the launch site. The chaser must also launch so that it is moving in the same direction as the target. This coupled with launch constraints (e.g., a vehicle can't launch southwest from the Cape) cuts the twice-a-day launch opportunities down to one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those once a day launch opportunities apply only to a rendezvous plan that can handle any phase difference between the chaser and target. The downside is a potentially long rendezvous process. The upside is launch opportunities occur once a day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's only one opportunity per year if the rendezvous plan can only accommodate a one degree span of phase angles. That's obviously not good. A rendezvous plan needs some slack in it so it can accommodate a reasonable span of phase changes. A plan that has the chaser transferring direct from orbit insertion to the target is a recipe for a plan with no slack.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, there's the separate concept of a launch window. What happens if there's a fixable problem during the pre-launch countdown? Can the launch occur a few minutes later than planned and still result in a successful rendezvous? Every minute delay results in about 1/8th degree of plane change and 4 degrees of phase change. At some point, the discrepancies become more than the vehicle can accommodate and the window closes.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Do they have a very precise estimate for the launch, and end up at the right phase when they match orbits?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The only way to end up at the right phase is to start somewhere near the right phase. A rendezvous plan must be able to handle some range of phase differences, but expecting a rendezvous plans to handle the full 360° range of phase differences inevitably means a very long rendezvous sequences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't have both a short rendezvous sequence and handle a large swath of phase differences. You can only launch during the launch window that surrounds a launch opportunity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your key problem is that you are trying to transfer directly from orbit insertion to the target vehicle. That's a &quot;Doctor! It hurts when I do this! 〈bonk〉&quot; type of situation. Don't do that then. You need to use phasing orbits are the mechanism by which errors are reduced and slack is introduced. Even the fast six hour rendezvous used by the Russians use phasing orbits.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Or is there a good algorithm for optimally synchronizing orbits once you are already in some orbit?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are lots of good algorithms. At some point, a real spacecraft will need to use Lambert targeting. Lambert's problem asks &quot;I'm here in some orbit at some point in time, but I want to be there in some other orbit at some later time. How do I accomplish that?&quot;&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-12-02T18:36:13.133" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6166" PostTypeId="2" ParentId="4295" CreationDate="2014-12-03T17:10:56.067" Score="10" Body="&lt;p&gt;NASA actually released &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/multimedia/scalemodel/index.html&quot; rel=&quot;noreferrer&quot;&gt;a set of measurements&lt;/a&gt; for a 1:100 ISS model. The detail is basic and the model is “as it was originally designed”, so it differs from the current configuration in places. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interesting part is the possibility of 3D printing.&#xA;You could input the measurements into your favourite CAD software, or you could use &lt;a href=&quot;https://support.thingiverse.com/thing:17114&quot; rel=&quot;noreferrer&quot;&gt;these ready made files&lt;/a&gt;  from Thingiverse.com. If you have access to a 3D printer you could just print it, or if not send off to someone like &lt;a href=&quot;https://www.3dprint-uk.co.uk/&quot; rel=&quot;noreferrer&quot;&gt;3DPrintUK&lt;/a&gt; or &lt;a href=&quot;http://www.shapeways.com/&quot; rel=&quot;noreferrer&quot;&gt;shapeways&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, you could resize the file. How about a 1/32 or 1/16 ISS? Plenty of space for motors and would just need a bit of detailing. &lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2014-12-03T17:10:56.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6167" PostTypeId="1" AcceptedAnswerId="6168" CreationDate="2014-12-03T18:42:01.083" Score="19" ViewCount="7508" Body="&lt;p&gt;I use TLE sets supplied on &lt;a href=&quot;https://celestrak.org/NORAD/elements/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this website&lt;/a&gt; to find the future position of satellites (well usually just the ISS) using the SGP4 propagation method implemented in C++.&lt;/p&gt;&#xA;&lt;p&gt;My question is: &lt;br&gt;&#xA;&lt;strong&gt;What is the accuracy of the satellite position in the supplied TLEs in terms of kilometers or error % ? And also if it is possible to estimate the error of the satellite's position after propagation with using SGP4 for a given time interval ?&lt;/strong&gt; &lt;br&gt;&lt;/p&gt;&#xA;&lt;p&gt;Any sources or references supporting the answers would also be very much appreciated.&lt;p&gt;&lt;/p&gt;&#xA;&lt;p&gt;Edit: &lt;br&gt;It is really not important for the error estimation to be precise and result of a rigorous analysis. But it should be based on some valid assumptions.&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2021-06-05T07:53:22.123" Title="What is the accuracy / uncertainty of Two Line Elements (TLEs)?" Tags="|iss|artificial-satellite|orbital-elements|calculation|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6168" PostTypeId="2" ParentId="6167" CreationDate="2014-12-03T19:21:35.630" Score="16" Body="&lt;p&gt;It depends on a lot of factors, including:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Time since epoch&lt;/li&gt;&#xA;&lt;li&gt;Altitude&lt;/li&gt;&#xA;&lt;li&gt;Atmospheric density variations&lt;/li&gt;&#xA;&lt;li&gt;Spacecraft maneuvers&lt;/li&gt;&#xA;&lt;li&gt;Accuracy, number and distribution of observations used to fit the TLE&lt;/li&gt;&#xA;&lt;li&gt;Fit span used for differential corrections&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Typical errors for a TLE for a non-maneuvering spacecraft at an altitude higher than 400km and with good observation data are approx 1 km error at epoch, growing at about 1 to 2 km per day.&lt;/p&gt;&#xA;&lt;p&gt;Many TLEs issued by JSpOC (which is the source of the data on Kelso's Celestrak website) fit those criteria, but many do not, and you can't generally tell just by looking at the TLE itself.  If there aren't enough observations or they are &amp;quot;cross-tagged&amp;quot; (identified as the belonging to an incorrect object) then the quality can be much worse, tens or hundreds of km in error even at epoch.&lt;/p&gt;&#xA;&lt;p&gt;Planet Labs operates a number of 3U cubesats, determines their orbits independently and publishes daily the RMS error of the JSpOC TLEs: &lt;a href=&quot;https://ephemerides.planet-labs.com/18SPCS_matches_v1.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ephemerides.planet-labs.com/18SPCS_matches_v1.txt&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2021-06-05T07:53:22.123" LastActivityDate="2021-06-05T07:53:22.123" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="6169" PostTypeId="1" CreationDate="2014-12-03T20:24:48.870" Score="12" ViewCount="12420" Body="&lt;p&gt;For calculating orbital state vectors of satellites, SGP4 (simplified perturbations model) is often used in conjunction with &lt;a href=&quot;http://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;two-line element sets&lt;/strong&gt;&lt;/a&gt; to calculate future positions of orbiting objects. What are the parameters and steps used in the SGP4 method to make these predictions? I am looking for an in depth source about how the model works. &lt;/p&gt;&#xA;" OwnerUserId="603" LastActivityDate="2022-02-25T10:14:57.743" Title="How does SGP4 work?" Tags="|orbital-mechanics|prediction|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6170" PostTypeId="1" AcceptedAnswerId="6172" CreationDate="2014-12-03T20:54:07.580" Score="14" ViewCount="4068" Body="&lt;p&gt;Were solids considered for the Saturn V?  If so, why was the idea discarded?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would guess this is due to a number of reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Inability to throttle&lt;/li&gt;&#xA;&lt;li&gt;Technology readiness in the era&lt;/li&gt;&#xA;&lt;li&gt;Launch escape&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="63" LastEditorUserId="3180" LastEditDate="2014-12-04T07:42:50.977" LastActivityDate="2016-12-30T15:12:34.357" Title="Why weren't SRBs used in the design of the Saturn V?" Tags="|saturn-v|solid-fuel|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6171" PostTypeId="2" ParentId="6169" CreationDate="2014-12-03T21:12:51.810" Score="13" Body="&lt;p&gt;The canonical references are &lt;a href=&quot;https://celestrak.org/NORAD/documentation/spacetrk.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacetrack Report #3&lt;/a&gt; which is the original documentation of the SGP4 algorithm, and Vallado + Kelso's &lt;a href=&quot;https://celestrak.org/publications/AIAA/2006-6753/AIAA-2006-6753-Rev2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Revisiting Spacetrack Report #3&lt;/a&gt; which has updates, analysis and modern* source code with discussion.&lt;/p&gt;&#xA;&lt;p&gt;* well, kinda :)&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2014-12-03T21:12:51.810" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6172" PostTypeId="2" ParentId="6170" CreationDate="2014-12-04T03:00:26.747" Score="11" Body="&lt;p&gt;According to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerojet#Florida_facility_and_canal&quot; rel=&quot;noreferrer&quot;&gt;Aerojet&lt;/a&gt; wiki, they were considered.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Aerojet solid fuel technology was under consideration for use in Saturn first stages. In the 1963, the U.S. Air Force gave Aerojet General $3 million to start construction of a manufacturing and testing site several miles southwest of Homestead, Florida. . . . .Between Sept. 25, 1965 and June 17, 1967, three static test firings [of the AJ-260 rocket] were done. SL-1 was fired at night, and the flame was clearly visible from Miami 50 km away, producing over 3 million pounds of thrust. SL-2 was fired with similar success and relatively uneventful. SL-3, the third and what would be the final test rocket, used a partially submerged nozzle and produced 2,670,000 kgf thrust, making it the largest solid-fuel rocket ever.&lt;a href=&quot;http://www.keysnet.com/2010/06/10/227720/hidden-in-the-glades-a-giant-relic.html&quot; rel=&quot;noreferrer&quot;&gt;[15]&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There was a problem with the third test. Near burnout, the rocket nozzle was ejected, causing propellant made of hydrochloric acids to be spread across wetlands in the Everglades and a few crop fields and homes in Homestead. Many residents of Homestead complained about the damage done, which included paint damage to their cars and killing thousands of dollars worth of crops.&lt;a href=&quot;http://www.abandonedfl.com/?p=627&quot; rel=&quot;noreferrer&quot;&gt;[16]&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;By 1969, NASA had decided to go with liquid-fueled engines for the Apollo’s Saturn V rockets. . .&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;More details &lt;a href=&quot;http://en.wikipedia.org/wiki/Solid-fuel_rocket#History&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The largest solid rocket motors ever built were Aerojet's three 260 inch monolithic solid motors cast in Florida.&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000033816.pdf&quot; rel=&quot;noreferrer&quot;&gt;[18]&lt;/a&gt; Motors 260 SL-1 and SL-2 were 261 inches in diameter, 80 ft 8in long, weighed 1,858,300 pounds and had a maximum thrust of 3.5M pounds. Burn duration was two minutes. The nozzle throat was large enough to walk through standing up. The motor was capable of serving as a 1-to-1 replacement for the 8-engine Saturn 1 liquid-propellant first stage but was never used as such. Motor 260 SL-3 was of similar length and weight but had a maximum 5.4M pounds thrust and a shorter duration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-12-04T03:07:26.160" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6173" PostTypeId="1" CreationDate="2014-12-04T04:50:29.883" Score="4" ViewCount="1534" Body="&lt;p&gt;I am an avid space advocate and even studied aerospace engineering in college, however, I still struggle with a &lt;strong&gt;great&lt;/strong&gt; answer to this question: &quot;Why explore space?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few days ago an old professor who I really respect posted on a public forum:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I am not concerned about space, the cosmos, or the unending vastness of our universe. I just want to see earth fixed first. Let's feed the hungry and house the homeless, live morally and love one another first. THEN let's worry about space, huh?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, I can think of many economical, social, and even political reasons that space exploration is important... but is there a way to suade people who reason in this way? It is easy to talk to people who love space about why we &quot;go there&quot; and &quot;do that&quot;, but how do you talk to people who are stuck on the idea that &quot;fixing&quot; the Earth comes first?&lt;/p&gt;&#xA;" OwnerUserId="5" LastEditorUserId="5" LastEditDate="2014-12-04T14:09:21.960" LastActivityDate="2014-12-16T00:05:43.493" Title="How to respond to the case against space?" Tags="|nasa|government|earth|" AnswerCount="4" CommentCount="10" ClosedDate="2014-12-04T13:02:11.220" ContentLicense="CC BY-SA 3.0" />
  <row Id="6174" PostTypeId="1" AcceptedAnswerId="6321" CreationDate="2014-12-04T05:20:07.970" Score="5" ViewCount="3754" Body="&lt;p&gt;In &lt;a href=&quot;http://phys.org/news/2014-11-musk-x-wing-style-fins-spaceport.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Musk is testing x-wing style fins, spaceport drone ship&lt;/a&gt; there is mention of two &#xA;Musk tweets:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Testing operation of hypersonic grid fins (x-wing config) going on next flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Grid fins are stowed on ascent and then deploy on reentry for 'x-wing' style control. Each fin moves independently for pitch/yaw/roll.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is the function of these fins?&lt;/p&gt;&#xA;" OwnerUserId="6753" LastEditorDisplayName="user10509" LastEditDate="2017-08-30T15:45:29.373" LastActivityDate="2017-08-30T15:45:29.373" Title="What is the function of the hypersonic grid fins on SpaceX's booster?" Tags="|spacex|rockets|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6175" PostTypeId="2" ParentId="6174" CreationDate="2014-12-04T06:09:05.187" Score="3" Body="&lt;p&gt;They are aerodynamic control surfaces.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2014-12-04T06:09:05.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6176" PostTypeId="2" ParentId="6173" CreationDate="2014-12-04T06:21:48.157" Score="2" Body="&lt;p&gt;Perhaps this professor (correctly) sees the majority of space exploration being performed by nation-states and therefore via taxation.  He also probably (incorrectly) sees the majority of the other things he mentions also being the responsibility of and performed by governments.  Finally, he also probably sees the world as a fixed sum -- meaning if we spend more on one, we must spend less on the other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first should be corrected -- hopefully with his help.  The second is wrong and should be pointed out.  The third is a philosophical viewpoint -- pessimistic to the extreme.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2014-12-04T06:21:48.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6177" PostTypeId="2" ParentId="6173" CreationDate="2014-12-04T07:13:10.577" Score="12" Body="&lt;p&gt;Because, first, we will never fix everything.  Humans are always naturally dissatisfied with their state of affairs, no matter what those are.  Reference &quot;first world problems&quot; (see image).  Compared to how things were hundreds of years ago, we have already solved all of the problems of that time and are now at what would have been to them an unimaginable level of prosperity.  So to them we have now earned the right to explore space, build particle accelerators, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/PwTID.jpg&quot; alt=&quot;Los Angeles angst&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, such an argument could, and to be consistent, must be used against many other things.  Art.  Movies.  Sports.  Restaurants.  Music.  Video games.  Personal automobiles.  Luxury items of any kind.  You'll go through most of that well before you get to the dollar level of any space program.  There are many things we should forgo until we have &quot;fixed the earth&quot;.  Ask the professor how he's doing on giving up all that, and by the way, what exactly has he done to fix the earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many arguments as to why we should invest some tiny, tiny fraction of our global disposable income on space exploration (which is what we do), all of which of course can be contested.  But this &quot;fixing the earth first&quot; assertion is far broader, and is clearly silly once those broad implications are exposed.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-12-16T00:05:43.493" LastActivityDate="2014-12-16T00:05:43.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6178" PostTypeId="2" ParentId="6173" CreationDate="2014-12-04T09:09:49.603" Score="5" Body="&lt;p&gt;Space has always been important for improving life conditions on Earth. The astronomical invention &quot;the calendar&quot; was not only necessary for agriculture, but even before that for migrating hunters and gatherers to time the seasons of suitable activities. Space has also always been necessary for navigation, since tens of thousands of years. And possibly even for some migrating animals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks to space flight, this has today been hugely expanded with weather satellites and GPS. And space flight has also become essential to a completely new area of benefit to life on Earth: communication.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your &quot;professor&quot; is obviously uneducated. What else does he say about which he has no knowledge? I recommend you to critically review everything else which this person has claimed. It might also be completely wrong. He seems to be lacking in judgement. Would you please post his identity here?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-04T09:09:49.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6181" PostTypeId="2" ParentId="6173" CreationDate="2014-12-04T12:37:39.350" Score="6" Body="&lt;p&gt;During a congressional debate regarding Fermilab's very first particle accelerator, Senator Pastore put the following question to the physicist Robert R. Wilson,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is there anything here that projects us in a position of being&#xA;  competitive with the Russians, with regard to this race?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To which Dr Wilson responded,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Only from a long-range point of view, of a developing technology.&#xA;  Otherwise, it has to do with: Are we good painters, good sculptors,&#xA;  great poets? I mean all the things that we really venerate and honor&#xA;  in our country and are patriotic about.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In that sense, this new knowledge has all to do with honor and country&#xA;  but it has nothing to do directly with defending our country except &lt;strong&gt;to&#xA;  help make it worth defending&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Along those same lines, we should indeed be interested in improving living conditions on earth. But ventures such as space exploration are what make life &lt;em&gt;worth&lt;/em&gt; living and &lt;em&gt;worth&lt;/em&gt; bettering.&lt;/p&gt;&#xA;" OwnerUserId="6756" LastActivityDate="2014-12-04T12:37:39.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6182" PostTypeId="1" CreationDate="2014-12-04T14:33:44.010" Score="5" ViewCount="268" Body="&lt;p&gt;I've heard that a different design of heat shield will be in place for the future Orion missions. How is the heat shield going to be different, and will the (Planned) Dec 4th launch of Orion adequately test the proposed changes to the heat shield?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-12-04T17:19:58.363" Title="Orion Heat Shield" Tags="|testing|orion-spacecraft|heat-shield|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6183" PostTypeId="1" AcceptedAnswerId="6190" CreationDate="2014-12-04T16:33:53.823" Score="15" ViewCount="4420" Body="&lt;p&gt;Usually gravity assist maneuvers are used to accelerate a probe without expending propellant. But could gravity assist maneuver be used to brake and insert into orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a formula to use for speed-deltaV-thrust that would be required at closest-approach for braking? consider the case of an interstellar probe to be able to brake in a target star using gravity assist. &lt;/p&gt;&#xA;" OwnerUserId="638" LastActivityDate="2014-12-05T14:24:08.403" Title="Gravity Assist braking" Tags="|interstellar-travel|gravity-assist|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="6184" PostTypeId="1" CreationDate="2014-12-04T16:52:54.870" Score="17" ViewCount="1757" Body="&lt;p&gt;The average surface temperature on Mars is -55°C and it can go down to -130°C. These are temperatures which sound deadly at first. But on the other hand, the atmospheric pressure on Mars is just 0.63% of that on Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't that mean that the thermal conductivity of the atmosphere would be very low? If yes, wouldn't this mean that the space suit wouldn't actually lose much thermal energy to the environment? So could an astronaut stay warm just with their own body heat and maybe a passive insulation layer, or would an active heating system be required?&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="49" LastEditDate="2014-12-04T17:57:24.710" LastActivityDate="2017-09-17T04:48:27.753" Title="How much temperature control would an EVA suit on Mars need?" Tags="|mars|spacesuits|temperature|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6185" PostTypeId="1" AcceptedAnswerId="10793" CreationDate="2014-12-04T17:02:05.397" Score="5" ViewCount="1078" Body="&lt;p&gt;This question is a follow up of &lt;a href=&quot;https://space.stackexchange.com/questions/6167/what-is-the-accuracy-uncertainty-of-two-line-elements-tles&quot;&gt;another question&lt;/a&gt; I have asked recently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sake of experiment I propagated the ISS's position (using SGP4) from epoch of first TLE to the epoch of next released TLE. The difference in time is a little over a day. The resulting position comes incredibly close to the one in the epoch of the second TLE. The difference in position vectors is in the range of meters. So, as strange as it may sound, instead of being happy about it, I am a bit concerned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That result makes me wonder &amp;mdash; is it possible that the propagation in the time span of a little over a day can be so accurate? Or is it due to the second TLE being produced from a series of previous TLEs? What is the most likely reason for my results? And what is the usual way of producing TLEs on the Celestrak website?&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="6816" LastEditDate="2017-11-03T02:45:15.383" LastActivityDate="2017-11-03T02:45:15.383" Title="How are Two Line Elements (TLEs) obtained?" Tags="|orbital-mechanics|artificial-satellite|orbit|orbital-elements|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6186" PostTypeId="2" ParentId="6174" CreationDate="2014-12-04T17:11:13.623" Score="8" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Grid_fin&quot;&gt;Grid fins&lt;/a&gt; can be lighter, lower drag, and improve maneuverability at supersonic speeds relative to conventional planar fins. Russian designers seem to like them.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-04T17:11:13.623" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6187" PostTypeId="2" ParentId="6182" CreationDate="2014-12-04T17:19:58.363" Score="4" Body="&lt;p&gt;My read of &lt;a href=&quot;http://spaceflightnow.com/2014/11/05/engineers-recommend-changes-to-orion-heat-shield/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; is that the material is the same, but rather than filling small cells in-place, they'll be building larger blocks separately and putting them together. So this upcoming flight test should still be a valid test of the material's ablation rate, as well as the other aspects of re-entry that aren't directly related to the heat shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Personally, I'd rather risk a finger-sized cell with less ablative fill than it's supposed to have, rather than have an entire block of heat shield fall off a la Columbia, but I will assume the Real Rocket Scientists are on top of it.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-04T17:19:58.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6188" PostTypeId="1" AcceptedAnswerId="6193" CreationDate="2014-12-04T17:27:56.597" Score="1" ViewCount="105" Body="&lt;p&gt;I am choosing a material for a spacecraft component, and I would like to estimate how it will perform under vibration, perhaps during launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does one estimate this performance? Are there archetypal design equations for this sort of question? I'm essentially looking for a way to compare two materials against each other.&lt;/p&gt;&#xA;" OwnerUserId="3082" LastActivityDate="2016-05-20T20:06:36.300" Title="Estimating vibration performance of a material" Tags="|materials|vibration|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6189" PostTypeId="1" AcceptedAnswerId="6663" CreationDate="2014-12-04T17:33:48.220" Score="6" ViewCount="394" Body="&lt;p&gt;I'm looking for a book about the Apollo program that specifically focuses on the logistics of the mission, as if it were a case study in project management. How were records kept, goals tracked, teams formed and coordinated, etc?&lt;/p&gt;&#xA;" OwnerUserId="6760" LastEditorUserId="1567" LastEditDate="2014-12-05T03:22:06.467" LastActivityDate="2015-02-27T15:38:13.483" Title="Book recommendation for Apollo program, very specific" Tags="|apollo-program|reference-request|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6190" PostTypeId="2" ParentId="6183" CreationDate="2014-12-04T18:17:39.060" Score="23" Body="&lt;p&gt;The speed of the probe doesn't change with regard to the assisting body. It is direction that is changed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a hyperbolic orbit about the sun comes in with a &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;noreferrer&quot;&gt;Vinfinity&lt;/a&gt; of 5 km/s, it will exit with a Vinfinity of 5 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless it happens to swing by a planet. From the planet's point of view incoming and outgoing speed are also the same. Again, it's direction that is changed. But a direction change with regard to a planet could be a speed change with regard to the sun. Thus a fly by of one of the gas giants might slow a hyperbolic about the sun to an elliptical capture orbit about the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an illustration of how a lunar swing by might change an hyperbolic asteroid's path to a capture about the earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WAIIA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the moon's neighborhood, earth escape velocity is around 1.5 km/s. Thus dropping the asteroid's speed from 1.98 wrt earth to 1.14 wrt earth brings the rock into an elliptical capture orbit about the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a similar fashion, a swing by of one of the gas giants might trap an object from outside our solar system into an elliptical capture orbit about the sun.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-12-04T18:17:39.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6191" PostTypeId="1" AcceptedAnswerId="6192" CreationDate="2014-12-04T18:35:47.950" Score="3" ViewCount="198" Body="&lt;p&gt;I have been following the &lt;a href=&quot;http://www.nasa.gov/orion/&quot; rel=&quot;nofollow&quot;&gt;Orion&lt;/a&gt; flight test this morning hoping that the engineers could finally manage to launch the spacecraft on time and I was disappointed when the scrubbing was communicated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question then came to my mind: why such a short mission has a launch window at all? There were no rendezvous scheduled, nor a trip to another celestial body, just two orbits and a splashdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What am I missing here?  &lt;/p&gt;&#xA;" OwnerUserId="6761" LastEditorUserId="49" LastEditDate="2014-12-04T19:02:01.463" LastActivityDate="2014-12-04T19:05:37.900" Title="Short EFT-1 missions launch window" Tags="|launch|nasa|procedure|orion-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6192" PostTypeId="2" ParentId="6191" CreationDate="2014-12-04T18:52:28.960" Score="3" Body="&lt;p&gt;The desired splashdown location in the Pacific constrains the timing and location of the high-apogee burn; they probably want that splashdown to be in daylight, and the orbit is inclined, not equatorial, so changing the timing would mean changing the inclination significantly, which I would guess has a lot of effects on the mission plan. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-04T19:05:37.900" LastActivityDate="2014-12-04T19:05:37.900" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6193" PostTypeId="2" ParentId="6188" CreationDate="2014-12-04T19:56:52.830" Score="3" Body="&lt;p&gt;Shooting from the hip, this area is known as NVH (Noise, Vibration and Harshness). I suspect a bare comparison between materials doesn't tell you much. NVH depends very much on the design of the component: bracing can reduce vibration, the size of a component determines how high the resonance frequency of the component will be.&lt;br&gt;&#xA;It may be possible to use the propagation speed of sound through a material to determine what the wavelength of a given frequency will be. From this you can derive at which size a panel will resonate to a given frequency.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket provider should be able to tell you which vibration frequencies occur during launch, those are the ones you want to avoid resonance in.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example: in the &lt;a href=&quot;http://www.arianespace.com/wp-content/uploads/2015/09/Ariane5_users_manual_Issue5_July2011.pdf&quot; rel=&quot;nofollow&quot;&gt;Ariane 5 user manual&lt;/a&gt; you'll find a section &quot;Environmental conditions&quot; which details the sound levels and vibration levels you can expect. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="11085" LastEditDate="2016-05-20T20:06:36.300" LastActivityDate="2016-05-20T20:06:36.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6194" PostTypeId="1" CreationDate="2014-12-04T20:03:43.067" Score="2" ViewCount="1327" Body="&lt;p&gt;I'm trying to follow the definition for converting Earth-centred inertial (ECI) coordinates (as output by SGP4) to Geocentric (GEOC) for later computation from this reference:&#xA; &lt;a href=&quot;http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA247550&quot; rel=&quot;nofollow&quot;&gt;Coordinate Systems for Space and Geophysical Application&lt;/a&gt;. However I can't find the alphaG value used in this equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\bar{X}=(\cos \alpha_G, \sin \alpha_G, 0)$$&#xA;$$\bar{Y}=\bar{Z} \times \bar{X}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume it's something based on the angle between the vernal equinox and Greenwich meridian but I'm not sure if I'm correct or how I would find this value. Any guidance/reference/advice welcome!&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="5963" LastEditDate="2014-12-04T23:18:07.670" LastActivityDate="2014-12-05T10:32:10.893" Title="Transformation from Earth-centered inertial (ECI) to Geocentric (GEOC) satellite coordinates" Tags="|artificial-satellite|ballistics|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6197" PostTypeId="1" AcceptedAnswerId="6198" CreationDate="2014-12-04T21:27:19.090" Score="9" ViewCount="2371" Body="&lt;p&gt;Although my asking this question now was inspired by &lt;a href=&quot;https://space.stackexchange.com/questions/6183/gravity-assist-braking&quot;&gt;this one&lt;/a&gt;, it's a question that's occurred to me before in the context of orbital capturing. Another way to put this might be: is there an equivalent &lt;a href=&quot;http://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Oberth effect&lt;/a&gt; for a retrograde burn?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wiki is ambiguous on this point. I also reviewed Hop David's excellent answer to &lt;a href=&quot;https://space.stackexchange.com/questions/4229/oberth-effect-for-earth-vehicles&quot;&gt;this question&lt;/a&gt;, but I can't seem to get my old brain to process it in reverse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the answer is yes, are there any instances where this has been used for orbital capture around some object?&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-03-01T12:13:38.557" Title="Is a braking maneuver more effective at periapsis?" Tags="|orbital-maneuver|oberth-maneuver|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6198" PostTypeId="2" ParentId="6197" CreationDate="2014-12-04T21:56:33.503" Score="12" Body="&lt;p&gt;Yes, it works exactly the same in reverse.  Every orbit insertion about a planet has taken advantage of this effect, by virtue of targeting each insertion to take place as close to the body as reasonable.  The Moon, Mars, Venus, Jupiter, Saturn, and Mercury.  Low-thrust missions like SMART-1 and DAWN don't get this Oberth advantage, since they can't generate a large impulse in a short time, so they have to start slowing down long before they can feel the gravity of the object.  However they get a high Isp advantage.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-12-04T22:24:34.967" LastActivityDate="2014-12-04T22:24:34.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6199" PostTypeId="2" ParentId="6183" CreationDate="2014-12-04T22:00:52.503" Score="12" Body="&lt;p&gt;You can not use a gravity assist around a body to enter orbit around the same body, because a gravity assist doesn't change your velocity relative to the body you fly by. It only changes your velocity relative to other bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, you can perform a gravity assist around a satellite of a body to enter the orbit of that body. You could, for example, perform a gravity assist around Ganymede to enter Jupiter orbit.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-12-04T23:09:26.363" LastActivityDate="2014-12-04T23:09:26.363" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6200" PostTypeId="1" CreationDate="2014-12-05T02:16:57.127" Score="2" ViewCount="1953" Body="&lt;p&gt;I'm currently working on a bit of code to calculate satellite passes for a ground station. I've found a variety of references that discuss the conversion form other coordinate reference frames to the local horizon (lat, long, radius to target) frame. Aside from other issues it would be really beneficial to have a worked step-by-step example of how to get from Earth-centered inertial (ECI - as output by SGP propagators) to the local horizon frame. Either a worked-example in an answer or a web link to a worked example would be great!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm currently using this reference:&#xA;&lt;a href=&quot;http://www.mssl.ucl.ac.uk/grid/iau/extra/local_copy/SP_coords/local.htm&quot; rel=&quot;nofollow&quot;&gt;Transformation to local horizontal coordinates&lt;/a&gt;, I'm stuck on step 3 - conversion matrix.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="49" LastEditDate="2014-12-05T08:04:37.343" LastActivityDate="2014-12-05T09:29:13.217" Title="Conversion of satellite position coordinates to local horizon reference frame (lat &amp; long)" Tags="|artificial-satellite|ground-station|reference-request|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6201" PostTypeId="2" ParentId="6194" CreationDate="2014-12-05T08:47:45.343" Score="3" Body="&lt;p&gt;The document you cited is fundamentally flawed. It has the z axes in the ECI and GEOC frames co-aligned. That's just wrong. It ignores precession and nutation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The full theory is amazingly complex. You probably don't need that. (You would have graduate advisors who would have already pointed you to the necessary software if you did need the full theory.) You do however need to account for precession and nutation to some extent if you want even single precision accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the full theory, you should read chapter 5 of &lt;a href=&quot;http://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn36.html&quot; rel=&quot;nofollow&quot;&gt;IERS Technical Node 36, IERS Conventions (2010)&lt;/a&gt;, which describes the transformation from the International Terrestrial Reference System (ITRS; what you call&quot;ECI&quot;) and the Geocentric Celestial Reference System (GCRS; what you call &quot;GEOC&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Software to compute the transformation matrix from ITRS to GCRS already exists. For that, I refer you to the International Astronomical Union's &lt;a href=&quot;http://www.iausofa.org&quot; rel=&quot;nofollow&quot;&gt;Standards Of Fundamental Astronomy&lt;/a&gt; (SOFA). Look at the left side of that page. There are cookbooks that instruct you on exactly how to use the relevant pieces of the SOFA software.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-12-05T08:47:45.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6202" PostTypeId="2" ParentId="6200" CreationDate="2014-12-05T09:29:13.217" Score="2" Body="&lt;p&gt;I couldn't comment on the question due to low reputation. So, I'd first like to clarify that Local Horizon frame and Long, Lat, Radial distance are two entirely different frames.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/North_east_down#mediaviewer/File:ECEF_ENU_Longitude_Latitude_relationships.svg&quot; rel=&quot;nofollow&quot;&gt;Local Horizon frame&lt;/a&gt; would be Earth-surface-centered (at Ground station) and principle axes being one towards local east (reason for the name 'Local horizon) , Other towards zenith and third one completing the right hand coordinate system and towards local North. While lat, long ,radial distance while specifically used for ECEF (Earth centered Earth Fixed) , can be generally used for any spherical coordinate frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trying to identify your task, I'd assume you'll need local horizon frame to record observed spacecraft position, &lt;a href=&quot;http://en.wikipedia.org/wiki/ECEF&quot; rel=&quot;nofollow&quot;&gt;ECEF or ECR&lt;/a&gt; (Non-inertial geo-centric and Earth fixed) &#xA;as intermediate frame to convert from/to ECI w.r.t the GS coordinates and local time of observation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For ECI-ECEF rotation matrix, you'll need single rotation matrix which is referenced with greenwich time and rotation angle being &lt;strong&gt;[A(s)+(We)(Tu)]&lt;/strong&gt; in Z axis rotation matrix. (Look up in wikipedia- Rotation matrix -under Basic 3x3 rotations)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, A(s) is rotation angle from Vernal Equinox to the sun given by time/day of the year i.e. position of earth in orbit around sun. &#xA;We- Angular velocity of Earth's rotation&#xA;Tu- Universal time&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, for subsequent change from ECEF to Local horizon frame you'd just need Ground station coordinates in spherical form i.e. latitude, longitude and distance for rotation matrix.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rotation matrix would be &#xA;R(ECEF_LH)= R(0.5pi)&lt;em&gt;R(lat-0.5pi)&lt;/em&gt; R(-long) &#xA;R being the rotation matrix as usual. You can refer to sign conventions of elementary rotation and order of elementary rotation in a composite rotation to self verify the matrix.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, with these conversions you can go from ECI to ECEF to LH while reverse conversion can be done by using inverse of the composite rotation matrix.&#xA;Referring to rules of combining simpler rotations might aid the understanding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S I'd really like to help if you could clarify the question more. You can however, use the answer to choose out particular transformation for your problem.&#xA;This is my first answer. So, Sorry for the lack of perfection in presentation. &lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2014-12-05T09:29:13.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6204" PostTypeId="1" CreationDate="2014-12-05T10:04:36.580" Score="4" ViewCount="351" Body="&lt;p&gt;I recently read the discussion in the question, &lt;a href=&quot;https://space.stackexchange.com/questions/6183/gravity-assist-braking&quot;&gt;Gravity Assist Braking&lt;/a&gt; that momentum degradation used while entering in the opposite direction to the body's rotation in the Hill's sphere of that body cannot be used to make the spacecraft orbit around it.&lt;/p&gt;&#xA;&lt;p&gt;I couldn't help but wonder about the &lt;a href=&quot;https://www.google.co.in/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0CBwQFjAA&amp;amp;url=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FLow-energy_transfer&amp;amp;ei=6HGCVMqNBIaNuASzv4GADQ&amp;amp;usg=AFQjCNEpSHzxRm-GEykM6E8BOuhNokZF9A&amp;amp;sig2=QbVyslA_tTataav9aRgm-Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;low energy transfers&lt;/a&gt; (complete low energy transfer introduced into practice during JAXA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Hiten&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hiten&lt;/a&gt; mission and consequently in NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Recovery_and_Interior_Laboratory&quot; rel=&quot;nofollow noreferrer&quot;&gt;Grail&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Genesis_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Genesis&lt;/a&gt; missions which utilizes Lissajous trajectories through manifolds and ideally assumes &lt;span class=&quot;math-container&quot;&gt;$\Delta v = 0$&lt;/span&gt; for ballistic capture around the body and completion of at least 1 orbit thereafter.)&#xA;Can that be related as an exception to gravity assist braking or is it a totally different case?&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="47149" LastEditDate="2022-06-07T23:01:53.790" LastActivityDate="2022-06-07T23:01:53.790" Title="Gravity braking and orbit around same body in low energy transfers" Tags="|orbital-mechanics|the-moon|gravity|low-energy-transfer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="6205" PostTypeId="2" ParentId="739" CreationDate="2014-12-05T10:25:02.553" Score="5" Body="&lt;p&gt;Talking about India It has one DSN station with S/X band support with 11,18 and 32m antennas. As earlier answer marked, except for time critical tracking and control operations, data set collected over series of passes or a larger time interval can suffice orbit determination needs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During critical mission phases of important missions like MOM, ISRO does get help from NASA DSN under conditions unknown. MOM was tracked simultaneously by Canberra or Goldstone station during the Mars insertion phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISRO also used a set of Ship-borne Terminals deployed in pacific for tracking the MOM until upper stage ignition.&#xA;Other TTC operations are coordinated by ISTRAC ground station network spread in and around India to track Earth orbiting spacecrafts and geo-centric phases of planetary spacecrafts.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2014-12-05T10:25:02.553" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6206" PostTypeId="2" ParentId="6194" CreationDate="2014-12-05T10:32:10.893" Score="0" Body="&lt;p&gt;Ignoring the effects of nutation and precession the conversion factor/angles can be determined ideally by projecting the angle between vernal equinox vector and the vector pointing the sun (Which is characteristic depending on time of the year and can also be determined by Mean difference of sidereal and solar time in angle multipled over number of days since spring equinox) over equitorial plane and adding an angular component of ( W(e)*T(u)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;W(e) being the angular velocity and T(u) being the universal time or time at greenwich.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Effects of Prescession and Nutation can also be introduced using Diurnal signature of Earth which is used by tracking stations to accurately transform the local coordinates to inertial. Here is an answer which gives information on Diurnal signature of the earth. I suppose that can be used to model Precession and Nutation effects in the conversion. Hope this Helps.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2014-12-05T10:32:10.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6207" PostTypeId="1" AcceptedAnswerId="6230" CreationDate="2014-12-05T11:12:31.397" Score="18" ViewCount="5780" Body="&lt;p&gt;In communication with interplanetary space probes, it is clear that one has to take the delay caused by speed of light into account. Besides that, what are current limits for the data upload and download bitrates? What is the main cause of the limit? &lt;/p&gt;&#xA;" OwnerUserId="6213" LastEditorUserId="25" LastEditDate="2018-11-28T14:34:36.680" LastActivityDate="2018-11-28T14:34:36.680" Title="What bitrates are currently achievable for communication with interplanetary space probes?" Tags="|communication|interplanetary|space-probe|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6208" PostTypeId="2" ParentId="6183" CreationDate="2014-12-05T14:24:08.403" Score="4" Body="&lt;p&gt;The maneuver you're looking for is not called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity assist&lt;/a&gt;. A gravity assist can be conceptualized as a 3-body problem: body 1 (your spacecraft) picks up or drops off angular momentum that body 2 (the planet/moon) has in relation to body 3 (the body being orbited).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No, you're not an idiot, this is actually a common pitfall encountered by many student astronavigators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The maneuver used to get captured into an orbit of your target planet or star is technically an &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Oberth maneuver&lt;/a&gt;, but is usually referred to as an &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_insertion&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital insertion&lt;/a&gt; burn. In your case, as you enter your stellar system's &lt;a href=&quot;https://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)&quot; rel=&quot;nofollow noreferrer&quot;&gt;sphere of influence&lt;/a&gt;, you adjust your course to approach the star on a &lt;a href=&quot;http://www.braeunig.us/space/orbmech.htm#conic&quot; rel=&quot;nofollow noreferrer&quot;&gt;hyperbolic&lt;/a&gt; trajectory. You'll want to make the periapsis of that trajectory as close as possible to the star itself, but not so close that you burn up. Do your burn during your closest approach (that's the &lt;a href=&quot;https://space.stackexchange.com/questions/6197/is-a-braking-maneuver-more-effective-at-periapsis&quot;&gt;most efficient time&lt;/a&gt;) to change that hyperbola to an ellipse, and you have capture. You'll probably want to have a secondary burn at apoapsis to circularize your orbit to avoid repeatedly roasting your craft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, if your target star has a gas giant in a suitable orbit, you might want to design your mission to use a gravity assist after all.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-12-05T14:24:08.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6210" PostTypeId="1" CreationDate="2014-12-05T15:44:10.250" Score="2" ViewCount="106" Body="&lt;p&gt;Say you are in a happy future where mother ships travel between Earth and the Moon, or Mars, or Venus on regular runs, and are met by shuttles at each end of their voyages that transfer people, cargo, and fuel to the ships, after which they continue on their way, looping back and forth. (As I understand it, if you can invest in the necessary infrastructure, this is the best way to do it. If not, another question is necessary.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would repeated aerobraking maneuvers when entering orbit add up to significant stress on the ship over time? If fuel was no longer a major concern (for instance because it is now produced off-world and is cheap and plentiful), would it be better to avoid such aerobraking  in order to extend the life of the ship?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-12-05T15:56:35.820" Title="Can aerobrake maneuvers for entering orbit stress a ship over time?" Tags="|interplanetary|reuse|aerobraking|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6211" PostTypeId="2" ParentId="6210" CreationDate="2014-12-05T15:56:35.820" Score="2" Body="&lt;p&gt;The simple answer here is given by the Space Shuttle flights - extensive data on stresses from re-entry / aerobraking show that this is going to cause significant stresses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://www.nasa.gov/centers/dryden/pdf/88084main_H-1382.pdf&quot; rel=&quot;nofollow&quot;&gt;This Paper&lt;/a&gt; and &lt;a href=&quot;http://www.nasa.gov/centers/dryden/pdf/88091main_H-1400.pdf&quot; rel=&quot;nofollow&quot;&gt;This Paper&lt;/a&gt; for 2 brief examples of identified stresses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assessment, repair and rebuild, along with replacement of tiles etc contributed to the high cost of the shuttle programme. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the shuttle was undergoing full reentry. Aerobraking may be possible at significantly lower stresses, reducing the maintenance requirement significantly. Additionally, if the craft is never intended to land, it can be built to different design specs, so could better accept stresses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;However&lt;/strong&gt; - your final paragraph is probably appropriate here. There are options other than aerobraking for the actual transport ship, including using fuel or more outlandish solutions (currently scifi) such as tethers, non-stop fly-bys etc.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2014-12-05T15:56:35.820" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6212" PostTypeId="1" CreationDate="2014-12-05T17:01:24.193" Score="1" ViewCount="92" Body="&lt;p&gt;So, I'm watching NASA's Orion splashdown coverage and I thought of something. The Navy is taking an awfully long time to recover the Orion command module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If braking engines (see Soyuz reeentry module) and parachutes are in use, why can't a space agency bring a command/descent module down on dry land or a floating platform designed for reentry? Wouldn't it be a lot quicker to recover? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the risks involved, aside from parachute failure and a crushing impact? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even with parachutes, does a descent module just travel too fast for a solid landing zone to be safe?&lt;/p&gt;&#xA;" OwnerUserId="6771" LastActivityDate="2014-12-05T17:07:48.423" Title="Why can't space agencies &quot;splashdown&quot; command modules on solid surfaces?" Tags="|spacecraft|reentry|" AnswerCount="0" CommentCount="1" ClosedDate="2014-12-05T19:04:11.787" ContentLicense="CC BY-SA 3.0" />
  <row Id="6214" PostTypeId="1" AcceptedAnswerId="6215" CreationDate="2014-12-05T17:16:34.003" Score="6" ViewCount="439" Body="&lt;p&gt;Orion's &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_Flight_Test_1&quot;&gt;Exploration Flight Test 1&lt;/a&gt; today appeared to go very well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only information on future Orion flights I was able to find suggests that the next step is an unmanned circumlunar flight launching on SLS in 2018, and there's some understandable skepticism around SLS development. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there really no plan over the next few years for longer-duration and manned test flights of Orion in LEO, launching on Delta IV, similar to Apollos 6 and 7?&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-08T17:30:20.443" Title="What's next for Orion?" Tags="|sls|delta-iv-heavy|orion-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6215" PostTypeId="2" ParentId="6214" CreationDate="2014-12-05T18:05:15.990" Score="11" Body="&lt;p&gt;The answer is more political than technical.  (Will be some opinion here, alas)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS exists as a booster without much of a mission.  Well it has Orion, since SLS is the only booster (except maybe Falcon Heavy once man-rated) that can launch Orion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Constellation there were several projects.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Ares-1 - SRB based manned booster for original Orion design.&lt;/li&gt;&#xA;&lt;li&gt;Ares-5 - Heavy lift for Moon/Mars/Asteroid missions. (Hab, upper stage, whatever)&lt;/li&gt;&#xA;&lt;li&gt;Orion - Actual crew vehicle (Service Module was key as well, since it did a lot)&lt;/li&gt;&#xA;&lt;li&gt;Altair - Lunar Lander - aha! A mission!&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;With the move away from Constellation to the current model, the first and last in my list were cancelled. So now the only mission for SLS is to launch Orion. Orion's mission has drifted and drifted and drifted.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moon? Mars? Asteroid? Who knows these days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way SLS and Orion were structured, politically was to keep people employed. (It sucks to say, and is not the 100% complete story, but it has sufficient elements of truth to need discussing). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus there was something of an underlying goal of long term jobs. As opposed to a strongly stated mission with a tight timeline.  I.e. No urgency, no goal, no real mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus you have the oddity of an SLS launch schedule, launching vaguely every 3-4 years. That means you have to pay the staff at the launch pad for 4 years, to launch once. That will never be cost effective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Worse, they will never be as 'good' at it, as a launch team that launches on a regular schedule. So now you likely need more time and people spent on training to make sure they stay ready.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But by developing it in this fashion, they ran out of money for funding other projects that would lead to missions, so they never quite happened. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It spirals out of control. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is why government large scale projects have weaknesses.  Compare with Commercial Cargo and Commercial Crew projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There, NASA wrote a spec, let companies bid, set milestones they had to meet, and paid upon their meeting milestones. Finally paying per mission after mission success.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is beyond question that Commercial Cargo was gigantic success. Helped SpaceX get over the funding hump, to bring out a new commercial booster that is likely to bring launcher dominance back to the US for the first time since the 1980s.  But they get the cargo they need to the ISS, cheaper than they could do it themselves. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Antares was an interesting choice, but since all launchers seem to have a failure every so often, (Sure Atlas V is doing well so far, and I only wish it well, but reality has this way of biting you in the tooshie. Same is true of Falcon 9).  But it was actually considered a conservative approach. Existing developed engine (if old, but &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerojet_Rocketdyne&quot;&gt;Aerojet Rocketdyne&lt;/a&gt; claimed they could refurbish them back to new) backed by an established rocket company, established first stage developer (&lt;a href=&quot;https://en.wikipedia.org/wiki/Yuzhnoye_Design_Bureau&quot;&gt;Yuzhnoye Design Bureau&lt;/a&gt;, makers of Zenit booster), upper stage from &lt;a href=&quot;https://en.wikipedia.org/wiki/Alliant_Techsystems&quot;&gt;ATK&lt;/a&gt; (Castors have long flight history), Service module from an establish satellite manufacturer, and a pressure vessel from &lt;a href=&quot;https://en.wikipedia.org/wiki/Thales_Alenia_Space&quot;&gt;Thales Alenia&lt;/a&gt; (made almost all the pressure modules on the ISS US side). But remember, it is still rocket science. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will CCtCAP (Commercial Crew) work out as well? One can hope. You have a now established vendor (SpaceX with a rocket with 10+ launches, and a vehicle that is an upgrade from a multiply flown version) and Boeing (the definition of 'established' in the space industry) with a new capsule design but on a booster with a very rich heritage (Atlas V with 50+ consecutive successes). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will SLS work? Hard to imagine how, except through largess. But if SLS goes away, Orion goes away since nothing else can launch it. All those jobs go away, so will it happen?  Hard to say.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2014-12-06T23:39:24.920" LastActivityDate="2014-12-06T23:39:24.920" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6216" PostTypeId="2" ParentId="4893" CreationDate="2014-12-05T18:23:33.983" Score="7" Body="&lt;p&gt;Geoffc gives all the main information, but it's worth adding that precision in such a landing is low. If you splashdown in an ocean, you have no risk of hitting something you wouldn't want to, like a tree, or a sharp boulder, or a car.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, a landing Soyuz normally hits the surface and then rolls for a bit, rather than hitting, sinking, and then bobbing back up, which i would imagine is less nauseating and less of a shock.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-12-05T18:23:33.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6217" PostTypeId="1" AcceptedAnswerId="6218" CreationDate="2014-12-05T19:18:19.553" Score="14" ViewCount="3896" Body="&lt;p&gt;I understand that there are flotation devices on the space capsule but I don't see them visibly deployed in this image&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://a57.foxnews.com/global.fncstatic.com/static/managed/img/fn2/feeds/Space.com/876/493/orion-water-recovery-test-navy.jpg?ve=1&amp;amp;tl=1&quot; alt=&quot;image&quot;&gt;. There are other images where they are deployed and clearly visible above the water surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://penguinsix.com/wp-content/uploads/2011/04/090408-F-4572F-108.jpg&quot; alt=&quot;images&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="2952" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.510" LastActivityDate="2022-02-18T22:38:53.147" Title="How do space capsules float on water?" Tags="|water|capsule|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6218" PostTypeId="2" ParentId="6217" CreationDate="2014-12-05T19:37:34.523" Score="21" Body="&lt;p&gt;Capsules like Apollo and Orion are mainly open space internally (the crew cabin); they have no problem floating by themselves (like a metal-hulled boat would).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The conical capsule shape by itself will float in either of two orientations: stable 1 (base down, nose up) and stable 2 (nose down). In stable 2, the crew is hanging upside down in their seats and the hatches are underwater, obviously not a desirable condition, so the inflatable floats at the top end are to right the capsule if it lands nose-down or turns over in rough seas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The larger ring float around the base both helps keep the capsule stable and insures against the capsule sinking due to water getting inside an open hatch after the crew gets out. The first Mercury capsules didn't have that kind of float, and were intended to be picked up by helicopter and flown to a recovery ship, but the heavier Gemini and Apollo capsules had to stay afloat longer, until the recovery ship could reach them and bring them aboard by crane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Gemini capsules had a different layout that appears to not have been nose-up stable, and its flotation system helps to keep it in the desired (hatches-up) orientation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QhvQ3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-05T20:47:23.163" LastActivityDate="2014-12-05T20:47:23.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6219" PostTypeId="1" AcceptedAnswerId="6220" CreationDate="2014-12-05T20:44:30.473" Score="6" ViewCount="631" Body="&lt;p&gt;This mission profile seems like a no-brainer to me, but I don't recall seeing it mentioned in Orion coverage. I know this may have more to do with politics than anything else, but I'm more interested in technical points such as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have ISS missions been discussed internally by the Orion design team?&lt;br&gt;&#xA;What remains to be done to achieve a manned rating for Orion?&lt;br&gt;&#xA;Are there any current US man-rated launch vehicles capable of sending it to the ISS?&lt;br&gt;&#xA;If not, what would qualifying a suitable launcher take?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is there some sort of non-compete clause in the CCtCAP program?&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2020-03-12T02:21:07.750" Title="What would be required for Orion to ferry astronauts to the ISS?" Tags="|iss|orion-spacecraft|crewed-spaceflight|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6220" PostTypeId="2" ParentId="6219" CreationDate="2014-12-06T01:28:13.030" Score="5" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Orion_(spacecraft)#Orion_CEV&quot; rel=&quot;noreferrer&quot;&gt;wikipedia&lt;/a&gt;, when Orion was conceived as part of the Constellation program, the ISS ferry role was considered; I would be surprised if that role wasn't kept in mind throughout development since then. In particular, Orion uses the ISS-compatible &lt;a href=&quot;http://en.wikipedia.org/wiki/NASA_Docking_System&quot; rel=&quot;noreferrer&quot;&gt;NASA Docking System&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the excitement around the Orion EFT-1 flight was that it was &quot;the first human-rated ship to leave LEO since Apollo&quot;; it's not clear to me whether than meant that Orion was actually, currently human-rated or if that was stretching to cover the Orion program's overall design. In any case, human-rating is mostly a matter of &quot;we are incorporating these necessary design features&quot;, so I would guess there's not much more to be done there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Human_Rating/HumanRatingAtlasVandDeltaIV.pdf&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; (possibly from 2008?), the Atlas and Delta launchers are very close to being human-rated, with Delta mostly needing automatic fault detection systems that could trigger an in-flight abort. Delta IV Heavy definitely has the capacity to send Orion to ISS, Atlas V probably not. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-06T01:28:13.030" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6222" PostTypeId="1" CreationDate="2014-12-06T16:18:56.957" Score="1" ViewCount="123" Body="&lt;p&gt;Watching footage of the Orion test launch on Dec 5th, one can see orange flames rise up from the base of the rocket in the few seconds before t=0 and before the engines appear to fully &quot;fire up&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nothing like it is visible on Space Shuttle launch footage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is happening in the launch sequence to produce these effects?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A description of the Saturn V mentions that ignition was accomplished by &quot;bursting a hypergolic cartridge&quot;. Does the Delta IV heavy do something similar?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="1142" LastEditDate="2014-12-06T20:09:34.920" LastActivityDate="2014-12-06T20:09:34.920" Title="What is happening in the few seconds surrounding t=0 of the Delta IV Heavy launch?" Tags="|launch|rockets|launch-sequence|delta-iv-heavy|" AnswerCount="0" CommentCount="2" ClosedDate="2014-12-07T16:06:18.707" ContentLicense="CC BY-SA 3.0" />
  <row Id="6223" PostTypeId="5" CreationDate="2014-12-06T17:43:09.833" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-12-06T17:43:09.833" LastActivityDate="2014-12-06T17:43:09.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6224" PostTypeId="4" CreationDate="2014-12-06T17:43:09.833" Score="0" Body="Questions on trajectories that allow spacecraft to change orbits using very little fuel." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-06T18:15:44.710" LastActivityDate="2014-12-06T18:15:44.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6225" PostTypeId="2" ParentId="1829" CreationDate="2014-12-06T17:48:46.003" Score="13" Body="&lt;p&gt;I found &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_orbiter#Shuttle_Orbiter_Specifications_.28OV-105.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;different specifications for the orbiter&lt;/a&gt; than those quoted from Artemis Project: max takeoff weight of 240,000 lbs (109 metric tons), and useful payload of 55,025 lbs (25 metric tons). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we fill the payload bay with fuel for the Shuttle's orbital maneuvering thrusters, and apply the rocket equation:&#xA;\begin{equation}\Delta v = v_e \ln\left(\frac{m_0}{m_f}\right)\end{equation}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using $v_e$ of 3096 m/s (derived from the 316 sec I&lt;sub&gt;sp&lt;/sub&gt; of the OMS rockets), we get about 800 m/s additional ∆v provided from the fuel in the payload bay. It takes about 4100 m/s to enter lunar orbit, and another ~900 m/s to return (assuming we can use aerobraking to slow down on arrival) so we're far short of what's needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main engines on the orbiter are somewhat more efficient by mass, so 1150 m/s might be possible there, but hydrogen fuel for the SSMEs is much less dense than the fuel needed by the OMS, so unlikely to physically fit in the payload bay -- and that's still far short of the required ∆v.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-05-31T19:08:26.237" LastActivityDate="2018-05-31T19:08:26.237" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="6226" PostTypeId="1" AcceptedAnswerId="6227" CreationDate="2014-12-06T18:07:54.027" Score="7" ViewCount="475" Body="&lt;p&gt;Since the CTS-100 &lt;a href=&quot;http://www.airspacemag.com/space/taxi-to-the-space-station-261647/?all&amp;amp;no-ist&quot;&gt;uses lithium-ion batteries&lt;/a&gt; to supply its electrical power, and lithium-ion batteries self-discharge over time, will the CST-100 need to recharge its batteries at the ISS? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so how will this be accomplished, will the astronauts pass a cable through the NDS and plug in, or does the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Docking_System&quot;&gt;NDS&lt;/a&gt; provide power transfer?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="1142" LastEditDate="2014-12-06T18:18:55.940" LastActivityDate="2014-12-06T19:22:11.903" Title="Will the CST-100 recharge its batteries?" Tags="|iss|power|battery|cst-100|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6227" PostTypeId="2" ParentId="6226" CreationDate="2014-12-06T18:24:17.280" Score="7" Body="&lt;p&gt;From the wiki on the NASA Docking System:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once mated the NDS interface can transfer power, data, commands, air, communication and in future implementations will be able to transfer water, fuel, oxidiser and pressurant as well.&lt;a href=&quot;http://dockingstandard.nasa.gov/Documents/AIAA_ATS_NDS-IDSS_Overview_Draft1.pdf&quot; rel=&quot;noreferrer&quot;&gt;[1]&lt;/a&gt;  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So they could certainly top off their charge if they need to (assuming they have a NDS installed by the time CST-100 gets there).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Boeing's &lt;a href=&quot;http://www.boeing.com/assets/pdf/defense-space/space/ccts/docs/Space_2011_Boeing.pdf&quot; rel=&quot;noreferrer&quot;&gt;Design Considerations&lt;/a&gt;, page 3:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The CST-100 can operate autonomously for up to 60 hours of free-flight, and is designed for Day One rendezvous with a Day Two backup opportunity. The vehicle can stay docked to a host complex for up to 210 days while provided with one kW of keep-alive power.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-06T19:22:11.903" LastActivityDate="2014-12-06T19:22:11.903" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6228" PostTypeId="5" CreationDate="2014-12-06T18:55:19.517" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-12-06T18:55:19.517" LastActivityDate="2014-12-06T18:55:19.517" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6229" PostTypeId="4" CreationDate="2014-12-06T18:55:19.517" Score="0" Body="The CST-100 (Starliner) capsule is Boeing's Crew Space Transportation designed to transport up to seven astronauts to orbit." OwnerUserId="49" LastEditorUserId="22326" LastEditDate="2020-02-10T16:22:27.517" LastActivityDate="2020-02-10T16:22:27.517" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="6230" PostTypeId="2" ParentId="6207" CreationDate="2014-12-06T23:02:02.200" Score="20" Body="&lt;p&gt;Speed-of-light delay is mostly irrelevant for data transmission rates.  Once you get out of Earth orbit, transmissions are generally &quot;fire and forget&quot;; if data gets garbled, you schedule a retransmission of the garbled portion at a later time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The limit for interplanetary communication rates is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Noisy-channel_coding_theorem&quot;&gt;Shannon limit&lt;/a&gt;: how fast you can send data while still being able to distinguish it from background noise.  This in turn is influenced by antenna gain (a bigger sending &lt;em&gt;or&lt;/em&gt; receiving dish lets you transmit faster), transmission power (a &quot;louder&quot; transmitter can send data faster), and distance (the further apart your antennas are, the more the signal fades towards background levels and the slower you can transmit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of how many variables there are, there is no one &quot;achievable bitrate&quot;.  I wouldn't be surprised to find that Arecibo could talk to a counterpart in Neptune orbit at gigabit or even terabit data rates, while the Galileo probe's non-directional antenna could talk to the Deep Space Network antennas at only 160 bits per second from Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to see what sort of data rates are in real-world use, NASA's &lt;a href=&quot;https://eyes.nasa.gov/dsn/dsn.html&quot;&gt;Deep Space Network status page&lt;/a&gt; will show you which probes are currently transmitting or receiving, and if you select &quot;more details&quot;, it will tell you the transmission rate (for example, as I write this, antenna 15 at Goldstone is receiving data from MRO at 1.5 Mbps).&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-12-06T23:02:02.200" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6231" PostTypeId="1" AcceptedAnswerId="6232" CreationDate="2014-12-06T23:05:06.483" Score="3" ViewCount="1419" Body="&lt;p&gt;I am wondering why only &lt;a href=&quot;http://www.universetoday.com/55512/how-many-people-have-walked-on-the-moon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;2 men out of 3 crew members&lt;/a&gt;, in every successful Apollo mission, made a lunar walk leaving only one man to remain inside the &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;lunar module&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 11, 12 ,14, 15, 16 and 17&lt;/a&gt; there were three crew members in each mission and only two men made a lunar walk. If that's true then what could be the reasons for not permitting the third member (the most unfortunate) to come out of the module?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it not the most unfortunate fate that can fall on a person who travels to the moon and doesn't get a chance to walk on its surface? &lt;/p&gt;&#xA;" OwnerUserId="6782" LastEditorUserId="21979" LastEditDate="2017-12-09T12:25:46.970" LastActivityDate="2017-12-09T12:25:46.970" Title="Why only 12 men have made lunar-walks and not all 18 men?" Tags="|apollo-program|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6232" PostTypeId="2" ParentId="6231" CreationDate="2014-12-06T23:07:57.140" Score="23" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot;&gt;Lunar Module&lt;/a&gt; only carried &lt;em&gt;two&lt;/em&gt; people.  The third astronaut stayed in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot;&gt;Command Module&lt;/a&gt;, which remained in lunar orbit for the duration of the landing mission.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-12-06T23:07:57.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6233" PostTypeId="1" AcceptedAnswerId="6235" CreationDate="2014-12-07T00:22:12.890" Score="4" ViewCount="599" Body="&lt;p&gt;Inspired by the comment of @AnthonyX to &lt;a href=&quot;https://space.stackexchange.com/a/1303/198&quot;&gt;this answer&lt;/a&gt;: Could the Apollo's CM pilot (who would normally have stayed in orbit) have switched to the role of LM pilot/commander if one of the LM personal would have developed a medical emergency (&quot;a really bad cold&quot;) enroute to the Moon that prohibited him from landing but permitted him manning the CM? Were they cross-trained?&lt;/p&gt;&#xA;" OwnerUserId="198" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-12-09T04:55:01.543" Title="Was the Apollo's CM pilot qualified for landing on the moon?" Tags="|apollo-program|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6234" PostTypeId="1" AcceptedAnswerId="6651" CreationDate="2014-12-07T02:54:28.443" Score="10" ViewCount="1185" Body="&lt;p&gt;All the atmospheric entry missions endure a communication blackout. What parameters except plasma penetration frequency affect the blackout time and how ? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, &lt;a href=&quot;http://en.wikipedia.org/wiki/Communications_blackout&quot;&gt;Comm Blackout&lt;/a&gt; states that space shuttle's shape created a hole on the rear side to allow relay communication through TDRSS. That raises a doubt if such holes could be as well be incorporated in modules say, Orion to maintain communication all the time when missions go real and there are humans inside.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2015-01-02T19:37:17.967" Title="Communication Blackout" Tags="|communication|spacecraft|space-shuttle|reentry|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6235" PostTypeId="2" ParentId="6233" CreationDate="2014-12-07T05:31:36.980" Score="1" Body="&lt;p&gt;No, they were not cross-trained and regardless the mission would never have proceeded in such a circumstance.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-12-07T05:31:36.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6236" PostTypeId="2" ParentId="5781" CreationDate="2014-12-07T14:01:12.993" Score="6" Body="&lt;p&gt;Having written a short article on this is my first year of uni, I concluded that the main problems with the probes would be reliability. Not only in terms of mechanical functionality but also in terms of programming. If you were to imagine that when each probe replicates there would be some degree of transmission error or corruption. This would normally be negligible, but if we scale this up to galactic or inter-galactic scales, even a minute error can grow and disrupt future generations of probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information I'd suggest you start with: &lt;a href=&quot;http://arxiv.org/abs/1206.0953v2&quot; rel=&quot;noreferrer&quot;&gt;Galactic exploration by directed Self-Replicating Probes, and its implications for the Fermi paradox - Martin Barlow&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6785" LastEditorUserId="49" LastEditDate="2014-12-07T14:13:26.627" LastActivityDate="2014-12-07T14:13:26.627" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6237" PostTypeId="1" CreationDate="2014-12-07T14:54:50.117" Score="0" ViewCount="563" Body="&lt;p&gt;I've been looking for close up or hi-res photos of the newer displays of the MCC21 (Mission Control Center - 21st Century) FCR (Flight Control Room), where the text is legible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't found any on Google Images.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is an older generation display, photographed by NASA.  This example has legible text on the flight control display.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IqlsF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here is the MCC21 display, but low resolution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JpJp1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="6786" LastActivityDate="2015-01-03T14:31:31.937" Title="MCC21 FCR Display Closeup or Hi-res" Tags="|nasa|software|design|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6238" PostTypeId="2" ParentId="6231" CreationDate="2014-12-07T15:51:27.023" Score="17" Body="&lt;p&gt;What happened in Apollo missions is that once they were in orbit around the Moon, the Lunar Module separated from the Command Module and descended to the surface. This meant that the mass of the lander was much lower, which saved tremendously on the size of rocket needed to launch the whole thing from Earth. Every kilogram they wanted to land on the Moon required something like 25 kg of rocket to launch it from Earth. It was most efficient to land only part of the spacecraft that reached the Moon on the surface. The rest stayed in orbit, with the heavy fuel, engines, heat shield, and life support systems needed to return home.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were a few reasons to leave one person aboard the Command Module. First, that person was very busy the whole time conducting observations and collecting data for science objectives that could only be done from orbit - they had a great view of the Moon. In this &lt;a href=&quot;https://what-if.xkcd.com/72/&quot;&gt;What If&lt;/a&gt; article, there are a couple of great quotes about what that was like for them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, that person was there in case there were problems. There are a few limited situations where they could have rendered assistance if the Lunar Module had trouble docking with the Command Module upon return. More to the point, if the astronauts that landed could not return and were stranded on the Moon, still the Command Module could make it back to Earth, and the mission would at least have some success. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-12-08T02:09:09.063" LastActivityDate="2014-12-08T02:09:09.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6241" PostTypeId="1" CreationDate="2014-12-08T01:17:02.660" Score="2" ViewCount="357" Body="&lt;p&gt;I found three videos (&lt;a href=&quot;https://www.youtube.com/watch?v=1IXYsDdPvbo&quot; rel=&quot;nofollow&quot;&gt;2009 Google Tech Talk&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/watch?v=s7RGSBOmsm4&quot; rel=&quot;nofollow&quot;&gt;2011 International Mars Society Convention&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/watch?v=Erax53WQX5s&quot; rel=&quot;nofollow&quot;&gt;2011 Quicklaunch Inc.&lt;/a&gt;) from Dr. John Hunter about using a hydrogen gas cannon to launch payloads into space (or at least as a first stage of launch). Dr. John Hunter wants to use a space cannon at sea level near the equator shooting east.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, would there be a significant benefit to building the &lt;a href=&quot;https://en.wikipedia.org/wiki/Quicklaunch&quot; rel=&quot;nofollow&quot;&gt;space gun&lt;/a&gt; into a tall mountain like Chimborazo in Ecuador (near the equator)? Would the muzzle velocity of the two space guns be very different, if one was at sea level and the other at 20,000 ft, both at the equator?&lt;/p&gt;&#xA;" OwnerUserId="6791" LastEditorUserId="49" LastEditDate="2014-12-08T01:38:02.790" LastActivityDate="2015-12-23T03:01:34.447" Title="Benefits of hydrogen cannon for first stage launch at sea level or 20,000 feet" Tags="|launch|atmospheric-drag|space-gun|hydrogen|high-altitude-launch|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6242" PostTypeId="2" ParentId="6241" CreationDate="2014-12-08T01:22:44.113" Score="5" Body="&lt;p&gt;The benefit of a high altitude exit from a high velocity gun is the reduced density of the atmosphere as the payload/projectile at full speed, hits the air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sea level is not a good place to suddenly hit the atmosphere at orbital velocity or even close to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;20,000 feet the air pressure reduces enough to matter.  Higher would be better. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is muzzle velocity differences, and obviously a sea level exit would require much higher velocity, which means greater heating, more mass for heat shielding.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus a sea level release would be less effective. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-08T01:22:44.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6243" PostTypeId="1" CreationDate="2014-12-08T02:51:21.777" Score="3" ViewCount="160" Body="&lt;p&gt;Assuming you have the budget to research &lt;a href=&quot;http://en.wikipedia.org/wiki/Electrodynamic_tether&quot; rel=&quot;nofollow noreferrer&quot;&gt;electrodynamic tether&lt;/a&gt; powered unmanned satellites, and you had a cheap way of getting them in to earth low earth orbit, would they be able to use electricity based propulsion alone to travel to stable orbits around the Sun (inside mercury for plasma fusion research), Jupiter (orbiting Europa), and Saturn (orbiting titan) for long term missions because of &lt;a href=&quot;http://www.astronomynotes.com/solarsys/plantblb.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;their strong magnetic fields&lt;/a&gt;? &lt;/p&gt;&#xA;" OwnerUserId="6792" LastEditorUserId="12102" LastEditDate="2020-01-24T07:49:37.847" LastActivityDate="2020-01-24T07:49:37.847" Title="Could electrodynamic tethers power long-term unmanned satellites around the Sun, Jupiter, or Saturn?" Tags="|jupiter|saturn|uncrewed-spaceflight|long-duration|electrodynamic-tether|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="6245" PostTypeId="1" AcceptedAnswerId="6246" CreationDate="2014-12-08T06:31:08.943" Score="21" ViewCount="10597" Body="&lt;p&gt;I was looking at some diagrams for the Apollo spacecraft, which I found to be interesting. It's clear that there's two separate enclosures that astronauts could reside in (the lunar module and command module). However, there is a massive rocket nozzle in between each habitable enclosure. This leads me to a few related questions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where and what kind of design was in place to allow the astronauts to safely move from enclosure to enclosure? How did the astronauts traverse them? Were there any restrictions? What sort of precautions and safety mechanisms were in place to keep the astronauts safe when a lower section of the craft was discarded through the various stages?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/11lCz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1531" LastEditorUserId="1531" LastEditDate="2014-12-08T12:55:54.620" LastActivityDate="2021-11-19T19:49:36.243" Title="How did astronauts traverse from module to module in the Apollo craft?" Tags="|apollo-program|spacecraft|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6246" PostTypeId="2" ParentId="6245" CreationDate="2014-12-08T06:39:01.590" Score="30" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Launch_escape_system&quot; rel=&quot;noreferrer&quot;&gt;launch escape system&lt;/a&gt; was jettisoned after launch.  Here is a picture of it being tested:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/G8f89.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the launch escape system out of the way, and the fairings released, the crew performed a &lt;a href=&quot;http://en.wikipedia.org/wiki/Transposition,_docking,_and_extraction&quot; rel=&quot;noreferrer&quot;&gt;transposition and docking maneuver&lt;/a&gt; on the way to the moon.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It was performed by the Command Module pilot (although, as a contingency, the Lunar Module pilot and commander were also trained to perform the maneuver), and involved separating the CSM from the S-IVB, pitching the CSM 180° and proceeding to dock with the Lunar Module, by inserting a probe at the top of the CSM into a drogue at the top of the Lunar Module. Then, the Apollo spacecraft stack would separate from the S-IVB, which would then either continue on to a heliocentric orbit or be deliberately steered into a crash landing on the Moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qkRIx.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="6805" LastEditDate="2014-12-08T17:53:50.113" LastActivityDate="2014-12-08T17:53:50.113" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="6247" PostTypeId="1" CreationDate="2014-12-08T10:24:20.220" Score="10" ViewCount="882" Body="&lt;p&gt;It seems like electric propulsion will be the most viable candidate for current and future interplanetary missions. However, I was wondering what type of research is currently being done on Hall Effect thrusters in particular, and what the current limitations of such systems are (besides their low levels of thrust which are unfortunately inherent to all EP devices). It seems like a lot of work is being done on limiting erosion rates along channel/anode walls since operation times need to be extended into thousands of hours, and am I correct in thinking that the development of accurate numerical models is also an active part of current research as well? I'd love to hear of anything else being done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lB1rD.jpg&quot; alt=&quot;HT-400 thruster in operation&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6794" LastActivityDate="2019-03-13T03:19:31.867" Title="Current Research Being Done on Hall Effect Thrusters" Tags="|advanced-propulsion|ion-thruster|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6248" PostTypeId="1" CreationDate="2014-12-08T11:11:27.017" Score="10" ViewCount="275" Body="&lt;p&gt;Are there any indications of animal loss during a launch (shuttle or rocket).  I'm referring to animals in the vicinity.  I'm sure that it's minimal, but does the exhaust/sound/etc. knock out a bunch of birds and rodents, for example?  Is there a big die-off radius or anything?  Maybe the noise prior to launch frightens most animals away or underground.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="6712" LastEditDate="2015-03-26T10:11:26.770" LastActivityDate="2015-03-26T10:11:26.770" Title="Are there any indicative statistics about animal loss during launches?" Tags="|launch-sequence|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6249" PostTypeId="1" AcceptedAnswerId="6255" CreationDate="2014-12-08T12:10:03.047" Score="21" ViewCount="3613" Body="&lt;p&gt;While it's obvious how a single gimbaling engine can control a rockets attitude, it is less clear how it could control roll.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/luFdn.jpg&quot; alt=&quot;&amp;quot;Delta 4&amp;quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nonetheless many rockets use just a single main engine with no fins or control vanes. Some examples include Ariane 5, and Delta 4 (pictured).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do these single engine rockets use the main engine to control roll?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they don't use the engine, what systems do they use?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-01-27T16:45:24.843" LastActivityDate="2015-01-27T16:45:24.843" Title="How do single engine rockets control roll?" Tags="|launch|rockets|guidance|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6250" PostTypeId="1" AcceptedAnswerId="6252" CreationDate="2014-12-08T12:49:17.633" Score="4" ViewCount="3388" Body="&lt;p&gt;The closer a body is to the Sun, the warmer it becomes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to find a certain trajectory around either the Sun or any star, on which the temperature is constantly between, for example, 20 and 60 degrees Celsius?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="6312" LastEditDate="2014-12-08T15:06:25.913" LastActivityDate="2017-09-28T23:18:44.297" Title="Is it possible to orbit around the Sun within a &quot;safe temperature zone&quot;?" Tags="|orbit|temperature|the-sun|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6252" PostTypeId="2" ParentId="6250" CreationDate="2014-12-08T13:19:45.830" Score="9" Body="&lt;p&gt;Yes, and no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all: Space has no temperature. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are no atoms to move, so nothing to have a temperature to be measured. However, any object in space will radiate its heat away very quickly. There simply isn't anything to stop it leaving, no insulation at all, so space &quot;sucks&quot; the heat energy from everything. For something that doesn't have a temperature, space is remarkably cold. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest problem with temperature in space is not its absolute value; we can make hardware to operate from -80 to 120 degrees Celsius if we have to. The problem is temperature &lt;em&gt;difference&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one side of our imaginary spacecraft is exposed to direct sunlight it will start heating, while the side facing out into deep space will cool to almost absolute zero. This will put a massive temperature differential on the spacecraft which left unchecked would warp the structure, cripple the electronics, and generally be unhelpful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So modern satellites use &lt;a href=&quot;http://en.wikipedia.org/wiki/Spacecraft_thermal_control&quot; rel=&quot;nofollow&quot;&gt;Thermal Control Systems&lt;/a&gt; to keep a constant (or at least managed) temperature throughout the craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Returning to your question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it is possible to find an orbit that (depending on the design of the spacecraft) would provide an average (of the entire craft) temperature of between 20 and 60 degrees Celsius. However most of the craft would not actually be between those limits unless some thermal control system is implemented.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately it's a balancing act between energy in and energy out. Get that right and you can have a comfortable temperature anywhere.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-01-26T09:28:14.087" LastActivityDate="2015-01-26T09:28:14.087" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6253" PostTypeId="1" AcceptedAnswerId="6258" CreationDate="2014-12-08T14:26:45.567" Score="29" ViewCount="5474" Body="&lt;p&gt;Why do spaceships have a frustum (portion of a cone) shape like e.g. the pressure capsule of the &lt;a href=&quot;http://www.spacex.com/dragon&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Dragon&lt;/a&gt; on the image below?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/hZsVn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think there is some engineering stuff behind this to do with aerodynamics during landing. Can someone explain why this is so? &lt;/p&gt;&#xA;" OwnerUserId="6798" LastEditorUserId="49" LastEditDate="2014-12-08T16:35:55.040" LastActivityDate="2019-01-30T21:46:01.167" Title="Why are spaceship capsules frustum shaped?" Tags="|rockets|spacecraft|reentry|technology|aerobraking|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6255" PostTypeId="2" ParentId="6249" CreationDate="2014-12-08T14:50:57.313" Score="19" Body="&lt;p&gt;The Ariane 5 launches with two strap-on solid rocket boosters. The thrust vector control on these solid rocket boosters provides the necessarily roll authority during the first 130 seconds of flight while the solid rocket engines are attached. After solid separation, roll control is performed by a set of 400 newton hydrazine thrusters located on the Vehicle Equipment Bay, which sits atop the cryogenic main stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RS-68 engine on the Delta 4 uses a gas-generator cycle. A portion of the fuel and oxidizer is diverted from the main engine to a preburner driving a gas turbine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary purpose of the turbine is to power the fuel pumps. &lt;strong&gt;The exhaust gas from the turbine is put to a secondary use, attitude control.&lt;/strong&gt; The exhaust is vented separately from the exhaust from the main engine. The vented turbine exhaust is thrust vector controlled and provides the requisite roll authority to keep the vehicle stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/showcase/ariane5-attitude-control-system.html&quot; rel=&quot;noreferrer&quot;&gt;Ariane 5 Attitude Control System&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The functions of the 400 N ACS include roll and pitch control of the launcher after jettisoning of the solid rocket boosters. Thereafter, the same 400 N thrusters are used for fine control manoeuvres and precision upper stage orientation before separation of one or more payloads.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.esa.int/esapub/achievements/Sc72s6.pdf&quot; rel=&quot;noreferrer&quot;&gt;￼Ariane-5&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;HTPB solid propellant of 68% ammonium perchlorate, 18% aluminium and 14% liner produced and cast in casings in Kourou; 3.4 m-long forward section shipped already loaded by BPD from Italy. Nozzle steering by two hydraulic actuators using flexbearing for 6° deflection.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The above are very explicit on roll control after solid rocket separation. Before? It's not so clear. Finding anything technical at ESA is a tooth-pulling chore. I'll have to add more on roll control while the solids are still attached later. On thing that is clear (but is never said): were it not for that thrust vectoring, the slightest misalignment on those solid rockets would send the vehicle spinning out of control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.rocket-propulsion.info/resources/articles/PropulsionForThe21stCentury-RS-68.pdf&quot; rel=&quot;noreferrer&quot;&gt;B. K. Wood, Propulsion for the 21st Century—RS-68, &lt;em&gt;AIAA 2002-4324&lt;/em&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Engine Specifics. The new RS-68 is capable of operating in—and transitioning between—full power level and minimum power level upon command from the vehicle. It also supplies pressurization gasses to vehicle fuel and oxidizer propellant tanks and thrust vector and roll control by gimbaling the thrust chamber assembly and the fuel turbine exhaust roll control nozzle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="288" LastEditDate="2015-01-27T07:43:46.333" LastActivityDate="2015-01-27T07:43:46.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6256" PostTypeId="1" AcceptedAnswerId="6257" CreationDate="2014-12-08T15:26:22.390" Score="22" ViewCount="5439" Body="&lt;p&gt;Considering that &lt;a href=&quot;https://aviation.stackexchange.com/a/10036/354&quot;&gt;there is plenty of oxygen in the air&lt;/a&gt; at the altitudes that the SRBs flew to (&amp;lt;50 KM), why did the STS SRBs use solid propellant. &lt;strong&gt;Specifically, why use a system that must carry its own oxidizer as opposed to a jet engine?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that solid fuel was used due to the amazing thrust those things provided (12,000 kN). However, a hypothetical jet engine would be able to throttle or even shut down, increasing crew safety. Unlike the first stages of other launch systems, the SRBs even had an exposed surface in the direction of flight, providing a convenient air intake.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose also that the salt-water recovery might also be an issue with jet engines.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:54.153" LastActivityDate="2015-08-06T14:32:16.670" Title="Why were jet engines not used in place of the solid fuel in the STS SRBs?" Tags="|launch|space-shuttle|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6257" PostTypeId="2" ParentId="6256" CreationDate="2014-12-08T15:32:01.133" Score="31" Body="&lt;p&gt;Rockets and jet engines are vaguely similar in that they both burn fuel and oxidizer and use the exhaust out the back end to generate thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Turns out rockets are just way better at it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, 2.8 million lbs of thrust per SRB is so mind-numbingly large an amount of thrust it is hard to visualize.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The largest liquid fueled engines ever really produced and used are the US F-1 (Saturn V had 5 of them)  at 1.5-1.8 million lbs of thrust and the Russian RD-170 (Ukranian Zenit used one, cut in half it is the RD-180 (Atlas V), in quarters it is the RD-190 (South Korean booster, and as RD-193 for export in Antares 201) at about 1.8 million lbs of thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Boeing 777 engines are in the 100 Klbs thrust level and they are some of the largest jet engines built (by thrust). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So 100,000 vs 2,800,000 which is at least one order of magnitude, and then a half again.  That is a HUGE difference. The size of the 777 engine is vaguely the diameter of a 737's main body. So imagine if it scaled linearly in terms of diameter.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRB segments are narrow enough that they can travel by train. So no more than 10-15 feet. How wide would a 2.8 million lb thrust jet engine be? Can you imagine the spinning blades issues? Are there actually materials strong enough to spin at the size and speed required that would not self destruct?  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-08-06T14:32:16.670" LastActivityDate="2015-08-06T14:32:16.670" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="6258" PostTypeId="2" ParentId="6253" CreationDate="2014-12-08T15:58:26.413" Score="44" Body="&lt;p&gt;The capsules designed to reenter the atmosphere have to slow down from about 8 km/s to zero by the time they get to the ground. They actually don't use the part that looks like a cone to do that. They all have flat bottoms that they face into the wind to do that. If you compare the Dragon capsule from your link to a &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_(spacecraft)#Reentry_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz&lt;/a&gt; capsule, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Orion_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion&lt;/a&gt; capsule, or the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle#Re-entry_and_landing&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle&lt;/a&gt;, their shapes are very different, but they all have a part that is more pointy (though not very much, in the case of the Soyuz), and a part that is flat. The pointier bit is used when they are launched, to lower the friction of passing through the atmosphere (the Soyuz is launched in the middle of the rocket, and so it doesn't need a point). The flat part is used to come back.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1e3od.png&quot; alt=&quot;blunt shapes of reentry vehicles&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The black lines in the picture show where the front of the blast wave occurs when these shapes pass through the air at very high speed. The blast wave is what is slowing the capsule down and you want it to be as big as possible. A sphere is actually the shape that makes the biggest blast wave - in fact, it works too well to be used on a capsule with people inside. It slows down so fast they feel crushed. All the shapes used are partly spherical, on the part that is facing into the wind, behind that they taper in, but how much is optional. The Space Shuttle was an exception - it had only a very slight curve on the bottom because that was best when the overall design was considered. It worked because the surface area it had for slowing down was so large compared to the size of the whole spacecraft.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice how the blast wave doesn't touch the craft, it moves along in front of it. The craft moves so fast there is a bubble of air trapped in front of it, compressed between the outside face of the blast wave and the spacecraft. That bubble is also very important. When air is compressed, it heats up. The faster it is compressed, the more it heats. The fastest compression is on the face of the blast wave, and so that part is hottest. The air bubble also gets very hot, but not as hot as that, so it protects the spacecraft from the worst heat.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And see the fuzzy section behind the craft? That is an area where the air is thinner, because the spacecraft is punching a hole through the air, and it takes some time for that hole to fill back in after it has passed. That creates suction pulling on the back of the craft, and that slows it down too. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2019-01-30T21:46:01.167" LastActivityDate="2019-01-30T21:46:01.167" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="6260" PostTypeId="2" ParentId="6241" CreationDate="2014-12-08T16:17:42.493" Score="1" Body="&lt;p&gt;The short answer is &quot;obviously yes&quot;. A sea-level launch accepts an additional penalty from the atmosphere. This has several very serious impacts:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Limits the minimum economic payload, and thus, minimum cost of the gun itself&lt;/li&gt;&#xA;&lt;li&gt;Demands that you use an ablative heat shield&lt;/li&gt;&#xA;&lt;li&gt;Requires more delta-v from the rocket because of drag losses&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The more difficult thing to explain is why someone might think the benefits of sea launch to outweigh the detriments in the first place. In Dr. Hunter's talks he mentions that you'll get sag if you built this device above ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's obviously important to keep in mind that this gun is 1 km in length. It seems pretty clear that you would have this supported at a large number of different places if built above-ground. The sag is then only something that happens between supports, but adding supports also adds cost. Not only that, but you will likely need adjustment at various supports in order to keep the laser-accurate alignment. Other people have pointed out that building a gun on the side of a mountain will help, but if you're looking for a 1 km segment, you've limited yourself to only a handful of potential sites throughout the globe. Also, since the characteristic height of Earth's atmosphere is on the order of 8 km, it's difficult to find a practical mountain high enough to decrease the initial air pressure more than 20%. 20,000 feet would bring that down by nearly 50%, but how many mountains have a smooth 1 km stretch, are above 20,000 feet, and are accessible for heavy trucks? Most South American nations don't even have a mountain of that altitude. Land rights for such a scarce geographic commodity are obviously a major hurtle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not the case that a higher altitude launch wouldn't need an ablative heat shield, but the challenges would be reduced. Also, the alignment issue favors the sea launch in several ways. After all, the benefit of higher altitude launches is smaller payloads, this will likely make the tolerances for alignment more strict.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Economically, sea launch is also vastly easier to service and has some synergy with shipyards, which might be where the tubes are produced anyway. If you can order tubes with twice the diameter in exchange for avoiding more permitting, sagging, and a political minefield, you'll definitely give it serious consideration.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="6214" LastEditDate="2015-12-23T03:01:34.447" LastActivityDate="2015-12-23T03:01:34.447" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6262" PostTypeId="2" ParentId="6256" CreationDate="2014-12-08T18:04:52.080" Score="36" Body="&lt;p&gt;One factor that would likely disqualify a jet engine for consideration is that they can't operate over such a large speed range. The SRBs propel the orbiter to about &lt;a href=&quot;http://www-pao.ksc.nasa.gov/kscpao/nasafact/ships.htm&quot;&gt;3,500 mph&lt;/a&gt; (Mach 5.3). Ramjets and scramjets can operate at this speed, but can not be operated at a zero airspeed when the vehicle needs an enormous amount of thrust to leave the pad.&lt;/p&gt;&#xA;" OwnerUserId="6804" LastEditorUserId="49" LastEditDate="2014-12-08T18:38:19.437" LastActivityDate="2014-12-08T18:38:19.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6263" PostTypeId="1" AcceptedAnswerId="6268" CreationDate="2014-12-08T19:13:17.710" Score="40" ViewCount="9039" Body="&lt;p&gt;I was looking at pictures of land rovers left on the moon (footage and what not).  &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_artificial_objects_on_the_Moon&quot; rel=&quot;noreferrer&quot;&gt;Then I visited this wiki page&lt;/a&gt; to see what derelict equipment is on the moon (just sitting around). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Which then raises the question:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Provided you have time and some understanding of electronics and mechanical devices. Is there enough material on the moon construct 'something useful' to either: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Get help?&lt;/li&gt;&#xA;&lt;li&gt;Get off the moon and get back to earth? (more unlikely).  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/w6v8W.jpg&quot; alt=&quot;Space Junk&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="37" LastEditorUserId="37" LastEditDate="2014-12-10T15:28:59.340" LastActivityDate="2015-09-09T02:34:27.817" Title="Is there enough man-made junk on the moon to help a stranded person get home?" Tags="|the-moon|rescue-missions|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6265" PostTypeId="2" ParentId="6263" CreationDate="2014-12-08T20:06:54.037" Score="22" Body="&lt;p&gt;&lt;strong&gt;No.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without even considering consumables such as food or oxygen, there is nothing on the moon to regulate human body temperature or to allow one to dispose of the waste that would accumulate in your space suit if you did have something to eat or breathe. More directly addressing the question, none of the 'junk' on the moon has any method of propelling itself off the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you could get off the moon, you have no way of escaping lunar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you could escape lunar orbit, you have no way of navigating to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you could navigate to Earth, you have no way of surviving reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you could survive reentry, you have no way of landing.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-12-08T20:06:54.037" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="6267" PostTypeId="2" ParentId="2698" CreationDate="2014-12-08T22:39:25.533" Score="5" Body="&lt;p&gt;There are many other reasons beyond the availability of jargon in Hindi, that contributes to English being used close to 90% in the organisation, officially, not only for radio communication.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the reason being, diversity of employees ISRO recruits. There are parts of India, mostly southern and far eastern where Hindi is as rare as any other language and only common way of communication is using English.  I do not have a source to cite, but major fraction or atleast a considerable number of the employees in every domain of ISRO's technical branches are from southern parts of India. Considering the ages, they belong to a time, when hindi was quite unpopular in these states.&#xA;So, Centres in the South-India like &lt;a href=&quot;http://en.wikipedia.org/wiki/Vikram_Sarabhai_Space_Centre&quot; rel=&quot;noreferrer&quot;&gt;VSSC&lt;/a&gt; ,&lt;a href=&quot;http://en.wikipedia.org/wiki/Master_Control_Facility_%28ISRO%29&quot; rel=&quot;noreferrer&quot;&gt;Master Control facility&lt;/a&gt; , The prime-branch for design, fabrication and testing of all Indian satellites &lt;a href=&quot;http://en.wikipedia.org/wiki/Indian_Space_Research_Organisation_Satellite_Centre&quot; rel=&quot;noreferrer&quot;&gt;ISAC&lt;/a&gt; and centre for ISRO's major launches &lt;a href=&quot;http://en.wikipedia.org/wiki/Satish_Dhawan_Space_Centre&quot; rel=&quot;noreferrer&quot;&gt;SDSC-SHAR&lt;/a&gt; have communication medium as English upto percentages close to 100%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other centres, mainly around Northern side have slightly lower usage, but English still dominates officially for communication in and between centres and also for any operation including radio communication.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Despite the fact, that Hindi has been crowned as a dominating language of use in parliamentary and other government works, it is not exactly taken into use by whole of the country equally, a result of independence given to each state to choose from one of the 22 recognized languages . &lt;a href=&quot;http://en.wikipedia.org/wiki/National_language#India&quot; rel=&quot;noreferrer&quot;&gt;This&lt;/a&gt; confusingly states that English, being not included in the Constitution (being of foreign origin) , is one of the official languages in India. &#xA;So English might as well be called psuedo-official language of the country.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2014-12-08T22:39:25.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6268" PostTypeId="2" ParentId="6263" CreationDate="2014-12-08T23:27:53.097" Score="43" Body="&lt;p&gt;Leave?  No.  By now, any residual fuel in various landers will have leaked off, and nothing has the sort of chemistry capability to produce fuel from lunar materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Get help?  Maybe.  Most of the landers still have working radios with radiation-hardened circuitry; it's just a matter of powering them up.  Your best bet for this is probably the Lunar Rover radios: those were designed to be powered from an external source, so it's just a matter of finding a 36-volt power supply.  Batteries aren't an option because virtually everyone used silver oxide batteries that fully self-discharge in less than 20 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My first choice resource-wise would be the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_12&quot;&gt;Apollo 12&lt;/a&gt; landing site.  You've got three choices for radio: &lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_3&quot;&gt;Surveyor 3's&lt;/a&gt; radio and directional antenna, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module#Descent_stage&quot;&gt;LM descent stage's&lt;/a&gt; parabolic high-gain antenna, or the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package&quot;&gt;ALSEP&lt;/a&gt; controller's radio and directional antenna.  You can try to use either Surveyor's solar panels or the ALSEP radiothermal generator for power if you don't have your own supply.  And as a last resort, you can trace out an SOS in the regolith and hope that, as one of the better-known parts of the Moon, somebody's orbiter will spot you during a survey.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2014-12-08T23:27:53.097" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6269" PostTypeId="2" ParentId="6233" CreationDate="2014-12-09T04:55:01.543" Score="7" Body="&lt;p&gt;As already noted by others, it was the commander and not the lunar module pilot, who actually flew the lunar module during landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Training for this was supported by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Landing_Research_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;Lunar Landing Training Vehicle&lt;/a&gt;.  From &lt;a href=&quot;http://history.nasa.gov/alsj/LLRV_Monograph.pdf&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Unconventional, Contrary, and Ugly: The Lunar Landing Research Vehicle&lt;/em&gt;&lt;/a&gt; (quoting MSC Director Robert R. Gilruth):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;blockquote&gt;&#xA;    &lt;p&gt;The present plans call for a minimum of 22 flights for each Commander and backup Commander assigned to lunar landing missions. This includes 11 flights for initial qualification and 11 additional full lunar simulation trajectories for proficiency. We do not intend to train any Lunar Module pilots in the LLTV...&lt;/p&gt;&#xA;    &#xA;    &lt;p&gt;Our current assessment of LLTV support for Apollo 13 shows adequate capability to provide proper training for the Commander, with marginal capability to support the backup Commander...&lt;/p&gt;&#xA;  &lt;/blockquote&gt;&#xA;  &#xA;  &lt;p&gt;Unlike LM commanders, who trained in the LLRV/LLTV, LM pilots received training in the Lunar Landing Research Facility (LLRF) at Langley. Training in this enormous outdoor structure was considered sufficient since the likelihood was remote that an emergency might develop during a lunar mission that would require the LM pilot to take over from the commander (who actually flew the LM) during the final 500 feet of the lunar-landing trajectory. Due to the extensive maintenance requirements on the LLTV and the shortage of skilled crews for operations, training of backup commanders was a low priority.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, in short: even the LM pilot was not really properly qualified to land the thing!&lt;/p&gt;&#xA;" OwnerUserId="6815" LastActivityDate="2014-12-09T04:55:01.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6270" PostTypeId="2" ParentId="6248" CreationDate="2014-12-09T06:11:25.173" Score="7" Body="&lt;p&gt;Yes, there was at least one.  Well, we're pretty sure he didn't make it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WnCLY.jpg&quot; alt=&quot;frog flying from Antares launch&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-12-09T06:11:25.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6271" PostTypeId="2" ParentId="6248" CreationDate="2014-12-09T09:52:56.470" Score="5" Body="&lt;p&gt;Another confirmed launch casualty is &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts119/launchbat.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Bat&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6vwxD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bravely going where no Bat had gone before.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-03-24T15:37:33.393" LastActivityDate="2015-03-24T15:37:33.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6273" PostTypeId="1" AcceptedAnswerId="6274" CreationDate="2014-12-09T12:17:44.607" Score="8" ViewCount="603" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/I9dIZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If SpaceX's plan to land the CRS-5 first stage on their Autonomous Spaceport Drone Ship goes perfectly, there will be a 50 (ish) meter rocket on a barge in the Atlantic. Leaving aside the interesting &lt;a href=&quot;https://space.stackexchange.com/questions/6111/through-what-mechanisms-would-spacex-test-and-examine-the-returned-crs-5-first-s&quot;&gt;question of checks and refurbishment&lt;/a&gt;, what happens next?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where will SpaceX take the ASDS, and how will they offload such an enormous cargo?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-12-31T02:31:40.267" Title="Where will SpaceX take CRS-5 after it lands?" Tags="|spacex|reuse|falcon-9|asds|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6274" PostTypeId="2" ParentId="6273" CreationDate="2014-12-09T12:35:46.727" Score="6" Body="&lt;p&gt;It is currently unknown (1 week before the Dec 16th launch/landing date) where they will do the checkout and refurbishment work.  So some questions to answer your question. (There may be an ethnic joke lurking in here)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First set of questions: Will they tow the barge back to port (Which port?) upright? Or will they somehow lower it onto it's side? Using a crane on another ship? Where will they keep the cradle to hold it before landing?  How will they fold the legs back up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second question: Which port will they unload it at? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third question: Where will they then move it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third one is likely the easiest to answer. They will most likely just mount it on a truck and drive it to the McGregor, TX site. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I say McGregor, TX site, since they have the ability to re-fire individual engines, or the entire stage. They have a component of the development team there working on engines. They have more room there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The factory in Hawthorne, CA is somewhat limited by space, since they are producing core stages at a furious rate, and likely are going to be space limited there at some point. (It is a huge factory space, used to build 747 fuselages, but not clear how much storage space they have left).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they hit their 40 core a year pace, they will need room to handle the flow of that many cores. Every picture we have seen of the factory floor shows it looking very full. Thus McGregor may make the most sense in terms of space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shipping components for additional specialized inspection back to Hawthorne is likely easy and common. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they wish to actually refire it, they need it at McGregor anyway, so may be worth just driving it there first. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;New information has come up.  The fine folks in the NasaSpaceFlight forums realized that the ASDS is docked in front of a cruise ship. Cruise ships often have webcams.  (PS: These guys are nuts!  Funny, but nuts!  Some guy overflew McGregor and took photos of Grasshoper. Another kayaked the river where the barge was built to get better pictures. They have sea/air recon assets now.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These webcams are often public. So now we have a running webcam pointed at the ASDS barge, when the cruise ship is in port.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VZqca.png&quot; alt=&quot;Cruise ship cam shot of ASDS Barge&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see to the right, there is a SpaceX trailer, and next to it, what appear to be F-9 mount points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has led to the speculation that the barge will return the stage standing upright, legs extended (mostly tied down. They have a good shot of the deck showing tie down points but I cannot find it right now).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then lift it off the barge, mount it on the structures, fold the legs, then move it where needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Close up of the structures, alas web cam is low res. (Wonder if we could crowdsource a better web cam for the front of that ship?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/glU9v.png&quot; alt=&quot;Close up of mount points on shore&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/mm9f9.jpg&quot; alt=&quot;Better close up of mount points&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So where will they initially take it?  Current docking port for ASDS barge. Fold the legs, then truck it to an appropriate facility (Hawthorne, or McGregor) for detailed examination.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-12-31T02:31:40.267" LastActivityDate="2014-12-31T02:31:40.267" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6275" PostTypeId="1" AcceptedAnswerId="6427" CreationDate="2014-12-09T12:37:38.933" Score="8" ViewCount="511" Body="&lt;p&gt;How much maneuvering can you do with a solar sail in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can you change your inclination (go from a polar orbit to an equatorial) in a single star system?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can this be done in a binary system?&lt;/p&gt;&#xA;" OwnerUserId="6824" LastEditorUserId="168" LastEditDate="2014-12-16T04:42:41.960" LastActivityDate="2014-12-16T13:33:37.803" Title="Can a solar sail craft change an orbit's inclination (from polar to equatorial)?" Tags="|orbital-maneuver|solar-sail|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6276" PostTypeId="2" ParentId="6263" CreationDate="2014-12-09T13:49:25.753" Score="12" Body="&lt;p&gt;If you are on a random point of the moon's surface, facing the earth, in the mid-latitudes, and your ascent stage goes boom while you are walkabout, you are going to die. Period. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lets assume there is a fully-functional Space-1999 level lunar facility at another point on the moon. How will you navigate there? No GPS, and the landscape is rather featureless. Transportation? Walking won't get you very far, the rovers were good for maybe 50km. Your suit won't sustain you for more than one day, and that's only if you don't exert yourself doing things like walking. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any kind of space travel requires &lt;em&gt;substantial&lt;/em&gt; advance planning, deviations from the plan by more than a tiny amount never end well, and the whole process is absolutely unforgiving of error. There is simply no way you could remain alive, even with the most advanced technology available today, long enough for someone else to come and get you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA fully expected to lose one of the Apollo crews, and estimated the Shuttle would fail once in 200 missions (0.5%). They were right in both cases, and everyone onboard accepted it.&lt;/p&gt;&#xA;" OwnerUserId="5933" LastActivityDate="2014-12-09T13:49:25.753" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6277" PostTypeId="1" AcceptedAnswerId="6278" CreationDate="2014-12-09T15:43:59.793" Score="-10" ViewCount="930" Body="&lt;p&gt;I was reading in the news that NASA was testing sending humans beyond the earth's orbit (higher than 200 miles I think ), so the test was &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;this program&lt;/a&gt; and they were happy because it succeeded to send an unmanned vehicle to 3200 miles, so I started wondering, if they had already sent a human to the moon why they were re-inventing the wheel? How could they send a human to 250000 km if they are happy to make him reach 5000 km?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any thought?&lt;/p&gt;&#xA;" OwnerUserId="6828" LastEditorUserId="5963" LastEditDate="2014-12-09T18:53:00.893" LastActivityDate="2014-12-09T18:53:00.893" Title="Is Orion space craft evident that NASA didnt send human beyond the earth orbit?" Tags="|spacecraft|orion-spacecraft|" AnswerCount="2" CommentCount="0" ClosedDate="2014-12-09T20:29:38.953" ContentLicense="CC BY-SA 3.0" />
  <row Id="6278" PostTypeId="2" ParentId="6277" CreationDate="2014-12-09T15:55:27.523" Score="5" Body="&lt;p&gt;The Apollo program was extremely expensive. After the American government felt they had achieved the aims of the program - which was ultimately meant to assure their superiority in space to avoid a military confrontation there with Russia - they cancelled the program. No more Saturn rockets were made after that, and no more modules designed for human deep space missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It isn't that they haven't before, it is that they stopped for a long time.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-12-09T15:55:27.523" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6279" PostTypeId="1" AcceptedAnswerId="6312" CreationDate="2014-12-09T16:10:30.800" Score="9" ViewCount="941" Body="&lt;p&gt;&lt;strong&gt;The Situation:&lt;/strong&gt; I have lost all functionality of my spaceship. I cannot thrust forward backwards, whatever....It is basically a floating metal container. I have power, communication abilities, I have oxygen, I have food and water. I am in decent physical shape but morale is low. I am essentially lost in space between the earth and the moon (somewhere). I have no important connections, I not famous nor am I influential. I am simply in trouble, my situation is dire and the world is watching. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The Question:&lt;/strong&gt; Would their be any effort to rescue me? and if so, who would I expect cracking open my tin can to pull me out alive? (hopefully alive).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The More Important, Realistic, Sub-Question:&lt;/strong&gt; More importantly, how do space organizations and government branches related to space missions qualify a rescue effort if one exists? And what individual in these organizations would be the ultimate decision-making people? Risk? Expense? How would the conversation go?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Additional Details:&lt;/strong&gt; I am U.S. citizen, and I ended up in my precarious position as the result of an effort between public and private organizations. I was launched from the United States with FAA authority.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tnX8w.png&quot; alt=&quot;Borked space craft...&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="37" LastEditorUserId="37" LastEditDate="2014-12-10T15:29:16.313" LastActivityDate="2014-12-10T15:33:11.547" Title="What country or flight team would be the most likely to rescue me from space?" Tags="|spacecraft|rescue-missions|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6280" PostTypeId="1" CreationDate="2014-12-09T16:16:00.990" Score="6" ViewCount="206" Body="&lt;p&gt;In order to save development costs, Mars2020 will basically be the same rover as MSL Curiosity, but with another set of instruments. NASA's next big Mars ambition seems to be sample return. But after that, are there plans to use a third copy of the same rover?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the same basic rover work on the Moon? Or do maybe temperature variations and lunar dust require another basic design? And could it be run in real time on the Moon? Is it the time lag to Mars, or its basic design, that makes it use only one batch of commands per day?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-09T17:27:21.377" Title="Would the MSL rover copy work on the Moon?" Tags="|the-moon|rovers|curiosity|teleoperations|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6281" PostTypeId="1" AcceptedAnswerId="6290" CreationDate="2014-12-09T16:21:48.863" Score="60" ViewCount="60568" Body="&lt;p&gt;Why is the US developing a new rocket and spacecraft as opposed to building the Saturn V and Apollo stacks again?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SLS / Orion development programs certainly can't cost less than the unit cost of building more Saturn Vs and Apollo spacecraft which are already designed, tested, and proven.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="1142" LastEditDate="2014-12-09T16:42:51.230" LastActivityDate="2015-08-17T15:48:11.503" Title="Why not build Saturn V's again?" Tags="|apollo-program|cost|saturn-v|sls|orion-spacecraft|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6282" PostTypeId="1" CreationDate="2014-12-09T16:24:28.570" Score="7" ViewCount="4639" Body="&lt;p&gt;Imagine that an astronaut during an EVA is cut loose from the space station and falls away from it in a tumbling way. Without any foreign object or air to interact with, could he stop tumbling and spinning and turn around to steadily face towards the space station again? The speed of the spinning can be changed by spreading our or curling up. But could the angular momentum be changed or redirected?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2020-06-30T22:31:00.503" Title="Can a free falling astronaut change his spin and orientation?" Tags="|microgravity|eva|attitude|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6283" PostTypeId="2" ParentId="6279" CreationDate="2014-12-09T16:26:11.987" Score="10" Body="&lt;p&gt;The answer depends on under what authority you launched from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you launched from the United States with FAA approval, then I would assume that the Americans would be the first ones to call for help. However, I assure you, there is nothing that they could do to help you. Even if the space shuttle or another manned craft were still flying out of the US, you would need to give them months of warning to mount a rescue mission. Note that the cost of designing and preparing the mission will be almost 1 billion US dollars, and it will exceed 1 billion USD if it does have to launch to actually perform the rescue. I do not believe that the US congress would fund that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you launched from the United States without FAA approval, then I would assume that you also didn't give NASA the months warning needed to prepare a rescue mission, so you are out of luck even more so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside the US, only the Russians and the Chinese currently have man-rated launchers. Perhaps with a few months notice they would be up to providing a rescue mission just for the PR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, you will need an airlock and additionally space suit that is compatible with the US / Russian / Chinese airlocks. With no airlock or the wrong space suit, you are out of luck.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, nobody on the planet &lt;strong&gt;could&lt;/strong&gt; help you. Furthermore, almost nobody on the planet &lt;strong&gt;would&lt;/strong&gt; help you even if they could.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-12-09T16:26:11.987" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6284" PostTypeId="2" ParentId="6282" CreationDate="2014-12-09T16:29:25.570" Score="7" Body="&lt;p&gt;There are only two ways he could do this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first is if he has something he can spin within his suit -- kind of like a personal CMG.  This would effectively &quot;absorb&quot; the angular momentum.  It would probably either have to spin fast or be a significant fraction of the astronaut's mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other is if he can throw something away as a reaction mass to counteract the spin.  The astronaut would have to throw the mass fairly precisely to zero out his spin.  This would have the (possibly) negative consequence of also imparting a change in linear momentum.  Not too difficult if it is a cold-gas thruster.  Harder if it is a tennis ball...&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2014-12-10T02:15:45.440" LastActivityDate="2014-12-10T02:15:45.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6285" PostTypeId="2" ParentId="6280" CreationDate="2014-12-09T16:36:48.213" Score="4" Body="&lt;p&gt;From what I gather, the main problems with putting MSL on the moon would be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lunar regolith is much coarser than Mars' dust.&lt;/li&gt;&#xA;&lt;li&gt;There is no atmosphere so the EDL mechanism would have to be completely redesigned.&lt;/li&gt;&#xA;&lt;li&gt;The rover is designed to perform science in the daylight, and moon nights are two weeks long.&lt;/li&gt;&#xA;&lt;li&gt;No more available plutonium for an RTG to power the rover.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some minor problems would be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The heat-sensitive components would have to be adapted.&lt;/li&gt;&#xA;&lt;li&gt;MSL is designed to communicate with a (relatively) low-altitude orbiter, and lunar orbits are unstable.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In conclusion, I believe that the project would work. &lt;strong&gt;However&lt;/strong&gt;, the &lt;strong&gt;goals&lt;/strong&gt; of the MSL are not compatible with the moon. We do not need to search for life or water on the moon. Therefore the science payload itself would probably have to be almost entirely scrapped and a moon-specific payload would have to be adapted. But the MSL as a &lt;strong&gt;platform&lt;/strong&gt; for delivering a science payload seems feasible.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-12-09T16:36:48.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6286" PostTypeId="2" ParentId="6277" CreationDate="2014-12-09T16:59:15.133" Score="2" Body="&lt;p&gt;They were happy because Orion is a new design. It's the natural reaction of a design team seeing its creation in action for the first time. You will see the same reaction at the first flight of a new aircraft, even if that first flight does not do anything really new. So their happiness is no evidence of anything.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion isn't reinventing the wheel, it is designed for far more ambitious missions than landing on the Moon: Orion is designed to go to Mars and/or to asteroids. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-12-09T16:59:15.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6287" PostTypeId="2" ParentId="6281" CreationDate="2014-12-09T17:06:24.753" Score="42" Body="&lt;p&gt;There are several reasons: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;We can do better these days. Saturn and Apollo were designed in the early 1960s, so the design tools used were mainly pen and paper, with some primitive computer tools thrown in here and there. These days CAD can be used to create a design that performs far better (because you can design parts closer to the strength they need, for example). Similarly, you'd want to replace all electronics from Saturn/Apollo anyway with modern equipment.  &lt;/li&gt;&#xA;&lt;li&gt;Politics. Unfortunately, the US Congress plays a large role in the design of SLS: choosing components based on where key manufacturers are located. ATK is an infamous example: they're located in the worst place possible for building large rocket components. The SRBs are transported on rail cars from Utah to Florida, and unavoidable tunnels through a mountain range in the route dictate the maximum length and diameter of the cargo. As a result, the SRBs have to be built in segments, adding a weak point to the design. Nevertheless, ATK solid boosters appear on every new design because of political factors.  &lt;/li&gt;&#xA;&lt;li&gt;Performance. This applies more to the spacecraft than the launcher: Apollo was designed for short trips to the Moon, and would have to be redesigned to support longer trips. Orion also has about 3 times the interior volume of Apollo.&lt;br&gt;&#xA;&lt;strong&gt;Edit&lt;/strong&gt;: there were some proposals to use Apollo for longer missions, see the comments. But those comments show that lots of systems would have to be changed to support long missions. These missions also needed a redesign of the S-IVB third stage, to allow its fuel tanks to be converted to habitable space in-flight.   &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-12-10T07:55:21.203" LastActivityDate="2014-12-10T07:55:21.203" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="6288" PostTypeId="2" ParentId="6280" CreationDate="2014-12-09T17:27:21.377" Score="8" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;But after that, are there plans to use a third copy of the same rover?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;After that is way, way in the future.  So any &quot;plans&quot; are more like notions.  They wouldn't mean anything.  It is not even clear what kind of rover would be used as part of a return of Martian samples after 2020.  If that happens at all.  The approval of such an effort is well into the future.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Could the same basic rover work on the Moon?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The fundamental issue is thermal control.  The rover is designed to work in the Martian atmosphere, with a Martian diurnal cycle of close to an Earth day.  No atmosphere and a 29-day diurnal cycle is very different.  The thermal control approach would have to be completely reconsidered and redesigned from the ground up.  Note that thermal control is not just about the electronics inside, but also all the actuators and other devices hanging out in the breeze, so to speak.  Wheel actuators, arm actuators, instruments, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The communications system would also need to be redesigned for more, higher-rate direct-to-Earth links.  Or you could send a relay orbiter along with the thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It wouldn't be the same instruments.  An entirely different set of instruments would be selected for a lunar investigation.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And could it be run in real time on the Moon? Is it the time lag to&#xA;  Mars, or its basic design, that makes it use only one batch of&#xA;  commands per day?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is some combination of the time lag, the opportunities for communication through a relay satellite, and the pace of activities on the rover along with energy limitations that lead to a daily (solly) command cycle for Mars rovers.  In principle joy-sticking a rover on the Moon should be possible, but it will still move slowly, and instrument activities will still take many hours or days.  I'd expect some benefit to traverse speeds from more frequent interactions with a rover on the Moon.  However you would be energy limited in how far you can go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, no, an exact copy would neither work, nor be very scientifically appropriate.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-12-09T17:27:21.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6289" PostTypeId="2" ParentId="6282" CreationDate="2014-12-09T17:27:55.250" Score="6" Body="&lt;p&gt;An addition to Erik's terrific answer, which is 100% correct, there is an additional theoretical method that the astronaut could use to change his spin and rotation.&lt;/p&gt;&#xA;&lt;p&gt;I really don't think that a human would come up with this by himself, but if an astronaut were already familiar with &lt;a href=&quot;https://en.wikipedia.org/wiki/Cat_righting_reflex&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Cat righting reflex&lt;/a&gt; then he could theoretically right himself by transferring angular momentum to different appendages and then changing the leverage of the appendage and returning the angular momentum. &lt;strong&gt;Basically, this means to extend the legs, twist the body, bring the legs back in, then extend the arms, twist the body, then bring the arms back in.&lt;/strong&gt; Repeat as necessary.&lt;/p&gt;&#xA;&lt;p&gt;Here is a video of &lt;a href=&quot;https://www.youtube.com/watch?v=y2OCwQJSHZM&quot; rel=&quot;nofollow noreferrer&quot;&gt;a cat performing the maneuver&lt;/a&gt;, and the &lt;a href=&quot;https://www.youtube.com/watch?v=RtWbpyjJqrU&quot; rel=&quot;nofollow noreferrer&quot;&gt;Smarter Every Day corollary video&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="12102" LastEditDate="2020-06-30T22:31:00.503" LastActivityDate="2020-06-30T22:31:00.503" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="6290" PostTypeId="2" ParentId="6281" CreationDate="2014-12-09T17:29:56.750" Score="81" Body="&lt;p&gt;Hobbes' answer focuses on why we might want to build SLS. There are also significant barriers to rebuilding Saturn/Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the (vast) amount of existing technical documentation on those designs, there's a (probably vaster) pool of knowledge that the individuals who actually built the things collected during the process. Nearly all of those people are dead or retired now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the individual manufacturers, subcontractors, and sub-subcontractors involved have merged or collapsed, and their internal process documents may have been lost; their documentation may have been very carefully stored in a warehouse somewhere, but the guy who knows where has retired as well! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Building Apollo and Saturn required particular manufacturing processes which are now obsolete. The tools needed to make the tools to make the rockets no longer exist. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To rebuild Saturn/Apollo, we'd first have to rebuild a substantial slice of the US aerospace industry as it existed circa 1965, and all that just to be able to carry out the same kind of missions we lost interest in 40 years ago. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-10T17:11:16.137" LastActivityDate="2014-12-10T17:11:16.137" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="6291" PostTypeId="2" ParentId="6282" CreationDate="2014-12-09T19:05:38.863" Score="10" Body="&lt;p&gt;Angular momentum is conserved.  Something has to be emitted or absorbed to change the angular momentum.  Spin rate can be changed by changing the moments of inertia of the object, as a skater does.  But the angular momentum is fixed unless they have a rocket or some other mass they can get rid of, a solar sail (and a lot of time), a way to react against Earth's magnetic field, or if someone throws something at them that they catch.  The gravity gradient of Earth can also apply torque against the angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They could slow their spin rate quite a bit with two masses on long strings that they play out.  If they then let go of the strings, that would emit the angular momentum transferred to the masses.  I would keep some of those in my pockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orientation is a different matter.  If the astronaut has roughly a zero spin rate, they can change which way they are facing without touching anything.  There are videos of this being done on the space station.  (I think you can find an example in another answer here somewhere.)  Cats do this as well in free fall.  You can do this even if you are spinning, but the result is harder to see.  The axis of their spin cannot change, since that is set by their angular momentum, but the orientation of the body relative to the spin axis can be changed.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-12-09T20:56:52.340" LastActivityDate="2014-12-09T20:56:52.340" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6292" PostTypeId="2" ParentId="6282" CreationDate="2014-12-09T19:20:35.547" Score="2" Body="&lt;p&gt;While Erik and Mark's answers are technically correct, an astronaut &lt;em&gt;might&lt;/em&gt; (at least, theoretically) try something risky in an emergency:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the astronaut had something sharp with which to make a hole in his/her suit, or else the suit has an attachment that could be ripped off, he might be able to use the pressurized air within the suit as a sort of make-shift thruster.  Pointed in the right direction (spinward), one could negate some small amount of spin (at the cost of some of the air in the suit, of course).  This is something you'd only try if the alternative was certain death, of course... &lt;/p&gt;&#xA;" OwnerUserId="6199" LastActivityDate="2014-12-09T19:20:35.547" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6293" PostTypeId="2" ParentId="6263" CreationDate="2014-12-09T20:14:40.450" Score="2" Body="&lt;p&gt;I think the biggest issue would be fuel. Trips to the Moon were very carefully calculated: they brought just enough fuel to make the trip. The amount of residual fuel left behind in any of the landers remaining on the surface is minimal. It was minimal the day they landed. In the 40+ years since then there has surely been some leakage, and probably chemical deterioration. Even if you somehow could visit all the vehicles on the surface and collect all their fuel ... well, I'm too lazy to do the research for a casual post like this, but I doubt it would be enough to get an adult human off the surface and escape from lunar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's the fact that none of the vehicles left on the Moon was built to take off. These are LANDERS, intended for one-way trips. Could they be modified into launch vehicles with sufficient time and effort? Maybe. I guess that's less impossible than finding fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could speculate about what a sufficiently ingenious person could do. Like Kirkaiya says, maybe a McGyver could figure out something brilliant. Could you somehow build a machine that would produce fuel from lunar resources using parts from various landers? I can't say that it's impossible, but it would be very, very difficult. Heck, would you even know how to start? If the hypothetical stranded person was an engineer who designed fuel refining equipment back on Earth, maybe he could. But most people, even most highly intelligent people, would likely have no idea how to go about building a fuel refining machine. I think I'm pretty clever and I have no idea. And people who do know would probably have no idea how to identify the type of rocks that include the appropriate raw materials, and vice versa. Modern technology is built on the work of many, many specialists, each contributing their own little piece to the complete system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if you are going to combine parts from multiple landers, how will you travel between them? Are we assuming you have a vehicle that can travel hundreds of miles across the lunar surface before you run out of air, food, and other supplies? If we're assuming you're on foot in a spacesuit with a few hours or even a few days of air, forget it, I think you're doomed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finding a working or repairable radio so you can call for help seems a lot more promising. Even without a radio, a clever person might come up with some way to send a message -- writing SOS in big letters with some available material, maybe using a small amount of fuel to send up a signal flare, using a mirror to flash light, etc. Most of these would require knowing when someone will be looking in your direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But even if you could signal for help, then what? Unless the folks back on Earth just happen to have a moon-rocket sitting around -- didn't I leave one in the garage last fall? -- how are they going to rescue you? By the time they built a rocket, launched it, and reached you, you are probably long dead.&lt;/p&gt;&#xA;" OwnerUserId="6839" LastActivityDate="2014-12-09T20:14:40.450" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="6295" PostTypeId="2" ParentId="209" CreationDate="2014-12-09T20:33:06.913" Score="4" Body="&lt;p&gt;As others have noted, there are a couple of international treaties that say that no nation owns or can own the Moon. Whether anyone will care about those treaties by the time technology reaches the point where it would actually be possible for anyone to live on the Moon or make use of it in any realistic way is a very different question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It wasn't until I read PearsonArtPhoto's post here that I learned that none of the countries who have actually launched spacecraft have ratified the treaty. (Hey, I learned something today!) So that makes it pretty irrelevant. But even if tomorrow the U.S., Russia, China, India, Japan, whomever, all ratified it, those nations may not even exist by the time someone wants to build a lunar colony, and if they do, they might simply renounce the treaty as out-dated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like, does anyone today pay any attention whatsoever to the Treaty of Tordesillas? Back in 1494, Spain and Portugal signed a treaty dividing the New World between them. Does anyone imagine that that treating is binding on the United States and Canada today?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Realistically, when technology reaches the point that a lunar colony is possible, I think someone will just plant the colony and declare that they own that little piece of the Moon. Then others will follow and divide it up. If we're lucky, the Moon will end up being carved up among various nations on a first-come/first-served basis. More likely, if it proves valuable enough economically or politically, wars will be fought to establish the borders. Barring some sudden and dramatic change in human nature unprecedented in history.&lt;/p&gt;&#xA;" OwnerUserId="6839" LastActivityDate="2014-12-09T20:33:06.913" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6296" PostTypeId="2" ParentId="2631" CreationDate="2014-12-09T21:34:37.317" Score="2" Body="&lt;p&gt;An observation: The large, dark maria are large lava flows. Lava tends to be pretty dense, as compared to layers of particulate matter deposited from being blasted out of impact craters, since there were few gaps in it when it was laid down, other than possibly gas bubbles, while layers of randomly piled rocks thrown from impacts will have many gaps and holes. So if large surface lava flows occurred, they would be &quot;heavy&quot; as compared to the surface of the rest of the moon. As such, the dynamics of tidal locking would tend to eventually result in these heavier areas facing the Earth, rather than away from it, since there is more mass there for the Earth's gravity to pull on as compared to other parts of the moon.&lt;/p&gt;&#xA;" OwnerUserId="6844" LastActivityDate="2014-12-09T21:34:37.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6297" PostTypeId="2" ParentId="6279" CreationDate="2014-12-09T22:00:36.503" Score="3" Body="&lt;p&gt;I don't know how well this would work, but there are Soyuz capsules attached to the ISS, one of which could come and get you, if the propulsion system was up to it. Russia and the US could split the cost (~200mUSD) - as dotancohan said, Russia could use the PR.&#xA;It is still a long shot though.&lt;/p&gt;&#xA;" OwnerUserId="6850" LastActivityDate="2014-12-09T22:00:36.503" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="6298" PostTypeId="1" CreationDate="2014-12-09T22:55:16.080" Score="6" ViewCount="250" Body="&lt;p&gt;We have the example of many Dragon capsules recovered by SpaceX, renting a ship (American Islander) for the duration of the mission each time. It is registered to &lt;a href=&quot;http://eandt.theiet.org/news/2012/oct/spacex-returns.cfm&quot; rel=&quot;nofollow&quot;&gt;American Marine Corp&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Contrast that with the EFT-1 Orion test flight, the &lt;a href=&quot;http://en.wikipedia.org/wiki/USS_Anchorage_%28LPD-23%29&quot; rel=&quot;nofollow&quot;&gt;USS Anchorage&lt;/a&gt;, LDP-23, a San Antonio class amphibious transport ship, with a crew of 396, plus NASA staff, which of course, did not have the 'paid for' directly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the real world cost of two days of operations (one day delay in launch of EFT-1 still required the vessel on call) vs the real world cost that SpaceX is incurring in recovery?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a price can be derived for the LDP-23 ships time, perhaps that could be compared to the cost of the ASDS to be used for landing first stages, plus tug time to move it around.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-12-09T23:28:43.583" LastActivityDate="2014-12-13T17:37:42.143" Title="How do the real costs of recovering a capsule compare, NASA vs commericial?" Tags="|nasa|spacex|cost|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6299" PostTypeId="2" ParentId="6281" CreationDate="2014-12-09T23:42:34.820" Score="11" Body="&lt;p&gt;A different approach might be to rebuild the Saturn V, with modern techniques, not as a one for one rebuild, but rather take the good parts, and make them better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the F-1 engine of the first stage, (5 used, 1.5 million lbs thrust) is being reconsidered as a more modern version with higher thrust. The nozzle was meticulously assembled in painstaking fashion. It consists of cooling tubes that fuel flows through to keep it from melting.  Nowadays a 3-D Printed version while expensive in capital costs (3-D printer that big for something like Inconel boggles the mind at cost) but still might be cheaper than the labour it takes to build the old way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The J-2 engine of the second stage (5 used, 232 Klbs thrust) and third stage (1 used) has been redeveloped as the J-2X for the Constellation program and then dropped. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The overall design of the various stages is still of interest. Even if the very specific minutaue of the control systems is not. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no need to reproduce the identical computer systems as they used back then. Modern computers are cheaper, and easier to program as SpaceX has shown with their development from scratch of the Falcon 9 computer control system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trick would be not to redevelop anything that does not need it, nor to hew exactly to the original design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now in theory that might work. In practice, if NASA is doing it, it is hard to imagine it happening in a cost effective fashion.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-09T23:42:34.820" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6301" PostTypeId="1" AcceptedAnswerId="30120" CreationDate="2014-12-10T02:17:52.090" Score="15" ViewCount="2776" Body="&lt;p&gt;Is there an online tool available to calculate &lt;a href=&quot;http://en.wikipedia.org/wiki/Porkchop_plot&quot; rel=&quot;noreferrer&quot;&gt;porkchop plots&lt;/a&gt; at arbitrary resolutions?  Real world only please -- I am aware of the &lt;a href=&quot;http://alexmoon.github.io/ksp/&quot; rel=&quot;noreferrer&quot;&gt;tools available&lt;/a&gt; to do this for, say, KSP.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/B734M.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/B734M.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="8693" LastEditDate="2015-12-17T23:34:46.117" LastActivityDate="2021-06-05T15:56:48.030" Title="Online Porkchop Plotter" Tags="|orbital-mechanics|delta-v|software|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6302" PostTypeId="2" ParentId="6281" CreationDate="2014-12-10T03:46:09.790" Score="28" Body="&lt;p&gt;&lt;strong&gt;NOTE&lt;/strong&gt; &lt;em&gt;All dollar values are in present day values accounting for inflation.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason we're not reusing the Saturn V is the same reason it was cancelled in the first place: cost.  The SLS is supposed to be half the cost per launch.  Whether that works out remains to be seen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V was expensive.  &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_18-16_Apollo_Program_Budget_Appropriations.htm&quot;&gt;The Saturn V program cost \$47 billion over 10 years for 13 launches&lt;/a&gt; which works out to something like \$3.6 billion per launch.  But the \$3.6 billion figure includes development costs.  Without dev costs, &lt;a href=&quot;http://history.nasa.gov/SP-4221/ch6.htm&quot;&gt;it's a mere \$1.2 billion per launch&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The assumption of many people is we can just dust off the plans and crank out Saturn V's without sinking any money into it.  Other answers have covered that's impossible, substantial cost would have to go into creating new facilities, re-creating manufacturing techniques, and re-engineering, not to mention the greatly increased cost of modern labor and re-engineering to modern safety margins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System#Program_costs&quot;&gt;SLS program &lt;em&gt;claims&lt;/em&gt; they will be able to get the cost down to $500 million per launch&lt;/a&gt;.  Many find this number to be fantasy.  Maybe SpaceX could, but a large government contract is more likely to balloon.  Nevertheless, this is the number they sold the government on and it is less than half what the Saturn V cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here I am going into speculation, but taking a page from aviation another consideration is how much room for future improvement there is left in the design.  The F-1 engine was late 50s technology.  The Saturn V was straining the limits of technology in the 60s.  After 13 launches NASA had enough experience with the rocket to make launches as efficient and safe as they were going to get.  The SLS, as a new design, will have a lot of room for improvement and flexibility.  It's not just about getting us to Mars, it's about providing the US with versatile heavy lift capacity for decades to come.&lt;/p&gt;&#xA;" OwnerUserId="4704" LastActivityDate="2014-12-10T03:46:09.790" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6303" PostTypeId="1" CreationDate="2014-12-10T04:17:43.517" Score="5" ViewCount="114" Body="&lt;p&gt;Based off what I can find online,  and in history books; only the first Atlas rocket could send crew into space, but there is potential for man rates Atlas v rockets (which man rating about that has been asked on here already). What about the other rockets in that family? Would it have been possible to send crew into orbit on one of those rockets given a crewed craft as its payload?&lt;/p&gt;&#xA;" OwnerUserId="5772" LastActivityDate="2014-12-10T04:17:43.517" Title="Were all the rockets in the Atlas family man-rated?" Tags="|crewed-spaceflight|human-rating|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6305" PostTypeId="2" ParentId="6282" CreationDate="2014-12-10T05:17:52.567" Score="0" Body="&lt;p&gt;NASA demonstrated that this can work, 'way back around 1970. If it happens, it must be possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For those who don't believe the physics: T.R. Kane and M.P. Scher of Stanford, California, in the International Journal of Solids and Structures: &quot;A Dynamical Explanation of the Falling Cat Phenomenon.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Websearch will turn up photos of astronauts (on trampolines, mostly, rather than free-fall) executing this move.&lt;/p&gt;&#xA;" OwnerUserId="3231" LastActivityDate="2014-12-10T05:17:52.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6308" PostTypeId="1" AcceptedAnswerId="6310" CreationDate="2014-12-10T09:33:05.933" Score="3" ViewCount="889" Body="&lt;p&gt;Would a space based magnifying lens (such as Hubble) be more efficient than a space based parabolic mirror to propel solar sails?&lt;/p&gt;&#xA;" OwnerUserId="6861" LastEditorUserId="6312" LastEditDate="2014-12-10T09:59:56.383" LastActivityDate="2014-12-10T11:20:12.480" Title="Could the Hubble mirror be used to create a laser to propel solar sails?" Tags="|energy|solar-sail|hubble|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6309" PostTypeId="1" CreationDate="2014-12-10T09:34:05.650" Score="18" ViewCount="7617" Body="&lt;p&gt;Google couldn't find the answer for me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Technically this is two very closely related questions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since no person has yet been much further from the Earth than the far side of the Moon I was wondering who has been the closest to and furthest from the Sun? It's possible (likely?) that it could be two different people or multiple people travelling together in the same capsule. Reasons why it could be different people: The Earth has an eccentric orbit so they would have to be closer to the Sun than the closest the Earth has been. Then they would have to wait six months for the Earth to be furthest from the Sun and be a bit further than that. This could be a single person in space for a long time, such as on the ISS but I don't believe its orbit is far enough from the Earth to be able to hold the record. The Apollo missions are a much more likely candidate, depending on the time of year. There haven't been any manned missions to another planet yet, which would make the question a lot easier to answer!&lt;/p&gt;&#xA;" OwnerUserId="6863" LastActivityDate="2021-02-19T16:06:07.263" Title="Who has been the closest to/furthest from the Sun?" Tags="|record|travel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6310" PostTypeId="2" ParentId="6308" CreationDate="2014-12-10T11:06:20.880" Score="12" Body="&lt;p&gt;The answer is a clear no because &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt; (HST) uses a fixed focal length of 57.6 m. The only thing you'd achieve by re-purposing it as a solar concentrator would be to melt its focal plane assembly and likely everything around it. HST is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassegrain_reflector&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassegrain reflector&lt;/a&gt; and its mirror assembly is utterly unsuitable to applications where adjustable focal point is required because they're fixed optics. HST actually even required an in-orbit repair because it was &lt;em&gt;out of focus&lt;/em&gt; due to one lens being out of position for 1.3 mm!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what is worth, never point any reflector telescopes towards the Sun without specially made front optical filters (like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/H-alpha&quot; rel=&quot;nofollow noreferrer&quot;&gt;H-Alpha&lt;/a&gt; filters). Use specially built refractor telescopes that don't use plastic parts that could easily melt, and only observe the Sun indirectly via a screen, say a so-called &lt;em&gt;&lt;a href=&quot;https://astronomy.stackexchange.com/questions/252/how-do-i-build-a-stargazing-aficionados-sun-funnel-to-observe-our-nearest-star&quot;&gt;Sun funnel&lt;/a&gt;&lt;/em&gt;. They &lt;em&gt;are&lt;/em&gt; optical concentrators, but their focal length is suitable for on-focal-plane observations of faint distant objects, not to redirect strong incident light to a distant location in a narrow beam. You can't make a laser-like assembly out of any telescopes. If you ever &lt;a href=&quot;https://en.wikipedia.org/wiki/Collimated_light&quot; rel=&quot;nofollow noreferrer&quot;&gt;collimated&lt;/a&gt; a Newtonian / Dobsonian primary and secondary mirrors using a &lt;a href=&quot;https://astronomy.stackexchange.com/a/281/13&quot;&gt;laser collimator&lt;/a&gt;, it would become pretty apparent why not. And Cassegrains use both hyperbolic and parabolic mirrors and refractors concave and convex lens assemblies. Their focal length is fairly limited because of this.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2014-12-10T11:20:12.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6311" PostTypeId="1" AcceptedAnswerId="6313" CreationDate="2014-12-10T14:11:10.680" Score="1" ViewCount="235" Body="&lt;p&gt;How would a crew of a spacecraft get home if 'everyone' on the spacecraft becomes incapacitated, immobilized or go unconscious? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew is alive and stable but unable to operate equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there automation in the system that can be invoked in a contingency? Can operators from the ground remote control the space craft back to earth? What about the reentry stage?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They could have suffered from some form of paralysis, or experienced an event that knocks them out or something else happened via poison, bacteria or virus.&lt;/p&gt;&#xA;" OwnerUserId="37" LastEditorUserId="37" LastEditDate="2014-12-11T03:06:33.293" LastActivityDate="2014-12-11T03:06:33.293" Title="If a flight crew becomes incapacitated some way, in space, can they still get home?" Tags="|spacecraft|rescue-missions|" AnswerCount="1" CommentCount="15" ClosedDate="2014-12-11T16:19:33.340" ContentLicense="CC BY-SA 3.0" />
  <row Id="6312" PostTypeId="2" ParentId="6279" CreationDate="2014-12-10T15:33:11.547" Score="13" Body="&lt;p&gt;Building off of @dotanchoen's &lt;a href=&quot;https://space.stackexchange.com/a/6283/6868&quot;&gt;answer&lt;/a&gt;, specifically this part:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In short, nobody on the planet &lt;strong&gt;could&lt;/strong&gt; help you. Furthermore, almost nobody on the planet &lt;strong&gt;would&lt;/strong&gt; help you even if they could.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;After the &lt;em&gt;Columbia&lt;/em&gt; accident, the Columbia Accident Investigation Board Report asked NASA if there were options for the safe return of the crew.&lt;/p&gt;&#xA;&lt;p&gt;The rest of this answer is taking snippets of the &lt;a href=&quot;http://arstechnica.com/science/2014/02/the-audacious-rescue-plan-that-might-have-saved-space-shuttle-columbia/1/&quot; rel=&quot;noreferrer&quot;&gt;Ars Technica article&lt;/a&gt; on the results of this report.&lt;/p&gt;&#xA;&lt;p&gt;In short, there was very little that that could have been done to save the crew. It was a race against time - the CO2 scrubbers were the piece of equipment that would eventually kill the crew if they couldn't return to the planet. Columbia has 69 of these scrubbers. NASA estimated that was enough to keep the air breathable for 30 days.&lt;/p&gt;&#xA;&lt;p&gt;The giant asterisk next to that &amp;quot;30 days&amp;quot; though depends on when the foam issue was discovered and what the crew did in the mean time. The earlier it is detected, the better. But, it needed to be detected by Flight Day 3 at the latest. But, in any case, the crew needed to immediately suspend any activity. Basically, they had to enter a long period of rest and waiting to slow the buildup of carbon dioxide, while NASA planned it's rescue mission.&lt;/p&gt;&#xA;&lt;p&gt;On the planet, NASA had a giant series of steps it needed to take as well. One positive NASA had in this situation is that &lt;em&gt;Atlantis&lt;/em&gt; was being prepared for it's own mission. It was already in the Orbital Processing Facility. It would need two weeks to get from this point to the Vehicle Assembly Building, where the solid rocket boosters would be attached.&lt;/p&gt;&#xA;&lt;p&gt;NASA estimated that they would need a minimum of &lt;a href=&quot;http://arstechnica.com/science/2014/02/the-audacious-rescue-plan-that-might-have-saved-space-shuttle-columbia/2/&quot; rel=&quot;noreferrer&quot;&gt;21 days&lt;/a&gt; to prepare Atlantis for launch. That is assuming that every accelerated test was perfect the first time. There could be no mistakes in preparation.&lt;/p&gt;&#xA;&lt;p&gt;This is also several times faster than a normal mission. Remember, the crew of &lt;em&gt;Atlantis&lt;/em&gt; had trained for years on another mission. They have 21 days to retrain for a rescue. This shows the standard shuttle prep times and what the shortened schedule would be compromising.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dRbkX.png&quot; alt=&quot;Standard Shuttle Prep time&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Another thing to be aware of is the crew of the rescue vehicle. &lt;em&gt;Atlantis&lt;/em&gt; has been training with a crew already, but a rescue mission is different than a science mission. NASA determined that &lt;em&gt;Atlantis&lt;/em&gt; would have to fly with a minimum crew of only 4, so that they could get Columbia's crew of 7 home. They also determined that only experienced crew (experienced in EVA, command and pilot) should be on the rescue mission. These 4 would only have a few weeks to train.&lt;/p&gt;&#xA;&lt;p&gt;Assuming that everything on the ground was successful - crew trained, and &lt;em&gt;Atlantis&lt;/em&gt; was prepared for launch - once in space the real challenge begins. &lt;em&gt;Atlantis&lt;/em&gt; would have to catch up to &lt;em&gt;Columbia&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Atlantis would arc upward into orbit, approaching Columbia from below in what's called an &amp;quot;R-bar approach&amp;quot;—that is, an approach along an imaginary radial line connecting Columbia with the Earth's center. (Contrast this to a V-bar approach, which would be an approach along Columbia's velocity vector—that is, from the front or back, rather than the top or bottom.) Columbia would already be oriented tail-first and &amp;quot;upside down&amp;quot; relative to the earth; Atlantis would approach &amp;quot;right side up&amp;quot; beneath it. Atlantis would swing slowly up into place, each shuttle growing larger and larger in the rendezvous windows in their respective flight decks' ceilings.&lt;/p&gt;&#xA;&lt;p&gt;Finally, Atlantis would ease to a halt 20 feet (six meters) from Columbia. Atlantis would be yawed 90 degrees to Columbia, pointing at three o'clock to the older orbiter's 12 o'clock, in order to keep their vertical stabilizers from striking.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Rq4ZU.png&quot; alt=&quot;Atlantis Approaching Columbia&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;They would be oriented like this in orbit&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sKDwb.png&quot; alt=&quot;Orbiters in rescue&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;This position would require that &lt;em&gt;Atlantis&lt;/em&gt; be under manual control constantly. Twenty feet doesn't sound like much, but it is enough for the two orbiters to drift apart if left unchecked.&lt;/p&gt;&#xA;&lt;p&gt;Finally, the rescue could begin. &lt;em&gt;Atlantis&lt;/em&gt; would take extra CO2 scrubbers over to &lt;em&gt;Columbia&lt;/em&gt;, in addition to two additional space suits. The report goes into explicit detail on how this rescue would occur, but the take away is that it will take at least 8 hours to transfer the crew of &lt;em&gt;Columbia&lt;/em&gt; to &lt;em&gt;Atlantis&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Prior to the last two members of &lt;em&gt;Columbia&lt;/em&gt; leaving the shuttle, would give ground control command of the guidance and maneuvering system. The shuttle would be deorbited (not landed) via Mission Control. Most likely, this would have occurred over the South Pacific in the coming hours or days.&lt;/p&gt;&#xA;&lt;p&gt;The eleven crew members in &lt;em&gt;Atlantis&lt;/em&gt; would prepare for reentry. This would have occurred with crew, literally, strapped to the floor.&lt;/p&gt;&#xA;&lt;p&gt;Cue Hollywood ending.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;In your scenario, all of this would have to go as planned as well. Unfortunately, in your scenario, you are already behind the eight ball. There isn't another vehicle being prepared for the next mission. Your CO2 scrubbers are doing their job now, but will eventually reach the point where they can't perform adequately.&lt;/p&gt;&#xA;&lt;p&gt;The time it takes for a mission is not as simple as rolling up another orbiter and firing it into orbit. There simply isn't enough time to save you at this point.&lt;/p&gt;&#xA;" OwnerUserId="6868" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-12-10T15:33:11.547" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6313" PostTypeId="2" ParentId="6311" CreationDate="2014-12-10T15:47:25.980" Score="10" Body="&lt;p&gt;Prior to 2006, STS could not be landed via Mission Control. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_orbiter#Landing_gear&quot;&gt;landing gear&lt;/a&gt; could only be operated manually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the &lt;em&gt;Columbia&lt;/em&gt; accident, the rest of the fleet was refitted with the &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-3xx#Remote_Control_Orbiter&quot;&gt;RCO IFM cable&lt;/a&gt;, a 28-foot braided cable that the flight crew could use to link the cockpit with the shuttle's avionics bay and give Mission Control access to the appropriate switches. &lt;/p&gt;&#xA;" OwnerUserId="6868" LastActivityDate="2014-12-10T15:47:25.980" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6314" PostTypeId="2" ParentId="3523" CreationDate="2014-12-10T15:50:02.490" Score="4" Body="&lt;p&gt;The moon is tidal-bound to the earth. As a consequence, one (&lt;em&gt;visible&lt;/em&gt;) half always faces earth. The other (&lt;em&gt;invisible&lt;/em&gt;) half never faces earth. The invisible hemisphere was unknown to us until space travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the &lt;strong&gt;invisible side&lt;/strong&gt; of the moon is not lit by the sun, it experiences an &lt;em&gt;&quot;earthless starlit night&quot;&lt;/em&gt;. So the illumination would be the same as &lt;em&gt;&quot;a moonless starlit night, which is measured at 0.0001 lux&quot;&lt;/em&gt; to quote OnoSendai's comment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the &lt;strong&gt;visible side&lt;/strong&gt; of the moon, the incident illumination would be fully determined by light radiated from earth (if the pertinent location is not getting any sunlight.) But I do not know how much light we emit.&lt;/p&gt;&#xA;" OwnerUserId="6870" LastActivityDate="2014-12-10T15:50:02.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6315" PostTypeId="2" ParentId="6309" CreationDate="2014-12-10T17:25:21.887" Score="29" Body="&lt;p&gt;Don't forget the lowly astronomer working at one of the observatories in the Atacama Desert, or one of the many indigenous peoples who preceded those astronomers. The combination of high altitude and latitude near the Tropic of Capricorn makes these people well-sited for being the Earth-bound people who are both closest to and furthest from the Sun at various times of the year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With regard to closest to the Sun, that would go to one of the astronauts on Apollo 8. With regard to furthest from the Sun, the Apollo missions are out of the question. A good candidate is some unnamed person near San Pedro de Atacama, Chile on July 4, 1731.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary constraint of the Apollo missions was to have the Sun at an inclination of five to twenty degrees at the landing site at the time of landing. An inclination lower than five degrees made for too many shadows, while an inclination higher than twenty degrees made for too few shadows. This meant the Sun had just risen or was about to set at the landing site. A setting Sun would not have worked for the rest of the mission. Landing was performed just after the Sun had risen, and that meant that lunar orbit insertion occurred with the Moon in waxing crescent. At waxing crescent, the Moon is closer to the Sun than is the Earth but is swinging away from the Sun. Departure took place a few days later, with the Moon at first quarter or waxing gibbous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This bodes well for an Apollo astronaut being the person closest to the Sun, but bodes ill for an Apollo astronaut being the person furthest from the Sun. For closest to the Sun, we need to look for a mission in December or January. That would be Apollo 8 and Apollo 14. Apollo 14 made it's lunar insertion a month after perihelion. Apollo 8 made it's lunar insertion a bit over a week before perihelion. Apollo 8 is the winner in this regard. The closest point to the Sun would have occurred about 20 minutes after lunar insertion. Which of the three Apollo 8 astronauts was closest to the Sun? Who knows. They were quite busy at that time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With regard to furthest from the Sun, the Apollo missions closest to aphelion (early July) were Apollo 11 and Apollo 15. The combination of departure time (in terms of lunar phase) and departure date (in terms of closeness to aphelion) means the Atacama Desert dweller on July third or fourth will have been further from the Sun than those astronauts were at their furthest points from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhap a vehicle in low Earth orbit? Whether the astronomer or astronaut is further from the Sun depends on solar beta angle. The solar beta angle needs to be between -20 and +20 degrees for the astronaut to be further from the Sun than the astronomer. Next, we need to look to the Moon on July 4. A new moon is ideal, a full moon, the opposite. New moon versus full moon makes for a swing of over 9000 km in distance between the Earth and the Sun. There have only been a few new moons in early July during the space age. To find an astronomer who was further from the Sun than an Earthbound person, you need to find a populated vehicle whose beta angle was close to zero at one of those few times when the moon was new at aphelion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not all new moons are created equal. The Moon's orbit is eccentric and is tilted with respect to the ecliptic and to the equator. An annular eclipse on July fourth over southern China is just about perfect for making a person in San Pedro de Atacama be furthest from the Sun. One candidate, as mentioned above, is July 4, 1731. I'm sure there are even better ones further in the past.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update: How I arrived at this answer&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My primary tool for verifying what I wrote was &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot;&gt;JPL's Horizons system&lt;/a&gt;. With this, I was able to verify that there is no way anyone has been closer to the Sun than the Apollo 8 astronauts and that there is no way anyone has been further from the Sun than some unnamed person near San Pedro de Atacama, Chile on July 4, 1731.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The website &lt;a href=&quot;http://stardate.org/nightsky/moon&quot;&gt;StarDate Online&lt;/a&gt; shows the phases of the Moon for any month since 1951. New moons very close to July 4 are needed to make a spot on the Earth, or in low Earth orbit, qualify as furthest from the Sun. The only years where this happened in the space age were 1962, 1967, 1970, 1978, 1981, 1989, 1992, 1997, 2000, and 2008. I checked all of them. None were anywhere close to the distance achieved on July 4, 1731. That was a red letter date.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What made me choose that date? &lt;a href=&quot;http://en.wikipedia.org/wiki/Lists_of_solar_eclipses&quot;&gt;Wikipedia&lt;/a&gt; has extensive lists of total solar eclipses, dating from the 20th century BCE to the 30th century CE. I was looking for an annular eclipse late June / early in July that occurred at around 23 north latitude, 110 east longitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why that search? The Moon's orbit is inclined by about five degrees with respect to the ecliptic. A solar eclipse occurs when the Earth, Moon, and Sun are perfectly aligned, which increases the distance between the Sun and Earth over nominal separation at new Moon. An annular eclipse means the Moon is abnormally far from the Earth, which in turn means the Earth is abnormally far from the Sun. The target latitude/longitude are diametrically opposed to my chosen point of San Pedro de Atacama. Why San Pedro de Atacama? The Andes are without parallel in the southern hemisphere. They make the tallest peaks in southern Africa and northern Australia look like hills. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were no good candidates this century, nor in the 20th century, nor in the 19th century. But then bingo! July 4, 1731 was almost perfect. On testing that date with Horizons, it was clear that no one has been further from the Sun since that date, including the Apollo astronauts, or any cosmonauts or astronauts in low Earth orbit. The Apollo astronauts who ventured furthest from the Sun were those on Apollo 11. (Apollo 15 was a close second.) An astronaut or cosmonaut in low Earth orbit on July 4, 2008 was considerably further from the Sun than those Apollo 11 astronauts. My unnamed person near San Pedro de Atacama was in turn considerably further from the Sun than those astronauts and cosmonauts on July 4, 2008, by over eight thousand kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Certainly there is some date even further in the past with an annular eclipse that occurs even closer to the nominal aphelion passage date and even closer to the target latitude / longitude than the date I found. The date I found was good enough to show that it was ordinary Earth-bound people rather than astronauts or cosmonauts who have been furthest from the Sun.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2014-12-11T04:09:34.943" LastActivityDate="2014-12-11T04:09:34.943" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6316" PostTypeId="1" AcceptedAnswerId="6318" CreationDate="2014-12-10T18:25:11.860" Score="12" ViewCount="1659" Body="&lt;p&gt;The possibility of a space elevator from the lunar surface is discussed in &lt;a href=&quot;https://space.stackexchange.com/questions/2158/between-a-space-elevator-on-mars-versus-the-moon-which-would-have-lower-materia&quot;&gt;this question&lt;/a&gt;. A lunar elevator for the purpose of return to Earth could be achieved at the L1 Lagrange Point between Earth and the moon. I'm wondering if we could acheive a lift from Low Earth Orbit up to the moon by extending such a space elevator far beyond L1 from the Moon toward Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I imagine it, when the elevator extends past L1, Earth gravity would take over, drawing the extension always toward Earth. The closer we get to Earth, the heavier the pull toward Earth and away from the Lunar surface. So by the time the structure is completed, the tension at the lunar surface pulling away from the surface toward the Earth would be very high. So the anchors to the Lunar surface would have to be strong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we can extend such a structure close to LEO in this fashion, would I be wrong to think that we could then launch to the moon merely by achieving that relatively low orbit, docking to the reverse elevator (delevator?) and then climbing the rest of the way toward the moon? Perhaps getting all the way to LEO would be too much for the strength of currently known materials like carbon nanotubes. But even bridging half the gap would cut down the energy required for a lunar commute dramatically, wouldn't it?&lt;/p&gt;&#xA;" OwnerUserId="945" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-09-27T22:24:54.577" Title="Reverse Lunar Space Elevator" Tags="|the-moon|lagrangian-points|space-elevator|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6318" PostTypeId="2" ParentId="6316" CreationDate="2014-12-10T19:24:46.897" Score="20" Body="&lt;p&gt;Here's a pic of the tether proposed by Liftport.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pV4Uc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Go 160,000 km beyond EML1 and drop a payload from that point. It will follow an elliptical path whose perigee grazes low earth orbit. A 3 km/s burn at perigee would circularize the orbit at LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doing a 3 km/s burn from LEO and you will get the same ellipse. At the apogee of this ellipse, the rocket is moving very nearly the same velocity as the tether, so little delta V is needed for rendezvous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Go farther towards the earth from that point and the tether will be moving slower than apogees of transfer orbits from LEO. So extending the tether further and boost delta V for rendezvous with the foot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pictured elevator has low enough stress that Kevlar or some other existing material might be used. If extended all the way to LEO, we're back to needing Bucky tubes.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2014-12-10T19:24:46.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6319" PostTypeId="1" CreationDate="2014-12-10T20:49:15.677" Score="7" ViewCount="195" Body="&lt;p&gt;Mars 2020 will drill and store several surface samples until a return-to-Earth vehicle arrives. The rover will then use its arm to move its sample box to onboard the return vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the rover in the same way physically transfer a digital memory unit for return to Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the radio-to-Earth bottleneck is bypassed by an interplanetary &lt;a href=&quot;https://en.wikipedia.org/wiki/Sneakernet&quot;&gt;&quot;sneakernet&quot;&lt;/a&gt;, then the instruments could, in the end, return much more data to Earth. MSL Curiosity seems to return about 250 megabyte per day. Not much compared to (radiation hardened!) physical memory, right?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-12T21:26:20.477" Title="Sneakernet: Would data return together with sample return be useful?" Tags="|data-transmission|data-storage|sample-return|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6320" PostTypeId="2" ParentId="6316" CreationDate="2014-12-10T20:59:37.637" Score="6" Body="&lt;p&gt;As you have figured out, space elevators are bidirectional.  They're useful for landing as well as taking off.  It's just that taking off is the bigger deal for the places that elevators are usually proposed--you can aerobrake down on any body with an atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2014-12-10T20:59:37.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6321" PostTypeId="2" ParentId="6174" CreationDate="2014-12-10T21:43:46.907" Score="5" Body="&lt;p&gt;Actually, they are deliberately draggy.&lt;br&gt;&#xA;During descent, they would produce significant drag given the waffle pattern producing vortices in the air around them (the hairs on a bee's wings have the same effect). In addition to attitude control, they could serve to reduce retrorocket burn and save a bit of fuel. They would definitely serve to keep the top of the rocket from heeling over when the thrusters ignite, acting like small drogue chutes.&lt;/p&gt;&#xA;" OwnerUserId="6878" LastEditorUserId="6816" LastEditDate="2015-01-14T04:50:35.143" LastActivityDate="2015-01-14T04:50:35.143" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6322" PostTypeId="2" ParentId="6319" CreationDate="2014-12-10T22:01:09.050" Score="8" Body="&lt;p&gt;The latency is killer, though -- 8 months to several years before mission controllers can analyze and respond to data they find. Curiosity's controllers are looking at the pictures it's sending, deciding which terrain features are interesting, and directing it to investigate, iteratively. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A somewhat smarter autonomous rover with a rack of removable SSDs could probably take a lot of pictures and movies for later sneakernetting, without restricting its material sample payload too painfully, but I suspect the non-visual instruments are providing more interesting data, and at a low enough data rate that transmission isn't a bottleneck. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-10T22:01:09.050" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6323" PostTypeId="1" CreationDate="2014-12-10T23:57:32.307" Score="14" ViewCount="2983" Body="&lt;p&gt;A little while ago, Ars Technica did a big story on the Firefly rocket under development.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://arstechnica.com/science/2014/11/firefly-space-systems-charges-full-speed-toward-low-earth-orbit/2/&quot;&gt;http://arstechnica.com/science/2014/11/firefly-space-systems-charges-full-speed-toward-low-earth-orbit/2/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A key design feature is that it will use its own propellant to do self-pressurization (autogenous). At first I thought I understood this well, but in latter online conversations about this, I found out that this detail is actually highly controversial. Here is the part of the article that raises questions, with the emphasis mine.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;heated fuel is turned into hot gas, which is then used to pressurize the fuel tanks. As noted earlier, the end result of this is a fuel tank filled with more burnable fuel, rather than an inert pressurant like helium. This makes Firefly’s aerospike &lt;strong&gt;fully self-pressurizing&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No one disputes that the method can pressurize the &lt;em&gt;fuel&lt;/em&gt; tank. The problem is the wording &quot;fully&quot;. Some people apparently believe that liquid Oxygen tank can't be pressurized by the same method, because this would result in high-temperature Oxygen gas or Oxygen-rich gas (this is my own understanding, which might have its flaws). &lt;a href=&quot;http://arstechnica.com/science/2014/11/firefly-space-systems-charges-full-speed-toward-low-earth-orbit/?comments=1&amp;amp;post=28054165&quot;&gt;The logic&lt;/a&gt; is that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hot oxygen will oxidize practically anything that's not already oxidized. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Checking some &lt;a href=&quot;http://link.springer.com/chapter/10.1007%2F978-1-4615-3368-9_61&quot;&gt;literature&lt;/a&gt; on the subject, they seem to have a point. Autogenous pressurization is addressed for a &lt;em&gt;hydrogen&lt;/em&gt; tank in this article, for an example. But there are still several elements I can't convince myself of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would autogenous pressurization necessarily raise the temperature of the tank anyway? Even if the Oxygen tank was being pressurized by an Oxygen-rich gas mixture of CO2 and/or H2O, this doesn't necessarily have to be extremely hot. You already have cryogenic fluids to exchange heat with, so maybe it's below room temperature and generally not a big issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I take the &quot;fully self-pressurizing&quot; statement to be a error of the journalist exclusively, then it seems like the Firefly design doesn't make any sense. If you're going to use Helium to pressurize the Oxygen tank, then using autogenous pressurization of the fuel tank clearly isn't going to decrease complexity. Two different methods of tank pressurization sounds fantastically complicated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what's the deal? Is there &lt;em&gt;any&lt;/em&gt; credible background behind designs for autogenous pressurization of the liquid Oxygen tank? Is this debacle just poor wording or a misguided startup?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="12102" LastEditDate="2021-11-23T00:06:59.523" LastActivityDate="2021-11-23T00:06:59.523" Title="Disposition of the Oxidizer Tank in Rockets with Autogenous Pressurization" Tags="|engine-design|oxidizer|firefly-aerospace|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6328" PostTypeId="1" AcceptedAnswerId="6329" CreationDate="2014-12-11T07:49:28.790" Score="3" ViewCount="688" Body="&lt;p&gt;Let's consider a satellite orbiting the earth.&#xA;From the centripetal acceleration we have $v=\sqrt{g_{earth}r}$. If we know the $r$ and $g$, we can get $v$, which is the velocity needed to orbit around the earth(is this correct?). My question is, what will happen if you exceed that velocity. Will you just leave the orbit after some time? What if you go slower. Will the earth suck you in?&lt;/p&gt;&#xA;" OwnerUserId="6886" LastActivityDate="2014-12-11T15:24:41.053" Title="Is the velocity of orbiting around the earth exact?" Tags="|low-earth-orbit|gravity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6329" PostTypeId="2" ParentId="6328" CreationDate="2014-12-11T09:19:04.807" Score="8" Body="&lt;p&gt;You have a slight misunderstanding in your question. There is no single orbit - there is effectively an infinite number of orbits your satellite could be in, with different radii (and if you include different ellipses)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you change your velocity, all that happens is you change your orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you increase your velocity such that you exceed escape velocity then you will escape Earth altogether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth is always &quot;sucking&quot; a satellite in - but that is almost the definition of orbiting. If the orbit intersects the atmosphere then atmospheric drag will change the orbit - and eventually the orbit will decay enough that it will fall to Earth.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2014-12-11T09:19:04.807" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6330" PostTypeId="2" ParentId="6316" CreationDate="2014-12-11T09:22:22.293" Score="10" Body="&lt;p&gt;I'm going to elaborate on the limitations of a Lunar elevator - things which can be done, but probably shouldn't, particulary in the context of bringing the elevator close to LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given sufficiently strong materials, there is no reason why a lunar elevator can't be dangled arbitrarily close to Earth's atmosphere - however there are very good reasons why you might not want to do this. The foot of the elevator will always 'orbit' the Earth in the same time as the Moon takes to orbit the earth, that is around 27 days. The Earth rotates in 24 hours, so in affect the foot of the elevator hangs in space, and the Earth rotates &lt;em&gt;under&lt;/em&gt; it. Nevertheless, the delta-v required to reach the foot of the elevator would be very low - just enough to get above the atmosphere, and counteract the velocity from the rotation of Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, while this would make it convenient for travel between the the Moon and Earth or vice-verca (at least for freight - crawlers would take a long time to make the journey), it does not make it even slightly easier to get materials from the Moon to LEO for orbital manufacturing. The reason is that orbital velocity in LEO is around 7.7km/s. The foot of the elevator, hanging in LEO, orbiting every 27 days, would be travelling at a stately 18m/s. This means the delta-v from the elevator's foot hanging in LEO, to orbiting in LEO, would be around 7.7km/s. Compare this with the numbers in HopDavid's answer, where dropping the payload from higher up requires only 3km/s delta-v to circularize the orbit (this is because the payload builds up the required velocity as it freefalls towards Earth). The foot of the elevator would also not be a microgravity environment as it is not in orbit, it would instead suffer nearly full earth gravity (and incidentally, the cable has to bear that weight).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides being impractical for inserting resources into LEO, the elevator has another huge disadvantage - it would sweep through the orbits of satellites from LEO to GEO. Actually the elevator darn near stays still, while the satellites zip past it (or into it!) at orbital velocities. It would also menace any Earth space elevator if one is ever built. Space is pretty roomy so these aren't necessarily insurmountable things, but every additional object up there complicates orbital traffic control, and a lunar elevator would be much worse than an Earth one, as the Earth one at least travels at orbital velocity in GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while lowering the cable to near LEO, would make it possible to travel cheaply from Earth to the Moon due to a low delta-v from Earth's surface to the elevator foot, the same low velocities would make it a very impractical thing to build, a menace to satellites and requiring much stronger materials due to the gravity it is subjected to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the advantages of a luna elevator are accomplished with a shorter one, with none of these hazards.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2014-12-11T10:56:16.613" LastActivityDate="2014-12-11T10:56:16.613" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6332" PostTypeId="2" ParentId="6328" CreationDate="2014-12-11T15:24:41.053" Score="4" Body="&lt;p&gt;Your equation is for a perfectly circular orbit. A body at a given altitude, moving tangentially to the Earth's surface below, at the speed given by the equation, orbits in a circle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is moving tangentially at a slower speed than that given by your equation, it will be at the high point of an elliptical orbit. If it's moving too slow, the low part of the elliptical orbit will intersect the atmosphere of the Earth, and will slow down due to atmospheric drag, and eventually fall (aerobraking). If it's moving much too slow, the low part of the orbit will intersect the Earth itself (lithobraking). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's moving tangentially at a higher speed than the circular orbit speed, it will be at the low point of an elliptical orbit. Higher speeds yield more eccentric ellipses with higher and higher altitudes at the other end, and a very high finite speed will cause the body to leave orbit and not return; this is called escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your body has a non-tangential velocity, that is, a velocity in a direction not exactly perpendicular to gravity's effect, then it's at some point on the ellipse other than the high or low point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In orbital scenarios, the high point is called the apogee (when you're talking about Earth orbit specifically) or apoapsis (when you're talking about any other body), and the low point is perigee or periapsis. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-11T15:24:41.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6334" PostTypeId="1" CreationDate="2014-12-11T20:20:16.200" Score="8" ViewCount="481" Body="&lt;p&gt;&lt;strong&gt;Extensive Edit&lt;/strong&gt; - I'm genuinely seeking a better understanding of this so I am putting up a new version of this question. I used an example design to make the matter easier to discuss, but it was pretty bad. I hope this one is good enough to place the topic within parameters that makes an analysis possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Design principles: A good design will restrict heat loss through conduction into the soil by placing thermal mass such that heat needs to traverse a large distance to reach the ground, and surface area of the thermal mass in direct contact with the ground is small. This implies building tall, thick walls. Let us consider a cylinder with height equal to diameter, being 40 m, with walls of 3 m thickness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is manageable through the use of &lt;a href=&quot;https://www.google.com.mx/search?q=earthbag&amp;amp;es_sm=122&amp;amp;source=lnms&amp;amp;tbm=isch&amp;amp;sa=X&amp;amp;ei=QlCMVIPQIc-XyASv84CgDA&amp;amp;ved=0CAgQ_AUoAQ&amp;amp;biw=1379&amp;amp;bih=967&quot; rel=&quot;nofollow noreferrer&quot;&gt;earthbag&lt;/a&gt; techniques. Only the fabric bags or tubes to be filled with regolith would need to be shipped from Earth, and the equipment needed to fill and place them is also simple and wouldn't be too heavy. Placing the walls would be slow and require the movement of the loose top layer of regolith from a large area, since that layer is only 10 to 20 cm deep. On the other hand, if your equipment runs on solar power and can be operated remotely, time is cheap compared to the cost of shipping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radiation penetrating such walls would be inconsequential, leaving only that entering through the transparent covering at the top to consider. For the sake of argument let that covering be a shallow dome of plastic sections with protective coatings, and a high-strength membrane beneath holding in the atmosphere. 50% of light passes through, reflected in by a mirror of thin aluminum sheets on a light armature. That mirror has dimensions matching that of the transparent roof, is backed with MLI (Multi-Layer Insulation that reflects EM radiation), and is lowered over the opening at night to hold in heat. During the day it tracks the sun. Let's suppose the structure is 60 degrees N or S of the equator. The transparent area of the greenhouse would receive 1700 kW if it was in full sun. At 60 degrees, it would get approximately half that amount directed into the greenhouse, between the mirror and what directly enters, over the course of a day. Half of that would be blocked by the membrane and plastic, so 425 kW enters on average.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Limiting heat loss by radiation is simple through use of MLI, which is light and so costs little to ship from Earth. Sheets of it would cover the exterior walls. Below is an illustration of how heat would spread outwards from such a structure. &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2010/12jul_rabbithole/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This article&lt;/a&gt; says temperatures in the ground are 'fairly constant' at depths of 2 m, and are 'around -30 to -40 degrees C'. Let's call 3 m the depth at which temperatures are constant. If there is a 3 m skirt of MLI around the base of the cylinder, the beginning of the gradient between the surrounding soil and the greenhouse is a figure following that 3 m distance from the structure's thermal mass. Temperatures outside that area have no bearing on interior temperatures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5Mxyl.jpg&quot; alt=&quot;moon greenhouse design cylinder transparent roof&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the math for how the heat in this system would move around over the course of many lunar days and nights before it found an equilibrium is very much beyond me, but with some rough calculations based on all the figures listed, it seems like that equilibrium could yield a fairly constant, comfortable temperature. Is that true?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've assumed that all the contents of the greenhouse add up to 5000 m3 of stuff (10% of volume) with an average density of 1200 kg/m3 and average specific heat of 2 kJ/kgK. The sunlight into this system would be enough to heat everything in it by about 8 Celsius in one lunar day once the heat was evenly distributed. At first some of that would get sucked into the ground under the greenhouse but soon that would be saturated with enough heat to stop it from doing that any more. Some would be lost through the transparent dome. I don't know how much that could be, but I don't think it would be enough to stop the greenhouse slowly heating up. How would I calculate that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then some heat would be lost over the course of the lunar night. Fine-tuning for cooling is easy, by maybe keeping part of the walls in shade uncovered by MLI so they radiate heat slowly, or turning the mirror a little so it reflects in a bit less light, but I don't know how much difference that would make.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A temperature fluctuation over one complete lunar day within 10 or 15 Celsius would be fine. As long as temperature doesn't change rapidly plants adapt well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Figures used for thermal properties and sunlight:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermal conductivity of regolith: &lt;a href=&quot;http://www.uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources13.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.015 W/mK to 0.22 W/mK&lt;/a&gt; (both figures were quoted in the linked paper from two different sources, on page 16)&#xA;Density of regolith: in the compacted layer below the powdery surface, &gt; &lt;a href=&quot;http://adsabs.harvard.edu/full/1972LPSC....3.3235M&quot; rel=&quot;nofollow noreferrer&quot;&gt;1.5 g/cm3&lt;/a&gt; (this was noted in the conclusions near the end of the paper, it seems good figures have been hard to establish because of the difficulty Apollo astronauts had getting drilling samples)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat capacity of regolith: about &lt;a href=&quot;http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.8 kJ/kg*K&lt;/a&gt; (this is taken from dry soil and sand, couldn't find anything better)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat capacity of water: 4.18 kJ/kgK, of moist soil: 1.48 kJ/kgK&#xA;Sunlight on lunar surface: average of 1360 W/m2&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2014-12-13T17:12:09.290" LastActivityDate="2014-12-13T17:12:09.290" Title="Might thermal mass, reflective insulation, and limiting light penetration allow heat control on the Moon? (reboot)" Tags="|the-moon|colonization|habitat|thermal-control|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6335" PostTypeId="1" AcceptedAnswerId="6381" CreationDate="2014-12-11T20:35:54.893" Score="2" ViewCount="120" Body="&lt;p&gt;Is there any free program (online or downloadable) to analyze the effects of the environment (radiation, micro-meteoroids...) on the spacecraft (after having defined the orbit)?&lt;/p&gt;&#xA;" OwnerUserId="6608" LastActivityDate="2014-12-13T13:26:55.893" Title="Free program to analyze environment effects on spacecrafts" Tags="|software|environment|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6336" PostTypeId="1" CreationDate="2014-12-11T23:53:53.820" Score="26" ViewCount="4679" Body="&lt;p&gt;When the Saturn V launch vehicles reached high altitudes and speeds, the exhaust plume looks drastically different compared to how it did at launch (it is much larger, and has a cone about halfway down the plume). Also, the flames seem to be creeping up the bottom of the S-IC stage. What causes these observations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Pr8Td.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6254" LastActivityDate="2016-09-07T04:01:07.363" Title="Saturn V Exhaust Plume" Tags="|launch|rockets|apollo-program|saturn-v|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6337" PostTypeId="2" ParentId="6336" CreationDate="2014-12-11T23:59:58.267" Score="16" Body="&lt;p&gt;The contained shape of the exhaust you see at sea level is being constrained by the atmosphere. It is expanding as much as it can, as it moves downwards. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the rocket goes higher, the air pressure decreases and the exhaust plume expands.  Watch a SpaceX Falcon 9 launch and it is really obvious.  Actually, watch any sea level launch from a good camera position and it is really obvious. SpaceX just happens to be flying cameras and posting videos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is why the nozzles on upper stage engines are longer to prevent expansion as long as possible to maximize performance. An aerospike engine is designed to expand as per atmospheric pressure.  I.e. Use the atmosphere as your nozzle, and it adjusts itself as the pressure changes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not sure why the exhaust appears to creep up the stage, but probably same reason. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-12-12T00:09:59.230" LastActivityDate="2014-12-12T00:09:59.230" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6338" PostTypeId="2" ParentId="2220" CreationDate="2014-12-12T02:49:47.440" Score="4" Body="&lt;p&gt;No evidence for identifiable scientific product(s) later used by the ISS and/or other manned spaceflight. However, researchers of biosphere two found the original crew's lowered blood pressure, blood cholesterol, and enhancement of there immune systems to be caused by Roy Walford's low calorie restriction diet (Walford, R., Mock D, Verdery R, MacCallum T, Calorie Restriction in Biosphere 2 Alterations in Physiologic, Hematologic, Hormonal, and Biochemical Parameters in Humans Restricted for a 2-Year Period). Perhaps, this could be interpreted as a model for outer space cuisine for astronauts. Probably not though. Currently, researchers at biosphere two study different biomes and there interactions.&lt;/p&gt;&#xA;" OwnerUserId="6881" LastActivityDate="2014-12-12T02:49:47.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6340" PostTypeId="2" ParentId="6335" CreationDate="2014-12-12T04:35:31.770" Score="1" Body="&lt;p&gt;Lockheed Martin has developed one internally for use in the Orion capsule design.  Clearly they are NOT sharing that code. It was referenced in a Space Show episode that I cannot find right now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much more sophisticated than just orbit, but also vehicle orientation in that orbit. And the layout of equipment, storage, and water supplies were taken into account.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-12T04:35:31.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6341" PostTypeId="2" ParentId="4703" CreationDate="2014-12-12T05:25:24.550" Score="10" Body="&lt;p&gt;From here: &lt;a href=&quot;http://www.spacetelescope.org/about/faq/&quot;&gt;http://www.spacetelescope.org/about/faq/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How do you protect and clean the lens of the Hubble Space Telescope?&#xA;  Hubble doesn’t have a lens. Like all large telescopes, Hubble uses a&#xA;  curved mirror to focus starlight. This mirror is located deep inside&#xA;  the telescope, protected by its long tube-like structure. As there is&#xA;  no atmosphere around Hubble, there is no risk of dust or corrosion&#xA;  reaching inside.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also, from here: &lt;a href=&quot;http://www.nap.edu/openbook.php?record_id=11169&amp;amp;page=12&quot;&gt;http://www.nap.edu/openbook.php?record_id=11169&amp;amp;page=12&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hubble was designed with an anticipated 15-year lifetime based on the&#xA;  expected integrity of the main mirror. It was believed that over HST’s&#xA;  15-year life the space environment in low Earth orbit would cause&#xA;  sufficient degradation of the mirror that the telescope’s&#xA;  light-gathering capabilities would be severely damaged by cosmic rays&#xA;  and orbital debris. To date, since the first shuttle servicing&#xA;  mission’s correction for a significant aberration in the mirror, there&#xA;  has been no measurable degradation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From these two sources, I'd say no, there is no measurable degradation of the mirrors.&lt;/p&gt;&#xA;" OwnerUserId="6881" LastActivityDate="2014-12-12T05:25:24.550" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6344" PostTypeId="1" AcceptedAnswerId="6358" CreationDate="2014-12-12T09:44:59.750" Score="8" ViewCount="263" Body="&lt;p&gt;By &quot;ideal&quot; I mean most nutritious, less maintenance as possible, less space consuming, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess this could be for any long-duration flights, as a means of self sustenance in addition to supplies, and to provide astronauts/cosmonauts with fresh food.&lt;/p&gt;&#xA;" OwnerUserId="3001" LastActivityDate="2014-12-12T20:34:07.510" Title="What would be the ideal hydroponic crops for long duration travels?" Tags="|long-duration|plants|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6345" PostTypeId="1" AcceptedAnswerId="6350" CreationDate="2014-12-12T09:53:10.417" Score="7" ViewCount="850" Body="&lt;p&gt;Pretty much what the question states, is SpaceX doing early work on that aspect of Mars' colonization or are they at the moment giving full focus to the launch system, craft and lander? (which would make sense of course, just curious)&lt;/p&gt;&#xA;" OwnerUserId="3001" LastEditorUserId="6537" LastEditDate="2017-12-12T23:34:36.733" LastActivityDate="2017-12-12T23:34:36.733" Title="Is SpaceX already researching, planning or designing, structures for Mars colonies?" Tags="|mars|spacex|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6346" PostTypeId="1" CreationDate="2014-12-12T10:16:50.723" Score="5" ViewCount="164" Body="&lt;p&gt;Given the &quot;recent&quot; (few years) exposure and access of astronauts/cosmonauts aboard ISS to social media in order to communicate with the general public, I was wondering if the decision to allow them to do so was aided by any psychological assessment. Probably on the positive impact of allowing them more open communication and also feel the support of the people on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there ongoing studies on this, and could there be any negative psychological aspects on this as well?&lt;/p&gt;&#xA;" OwnerUserId="3001" LastActivityDate="2014-12-12T10:16:50.723" Title="Has the psychological impact of social media exposure for astronauts aboard ISS been studied?" Tags="|iss|psychology|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6347" PostTypeId="1" AcceptedAnswerId="6402" CreationDate="2014-12-12T11:11:31.070" Score="7" ViewCount="444" Body="&lt;p&gt;I recently read about Orion's touch-screen instrument panels designs, and remember seeing this first on the Dragon v2 unveiling (with a 4 panel design instead of 3).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was this design influenced in any way by one or the other? (Dragon influencing Orion's, or vice-versa)&lt;/p&gt;&#xA;" OwnerUserId="3001" LastEditorUserId="51" LastEditDate="2014-12-12T13:50:48.227" LastActivityDate="2023-06-22T18:02:30.770" Title="Has Orion's touch-screen instrument panel been influenced by Dragon's v2 panel? (or vice-versa)" Tags="|spacex|orion-spacecraft|dragon|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6348" PostTypeId="1" CreationDate="2014-12-12T11:25:23.453" Score="12" ViewCount="455" Body="&lt;p&gt;Just as seen on Orion and Dragon designs (I haven't checked on other current spacecrafts), there are a lot of benefits from using touch-sensitive panels instead of traditional electrical switches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some benefits might be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Less wiring and less clutter, hence more space in the module and less&#xA;weight &lt;/li&gt;&#xA;&lt;li&gt;Less risk of electrical/mechanical malfunction, electrical&#xA;fire, etc. &lt;/li&gt;&#xA;&lt;li&gt;More intuitive interface&lt;/li&gt;&#xA;&lt;li&gt;Ability to update firmware/software&lt;/li&gt;&#xA;&lt;li&gt;Language translations on UI (could come in handy for multi-cultural teams)&lt;/li&gt;&#xA;&lt;li&gt;Many others...&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now, having said that, what could be some potential dangers or drawbacks from using this design instead of classical electrical switches?&lt;/p&gt;&#xA;" OwnerUserId="3001" LastEditorUserId="12102" LastEditDate="2020-05-30T05:41:45.513" LastActivityDate="2020-05-30T05:41:45.513" Title="Are there any drawbacks or potential dangers on using touch-sensitive instrument panels on spacecraft?" Tags="|spacex|spacecraft-development|dragon|orion-spacecraft|cst-100|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6349" PostTypeId="2" ParentId="6334" CreationDate="2014-12-12T12:57:21.510" Score="3" Body="&lt;p&gt;It would certainly be possible to use internal thermal mass to regulate the temperature. I couldn't find much about the properties of regolith but I did find &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2010/12jul_rabbithole/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Once you get down to 2 meters under the surface of the Moon, the temperature remains fairly constant, probably around -30 to -40 degrees C.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The idea of storing heat long term is called &lt;a href=&quot;http://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage&quot; rel=&quot;nofollow&quot;&gt;seasonal thermal energy storage&lt;/a&gt;, there are also passive versions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For passive temperature regulation, it is ideal to use &lt;a href=&quot;http://en.wikipedia.org/wiki/Phase-change_material&quot; rel=&quot;nofollow&quot;&gt;Phase Change Material&lt;/a&gt;, these are substances with a melting point at room temperature when they melt they absorb a great deal of heat, and as they freeze they release a great deal of heat, this tends to maintain temperature at the melting point of the PCM. PCM's can be very effective, from wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Within the human comfort range between 20–30 °C, some PCMs are very&#xA;  effective. They store 5 to 14 times more heat per unit volume than&#xA;  conventional storage materials such as water, masonry or rock.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Therefore a PCM could be used to finely regulate the temperature without any need for active controls. Unfortunately as they are usually carbon based materials, it might not be possible to create them in-situ.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without a PCM or active thermal storage using pumps, the system will necessarily undergo temperature fluctuations, it will warm up at least somewhat during the luna day, and cool down at least somewhat during the luna night. This could be kept to almost arbitrarily low levels by making the system big enough, if there is enough internal thermal mass then temperatures will remain fairly regular. &#xA;However with modern passive heated houses such as &lt;a href=&quot;http://earthship.com/Systems/comfort-in-any-climate&quot; rel=&quot;nofollow&quot;&gt;Earthship&lt;/a&gt; temperature regulation still relies somewhat on selectively admiting or blocking sunlight, and exploiting the coolness of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's impossible to say just how big a purely passive system would have to be, that's because it depends on how regular you want the temperature to be - and for an intensive greenhouse you probably want very regular. The problem isn't so much keeping the heat in - that's mainly a matter of using enough insulation. The problem is getting rid of excess heat during the day as the dome on the whole would be well insulated. It means there must be enough thermal mass to soak up the sun's heat for 15 days, without significantly increasing in temperature, or more of the sun's heat must be rejected by radiators, or by selectively reflecting more of the sunlight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However there is a serious hazard in any system which is exposed directly to the sun/space - that is cosmic and solar radiation. Generally radiation is not harmful to plants, but it would be extremely hazardous to humans and animals working or living in the dome. Since the LED lights are already required anyway, we may as well avoid the radiation hazard and expense and complexity of the shutters and just make the greenhouse fully isolated from the sun, whatever generates electricity for the night time lights would probably generate enough waste heat to heat the greenhouse too - or you could store some heat using a solar water heater to heat water which is pumped into an insulated tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of a passive dome would work quite well for large domes, large enough to have enough mass of air to provide a barrier to radiation, and to have soil, lakes and buildings for internal thermal mass, and large enough to justify using orbiting mirrors to provide sunlight during the luna night. The dome would need to be quite large - for adequate radiation shielding a dome with a radius of 1km with an air pressure of 1 bar would be appropriate (this reduces radiation by 99% in the center of the dome - less towards the edges), but we're talking here a city rather than a greenhouse. So my conclusion is that it's a practical idea at the scale of mega-engineering, but not at a small scale.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2014-12-12T12:57:21.510" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6350" PostTypeId="2" ParentId="6345" CreationDate="2014-12-12T13:59:52.800" Score="4" Body="&lt;p&gt;This is an interesting behavior on SpaceX's part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are very vocal about near term development, showing off hardware early, building excitement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Longer term, the booster (BFR - Big F-ing Rocket), its engine (Raptor), and the transit vehicle (MCT - Mars Colonial Transporter) have been named, hints given, but not much more. Well Raptor at least we know its propellants (Methane/LOX), its vague thrust range (Currently around 1 mlbs, but that has changed numerous times), and where they plan to test it (Stennis test stand being modified for Methane ops).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We do not know how big (width of the core, number of Raptor engines) that will be on the BFR, nor much of anything about MCT.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the horse before the cart. Need the engine to get there. Planning for actual logistics on the surface, is even farther out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have little doubt there is a small team working on ideas and concepts, or even just reviewing the literature for the best ideas to liberate.  (Steal one sentence of text from someone else, it is plagarism. Steal 100 ideas from 100 other places and it is called research). But there has been little to nothing said. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recall hearing Elon mention (but cannot find the reference) that he hopes other companies will spring up, once he has serious work done on BFR/MCT and the likelyhood or a Mars colony is real, to work on Mars colonial logistics. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-12T13:59:52.800" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6351" PostTypeId="1" AcceptedAnswerId="6362" CreationDate="2014-12-12T14:03:54.647" Score="5" ViewCount="507" Body="&lt;p&gt;We have come a long way since the first space missions, and now we have state of the art avionics systems on board our spacecrafts. The more deep we go into space, the more autonomy our robotic spacecrafts need.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By autonomy, I mean to elaborate my question on two main conditions,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Autonomous Control over unfamiliar conditions (Self Repair, GNC without human interference)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Automatic scientific inference and hypothesis and decisions based on the inference&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Are these problems already solved ?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, then&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question then comes to the following -:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What are some of the unsolved problems in robotics and avionics that need to be solved for such kinds of missions ?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6905" LastEditorUserId="12102" LastEditDate="2023-10-27T19:33:38.310" LastActivityDate="2023-10-27T19:33:38.310" Title="Advances in AI and Avionics needed for deep space exploration?" Tags="|spacecraft|spacecraft-development|robotic-missions|avionics|artificial-intelligence|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6352" PostTypeId="1" CreationDate="2014-12-12T14:37:15.900" Score="29" ViewCount="15098" Body="&lt;p&gt;Some rockets have black-and-white patterns painted on them. For example, the Saturn V has them at the bottom of the upper stages and the SLS block 2 is supposed to have them below the payload. In contrast, the Delta rockets do not have these patterns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least the prototype of the German Aggregat 4 rocket had similar patterns for the purpose of estimating radial motion visually. I suppose that this cannot be the sole purpose of these patterns on more modern rockets since telemetry would provide this kind of data.&lt;/p&gt;&#xA;" OwnerUserId="6906" LastEditorUserId="51" LastEditDate="2015-07-02T20:51:34.813" LastActivityDate="2015-07-02T20:51:34.813" Title="What is the purpose of the black-and-white patterns on some rockets?" Tags="|launch|rockets|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6353" PostTypeId="2" ParentId="6348" CreationDate="2014-12-12T14:55:24.130" Score="8" Body="&lt;p&gt;If you look at the SpaceX design, you see that there are the large touch screen displays. But there are also standard physical buttons for manual controls. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As noted in the comments, while under thrust at 3-5G's a touch screen just isn't going to cut it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, Dragon V2 is meant to have an almost entirely automated flight regime. The crew are not meant to fly the vehicle in normal cases.  As a consequence the small set of physical switches are meant to be used only under exceptional circumstances. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-12T14:55:24.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6354" PostTypeId="2" ParentId="6352" CreationDate="2014-12-12T15:10:07.907" Score="12" Body="&lt;p&gt;Additionally the various ISS modules are covered in similar items, meant as targets/locators/callibrators for computer based vision systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JWEjC.jpg&quot; alt=&quot;Unity, Node 1 covered in target spots&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image is from this &lt;a href=&quot;http://www.edn.com/electronics-blogs/edn-moments/4402159/International-Space-Station-Unity-Module-is-launched--December-4--1998&quot; rel=&quot;noreferrer&quot;&gt;site.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have you ever watched Mythbusters the TV show?  When they want to calculate speed of fast moving things, they paint a 'ruler' behind it, tape it on high speed camera, and then calculate the speed by how many frames it takes to move a distance shown behind it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Building a 'ruler' behind a Saturn V, that can be used, and is able to withstand a Saturn V taking off boggles the mind. It is easier to cover the Saturn V in stuff that is easy to track. Now not specifically the same use case as in MythBusters, (raw speed), but more likely for roll tracking, movements, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS modules are covered in targets for a slightly different reason, to allow the CanadaArm system to operate a bit more autonomously. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-12T15:10:07.907" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6355" PostTypeId="2" ParentId="6348" CreationDate="2014-12-12T15:20:43.877" Score="7" Body="&lt;p&gt;Anything that goes into space needs to be robust enough to withstand cosmic rays. That's why computer chips that go into space are usually of the older variety with thicker connections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mechanical devices such as buttons and switches are more intuitive. You can feel them when they click into place. They don't change what they do because someone updated the software.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of this has to do with industrial psychology, for example, beer handles in bars to ensure that the bartender never pours the wrong beer. Those beer handles were traditionally installed in nuclear power plant control rooms (attached with electrical tape) to make sure somebody didn't pull the wrong switch at the wrong time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can come up with a touch-screen design that has seen at least a billion hours of service (cumulative for all users) with absolutely no changes to the hardware or software, and if it's performed well, then maybe there would see the beginnings of a discussion.&lt;/p&gt;&#xA;" OwnerUserId="6881" LastActivityDate="2014-12-12T15:20:43.877" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6356" PostTypeId="2" ParentId="6345" CreationDate="2014-12-12T15:55:43.197" Score="4" Body="&lt;p&gt;Elon Musk seems to have a long term plan. His other main investments, except for the obvious rocket launchers and spacecrafts (SpaceX), are in solar power, battery technology and electric vehicles (SolarCity and Tesla). All of that, and maybe some hyperloop tech, would be very useful on Mars. If you look at it in terms of laying the base for key Martian technologies to arrive in a decade or two, I think you can say that he is seriously preparing for a colony on Mars.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-12T15:55:43.197" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6357" PostTypeId="2" ParentId="2339" CreationDate="2014-12-12T16:29:08.540" Score="6" Body="&lt;p&gt;I have an arts degree and am not an engineer, but I work in technical communications and have an aptitude for problem solving, so I'll give it a go. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Marek makes a good point about the acceptable leak rate. I've seen comments from others posted elsewhere that suggest that the seals have to cause friction, or the crew must be in suits to transfer between sections if the seals are loose. That is not necessarily the case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the SF tales I'm developing, I accept the limitations of labyrinth seals for preventing leaks of atmosphere and most ships/stations do not normally have the volumes adjacent to the transfer areas pressurized at all. With the air held behind bulkheads and the robust seals of standard hatches, there is no air to leak out through the seals that protect the juncture of the ship sections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some options for getting crew through the juncture without them  getting into suits. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Use a pressurized tube with appropriate hatches at the ends that can couple to and detach from the rotating and non-rotating portions of the ship. Crew open one end, enter and close that hatch behind them. The tube detaches and its rotation is changed to match that of the other section, to which it is then connected. The crew open that hatch and exit the transfer tube. No pressure suits or major hassle. Power/data is transferred through slip rings elsewhere, or through cables in the transfer tube wall and slip rings.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Instead of just a tube that changes spin, a pod could be transferred through an unpressurized junction and spun to connect with the ship sections as needed, sealing to hatches directly or moving in and out of volumes that can be evacuated and pressurized. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A non-rotating tube extends into the rotating section. The adjacent chamber -- perhaps the size of a small room -- is unpressurized when not in use for transferring crew. When that chamber is in use, it is sealed and air is pumped in. Some will leak out, but at a slow rate, and only from that limited volume. When not in use, the transfer tube volume is pumped out to conserve the atmosphere. Again, no air there means no air to leak out through the seals. Some is lost, but at a much slower rate than if it was pressurized at all times. The small cumulative loss of air from the transfer section is balanced with air from the much larger volumes in the rest of the station/vessel. If you really need to pipe water into and out of the rotating section, a pipe or hose could run through an axial transfer tube, down the center in the case of a larger tube or in the wall of the tube so long as it connected to the rotating section at the actual axis with enough room that crew transfer isn't hampered.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The tube is optional. Instead, have the two sections meet with two hatches that can be sealed, perhaps a meter apart, or even less. The smaller the volume the faster it can be pumped full or evacuated. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In options 3 and 4, the crew start by pumping air from tanks into the transfer tube section, then open the hatches one at a time or simultaneously, move through into the other section, then close the hatches. If they need to go back and forth, they keep it pressurized, but perhaps closed. If not, they pump the air back into tanks, minus what little amount leaked out in the short time it was in use. Just don't have a phobia about not being able to breathe...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Leakage is not a problem if there is nothing there to leak out. &lt;/p&gt;&#xA;" OwnerUserId="6908" LastEditorUserId="6908" LastEditDate="2014-12-12T19:13:04.693" LastActivityDate="2014-12-12T19:13:04.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6358" PostTypeId="2" ParentId="6344" CreationDate="2014-12-12T16:37:16.837" Score="5" Body="&lt;p&gt;Allow me to give the obvious answer of &lt;strong&gt;spirulina&lt;/strong&gt;. Here's a NASA report from 1988 on the subject of growing food during space missions. I don't know if it's particularly different from any other NASA report on the subject:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19890016190&quot; rel=&quot;nofollow&quot;&gt;http://ntrs.nasa.gov/search.jsp?R=19890016190&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The abstract is unhelpful, but the entire introduction is highly relevant to your question, and provides a broad overview of associated science. This was part of the effort for a particular program at that time. I don't imagine they use the same program name anymore, but a number of species have certainly been cultivated in space since that time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll offer some of the most relevant quotes which answer your question.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Conventional food sources consist of higher plants and animals. Unconventional food sources for human consumption are photosynthetic algae and bacteria and non-photosynthetic bacteria, yeasts and fungi. Conventional food sources are highly palatable, but require a long time to produce. The photosynthetic energy efficiency of higher plants is less than 3%. Algae, on the other hand, grow rapidly; their metabolism can be controlled; they produce a high ratio of edible to nonedible biomass; and their gas-exchange characteristics are compatible with human requirements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;tl;dr: Conventional food is far too inefficient to use, but algae tastes disgusting.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nutritional quality of all cyanobacteria which have been tested (See Appendix) appears to be very high. For example, Spirulina, in addition to being the richest known source of vitamin B12, also contains significant amounts of vitamins B1 and B2. Similarly, one gram of Spirulina contains one-half of the adult daily requirements of vitamin A (B-carotene). The trace elements and iodine found in cyanobacteria are also important when considering the nutritional quality cyanobacteria.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;tl;dr: cyanobacteria (a type of algae) does just fine nutritionally&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The most difficult problem in using algae as food is the conversion of algal biomass into products that a space crew could actually eat over a long period of time. If algae are to be considered as a primary food source, it will be necessary to determine that they can be converted into a wide enough range of a palatable complete diet. Therefore, Spirulina, an edible alga with less nucleic acids and no cell walls, offers a good prospect for further studies by manipulating growth parameters.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;tl;dr: Spirulina is basically the least disgusting cyanobacteria we can find&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="3065" LastEditDate="2014-12-12T20:34:07.510" LastActivityDate="2014-12-12T20:34:07.510" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6359" PostTypeId="1" CreationDate="2014-12-12T16:48:58.173" Score="13" ViewCount="1310" Body="&lt;p&gt;Both &lt;a href=&quot;http://www.mars-one.com/mission/roadmap&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars One&lt;/a&gt; and &lt;a href=&quot;http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; have suggested sending uncrewed supply ships to Mars as part of a manned mission or colonization. These payloads can take all the time they like, not being constrained by human radiation exposure time limits. Instead, what you want to minimize is the total $\Delta v$ needed to reach Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can think of two possibilities that have already been used:&lt;br&gt;&#xA;1) &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission#Orbit_raising_manoeuvres&quot; rel=&quot;nofollow noreferrer&quot;&gt;MOM&lt;/a&gt;-style multiple burns at perigee, culminating in trans-Martian injection.&lt;br&gt;&#xA;2) &lt;a href=&quot;https://space.stackexchange.com/questions/4027/has-luna-been-used-for-gravity-assist-for-interplanetary-probes&quot;&gt;Lunar slingshots&lt;/a&gt;, perhaps in multiples. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible these two approaches can be combined. There might also be other possibilities, such as starting with a low-energy transfer to EML-2, which is apparently the &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/04/cartoon-delta-v-map.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;lowest $\Delta v$ destination in local space&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How do you minimize $\Delta v$ to get to Mars?&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt;&#xA;As Mark Adler points out, it may very well be more efficient to use electric propulsion, which would require higher $\Delta v$ but would result in lower launch mass. However, to avoid making this question too broad, let's assume you're using a high-thrust motor. Perhaps later I'll figure out how to properly ask the underlying question, &quot;What's the cheapest way to send 1000 kg to Mars?&quot;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2014-12-28T00:53:02.443" Title="What is the least energetic trajectory design to send uncrewed supplies to Mars?" Tags="|mars|trajectory|mission-design|uncrewed-spaceflight|low-energy-transfer|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6360" PostTypeId="2" ParentId="6359" CreationDate="2014-12-12T17:07:08.213" Score="10" Body="&lt;p&gt;I don't think that's really the question you want to ask.  The lowest energy trajectories are ones like you suggest, which achieve most of the energy to get to Mars with short impulsive burns very close to Earth.  That requires chemical or nuclear thermal propulsion systems that can expend much of the propellant over a very short time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you want for Mars cargo are low-thrust trajectories that allow you to use much more efficient electric propulsion systems.  Those take more energy and $\Delta V$ overall than those that can use the Oberth effect, but despite that require lower launch mass due to the very high $I_{sp}$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-12-12T17:07:08.213" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6361" PostTypeId="1" AcceptedAnswerId="6364" CreationDate="2014-12-12T17:38:32.877" Score="15" ViewCount="10119" Body="&lt;p&gt;Is it known whether probabilities of success vs failure (and specifically fatal failures) were estimated at the time for the Apollo XI mission? &#xA;Or perhaps some current estimates?&lt;/p&gt;&#xA;" OwnerUserId="6911" LastActivityDate="2017-05-21T23:51:24.133" Title="Success-fail probabilities for Apollo XI?" Tags="|apollo-program|risk-management|apollo-11|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6362" PostTypeId="2" ParentId="6351" CreationDate="2014-12-12T18:15:02.410" Score="6" Body="&lt;p&gt;As far as autonomous control is concerned, much of this has already been developed. It had to be given the long time delays between Earth and even &quot;nearby&quot; destinations such as Mars. The Mars rovers, for example, are able to navigate to nearby locations on their own (&lt;a href=&quot;http://www.extremetech.com/extreme/143884-how-nasa-drives-mars-rover-curiosity&quot; rel=&quot;noreferrer&quot; title=&quot;Curiosity article&quot;&gt;Curiosity article&lt;/a&gt;). This is extremely limited in comparison to the autonomous vehicles entered into the DARPA grand challenges. The biggest road blocks in bringing that tech into space is the amount of computation needed and senors required. It can be done, but the DARPA challenge vehicles had a lot more power and mass available. For more information on the research side of this, look for SLAM (&lt;a href=&quot;http://en.wikipedia.org/wiki/Simultaneous_localization_and_mapping&quot; rel=&quot;noreferrer&quot;&gt;simultaneous localization and mapping&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives you a system that can move from point A to point B. That's great, but your question gets at the more difficult problem of how to decide where you want to go in the first place. This is starting to get into the domain of Artificial Intelligence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many examples of artificial intelligence that work very well in specific domains. For example, NASA's &lt;a href=&quot;http://en.wikipedia.org/wiki/Deep_Space_1&quot; rel=&quot;noreferrer&quot;&gt;Deep Space 1&lt;/a&gt; mission demonstrated Remote Agent. Remote Agent was a software system that could do planning (for example, what sequences of actions are required to perform a task) as well as diagnose faults within the spacecraft (such as, why didn't I get a response after transmitting my data?). For this, Remote Agent has a model of the underlying hardware, and maintains &quot;knowledge&quot; of it such as sensor readings and actuation commands. It works well, however the system is limited. The underlying mathematical model is a &lt;a href=&quot;http://en.wikipedia.org/wiki/Partially_observable_Markov_decision_process&quot; rel=&quot;noreferrer&quot;&gt;POMDP&lt;/a&gt;, which can be solved for an optimal action given the state of the system (even given that some of this state is not know). But solving a POMDP becomes intractable for large / complex systems. So it does not scale well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Making inferences from scientific measurements that direct future actions falls into the intractable domain. In fact, I would say that it is getting very close to the goal of build a general purpose AI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for the first part of your question: these problems have demonstrate-able solutions. It is not easy, but with enough resources and effort robots that can navigate autonomously are possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the the second part: unless the domain can be greatly restricted, these problems are generally intractable. However, approximate solutions that are probably &quot;good enough&quot; may be a viable.&lt;/p&gt;&#xA;" OwnerUserId="4686" LastActivityDate="2014-12-12T18:15:02.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6364" PostTypeId="2" ParentId="6361" CreationDate="2014-12-12T18:38:08.493" Score="25" Body="&lt;p&gt;Neil Armstrong thought &lt;a href=&quot;http://www.blastr.com/2012/05/why_armstrong_thought_the.php&quot; rel=&quot;noreferrer&quot;&gt;they had a 90% chance of survival, but only a 50-50 chance of landing&lt;/a&gt; on the first attempt. I've seen one source that says Aldrin was less optimistic, estimating 2/3 chance of survival and 1/3 chance of success. At that time, Apollo spacecraft had flown several missions safely, including two lunar orbit missions, so presumably he was worried about the landing proper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen multiple references to &lt;a href=&quot;http://amyshirateitel.com/2011/03/19/spaceflight-risky-business/&quot; rel=&quot;noreferrer&quot;&gt;a &quot;three nines&quot; goal&lt;/a&gt; per mission for crew safety, and either 90% or 99% for mission success. Presumably they got to those reliability estimates &quot;on paper&quot; by the time Apollo 11 flew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both those numbers seem pretty optimistic to me. In the end, the Apollo flights from 11 on had a 100% safety rate and 85.7% mission success rate, and Apollo 11 was probably the riskiest of that series. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an &lt;a href=&quot;https://archive.org/stream/nasa_techdoc_19790077142/19790077142_djvu.txt&quot; rel=&quot;noreferrer&quot;&gt;early-1965 QA report&lt;/a&gt; giving the then-current probabilities of success as 0.73 and safety as 0.96, and presumably they got those numbers up over the next four years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the individual failures experienced on the actual Apollo missions generally fell into the &quot;unknown unknowns&quot; category of interactions between individually-reliable subsystems, so I feel like the actual chances could have been quite a bit worse than what the analysts might have calculated. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-14T20:56:36.887" LastActivityDate="2014-12-14T20:56:36.887" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="6366" PostTypeId="2" ParentId="6352" CreationDate="2014-12-12T19:19:17.423" Score="22" Body="&lt;p&gt;The stripe patterns (generically called &lt;a href=&quot;https://en.wikipedia.org/wiki/Fiducial_marker&quot;&gt;fiducials&lt;/a&gt;) are indeed for accurate visual positioning when reviewing film and photos. Telemetry can drop out, and accelerometers can fail, especially in the middle of catastrophic events, and estimating a vehicle's speed from inside the vehicle by integrating acceleration is highly error prone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fewer modern rockets seem to have those large black-and-white areas, I would guess because modern computer image processing techniques can easily locate smaller reference points. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-12T23:38:08.790" LastActivityDate="2014-12-12T23:38:08.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6367" PostTypeId="1" AcceptedAnswerId="6369" CreationDate="2014-12-12T19:43:32.173" Score="18" ViewCount="2623" Body="&lt;p&gt;Ion thrusters are capable of providing the same amount of delta-V for far less fuel (e.g. Falcon 9 v1.1 second stage specific impulse is 340 seconds, while some ion-thrusters have a specific impulse of over 10000 seconds).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ion thrusters have excellent life expectancy. NASA's NEXT ion drive continuously operated for over 5.5 years and after that the engine showed little signs of degradation. Rocket motors are not that reliable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ion thrusters have been tested and performed very well in numerous space missions, including Dawn, Hayabusa 1 and 2, Smart 1, GOCE, Deep Space 1 and more are upcoming. This concept isn't particularly new or experimental.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, ion thrusters are awesome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why don't we use them to lift everything from LEO? They can use ~30 times less fuel, and fuel in space is very expensive. Why do we still need rockets going above LEO for unmanned spaceflight? Why develop super-complicated harsh-reentry capable reusable rocket technology when we can have spacecraft lasting for decade(s)in a stable space environment? They proved to be powerful enough to withstand air drag at very low orbit (only 255 km above the surface of Earth). What are the problems?&lt;/p&gt;&#xA;" OwnerDisplayName="user6738" LastEditorUserId="51" LastEditDate="2014-12-12T19:58:36.947" LastActivityDate="2021-09-05T17:48:47.933" Title="Why aren't there any space tugs in use?" Tags="|propulsion|spacecraft|reuse|ion-thruster|" AnswerCount="6" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6368" PostTypeId="2" ParentId="4793" CreationDate="2014-12-12T20:01:23.923" Score="9" Body="&lt;p&gt;The accepted answer assumes a single hydrogen-fueled stage for the whole TLI-to-landing thing, which isn't how it's normally done, because cryogenic hydrogen would boil off along the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a more accurate estimate, you have to split the LEO-to-moon delta-V requirement into 3300 m/s for the translunar injection (to be done on a hydrogen-fueled upper stage with an ISP in the 400s) and 2600 m/s orbital insertion and landing (to be done on a non-cryogenic hypergolic with an ISP in the 300s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you run the rocket equation once for each leg of the trip, and find you need about 2.1 tons of TLI stage to get 1 ton from LEO to the lunar intercept, and about 3.6 tons of orbiter/lander at lunar intercept to soft-land 1 ton. So there's a total mass ratio of about 7.5 involved. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-12T20:01:23.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6369" PostTypeId="2" ParentId="6367" CreationDate="2014-12-12T20:02:10.273" Score="22" Body="&lt;p&gt;While the ISP on Ion thrusters is awesome, the overall thrust is pretty low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the transit time from LEO to GEO would be quite long and slow. In some cases this matters. If it takes an extra year to get in service, that is a year of lost service while in transit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact a critique of the Falcon 9's ability to do dual launches is that only the smaller electric propulsion (Ion as you like) satellites will fit, and while they WILL be dropped close to GEO/GTO orbits, they will need another 6 months to finalize the orbit on their electric thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about payloads that are not time sensitive? Well the other problem is that while you are slowly raising your orbit you pass through the Van Allen belts ALOT.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Passing through once, quickly, is not terrible, and survivable. But repeated passes, over the course of many months requires additional hardening and work to make it work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said all that, Russia has sort of had a tug in use. The modules that delivered some of the ISS modules are technically tugs, that never got reused.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a while there, (Cannot find the &lt;a href=&quot;http://www.russianspaceweb.com/&quot;&gt;http://www.russianspaceweb.com/&lt;/a&gt; reference off hand right now) they proposed a manned or cargo launcher that relied on docking to an already orbited tug to manuever to final destination. That would have been a traditional propulsion system, not ion/electric propulsion. Thus fewer missions and a shorter lifespan, but they still seriously considered it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-12T20:02:10.273" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6370" PostTypeId="2" ParentId="6367" CreationDate="2014-12-12T20:06:21.060" Score="11" Body="&lt;p&gt;A single-use, disposable ion drive can be made no larger than it needs to be, thus uses no more fuel than it needs to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A reusable ion drive tug, first of all, has to take its payload to destination, and then come back; due to the exponential increase in fuel requirements for linear increases in delta-v, this can be more than twice as much fuel as a one-way mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, to go anywhere interesting on an ion drive takes months or years; a tug doing a Jupiter mission is going to miss a bunch of launch windows for Mars missions.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-13T04:47:13.287" LastActivityDate="2014-12-13T04:47:13.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6371" PostTypeId="2" ParentId="6344" CreationDate="2014-12-12T20:08:55.607" Score="2" Body="&lt;p&gt;A group at MIT did a review of the Mars One proposal, and in the mode that grew its own food supplies found that they died 61 days in, because of too MUCH oxygen production by the plants!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is counter intuitive, but is related in fact to nitrogen, as opposed to oxygen. The plants produced oxygen, and to keep the atmosphere less reactive (I.e. Not catch on fire at the slightest spark) they needed nitrogen. So to keep the oxygen level low enough they planned to vent excess atmosphere, which means they lost nitrogen. Eventually they had too much oxygen, too little nitrogen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interesting counter intuitive problem for using plants in transit.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-12T20:08:55.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6373" PostTypeId="2" ParentId="6352" CreationDate="2014-12-12T21:06:48.630" Score="34" Body="&lt;p&gt;Those markings are called &lt;a href=&quot;https://blogs.nasa.gov/Constellation/2009/03/13/post_1236964869295/&quot; rel=&quot;noreferrer&quot;&gt;roll patterns&lt;/a&gt;. They make it easy to spot if the rocket is rolling (i.e. turning about its vertical axis). They were common on early rockets as a visual backup to not-so-advanced acceleration sensors; you can imagine it's difficult to detect a bit of roll in the middle of huge vibrations plus enormous vertical acceleration.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The patterns are so large they're visible from enormous distances. This photo was made from 100 km away:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Saturn V Stage Separation during the Launch of Apollo 11, July 16th,&#xA;  1969. The Image was Taken with a 70mm Camera Mounted on the Cargo Door of a U.S. Air Force EC-135N Aircraft. The Rocket is 55 Miles&#xA;  Downrange, at an Altitude of 38 Miles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Psh0B.jpg&quot; alt=&quot;Saturn V Stage Separation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA still uses roll patterns. Ares I-X had a Z-shaped pattern that works the same way: the vertical distance between the diagonal and the horizontal bar tells you how the rocket is oriented:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/T9mdy.jpg&quot; alt=&quot;Ares I-X launch&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The base color is usually white because that absorbs less heat than any other color. This reduces propellant boiloff. According the book &lt;em&gt;N-1 For the Moon and Mars&lt;/em&gt; (page 13), the designers had to change the color of the N-1 from dark gray to white to keep the interior temperatures at reasonable levels. The first vehicle, N-1 L3 was mostly dark gray, N-1 L8 (the fifth planned vehicle) would have been overall gloss white.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The black areas of the roll pattern are black because the high contrast makes it easy to see the patterns. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-12-13T09:44:30.393" LastActivityDate="2014-12-13T09:44:30.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6374" PostTypeId="2" ParentId="6359" CreationDate="2014-12-12T21:15:38.063" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How do you minimize Δv to get to Mars?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The answer is simple: Wait until 2018 or 2035. Those are the local minima in the Δv needed to get to Mars. The required minimum Δv varies by a large amount. There's a local minimum roughly every two years where transfers to Mars become feasible, but even this quantity varies considerably. There's a ~15 year variation on top of this ~2 year cycle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting to Mars can cost a lot if you are extremely persistent about getting to Mars right now. The Δv goes down drastically if you are willing to wait a couple of years. The Δv drops by another factor of two on top of that already drastic decline if you are willing to wait a couple of decades.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="49" LastEditDate="2014-12-28T00:53:02.443" LastActivityDate="2014-12-28T00:53:02.443" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6375" PostTypeId="2" ParentId="6319" CreationDate="2014-12-12T21:26:20.477" Score="3" Body="&lt;p&gt;It's been over forty years since the last sample was returned from the Moon. Yet to this very day, Earth-bound scientists are managing to extract meaningful data from those returned samples. Those returned samples were and still are a treasure trove of information. This is what makes a sample return from Mars the holy grail of Mars exploration, much so than terabytes of data from a rover that is inherently limited in power, size, and analytic capabilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The devices we humans send to Mars are extremely limited in capability. There's only so much sense a scientist can make out of remotely sensed data sent from a limited device (e.g., a Mars rover). It's a bit of a matter of garbage in, garbage out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bringing back samples from Mars would be a game-changing event. This would enable scientists who are stuck on the surface of the Earth to have decades to make sense of those returned samples. As I mentioned above, scientists are still managing to garner information from the samples returned from the Moon. The same would apply to samples returned from Mars, in spades. (Soooory, I just couldn't resist the pun.)&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-12-12T21:26:20.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6376" PostTypeId="2" ParentId="6359" CreationDate="2014-12-12T22:31:34.813" Score="3" Body="&lt;p&gt;In addition to possibilities outlined in your post, you can also take advantage of the fact that (gross simplification ahead) orbits around Lagrange points are possible, but very unstable, letting you &quot;choose the instability you want&quot; and leave the Lagrange point in almost any direction, without much fuel needed. The downside is that those trajectories can take very long. See wiki: &lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot; rel=&quot;nofollow&quot;&gt;Interplanetary Transport Netowrk&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Low-energy_transfer&quot; rel=&quot;nofollow&quot;&gt;Low energy transfer&lt;/a&gt; for further details.&lt;/p&gt;&#xA;" OwnerUserId="6917" LastActivityDate="2014-12-12T22:31:34.813" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6378" PostTypeId="1" AcceptedAnswerId="6380" CreationDate="2014-12-13T09:35:35.947" Score="-2" ViewCount="357" Body="&lt;p&gt;When a spacecraft enters earth's atmosphere, it is totally destroyed and comes down as a single probe, so how can a spacecraft be reusable for new missions?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Soyuz spacecraft is launched on a Soyuz rocket, the most frequently used and most reliable launch vehicle in the world to date.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6900" LastEditorUserId="13208" LastEditDate="2017-02-27T21:47:17.267" LastActivityDate="2017-02-27T21:47:17.267" Title="How can spacecraft be reusable for new missions?" Tags="|rockets|reentry|reuse|soyuz-spacecraft|soyuz-launcher|" AnswerCount="1" CommentCount="1" ClosedDate="2014-12-23T11:21:08.697" ContentLicense="CC BY-SA 3.0" />
  <row Id="6379" PostTypeId="2" ParentId="6367" CreationDate="2014-12-13T10:49:20.520" Score="8" Body="&lt;p&gt;Tugging would require the satellites to be built for docking or berthing, which adds mass and costs. And preferably in a standardized way, which is difficult to establish in the beginning. The extra operations increases the risk. The delay of tugging can have very high opportunity costs, for example that a new communication service is delayed while competitors win market shares, which needs to be weighed against the saved launch costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I wonder if there are drawbacks with having a satellite built for GEO waiting in LEO. Such as space debris collision risk, short Earth communication windows, longer and more frequent periods without solar power and with it temperature variations. Radiation problems with slowly traversing the van Allen belts has already been mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there might be competition. The upper stage for the Falcon 9 &lt;a href=&quot;http://shitelonsays.com/transcript/elon-musk-at-mits-aeroastro-centennial-part-1-of-6-2014-10-24&quot; rel=&quot;nofollow&quot;&gt;will not be reused&lt;/a&gt; as in soft landed on Earth. But maybe it, or any upper stage, could be redesigned to be reused as a tug (once), coasting in the orbit of the payload it originally launched, with the rest of its fuel. If a suitable opportunity arises, it could tug a satellite from LEO to GEO, or do station keeping on a satellite in GEO, before it ends its life. I don't see how it would be profitable to refuel a tug, rather than launching a new tug together with that fuel. There seems to be no lack of used rocket engines in Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now and then a valuable payload like Fobos-Grunt doesn't reach high enough orbit and is doomed to soon burn up in the atmosphere. A flexible tug ready to assist could then perform a very much more valuable service than just saving launch fuel. If it is fast and flexible enough and able to interface with almost any cargo.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-12-15T08:14:17.590" LastActivityDate="2014-12-15T08:14:17.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6380" PostTypeId="2" ParentId="6378" CreationDate="2014-12-13T11:52:52.190" Score="7" Body="&lt;p&gt;That quote &quot;the most frequently used&quot; just means they built and launched lots of Soyuz rockets (more than 2000). None of them were reused.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reuse is still a new concept. The Space Shuttle was the first reusable launch system: the orbiter and the solid boosters were designed to land intact. As it turns out, it took a lot of work to refurbish them, so a Shuttle launch was still really expensive.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX have designed their Dragon capsule to be reusable. Now the Dragon consists of two parts: the crew module (which lands intact and can be reused) and the trunk (which cannot land and will burn up during reentry). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX are working on making the first stage reusable as well: the first stage is equipped with landing legs and it is meant to make a soft landing on a barge in the Atlantic ocean, or even fly back to the launch site and land there. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-12-13T11:52:52.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6381" PostTypeId="2" ParentId="6335" CreationDate="2014-12-13T13:26:55.893" Score="2" Body="&lt;p&gt;I finally found an online program to analyze the effects of the environment.&lt;/p&gt;&#xA;&lt;p&gt;It is &amp;quot;distributed&amp;quot; by ESA and to use it you need to register or ask for a student account.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;With SPENVIS, one can generate a spacecraft trajectory or a coordinate grid and then calculate:&lt;/p&gt;&#xA;&lt;p&gt;geomagnetic coordinates&lt;/p&gt;&#xA;&lt;p&gt;trapped proton and electron fluxes and solar proton fluences&lt;/p&gt;&#xA;&lt;p&gt;radiation doses (ionising and non-ionising) for simple geometries&lt;/p&gt;&#xA;&lt;p&gt;a sectoring analysis for dose calculations in more complex geometries&lt;/p&gt;&#xA;&lt;p&gt;damage equivalent fluences for Si, GaAs and multi-junction solar cells&lt;/p&gt;&#xA;&lt;p&gt;Geant4 Monte Carlo analysis for doses and pulse height rates in planar and spherical shields&lt;/p&gt;&#xA;&lt;p&gt;ion LET and flux spectra and single event upset rates&lt;/p&gt;&#xA;&lt;p&gt;trapped proton flux anisotropy&lt;/p&gt;&#xA;&lt;p&gt;atmospheric and ionospheric densities and temperatures&lt;/p&gt;&#xA;&lt;p&gt;atomic oxygen erosion depths&lt;/p&gt;&#xA;&lt;p&gt;Magnetic field line tracing is implemented, as well as the generation of world maps and altitude dependence plots of the magnetic field and the current models of the neutral atmosphere and the ionosphere.&lt;/p&gt;&#xA;&lt;p&gt;Models for spacecraft charging, both surface charging and internal charging, are available.&lt;/p&gt;&#xA;&lt;p&gt;A tool to visualise satellite data produces panel plots of measured quantities in combination with geomagnetic and solar indices.&lt;/p&gt;&#xA;&lt;p&gt;Micrometeoroid and space debris models are implemented, and an impact risk analysis module is currently under development.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.spenvis.oma.be/intro.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.spenvis.oma.be/intro.php&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6608" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-12-13T13:26:55.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6382" PostTypeId="1" CreationDate="2014-12-13T13:34:30.613" Score="8" ViewCount="543" Body="&lt;p&gt;With experience of the ISS and hopefully soon the Orion and SLS, it seems to me that a Lunar Orbital Station (LOS) could be the next step.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the main challenges of building and running a LOS compared to ISS? (radiation, eclipses, time delay, travel distance, gravity field...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could modules from the ISS be disconnected and tugged to Lunar orbit? Could the asteroid tug of the &lt;a href=&quot;http://www.lpi.usra.edu/sbag/meetings/jul2014/presentations/1030_Wed_Muirhead_ARM_OptionA.pdf&quot;&gt;ARRM&lt;/a&gt; be used to do it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of orbit would be suitable for a LOS, given the heterogenous Lunar gravity field and the desire for logistics with Earth and the Lunar surface (esp. poles and far side)? What are the pros and cons of a Lagrange point 60,000 km away from the Moon compared to a Lunar orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In what way would a Lunar orbital station be useful? Both before and after there is a Lunar lander available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roscosmos has a vague &lt;a href=&quot;http://www.russianspaceweb.com/los.html&quot;&gt;plan to build a LOS&lt;/a&gt; in the 2030's.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-07-21T19:48:32.487" LastActivityDate="2015-07-21T21:01:22.940" Title="What are the challenges and utility of a Lunar Orbital Station?" Tags="|iss|the-moon|orbit|space-station|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6383" PostTypeId="1" CreationDate="2014-12-13T14:05:24.497" Score="-2" ViewCount="872" Body="&lt;p&gt;How Soyuz spacecraft still in service, even though one of it module displayed at museum?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One of the soyuz module is displayed clearly at museum.&#xA;  &lt;img src=&quot;https://i.stack.imgur.com/3dgcN.jpg&quot; alt=&quot;Soyuz T-10 Module displayed at Musuem&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Info-box of Soyuz Spacecraft still showing in service. &#xA;  &lt;img src=&quot;https://i.stack.imgur.com/pAhj4.jpg&quot; alt=&quot;Info-box of Soyuz Spacecraft still showing in service.&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6900" LastEditorUserId="6900" LastEditDate="2014-12-13T14:10:47.530" LastActivityDate="2014-12-13T15:11:36.507" Title="How Soyuz spacecraft still in service, even though one of it module displayed at museum?" Tags="|spacecraft|soyuz-spacecraft|" AnswerCount="1" CommentCount="5" ClosedDate="2014-12-13T21:46:30.710" ContentLicense="CC BY-SA 3.0" />
  <row Id="6384" PostTypeId="2" ParentId="6383" CreationDate="2014-12-13T14:29:27.620" Score="5" Body="&lt;p&gt;Soyuz spacecraft are not reused. So at the end of the mission, the descent capsule is no longer useful. The Russians have decided not to try reusing them, they just build a new descent capsule for each mission.&lt;br&gt;&#xA;There have been hundreds of Soyuz missions, so there are hundreds of capsules going spare. Some of these end up in museums.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soyuz system is still in use. The capsule has been updated a few times, the current version is the TMA capsule. In 2014, there were &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_R-7_launches_%282010%E2%80%9314%29&quot; rel=&quot;nofollow&quot;&gt;4 manned Soyuz launches&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there is no contradiction between one capsule at a museum and the Soyuz still being listed as &quot;in service&quot;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-12-13T15:11:36.507" LastActivityDate="2014-12-13T15:11:36.507" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6385" PostTypeId="1" CreationDate="2014-12-13T16:22:53.543" Score="3" ViewCount="190" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;nofollow&quot;&gt;Dusk/dawn Sun-synchronous orbits&lt;/a&gt; are possible at Earth thanks to the fact that Earth is a bit flattened. Would it be possible with a similar orbit around the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flattening of the Moon seems to be only about 1/30 of that of Earth. But an advantage might be that it would have the same distance to Earth as the Moon has, so the Earth should not disturb the orbit of such a spacecraft as much as if it were in an equatorial orbit. Would this kind of orbit be more or less demanding than other orbits?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An orbit over the Lunar terminator would provide constant sunlight, cover the entire Lunar surface since it is a polar orbit, and the dusk/dawn phenomena where the solar wind is shut down and turned on might be scientifically interesting to study.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-13T16:22:53.543" Title="Is there a Lunar Sun-synchronous orbit over the terminator?" Tags="|orbital-mechanics|the-moon|sun-synchronous|" AnswerCount="0" CommentCount="1" ClosedDate="2018-01-28T13:28:57.170" ContentLicense="CC BY-SA 3.0" />
  <row Id="6386" PostTypeId="1" CreationDate="2014-12-13T17:35:34.240" Score="3" ViewCount="423" Body="&lt;p&gt;NASA is planning to bring an asteroid into a lunar orbit. As I understand, it's a tremendous task, even when using high specific impulse ion drives. It seems nearly impossible to safely land one on Earth without using aerobraking: the ion thrusters cannot be used anymore because of fast action required and therefor additional rocket engines with super reliability, high maneuverability, vast thrust power variability and huge amounts of fuel have to take over (seems pretty expensive and risky). Aerobraking would greatly reduce the total delta-V requirements, risk, price and landing complexity. But all the heat shields I've seen are robust, solid and far away from &quot;flexible&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any atmosphere entry protection capable cloths available? Would they tightly surround an asteroid without any holes and leaks under atmospheric pressure alone or some junctions before entry would be required? Could these cloths be strong enough to survive tearing force from the parachutes above or the parachutes should be anchored to the asteroid itself?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, I'm thinking about commercial (not scientific) purpose landings of super-valuable space rocks, therefor a &quot;safe landing&quot; can be declared when the asteroid touches the ground (not burns up or shatters above ground) and it's parts remain within a small radius (1km or mile). I mean, it can reach the ground at 100 m/s and leave a fair crater, but as long as we can collect all the valuable material (platinum, may it be) scattered in a reasonably sized area, it's okay. Maybe even the parachutes aren't a necessity then, if terminal velocity of an asteroid (now meteorite) is small enough for this task (is it?).&lt;/p&gt;&#xA;" OwnerDisplayName="user6738" LastActivityDate="2018-12-14T12:58:15.887" Title="Are there any techniques of heat shielding an irregularly shaped object?" Tags="|asteroid|reentry|mining|atmospheric-drag|heat-shield|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6387" PostTypeId="2" ParentId="6298" CreationDate="2014-12-13T17:37:42.143" Score="2" Body="&lt;p&gt;SpaceX likely recognises that they are in the space business, not the marine business, and is focussing on core competencies. I understand that you are asking specifically about monetary cost, for which I have no answer. However, the idea that SpaceX would recover their own capsules has a very high opportunity and time cost as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A ship owned by SpaceX used for recovery would be idle for 50 weeks out of the year currently. During that time, SpaceX would have to pay for maintenance of craft and crew. And that crew would be sorely lacking in real world experience, even if they kept doing practice drills for the 50 weeks of idle time. Additionally, SpaceX would have to manage a marine services division complete with human resource structure. It would have to develop hiring and training materials, pensions, and account for expected promotion and mobility within the company, at the expense of opportunities for the engineers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see a lot of &quot;the Unix philosophy&quot; at SpaceX (do one thing, and do it well). Not recovering their own capsules is an obvious extension of that.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2014-12-13T17:37:42.143" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6388" PostTypeId="1" AcceptedAnswerId="6389" CreationDate="2014-12-13T19:42:33.753" Score="9" ViewCount="821" Body="&lt;p&gt;One of the &lt;a href=&quot;https://space.stackexchange.com/questions/2869/how-much-does-it-cost-to-operate-a-satellite-in-orbit&quot;&gt;operational costs for a satellite in orbit&lt;/a&gt; is maintaining a receiver ground station. Would it be feasible to send an Iridium transceiver along with my satellite, and then operate it from my personal computer via the Internet?&lt;/p&gt;&#xA;" OwnerUserId="759" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-12-14T00:16:39.100" Title="Can I use Iridium as a receiver network for my satellite?" Tags="|communication|communication-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6389" PostTypeId="2" ParentId="6388" CreationDate="2014-12-13T20:44:52.490" Score="9" Body="&lt;p&gt;Yes, if your satellite will orbit in Low Earth Orbit (LEO) below Iridium Constellation's orbital altitude (about 780 km), then this could work as has been experimentally demonstrated by the &lt;a href=&quot;http://www.nasa.gov/directorates/spacetech/small_spacecraft/phonesat.html&quot;&gt;“Bell” PhoneSat 1.0&lt;/a&gt; launched in April, 2013. But you better complete all the paperwork before the launch not to share the same fate as the &lt;a href=&quot;http://amsat-uk.org/2012/02/24/techedsat-to-use-satphone/&quot;&gt;TechEdSat&lt;/a&gt; that was supposed to also use Iridium frequencies but its operators had to disable that module and only keep the Orbcomm SatPhone module because they didn't get both its frequency ranges approved by FCC in time. From the latter linked to source:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“We were forced to disable the Iridium modem as our FCC license did&#xA;  not come in time. As usual, building the satellite is the easy part.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-12-13T20:44:52.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6391" PostTypeId="1" CreationDate="2014-12-13T22:33:58.233" Score="16" ViewCount="2096" Body="&lt;p&gt;Is there any assessment of the annual cost of orbital debris for satellite operators?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am interested in all kind causes of costs, such as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;damages to satellites in orbit&lt;/li&gt;&#xA;&lt;li&gt;human-hours spent on investigating orbital debris mitigation and prevention schemes&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I am aware that the question is broad but I would like to get a sense of the total cost and its repartition amongst different causes and satellite operators.&lt;/p&gt;&#xA;" OwnerUserId="1111" LastEditorUserId="25" LastEditDate="2014-12-16T03:01:02.497" LastActivityDate="2015-05-14T13:41:17.690" Title="How much do orbital debris cost annually to satellite operators?" Tags="|artificial-satellite|debris|cost|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6392" PostTypeId="2" ParentId="6391" CreationDate="2014-12-13T23:50:24.667" Score="15" Body="&lt;p&gt;The number of actual collisions with orbital debris is incredibly low, all things considered. (Number of actual debilitating impacts vs number of active satellites divided by time. Odds are very very low).  Sure flecks of paint have hit the Shuttle and ISS, but really a very small number of actual collisions causing damage, to a fairly large number of satellites, over many decades of operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To date, there have been 4 confirmed collisions between tracked objects:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1991-12-23 (discovered in 2005): COSMOS 1934 (18985) vs. COSMOS 1275 DEB (13475)&lt;/li&gt;&#xA;&lt;li&gt;1996-07-24: CERISE (23606) vs. ARIANE 1 DEB (18208), Cerise was recovered and continued operations&lt;/li&gt;&#xA;&lt;li&gt;2005-01-17: THOR BURNER 2A R/B (7219) vs. CZ-4 DEB (26207)&lt;/li&gt;&#xA;&lt;li&gt;2009-02-10: Iridium 33 (24946) vs. COSMOS 2251 (22675)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are other likely collisions, such as BLITS in January 2013, however the secondary in that case is likely to have been an uncataloged object.  Several other satellite failures may be due to collisions with untracked/untrackable debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A real cost that comes up is the use of fuel to change orbit to avoid feared collisions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a satellite this shortens the useful lifespan, a direct cost. For the ISS this requires fuel that is brought up by Progress or ATV. It might even be possible to put an actual cost to that. But would be hard, need lots and lots of data to do it meaningfully, most of which would be considered proprietary.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="4709" LastEditDate="2014-12-16T19:47:15.443" LastActivityDate="2014-12-16T19:47:15.443" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6393" PostTypeId="2" ParentId="5812" CreationDate="2014-12-14T03:44:54.453" Score="11" Body="&lt;p&gt;Another huge disadvantage to 'building' something vs. assembling something, is working in a spacesuit.  Tool use and everything else is much, much, much harder than in a shirtsleeve environment (just the amount of effort you need to do to close your hands while working through 5 pairs of gloves that're pressurized, while hauling around the weight of micro-heaters on each finger).  Just suiting up and unsuiting eats a fair amount into each work day.  Once you start working longer hours to try and make up for that, you start making errors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Diagram of spacesuit glove with layers:&lt;br&gt;&#xA;&lt;img src=&quot;https://msis.jsc.nasa.gov/images/Section14/Image362.gif&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/9217-spacesuit-gloves-astronauts-fingernails-fall.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Using spacesuit gloves can cause your fingernails to fall off&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/hubble/servicing/series/cats.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Working on the Hubble&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which talks about the problems of merely unscrewing(!!) a connector (makes micro metal debris which can short out / impair your electronics).  On Earth; gravity, wind and vacuum can all be used to get rid of that debris and make it into dust which just 'magically' goes 'away'.&lt;/p&gt;&#xA;" OwnerUserId="6893" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.527" LastActivityDate="2014-12-14T23:03:52.880" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6394" PostTypeId="1" AcceptedAnswerId="6398" CreationDate="2014-12-14T12:55:20.253" Score="9" ViewCount="764" Body="&lt;p&gt;&lt;a href=&quot;http://www.bigelowaerospace.com/&quot; rel=&quot;nofollow&quot;&gt;Bigelow Aerospace&lt;/a&gt; develops expandable (inflatable/foldable) space stations. Their flagship is their cylindrical &lt;a href=&quot;http://www.bigelowaerospace.com/ba330.php&quot; rel=&quot;nofollow&quot;&gt;BA330&lt;/a&gt; which has the dimensions 13.7 by 6.7 meter when expanded. Current ISS modules seem to have a diameter of less than 5 meter. But this is not far from the dimensions of fairings of &lt;a href=&quot;http://www.ulalaunch.com/products_atlasv.aspx&quot; rel=&quot;nofollow&quot;&gt;Atlas V&lt;/a&gt;, &lt;a href=&quot;http://www.boeing.com/boeing/defense-space/space/delta/delta4/delta4.page&quot; rel=&quot;nofollow&quot;&gt;Delta IV Heavy&lt;/a&gt;, &lt;a href=&quot;http://www.khrunichev.ru/main.php?id=42&amp;amp;lang=en&quot; rel=&quot;nofollow&quot;&gt;Proton&lt;/a&gt; and &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Launch_vehicles/Ariane_5_ECA2&quot; rel=&quot;nofollow&quot;&gt;Ariane 5&lt;/a&gt;. They have 5.0-5.4 meter fairing diameter. The mass of the BA330 (20-23 tons) is on the limit of what those rockets can launch to LEO. &lt;a href=&quot;http://www.nasaspaceflight.com/2012/11/nasa-payload-fairings-options-multi-mission-sls-capability/&quot; rel=&quot;nofollow&quot;&gt;SLS&lt;/a&gt; might get fairings with up to 10 meter diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why go through the extra trouble of making their space station expandable? Is the extra 1.3-1.7 meter diameter so important? Wouldn't a static structure, maybe made by the same materials, be much simpler and cheaper to develop and manufacture, lighter and less risky to deploy in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the BA330 is primarily a technology demonstration, what would be the use of space station modules which have larger diameter than the payload fairings of today's and near-future launchers?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="12865" LastEditDate="2016-07-24T11:18:02.767" LastActivityDate="2019-01-10T08:29:29.870" Title="Why is Bigelow BA330 expandable instead of fitting fairings?" Tags="|space-station|bigelow|expandable|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6398" PostTypeId="2" ParentId="6394" CreationDate="2014-12-14T22:20:13.410" Score="22" Body="&lt;p&gt;That 5.0 meters is the outer diameter of the payload fairing. Payloads obviously cannot be that large. In fact, they are constrained to be a bit smaller. The figures below depicts the Delta IV Heavy and Atlas V static volume (volume that a payload can occupy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www-eng.lbl.gov/~lafever/SNAP/OldFiles/DELTA%20PAYLOAD%20OVERVIEW_files/d4hfairings.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www-eng.lbl.gov/~lafever/SNAP/OldFiles/DELTA%20PAYLOAD%20OVERVIEW_files/d4hfairings.gif&lt;/a&gt;&lt;br&gt;&#xA;&lt;sup&gt;Image source: &lt;a href=&quot;http://www-eng.lbl.gov/~lafever/SNAP/OldFiles/DELTA%20PAYLOAD%20OVERVIEW.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www-eng.lbl.gov/~lafever/SNAP/OldFiles/DELTA%20PAYLOAD%20OVERVIEW.htm&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bU1LC.jpg&quot; alt=&quot;&quot;&gt;&lt;br&gt;&#xA;&lt;sup&gt;Image source: &lt;a href=&quot;http://www.ulalaunch.com/products_atlasv.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ulalaunch.com/products_atlasv.aspx&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the Delta IV Heavy and Atlas V both constrain payloads to 4.57 meters in diameter. One would think that the Ariane 5 with its 5.4 meter fairing would accommodate a wider payload. This is not the case. The Ariane 5 accommodates the same size payload as does a Delta IV Heavy or Atlas V. A Proton M is in the same class.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That 4.57 meters includes space for the un-deployed solar arrays and for vibrations. One would be hard pressed to launch a standalone rigid structure much bigger than the 4.2 meter diameter &lt;a href=&quot;http://en.wikipedia.org/wiki/Destiny_(ISS_module)&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Destiny module&lt;/a&gt; on an existing launch vehicle. Yes, we can wait (and wait and wait) for the Senate Launch System to come on line, but it's going to be a long wait.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That increase from 4.2 meters to 6.7 meters in diameter alone is quite significant. That's a 60% increase in diameter, or a 154% increase in cross section area. The volume increases even more with inflatables thanks to length. The BA330 will have over three times the pressurized volume of the Destiny module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Section 6, &quot;Payload Fairings,&quot; of the &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Launch_Vehicles/Delta_IV_Users_Guide_June_2013.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta IV Users Guide&lt;/a&gt; describes the constraints on payload size on a Delta IV.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Section 6, &quot;Payload Fairings,&quot; of the &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/AtlasVUsersGuide2010.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas V Launch Services User's Guide&lt;/a&gt; describes the constraints on payload size on an Atlas V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Annex 5, &quot;Usable volume under fairing and SYLDA 5,&quot; of the &lt;a href=&quot;http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5 User's Manual&lt;/a&gt; describes the constraints on payload size on an Ariane 5.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="10273" LastEditDate="2019-01-10T08:29:29.870" LastActivityDate="2019-01-10T08:29:29.870" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="6399" PostTypeId="1" AcceptedAnswerId="6409" CreationDate="2014-12-14T22:47:11.897" Score="7" ViewCount="141" Body="&lt;p&gt;The majority of Earth Observation satellites, whether for meteorology or land analysis, downlink their data in the X-Band to either government-run or privately-owned ground stations. In Europe, the company Tesat has developed laser terminals that have been successfully tested in orbit, from one EO satellite to one geostationary communications satellite, with TerraSAR-X and recently with Sentinel-1A. Apart from the obvious - all types of lasers interact with the atmosphere (absorption, attenuation and scattering) - and the pointing accuracy required, what would be the main technological challenges in using lasers for direct downlinks to the ground?&lt;/p&gt;&#xA;" OwnerUserId="6897" LastEditorUserId="49" LastEditDate="2014-12-14T23:17:32.733" LastActivityDate="2014-12-15T08:44:04.590" Title="Technological challenges in substituting EO Satellites X-band downlinks with laser downlinks?" Tags="|low-earth-orbit|data-transmission|laser|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6400" PostTypeId="1" CreationDate="2014-12-15T00:15:35.667" Score="7" ViewCount="2730" Body="&lt;p&gt;For the kinds of tanks used in upper stages, what is the ratio of the weight of the tank to the weight of the hydrogen? Are larger tanks more efficient?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is for considering transport of hydrogen to be used as fuel in combination with local oxygen on the moon - so, quite hypothetical but I want to postulate such a system as accurately as I can. I presume the kinds of tanks used in for instance the Centaur would be the thing to use?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-12-15T01:10:21.337" Title="How much does a hydrogen tank weigh compared to the hydrogen in it?" Tags="|fuel-depots|hydrogen|fuel-system|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6401" PostTypeId="2" ParentId="6400" CreationDate="2014-12-15T00:38:48.400" Score="6" Body="&lt;p&gt;Even with hydrogen, the tank weight is quite small compared to the content weight. For some examples, you can look here (you may need to do some math to get the actual numbers):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750004950.pdf&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750004950.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are right that the heat loss is less of a problem for larger tanks, but structural integrity, especially when facing the aerodynamic forces of launch, becomes a larger problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Transporting liquid hydrogen to the moon is probably not a good option. Apart from the high tank volume and weight (compared to other fluids) it also boils away very quickly under thermal radiation. On a path to the moon, the spaceship has to coast for several days, all the while losing fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This usually works the following way: The tank is kept at a certain pressure. If this pressure is exceeded, a valve opens to vent some gas. This way the liquid is constantly boiling: If a bit of heat enters the tank, a bit of hydrogen evaporates and escapes through the valve. The amount of evaporated hydrogen is the entered heat over the specific evaporation heat of hydrogen at its current pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say we set the valve to a higher pressure. Then the hydrogen would keep boiling, and thereby pressurizing the tank, until that higher pressure is reached and the valve opens again. Now we have a bit higher temperature on the tank because the boiling temperature rises with pressure (up to a certain point). Therefore there is less of a temperature difference between the cold hydrogen inside and the solar radiation on the outside and less heat will enter the tank. However, we need to build a thicker tank, and this is generally not worth it. As seen in the document linked above, hydrogen tanks are designed to operate at a modest pressure level of a few bar. (and even this pressure is not because of the temperature in the tank, but because of the pressure requirement of the pump inlet)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The general idea to bring fuel to the moon may be viable, although a lot of energy would be needed to generate sufficient amounts of oxygen for propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is unless there is abundant water on the moon, in which case generating fuel and oxidizer would be simpler, but that is still under discussion.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2014-12-15T01:10:21.337" LastActivityDate="2014-12-15T01:10:21.337" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6402" PostTypeId="2" ParentId="6347" CreationDate="2014-12-15T00:55:14.763" Score="2" Body="&lt;p&gt;More likely the Dragon interface was influenced by the Tesla car interface. The Tesla Model S has a 17 inch touch screen that contains almost all of the cars controls. It is designed to be used while driving, with minor accelerations, compared to space launch. But this is much more concern than the common touch interface of a phone or tablet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Orion team and the SpaceX team have been nicely isolated and unlikely to have influenced each other. But they both looked at the state of the art today, looked at their options and made decisions.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-15T00:55:14.763" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6403" PostTypeId="1" AcceptedAnswerId="15214" CreationDate="2014-12-15T00:57:54.970" Score="13" ViewCount="4186" Body="&lt;p&gt;The ASDS barge is a large flat surface. A large tall skinny structure will land on it. This is not likely to be very stable on its own. The center of mass is pretty low, since the empty stage tanks are light and fluffy, with all the dense mass of the engines at the bottom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The legs have a 60+ foot span, so it has a pretty good base.  But you probably do not want it to slide around either as the sea conditions change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How will SpaceX secure it for transport back to shore?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-12-15T01:03:10.433" LastActivityDate="2016-05-10T18:46:58.403" Title="How will SpaceX tie down a landed stage on the ASDS barge after landing?" Tags="|spacex|landing|" AnswerCount="6" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6404" PostTypeId="1" AcceptedAnswerId="6410" CreationDate="2014-12-15T01:46:14.250" Score="12" ViewCount="904" Body="&lt;p&gt;I was watching &lt;a href=&quot;https://www.youtube.com/watch?v=M2_NeFbFcSw&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; and I realized that all the instruments on the Soyuz spacecraft is in Russian. So how can the non-Russian crew use those instruments? Do they also study Russian before the launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/M2_NeFbFcSw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6938" LastEditorUserId="49" LastEditDate="2015-06-02T00:43:02.993" LastActivityDate="2015-06-02T00:43:02.993" Title="Do the ISS crew also study Russian language?" Tags="|iss|soyuz-spacecraft|language|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6405" PostTypeId="2" ParentId="6403" CreationDate="2014-12-15T03:41:24.227" Score="10" Body="&lt;p&gt;My understanding (sorry, I have no citations yet) is that a lot of the same technologies used to position and control drillships is being used to position SpaceX's barge.  Drillships use dynamic positioning which handle translation and rotation of the barge in the plane of the surface.  I believe heave, sway, and surge are being handled by articulation of the landing surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of this, as well as launch commit criteria, is adequate for the period of time required to get the vehicle lashed down I assume.  I suspect that the launch commit criteria is the primary way they are dealing with the sea state issue.  Remember, SpaceX isn't planning on landing on a barge forever -- this is just being used until they can cajole the range safety folks to allow a land landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless of the outcome, the attempted landing will be quite a show...&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2014-12-15T03:59:10.990" LastActivityDate="2014-12-15T03:59:10.990" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6406" PostTypeId="1" CreationDate="2014-12-13T17:39:22.690" Score="4" ViewCount="1017" Body="&lt;p&gt;Will we be able to take the amount of water that will be needed for this 18 month trip, or will they have to recycle the water that they take with them? I imagine that this will be quite a challenge because water is very heavy?&lt;/p&gt;&#xA;" OwnerUserId="44083" OwnerDisplayName="Peter U" LastEditorUserId="49" LastEditDate="2014-12-15T16:48:33.250" LastActivityDate="2014-12-15T16:48:33.250" Title="Water needs for a manned journey to Mars" Tags="|mars|water|long-duration|crewed-spaceflight|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6407" PostTypeId="2" ParentId="6406" CreationDate="2014-12-13T19:51:37.127" Score="5" Body="&lt;p&gt;It's likely that a Mars mission will take a total of 32 months, not 18; the launch window for a fuel-efficient return journey requires a wait of over a year after arrival.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Humans need 2-4 liters of water per day. Assume a crew of 5; that works out to about 10-20 tons of water if you don't recycle it. That's a significant chunk of the overall mass requirement of the mission (NASA thinks the crew habitat and command module for a Mars mission will mass about 50 tons), so it's obviously attractive to consider a small water supply and a good recycling system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, one of the major concerns for a manned Mars mission is radiation exposure over the journey, and water makes a decent radiation shield. If you have to carry 20 tons of shielding anyway, you could do worse than carrying it in the form of water. &lt;/p&gt;&#xA;" OwnerUserId="195" OwnerDisplayName="Russell Borogove" LastActivityDate="2014-12-13T19:51:37.127" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6408" PostTypeId="2" ParentId="6367" CreationDate="2014-12-15T06:00:32.290" Score="2" Body="&lt;p&gt;The Ion Thruster is still a very low-thrust engine which would need many of them to be used as a tug. The Thrust-to-Weight ratio of an ion engine is too low to be used in LEO effectively, so it would be too power consuming to power the Ion Thruster. Even if a particular spacecraft had the power sources available, the amount of maintenance needed to use the power would be so extreme that a craft may need a crew member or two made to just EVA everyday to repair the power source. (This is for solar) Also, if a space craft used RTGs (Radioisotope Thermoelectric Generators) like the ones Voyager and Dawn used, it would be too expensive and dangerous for the craft as the intense radiation could damage the electronics and give the crew fatal doses of radiation. That is why the Ion Thruster is only used on light weight and unmanned probe craft.&lt;/p&gt;&#xA;" OwnerUserId="6881" LastActivityDate="2014-12-15T06:00:32.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6409" PostTypeId="2" ParentId="6399" CreationDate="2014-12-15T08:34:41.580" Score="6" Body="&lt;p&gt;The technical challenges aren't the main barrier; several demonstration missions have demonstrated laser downlink to the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main obstacles are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Operational: Clouds necessitate ground stations on mountain tops and in deserts, which generally means expensive + poor backhaul for the data as well as making it difficult to achieve 99% uptime without many spare stations all over the world&lt;/li&gt;&#xA;&lt;li&gt;Regulatory: All currently planned systems rely on a powerful laser uplink &quot;beacon&quot; for the satellite to lock onto; that can be a hazard for aircraft and potentially to other satellites, requiring permission to be obtained days in advance of each comm session from the &lt;a href=&quot;http://www.aps.org/publications/apsnews/200910/optics.cfm&quot;&gt;Laser Clearing House&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Necessity: X-band works pretty well!&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2014-12-15T08:44:04.590" LastActivityDate="2014-12-15T08:44:04.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6410" PostTypeId="2" ParentId="6404" CreationDate="2014-12-15T09:49:40.243" Score="9" Body="&lt;p&gt;They all need to study English/Russian as part of their basic training, and have some basic technical grasp of the language to suffice for communicating with crew-mates and handling some equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been cases though, where cosmonauts, mostly, had poor (or under the 'desired' level) English language skills. In these cases, this has been amended by having another cosmonaut in the team with good English language skills, or having most of the astronauts speak basic/good Russian.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I remember the case with one of the last space shuttle missions to Mir station before it was decommissioned, where a cosmonaut (can't remember his name right now), could barely speak English, and they bypassed this by having the rest of the crew communicate with him in a mix of basic Russian and hand signs, and he also had another cosmonaut to rely on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;His very presence on this mission was very polemic, but this is aside from your question, as he kind of 'forced' himself into the mission by some political leverage. He was one of the chief designers of Mir and wanted to see it once before it being de-orbited.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="3001" LastActivityDate="2014-12-15T09:49:40.243" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6411" PostTypeId="1" AcceptedAnswerId="6416" CreationDate="2014-12-15T10:00:40.687" Score="6" ViewCount="1926" Body="&lt;p&gt;What are the calculations involved in choosing the thickness of a micrometeoroid shield during the tradeoff study phase?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where can I find publicly available data on the distribution of micrometeoroid hazard by projectile energy (and possibly, size) for Earth, Moon, Mars orbits and interplanetary space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do a spacecraft's attitude and attitude rates matter for real-life mission analysis (say, constantly Sun- or nadir-facing, &quot;barbecue roll&quot; etc.) (thus presenting different cross-sections to different sky segments)?&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="49" LastEditDate="2014-12-15T15:13:42.827" LastActivityDate="2015-08-06T00:54:52.143" Title="Choosing thickness of a Whipple shield" Tags="|debris|design|environment|shielding|meteoroid|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6412" PostTypeId="1" CreationDate="2014-12-15T10:53:30.700" Score="6" ViewCount="329" Body="&lt;p&gt;NASA's human space flight program plans to within 5 years or so have access to Orion and a launcher for it. Without a Lunar or Martian lander, the targets for Orion missions are limited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But given that an ISS derived interplanetary habitation module (instead of a lunar lander) is developed next to take care of the 6-8½ month travel time to Mars capture orbit, could the Orion then be used to transport astronauts to the surface of the Martian moons Phobos and Deimos? According to the planned ARM mission, Orion will approach a small asteroid and do an EVA at it. Is it suitable to approach the surface of a Martian moon with the Orion spacecraft? Could it even soft land on its service module in that low surface gravity (0.0006g and 0.0003g)? Orion is designed to manage one EVA and then restore the atmosphere once. But with a mothership habitation module nearby, they might maybe do two EVA's and return to it suited up in an Orion empty of air.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-12-15T13:09:45.960" LastActivityDate="2014-12-15T15:19:18.723" Title="Could Orion be used for human exploration of Phobos and Deimos?" Tags="|eva|orion-spacecraft|crewed-spaceflight|phobos|deimos|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6413" PostTypeId="5" CreationDate="2014-12-15T11:12:00.347" Score="0" Body="&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; For mission planning use &lt;a href=&quot;/questions/tagged/mission-design&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;mission-design&amp;#39;&quot; rel=&quot;tag&quot;&gt;mission-design&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question is particularly appropriate for questions relating to the various fields of &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerospace_engineering&quot; rel=&quot;nofollow&quot;&gt;Aerospace Engineering&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="10132" LastEditDate="2015-07-30T05:36:49.793" LastActivityDate="2015-07-30T05:36:49.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6414" PostTypeId="4" CreationDate="2014-12-15T11:12:00.347" Score="0" Body="Questions about how space vehicles and related hardware are designed. See also [structural-design] and [engine-design]." OwnerUserId="25" LastEditorUserId="58" LastEditDate="2019-09-19T15:22:44.543" LastActivityDate="2019-09-19T15:22:44.543" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="6415" PostTypeId="2" ParentId="6412" CreationDate="2014-12-15T11:59:45.817" Score="5" Body="&lt;p&gt;Orion itself is meant to be the launch and reentry vehicle for crew on the mission. For pretty much everything else it is not that useful. It has a slightly longer than short term duration life support system.  21 days is long enough for lunar missions, but not long enough for much else. Not enough room for much longer anyway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The expectation is some kind of hab module for transit.  Some kind of lander for landing.  Some kind of ascent vehicle for getting back off the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion itself has next to no useful provisions beyond that. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2014-12-15T15:19:18.723" LastActivityDate="2014-12-15T15:19:18.723" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6416" PostTypeId="2" ParentId="6411" CreationDate="2014-12-15T13:18:37.483" Score="8" Body="&lt;p&gt;This is an incredibly complex problem, so the only way I can think of usefully answering your question is by referring you to NASA's &lt;a href=&quot;http://www.techbriefs.com/component/content/article/6-ntb/tech-briefs/software/17221&quot; rel=&quot;nofollow noreferrer&quot;&gt;Micrometeoroid and Orbital Debris (MMOD) Shield Ballistic Limit Analysis Program&lt;/a&gt; and &lt;a href=&quot;http://ston.jsc.nasa.gov/collections/TRS/_techrep/TM-2009-214789.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;its documentation&lt;/a&gt; (PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This software complements the NASA's &lt;a href=&quot;http://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-23774-1-bumper.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;BUMPER-II&lt;/a&gt; risk assessment software package that was used for the Orion project (&lt;a href=&quot;http://www.nasa.gov/offices/nesc/home/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NESC&lt;/a&gt; - NASA Engineering and Safety Center - wasn't happy with Orion MMOD Risk Assessment and produced their own &lt;a href=&quot;http://www.nasa.gov/offices/nesc/home/Feature_Orbital_Debris_Risk.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;independent review&lt;/a&gt;) and supports analysis of various &lt;a href=&quot;https://en.wikipedia.org/wiki/Whipple_shield&quot; rel=&quot;nofollow noreferrer&quot;&gt;Whipple shield&lt;/a&gt; materials and configurations, from metallic dual plate to stuffed Whipple and honeycomb triple walls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some possible shielding configurations supported in the MMOD Shield Ballistic Limit Analysis Program:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9N0T5m.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/jcyZkm.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/dFPGcm.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/U38rpm.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/6wwZCm.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/VXJUgm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of its &lt;a href=&quot;http://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-24582-1-mmod.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;quoted applications and benefits&lt;/a&gt; include:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Benefits&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Effective: Allows for simple application of the MMOD shield design and analysis equations without requiring expert knowledge in the field&lt;/li&gt;&#xA;  &lt;li&gt;Efficient: Enables simple evaluation of competing shielding concepts in the preliminary design phase &lt;/li&gt;&#xA;  &lt;li&gt;Reliable: Provides consistency among users in the selection and application of relevant damage equations&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Applications&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Preliminary design and risk assessment for MMOD analysis&lt;/li&gt;&#xA;  &lt;li&gt;Spacecraft and satellite design&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;MMOD Shield Ballistic Limit Analysis Program's underlying data relies on the &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/model/engrmodel.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ORDEM2000&lt;/a&gt; (Orbital Debris Engineering Models) modelling debris environment of objects between 10 µm to 1 m in size, and authors produced own ballistic limit curves using NASA's &lt;a href=&quot;http://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-23774-1-bumper.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;BUMPER-II&lt;/a&gt; and CSI's &lt;a href=&quot;http://www.csiamerica.com/products/sap2000&quot; rel=&quot;nofollow noreferrer&quot;&gt;SAP2000&lt;/a&gt; Ballistic Limit Analysis Program for various configurations and materials. See &lt;a href=&quot;http://ston.jsc.nasa.gov/collections/TRS/_techrep/TM-2009-214789.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Page 44 &lt;em&gt;Appendix: Validation of Program Output&lt;/em&gt; in NASA/TM–2009–214789&lt;/a&gt; (PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example ballistic limit curves (representative titanium single-wall MMOD shield calculated using the published BLE and the Ballistic Limit Analysis Program):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gLd5s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All images courtesy of NASA / Johnson Space Center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sadly, the licensing for this software, made available through &lt;a href=&quot;http://www.nasa.gov/centers/johnson/techtransfer/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JSC's Technology Transfer and Commercialization Office&lt;/a&gt;, limits its use to U.S. persons only. &lt;sup&gt;*&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;*&lt;/sup&gt; For those from ESA member states that can't get access to ORDEM, I'd suggest ESA's &lt;a href=&quot;https://sdup.esoc.esa.int/web/csdtf/home&quot; rel=&quot;nofollow noreferrer&quot;&gt;MASTER&lt;/a&gt; (Meteoroid and Space Debris Terrestrial Environment Reference, reference model for space debris and meteoroid environment, suitable for evaluation of incident flux due to the particulate environment in Earth orbits), coupled with DRAMA (Debris Risk Assessment and Mitigation Analysis) and DISCOS (Database and Information System Characterising Objects in Space), all of which can be obtained via their &lt;a href=&quot;https://sdup.esoc.esa.int/group/csdtf/home&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Debris User Portal&lt;/a&gt; (SDUP).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-08-06T00:54:52.143" LastActivityDate="2015-08-06T00:54:52.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6417" PostTypeId="2" ParentId="2017" CreationDate="2014-12-15T14:15:21.450" Score="18" Body="&lt;p&gt;What you see is the first Mach disk (a standing shock wave), which causes a sudden increase in temperature, pressure, and density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the nozzle exit, the exhaust gases have a comparably low temperature due to the high expansion ratio. In case of your SSME, the temperature is about 1200 K. The hydrogen-oxygen flame plume does not soot, and the water vapor or the other combustion products are not hot enough to radiate in the visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over the shock, the temperature jumps above 3000 K, which causes thermal excitation of the water vapor. You can sometimes see this as a faint reddish glow. The origin of the dominant broad-band blue radiation from hydrogen flames is actually unclear. Most likely, it is a chemiluminescence recombination of radicals present in the plume. This is suddenly enhanced over the shock because of the high density and temperature, causing a great number of molecular collisions.&lt;/p&gt;&#xA;" OwnerUserId="6939" LastActivityDate="2014-12-15T14:15:21.450" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="6418" PostTypeId="1" AcceptedAnswerId="6419" CreationDate="2014-12-15T17:14:51.307" Score="5" ViewCount="212" Body="&lt;p&gt;All known bodies of the space rotate around their axis. For example, Earth completes a full rotation cycle in about 24 hours. However, the axial tilt seems to be constant, even though it changes a little bit over a long (in comparison with rotational period) time.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/SXQbs.png&quot; alt=&quot;Predicted variation of Earth&amp;#39;s axial tilt in next million years&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What causes the tilt to stop getting smaller or bigger after some limit?&lt;br/&gt;&#xA;Why is the axial tilt variation so slow compared to the rotation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One article regarding the 177.3° axial tilt of Venus suggests that &quot;it’s possible that Venus was knocked upside down by a massive impact early in its history&quot;. Let's assume that the impact happened. What caused Venus to stop after 177.3°?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why doesn't our South pole appear in the equator after some time, and after some more become the North pole?&lt;/p&gt;&#xA;" OwnerDisplayName="user6738" LastEditorUserId="49" LastEditDate="2014-12-15T19:01:16.370" LastActivityDate="2014-12-15T19:01:16.370" Title="What stabilises the axial tilt of planets?" Tags="|planetary-science|rotation|celestial-mechanics|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6419" PostTypeId="2" ParentId="6418" CreationDate="2014-12-15T17:36:46.637" Score="4" Body="&lt;p&gt;Think of the earth as a 12,800-km diameter gyroscope.  It takes an enormous amount of energy to change the axis of rotation.  The blips that are there are due to gyroscopic precession, primarily due to gravity gradient torques from the sun and the moon.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2014-12-15T17:36:46.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6420" PostTypeId="1" AcceptedAnswerId="6421" CreationDate="2014-12-15T19:28:08.600" Score="38" ViewCount="5881" Body="&lt;p&gt;The Moon has about 1/6th of Earth's gravity, which (it is said) would perhaps lead to muscle degeneration for humans living there long-term.  However, what if those humans wore bodysuits weighted to increase their weight 6 times?  Would that make walking (not jumping) on the Moon similar to walking on Earth?  Would it exercise the muscles as much, or would the reduced Moon gravity still mean that muscles didn't get the right amount of exercise?&lt;/p&gt;&#xA;" OwnerUserId="6942" LastActivityDate="2014-12-16T20:50:11.370" Title="Could weights make walking on the Moon feel like walking on Earth?" Tags="|the-moon|gravity|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6421" PostTypeId="2" ParentId="6420" CreationDate="2014-12-15T19:54:14.207" Score="43" Body="&lt;p&gt;It's complicated! Keep in mind the distinction between &lt;em&gt;weight&lt;/em&gt; and &lt;em&gt;mass&lt;/em&gt;. On the moon, weight is 1/6 what it is on Earth, but mass is the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you hold your arm straight out, you have to exert force equivalent to its weight to hold it in place. So on the moon, that force is much reduced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you move your arm, you have to exert force proportional to its mass to accelerate it, then more force proportional to its mass to stop it. On the moon, that force is unchanged!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So consider adding 5 times your mass in a bodysuit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would make &quot;just standing around&quot; on the moon feel somewhat more like it does on Earth, because the weight supported by your skeleton and musculature would be your Earth-standard weight, but it would make moving vastly, vastly harder, because you'd have to accelerate and decelerate 6 times your normal mass whenever you moved a limb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to get an equivalent amount of muscular exercise, you'd need somewhere between 0 and 5 times your weight in that bodysuit, depending on how active you were. The weight distribution likely wouldn't match your natural weight distribution, either, so moving would definitely be awkward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One other thing such a body suit might be useful for would be to slightly compress you to increase your blood pressure, in order to force your heart to work a little harder to circulate blood. Otherwise, your heart musculature would likely atrophy over long periods of time.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-15T21:47:43.027" LastActivityDate="2014-12-15T21:47:43.027" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6423" PostTypeId="2" ParentId="2097" CreationDate="2014-12-15T23:52:59.910" Score="9" Body="&lt;p&gt;Performing the experiment on the Moon would make it more accurate.  The lack of atmosphere means there's no drag on the falling objects to interfere with the measurements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Galileo was smart/lucky enough to pick two balls which would minimize the effect of then very poorly understood drag.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo%27s_Leaning_Tower_of_Pisa_experiment&quot; rel=&quot;nofollow noreferrer&quot;&gt;There was a small discrepancy due to drag when the two landed&lt;/a&gt;, but Aristotle had stated things fall with a velocity proportional to their weight, so this discrepancy should have been huge.  Had Galileo picked something else with more drag, the discrepancy might have been larger and the march of science might have been set back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=E43-CfukEgs&quot; rel=&quot;nofollow noreferrer&quot;&gt;We have vacuum chambers on Earth to repeat the experiment.  Feathers and bowling balls are used to underscore the point&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="4704" LastEditorUserId="4704" LastEditDate="2019-07-31T23:05:11.500" LastActivityDate="2019-07-31T23:05:11.500" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="6424" PostTypeId="5" CreationDate="2014-12-16T03:52:35.000" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2014-12-16T03:52:35.000" LastActivityDate="2014-12-16T03:52:35.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6425" PostTypeId="4" CreationDate="2014-12-16T03:52:35.000" Score="0" Body="Questions on a place or enclosure in which humans and/or other lifeforms live." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-16T07:13:24.653" LastActivityDate="2014-12-16T07:13:24.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6426" PostTypeId="1" AcceptedAnswerId="6440" CreationDate="2014-12-16T05:40:40.920" Score="15" ViewCount="1651" Body="&lt;p&gt;When I reflect upon the unfortunate circumstance of STS-107 in 2003, I seem to have memories of hearing reports of the foam striking the underside of the vehicle before we even learned of the reentry mishap. Sometimes we have false memories though, so I was trying to find old news reports of the mission from the timespan between launch on Jan 16 and the disaster on Feb 1 a couple weeks later.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did find &lt;a href=&quot;http://abcnews.go.com/Technology/columbia-shuttle-crew-told-problem-reentry/story?id=18366185&quot;&gt;this article&lt;/a&gt; on an ABC news site that claims Wayne Hale had posted this on a blog some years later:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;After one of the MMTs (Mission Management Team) when possible damage to the orbiter was discussed, he (Flight Director Jon Harpold) gave me his opinion: 'You know, there is nothing we can do about damage to the TPS (Thermal Protection System). If it has been damaged it's probably better not to know. I think the crew would rather not know. Don't you think it would be better for them to have a happy successful flight and die unexpectedly during entry than to stay on orbit, knowing that there was nothing to be done, until the air ran out?&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe that statement confirms that NASA officials at least suspected a potential damage issue, which they had no way of verifying. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question is: were there any reports to the public in the week or two after launch that the space shuttle had been struck by debris on its launch and that damage to the tiles was suspected?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was not able to find anything via web searches, but then I cannot find many old newspaper stories or television broadcasts at all.&lt;/p&gt;&#xA;" OwnerUserId="6392" LastEditorUserId="29286" LastEditDate="2020-08-31T18:13:51.683" LastActivityDate="2020-08-31T18:13:51.683" Title="Was the possibility of damage to the SS Columbia made public before it reentered on Feb 1, 2003?" Tags="|space-shuttle|failure|shuttle-columbia|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6427" PostTypeId="2" ParentId="6275" CreationDate="2014-12-16T12:09:08.923" Score="4" Body="&lt;p&gt;In general, inclination change is the costliest component of orbital maneuvers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For solar sails, like for every other craft, the easiest solution is launching directly into the required orbit. Barring that, we have to expend resources on plane change. Here the differences between normal spacecraft and sailcraft begin to emerge:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;a normal spacecraft has limited propellant supply, and our proposed solution should focus on minimizing propellant use and hence, $\Delta V$ (operational lifetime may be also constraining our maneuver schedule);&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;an ideal solar sailcraft has &quot;unlimited fuel&quot;, and does not use any propellants for attitude control (i.e. is equipped with variable reflectivity solar vanes, or magnetorquers for LEO operations). We would like to minimize time till maneuver completion.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;a more realistic sailcraft is characterized by solar sail and attitude control degradation (due to solar wind and galactic cosmic rays, and finite resource of attitude control surfaces/actuators). Here, the most relevant objective to minimize may be a combination of time-to-completion and number of attitude changes.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The 90 degrees' plane change stipulated in the question has been shown to be achievable through three burns for high-thrust spacecraft (we'll adapt the scheme to low-thrusters shortly):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;boost the orbit's apoapsis by a prograde burn so that the necessary node for plane change (ascending or descending) is also moved higher, away from your planet's influence;&lt;/li&gt;&#xA;&lt;li&gt;at the necessary node, fire in the normal/antinormal direction;&lt;/li&gt;&#xA;&lt;li&gt;at the next periapsis do a retrograde burn to get back to target orbit (equatorial in your case).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some spacecraft with thermal protection and half-decent brick-like aerodynamics can do plane changes in the upper atmosphere. Because of mass constraints, most spacecraft including solar sailcraft can not afford this (nor do they really need it). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low-thrusters suffer from gravity losses so raising apoapsis takes many more revolutions. Yet, the underlying sequence of steps stays more or less the same:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;first, get the periapsis as close to the planet as possible and at the right argument of periapsis (taking care to stay outside the influence of atmospheric drag) to get the most of Oberth effect during subsequent revolutions. Altitude of periapsis (PeA) and argument of periapsis (AgP) are &lt;strong&gt;crucial&lt;/strong&gt; for efficiency of the maneuver and they will have to stay mostly the same (you have to take into account the planet's oblateness - the $J_2$ term of the gravity field will perturb AgP). Timing also matters: you want periapsis to be located over the sunny hemisphere of the planet.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;second, raise the apoapsis aiming to maximize the prograde component of solar pressure (here, efficiency $\frac{dApA}{dt}$ depends on the solar angle ($\beta$) and the maximum sustained attitude rate allowed by your attitude control subsystem) while passing the periapsis. Somewhere between the apses there will be a region for another attitude change so that during the apoapsis you can contribute to the maneuver's objectives by a) changing inclination (every little bit helps), b) maintaining altitude of the periapsis, c) getting into the right prograde attitude for the next periapsis pass. The exact sequence depends on the location of the node you need.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In general, the whole maneuver has to be optimized in a special piece of software. Again, I urge you to consider injecting the craft into the right orbit right from the start (or doing midcourse corrections in case of interplanetary/interstellar flights).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a binary system (let's just call the planet you're orbiting Tattooine), the nearest star will contribute the most pressure, and the resulting force will be a geometric sum of the three vectors (pressure from the 1st sun, 2nd sun, and the reflected sunlight from Tattooine) instead of two. Thermal control may be complicated because of this. There may be other interesting corner cases I haven't yet thought through.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-12-16T13:33:37.803" LastActivityDate="2014-12-16T13:33:37.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6428" PostTypeId="2" ParentId="6420" CreationDate="2014-12-16T13:05:39.920" Score="9" Body="&lt;p&gt;The straight answer is no. It has to do with the gravitational pull of the Moon and the parabolic paths of a mass in motion around it. First, think of dropping a textbook and a penny on the earth.  Both fall with an acceleration of 9.8 m/s^2.  On the Moon that acceleration would be 1.625 m/s^2.  The thing is, that acceleration does not depend on the mass of the object, as the textbook/penny experiment demonstrates. As you walk, your center of mass rises and falls, and your walk is affected by the gravitational acceleration. It would always be awkward no matter how much mass you had stacked on your body.&lt;/p&gt;&#xA;" OwnerUserId="6948" LastEditorUserId="-1" LastEditDate="2014-12-16T20:50:11.370" LastActivityDate="2014-12-16T20:50:11.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6429" PostTypeId="2" ParentId="6426" CreationDate="2014-12-16T13:44:57.410" Score="6" Body="&lt;p&gt;I'm not sure about reports to the public, but NASA knew about the debris strike. According to the &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/archives/sts-107/investigation/CAIB_medres_full.pdf&quot; rel=&quot;nofollow&quot;&gt;Columbia Accident Investigation Board (CAIB) report&lt;/a&gt;, page 34:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Post-launch photographic analysis showed that one large&#xA;  piece and at least two smaller pieces of insulating foam&#xA;  separated from the External Tank left bipod (–Y) ramp area&#xA;  at 81.7 seconds after launch. . . .&#xA;  Further photographic analysis conducted the day after&#xA;  launch revealed that the large foam piece was approximately&#xA;  21 to 27 inches long and 12 to 18 inches wide, tumbling at&#xA;  a minimum of 18 times per second, and moving at a relative&#xA;  velocity to the Shuttle Stack of 625 to 840 feet per second&#xA;  (416 to 573 miles per hour) at the time of impact.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is elaborated on pages 37-38:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After discovering the strike, the Intercenter Photo Working&#xA;  Group prepared a report with a video clip of the impact and&#xA;  sent it to the Mission Management Team, the Mission Evaluation&#xA;  Room, and engineers at United Space Alliance and&#xA;  Boeing. In accordance with NASA guidelines, these contractor&#xA;  and NASA engineers began an assessment of potential&#xA;  impact damage to Columbiaʼs left wing, and soon formed a&#xA;  Debris Assessment Team to conduct a formal review.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Debris Assessment Team requested photographic imagery of the strike area, but was turned down. The matter was apparently discussed internally:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Even after the Debris Assessment Teamʼs conclusion had&#xA;  been reported to the Mission Management Team, engineers&#xA;  throughout NASA and Mission Control continued to exchange&#xA;  e-mails and discuss possible damage. These messages&#xA;  and discussions were generally sent only to people within&#xA;  the sendersʼ area of expertise and level of seniority.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-17T00:41:48.637" LastActivityDate="2014-12-17T00:41:48.637" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6430" PostTypeId="1" CreationDate="2014-12-16T15:20:18.467" Score="2" ViewCount="165" Body="&lt;p&gt;A recent BBC news article reports on finding &lt;a href=&quot;http://www.bbc.com/news/science-environment-30489814&quot; rel=&quot;nofollow&quot;&gt;microbes living 2,400m beneath the seabed off Japan&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The team found that microbes, despite having no light, no oxygen, barely any water and very limited nutrients, thrived in the cores.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is this a significant enough difference from our expectations of habitable environments to alter how and where we might search for life in our solar system? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2014-12-16T15:20:18.467" Title="How does the recent discovery of &quot;microbes living 2,400m beneath the seabed off Japan&quot; impact the search for life in our solar system?" Tags="|life|solar-system|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6431" PostTypeId="2" ParentId="6391" CreationDate="2014-12-16T18:57:25.830" Score="4" Body="&lt;p&gt;Here are a few cost vectors (and I'm sure I'm missing some)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Debris mitigation in spacecraft and mission design.&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This includes things like &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Reduction of launch debris, stages left on orbit, etc.&lt;/li&gt;&#xA;&lt;li&gt;end of life disposal capability&lt;/li&gt;&#xA;&lt;li&gt;passivisation (ie. venting unused fuel, etc. to reduce risk of breakups)&lt;/li&gt;&#xA;&lt;li&gt;reducing operational debris (stuff intentionally released from spacecraft during operations - lens caps, retaining devices, etc.)&lt;/li&gt;&#xA;&lt;li&gt;collision avoidance, including initial orbit selection.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are international, national and organizational standards governing this&#xA;   sort of thing.  Here are a few examples:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.iadc-online.org/References/Docu/USG_OD_Standard_Practices.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.iadc-online.org/References/Docu/USG_OD_Standard_Practices.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/mitigate/mitigation.html&quot; rel=&quot;nofollow&quot;&gt;http://orbitaldebris.jsc.nasa.gov/mitigate/mitigation.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.iadc-online.org/References/Docu/Space_Debris_Mitigation_Guidelines_COPUOS.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.iadc-online.org/References/Docu/Space_Debris_Mitigation_Guidelines_COPUOS.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Conjunction Analysis Screening&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the big picture ongoing calculation and assessment of potential satellite collisions.  While the calculation of the potential conjunction events is relatively straightforward, it requires as input, the full high accuracy space catalog.  In the US, this is closely held by the Air Force at the Joint Space Operations Center (JSpOC).  As such, they typically perform the high level screenings and provide warnings to interested satellite operators around the world as a free service.  The Space Data Alliance - an industry consortium - also provides a conjunction screening service using owner/operator ephemeris from their members, which may be more accurate then the catalog maintained by the JSpOC, particularly for actively maneuvering objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Owner/Operator Conjunction Analysis &amp;amp; Response&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given an externally provided warning, an owner/operator must then analyze the event and decide on a course of action.  This can incur significant technical manpower and may result in the execution of a spacecraft maneuver with a corresponding loss of operational life.  Not all spacecraft have a maneuver capability, and not all operators of those that do perform active collision avoidance.  Many operators subscribe to the big sky theory of collision avoidance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Business Strategy&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would include costs such as constellation redundancy, insurance, etc. to protect the business operations in the face of various risks, including loss of spacecraft due to collision events.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastActivityDate="2014-12-16T18:57:25.830" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6432" PostTypeId="1" AcceptedAnswerId="6498" CreationDate="2014-12-16T22:14:33.973" Score="18" ViewCount="6818" Body="&lt;p&gt;In &lt;a href=&quot;http://youtu.be/b6FyejiWBnM?t=32m52s&quot;&gt;this interview&lt;/a&gt; (part of the extra material from the Apollo 13 film), Tom Kelly of Grumman talks about the construction of the Apollo LEM. In particular he mentions that the &quot;skin&quot; of the LEM was made of 0.012&quot; aluminum and that &quot;You could easily, if you were careless, put  your boot or your foot right through that wall&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is he talking about the outer pressure hull, or could a careless astronaut really have put their foot through the side of the spaceship?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2014-12-23T08:51:10.677" Title="Could an Apollo astronaut have put their foot through the hull of the LEM?" Tags="|apollo-program|apollo-13|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6433" PostTypeId="2" ParentId="6403" CreationDate="2014-12-16T22:17:10.120" Score="2" Body="&lt;p&gt;Before an actual landing most everything is speculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;General speculations seems to float between two extremes. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Leave the stage standing as it on the barge. Let it cool down, send in small crew in a boat, tie down the stage so it won't slide. Safe it and sail back to port in that orientation.&lt;/li&gt;&#xA;&lt;li&gt;After landing, approach the stage in a boat with a crane, lift the stage, retract the legs (unclear how, must be possible though) and lay it down in a cradle on the crane boat (faster transit back to port) or else on a cradle moved onto the barge. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The actual stage seems unlikely to tip over. It is a 140+ feet tall, but it is almost all empty tank, except for the engines and legs at the bottom. Center of gravity is very low, and the legs extend out 60 feet, so it should be fairly stable from tipping. Sliding is a bigger concern, since whatever the barges surface is made of, it is likely very durable since a rocket is landing under power on top of it. Throttled down Merlin, say 70% of 165Kbs thrust is on the order of 115 KLbs thrust. That will melt many things that be considered non-slip.  Thus securing it in wave action will be critical, whether that is standing up, or on its side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the first two landings, the evidence seems to be two steps taken. Once landed, the crew comes in via boat.  They use airplane jacks, and mount then under the engines, and lift the stage on them. They weld the jacks to deck to secure them, wherever the stage lands. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then they weld caps over the tips of the legs as well to hold it in place. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-05-09T20:17:36.590" LastActivityDate="2016-05-09T20:17:36.590" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6434" PostTypeId="1" CreationDate="2014-12-17T00:14:19.550" Score="1" ViewCount="250" Body="&lt;p&gt;What would be someone's terminal velocity based on the gravity of Europa (.1335G) but given the drag coefficient and air density of Earth? I have a protagonist who is currently plummeting about four hundred feet, but I want to know how long his one hundred pound body will be aiborne and if his landing will pulverize him into protagonist soup. I may need to tweak things based on your answer. Thanks, A &lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS--couldn't make a terminal velocity tag because I don't have enough reputation points. &lt;/p&gt;&#xA;" OwnerUserId="2802" LastActivityDate="2014-12-17T01:06:40.560" Title="Terminal velocity on Europa" Tags="|mathematics|europa|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6435" PostTypeId="2" ParentId="6434" CreationDate="2014-12-17T00:18:35.497" Score="3" Body="&lt;p&gt;Terminal velocity is proportional to the square root of gravitational acceleration, if other factors are held equal.  So it will be about 120 mph $\times \sqrt{0.1335} \approx $ 44 mph.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-12-17T00:18:35.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6436" PostTypeId="2" ParentId="6434" CreationDate="2014-12-17T01:06:40.560" Score="1" Body="&lt;p&gt;There are two formulas for &quot;resistive forces&quot; that may be of use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;#1: Velocity is linearly related to force (Slow speeds).&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, the resistive force $R$ is defined as&#xA;$$R=-bv$$&#xA;where $v$ is velocity and $b$ is a constant that depends on the object and the medium. Using Newton's second law,&#xA;$$\sum F=ma$$&#xA;The other force to take into account is gravity:&#xA;$$\sum F=mg-bv=ma$$&#xA;$$\frac{dv}{dt}=\frac{mg}{m}-\frac{bv}{m}$$&#xA;$$\frac{dv}{dt}=g-\frac{bv}{m}$$&#xA;When an object reaches terminal velocity, $\frac{dv}{dt}=0$. so&#xA;$$0=g-\frac{bv}{m}$$&#xA;$$g=\frac{bv}{m}$$&#xA;$$v=v_T=\frac{mg}{b}$$&#xA;$b$ will most likely have to be determined experimentally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;#2: Velocity is non-linearly related to force (High speeds).&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other equation (and the one I find is more common) is&#xA;$$R=\frac{1}{2}D \rho Av^2$$&#xA;where $D$ is a constant, $\rho$ is the density of the medium, and $A$ is the cross-sectional area of the object. Again, using Newton's second law,&#xA;$$\sum F=ma=mg-\frac{1}{2}D \rho Av^2=ma$$&#xA;$$\frac{dv}{dt}=\frac{mg}{m}-\frac{D \rho Av^2}{2m}$$&#xA;Setting $\frac{dv}{dt}$ to $0$, we have&#xA;$$g=\frac{D \rho Av^2}{2m}$$&#xA;$$\frac{2mg}{D \rho A}=v^2$$&#xA;$$v=v_T=\sqrt{\frac{2mg}{D \rho A}}$$&#xA;Once again, $D$, like $b$ before, must be determined experimentally.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So without knowing the exact conditions and modeling them - or conducting them in an experiment on Europa - the precise velocities can not be determined.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2014-12-17T01:06:40.560" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6437" PostTypeId="1" AcceptedAnswerId="6438" CreationDate="2014-12-17T03:18:25.577" Score="18" ViewCount="3372" Body="&lt;p&gt;We could have sent seeds of life (say a few ounces of microbes) along with every planetary exploration craft. Why we haven't done that so far? &lt;/p&gt;&#xA;" OwnerUserId="2722" LastEditorUserId="168" LastEditDate="2015-01-11T09:12:51.373" LastActivityDate="2015-01-11T09:12:51.373" Title="Why haven't we sent seeds of life to other planets yet?" Tags="|life|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6438" PostTypeId="2" ParentId="6437" CreationDate="2014-12-17T03:22:08.700" Score="28" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_protection&quot;&gt;Planetary protection&lt;/a&gt; is the principle that on the contrary to your proposal, we should take steps to avoid any contamination of planetary bodies with life from Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are various cited reasons for this, one of the most important being that since we don't yet know whether extraterrestrial life exists on these planets, if we accidentally or deliberately introduce terrestrial life that could confound future experiments and make it much harder to determine whether extraterrestrial life is present.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-12-17T03:22:08.700" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6440" PostTypeId="2" ParentId="6426" CreationDate="2014-12-17T04:23:23.263" Score="17" Body="&lt;p&gt;Yes, this information was publicly available.  In fact, the only reason that the STS-107 crew was informed of the debris strike during the mission was that NASA management was concerned that the press might ask them about it during an on-orbit press conference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &quot;Columbia Accident Investigation Board Report&quot;, Volume 1, page 158:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mission Control personnel thought they should tell Commander Rick Husband and Pilot William McCool about the debris strike, not because they thought that it was worthy of the crewʼs&#xA;  attention but because the crew might be asked about it in an upcoming media interview. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The CAIB report is available online &lt;a href=&quot;http://www.nasa.gov/columbia/home/CAIB_Vol1.html&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2014-12-17T04:23:23.263" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6442" PostTypeId="1" CreationDate="2014-12-17T09:43:53.080" Score="9" ViewCount="1441" Body="&lt;p&gt;I wonder if SpaceX will cover the barge lading of the Falcon 9 first stage live during the upcoming launch for the fifth SpaceX cargo mission to the International Space Station or not? NASA TV's &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/news/spacex_crs5_briefings_events.html&quot;&gt;CRS-5 launch coverage announcement&lt;/a&gt; isn't clear on that.&lt;/p&gt;&#xA;" OwnerUserId="6421" LastEditorUserId="49" LastEditDate="2015-12-22T06:02:16.237" LastActivityDate="2015-12-22T06:02:16.237" Title="Will we see the barge landing live during the SpaceX CRS-5 webcast?" Tags="|launch|spacex|landing|falcon-9|booster-flyback|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6443" PostTypeId="2" ParentId="6442" CreationDate="2014-12-17T10:14:07.633" Score="8" Body="&lt;p&gt;As of yet (December 17, 2014), there was no official announcement of live webcast of the Falcon 9 booster landing on the barge (aka &lt;em&gt;autonomous spaceport drone ship&lt;/em&gt;). Here's SpaceX' &lt;a href=&quot;http://www.spacex.com/news/2014/12/16/x-marks-spot-falcon-9-attempts-ocean-platform-landing&quot;&gt;latest update&lt;/a&gt; on this, stating:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The odds of success are not great—perhaps 50% at best. However this test represents the first in a series of similar tests that will ultimately deliver a fully reusable Falcon 9 first stage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So all things considered, it is highly unlikely that the booster stage landing and recovery attempt will be webcast live. According to &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=35853.225&quot;&gt;comments on NASASpaceFlight forum&lt;/a&gt;, I agree with &lt;em&gt;Kabloona's&lt;/em&gt; take that there is no real benefit for SpaceX to provide live webcast of the landing attempt. They will most certainly have live feeds from the booster stage, the barge and the support ship themselves (they registered &lt;a href=&quot;https://apps.fcc.gov/oetcf/els/reports/STA_Print.cfm?mode=current&amp;amp;application_seq=62792&quot;&gt;two additional broadcast frequencies for the sub-orbital first-stage&lt;/a&gt; with FCC), but the general public will most likely see only a recording of the landing event at SpaceX' discretion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt; - I wouldn't hold my breath for it. But considering SpaceX provided us with videos and stills of previous attempts of soft-splashing Falcon 9 booster stage into the ocean the best they could (with help of spaceflight enthusiasts), I'm sure they'll give us &lt;em&gt;something&lt;/em&gt; after the fact. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-12-17T10:14:07.633" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6444" PostTypeId="2" ParentId="4354" CreationDate="2014-12-17T11:44:34.297" Score="4" Body="&lt;p&gt;In Dec 2014, &lt;a href=&quot;http://aviationweek.com/space/antares-upgrade-will-use-rd-181s-direct-buy-energomash&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital selected&lt;/a&gt; the Russian Energomash RD-181, which is basically an export version of the &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-191&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-191&lt;/a&gt; used in the Angara booster.&lt;/p&gt;&#xA;&lt;p&gt;This is also a LOX/kerosene engine, derived from the &lt;a href=&quot;http://www.b14643.de/Spacerockets_2/Diverse/Energomash_RD-170/index.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-170&lt;/a&gt;.  The RD-170 is a 1.6 MLbs thrust engine used on Zenit boosters (which were initially strap on boosters for Energia) that has 4 thrust chambers.  The two thrust chamber and about 800 Klbs thrust version is known as the &lt;a href=&quot;http://www.astronautix.com/engines/rd180.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-180&lt;/a&gt; of political fame and Atlas V usage.&lt;/p&gt;&#xA;&lt;p&gt;The single thrust chamber version cuts thrust by half again to about 420 Klbs thrust and is being used on the Angara common core booster as the RD-191. The export version of this, RD-181 will be used in a set of two on the upgraded Antares.&lt;/p&gt;&#xA;&lt;p&gt;They will get a payload boost out of the upgrade, and access to an in-production engine that should be manufactured at a reasonably high rate. The Russians are testing an Angara 5 configuration in Dec 2014 that is meant to replace the Proton booster. The Angara family is meant to replace almost all Russian boosters, so the engine should be in production, and reasonably high volume production, for the forseeable future.&lt;/p&gt;&#xA;&lt;p&gt;The choice of a Russian booster in a time of political turmoil with Russian behavior and congressional disapproval of the use of RD-180 engines on Atlas V from the same vendor makes this either a very brave or very foolish choice.  Time will tell.&lt;/p&gt;&#xA;&lt;p&gt;Edit 2022: With the conflict in Ukraine/Russia, it looks like time has told a tale of foolishness on this choice, since the engines are now reported to not be available to the west due to political and other reasons.&lt;/p&gt;&#xA;&lt;p&gt;I suppose we should re-raise the question as to what OTHER other engine options are available.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2022-05-30T11:18:38.430" LastActivityDate="2022-05-30T11:18:38.430" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="6445" PostTypeId="2" ParentId="6442" CreationDate="2014-12-17T11:49:40.170" Score="6" Body="&lt;p&gt;The barge itself has an FCC license for its systems and antennae that do not seem to include anything powerful enough to send video live. However SpaceX has access to a jet (Elon Musks N900SX) and they know where the booster will be landing with greater accuracy this time, so it is within the realm of possibility.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The support ship will be nearby and may be able to record its own video, or relay from the barge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But most likely live coverage will not happen, which is a shame since it would be so thrilling to watch. Even if it failed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From SpaceX's perspective a success is great, but showing it a week later vs live has little downside, compared to showing a failure live. Alas news media in this day and age is largely ill informed and mostly incapable of nuance. The headline &quot;SpaceX rocket crashes&quot; is easier to write than &quot;First ever attempt at soft landing a stage comes close to success, they will get it next time&quot;. Why take the risk of bad publicity, since they know it is not yet a certainty of success.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at the media coverage of the F9R-Dev1 termination of flight.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-17T11:49:40.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6446" PostTypeId="2" ParentId="6437" CreationDate="2014-12-17T15:14:58.067" Score="7" Body="&lt;p&gt;As noted, there is a strong desire to NOT send terrestrial 'anything' to other planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They spend a LOT of money and time properly sterilizing space craft that land or might crash land into other planets.  Now you might say, this vehicle will be in interplanetary space for 1-2 years in transit, surely nothing can survive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet amusingly odd stuff has been found to survive on the surface of the ISS for years since launch.  (Something about sea plankton DNA?). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk has told the story many many times, that his initial plan before starting SpaceX was to build a contained greenhouse as part of a Mars lander, so that on landing it would add water and starting growing green things on the red planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once he realized the issue was not enthusiasm but rather launch costs, he abandoned that approach and instead focused on launch costs via SpaceX.  (Which he has succeeded at spectacularly so far!  Everyone is forced to drop launch costs, Chinese do not think they can compete. Ariane 6 is designed specifically to try to compete. If the landing succeeds on any of the launches in the coming years, and they are able to refly first stages it is likely his success will become only greater). &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-17T15:14:58.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6448" PostTypeId="1" CreationDate="2014-12-17T16:12:41.910" Score="12" ViewCount="696" Body="&lt;p&gt;The news that Boeing will bid the CST-100 as a Commercial Resupply Services 2 (CRS-2) vehicle raises a question: will they be using a docking port, to a &lt;a href=&quot;http://en.wikipedia.org/wiki/Pressurized_Mating_Adapter&quot; rel=&quot;nofollow noreferrer&quot;&gt;PMA&lt;/a&gt; or berthing to a &lt;a href=&quot;http://spacecraft.ssl.umd.edu/design_lib/ICES01-2435.ISS_CBM.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;CBM&lt;/a&gt; port?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that NASA wants two open ports when you attempt to connect.  So there is primary CBM (earth facing Node 2), and backup CBM (sky facing Node 2).  They are rearranging the station so that the up-facing CBM will get PMA-3 + NDS interface attached and will be PMA/docking backup.  And MPLM will be moved from Node 1 earth facing onto Node 3 (forward facing port), freeing up the CBM as backup.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see a lot more about this and the reconfiguration in &lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;this question and its answers&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if a crew vehicle is on station using PMA-2 (forward facing Node-2) then could a cargo dock to PMA-3? And would that mean the cargo vehicle has no backup if an issue with PMA-3 occurs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is true if SpaceX bids Dragon V2 for cargo as well. NASA did allow in the RFP for PMA docking for cargo, but did not seem to address this issue. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew vehicle also serves as lifeboat so will stay on station 6 months or longer. Would that then block cargo vehicles from using PMA-3, or will this restriction be relaxed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Russian side this is less of an issue since there are 4 possible ports, and one is almost always a Progress, so they could dump that if a port fails to allow the new vehicle to dock.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-01-06T20:38:21.940" Title="What will be NASA Policy on backup ISS docking/berthing ports, once commercial crew is up and running?" Tags="|iss|spacex|docking|boeing|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6449" PostTypeId="1" AcceptedAnswerId="6450" CreationDate="2014-12-17T16:29:07.347" Score="5" ViewCount="245" Body="&lt;p&gt;Geology &lt;a href=&quot;http://www.spudislunarresources.com/&quot; rel=&quot;nofollow&quot;&gt;Dr. Paul Spudis&lt;/a&gt; (who was on the Clementine Lunar orbiter mission) has an interesting &lt;a href=&quot;http://www.spudislunarresources.com/Bibliography/p/102.pdf&quot; rel=&quot;nofollow&quot;&gt;step-by-step proposal&lt;/a&gt; for exploration of potentially extractable water ice at the Lunar poles. It includes communication satellites in Lunar polar orbit. I wonder if it would be practical to replaced them with a cable. I think that the communication satellites raise the threshold for such a project, since they would have to be the very first investment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A stationary lander on a crater ridge in (nearly) &lt;a href=&quot;https://en.wikipedia.org/wiki/Peak_of_eternal_light&quot; rel=&quot;nofollow&quot;&gt;constant sunlight&lt;/a&gt; with a large turning photovoltaic array could produce tens of kilowatts (tens times the power of the Curiosity rover on Mars). I presume that it could have constant line-of-sight to Earth too and hence constant communication. A rover in an eternally shadowed crater has none of that on its own. I wonder if it would be practical to have a rover connected to such a stationary lander with a cable for power supply and communication with, and real time teleoperation from, Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rover would have to carry the entire cable and unwind it, or else it would have to be dragged from the station. If the rover turns back in a curve, the cable would have to be rewound or dragged. Dragging a cable means that it will get stuck at rocks. And the lunar soil might be electrostatic and interfere with power and communication in the cable. Does a corded rover sound like a good idea, or would the mass and hassle with the cable be worse than communication satellites and big batteries or RTG (which are pretty expensive alternatives)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that after the first surveyor rover has struck gold, a heavy extraction rover could precision land near the power/communication station and connect its own cable to it. Of all the unrealized plans for space exploration, has something like this been proposed and analyzed?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-12-17T17:22:55.413" LastActivityDate="2014-12-17T20:17:52.170" Title="Would it be practical to use a rover with a cable to a power and communication station?" Tags="|the-moon|communication|rovers|solar-power|isru|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="6450" PostTypeId="2" ParentId="6449" CreationDate="2014-12-17T16:50:38.173" Score="8" Body="&lt;p&gt;There are several problems with this idea: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Snagging the cable on a protrusion. You'd have to constantly monitor the cable to make sure it didn't snag, and/or be very careful when driving: always return along the same path you took to get somewhere. If you've ever tried to mow the lawn with an electric mower, you know how annoying this gets. &lt;/li&gt;&#xA;&lt;li&gt;Moon rock and dust are incredibly abrasive. Any movement of the cable would wear down the insulation. This includes winding the cable on and off a storage drum, so this would happen even if you're very careful not to disturb the cable.&lt;/li&gt;&#xA;&lt;li&gt;Weight of the cable. Power cable is thick and heavy. I've got a 16 Amp cable that's maybe 25 m long, it weighs something like 7 kg. Because of 1. and 2. you need to install the cable drum on the rover, adding a lot of weight to the rover.  &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This method is used sometimes on Earth, e.g. in mining: some bucket wheel excavators are cable-powered. The one in the picture weighs some 10,000 tons. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/eqgan.jpg&quot; alt=&quot;Schaufelradbagger&quot;&gt;&lt;br&gt;&#xA;In these applications, the machine is stationary for longer periods and when it moves, personnel can keep an eye on the cable. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-12-17T20:17:52.170" LastActivityDate="2014-12-17T20:17:52.170" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6452" PostTypeId="1" AcceptedAnswerId="6453" CreationDate="2014-12-17T17:46:00.167" Score="7" ViewCount="329" Body="&lt;p&gt;The solar electric ion propulsion engine of the Dawn spacecraft to Vesta and Ceres used &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_%28spacecraft%29#Cruise&quot;&gt;Mars for gravity assist&lt;/a&gt;. Would it be gainful to complement an ion engine with a high thrust (solid) chemical rocket to fire only during such a gravity assist in order to make maximum use of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot;&gt;Oberth effect&lt;/a&gt;? Or can it be indicated that the burden of launching and carrying its mass would be difficult to make up for in that way?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-17T18:32:58.243" Title="Does the Oberth effect motivate complementing ion electric propulsion with chemical rocketry?" Tags="|gravity-assist|advanced-propulsion|solid-fuel|oberth-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6453" PostTypeId="2" ParentId="6452" CreationDate="2014-12-17T18:24:56.680" Score="6" Body="&lt;p&gt;Solid rocket motors &lt;strong&gt;have&lt;/strong&gt; been used successfully to exploit the Oberth effect during the most critical phase of an interplanetary mission - target body orbit insertion. NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Magellan_%28spacecraft%29#Attitude_control_and_propulsion&quot; rel=&quot;noreferrer&quot;&gt;Magellan mission&lt;/a&gt; to Venus used a standard Thiokol/ATK &lt;a href=&quot;https://en.wikipedia.org/wiki/Star_48&quot; rel=&quot;noreferrer&quot;&gt;Star 48&lt;/a&gt; propulsion module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This same module was employed as an upper stage in the New Horizons flight to Pluto and beyond.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A mass-conscious mission designer is always aware of solids' low specific impulse, yet may use them for their reliability in single-burn maneuvers. Powered gravity assists are mostly done in several burns (Venus-Earth-Earth, for instance), and require very high precision as far as burn $\Delta V$ is concerned. It may be also desirable to re-program burn length in-flight, which is not possible with solids. Hypergolic-based thrusters are much more convenient, well-tested, and flexible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To sum it up, a mission with an ion thruster &lt;strong&gt;could&lt;/strong&gt; use a solid rocket motor for orbit insertion, if the approach velocity is simply too high for a low-thrust capture. Multiple powered gravity assists - not really. One should note that powered assists are much less desirable mass-wise than unpowered ones.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-12-17T18:32:58.243" LastActivityDate="2014-12-17T18:32:58.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6454" PostTypeId="1" AcceptedAnswerId="6455" CreationDate="2014-12-17T19:33:30.250" Score="21" ViewCount="10495" Body="&lt;p&gt;Obviously there are many factors that go into the design of a rocket. However, to me, many rockets seem &lt;em&gt;very&lt;/em&gt; tall and skinny. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I mean is that an ideal rocket would have as little mass used for the tank structure, and hold the maximum amount of fuel. Which, if it operated in a vacuum, would call for a spherical tank. I.e., Surface area (dry mass) to volume (fuel mass) ratios would be worse in a tall skinny rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Launching a rocket requires passing through the atmosphere. &lt;/li&gt;&#xA;&lt;li&gt;An airfoil shape has less drag than a sphere. &lt;/li&gt;&#xA;&lt;li&gt;Fabricating a sphere is much harder than fabricating a cylinder &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, for a single stage rocket, I would &lt;em&gt;assume&lt;/em&gt; a bullet shaped rocket with something like a 3:1 length to diameter ratio would be ideal for an Earth launched rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And for a multistage rocket, the top stage would have something like a 3:1, and the lower stages something like 1:1 - 2:1 length to diameter ratios.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, rocket designers are reasonably smart folks, and have probably thought of this; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So what am I missing? Where in my broad estimations / calculations have I made naïve assumptions?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="3460" LastEditorUserId="49" LastEditDate="2015-05-04T10:28:33.257" LastActivityDate="2019-02-27T09:16:50.947" Title="What is the ideal shape for a rocket?" Tags="|launch|rockets|design|engineering|atmospheric-drag|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6455" PostTypeId="2" ParentId="6454" CreationDate="2014-12-17T20:05:02.787" Score="26" Body="&lt;p&gt;For supersonic flow, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sears%E2%80%93Haack_body&quot; rel=&quot;noreferrer&quot;&gt;Sears-Haack body&lt;/a&gt; offers less drag than the shorter teardrop that's optimal in the subsonic regime. Sears-Haack is pretty similar to the German V-2 rocket body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wDigW.png&quot; alt=&quot;slender pointed ended shape&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that the &lt;em&gt;proportions&lt;/em&gt; of this particular example aren't part the definition of the Sears-Haack shape; for minimal drag you'd have to have an impractical body of infinite length and infinitesimal cross section.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tapered &quot;boat-tail&quot; at the back of the Sears-Haack shape, though, moves the center of aerodynamic pressure forward. For stability, you want to keep the center of pressure behind the center of gravity -- this is why darts, arrows, and many rockets have fins at the back; the fins add drag at the base, moving the center of pressure back and keeping the projectile stable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping the bottom end of the Sears-Haack shape cylindrical, or even flaring it out slightly, rather than boat-tailing, improves stability dramatically. Also, you need to put the engines there. So that's where the basic familiar rocket shape comes from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you're making a really big rocket, other constraints, like how high your factory roof is or how wide your train tunnels are, start to come into play, and you may have to get longer and skinnier than the aerodynamically optimal shape as well.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-06-02T01:49:43.230" LastActivityDate="2015-06-02T01:49:43.230" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6456" PostTypeId="2" ParentId="6454" CreationDate="2014-12-17T20:33:30.443" Score="25" Body="&lt;p&gt;I think you answered your question in the first sentence.  I am reminded of this image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://aviatormag.com.au/wp/wp-content/uploads/2012/05/IntelligentDesignMain.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://aviatormag.com.au/wp/wp-content/uploads/2012/05/IntelligentDesignMain.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerodynamics is but one (albeit a large one) of many concerns in the systems engineering of a rocket.  Others include manufacturability, propulsion, changing flight regimes, safety, structures, economics, etc...  All of these seem to have converged on a simple, more-or-less cylindrical design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sake of innovation however, I encourage you to try something new!&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="10273" LastEditDate="2019-02-27T09:16:50.947" LastActivityDate="2019-02-27T09:16:50.947" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="6457" PostTypeId="2" ParentId="5409" CreationDate="2014-12-17T22:19:08.773" Score="18" Body="&lt;p&gt;The other answers address the case of nominal entries.  Since the question references aborts, perhaps the intent is to ask about ascent aborts which result in a stable entry that does not reach a runway.  In this case a special autopilot mode would be engaged which basically held the wings level, the side hatch would be jettisoned and the crew would use the &quot;escape pole&quot; to bail out.  The orbiter would eventually crash into the water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this was an extremely unlikely situation due to ascent flight design closing most of the black zones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/escape/inflight.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, you can read the bailout flight rules &lt;a href=&quot;https://archive.org/details/flight_rules_generic&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; (Paragraph A2-251)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-02-02T23:29:08.807" LastActivityDate="2019-02-02T23:29:08.807" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="6458" PostTypeId="2" ParentId="6174" CreationDate="2014-12-18T07:27:47.827" Score="1" Body="&lt;p&gt;Also since the rocket is only using one engine it has no control over rotation about the long axis of the rocket the fins provide that.&lt;/p&gt;&#xA;" OwnerUserId="6981" LastActivityDate="2014-12-18T07:27:47.827" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6460" PostTypeId="1" CreationDate="2014-12-18T10:56:14.107" Score="5" ViewCount="516" Body="&lt;p&gt;Used upper stages seem to be a big &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_debris#Boosters&quot; rel=&quot;noreferrer&quot;&gt;contributor to space debris&lt;/a&gt;. Wouldn't it be fairly easy and cheap to instead have an upper stage somehow physically lock onto a big piece of space debris and push it down into the atmosphere, or to the graveyard orbit outside of GEO? Sooner or later, the big pieces of junk in orbit will collide and fragment into thousands of dangerous pieces, so removing a small number of the most massive ones should be very helpful in the long run. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launchers can obviously not always carry a payload which makes use of all their capacity. So there's often place for extra fuel for a small secondary mission after the payload has been inserted in its orbit or trajectory. That's why there's a market for secondary payloads. Even Curiosity's sky crane on Mars had &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=29612.450&quot; rel=&quot;noreferrer&quot;&gt;140 kg of unused fuel&lt;/a&gt; after landing. And &lt;a href=&quot;http://www.ecns.cn/2014/12-03/145158.shtml&quot; rel=&quot;noreferrer&quot;&gt;Chang'e 5T is touring cis-lunar space&lt;/a&gt; after having delivered its return capsule to Earth. Upper stages, of course, often end their primary missions in the most commonly used orbits, where it is most important to remove debris. Large maneuvers would generally not be necessary. This concept could be applied opportunistically for launches with spare capacity and in orbits similar to a specific big piece of debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the problems with this? Is the cost of filling up a rocket to the top prohibitive? Is it dangerous to approach and grab a dead spinning debris object, maybe it actually risks causing a collision and a new field of debris? Or is it an economic problem of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tragedy_of_the_commons&quot; rel=&quot;noreferrer&quot;&gt;tragedy of the commons&lt;/a&gt; which prevents even small investments in removing space debris?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-09-30T02:54:27.937" Title="Could upper stages take down space debris as a 2nd mission?" Tags="|debris|orbital-maneuver|upper-stage|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6461" PostTypeId="1" CreationDate="2014-12-18T11:14:57.210" Score="9" ViewCount="10037" Body="&lt;p&gt;What G-force is the payload exposed to on different launchers? Are there more gentle and more rough rockets? And do other factors, like vibrations, differ significantly too? Some scientific payloads are extremely sensitive, I wonder if they prefer a certain rocket over others, and if the payload is designed with a certain launch system in mind from the beginning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From watching videos, I get the impression that an Ariane 5 races much faster from the launch pad than does a Falcon 9. I don't know if that says something about the G-forces involved during the launch sequence as a whole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is the question basically a non issue, maybe all launchers use about the same launch profile with respect to G-forces?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-12-18T12:35:17.867" LastActivityDate="2016-05-26T03:52:23.737" Title="What G-forces do different launchers cause?" Tags="|launch|payload|vibration|g-force|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6463" PostTypeId="1" CreationDate="2014-12-18T11:54:25.167" Score="4" ViewCount="148" Body="&lt;p&gt;A femto probe is a probe that weights less than 100 grams. I would expect that it's not much more than:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Omnidirectional antenna (locating Earth would require relatively heavy equipment, right?)&lt;/li&gt;&#xA;&lt;li&gt;Power source (solar cell or an unrechargable battery, depends on destination)&lt;/li&gt;&#xA;&lt;li&gt;Micro computer chip&lt;/li&gt;&#xA;&lt;li&gt;Super tiny flywheels for rotation&lt;/li&gt;&#xA;&lt;li&gt;Thermal control system&lt;/li&gt;&#xA;&lt;li&gt;Ultralight sensors (thermistor, photoresistor, could you suggest more?)&lt;/li&gt;&#xA;&lt;li&gt;Single scientific device. E.g. I found an of the shelf camera that weighted a little more than 1 gram&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Could it do some useful science? If yes, why no space exploration agency has tried sending a bunch of them? Seems to be cheaper, more reliable (redundancy) and can be at many locations at once.&lt;/p&gt;&#xA;" OwnerDisplayName="user6738" LastActivityDate="2014-12-18T12:35:14.953" Title="What could femto probes do?" Tags="|probe|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6464" PostTypeId="2" ParentId="6463" CreationDate="2014-12-18T12:22:56.877" Score="5" Body="&lt;p&gt;Their primary advantage is in numbers. A single such probe would hardly be useful. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a swarm, they could cover a large area, network together (just relay communication to next probe in range) and depend on a larger probe to pass data back to Earth. They could be used to map various fields - magnetic field, gas distribution, acting as a really big radiotelescope array, map chemical composition of objects (asteroids etc) and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike sensors that can either depend on reflected radiation (camera, radar, ultrasound, emission spectrum) or examine just local properties of given object/space right where they are, a swarm of nanoprobes is good for mapping large area for properties that can't be sensed remotely, or for compiling a wide-angle reception of reflected radiation that varies with angle. Also, thanks to numbers, they'd be efficient in pinpointing interesting locations for more detailed analysis - e.g. finding unique chemicals in the vicinity, and monitoring large areas for changes.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2014-12-18T12:35:14.953" LastActivityDate="2014-12-18T12:35:14.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6465" PostTypeId="2" ParentId="6461" CreationDate="2014-12-18T12:54:36.217" Score="11" Body="&lt;p&gt;It is necessary to have a look at LV manuals (see this &lt;a href=&quot;https://space.meta.stackexchange.com/a/319/168&quot;&gt;meta post&lt;/a&gt; for the links to actual PDFs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, Ariane 5 has the following longitudinal acceleration profile:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ifNeX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Minotaur I:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/u7jmN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5rIZb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iMCv6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roughness of the ride varies greatly - from ICBMs that aim for maximum acceleration at launch to gentle heavy riders. This includes jerk rates, vibration at various frequencies, lateral acceleration profiles. Some launchers suffer from the pogo problem, some experience unexpectedly high lateral and axial accelerations at separation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yes, choice of launch vehicles may be constrained by the loads the equipment can bear. Sometimes design of the equipment/spacecraft itself (as a bespoke job, I'm not talking about mass-produced sat platforms here) co-evolves with the selection of launch vehicle (to avoid the most dangerous vibration frequencies, for instance).&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:04.403" LastActivityDate="2016-05-26T02:41:13.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6466" PostTypeId="1" AcceptedAnswerId="6467" CreationDate="2014-12-18T13:03:28.730" Score="14" ViewCount="2721" Body="&lt;p&gt;I recall seeing LOTS of water usage on a tour of Kennedy Space Center as a kid: during launches, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking only for a magnitude of order, so I would be satisfied with &quot;hella&quot; or &quot;not much more than a fish farm&quot; or whatever.  This does  not  include regular use (the bathrooms, landscaping, etc.) but just the operations, and whatever timeframe that is convenient for you; I can extrapolate from there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus Question - if it's a lot, is it fresh water, distilled water, sea water?&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="6712" LastEditDate="2014-12-18T14:18:31.267" LastActivityDate="2014-12-18T20:44:28.450" Title="How much water is used at the Kennedy Space Center (KSC)?" Tags="|launch-site|kennedy-space-center|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6467" PostTypeId="2" ParentId="6466" CreationDate="2014-12-18T13:21:24.117" Score="18" Body="&lt;p&gt;Well, I must say a &quot;hella&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only figures I can quote are for the sound suppression system:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An elevated water tank near each pad provided sound buffering protection for the launching spacecraft. Part of the Sound Suppression Water System (SSWS), the 290-foot (88 m) water towers stored 300,000 gallons (1.1 Megalitres) of water, which was released just before engine ignition. The water discharged onto the launch platform during lift-off muffled the intense sound waves produced by the first stage Rocketdyne F-1 engines. Due to heating of the water, a large quantity of steam was produced during launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I think right now there are 2 launchpads in operation, so that would give you an idea. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/scJUe.jpg&quot; alt=&quot;sound suppression system test&quot;&gt;&#xA;Sound suppression system test on Launchpad 39A in 2004.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Quoted source: &lt;a href=&quot;http://en.wikipedia.org/wiki/Kennedy_Space_Center_Launch_Complex_39#Sound_suppression_water_system&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3001" LastEditorUserId="3001" LastEditDate="2014-12-18T15:52:15.490" LastActivityDate="2014-12-18T15:52:15.490" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6468" PostTypeId="1" AcceptedAnswerId="6469" CreationDate="2014-12-18T14:20:23.497" Score="12" ViewCount="1722" Body="&lt;p&gt;I know one of the main advantages of the suitport is to not need an airlock, hence saving a lot of production costs, launch mass, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But why are the EVA suits completely exposed to the environment? Wouldn't that increase the risks on the suits potentially getting damaged?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Couldn't the suits be in an unpressurized chamber just to shield them of any environmental dangers/degradation while not in use?&lt;/p&gt;&#xA;" OwnerUserId="3001" LastActivityDate="2017-01-06T20:25:59.890" Title="Why are the EVA suits in the suitport (concept) exposed to the environment?" Tags="|spacesuits|environment|suitport|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6469" PostTypeId="2" ParentId="6468" CreationDate="2014-12-18T14:30:26.267" Score="12" Body="&lt;p&gt;You answered that in your question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic notion described is often seen on Mars rover style suits as in this image from Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/S0LvX.jpg&quot; alt=&quot;Mars Rover Suitport notion&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The suit itself is the airlock. To go EVA, you climb into the suit from the back, (sort of like an Orlan space suit right now).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CDjZY.jpg&quot; alt=&quot;Entering an Orlan suit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference is that the suit itself seals to the 'airlock' but does not really require the mass/equipment of an airlock. Rather the back of the suit seals to an interface, hopefully simpler than a full airlock and allows entrance/egress to the suit and thence to the outside. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way of looking at is, a spacesuit needs a airtight seal that can open and close anyway. Why not use that as the outer door of the airlock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the real question is why not surround that in yet another structure for protection from UV/cosmic radiation/impacts? Probably you would build your suit port in a somewhat sheltered area of your structure as a first draft approach to this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not think the real use cases have provided far enough to identify if these are real risks or not to a suit hanging out in space for long periods of time. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-18T14:30:26.267" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6470" PostTypeId="2" ParentId="6468" CreationDate="2014-12-18T14:59:13.300" Score="11" Body="&lt;p&gt;You have to consider that this is a concept that is still under development. If the suitports will really be used (and I don't see why not, it solves many problems at the same time) on Mars, or anywhere else where dusty environment is a bigger concern than during testing on Earth, they would likely use some sort of a retractable tunnel leading to the ports themselves to additionally protect them from extremely fine-powder airborne dust, corrosive vapors and UV radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tighter solid chambers wouldn't be suitable because of accessibility (there's still quite some maneuvering involved with suitport ingress and egress), additional mass and being difficult to replace, while you can run light and retractable tunnels that would still do the job and can be extended on the spot during first egress. The tunnel cloth doesn't even have to seal perfectly, as long as the material stays somewhat loose on its support structure (not extended all the way). And a more solid structure, if damaged, could bend to block access or even damage rather bulky EVA suits with suitlock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For now though, during testing on Earth, such structures would only impair movement and visual inspection.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;FWIW retractable tunnels aren't a new concept when it comes to extravehicular suit systems. Here's one NASA's patent from 60's: &lt;a href=&quot;https://www.google.com/patents/US3537668&quot;&gt;Extravehicular tunnel suit system, US 3537668 A&lt;/a&gt;. This is however a tunnel leading from the vehicle (or any other pressurized environment) to the pressure suit. But it demonstrates that such systems have been in development for a long time. And if we look at the &lt;a href=&quot;https://www.google.com/patents/US4842224#backward-citations&quot;&gt;US 4842224 A Suitport patent's citations&lt;/a&gt;, it also references US 3537668 A. So there's a good chance that actual Suitport system will use some type of a retractable tunnel also.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-12-18T15:14:56.847" LastActivityDate="2014-12-18T15:14:56.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6471" PostTypeId="1" AcceptedAnswerId="6474" CreationDate="2014-12-18T15:17:02.017" Score="7" ViewCount="999" Body="&lt;p&gt;There are a number of places you can find details about currently operational satellites, and a few sources for past values. Is there a reliable source that has the dates of each launch and how many satellites each launch launched?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently the only method of getting this information I can think of is by getting TLE file for each object and fining the launch date and part, but this seems a little cumbersome. Any suggestions/references would be an ideal answer.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1142" LastEditDate="2014-12-19T15:18:40.857" LastActivityDate="2020-11-07T03:11:33.397" Title="Where is the best place to get a time history of launches and satellite numbers" Tags="|launch|artificial-satellite|reference-request|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6472" PostTypeId="2" ParentId="6460" CreationDate="2014-12-18T15:26:58.633" Score="7" Body="&lt;p&gt;Rendezvous and docking is not a trivial problem &amp;mdash; you're talking about the addition of essentially an additional complex payload to the upper stage.  Sensors, attitude control, comms, and some means to actually grapple and hold on to the likely tumbling target once you've gotten there.  You may find a target of opportunity that doesn't need much plane change to approach, but even small plane changes are expensive in terms of $\Delta V$.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a challenging problem even for a purpose-built spacecraft &amp;mdash; not sure it's feasible as an add-on to an upper stage.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="6816" LastEditDate="2017-09-26T20:23:22.277" LastActivityDate="2017-09-26T20:23:22.277" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6473" PostTypeId="2" ParentId="6471" CreationDate="2014-12-18T15:52:41.737" Score="3" Body="&lt;p&gt;The [SATCAT][1] on the Celes Trak site was last updated yesterday and does not require registration to use. It allows you to search for satellites by name, by their International Designator code or by NORAD designation. You may use the year(s) in the International Designator blank to search by time period.&lt;/p&gt;&#xA;&lt;p&gt;Here is a typical entry:&lt;br /&gt;&#xA;&lt;strong&gt;International Designator: 1971-093A&lt;/strong&gt;&lt;br /&gt;&#xA;&lt;strong&gt;NORAD Catalog Number: 05580&lt;/strong&gt;&lt;br /&gt;&#xA;&lt;strong&gt;Name: PROSPERO (BLACK ARROW)&lt;/strong&gt; [satellite (launch vehicle)]&lt;br /&gt;&#xA;&lt;strong&gt;Source: UK&lt;/strong&gt; [country or organization]&lt;br /&gt;&#xA;&lt;strong&gt;Launch Date: 1971-10-28&lt;/strong&gt;&lt;br /&gt;&#xA;&lt;strong&gt;Launch Site: WOMRA&lt;/strong&gt; [Australia]&lt;br /&gt;&#xA;&lt;strong&gt;Decay Date:&lt;/strong&gt; [blank, still in orbit]&lt;br /&gt;&#xA;&lt;strong&gt;Status: ?&lt;/strong&gt; [unknown]&lt;br /&gt;&#xA;&lt;strong&gt;TLE&lt;/strong&gt; [link to latest TLE]&lt;/p&gt;&#xA;&lt;p&gt;Other status codes are:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Operational&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Nonoperational&lt;br /&gt;&#xA;P   Partially Operational&lt;br /&gt;&#xA;B   Backup/Standby&lt;br /&gt;&#xA;S   Spare&lt;br /&gt;&#xA;X   Extended Mission&lt;br /&gt;&#xA;D   Decayed&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;pre&gt;&lt;code&gt;&#xA;Another [site for satellite launch data][2] is the one maintained by the [United Nations Office of Outer Space Affairs][3] (UNOOSA) in Vienna, based on data submitted to the committee by member states. You are allowed to search for any one or more of the following parameters:&#xA;&#xA;**Designation** (International Designator or Name)  &#xA;**State/Organisation**  &#xA;**Date of Launch**  (will accept full date, month/year or year)  &#xA;**Place of Launch** (Launch Facility)  &#xA;**Launch Vehicle**  &#xA;**Geostationary Satellites** (includes functional status)  &#xA;**Nuclear Power Sources**  &#xA;**UN Registration**  &#xA;**Status of Space Objects** (Presently in Space and Location Status)  &#xA;**Manned Spaceflight** (either Spacecraft or Space Station)  &#xA;**Satellite Constellations**  &#xA;&#xA;A query for PROSPERO includes the following under 'Function of Spacecraft':&#xA;&amp;gt;To test in space equipment for future satellites and to conduct a scientific experiment to measure the incidence of micro-meteoroids. Launch time 0409 GMT, injection point 13.7 deg S, 137.9 deg E. Anticipated life 100 years.  &#xA;&#xA;Satellite entries also include links to relevant websites, if available. &#xA;&#xA;&#xA;  [1]: https://celestrak.org/satcat/search.asp&#xA;  [2]: http://www.oosa.unvienna.org/oosa/showSearch.do&#xA;  [3]: https://en.wikipedia.org/wiki/United_Nations_Office_for_Outer_Space_Affairs&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2014-12-19T23:54:11.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6474" PostTypeId="2" ParentId="6471" CreationDate="2014-12-18T15:55:18.580" Score="8" Body="&lt;p&gt;What you want is the Satellite Catalog or &quot;SATCAT&quot; file.  The original public source for this is the Satellite Situation Report (SSR) published daily on Space-Track (&lt;a href=&quot;http://www.space-track.org&quot; rel=&quot;nofollow&quot;&gt;http://www.space-track.org&lt;/a&gt; - free registration required).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This file contains a record for every catalogued object which includes the launch date, launch site, object name, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a typical record:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1957-001B           2   CIS        96.1    65.0      945        227         N/A&#xA;SPUTNIK 1                              Launched (10/04/1957)  Decayed [01/03/1958]&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Breaking it down...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1957-001B&lt;/strong&gt; - International Designator, 1957 is the launch year, 001 indicates that this was the first launch of that year and B indicates it was the 2nd piece from that launch (A was the upper stage, which also achieved orbit)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2&lt;/strong&gt; - The object catalog number, aka NORAD id&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;CIS&lt;/strong&gt; - Responsible country or organization, in this case the Commonwealth of Independent States, AKA Soviet Union.  Space-Track has a key for these abbreviations&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;96.1&lt;/strong&gt; - Orbital period in minutes&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;65.0&lt;/strong&gt; - Orbital inclination in degrees&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;945&lt;/strong&gt; - Apogee altitude in Km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;227&lt;/strong&gt; - Perigee altitude in Km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;N/A&lt;/strong&gt; - This position would hold the RADAR Cross Section (RCS) in m^2, but it's not available for this object&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;SPUTNIK 1&lt;/strong&gt; - Object common name&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launched (10/04/1957)&lt;/strong&gt; - UTC launch date&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Decayed [01/03/1958]&lt;/strong&gt; - UTC decay date&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just noticed that the SSR doesn't have the launch site in it - I guess that's only in a separate SATCAT data product, which isn't available for general users on Space-Track.  The information is available via the site, but not as a file.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several of these fields may not be available for every object (inclination, period, apogee, perigee, RCS) but the launch date and international designator is always there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few other things about the SATCAT...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SATCAT records for an object aren't static - they are updated as orbits change or decay.  So the current inclination, apogee, perigee tell you where it is now, but not where it's been.  You would need the SATCAT or TLE history of the object to understand that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Debris objects can be correlated to the original object via piece component of the international designator.  This is also true for objects deployed from other objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Objects which don't remain in earth orbit have a note to that effect in place of the orbit shape/orientation parameters.  Here an example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1969-059A        4039   US     CIRCUMLUNAR                           N/A&#xA;APOLLO 11 CM (COLUMBIA)                Launched (07/16/1969)  Decayed [07/24/1969]&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="4709" LastEditDate="2014-12-18T16:18:24.360" LastActivityDate="2014-12-18T16:18:24.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6475" PostTypeId="2" ParentId="6403" CreationDate="2014-12-18T16:14:20.503" Score="5" Body="&lt;p&gt;According to this &lt;a href=&quot;http://www.nasaspaceflight.com/2014/12/spacex-static-fire-falcon-9-crs-5/&quot; rel=&quot;noreferrer&quot;&gt;article on NASASpaceFlight.com&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is also understood that the ASDS [Autonomous Spaceport Drone Ship -&#xA;  the barge] will have the ability to refuel returned stages, allowing&#xA;  them to make the hop back to land for future reuse.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it lands, is refueled, then flies by itself to wherever they want it. Which I think is pretty audacious.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't suppose they'll try to do that on the very first landing attempt (now scheduled for January 6), tho.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I think about it, this makes more sense than I first thought. My impression is that SpaceX wanted to land at the Cape but NASA wouldn't let them (safety concerns), so the barge was their workaround.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But they can position the barge way downrange, and use less fuel flying back up-range to land at the Cape (or Brownsville, whatever). Then refuel and fly back to base.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that leaves more fuel available for boosting the payload - more payload or more delta V. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe the barge makes sense even without NASA's safety concerns. [Ha - maybe they thought of this only because NASA wouldn't let them land at the Cape... wouldn't that be ironic?]&lt;/p&gt;&#xA;" OwnerUserId="6986" LastEditorUserId="49" LastEditDate="2014-12-18T16:39:58.293" LastActivityDate="2014-12-18T16:39:58.293" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6476" PostTypeId="2" ParentId="1314" CreationDate="2014-12-18T17:01:41.657" Score="21" Body="&lt;p&gt;&lt;em&gt;[Background: I'm writing this as a developer whose firmware's in flight on several substantial satellite missions. I've developed attitude control systems, working directly with RW hardware engineers.]&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Hash says, there is a lubrication distribution issue with conventional mechanical-bearing reaction wheels. The result is increased wear, leading to wear particulates that become embedded into the lubricant, further accelerating the wear process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a solution to this: magnetic-bearing RWs. These have no physical contact between the rotor and the static assembly, so have a mechanically indefinite lifetime. They also have the benefit of greater torque, and less micro-vibration. Unfortunately, these are both much more expensive, and can be rather heavier, than mechanical designs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I may expand the scope of the question slightly (but remain on topic): the situation is compounded by the finance model for space missions. In order to be able to declare a mission success (and hence funding for future projects), mission designers set a very low bar for success. Many, perhaps most, missions are designed with an unspoken expectation that they will exceed the success threshold by a very substantial margin. The problem is that the mission manifest will be based upon the bill of materials to achieve that low success threshold, something that will be under heavy scrutiny from budget-conscious managers and/or customers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, reaction wheels will be chosen that satisfy the base mission requirements, but are not necessarily fit for purpose for a substantially extended mission. Unsurprisingly, we then see early failures during the extended missions.&lt;/p&gt;&#xA;" OwnerUserId="6989" LastActivityDate="2014-12-18T17:01:41.657" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6477" PostTypeId="2" ParentId="6466" CreationDate="2014-12-18T17:15:11.327" Score="13" Body="&lt;p&gt;While KSC does use a lot of water for a launch, as CodingDuckling's answer points out, I'd have to say that, for comparison purposes, the answer is &quot;much less than a fish farm.&quot; &lt;a href=&quot;http://gallery.usgs.gov/audios/162#.VJMI6cCcA&quot;&gt;According to USGS&lt;/a&gt;, as of the year 2000, the state of Idaho alone used 1,970,000,000 gallons of water &lt;strong&gt;per day&lt;/strong&gt; for fish farming. By comparison, &lt;a href=&quot;http://en.wikipedia.org/wiki/Kennedy_Space_Center_Launch_Complex_39#Sound_suppression_water_system&quot;&gt;NASA's 300,000 gallons per launch&lt;/a&gt; isn't as impressive of a figure. Also &lt;a href=&quot;http://water.usgs.gov/watuse/wuto.html&quot;&gt;according to USGS&lt;/a&gt;, the U.S. as a whole used about 355,000,000,000 gallons of water per day, which is over 1,000 gallons per day per capita. So, an Apollo launch from KSC only used about as much water as is used per 300 people in the U.S. on an average day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As disappointing as it may seem, I'd honestly be pretty surprised if the average orbital launch didn't use more water for the bathrooms and drinking water for all of the people involved for the years of work involved to make that launch happen than is used for the actual launch itself and other purposes directly related to the rocket itself.&lt;/p&gt;&#xA;" OwnerUserId="2900" LastEditorUserId="2900" LastEditDate="2014-12-18T20:44:28.450" LastActivityDate="2014-12-18T20:44:28.450" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6478" PostTypeId="2" ParentId="2598" CreationDate="2014-12-18T19:16:13.133" Score="9" Body="&lt;p&gt;No idea on the Russian or Chinese, but the EMU's (Extravehicular Mobility Unit) Lithium-ion batteries (or LIB) have a 20.5 V (Volt) and 40 Ah (Ampere-hour) capacity. Previously, Silver-Zinc batteries with same voltage and 45 Ah current capacity were used, but were replaced for LIB. Since an EVA typically lasts no more than eight hours, that gives you an idea of the power loading:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;20.5 V * 40 Ah = 820 Wh (Watt-hours) total power available. Divided by 8 hours of operation, this gives an average power draw of no more than 102.5 watts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the &lt;a href=&quot;https://archive.org/details/nasa_techdoc_20050217305&quot; rel=&quot;noreferrer&quot;&gt;article about the &quot;new&quot; EMU battery&lt;/a&gt; quotes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The LIB delivers a maximum of 3.8 A on the average, for seven&#xA;  continuous hours, at voltages ranging from 20.5 V to 16.0 V and it&#xA;  should be capable of supporting transient pulses during start up and&#xA;  once every hour to support PLSS [Portable Life Support System] fan and&#xA;  pump operation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So in quoted 7 hours, it would consume between 425.6 and 545.3 Wh with roughly 33% spare capacity when the LIB are new.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="49" LastEditDate="2014-12-18T19:43:46.380" LastActivityDate="2014-12-18T19:43:46.380" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6479" PostTypeId="2" ParentId="4535" CreationDate="2014-12-18T19:24:10.460" Score="2" Body="&lt;p&gt;Rumor had it that the air-start of the SSME at stage separation was doable, but restarting it for any kind of orbital burn was not.  I have never seen this openly discussed, and can hardly believe it wasn't addressed in the conceptual design, but &lt;em&gt;something&lt;/em&gt; made them switch to J-2X with the subsequent dire effects on the program.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2014-12-18T19:24:10.460" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6480" PostTypeId="2" ParentId="6460" CreationDate="2014-12-18T20:04:39.723" Score="5" Body="&lt;p&gt;Many satellites have very specific narrow launch windows, of some minutes / day when all constraints are met for reaching the target orbit. Such constraints will usually not match well to a rendezvous soon after launch to an existing specific debris. If launch slips (a common event), the new launch window for primary payload will not in general still be compatible with a rendezvous with the same debris target. (The launch windows to ISS, for instance, are a few minutes wide.) A low fuel cost rendezvous at some date possibly many months after launch might be possible, but that would mean a need for the stage to be operable for that long (which means many changes to the stage; most stages operate for a few hours, not days or weeks). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as already said, almost all rendezvous have been with controlled targets designed with rendezvous in mind, not inert unpowered tumbling vehicles. &lt;/p&gt;&#xA;" OwnerUserId="6993" LastEditorUserId="6816" LastEditDate="2017-09-26T20:25:36.097" LastActivityDate="2017-09-26T20:25:36.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6481" PostTypeId="2" ParentId="4535" CreationDate="2014-12-18T22:21:52.877" Score="7" Body="&lt;p&gt;SSME wouldn't have been insane. RL-10 is used on relatively lightweight upper stages: Centaur and DCSS plus their payloads are in the 20-40 ton range, and the thrust-to-weight ratio winds up being in the ballpark of 0.3:1 -- not enough to lift off, but reasonable for circularizing your orbit once the first stage gets you most of the way to altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you've got 10 times the thrust, you can push a much larger payload at the same acceleration, so Ares-I second stage would be a much larger beast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know exactly how big the conceptual SSME-driven Ares-I second stage would have been, but you know how I love back-of-the-envelope math:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a shuttle SRB at 12000kN thrust, a total weight of 800 tons could have started off the pad at 1.5:1 thrust-to-weight, which is respectable. SRB is 600t, so second stage plus Orion would have come to 200t. SSME at 1860kN pushes that at just about 1:1 TWR when it lights up, increasing as fuel burns off, which is indeed much higher than most RL-10 stages, but not unacceptably high. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia, it seems like Orion was going to eat too much of that 200 tons mass budget:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Shortly after the initial design was approved, additional tests&#xA;  revealed that the Orion spacecraft would be too heavy for the&#xA;  four-segment booster to lift&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the design evolved from from a 4-segment SRB + SSME stage to a 5-segment SRB + J-2. The five-segment booster increases thrust to 16000kN (and presumably weight to about 750t), giving you 1088t off the pad at 1.5:1. 338t upper stage, J-2X at 1310kN, yields a much lower TWR at separation, about 0.4:1 -- very similar to what you see for RL-10-driven upper stages. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-18T22:21:52.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6482" PostTypeId="1" AcceptedAnswerId="6484" CreationDate="2014-12-19T01:05:33.180" Score="6" ViewCount="3146" Body="&lt;p&gt;Where can I find the location of the center of gravity (CoG) of any rocket? I need any popular rocket, such as the Soyuz or Space Shuttle, in any typical condition, like fully fueled on the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since many assemblies are symmetrical, it will be enough to know CoG height from the ground.&lt;/p&gt;&#xA;" OwnerUserId="5734" LastEditorUserId="1567" LastEditDate="2014-12-19T13:48:05.627" LastActivityDate="2015-12-09T02:23:01.783" Title="Center of mass location on any popular rocket in any typical condition?" Tags="|rockets|ballistics|reference-request|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6483" PostTypeId="2" ParentId="6482" CreationDate="2014-12-19T01:09:51.750" Score="7" Body="&lt;p&gt;I don't know how commonly rocket CGs are published. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can estimate it reasonably well from published weights and dimensions by modeling engines, fuel tank, and oxidizer tank as cylinders of uniform density, and computing weighted averages of the centers of those cylinders. If you don't know what order the fuel and oxidizer tanks are stacked in, assume the denser one is on top to keep the CG high; this will make more difference in hydrogen-fueled rockets with their very low fuel densities than in others.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2014-12-19T01:09:51.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6484" PostTypeId="2" ParentId="6482" CreationDate="2014-12-19T02:55:54.503" Score="16" Body="&lt;p&gt;Here is the Apollo 11 (Columbia and Eagle) graph of center of gravity during the AS-506 flight, taken from the &lt;a href=&quot;https://ia600505.us.archive.org/3/items/nasa_techdoc_19920075301/19920075301.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;APOLLO/SATURN V POSTFLIGHT TRAJECTORY - AS-506&lt;/a&gt; (rather large scanned PDF):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Varvo.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I selected Apollo 11 flight for historical significance, but you could find many other postflight telemetry and flight analysis documents online for various launch vehicles, or approximate it yourself using e.g. such calculations (and technical data of specific launch vehicle configurations that can often be found online, say on Wikipedia):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/TRATB.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Determining Center of Gravity - cg. Image source: &lt;a href=&quot;http://exploration.grc.nasa.gov/education/rocket/shortr.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Rocket Fundamentals&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where weight $w= m \times g$ (mass times &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational constant&lt;/a&gt;) and $d$ is distance from reference location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do note though that &lt;a href=&quot;http://exploration.grc.nasa.gov/education/rocket/rktcg.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;center of gravity&lt;/a&gt; does depend on individual vehicle's specific configuration, payload mass distribution, flight profile, that it will shift vertically during flight as stages consume propellants and during staging, and perhaps more significantly that this data is on its own only perhaps relevant to static stability of the launch vehicle. For anything else, i.e. dynamic / flight stability, you'll at the minimum also require values for &lt;a href=&quot;http://exploration.grc.nasa.gov/education/rocket/rktcp.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;center of pressure&lt;/a&gt; as a function of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a graph showing movement of both along the long axis for the first 140 seconds of a Saturn V flight, taken from &lt;a href=&quot;http://www.ibiblio.org/apollo/Documents/tn_d-5869_1970023342.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Description and Performance of the Saturn Launch Vehicle's Navigation, Guidance, and Control System&lt;/a&gt;, Walter Haeussermann, MSFC 1970 (PDF):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/d2eSh.png&quot; alt=&quot;Variations of center of pressure and center of mass during rocket flight&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Variations of center of pressure and center of mass during flight.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're right though that typically CG only moves along the long axis (vertically) and that the mass distribution of launch vehicles and their payloads are balanced to remain centered along this axis. For launch vehicles, mass distribution and center of gravity would most commonly be established during so-called &lt;a href=&quot;https://en.wikipedia.org/wiki/Boilerplate_%28spaceflight%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;boilerplate&lt;/em&gt; mass simulators&lt;/a&gt;, and for their payloads also during payload integration. Oftentimes, ballasts weights (or even secondary piggyback payloads - like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/OSCAR&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSCAR&lt;/a&gt; satellites) would be used to assure that launch vehicles and payloads are spin stable.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-09T02:23:01.783" LastActivityDate="2015-12-09T02:23:01.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6486" PostTypeId="1" AcceptedAnswerId="6487" CreationDate="2014-12-19T04:18:50.397" Score="18" ViewCount="2580" Body="&lt;p&gt;Here is a picture of LC-40 at the Cape.  The four tall towers are basically lightning rods.  Why are four required?  Wouldn't one be sufficient?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/idnCg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2016-12-09T10:45:25.027" Title="Why multiple lightning towers at launch sites?" Tags="|launch-site|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6487" PostTypeId="2" ParentId="6486" CreationDate="2014-12-19T04:46:38.647" Score="19" Body="&lt;p&gt;No, one wouldn't be sufficient unless it would be a really tall and massive (very conductive) structure stretching far above the launch vehicle (&lt;a href=&quot;https://space.stackexchange.com/questions/1968/do-rocket-exhaust-plumes-really-provide-a-pathway-for-lightning&quot;&gt;apparently higher than the Saturn V's launch tower&lt;/a&gt;), because lightnings can be rather unpredictable from which direction they will strike. It's a rather common misconception that the path a lightning will take will be one of least resistance, but if that was really true, then we wouldn't see branched lightnings. See e.g. this fascinating &lt;a href=&quot;https://www.youtube.com/watch?v=W9xzU0xjlhE&quot; rel=&quot;nofollow noreferrer&quot;&gt;video of slow motion lightning strikes&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens though is that the lightning discharges through an ionized (plasma) conduit that branches out rather randomly through smaller, more localized discharges. When the two ends (the &lt;em&gt;source&lt;/em&gt; and the &lt;em&gt;sink&lt;/em&gt;) become connected with this now far more conductive conduit than the surrounding medium, i.e. reaches the ground or anything tall and conductive sticking out of it, the main discharge between two charged areas (the lightning) happens that could damage even a well grounded launch vehicle due to rapid &lt;a href=&quot;https://en.wikipedia.org/wiki/Joule_heating&quot; rel=&quot;nofollow noreferrer&quot;&gt;resistive heating&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So they use four lightning towers all around the launch vehicle that are also connected between them via stainless steel wires to form a very effective &lt;s&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Faraday_cage&quot; rel=&quot;nofollow noreferrer&quot;&gt;Faraday cage&lt;/a&gt;&lt;/s&gt; &lt;a href=&quot;https://www.youtube.com/watch?v=p_uCN1jn8bw&quot; rel=&quot;nofollow noreferrer&quot;&gt;FARADAY CAGE!&lt;/a&gt; from all directions and can connected together handle higher current density due to smaller resistance with their joint thickness (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Ohm%27s_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ohm's law&lt;/a&gt;) and provide a highly conductive medium. Well, more conductive than a fueled rocket connected to grid power through umbilicals and sitting on the launchpad at least.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-12-19T05:26:31.890" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6488" PostTypeId="2" ParentId="6460" CreationDate="2014-12-19T10:29:59.287" Score="0" Body="&lt;p&gt;Assuming that the debris is large enough to pick up in a meaningful fashion, which is &lt;em&gt;far&lt;/em&gt; from always the case (orbital-velocity flakes of paint anyone?), and besides the problems with launch windows, delta-v requirements both for orbital adjustments for rendezvous (and for deorbiting), the rendezvous itself, docking and grappling which have already been mentioned (I'll just handwave those away here, deferring to the existing answers for discussion on how the previous factors are difficult), there is an additional complication. &lt;strong&gt;Getting the thing back down to Earth in a safe manner.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, you could aim for a big pond of water and just tell people to stay out of the way through appropriate channels. (For aircraft there are NOTAMs, and I'm certain there is something similar for ocean vessels.) I'm sure &lt;em&gt;more than one&lt;/em&gt; spacecraft has been deliberately sent into the Atlantic or the Pacific. But even something like that adds complexity as well. I doubt that the state of a particular piece of space debris is perfectly known, but mass makes a &lt;em&gt;big&lt;/em&gt; difference for rocket handling in a freefall/microgravity environment. Being off by a few percent of mass could easily throw your deorbit burn off completely, causing you to hit the Earth in some very different location than you planned. And like someone pointed out in some other answers to another question, about the only certainty is that once you perform a deorbit burn, you &lt;em&gt;are&lt;/em&gt; going to be on an orbit that intersects ground in some fashion, somewhere. Best hope there isn't anyone in the way where that happens.&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2014-12-19T10:29:59.287" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6489" PostTypeId="2" ParentId="5525" CreationDate="2014-12-19T11:30:40.833" Score="0" Body="&lt;p&gt;If NASA needed to reboost ISS a Dragon could be fitted with a propulsive trunk. 4/8 Super Dracos mounted on its sidewalls &amp;amp; fuel tanks in the unpressurised cargo space. &lt;/p&gt;&#xA;" OwnerUserId="7001" LastActivityDate="2014-12-19T11:30:40.833" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6490" PostTypeId="1" AcceptedAnswerId="6509" CreationDate="2014-12-19T12:03:49.817" Score="10" ViewCount="5408" Body="&lt;ul&gt;&#xA;&lt;li&gt;At NASA Wallops the &lt;a href=&quot;http://www.nasa.gov/content/nasa-wallops-visitor-center-launch-viewing/&quot;&gt;visitor center&lt;/a&gt; is 4 miles from the pad.&lt;/li&gt;&#xA;&lt;li&gt;Families of shuttle crews could watch from 3 miles away as their&#xA;loved ones where hurled into orbit.&lt;/li&gt;&#xA;&lt;li&gt;At Biakonur I can &lt;a href=&quot;http://www.worldtimezone.com/travel/baikonur-tour.html&quot;&gt;apparently&lt;/a&gt; get to within 1.1 miles of the launch.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But is that the closest I could get, as a civilian not affiliated with the crew? Is there anywhere in the world that allows viewing from even closer? Moreover, what's the closest &lt;em&gt;anyone&lt;/em&gt; has been to an orbital launch. (including bunkers)&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="415" LastEditDate="2014-12-19T12:26:37.533" LastActivityDate="2014-12-20T14:31:50.953" Title="How close can I get to a rocket launch?" Tags="|launch|launch-site|launchpad|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6492" PostTypeId="1" AcceptedAnswerId="6495" CreationDate="2014-12-19T14:00:41.903" Score="7" ViewCount="1155" Body="&lt;p&gt;Cubesats and femto-sats are a nice place for universities and small enterprises to set their sights on. There are a number of sources of hardware and software (some even available at reasonably low prices), but outside of specific projects (launch many at once together) I can't seems to find any accurate price model. Data please!&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="9092" LastEditDate="2021-03-26T14:54:40.910" LastActivityDate="2021-03-26T14:54:40.910" Title="Barriers to launching a cubesat" Tags="|launch|cubesat|reference-request|secondary-payload|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6493" PostTypeId="2" ParentId="6492" CreationDate="2014-12-19T14:26:06.470" Score="5" Body="&lt;p&gt;Edit: This answer was written before the question was clarified to specifically address launch costs, and as such does not fully answer the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a good &lt;a href=&quot;http://www.satmagazine.com/story.php?number=602922274&quot; rel=&quot;nofollow&quot;&gt;price breakdown/analysis on SatMagazine&lt;/a&gt; for some generic numbers. However, it was written in 2009 so may be a little out of date. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might also take a look at &lt;a href=&quot;http://www.clyde-space.com/&quot; rel=&quot;nofollow&quot;&gt;Clyde-Space&lt;/a&gt;, they have a lot of products with clear pricing, and more importantly no ITAR restrictions. &lt;a href=&quot;http://www.cubesat.org/index.php/collaborate/suppliers&quot; rel=&quot;nofollow&quot;&gt;Here's&lt;/a&gt; another good list of potential component suppliers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure there's better data out there, but that's the best I know of. Hope it's helpful.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-03-24T15:40:16.800" LastActivityDate="2015-03-24T15:40:16.800" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6495" PostTypeId="2" ParentId="6492" CreationDate="2014-12-19T15:04:04.593" Score="10" Body="&lt;p&gt;Launching a CubeSat is by their nature a difficult process. They are designed as a secondary payload, which means that the primary payload dictates the entire launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some programs such as NASA's &lt;a href=&quot;http://www.nasa.gov/directorates/heo/home/CubeSats_initiative.html#.VJQ7YAT1I&quot;&gt;CubeSat initiative&lt;/a&gt;, which provide free rides to educational projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For commercial launches, data is spotty. One CubeSat reportedly launched for 100k (&lt;a href=&quot;http://www.southgatearc.org/news/april2013/ossi_1_amateur_radio_cubesat_launched.htm#.VJRO-AT1I&quot;&gt;OSSI-1&lt;/a&gt;). One company charges about 300k for a 3u (&lt;a href=&quot;http://spaceflightservices.com/pricing-plans/&quot;&gt;Space Flight Services&lt;/a&gt;). So 100k per Unit seems like a good ballpark. Both quotes are for LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recently many CubeSats have been shipped to the ISS, and then released from there with the &lt;a href=&quot;http://nanoracks.com/products/smallsat-deployment/&quot;&gt;NanoRacks deployer&lt;/a&gt;. However, there have been some problems with the launcher (&lt;a href=&quot;http://news.discovery.com/space/space-stations-cubesat-cannon-has-mind-of-its-own-140905.htm&quot;&gt;&lt;em&gt;Discover&lt;/em&gt; news article&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been a few proposals of small rockets designed to just launch CubeSats, but as far as I know none of these have come to fruition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is that CubeSats are by design just catching a ride, so there is no real price model or launching service.&lt;/p&gt;&#xA;" OwnerUserId="4686" LastEditorUserId="1142" LastEditDate="2014-12-19T16:30:18.823" LastActivityDate="2014-12-19T16:30:18.823" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6496" PostTypeId="2" ParentId="2080" CreationDate="2014-12-19T16:01:52.103" Score="5" Body="&lt;p&gt;On March 7, 1970 a total solar eclipse passed directly over Wallops Island, VA. In conjunction with quite a few university groups NASA launched about 36 sounding rockets within a 4 hour period.  I was there, sitting on the beach about one mile from the pads.  Preparations and rehearsals for this extravaganza of launches started a couple of years before the event, and NASA Wallops borrowed practically every portable radar and theodolite they could find in the country.&lt;/p&gt;&#xA;" OwnerUserId="7006" LastActivityDate="2014-12-19T16:01:52.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6498" PostTypeId="2" ParentId="6432" CreationDate="2014-12-19T21:02:17.250" Score="12" Body="&lt;p&gt;0.012 inches (or approximately 0.305 mm) is actually fairly sturdy, under the right circumstances. See &lt;a href=&quot;https://space.stackexchange.com/a/933/415&quot;&gt;this related answer&lt;/a&gt; as well, but &lt;a href=&quot;http://www.canmaker.com/news/index.php?option=com_content&amp;amp;view=article&amp;amp;id=1504&amp;amp;Itemid=126&quot; rel=&quot;nofollow noreferrer&quot;&gt;a soda can may have a wall thickness of 0.1 mm or even less&lt;/a&gt; and is capable of containing &lt;a href=&quot;http://hypertextbook.com/facts/2000/SeemaMeraj.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;30 pounds per square inch (psi)&lt;/a&gt; against a &lt;a href=&quot;https://en.wikipedia.org/wiki/ICAO_Standard_Atmosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;standard atmosphere's 14.7 psi pressure&lt;/a&gt; on the outside (a 15.3 psi positive pressure). (The ICAO definition of a standard ground-level atmosphere according to Wikipedia is &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Standard_Atmosphere#ICAO_Standard_Atmosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;1013.25 hPa = 101325 Pa&lt;/a&gt;, &lt;a href=&quot;https://www.google.com/search?q=what+is+101325+Pascals+in+psi%3F&quot; rel=&quot;nofollow noreferrer&quot;&gt;which Google converts to just under 14.7 psi&lt;/a&gt;.) The LM was not tasked with such a pressure difference: according to &lt;a href=&quot;http://history.nasa.gov/SP-350/ch-4-4.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo Expeditions to the Moon, chapter 4.4&lt;/a&gt; and &lt;a href=&quot;http://ocii.com/~dpwozney/apollo2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo Oxygen And Two-Gas Environment Problems&lt;/a&gt;, Apollo missions used a cabin pressure of approximately 4.8-5.0 psi in flight. (NASA states &quot;5 psi&quot; on the former page linked whereas the latter page linked states a more exact &quot;4.8 psi&quot;; whether it is more &lt;em&gt;accurate&lt;/em&gt;, I don't know, but the difference between the two numbers is small enough to be insignificant for our purposes.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under a positive pressure, materials tend to expand. Provided that the strength of the material is adequate to hold in the positive pressure (if it wasn't, the LM would have ruptured violently most likely during ascent tucked away inside the Saturn V as the pressure difference between its interior and exterior grew), this also tends to make them rigid. As even a thought experiment, &lt;strong&gt;take an unopened soda can and deliberately try to puncture it.&lt;/strong&gt; You may succeed in puncturing the can, but it's likely to take a fair amount of deliberate effort. The ratio of positive pressure inside the soda can to the thickness of its wall (about 15 psi to 0.1 mm, or 150 psi per mm of thickness) is &lt;strong&gt;significantly higher compared to that seen by the LM pressure vessel while in flight in space,&lt;/strong&gt; which is the only thing it was designed to do (about 5 psi to 0.3 mm, or 15 psi per mm of thickness).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while 0.3 mm might sound very thin, and at the face of it sound like it is at risk of being punctured, by the time you consider what it really needed to do and what conditions it faced, it turns out that it most likely was quite adequate for the job. Going with thinner materials also saved a lot of weight, which made quite a big difference in the difficulty in going to the Moon at all. If the people designing and building the LM had used something &quot;obviously sturdy enough&quot;, it's entirely possible that there would have been no need for the LM at all because the tyranny of the rocket equation might have meant the additional mass made just the difference that killed the idea of the Trans-Lunar Injection (or even earlier). It certainly would have required the Saturn V to be even larger than it already was, to get the additional mass into a transfer orbit to the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And of course, if the hole was small enough, &lt;a href=&quot;https://space.stackexchange.com/a/933/415&quot;&gt;you could just patch it up&lt;/a&gt;. Even &lt;a href=&quot;https://space.stackexchange.com/a/875/415&quot;&gt;a &quot;simple&quot; space suit can handle a puncture&lt;/a&gt;. Only in movies do things violently blow up or decompress because of a small hole. Heck, people have survived &lt;a href=&quot;https://space.stackexchange.com/a/3460/415&quot;&gt;being exposed to very low air pressure&lt;/a&gt;, including &lt;a href=&quot;https://space.stackexchange.com/a/126/415&quot;&gt;during Apollo equipment testing&lt;/a&gt; as well.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-12-23T08:51:10.677" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6499" PostTypeId="2" ParentId="6432" CreationDate="2014-12-19T21:05:27.693" Score="10" Body="&lt;p&gt;The shear strength of 6061-T6 AL is about 30,000 psi.  So if we conservatively estimate the perimeter of a boot to be about 24&quot;, and assume a rigid boot applying its load uniformly, we get about 30,000 psi x 24&quot; x 0.012&quot; = 8,640 lbf of pressure to yield the wall.  This is often called &lt;a href=&quot;http://baeresources.com/resources/punching-shear-stress/&quot; rel=&quot;nofollow noreferrer&quot;&gt;punching shear stress&lt;/a&gt;.  More (much more) would be required to fracture the wall.  Realistically, the wall would start to bend and the failure mode wouldn't be pure shear.  This of course ignores other loads (pressure for example) that are being applied to the wall at the same time and/or stress concentrators (like attach fittings and holes) -- which would further complicate the failure mode.  But 8K+ lbf seems like a lot of margin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this really depends on the radius of curvature of the boot edge and the boot's hardness.  &lt;a href=&quot;https://space.stackexchange.com/a/6498/415&quot;&gt;As Michael Kjörling pointed out&lt;/a&gt;, 12 mils is pretty strong under uniform pressure.  However, if I subject this to a concentrated point load I can, like a soda can, easily &quot;put my foot through it.&quot;  You can see this by taking a reasonably sharp awl (small radius of curvature and high hardness) to a soda can.  I can puncture it with very little effort.  Boots are soft and compliant though compared to an awl.  And I suspect that the astronauts weren't wearing spurs...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a side note, this is a critical load case for the design of the panel that runs down the floor of the aisle in commercial jets.  High heels are rough...&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2014-12-19T22:38:52.917" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="6500" PostTypeId="2" ParentId="6471" CreationDate="2014-12-20T00:39:46.073" Score="1" Body="&lt;p&gt;&lt;a href=&quot;http://planet4589.org/jsr.html&quot; rel=&quot;nofollow&quot;&gt;Jonathan's Space Report&lt;/a&gt; and his &lt;a href=&quot;http://planet4589.org/space/log/launch.html&quot; rel=&quot;nofollow&quot;&gt;Launch Log&lt;/a&gt; are also great resources for analysis.  Example entry from the Launch Log:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2014-069     2014 Oct 30 0142:52        2014-069A      Meridian No. 17L               Meridian No. 17L          S40296   Soyuz-2-1A             76058-  174/102 GIK-1    LC43/4                  S    nasaspaceflight.com&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Indicating that the 69th launch of 2014, launched on October 30 at 01:42:52 UTC had COSPAR ID 2014-069A, primary payload &quot;Meridian No. 17L&quot;, with the same original name for the payload.  The launch was successful and was assigned SATCAT ID 40296.  It flew on a Soyuz-2-1A rocket with serial number 76058-174/102 from the Plesetsk launch site (code GIK-1), pad number LC43/4.  The source for the information was &lt;a href=&quot;http://www.nasaspaceflight.com&quot; rel=&quot;nofollow&quot;&gt;nasaspaceflight.com&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nice thing about the Launch Log is that it's readily parsed with UNIX tools such as awk and cut.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2014-12-20T00:39:46.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6501" PostTypeId="1" AcceptedAnswerId="6508" CreationDate="2014-12-20T01:44:58.297" Score="12" ViewCount="1046" Body="&lt;p&gt;I have a distinct feeling that there's some apparent &lt;em&gt;no-go&lt;/em&gt; on the question in the title, but currently can't put my finger on it;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the lifting mass to Low Earth Orbit (LEO), let's assume that &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot;&gt;SLS&lt;/a&gt; (Space Launch System) Block 1 is completed (crew and cargo) and for payload size that the missions could use ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Automated_Transfer_Vehicle&quot;&gt;ATV&lt;/a&gt;-derived &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_Service_Module&quot;&gt;Orion Service Module&lt;/a&gt;. Obviously, &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot;&gt;Orion Crew Module&lt;/a&gt; would be used with the maximum crew size of four, where the HST-STS missions had a crew of seven. Let's say that we need to launch at least as numerous crew as the number of Extravehicular Activity (EVA) astronauts during each HST-STS mission. All other mission parameters would stay identical to the &lt;em&gt;&quot;4½&quot;&lt;/em&gt; (OK, 5) &lt;a href=&quot;http://www.nasa.gov/mission_pages/hubble/servicing/index.html&quot;&gt;Hubble Space Shuttle servicing missions&lt;/a&gt; (&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-61&quot;&gt;STS-61&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-82&quot;&gt;STS-82&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-103&quot;&gt;STS-103&lt;/a&gt; + &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-109&quot;&gt;STS-109&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-125&quot;&gt;STS-125&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So could Orion eventually pull something like this off, even if multiple launches are used for each individual mission?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-12-20T01:50:26.403" LastActivityDate="2021-07-13T10:14:44.793" Title="Could Orion repeat any of the Hubble Space Shuttle servicing missions?" Tags="|space-shuttle|orion-spacecraft|sls|hubble|crew-size|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6504" PostTypeId="1" AcceptedAnswerId="6510" CreationDate="2014-12-20T04:51:52.410" Score="10" ViewCount="1625" Body="&lt;p&gt;I can't seem to find any good sources online for this, all I get are documents on how Nice model was used to compute Neptune's gravity capture of Triton via a binary dissociation, possibly because of similar terminology. So to the question;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To what extent, in terms of % delta-v, could Triton's retrograde orbit around Neptune gravitationally assist in slowing down a direct Hohmann transfer from Earth to Neptune for Neptune Orbit Insertion (NOI) during a single close-shave flyby of Triton?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize that the transfer would take about good three decades (I calculated 30.61652 years at Hohmann semi-major axis of 15.53545 AU using &lt;a href=&quot;http://www-istp.gsfc.nasa.gov/stargaze/Skeplaws.htm&quot;&gt;Kepler's third law&lt;/a&gt; and semi-major axes of Earth and Neptune), transfer time is not a concern, but I'm stuck with calculating velocity relative to Neptune at Triton's altitude for this transfer (I'm assuming Triton to be at the right place at the right time) and I didn't yet get to calculate delta-v that a low altitude (100 km above surface) gravity assist flyby of Triton could shed, so I'm not sure I'd know how to do that either. Neptune NOI would be for a Neptune orbit with semi-major axis of roughly 5,000 km above its &lt;em&gt;surface&lt;/em&gt; (at 1 atm pressure). There wouldn't be any plane change for the target orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is not a homework question. It's been about 20 years since I last &lt;em&gt;had to&lt;/em&gt; do such calculations back in the college years and I'd appreciate a bit of help in brushing up on this. I could plug this in some software but, call me masochistic, I wanted to do it the old-fashioned way. Back-of-an-envelope calculations would do, preferably discussing any shortcuts there might be in getting first-order approximations faster. No need to be too academic either, but I would like to see some calculations here, if that's not terribly inconvenient, in which case I guess a good reference with a short-ish writeup would do, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you'll opt for the latter option using external references, please also include optimal launch dates, delta-v and Hohmann transfer times.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-12-20T17:18:59.567" Title="To what extent could a single Triton flyby slow down a direct Hohmann transfer to Neptune for NOI?" Tags="|orbital-mechanics|gravity-assist|hohmann-transfer|neptune|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6505" PostTypeId="1" CreationDate="2014-12-20T07:32:28.183" Score="10" ViewCount="3317" Body="&lt;p&gt;Just had 2 evenings when the ISS was visible twice on successive passes.&#xA;Times for appearance were 94 minutes apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I use the quoted value of g at 400 km above the earth of 0.89m/s/s I can calculate orbit time of 91 minutes which is pretty close BUT it makes no allowance for the earth's rotation in the 90 minutes which is not insignificant - about 22deg.  For the ISS to 'catch up' with the earth's rotation surely it should need another 6 minutes or so.&#xA;Where is the flaw in my thinking?&lt;/p&gt;&#xA;" OwnerUserId="7012" LastEditorUserId="168" LastEditDate="2014-12-20T08:51:41.690" LastActivityDate="2014-12-20T19:25:59.237" Title="ISS: time between two successive observations and orbital period" Tags="|iss|orbit|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6506" PostTypeId="1" CreationDate="2014-12-20T09:01:46.650" Score="14" ViewCount="2533" Body="&lt;p&gt;Why is it that Controlled Re-entry Vehicles (like the most recent Orion &amp;amp; Dragon) do not use a strong magnetic field during re-entry to &quot;shield&quot; the blunt shaped end from plasma ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reasoning: Charged particles (plasma) are deflected by magnetic fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnetic field will only have to be activated for a few minutes &amp;amp; maybe can also pivot the vehicle by controlling its magnitude ever so slightly. It is mainly used to reduce the heat load/intensity on the heat shield, control descent speed and provide maneuverability &#xA;Newtons 3rd law states that the magnetic field will slow down the vehicle depending on its strength.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quick calculation using $F=qvB$, where $v=8900\text{ m/s}$ (highest velocity on reentry), $F=255\text{ kN}$ (3g deceleration on orion crew module $8.5 \text{ tons}$), $q=10\text{ C}$ (Arbitrary) implies $B=2 \text{ Tesla}$. Which is possible to build light.The charge of plasma was chosen randomly, a more accurate charge will be very helpful!&lt;/p&gt;&#xA;" OwnerUserId="7013" LastEditorUserId="5963" LastEditDate="2014-12-20T21:39:43.293" LastActivityDate="2021-03-27T17:04:14.713" Title="Re-entry Heat Shield Alternative" Tags="|reentry|magnetic-field|atmospheric-drag|heat-shield|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6507" PostTypeId="2" ParentId="6506" CreationDate="2014-12-20T09:34:27.023" Score="10" Body="&lt;p&gt;There are several misconceptions in your question/proposal:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strike&gt;The Orion and the Dragon as well as other reentry capsules do not fly with the cone tip forward. They fly with the blunt &quot;bottom&quot; side forward.&lt;/strike&gt; (&lt;em&gt;fixed in the revised question&lt;/em&gt;)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Plasma is created by the very passage of the vehicle through the air at high hypersonic velocities. At lower velocities, ionization is far less, but the resistance from the air is much more - imagine hitting a brick wall at 60 mph.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Magnetic field requires bulky generators and heavier batteries, which makes launches much more expensive.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Reentry means using the Earth's atmosphere to decelerate the craft. If you somehow remove the source of drag, you are nullifying the advantages the atmosphere provides.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;However, your question has the seeds of usable engineering devices, albeit for other phases of flight or purposes:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Launch through the lower, densest atmosphere does benefit from drag reduction. Hence, aerospikes have been invented.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Designers of reentry vehicles in less civilian settings may find it necessary to reduce their radar signature, and may resort to &quot;electromagnetic cloaking&quot;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-12-20T12:35:35.680" LastActivityDate="2014-12-20T12:35:35.680" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6508" PostTypeId="2" ParentId="6501" CreationDate="2014-12-20T11:14:29.813" Score="12" Body="&lt;p&gt;Without arm and cargo bay and with only one EVA because the Orion has no airlock, the cabin can be refilled with air once, there would be some serious limitations. Orion could maybe be extended with an extra module for such a mission. Orion is not really a complete spaceship like the shuttle was, it is an exploration vehicle component which needs a larger system. In order to be useful in the long run it will need complementary modules for lunar landing, deep space habitation and why not satellite handling equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion is not made for LEO. A future Dragon or CST-100 would be more suitable for servicing a satellite in LEO, if complemented with extra equipment. Not more capable but at least cheaper. Orion could possibly service telescopes at Lagrange points, where Gaia is today and where James Webb will be sent. Those telescopes have however not been designed to be upgradable or serviced. The advantage of Orion is that it can reach the Moon-Earth and (barely) Sun-Earth Lagrange points 1 and 2. If it will service satellites at all, it will happen there.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2014-12-20T11:14:29.813" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6509" PostTypeId="2" ParentId="6490" CreationDate="2014-12-20T13:51:54.993" Score="12" Body="&lt;p&gt;The closest launch to the public for an orbital launch is almost certainly the one you listed, Baikonur. I'm quite confident that for people outside, that is probably about the closest you can get as well. Quite frankly, a mile away isn't very safe to watch a launch...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for just how close can you get in a protected environment, outside of the spacecraft? There are a few items which I can't determine which would let you be the closest. Contenders are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;I believe I've heard of underground bunkers where some engineers sit during a launch, to do things like stop charging batteries right before launch. &lt;/li&gt;&#xA;&lt;li&gt;There is the infamous &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rubber_room_%28bunker%29&quot; rel=&quot;noreferrer&quot;&gt;Rubber Room&lt;/a&gt;&quot;, which was designed to protect astronauts and staff in the event of an emergency with a fully fueled Apollo 5. To my knowledge, this was never used during a launch, but it could have been, and was indeed very close.&lt;/li&gt;&#xA;&lt;li&gt;Submarines often launch orbital vehicles. The crew is presumably very close during launch.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2014-12-20T14:31:50.953" LastActivityDate="2014-12-20T14:31:50.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6510" PostTypeId="2" ParentId="6504" CreationDate="2014-12-20T17:18:59.567" Score="8" Body="&lt;p&gt;I'll try to get you started anyway.  From the frame of reference of the assisting body, the trajectory of the probe is hyperbolic, with the same $v_\infty$ going out as coming in, but in a different direction.  The trajectory is simply bent.  The bend angle is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\delta=2\sin^{-1}\left(1\over e\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $e$ is the eccentricity of the hyperbola.  You can get $e$ from $v_\infty$, the closest approach radius $r$ (measured from the center of the body), and the $GM$ of the body, $\mu$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$e={r\,v^2_\infty\over\mu}+1$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$e$ goes from $1$ to $\infty$, where $1$ is effectively an infinite gravity assist that reverses the direction of the velocity by 180°, and $\infty$ is no assist at all with a bend of 0°.  The closer in you can get and the slower you can go, the greater the bend angle is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the vector addition of the velocity of the assisting body (Triton) in the frame of the body it is orbiting (Neptune), you can see how the velocity of the probe is changed by the flyby in the Neptune frame, which is what you care about.  You will need to do the reverse to get the $v_\infty$ approaching Triton, i.e. subtract the velocity vector of Triton from the approach velocity vector in the frame of Neptune.  To get &lt;em&gt;that&lt;/em&gt;, you subtracted the velocity vector of Neptune from the velocity in the Sun frame of the Hohmann transfer.  Lots of frame changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a range of geometries for the flyby you can try, depending on where in its orbit Triton is at the time, the altitude of the flyby, and whether the flyby closest approach is on the leading or trailing side of Triton in its orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2014-12-20T17:18:59.567" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6511" PostTypeId="2" ParentId="6505" CreationDate="2014-12-20T17:25:46.593" Score="5" Body="&lt;p&gt;&lt;em&gt;Flaw in your thinking&lt;/em&gt; (I wouldn't really call it like that, it's an honest question) is that when the station is observable during two consecutive orbits, it would be observable during the first pass eastward from your stationary location, and on the second pass westward. You see, the station's ground track moves westward (about 2,505 km or 1,556 miles at the equator) as the Earth rotates towards the east. So the Earth's rotation is, in a sense, &lt;em&gt;caching up&lt;/em&gt; with the first observation's ground track.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if the second pass will go closer to zenith than the first one did (makes an apparent movement arc more directly overhead), then you'll be able to also see it a bit earlier in its orbital position that you did the first time around. Depending on your location relative to the station's first pass ground track, this slight variation in observable times could also go the other way around (like it seems it happened in your case), and you happen to have caught two consecutive passes when the first pass was closer to you (it would also be observable for a longer time, the station would appear brighter and it would have an eastward azimuth compared to the second pass).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, you probably also slightly miscalculated the station's orbital period, might be due to rounding input values, which in your case only increased the perceived difference in station passes timetable. Station's average orbital altitude is 420 km above mean sea-level, higher than your input of 400 km, so its true orbital period would also be slightly longer than 91 minutes. Let's do the calculations again:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If orbital period (in seconds) is given by $T = 2\pi\sqrt{a^3/\mu}$ (derived from Kepler's third law saying that the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit), $a$ is semi-major axis of the orbit (6798.1 km from Earth's center), and $\mu$ is &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;noreferrer&quot;&gt;standard gravitational parameter&lt;/a&gt; (398,600.4418 km&lt;sup&gt;3&lt;/sup&gt;s&lt;sup&gt;−2&lt;/sup&gt; for Earth), then:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T = 2\pi\sqrt{314,168,505,637.141\ km^3/398,600.4418\ km^3s^{-2}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T = 2\pi\sqrt{788,179.02\ s^{-2}} = 6.28318531 \times 887.7945\ s = 5,578.18\ s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or 92.97 minutes. So the westward drift you observed on the second pass caused that the station was from your stationary position observable about one minute later than on the first pass, i.e. the station must have passed more directly overhead to your location on the first pass than it did on the second. Note that part of this might also be due to observational conditions, such as natural obstacles limiting your horizon towards the first and the second pass differently, and if your observations were just after dusk, it would be slightly darker on the second pass and you might be able to spot the station sooner because of that, too. So there's many different factors sliding this observational time up to a few minutes forward or backward in stations orbital position relative to your stationary location.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-12-20T17:35:48.420" LastActivityDate="2014-12-20T17:35:48.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6512" PostTypeId="2" ParentId="4354" CreationDate="2014-12-20T18:59:10.240" Score="4" Body="&lt;p&gt;I was looking for possible answers for an US-sourced engine, and was going to float the &lt;a href=&quot;http://en.wikipedia.org/wiki/Rocketdyne_RS-27A&quot; rel=&quot;nofollow&quot;&gt;RS-27A&lt;/a&gt; as a possibility (though it's been out of production for over a decade) -- you'd probably need a cluster of 4 to substitute for a pair of AJ-26.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then I saw this reference to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerojet_Rocketdyne#AR-1&quot; rel=&quot;nofollow&quot;&gt;Aerojet Rocketdyne AR-1&lt;/a&gt; proposal. At 2200 kN, a pair of them would be up in the RD-180 class, and it looks like they're expecting to use them &lt;a href=&quot;http://www.rocket.com/ar1-booster-engine&quot; rel=&quot;nofollow&quot;&gt;in pairs&lt;/a&gt;. And lo! &lt;a href=&quot;http://aviationweek.com/defense/aerojet-rocketdyne-targets-25-million-pair-ar-1-engines&quot; rel=&quot;nofollow&quot;&gt;One article&lt;/a&gt; mentions it in the Antares context!&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-20T19:18:41.127" LastActivityDate="2014-12-20T19:18:41.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6513" PostTypeId="2" ParentId="6461" CreationDate="2014-12-20T19:11:48.730" Score="6" Body="&lt;p&gt;There are a number of stressing environments that payloads are subjected to during a rocket launch, and g-loads, often referred to as quasi-static loads/accelerations, are just one of them, and often not the most stressing. Other key environments include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;random vibration&lt;/li&gt;&#xA;&lt;li&gt;shock&lt;/li&gt;&#xA;&lt;li&gt;thermal&lt;/li&gt;&#xA;&lt;li&gt;acoustic&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These environments differ greatly between launch vehicles, and are typically published in Payload User's Guides as general guidance and provided to a payload during the launch systems integration process, where compatibility of the payload and rocket is established.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typically a combination of test and analysis is used to ensure the payload can survive the environments and it can be one of the most expensive and time-consuming parts of developing spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Quasi-static loads&lt;/strong&gt; can be thought of as constant accelerations. They are a result of the bulk acceleration of the entire rocket as well as the contribution of any random vibration significantly below the natural frequency of the payload, where it will respond as a rigid body. QS loads are typically specified as axial (along the central axis of the vehicle) and lateral (perpendicular to axial). Axial loads tend to be highest at stage burnout, just prior to separation events. Several g's is typically, above 10 is not uncommon. Vehicles are designed with factors of safety, so a vehicle may need to withstand, for example, above 20-g of acceleration. Lateral g's are typically driven by aerodynamic loading or other vehicle maneuvers. They tend to be less (horizontally launched and spin-stabilized rockets are a notable exception because they have notably different trajectories and/or body dynamics than vertically launched rockets). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Random_vibration&quot; rel=&quot;noreferrer&quot;&gt;Random vibration&lt;/a&gt;&lt;/strong&gt; is typically depicted as as a power spectral density function, which is a visualization of vibrational energy in frequency space (e.g. an fast Fourier transform of the time history random vibration), and as a gRMS (g, root-mean-square) value. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/314d5.gif&quot; alt=&quot;RV PSD example&quot;&gt; These are often the most stressing environments because amplification at specific natural frequencies can result in the effective load at certain locations in the vehicle to be many tens of g's equivalent (I've seen things as high at 70 or 80 g's easily). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Shock&lt;/strong&gt; is also depicted as a PSD, but at much higher frequencies than RV. Shock loads are a result of events like stage or fairing separation and is caused by things like explosive bolts. It results in a high-frequency ring through the system. Structures tend to be less susceptible to shock than RV or QS loads, but they can be murder on mechanisms, electronics, and finely calibrated instruments (including optics).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Acoustic&lt;/strong&gt; is sound loading. It tends to put energy into large area, lightweight structures like panels or antennas. It excites these structures which feed random vibration into the rest of the structure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Thermal&lt;/strong&gt; environments can be extreme because of heating on the fairing during ascent, which then radiates the payload. &lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2014-12-20T19:11:48.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6514" PostTypeId="2" ParentId="6505" CreationDate="2014-12-20T19:25:59.237" Score="14" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Where is the flaw in my thinking?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is a good question, but there are a few flaws in your thinking.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If I use the quoted value of g at 400 km above the earth of 0.89m/s/s I can calculate orbit time of 91 minutes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Your calculation of the ISS' orbital period is a bit off. The ISS is currently (Dec. 20) in a 404 x 415 km orbit, or a mean altitude of 409.4 km. That corresponds to an orbital period of &lt;a href=&quot;http://www.wolframalpha.com/input/?i=2*pi%2Fsqrt%28%28earth+geopotential%29%2F%286378.137+km+%2B+409.4+km%29%5E3%29&quot;&gt;92.75 minutes&lt;/a&gt;. Even using a 400 km orbit, you should have computed an orbital period of 92.56 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This of course makes your perceived problem even worse. With this correction, there's but a ~1.25 minute difference between the 92.75 minute long orbit and the 94 minutes between passes.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Times for appearance were 94 minutes apart.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I suspect you subtracted the start times of the two passes, and that the first pass was short in duration while the second pass was a nice long one. The difference between consecutive start times can vary from about 94 minutes to 99 minutes. The difference between zenith times on consecutive passes has a much smaller variance, from a bit under 96 minutes to a bit over 97 minutes, with a mean of about 96.75 minutes. This corresponds to about four minutes in excess of the orbital period.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the ISS to 'catch up' with the earth's rotation surely it should need another 6 minutes or so.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That would be the case if the ISS was in an equatorial orbit. An equatorial observer of a vehicle with a 92.75 minute prograde equatorial orbit would see successive passes separated by about 99 minutes. All passes would be more or less the same, with the vehicle moving from west to east and passing straight overhead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS is instead in an orbit inclined by about 51.6°. This changes the geometry considerably. An observer might see the first of two consecutive passes starting in the south and disappearing in the southeast, while the second pass might start in the southwest and disappear due east. The zenith passage of the two events corresponds to a 360°+15.5°change in true anomaly rather than the 360°+25° change for the equatorial orbiter.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-12-20T19:25:59.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6515" PostTypeId="2" ParentId="6506" CreationDate="2014-12-21T04:22:58.337" Score="4" Body="&lt;p&gt;There have been suggestions for using a heat shield that has many small holes to allow flowing water through it.  This would let the energy transpire the water instead of melting the materials.  But you have to carry enough water for landing in that case.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-21T04:22:58.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6516" PostTypeId="1" AcceptedAnswerId="6522" CreationDate="2014-12-21T04:40:31.537" Score="4" ViewCount="1279" Body="&lt;p&gt;I've heard that during the space race era, many of the Soviet cosmonauts were lost in space. Or at least, &lt;a href=&quot;https://en.wikipedia.org/wiki/Lost_Cosmonauts&quot; rel=&quot;noreferrer&quot;&gt;such were the claims&lt;/a&gt; of some. So I wonder, is there any truth in these claims, or was that merely cold war U.S. anti-communist propaganda against the Soviet Union? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has it really happened, for example, that cosmonauts would become untethered during extravehicular activity and were lost in orbit or to outer space?&lt;/p&gt;&#xA;" OwnerUserId="6782" LastEditorUserId="49" LastEditDate="2014-12-21T05:24:54.360" LastActivityDate="2015-01-07T03:35:59.873" Title="Is there any truth to the claims of cosmonauts lost in space?" Tags="|history|crewed-spaceflight|myth-busting|space-race|soviet-union|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6517" PostTypeId="1" CreationDate="2014-12-21T07:52:20.000" Score="2" ViewCount="571" Body="&lt;p&gt;It seems to me that heated helium with syphoned pressure regulated to 1000 mbar or less would get you halfway to the Karman line prior to expenditure of precious fuel. Has anyone thought of this? &lt;/p&gt;&#xA;" OwnerUserId="7014" LastEditorUserId="1142" LastEditDate="2014-12-23T16:14:18.573" LastActivityDate="2014-12-23T16:14:18.573" Title="Has anyone proposed a serious space dirigible design?" Tags="|spacecraft-development|balloons|" AnswerCount="1" CommentCount="7" ClosedDate="2014-12-22T10:06:08.387" ContentLicense="CC BY-SA 3.0" />
  <row Id="6519" PostTypeId="2" ParentId="6517" CreationDate="2014-12-21T09:21:51.830" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www.jpaerospace.com/&quot; rel=&quot;noreferrer&quot;&gt;JP Aerospace&lt;/a&gt; are thinking &lt;a href=&quot;http://www.jpaerospace.com/atohandout.pdf&quot; rel=&quot;noreferrer&quot;&gt;about it&lt;/a&gt;. It would be assisted by a balloon station at 140,000 feet, from where it will start to slowly accelerate to orbital velocity using an ion engine. They are running airships commercially and have the altitude record. But I think that the Airship to Orbit is part of their under-financed not-for-profit activities.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-12-21T09:21:51.830" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6520" PostTypeId="2" ParentId="5835" CreationDate="2014-12-21T09:49:58.630" Score="4" Body="&lt;p&gt;There were several instances of cooperation during the space-race; Russia was not entirely isolated. Russia (USSR, to be precise) was an active participant in the &lt;a href=&quot;http://en.wikipedia.org/wiki/International_Astronautical_Federation&quot; rel=&quot;nofollow&quot;&gt;IAF&lt;/a&gt;, and UN programmes such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Committee_on_Space_Research&quot; rel=&quot;nofollow&quot;&gt;COSPAR&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote Brian Harvey from &quot;Rebirth of the Russian Space Program&quot;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Soviet Union ran cooperative programs with the United States from 1965, especially in space biology and together the two countries hosted annual conferences on lunar and planetary exploration&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You'll find more about the complex relationship in the space arena between the mentioned protagonists in that book, and the link provided by Noordung. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(+: Google is your friend&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2014-12-21T09:49:58.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6521" PostTypeId="1" CreationDate="2014-12-21T10:17:04.690" Score="6" ViewCount="1667" Body="&lt;p&gt;A quick follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/6506/re-entry-heat-shield-alternative&quot;&gt;Re-entry Heat Shield Alternative&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of looking for an alternative to heat-shield, I'd like to draw from analogous naval tactics I heard about; Back in the 1950's to 1960's, or thereabouts, in heavy weather, ships stream (or used to stream) oil over the waters for ease of passage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be possible for a re-entry capsule to use something similar to reduce it's re-entry velocity to relatively manageable levels? Say, nylon/silk threads/sheet after them - sort of like deploying a drogue, or even the chute in thin atmosphere. Pack multiple chutes, and discard each in stages as a function of air-speed, and atmosphere density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the maximum re-entry velocity (I know this will vary depending upon the mass &amp;amp; form of the capsule - but say we're talking about Soyuz TMA-M) that would allow heat generated by passage of air to be dissipated without special shielding? &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-01-16T18:24:25.317" Title="What is the maximum velocity at which Soyuz TMA-M may transit through Earth' atmosphere at reentry without a heat-shield?" Tags="|reentry|soyuz-spacecraft|heat-shield|thermal|drag-augmentation|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="6522" PostTypeId="2" ParentId="6516" CreationDate="2014-12-21T12:05:42.947" Score="6" Body="&lt;p&gt;As the Wikipedia article you refer to says, there is no conclusive evidence any cosmonauts were lost, and most of the 'lost cosmonaut' stories have been disproven.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The evidence cited to support Lost Cosmonaut theories is generally not regarded as conclusive, and several cases have been confirmed as hoaxes. In the 1980s, American journalist James Oberg researched space-related disasters in the Soviet Union, but found no evidence of these Lost Cosmonauts. Since the collapse of the Soviet Union in the early 1990s, much previously restricted information is now available, including on Valentin Bondarenko, a would-be cosmonaut whose death during training on Earth was covered up by the Soviet government. Even with the availability of published Soviet archival material and memoirs of Russian space pioneers, no hard evidence has emerged to support the Lost Cosmonaut stories.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-12-21T12:05:42.947" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6523" PostTypeId="1" CreationDate="2014-12-21T14:09:31.777" Score="14" ViewCount="518" Body="&lt;p&gt;I know that on the forthcoming SpaceX attempt at sea landing the first stage, they have added hypersonic fins to the first stage. I was curious as to what altitude transonic compressibility is an issue, and whether the first stage upon reentry would be affected by such, and whether these fins could become a standard feature for reentry craft, given their potential to create drag at hypersonic speeds, thus possibly lowering requirements for heat shielding. &lt;/p&gt;&#xA;" OwnerUserId="7014" LastEditorUserId="49" LastEditDate="2014-12-22T11:09:50.117" LastActivityDate="2017-12-19T13:19:45.167" Title="At what altitude does transonic compressibility become a non-issue?" Tags="|spacex|reentry|atmosphere|spacecraft-development|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6524" PostTypeId="1" AcceptedAnswerId="6529" CreationDate="2014-12-21T15:39:32.520" Score="12" ViewCount="1955" Body="&lt;p&gt;This might be a beginner's question, but I cannot find a full answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to clarify the theoretical speeds that change the nature of the trajectory of an object launched at some altitude on Earth, horizontally and in the equator plane, assuming no atmosphere, without any propulsion mean after the launch. My incomplete reasoning:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;If the object is launched with an initial horizontal move with a low speed it will fall free (parabolic trajectory)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If the speed is increased, the object may start to move in elliptical orbit, such that it will just reach the sea level at the opposite point of the launch (perigee) and return to apogee at the launch point.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Increasing further the speed, the orbit should become circular.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Increasing again, the orbit will be elliptical again, with the perigee being at the launch point, and the apogee at the opposite point.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Increasing again, up to what I believe is the &quot;critical speed&quot;, the ellipse change back to a parabolic curve, the trajectory is open, and the object will not return (the null speed being reached only at an infinite distance).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Increasing past this speed, the trajectory continue to be open, but the curve is now hyperbolic.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;A common illustration of the experience:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wzgCq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To sum up&lt;/strong&gt;, what I think true, and what I don't know about launch speeds:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$0 \leq s &amp;lt; s_1$, free fall / parabolic&lt;/li&gt;&#xA;&lt;li&gt;$s_1 \leq s &amp;lt; s_2$, elliptical, apogee at launch point&lt;/li&gt;&#xA;&lt;li&gt;$s = s_2$, circular&lt;/li&gt;&#xA;&lt;li&gt;$s_2 \leq s &amp;lt; s_3$, elliptical, perigee at launch point&lt;/li&gt;&#xA;&lt;li&gt;$s = s_3$, parabolic, open&lt;/li&gt;&#xA;&lt;li&gt;$s_3 &amp;lt; s$, hyperbolic, open&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question&lt;/strong&gt;: What are the names and values (for Earth) of $s_1$, $s_2$ and $s_3$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The kind of information I've got, which have been useful: &lt;a href=&quot;http://web.ics.purdue.edu/~nowack/geos105/lect5-dir/lecture5.htm&quot; rel=&quot;noreferrer&quot;&gt;At Purdue&lt;/a&gt;, at &lt;a href=&quot;http://www.astro.virginia.edu/class/oconnell/astr1210/guide08.html&quot; rel=&quot;noreferrer&quot;&gt;University of Virginia&lt;/a&gt;, at &lt;a href=&quot;http://scienceblogs.com/startswithabang/2013/10/09/how-to-reach-the-outer-solar-system/&quot; rel=&quot;noreferrer&quot;&gt;ScienceBlogs&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;From the answers and comments provided (thanks!) and additional searches:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Orbital speed $s_2$, the speed for injection into a circular orbit, depends on the altitude above the sea level: 7.91 km/s at sea level, 7.73 at 300 km.&lt;/li&gt;&#xA;&lt;li&gt;Escape velocity ($s_3$) is the speed at which an object will not orbit the Earth, but escape it along a parabolic trajectory. 11.18 km/s at sea level, 10.93 at 300 km/s.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Below escape velocity, the object will move along an ellipse, if ellipse doesn't intersect the ground surface (free fall in this case).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The circular orbit is a particular case of elliptical orbit. We learned at school that the free fall trajectory is a parabola, this is not valid in real life where the Earth is not flat, and the gravitational force varies with the distance to the Earth center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For orbits, below $s_2$, the launch point becomes the apogee and above $s_2$, it becomes the perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above escape velocity the curve is hyperbolic, with no return.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also: &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;noreferrer&quot;&gt;Orbital speed&lt;/a&gt; on wikipedia.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2014-12-22T11:24:07.483" LastActivityDate="2014-12-22T15:18:01.110" Title="What are speed limits for free fall, orbital trajectory, and unbound trajectory?" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6525" PostTypeId="2" ParentId="5403" CreationDate="2014-12-21T17:14:57.033" Score="21" Body="&lt;p&gt;It's to protect the engine from the low temperature.&#xA;The cover is removed before the deorbit burn:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hHUPc.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Source: &lt;a href=&quot;http://www.kosmonavtika.com/vaisseaux/soyouz/tech/kdu/kdu.html#2&quot; rel=&quot;noreferrer&quot;&gt;Kosmonavtika&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\rightarrow$ During mission &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_32&quot; rel=&quot;noreferrer&quot;&gt;Soyuz-32&lt;/a&gt; in 1979, the cosmonauts forgot to open the cover before firing the SKD docked to &lt;a href=&quot;http://en.wikipedia.org/wiki/Salyut_6&quot; rel=&quot;noreferrer&quot;&gt;Salyut-6&lt;/a&gt; space station. This was a test burn. The cover was destroyed, and the engines left unprotected. As a protective measure, the station was reoriented so that the Soyuz temperature variations were limited.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2014-12-23T09:03:02.403" LastActivityDate="2014-12-23T09:03:02.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6526" PostTypeId="2" ParentId="6524" CreationDate="2014-12-21T17:57:02.223" Score="13" Body="&lt;p&gt;Using your enumerated list:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;This is only approximately a parabola for short falls.  It is really an elliptical orbit whose periapsis is below the surface.  Therefore the &quot;orbit&quot; comes to an abrupt stop when it intersects the surface.  The parabola approximation is for a constant gravitational acceleration on a flat Earth.  However the gravitational acceleration is not constant, and diminishes as $1/r^2$, which gives an elliptical path.  Also Earth is not flat.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Also an elliptical orbit that just barely misses the surface of the hypothesized airless body.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Also an elliptical orbit that is circular.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Also an elliptical orbit.  All of 1 through 4 have a negative total energy relative to the body.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;This one is in fact parabolic, but not at all like the parabola that is the approximation for short falls.  This intermediate state between an ellipse and a hyperbola is not useful, since it has zero measure.  It only exists when the total energy is &lt;em&gt;exactly&lt;/em&gt; zero.  Once it has been derived as one of the solutions, it can be safely ignored from then on.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Now the total energy is positive, so the trajectory is a hyperbola.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;All of the above can be represented in a single equation, where $\mathcal{E}$ is the total specific energy of the object in flight, i.e. the energy per unit mass in units of distance squared per time squared:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathcal{E}={v^2\over 2}-{\mu\over r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}$ is a conserved quantity that is the same at all points in the ellipse or hyperbola, so long as there are no impacts with atmospheres or surfaces, the object isn't firing any rockets, and there are no other forces on the object except for the gravity of the body.  At any given point in the trajectory, $v$ is the magnitude of the velocity at that point relative to the body, and $r$ is the radius, or distance from the center of the body at that point.  $\mu$ is the gravitational parameter for the body, $\mu=GM$, where $M$ is the mass of the body and $G$ is Newton's gravitational constant.  Aside from the notational convenience, $\mu$ is what is actually measured, and can almost always be determined much more accurately than $M$, which is limited by our knowledge of $G$.  (The exception is when we can see nothing else affected by the gravity of the body, in which case crude estimates of $M$ are made using the size of the object and assumptions about its density.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When $\mathcal{E}$ is negative, you have your cases 1..4.  When $\mathcal{E}$ is zero, you have your case 5.  when $\mathcal{E}$ is positive, you have your case 6.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For $\mathcal{E}$ negative, you have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathcal{E}=-{\mu\over r_p+r_a}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $r_p$ is the periapsis radius and $r_a$ is the apoapsis radius of the elliptical orbit.  Radii are measured from the center of the body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For $\mathcal{E}$ positive, you have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathcal{E}={v_\infty^2\over 2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $v_\infty$ is the velocity magnitude at infinity, i.e. how fast the departing object will be moving relative to the body once it is far enough away that the gravitational acceleration of the body is negligible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When $\mathcal{E}=0$, you have just reached escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You now have all you need to compute all of your velocities with just a calculator.  (Assuming that the calculator has square root.)  For Earth, $\mu_e\approx 398600\,\mathrm{km^3/s^2}$ and the radius of the surface at the equator is $r_e\approx 6378\,\mathrm{km}$.  Go to town.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-12-21T19:24:00.590" LastActivityDate="2014-12-21T19:24:00.590" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6527" PostTypeId="2" ParentId="4423" CreationDate="2014-12-21T19:20:49.177" Score="8" Body="&lt;p&gt;I'm not a specialist, but from the documentation available online, it seems snoring is reduced in microgravity.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Microgravity was associated with a 55% reduction in the&#xA;  apnea–hypopnea index (AHI), which decreased from a preflight value of&#xA;  8.3 $\pm$ 1.6 to 3.4 $\pm$ 0.8 events/h inflight. This reduction in AHI was accompanied by a virtual elimination of snoring, which fell&#xA;  from 16.5% $\pm$ 3.0 of total sleep time preflight to 0.7% $\pm$ 0.5&#xA;  inflight&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source:&#xA;&lt;a href=&quot;http://temec.com/Documents/Microgravity%20humans.pdf&quot; rel=&quot;noreferrer&quot;&gt;Microgravity Reduces Sleep-disordered Breathing in Humans&lt;/a&gt;&#xA;ANN. R. ELLIOTT, STEVEN A. SHEA, DERK-JAN DIJK, JAMES K. WYATT, EYMARD RIEL, DAVID F. NERI, CHARLES A. CZEISLER, JOHN B. WEST, and G. KIM PRISK.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2014-12-22T13:12:23.647" LastActivityDate="2014-12-22T13:12:23.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6528" PostTypeId="1" AcceptedAnswerId="28337" CreationDate="2014-12-21T19:28:14.250" Score="11" ViewCount="334" Body="&lt;p&gt;It is said that for Precise Orbit Determination (POD) of interplanetary spacecrafts, generally, during the cruise phase for aiding accurate TCM (Trajectory Correction Maneuvers) planning, long arc of tracking observables is required. &#xA;During Cruise Phase of ISRO's MOM mission, its Facebook fan page updated:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;TCM has been executed and tracking details suggest successful TCM. However, we will have to wait for another 5-6 hours for accurate orbit details. (Result of POD)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is there any such record of distinct tracking time required in missions? Since Apollo program has been quite elaborately known, I hope there must be record of tracking details or any other mission for that case. I am just looking at a general trend of interplanetary spacecrafts' orbits and consequent time intervals required to get sufficiently accurate orbit details (by employing tracking by common Delta-DOR or Single station Doppler), to draw raw relation between orbits and measurement arcs required for POD without having to go into error and accuracy analysis.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="49" LastEditDate="2014-12-22T11:12:53.830" LastActivityDate="2018-07-12T17:08:42.263" Title="Tracking Arc and Time for Precise Orbit Determination" Tags="|interplanetary|tracking|orbit-determination|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6529" PostTypeId="2" ParentId="6524" CreationDate="2014-12-21T19:51:48.093" Score="13" Body="&lt;p&gt;What they teach in high school physics is wrong. When you toss a penny, it does &lt;strong&gt;not&lt;/strong&gt; follow a parabolic path. Here is a picture of  the penny's actual path (neglecting friction from air and rock and neglecting the fact that the Earth is not a point mass at the center):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yExpe.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you tossed at a 45 degree angle, one focus of the ellipse will be in front of you. The other focus of the ellipse is at earth's center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The eccentricity of this ellipse is around .9999. The eccentricity of a parabola is 1. So the penny toss is very to close to parabola shaped, especially from the scale of the penny tosser.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wish they wouldn't keep teaching this. It confused me when I was first studying orbital mechanics. I thought &quot;How can a penny toss be parabolic? it's nowhere near 11.2 km/s&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So from the earth's surface (neglecting friction from air and rocks):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0 to less than 7.9 km/s is elliptical orbit (but with perigee below earth's surface).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;7.9 is circular.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More than 7.9 to less than 11.2 is elliptical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;11.2 is parabolic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More than 11.2 km/s is hyperbolic&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2014-12-22T15:18:01.110" LastActivityDate="2014-12-22T15:18:01.110" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6530" PostTypeId="2" ParentId="3424" CreationDate="2014-12-21T19:57:43.050" Score="10" Body="&lt;p&gt;Contrary to small animals, now that the human being has stand up on their feet, their brain is located much higher than their heart, but the brain still continuously needs blood.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under increased vertical acceleration, the blood pressure must be increased to maintain brain irrigation. This has been studied in the past:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://books.google.fr/books?hl=fr&amp;amp;id=Lqz-4XU5m28C&amp;amp;q=Human%20volunteers%20have%20tolerated#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow&quot;&gt;The Biology of Human Survival&lt;/a&gt;: Life and Death in Extreme Environments, Claude A. Piantadosi:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Human volunteers have tolerated 1.5g for seven days with no apparent&#xA;  ill effects. However, after just twenty-four hours at 2g, evidence of&#xA;  significant fluid imbalance is detectable. At 3g to 4g fatigue is&#xA;  limiting, and above 4g cardiovascular factors limit g tolerance.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-09-24T17:48:33.537" LastActivityDate="2015-09-24T17:48:33.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6531" PostTypeId="1" CreationDate="2014-12-21T21:12:45.913" Score="3" ViewCount="1667" Body="&lt;p&gt;SpaceX's next big booster is notionally known as BFR - Big F-ing Rocket [or Big Falcon Rocket]. (Like the B-52 is the BUFF - Big Ugly Fat F-er, it is a term of enderarment). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know that it will use a cluster of Raptor engines, running on LOX/Methane.  Raptor has been changing over time but appears likely to be in the 1 Million lb thrust or higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know they want it to be reusable which in their current mode of operation requires a center engine to keep thrust levels manageable for landing, so likely a cluster of 3, 5, or 9 engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That puts some boundaries on likely sizes of the core. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What will the BFR look like?  This question may have to evolve over time as more information is released.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="1457" LastEditDate="2014-12-21T21:19:40.363" LastActivityDate="2014-12-21T21:19:40.363" Title="What will the SpaceX BFR booster look like?" Tags="|spacex|raptor|" AnswerCount="0" CommentCount="7" ClosedDate="2014-12-22T10:10:36.523" ContentLicense="CC BY-SA 3.0" />
  <row Id="6533" PostTypeId="2" ParentId="6047" CreationDate="2014-12-22T06:56:29.970" Score="3" Body="&lt;p&gt;I'm going to attempt an answer here; some of this is &lt;strong&gt;conjecture!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How could a probe detect and map voids? Are there techniques for this?&#xA;  Ground-penetrating radar?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From what I've read on ground-penetrating radar it's unlikely to be a great help. The depth of GPR is dependent on a lot of factors, but &lt;a href=&quot;http://en.wikipedia.org/wiki/Ground-penetrating_radar&quot; rel=&quot;nofollow noreferrer&quot;&gt;according to Wikipedia&lt;/a&gt; the chances of getting to more than 15 meters or so are slim, if the material is dry. However, since we're looking for potentially large openings, we don't really care too much about having a great resolution, so if we were to use an ultra low frequency we might have a chance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An idea that struck me when thinking of this answer was the application of a 'swarm' of small orbiters. They could transmit signals into Phobos, and if that signal happens upon a a pore at the right interface angle, then the signal will be reflected! If you have enough of satellites in your swam, you could pick up this reflected signal and get the depth of the pore based on travel time of the signal. For reference:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TUv0t.png&quot; alt=&quot;Reflection Example&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you can see if the angle theta is greater than the critical angle then we could receive reflected signal.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would the benefits of a pre-existing cave make it worthwhile to convert it into a habitat, or is that just more work than building something on the surface?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This question depends on a lot of other factors. To name a few:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How frequent and of what magnitude is the seismic activity&lt;/li&gt;&#xA;&lt;li&gt;How much science we want done on Phobos itself&lt;/li&gt;&#xA;&lt;li&gt;The cost, this seems like the kind of idea that would save money in&#xA;the long run - the very long run&lt;/li&gt;&#xA;&lt;li&gt;Technology when this idea is brought to funding stage&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There's a lot that can be said about building under the surface of a moon. But personally I'd prefer we avoid &lt;a href=&quot;http://doom.wikia.com/wiki/Phobos&quot; rel=&quot;nofollow noreferrer&quot;&gt;digging up too much on Phobos&lt;/a&gt;. :)&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="49" LastEditDate="2014-12-23T12:20:24.560" LastActivityDate="2014-12-23T12:20:24.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6534" PostTypeId="2" ParentId="6047" CreationDate="2014-12-22T08:44:41.127" Score="4" Body="&lt;p&gt;I'll go with a longshot here, please bear in mind this is pure speculation and conjecture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @HopDavid said, probably 3D wall mappers could be a good choice, so I'd say a swarm of autonomous probes using some technology like &lt;a href=&quot;https://www.google.com/atap/projecttango/#project&quot; rel=&quot;nofollow&quot;&gt;Google's Project Tango&lt;/a&gt; could prove useful for doing this. Supposing they have been appropriately fitted, rugged, and protected from radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;-Edit: Funny I just came across this Google Tango and NASA collaboration on Spheres w/Tango: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Vc5YyLl1Ksg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; -&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Small voids could be detected using GPR (&lt;em&gt;ground penetrating radar&lt;/em&gt;) or SIR (&lt;em&gt;subsurface interface radar&lt;/em&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Big voids on Earth can be detected non-intrusively by micro-gravity surveying equipment, although I am not sure if this technique could be &quot;fine-tuned&quot; to work with Phobos' gravity.&lt;/p&gt;&#xA;" OwnerUserId="3001" LastEditorUserId="3001" LastEditDate="2015-01-23T21:36:22.467" LastActivityDate="2015-01-23T21:36:22.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6536" PostTypeId="1" CreationDate="2014-12-22T10:14:27.510" Score="12" ViewCount="1765" Body="&lt;p&gt;I read that &lt;a href=&quot;http://russianspaceweb.com/angara5_flight1.html&quot;&gt;Angara 5&lt;/a&gt; will carry a &quot;payload simulator&quot; on its first test flight tomorrow Dec. 23, 2014. But the Indian GSLV mk.III recently flew for the first time with a prototype of the ISRO crewed spacecraft as payload. And SLS is planned to carry an uncrewed Orion around the Moon on its premiere launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not always use a test launch to test some real payload? Or is a simulated payload just another name for doing precisely that?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="2752" LastEditDate="2014-12-22T12:00:41.013" LastActivityDate="2014-12-22T22:29:59.387" Title="What is simulated payload and why are they used?" Tags="|launch|testing|payload|angara|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6537" PostTypeId="2" ParentId="4466" CreationDate="2014-12-22T10:47:34.423" Score="3" Body="&lt;p&gt;The last water landing was about six miles down range from the launch site. About as close as they wanted to get. The goal is to come back to the same launch site, and have the mobile launch tower go and get it very soon after landing. This might be a few hundred feet from the assembly and launch site. I did see where the planned landing sight is at the California complex. I asked as was assured that SpaceX increased the Falcon 9 from the ten to 13 metric tons was so they could fulfill commitments but use the fly-back system and it might lower the payload buy up to that much because of fuel needs to do fly back to the launch sight. I would have designed it so that it could land a few hundred mile down range close to a significant airport that could handle heavy air transport and use that to bring the stage back to the launch sight. It would use the tired wheeled launch tower as in in California instead of the first one in Florida on rails. The erection and launch tower can drive over to where the booster has landed, grab it, bring it down, drive back to the horizontal assembly tower, or possibly drive into a transport boat, train, and or plane. At the assembly area they would check it out, attach new payloads, possibly sometimes change some of the engines, do any other maintenance needed, and reuse it as a new rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The back and bottom of the booster does have a very thin heat shielding material. Early plans called for this supplying most of the breaking at reentry, but current plans are to use fuel to do most of that, which is one of the reasons a larger booster and more return fuel is needed. It might take up to thirty percent of the payload as fuel to return.&lt;/p&gt;&#xA;" OwnerUserId="7021" LastEditorUserId="1521" LastEditDate="2015-01-09T14:24:50.687" LastActivityDate="2015-01-09T14:24:50.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6538" PostTypeId="2" ParentId="6047" CreationDate="2014-12-22T11:08:34.020" Score="11" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;How could a probe detect and map voids? Are there techniques for this?&#xA;Ground-penetrating radar?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Radar, either GPR (Ground-Penetrating Radar) or SIR (Subsurface Interface Radar) would seem the obvious solution. However, the shortcomings of those techniques (as pointed out in @FraserOfSmeg's answer) may mean that they are of limited use for finding deeper caves. A technique similar to &lt;a href=&quot;http://en.wikipedia.org/wiki/CONSERT&quot; rel=&quot;noreferrer&quot;&gt;CONSERT&lt;/a&gt; might be the solution. However, at the moment CONSERT doesn't have good enough resolution to accurately map caves, just give a good indication of where to look.&lt;/p&gt;&#xA;&lt;p&gt;For really deep mapping of voids we might turn to seismic or sonic based techniques such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Seismic_tomography&quot; rel=&quot;noreferrer&quot;&gt;seismic tomography&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Reflection_seismology&quot; rel=&quot;noreferrer&quot;&gt;reflection seismology&lt;/a&gt;, or direct sonic mapping. Several small “Philea-class” probes could be landed to listen to Phobos. They could either listen for natural seismic activity, or an orbiter could bombard Phobos with impactors, rather like &lt;a href=&quot;http://en.wikipedia.org/wiki/Deep_Impact_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Deep Impact&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;There's a &lt;a href=&quot;http://www.enviroscan.com/html/void_detection.html&quot; rel=&quot;noreferrer&quot;&gt;good article on void detection and mapping&lt;/a&gt; by a company called EnviroScan that outlines most of the techniques used on Earth:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Large volume voids such as significant washouts, mine workings,&#xA;tunnels, and karst-related cavities are excellent targets for&#xA;microgravity surveys.  The “missing” mass of the void creates a&#xA;measurable disturbance in the earth’s gravitational field, with the&#xA;magnitude of the disturbance directly proportional to the volume of&#xA;the void.  This relationship has allowed Enviroscan to use&#xA;microgravity surveys to assist engineers in the effective design and&#xA;accurate cost estimation of grouting programs. Other examples of&#xA;Enviroscan’s application of microgravity are mapping of undocumented&#xA;mine workings, location of abandoned or clandestine tunnels, and&#xA;delineation of areas of unsupported floor slab.  Dramatic Microgravity&#xA;Anomaly Over a Cave System Beneath a Factory Since gravity surveys are&#xA;immune to many common sources of electronic or acoustic noise that can&#xA;impede many geophysical techniques, they are particularly suited to&#xA;highly developed or industrial sites.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ykrYF.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Smaller shallow voids can be detected using ground penetrating radar&#xA;(GPR) or subsurface interface radar (SIR). Deeper voids are sometimes&#xA;best detected and measured using seismic reflection.  Deep,&#xA;water-filled cavities (e.g. caves or tunnels below the below the water&#xA;table) are particularly good targets for seismic shear wave imaging&#xA;since water (or any fluid) cannot transmit shear waves - causing&#xA;water-filled voids to appear as characteristic “blank spots” or&#xA;shadows on a shear wave record.&lt;/p&gt;&#xA;&lt;p&gt;Where boreholes are available, Enviroscan can perform crosshole&#xA;seismic or electrical tomography.  Beneath water, sub-bottom sonar, as&#xA;well as gravity and sometimes radar can be used to detect voids such&#xA;as tunnels or bridge pier scour features.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Would the benefits of a pre-existing cave make it worthwhile to&#xA;convert it into a habitat, or is that just more work than building&#xA;something on the surface?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Much harder to answer. But we can speculate on a few points;&lt;/p&gt;&#xA;&lt;p&gt;The seismic instability, depending on severity, could render the direct conversion of a cave impossible, but that doesn't mean the cave couldn't still be useful. &lt;a href=&quot;http://en.wikipedia.org/wiki/North_American_Aerospace_Defense_Command&quot; rel=&quot;noreferrer&quot;&gt;NORAD&lt;/a&gt; is constructed on giant springs to survive the shock of a nuclear explosion, and a similar technique could be applied to a habitat in a Phobian cave to help it survive Phobos' quakes.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3J15I.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;This would help reduce the risk of seismic activity rupturing the habitat, but unless the seismic nature of Phobos becomes &lt;em&gt;very&lt;/em&gt; well understood it would probably still be considered too dangerous.&lt;/p&gt;&#xA;&lt;p&gt;If seismic activity isn't an issue the advantages to converting a cave are the same as anywhere else; cheap habitable space and protection from radiation. Depending on how deep the cave is it could provide ample shielding from solar radiation.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-01-20T09:29:07.163" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6539" PostTypeId="2" ParentId="6536" CreationDate="2014-12-22T12:01:54.437" Score="15" Body="&lt;p&gt;Launching on a risky booster, is risky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can define a risky booster as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;First launch of a new booster.&lt;/li&gt;&#xA;&lt;li&gt;Return to flight after a failure. &lt;/li&gt;&#xA;&lt;li&gt;Booster with record of failure&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So you feeling lucky punk? How much development money are you willing to risk on any of those cases?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did ISRO take the risk on the GSLV launch and Angara5 won't? I would assume that ISRO wanted to launch that payload, their boosters are not cheap, their budget is not unlimited, and they needed a payload anyway to demonstrate it. If they do not demonstrate confidence in their own booster, who will be confident enough to use it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians launch often enough on so many different classes of vehicles that they are not lacking options for payloads. They may not have had some project that needed a cheap-to-free launch giveaway.  Also Angara-5 is a fairly large payload booster (49,000 lbs to LEO), so to really test it out you need a fairly big payload.  Big payloads are usually high value payloads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At that point it becomes a better choice to literally launch a ball of steel, or container of water. Something cheap, with good sensors to characterize the details of the environment inside the launch fairing so customers know what to expect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if you had say a propellant depot in orbit, and you had a tug, then it would have made a ton of sense to launch propellant for the depot.  Value is low if lost, but nice freebie if it worked.  But we don't. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for SLS, it is so expensive, and will be launching so infrequently it almost cannot be 'wasted' on a test flight. That to me is a design flaw. It seems like a full up series of test flights before humans fly on it would be part of man-rating. Atlas V will have had 60 or 70 flights by the time a human flies in a CST-100.  Falcon 9 will likely be at 20-30 flights before a human flies in a Dragon V2. Why is SLS going to do it on the second flight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will be interesting to see what SpaceX launches on Falcon Heavy as its first payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I personally would love to see Elon Musk's original goal, a greenhouse on Mars, as the first payload (Huge overkill since that was planned to be tiny, and F-H has a huge payload ability). That is the sort of 'waste' of money they could do as a publicity stunt for vaguely cheap inhouse if the booster needed a payload no one else would pay for. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2014-12-22T12:01:54.437" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="6540" PostTypeId="2" ParentId="2992" CreationDate="2014-12-22T12:59:15.450" Score="2" Body="&lt;ol&gt;&#xA;&lt;li&gt;&quot;Angle between the sail force vector and the radial from the sun&quot;: the sail is moved by the incident photons from the Sun, &lt;strong&gt;and&lt;/strong&gt; by the reflected photons (reaction).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IIQez.png&quot; alt=&quot;enter image description here&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(source: &lt;a href=&quot;http://spaceplan2020.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceplan2020.com&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whatever the angle of the Sun, the resultant force will be close to the normal to the sail surface (normal only if 100% of the incident photons are reflected). This means there is an angle between the radial from the Sun and the sail force (direction of thrust) which depends on the reflectivity and also, more important, on the sail orientation.&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Coefficients 0.349, 0.662 and 0.011 are for a &lt;strong&gt;square&lt;/strong&gt; sail. &lt;a href=&quot;https://books.google.fr/books?id=vfMilANfARYC&amp;amp;pg=PA53&amp;amp;lpg=PA53#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;More on this&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Some clues:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\frac{1}{r^2}$ is the effect of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Inverse-square_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;inverse square law&lt;/a&gt; (because the light from the Sun is emitted on a sphere, which total surface area is proportional to the square of distance.&lt;/li&gt;&#xA;&lt;li&gt;$\cos \theta$ reflects the ratio between the actual surface area of the sail, and its apparent surface area as seen from the Sun (1 when the angle is null, then decrease down to 0 when the angle is 90°).&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Variables are the solar pressure exerted on the sail when the sail is oriented in the Sun direction ($F_0$), the angle between the Sun and the normal to the sail ($\theta$) and the distance to the Sun ($R$).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="7017" LastActivityDate="2014-12-22T12:59:15.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6541" PostTypeId="2" ParentId="3265" CreationDate="2014-12-22T19:47:53.213" Score="7" Body="&lt;p&gt;This is a suggestion, I'm not sure this was the end of the story, but...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2008, a &lt;a href=&quot;http://www.spaceflightnow.com/shuttle/sts126/080925pyrobolt/&quot; rel=&quot;noreferrer&quot;&gt;problem of arcing&lt;/a&gt; near the Pyro bolts was proposed as an explanation by the Russians.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Pyro bolt was later &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition17/eva20a.html&quot; rel=&quot;noreferrer&quot;&gt;retrieved from a docked Soyuz&lt;/a&gt; (TMA-12 -- Also &lt;a href=&quot;http://spaceflightnow.com/station/exp17/080709evapre/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;), and returned to earth for examination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ttSNx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;(source: ibid)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TMA-12 undocked with the remaining Pyro bolts and returned normally on Earth. The returned Pyro bolt examination was not conclusive, but other data existed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.nasaspaceflight.com/2009/04/soyuz-investigation-findings-backed-by-nominal-soyuz-tma-13-return/&quot; rel=&quot;noreferrer&quot;&gt;Nasa Spaceflight&lt;/a&gt; Russians engineers stated:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Following the investigation, the Commission found that the most likely&#xA;  cause of the failure to open the latch located at “Plane I” of the&#xA;  interface, “was a failure of the 8 X 55 explosive bolt, caused by&#xA;  combined long-term exposure, under space flight conditions, to&#xA;  electrical discharges resulting from the difference in potential&#xA;  between the station hull and the surrounding plasma, which increased&#xA;  in magnitude after the electrical power capacity of the ISS was&#xA;  augmented.”&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The findings relating to the long-term exposure to electromagnetic&#xA;  emissions on-orbit, and their potential to cause issues with the pyro&#xA;  bolts, came after an extensive investigation that included the removal&#xA;  and return of one pyro bolt from Soyuz TMA-12.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA confirmed somehow the assessment:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Our independent analysis agreed well with Russian provided data.&#xA;  Pyrotechnic and environments experts believe the stated most probable&#xA;  cause is possible, however unlikely in our experience.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="7017" LastActivityDate="2014-12-22T19:47:53.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6542" PostTypeId="1" AcceptedAnswerId="6544" CreationDate="2014-12-22T20:28:15.047" Score="9" ViewCount="1743" Body="&lt;p&gt;According to the ESA movie &lt;a href=&quot;http://youtu.be/M2_NeFbFcSw?t=9m55s&quot;&gt;Soyuz rendezvous and docking explained&lt;/a&gt;, Soyuz uses a &lt;a href=&quot;http://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot;&gt;bi-elliptic transfer&lt;/a&gt; to move from the phasing orbit to the ISS orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this more complex transfer used instead of a &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot;&gt;Hohmann transfer&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2018-11-07T10:15:56.017" Title="Reasons for using a bi-elliptic transfer for Soyuz-ISS rendezvous" Tags="|iss|orbital-mechanics|soyuz-spacecraft|hohmann-transfer|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6543" PostTypeId="2" ParentId="2819" CreationDate="2014-12-22T21:06:09.190" Score="4" Body="&lt;p&gt;Since you have no answer so far, here is the contribution of a non expert. This may not be of great help. It should be a comment, but more space is required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Documentation relating to your interests:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;A summary of &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140004426.pdf&quot; rel=&quot;nofollow&quot;&gt;health monitoring in space&lt;/a&gt; from NASA. Page 10 is&#xA;titled &quot;Medical Complaints in Space&quot; and include some information.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Headaches may be caused by the &lt;a href=&quot;http://www.airspacemag.com/daily-planet/why-living-space-can-be-pain-head-180951507/&quot; rel=&quot;nofollow&quot;&gt;carbon-dioxyd aboard ISS&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.mdpi.com/2075-1729/4/4/621/htm&quot; rel=&quot;nofollow&quot;&gt;Microgravity-Induced Fluid Shift and Ophthalmic Changes&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Head Down Tilt Bed Rest experiments to simulate microgravity, like &lt;a href=&quot;http://www.pubfacts.com/detail/24875927/Space-headache-on-Earth:-Head-down-tilted-bed-rest-studies-simulating-outer-space-microgravity.&quot; rel=&quot;nofollow&quot;&gt;this one&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.medicaldaily.com/nasa-astronauts-brain-scans-reveal-tissue-and-eye-damage-after-space-flight-239926&quot; rel=&quot;nofollow&quot;&gt;NASA Astronauts Brain Scans Reveal Tissue and Eye Damage After Space Flight&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Longitudinal Study of Astronaut Health - &lt;a href=&quot;http://lsda.jsc.nasa.gov/refs/LSAH/Vol_12_Issue_1_Jul_031_Newsletter_02_06_2014_08_35_25.pdf&quot; rel=&quot;nofollow&quot;&gt;LSAH&lt;/a&gt;. Access to &lt;a href=&quot;http://lsda.jsc.nasa.gov/lsah_home1.aspx&quot; rel=&quot;nofollow&quot;&gt;data&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Committee on Space Biology and Medicine (&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0309071267&quot; rel=&quot;nofollow&quot;&gt;CSBM&lt;/a&gt;) (also at Google Scholar -- link not allowed on SE).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also &lt;a href=&quot;http://www.researchgate.net/publication/51802378_The_headache_of_high_altitude_and_microgravity--similarities_with_clinical_syndromes_of_cerebral_venous_hypertension/links/09e4150289504ce9ae000000&quot; rel=&quot;nofollow&quot;&gt;The Headache of High Altitude and Microgravity—Similarities with&#xA;   Clinical Syndromes of Cerebral Venous Hypertension&lt;/a&gt; (Mark H.&#xA;   Wilson, Christopher H.E. Imray and Alan R. Hargens)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this study:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There have been a number of reports recently of astronauts suffering&#xA;  headache and loss of peripheral vision.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The study also refers to:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Watenpaugh DE and Hargens AR. (1996). The Cardiovascular&#xA;System in Microgravity. In Handbook of Physiology: Section4,&#xA;Environmental Physiology. Fregly MJ, Blatteis CM, eds. Oxford&#xA;University Press, New York. pp. 631–674.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hu Z, Zhao G, Xiao Z, Chen X, Zhong C, and Yang J. (1999).&#xA;Different responses of cerebral vessels to - 30 degrees headdown&#xA;tilt in humans. Aviat Space Environ Med 70:674–680.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Iwase S and Mano T. (2000). [Microgravity and autonomic nervous&#xA;system]. Nippon Rinsho 58:1604–1612.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2014-12-22T23:05:06.847" LastActivityDate="2014-12-22T23:05:06.847" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6544" PostTypeId="2" ParentId="6542" CreationDate="2014-12-22T21:37:50.223" Score="14" Body="&lt;p&gt;First off, Hohmann transfers are a fiction. A very useful fiction, but a fiction nonetheless. Hohmann transfers are for transferring from one perfectly circular Keplerian orbit about a planet (or star) with a perfectly spherical mass distribution to another perfectly circular Keplerian orbit. There is no such thing as a perfectly circular Keplerian orbit. Orbits always have some eccentricity, and there are always perturbing gravitational bodies about. Moreover, the Earth does not have a spherical mass distribution; it most notably has an equatorial bulge. This alone pretty much ruins the idea of a Hohmann transfer in low Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another problem with Hohmann transfers is that while they are great (in theory) for transferring to another orbit, rendezvous demands more than just reaching the ISS's orbit. The transfer has to reach an exact point on that orbit at an exact point in time. This is a rather demanding requirement. A Hohmann transfer (assuming they exist) would have to be performed at a very specific point in time, with very little margin for error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A generalization of the concept of a Hohmann transfer is that of a two burn transfer. There are targeting algorithms (Lambert's problem) that solve for reaching an exact point in space at an exact time. Suppose the first burn is off by a little. A correction burn halfway through the transfer to ensure that the vehicle will still reach the target can be rather expensive. There are always correction burns because nothing in space ever goes exactly according to plan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A three burn transfer is a tiny bit more expensive in terms of delta-V compared to the non-corrected two burn transfer. However, that midpoint burn can absorb a lot of slop without a huge delta-V penalty. This is one reason to prefer this slightly suboptimal solution (and it's only slightly suboptimal).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason, as noted in the comments, is that the final burn of a two burn transfer can be rather large. Adding that intermediate point, coupled with making the transfer take a full orbit rather than half an orbit, can significantly reduce the final burn. The Space Station is a very, very expensive international asset. Expending a bit extra fuel to ensure the safety of the ISS is a worthwhile expense.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2014-12-22T21:37:50.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6545" PostTypeId="2" ParentId="6536" CreationDate="2014-12-22T22:29:59.387" Score="10" Body="&lt;p&gt;In my time in Aerospace, we used &quot;mass simulators&quot; (lead or brass weights or water) for the simple reason that the actual payload (a satellite) wasn't ready yet. We try to get the launch vehicle and the payload ready for launch around the same time, which means both things are in testing at the same time, typically at different facilities, and they never meet until the final launch vehicle is assembled. For testing launch vehicles, the answer above is absolutely correct (cheaper to blow up a bucket of water than a satellite) - but for most uses of mass simulators, it's because the actual device isn't available. You can be sure that &quot;uncrewed Orion module&quot; is mostly mass simulators.&lt;/p&gt;&#xA;" OwnerUserId="6585" LastActivityDate="2014-12-22T22:29:59.387" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6546" PostTypeId="1" AcceptedAnswerId="6547" CreationDate="2014-12-22T23:31:42.860" Score="14" ViewCount="2148" Body="&lt;p&gt;I think it is a well-known fact here that the Earth is slowly losing rotational velocity over time (some 1.4 milliseconds of the day are gained per hundred years). Eventually, this means that the Earth will become tidally locked with the Sun, meaning that one face of the Earth will always face the Sun and one will face away, much like our relationship with our own moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question may perhaps be more related to physics, but why does the Earth lose rotational velocity? We know from Newton's laws that an object in motion stays in motion unless acted upon by an outside force. Well, what is the outside force? Is it the Sun's pull on us? Is it the Moon's pull? Is it another planet?&lt;/p&gt;&#xA;" OwnerUserId="7027" LastEditorUserId="49" LastEditDate="2014-12-23T11:52:32.133" LastActivityDate="2016-08-04T20:06:24.533" Title="Why does the Earth lose rotational velocity in the vacuum of space?" Tags="|physics|rotation|celestial-mechanics|earth|tides|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6547" PostTypeId="2" ParentId="6546" CreationDate="2014-12-23T00:04:19.307" Score="17" Body="&lt;p&gt;One of your instincts was correct; it is indeed the influence of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Earth%27s_rotation#Changes_in_rotation&quot;&gt;Wikipedia&lt;/a&gt; notes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Over millions of years, the rotation is significantly slowed by gravitational interactions with the Moon; both rotational energy and angular momentum are being slowly transferred to the Moon: see tidal acceleration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_acceleration&quot;&gt;here&lt;/a&gt; is the general case of a satellite influencing the rotation of the primary:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon), and the primary planet that it orbits (e.g. Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking of the smaller first, and later the larger body. The Earth–Moon system is the best studied case.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Pictorially it is described &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/7/79/Tidal_braking.svg&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/7/79/Tidal_braking.svg&quot; alt=&quot;Tidal bulge offset&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that the tidal bulge is moved relative to the Moon causes a &lt;a href=&quot;https://en.wikipedia.org/wiki/Torque&quot;&gt;torque&lt;/a&gt; between the two bodies, leading to the slowing of Earth's rotation and the recession of the Moon. As torque is related to angular acceleration $\alpha$ via the relation&#xA;$$\tau=I \alpha \to \alpha=\frac{\tau}{I}$$&#xA;and torque is defined as &#xA;$$\tau = r F \sin \theta$$&#xA;where $I$ is inertia, we can calculate the angular acceleration if we know the tidal forces at work.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-12-23T00:15:36.090" LastActivityDate="2014-12-23T00:15:36.090" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6548" PostTypeId="1" AcceptedAnswerId="8614" CreationDate="2014-12-23T01:42:17.433" Score="37" ViewCount="5955" Body="&lt;p&gt;By one of those weird coincidences, I had been on &lt;a href=&quot;http://www.jpaerospace.com/&quot; rel=&quot;noreferrer&quot;&gt;JP Aerospace&lt;/a&gt;'s site mere hours before reading &lt;a href=&quot;https://space.stackexchange.com/questions/6517/has-anyone-proposed-a-serious-space-dirigible-design&quot;&gt;this question about space dirigibles&lt;/a&gt; here. Their &lt;a href=&quot;http://www.jpaerospace.com/atohandout.pdf&quot; rel=&quot;noreferrer&quot;&gt;mission plan&lt;/a&gt; seems too good to be true.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are proposing a three part infrastructure&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Earth to the top of the atmosphere.&lt;/strong&gt;&lt;br&gt;&#xA;  High Altitude airships fly from the ground to the station at&#xA;  140,000 feet.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Transfer point at the edge of space.&lt;/strong&gt;&lt;br&gt;&#xA;  A two mile wide station parked at 140,000 feet is the new way station to space. The station acts not only as a port for the orbital airship but also as a research center, construction site and tourist destination. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;From the edge of space to orbit.&lt;/strong&gt;&lt;br&gt;&#xA;  This 6,000 foot long vehicle never touches the ground. This&#xA;  airship flies from the upper atmospheric station to orbit. It&#xA;  uses hybrid chemical/electric propulsion to slowly&#xA;  accelerate and reach orbit. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MtYI3.jpg&quot; alt=&quot;airship, station, orbitor&quot;&gt;&lt;br&gt;&#xA;&lt;em&gt;L to R: Airship, Station and Orbitor (not to scale).&lt;/em&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Set aside, for the moment, the engineering &amp;amp; technical challenges of establishing a permanent, crewed &quot;suborbital space station&quot; two miles wide at 140,000 feet. The company does, after all, hold the world's record for the &lt;a href=&quot;http://www.aero-news.net/index.cfm?do=main.textpost&amp;amp;id=0fcb3845-ad1f-4b08-b6f1-8a7e5d0c66e6&quot; rel=&quot;noreferrer&quot;&gt;highest flying airship&lt;/a&gt;. What really lifted my eyebrow was their Orbital Ascender:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WNYXC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;This is a huge (a mile or more long) lighter-than-air craft to be assembled at 140,000 feet.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The airship uses buoyancy to climb to 200,000 feet. From there it uses hybrid electric/chemical propulsion to slowly accelerate. As it accelerate it dynamically climbs. In nearly nine hours it achieves orbital velocity.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They're a bit cagey about their &quot;hybrid electric/chemical propulsion,&quot; but the ion engine (or whatever) is to be powered by solar cells overlaying most of the top surface.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there anything about high-altitude dynamics or orbital mechanics that renders the concept of an airship reaching LEO orbit unlikely? Or are there any other show-stoppers to this concept?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're interested, there's a book by the concept's originator:&lt;br&gt;&#xA;&lt;em&gt;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1894959736&quot; rel=&quot;noreferrer&quot;&gt;Floating to Space: The Airship to Orbit Program&lt;/a&gt;&lt;/em&gt;, by John M. Powell (Apogee Books Space Series, 2008)&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="18879" LastEditDate="2018-04-06T12:09:00.043" LastActivityDate="2018-04-06T12:09:00.043" Title="Is the &quot;airship to orbit&quot; mission profile feasible?" Tags="|orbital-mechanics|atmosphere|balloons|uncrewed-spaceflight|air-launch|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6550" PostTypeId="1" CreationDate="2014-12-23T12:23:15.773" Score="3" ViewCount="7950" Body="&lt;p&gt;Lockheed Martin, mostly, develop both he &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Orion&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/CST-100&quot; rel=&quot;nofollow&quot;&gt;CST-100&lt;/a&gt; for NASA. I feel like I'm a bit late at learning here, but are they basically the same designs? The LEO and the Beyond-LEO version. CST-100 as a kind of evolution from the now discontinued an Orion lite project. And with Bigalow involved!? What a small world.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-12-23T13:59:19.387" LastActivityDate="2016-08-15T19:11:38.050" Title="Are the Orion and the CST-100 basically the same spacecraft?" Tags="|orion-spacecraft|cst-100|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6551" PostTypeId="1" AcceptedAnswerId="6553" CreationDate="2014-12-23T12:33:24.060" Score="20" ViewCount="1494" Body="&lt;p&gt;I've read several articles about the new Angara rocket, that called it clean/eco-friendly. I know it runs on kerosene (and LOX), which is not super-toxic like hydrazine, but still... it's polluting the air, right? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I've never heard Falcon/Atlas/Delta or any other kerosene rocket called eco-friendly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question: Are there any aspects of Angara which deserves the eco-friendly term, or is it just &quot;marketing&quot;?&lt;/p&gt;&#xA;" OwnerUserId="6992" LastEditorUserId="51" LastEditDate="2015-07-02T20:47:01.680" LastActivityDate="2015-07-02T20:47:01.680" Title="Why is the Angara rocket called eco-friendly?" Tags="|launch|environment|fuel|angara|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6552" PostTypeId="2" ParentId="6550" CreationDate="2014-12-23T13:24:36.510" Score="7" Body="&lt;p&gt;They might be externally alike (as in design/looks), but they are very different spacecrafts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CST-100's design is for commercial use, and for use within LEO, whereas Orion is intended for use beyond LEO. This alone involves a lot of differences between the spacecrafts, such as heat shield design (Orion needs a more resistant heat shield for entrances from higher altitudes at higher speeds), modules (Orion will have a service module), etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other notable difference is the splashdown: CST-100 uses airbags, while Orion splashes on the sea (like the Apollo spacecrafts).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Orion Lite project cancellation gave place for the development of the CST-100 if I am not mistaken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, quote from &lt;a href=&quot;http://en.wikipedia.org/wiki/CST-100&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The CST-100 has no Orion heritage, but it is sometimes confused with the earlier and similar Orion-derived Orion Lite proposal that Bigelow was reportedly working on with technical assistance from Lockheed Martin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;It would be interesting to elaborate on all the major differences, I guess there must be a comparison list somewhere between Orion, CST-100 and Dragon v2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; &lt;a href=&quot;http://www.reddit.com/r/space/comments/2id53v/what_are_the_differences_in_the/&quot;&gt;Here's a link to an interesting discussion on Reddit about the differences in all 3 major new american spacecrafts (Orion, CST-100, Dragon v2)&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="3001" LastActivityDate="2014-12-23T13:24:36.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6553" PostTypeId="2" ParentId="6551" CreationDate="2014-12-23T13:51:02.840" Score="24" Body="&lt;p&gt;When you compare the waste products of LOX/Kerosene vs hydrazine/MMDH/etc, it is actually not too terrible at full combustion.  (End up with CO2 and water, or CO2, Ammonia, and water).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if say, a fully fueled Proton looses control say 50 seconds into flight and collides with the ground, literally a million lbs of hydrazine is the definition on Unenvironmentally friendly.  While the Proton is very reliable, this has happened twice recently, once in &lt;a href=&quot;https://en.wikipedia.org/wiki/Blok_DM-03#Failures&quot;&gt;July 2013&lt;/a&gt; and again in &lt;a href=&quot;https://en.wikipedia.org/wiki/Proton-M#Reliability&quot;&gt;May 2014&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also since Russia has no good coastal launch sites, they have no choice but to drop stages that never have completely used all their fuel over land, which is no fun wherever it lands. (Mostly in barren desert). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is fundamentally the difference in the fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now technically LOX/H2 is better than LOX/Kerosene, but Hydrazine is so bad, it is worthy of the designation for switching to Kerosene.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="4704" LastEditDate="2014-12-24T01:35:09.793" LastActivityDate="2014-12-24T01:35:09.793" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6554" PostTypeId="2" ParentId="6550" CreationDate="2014-12-23T14:32:30.753" Score="7" Body="&lt;p&gt;A big difference is that Lockheed Martin has spent a lot of money on radiation management in the Orion vehicle. That is, they have looked at shielding, placement of bulkheads, food and water storage, etc, so as to quantify the level of radiation you would take in the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, should there be a solar storm, where is the safe spot in the vehicle, and how to orient the vehicle for maximum protection.  Face the Service Module into the direction and get everybody huddling in the bottom, to allow fuel in the SM and storage supplies to offer as much protection as possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an area SpaceX, Boeing, and Sierra Nevada have been able to mostly ignore to date. It is apparently a very complex and expensive task to accomplish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond that, they really just share the same generic shape, a capsule.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion is: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Larger than CST-100. &lt;/li&gt;&#xA;&lt;li&gt;Lands in water (vs. airbags on land for CST-100).&lt;/li&gt;&#xA;&lt;li&gt;21 day endurance for crew of 4 (vs. couple of days, then docked to ISS for power/etc).&lt;/li&gt;&#xA;&lt;li&gt;Orion Service Module has significantly more delta V.&lt;/li&gt;&#xA;&lt;li&gt;Washroom facilities. (CST-100 is most likely to use diapers).&lt;/li&gt;&#xA;&lt;li&gt;Radiation protection.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="51" LastEditorUserId="574" LastEditDate="2016-08-15T19:11:38.050" LastActivityDate="2016-08-15T19:11:38.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6555" PostTypeId="1" AcceptedAnswerId="6557" CreationDate="2014-12-23T15:29:22.857" Score="4" ViewCount="17700" Body="&lt;p&gt;I live in a Maryland suburb of Washington, D.C., and the weatherman thought we'd be able to see a rocket launched from Wallops Island (alas, it blew up instead) which by car is 166 miles away.  Assuming that I have a clear view of the horizon (I don't really, but perhaps I could go to a nearby 19-story apartment building), and clear skies, how far from the launch pad would the launch be visible to the naked eye?&lt;/p&gt;&#xA;" OwnerUserId="7032" LastEditorUserId="7062" LastEditDate="2022-11-15T19:33:59.647" LastActivityDate="2022-11-16T22:23:02.893" Title="From how far away can one see a NASA rocket launch?" Tags="|launch|launch-site|visibility-of-spacecraft|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="6556" PostTypeId="1" AcceptedAnswerId="6563" CreationDate="2014-12-23T15:39:34.317" Score="10" ViewCount="6303" Body="&lt;p&gt;I lived in Hawaii when Apollo 15 returned from the moon.  I saw the crew return by helicopter from the ship to Hickham A.F.B. and when the helicopter carrier, the USS Okinawa (LPH 3), returned to Pearl Harbor, my father (a USAF full-bird colonel) arranged for my family to get a tour of the ship from one of the frogmen who helped keep the command module afloat and pull out the astronauts.  He gave me a small piece of the gold-colored mylar foil which he said he and other sailors stripped off the outside of the command module as souveniers (I saw the capsule on the hanger deck).  The sailor, I saw, had retained a piece of foil about 9 inches in diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The capsule did look clean as I saw it on the hanger deck, not like this &lt;a href=&quot;http://www.redorbit.com/images/pic/2772/apollo-11-spacecraft-hoisted-aboard-u-s-s-hornet/&quot; rel=&quot;nofollow noreferrer&quot;&gt;photograph of Apollo 11 as it was hoisted onboard the Hornet&lt;/a&gt;; rather it looked more like the &lt;a href=&quot;http://fc03.deviantart.net/fs70/i/2013/026/d/7/apollo_11_capsule_by_generaltate-d5svdh3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;cleaned-up version of Apollo 11&lt;/a&gt; in the Air and Space Museum.  I suspect that the mylar foil was on the underside of white paint that burned off during reentry, and that the sailors finished the job.  But I do not know this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this piece of mylar I have MLI (&lt;em&gt;Multi-Layer Insulation&lt;/em&gt;)? Is it possible to have that verified? If it is MLI, what material would have covered it so that it did not look gold on liftoff?  Was it always that color, or did its color change?  Is it worth anything today?&lt;/p&gt;&#xA;" OwnerUserId="7032" LastEditorUserId="19586" LastEditDate="2019-12-19T15:24:38.670" LastActivityDate="2019-12-19T15:24:38.670" Title="What was mylar used for on the Apollo Command Module space craft? Why is it gold after returning from space?" Tags="|apollo-program|materials|heat-shield|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="6557" PostTypeId="2" ParentId="6555" CreationDate="2014-12-23T15:43:44.543" Score="7" Body="&lt;p&gt;Here's a map that shows the general visibility for Antares launches (that was the rocket the weatherman was talking about, although it won't be flying again until 2016):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ap6p8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I once watched a Minotaur V launch from Wallops down on Brighton Beach in New York. It was clearly visible. It was also a night launch, it probably wouldn't have been visible during daytime. Minotaur V is less powerful than Antares, so if you had a clear view of the horizon, you should definitely be able to see an Antares launch from DC.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, it's not a NASA rocket. The Antares (and the Minotaur V) is built by Orbital Sciences Corporation. Currently all manifested missions of the Antares are under contract with NASA.&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="1255" LastEditDate="2014-12-23T15:49:02.353" LastActivityDate="2014-12-23T15:49:02.353" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6558" PostTypeId="1" AcceptedAnswerId="6593" CreationDate="2014-12-23T16:22:35.187" Score="0" ViewCount="257" Body="&lt;p&gt;If humanity ever got to the point of being able to explore new planets (assuming we could land there), are there any known entities in the universe that are so massive that gravitational time dilation could really become a serious issue? In other words, if the explorers landed on the planet/entity, could it create a potential disaster as their time would slow down so much that the cooresponding operations on earth would have long since expired? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I read, most gravitational time dilation effects are minimal, nanoseconds. When/where does it become a real problem? Short of black holes, because of the obvious obstacles surrounding them.&lt;/p&gt;&#xA;" OwnerUserId="6627" LastActivityDate="2014-12-27T11:23:40.913" Title="Theoretically, could gravitational time dilation ever be an issue when visiting an alien planet?" Tags="|interstellar-travel|interplanetary|time|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6559" PostTypeId="1" CreationDate="2014-12-23T19:14:30.827" Score="22" ViewCount="1124" Body="&lt;p&gt;It is well known that Pan American World Airways took &lt;a href=&quot;http://articles.latimes.com/1985-02-10/business/fi-3559_1_public-interest&quot;&gt;more than 90,000 reservations&lt;/a&gt; for future passenger flights to the Moon from 1968 until its fall into bankruptcy. How far beyond that did Pan Am go? Did they hire engineers to determine, at least, what would be necessary to build a space plane capable of carrying passengers to the Moon? Did any of the aircraft manufacturers start work on the vehicle?&lt;/p&gt;&#xA;" OwnerUserId="7032" LastEditorUserId="49" LastEditDate="2014-12-23T19:22:50.077" LastActivityDate="2014-12-24T08:48:26.870" Title="How far did Pan Am go with its plans to have passenger trips to the Moon?" Tags="|the-moon|history|commercial|feasibility|tourism|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6563" PostTypeId="2" ParentId="6556" CreationDate="2014-12-23T21:02:00.523" Score="12" Body="&lt;p&gt;The outside of the Command Module is covered by a heat shield. According to &lt;a href=&quot;https://www.hq.nasa.gov/alsj/CSM06_Command_Module_Overview_pp39-52.pdf&quot; rel=&quot;noreferrer&quot;&gt;this report&lt;/a&gt; (page 5), the forward portion of the heat shield (i.e. the part covering the cone-shaped part of the CM) is a 0.5&quot; thick layer of phenolic resin. It is covered by several outer layers: a pore seal, a moisture barrier (which is white) and the outer layer is a silvered Mylar reflective coating. This looks like aluminium foil initially, as you can see in this photo:&lt;br&gt;&#xA;  &lt;img src=&quot;https://i.stack.imgur.com/pPtgq.jpg&quot; alt=&quot;Apollo CM in flight&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After reentry and landing the outer layers are partially burned. This is the Apollo 13 CM during recovery:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/G07ue.jpg&quot; alt=&quot;Apollo 13 CM post-flight&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The charring could have caused the Mylar to yellow, or the Mylar was yellow to begin with and burning off the reflective coating revealed its original color.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After landing, the CM would have been cleaned up for display, the charred outer layers removed to reveal the phenolic resin beneath:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/3bgQH.jpg&quot; alt=&quot;CM in Science Museum&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In conclusion, the piece of Mylar you have is not MLI.&lt;br&gt;&#xA;On liftoff, the CM was covered by the white Boost Protective Cover which was attached to the Lanch Escape System. Once the rocket was above the atmosphere, the BPC and LES were jettisoned.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2014-12-23T21:10:54.977" LastActivityDate="2014-12-23T21:10:54.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6564" PostTypeId="2" ParentId="2123" CreationDate="2014-12-24T05:03:03.950" Score="7" Body="&lt;p&gt;With the right google search you can find just about anything. This link has details about the leak from each module and the volume of each module. You can average the values or find whichever you think is most appropriate for your purpose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110012997.pdf&quot; rel=&quot;nofollow&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110012997.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's just a shame this question is over a year old -_-&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2014-12-24T13:54:05.113" LastActivityDate="2014-12-24T13:54:05.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6565" PostTypeId="2" ParentId="6559" CreationDate="2014-12-24T08:48:26.870" Score="12" Body="&lt;p&gt;Interesting, been reading about this and it looks like the first moon flight reservation for Pan Am was made in 1964 actually, and then the boom in reservations came in 1968 as you say. *¹&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;August 1964 Pan Am accepted the reservation of Gerhart Pistor, journalist from Vienna, Austria, as the No. 1 for future flights to the moon. He paid a deposit of 20 US $. About 93.000 people followed on the Pan Am waiting list, called &quot;First Moon Flights Club&quot;. Pan Am expected the flight to depart about the year 2000 &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's not much out there on the technology, I believe it didn't even made it past marketing maybe, just a publicity stunt or a placeholder *²&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A Pan Am spokesperson acknowledged that a few problems would need to be sorted out before lunar flights could get off the ground—among them, the high cost of fuel, passenger comfort during launch, and lack of accommodations on the moon. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/LQYTo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;New members got a glossy membership card signed by the vice-president of sales and a letter that helpfully explained what would happen next.  “Starting date of service is not yet known,” it warned moon-flight-clubbers, but asked for patience &lt;strong&gt;while the “essentials are worked out”&lt;/strong&gt; – something that would surely be happening soon, right?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also it looks like they had nailed down the looks of the space plane *³&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BqwEH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trans World Airlines was also a competitor collecting about 6,000 reservations for moon flights, which they later canned in a warehouse. *⁴&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Trans World Airlines, caught up in the same space exploration fever, also began taking reservations about then. But after collecting about 6,000, TWA found that public interest had waned and now stores the applications in a warehouse.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Finally, here's an interesting thread on History SE about this: &lt;a href=&quot;https://history.stackexchange.com/questions/9656/how-much-did-pan-am-tickets-to-the-moon-cost&quot;&gt;How much did Pan Am tickets to the moon cost?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, tl;dr - I'm pretty certain this did not get past a publicity stunt or just some very early initial artist concept designs (like the one in the picture above).&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;¹ &lt;a href=&quot;http://en.wikipedia.org/wiki/Pan_American_World_Airways#In_popular_culture&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pan Am article on Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;² &lt;a href=&quot;http://www.airspacemag.com/not-categorized/space-tourisms-first-small-steps-20428780/#ixzz3Mnqg2WDY&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Tourism's First Small Steps&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;³ &lt;a href=&quot;http://articles.latimes.com/1985-02-10/business/fi-3559_1_public-interest&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pan Am Has 90,002 Reservations : Public Interest Grows in Flights to the Moon&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;⁴ &lt;a href=&quot;http://backstoryradio.org/2013/08/19/pan-am-and-the-waiting-list-for-the-moon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;PAN AM AND THE WAITING LIST FOR THE MOON…&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3001" LastEditorUserId="-1" LastEditDate="2017-04-13T12:47:53.127" LastActivityDate="2014-12-24T08:48:26.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6566" PostTypeId="2" ParentId="6521" CreationDate="2014-12-24T11:27:55.300" Score="4" Body="&lt;p&gt;&lt;strong&gt;1. About maximum reentry speed, unshielded.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Not an answer, just some additional elements...&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmospheric_entry#Shock_layer_gas_physics&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;An approximate rule-of-thumb used by heat shield designers for&#xA;estimating peak shock layer temperature is to assume the air&#xA;temperature in kelvins to be equal to the entry speed in meters per&#xA;second.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(for vehicle speed see figure below).&lt;/p&gt;&#xA;&lt;p&gt;Then it depends on the spacecraft material exposed to heat. The material must maintain its mechanical properties, and the temperature inside must be acceptable.&lt;/p&gt;&#xA;&lt;p&gt;For &lt;a href=&quot;http://suzymchale.com/ruspace/soyland.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;current deorbit schedule&lt;/a&gt;, the reentry interface occurs at 102.3 km altitude, at 7,591 m/s. How much time will be needed for the compounds to melt at 7,823 °C (or the inside temperature unacceptable)?&lt;/p&gt;&#xA;&lt;p&gt;figures will change when the reentry interface speed will be decreased, and so the landing trajectory.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;2. About alternative atmosphere reentry methods&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Alexander_Bolonkin&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alexander Bolonkin&lt;/a&gt; wrote &lt;a href=&quot;http://arxiv.org/pdf/physics/0701094&quot; rel=&quot;nofollow noreferrer&quot;&gt;an article&lt;/a&gt; (another version &lt;a href=&quot;http://gsjournal.net/Science-Journals/Essays/View/5289&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;) in which he proposes to use high-altitude parachutes to supplement or replace the heat shields.&lt;/p&gt;&#xA;&lt;p&gt;Warning: The author looks like &lt;a href=&quot;http://2012.gf2045.com/read/111/&quot; rel=&quot;nofollow noreferrer&quot;&gt;controversial&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;The widespread production of high temperature fibers and whiskers&#xA;allows us to design high temperature tolerant parachutes, which may be&#xA;used by space apparatus of all types for braking in a rarefied planet&#xA;atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;The parachute has open backside surface that rapidly emits&#xA;the heat radiation to outer space thereby quickly decreasing the&#xA;parachute temperature.&lt;/p&gt;&#xA;&lt;p&gt;The proposed new method significantly decreases&#xA;the maximum temperature and heat flow to main space apparatus. That&#xA;decreases the heat protection mass and increases the useful load of&#xA;the spacecraft.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In the same article, he shows the effect on the STS orbiter (with different parachute sizes):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/U9CPw.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I don't know if this has been researched further, or even confirmed as valid.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2014-12-24T13:06:55.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6567" PostTypeId="1" CreationDate="2014-12-24T14:55:16.977" Score="20" ViewCount="2105" Body="&lt;p&gt;When Orbital Sciences' &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_CRS_Orb-3&quot;&gt;Antares rocket exploded at Wallops Island&lt;/a&gt; just six seconds after launch, the loss of the rocket and spacecraft was estimated at more than $200 million, not including damage to the launch pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.forbes.com/sites/alexknapp/2014/10/28/antares-rocket-explodes-on-launch-pad/&quot;&gt;Forbes&lt;/a&gt;, some of the loss was insured, although Forbes did not say how much of the loss was covered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much risk of commercial spacecraft launches are insurance companies willing to take, and how much of the potential losses must be considered a cost of doing business for the new commercial space industry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, how much of the cost of a commercial launch goes for insurance premiums?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, what conditions have insurance companies placed on the commercial space industry as a prerequisite for underwriting launches?   &lt;/p&gt;&#xA;" OwnerUserId="7032" LastEditorUserId="7017" LastEditDate="2015-06-23T12:25:21.877" LastActivityDate="2015-06-23T12:25:21.877" Title="How much of a commercial space launch can be insured?" Tags="|commercial|space-industry|risk-management|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6568" PostTypeId="2" ParentId="6184" CreationDate="2014-12-24T15:24:56.750" Score="4" Body="&lt;p&gt;You could certainly design an EVA suit that has a low enough thermal conductivity (or effective thermal conductivity) such that it has a loss equal to the typical thermal output from a human body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can estimate heat output from the human body based on the typical (suggested) calorific intake.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$2,000\text{ cal} = 8.37\times10^6\text{ J}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we assume all this is turned into heat energy, then we can design a suit to match this energy input. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Body_surface_area&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; the average surface area of the human body is $1.73\ m^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we would want to design a suit with a thermal flux of $4.84\times10^6\text{ J}/m^2/\text{day}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Joules/day is kind of a messy unit system. We can tidy that up and turn into joules / second, or watts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$4.84\times10^6\text{ J}/m^2/\text{day} = 4.84\times10^6\times(1/(60\times60\times24))\mathrm{\ W}/m^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves us with a value of almost exactly $56\mathrm{\ W}/m^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to design for the worst case scenario (coldest surroundings) you have both conduction from the wall to the atmosphere, and radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The convection from the outside wall of the suit is (derived from Fourier's law):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathrm{Q} = κA(T_{\mathrm{hot}}-T_{\mathrm{cold}})/d$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\mathrm{Q}$ = rate of heat transfer (in watts)&lt;/li&gt;&#xA;&lt;li&gt;$κ$ = thermal conductivity of the wall (in watts per meter kelvin)&lt;/li&gt;&#xA;&lt;li&gt;$A$ = heat transfer surface area (in square meters)&lt;/li&gt;&#xA;&lt;li&gt;$T_{\mathrm{hot}}$ = hot temperature (in kelvins)&lt;/li&gt;&#xA;&lt;li&gt;$T_{\mathrm{cold}}$ = cold temperature (in kelvins)&lt;/li&gt;&#xA;&lt;li&gt;$d$ = distance to ambient temperature (in meters)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The radiation can be calculated with the following equation (derived from &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Stefan-Boltzmann law&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$q = \sigma \times A \times \Delta T^4$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$q$ = heat transfer per unit time (in watts)&lt;/li&gt;&#xA;&lt;li&gt;$\sigma$ = &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stefan–Boltzmann constant&lt;/a&gt; ($5.670373 \times 10^{-8}\, \mathrm{J\, s^{-1}m^{-2}K^{-4}}$)&lt;/li&gt;&#xA;&lt;li&gt;$A$ = total surface area of the emitting body (in square meters)&lt;/li&gt;&#xA;&lt;li&gt;$\Delta T$ = $T_{\mathrm{hot}}-T_{\mathrm{cold}}$ (in kelvins)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Depending on the design of your suit the internal heat transfer is going to happen in one of two ways, either conduction though material or radiation, if the design is &lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-layer_insulation&quot; rel=&quot;nofollow noreferrer&quot;&gt;MLI&lt;/a&gt; based. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So is it possible to build an EVA suit that doesn't require an external heat source? Yes. It's a question of the mass of the resulting suit and whether the human inside could move around in the potentially heavy suit.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="6816" LastEditDate="2017-01-06T22:56:37.360" LastActivityDate="2017-01-06T22:56:37.360" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6572" PostTypeId="1" AcceptedAnswerId="6573" CreationDate="2014-12-24T20:28:36.273" Score="7" ViewCount="703" Body="&lt;p&gt;According to the “NASA Earth Science Instruments” section of noaa.gov's &lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSCOVR&lt;/a&gt; page, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;DSCOVR will make unique space measurements from the first sun-Earth Lagrange point (L1). ... 1.5 million kilometers ... sunward from Earth ... The spacecraft will be &lt;strong&gt;orbiting this point in a six-month orbit&lt;/strong&gt; with a spacecraft-Earth-sun angle varying between 4 and 15 degrees. &lt;em&gt;[emphasis added]&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It seems to me that if the orbit is perpendicular to the Earth-Sun line, the range of spacecraft-Earth-Sun angles would include negative angles, rather than ranging +4° to +15°.  Or, it seems to me that if it is a long ellipse parallel to that line (which would allow a range of positive angles) it might not be a stable orbit (without using fuel)&lt;a href=&quot;https://space.stackexchange.com/questions/5871/how-can-i-calculate-the-characteristic-energy-to-sun-earth-l1&quot;&gt;.&lt;/a&gt;  Are these ideas incorrect?  More concretely, what plane will the six-month orbit be in, and what will its shape be?&lt;/p&gt;&#xA;" OwnerUserId="7043" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-11-20T10:59:54.037" Title="What plane will DSCOVR’s orbit be in?" Tags="|orbit|lagrangian-points|dscovr|noaa|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6573" PostTypeId="2" ParentId="6572" CreationDate="2014-12-25T00:06:43.427" Score="6" Body="&lt;p&gt;The orbit's probably close to perpendicular to the Earth-Sun line, since its imaging target is the Earth. The reason the angles referenced are all positive is because they measure the divergence of two lines, Earth-Sun (passing through SEL-1) and Earth-spacecraft. In essence, they're describing the distance from SEL-1, always a positive value, in terms of the Earth-based observational angle between DSCOVR and the sun.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the ranges were closer together, they would be tracing out a more circular orbit, but instead it's in a quasi-periodic orbital trajectory known as a &lt;a href=&quot;http://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lissajous orbit&lt;/a&gt;, which means it looks something like this:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/lQPch.jpg&quot; alt=&quot;Lissajous orbits&quot;&gt;&#xA;&lt;em&gt;ARTEMIS Lissajous Trajectory Design&lt;/em&gt;&lt;br&gt;&#xA;(Source: &lt;em&gt;&lt;a href=&quot;http://www.igpp.ucla.edu/public/THEMIS/SCI/Pubs/artemis/artemis_woodward.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARTEMIS: The First Mission to the Lunar Libration Orbits&lt;/a&gt;&lt;/em&gt;, Mark Woodard, David Folta, Dennis Woodfork (Goddard Space Flight Center), page 9.)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The +15 occurs near the end of the bean (as it were), the +4 the inside &quot;belly&quot; of the bean.  &lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-25T14:54:16.757" LastActivityDate="2014-12-25T14:54:16.757" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6574" PostTypeId="2" ParentId="6548" CreationDate="2014-12-25T02:02:33.467" Score="11" Body="&lt;p&gt;To float something at 140,000 feet (&gt; 42 km) requires an impossibly low density structure.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 40 km the density of air is less than 4 grams per cubic meter. Barely attainable with the lightest of balloons. The proposed added structures, engines etc make it quite impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to making a balloon structure ascend to orbit, again impossible. The drag will be orders of magnitude too great.&lt;/p&gt;&#xA;" OwnerUserId="7044" LastEditorUserId="49" LastEditDate="2014-12-25T03:22:19.557" LastActivityDate="2014-12-25T03:22:19.557" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6575" PostTypeId="1" AcceptedAnswerId="6580" CreationDate="2014-12-25T02:37:08.847" Score="6" ViewCount="734" Body="&lt;p&gt;Amateur radio hardware is reasonably expensive. Are there any real scientific benefits for universities to build these stations (assuming the don't have a satellite in orbit)? I know there's some clever science that can be done with a ground station - but if it's already been done why are so many universities jumping on this now?&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="168" LastEditDate="2014-12-25T07:24:00.843" LastActivityDate="2014-12-25T12:48:25.517" Title="What are the benefits of building an amateur satellite ground station?" Tags="|ground-station|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6576" PostTypeId="1" AcceptedAnswerId="6578" CreationDate="2014-12-25T10:20:19.463" Score="3" ViewCount="209" Body="&lt;p&gt;Have there been any specifically Christmas related events in space? I assume there's a 'Christmas dinner' on the ISS, but is there anything else?&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-12-25T11:16:49.093" Title="What Christmas events have happened in space?" Tags="|reference-request|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6577" PostTypeId="1" AcceptedAnswerId="6579" CreationDate="2014-12-25T11:03:54.463" Score="21" ViewCount="2614" Body="&lt;p&gt;Many NASA inventions have had subsequent lives in the commercial marketplace. Have the Space Shuttle's heat-shielding tiles also had a second life?&lt;/p&gt;&#xA;" OwnerUserId="7032" LastEditorUserId="49" LastEditDate="2014-12-26T13:51:56.927" LastActivityDate="2014-12-26T13:51:56.927" Title="Is the technology behind the Shuttle's heat-shielding tiles used in any product today?" Tags="|space-shuttle|technology|materials|heat-shield|spinoff|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6578" PostTypeId="2" ParentId="6576" CreationDate="2014-12-25T11:16:49.093" Score="8" Body="&lt;p&gt;The most famous Christmas-related event was on the &lt;a href=&quot;http://www.space.com/28098-christmas-in-space-apollo8.html&quot;&gt;Apollo 8&lt;/a&gt; mission. Apollo 8 orbited the Moon on Christmas Eve and into the morning of December 25th.&lt;br&gt;&#xA;On Christmas Eve, the crew read &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_8_Genesis_reading&quot;&gt;the first 10 verses from Genesis 1&lt;/a&gt; on TV for what was then the largest TV audience ever. They closed their broadcast with:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-12-25T11:16:49.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6579" PostTypeId="2" ParentId="6577" CreationDate="2014-12-25T11:53:53.670" Score="25" Body="&lt;p&gt;There's a &lt;a href=&quot;http://www.nasa.gov/centers/marshall/news/background/facts/SpinoffsFromTheShuttleFS.html&quot;&gt;Spinoffs from the Space Shuttle Program&lt;/a&gt; page hosted on the &lt;a href=&quot;http://www.nasa.gov/centers/marshall/home/index.html&quot;&gt;NASA Marshall Space Flight Center&lt;/a&gt; portal. According to it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Jewelry Design&lt;/strong&gt; — Jewelers no longer have to worry about inhaling dangerous asbestos fibers from the blocks they use as soldering bases.&#xA;  Space Shuttle heat shield tiles offer jewelers a safer soldering base&#xA;  with temperature resistance far beyond the 1,400 degrees Fahrenheit&#xA;  generated by the jeweler's torch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is also:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Automotive Insulation&lt;/strong&gt; — Materials from the Space Shuttle thermal protection system are being used on NASCAR racing cars to protect&#xA;  drivers from the extreme heat generated by the engines.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But the latter isn't specific about the Space Shuttle heat shield tiles (aka HRSI - High-temperature Reusable Surface Insulation) and I believe they're referring to the use of Reinforced Carbon–Carbon (RCC) composites and Flexible Insulation Blankets (FIB) here. &lt;a href=&quot;http://www.nasa.gov/topics/nasalife/features/nasa-nascar.html&quot;&gt;NASA at NASCAR&lt;/a&gt; page isn't much more specific on this, but it does list some other &lt;a href=&quot;http://spinoff.nasa.gov/&quot;&gt;NASA Spinoff&lt;/a&gt; technologies that benefit the National Association for Stock Car Auto Racing, from fire-resistant materials for race car drivers and pit crew suits to better brakes and safer tires.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an additional, indirect benefit to the industry developed for the HRSI tiles:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Product Labeling&lt;/strong&gt; — NASA needs to identify, track, and keep records on each of the thousands of heat-shield tiles on the Space Shuttle.&#xA;  This required a labeling system that could be put on ceramic material&#xA;  and withstand the rigors of space travel to be readable after a&#xA;  flight. NASA developed high data-density, two-dimensional,&#xA;  machine-readable symbol technology used to mark individual tiles. This&#xA;  novel method of labeling products with invisible and virtually&#xA;  indestructible markings can be used on electronic parts,&#xA;  pharmaceuticals and livestock -- in fact on just about anything.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://fas.org/news/usa/2000/usa-001012.htm&quot;&gt;Space Shuttle Program Benefits Industry and Health&lt;/a&gt;. You can read more about this laser-etched symbol coding, dubbed &lt;em&gt;&quot;compressed symbologies&quot;&lt;/em&gt;, now commercialized labeling / identification system through a &lt;a href=&quot;http://www.nasa.gov/open/plan/space-act.html&quot;&gt;NASA Space Act&lt;/a&gt; agreement, on &lt;a href=&quot;http://spinoff.nasa.gov/spinoff1998/ch1.htm&quot;&gt;NASA Spinoff&lt;/a&gt;. I'm reproducing a shot excerpt here for convenience:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Compressed symbologies can withstand extreme fluctuations of&#xA;  temperatures, up to 2,200 degrees Fahrenheit and an air flow exceeding&#xA;  18,000 miles per hour. That is the heat load and speed associated with&#xA;  a Shuttle orbiter during a space mission.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The coding technology provides up to 100 times as much information as&#xA;  linear bar coding symbology in the same or less space. Markings can&#xA;  range in size from a mere four microns (read microscopically) to as&#xA;  large as two square feet (read telescopically).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-12-25T15:26:16.490" LastActivityDate="2014-12-25T15:26:16.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6580" PostTypeId="2" ParentId="6575" CreationDate="2014-12-25T12:48:25.517" Score="5" Body="&lt;p&gt;Many universities might be on the &lt;em&gt;cheap spaceflight&lt;/em&gt; waiting list and might be building their own CubeSats or similar microsatellites that could potentially fly &lt;em&gt;on the cheap&lt;/em&gt; as secondary payloads, but so far weren't as lucky yet. Such waiting lists can stretch for many years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So they would want to be prepared and with a ground team having some hands-on experience with the tracking and communications equipment when some space launch provider is ready to accept their payload and launch vehicle integration can finally begin. There is also a lot of the paperwork involved, including getting FCC license for radio frequencies used, and with actual equipment on premise, both the satellite and the radio equipment needed to communicate with it, you can test and optimize radio link between them and the software that runs it. If this means changing frequencies, you'll have to go back to the FCC again. All this takes time and requires having actual equipment that will be later used. It would also be the time to certify your ground crew, secure and manage access to equipment, and organize any other of your project's support activities, if any of that is a requirement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of this needs to happen before an actual launch and later having your satellite in Earth's orbit or beyond. If your team also competes for some challenge, there's also going to be a lot of other terms and conditions that your team will try to adhere to the best they can and manage their way through the selection process. Of course, having on premise equipment early can also help promote university's capabilities and give potential sponsors from the industry ample time to help finance, provide, build and test any equipment and facilities, and they can use that for their own promotional purposes, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there is any point to having an amateur satellite ground station without even having any ambitions to launch, that I wouldn't know. Perhaps through cooperation with other colleges and being able to organize interesting STEM-related workshops (that might be government sponsored), there would still be some point to having such facilities. And if you receive them as a donation, then why not?&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-12-25T12:48:25.517" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6582" PostTypeId="1" AcceptedAnswerId="6588" CreationDate="2014-12-25T18:56:15.740" Score="19" ViewCount="5469" Body="&lt;p&gt;A lot of space probes use gravity assists from various space objects to boost their speed and save on propellant requirements. On what factors does the increase of delta-v depend? My guesses are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mass of object (the more massive, the better)&lt;/li&gt;&#xA;&lt;li&gt;Distance from object to spacecraft (the closer, the better)&lt;/li&gt;&#xA;&lt;li&gt;Time spent behind (or ahead, if you want to decelerate) object (the longer, the better)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, I'm not sure about my speculations, as they come solely from free-time reading and experience gained playing the Kerbal Space Program game. Could someone more confident widen my perception of gravitational slingshot?&lt;/p&gt;&#xA;" OwnerDisplayName="user6738" LastEditorUserId="49" LastEditDate="2014-12-25T19:17:04.737" LastActivityDate="2014-12-29T01:32:26.593" Title="How much delta-v can we squeeze out of a gravitational slingshot and what factors limit it?" Tags="|orbital-mechanics|delta-v|gravity-assist|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="6583" PostTypeId="1" CreationDate="2014-12-25T18:56:30.870" Score="5" ViewCount="1265" Body="&lt;p&gt;How do astronauts walking on the moon keep their waste body fluids from freezing in the extremely cold environment? I understand that if they need to release waste body fluids, they do so into specifically prepared &quot;diapers&quot; or some other such system. But still, all body fluids, such as blood, cranial, urinary, etc., must be kept at warm enough temperature to flow. How is this done? Are radioisotope heat generators used in any way?&lt;/p&gt;&#xA;" OwnerUserId="7047" LastActivityDate="2016-05-09T02:29:17.380" Title="Lunar astronauts and freezing temperatures" Tags="|the-moon|astronauts|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6584" PostTypeId="2" ParentId="6583" CreationDate="2014-12-25T19:26:07.260" Score="10" Body="&lt;p&gt;The challenge of keeping an astronaut in a suit on the moon comfortable turns out to be in keeping him cool, not keeping him warm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat is transferred in three ways: (infrared) radiation, conduction (direct contact), and convection (transfer via a fluid medium like air). For a suited astronaut, conduction is minimal and convection is nonexistent; the only way the suit loses heat is by radiating it. The exterior of the suit is white to minimize heating in direct sunlight, but that also minimizes radiative ability. So all the heat produced by the astronaut's body in the suit stays in the suit, and that's a substantial amount. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you actually have to circulate coolant through tubes in an undergarment of the space suit to draw heat out. Freezing urine is not at all a concern!&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-05-09T02:29:17.380" LastActivityDate="2016-05-09T02:29:17.380" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6585" PostTypeId="1" AcceptedAnswerId="6622" CreationDate="2014-12-25T19:37:01.673" Score="6" ViewCount="1956" Body="&lt;p&gt;The geostationary orbit of about 35,786 km above mean sea-level allows satellites to rotate at the same speed as the Earth on its axis, making them seem as if they don't move relative to the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do any other orbital altitudes have particular advantages, for example, why was the orbital height of 431 km chosen for the International Space Station? Why are satellites placed in certain orbits? To limit the scope of this question, I am not really talking about inclination (e.g. polar, retrograde, prograde, etc.).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If any particular orbital altitudes have technical names, that would be a bonus.&lt;/p&gt;&#xA;" OwnerUserId="65" LastEditorUserId="4660" LastEditDate="2014-12-26T00:47:33.253" LastActivityDate="2014-12-29T11:36:37.450" Title="Orbital altitudes, are some better than others and why?" Tags="|orbit|geostationary|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6586" PostTypeId="2" ParentId="6585" CreationDate="2014-12-25T19:48:04.670" Score="5" Body="&lt;p&gt;Different orbits allow for different benefits. For example lower orbital altitudes have a lower space debris population (or at least guarantee debris will de-orbit faster), also specific inclination and altitude combinations allow for sun-synchronisation, so you can perform Earth observations and guarantee the sun is is the same relative position with every pass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to get a general idea of which altitudes have specific benefits you could look at space debris populations or just look at where things are right now in the satcat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS as you specifically asked about was placed in the orbit because it minimises risk from space debris without having such significant drag that the Fuel requirements for station keeping are too high!&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2014-12-25T19:48:04.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6587" PostTypeId="1" AcceptedAnswerId="8724" CreationDate="2014-12-25T19:51:14.123" Score="19" ViewCount="30709" Body="&lt;p&gt;There was a question regarding the closest distance regular people are &lt;strong&gt;allowed&lt;/strong&gt; to be near a rocket. At that distance (Baikonur I, 1.1 miles, 1.8 km), even if a rocket explodes, there is a small chance someone is getting killed.&lt;/p&gt;&#xA;&lt;p&gt;What I ask you is how close can I be near a rocket assuming a successful launch and survive?&#xA;And what thing will be most likely to kill me? Heat? Rocket debris? Little rocks flying away? Toxic gas?&lt;/p&gt;&#xA;&lt;p&gt;Of course, the specific answer highly depends on a variety of factors, perhaps most importantly the type of a rocket, therefor I only want an approximate answer (like 100m, 50m, 10m, most probably heat).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;NO MEANS OF PROTECTION ALLOWED!&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user6738" LastEditorUserId="7062" LastEditDate="2022-08-22T06:36:59.973" LastActivityDate="2022-12-10T16:10:23.517" Title="How close can I get to a rocket launch and not get killed?" Tags="|launch|launch-site|launchpad|" AnswerCount="6" CommentCount="14" ContentLicense="CC BY-SA 4.0" />
  <row Id="6588" PostTypeId="2" ParentId="6582" CreationDate="2014-12-25T20:25:28.727" Score="20" Body="&lt;p&gt;Yes, those are the three factors.  Your third factor shows up as the $v_\infty$ of the spacecraft relative to the object.  The first two are the GM of the object, $\mu$ and the closest approach distance $r$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $\Delta V$ you can get is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$2\,v_\infty\over 1+{r\,v_\infty^2\over\mu}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you surmised, lower $v_\infty$ is good since you spend more time under the influence, so to speak.  But not too low.  The $\Delta V$ goes up as $v_\infty$ drops towards $\sqrt{\mu\over r}$, but below that, the $\Delta V$ starts going down again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5Tuzu.png&quot; alt=&quot;Delta V has a maximum&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the left side of the curve, there's not much velocity there to change.  Note that the change in velocity comes entirely from a change in direction in the reference frame of the body doing the slinging.  The magnitude of the $v_\infty$ going out is exactly the same as the magnitude going in.  The change in direction is called the bend.  The bend angle at the maximum $\Delta V$ of $\sqrt{\mu/r}$ is 60°.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2014-12-26T04:26:03.163" LastActivityDate="2014-12-26T04:26:03.163" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="6590" PostTypeId="1" AcceptedAnswerId="9465" CreationDate="2014-12-25T22:13:46.700" Score="8" ViewCount="925" Body="&lt;p&gt;The barge, the ASDS (Autonomous Spaceport Drone Ship) that SpaceX plans to demonstrate landing with is based out of the east coast of the US.  But the landing site will be well downrange off the coast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How will it get to the proper location?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will it be towed, or will it use the positioning thrusters to travel?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-10T19:42:33.407" Title="How will the SpaceX landing barge (ASDS) get to the proper location?" Tags="|spacex|landing|asds|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6591" PostTypeId="2" ParentId="6585" CreationDate="2014-12-26T01:37:27.203" Score="4" Body="&lt;p&gt;So let's talk about geostationary first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you pointed out, geostationary altitudes let the satellites 'hover' over a certain area of the Earth, this is useful for weather monitoring, communications (a single satellite can be dedicated to an entire continent, permanently), and probably other things I haven't thought of. One disadvantage though, is signal latency. Some satellite internet providers have satellites at this altitude, and while they provide good enough coverage for doing something like reading StackExchange, forget about trying to play multiplayer games.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there's two considerations of altitude, coverage and lag. The higher your altitude, the higher your coverage (meaning less total satellites needed to cover the globe, if that's your intent), but it comes at the cost of increased lag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GPS satellites are a good example of high-altitude satellites that aren't geostationary. Their high altitude (20000 km) allows for a reduced number of satellites, but they're also low enough that signal lag is acceptable. If they were to orbit much lower, signal latency would be improved, but more satellites would be required, which would be more expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you get lower, photo resolution would improve, which is obviously going to be very important for spy satellites and other imaging satellites. One example is the KH-11 series of satellites, which orbited in 300x1000km orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you get even lower, different factors come into play. As FraserOfSmeg noted, the lower altitude orbits (i.e. a couple hundred kilometers) will decay faster. Sometimes that's good. If you want to launch a satellite that only requires a few weeks of testing, if you launch it into a low orbit you'll have time to test it, and then it will fall back into the atmosphere, reducing the problem of space debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another important consideration of orbital altitude is if you want to go somewhere multiple times, like Hubble or ISS. The higher the orbit, the harder it is to get there, and the less stuff (science, food, equipment, etc.) you can take with you. Again, at the lower orbits you also have to balance the cost of re-boosting the orbit when it decays too much. The ISS isn't set at 431km like you said in your question, the orbit is constantly decaying and getting reboosts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Geostationary altitude is the only one that I'm aware of that has a special name, but beyond that orbits are generally divided into Low Earth Orbit (LEO, generally anything below 1000km, but that definition varies depending on who you ask), Medium Earth Orbit (MEO, above LEO but below geostationary) and High Earth Orbit (HEO, above geostationary.)&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2014-12-26T01:37:27.203" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6592" PostTypeId="1" AcceptedAnswerId="6595" CreationDate="2014-12-26T03:44:36.123" Score="11" ViewCount="3246" Body="&lt;p&gt;In the &lt;a href=&quot;http://www.radiolab.org/story/91520-space/&quot; rel=&quot;noreferrer&quot;&gt;Radiolab podcast Space episode (2004)&lt;/a&gt; Ann Druyan mentioned that the contents of the Voyager Golden Record will last for 1 billion years before the contents of the Record becomes unreadable. This raises some questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How will the Golden Record outlast everything mankind has created? What are the special features of the Record to reduce its deterioration? How did the billion year lifespan come up?&lt;/li&gt;&#xA;&lt;li&gt;I have read that the Record is electropated with U-238 for alien civilizations to date the age of the Record. However U-238 is radioactive with a half-life of 4.4ish billion years. Does this radioactive decay of the U-238 reduce the potential lifespan on the Record?&lt;/li&gt;&#xA;&lt;li&gt;What actually causes the Record to decay into unreadability in the near vacuum and emptiness of space?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;(note: I have read the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/4743/for-how-long-will-the-voyager-records-remain-playable&quot;&gt;this question&lt;/a&gt; but it doesn't exactly address my question. I do take that it is related though :) )&lt;/p&gt;&#xA;" OwnerUserId="733" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-12-26T16:00:43.807" Title="How will the Voyager Golden Record last for 1 billion years?" Tags="|probe|voyager|astrobiology|record|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6593" PostTypeId="2" ParentId="6558" CreationDate="2014-12-26T03:50:09.947" Score="1" Body="&lt;p&gt;You'll need a &lt;strong&gt;really&lt;/strong&gt; big black hole to have any substantial effects.  I'm out of my depth on the math but plugging in numbers for an Earth-type planet orbiting at the Roche limit of a 10 billion solar mass star and getting garbage because I don't have the Einsteinian equations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting the sort of time ratios you are talking about would be impossible, though.  &lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2014-12-27T11:23:40.913" LastActivityDate="2014-12-27T11:23:40.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6594" PostTypeId="2" ParentId="6585" CreationDate="2014-12-26T04:51:51.093" Score="5" Body="&lt;p&gt;The ideal orbit of a space station should be high enough that atmospheric friction is low and reboosts don't need to happen too often, but also low enough that supply flights don't become too expensive. The ISS orbital height is a compromise between these two requirements. Its inclination is a compromise between the ideal inclinations for the two launch sites Baikonur and Kennedy Space Center. It was chosen to be slightly better for Baikonur. A higher inclination also allows the ISS to observe a larger part of the Earth surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another interesting orbit is the &lt;strong&gt;heliosynchrous orbit&lt;/strong&gt;. This is a highly inclined orbit which always passes the equator at the exact same local time. Or in other words: when it passes over the equator, the sun is at exactly the same angle. Such satellites are used for sun observation and for meteorological purposes. There are multiple such orbits with different inclinations and different heights (which also means a different number of orbits per earth-day).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you want the opposite of a geostationary satellite - one which moves over the ground as fast as possible - you would put it into a low &lt;strong&gt;retrograde orbit&lt;/strong&gt;. The difference between a retrograde and a usual (prograde) orbit is that the satellite orbits the earth in the opposite direction the earth is rotating. Instead of eastwards, the satellite orbits westwards. This kind of satellite is quite rare in practice because it requires more energy to launch (the rotational speed of the ground which usually assists rocket launches now works against it). Also, very few launch sites allow westward launches. The Kennedy Space Center, for example, isn't allowed to perform such launches for safety reasons: A launch failure would endanger the Orlando metropole region.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2014-12-26T05:08:04.743" LastActivityDate="2014-12-26T05:08:04.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6595" PostTypeId="2" ParentId="6592" CreationDate="2014-12-26T10:47:15.527" Score="12" Body="&lt;ol&gt;&#xA;&lt;li&gt;NASA made the record from stable, inert materials, and placed it in the vacuum of space where erosion is not an issue. The record sits underneath an &lt;a href=&quot;http://www.cedmagic.com/featured/voyager/voyager-record.html&quot;&gt;aluminium cover&lt;/a&gt;. The only sources of decay are cosmic radiation and the interplanetary/interstellar medium. Cosmic radiation takes a long, long time to create visible distortion (basically it affects an area one atom wide). The &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_medium&quot;&gt;interplanetary&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_medium&quot;&gt;interstellar&lt;/a&gt; medium consist mostly of monoatomic gases at very low density. Again, one atom at a time takes a long time to make a noticeable dent. &lt;/li&gt;&#xA;&lt;li&gt;The comment about U-238 surprised me as I'd never heard about it before, but &lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/goldenrec1.html&quot;&gt;it's correct&lt;/a&gt;. As in point 1, atomic decay products only affect one atom at a time. Parts of the record may transmute, but that doesn't really affect the profile of the grooves in the record. The amount of U-238 is tiny, and it decays very slowly, so it can only affect a small fraction of the atoms in the record. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I haven't been able to find a formal calculation for the record's lifespan. It seems likely the 'one billion' was an offhand remark. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2014-12-26T10:47:15.527" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6596" PostTypeId="2" ParentId="6029" CreationDate="2014-12-26T11:30:45.320" Score="0" Body="&lt;p&gt;Theoretically, no height is requires at all. Of course, the atmosphere makes that impractical. I would say most flights will be above the atmosphere most of the time, so probably about 30 mile minimum. 100 or more is likely as well.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2014-12-26T11:30:45.320" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6597" PostTypeId="5" CreationDate="2014-12-26T14:14:51.630" Score="0" Body="&lt;p&gt;Commercialized technologies and civilian goods that were initially developed for space exploration, aviation or military purposes, their R&amp;amp;D funded by military or governmental research and were later licensed for commercial use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This often involves releasing government held intellectual property into public domain and declassifying related technologies. E.g. NASA licenses spin-off technologies to commercial US industries through NASA Space Act agreement.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-18T01:01:18.880" LastActivityDate="2015-01-18T01:01:18.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6598" PostTypeId="4" CreationDate="2014-12-26T14:14:51.630" Score="0" Body="Commercialized technologies and civilian goods that were initially developed for space exploration, aviation or military purposes, their R&amp;D funded by military or governmental research and were later licensed for commercial use." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-18T01:01:18.880" LastActivityDate="2015-01-18T01:01:18.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6599" PostTypeId="2" ParentId="6590" CreationDate="2014-12-26T16:00:45.250" Score="3" Body="&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: &lt;a href=&quot;http://spaceflightnow.com/2015/01/11/photos-spacexs-rocket-landing-platform-back-in-port/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This article&lt;/a&gt; mentions a tug.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not certain. IANAS(ailor) but I believe it could technically do either. This &lt;a href=&quot;http://www.reddit.com/r/spacex/comments/2oakm0/45th_space_wing_confirms_the_offending_vessel/cmlanlc&quot; rel=&quot;nofollow noreferrer&quot;&gt;brief Reddit discussion&lt;/a&gt; backs that up a little, but it's never been officially answered to my knowledge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflightinsider.com/missions/commercial/spacex-crs-5-landing-barge-grid-fins/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This SFI article&lt;/a&gt; goes into some detail about the ASDS, including some information about the company supplying the dynamic positioning system: &lt;a href=&quot;http://www.thrustmaster.net/out-drive-propulsion-unit/portable-dynamic-positioning-system/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thrustmaster&lt;/a&gt;'s Portable Dynamic Positioning System. All of the press releases, Twitter mentions, and publicly-available info on the ASDS have discussed the fact that it can maintain position autonomously, but not how it &lt;em&gt;gets to that position&lt;/em&gt; exactly. Of the two problems, maintaining a position is by far harder than moving to one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An additional consideration is the &lt;a href=&quot;https://space.stackexchange.com/questions/6273/where-will-spacex-take-crs-5-after-it-lands&quot;&gt;question&lt;/a&gt; of what they are planning to do once the stage is secured. If they are keeping the stage on the barge and then moving everything back to shore, a tugboat may make more sense because there is additional weight involved. But again, my knowledge of boats &gt; my knowledge of spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Tl;dr&lt;/strong&gt; - I think it will probably be towed by a tugboat because that's usually how barges move, but I'm not certain.&lt;/p&gt;&#xA;" OwnerUserId="6772" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-01-11T20:44:24.143" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6600" PostTypeId="1" CreationDate="2014-12-27T00:18:03.607" Score="5" ViewCount="319" Body="&lt;p&gt;For example, how were the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Stardust&lt;/a&gt; particle collector samples, that were suspended in brittle and extremely low density (0.014 g/cm&lt;sup&gt;3&lt;/sup&gt;) silica &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerogel&quot; rel=&quot;noreferrer&quot;&gt;aerogel&lt;/a&gt;, secured prior to atmospheric reentry so they withstand g-loads and vibration during deceleration and touchdown, and to prevent sample contamination, both external as well as by the enclosure hatch itself?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/7tJLt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The Stardust dust collector with silica aerogel blocks (image credit: &lt;a href=&quot;http://stardust.jpl.nasa.gov/photo/spacecraft.html&quot; rel=&quot;noreferrer&quot;&gt;NASA / JPL&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quoting from &lt;a href=&quot;http://stardust.jpl.nasa.gov/tech/aerogel.html&quot; rel=&quot;noreferrer&quot;&gt;Aerogel: Catching Comet Dust&lt;/a&gt; article (&lt;a href=&quot;http://stardust.jpl.nasa.gov/tech/aerogel.html&quot; rel=&quot;noreferrer&quot;&gt;NASA / JPL Stardust&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To collect particles without damaging them, Stardust uses an&#xA;  extraordinary substance called aerogel. This is a silicon-based solid&#xA;  with a porous, sponge-like structure in which 99.8 percent of the&#xA;  volume is empty space. By comparison, aerogel is 1,000 times less&#xA;  dense than glass, which is another silicon-based solid. When a&#xA;  particle hits the aerogel, it buries itself in the material, creating&#xA;  a carrot-shaped track up to 200 times its own length. This slows it&#xA;  down and brings the sample to a relatively gradual stop. Since aerogel&#xA;  is mostly transparent - with a distinctive smoky blue cast -&#xA;  scientists will use these tracks to find the tiny particles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Additionally, were in Stardurst particulate samples observed any undesired contaminants, or perhaps deformation of their &quot;carrot-shaped&quot; impact tracks embedded within the aerogel that could be attributed to reentry, touchdown or even later retrieval of the sample return capsule? Perhaps during the analysis by the citizen science project &lt;a href=&quot;http://www.citizensciencecenter.com/stardusthome/&quot; rel=&quot;noreferrer&quot;&gt;Stardust@Home&lt;/a&gt;, during which participants literally forensically examined each individual particle? Were some common anomalies detected in tracks they made through the silica aerogel? Or was their mass too insignificant to be disturbed by g-loads of what I'd expect to be in the range of 3-4 g at maximum (is this correct?), despite the extremely low density medium they were embedded into?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-12-27T00:52:41.323" LastActivityDate="2014-12-28T01:56:03.780" Title="How are ultra-sensitive aerogel suspended samples secured for Earth reentry and touchdown?" Tags="|reentry|sample-return|contamination|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6601" PostTypeId="2" ParentId="6029" CreationDate="2014-12-27T02:39:49.557" Score="10" Body="&lt;p&gt;The altitudes are not what I would call ridiculous.  Though you seem prone to using unjustified superlatives in your questions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The optimum, i.e. minimum injection velocity sub-orbit &lt;a href=&quot;https://space.stackexchange.com/a/4190/265&quot;&gt;was provided by HopDavid in this answer&lt;/a&gt;.  With some manipulation, you get that the maximum altitude of the optimal trajectory is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${r\over 2}\left(\sin{\alpha\over 2}+\cos{\alpha\over 2}-1\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $r$ is the radius of the injection and $\alpha$ is the central angle of the traverse.  The optimal sub-orbits look like this:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pLb9k.gif&quot; alt=&quot;animation of sub-orbits&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The green lines show the central angle traversed, and the solid orange curve is the trajectory.  The dotted orange curve completes the rest of the sub-orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The maximum altitude is at a traverse of $\pi/2$, or a fourth of the way around the planet, about the distance between Los Angeles and Moscow (since we're talking about ICBMs).  That maximum altitude is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${r\over 2}\left(\sqrt{2}-1\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or about $0.207\,r$.  I would consider a reasonable injection altitude to be 50 km, so this gives a maximum ICBM altitude of 1330 km.  That would nick the bottom of the inner van Allen belt, which starts at around $0.2\,r$.  For most traverse angles you will be lower and should miss the belt.  (Though watch out for the South Atlantic Anomaly if you're going from, say, Los Angeles to Port Elizabeth, South Africa.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you accept a penalty in the injection velocity, you can lower the peak altitude.  For example, to reduce the peak altitude for a $\pi/2$ traverse by a factor of two to 665 km, you would need to increase the injection velocity from 7.17 km/s to 7.29 km/s.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2014-12-28T16:27:25.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6602" PostTypeId="1" CreationDate="2014-12-27T05:27:22.817" Score="6" ViewCount="659" Body="&lt;p&gt;Pertaining to Mars habitation, eventually, could the answer be in burrowing into the ancient volcanoes instead of building the habitation outside in the open? Since Mars does not have magnetic shielding like the Earth, could this help in mitigating the harmful radiation from the Sun?&lt;/p&gt;&#xA;" OwnerUserId="7054" LastEditorUserId="49" LastEditDate="2014-12-27T13:20:55.093" LastActivityDate="2017-03-21T22:57:04.397" Title="On building future sustainable Mars habitat" Tags="|mars|colonization|construction|habitat|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6603" PostTypeId="2" ParentId="6602" CreationDate="2014-12-27T12:17:46.003" Score="2" Body="&lt;p&gt;I'm no Martian geologist but I would assume the rock composition in the volcanoes is different to the average for the whole of Mars - I wonder if this is preferable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Certainty using solid rock for radiation insulation &lt;a href=&quot;http://cosmology.com/Mars130.html&quot; rel=&quot;nofollow&quot;&gt;has been suggested before&lt;/a&gt; (that link is even about lava caves!). It could be helpful in mitigating the harmful radiation, any material (solid, liquid, or gas) is helpful for this purpose &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_protection#Shielding&quot; rel=&quot;nofollow&quot;&gt;if you have enough it it&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="49" LastEditDate="2014-12-27T13:13:45.817" LastActivityDate="2014-12-27T13:13:45.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6604" PostTypeId="2" ParentId="6602" CreationDate="2014-12-27T12:33:36.110" Score="11" Body="&lt;p&gt;Yes, eventually, but we would first have to establish structural stability of any such rocks, get a sense of Martian subsurface seismic activity, seasonal changes to its thermal characteristics, how fast would aeolian processes block access to it, amount of volatiles embedded within the rock that could collapse it during drilling or with higher temperatures of the habitat, get the heavy equipment needed to do that there or make it on site once we have the other needed facilities to do so and learn to use Martian resources to support such industry, and we might not need to resort to such drastic measures either, if we learn to use the Martian regolith as &lt;em&gt;bioshielding&lt;/em&gt; mass instead. Let's expand a bit on all these points;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, ancient volcanoes might not be the best choice as they tend to be rather porous and unstable due to the many pathways that the flowing lava would take through them, and the pressure building within it prior to eruptions also tends to form deep cracks. With Martian nearly three times weaker gravity than on Earth, the sheer weight of such formations isn't compacting the structure as fast as we'd expect here and they tent to be taller due to it. Much taller. Not as much of weathering and no tectonic activity on Mars slows this settling of rock even further. The residual heat after volcanoes being active could also allow volatiles to flow deep into these cracks and fill them. As they eventually freeze, they might also expand (water) or contract (dry ice) and crack or loosen the rock further. A better choice might be calderas that already collapsed and were later filled with once flowing and later solidified lava. If this process happened in Martian ancient times when it still had some planetary magnetic field, such solidified lava beds would also remain slightly magnetic, forming what we refer to as &lt;em&gt;remnant magnetic field&lt;/em&gt;. Mars has spots on its surface where this magnetic field is present, but sadly, at perhaps up to a couple microtesla at its best, it is rather weak (about 30 times weaker than the Earth's magnetic field at its surface) and fairly localized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second is that Martian surface is active and the planet has seasons just like the Earth has, only even longer and with bigger surface temperature differences. Perhaps the most obvious surface processes are winds that can cause global dust storms, and they cause various aeolian formations, both wind erosion as well as sedimentation. This could mean that entrances to caves could get blocked by sand. But perhaps still less well known are the seasonal fans that come to life during Martian springs, formed by dry ice (frozen carbon dioxide) embedded into regolith during winters and later warmed during springs. This can cause violent (read: explosive) release of volatiles beneath the regolith and forms dry ice fans that were observed on Mars. Granted, we could select areas where this isn't an issue, but what I wanted to demonstrate is what the large temperature difference between seasons can cause. Obviously, rocks would also be a subject to these seasonal temperature swings. Depending on their thermal characteristics, it could cause additional loosening of its materials closer to the surface. And if they embed much of volatile materials, as discussed earlier that they could, such environment can quite literally be explosive. Especially if we try to build a habitat on top or within it, that could act as a pressure cooker's lid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third is of course that we'd require quite a bit of fairly heavy equipment to dig any such burrows. We don't have that (the heaviest piece of equipment we so far landed there was about two metric tons heavy MSL), and nor we'll have it any time soon on Mars. But if you're ready to wait, then I guess we could eventually establish Martian industry and learn to use its many resources to build the heavy machinery needed. But the point I wanted to make is that by the time we'll be ready to burrow into Martian mountains, we'll have to have ways of protecting our habitats from dangerous radiation in different, more accessible ways. And luckily, Mars has a lot of sand, that could at first be used just as it is to provide the shielding material, and perhaps later be mixed with liquids to improve its strength and could be shaped for easier construction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for subsurface seismic activity on Mars, we'll only get a fair overview once the NASA's &lt;a href=&quot;http://www.nasa.gov/mission_pages/insight/main/index.html&quot;&gt;InSigth&lt;/a&gt; (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) lander gets to the Martian surface, presumably in late 2016. So, until we have this data, it might be more prudent to search for more structurally stable natural caves, perhaps close subsurface lava tubes that can be many meters wide and, where flowing lava already started cooling down, can form thick and strong silicate walls surrounding them. some might also have natural exits into the open, and if they formed on a slightly inclined slope (they often are), wouldn't get blocked by dust as easily. Mind, we're still talking of untraversed, largely unweathered terrain here, and we'd be disturbing its millions of years undisturbed natural cycles (diurnal, seasonal,...) with human activity, including, quite possibly, heavy machinery and explosives. Each of such lava tubes would have to be first inspected on site, and might need structural reinforcing. Make sure you bring along many structural engineers, geologists and seismologists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, TL;DR - We might one day learn to dig deep burrows into Martian slopes of this and that variety, but we'll have to learn to do without that anyway, if humans are to colonize Mars at all.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2014-12-27T13:54:39.990" LastActivityDate="2014-12-27T13:54:39.990" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6605" PostTypeId="2" ParentId="6323" CreationDate="2014-12-27T14:02:15.100" Score="5" Body="&lt;p&gt;In describing an instance of an autogenous pressurization system, Appendix E of &lt;strong&gt;A Review of United States Air Force and Department of Defense Aerospace Propulsion Needs&lt;/strong&gt; (downloaded from: &lt;a href=&quot;http://www.nap.edu/catalog/11780.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nap.edu/catalog/11780.html&lt;/a&gt;) states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Vapor Pressurization] utilizes the internal energy of a liquid stored in a closed container to perform the work required to expel the liquid from the container. Before starting, the bulk liquid temperature is adjusted so that the vapor pressure equals the desired tank pressure. The liquid is in thermal equilibrium with the saturated vapors present in the tank ullage (other gases excluded). When the tank valve is opened, draining the liquid or vapor, the tank pressure drops, upsetting the vapor-liquid equilibrium. At this point, the liquid boils, creating additional vapor and tending to counteract the pressure reduction.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This would mean that the gaseous oygen would not be appreciably warmer that the LOX beneath it in the tank.&lt;/p&gt;&#xA;" OwnerUserId="6214" LastEditorUserId="127" LastEditDate="2017-08-27T02:35:44.327" LastActivityDate="2017-08-27T02:35:44.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6606" PostTypeId="1" CreationDate="2014-12-27T15:48:04.860" Score="9" ViewCount="666" Body="&lt;p&gt;Drilling seems to require heavy equipment, preferably with humans on site for maintenance, much energy and might be even more problematic in low gravity and unknown geology. &lt;a href=&quot;https://space.stackexchange.com/questions/4446/the-maximum-depth-apollo-astronauts-dug-into-lunar-regolith&quot;&gt;Deepest hole&lt;/a&gt; drilled in space was 292 cm by Apollo 17, manually operated on site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a heavy drill be replaced by a rover dropping off a much lighter shaped charge, taking cover when it explodes, and returning to examine the debris from the deep which landed on the surface? Couldn't a modern shaped charge even cause a deep narrowish hole, rather than a flat crater, which could be inspected by &lt;a href=&quot;https://en.wikipedia.org/wiki/Chemistry_and_Camera_complex&quot; rel=&quot;noreferrer&quot;&gt;&quot;a ChemCam&quot;&lt;/a&gt; and even by physically lowering down small instruments and tools? And then even by lowering down another explosive, much like construction and mining drilling on Earth often is done only in order to place explosives more favourably.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By blasting out a bay shaped cavity in a Martian or Lunar hill, radiation shielding and maybe thermal benefits might be provided for a habitat module later placed there. Explosives might make subsurface water ice much more available to a human space colony. With the advantage of using only a few tons of explosives which require almost no operational overhead, other than being placed by a rover. Even if a detonation gives an unwanted result, the investment lost was low compared to a heavy drilling station, and could be replaced by a new attempt on another site nearby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the main problems which I optimistically leave out here?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Actual examples of explosives in space:&lt;/strong&gt;&#xA;&lt;a href=&quot;http://global.jaxa.jp/projects/sat/hayabusa2/&quot; rel=&quot;noreferrer&quot;&gt;Hayabusa2&lt;/a&gt; will use a shaped explosive charge to create a crater from where it will land and take a sample (&lt;a href=&quot;http://jda.jaxa.jp/result_strm.php?lang=e&amp;amp;id=2e032c23f72832dba634998602294f69&quot; rel=&quot;noreferrer&quot;&gt;animation&lt;/a&gt; and &lt;a href=&quot;http://www.researchgate.net/publication/256935344_Small_carry-on_impactor_of_Hayabusa2_mission&quot; rel=&quot;noreferrer&quot;&gt;an abstract&lt;/a&gt; of a paywalled article). That is more of a replacement for an impactor like &lt;a href=&quot;http://www.nasa.gov/mission_pages/deepimpact/mission/#.VJ7JUUBA&quot; rel=&quot;noreferrer&quot;&gt;Deep Impact&lt;/a&gt;, rather than for a drill. Except for explosive bolts, the only other use of (non-military) explosives in space I know of is &lt;a href=&quot;http://www.lpi.usra.edu/lunar/missions/apollo/apollo_17/experiments/lspe/&quot; rel=&quot;noreferrer&quot;&gt;Apollo 14, 16, 17&lt;/a&gt; and that was for seismic purposes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;&lt;em&gt;Is there a way to roughly compare the launch mass and useful energy delivered of explosive devices compared to mining devices on the Moon, Mars, Asteroids given some performance criteria such as depth reachable or mass of rocks cleared? I imagine that explosives are pretty energy intensive payloads compared to solar powered machines.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2014-12-27T19:36:42.640" LastActivityDate="2014-12-28T03:46:47.747" Title="Could explosives replace drilling for science and construction in space?" Tags="|construction|explosives|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6607" PostTypeId="1" AcceptedAnswerId="7923" CreationDate="2014-12-27T16:05:35.423" Score="18" ViewCount="4856" Body="&lt;p&gt;The hypothetical swimming pool is inside a habitat at atmospheric pressure, and on Mars at 0.38g of Earth. What are the major differences regarding water properties (viscosity, surface tension, cohesion, does it feel like gel...) and how would the swimming astronaut compare the experience to Earth.&lt;/p&gt;&#xA;" OwnerUserId="7057" LastEditorUserId="12102" LastEditDate="2016-09-28T04:20:49.773" LastActivityDate="2016-09-28T05:55:45.507" Title="How would swimming on Mars feel, given the lower gravity?" Tags="|mars|water|physical-activity|reduced-gravity-sports|" AnswerCount="4" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="6608" PostTypeId="2" ParentId="6606" CreationDate="2014-12-27T16:38:12.943" Score="3" Body="&lt;p&gt;For scientific purposes, like exposing near-surface materials, probing with seismic explosives, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Prospecting&quot; rel=&quot;nofollow&quot;&gt;prospecting&lt;/a&gt; for pockets or veins of subsurface materials, I guess it could work, and you already give a few good examples in your question. But for industrial purposes, on its own and without rolling in heavy machinery once you, say, reduce some heavy boulder into rubble, not very likely. Precision required is too great and there's too many uncertainties involved, some, you'd only make worse with the use of explosives;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Explosives are otherwise used all the time in both subsurface and surface blast-mining on Earth. Quarries would also be one such example. But laying and precise detonating charges in sequence so they also clear the resulting craters off ruble would be incredibly difficult and it wouldn't result in perfectly clear tunnels to roll into them some form of autonomous or remotely operated vehicle to place new charges either. No rock is exactly homogenous and fault-free, and with each explosion you're deepening these cracks that can give-in at any time. So you'd be laying support structures (roofing and walls support) that requires heavy machinery, after you managed to clear the path for them, usually using smaller, but still heavy loaders and trucks or carts (which would also require rails, again, laid by heavy machinery). The deeper you go, the tougher this gets and you're constantly laying your charges along its way, increasing stress with each detonation. But, I guess, you could start like this, then roll in heavy machinery as you need to deepen the hole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in a nutshell, not really. At least not for construction work or heavy mining operations. But you could manage to crack open tougher regolith to expose subsurface pockets of water ice, use smaller amounts of it as seismology charges, explosive bolts, and so on.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-12-27T16:38:12.943" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6609" PostTypeId="2" ParentId="6359" CreationDate="2014-12-27T18:03:20.733" Score="5" Body="&lt;p&gt;According to this recent &lt;a href=&quot;http://www.scientificamerican.com/article/a-new-way-to-reach-mars-safely-anytime-and-on-the-cheap/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scientific American article&lt;/a&gt;, there is a cheaper way to get to Mars than the traditional &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann transfer&lt;/a&gt;. The mission design relies on a ballistic capture to do away with the Mars orbital insertion burn.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The premise of a ballistic capture: Instead of shooting for the location Mars will be in its orbit where the spacecraft will meet it, as is conventionally done with Hohmann transfers, a spacecraft is casually lobbed into a Mars-like orbit so that it flies ahead of the planet. Although launch and cruise costs remain the same, the big burn to slow down and hit the Martian bull's-eye—as in the Hohmann scenario—is done away with. For ballistic capture, the spacecraft cruises a bit slower than Mars itself as the planet runs its orbital lap around the sun. Mars eventually creeps up on the spacecraft, gravitationally snagging it into a planetary orbit.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UefM6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;em&gt;Structure of the ballistic capture transfers to Mars.&lt;/em&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As this adds at least several months to total travel time, probably not the best bet for manned missions, but for pre-positioning your 100 metric tons of water &amp;amp; construction equipment in Mars orbit, it should do nicely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a couple of caveats about the Scientific American article. The author says that &quot;ballistic capture&quot; is synonymous with &quot;low energy transfer,&quot; whereas AFAIK the former is a merely subset of the later. Also, he seems to imply that this approach does away altogether with the several-days-every-two-years launch window, but the paper actually only states that the launch window is more flexible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The underlying paper is Edward Belbruno &amp;amp; Francesco Topputo, &lt;a href=&quot;http://arxiv.org/pdf/1410.8856.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth–Mars Transfers with Ballistic Capture&lt;/a&gt; (2014)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further reading:&lt;br&gt;&#xA;Walter Hohmann, &lt;a href=&quot;http://archive.org/stream/nasa_techdoc_19980230631/19980230631#page/n0/mode/2up&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Attainability of Heavenly Bodies&lt;/a&gt; (Washington: NASA Technical Translation F-44, 1960)&#xA;Koon, Lo, Marsden &amp;amp; Ross, &lt;a href=&quot;http://www.cds.caltech.edu/~koon/book/KoLoMaRo_DMissionBk.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dynamical Systems, the Three-Body Problem and Space Mission Design&lt;/a&gt; (2006)&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-27T19:19:07.783" LastActivityDate="2014-12-27T19:19:07.783" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6610" PostTypeId="2" ParentId="6606" CreationDate="2014-12-27T21:06:20.380" Score="4" Body="&lt;p&gt;There's one quite peculiar application where explosives will not require much &lt;strong&gt;post-processing&lt;/strong&gt; - using a nuclear charge to create a cavern under ground. While it may be considered illegal in terms of &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;nofollow&quot;&gt;international treaties&lt;/a&gt;, it is a viable construction technique. Melt cavity radius would be roughly proportional to the cube root of the device's yield :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$r_{\textrm{(in meters)}} \approx 16.76 Y_{\textrm{(in kilotons)}}^{\frac{1}{3}} $ for &lt;a href=&quot;http://www.princeton.edu/~ota/disk1/1989/8909/8909.PDF&quot; rel=&quot;nofollow&quot;&gt;some of Earth's rock types&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, a fully contained cavity &lt;strong&gt;at the current technology level requires drilling and concrete-filling&lt;/strong&gt; as pre-processing steps. It might be possible to devise a ballistic/rocket-assisted penetrator contraption to dig into the ground from a hyperbolic orbit, but there's always the chance (4 per cent for the Earth, higher for less-studied planetary bodies) that the hole may be unsuitable, and another risk is penetration failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To sum it up: at the current level of technology nuclear explosions will likely require drilling. Future advances may replace drilling with ballistic penetrators.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;For the record, I am not insensitive to the environmental and biological risks posed by nuclear explosions. An unsuccessful launch abort may set up the stage for another &lt;a href=&quot;https://en.wikipedia.org/wiki/Broken_Arrow_%28nuclear%29&quot; rel=&quot;nofollow&quot;&gt;Broken Arrow&lt;/a&gt;. The explosion may poison aquifers (if they exist) or vent contaminants into the atmosphere, or kill local life. The real reason I added this answer is we have to always keep in mind the whole gamut of techniques the humanity possesses. Nukes pack enormously large energy in low volume and mass.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2014-12-27T23:32:47.540" LastActivityDate="2014-12-27T23:32:47.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6611" PostTypeId="2" ParentId="6582" CreationDate="2014-12-28T01:46:10.790" Score="3" Body="&lt;p&gt;As stated in Mark Adler's excellent answer, the maximum DeltaV possible occurs at the following condition:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{\infty \text{ for maximum } \Delta \text{V}} = \sqrt{\mu/r}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tabulated values for this quantity are difficult to find. But the escape velocity at distance $r$ is given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\text{Escape Velocity} = \sqrt{2\mu/r}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tablulated values for escape velocities at the surface (although not directly relevant to slingshot) are much easier to find, and all we have to do to convert them is to divide by $\sqrt{2}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/&quot; rel=&quot;nofollow&quot;&gt;http://nssdc.gsfc.nasa.gov/planetary/factsheet/&lt;/a&gt; gives escape velocities for all the planets of the solar system, plus the moon. It also gives their orbital velocities (around the sun for the planets and around the earth for the moon.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the terrestrial planets, the orbital velocity about the sun is several times greater than the planet's escape velocity, and it is possible to conceive a situation where $V_{\infty}$ is equal (or greater than)  $\sqrt{\mu/r}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, for the giant planets (Jupiter, Saturn, Uranus, Neptune) the orbital velocity about the sun is several times less than the planet's escape velocity. It is difficult to conceive a trajectory where a spacecraft from Earth would approach one of these planets with a relative velocity much greater than the orbital velocity of the planet[1]. In practice this may make it difficult for $V_{\infty}$ to get near the limit of $\sqrt{\mu/r}$[1], so it may be difficult to take advantage of all the $\Delta V$ available from the planet's gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However we can get plenty of change of direction from the planet at lower $V_{\infty}$ (potentially up to nearly 180 degrees for the lowest $V_{\infty}$ values.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1] EDIT: to qualify further, add &quot;at a convenient angle.&quot; See comments (obviously this depends on the exact mission, all missions are different.)&lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="6117" LastEditDate="2014-12-29T01:32:26.593" LastActivityDate="2014-12-29T01:32:26.593" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6612" PostTypeId="2" ParentId="6600" CreationDate="2014-12-28T01:50:14.140" Score="4" Body="&lt;p&gt;Short answer: The aerogel itself is sufficient to restrain the samples.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The captured particles are tiny and therefore have a very high surface area to volume ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that much like dust or cloud droplets in an atmosphere, they have a very low &quot;terminal velocity&quot; in the aerogel under the relatively modest accelerations of reentry (modest compared to the millions of gs that they experienced when they collided with the aerogel to begin with).  This terminal velocity is sufficiently small that there is essentially zero movement of the particles with respect to the aerogel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The aerogel itself does need to be sufficiently cushioned to prevent it breaking under the forces of reentry and landing.  That part didn't work out so well for Genesis.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2014-12-28T01:56:03.780" LastActivityDate="2014-12-28T01:56:03.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6613" PostTypeId="2" ParentId="6606" CreationDate="2014-12-28T03:46:47.747" Score="3" Body="&lt;p&gt;It depends on what you are trying to achieve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the condition of what is being drilled, drilling a hole will produce a neat hole of uniform diameter. Explosives however, always produce a significant zone of fractured material around where the explosive was detonated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to take a sample of anything, drilling is the best option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to produce holes for foundations for stilts, drilling is still the best option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to maximize a crater it is still better to drill a hole, or holes, to the required depth, fill the holes with explosives, as required and then blast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By burying an explosive in a drilled hole all the energy of the blast is direct to fracturing the rock and moving the rock. By detonating an explosive on the surface a significant portion of the energy of the explosive will be lost, even in a vacuum, which is inefficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth we tend to be wasteful because it is easy to obtain resources. In Space however, resources are scarce and their efficiency needs to be maximized.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2014-12-28T03:46:47.747" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6614" PostTypeId="2" ParentId="6587" CreationDate="2014-12-28T04:09:46.030" Score="3" Body="&lt;p&gt;It would vary for each type of rocket. The more powerful a rocket the greater the safe stand-off distance. A rocket like the Saturn V would require a larger stand-off distance than a Proton rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would also depend on what exhaust directing structures exist below the rocket at launch and where the person wanted to be in relation to that. A stand-off distance perpendicular to the exhaust vents would be less than a stand-off distance in the path of the exhaust from the vents.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2014-12-28T04:09:46.030" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6615" PostTypeId="2" ParentId="6516" CreationDate="2014-12-28T04:45:16.413" Score="5" Body="&lt;p&gt;In the book '&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0312308663&quot; rel=&quot;nofollow&quot;&gt;Two Sides of the Moon&lt;/a&gt;', co-written by David Scott (Apollo 15) &amp;amp; Alexei Leonov (first to walk in space), Leonov discusses the Bondarenko accident. but does not mention anything about lost cosmonauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the Apollo 1 accident is mentioned, Leonov expressed his surprise that the accident happened given that the Bondarenko accident occurred a number of years previous to the Apollo 1 launch pad accident. Leonov wrote that he thought the Americans would have known better because those involved with the Soviet space program thought the Americans had spies within the Soviet program and they would have told the Americans about Bondarenko and why a 100% oxygen atmosphere was a bad idea.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="1142" LastEditDate="2015-01-07T03:35:59.873" LastActivityDate="2015-01-07T03:35:59.873" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6616" PostTypeId="2" ParentId="6323" CreationDate="2014-12-28T09:57:58.810" Score="9" Body="&lt;p&gt;If the question is whether such a system is feasible, the answer is yes, it is 100% feasible, because STS used this exact system.  The LOX and LH2 tanks in the ET were pressurized on the pad by GSE-supplied helium. After SSME start the tanks were pressurized by gaseous propellants tapped from the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/prop/et.html&quot; rel=&quot;noreferrer&quot;&gt;1985 NASA shuttle press reference&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once the liquid oxygen is loaded and ready for main engine ignition,&#xA;  the liquid oxygen tank's vent and relief valve is closed, and the tank&#xA;  is pressurized to 21 psig by GSE-supplied helium. During SSME&#xA;  thrusting, liquid oxygen flows out of the external tank through the&#xA;  orbiter/external tank umbilical into the orbiter MPS and to each SSME.&#xA;  &lt;strong&gt;Pressurization in the tank is maintained by gaseous oxygen tapped from&#xA;  the three main engines and supplied to the liquid oxygen tank through&#xA;  the orbiter/external tank gaseous oxygen umbilical.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When the liquid hydrogen is loaded and ready for main engine ignition,&#xA;  the liquid hydrogen tank's vent and relief valve is closed, and the&#xA;  tank is pressurized to 42.5 psia by GSE-supplied helium.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Approximately 45 minutes after loading starts, three electrically&#xA;  powered liquid hydrogen pumps in the orbiter begin to circulate the&#xA;  liquid hydrogen in the external tank through the three SSMEs and back&#xA;  to the external tank through a special recirculation umbilical. This&#xA;  recirculation chills down the liquid hydrogen lines between the&#xA;  external tank and the high-pressure fuel turbopump in the SSMEs so&#xA;  that the path is free of any gaseous hydrogen bubbles and is at the&#xA;  proper temperature for engine start. Recirculation ends approximately&#xA;  six seconds before engine start. During engine thrusting, liquid&#xA;  hydrogen flows from the external tank and through the orbiter/external&#xA;  tank liquid hydrogen umbilical into the orbiter MPS and to the main&#xA;  engines. &lt;strong&gt;Tank pressurization is maintained by gaseous hydrogen tapped&#xA;  from the three SSMEs and supplied to the liquid hydrogen tank through&#xA;  the orbiter/external tank gaseous hydrogen umbilical.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2014-12-30T23:54:03.673" LastActivityDate="2014-12-30T23:54:03.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6617" PostTypeId="2" ParentId="6348" CreationDate="2014-12-28T14:10:28.107" Score="2" Body="&lt;p&gt;The other thing to consider is in the zero gravity environment of outer space you don't want a floating or a fast flying item to accidentally hit a touch panel and inadvertently activate any of the craft's controls.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2014-12-28T14:10:28.107" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6620" PostTypeId="2" ParentId="5771" CreationDate="2014-12-29T05:09:59.360" Score="14" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Does the entire crew fit into the Soyuz life boat they have today?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Each crew member comes with a Soyuz which remains docked and can be used as a shelter or to return to Earth (actually the oldest docked Soyuz returns first, the hydrogen peroxide of the reaction control system degrades with time).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;For example: &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-11M&quot; rel=&quot;noreferrer&quot;&gt;TMA-11M&lt;/a&gt; (carrying the Olympic flame...) docked to the ISS on Nov. 7, 2013 with 3 additional crew members, making the total aboard to 9 persons. At this time 3 Soyuz were docked: &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-09M&quot; rel=&quot;noreferrer&quot;&gt;TMA-09M&lt;/a&gt; (initially on Rassvet but relocated on Zvesda to make room for TMA-11M), &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TMA-10M&quot; rel=&quot;noreferrer&quot;&gt;TMA-10M&lt;/a&gt; (on Poisk) and TMA-11M (on Rassvet).&lt;br&gt;&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/5JZ8u.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;(source: &lt;a href=&quot;http://www.microsiervos.com/archivo/ciencia/recolocada-soyuz-estacion-espacial-internacional-por-antorcha-olimpica.html&quot; rel=&quot;noreferrer&quot;&gt;microsiervos&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.youtube.com/watch?v=LTflOstC1Ec&quot; rel=&quot;noreferrer&quot;&gt;relocation of TMA-09M&lt;/a&gt; was done with its 3 crew members &lt;strong&gt;onboard&lt;/strong&gt;.&#xA;The same crew members (and the flame) returned to Earth with TMA-09M on Nov 11.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Do they have to get dressed and prepare in other ways?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Most of the equipment of the ISS can be operated remotely from the ground, so there is not much to prepare before evacuating. The astronauts and cosmonauts &lt;a href=&quot;https://www.youtube.com/watch?v=-l7MM9yoxII&quot; rel=&quot;noreferrer&quot;&gt;put their suits on while in the Soyuz&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soyuz vehicle can be used even if the communications with Earth are lost. The crew of three inside it &lt;a href=&quot;http://www.spaceanswers.com/space-exploration/what-would-happen-if-the-iss-permanently-lost-all-contact-with-earth/&quot; rel=&quot;noreferrer&quot;&gt;can manually return to Earth&lt;/a&gt; by themselves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Soyuz can be separated from the ISS within three minutes. Typical time from decision to leave until landing in Kazakhstan is just under 3.5 hours (&lt;a href=&quot;http://www.quora.com/Does-the-International-Space-Station-ISS-have-an-emergency-escape-pod&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="7017" LastActivityDate="2014-12-29T05:09:59.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6621" PostTypeId="2" ParentId="2570" CreationDate="2014-12-29T05:34:34.213" Score="3" Body="&lt;p&gt;During daylight, it may be possible to adapt a technique used on Earth for locating North that requires an analogue watch/clock and for the sun to be visible. Of course, for Mars, the clock would need to be made for Martian Sols, not Earth days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The technique for locating North on Earth is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;In the Southern Hemisphere&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hold the watch horizontal with the '12' pointing directly towards the sun.&#xA;Look at the hour hand, half the angle between '12' and the hour hand points towards north.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;In the Northern Hemisphere&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hold the watch horizontal with the hour hand pointing directly towards the sun.&lt;br&gt;&#xA;Half the angle between '12' and the hour hand points towards south.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Don't forget to make allowances for daylight savings.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2014-12-29T05:34:34.213" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6622" PostTypeId="2" ParentId="6585" CreationDate="2014-12-29T11:36:37.450" Score="4" Body="&lt;p&gt;First, lets talk about simple (near to) circular orbits. Usually, you can divide them into Low, Medium and High.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All geocentric orbits with an altitude up to 2,000km (1,240mi) are classified as a &lt;strong&gt;Low Earth Orbit&lt;/strong&gt; (&lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;nofollow&quot;&gt;LEO&lt;/a&gt;). Any object that is supposed to stay in space for a longer period of time (such as satellites or the ISS) usually has an altitude of &gt;300km, since orbits below that altitude would be impractical due to atmospheric drag in the Thermosphere, causing orbital decay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although the altitude of the ISS is higher than that, it still &lt;a href=&quot;http://www.heavens-above.com/IssHeight.aspx&quot; rel=&quot;nofollow&quot;&gt;loses ~50-100m a day and has to do orbital boost burns every so often.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEOs are &lt;em&gt;better&lt;/em&gt; than higher-altitude orbits because they require less Delta-V to reach, so it is cheaper to build space stations there. They are &lt;em&gt;worse&lt;/em&gt; because you can see objects in LEO only from a small portion of the Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Geocentric) orbits above 2,000km to just below geosynchronous orbits (35,786km) are classified as &lt;strong&gt;Medium Earth Orbits&lt;/strong&gt; (MEO), &lt;a href=&quot;http://gcmd.nasa.gov/add/ancillaryguide/platforms/orbit.html&quot; rel=&quot;nofollow&quot;&gt;&quot;most commonly at 20,200 km or 20,650, with an orbital period of 12 hours&quot;&lt;/a&gt;, as used by the GPS satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MEOs are &lt;em&gt;good&lt;/em&gt; because with an orbital period of 12 hours, you can easily calculate when the satellite is going to pass over you and you still have a reasonable viewing angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Geocentric) orbits above 35,786km are classified as &lt;strong&gt;High Earth Orbits&lt;/strong&gt; (&lt;em&gt;not HEO!&lt;/em&gt;). Weather and communications satellites &lt;a href=&quot;http://earthobservatory.nasa.gov/Features/OrbitsCatalog/&quot; rel=&quot;nofollow&quot;&gt;tend to be in High Earth Orbits&lt;/a&gt; because they can &quot;see&quot; a large portion of the earths surface from this altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some examples for interesting elliptic orbits are the Tundra and the Molniya orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Geosynchronous orbits obviously only work when placed above the equator. If you want to have a satellite stationary over very high latitudes, they don't work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So instead you use one of the two orbits. They are highly elliptical and have an inclination of ~63.4°. The high eccentricity means that a satellite has a long apogee dwell, so it remains around a certain point on the surface for most of its orbit, then quickly swoops around the earth and comes into vision again after a short time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Tundra_orbit&quot; rel=&quot;nofollow&quot;&gt;Tundra orbit&lt;/a&gt; has a perigee of 1,000km and an apogee of 70,580, giving it an orbital period of 24 hours. The only current user of Tundra orbits is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sirius_Satellite_Radio&quot; rel=&quot;nofollow&quot;&gt;Sirius Satellite Radio&lt;/a&gt;. It uses a constellation of three satellites, with which it can provide all of North America with its stream for 24 hours a day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Molniya orbit has a perigee of 1,000km and an apogee of ~40,000, giving it an orbital period of 12 hours. They were primarily used by the Molniya communications satellites and some spy satellites during the Cold War.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another point to consider is that orbits intended for manned spacecraft shouldn't cross the &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;nofollow&quot;&gt;Van Allen radiation belt&lt;/a&gt; (although that will be hardly avoidable when going to the moon), so that excludes orbits of around 1,000-60,000km from the list for manned craft.&lt;/p&gt;&#xA;" OwnerUserId="7067" LastActivityDate="2014-12-29T11:36:37.450" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6623" PostTypeId="1" AcceptedAnswerId="6624" CreationDate="2014-12-29T14:02:06.870" Score="6" ViewCount="614" Body="&lt;p&gt;I am working in a team designing a nano-satellite for a remote sensing mission. I am a computer engineering student so I am not really familiar with space stuff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that we have been told that an overheating problem could occur, if we put the de-orbiter (gossamer sail) right next to the avionics EPS (Electrical Power System) as shown in the picture. what do you recommend to avoid such a problem?&#xA;The satellite is a 3U (3 Unit) satellite with a mass around ~2 Kg. The mission is supposed to last 5 years. The altitude is 604.5 Km. The inclination is 98.43 degree. We don't use the sail to control and it's deployed as soon as it's in the nominal orbit &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SdcUI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="7068" LastEditorUserId="7068" LastEditDate="2015-01-01T20:20:35.920" LastActivityDate="2015-01-01T20:20:35.920" Title="Overheating problem in a nano-satellite due to proximity of system components" Tags="|artificial-satellite|design|remote-sensing|drag-augmentation|avionics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6624" PostTypeId="2" ParentId="6623" CreationDate="2014-12-29T16:19:40.670" Score="9" Body="&lt;p&gt;Spacecraft thermal control analysis employs modelling programs that can analyze combinations of conduction, convection, and radiation heat transfer in complex structures. An example is &lt;em&gt;Sinda&lt;/em&gt; by MCS Software. I have no association with MSC Software but I did work on thermal analysis problems for the Apollo Service Model in the early 1960s using primitive versions of what are now much more advanced (I am sure) software.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Complex structures are analyzed by developing thermal networks and are solved through analogies to electrical networks. Using these programs you can analyze a proposed system structure throughout a mission cycle including transient direct solar, earth emitted and earth reflected radiation as the satellite transits in an orbit in some orientation.  The thermal models consider as well internal thermal generation. Conduction/radiation paths between components would be constructed from the proposed geometry. If the proposed component layout shows a thermal problem, i.e. some structure gets too hot or cold, you can examine the need to relocate the component, alter thermal pathways, or provide some means of thermal control. You can then re-analyze your changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Apollo service module verification of the thermal calculations was so important that a thermal simulator was built that housed a service module and simulated radiant thermal inputs to simulate various attitudes in orbit and lunar transcoast. Comparison between calculations and simulations was not too bad even then. Now I'm sure it would be much better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know the magnitude of your nano-satellite program but some thermal analysis is likely necessary using &lt;em&gt;Sinda&lt;/em&gt; or some other equivalent to provide confidence that components can remain within satisfactory temperature ranges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a preliminary step take a look at the proposed layout, heat inputs, heat generation and do some simple analyses to see if there is some obvious likely problems.&lt;/p&gt;&#xA;" OwnerUserId="6501" LastEditorUserId="4703" LastEditDate="2014-12-29T20:01:31.213" LastActivityDate="2014-12-29T20:01:31.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6625" PostTypeId="2" ParentId="2570" CreationDate="2014-12-29T16:34:40.637" Score="13" Body="&lt;p&gt;I'm a little surprised nobody has mentioned markers. As in flags, signposts, and splashes of paint on rocks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronauts on the first missions to Mars are not going to go all that far from their base. Vehicles won't have the range and safety considerations would limit them. Vehicle tracks remain visible for a very long time, &lt;a href=&quot;http://en.wikipedia.org/wiki/Climate_of_Mars#Effect_of_dust_storms&quot;&gt;dust storms&lt;/a&gt; lay down a thin layer of dust over them but don't rub them out. So there will quickly be a network of trails to follow. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As activity expands on Mars the network of trails and the distribution of markers will expand with it. These are by far the most reliable navigation tools and I'm sure mission members will be careful to create a dense, easy to use, widely distributed array of them. They don't unexpectedly break, or suffer from interference, or run out of power. Finding them in the reduced visibility of a dust storm might be hard - it might be prudent to set up networks of blinking light posts and beacons for such cases. But any radio-based system would have trouble at such times too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were so far away from any explored area with a base that there were no markers at all, about the only way that could happen is if you landed off course. Even in that case, you probably wouldn't strike out on your own, you'd wait to be found. You'd activate a radio beacon and try to establish radio contact, which you'd likely then use to tell the base where you figure you are based on how you came in and what you can see around you. (And quite likely, figuring out where you were would be the least of your problems.)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2014-12-29T16:34:40.637" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6626" PostTypeId="2" ParentId="411" CreationDate="2014-12-29T16:47:48.277" Score="7" Body="&lt;p&gt;Combustion analysis computer programs and specifically exact reacting gas chemical kinetics programs were developed on the Apollo program to analyze and optimize rocket nozzle designs for efficiency improvements. Adaptation of these programs to other than rocket fuels provided the ability to analyze the mechanisms of NOx formation in fuel/air combustion processes. The programs analyze millisecond chemical species formation rates from many competing reactions. This never could be done before. Improved mathematics and computational capabilities provided the push.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In turn, this capability was employed in combustion devices like fossil fuel fired utility boilers, gas turbines and mobile sources to make significant reductions in the formation of NOx emissions. The technologies used for NOx control worldwide emanated from the computational chemistry calculations that were given the initial boost by the Apollo efforts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We built a company based upon these spin-offs.&lt;/p&gt;&#xA;" OwnerUserId="6501" LastEditorUserId="4703" LastEditDate="2014-12-29T21:07:04.487" LastActivityDate="2014-12-29T21:07:04.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6627" PostTypeId="1" CreationDate="2014-12-29T18:39:30.573" Score="9" ViewCount="406" Body="&lt;p&gt;I was reading in '&lt;a href=&quot;https://space.stackexchange.com/q/2570/4660&quot;&gt;How could one navigate on Mars&lt;/a&gt;' about communications satellites, and it got me thinking about radio relay and navigation infrastructure. Pure nitrogen is an inert lifting gas in the carbon dioxide atmosphere there. Carbon monoxide is too and could be refined from the air. Hydrogen could be gotten from the water in the soil but would slowly leak from the balloon, though it is so much lighter and plentiful it would maybe still be a better choice. With the very thin atmosphere it would take very little to fill a balloon - but it would also mean a balloon would have to be extremely large to have much lifting capacity. Still, if all it is is an antenna for a radio transceiver it is tethered to, and a navigational aid, maybe it could be a good way to cheaply set up reliable radio contact on Mars and an array of landmarks for triangulation visible over great distances. Wind loads are a lot lower on Mars, there aren't any thunderstorms - though maybe static discharges would be a problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could this be useful? How high would they need to be to get above most of the dust in a storm and thus work despite the static interference during such storms? What kind of calculations do you need to do to analyze such a system?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2014-12-31T00:41:49.247" Title="How useful could lighter-than-air tethered balloons be on Mars for radio and navigation?" Tags="|mars|colonization|communication|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6628" PostTypeId="2" ParentId="6627" CreationDate="2014-12-29T19:49:55.043" Score="8" Body="&lt;p&gt;During day they could be, but the main problem on Mars is the proverbial &lt;em&gt;in troposphere during day, in stratosphere during night&lt;/em&gt; issue, due to extreme diurnal air temperature variations and air saturation swings between fully saturated during nights and extremely dry during days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Height of dust storms is tough to appreciate with a few single numbers because of the so-called &lt;a href=&quot;https://en.wikipedia.org/wiki/Lee_wave&quot; rel=&quot;noreferrer&quot;&gt;lee-wave clouds&lt;/a&gt; that rise and fall due to change in atmospheric pressure and local topography (elevations, depressions). For example, from NASA's &lt;a href=&quot;http://www.nasa.gov/mission_pages/mars/news/mars20121121.html&quot; rel=&quot;noreferrer&quot;&gt;Spacecraft Monitoring Martian Dust Storm&lt;/a&gt; article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Starting on Nov. 16, the Mars Climate Sounder instrument on the Mars&#xA;  Reconnaissance Orbiter detected a warming of the atmosphere at about&#xA;  16 miles (25 kilometers) above the storm. Since then, the atmosphere&#xA;  in the region has warmed by about 45 degrees Fahrenheit (25 degrees&#xA;  Celsius). This is due to the dust absorbing sunlight at that height,&#xA;  so it indicates the dust is being lofted well above the surface and&#xA;  the winds are starting to create a dust haze over a broad region.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the atmospheric dust is widely dispersed in dozens of kilometers high layers, especially during the global dust storms, but also with more localized dust devils that can reach 20 kilometers high above ground:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/t9DAr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;A Monster Dust Devil Stalks the Martian Landscape (Image source: &lt;a href=&quot;http://www.uahirise.org/ESP_026394_2160&quot; rel=&quot;noreferrer&quot;&gt;HiRISE&lt;/a&gt;, Credit: NASA/JPL/University of Arizona)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fast winds are also not to be dismissed of, despite their seeming lack of force, because of the surface area of your lighter-than-air tethered balloons exposed to it. So it would have to be quite a system, though in my opinion not out of the question with on the cusp materials science. It would however at first likely have questionable reliability, or at least it would have to be established in-situ, so shouldn't be depended on for basic needs such as primary communications channels, or navigation markers / beacons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculations would be more statistical and procedural in nature, from tracking atmospheric conditions over years, to evaluating the need for them and establishing reliability of such systems. But you'll first have to have exact designs for them, or even do trial runs in simulated environments.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-12-29T19:49:55.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6629" PostTypeId="1" CreationDate="2014-12-29T20:03:55.250" Score="9" ViewCount="1022" Body="&lt;p&gt;SpaceX will be doing a Pad Abort test at LC-40 in Florida for the Dragon V2 vehicle to close out the CCiCAP criteria.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They will not be using a real first or second stage, instead building some kind of truss to hold the Dragon V2 at about the right height and then trigger the abort from there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps the test would destroy the second stage, but why not use a real first stage, mount an appropriate space to represent the second stage, and then do the abort?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-01-27T20:11:06.783" LastActivityDate="2015-01-27T20:11:06.783" Title="Why will SpaceX do Pad Abort test from a truss, not a real first and second stage?" Tags="|spacex|testing|dragon|abort|dragon-v2|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6630" PostTypeId="2" ParentId="6629" CreationDate="2014-12-29T20:11:33.820" Score="13" Body="&lt;p&gt;It's got to be hugely expensive to transport and erect a Falcon even if you don't intend to launch it. The potential for damaging part of the rocket in the test makes it even less attractive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why use a real rocket if a truss suffices?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After Apollo's first abort test from a low platform in the desert, they also did several in-flight abort tests by launching a boilerplate CSM on a stubby Little Joe II rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given SpaceX's demonstrated preference for combining non-critical-path R&amp;amp;D tests with commercial missions, I wouldn't be surprised if they combine a Dragon V2 in-flight abort test with a Falcon first-stage powered landing and/or flyback test at some point in the future. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2014-12-29T20:36:22.117" LastActivityDate="2014-12-29T20:36:22.117" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6631" PostTypeId="1" CreationDate="2014-12-29T22:14:59.067" Score="8" ViewCount="263" Body="&lt;p&gt;In the continuation of &quot;&lt;a href=&quot;https://space.stackexchange.com/questions/1527/are-there-any-humanoid-robots-on-board-the-iss?rq=1&quot;&gt;are there any humanoid robots on board the ISS?&lt;/a&gt;&quot;...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we have more information, further than &lt;a href=&quot;http://www.telegraph.co.uk/news/science/space/10529852/Kirobo-robot-has-first-space-chat-on-board-ISS.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;marketing&lt;/a&gt;, e.g:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Do we have firm &lt;strong&gt;plans&lt;/strong&gt; for &lt;strong&gt;cognitive&lt;/strong&gt; robots on board the ISS?&lt;/li&gt;&#xA;&lt;li&gt;If we have plans, what are the &lt;strong&gt;expectations&lt;/strong&gt;?&lt;/li&gt;&#xA;&lt;li&gt;So far, &lt;strong&gt;what robots have been tested / used for&lt;/strong&gt; in the ISS?&lt;/li&gt;&#xA;&lt;li&gt;What are the current &lt;strong&gt;results / benefits&lt;/strong&gt;?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mh0nh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-12-31T00:13:01.830" Title="Robots on board the ISS: Expectations and results?" Tags="|iss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6632" PostTypeId="2" ParentId="6607" CreationDate="2014-12-30T05:39:10.923" Score="12" Body="&lt;p&gt;&lt;strong&gt;TLDR&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its much harder to swim fast, even though it might be easier to swim slowly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;WHY&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your main difference will be due to a reduced hull speed of the swimmer. As noted in the comments section, the phase velocity of a wave would be reduced. This forms a hard limit on the speed that one can swim whilst displacing water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you swim, you generate a wave. If you swim at the speed of the wave, you will be swimming in the trough of the wave, which is basically flat. As you try to speed up, you find that you will need to swim &quot;up hill&quot; against the wave. This is what we call &quot;aqua-planing&quot; or as we boaters like to call it &quot;planing&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point the equations of motion become much more complicated, but suffice to say  drag increases significantly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://en.wikipedia.org/wiki/Hull_speed&quot;&gt;http://en.wikipedia.org/wiki/Hull_speed&lt;/a&gt; for more information (note, the calculations there assume 9.81m/s^2 local gravity).&lt;/p&gt;&#xA;" OwnerUserId="6157" LastEditorUserId="6157" LastEditDate="2014-12-30T10:44:14.633" LastActivityDate="2014-12-30T10:44:14.633" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6634" PostTypeId="2" ParentId="6567" CreationDate="2014-12-30T17:28:06.333" Score="9" Body="&lt;p&gt;You may want to read: &lt;a href=&quot;http://www.kouskouvelis.gr/site/files/Space_Insurance.pdf&quot; rel=&quot;noreferrer&quot;&gt;Legal Aspects of Space Commercialization - K. Tatsuzawa&lt;/a&gt;. A few quotations follow...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Costs have been insured:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In 1982, ESA insured the Marecs-A and B satellites for \$90 million,&#xA;  for a premium of \$6.7 million (7.5% of the insured capital). Insurance would cover&#xA;  only a second failure, meaning that payment would occur if both&#xA;  satellites malfunctionned or were de stroyed during launching. Thus,&#xA;  having successfully launched Marecs-A, the second satellite was not&#xA;  covered.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Profit loss has been insured:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Japan's Space Communications Corp. (SCC) was forced to shut down its&#xA;  Superbird-A which has been used by 20 companies and was providing TV&#xA;  for a 100,000 homes in Japan, in December 23, 1990, and is filing a&#xA;  \$150 million insurance claim for the vehicle and additional insurance&#xA;  filings are likely by users of the satellite. The inability to provide&#xA;  communications services could cost the SCC millions of dollars in&#xA;  revenue.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Risks insured are actually various and multiple:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first client to take an insurance policy was a telecommunications&#xA;  satellite — COMSAT's Early Bird in 1965. COMSAT had taken a&#xA;  third-party liability, for damages during the satellite's launching&#xA;  and its orbital life, and a pre-launch insurance for damage before&#xA;  launching, while the satellite was still on the ground.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Life insurance has existed since Apollo program:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There has even been a life space insurance policy, insuring the crew&#xA;  of the Apollo 11 mission. This “astropolicy”, made by a Greek&#xA;  insurance pool was hailed by the insurance industry for its&#xA;  inventiveness and innovation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Space insurance is comparable to maritime insurance, with reinsurance:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Insurance companies usually insure the space risk and then reinsure&#xA;  themselves through insurance pools.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Types of contracts that have been reviewed by the authors:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pre-launch, and in-orbit insurance.&lt;/li&gt;&#xA;&lt;li&gt;Economic losses insurance.&lt;/li&gt;&#xA;&lt;li&gt;Damages to third parties.&lt;/li&gt;&#xA;&lt;li&gt;Re-entry liability.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Other elements you mention (e.g. constraints for launch) seem to be the result of a negotiation between the owner and the insurer, as explained in the document.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From OECD (2011), “&lt;a href=&quot;http://dx.doi.org/10.1787/9789264113565-12-en&quot; rel=&quot;noreferrer&quot;&gt;Insurance market for space activities&lt;/a&gt;”, in The Space Economy at a Glance 2011, OECD Publishing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4SPlQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2014-12-30T23:43:27.117" LastActivityDate="2014-12-30T23:43:27.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6635" PostTypeId="1" AcceptedAnswerId="6636" CreationDate="2014-12-30T07:38:01.653" Score="8" ViewCount="949" Body="&lt;p&gt;I've started to notice more and more as I study the ISS, that I can only ever find pictures of &lt;strong&gt;one&lt;/strong&gt; Soyuz docked. Where is the other one?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lets say their sending up a crew of 6 for an expedition.&lt;br&gt;&#xA;A Soyuz craft can only carry 3 at a time (&lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_%28spacecraft%29&quot;&gt;&lt;strong&gt;wiki&lt;/strong&gt;&lt;/a&gt;), thus meaning another one has to launch later.&lt;br&gt;(Which is why theres always a period where only 3 of the 6 are on the station)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What modules&lt;/strong&gt; do &lt;strong&gt;both&lt;/strong&gt; the Soyuz dock to?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also feel that I might be looking at this the wrong way. Please correct me if I'm wrong that both are docked at the same time?&lt;/p&gt;&#xA;" OwnerDisplayName="user7078" LastEditorUserId="5963" LastEditDate="2014-12-30T18:04:05.630" LastActivityDate="2014-12-30T18:14:41.120" Title="What module(s) on the ISS do the Soyuz dock to?" Tags="|iss|soyuz-spacecraft|docking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6636" PostTypeId="2" ParentId="6635" CreationDate="2014-12-30T09:52:43.823" Score="11" Body="&lt;p&gt;Given the International Space Station's modular design, it appears spacecraft such as Soyuz and resupply freighters such as Progress can attach to many different modules. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This NASA diagram from 2008 shows Soyuz can be attached to four modules:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;MRM1 (Mini Research Module - Rassvet)&lt;/li&gt;&#xA;&lt;li&gt;MRM2 (Poisk)&lt;/li&gt;&#xA;&lt;li&gt;Pirs&lt;/li&gt;&#xA;&lt;li&gt;Zvezda&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7Pvy7.jpg&quot; alt=&quot;NASA diagram&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here's a &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2008/11/ESA_s_Columbus_laboratory_Node-2_and_Japan_s_Kibo_module_at_the_ISS_main_truss_at_rear_two_Soyuz_vehicles_lower_centre&quot; rel=&quot;nofollow noreferrer&quot;&gt;2008 NASA photograph&lt;/a&gt; showing two Soyuz craft attached to the ISS. The angle makes it look like they might be attached to one of the US section modules, but in fact they are docked to Pirs and Rassvet. The caption reads:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;ESA's Columbus laboratory, Node-2 and Japan's Kibo module at the ISS (main truss at rear, two Soyuz vehicles lower centre)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dyxpI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here's &lt;a href=&quot;http://www.nasa.gov/content/crew-gears-up-for-spacecraft-traffic-at-the-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;another NASA image from April 2014&lt;/a&gt; showing two attached Soyuz, captioned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are now four vehicles, two Soyuz spacecraft and two Progress space freighters, docked to the International Space Station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5AY1K.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good question and answer similar to this is the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;Will the ISS need more docking ports?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some details on the Russian and US segments future plans as well as current situation is available in that question.&lt;/p&gt;&#xA;" OwnerUserId="14595" OwnerDisplayName="Hugo" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2014-12-30T18:14:41.120" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6637" PostTypeId="2" ParentId="6631" CreationDate="2014-12-30T19:11:23.157" Score="4" Body="&lt;p&gt;The most useful robots onboard currently are the SSRMS and JEMRMS, both are teleoperated robotic systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The JEMRMS (Japanese Experiment Module Remote Manipulator system) is &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a 10m long robotic arm, mounted at the port cone of the PM, intended&#xA;  to service the EF and to move equipment from and to the ELM. The RMS&#xA;  control console was launched while inside the ELM-PS. The main arm was&#xA;  launched with the PM. The &quot;Small Fine Arm&quot;, is 2m long and attaches to&#xA;  the end effector of the main arm, was launched aboard.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(from &lt;a href=&quot;https://en.wikipedia.org/wiki/Kibo_%28ISS_module%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;)  Acronyms: PM - Pressurized Module, EF - Exposed Facility, ELM - External Logistics Module&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSRMS (Space Station Remote Manipulator System) &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Launched on STS-100 in April 2001, this second generation Canadarm is&#xA;  a larger, more advanced version of the space shuttle's original&#xA;  Canadarm. Canadarm2 is 17.6 m (58 ft) when fully extended and has&#xA;  seven motorized joints. It has a mass of 1,800 kg (4,000 lb) and a&#xA;  diameter of 35 cm (14 in). The arm is capable of handling large&#xA;  payloads of up to 116,000 kg (256,000 lb) and was able to assist with&#xA;  docking the space shuttle. Officially known as the Space Station&#xA;  Remote Manipulator System (SSRMS), it is self-relocatable and can move&#xA;  end-over-end to reach many parts of the Space Station in an&#xA;  inchworm-like movement. In this movement, it is limited only by the&#xA;  number of Power Data Grapple Fixtures (PDGFs) on the station. PDGFs&#xA;  located around the station provide power, data and video to the arm&#xA;  through its Latching End Effectors (LEEs). The arm can also travel the&#xA;  entire length of the space station truss using the Mobile Base System.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Most of the time the arm operators see what they are doing by looking&#xA;  at the three Robotic Work Station (RWS) LCD screens. The MSS has two&#xA;  RWS units: one located in the Destiny module (US Lab module) and the&#xA;  other in the Cupola. Only one RWS controls the MSS at a time. The RWS&#xA;  has two sets of control joysticks: one Rotational Hand Controller&#xA;  (RHC) and one Translational Hand Controller (THC). In additional to&#xA;  this is the Display and Control Panel (DCP) and the Portable Computer&#xA;  System (PCS) laptop.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Mobile_Servicing_System&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that no one involved with the SSRMS &lt;em&gt;ever&lt;/em&gt; refers to it as the Canadarm.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2014-12-30T23:00:38.267" LastActivityDate="2014-12-30T23:00:38.267" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6638" PostTypeId="1" CreationDate="2014-12-31T02:18:39.357" Score="20" ViewCount="3732" Body="&lt;p&gt;Measures of the vertical and horizontal vectors of athletes running shows that once they get up to speed the ratio of vertical to horizontal exertion is about 4:1. This paper by &lt;a href=&quot;http://www.aspetar.com/journal/upload/PDF/201312187352.pdf&quot; rel=&quot;noreferrer&quot;&gt;Jean-Benoit Morin Ph.D.&lt;/a&gt; gave data using the latest measurement techniques:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zqBFz.jpg&quot; alt=&quot;force vectors during sprinting&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, imagining that on the Moon, it would take you 1/6 of the force you need to apply vertically on the Earth to achieve the same vertical vector. Then the question is how efficiently you can apply force horizontally - how much of the effort you save on the vertical element you can put into the horizontal element. This must imply a different running stance, which implies an entirely different biomechanical model. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume this can be done fairly efficiently, albeit with a very different running stance which may be supported by the use of different gear - different shoes, perhaps poles somewhat analogous to ski poles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air friction needs to be considered, for a model for within a habitat . I've looked at the formula $F_D\, =\, \tfrac12\, \rho\, v^2\, C_D\, A$ . I don't know how to properly use this formula for this case, though I've looked up the figures. Indoors, in fairness a lower air pressure than on Earth would likely be chosen. Let us say &lt;a href=&quot;http://en.wikipedia.org/wiki/Effects_of_high_altitude_on_humans#Effects_as_a_function_of_altitude&quot; rel=&quot;noreferrer&quot;&gt;half&lt;/a&gt; the air pressure with twice the oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Traction would be an issue. How does one incorporate that into a calculation? Also, if the environment was designed for running, some solution for lack of traction could surely be found through the use of a custom surface and shoes with cleats.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Put together, what top speeds could a person achieve INDOORS in 0.5 atmospheres, and OUTDOORS, in a vacuum?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-07T18:59:58.553" LastActivityDate="2015-02-15T02:24:04.127" Title="How fast could a person run on the Moon?" Tags="|the-moon|colonization|gravity|physical-activity|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6639" PostTypeId="1" AcceptedAnswerId="6641" CreationDate="2014-12-31T04:29:09.747" Score="15" ViewCount="1885" Body="&lt;p&gt;If you look at &lt;a href=&quot;http://www.astronoo.com/en/articles/apsis-objects-celestial.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;this&lt;/strong&gt;&lt;/a&gt; site, it shows some of our common bodies' apsidal names, as shown below:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&#xA;Objects     Periapsis       Apoapsis&#xA;&#xA;Galaxy      Perigalacticon  Apogalacticon&#xA;Black hole  Perimélasma     Apomelasma&#xA;Star        Periastron      Apoastron&#xA;Sun         Perihelion      Aphelion&#xA;Mercury     Perihermion     Apohermion&#xA;Venus       Pericytherion   Apocytherion&#xA;Earth       Perigée         Apogee&#xA;Moon        Periselene      Aposelene&#xA;Mars        Periareion      Apoareion&#xA;Jupiter     Perizene        Apozene&#xA;Saturn      Perikrone       Apokrone&#xA;Uranus      Periuranion     Apouranion&#xA;Neptune     Periposeidion   Apoposeidion &#xA;Pluto       Perihadion      Apohadion&#xA;&lt;/pre&gt; &#xA;&#xA;&lt;p&gt;My question is, &lt;strong&gt;how&lt;/strong&gt; do scientists, astronomers or whoever, come up with these names?&lt;br&gt;&#xA;Is there a list waiting for discovered bodies? Do they make them up as they go?&lt;/p&gt;&#xA;" OwnerDisplayName="user7078" LastEditorUserId="-1" LastEditDate="2018-02-01T13:47:22.320" LastActivityDate="2018-02-01T13:47:22.320" Title="How do apsides of celestial bodies get their names?" Tags="|orbit|terminology|planet|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6640" PostTypeId="2" ParentId="6639" CreationDate="2014-12-31T05:33:13.737" Score="7" Body="&lt;p&gt;They are generally composed by using the adjectival form of the Greek god name:&lt;br&gt;&#xA;Star = Asteria&lt;br&gt;&#xA;Sun = Helios&lt;br&gt;&#xA;Mecury = Hermes&lt;br&gt;&#xA;Venus = Cytherea&lt;br&gt;&#xA;Earth = Gaia&lt;br&gt;&#xA;Moon = Selene&lt;br&gt;&#xA;Mars = Ares&lt;br&gt;&#xA;Jupiter = Zeus&lt;br&gt;&#xA;Saturn = Kronos&lt;br&gt;&#xA;Neptune = Poseidon&lt;br&gt;&#xA;Pluto = Hades  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Uranus was never known by its Latin equivalent, Caelus, as it was not a planet known to the ancients. Uranus was the Greek &quot;Father Sky,&quot; the consort of &quot;Mother Earth,&quot; or Gaia.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Galacticon&quot; is a (rather awkward) back formation from &quot;galaxy.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which leaves &quot;melasma,&quot; which has nothing to do with the &lt;a href=&quot;http://en.wikipedia.org/wiki/Melasma&quot; rel=&quot;noreferrer&quot;&gt;skin condition&lt;/a&gt;. It is Greek for &quot;black spot.&quot;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2014-12-31T06:14:37.113" LastActivityDate="2014-12-31T06:14:37.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6641" PostTypeId="2" ParentId="6639" CreationDate="2014-12-31T05:37:09.633" Score="10" Body="&lt;p&gt;There is no generally agreed upon naming convention that I'd know of. Since the prefixes for the closest (peri-) and farthest (apo-) orbital point to the parent body are of ancient Greek origins, most commonly found forms use ancient Greek names for celestial bodies. Either ancient Greek gods that are equivalent to Roman gods that the celestial bodies were named after (e.g. for Latin / Roman &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_%28mythology%29&quot;&gt;Mars&lt;/a&gt;, ancient Greek equivalent is &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares&quot;&gt;Ares&lt;/a&gt;, both gods of war), any objects, places and so on that they were associated with, or even their alternative names. Similar goes for adjectives. But the names used both for apsides as well as adjectives can also come from Latin, for example, for the Moon: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Adjectives: lunar (from Latin &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_%28goddess%29&quot;&gt;Luna&lt;/a&gt;), selenic / selenian (from Greek &lt;a href=&quot;https://en.wikipedia.org/wiki/Selene&quot;&gt;Selene&lt;/a&gt;) or alternative cynthean (from Greek Cynthia, epithet of the Greek goddess of the moon &lt;a href=&quot;https://en.wikipedia.org/wiki/Artemis&quot;&gt;Artemis&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Apisides: (peri-/apo-) lune, selene, cynthion&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Forms from Artemis are missing, perhaps because it would make such named apsides &lt;a href=&quot;http://medical-dictionary.thefreedictionary.com/periarteritis&quot;&gt;sound like a bad medical condition&lt;/a&gt;. Some of the remaining forms are more commonly used for artificial satellites (e.g. -cynthion), others for natural satellites (e.g. -selene) and so on. But their use might depend on something as simple as which one sounds better or catches on in scientific and popular literature. And yes, such naming can get pretty inventive, like for example for black holes (-melasma/-bothra/-nigricon) or Venus (-cytherion/-cytherean/-krition).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia &lt;a href=&quot;https://en.wikipedia.org/w/index.php?title=Apsis&amp;amp;oldid=633890504#Terminology&quot;&gt;used to have&lt;/a&gt; a table of most common names of apsides, but that's sadly &lt;a href=&quot;https://en.wikipedia.org/wiki/Apsis#Terminology&quot;&gt;gone with new revisions&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Body        Closest approach                            Farthest approach&#xA;------------------------------------------------------------------------------------------&#xA;General     Periapsis/Pericenter                        Apoapsis/Apocenter&#xA;Galaxy      Perigalacticon                              Apogalacticon&#xA;Star        Periastron                                  Apastron&#xA;Black hole  Perimelasma/Peribothra/Perinigricon         Apomelasma/Apobothra/Aponigricon&#xA;Sun         Perihelion                                  Aphelion&#xA;Mercury     Perihermion                                 Aphermion&#xA;Venus       Pericytherion/Pericytherean/Perikrition     Apocytherion/Apocytherean/Apokrition&#xA;Earth       Perigee                                     Apogee&#xA;Moon        Periselene/Pericynthion/Perilune            Aposelene/Apocynthion/Apolune&#xA;Mars        Periareion                                  Apoareion&#xA;Jupiter     Perizene/Perijove                           Apozene/Apojove&#xA;Saturn      Perikrone/Perisaturnium                     Apokrone/Aposaturnium&#xA;Uranus      Periuranion                                 Apouranion&#xA;Neptune     Periposeidion                               Apoposeidion&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="49" LastActivityDate="2014-12-31T05:37:09.633" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="6642" PostTypeId="1" AcceptedAnswerId="6643" CreationDate="2014-12-31T17:25:56.037" Score="10" ViewCount="1298" Body="&lt;p&gt;I spoke once with a USAF officer involved in keeping our satellites operational and in the proper orbit.  He told me that satellites in geo-synchronous orbit are in little danger of colliding with space junk or with each other as there is a great deal of parking space between objects in orbits 26,199 miles above the earth.  But it occurs to me that other types of satellites, especially spy satellites, and the ISS, fly much lower orbits and, therefore, have less room to themselves.  So that leads me to a few questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is there an international agency that coordinates satellite orbits in order to lessen possibility of collisions?&lt;/li&gt;&#xA;&lt;li&gt;At what distance above the earth is a satellite most in danger of collision with other objects, including space junk?&lt;/li&gt;&#xA;&lt;li&gt;At what distance does the threat of collission become relatively negligible?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="7032" LastActivityDate="2014-12-31T21:46:29.257" Title="Collision with space junk - Where is it safe? Where is it most dangerous?" Tags="|artificial-satellite|low-earth-orbit|debris|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6643" PostTypeId="2" ParentId="6642" CreationDate="2014-12-31T17:43:24.653" Score="14" Body="&lt;p&gt;Here are two graphs$^1$ regarding the amount of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_debris&quot;&gt;space debris&lt;/a&gt; at different altitudes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/File:Spacedebris_small.png&quot;&gt;&lt;strong&gt;#1: ESA report on debris in 2001&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/4/49/Spacedebris_small.png&quot; alt=&quot;ESA report; 2001&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/File:Spacedebris_upd_2011.jpg&quot;&gt;&lt;strong&gt;#2: NASA report on debris in 2011&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/b/be/Spacedebris_upd_2011.jpg&quot; alt=&quot;NASA report; 2011&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sharp spikes in the second graph are from &lt;a href=&quot;http://www.space.com/19450-space-junk-worst-events-anniversaries.html&quot;&gt;the collision between two satellites (Iridium 33 and Cosmos 2251) and a Chinese anti-satellite test&lt;/a&gt;. Both occurred between 2001 and 2011 - hence the reason for their non-existence in the first graph.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At what distance above the earth is a satellite most in danger of collision with other objects, including space junk?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Roughly 700-1000 kilometers in altitude. The debris at 800 km was greatly increased after the collision and the Chinese test. There's also a decent spike as we get even higher up, to geostationary orbit. It's crowded there with operational satellites, too.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At what distance does the threat of collision become relatively negligible?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You have to go above geosynchronous orbit (GEO) for it to become negligible, because even at the lowest levels on the graphs, there are still &lt;em&gt;some&lt;/em&gt; objects. There isn't any altitude where there's a huge chance of collision, but there's always a chance, unless you get to past High Earth Orbit. Go for the far right side of &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/8/82/Orbitalaltitudes.jpg&quot;&gt;this&lt;/a&gt; diagram:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/8/82/Orbitalaltitudes.jpg&quot; alt=&quot;Orbits&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The red area is near High Earth Orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main organization that tracks space debris is the &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/&quot;&gt;NASA Orbital Debris Program Office&lt;/a&gt;. This is a US agency; however, other space agencies (such as the ESA) also track space debris. The NASA site is pretty comprehensive, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, you can just go to the poles. &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/a/a1/Debris-GEO1280.jpg&quot;&gt;This&lt;/a&gt; shows the distribution of objects around Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/a/a1/Debris-GEO1280.jpg&quot; alt=&quot;Worldwide distribution&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The objects close to Earth are in Low Earth Orbit; the ring is objects in GEO.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;$^1$ As pointed out by Russell Borogove, the graphs differ by quite a lot along the $y$-axis (spatial density). We both suspect that it may be due to different sizes of objects being compared. I'm working on that and trying to figure out if NASA or the ESA has any information. Fortunately, the peaks (even accounting for the Iridium-Cosmos collision and the Chinese anti-satellite test) align at roughly the same altitudes, so the graphs are still applicable.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="5963" LastEditDate="2014-12-31T21:46:29.257" LastActivityDate="2014-12-31T21:46:29.257" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6644" PostTypeId="1" CreationDate="2014-12-31T18:49:26.540" Score="5" ViewCount="707" Body="&lt;p&gt;The calculations for the taper ratio of a space elevator don't seem to be all that difficult and it is easy to find &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_elevator#Cable_section&quot; rel=&quot;noreferrer&quot;&gt;some equations to use&lt;/a&gt;. You can even find an expression written out for the total mass (which involves more complicated mathematical functions).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if we have an orbiting, tidally locked, partial space elevator what would the equations for those be? Searching around, I can't find any material on this. Presumably it would be slightly more complicated than a full geosynchronous space elevator because you have an additional independent variable. Does anyone have either an approach to this problem with some of the basic math hashed out, or just a straightforward equation for taper ratio?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2014-12-31T21:13:57.820" Title="Taper ratio for partial space elevator" Tags="|materials|space-elevator|mathematics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6645" PostTypeId="2" ParentId="6644" CreationDate="2014-12-31T19:53:58.860" Score="7" Body="&lt;p&gt;I was over-complicating. It's the same equation as for a geostationary space elevator. In fact, the geostationary space elevator is just one specific case of a general equation for taper ratio. As a mental experiment, pretend that the space elevator equations were applied for a planet of the same mass but with a different rotational period. Clearly the geostationary point would lie at a different point, and you could pretend that such an imaginary planet exists for every non-geostationary orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Borrowing from &lt;a href=&quot;https://physics.stackexchange.com/questions/33547/space-elevator-on-mars-with-todays-technology-possible&quot;&gt;a previous answer&lt;/a&gt;, you could do this simply as follows. The one thing we're introducing is a new criteria for the neutral circular orbit based on the radius, r0, at the middle of the partial space elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ U(r) = - \frac{ GM}{r } - \frac{1}{2} \omega^2 r^2 \\&#xA;\frac{ GM}{r_0^2} = \omega^2 r_0 \\&#xA;U(r) = - \frac{ GM}{r } - \frac{1}{2} \frac{ GM}{r_0^3 } r^2 \\&#xA;\frac{ \lambda(r_{0}) }{ \lambda(r) }  = \exp{ \left(  \frac{ U(r_{0}) - U(r) } { \left( \frac{ \sigma}{ \rho} \right) }  \right) } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two independent variables are r0 and r. The final expression is the taper ratio. You need to know the free-floating central point and you need to know how far it extends in either direction. Yes, it does need to be balanced on both ends, and that would be the more difficult requirement. In order to find the length on the &quot;other side&quot; that corresponded to a radius selection you would see to set the integral of the linear mass-thickness on both sides to be equal.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2014-12-31T19:53:58.860" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6647" PostTypeId="2" ParentId="5610" CreationDate="2014-12-31T22:57:53.240" Score="12" Body="&lt;p&gt;In November 2010, &lt;a href=&quot;http://swfound.org/&quot;&gt;Secure World Foundation&lt;/a&gt; (SWF) published their &lt;a href=&quot;http://swfound.org/media/1791/swf_x-37b_otv_fact_sheet_updated_2012.pdf&quot;&gt;X-37B Orbital Test Vehicle Fact Sheet&lt;/a&gt; (PDF) that is a conjecture on the purpose of the two &lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_X-37&quot;&gt;Boeing built X-37B OTV&lt;/a&gt; (Orbital Test Vehicles) currently operated by U.S. Air Force. It also gives a background on vehicles, their known technical characteristics, and describes official objectives they're supposed to serve by quoting the then &lt;a href=&quot;http://www.af.mil/AboutUs/FactSheets/Display/tabid/224/Article/104539/x-37b-orbital-test-vehicle.aspx&quot;&gt;“X-37 Orbital Test Vehicle,” U.S. Air Force Fact Sheet&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Official objectives of the X-37B program include &quot;space&#xA;  experimentation, risk reduction and concept of operations development&#xA;  for reusable space vehicle technologies.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The U.S. Air Force page linked since changed in contents and URL address, but the new one isn't any more forthcoming, I'd say it's even less so. So all we have is speculation on its possible uses, based on its equally speculated performance characteristics. Educated guessing galore, here's some of the additional uses that the SWF paper suggested, including their estimated feasibility:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;X-37B as an on-orbit sensor platform and test bed (Feasibility: high)&lt;/li&gt;&#xA;  &lt;li&gt;X-37B as a deployment platform for ORS satellites (Feasibility: medium)&lt;/li&gt;&#xA;  &lt;li&gt;X-37B as an on-orbit repair vehicle (Feasibility: low)&lt;/li&gt;&#xA;  &lt;li&gt;X-37B as an on-orbit inspection or ASAT platform (Feasibility: low)&lt;/li&gt;&#xA;  &lt;li&gt;X-37B as a Conventional Prompt Global Strike (CPGS) weapon or delivery system (Feasibility: zero)&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: In April 2015, the first speculated possible use in the list was finally &lt;a href=&quot;http://spaceflightnow.com/2015/04/27/x-37b-launch-date-firms-up-as-new-details-emerge-about-experiment/&quot;&gt;confirmed with the DoD news release&lt;/a&gt; that X-37B will start testing Hall thruster technology aboard the vehicle that is currently scheduled for launch from Cape Canaveral on May 20, 2015 atop Atlas 5 rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This does increase future possibility of the second and fourth items in the list too, to use X-37B as ORS satellite deployment platform since Hall thrusters, if successfully tested, could greatly reduce their weight and increase maneuvering capability, which could also enable various ASAT (Anti-Satellite) options. Same goes for X-37B vehicles themselves, as &lt;a href=&quot;http://foxtrotalpha.jalopnik.com/we-finally-know-something-about-what-the-shadowy-x-37b-1700434054&quot;&gt;speculated by Foxtrot Alpha&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This technology could also help in the development of killer satlets,&#xA;  parasites and other foreign satellite manipulation technologies as it&#xA;  would allow these small satellites to pack more maneuvering capability&#xA;  while also possessing a finer degree of control.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Then there is the unmanned X-37B itself, which is a clear candidate&#xA;  for such a technology as it is basically a long endurance space truck.&#xA;  On its last mission it spent 675 days in orbit. With this new thruster&#xA;  technology, its endurance could be enhanced even longer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-04-30T09:47:04.750" LastActivityDate="2015-04-30T09:47:04.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6649" PostTypeId="1" AcceptedAnswerId="6650" CreationDate="2015-01-01T22:21:29.537" Score="3" ViewCount="1216" Body="&lt;p&gt;I looked at a presentation of Robert Zubrin. In this presentation, he first about his (old) Mars Direct plan, then he talks a bit about Mars Semi Direct with the Falcon Heavy Rocket. He want to use every two years three of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, does someone know a &lt;strong&gt;link&lt;/strong&gt;, which has some &lt;strong&gt;details&lt;/strong&gt; about this plan?&lt;/p&gt;&#xA;" OwnerUserId="7035" LastEditorUserId="7035" LastEditDate="2015-01-01T22:33:00.790" LastActivityDate="2015-01-06T08:40:20.713" Title="Mars Semi Direct" Tags="|mars|nasa|spacex|crewed-spaceflight|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6650" PostTypeId="2" ParentId="6649" CreationDate="2015-01-01T22:52:31.257" Score="3" Body="&lt;p&gt;Not really a good question. So not really a good answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is &lt;a href=&quot;https://www.youtube.com/watch?v=EKQSijn9FBs&quot; rel=&quot;nofollow noreferrer&quot;&gt;a great presentation&lt;/a&gt; Zubrin did, where he discusses Mars Direct / Semi Direct. Very compelling discussion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/EKQSijn9FBs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Video was recorded in July 10, 2014 during the NASA Ames Research Center Director's Colloquium at Moffett Field, California. Here are some numbers for the Semi Direct plan, pulled from the video (&lt;a href=&quot;https://www.youtube.com/watch?feature=player_detailpage&amp;amp;v=EKQSijn9FBs#t=1724&quot; rel=&quot;nofollow noreferrer&quot;&gt;roughly 29 minutes into it&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/iIMpQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zubrin commented that the margins for this Mars Semi Direct are tight, but doable with water recycling that is the key to payload mass savings.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some additional sources to consider:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Direct&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Direct Wikipedia page&lt;/a&gt; is pretty good. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Nice &lt;a href=&quot;http://www.nss.org/settlement/mars/zubrin-promise.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Direct article from 1996&lt;/a&gt; (Pre-Falcon Heavy).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/376589main_04%20-%20Mars%20Direct%20Power%20Point-7-30-09.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;2009 Presentation that was used in the video&lt;/a&gt;, for the Mars Direct part.  Still no Falcon Heavy version in it. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Jeff Foust (Space Review) has a nice 2011 article on &lt;a href=&quot;http://www.thespacereview.com/article/1850/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Semi-Direct using Falcon Heavy&lt;/a&gt;. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-01-06T08:40:20.713" LastActivityDate="2015-01-06T08:40:20.713" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6651" PostTypeId="2" ParentId="6234" CreationDate="2015-01-02T15:55:37.260" Score="12" Body="&lt;p&gt;&lt;strong&gt;Blackout duration&lt;/strong&gt; on Earth is dependent on the angle of re-entry, the more horizontal the re-entry, the longer the blackout, because of the longer time for speed to exit the plasma creation conditions. See &lt;a href=&quot;https://archive.org/details/nasa_techdoc_19640019602&quot; rel=&quot;noreferrer&quot;&gt;A rough estimate of the 'blackout' time in re-entry communications&lt;/a&gt; (1962).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has been visible for &lt;a href=&quot;http://en.wikipedia.org/wiki/Communications_blackout#Spacecraft_reentry&quot; rel=&quot;noreferrer&quot;&gt;Apollo 13 re-entry&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to the mission log maintained by Gene Kranz, the Apollo 13&#xA;  re-entry blackout lasted around 6 minutes [...], and was 1 minute 27 seconds longer&#xA;  than had been predicted.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Blackout is a negative side-effect of a design choice: The re-entering spacecraft is decelerated using the atmosphere passive resistance, rather than other means, because this is the cheapest method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;Being a capsule-shaped vehicle&lt;/strong&gt;, Orion experiments a re-entry blackout, though &lt;a href=&quot;http://www.spaceflight101.com/orion-eft-1-mission-updates.html&quot; rel=&quot;noreferrer&quot;&gt;limited&lt;/a&gt;, less than 2:30 mn for first flight test:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At 16:18 UTC, Orion slammed into the atmosphere, starting to slow down&#xA;  from 32,000 Kilometers per hour. Plasma started building up around the&#xA;  vehicle, causing a two and a half minute blackout in communications.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;Re-entry blackout is still a problem&lt;/strong&gt; for capsule-shaped vehicles, both manned and unmanned. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Genesis_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Genesis mission&lt;/a&gt; samples have been lost because the control center had no mean to force the deployment of the parachute, due to communication blackout, after the inboard sensor didn't trigger the automated sequence properly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;The most beautiful solution&lt;/strong&gt; to the re-entry blackout could be using the plasma layer as an antenna. A team of scientists stated having found a way to do this:&#xA;&lt;a href=&quot;http://arxiv.org/pdf/0704.3103v2.pdf&quot; rel=&quot;noreferrer&quot;&gt;Communication through plasma sheaths - Korotkevich, Newell, Zakharov&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We wish to transmit messages to and from a hypersonic vehicle around&#xA;  which a plasma heath has formed. For long distance transmission, the&#xA;  signal carrying these messages must be necessarily low frequency,&#xA;  typically 2 GHz, to which the plasma sheath is opaque. The idea is to&#xA;  use the plasma properties to make the plasma sheath appear transparent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Looks good on the paper, now needs to be experimented.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;Other solutions&lt;/strong&gt;: Besides using the natural plasma-free tunnel created by the shape of the vehicle, different ways to reduce the plasma layer effects have been investigated...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(Source: &lt;a href=&quot;http://www.academia.edu/413990/Numerical_Modeling_of_Plasma_Manipulation_Using_an_ExB_Layer_in_the_Hypersonic_Boundary_Layer&quot; rel=&quot;noreferrer&quot;&gt;American Institute of Aeronautics and Astronautics&lt;/a&gt;)&lt;br&gt;&#xA;  ...including aerodynamic shaping, quenchant injection, high radio&#xA;  frequencies, high transmission power, and Raman scattering. One of the&#xA;  most promising methods is the magnetic window method where a strong DC&#xA;  magnetic ﬁeld is used to allow radio wave propagation through the&#xA;  plasma layer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The latter method can be improved by adding an electric field to the magnetic field to further reduce the electron density. Such combination of fields is known as ExB.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;ExB electron density reduction&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Yo5PT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea to to use to crossed fields to generate a so-called &lt;em&gt;E$\times$B drift&lt;/em&gt; of the plasma, and use the resulting low density corridor to communicate. Drift velocity according to Chen's equation: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{V}d=\frac{\mathbf{E} \times \mathbf{B}}{B^2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simulation gives the following results:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qQayO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(Source: AIAA)&lt;br&gt;&#xA;  For the S-band communication blackout, the ExB layer&#xA;  with a 0.3 T magnetic ﬁeld provides a sufficient plasma density&#xA;  reduction to enable communication through the plasma layer. The ExB&#xA;  layer with 0.5 T and - 1000 V potential shows the possibility for&#xA;  communication through the plasma layer for the L band and GPS&#xA;  frequency. The applied ExB layer gives another eﬀect beside&#xA;  manipulation of plasma density.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For a extensive presentation of the technique, see also:&#xA;&lt;a href=&quot;http://ngpdlab.engin.umich.edu/files/papers/Kim.pdf&quot; rel=&quot;noreferrer&quot;&gt;Manipulation of plasma layer for re-entry blackout mitigation - Min Kwan Kim.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This technique was demonstrated in the &lt;a href=&quot;http://global.jaxa.jp/projects/rockets/orex/index.html&quot; rel=&quot;noreferrer&quot;&gt;Orbital Re-entry Experiment (OREX)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-01-02T19:37:17.967" LastActivityDate="2015-01-02T19:37:17.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6653" PostTypeId="2" ParentId="6237" CreationDate="2015-01-03T00:57:36.830" Score="6" Body="&lt;p&gt;Online pictures of rooms new configuration are actually quite rare at the moment, especially close-ups. It's also a bit difficult to state which FCR is depicted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Johnson Space Center (JSC) has &lt;a href=&quot;http://en.wikipedia.org/wiki/Christopher_C._Kraft_Jr._Mission_Control_Center&quot; rel=&quot;nofollow noreferrer&quot;&gt;5 FCR&lt;/a&gt;, which roles have evolved and still do. The Shuttle and the ISS FCR once have used to be controlled respectively from the &lt;a href=&quot;http://en.wikipedia.org/wiki/Christopher_C._Kraft_Jr._Mission_Control_Center#White_FCR_.281998.E2.80.932011.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;White&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Christopher_C._Kraft_Jr._Mission_Control_Center#Blue_FCR_.281998.E2.80.932006.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Blue&lt;/a&gt; FCR, but ISS control has switched to FCR1, and White and Blue FCR are converted to MCC-21.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/j6z2N.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;(Source: &lt;a href=&quot;http://home.earthlink.net/~mike_scott/JSC-Mission%20Control%20Center.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nasa, Mike Scott&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;'Houston' is not the only location for ISS-related activities:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;US launches from KSC &lt;a href=&quot;http://en.wikipedia.org/wiki/Kennedy_Space_Center_Launch_Complex_39&quot; rel=&quot;nofollow noreferrer&quot;&gt;p&#xA;Pad-39&lt;/a&gt; are controlled from KSC &lt;a href=&quot;http://enterfiringroom.ksc.nasa.gov/tour.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;LCC&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Russian launches from &lt;a href=&quot;http://www.russianspaceweb.com/baikonur_r7_1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Site-1/Gagarinskiy&lt;/a&gt; are controlled from &lt;a href=&quot;http://www.mcc.rsa.ru/English/osn_upr_ka.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;TsNIImash&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;European launches from &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Europe_s_Spaceport/Europe_s_Spaceport2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kourou&lt;/a&gt; are controlled from the &lt;a href=&quot;http://msg-3launchcampaign.blogspot.fr/2012/06/dress-rehearsal.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter&lt;/a&gt; room.&lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;http://www.russianspaceweb.com/iss_russia.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Russian segment&lt;/a&gt; of the ISS is now controlled from &lt;a href=&quot;http://www.mcc.rsa.ru/English/osn_upr_ka.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;TsNIImash&lt;/a&gt; MCC too.&lt;/li&gt;&#xA;&lt;li&gt;ATV activities are controlled from in &lt;a href=&quot;http://www.cnes.fr/web/CNES-en/6361-controlling-the-atv-from-toulouse.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;ATV-CC&lt;/a&gt; in France.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That means control centers must be interlocked to work on activities such the ISS missions. More on MCCs &lt;a href=&quot;http://www.quora.com/How-do-mission-control-centers-work&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/eft1_mcc_staffing.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;EFT-1&lt;/a&gt; was an opportunity to discover a part of MCC-21. Unfortunately no image displays a legible content, as you already stated. The best I could find:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FH0XJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://pbs.twimg.com/media/B1zq0JfIUAElpZn.jpg:large&quot; rel=&quot;nofollow noreferrer&quot;&gt;[Large image]&lt;/a&gt;.&#xA;(Source: &lt;a href=&quot;https://twitter.com/ShuttleAlmanac/status/530557040230690817/photo/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Twitter / SpaceShuttleAlmanac&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tslOH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://i.warosu.org/data/sci/img/0065/67/1401692389329.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;[Large image]&lt;/a&gt;.&#xA;(Source: &lt;a href=&quot;https://warosu.org/sci/thread/6567899&quot; rel=&quot;nofollow noreferrer&quot;&gt;Warosu.org&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wQVqe.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://bgbwp.malibulist.com/wp-content/uploads/2013/07/NASA-070.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;[Large image]&lt;/a&gt;.&#xA;(Source: &lt;a href=&quot;http://www.bobgreenberger.com/index.php/2013/07/02/feeling-like-a-vip-at-nasa/&quot; rel=&quot;nofollow noreferrer&quot;&gt;BobGreenberger.com&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/PVlMf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://1.bp.blogspot.com/-cRYtbgII-hQ/Urh8NTrWywI/AAAAAAAAJeI/zY-iNeWeGV4/s1600/2013-12-08+10.35.03.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;[Large image]&lt;/a&gt;.&#xA;(Source: &lt;a href=&quot;http://www.ohsnaplizzie.com/2013/12/houston-citypass-adventure-1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;OhSnapLizzie.com&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/users/49/tildalwave&quot;&gt;TidalWave&lt;/a&gt; provides this link to &lt;a href=&quot;http://spacestationlive.jsc.nasa.gov/displays/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS MCC&lt;/a&gt; consoles. This is a live content, definitely invaluable. As an example, the &lt;a href=&quot;http://spacestationlive.nasa.gov/educators/Physics-MC-ADCO-MC.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;CMG system&lt;/a&gt; view, providing CMG status and active momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/f38DK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Thanks for the idea!)&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-01-03T14:31:31.937" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6655" PostTypeId="1" CreationDate="2015-01-04T05:25:17.830" Score="7" ViewCount="1733" Body="&lt;p&gt;Does anyone have quantitative information on the delta-v overhead involved with low thrust interplanetary transfers? Looking at accelerations of roughly 0.00032 m/s/s from LEO to the Earth-Mars region. Paper references or handbooks would be useful.&lt;/p&gt;&#xA;" OwnerUserId="7106" LastEditorUserId="168" LastEditDate="2015-01-04T10:05:55.830" LastActivityDate="2016-08-24T18:38:54.347" Title="Low Thrust Interplanetary Transfers" Tags="|mars|spacecraft|trajectory|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="6656" PostTypeId="2" ParentId="6655" CreationDate="2015-01-04T08:31:10.663" Score="3" Body="&lt;p&gt;To get the delta V of a slow, low thrust spiral from one circular orbit to another coplanar circular orbit, use the difference between the speeds of the two circular orbits. &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Low-thrust_transfer&quot; rel=&quot;nofollow&quot;&gt;See this Wikipedia article&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speed of earth orbit is about 30 km/s. Mars is about 24 km/s. Delta V would be (30-24) km/s, about 6 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEO to escape would be a 7.7 km/s orbit to a 0 km/s orbit at infinity. Though you wouldn't have to go infinity for escape, more like just the edge of earth's sphere of influence about a million kilometers out. So I'd say more like 7 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low Mars orbit is about 3.4 km/s. Spiraling in from the edge of Mars sphere of influence to low Mars orbit would take around 3 km/s, I believe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;7+6+3 is 16. 16 km/s is my guestimate for a very low thrust trip from Low Earth Orbit to Low Mars Orbit.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-08-22T03:12:46.220" LastActivityDate="2016-08-22T03:12:46.220" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6657" PostTypeId="2" ParentId="4301" CreationDate="2015-01-04T08:31:58.653" Score="6" Body="&lt;p&gt;Iron and aluminium are very common on Earth, but what makes them mineable is that at certain locations each metal is concentrated in sufficient quantities to make mining viable. The difference between a mineral deposit and an orebody is economics. A deposit containing 15 percent iron is uneconomic, but one containing 66 percent is &lt;strong&gt;potentially&lt;/strong&gt; economic, depending on the quantities of any associated undesirable elements (gangue minerals) such as aluminium silicates and phosphorus. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the issues of smelting Fe and Al in Space is what minerals of each metal will be smelted. Different mineralogies dictate different smelting methods. Most open cut mines in Brazil and Australia produce an iron ore blended product of about 60 percent iron. Sweden mines iron ore from underground which contains 35 percent iron. This is because they are mining different iron minerals: mainly haematite rich ores in Brazil and Australia and predominantly magnetite in Sweden.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Space, mineralogies may be discovered that are unknown on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The general process to get smelted metal is:&#xA;Mine minerals containing the metal&#xA;Concentrate the minerals required and remove as much of the gangue minerals as possible&#xA;Smelt the concentrated minerals to produce metal&#xA;Each step produces its own quantities of waste and product. If smelting is undertaken in Space, what is to be done with the waste products of smelting? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is most likely that any concentrated forms of metals, available for smelting, will be found in clumps similar to the asteroids of our solar system or on planets or moons. For any space based smelter, transporting the raw materials will require a lot of effort, equipment and energy. Having a planet/moon based smelter for planet/moon minerals would be the most feasible option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other things to consider are why do you want to establish a Space based iron or aluminium smelter? What and where will its product be used? Will the smelter be operated on a continuous basis or a campaign basis during its production life and what will be the production life of the smelter? Do you expect the organization owing the smelter to operate it at a profit? Or will it be a government owned item of infrastructure that will be solely used for the advancement of science?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One issue specifically for smelting aluminium is that with our current Earth based smelting techniques a lot of electricity is required and it wasn't until electricity was produced on an industrial scale that aluminium became more widely available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe 3D printing could be an option for iron and aluminium as it is for titanium now: &lt;a href=&quot;http://www.smh.com.au/technology/sci-tech/csiro-builds-sophie-a-dragon-20140110-30lpf.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.smh.com.au/technology/sci-tech/csiro-builds-sophie-a-dragon-20140110-30lpf.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-01-04T08:31:58.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6658" PostTypeId="1" AcceptedAnswerId="6659" CreationDate="2015-01-04T12:31:03.100" Score="6" ViewCount="2236" Body="&lt;p&gt;After watching &lt;a href=&quot;http://en.wikipedia.org/wiki/Chris_Hadfield&quot; rel=&quot;nofollow&quot;&gt;&lt;strong&gt;Chris Hadfield&lt;/strong&gt;&lt;/a&gt; make a &lt;a href=&quot;https://www.youtube.com/watch?v=f8-UKqGZ_hs&quot; rel=&quot;nofollow&quot;&gt;&lt;strong&gt;Space Burrito&lt;/strong&gt;&lt;/a&gt;, I saw his bits and pieces floating away here and there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inside the ISS, and any other manned craft, there is Oxygen (and other gases) which is obviously to keep them alive. On Earth, when someone throws a ball, it's not only pulled down by gravity, but is slowed down because of drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, the ISS is &lt;em&gt;also&lt;/em&gt; pulled down towards Earth indefinitely because of &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_decay&quot; rel=&quot;nofollow&quot;&gt;&lt;strong&gt;Orbital Decay&lt;/strong&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I was to slightly prod that Space Burrito inside a manned ship in any &quot;direction&quot;, will it eventually slow down to the point where it's moving relative to the ship itself because of the &lt;em&gt;Oxygen&lt;/em&gt; (and again, other gases) which is creating drag against it?&lt;br&gt;&#xA;Or will it continue floating until it hits the wall?&lt;/p&gt;&#xA;" OwnerDisplayName="user7078" LastEditorDisplayName="user7078" LastEditDate="2015-01-04T21:03:01.087" LastActivityDate="2015-01-05T09:57:30.113" Title="Is there drag inside a Space craft?" Tags="|iss|spacecraft|atmospheric-drag|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6659" PostTypeId="2" ParentId="6658" CreationDate="2015-01-04T13:04:11.663" Score="16" Body="&lt;p&gt;There is air inside the space station so, yes, anything moving within that air will experience air resistance.  Whether that air resistance is enough to stop a thrown object before it hits the wall depends entirely on the object and how hard it is thrown. If the object is very aerodynamic (i.e., experiences little air resistance) and/or is thrown hard, it will hit the wall. If the object is very unaerodynamic and/or is thrown gently, it may well stop before it hits the wall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an equivalent experiment you could do on earth.  If you start throwing burritos around in your kitchen, some of them will hit the floor because of gravity.  To take that out of the equation, suppose you have two very tall towers, close enough together that you could throw a burrito from one to the other.  The idea is that the ground is so far away that all the interesting stuff will happen before any burrito splats on the pavement.  Assume there's no wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you stand on top of one of the towers and throw your burrito horizontally and hard enough it will splat on the other tower.  If throw it a little more gently, it will still hit the other tower, but lower down.  However, if you throw it significantly more gently, air resistance will slow its horizontal velocity to zero before it reaches the other tower and, from that point, it will fall vertically and splat on the pavement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, suppose that you jumped off your tower at the moment you throw the burrito. Let's assume that you are also a burrito so you both fall at the same rate.  What would you see? Well, in the first two cases, you'd see the other burrito moving horizontally away from you, slowing down until it hits the wall. In the third case, you'd see it slow down and eventually stop without hitting the other wall, exactly as Chris Hadfield would see in the ISS.  Think about that for a moment but don't forget to open your parachute before you hit the ground.&lt;/p&gt;&#xA;" OwnerUserId="2784" LastActivityDate="2015-01-04T13:04:11.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6660" PostTypeId="2" ParentId="6658" CreationDate="2015-01-04T13:05:24.080" Score="4" Body="&lt;p&gt;Yes it experiences drag. The burrito is also experiencing the pull of gravity but due to it's velocity relative to the ISS this would be equal and hence unnoticeable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will it hit the wall? That depends on how hard you throw it! You can work out how far it will travel by calculating the &lt;a href=&quot;http://en.wikipedia.org/wiki/Drag_%28physics%29&quot; rel=&quot;nofollow&quot;&gt;drag force&lt;/a&gt; and then &lt;a href=&quot;http://www.physics.usyd.edu.au/~cross/TRAJECTORIES/Trajectories.html&quot; rel=&quot;nofollow&quot;&gt;integrating through time&lt;/a&gt;. If it requires a longer distance to stop than the longest inside length of the ISS then it'll hit the wall.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-01-04T13:05:24.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6662" PostTypeId="1" AcceptedAnswerId="6665" CreationDate="2015-01-04T14:22:59.787" Score="12" ViewCount="1947" Body="&lt;p&gt;How is it possible for a ion thruster powered spacecraft like &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_%28spacecraft%29&quot;&gt;Dawn&lt;/a&gt; to decelerate to orbit Ceres? Does it have a secondary engine to accomplish this task?  &lt;/p&gt;&#xA;" OwnerUserId="7108" LastEditorUserId="49" LastEditDate="2015-01-04T14:42:57.210" LastActivityDate="2015-03-03T02:05:49.217" Title="How will the ion thruster powered Dawn spacecraft enter orbit around Ceres?" Tags="|orbital-maneuver|advanced-propulsion|ion-thruster|ceres|dawn-mission|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6663" PostTypeId="2" ParentId="6189" CreationDate="2015-01-04T15:25:24.227" Score="8" Body="&lt;p&gt;This NASA technical report seems to correspond to your focus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19720025304&quot;&gt;Project management in the Apollo program: An interdisciplinary study&lt;/a&gt;&lt;br&gt;&#xA;Drucker, Pooler, Wilemon, Wood.&lt;br&gt;&#xA;Publication Date: Jan 01, 1972&lt;br&gt;&#xA;Document ID: 19720025304  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Findings concerning project management in the NASA Apollo program are&#xA;  presented. The Apollo program in the context of the total NASA&#xA;  organization is examined along with the nature of project management&#xA;  and the manner in which project managers functioned in the Apollo&#xA;  program. The utilization of the in-house technical competence in the&#xA;  support of the Apollo program, and the formal and informal&#xA;  relationships between Apollo managers and the contractors are&#xA;  discussed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is a list of references that seems also interesting (appendix B, starting page 165).  &lt;/p&gt;&#xA;" OwnerUserId="7017" LastActivityDate="2015-01-04T15:25:24.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6664" PostTypeId="2" ParentId="6662" CreationDate="2015-01-04T15:40:32.743" Score="3" Body="&lt;p&gt;Dawn indeed uses its ion thrusters to enter orbit. From &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2011index/IEPC-2011-326.pdf&quot; rel=&quot;nofollow&quot;&gt;this PDF&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An additional 210 hours of IPS thrusting was used to enter the first Vesta science orbit, called Survey orbit, on August 3, 2011 at an altitude of approximately 2,735 km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="472" LastActivityDate="2015-01-04T15:40:32.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6665" PostTypeId="2" ParentId="6662" CreationDate="2015-01-04T16:38:56.340" Score="21" Body="&lt;p&gt;Does it have any additional thrusters? Not to thrust towards its targets. For that, it's &lt;a href=&quot;http://dawn.jpl.nasa.gov/mission/ion_prop.asp&quot; rel=&quot;noreferrer&quot;&gt;100% ion thruster propelled&lt;/a&gt;. It does also have a set of 12 &lt;a href=&quot;http://www.astronautix.com/engines/mr103.htm&quot; rel=&quot;noreferrer&quot;&gt;MR-103G&lt;/a&gt; variable thrust (0.9 N maximum) RCS (Reaction Control System) &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrazine&quot; rel=&quot;noreferrer&quot;&gt;hydrazine&lt;/a&gt; monopropellant thrusters that launched with only 46 kg of propellants (read: total thrust of its RCS doesn't provide the spacecraft with nearly enough velocity change to control its trajectory with them), and it also uses reaction wheels (now two out of four still operational) as well as its three gimballed Xenon ion thrusters for &lt;a href=&quot;http://dawn.jpl.nasa.gov/technology/navigating.asp&quot; rel=&quot;noreferrer&quot;&gt;attitude control&lt;/a&gt;. Actually, even its huge solar panels are rotatable, gimballed, even foldable / extendable, so it could technically also use that to change its orientation through change in center of mass, if all else failed and the power they provide at less than ideal angle towards the Sun would be a secondary concern. But only ion thrusters are used to provide the spacecraft with required change in velocity relative to its targets.&lt;/p&gt;&#xA;&lt;p&gt;How is it possible? Very gracefully. Low-thrust but high specific impulse (&lt;span class=&quot;math-container&quot;&gt;$I_\mathrm{sp}$&lt;/span&gt;) propulsion calls for slightly different trajectories than high-thrust near-instantaneous (impulse) but lower &lt;span class=&quot;math-container&quot;&gt;$I_\mathrm{sp}$&lt;/span&gt; thrusters. Their downside is that they are, as such, unsuitable for high-thrust applications such as launching off the surface of strong gravity bodies, landing, or any other applications where large change in velocity (&lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;) is required in a short period of time (read: fast acceleration / deceleration), but their upside is that they are highly efficient with their reaction mass (Dawn uses only about a kilo of its Xenon propellant per week with all three of its ~ 90 mN thrusters), and can reach far greater total &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; throughout their mission.&lt;/p&gt;&#xA;&lt;p&gt;Dawn has &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2014-443&quot; rel=&quot;noreferrer&quot;&gt;recently began its approach to Ceres&lt;/a&gt; and will continue to thrust after leaving Vesta (with a slight delay due to problems with one of its reaction wheels) to roughly match the orbit of Ceres in speed, inclination, eccentricity, and do that so it also coincides with its orbital position around March, 2015. Its final approach trajectory has since also been changed (read: optimized) in a way that will allow it to circularize initial high orbit around Ceres faster, and use Ceres' own gravity on top of its ion thrusters to gracefully &amp;quot;fall&amp;quot; in a spiral path closer towards it. To do this, it will have slight hyperbolic excess velocity (&lt;span class=&quot;math-container&quot;&gt;$v_\infty$&lt;/span&gt;) with respect to Ceres as it passes it in its heliocentric orbit, and will then begin slowly to decelerate with respect to it. From one of Dawn journals (&lt;a href=&quot;http://dawnblog.jpl.nasa.gov/2014/11/28/dawn-journal-november-28/&quot; rel=&quot;noreferrer&quot;&gt;November 28, 2014&lt;/a&gt;):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Tzh3C.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;North is at the top of this figure and the sun is far to the left.&#xA;Ceres orbital motion around the sun carries it straight into the&#xA;figure. The original approach took Dawn over Ceres’ south pole as it&#xA;spiraled directly into RC3. On the new approach, it looks here as if&#xA;it flies in over the north pole, but that is because of the flat&#xA;depiction. As the previous figure shows, the approach takes Dawn well&#xA;ahead of Ceres. The upper part of the green trajectory is not in the&#xA;same plane as the original approach and RC3; rather, it is in the&#xA;background, “behind” the graphic. As Dawn flies to the right side of&#xA;the diagram, it also comes forward to the plane of the figure to align&#xA;with the targeted RC3. As before, the circles, spaced at intervals of&#xA;one day, indicate the spacecraft’s speed; where they are closer&#xA;together, the ship travels more slowly. (You can think of this&#xA;perspective as being from the side and the previous figure as showing&#xA;the view from above, off the top of this graphic.) Credit: NASA/JPL&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;New trajectory, by approaching Ceres from ahead of its orbital path, was chosen to reduce eccentricity of resulting orbit after orbital injection faster. Exact approach to Ceres, insertion into its high orbit (13,500 km), and later spiraling to a lower (4,430 km) orbit are all described in detail in &lt;a href=&quot;http://dawnblog.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Dawn Blog&lt;/a&gt; articles.&lt;/p&gt;&#xA;&lt;p&gt;But the point is that all these changes in Dawn's trajectory are made far in advance to being within the Cererean &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;noreferrer&quot;&gt;Hill sphere&lt;/a&gt; (area around it where its gravitational attraction dominates over gravitational attraction of other celestial bodies) where large changes to its trajectory would be more prohibiting due to the time it would take to make them and not being able to counter gravitational pull (say, if trajectory was way off) with such low thrust available to it. So it approaches celestial bodies with grace, and such words as &amp;quot;celestial ballet&amp;quot; were used to describe this by Marc Rayman, Dawn Chief Engineer/ Mission Director.&lt;/p&gt;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;https://www.youtube.com/watch?v=WLu4P4goZ1s&amp;amp;t=79s&quot; rel=&quot;noreferrer&quot;&gt;simulation video&lt;/a&gt; of Dawn's final approach and injection into orbit around Ceres, just published by NASA Jet Propulsion Laboratory (JPL) via &lt;a href=&quot;https://www.youtube.com/channel/UCryGec9PdUCLjpJW2mgCuLw&quot; rel=&quot;noreferrer&quot;&gt;their YouTube channel&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/WLu4P4goZ1s?start=79&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Edit to add&lt;/strong&gt;: For intricacies of Dawn's propulsion system and its &lt;em&gt;celestial ballet&lt;/em&gt; through the Solar system and orbits around Vesta and Ceres, I can't recommend warmly enough watching &lt;a href=&quot;http://www.jpl.nasa.gov/events/lectures_archive.php?year=2014&amp;amp;month=12&quot; rel=&quot;noreferrer&quot;&gt;The von Kármán Lecture on NASA's Dawn Mission to the Asteroid Belt&lt;/a&gt;. Dr. Rayman is a fantastic speaker and I'm sure his explanations will pull you in a lot better than I'm capable of. He answers &lt;a href=&quot;https://www.youtube.com/watch?v=Qa5fQ0dkOW0&amp;amp;t=3825s&quot; rel=&quot;noreferrer&quot;&gt;a similar question to yours&lt;/a&gt; in the Q&amp;amp;A session following the lecture, roughly 1 hour and 3 and a half minutes into the recording.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h3&gt;Lecture: &lt;a href=&quot;http://www.jpl.nasa.gov/events/lectures_archive.php?year=2014&amp;amp;month=12&quot; rel=&quot;noreferrer&quot;&gt;NASA's Dawn Mission to the Asteroid Belt&lt;/a&gt;&lt;/h3&gt;&#xA;&lt;p&gt;&lt;em&gt;Friday, December 5&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;The ambitious and exciting Dawn mission is one of NASA's most&#xA;remarkable ventures into the solar system. After more than seven years&#xA;of interplanetary spaceflight, the probe is just a few months away&#xA;from the mysterious world Ceres. The spacecraft has already completed&#xA;a spectacular exploration of Vesta. These were among the last&#xA;uncharted worlds in the inner solar system prior to Dawn. They are the&#xA;two most massive residents of the main asteroid belt, that vast&#xA;collection of bodies between Mars and Jupiter. Ceres is so large that&#xA;it is included in the category of dwarf planets, along with Pluto. The&#xA;alien landscapes Dawn reveals provide humankind with a new perspective&#xA;on the solar system. Remnants from the time that planets were formed,&#xA;Ceres and Vesta hold clues that will help scientists understand the&#xA;dawn of the solar system. Dawn orbited Vesta from July 2011 to&#xA;September 2012 and returned astonishing views of this fascinating&#xA;world. It is the only spaceship ever to orbit an object in the&#xA;asteroid belt and is the first ever targeted to orbit any two&#xA;extraterrestrial destinations. Such a mission would be truly&#xA;impossible without the use of ion propulsion, a technology that has&#xA;mostly been in the domain of science fiction. Dr. Marc Rayman will&#xA;give a fascinating and entertaining presentation on the Dawn mission&#xA;and its two exotic destinations as well as its use of ion propulsion.&#xA;He also will share the excitement and profundity of controlling a&#xA;robotic ambassador from Earth in deep space.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Speaker&lt;/strong&gt;: Dr. Marc Rayman, Dawn Project Mission Director&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Qa5fQ0dkOW0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-03-03T02:05:49.217" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6666" PostTypeId="1" AcceptedAnswerId="6668" CreationDate="2015-01-04T16:41:17.743" Score="7" ViewCount="358" Body="&lt;p&gt;In video documents where astronauts are in micro-gravity, we often see them moving objects without paying attention for the nearby instruments or buttons panels (either aboard spacecrafts or aboard the ISS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2656466/World-Cup-fever-hits-ISS-Astronauts-microgravity-football-skills-SPACE.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astronauts playing football&lt;/a&gt; (nice actions)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/spjhK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why don't they fear those objects will move some switch or unplug some connector, due to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Their initial speed provided by the astronaut.&lt;/li&gt;&#xA;&lt;li&gt;A later impact between the object and a moving astronaut.&lt;/li&gt;&#xA;&lt;li&gt;A later acceleration from the CMG or a re-boost from a Soyuz.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I believe the kinetic energy of these objects is sufficient for such involuntary action.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-01-04T16:46:33.313" LastActivityDate="2015-01-04T18:01:48.417" Title="Playing football in microgravity: is it hazardous for instruments?" Tags="|microgravity|impact|movement|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6667" PostTypeId="2" ParentId="2648" CreationDate="2015-01-04T17:03:47.040" Score="-1" Body="&lt;p&gt;It would be armed prior to destruction, not as part of launch sequence but only in emergency.&#xA;Don't forget that, in theory, it's possible to bail out before the RSO (Range Safety Officer) initiates self-destruct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if the crew see this light, they can always try to bail out. And astronauts should be always informed about everything, just like airplane pilots and soldiers on battlefield. &lt;/p&gt;&#xA;" OwnerUserId="7110" LastEditorUserId="49" LastEditDate="2015-01-04T17:18:16.247" LastActivityDate="2015-01-04T17:18:16.247" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6668" PostTypeId="2" ParentId="6666" CreationDate="2015-01-04T17:51:37.497" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why don't they fear those objects will move some switch or unplug some&#xA;  connector?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Because none of the crucial switches aboard the station are installed without some sort of a safety mechanism, require multiple different actions to initiate them or aren't as easy to move out of position (such as rotating levers for bulkhead doors - sometimes, they would even be intentionally missing and you'd require a torque wrench to rotate it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image you attach to your question displays a few of such initiators. Some are recessed into the structure, include safety pins, are covered by protective (non-shattering, say plexi or laminated) glass or in own enclosure with access doors, need to be pressed or pulled then turned to lock in position, and none are touch or push initiated alone. And console controlled equipment can implement all kinds of other checks, including multiple confirmation buttons, entering access codes, need to be confirmed by additional crew, or as simple as checking with other system settings if a change to some of them even makes sense from a procedural standpoint or doesn't breach some safety standards and regulations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, crew would be required to secure any equipment that could withstand damage due to any of their activities, as soon as they finish with their work on them. The so-called &lt;em&gt;clean desk policy&lt;/em&gt; requirements are constantly in effect. These are pretty standard health and safety requirements in any hazardous work environments, and would be combined with a plethora of other regulations (the stations is still international and not all of its members would be allowed even physical access to just any of its consoles, and so on). Also, reboosts are scheduled and initiated by the crew aboard the station. They would all know when it's time to hold onto something and mind that sucker in the toilet because it'll soon go temporarily out of pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if astronauts forget themselves or get too excited during some such activities, they're also constantly monitored by ground control (not as intrusive as it might sound, but in common crew quarters and laboratories there are cameras, microphones and so on ... but they do get to do number one, number two and sleep without constant CCTV monitoring), and they can always remind them to play nice, no foul-play and to mind that rack with lettuce. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-04T18:01:48.417" LastActivityDate="2015-01-04T18:01:48.417" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6669" PostTypeId="1" AcceptedAnswerId="6671" CreationDate="2015-01-05T07:40:05.103" Score="5" ViewCount="1526" Body="&lt;p&gt;Is this the current configuration of the ISS (ignoring visiting vehicles)?&#xA;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0VEZw.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;(source: &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/ISS_configuration_2011-05_en.svg/1257px-ISS_configuration_2011-05_en.svg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/STS-134&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-134/UL6&lt;/a&gt; delivered the last elements.  &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Nauka_%28ISS_module%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nauka&lt;/a&gt; still to launch.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/kvwRl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;(source: &lt;a href=&quot;http://spaceflight.nasa.gov/gallery/images/station/issartwork/hires/jsc2011e072139.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nasa&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SqyLU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;(source: &lt;a href=&quot;http://i.space.com/images/i/000/010/101/original/photo-shuttle-endeavour-space-station-tilt-belly-sts134.jpg?1307485436&quot; rel=&quot;nofollow noreferrer&quot;&gt;space.com&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-01-05T08:06:22.407" LastActivityDate="2015-01-08T21:54:12.970" Title="Current ISS configuration (Dec. 2014)?" Tags="|iss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6670" PostTypeId="2" ParentId="6658" CreationDate="2015-01-05T09:57:30.113" Score="1" Body="&lt;p&gt;From the other answers I make out that if the speed changes, the drag force changes quadratically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without going into the calculations, I believe that it means the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Yes, the air slows down the object, and keeps slowing it down&lt;/li&gt;&#xA;&lt;li&gt;No, the object does not ever have to stop due to drag force as the decrease in speed decreases too fast.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;My intuition: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the wall is far enough it may never be reached, but the object will keep moving forever.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;It would be nice if someone could expand or refute this answer with a calculation.&lt;/p&gt;&#xA;" OwnerUserId="7113" LastActivityDate="2015-01-05T09:57:30.113" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6671" PostTypeId="2" ParentId="6669" CreationDate="2015-01-05T11:18:24.747" Score="4" Body="&lt;p&gt;In terms of modules, and major components, mostly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX CRS-7 and CRS-8 are supposed to bring up add ons for the PMA docking adapaters to convert them to NDS (NASA Docking System) standard. Another SpaceX CRS will bring up the Bigelow Experimental Module (BEAM) in 2016 or so to dock to Node 3 aft facing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They will be moving some of the US modules around to free up the Node 1 nadir port as CBM/berthing backup for cargo as they will be moving PMA-3 to Node 2 zenith to server Commercial crew as a backup docking port.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see some good description and pictures in the answer to: &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;Will the ISS need more docking ports?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MPLM (now renamed PLM) that is docked to Node 1 nadir has to be moved to forward facing port on Node 3 so that the second CBM port can be available as a backup for crew, once the zenith Node 2 port is taken up the PMA for Commercial Crew. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russians are all over the place on what they will actually deliver for real.  Nauka, UM (Docking ball module), the rest, or take their toys and use them to start a new station. They fluctuate quite a bit on their planning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact your image (as you called out) has Nauka docked on nadir port of Zvezda, with Pirs discarded already. But that has actually not yet happened.  Pirs was supposed to be discarded in March 2015 last I heard, but that is on delay due to issues with Nauka. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-01-08T21:54:12.970" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6672" PostTypeId="1" CreationDate="2014-12-24T01:44:48.470" Score="33" ViewCount="1338" Body="&lt;p&gt;I was wondering for some time how we know that the scientific data received by the Voyager spacecrafts are correct?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, I'm wondering about &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2014/16dec_voyagercme/&quot;&gt;the recent &quot;tsunami-wave&quot;&lt;/a&gt; (frequency of ionized matter) detected by Voyager1 - couldn't it just be a malfunction of a 37 years old instrument?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or also the impressive data about the edge of our solar system, which were recorded in &lt;a href=&quot;http://www.nature.com/nature/journal/v474/n7351/full/nature10115.html&quot;&gt;2011&lt;/a&gt; (they detect a linear decrease of ion velocity over a few years) and in &lt;a href=&quot;http://www.sciencemag.org/content/341/6142/144.full&quot;&gt;2012&lt;/a&gt; (where they show that they found - unexpected - that even with rotating the spacecrafts plasma sensor, they find no flow). Could this also be explained by a broken plasma detector which lost sensitivity over the time of a few years, up to zero?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not questioning these results, I'm sure NASA scientists do an excellent job, I just want to understand how they distinguish between those scenarios. Those data are important as it would take quite a few years until they can be verified by other spacecrafts (when will voyager 2 reach that level? Or New Horizons?) - and until that time, Voyager1's data are a main-source for our understanding of the edge of the solar system.&lt;/p&gt;&#xA;" OwnerUserId="7118" OwnerDisplayName="NicoDean" LastEditorUserId="7118" LastEditDate="2015-01-06T03:34:54.923" LastActivityDate="2019-08-15T05:16:38.090" Title="How do we know that Voyager's data is correct?" Tags="|probe|voyager|solar-system|scientific-data|reliability|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6673" PostTypeId="1" AcceptedAnswerId="6674" CreationDate="2014-12-29T11:52:00.893" Score="8" ViewCount="1521" Body="&lt;p&gt;Voyager 1 and 2 have been gone for a LONG time now (decades) and presumably we can produce bigger, faster and better probes these days that could tell us a lot more about the heliosphere. I've not found any articles or pieces about any planned missions to exit the solar system in the future.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So was that it? Voyager 1 and 2 were our only efforts? Or are there other missions planned to exit the system? I'm not talking about &lt;a href=&quot;http://en.wikipedia.org/wiki/Kepler_%28spacecraft%29&quot;&gt;Kepler&lt;/a&gt; which to its credit is locating extra-solar planets and systems, but more about craft which will actually &lt;em&gt;exit&lt;/em&gt; the solar system as we know it.&lt;/p&gt;&#xA;" OwnerUserId="328" OwnerDisplayName="Mark Mayo" LastEditorUserId="49" LastEditDate="2015-01-05T15:59:30.457" LastActivityDate="2015-01-05T18:17:44.320" Title="Are there any planned extra-solar missions by NASA or others?" Tags="|voyager|future-missions|planning|interstellar-travel|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6674" PostTypeId="2" ParentId="6673" CreationDate="2014-12-29T13:53:39.147" Score="16" Body="&lt;p&gt;You are forgetting Pioneer 10, Pioneer 11, and New Horizons. Pioneers 10 and 11 respectively operated for 28 years and 16 years after completing their primary missions. New Horizons, which has yet to perform its primary mission (Pluto flyby in July 2015) or secondary mission (TBD flyby of a Kuiper belt object) will reach the heliopause in about 2047.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No future extra-solar missions are currently planned by NASA or by ESA. There were some initial investigations toward a New Horizons follow-on, but nothing substantive came of those investigations. It was deemed better to wait for New Horizons to successfully complete its primary mission before even starting to consider a follow-on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's too little science return compared to the expense to justify making escaping the solar system the primary mission of a spacecraft. The extra-solar missions of those five escaping satellites were/are tertiary missions. The Voyager missions would have been deemed a complete success had both those spacecraft failed immediately after completing their grand tours. &lt;/p&gt;&#xA;" OwnerUserId="2752" OwnerDisplayName="David Hammen" LastActivityDate="2014-12-29T13:59:45.160" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6676" PostTypeId="2" ParentId="6673" CreationDate="2015-01-05T15:25:04.557" Score="3" Body="&lt;p&gt;As David Hammen stated in his previous answer, neither NASA, ESA or any other space program has any extra-solar mission in the works. However, there are many studies of interstellar and interstellar precursor missions, some of them ongoing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps the best known and most advanced interstellar probe is &lt;a href=&quot;http://www.icarusinterstellar.org/&quot; rel=&quot;nofollow&quot;&gt;Project Icarus&lt;/a&gt; &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Icarus_(interstellar)&quot; rel=&quot;nofollow&quot;&gt;(wiki)&lt;/a&gt;. This is a modern redesign of an earlier project from the 70s known as &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Daedalus&quot; rel=&quot;nofollow&quot;&gt;Project Daedelus&lt;/a&gt;. Icarus Interstellar is a collaborative project of the &lt;a href=&quot;http://www.tauzero.aero/&quot; rel=&quot;nofollow&quot;&gt;Tau Zero Foundation&lt;/a&gt; and the &lt;a href=&quot;http://en.wikipedia.org/wiki/British_Interplanetary_Society&quot; rel=&quot;nofollow&quot;&gt;British Interplanetary Society&lt;/a&gt;, the latter of which came up with Daedelus. Both ship designs rely on nuclear propulsion to attain an appreciable fraction of the speed of light in order to reach a close star system within a century.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA produced a study on what they called the &lt;a href=&quot;http://en.wikipedia.org/wiki/Innovative_Interstellar_Explorer&quot; rel=&quot;nofollow&quot;&gt;Innovative Interstellar Explorer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This study focused on the elusive quest to reach and measure the interstellar medium, the region outside the influence of the nearest star, the Sun. It proposes to use a radioisotope thermal generator with ion thrusters.&lt;a href=&quot;http://interstellarexplorer.jhuapl.edu/mission/implementation.html&quot; rel=&quot;nofollow&quot;&gt;(1)&lt;/a&gt; The project is a study of a proposed interstellar precursor mission that would probe the nearby interstellar medium and measure the properties of magnetic fields and cosmic rays and their effects on a craft leaving the Solar System.&lt;a href=&quot;http://www.space.com/businesstechnology/050706_star_voyage.html&quot; rel=&quot;nofollow&quot;&gt;(2)&lt;/a&gt;  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Using an ion drive with xenon propellant, it could attain 1000 AU within 100 years of launch (updates &lt;a href=&quot;http://www.centauri-dreams.org/?p=21050&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://www.centauri-dreams.org/?p=21312&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, slide show &lt;a href=&quot;http://www.niac.usra.edu/files/library/meetings/misc/trieste_may02_mtg/McNutt_Ralph.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another interstellar precursor probe is &lt;a href=&quot;http://www.centauri-dreams.org/?p=785&quot; rel=&quot;nofollow&quot;&gt;Project FOCAL&lt;/a&gt;, a proposal by Italian physicist &lt;a href=&quot;http://en.wikipedia.org/wiki/Claudio_Maccone&quot; rel=&quot;nofollow&quot;&gt;Claudio Maccone&lt;/a&gt;. The idea is to send a probe out to 550 AU and beyond in order to use the Sun as a &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravitational_lens&quot; rel=&quot;nofollow&quot;&gt;gravitational lens&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would be remiss if I did not mention the &lt;a href=&quot;http://en.wikipedia.org/wiki/100_Year_Starship&quot; rel=&quot;nofollow&quot;&gt;100 Year Starship Project&lt;/a&gt;, headed by American physician and former NASA astronaut &lt;a href=&quot;http://en.wikipedia.org/wiki/Mae_Jemison&quot; rel=&quot;nofollow&quot;&gt;Mae Jemison&lt;/a&gt;. This is a DARPA sponsored project to come up with a business plan for research on interstellar travel over the next hundred years.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2015-01-05T18:17:44.320" LastActivityDate="2015-01-05T18:17:44.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6677" PostTypeId="1" AcceptedAnswerId="6678" CreationDate="2015-01-05T17:34:35.663" Score="11" ViewCount="1582" Body="&lt;p&gt;What is the next excellent year for a interstellar mission (beyond Neptune). The main acceleration for those missions are gravity-assists with Jupiter, Saturn, Uranus and Neptune.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 2 managed the gravity-assist for each of the 4 gas-giants, Voyager 1 used only Jupiter and Saturn. New Horizons, even through it started at a significantly higher speed, will never reach Voyager 2 because it only visited Jupiter (see &lt;a href=&quot;https://space.stackexchange.com/questions/3520/when-will-new-horizons-overtake-voyager-1-in-distance-from-the-sun&quot;&gt;the related question here&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder what is the next year with an ideal constellation of (at least) the four gas giants to speed up a space-craft, for reaching interstellar space faster?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A safe upper limit should be ($OP$=Orbital Period in years) $$t =OP_{\text{Jupiter}} \times OP_{\text{Saturn}} \times OP_{\text{Uranus}} \times OP_{\text{Neptune}} ~= 4.8 \text{ million years}$$. That's the maximum time needed for reaching the exact same configuration. But of course space-crafts are flexible, thus the least common multiple should be much smaller.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in the next good year (and also on how to calculate that).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Added later:&lt;/strong&gt; Part of the question has already been answered &lt;a href=&quot;https://space.stackexchange.com/questions/5075/when-is-the-next-outer-planet-lineup-voyager&quot;&gt;here&lt;/a&gt;, and also &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow noreferrer&quot;&gt;on Wikipedia on gravity assist&lt;/a&gt;. I would like to understand how to calculate the year (&lt;em&gt;teach a man how to fish...&lt;/em&gt;), and what the years for three-planet line-up is.&lt;/p&gt;&#xA;" OwnerUserId="7118" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-01-07T03:18:03.280" Title="Good year for start of an interstellar mission, due to gravity-assist" Tags="|voyager|future-missions|gravity-assist|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="6678" PostTypeId="2" ParentId="6677" CreationDate="2015-01-05T18:32:14.587" Score="10" Body="&lt;p&gt;An &lt;strong&gt;interstellar&lt;/strong&gt; mission does not need to visit more than two planets, since your interest by definition lies in objects beyond the Solar System. The Voyagers were first and foremost planetary exploration craft, albeit with symbolic messages to alien civilizations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I wrote in an &lt;a href=&quot;https://space.stackexchange.com/a/971/168&quot;&gt;answer&lt;/a&gt; to another question, the best trajectory (a.k.a. the &lt;a href=&quot;https://en.wikipedia.org/wiki/Krafft_Arnold_Ehricke&quot; rel=&quot;nofollow&quot;&gt;Krafft Arnold Ehricke&lt;/a&gt; trajectory) to get out of Solar System with the highest velocity $V_\infty$ will have to use:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A gravity assist from Saturn&lt;/li&gt;&#xA;&lt;li&gt;A gravity assist from Jupiter&lt;/li&gt;&#xA;&lt;li&gt;A perihelion propulsive maneuver (aka firing engines) near the Sun (as close as allowed by the spacecraft's thermal control system).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Claudio_Maccone&quot; rel=&quot;nofollow&quot;&gt;Claudio Maccone&lt;/a&gt; proposed a following trajectory yielding an exit velocity of 51 km/s:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A gravity assist at Jupiter&lt;/li&gt;&#xA;&lt;li&gt;A powered maneuver near the Sun&lt;/li&gt;&#xA;&lt;li&gt;Another Jovian gravity assist.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The opportunities for a multiple-assist + propulsive maneuver mission are much more frequent due to the flexibility that powered swingbys give.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://gregmatloff.com/biography.html&quot; rel=&quot;nofollow&quot;&gt;Gregory L. Matloff&lt;/a&gt;. Deep Space Probes: To the Outer Solar System and Beyond. 2nd ed., 2005. P.46-48.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Claudio Maccone. Solar Foci Missions, 2nd Int'l Conf. on Low-Cost Planetary Missions, Laurel, MD, 1996. IAA-L-0604.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Krafft Arnold von Ehricke, &quot;Saturn-Jupiter Rebound. A method of high-speed spacecraft ejection from the Solar System&quot;. Journal of the British Interplanetary Society, Vol.25, 1972. Pp.561-571.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="1142" LastEditDate="2015-01-07T03:18:03.280" LastActivityDate="2015-01-07T03:18:03.280" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6679" PostTypeId="1" AcceptedAnswerId="6680" CreationDate="2015-01-06T13:37:31.413" Score="18" ViewCount="6159" Body="&lt;p&gt;What mechanisms and technologies will be used to control the descent and movement of the falcon 9 rocket's booster stage on its way to the landing pad?&lt;/p&gt;&#xA;" OwnerUserId="3384" LastEditorUserId="49" LastEditDate="2015-01-06T14:23:45.093" LastActivityDate="2018-02-19T14:56:11.830" Title="How will SpaceX achieve landing of the Falcon 9 first stage?" Tags="|rockets|spacex|landing|reuse|falcon-9|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6680" PostTypeId="2" ParentId="6679" CreationDate="2015-01-06T13:43:36.907" Score="24" Body="&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/csOwN.png&quot; alt=&quot;Launch and Landing profile&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Key phases in the launch-and-landing plan for SpaceX Falcon 9 rocket. Image credit: Jon Ross, &lt;a href=&quot;http://www.nbcnews.com/science/space/spacex-does-reality-check-its-falcon-9-rocket-landing-plan-n270246&quot; rel=&quot;noreferrer&quot;&gt;NBC News.com&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the first stage main engine cutoff, cold gas N2 thrusters are used to rotate the booster into the direction of flight, they reignite 3 of the Merlin 1D engines.  (With 9 in the &lt;a href=&quot;http://www.spacex.com/falcon9&quot; rel=&quot;noreferrer&quot;&gt;Octaweb&lt;/a&gt; alignment, the center and two on either side of it allow a 'line' of engines to fire).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They use this to kill forward momentum. They continue arcing upwards (since vertical and horizontal momentum components are independent) until they start falling down again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They then fire three &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29#Merlin_1D&quot; rel=&quot;noreferrer&quot;&gt;Merlin 1D&lt;/a&gt; engines to control descent through the hypersonic regime as they hit thick enough atmosphere to be problematic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally they use the &lt;a href=&quot;https://twitter.com/elonmusk/status/536258543675252739&quot; rel=&quot;noreferrer&quot;&gt;hypersonic grid fins&lt;/a&gt; at the top of the upper stage, the cold gas N&lt;sub&gt;2&lt;/sub&gt; thrusters in combination to try and steer the booster to its target. Time will tell if they have sufficient control to hit a small target that's 300 feet long by 170 feet wide (91 by 52 meters) precisely enough for Falcon 9's leg span of about 70 feet (21 meters). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once they approach their landing surface, in the last few seconds (10-30 seconds?) they re-ignite the central Merlin 1D to decelerate from terminal velocity to a landing.  Hopefully down to 4.5 mph (7.2 km/h) at the time of touchdown, a number that was mentioned during the recent CRS-5 pre-launch news conference. If not, bad things happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing legs will deploy during the final burn, according to SpaceX representatives. Here is a &lt;a href=&quot;https://www.youtube.com/watch?v=CfMuvsC9k2U&quot; rel=&quot;noreferrer&quot;&gt;rendered video of this whole launch profile&lt;/a&gt;, from launch to landing of the Falcon 9 first stage on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;noreferrer&quot;&gt;Autonomous Spaceport Drone Ship&lt;/a&gt; (ASDS):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/CfMuvsC9k2U?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-01-06T14:20:48.063" LastActivityDate="2015-01-06T14:20:48.063" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="6681" PostTypeId="1" AcceptedAnswerId="6683" CreationDate="2015-01-06T16:18:00.963" Score="7" ViewCount="1685" Body="&lt;p&gt;I figured this is probably an okay place to post this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9L1ZA.jpg&quot; alt=&quot;From Rocket Propulsion Elements: Chapter 2, Problems&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I managed to get (a), (c) and (d) (quite simply), but I have absolutely no idea why I can't work out (b).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used $$ I_{sp}=\frac{I_t}{g_0\dot{m}\Delta t} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assumed thrust was constant (is this wrong?). I did also check my units.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks. :)&lt;/p&gt;&#xA;" OwnerUserId="7126" LastEditorUserId="4660" LastEditDate="2015-01-06T18:55:39.750" LastActivityDate="2015-01-06T18:55:39.750" Title="Rocket Propulsion Elements: Total Impulse Problem" Tags="|propulsion|rockets|measurement|specific-impulse|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6682" PostTypeId="2" ParentId="6681" CreationDate="2015-01-06T17:22:05.753" Score="-2" Body="&lt;p&gt;You take the effective exhaust velocity and multiply it by the mass flow. That gives you the thrust. Multiply that by the operating time and you get the total impulse.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-01-06T17:22:05.753" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="6683" PostTypeId="2" ParentId="6681" CreationDate="2015-01-06T17:39:27.747" Score="10" Body="&lt;p&gt;You got caught by those silly English units.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your expression, $I_t = I_{sp}g_0 \dot m \Delta t$, works just fine when you use metric units. The mass flow rate, in metric units, is 127 kg/s. The total impulse, in metric units, is&#xA;$$I_t = (217.5\,\text{s})(9.80665\,\text{m}/\text{s}^2)(127\, \text{kg}/\text{s})(65\,\text{s}) = 1.76\times 10^7\,\text{N}\,\text{s}$$&#xA;Converting this to customary units yields $3.96\times 10^6\,\text{lbf}\,\text{s}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doing the exact same calculation, this time using customary units, yields&#xA;$$I_t = (217.5\,\text{s})(32.17405\,\text{ft}/\text{s}^2)(280\, \text{lbm}/\text{s})(65\,\text{s}) = 1.2736\times 10^8\,\text{but in what units?}$$&#xA;The dimensional analysis says the units of this expression are lbm·ft/s. Those aren't the desired units; you want lbf·s. The metric F=ma becomes F=kma in customary units. Dividing by the numerical value of &lt;em&gt;g&lt;/em&gt;&lt;sub&gt;0&lt;/sub&gt; converts lbm·ft/s&lt;sup&gt;2&lt;/sup&gt; to lbf. That same division here converts lbm·ft/s to lbf·s. And indeed, 1.2736×10&lt;sup&gt;8&lt;/sup&gt;/32.17405 is 3.96×10&lt;sup&gt;6&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This suggests an alternate expression, $I_t = I_{sp} \dot m \Delta t$, where $I_{sp}$ is in seconds.This yields a numerical value of 217.5·280·65=3.96×10&lt;sup&gt;6&lt;/sup&gt;. That's the right value (at least numerically).  Note well: This alternate expression does not work in metric units. In metric, this calculation yields a value of 1.80×10&lt;sup&gt;6&lt;/sup&gt; kg·s. The correct value is 1.76×10&lt;sup&gt;7&lt;/sup&gt; N·s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Strictly speaking, the above does not have the correct units; it has units of mass*time. Numerically, it yields the right value in customary units because dividing by the numerical value of &lt;em&gt;g&lt;/em&gt;&lt;sub&gt;0&lt;/sub&gt; cancels the explicit use of &lt;em&gt;g&lt;/em&gt;&lt;sub&gt;0&lt;/sub&gt; in the numerator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This calculation is not correct in metric units. To get the right metric units from this calculation one needs to multiply the numerical value of that result (1.80×10&lt;sup&gt;6&lt;/sup&gt; kg·s) by the numerical value of &lt;em&gt;g&lt;/em&gt;&lt;sub&gt;0&lt;/sub&gt;. Indeed, 1.80×10&lt;sup&gt;6&lt;/sup&gt; * 9.80665 = 1.76×10&lt;sup&gt;7&lt;/sup&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-01-06T18:10:18.773" LastActivityDate="2015-01-06T18:10:18.773" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="6684" PostTypeId="2" ParentId="6590" CreationDate="2015-01-06T21:04:06.160" Score="0" Body="&lt;p&gt;It appears to be motoring under its own power in the photo posted by SpaceX saying it has left the port on Jan 5. There was a statement posted that it would be approximately 200 miles east of Jacksonville. (did not record where I got this). I believe that this is true and that the booster will never backtrack because of the extra fuel to turn it back. My conjecture is that until landings back at the cape are approved,this will be the operating mode to simply secure it to barge and motor back to shore. Probably less than a 48 hour trip. If barge landings turn out to be successful, why would you ever fly back to shore , taking a chance of loosing the booster plus using precious engine life??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just saw the pic of barge in port at cape with tools to rotate booster from vertical to horizontal and a trailer to haul it to where it will be checked out. I would drive to port to get a pic, but based on discussion 5, it will be well covered :&gt;)&lt;/p&gt;&#xA;" OwnerUserId="7120" LastEditorUserId="7120" LastEditDate="2015-01-06T21:16:45.643" LastActivityDate="2015-01-06T21:16:45.643" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6685" PostTypeId="2" ParentId="667" CreationDate="2015-01-06T21:34:09.947" Score="-1" Body="&lt;p&gt;The booster is probably stable without controlled thrust once the landing legs are deployed, the spaceX description says the engine(s) will be fired 3 times after separation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The booster will only turn around backwards to direction of flight during the decel and  landing phases. It will use aerodynamic drag to reduce the majority of its speed and not use  extra fuel.&lt;/p&gt;&#xA;" OwnerUserId="7120" LastEditorUserId="7120" LastEditDate="2015-01-06T21:39:17.323" LastActivityDate="2015-01-06T21:39:17.323" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="6687" PostTypeId="2" ParentId="6672" CreationDate="2015-01-06T22:27:16.547" Score="17" Body="&lt;p&gt;This may answer some of your questions, but not all. Additional information may exist in the book.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also take into account that both Voyager spacecrafts could perform the same measurements before they diverged. The results could be compared.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Following excerpts are from the book &lt;strong&gt;&lt;a href=&quot;https://books.google.com/books?id=Gp-2B7AUPVUC&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Craft: An Overview of Interplanetary Flight&lt;/a&gt;&lt;/strong&gt;, Dave Doody, Springer Science &amp;amp; Business Media, 2010, parts of which are available in the free preview (use chapter links to the source of the two excerpts):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://books.google.com/books?id=Gp-2B7AUPVUC&amp;amp;pg=PA176&amp;amp;lpg=PA176&quot; rel=&quot;noreferrer&quot;&gt;5.6.2 Extensible Booms&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...] A potential source of error in the magnetometer instruments’&#xA;  measurements is the amount of twist in the long fiberglass&#xA;  magnetometer boom established after deployment. To compensate, a&#xA;  magnetic coil, which can be energized on command to create a magnetic&#xA;  field of known strength and orientation to calibrate the instruments,&#xA;  is mounted on the spacecraft.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://books.google.com/books?id=Gp-2B7AUPVUC&amp;amp;pg=PA226&amp;amp;lpg=PA226&quot; rel=&quot;noreferrer&quot;&gt;6.4.8 Calibrations and Ground Truth&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Scientific measurements are made by using instruments that have&#xA;  quantifiable error. Calibrations are carried out by instruments on a&#xA;  spacecraft to acquire baseline data for comparison with an actual&#xA;  observation, thus allowing instrument errors to be quantified.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Prior to carrying out an infrared spectral measurement of a target,&#xA;  the IR spectrometer will be aimed toward a spot of deep space free of&#xA;  any bright objects in its field of view. An absolute reference value&#xA;  is obtained, and any defects in the instrument’s sensors can be&#xA;  recorded and later included in data analysis. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;For the same reason, radio science experiments always begin and end&#xA;  with a measurement of the spacecraft’s unobstructed, unmodulated radio&#xA;  tones lasting tens of minutes before and after encountering the&#xA;  target. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Many imaging instruments may be aimed toward a special calibration&#xA;  target mounted on the spacecraft bus. Voyager’s calibration target was&#xA;  a rectangular plate mounted below the bus (see Appendix A, page 294)&#xA;  coated with a material of known grayscale and albedo values. The&#xA;  spacecraft’s scan platform could aim the cameras so that the target&#xA;  plate would fill the field of view for calibration. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The same target plate on Voyager serves as the thermal radiator for&#xA;  the spacecraft’s electrical system regulator, so Voyager’s infrared&#xA;  instrument, the infrared radiometer-spectrometer (IRIS) could be&#xA;  calibrated. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The operation of every science instrument and experiment includes some&#xA;  sort of procedure or other means for calibrating its measurements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/uc190.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="1142" LastEditDate="2015-01-07T00:35:28.733" LastActivityDate="2015-01-07T00:35:28.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6688" PostTypeId="1" CreationDate="2015-01-06T23:50:37.413" Score="9" ViewCount="1324" Body="&lt;p&gt;I am interested in the possibility of sending unmanned reconnaisance spacecraft to study exoplanets, especially Earth-like ones with the ultimate goal of detecting alien life. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Certainly earth-based telescopes will become incredibly powerful in the next century, allowing us to study exoplanets in much greater detail. But I would think an unmanned spacecraft sent for a flyby of an expolanet would be a revolutionary step in this field. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;What kinds of technologies would we need for a flyby of an expolanet. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What are the different approaches or methods for accomplishing a flyby. Please mention time to get to destination (ex: can reach alpha centauri in 60 years)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Please mention which technologies/approaches are the most feasible.&lt;/p&gt;&#xA;" OwnerUserId="6679" LastEditorUserId="6679" LastEditDate="2015-01-07T02:38:44.070" LastActivityDate="2015-05-07T04:03:23.640" Title="Technology for unmanned spacecraft to exoplanets" Tags="|spacecraft|interstellar-travel|exoplanet|uncrewed-spaceflight|" AnswerCount="6" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="6689" PostTypeId="1" CreationDate="2015-01-07T00:59:06.743" Score="5" ViewCount="164" Body="&lt;p&gt;I'm playing around with some machine learning software I've written. It would be theoretically possible to code this to answer real word questions (like those asked on this site). Of course any answers are only as complete as the sources the answer is constructed from. Are there any plain text document repositories for the field of space exploration?&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1142" LastEditDate="2015-01-07T05:32:00.603" LastActivityDate="2018-10-22T22:32:30.647" Title="Are there any open access space journal repositories?" Tags="|reference-request|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="6690" PostTypeId="2" ParentId="6688" CreationDate="2015-01-07T02:25:42.217" Score="15" Body="&lt;p&gt;There are incredible, mindboggling distances involved in merely reaching an exoplanet. &lt;a href=&quot;http://en.wikipedia.org/wiki/Alpha_Centauri#Alpha_Centauri_Bb&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alpha Centauri Bb&lt;/a&gt;, the closest known exoplanet, is 4.365 light years away. That's 41,295,000,000,000 km (25,660,000,000,000 mi), or 276,000 AU (the distance from the Earth to the Sun).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 1 has left most of our solar system behind and &lt;a href=&quot;http://heasarc.nasa.gov/docs/cosmic/nearest_star_info.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;is traveling a blistering 17.3 km/s&lt;/a&gt;. If it were headed toward Alpha Centauri Bb, it would take nearly 75,700 years to get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At interstellar distance scales, the technical challenge is all about attaining speed. &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Icarus_%28interstellar%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Icarus&lt;/a&gt; is proposing to use a nuclear fusion to attempt interstellar speeds. &lt;a href=&quot;http://en.wikipedia.org/wiki/Interstellar_travel#Beamed_propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Beamed propulsion&lt;/a&gt; is a proposal to use light sails powered by gigantic lasers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way to get a boost is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Krafft_Arnold_Ehricke&quot; rel=&quot;nofollow noreferrer&quot;&gt;Krafft Arnold Ehricke&lt;/a&gt; trajectory, recently mentioned in &lt;a href=&quot;https://space.stackexchange.com/questions/6677/good-year-for-start-of-an-interstellar-mission-due-to-gravity-assis&quot;&gt;this question&lt;/a&gt;, from which I quote:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;. . . the best trajectory . . . to get out of Solar System with the highest velocity $V_\infty$ will have to use:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;A gravity assist from Saturn  &lt;/li&gt;&#xA;  &lt;li&gt;A gravity assist from Jupiter  &lt;/li&gt;&#xA;  &lt;li&gt;A perihelion propulsive maneuver (aka firing engines) near the Sun (as close as allowed by the spacecraft's thermal control system).  &lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Even the most optimistic predictions say perhaps within a hundred years we might attain speeds sufficient to get our probes there in hundreds of years. Thus another technical challenge will be designing a probe that will last that long, will still have a functioning power system upon arrival, will still have functioning instruments, radios, etc. This is, of course, complicated by the fact that there is not truly anything like empty space anywhere. The particles in interstellar space are extremely sparse, but when one is traveling at an appreciable fraction of the speed of light, the slightest mote has great damage potential.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's the question of how to decelerate so that when we get to the destination, we don't flash by so quickly that no practical amount of data can be absorbed by our instruments. Most designs call for accelerating to speed, then decelerating. &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetic_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;Magnetic sails&lt;/a&gt; have been put forth as a possible deceleration technique, but that presupposes sufficient magnetic flux in the interstellar medium to render that feasible.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting the data from the exoplanet flyby back to Earth is itself a technical challenge. Assuming your probe manages to capture a scientifically significant cache of data about the target system and planet(s), the vast distance the telemetry has to transmit means a significantly attenuated signal by the time it gets to any receiver in the solar system. One approach might be to send a series of relay probes after the primary vehicle to receive, boost, and then relay the signal.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-01-27T06:42:09.767" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="6691" PostTypeId="2" ParentId="6688" CreationDate="2015-01-07T02:33:48.643" Score="7" Body="&lt;p&gt;The most feasible that I've seen, which isn't all that high on the feasibility scale, is a very large, very low mass wire mesh containing sensors that is accelerated and powered by a microwave beam from Earth.  See &lt;a href=&quot;http://en.wikipedia.org/wiki/Starwisp&quot;&gt;Starwisp&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-01-07T02:33:48.643" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7690" PostTypeId="1" AcceptedAnswerId="7705" CreationDate="2015-01-07T09:31:49.513" Score="19" ViewCount="3828" Body="&lt;p&gt;In the book on the ill-fated Apollo 13 mission there is a chapter that contains an overview of the consumables on board the spacecraft, and whether or not they might pose a problem to the survival of the astronauts. Because the CDR and LMP were scheduled to perform two EVAs on the lunar surface, they had ample oxygen (the atmosphere of the LEM would be vented and completely replaced twice for these EVAs) but the LEM could not support the three men because the CO2-scrubbers were insufficient. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With them having sufficient oxygen I was wondering if the crew might have survived if they had all donned their space suits (or at least the CDR and LMP, taxing the scrubbers less). My reasoning is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The suits can be used inside the spacecraft using umbilical connections that draw on the LEM's ample oxygen supplies&lt;/li&gt;&#xA;&lt;li&gt;For suits worn only inside the craft cooling water (another thing they did not have in excess) is not needed as the flow of oxygen provides cooling&lt;/li&gt;&#xA;&lt;li&gt;If they used the PLSS backpack the CDR and LMP could even stretch the LEMs resources another 14 hours&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The only way I can think of that would invalidate this approach was if the suits umbilical connectors also relied on the scrubbers on the LEM to function, but I can't find anything to confirm or deny this.&lt;/p&gt;&#xA;" OwnerUserId="8132" LastEditorUserId="6214" LastEditDate="2015-01-12T19:29:25.347" LastActivityDate="2021-06-27T11:49:52.030" Title="Could the Apollo 13 crew have survived the CO2 issue using their spacesuits" Tags="|apollo-program|life-support|apollo-13|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7691" PostTypeId="1" AcceptedAnswerId="7695" CreationDate="2015-01-07T13:59:25.353" Score="2" ViewCount="512" Body="&lt;p&gt;I am working on a story where a spaceship is on a mission to colonize the exoplanet &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler-186f&quot; rel=&quot;nofollow&quot;&gt;Kepler-186f&lt;/a&gt;. However, after hitting of small meteor, they need to find a closer candidate for the new colony. I want this to be a known, potentially-habitable exoplanet candidate. How can I find such a planet?&lt;/p&gt;&#xA;" OwnerUserId="5913" LastEditorUserId="5963" LastEditDate="2015-01-07T23:23:33.773" LastActivityDate="2017-10-05T13:22:12.953" Title="How could I find a potentially habitable exoplanet in same direction as Kepler-186f?" Tags="|interstellar-travel|exoplanet|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7692" PostTypeId="2" ParentId="7691" CreationDate="2015-01-07T15:02:27.077" Score="2" Body="&lt;p&gt;I think the premise is flawed to be honest. If your spacecraft took damage such that it couldn't make it to kepler-186 it probably isn't going to make it anywhere else either. What makes more sense is that something breaks, for instance a problem develops in the biological support systems or a vital mechanical system starts to slowly degrade. Another possibility is that once they get closer to kepler-186 they realize that there are no habitable planets there after all, and need to go elsewhere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case the mission would benefit from:&#xA; - Being much closer to the planets in the area: a large telescope (presumably being part of the spacecraft) would be able to see things in much better detail and find suitable planets that would not be visible from earth&#xA; - Having a different angle of view: Once the spacecraft moves away from the earth it will be viewing things from a different plane, this may bring object into view which are not visible from the earth&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So perhaps your explorers see a planet that was from a star hidden by a dust cloud or nebula, or maybe they see a planet that rotates on a plane you couldn't see from earth, or simply could not see until approaching more closely. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-01-07T15:02:27.077" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7693" PostTypeId="1" AcceptedAnswerId="7697" CreationDate="2015-01-07T15:11:21.533" Score="14" ViewCount="1781" Body="&lt;p&gt;Was the implementation of artificial gravity ever seriously considered for use on board the ISS and its derivatives? If not, why?&lt;/p&gt;&#xA;" OwnerUserId="3384" LastEditorUserId="8136" LastEditDate="2015-01-07T19:55:20.327" LastActivityDate="2015-01-07T19:55:20.327" Title="Artificial Gravity on board the ISS and its derivatives" Tags="|iss|space-station|artificial-gravity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7695" PostTypeId="2" ParentId="7691" CreationDate="2015-01-07T15:20:08.370" Score="8" Body="&lt;p&gt;Here is a map of all &lt;a href=&quot;http://www.hpcf.upr.edu/~abel/phl/exomaps/all_stars_with_exoplanets_white.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;known exoplanets&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/i7fnj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Go to the link and click in the tight bunch of dots in the upper left - that is the section of sky Kepler is searching. Kepler 186 is at &lt;a href=&quot;http://en.wikipedia.org/wiki/Kepler-186&quot; rel=&quot;nofollow noreferrer&quot;&gt;19h54m37s by 43d57m&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Damage due to a micro-meteor in interstellar space is extremely unlikely. Interstellar space is very empty compared to a solar system. Even passing through the asteroid belt in our solar system, hitting something is extremely unlikely. It can still happen of course, but you might want to include dialogue to that effect. And if you were struck by one, it must have breached the hull in order to do damage you need to be concerned about, right? In that case, it would also have had to do something extraordinarily unlikely if it damaged systems enough that the full journey can't be completed and an alternate needs to be found, but it didn't do enough damage to doom the mission entirely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over the time-scale of an interstellar journey, the locations of stars shift due to their orbit around the galactic core, and their dance around each other due to their mutual gravitational influence. This movement is chaotic, there is no way to predict it. For that reason, your ship would already have needed to have enough delta V to do quite a lot of course correction once it was close enough to Kepler 186 to establish what those corrections would be. In that case, it should be able to aim for another system clustered in the Kepler field of view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has a list of &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_exoplanets_discovered_using_the_Kepler_spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;all exoplanets discovered by Kepler&lt;/a&gt;, in a chart that includes distances. Most are farther away than Kepler 186. Many entries don't have the distance data - there are so many of them now, I guess people got tired of filling it all in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also consider that the planets detectable by current methods are only a small portion of all the planets there are. Kepler uses the transit of planets in front of their stars to detect them. If a star system is tilted even a little from our point of view this technique doesn't work. Another successful method is to measure the changes over time in the Doppler shift of a star's light as it wobbles due to the gravitational effect of its planets. This also doesn't work if the star system is close to perpendicular to us from our point of view, and can't find rocky planets like Earth farther away than about 160 light years because of the limits of instrument sensitivity. There are most certainly many other planets on the way to Kepler 186 that we don't know about.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-01-07T18:48:23.307" LastActivityDate="2015-01-07T18:48:23.307" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7696" PostTypeId="2" ParentId="6688" CreationDate="2015-01-07T15:39:19.207" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;But I would think an unmanned spacecraft sent for a flyby of an expolanet would be a revolutionary step in this field.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Better said: Impossible, at least using anything close to current technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting to Alpha Centauri in 60 years would require an average velocity of nearly 22,000 kilometers per second, or 0.5 astronomical units per hour. That high of a velocity would give the vehicle just hours to conduct its mission. That pretty much rules out a flyby mission. The vehicle needs to come to rest at the target star, which ups the ante even further on the &quot;we don't know how to do that&quot; scale (aka the technology readiness level). The technologies that might be able to do that are optimistically rated at TRL 3. That's very low, and even that number requires looking at things through very rose colored glasses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if we had the propulsion technology to do that, what good will it do if the vehicle is dead on arrival or if it can't communicate with Earth? The vehicle needs a power system that will last for 60 years (we don't know how to do that), a computer system that can survive for 60 years (we have some idea how to do that), and a communications system that can broadcast at a reasonable rate across 4.365 light years (we don't know how to do that).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To illustrate the last problem, the Voyager satellites have had to regularly throttle back its transmission rate as they has receded further and further from the solar system. Some time this year, Voyager 1 will have to reduce its high rate transmission to 1.4 kilobits per second. That will mark the end of those high rate transmissions; the playback system on the spacecraft can't work at that low a rate. That 1.4 kilobits per second at 132 AU becomes 28 bits per day at 4.365 light years. It would take almost a year to transmit a single 256x256 gray scale image compressed by a factor of 10:1 at that rate. We need a communication system three orders of magnitude better than that to be useful. One order of magnitude improvement is doable. Three orders of magnitude? We don't know how to do that.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-01-07T15:39:19.207" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="7697" PostTypeId="2" ParentId="7693" CreationDate="2015-01-07T15:53:07.733" Score="15" Body="&lt;p&gt;Yes, as far as the ISS is concerned, there were at least two such projects in the past that I know of:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Nautilus-X#ISS_centrifuge_demonstration&quot;&gt;ISS centrifuge demonstration&lt;/a&gt; as part of the Nautilus-X (Non-Atmospheric Universal Transport Intended for Lengthy United States Exploration)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module&quot;&gt;Centrifuge Accommodations Module&lt;/a&gt; that was designed for NASA by JAXA&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;None of these seem to be going anywhere, for now, but you can say that they were &lt;em&gt;seriously considered&lt;/em&gt;. The latter (CAM) was even partially built and is now sitting as a sad reminder to budgetary cuts and plan changes as administrations change at the Tsukuba Space Center's outdoor exhibit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were also some non-related to the station proposals, such as Zubrin's &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Direct#Revisions&quot;&gt;Mars (Semi-) Direct&lt;/a&gt; variants that suggest rotating a tethered formation using a spent upper stage as counterbalance mass, and Nautilus-X on its own as MMSEV (Multi-Mission Space Exploration Vehicle). There might be many more similar concepts under consideration that I can't think of now. Of course, there are always the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rotating_wheel_space_station&quot;&gt;&quot;space wheel&quot; stations&lt;/a&gt; that popup in various discussions, but I'm not aware of any being &lt;em&gt;seriously considered&lt;/em&gt; (whatever that means, let's settle for some significant money being raised or spent on it) for the time being.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-01-07T15:53:07.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7698" PostTypeId="1" CreationDate="2015-01-07T17:41:57.553" Score="-11" ViewCount="3386" Body="&lt;p&gt;If you spin a generator in weightless space could you generate infinite power? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course the friction would be a issue but couldn't there be ways to reduce this in a weightless environment?&lt;/p&gt;&#xA;" OwnerUserId="8135" LastActivityDate="2016-05-28T12:58:45.937" Title="Generator in space" Tags="|microgravity|power|" AnswerCount="2" CommentCount="2" ClosedDate="2016-10-06T13:52:35.557" ContentLicense="CC BY-SA 3.0" />
  <row Id="7699" PostTypeId="2" ParentId="7698" CreationDate="2015-01-07T17:45:51.473" Score="5" Body="&lt;p&gt;No. Although it is weightless, its mass remains the same. Therefore, the effective angular momentum &amp;amp; energy required to spin it up to speed is not changed.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2015-01-07T17:45:51.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7700" PostTypeId="2" ParentId="7698" CreationDate="2015-01-07T18:31:44.927" Score="6" Body="&lt;p&gt;Friction is ultimately where most power is lost in a generator system or any other mechanical system. The pull of gravity against interlocking objects, pressing them against each other, certainly causes some friction. The medium in which objects move also causes friction. In the weightless vacuum of space, both of these kinds of friction can be reduced to nearly zero, which is why we can have things like relatively stable orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, engineers used to try to produce perpetual motion machines. To do this, the friction that would otherwise gradually slow down the machine must be reduced to a level that is negligible compared to the momentum in the machine. Even when this is achieved, you don't really have &lt;em&gt;perpetual&lt;/em&gt; motion, unless friction is actually zero, but you could have the appearance of perpetual if the ratio of friction to momentum is low enough. This is what can be achieved in the vacuum and weightlessness of space - &lt;em&gt;apparently&lt;/em&gt; perpetual motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perpetual motion does not mean infinite energy. With your generator idea, you are trying to imagine something that escapes the limitations of the Law of Conservation of Energy. You are imagining a system in motion which stays in motion, and you recognize that motion is a kind of energy. But what you want to do next is to remove some of that energy and apply it to something else. When you do so, your system will slow down by &lt;em&gt;at least&lt;/em&gt; as much as the energy you have used. When you generate electricity by spinning a magnet inside a coil of wire, the energy you get comes from pushing electrons down that wire. But what pushes the electrons? Have you ever used a device that generates some kind of power by turning a hand-crank? Do you ever wonder why there is so much resistance to turning that crank? It's essentially because you are pushing electrons &lt;em&gt;with your hand.&lt;/em&gt; By turning the magnet, you induce a current in the wire. By inducing the current, you generate a magnetic field. That new magnetic field resists the field of the magnet you are turning. Ultimately, what you are feeling is the resistance of the momentum of those electrons wanting to remain &quot;at rest&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But simply moving them is not the only work you have to do actually. Did you know there is friction at that atomic level? If you put a lot of power through a wire, the wire gets hot. That's from a kind of friction called electrical resistance. So what you would find if you have a &quot;perpetually&quot; spinning magnet in a space station is that as soon as you move a coil of wire over it, it will very quickly stop. If you had a light bulb attached to that wire in a completed circuit, it would glow briefly. You would also feel a tug on that wire. Then your energy is spent. The motion was transformed into  heat, light, and the tug you felt on the wire. The end.&lt;/p&gt;&#xA;" OwnerUserId="945" LastEditorUserId="945" LastEditDate="2015-01-08T15:50:10.397" LastActivityDate="2015-01-08T15:50:10.397" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7701" PostTypeId="1" AcceptedAnswerId="7703" CreationDate="2015-01-07T18:49:39.267" Score="38" ViewCount="44260" Body="&lt;p&gt;Is there any scientific name for &quot;The Moon&quot;, Earth's satellite, when talking about it apart from, but in context of, other moons? Google simply states that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Earth's only natural satellite is simply called the moon because people didn't know other moons existed until Galileo Galilei discovered four moons orbiting Jupiter in 1610. Other moons in our solar system are given names so they won't be confused with each other.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It seems sort of dumb to just call the moon &quot;The Moon&quot; because we never had a classification for what it is/was. I'm guessing it has some sort of astronomical name. The main difference between The Sun and The Earth and their English naming conventions is that we don't call other planets &quot;Earths&quot; and other stars &quot;Suns&quot;, at least not in a scientific sense; we tend to use it informally rather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, does The Moon have an agreed upon international astronomical name, such as Luna or something like that? And while we're at it, there must also be a name for &quot;The Sun&quot; (It might be Sol) and our solar system?&lt;/p&gt;&#xA;" OwnerUserId="7027" LastEditorUserId="7027" LastEditDate="2016-06-17T15:17:07.717" LastActivityDate="2016-06-17T15:17:07.717" Title="Does the moon have a name?" Tags="|the-moon|terminology|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="7702" PostTypeId="2" ParentId="7701" CreationDate="2015-01-07T19:06:58.593" Score="13" Body="&lt;p&gt;Because the moon was so well known before international agreement on astronomical naming was a thing, different languages have different words for it. In English it's &quot;the Moon&quot;; in the Romance languages it's usually &quot;Lune&quot; or &quot;Luna&quot; from the Latin. The modern dominance of English and the historical dominance of Latin in scientific communities pretty much guarantees that either one is recognized, and there's no particular reason to force people to use one or the other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similar reasoning applies to &quot;Sun&quot; versus &quot;Sol&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-01-07T19:06:58.593" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7703" PostTypeId="2" ParentId="7701" CreationDate="2015-01-07T19:16:29.513" Score="47" Body="&lt;p&gt;It has many names, depends on who you ask. In English, one name for our moon is simply &lt;em&gt;&lt;strong&gt;the Moon&lt;/strong&gt;&lt;/em&gt;. Notice the article &lt;em&gt;the&lt;/em&gt; and the capitalization making it a &lt;a href=&quot;https://en.wikipedia.org/wiki/Proper_noun&quot; rel=&quot;noreferrer&quot;&gt;proper noun&lt;/a&gt;. This is similar in convention as &lt;em&gt;the Galaxy&lt;/em&gt; for our own galaxy that is otherwise also known as the Milky Way. But the word &lt;em&gt;galaxy&lt;/em&gt; actually already implies &lt;em&gt;something related to milk&lt;/em&gt; in Greek anyway (galakt / γάλακτ = milk), so some linguistic purist frown upon the use &lt;em&gt;the Milky Way galaxy&lt;/em&gt; as redundant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another name for our moon could be &lt;strong&gt;&lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_%28goddess%29&quot; rel=&quot;noreferrer&quot;&gt;Luna&lt;/a&gt;&lt;/em&gt;&lt;/strong&gt; by the ancient Roman deity that is, according to ancient beliefs, embodiment of the Moon. Roman / Latin names for celestial bodies (and sometimes &lt;a href=&quot;https://space.stackexchange.com/q/6639/49&quot;&gt;their adjectives&lt;/a&gt;) are frequently used. In fact, we all know e.g. the planets Mars, Neptune, Venus,... by their Latin names for a god of war, god of fresh waters and the sea, and goddess of love, respectively, and less frequently use their Greek equivalents Ares, Poseidon, and Aphrodite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the Moon / Luna is known by many other names in other cultures. For example, in Greek mythology, it would be &lt;strong&gt;&lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Selene&quot; rel=&quot;noreferrer&quot;&gt;Selene&lt;/a&gt;&lt;/em&gt;&lt;/strong&gt;, goddess of the Moon, or rarer, &lt;strong&gt;&lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cynthia&quot; rel=&quot;noreferrer&quot;&gt;Cynthia&lt;/a&gt;&lt;/em&gt;&lt;/strong&gt; or even &lt;strong&gt;&lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Artemis&quot; rel=&quot;noreferrer&quot;&gt;Artemis&lt;/a&gt;&lt;/em&gt;&lt;/strong&gt;, both the names of the Greek goddess of the hunt, forests and hills, the Moon, and archery. Obviously, this implies another name for the Moon - &lt;strong&gt;&lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Diana_%28mythology%29&quot; rel=&quot;noreferrer&quot;&gt;Diana&lt;/a&gt;&lt;/em&gt;&lt;/strong&gt;, the Roman goddess of the hunt, the moon and birthing. And, say, Chinese equivalent would be &lt;strong&gt;&lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e&quot; rel=&quot;noreferrer&quot;&gt;Chang'e&lt;/a&gt;&lt;/em&gt;&lt;/strong&gt;, their mythological goddess of the Moon (relevant to space exploration since the China National Space Administration chose this name for their lunar exploration program, and you might have heard of some of the Chang'e lunar orbiters and landers, e.g. the Chang'e 3 that delivered a small rover named &lt;a href=&quot;https://space.stackexchange.com/questions/3149/what-is-the-significance-of-the-name-jade-rabbit&quot;&gt;Yutu&lt;/a&gt;, in English &lt;em&gt;Jade rabbit&lt;/em&gt;, to the Moon). The list could go on, as evidenced for example by this &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_lunar_deities&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia's list of lunar deities&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, when it comes to terminology and naming conventions (nomenclature), you should never neglect to mention what language, culture and use you're interested in. I know you did, I'm just trying to additionally emphasize that it makes a difference. But, to cut the long story short and answer your question more directly:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Does The Moon have an agreed upon international astronomical name?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes. The Moon. And this is the &lt;a href=&quot;http://www.iau.org/public/themes/naming/#spelling&quot; rel=&quot;noreferrer&quot;&gt;English language naming convention recommended by the International Astronomical Union&lt;/a&gt; (IAU):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The IAU formally recommends that the initial letters of the names of&#xA;  individual astronomical objects should be printed as capitals; e.g., Earth, Sun, Moon, etc. &quot;The Earth's&#xA;  equator&quot; and &quot;Earth is a planet in the Solar System&quot; are examples of&#xA;  correct spelling according to these rules.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Additionally, if I may, what you posit in the penultimate paragraph of your question seems somewhat flawed. There is a distinction between &lt;em&gt;a sun&lt;/em&gt; and &lt;em&gt;the Sun&lt;/em&gt; in that while they both suggest a star that is a parent to planets, i.e. a planetary system and not merely &lt;em&gt;a star&lt;/em&gt;, the former form is both frequently used (saying, e.g., &lt;em&gt;&quot;This planet has two suns because it orbits a binary star.&quot;&lt;/em&gt; would be perfectly acceptable, why not?) as well as both being distinct enough with the use of articles &lt;em&gt;a&lt;/em&gt; or &lt;em&gt;the&lt;/em&gt;, so they cannot be ambiguous with proper use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it helps make a difference, here's a quote from &lt;a href=&quot;http://curious.astro.cornell.edu/question.php?number=289&quot; rel=&quot;noreferrer&quot;&gt;Curious About Astronomy post on What are the names of the earth, moon, sun, and solar system?&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The name of our planet is the Earth. The name of our moon is the Moon.&#xA;  The name of our solar system is the Solar System.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Notice that I capitalize them, because when used as names, they are&#xA;  proper nouns. This also helps us distinguish between the planet Earth&#xA;  and earth (meaning soil), between the Earth's Moon and moon (meaning&#xA;  the natural satellite of a planet), and between our Solar System and&#xA;  any other solar systems (since any system containing a star and a&#xA;  planet or a planet-forming disk can be called a solar system.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia page you quote seems only partially correct and neglects to mention capitalization of proper nouns. Which is somewhat amusing considering its own page on proper nouns clearly stating:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In English and many other languages, proper names and words derived&#xA;  from them are associated with capitalization;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-01-07T20:19:35.073" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="7704" PostTypeId="2" ParentId="5635" CreationDate="2015-01-08T02:36:33.350" Score="-2" Body="&lt;p&gt;I've thought about this myself and have wondered as well, can you cheat g's if you are suspended in a mostly non compressible liquid that keeps you at 0 buoyancy (or close to it, water will do fine). Of course there will be hollow spaces like your lungs, but you could hypothetically withstand a substantially greater g load if it worked.&lt;/p&gt;&#xA;&lt;p&gt;I don't recommend doing this, but &lt;em&gt;&lt;strong&gt;imagine&lt;/strong&gt;&lt;/em&gt; having a goldfish in a bowl tied securely to a rope, and you were to swing it around, would the goldfish experience much more if any than 0 g's?  Correct me if I'm wrong, but the answer would be no, because the force is being paid for by the bottom of the container and rope holding it, but not to its occupant (unless of course an accident happens.)&lt;/p&gt;&#xA;" OwnerUserId="8138" LastEditorUserId="46507" LastEditDate="2023-03-12T23:32:15.710" LastActivityDate="2023-03-12T23:32:15.710" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="7705" PostTypeId="2" ParentId="7690" CreationDate="2015-01-08T03:18:25.497" Score="9" Body="&lt;p&gt;The CO2 scrubbers were certainly an issue to be solved on Apollo 13, but I believe the most difficult problem was that of power.  The fuel cells on the spacecraft needed oxygen that was lost in the tank that ruptured.  I doubt there would have been enough power available to operate the suits.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-01-08T03:18:25.497" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7706" PostTypeId="2" ParentId="6688" CreationDate="2015-01-08T03:43:20.107" Score="3" Body="&lt;p&gt;Joshua,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The responses from Jerard Pucket and David Hammond are very good.  As Jerard Pucket indicated, most people do not realize the great exponential increase in distances between the practical planetary/planetoid edge of our solar system, (such as Pluto and Sedna) and even the closet stars.  The layman analogy that lays it out mathematically scaled down but proportional to the distances involved is the Football Stadium analogy for our Solar System... Imagine the Sun as a Clementine at the 50 Yard line, center field. Earth is 10 feet away.  At this scale, Pluto is at the stadium edge, the farthest seats, where the lowest cost ticketed fans are.  To go to the nearest star at this scale ?  It's an Airline flight or a long car ride... Alpha Centauri is approx. a 1,000 miles at that scale.  Gliese 581c, 21 Light years away... If the stadium is in Cleveland, then Gliese 581 is near Scotland, across the Atlantic.  Pluto is about 7 to 9 years away from Earth with current tech (to the fans in the far stadium seats)... Get the picture... going the Gliese 581 in Scotland when its 7 to 9 years to reach the stadium edge.  Marc Millis of Tau Zero Foundation has a good article on estimated resources and time lines for sending an interstellar probe... see:  &lt;a href=&quot;http://arxiv.org/ftp/arxiv/papers/1101/1101.1066.pdf&quot; rel=&quot;nofollow&quot;&gt;http://arxiv.org/ftp/arxiv/papers/1101/1101.1066.pdf&lt;/a&gt;    - also see also the  &quot;incessant obsolescence postulate&quot; (again from Marc Millis, Tau Zero Foundation)      &lt;a href=&quot;http://www.tauzero.aero/discoveries-log/getting-there/motives/&quot; rel=&quot;nofollow&quot;&gt;http://www.tauzero.aero/discoveries-log/getting-there/motives/&lt;/a&gt;&lt;br&gt;&#xA;David Hammond gave a good basic review on radio transmissions back the Sol (Earth) from Alpha Centauri.  Here is some more &quot;sobering&quot; info derived from an article written by David Woolley with references from Project Cyclops, (ISBN 0-9650707-0-0), Reprinted 1996, by the SETI League and SETI Institute; and Radio Astronomy, John D. Kraus, 2nd edition, Cygnus-Quasar Books, 1986. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How BIG should this interstellar probe be?&lt;br&gt;&#xA; How much power should it generate for communications ?     For propulsion ? (to reach Alpha Centauri in a life-time)      &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Transmitter/Receiver (and it's size):  Even if we took the biggest radio dish telescope today, the 1,000 foot Arecibo Radio Telescope and &quot;magically&quot; transported in orbit of Alpha Centauri Bb, we could NOT detect ANY Television, Radio, Radar or any other main stream communication from Earth !  Using the S- Band at 2.380 GHz with a band width of 0.1Hz (possible today, but would limit the data stream transfer rate) add a ludicrous transmission power of one giga-watt (well beyond anything we presently do, but possible if truly required). All this would have a detection range of 5 Light years with the Arecibo dish. A very precise pointing laser communications system is another option, with reduced power, but still power outputs in the multi-mega watt range and it would represent developing technology, but likely possible in your lifetime. For the Probe to reach Alpha Centauri Bb in 90 years, the probe must achieve a max speed of 10 % of light, decelerate and then orbit.  The bottom line would be a probe utilizing a VERY robust nuclear fusion generator with lots of fuel (which we do NOT yet have nuclear fusion, it is still beyond us).  The probe would be several times bigger than the USS Aircraft Carrier Nimitz (mainly for fusion fuel to reach 10 % of light speed twice), with just the communication system having enough power generation to run a large city.  This probe's propulsion system would generate power which would DRAWF the entire World's energy production capacity today...  The cost:  The economics of building it would bankrupt the world many times over at this point in history, and oh yes, the technology is NOT yet available, so the whole point is mute...  However, read the information from Marc Millis as today's stepping stone technologies could lead to building such a probe... in several hundred years.  Additionally, the probe's  size would come down an order of magnitude with advancing far future technologies, but the probe would still be larger than anything we have placed into orbit at this point. &lt;/p&gt;&#xA;" OwnerUserId="8139" LastEditorUserId="8139" LastEditDate="2015-01-08T04:58:37.603" LastActivityDate="2015-01-08T04:58:37.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7707" PostTypeId="2" ParentId="6688" CreationDate="2015-01-08T05:24:06.060" Score="1" Body="&lt;p&gt;Things aren't quite as negative as the other posters are making it out to be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While we simply can't do it yet we aren't as far away from such a probe.  Look at Robert L. Forward's &lt;a href=&quot;http://en.wikipedia.org/wiki/Starwisp&quot; rel=&quot;nofollow&quot;&gt;Starwisp&lt;/a&gt; concept--both the boosting and the power for the flyby are done with beamed power, greatly simplifying matters.  The construction of the probe is beyond current technology but it doesn't require fundamental breakthroughs like any fuel-carrying rocket capable of that kind of speed would.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starwisp also has the advantage that a big part of the system (the huge solar power -&gt; microwave satellite) stays here and can be used for launch after launch after launch.  This makes additional launches &lt;strong&gt;much&lt;/strong&gt; cheaper, if you're going to send out such probes you can send them to every star that's close enough.  On the other hand, they can't communicate without being energized by the beam--if you want to know if the probe has survived you have to direct a power beam to it--and the design inherently can't have any sort of shield against the interstellar gas and dust.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-01-08T05:24:06.060" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7708" PostTypeId="2" ParentId="7701" CreationDate="2015-01-08T06:06:28.053" Score="8" Body="&lt;p&gt;In most languages, including English, before Galileo, Kepler and Newton, there was only one thing called &lt;em&gt;Sun&lt;/em&gt; and just one thing called &lt;em&gt;Moon&lt;/em&gt;. After Galileo and Kepler, for some time, there was only one thing called &lt;em&gt;Solar System&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then Galileo (and/or Simon Marius) discovered Jupiter's satellites. They are things related to Jupiter the same as The Moon is related to Earth. So, each one of them is &lt;strong&gt;like&lt;/strong&gt; a Moon, but orbiting Jupiter instead of Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quickly, saying something like that &quot;&lt;em&gt;Io is like the Moon, but orbiting Jupiter&lt;/em&gt;&quot;, was reduced to the similar, but quite different &quot;&lt;em&gt;Io is one of Jupiter's moons&lt;/em&gt;&quot;. This means that the noun &lt;em&gt;moon&lt;/em&gt; had its meaning changed. It used to denote solely to Earth's Moon before, and then it started to denote a satellite of a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same change of meaning happened to the noun &lt;em&gt;sun&lt;/em&gt; also. And the same for &lt;em&gt;solar system&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To disambiguate that in a pedantic manner, you should use the noun &lt;em&gt;satellite&lt;/em&gt; or &lt;em&gt;natural satellite&lt;/em&gt; instead of the noun &lt;em&gt;moon&lt;/em&gt; to refer to unnamed, unspecified, generic or collective entities orbiting planets, including Earth's Moon itself, reserving the word &lt;em&gt;Moon&lt;/em&gt; just to Earth's natural satellite. The same for &lt;em&gt;star&lt;/em&gt; and &lt;em&gt;Sun&lt;/em&gt; and for &lt;em&gt;planetary system&lt;/em&gt; and &lt;em&gt;Solar System&lt;/em&gt;. This also returns the terms &lt;em&gt;Moon&lt;/em&gt;, &lt;em&gt;Sun&lt;/em&gt; and &lt;em&gt;Solar System&lt;/em&gt; to their original meanings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, even if you start to disambiguate the terms in a pedantic manner, returning to the original meaning, don't expect that other people would follow you. Calling Io, Europa and Titan as moons is easy and common, even if it is in truth a potentially confusing and possibly wrong name-bending and using the term &lt;em&gt;satellites&lt;/em&gt; would be the correct.&lt;/p&gt;&#xA;" OwnerUserId="8140" LastEditorUserId="8140" LastEditDate="2016-05-20T14:06:02.030" LastActivityDate="2016-05-20T14:06:02.030" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7710" PostTypeId="2" ParentId="1990" CreationDate="2015-01-08T16:20:38.040" Score="4" Body="&lt;p&gt;You can't &quot;buy&quot; the land as no one so far owns any lunar property so there are no sellers.  However, you can own lunar property by &lt;a href=&quot;http://en.wikipedia.org/wiki/Homestead_principle&quot; rel=&quot;nofollow&quot;&gt;homesteading&lt;/a&gt; the property.  &lt;strong&gt;Private property rights are natural, inalienable, and are not granted by any nation.&lt;/strong&gt;  You, of course, will be responsible for protecting your own liberties once you get there.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-06-09T15:26:31.817" LastActivityDate="2015-06-09T15:26:31.817" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7711" PostTypeId="1" AcceptedAnswerId="7713" CreationDate="2015-01-08T21:03:57.323" Score="4" ViewCount="456" Body="&lt;p&gt;I was just wondering to myself &quot;If Human colonization is possible on the Moon , will Mars be colonised in the near future?&quot;&#xA;The two primary documents regarding legal niceties on the Moon and often mentioned are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the outer Space Treaty of 1967, which according to Wikipedia is essentially ratified by any country with a chance of getting there.&#xA;Also, Mars is the focus of much speculation and serious study about possible human colonization. Its surface conditions and the likely availability of water make it arguably the most hospitable of the planets, other than Earth, it also requires less energy per unit mass to reach from Earth than any other planet apart from Venus, according to Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, will human colonisation commence on Mars in the (somewhat) near future?&lt;/p&gt;&#xA;" OwnerUserId="8149" LastEditorUserId="4660" LastEditDate="2015-01-09T03:38:54.320" LastActivityDate="2015-01-09T16:21:38.807" Title="Will human colonisation be introduced/commenced in the (somewhat) near future?" Tags="|mars|the-moon|colonization|crewed-spaceflight|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7712" PostTypeId="2" ParentId="7711" CreationDate="2015-01-08T21:19:53.147" Score="3" Body="&lt;p&gt;There is a project called &lt;a href=&quot;http://www.mars-one.com/&quot; rel=&quot;nofollow&quot;&gt;Mars One&lt;/a&gt; that is trying to start the process of colonizing Mars. However they need to raise fairly large amounts of money and have not. They have not built any hardware (They do plan to rely on commercial providers where possible).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond that Elon Musk of SpaceX has repeatedly said that his goal with SpaceX is to colonize Mars. He is demonstrably farthest along for several reasons.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;He has capital to fund at least some of it personally. (Tesla valuation puts him in the $10 billion range). He HAS used his personal capital to save both Tesla and SpaceX when they needed it, in the several hundred million dollar range.&lt;/li&gt;&#xA;&lt;li&gt;He has a valid business model making money (SpaceX and satellite launch) working in the right direction.&lt;/li&gt;&#xA;&lt;li&gt;He has a space related company building relevant hardware needed to colonize Mars. They have a seemingly reasonable plan to move forward. They are developing an engine (Raptor) and eventually a booster.&lt;/li&gt;&#xA;&lt;li&gt;He has a record of success.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-01-08T21:19:53.147" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="7713" PostTypeId="2" ParentId="7711" CreationDate="2015-01-08T22:50:56.843" Score="6" Body="&lt;p&gt;This has to be answered with speculation, because it comes down to a question of will. We have the technology to colonize the Moon, or Mars, or actually &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030022668.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus&lt;/a&gt; as well (by placing floating habitats in the atmosphere). The issue is expense and motivation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once Heavy Lift vehicles again exist, sometime in the 2020s, we shall see what the market is inclined to do with them. NASA's &lt;a href=&quot;http://www.nasa.gov/exploration/systems/sls/#.VK8Efd-c30o&quot; rel=&quot;nofollow noreferrer&quot;&gt;SLS&lt;/a&gt; rockets and/or SpaceX's Falcon Super Heavy rockets are needed to even begin considering creation of off-world colonies. The SLS rockets would be capable of landing about 20 tons of payload on the Moon. The capacity of the Falcon Super Heavy is not yet clear. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no &lt;em&gt;financial&lt;/em&gt; profit in going to Mars. SpaceX &lt;em&gt;may&lt;/em&gt; be able to fund &lt;a href=&quot;http://aeon.co/magazine/technology/the-elon-musk-interview-on-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;an expedition&lt;/a&gt; anyhow, out of their own pockets with the long-term hope of breaking even. See &lt;a href=&quot;https://space.stackexchange.com/q/6000/4660&quot;&gt;What is the largest hurdle of the mission to Mars?&lt;/a&gt; for extensive commentary on this. Colonizing Mars has to be entirely justified as something with sufficient merit in itself, and that is the argument SpaceX CEO Elon Musk makes - it would protect the human race against extinction in the event of catastrophe on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The long-term profitability of a colony on the Moon is probable. The approach would be media and tourism. This has not been explored formally as far as I know, however the entertainment industry in US$ is close to &lt;a href=&quot;http://www.billboard.com/biz/articles/news/global/1565728/study-global-entertainment-industry-poised-to-top-2-trillion-in&quot; rel=&quot;nofollow noreferrer&quot;&gt;2 trillion&lt;/a&gt; and tourism is over &lt;a href=&quot;http://en.wikipedia.org/wiki/World_Tourism_rankings&quot; rel=&quot;nofollow noreferrer&quot;&gt;1 trillion&lt;/a&gt;. Annual revenues sufficient to support development of a Moon colony is highly debatable, however something under 1% of this combined market should do nicely. The environment of the Moon offers ample possibilities for highly entertaining broadcast or film productions, and has obvious appeal as a destination for the super rich. It is only 3 days away at any time, and signals reach Earth from there in under 2 seconds. Neither Mars nor Venus could support such businesses, but the Moon can. The Moon would also be very useful in the development of other bases in the Earth-Moon system, such as space stations and fuel depots at the Lagrange points or in orbit around either body. Over the very long term, the Moon would be a hub for traffic around the solar system.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-01-09T15:08:24.080" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="7714" PostTypeId="1" CreationDate="2015-01-09T03:05:57.480" Score="10" ViewCount="32663" Body="&lt;p&gt;Mars has a lot of carbon dioxide in its atmosphere. It also gets a lot of sunlight. On Earth, plants use carbon dioxide and sunlight for photosynthesis. So, why can't plants live on Mars?&lt;/p&gt;&#xA;" OwnerUserId="8153" LastEditorUserId="49" LastEditDate="2015-05-17T04:30:05.980" LastActivityDate="2015-09-28T20:50:13.547" Title="Why can't plants grow on Mars?" Tags="|mars|water|astrobiology|plants|climate|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7715" PostTypeId="2" ParentId="7714" CreationDate="2015-01-09T03:31:27.957" Score="8" Body="&lt;p&gt;Life may once have existed on Mars, which is largely why so much exploration of it is being done now. There is evidence of &lt;a href=&quot;http://spacescience.arc.nasa.gov/mars-climate-modeling-group/past.html&quot; rel=&quot;noreferrer&quot;&gt;flowing water&lt;/a&gt;, which implies a warmer climate and thicker atmosphere. But that atmosphere was stripped away as the sun grew hotter, and now Mars is frozen. It doesn't have enough gravity or a magnetic field strong enough to hold in an atmosphere thick enough to keep it warm, with the sun the way it is now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plants need liquid water. Mars today has none. Well, actually it does, in the summer, for a few hours each day. Because of the presence of &lt;a href=&quot;http://en.wikipedia.org/wiki/Perchlorate#Perchlorate_on_Mars&quot; rel=&quot;noreferrer&quot;&gt;perchlorates&lt;/a&gt; in the soil, the melting temperature of water is lower, it works like antifreeze. This is enough to allow it to exist briefly in a liquid state. But then it freezes again. That brings up the matter of the extreme swings in temperature between day and night, because there isn't enough atmosphere to even temperatures out. At the equator, temperatures vary by about &lt;a href=&quot;http://www.space.com/16907-what-is-the-temperature-of-mars.html&quot; rel=&quot;noreferrer&quot;&gt;100&lt;sup&gt;o&lt;/sup&gt;C&lt;/a&gt; each day and in mid-latitudes by about &lt;a href=&quot;http://quest.nasa.gov/aero/planetary/mars.html&quot; rel=&quot;noreferrer&quot;&gt;60&lt;sup&gt;o&lt;/sup&gt;C&lt;/a&gt;. Aside from water issues, such big temperature swings present a challenge to any life form, though some can handle it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As said by cpast, plants also make use of oxygen and will die without it. They only need a little. It is used to &lt;a href=&quot;http://scienceline.ucsb.edu/getkey.php?key=760&quot; rel=&quot;noreferrer&quot;&gt;process nutrients&lt;/a&gt; drawn from the soil. On Earth, the evolution of plants began after cyanobactria began to produce oxygen as a metabolic byproduct, which eventually caused atmospheric oxygen to build up. &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/20120515-earth-life-survive-mars.html&quot; rel=&quot;noreferrer&quot;&gt;Cyanobacteria&lt;/a&gt; seem able to survive on Mars, so if there weren't other problems, this mechanism could also have occurred there. Alas, the other problems already mentioned made that impossible.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-01-10T17:00:49.063" LastActivityDate="2015-01-10T17:00:49.063" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7716" PostTypeId="2" ParentId="7701" CreationDate="2015-01-09T04:59:32.100" Score="6" Body="&lt;p&gt;In English, the name of the Sun is &lt;a href=&quot;http://en.wikipedia.org/wiki/Sun#Name_and_etymology&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Sun&lt;/em&gt;&lt;/a&gt;, and the name of the Moon is &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon#Name_and_etymology&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Moon&lt;/em&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your question stems from using the word &lt;em&gt;moons&lt;/em&gt; but not &lt;em&gt;earths&lt;/em&gt; or &lt;em&gt;suns&lt;/em&gt;. This is really a question of etymology, and to understand how we got to this point you have to put yourself in the perspective of the ancients.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To begin with, you point out that &lt;em&gt;planet&lt;/em&gt; and &lt;em&gt;star&lt;/em&gt; are generic names for the kinds of objects that the Earth and the Sun are. Not using &lt;em&gt;Earth&lt;/em&gt; or &lt;em&gt;Sun&lt;/em&gt; for the classes of objects is quite understandable, since there was nothing to indicate that the Earth and Sun were instances of these types. So a &lt;em&gt;star&lt;/em&gt; (to use the word how it has eventually evolved in modern English) was something entirely different from the Sun and the Moon, and a &lt;em&gt;planet&lt;/em&gt; was just a special kind of star: &lt;a href=&quot;http://www.etymonline.com/index.php?term=planet&quot; rel=&quot;noreferrer&quot;&gt;the word is derived from the Greek word meaning &quot;wander&quot;&lt;/a&gt;, since these stars moved through the sky relative to the other stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sun and the Moon were not members of a class of objects; they were two objects with names, and if there was anything else like them out there, we didn't know about it. Their names express the role they played in human activity. &lt;em&gt;Sun&lt;/em&gt; (along with its cognates throughout the Indo-European languages, including &lt;a href=&quot;http://www.etymonline.com/index.php?term=sun&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Sonne&lt;/em&gt;, &lt;em&gt;zon&lt;/em&gt;&lt;/a&gt;, &lt;a href=&quot;http://www.etymonline.com/index.php?term=Sol&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;sol&lt;/em&gt;, &lt;em&gt;soleil&lt;/em&gt;&lt;/a&gt;, etc.) is derived from a word meaning &quot;shine&quot;. The Sun is that bright thing that shines, lighting up our world. &lt;a href=&quot;http://www.etymonline.com/index.php?term=moon&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Moon&lt;/em&gt; is derived from the same root as &lt;em&gt;month&lt;/em&gt;&lt;/a&gt;, ultimately deriving from a word meaning &quot;measure&quot;. The Moon was (and sometimes still is!) how people measured time, and a major unit of time is how long it takes for one cycle through its phases.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Nowadays we know that the Sun is the closest example of the stars we see. But we already had the word &lt;em&gt;star&lt;/em&gt;, so we just kept using that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nowadays we know that there are other objects orbiting stars like Earth does. But we already had the word &lt;em&gt;planet&lt;/em&gt;, so we just kept using that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And now we've learned that other planets can have objects orbiting them like our Moon does us! But this is brand-new; no ancient had any idea about this moon-planet relationship. So with no other word to turn to, we just repurposed &lt;em&gt;Moon&lt;/em&gt; as a common noun meaning &quot;Moon-like object&quot;. (NB: this repurposing came at approximately the same time as &lt;a href=&quot;http://www.etymonline.com/index.php?term=satellite&quot; rel=&quot;noreferrer&quot;&gt;extending the meaning of &lt;em&gt;satellite&lt;/em&gt;&lt;/a&gt; to refer to these celestial bodies. But &lt;em&gt;satellite&lt;/em&gt; has taken on a more general meaning.)&lt;/p&gt;&#xA;" OwnerUserId="6974" LastEditorUserId="6974" LastEditDate="2015-01-09T05:11:38.463" LastActivityDate="2015-01-09T05:11:38.463" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7717" PostTypeId="2" ParentId="7714" CreationDate="2015-01-09T07:10:18.190" Score="15" Body="&lt;p&gt;Plants don't just need carbon dioxide; like most organisms, they need oxygen to survive as well. They can produce oxygen from carbon dioxide, but that doesn't help cells that aren't exposed to sunlight; if you put a plant in a very low-oxygen environment, it will die.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are organisms that can survive on little to no oxygen (e.g. cyanobacteria, which can directly supply their respiration cycle from their photosynthesis cycle), but as a rule plants aren't among them.&lt;/p&gt;&#xA;" OwnerUserId="3316" LastActivityDate="2015-01-09T07:10:18.190" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7718" PostTypeId="1" AcceptedAnswerId="7725" CreationDate="2015-01-09T11:12:08.700" Score="67" ViewCount="39410" Body="&lt;p&gt;According to what I've read, SpaceX tried to use parachutes to recover the first stage of the Falcon 9, but it did not survive reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now they plan to use 2 separate rocket burns to land:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Reentry burn&lt;/li&gt;&#xA;&lt;li&gt;Landing burn&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I understand the reentry burn in light of the previous failure, but my question is why not still use parachutes instead of a &lt;strong&gt;landing burn&lt;/strong&gt;?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2021-03-13T09:34:08.993" Title="Why would SpaceX not use parachutes for the final descent of the first stage?" Tags="|spacex|falcon-9|reuse|first-stage|" AnswerCount="8" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7719" PostTypeId="1" AcceptedAnswerId="7720" CreationDate="2015-01-09T11:37:24.867" Score="8" ViewCount="1566" Body="&lt;p&gt;Very excited about the upcoming test to land the first stage of the next Falcon 9 on an ocean platform. Elon Musk &amp;amp; Co. make it sound incredibly difficult though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm assuming that the first stage of the Falcon 9 could potentially be split into 2 parts:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A very large and simple section containing most of the fuel&lt;/li&gt;&#xA;&lt;li&gt;A relatively smaller and more complex section containing all of the expensive stuff like the engines etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it be much easier to just dump part containing the fuel (Assuming this part would be relatively cheap), and only try to land the smaller (And presumably more expensive) section?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See my sketch below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/izTsU.png&quot; alt=&quot;Split first stage into 2 parts&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-01-09T14:29:24.740" Title="Would it not be easier for SpaceX to make their first stage only partly reusable?" Tags="|spacex|falcon-9|reuse|first-stage|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7720" PostTypeId="2" ParentId="7719" CreationDate="2015-01-09T12:12:43.323" Score="15" Body="&lt;p&gt;There are several problems with your proposal:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need fuel to land the engine section, so in your proposal the stage would need two sets of fuel tanks: one disposable set and a second, smaller set that stays with the engine. This (plus the separation mechanism) makes the first stage much more complex.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the empty tank also helps the stability: the first stage has most of its weight at the bottom, which is the most stable configuration. The engine section would have its center of gravity halfway up, which makes it more likely the stage will start to tumble.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel tanks are mostly empty space, but they are made from spacecraft-grade alloys and are welded on expensive machines. This is not a coke can you're throwing away. If you're going to reuse the first stage, it makes sense to reuse as much of it as possible.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you remove the tanks, you reduce the weight by at most half, so removing them doesn't make the landing much easier. They have plenty of power available to land the weight. The complexity is in controlling the descent to land in exactly the right spot. By making the stage more stable, the fuel tanks actually help make the landing simpler. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-01-09T12:12:43.323" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7721" PostTypeId="2" ParentId="7718" CreationDate="2015-01-09T12:21:28.210" Score="22" Body="&lt;p&gt;The Falcon 9 first stage uses 3 engine burns in its landing trajectory:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Boostback, to kill its forward velocity and return in the general direction of the landing pad. This decelerates the stage from 5000 km/h to 0, plus a bit of return speed. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Reentry, to reduce its speed as it enters the atmosphere. Starts at 45 km up, the stage is supersonic at the start of this burn. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Landing, to steer the stage toward the landing pad and kill off its remaining speed. Note: if the engine fails at this stage, the current trajectory will have the rocket crash into the ocean. &#xA;So, how much remaining speed are we talking about? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;According to the &lt;a href=&quot;http://shitelonsays.com/transcript/postlanding-teleconference-with-elon-musk-2015-12-22&quot; rel=&quot;noreferrer&quot;&gt;post-landing teleconference&lt;/a&gt;, the sonic booms arrived at a point 4 miles from the landing pad at about the same time that the rocket landed. Taking a few shortcuts, that means the booms originated 20 seconds before landing, or well into the landing burn (which started &lt;a href=&quot;https://space.stackexchange.com/questions/13139/how-did-the-falcon-9s-1st-stage-return-back-to-cape-canaveral&quot;&gt;30 s before landing&lt;/a&gt;). So the landing burn starts at a speed of 400-500 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if you want to replace just the landing burn with a parachute, you'll have to do something about that initial speed. Parachutes that work at supersonic speeds are rare: the only ones I know of have been built for Mars, not Earth. So you need to extend the reentry burn to decelerate the stage to something like 150 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd also need just about the &lt;a href=&quot;http://www.prnewswire.com/news-releases/worlds-largest-rocket-stage-recovery-parachute-test-is-successful-58323952.html&quot; rel=&quot;noreferrer&quot;&gt;largest parachute ever built&lt;/a&gt; to decelerate the 23-ton stage. This parachute weighs about 1 ton. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So instead of a rocket burn with a delta-V of 500 m/s, you get a rocket burn with a delta-V of 350 m/s (1) plus a deployment of the largest parachute ever made, plus another rocket burn to cushion the landing. &#xA;This seems to add a lot of complication for little benefit, especially when you add in the fact that you also need to steer that parachute because your existing steering mechanism (rocket nozzle plus grid fins) don't work anymore when you're under a parachute.&lt;br&gt;&#xA;You've also inserted more points of failure into the system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, parachutes add a lot of work to refurbishing the rocket: they have to be inspected, carefully packed etc. And they add complexity to the landing: say the rocket lands on a barge in the ocean. The rocket's upright, but the parachutes float down into the water where they quickly fill up and start pulling the top of the rocket sideways, threatening to tip it over. You'd have to have a quick and foolproof way of disconnecting the parachute from the rocket while not letting it sink to the bottom of the ocean. Remember the barge is unmanned during the landing so this would have to be done automatically. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1: assuming the rocket speed at the end of the reentry burn is 500 m/s&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-12-27T20:54:59.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7722" PostTypeId="2" ParentId="7718" CreationDate="2015-01-09T12:57:07.863" Score="16" Body="&lt;p&gt;Parachute descents cannot be guided with any accuracy, if you pop the chutes at high altitude the stage would get carried miles away from the landing pad and the stage would not have enough residual fuel for that much lateral movement. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-01-09T12:57:07.863" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7723" PostTypeId="2" ParentId="7719" CreationDate="2015-01-09T14:20:22.450" Score="8" Body="&lt;p&gt;Let me add a few details that tears apart the justification a little more. But I'll have to introduce some speculation about the motives of reusing only part of the first stage, then I can go about tearing them down:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Perhaps the OP thinks it will be easier to maneuver without the top tank, or at least less likely to break&lt;/li&gt;&#xA;&lt;li&gt;Perhaps only recovering the bottom part will require either less fuel or less thrust, making it more viable&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I will argue that the real effect of both of these are actually &lt;em&gt;opposite&lt;/em&gt; of the intention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One analogy is that that balancing the first stage is like balancing a baseball bat on the top of your hand. But I think this ignores several aerodynamic factors. The center of gravity is higher with the top of the tank attached, but the center of pressure also moves in a stabilizing mode. If you drop a pipe with a weighted end, then the natural forces will tend to keep it upright. This doesn't change the need for active control once you fire the engines, but you need that anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, regarding the point #2, consider the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://cosmiclog.nbcnews.com/_news/2013/03/09/17251376-spacexs-elon-musk-shows-off-grasshopper-test-rockets-latest-hop?lite&quot;&gt;http://cosmiclog.nbcnews.com/_news/2013/03/09/17251376-spacexs-elon-musk-shows-off-grasshopper-test-rockets-latest-hop?lite&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Grasshopper touched down with its most accurate thus far on the centermost part of the launch pad,&quot; SpaceX said. &quot;At touchdown, the thrust-to-weight ratio of the vehicle was greater than one, proving a key landing algorithm for Falcon 9.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;When the first stage lands, its thrust to weight ratio is greater than 1. If that sounds crazy to you, you read it right. As such, it would actually be more convenient to have the stage weigh &lt;em&gt;more&lt;/em&gt; when it lands. Saving mass at this point isn't a benefit, it's a detriment. You can't throttle the engine down enough to get a perfect hovering type of landing. If you shed more mass, then you'll have to hit the ground harder, and with less margin of error.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;TANGENT: This scheme doesn't make any sense for the first stage, true. But maybe it makes sense for the 2nd, or final, stage? If this rocket reaches orbit, then you have a big problem for making it back in through the atmosphere and propulsive reentry is going to be decimated by propellent budget. As such, I think there is growing acceptance that final stage re-usability can not be anything like first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speaking extremely speculatively, a fix for this might possibly be to reenter some of the final stage's hardware inside of a capsule and heat shield sent up separately. The idea is that you tear down the rocket in an orbital workshop and prioritize the most valuable and easy to reuse parts. If you can pack those parts from several launches into a single capsule, then it could possibly make economic sense. This would be a form of re-usability for the engines, but not the tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know if it's a very good idea, but it could be coherent.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2015-01-09T14:29:24.740" LastActivityDate="2015-01-09T14:29:24.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7724" PostTypeId="2" ParentId="6403" CreationDate="2015-01-09T14:21:52.767" Score="7" Body="&lt;p&gt;For this first flight: &lt;a href=&quot;http://www.inc.com/oscar-raymundo/elon-musk-to-unveil-spacex-mars-ship-spacesuit-2015.html&quot;&gt;&quot;We are going to weld steel shoes over the landing feet as a precautionary measure.&quot; -- Elon Musk&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know that they would do that every time, but they are going to the first time.&lt;/p&gt;&#xA;" OwnerUserId="8159" LastActivityDate="2015-01-09T14:21:52.767" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7725" PostTypeId="2" ParentId="7718" CreationDate="2015-01-09T15:57:26.807" Score="65" Body="&lt;p&gt;There are many key points to this, probably none on their own sufficient to ditch the parachutes approach (except economics, those are good enough on their own), but together they make for a compelling case against it;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Descent control&lt;/strong&gt;: As already mentioned, there's a significant guidance uncertainty with the use of a parachute system. Some of it comes from the weather in the lower atmosphere, and some from the geometry of booster stage and where the forces during descent are applied to it. Weather uncertainties mean that you'll have a wide and long best effort landing ellipse where the stage could be retrieved, and with slightly unfavorable weather downrange, further limits your launch opportunities. This might be excusable for NASA's human spaceflight endeavors (I personally don't share that opinion, STS could barely launch at all due to all the safety constraints imposed on it, and such parameters don't seem to stop Russians so much from launching). The other problem is that you can't correct for any such drift as easily with propulsive system due to stage geometry.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Structural stability&lt;/strong&gt;: Geometry of the stage, being a long cylinder with its center of gravity low and close to the engines with only a certain percentage of propellants remaining, means that parachutes will apply force on the stage at the opposite end of it while its descent rate is reduced. This is not optimal. Deploying parachutes is also rather violent in terms of achievable maximum g-forces even during staged parachute deployment. This could introduce significant lateral forces to the stage if the parachutes deployed while the stage's long axis isn't aligned with its velocity vector. Rocket stages are not constructed to withstand much lateral force so they can remain light, and most forces associated with their launch and during staging are along their long axis, not lateral.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;System complexity and reliability&lt;/strong&gt;: Adding an additional, complex system of pilot, drogue and main chutes adds to the overall complexity of the already complex system, increasing chances of something going wrong. Reliability of parachute descent systems and their final design and reefing profile, is also hard to establish without first sacrificing a great deal worth of flight hardware. This is a rather tentative process with many uncertainties, and unless you have long years of testing and development behind it (like, say, NASA's Orion that has nearly 60 years long legacy in Apollo program), and can design your vehicle to withstand structural loads, it's not very likely that you'll get it right for an entirely different system before your budget runs out.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Weight (argumentative)&lt;/strong&gt;: Depending on parachutes wouldn't save much weight, if any. It's a complex system with many components, including (but not limited to) parachute fairing that would protect it during ascent, staging and hypersonic part of the descent, cabling for pilot, rogue and main chutes and a system to cut the lines when needed, canopies themselves and many additional sensors and actuators needed to make it happen. The stage would also require structural reinforcements to provide parachute attach points and withstand forces they'd introduce to the descent profile. Using parachutes also doesn't get rid of de-orbit, de-spin / stage rotation, and final landing cushioning retrofires. Such system might also require additional inflatable airbags to cushion forces associated with landing. Keeping some 10% of propellants in your tanks for the final landing burn and slowing down from stage's terminal velocity to near zero vertical speed seems pretty close to the added weight a parachute descent system would introduce.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Manufacturing volume&lt;/strong&gt;: SpaceX is keen on lowering price of launching to space wherever possible. Serializing manufacturing processes while keeping them flexible and easy to implement future improvements (often by introducing innovative manufacturing techniques such as additive manufacturing, aka 3D printing) is key to keeping non-operational costs down. So while they offer to their customers an option - fly on expendable, more capable hardware and pay more, or fly on reusable, slightly less capable hardware and pay less - they do this, or rather - will do, with exactly the same parts as far as manufacturing is concerned. There are but minor differences between F9E and F9R, most of them modular, such as addition of the grid fins and landing legs to otherwise same, or nearly same stage. Using parachutes on F9R and no parachutes on F9E would go against such doctrine and require two different stages, one reinforced to support parachutes, and an expendable, lighter version that wouldn't require that. So you'd have two manufacturing lines instead of one. Not economical.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And there are some other, finer points that I'll omit to keep the length of this answer down. But I believe we can agree that going for a system that's already an essential part of the stage was the better option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's another way of looking at it, tho. Reusable hardware has one inherent property that it is worthless unless proven to still work. So, if you fail at recovering the stage due to, say, descent engine failure, and there's not enough time to compensate with the other of the nine Merlin 1D engines because they also didn't fire in time, then most of the value of your hardware you were trying to recover is near worthless anyway. How exactly will SpaceX be discarding reusable hardware once it's tested to no longer be suitable for reuse is still somewhat a mystery, but failure to recover the stage would be obviously one of the ways to do that.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For more information also see a related question &lt;a href=&quot;https://space.stackexchange.com/questions/3911/what-was-spacexs-original-vision-for-booster-reusability&quot;&gt;What was SpaceX&amp;#39;s original vision for booster reusability?&lt;/a&gt; and I'd also recommend &lt;a href=&quot;http://www.nasa.gov/connect/ebooks/coming_home_detail.html&quot; rel=&quot;noreferrer&quot;&gt;Coming Home: Reentry and Recovery from Space&lt;/a&gt; (free download in EPUB / MOBI / PDF formats via NASA e-Books page) as a general reference book.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-01-09T18:01:12.573" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="7726" PostTypeId="2" ParentId="7711" CreationDate="2015-01-09T16:21:38.807" Score="0" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;If Human colonization is possible on the Moon , will Mars be&#xA;  colonised in the near future?&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you made an expedition to Sahara (the Moon), then you'll probably learn many things about operations and such which is useful in order to make an expedition to the Antarctic (Mars) next. But I think you'd have to redevelop almost all of your equipment from scratch, at least:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Transfer spacecraft&lt;/strong&gt; - because of 100:1 travel time Mars:Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Landing system&lt;/strong&gt; - Mars has an atmosphere and twice the gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Surface habitat&lt;/strong&gt; - temperature variations are smaller on Mars. Gravity load differs by a factor of 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Energy (solar)&lt;/strong&gt; - the Moon has 14½ Earth day long nights. Mars surface has about 1/3 the solar intensity per square meter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Communication&lt;/strong&gt; - from the Moon, Earth is always in sight and 1½ radio second away. But Mars is on average about 1,000 times further away and rotates to hide line of sight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Resources on site&lt;/strong&gt; - Primarily water on the Moon, primarily methane from the CO2 atmosphere on Mars. That's fuel for two different rocket engines or surface power plants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Self-sufficiency&lt;/strong&gt; - a Moon mission can be recalled to Earth, or resupplied if prepared, within a few days. That takes up to 2 years for Mars. Real-time remote control is available from Earth to the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Setting up an infrastructure for going to the Moon would make it cheaper and more useful to go to the Moon again. The cliff between going to the Moon or going to Mars, in the eye of the commercial/settlement people, would increase with every new investment in the Lunar infrastructure. While it would facilitate going to Mars, it would facilitate returning to the Moon even more in the early stages. On the other hand, if science and enthusiasm takes the upper hand, then the opposite effect would dominate and the relatively lesser known Mars would become more and more attractive and valuable. We'll see where the money is, I think I know ;-)&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-01-09T16:21:38.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7727" PostTypeId="2" ParentId="7718" CreationDate="2015-01-09T16:32:22.230" Score="22" Body="&lt;p&gt;Because the F9 rocket's first stage already has nine rocket engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why isn't a helicopter landed with parachutes? Because it has an engine and rotating wings. It can use them to start, fly and land. Same thing with rocket engines.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-01-09T16:32:22.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7728" PostTypeId="1" CreationDate="2015-01-09T17:42:47.630" Score="6" ViewCount="457" Body="&lt;p&gt;Curious as to how gyroscopic sensors, accelerometers and also magnetometers that are internal to the Nexus S smartphone on-board the ISS would work in a zero gravity environment. &lt;/p&gt;&#xA;" OwnerUserId="8165" LastActivityDate="2015-01-09T17:55:58.540" Title="Will the accelerometer, gyroscope and magnetometer in a smartphone work on the ISS?" Tags="|iss|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7729" PostTypeId="2" ParentId="7728" CreationDate="2015-01-09T17:55:58.540" Score="5" Body="&lt;p&gt;The sensors should all function properly.  I guess you're asking how the phone would interpret them?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The phone is in a near-zero acceleration environment (freefall), so the accelerometers should show zero when it's not being pushed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gyroscopes would work normally to sense rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The weaker magnetic field of the earth should be able to be sensed by the magnetometer (assuming it's not overwhelmed by nearby equipment), but the different shape at that altitude is going to make it horribly inaccurate.  &lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2015-01-09T17:55:58.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7730" PostTypeId="1" CreationDate="2015-01-09T20:30:41.190" Score="0" ViewCount="62" Body="&lt;p&gt;The idea is to use massive cylindrical balloons to raise the vehicle into the upper atmosphere, then reel in the balloons, burning the hydrogen. &lt;/p&gt;&#xA;" OwnerUserId="8167" LastActivityDate="2015-01-09T20:30:41.190" Title="Are hydrogen balloons a feasible launch device?" Tags="|launch|" AnswerCount="0" CommentCount="2" ClosedDate="2015-01-09T20:57:27.853" ContentLicense="CC BY-SA 3.0" />
  <row Id="7733" PostTypeId="1" AcceptedAnswerId="7734" CreationDate="2015-01-10T00:59:54.937" Score="7" ViewCount="763" Body="&lt;p&gt;Will the &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot;&gt;New Horizons&lt;/a&gt; space probe be able to discover whether Pluto is an extra solar body captured by our sun or not?&lt;/p&gt;&#xA;" OwnerUserId="7108" LastEditorUserId="5963" LastEditDate="2015-01-10T01:16:18.280" LastActivityDate="2019-03-18T16:12:35.437" Title="Will New Horizons be able to tell if Pluto is from outside the Solar System?" Tags="|extra-solar-object|pluto|new-horizons|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7734" PostTypeId="2" ParentId="7733" CreationDate="2015-01-10T02:14:29.270" Score="11" Body="&lt;p&gt;&lt;strong&gt;No.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, a Pluto-like object from outside the Solar System should look pretty much like a Pluto-like object from inside the Solar System - small and cold. When it comes down to it, one relatively tiny rock from space looks like just about any other tiny rock from space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What &lt;em&gt;could&lt;/em&gt; indicate whether or not Pluto came from outside the Solar System would be the presence of one of a few things:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A composition not consistent with that of other Kuiper Belt objects&lt;/li&gt;&#xA;&lt;li&gt;A highly eccentric orbit&lt;/li&gt;&#xA;&lt;li&gt;An age unlike that of other objects in the Solar System&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What &lt;em&gt;is&lt;/em&gt; known of Pluto's composition (&lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto#Structure&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Pluto#Structure&lt;/a&gt;), rock and ice, is not enough to determine whether it is from the Kuiper belt or beyond. While other objects in the Kuiper belt would have that composition, so would an object from a belt around another star or from interstellar space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know it doesn't have a &lt;em&gt;highly&lt;/em&gt; eccentric orbit because of observations, that its orbit is in resonance with that of Neptune. Still, this only apply to the current era, it might have had a more eccentric orbit in the past that has become less eccentric because of the influence of the outer planets until it entered its current resonance with Neptune.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last option is more interesting. We know the &lt;a href=&quot;https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System#Chronology&quot; rel=&quot;nofollow noreferrer&quot;&gt;age of the Solar System&lt;/a&gt; through &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiometric_dating&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiometric dating&lt;/a&gt;. Unfortunately, New Horizons will not be able to collect samples or rock from Pluto.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if Pluto did form outside the Solar System, chances are good that it would have the other two things - which it doesn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons &lt;em&gt;will&lt;/em&gt; be able to (among other things):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Test the chemical composition of Pluto and Charon&lt;/li&gt;&#xA;&lt;li&gt;Test the atmospheres of Pluto and Charon&lt;/li&gt;&#xA;&lt;li&gt;Figure out the surface temperatures of Pluto and Charon&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;None of these will help figure out if Pluto was formed outside the Solar System. For details of the ability of New Horizons, see &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/NHMissionFS082114HiPrint.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; NASA page.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="-1" LastEditDate="2019-03-18T16:12:35.437" LastActivityDate="2019-03-18T16:12:35.437" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="7735" PostTypeId="2" ParentId="4482" CreationDate="2015-01-10T02:43:10.510" Score="2" Body="&lt;p&gt;To minimize &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_drag&quot; rel=&quot;nofollow&quot;&gt;gravity drag&lt;/a&gt; you want to minimize fighting gravity. I basically means that you want to transfer to an orbit just above the surface. Depending on you initial trajectory this might be a &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Hohmann transfer&lt;/a&gt;, or maybe a &lt;a href=&quot;http://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot; rel=&quot;nofollow&quot;&gt;bi-elliptical transfer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After this you would want to perform a constant altitude landing. I am not sure if this is the correct technical term, since I have only heard this being by Kerbal Space Program players, when I into this myself. This was also being discussed in this &lt;a href=&quot;http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse&quot; rel=&quot;nofollow&quot;&gt;forum thread&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of fuel/change in velocity this would require would depend on your (initial) thrust to weight ratio (TWR) and engine efficiency (since less efficient engine would burn more mass, making it lighter, thus its TWR will increase faster assuming a constant exerted force). For this I also assume that the rate at which the pitch/direction of the thrust vector of the satellite has to changes would not be a problem. So the thrust should be angled in such a way that all vertical &quot;force&quot; add to zero (using a rotating reference frame, thus also include the centripetal force). So the angle of the thrust with the vertical, $\theta$, can be calculated as follows,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{F}{m}\sin{\theta}=\frac{\mu}{r^2}-\frac{v^2}{r},&#xA;$$&#xA;where $F$ is the thrust, $m$ the mass, $v$ the orbital velocity (so not necessarily relative to the surface) and $r$ the radius of the trajectory of the satellite relative to the center of the celestial body, and $\mu$ the &lt;a href=&quot;http://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow&quot;&gt;gravitational parameter&lt;/a&gt; of the celestial body. So when solving for $\theta$ yields,&#xA;$$&#xA;\theta=\sin^{-1}\left(\frac{m}{F}\left(\frac{\mu}{r^2}-\frac{v^2}{r}\right)\right),&#xA;$$&#xA;however $m$ and $v$ are time dependent, so a differential equation will have to be solved. Namely by using the rate at which &lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_impulse#General_definition&quot; rel=&quot;nofollow&quot;&gt;fuel is burned&lt;/a&gt;,&#xA;$$&#xA;\dot{m} = \frac{F}{v_e},&#xA;$$&#xA;where $v_e$ is the effective exhaust velocity. And horizontal component of the thrust,&#xA;$$&#xA;\dot{v} = \frac{F}{m}\cos{\theta}.&#xA;$$&#xA;I do not believe this differential equation can be solved analytically, so numerical methods will have to be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How I currently formulated these equations would assume that you are landing along the equator of the celestial body, either against or with its rotation, since you would have to use a stopping criteria for $v$ when your relative velocity with respect to the surface is zero. I also assumed that there are no peaks and valleys in the terrain (the celestial body is completely in &lt;a href=&quot;http://en.wikipedia.org/wiki/Hydrostatic_equilibrium&quot; rel=&quot;nofollow&quot;&gt;hydrostatic equilibrium&lt;/a&gt;). If there would be then you either would have to end up exactly on top of a peak, or drop down a bit into a valley and perform an additional burn to slow down from that.&lt;/p&gt;&#xA;" OwnerUserId="1086" LastActivityDate="2015-01-10T02:43:10.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7736" PostTypeId="2" ParentId="5767" CreationDate="2015-01-10T09:29:51.080" Score="5" Body="&lt;p&gt;According to a Virgin Galactic spokesman, &lt;a href=&quot;http://news.discovery.com/space/private-spaceflight/virgin-galactic-test-flights-to-restart-this-year-150109.htm&quot; rel=&quot;noreferrer&quot;&gt;the NTSB has determined&lt;/a&gt; the pilot unlocked the feathering mechanism too early. The aircraft's speed was too low to keep the feather in place, so it deployed, causing so much drag that structural failure ensued.&lt;br&gt;&#xA;As of 2015-01-10, Scaled Composites has built most of the second airframe. They are proceeding with the construction of the airframe. It is not known yet if the NTSB will mandate any design changes, but Virgin expects they'll be able to accommodate them in the build.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-01-10T09:29:51.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7737" PostTypeId="1" AcceptedAnswerId="7738" CreationDate="2015-01-10T10:10:24.940" Score="28" ViewCount="3751" Body="&lt;p&gt;I'm currently watching the &lt;a href=&quot;http://www.spacex.com/webcast/&quot; rel=&quot;noreferrer&quot;&gt;live stream&lt;/a&gt; of the &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX_CRS-5&quot; rel=&quot;noreferrer&quot;&gt;CRS-5&lt;/a&gt; launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They've been cycling through several different camera angles including one which looks a lot like bubbles in water, or perhaps blobs of water floating without gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering what this footage is showing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you watch that video there's an example at about 34:26, I also took some print screens:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Bflm6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/gpYaA.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/hGFl4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They look a lot more like bubbles when the footage is in motion. Here's a &lt;a href=&quot;https://www.youtube.com/watch?v=p7x-SumbynI&amp;amp;feature=player_detailpage#t=1815&quot; rel=&quot;noreferrer&quot;&gt;video of the SpaceX/CRS-5 launch&lt;/a&gt; showing this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/p7x-SumbynI?start=1575&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="8178" LastEditorUserId="4660" LastEditDate="2015-01-10T14:55:53.670" LastActivityDate="2015-01-30T17:55:02.240" Title="SpaceX CRS-5 Launch: What is the footage that appears to contain bubbles?" Tags="|launch|spacex|upper-stage|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7738" PostTypeId="2" ParentId="7737" CreationDate="2015-01-10T10:38:55.843" Score="27" Body="&lt;p&gt;That's from within the propellants tanks of the upper stage after Dragon separation and the microgravity environment saying hello. Lack of acceleration (stage engine is shut off during separation) acting on propellants when looking at them from inertial frame of reference (the tank itself) means they're essentially in free-fall together with the stage propellant tank they're in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a slight &lt;em&gt;kick&lt;/em&gt; of the Dragon separation, propellants have sloshed and that produced droplets of the liquid fuel and oxidizer (the first image shows the &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot;&gt;RP-1&lt;/a&gt; fuel tank, the other two the liquid cryogenic oxygen, or LOX tank, while the embedded video interchangeably shows both) within the tanks, interacting with each other while no other external force acts on them any different than the stage itself. Once the stage engine is reignited, some acceleration is introduced back to the tank and it essentially &lt;em&gt;catches up&lt;/em&gt; with the propellants, forcing them towards the aft of tanks in the opposite direction to the stage's acceleration vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a corollary, this is the reason why some upper stages require separate &lt;a href=&quot;https://en.wikipedia.org/wiki/Ullage_motor&quot;&gt;ullage motors&lt;/a&gt; to push propellants back towards the aft of the tank, while others might use different systems to achieve that, like with &lt;a href=&quot;https://en.wikipedia.org/wiki/Attitude_control&quot;&gt;attitude control&lt;/a&gt; system's &lt;a href=&quot;https://en.wikipedia.org/wiki/Vernier_thruster&quot;&gt;vernier thrusters&lt;/a&gt; for a slight stage &lt;em&gt;kick&lt;/em&gt;, a wick within the tank that supplies fuel through their surface tension, or pressurant ullage gas and a membrane (aka a bladder) to provide propellants pressure in the feed line.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For additional explanation on how liquids slosh in microgravity, see this NASA's &lt;a href=&quot;https://www.youtube.com/watch?v=AKrmrbCTNxc&quot;&gt;ScienceCasts: The Strange Way Fluids Slosh on the International Space Station&lt;/a&gt; video presentation of the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/980.html&quot;&gt;SPHERES-Slosh experiment&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/AKrmrbCTNxc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-30T17:55:02.240" LastActivityDate="2015-01-30T17:55:02.240" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7739" PostTypeId="2" ParentId="4482" CreationDate="2015-01-10T14:00:12.277" Score="2" Body="&lt;p&gt;Just to add some $\Delta v$ preliminary estimation in the case of a landing performed with a &quot;Hohmann-like&quot; 2-impulses manoeuvre.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume we start from a 100 km parking orbit and we want to land on the Moon (assumed as perfectly spherical).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v_{tot} = \Delta v_{1} + \Delta v_{2}$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v_{1} = \sqrt{\dfrac{\mu}{r_{park}}} - \sqrt{2 \mu \biggl( \dfrac{1}{r_{park}} - \dfrac{1}{r_{park}+r_{moon}} \biggr)} = 22.99 \ m/s $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v_{2} =  \sqrt{2 \mu \biggl( \dfrac{1}{r_{moon}} - \dfrac{1}{r_{park}+r_{moon}} \biggr)} = 1702.40 \ m/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v_{tot} = 1725.39 \ m/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i64.photobucket.com/albums/h182/Olgola/hohmann.jpg&quot; alt=&quot;hohmann&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issues related to this manoeuvre are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;too tight trajectory&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;not vertical touchdown (the $\Delta v$ will not be directed in the vertical direction)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A better approach would be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;impulsive de-orbiting burn on an intermediate transfer orbit&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;coasting phase (ballistic)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;powered descent phase&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i64.photobucket.com/albums/h182/Olgola/powered.jpg&quot; alt=&quot;powered_descent&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6608" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.540" LastActivityDate="2015-01-10T19:33:21.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7743" PostTypeId="2" ParentId="7714" CreationDate="2015-01-10T15:34:48.920" Score="11" Body="&lt;p&gt;All life as we know it requires liquid water as a medium (a polar solvent) in which the chemistry of life can take place. Mars surface conditions pretty much preclude the existence of liquid water. The atmospheric pressure is at or below the triple point of water, meaning that water can only exist as ice or gas (vapor). Even though Martian soil probably contains all the elements necessary for plant growth, and it appears as though Mars once had liquid water on its surface (it would have had a thicker atmosphere back then), plants could not survive in current Martian conditions due to the absence of liquid water. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the Martian atmosphere were a bit thicker, it could be a different situation, but, as pointed out in one of the other answers, plants actually need oxygen as well as carbon dioxide. In the presence of sunlight, plants consume carbon dioxide and produce oxygen as a &quot;waste product&quot;. In darkness, plants still have to support their metabolism and consume oxygen from the atmosphere to do so. Apart from that, plants are not islands of life; they have symbiotic relationships with other organisms including bacteria. Some of those organisms require oxygen too.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-01-10T15:34:48.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7744" PostTypeId="1" AcceptedAnswerId="7746" CreationDate="2015-01-10T17:22:20.873" Score="5" ViewCount="4533" Body="&lt;p&gt;Diagrams and descriptions of the F1 engine as used on the Saturn V booster identify a turbine exhaust manifold which leads down from the fuel turbopump and wraps around the main nozzle. As it winds around the nozzle it gets progressively smaller in diameter. This feature can be seen in images and models of the engine. What exactly is the path of the turbine exhaust - where/how does it discharge from the manifold? Does it enter the nozzle to join the main thrust exhaust? Why was it designed as it was?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2015-01-10T17:43:04.350" LastActivityDate="2015-01-10T18:19:04.837" Title="Where does the turbine exhaust go on the Saturn V F1 engine?" Tags="|rockets|saturn-v|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7745" PostTypeId="1" AcceptedAnswerId="7955" CreationDate="2015-01-10T17:25:09.693" Score="11" ViewCount="773" Body="&lt;p&gt;I'm a big fan of the &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030022668.pdf&quot; rel=&quot;nofollow&quot;&gt;floating aerostat&lt;/a&gt; concept developed by &lt;a href=&quot;http://www.geoffreylandis.com/&quot; rel=&quot;nofollow&quot;&gt;Geoffrey Landis&lt;/a&gt; for &lt;a href=&quot;http://spectrum.ieee.org/aerospace/space-flight/nasa-study-proposes-airships-cloud-cities-for-venus-exploration&quot; rel=&quot;nofollow&quot;&gt;colonizing Venus&lt;/a&gt;. But how would you supply a floating base?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be an awfully small target to aim a vessel at. The Space Shuttle managed to make it to a runway - but it needed a runway. The base would not be a heavy or rigid object capable of assisting much in vehicle braking. The winds would be greater than hurricane force all the time. At the height of the clouds, about &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation&quot; rel=&quot;nofollow&quot;&gt;60 to 70 km&lt;/a&gt; above the surface, they are measured at roughly &lt;a href=&quot;http://www.astronomy.com/news/2013/06/the-fast-winds-of-venus-are-getting-faster&quot; rel=&quot;nofollow&quot;&gt;300 to 400 km/h&lt;/a&gt;. The base would be carried along by these winds, stationed at a height of about 50 km. What sort of vessel and approach can deal with those conditions?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="109" LastEditDate="2019-02-26T19:10:11.423" LastActivityDate="2019-02-26T19:10:11.423" Title="How could the proposed floating base in the atmosphere of Venus receive supplies?" Tags="|reentry|colonization|venus|balloons|supply-mission|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="7746" PostTypeId="2" ParentId="7744" CreationDate="2015-01-10T17:35:47.340" Score="7" Body="&lt;p&gt;The F-1 exhaust consists of 2 parts: the nozzle and the nozzle extension. The manifold sits at the point where they meet.&lt;br&gt;&#xA;The exhaust gases from the turbine are fed through the manifold into the nozzle extension. The passage was designed to guide this gas along the wall of the nozzle extension. This gas was cooler than the main rocket exhaust gas, so this design reduced the thermal load on the nozzle extension.&lt;br&gt;&#xA;From the &lt;a href=&quot;http://history.msfc.nasa.gov/saturn_apollo/documents/F-1_Engine.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA fact sheet&lt;/a&gt; (page 3):  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The interior of the nozzle extension is protected from the engine exhaust gas environment (5800 F) by film cooling, using the turbine exhaust gases (1200 F) as the coolant. The gases enter the extension between a continuous outer wall and a shingled inner wall, pass out through injection slots between the shingles, and flow over the surfaces of the shingles forming a boundary layer between the inner wall of the nozzle extension and the hotter exhaust gases exiting from the main engine combustion chamber. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here's a cross-section of the manifold (&lt;a href=&quot;http://heroicrelics.org/info/f-1/f-1-thrust-chamber.html&quot; rel=&quot;noreferrer&quot;&gt;borrowed from Herorelics.org&lt;/a&gt;):&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/XygUc.jpg&quot; alt=&quot;F-1 manifold&quot;&gt;&lt;br&gt;&#xA;And another view:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/hktrh.jpg&quot; alt=&quot;F-1 manifold 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nozzle is cooled by fuel: the fuel is fed through channels in the nozzle wall. The channels end at the top of the nozzle, where the fuel is fed though the injector plate to be burned. Here's a diagram: &#xA;&lt;img src=&quot;https://i.stack.imgur.com/LOxgJ.jpg&quot; alt=&quot;F-1 nozzle cooling&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://heroicrelics.org/resources.html#f-1&quot; rel=&quot;noreferrer&quot;&gt;Pile of F-1 info&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-01-10T18:19:04.837" LastActivityDate="2015-01-10T18:19:04.837" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7747" PostTypeId="1" AcceptedAnswerId="7748" CreationDate="2015-01-10T17:40:53.417" Score="4" ViewCount="5949" Body="&lt;p&gt;In the Apollo era, whenever there was a significant event about to occur, the flight director (I can recall the voice of Gene Kranz) would do a sort of go/no-go &quot;roll call&quot;. He would call off names like &quot;fido&quot;, &quot;gido&quot;(?), etc. (obviously shortened from a more meaningful title), to which someone would respond with a &quot;go&quot; (don't ever recall hearing a &quot;no go&quot;). Views of the controller room showed perhaps dozens of people; the director called off maybe 7 names.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What were the short names short for (what was the full title and scope of responsibility), and how was a room full of controllers organized so that only a short call-off list properly represented an over-all &quot;go&quot;/&quot;no-go&quot; status?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="49" LastEditDate="2015-02-20T01:03:57.807" LastActivityDate="2019-07-09T13:51:04.793" Title="What exactly were the controller roles in the Apollo go/no-go calls?" Tags="|nasa|apollo-program|terminology|procedure|launch-sequence|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7748" PostTypeId="2" ParentId="7747" CreationDate="2015-01-10T18:06:42.437" Score="7" Body="&lt;p&gt;That &quot;GIDO&quot; would probably be &quot;GUIDO&quot; (Guidance Officer). Here's some of the most common Apollo program &lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller&quot; rel=&quot;noreferrer&quot;&gt;flight controller&lt;/a&gt; station call signs (from linked to Wikipedia page):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Common_flight_control_positions&quot; rel=&quot;noreferrer&quot;&gt;Common flight control positions&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Flight_Director_.28FLIGHT.29&quot; rel=&quot;noreferrer&quot;&gt;Flight Director (FLIGHT)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Mission_Operations_Directorate_.28MOD.29&quot; rel=&quot;noreferrer&quot;&gt;Mission Operations Directorate (MOD)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Capsule_Communicator_.28CAPCOM.29&quot; rel=&quot;noreferrer&quot;&gt;Capsule Communicator (CAPCOM)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Flight_Surgeon_.28SURGEON.29&quot; rel=&quot;noreferrer&quot;&gt;Flight Surgeon (SURGEON)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Public_Affairs_Officer_.28PAO.29&quot; rel=&quot;noreferrer&quot;&gt;Public Affairs Officer (PAO)&lt;/a&gt;&#xA;  &lt;br /&gt;&lt;br /&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Apollo_flight_control_positions&quot; rel=&quot;noreferrer&quot;&gt;Apollo flight control positions&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Booster_Systems_Engineer_.28BOOSTER.29&quot; rel=&quot;noreferrer&quot;&gt;Booster Systems Engineer (BOOSTER)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Control_Officer_.28CONTROL.29&quot; rel=&quot;noreferrer&quot;&gt;Control Officer (CONTROL)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Electrical.2C_Environmental_and_Consumables_Manager_.28EECOM.29&quot; rel=&quot;noreferrer&quot;&gt;Electrical, Environmental and Consumables Manager (EECOM)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Flight_Activities_Officer_.28FAO.29&quot; rel=&quot;noreferrer&quot;&gt;Flight Activities Officer (FAO&lt;/a&gt;)&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Flight_Dynamics_Officer_.28FDO_or_FIDO.29&quot; rel=&quot;noreferrer&quot;&gt;Flight Dynamics Officer (FDO or FIDO)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Guidance_Officer_.28GUIDANCE_or_GUIDO.29&quot; rel=&quot;noreferrer&quot;&gt;Guidance Officer (GUIDANCE or GUIDO)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Guidance.2C_Navigation.2C_and_Controls_Systems_Engineer_.28GNC.29&quot; rel=&quot;noreferrer&quot;&gt;Guidance, Navigation, and Controls Systems Engineer (GNC)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Instrumentation_and_Communications_Officer_.28INCO.29&quot; rel=&quot;noreferrer&quot;&gt;Instrumentation and Communications Officer (INCO)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Network_.28NETWORK.29&quot; rel=&quot;noreferrer&quot;&gt;Network (NETWORK)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Organization_and_Procedures_Officer_.28O.26P.29&quot; rel=&quot;noreferrer&quot;&gt;Organization and Procedures Officer (O&amp;amp;P)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Retrofire_Officer_.28RETRO.29&quot; rel=&quot;noreferrer&quot;&gt;Retrofire Officer (RETRO)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Telemetry.2C_Electrical.2C_EVA_Mobility_Unit_Officer_.28TELMU.29&quot; rel=&quot;noreferrer&quot;&gt;Telemetry, Electrical, EVA Mobility Unit Officer (TELMU)&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Please see the links for stations' descriptions. Also see &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_status_check#Apollo_missions&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Launch status check&lt;/a&gt; (maybe it would be a better source for the quote above... &lt;em&gt;shrug&lt;/em&gt;).&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-01-10T18:06:42.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7749" PostTypeId="1" AcceptedAnswerId="7750" CreationDate="2015-01-10T19:47:52.823" Score="4" ViewCount="9027" Body="&lt;p&gt;I've seen in some videos of astronauts that they used to use a jetpack type system to maneuver in the vacuum of space around the ISS and other satellites using pressurized gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Manned_Maneuvering_Unit&quot; rel=&quot;nofollow&quot;&gt;From Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Gaseous nitrogen was used as the propellant for the MMU. Two aluminium tanks with Kevlar wrappings contained 5.9 kilograms of nitrogen each, enough propellant for a six-hour EVA depending on the amount of maneuvering done. Typical MMU delta-v (velocity change) capability was about 80 feet per second (25 m/s). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is perhaps a convoluted question because of the circumstances that surround my asking of this question. I was explaining to a friend that we don't really &quot;pilot&quot; shuttles to the moon or other objects, rather it is like a calculated throw, a one time launch that will get us to the object because of our velocity, direction, and the gravitational pull of objects in space. To that, she raised the question, &quot;Well why do jetpacks work in space if there is nothing to push against?&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, there &lt;em&gt;isn't&lt;/em&gt; anything for the gas to push against. The best answer I could muster was that expelling gas from a pressurized container still results in a force, and according to Newton's Third Law, there is an opposite force pushing the astronaut away. This can also theoretically be accomplished by throwing an object to propel yourself away from the direction of the thrown object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to know more specifically how a pressurized container creates force if there is nothing to push against, if indeed there &lt;em&gt;is&lt;/em&gt; nothing to push against. Another question may arise from this answer, but I will save it for it's own thread.&lt;/p&gt;&#xA;" OwnerUserId="7027" LastEditorUserId="26446" LastEditDate="2021-07-05T03:51:14.517" LastActivityDate="2021-12-18T07:13:24.753" Title="How do jetpacks work in space?" Tags="|physics|astronauts|astronaut-propulsion|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="7750" PostTypeId="2" ParentId="7749" CreationDate="2015-01-10T20:24:55.813" Score="16" Body="&lt;p&gt;As you said, action = reaction, &lt;a href=&quot;http://www.physicsclassroom.com/class/newtlaws/lesson-4/newton-s-third-law&quot; rel=&quot;noreferrer&quot;&gt;Newton's third law&lt;/a&gt;. In a rocket engine, a fuel is burned, creating gas that expands. Now this gas wants to expand equally in all directions. In the 'front' and 'side' directions, the gas encounters the rocket nozzle and pushes against it. In the 'rear' direction, the gas goes out the end of the nozzle and into the vacuum of space.&lt;br&gt;&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/4108/how-does-a-thin-metal-rocket-engine-not-get-crushed-by-the-weight-of-the-flying/5509#5509&quot;&gt;The gas pushing against the sides of the nozzle produces no net force:&lt;/a&gt; the various directions cancel each other out. But the gas pushing against the front of the nozzle has no counterpart, so there is a net force on the nozzle pointing forward.&lt;br&gt;&#xA;Here's a diagram of the forces exerted by the expanding gas inside the engine:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Efr3t.jpg&quot; alt=&quot;diagram&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-01-10T20:24:55.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7751" PostTypeId="1" AcceptedAnswerId="7756" CreationDate="2015-01-10T20:38:08.037" Score="16" ViewCount="7165" Body="&lt;p&gt;The flame exiting the space shuttle main engine is obviously transparent. But, you can't see objects through either the Saturn V booster flame or that of the space shuttle solid rocket boosters. Is it that they are actually just as transparent as the hydrogen-fueled flame of the shuttle main engines, but emitting so brightly in visible wavelengths that transmitted light is just too dim by comparison, or is there actually material in the exhaust of a type and at sufficient density to obscure any transmitted light?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2015-01-10T22:19:27.310" LastActivityDate="2016-05-30T15:06:14.580" Title="Are rocket exhaust flames ever opaque?" Tags="|rockets|exhaust|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7752" PostTypeId="2" ParentId="7745" CreationDate="2015-01-10T20:39:40.710" Score="5" Body="&lt;p&gt;According to &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030022668.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Colonization of Venus&lt;/a&gt;, by Geoffrey A. Landis (2003), most resources necessary for sustaining the colonies will be accessed &lt;em&gt;in situ&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A permanent settlement will need access to the resources required for human life and for greenhouses to provide food and oxygen, and the atmosphere of Venus has these in abundance. Atmospheric carbon dioxide and nitrogen are a plentiful resource. Along with hydrogen reaped from condensing atmospheric sulfuric acid droplets, the basic elements needed for human survival can be found in the atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;A settlement will require structural and industrial materials as well. These materials, such as silicon, iron, aluminum, magnesium, calcium, potassium, sodium etc. can be mined from the surface material, which is apparently primarily a basaltic silicate. Access to the surface is relatively simple from an aerostat, since the thick atmosphere allows flight by airplanes (&lt;a href=&quot;http://media.cleveland.com/science_impact/other/Landis%20Venus%20atmospheric%20flight%20study%202002.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Landis 2001&lt;/a&gt;) or balloons (already demonstrated on Venus during the Russian VEGA mission [Bougher, Hunten and Phillips 1997]).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As Nick2253 points out in his comment below, the surface may be accessible enough, but conditions for mining are extremely challenging. Air pressure at the planet's surface is about 92 times that at Earth's surface, while temperatures run at least 462&lt;span class=&quot;math-container&quot;&gt;$^o$&lt;/span&gt; C (864&lt;span class=&quot;math-container&quot;&gt;$^o$&lt;/span&gt; F). Perhaps by the time such colonies exit the technology could be hardy enough, but it's a good bet many such materials will have to be imported.&lt;/p&gt;&#xA;&lt;p&gt;With sufficient hydrogen and all that CO2, via the Sabatier reaction, comes methane, useful as fuel.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Breathing oxygen for life support can be easily provided by separation of oxygen from atmospheric carbon dioxide, either by zirconia electrolysis or by Sabatier processes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Landis uses breathable air as part of his lifting support.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On Venus, breathable air (i.e., oxygen/nitrogen mixture at roughly 21:78 mixture ratio) is a lifting gas. The lifting power of breathable air in the carbon dioxide atmosphere of Venus is about half kg per cubic meter. Since air is a lifting gas on Venus: the entire lifting envelope of an aerostat can be breathable gas, allowing the full volume of the aerostat to be habitable volume. For comparison, on Earth, helium lifts about one kg per cubic meter, so a given volume of air on Venus will lift about half as much as the same&#xA;volume of helium will lift on Earth.).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I can imagine it as living in a bubble with a partially dirt deck where the tomatoes grow.&lt;/p&gt;&#xA;&lt;p&gt;I don't think it's speculation that cargo ships with supplies of LOX, cargo containers of ice, and the mail, among other things,  would spring up to service such floating Venusian colonies. The logical infrastructure (to my mind) would be &amp;quot;space supertankers&amp;quot; that are captured into low Venus orbit, serviced by optimized craft designed to ferry goods from orbit to aerostat.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-02-01T01:08:27.127" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7753" PostTypeId="1" CreationDate="2015-01-10T21:28:29.597" Score="34" ViewCount="20886" Body="&lt;p&gt;I understand the benefits of reusability for the rocket booster. But I don't understand why they're landing it on a barge. I've read tons of articles about this but not one has even hinted at an answer. Why is landing on a barge preferable over a launchpad or any other large flat surface that isn't moving with the waves?&lt;/p&gt;&#xA;" OwnerDisplayName="user379" LastActivityDate="2017-12-19T13:35:21.610" Title="Why did SpaceX attempt to land the booster on a barge instead of somewhere on land?" Tags="|spacex|landing|falcon-9|reuse|" AnswerCount="6" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7754" PostTypeId="5" CreationDate="2015-01-10T21:29:47.837" Score="0" Body="&lt;p&gt;Nuclear thermal rockets have a higher performance than traditional chemical rockets, despite not heating the propellant more. This is achieved because you can choose a propellant freely, allowing the use of low molar mass molecules like $H_2$. A NTR has higher efficiency, but less thrust, making it most useful for upper-stages or space tugs. No NTRs are currently operational, but prototypes were developed by both the US and the Soviet union. See &lt;a href=&quot;https://en.wikipedia.org/wiki/NERVA&quot; rel=&quot;nofollow&quot;&gt;NERVA&lt;/a&gt; and &lt;a href=&quot;http://www.astronautix.com/engines/rd0410.htm&quot; rel=&quot;nofollow&quot;&gt;RD-0410&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-26T08:27:09.390" LastActivityDate="2016-01-26T08:27:09.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7755" PostTypeId="4" CreationDate="2015-01-10T21:29:47.837" Score="0" Body="NTR or Nuclear thermal rocket is a subtype of rocket propulsion wherein gas/fluid is heated inside a special-purpose nuclear reactor and then expelled at large velocity." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-01-10T21:46:48.797" LastActivityDate="2015-01-10T21:46:48.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7756" PostTypeId="2" ParentId="7751" CreationDate="2015-01-10T21:37:48.600" Score="32" Body="&lt;p&gt;The reason for the transparency of the Space Shuttle Main Engine's flame is the fuel it's burning. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SSMEs burn liquid hydrogen and liquid oxygen, and when hydrogen and oxygen burn the &lt;a href=&quot;http://en.wikipedia.org/wiki/Hydrogen#Combustion&quot; rel=&quot;noreferrer&quot;&gt;flame is nearly invisible to the human eye&lt;/a&gt;. i.e. it's hardly emitting any visible light, the gases themselves are colorless, and the product of the reaction, H&lt;sub&gt;2&lt;/sub&gt;O (water vapor), is also invisible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sgCom.jpg&quot; alt=&quot;Space Shuttle Main Engine test firing&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(digression)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another good example of this kind of “invisible” flame is the combustion of hydrazine with tetroxide, à la SpaceX SuperDraco engines (pictured).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q6SkR.jpg&quot; alt=&quot;SpaceX SuperDraco engine test firing&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(/digression)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both of your other two examples, the Saturn V first stage and the space shuttle solid boosters used much dirtier fuels. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RP-1 that the F-1 engines on the Saturn V first stage used is a highly refined form of kerosene that burns with a very bright flame. Just like almost every engine, the F-1s didn't completely burn all their fuel before ejecting it as exhaust, add to that minute imbalances in the fuel/oxygen mix and the result is the long plume of flame that is both partially opaque from the fuel and soot from the engines, and emitting millions of Lumens of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nabSt.jpg&quot; alt=&quot;Saturn V rocket ascending from the launch pad&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space shuttle solid rocket boosters burned aluminium with an oxidizer (ammonium perchlorate), along with a &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#Propellant&quot; rel=&quot;noreferrer&quot;&gt;whole bunch of other compounds&lt;/a&gt; included in the propellant. What sets the solid booster exhaust apart from our other two examples is the amount of particulates it contained. It was basically chucking tons of powdered aluminium oxide (a bright white compound) into the air, thus the exhaust was completely opaque.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cOnXc.jpg&quot; alt=&quot;Space Shuttle Discovery just after lift-off&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="1419" LastEditDate="2016-05-30T13:45:39.123" LastActivityDate="2016-05-30T13:45:39.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7757" PostTypeId="2" ParentId="7753" CreationDate="2015-01-10T22:15:26.960" Score="18" Body="&lt;p&gt;Most people would cite danger to populations and property as the foremost reason to stay away from land. You're setting down a rocket on a tail of flame like a 50s sci-fi film, after all. But the economics of the launch physics is the overwhelming factor.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;flying over populated areas&quot; question is pretty much solved by their options to launch, currently coastal areas. By the time of the first stage &lt;a href=&quot;http://www.daviddarling.info/encyclopedia/M/MECO.html&quot; rel=&quot;nofollow&quot;&gt;MECO (Main Engine Cut Off)&lt;/a&gt; and separation, the Falcon 1st stage is significantly northeast of Canaveral, with nothing but water underneath.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk did pitch trying to fly back to the launch pad, but the tyranny of the rocket equation makes the price exacted on payload unacceptable. Even if some of the booster cores from the Falcon Heavy make it back to shore (and this remains to be seen), they still plan to land the center core on a barge.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2016-01-19T04:43:44.480" LastActivityDate="2016-01-19T04:43:44.480" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7758" PostTypeId="5" CreationDate="2015-01-10T22:31:05.430" Score="0" Body="&lt;p&gt;Mission design (more broadly, mission analysis and design) is the (typically, iterative and gradual, yet systematic) process of finding an engineering solution to a complex scientific, technical, military, political, or social problem that potentially involves a spacecraft. It is meant to minimize the cost of a space mission or the &quot;system of systems&quot; where it belongs while meeting the objectives and constraints set by mission planners and customers.&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="25" LastEditDate="2016-06-12T13:20:41.250" LastActivityDate="2016-06-12T13:20:41.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7759" PostTypeId="4" CreationDate="2015-01-10T22:31:05.430" Score="0" Body="Mission design is the process of designing a space mission to fulfill a particular objective." OwnerUserId="168" LastEditorUserId="40257" LastEditDate="2022-05-03T14:43:50.957" LastActivityDate="2022-05-03T14:43:50.957" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="7760" PostTypeId="2" ParentId="7753" CreationDate="2015-01-10T22:52:29.003" Score="5" Body="&lt;p&gt;The only reason SpaceX is landing on the barge is to demonstrate to the range safety community (and themselves) that they can safely and reliably land the first stage.  There is a very little propulsive advantage to landing on the barge where SpaceX is positioning it vs. returning to the launch site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good video showing SpaceX's aspirations in this area is shown in this &lt;a href=&quot;http://youtu.be/4Ca6x4QbpoM&quot; rel=&quot;nofollow&quot;&gt;video&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-01-27T20:38:47.020" LastActivityDate="2015-01-27T20:38:47.020" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="7763" PostTypeId="1" AcceptedAnswerId="7764" CreationDate="2015-01-11T06:07:58.547" Score="4" ViewCount="229" Body="&lt;p&gt;What was the on station position (lat, long) of the barge for the January 10th launch?&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-01-11T12:12:41.647" Title="Position of the SpaceX barge for the January 10th launch" Tags="|spacex|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7764" PostTypeId="2" ParentId="7763" CreationDate="2015-01-11T06:30:46.070" Score="7" Body="&lt;p&gt;The &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=36326.750&quot; rel=&quot;nofollow noreferrer&quot;&gt;ASDS forum&lt;/a&gt; over at NasaSpaceflight.com were lamenting the lack of satellite data for a precise fix, but they followed the GoQuest escort boat until it went out of range on vesselfinder.com:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/pQka1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;Closer to launch time someone on the forum reported wave height at this closest buoy:&lt;br&gt;&#xA;&lt;a href=&quot;http://www.ndbc.noaa.gov/station_page.php?station=41010&amp;amp;unit=M&amp;amp;tz=STN&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ndbc.noaa.gov/station_page.php?station=41010&amp;amp;unit=M&amp;amp;tz=STN&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ships appear to be headed back in now:&lt;br&gt;&#xA;&lt;a href=&quot;http://www.vesselfinder.com/?mmsi=367017460&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.vesselfinder.com/?mmsi=367017460&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflight101.com/dragon-spx-5-mission-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt; placed the ASDS here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UjLu0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="49" LastEditDate="2015-01-11T12:12:41.647" LastActivityDate="2015-01-11T12:12:41.647" CommentCount="0" CommunityOwnedDate="2015-01-11T06:30:46.070" ContentLicense="CC BY-SA 3.0" />
  <row Id="7766" PostTypeId="2" ParentId="7737" CreationDate="2015-01-11T08:22:55.097" Score="11" Body="&lt;p&gt;While TildalWave's answer is mostly correct, he missed one important thing. This is not the RP-1 fuel, it's &lt;strong&gt;Liquid Oxygen&lt;/strong&gt;, the oxidizer - floating in microgravity after the Merlin 1D Vacuum engine shutoff. There are two very clear giveaways for this.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;RP-1 has a yellowish hue, whereas Liquid Oxygen has a more characteristic blue color, which can be seen in these images taken from the top of the LOX tank.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The RP-1 tank has an insulated pipe from the LOX tank above running through it, to facilitate the delivery of LOX to the Merlin engine. No such pipe is seen in this picture.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;And yes, as TildalWave also mentioned, small ullage motors are commonly used to &quot;push&quot; the fuel back down to the outlet of the tank, by inducing a tiny forward accelaration on the stage. In Falcon 9's case, pressurized Nitrogen delivered via cold gas thrusters is used to provide this kick. Some rockets, such as the Delta IV, use solid motors. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-01-11T08:22:55.097" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7767" PostTypeId="2" ParentId="7753" CreationDate="2015-01-11T11:26:51.217" Score="5" Body="&lt;p&gt;I see nobody seems to have mentioned the simplest reason why they're not landing on land: &lt;em&gt;there isn't any&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They launch east from Florida, and there aren't any islands in the Atlantic out there. Without a major course change from the first stage, which as other people point out would use a prohibitive amount of fuel, then it's basically going to come down in vast stretches of empty ocean. Hence the barge.&lt;/p&gt;&#xA;" OwnerUserId="8192" LastActivityDate="2015-01-11T11:26:51.217" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7768" PostTypeId="1" CreationDate="2015-01-11T11:32:08.140" Score="5" ViewCount="2178" Body="&lt;p&gt;So Kerbal Space Program's map view is one of the greatest tools for visualising trajectories and the relative position of bodies and spacecraft I've ever seen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know if someone's produced a similar tool for the &lt;em&gt;real&lt;/em&gt; solar system, regularly updated with information for all the various spacecraft and probes that we've launched so far? It would make it so much simpler to see at a glance, e.g., the F9 station rendezvous mission profile (which I've just failed to find information on online).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screenshot follows; for those who don't know KSP, the blue arc is the currently selected object's trajectory, the asterisk marks a manoeuvre, the dotted arc is the trajectory after the manoeuvre, the green arc is the target object, the red and blue markers show various closest-approach information, etc, etc. The player here is planning an orbital rendezvous. The circular object at the bottom is for controlling the player's ship and not relevant to the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://blog.asgaard.co.uk/assets/img/2013-08-27/kerbal-map-view.jpg&quot; alt=&quot;ObScreenshot&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8192" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.557" LastActivityDate="2017-09-17T04:20:16.440" Title="KSP-like interactive map of the real solar system?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="7769" PostTypeId="1" CreationDate="2015-01-11T14:24:41.143" Score="11" ViewCount="884" Body="&lt;p&gt;In case of necessity -- for example in case of fire or contamination -- is it possible for any pressurized element of the ISS to be exposed to vacuum?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If all compartments and airlocks were to be depressurized, I assume it would be possible for astronauts to take refuge in the Soyuz until the station is pressurized again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this something that is feasible without damages to the elements? if so:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Has it been anticipated for specific eventualities?&lt;/li&gt;&#xA;&lt;li&gt;What would be the time required to repressurize the whole station?&lt;/li&gt;&#xA;&lt;li&gt;Would that require an immediate re-supply for food, water, etc?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edited&lt;/strong&gt;. About Zvezda:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Much of the equipment necessary for maintaining ISS on-orbit, in the&#xA;  SM, is air-cooled and/or not vacuum-compatible, and will therefore&#xA;  eventually fail after a depressurization&quot;&lt;br&gt;&#xA;  (source: &lt;a href=&quot;http://files.seds.org/pub/info/newsletters/spaceviews/text/20000605.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceViews&lt;/a&gt;, 2000 - Citing Nasa ISS Safety Noncompliance Report)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Related information, but not duplicate questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/881/are-there-any-safety-procedures-in-place-on-the-iss-in-case-of-puncture&quot;&gt;Are there any safety procedures in place on the ISS in case of puncture?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2576/what-would-happened-if-the-iss-was-depressurized-while-astronauts-were-shelterin?lq=1&quot;&gt;What would happened if the ISS was depressurized while astronauts were sheltering?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-12-14T19:05:08.753" Title="Could the ISS pressurized elements withstand a full depressurization?" Tags="|iss|life-support|construction|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="7771" PostTypeId="1" AcceptedAnswerId="7772" CreationDate="2015-01-11T15:05:37.600" Score="35" ViewCount="10418" Body="&lt;p&gt;According to many news articles, the crash of the Falcon 9 during landing on 1/10/2015 was due to running out of hydraulic fluid for the steerable hypersonic grid fins.  This was seemingly confirmed the Musk's tweet &quot;Upcoming flight already has 50% more hydraulic fluid, so should have plenty of margin for landing attempt next month.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why, oh why, do the fins consume hydraulic fluid??  Hydraulic fluid is normally used in a closed system that only consumes the fluid via leaks, right?  Are the leaks due to temperature changes so bad, and so unsolvable, that the only solution is to just have more in reserve?&lt;/p&gt;&#xA;" OwnerUserId="8196" LastEditorUserId="10612" LastEditDate="2015-12-21T00:57:57.350" LastActivityDate="2020-09-12T18:12:45.963" Title="Why does the Falcon 9 consume hydraulic fluid?" Tags="|spacex|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7772" PostTypeId="2" ParentId="7771" CreationDate="2015-01-11T15:22:57.820" Score="38" Body="&lt;p&gt;Normal hydraulic systems are closed. When hydraulic fluid is squeezed out of a cylinder, it returns to a holding tank ready to be pressurized again by the pump.&lt;br&gt;&#xA;SpaceX have confirmed the Falcon 9 uses an &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/2s1lq9/my_guess_about_the_hydraulic_system/&quot;&gt;open system&lt;/a&gt; instead: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hydraulics are usually closed, but that adds mass vs short acting open systems. F9 fins only work for 4 mins. We were ~10% off.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In a rocket, you don't want to run a pump (because pumps consume a lot of power, and their drive system is heavy). Instead, you use a tank of e.g. nitrogen to pressurize the fluid. This means you can't return the used fluid to the pressurized reservoir (you'd have to use a pump to get the fluid to a higher pressure than the reservoir, which defeats the point). So in this case, the used fluid can't be reused.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are exceptions. The Shuttle SRBs used a &lt;a href=&quot;http://www.spaceflight101.com/solid-rocket-boosters.html&quot;&gt;hydrazine-powered power unit&lt;/a&gt; to drive a hydraulic pump for nozzle control. But hydrazine is something you want to avoid if you can (it's nasty stuff). There's a tradeoff between a simple (but limited-life) nitrogen pressurization system, or a (more complex but doesn't run out of hydraulic fluid) hydrazine &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-apu.html#sts-apu&quot;&gt;power unit&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-01-11T15:35:06.103" LastActivityDate="2015-01-11T15:35:06.103" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="7773" PostTypeId="1" AcceptedAnswerId="7780" CreationDate="2015-01-11T16:04:45.933" Score="6" ViewCount="1273" Body="&lt;p&gt;With the present &quot;state-of-the-art&quot; of space technology what is possible to do to avoid an Earth impact by rocky bodies 10 km to 100 km in diameter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any international project to develop a planetary defense system in the near future?&lt;/p&gt;&#xA;" OwnerUserId="7108" LastEditorUserId="1142" LastEditDate="2015-01-12T10:32:19.173" LastActivityDate="2015-11-22T00:36:49.137" Title="Extinction asteroid (10-100 km) impact avoidance" Tags="|impact|earth|planetary-defense|near-earth-object|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="7774" PostTypeId="1" AcceptedAnswerId="7796" CreationDate="2015-01-11T16:39:02.907" Score="6" ViewCount="809" Body="&lt;p&gt;I'm a Sci-Fi author, trying to keep the physics real in my books. I am in the process of moving a space station to the Earth-Moon &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#L1&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt;&lt;/a&gt;. I've read that a vessel can't just sit at that point, but has to orbit it. Can anyone tell me the characteristics of that 'orbit' relative to an Earth-Moon frame of reference? What are its period and velocity? The usual equations don't seem to apply. Coming 'up' from LEO, is there a particular orbit which would be easier to get into, or more favourable for customers, who will be using the station as a waypoint on trips to the Moon?&lt;/p&gt;&#xA;" OwnerUserId="8198" LastEditorDisplayName="user20636" LastEditDate="2020-11-28T03:13:56.330" LastActivityDate="2020-11-28T03:13:56.330" Title="'Orbiting' Earth-Moon L1" Tags="|orbital-mechanics|lagrangian-points|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="7776" PostTypeId="1" AcceptedAnswerId="7778" CreationDate="2015-01-12T01:46:03.087" Score="27" ViewCount="9577" Body="&lt;p&gt;I read that the New Horizon's probe will send data to back to Earth at 3000 bits per second by the time it reaches Pluto. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't understand why a spacecraft has to have a lower data tranfer rate the further away it gets from Earth. The only reason I can think of is that the further away from Earth, the less remaining power for the spacecraft to send data.&lt;/p&gt;&#xA;" OwnerUserId="6679" LastActivityDate="2015-07-25T20:18:20.217" Title="Why does data transfer rate decrease with distance" Tags="|spacecraft|data-transmission|new-horizons|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7777" PostTypeId="2" ParentId="7776" CreationDate="2015-01-12T02:48:31.260" Score="5" Body="&lt;p&gt;The transmitter has only so much power to work with, no matter what the mission phase. Of course the power budget would probably drop over time. Probably more significantly, the greater the distance, the less signal strength at the receiving station according to the inverse square law, so the less it stands out against background noise. Reducing the data rate allows the receiver to &quot;integrate&quot; the incoming signal (filter out the  noise) so that the original signal can be recovered.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-01-12T02:48:31.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7778" PostTypeId="2" ParentId="7776" CreationDate="2015-01-12T05:24:21.697" Score="27" Body="&lt;p&gt;Look up the inverse square law.  With the same power available at the spacecraft, the signal strength received at Earth diminishes as the square of the distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KCn59.gif&quot; alt=&quot;inverse square law&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the vehicle is solar powered, then it is possible that it will also reduce the transmitted power as it goes farther from the Sun.  However it is more likely to reduce the transmission &lt;em&gt;duration&lt;/em&gt; by drawing on the batteries, since you'd like the best signal to noise ratio possible.  Either way, the effective result is that if all the power is going to the radio, then the average data rate integrated over the time spent transmitting and not transmitting will go down as the &lt;em&gt;fourth&lt;/em&gt; power of distance (assuming that the distance from the Sun and the distance from the Earth are about the same, which it is for a probe at Pluto).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons is not solar powered.  Now you know why.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-01-12T05:24:21.697" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="7780" PostTypeId="2" ParentId="7773" CreationDate="2015-01-12T12:40:12.390" Score="5" Body="&lt;p&gt;The threat posed by such near-Earth objects can be illustrated by the most famous extinction-level asteroid, the one responsible for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Chicxulub_crater&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chixclub crater&lt;/a&gt; of the Yucatán Peninsula. This bolide, at least 10 km wide, is almost universally credited with the demise of the dinosaurs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Defensive measures against such objects entirely depends upon finding it in time to do something about it. Should we discover an asteroid on an impact trajectory only months before D-Day, we could do little except hunker down. From the wiki on &lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Deflection_efforts&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asteroid impact avoidance&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“REP. STEWART: ... are we technologically capable of launching something that could intercept [an asteroid]? ... DR. A'HEARN: No. If we had spacecraft plans on the books already, that would take a year ... I mean a typical small mission ... takes four years from approval to start to launch ...”&lt;br&gt;&#xA;  —Rep. Chris Stewart (R,UT) and Dr. Michael F. A'Hearn, 10 April 2013, United States Congress&lt;a href=&quot;http://www.gpo.gov/fdsys/pkg/CHRG-113hhrg80552/pdf/CHRG-113hhrg80552.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;(2)&lt;/a&gt;  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Most deflection efforts for a large object require from a year to decades of warning, allowing time to prepare and carry out a collision avoidance project, as no known planetary defense hardware has already been developed.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The best place to go for data on dangerous NEOs is NASA/JPLs &lt;a href=&quot;http://neo.jpl.nasa.gov/risk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sentry Risk Table&lt;/a&gt;. This upcoming Thanksgiving, when the news media starts talking about 2007 VE191, you can bet they'll be interviewing someone from the Sentry team (fortunately this object has only a 1 in 63,000 chance of striking Earth). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As mentioned by TildalWave in his comment above, the &lt;a href=&quot;http://en.wikipedia.org/wiki/B612_Foundation&quot; rel=&quot;nofollow noreferrer&quot;&gt;B612 Foundation&lt;/a&gt; is a private nonprofit dedicated to planetary defense against near-Earth object (NEO) impacts. In addition to publicizing the rate and magnitude of the problem posed by planetary impactors, it assisted the UN in establishing the &lt;a href=&quot;http://www.unoosa.org/oosa/en/COPUOS/stsc/wgneo/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;International Asteroid Warning Network&lt;/a&gt;. The B612 Foundation is also designing and building (with Ball Aerospace) a privately financed asteroid-finding space observatory, the &lt;a href=&quot;http://sentinelmission.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sentinel Space Telescope&lt;/a&gt;, to be launched in 2017–2018. From a heliocentric orbit it will use a supercooled infrared detector to survey space for potentially dangerous NEOs. An interesting article from their site by one of Sentinel's mission scientists, including a video simulation, can be found &lt;a href=&quot;http://sentinelmission.org/research/pathways-to-flying-the-sentinel-mission-soda-anyone/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are various other surveys and databases trying to keep track of threats &lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Ongoing_projects&quot; rel=&quot;nofollow noreferrer&quot;&gt;summarized here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we discover an extinction-size asteroid bearing down on Earth, and we have time to do something about it, should we call Bruce Willis? The pros and cons of the &lt;em&gt;Armageddon&lt;/em&gt; approach, boring a hole deep into the asteroid and dropping in a nuke in order to shatter it, are pretty well addressed in &lt;a href=&quot;https://space.stackexchange.com/questions/5617/is-it-or-why-is-it-worse-to-break-up-a-asteroid-on-a-collision-course-with-ear&quot;&gt;Is it (or why is it) worse to break up a asteroid on a collision course with Earth?&lt;/a&gt; There may be more utility in a well-timed and positioned surface blast or even a standoff explosion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other &lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Collision_avoidance_strategies&quot; rel=&quot;nofollow noreferrer&quot;&gt;collision avoidance strategies&lt;/a&gt; include:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Kinetic impact, essentially ramming it with a spacecraft  &lt;/li&gt;&#xA;&lt;li&gt;Gravity tractor, hovering a heavy spacecraft near an asteroid to gravitationally deflect it  &lt;/li&gt;&#xA;&lt;li&gt;Conventional rocket engine attached to the asteroid  &lt;/li&gt;&#xA;&lt;li&gt;Solar sails  &lt;/li&gt;&#xA;&lt;li&gt;Laser ablation&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-11-19T14:54:22.597" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7782" PostTypeId="5" CreationDate="2015-01-12T18:21:18.167" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-01-12T18:21:18.167" LastActivityDate="2015-01-12T18:21:18.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7783" PostTypeId="4" CreationDate="2015-01-12T18:21:18.167" Score="0" Body="Questions regarding NASA latest manned spacecraft program currently under development. Orion spacecraft will be used for missions beyond Earth orbit. For questions related to the Project Orion (propulsion with atomic bombs exploded underneath the spacecraft) please use the nuclear-pulse-propulsion tag." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-01-12T18:38:34.777" LastActivityDate="2015-01-12T18:38:34.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7784" PostTypeId="5" CreationDate="2015-01-12T18:36:10.217" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-01-12T18:36:10.217" LastActivityDate="2015-01-12T18:36:10.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7785" PostTypeId="4" CreationDate="2015-01-12T18:36:10.217" Score="0" Body="Questions about manual work and exertion during extra- and intravehicular activity, and physical exercises for recreation and body conditioning aboard spacecraft and in extraterrestrial habitats." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-01-12T18:38:38.450" LastActivityDate="2015-01-12T18:38:38.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7786" PostTypeId="2" ParentId="7753" CreationDate="2015-01-12T19:35:55.033" Score="22" Body="&lt;p&gt;Edit Jan. 25, 2015: Geoffcc recently provided a &lt;a href=&quot;http://shitelonsays.com/transcript/spacex-press-conference-september-29-2013-2013-09-29&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt; to words straight from the Horse's mouth, Elon Musk. Musk says the payload hit for RTLS (Return To Launch Site) is 30% vs a 15% payload hit for landing on a downrange ocean platform. Musk also said said safety is a reason for having an ocean landing platform -- &quot;As far as the safety aspect of the return to launch site of the first stage that's part of why we want to do it first in the ocean just to make sure that things will be fine. For any landing area that we would have, the landing ellipse, the sort of error that the stage could encounter would be an unpopulated region. So we would aim to have a landing site that's unpopulated with a radius of a couple of miles (which can be achieved in Cape Canaveral and Vandenberg).&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it seems safety as well as delta V are reasons for the barge landing&lt;/p&gt;&#xA;&#xA;&lt;p&gt;End Edit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get back to the launch pad, a booster stage would need to kill it's eastward velocity and then do a westward burn. This extra delta V &quot;boost back&quot; expense inflicts a penalty on payload mass as others have said. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However a Jon Ross info graphic from &lt;a href=&quot;http://www.nbcnews.com/science/space/liftoff-aborted-spacex-attempt-historic-landing-delayed-n280391&quot; rel=&quot;nofollow noreferrer&quot;&gt;an NBC story&lt;/a&gt; indicates a boost back burn to get to the barge:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3zJ0c.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had imagined a near parabolic trajectory for the booster's path from launch site to barge but Ross' graphic indicates a boost back burn as well as a reentry burn and landing burn. I haven't been able to get actual numbers but the delta V savings of this path doesn't seem that great.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, if this sort of barge landing offers substantial delta V savings over RTLS, then why is &lt;a href=&quot;http://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Launch_Complex_13#Future_use&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX still negotiating with the air force for the use of SLC 13 for RTLS&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="25" LastEditDate="2017-12-19T13:35:21.610" LastActivityDate="2017-12-19T13:35:21.610" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="7787" PostTypeId="1" CreationDate="2015-01-12T21:16:19.013" Score="5" ViewCount="342" Body="&lt;p&gt;Apparently, no information could pass through plasma, therefore during descent of spacecraft, no information can be transmitted. Why so?&lt;/p&gt;&#xA;" OwnerUserId="8204" LastEditorUserId="49" LastEditDate="2015-01-12T21:24:26.813" LastActivityDate="2015-01-14T09:46:07.180" Title="Why can't infomation be transmitted during descent?" Tags="|spacecraft|communication|reentry|atmosphere|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7788" PostTypeId="1" AcceptedAnswerId="7791" CreationDate="2015-01-12T22:04:27.907" Score="9" ViewCount="2816" Body="&lt;p&gt;When Elon Musk unveiled Dragon v2 last year, we got our first truly good look at what SpaceX's crew vehicle is like. Apart from the ability to propulsively land, one of the big changes was the lack of extendable solar panels on the vehicle, rather they're now wrapped radially around the trunk. As seen here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hjiNV.png&quot; alt=&quot;Dragon v2 concept&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can only assume this is for reliability reasons and to eliminate more moving components. Something I've wondered however, is how will the solar panels be encapsulated for protection against the aerodynamic forces that occur during launch? They're even present on the fins of the trunk, which are aerodynamic surfaces themselves (!), designed to keep the capsule oriented correctly in an abort scenario.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a second, take a look at Dragon, both &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/9/9a/SpaceX_Dragon_C2%2B_vehicle_approaches_ISS_(ISS031-E-071140).jpg&quot; rel=&quot;noreferrer&quot;&gt;in orbit&lt;/a&gt;, and &lt;a href=&quot;http://www.wired.com/images_blogs/autopia/2012/05/Falcon-9-Rocket-and-Dragon-spacecraft-5-12-1.jpg&quot; rel=&quot;noreferrer&quot;&gt;attached to Falcon 9&lt;/a&gt;. The latter is significantly more aerodynamic than how it looks in space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a few main differences:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The Draco engines are covered with a plastic-like cover which get ripped off either during launch or on the first thruster firing.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The nose cone hides the CBM inside and is jettisoned around the time a Falcon 9 fairing is.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Most importantly, the solar panels on Dragon v1 are encapsulated in an aerodynamic fairing that presumably gets jettisoned during the flight up. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The first two are easily solved. For #1, I can only assume the Draco thrusters will be similarly covered up, and the SuperDracos are at an angle where they shouldn't need covering. And for the nose cone, that stays attached to Dragon and is reusable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, &lt;strong&gt;how will the solar panels on the Dragon v2 be protected from the elements that occur during launch?&lt;/strong&gt; Some kind of tempered glass, fairing to be jettisoned? Or will the be left exposed? &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="51" LastEditDate="2015-07-12T14:00:36.903" LastActivityDate="2015-07-12T14:00:36.903" Title="How will Dragon v2's solar panels be protected during launch on top of Falcon 9?" Tags="|spacex|falcon-9|dragon|solar-power|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7789" PostTypeId="1" CreationDate="2015-01-12T22:12:32.387" Score="4" ViewCount="570" Body="&lt;p&gt;Most current extravehicular activity spacesuits are very thick and chunky, like &lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit&quot; rel=&quot;nofollow&quot;&gt;EMU&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Orlan_space_suit&quot; rel=&quot;nofollow&quot;&gt;Orlan&lt;/a&gt;. Even though NASA/MIT work on something thinner (&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_activity_suit#MIT_Bio-Suit&quot; rel=&quot;nofollow&quot;&gt;Bio Suit&lt;/a&gt;), it's not for EVA in vacuum of space, but for surface activity on Mars, where there is some atmospheric pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what are the components that make spacesuits so thick and bulky? What if we had a layer of graphene composite, custom-shaped to the size of the wearer? I mean if we can make one, would the thinner outer layer eliminate the bulkiness, and enable thinner suits only requiring temperature regulators, so astronauts could handle more delicate equipment and improve mobility in space?&lt;/p&gt;&#xA;" OwnerUserId="8204" LastEditorUserId="49" LastEditDate="2015-01-12T23:29:04.047" LastActivityDate="2022-10-24T03:01:16.093" Title="What factors affect the thickness of EVA spacesuits?" Tags="|materials|spacesuits|temperature|feasibility|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7791" PostTypeId="2" ParentId="7788" CreationDate="2015-01-12T23:17:40.320" Score="4" Body="&lt;p&gt;Most likely answer is designed to be sufficiently robust to survive the ascent environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was going to say that the cover on the Dragon V1's solar panels do not appear to be covers as such, more like the back of the panels themselves, but then as I recall the various video feeds of the solar array deploy, the covers are no where to be seen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Proton, launching certain manned space payloads, could not fit a fairing around it. They instead encapsulated parts of the vehicle for launch, with a top fairing, bottom fairing, but no middle fairing.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-01-12T23:17:40.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7792" PostTypeId="1" CreationDate="2015-01-13T00:29:39.067" Score="40" ViewCount="5316" Body="&lt;p&gt;Among a variety of alternatives considered at the time, the Apollo configuration of 3-person CSM and 2-person LM, launched atop a single vehicle was considered the optimal choice for its day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With today's accumulated experience in spaceflight and almost 50 years of technological advancement, would a mission mounted today to fulfill Kennedy's mandate &quot;land a man on the Moon and return him safely to the Earth&quot; look similar?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, let's exclude from consideration any spaceflight systems currently existing or under development capable of transporting humans beyond low Earth orbit (we can consider the Space Shuttle, but not Orion).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say that the mission must deliver at least one person to the lunar surface such that the astronaut can walk on the surface, and perform physical tasks similar to what was done during Apollo. Let's also say that the science payload (all equipment delivered to surface and samples returned) are mass equivalent, and the durations of both moonwalk and surface stay are similar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would such a mission have the same complement (3 to lunar orbit, 2 landing), or would it be more, or less?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would there be a separate CM and LM at all (perhaps a combined CM/LM, leaving only an unmanned SM in lunar orbit to rendezvous with after lunar ascent and power the ride home)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the mission vehicle(s) be similar in size and configuration? Would a single launch vehicle for the entire mission requirement be optimal or would multiple launch with orbital rendezvous be favoured? Would the Space Shuttle (if not retired) have been a practical way to deliver as far as LEO (by multiple launches) the people and equipment required?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point of the question is whether 50 years of technological advancement and spaceflight experience has &quot;changed the equation&quot; which was calculated back then, or whether the best answer then would still be the best answer today.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="25" LastEditDate="2015-07-29T12:17:02.067" LastActivityDate="2019-09-28T23:38:34.233" Title="If the Apollo mandate were delivered today, would the mission vehicle(s) and profile be similar?" Tags="|launch|the-moon|propulsion|apollo-program|mission-design|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="7793" PostTypeId="1" AcceptedAnswerId="7798" CreationDate="2015-01-13T00:53:50.083" Score="7" ViewCount="4683" Body="&lt;p&gt;What some factors to consider when they choose launch sites like Cape Canaveral or Baikonur Cosmodrome? Is it possible to have a launch site anywhere in the world (if so why does the ESA launch its Ariane rockets from French Guiana?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is a big question could the factors only be briefly laid out so further research can be performed!&lt;/p&gt;&#xA;" OwnerUserId="65" LastEditorUserId="168" LastEditDate="2015-01-13T14:21:37.260" LastActivityDate="2020-09-18T17:36:26.083" Title="Briefly, what are the factors to consider when choosing a launch site?" Tags="|rockets|launch-site|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7796" PostTypeId="2" ParentId="7774" CreationDate="2015-01-13T01:05:58.160" Score="8" Body="&lt;p&gt;All first three Lagrange points are unstable, namely the effective potential near these points are saddle points. The L4 and L5 points are stable when the mass ratios are big enough. The unstable directions of those saddle points are inline with the vector connecting the two celestial bodies, however the forces in the plane normal to this direction will be directed towards the respective Lagrange point. So if you move in this plane away from such a point and give yourself a relative velocity perpendicular to the attractive force, you could orbit that point, but you would still have to correct the unstable direction, because any perturbation will build op exponentially over time if you don't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to know the periods of these orbits as a function of the radius (limiting to circular orbits) you have to look at the forces involved, which should act as the centripetal force. I have to make the assumption that the velocity, with which the L1 point is being orbited, is small compared to the rotational velocity of the rotating reference frame, such that the fictitious centripetal force would only be a function of the position. The variables that I will be using are illustrated in the following figure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VDX62.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $m$ is the Moon, $M$ is the Earth, the $\times$ illustrates the center of mass of the two and $L_1^*$ is a translation of distance $r$ of the $L_1$ point in the perpendicular plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The resulting acceleration, towards $L_1$, can be calculated as follows,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;a = \frac{\mu_m r}{\left(R_m^2+r^2\right)^{3/2}} + \frac{\mu_M r}{\left((R_C+R_M)^2+r^2\right)^{3/2}} - \omega^2r,&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mu_i$ is the gravitational parameter of body $i$ and $\omega$ is equal to the angular velocity with which the two bodies rotate around each other. If $r$ is small compared to the other distances, then this equation can be approximated with,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;a \approx r \left(\frac{\mu_m}{R_m^3} + \frac{\mu_M}{(R_C+R_M)^3} - \omega^2\right).&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This should be equal to the centripetal acceleration, thus the required velocity, $v$, and corresponding period, $T$, can be expressed as,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;v = r \sqrt{\frac{\mu_m}{R_m^3} + \frac{\mu_M}{(R_C+R_M)^3} - \omega^2},&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;T = \frac{2\pi}{\sqrt{\frac{\mu_m}{R_m^3} + \frac{\mu_M}{(R_C+R_M)^3} - \omega^2}}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the period would be independent of $r$ (if sufficiently small) and for the Earth-Moon L1 point would be roughly equal to 13.4 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not an expert in this field and did a few simplifications, so I also looked for a reference and according to &lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/ARI/ARI%20Study%20Report/ACT-RPT-MAD-ARI-03-4103b-InterplanetaryHighways-Milano.pdf#page=45&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt; for a small $r$ the period would be closer to 11.94 days, so I might have used wrong values for the involved parameters and/or simplified the problem to much, but hopefully this does give some insight into the physics involved, or the low eccentricity of the Moon also has an influence. I also tried to calculate it for the L2 point and for both points in the Sun-Earth system, but kept getting about 1.2 times too high periods compared to my reference.&lt;/p&gt;&#xA;" OwnerUserId="1086" LastEditorUserId="1086" LastEditDate="2015-01-13T04:13:05.140" LastActivityDate="2015-01-13T04:13:05.140" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7797" PostTypeId="2" ParentId="7792" CreationDate="2015-01-13T01:22:57.967" Score="66" Body="&lt;p&gt;A big difference is that you wouldn't need to leave someone in lunar orbit.  We now have experience and confidence in the remote operation of an uncrewed vehicle.  So you could have a crew of two instead of three.  Or perhaps a crew of three to the surface with a larger LM.  Overall, there would be much more automation, especially for the landing process, pinpoint landing, and hazard avoidance.  Also rendezvous and docking would be automated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The overall architecture would be quite similar.  If you don't have a big enough launch vehicle handy, you could add an Earth orbit rendezvous to launch the CM/SM and LM separately.  You could consider electric propulsion to put the LM in lunar orbit on the slow boat, ahead of the CM/SM, for mass savings or more capability to the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would need something like Orion or crewed Dragon, qualified to enter at lunar return velocities.  You can't &quot;exclude from consideration&quot; new systems, since we would have to recreate the capabilities of most of the Apollo systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our management of large engineering projects has advanced considerably, so with all the management processes we now engage in, it will take 20 years to complete the project instead of eight.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-07-29T03:13:49.077" LastActivityDate="2015-07-29T03:13:49.077" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="7798" PostTypeId="2" ParentId="7793" CreationDate="2015-01-13T02:32:39.503" Score="17" Body="&lt;p&gt;Some factors might be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Proximity to the equator.&lt;/strong&gt; The closer the launch site to the equator, the easier it is to launch to equatorial orbit, and the more help you get from Earth's rotation for a prograde orbit. Unless launching to a polar orbit, there's at least some benefit to launching from as close to the equator as you can get.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Downrange safety.&lt;/strong&gt; You want to be sure that spent stages dropped along the launch path or failed launches don't fall on people or property. Eastern coasts are good for that, as long as you can exclude marine and air traffic along the launch path. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every launcher in the world, except for polar orbits, launches to the east to take advantage of the rotation of the Earth. That is except Israel's Shavit, which launches to the west (Overflying its eastern neighbours looks like an attack).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus an eastern coast is a good choice, with lots of ocean downrange for dropping stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For polar/sun synchronous orbits a mostly northern direction, which in the US is usually from Vandenberg AFB in California. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian sites use large expanses of desert to simulate downrange ocean. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launch site safety/security.&lt;/strong&gt; There needs to be an exclusion zone of some size (depending on the size of your vehicle) in all directions around your launch site to keep unauthorized people away from your launch facility, and keep things/people out of harm's way if the vehicle has a catastrophic failure at or close to the launch pad. Even if all goes well, exhaust from a big rocket can do serious damage to things on the ground from just the acoustic energy (shock waves, vibration).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Proximity to transportation infrastructure.&lt;/strong&gt; Your launch site needs to be well clear of anything a failed launch can damage, but still close enough to roads/rails/docks to be able to bring people and material to or from it in support of launch activities.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="51" LastEditDate="2015-01-14T01:04:37.933" LastActivityDate="2015-01-14T01:04:37.933" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7799" PostTypeId="2" ParentId="5857" CreationDate="2015-01-13T03:50:38.467" Score="3" Body="&lt;p&gt;Based on the information on the following websites, my answer would be 'enticingly potential':&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.britannica.com/EBchecked/media/73247/Global-distribution-of-iron-on-the-Moons-surface-based-on&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.britannica.com/EBchecked/media/73247/Global-distribution-of-iron-on-the-Moons-surface-based-on&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://lunarnetworks.blogspot.com.au/2011/10/new-map-of-lunar-titanium-and-iron.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://lunarnetworks.blogspot.com.au/2011/10/new-map-of-lunar-titanium-and-iron.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.permanent.com/lunar-geology-minerals.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.permanent.com/lunar-geology-minerals.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Cw4HA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Source: &lt;a href=&quot;http://www.bibliotecapleyades.net/luna/esp_luna_62.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.bibliotecapleyades.net/luna/esp_luna_62.htm&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there were be an iron extraction industry on the Moon it would be on the near side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The greatest concentrations of iron occur in the Maria (the flat plains). In these locations the concentration tends to be about 15%, but can be as high as 25%. By comparison, the major iron ore mines of Australia and Brazil tend to mine iron ore grading 55% to 65%. China and Sweden however mine lower grade ore averaging 35% Fe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Iron on the Moon occurs in two main forms: combined with other elements such as ilmenite (FeTiO3), originating from Moon rock and there is free iron originating from meteors impacting on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The free iron is already in powdered form and could easily be collected using magnets. Iron in combination with other elements will require some form of mineral processing to liberate the iron.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lateral extent of iron on the Maria of the Moon is large. One of the main things that will determine the viability of an iron extractive industry on the Moon will be the depth of the iron rich deposits. The other factors will be the easy or difficulty with which the deposits can be mined and then processed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other factors will be the availability of energy to power the mining and processing equipment and the impact of the abrasiveness of Moon dust on such equipment. The equipment will need to work when required, not be idle due maintenance and overhaul issues resulting from excessive abrasive wear and tear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Equipment that will be used to mine and process iron will need to designed and manufactured to prevent the fine iron dust from short circuiting electrical components.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="168" LastEditDate="2015-01-13T10:36:15.283" LastActivityDate="2015-01-13T10:36:15.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7800" PostTypeId="1" AcceptedAnswerId="7801" CreationDate="2015-01-13T06:53:26.620" Score="20" ViewCount="6216" Body="&lt;p&gt;Been looking at the amazing success of SpaceX and from what I found, the contracts they get awarded don't seem to make much sense. It looks like they are simply getting paid less each time NASA awards a contract. Examples that I found:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Commercial Resupply Services&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SpaceX - \$1.6b for 12 flights on Falcon 9 (13150kg payload capacity)&lt;/li&gt;&#xA;&lt;li&gt;Orbital - \$1.9b for 8 flights on Antares (6120kg payload capacity)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;That roughly works out to \$39k per kg for Orbital, but only \$10k per kg for SpaceX. Even if SpaceX allocates less than half their capacity to the primary CRS payload, they still deliver much more than Orbital for the same amount of money. Am I missing something?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Commercial Crew Development&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Not even part of the 5 companies selected for CCDev 1 even though they did put in a proposal. CCDev 2, they did get selected as one of 4, but got less funding than Boeing and Sierra Nevada Corporation. CCiCap, they were selected as one of 3, but still got less funding than Boeing.&lt;/p&gt;&#xA;&lt;p&gt;In September, they announced SpaceX and Boeing received contracts to provide crewed launch services to ISS.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Boeing got \$4.2b&lt;/li&gt;&#xA;&lt;li&gt;SpaceX got \$2.6b&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;From Wikipedia &amp;quot;Both Boeing and SpaceX were awarded for the same set of requirements&amp;quot;. If they need to do exactly the same thing, why did Boeing get awarded some 61% more money?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="47149" LastEditDate="2022-06-03T21:55:30.860" LastActivityDate="2022-06-03T21:55:30.860" Title="Why does SpaceX appear to get less funding from NASA for the same thing as competitors?" Tags="|spacex|nasa|commercial|cost|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="7801" PostTypeId="2" ParentId="7800" CreationDate="2015-01-13T08:15:51.973" Score="29" Body="&lt;p&gt;This is due to a variety of reasons which I will detail further down in my answer, but let me first correct a few of the points you raise.&lt;/p&gt;&#xA;&lt;h1&gt;Corrections&lt;/h1&gt;&#xA;&lt;h3&gt;Contract Awarding&lt;/h3&gt;&#xA;&lt;p&gt;These contracts represent the amount of funds &lt;strong&gt;requested&lt;/strong&gt; by each company, and is essentially their &amp;quot;bidding price&amp;quot; for the services they can provide. NASA then selects companies from those that applied based on their technical &amp;amp; managerial performance, as well as their bid value.In both instances, for CRS &amp;amp; CCDev, SpaceX simply underbid the competitors. NASA is not giving SpaceX a &amp;quot;hard time&amp;quot;, rather, &lt;em&gt;SpaceX is bidding what they believe they can accomplish the contract for&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;h3&gt;CRS Specifics&lt;/h3&gt;&#xA;&lt;p&gt;Also, you misunderstand the CRS contracts. They don't state that Orbital Sciences should carry up 6120kg on each of its 8 flights, and likewise, they don't state that SpaceX should carry up 13,140kg on each of its 12. Rather, the agreements state both companies are required to fly up a minimum cargo mass of 20,000kg over their requested number of flights. Therefore, the correct cost/kg value is 80,000/kg for SpaceX &amp;amp; 95,000/kg for Orbital Sciences. There is no downmass requirement, even though Dragon can carry down cargo. If you don't mind reading through dense documents, &lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/418857main_sec_nnj09ga04b.pdf&quot; rel=&quot;noreferrer&quot;&gt;here's SpaceX's CRS contract&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;h3&gt;Payload Capacity&lt;/h3&gt;&#xA;&lt;p&gt;You also confuse the capacity of Cygnus &amp;amp; Dragon - which are the cargo vehicles which actually berth to the ISS. You're stating the maximum payload of Antares &amp;amp; Falcon 9 to a 200x200km Low Earth Orbit, the rockets which carry both cargo vehicles uphill, respectively. Both the Cygnus &amp;amp; Dragon vehicles weigh a significant amount and only so much can be packed inside.&lt;/p&gt;&#xA;&lt;p&gt;In fact, both craft are actually rather complementary to each other and serve the CRS program well. Due to the lower power of the Antares rocket, Cygnus was only able to carry up 2,000kg to the ISS per flight, and all of it had to be pressurized - there was no storage for unpressurized items of NASA's manifests. Cygnus also doesn't return to Earth, instead it can dispose of 1200kg via reentry, a capability Dragon does not have. Additionally &amp;amp; crucially, Cygnus is a very large vehicle with a spacious interior, it has nearly 19m^3 of volume. You'll see why this bit comes in useful later.&lt;/p&gt;&#xA;&lt;p&gt;Dragon on the other hand can haul up &amp;amp; down 3,310kg of cargo. It can also be a mix of unpressurized and pressurized components. A unique selling point of Dragon is it’s the only ISS visiting vehicle that can return significant amounts of cargo back to Earth intact. Sounds great, right? Here's where it falls flat, a bit. Dragon only has a pressurized volume of 11.2m^3. On nearly all flights, Dragon has been &lt;strong&gt;volume limited&lt;/strong&gt;. ISS cargo takes up a lot of room, and actually doesn't weigh all that much. Because of this, Dragon has never been utilized to its mass capacity, its critical constraint is volume. &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/SpaceX_CRS-4_Mission_Overview-1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Here's a cargo manifest for CRS-4&lt;/a&gt;, showing that Dragon only launched 2216kg uphill.&lt;/p&gt;&#xA;&lt;h1&gt;SpaceX's Cost Advantage&lt;/h1&gt;&#xA;&lt;p&gt;Why SpaceX tends to be cheaper is because they're what is called a &amp;quot;new space&amp;quot; company. Rather than being founded on a traditional corporate structure, they tend to follow a more agile development strategy similar to startup technology companies - Musk himself was one of the cofounders of PayPal.&lt;/p&gt;&#xA;&lt;h3&gt;Flat organizational structure&lt;/h3&gt;&#xA;&lt;p&gt;SpaceX has very little middle management. There's no more than 1-2 layers between Elon Musk and Joe Bloggs, our propulsion intern. In fact, right up to the last few minutes before a launch, any employee can email Elon personally and ask for it to be scrubbed if they have reason to believe launching may be dangerous - he even works in a cubicle like everyone else (&lt;a href=&quot;https://www.youtube.com/watch?v=yFbrK_sRV9Q&quot; rel=&quot;noreferrer&quot;&gt;check out their offices in this video&lt;/a&gt;, although it's getting a bit dated now). This drastically cuts down on the inefficiencies across the company, resulting in quicker communication &amp;amp; less paychecks needed to be handed out. SpaceX is a very lean piece of meat.&lt;/p&gt;&#xA;&lt;p&gt;This can be contrasted with an &amp;quot;old space&amp;quot; company like Boeing, where a far larger fraction of the employees are in managerial positions, less are engineers and technicians. That's not to say they don't get work done, but it's slower and more costly than SpaceX's approach.&lt;/p&gt;&#xA;&lt;h3&gt;Off the shelf components &amp;amp; vertical integration&lt;/h3&gt;&#xA;&lt;p&gt;Traditionally aerospace has been a game of subcontracting. A launch provider will handle the rocket and assemble the components provided by everyone else, who requested the parts from others. This adds transportation costs and can result in price gouging. SpaceX build the entire rocket themselves. They build their engines, they build their tanks, they build their software, they build their equipment. As Bill Nye said: &lt;a href=&quot;https://www.youtube.com/watch?v=cos2CBkg8kY&quot; rel=&quot;noreferrer&quot;&gt;&amp;quot;Metal comes in one door, rockets roll out another&amp;quot;&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;SpaceX also ditched the idea of using &amp;quot;aerospace grade&amp;quot; components in their rockets - much to the annoyance of the Air Force &amp;amp; NASA. &lt;a href=&quot;https://i.stack.imgur.com/zpNmL.jpg&quot; rel=&quot;noreferrer&quot;&gt;Check out this album of photos&lt;/a&gt; of a washed up Falcon 9 fairing on the Eastern seaboard, you'll see lots of plastic clip ties amongst the mangled carbon fibre. No other aerospace company employs clip ties whatsoever! Often, SpaceX found that the aerospace grade components costs an order of magnitude more and did not deliver much extra performance. &lt;a href=&quot;http://qz.com/281619/what-it-took-for-elon-musks-spacex-to-disrupt-boeing-leapfrog-nasa-and-become-a-serious-space-company/&quot; rel=&quot;noreferrer&quot;&gt;Toilet stall handles as Dragon parts, for example&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;h3&gt;Modern equipment with no legacy costs&lt;/h3&gt;&#xA;&lt;p&gt;Everything SpaceX operates has been conceived and built in the last 10 years. They've benefitted massively from Computer Automated Design tools and modern technology, embracing concepts such as &lt;a href=&quot;http://www.spacex.com/news/2014/07/31/spacex-launches-3d-printed-part-space-creates-printed-engine-chamber-crewed&quot; rel=&quot;noreferrer&quot;&gt;3D printing&lt;/a&gt;. This makes their rockets cheaper, and therefore their bids by extension. They don't have to maintain old software of old design specifications like others, for them to migrate to a newer system would be extremely expensive.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;All this results in a highly optimized company that can deliver a bid price well below that of their competitors.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-19T16:16:54.313" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="7802" PostTypeId="1" AcceptedAnswerId="7803" CreationDate="2015-01-13T09:50:50.447" Score="16" ViewCount="5509" Body="&lt;p&gt;For a GPS receiver to estimate its position, it first receives signals from at least 4 satellites. Does the receiver calculate the distance that separates it from each satellite, or does it just calculate the distance difference between each pair of satellites? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, does the GPS system use &lt;a href=&quot;https://en.wikipedia.org/wiki/Time_of_flight&quot;&gt;TOF&lt;/a&gt; (time of flight) or TDOA (time difference of arrival, aka &lt;a href=&quot;https://en.wikipedia.org/wiki/Multilateration&quot;&gt;multilateration&lt;/a&gt;) technique to calculate position and how are these techniques applied?&lt;/p&gt;&#xA;" OwnerUserId="8209" LastEditorUserId="49" LastEditDate="2015-01-13T13:03:56.860" LastActivityDate="2022-12-06T13:48:31.277" Title="How does GPS work exactly?" Tags="|navigation|gps|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7803" PostTypeId="2" ParentId="7802" CreationDate="2015-01-13T10:37:25.807" Score="9" Body="&lt;p&gt;I think GPS receivers do not exclusively need at least 4 satellites, but rather 3 as a minimum for trilateration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bkOKG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A fourth satellite signal is necessary to synchronize the receiver clock with the satellite clocks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for &lt;strong&gt;TOF/TOA vs. TDOA&lt;/strong&gt;, I believe the difference relies whether on if the GPS receiver has an internal clock synchronized with the satellite transmitters or not. In other words, whether if you know the time on the satellite or not (in which case, you measure the range differences).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, user-end GPS systems probably use TDOA as they wouldn't have a synchronized clock (would be too expensive).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Details on the GPS positioning calculations:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BH5rW.gif&quot; alt=&quot;GPS position calculation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GPS calculation in the receiver uses four equations in the four unknowns &lt;strong&gt;x, y, z, t&lt;sub&gt;c&lt;/sub&gt;&lt;/strong&gt;, where &lt;strong&gt;x, y, z&lt;/strong&gt; are the receiver's coordinates, and &lt;strong&gt;t&lt;sub&gt;c&lt;/sub&gt;&lt;/strong&gt; is the time correction for the GPS receiver's clock. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equation is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{equation}&#xA;\ d_n = c(t_{t,n} - t_{r,n} + t_c) = \sqrt[]{(x_n-x)^2+(y_n-y)^2+(z_n-z)^2}&#xA;\end{equation}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;n&lt;/strong&gt; would be: &lt;em&gt;GPS 1,2,3,4&lt;/em&gt; &lt;em&gt;(for each of the 4 equations respectively)&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;c&lt;/strong&gt; = speed of light (3x10&lt;sup&gt;8&lt;/sup&gt; m/s)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;t&lt;sub&gt;t,1&lt;/sub&gt;, t&lt;sub&gt;t,2&lt;/sub&gt;, t&lt;sub&gt;t,3&lt;/sub&gt;, t&lt;sub&gt;t,4&lt;/sub&gt;&lt;/strong&gt; = times that &lt;strong&gt;GPS satellites 1, 2, 3, and 4,&lt;/strong&gt; respectively, transmitted their signals (these times are provided to the receiver as part of the information that is transmitted).&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;t&lt;sub&gt;r,1&lt;/sub&gt;, t&lt;sub&gt;r,2&lt;/sub&gt;, t&lt;sub&gt;r,3&lt;/sub&gt;, t&lt;sub&gt;r,4&lt;/sub&gt;&lt;/strong&gt; = times that the signals from &lt;strong&gt;GPS satellites 1, 2, 3, and 4,&lt;/strong&gt; respectively, are received (according to the inaccurate GPS receiver's clock)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;x&lt;sub&gt;1&lt;/sub&gt;, y&lt;sub&gt;1&lt;/sub&gt;, z&lt;sub&gt;1&lt;/sub&gt;&lt;/strong&gt; = coordinates of &lt;strong&gt;GPS satellite 1&lt;/strong&gt; (these coordinates are provided to the receiver as part of the information that is transmitted); similar meaning for x&lt;sub&gt;2&lt;/sub&gt;, y&lt;sub&gt;2&lt;/sub&gt;, z&lt;sub&gt;2&lt;/sub&gt;, etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TOA&lt;/strong&gt; = Time of arrival, &lt;strong&gt;TOF&lt;/strong&gt; = Time of flight, &lt;strong&gt;TDOA&lt;/strong&gt; = Time differences of arrival  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://www.courses.psu.edu/aersp/aersp055_r81/satellites/gps_details.html&quot; rel=&quot;noreferrer&quot;&gt;https://www.courses.psu.edu/aersp/aersp055_r81/satellites/gps_details.html&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.adv-radio-sci.net/9/203/2011/ars-9-203-2011.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.adv-radio-sci.net/9/203/2011/ars-9-203-2011.pdf&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://www.maa.org/sites/default/files/pdf/upload_library/22/Polya/Kalman.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://www.maa.org/sites/default/files/pdf/upload_library/22/Polya/Kalman.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3001" LastEditorUserId="3001" LastEditDate="2015-01-21T15:14:30.127" LastActivityDate="2015-01-21T15:14:30.127" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="7805" PostTypeId="2" ParentId="6174" CreationDate="2015-01-13T14:36:23.723" Score="0" Body="&lt;p&gt;I am not an aerospace engineer, but it seems to me that because each grid fin can be independently controlled, then each can be rotated at extremely high speeds to create small vortexes which would allow discrete control of aerodynamic forces on the overall moment of the inherently unstable vertical proportion of the Falcon booster.  A really eureka solution to creating means to achieve a soft landing in atmosphere...gyroscopically controlled rockets might require more fuel and weight than these fins.  Very very impressed with what SpaceX and Musk are accomplishing...just hope that they partner up with Sierra Nevada and Dream Chaser to provide a more complete U.S. space capacity...after reading another article it seems the grid fins do not rotate at high speeds...rather they operate to assist in stabilizing and steering the booster as it descends at hypersonic then transonic velocities...sorry for the confusion but am not a rocket scientist...&lt;/p&gt;&#xA;" OwnerUserId="8215" LastEditorUserId="8215" LastEditDate="2015-01-13T19:47:12.573" LastActivityDate="2015-01-13T19:47:12.573" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7806" PostTypeId="2" ParentId="7792" CreationDate="2015-01-13T16:30:51.970" Score="19" Body="&lt;p&gt;There's an assumption in your question.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the Apollo configuration of 3-person CSM and 2-person LM, launched&#xA;  atop a single vehicle was considered the optimal choice for its day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The simplest mission profile would have been a direct flight. The docking required for both EOR and LOR schemes seems to have been distrusted at the time; hence a major objective of the Gemini flights was to demonstrate rendezvous and docking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As every popular history points out, the Saturn 5 was &quot;not large enough&quot; to launch a direct flight, thus the LOR scheme. There's an unmentioned assumption in this statement...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen a photo of a concept model from the 1960s, a Saturn 8. Basically a fatter Saturn V with 8 engines in stages 1 and 2 (don't remember what stage 3 was). It was conceived to launch a direct flight with the 3-man CM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wait a minute! If that's the needed size for that mission, might it be possible to scale the CM down for 2 crew and launch a direct flight with a Saturn 5?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer is &quot;yes&quot;. The CM wasn't a mass-efficient design even for 3 crew.&#xA;The Lunar Gemini proposal used this approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, NASA settled on the CM design too early. Then settling on a Saturn 5 too small to launch a direct flight with that, they were forced to a rendezvous scheme.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, a similar lock-in situation occurred in the Soviet Moon program. In their case, with the booster. They had designs for all-new Moon rockets, but they rushed into the program and decided to repurpose an already-designed but not yet built rocket: the infamous N1. It was not sized for a lunar landing, having been designed around a manned Mars-and-Venus flyby mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Broadly, we're in a similar situation now in terms of available options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some possible differences, major and minor:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would we settle for the low science payload and surface stay time of Apollo? Particularly the latter could be increased at little cost in mass. And I expect a rover would be there for the first landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch vehicle might have solid boosters or an all-solid first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EOR might be more accepted. However, I expect this would still use launch vehicles larger than those currently in use. The Shuttle is horribly inefficient in terms of payload fraction, and I assume any EOR scheme would be limited to rendezvous and docking rather than assembly needing workers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect LOR might be replaced by L1R - using Earth-Moon Lagrange 1. Easier access to high-latitude landing sites, can launch from Moon at any time and still rendezvous, and the craft left in space can maintain line-of-sight communication with Earth.&lt;/p&gt;&#xA;" OwnerUserId="8216" LastActivityDate="2015-01-13T16:30:51.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7808" PostTypeId="2" ParentId="7788" CreationDate="2015-01-13T20:28:43.350" Score="3" Body="&lt;p&gt;On Dragon v1, the solar panels are folded up (there are 4 panels on each side) and protrude outside the rocket body. The fairing improves the aerodynamics and reduces the (drag) load on the panels (and the hardware that attaches them to each other).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Dragon v2, there are no hinges. The panels are attached to the trunk. This means there's no aerodynamic loading, just some friction heating during the first minute of the launch. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-01-13T20:28:43.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7809" PostTypeId="2" ParentId="7788" CreationDate="2015-01-13T21:04:50.993" Score="1" Body="&lt;p&gt;Solar cells in space are rarely left completely exposed.  There is usually some kind of coating or cover glass to protect the cells from damage by UV radiation and atomic oxygen.  Potentially, SpaceX can just make that coating or cover glass sturdy enough to deal with aero loads and heating during ascent, which are significant but aren't too severe.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2015-01-13T21:04:50.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7810" PostTypeId="2" ParentId="7787" CreationDate="2015-01-14T09:46:07.180" Score="3" Body="&lt;p&gt;Plasma is &quot;electrically neutral medium of unbound positive and negative particles&quot;. As it's got charge carriers floating about, it's electically conductive. In a sense, you can think of it as a metal sheet. If a metal sheet fully encloses the spacecraft, you'd get a Faraday Cage. Similarly, if the plasma sheath fully encloses the space craft, you end up with a Faraday Cage. Faraday Cages massively attenuate EM signals attempting to pass through them, resulting in loss of RF communications.&lt;/p&gt;&#xA;" OwnerUserId="8230" LastActivityDate="2015-01-14T09:46:07.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7811" PostTypeId="1" AcceptedAnswerId="7812" CreationDate="2015-01-14T10:23:10.757" Score="13" ViewCount="1285" Body="&lt;p&gt;I believe I read in a book once a description of a kind of interplanetary ferry that would orbit the sun and regularly pass different planets like Mars and Earth. The idea of the ferry is to have a place to stay for astronauts on their long journey apart from being strapped into the confines of a single space ship. I find it hard, however, to find anything on the web on this idea. Would it be possible for such a ferry to frequent more than two planets with a single orbit? &lt;/p&gt;&#xA;" OwnerUserId="6797" LastActivityDate="2015-01-14T14:06:49.413" Title="Are there any proposals for a permanent orbital interplanetary ferry?" Tags="|mars|interplanetary|travel|earth|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="7812" PostTypeId="2" ParentId="7811" CreationDate="2015-01-14T13:38:34.203" Score="12" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_cycler&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars cyclers&lt;/a&gt; have already been mentioned in the comments, with the Aldrin cycler being one example of them. The &lt;em&gt;castle&lt;/em&gt; parts of them could be seen as &lt;em&gt;ferries&lt;/em&gt;, and the visiting &lt;em&gt;taxis&lt;/em&gt; as means of embarking and disembarking during visits of planets along their routes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And three or more bodies cyclers are also possible, for example the so-called &lt;a href=&quot;http://starsdestination.blogspot.com/2012/05/resonant-cyclers.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;resonant cyclers&lt;/a&gt; of which Laplace resonant cyclers in the Jupiter system might be the best example and are made possible due to the Laplace orbital resonance between Io, Europa and Ganymede (see e.g. &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576511000683&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laplace-resonant triple-cyclers for missions to Jupiter&lt;/a&gt;, A. E. Lynam et al.). These can be either &lt;em&gt;gravity assist&lt;/em&gt; or &lt;em&gt;visit&lt;/em&gt; cyclers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There could also be &lt;em&gt;visit&lt;/em&gt; only non-resonant cyclers with highly elliptical heliocentric orbits that stretch orbital altitude of several planet but never intersect their orbits (slight inclanation of the cycler w.r.t. orbital plane of involved planets), but those would be less useful in terms of delta-v savings for the &lt;em&gt;taxis&lt;/em&gt; to and fro planets, tho they could still provide significant mass savings with their reusable &lt;em&gt;castle&lt;/em&gt; parts, depending on utility they could provide (see e.g. &lt;a href=&quot;https://space.stackexchange.com/q/3880/49&quot;&gt;this thread&lt;/a&gt; for some suggested uses).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, there might be an infinite number of cyclers and other periodic or quasi-periodic orbits between multiple celestial bodies. Problem with finding those is the computational intensity mostly limiting analysis to non-exhaustive R3BP (restrictive three-body problem) and taking &lt;em&gt;shortcuts&lt;/em&gt; such as restricting search to best possible candidates, natural equilibrium between celestial bodies, and using precomputed invariant manifolds in (simplified) R3BP space, then &lt;em&gt;stitching&lt;/em&gt; these together to design more complex trajectories involving multiple targets. Adding more bodies to such analysis greatly intensifies search for additional possible resonant cyclers (or other low energy trajectories, often the lot of them known under a moniker of &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Interplanetary Transport Network&lt;/a&gt;&lt;/em&gt;) within our Solar system, and is still one of the areas where human intuition, for lack of a better word, often trumps all the supercomputers that we task with solving such problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Throw at this problem also the possibility of using propulsion methods that don't consume any reaction mass, such as momentum exchange tethers, solar or magnetic sails, beam powered propulsion, ramjets and similar, or settle for shorter term use or that your &lt;em&gt;ferry&lt;/em&gt; system doesn't have to be self-sufficient (dependent on supply of consumables and would require maintenance), and the number of possibilities becomes infinite.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-01-14T14:06:49.413" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7813" PostTypeId="1" AcceptedAnswerId="12584" CreationDate="2015-01-14T18:39:33.320" Score="19" ViewCount="648" Body="&lt;p&gt;The ISS had &lt;a href=&quot;http://www.washingtonpost.com/national/health-science/its-spacewalk-no-2-for-nasas-1-year-spaceman/2015/11/06/b4d2031c-847a-11e5-8bd2-680fff868306_story.html&quot; rel=&quot;noreferrer&quot;&gt;problems&lt;/a&gt; related to ammonia &lt;a href=&quot;http://www.nbcnews.com/science/space/nasa-tracks-down-glitchy-ammonia-leak-alarm-space-station-n285906&quot; rel=&quot;noreferrer&quot;&gt;more&lt;/a&gt; &lt;a href=&quot;http://www.universetoday.com/107489/astronauts-brave-brief-ammonia-snowstorm-as-they-conclude-fix-to-space-station/&quot; rel=&quot;noreferrer&quot;&gt;than&lt;/a&gt; &lt;a href=&quot;http://www.space.com/8900-faq-international-space-station-cooling-system-pump-failure.html&quot; rel=&quot;noreferrer&quot;&gt;once&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These problems happened in different calender years, and it seems like almost every &quot;near miss&quot; that makes the news about the ISS is connected to the ammonia loop. This system exists to transfer heat from the station to the radiators, which I've heard compared to a car radiator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When one single subsystem &lt;em&gt;consistently&lt;/em&gt; puts the entire station at risk, I would predict that NASA makes it a high priority for technology improvement. Particularly because they've actively been talking about BEO space stations quite seriously (which I'm sure would multiply the complications).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any competing technologies for the cooling systems of the next generation of NASA or commercial space stations? Hopefully options less likely to leak... Or is it generally accepted that we'll still use the ammonia coolant in spite of its track record?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't mean the distant future either, I want to limit this to concepts that a mission designer could &lt;em&gt;currently&lt;/em&gt; take seriously.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="12102" LastEditDate="2021-08-09T01:33:31.317" LastActivityDate="2021-08-09T01:33:31.317" Title="Are there any promising next-generation space station cooling technologies?" Tags="|space-station|thermal|radiators|ammonia|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7814" PostTypeId="1" AcceptedAnswerId="7821" CreationDate="2015-01-14T19:31:08.967" Score="98" ViewCount="140588" Body="&lt;p&gt;In a &lt;a href=&quot;http://www.youtube.com/watch?v=p7x-SumbynI&quot;&gt;Falcon 9 launch&lt;/a&gt;, the second stage looks like it achieves orbit along with the payload. But is that impression accurate? Does SpaceX need to do anything to deorbit the stage?&lt;br&gt;&#xA;I can see a few possibilities, but haven't been able to find which one's correct:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The stage is in a suborbital trajectory and will come down on the first orbit.&lt;/li&gt;&#xA;&lt;li&gt;The stage achieves orbit and is left there until its orbit decays.&lt;/li&gt;&#xA;&lt;li&gt;The stage achieves orbit and there's a deorbit burn. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="51" LastEditDate="2015-07-09T02:31:27.190" LastActivityDate="2020-08-23T15:57:37.640" Title="What happens to the Falcon 9 second stage after payload separation?" Tags="|launch|spacex|falcon-9|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7815" PostTypeId="2" ParentId="7814" CreationDate="2015-01-14T19:45:06.373" Score="23" Body="&lt;p&gt;It depends on trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A mission to LEO obviously leaves the second stage in a very different orbit than a GEO mission, where perhaps they boosted to some odd 80,000M X 250M orbit, where the satellite bus will correct the orbit after it is done with the second stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX would love to recover the second stage, which they had planned. But it sounds like they have given up on that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://shitelonsays.com/transcript/elon-musk-at-mits-aeroastro-centennial-part-1-of-6-2014-10-24&quot; rel=&quot;noreferrer&quot;&gt;Transcript - Elon Musk at MIT's Aero/Astro Centennial (part 1 of 6)&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[What about the second stage?] The next generation vehicles after the Falcon architecture will be designed for full reusability. I don't expect the Falcon 9 to have a reusable upper stage, just because the - with a kerosene-based system, the specific impulse isn't really high enough to do that, and a lot of the missions we do for commercial satellite deployment are geostationary missions. So, we're really going very far out. These are high delta-velocity missions, so to try to get something back from that is really difficult.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Reality bit them harder than expected, and they are focused on the next booster (BFR - Big F-ing Rocket, using Raptor engines) and its second stage for reuse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second stage left in LEO orbit is very easy to deorbit and most likely would on its own fairly quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second stage left in GTO (GEO Transfer Orbit) is a different kettle of fish.  If it is left in a highly elliptical transfer orbit, it usually has a fairly low perigee, which means minor alterations at the low point can force a reentry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a Reddit page that tracks the current location of the various second stages left in orbit from GTO launches.&#xA;&lt;a href=&quot;https://www.reddit.com/r/spacex/comments/49v7cn/falcon_9_gto_stage_2_orbit_and_decay_history_post/&quot; rel=&quot;noreferrer&quot;&gt;Table of Second stages&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-06-27T01:38:53.110" LastActivityDate="2016-06-27T01:38:53.110" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7817" PostTypeId="2" ParentId="5938" CreationDate="2015-01-14T21:13:28.230" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;By default Houston Centre is mission control for the ISS. Yet control&#xA;  may be transferred to Moscow in case Houston centre is not available.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Houston &lt;a href=&quot;http://en.wikipedia.org/wiki/Christopher_C._Kraft_Jr._Mission_Control_Center&quot; rel=&quot;nofollow&quot;&gt;JSC ISS MCC -- MCC-H&lt;/a&gt; has control for the &lt;a href=&quot;http://en.wikipedia.org/wiki/US_Orbital_Segment&quot; rel=&quot;nofollow&quot;&gt;US segment&lt;/a&gt; only. &lt;a href=&quot;http://www.mcc.rsa.ru/English/cup.htm&quot; rel=&quot;nofollow&quot;&gt;Roskosmos TsNIIMASH -- MCC-M&lt;/a&gt; has control for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Russian_Orbital_Segment&quot; rel=&quot;nofollow&quot;&gt;Russian segment&lt;/a&gt;. They are interlocked. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many ISS control systems are in the Russian segment, including attitude control and reboost (&lt;a href=&quot;http://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot; rel=&quot;nofollow&quot;&gt;Zvezda Service Module&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other systems (payload aboard ISS) are controlled by other control centers (ESA, JAXA, etc). There are &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/thumb/2/2d/ISS_Centers.svg/1920px-ISS_Centers.svg.png&quot; rel=&quot;nofollow&quot;&gt;many control centers&lt;/a&gt; for ISS and related elements (source: Wikipedia).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that ISS operations are subject to a number of ad-hoc agreements, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Politics_of_the_International_Space_Station&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Politics of the International Space Station&lt;/a&gt;. One agreement states than each module of the ISS remains under the control of the owning state.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How is control transferred? Is it merely a hand-off the way an aircraft&#xA;  is transferred from one tower to another?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Aircrafts are handed off between &lt;a href=&quot;http://en.wikipedia.org/wiki/Area_Control_Center&quot; rel=&quot;nofollow&quot;&gt;Area Control Centers&lt;/a&gt; (not Towers which are used for ATC in the vicinity of the airport only), but &lt;a href=&quot;http://en.wikipedia.org/wiki/Air_traffic_control&quot; rel=&quot;nofollow&quot;&gt;ATC&lt;/a&gt; is not monitoring the aircraft functions, only the aircraft position and altitude. ATC is not responsible for flying the aircraft or performing navigation. Services provided are information, collision avoidance, and SAR. Hand over consists merely on transferring data about crossing of control points time and altitude, which is performed using computer networks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand Mission Control Centers for the ISS monitor ISS position, systems status and alerts. In addition MCC are able to switch and command most systems aboard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As there are systems in both ISS segments, both MCC and related control teams are already interlocked. Aboard ISS critical systems control can be transferred from one segment to the other. From &lt;a href=&quot;http://spaceflight.nasa.gov/feedback/expert/answer/mcc/exp7/08_05_15_11_74.html&quot; rel=&quot;nofollow&quot;&gt;Nasa&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;There are data and voice links for direct communication between the control centers. Additionally, there is overlap in both control center manning and vehicle segment control.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Support groups work in each center. There is a Houston Support Group that works in MCC-M, and a Russian Support Group that works in MCC-H. These groups provide a working-level interface between the different control centers.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;At all times, MCC-M and MCC-H can interface on a higher-authority level through the flight directors.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What are the mission control backup options on a&#xA;  satellite/space-craft?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For spacecrafts other than the orbital station (ISS visiting vehicles, rockets, satellites, science probes, etc), I believe the organization remains at discretion of the operator. Operators may rely on ground segments which are more or less control rooms, transmitters and antennas, and networks on the ground. Those ground segments may serve multiple operators and spacecrafts at the same time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A couple of ground segments:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Intelsat&quot; rel=&quot;nofollow&quot;&gt;Intelsat&lt;/a&gt;&lt;/strong&gt;: Spacecraft operations are controlled through ground stations in Hagerstown, Maryland (USA), Riverside, California (USA), and Fuchsstadt, Germany. (source: &lt;a href=&quot;http://en.wikipedia.org/wiki/Intelsat&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;). Services provided include backup centers (source: &lt;a href=&quot;http://www.intelsat.com/services/satellite-related/satellite-payload-operations/&quot; rel=&quot;nofollow&quot;&gt;Intelsat&lt;/a&gt;).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;http://satellite.tmcnet.com/topics/satellite/articles/90338-globalstar-upgrades-satellite-operations-control-center-operations.htm&quot; rel=&quot;nofollow&quot;&gt;Globalstar&lt;/a&gt;&lt;/strong&gt; (aeronautical communications) use a Thales infrastructure, which includes a control center backup: &quot;Globalstar has a 24/7 fully operational back up Ground Control Operations Center in El Dorado Hills in the event of an outage at its primary GOCC facility&quot; (source: &lt;a href=&quot;http://www.globalstar.info/globalstar_background.html&quot; rel=&quot;nofollow&quot;&gt;Globalstar&lt;/a&gt;).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is there legislation making it mandatory for a satellite/launch&#xA;  vehicle owner/provider to have a fully manned back-up control center?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Laws for space operations are in their infancy -- after all, space operations have been conducted for a long time by State organizations only, -- though treaties have been agreed as soon as Sputnik 1 was launched. One of them states that the launcher of a space object is liable for any damages it causes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think they are treaties or local regulations for having backup control centers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For additional information, see:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;UN document &quot;&lt;a href=&quot;http://www.unoosa.org/pdf/limited/c2/AC105_C2_2012_CRP03E.pdf&quot; rel=&quot;nofollow&quot;&gt;Status of International Agreements relating to activities in outer space&lt;/a&gt;&quot;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Space_law&quot; rel=&quot;nofollow&quot;&gt;Space law&lt;/a&gt; on Wikipedia.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-01-14T21:34:35.057" LastActivityDate="2015-01-14T21:34:35.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7819" PostTypeId="2" ParentId="7814" CreationDate="2015-01-14T22:28:38.270" Score="3" Body="&lt;p&gt;SpaceX's second stages probably always achieve orbit. Typically they do a post-deployment burn to deorbit the stage, although they don't always do that. I know most/all LEO missions are done that way, I'm not sure about GEO missions. Basically, if they can deorbit it deliberately, they will.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2020-08-23T15:57:37.640" LastActivityDate="2020-08-23T15:57:37.640" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="7820" PostTypeId="1" AcceptedAnswerId="7822" CreationDate="2015-01-14T23:41:57.267" Score="13" ViewCount="5189" Body="&lt;p&gt;Question is fairly self-explanatory, but I couldn't find an answer on Google. I would expect ion thrusters because of their efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="8239" LastEditorUserId="49" LastEditDate="2015-01-15T00:56:48.837" LastActivityDate="2021-12-30T11:39:50.080" Title="What kind of thrusters will the James Webb Space Telescope use for station keeping?" Tags="|propulsion|fuel|attitude|station-keeping|james-webb-telescope|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7821" PostTypeId="2" ParentId="7814" CreationDate="2015-01-14T23:48:50.460" Score="108" Body="&lt;p&gt;There's actually a few outcomes of the second stage that can occur (and some interesting tales to go along with them), but as geoffc has mentioned, second stage reuse is no longer planned for Falcon as &lt;a href=&quot;http://shitelonsays.com/transcript/elon-musk-at-mits-aeroastro-centennial-part-1-of-6-2014-10-24&quot; rel=&quot;noreferrer&quot;&gt;Musk thinks the resources to develop it are better spent elsewhere&lt;/a&gt;. It's not an insurmountable technical challenge.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Intentional Deorbit&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;This is done for missions where the upper stage has enough remaining fuel reserves to ensure an intentional decay can occur safely. This has been done on every &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;LEO&lt;/a&gt; mission since CRS-3 (including Orbcomm OG2), and usually results in the stage being deorbited Southsouthwest of Australia in the Indian Ocean (close to the area where MH370 was lost). We know this because occasionally SpaceX will post a NOTAM declaring the zone unsafe for a certain time. Here's the CRS-3 NOTAM, for example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/46DHs.jpg&quot; alt=&quot;CRS-3 Deorbit Zone&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Left in GTO to decay&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;So far, this has been standard operating procedure for all 4 Falcon 9 upper stages that have delivered communications satellites to &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_transfer_orbit&quot; rel=&quot;noreferrer&quot;&gt;GTO&lt;/a&gt;. At this time, none remain in orbit, as the periapsis of each is so low (~200-300km) that they decay within 2-6 months. No deorbit profile has been attempted as it presents SpaceX with a liability if the stage decides to explode, scattering debris in GTO (additionally, if not enough fuel remains, they cannot safely deorbit either).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The standard apoapsis for GTO is about 35,000km, but for the first two GTO missions (SES-8 &amp;amp; Thaicom 6), they were injected into what is known as a &quot;Supersynchronous Orbit&quot; with an apoapsis beyond 35,000km. In the latter, it was in excess of 90,000km. &lt;a href=&quot;http://www.n2yo.com/satellite/?s=39501&quot; rel=&quot;noreferrer&quot;&gt;It was launched on January 6 and decayed May 28&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd also expect the stage to disintegrate upon reentry, wouldn't you. Not always! The upper stage of &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/2qwrnq/from_nsf_possible_falcon_9_second_stage_found_in/&quot; rel=&quot;noreferrer&quot;&gt;AsiaSat 6 reentered over Brazil and parts of it were found scattered in an open field&lt;/a&gt;. It made quite the fireworks on reentry though:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CCLtJ.jpg&quot; alt=&quot;AsiaSat 6 reentry&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the largest items found intact were the Composite Overwrapped Pressure Vessels (COPV's), which stored helium. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Left in LEO to decay&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;This has been the case for Falcon 1 Flights 4 &amp;amp; 5 which remain in a nearly equatorial LEO to this day, as well as the first 5 Falcon 9 flights - of which &lt;a href=&quot;http://spacexstats.com/mission.php?launch=7&quot; rel=&quot;noreferrer&quot;&gt;COTS1 performed an unannounced upper stage restart&lt;/a&gt; boosting it into a 290x10,700km orbit &amp;amp; CASSIOPE, which attempted a &quot;sideways&quot; upperstage restart which failed, stranding it in a 900km polar orbit. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Solar (Heliocentric Orbit)&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;This hasn't happened yet, but the next mission, launching DSCOVR (currently scheduled for January 29, likely to be delayed), will see the upperstage place the 570kg payload in Earth-Sun L1, before entering a solar orbit, becoming the first piece of SpaceX hardware to leave Earth's orbit. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="595" LastEditDate="2017-02-20T01:11:30.160" LastActivityDate="2017-02-20T01:11:30.160" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7822" PostTypeId="2" ParentId="7820" CreationDate="2015-01-15T00:27:48.720" Score="14" Body="&lt;p&gt;According to the &lt;a href=&quot;http://www.ai-solutions.com/Portals/0/AI%20Docs/Technical%20Paper%20Library%20pdf/2014/ISSFD24_Petersen_et_al_JWST_MCC_Monte_Carlo.pdf&quot; rel=&quot;noreferrer&quot;&gt;James Webb Space Telescope Initial Mid-Course Correction Monte Carlo Implementation using Task Parallelism&lt;/a&gt;, J. Petersen et al. (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;h3&gt;3.1 Propulsion System Overview&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;Two sets of thrusters comprise the observatory’s propulsion system.&#xA;  The first is a set of Secondary Combustion Augmented Thrusters (SCATs)&#xA;  that are the main thrusters for the MCC maneuvers. The SCATs are&#xA;  bi-propellant thrusters and draw from two separate tanks for a&#xA;  hypergolic reaction. Two pairs of SCATs exist: one for MCC-1a and -1b,&#xA;  and one for MCC-2. The MCC-2 SCAT is also employed in station keeping&#xA;  throughout the life of the mission. Two pairs are required because the&#xA;  center of mass of the observatory changes between MCC-1b and MCC-2&#xA;  because of the sun shield deployment; the thrust vector from a SCAT is&#xA;  directed through the center of mass at the time of that maneuver. Each&#xA;  pair is composed of a primary and a redundant thruster (the respective&#xA;  thrusters are coupled to the prime and redundant flight systems; only&#xA;  one system is on at a time). SCAT 1 and 2 are the primary and&#xA;  redundant pair for MCC-1a/b while SCAT 3 and 4 are the primary and&#xA;  redundant pair for MCC-2 and stationkeeping. For a given maneuver only&#xA;  one SCAT is on throughout that maneuver. The second set of thrusters&#xA;  is composed of eight Dual Thruster Modules (DTMs), each comprising of&#xA;  a primary and redundant Monopropellant Rocket Engine, 1 lbf, (MRE-1)&#xA;  thruster. A schematic of the propulsion system appears in Fig. 3. The&#xA;  MRE-1s consume hydrazine fuel only (and no oxidizer) from the same&#xA;  tank as the SCATs. The MRE-1s are subdivided into two sets: MRE-1s 1&#xA;  through 4, which are directed generally along the observatory J3 axis&#xA;  for pitch and roll control, and MRE-1s 5 through 8, which are directed&#xA;  radially about the J3 axis for yaw control &lt;code&gt;[3]&lt;/code&gt;. A picture of the&#xA;  spacecraft bus with the body J frame and orientation of the SCATs&#xA;  relative to the J frame appears in Fig. 4. Together, the MRE-1s&#xA;  on-pulse throughout a maneuver to control the attitude. Because of the&#xA;  location and alignment of MRE-1s 1 through 4, any firings to control&#xA;  pitch and roll during a maneuver may result in an additional ΔV to the&#xA;  MCC maneuver. Therefore, thrust contributions from the MRE-1s must be&#xA;  included in the design of the MCC maneuver.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UYSzy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Figure&#xA;  3. Schematic of the JWST propulsion system &lt;code&gt;[4]&lt;/code&gt;.&lt;/strong&gt;&lt;/sup&gt;&#xA;  &lt;br /&gt;&lt;br /&gt;&#xA;  &lt;img src=&quot;https://i.stack.imgur.com/GjwZK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;&lt;strong&gt;Figure 4. An independent view of the spacecraft bus showing the&#xA;  orientation of the body J frame along with the orientation of the&#xA;  SCATs relative to the spacecraft bus. The +J1 axis points in the&#xA;  general direction of the OTE boresight. The +J3 axis is roughly&#xA;  perpendicular to the multilayer sunshield and pointing parallel to the&#xA;  primary mirror (Image credit: jwst.nasa.gov)&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;h3&gt;References&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;&lt;code&gt;[3]&lt;/code&gt; Gidanian, D. 7.0. ACS design &amp;amp; analysis: Thruster Configuration&#xA;  and Modeling. In James Webb Space Telescope (JWST) Spacecraft Attitude&#xA;  Control Subsystem Hardware Critical Design Audit. Northrop Grumman, 28&#xA;  August 2013.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;code&gt;[4]&lt;/code&gt; Hammann, Jeff. JWST Propellant Budget Document, Northrup&#xA;  Grumman. 19 July 2013. Document Number D40258&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wanted to share this as it describes JWST stationkeeping thrusters in detail, and as you can see in text and the attached images:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SCAT (Secondary Combustion Augmented Thrusters) use &lt;a href=&quot;https://en.wikipedia.org/wiki/Dinitrogen_tetroxide&quot; rel=&quot;noreferrer&quot;&gt;dinitrogen tetroxide&lt;/a&gt; (N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;) as oxidizer and &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrazine&quot; rel=&quot;noreferrer&quot;&gt;hydrazine&lt;/a&gt; (N&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;4&lt;/sub&gt;) as fuel components of its bipropellant, and&lt;/li&gt;&#xA;&lt;li&gt;DTMs (Dual Thruster Modules) each use two MREs (Monopropellant Rocket Engines) propelled only by hydrazine as low-thrust monopropellant and each providing 1 lbf (4.45 N) of thrust&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also see &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140008865&quot; rel=&quot;noreferrer&quot;&gt;James Webb Space Telescope Orbit Determination Analysis&lt;/a&gt;, Yoon et al., NTRS 20140008865 (PDF) for explanation of JWST's three mid-course correction (MCC) maneuvers mentioned in the quote above.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-15T00:51:19.343" LastActivityDate="2015-01-15T00:51:19.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7824" PostTypeId="1" CreationDate="2015-01-15T08:24:38.600" Score="3" ViewCount="382" Body="&lt;p&gt;I know turbulence is a thing for aircraft etc. but I am not sure how even the dense clouds affect the flight path of a rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know the storm winds might affect the flight path of a rocket but if there are dense clouds and there are heavy winds how much they will effect the flight of rocket as it don't take much time to pass through the upper layers of the atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="8242" LastEditorUserId="6537" LastEditDate="2015-01-15T08:42:26.890" LastActivityDate="2019-02-02T23:27:15.633" Title="How could dense clouds or storms change the trajectory of rocket?" Tags="|rockets|trajectory|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7825" PostTypeId="2" ParentId="7824" CreationDate="2015-01-15T08:43:04.000" Score="3" Body="&lt;p&gt;Many rockets (for example Falcon 9) will have a Thick Cloud Rule as one of the launch constraints — if the density of height of a cloud exceeds a particular threshold, the launch will be postponed until the constraint is &lt;strong&gt;GO&lt;/strong&gt; - making your question nonanswerable in some instances. Mission managers realize it can pose a danger so they will scrub. &lt;a href=&quot;http://en.wikipedia.org/wiki/Launch_commit_criteria#Falcon_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here are the Falcon 9 weather commit criteria&lt;/a&gt;. These conditions are generally measured in the days and hours preceding a launch by the 45th Air Force Weather Squadron (at CCAFS). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wind shear actually played a small part in the destruction of Space Shuttle Challenger in 1986 (although it was essentially doomed from the start):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...However, at around T+36 seconds and an altitude of just over 10,000 feet (3,000 m), Challenger experienced the strongest wind shear ever felt by a Space Shuttle launch. The pitch and yaw commanded by the shuttle's computers in order to counter this wind caused the solid fuel plug to become dislodged from the field joint on the right SRB...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Taken from the &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-51-L_Mission_timeline&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-51-L Mission Timeline&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In general terms, a rockets outcome can be expressed as a boolean: either the launch is a success and the payload reaches orbit, or the launch is not a success and the rocket will either explode, veer off trajectory (to be destroyed by range safety), or deliver a payload to a suboptimal orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that the most dangerous winds will be found when the vehicle isn't very high or very downrange, if the vehicle cannot self correct its trajectory to account for the wind shear or clouds by either throttling up slightly or gimballing its engines, it will veer outside its preplanned flight corridor, and as I mentioned, by automatically terminated by range safety. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2017-12-19T02:09:37.153" LastActivityDate="2017-12-19T02:09:37.153" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7826" PostTypeId="2" ParentId="2954" CreationDate="2015-01-15T10:40:24.407" Score="31" Body="&lt;p&gt;Although this has indeed &quot;worked to bits&quot; on the Physics and other SE sites it's worth looking at, for the sake of Space Exploration, the interesting history behind the analysis of the falling cat. For the fully rigorous description of the cat's righting reflex - perfectly in keeping with conservation of angular momentum - &lt;em&gt;only&lt;/em&gt; came about because it was prompted precisely by research that was done in the late 1950s and early 1960s into &lt;em&gt;how the human body would deal with the environment it met in outer space&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main researcher here was Professor Thomas Kane, who &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Found the correct theoretical equations describing the cat's shift in orientation in response to its &quot;shape shifting&quot; during the righting reflex;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Experimentally showed that humans could&lt;/strong&gt;, with some training, make the righting reflex motions and flip over &lt;strong&gt;exactly like a cat&lt;/strong&gt;. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The primary reference here is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pentagono.uniandes.edu.co/~jarteaga/geosem/taller7/minicursoJK-Uniandes/robotic%20examples/kane.pdf&quot; rel=&quot;noreferrer&quot;&gt;T. R. Kane and M. P. Scher, “A Dynamical Explanation of the Falling Cat Phenomenon“, Int. J.Solids and Structures, 5, pp663-670, 1969&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Indeed Thomas Kane trained people to do this in 1968 in Apollo spacesuits, as shown below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zOEUk.jpg&quot; alt=&quot;Cat and Astronaut&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The left falling cat sequence was taken from the work of physiologist Étienne-Jules Marey (1830-1904) (famous for the development of motion photography for the study of high speed movements); the one on the right was taken during Thomas Kane's 1968 experiments with a trampolinist in an Apollo like spacesuit. Étienne-Jules Marey was a physiologist who did some of the little serious research into the cat's righting reflex before the outer space prompted research of Thomas Kane. Marey, unlike many of his contemporaries, clearly understood that the cat's motion was torque free (see footnote) and indeed used his photography to rule out a commonly held theory that the cat pushes off whatever it falls from. The collage was taken from&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.theatlantic.com/technology/archive/2011/09/video-deducing-the-physics-of-how-cats-fall/244842/&quot; rel=&quot;noreferrer&quot;&gt;Alexis C. Madrigal, “Video: Deducing the Physics of How Cats Fall“, The Atlantic Magazine, 9th September, 2011&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now to clear up some popular misconceptions about the cat righting reflex, particularly applied to astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rollover motion is not particularly burdensome or clumsy for humans to do, as Thomas Kane's experiments showed. It's very much like a hula hoop motion. OK so my CGI skills are crap - this is the best cat animation I can make with basic solids in Mathematica, but this movement will roll you over in space, whether you be cat or human, with or without a tail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hyoyl.gif&quot; alt=&quot;Hula Hoop Motion&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only differences between cat and human for this motion are (1) the cat's spine is much bendier than ours, so that the flipover can be done in fewer &quot;hula hoop&quot; cycles by the cat and (2) the exquisite sensitivity of the cat's vestibular system and lightning swift reflexes compared to ours. Point (2) is irrelevant when one is making a planned rotation in a freefall (gravity free) state in space, as opposed to flipping oneself over in limited time as one falls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another common misconception is that the cat needs its tail to flip over: this is wrong as shown by Thomas Kane's experiments that show tailless humans can make the righting motion. Indeed for our housecats, the tail is actually not used much for the reflex at all. I  cite here solid observational evidence of my own here: my own cat has been tailless since she was run over by a car in 2004 and has no difficulty whatsoever righting herself when she falls off things, which she does often owing to her somewhat clumsy nature – typically when she falls asleep with her head drooping too far over the edge of our bed. Even though twelve years old, she manages to rouse herself and make the righting reflex within the time it takes her to fall the 40cm or so to the ground from off our bed. Moreover, just after the time she had the accident, I saw her make the righting reflex falling asleep in this way when she had healed barely well enough to walk properly. So it would seem that she needed very little &quot;retraining&quot; to adjust for her new lack of tail. This also comes out of a theoretical analysis, as I show in my article cited below. Indeed the analysis of the video below from Wikipedia, showing the isosceles triangle addition of the two angular momentum vectors is valid only for a &quot;symmetric&quot; cat (owing to the isosceles shape of the vector addition diagram) with no tail (&lt;em&gt;i.e.&lt;/em&gt; a cat whose hinder half has the same inertia tensor about the origin as the forward half).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pVoDX.gif&quot; alt=&quot;Falling Cat Explanation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Source: &lt;a href=&quot;http://en.wikipedia.org/wiki/Cat_righting_reflex&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia &quot;Cat Righting Reflex&quot; Page&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some wild cats, notably the Asian Clouded Leopard and the Asian Marbled Cat have huge tails, much more like a club than the elegant, slender (and very small mass moment of inertia) tail of the housecat (Felis Sylvestris) and this &lt;em&gt;is indeed&lt;/em&gt; very much used to control the animal's orientation in space, but the tail lets the animal reorient itself freely about all three axes &lt;em&gt;i.e.&lt;/em&gt; it can pitch and yaw as well as roll pretty much at will - in contrast to simply flipping over in the cat righting reflex, which is essentially a one-axis motion. This is a useful ability for tree dwelling predators as they leap from tree to tree and also precision dive-bomb their prey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I discuss all this in much more detail in my article (including the analysis of &quot;symmetric&quot; (tailless) cats on my website:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.wetsavannaanimals.net/wordpress/of-cats-and-their-most-wonderful-righting-reflex/&quot; rel=&quot;noreferrer&quot;&gt;&quot;Of Cats and their most Wonderful Righting Reflex&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Footnote: The primary source for Marey's 1894 studies is the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://sciences.gloubik.info//spip.php?article134&quot; rel=&quot;noreferrer&quot;&gt;Étienne-Jules Marey, “Des mouvements que certains animaux exécutent pour retomber sur leurs pieds, lorsqu’ils sont précipités d’un lieu élevé“, La Nature, 1119, 10 Novembre 1894&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Near the end of this article he makes the following definitive statement (translation mine, so apologies to French speakers): &quot;&lt;em&gt;First of all, the inspection of these figures [photos of falling cats] rules out the notion that the animal imparts a rotational motion on itself by thrusting against the hands of the experimenter. [This conclusion follows] because the first frames of the two series [of photos of a falling cat] show that in the first instants of it its fall, the cat as yet has no tendency to turn from one side nor the other. Its rotation only begins with the twisting of its waist.&lt;/em&gt;&quot;&lt;/p&gt;&#xA;" OwnerUserId="6005" LastEditorUserId="6005" LastEditDate="2015-04-22T01:15:46.907" LastActivityDate="2015-04-22T01:15:46.907" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7827" PostTypeId="1" AcceptedAnswerId="7913" CreationDate="2015-01-15T16:24:14.823" Score="15" ViewCount="43024" Body="&lt;p&gt;I've been reading about &quot;cryogenic&quot; vs. &quot;storable&quot; propellants, and specifically the issue of boil-off: If the storage temperature gets above the propellant's boiling point, the propellant boils off and is lost. Apparently this is why &quot;storable&quot; propellants like hydrazine are used for satellites, where there's no support for keeping cryogenic propellants cool.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean the thermal control systems of a typical satellite can't radiate away enough heat to keep the propellant storage cool enough to prevent boil-off? What's the typical temperature of a satellite orbiting the Earth, and why can't it be kept low enough to use cryogenic propellants?&lt;/p&gt;&#xA;" OwnerUserId="8245" LastActivityDate="2018-11-06T20:22:11.170" Title="What's the typical temperature of a satellite orbiting the Earth?" Tags="|temperature|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7828" PostTypeId="1" CreationDate="2015-01-15T17:03:15.703" Score="8" ViewCount="180" Body="&lt;p&gt;Soon after the Philae landing on the comet Churyumov - Gerasimenko, there were news that it detected organic molecules on the surface. Does anybody know which ones? Were they the basic ones, like CO, that are so common in the universe? If so, of what interest is it to mankind?  &lt;/p&gt;&#xA;" OwnerUserId="8246" LastEditorUserId="25" LastEditDate="2015-01-15T21:07:48.613" LastActivityDate="2015-01-20T23:03:07.863" Title="Philae - organic molecules?" Tags="|rosetta|philae|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7829" PostTypeId="1" CreationDate="2015-01-15T17:47:15.447" Score="24" ViewCount="36617" Body="&lt;p&gt;This topic: &lt;a href=&quot;https://space.stackexchange.com/questions/6461/what-g-forces-do-different-launchers-cause&quot;&gt;What G-forces do different launchers cause?&lt;/a&gt; indicates that current satellite launchers are limiting peak acceleration to about 4g. I'm pretty sure the STS (Shuttle) did the same. My recollection from the Apollo days is that they threw 6g and 8g figures around for peak launch accelerations, or possibly peak re-entry forces.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What are typical peak accelerations of manned launchers? Soyuz, planned SLS (Orion), planned Dragon? What about re-entry?&lt;/li&gt;&#xA;&lt;li&gt;What were values in the 60's? What was the astronaut experience then? How critical was it that we reduce the forces? &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="8247" LastEditorUserId="8247" LastEditDate="2015-01-16T18:29:14.047" LastActivityDate="2021-03-07T23:44:45.427" Title="Launch Accelerations: Values, history" Tags="|launch|rockets|apollo-program|crewed-spaceflight|uncrewed-spaceflight|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7830" PostTypeId="1" AcceptedAnswerId="7848" CreationDate="2015-01-15T18:34:58.623" Score="1" ViewCount="9255" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3083/how-is-heat-dissipated-from-a-satellite-or-any-metal-in-space&quot;&gt;This&lt;/a&gt; doesn't make sense to me. How is this radiation given off? Is it a natural phenomenon helped along by using certain materials, or an active system? How are &quot;heat radiating fins&quot; possible in deep space, where there are few particles which can transfer heat away from a spacecraft? And particularly in a ship/station with humans, where certain types of radiation need to be shielded against and internal energy must be conserved, how does this radiation transfer work?&lt;/p&gt;&#xA;" OwnerUserId="8248" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-01-16T17:53:06.977" Title="How is heat in space dissipated?" Tags="|radiation|temperature|thermal-control|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7831" PostTypeId="2" ParentId="7830" CreationDate="2015-01-15T18:57:07.957" Score="5" Body="&lt;p&gt;There are three ways heat can be moved:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Convection&quot;&gt;convection&lt;/a&gt;, basically the heat moves because the object itself moves&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Thermal_conduction&quot;&gt;conduction&lt;/a&gt;, the most intuitive one: when two objects touch, heat is transfered from the hotter to the colder&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_radiation&quot;&gt;radiation&lt;/a&gt;, which always happen, is simply the natural emission of some electromagnetic waves, which takes energy, i.e. heat&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Convection is only interesting when considering fluid dynamics and we don't touch anything (well, virtually nothing) while floating in space. However, electromagnetic waves don't need a support to travel; this is why light can travel from the Sun to us and we can communicate with our satellites. In other words, heat dissipation can and does happen, even in outer space.&lt;/p&gt;&#xA;" OwnerUserId="8249" LastEditorUserId="5963" LastEditDate="2015-01-15T19:20:03.857" LastActivityDate="2015-01-15T19:20:03.857" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7832" PostTypeId="2" ParentId="7802" CreationDate="2015-01-15T19:23:59.830" Score="2" Body="&lt;p&gt;GPS satellites can be used to determine the attitude of other satellites also. I am not an expert in this field, but it might work the same way for tracking of people/cars on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, this is how it works for satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i64.photobucket.com/albums/h182/Olgola/GPS_scheme.jpg&quot; alt=&quot;gps&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GPS sensors can be used to determine attitude provided that &#xA;at least 3 antennas are vailable. The 4th is needed for timing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slave and master are 2 antennas mounted on the s/c. If the distance $b$&#xA; is known we can detect the s/c orientation in space. Taking the &#xA;projection of $\underline{b}$ on the direction $\underline{S}$ of the incoming GPS signal &#xA;we obtain the path difference of the signal received by the 2 antennas as $\underline{S}^T \underline{b}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The position of the GPS satellite is known, therefore $\underline{S}$ is known in the geocentric reference frame if we know the position of the master antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Measuring the path difference&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta r = \underline{S}^T \underline{b}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and transforming the vector $\underline{S}$ from geocentric to body frame&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\underline{S} = AS$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta r = S^T A^T \underline{b} $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;the unknown is the rotation matrix A.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having 3 independent measurements:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta r_{11} = S_1^T A^T \underline{b}_1 $ baseline 1 GPS satellite 1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta r_{21} = S_2^T A^T \underline{b}_1 $ baseline 1 GPS satellite 2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta r_{12} = S_1^T A^T \underline{b}_2 $ baseline 2 GPS satellite 1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta r_{22} = S_2^T A^T \underline{b}_2 $ baseline 2 GPS satellite 2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2 baselines are required, otherwise if you have a single baseline&#xA;you cannot determine the rotation around that baseline.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To determine attitude minimize the cost function:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$J = \sum_{i = 1}^{N_S} \sum_{j = 1}^{N_b} (\Delta r_{ij} - S_i^T A^T b_j)$&lt;/p&gt;&#xA;" OwnerUserId="6608" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.567" LastActivityDate="2015-01-16T06:42:59.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7833" PostTypeId="2" ParentId="7824" CreationDate="2015-01-15T22:18:42.463" Score="4" Body="&lt;p&gt;Thick clouds in general are not a &lt;em&gt;trajectory&lt;/em&gt; issue but can pose a threat of &lt;a href=&quot;http://articles.orlandosentinel.com/1987-04-17/news/0120270171_1_rocket-lightning-space-agency&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket-triggered lightning&lt;/a&gt;.  For STS &lt;a href=&quot;https://archive.org/details/flight_rules_generic&quot; rel=&quot;nofollow noreferrer&quot;&gt;flight rules&lt;/a&gt; were in place to mitigate the risk of lightning strikes (see table A2-6-III). &lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-02-02T23:27:15.633" LastActivityDate="2019-02-02T23:27:15.633" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="7834" PostTypeId="2" ParentId="623" CreationDate="2015-01-15T22:22:53.537" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Presumably this is because solar power isn't feasible at large&#xA;  distances from the Sun.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is a possibility to use solar energy as long as the arrays receive a quantity of energy greater than the working level of a photo voltaic cell. This includes the full solar system. The solar cell usability under low intensity is constantly improving. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, right... being able to collect only very little energy is not sufficient to power any space probe for the time being.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With the current technology available, what is considered the &quot;safe&#xA;  zone&quot; that solar arrays can be used as a reliable source to power a&#xA;  space craft?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The energy required for a given mission can be obtained by adjusting the size of the solar arrays, but this adjustment has an upper limit. The other way it to use more efficient cells:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_cell_efficiency&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hfbog.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/a/a5/PVeff%28rev141208%29.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Larger image&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;Cell efficiencies are measured under standard test conditions (STC) unless stated otherwise. STC specifies a temperature of 25 °C and an irradiance of 1000 W/m² with an air mass 1.5 (AM1.5) spectrum. [...] This represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cells work outside of STC very well, as soon as working conditions are taken into account in the design:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;Inner planetary missions and missions to study the sun within a few solar radii require solar arrays capable of withstanding temperatures above 450 °C and functioning at high solar intensities (HIHT). Outer planetary missions require solar arrays that can function at low solar intensities and low temperatures (LILT). In addition to the near-sun missions, missions to Jupiter and its moons also require solar arrays that can withstand high-radiation levels.&quot;&lt;/em&gt;&#xA;(Source: &lt;a href=&quot;http://catalogue.polytechnique.fr/site.php?id=733&amp;amp;fileid=7697&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Solar Cells and Arrays&lt;/a&gt; - Bailey, Raffaelle)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also different possibilities to concentrate light on cells to prevent low intensity efficiency degradation, and to obtain more energy from the same cell area:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spinoff.nasa.gov/Spinoff2012/ee_6.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Focusing&lt;/a&gt; technology  &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070010544.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stretched Lens Array SquareRigger&lt;/a&gt; -- SLASR&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Practical usuability of solar arrays in space&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall, this &lt;a href=&quot;http://www.lpi.usra.edu/opag/nov_2007_meeting/presentations/solar_power.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nasa's study (2007)&lt;/a&gt; assume solar arrays are practically usable as far as the Jupiter orbit (5.2 AU, &lt;a href=&quot;http://nmp.jpl.nasa.gov/st8/tech/solar_array3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ultraflex&lt;/a&gt; products), and that Saturn (10 AU) mission will be achievable in near-term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lPpHo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;(Juno mission for Jupiter)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;Near term Ultraflex arrays and state-of-art multi junction cells can provide capability to perform low power (200-300 W) missions out to 10 AU.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However several factors are to be taken into consideration.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Size of the solar arrays&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The quantity of energy received at some distance from the Sun is driven by an inverse square law. See &lt;a href=&quot;https://physics.stackexchange.com/questions/36041/how-far-from-the-sun-is-a-photovoltaic-effective&quot;&gt;this question on Physics.SE&lt;/a&gt; for more details:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;PV works great near the Earth, at 1 AU from the Sun, where we receive about 1400 Watts per square meter [...] At Saturn, nearly 10AU from the Sun, there's 1/100th power. Fine, if a spacecraft carries solar arrays 100 times bigger than would be used near Earth.&quot;&lt;/em&gt; -- For &lt;a href=&quot;http://www.nasa.gov/mission_pages/juno/main/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno mission&lt;/a&gt;: &lt;em&gt;&quot;Its 45 m² planar array produces 9.6 kW BOL at 1 AU and 414 W at 5.5 AU&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BOL / begin of life: Cells efficiency decreases with time, as they are exposed to radiations (protons, UV, IR, etc).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first problem arises in term of arrays size to deliver the electrical energy you need, and whether the spacecraft can accommodate such size or not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Eclipse&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft in orbit around a celestial body will not receive Sun light when behind this body. Some energy storage mean is required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rXr8g.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;(Source: &lt;a href=&quot;http://global.britannica.com/EBchecked/topic/525169/Saturn&quot; rel=&quot;nofollow noreferrer&quot;&gt;Britannica&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Planetary albedo&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A celestial body may reflect Sun light to the probe arrays, increasing energy production.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Robustness of the arrays&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Arrays may be destroyed during launch, or in orbit by debris. As they become larger their robustness is difficult to maintain without adding mass to the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Cost of the launch&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The larger the energy required, or the further the spacecraft from the Sun, the more expensive the arrays due to their size. The cost of the launch is also impacted due to the corresponding variation in mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some point other sources of energy will become cheaper to build and to launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Maximum current output&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the mission has a need for more current than the arrays are able to produce, and it's not suitable to increase the arrays size then energy must be stored at the rate the array can deliver it, then consumed at the higher rate required until the battery is empty, and then wait for the battery to be charged again. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Working discontinuously may be acceptable or not. In addition battery effectiveness decreases over time, and dust or propellant may dim the solar radiations. Long missions may not be able to accommodate these issues.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-01-16T19:13:00.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7835" PostTypeId="1" AcceptedAnswerId="7840" CreationDate="2015-01-15T22:26:47.517" Score="7" ViewCount="865" Body="&lt;p&gt;Opportunity, the rover on Mars, is on its tenth year. Wow! But its solar panels are fixed flat on top of it. As are those of all interplanetary landers which I know of. Why aren't they designed to mechanically turn their photovoltaic cells towards the Sun? And the same question about their communication radio antennas towards Earth. They just seem to sit there and waste geometric possibilities. Why? To keep it simple?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-01-16T05:05:46.573" Title="Why aren't landers designed to point their solar panels and radio antennas to Sun and Earth?" Tags="|communication|lander|solar-power|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7836" PostTypeId="2" ParentId="4814" CreationDate="2015-01-16T00:53:49.440" Score="1" Body="&lt;p&gt;I can describe the once-existing method that was used on the Shuttle Orbiter.  Large orbital changes were made using the 2 Orbital Maneuvering System (OMS) engines (6000 lbf each).  These engines were gimbaled and positioned by a set of electric gimbal actuators to keep them pointing through the Orbiter's CG.  The gimbaling was handled by the Digital Autopilot (DAP) as described &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rhc.html#sts-gnnc-autopilot&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the OMS thrusting period, the orbital state (position and vector)&#xA;  is produced by navigation incorporating inertial measurement unit&#xA;  delta velocities during powered and coasting flight. This state is&#xA;  sent to guidance, which uses target inputs through the CRT to compute&#xA;  thrust direction commands and commanded attitude for flight control&#xA;  and thrusting parameters for CRT display. Flight control converts the&#xA;  commands into OMS engine gimbal angles (thrust vector control) for an&#xA;  automatic thrusting period. OMS thrust vector control for normal&#xA;  two-engine thrusting is entered by depressing the orbital DAP auto&#xA;  push button light indicator with both RHCs within software detents.&#xA;  OMS manual thrust vector control for both OMS engines is entered by&#xA;  depressing the orbital man DAP push button light indicator or by&#xA;  moving the commander's or pilot's RHC out of detent; the flight crew&#xA;  supplies the rate commands to the TVC system instead of guidance. The&#xA;  manual RHC rotation requests are proportional to RHC deflections and&#xA;  are converted into gimbal angles. OMS thrust in either case is applied&#xA;  through the spacecraft's center of gravity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Acronym deciphering: CRT = cathode ray tube, the crew display (this manual is old), RHC = rotational hand controller aka the joystick, TVC = thrust vector control&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to read more about the OMS itself, that's &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-oms.html#sts-oms&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-01-16T00:53:49.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7837" PostTypeId="2" ParentId="7835" CreationDate="2015-01-16T01:07:35.483" Score="4" Body="&lt;p&gt;In a way you've answered your own question, &quot;to keep it simple&quot;. Don't forget, simple is also cheaper and less risky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having solar panels track the Sun to maximize energy capture makes sense, but someone has to design the system and the system has to be built. This takes time and money and it will increase the weight of the lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other thing is the more complex something is the greater the chance of something going wrong. Space probes and landers cost a lot money to make and put into position. The number of failed probes and landers is well documented. A malfunctioning tracking system for solar panels could render a probe/lander useless or severely limit its functionality.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-01-16T01:07:35.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7838" PostTypeId="2" ParentId="2318" CreationDate="2015-01-16T03:34:29.400" Score="10" Body="&lt;p&gt;There is no emergency escape procedure onboard the International Space Station (ISS) that would involve Extravehicular Activity (EVA). If you'd be &lt;em&gt;racing&lt;/em&gt; in your space suit, be it the Extravehicular Mobility Units (EMU) or the Russian Orlan-M space suit, it would be from your EVA back towards the ISS's Joint Airlock &quot;Quest&quot;, like the Italian ESA (European Space Agency) astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Luca_Parmitano&quot; rel=&quot;noreferrer&quot;&gt;Luca Parmitano&lt;/a&gt; did during his second EVA when his EMU helmet &lt;a href=&quot;https://space.stackexchange.com/a/1626/49&quot;&gt;started filling with cooling water originating from his PLSS&lt;/a&gt; (Portable Life Support Unit):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/e6h56.jpg&quot; alt=&quot;Joint Airlock &amp;quot;Quest&amp;quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;View of the Quest Airlock (left) taken from Space Shuttle Atlantis during docking procedures on 9 April 2002 as part of the&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;STS-110 mission to the International Space Station. (Image: NASA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which space suits are onboard the ISS was also already &lt;a href=&quot;https://space.stackexchange.com/a/1638/49&quot;&gt;covered in this answer&lt;/a&gt;, and I'm saying all of this so there won't be any expectation that the procedure of getting into one's space suit would be a timed or a life-saving racing event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before the airlock hatch is opened and the EVA gets underway there are lots of preparations that have to take place to make sure that the EVA proceeds seamlessly. We'll be the most interested in the EVA donning procedure, but for the sake of completeness, I'll include shortened versions of the complete Standard EVA Preparations, taken from the &lt;a href=&quot;http://esamultimedia.esa.int/docs/celsius/infokit/english/05_EVASupportInfo.pdf&quot; rel=&quot;noreferrer&quot;&gt;ESA's EVA support information brochure&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Airlock Preparation&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The day before the first &#xA;  EVA starts, the ISS Quest Joint Airlock needs to be &#xA;  configured and activated. The equipment has to be &#xA;  laid out to be easily accessible to the EVA astronauts during the EVA. This includes &#xA;  hardware for installation during the EVA and tools &#xA;  needed to carry out the relevant work, which will &#xA;  also need configuring before the start of the EVA &#xA;  (and during).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;EVA Suit Checkout&lt;/strong&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The EVA suits are known as Extravehicular &#xA;  Mobility Units or EMUs. These procedures are &#xA;  performed at least 1 day before the EVA. The purpose of EMU checkout is to ensure the integrity &#xA;  of the suits. This can include tasks such as &#xA;  powering up and installing the suits batteries &#xA;  necessary during the EVA, checking the Life &#xA;  Support Systems and Simplified Aid for EVA &#xA;  Rescue (SAFER) units that allow an astronaut to &#xA;  return to the ISS if he comes untethered during the &#xA;  EVA and checking that the suits communication &#xA;  devices are working. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Camping Out (Nitrogen Purge)&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Astronauts have to be in very good physical &#xA;  condition in order to undertake an extravehicular &#xA;  activity. One of the potential risks relating to EVA &#xA;  work is decompression sickness. For this reason, &#xA;  prior to the EVA the astronauts go through a &#xA;  regime of breathing pure oxygen in order to purge &#xA;  nitrogen out of their blood systems.&lt;br /&gt;&lt;br /&gt;The day before the EVA, the relevant astronauts &#xA;  will sleep in the airlock, which will be sealed and &#xA;  the pressure reduced from 1 bar to 0.7 bar. 1 bar &#xA;  is normal ISS (and earth sea-level) pressure. This &#xA;  process is known as camping out.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Donning EVA suits&lt;/strong&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The EVA suits known as Extravehicular Mobility &#xA;  Units or EMUs are extremely complex, containing &#xA;  many different layers and systems in order to &#xA;  provide the astronaut with an as safe and &#xA;  comfortable environment during EVAs whilst &#xA;  remaining functional for performing the tasks at &#xA;  hand. The EVA astronauts are usually assisted in &#xA;  donning their suits by one or more astronauts, &#xA;  with relevant checks being carried out during this &#xA;  procedure.&lt;br /&gt;&lt;br /&gt;When suiting up the astronaut first puts on the &#xA;  urine collection device and then a Liquid Cooling &#xA;  and Ventilation Garment. This spandex garment &#xA;  has water-cooling tubes running through it and &#xA;  also supports a network of ducting that draws &#xA;  ventilation gas from the suit extremities and routes &#xA;  it back to the primary life support system.&lt;br /&gt;&lt;br /&gt;The astronaut now gets into the Lower Torso &#xA;  Assembly of the space suit and then rises into the &#xA;  Hard Upper Torso section, which is attached to the airlock wall by an adaptor. The Lower Torso &#xA;  Assembly can be seen as the waist, trousers and &#xA;  boots of the EMU and has separation joints above &#xA;  the knee and above the ankle. The flexible waist &#xA;  section and waist bearing afford the astronaut a &#xA;  large degree of movement about the waist, e.g. &#xA;  bending and hip rotation.&lt;br /&gt;&lt;br /&gt;&lt;img src=&quot;https://i.stack.imgur.com/UxjzX.png&quot; alt=&quot;Donning EVA suits&quot;&gt;&lt;br /&gt;&lt;sup&gt;ISS Expedition 12 Flight Engineer Valery Tokarev being &#xA;  assisted into the Hard Upper Torso section of an EMU by &#xA;  Expedition&lt;br /&gt;12 Commander Bill McArthur on 23 October 2005. &#xA;  (Image: NASA)&lt;/sup&gt;&lt;br /&gt;The Hard Upper Torso is a rigid fibreglass vest &#xA;  onto which the Lower Torso Assembly attaches. It &#xA;  also acts as the attachment point for the helmet &#xA;  and the flexible arm sections, which have an arm &#xA;  bearing to allow for arm rotation. The Life Support &#xA;  System is attached to the back of this assembly &#xA;  with Life Support controls mounted to the front in &#xA;  easy reach of the astronaut. Connections &#xA;  between the two parts must be aligned to enable &#xA;  circulation of water and gas into the Liquid &#xA;  Cooling Ventilation Garment and return. The Life &#xA;  Support System provides the crew member with &#xA;  pure oxygen to breath, removes carbon dioxide &#xA;  exhaled, regulates the temperature in the suit, and &#xA;  keeps the pressure during EVA at 0.3 bar, this is &#xA;  30% of the air pressure at sea level on Earth and &#xA;  30% of the normal ISS air pressure. This low pressure is necessary to maintain suit flexibility. If &#xA;  the pressure level was higher the suit would &#xA;  become too stiff to work.&lt;br /&gt;&lt;br /&gt;Once the upper torso section is donned the &#xA;  astronauts put on their communications headset &#xA;  otherwise known as a snoopy cap with &#xA;  headphones and microphones for two-way &#xA;  communications between crew members and to &#xA;  Mission Control. This is followed by the gloves and &#xA;  lastly the extravehicular visor and helmet assembly.&lt;br /&gt;&lt;br /&gt;This provides protection from micrometeoroids &#xA;  and from solar ultraviolet and infrared radiation. &#xA;  This is made of a rugged, impact resistant &#xA;  polycarbonate material. A vent assembly, bonded &#xA;  to the inside rear of the polycarbonate shell, &#xA;  serves to diffuse the incoming gas over the &#xA;  astronaut's face.&lt;br /&gt;&lt;br /&gt;The Extravehicular Visor Assembly is a light-and-heat-attenuating shell which fits over the Helmet &#xA;  Assembly. It is designed to provide protection &#xA;  against micrometeoroid activity and accidental &#xA;  impact damage, plus protect the crewmember from solar radiation. A special coating gives the &#xA;  sun visor optical characteristics similar to those of &#xA;  a two-way mirror; it reflects solar heat and light, &#xA;  yet permits the astronaut to see. Adjustable &#xA;  eyeshades may be pulled down over the visor to &#xA;  provide further protection against sunlight and &#xA;  glare.&lt;br /&gt;&lt;br /&gt;An extra unit that is attached to the EMU once it is &#xA;  donned is the SAFER unit. This is a small, self-contained, propulsive backpack system used to &#xA;  provide a free-flying self-rescue capability for an &#xA;  EVA crewmember if he becomes separated from &#xA;  the ISS during an EVA.&lt;br /&gt;&lt;br /&gt;So as not unnecessarily use up EMU battery &#xA;  power the EMUs will remain plugged into the ISS &#xA;  electrical power supply via an umbilical. The &#xA;  spacesuits will then be ventilated with pure &#xA;  oxygen and the airlock will be re-pressurised to 1 &#xA;  bar. The EVA crew members will continue the prebreathe of pure oxygen inside their spacesuits for &#xA;  50 minutes. The EVA astronauts will go into the &#xA;  crewlock of the Quest airlock where the hatch will &#xA;  be closed. The depressurisation of the crewlock &#xA;  will now be initiated.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Depressurisation&lt;/strong&gt;: &#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The usual pressure inside the ISS is 1 bar, though &#xA;  in the Quest airlock this is 0.7 bar during &#xA;  depressurisation in connection with nitrogen &#xA;  purging. When the astronauts are in the crewlock ready to start their EVA this pressure is reduced &#xA;  first to 0.35 bar when a leak check is performed &#xA;  on the suits. If this is ok the crewlock is reduced in &#xA;  pressure down to 0.2 bar. The final &#xA;  depressurisation to vacuum occurs through &#xA;  venting through a valve in the EVA hatch. The &#xA;  hatch can now be opened and the EVA can begin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I've somewhat shortened the other points we're not as interested in to answer your questions, but the full description and many more photographs explaining the procedure are available in the linked to brochure. As you can see, these procedures are laborious and time consuming. Donning of the spacesuit might be the shortest of the procedures, but would in reality still take hours, with the soon to EVA astronauts possibly already in the Lower Torso Assembly long before donning the rest of the space suit and all the equipment that attaches to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to conclude, we could say that all procedures involved take at least one day, and the donning of the spacesuit itself maybe a few hours? Nobody would be racing either, there's way too many things to check and recheck, complete spacesuits are essentially your mini personal spaceships with all the bells and whistles and that means they're complex while trying to be as ergonomic as possible once you're in them. Comfort of getting in and out of them was somewhat sacrificed for that. And you can't just &lt;em&gt;walk out of the station&lt;/em&gt; either, once in the suit. Depressurization of the airlock prior to egress will take some time, and so will repressurization and sometimes purging of possible contaminates (like ammonia that is used as external cooling liquid and those systems are often worked on during EVAs, if it couldn't have been &lt;em&gt;baked-off&lt;/em&gt; during EVA with exposure to the Sun or airlock sensors still detected some despite attempts to not bring any in) during ingress.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-01-16T03:34:29.400" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7839" PostTypeId="2" ParentId="5178" CreationDate="2015-01-16T03:39:14.463" Score="0" Body="&lt;p&gt;There is currently a means to modify kerosene into a more energetic, more completely burning substance. &lt;a href=&quot;http://www.clearrefining.com/%20http://www.clearrefining.com/newfactsheet1211.pdf&quot; rel=&quot;nofollow&quot;&gt;Advanced Refining Concepts&lt;/a&gt; of Reno NV is producing a modified diesel fuel and heating oil using methane, a catalyst and a low temperature low pressure process. The molecules are modified so that their lengths are more uniform. The final product burns more easily and cleanly, releasing more energy than the standard petroleum products. Using this fuel for the kerosene portion of RP1 may make more energy available per volume of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a link that describes the clearrefining process. &lt;a href=&quot;http://www.purefuelusa.com/Pages/The-ClearRefining-Process.html&quot; rel=&quot;nofollow&quot;&gt;http://www.purefuelusa.com/Pages/The-ClearRefining-Process.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The material safety sheet says the specific gravity for GDiesel is .8158. &lt;a href=&quot;http://www.clearrefining.com/MSDS%20-%20GDiesel%203_30_09.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.clearrefining.com/MSDS%20-%20GDiesel%203_30_09.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8253" LastEditorUserId="6312" LastEditDate="2015-01-16T20:56:36.723" LastActivityDate="2015-01-16T20:56:36.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7840" PostTypeId="2" ParentId="7835" CreationDate="2015-01-16T05:05:46.573" Score="16" Body="&lt;p&gt;The Opportunity, Spirit (RIP), and Curiosity rovers all have high-gain antennas that point at Earth when in use, using a two-axis gimbal.  They are used mostly to receive the command loads every sol from Earth.  The rovers also have low-gain UHF antennas, basically like a car antenna but shorter, for communicating with relay orbiters around Mars.  That path is used to send back the vast majority of the data to Earth.  You can get very high data rates to an orbiter due to the short range, so you don't need to point the UHF antenna at the orbiters, and try to slew them accurately as the orbiter flies from horizon to horizon in about 15 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for articulating the solar panels, the motors would be massive, and the motors would have to be heated to operate, taking some of the energy benefit from pointing.  It is less mass to add more solar cells than to articulate the panels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As it turns out, we &lt;em&gt;can&lt;/em&gt; point the panels at the Sun, and in fact that is being done as we speak.  Opportunity is being driven to terrain with the proper slope and direction to point the panels towards the Sun as it gets closer to the horizon in Winter.  This strategy has been used every Winter to keep the rovers alive as the Sun gets lower in the sky.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-01-16T05:05:46.573" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7841" PostTypeId="1" AcceptedAnswerId="7847" CreationDate="2015-01-16T08:50:28.777" Score="28" ViewCount="8136" Body="&lt;p&gt;What are the reasons for using 9 engines in the first stage of the Falcon 9? Why not 8 or 10? Looking at the Raptor engine, they seem to be looking at 9 engines again. Is there some specific advantage to using 9 engines?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="51" LastEditDate="2015-07-02T20:49:14.130" LastActivityDate="2016-10-01T05:46:07.440" Title="Why does the Falcon 9 have 9 engines?" Tags="|launch|spacex|falcon-9|design|first-stage|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7842" PostTypeId="1" AcceptedAnswerId="7845" CreationDate="2015-01-16T08:57:47.380" Score="12" ViewCount="1036" Body="&lt;p&gt;For example, when the Antares rocket, built and launched by commercial Orbital Sciences Corp burst into flames on October 28, 2014, it carried a lot of expensive payload. Did the company have to refund the loss of payload? Did they only return money received for the launch itself? Who takes the risk - The launch company, or the customer?&lt;/p&gt;&#xA;" OwnerDisplayName="user6738" LastEditorUserId="6214" LastEditDate="2015-01-17T04:13:17.433" LastActivityDate="2015-01-17T04:13:17.433" Title="Do space launch service providers have to refund the loss of payload if the rocket explodes?" Tags="|launch|failure|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7843" PostTypeId="2" ParentId="7841" CreationDate="2015-01-16T11:37:13.680" Score="4" Body="&lt;p&gt;As you increase the number of rocket engines certain problems begin to arise. Back pressure increases, vibration increases, - In both cases I think that this is owing to the fact that all the engines must, at some point, all lead to a single fuel line from the fuel tank where the effects of a single engine all start to become compounded - I suppose stability in general. In summary, you cannot simply add more engines and expect more power without serious negative implications -  It's very much a compromise. Hopefully someone with more expertise can elaborate on my answer.&lt;/p&gt;&#xA;" OwnerUserId="3384" LastActivityDate="2015-01-16T11:37:13.680" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7844" PostTypeId="2" ParentId="7841" CreationDate="2015-01-16T11:52:17.397" Score="9" Body="&lt;p&gt;Many rocket manufacturers use different engines for different stages which are tailor-made for their purpose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX, on the other hand, tries to save cost by using as many standardized parts as possible. So instead of developing new engines from scratch for every single purpose, they just developed one rocket engine for the lower stages of all of their rockets: &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1A&quot;&gt;the Merlin&lt;/a&gt;. Yes, there are the variants 1A, 1B, 1C and 1D, but these are incremental engineering improvements for an engine which all serve the same purpose. The only variant of the Merlin which is a spinoff and not an incremental improvement is the vacuum version of the C and D engine, which are in many ways very similar to the normal version.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what do they do when their engine is too weak and they require more power, like on the first stage of a medium-lift rocket like the falcon 9? They just use more of them. 9 engines happens to be the ideal thrust-to-weight ratio for the first stage of the Falcon 9 rocket.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2015-01-16T12:23:53.320" LastActivityDate="2015-01-16T12:23:53.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7845" PostTypeId="2" ParentId="7842" CreationDate="2015-01-16T12:15:59.130" Score="9" Body="&lt;p&gt;It is common practice to have an insurance which covers the loss in case of a launch failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who pays for the insurance (customer or launch provider) is a contract detail.&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2015-01-16T12:15:59.130" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7846" PostTypeId="1" AcceptedAnswerId="7859" CreationDate="2015-01-16T13:20:20.780" Score="7" ViewCount="2835" Body="&lt;p&gt;I am not sure if this question is appropriate for this SE - Ideally this should be posted in Engineering SE but as far as I'm aware it does not exist! (I can only assume that the scope of engineering is far too broad for one sub-domain.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the 1960s and 70s computers such as the CDC-6600, and its successor the CRAY-1, were considered the pinnacle of computing power. Now, in my understanding of computers and FEA/CFD etc., the power of these computers (60-80MHz-ish) would be nowhere near powerful enough to perform FEA/CFD calculations within realistic time frames and to a reasonable degree of accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what tools did NASA, and other entities with similar requirements (be it in construction, automotive, aerospace industries), use to perform their stress analysis, aerodynamics, engine fluid flows, pressures etc. before constructing and indeed launching their vehicles (, implementing their designs in general)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an engineer I can appreciate the importance of these steps in a design process so I'm truly stumped.&lt;/p&gt;&#xA;" OwnerUserId="3384" LastActivityDate="2020-07-01T22:15:37.173" Title="What sort of analysis was performed before &quot;modern&quot; computing and the invention of finite element analysis and computational fluid dynamics?" Tags="|engine-design|design|analysis|" AnswerCount="5" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7847" PostTypeId="2" ParentId="7841" CreationDate="2015-01-16T13:38:13.300" Score="43" Body="&lt;p&gt;SpaceX initially was looking to buy an engine but could not find one on the market that would allow them to meet their goals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once they decided to develop their own, they had to consider their goals: Reliability, and reduced costs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nine engines means at most any time in the flight profile, you can handle an engine out event. The Saturn V with 5 F-1 engines experienced this, and took advantage of it. A SpaceX Falcon 9 1.0 on a CRS mission lost an engine, and continued to deliver the primary payload to orbit.  (Secondary payload could have been delivered, but NASA safety rules around ISS orbits meant they did not meet the criteria to release the payload with sufficient certainty it would not impact the ISS if things went wrong). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond simple launch, they have planned on recovery for a long time. The theory being you cannot reduce prices seriously if you keep throwing away the hardware each flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to land a first stage you need to get it through the atmosphere slow enough to survive. Parachutes did not work, but retrothrust did. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A first stage, at launch, needs all the thrust it can get as it is over 90% fuel and oxidizer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A first stage on landing is only 10% the weight, and in fact the problem is you cannot throttle the engines low enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But by using a large (9) number of engines, you can control thrust by throttling all nine as needed, and shutting down engines as needed. In fact during launch, they shut off two engines midway through the flight to reduce thrust as the vehicle burns off fuel and oxidizer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For first stage recovery they fire three engines (It is unclear if the first two of three burns use 3 engines, or just the first), and finally one engine, which throttled down as low as it goes, still has too much thrust to actually hover. But it allows them to do the 'hover-slam' approach they use. (Come in fast, thrusting to slow down so that you hit the ground at 0 m/s). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The use of 9 engines (or 5, or 3) gives them a center engine, that is balanced for landing. It allows throttling. It allows engine out capability. It is not crazy like the N-1 with 30 engines. It is a reasonable tradeoff that seems to be working well with a dozen flights so far.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2015-05-09T09:38:19.873" LastActivityDate="2015-05-09T09:38:19.873" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="7848" PostTypeId="2" ParentId="7830" CreationDate="2015-01-16T17:12:14.547" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How are &quot;heat radiating fins&quot; possible in deep space, where there are few particles which can transfer heat away from a spacecraft?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The particles that transfer heat away from spacecraft are photons, and &lt;strong&gt;they are created from nothing&lt;/strong&gt;. The molecular motion on the surface of a solid material creates electromagnetic radiation (in form of photons) because it contains oscillating electric charge. In this case, the oscillation is the thermal motion of atoms in the material.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Addition: To elaborate on the &quot;from nothing&quot; part, I mean this in the sense of particle physics. The number of photons are not constant. They carry energy, and they are the means by which thermal energy escapes the radiator. This is different from and electron, for instance. You could make an electron from pure energy, but you would also need to make a positron in order to conserve electric charge. Conservation of the quantum numbers gets more complicated with protons and other matter. Materials are constantly creating new photons, and many of the photons from a space radiator will likely travel forever.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is it a natural phenomenon helped along by using certain materials, or an active system?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Some materials are vastly more radiative than others. The theoretically perfect radiator is a blackbody (which is an academic physics concept). All materials fall short of this to some degree, but materials selected for a space radiators get pretty close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, some materials are very bad at emitting thermal radiation and very good at reflecting thermal radiation. These have their own use if you're trying to insulate something.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;in a ship/station with humans, where certain types of radiation need to be shielded against and internal energy must be conserved, how does this radiation transfer work?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We shield against ionizing radiation. Some ionizing radiation particles are photons. Some are protons, electrons, and other particles. It's soup of different particles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By definition, ionizing radiation will destroy chemical bonds. Particularly, we would like to minimize the disruption that this radiation causes to astronaut's DNA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While thermal radiation and ionizing radiation can both be photons, that's like comparing a baseball to a speeding bullet. It's completely possible to both shield astronauts from ionizing radiation while dissipating their body heat (and energy use) into space. The spacecraft's heat is transferred to the radiator via a working fluid. This fluid passes from the interior through the shielding to the radiator and back again.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2015-01-16T17:53:06.977" LastActivityDate="2015-01-16T17:53:06.977" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7849" PostTypeId="1" AcceptedAnswerId="43548" CreationDate="2015-01-16T17:57:23.733" Score="19" ViewCount="7364" Body="&lt;p&gt;Saturn being at 10 A.U. means sunlight on Titan's cloud tops is about 1/100 that on Earth's.  That's &lt;a href=&quot;http://blogs.discovermagazine.com/badastronomy/2012/08/27/bafact-math-the-sun-is-400000-times-brighter-than-the-full-moon/&quot;&gt;4000 times&lt;/a&gt; the illumination of Earth's moon.  Titan's atmosphere is described as &lt;a href=&quot;http://en.wikipedia.org/wiki/Titan_%28moon%29#Atmosphere&quot;&gt;opaque smog&lt;/a&gt;.  If it were 99.97% opaque the surface would still be illuminated like a full moon.  Photos from Huygens seem to show &lt;a href=&quot;http://apod.nasa.gov/apod/ap150116.html&quot;&gt;some light&lt;/a&gt; gets through.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a (warmly-insulated) astronaut see pitch black?  A diffusely glowing brown sky?  A foggy city streetlamp?  I'm finding it hard to picture.&lt;/p&gt;&#xA;" OwnerUserId="213" LastEditorUserId="49" LastEditDate="2015-01-16T18:17:10.037" LastActivityDate="2020-04-20T14:20:01.237" Title="How much sunlight gets to the surface of Titan? What would astronauts see?" Tags="|planetary-science|atmosphere|titan|insolation|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7850" PostTypeId="1" CreationDate="2015-01-16T18:33:18.340" Score="4" ViewCount="213" Body="&lt;p&gt;Random synapses firing reading across posted questions here on SEx.SE ...&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Martian gravity is weaker than Terran&lt;/li&gt;&#xA;&lt;li&gt;Martian magnetic field almost non-existent&lt;/li&gt;&#xA;&lt;li&gt;Martian atmosphere is far thinner than Terran &lt;/li&gt;&#xA;&lt;li&gt;Traces of water/moisture were reportedly discovered on Mars&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Facts as I understand&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A magnetic field helps retard the impact of Sunspots/CME, and contain the atmosphere of a planet/body.&lt;/li&gt;&#xA;&lt;li&gt;Above is assisted by the planet's gravity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;From the above, and knowing what we do about Mars at present ...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How would the nature of the Martian atmosphere, in it's heyday, differ from Earth by virtue (or vice!) of the weaker planetary gravity?&lt;/li&gt;&#xA;&lt;li&gt;Could the Martian atmosphere, in it's heyday, have caused liquid water in oceanic/sub-oceanic volumes?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="288" LastActivityDate="2015-01-19T05:34:58.347" Title="Could the Martian atmosphere, in it's heyday, have caused liquid water in oceanic/sub-oceanic volumes?" Tags="|mars|planetary-science|water|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7851" PostTypeId="2" ParentId="7749" CreationDate="2015-01-16T18:35:15.660" Score="2" Body="&lt;p&gt;By &lt;strong&gt;conservation of the center of momentum&lt;/strong&gt;.  The center of mass of a jetpack plus it's propellant stays on a constant course (along an inertial frame of reference, e.g. the ISS orbit).  This remains true whether the propellant is inside or outside the jetpack.  By moving a mass of propellant one way, the jet-pack-wearing astronaut moves the other way, according to this relation, the essence of rocket science:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;	m dv = -v_e dm&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$m$ - mass of astronaut + jetpack + remaining propellant&lt;/li&gt;&#xA;&lt;li&gt;$dv$ - change in astronaut's velocity&lt;/li&gt;&#xA;&lt;li&gt;$v_e$ - velocity of the ejected propellent, relative to jetpack&lt;/li&gt;&#xA;&lt;li&gt;$dm$ - mass of ejected propellent, also the change in stored propellent&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;By &lt;a href=&quot;http://en.wikipedia.org/wiki/Noether&amp;#39;s_theorem&quot; rel=&quot;nofollow&quot;&gt;Noether's Theorem&lt;/a&gt; this conservation law is the same as translational symmetry, that moving through space doesn't change your laws of physics.&lt;/p&gt;&#xA;" OwnerUserId="213" LastActivityDate="2015-01-16T18:35:15.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7852" PostTypeId="1" AcceptedAnswerId="7853" CreationDate="2015-01-16T19:31:42.113" Score="2" ViewCount="3186" Body="&lt;p&gt;As we know, the moon has a very different orbit relative to the Sun than the Earth does. So I imagine a colony on the moon might have drastically different &quot;days&quot; depending on where it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'm wondering if Astronauts or NASA would or does have a preferred location for where a lunar colony should be and what the reasons are for that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it turns out that any or all NASA reasoning has more do to with other factors (such as the ground, nearby mountains, ease of returning to Earth, etc.) then I would like the answer to focus on the pros and cons of being in the various areas (such as near side, far side, one of the poles, etc.)&lt;/p&gt;&#xA;" OwnerDisplayName="user6587" LastEditorDisplayName="user6587" LastEditDate="2015-01-18T22:29:31.653" LastActivityDate="2018-10-08T13:28:03.693" Title="What is the best (NASA preferred) location for a lunar colony?" Tags="|the-moon|nasa|colonization|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7853" PostTypeId="2" ParentId="7852" CreationDate="2015-01-16T20:15:49.923" Score="6" Body="&lt;p&gt;I don't know about NASA's official preferred location if they have one at all, but the most considered locations (also by NASA) for a permanent settlement on the moon are, based on our current knowledge, the two poles of the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the south pole of the moon there is &lt;a href=&quot;http://en.wikipedia.org/wiki/Shackleton_(crater)&quot; rel=&quot;noreferrer&quot;&gt;Shackleton Crater&lt;/a&gt;. It is one of the possible sites for a lunar base , in particular its rim. The Pole is almost permanently sunlight and thus one can sustain reliable solar power with very few solar arrays. This also means that the temperatures average at a more human-friendly -50°C, other than the huge temperature fluctuations you experience in equatorial regions.&#xA;The inside of the crater rarely to never sees direct sunlight, and it is believed that there might be a valid source of frozen water inside. Another advantage of this spot is, that it is proposed as a possible site for an infrared telescope, because the low temperatures inside the shady crater are ideal for infrared exploration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the north pole there is &lt;a href=&quot;http://en.wikipedia.org/wiki/Peary_(crater)&quot; rel=&quot;noreferrer&quot;&gt;Peary Crater&lt;/a&gt;. It's advantages are basically identical to it's southern twin, Shackleton Crater.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A common limitation on both poles however might be solar wind, which can cause electrical charges on the surface of the crater rims. Those charges could harm or distort electrical equipment and signals, and erode the surface.&#xA;Of course, the other problems that come with the absence of an atmosphere will not be solved by those locations. There is no protection from meteors whatsoever, and cosmic radiation is not limited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this helps to give you a little bit of an idea of what the pros and cons of those areas are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: To add to the answer of the question, the ESA has a concept (though not a definite plan, at least to my knowledge) up their pipes, considering the southern pole as the most likely site. It is their concept for a 3D printed lunar base. Recommend a look at it.&lt;/p&gt;&#xA;" OwnerUserId="8237" LastActivityDate="2015-01-16T20:15:49.923" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7855" PostTypeId="2" ParentId="7841" CreationDate="2015-01-16T20:41:46.470" Score="16" Body="&lt;p&gt;&lt;a href=&quot;http://www.spacelaunchreport.com/falcon9.html&quot; rel=&quot;noreferrer&quot;&gt;After the Falcon 1&lt;/a&gt;, SpaceX initially planned a &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_5&quot; rel=&quot;noreferrer&quot;&gt;5-engined rocket&lt;/a&gt; (around 2003). This Falcon 5 would be the minimum configuration with engine-out capability based on the Merlin engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Around 2005, plans changed. Due to customer requirements SpaceX wanted a version with 9 engines. They extended the 1+3+1 configuration to a 3x3 grid, giving maximal commonality.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/LSUOL.jpg&quot; alt=&quot;Falcon 5&quot;&gt;&lt;br&gt;&#xA;The Falcon 5 was cancelled eventually, but by then the Falcon 9 design was close enough to finished that they kept using the 3x3 grid arrangement for the first few launches.&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Z6bOi.jpg&quot; alt=&quot;F9 v1.0&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The arrangement was later changed to the 8+1 &quot;Octaweb&quot; we see on the Falcon 9v1.1, which uses a more efficient (lighter) structure to transfer the thrust from the engines to the fuselage.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-01-16T20:41:46.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7856" PostTypeId="2" ParentId="6442" CreationDate="2015-01-16T20:44:43.063" Score="3" Body="&lt;p&gt;Live, no. But they did release the video today, only a few days after the launch. See &lt;a href=&quot;https://vine.co/v/OjqeYWWpVWK&quot; rel=&quot;nofollow&quot;&gt;this link&lt;/a&gt;, from &lt;a href=&quot;http://spaceflightnow.com/2015/01/16/elon-musk-shares-images-of-falcon-9s-crash-landing/&quot; rel=&quot;nofollow&quot;&gt;SpaceFlightNow&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-01-16T20:44:43.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7857" PostTypeId="2" ParentId="7829" CreationDate="2015-01-17T00:21:24.787" Score="29" Body="&lt;h3&gt;Ascent G-Forces&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.ibiblio.org/apollo/Documents/lvfea-AS506-Apollo11.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Apollo 11 AS-506 launcher flight report&lt;/a&gt; contains a nice graph of the G-force curve of that famous Saturn V launch:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J38be.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J38be.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;From this chart you can see that, off the pad, the Saturn V first stage is doing about 1.2g; this climbs rapidly as atmospheric drag falls and fuel mass is consumed. The center engine is intentionally shut down to limit acceleration, and the outboard four keep pushing to a max of about 3.9g. This is the highest acceleration in the mission until re-entry and landing.&lt;/p&gt;&#xA;&lt;p&gt;The upper stages are less dramatic in their acceleration but follow similar increasing curves; the second stage curve steps down once for the center engine cutoff and once again when the fuel-to-oxidizer ratio is switched (&amp;quot;EMR Shift&amp;quot; on the graph, for Engine Mixture Ratio) -- this is done to optimize I&lt;sub&gt;sp&lt;/sub&gt; in vacuum, with the timing dynamically chosen to ensure simultaneous depletion of fuel and oxidizer. The second stage center engine early cutoff is done to reduce longitudinal (pogo) vibrations rather than to limit acceleration; this was instituted starting with the Apollo 10 flight.&lt;/p&gt;&#xA;&lt;p&gt;The third stage doesn't use all its fuel in this portion of the mission; most of the fuel load is for the later lunar injection burn, and that's why its acceleration curve is so flat in comparison to the others.&lt;/p&gt;&#xA;&lt;p&gt;Mercury-Atlas missions were more dramatic: 1.35g off the pad, peaking around 7g just before the booster engines shut down and dropped away, climbing again to almost 8g before the sustainer ran out of fuel.&lt;/p&gt;&#xA;&lt;p&gt;Here's &lt;a href=&quot;https://www.jsc.nasa.gov/history/mission_trans/MA07_TE2.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mercury-Atlas 7&lt;/a&gt;:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/YnHvL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YnHvL.jpg&quot; alt=&quot;Acceleration time series plot from 1.4g at 0:00 to 2.1g at 0:55, then in a steepening curve up to 6.8g at booster cutoff at 2:10; rising again from 1.3g to 7.8g at sustainer cutoff at 5:10&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Gemini-Titan peaked above 7g on the second stage. Here's a plot from the Gemini VIII mission report:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JQEKm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JQEKm.png&quot; alt=&quot;g force time series plot, increasing from about 1.25 g at liftoff in an inverse-linear curve to booster cutoff at around 155 seconds, 5.5 g, rising again from 1.35 g at second stage ignition to nearly 7.5g at second stage cutoff at around 335 seconds&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Both Atlas and Titan were designed as ICBMs, so not really optimized for human comfort.&lt;/p&gt;&#xA;&lt;p&gt;The Space Shuttle was much more gentle in comparison; at solid booster burnout it reached the first peak of 2.5g, briefly falling a bit &lt;em&gt;below&lt;/em&gt; 1g then slowly picked back up to 3g on the main engines; the mains were repeatedly throttled down to hold about 3g for a little over a minute.&lt;/p&gt;&#xA;&lt;p&gt;I think Soyuz does under 4g on launch.&lt;/p&gt;&#xA;&lt;p&gt;Other things being equal, a higher-g launch can be more fuel efficient, because less energy is lost to gravity by getting to orbit more quickly, and gravity losses normally dominate over drag losses. Keeping STS down to 3g was a challenging design goal - it's hard to build deep throttling capability into an engine, but the shuttle was designed to carry relatively fragile payloads. Soyuz is a bit of a compromise there.&lt;/p&gt;&#xA;&lt;p&gt;Falcon 9 starts at about 1.15g, and depending on payload would have a first-stage peak acceleration of around 4.5g, but it appears to throttle its engines back toward the end of the first-stage burn to maintain closer to 3.5g.&lt;/p&gt;&#xA;&lt;h3&gt;Re-entry and landing G-Forces&lt;/h3&gt;&#xA;&lt;p&gt;I haven't found a good time series graph of reentry force, but the peaks are relatively brief -- force increases as the capsule descends into denser air, but decreases as the capsule slows, so the higher the decelerating g-force, the shorter it's going to last.&lt;/p&gt;&#xA;&lt;p&gt;Mercury astronauts took about 11g peak force on re-entry, Apollo about 6.5-7g. The space shuttle was amazingly gentle, &lt;a href=&quot;https://space.stackexchange.com/a/50619/195&quot;&gt;with reentry force peaking at just 1.6g.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Again, Soyuz does about 4g here, I think.&lt;/p&gt;&#xA;&lt;p&gt;There may be a pretty good jolt at touchdown/splashdown, too. Some of the Apollos hit rising waves at the end of the ride for very brief 15g bump.&lt;/p&gt;&#xA;&lt;p&gt;STS and Soyuz g-forces are necessarily low, again, because they carry civilian crews. In the case of the shuttle, again, it's a major design consideration: the gentle re-entry means the ship has to deal with a prolonged period of high thermal load, which requires fancy and vulnerable ceramic tiles rather than a simple ablative heat shield.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-03-07T23:44:45.427" LastActivityDate="2021-03-07T23:44:45.427" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="7858" PostTypeId="2" ParentId="7849" CreationDate="2015-01-17T02:55:46.733" Score="7" Body="&lt;p&gt;The Huygens probe that landed on Titan back in 2005 had a special flood-light affixed to its cameras because it's supposedly too dark to take pictures with any detail during Titans day without it.&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/lytHs.jpg&quot; alt=&quot;Titan&amp;#39;s surface from Huygens lnder (courtesy ESA/NASA/University of Arizona&quot;&gt;&lt;br&gt;&#xA;&lt;em&gt;Images from the DISR Side-Looking Imager and from the Medium Resolution Imager, acquired after Huygens' landing on Titan, were merged to produce this image.&lt;/em&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Little sunlight reaches the surface, due to its thick haze, and large distance from the sun. I would imagine it being like an extremely overcast, late afternoon here on earth, everyday.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1BV5f.jpg&quot; alt=&quot;Huygen&amp;#39;s cameras with spotlight.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above is a picture of the cameras that Huygens used to snap the pictures. They were aided by the spotlight in the middle (the larger, goldish disc)&lt;/p&gt;&#xA;" OwnerUserId="6627" LastEditorUserId="5963" LastEditDate="2015-01-17T19:57:54.967" LastActivityDate="2015-01-17T19:57:54.967" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7859" PostTypeId="2" ParentId="7846" CreationDate="2015-01-17T05:07:05.980" Score="15" Body="&lt;p&gt;As an engineer who has used slide rules, logarithmic tables, calculators, computers and done designs on paper and using computers, in the paper design era engineers concentrated on the critical areas of designs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These days, computers and mathematical techniques such as FEA &amp;amp; boundary element analysis, etc. allow engineers to consider a larger number of options and to consider a larger number of &quot;what if&quot; scenarios. These days, engineers can be bogged down in data, detail and decimal point accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the pre computerized era engineers also used graphs that others had developed such as psychrometric charts and where they didn't know they did laboratory and larger scale testing, such as wind tunnels with smoke and streamer and hydraulic channels and dye for hydraulics and streamlining.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To test the streamlining of high performance cars, small streamers were placed all over the body to be able to view how the air flowed over the car.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One method used to envisage stresses and stress flow was to cut shapes into perspex sheets and to view the prismatic light patterns in the perspex when it was placed under different stress regimes (it was a bit like stress contours). It didn't give any numerical values but people could get a better idea of where adverse stress concentrations could occur and then alter the design accordingly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Prior to the mid 1970s engineers had to know mathematics and hand calculation shortcuts, i.e. to approximate $\pi$ to 2 or 3 decimal places using 22/7; particularly for junior engineers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neil Armstrong &amp;amp; Buzz Aldrin took slide rules to the Moon as part of NASA standard issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.worthpoint.com/worthopedia/apollo-11-slide-rule-neil-armstrong-77275561&quot; rel=&quot;noreferrer&quot;&gt;http://www.worthpoint.com/worthopedia/apollo-11-slide-rule-neil-armstrong-77275561&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JbJTU.jpg&quot; alt=&quot;Buzz Aldrin&amp;#39;s slide rule (sliderulemuseum.com)&quot;&gt;&lt;br&gt;&#xA;&lt;em&gt;Buzz Aldrin's slide rule from Apollo 11 (courtesy of sliderulemuseum.com)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the early part of the Cold War scientists and engineers in the West had access to powerful computers. In the Soviet Bloc countries scientists and engineers didn't have such access, they developed mathematical techniques to be able to get answers quickly.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="8693" LastEditDate="2016-01-13T13:07:43.453" LastActivityDate="2016-01-13T13:07:43.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7860" PostTypeId="1" AcceptedAnswerId="15220" CreationDate="2015-01-17T15:34:35.680" Score="6" ViewCount="944" Body="&lt;p&gt;As I see it, Falcon 9 has to land on the barge in a vertical orientation and with no horizontal drift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has fins that are ineffective at low speed. It has a rocket motor that can probably vector some, but doing so it gives a rotating (yawing) moment to the craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there some more tools, like horizontal nozzles at the top that can counteract wind pressure?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, can it land at an angle from vertical on two legs only?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New thinking:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a dynamic approach be possible? Coming in towards the platform from downwind (against the wind) at a higher speed than wind speed with the base of the rocket pointing slightly into the wind. So the rocket retards to wind speed and rotates to vertical exactly at touchdown.&lt;/p&gt;&#xA;" OwnerUserId="8278" LastEditorUserId="8278" LastEditDate="2015-01-17T20:18:11.477" LastActivityDate="2016-05-11T08:18:41.470" Title="How can Falcon 9 first stage land vertical in a strong wind?" Tags="|spacex|landing|falcon-9|weather|vtvl|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7861" PostTypeId="2" ParentId="7860" CreationDate="2015-01-17T15:40:14.637" Score="1" Body="&lt;p&gt;It probably can't.  Where did you see that it can?  Launch commit criteria prevents launching if the conditions at the first stage landing site are prohibitive.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-01-17T15:40:14.637" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7862" PostTypeId="1" CreationDate="2015-01-17T16:13:01.373" Score="9" ViewCount="1420" Body="&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/RMAtCQEPLeQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the video of the Falcon 9 first stage attempting a landing on SpaceX's barge. Why is the rocket moving so fast at impact?&lt;/p&gt;&#xA;" OwnerUserId="8221" LastEditorUserId="8162" LastEditDate="2015-01-17T18:21:55.280" LastActivityDate="2015-05-21T22:16:35.487" Title="SpaceX Landing Platform video: why is the rocket moving so fast?" Tags="|spacex|landing|falcon-9|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7863" PostTypeId="1" AcceptedAnswerId="7870" CreationDate="2015-01-17T16:13:52.603" Score="1" ViewCount="182" Body="&lt;p&gt;Does this mean that, for all the fanfare, the USA's actual contribution to the future of human-flight space exploration based on this particular vehicle is now rather less than is economically-sustainable and amounts to a few us-based private contractors, some design work and a 'flag on the hull' while other nations do the real work and benefit from the real fruits of their own research?&lt;/p&gt;&#xA;" OwnerUserId="8279" LastActivityDate="2015-01-17T18:25:29.630" Title="How much of the Orion vehicle will be built or provided by Airbus/ESA?" Tags="|nasa|esa|orion-spacecraft|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7864" PostTypeId="2" ParentId="7862" CreationDate="2015-01-17T16:19:42.927" Score="8" Body="&lt;p&gt;It is actually moving quite slow vertically.  It's speed comes from the horizontal component -- which is a result of the vehicle trying (unsuccessfully) to correct its horizontal position.  The vehicle was unable to accomplish this with the aerodynamic control surfaces as intended.  The control surfaces that were responsible for this ran out of hydraulic fluid.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-01-17T16:19:42.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7866" PostTypeId="1" CreationDate="2015-01-17T17:17:43.800" Score="7" ViewCount="754" Body="&lt;p&gt;Why must you land the boosters on a flat surface? Why not aim for a &lt;strike&gt;cone&lt;/strike&gt; a deep net that can accommodate rough seas, winds, etc?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A picture is below - obviously I am not an artist nor an engineer.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/bxywe.jpg&quot; alt=&quot;Capturing a booster in a depression&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8280" LastEditorUserId="5963" LastEditDate="2015-01-18T21:38:55.607" LastActivityDate="2015-12-21T16:36:57.343" Title="Why not land booster stages in a cone or net?" Tags="|spacex|landing|falcon-9|feasibility|vtvl|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7867" PostTypeId="1" CreationDate="2015-01-17T17:18:41.257" Score="2" ViewCount="341" Body="&lt;p&gt;The human body has a limit of being able to withstand, roughly, 2g for very long periods without causing too many health problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An engine technology, that could produce constant thrust producing acceleration of up to this limit for a crew, would be able to &lt;strong&gt;accelerate&lt;/strong&gt; a craft toward a target in our solar system then have to turn around on its trajectory and &lt;strong&gt;decelerate&lt;/strong&gt; that craft for arrival to orbit or for a surface landing, all the time &lt;strong&gt;NEVER&lt;/strong&gt; exceeding the safe acceleration limits for a living crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How fast could &lt;strong&gt;safe&lt;/strong&gt; journeys actually be made in our solar system and what's the safe minimum transit times to the major planets or interesting minor planets?&lt;/p&gt;&#xA;" OwnerUserId="8279" LastEditorUserId="49" LastEditDate="2015-01-17T18:12:43.753" LastActivityDate="2015-01-17T18:12:43.753" Title="Given limits on acceleration for human space flight, what are the minimum travel times in our solar system for real powered journeys?" Tags="|trajectory|interplanetary|crewed-spaceflight|hypergravity|" AnswerCount="0" CommentCount="2" ClosedDate="2015-01-18T13:55:58.693" ContentLicense="CC BY-SA 3.0" />
  <row Id="7868" PostTypeId="1" AcceptedAnswerId="7886" CreationDate="2015-01-17T17:24:29.087" Score="3" ViewCount="432" Body="&lt;p&gt;Just about every time I watch a sci-fi show or movie about space, there always seems to be &quot;regular&quot; food on board. That is, stuff that we on earth (and more specifically Americans) recognize as &quot;home cooking&quot;, things with potatoes, beef and chicken, soups, and a wide variety of other things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I know, most of the food the astronauts use today (please correct me on this) is dehydrated or altered in such a way to make it feasible to bring into space (like a backpacking meal).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm curious of what technological costs would go into a spacecraft being able to carry a kitchen capable of cooking raw foods, specifically for the spacecraft and not for transport to colonies or other such things. I think, obviously, one day we may have such technologies, but I'm more curious about the next 100 years or so. Will we be able to accomplish this, or are the next few generations of astronauts doomed to eat &quot;space food&quot;?&lt;/p&gt;&#xA;" OwnerUserId="7027" LastEditorUserId="12102" LastEditDate="2017-05-15T16:24:10.483" LastActivityDate="2017-05-15T16:24:10.483" Title="In the near future, will spacecraft be able to carry the means to cook and serve traditional meals?" Tags="|crewed-spaceflight|spacecraft|feasibility|food|reduced-gravity-cooking|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7869" PostTypeId="2" ParentId="7868" CreationDate="2015-01-17T17:49:14.913" Score="10" Body="&lt;p&gt;Several points come to mind:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;In most sci-fi there is gravity. At the moment we do not have a non-earth location where humans can go with gravity. If we have habitats on the Moon or Mars then there will be a degree of gravity and the situation can change. We have not yet constructed a vast spinning space station with artificial gravity as depicted in 2001 A Space Odyssey. This makes the handling of fluids (like water), boiling in water, peeling vegetables etc very difficult. Food fragments floating in low gravity is also a safety hazard, which is why no bread, chips or other fragmenting items.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Frequency of supplies. It takes considerable cost and effort to ship food items to space from earth. Therefore supplies are not frequent. Thus they need to be preserved. However, they are not all dehydrated or altered. The Russian food on the ISS is frequently canned and the cosmonauts and astronauts enjoy much canned fish, meat, vegetables and fruit, much as they would on an earth expedition away from civilization.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2015-01-17T17:49:14.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7870" PostTypeId="2" ParentId="7863" CreationDate="2015-01-17T18:09:08.423" Score="2" Body="&lt;p&gt;ESA and NASA have reached an agreement where ESA will provide a &lt;a href=&quot;http://www.bbc.com/news/science-environment-30094942&quot; rel=&quot;nofollow noreferrer&quot;&gt;service module based on the ATV&lt;/a&gt;. NASA will get this module for free: it is basically a way for ESA to pay for its share of the ISS.&lt;br&gt;&#xA;This agreement has many benefits: NASA pays a lot less for its spacecraft, ESA gets to keep working on the technology they developed for ATV. The cooperation also means it becomes harder for politicians to cancel Orion. This is not unimportant, in view of what happened to &lt;a href=&quot;https://en.wikipedia.org/wiki/Constellation_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Constellation&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, Lockheed is building the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion crew capsule&lt;/a&gt; for NASA. This is a major part of the work, being done by an American company. The SLS rocket that will launch Orion is also designed and built by American companies.&lt;br&gt;&#xA;The service module will also contain American parts: its main engine is derived from the Shuttle OMS.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tBDA0.png&quot; alt=&quot;Gratuitous Orion image&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, is Orion economically sustainable? The European contribution has made the program a lot cheaper. Even if NASA has to pay for future service modules, it'll still be cheaper than having to develop an all-new service module.&lt;br&gt;&#xA;Orion isn't meant to be built in large numbers, it will be used for rare flagship missions that require a large, long-range spaceship. So the development investment will have to be paid back over only a few missions. Reusing existing technology makes sense.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The service module won't be the only example of international cooperation on these flagship missions, I suspect. Much like the ISS, missions to e.g. Mars or an asteroid are immensely expensive, it makes sense to share the cost. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-01-17T18:25:29.630" LastActivityDate="2015-01-17T18:25:29.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7871" PostTypeId="2" ParentId="7866" CreationDate="2015-01-17T18:25:01.043" Score="2" Body="&lt;p&gt;Your proposal wouldn't help. When the rocket hits the edge of the cone, the rocket will still be damaged. The cone also traps the exhaust gases so you'd have to add insulation to the rocket.&lt;br&gt;&#xA;Rockets are flimsy: they are built from the lightest materials they can get away with. This means you have to be careful when you transport the rocket. Some rockets need to have their fuel tanks pressurized, otherwise they will collapse under their own weight.&lt;br&gt;&#xA;Making the rocket heavier (so it can withstand a rough landing) means it will carry less payload. For now, this is a big issue. If rockets become fully reusable, it's likely they will become heavier and sturdier too, but until then SpaceX has to design a landing strategy that works with the rocket they have. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-01-17T18:25:01.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7872" PostTypeId="2" ParentId="7866" CreationDate="2015-01-17T18:25:19.920" Score="1" Body="&lt;p&gt;Landing a rocket on a flat surface can be tricky. An airplane pilot could tell you all the different factors necessary for landing an aircraft: angle of attack, horizontal speed, vertical speed, &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobraking#Aerodynamic_braking&quot; rel=&quot;nofollow&quot;&gt;braking&lt;/a&gt; - and these are just the things that can be controlled. When it comes to landing a plane, there's one thing in your favor: it's tough for the plane to fall over. Yes, it's happened in the past, with disastrous consequences, but in general, a planet is more likely to stay at the angle it's landing at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets can be different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking from above, their center of gravity is in the middle of a very narrow cylinder. Tilt the rocket too much one way and its trajectory could change. When you're aiming for the &lt;em&gt;ground&lt;/em&gt;, a slip like this could mean that the rocket falls over sideways and explodes. That's &lt;em&gt;not&lt;/em&gt; the optimal arrangement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's also the problem with your setup. You show a conical depression in the barge, with the rocket leaning against it. That kind of landing would be the worst possible, because the rocket is going to come down vertically - as planned - and then fall sideways and hit the wall of the chamber, most likely resulting in severe harm, if not total destruction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could make the chamber in the shape of an upside-down conical &lt;a href=&quot;https://en.wikipedia.org/wiki/Frustum&quot; rel=&quot;nofollow&quot;&gt;frustum&lt;/a&gt;, but at this point we're back to having it land on what is essentially a flat surface.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastEditorUserId="195" LastEditDate="2015-01-18T03:22:02.743" LastActivityDate="2015-01-18T03:22:02.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7873" PostTypeId="2" ParentId="2456" CreationDate="2015-01-17T18:35:48.103" Score="2" Body="&lt;p&gt;Efficiency of the engine bell at the nozzle end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bell-shape allows the gas to expand &lt;strong&gt;but&lt;/strong&gt; this shape is usually tuned for the region in which the engine operates, namely low altitude + thick air or high altitude + thinner air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bell-shape is usually a compromise, since as the rocket climbs the air thins.&#xA;So what's best? Efficiency at low altitudes that gets worse as the rocket climbs ever higher... or low efficiency that improves? The mission designer and the engine designer figured that out in the 1950s!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engines work best when there's no air for the expanding gas to push against, wasting thrust, namely '&lt;em&gt;in space&lt;/em&gt;'.&lt;/p&gt;&#xA;" OwnerUserId="8279" LastActivityDate="2015-01-17T18:35:48.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7874" PostTypeId="5" CreationDate="2015-01-17T18:45:09.067" Score="0" Body="&lt;p&gt;Questions about VTVL (Vertical Takeoff Vertical Landing) capable rockets, use and means to achieve these capabilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;VLVTs often employ same main engine(s) thrust to accomplish both vertical takeoff and landing.  They differ from the similar VTOL (Vertical Take-Off and Landing) vehicles in that VTVLs do not depend on aerodynamic lifting to a significant degree, and are capable of sustaining their own  deceleration with engines and atmospheric drag alone, while maintaining descent attitude.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="1142" LastEditDate="2015-01-18T01:52:03.000" LastActivityDate="2015-01-18T01:52:03.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7875" PostTypeId="4" CreationDate="2015-01-17T18:45:09.067" Score="0" Body="Questions about VTVL (Vertical Takeoff Vertical Landing) capable rockets, use and means to achieve these capabilities." OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-18T01:00:37.800" LastActivityDate="2015-01-18T01:00:37.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7876" PostTypeId="2" ParentId="7862" CreationDate="2015-01-17T19:03:06.470" Score="4" Body="&lt;p&gt;The Falcon 9 can't hover--even one engine at minimum is too much thrust.  Thus it must do what KSP calls a suicide burn.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-01-17T19:03:06.470" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7877" PostTypeId="1" AcceptedAnswerId="12068" CreationDate="2015-01-18T01:17:04.747" Score="3" ViewCount="2880" Body="&lt;p&gt;I'm trying to understand the terminology used in the specifications of several satellite systems, such as those outlined on pages 9-11 &lt;a href=&quot;http://www.its.bldrdoc.gov/media/30335/red_s.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm guessing a mobile uplink/downlink is RF uplink and downlink that can communicate with mobile phones, but I can't find anything in my searches that defines feeder downlink.&lt;/p&gt;&#xA;" OwnerUserId="6570" LastActivityDate="2015-09-22T12:41:41.097" Title="What is a feeder uplink/downlink?" Tags="|cubesat|communication-satellite|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7878" PostTypeId="5" CreationDate="2015-01-18T01:23:23.090" Score="0" Body="&lt;p&gt;Insolation, aka solar irradiation, is the total amount of solar radiation that any given surface is exposed to in some period of time. This tag might be relevant to questions about surface temperature and heat absorption, sources of solar and thermal energy, radiation exposure, climate,... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we assume it's emitted from a point source and transmitted through vacuum, its intensity decreases in inverse square law relationship with distance from its point source.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-18T01:23:23.090" LastActivityDate="2015-01-18T01:23:23.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7879" PostTypeId="4" CreationDate="2015-01-18T01:23:23.090" Score="0" Body="Insolation, aka solar irradiation, is the total amount of solar radiation that any given surface is exposed to in some period of time. This tag might be relevant to questions about surface temperature and heat absorption, sources of solar and thermal energy, radiation exposure, climate,... &#xD;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-01-18T01:23:23.090" LastActivityDate="2015-01-18T01:23:23.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7880" PostTypeId="1" AcceptedAnswerId="7883" CreationDate="2015-01-18T06:09:40.740" Score="9" ViewCount="1679" Body="&lt;p&gt;So we had the shuttle, and they obviously reused it many times, but it always needed new rockets to get it out of Earth's atmosphere. Why is it so difficult to create a spacecraft that is entirely reusable? Is it simply because the fuel to weight ratio is so large and we need that enormous fuel tank? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What prevents us specifically from creating an all-in-one craft that can take off and land without so many obstacles/costs associated with its use? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How close are we, if at all, to finding a solution to this problem? &lt;/p&gt;&#xA;" OwnerUserId="6627" LastEditorUserId="49" LastEditDate="2015-01-18T14:25:07.797" LastActivityDate="2015-01-18T15:33:26.973" Title="Why have no reusable launch technologies been developed?" Tags="|spacecraft|spacecraft-development|reuse|space-industry|ssto|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7881" PostTypeId="1" CreationDate="2015-01-18T06:40:59.320" Score="1" ViewCount="112" Body="&lt;p&gt;I'd like know how sites like helloearth.us or satflare.com/iss calculate the time until the ISS enters/leaves darkness (sunrise/sunset).&lt;/p&gt;&#xA;" OwnerUserId="8288" LastActivityDate="2015-01-18T06:40:59.320" Title="How does one estimate the time until ISS enters/leaves darkness with available data" Tags="|iss|" AnswerCount="0" CommentCount="2" ClosedDate="2015-01-25T13:28:09.537" ContentLicense="CC BY-SA 3.0" />
  <row Id="7882" PostTypeId="2" ParentId="7880" CreationDate="2015-01-18T15:26:49.717" Score="8" Body="&lt;p&gt;The basic reason is it that it takes a lot of velocity to put an object in orbit. For example the orbital velocity at LEO is around 7.8 km/s. What this necessarily means, is that the final stage of the rocket - the one that actual releases the satellite in it's intended orbit - will be moving at nearly orbital velocity. If you want to recover that stage, you need to &lt;em&gt;slow it back down&lt;/em&gt; so it can re-enter atmosphere which can be done most economically through a combination of fuel for a re-entry burn, a heat shield to survive re-entry and a parachute or more fuel for a gentle touchdown. The problem is that all of those add significant weight, so instead of only launching the payload into orbit, you're now launching the payload, fuel, heat shield and a parachute into orbit. This is why it will not soon be practical to recover the upper stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was proposed to re-use the space shuttle tanks by boosting them into orbit. This would have been a lot cheaper than reusing them on the ground, as the extra fuel required to boost them into a safe orbit would have been much less weighty than the fuel, heat shield and parachute for getting them back to the Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recovery and re-use of lower stages is more practical because they aren't travelling nearly so fast and might even still be in the atmosphere making parachutes very practical. The less delta-v required for recovery, the cheaper and more practical it is. This is why the space shuttle's booster rockets were recovered. Nevertheless recovery of the first stage is not without serious challenges as demonstrated by SpaceX's exercises in first stage recovery:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Following the booster loft of the second stage and payload on its orbital trajectory, SpaceX conducted a successful flight test on the spent first stage. The first stage successfully decelerated from hypersonic velocity in the upper atmosphere, made a successful reentry, landing burn, deployment of its landing legs, and touched down on the ocean surface. The first stage was not recovered for analysis as the hull integrity was breached, either on landing or on the subsequent &quot;tip over and body slam&quot;. Results of the post-landing analysis showed that the hull integrity was lost as the 46-metre (150 ft)-tall booster rocket body fell horizontally, as planned, onto the ocean surface following the landing.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;-- &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX_reusable_launch_system_development_program#Test_flights&quot;&gt;wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is not easy to engineer a structure to both withstand the vertical forces of gravity/acceleration and very different forces involved in tipping over onto it's side. Even with a parachute you can't just drop a 15 story rocket in the ocean and hope for the best. This is one reason why SpaceX is attempting to land the first stage upright as it would be much cheaper to do this than to add the extra structural reinforcements required to have such a large rocket withstand tipping over into water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nevertheless, there is no reason in principle why a fully reusable rocket can't be built, the rocket just has to be significantly bigger (in terms of fuel) and more complex than the equivalent disposable rocket, so far it has not been economically viable to make such larger and more complex rockets.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-01-18T15:26:49.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7883" PostTypeId="2" ParentId="7880" CreationDate="2015-01-18T15:33:26.973" Score="12" Body="&lt;p&gt;&lt;a href=&quot;http://hopsblog-hop.blogspot.com/2012/08/mf-is-mofo-tyranny-of-rocket-equation_21.html&quot; rel=&quot;noreferrer&quot;&gt;Mass fraction&lt;/a&gt; and Tsiolkovsky's rocket equation make for major hurdles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mf, or the ratio of propellent to dry mass is given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$Mf=1-e^{-delta V/V_{exhaust}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To climb out of a steep gravity well we need a high thrust propellent. So we use chemical rockets. The $V_{exhaust}$ for the higher ISP chemical propellents is around 4 km/s. Delta V to get to LEO is around 9 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This sort of delta V combined with Tsiolkovsky's rocket equation means around 90% propellent and 10% dry mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now some of the dry mass budget must go to rocket engines, structure, avionics, power source and payload. So as the delta V budget climbs we're left with fuel tanks about as tenuous as an aluminum coke can. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MoSwY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/centers/marshall/michoud/shuttle_et119_1.html&quot; rel=&quot;noreferrer&quot;&gt;Shuttle tank photo from NASA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boost the delta V budget more and you need fueltank walls of cellophane and moon beams.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the ways to achieve the mass fractions mandated by the rocket equation is to throw away dry mass enroute: expendable multi-stage rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then there's re-entry. So now we have a spaceship about as robust as an eggshell re-entering earth's atmosphere at 8 km/s. Re-entry subjects the ship to very extreme conditions: temperatures in 1000s of degrees, dynamic pressure that make a class 5 hurricane look like a gentle breeze.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I give SpaceX better than even odds that they'll reuse a booster stage. If the booster stage has a delta V budet of only 4 or 5 km/s, that allows for a sturdier structure. Also the booster won't be re-entering the atmosphere at orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Re-using a capsule might also be doable. A capsule re-enters the atmosphere at orbital velocity. But it has a low delta V budget. The lower delta V budget allows a mass fraction that permits sturdy structure and Thermal Protection System (TPS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I give less than even odds the 2nd stage can be re-used. The 2nd stage needs to provide around 8 km/s and needs to re-enter at 8 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I give better than even odds that the SpaceX capsule and booster can be re-used, I give a little less than even odds these can be re-used &lt;em&gt;economically&lt;/em&gt;. The Space Shuttle re-used some parts but the savings were largely eaten up in refurbishment costs. What will SpaceX refurbishment costs be? this remains to be seen.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-01-18T15:33:26.973" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7884" PostTypeId="2" ParentId="7866" CreationDate="2015-01-18T15:47:16.320" Score="2" Body="&lt;p&gt;I'll second the earlier answers by @Hobbes, and @HDE226868. Aiming for a hole in the ground would be a trickier task, to say naught of the potential damage to the fuselage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of a net could be &lt;em&gt;probably&lt;/em&gt; be used on a flat surface though. Here are a couple of thoughts to begin with &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Engineered_materials_arrestor_system&quot; rel=&quot;nofollow&quot;&gt;Arrestor Systems&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Arresting_gear#Barricade&quot; rel=&quot;nofollow&quot;&gt;Arresting Barricade&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Obviously the above two systems are passive devices. An active device could &lt;em&gt;probably&lt;/em&gt; be used to flip a net &lt;em&gt;over&lt;/em&gt; the fuselage to catch it before it topples; a glorified &lt;a href=&quot;http://en.wikipedia.org/wiki/Net_gun&quot; rel=&quot;nofollow&quot;&gt;snare gun&lt;/a&gt; say. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;p.s. Obviously I may be talking through me hats here. Feel free to down-vote/delete!&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2015-01-18T15:47:16.320" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7885" PostTypeId="2" ParentId="7877" CreationDate="2015-01-18T16:31:53.777" Score="0" Body="&lt;p&gt;Feeder link is from the Gateway to the satellite&#xA;Check the image &lt;a href=&quot;http://csivc.csi.cuny.edu/engsci/files/ens466/ENS466_P2.pdf&quot; rel=&quot;nofollow&quot;&gt;on Slide#3&lt;/a&gt; of the link&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2015-01-18T16:31:53.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7886" PostTypeId="2" ParentId="7868" CreationDate="2015-01-18T17:35:14.580" Score="8" Body="&lt;p&gt;In addition to Brian's points (which are both correct), it's actually a misnomer that astronauts eat only dehydrated &quot;space food&quot;. Modern food that is shipped to the ISS, for example, is actually quite good. It's still tricky to cook, but they get plenty of raw fruits and vegetables, among other things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://content.time.com/time/health/article/0,8599,1911617,00.html&quot;&gt;this TIME article&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Today, the most elaborate outer-space meals are consumed in the International Space Station (ISS), where astronauts enjoy everything from steak to chocolate cake. They even have a small beverage chiller that can serve cold drinks. The ISS is a joint venture between the U.S. and Russia, and diplomatic guidelines dictate the percentage of food an astronaut must eat from each country. NASA's food laboratory has 185 different menu items, Russia offers around 100, and when Japan sent up its first crew member in 2008, about 30 dishes came with him. Kloeris says that the freeze-dried shrimp cocktail, served with horseradish-infused powdered sauce, is the most popular dish.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the far future, when we have permanent human settlements on other worlds, things like hydroponics will likely be a source of food. There's already &lt;a href=&quot;http://www.nasa.gov/missions/science/biofarming.html&quot;&gt;research being done&lt;/a&gt; on that today. &quot;Space food&quot; is already partly a relic of the Apollo era, and will be moreso in the future.&lt;/p&gt;&#xA;" OwnerUserId="6772" LastActivityDate="2015-01-18T17:35:14.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7888" PostTypeId="2" ParentId="7850" CreationDate="2015-01-19T05:34:58.347" Score="3" Body="&lt;p&gt;It's not yet proven, and other explanations are still possible, but evidence for a paleo-ocean in Mars's northern hemisphere has been mounting for the past several decades.&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_ocean_hypothesis&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Research published in 2009 shows a much higher density of stream channels than formerly believed. Regions on Mars with the most valleys are comparable to what is found on the Earth. In the research, the team developed a computer program to identify valleys by searching for U-shaped structures in topographical data. The large amount of valley networks strongly supports rain on the planet in the past. The global pattern of the Martian valleys could be explained with a big northern ocean. A large ocean in the northern hemisphere would explain why there is a southern limit to valley networks; the southernmost regions of Mars, farthest from the water reservoir, would get little rainfall and would develop no valleys. In a similar fashion the lack of rainfall would explain why Martian valleys become shallower from north to south.&lt;/p&gt;&#xA;&lt;p&gt;A 2010 study of deltas on Mars revealed that seventeen of them are found at the altitude of a proposed shoreline for a Martian ocean. This is what would be expected if the deltas were all next to a large body of water.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="157" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-01-19T05:34:58.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7889" PostTypeId="1" AcceptedAnswerId="7892" CreationDate="2015-01-19T10:19:39.967" Score="5" ViewCount="1317" Body="&lt;p&gt;Most launch vehicles define an upper limit on the payload they may carry. &lt;em&gt;For instance - Falcon 9, prepped for launch, has a defined max. payload capability of 13,150 kg.&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to know the minimum weight above the rated max. payload capability which would be likely to cause a failure/to stop it even getting off the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By how much may the maximum payload vary without causing a launch failure?&#xA;What would happen if a launch was attempted with extra mass on board?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is purely hypothetical, so no need to delve into the reasons why there was extra mass on board. The question is also in a general context and not specific to the Falcon.&lt;/p&gt;&#xA;" OwnerUserId="3384" LastEditorUserId="3384" LastEditDate="2015-01-21T10:53:14.557" LastActivityDate="2015-01-21T10:53:14.557" Title="By how much may the maximum payload mass vary without causing a launch failure?" Tags="|launch|rockets|failure|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7891" PostTypeId="2" ParentId="7889" CreationDate="2015-01-19T16:30:15.430" Score="2" Body="&lt;p&gt;Like Tildal says, this question doesn't really make sense in the real world, as the flight profile depends on the precise mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the payload changed, the launch would be aborted. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it went undetected before launch, the extra payload would be rapidly detected as it would immediately affect the flight profile. If too heavy, bringing the Thrust/Weight ratio below 1, then it wouldn't even launch, but in any case there would be a measurable effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether or not it impacted the delivery of payload to the correct orbit would depend on propellant tolerances. It may impact the lifespan of the satellite, if it had to use extra propellant to maneouvre to a correct orbit, etc.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2015-01-19T16:37:43.500" LastActivityDate="2015-01-19T16:37:43.500" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7892" PostTypeId="2" ParentId="7889" CreationDate="2015-01-19T17:40:23.150" Score="9" Body="&lt;p&gt;Normally, rocket manufacturers show capability as &quot;payload to LEO&quot; or &quot;payload to GTO&quot;. These aren't super-precise; LEO means roughly between 160 and 2000km altitude, which is a pretty broad range. If a rocket can deliver 13,000kg to the 400km altitude of the ISS, it can deliver slightly more than that to 200km, and slightly less than that to 2000km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Weather conditions and manufacturing variations in individual rocket engines means there has to be a few percent of margin in the specified payload.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flight control systems know their altitude, speed, and so on, and continually compensate for it; if the rocket is accelerating 0.1% less quickly than it's supposed to because of a 10kg stowaway, it may be able to compensate by firing the orbital insertion stage for a bit longer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of uncertainties in the flight profile (weather, hardware variation, imperfect timing of engine cutoff) it's common for the actual orbit reached to be slightly different from the one intended. This is not a big deal in most cases, and if positioning is critical, the payload will have thrusters of its own to fine-tune its trajectory. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you're talking about a human-sized stowaway, that's about 1% of Falcon 9's payload. That would probably make enough difference that the mission controllers would actually notice, but I'd be shocked if there wasn't at least sufficient margin of fuel tankage to handle that -- I'd call it negligence otherwise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typically these rockets have an thrust-to-weight ratio at launch of 1.15 or above. For Falcon 9, you'd need 75 tons of additional payload to keep it from leaving the pad entirely. Even then, once you'd burned off some fuel, the rocket would conceivably fly -- except you'd destroy the launch pad first, and that would doubtless destroy the rocket as well. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-01-19T21:28:02.230" LastActivityDate="2015-01-19T21:28:02.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7893" PostTypeId="2" ParentId="7889" CreationDate="2015-01-19T19:18:25.440" Score="3" Body="&lt;p&gt;Margins vary from launch vehicle to launch vehicle and from mission to mission, but to put a lower bound on it, the final dry mass of the upper stage + payload is often not even known to better than about 0.5%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a typical flight (say 5000 kg to LEO), extra (or missing) mass would have to be several tens of kilograms to even be noticeable in the telemetry / trajectory corrections.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2015-01-19T19:18:25.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7894" PostTypeId="1" AcceptedAnswerId="7900" CreationDate="2015-01-19T21:48:22.603" Score="12" ViewCount="1635" Body="&lt;p&gt;In a typical, modern satellite launch, what triggers the cutoff of the orbital insertion stage's rocket engines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can think of three basic possibilities:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Compute how much fuel you need for the trajectory, load exactly that much, and burn all the fuel&lt;/li&gt;&#xA;&lt;li&gt;Bring a little extra fuel and cut off the engines at a specific time&lt;/li&gt;&#xA;&lt;li&gt;Bring a little extra fuel and cut off the engines when telemetry indicates you're at the desired altitude and speed&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The first two seem like they'd be susceptible to small variations in atmospheric conditions, engine performance, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A historical example: Apollo 13's second stage. The center engine cut off early due to pogo oscillation, when the fuel pressure dropped below a shutdown threshold which was intended to make sure the engine stopped cleanly when the stage started to run out of fuel (which itself implies that option 1 is a little uncertain; not all the fuel is going to get burned). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To compensate, they simply ran the second stage longer. Was that a natural result of consuming all the remaining fuel through four engines instead of 5 (implying option 1), or did they have to explicitly command a later cutoff time (implying option 2), or was the compensation completely automatic (option 3)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that I'm asking about the initial launch and orbital insertion phase, not about fine-tuning the orbit and/or rendezvousing with a target via multiple incremental burns.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-01-20T03:36:42.380" LastActivityDate="2015-01-20T03:36:42.380" Title="How is rocket engine cutoff controlled?" Tags="|launch|low-earth-orbit|stages|flight-profile|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7895" PostTypeId="1" AcceptedAnswerId="7898" CreationDate="2015-01-19T21:48:48.337" Score="10" ViewCount="487" Body="&lt;p&gt;In this &lt;a href=&quot;http://youtu.be/_UFjK_CFKgA&quot;&gt;video&lt;/a&gt;, you can see the Falcon 9 first stage flying through the second stage's plume post-separation.  I would assume that this would alter the first stage's trajectory -- potentially causing a tumble.  Why not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that either the plume is rarified enough to not cause this or the first stage has an active system (first stage nitrogen thrusters?) that is activated at that time to prevent it.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="51" LastEditDate="2015-01-19T21:58:46.260" LastActivityDate="2015-01-19T22:35:05.633" Title="Falcon 9 second stage impact on first stage trajectory" Tags="|spacex|falcon-9|reuse|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7897" PostTypeId="1" CreationDate="2015-01-19T22:24:02.893" Score="15" ViewCount="2507" Body="&lt;p&gt;Falcon Heavy is expected to take 4 times more mass to LEO than Falcon 9 (53 instead of 13 tons). Are bigger rockets generally about 4/3 times more capable per fuel mass and engine thrust? It doesn't seem to be true for Delta IV Heavy compared with Delta IV. Its 3 common core boosters give 3 times more payload mass to LEO. (9 and 27 tons).&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2015-01-19T22:33:13.397" LastActivityDate="2018-02-02T15:54:40.427" Title="What makes Falcon Heavy with 3 booster 4 times more capable than single booster F9?" Tags="|rockets|spacex|delta-iv-heavy|falcon-heavy|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7898" PostTypeId="2" ParentId="7895" CreationDate="2015-01-19T22:35:05.633" Score="9" Body="&lt;p&gt;You're correct on both counts. At the time of stage separation, the rocket is approximately 80km in altitude (although it varies based on the specific flight path) where the atmospheric pressure is &lt;a href=&quot;http://www.endmemo.com/physics/pressurealtitude.php&quot;&gt;around 1 Pascal&lt;/a&gt;, essentially a vacuum - this results in a very rarified plume which expands quickly. Combine that with the empty weight of the first stage being in excess of &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot;&gt;~18 metric tonnes&lt;/a&gt; due to the fuel remaining required for the three relanding burns, and you can quickly see that any effects due to the second stage plume would be aerodynamically minimal on an object carrying that much inertia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Additionally,&lt;/strong&gt; following stage separation in the video you link, you can actually see the pulses of the cold Nitrogen gas thrusters actively controlling the attitude of the vehicle. Furthermore, this is a technique SpaceX has used as far back as CASSIOPE (Falcon 9 v1.1) which you can see &lt;a href=&quot;http://youtu.be/uFefasS6bhc?t=3m8s&quot;&gt;in this video&lt;/a&gt;, as viewed from the second stage. Not only that, but it's &lt;a href=&quot;http://youtu.be/eWtfE1GHNjg?t=1m48s&quot;&gt;been tried on Falcon 1 too&lt;/a&gt;! &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-01-19T22:35:05.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7899" PostTypeId="2" ParentId="7897" CreationDate="2015-01-19T22:36:22.580" Score="18" Body="&lt;p&gt;The 53 metric tonne number comes from the use of &lt;a href=&quot;http://en.wikipedia.org/wiki/Propellant_crossfeed&quot; rel=&quot;noreferrer&quot;&gt;propellant cross feed&lt;/a&gt;.  And only when using propellant cross feed.  When not using it, payload to LEO is lower, but I cannot find a reference to that number.&lt;/p&gt;&#xA;&lt;p&gt;Cross feed means that the center core has connections to the side cores, sending fuel and oxidizer over from each side to the engines of the center core.  This way, when the two side boosters burn out, the center core is mostly full of fuel/oxidizer, converting from a 2 stage vehicle to a functional three stage vehicle. The side boosters only feed the three engines closest to the side booster.  Thus 6 of the 9 engines are cross fed, and the middle three of the middle core are fed from its native tankage.&lt;/p&gt;&#xA;&lt;p&gt;Then the center core, acting as a second stage, can burn most of a full Falcon core's worth to boost the upper stage (now really a third stage) to orbit for it to finish the mission.&lt;/p&gt;&#xA;&lt;p&gt;It is amazing how big a deal cross feed makes. Alas, it turns out that SpaceX decided it was not worth the effort. Instead they went with launching with all 27 engines at full thrust, and soon after they are moving, throttle down the 9 engines from the middle core, so that the side cores burn out sooner and still give a staging effect.&lt;/p&gt;&#xA;&lt;p&gt;Delta 4 Heavy, burns all three cores at basically the same rate, though they might be considering full thrust to start, throttle down the center core, separate the cores, and burn the center core a bit longer to get a pseudo third stage effect.&lt;/p&gt;&#xA;&lt;p&gt;Angara 3 or 5 which will be flying with multiple strapons may have the ability to do something similar with throttling, but unless it is built in from the beginning, cross feed is very hard to add afterwards.&lt;/p&gt;&#xA;&lt;p&gt;But throttling down the center core to burn longer after the side cores separate is not as effective of emptying the side boosters much faster, staging earlier, throwing away all that extra mass, and then burning the lighter 'second' stage longer. However, SpaceX decided to go with throttling instead of cross feed in the end.&lt;/p&gt;&#xA;&lt;p&gt;Most likely this is because no paying customer actually needed the full payload capacity for a mission. Thus they get close enough with throttling. Additionally, between the time they announced the first iteration of Falcon Heavy, and the first launch, the Falcon core booster greatly improved.&lt;/p&gt;&#xA;&lt;p&gt;The performance of the Falcon 9 1.0 (the first 5 boosters) differs greatly from the Block 5 Falcon 9. They went from a LEO payload of 10,450kg to 22,800kg and the penultimate Falcon Heavy will be using the newer boosters.&lt;/p&gt;&#xA;&lt;p&gt;The first Falcon Heavy mission actually uses Block 3 side boosters (They are previously flown Block 3 boosters). The center core is unique and new, but is thought (Not seen any evidence yet) to not be quite the Block 5 design. The future flights are expected to use three Block 5 cores.&lt;/p&gt;&#xA;&lt;p&gt;Much of that performance boost is Merlin related, where the Merlin 1A/1B from the original Falcon 1 was only around 75,000 lbs of thrust.&lt;/p&gt;&#xA;&lt;p&gt;The Merlin 1C used in the Falcon 9 1.0 was a great improvement at closer to 125,000lbs of sea level thrust.&lt;/p&gt;&#xA;&lt;p&gt;The Merlin 1D used in Falcon 9 1.2/Full Thrust is closer to 170,000lbs of thrust.&lt;/p&gt;&#xA;&lt;p&gt;The final version expected to be used in Block 5 (No name change officially yet) is expected to increase again, closer to 190,000lbs.&lt;/p&gt;&#xA;&lt;p&gt;Thus they more than doubled single stick payload performance between the time Falcon Heavy was announced and its first launch.&lt;/p&gt;&#xA;&lt;p&gt;This allowed them to get payload back to what they originally predicted, without the need for the more complex cross feed.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-02-02T15:54:40.427" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="7900" PostTypeId="2" ParentId="7894" CreationDate="2015-01-20T03:22:26.653" Score="7" Body="&lt;p&gt;Option #1 is out. You always bring more fuel than you think you need. Then you bring a little more yet. SpaceX just showed us what happens when you don't do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Option #2 is still used to some extent. Rockets initially did use timed burns, but improved sensors and onboard computers lead to a better way. Russia still uses programmed burns early on. It uses other approaches when finer control is needed. The US likes more extravagant approaches throughout. This costs more but is more accurate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Option #3 is used to some extent, during some phases of flight. There is a problem: It can't be done as stated in the question. If the rocket gets to the desired altitude it won't be at the desired velocity, and if it gets to the desired velocity it won't be at the desired altitude. What happens is instead that along the way, the vehicle adjusts its trajectory so that it will come close to having the right velocity when the vehicle reaches the desired altitude. Then it shuts down when it thinks it has reached the target altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A related option is to calculate the delta v needed to get from the current orbit to the desired orbit and stop thrust when the sensed accumulated delta v reaches the desired value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an additional problem: The vehicle isn't where it thinks it is, nor is it going at the velocity it thinks it is. The sensed accumulated delta v is erroneous as well. This brings up the need for additional options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Option #4: Make corrections along the way. Burns tend to be short once the vehicle is on orbit. The vehicle is only thrusting all the time during launch. After that, space vehicles use a burn-coast-burn strategy. This gives the vehicle time to figure out that the burn that started the transfer wasn't quite right. Correction burns put the vehicle back on a trajectory that will more or less bring the vehicle to the desired place, at the desired velocity, and at the right time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Option #5: Don't do it all at once. Nobody goes from launch to the target orbit. Everyone sneaks up on the target. A direct flight from ground to the International Space Station would take about ten minutes. The Automated Transfer Vehicle sometimes takes many days to get from the ground to the the International Space Station. Even the Soyuz rapid launch takes six hours.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-01-20T03:22:26.653" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7901" PostTypeId="1" AcceptedAnswerId="7902" CreationDate="2015-01-20T13:47:13.600" Score="4" ViewCount="1270" Body="&lt;p&gt;I was wondering why low earth orbit satellites can not be seen from the same place all the time? For instance, the ISS passes over my house twice in a month. Why does it happen only two times?&lt;/p&gt;&#xA;" OwnerUserId="1267" LastEditorUserId="1142" LastEditDate="2015-01-20T14:24:19.907" LastActivityDate="2016-02-27T15:32:20.287" Title="Why are low earth orbit satellites not visible from the same place all the time?" Tags="|iss|low-earth-orbit|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7902" PostTypeId="2" ParentId="7901" CreationDate="2015-01-20T15:23:39.910" Score="9" Body="&lt;p&gt;The ISS orbits approximately once every 90 minutes.  In that 90 minutes, the earth rotates about 15 degrees.  Thus, your position under the orbital track of the ISS changes each orbit.  Combine that with the ISS' position in its orbit when you are on the ground track and you get the result you are observing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As mentioned in the comments, inclination matters.  If the ISS was orbiting with an inclination of 0 degrees and you lived on the equator, it would pass over you every orbit (every 90 minutes).  But, the ISS orbits at about 51 degrees inclination and I suspect you don't live on the equator.  The higher the object's inclination, the more you will move under the ground track each orbit.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-01-20T16:05:49.930" LastActivityDate="2015-01-20T16:05:49.930" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7903" PostTypeId="2" ParentId="7901" CreationDate="2015-01-20T15:57:09.647" Score="2" Body="&lt;p&gt;This has to do with orientation of the satellite's orbit. The ISS does not orbit the Earth along its equator, instead it makes an angle of 51.65 degrees with the equator. Like Erik said, it is orbiting faster than the Earth is rotating which means that it passes over a different swath of land every orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So long story short, between the Earth rotating at one speed, the ISS orbiting at another, and the plane inclination of the ISS's orbit, there are a lot of things that have to line up in order for the ISS to pass over your head.&lt;/p&gt;&#xA;" OwnerUserId="6354" LastActivityDate="2015-01-20T15:57:09.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7904" PostTypeId="2" ParentId="7829" CreationDate="2015-01-20T16:12:20.280" Score="14" Body="&lt;p&gt;To answer your second question on the astronauts' experience and how much thought went into adjusting the g-force profile of a launch, NASA published a document that contains information on the g-force survivability range of a human. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/conghand/mannedev.htm&quot; rel=&quot;noreferrer&quot;&gt;Here is the Paper&lt;/a&gt;, the relevant figure you want is Figure 5 which is about halfway down the page. The figure is a plot of g-forces in the y axis and time in the x axis with highlighted regions of survivability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jaQ0H.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;em&gt;Fig. 5 - Human time-tolerance: acceleration&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="6354" LastEditorUserId="1142" LastEditDate="2015-01-20T17:51:14.030" LastActivityDate="2015-01-20T17:51:14.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7906" PostTypeId="2" ParentId="7828" CreationDate="2015-01-20T19:20:54.943" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://rosettaptolemy.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ptolemy instrument&lt;/a&gt; sniffed water and 'organic' molecules before hibernation.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xPh0X.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;(source: &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/12/02/the-quest-for-organic-molecules-on-the-surface-of-67pc-g/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Organic_compound&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An organic compound is any member of a large class of gaseous, liquid,&#xA;  or solid chemical compounds whose molecules contain carbon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Exact composition of Ptolemy detected organic molecules has not yet been announced, and according to some recent explanations such as during the &lt;a href=&quot;http://www.ustream.tv/recorded/56575467&quot; rel=&quot;nofollow noreferrer&quot;&gt;2014 AGU Fall Meeting Rosetta Comet Science Results&lt;/a&gt;, the team is still working on identifying precisely which molecules were detected. That on its own means there's a good chance that all this excitement surrounding Ptolemy findings isn't merely about simple carbon compounds such as carbon oxides, and perhaps even less likely because &lt;a href=&quot;http://www.ifsi-roma.inaf.it/virtis/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta's VIRTIS instrument&lt;/a&gt; already detected presence of carbon dioxide in comet's coma before the Philae lander even departed it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/kgtt5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image source: &lt;a href=&quot;http://tvdownload.esa.int/Philae%20Landing%2012%20Nov%202014/Still%20images/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA TV exchange FTP site&lt;/a&gt;, shared Nov. 9, 2014 (Philae detached from Rosetta on Nov. 12, 2014)&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="49" LastEditDate="2015-01-20T23:03:07.863" LastActivityDate="2015-01-20T23:03:07.863" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7908" PostTypeId="2" ParentId="3889" CreationDate="2015-01-20T20:03:54.833" Score="8" Body="&lt;p&gt;Page 10 of the article: &lt;em&gt;&quot;Earth-return trajectory that has a period of 1/2 month (or 1/3 month is some cases) in order to ensure return to the Moon after 2 (or 3) revolutions of the Cycler in its Earth-return orbit&quot;&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those revolutions are not shown in the diagram; &quot;Earth Return Trajectory&quot; and &quot;Translunar Trajectory&quot; only show half of their respective orbits. After perigee with Earth, the Cycler makes 1 (or 2) highly elliptical orbits whose apogee is at lunar orbit, but with the Moon 180° (or 120°) out of phase. Then on the 2nd (or 3rd) apogee, the Cycler is in sync with the Moon again and may perform another BackFlip.&lt;/p&gt;&#xA;" OwnerUserId="3466" LastActivityDate="2015-01-20T20:03:54.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7910" PostTypeId="1" CreationDate="2015-01-21T13:05:13.650" Score="7" ViewCount="302" Body="&lt;p&gt;I was reading an Aviation Week article about the &lt;a href=&quot;http://aviationweek.com/space/aerojet-rocketdyne-cranking-expendable-ssme?NL=AW-19&amp;amp;Issue=AW-19_20150114_AW-19_165_CPY1&amp;amp;sfvc4enews=42&amp;amp;cl=article_2&amp;amp;YM_RID=CPEN1000000228470&amp;amp;YM_MID=1556&quot;&gt;restart of SSME (RS-25E) production for SLS&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a paragraph in it that is interesting to me. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the SLS, the engine inlets will receive colder liquid oxygen at higher pressures because the LOX tank is taller and the vehicle will have higher acceleration than the shuttle. Plumes from the SLS solid boosters will be much closer to the liquid-fuel engines, so the RS-25 nozzles will need shielding from the heat.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I get how you simulate the higher pressure/colder LOX intake on a test stand. That is just 'mechanics', or as Sheldon would say the doodads that engineers twiddle their time with. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But building a test stand that can simulate a pair of SRB's at 2.8 Mlbs thrust firing nearby?  Good grief that is a LOT of energy to try and simulate.  Plus 3 other SSME's in close proximity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is an awe inspiring amount of energy at play at the base of the SLS.  (2.8 X 2 + .66 X 4 = 8.24 Mlbs of thrust).  That is an amazing amount of heat to try and play around with.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The test stand for the SSME's is pretty robust, but it seems like this is an order of magnitude harder. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How will they test this extra shielding need in a meaningful way? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-01-21T15:18:57.197" LastActivityDate="2015-07-14T18:01:16.630" Title="How do they simulate the SLS engine environment?" Tags="|testing|sls|ssme|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7912" PostTypeId="2" ParentId="7846" CreationDate="2015-01-21T16:28:05.297" Score="10" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;What sort of analysis was performed before “modern” computing and the invention of finite element analysis and computational fluid dynamics?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Techniques such as computational fluid dynamics (CFD) and finite element method (FEM) are older than you think and were used early on in the space age. Although CFD is very computationally demanding, CFD predates digital computers. The development of the finite element method post-dates the development of digital computers, but not by much. M. Jon Turner at Boeing is generally credited with being one of the key inventors of the FEM during the 1950s. Precursors to FEM also pre-date digital computing.&lt;/p&gt;&#xA;&lt;p&gt;Below is a picture of a roomful of highly parallel computer processors from the pre-digital age. This image also depicts an old-style data storage system, the box full of nicely filed paper at the bottom right.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iVNLN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iVNLN.png&quot; alt=&quot;Human computers - &amp;quot;computers&amp;quot; used to be human&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;br&gt;&#xA;&lt;p&gt;Prior to the widespread use of digital computers, analog computation provided an alternative to hand calculation computation. Analog computers were heavily used to simulate a number of physical processes, including engines and rockets. The next two images show Vannevar Bush's differential analyzer, a mechanical analog computer that could solve up to sixth order differential equations, and the Beckman Instruments EASE analog computer, which the Allison division of General Motors used to design jet engines. Until the 1960s, many saw analog computers as being superior to digital computers.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oVMlT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oVMlT.png&quot; alt=&quot;Vennevar Bush's differential analyzer&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/33kqi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/33kqi.png&quot; alt=&quot;Beckman Instruments EASE analog computer&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2020-07-01T12:56:19.087" LastActivityDate="2020-07-01T12:56:19.087" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="7913" PostTypeId="2" ParentId="7827" CreationDate="2015-01-21T23:49:55.270" Score="18" Body="&lt;p&gt;&lt;strong&gt;Short answer&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall temperature of a satellite around Earth is more or less the same than on the ground due to heat from the Sun and heat from systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cryogenic propellants must be stored at very low temperature, or they evaporate. This is not effective for a satellite which must stay in orbit for years (unless the satellite orbits Saturn where the equilibrium temperature starts to be very low, and below boiling points).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They must be kept in heavy tanks to avoid deformation caused by gas pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Storable propellants are more easy to use, and are robust in case the cooling system is temporary ineffective.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;More details:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closer the spacecraft to the Sun, the hotter it may become.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SlrzE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;Black body temperature according to its distance from the Sun (ignoring planet albedo.)&lt;br&gt;&#xA;(source: &lt;a href=&quot;http://www.aerospace.org/publications/books-by-aerospace/spacecraft-thermal-control-handbook/&quot; rel=&quot;noreferrer&quot;&gt;Spacecraft Thermal Control Handbook&lt;/a&gt;, David G. Gilmore et.al.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Off the protection of the atmosphere, this will really be a problem, the spacecraft must radiate the heat received, in addition of radiating any heat produced by the equipments on-board.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Temperature inside a spacecraft is the result of two opposite effects:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;heat radiation capture (from Sun and planets albedo) and own heat production&lt;/li&gt;&#xA;&lt;li&gt;internal heat extraction and rejection by radiation (by thermal control).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/I08Nr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;(Source: &lt;a href=&quot;https://books.google.com/books?id=_dpkAgAAQBAJ&quot; rel=&quot;noreferrer&quot;&gt;Spacecraft Thermal Control&lt;/a&gt;). Note that energy received from orbited planet will vary with altitude (e.g. GEO vs LEO), this can be ignored here for simplification. Effects like Sun eclipse and satellite spin will increase complexity for the regulation, but will be ignored too here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The result depends on how much heat is captured, or produced, and how efficient is the heat extraction (by conduction) and heat rejection. This resulting temperature must be in the range required by the systems aboard: e.g. sensors, solar arrays and communication equipment. Actually different areas of the spacecraft may have different temperatures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OLKbe.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;(source: &lt;a href=&quot;https://books.google.fr/books?id=uTwb7d8PTXMC&amp;amp;pg=PA424&quot; rel=&quot;noreferrer&quot;&gt;Fundamentals of Space Systems&lt;/a&gt;, Vincent L. Pisacane)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whatever this temperature is, it must be regulated.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If there is any problem with the cooling system, any usual liquid propellant will start to boil. A portion of the gas must be released to vacuum to avoid the tank destruction. This is similar to what happens on a pad while the vehicle is waiting its launch, but there is no possibility to refill the tank.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YQYoi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;STS external tank with gaseous oxygen evaporation under beanie cap.&lt;br&gt;&#xA;(source: &lt;a href=&quot;http://www.nasa.gov/images/content/161054main_stack-lg.jpg&quot; rel=&quot;noreferrer&quot;&gt;Nasa&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LH2 has a boiling temperature of -183°C, this is indeed a cryogenic fuel. Hydrazine (UDMH) boils only at 63°C. It is storable at Earth temperature.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;More LH2 will be released in case of temporary cooling failure. The tank must be more robust to contain the greater pressure which results from evaporation. Robustness means usually additional mass. For these reasons, a storable propellant is used for attitude control and reboost.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In addition the propellant may need to be oxidized to produce energy. There are two types of oxidization: either spontaneous -- hypergolic -- like for UDMH with N2O4 or requiring an igniter like for LH2 with LO2. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Non-hypergolic combustion adds complexity, an engine is required to mix the fuel and the oxidizer and ignite the mixture. This is a problem when the engine must be fired a lot of times during the mission to maintain attitude and/or altitude. Failures would occur, for this reason only hypergolic are used for satellites.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Some propellants don't require additional oxidizer to release energy. They are named &lt;a href=&quot;http://en.wikipedia.org/wiki/Monopropellant&quot; rel=&quot;noreferrer&quot;&gt;monopropellants&lt;/a&gt;, and work by exothermic chemical decomposition.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The propellant in the end is ejected thru the thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5CGg9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;Monopropellant thruster - Source: &lt;a href=&quot;http://www.moog.com/products/thrusters/bipropellant-thrusters/&quot; rel=&quot;noreferrer&quot;&gt;Moog&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two types of thruster feeding: tank pressure and pumps.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Tank pressure is the only system used for orbit / attitude control of satellites. Pumps are too complex and to heavy for this use.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EeQYL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;Hydrazine bladder tank, returned to the sender (source: &lt;a href=&quot;http://www.dailymail.co.uk/news/article-2077708/Metallic-spaceball-Metal-sphere-fell-Earth-sparks-alien-speculation.html&quot; rel=&quot;noreferrer&quot;&gt;Daily Mail Online&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Designers will usually choose a mean of propulsion which is storable and hypergolic and can work without pumps. Taking into account that the propellant must be efficient (&lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;noreferrer&quot;&gt;specific impulse&lt;/a&gt;) some combinations are often encountered on satellites:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astronautix.com/props/hydazine.htm&quot; rel=&quot;noreferrer&quot;&gt;Hydrazine&lt;/a&gt; (monopropellant)&lt;/li&gt;&#xA;&lt;li&gt;UDMH/N2O4 (&lt;a href=&quot;http://www.astronautix.com/props/n2o4udmh.htm&quot; rel=&quot;noreferrer&quot;&gt;unsymmetrical dimethylhydrazine&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;MON-3/MMH (&lt;a href=&quot;http://www.astronautix.com/props/mon3mmh.htm&quot; rel=&quot;noreferrer&quot;&gt;monomethylhydrazine&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Nitrogen (cold gas)&lt;/li&gt;&#xA;&lt;li&gt;Helium (cold gas)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Use:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Bipropellants are most commonly used on GEO orbit spacecrafts, they are used for transfer from launch orbit to GEO.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Monopropellant systems are more simple, but they are less powerful.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Cold gas are ever more simple, but very less powerful.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-01-23T17:53:06.697" LastActivityDate="2015-01-23T17:53:06.697" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7914" PostTypeId="1" AcceptedAnswerId="7916" CreationDate="2015-01-22T05:40:29.547" Score="8" ViewCount="1061" Body="&lt;p&gt;The Dawn spacecraft has three ion engines that are mounted such that their thrust vectors are at significant angles.  You can see this in this image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ri6Dq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the image, one of the three engines is active.  Two questions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Why three engines?  I suspect this is for redundancy, but seems very wasteful.  In addition, several other missions use a single propulsion system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Why different thrust vectors?  I suspect this has something to do with redundancy and making sure that the thrust vector passes through the center of mass.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="4660" LastEditDate="2015-05-16T14:10:15.813" LastActivityDate="2015-05-16T14:10:15.813" Title="Why does the Dawn spacecraft have three ion engines at different angles?" Tags="|propulsion|probe|engines|dawn-mission|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7916" PostTypeId="2" ParentId="7914" CreationDate="2015-01-22T08:04:02.967" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Each of Dawn's three 30-centimeter-diameter (12- inch) ion thrust&#xA;  units is movable in two axes to allow for migration of the&#xA;  spacecraft's center of mass during the mission. This also allows the&#xA;  attitude control system to use the ion thrusters to help control&#xA;  spacecraft attitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The same article writes further to say &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Two ion propulsion engines are required to provide enough thruster&#xA;  lifetime to complete the mission, and the third engine serves as a&#xA;  spare.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;See more at: &lt;a href=&quot;http://dawn.jpl.nasa.gov/technology/ion_prop.asp#sthash.IOhMDaGr.dpuf&quot;&gt;http://dawn.jpl.nasa.gov/technology/ion_prop.asp#sthash.IOhMDaGr.dpuf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="288" LastEditDate="2015-01-22T17:01:48.650" LastActivityDate="2015-01-22T17:01:48.650" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="7917" PostTypeId="1" AcceptedAnswerId="7920" CreationDate="2015-01-22T09:08:40.017" Score="7" ViewCount="245" Body="&lt;p&gt;What are the rules governing international use of water areas to dump used spacecrafts from orbit or launch trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sm8a4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;(source: &lt;a href=&quot;http://de.wikipedia.org/wiki/Datei:Apollo_7_Rondevouz.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia / Nasa&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would apply to rocket stages, boosters, tanks, decommissioned satellites, space station returning cargos, etc. Some may burn during re-entry, some may not, and some may be dumped before reaching any orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that formally allowed to dump space stuff into oceans, are there specific areas empty of human activities reserved for such purpose?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any international agreement on what could return to Earth (e.g. hazardous propellant like hydrazine)&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-01-22T10:21:59.253" LastActivityDate="2015-01-22T22:13:22.750" Title="Regulation governing the use of ocean areas for the return of space objects?" Tags="|reentry|debris|stages|ballistics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7919" PostTypeId="1" AcceptedAnswerId="7921" CreationDate="2015-01-22T20:54:11.630" Score="8" ViewCount="1615" Body="&lt;p&gt;Just a quick question: Is a &lt;a href=&quot;http://en.wikipedia.org/wiki/Lambert&amp;#39;s_problem&quot;&gt;Lambert solver&lt;/a&gt;  a generalization of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Clohessy-Wiltshire_equations&quot;&gt;Clohessy-Wiltshire equations&lt;/a&gt; when regarding, for example, orbital rendezvous around Earth?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="5963" LastEditDate="2015-01-22T22:54:35.093" LastActivityDate="2015-01-24T19:50:51.810" Title="Clohessy-Wiltshire equations vs. Lambert solver for low-Earth Orbital Rendezvous" Tags="|rendezvous|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7920" PostTypeId="2" ParentId="7917" CreationDate="2015-01-22T22:13:22.750" Score="5" Body="&lt;p&gt;Short answer: none that are binding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Longer answer: Most space-faring countries and organizations voluntarily implement standards that abide by the principles in the space debris mitigation guidelines&#xA;of the Scientific and Technical Subcommittee of the United Nations Committee on the Peaceful Uses of Outer Space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of NASA, this is implemented by &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/mitigate/technicalstandard.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Technical Standard 8719.14&lt;/a&gt; -- specifically section 4.7 of that standard called &quot;Survival of Debris From the Postmission Disposal Earth Atmospheric Reentry Option.&quot;  In summary, a risk assessment is performed to ensure that the chance of a human casualty below 1 in 10,000.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-01-22T22:13:22.750" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7921" PostTypeId="2" ParentId="7919" CreationDate="2015-01-23T03:55:36.167" Score="15" Body="&lt;p&gt;They are very different things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Lambert solver is one way to bring a spacecraft close enough to another spacecraft where that first spacecraft can use the Clohessy-Wiltshire (CW) equations to guide the way through the rendezvous process. The chaser (the active spacecraft in the rendezvous) can't use CW to make the transition from far away / far below to near field rendezvous. On the flip side, the chaser can't use Lambert's technique to guide the final rendezvous. The two techniques address two very different problems in two very different ways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lambert's problem addresses how to transfer from point $A$ at time $t_A$ to point $B$ at time $t_B$ subject to the influence of a gravitational field, plus some means to change velocity to accomplish the transfer. The CW equations don't address this problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CW equations are instead a linearization of the rotating frame equations of motion in the vicinity of a spacecraft, where the rotating frame is the target vehicle's Local Vertical / Local Horizontal (LVLH) frame. The linearization makes for easily computable second derivatives, which makes it very useful in onboard flight software. While this approximation is fairly accurate at close distances, it's terrible at large distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;Update:&lt;/strong&gt; Answers to two questions raised in a comment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Is there a reason why a Lambert solver can't be used for the final part of the rendezvous?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's not just one reason. There are many reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Lambert solver yields a burn-coast-burn transfer, where both burns are impulsive. This isn't how proximity operations work. Prox ops instead involves a (potentially large) number of tiny burns that keep the vehicle on a pre-planned trajectory to the docking port (or to the capture box in the case of HTV, Dragon, and Cygnus). The pre-planned trajectory is designed in part with those numerous small burns in mind. A Lambert solver may well yield a solution that is mind numbingly bad in terms of delta V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even more important is the primary reason for that pre-planned trajectory: It keeps things safe. A trajectory that intersects with the target along the way from point A to point B is a monumentally bad idea. A Lambert solver doesn't address the nature of the space that intervenes from point A to point B; that space is implicitly assumed to be empty space. That assumption doesn't apply in the vicinity of the target vehicle. Lambert cannot be used during prox ops for this reason alone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet another reason is pluming. Rocket exhaust inevitably contains a small portion of unburnt fuel, unburnt oxidizer, or both. Pluming the Space Station with hydrazine and/or nitric acid is another one of those monumentally bad ideas. Lambert solutions don't address where the exhaust goes; it presumably goes into empty space. That assumption fails during proximity operations. A Lambert solution not only can but will plume the target, particularly on final approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet another reason is computation. Lambert's problem is a boundary value problem; these are typically computationally expensive to solve. Lambert's problem is not an exception to this rule. This is not a problem when the chaser is far away from the target. Simply enable the Lambert solver tens of seconds (or whatever it takes) before the solution is needed, thereby giving the solver time to find the solution. This doesn't work during prox ops, where algorithms need to be fast and timely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Numerical precision is another computational issue. Lambert solvers work in Earth centered inertial coordinates. CW-based techniques work in local LVLH coordinates. That is by far the preferred frame for proximity operations. Going from local LVLH to ECI and back to local LVLH sacrifices over half of the precision of a double precision number. With single precision numbers, the precision loss is nearly 100%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all, using a Lambert solver during proximity operations is worse than monumentally bad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;em&gt;Are the CW equations used by real spacecraft rendezvous systems, or is something more advanced utilized?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CW is the dominant approach used to rendezvous with the Space Station. (I'm tempted to say it's the only approach used for ISS rendezvous.) The linear nature of the equations make them fast and make them fit very well with a Kalman filter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Clohessy-Wiltshire equations make two key explicit assumptions: The small angle assumption and a circular target orbit assumption. The small angle assumption means that CW becomes less accurate with increasing distance. That's not a problem. You don't use CW at large distances. They're used for near field rendezvous and proximity operations. The second assumption isn't a problem with the Space Station. It's eccentricity is typically less than 0.003, which is extremely low. The assumption of a circular orbit is a problem for a formation of satellites in an elliptical orbit. Nonetheless, a CW-like technique is still used. There are a number of techniques that yield equations that look very much like the CW equations for elliptical orbits.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-01-24T19:50:51.810" LastActivityDate="2015-01-24T19:50:51.810" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7922" PostTypeId="2" ParentId="601" CreationDate="2015-01-23T11:25:33.670" Score="0" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The interstellar mission could consist of a continuous convoy of ships which form a chain of communication. If the interstellar ships receive beamed power from Earth, the ships in such a convoy might also be used to refocus that power beam to the next ship. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;http://www.kiss.caltech.edu/study/science/FOCAL%20Mission%20Concept%20JOHNSON.pdf&quot; rel=&quot;nofollow&quot;&gt;gravitational focus&lt;/a&gt; of the Sun could be used to lens the radio signals. The gain would be stupendous. We'd probably like to do it anyway in order to telescopically investigate the interstellar destination in detail, and measuring the amount of dust along the way, before launching any voyage there. The focus is a line, not a dot, so the radio focusing communication spacecraft never needs to decelerate. Geometrically it starts 550 AU from the Sun, but because of the corona one might have to go 700 to 1000 AU away (in the opposite direction from the destination) before it works practically. That is nearby compared to the nearest star 270,000 AU away.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-01-23T11:30:41.480" LastActivityDate="2015-01-23T11:30:41.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7923" PostTypeId="2" ParentId="6607" CreationDate="2015-01-23T12:37:39.180" Score="16" Body="&lt;p&gt;Shortly after this question was asked, it was answered (in the case of the moon, not Mars) on &lt;a href=&quot;http://what-if.xkcd.com/124/&quot; rel=&quot;noreferrer&quot;&gt;XKCD's what-if&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Summarizing that article, for most average people &lt;em&gt;swimming&lt;/em&gt; would be the same, as the buoyancy effects are an order of magnitude more dominant than the gravitational effects. Extraordinarily skilled swimmers would notice small difference, but the real differences would be in &lt;strong&gt;activities that break the surface&lt;/strong&gt;. For instance, humans could leap out of the water like dolphins. Jumping out of the pool would be an actual jump. And with a monofin, you could high dive &lt;strong&gt;&lt;em&gt;onto&lt;/em&gt;&lt;/strong&gt; the diving board:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sp3hN.png&quot; alt=&quot;High dive&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2015-01-23T12:37:39.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7924" PostTypeId="2" ParentId="601" CreationDate="2015-01-23T14:45:49.210" Score="1" Body="&lt;p&gt;It may be worth noting that physically sending back 'memory cards' (or in this case, film canisters) has been done by the recently declassified KH-9 Hexagon spy satellites, each of which was equipped with 4 to 5 re-entry capsules that were packed with film for recovery. These satellites operated in Low Earth Orbit for only a few months each, so the Delta-V and return vehicle complexity requirements are significantly lower than returning a capsule from a solar escape trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lKGrF.jpg&quot; alt=&quot;KH-9 Hexagon&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the points Hobbes raised, you would also need to consider:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The return trajectory design of such a vehicle.&lt;/li&gt;&#xA;&lt;li&gt;The risks associated with a physical return - remember the &lt;a href=&quot;http://en.wikipedia.org/wiki/Genesis_%28spacecraft%29#Recovery_phase&quot; rel=&quot;nofollow noreferrer&quot;&gt;Genesis crash&lt;/a&gt;? Both Hexagon and Genesis planned mid-air retrievals (using a modified C-130 and a helicopter respectively) of reentry capsules. A long term trickle of data may be preferable to a high-risk, piecemeal return of greater volumes.&lt;/li&gt;&#xA;&lt;li&gt;Given that mass constrains are already very tight, it seems unlikely that mass would be spent on a return system given that any such mission would require a capable transmitter/receiver in any case.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia: KH-9 Hexagon&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.thespacereview.com/article/2392/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Working in the shadows: Phil Pressel and the Hexagon spy camera&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8323" LastActivityDate="2015-01-23T14:45:49.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7925" PostTypeId="2" ParentId="4646" CreationDate="2015-01-23T16:57:54.847" Score="4" Body="&lt;p&gt;ISS and a Moon base are equally safe. As long as they don't suddenly explode or otherwise become death traps because of some technical malfunction in the human artefacts themselves, regardless of the space environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Among all the innovative suggestions about what could be dangerous, I want to remind of the fact that &lt;em&gt;the only thing&lt;/em&gt; which has &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spaceflight-related_accidents_and_incidents#Astronaut_fatalities_during_spaceflight&quot; rel=&quot;noreferrer&quot;&gt;killed astronauts&lt;/a&gt; thus far, is &lt;strong&gt;technical malfunction&lt;/strong&gt;! That goes for all the 18-19 who died in space (or rather, in Earth atmosphere during launch or reentry!) as well as for all the 5 astronauts (which I know of) killed on the ground during space specific training. Add to that a few of the about 500 who have been in space, are said to suffer lasting but minor health problems from microgravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of a ton extra launch mass for increased radiation shielding, maybe a ton extra of redundancy for technical systems has a much greater life saving effect?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-01-23T19:08:05.210" LastActivityDate="2015-01-23T19:08:05.210" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7926" PostTypeId="2" ParentId="601" CreationDate="2015-01-23T19:17:52.513" Score="1" Body="&lt;p&gt;You mentioned Voyager. This is actually an example of an alternative data transfer strategy. Voyager was built for flyby missions: it whizzed by a planet at high speed, gathering science data at a much higher rate than could be transmitted. &#xA;This data was stored onboard and sent to Earth later, during the long, boring cruise to the next planet.&lt;br&gt;&#xA;New Horizons will do the same during the Pluto flyby. In fact, it can't transmit during the flyby because it has to point its science instruments at Pluto, which points the radio antenna away from Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interstellar probes could follow a similar strategy, although it is hard to imagine a probe that has a radio transmitter capable of sending signals over a distance measured in lightyears. The power required boggles the mind. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-01-23T19:17:52.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7927" PostTypeId="1" CreationDate="2015-01-24T02:11:55.407" Score="9" ViewCount="1433" Body="&lt;p&gt;What is a good estimate for SLS' annual fixed cost and variable per flight cost?  What references are used for these costs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen uncited figures of $1B+/launch variable cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia quotes a NASA deputy project manager as saying that $500M/launch is a target.  This seems low as the Delta IV Heavy used in the Orion test launch was in that range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other estimates cited on Wikipedia range up to $1.9B/launch.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-01-24T03:28:58.203" LastActivityDate="2016-06-01T11:13:21.740" Title="Space Launch System (SLS) costs" Tags="|sls|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7929" PostTypeId="1" AcceptedAnswerId="12574" CreationDate="2015-01-24T13:38:20.663" Score="18" ViewCount="3286" Body="&lt;p&gt;Which are the main control theories behind the landing maneuvers of the Falcon 9? For the hovering part, as it approaches the landing surface, I guess the &lt;a href=&quot;http://en.wikipedia.org/wiki/Inverted_pendulum&quot; rel=&quot;nofollow&quot;&gt;inverted pendulum&lt;/a&gt; control approaches would do it. But what about the control from stage separation till it the reentry burn? Any textbook references appreciated!    &lt;/p&gt;&#xA;" OwnerUserId="8328" LastEditorUserId="1142" LastEditDate="2015-08-31T00:25:30.817" LastActivityDate="2015-11-05T03:06:01.173" Title="What control engineering techniques are used for the landing maneuvers of Falcon 9-R?" Tags="|spacex|landing|attitude|falcon-9|aerodynamics|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="7931" PostTypeId="2" ParentId="4969" CreationDate="2015-01-24T20:19:18.393" Score="2" Body="&lt;p&gt;Rendezvous can be divided into 4 main phases:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Phasing: to approach the target orbit&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Initial approach: to acquire a stable orbit with respect to the target (distance in the order of magnitude of $10^3$)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Final approach: to get closer to the target (distance in the order of magnitude of $10^2$)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) Final translation: to finalize the contact (distance in the order of magnitude of $10^1$)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually every phase starts with a $\Delta v$: therefore the propulsion system is turned on (impulsive manoeuvres) to change the velocity (in magnitude and/or direction) of the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i64.photobucket.com/albums/h182/Olgola/RV.jpg&quot; alt=&quot;phases&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6608" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.580" LastActivityDate="2015-01-24T20:19:18.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7932" PostTypeId="2" ParentId="5964" CreationDate="2015-01-24T20:46:07.633" Score="2" Body="&lt;p&gt;As someone pointed out, specially on the TMA vehicle the white parts are the LOX's tank skin. That color on those tank may reduce heat absortion (from solar light), reducing LOX boil off and thus, increasing laundpad residence time. It could be very important to deal with some launch delays/scrubs.&#xA;The grey parts are the RP-1 tank's skin. I can't guess now a reason for that color, but the necessities of those tanks are quite different from the necessities of the LOX tanks.&lt;/p&gt;&#xA;" OwnerUserId="8330" LastActivityDate="2015-01-24T20:46:07.633" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7934" PostTypeId="2" ParentId="5964" CreationDate="2015-01-25T01:32:41.070" Score="3" Body="&lt;p&gt;I think looks are a serious matter to consider, as Andrew Thompson pointed out in the comments. Satellite operators have to decide which launch provider to use, and for many missions, there are no definitive criteria to pick one over the other. Looking modern and &quot;high-tech-y&quot; may secure some contracts. In any event, because the payload fraction is low on the Soyuz to begin with, carrying a little paint will not hurt much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a soviet-era poster in my bedroom of a Soyuz on the launch pad. The poster is black and white and also faded, but it the rocket seems to be single-colored, apparently white.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-01-25T01:32:41.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7935" PostTypeId="2" ParentId="3187" CreationDate="2015-01-25T08:10:00.283" Score="0" Body="&lt;p&gt;I think rubber tyres are a no go, adding additional complexity (that can fail) is too risky.  Metal is definitely the way to go.  I do however think firstly the type of metal used was a very bad idea as it is inherently soft, though light (which is why they chose aluminium) they should have gone with titanium, yes, a bit more weight (not much) but almost indestructible.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, they should have given greater thought to the thread pattern, the one they went with is not great at shifting soft sand to the side and out.  They really could have done a lot better than they did.  If curiosity fails, and I hope it does not, it will probably be because of the design flaws on the wheels.&lt;/p&gt;&#xA;" OwnerUserId="8333" LastActivityDate="2015-01-25T08:10:00.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7936" PostTypeId="2" ParentId="5964" CreationDate="2015-01-25T15:15:05.683" Score="7" Body="&lt;p&gt;Most rockets are painted white (some have &lt;a href=&quot;https://space.stackexchange.com/questions/6352/what-is-the-purpose-of-the-black-and-white-patterns-on-some-rockets&quot;&gt;black stripes&lt;/a&gt;). White paint &lt;a href=&quot;http://energy.gov/energysaver/articles/cool-roofs&quot; rel=&quot;nofollow noreferrer&quot;&gt;reflects more heat&lt;/a&gt; than other colors, so this reduces propellant boiloff. The first N-1 was dark gray, later launchers were painted in increasingly light colors because temperatures rose too high to work inside the rocket when it was sitting on the launch pad (from the book N-1: for the Moon and Mars). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The R-7 is an outlier. There is some speculation that its green color is due to it being derived from an ICBM, but there are issues with that theory:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It would have made sense to paint them all green if rockets were manufactured and painted before it became clear whether they'd be used as an ICBM or space launcher. The numbers make that an unlikely story: thousands of R-7s have been built for space launches. &lt;a href=&quot;https://en.wikipedia.org/wiki/R-7_Semyorka&quot; rel=&quot;nofollow noreferrer&quot;&gt;Only ~10&lt;/a&gt; were ever operational as ICBMs. By 1968, the R-7 ICBM was phased out. &lt;/li&gt;&#xA;&lt;li&gt;Some early R-7s were gray or white instead of green.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I have not been able to find a good explanation for the green paint of the R-7.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other outlier is the &lt;a href=&quot;https://space.stackexchange.com/questions/2449/why-do-the-shuttles-external-tanks-appear-to-be-rusted&quot;&gt;Shuttle external tank&lt;/a&gt;. This had a thick layer of insulation to prevent ice formation on the outside of the tank; ice shedding during the launch could have easily damaged the Shuttle's heat shield tiles. The insulation was left in its natural color after the first few launches, when it was found the heat load was acceptable without paint. This saved a few hundred kg. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-01-25T15:15:05.683" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7938" PostTypeId="2" ParentId="2648" CreationDate="2015-01-25T19:48:21.750" Score="-1" Body="&lt;p&gt;Don't forget that you can always shut down engine to terminate thrust (its possible from ground and there are also button in cockpit, those buttons are on center panel between commander's and pilot's seats).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SRB can be jettissoned, normally its automatic but you can also do it manually in emergency (using ground command and there is switch + button on center panel between seats).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ET (External Tank) can also be jettissoned manually (switch + button on center console between seats, i am not sure about ground command).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So its very easy to terminate thrust and turn this &quot;rocket&quot; into glider then blow SRB away from Space Shuttle itself.&lt;/p&gt;&#xA;" OwnerUserId="8335" LastActivityDate="2015-01-25T19:48:21.750" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7939" PostTypeId="2" ParentId="2648" CreationDate="2015-01-25T20:58:14.793" Score="1" Body="&lt;p&gt;Here is a cockpit photo from Wikipedia. I added some labels to show what some of the switches and buttons doing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am showing the new location of RANGE ARM SAFE light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uId5c.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uId5c.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if I would be an astronaut I would like to know what is going to happen with me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;&lt;a href=&quot;http://history.nasa.gov/rogersrep/v1ch9.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://history.nasa.gov/rogersrep/v1ch9.htm&lt;/a&gt;&#xA;This explains a lot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all SRB may hang out on aft attachment points when you try to jettison them while they still provide thrust...&#xA;But when SRBs are burned out you can do a fast separation and quickly separate the ET from shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looks like that this not confirms that shuttle itself has a charges.&#xA;On this picture you can see only SRB and ET linear charges. Note that IIRC charges on ET was later removed.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Since the current range safety system does not allow for selective&#xA;  destruction of components, the Commission believes that NASA and the&#xA;  Air Force should critically re-examine whether the destruct package on&#xA;  the External Tank might be removed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Looks like i was wrong and there is no selective destruct.&#xA;Now after ET charges was removed probably only SRB have charges as they are most dangerous because of uncontrollable flight when detached from rest of package.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And OFC there is no need for charges inside shuttle itself as its the safest part of whole package. It's rather predictable, the thrust of SSME can be terminated by command, it had not so much dangerous fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ET itself is quite dangerous, its flight is predictable but when it hit ground there will be huge explosion. IMO the decision to remove charges from ET was not so good. Implementing selective destruct would be much better IMO.&lt;/p&gt;&#xA;" OwnerUserId="8335" LastEditorUserId="10273" LastEditDate="2019-08-26T00:56:19.517" LastActivityDate="2019-08-26T00:56:19.517" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="7940" PostTypeId="2" ParentId="2648" CreationDate="2015-01-26T01:22:17.130" Score="16" Body="&lt;p&gt;The Range Safety Armed light was a cue for the crew to eject during the Orbital Test Flight period of the Shuttle program.  After the ejection seats were removed from Columbia, the light was still there.&lt;/p&gt;&#xA;&lt;p&gt;See &lt;a href=&quot;http://history.nasa.gov/rogersrep/v1ch9.htm&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Range Safety System&lt;/a&gt; in the Rogers Commission Report.&lt;/p&gt;&#xA;&lt;p&gt;The location of the indicator changed after the Orbiter 'glass cockpit' upgrade, but it was in the vehicles until the end of the program.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KusDr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KusDr.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/KRcWw.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KRcWw.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;(Personal photo taken in Orbiter cockpit in May 2008, drawing from SCOM, see link below)&lt;/p&gt;&#xA;&lt;p&gt;For the latter part of the program, the only Range Safety System left on the stack was in the Solid Rocket Boosters.  So if Range Safety actions had to be taken in 2nd stage, the illumination of this light served as a backup or no-voice-comm cue to the crew to take the actions they had been briefed on.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Controllability&lt;/p&gt;&#xA;&lt;p&gt;During first-stage, trajectory deviations may lead to a violation of a&#xA;destruct line by a shuttle that is still under control. It may be&#xA;possible to return the vehicle toward its nominal trajectory or to&#xA;safely execute an abort. Therefore, the Flight Director (FD) and&#xA;Flight Dynamics Officer (FDO) are in voice communication with the&#xA;Flight Control Officer (FCO) (formerly Range Safety Officer) during&#xA;ascent. If the FCO detects a violation, the FDO and FD are immediately&#xA;informed. The FD must determine whether the shuttle is controllable or&#xA;uncontrollable, and inform the FCO. As long as the FD declares the&#xA;shuttle controllable, the FCO takes no action to terminate the flight&#xA;for trajectory deviations alone.&lt;/p&gt;&#xA;&lt;p&gt;Range Safety Limit Avoidance&lt;/p&gt;&#xA;&lt;p&gt;A deviation from nominal trajectory that is large enough to warrant&#xA;FCO action must be corrected. First-stage options are:&lt;/p&gt;&#xA;&lt;p&gt;• State vector&#xA;update&lt;/p&gt;&#xA;&lt;p&gt;• Select CSS (after 90 seconds MET)&lt;/p&gt;&#xA;&lt;p&gt;• Engage BFS&lt;/p&gt;&#xA;&lt;p&gt;In second stage, the options are:&lt;/p&gt;&#xA;&lt;p&gt;• State vector update&lt;/p&gt;&#xA;&lt;p&gt;• ATO selection&lt;/p&gt;&#xA;&lt;p&gt;• Select CSS&#xA;• TAL or RTLS&#xA;selection&lt;/p&gt;&#xA;&lt;p&gt;• Engage BFS&lt;/p&gt;&#xA;&lt;p&gt;• SSME manual cutoff. &lt;strong&gt;For loss of comm, FCO&#xA;will send the ARM command as a cue for manual MECO&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Options are listed in order of priority. The priorities are&#xA;established such that higher priority options preserve lower priority&#xA;options and involve fewer new dangers to the crew and shuttle when&#xA;possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&quot; rel=&quot;noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt;, page 6.2-5 (emphasis mine)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-07-17T16:23:13.233" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="7943" PostTypeId="1" CreationDate="2015-01-26T12:11:33.550" Score="16" ViewCount="411437" Body="&lt;p&gt;I'm writing a story about Mars and I'm really confused as to how one should use the word 'Sol when talking about the passage of time there. For example, Would one say to a colleague: &quot;Today is Sol 345, Jack&quot;&lt;/p&gt;&#xA;" OwnerUserId="8340" LastEditorUserId="6312" LastEditDate="2015-01-26T12:28:31.943" LastActivityDate="2015-10-17T21:26:35.687" Title="The true meaning of Sol regarding time on Mars" Tags="|mars|time|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7944" PostTypeId="2" ParentId="7943" CreationDate="2015-01-26T12:26:56.397" Score="13" Body="&lt;p&gt;From the &lt;a href=&quot;http://en.wikipedia.org/wiki/Timekeeping_on_Mars#Sols&quot;&gt;Wikipedia page on Martian timekeeping&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The term sol is used by planetary astronomers to refer to the duration&#xA;  of a solar day on Mars.&lt;a href=&quot;http://www.nasa.gov/mission_pages/mer/images/pia01892.html&quot;&gt;[7]&lt;/a&gt; A mean Martian solar day, or &quot;sol&quot;, is 24&#xA;  hours, 39 minutes, and 35.244 seconds.&lt;a href=&quot;http://www.giss.nasa.gov/tools/mars24/help/notes.html&quot;&gt;[6]&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;“Sol” is often used as a direct replacement for “Day” when concerning Mars. Mission duration for Mars missions is measured in Sols, so saying “Today is Sol xyz” would be normal, but I’m not sure if anyone would say “what a wonderful Sol tomorrow is going to be”.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2015-01-26T12:26:56.397" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="7946" PostTypeId="2" ParentId="4325" CreationDate="2015-01-26T16:02:50.023" Score="20" Body="&lt;p&gt;I believe the best answer to this question is to deny its premise, &quot;Buzz Aldrin is credited (or was tasked) with being the Apollo 11 Lunar Module pilot.&quot; A better phrasing is that he was &lt;em&gt;billeted&lt;/em&gt; as being the LMP. But that was neither what he was credited for nor what he was tasked with doing. It was simply a great title for someone whose job in every non-contingency case was to assist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that as a general principle if the CM or LM needed to be piloted and the commander was present then he would usually be the one to do it. A major exception was re-entry, since it was possible in the case where lunar rendezvous failed that the Command Module Pilot would then be the only person in the re-entering spacecraft; therefore it was his job to pilot the re-entry. Transposition and LM extraction was also the CMP's job, presumably because CM docking was necessarily his specialty.&lt;/p&gt;&#xA;" OwnerUserId="8343" LastActivityDate="2015-01-26T16:02:50.023" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7947" PostTypeId="2" ParentId="646" CreationDate="2015-01-26T16:11:19.187" Score="17" Body="&lt;p&gt;&lt;a href=&quot;http://orekit.org&quot;&gt;Orekit&lt;/a&gt; is the best space mechanics tool I know. Developed in Java (cross-platform), &lt;strong&gt;Orekit&lt;/strong&gt; is a &lt;strong&gt;space dynamics open source library&lt;/strong&gt;, based on Common Apache Math. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite the fact it has no visualisation tool so far, the different force model it contains make it a really good choice if your plan is to solve accurate flight dynamics problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orekit includes all available IERS convention for frame definition. It includes orbit propagators of 3 types : &lt;br&gt;&#xA;- Analytical (Kepler, Eckstein-Heschler, SDP4/SGP4 with 2006 corrections) &lt;br&gt;&#xA;- Numerical (with customizable force models) &lt;br&gt;&#xA;- semi-analytical propagation based on Draper Semianalytic Satellite Theory (DSST) with customizable force models. &lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For information, you'll find on the same address above the Rugged add-on. Rugged is a sensor-to-terrain mapping tool which takes into account Digital Elevation Models (DEM) in its line of sight computation. It is a free software intermediate-level library written in Java and implemented as an add-on for Orekit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some of the features Orekit provides :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;high accuracy absolute dates&#xA;time scales (TAI, UTC, UT1, GPS, TT, TCG, TDB, TCB, GMST, GST ...)&#xA;transparent handling of leap seconds&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Geometry&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;frames hierarchy supporting fixed and time-dependent (or telemetry-dependent) frames&#xA;predefined frames (EME2000/J2000, ICRF, GCRF, ITRF93, ITRF97, ITRF2000, ITRF2005, ITRF2008 and intermediate frames, TOD, MOD, GTOD and TOD frames, Veis, topocentric, tnw and qsw local orbital frames, spacecraft body, Moon, Sun, planets, solar system barycenter, Earth-Moon barycenter)&#xA;user extensible (used operationally in real time with a set of about 60 frames on several spacecraft)&#xA;transparent handling of IERS Earth Orientation Parameters (for both new CIO-based frames following IERS 2010 conventions and old equinox-based frames)&#xA;transparent handling of JPL DE 4xx (405, 406 and more recent) and INPOP ephemerides&#xA;transforms including kinematic combination effects&#xA;composite transforms reduction and caching for efficiency&#xA;extensible central body shapes models (with predefined spherical and ellipsoidic shapes)&#xA;cartesian and geodesic coordinates, kinematics&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Spacecraft state&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Cartesian, Keplerian (including hyperbolic), circular and equinoctial parameters&#xA;Two-Line Elements&#xA;transparent conversion between all parameters&#xA;automatic binding with frames&#xA;attitude state and derivative&#xA;Jacobians&#xA;mass management&#xA;user-defined associated state (for example battery status, or higher order derivatives, or anything else)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Propagation&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;analytical propagation models:&#xA;    Kepler&#xA;    Eckstein-Heschler&#xA;    SDP4/SGP4 with 2006 corrections&#xA;numerical propagation with:&#xA;    customizable force models:&#xA;        central attraction&#xA;        gravity models (automatic reading of ICGEM (new Eigen models), SHM (old Eigen models), EGM and GRGS gravity field files formats, even compressed)&#xA;        atmospheric drag (DTM2000, Jacchia-Bowman 2006, Harris-Priester and simple exponential models) and Marshall solar Activity Future Estimation&#xA;        third body attraction (with data for Sun, Moon and all solar systems planets)&#xA;        radiation pressure with eclipses&#xA;        solid tides, with or without solid pole tide&#xA;        ocean tides, with or without ocean pole tide&#xA;        general relativity&#xA;        multiple maneuvers&#xA;    state of the art ODE integrators (adaptive stepsize with error control, continuous output, switching functions, G-stop, step normalization ...)&#xA;    computation of Jacobians with respect to orbital parameters and selected force models parameters&#xA;    serialization mechanism to store complete results on persistent storage for later use&#xA;semi-analytical propagation based on Draper Semianalytic Satellite Theory (DSST) with customizable force models:&#xA;    central body with full gravity model&#xA;    third body attraction&#xA;    atmospheric drag&#xA;    radiation pressure with eclipses&#xA;tabulated ephemerides:&#xA;    file based&#xA;    memory based&#xA;    integration based&#xA;unified interface above analytical/numerical/semianalytical/tabulated propagators for easy switch from coarse analysis to fine simulation with one line change&#xA;all propagators can be used in several different modes:&#xA;    slave mode: propagator is driven by calling application&#xA;    master mode: propagator drives application callback functions&#xA;    ephemeris generation mode: all intermediate results are stored during propagation and provided back to the application which can navigate at will through them, effectively using the propagated orbit as if it was an analytical model, even if it really is a numerically propagated one, which is ideal for search and iterative algorithms&#xA;handling of discrete events during integration (models changes, G-stop, simple notifications ...)&#xA;predefined discrete events:&#xA;    eclipse (both umbra and penumbra)&#xA;    ascending and descending node crossing&#xA;    apogee and perigee crossing&#xA;    alignment with some body in the orbital plane (with customizable threshold angle)&#xA;    raising/setting with respect to a ground location (with customizable triggering elevation)&#xA;    date&#xA;    altitude crossing&#xA;    target detection in sensor field of view (circular or dihedral)&#xA;    complex geographic zones traversal&#xA;    impulse maneuvers occurrence&#xA;possibility of slightly shifting events in time (for example to switch from solar pointing mode to something else a few minutes before eclipse entry and reverting to solar pointing mode a few minutes after eclipse exit)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Attitude&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;extensible attitude evolution models&#xA;predefined laws:&#xA;    central body related attitude (nadir pointing, center pointing, target pointing, yaw compensation, yaw-steering)&#xA;    orbit referenced attitudes (LOF aligned, offset on all axes)&#xA;    space referenced attitudes (inertial, celestial body-pointed, spin-stabilized)&#xA;    tabulated attitudes&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Orbit file handling&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;loading of SP3-a and SP3-c orbit files&#xA;loading of CCSDS orbit data messages&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Atmosphere models&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;tropospheric delay (modified Saastamoinen)&#xA;geomagnetic field (WMM, IGRF)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Customizable data loading&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;loading from local disk&#xA;loading from classpath&#xA;loading from network (even through internet proxies)&#xA;support for zip archives&#xA;support from gzip compressed files&#xA;plugin mechanism to delegate loading to user defined database or data access library&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Localized in several languages&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;English&#xA;French&#xA;Galician&#xA;German&#xA;Greek&#xA;Italian&#xA;Norwegian&#xA;Romanian&#xA;Spanish&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="8342" LastActivityDate="2015-01-26T16:11:19.187" CommentCount="0" CommunityOwnedDate="2015-01-26T16:11:19.187" ContentLicense="CC BY-SA 3.0" />
  <row Id="7948" PostTypeId="1" CreationDate="2015-01-26T19:56:01.047" Score="12" ViewCount="6190" Body="&lt;p&gt;I thought it would be fun to make a basic simulation of a first-stage fly-back of SpaceX's Falcon 9, but was wondering what mathematics is used to perform something like this. Looking at this page: &lt;a href=&quot;http://en.wikipedia.org/wiki/Trajectory_of_a_projectile&quot;&gt;http://en.wikipedia.org/wiki/Trajectory_of_a_projectile&lt;/a&gt;, there is a nice equation for calculating the distance a projectile will cover, but this is based on a flat earth, constant gravitational acceleration and an absence of drag. Furthermore, using this formula would also imply an instantaneous delta-v change for it to be accurate (unless we re-calculate $d$ every second or so during engine burn in order to get a more accurate result), which is also rather unrealistic considering the time scales involved. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is this: If we consider the Falcon 9 first-stage to be a point mass so that we don't need to worry about 6 DOF attitude dynamics, and also only look at 2 dimensions instead of having to worry about a 3D spherical Earth, what equations would be used in order to accurately calculate the required burn time needed to get the Falcon 9 first-stage to within a few kilometres of its intended surface target when taking atmospheric drag and a circular Earth into consideration. Any links to informative websites covering something like this, or academic papers would be greatly appreciated. Thanks!&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="51" LastEditDate="2015-08-07T23:18:16.373" LastActivityDate="2016-08-10T22:39:53.050" Title="Mathematics used for SpaceX first stage re-entry burn guidance?" Tags="|spacex|falcon-9|reentry|trajectory|reuse|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7949" PostTypeId="1" AcceptedAnswerId="7954" CreationDate="2015-01-27T08:17:02.987" Score="62" ViewCount="19687" Body="&lt;p&gt;As alternatives to &lt;em&gt;vertical&lt;/em&gt; retro-engines landing:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;horizontal landing using &lt;em&gt;wings&lt;/em&gt;, already discussed in &lt;a href=&quot;https://space.stackexchange.com/questions/5103/why-is-it-preferable-for-spacex-to-land-their-booster-vertically-rather-than-fly&quot;&gt;Why is it preferable for SpaceX to land their booster vertically rather than fly it down with wings?&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;horizontal landing using using &lt;em&gt;airbags&lt;/em&gt;, not yet discussed.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;TSTO, the India project for reusable launcher, plans to test both vertical and horizontal options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/boqDU.jpg&quot; alt=&quot;enter image description here&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z1yon.png&quot; alt=&quot;enter image description here&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(source: &lt;a href=&quot;http://isp.justthe80.com/launchers/reusable-launch-vehicle---technology-demonstrator-rlv-td&quot; rel=&quot;noreferrer&quot;&gt;JustThe80.com&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is trying to shorten the distance between the landing site and the launch site for shorter launch cycles, so the use of the controlled landing, but it seems also possible to land horizontally without losing this advantage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To prevent a crash when reaching the ground, the sea or the barge, why not just switch, 30s before the landing, from thrust-controlled trajectory to an unpowered horizontal landing with &lt;strong&gt;parachutes and airbags&lt;/strong&gt;)&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2018-12-27T05:21:43.603" Title="Why does the SpaceX reusable launcher use vertical rather than horizontal landing?" Tags="|spacex|landing|falcon-9|" AnswerCount="6" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7950" PostTypeId="2" ParentId="7949" CreationDate="2015-01-27T08:31:16.207" Score="23" Body="&lt;p&gt;SpaceX have tried &lt;a href=&quot;https://space.stackexchange.com/questions/7718/why-would-spacex-not-use-parachutes-for-the-final-descent-of-the-first-stage&quot;&gt;using parachutes&lt;/a&gt; for landing the first stage, but they weren't happy with the results.&lt;br&gt;&#xA;A landing with airbags is more complicated: you need to attach parachutes to at least two locations (top and bottom of the stage). You still don't have fine control over the landing location. You need to land on a barge to prevent soaking the stage in seawater (which would add a lot of processing time to clean the stage). A barge landing requires exact control.&lt;br&gt;&#xA;Landing horizontally also means you need more space for the landing: in a vertical landing the footprint is 2x the length of the landing legs, in a horizontal landing you need an area at least as long as the stage.&lt;br&gt;&#xA;Airbags have never been used for landings of this size: the largest airbags I know of were for the Mars rovers Spirit and Opportunity. For Curiosity, NASA switched to a powered landing because they didn't think they could make airbags work. The Falcon 9 first stage is at least 10x heavier than Curiosity. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-01-27T08:31:16.207" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7951" PostTypeId="1" CreationDate="2015-01-27T09:26:15.227" Score="0" ViewCount="81" Body="&lt;p&gt;A high-altitude airship hitched to solid rocket boosters  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the helium-filled airship reaches its ceiling, the boosters would initiate a launch sequences, thus sending the airship into orbit. &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_airship&quot; rel=&quot;nofollow&quot;&gt;Orbital Airship&lt;/a&gt; &lt;a href=&quot;http://www.jpaerospace.com/atohandout.pdf&quot; rel=&quot;nofollow&quot;&gt;jpaerospace&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="7075" LastActivityDate="2015-01-27T09:26:15.227" Title="Theoretical: Could a heavily modified airship enter low Earth orbit?" Tags="|launch|rockets|launch-site|booster|" AnswerCount="0" CommentCount="2" ClosedDate="2015-01-27T14:01:52.550" ContentLicense="CC BY-SA 3.0" />
  <row Id="7952" PostTypeId="2" ParentId="7949" CreationDate="2015-01-27T09:49:18.650" Score="34" Body="&lt;p&gt;It is unnecessary to use anything but the rockets. For the same reason a helicopter uses its engine and rotor to land. It would be a bit ridiculous to land a helicopter with parachutes and airbags, wouldn't it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boeing CST-100 will use parachutes and airbags to land, and Elon Musk made fun of it in an interview, saying that its &lt;a href=&quot;https://www.youtube.com/watch?v=4Wrz8Yz3TtU&quot;&gt;drop test&lt;/a&gt; landing looked like a crash and that it is not the way to land a spaceship in the 21st century:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4Wrz8Yz3TtU?start=28&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vertical rocket powered landing is more reliable than parachutes and only adds fuel, no extra hardware mass. It also saves the runway and gives total flexibility to land anywhere anytime, also on airless bodies. It is obviously the best way to do it. The Russian &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikal_%28rocket_booster%29&quot;&gt;Baikal&lt;/a&gt; concept takes the horizontal landing concept to its extreme, I think. Adding jet engines, wings, landing gear and whatnot to the rocket boosters. It's like putting a booster on an ordinary airplane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To shorten the launch cycle, the landed first stage maybe could be lifted from the barge to land by a helicopter? The &lt;a href=&quot;https://en.wikipedia.org/wiki/Mil_Mi-26&quot;&gt;Mi-26&lt;/a&gt; is capable of carrying 20 tons 800 kilometers; the dry weight of Falcon 9's first stage is 16 tons.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="4592" LastEditDate="2015-01-27T20:04:23.227" LastActivityDate="2015-01-27T20:04:23.227" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7953" PostTypeId="1" CreationDate="2015-01-27T11:25:58.127" Score="5" ViewCount="137" Body="&lt;p&gt;How is accurate time of round-travel determined from downlinked ranging signal?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK Ranging signal, used to determine distance of a spacecraft is usually a reference wave modulated and uplinked, passed through PLL (Phase Locked Loop) thus demodulated and then again modulated to a downlink signal which is demodulated by PLL at receiving ground station.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the steps that follow in calculating the distance of the spacecraft?&#xA;How exactly is time of round trip calculated accurately ? How is time tagged to the wave downlinked?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A brief reference to &lt;a href=&quot;http://www.free-ebooks.com/book/189440/radiometric-tracking-techniques-deep-space-navigation-c-thornton-j-border-wiley-&quot;&gt;Radiometric Tracking Techniques for Deep Space Navigation&lt;/a&gt; - C Thornton, J Border (Wiley, 2003) helped me to get this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A Phase Locked Loop at receiving station produces a reference signal coherent with received signal. This reference signal is used by the ranging assembly to demodulate the downlink signal. The received range code is compared against a model of the transmitted range code to determine the round trip transit time. range measurements are quantized in steps referred to as range units(RU). The size of an RU depends on the frequency of the highest component of code, and is currently about 28cm. Doppler data are obtained by differencing the received reference signal with the station frequency reference.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Maybe I am connecting the dots wrong and completely unable to understand the process.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="1142" LastEditDate="2015-01-27T13:33:26.897" LastActivityDate="2015-01-28T14:23:01.083" Title="Range Measurement from Range signal downlink" Tags="|communication|tracking|ground-station|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7954" PostTypeId="2" ParentId="7949" CreationDate="2015-01-27T14:15:16.990" Score="64" Body="&lt;p&gt;In two words: Precision landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Underlying all of SpaceX's decisions is the desire to go to other planets, especially Mars.  For exploring the solar system, Elon Musk feels that precision landings are extremely important.  The precision landing requirement means that you want to start slowing the spacecraft before you reach the surface, which points towards wings or rockets.  Unfortunately, not every moon/planet in the solar system has an atmosphere capable of supporting flight (or at least, flight of heavy aircraft at reasonable speeds), which makes rockets a more appealing option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To hear Musk discuss this in his own words, watch &lt;a href=&quot;https://www.youtube.com/watch?v=PULkWGHeIQQ&quot;&gt;his talk&lt;/a&gt; at the &lt;a href=&quot;http://aeroastro.mit.edu/aeroastro100/centennial-symposium&quot;&gt;MIT AeroAstro Centennial Symposium&lt;/a&gt; in October 2014. He discusses wings vs rockets at &lt;a href=&quot;https://www.youtube.com/watch?v=PULkWGHeIQQ&amp;amp;t=4m37s&quot;&gt;4:37&lt;/a&gt;, and SpaceX vs Boeing (rockets vs airbags) at &lt;a href=&quot;https://www.youtube.com/watch?v=PULkWGHeIQQ&amp;amp;t=48m15s&quot;&gt;48:15&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/PULkWGHeIQQ?start=277&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="8347" LastEditorUserId="49" LastEditDate="2015-01-27T14:38:27.607" LastActivityDate="2015-01-27T14:38:27.607" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7955" PostTypeId="2" ParentId="7745" CreationDate="2015-01-27T17:21:40.650" Score="6" Body="&lt;p&gt;The winds are believed to be relatively constant, so the speed of the outpost relative to the surface should not be a factor.  If there are major wind shears at that altitude, then the outpost has other problems.  In fact, landing on something that naturally moves with the wind is easier than landing on a runway that is fixed relative to the wind.  A runway will often have crosswinds, whereas a floating outpost cannot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle could land on a little runway in Florida entering from orbit, so the same approaches (literally) could be used at Venus.  Landing at 1 bar at Venus would be a lot like landing at 1 bar at Earth.  You can imagine many approaches depending on the mass of the supplies, such as steered parachutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heat shield will need to be beefier, depending on if you are entering from a hyperbolic approach or from orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vehicle could use retrorockets to arrest the final approach speed.  Shuttle approach speeds were about 100 m/s, which isn't a huge amount of $\Delta V$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think that this is a problem we're going to have to solve anytime soon.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-01-27T17:21:40.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7956" PostTypeId="1" CreationDate="2015-01-27T19:45:38.257" Score="4" ViewCount="1141" Body="&lt;p&gt;After an overflight of the McGregor site, in a &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=35426.140&quot; rel=&quot;nofollow noreferrer&quot;&gt;thread at NASA Space Flight.com&lt;/a&gt; someone noticed an interesting structure:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RiVXA.jpg&quot; alt=&quot;McGregor, TX structure&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The theory being presented is that the size and hexagon pattern at the top resemble the trunk of a Dragon.  Which is very obvious from this photo:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ed2Q2.jpg&quot; alt=&quot;Inside a Dragon trunk&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The theories being postulated are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;DragonFly launch mount.&lt;/li&gt;&#xA;&lt;li&gt;Pad Abort test structure.&lt;/li&gt;&#xA;&lt;li&gt;Something unrelated.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The overall picture which also shows a Grasshopper mount structure, what looks like a Transporter/Erector extension is below (well cropped version, original was too big to upload):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gsSD1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-05-05T11:30:40.720" LastActivityDate="2018-07-30T01:23:20.400" Title="What is this structure at the SpaceX McGregor, TX test site?" Tags="|launch|spacex|dragon|testing|mcgregor-tx|" AnswerCount="2" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7957" PostTypeId="2" ParentId="7948" CreationDate="2015-01-27T22:38:27.623" Score="2" Body="&lt;p&gt;You're not facing a mathematical problem, really, this is physics. Mathematics is just one of the tools needed. You also need drag models (aerodynamics) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And realistically this is solved by numerical methods. You calculate the time series &lt;code&gt;x[t], y[t], vx[t], vy[t], Fx[t], Fy[t], m[t]&lt;/code&gt; for every time step &lt;code&gt;dt&lt;/code&gt;. This replaces the calculation of the related differential equations and integrals.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2015-01-27T22:38:27.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7958" PostTypeId="2" ParentId="7948" CreationDate="2015-01-27T23:59:57.497" Score="9" Body="&lt;p&gt;You really want to break down your exploration into a number of projects.  I'm going to assume you have some mathematical and physics background.  Otherwise, getting those is task #0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, you want to build a Runge Kutta integrator that will numerically integrate a system of differential equations from an initial condition state vector forward to an arbitrary moment in time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, you want to derive the equations of motion for your vehicle.  I would start with simply dropping an uncontrolled mass with some initial velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, implement your control system of choice into the system of equations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, employ optimal control theory to minimize a cost function of choice (minimize propellent required perhaps?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No, these are not necessarily simple things to do.  But important things rarely are.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-01-27T23:59:57.497" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7959" PostTypeId="2" ParentId="7948" CreationDate="2015-01-28T04:32:29.587" Score="5" Body="&lt;p&gt;Chapter 3 of Hicks' text &lt;em&gt;Introduction to Astrodynamic Reentry&lt;/em&gt; provides an overview of the derivation of the reentry equations of motion.  The text can be found at &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a505342.pdf&quot; rel=&quot;noreferrer&quot;&gt;this link&lt;/a&gt;, and also provides an introduction into reentry control methods such as aerobraking.  The translational reentry equations are highly-coupled, non-linear ordinary differential equations that require numerical routines (e.g., RK integrators) for solutions, unless some very simplifying assumptions are made (e.g., planar entry, constant bank angles, etc.).  This &lt;a href=&quot;https://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section%20III.4.1.7%20Returning%20from%20Space.pdf&quot; rel=&quot;noreferrer&quot;&gt;reference&lt;/a&gt; at FAA.gov also provides a reentry discussion using a kinetic / potential energy tradeoff formulation, which could also be a good way to start thinking about your project.&lt;/p&gt;&#xA;" OwnerUserId="1478" LastActivityDate="2015-01-28T04:32:29.587" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7960" PostTypeId="1" AcceptedAnswerId="7961" CreationDate="2015-01-28T05:00:45.887" Score="10" ViewCount="736" Body="&lt;p&gt;If we discovered a sizeable (Tunguska-like mass or larger) near-earth object tomorrow that was on a definite collision course with Earth in the near future, could we deal with it with our current levels of technology?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretical asteroid avoidance or deflection protocols are interesting to analyze, but I'm curious about a situation where we get caught unawares, Chelyanbinsk-style, and suddenly discover a previously-unknown large object that's aimed right for us. What, if anything, could we do with the technology and space programs available today?&lt;/p&gt;&#xA;" OwnerUserId="6772" LastActivityDate="2015-01-29T00:06:37.710" Title="Could we deal with an asteroid threat given current technology?" Tags="|asteroid|technology|impact|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7961" PostTypeId="2" ParentId="7960" CreationDate="2015-01-28T08:44:42.047" Score="13" Body="&lt;p&gt;At this point in time it seems doubtful that we could intercept and deflect an asteroid large enough to justify being deflected. This is simply a matter of momentum, a large asteroid has a great deal of momentum and the puny little spaceships we can presently intercept an asteroid with can impart only very little momentum. The only way we can substantially alter this formula is by using nuclear weapons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Ongoing_projects&quot;&gt;wikipedia article&lt;/a&gt; states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If the object is very large but is still a loosely held together&#xA;  rubble pile, a solution is to detonate a series of nuclear explosive&#xA;  devices alongside the asteroid, far enough away as not to fracture the&#xA;  potentially loosely held together object. Providing this stand-off&#xA;  strategy was done far enough in advance, the force from any number of&#xA;  nuclear blasts would be enough to alter the object's trajectory to&#xA;  avoid an impact. By the 2020s NASA has concluded that 1 mission&#xA;  utilizing nuclear stand off, can deflect NEOs of 100–500-metre&#xA;  (330–1,640 ft) diameters two years before the estimated Earth impact,&#xA;  and larger NEOs with a five year warning.[38]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A NASA analysis of deflection alternatives, conducted in 2007,&#xA;  stated:[39]&lt;/p&gt;&#xA;  &#xA;  &lt;blockquote&gt;&#xA;    &lt;p&gt;Nuclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives analyzed in this study.&#xA;    Other techniques involving the surface or subsurface use of nuclear&#xA;    explosives may be more efficient, but they run an increased risk of&#xA;    fracturing the target NEO. They also carry higher development and&#xA;    operations risks.&lt;/p&gt;&#xA;  &lt;/blockquote&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Nuclear standoff (detonating a nuclear warhead near the asteroid) would seem to be feasible, but would require at least 2 years warning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one way to deal with an incoming asteroid, not actually precluded by the question, that we definitely have the technology for - evacuate the area and let it hit us. These asteroids, while destructive, are not &lt;em&gt;that&lt;/em&gt; destructive. The Tunguska event, for all it's boom, was only like a large nuclear bomb. Humans have detonated larger nuclear bombs on the surface of Earth, most famously the Castle Bravo nuclear test, and the Tzar Bomb. We can survive being hit by an asteroid. Most of Earth's surface is sparsely or unpopulated and an asteroid impact would carry little danger to humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even much, much larger asteroid impacts leave no evidence in the fossil record of species extinction. For example the &lt;a href=&quot;http://en.wikipedia.org/wiki/Manson_crater&quot;&gt;Manson Crater&lt;/a&gt; in USA:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Most researchers agree that the 35-km-diameter Manson Crater of Iowa, a possible source of shocked quartz in the K-T boundary sections, was too small and insignificant to have caused mass extinctions. &#xA;  &lt;a href=&quot;https://books.google.bg/books?id=viHwAAAAMAAJ&amp;amp;pg=PP28&amp;amp;lpg=PP28&amp;amp;dq=manson+crater+no+recorded+extinctions&amp;amp;source=bl&amp;amp;ots=czCwEO6B8g&amp;amp;sig=QhfjyE0abp0D8j9_IxNdbY8vsSE&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=-ZbIVPmNLYbVygOrkIKgCg&amp;amp;ved=0CCUQ6AEwAQ#v=onepage&amp;amp;q=manson%20crater%20no%20recorded%20extinctions&amp;amp;f=false&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The impactor in that case is thought to have been a 2km diameter stony meteorite, much larger than the Tunguska event impactor, and it is still small and insignificant when it comes to causing extinction. Now this is not to say that such a large impact would not be incredibly disruptive, but it would certainly be survivable, it is not like there is an imperative to avoid it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This makes evacuation a legitimate strategy, the impact is only going to be extremely lethal at ground zero, or where it raises tsunamis if it lands in the ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With forewarning - even only weeks or days - there would be time to relocate people who would be at ground zero. Those areas which would be at risk from secondary effects such as ejecta, tsunamis, earthquakes and volcanic eruption could also be evacuated, or at least emergency services could be on heightened alert.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that if the asteroid is broken up rather than deflected while still somewhat distant from Earth, it would cause much, much more damage. This is the MIRV warhead principle, a lot of smaller explosions spread over a large area causes much more damage than one large explosion. Thus intercept is not without risk and it might be safer to take the hit than to risk breaking up a 'rubble pile' asteroid with a nuclear explosion. Nevertheless, if the asteroid is aimed for a major city, a mission to deflect it might well be cheaper than the costs of dealing with a natural disaster of that magnitude. For example cleanup/reconstruction costs for large earthquakes which level a major city measure in 10's of billions, while space missions are often only in the 100's of millions or a few billion. In the event of a major impactor, it would come down to a cost analysis like this, how much would it cost to deflect it, versus the costs of dealing with the damage an impact would cause?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In sum, early detection may well be the best form of preparation we have, as it gives time for orderly evacuation of the danger zone, and the relevant emergency services can be ready to respond.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-01-28T08:44:42.047" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7962" PostTypeId="2" ParentId="7960" CreationDate="2015-01-28T09:46:13.400" Score="3" Body="&lt;p&gt;It is possible we could deal with an asteroid large enough to warrant action, but only if we had many years of warning. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We cannot destroy a large asteroid or comet, that's science fiction now, the best we can do is deflect it. The technology we have now is only capable of causing very small course changes to large objects, however a small change is still enough if it happens soon enough as even small changes will add up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key here would be very early detection which is the major problem. Technologically we can do much better than we are at detecting dangerous space objects, however IMO there is a lack of investment which means we don't use what we have to best advantage. The technology could also be substantially improved with investment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also no deflection system ready to go, if we did discover an asteroid which poses a threat it would take months or years of precious time to design, build, and launch one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What this all adds up to is that it is possible that we could detect and deflect an asteroid with today's technology, but unlikely it would actually happen. We'd have to get lucky. With investment in developing the technology and having something ready to do quickly we could substantially improve our chances and luck would have less to do with it. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-01-28T09:46:13.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7964" PostTypeId="2" ParentId="7953" CreationDate="2015-01-28T14:23:01.083" Score="3" Body="&lt;p&gt;From page 144 of &lt;a href=&quot;http://descanso.jpl.nasa.gov/dstse/DSTSE.pdf&quot; rel=&quot;nofollow&quot;&gt;Yuen (1983), &quot;Deep space telecommunication systems engineering,&quot; &lt;em&gt;Plenum Press&lt;/em&gt;&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The transmitted ranging signal is generated by coherent frequency division from the frequency reference used to generate the uplink carrier. Usually a code consisting of a succession of frequencies is generated, starting with approximately 1 MHz and decreasing by factors of two to as low as approximately 1 Hz.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The spacecraft returns this uplinked time-varying code back to Earth. That the highest frequency is 1 MHz limits the range determination to 300 meter accuracy (at best). That's not bad for a spacecraft that is many astronomical units from Earth. The received downlink from the satellite provides the basis by which the receiving station determines the range to the satellite. Simply look for the transition between frequencies, and voila! the range is determined. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, it's not as simple as that. The linked document goes into details as to why it's not that simple. But that is the basic concept.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-01-28T14:23:01.083" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7965" PostTypeId="1" AcceptedAnswerId="7966" CreationDate="2015-01-28T14:27:15.777" Score="7" ViewCount="2350" Body="&lt;p&gt;What is a &quot;payload adapter&quot;? For example, from the following page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The IXV Intermediate eXperimental Vehicle installed on its &lt;strong&gt;payload adapter&lt;/strong&gt;, on 26 January 2015 at Europe's Spaceport in Kourou, French Guiana.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&amp;mdash; &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2015/01/IXV_installed_on_its_payload_adapter2&quot; rel=&quot;nofollow&quot;&gt;IXV Installed on its Payload Adapter&lt;/a&gt;, ESA Space in Images&lt;/p&gt;&#xA;" OwnerUserId="6753" LastEditorUserId="881" LastEditDate="2015-01-28T20:06:51.620" LastActivityDate="2015-01-28T20:06:51.620" Title="What is a &quot;payload adapter&quot;?" Tags="|payload|integration|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7966" PostTypeId="2" ParentId="7965" CreationDate="2015-01-28T14:38:06.833" Score="12" Body="&lt;p&gt;Payloads come in different shapes and sizes. To put them on a specific launch vehicle, you need to either design the payload to fit perfectly onto the rocket or you need some sort of &lt;strong&gt;adapter&lt;/strong&gt; to mount it in place. It's also common to launch multiple payloads with a single launch, so you need some mechanism to separate the different payloads and release them into their respective orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information, see &lt;a href=&quot;http://en.wikipedia.org/wiki/EELV_Secondary_Payload_Adapter&quot;&gt;the Wikipedia article on the ESPA&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-01-28T14:38:06.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7967" PostTypeId="1" AcceptedAnswerId="7968" CreationDate="2015-01-28T15:45:47.137" Score="7" ViewCount="1819" Body="&lt;p&gt;When I was flying in the plane the other day, I just noticed how many systems checks are completed before the plane even takes off. This makes sense, considering how many human lives depend on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, being software tester, I know how this &lt;em&gt;routine&lt;/em&gt; can affect your daily performance. It doesn't matter if I'm check multi-billion dollar business software; after several rounds of testing, it becomes &lt;em&gt;very&lt;/em&gt; routine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For plane operators or astronauts, this feeling can be &lt;em&gt;deadly.&lt;/em&gt; So what do astronauts do in order to avoid that &lt;em&gt;routine feeling?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="68" LastEditDate="2015-01-28T17:11:21.533" LastActivityDate="2015-12-11T04:45:44.983" Title="How do astronauts overcome the boredom from &quot;the daily routine&quot;?" Tags="|astronauts|psychology|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7968" PostTypeId="2" ParentId="7967" CreationDate="2015-01-28T21:28:54.613" Score="4" Body="&lt;p&gt;Mary Roach's &lt;a href=&quot;http://www.maryroach.net/packing-for-mars.html&quot; rel=&quot;nofollow&quot;&gt;book&lt;/a&gt; &lt;em&gt;Packing for Mars:  The Curious Science of Life in the Void&lt;/em&gt; gives an interesting look at almost this exact question (with a humorous undertone).  It's a very easy read with some pop-flair to it that addresses specific experiments that have been done to combat the inherent psychological issues of isolation and physiological effects as well.  Like Nick T commented above, a daily schedule is managed by the ground to include scientific experiments, PA time, relaxation, etc.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the job titles likely involved in vetting / constructing this schedule can be found &lt;a href=&quot;http://www.nasa.gov/audience/foreducators/k-4/features/F_People_Behind_the_Astronauts.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; and include the Ground Controller, Flight Activities Officer, Flight Dynamics Officer, and Flight Director, among other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA also has a &lt;a href=&quot;http://www.spaceflight.nasa.gov/living/spacefun/index.html&quot; rel=&quot;nofollow&quot;&gt;Space Fun page&lt;/a&gt; that lists some activities...where &quot;looking out the window&quot; is a popular pastime.  If you're ever curious, you &lt;a href=&quot;http://www.nasa.gov/multimedia/nasatv/iss_ustream.html&quot; rel=&quot;nofollow&quot;&gt;can watch the crew on the ISS&lt;/a&gt; (when available) or check astronaut schedules and current activity via &lt;a href=&quot;http://spacestationlive.jsc.nasa.gov/timeline/timelineAstronaut.html&quot; rel=&quot;nofollow&quot;&gt;ISSLive!&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1478" LastEditorUserId="49" LastEditDate="2015-01-28T22:21:49.037" LastActivityDate="2015-01-28T22:21:49.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7969" PostTypeId="2" ParentId="7949" CreationDate="2015-01-28T22:15:22.780" Score="4" Body="&lt;p&gt;Adding to the advantages that others have mentioned ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long ago the head of the now long defunct ROTRON project commented that &quot;Wings are the most expensive form of recovery&quot; - and then explained that by using a &quot;rocket powered helicopter&quot; for the in-atmosphere portion of ascent they were effectively delivering a set of wings to orbit at zero net energy cost so they could use them for descent. If your wings 'go along for the ride' without 'paying their way' as ROTRON's did, they represent a net expense on the mass budget. 1st stage recovery does not require taking your descent mechanism to orbit but is still a cost to orbitable payload. Given that orbited payload as a % of launch mass is typically ijn the 1%-5% range, every saved kg helps. (Every saved pound helps too :-) ). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost of using fuel in place of wings, parachutes, airbags etc is slightly more tankage, the fuel mass required and possibly added motor reliability and run-life.&lt;br&gt;&#xA;Those who have pursued this path have always claimed that the gains outweigh the costs.&lt;br&gt;&#xA;But, then, they would :-). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gains are percent payload plus all the things others have listed.&lt;/p&gt;&#xA;" OwnerUserId="8362" LastActivityDate="2015-01-28T22:15:22.780" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7970" PostTypeId="2" ParentId="7960" CreationDate="2015-01-29T00:06:37.710" Score="0" Body="&lt;p&gt;Your question doesn't limit it nearly well enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We almost certainly could intercept a Tunguska-sized rock on a notice of days.  Such a rock is small enough that brute force at the last instant will do the job, fit a standoff detonator to an ICBM and launch it into the path of the rock--you need just enough standoff distance that the warhead goes off before it's crushed by the impact.  The rock is blown apart, the fragments burn, the biggest threat is the EMP from the warhead.  (Which might actually make this not a good idea.)  Note that it merely needs to get in the path of the rock, it doesn't need to match velocities--hence the need of a standoff detonator as it's going to be smacked at several miles per second.  Sure, the fragments will still come down pretty close to each other but remember Chelyabinsk didn't do anything beyond break glass--a bunch of Chelyabinsks in close proximity would likewise not be a big issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Blowing the rock apart is only an answer for small stuff where the fragments will burn without causing devastation.  Blowing apart a bigger rock is worse than doing nothing at all--blowing a large rock into 8 equal pieces approximately doubles the damage it causes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, there's a political factor here.  What we could accomplish and what would be accomplished aren't necessarily the same thing.  Politicians love to cut corners.  Suppose we are going to nudge it aside with nukes.  The blasters say we need 30 bombs on target.  The rocketeers say there's an 80% chance a booster functions correctly and fires it's warhead at exactly the right spot.  How many boosters will fly?  I would be surprised if more than 40 rockets fly despite the fact that that gives nothing like certainty of deflection.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-01-29T00:06:37.710" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7972" PostTypeId="1" CreationDate="2015-01-29T01:45:56.250" Score="5" ViewCount="362" Body="&lt;p&gt;The new SpaceX Heavy animation shows it being launched from LC-39A at Cape Canaveral, FL then all F9 cores returning to the Cape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4Ca6x4QbpoM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does SpaceX plan on doing a full 180° and returning to the same launch facility? I thought that the land in south Texas they started developing was for launch, and down-range in Florida  the boosters could land, as they were basically headed that way anyhow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The penalty of doing the 180° (kill &lt;em&gt;all&lt;/em&gt; forward v, then burn about that much again to make it back) seems extreme versus a smaller retro burn that tweaks your &quot;orbit's&quot; intersection point with the surface.&lt;/p&gt;&#xA;" OwnerUserId="881" LastActivityDate="2015-01-29T03:54:38.350" Title="Are the SpaceX boosters slated to return to their launch site or somewhere downrange?" Tags="|spacex|launch-site|reuse|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7973" PostTypeId="2" ParentId="7972" CreationDate="2015-01-29T03:54:38.350" Score="7" Body="&lt;p&gt;SpaceX seems to have developed a launch system that has a great deal of flexibility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fully expendable mode, the F9 and FH are very capable boosters. But more expensive if you throw them away after one use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fully reusable mode, with Return to Launch Site (RTLS) you waste a lot of potential payload, on fuel for RTLS. (Up to 30% payload hit on a Falcon 9.  Unclear on a FH).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But they have the ability to go somewhere in between as well.  With the ASDS they potentially can position it downrange so that a minimized amount of fuel needs to be reserved for recovery.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a Falcon Heavy, the outer two core boosters separate earlier, thus lower and slower than a normal Falcon 9 first stage. Thus the penalty for RTLS is lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But on a cross fed, full payload mission the center core is going much higher and much faster than a F9 first stage, so the RTLS penalty would be higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again the flexibility options remain to eat the cost, and ditch the center core, or else position the ASDS as far downrange as possible to reduce payload reduction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the specific answer is sometimes yes, sometimes no, depends on the payload requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS: Mighy dang fine video too by the way.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-01-29T03:54:38.350" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7977" PostTypeId="2" ParentId="5380" CreationDate="2015-01-29T16:45:38.487" Score="7" Body="&lt;p&gt;A connection between two different periodic orbits around Lagrange / libration points is often termed a heteroclinic connection.  Although the L1 and L2 points in the Earth-Moon system are themselves unstable equilibrium points, there are stable periodic (or quasi-periodic) orbits that can be found around each point.  From these orbits, it is possible to find trajectories (manifolds) that connect the orbits and provide cheap transfers.  These manifolds are termed stable or unstable depending on whether they are directed to or away from the periodic orbit, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider this scenario that could result in a cheap transfer:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Establish a periodic orbit about a Lagrange point&lt;/li&gt;&#xA;&lt;li&gt;Perform a maneuver to approximate being on an unstable manifold from this orbit that is also a heteroclinic connection to another periodic orbit's stable manifold&lt;/li&gt;&#xA;&lt;li&gt;Ride this heteroclinic connection for free until reaching the vicinity of the other periodic orbit&lt;/li&gt;&#xA;&lt;li&gt;Perform a maneuver that places you on the other periodic orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For a specific example, consider the &lt;a href=&quot;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Journals/2001_JAS_Genesis.pdf&quot;&gt;Genesis mission design&lt;/a&gt;, which employs manifold transfers between periodic orbits.  Technically, there is some math that says it takes infinite time to arrive from the manifold, so if we could wait an infinite time, it would be &quot;free&quot;--but we don't have time for that, so maneuvers will solve that issue.  Here's a &lt;a href=&quot;http://www.cds.caltech.edu/~koon/papers/hetero_halos_4-12-00.pdf&quot;&gt;paper from CalTech&lt;/a&gt; that looks at heteroclinic connections between perioidic orbits--but not specifically in the Earth-Moon system (but, as an aside, the great thing about the system dynamics in the circular-restricted three body problem, the only real free parameter is the system mass ratio, usually $\mu$, that you can easily change to represent the system)&lt;/p&gt;&#xA;" OwnerUserId="1478" LastActivityDate="2015-01-29T16:45:38.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="7978" PostTypeId="1" AcceptedAnswerId="9078" CreationDate="2015-01-29T23:00:29.233" Score="19" ViewCount="3234" Body="&lt;p&gt;Agencies/organizations who own/operate satellites want to know their future positions after a certain amount of time. We know that models of satellite dynamics are imperfect and also perturbations effects are often hard to predict/quantize.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the present (January 2015) technology, state of research, knowledge, data we can acquire..., &lt;strong&gt;how accurately can future positions of Earth orbiting satellites be predicted? Or in other words, maximal possible accuracy of prediction we can acquire presently? (accuracy of propagated trajectory)&lt;/strong&gt; Given the initial state of the satellite, which is for this example considered 100% accurate reference. Also assuming that we get the best data on perturbation effects that is presently possible to acquire. Answer in terms of distance units would be most helpful, otherwise order of magnitude is all right as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time intervals in question could be 1 hour, 12 hours and 24 hours, between present satellite position and the final one we are interested in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if the answer depends on orbit type, then I'd like to know for say 3 typical reference orbits, like:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ISS orbit&lt;/li&gt;&#xA;&lt;li&gt;GPS satellites orbit&lt;/li&gt;&#xA;&lt;li&gt;Geostationary orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="6750" LastEditDate="2015-01-30T11:36:22.800" LastActivityDate="2018-08-13T20:51:31.813" Title="How accurately (maximum possible accuracy) can future satellite positions be predicted?" Tags="|orbital-mechanics|artificial-satellite|orbit|calculation|prediction|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="7980" PostTypeId="1" CreationDate="2015-01-30T05:39:43.717" Score="8" ViewCount="1951" Body="&lt;p&gt;My understanding of the Falcon Heavy cross feed system is that the outer cores feed six of the nine engines directly on the center core.  Thus, the cross feeds don't pump propellent and oxidizer into the center core tanks but rather feed six of the nine center core engines directly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is very similar to how the Space Shuttle worked.  The Shuttle's cross feed between the External Tank and the Orbiter was 17&quot; in diameter -- one for propellent, one for oxidizer.  A project to reduce the size of the cross feed to 14&quot; was cancelled.  A video of the disconnect being tested can be seen here: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/JD5uWlxDjKo?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some questions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How large are the cross feeds between the Falcon Heavy's outer cores and the inner core?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there four (2 for propellent, 2 for oxidizers)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has cross-feeding been done on any other vehicles other than the Shuttle and the Falcon Heavy?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the pumps reside on the center core?&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="6537" LastEditDate="2015-03-30T18:26:38.957" LastActivityDate="2015-09-01T11:00:24.733" Title="Questions regarding Falcon Heavy's crossfeed system" Tags="|launch|spacex|falcon-heavy|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7981" PostTypeId="1" CreationDate="2015-01-30T07:49:49.423" Score="2" ViewCount="9269" Body="&lt;p&gt;Why does a spacecraft use a curved orbit to go to its destination, instead of travelling to its destination directly? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GIsGT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8382" LastEditorUserId="1142" LastEditDate="2015-02-07T23:56:30.990" LastActivityDate="2015-02-09T13:26:48.160" Title="Why does a spacecraft use a curved orbit to go to its destination, instead of traveling in a straight line?" Tags="|artificial-satellite|orbit|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="7982" PostTypeId="2" ParentId="7981" CreationDate="2015-01-30T09:41:54.803" Score="21" Body="&lt;p&gt;This field of study is called &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_mechanics&quot;&gt;Orbital Mechanics&lt;/a&gt;. An object in orbit behaves in ways that are counterintuitive if you don't realize fully what's going on.&lt;br&gt;&#xA;The orbit you've drawn is almost impossible to achieve. When a spacecraft travels from Earth to Mars, it orbits the Sun. Anything it does is influenced by the gravitational attraction of the Sun. When you leave Earth and you accelerate, you won't fly in a straight line: your path will be curved because the Sun keeps pulling on you. This is the curve you see in the MOM trajectory.&lt;br&gt;&#xA;If you really want a trajectory that is straight, you'll have to expend lots of fuel counteracting the Sun's pull. This takes much more fuel than a curved orbit.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-01-30T09:41:54.803" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7983" PostTypeId="2" ParentId="7980" CreationDate="2015-01-30T12:15:12.977" Score="7" Body="&lt;p&gt;Cross-feeding has been used (sort of) on the &lt;a href=&quot;https://en.wikipedia.org/wiki/SM-65_Atlas&quot;&gt;early versions of Atlas&lt;/a&gt;: these had booster engines that were fed from the first stage tanks. These engines were jettisoned while the first stage was still running. Of course this is a simpler construction than the proposed design for Falcon Heavy.&lt;br&gt;&#xA;The Space Shuttle also isn't really comparable: the SSME's are fed only from the ET, there is no switchover to another tank.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will be the critical point for Falcon Heavy: they need to switch the engines on the center core from one set of tanks to the other, without interrupting the flow of oxygen and propellant. High-speed turbopumps are sensitive to flow issues, and inserting a gas bubble into the liquid flow could be disastrous.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, when you close a valve in a high-speed flow of liquid, it's difficult to avoid creating a &lt;a href=&quot;https://en.wikipedia.org/wiki/Water_hammer&quot;&gt;fluid hammer&lt;/a&gt; (a pressure spike traveling upstream from the valve). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-01-30T12:15:12.977" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7984" PostTypeId="2" ParentId="7980" CreationDate="2015-01-30T14:25:25.777" Score="6" Body="&lt;p&gt;There is an image floating around, I do not recall where I got, called the LOXtopus, which is the manifold at the bottom of the tank showing the plumbing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MUpEd.jpg&quot; alt=&quot;LoxToPus Manifolfd&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a total of ten connections.  Nine are for the nine engines hosted locally.  The tenth is for draining/filling.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="63" LastEditDate="2015-03-30T18:07:33.810" LastActivityDate="2015-03-30T18:07:33.810" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="7985" PostTypeId="1" AcceptedAnswerId="8180" CreationDate="2015-01-30T18:27:39.337" Score="9" ViewCount="751" Body="&lt;p&gt;Today I was playing Alien: Isolation and saw a poster regarding lice and how you ought to get checked to reduce the risk of spreading it. (I give you this context to let you know that I am generally thinking towards the future, but my question is in regards to small spaces in the context of space travel and should be applicable today.) My thought initially was, &quot;Well that's a good idea, disease, infection, viruses, and things like lice must be a big problem due to the isolated and cramped nature of space travel&quot;. But then I thought that perhaps that was also working in its favor. Might it be easier to control if fewer people were coming and going and it remained &quot;contained&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at &lt;a href=&quot;https://space.stackexchange.com/a/2432/7027&quot;&gt;this answer&lt;/a&gt;, I can see that there are considerable precautions to eliminating infection aboard the ISS. I think this is smart because it is very expensive to bring supplies up and much cheaper to make sure your astronauts are healthy before they go. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can see that outbreaks happen in close public places, like &lt;a href=&quot;http://www.nbcnews.com/health/health-news/disney-measles-outbreak-came-overseas-cdc-says-n296441&quot; rel=&quot;nofollow noreferrer&quot;&gt;Disneyland&lt;/a&gt; but also in more common places like a house with several people inside. I think we've all experienced this at some point; someone in your family or house gets sick and it jumps from person to person and can take a long time to &quot;get rid of&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is my question: is a disease/virus/infection more hazardous in small spaces like the ISS or any other space vehicle, or does the isolation make it potentially easier to deal with? I'm not interested in the economic costs, because I think that it is rather obvious that is far more expensive to have an infected crew than a healthy one. I'm only interested in the potential risks to the health of the crew for current and future spaceflight.&lt;/p&gt;&#xA;" OwnerUserId="7027" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-02-15T18:43:23.913" Title="How serious is a contagion in space flight?" Tags="|iss|health|travel|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7986" PostTypeId="1" CreationDate="2015-01-30T21:30:20.217" Score="17" ViewCount="4092" Body="&lt;p&gt;(The question &lt;a href=&quot;https://space.stackexchange.com/questions/844/day-versus-night-launches/&quot;&gt;Day versus night launches&lt;/a&gt; is similar but the answers there, that time of day is irrelevant, seem to conflict with my data which shows clear biases towards certain times)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I feel like most of the interesting NASA launches happen before I get up in the morning: I live out West and like to sleep in, and it seems that whenever I start seeing excitement on NASA's Twitter accounts or on NASA TV for an upcoming launch and I look up details, I find that the launch is going to happen before my alarm clock goes off.  I was curious if this was true so I tried to make a graph of launch-local time:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5dhqz.png&quot; alt=&quot;launches per hour&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, with data from &lt;a href=&quot;http://planet4589.org/space/log/launch.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JSR's launchlog&lt;/a&gt; and looking just at launches from Cape Canaveral and Kennedy Space Center, I partly confirm my hunch by seeing that a big chunk of launches happen in the morning with a significant drop-off at 9am Pacific time.  However I also see that there's an even bigger component in their evening (yay for me!) and this leads me to ask why these two chunks of time (7am-noon and 6pm-9pm) seem to be the most popular times for launches.  And a follow-on, why the evening hours seem to be the most popular with launches from 6-7pm happening at almost double the rate of morning launches and about four times as often as afternoon and night launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I won't pretend my data is accurate or complete, but it passes my sniff test: lots of launches in those morning hours, and fewer launches at night than in daylight.  But I wouldn't be surprised if someone can get more accurate data that shows something different or finds a mistake in my methodology:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After downloading the 1.6MB (don't be put off by the &quot;LARGE&quot; warning) launchlog.txt, I ran these commands (they're pretty hacked-together so don't judge me too harshly, and among other things they rely on both launch sites having at least 1 launch per hour, which is part of why I didn't include Wallops launches in my graph) to get just the Canaveral &amp;amp; KSC launches, convert UTC to Eastern, and plot them by hour:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;sites=&quot;CC KSC&quot;; for launchsite in $sites; do echo trying $launchsite; cut -b 14-29,160-163 launchlog.txt | grep -E $launchsite[[:space:]]*$ | awk '{print $2, $3, $1, $4}' | while read utc; do TZ='America/New_York' date -d &quot;$utc UTC&quot; +%H; done | sort -n | uniq -c &amp;gt; site$launchsite.out; done; gnuplot -e &quot;set terminal pngcairo; set xrange[-1:24]; set boxwidth 0.75; set style data histograms; set style fill solid; set style histogram rowstacked; set multiplot; set xlabel 'hour of day'; set ylabel '# of launches'; plot 'siteCC.out' using 1 , 'siteKSC.out' using 1&quot; &amp;gt; launch_tod.png&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;hr&gt;&#xA;There was a very good point made that this data goes back to the very earliest rocket launches, and launch operations have matured significantly over that time.  I would have liked to include only more-recent launches, but there just aren't that many.  Here is a harder-to-read graph that shows the top 7 launch sites from Jan 1st 2000 through the first launch of 2015: Baikonur (304 launches), Canaveral (148), French Guiana (109), Plesetsk (96), Vandenberg (63), Xichang (58), and Jiuquan (48).  If I did everything right, times are for launchsite-local time (the extra launch at '0' is a CSG launch with a date but no hour).  A couple sites appear to have some trends towards preferred times but I think there's just too little data to make any real conclusions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BjklM.png&quot; alt=&quot;launches since 2000, top 7 sites&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pastebin.com/nu2xTf9K&quot; rel=&quot;nofollow noreferrer&quot;&gt;data since 2000: launches for all launch sites and top-7 hourly numbers on pastebin&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user1556" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-12-29T18:02:43.997" Title="Why do certain times of day have more launches than others?" Tags="|launch|nasa|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7987" PostTypeId="2" ParentId="5742" CreationDate="2015-01-31T04:03:52.100" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;then why was this point in flight obviously destructive when the&#xA;  attitude changed?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The vehicle was just past &lt;em&gt;maximum&lt;/em&gt; dynamic pressure (&lt;a href=&quot;https://en.wikipedia.org/wiki/Max_Q&quot; rel=&quot;noreferrer&quot;&gt;max Q&lt;/a&gt;) so the aerodynamic loads on the launch vehicle and orbiter were near maximum when the breakup began.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I've read, both the hydrogen and oxygen tanks were breached at approximately the same time thus, the breakup of the ET happened very quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the ET suddenly disintegrated, the attitude of the orbiter, with respect to the velocity vector changed, which, in combination with stresses from the attach points to the disintegrating ET, quickly broke the orbiter apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you carefully watch a slow motion video of the initial moments of the ET breakup, you can see the nose and crew compartment rotate 'down' in a way that it couldn't possibly do if it were still attached to the payload bay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a contemporary example, consider the &lt;a href=&quot;https://en.wikipedia.org/wiki/VSS_Enterprise_crash&quot; rel=&quot;noreferrer&quot;&gt;recent break-up of Spaceship 2&lt;/a&gt; when its attitude (evidently) suddenly changed with respect to its velocity vector due to the tail booms prematurely feathering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm reminded of a quote by Story Musgrave that the shuttle is ...&lt;em&gt;&quot;very fragile. A butterfly bolted onto a bullet, you know.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If it has a heat shield which can withstand the tremendous&#xA;  aerodynamics of reentry&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;structural&lt;/em&gt; loads on the orbiter are what are relevant here.  The loads during (a nominal) re-entry are quite a bit less than the loads during ascent.&lt;/p&gt;&#xA;" OwnerUserId="1035" LastEditorUserId="1035" LastEditDate="2015-01-31T04:09:20.407" LastActivityDate="2015-01-31T04:09:20.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7990" PostTypeId="1" CreationDate="2015-01-31T21:53:44.093" Score="5" ViewCount="17112" Body="&lt;p&gt;I know that rocket launches are extremely complex. From watching a few, I get the sense that there are some things that happen in a standard way in all rocket launches, maybe depending on propellants, booster combinations, and payloads they are more variable than I think. I feel like if I understood what those steps are about, at least a little, it would give me a sense of how the rocket works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, loading of propellants happens at a certain time, why then? I can only find two run-downs of pre-launch sequences, one for Falcon and one for the Space Shuttle. Falcon:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;T-0:02:00   Align Flight Computers to Self Alignment    &#xA;T-0:02:00   Range Verification  &#xA;T-0:02:00   Flight Control to Self Alignment    &#xA;T-0:01:35   Helium Loading Termination  &#xA;T-0:01:30   Final Engine Chilldown, Pre-Valves/Bleeders Open    &#xA;T-0:01:20   Engine Purge    &#xA;T-0:01:00   Flight Computer to start-up &#xA;T-0:01:00   Pad Deck Water Deluge System Activation &#xA;T-0:00:50   First Stage Thrust Vector Actuator Test &#xA;T-0:00:40   Propellant Tank Pressurization  &#xA;T-0:00:25   All Tanks at Flight Pressure    &#xA;T-0:00:15   Arm Pyrotechnics    &#xA;T-0:00:10   Latest VC Abort &#xA;T-0:00:03   Merlin Engine Ignition  &#xA;T-0:00:00   LIFTOFF&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Shuttle:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;T-9 minutes and counting&#xA;Start automatic ground launch sequencer&#xA;Retract orbiter access arm (T-7 minutes, 30 seconds)&#xA;Start auxiliary power units (T-5 minutes, 0 seconds)&#xA;Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds)&#xA;Start orbiter aerosurface profile test, followed by main engine gimbal &#xA;profile test (T-3 minutes, 55 seconds)&#xA;    Retract gaseous oxygen vent arm, or &quot;beanie cap&quot; (T-2 minutes, 55 seconds)&#xA;    Crew members close and lock their visors (T-2 minutes, 0 seconds)&#xA;    Orbiter transfers from ground to internal power (T-50 seconds)&#xA;    Ground launch sequencer is go for auto sequence start (T-31 seconds)&#xA;    Activate launch pad sound suppression system (T-16 seconds)&#xA;    Activate main engine hydrogen burnoff system (T-10 seconds)&#xA;    Main engine start (T-6.6 seconds)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;These imply a raft of questions but it's a bit overwhelming. Isn't there a general scheme to things to make it easier to grapple with? An overview, and from there more specific questions can be asked? For an uncrewed launch vehicle, what is the basic outline of the things that need to be done before launch, and the basic reasons?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: The pre-launch checklists included are mostly as examples, a general explanation from maybe once a rocket is on the pad would be great.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-02T00:15:18.800" LastActivityDate="2015-02-02T00:15:18.800" Title="What are the main steps in the lead-up to a rocket launch?" Tags="|launch|rockets|launchpad|launch-sequence|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="7991" PostTypeId="1" AcceptedAnswerId="8106" CreationDate="2015-01-31T22:07:39.977" Score="14" ViewCount="6020" Body="&lt;p&gt;The video below mentions that Soyuz boosters consistently land a certain distance away from the Baikonur Cosmodrome, in deserted areas. What happens to those boosters? I'd sure like to have an interesting chunk of booster...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/AVvgpKt5uCA?start=429&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This article from &lt;a href=&quot;http://www.russianspaceweb.com/baikonur_downrange.html&quot; rel=&quot;noreferrer&quot;&gt;russianspaceweb.com&lt;/a&gt; mentions there used to be recovery, but that has stopped. &lt;a href=&quot;http://forum.kerbalspaceprogram.com/threads/17495-Soyuz-Debris&quot; rel=&quot;noreferrer&quot;&gt;This thread&lt;/a&gt; on the Kerbal Space Program forum says 'rocket part hunters' harvest the scrap and sell it in China. (If so, they are missing a significant hobbyist market opportunity, I think...) Does anyone know more?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="13208" LastEditDate="2017-02-27T21:35:00.807" LastActivityDate="2020-04-12T19:43:29.337" Title="What happens to the Soyuz boosters that land in the Kazakhstan desert?" Tags="|booster|recovery|soyuz-launcher|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="7992" PostTypeId="1" AcceptedAnswerId="8003" CreationDate="2015-01-31T22:44:00.767" Score="18" ViewCount="15299" Body="&lt;p&gt;Russian rockets look like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GwW4F.jpg&quot; alt=&quot;Russian rocket designs&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They flare them out at the bottom. With their newest rocket, the Proton, the flared shape is gone but the boosters still have caps that angle in towards the main rocket so the shapes fuse together. American rockets look like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MfqKu.jpg&quot; alt=&quot;American rocket designs&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The boosters look like mini-rockets, with their own nose cones, stages are straight, not flared out. The Chinese do the same. Europe takes the Russian approach with Ariane. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why the different approach in booster design?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-01T01:25:09.440" LastActivityDate="2016-12-22T01:33:46.427" Title="Why is the Russian approach to the aerodynamics of their rockets different?" Tags="|rockets|engine-design|russia|atmospheric-drag|booster|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="7994" PostTypeId="1" AcceptedAnswerId="12179" CreationDate="2015-02-01T01:14:24.820" Score="9" ViewCount="1218" Body="&lt;p&gt;I'm curious, if a space station were rotating to produce a gravity-like force, would being able to see large portions of the sky disorient human beings? Is there any research on the maximum rotation of the &quot;sky&quot; before human beings become disoriented, if at all?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The picture below sparked this curiousity, although I don't think it actually has windows to see into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've made a few calculations with &lt;a href=&quot;http://www.artificial-gravity.com/sw/SpinCalc/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpinCalc&lt;/a&gt;. A 200 meter radius ship needs to spin at 2.11 RPM. A &lt;a href=&quot;https://en.wikipedia.org/wiki/Stanford_torus&quot; rel=&quot;nofollow noreferrer&quot;&gt;1km radius ship&lt;/a&gt; rotates 0.9456 times per minute. This is VERY fast compared to the earth's rotation speed of .000694... rotations per minute. That's over 1300 times faster.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/OyBsB.jpg&quot; alt=&quot;Unknown artist&quot;&gt; &lt;/p&gt;&#xA;" OwnerUserId="7027" LastEditorUserId="7027" LastEditDate="2015-09-29T20:24:42.967" LastActivityDate="2015-10-02T00:26:55.040" Title="Would a rotating spacecraft disorient those looking out of it?" Tags="|colonization|spacecraft|artificial-gravity|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="7996" PostTypeId="2" ParentId="7990" CreationDate="2015-02-01T04:05:06.450" Score="5" Body="&lt;p&gt;I can explain the STS entries.  I'm gonna go off my own knowledge here but if I get ambitious I'll come back with cites.  'll try to make it past tense because no more STS but I'll probably slip up on that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Start automatic ground launch sequencer&lt;/strong&gt;&#xA;  The GLS was the software that automatically commanded the final phase of the countdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Retract orbiter access arm (T-7 minutes, 30 seconds)&lt;/strong&gt;&#xA;   The orbiter access arm was the swing arm that reached from the launch tower to the crew hatch.  It's where the 'white room' was.  If there was an emergency after this point that required the crew to evacuate the Orbiter, it could swing back very quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Start auxiliary power units (T-5 minutes, 0 seconds)&lt;/strong&gt;&#xA;  The pilot starts the 3 hydrazine fueled APUs that provide hydraulic power to the Orbiter.  It's done as late as possible to save fuel.  They must be up and running by T-4 minutes when hydraulic pressure is needed to move a valve in the shuttle main engines, when they enter Purge Sequence 4.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds)&lt;/strong&gt;&#xA;  Mechanical devices that break the firing chain in the SRB destruct devices are rotated to the non-safe position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Start orbiter aerosurface profile test, followed by main engine gimbal &#xA;profile test (T-3 minutes, 55 seconds)&lt;/strong&gt;&#xA;  Now that hydraulic power is available, the elevons and body flap are exercised, and then the SSME gimbals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Retract gaseous oxygen vent arm, or &quot;beanie cap&quot; (T-2 minutes, 55 seconds)&lt;/strong&gt;&#xA;   This is another arm from the launch tower to the tip of the ET.  It carries a duct that lets any GOX vented from the ET LOX tank be released away from the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Crew members close and lock their visors (T-2 minutes, 0 seconds)&lt;/strong&gt;&#xA;  This seals the launch and entry suits in case of a bird strike during first stage busting the windows.  It is done as late as possible in the count because the suits vent O2 into the cabin in this configuration and you don't want the O2 concentration getting too high.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Orbiter transfers from ground to internal power (T-50 seconds)&lt;/strong&gt;&#xA;  The voltage on the power feeds from the mobile launcher to the orbiter is dropped so the orbiter fuel cells pick up all the power loads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ground launch sequencer is go for auto sequence start (T-31 seconds)&lt;/strong&gt;&#xA;  If all is well, the onboard launch sequencer in the orbiter is started to control the last few seconds of the count.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Activate launch pad sound suppression system (T-16 seconds)&lt;/strong&gt;&#xA;  The 'rain birds' start gushing water onto the pad to prevent damage from the acoustic power blasted out by the SRBs (changed after STS-1 when a strut was bent in the fwd RCS because of this)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Activate main engine hydrogen burnoff system (T-10 seconds)&lt;/strong&gt;&#xA;  The sparklers (ROFIs) ignite to burn up excess Gh2 vented from the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Main engine start (T-6.6 seconds)&lt;/strong&gt;&#xA;  Those big old water vapor generators start going.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The good old &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/stsover-launch.html#stsover-prelnfinal&quot; rel=&quot;nofollow&quot;&gt;Space Shuttle News Reference&lt;/a&gt; is a great starting point to read about this.  My link points to the countdown sections. It's old (dated before we started flying again after the 1st accident) but still has lots of great info.  Hit the links in the document for background on a lot of the stuff I talk about. &lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-02-01T04:17:38.133" LastActivityDate="2015-02-01T04:17:38.133" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="7997" PostTypeId="1" AcceptedAnswerId="7998" CreationDate="2015-02-01T04:56:59.913" Score="10" ViewCount="1479" Body="&lt;p&gt;The surface of Curiosity's wheels have picked up quite a lot of damage with significant holes beyond the &quot;JPL&quot; holes that were designed in. By contrast the surface of Opportunity wheels have held up pretty well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any assessment of what factors have contributed to the greater damage on Curiosity? For example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;rover weight&lt;/li&gt;&#xA;&lt;li&gt;driving technique&lt;/li&gt;&#xA;&lt;li&gt;terrain&lt;/li&gt;&#xA;&lt;li&gt;wheel material&lt;/li&gt;&#xA;&lt;li&gt;speed&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8392" LastActivityDate="2016-08-27T21:32:54.450" Title="Curiosity wheel damage" Tags="|rovers|curiosity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7998" PostTypeId="2" ParentId="7997" CreationDate="2015-02-01T06:33:46.057" Score="13" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2014/08190630-curiosity-wheel-damage.html&quot;&gt;this Planetary Society blog post&lt;/a&gt;, it's a combination of two things:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Wheel design.  The wheels have a different tread pattern than Spirit and Opportunity, for better traction on soft sand.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Terrain.  Tears in the wheel surface come from driving on bare bedrock, where the tread pattern causes excess flexing of the wheel skin, leading to fatigue failure.  Punctures come from driving on pointy rocks firmly embedded in sand (think: caltrops), a type of terrain they'd never seen before.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2015-02-01T06:33:46.057" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="7999" PostTypeId="1" CreationDate="2015-02-01T07:56:19.670" Score="5" ViewCount="238" Body="&lt;p&gt;The fine U. Washington article &lt;a href=&quot;http://www.washington.edu/news/2015/01/28/some-potentially-habitable-planets-began-as-gaseous-neptune-like-worlds/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Some potentially habitable planets began as gaseous, Neptune-like worlds&lt;/a&gt; mentions twice that &lt;em&gt;tidal forces can induce inward planet migration.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Two phenomena known to inhibit the potential habitability of planets — tidal forces and vigorous stellar activity — might instead help chances for life on certain planets orbiting low-mass stars, University of Washington astronomers have found.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.liebertpub.com/doi/abs/10.1089/ast.2014.1215&quot; rel=&quot;nofollow noreferrer&quot;&gt;In a paper&lt;/a&gt; published this month in the journal Astrobiology, UW doctoral student Rodrigo Luger and co-author Rory Barnes, research assistant professor, say the two forces could combine to transform uninhabitable “mini-Neptunes” — big planets in outer orbits with solid cores and thick hydrogen atmospheres — into closer-in, gas-free, potentially habitable worlds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It links to the 2015 &lt;em&gt;open access&lt;/em&gt; article published in Astrobiology &lt;a href=&quot;https://www.liebertpub.com/doi/abs/10.1089/ast.2014.1215&quot; rel=&quot;nofollow noreferrer&quot;&gt;Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The discussion is in the context of gaseous planets between the size of Earth and Neptune. &lt;strong&gt;What is the mechanism behind this?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;For rocky worlds, tidal forces sap the energy of rotation, resulting in a tidally-locked planet. As this may not be practical for gaseous worlds, where does the energy of tidally-heating the planet (due to stretching caused by the tidal forces) come from? Does it sap energy from the orbital velocity, migrating gaseous worlds closer to their suns? Is there some other mechanism at play?&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-10-31T03:54:28.990" Title="Planetary migration due to tidal forces" Tags="|orbit|planet|gas-giant|celestial-mechanics|tidal-locking|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="8000" PostTypeId="2" ParentId="7990" CreationDate="2015-02-01T10:32:09.867" Score="2" Body="&lt;p&gt;Generally speaking, a rocket launch has these steps: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Integration: stacking the rocket stages and the payload together. &lt;/li&gt;&#xA;&lt;li&gt;Rollout: moving the rocket to the launch pad. &lt;/li&gt;&#xA;&lt;li&gt;Fueling.&lt;/li&gt;&#xA;&lt;li&gt;Pre-launch checks.&lt;/li&gt;&#xA;&lt;li&gt;Starting up the rocket: switching on its internal power sources, starting the rocket's control systems etc. &lt;/li&gt;&#xA;&lt;li&gt;Starting the engines and checking if they work properly.&lt;/li&gt;&#xA;&lt;li&gt;Launch. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;You want to do as much of the work as possible while the rocket is still in a hangar and easy to work on, but some steps are unavoidable on the launch pad.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fueling for instance: you want to pump fuel and oxidizer into the rocket as late as possible, to reduce the risk. So for liquid-fuel rockets this is done on the launch pad. For solid boosters you can't do that, so you have to fuel those at an earlier stage.&lt;br&gt;&#xA;When fueling has finished, you have to disconnect the fuel lines (umbilicals) from the rocket and stow them so they won't be burned by the rocket. There may be other steps to secure the launch tower at this stage.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you want to make sure all the internal systems work properly. Some checks can't be done beforehand (e.g. because this is the first time the rocket's been fueled). Other checks are on general principle: better to test 10 times than to lose a payload.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until now, the rocket has been receiving power from the launch pad. This must be switched to an internal power source (batteries, generator, fuel cell). Then you have to confirm again that everything works on internal power.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last step is to start the engines, holding down the rocket for a few seconds so you can make sure the engines work properly. Then release the clamps and launch.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is only a general overview: each step is split into hundreds of checkpoints and actions, depending on the rocket type. The Shuttle had a much more complicated launch procedure than the Falcon, for instance. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-02-01T10:32:09.867" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8001" PostTypeId="1" AcceptedAnswerId="8010" CreationDate="2015-02-01T19:11:33.877" Score="9" ViewCount="2108" Body="&lt;p&gt;With current technologies, are there practical limits on the number of times a rocket engine can be restarted while still remaining in orbit? Given sufficient fuel, could an engine be designed today that could do 1000 short orbital burns? What about 100 burns, each lasting no more than two minutes or so?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For my purposes, you can exclude engines that de-orbit and then go back up. I am only interested in high-thrust rockets, be they conventional, hybrid or other types; I am thinking about something that could reasonably move major tonnage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would also be interested in current record-holding designs in terms of how many times they have successfully refired on a single mission. A reasonable analysis of what subsystem would likely fail first would also be appreciated.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2016-03-01T02:54:47.767" LastActivityDate="2016-03-01T02:54:47.767" Title="Practical limits to a restarting a main rocket engine in orbit?" Tags="|rockets|engine-design|restartable-engine|" AnswerCount="1" CommentCount="12" ClosedDate="2016-03-01T05:49:45.667" ContentLicense="CC BY-SA 3.0" />
  <row Id="8002" PostTypeId="1" AcceptedAnswerId="27459" CreationDate="2015-02-01T19:17:58.690" Score="11" ViewCount="3099" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Venus&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; there are just 150 ppm of sulphur dioxide and 20 ppm of water in Venus' atmosphere. At the same time it is known that there is a considerable amount of sulphuric acid in the clouds. Venus' clouds are made of concentrated sulphuric acid, if I understand it correctly. This would suggest that the amount of sulphuric acid on Venus should be higher than the amount of water (otherwise sulphuric acid would absorb the water since it is strongly hygroscopic). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, if data cited on Wikipedia are from some spectrographic measurements, this may be affected by some bias: above the clouds there is probably a much lower concentration of sulphuric acid vapors than under the clouds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I ask especially concerning possible colonization and local resource utilization on Venus. Two of the limiting substances are water and hydrogen, and a lot of $H_2SO_4$ would be helpful as a source of both hydrogen and water.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be more specific with the question.&#xA;20 ppm of water means $9.6 \times 10^{15} \text{ kg } H_2O$ in total (Venus' atmosphere's mass is $4.8 \times 10^{20} \text{ kg}$). &lt;strong&gt;Is there less or more sulphuric acid?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just for scale ... the amount of various sources of carbon on Earth follows:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; CO2 in Earth atmosphere     0.810e+15 kg&#xA; Earth coal reserves         0.9e+15 kg (proven recoverable)&#xA; Earth biosphere             1.9e+15 kg &#xA; CO2 in Earth sea water     36e+15kg&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="8393" LastEditorUserId="58" LastEditDate="2018-05-25T19:05:21.497" LastActivityDate="2021-07-23T15:21:42.383" Title="How much sulphuric acid is on Venus?" Tags="|planetary-science|atmosphere|venus|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8003" PostTypeId="2" ParentId="7992" CreationDate="2015-02-01T20:10:12.260" Score="20" Body="&lt;p&gt;The Soyuz uses conical boosters because there's an aerodynamic advantage.&lt;br&gt;&#xA;According to &lt;a href=&quot;https://books.google.nl/books?id=iVilfSI_5sAC&amp;amp;pg=PA275&amp;amp;lpg=PA275&amp;amp;dq=rocket%20conical%20booster%20%20aerodynamics&amp;amp;source=bl&amp;amp;ots=xa-itclcon&amp;amp;sig=GtLu-8Rl9tzL6t28gtxSwZHd7yU&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=MoDOVKbkFITgOJrVgFg&amp;amp;ved=0CF4Q6AEwCg#v=onepage&amp;amp;q=rocket%20conical%20booster%20%20aerodynamics&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;The Red Rockets' Glare&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Engineers gravitated to a conical shape primarily because of the&#xA;  aerodynamic advantages ...but also for 3 other reasons: the large size&#xA;  of the engines at the tail end, the possibility of imparting&#xA;  additional thrust to the central sustainer by the shape of the&#xA;  booster, and the opportunity of decreasing wall thickness.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://gaia.ub.edu/?p=2876&quot; rel=&quot;noreferrer&quot;&gt;Another source&lt;/a&gt; claims: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But it turned out that the conical-shaped boosters of the Soyuz are an advantage for the aerodynamics, especially the gap between the boosters has an effect of a “negative” wing.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At first glance a cone wouldn't appear to have an advantage: &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_equation&quot; rel=&quot;noreferrer&quot;&gt;drag depends mostly on frontal area&lt;/a&gt;, and a cone has a larger frontal area than a cylinder of the same volume.&lt;br&gt;&#xA;But a cylinder of this diameter would be half as long as the conical booster, giving a short, stubby shape that is less efficient than a long, thin cylinder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there's a more subtle effect going on here, one specific to having 4 boosters close to each other. Rockets with 2 boosters wouldn't benefit from this effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other conical rocket is the N-1. In this case, there's no aerodynamic advantage. The rocket's shape is due to limitations of manufacturing technology: a spherical tank was &lt;a href=&quot;https://books.google.nl/books?id=Sr6JtOoWghkC&amp;amp;pg=PA174&amp;amp;lpg=PA174&amp;amp;dq=n-1%20rocket%20spherical%20tanks&amp;amp;source=bl&amp;amp;ots=gZwPb6MmN9&amp;amp;sig=sIMA5KVdCIC8fUFHWfeAUeL7zCk&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=cZbPVJudBcflUsWMhMgH&amp;amp;ved=0CEEQ6AEwBQ#v=onepage&amp;amp;q=n-1%20rocket%20spherical%20tanks&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;easier to manufacture&lt;/a&gt; than a cylindrical one, so they used spheres. But they needed two spheres of different diameters to get the fuel:oxidiser ratio correct, so they ended up with conical stages.&lt;br&gt;&#xA;According to &lt;a href=&quot;https://books.google.nl/books?id=MYasNyKQIxMC&amp;amp;pg=PA201&amp;amp;lpg=PA201&amp;amp;dq=n-1%20spherical%20tanks&amp;amp;source=bl&amp;amp;ots=KL4DEVjufT&amp;amp;sig=4b2pAz0snx9IBxSs57ipi2dKboo&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=6JPPVOqZDcjvUPX8gpgF&amp;amp;ved=0CDgQ6AEwBQ#v=onepage&amp;amp;q=n-1%20spherical%20tanks&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Foothold in the Heavens: The Seventies&lt;/a&gt;, the Soviets were unable to produce cylindrical tanks that were part of the rocket's load-bearing structure: they couldn't manufacture aluminium plate that was thick enough for the job. So they had to separate the rocket's skin from the tank. They chose to use a spherical tank instead of having two cylinders inside one another, to minimize contact between the tank and the outer skin. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://www.starbase1.co.uk/pages/N1-Galleries/Diagrams%20&amp;amp;%20Plans/index.html#n1-top-interior.jpg&quot; rel=&quot;noreferrer&quot;&gt;website linked to the book N-1 for the Moon and Mars&lt;/a&gt;:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/dNbsQ.jpg&quot; alt=&quot;N-1 diagram&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-02-02T15:25:15.193" LastActivityDate="2015-02-02T15:25:15.193" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8005" PostTypeId="5" CreationDate="2015-02-02T02:22:47.770" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-02T02:22:47.770" LastActivityDate="2015-02-02T02:22:47.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8006" PostTypeId="4" CreationDate="2015-02-02T02:22:47.770" Score="0" Body="A rocket engine, or other impulse engine, which is designed to be fired more than once." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2015-02-02T02:34:07.480" LastActivityDate="2015-02-02T02:34:07.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8009" PostTypeId="1" CreationDate="2015-02-02T15:51:12.257" Score="6" ViewCount="3001" Body="&lt;p&gt;SpaceX is apparently uprating the thrust of the Merlin 1D engine used on the Falcon 9, by about 15-20%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are they changing (in the engine, in the fuel) to achieve this new thrust level?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been suggested they will super cool the RP-1 to decrease its volume, thus increasing it's density.  What else? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacenews.com/ses-rethinking-being-first-to-fly-on-a-full-throttle-falcon-9/&quot; rel=&quot;nofollow&quot;&gt;SES is willing&lt;/a&gt; to forgo a launch slot in order to let someone else be first to fly on the new uprated engines in Spring of 2015. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You know SpaceX is introducing into their manifest ... a modification of the current engine, with about a 20 percent increase in thrust,&quot; said Martin Halliwell, SES’s chief technical officer. &quot;We’re making a decision internally as to whether we want to be the first to fly it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It makes sense to do if possible, since it helps them recover payload capability they lost by reusing a stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back in 2013 &lt;a href=&quot;http://shitelonsays.com/transcript/spacex-ses-8-pre-launch-conference-2013-11-24&quot; rel=&quot;nofollow&quot;&gt;we heard Elon say:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With respect to the future potential for the rocket, I do think we've got.. I'm really happy with this rocket design. It's an incredibly capable vehicle. It's actually one of the biggest rockets in the world, it's worth noting, at about 1.3 million pounds of thrust, and we're only actually operating the engines at about 85% of their potential. Down the road, in future missions, we anticipate being able to crank them up to their full thrust capability, which would give about 165,000 pounds of sea-level thrust per engine. Anyway, it really is something that is, I think, going to serve really well for the commercial launch market, for government satellites and for Dragon, both crew and cargo. I believe its inherent reliability potential is better than any other rocket in the world. It will be up to us to show that it lives up to that reliability potential.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-07-10T02:56:03.423" LastActivityDate="2015-07-10T02:56:03.423" Title="What is changing between the Merlin 1D engine and the uprated Merlin 1D+ engine?" Tags="|spacex|falcon-9|merlin-1d|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8010" PostTypeId="2" ParentId="8001" CreationDate="2015-02-02T17:17:25.000" Score="3" Body="&lt;p&gt;For the purposes of your question, I will subdivide rocket engine designs into two categories, and some sub-categories&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Designs using hypergolic propellants&lt;/li&gt;&#xA;&lt;li&gt;Designs using non-hypergolic propellants&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Designs using a torch igniter or something similar&lt;/li&gt;&#xA;&lt;li&gt;Designs using non-reusable ignition mechanisms (e.g. hypergolic slugs, solid fuel igniters, probably others I haven't thought of)&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Designs of type 1 can be restarted any number of times. Hypergolic propellants ignite on contact with each other, meaning no separate ignition mechanism is necessary. For this reason you see them used in Reaction Control Systems (RCS) on the space shuttle, SpaceX's Dragon, and other vehicles&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Designs of type 2.1 can also be restarted any number of times, theoretically, but they rely on an ignition mechanism that has moving parts (i.e. valves) that can fail, so the upper limit on the number of times it can restart would depend on the reliability of the mechanical components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Designs of type 2.2 can only be restarted a fixed number of times, in some cases they can't be restarted at all. Designs using hypergolic 'slugs' place a 'slug' of hypergolic propellant in front of the 'normal' propellant, so that the slugs ignite and provide the heat to ignite the rest of the propellant. Here, you're limited to the number of slugs you take with you. This will typically be a single-digit number. I believe this is the method SpaceX uses for their upper stage engines when they need to deliver satellites to higher orbits. And as Rikki-Tikki-Tavi pointed out, other engines use solid fuel mechanisms to ignite their fuel, and in that case you're limited to the number of solid fuel igniters you bring with you.&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="1255" LastEditDate="2015-02-02T18:17:25.947" LastActivityDate="2015-02-02T18:17:25.947" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8011" PostTypeId="2" ParentId="8009" CreationDate="2015-02-02T18:06:03.087" Score="3" Body="&lt;p&gt;There isn't a lot of publicly available information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think they are planning to subcool the LOX in addition to the fuel.  The increased density results in an increased propellant mass flow rate for the same turbopump speed.  This will give increased chamber pressure and thus higher thrust and probably higher Isp.  Hardware changes to the engine might include stronger thrust chamber structure - or they might use up some of the margin built into the existing structure.  Likewise, perhaps the &lt;a href=&quot;http://en.wikipedia.org/wiki/Regenerative_cooling_%28rocket%29&quot; rel=&quot;nofollow&quot;&gt;regenerative cooling&lt;/a&gt; will need to be improved.  They might or might not increase the turbopump speed also, to further increase propellant mass flow rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the thrust increase was always planned for, some or all of these changes may have been anticipated at the time of the original engine design so they might not need to be made.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2015-02-03T00:13:37.053" LastActivityDate="2015-02-03T00:13:37.053" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8012" PostTypeId="1" CreationDate="2015-02-02T21:13:51.157" Score="2" ViewCount="377" Body="&lt;p&gt;I know all about the plans for reusing the 1st stage, ocean landing vs. fly back, etc. But, Musk wants to make stages 2 and 3 reusable as well. Surely they would have far too much velocity and height to turn around and come back? Or, would they just go for a long ballistic arc to somewhere in Africa on the 2nd, and circumnavigating the planet with the 3rd, to return to the launch site?&lt;/p&gt;&#xA;" OwnerUserId="8396" LastEditorUserId="238" LastEditDate="2015-02-02T21:21:58.087" LastActivityDate="2015-02-02T21:41:57.307" Title="SpaceX: Recovery/reuse of upper stages - where would they land?" Tags="|spacex|reuse|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8013" PostTypeId="2" ParentId="8012" CreationDate="2015-02-02T21:33:35.413" Score="3" Body="&lt;p&gt;Elon Musk has said that they have basically given up on Falcon 9 upper stage recovery, focusing on first stage recovery for Falcon 9, and whatever you want to call Falcon Heavy's side boosters and center core.  The center core is not quite a second stage, but is kind of sort of a second stage.  Stage 1.5?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the upper stages on a LEO mission, it is basically in orbit.  On a GEO/GTO mission it is in a pretty high orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting that back is basically the same problem as a Dragon recovery, which is probably going to add a major payload hit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For upper stage recovery they have decided that BFR (Big F***ing Rocket) they plan for Mars missions, using the a cluster of Raptor (million lb thrust LOX/CH4 engine) will be reusable on first stage and second stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has been some confusion and suggestions that in fact, the second stage of the BFR will be the MCT (Mars Colonial Transport).  That is, the first stage delivers the second stage which proceeds to orbit.  Possibly is refueled by other BFR launches, and finally departs for Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case, many BFR/MCT launches will likely be needed to refuel the first MCT waiting to go to Mars. So upper stage reuse of the MCT will be critical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to the actual question, where would they land? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well they will basically be in orbit once they are ready to land, so going once around and aiming for the launch site is likely possible, assuming their orbital inclination did not move them too far away from the launch site. If they have sufficient aerodynamic control they can likely make it back to the launch site. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Smartest place to land is wherever they intend to refurb them for reuse.  Likely the launch site, but that is unclear. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-02-02T21:41:57.307" LastActivityDate="2015-02-02T21:41:57.307" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8014" PostTypeId="1" AcceptedAnswerId="8017" CreationDate="2015-02-02T22:19:21.520" Score="12" ViewCount="2892" Body="&lt;p&gt;That particular quote comes from an article about the Proton rocket (&lt;a href=&quot;http://www.nasaspaceflight.com/2015/02/ils-proton-m-inmarsat-5-f-2-launch/&quot;&gt;http://www.nasaspaceflight.com/2015/02/ils-proton-m-inmarsat-5-f-2-launch/&lt;/a&gt;), but I've seen this used elsewhere. The wiki page for SpaceX's Merlin engines also mentions something similar ('SpaceX uses a triple-redundant design in the Merlin flight computers.' - &lt;a href=&quot;http://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29&quot;&gt;http://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it mean that for every relay, there's 3 sets of wires going to it, one from each computer? Does only one computer run at a time, and the others only pick up if there's a failure? How are failures detected?&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2015-02-04T05:21:52.047" Title="What does 'triple redundant closed-loop digital avionics system' mean?" Tags="|avionics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8015" PostTypeId="2" ParentId="8014" CreationDate="2015-02-02T22:42:15.423" Score="5" Body="&lt;p&gt;Redundancy is implemented in various ways.  Generally speaking, triple redundancy means that the system can withstand two failures without having the system fail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In greatly simplified terms, redundant avionic systems have multiple computers running and comparing results -- voting out a system that gives a different answer.  This voting is then effectively the way failures are detected.  Sometimes, a completely separate system with completely different software is available as a final backup.  The Shuttle, for example, had a backup flight software (BFS) system available to it for this purpose.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-02-02T23:50:25.960" LastActivityDate="2015-02-02T23:50:25.960" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8016" PostTypeId="1" CreationDate="2015-02-03T00:32:43.750" Score="21" ViewCount="1886" Body="&lt;p&gt;I know Earth's atmosphere is a show stopper for rail gun launch to orbit from Earth's surface.&lt;/p&gt;&#xA;&lt;p&gt;The vacuum of the Moon is often cited by lunar enthusiasts who point out lunar mass drivers could provide a lot of delta V.&lt;/p&gt;&#xA;&lt;p&gt;But how about a rail up Olympus Mons? At &lt;span class=&quot;math-container&quot;&gt;$0.02 \ kg/m^3$&lt;/span&gt; &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars surface atmosphere&lt;/a&gt; is already rarefied. And at 22 km, &lt;a href=&quot;http://en.wikipedia.org/wiki/Olympus_Mons&quot; rel=&quot;nofollow noreferrer&quot;&gt;Olympus Mons&lt;/a&gt; is a very tall mountain. At 3.5 km/s, Mars orbital velocity is a little less than half that of earth.&lt;/p&gt;&#xA;&lt;p&gt;Is the air at the top of Olympus Mons thin enough to allow rail gun launches to orbit?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="7062" LastEditDate="2023-11-28T05:29:30.437" LastActivityDate="2023-12-05T06:28:33.403" Title="Is a railgun up Olympus Mons possible?" Tags="|mars|mass-driver|olympus-mons|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8017" PostTypeId="2" ParentId="8014" CreationDate="2015-02-03T05:28:12.483" Score="14" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Never go to sea with two chronometers; take one or three.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Dual redundancy presents a big problem. Suppose you bring two chronometers (or two computers, or two inertial measurement units). What do you do when the two devices disagree? Which do you trust? Triple redundancy solves that problem. Ignoring the possibility that two of the devices go whacko at the same time and in the same way, when one device goes whacko, the other two will still be in agreement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sans some very smart hardware/firmware/software, triple redundancy protects against but a single failure. Dual fault tolerance requires very smart computers or requires at least quad redundancy.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this update, I will focus on fault tolerance versus fault safety, and on common mode failures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fault tolerance (in NASA) means that the mission succeeds despite a failure. Dual fault tolerance means that the mission succeeds despite two independent failures. Fault safety means that a failure does not result in irreparable harm, but it does not necessarily mean mission success. There's a big difference between the two. A mission that succeeds despite a failure or two is fault tolerant. Fault safety might mean sending the astronauts/cosmonauts home. While the mission would be a failure, the astronauts/cosmonauts would still be alive after the abort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, as far as NASA and Roscosmos are concerned, a vehicle bringing cargo to the International Space Station would be deemed fail-safe if a failure occurs on the vehicle and the vehicle intentionally splashes itself into the Pacific as a result. A few tens of millions of dollars of cargo might be lost, but the ISS will remain unharmed. Visiting vehicles to the ISS are required to be safe against two failures, meaning no harm to the ISS. Fault tolerance? NASA and Roscosmos don't care. Their primary concern is the continued safety of that trillion dollar asset.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Redundancy_(engineering)&quot;&gt;wikipedia article on redundancy&lt;/a&gt; is very wrong. It implies that triple redundancy is the be-all and end-all of redundancy, and that triple redundancy implies dual fault tolerance. Triple redundancy works against &lt;strong&gt;some&lt;/strong&gt; single failures. Dealing with two failures is a much harder problem. A number of aircraft and space vehicles are quad redundant (e.g., the Space Shuttle, the F-16, the 747, and Orbital's Cygnus). SpaceX's Dragon uses massive redundancy (with non-rad hardened flight computers) to achieve two fail safety. Dual fault tolerance is an even tougher nut to crack. Sometimes, two failures means ending the mission and sending the crew home, or splashing an unmanned payload into the Pacific.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, triple redundancy accomplishes nothing if every one of those triply-redundant systems exhibits the same common error. A good number of mishaps in space have been attributed to bad flight software or to bad commands issued to the flight software. It doesn't matter if there are one hundred flight computers if each and every one of them has the same bad code or is given the same bad command. Something bad will happen. Common mode failure is the thing that scares safety engineers the most.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Carbon-copy redundancy offers zero protection against common mode failures. The Space Shuttle used quad redundancy in its primary system to address the problem of two fault tolerance (to some failures). To combat the problem of common mode failures, the Space Shuttle had a fifth Backup Flight System. The Shuttle BFS software was built by a completely different contractor than that responsible for building the primary avionics software system. The job of the BFS (which was never used) was to bring the vehicle back to Earth. While the mission would have been a failure, the astronauts would still have been alive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other spacecraft have taken things even further. The Japanese HTV Abort Control Unit runs a separate set of software dedicated toward keeping the Space Station safe. The ACU uses distinct attitude sensors from those used by the primary system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet others argue that the idea of a distinct backup system written by a different organization is a bad idea. Per this line of reasoning, that distinct backup removes pressure on the designers of the primary system to write their software correctly and is an extra expense that would be better spent on the primary software. Moreover, the mistakes aren't so much in the code as in the requirements and design. The designers of the backup are likely to make those very same mistakes.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-02-04T05:21:52.047" LastActivityDate="2015-02-04T05:21:52.047" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8018" PostTypeId="2" ParentId="8016" CreationDate="2015-02-03T06:21:16.240" Score="18" Body="&lt;p&gt;I've been wondering this for years, not knowing how to approach the problem.&lt;/p&gt;&#xA;&lt;p&gt;Then at a &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=24169.msg1308624#msg1308624&quot; rel=&quot;nofollow noreferrer&quot;&gt;NasaSpaceFlight thread&lt;/a&gt; a poster using the name R7 told me about &lt;a href=&quot;http://en.wikipedia.org/wiki/Dynamic_pressure&quot; rel=&quot;nofollow noreferrer&quot;&gt;dynamic pressure&lt;/a&gt;, also known as q.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$q \ = \ 0.5*\rho*v^2$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;q is measured in pascals. R7 told me the expression Max-Q is the maximum dynamic pressure a space craft endures during it's flight.&lt;/p&gt;&#xA;&lt;p&gt;Page 109 of &lt;em&gt;&lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/com/1420075284&quot; rel=&quot;nofollow noreferrer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Introduction to Rocket Science and Engineering&lt;/a&gt;&lt;/em&gt; by Travis S. Taylor says the Shuttle's Max-Q was at an altitude of 11 km at a speed of 442 m/s. Max-Q was about 35,000 pascals.&lt;/p&gt;&#xA;&lt;p&gt;What's the density of Martian air at the top of Olympus Mons? According to the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Fact Sheet&lt;/a&gt; the density of Mars surface atmosphere is &lt;span class=&quot;math-container&quot;&gt;$ 0.02 kg/m^3$&lt;/span&gt; and the &lt;a href=&quot;http://en.wikipedia.org/wiki/Scale_height&quot; rel=&quot;nofollow noreferrer&quot;&gt;scale height&lt;/a&gt; is 11.1 km. Since Olympus Mons is 22 km high, it seems to me the the density at the mountain top would be &lt;span class=&quot;math-container&quot;&gt;$e^{-22/11.1}*0.02kg/m^3$&lt;/span&gt; where e is Euler's number, about 2.72. That gives a density of about &lt;span class=&quot;math-container&quot;&gt;$ 0.0028 kg/m^3 $&lt;/span&gt;. Mars orbital speed at 22 km altitude is about 3,540 m/s. Plugging these numbers into&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$q \ = \ 0.5*\rho*v^2$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;I get a dynamic pressure is about 17,268 pascals, or about half of the shuttle's Max-Q.&lt;/p&gt;&#xA;&lt;p&gt;So at this point I believe Mars' atmosphere wouldn't be a showstopper for an Olympus Mons rail gun.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="7062" LastEditDate="2023-11-28T05:31:58.443" LastActivityDate="2023-11-28T05:31:58.443" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="8019" PostTypeId="2" ParentId="8002" CreationDate="2015-02-03T10:17:40.280" Score="3" Body="&lt;p&gt;OK, so I found something which more or less answers the question. Presentation &#xA;&lt;a href=&quot;http://www.lpi.usra.edu/vexag/meetings/archive/vexag_4th/nov_2007/presentations/toon.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aerosols and Clouds on Earth and Venus&lt;/a&gt; from Owen Brian Toon of  Department of Atmospheric and Oceanic Sciences&#xA;and the abstract of &lt;a href=&quot;http://www.hou.usra.edu/meetings/venus2014/pdf/6005.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;BEYOND SULPHURIC ACID – WHAT ELSE IS IN THE CLOUDS OF VENUS?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short the density of the clouds is about 1-50 $mg / m^3$ of liquid phase. Clouds are formed from 75-90% $H_2SO_4$. $6 \times 10^{13} \text{ kg }H_2SO_4$ is produced each year by photochemical processes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But still it does not answer exactly what is the total amount.&lt;/p&gt;&#xA;" OwnerUserId="8393" LastEditorUserId="19335" LastEditDate="2018-05-27T09:24:33.947" LastActivityDate="2018-05-27T09:24:33.947" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="8020" PostTypeId="1" AcceptedAnswerId="8021" CreationDate="2015-02-03T10:55:01.923" Score="4" ViewCount="347" Body="&lt;p&gt;Are the space stations and habitation modules of Bigelow Aerospace (Genesis I &amp;amp; II and the planned BEAM and B330) blown up by air like balloons? Or are they unfolded by mechanical or other means? They themselves use the word &quot;expandable&quot; but most others say &quot;inflatable&quot;.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-02-03T15:33:05.237" Title="Are Bigelow's habitats inflatable, or expandable with other methods?" Tags="|space-station|bigelow|expandable|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8021" PostTypeId="2" ParentId="8020" CreationDate="2015-02-03T12:36:37.397" Score="7" Body="&lt;p&gt;Genesis I and II were technology demonstrators and inflated by increasing pressure internally with Nitrogen gas. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;BEAM because it will inflate while attached the ISS will inflate with a breathable mix. Almost certainly not pure oxygen (fire risk), likely with a mix that matches the ISS atmosphere. (In fact the ISS needs large tanks of Nitrogen, since it leaks away with each spacewalk. Splitting water into Hydrogen and Oxygen is easy, getting nitrogen gas is harder.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BA330 will have the same issues, since it will be a standalone space station, but since people will need to occupy it, most certainly a breathable mix and since you need to fill it with breathable air anyway, why not use it for two things at the same time.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-02-03T15:33:05.237" LastActivityDate="2015-02-03T15:33:05.237" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8022" PostTypeId="1" CreationDate="2015-02-03T14:41:04.283" Score="4" ViewCount="437" Body="&lt;p&gt;From &lt;a href=&quot;http://www.bbc.com/news/science-environment-31113216&quot; rel=&quot;noreferrer&quot;&gt;Obama seeks to raise NASA Funding (BBC News)&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nasa also wants to launch a free-flying infrared sensor to support the Landsat observations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is this &quot;free-flying infrared sensor&quot; they are referring to?  I'm not sure what &quot;free flying&quot; means in the context of satellites, if anything.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2015-02-03T18:24:28.150" Title="What is the NASA &quot;free flying infrared sensor&quot; that BBC News is referring to?" Tags="|nasa|earth-observation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8023" PostTypeId="2" ParentId="8022" CreationDate="2015-02-03T18:24:28.150" Score="5" Body="&lt;p&gt;Free-flying means the IR sensor will be carried by a dedicated satellite, as opposed to being added to an existing satellite. &#xA;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Landsat_8&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In August 2004, a memorandum from the White House Office of Science&#xA;  and Technology Policy (OSTP) directed Federal agencies to place&#xA;  Landsat-type sensors on the National Polar-orbiting Operational&#xA;  Environmental Satellite System (NPOESS) platform.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That plan has been scuttled in favor of building a new Landsat 8 satellite specifically for the Landsat mission. This became operational in 2013. &#xA;For Landsat 9 &lt;a href=&quot;http://spacenews.com/35116despite-past-failures-nasa-again-pursuing-landsat-alternative/&quot; rel=&quot;noreferrer&quot;&gt;a similar discussion has been taking place&lt;/a&gt;, and the news article indicates that NASA again prefers to have a dedicated satellite.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-02-03T18:24:28.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8024" PostTypeId="1" CreationDate="2015-02-03T18:52:11.773" Score="22" ViewCount="20957" Body="&lt;p&gt;How do the overall dimensions, internal volume, mass of the various manned capsules being developed compare?&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Apollo (as a good baseline)&#xA;Orion&#xA;Dragon&#xA;Dragon V2&#xA;CST-100&#xA;Soyuz&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;For bonus points, ISS cargo vehicles.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Cygnus&#xA;ATV&#xA;HTV&#xA;Progress&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2015-02-04T21:21:07.450" LastActivityDate="2017-02-10T11:45:24.023" Title="How do the sizes of the various proposed manned capsules differ?" Tags="|spacex|crewed-spaceflight|roscosmos|boeing|lockheed-martin|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8025" PostTypeId="2" ParentId="8024" CreationDate="2015-02-03T18:55:38.103" Score="32" Body="&lt;p&gt;Found a couple of good photos of at least three of the vehicles side by side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CST-100 - Orion - Dragon&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/T3zAY.jpg&quot; alt=&quot;CST-100 - Orion - Dragon&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Graphic of Apollo - Orion - CST-100 - Dragon&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YJlOf.png&quot; alt=&quot;Apollo - Orion - CST-100 - Dragon&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A great picture from User3, who found it on reddit, showing the various vehicles all side by side to scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/R1kbs.jpg&quot; alt=&quot;Side by side of manned vehicles&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another good one, doing a better job of showing manned vs reentry vs service modules parts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UUFnw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Great summary of the various vehicles with sizes and internal volumes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://lasttechage.wordpress.com/2012/06/26/the-web-of-privatized-space-1-crew-and-cargo/&quot;&gt;https://lasttechage.wordpress.com/2012/06/26/the-web-of-privatized-space-1-crew-and-cargo/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not to forget the &lt;a href=&quot;http://www.astronautix.com/craft/orbhicle.htm&quot;&gt;Indian Orbital Vehicle&lt;/a&gt;, here it is laid out next to some other vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zdZXP.jpg&quot; alt=&quot;From Astronautix.com&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0I0Ya.jpg&quot; alt=&quot;From Astronautix.com&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then for the granddaddy of them all, how much of the shuttle is really crew space.  (Really just a portion of the nose)&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.scientificindians.com/images/crew_cabin.gif&quot;&gt;Shuttle Crew Cabin&lt;/a&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/RTNEX.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.space.com/12725-russia-progress-cargo-spacecraft-infographic.html&quot;&gt;good infographic from Space.com&lt;/a&gt; has this image:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/IcsMl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That shows the relative sizes of Progress, Soyuz, ATV, and HTV at the bottom, overlain on a space shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.space.com/15429-spacex-dragon-space-capsule-manned-infographic.html&quot;&gt;great Space.com infographic&lt;/a&gt; includes Dragon, Progress, Shenzou, ATV, and Orion:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/ihULT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-02-05T03:01:39.753" LastActivityDate="2015-02-05T03:01:39.753" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8026" PostTypeId="1" AcceptedAnswerId="8028" CreationDate="2015-02-03T19:57:21.413" Score="21" ViewCount="5734" Body="&lt;p&gt;Let's say you launched a rocket straight up, not intending to go into orbit.  At its apogee, the rocket is (say) 200 km above the earth: high enough (though certainly not fast enough) for LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would this rocket need a heat shield on its descent, or would it reenter the atmosphere slowly enough that parachutes would be enough to slow it down?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This question is basically equivalent to asking &quot;Do sounding rockets need heat shielding to reenter the atmosphere?&quot;.  (Thanks to &lt;a href=&quot;https://space.stackexchange.com/users/6312/forgemonkey&quot;&gt;ForgeMonkey&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-02-21T21:10:36.007" Title="Do you need a heat shield to enter the atmosphere from non-orbital speeds?" Tags="|reentry|sub-orbital|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8027" PostTypeId="2" ParentId="8026" CreationDate="2015-02-03T20:53:45.257" Score="1" Body="&lt;p&gt;What really matters is the heat generated by flying at high speed inside the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As in space there is not any kind of drag, you will accelerate by gravity, without limits, until you reached the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Without worrying about the atmosphere&lt;/strong&gt;, when reaching the ground you will be traveling at about 2km/s. &#xA;Thanksfully, we have an atmosphere which will provide drag, at the cost of generating heat.&#xA;From that fall, the object would enter the upper layers of the atmosphere at about 1.5km/s, and atmosphere will start to slow you down, generating a considerable ammount of heat, that could break tons of equipment, and possibly, melt the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, you need a heat shield, not as strong as an orbit return one, but you need atleast a &quot;casing&quot; that can survive high temperatures. (Not sure how hot will the object be, but it can be calculated)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The slowdown can then be caused by parachutes, or even, a lifting body or wings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cheers!&lt;/p&gt;&#xA;" OwnerUserId="8401" LastActivityDate="2015-02-03T20:53:45.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8028" PostTypeId="2" ParentId="8026" CreationDate="2015-02-03T21:10:39.403" Score="22" Body="&lt;p&gt;I'm going to make some educated guesses and grossly oversimplify the problem:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, we'll ignore the change in gravitational acceleration over the 200km fall (it's only a 5% difference, so who cares). Next we'll assume that our spacecraft encounters negligible atmospheric resistance until it reaches the &lt;a href=&quot;http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow&quot;&gt;Karman line&lt;/a&gt; at 100km. Finally we'll assume that it begins to decelerate once it has reached the Karman line. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After a 100km fall the speed of the craft can be found with the equations of motion thus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$s=ut+\frac{at^2}{2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking $$g = 9.8 ms^{-2}, u = 0 ms^{-1}, s = 100 km$$ and solving for t gives: $$t = 140s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting this value into:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v = u + at$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and solving for v we get: $$v = 1400 m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is only 3100 mph, or Mach 4 (at sea level). For comparison, orbital velocity starts around 16,800 mph (Mach 22).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So from this calculation we can state that no, provided the craft is reasonably aerodynamic it doesn't need a heat shield to survive reentry. The craft would get a little warm, but it would be possible to work with it. Once it lost enough energy to go sub-sonic it would be easy enough to bring the craft down on parachutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, even if our assumption that the spacecraft would start to decelerate at the Karman line was wrong, and the craft didn't start slowing until 0 altitude (maybe they where aiming at the Dead Sea), the craft would still only be going at 4400 mph (Mach 6). With a little cooling, some thermal paint, and smooth lines we'd still be fine without a heat shield. (Although faced with the bigger problem of surviving a 4400 mph collision with a medium sized planet)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A real world example of this kind of sub-orbital reentry can be found in &lt;a href=&quot;http://en.wikipedia.org/wiki/Sounding_rocket&quot; rel=&quot;nofollow&quot;&gt;sounding rockets&lt;/a&gt;. These craft routinely reenter the atmosphere from apogees of 1,000km to 1,500km and generally suffer greater heating on the way up than they do coming back down. Most don't have a dedicated heat shield, just thermal paint.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-02-21T21:10:36.007" LastActivityDate="2015-02-21T21:10:36.007" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8029" PostTypeId="2" ParentId="8024" CreationDate="2015-02-03T21:31:09.810" Score="13" Body="&lt;p&gt;Here's one more comparison of relative sizes of manned vehicles. &lt;a href=&quot;https://i.stack.imgur.com/3ad7C.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://i.stack.imgur.com/3ad7C.jpg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4sp3U.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="3082" LastActivityDate="2015-02-03T21:31:09.810" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8030" PostTypeId="2" ParentId="8026" CreationDate="2015-02-04T11:19:15.183" Score="8" Body="&lt;p&gt;The &lt;a href=&quot;http://www.spacex.com/falcon9&quot; rel=&quot;noreferrer&quot;&gt;Falcon 9 rocket&lt;/a&gt; first stage leaves the atmosphere, reaches about 140 KM max altitude, and then reenters without a heat shield. So no, &lt;strong&gt;heat shields are not strictly necessary for sub-orbital reentry.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://www.youtube.com/watch?v=_UFjK_CFKgA&amp;amp;feature=youtu.be&quot; rel=&quot;noreferrer&quot;&gt;this terrific Youtube video&lt;/a&gt; which actually shows a Falcon 9 first stage perform the reentry maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nJNUG.png&quot; alt=&quot;Falcon 9 reentry&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="472" LastEditDate="2015-02-05T07:39:01.393" LastActivityDate="2015-02-05T07:39:01.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8031" PostTypeId="1" CreationDate="2015-02-04T12:48:58.790" Score="1" ViewCount="1260" Body="&lt;p&gt;There's been a lot of talk in the last years about a trip to Mars (with SpaceX having the most serious intentions). I can understand that it has a less harsh environment and it will be easier to land and take off from the surface due to lower gravity, but Venus has lots of advantages too!&lt;br/&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My comparison:&lt;br/&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Gravity&lt;/em&gt;&lt;br/&gt;&#xA;Lower on Mars - easier landing and take-off, lower energy consumption for everyday operations.&lt;br/&gt;&#xA;On Venus almost same as on Earth - potentially MUCH healthier for humans. This single feature could outweigh all the other advantages on Mars in long term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Environment (atmospheric conditions)&lt;/em&gt;&lt;br/&gt;&#xA;Milder on Mars - easier to build reliable housing and suits for outdoor activities.&lt;br/&gt;&#xA;Harsher on Venus - because of temperatures and acids in atmosphere probably classic suits won't be enough. Will probably need to design mecha-like, exoskeletal-assisted vehicles &lt;a href=&quot;https://static.squarespace.com/static/51b3dc8ee4b051b96ceb10de/51ce6099e4b0d911b4489b79/51ce6197e4b0d911b4499697/1303346867025/1000w/Heavy_Mech.jpg&quot; rel=&quot;nofollow&quot;&gt;https://static.squarespace.com/static/51b3dc8ee4b051b96ceb10de/51ce6099e4b0d911b4489b79/51ce6197e4b0d911b4499697/1303346867025/1000w/Heavy_Mech.jpg&lt;/a&gt;.&#xA;But nothing an engineer couldn't do these days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Sustainable energy&lt;/em&gt;&lt;br/&gt;&#xA;Much less energy from the Sun on Mars (because farther from the Sun).&lt;br/&gt;&#xA;A hell lot of direct solar energy on Venus, plus the environment is so hot that there is no need to use solar panels. Just put out the turbines and they will run on the hot Venusian air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Terraforming&lt;/em&gt;&lt;br/&gt;&#xA;Mars probably much easier to terraform than Venus (I mean mainly the atmosphere). But in both cases this procedure is so long-term it doesn't really matter for us in the near future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Natural resources&lt;/em&gt;&lt;br/&gt;&#xA;Probably different amount of different resources but still same overall potential on both planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Further space exploration&lt;/em&gt;&lt;br/&gt;&#xA;A bit easier from Mars because closer to other planets (and the outer bounds of the solar system).&lt;/p&gt;&#xA;" OwnerUserId="6065" LastActivityDate="2015-02-06T12:47:01.480" Title="Why is Mars favored over Venus?" Tags="|mars|future-missions|venus|terraforming|" AnswerCount="1" CommentCount="7" ClosedDate="2015-02-04T13:47:45.213" ContentLicense="CC BY-SA 3.0" />
  <row Id="8032" PostTypeId="2" ParentId="8031" CreationDate="2015-02-04T13:16:38.040" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Will probably need to design mecha-like, exoskeletal-assisted vehicles &lt;a href=&quot;https://static.squarespace.com/static/51b3dc8ee4b051b96ceb10de/51ce6099e4b0d911b4489b79/51ce6197e4b0d911b4499697/1303346867025/1000w/Heavy_Mech.jpg&quot; rel=&quot;nofollow&quot;&gt;https://static.squarespace.com/static/51b3dc8ee4b051b96ceb10de/51ce6099e4b0d911b4489b79/51ce6197e4b0d911b4499697/1303346867025/1000w/Heavy_Mech.jpg&lt;/a&gt;. But nothing an engineer couldn't do these days.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Please show me these massive self-contained mechanical suits that are somehow easy to build?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus is hot - about 467°C.  How do you plan to live in an environment that's considerably hotter than molten lead?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus's atmosphere is under extreme pressure - about 93 times that of Earths atmosphere.  For comparison the world record scuba dive is about a third of that.  This is caused by less than 1 atmosphere of differance:  &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Zz95_VvTxZM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; and even nuclear attack submarines would collapse at around &lt;a href=&quot;http://en.wikipedia.org/wiki/Submarine_depth_ratings&quot; rel=&quot;nofollow&quot;&gt;74 atmospheres.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus faces constant rain made of sulfuric acid.... enough said.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suggest taking a look at how much submarines cost, how little comfort they can afford, and then consider that they aren't in an oven, aren't exposed to acid and have ready access to water, air, cooling and supplies!&lt;/p&gt;&#xA;" OwnerUserId="6250" LastEditorUserId="6250" LastEditDate="2015-02-06T12:47:01.480" LastActivityDate="2015-02-06T12:47:01.480" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="8033" PostTypeId="2" ParentId="7981" CreationDate="2015-02-05T00:42:30.457" Score="15" Body="&lt;p&gt;Since you have asked a basic question, I will answer in a non mathematical way. Bear with me, there are few things to explain before we get to the answer you seek.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mass and Gravity&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every object that has mass is attracted to other objects that have mass. Just as you are attracted by the gravity of the Earth. The amount you are attracted depends directly on your mass. Weight is the name we give to the amount of attraction between an object and the Earth. Weight is actually a force; a 1kg mass &quot;weighs&quot; 9.8 Newtons at the surface of the Earth. A Newton is a unit of force (named after &lt;a href=&quot;http://en.wikipedia.org/wiki/Isaac_Newton&quot;&gt;Isaac Newton&lt;/a&gt;) - but kitchen and bathroom scales are calibrated in Kg for convenience; it saves people having to work out how much butter they have each time they want to measure it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Inverse square law&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the gravitational attraction between two masses varies with the distance between them. If we double the distance between two objects, their gravitational attraction is one quarter of what it was. If we triple the distance between two objects, their gravitational attraction is one ninth of what it was. This is called an &lt;a href=&quot;http://en.wikipedia.org/wiki/Inverse-square_law&quot;&gt;inverse square law&lt;/a&gt;, because if you say the distance is equal to $D$, then the gravitational force will be equal to $\frac 1 {D^2}$. Inverse square laws occur in many places in physics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's get back to our 1kg mass at the surface of the Earth. For convenience, lets say it is at the North Pole (by the end of this, you should be able to say why we made this simplification). Our 1kg mass at the North Pole is actually &lt;a href=&quot;http://en.wikipedia.org/wiki/Earth_radius&quot;&gt;6,357 km&lt;/a&gt; from the center of the Earth. As we already know, it weighs 9.8 Newtons. But if we lift it to double that distance (12,714km) from the center of the Earth it would exert one quarter of the force - 2.45 Newtons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The problem with hovering&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we wanted our 1kg mass to stay at that 12,714km distance, we would need to apply a continuous force of 2.45 Newtons to keep pushing it away from the Earth. If we stop pushing, it will fall to Earth. Now, since there is no air at that distance, the only way we know of to create the pushing force is to use a rocket engine. There are different kinds of rockets, but they all use propellant (to do the propelling, or pushing), and sooner or later the propellent will be exhausted. So then our 1kg mass will fall back to Earth. We need a different way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Acceleration&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, you are sitting a powerful car, stopped at the traffic lights. The driver releases the brake and presses the accelerator pedal. The car zooms away, and you feel pressed back into your seat. What you feel is the force of the seat pushing against your mass, accelerating &lt;em&gt;you&lt;/em&gt;. Now, you know what happens if the car turns quickly: you will feel pushed to the side. What you feel is the force of the side pushing against your mass, accelerating you. The difference is that the first time the acceleration was in a straight line, and the second time the acceleration was to the side. This force to the side is what changes your direction; without it you would continue in a straight line. This is due to &lt;a href=&quot;http://en.wikipedia.org/wiki/Inertia&quot;&gt;inertia&lt;/a&gt;, a basic feature of matter, described by Newton's &lt;a href=&quot;http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot;&gt;first law of motion&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;A rock on a string&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, imagine you have rock tied to the end of a string. You can actually do this experiment. If you quickly whirl the rock around in a circle you can feel the rock pulling at the string, which is pulling at your hand. If you let go of the string the rock will fly away in a straight line. While you keep pulling (applying a force), the rock will fly in a circle. By applying the force to the rock, you are causing it to accelerate in the direction of the force. But the force is always toward the center of the circle, along your string, so the rock flies in a circle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How hard you have to pull on the string depends on two things: how heavy is the rock, and how fast is it going.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;An orbit is like a rock on a string&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's go back to our 1kg mass above the North pole. If it was stationary, then it would quickly start falling toward the Earth, accelerated by the force of gravity. But if, like our rock on the string, it was zooming to the side, then it could follow a different path. The Earth's gravity will pull the mass towards the Earth, just as the string pulls our rock. So if our 1kg mass was moving at just the right speed and direction, then the Earth's gravity will keep bending its path into a circular &lt;em&gt;orbit&lt;/em&gt;. The amazing thing is that it will keep going indefinitely, because there is no air to create friction. This is exactly how the space station or the Moon orbits the Earth, and how the Earth and other planets orbit the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Orbits are really ellipses&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Johannes_Kepler&quot;&gt;Johannes Kepler&lt;/a&gt; discovered that the paths followed by the planets are &lt;a href=&quot;http://en.wikipedia.org/wiki/Ellipse&quot;&gt;ellipses&lt;/a&gt;, not circles. A circle is a special kind of ellipse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When following an elliptical path around the Sun, a body or vehicle goes faster when it is closer to the Sun, and slower when it is further away. You can see the same effect using the rock on a string experiment. If you tie the string to a tree branch and push the rock to go almost in a circle, then you see it going faster when it is nearer to the ground and slower when it's higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How to get from one planet to another&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have a problem. In comparison to the distances between the planets, and the limited time we want use for the journey, our rockets are really pathetically puny and weak. It takes &lt;em&gt;HUGE&lt;/em&gt; amounts of energy to get into Earth orbit. It then takes a &lt;em&gt;LOT&lt;/em&gt; of energy to get out of Earth orbit into orbit around the Sun. And then it takes a lot more energy to lift (or drop) our orbit to match another planet. Then we have to use more energy to get into orbit around the target planet, and still more to land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we have to go the way that takes the least energy. And that is to use our rocket engines to accelerate the vehicle to be on an elliptical orbit, one that passes through our current position and where the planet will be when we get there. This kind of orbit is sometimes called a &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot;&gt;transfer orbit&lt;/a&gt;. After we use our engines to get into the transfer orbit, we can turn them off and just coast along, until we get near the target planet. Then we will use the engines again to get into orbit around the planet, and later to land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, to answer your question&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We cannot currently go in the straight line you draw, because we don't have rocket engines that are powerful enough. But also, the straight line path would be wasteful. And finally, even with engines that could fire all the way, the quickest path would not actually be a strait line. But &lt;em&gt;that&lt;/em&gt; explanation would be &lt;em&gt;really&lt;/em&gt; long!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Some further reading&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion&quot;&gt;Kepler's laws of planetary motion&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot;&gt;Newton's laws of motion&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Orbit&quot;&gt;Orbit&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot;&gt;Hohmann transfer orbit&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Gravitation&quot;&gt;Gravity&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user8406" LastEditorUserId="1142" LastEditDate="2015-02-07T03:24:19.837" LastActivityDate="2015-02-07T03:24:19.837" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8034" PostTypeId="2" ParentId="6638" CreationDate="2015-02-05T06:11:19.480" Score="2" Body="&lt;p&gt;This is a 3 part question imo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Part 1: Force&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on the graphic in the question it looks (as you've surmised) as if the steady state top speed is based on avoiding the ground more than overcoming drag. So the question then becomes do large hops lead to higher speed? Intuitively I'd guess no. Larger hopes mean less time in contact with the ground and hence less power (energy per second) going into forward motion. You want to be in the air long enough for your leg to get in position for the next step, no longer. So you'd probably want a similar step frequency as that on Earth (looks like 4hz-ish). This means you want less energy going into upward motion and hence more going into forward motion. The 2000N vertical force in the graphic could be reduced to 1/6th of the value (333N) and the remaining 1667N could be put into forward motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Part 2: Drag&#xA;The drag equation you've stated is the appropriate equation. We can even calculate the drag experienced by the runner in the graphic easily from equilibrium conditions. The running speed is approximately constant towards the end of the graphic, with a horizontal force of about 500N. Therefore the drag must be equivalent. If we can estimate the horizontal force per time this should give a value of drag per time. To me it looks like it about 500N for about a quarter of the period of one step. So lets say an average of 125N/step. We said earlier there's about 4 steps per second so that's about 500N per second. We now know our drag must be 500N each second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we look at the drag equation we can make a few assumptions. First lets say the drag area ($A$) of the runner stays the same. Lets also say the coefficient of drag ($C_d$) stays the same (this may not quite be the case). We know the 1/2 stays the same. So we're left with the $V$ and the $ \rho\ $. On Earth, at sea level $ \rho\ $ is approximately $1.225\text{ kg/m}^3$. The top speed of the Earth runner looks like its around $6.5\text{ m/s}$. We can now get a rough value for our $C_d \times A$. $$C_d \times A = F/( \rho\ \times V^2 \times 1/2) = 19.3$$ (units here would be $\text{m}^2$ but they're somewhat meaningless).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Part 3: Lunar air density&#xA;You've said the habitat would have a lower air pressure than Earth, but you've not specified a value. Lets assume an ISS style habitat, air pressure of 101.3kpa. The air density can be calculated from $\text{rho} = p /(R \times T)$ where $R$ is specific gas constant and $T$ is temperature (Kelvin). So we have a density of $1.17\text{ kg/m}^3$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The results:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on the above it looks as if running on the moon will have a similar drag profile to that on Earth (at least in my chosen habitat). So it becomes a question of calculating the equilibrium where the horizontal force (of 500 + 1667 = 2167N) is equal to the drag force. So simply $$V = \sqrt{2167/(1.17 \times 1/2 \times 19.3)} = *13.85\text{ m/s}*$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is based on a lot of estimations though, so anywhere from 10 - 15 m/s wouldn't be unreasonable.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="4660" LastEditDate="2015-02-15T02:24:04.127" LastActivityDate="2015-02-15T02:24:04.127" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8035" PostTypeId="2" ParentId="3194" CreationDate="2015-02-05T06:26:13.577" Score="1" Body="&lt;p&gt;As difficult to answer as this question is (how can one know that a government hasn't and just not publicised it well?) here's a good first step in looking for the answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.google.co.uk/search?q=link%3Awww.mars-one.com+inurl%3A.gov&amp;amp;oq=link%3Awww.mars-one.com+inurl%3A.gov&quot; rel=&quot;nofollow&quot;&gt;http://www.google.co.uk/search?q=link%3Awww.mars-one.com+inurl%3A.gov&amp;amp;oq=link%3Awww.mars-one.com+inurl%3A.gov&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That google search looks for any webpage that has .gov in it's url that links to the mars one website. Currently it brings up a single youtube video of a mars one applicant (god knows how that has .gov in the url).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This suggests to me that no government has released anything about mars one. That's been index by google. That has a link to the mars one website. That they've put in their .gov site. - hey I said it was a first step, not perfect!&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2015-02-05T07:04:55.587" LastActivityDate="2015-02-05T07:04:55.587" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8036" PostTypeId="2" ParentId="176" CreationDate="2015-02-05T15:15:06.323" Score="1" Body="&lt;p&gt;Mars is a no-go because it's core has cooled and stopped it's magnetosphere has collapsed exposing it to radiation and the problem that even if you could build an atmosphere, solar windows would constantly strip it away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you could achieve couple of epic engineering challenges Venus is possible;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) First you would need to increase it's spin rate which would give it a magnetosphere, by somehow getting mercury to fly by and then lock in as a moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Second you would need to deal with it's atmosphere, now there all sorts of complex solutions proposed, but basically you need to dump a huge amount of hydrogen on it (e.g. from Jupiter), this would convert all the CO2 into oceans (80% coverage) and a 3 bar atmosphere.&lt;/p&gt;&#xA;" OwnerDisplayName="user8414" LastActivityDate="2015-02-05T15:15:06.323" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8037" PostTypeId="1" AcceptedAnswerId="8039" CreationDate="2015-02-05T17:59:19.410" Score="12" ViewCount="1209" Body="&lt;p&gt;One of my dreams is to go into space. I believe that humanity is headed in such a direction that before I die (in the next 60-80 years) I should be able to pay for a ticket into orbit, or even to the moon. I'd like more data on this, to back it up. However, &quot;How close are we to commercial space flights on the regular&quot; is extremely vague.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I'll try and be specific.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many companies are actively pursuing commercial manned space flight? In-depth answers are appreciated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/8048/how-many-companies-are-active-in-industrial-space-exploration&quot;&gt;Here&lt;/a&gt; is a question related to creating industry in space, like mining.&lt;/p&gt;&#xA;" OwnerUserId="7027" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-02-06T21:52:00.250" Title="How many companies are active in commercial space exploration" Tags="|launch|crewed-spaceflight|commercial|travel|" AnswerCount="1" CommentCount="3" ClosedDate="2015-02-08T12:11:35.980" ContentLicense="CC BY-SA 3.0" />
  <row Id="8038" PostTypeId="1" CreationDate="2015-02-05T18:15:49.510" Score="8" ViewCount="3435" Body="&lt;p&gt;I'm helping some middle-school students write a spacecraft simulator.  As part of that, I'm trying to wrap my head around orbital mechanics today, mainly using this excellent guide:  &lt;a href=&quot;http://www.braeunig.us/space/orbmech.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.braeunig.us/space/orbmech.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But of the six orbital elements it says are required, there's one I don't understand: argument of periapsis.  This is defined as the angle between the ascending node and the periapsis point.  But how could this angle ever be anything other than 90°?  I'm having real trouble picturing that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More to the point, I guess, is: what do I do with this parameter to actually calculate a position (in 3D space) on an orbit?  Currently I'm doing this:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;public Vector3 PointAtAngle(float radians) {&#xA;    float r = RadiusAtAngle(radians);&#xA;&#xA;    // Start by finding the orbital point in the XZ plane&#xA;    // (i.e., the plane against which the inclination is measured),&#xA;    // around a focus of zero, and aligned on the X axis.&#xA;    Vector3 p = new Vector3(r * Mathf.Cos(radians), 0, r * Mathf.Sin(radians));&#xA;&#xA;    // Then rotate this by the inclination around the Z axis.&#xA;    p.Set(p.x * Mathf.Cos(i), p.x * Mathf.Sin(i), p.z);&#xA;&#xA;    // Rotate around the Y axis according to the longitude of ascending node.&#xA;    float cosYRot = Mathf.Cos(omega);&#xA;    float sinYRot = Mathf.Sin(omega);&#xA;    p.Set(p.x * cosYRot - p.z * sinYRot, p.y, p.x * sinYRot + p.z * cosYRot);&#xA;&#xA;    // Finally, add the focus to shift to whatever we're orbiting around.&#xA;    return focus + p;&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Here, RadiusAtAngle just computes the distance from the focus point from the angle, semimajor axis (a), and eccentricity (e).  I'm pretty confident in that.  Then I do a bit of trig to find a point around the origin in the XZ (ecliptic) plane, rotate this up (around the Z axis) by the inclination (i), and rotate it around the Y axis by the longitude of the ascending node (omega).  And finally, add in the focus point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This all appears to work great, and by tweaking a, e, i, and omega, I can make any-shaped orbit I can imagine.  But this is probably a failure of imagination on my part.  How does this argument of periapsis come into it?&lt;/p&gt;&#xA;" OwnerUserId="8415" LastActivityDate="2015-02-05T21:29:19.850" Title="Why wouldn't argument of periapsis always be 90?" Tags="|orbital-mechanics|orbital|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8039" PostTypeId="2" ParentId="8037" CreationDate="2015-02-05T18:32:12.143" Score="13" Body="&lt;p&gt;&lt;strong&gt;Roscosmos&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians right now, will sell you a flight on a Soyuz, and you will fly, in the next few years if you have the money. Oddly the former Communists are most capitalistic about this endeavour. They are the only actual space tourist option that has actually flown real tourists to space. Once the Shuttle was retired, Soyuz missions became the only option for crew rotation, so all three seats were needed for long duration ISS crew members, and the spare middle seat was no longer available. That will change as a two person team will aim for a 1 year mission, meaning that for at least two or three crew rotations, a spare seat will be available again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Boeing/Lockheed Martin&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are the big boys in US space. They are the incumbents. They build Orion, SLS, CST-100, Atlas V, Delta 4. They clearly are interested in space if NASA or the government will pay them to do the work. But it won't be cheap. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of real hardware, they have two different active boosters (Atlas V and Delta 4 families), working on a third (SLS), working on two capsules (Orion, CST-100). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Space Exploration Technologies&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is probably the closest to actually being able to sell something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have a booster (Falcon 9) that is for sale commercially with a backlog of 40-50 flights booked (Depending how you count).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have an operational cargo vehicle fulfilling a contract for cargo to the ISS. (Dragon).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have a contract with NASA to build a manned version of Dragon (Dragon V2).  Bigelow has been touted as a secondary customer for Dragon V2. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bigelow Aerospace&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bigelow has demonstrated two scale models in orbit (Genesis I and Genesis II) of their inflatable habitats. They have been hampered by the lack of vehicle to bring crew/cargo to an operational station at a price that can be afforded by their customers.  No point in launching a habitat that is unreachable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bigelow has said they will use whoever offers services, (Boeing CST-100 on a Lockheed Martin Atlas V, or SpaceX Dragon V2 on a Falcon 9 or anyone else who comes along (probably SNC's best hope)). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of commercial space access, if you mean orbital, then a SpaceX Dragon to a Bigelow habitat is your best bet. Still not cheap, but enough for sovereign country customers to afford. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Virgin Galactic&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Virgin is working on a suborbital tourist vehicle (SpaceShip 2)at a price point that is potentially affordable to some. But only suborbital, which technically counts as space, but we all know that orbit is where it is at. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Blue Origin&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who the heck knows what they are doing at the moment, they are not talking. They have a capsule (Biconic design) and a booster design that when complete will allow them to do manned missions, but no clear evidence in sight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However they have funding from a billionaire, so they may succeed if the money does not run out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sierra Nevada Corp (SNC)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SNC lost on the CCtCAP contract with NASA (Boeing and SpaceX won), and now must try and find funding to complete the project if possible. If so, they would be able to offer orbital missions, launching on an Atlas V booster.  Bigelow is probably their best hope. If they can be assured of launches, they can probably get funding to complete.  (NASA suffices as the market/funder for Dragon/CST-100 in this example)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Orbital Sciences Corp&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital launched 3 Cygnus unmanned cargo capsules to the ISS on their Antares booster. But they had a bad launch failure and are in recovery mode now. One or two next launches on Atlas V, before they can transition to a new engine (pair of RD-181s, aka export version of RD-193 used on Angara). They will likely compete and may win the next commercial cargo contract (since NASA wants 2 and SpaceX is pretty much a lock for one slot). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They do not have any real world manned flight plans. (They may have internal designs, but nothing beyond paper studies).  They also have to recover from the booster loss, and transition to new engines and get back on track for cargo delivery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;National Programs&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;India seems to be slowly considered manned flight.  But it won't be commercial. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russia may replace the Soyuz booster one day with Angara, and maybe a new crew design, but that is many years down the road. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;China has a manned program, but it is not commercial at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Everyone else&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lots of little guys, but none seem very realistic. These are the big ones. If you find a big one I missed, edit it in please.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-02-05T19:00:09.360" LastActivityDate="2015-02-05T19:00:09.360" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8040" PostTypeId="1" AcceptedAnswerId="8041" CreationDate="2015-02-05T19:45:52.037" Score="12" ViewCount="1394" Body="&lt;p&gt;A Long Distance Look from LORRI images &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150204&quot; rel=&quot;noreferrer&quot;&gt;were published recently&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto and Charon look like shrunken ovals here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/glrca.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it due to Pluto and Charon's phases? Is the distance enough to observe not full phase?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I calculated that since the speed of Pluto is 4.6 km/s and the speed of New Horizons is 14.6 km/s, then the angle it observed is 17 degrees. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VZqgb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this correct and is this enough to observe not full phase?&lt;/p&gt;&#xA;" OwnerUserId="5734" LastEditorUserId="5734" LastEditDate="2015-02-06T04:47:38.710" LastActivityDate="2015-02-06T04:47:38.710" Title="Why isn't Pluto a circle on LORRI images?" Tags="|new-horizons|pluto|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8041" PostTypeId="2" ParentId="8040" CreationDate="2015-02-05T20:04:52.980" Score="22" Body="&lt;p&gt;Per New Horizons science team member John Spencer, &lt;a href=&quot;https://twitter.com/elakdawalla/status/563376682497605632&quot;&gt;via Emily Lakdawalla&lt;/a&gt;, the spacecraft is &lt;a href=&quot;http://astro.cornell.edu/academics/courses/astro201/diff_limit.htm&quot;&gt;not yet close enough&lt;/a&gt; to fully resolve the the shapes of Pluto and Charon.  Their apparent shape is dominated by the camera / telescope's &lt;a href=&quot;http://en.wikipedia.org/wiki/Point_spread_function&quot;&gt;point spread function&lt;/a&gt;, which is &quot;slightly teardrop shaped, giving that gibbous appearance&quot;.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2015-02-05T20:04:52.980" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8042" PostTypeId="2" ParentId="8038" CreationDate="2015-02-05T21:29:19.850" Score="8" Body="&lt;p&gt;The periapsis is the point at which the satellite is closest to the central body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ascending node is where the orbit crosses the equatorial plane of the central body (this can be defined in a number of way but we'll stick with equator at the moment).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, there's no reason the periapsis can't be at any point in the orbit, so lets discuss a few cases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1)periapsis coincides with ascending node - this means that when the satellite crosses the equator (going upward) it is also at the closest point to the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) periapsis is a little after the ascending node - in this case the satellite is close to the earth when crossing the equator, but it's still getting closer. This is the case when the argument of periapsis is between 0 and pi radians (0-180degs)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) periapsis is before the ascending node - in this case when the satellite passes the equator it has already just been at it's closest point and it currently getting further from the earth. This is the case when the argument of periapsis is between pi and 2pi (180-360deg).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good way for you to test the implementation of your argument of periapsis code would be yo have an orbit with a non zero eccentricity, and a zero inclination (if you view the orbit from above, pi/2 if from the side). This way the changing value of the argument of perigee will change where the closest point of the orbit is.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-02-05T21:29:19.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8044" PostTypeId="1" AcceptedAnswerId="8050" CreationDate="2015-02-06T11:02:42.233" Score="6" ViewCount="533" Body="&lt;p&gt;Let's say that we want constant communication between the Lunar north pole and Earth. The 5.145° orbital inclination of the Moon means that the (mountain at) the Lunar north pole will have line-of-sight with Earth for direct communication during only about half of each monthly Lunar revolution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to maintain uninterrupted communication with Earth, at least 3 communication satellites in polar orbits would be required. Could this instead be accomplished by using only 1 satellite in Earth orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the communication satellite be put in an orbit which is very similar to that of the Moon, except for having another orbital inclination (say, twice that of the Moon). Same nodes, same semi-major axis, same eccentricity or almost. Couldn't it then remain straight &quot;above&quot; the Lunar north pole during the half of every orbit when the Earth is out of sight from the pole? Could it be achieved with minimal station keeping, maybe using a solar sail?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-02-06T13:22:07.453" LastActivityDate="2016-01-28T22:45:00.037" Title="Could a (differently) inclined orbit remain above a Lunar pole half of every revolution?" Tags="|orbital-mechanics|the-moon|communication-satellite|pole|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8045" PostTypeId="1" CreationDate="2015-02-06T13:30:43.370" Score="3" ViewCount="871" Body="&lt;p&gt;The most common type of asteroid in the inner solar system are &lt;a href=&quot;https://en.wikipedia.org/wiki/C-type_asteroid&quot; rel=&quot;nofollow noreferrer&quot;&gt;carbonaceous (C-type)&lt;/a&gt;, while the most common type of object in the outer solar system are comets. Carbonaceous asteroids are very rich in carbon and other technologically very valuable elements (rare metals, semiconductors). Comets are mostly made of water ice or other solidified volatiles (&lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$NH_3$&lt;/span&gt;). Saturn's rings are made mostly of water ice.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Can I assume that &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Saturn&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn's rings&lt;/a&gt; are made of basically the same material as comets?&lt;/li&gt;&#xA;&lt;li&gt;Can I assume that carbonaceous asteroids are just comets which have lost most of their volatiles? So that most comet and Saturn ring material would be just as rich in carbon, metals and semiconductors as carbonaceous asteroids, if the volatiles were evaporated.&lt;/li&gt;&#xA;&lt;li&gt;I'd appreciate a table with the average elemental composition of comets and Saturn rings (I already have such a table for carbonaceous asteroids).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;background&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;I'm a big fan of colonization of asteroids, because free space has many advantages for industry over planets. (High-quality vacuum, cryogenic temperatures, zero gravity, lots of space for big structures.) This is exactly what most high-tech instruments (like particle accelerators, superconductors, plasma chambers) need.&lt;/p&gt;&#xA;&lt;p&gt;To build industry in free space, we need material. A lot of useful material (carbon, transition metals and semiconductors) is in asteroids in the main asteroid belt - especially carbonaceous asteroids. The problem of asteroids is that they are very far apart and a lot of time and &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; is needed to go from one asteroid to another. This would limit development of large-scale civilization. Asteroids also lack volatiles like nitrogen compounds, which is a big problem for industry, life support and production of rocket fuel.&lt;/p&gt;&#xA;&lt;p&gt;Saturn's rings seem very attractive from this perspective. They are very compact (lot of material within low time and &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; requirements from each other). They are also rich in volatiles. But I'm not so sure if they are also rich enough in other materials (like carbon, metals and semiconductors) to support the full spectrum of industry and technological civilization.&lt;/p&gt;&#xA;" OwnerUserId="8393" LastEditorUserId="37047" LastEditDate="2020-11-28T08:08:23.477" LastActivityDate="2020-11-28T23:52:56.490" Title="Valuable materials in Saturn rings and comets vs. C-type asteroids" Tags="|colonization|asteroid|comet|rings|chemistry|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="8046" PostTypeId="2" ParentId="8045" CreationDate="2015-02-06T18:49:04.370" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rings_of_Saturn#Physical_characteristics&quot; rel=&quot;nofollow noreferrer&quot;&gt;According to Wikipedia&lt;/a&gt;, Saturn's rings are 99.9% water ice.&lt;/p&gt;&#xA;&lt;p&gt;It also takes a lot of &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt; to reach. From Low Earth Orbit (LEO), trans Saturn insertion is more than 7 km/s. Earth to Saturn Hohmann trip time is around 6 years.&lt;/p&gt;&#xA;&lt;p&gt;Another problem is lack of solar energy. at ~9.6 A.U. from the Sun, the sunlight in that neighborhood is about 1/90 as strong as sunlight here on Earth.&lt;/p&gt;&#xA;&lt;p&gt;So at first glance, Saturn's rings are lacking as a possible location for human settlements.&lt;/p&gt;&#xA;&lt;p&gt;Should we overcome lack of solar energy and the big &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt; it takes to leave Earth, I believe the gas giants' moon systems have a lot of potential. I call these &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/01/mini-solar-systems.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;mini solar systems&lt;/a&gt;. Many of the moons are small enough that their gravity well isn't a huge hurdle and if elevators through planet-moon Lagrange 1 and 2 regions are possible, &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt; is even less of a concern.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="33864" LastEditDate="2020-11-28T23:52:56.490" LastActivityDate="2020-11-28T23:52:56.490" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="8047" PostTypeId="1" AcceptedAnswerId="8056" CreationDate="2015-02-06T20:48:19.330" Score="9" ViewCount="2345" Body="&lt;p&gt;It seems that launching rockets to orbit is already pretty mature technology. Yet, costs of payload delivered are very high (around &lt;a href=&quot;https://space.stackexchange.com/questions/2015/what-is-the-current-cost-per-kg-to-send-something-into-gso-geo&quot;&gt;50,000 USD/kg for GEO&lt;/a&gt;, around &lt;a href=&quot;https://space.stackexchange.com/questions/1989/what-is-the-current-cost-per-pound-to-send-something-into-leo&quot;&gt;4,000 USD/kg for LEO&lt;/a&gt;). I wonder what is the main cost factor. Could you give a cost breakdown of a Delta IV Heavy launch? (Cost breakdown of any similar launch is also interesting for me.) Thank you!&lt;/p&gt;&#xA;" OwnerUserId="6213" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-02-08T19:12:45.187" Title="Cost breakdown of Delta IV Heavy launch" Tags="|launch|economics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8049" PostTypeId="2" ParentId="6638" CreationDate="2015-02-07T19:48:31.857" Score="7" Body="&lt;p&gt;Because the biomechanics of running would change a lot in the moon environment, any answer is a preliminary one, pending actual experimentation on the moon. :D &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the indoor case can still be calculated pretty accurately due to the fact air resistance is by far the most important factor. &lt;a href=&quot;http://iopscience.iop.org/0143-0807/34/5/1227/article&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; calculated that Usain Bolt was using &lt;a href=&quot;http://iopscience.iop.org/0143-0807/34/5/1227/article&quot; rel=&quot;noreferrer&quot;&gt;92.5%&lt;/a&gt; of his effort to overcome drag when he was at his top speed of &lt;a href=&quot;http://sportsscientists.com/2008/08/beijing-2008-men-100m-race-analysis/&quot; rel=&quot;noreferrer&quot;&gt;12.2 m/s&lt;/a&gt;, or 44 km/h. Since the force required to overcome drag increases with the cube of velocity, even in 0.5 atmospheres he could only go slightly faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The case in a vacuum depends entirely on how much of the savings on vertical force can be applied as horizontal force, and how well a runner can manage the increased speed. The graph in the question suggests that if all the saved vertical effort could be applied horizontally, that force would increase by a factor of 3.3. Bolt would then be able to run at 40 m/s, or 144 km/h. You may already see the problem, but before getting into that, let us note that since there is no air resistance, if there wasn't that other problem, he could run even faster. Nothing is slowing his forward momentum, and he only needs to apply a small fraction of his force to overcome gravity, so as long as he could continue to apply the remaining force against the ground, he could continue to accelerate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But he has to be able to swing his leg faster than the ground is already passing under him in order to increase his speed. The force that represents the difference between how fast he can swing his leg, and how fast he is already moving, would be applied as acceleration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I took a little video of me swinging my leg as fast as i could, just above the ground, above a measuring tape. It took very close to two frames of the video for my foot to cover 50 cm. That is a speed of 15 m/s, 54 km/h. My leg measures 88 cm. I can't find a figure for Bolt's legs, but he is 13 inches taller than me and has a much longer leg relative to the length of his torso. I'm going to estimate his leg is 108 cm. If he can also swing his leg through the air at my top speed, he would be moving it at 66 km/h. He can probably do it faster, but maybe not by much since this is not a simple question of strength. I'll call it 70 km/h. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If he is moving forward faster than he can move his legs, he is going to fall. But moving at the top speed his legs can move is only going to occupy about 60 % of his strength. It seems reasonable to think that he could do that despite the biomechanical problems. For one thing, he could slightly increase the force he is applying vertically so that he is hopping high enough that he has time to position himself exactly how he wants for the next push against the ground. Also, one could surely maintain a pace that only requires 60 % of your strength for a long time. You could probably run a whole marathon at your top speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;His main problem would be learning not to overshoot the speed his legs can handle, and having adequate protection for when he does. Falling at over 70 km/h is no fun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Notes - This would be on the hard-packed lunar soil 10 to 15 cm under the surface - a track would have to be prepared. It is assumed that through the use of adequate cleats the lack of traction due to low weight could be overcome. Matters not covered: how to turn.)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-02-07T19:48:31.857" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8050" PostTypeId="2" ParentId="8044" CreationDate="2015-02-07T20:34:53.597" Score="8" Body="&lt;p&gt;Could an relayer's orbit remain north of the moon? No. Such an orbit would have a center north of earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/NIl5N.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbits about the earth have a center within the earth. An inclined relayer orbital plane must intersect earth's plane along a line passing through the center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A circular relayer orbit would spend half the time north of the moon's orbital plane and half the time south. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the relayer is north of the orbital plane does it enjoy line of sight with a northern peak? Not necessarily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pictured is a northern lunar communications tower. As the satellite moves towards the moon's equatorial plane, it can fall into a moon shadow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dFYrO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the tower's point of view, something within the moon shadow would be below the horizon and out of line of sight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For looking at eclipses, line of sight and similar problems I like to think of &lt;a href=&quot;http://clowder.net/hop/Dandelin/Dandelin.html&quot; rel=&quot;noreferrer&quot;&gt;Dandelin Spheres&lt;/a&gt;. Borrowing from Dandelin Sphere models, the moon shadow falls within a tangent cone to the spherical moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most orbits having the same periods as the moon would be quickly destabilized by the moon's influence. Exceptions are the Lagrange regions.  EML4 and 5 are fairly stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a EML4 or 5 probe where crossing through the moon's equatorial plane, how tall would a polar tower need to be enjoying line of sight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OYt7b.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EML4 distance from moon's center is 1 lunar distance, 384,400 km. Moon's radius is 1738 km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;sin(angle)=opposite leg/hypotenuse. So the angle pictured is asin(1738/384400) or about .2º.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An EML4 relayer sitting on the moon's equatorial plane will be able to see surface points to within about .2º of the moon's north pole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course a mountain would be visible even if closer to the pole. Let's imagine a mountain right on the pole. How tall would it have to be to be visible to the relayer?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2m80o.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1738.009 km - 1738 km is about 9 meters. So if your mountain is more than 9 meters high, it's top would be visible to the EML4 relayer sitting on the moon's equatorial plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's trojan &lt;a href=&quot;http://en.wikipedia.org/wiki/2010_TK7&quot; rel=&quot;noreferrer&quot;&gt;2010 TK7&lt;/a&gt; is inclined about 21 degrees to earth's orbital plane. So I would think it's possible for a lunar trojan inclined to the lunar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angle between the moon's orbital plane and equatorial plane (a.k.a. &lt;a href=&quot;http://en.wikipedia.org/wiki/Axial_tilt&quot; rel=&quot;noreferrer&quot;&gt;obliquity&lt;/a&gt;) is 6.58º. See &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbit_of_the_Moon&quot; rel=&quot;noreferrer&quot;&gt;Orbit of the Moon&lt;/a&gt;. I am imagining a lunar trojan orbit inclined 6.58º with regard to the moon's orbital plane so that the relayer's plane is parallel to the moon's orbital plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bBsdz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I enlarged the moon to try to make the moon's equatorial plane more visible. When the moon passes through the line where the two planes intersect, they will be one and the same plane. At this point the relayer would be able to see a 9 meter tower on the north pole as well as a 9 meter tower on the south pole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other times the relayer would be either north or south of the moon's equatorial plane. So polar towers would be visible a little more than half the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me there's two possible trojan orbits that see the towers more than half the time: An EML4 orbit parallel to the moon's equatorial plane and an EML5 orbit also parallel to the moon's equatorial plane. But these two relayers would only be 120º apart. At first glance it seems to me they'd need to be 180º apart to provide a given pole with constant line of sight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So at this time I can't imagine a pair trojan relayers that could provide constant line of sight to the poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Halo orbits about L1 and L2 are also possibilities. But these orbits are a lot closer than L4 and L5 and thus more vulnerable to falling into the moon's shadow as they approach the moon's equatorial plane. Thus less than half of their orbits would enjoy line of sight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me 3 is the minimum number in any case.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-02-09T17:57:39.467" LastActivityDate="2015-02-09T17:57:39.467" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8051" PostTypeId="1" CreationDate="2015-02-07T21:53:02.757" Score="14" ViewCount="687" Body="&lt;p&gt;&lt;a href=&quot;http://www.virgingalactic.com/&quot;&gt;Virgin Galactic&lt;/a&gt;, &lt;a href=&quot;http://www.spacex.com/&quot;&gt;SpaceX&lt;/a&gt;, &lt;a href=&quot;http://www.stratolaunch.com/&quot;&gt;Stratolaunch Systems&lt;/a&gt;, &lt;a href=&quot;http://www.xcor.com/&quot;&gt;XCOR&lt;/a&gt;, and &lt;a href=&quot;http://www.blueorigin.com/&quot;&gt;Blue Origin&lt;/a&gt;. These companies have all been formed since 1999, and together are the bulk of the truly private space industry (i.e. are not dependent on government contracts or subsidies and intend to develop space for commercial purposes). There are also very well-funded startups looking beyond LEO for business - &lt;a href=&quot;http://www.planetaryresources.com/&quot;&gt;Planetary Resources&lt;/a&gt; and &lt;a href=&quot;http://bigelowaerospace.com/&quot;&gt;Bigelow Aerospace&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These companies have big bucks behind them, much more than earlier attempts. The success of SpaceX (admittedly the only commercial success so far, and still greatly bolstered by government business) makes the success of the others seem likely for the first time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this happening now?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One answer is the sudden flurry of nerd billionaires. A gaggle of Google billionaires, two dot-commers, and two MicroSoft billionaires are responsible for the bulk of this. They've got lots of money and they like space. But I wonder if the maturity of certain key technologies have now brought us to a tipping point. Is it the micro-sensors, micro-controllers, and microchips? Is it the vast power of CGI design software? Are there other things?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-08T03:47:16.900" LastActivityDate="2022-10-23T12:05:57.820" Title="Why is private spaceflight suddenly expanding so quickly?" Tags="|launch|commercial|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8052" PostTypeId="7" CreationDate="2015-02-08T00:03:26.860" Score="0" Body="&lt;ul&gt;&#xA;    &lt;li&gt;Specific aspects of space exploration&lt;/li&gt;&#xA;    &lt;li&gt;Satellite design and operation&lt;/li&gt;&#xA;    &lt;li&gt;Systems with potential use in space&lt;/li&gt;&#xA;    &lt;li&gt;Launch systems, facilities and support infrastructure&lt;/li&gt;&#xA;    &lt;li&gt;Scientific discoveries made by space probes&lt;/li&gt;&#xA;    &lt;li&gt;Heliophysics and planetary science&lt;/li&gt;&#xA;    &lt;li&gt;History of spaceflight&lt;/li&gt;&#xA;    &lt;li&gt;Trajectory design, orbital and celestial mechanics&lt;/li&gt;&#xA;    &lt;li&gt;Space policy, legislation, procedures, terminology&lt;/li&gt;&#xA;    &lt;li&gt;Scientific experiments done in microgravity environment&lt;/li&gt;&#xA;    &lt;li&gt;Enabling technologies and advances in medical science pertaining to space exploration&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="49" LastEditDate="2015-08-17T23:56:34.050" LastActivityDate="2015-08-17T23:56:34.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8053" PostTypeId="7" CreationDate="2015-02-08T00:05:37.517" Score="0" Body="&lt;ul&gt;&#xA;    &lt;li&gt;Anything not directly related to space exploration&lt;/li&gt;&#xA;    &lt;li&gt;Questions that are primarily opinion-based&lt;/li&gt;&#xA;    &lt;li&gt;Questions with too many possible answers or that would require an extremely long answer&lt;/li&gt;&#xA;    &lt;li&gt;The study of exosolar objects, except as they relate to space travel&lt;/li&gt;&#xA;    &lt;li&gt;Physical sciences, such as geology, astronomy, cosmology, meteorology, except as they relate to spacecraft and space exploration&lt;/li&gt;&#xA;    &lt;li&gt;Questions relating to amateur experiments that are extremely dangerous, such as home-brew rocket propellant&lt;/li&gt;&#xA;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;WARNING&lt;/strong&gt;: This community does not condone such experiments as there may be risk to your health (or even risk of death), and they may even be illegal in your jurisdiction. If you are considering making your own rocket propellant/fuel, please &lt;strong&gt;DO NOT DO THIS&lt;/strong&gt;! Rocket propellant is extremely dangerous, and the protection and storage requirements are best left to professionals.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Are you looking for reference materials or other lists of resources? Check out these resources on our meta site:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.meta.stackexchange.com/questions/249/resources-and-references-on-the-topic-of-space-exploration&quot;&gt;Resources and references on the topic of space exploration&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://space.meta.stackexchange.com/questions/472/collection-of-space-exploration-related-infographics&quot;&gt;Collection of space exploration related infographics&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://space.meta.stackexchange.com/questions/790/software-tools-for-space-exploration-available-online&quot;&gt;Software tools for space exploration available online&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="58" LastEditDate="2018-06-27T16:57:12.863" LastActivityDate="2018-06-27T16:57:12.863" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8054" PostTypeId="2" ParentId="8051" CreationDate="2015-02-08T02:15:52.880" Score="9" Body="&lt;p&gt;The most likely cause of the flurry of recent actual developments has to be the funding source. There have been development companies that have tried and failed, (Rotary Rocket, Kistler) due to lack of funding at crucial times. When you have a sugar daddy willing to fund you through the tough times, it is much easier to get through them.  That is a very major contributor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the Cubesat level, we are seeing basic smart phones sufficing as computer/control systems with sufficient sensors to do a lot of work that cost thousands to millions of dollars in the past. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That obviously is immaterial for the most part to a product like a Falcon 9, but does not hurt. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another possibility is the crowd sourcing model is allowing small contributions from many people to fund largish projects. It has its limits but can help companies get over that first hump. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;American companies get a freebie boost, since NASA documents are available to them. For example SpaceX credits the Apollo projects notes on the parachutes used back then to seriously reducing costs of developing chutes for the Dragon capsule. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-02-08T02:15:52.880" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8056" PostTypeId="2" ParentId="8047" CreationDate="2015-02-08T19:06:35.097" Score="10" Body="&lt;p&gt;&lt;a href=&quot;http://www.ulalaunch.com/about_Bios_SBruno.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;Salvatore T. &quot;Tory&quot; Bruno&lt;/a&gt;, president and chief executive officer for United Launch Alliance (ULA), shared this &lt;em&gt;&quot;Launch Vehicle Weight and Cost By Major Elements&quot;&lt;/em&gt; infographic in &lt;a href=&quot;https://twitter.com/torybruno/status/561933155951595520&quot; rel=&quot;nofollow noreferrer&quot;&gt;one of his recent tweets&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/aY2w2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aY2w2.jpg&quot; alt=&quot;ULA Launch Vehicle Weight and Cost By Major Elements&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was no additional description posted for it that I could find. It seems to be for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas V&lt;/a&gt; though (judging by the vehicle's and payload fairing shape) so for a launch vehicle utilizing &lt;a href=&quot;https://en.wikipedia.org/wiki/Common_Core_Booster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Common Core Booster&lt;/a&gt; (CCB) and not &lt;a href=&quot;https://en.wikipedia.org/wiki/Common_Booster_Core&quot; rel=&quot;nofollow noreferrer&quot;&gt;Common Booster Core&lt;/a&gt; (CBC) that &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta IV&lt;/a&gt; launch vehicles use. It should give you an overview though. Delta IV Heavy of course uses three CBC's, two as boosters and central one as its first stage. It also uses an extended payload fairing.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-02-08T19:12:45.187" LastActivityDate="2015-02-08T19:12:45.187" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8058" PostTypeId="1" AcceptedAnswerId="8061" CreationDate="2015-02-08T23:18:19.893" Score="8" ViewCount="650" Body="&lt;p&gt;The Air Force runs the range for launches out of Florida and it is aging hardware, with long reset times, radars dropping off (Like first attempt of DSCOVR, and SMAP launch).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot find a reference but I recall the cost to upgrade the range was some crazy amount of money. A 10 year deal to maintain the range is on the order of &lt;a href=&quot;http://spacenews.com/42473raytheon-team-wins-2-billion-air-force-range-support-contract/&quot; rel=&quot;noreferrer&quot;&gt;two billion.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping that in mind, what kind of range will SpaceX be building for the Boca Chica/Brownsville space port they are building in Texas? &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-01-16T07:00:48.197" Title="For SpaceX's Texas launch site, what kind of range radar/tracking will they have?" Tags="|launch|spacex|launch-site|tracking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8059" PostTypeId="2" ParentId="6133" CreationDate="2015-02-08T23:36:29.187" Score="7" Body="&lt;p&gt;Yes, they can, at least they are designed to do such. Here's some links supporting that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.space.com/21541-nasa-orion-spacecraft-reusable.html&quot;&gt;http://www.space.com/21541-nasa-orion-spacecraft-reusable.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Orion_%28spacecraft%29#Crew_module_.28CM.29&quot;&gt;http://en.wikipedia.org/wiki/Orion_%28spacecraft%29#Crew_module_.28CM.29&lt;/a&gt; &quot; The CM is designed to be refurbished and reused.&quot;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.informationweek.com/government/leadership/nasa-orion-space-capsule-has-surprising-brain/d/d-id/1297427&quot;&gt;http://www.informationweek.com/government/leadership/nasa-orion-space-capsule-has-surprising-brain/d/d-id/1297427&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As for how re-usable it will be, well, only time will tell. There was mention in the above articles of Orion having a kind of plug and play architecture where parts could easily be swapped, meaning that it should be easy to refurbish an Orion, but only time will tell just how easy it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.bizjournals.com/denver/blog/boosters_bits/2015/02/lockheed-martin-reports-on-orions-first-space.html?ana=yahoo&amp;amp;page=all&quot;&gt;this article&lt;/a&gt;, comes the phrase:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ultimately, LMSS wants each Orion capable of flying on half a dozen missions or more.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I believe LMSS is Lockheed Martin Space Systems, the prime contract on Orion.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-02-22T20:41:20.700" LastActivityDate="2015-02-22T20:41:20.700" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8060" PostTypeId="1" AcceptedAnswerId="13149" CreationDate="2015-02-08T23:42:37.007" Score="20" ViewCount="12280" Body="&lt;p&gt;In reading about SLS, I learned that the reason they won't reuse their rocket is that the weight of the parachutes to enable recovery is too much of a sacrifice. How is the weight of the hardware and fuel NOT too much of a sacrifice for Falcon 9?&lt;/p&gt;&#xA;" OwnerUserId="8436" LastEditorUserId="51" LastEditDate="2015-02-09T00:27:15.987" LastActivityDate="2022-09-06T16:31:18.880" Title="Why can the Falcon 9 be reused while the SLS cannot?" Tags="|spacex|falcon-9|reuse|sls|recovery|" AnswerCount="7" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8061" PostTypeId="2" ParentId="8058" CreationDate="2015-02-09T00:05:07.293" Score="8" Body="&lt;p&gt;SpaceX is teaming up with the University of Texas Brownsville, UT  Rio Grande Valley and local development agencies on STARGATE - a combination education, research, tech development and space tracking center using phased array antennas. It'll be situated next to their Mars Crossing mission control centers at the Texas Launch Site. From &lt;a href=&quot;https://web.archive.org/web/20160427163049/http://www.utb.edu/stargate/Pages/STARGATE.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;UT Brownsville page on STARGATE&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As a research center adjacent to SpaceX's future command and control&#xA;  center, STARGATE will developing new radio frequency based (RF)&#xA;  technologies for a wide range of academic and commercial applications.&#xA;  STARGATE will enable our students to be directly involved in all&#xA;  aspects of a space mission, including the design of the spacecraft,&#xA;  testing, launch, and orbital operations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Initially the site will be a Falcon 9/Falcon Heavy launch complex similar to their Vandenberg SLC-4E and its neighboring landing pads.&lt;/p&gt;&#xA;" OwnerUserId="8438" LastEditorUserId="9060" LastEditDate="2017-01-16T07:00:48.197" LastActivityDate="2017-01-16T07:00:48.197" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8062" PostTypeId="2" ParentId="8060" CreationDate="2015-02-09T00:27:02.677" Score="13" Body="&lt;p&gt;The initial SpaceX plan for recovering a first stage was parachutes, into the water. After all they had tons of data available from NASA on SRB recoveries (266 of them?). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But after the tried on the first few flights of the Falcon 9 1.0 they realized they needed to slow the rocket down, before it hit atmosphere (where parachutes would work) or else they got too hot on the way down and burned up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They took the lessons learned from the Falcon 9 v1.0 and rolled them into the Falcon 9 v1.1 which is what you see launching today. (Of course, they have since added grid fins, changed the amount of control fluid for those fins, are about to uprate the Merlin 1D engine in the next few flights, and probably many more changes they won't tell us about).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That should give you a pattern on how they approached reuse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS is planning on launching payloads once every 4 years if we are lucky. It is unlikely they will be willing to experiment, learn, react, and update the same way in that kind of a model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When SLS was designed, recovery was not considered a priority. So while in the Shuttle days, the SRB's were at least recovered, and mostly reused (after a LOT of rework, to the point, there was almost no cost savings) for SLS they are being thrown away, no attempts at recovery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whereas the SSME (RS-25D engines) that the Shuttle used could be reused (after significant and expensive refurbishment work) the SLS will not be recovering them, instead throwing them away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am known to be cynical about SLS in this aspect, so take my perspective with that grain of salt, as I suggest that cost effectiveness was never a concern for SLS, only the number of jobs it would maintain in crucial states. (I think NASA is a great research organization. I think they are a terrible launch provider. Big difference.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you consider that versus the SpaceX model of trying to make money, and in the process bring launch dominance back to the US, increase humanities presence in space then it is obvious why. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="238" LastEditDate="2015-02-09T06:16:19.670" LastActivityDate="2015-02-09T06:16:19.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8063" PostTypeId="5" CreationDate="2015-02-09T00:31:38.663" Score="0" Body="&lt;p&gt;The Space Launch System (SLS) is meant to use Shuttle legacy hardware to build a larger launch vehicle for missions beyond earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core is powered by four RS-25D or E engines. (RS-25D are leftover SSME's from the Shuttle program.  The RS-25E is meant to be a cheaper to build non-reusable version of the SSME). The center core booster is based around the diameter and design of the Space Shuttle External Tank, stretched and strengthened to carry a load directly above, instead of hanging off the side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRBs are 5 segment versions of the Shuttle 4 segment SRBs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of it is recoverable, differing from the Shuttle program. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initial payload, without a better upper stage will be in the 70 metric tonne range. With a future high ISP upper stage, the expectation is reaching 130 metric tonnes, allowing single launches to Mars of large enough payloads to be useful for humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch rate is expected to be very low, as infrequently as once every 2-4 years which will be a difficulty to overcome by the system.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-02-09T00:31:38.663" LastActivityDate="2015-02-09T00:31:38.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8064" PostTypeId="4" CreationDate="2015-02-09T00:31:38.663" Score="0" Body="The Space Launch System (SLS) is NASA's rocket replacement for the Space Shuttle for large payloads. Use for questions about the vehicle and the launching of any payloads." OwnerUserId="51" LastEditorUserId="22326" LastEditDate="2022-11-29T19:14:55.413" LastActivityDate="2022-11-29T19:14:55.413" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8065" PostTypeId="2" ParentId="8060" CreationDate="2015-02-09T00:34:56.930" Score="22" Body="&lt;p&gt;SpaceX has decided that reusability is a critically important goal to keep long-term costs down, therefore they've designed in the ~15%-30% performance margin they need to achieve reusability (the lower figure for mid-Atlantic barge landing; the higher figure for boost-back to the launch site). Cost is critical for SpaceX because Falcon 9 has to compete commercially with other medium-lift boosters like Delta IV, Atlas V, and Soyuz. If they achieve the reusability they hope to, the increased cost of a 15% more powerful booster will be more than covered.&lt;/p&gt;&#xA;&lt;p&gt;SLS is a much bigger rocket, carrying 5-10 times the payload. The only launches that really require that are crewed missions beyond LEO. Such missions are few and far between and don't have to be cost-effective in the same way that a communications satellite launch has to be.&lt;/p&gt;&#xA;&lt;p&gt;Falcon 9's reusability plan is based on precision powered vertical landing. SLS's core stage is much larger and heavier, and landing it in a similar way would be a substantially harder problem.  Parachute landings would probably be feasible, but that makes recovery and refurbishment more time-consuming and difficult.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2022-09-04T10:53:22.403" LastActivityDate="2022-09-04T10:53:22.403" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="8066" PostTypeId="1" CreationDate="2015-02-09T00:50:17.600" Score="10" ViewCount="363" Body="&lt;p&gt;In recent photos of &lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/spacecraft.html#&quot; rel=&quot;noreferrer&quot;&gt;DSCOVR&lt;/a&gt; I've noticed it appears to be missing eight solar cells in its array.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Twwgd.jpg&quot; alt=&quot;DSCOVR undergoing testing&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4CUdQ.jpg&quot; alt=&quot;Photo of the DSCOVR spacraft at NASA/GSFC prior to shipment to Cape Canaveral, FL (image credit: NASA)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In photos from the &lt;a href=&quot;http://spaceflightnow.com/news/n0903/01dscovr/&quot; rel=&quot;noreferrer&quot;&gt;original &quot;unboxing&quot; from storage&lt;/a&gt; it appears to have all its cells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gqlER.jpg&quot; alt=&quot;DSCOVR in Feb 2009.  Credit: Stephen Clark/Spaceflight Now&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this?&lt;/p&gt;&#xA;" OwnerUserId="4704" LastEditorUserId="4704" LastEditDate="2015-02-09T05:10:31.480" LastActivityDate="2015-02-09T06:22:38.577" Title="Why does DSCOVR appear to be missing solar cells?" Tags="|artificial-satellite|spacecraft|solar-power|dscovr|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8067" PostTypeId="5" CreationDate="2015-02-09T00:52:34.127" Score="0" Body="&lt;p&gt;The &lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/&quot; rel=&quot;nofollow&quot;&gt;Deep Space Climate Observatory&lt;/a&gt; is a collaboration between NOAA and the USAAF to monitor space weather and the solar wind.  Its primary mission is to provide warning of activity which may be harmful to public infrastructure.&lt;/p&gt;&#xA;" OwnerUserId="4704" LastEditorUserId="4704" LastEditDate="2015-02-09T01:27:01.847" LastActivityDate="2015-02-09T01:27:01.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8068" PostTypeId="4" CreationDate="2015-02-09T00:52:34.127" Score="0" Body="NOAA's &amp; USAAF Deep Space Climate Observatory for monitoring space weather." OwnerUserId="4704" LastEditorUserId="4704" LastEditDate="2015-02-09T01:26:58.163" LastActivityDate="2015-02-09T01:26:58.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8069" PostTypeId="2" ParentId="8066" CreationDate="2015-02-09T06:07:29.750" Score="5" Body="&lt;p&gt;@TildalWave mentioned DSCOVR was originally Triana built around the SMEX-Lite bus.  I think the key is that Triana originally had all its cells, but DSCOVR does not.  I believe, as @RussellBorogove commented, the original solar array has been replaced with an upgrade.  The cells have been left off to save weight, cost and heat dissipation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SMEX-Lite, when proposed in 1996, was said to be able to deliver 150W to the instruments via its solar arrays.  Schematics for DSCOVR say 600W.  SMEX-Lite was specifically designed with modular solar arrays.  I can only assume with the advances in solar power in the last 10 years they have taken advantage of this and upgraded them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SMEX-Lite also talks about the problem of excess power...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Excess power is dissipated in shunts mounted on the backside of the solar arrays. TRACE and WIRE are working towards the total elimination of the shunts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I agree with @RussellBorogove that modern solar cells are so much better than the originals that a full array would produce excess power, so they simply eliminated them.  This eliminates the need for shunts and reduces the weight of the array.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://klabs.org/DEI/References/avionics/small_sat_conference/1996/smexlite.pdf&quot; rel=&quot;noreferrer&quot;&gt;SMEX•LITE - NASA’s NEXT GENERATION SMALL EXPLORER&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www-pm.larc.nasa.gov/site/doc-library/references/DSCOVR/NAS.Triana.report.12.99.pdf&quot; rel=&quot;noreferrer&quot;&gt;Triana - a Deep Space Earth and Solar Observatory&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/pdf/DSCOVR-facts-Jan15.pdf&quot; rel=&quot;noreferrer&quot;&gt;NOAA DSCOVR Fact Sheet&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceflight101.com/dscovr.html&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://public.ccsds.org/sites/databases/Lists/CCSDSProducts/Attachments/23/SwalesSMEX.pdf&quot; rel=&quot;noreferrer&quot;&gt;Swales Aerospace SMEX-Lite&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="4704" LastEditorUserId="4704" LastEditDate="2015-02-09T06:22:38.577" LastActivityDate="2015-02-09T06:22:38.577" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8070" PostTypeId="2" ParentId="8060" CreationDate="2015-02-09T06:14:55.130" Score="17" Body="&lt;p&gt;&lt;strong&gt;Design criteria&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SLS was designed to be a backstop to replace the Shuttle in the near term, and to support missions to Mars in the longer term. Reusability was not on the agenda then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 is a step in Elon Musk's vision of enabling self sustaining human civilization of Mars. So his underlying design philosophy is different: it's more long term. This is explained by Musk himself, in a One on One interview &lt;a href=&quot;https://www.youtube.com/watch?v=PULkWGHeIQQ&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt; previously &lt;a href=&quot;https://space.stackexchange.com/a/7954/8406&quot;&gt;posted&lt;/a&gt; by user flangford.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Musk's view is that to land anywhere except Earth requires rocket powered soft landings, so SpaceX had better become good at them. Also he believes that competition will drive prices down, making space more accessible. Reuse is a key component of this, so implementing a feature now that will be needed eventually anyway serves both purposes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;noreferrer&quot;&gt;SpaceX&lt;/a&gt; an F9 launch is currently priced at \$61.2M, delivering 13,150kg to LEO at \$4654/kg. In &lt;a href=&quot;https://space.stackexchange.com/a/1991/8406&quot;&gt;this 2013 answer&lt;/a&gt; PearsonArtPhoto quotes Falcon 9 at \$4,109, Ariane 5 at \$10,476, Delta IV at \$13,072, and Atlas V at $13,182. In round numbers SpaceX is quoting 30-40% of the price of other Western launchers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the price competition gets stronger with Falcon Heavy. The same &lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;noreferrer&quot;&gt;SpaceX page&lt;/a&gt; quotes a Falcon Heavy launch at \$85M, delivering 53,000kg to LEO at \$1604/kg, 1/8th the cost by a Delta IV (and also comparing 2013 dollars to 2015 dollars).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;a href=&quot;https://www.youtube.com/watch?v=PULkWGHeIQQ&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt; Musk also mentions that SpaceX is planning a bigger vehicle to follow Falcon Heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ESA has responded with the &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Launch_vehicles/Adapted_Ariane_5_ME&quot; rel=&quot;noreferrer&quot;&gt;Ariane 5 Midlife Evolution&lt;/a&gt;. The current &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Launch_vehicles/Ariane_6&quot; rel=&quot;noreferrer&quot;&gt;Ariane 6&lt;/a&gt; proposal does not include reuse, but does mention price competition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Conclusion&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference is due to completely different design objectives, driven by different corporate philosophies. Competition between launch systems is expected to drive costs down, and as costs drop then more missions will become practical, leading to an expansion of the launch market. In the SpaceX view, reuse is key.&lt;/p&gt;&#xA;" OwnerDisplayName="user8406" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-02-09T06:14:55.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8072" PostTypeId="2" ParentId="7981" CreationDate="2015-02-09T12:10:07.440" Score="4" Body="&lt;p&gt;The short answer is: a straight line in space is only possible with &lt;strong&gt;continuous acceleration&lt;/strong&gt; (because without acceleration the Sun forces the path to be an elliptical or hyperbolic orbit, depending on total energy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do the math (and mission design engineers do it all day) you will also find that the energy required (read &lt;em&gt;fuel&lt;/em&gt;) would increase compared to a flight path (&lt;em&gt;trajectory&lt;/em&gt; in space lingo) with coasting along an elliptical path. The most energy efficient trajectory between two circular orbits is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Hohmann Transfer Orbit&lt;/a&gt;. This trajectory basically consists of an initial short push (&lt;em&gt;delta v&lt;/em&gt; in space lingo) to change the orbit from circular to elliptic, then a looooooong coast to the large circular orbit, then another short push to turn the elliptic orbit into a circular once again.&lt;/p&gt;&#xA;" OwnerUserId="716" LastEditorUserId="716" LastEditDate="2015-02-09T13:26:48.160" LastActivityDate="2015-02-09T13:26:48.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8073" PostTypeId="1" AcceptedAnswerId="8075" CreationDate="2015-02-09T12:51:24.410" Score="16" ViewCount="16021" Body="&lt;p&gt;My wife shared this picture with me today:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UV81O.gif&quot; alt=&quot;picture of a cartoon corgi doing the doggie paddle in space and moving forward&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adorable, but it got me thinking about that trope - swimming in space.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen it before, mostly in cartoons and not live-action shows, the latter of which tend to have either more realistic space stations that astronauts pull themselves along on rails to get around in, or fictional artificial gravity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question then is three-part:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Can a person actually 'swim' to get around in a real spacecraft?&lt;/li&gt;&#xA;&lt;li&gt;Could they do this in the vacuum of space? (Assuming they have proper space protection)&lt;/li&gt;&#xA;&lt;li&gt;Could a Pembroke Welsh Corgi on the ISS do a doggie paddle to get around?  &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="8442" LastEditorUserId="8442" LastEditDate="2018-03-30T13:16:22.707" LastActivityDate="2021-06-17T15:35:32.920" Title="Can you swim in space?" Tags="|movement|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8074" PostTypeId="1" CreationDate="2015-02-09T12:58:59.500" Score="25" ViewCount="5200" Body="&lt;p&gt;Space Shuttle was designed to be partially reusable but was later &lt;a href=&quot;https://space.stackexchange.com/q/1173/2948&quot;&gt;shut down&lt;/a&gt; because it turned out to be very expensive to run compared to disposable spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now Falcon 9 is suddenly reusable to a large extent &lt;em&gt;and&lt;/em&gt; presumably not that expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does Falcon 9 do it after Space Shuttle failed to do the same?&lt;/p&gt;&#xA;" OwnerUserId="2948" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2018-02-13T12:48:43.937" Title="How can the reusable Falcon 9 succeed when the reusable Space Shuttle failed?" Tags="|launch|spacex|falcon-9|space-shuttle|reuse|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8075" PostTypeId="2" ParentId="8073" CreationDate="2015-02-09T13:07:07.153" Score="13" Body="&lt;p&gt;You can sort of swim, but it would be very slow. The viscosity of air vs water is very low. Therefore the 'scoop' your hands or feet can get of the fluid, to propel it, and thus move you around would need to be much higher. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you had fans (or 'wings') on your arms you would be more effective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the corgi?  Not likely, or at least very, very slowly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be more effective inhaling with a wide open mouth slowly, then quickly exhaling through a small mouth in the direction opposite you wish to go.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-02-09T13:07:07.153" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="8076" PostTypeId="2" ParentId="8060" CreationDate="2015-02-09T13:08:51.167" Score="5" Body="&lt;p&gt;&lt;strong&gt;They are placing their bets differently&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While SpaceX is quite innovative with their reusable launch system, NASA has a more conservative approach and builds a fully expendable launch system because that's what has proven to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After several failures, SpaceX managed to pull it off and fly the first stage back to the launchpad. But if this feat can be replicated reliably and if it is actually cheaper is still open. So the future will tell us which approach will prevail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the SLS is trying to break the record for the most powerful launch system ever built. A fully expendable system will always be more powerful than a reusable one with the same launch-mass because you don't need any landing equipment and no part needs to be built more sturdy than required to survive just a single launch.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2015-12-22T11:46:01.323" LastActivityDate="2015-12-22T11:46:01.323" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8077" PostTypeId="2" ParentId="8074" CreationDate="2015-02-09T13:27:08.130" Score="33" Body="&lt;p&gt;There are many contributing factors to this problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle was designed in the 1970's and technology has matured since then.  Additionally, the issues the Shuttle ran into (While in truth may have been predictable at the time) are now more obvious and a new design can try to avoid them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider the simple case of the heat shield on the Shuttle. It was composed of 10's of thousands of small tiles. It was not clear they could make large tiles, of the right shape, that could handle the bending loads and whatnot. But that added a monstrous manpower cost on refurbishing the orbiter after every launch. You saw that NASA tried replacing some of the tiles where they could with larger blankets later in the program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX went with a known workable system (PICA) refreshed for the new century, and used fairly large pieces. (Also a capsule is much smaller to coat than something the size of the Orbiter)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engines on the Shuttle were top of the line, possibly highest performing engines ever built and used in production. But along with that comes maintenance costs which again were being mitigated somewhat at the end of the program with various upgrades to the SSMEs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX went with the most reliable design they could find in the Merlin family, a pintle injector and spent a lot of time and effort on making the engine affordable and performant. They also iterated faster. While it is true elements of the SSME were upgraded over the life of the program, the Merlin family progressed in a much shorter time from the Merlin 1A, 1B, 1C, and 1D with a further uprating of the 1D pending soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME are in the 660Klb thrust range, whereas the Merlin family started in the 70Klb range and has upgraded to the 195Klb (and on its way higher again with the Block 5 Falcon 9), which is a much simpler problem, and easier to design, and easier to reuse in theory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider size. The STS system is huge, with 7 million lbs of thrust on liftoff vs a Falcon 9 with only 1.3 million lbs of thrust. Obviously mass of the system is vaguely similar, since you need a T/W of greater than 1 to liftoff. (Else you sit on the pad burning propellant).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX's system is much smaller, and thus much much easier to recover. Consider the problem of recovering a capsule (Dragon) vs the Orbiter. Then consider reuse. (Which to be fair, SpaceX has yet to reuse a Dragon capsule, regardless of plans to do so, Edit: 2 years later, 2 Dragons have reflown and the remaining 10 CRS missions are expected to use reused Dragons). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a design decision the choice of wings vs vertical landing is a huge differentiator. Only time will tell if SpaceX made the right choice. NASA clearly did not make the right choice with their overall approach. (SNC would argue that wings can be done properly, time would tell, but not clear they will ever launch).  (Edit: 2 years later they have landed 23 first stages without wings pretty much without fail. So probably a good design choice.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle needed a standing army of something like 24,000 people to maintain the system.  SpaceX entire employment is still under 5000. Consider the cost of salaries for 24,000 people vs 5,000 and divide that over the number of launches a year. This is a large contributor to system costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For political reasons the Shuttle was built across many states, much like the European Ariane 5 is built. This way different senators and congress-critters would be willing to vote for it, as it provided copious numbers of jobs in their districts. This is a recipe for inefficiency. As has been shown by those two programs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX builds almost everything in one site (Hawthorne, CA), tests in a second (McGregor, TX), and launches from a third (LC-40). (Soon to be fourth (LC-39A) and fifth (Brownsville, TX). For Polar orbits they launch from Vandenberg Air Force Base in California (LC-4). When they discuss their next generation launcher, they suggest they plan to build and test it next to the launch site.  Mostly because it will be too big to transport.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could probably go on for longer. Suffice to say this is comparing peanuts to watermelons.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-02-13T11:56:22.357" LastActivityDate="2018-02-13T11:56:22.357" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8078" PostTypeId="1" CreationDate="2015-02-09T16:41:28.860" Score="8" ViewCount="391" Body="&lt;p&gt;Dawn visits both Vesta and Ceres, thanks to its patient solar electric ion propulsion. AFAIK more xenon and perhaps a little more hydrazine for the attitude thrusters, would've sufficed to give it power to visit even more targets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what is the geography of a multi asteroid trip like? Looking at the diagrams below, asteroid families are identified with similar semi-major axis, inclination and eccentricity. Could a SEP spacecraft like Dawn+ visit several asteroids a year within such a family? Massalia and Koronis look pretty well concentrated. And aren't most NEA thought to come from one single family with low inclination in the inner part of the Main Belt and wouldn't it be interesting and easier to visit them out there than trying to catch them at perihelion where they are more spread out due to differences in eccentricities and higher velocities?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Couldn't Dawn make even one single near passage of any other asteroid during its long trip from Vesta to Ceres? Several other spacecrafts, like Rosetta and Chang'e 3 (upper stage) and New Horizons have done asteroid flybys without even being Main Belt missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To understand the scale in these diagrams, 0.1 AU is about 250 times the distance Earth-Moon. The distance between the perihelion Dwarf-Ceres is 0.5 AU which is about the distance between Earth and Mars when at the closest. The inclination of the Moon is about 1.5 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Inclination and semi-major axis:&lt;/em&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/ehNVV.png&quot; alt=&quot;Inclination and semi-major axis&quot;&gt;&#xA;&lt;em&gt;Inclination and eccentricity:&lt;/em&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/xrrJ7.png&quot; alt=&quot;Inclination and eccentricity&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-02-09T17:20:21.693" LastActivityDate="2017-01-09T13:19:57.010" Title="How does one spacecraft best visit multiple asteroids?" Tags="|orbital-mechanics|mission-design|dawn-mission|asteroid-belt|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8079" PostTypeId="2" ParentId="8074" CreationDate="2015-02-09T17:02:59.100" Score="24" Body="&lt;p&gt;To avoid the hazard of going off-topic, I will start by re-framing the question into 2 different parts. This is done so that all the evidence &lt;em&gt;directly&lt;/em&gt; builds to an answer to the question. In this case, the question is:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Why did the Shuttle's effort at economic reusability fail?&lt;/li&gt;&#xA;&lt;li&gt;Why do many experts believe that the current SpaceX attempt will not fail in the same way?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Firstly, we need to define &quot;success&quot;. For this definition, I will use the same reference which is used to make the overall argument. My own reference also uses a reference to make its argument, which is much more esoteric, so here they both are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Secondary reference: &lt;a href=&quot;http://history.nasa.gov/SP-4221/ch6.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Space Shuttle Decision&lt;/a&gt; by T. A. Heppenheimer, Ch 6&lt;/li&gt;&#xA;&lt;li&gt;Primary reference: Budget Bureau, &quot;NASA Issues Paper,&quot; August 1969: attachment, &quot;Space Transportation System,&quot; August 22, 1969.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You can see in their tables that different scenarios were analyzed and compared to the alternative option of the Titan III. For our purposes, Titan III is the alternative expendable launch system. Thus, I can reasonably claim that success was evaluated (and should be evaluated) on the basis of the &lt;strong&gt;cost of the reusable launch system versus the alternative expendable system&lt;/strong&gt;. The degree of success is defined as the rate of return using standard financial analysis. A rate of return of 0% or less is a &quot;no go&quot;, but the competitive rate of return is a matter for debate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will volunteer the fact that, at that year, the design of the space shuttle wasn't finalized anyway. So this doesn't necessarily apply to the final version. However, the underlying claim is &lt;em&gt;so strong&lt;/em&gt; that I think you'll accept that it probably won't matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Somewhat trivially, the economics of a reusable system depends on the number of launches. Firstly you have a capital investment of R&amp;amp;D and hardware, then that cost is later recouped from reduced needs of manufacturing. The studies we have from the design of the Space Shuttle reflect exactly this. They give the competitiveness of the launch system as a function of launch rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual launch rate of the space shuttle was 4.5 launches per year, as historical fact. The above study found that the system would break even (defined as 0% return rate, but still recouping the investment) if they launched a few less than 28 times per year. There were more studies than just this, but all had the same general thinking. The flight rates studied tended to &lt;em&gt;start&lt;/em&gt; at 40-or-so per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We come to a simple conclusion - anyone rationally reading the literature at the time would have concluded that the Space Shuttle would be horrendously uneconomic at the flight rates for which it was used at. The fact that it was still developed probably reflects 1) over-optimism by the designers of Congress' demand for spaceflight and also 2) over-optimism by the policymakers of the technology's performance and flexibility. In the end, the USA used a solution to solve a problem that it wasn't exactly designed for. That's mostly because the intended use evolved over time, and the design was too inflexible to adapt (partially because of enormous sunk costs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why is the F9R different?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let us apply the same logical approach here. What is the expendable alternative to the F9R (reusable Falcon 9)? That is simply the F9. Similar to how a &quot;broken&quot; escalator becomes stairs, a failed reusable launch system simply becomes an expendable launch system. This happened in January 2015 when the first stage recovery effort failed after it boosted a successful launch. The shuttle program, on the other hand, was worlds apart from its alternative expendable system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, there are lots of reasons to think the F9R will fare much better:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it can be an incremental development process&lt;/li&gt;&#xA;&lt;li&gt;without people aboard the recovered stage, enormous costs are cut&lt;/li&gt;&#xA;&lt;li&gt;new customers can be brought in if costs are reduced&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;NASA was a massive space program which developed a reusable launch system, and then shrunk into a smaller space program. SpaceX is a tiny launch service company that is hoping that reusability will grow it into a giant space company.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of these arguments are robust, but the demand-side factors may still plague SpaceX in the same way they did for the Shuttle. The break-even launch rate for the F9R over the F9 is lower than what it was for the Shuttle, but it might still be greater than their current rate. In that sense their success is not guaranteed, but anyone with demand-side optimism would reasonably predict success. In the worst case scenario, this means their investment was wasted. The worst case scenarios for the Shuttle were much more dire.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="415" LastEditDate="2017-03-31T09:08:59.937" LastActivityDate="2017-03-31T09:08:59.937" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8080" PostTypeId="2" ParentId="8073" CreationDate="2015-02-09T18:29:56.053" Score="5" Body="&lt;p&gt;A person &lt;em&gt;could&lt;/em&gt; swim. But that would take many, many effort to generate even a little of velocity. The viscosity of air is much lower than the viscosity of water. When you swim in water you are practically pushing the water away, creating a force that makes you go the direction you're swimming to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine this with air. You could simulate this yourself too: Standing on the bottom of a swimming pool and make a swimming movement with your arms and feel the effect of you pushing forward.&#xA;Then try it in normal air, almost nothing would happen. Then again with a propeller it's a different story.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the vacuum of space you can't do this of course. Since there's no mass to push away, nothing to swim in, nothing can make you push forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Corgi in the ISS could theoretically do that. But, that would be more of an disappointment if you compare that with the moving picture. It would take a long time a little, so I don't think the dog will use that to move itself around.&lt;/p&gt;&#xA;" OwnerUserId="648" LastActivityDate="2015-02-09T18:29:56.053" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8081" PostTypeId="1" CreationDate="2015-02-09T18:43:04.490" Score="16" ViewCount="1654" Body="&lt;p&gt;I was watching the webcast for Falcon 9 Flight 15 (launching &lt;a href=&quot;http://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory&quot;&gt;DSCOVR&lt;/a&gt;) when they scrubbed their first launch attempt due to some issues during the terminal countdown. Before the scrub, the narrator noted that if the terminal count had to be held for any reason, the launch would be postponed to a backup date because the launch window was instantaneous - which is what ended up happening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm curious &lt;strong&gt;why the launch window was instantaneous&lt;/strong&gt; in a mission like this (to Sun-Earth L1). I'm familiar with how launch windows for ISS resupply missions are pretty slim, due to the timing of having to catch up to the station. What factors would cause such a small launch window for a mission of this type?&lt;/p&gt;&#xA;" OwnerUserId="6772" LastEditorUserId="51" LastEditDate="2015-07-09T21:09:17.927" LastActivityDate="2021-02-24T13:58:57.027" Title="Why would a mission to Sun-Earth L1 have an instantaneous launch window?" Tags="|launch|spacex|falcon-9|lagrangian-points|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8082" PostTypeId="2" ParentId="8078" CreationDate="2015-02-09T18:55:24.093" Score="4" Body="&lt;p&gt;Those charts don't tell the whole story. Keep in mind that inclination, eccentricity, and SMA are only three of six &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;nofollow&quot;&gt;orbital elements&lt;/a&gt; that define the position of an orbiting body. Two bodies in the exact same orbital path (inclination, eccentricity, SMA, argument of periapsis, and LAN) could be in different positions on that orbit, on opposite sides of the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, the plan is to leave Dawn in Ceres orbit at the conclusion of the mission, rather than continue on to other targets. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-02-09T18:55:24.093" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8083" PostTypeId="1" AcceptedAnswerId="8096" CreationDate="2015-02-09T21:27:48.557" Score="20" ViewCount="2133" Body="&lt;p&gt;This &lt;a href=&quot;http://en.wikipedia.org/wiki/Delta_II#Vehicle_description&quot; rel=&quot;noreferrer&quot;&gt;diagram&lt;/a&gt; of the Delta II 74xx-series shows 4 booster rockets mounted rather asymmetrically. All other things being equal, it looks like the net thrust of the set would push the rocket off-axis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bu3jy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why are the boosters not mounted in a radially symmetrical pattern?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;@pericynthion points out that as long as all the booster thrust axes point through the center of mass, there's no net rotation. However, center of mass is going to shift as core stage propellant is consumed, so (assuming the booster nozzles are fixed) it's not going to be exactly zero. Furthermore, it makes the net thrust of the whole rocket not aligned with the vertical axis, which is going to produce some (small) amount of aerodynamic shear force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is anything done to correct for these imbalances, or are they just small enough to ignore? Do the individual boosters have differently-canted nozzles, or is it corrected by center engine gimbaling or some other mechanism?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Update: TIL there are Atlas V configurations with a &lt;em&gt;single&lt;/em&gt; radially attached solid rocket booster! The core engine and SRB both just vector to keep everything upright and it accepts the high AOA. &lt;a href=&quot;https://www.youtube.com/watch?v=dME5amgp0Ug&quot; rel=&quot;noreferrer&quot;&gt;Check it out.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cGpu5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cGpu5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-04-25T19:45:04.747" LastActivityDate="2016-04-25T19:45:04.747" Title="Why were Delta II SRBs asymmetrically mounted?" Tags="|launch|rockets|history|solid-fuel|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="8085" PostTypeId="2" ParentId="8081" CreationDate="2015-02-09T23:24:32.883" Score="1" Body="&lt;p&gt;While I am no expert, one of the reasons is the limited restart capability of the second stage. Once the second lights, it is staying lit. This means it can't fine tune the transfer orbit nor correct any errors with manoeuvres that require another burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically it is a direct accent to the transfer orbit.&lt;/p&gt;&#xA;" OwnerUserId="1076" LastActivityDate="2015-02-09T23:24:32.883" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8086" PostTypeId="1" AcceptedAnswerId="8090" CreationDate="2015-02-09T23:46:59.830" Score="5" ViewCount="1656" Body="&lt;p&gt;In the article &lt;a href=&quot;http://arxiv.org/pdf/1201.2687v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nomads of the Galaxy&lt;/a&gt; (or &lt;a href=&quot;http://www.fromquarkstoquasars.com/planet-go-rogue-video/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt;), it is said that there are possibly 100000x more &quot;free planets&quot; (not orbiting a particular star) than stars. If the size distribution of &quot;free objects&quot; is similar to &lt;a href=&quot;http://astro.uwo.ca/~wiegert/papers/2007AJ.133.1609.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;size distribution of asteroids&lt;/a&gt; (some power law - much more small object than large objects), then it could be expected that there are a considerable amount of smaller objects which migrate between the sphere of influence of one star to another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In such a case it would be better to describe the situation as star system is immersed in some continuum (like gas) of free objects with some non-negligible background density, rather than to speak about discrete Oort clouds.&#xA;Is there any model or other work which addresses this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I'm interested in the connection to the possibility of interstellar travel)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is a graphical illustration of my question. Which is more realistic, the upper or the lower chart?&#xA;&lt;img src=&quot;https://i.stack.imgur.com/i8iKx.png&quot; alt=&quot;mass distribution vs. distance&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8393" LastEditorUserId="1142" LastEditDate="2015-02-10T04:58:04.253" LastActivityDate="2015-02-10T04:58:04.253" Title="Are Oort clouds of stars rather discrete or connected?" Tags="|interstellar-medium|oort-cloud|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8087" PostTypeId="1" CreationDate="2015-02-10T00:10:34.453" Score="28" ViewCount="7929" Body="&lt;p&gt;If you have a rough map of Mars a picture would help, but how far away are the three big rovers on Mars from each other? Is there any possibility of &quot;roadside assistance&quot; on Mars to &quot;unstick&quot; and recharge Spirit? Curiosity, being as big as she is, should have no problem nudging Spirit out of her predicament and angling her toward the sun to recharge, get back to work, and quit slacking off!!&lt;/p&gt;&#xA;" OwnerUserId="8450" LastEditorUserId="25" LastEditDate="2015-02-10T11:37:50.687" LastActivityDate="2021-02-26T13:09:38.597" Title="Can either Opportunity or Curiosity explore the state of Spirit?" Tags="|mars|rovers|curiosity|spirit|opportunity|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8089" PostTypeId="2" ParentId="8087" CreationDate="2015-02-10T00:44:41.980" Score="34" Body="&lt;p&gt;Not a chance. From NASA, here's the locations of everything that's landed on Mars (Well, NASA missions)&lt;/p&gt;&#xA;&lt;p&gt;Curiosity is kind of close to Spirit, but keep this in mind. The equatorial circumference is about 13,000 miles. I'd guess the distance is over 1000 miles, far further than any rover has gone before, or is likely to ever go. There's just much more interesting things to do than go bail out a dead rover across the planet. &lt;a href=&quot;https://www.planetary.org/space-images/mars_landing_site_map_lakdawalla&quot; rel=&quot;nofollow noreferrer&quot;&gt;This map&lt;/a&gt; from Emily Lakdawalla of the Planetary Society shows where all of the spacecraft are on Mars, including failed ones.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qOD0j.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qOD0j.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Keep in mind they have multiple landers so that one can learn about different areas of Mars. Otherwise, they'd only need one rover... And while a theoretical mission could be done to save Spirit, it's just not practical.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2021-02-26T13:09:38.597" LastActivityDate="2021-02-26T13:09:38.597" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="8090" PostTypeId="2" ParentId="8086" CreationDate="2015-02-10T00:58:22.617" Score="6" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_comet&quot;&gt;Interstellar comets&lt;/a&gt; are icy bodies traveling in the areas between stars. They're just like the comets in the Oort Cloud, with the exception that they've been ejected from whatever stellar system they formed in. Wikipedia cites a good paper that analyzes the distribution of these comets, &lt;a href=&quot;http://iopscience.iop.org/0004-637X/635/2/1348/pdf/62880.web.pdf&quot;&gt;&lt;em&gt;The Demographics of Long-Period Comets&lt;/em&gt; by Paul J. Francis&lt;/a&gt;. Francis gives an estimate of a number density of $4.5 \times 10^{-4} \text{ comets/AU}^3$. Francis used data from the Lincoln Near-Earth Asteroid Research survey (LINEAR).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Oort_cloud&quot;&gt;Oort Cloud&lt;/a&gt;, on the other hand, has trillions of objects within a sphere of about 50,000 AU in radius, which, at a lower bound of one trillion objects, gives a number density or about $1.91 \times 10^{-3} \text{ comets/AU}$ - and that's a conservative estimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even though the density of interstellar comets is much lower than the density of comets in the Oort Cloud, the environment is more &quot;continuous&quot; rather than &quot;discrete.&quot;&lt;/p&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2015-02-10T00:58:22.617" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8091" PostTypeId="2" ParentId="8087" CreationDate="2015-02-10T01:04:56.310" Score="31" Body="&lt;p&gt;Opportunity is roughly 10,000 km away from Spirit in a straight line.  At its top speed of 0.05 m/s (0.18 km/h), it would take approximately 2300 days to reach Spirit -- not that Opportunity can maintain that speed.  Between limited electrical power, needing to slow down to navigate, and being unable to drive at night, driving is much slower.  Based on the performance of &lt;a href=&quot;http://mars.nasa.gov/mer/mission/status_opportunityAll_2010.html#sol2403&quot; rel=&quot;noreferrer&quot;&gt;the week of October 27, 2010&lt;/a&gt;, a more reasonable speed estimate is 60 meters per day, or about 450 years to reach Spirit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity is much closer, at about 2500 km.  It has a rated travel speed of 200 meters per day, but this is mainly due to navigation restrictions rather than power limitations.  Improved techniques might be able to get it up to its rated driving speed of 90 meters per hour, and since the rover's RTG operates just fine at night (unlike Opportunity's solar panels), continuous driving might be possible.  In this case, it would take about 1200 days for Curiosity to reach Spirit, assuming it didn't need to detour around any major obstacles.  This is, of course, assuming that &lt;a href=&quot;https://space.stackexchange.com/questions/7997/curiosity-wheel-damage&quot;&gt;the wheels don't wear out&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-02-10T01:04:56.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8092" PostTypeId="2" ParentId="8087" CreationDate="2015-02-10T02:10:00.833" Score="14" Body="&lt;p&gt;According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Spirit_(rover)#Mission_end&quot;&gt;Wikipedia&lt;/a&gt;, it's likely that Spirit has suffered permanent damage from cold as a result of insufficient power to run its heaters; any future rover which had the ability to fix that sort of thing would be so much more sophisticated than our current rovers that there would be no point in doing so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, Spirit operated more than 20 times longer than it was designed to, which seems enough to earn it a permanent rest; maybe some day we'll be able to put it in a museum on Mars.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-02-10T02:10:00.833" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8093" PostTypeId="1" CreationDate="2015-02-10T06:02:58.500" Score="7" ViewCount="836" Body="&lt;p&gt;With a huge robust ship you could go get an ice-teroid. With effectively unlimited reaction mass, finely balanced trajectories become unnecessary and launch windows a thing of the past. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A pulsed nuclear launch would provoke a political furore, but the military advantage of being first to have a robust ship and substantial reaction mass is incalculable. It would be tremendously enabling from both military and industrial perspectives. In particular it would enable preventing anyone &lt;em&gt;else&lt;/em&gt; from acquiring the capability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why aren't the superpowers in a frantic scramble to acquire an ice asteroid?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Stuff I mistakenly thought was obvious:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pulsed nuke is only for getting a big heavy ship into orbit and going to get the ice. &lt;/li&gt;&#xA;&lt;li&gt;Once you have the ice, heat from a reactor produces steam for a steam thrusters.&lt;/li&gt;&#xA;&lt;li&gt;You don't carry all of it on one ship, if you did that you would have to use a ridiculous amount to move. You park stockpiles orbiting planets and LaGrange points.&lt;/li&gt;&#xA;&lt;li&gt;You freeze it around ships as ablative armour, radiation shielding and thermal mass, and spin the ship fairly fast to get both gravity and temperature distribution buffered by the ice.&lt;/li&gt;&#xA;&lt;li&gt;For sudden high acceleration you could use steam to throw a lump of ice. This would also make a good weapon when others try to become space powers.&lt;/li&gt;&#xA;&lt;li&gt;I don't know about further out but between Earth and the sun, UV radiation with platinum catalysts can split water into hydrogen and oxygen. With ready supplies of fresh oxygen you could simply dump excess CO2 until there is industrial demand to reclaim it.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="4586" LastEditorUserId="4586" LastEditDate="2015-02-10T22:50:38.493" LastActivityDate="2015-02-13T17:28:58.317" Title="Why isn't Project Orion the new space race?" Tags="|economics|military|nuclear-pulse-propulsion|" AnswerCount="2" CommentCount="11" ClosedDate="2022-05-18T11:11:22.477" ContentLicense="CC BY-SA 3.0" />
  <row Id="8094" PostTypeId="2" ParentId="8093" CreationDate="2015-02-10T08:55:16.213" Score="8" Body="&lt;p&gt;Let's start from the misconceptions in your question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;Unlimited reaction mass&quot; - there's no unlimited reaction mass in pulsed nuclear propulsion - you can go as far as your bomb supply lasts, but no further. Specifically, you cannot manufacture nuclear devices on board.&lt;/li&gt;&#xA;&lt;li&gt;&quot;Military advantage of getting an ice-teroid&quot; - there's no advantage to be gained by capturing water off-world, military or commercial. The only things that can be done with ice asteroids are exploration/in-situ extraction of water for life support, or hydrogen/oxygen for propulsion. If you want to blast someone with an asteroid, you can do it better and cheaper with nukes. You cannot use the Orion ship to fight terrorists/insurgents/whatever, either.&lt;/li&gt;&#xA;&lt;li&gt;&quot;Preventing others from developing the capability&quot; - exactly the opposite is true. Once the blueprints are out there, it's easier to steal them than to develop anew.&lt;/li&gt;&#xA;&lt;li&gt;&quot;Superpowers&quot; - as noted in the comments, an old-fashioned and slightly incorrect term.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Otherwise, the question has two parts: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;why nobody seems to be in a race for a very heavy launcher, &lt;/li&gt;&#xA;&lt;li&gt;and why Project Orion is mothballed.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are nice answers to the first part elsewhere on the site, and you have been pointed to Test Ban and Outer Space Treaties. I'm only going to summarize the two main objections:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;a very heavy launcher is uneconomical due to extremely high fixed costs of R&amp;amp;D and infrastructure maintenance and very unfrequent launches (there are no commercial or military payloads that have to be launched to LEO);&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;there exist high specific impulse propulsion technologies not requiring the huge dead weight of the pusher plate (like nuclear thermal rockets).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For a high thrust launch solution, we're still bound to think about Environmental Impact Assessments, and while chemical rockets are not fluffy cute animals due to ozone depletion and (for solids) large plumes of not really healthy particulate matter, they are better than injecting radioactive fallout and unburnt plutonium into the stratosphere for (un)timely transport to millions of civvies around the world by jet streams.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An obligatory reference to Winchell Chung's site: &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/index.php&quot;&gt;Project Rho&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-02-10T10:09:56.287" LastActivityDate="2015-02-10T10:09:56.287" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8095" PostTypeId="1" CreationDate="2015-02-10T15:42:30.240" Score="7" ViewCount="953" Body="&lt;p&gt;I know this sounds off topic, but bear with me.   This is a question about real world space agencies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it well established that Star Trek follows naval tradition.  For example its ranks come from USN.  Ships share names with real Earth vessels, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was surprised that the contingency speech on the death of the Apollo 11 astronauts contained&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;AFTER THE PRESIDENT'S STATEMENT, AT THE POINT WHEN NASA ENDS COMMUNICATIONS WITH THE MEN:&#xA;  A clergyman should adopt the same procedure as a burial at sea, commending their souls to &quot;the deepest of the deep,&quot; concluding with the Lord's Prayer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://www.lettersofnote.com/2010/11/in-event-of-moon-disaster.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.lettersofnote.com/2010/11/in-event-of-moon-disaster.html&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this led me to the question:  This speech was from after the Star Trek aired.  So were there other established traditions for using naval terminology and procedures at NASA before Star Trek (1966) ?&lt;/p&gt;&#xA;" OwnerUserId="8390" LastEditorUserId="49" LastEditDate="2015-02-10T16:38:30.250" LastActivityDate="2015-02-10T16:38:30.250" Title="Does Star Trek's adherence to naval tradition have roots in NASA or earlier real organizations?" Tags="|nasa|history|terminology|procedure|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8096" PostTypeId="2" ParentId="8083" CreationDate="2015-02-10T16:06:06.027" Score="20" Body="&lt;p&gt;The Delta II first stage thrust structure is designed with attach points for up to nine strap-on boosters, of which either three, four, or all nine are used.  If we number the attach points sequentially around the circumference of the first stage (they aren't, but let's pretend anyway), three boosters might use attach points 1, 4, and 7, leaving two empty slots in between each.  If they use four boosters, they might use points 1, 3, 5, and 7, leaving 1, 1, 1, and 2 slots between them, hence the asymmetric look.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See these images for example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.americaspace.com/wp-content/uploads/2015/01/SMAP-Roll-MK-31.jpg&quot;&gt;Two spaces between the boosters visible here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/f/fd/Delta_II_7425_carrying_Mars_Polar_Lander_%28KSC-99PC-1%29.jpg&quot;&gt;One space visible here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2015-07-27T22:33:21.313" LastActivityDate="2015-07-27T22:33:21.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8097" PostTypeId="2" ParentId="8095" CreationDate="2015-02-10T16:08:20.867" Score="9" Body="&lt;p&gt;Well yes, most early NASA astronauts / pilots were naval aviators, so there would be many US Navy terms and procedures adopted from there, for example the &lt;a href=&quot;https://space.stackexchange.com/a/4314/49&quot;&gt;go/no-go polling&lt;/a&gt; that was used on aircraft carriers to scramble war birds into the air. That was already used during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Mercury&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Mercury&lt;/a&gt; and later &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Gemini&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Gemini&lt;/a&gt; that ended in 1966. So both were before 1966 and Star Trek.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Just to clarify, when I say &quot;most were naval aviators&quot;, I mean with that US Navy and US Marine Corps (and &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_naval_aviator&quot; rel=&quot;nofollow noreferrer&quot;&gt;technically also US Coast Guard&lt;/a&gt;, but none of the early NASA astronauts came from there). Here's a list of the &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury_Seven&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mercury Seven&lt;/a&gt;&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Malcolm Scott Carpenter (1925–2013), USN&lt;/li&gt;&#xA;&lt;li&gt;Leroy Gordon &quot;Gordo&quot; Cooper, Jr. (1927–2004), USAF&lt;/li&gt;&#xA;&lt;li&gt;John Herschel Glenn, Jr. (1921–), USMC&lt;/li&gt;&#xA;&lt;li&gt;Virgil Ivan &quot;Gus&quot; Grissom (1926–1967), USAF&lt;/li&gt;&#xA;&lt;li&gt;Walter Marty &quot;Wally&quot; Schirra, Jr. (1923–2007), USN&lt;/li&gt;&#xA;&lt;li&gt;Alan Bartlett Shepard, Jr. (1923–1998), USN&lt;/li&gt;&#xA;&lt;li&gt;Donald Kent &quot;Deke&quot; Slayton (1924–1993), USAF&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So out of these seven, four were naval aviators, and three were US Air Force pilots. Slightly in favor of naval aviators. During Project Gemini, this &lt;em&gt;score&lt;/em&gt; shifted in favor of USAF pilots:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;L. Gordon Cooper, USAF&lt;/li&gt;&#xA;&lt;li&gt;Virgil &quot;Gus&quot; Grissom, USAF&lt;/li&gt;&#xA;&lt;li&gt;Walter M. Schirra, USN&lt;/li&gt;&#xA;&lt;li&gt;Neil A. Armstrong, Civilian&lt;/li&gt;&#xA;&lt;li&gt;Frank Borman, USAF&lt;/li&gt;&#xA;&lt;li&gt;Charles &quot;Pete&quot; Conrad, USN&lt;/li&gt;&#xA;&lt;li&gt;James A. Lovell, USN&lt;/li&gt;&#xA;&lt;li&gt;James A. McDivitt, USAF&lt;/li&gt;&#xA;&lt;li&gt;Thomas P. Stafford, USAF&lt;/li&gt;&#xA;&lt;li&gt;Edward H. White II, USAF&lt;/li&gt;&#xA;&lt;li&gt;John W. Young, USN&lt;/li&gt;&#xA;&lt;li&gt;Edwin &quot;Buzz&quot; Aldrin, USAF&lt;/li&gt;&#xA;&lt;li&gt;Eugene A. Cernan, USN&lt;/li&gt;&#xA;&lt;li&gt;Michael Collins, USAF&lt;/li&gt;&#xA;&lt;li&gt;Richard F. Gordon, USN&lt;/li&gt;&#xA;&lt;li&gt;David R. Scott, USAF&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So Project Gemini had 9 USAF pilots, 6 USN and one civilian pilot. First three in the list were Project Mercury veterans. But naval aviators still remain well represented all the way to 1966.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-02-10T16:32:25.013" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8098" PostTypeId="1" AcceptedAnswerId="8099" CreationDate="2015-02-10T17:19:16.063" Score="8" ViewCount="2906" Body="&lt;p&gt;When SpaceX recovers the Falcon 9 first stage at sea, roughly how much time passes between lift-off and soft touchdown on the barge? &lt;/p&gt;&#xA;" OwnerUserId="3082" LastActivityDate="2015-02-10T17:50:00.350" Title="How long does it take to recover the first stage of the Falcon 9?" Tags="|spacex|falcon-9|recovery|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8099" PostTypeId="2" ParentId="8098" CreationDate="2015-02-10T17:44:51.687" Score="9" Body="&lt;p&gt;This depends on the exact flight profile. For the upcoming (&lt;a href=&quot;http://www.spacex.com/webcast/&quot;&gt;hopefully today's&lt;/a&gt;) DSCOVR mission launch, expected first stage landing is at &lt;a href=&quot;http://www.spaceflight101.com/dscovr-mission-updates.html&quot;&gt;T+09:20&lt;/a&gt; (liftoff is at T-0, which is not exactly the case for all launch vehicles, but the difference would still be up to a few seconds at most). For the SpX-5 cargo mission to the station, this ETA was at &lt;a href=&quot;http://www.spaceflight101.com/dragon-spx-5-mission-updates.html&quot;&gt;T+09:45&lt;/a&gt; (reentry was slower from insertion into LEO that it will be from the flight profile for the higher energy trajectory towards Sun-Earth Lagrange point 1's insertion, so next reentry will reach roughly 2x the dynamic pressure than the previous one - i.e. initial first stage descent will be faster too).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, on average, the Falcon 9 first stage is expected to land downrange on ASDS (Autonomous Spaceport Drone Ship) roughly 9 and a half minutes after liftoff.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-02-10T17:50:00.350" LastActivityDate="2015-02-10T17:50:00.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8100" PostTypeId="2" ParentId="8093" CreationDate="2015-02-10T18:11:18.070" Score="3" Body="&lt;p&gt;Bringing propellent rich asteroids to the earth moon neighborhood is an interesting idea. There are a number of entities pursuing this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However most proposals suggest solar powered ion rocket engines for asteroid retrieval rather than a vehicle driven by a series of nuclear blasts against a pusher plate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The U. S. is thinking of developing a solar powered asteroid retrieval vehicle. It would be part of the Asteroid Redirect Mission (A.R.M.). The A.R.M. vehicle is expected to be modeled after the vehicle described in the &lt;a href=&quot;http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf&quot; rel=&quot;nofollow&quot;&gt;Keck Report&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However A.R.M. is not being sold as an enabler for using asteroid resources. The mission is promoted as an intermediate step to Mars (a foolish sales pitch, in my opinion).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Critics will say politicians embrace A.R.M. because it's doable by SLS and they want to justify the SLS pork barrel program that keeps folks employed in certain congressional districts. They further contend that SLS missions to an asteroid in lunar orbit do little to get us closer to Mars. I believe the critics are correct. But what the politicians as well the critics miss is the potential usefulness of the A.R.M. SEP vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planetary Resources has made a &lt;a href=&quot;https://www.youtube.com/watch?v=VLouRKHknOU&amp;amp;feature=youtu.be&quot; rel=&quot;nofollow&quot;&gt;nice video&lt;/a&gt; giving a layman's explanation of Tsiolkovsky's rocket equation and delta V budgets:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/VLouRKHknOU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aseroids aren't the only possible source of extra-terrestrial propellent. &lt;a href=&quot;http://www.spudislunarresources.com/Papers/Affordable_Lunar_Base.pdf&quot; rel=&quot;nofollow&quot;&gt;Paul Spudis is an advocate of using lunar water&lt;/a&gt; as a propellent source. There may be large deposits of ice at the lunar poles. Another advocate of lunar propellent is Bill Stone of &lt;a href=&quot;http://www.shackletonenergy.com&quot; rel=&quot;nofollow&quot;&gt;Shackleton Energy Company&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Extraterrestrial propellant, whether asteroidal or lunar in origin, would require substantial mining and processing infra-structure. This is an extremely expensive barrier. It may be a large enough hurdle that we will never make use of extraterrestrial water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are those who argue captured rocks offer no military advantage. It's true kinetic kill weapons would be horrendously expensive. Nuclear bombs could wipe out a city for a small fraction of the cost. But those using this very obvious argument are stuck in 1970's science fiction. There are other possible military uses for space resources than throwing rocks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What extraterrestrial propellent might offer is routine access to orbital assets in cislunar space. This includes surveillance satellites, GPS, communication satellites and weather sats. Easy access to these would indeed be of incalculable military value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As suggested I will elaborate on what I mean by routine access. Presently a ship from earth's surface to LEO has a 9 km/s delta V budget. Returning to earth's surface entails enduring the extreme conditions of an 8 km/s re-entry through earth's atmosphere. So far economical reusable vehicles haven't been realized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast vehicles  between earth orbits would have 3 to 4 km/s delta V budgets. Most wouldn't have to endure the high temperatures and dynamic pressures of re-entry. Those ships with LEO destinations might use aerobraking for circularizing at LEO but the aerobraking would be much less extreme than an 8 km/s re-entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the present paradigm the routine is launch, use and discard. With cheaper transportation between orbits it would be possible to upgrade and maintain existing assets rather than letting them die and add to the population of orbital debris. It would also be possible to salvage dead sats.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-02-13T17:28:58.317" LastActivityDate="2015-02-13T17:28:58.317" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="8101" PostTypeId="2" ParentId="8081" CreationDate="2015-02-10T18:42:30.067" Score="10" Body="&lt;p&gt;Found an answer on the &lt;a href=&quot;http://www.reddit.com/r/spacex/comments/2vb5kv/1second_launch_window_please_explain/&quot;&gt;SpaceX subreddit&lt;/a&gt; from &lt;a href=&quot;http://www.reddit.com/r/spacex/comments/2vb5kv/1second_launch_window_please_explain/cog1o3p&quot;&gt;user cwhitt&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Basically one specific moment in Earth's 24-hour rotation is best for the trajectory to get from Florida to the DSCOVR destination (beyond earth orbit) [...]&#xA;  In this case it's slightly different than with Dragon/CRS launches, but the concept is the same: alignment of launch point and destination wrt to Earth's rotational position.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And further explanation &lt;a href=&quot;http://www.reddit.com/r/spacex/comments/2vb5kv/1second_launch_window_please_explain/cogfo0w&quot;&gt;from atrain728&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[...] in DSCOVR's case, every second that is in delay or advance of the launch moment is raw delta-v that will have to be made up in flight. There's no amount of thrust regulation that can solve that problem, it will simply take more fuel. And that's why the window is so short.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the short launch window is a matter of efficiency with regards to the trajectory to SEL1 and minimizing any need to change plane/inclination.&lt;/p&gt;&#xA;" OwnerUserId="6772" LastActivityDate="2015-02-10T18:42:30.067" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8102" PostTypeId="1" CreationDate="2015-02-10T23:12:08.097" Score="4" ViewCount="161" Body="&lt;p&gt;This question is related to reuse possibilities. One of the things that affects reuse is thermal cycling of the fuel tanks. (Consider how microfractures in the X-33 composite tank killed that project. Ok, political lack of will killed it, but that was the excuse).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the question. With the DSCOVR Falcon 9 launch, Feb. 2015, we saw the core do the following fuel/defuel events:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;McGregor testing: 1 fuel/burn it out. (We know they are all tested with a burn at Texas site)&lt;/li&gt;&#xA;&lt;li&gt;Wet dress rehearsal: 1 fuel/defuel, 1-3 second burn.&lt;/li&gt;&#xA;&lt;li&gt;First attempt, Feb. 8: 1 fuel/defuel&lt;/li&gt;&#xA;&lt;li&gt;Second attempt, Feb. 10: 1 fuel/defuel&lt;/li&gt;&#xA;&lt;li&gt;Third attempt, Feb. 11: 1 fuel (hopefully 1 launch, recovery, and eventually reuse).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Which F9 core has done the most of these cycles? There have been lots of scrubs, delays, tests, failed wet rehearsals, etc.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="195" LastEditDate="2015-02-10T23:38:18.263" LastActivityDate="2015-03-14T21:23:15.177" Title="Which Falcon 9 (1.0 or 1.1) core has fuel cycled the most?" Tags="|spacex|falcon-9|reuse|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8103" PostTypeId="1" CreationDate="2015-02-11T01:57:08.683" Score="1" ViewCount="356" Body="&lt;p&gt;What is a reasonable cost estimate for the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;A single space solar power station beaming power to earth via laser or maser and one associated receiving station?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The following complex:  A moon colony used to mine material and launch it into space, presumably with a solar powered electrically driven ramp, an orbital station used to construct space solar power satellites and enough solar power satellites to meet 50% of the USA's electrical needs along with associated ground stations.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I saw an estimate, 30-40 years out of date, of trillion dollars for #2.  Unfortunately, I cannot recall the source of this estimate.&lt;/p&gt;&#xA;" OwnerUserId="5836" LastEditorUserId="5836" LastEditDate="2016-03-14T22:51:39.380" LastActivityDate="2016-03-15T01:39:41.123" Title="What is a reasonable cost estimate for space solar power (SSP)?" Tags="|the-moon|artificial-satellite|solar-power|" AnswerCount="2" CommentCount="4" ClosedDate="2016-03-15T21:12:45.990" ContentLicense="CC BY-SA 3.0" />
  <row Id="8104" PostTypeId="2" ParentId="2992" CreationDate="2015-02-11T04:13:43.710" Score="3" Body="&lt;p&gt;This is a partial answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given a flat reflector, as shown below, the force resulting from the incident photons will always be normal (90°) to the reflector plane. This is because the angle of reflection is always equal to the angle of incidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angle between the incident photons and the force vector is labelled θ. The magnitude of the force will vary according to cos θ. When sail is at right angles to the incident photons, as when accelerating away from the Sun, θ will be zero, and the sail will deliver the maximum force (cos 0 is 1).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DKmpc.png&quot; alt=&quot;Solar Sail (after Jerry Wright, Wikipedia)&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;θ increases as the sail is oriented away from the Sun, and the thrust obtained from every photon is reduced. For example, with the sail oriented at 45°: cos 45° is $\sqrt{2}$, about 0.71. But the area of the sail available to collect photons is reduced also; it also varies according to cos θ. Hence the $\cos^2 {\theta}$ term in the ideal formula.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar sail&lt;/a&gt; article you mention does not clearly explain how the actual square sail model is derived, nor does the associated &lt;a href=&quot;http://en.wikipedia.org/wiki/Radiation_pressure&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radiation pressure&lt;/a&gt; article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All I can add at this point is that the performance of a real solar sail will be affected by&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the sail reflectivity; the linked articles assume 90% is feasible.&lt;/li&gt;&#xA;&lt;li&gt;the sail re-radiation of absorbed energy.&lt;/li&gt;&#xA;&lt;li&gt;the sail mass per unit area; the linked articles say&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At a thickness of 20 nm, lithium has an areal density of 0.011 g/m2.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;the sail's surface accuracy (how flat it is).&lt;/li&gt;&#xA;&lt;li&gt;the efficiency with which the force on the sail is translated to the spacecraft (eg, is there a turning moment that must be balanced).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user8406" LastActivityDate="2015-02-11T04:13:43.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8105" PostTypeId="1" AcceptedAnswerId="8112" CreationDate="2015-02-11T08:26:33.037" Score="10" ViewCount="2493" Body="&lt;p&gt;Which spacecraft / satellites currently deployed in space (2015) use ion thruster propulsion and how do they compare? The &lt;a href=&quot;http://en.wikipedia.org/wiki/Ion_thruster#Comparisons&quot;&gt;wikipedia article&lt;/a&gt; have some comparative numbers, but some of the engines on the list don't seem to have even left the lab yet and a lot of the info is missing. What I would like to see is a list of spacecraft using ion thrusters with details like:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Number of thrusters&lt;/li&gt;&#xA;&lt;li&gt;Mass per thruster&lt;/li&gt;&#xA;&lt;li&gt;Fuel type and mass&lt;/li&gt;&#xA;&lt;li&gt;Required power per thruster&lt;/li&gt;&#xA;&lt;li&gt;Specific impulse&lt;/li&gt;&#xA;&lt;li&gt;Thrust per thruster&lt;/li&gt;&#xA;&lt;li&gt;Period operational and any thruster failures&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="8157" LastEditDate="2015-02-11T13:36:15.863" LastActivityDate="2015-02-11T22:24:42.467" Title="What is the performance of ion thrusters in actual deployed spacecraft?" Tags="|propulsion|advanced-propulsion|ion-thruster|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8106" PostTypeId="2" ParentId="7991" CreationDate="2015-02-11T10:06:59.740" Score="14" Body="&lt;p&gt;Local scrap metal dealers collect and recycle them. See &lt;a href=&quot;http://www.eurasianet.org/departments/culture/articles/eav041902.shtml&quot; rel=&quot;noreferrer&quot;&gt;this link&lt;/a&gt; for some more information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vFkG7.jpg&quot; alt=&quot;eurasianet.org 1&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Omi2K.jpg&quot; alt=&quot;eurasianet.org 2&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/YsCy9.jpg&quot; alt=&quot;eurasianet.org 3&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-02-11T10:06:59.740" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8107" PostTypeId="1" AcceptedAnswerId="8108" CreationDate="2015-02-11T12:24:01.087" Score="19" ViewCount="11756" Body="&lt;p&gt;Why do spaceships almost make a straight line in the atmosphere when coming back to earth? This makes the ship undergo high stress and temperature. Why don't they make a spiral trajectory so that they can slowly brake?&lt;/p&gt;&#xA;" OwnerUserId="8470" LastEditorUserId="1142" LastEditDate="2015-12-24T23:47:37.933" LastActivityDate="2022-06-07T11:10:33.377" Title="Why do spacecraft enter the atmosphere violently instead of a smooth spiral?" Tags="|reentry|atmospheric-drag|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8108" PostTypeId="2" ParentId="8107" CreationDate="2015-02-11T12:50:29.363" Score="35" Body="&lt;p&gt;Spiraling down in the sense you mean is not possible, the reason is that when a spaceship is orbiting Earth, it is travelling extremely fast relative to the surface, it is not that space is so high up, but that a spaceship needs to travel very fast in order to orbit. So in order to reenter, it is not the velocity of &lt;em&gt;falling&lt;/em&gt; that needs to be shed, but the velocity of &lt;em&gt;orbiting&lt;/em&gt;. Think of the spaceship skimming very fast over the surface of the Earth but not that high up - that is orbiting. Now think about what it would need to do in order to spiral, it would need to shed all of that orbital velocity - all of it - and start moving in the opposite direction, in other words it would have to have already shed all that orbital velocity, before it could spiral! That is why it is simply not possible to spiral down from orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When they reenter, spaceships enter at a very shallow angle so they travel a long way through the atmosphere, with density gradually increasing. If the angle is too shallow it would skip off back into space, if the angle is too steep it will be destroyed by heat and gforces. So spaceships already enter in a way which is as gentle as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See for example &lt;a href=&quot;http://www.aerospaceweb.org/question/spacecraft/q0218.shtml&quot; rel=&quot;noreferrer&quot;&gt;this page on reentry corridors&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/y1Qkj.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y1Qkj.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see from the image, in a sense a spaceship does spiral down when it re-enters, but it spirals down around the globe of the Earth. But that is because the Earth is a globe, considered from the frame of reference of the surface of the Earth, the reentry trajectory is rather like a straight line.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorDisplayName="user20636" LastEditDate="2019-02-28T22:30:45.713" LastActivityDate="2019-02-28T22:30:45.713" CommentCount="16" ContentLicense="CC BY-SA 4.0" />
  <row Id="8109" PostTypeId="2" ParentId="8107" CreationDate="2015-02-11T12:59:21.040" Score="15" Body="&lt;p&gt;There are three major constraints that have to be taken into account:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;maximum deceleration&lt;/strong&gt; (equipment and structural elements can withstand much higher g's than the crew, so it's about &lt;a href=&quot;https://space.stackexchange.com/questions/6154/maximum-survivable-long-term-g-forces&quot;&gt;medical limits&lt;/a&gt;);&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;peak heat flux&lt;/strong&gt; which allows one to determine the worst case temperatures that the spacecraft structure is heated to during re-entry (heat transfer and dissipation are not instantaneous);&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;total heat load&lt;/strong&gt; - the spacecraft accumulates heat faster than it radiates/convects it away, raising interior temperatures, possibly beyond what the crew and avionics can tolerate. Heat sinks are designed to alleviate this problem.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The total heat load to be absorbed/re-emitted is lower during a normal re-entry than that in a slow &quot;spiral&quot; trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please also note that to &lt;strong&gt;maintain a spiral trajectory you need some source of ever increasing lift, since the slower you go, the less lift your craft generates&lt;/strong&gt;. Adding engines and fuel to maintain lift increases initial mass to be put into orbit several orders of magnitude, thus making the whole affair much more costly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When entering the atmosphere while returning from the Moon (or other planets), it is indeed possible to split the heat load into at least two phases (with some radiative cooling in between), viz. &lt;a href=&quot;https://space.stackexchange.com/questions/3068/how-does-skipping-off-the-atmosphere-work&quot;&gt;Skip re-entry&lt;/a&gt;, however the resulting trajectory is not a spiral.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="288" LastEditDate="2015-02-12T16:07:29.567" LastActivityDate="2015-02-12T16:07:29.567" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8110" PostTypeId="1" AcceptedAnswerId="8111" CreationDate="2015-02-11T14:25:28.490" Score="8" ViewCount="1171" Body="&lt;p&gt;The F9R is planning to safely land on platforms, using propulsive landing, which requires that the rocket has extra fuel leftover from the primary launch mission, to be able to have a propulsive landing. Shorter missions such as CRS-5 and DSCOVR will have enough extra fuel that they can attempt propulsive landing, however, some commercial satellite launches will not have enough leftover fuel, that the booster has to be dropped into the ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, is there any way that the nearly empty rocket booster could dock in space (to ISS, or Shell-Space-Petrol-Station, Propellant Depot), refill the kerosene tank, and land on a launch pad on Earth? I understand that the moon hasn't proved its oil deposits yet, but, just wondering if there were options.&lt;/p&gt;&#xA;" OwnerUserId="8472" LastEditorUserId="51" LastEditDate="2015-02-11T14:46:09.020" LastActivityDate="2015-02-11T15:11:49.100" Title="Could a rocket booster be refueled in space?" Tags="|spacex|reuse|falcon-9|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8111" PostTypeId="2" ParentId="8110" CreationDate="2015-02-11T14:45:35.750" Score="12" Body="&lt;p&gt;Problem the first: The first stage when finished with its mission is not in orbit, nor even close. It somewhat depends, and specifically with a Falcon 9 there is a wider range than usual of first stage MECO stats. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They can run it as a F9 expendable, where it is max payload, max thrust, empty tanks. They can run it as a F9 reusable, where they save 15-30% of fuel for recovery. With the ASDS allowing down range recovery, they can also vary between RTLS (Return to Launch Site) or land down range on the barge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More 'traditional' first stages mostly delivered as much payload as possible for the mission. Thus mission trajectories differ, and thus conditions at MECO differ, but in all those cases, none are really close to Orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I saw a calculation (but cannot find it now) of what Grasshopper (Falcon 9 1.0 first stage, no second stage or interstage, with heavy boilerplate legs) could do as an SSTO. It was not really capabale. The F9R-Dev1 that was lost in testing with 9 engines, could actually deliver a low couple of hundred pounds as an SSTO but would not be recoverable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus getting that first stage to the ISS (which is in a lousy orbit for US launches) is pretty much out of the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem the second: You now have to run a bunch more launches to get the needed fuel and oxidizer to the ISS to recover this one stage, that can actually launch cargo on its own (Dragon mode) and recover itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem the third: The mass of fuel required, is estimated at 15-30% of the fuel load for a normal launch is on the order of several hundred thousand pounds. Current deliveries to the ISS top out around 10,000 lbs a mission. (ATV I think is the biggest payload vehicle right now, though not anymore since it is finished, no more to be built.  So HTV then as number 2). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem the fourth: Fluid transfer in space is not well understood.  Right now, Progress and ATV do it, to provide fuel for the ISS Russian segment engines. But those are hypergolics that are not cryogenic. That would in theory be analogous to Kerosene/RP1, but would be new development for LOX. It should not be insurmountable, but it is not something that is off the shelf either. (Liquid Hydrogen is the holy grail, with its much better ISP for orbital operations. But that is much harder due to needs for active cooling and boiloff).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem the fifth: Related to the second and the third. As Steven Wright used to say &quot;You can't have everything, where would you put it?&quot;.  The volume of fuel and oxidizer you would need to store somewhere (a depot or somewhere on the ISS) would be fairly large, and of course no such system exists. Not to say it could not be built, (in fact I would argue it really should, but different answer to a different question) but it does not yet exist, nor are there any serious plans (as in bent metal) for such a thing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3709/does-any-group-have-a-serious-proposal-to-build-maintain-a-station-at-l1&quot;&gt;Does any group have a serious proposal to build &amp;amp; maintain a station at L1?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5816/why-would-orbital-fuel-depots-make-sense&quot;&gt;Why would orbital fuel depots make sense?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very cost in-effective across the board. The numbers just do not work.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-02-11T15:11:49.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8112" PostTypeId="2" ParentId="8105" CreationDate="2015-02-11T18:13:32.030" Score="12" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot;&gt;Dawn&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Deep_Space_1&quot;&gt;Deep Space 1&lt;/a&gt; both use the NSTAR ion engine - I got my stats from a mix of sources so there may be small differences between the engines used on the two spacecraft, but they seem to be pretty similar.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dawn has 3 redundant NSTAR thrusters (not intended to be used together); DS1 has 1. &lt;/li&gt;&#xA;&lt;li&gt;Thruster mass is 8.2kg, power processing unit and control unit bring it to ~25.5kg total&lt;/li&gt;&#xA;&lt;li&gt;Xenon fuel, 425kg on Dawn, 82kg on DS1&lt;/li&gt;&#xA;&lt;li&gt;2100-2300 watts at full power&lt;/li&gt;&#xA;&lt;li&gt;ISP 3100s at full power &lt;/li&gt;&#xA;&lt;li&gt;90mN at full power &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;DS1's thruster failed after a few minutes due to crud on the ion grids, but was eventually restarted. Launched in October 1998, the thruster ran for 1800 hours, with 34 restarts, through April 1999 for its primary mission. Its mission was extended and the thruster &lt;a href=&quot;http://www.grc.nasa.gov/WWW/ion/past/90s/nstar.htm&quot;&gt;ran a total of something like 10,000 hours&lt;/a&gt; before being shut down at the end of 2001.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of 2010, &lt;a href=&quot;http://www.space.com/8579-nasa-spacecraft-breaks-speed-boost-record.html&quot;&gt;Dawn had fired its thrusters for ~15000 cumulative hours&lt;/a&gt; of acceleration and managed 4.3 km/s of delta-v, an average of something like 8 micro-g acceleration over that time period if I did my math right. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Japan's Hayabusa 2 is ion-propelled, but I had a harder time finding stats:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;4 thrusters&lt;/li&gt;&#xA;&lt;li&gt;Xenon fuel (Hayabusa 1 carried 65kg)&lt;/li&gt;&#xA;&lt;li&gt;ISP 2800s&lt;/li&gt;&#xA;&lt;li&gt;1000 watts?&lt;/li&gt;&#xA;&lt;li&gt;28mN thrust each? (another source says 10mN each?)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Hughes/Boeing/L-3 Communications Electron Technologies XIPS units are flying on a large number of Boeing 601- and 702-series communication satellites. I can't find mass figures for them, but they're quite compact and probably in a similar ballpark to the others listed. In this role they're fired periodically for orbital stationkeeping, for up to 30 minutes (702) or 5 hours (601) per day.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;601 series: 500 watts, ISP 2568s, 18mN thrust, 13cm grid&lt;/li&gt;&#xA;&lt;li&gt;702 series: 4500 watts, ISP 3500s, 165mN thrust, 25cm grid (14 satellites, 56 thrusters in operation as of 2007?)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Aerojet &lt;a href=&quot;http://www.astronautix.com/engines/bpt4000.htm&quot;&gt;BPT-4000&lt;/a&gt; Hall effect thruster is used on a the &lt;a href=&quot;http://en.wikipedia.org/wiki/Advanced_Extremely_High_Frequency&quot;&gt;AEHF USAF communications satellite&lt;/a&gt; -- 3 of them seem to be currently operational.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Thruster mass is 7.5kg&lt;/li&gt;&#xA;&lt;li&gt;Xenon fuel&lt;/li&gt;&#xA;&lt;li&gt;4500 watts&lt;/li&gt;&#xA;&lt;li&gt;ISP 1950s&lt;/li&gt;&#xA;&lt;li&gt;270mN&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Other Hall effect thrusters have been used (e.g. on &lt;a href=&quot;http://en.wikipedia.org/wiki/SMART-1&quot;&gt;SMART-1&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/TacSat-2&quot;&gt;TacSat-2&lt;/a&gt;, which are no longer in operation). SMART-1's Snecma PPS-1350G:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;5.3kg thruster (including Xenon flow control systems), 29kg (complete propulsion system)&lt;/li&gt;&#xA;&lt;li&gt;Xenon fuel, 82kg&lt;/li&gt;&#xA;&lt;li&gt;1200 watts&lt;/li&gt;&#xA;&lt;li&gt;ISP 1640s&lt;/li&gt;&#xA;&lt;li&gt;68mN thrust (thruster rated 88mN at 1500 watts)&lt;/li&gt;&#xA;&lt;li&gt;Operated for 5000 hours&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Other Hall thrusters from Russia's OKB Fakel range from 350-5000 watts, 1100-1750 seconds ISP, and 20-300mN thrust.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-02-11T22:24:42.467" LastActivityDate="2015-02-11T22:24:42.467" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8113" PostTypeId="2" ParentId="3286" CreationDate="2015-02-11T19:44:40.553" Score="2" Body="&lt;p&gt;I've been wondering how much work would need to be done.   &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fueling is done at the bottom of the first stage, so there's no need to run any piping up the tower for fueling the side boosters. &lt;/li&gt;&#xA;&lt;li&gt;They may need to support the top of the side boosters during erection to limit the load on the center core. The bottom should be adequately supported by the hold-down posts, so no need for any structure there. So no big changes here. (BTW this means the shoulders serve some other purpose.)    &lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Falcon 9 is held down by 4 support posts at 90° spacing. Two of these would be in the way of a Falcon Heavy. The post on the right contains the fueling connectors:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/gfXcV.jpg&quot; alt=&quot;F9 launch pad&quot;&gt;&lt;br&gt;&#xA;So they'd need to change the orientation of the rocket (which would create problems with the umbilicals higher up the tower), or fuel all 3 cores from a single point, or change the design of the first stage to accept fueling from a new location. &#xA;They also need to change the support posts themselves: if they need 4 per core, they need to make them more compact to fit 12. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The tower needs a new base structure with space for 3 cores. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, few changes to the tower, but big changes to the base. What they'll do exactly depends on e.g. whether they'll change the F9 design for commonality with the FH.&lt;br&gt;&#xA;I suspect they'll end up building a new tower for FH rather than rebuild this one. It's just ironmongery anyway so not very expensive, and it'll be handy to have more than one around so they can e.g. quickly swap a rocket should the one on the pad develop a fault. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-02-11T19:44:40.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8114" PostTypeId="1" CreationDate="2015-02-11T21:09:08.323" Score="6" ViewCount="184" Body="&lt;p&gt;Feather configuration for reentry used in SpaceshipOne, they say, significantly reduces heat loads on reentry by encountering most drag and significantly reducing a large amount kinetic energy at much higher altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does this configuration work at lower air density? Efficiently enough to reduce intensive thermal encounter later?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is its feasibility for different reentry speed ranges in other applications?&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="49" LastEditDate="2015-02-11T21:44:21.767" LastActivityDate="2016-01-30T18:53:06.680" Title="Is high velocity feathered configuration reentry possible?" Tags="|reentry|technology|sub-orbital|feasibility|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8115" PostTypeId="2" ParentId="1934" CreationDate="2015-02-11T23:05:48.593" Score="2" Body="&lt;p&gt;I always thought the first man made object to leave earth and make it past Jupiter was a &lt;a href=&quot;https://en.wikipedia.org/wiki/Operation_Plumbbob#Propulsion_of_steel_plate_cap&quot; rel=&quot;nofollow noreferrer&quot;&gt;manhole cover&lt;/a&gt;. I assume its further out than Voyager now. Under ground nuclear tests launched a manhole cover at escape velocity, but this is all I know, hopefully an expert can clarify this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; Scott Manley has come to the rescue! In his video ** he describes the event in detail. He also shows a link to io9's &lt;a href=&quot;https://io9.gizmodo.com/no-a-nuclear-explosion-did-not-launch-a-manhole-cover-1715340946&quot; rel=&quot;nofollow noreferrer&quot;&gt;No, a Nuclear Explosion Did Not Launch a Manhole Cover into Space&lt;/a&gt; who's title suggest that it didn't happen quite like this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The explanation is that the original velocity estimation was not intended to be the starting point of a trajectory, and in Earth's surface atmospheric density it would not have survived:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It’s clear they weren’t on the same page. Brownlee wasn’t interested in what happened to the cap, and so pretended that the atmosphere didn’t exist. When the cap wasn’t found, he put it out of his head, figuring it had vaporized in the atmosphere. It was only later, when he both got credit for the world’s first space launch and criticism for not taking the atmosphere into account when he calculated the velocity of the cap, that he realized the legend of the cap that got launched at “six times the escape velocity from earth,” had taken on a life of its own.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/pk_ymR0XFDg?start=219&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="8478" LastEditorUserId="12102" LastEditDate="2019-08-24T06:29:05.647" LastActivityDate="2019-08-24T06:29:05.647" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="8116" PostTypeId="1" CreationDate="2015-02-12T01:42:38.687" Score="7" ViewCount="296" Body="&lt;p&gt;&lt;a href=&quot;http://www.shatters.net/celestia/&quot; rel=&quot;noreferrer&quot;&gt;Celestia&lt;/a&gt; supports importing spacecraft data, but I didn't see any data for &lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/&quot; rel=&quot;noreferrer&quot;&gt;DSCOVR&lt;/a&gt;. Is there any way to see where it is now so we can track it on it's way to Sun-Earth L1?&lt;/p&gt;&#xA;" OwnerUserId="8481" LastEditorUserId="1142" LastEditDate="2015-02-12T03:52:06.557" LastActivityDate="2017-12-20T12:03:09.793" Title="How can we track where DSCOVR is now?" Tags="|trajectory|tracking|dscovr|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8117" PostTypeId="2" ParentId="8103" CreationDate="2015-02-12T05:50:15.773" Score="4" Body="&lt;p&gt;You probably read an Omni story, or &quot;The High Frontier: Human Colonies in Space&quot; by Gerard K. O'Neil (also wrote a nice General Relativity book). There is a 2013 book, &lt;em&gt;&lt;a href=&quot;http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA573816&quot; rel=&quot;nofollow&quot;&gt;Spaced-Based Solar Power System Architecture&lt;/a&gt;&lt;/em&gt; by Brain C. Busch (Naval Postgraduate School). That might have some good stuff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The numbers back then looked good to some experts and impossible to others. I have not seen much serious work in ages. I fiddled around once and found that an underground (undermoon?) habitat with 1 atmosphere of air pressure could hold up 30 feet of lunar regolith on the air pressure alone. So huge underground habitats are possible from a simple engineering point of view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The power sat does not need laser or maser. Simple microwave arrays that maintain coherence from feed back from the receiver are fine. They could use robust tubes (without the tube) instead of semiconductors. The idea is that if the sat power wondered off the target area the coherence is lost and the beam spreads out enough to be harmless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I always liked the idea. The Luddites and Greens won't because the power comes from off-Earth and will contribute to the total heat content of the atmosphere. On the other hand, if construction takes place in Lunar orbit, you save on all that. The trade-offs are many and depend on  politics and the current thinking on warming/cooling, which is subject to change any time now.&lt;/p&gt;&#xA;" OwnerUserId="1432" LastEditorUserId="1142" LastEditDate="2016-03-15T01:39:41.123" LastActivityDate="2016-03-15T01:39:41.123" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8118" PostTypeId="1" AcceptedAnswerId="8119" CreationDate="2015-02-12T09:59:22.560" Score="8" ViewCount="4169" Body="&lt;p&gt;I am new in the field of Spacecraft Attitude Dynamics and Control, and came across the terminology &quot;Y-Thomson spin&quot;. What is it? Where can I read more about this attitude control mode?&lt;/p&gt;&#xA;" OwnerUserId="8321" LastEditorUserId="49" LastEditDate="2015-02-14T13:33:43.240" LastActivityDate="2015-02-14T13:33:43.240" Title="What is a &quot;Y-Thomson spin&quot;?" Tags="|orbital-mechanics|artificial-satellite|orbital-maneuver|attitude|terminology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8119" PostTypeId="2" ParentId="8118" CreationDate="2015-02-12T11:34:33.697" Score="9" Body="&lt;p&gt;Here's a description of Y-Thomson spin from a &lt;a href=&quot;http://hanspeterschaub.info/Papers/grads/BradyYoung.pdf&quot; rel=&quot;nofollow&quot;&gt;thesis&lt;/a&gt; on the attitude control systems employed for the &lt;a href=&quot;http://en.wikipedia.org/wiki/Drag_and_Atmospheric_Neutral_Density_Explorer&quot; rel=&quot;nofollow&quot;&gt;Drag and Atmosphere Neutral Density Explorer (DANDE)&lt;/a&gt;, a student nanosat developed by the University of Colorado Boulder:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The y-axis spin rate is chosen such that, when a boom is deployed along the z-axis in a later phase, the y-axis spin rate coincides with the angular change of the spacecraft in orbit about the Earth. In a nearly circular orbit, this conveniently fixes the spacecraft body axes to the orbit tangent-normal-radial (TNR) reference frame. If deployed with the boom pointing to nadir, the gravity gradient restoring forces stabilize the system as well.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In this reference frame, X is the line tangential to the forward motion of the spacecraft. Z is the radial line pointing toward the center of the Earth. Y is the line 90° to the other two axes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you make your spacecraft spin exactly once about the Y axis during the period of its orbit, the &quot;bottom&quot; of the spacecraft always faces Earthward. This is convenient for attitude stabilization because, if you extend a long boom along the Z axis (&quot;down&quot;), you can take advantage of the difference in gravitational pull from the top of the spacecraft to the end of the boom (the gravity gradient) to keep the spacecraft stabilized in that position with relatively little stationkeeping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen it also cited as &quot;y-Thompson.&quot; A Google search will give you several papers. TildalWave recommended &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2190&amp;amp;context=smallsat&quot; rel=&quot;nofollow&quot;&gt;this one&lt;/a&gt;, a small satellite using a combination of magnetorquers, reaction wheels, and thrusters for attitude control. He also dug up the original paper by Thomson: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thomson W.T., &lt;em&gt;Spin Stabilisation of Attitude against Gravity Torque&lt;/em&gt;, Journal of Astronautical Science, No.9, pp.31-33, 1962.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2015-02-13T17:16:57.113" LastActivityDate="2015-02-13T17:16:57.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8120" PostTypeId="1" AcceptedAnswerId="8121" CreationDate="2015-02-12T13:23:01.737" Score="7" ViewCount="568" Body="&lt;p&gt;After Rosetta released the lander, it bounced several times upon hitting the comet's surface and its two harpoons did not deploy. Why did the Philae's grabbing mechanism fail?&lt;/p&gt;&#xA;" OwnerUserId="8486" LastEditorUserId="49" LastEditDate="2015-02-12T14:13:59.460" LastActivityDate="2023-12-14T17:59:55.213" Title="Why did Philae the comet lander bounce?" Tags="|landing|rosetta|comet|philae|lander|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8121" PostTypeId="2" ParentId="8120" CreationDate="2015-02-12T14:54:48.283" Score="7" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.seti.org/weeky-lecture/rosetta-lander-philae-mission-landing-comet-67pchuryumov-gerasimenko&quot;&gt;this SETI Talk&lt;/a&gt; 2014 Dec 16 by Jens Biele at German DLR, the &quot;grabbing mechanisms&quot; failed in several ways. The cold gas thruster that should've pushed it down gave an error indication even before landing. But since there was nothing they could do about it they landed as planned anyway. They had wired the harpoons wrong so they did not fire on command. Now they have figured out how to fire them and might do so if they get contact with Philae again, which they seem hopeful to do in July-September. The harpoons are scientific instruments which could measure the comet's density and temperature down to a couple of meters, and they might move the lander to a sunnier place. The ice screws on the landing legs might not have had the proper angle or ground texture to do the grabbing job on their own. All three legs seem to have hit the ground at the first bounce, and left visible tracks indicating that the ground was very soft there.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-02-12T14:54:48.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8122" PostTypeId="1" CreationDate="2015-02-12T18:13:05.463" Score="-2" ViewCount="1273" Body="&lt;p&gt;Electric propulsion is driven by power. Are there any theoretical limitations to the amount of power that can be used for electric propulsion? If limits do exist, do they rule out electric propulsions as a launch mechanism? For simplicity, in talking about a vertical launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The perfect answer would also give details on power storage/production and if this rules out the method for launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clarifying statement: My intension for the question was ion engines.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2015-02-13T20:27:38.893" LastActivityDate="2015-06-02T22:21:51.410" Title="Can electric propulsion be used for launch (theoretically)" Tags="|propulsion|advanced-propulsion|" AnswerCount="2" CommentCount="5" FavoriteCount="0" ClosedDate="2015-06-03T17:35:22.993" ContentLicense="CC BY-SA 3.0" />
  <row Id="8123" PostTypeId="2" ParentId="8122" CreationDate="2015-02-12T18:32:33.597" Score="4" Body="&lt;p&gt;Edit: on second thought, you're probably referring to an ion drive instead of firing the spacecraft out of a rail gun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could use a rail gun on a spacecraft as a mass driver: gun fires a rock out the back end, and the reaction force drives the ship. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're talking about firing the spacecraft out of a rail gun: the main limits to this are common to &lt;a href=&quot;https://space.stackexchange.com/questions/51/is-there-any-research-going-on-launching-into-orbit-with-space-guns-why-not&quot;&gt;all gun launch methods&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;either huge G-forces or very long barrels needed (500 m at 1000 G). G-forces sustainable for a human mean a gun that's hundreds of km long.&lt;/li&gt;&#xA;&lt;li&gt;atmospheric drag means you have to launch at speeds much higher than orbital speed.&lt;/li&gt;&#xA;&lt;li&gt;if the barrel is not a vacuum, you have to provide holes in the barrel so the air ahead of the projectile can escape. The supersonic shockwave means you can't build the gun near inhabited areas. Try drawing an uninhabited area 30 km wide and 300 km long anywhere in the First World. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Railgun&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rail guns&lt;/a&gt; exist. The US Navy is experimenting with one that will fire a 3-kg projectile at 2.5 km/s. The most difficult part of this is getting the gun to not destroy itself. This gun works by placing a conductive dart on top of two parallel rails. The current goes up one rail, through the dart and down the other. The rails are heated by the huge current needed (on the order of 1 MA), to the point where they initially had to swap out the rails after a few shots. You're also setting up huge EM fields around the projectile.&lt;br&gt;&#xA;There are other electromagnetic gun principles, e.g. you can place coils in series and pulse current through them to propel a metal projectile. I used to have fun firing metal pins from a coil scavenged from a set of toy train points.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-02-12T18:32:33.597" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8124" PostTypeId="1" CreationDate="2015-02-12T18:46:55.333" Score="14" ViewCount="2185" Body="&lt;p&gt;I am interested in knowing how accurately a solar panel (or for that matter an RF antenna) can be physically oriented as a satellite in a geocentric orbit travels about the earth. Within 1 degree? Worse? Better? I am interested in both what is typical and what is &quot;best in class&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am sure that it would be a major challenge to set up a rotation of the satellite about its own center of mass that would perfectly offset the angular rotation about the earth, so there must be some kind of &quot;fine tuning&quot; done as it moves.  I am also surprised that any motion of a gimbal does not cause the entire craft to rotate -- as it must by conservation of angular momentum ....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please enlighten me!&lt;/p&gt;&#xA;" OwnerUserId="8488" LastEditorUserId="603" LastEditDate="2015-02-12T19:08:20.010" LastActivityDate="2022-03-16T21:05:33.217" Title="How accurately can solar panels be continuously oriented toward the sun on a typical satellite?" Tags="|orbital-mechanics|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8125" PostTypeId="1" CreationDate="2015-02-12T18:47:14.973" Score="15" ViewCount="1870" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Soviet_Union&quot; rel=&quot;noreferrer&quot;&gt;Soviet Union's&lt;/a&gt; (in cooperation with some European governments like France and West Germany) &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_program&quot; rel=&quot;noreferrer&quot;&gt;Vega missions&lt;/a&gt; put balloons in the atmosphere of Venus back in 1985. It was 30 years ago, the last successful interplanetary mission of Soviet/Russia. There are advanced paper plans for airships in the atmosphere of Venus. But I've never heard of anything similar for Uranus or Neptune. I actually hear very little about any mission plan for Uranus or Neptune.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be more difficult to put an airship in the atmosphere of the gas giants, compared to Venus? It is colder and another composition and probably another density profile, and overall less known. What difficulties would arise, and how might they be overcome? Would radio communication get difficult? I note that all three mentioned planets have about the same surface gravity, very similar to that on Earth (although &quot;surface&quot; might have a fuzzy definition on a gas giant). Of course it is much more difficult to get to Uranus, though its deeper gravity well and aerobraking helps, but I ask about what can be done with an airship once a probe has entered orbit there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to add the side question if it would be valuable for exoplanet research to examine especially the atmospheres of Venus, Uranus, Neptune since the most common exoplanet size seems to lie between them, and their atmospheres might be analyzed with telescopes planned for the next few years or a decade. Is there a sensible synergy between studying those three planets here, and exoplanets?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="286" LastEditDate="2015-02-13T21:37:10.843" LastActivityDate="2020-09-22T06:22:42.117" Title="Is it more challenging to put an airship in the Uranian than in the Venusian atmosphere?" Tags="|atmosphere|venus|balloons|neptune|uranus|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="8126" PostTypeId="2" ParentId="7718" CreationDate="2015-02-12T18:52:23.380" Score="4" Body="&lt;p&gt;If you used a drogue parachute that is large enough to slow the vehicle down but small enough not to induce drift you could jettison that before landing under booster power.&lt;/p&gt;&#xA;" OwnerUserId="8489" LastActivityDate="2015-02-12T18:52:23.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8127" PostTypeId="2" ParentId="8125" CreationDate="2015-02-12T19:33:08.460" Score="5" Body="&lt;p&gt;My first concern was the magnetosphere, but &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Venus#Induced_magnetosphere&quot; rel=&quot;noreferrer&quot;&gt;Venus seems to have little or no intrinsic magnetic field&lt;/a&gt;, and Uranus's is &lt;a href=&quot;http://en.wikipedia.org/wiki/Uranus#Magnetosphere&quot; rel=&quot;noreferrer&quot;&gt;somewhat stronger&lt;/a&gt; than Earth's, but not ridiculously so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The temperature and pressure of &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Venus#Troposphere&quot; rel=&quot;noreferrer&quot;&gt;Venus' atmosphere at 50-65km altitude&lt;/a&gt; are pretty manageable, not unlike conditions that can be found near Earth's surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Uranus's atmosphere seems to be &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmosphere_of_Uranus#Structure&quot; rel=&quot;noreferrer&quot;&gt;much colder&lt;/a&gt; at comparable pressures. Keeping your airship heated (at high altitudes) and its envelope pressurized (at lower altitudes) is going to be the challenge there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radio communication is hard enough at Uranus' distance outside the atmosphere; I imagine you'd want the airship to have a low-power radio link to an orbiter (using frequency bands selected for minimal atmospheric absorption) and the orbiter would have a higher-powered comm link back to Earth. A similar strategy might be necessary in Venusian atmosphere too, if you're deep enough; sending an orbiter with a separate atmospheric probe probably has a lot of other advantages anyway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding exoplanets: big exoplanets currently appear to be the &quot;most common&quot; because massive planets in close orbits around their stars are the easiest to detect. While we might learn something about Uranian-type exoplanets from studying Uranus, they still aren't going to be as important to us in the long term as more terrestrial-type planets.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-02-12T19:33:08.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8128" PostTypeId="2" ParentId="8124" CreationDate="2015-02-12T19:45:37.773" Score="13" Body="&lt;p&gt;Solar panels don't have to be aimed very accurately. Solar panel efficiency is going to be proportional to the cosine of the error angle, so even if you're 10 degrees off your panels are still 98.5% effective!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;High-gain RF antennas do care about pointing accuracy; it's dependent on the wavelength and the size of the antenna -- the larger the dish the narrower the beam. Typically the effective beam width is on the order of a degree.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sstl.co.uk/Products/Subsystems/Communication/Antennas/High-Gain-X-Band-Antenna-Pointing-Mechanism&quot; rel=&quot;noreferrer&quot;&gt;Here's an antenna pointer&lt;/a&gt; that's spec'd for less than a quarter of a degree. I believe the satellite it's mounted on would use reaction wheels or small attitude control thrusters to hold a stable platform while the antenna moved. You'd obviously need comparable accuracy in the control system in order to make use of accurate pointing hardware. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-02-13T15:48:59.850" LastActivityDate="2015-02-13T15:48:59.850" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8130" PostTypeId="2" ParentId="8125" CreationDate="2015-02-12T20:30:17.447" Score="27" Body="&lt;p&gt;Buoyancy is a big problem.  To stay aloft, the average density of the balloon envelope, lifting gas and gondola must be &amp;lt;= the density of the surrounding atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;The pressure inside a balloon must be equal to or slightly greater than the surrounding atmosphere, otherwise the balloon will collapse.  If you look at the ideal gas law you'll see that see that there are two ways of satisfying these two conditions:  Either the molar mass of the lifting gas must be (substantially) less than the molar mass of the atmosphere, or the temperature must be (substantially) higher than that of the atmosphere, or both.&lt;/p&gt;&#xA;&lt;p&gt;On Venus the atmosphere is primarily CO&lt;span class=&quot;math-container&quot;&gt;$_2$&lt;/span&gt; with a molar mass of 44.  The Vega balloons were filled with He, molar mass 4.  The buoyancy is proportional to the difference (not the ratio) of the molar masses, thus 40 grams per mole - quite a lot of lift, in fact more than a helium balloon has on Earth for the same ambient pressure.&lt;/p&gt;&#xA;&lt;p&gt;Uranus' upper troposphere is about 85% hydrogen, 15% helium for an average molar mass of approx 2.3.  If you use the lightest lifting gas possible - pure hydrogen - the buoyancy is only 0.3 grams per mole, vastly less than the Vega balloons.  It would be extremely challenging (basically impossible) to build a light enough balloon envelope, gondola and payload to be supported by that.  Hydrogen is also rather more difficult to transport across the solar system than helium.&lt;/p&gt;&#xA;&lt;p&gt;The solution is probably to use a hot air balloon, with the air being the Uranian atmosphere.  It will still be challenging - you still need a much greater balloon volume for the same payload weight than on Venus, and you also have to bring a compact, lightweight heat source to heat all that atmosphere.  These technologies have yet to be developed.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2020-09-21T19:57:35.130" LastActivityDate="2020-09-21T19:57:35.130" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="8131" PostTypeId="2" ParentId="8102" CreationDate="2015-02-12T20:32:01.847" Score="2" Body="&lt;p&gt;Building on TidalWave's observation, I've found the following from wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Grasshopper 1.0 flew 8 times. I would presume that it also underwent an acceptance test, so there's &lt;strong&gt;9&lt;/strong&gt; propellant cycles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;F9R dev 1 flew 5 times before it self-destructed, so assuming it also underwent an acceptance test, there's &lt;strong&gt;6&lt;/strong&gt; times for the 1.1 architecture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure about specific missions that might have required more cycles due to scrubs, like the DSCOVR mission you pointed out.&lt;/p&gt;&#xA;" OwnerUserId="1255" LastActivityDate="2015-02-12T20:32:01.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8133" PostTypeId="1" AcceptedAnswerId="8149" CreationDate="2015-02-13T00:34:23.637" Score="11" ViewCount="477" Body="&lt;p&gt;We can read about Moon exploitation and making bases on there for long-term colonies. But bringing materials to make buildings there does not makes economical sense, and a large spaceship may be a hard bet. It seems to make much more sense to make buildings out of local resources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there enough resources on the Moon to make a concrete-like building to host operations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder what kind of materials could be used to make walls and floors and ceilings. On Earth, we use basically sand, water, etc.&lt;/p&gt;&#xA;" OwnerUserId="8383" LastEditorUserId="7062" LastEditDate="2021-10-27T23:16:19.707" LastActivityDate="2021-10-27T23:16:19.707" Title="Are there relevant resources on the Moon for civil engineering?" Tags="|the-moon|colonization|materials|natural-resources|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8135" PostTypeId="2" ParentId="8125" CreationDate="2015-02-13T00:57:05.197" Score="7" Body="&lt;p&gt;Venus' atmosphere is mostly CO2 which has a higher molar weight than N2 or O2 so a balloon filled with our atmosphere would be buoyant. Balloons filled with H2 or He even more so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But Uranus' atmosphere is mostly hydrogen and helium. Balloons that float would be harder. For a more thorough explanation of this problem, see pericynthion's excellent answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other problems than make Uranian balloon probes a lot harder than Venusian balloon probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital velocity just above Venus' atmosphere is about 7 km/s vs about 15 km/s for Uranus. Energy scales with velocity squared. (15/7)^2 is about 4.6. So entry into Uranus' atmosphere would entail shedding more than 4 times as much energy as Venus atmospheric entry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also it takes a lot more delta V to reach Uranus than it does Venus. From Low Earth Orbit (LEO) it would take about 8 km/s to send a probe along a Hohmann transfer to Uranus. Tran Venus Insertion from LEO is about 3.5 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If coming from earth along a Hohmann orbit, the probe would be moving about 22 km/s when it reaches Uranus' atmosphere. And about 11 km/s when reaching Venus' atmosphere. Assuming a Hohmann path, Uranus atmospheric entry is a bigger problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth to Uranus Hohmann trip time is about 16 years. Earth to Venus Hohmann trip is about 5 months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar power is an option for Venusian probes, Venus receives about twice earth's insolation. At ~20 AU, Uranus receives about 1/400 of the insolation that earth enjoys. The Uranian probe would need to be nuke powered.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-02-14T03:02:21.137" LastActivityDate="2015-02-14T03:02:21.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8136" PostTypeId="2" ParentId="5898" CreationDate="2015-02-13T04:33:35.153" Score="6" Body="&lt;p&gt;I would say chemical engineers play less of a role in space exploration than aero/astro, mechanical, software, and electrical engineers. I'll try to back this up with evidence before I share my anecdote. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the &lt;a href=&quot;http://www.bls.gov/ooh/architecture-and-engineering/chemical-engineers.htm#tab-3&quot; rel=&quot;noreferrer&quot;&gt;Bureau of Labor Statistics&lt;/a&gt;, most chemical engineers work in manufacturing (of chemicals). Designing new chemical products (like fuels or composite materials) is more of a scientist's role rather than an engineer's and typically requires a master's degree or PhD.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A typical chemical engineer in a non-research role (so probably with a bachelor's degree) works as a &lt;a href=&quot;http://en.wikipedia.org/wiki/Process_engineering&quot; rel=&quot;noreferrer&quot;&gt;process engineer&lt;/a&gt;. A process engineer is usually tasked with profitably mass producing a product. That entails using thermodynamics, fluid mechanics, heat/mass transfer, and chemical kinetics to design, optimize, and operate a chemical plant or refinery. &lt;a href=&quot;http://www.bls.gov/ooh/architecture-and-engineering/aerospace-engineers.htm#tab-3&quot; rel=&quot;noreferrer&quot;&gt;Aerospace engineers&lt;/a&gt;, however, largely go into the space industry since they've specialized in things like aerodynamic fluid flow; structural design; guidance, navigation, and control; instrumentation and communication; robotics; and propulsion and combustion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I certainly wouldn't say it will be impossible for you to get a job in the space industry with a background in chemical engineering. You can do anything if you work hard and focus on developing the right skills. I do think majoring in something that is already more common in the space industry will increase your chances of getting a job right out of school. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've gone down a career path similar to the one you're considering. I majored in chemical engineering for my BS, but I really want my career to have something to do with advancing space exploration. This has proven difficult so far. Since I haven't yet successfully switched to the space industry, I'm trying to develop skills relevant to space companies (CFD, propulsion knowledge) and working toward an MS in aero/astro engineering. I'm sure I'll make the transition eventually, but I think if space exploration is really what you want to do, you might save some time and effort focusing on the right things earlier than I did. &lt;/p&gt;&#xA;" OwnerUserId="3082" LastActivityDate="2015-02-13T04:33:35.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8137" PostTypeId="2" ParentId="8124" CreationDate="2015-02-13T06:11:38.153" Score="8" Body="&lt;p&gt;Three-axis controlled spacecraft can have very fine attitude control. Tens of arc seconds is well within state of the art for even low-cost microsatellites using technology no more advanced than reaction wheels and star trackers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That level of precision is totally unnecessary to point solar panels, as the cosine losses at even up to 10° are essentially negligible. Even high gain antennas (e.g. 15 dB) have a 3 dB beam width on the order of 10–20 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An obvious application for high pointing accuracy is imaging. To make the math easy, at an altitude of 1000 km, 0.01 radians (0.57°) of pointing error results in 10 km of error. This could be a significant fraction of your sensor field of view for a payload like a high-resolution imaging telescope, and thus likely unacceptable. &lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2015-02-13T06:11:38.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8138" PostTypeId="1" CreationDate="2015-02-13T10:56:21.220" Score="3" ViewCount="73" Body="&lt;p&gt;How is the money which is spent on space flight, in terms of launch costs, distributed between different purposes? I suppose that it would be unwieldy to consider the money spent on the development and construction of the equipment launched. If so, let's focus on launch costs alone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below I suggest a partitioning, but any categorization for which statistics exist would be very interesting. Global or US only are both of interest, depending on availability of data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Military&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Civilian&lt;/strong&gt; (the ISS, communication, navigation, weather)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Astronomy&lt;/strong&gt; (space telescopes, interplanetary probes, student's cubesats)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Consumer&lt;/strong&gt; (tourism, funerals, time capsules)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launch development&lt;/strong&gt; (testing launch vehicles, launch escape systems et c)&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-02-13T11:01:29.363" LastActivityDate="2015-02-13T11:01:29.363" Title="What sectors spend how much on space launches?" Tags="|launch|economics|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8139" PostTypeId="2" ParentId="8124" CreationDate="2015-02-13T13:55:41.530" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How accurately can solar panels be continuously oriented toward the sun on a typical satellite?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is no such thing as a typical satellite orbiting the Earth. Satellites range in size from 10 cm cubesats (or even smaller!) to the 100 meter long International Space Station. The orbits range in altitude of a few hundreds of kilometers to beyond geostationary, with inclinations ranging from near 0° to over 90°. Some satellites point at the Sun, others point at space, but most point at the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to look at this question is in terms of the number of rotational degrees of freedom used to control the orientation of the solar arrays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An Earth-orbiting satellite whose orientation with respect to the Earth is fully constrained needs to have solar arrays with two degrees of freedom to achieve 100% optimal power production. For example, the International Space Station has two alpha joints and eight beta joints that collectively work to keep the ISS close to optimal in terms of Sun pointing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those joints represent a good amount of weight and complexity, and also a number of single points of failure. Other satellites make do with less. At the opposite extreme, a number of satellites have fixed solar arrays. This is the only way to go with a satellite whose job is to observe the Sun. The pointing sensitivity of the sensors is much, much greater than the pointing sensitivity of the solar arrays. This is also how most cubesats (and surprisingly, some very expensive satellites) operate. There's a lot to be said for not having to rotate the solar panels. The drive mechanisms that do this are necessarily single points of failure. Getting rid of these mechanisms simplifies operations, simplifies design, and eliminates single points of failure. There's a lot to be said for simplicity, even if this means that the solar cells are almost never producing at 100% of optimal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In between these two extremes are those satellites whose solar arrays have one rotational degree of freedom. If there is such a thing as a &quot;typical&quot; satellite, this is it. In some cases (e.g., sun-synchronous orbiters), that single degree of freedom is enough to provide optimal power.  In others, the little bit of added power offered by a second degree of freedom is more than offset by added weight, added complexity, and reduced reliability. For example, most geostationary satellites have one degree of freedom solar arrays. (A few have fixed arrays or solar cells attached directly to the satellite.) Twice a year, the solar arrays of a geostationary satellite with one degree of rotational freedom will be misaligned from optimal by at least 23.5 degrees. However, the cosine of that angle is 0.917, which means the solar arrays will operate at more than 90% of optimal even under worst conditions. Getting that last 10% isn't worth it.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-02-13T13:55:41.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8140" PostTypeId="1" AcceptedAnswerId="10871" CreationDate="2015-02-13T14:46:10.233" Score="10" ViewCount="686" Body="&lt;p&gt;I was quite surprised to read in &lt;a href=&quot;http://earthobservatory.nasa.gov/IOTD/view.php?id=85288&quot;&gt;today's Earth Observatory Picture of the Day&lt;/a&gt; that the &lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/&quot;&gt;DSCOVR&lt;/a&gt; spacecraft, currently en route to the $L_1$ Lagrange point, will be the first spacecraft able to see the entire day-lit hemisphere of Earth, and take pictures of it, since Apollo 17 took the original &lt;a href=&quot;http://earthobservatory.nasa.gov/IOTD/view.php?id=1133&quot;&gt;Blue Marble photograph&lt;/a&gt;. Why is this? There certainly &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_objects_at_Lagrangian_points#L1&quot;&gt;are and have been&lt;/a&gt; missions to $L_1$; were they not equipped with cameras? Is it that hard to get far enough from Earth in that particular direction if you're not explicitly headed for $L_1$? I simply find it strange that in the 40+ years since Apollo 17 this has not been possible again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is this assertion, in fact, incorrect? Are there other previous blue-marble shots of the entire Earth in a single frame?&lt;/p&gt;&#xA;" OwnerUserId="653" LastEditorUserId="653" LastEditDate="2015-02-13T19:37:33.430" LastActivityDate="2015-09-01T15:25:52.160" Title="Single-shot Blue Marble pictures" Tags="|lagrangian-points|photography|earth-observation|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8141" PostTypeId="2" ParentId="8140" CreationDate="2015-02-13T19:11:45.547" Score="4" Body="&lt;p&gt;The full view of an hemisphere requires that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The camera is between the Sun and the Earth, really between!  Else some shadow fringe will be visible. For an orbit which is not in the ecliptic plan, this can happen only when the orbit crosses the ecliptic.&lt;/li&gt;&#xA;&lt;li&gt;This camera is far enough from the Earth. &lt;a href=&quot;https://physics.stackexchange.com/questions/64253/distance-away-from-earth-to-see-it-as-a-full-disk&quot;&gt;How far?&lt;/a&gt; 40,000 km -- about the GEO -- ensures nearly 50% of the sphere are visible (actually &lt;a href=&quot;http://www.neoprogrammics.com/spheres/visible_fraction_of_surface.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;43%&lt;/a&gt; of the surface). Further distance will improve the visible area, only a small improvement, but will make the Earth disk smaller in the picture.&lt;/li&gt;&#xA;&lt;li&gt;No shadow is cast by the Moon.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is not easily found within the frame of a short mission, to the point that no Apollo crew, except Apollo 17, has been in the required conditions for a &quot;full Earth&quot; picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the well known Blue Marble from Apollo 17 (Cernan, Schmitt, Evans) may not be the only full view of the illuminated Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems &lt;a href=&quot;http://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa&lt;/a&gt; also &lt;a href=&quot;http://www.planetary.org/multimedia/space-images/earth/earth_hayabusa_ST_1038697115_wvb.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;shot a full hemisphere&lt;/a&gt;. Hayabusa was an asteroid mission from Japan JAXA, to return samples on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Henp1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hayabusa snapped this image of Earth during its flyby on May 18, 2004&#xA;  at 15:00 UTC&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-02-14T08:07:37.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8142" PostTypeId="1" AcceptedAnswerId="8146" CreationDate="2015-02-13T20:16:24.720" Score="8" ViewCount="2080" Body="&lt;p&gt;The escape velocity of earth is about 11.2 km/s. Let's say it was, for the sake of argument, exactly 11.2 km/s. If I go 11.199999 km/s, I'm going to fall back down to earth. But how can I determine how far I will get before I do so? What pieces of data do I need for this?&lt;/p&gt;&#xA;" OwnerUserId="1537" LastEditorUserId="4660" LastEditDate="2015-02-14T20:11:33.183" LastActivityDate="2015-02-15T01:58:41.757" Title="How far away will I get if I'm --just under-- escape velocity?" Tags="|launch|orbital-mechanics|escape-velocity|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8143" PostTypeId="1" AcceptedAnswerId="8150" CreationDate="2015-02-13T20:55:43.307" Score="3" ViewCount="2134" Body="&lt;p&gt;In doing some research with my son about solar sails, we stumbled on &lt;a href=&quot;http://science.howstuffworks.com/solar-sail.htm&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; that claims (on &lt;a href=&quot;http://science.howstuffworks.com/solar-sail2.htm&quot; rel=&quot;nofollow&quot;&gt;page 3&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You might be wondering what happens when the spacecraft finds itself far from sunlight. An onboard laser could take over providing the necessary propulsion to the sails.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Doesnt this break the some kind of physics law of conservation?  It seems a bit like blowing on the sail of the boat you were in.  You being pushed back as much as the boat was being pushed forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is something like this possible?&lt;/p&gt;&#xA;" OwnerUserId="3510" LastActivityDate="2015-02-14T03:01:11.087" Title="Can an onboard laser propel a solar sail?" Tags="|propulsion|solar-sail|" AnswerCount="2" CommentCount="2" ClosedDate="2015-02-14T06:03:43.240" ContentLicense="CC BY-SA 3.0" />
  <row Id="8144" PostTypeId="2" ParentId="8140" CreationDate="2015-02-13T21:08:01.173" Score="4" Body="&lt;p&gt;Strictly speaking, even DSCOVR won't see the the entire day-lit hemisphere of Earth. From a distance of 1.5 million kilometers, it will see 49.8% of the Earth's surface. Getting that last 0.2% would require an even more remote camera. The Hayabusa picture shown in mins' answer was shot from a distance of 295,000 km and encompasses about 49.2% of the globe. The famous Blue Marble picture was taken from a distance of 45,000 kilometers and thus shows 42.9% of the surface. That's not much further than geostationary, which see 42.4% of the surface.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why is this?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What's the point? The Earth-imaging part of this satellite was Al Gore's idea back in 1998, and it was widely perceived as a publicity stunt. Geostationary satellites daily provide a fully-lit view of almost half of the Earth's surface. That extra 7.6% coverage provided by DSCOVR comes at the cost of a rather expensive telescope that nonetheless has rather low resolution.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-02-13T21:08:01.173" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8145" PostTypeId="2" ParentId="8142" CreationDate="2015-02-13T21:09:46.727" Score="7" Body="&lt;p&gt;You wouldn't get far, since that wouldn't quite get you escaped from the Earth-Moon system.  Your escape velocity is what you would need at the surface of the Earth just to escape Earth.  (I get $11.18\,\mathrm{km/s}$.)  To escape the Earth-Moon system, you would need $11.25\,\mathrm{km/s}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an escape from a body, if you depart radius $r$ with $1-\epsilon$ times the escape velocity, and you are going directly away from the center of the body with that velocity, then for small $\epsilon$ you will get to $r\over 2\epsilon$ before falling back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Derivation, starting with the velocity $v$ at radius $r$, just shy of the escape velocity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_e^2={2\mu\over r}$$&#xA;$$v=v_e\left(1-\epsilon\right)$$&#xA;$$v^2={2\mu\over r}\left(1-\epsilon\right)^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Relating the velocity to periapsis and apoapsis using the conservation of energy:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v^2=2\mu\left({1\over r}-{1\over r_p+r_a}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a trajectory directly away from the center of the body, use a periapsis of zero:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v^2=2\mu\left({1\over r}-{1\over r_a}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${2\mu\over r}\left(1-\epsilon\right)^2=2\mu\left({1\over r}-{1\over r_a}\right)$$&#xA;$${1\over r}\left(1-\epsilon\right)^2={1\over r}-{1\over r_a}$$&#xA;$$\left(1-\epsilon\right)^2=1-{r\over r_a}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For small $\epsilon$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$1-2\epsilon\approx 1-{r\over r_a}$$&#xA;$$2\epsilon\approx {r\over r_a}$$&#xA;$$r_a\approx {r\over 2\epsilon}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in the case where you make the mistake of using the Earth escape velocity without taking into account the Moon, then $\epsilon$ is about $0.006$.  So you would get about $500,\!000\,\mathrm{km}$ from Earth before falling back.  A little farther than the Moon.  That assumes however that the Moon is not close to your path.  If it is, then it might either a) give you enough energy to completely escape, or b) send you back to the Earth with less energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the spirit of your example, e.g. if you assume that $11.2\,\mathrm{km/s}$ is &lt;em&gt;exactly&lt;/em&gt; the escape velocity, then your $\epsilon$ is about $10^{-7}$.  So you would get about 36 billion kilometers away (240 AU!).  However other bodies such as the Sun, Venus, Jupiter, and re-encountering the Earth-Moon system would alter your course well before that.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-02-15T01:58:41.757" LastActivityDate="2015-02-15T01:58:41.757" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8146" PostTypeId="2" ParentId="8142" CreationDate="2015-02-13T21:18:21.417" Score="11" Body="&lt;p&gt;You are talking about an elliptical orbit about the earth. Two points on this ellipse are of interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perigee - point on the orbit closest to earth.&#xA;Apogee - point on the orbit farthest from earth&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're launching from earth's surface, the perigee would be 6378 km from earth's center. (6378 is earth's radius). For this problem I will pretend there's no earth atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The apogee is what you're interested in, the farthest point from the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;noreferrer&quot;&gt;vis viva equation&lt;/a&gt; is a nice tool for finding velocity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v=\sqrt{Gm(2/r-1/a)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is a? That's the semi-major axis. We can find a by taking the average of perigee distance from center and apogee distance from earth's center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a pic of the stuff I've described so far:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JkVPn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We're interested in the speed when the object leaves earth so r would be 6378 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take a look at the vis viva question again:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v=\sqrt{Gm(2/r-\mathbf{1/a})}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Noticed I bolded the term that uses the semi-major axis quantity. What happens when a gets real big? 1/a gets closer and closer to zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for high apogees you have something very close to&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v=\sqrt{Gm(2/r)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Just a hair under escape&quot; describes a multitude of ellipses!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example let's imagine an ellipse whose apogee reaches EML1, about 5/6 of the way to the moon. Now let's compare it to an ellipse whose apogee extends all the way to Sun-Earth L2, about 1.5 million kilometers away. The perigee speeds of these two ellipses differ by only .11 km/s!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But when you get apogees that high, 2 body mechanics is no longer a good model. The gravitational influences of the moon and sun are enough to bend the object out of an elliptical path. Launch something as high as SEL2 and it is likely the sun will wrest the object from earth's sphere of influence.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="8499" LastEditDate="2015-02-13T23:48:58.453" LastActivityDate="2015-02-13T23:48:58.453" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8147" PostTypeId="2" ParentId="8143" CreationDate="2015-02-13T22:18:36.887" Score="3" Body="&lt;p&gt;If the sail is a mirror (which you would expect) it is the same as if you pointed the laser to the rear, like a rocket engine. No need for the sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if the laser is in a fixed position, and to save mass by avoiding unneeded equipment, the sail can be used as the mirror to steer the laser energy and change trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, blowing on a sail would work fine if the sail were a &quot;wind mirror&quot; and the energy didn't dissipate in turbulence. It would be like blowing through a tube that directed the air to the rear. If it bounced the air right back at you, then there would be no net work.&lt;/p&gt;&#xA;" OwnerUserId="1432" LastActivityDate="2015-02-13T22:18:36.887" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8148" PostTypeId="2" ParentId="8140" CreationDate="2015-02-14T00:21:08.513" Score="4" Body="&lt;p&gt;Russian Electro-L meteorological satellite is capable of producing whole hemisphere shots from GEO:&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Elektro%E2%80%93L&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Elektro%E2%80%93L&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/irQWG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Timelapse shots converted to &lt;a href=&quot;http://www.youtube.com/watch?v=b8v0LCOJ1Vk&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/b8v0LCOJ1Vk?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6491" LastEditorUserId="49" LastEditDate="2015-02-14T08:35:29.220" LastActivityDate="2015-02-14T08:35:29.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8149" PostTypeId="2" ParentId="8133" CreationDate="2015-02-14T02:57:59.730" Score="10" Body="&lt;p&gt;Paul Spudis notes the moon has lots of aggregate. Lunar regolith is shattered bedrock with granules of all different sizes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spudis suggests &quot;Instead of water-set lime-based cement, we can use glass to cement particulate material together.  Regolith can be sintered into bricks and blocks, as well as roads and landing pads, using thermal energy (passive solar, concentrated by focusing mirrors) or microwaves that can melt grain edges into a hard, durable ceramic.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://www.airspacemag.com/daily-planet/regolith-the-other-lunar-resource-156943194/&quot;&gt;Regolith, the &quot;Other&quot; Lunar Resource&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-02-14T02:57:59.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8150" PostTypeId="2" ParentId="8143" CreationDate="2015-02-14T03:01:11.087" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Can an onboard laser propel a solar sail?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In theory, a tiny, tiny, tiny little bit. In practice, no. You'd be better off without the sail and pointing the laser backwards, but you'd be even much better off without the laser.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Warning: sarcasm follows.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A one point twenty-one gigawatt laser would provide a paltry &lt;a href=&quot;http://www.wolframalpha.com/input/?i=%281.21+gigawatts%29+%2F+%28speed+of+light%29&quot; rel=&quot;nofollow&quot;&gt;four newtons of thrust&lt;/a&gt;. There is no such thing as a continuous wave gigawatt laser; we'll need a thousand or so one point twenty-one megawatt lasers (they don't exist, either). Lasers aren't exactly efficient; I'll be very generous and assume a 25% efficiency. All we need is four Mr. Fusions and voila! our thousand laser farm has power. Being even more generous and assuming each of our non-existent megawatt lasers weighs a metric ton, that's an acceleration of 4 microns/second&lt;sup&gt;2&lt;/sup&gt;. (That's a microscopically tiny acceleration.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Mr. Fusion doesn't exist, we need some other power source. Mr. Fusion was nice and compact. The world's largest nuclear reactor, the Kashiwazaki-Kariwa Nuclear Power Plant, was rated at 8.21 gigawatts, two more than we need. Since Japan no longer wants their nuclear reactors, all we need to do is package that plant for use in space. One minor problem: The extreme mass will make our already paltry 4 microns per second&lt;sup&gt;2&lt;/sup&gt; orders of magnitude smaller yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Mr. Fusion doesn't exist, and nuclear reactors are a lousy power source for a space-based laser, perhaps solar arrays (very, very big solar arrays) might work? The answer is no, they won't. Now we're stuck with the exact same problem that makes solar sails rather useless after five AU or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line: You'd be much better off without the laser.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;What about the idea raised in &lt;a href=&quot;http://ykbcorp.com/downloads/Bae_photon_propulsion_STAIF2_Paper_Circulation.pdf&quot; rel=&quot;nofollow&quot;&gt;Prospective of Photon Propulsion for Interstellar Flight&lt;/a&gt;, which was pointed out in a comment to the question?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note very well: This laser is not mounted on the vehicle. Even the author of this conference paper knows that mounting the laser on the spacecraft is sheer nuttiness. What he doesn't know is that his idea is sheer nuttiness, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;His idea is based on the 2001 conference paper &lt;a href=&quot;http://scitation.aip.org/content/aip/proceeding/aipcp/10.1063/1.1357953&quot; rel=&quot;nofollow&quot;&gt;Metzger and Landis (2001) &quot;Multi-bounce laser-based sails,&quot; &lt;em&gt;Space Technology and Applications International Forum-2001&lt;/em&gt;, Vol. 552:1&lt;/a&gt;. This paper has been cited a whole seven times since 2001, with six of those citation coming from the author of the paper in question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic idea is to have a fixed laser source such as on the Moon and have a mirror at the laser source and a mirror on the spacecraft. The laser hits the spacecraft mirror, bounces straight back to the source mirror, which bounces straight back to the spacecraft mirror, and so on, a thousand times over.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This idea is completely divorced from reality. This places pointing accuracy requirements on the spacecraft that makes the pointing of the Hubble Space Telescope look absolutely amateurish. This places flatness and reflectivity requirements on the mirrors that are far, far beyond our dreams of Mr. Fusion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, if by some miracle this idea does work, do you really want to be on the receiving end of the equivalent of one point twenty one terawatts of laser power?&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-02-14T03:01:11.087" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8151" PostTypeId="1" AcceptedAnswerId="8173" CreationDate="2015-02-14T10:35:49.620" Score="9" ViewCount="278" Body="&lt;p&gt;To position a spacecraft around an asteroid, one must know the orbital parameters and orientation of the asteroid and the spacecraft. Formation flying will be more complex in terms of positioning relative to an asteroid. In case of formation flying in LEO, one uses GPS to accurately determine the position of the spacecraft. Since GPS is not useful in deep space, formation flying with local navigation system / positioning system is more important. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering two or more bodies flying in a swarm configuration relatively to one another, what are the possibilities of using wireless ad hoc networks, or other known similar technologies that can be useful to position the spacecraft autonomously?&lt;/p&gt;&#xA;" OwnerUserId="8501" LastEditorUserId="12102" LastEditDate="2017-06-03T09:53:38.157" LastActivityDate="2017-06-03T09:53:38.157" Title="Could a swarm of probes autonomously establish orbit around an asteroid?" Tags="|asteroid|technology|navigation|computing|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8152" PostTypeId="1" CreationDate="2015-02-14T17:05:50.163" Score="12" ViewCount="320" Body="&lt;p&gt;There were many studies in the 1960s, and a lesser number of later ones, for launch vehicles with nuclear-thermal upper stages. I have rarely seen mention of what emergency procedures were considered for launch failures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4560/dangers-of-launching-a-nuclear-thermal-rocket&quot;&gt;Dangers of launching a nuclear thermal rocket&lt;/a&gt;  suggests the hazard is not extreme - for the case of an NTR started in orbit. A nuclear upper stage has the possibility of running for some time but failing to reach orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even in the pre-Chernobyl era, there has to have been serious consideration of this. What studies were done, and is there any available summary of the results?&lt;/p&gt;&#xA;" OwnerUserId="8216" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-12-13T17:17:37.757" Title="What were emergency procedures for failure of launch vehicles with nuclear upper stages?" Tags="|abort|ntr|risk-management|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8153" PostTypeId="1" CreationDate="2015-02-14T17:16:46.943" Score="7" ViewCount="373" Body="&lt;p&gt;Magazines will only get you so far, Google searches don't usually yield technical papers, or not many or the best or ones that can be read in full. NASA's public website is also limited and somewhat general, ESA's too. Yet many times when I've asked a question here I am directed to a pdf of a great technical paper on exactly that topic, sometimes within minutes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Through this assistance I have found a couple of mother lodes. There may be more, and there may be an approach to using them I haven't figured out. What are the key resources, and are there any important tips on how to use them?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: this question has been designated a community wiki. So, go ahead and add whatever you've got to the one answer below, so the resources are all in one well organized answer.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-15T03:35:37.300" LastActivityDate="2015-12-14T22:00:20.510" Title="What are the best places to go to research a space exploration matter online (in depth)?" Tags="|research|" AnswerCount="2" CommentCount="2" CommunityOwnedDate="2015-02-14T18:04:04.083" ContentLicense="CC BY-SA 3.0" />
  <row Id="8155" PostTypeId="1" AcceptedAnswerId="8172" CreationDate="2015-02-14T17:52:50.687" Score="21" ViewCount="3606" Body="&lt;p&gt;It's only for 5 seconds, but that is an &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_7K-ST_No._16L#Mission_highlights&quot;&gt;awful lot of force&lt;/a&gt;. The Falcon system is similar and presumably also involves very high-g forces. Could injury result from the abort itself in either case?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-14T21:52:05.380" LastActivityDate="2021-03-15T22:43:25.627" Title="How hard is it on the crew to go through the 14+ g's of a launchpad abort of the Soyuz, or the Falcon system?" Tags="|health|failure|abort|launch-escape-system|g-force|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8156" PostTypeId="5" CreationDate="2015-02-14T18:41:51.310" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-14T18:41:51.310" LastActivityDate="2015-02-14T18:41:51.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8157" PostTypeId="4" CreationDate="2015-02-14T18:41:51.310" Score="0" Body="Questions on taking images in space or of spacecraft. " OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-14T21:29:42.510" LastActivityDate="2015-02-14T21:29:42.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8158" PostTypeId="5" CreationDate="2015-02-14T18:46:55.190" Score="0" Body="&lt;p&gt;How passage of time impacts space activities.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-14T20:48:14.123" LastActivityDate="2015-02-14T20:48:14.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8159" PostTypeId="4" CreationDate="2015-02-14T18:46:55.190" Score="0" Body="How passage of time impacts space exploration activities." OwnerUserId="4660" LastEditorUserId="1142" LastEditDate="2015-02-14T21:29:28.663" LastActivityDate="2015-02-14T21:29:28.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8160" PostTypeId="2" ParentId="8155" CreationDate="2015-02-14T18:53:27.240" Score="12" Body="&lt;p&gt;When I worked with Vladimir Titov on STS-86 he was fine after experiencing such an abort years earlier.  If your question is &lt;em&gt;could&lt;/em&gt; an injury happen, sure, anything &lt;em&gt;can&lt;/em&gt; happen.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-02-14T18:53:27.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8161" PostTypeId="5" CreationDate="2015-02-14T19:04:57.977" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-14T19:04:57.977" LastActivityDate="2015-02-14T19:04:57.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8162" PostTypeId="4" CreationDate="2015-02-14T19:04:57.977" Score="0" Body="How heat affects spacecraft and space missions, or materials and methods to deal with heat." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-14T20:48:37.203" LastActivityDate="2015-02-14T20:48:37.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8163" PostTypeId="5" CreationDate="2015-02-14T19:08:37.780" Score="0" Body="&lt;p&gt;A branch of science that deals with all matter and energy. Physics includes mechanics, heat, light and other radiation, sound, electricity, magnetism, the structure of atoms, and more.&lt;/p&gt;&#xA;" OwnerUserId="46507" LastEditorUserId="46507" LastEditDate="2023-03-23T14:13:43.407" LastActivityDate="2023-03-23T14:13:43.407" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8164" PostTypeId="4" CreationDate="2015-02-14T19:08:37.780" Score="0" Body="How physics applies to a particular activity in space or to getting to and from space." OwnerUserId="4660" LastEditorUserId="46507" LastEditDate="2023-03-23T14:10:05.433" LastActivityDate="2023-03-23T14:10:05.433" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8165" PostTypeId="5" CreationDate="2015-02-14T19:10:53.540" Score="0" Body="&lt;p&gt;Feasibility refers to the assessment of whether an idea is technologically, financially, and operationally practical. Feasibility studies are generally conducted to determine whether it is possible to achieve its objectives with the current limitations.&lt;/p&gt;&#xA;" OwnerUserId="46507" LastEditorUserId="46507" LastEditDate="2023-03-21T16:59:59.520" LastActivityDate="2023-03-21T16:59:59.520" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8166" PostTypeId="4" CreationDate="2015-02-14T19:10:53.540" Score="0" Body="Used when the question pertains to how practical something is. It will also include an economic review of the what is being considered. It may be possible for something to be technically possible but the cost might be prohibitive." OwnerUserId="4660" LastEditorUserId="7062" LastEditDate="2023-03-21T11:32:03.577" LastActivityDate="2023-03-21T11:32:03.577" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8167" PostTypeId="5" CreationDate="2015-02-14T19:12:57.813" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-14T19:12:57.813" LastActivityDate="2015-02-14T19:12:57.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8168" PostTypeId="4" CreationDate="2015-02-14T19:12:57.813" Score="0" Body="Questions about making equipment that can be used more than once, or the performance of such equipment. " OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-14T21:07:53.697" LastActivityDate="2015-02-14T21:07:53.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8169" PostTypeId="5" CreationDate="2015-02-14T19:17:20.360" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-14T19:17:20.360" LastActivityDate="2015-02-14T19:17:20.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8170" PostTypeId="4" CreationDate="2015-02-14T19:17:20.360" Score="0" Body="Questions about the steps in a process, or what procedures exist for a space exploration activity." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-14T21:05:18.713" LastActivityDate="2015-02-14T21:05:18.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8171" PostTypeId="2" ParentId="8142" CreationDate="2015-02-14T19:55:39.523" Score="2" Body="&lt;p&gt;The easiest way how to compute this is to use &lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_orbital_energy&quot; rel=&quot;nofollow&quot;&gt;specific orbital energy&lt;/a&gt;. The following quantity is preserved over time:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{v^2}{2} - \frac{G(M+m)}{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;since the mass of a spaceship is negligible compared to the mass of the Earth, we can omit $m$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The furthest point where we can get is when we consume all kinetic energy, so $v=0$. Knowing the velocity $v$ at the Earth surface $R$, we get&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{equation}&#xA;\frac{v^2}{2} - \frac{GM}{R} = - \frac{GM}{r_{\mbox{max}}}&#xA;\end{equation}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the escape velocity $v_e$ we have $r_{\mbox{max}}=\infty$ so&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{v_{e}^2}{2} - \frac{GM}{R} = 0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which leads to&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{GM}{R} = \frac{v_{e}^2}{2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $R$ is the radius of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting this into the equation for $r_{\mbox{max}}$, we obtain&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{v^2}{2} - \frac{v_{e}^2}{2} = - \frac{GM}{r_{\mbox{max}}}&#xA;$$&#xA;and&#xA;$$&#xA;r_{\mbox{max}} = \frac{2GM}{v_{e}^2 - v^2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting the velocity you asked for and the Earth's &lt;a href=&quot;http://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow&quot;&gt;standard gravitational parameter&lt;/a&gt; we get (in km)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;r = \frac{2 \times 398600.4419}{11.2^2-11.199999^2} \doteq 3.56\times 10^{10}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which is approximately $238 \mbox{AU}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Of course we completely disregarded the effect of all the other celestial bodies.)&lt;/p&gt;&#xA;" OwnerUserId="8505" LastEditorUserId="8505" LastEditDate="2015-02-14T20:37:34.847" LastActivityDate="2015-02-14T20:37:34.847" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8172" PostTypeId="2" ParentId="8155" CreationDate="2015-02-14T19:58:59.810" Score="28" Body="&lt;p&gt;14G sounds like a lot. To put it in context:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/G-force#Human_tolerance_of_g-force&quot; rel=&quot;noreferrer&quot;&gt;the acceleration you can endure depends on the force vector&lt;/a&gt;. The position in which we can withstand G-forces best is forward (using the directions from the XKCD diagram), so spacecraft seats are placed to take advantage of this. When the rocket sits on the pad, the astronauts lie on their back. &lt;/li&gt;&#xA;&lt;li&gt;fighter pilots are trained to function at peaks of 9G upward. They use special flight suits to achieve this much. Much higher than that and people lose consciousness. &lt;/li&gt;&#xA;&lt;li&gt;we know from tests (&lt;a href=&quot;http://www.livescience.com/34128-limits-human-survival.html&quot; rel=&quot;noreferrer&quot;&gt;John Stapp&lt;/a&gt;) that a human can survive peak loads of 45G (backward, or 'eyeballs out') but he suffered vision damage at these loads. People have survived higher loads in car crashes, but injuries are likely.&lt;/li&gt;&#xA;&lt;li&gt;untrained humans can survive 20G for 10 seconds. &lt;/li&gt;&#xA;&lt;li&gt;fighter pilots who have to eject, experience 12-22G peak loads upward and often have some health problems afterward, and are usually grounded after their second ejection to prevent worse. &lt;/li&gt;&#xA;&lt;li&gt;NASA did some &lt;a href=&quot;http://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf&quot; rel=&quot;noreferrer&quot;&gt;research into parachute harnesses&lt;/a&gt;. The opening of a parachute also induces a large G-load upward, and they found:  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For such harnesses the NASA/AGARD researches indicate a 5% injury risk at 12.1G,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sitting in a well-designed seat the forces are spread out much better than in a harness. I suspect the injury risk at 14G is pretty low. The crew won't be able to move for those few seconds, but they should be okay. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://what-if.xkcd.com/116/&quot; rel=&quot;noreferrer&quot;&gt;the XKCD site&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zxuik.png&quot; alt=&quot;Obligatory XKCD&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-02-16T08:21:58.977" LastActivityDate="2015-02-16T08:21:58.977" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8173" PostTypeId="2" ParentId="8151" CreationDate="2015-02-14T21:44:30.083" Score="12" Body="&lt;p&gt;Let's break your question into separate tasks:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Autonomous orbit determination (autonomous because the DSN won't be there to help you when you need it)&lt;/li&gt;&#xA;&lt;li&gt;Autonomous attitude determination&lt;/li&gt;&#xA;&lt;li&gt;Situational awareness for formation flying (relative positions, velocities, attitudes and attitude rates), most efficiently done in a cooperative manner&lt;/li&gt;&#xA;&lt;li&gt;Cooperative collision avoidance&lt;/li&gt;&#xA;&lt;li&gt;Non-cooperative collision avoidance&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Let's state it from the start: you can't rely on other craft in the swarm to fulfil your mission since losing one craft implies at best, degraded capabilities for all others, and at worst, mission failure.&lt;/p&gt;&#xA;&lt;p&gt;Hence, you can't rely on cooperative ways of finding out where the heck your own craft is, and this means extracting range, range rate, attitude, attitude rate information from comms links is a bonus, but never a primary method of running the show. There are some mitigating factors, but they are marginal at best.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Autonomous orbit determination&lt;/strong&gt; has to contend with uncertainties in:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;asteroid's terrain&lt;/li&gt;&#xA;&lt;li&gt;asteroid's gravity&lt;/li&gt;&#xA;&lt;li&gt;asteroid's rotation parameters&lt;/li&gt;&#xA;&lt;li&gt;all extra perturbations&lt;/li&gt;&#xA;&lt;li&gt;systematic biases in the craft's instruments&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;To overcome all this and more, you've got to use several physical principles (I don't really know the mission you have in mind, so I am not concerned by the exact mass budget) and fuse data from all of them in a smart way (possibly, to uncover systematic errors and compensate for equipment malfunctions):&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;passive optical triangulation and ranging (a lot of thought goes into pattern recognition here, luckily you &lt;strong&gt;have installed&lt;/strong&gt; a camera on your craft, haven't you?);&lt;/li&gt;&#xA;&lt;li&gt;active optical (ranges, range rates, angles) with a lidar;&lt;/li&gt;&#xA;&lt;li&gt;active radar (nice if you can get synthetic aperture pictures from the asteroid, otherwise you are forced to use a terrain model obtained by other means which introduces the possibility of undetected misalignment - your camera and your radar, for instance, are looking at slightly different spots).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;For &lt;strong&gt;autonomous attitude determination&lt;/strong&gt;, there's nothing new here: star and Sun trackers, laser gyros, and MEMS accelerometers.&lt;/p&gt;&#xA;&lt;p&gt;For &lt;strong&gt;formation flying and collision avoidance&lt;/strong&gt;, you have to think about corner cases when one of the sats in the swarm has lost attitude control and is outgassing, threatening to collide with others. You can use optical reflectors and targets to make combined lidar/camera measurements.&lt;/p&gt;&#xA;&lt;p&gt;If the satellites in your formation have comm links on, you &lt;strong&gt;can&lt;/strong&gt; obtain extra information to feed into your fusion algorithm from the low-level params read from the comms card:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Doppler frequency shift&lt;/li&gt;&#xA;&lt;li&gt;time difference of arrival, including TDOA sent back to your sat by others&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Needless to say, run-of-the-mill industrial and retail wireless protocols and cards don't usually cater to this niche segment. You have to design your own protocol and find ways to implement it in hard- (FPGA) and software (SDR).&lt;/p&gt;&#xA;&lt;h2&gt;References:&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Grelier et al. Formation flying radio frequency instrument: First flight results from the PRISMA mission. 5th ESA Workshop on Satellite Navigation Technologies and European Workshop on GNSS Signals and Signal Processing (NAVITEC). 2010. &lt;a href=&quot;http://dx.doi.org/10.1109/navitec.2010.5708059&quot; rel=&quot;noreferrer&quot;&gt;DOI&lt;/a&gt; (paywalled).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Another look at the PRISMA mission: &lt;a href=&quot;http://issfd.org/ISSFD_2009/FormationFlyingI/Delpech.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://issfd.org/ISSFD_2009/FormationFlyingI/Delpech.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h2&gt;TL;DR&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Commercial off-the-shelf wireless won't be able to provide navigation data to swarms operating near an asteroid&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Specially designed wireless comms networks will be able to supplement but not replace autonomous navigation and attitude determination equipment&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Optical, radar, and wireless comms technologies can and should be used in synergy to improve navigation and formation flying accuracy.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-02-15T18:02:42.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8174" PostTypeId="5" CreationDate="2015-02-14T21:49:29.403" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-14T21:49:29.403" LastActivityDate="2015-02-14T21:49:29.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8175" PostTypeId="4" CreationDate="2015-02-14T21:49:29.403" Score="0" Body="Questions about the stage(s) of a rocket after the first, used to establish an orbit, or a trajectory beyond Earth orbit." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-14T21:59:42.643" LastActivityDate="2015-02-14T21:59:42.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8177" PostTypeId="5" CreationDate="2015-02-14T21:54:54.117" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-14T21:54:54.117" LastActivityDate="2015-02-14T21:54:54.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8178" PostTypeId="4" CreationDate="2015-02-14T21:54:54.117" Score="0" Body="Questions about engines used in space that accelerate ions to create thrust." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-02-14T21:59:49.520" LastActivityDate="2015-02-14T21:59:49.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8179" PostTypeId="2" ParentId="8153" CreationDate="2015-02-14T23:25:28.680" Score="3" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://scholar.google.com/?hl=en&quot; rel=&quot;nofollow&quot;&gt;Google Scholar&lt;/a&gt; is a good source. You can set up alerts to be sent to your mailbox for a particular query string.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Another good tip is to use Google Advanced Search for file type .pdf.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://scholar.google.com/?hl=en&quot; rel=&quot;nofollow&quot;&gt;NASA Technical Reports Server&lt;/a&gt; - which includes tabs in its menu&#xA;bar to tutorials and search tips&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The NTRS integrates the following three NASA collections and enables&#xA;  search and retrieval through a common interface:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NACA Collection: Citations and reports from the National Advisory&#xA;  Committee for Aeronautics period lasting from 1915 to 1958. NASA&#xA;  Collection: Citations and documents created or sponsored by NASA&#xA;  starting in 1958 and continuing to the present. NIX Collection:&#xA;  Citations and links to the images, photos, movies, and videos from the&#xA;  discontinued NASA Image eXchange (NIX).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://arxiv.org/&quot; rel=&quot;nofollow&quot;&gt;arXiv&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Open access to 1,012,588 e-prints in Physics, Mathematics, Computer&#xA;  Science, Quantitative Biology, Quantitative Finance and Statistics&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;which has a section for astrophysics, with a number of subsections.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="4660" LastEditDate="2015-02-15T02:43:23.937" LastActivityDate="2015-02-15T02:43:23.937" CommentCount="0" CommunityOwnedDate="2015-02-14T23:25:28.680" ContentLicense="CC BY-SA 3.0" />
  <row Id="8180" PostTypeId="2" ParentId="7985" CreationDate="2015-02-15T18:36:11.033" Score="6" Body="&lt;p&gt;A space station has the advantage that it can tightly control what comes in.  It is the perfect quarantine.  Unlike the cargo hold of a ship, every gram of weight is meticulously accounted for.  Unfortunately, there's no way to fully clean a human (without killing them) and once you've sealed them inside the space station, the advantages end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Living on a space station would share in common the same health risks as being in other sealed environments, but much much worse.  There's even a term for it, &lt;a href=&quot;http://www.epa.gov/iaq/pdfs/sick_building_factsheet.pdf&quot; rel=&quot;noreferrer&quot;&gt;BRI or Building Related Illnesses aka Sick Building Syndrome&lt;/a&gt;.  One of the biggest problems is air circulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a plane or poorly ventilated office, the air can stagnate.  You're literally breathing each other's air giving air and particle (sneezing, coughing) borne contagions a better chance to reach more victims.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cdc.gov/niosh/topics/indoorenv/buildingventilation.html&quot; rel=&quot;noreferrer&quot;&gt;The HVAC system itself can be a source of health problems&lt;/a&gt; if it is poorly maintained.  It can accumulate and spread smoke, mold, chemicals, and germs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everything smells, and some smells can hurt you.  You, your shoes, your clothes, the wood in the chair you're in, the plastic in your computer, the solvents used to clean it, paint, perfumes, food, mold... this is known as &quot;outgassing&quot; and normally its at such low volume and the ventilation is good enough it just dissipates.  In a poorly ventilated office building this is bad.  In a sealed space station this can be debilitating.  &lt;a href=&quot;http://www.nasa.gov/topics/technology/features/outgas-tech.html&quot; rel=&quot;noreferrer&quot;&gt;NASA employs people whose job is to literally ensure that nothing will smell too bad in space&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Airplanes and buildings can refresh their air from the outside.  With no outside air supply, a space station must recycle all its air requiring very good filters and CO2 scrubbers.  If they degrade, all these problems become more acute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having a cold in space would probably be horrible for the human, and not good for the technology.  It's worse because being in space messes up the immune system.  I can't imagine what would happen to the excess sinus fluid in your head in zero-gravity, but it can't be nice.  On top of that, NASA can't afford a day off, you have to keep working.  Sneezing and coughing will produce a fine mist of droplets which will continue until they impact something: a fellow crew member, a surface, a piece of electrical equipment it might short out...  &lt;a href=&quot;http://science.howstuffworks.com/can-you-sneeze-in-space.htm&quot; rel=&quot;noreferrer&quot;&gt;This article goes into some detail about being sick in space&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the simplest ways to cure BRI is to leave the building.  Not possible on a space station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The typical grimy space station in sci-fi movies makes this all much, much worse.  It's probably old, poorly maintained and filthy (a lot of that being human skin and oils).  The cargo is never meticulously checked, and the humans rarely undergo quarantine or any sort of scrubbing.  The Alien series even has a cat.  On the up side, sci-fi spaceships are typically much, much, much larger than real world spaceships and have far more air volume.&lt;/p&gt;&#xA;" OwnerUserId="4704" LastEditorUserId="4704" LastEditDate="2015-02-15T18:43:23.913" LastActivityDate="2015-02-15T18:43:23.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8181" PostTypeId="2" ParentId="8151" CreationDate="2015-02-15T18:46:47.007" Score="8" Body="&lt;p&gt;With the right sensors and software, &lt;em&gt;one&lt;/em&gt; probe could autonomously establish an &quot;orbit&quot; around an asteroid.  I put that in quotes, since the gravity field will make the trajectory more complicated than the orbit you may be thinking of, depending on the distance from the asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A wide range of sensors might be employed, including things like LIDARs and RADARs.  Though it could be done with just cameras and an IMU.  Cameras can provide attitude information using stars, &lt;a href=&quot;http://www.spaceops2012.org/proceedings/documents/id1267135-Paper-001.pdf&quot;&gt;orbit determination using the target asteroid and other asteroids&lt;/a&gt;, and distance and location relative to the shape and surface features on the target asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The software would at first have the probe do distant flybys and then closer over time in order to map the gravity field, spin rate, and take images of the asteroid.  As those maps are built up, the probe can reduce the distance of the flybys safely and enter orbit-like trajectories.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have lots of probes at lots of asteroids, then it would make sense to have all the software and smarts on board.  Initially though, you would use and develop software mostly on the ground for these tasks as you learn how to do it reliably and efficiently.  Later you would migrate the software to the probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several probes at the same asteroid would be able to map it much faster, exchanging accumulated map and gravity field information between the probes.  Though now you have the added complication of collision avoidance with other probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know that there would be much value in collecting inter-probe data types, e.g. inter-probe doppler or range.  They can each do their own navigation independently using maps of the asteroid, and report their trajectories to each other just for the purposes of collision avoidance.  (&lt;a href=&quot;http://en.wikipedia.org/wiki/Traffic_collision_avoidance_system&quot;&gt;TCAS&lt;/a&gt; around the asteroid.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one of the probes dies, you need to predict where that one could be from its last reported position and velocity.  Hopefully it will soon leave the system or crash into the asteroid.  Otherwise the uncertainty on its position will grow over time.  Then you will just have to rely on &lt;a href=&quot;http://en.wikipedia.org/wiki/Big_sky_theory&quot;&gt;Big Sky Theory&lt;/a&gt; for collision avoidance with that one.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="1142" LastEditDate="2015-02-15T19:16:13.237" LastActivityDate="2015-02-15T19:16:13.237" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8182" PostTypeId="5" CreationDate="2015-02-15T19:07:53.693" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-15T19:07:53.693" LastActivityDate="2015-02-15T19:07:53.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8183" PostTypeId="4" CreationDate="2015-02-15T19:07:53.693" Score="0" Body="Questions regarding the movements and interactions of astronomical bodies as it relates to spacecraft, their trajectories, and their maneuvers. For the motion of spacecraft themselves, see [orbital-mechanics]." OwnerUserId="1142" LastEditorUserId="58" LastEditDate="2019-09-19T15:28:04.963" LastActivityDate="2019-09-19T15:28:04.963" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8184" PostTypeId="1" AcceptedAnswerId="8193" CreationDate="2015-02-16T06:15:36.200" Score="9" ViewCount="523" Body="&lt;p&gt;I suspect the question may be a trifle subjective; dependent upon the nature of the propulsion system, the energy levels, as also the nature of the object impinged&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exhaust from a space-craft is very high velocity (&lt;a href=&quot;https://en.wikipedia.org/wiki/Spacecraft_propulsion&quot;&gt;Wikipedia&lt;/a&gt; says as high as Mach 10. In atmosphere (perhaps as high as LEO) the exhaust probably loses energy relatively quickly, or at near distance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When travelling in deep-space, how long/far would it take for the ejecta to lose energy? How closely could space-craft, as depicted in popular literature, 'fly' in 'formation'?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say, a space-craft is under assembly in orbit, how closely could an astronaut equipped with an MMU follow his/her/it's buddy?&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2019-02-02T23:29:39.563" Title="In space, how far from the nozzle is the exhaust from a space-craft capable of causing damage?" Tags="|propulsion|eva|nozzle|exhaust|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8185" PostTypeId="1" CreationDate="2015-02-16T06:16:04.360" Score="8" ViewCount="1797" Body="&lt;p&gt;The Chinese have accomplished quite a bit with the service module left in space following the return of their latest lunar mission. I was wondering if something similar could be done using the 'trunk' section of a SpaceX Dragon cargo capsule. The DragonLab write-up says the solar arrays can supply up to 4KW, with a 1 year on-orbit lifetime. If it was left at the ISS it could become the basis for a good-sized free-flying satellite.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this is such an obvious idea, I'm wondering if I have overlooked something …  &lt;/p&gt;&#xA;" OwnerUserId="6214" LastEditorUserId="51" LastEditDate="2015-02-16T12:10:26.193" LastActivityDate="2015-02-16T12:16:39.697" Title="Can the SpaceX Dragon leave the 'trunk' in orbit?" Tags="|spacex|reentry|dragon|orbital-assembly|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8186" PostTypeId="2" ParentId="8155" CreationDate="2015-02-16T07:17:46.720" Score="3" Body="&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;: I had missed that this was about &lt;em&gt;abort&lt;/em&gt;, not launch. The answer below has been changed to reflect that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The G-force estimations here seem to generally be expecting the feet of the crew to be toward the pull. This is not so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Crew are positioned with their backs to the ground at launch in all spacecraft designs I've ever seen (almost always in the descender part of the craft), specifically because this is the best position from which to resist high G forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few fighter ejection seats are rated for a 14G force, though for lesser duration, with the additional detrimental points that it &lt;em&gt;is&lt;/em&gt; a downward directed force and subjects the pilot to windblast (which, if anything kills them, this or a collision is the most likely thing to do it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this taken into account, 5 seconds at 14Gs is not pleasant, but its not going to cripple or incapacitate a healthy human, as evidenced by all the healthy folks walking around the planet who have ejected from fighter aircraft. Of course, there may be side effects, but its a lot better than the alternative.&lt;/p&gt;&#xA;" OwnerUserId="6370" LastEditorUserId="6370" LastEditDate="2015-02-16T09:39:31.023" LastActivityDate="2015-02-16T09:39:31.023" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8187" PostTypeId="1" AcceptedAnswerId="8188" CreationDate="2015-02-16T07:28:54.320" Score="9" ViewCount="5492" Body="&lt;p&gt;Here's neat &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle#mediaviewer/File:Space_shuttles_Atlantis_%28STS-125%29_and_Endeavour_%28STS-400%29_on_launch_pads.jpg&quot; rel=&quot;noreferrer&quot;&gt;image from Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/i3GiNl.jpg&quot; alt=&quot;two Space Shuttles and two towers&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the right there's a tall white tower with a rounded tank on top looking much like a water tower. There's another one right behind the first one opposing the other spaceship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's that tower and what is it for?&lt;/p&gt;&#xA;" OwnerUserId="2948" LastActivityDate="2016-02-05T11:39:21.767" Title="What's the white tall tower with a tank near Space Shuttle launch pad?" Tags="|space-shuttle|launch-site|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8188" PostTypeId="2" ParentId="8187" CreationDate="2015-02-16T08:08:53.873" Score="12" Body="&lt;p&gt;This tower is 290 ft. high. It has storage capacity of 300000 gallons of water used during STS launches to protect the Orbiter and its payload from acoustic vibrations due to SRB's firing. The pipes beneath the tower are the main distribution area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information, including a virtual walkaround, please visit &lt;a href=&quot;http://nasatech.net/ntSubPad39A_PAGE.html&quot; rel=&quot;nofollow&quot;&gt;http://nasatech.net/ntSubPad39A_PAGE.html&lt;/a&gt;, in particular &lt;a href=&quot;http://nasatech.net/ntSubPad39A/Pad39AH2OSSSOverDistrib130412/&quot; rel=&quot;nofollow&quot;&gt;this view&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2016-02-05T11:39:21.767" LastActivityDate="2016-02-05T11:39:21.767" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8189" PostTypeId="2" ParentId="8185" CreationDate="2015-02-16T08:27:04.303" Score="2" Body="&lt;p&gt;The current trunk design has no thrusters and no flight control system. It's basically an empty shell that needs a Dragon to control it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vcUEG.jpg&quot; alt=&quot;Dragon trunk interior&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trunk doesn't have a means to attach it to the ISS. You'd need a parking spot on the ISS and an attachment mechanism on both the ISS and trunk. Doable, but not present in the current version.&lt;br&gt;&#xA;The trunk is not built for on-orbit disassembly. If you want to work on it in orbit, you'd want to optimize the design to make it easy to remove e.g. the solar arrays, with big, easily-accessible bolts instead of tiny, hidden ones. Add some hand/footholds, etc. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-02-16T10:00:31.520" LastActivityDate="2015-02-16T10:00:31.520" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8190" PostTypeId="2" ParentId="8185" CreationDate="2015-02-16T12:16:39.697" Score="2" Body="&lt;p&gt;The Dragon trunk is mostly a simple structure used to provide an interface between the second stage and the Dragon. It carries the solar panels, and allows for carrying bulky unpressurized cargo. It has no orbital maneuvering system, docking module, or anything else of use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand the Chinese Shenzou design is heavily influenced by the Soviet/Russian design of the Soyuz. In that model, the spacecraft has three modules, a propulsion module that is unpressurized at its base. The middle module is the reentry module, and the top module is the orbital module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A pedestrian benefit of this design is that there are two rooms, while in orbit, with the washroom in one for some modicum of privacy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a Soyuz or Shenzou is done with its mission it discards both the Orbital and Propulsion modules and only the re-entry module returns. The Chinese advancement over Soyuz was to leave that Orbital module behind for science missions. A clever use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems unlikely that they could 'stack' them up, since the interface between the modules does not seem to be the kind that would allow reconnecting modules in orbit after use.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-02-16T12:16:39.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8191" PostTypeId="1" CreationDate="2015-02-16T13:19:16.210" Score="3" ViewCount="533" Body="&lt;p&gt;I have been unable to find any information on the masses of the docking / berthing mechanisms used by the ISS. What are the masses of the APAS-95, Active / Passive Common Berthing Mechanism, and Russian probe / drogue docking systems? The best I can find is the mass of the PMA's, the lightest one is &lt;a href=&quot;http://www.nasa.gov/externalflash/ISSRG/pdfs/pma.pdf&quot; rel=&quot;nofollow&quot;&gt;1183 kg&lt;/a&gt;, which consists of a passive CBM + permanently passive APAS + additional structure.&lt;/p&gt;&#xA;" OwnerUserId="7034" LastActivityDate="2022-01-14T03:52:40.320" Title="What are the masses of the docking / berthing mechanisms used by the ISS?" Tags="|iss|docking|docking-port|mass|berthing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8192" PostTypeId="1" AcceptedAnswerId="8221" CreationDate="2015-02-16T13:32:13.047" Score="5" ViewCount="202" Body="&lt;p&gt;When the CanadArm catches the Dragon grapple fixture and attaches/berths it to the CBM on the ISS, is the orientation of the Dragon in the berthing port fixed and the same every berthing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or do they position it specific to the ISS orbital paths. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-02-21T04:57:48.733" Title="Dragon berthing - Is the orientation specific to the orbital conditions, or always the same?" Tags="|spacex|dragon|berthing|cbm|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8193" PostTypeId="2" ParentId="8184" CreationDate="2015-02-16T14:23:24.243" Score="11" Body="&lt;p&gt;It varies tremendously based on the engine involved and what's being protected.  Back in the day when Shuttle deployed comsats with solid fuel boost stages, the satellite would be tens of miles away at ignition but the Shuttle would maneuver to protect the windows.  For Shuttle EVA, the &lt;a href=&quot;https://archive.org/details/flight_rules_generic&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Flight Rules&lt;/a&gt; show that the safe distance for a suited crewmember was 27 feet for the main jets, 3 feet for the vernier jets, and 3 feet for the APU exhaust.  It's rule A-15-22 in the document.  This rule is probably a simplified version of what would actually be managed to; I remember seeing &quot;Keep Out Zones&quot; (KOZ) defined for all kinds of things on the Orbiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The buzzword for this is &quot;plume impingement&quot; if you want to google for other specific cases.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-02-02T23:29:39.563" LastActivityDate="2019-02-02T23:29:39.563" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="8194" PostTypeId="1" AcceptedAnswerId="8199" CreationDate="2015-02-16T22:18:07.360" Score="11" ViewCount="3360" Body="&lt;p&gt;I've seen &lt;a href=&quot;https://www.youtube.com/watch?v=Z3espRWmLbw&quot; rel=&quot;noreferrer&quot;&gt;this movie&lt;/a&gt;: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Z3espRWmLbw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It shows Sun-synchronized orbit of a satellite. What causes this orbit to be Sun-synchronized? I mean, from what I know, it shouldn't rotate as it does. Twice a year it should be perpendicular to the Sun, and twice a year it should be parallel. Apparently I'm wrong, so why does it rotate?&lt;/p&gt;&#xA;" OwnerUserId="8521" LastEditorUserId="49" LastEditDate="2015-02-16T22:46:50.567" LastActivityDate="2015-02-17T10:57:14.553" Title="Why does the Sun-synchronous orbital plane rotate?" Tags="|orbital-mechanics|low-earth-orbit|sun-synchronous|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8195" PostTypeId="2" ParentId="8194" CreationDate="2015-02-16T22:30:47.987" Score="6" Body="&lt;p&gt;Sun-synchronous orbits are possible because the Earth is not a perfect sphere. The Earth is best described as an oblate spheroid. This non-spherical shape and the resulting gravity field can be used to alter the orbit in a predictable way ― specifically to precess the orbit with a period of one year.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="49" LastEditDate="2015-02-16T22:48:37.170" LastActivityDate="2015-02-16T22:48:37.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8196" PostTypeId="5" CreationDate="2015-02-17T05:55:03.350" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-17T05:55:03.350" LastActivityDate="2015-02-17T05:55:03.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8197" PostTypeId="4" CreationDate="2015-02-17T05:55:03.350" Score="0" Body="Questions pertaining to the launch vehicle developed by Wernher von Braun's team under Arthur Rudolph's supervision at Marshall Space Flight Center for carrying astronauts to the Moon. Saturn V (or C-5 configuration) had three stages with clustered engines F-1 and J-2 and an instrument unit." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-02-17T09:31:09.723" LastActivityDate="2015-02-17T09:31:09.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8198" PostTypeId="1" AcceptedAnswerId="8227" CreationDate="2015-02-17T06:48:40.180" Score="9" ViewCount="1447" Body="&lt;p&gt;Is it feasible to design a flight computer for a small CubeSat spacecraft destined for lunar orbit that is controlled exclusively by one or more non-radiation-hardened processors? What are the main design considerations to make such an architecture radiation tolerant, without using special radiation hardened components?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the expected risk of single-event upsets and total ionizing dose effects over a one-year time span?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What type of redundancy or voting scheme, if any, would be recommended for the processors?&lt;/p&gt;&#xA;" OwnerUserId="8523" LastEditorUserId="6214" LastEditDate="2015-02-21T05:53:44.523" LastActivityDate="2015-02-22T02:44:04.723" Title="Using non-radiation-hardened processors in lunar orbit …" Tags="|the-moon|artificial-satellite|radiation|cubesat|flight-computer|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8199" PostTypeId="2" ParentId="8194" CreationDate="2015-02-17T08:01:17.847" Score="10" Body="&lt;p&gt;Sun-synchronous orbits are those whose orbital plane makes a constant angle with the radial from the Sun. For that to occur, the orbital plane of geocentric satellites must rotate in inertial space with the angular velocity of the Earth in its orbit around the Sun, which is $360^\circ$ per $365.26$ days, or $0.9856$ degrees per day. With the orbital plane precessing eastward at this rate, the ascending node will lie at a fixed local time, or equivalently, at a fixed direction relative to the Sun-Earth radial line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As pointed out by @Erik, this precession is due to the oblateness of the Earth (the Earth's equatorial bulge). The rate at which the line of nodes moves owing to this bulge is given by&#xA;$$&#xA;\dot\Omega = -\frac{3}{2}J_2\left(\frac{r_E}{ℓ}\right)^2 n\cos\iota&#xA;$$&lt;br&gt;&#xA;where $J_2$ is the is the zonal harmonic coefficient ($1.08262668\times10^{-3}$ for Earth), $r_E$ is the body's equatorial radius ($6,378,137$ m for Earth), $ℓ$ is the orbit parameter (the semi-latus rectum), $n$ is the mean motion, and $\iota$ is the inclination of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a given orbit parameter ($ℓ$) and mean motion ($n$), the inclination of a geocentric satellite orbit can be selected to obtain a Sun-synchronous orbit.&lt;/p&gt;&#xA;" OwnerUserId="8321" LastEditorUserId="49" LastEditDate="2015-02-17T10:57:14.553" LastActivityDate="2015-02-17T10:57:14.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8201" PostTypeId="1" CreationDate="2015-02-17T09:53:04.997" Score="5" ViewCount="1100" Body="&lt;p&gt;So, usually a few days after every launch, I go out and check out the published &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE data&lt;/a&gt; for each satellite, made public courtesy of CelesTrak. It generally includes all the orbital parameters for each satellite, and more importantly, the rocket body, as I like to see how accurately SpaceX places their payloads into the predetermined orbits.&lt;/p&gt;&#xA;&lt;p&gt;However, for the launch of DSCOVR, no TLE (or more correctly, an incomplete TLE) has been published for either. Plugging in the International Designators for DSCOVR and the Falcon 9 R/B (which are &lt;code&gt;2015-007A&lt;/code&gt; and &lt;code&gt;2015-007B&lt;/code&gt;, respectively) into the &lt;a href=&quot;https://celestrak.org/satcat/search.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;SatCat search form&lt;/a&gt;, which I acquired from the raw SatCat database &lt;a href=&quot;https://celestrak.org/pub/satcat.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (warning, large text file), returns this:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;No TLE found&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Furthermore, this is reflected on the &lt;a href=&quot;http://www.n2yo.com/satellite/?s=40390&quot; rel=&quot;nofollow noreferrer&quot;&gt;n2yo.com&lt;/a&gt; tracking site, which does not show any orbital path for DSCOVR or the Falcon 9 R/B.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Why is this? Is it simply a case of the TLE having not been published yet, or will they not publish it as it is beyond Earth's SOI (which is not technically true, the Falcon 9 R/B is in Earth orbit still).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If they don't track objects beyond Earth's SOI, how does one follow their orbital positions?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2019-04-15T18:09:21.247" Title="Why has no TLE been published for the DSCOVR satellite and the Falcon 9 R/B?" Tags="|orbital-mechanics|spacex|falcon-9|tracking|dscovr|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8202" PostTypeId="1" AcceptedAnswerId="8203" CreationDate="2015-02-17T10:20:06.763" Score="8" ViewCount="284" Body="&lt;p&gt;If, hypothetically, there was to be an experiment that would look for solar activity induced increases in infrared radiation on the International Space Station, would the infrared radiation produced on Earth or from other sources (Cosmic Background Radiation) make the data hard to determine?&lt;/p&gt;&#xA;" OwnerUserId="6053" LastEditorUserId="6053" LastEditDate="2015-02-17T20:52:54.553" LastActivityDate="2015-02-21T15:30:13.327" Title="Would measuring the Infrared radiation output of the sun be difficult, due to the output of other sources (Earth) interfering with the results?" Tags="|iss|radiation|the-sun|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8203" PostTypeId="2" ParentId="8202" CreationDate="2015-02-17T13:53:32.217" Score="9" Body="&lt;p&gt;In fact the ESA observatory &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Columbus/SOLAR&quot;&gt;SOLAR&lt;/a&gt; has been monitoring the Sun activity from the ISS Columbus Laboratory since 2008. In particular the SOVIM science instrument is in charge to monitor the infrared radiation. Further infrared radiation sources can be handled provided they are well known.&lt;/p&gt;&#xA;" OwnerUserId="8528" LastActivityDate="2015-02-17T13:53:32.217" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8204" PostTypeId="2" ParentId="8202" CreationDate="2015-02-17T16:05:28.090" Score="5" Body="&lt;p&gt;As a first approach:&lt;br&gt;&#xA;total amount of power used on Earth: &lt;a href=&quot;http://en.wikipedia.org/wiki/World_energy_consumption&quot; rel=&quot;nofollow&quot;&gt;150 TWh/year&lt;/a&gt;. Pretty much all of this ends up as heat, so it's a decent proxy for amount of IR produced.&lt;br&gt;&#xA;total amount of solar energy received on Earth: &lt;a href=&quot;http://en.wikipedia.org/wiki/Insolation&quot; rel=&quot;nofollow&quot;&gt;250 W/m$^2$ times 510 million km$^2$&lt;/a&gt; is 1.275 EW, or 1.11 ZWh/y, or 2 million times as much.&lt;br&gt;&#xA;So human production is only a factor if you want to measure to an accuracy of 0.05% or better. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-02-17T20:48:02.470" LastActivityDate="2015-02-17T20:48:02.470" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8205" PostTypeId="2" ParentId="8201" CreationDate="2015-02-17T16:46:27.080" Score="10" Body="&lt;p&gt;TLEs are (as far as I know) only issued for Earth-orbiting spacecraft. Since DSCOVR is placed in L1 and not in a near-Earth orbit that's probably why its TLE are missing. I checked for TLEs of SOHO and ACE, which are also placed in L1, and SatCat gives the same output.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S: The fact that there's no TLEs does not mean that the spacecraft is not tracked. TLEs are just a type of low-precision ephemerides (+ something else such as information on the orbital lifetime) kindly issued by NORAD to the general public. There's more options to retrieve spacecraft position and velocity, such as &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot;&gt;HORIZONS&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8529" LastEditorUserId="8529" LastEditDate="2015-02-18T00:50:39.647" LastActivityDate="2015-02-18T00:50:39.647" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8206" PostTypeId="1" CreationDate="2015-02-17T18:14:29.527" Score="13" ViewCount="793" Body="&lt;p&gt;One of the challenges that faced the Apollo moon landing crews, and particularly the longer-stay missions of the later flights, was going through a full 7 hours of extra-vehicular activity in a locked pressure suit with only a single energy bar to eat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KJRQm.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;a href=&quot;https://www.hq.nasa.gov/alsj/alsj-DrinkFood.html&quot; rel=&quot;noreferrer&quot;&gt;(source)&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears, though, that some pressurized suits now allow jet pilots to eat through a feeder tube. Specifically, &lt;a href=&quot;http://www.wired.com/2014/02/u2/#slide-id-152551&quot; rel=&quot;noreferrer&quot;&gt;in newer U2 spy planes&lt;/a&gt;, pilots can now have a range of flavours, and most importantly, eat multiple times during a flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tdsmP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;PILOTS' TUBE FOOD used to be just mush. New formulations deliver lunch in texturized layers of flavors (bacon with hash browns!) that squirt into the feeding port.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Does this technology extend to a complete vacuum? Could future Moon or Mars (or whatever) explorers use this to extend their autonomy during EVAs? Is it being used now in Earth orbit? (Or are the EVAs simply not long enough to merit it?)&lt;/p&gt;&#xA;" OwnerUserId="653" LastActivityDate="2015-03-19T16:36:08.940" Title="Could future astronauts eat during EVAs?" Tags="|eva|food|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8207" PostTypeId="1" AcceptedAnswerId="8210" CreationDate="2015-02-17T20:39:11.007" Score="6" ViewCount="393" Body="&lt;p&gt;I used to think that there was a point at which you escaped the Earth's gravitational pull. This is technically false, since every object in the universe is attracted towards every other object. Is there a term for the point at which we consider it negligible? I think of the Oort Cloud and it's boundaries around the Sun's significant gravitational pull&lt;/p&gt;&#xA;" OwnerUserId="7027" LastActivityDate="2015-02-19T08:44:31.050" Title="Is there a term for the distance from an object at which it's gravitational pull is negligible?" Tags="|gravity|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8208" PostTypeId="1" AcceptedAnswerId="8242" CreationDate="2015-02-17T21:48:08.053" Score="7" ViewCount="402" Body="&lt;p&gt;Rockets are very loud, and seems to have a lot of vibration caused by turbulence. They're also run by microchips, which I tend to think of as fragile. Do the microchips need to be specially designed or encased to withstand these forces?&lt;/p&gt;&#xA;" OwnerUserId="6287" LastEditorUserId="6287" LastEditDate="2015-02-18T00:18:19.740" LastActivityDate="2015-02-23T17:34:49.830" Title="Do processors need to be hardened against vibration?" Tags="|vibration|sound|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8209" PostTypeId="2" ParentId="8207" CreationDate="2015-02-18T01:53:09.783" Score="3" Body="&lt;p&gt;You could say you are &lt;strong&gt;outside the Hill sphere&lt;/strong&gt; of another object. From &lt;a href=&quot;http://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. [...] One simple view of the extent of the Solar System is the Hill sphere of the Sun with respect to local stars and the galactic nucleus.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6772" LastActivityDate="2015-02-18T01:53:09.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8210" PostTypeId="2" ParentId="8207" CreationDate="2015-02-18T15:51:37.843" Score="10" Body="&lt;p&gt;Although related to similar concepts, the terms &lt;em&gt;sphere of influence&lt;/em&gt; and &lt;em&gt;Hill sphere&lt;/em&gt; are used in different contexts, as already pointed out by someone answering the following question on the astronomy branch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://astronomy.stackexchange.com/questions/6348/what-is-the-difference-between-sphere-of-influence-and-hill-sphere&quot;&gt;What is the difference between Sphere of Influence and Hill sphere?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically the Hill sphere is related to the stability of an object (no. 2) orbiting another one (no. 1), subjected to the perturbations of a third body (no. 3). If the orbit of (2) lies outside the Hill sphere of (1), then the gravitational perturbations induced by (3) will eventually pull out (2) from its orbit about (1). Otherwise the orbit of (2) is said to be stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand the sphere of influence is mainly used in the context of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Patched_conic_approximation&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;patched conic approximation&lt;/em&gt;&lt;/a&gt;, a method used to simplify the trajectory computations of a spacecraft in a complex multiple-body gravitational field.&#xA;The idea is to discretize the space in different zones in which one can consider the spacecraft to be subjected only to the gravitational field of a single object, neglecting the other bodies. These zones are the spheres of influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given this distinction, I would say that the answer to the original question is &lt;em&gt;sphere of influence&lt;/em&gt; and yes, it false that at a precise point in space a probe escapes the Earth's gravity field, but it is an acceptable simplification.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like also to underline a concept that often seems to be taken lightly. Both the Hill sphere and the sphere of influence of a body in space (usually a planet) can only be defined in relation with another celestial body (usually the Sun).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this could be useful.&lt;/p&gt;&#xA;" OwnerUserId="8528" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2015-02-19T08:44:31.050" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8211" PostTypeId="2" ParentId="2095" CreationDate="2015-02-18T22:54:25.273" Score="7" Body="&lt;p&gt;No, but maybe yes. U.S. President Carter signed treaty's on sharing the Moon, the ocean floors, and a number of other similar things, like Antarctica. They have actually caused plans to utilize resources like manganese nodules to come to a halt due to requirements for sharing resources - at least for the signee countries. Other countries will do as they please.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the Lunar Rovers, I think they fall in the same category as equipment in Antarctica. If it isn't yours, and it isn't an emergency, don't touch it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the truth is, the first country to land a gun like a AAA on the Moon is going to own it. A simple automated gun could hit you anywhere in any Lunar orbit. Game over. Who will be first?&lt;/p&gt;&#xA;" OwnerUserId="1432" LastActivityDate="2015-02-18T22:54:25.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8212" PostTypeId="2" ParentId="8202" CreationDate="2015-02-19T01:28:41.283" Score="4" Body="&lt;p&gt;The infrared coming from the Earth, or activities on the International Space Station, won't interfere with an instrument intended to observe infrared light produced by the Sun. The Sun's infrared output is primarily in wavelengths shorter than 1.75 microns. Thermal infrared is at wavelengths longer than 3 microns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With regard to the CMBR, there's no overlap whatsoever between those long radio waves and the short near infrared produced by the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The James Webb Telescope will have instruments that look at wavelengths as long as 28.6 microns. That will require &lt;a href=&quot;http://webbtelescope.org/webb_telescope/technology_at_the_extremes/keep_it_cold.php&quot; rel=&quot;nofollow&quot;&gt;extensive cooling&lt;/a&gt;, but the Webb isn't intended as a solar observatory.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="4660" LastEditDate="2015-02-21T15:30:13.327" LastActivityDate="2015-02-21T15:30:13.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8213" PostTypeId="1" CreationDate="2015-02-19T05:36:41.277" Score="8" ViewCount="2324" Body="&lt;p&gt;I'm trying to find out if there are reliable values for the solar flux received on Earth. I realise this will change with location, time of day, season, weather etc. but I'm surprised that I can find anything at all. For space calculations of solar flux on solar panels I typically use a value of 1372 W/m2 at 1 AU from the Sun. I understand that this value will be significantly reduced once the light has travelled through the atmosphere but I can't find a lookup table or anything. &lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-02-19T07:25:25.607" Title="Solar flux on (not at) Earth" Tags="|solar-power|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8214" PostTypeId="2" ParentId="2095" CreationDate="2015-02-19T05:44:08.713" Score="47" Body="&lt;p&gt;Probably no.&lt;/p&gt;&#xA;&lt;p&gt;First there are the technical issues that have already been nicely covered.&lt;/p&gt;&#xA;&lt;p&gt;There are also political issues, which are more driving than the technical ones. There are treaties, space law, and maritime law to serve as a precedence where space law doesn't exist.&lt;/p&gt;&#xA;&lt;h3&gt;NASA's stance&lt;/h3&gt;&#xA;&lt;p&gt;As far as NASA is concerned, those lunar rovers are not abandoned property. To the contrary, NASA has been quite adamant that they are still NASA's property and that they are not abandoned. This has come to the forefront lately with the Google Lunar X Prize. The Lunar X Prize has multimillion bonuses for contestants that go visit one of the legacy lunar landing sites. &lt;a href=&quot;http://www.nasa.gov/pdf/617743main_NASA-USG_LUNAR_HISTORIC_SITES_RevA-508.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA has issued guidelines&lt;/a&gt; with regard to what those contestants could do to help NASA should they go to one of those sites, and also what they absolutely should not do.&lt;/p&gt;&#xA;&lt;h3&gt;Space law&lt;/h3&gt;&#xA;&lt;p&gt;Space law is about as sparse as space itself. There are however two key provisions that have been signed and ratified by every space faring nation, the 1967 &lt;a href=&quot;http://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;noreferrer&quot;&gt;Outer Space Treaty&lt;/a&gt; and the 1972 &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Liability_Convention&quot; rel=&quot;noreferrer&quot;&gt;Space Liability Convention&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The Outer Space Treaty holds that the Moon and other celestial bodies are the common heritage of mankind. Sovereign nations cannot make claims of property rights on the Moon or any other celestial body. It also addresses non-government entities. The activities of non-governmental entities in outer space requires authorization and continuing supervision by the appropriate state party to the treaty. Finally, &lt;a href=&quot;http://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967#Article_VIII&quot; rel=&quot;noreferrer&quot;&gt;Article VIII&lt;/a&gt; of the Outer Space Treaty covers objects sent into space, including onto the Moon and other celestial bodies. While states cannot make property claims on celestial bodies, vehicles are still the property of the state, even if apparently abandoned.&lt;/p&gt;&#xA;&lt;p&gt;The Space Liability Convention adds a tiny bit of teeth to some of the vaguer aspects of the Outer Space Treaty. Basically, if something in space isn't yours, you had better not touch it. This raises issues with regard to the growing problem of space debris.&lt;/p&gt;&#xA;&lt;h3&gt;Maritime law&lt;/h3&gt;&#xA;&lt;p&gt;Maritime law also comes into play here. With one exception, maritime law has a concept of abandoned ships being salvageable, even if the owner hasn't acknowledged the abandonment. That one exception is ships owned by sovereign nations. Consider the case of &lt;a href=&quot;https://law.resource.org/pub/us/case/reporter/F3/221/221.F3d.634.-98-281-2.99-2036.99-2035.html&quot; rel=&quot;noreferrer&quot;&gt;Sea Hunt, Incorporated v. The unidentified shipwrecked vessel or vessels, their apparel, tackle, appurtenances,and cargo&lt;/a&gt;, and the corresponding countersuit by the Kingdom of Spain. A long, long time ago (1750 and 1802), Spain lost two ships off the coast of Virginia. Sea Hunt, Inc. found them and submitted claims. A lower court found that Spain had never explicitly abandoned the latter ship to sink but had implicitly abandoned the former in the Treaty of Paris. Per this lower court decision, the 250 year old shipwreck was Sea Hunt's to salvage, but the 200 year old shipwreck was still Spain's. The upper court overturned the first decision but affirmed the second. Both ships still belong to Spain. Sea Hunt has no claim.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-02-19T05:44:08.713" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8215" PostTypeId="2" ParentId="8213" CreationDate="2015-02-19T06:24:00.607" Score="5" Body="&lt;p&gt;You are asking about the Earth's radiation budget. (Google that phrase. There's a lot of information out there on this topic.) NASA and NOAA have been using remote sensing to study the Earth's radiation budget for at least forty years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/b/bb/The-NASA-Earth%27s-Energy-Budget-Poster-Radiant-Energy-System-satellite-infrared-radiation-fluxes.jpg/1552px-The-NASA-Earth%27s-Energy-Budget-Poster-Radiant-Energy-System-satellite-infrared-radiation-fluxes.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the above has the incoming solar radiation at 340.4 watts/meter&lt;sup&gt;2&lt;/sup&gt;. Compare that with your value of 1372 (which is a bit high). Your value and the NASA value differ by a factor of four. The reason is simple: The flux intercepted by the Earth is proportionate to $\pi {R_e}^2$, but the surface area of the Earth is  $4 \pi {R_e}^2$. Dividing the solar constant by four yields the incoming solar flux to the top of the atmosphere, averaged over ten years and over the surface of the Earth. About 29.3% of the incoming solar radiation is reflected back into space. Most of the reflection is by clouds. The atmosphere isn't perfectly clear. It absorbs another 22.6% of the incoming solar radiation. A bit less than half (48.0%) makes it's way all the way to the surface.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.593" LastActivityDate="2015-02-19T06:24:00.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8216" PostTypeId="2" ParentId="8213" CreationDate="2015-02-19T06:26:33.910" Score="7" Body="&lt;p&gt;You can use an online calculator like the one at &lt;a href=&quot;http://www.meteoexploration.com/products/SolarCalculator.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Meteo Exploration&lt;/a&gt;. Input parameters are classic lat / lon / alt / time and:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Visibility&lt;/strong&gt; (km): it is the maximum distance in km where large objects can be distinguished in the horizon. The value given (50 km)&#xA;  is for a clean atmosphere. This is a proxy for atmospheric turbidity,&#xA;  it is choosen as many airports report visibility, while other&#xA;  parameters, such as Linke Turbidity, are more difficult to obtain.&#xA;  Airport reports (METAR) are given here (choose 'Decoded').&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Temperature&lt;/strong&gt; °C: temperature in degrees centigrade, use CT to convert from Farenheit. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;RH&lt;/strong&gt; (0-100): Relative Humidity in percentage,&#xA;  values from 0 to 100. Airport reports (METAR) normally include RH.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Ozone thickness&lt;/strong&gt;: ozone layer thickness in cm. Divide Dobson Units by 1000 to get the equivalent in cm. Check the TOMS (Total Ozone&#xA;  Mapper Spectrometer) pages for current values.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Albedo ground&lt;/strong&gt; (0-1): albedo of the surrounding terrain, e.g. 0.8 to 0.95 for fresh snow, 0.17 for deciduous forest, 0.35 for sand, etc.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Timezone&lt;/strong&gt; (timezone map): the timezone, a good approximation is (-1)*longitude/15, longitude in degrees and west is negative.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Slope Orientation&lt;/strong&gt; (0-360): orientation of the surface, solar panel, roof, etc. If looking north it will be 0°, if looking south&#xA;  will be 180°. Range 0 to 360 degrees&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Slope Tilt&lt;/strong&gt; (0-90): inclination of the surface or panel with respect to the horizontal, 0° is flat 90° is completely vertical.&#xA;  Range is from 0 to 90 degrees.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Wikipedia article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Insolation&quot; rel=&quot;nofollow noreferrer&quot;&gt;insolation&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Wikipedia article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Earth%27s_energy_budget&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth's energy budget&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/13256/how-much-solar-energy-hits-a-rooftop-in-israel?rq=1&quot;&gt;How much solar energy hits a rooftop in Israel?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note that the Earth receives energy from the Sun, but also radiates energy (infrared). Some of this radiated energy is reflected by the atmosphere (greenhouse effect) and is reabsorbed by the Earth. The infrared energy radiated is indeed usable. See &lt;a href=&quot;http://earthobservatory.nasa.gov/Features/EnergyBalance/page6.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Climate and Earth’s Energy Budget&lt;/a&gt; at Nasa:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ET1iY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To convert between the energy radiated by the Sun at 1AU and the insolation, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; on Wikipedia. Extract:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Earth receives a total amount of radiation determined by its cross&#xA;  section (π·RE²), but as it rotates this energy is distributed across&#xA;  the entire surface area (4·π·RE²). Hence the average incoming solar&#xA;  radiation, taking into account the angle at which the rays strike and&#xA;  that at any one moment half the planet does not receive any solar&#xA;  radiation, is one-fourth the solar constant (approximately 340 W/m²).&#xA;  The amount reaching the Earth's surface (as insolation) is further&#xA;  reduced by atmospheric attenutation, which varies. At any given&#xA;  moment, the amount of solar radiation received at a location on the&#xA;  Earth's surface depends on the state of the atmosphere, the location's&#xA;  latitude, and the time of day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-02-19T07:25:25.607" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8217" PostTypeId="1" AcceptedAnswerId="8218" CreationDate="2015-02-20T10:16:58.617" Score="4" ViewCount="2911" Body="&lt;p&gt;Here is rocket engine drawing. The combustion chamber made from copper and the outer shell from stainless steel. The coolant, either water or gasoline will be between&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GaFaa.jpg&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;engine&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GaFaa.jpg&quot; alt=&quot;engine&quot; title=&quot;engine&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://img.wonderhowto.com/img/74/78/63427422815856/0/be-your-own-spacex-design-build-test-liquid-fueled-rocket-engines.w654.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;wonderhowto.com&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;With either pressurised water or propellant,  used as the coolant how can these two components be hermetically joined? Can I use welding or something else to create a sealed join between steel and copper?&lt;/p&gt;&#xA;&lt;p&gt;I can't use rubber seals because of the heat produced by the combustion chamber. I thought about a threaded connection. Can it be used here? Or maybe there is a better  way to join the two?&lt;/p&gt;&#xA;" OwnerUserId="8544" LastEditorUserId="10273" LastEditDate="2021-01-13T20:37:12.930" LastActivityDate="2021-01-13T20:37:12.930" Title="How to seal the connection between the combustion chamber and outer shell in this rocket engine model?" Tags="|engine-design|liquid-fuel|engineering|model-rocketry|krzycki-text|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="8218" PostTypeId="2" ParentId="8217" CreationDate="2015-02-20T12:43:33.660" Score="10" Body="&lt;p&gt;Firstly &lt;strong&gt;tl/dr&lt;/strong&gt; - if you need to ask, then this will go RUDE* and may take out your garden, pets, family, etc...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From someone who knows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;it's gonna go &quot;boom&quot; and spray propellants at about 3000 Kelvin or more&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, you need to think about what you have here - &lt;strong&gt;Pressurised propellant&lt;/strong&gt; will find a way out if it can. And then it will ignite. Rapidly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A threaded connection will not be enough - that nozzle is going to get very hot, and like most things, metal expands when heated. So that threading is going to move against the cooler outer portion of the engine. This is &lt;strong&gt;not a good thing&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Welding is a much better idea, but you are going to need to be able to deliver a weld that can cope with those temperatures. The book @Tildal linked to has an entire section on Safety, and the section on Fabrication appears to give you what you need in terms of materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Rapid Unplanned Disassembly Event (from Elon Musk, who quite possibly knows what he is talking about)&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2015-02-21T08:56:37.530" LastActivityDate="2015-02-21T08:56:37.530" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8219" PostTypeId="2" ParentId="8208" CreationDate="2015-02-21T00:22:55.660" Score="2" Body="&lt;p&gt;As Russell said, it's the connections generally speaking that are difficult. The environment for launching is very stressful, and sometimes the connectors come loose. The trick is to make sure the chips are on solidly, and most importantly, to do a vibration test to ensure that the satellites can manage the stress of launching to orbit.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-02-21T00:22:55.660" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8221" PostTypeId="2" ParentId="8192" CreationDate="2015-02-21T04:57:48.733" Score="7" Body="&lt;p&gt;So far it has always been the same&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CRS-1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vNo0B.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CRS-2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WIR7F.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CRS-3&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/E45vC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CRS-4&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sKG3O.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CRS-5&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bxhnG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-02-21T04:57:48.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8222" PostTypeId="1" AcceptedAnswerId="8223" CreationDate="2015-02-21T16:14:18.847" Score="15" ViewCount="3437" Body="&lt;p&gt;I've watched this (excellent) panel discussion titled 'Our Future in Space'. Bill Nye says at 22:48 that SpaceX got their engine design from work done by NASA, which they developed into a working production model. What design was that and what documents did they use? And can I get those documents?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/_cHafSg6dw0?start=1368&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2015-04-17T13:09:18.507" LastActivityDate="2017-08-06T21:53:39.343" Title="What NASA documents did SpaceX use to design their engines?" Tags="|spacex|nasa|engine-design|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8223" PostTypeId="2" ParentId="8222" CreationDate="2015-02-21T19:14:39.117" Score="22" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29&quot;&gt;Merlin&lt;/a&gt; uses a &lt;a href=&quot;https://en.wikipedia.org/wiki/Pintle_injector&quot;&gt;pintle injector&lt;/a&gt;, a design first used in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Descent_Propulsion_System&quot;&gt;Lunar Module Descent Engine&lt;/a&gt;, developed from original work at Caltech and JPL. Its design was publicized as &lt;a href=&quot;https://www.google.com/patents/US3699772&quot;&gt;U.S. Patent 3,699,772&lt;/a&gt;. It's a &lt;a href=&quot;http://www.usbid.com/datasheets/usbid/2001/2001-q1/pintleenginepaperaiaafinal.pdf&quot;&gt;design&lt;/a&gt; (PDF) that was used a lot by patentor TRW.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.popularmechanics.com/space/rockets/a5073/4328638/&quot;&gt;TRW employee&lt;/a&gt;, Tom Mueller, got &lt;a href=&quot;http://science.slashdot.org/comments.pl?sid=3383773&amp;amp;threshold=-1&amp;amp;commentsort=0&amp;amp;mode=thread&amp;amp;pid=42592505&quot;&gt;bored with his day job&lt;/a&gt; and started working in his spare time on engines for model rockets. Around 2001, he was working on an engine capable of 13,000 lb of thrust (the biggest model rocket engine in the world). Elon Musk hired him and put him to work building &quot;something bigger&quot;. &lt;a href=&quot;https://linuxacademy.com/blog/2013/01/14/under-the-hood-with-the-spacex-merlin-engine/&quot;&gt;Mueller&lt;/a&gt; became one of the founding employees of SpaceX and was Vice President of Propulsion Development as of 2013.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Musk sought Mueller out in 2001 when Musk decided to build his own rockets instead of buying some from the Russians. Musk caught wind of a rocket engine Mueller built in his garage and “apparently had a religious experience” once he saw it. If you didn’t know, Elon Musk used $100 million of his Paypal money to start SpaceX. That money was used to build the Merlin engine Mueller had designed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-02-21T19:14:39.117" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8224" PostTypeId="1" AcceptedAnswerId="19959" CreationDate="2015-02-21T21:53:02.190" Score="9" ViewCount="1434" Body="&lt;p&gt;In the last few years amazing images of the sun have been around. They are from the &lt;a href=&quot;https://sdo.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Dynamics Observatory&lt;/a&gt;'s &lt;a href=&quot;http://aia.lmsal.com/public/instrument.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;AIA&lt;/a&gt; instrument. The one below shows super-hot iron atoms. So, am I looking at field lines of the sun's magnetosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Jcuj9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The light is in the extreme ultra-violet range and was collected through filters. What are those filters made of? What is the exposure time for these photos? Does the unit use the kind of light recording surface a digital camera does, or is it a surface capable of recording in the EUV range?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole thing stirs a bunch of other questions, I thought I'd just test the waters...&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2017-01-31T03:52:17.800" LastActivityDate="2017-01-31T03:52:17.800" Title="How was this image of the sun taken and what does it show?" Tags="|the-sun|imaging|space-weather|solar-dynamic-observatory|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8225" PostTypeId="1" CreationDate="2015-02-22T00:18:28.140" Score="12" ViewCount="694" Body="&lt;p&gt;Looking at a great answer to a question about &lt;a href=&quot;https://space.stackexchange.com/questions/8192/dragon-berthing-is-the-orientation-specific-to-the-orbital-conditions-or-alwa&quot;&gt;Dragons berthing orientation at the ISS&lt;/a&gt; which shows images of the 5 CRS Dragons berthed to the ISS one thing struck me. There looks to be no easy way to tell which mission was which.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They make a badge for each mission, (you can see it by the Webcast page when they broadcast it).  But they do not paint it on the side of the Dragon.  Nor any visible wording.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/v5vSM.jpg&quot; alt=&quot;CRS-1&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/gFhjJ.jpg&quot; alt=&quot;CRS-2&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/fFrY7.png&quot; alt=&quot;CRS-3&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/YSmQR.png&quot; alt=&quot;CRS-4&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/68Vhw.jpg&quot; alt=&quot;CRS-5&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ESA's ATV has badges and names on the side of them.  Soyuz/Progress have mission numbers, not sure if they have it painted on the side. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a reason for that? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-01-30T08:59:05.753" Title="Why does SpaceX not label the Dragon capsules externally?" Tags="|iss|spacex|commercial|dragon|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8226" PostTypeId="1" CreationDate="2015-02-22T02:37:35.290" Score="4" ViewCount="417" Body="&lt;p&gt;After other recent questions, I'm more aware than ever how much research NASA has documented, and how much interplay there is between its many facilities and huge staff, and private aerospace enterprises. So how much is that data tapped, and by how large a group of people? Is it enough to be a student at an accepted institution? Do you have to register yourself or request it some way? Is it a hard rule that only Americans can get access to the portion that isn't public?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-03-03T17:46:30.807" Title="What gets you full access to NASA documents, and what is their role in private aerospace ventures?" Tags="|nasa|research|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8227" PostTypeId="2" ParentId="8198" CreationDate="2015-02-22T02:44:04.723" Score="11" Body="&lt;p&gt;Yes, it’s probably possible, but to determine specific feasible approaches more information is needed about your mission. Key questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;What is the transfer and operational orbit? This information is needed to make analytical predictions about the probability of &lt;a href=&quot;http://radhome.gsfc.nasa.gov/radhome/see.htm&quot;&gt;single-event-effects (SEE)&lt;/a&gt; and the total ionizing dose (TID) as a function of mission time. &lt;a href=&quot;http://www.jhuapl.edu/techdigest/TD/td2801/Maurer.pdf&quot;&gt;Good overview of radiation from JHU.&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What shielding, if any, does the larger spacecraft provide? Shielding knocks down the derived environments to those the processor and other susceptible electronics will actually see.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What type of computer is required? Key requirements include operations per second, memory, storage, etc. Different types components and technologies respond to radiation environments differently. Your performance requirements may rule out entire types of technology.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;When does your computer need to operate? Is it on during launch, transit through the South Atlantic Anomaly, Van Allen Belts, or other localized areas of high radiation environments? It’s probably an easier mission if it’s only turned on once it reaches Lunar orbit.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Probably others I’m forgetting that I’ll add based on comments.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Now that we've enumerated some complexities, let's blithely brush that away and talk about a potential architecture. You specifically want a processor that's not radiation hardened, but there are a variety of other radiation tolerance approaches you can use.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;More shielding. This time around the computer or processor. Aluminum is a good, cheap, and lightweight shield material. Tantalum is another high-performance shield material.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;De-rating. Use components below their specified values. This makes them more radiation tolerate. MIL-STD-975M has de-rating advice.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Voting. Have multiple copies of your most susceptible components (probably processors or memory) that vote to determine truth. Voted EEPROMS can be a good way to store a golden master of key information that can be restored if error-detecting memory finds a flipped bit that it cannot repair. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Watchdog timers. Latch-up is a radiation failure mode that can be recovered from by power-cycling. Have a robust analog watch dog timer that isn't radiation susceptible that must be pet periodically by the processor. If the processor hangs the timer resets the power to clear any latches. Of course this requires your mission be robust to having the computer go down for a little bit as things reboot.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If you can't have the computer go down you may want a redundant processor running in a shadow mode and automatic fail-over to the secondary computer. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You should test the snot out of all of this.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="127" LastActivityDate="2015-02-22T02:44:04.723" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8228" PostTypeId="1" AcceptedAnswerId="8229" CreationDate="2015-02-22T03:38:24.663" Score="24" ViewCount="5323" Body="&lt;p&gt;Short-duration flights (Vostok, Voskhod, Mercury) use several-kWh batteries. Space stations use photovoltaic cells for their high power-to-weight ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The longest mission duration accomplished with a fuel cell seems to be a few weeks: Gemini 7 (13 days 8:35:01), Apollo 17 (12 days 13:51:59) and STS-80 (17 days, 15:53). (&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-LOK&quot;&gt;Soyuz 7K-LOK&lt;/a&gt; would have used fuel cells for its moon landing mission, as well.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz and Shenzhou both use PV cells. SpaceX Dragon is the &lt;a href=&quot;http://www.spacex.com/news/2013/02/09/going-solar&quot;&gt;first American transport vehicle to use solar arrays&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What advantage do fuel cells have over solar for 12-18 day missions, and why didn't Gemini, Apollo, and STS use PV instead?&lt;/p&gt;&#xA;" OwnerUserId="759" LastEditorUserId="23891" LastEditDate="2019-07-10T22:31:28.270" LastActivityDate="2019-07-10T22:31:28.270" Title="Why didn't Gemini, Apollo or STS use solar panels?" Tags="|crewed-spaceflight|solar-power|fuel-cell|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8229" PostTypeId="2" ParentId="8228" CreationDate="2015-02-22T04:59:00.870" Score="22" Body="&lt;p&gt;The high power levels of Shuttle and its need to return to an aerodynamic configuration would have necessitated a very large array that would need to be deployed and retracted each mission.  This would have complicated the design considerably. reduced the maneuverability of the Shuttle, and introduced other failure modes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I am not as familiar with them, I imagine some of the same considerations apply to the other vehicles you mention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, Discovery's maiden flight, &lt;a href=&quot;http://en.wikipedia.org/wiki/STS-41-D&quot; rel=&quot;noreferrer&quot;&gt;STS-41-D&lt;/a&gt;, deployed a large solar array for test purposes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FXcSL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/c8Ktu.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="472" LastEditDate="2015-02-22T14:21:42.940" LastActivityDate="2015-02-22T14:21:42.940" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8230" PostTypeId="5" CreationDate="2015-02-22T08:39:06.437" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-22T08:39:06.437" LastActivityDate="2015-02-22T08:39:06.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8231" PostTypeId="4" CreationDate="2015-02-22T08:39:06.437" Score="0" Body="Should be used whenever one specific online resource or printed book/tech.report/article or other space exploration-related media is required. Care should be taken to avoid asking for broad, open-ended lists. It is better to ask for references fitting specific criteria rather than for &quot;best resources&quot;." OwnerUserId="168" LastEditorUserId="12102" LastEditDate="2021-06-27T09:41:45.857" LastActivityDate="2021-06-27T09:41:45.857" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8232" PostTypeId="5" CreationDate="2015-02-22T08:58:15.440" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-02-22T08:58:15.440" LastActivityDate="2015-02-22T08:58:15.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8233" PostTypeId="4" CreationDate="2015-02-22T08:58:15.440" Score="0" Body="Questions regarding exploration of the Pluto-Charon dwarf planetary system." OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-02-22T09:52:31.107" LastActivityDate="2015-02-22T09:52:31.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8234" PostTypeId="2" ParentId="5329" CreationDate="2015-02-22T11:55:44.843" Score="17" Body="&lt;p&gt;Settling dust is not a problem. Keep in mind that dust in space does not settle on surfaces like it does on Earth--it's not slowly sinking, it hits the surface at relative speeds measured in km/s. It's basically a micro-meteoroid bombardment and the allowed degradation is derived from the performance requirements over the design lifetime of the solar array.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an ESA image of dust grain impact on aluminum (from &lt;a href=&quot;http://space-env.esa.int/madweb/hdebris.php&quot; rel=&quot;noreferrer&quot;&gt;http://space-env.esa.int/madweb/hdebris.php&lt;/a&gt;) to give you an idea what space dust does to thin foils.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lAUAD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, there are situations where specific satellites take precautions to not expose delicate parts to excess degradation. In particular, satellites with optical components (think telescopes, laser communication) which also use thrusters for attitude control will protect mirrors from thruster exhaust contamination by covering their aperture prior to firing attitude thrusters.&lt;/p&gt;&#xA;" OwnerUserId="716" LastEditorUserId="716" LastEditDate="2016-06-22T11:07:02.687" LastActivityDate="2016-06-22T11:07:02.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8236" PostTypeId="2" ParentId="8224" CreationDate="2015-02-22T12:44:22.873" Score="9" Body="&lt;p&gt;OK I'll bite, despite that Dean Pesnell of the SDO team &lt;a href=&quot;http://sdoisgo.blogspot.com/2013/12/was-comet-ison-seen-in-aia-4500.html&quot;&gt;must really love me by now&lt;/a&gt;. :)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Most of the info you're after is available on &lt;a href=&quot;http://sdo.gsfc.nasa.gov/data/&quot;&gt;SDO Data&lt;/a&gt; pages, if you click on the info icon. Also note that the Fe (Iron) channel composite in your question is equipped with color-keyed channel info bottom left of it, tho it is rather difficult to see at the resolution you used. Here's the description for the composite image:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This image combines three images with different, but very similar,&#xA;  temperatures. The colors are assigned differently than in the single&#xA;  images. Here AIA 211 is red, AIA 193 is green, and AIA 171 is blue.&#xA;  Each highlights a different part of the corona.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the composite is generated by taking the three different wavelength (given in their names in angstroms) images and combining them per color channel into a single color image. The three inputs are described as (using latest SDO data thumbnails as examples):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;AIA 211 (Red channel in Fe composite)&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/189a2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This channel (as well as AIA 335) highlights the active region of the&#xA;  outer atmosphere of the Sun - the corona. Active regions, solar&#xA;  flares, and coronal mass ejections will appear bright here. The dark&#xA;  areas - called coronal holes - are places where very little radiation&#xA;  is emitted, yet are the main source of solar wind particles.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Where&lt;/strong&gt;: Active regions of the corona&lt;br /&gt;&#xA;  &lt;strong&gt;Wavelength&lt;/strong&gt;: 211 angstroms (0.0000000211 m) = Extreme Ultraviolet&lt;br /&gt;&#xA;  &lt;strong&gt;Primary ions seen&lt;/strong&gt;: 13 times ionized iron (Fe XIV)&lt;br /&gt;&#xA;  &lt;strong&gt;Characteristic temperature&lt;/strong&gt;: 2 million K (3.6 million F)&lt;/p&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;AIA 193 (Green channel in Fe composite)&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eGNX9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This channel highlights the outer atmosphere of the Sun - called the&#xA;  corona - as well as hot flare plasma. Hot active regions, solar&#xA;  flares, and coronal mass ejections will appear bright here. The dark&#xA;  areas - called coronal holes - are places where very little radiation&#xA;  is emitted, yet are the main source of solar wind particles.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Where&lt;/strong&gt;: Corona and hot flare plasma&lt;br /&gt;&#xA;  &lt;strong&gt;Wavelength&lt;/strong&gt;: 193 angstroms (0.0000000193 m) = Extreme Ultraviolet&lt;br /&gt;&#xA;  &lt;strong&gt;Primary ions seen&lt;/strong&gt;: 11 times ionized iron (Fe XII)&lt;br /&gt;&#xA;  &lt;strong&gt;Characteristic temperature&lt;/strong&gt;: 1.25 million K (2.25 million F)&lt;/p&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;AIA 171 (Blue channel in Fe composite)&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/F3oX3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This channel is especially good at showing coronal loops - the arcs&#xA;  extending off of the Sun where plasma moves along magnetic field&#xA;  lines. The brightest spots seen here are locations where the magnetic&#xA;  field near the surface is exceptionally strong.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Where&lt;/strong&gt;: Quiet corona and upper transition region&lt;br /&gt;&#xA;  &lt;strong&gt;Wavelength&lt;/strong&gt;: 171 angstroms (0.0000000171 m) = Extreme Ultraviolet&lt;br /&gt;&#xA;  &lt;strong&gt;Primary ions seen&lt;/strong&gt;: 8 times ionized iron (Fe IX)&lt;br /&gt;&#xA;  &lt;strong&gt;Characteristic temperature&lt;/strong&gt;: 1 million K (1.8 million F)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I've also tried to generate my own composites out of AIA 211, AIA 193, and AIA 171. These are the two tries:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/j9pZS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image above is a RGB channel combine from direct grey scale conversion. It looks pretty nice, but too much red. OK, next try:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/9yLKV.jpg&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's better, but still not as good as from the SDO team of course. What I did here is to convert input channels into B&amp;amp;W by using the portrait profile in Photoshop, then recombined as RGB components just like before, and also adjusted output color balance a bit. I opted for 6500 K color temperature. And there was a bit of contrast adjustment too. It's still not exactly like the composites from SDO tho:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/913P3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;OK, enough of playing with data. Let's try to answer your questions more directly:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What are those filters made of?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have no idea. They're &lt;a href=&quot;http://adsabs.harvard.edu/abs/2012SoPh..275...17L&quot;&gt;multilayer-coated optical filters&lt;/a&gt; that let through incident light in wavelengths as described above. I presume they're high quality and were really expensive though. :) My best guess would be they were made by multiple vacuum deposition layers of partially reflecting dielectric compounds on a glass substrate. This would be the so-called &lt;em&gt;&quot;traditional coating&quot;&lt;/em&gt;, but there are other processes that could have been uses for AIA.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What is the exposure time for these photos?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you look at individual channel timestamps in the image, they're at most a few seconds apart. AIA is capable of taking &lt;a href=&quot;http://sdo.gsfc.nasa.gov/mission/instruments.php&quot;&gt;at least one image each second&lt;/a&gt;. But changing optical path (AIA uses four telescopes) and filters will take a few seconds, which will reflect in timestamps. And each filter will likely require slightly different exposure times. The browse data you're looking at (&lt;em&gt;Level 2&lt;/em&gt;) are processed from raw (&lt;em&gt;Level 0&lt;/em&gt;) data stream and corrected to a standardized exposure time. Refer to the Guide to &lt;a href=&quot;https://www.lmsal.com/sdodocs/doc/dcur/SDOD0060.zip/zip/entry/&quot;&gt;SDO Data Analysis documentation&lt;/a&gt; for more info.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Does the unit use the kind of light recording surface a digital camera&#xA;  does, or is it a surface capable of recording in the EUV range?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;AIA uses 4Kx4K resolution &lt;a href=&quot;https://en.wikipedia.org/wiki/Charge-coupled_device&quot;&gt;CCD&lt;/a&gt; (Charge-Doupled Device) sensors that operate at about -70° C (&lt;a href=&quot;http://sdoisgo.blogspot.com/2010/04/sdo-day-78-just-ccd-facts-ma.html&quot;&gt;source&lt;/a&gt;). Keeping sensors at low enough temperatures enables them to record in visible and UV range with low thermal noise. CCDs are a bit expensive to make, and you'd usually find cheaper &lt;a href=&quot;https://en.wikipedia.org/wiki/CMOS&quot;&gt;CMOS&lt;/a&gt; sensors in your consumer grade appliances. But they are widely used in various professional markets, including astronomy.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some additional sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sdo.gsfc.nasa.gov/&quot;&gt;Solar Dynamics Observatory&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sdoisgo.blogspot.com/&quot;&gt;SDO Blog&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.swpc.noaa.gov/&quot;&gt;Space Weather Prediction Center&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://aia.lmsal.com/&quot;&gt;SDO's Atmospheric Imaging Assembly (AIA)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://hmi.stanford.edu/&quot;&gt;Helioseismic and Magnetic Imager (HMI)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/channel/UCn69TGTR1nVI-YhtPSv3pwA&quot;&gt;Solar Dynamics Observatory's YouTube channel&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-02-22T12:52:33.713" LastActivityDate="2015-02-22T12:52:33.713" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8237" PostTypeId="2" ParentId="8226" CreationDate="2015-02-22T15:43:42.300" Score="3" Body="&lt;p&gt;Generally speaking, NASA documents aren't classified and are available to the public on request -- although they are subject to International Traffic in Arms Regulations (ITAR) restrictions.  This doesn't restrict many documents, but verifying this can slow release.  Many times, something has to be requested before it is made available online.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has lots of its materials online: NASA Documents Online (&lt;a href=&quot;http://www.hq.nasa.gov/office/hqlibrary/find/nasadoc.htm&quot; rel=&quot;nofollow&quot;&gt;http://www.hq.nasa.gov/office/hqlibrary/find/nasadoc.htm&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the summary:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The following is a list of NASA publications that have been put online&#xA;  in either full-text or hypertext format; also included in this list&#xA;  are some frequently requested non-NASA documents. This page will be&#xA;  updated as often as possible and we welcome your comments. If there is&#xA;  a NASA document you are looking for and cannot locate it here, try&#xA;  searching the NASA Aeronautics and Space Database, its public version&#xA;  the NASA Technical Report Server, our library catalog NASA GALAXIE,&#xA;  and the Johnson Space Center Document Index System, or NASA&#xA;  Headquarters employees can request a search.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The NASA Scientific and Technical Information (STI) Program has a nice twitter feed of document releases at &lt;a href=&quot;https://twitter.com/nasa_sti&quot; rel=&quot;nofollow&quot;&gt;https://twitter.com/nasa_sti&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding their role in private aerospace ventures: I'm not sure what you are asking for.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-02-22T16:08:11.363" LastActivityDate="2015-02-22T16:08:11.363" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8239" PostTypeId="1" CreationDate="2015-02-23T14:54:16.330" Score="11" ViewCount="890" Body="&lt;p&gt;My local paper has an article suggesting a possible event with &lt;a href=&quot;http://www.post-gazette.com/business/tech-news/2015/02/23/Pittsburgh-Astrobotic-CMU-first-competitor-to-send-payload-to-the-moon/stories/201502230027&quot;&gt;several robotic rovers leaving the same lander on the moon and trying to drive off for 500 meters (about a 1/3 of a mile) as fast as they can:&lt;/a&gt; the article makes it all seem fairly possible. They are competing for the &lt;a href=&quot;http://lunar.xprize.org/&quot;&gt;Google Lunar XPRIZE&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So does this race scenario have an realistic chances of occurring or is all hype? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2015-02-23T14:59:14.027" Title="How realistic are robot races on the moon this decade (circa 2015)?" Tags="|lunar-landing|robotic-missions|xprize|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8240" PostTypeId="2" ParentId="8239" CreationDate="2015-02-23T14:59:14.027" Score="8" Body="&lt;p&gt;If you mean within the next 10 years, with 2015 as a starting date, then it is not outside the realm of possibility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The advent of cheaper launches does appear to be upon us. SpaceX is very close to recovering a first stage. Whether they can refly it cheaply is the key question. They are close to launching a Falcon Heavy, again if they can succeed and also recover/refly is the key question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they succeed, space flight costs will drop dramatically. At that point, cheaper missions become very possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a moon launch was \$200 million (Say Atlas V launched) for launch services and with reuse of first stages drops to \$20-30 million a launch, the economics of it clearly change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google has been pushing for lunar rovers, and people are actively working on them. Will they succeed?  An interesting question.  The moon is a harsh mistress, so it is hard to know.  Odds seem to favour at least one succeeding out of the pack. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can all these things come together within the next ten years? At the moment it looks likely, but many things can happen between now and then.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-02-23T14:59:14.027" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8241" PostTypeId="2" ParentId="8226" CreationDate="2015-02-23T16:28:30.303" Score="1" Body="&lt;p&gt;I can also not provide a reference that described the direct influence this had on the development on commercial rockets, but I find the NASA technical report system to be a very valuable source for this kind of information:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://trs.nis.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;http://trs.nis.nasa.gov/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This website includes the database that briligg linked to in her comment as well as two other NASA databases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US taxpayers pay for these developments, so they are entitled to seeing the results. And once something is published within the USA, its futile to try to keep it out of the hands of other countries' citizens. Better to release it publicly to make a showing of generosity and technological leadership.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way: Russian textbooks on the topic are just as freely available and cover the topics in a similar degree of detail. Often, they are more up to date.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2015-03-01T16:12:39.403" LastActivityDate="2015-03-01T16:12:39.403" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8242" PostTypeId="2" ParentId="8208" CreationDate="2015-02-23T17:34:49.830" Score="5" Body="&lt;p&gt;Well, yes and no. The most important feature for modern space-bound CPUs (aside from 100% reliability) is radiation hardening. Radiation hardening is considered at every step of the design process from the materials used to the configuration of the transistors in each internal circuit. These chips are &lt;strong&gt;specifically designed&lt;/strong&gt; to withstand any bombardment of radiation that can be thrown at them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, there have been some improvements considering susceptibility to vibration in recent history. Traditionally processor circuits for devices meant to live in the harshness of space contain a simple (or in the case of the Voyager 10/11 crafts completely discrete logic) devices that are supported by a large array of discrete logic. This was done, again, in pursuit of radiation hardening. Such circuits were composed of a large number of components with exposed leads. Below is an image one of the main computer boards for the (now retired) space shuttle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qmecm.jpg&quot; alt=&quot;IBM AP-101S Space Shuttle General Purpose Computer&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nowadays the dense CPUs (such as the CPU in the &lt;a href=&quot;https://en.wikipedia.org/wiki/RAD750&quot; rel=&quot;nofollow noreferrer&quot;&gt;BAE RAD750 single board computer&lt;/a&gt;) are designed with some variety of a leadless (the pins are underneath the component) footprint. &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.202.7065&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This document&lt;/a&gt; shows the features of the CPU contained within the RAD750, and there is a section on the device's reliability: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The RAD750 will be packaged in a Ceramic Column&#xA;  Grid Array (CGA) package, constructed by attaching&#xA;  extended columns to the original Ball Grid Array (BGA)&#xA;  360 pin package employed for the 0.25 micron&#xA;  PowerPC 750. The CGA package has been adopted &#xA;  because it is better suited to the stresses of launch&#xA;  and the space environment. The CGA has&#xA;  demonstrated significantly increased reliability during&#xA;  temperature cycling and stress, when compared to&#xA;  BGA packages, as shown in Figure 3. The CGA&#xA;  package has also passed shock and vibration testing&#xA;  for the space launch environment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/b7S1L.jpg&quot; alt=&quot;CGA vs BGA testing&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can imagine, the column grid array components are much more rigid than the leaded components. More information on CGA can be found &lt;a href=&quot;http://electroiq.com/blog/2000/08/column-grid-array-high-reliability-option-for-packaging/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="630" LastActivityDate="2015-02-23T17:34:49.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8244" PostTypeId="2" ParentId="4678" CreationDate="2015-02-23T19:52:05.803" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How far could one fall on the moon before sustaining injuries?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As on Earth, it really depends how you land. High-jumpers land on crash mats, therefore it seems reasonable to suppose that on Earth it's possible to jump high enough to sustain injuries on landing, anyway if you choose to land on the back of your shoulders and head. I certainly would not choose to leap head-first at the ground, I reckon that would mess me up ;-)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As others have pointed out, the strength of gravity doesn't make a difference here, you land with whatever speed you can jump at, so the same would apply on the Moon. You can't jump so high that you'll injure yourself if you land &lt;em&gt;well&lt;/em&gt;, because humans are well-equipped to jump and land. Our legs can handle take-off speed in reverse. Any of our potential ancestors whose legs couldn't do that, probably broke their legs early and never reproduced...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might be harder to jump into the air and land on your feet on the Moon than it is on Earth. Aside from being unfamiliar with low gravity, the jump takes longer (6 times longer), so you have to jump more accurately to avoid rotating significantly in the air. So take it a bit easy before you go bounding around the place, your risk of landing on your head is increased compared with Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about heights we can't jump to? On earth, a 4m fall onto something hard is pretty nasty if you don't land properly, &lt;em&gt;and&lt;/em&gt; it's difficult-ish to land properly. This is why you don't jump out of an upstairs window onto a concrete drive unless there's something even worse in the house, like a fire. Since the Moon has 1/6 the gravity, and energy is equal to force times distance, you'd hit the ground after a 24m drop with the same kinetic energy (and hence the same velocity). This should therefore be considered a dangerous height, just like upstairs windows are dangerous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The breaking force for the femur, though, is liable to be achieved through unfortunate landings rather than sheer speed. People break legs falling off swings or ladders, never mind upstairs windows. The issue is what's the maximum force put through the bone at any instant during the landing (and the angle the force is applied: bones are stronger in compression than they are in torsion). A landing &lt;em&gt;will&lt;/em&gt; bring you to a halt, and a good landing does so over as long a period as possible, with as close as possible to constant force, so as to get the max force as low as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual process of landing with a given velocity is roughly the same on the Moon as on the Earth. The hardness of the surface, the angle and posture you hit it, and your ability to bend yourself in order to prolong the landing, all contribute. A bulky vacuum suit might make it harder to land &quot;well&quot; on the Moon than it is on Earth, especially if it restricts your flexibility at the knees and hips.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;49 m/s&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's easy in a vacuum - velocity of 49m/s requires kinetic energy ($1/2 m v^2$) of 1200 Joules per kilogram, which requires a height of 122m on Earth (where $g$ is about 9.8, although there's nowhere on earth you can fall that far in a vacuum), and 6 times that on the Moon, 730m or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that on Earth you can increase terminal velocity by falling head-first or feet-first, the figure you give is for a skydiver pose. In a vacuum there is no terminal velocity and your posture is irrelevant.&lt;/p&gt;&#xA;" OwnerUserId="6946" LastEditorUserId="6946" LastEditDate="2015-03-16T12:34:31.127" LastActivityDate="2015-03-16T12:34:31.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8247" PostTypeId="1" AcceptedAnswerId="8248" CreationDate="2015-02-24T04:44:49.337" Score="6" ViewCount="1467" Body="&lt;p&gt;Logically the problem of a satellite in orbit around the Earth can be fully defined by the position and velocity of the satellite (assuming a perfectly spherical Earth etc.). However when I try and get from a velocity and radius I can't calculate much more than the energy in the orbit and the semimajor axis via the &lt;a href=&quot;http://en.wikipedia.org/wiki/Vis-viva_equation&quot;&gt;vis-viva equation&lt;/a&gt;. Am I missing some underlying issue with this problem or have I just not found the correct equation?&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-02-25T16:59:22.733" Title="Calculating orbital properties after an instantaneous burn not at apogee/perigee" Tags="|orbital-mechanics|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8248" PostTypeId="2" ParentId="8247" CreationDate="2015-02-24T07:11:23.553" Score="8" Body="&lt;p&gt;As Yoda once said about the constants of motion, &quot;There is another.&quot;  He was referring to angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The specific angular momentum vector is the cross product of the position and velocity vectors.  That vector is a constant through the entire orbit.  With that and the semi-major axis $a$ (you figured that part out already), you can get the eccentricity $e$.  Here $\mathcal{M}$ is the specific angular momentum vector and $\mu$ is the $GM$ of the central body.  Note that for the cross product, $\overrightarrow{r}$ and $\overrightarrow{v}$ are three-dimensional vectors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\overrightarrow{\mathcal{M}}=\overrightarrow{r}\times\overrightarrow{v}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\overrightarrow{\mathcal{M}}\cdot\overrightarrow{\mathcal{M}}=\mu a\left(1-e^2\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are working in the plane of the orbit, then the angular momentum vector is in the &lt;em&gt;Z&lt;/em&gt; direction, and $\overrightarrow{\mathcal{M}}\cdot\overrightarrow{\mathcal{M}}$ is simply $\left(x v_y - y v_x\right)^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Determining the orientation of the orbit, e.g. the argument of periapsis in two dimensions or the three Euler angles in three dimensions, and the current eccentric, true, or mean anomaly position in the orbit is left as an exercise for the reader.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-02-25T16:59:22.733" LastActivityDate="2015-02-25T16:59:22.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8249" PostTypeId="2" ParentId="8247" CreationDate="2015-02-24T09:44:16.053" Score="0" Body="&lt;p&gt;Generally speaking, the problem of a satellite in orbit around the Earth is fully defined as long as you take into account the radius and velocity vectors of the satellite, not just their magnitude.&lt;/p&gt;&#xA;" OwnerUserId="8528" LastActivityDate="2015-02-24T09:44:16.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8250" PostTypeId="2" ParentId="4704" CreationDate="2015-02-24T10:27:05.653" Score="8" Body="&lt;p&gt;To expand on the currently accepted answer: The &quot;unconditional gifts only&quot; rule is not NASA-driven policy, it's a matter of federal law. Specifically, three laws control this. The Miscellaneous Receipts Act requires that any money that a federal agency receives, absent any other statutory authority, has to go into the general treasury -- unless a federal law specifically allows an agency to collect money, whatever you give them is treated as a gift to the United States. The Antideficiency Act says that an agency can't spend money that was not appropriated for it under most circumstances (important exception: agency trust funds). So, for a generic federal agency, if you donate money they have to forward it to the general treasury. If you donate money to the US under the condition it go to that agency, that condition is binding, but just means the government can't accept the gift without Congressional authorization (as Congress must approve all gifts that impose some duty on the US).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, NASA is not a generic federal agency: they have specific statutory authority to accept gifts. But that authority only goes so far as unconditional gifts -- they can be accepted and are auto-appropriated for NASA by going into the NASA trust fund, but nothing else can be accepted. They can also enter into contracts, but they can't make profit off those contracts - it's limited to reimbursement. So, because NASA lacks statutory authority to accept conditional gifts, they cannot do so without the consent of Congress -- otherwise it'd involve putting money in their trust fund that they have no authority to keep, as well as spending money they have not been appropriated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, for &lt;em&gt;why&lt;/em&gt; federal agencies need special authority to accept gifts, there are various solid public policy reasons involving checks and balances. But that's not on-topic here: the point is it's illegal for NASA to accept conditional gifts, not because of their own policy, but by act of Congress.&lt;/p&gt;&#xA;" OwnerUserId="3316" LastActivityDate="2015-02-24T10:27:05.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8251" PostTypeId="2" ParentId="8247" CreationDate="2015-02-24T10:30:42.290" Score="8" Body="&lt;p&gt;Let $r_o$, $v_o$, and $\gamma_o$ be the instantaneous position, speed, and flight path angle of a satellite in a  &lt;a href=&quot;http://en.wikipedia.org/wiki/Kepler_orbit&quot;&gt;Keplarian orbit&lt;/a&gt; around a perfecly spherical Earth with  &lt;a href=&quot;http://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot;&gt;gravitational parameter&lt;/a&gt; $\mu=GM$.&#xA;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_relative_angular_momentum&quot;&gt;specific relative angular momentum&lt;/a&gt;&#xA;$$&#xA;h = rv\cos\gamma = r_ov_o\cos\gamma_o&#xA;$$&#xA;is constant anywhere on the trajectory.&#xA;This can be used to calculate the orbit parameter (semi-latus rectum), $ℓ=h^2/\mu$ of the orbit equations &#xA;$$&#xA;r = \frac{ℓ}{1+e\cos\nu}&#xA;$$&#xA;What remains is to determine $e$, the eccentricity of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_orbital_energy&quot;&gt;Specific orbital energy&lt;/a&gt; is also constant throughout the orbit so that, at periapsis (subscript $p$) &#xA;$$&#xA;v^2_p - v^2_o = 2\mu\left(\frac{1}{r_p}-\frac{1}{r_o}\right)&#xA;$$&#xA;At periapsis, $\gamma=0$ and $r=r_p$ so that, from the specific relative angular momentum relationship, &#xA;$$&#xA;v_p=\frac{r_ov_o}{r_p}\cos\gamma_o&#xA;$$&#xA;Substitute this into the preceeding equation and re-arrange to obtain a quadratic equation in $r_p/r_o$:&#xA;$$&#xA;(2c_o-1)\left(\frac{r_p}{r_o}\right)^2&#xA;-2c_o\left(\frac{r_p}{r_o}\right)&#xA;+\cos^2\gamma_o &#xA;= 0&#xA;$$&#xA;where&#xA;$$&#xA;c_o = \frac{\mu}{r_ov^2_o}.&#xA;$$&#xA;The two solutions of the quadratic equation are&#xA;$$&#xA;r_p,r_a = r_o\frac{c_o\pm\sqrt{c^2_o-(2c_o-1)\cos^2\gamma_o}}{&#xA;2c_o-1},&#xA;$$&#xA;The smallest solution is the periapsis distance, $r_p$, and the larger one is the apoapsis distance, $r_a$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At periapsis, $\nu=0$ so that the eccentricity of the orbit can be obtained from the orbit equations at that point:&#xA;$$&#xA;r_p = \frac{ℓ}{1+e}&#xA;$$&#xA;or&#xA;$$&#xA;e = \frac{ℓ}{r_p}-1.&#xA;$$&#xA;If $e=0$, the orbit is circular; if $0\leq e&amp;lt;1$, it is elliptical; if $e=1$, it is parabolic; and if $e&amp;gt;1$, the orbit is hyperbolic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flight path angle can be calculated using&#xA;$$&#xA;\tan\gamma = \frac{e\sin\nu}{1+e\cos\nu}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope the sheds more light on your problem.&lt;/p&gt;&#xA;" OwnerUserId="8321" LastEditorUserId="8321" LastEditDate="2015-02-24T10:46:59.117" LastActivityDate="2015-02-24T10:46:59.117" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8252" PostTypeId="2" ParentId="575" CreationDate="2015-02-24T12:08:42.330" Score="5" Body="&lt;p&gt;According to this 2013 paper: &lt;a href=&quot;http://www.academia.edu/5903667/Prospecting_for_native_metals_in_lunar_polar_craters&quot; rel=&quot;noreferrer&quot;&gt;Prospecting for Native Metals in Lunar Polar Craters&lt;/a&gt; (Platts, Boucher, Gladstone) which uses spectra captured from the Lunar orbiter LCROSS, they give the measured/estimated mass abundances:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Silver Ag 0.11%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gold Au 0.52%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mercury Hg 0.53% (0.39% according to estimates from the orbiter LRO).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this is &lt;strong&gt;only for polar craters&lt;/strong&gt; which are always in the shadow. They propose electrostatic dust transport of metals which are then trapped in such cold craters, along with volatiles like water. These are small areas compared with the entire Moon, but if you consider early human resource utilization on the Moon, it might be a good place to start.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-02-24T12:08:42.330" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8253" PostTypeId="2" ParentId="8228" CreationDate="2015-02-24T12:34:52.110" Score="13" Body="&lt;p&gt;Fuel cells produce water. The output of the &lt;a href=&quot;http://www.ibiblio.org/apollo/ApolloProjectOnline/Documents/SMA2A-03-BLOCK%20II%20Volume%201%2019691015/aoh-v1-2-06-eps.pdf&quot;&gt;Apollo fuel cells&lt;/a&gt; (PDF on Apollo power supply system design) was used as drinking water and as a coolant in the environmental control system.&lt;br&gt;&#xA;If you use solar cells, you need rechargeable batteries to supply the spacecraft when it's on the night side of the planet.&lt;br&gt;&#xA;I suspect the choice came down to the total weight of the alternatives: fuel cells + H2 and O2 tanks versus solar cells + batteries + drinking water. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="168" LastEditDate="2015-02-24T14:05:09.580" LastActivityDate="2015-02-24T14:05:09.580" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8257" PostTypeId="1" CreationDate="2015-02-24T19:59:02.370" Score="4" ViewCount="186" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/744/effect-of-atmospheric-drag-on-rocket-launches-and-benefits-of-high-altitude-laun&quot; title=&quot;this&quot;&gt;this&lt;/a&gt; question, higher altitude launches are discussed, but no one seems to address fuel consumption. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take the Saturn V for example. A Saturn V rocket on the launch pad weighed approximately 3,350 tons. With the launch at sea level, it took about 39 seconds to reach 2200 m (the altitude of Santa Fe, New Mexico). At approximately 15 tons of fuel per second, the Saturn V consumes 585 tons of fuel (17.5% of its launchpad weight) just to get to an altitude that we can reach with a train. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other high altitude cities in the U.S., some of which are near the ocean (if that's needed). Even a not-so-high city like Salt Lake City, Utah saves the Saturn V 31 seconds of burn time and 465 tons of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've heard arguments about transporting parts and fuel, but remember that a lot of these things were already shipped across the country just to put them in Florida.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, from a weight and fuel consumption standpoint, why do we not attempt more high altitude launches?&lt;/p&gt;&#xA;" OwnerUserId="8565" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-02-24T19:59:02.370" Title="Launch altitude and fuel cost" Tags="|launch|saturn-v|" AnswerCount="0" CommentCount="6" ClosedDate="2015-08-26T22:18:07.193" ContentLicense="CC BY-SA 3.0" />
  <row Id="8258" PostTypeId="1" AcceptedAnswerId="8259" CreationDate="2015-02-24T20:46:30.887" Score="5" ViewCount="734" Body="&lt;p&gt;Dragon of SpaceX has been very successfully tested and used.&#xA;But Dragon v2 seems to be differently designed and has 8 superdraco engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has the v2 been drop tested from a balloon or helicopter?&#xA;Have the 8 superdraco engines of the v2 been test fired together all at once in assembly, flying or on the ground?&#xA;(Does SpaceX perform such major system tests out of public insight?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that a prototype of the Dragon v2, within a year, goes directly from a showroom exhibit to the first launch pad abort test in 50 years.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="51" LastEditDate="2015-02-24T21:17:38.067" LastActivityDate="2015-02-24T21:17:38.067" Title="How has the Dragon v2 been tested before its launch pad abort test?" Tags="|spacex|dragon|testing|abort|dragon-v2|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8259" PostTypeId="2" ParentId="8258" CreationDate="2015-02-24T21:10:33.253" Score="7" Body="&lt;p&gt;The parachute configuration for abort was determined to require changes from the parachute configuration for a normal reentry.  On a normal reentry, the parachutes have more time to deploy at a higher altitude. In an abort case, they need to be shot out with a mortar and deploy faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was tested in 2014 after being dropped from a helicopter. The test unit was not exactly the Dragon V2 configuration, but was designed to test the new parachute configuration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SuperDraco engines have been tested singly and in pairs at the McGregor, TX test facility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/AMqaY.jpg&quot; alt=&quot;Single SuperDraco firing&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=bTZPax_kRhc&quot; rel=&quot;noreferrer&quot;&gt;Test fire of SuperDraco 'pack' (set of two engines)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is now time to try the whole thing and get this baby off the ground. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eV4Rz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two tests scheduled for the full up Dragon V2 testing.  First a pad abort, from a representative structure (but not a real first/second stage).  Then later in 2015 the plan is to launch a Falcon 9 with a dummy second stage, and abort the Dragon V2 around Max-Q, sort of the hardest abort case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the abort tests, the SuperDracos are used to get away, not to land, rather it will land, in the water, under parachutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The McGregor, TX test site has a completed environmental assessment for the DragonFly test vehicle. SpaceX will be testing powered landings and other SuperDraco/Dragon operations (Land on land with parachutes. Land on power. Take off on powered hop, land on parachutes, ect).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-02-24T21:17:25.863" LastActivityDate="2015-02-24T21:17:25.863" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8260" PostTypeId="1" CreationDate="2015-02-24T22:20:20.893" Score="10" ViewCount="1533" Body="&lt;p&gt;Looking at the numbers that are given, for example, at the SpaceX launches, it seems that the velocities at first stage separation are a bit above 2 km/s. The second stage does all the rest of the work, accelerating to the final velocity. So, the first stage delivers just a bit more of 25% of total delta-v on the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By comparison, the Saturn V reached around 2.7 km/s with its first stage, quite a lot more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why was it designed that way for the Falcon 9?  Of course, the Tsiolkovsky equation governs this (plus the effect of drag and gravity), but I assume some cost optimization must have gone into it as well.&lt;/p&gt;&#xA;" OwnerUserId="8566" LastEditorUserId="51" LastEditDate="2015-07-10T02:55:40.077" LastActivityDate="2015-07-10T02:55:40.077" Title="Optimum velocity for first stage separation in a two-stage launcher" Tags="|spacex|falcon-9|cost|stages|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8261" PostTypeId="1" AcceptedAnswerId="8263" CreationDate="2015-02-25T03:10:46.307" Score="3" ViewCount="1172" Body="&lt;p&gt;My co-workers and I had a debate if there is an &quot;orbital speed limit&quot; for a given altitude over a celestial body.&#xA;According to Kepler's laws it appears you cannot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My co-worker proposed the following thought experiment to prove you could if you were also thrusting towards the planet.&#xA;If you had infinite fuel, could you orbit around an arbitrary point in deep space (No gravity effects) with any given orbital radius?&#xA;For example orbit a point going the nearly speed of light with a 1cm orbital radius as long as you had, and kept adjusting your thrust vector and had enough thrust to arrest and reverse your velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We understand and agree if you turned you engines off you would fly away in a straight line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I am under the impression that at any given speed you have a minimum orbital radius which you could never go under without slowing your orbital velocity down regardless if you are actively changing your thrust vector to achieve impossible natural orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I correct?&lt;/p&gt;&#xA;" OwnerUserId="8567" LastEditorUserId="8567" LastEditDate="2015-02-25T14:21:19.637" LastActivityDate="2015-04-17T09:08:38.030" Title="Speed limit while &quot;Orbiting&quot; a fixed point in deep space" Tags="|orbital-mechanics|physics|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8262" PostTypeId="1" CreationDate="2015-02-25T03:18:32.673" Score="10" ViewCount="626" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Soil_Moisture_Active_Passive&quot; rel=&quot;noreferrer&quot;&gt;SMAP&lt;/a&gt; (Soil Moisture Active/Passive) satellite &lt;a href=&quot;http://spaceflightnow.com/2015/02/25/smap-observatory-unfurls-science-antenna-structure/&quot; rel=&quot;noreferrer&quot;&gt;just deployed&lt;/a&gt; its 6 meters (19.7-foot) mesh reflector antenna today. The satellite orbits the Earth in Sun-synchronous retrograde near-polar, near circular orbit with inclination of 98.116°, altitude of about 660 km (410 mi), orbital period of ~ 98 minutes and is essentially riding the Solar terminator:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/g55pc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Ground track and orbit of SMAP. Source: &lt;a href=&quot;http://www.satflare.com/track.asp?q=40376#TOP&quot; rel=&quot;noreferrer&quot;&gt;Satflare&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will repeat its ground track every two-to-three-days and you can see the table of its visible passes e.g. &lt;a href=&quot;http://www.n2yo.com/passes/?s=40376&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. This means that it could be visible from nearly any location on Earth around dawn and dusk. Its relatively large (for a LEO satellite) reflector antenna is constructed out of gold-plated molybdenum wire mesh, and rotates on a long boom at constantly changing beta angle relative to the Sun and the ground:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/DDXvL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Artists rendition of SMAP in orbit. Image: NASA/JPL&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here's a &lt;a href=&quot;https://www.youtube.com/watch?v=-7_YdsNb2w4&amp;amp;t=54s&quot; rel=&quot;noreferrer&quot;&gt;video simulation of SMAP at work&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/-7_YdsNb2w4?start=54&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mesh antenna is see-through, but given proper conditions, I imagine it should still reflect sufficient light - perhaps also with its structure and spacecraft's truss - to be visible to a naked eye:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/8f7Ba.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Unfurled SMAP mesh reflector antenna. Image: NASA/Northrop Grumman&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now that we know a bit about SMAP's configuration, orbital regime and that its reflector antenna successfully unfurled, back to my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Will SMAP produce periodic &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;noreferrer&quot;&gt;satellite flares&lt;/a&gt;? Has this chance of &lt;em&gt;flaring&lt;/em&gt; been simulated before deployment, perhaps out of curiosity of someone working on the mission?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, is ground visibility of non-classified artificial satellites ever established before deployment, perhaps already in the mission design phase? Is it ever a requirement and if, for what (non-stealth) reasons?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-02-25T03:29:31.143" LastActivityDate="2016-01-30T19:12:16.020" Title="Will SMAP produce predictable satellite flares visible to the naked eye from the surface of the Earth?" Tags="|artificial-satellite|nasa|observation|earth-observation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8263" PostTypeId="2" ParentId="8261" CreationDate="2015-02-25T03:36:51.017" Score="7" Body="&lt;p&gt;No, you are not correct.  You could simply thrust towards the point to keep going in a circle at any speed (below c).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However I would recommend that instead of wasting all that fuel, you put enough of that fuel in a separate tank so that you split your mass in half, and put that tank on a rope in the opposite position around the point.  The speed of rotation will determine the required strength and therefore mass of the rope, but it will be less than an infinite amount of fuel.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-02-25T03:36:51.017" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8264" PostTypeId="2" ParentId="8261" CreationDate="2015-02-25T04:25:28.167" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I am under the impression that at any given speed you have a minimum&#xA;  orbital radius which you could never go under without slowing your&#xA;  orbital velocity down.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Your impression is wrong. Since you seem to appreciate thought experiments, let's say it this way; Spin your craft along its axis. Any axis, at any speed you want. Assuming your craft can take the stress (being pulled apart by centrifugal force as it's not a point mass and the farther from its axis of rotation the higher the force in a vector away from it), it will orbit itself at essentially zero orbital altitude at the rotation rate it can still tolerate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, that's cheating, right? But is it? It matches your requirements of infinitesimally small orbital radius at possibly ludicrous speeds at its extreme ends. As long as you can find that unobtainium to build your spacecraft of imagination out of, so it can tolerate shear stress and it won't rip it apart. And you don't even need all that much of propellants to achieve that. You just cant two engines at 90° angle to the craft (or one with two nozzles), each facing in the other direction and &lt;em&gt;let it rip&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;My co-worker proposed the following though experiment to prove you&#xA;  could if you were also thrusting towards the planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So here's another thought experiment. What in the spin I describe, that cheating &lt;em&gt;self-orbit&lt;/em&gt; one, is different from what we'd usually consider &lt;em&gt;an orbit&lt;/em&gt;? Can you now see what your co-worker proposed with continuous thrust towards the orbital focus? It's essentially &lt;em&gt;keeping things together&lt;/em&gt;, replacing strength of atomic bonds of our spacecraft of imagination's materials at micro scale with macro scale propulsion of imagination, counteracting centrifugal force that is the product of your mass and acceleration, which in our case is v&lt;sup&gt;2&lt;/sup&gt;/r. As long as the thrust towards focus negates centrifugal force, you can orbit at any speed below &lt;em&gt;c&lt;/em&gt;. Your co-worker is correct.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-02-25T04:25:28.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8265" PostTypeId="2" ParentId="8261" CreationDate="2015-02-25T09:12:50.760" Score="4" Body="&lt;p&gt;With sufficient thrust you can achieve an arbitrary speed at an arbitrary radius. The magnitude of the required thrust, in the direction of the centre point of the orbit, is given by:&#xA;$$&#xA;F = \frac{mv^2}{r},&#xA;$$&#xA;$m$ being the mass of the spacecraft, $v$ being the speed of the spacecraft and $r$ being the radius of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the more reaction mass you have, the larger the thrust, the more reaction mass you have to use. Which is part of the fundamental problems with rockets. Most of the fuel is used to get the fuel in orbit (or in this case, to keep it in orbit).&lt;/p&gt;&#xA;" OwnerUserId="8570" LastActivityDate="2015-02-25T09:12:50.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8266" PostTypeId="2" ParentId="8261" CreationDate="2015-02-25T14:51:39.890" Score="-4" Body="&lt;p&gt;Yes, you're right. The other answers already noted that you may need a &lt;strong&gt;lot&lt;/strong&gt; of fuel. That's an understatement. You've got the tyranny of the rocket equation working against you, with the exponential factor. Just for a single orbit at speed v, you need to a delta-v of 2*pi*v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, this means that you may need an initial weight (including fuel) just to complete one orbit that would cause your whole ship to turn into a black hole. I'd call that a failure.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2015-02-25T14:51:39.890" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8267" PostTypeId="1" AcceptedAnswerId="8268" CreationDate="2015-02-25T20:03:10.093" Score="8" ViewCount="584" Body="&lt;p&gt;I've been playing with Hohmann transfers from one circular orbit to another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been calling the radius of the departure orbit 1 and the radius of the arrival orbit &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; with &lt;span class=&quot;math-container&quot;&gt;$r&amp;gt;1$&lt;/span&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two burns:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Departure burn to leave circular departure orbit and enter Hohmann transfer orbit.&lt;/li&gt;&#xA;&lt;li&gt;Arrival burn to exit Hohmann transfer and enter circular arrival orbit (aka a circularize burn)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The total &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt; is the sum of the &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt;s these two burns take.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; increases, total &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt; increases up until a certain point. &lt;em&gt;Then total &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt; starts falling!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/asUgI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If my calculus weren't so rusty I'd try to solve for &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; where &lt;span class=&quot;math-container&quot;&gt;$f'(r)=0$&lt;/span&gt;. But my brute force numerical efforts seem to indicate at the top of this hill &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; is roughly 15.5817.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a name for this number?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="10273" LastEditDate="2018-09-30T21:40:32.733" LastActivityDate="2018-09-30T23:52:50.653" Title="~15.5817 -- Is there a name for this number?" Tags="|hohmann-transfer|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="8268" PostTypeId="2" ParentId="8267" CreationDate="2015-02-25T20:08:31.447" Score="9" Body="&lt;p&gt;I don't think it has a particular name other than &lt;a href=&quot;http://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Worst_case.2C_maximum_delta-v&quot; rel=&quot;noreferrer&quot;&gt;&quot;worst-case Hohmann transfer&quot;&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-02-25T20:08:31.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8269" PostTypeId="1" AcceptedAnswerId="8270" CreationDate="2015-02-25T20:10:10.290" Score="51" ViewCount="21454" Body="&lt;p&gt;According to &lt;a href=&quot;http://artifacts.nasa.gov/shuttle_facts_figures.htm&quot;&gt;this NASA article&lt;/a&gt; the tires are inflated to 340 psi (main gear) and 300 psi (nose gear). At landing, there is significant strain, but what about in space? Are the tires exposed to vacuum or are they in a pressurized compartment?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like the stress of being in a very cold and non-pressurized environment would be huge, and completely different from the strain they are under when landing. Why don't the Space Shuttle Orbiter's tires explode in the vacuum of space?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="4660" LastEditDate="2015-04-22T23:25:49.557" LastActivityDate="2019-03-20T09:12:47.433" Title="Why don't the Space Shuttle's tires explode in the vacuum of space?" Tags="|nasa|space-shuttle|environment|physics|landing-gear|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8270" PostTypeId="2" ParentId="8269" CreationDate="2015-02-25T20:25:06.227" Score="81" Body="&lt;p&gt;Standard atmospheric pressure at sea-level Earth is &lt;em&gt;just&lt;/em&gt; 14.696 psi. Compare that to 340 or 300 psi (23.14 and 20.42 amt, respectively). The difference in internal tire pressure in Earth's atmosphere and absence of atmospheric pressure in vacuum of space is only 4.3 - 4.9%. Tires would experience far more dynamic pressure environment due to friction heating and pressure of weight of the orbiter during landing (240,000 lbs / 109,000 kg) at contact speeds up to 260 mph (418 km/h) than those ~ 5%. So it's reasonable to assume they were built and tested to withstand more than a pressure difference during a trip to LEO and back.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-02-25T20:48:08.990" LastActivityDate="2015-02-25T20:48:08.990" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8271" PostTypeId="1" AcceptedAnswerId="8279" CreationDate="2015-02-25T20:33:35.247" Score="31" ViewCount="8015" Body="&lt;p&gt;US Companies have been using Russian rocket engines for their rockets for a long time, even during sanctions. But as I was going through the European rocket launches, I felt European rocket engines are not inferior to Russian. But then why are US companies continuing to rely on Russian rocket engines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My assumptions are,&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Legacy rocket design. US companies are following the mantra, &quot;Don't fix what's not broken.&quot; They will continue using their design unless they hit a technological limitation.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;European countries don't export their rocket engines.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;My question is, does anyone knows anything about why US companies don't use European rocket engines?&lt;/p&gt;&#xA;" OwnerUserId="6415" LastEditorUserId="8383" LastEditDate="2015-02-28T04:25:28.337" LastActivityDate="2019-12-31T11:02:40.313" Title="Why doesn't the US use European rocket engines?" Tags="|launch|rockets|engine-design|engines|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8272" PostTypeId="2" ParentId="8271" CreationDate="2015-02-25T20:42:56.670" Score="33" Body="&lt;p&gt;The primary reason is one of competence.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians have a series of pretty darn good rockets.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RD-171, a 4 engine bell, one turbopump engine (used on Zenit first stage) is an impressive Kerosene/LOX engine. It is a competitor for the F-1 used on the Saturn V in terms of capability.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RD-180 is a 2 engine bell, smaller turbopump version of the RD-171 and as Atlas V has shown, a pretty darn good engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RD-193/181 is the 1 engine bell version of the same.  Orbital will be using 2 of these to replace the NK-33s on the Antares booster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heck, even the NK-33 built in the 1960s and 1970s is a darn good engine for today, which is why Orbital started with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians do not do as well with LH2/LOX engines; there the US is world class. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus there are a number of useful engines made by the Russians that they are happy to export. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the European side, there is the Vulcain engine used on the Ariane V, and not much else. The SRBs they use on Ariane V boosters and on the Vega light launcher are mostly equivalent to the solids that ATK can deliver. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a national system, buying American is the best way to get funding, unless there is a really good alternative you want to use. For the Atlas V, there really is no good American alternative.  Same for the RD-181 for Antares.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it is really pragmatism. The Russians have some great engines using LOX/Kerosene and the Americans do not have good alternatives.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2015-02-26T12:18:50.573" LastActivityDate="2015-02-26T12:18:50.573" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8273" PostTypeId="1" CreationDate="2015-02-25T22:41:58.937" Score="5" ViewCount="693" Body="&lt;p&gt;The Atlas rockets and Centaur upper stages (often used in combination) successfully use(d) pressure-stabilized tanks to minimize dry mass. This technology was not used in any other rocket stages I'm familiar with. Given the high value of a pound of payload, why?&lt;/p&gt;&#xA;" OwnerUserId="8216" LastActivityDate="2015-02-25T22:57:39.377" Title="Why are pressure-stabilized rockets not more common?" Tags="|technology|mass-fraction|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8274" PostTypeId="2" ParentId="8273" CreationDate="2015-02-25T22:57:39.377" Score="5" Body="&lt;p&gt;The high price of payload to orbit is precisely the point: Pressure stabilization means that the entire transport of the tanks has to be done with the greatest care. Any dropped tool can easily punch a hole through the paper-thin tank walls. They also have to be constantly wetted with an oil film to prevent corrosion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The additional labour adds greatly to the cost of the launch system.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-02-25T22:57:39.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8275" PostTypeId="2" ParentId="439" CreationDate="2015-02-26T00:44:54.170" Score="0" Body="&lt;p&gt;Just to add a bit of perspective.  The high pressure fuel turbo-pump operates with a power of about 69,000 HP, and the ox is 25,000 HP &lt;a href=&quot;http://www.rocket.com/space-shuttle-main-engine&quot; rel=&quot;nofollow&quot;&gt;http://www.rocket.com/space-shuttle-main-engine&lt;/a&gt;.  So, just pumping high pressure cryogenics takes ~100,000 HP on each shuttle engine.  (On the order of 500-1000 cars engines)&lt;/p&gt;&#xA;" OwnerUserId="8579" LastEditorUserId="8579" LastEditDate="2015-02-26T14:17:16.120" LastActivityDate="2015-02-26T14:17:16.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8276" PostTypeId="2" ParentId="8262" CreationDate="2015-02-26T03:00:46.857" Score="1" Body="&lt;p&gt;SMAP will produce visible &lt;em&gt;Satellite Flares&lt;/em&gt; but not because of the antenna.  As you can see in the photo of the completed reflector, even at a very offset angle, little visible light is reflected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar panels will reflect the most visible light to the Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://www.heavens-above.com/PassSummary.aspx?satid=40376&amp;amp;lat=46.0569&amp;amp;lng=14.5058&amp;amp;loc=Ljubljana&amp;amp;alt=297&amp;amp;tz=CET&quot; rel=&quot;nofollow&quot;&gt;&lt;strong&gt;Heavens Above&lt;/strong&gt; SMAP visibilty prediction for your location&lt;/a&gt; or the references that you provide.&lt;/p&gt;&#xA;" OwnerUserId="1225" LastEditorUserId="1225" LastEditDate="2015-03-11T06:52:07.613" LastActivityDate="2015-03-11T06:52:07.613" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8277" PostTypeId="1" AcceptedAnswerId="8278" CreationDate="2015-02-26T07:14:49.493" Score="18" ViewCount="1682" Body="&lt;p&gt;The multi-target orbiter Dawn spacecraft, launched in 2007, is now approaching the dwarf planet Ceres and had recently sent high resolution images of Ceres from a distance of 46,000 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CDXXD.jpg&quot; alt=&quot;Ceres from 46,000km away, by DWAN spacecraft&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;What is so interesting about Ceres that we decided to send a spacecraft to explore it?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Did the fact that Ceres is the largest body in the asteroid belt contribute to planning a mission to Ceres instead of other bodies? Perhaps being a more feasible target to orbit due to higher gravity?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="3456" LastEditorUserId="49" LastEditDate="2015-02-26T10:18:00.903" LastActivityDate="2015-08-11T01:00:27.520" Title="Why explore Ceres?" Tags="|mission-design|feasibility|dawn-mission|asteroid-belt|ceres|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8278" PostTypeId="2" ParentId="8277" CreationDate="2015-02-26T07:26:33.143" Score="25" Body="&lt;p&gt;Dawn has several mission objectives, including &lt;em&gt;to continue&lt;/em&gt; testing the Ion Thruster.  But why Ceres?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ceres and Vesta were chosen, because they have &lt;strong&gt;contrasting content&lt;/strong&gt;, one icy and one rocky.  Also, they are among the &lt;a href=&quot;http://en.wikipedia.org/wiki/Protoplanet&quot; rel=&quot;noreferrer&quot;&gt;protoplanets&lt;/a&gt; that &lt;strong&gt;remain intact since formation&lt;/strong&gt;, which (hopefully) leads to a better understanding of the &lt;strong&gt;formation of our solar system&lt;/strong&gt;, more specifically the different roles water and rock have.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We live in exciting times.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="6712" LastEditDate="2015-08-11T01:00:27.520" LastActivityDate="2015-08-11T01:00:27.520" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8279" PostTypeId="2" ParentId="8271" CreationDate="2015-02-26T08:31:50.257" Score="30" Body="&lt;p&gt;Initially, every country that built rockets built their own. After WW2, the design of ICBMs and space rockets was related closely enough that the capability to design and build rockets was seen as having strategic value. This situation persisted throughout the Cold War.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the Soviet Union collapsed in 1989, the &lt;a href=&quot;http://history.nasa.gov/SP-4225/hhs/hhs.htm&quot;&gt;US government worried&lt;/a&gt; about the possible consequences of lots of Russian rocket designers getting fired. What if they ended up working for regimes like Iran or North Korea?  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Moreover, several on the Space Council, as well as others in the Bush Administration, saw another reason to engage the post-Soviets in a cooperative space venture: as a way to help hold the Russian nation together at a time when the Russian economy was faltering and its society was reeling. In the words of Brian Dailey, Albrecht's sucessor, &quot;If we did not do something in this time of social chaos … in Russia, … then there would be potentially a hemorrhaging of technology … ‘away from Russia’ … to countries who may not have a more peaceful intention behind the use of those technologies.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So they started various programs to keep Russian space companies and the Russian space program afloat. This started with Space Shuttle visits to Mir. The biggest of these projects was the &lt;a href=&quot;https://www.wsws.org/en/articles/1998/12/spac-d31.html&quot;&gt;ISS&lt;/a&gt;, but US rocket companies were also encouraged to use Russian technology. US companies recognized a bargain when they saw one, and several American rockets ended up with Russian engines. There was some really good stuff available (see Geoffc's answer) for much lower prices than a newly developed engine would cost.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2014, the political situation changed. US-Russian relations cooled, and US companies started to look for alternatives to Russian-sourced engines. This includes license-building of Russian engines in the US, and new development. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-02-26T19:20:12.967" LastActivityDate="2015-02-26T19:20:12.967" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8281" PostTypeId="1" CreationDate="2015-02-26T12:37:56.087" Score="28" ViewCount="18937" Body="&lt;p&gt;My take is that, like so many bad, and good, NASA ideas, the SLS will get the axe after spending billions on the NASA subcontractor(s). Regardless if the SLS comes to full life cycle, NASA will not fly/build many rockets - once SpaceX proves the Falcon 9 is reusable, and the Falcon Heavy launches successfully. Which will be this year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The axe chopping probably will be with the next president. Obama already tried to kill SLS, and pushed for increases in Commercial Crew, which the senate lowered. The fight on that is over for this term, I believe.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a fully reusable Falcon Heavy, can Boeing's SLS product ever compete?&lt;/p&gt;&#xA;" OwnerUserId="8585" LastEditorUserId="567" LastEditDate="2016-06-27T15:32:34.413" LastActivityDate="2021-01-18T16:50:16.923" Title="Is the SLS irrelevant if the Falcon Heavy launch is successful this year?" Tags="|rockets|spacex|nasa|sls|boeing|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8282" PostTypeId="2" ParentId="8281" CreationDate="2015-02-26T12:50:15.943" Score="28" Body="&lt;p&gt;I am not a fan of the SLS.  So take my comments with that as a grain of salt.&lt;/p&gt;&#xA;&lt;p&gt;SLS is fundamentally a jobs program. It has no mandate to be affordable. It also has no real mission at the moment.  Thus there is nothing really pushing its development other than jobs in states of politicians.&lt;/p&gt;&#xA;&lt;p&gt;Thus logic and rationality do not apply to SLS.  It will be cancelled when the politicians stop fighting for it.&lt;/p&gt;&#xA;&lt;p&gt;Now on a technical level, Falcon Heavy is pretty powerful, but it is not actually powerful enough to replace SLS, phase 1 or phase 2.  Falcon Heavy is about 54 Tonnes (Metric Tonne) to LEO.&lt;/p&gt;&#xA;&lt;p&gt;Crossfeed, which was initially discussed has been discarded as the performance of the base Falcon 9 core improved. The first Falcon Heavy will launch with Block 3 side boosters. Unclear if they built a Block 5 center core, but it is a slightly different booster as the center core. Once they transition to all being Block 5 cores, performance is expected to hit their old numbers without needing crossfeed. It also turned out to be way harder than they expected, and then became unnecessary and they pivoted. (edit: 2021 - 3 launches so far, 2 coming in 2021 of F-H, 0 SLS so far, with one planned in 2021).&lt;/p&gt;&#xA;&lt;p&gt;SLS is 70T (Block 1) going up to 130T (Block 2).  That is a big differential and in this space, it makes a big difference.&lt;/p&gt;&#xA;&lt;p&gt;But if you can launch a Falcon Heavy, fully reusable for the prices SpaceX is quoting (\$90-\$125 million), several times a year, vs SLS once every 2-4 years for billions of dollars a launch, the choice seems obvious.  Build smaller payloads in pieces and actually get your payload launched. Cadence matters almost as much as payload.&lt;/p&gt;&#xA;&lt;p&gt;As currently designed, although the Falcon Heavy has a 54 mT payload, the actual payload adapter that SpaceX produces is limited to much less mass.  It is unlikely in the near future we will see any LEO boosts with the full payload mass, but if a customer wants to I am sure SpaceX will modify the payload adapter.&lt;/p&gt;&#xA;&lt;p&gt;On the other hand, the fairing on Falcon Heavy is the same as the Falcon 9 fairing and is in fact, possibly one of the most limiting factors. It is a 3m core, with a hammerhead fairing, extending about a meter on either side, for a 5m wide fairing. Alas, there is 'stuff' on the inside that reduces the size enough to differentiate from the Atlas booster's 5m fairing which has more internal space.  SpaceX faces this issue with launching their 4400 satellite Starlink constellation, where they find the booster can launch the needed mass, but the fairing cannot hold enough satellites.&lt;/p&gt;&#xA;&lt;p&gt;SLS will have MUCH larger fairings around 8.5 to 10 meters wide, for which there may certainly be good use cases.  However you have to trade off the cost, low flight rate, against building the payload in smaller pieces.&lt;/p&gt;&#xA;&lt;p&gt;One of SLS's big problems is lack of a mission. Thus they need to find something really important to do with SLS. A realist who wants their mission launched would be wiser to consider Falcon Heavy with multiple launches and in space assembly to actually get launched.&lt;/p&gt;&#xA;&lt;p&gt;The bigger telltale will be when you see a push to launch Orion on Falcon Heavy. That will be the death knell for SLS.&lt;/p&gt;&#xA;&lt;p&gt;My personal prediction is SLS will actually launch (late), at least twice, which takes us out to at least 2020, but not too many times more. Thus to the specific question asked, the answer would be no.  (Edit in 2021: Still no SLS launches, 2021 is scheduled to have one test flight in Sept. But any delays, of which there shall of course be, will likely push it to 2022 realistically. AND the test fire of SLS only managed 1 minute out of an 8 minute test, so expect delays.).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/BFR_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;BFR/BFS&lt;/a&gt; from SpaceX will be the SLS killer, if they succeed at developing it. That looks to be available in the same time frame, and is more likely the SLS killer.&lt;/p&gt;&#xA;&lt;p&gt;To give some context around Starship/Super Heavy (renamed since I wrote this originally), they are SN9 (Serial Number 9) of test vehicles.  They have flown 4 vehicles (not very far, 3 at 150m, one to 12.5K (though it failed to land). They are rapidly building more prototypes and are at least engine #46 from spy photos.&lt;/p&gt;&#xA;&lt;p&gt;The current state of the Starship build as of Jan 2021 is nicely summarized in this infographic.  (Yes it is gratuitous, but I really like this image so I am going to share it).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/czKmJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/czKmJ.jpg&quot; alt=&quot;StarPrototype Status&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Thus you can see the progress, vs a single SLS core stage under test (that did not properly complete).  Which will work in the end?  Unclear but fun to watch! (I have my theories).&lt;/p&gt;&#xA;&lt;p&gt;A commenter notes that if the Grey Dragon mission happens (edit 2021: It did not), two paying customers, on a loop around the moon, (Almost Apollo 8, but not actually entering Lunar orbit, rather a simple free return mission), to expect SLS to be cancelled. I am not so sure, but it would certainly make supporting SLS that much harder.&lt;/p&gt;&#xA;&lt;p&gt;2021 Update: The Grey Dragon idea did not happen, but a Japanese billionaire did pay to fly 10 people or so, around the moon in the 2023 time frame on Starship. Still looks like this is vaguely on track. With a likely delay of the first SLS demo flight to 2022, we shall see who is first with humans around the moon.  Second SLS will need at least 1 year, if nor more for second flight. So 2023 is likely the earliest a manned SLS could fly, but seems unlikely based on past performance.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2021-01-18T16:50:16.923" LastActivityDate="2021-01-18T16:50:16.923" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="8283" PostTypeId="1" CreationDate="2015-02-26T12:51:08.227" Score="5" ViewCount="221" Body="&lt;p&gt;I know the systems being used are very new and being used in very small satellites, but if a system could be scaled up in the near future, that would certainly help keep the station afloat. Thoughts? &lt;/p&gt;&#xA;" OwnerUserId="8585" LastEditorUserId="49" LastEditDate="2015-02-26T13:51:06.733" LastActivityDate="2015-02-26T13:54:43.537" Title="Will the International Space Station's propulsion system be changed to electric propulsion?" Tags="|iss|propulsion|orbital-maneuver|technology|ion-thruster|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8284" PostTypeId="2" ParentId="8283" CreationDate="2015-02-26T12:55:28.570" Score="6" Body="&lt;p&gt;Ad Astra is scheduled to deliver a &lt;a href=&quot;http://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket&quot; rel=&quot;noreferrer&quot;&gt;VASIMR 200 kW&lt;/a&gt; unit to the ISS if they secure funding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It only provides 5 Newtons of thrust, at very high specific impulse ($I_\mathrm{sp}$). Due to power requirements, it will use batteries with charge capacity sufficient for 15 minutes of usage. Could be helpful, but is not clear if it will suffice for station re-boost needs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it uses any excess power on the station to charge its batteries, then it would be worthwhile as – if we don't count Argon gas propellant that is a consumable and would still need to be delivered to the station, but so have to be propellants for less efficient re-boost thrusters currently in use – basically a freebie.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-02-26T13:54:43.537" LastActivityDate="2015-02-26T13:54:43.537" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8285" PostTypeId="1" CreationDate="2015-02-26T12:59:02.153" Score="5" ViewCount="527" Body="&lt;p&gt;What is the cost, in man-hours per month, of ISS maintenance? I am looking for the actual time, not the dollar figure. What the governments of the world pay versus what the private sector will be able to pay in the very near future will be drastically different.&lt;/p&gt;&#xA;" OwnerUserId="8585" LastEditorUserId="49" LastEditDate="2015-02-26T14:19:16.530" LastActivityDate="2015-02-26T21:10:45.300" Title="How many man-hours are spent on International Space Station's maintenance?" Tags="|iss|crewed-spaceflight|cost|maintenance|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8287" PostTypeId="2" ParentId="8281" CreationDate="2015-02-26T13:31:52.603" Score="18" Body="&lt;p&gt;Short answer, no. Falcon Heavy and SLS Blocks 1B and 2 aren't even in the same lifting capacity category. The former is a heavy lift LV and latter are super-heavy LV. There also seems to be an increasing interest in SLS from the various scientific communities, for example, this slide was shared during the recent MEPAG (Mars Exploration Program Analysis Group) meeting #30:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/aa4gI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer is a bit more convoluted. SpaceX is progressively increasing FH capability with evolution of its Merlin engines, fuel cross-feed between the two boosters and central core stage, extended capacity upper stage, and so on. It won't match SLS lifting capacity, but they are decreasing that margin. To close the gap (OK, this sounds odd as there isn't any gap to close yet), a whole new rocket will be needed. They also need to build a commercial interest for them, something NASA doesn't have to, so it might not be as trivial as saying Elon can do it. I'm sure he can, but he's not stupid. FH could take a fair share of satellite launches to GEO and Earth-Moon and Sun-Earth libration points. That's a nice business to have, but at least initially, it won't be human rated. That's what they're trying to achieve first with Falcon 9. On the other hand, human rating of SLS is one of its core design requirements. So with it, NASA would be two classes of launch vehicles ahead of SpaceX if SLS, even Block 1B qualifies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS is also a whole lot more than merely a launch vehicle. Even its name (Space Launch System) suggests it's a whole system of components. OK, launch vehicles and recently the Orion Crew Module of course enjoy most interest of the public, but another one of them that is essential to human cislunar space exploration is also Solar Electric Propulsion (SEP) enabled upper stage. SpaceX hasn't anything like it, and while I don't doubt they could eventually produce a competing solution, nothing is really on the drawing board as far as I can tell. Can more traditional propulsion upper stages compete in this department? Not very likely, at least not for complex multiple burn and large delta-v trajectory missions they can't without sacrificing a great deal of lifting capacity for it and lowering your effective payload mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So OK, we could diss SLS and call it a Senate Launch System or whatnot. But in reality, while there's many things that we'd still want from it and some might be losing patience with lack of real missions for it, it's a whole different ballgame. It also enables development of a multitude of new technologies that will eventually benefit even commercial companies like SpaceX. Remember, even Merlin 1x engines &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29#Merlin_1A&quot; rel=&quot;noreferrer&quot;&gt;greatly benefited from NASA Fastrac engine program&lt;/a&gt; (and that was eventually a scrubbed program) and share many same or really similar components and designs. So I'm not &lt;em&gt;that&lt;/em&gt; pessimistic about SLS. I think of it in a sense that even if it fails, it will produce many spinoffs and advanced technologies that US commercial sector will benefit from, while at the same time keeping highly trained professionals at work. And if it succeeds, and I don't doubt that either, it's really just a question of how well we can use it once it's here, then even better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I.e. what I'm saying is that in my opinion, SLS can't fail at all. And I realize that this might not be the most popular opinion, but it will also enable future technologies that SpaceX will be able to use with more confidence because they'll be mature technologies by then. And if they scrub SLS, well then SpaceX knows where to get the next big expansion batch of highly trained professionals from. And there might be a whole lot of infrastructure to rent instead of having to build one's own from scratch.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-02-26T13:31:52.603" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8288" PostTypeId="2" ParentId="8285" CreationDate="2015-02-26T14:12:41.233" Score="9" Body="&lt;p&gt;The ISS has, quite literally, thousands of people working to support it.  What the total man-hour cost is, I couldn't say, but my building alone, most of which supports ISS in some form, has over a thousand people in it.  That's outside the JSC campus, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To paint a more complete picture, the ISS workforce, in addition to federal civil servants, is spread over a large number of contractors, many of whom would probably be reluctant to give up that information to just anybody. Including commercial spaceflight funding, which more or less supports ISS anyway, NASA's annual ISS budget is about $4B.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2015-02-26T21:10:45.300" LastActivityDate="2015-02-26T21:10:45.300" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8289" PostTypeId="1" CreationDate="2015-02-26T14:31:01.937" Score="9" ViewCount="2056" Body="&lt;p&gt;As the ISS has shown the ability for human habitation for extended periods, might we be able to add a propulsion unit to the ISS that would be capable of providing delta-v to send ISS to a martian orbit? Probably a low acceleration system that would allow for the maintenance of structural integrity would be needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A supply module with supplies sufficient for the Mars transit period would have to be added with the necessary supplies for the mission. If this presents structural or other problem, perhaps there could be resupply missions that could occur in Mars transit, or perhaps supply repositories placed in path of the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likely, its power generating system like solar panels might have to be upgraded to meet internal energy needs at increased distance from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now a bit of an editorial for which I have been chastised in the past. This approach would take advantage of our proven capability of assembling a major structure in orbit using many lifts of smaller modules, rather than the one shot approach that NASA is employing in developing larger lift capability and the one shot to space approach of the Orion unit. Orion must fit within an envelop needed for launch.&lt;/p&gt;&#xA;" OwnerUserId="6501" LastEditorUserId="49" LastEditDate="2015-02-26T14:40:53.727" LastActivityDate="2022-02-13T01:29:14.137" Title="Could the ISS be propelled to Mars?" Tags="|mars|iss|interplanetary|future-missions|feasibility|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8290" PostTypeId="2" ParentId="8289" CreationDate="2015-02-26T15:45:29.317" Score="7" Body="&lt;p&gt;The ISS is not designed for the kind of high thrust to escape Earth orbit, nor the duration of time it would take. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS is not designed to operate far from Earth, as it requires constant resupply.  If we have trouble resupplying it in low earth orbit, how hard will it be on the way to Mars, and then in Mars orbit?  That is a lot of supplies, which currently are provided by 3-4 Progress, 3-4 Dragon, 1-2 Cygnus, maybe an HTV or ATV launch a year. That is a reasonably large amount of payload, that would be very expensive and hard to send on to Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The medical facilities on the ISS depend on return to Earth being available within a day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Mark Adler notes, the ISS design is carefully tuned for the thermal environment of low-Earth orbit. It would not work at Mars. The Earth is quite 'warm' compared to space, and Mars is not quite as warm. Solar influx at Mars would be lower than at Earth due to distance from the Sun.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-02-26T16:18:39.100" LastActivityDate="2015-02-26T16:18:39.100" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8291" PostTypeId="1" CreationDate="2015-02-26T16:23:23.693" Score="5" ViewCount="466" Body="&lt;p&gt;Can launchers reasonably be equipped with wider payload fairings in order to carry larger (not heavier) payload? What would be the main problems with doing so, air resistance during the early slow part of the launch maybe? Or the balancing of the center of mass of the payload? Or is it just rarely demanded and therefore uneconomical to provide?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, everyone (except for the professionals involved it seems officially) get worried when they see animations of the complicated &lt;a href=&quot;https://www.youtube.com/watch?v=vpVz3UrSsE4&quot; rel=&quot;nofollow noreferrer&quot;&gt;unfolding&lt;/a&gt; planned for the James Webb Space Telescope in space. The mirror is 6.5 meters in diameter. What would it take to launch that unfolded? Its 12.2 meter diameter &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/James_Webb_telescope_sunshield.jpg/1280px-James_Webb_telescope_sunshield.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;sunshield&lt;/a&gt; would be the next step.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a follow up on this similar &lt;a href=&quot;https://space.stackexchange.com/questions/6394/why-is-bigalow-ba330-expandable-instead-of-fitting-fairings&quot;&gt;question/answer&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MIECy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-02-26T16:28:22.577" LastActivityDate="2015-04-24T16:27:39.823" Title="Could current (+SLS+FH) launchers use much larger diameter payload fairings?" Tags="|spacex|sls|payload|falcon-heavy|launcher-integration|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8292" PostTypeId="2" ParentId="8260" CreationDate="2015-02-26T17:41:27.920" Score="5" Body="&lt;p&gt;Short answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can get pretty close with the rocket equation and optimizing for a rocket of given mass to lift a maximum payload to a specified velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I finally tried some math (though in a slightly different way than the link above).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, I made the problem dimensionless&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;dividing all masses by the total mass of the rocket without the payload.  The mass of the rocket is then equal to one.&lt;/li&gt;&#xA;&lt;li&gt;introducing a factor $\phi$ which equals the total mass of the first stage divided by the total mass of the rocket. $\phi=0$ would mean a zero stage of no mass (rather silly) and $\phi=1$ would mean a second stage of zero mass (equally silly).&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Calling the dry mass fraction of the first stage $e_1$ and the dry mass fraction of the second stage $e_2$&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Assuming a mass $m_3$ which is the payload, which comes on top&lt;/li&gt;&#xA;&lt;li&gt;Dividing all velocities by the target velocity $v_t$ (in this case, needed to reach LEO).&lt;/li&gt;&#xA;&lt;li&gt;Dividing effective exhaust velocities, $I_{sp} g$, by the required velocity, to get $v_{e,1} = I_{sp,1} g / v_t $ as the effective dimensionless velocity for the first stage and $v_{e,2} = I_{sp,2} g / v_t$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Plugging this into the rocket equation gives $$\Delta v_1 = v_{e,1} \ln\left(\frac{1+m_3}{\phi * e_1 + 1 - \phi + m_3}\right) $$&#xA;$$\Delta v_2 = v_{e,2} \ln \left( \frac{1-\phi+m_3}{(1-\phi) e_2 + m_3} \right) $$ with $\Delta v = \Delta v_1 + \Delta v_2$.  Plugging in the parameters and solving the equation $$\Delta v_1 + \Delta v_2 = 1$$ (iteratively) for $m_3$ will give you the payload for a given rocket mass, which should be maximized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I plugged in some values for a Falcon 9:&#xA;$$v_t = 9.5 km/s$$&#xA;$$I_{sp,1} = 290s$$&#xA;$$v_{e,1}=0.305$$&#xA;$$v_{e,2}=0.358$$&#xA;$$I_{sp,2}=340 s$$&#xA;$$e_1=18 t/385 t = 0.0468$$&#xA;$$e_2= 4.9t/90t=0.0544$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After varying $phi$ and also calculating the velocity of the second stage, I got&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;    phi     m_3      delta v 2&#xA;    0.2    0.0160      0.895&#xA;    0.4    0.0241      0.858&#xA;    0.6    0.0308      0.753&#xA;    0.7    0.0331      0.683&#xA;    0.8    0.0339      0.592&#xA;    0.9    0.0313      0.456&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So, there is a relatively broad optimum where the first stage has around 80% of the mass of the total rocket and the second stage is responsible for around 60% of the total $\Delta v$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plug this into the original assumption of a $\Delta v$ of around 9.5 km/s, and you get to the (approximate) 6 km/s for the second stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why doesn't the first stage become faster?  Probably because it is the one which has to fight gravity and drag. 180 s at 10 m/s^2 could cost you around 1.8 km/s if you were firing straight up (which nobody is doing, I know...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare this to the actual mass fraction of the first stage of a Falcon 9, which is around 0.809... I probably came out a bit closer than I had a right to expect.&lt;/p&gt;&#xA;" OwnerUserId="8566" LastEditorUserId="8566" LastEditDate="2015-02-26T18:06:20.650" LastActivityDate="2015-02-26T18:06:20.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8293" PostTypeId="2" ParentId="8291" CreationDate="2015-02-26T18:26:27.187" Score="3" Body="&lt;p&gt;I believe the main problems are aerodynamic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A wider payload fairing produces more drag, so you'd be able to take less payload mass to orbit; worse, the added drag is above the center of mass of the rocket, which decreases stability; at some point you'd need fins on the base of the rocket to compensate, adding still more drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The back end of the payload fairing has to be tapered back to the rocket body diameter to avoid turbulence, so the fairing is going to be longer, thus bigger and heavier, than a narrower equivalent-payload-volume fairing.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-04-24T16:27:39.823" LastActivityDate="2015-04-24T16:27:39.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8296" PostTypeId="1" AcceptedAnswerId="8298" CreationDate="2015-02-27T09:58:51.713" Score="22" ViewCount="1831" Body="&lt;p&gt;Orbital periods in LEO are quite short, about 92 minutes for the ISS. So every other almost 46 minutes it is in Earth's shadow. Not only is it then out of direct sunlight, I suppose that the night side of Earth doesn't reflect any useful light either. Many EVA's last several hours and are thus conducted in the shadow about half of the EVA time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is an EVA affected when in the shadow? Is it naturally pitch black? What light sources do the astronauts have? Is their use of electricity restricted while on batteries? Has this been managed differently for different spacecraft in LEO, like the ISS, MIR, STS?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-02-05T15:01:20.227" Title="How are EVA's in LEO affected by being at the night side of Earth?" Tags="|iss|low-earth-orbit|eva|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8297" PostTypeId="1" CreationDate="2015-02-27T10:09:04.693" Score="9" ViewCount="193" Body="&lt;p&gt;Given the complexity of liquid propellant rocket engine development, I wonder if (and how much) modern &lt;a href=&quot;https://en.wikipedia.org/wiki/Computational_fluid_dynamics&quot; rel=&quot;nofollow&quot;&gt;Computational Fluid Dynamics&lt;/a&gt; (CFD) codes help to reduce overall development costs. Do they actually cut the number of test firings?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would be nice if the answers highlighted the number of firings and burn time in engine programs with and without CFD modeling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Preliminary bibliography for would-be posters:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://enu.kz/repository/2010/AIAA-2010-1518.pdf&quot; rel=&quot;nofollow&quot;&gt;http://enu.kz/repository/2010/AIAA-2010-1518.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.fe-net.org/downloads/FENet_Meetings/Noordwijk_Netherlands_Oct_2003/FENET_Noordwijk_Oct2003_MPA_Steelant.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.fe-net.org/downloads/FENet_Meetings/Noordwijk_Netherlands_Oct_2003/FENET_Noordwijk_Oct2003_MPA_Steelant.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ufdcimages.uflib.ufl.edu/UF/E0/02/13/03/00001/mack_y.pdf&quot; rel=&quot;nofollow&quot;&gt;http://ufdcimages.uflib.ufl.edu/UF/E0/02/13/03/00001/mack_y.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20010037826_2001054693.pdf&quot; rel=&quot;nofollow&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20010037826_2001054693.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.medwelljournals.com/ref.php?doi=rjasci.2014.660.663&quot; rel=&quot;nofollow&quot;&gt;http://www.medwelljournals.com/ref.php?doi=rjasci.2014.660.663&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://people.eng.unimelb.edu.au/imarusic/proceedings/19/169.pdf&quot; rel=&quot;nofollow&quot;&gt;http://people.eng.unimelb.edu.au/imarusic/proceedings/19/169.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-02-28T16:43:51.393" LastActivityDate="2015-12-18T06:36:18.980" Title="Do modern CFD codes reduce the number of test firings in an engine program?" Tags="|propulsion|engine-design|engineering|physics|computing|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8298" PostTypeId="2" ParentId="8296" CreationDate="2015-02-27T12:04:06.917" Score="22" Body="&lt;p&gt;Spacesuit designers and extravehicular activity (EVA) planners would probably prefer if spacewalks only took part in the Earth's shadow. From the spacesuit design perspective, one of the biggest issues is heat rejection, not heat retention. This is because of nearly nonexistent &lt;a href=&quot;https://en.wikipedia.org/wiki/Convective_heat_transfer&quot; rel=&quot;nofollow noreferrer&quot;&gt;convective heat transfer&lt;/a&gt; (conduction and advection) in near vacuum in LEO so the only way to keep suit internal environment's temperature at acceptable levels is by either &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_radiation&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiating&lt;/a&gt;, rejecting or storing it. Large (and massive) radiators on mobility units obviously wouldn't be very welcome, so while some smaller heat sinks and heat exchanging radiators are used internally in the PLSS (Portable Life Support System) units, we're only really left with heat rejection and excess heat storage options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rejection of external environmental heat is pretty straightforward. Spacesuits are mostly white or use reflective surfaces to prevent absorption and reflect as much of incident light from the Sun and &lt;em&gt;Earthshine&lt;/em&gt; (Earth's albedo) before it's converted to photothermal, or if you want, photoexcitation heat. This also helps prevent &lt;a href=&quot;https://en.wikipedia.org/wiki/Photoelectric_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;photoelectric charging&lt;/a&gt; of suit's outer layers, which could also be dangerous if too much charge built up and would be rapidly (sometimes referred to as &lt;em&gt;catastrophically&lt;/em&gt;) discharged upon touching the station's truss, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suit internal thermal environment is a bit more tricky. Design parameters on average work with 1,200 &lt;a href=&quot;https://en.wikipedia.org/wiki/British_thermal_unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;BTU&lt;/a&gt; (British Thermal Units) per hour, about 350 W if memory serves correctly, of so-called &lt;em&gt;metabolic rate&lt;/em&gt;. In simpler terms, this is design ceiling heat output per hour during an active spacewalk. All this heat, with spacewalks sometimes lasting even over 8 hours (current Russian spacesuits can support longer spacewalks than American, but are more clumsy with umbilicals - i.e. they're less autonomous) has to thus be transferred to cooling water and rejected using the sublimator in the PLSS &lt;a href=&quot;http://quest.nasa.gov/space/teachers/suited/5emu4.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://quest.nasa.gov/space/teachers/suited/5emu4.html&lt;/a&gt;  One limit to EVA time is the amount of water remaining.  As mentioned, Russian spacesuits (&lt;a href=&quot;https://en.wikipedia.org/wiki/Orlan_space_suit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orlan&lt;/a&gt;) can extend their autonomy with the use of umbilical connections to the station, while American (Enhanced &lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;EMU&lt;/a&gt; w/ &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_Aid_for_EVA_Rescue&quot; rel=&quot;nofollow noreferrer&quot;&gt;SAFER&lt;/a&gt;) can last 8 hours plus 30 minutes of backup (redundant life support), having &lt;a href=&quot;https://space.stackexchange.com/a/6478/49&quot;&gt;about 820 Wh&lt;/a&gt; of total power available to them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So when EVA is taking place in the Earth's shadow, despite spacewalkers having to switch on a few lights mounted on their helmets, PLSS active heat rejection system has to work less with no external, incident heat sources and their overall power consumption is during those times lower. So from suit design perspective, being in the Earth's shadow is an advantage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From EVA planner's perspective, it's a bit of a mixed bag. In Earth's shadow, there's no worry of increased solar flux (solar storms and other events), but it might be a bit more difficult for astronauts to retreat back to the airlock in case of an emergency, and they might work a bit slower too due to reduced visibility. So if all the duration of an EVA took place in the shadow, it would be an advantage, but as transits in and out of them take from EVA time (astronauts would be adjusting their PLSS for thermals, work more with their suit controls like switching lights on and off, adjusting cameras,...), it's not really an advantage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for astronauts themselves, they would probably prefer all EVA take place during daylight. They don't have to take so much care about external fluid leaks (such as ammonia from the station's cooling system) since they'd &lt;em&gt;bake-off&lt;/em&gt; (outgas) faster on the &lt;em&gt;sunny side&lt;/em&gt;. They also see what they're doing much better, can work faster and perhaps even progress to tasks otherwise scheduled for next EVAs, and the view is much nicer too. This is when they'd take their cameras out and take a few EVA portrait shots, &lt;em&gt;selfies&lt;/em&gt; or whatever is now trending. Having watched quite some spacewalks (including &lt;a href=&quot;http://www.nasa.gov/press/2015/february/nasa-tv-previews-and-broadcasts-space-station-us-spacewalks/&quot; rel=&quot;nofollow noreferrer&quot;&gt;recent Wilmore &amp;amp; Virts ones&lt;/a&gt;), I'd say they also become more &lt;em&gt;chatty&lt;/em&gt; and generally the working &lt;em&gt;atmosphere&lt;/em&gt; (pardon the pun) seems more &lt;em&gt;uplifting&lt;/em&gt; (there I go again, sorry). :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Xvlk9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;The Enhanced EMU Suit (with lights on). The suits are white to reflect heat and to stand out against the blackness of space; the red&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;stripes serve to differentiate astronauts. (Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit#mediaviewer/File:STS-118_EVA_EMU_Suit.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, Credit: &lt;a href=&quot;http://www.spaceflight.nasa.gov/gallery/images/shuttle/sts-118/html/s118e06304.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some further sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eclss/emu.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Human Spaceflight article on Extravehicular Activity Mobility Units&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120009158.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced EMU Portable Life Support System (PLSS) and Shuttle/ISS EMU Schematics, a Comparison&lt;/a&gt;, Colin Campbell, NASA JSC, 2012 (PDF)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-02-27T15:32:09.093" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8299" PostTypeId="2" ParentId="6189" CreationDate="2015-02-27T15:38:13.483" Score="2" Body="&lt;p&gt;A brace of studies (intentionally made community wiki):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Stephen B. Johnson. The Secret of Apollo: Systems Management in American and European Space Programs. Johns Hopkins Univ., 2002. &lt;a href=&quot;http://www.worldcat.org/title/secret-of-apollo-systems-management-in-american-and-european-space-programs/oclc/51504397&quot; rel=&quot;nofollow&quot;&gt;About the book&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Thomas J. Kelly - Moon Lander: How We Developed the Apollo Lunar Module. Wash., D.C, Smithsonian Books, 2001. &lt;a href=&quot;http://www.worldcat.org/title/moon-lander-how-we-developed-the-apollo-lunar-module/oclc/44794944&quot; rel=&quot;nofollow&quot;&gt;About the book&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/SP-4102/contents.htm&quot; rel=&quot;nofollow&quot;&gt;SP-4102 Managing NASA in the Apollo Era, NASA History Office&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastActivityDate="2015-02-27T15:38:13.483" CommentCount="0" CommunityOwnedDate="2015-02-27T15:38:13.483" ContentLicense="CC BY-SA 3.0" />
  <row Id="8300" PostTypeId="2" ParentId="8296" CreationDate="2015-02-27T18:33:06.833" Score="11" Body="&lt;p&gt;I have heard, no citation, that while what TildalWave discusses about heat management while on EVA, the fingers of the gloves are almost the opposite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When in night, the gloves get cold and the astronauts fingers have issues. Whereas in light they warm up.  A nice example of how even the general issue has minor/local differences. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;References &lt;a href=&quot;http://papers.sae.org/1999-01-1969/&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/11506232&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="690" LastEditDate="2015-03-01T08:55:09.397" LastActivityDate="2015-03-01T08:55:09.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8301" PostTypeId="1" AcceptedAnswerId="8302" CreationDate="2015-02-28T02:40:34.443" Score="4" ViewCount="685" Body="&lt;p&gt;I read &lt;a href=&quot;http://www.eetimes.com/document.asp?doc_id=1262350&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; about &quot;dream mode&quot; in the Curiosity rover. In the article they mention:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... implemented on about 1.2 million logic gates ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How large would the avionics hardware need to be in order to house 1.2 million logic gates? Are there any pictures which highlight what the hardware looks like relative to the rest of the rover?&lt;/p&gt;&#xA;" OwnerUserId="8184" LastEditorUserId="8184" LastEditDate="2015-02-28T02:50:28.063" LastActivityDate="2015-10-19T22:06:29.743" Title="How large are the Curiosity rover's FPGAs which enable &quot;dream mode&quot;?" Tags="|rovers|curiosity|engineering|avionics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8302" PostTypeId="2" ParentId="8301" CreationDate="2015-02-28T03:51:25.223" Score="3" Body="&lt;p&gt;I am not an expert in electronics or FPGAs, but a couple of these would fit the bill, I think:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.xilinx.com/products/silicon-devices/fpga/kintex-7q.html&quot; rel=&quot;nofollow&quot;&gt;http://www.xilinx.com/products/silicon-devices/fpga/kintex-7q.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These aerospace grade fpga-chips fit 480,000 in a tiny IC package. I don't think the physical size is much of an issue here, even if we consider that the rover was designed a few years ago, when density wasn't quite as high. The packaging of this particular chip is a 23 mm square; 2.5 mm high.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-02-28T03:51:25.223" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8304" PostTypeId="1" CreationDate="2015-03-01T02:57:59.660" Score="8" ViewCount="791" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Volatiles&lt;/a&gt; in polar craters on the Moon are great, but so far apparently it is unclear how much is there. Though I still have a lot of reading to do, so far I haven't found any information about what the Moon's bedrock might be like. Down there where the surface hasn't been churned by meteor strikes into dust, which has then been baked by solar radiation for eons, can we say whether or not the rock will have some of those volatiles bound into their minerals? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a case made in &lt;a href=&quot;https://space.stackexchange.com/a/3365/4660&quot;&gt;an answer&lt;/a&gt; to a recent question on Mars that nitrogen has to be on Mars. It is very common and the whole solar system was formed of the same basic stuff - that's the part that also applies to the Moon. That seems a solid argument, alright (though I'm looking for more sources to get the details, because I'd like to know more). But the Moon's history is so different. Is there good reason to hope that when we make it into a good &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2010/12jul_rabbithole/&quot; rel=&quot;nofollow noreferrer&quot;&gt;lava tube&lt;/a&gt;, there will be nitrogen-containing minerals to be found? Some hydrogen minerals? Perhaps some carbon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the interiors of rock samples from the moon really represent the composition of the crust, or have they been altered by the impact churning process? Since all of the rocky planets were formed from the same accretion disk, shouldn't a certain percentage of the elements missing from the Moon's surface be there, at some distance below the surface? If not, how were they stripped off?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-03-03T04:01:48.160" Title="How much can be said about the bedrock of the Moon?" Tags="|the-moon|natural-resources|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8307" PostTypeId="1" AcceptedAnswerId="8316" CreationDate="2015-03-01T10:13:49.380" Score="15" ViewCount="1409" Body="&lt;p&gt;I wonder if the Dragon V2 heat shield could withstand the extreme speed of a reentry from the moon. Or is it only capable of LEO flights?&lt;/p&gt;&#xA;" OwnerUserId="8606" LastActivityDate="2015-07-08T07:18:09.393" Title="Could the current dragon v2 heat shield withstand a reentry from the moon?" Tags="|the-moon|spacex|reentry|dragon|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8308" PostTypeId="1" AcceptedAnswerId="8313" CreationDate="2015-03-01T12:51:41.313" Score="4" ViewCount="12508" Body="&lt;p&gt;&lt;a href=&quot;http://www.universetoday.com/15303/does-saturn-have-a-solid-core/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This page&lt;/a&gt; argues that Saturn's density tells scientists that it has a liquid metal core with maybe some rocky chunks:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The core region of Saturn may never be directly observed. Neither has&#xA;  the Earth’s. Despite that, scientists are fairly certain that, while&#xA;  Saturn has a core, it is not a solid mass of rock or metal, but a&#xA;  liquid metallic mixture similar to all of the gas giants.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.windows2universe.org/saturn/interior/S_int_compo_overview.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Some&lt;/a&gt;, &lt;a href=&quot;http://pages.uoregon.edu/jimbrau/astr121/Notes/Chapter12.html#int&quot; rel=&quot;nofollow noreferrer&quot;&gt;not all&lt;/a&gt;, Google image diagrams show a solid layer of ice around the cores of our gas giants:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q3kCE.gif&quot; alt=&quot;Solid over liquid&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ne1sL.jpg&quot; alt=&quot;Only gas over liquid&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;https://space.stackexchange.com/questions/2416/can-i-harvest-diamonds-from-the-rain-of-saturn-and-jupiter/&quot;&gt;this QA&lt;/a&gt; implies that that core would be completely covered by solid diamonds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, would there be anything solid to land on from the air inside Saturn, and what would that be?&lt;/p&gt;&#xA;" OwnerUserId="2649" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2023-07-19T03:44:10.843" Title="Does Saturn have a solid surface?" Tags="|planetary-science|planet|saturn|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="8309" PostTypeId="1" CreationDate="2015-03-01T13:48:39.700" Score="10" ViewCount="1591" Body="&lt;p&gt;It is well-known that the Earth's magnetic field may flip (&lt;a href=&quot;https://en.wikipedia.org/wiki/Geomagnetic_reversal&quot; rel=&quot;nofollow noreferrer&quot;&gt;geomagnetic reversal&lt;/a&gt;), and some data suggests it is weakening or distorting. The following shows the Earth's magnetic field at its surface in June 2014 (&lt;a href=&quot;http://www.esa.int/Our_Activities/Observing_the_Earth/Swarm/Swarm_reveals_Earth_s_changing_magnetism&quot; rel=&quot;nofollow noreferrer&quot;&gt;by ESA&lt;/a&gt;) which shows some distortion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GsLgA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, can such changes in the Earth's magnetic field affect the space exploration systems and satellites and possibly cause troubles for them? &lt;/p&gt;&#xA;" OwnerUserId="8607" LastEditorUserId="8607" LastEditDate="2015-03-07T20:07:12.330" LastActivityDate="2015-03-09T14:25:04.317" Title="How would changes in the Earth's magnetic field affect space exploration?" Tags="|artificial-satellite|orbit|magnetic-field|earth|geomagnetic-reversal|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8310" PostTypeId="1" AcceptedAnswerId="8312" CreationDate="2015-03-01T13:58:32.923" Score="15" ViewCount="6971" Body="&lt;p&gt;In the first seconds of &lt;a href=&quot;https://www.youtube.com/watch?v=GuzDsPgVrPs&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; you can see/hear ISS astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Samantha_Cristoforetti&quot; rel=&quot;noreferrer&quot;&gt;Samantha Cristoforetti&lt;/a&gt; prepare for a PR event:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/GuzDsPgVrPs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the preparation she uses two different microphones. A small, blue box-like one for communication with mission control, and a larger, cylindrical one for talking with the interviewer. The latter has a much superior voice quality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having two different microphone systems on the station seems quite redundant. Why don't the International Space Station (ISS) astronauts always use these high quality microphones?&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="49" LastEditDate="2015-03-01T18:13:26.687" LastActivityDate="2016-12-06T13:09:26.183" Title="Why do ISS astronauts use lower quality microphones for official radio communication than for PR events?" Tags="|iss|communication|technology|crewed-spaceflight|procedure|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8311" PostTypeId="2" ParentId="8310" CreationDate="2015-03-01T14:47:29.440" Score="1" Body="&lt;p&gt;Pure speculation on my part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To anybody &lt;em&gt;not&lt;/em&gt; habituated to listening to radio communication, static/radio-interference may necessitate multiple 'takes'. This may be the case with the majority of viewers. The larger microphone probably carries some on-board signal processing, and high-gain instrumentation. In short, investing in the microphone potentially helps keep down the cost of the PR (+:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, mission control probably communicate over the radio day-in, and day-out. They can do without the overheads mentioned above. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;p.s. As I said, pure speculation. Feel free to down-vote!&lt;/p&gt;&#xA;" OwnerUserId="288" LastActivityDate="2015-03-01T14:47:29.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8312" PostTypeId="2" ParentId="8310" CreationDate="2015-03-01T15:02:27.853" Score="14" Body="&lt;p&gt;That &lt;em&gt;blue box&lt;/em&gt; is a wired receiver unit for the wireless microphone (they currently use a &lt;a href=&quot;https://www.facebook.com/media/set/?set=a.10151205084315583.438721.285915800582&quot; rel=&quot;noreferrer&quot;&gt;Sennheiser handheld with SKP-100 wireless transmitter&lt;/a&gt;, but they did or still do use also products from other companies, inc. e.g. Shure that I know they flew during some STS missions to the station). &lt;em&gt;The box&lt;/em&gt; also hosts its own microphone because the wireless microphone does need charging (EPO - Education &amp;amp; Public Outreach events usually last about half an hour or less, while they're in contact with CAPCOM throughout the working day on the station and the small battery pack it has wouldn't last that long without requiring a few battery swaps) and it might be awkward to use in some circumstances and not having to worry about a free-floating microphone might be easier too. For example, here's Karen Nyberg speaking into a microphone while conducting a session with the Advanced Colloids Experiment in the Destiny laboratory:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/fx1Vd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;NASA astronaut Karen Nyberg speaks into a microphone while conducting a session with the Advanced Colloids Experiment&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;(ACE-1) sample preparation at the Light Microscopy Module (LMM) in the Fluids Integrated Rack/Fluids Combustion Facility, on&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;June 24, 2013. Image: NASA&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;The box&lt;/em&gt; has two large buttons that while I can't seem to find any supporting document to their purpose can really only be to toggle the wired and wireless microphones on and off and adjust their volume during communications with &lt;a href=&quot;http://spacestationlive.jsc.nasa.gov/additional/index.html#capcom&quot; rel=&quot;noreferrer&quot;&gt;CAPCOM&lt;/a&gt; (Capsule Communicator):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;CAPCOM communicates through the use of voice loops (collections of&#xA;  audio communication from individuals which are tied together on an&#xA;  intercom through a satellite link). CAPCOM also coordinates crew&#xA;  communications with other spacecraft communicators supporting the&#xA;  mission which may be in other locations - such as Huntsville, Alabama;&#xA;  Munich, Germany; Tskuba, Japan or Moscow, Russia.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here's a photo of Shannon Walker on the station showing &lt;em&gt;the box&lt;/em&gt; with the two buttons and a microphone behind her a bit better:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/9/99/ISS-24_Shannon_Walker_in_front_of_the_robotic_workstation.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CEPZE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;NASA astronaut Shannon Walker, Expedition 24 flight engineer, is pictured near a robotic workstation in the Destiny laboratory of&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;the International Space Station. Credit: NASA, Source: &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:ISS-24_Shannon_Walker_in_front_of_the_robotic_workstation.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt; (&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/9/99/ISS-24_Shannon_Walker_in_front_of_the_robotic_workstation.jpg&quot; rel=&quot;noreferrer&quot;&gt;click for higher resolution&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in the video you link to, Samantha Cristoforetti would first have to reach for the receiver unit to enable both microphones to pass through the intercom voice loop anyway. But in majority of cases, having watched quite many public events from the station (available either live on &lt;a href=&quot;http://www.nasa.gov/multimedia/nasatv/&quot; rel=&quot;noreferrer&quot;&gt;NASA TV&lt;/a&gt; or recorded on &lt;a href=&quot;https://www.youtube.com/channel/UCLA_DiR1FfKNvjuUpBHmylQ&quot; rel=&quot;noreferrer&quot;&gt;NASA YouTube channel&lt;/a&gt;), station astronauts would mostly use the better quality wireless microphone.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-03-01T16:15:11.213" LastActivityDate="2015-03-01T16:15:11.213" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="8313" PostTypeId="2" ParentId="8308" CreationDate="2015-03-01T15:15:27.753" Score="6" Body="&lt;p&gt;&lt;strong&gt;No.&lt;/strong&gt; The surface of Saturn is not solid. Saturn is too hot to support solid ice, and not just because of solar radiation:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Also like Jupiter, Saturn gives off almost twice as much energy as it&#xA;  receives from the Sun, because it has its own internal heat source,&#xA;  powered by the &lt;a href=&quot;http://coolcosmos.ipac.caltech.edu/page/jupiter_saturn&quot; rel=&quot;nofollow noreferrer&quot;&gt;slow gravitational collapse&lt;/a&gt; that started when the&#xA;  planet first formed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Given that Uranus and Neptune have an &lt;a href=&quot;http://yellowmagpie.com/planet-saturn-about/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ice mantle&lt;/a&gt;, Saturn's sheer size and accretion must have extended the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/psns0.jpg&quot; alt=&quot;Jupiter and Saturn have no ice mantle&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any solid ice present on Saturn is high up in the atmosphere or deep beneath dense liquid gas along with the other solids. Any diamonds created by atmospheric pressure would eventually &lt;a href=&quot;http://www.bbc.com/news/science-environment-24477667&quot; rel=&quot;nofollow noreferrer&quot;&gt;melt into a liquid sea in the planet's hot core&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://phys.org/news/2015-03-dense-cores-giant-planets.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shock wave experiment provides the best look yet at 'Warm dense matter' at cores of giant planets&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2649" LastEditorUserId="2649" LastEditDate="2017-03-01T18:44:45.380" LastActivityDate="2017-03-01T18:44:45.380" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8314" PostTypeId="2" ParentId="8310" CreationDate="2015-03-01T15:42:50.507" Score="8" Body="&lt;p&gt;The high-fidelity microphones (as used in broadcast media, for example) are all about relaying the highest quality sound signal, and may compromise other aspects such as reliability, tolerance of extreme environments, battery life, etc. because the primary market for them can accommodate their limitations one way or another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, in space, reliability is king, along with minimizing weight. Mission control doesn't need to hear music from the ISS; as long as the microphone can reliably convey an intelligible speech signal, that's all that counts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hi-fi mikes are good for music (any decent one should handle something like 20Hz-20kHz, a typically quoted range of human hearing); the useful information in human speech is conveyed in a band from 300Hz-3kHz. Frequencies outside that band can be completely removed from a sound signal and we can understand the speech content within it. Getting good reproduction over 20-20kHz is not that easy, and a mike which can do it will probably need to be handled with some care. On the other hand, mikes capable of speech bandwidth can be made relatively easily and can be quite robust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For everyday communications, and especially when it's just got to work, the lower-fi higher-reliabilty mikes would win out. When it's a press event, makes sense to bring out the media mike, with the hi-rel always there to back it up.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-03-01T15:42:50.507" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8315" PostTypeId="2" ParentId="8304" CreationDate="2015-03-01T16:51:19.673" Score="3" Body="&lt;p&gt;There are some lunar phenomena that suggest volatile deposits beneath the lunar crust. Spudis believes &lt;a href=&quot;http://www.airspacemag.com/daily-planet/its-a-gas-man-2-101124825/?no-ist&quot; rel=&quot;nofollow&quot;&gt;LTP&lt;/a&gt;s might be from outgassing of such deposits. If such outgassings do occur, most of would spread across the moon's surface and eventually escape from the moon's gravity well. But perhaps some of these volatiles would find their way to the lunar cold traps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Francis McCubbins &lt;a href=&quot;http://www.space.com/8593-moon-100-times-water-study-suggests.html&quot; rel=&quot;nofollow&quot;&gt;re-analyzed lunar samples&lt;/a&gt; and found lunar water may be more common than was earlier thought. However his samples were still very dry -- 64 to 5 parts per billion. &lt;a href=&quot;http://www.bbc.com/news/science-environment-26696856&quot; rel=&quot;nofollow&quot;&gt;Dr. Boyce has argued&lt;/a&gt; that the lunar magma may be drier than what McCubbins estimated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar wind might be the source of some lunar water. The protons from the sun are hydrogen ions. Most of the lunar crust is oxygen compounds, for example silica and alumina. The hydrogen ions from the solar wind might interact with these oxygen compounds to make water. Some of these water molecules might find their way to the lunar cold traps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there are substantial volatiles at the lunar poles, I would bet they come from asteroid and comet impacts on the lunar surface. Some of the volatiles released at impact would spread across the lunar surface, most eventually escaping the moon. But some fraction of the impact volatiles might find their way to the cold traps and remain there when the 30 to 40 k temperatures turn them to ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, the origin of the Moon's polar volatiles remains an open question.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-03-01T16:51:19.673" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8316" PostTypeId="2" ParentId="8307" CreationDate="2015-03-02T00:08:20.260" Score="15" Body="&lt;p&gt;Yes. According to SpaceX; PICA-X, which is SpaceX's proprietary derivative of the PICA (Phenolic Impregnated Carbon Ablator) heatshield designed by NASA, can withstand reentries from &quot;Lunar and Martian Velocities&quot;. Martian velocities being even higher, of course. &lt;a href=&quot;http://www.spacex.com/news/2013/04/04/pica-heat-shield&quot;&gt;Source from SpaceX here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PICA itself was used on the Stardust spacecraft which reentered at a speed of 46,500km/h. Apollo craft generally reentered at a speed of ~40,000km/h. An Earth reentry from Mars would, depending on how much energy you choose to expend, be between 49,000km/h-77,000km/h. &lt;a href=&quot;http://www.wired.com/2012/07/interplanetary-reentry-tests-1966/&quot;&gt;Wired article&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another source is testimony &lt;a href=&quot;http://science.house.gov/sites/republicans.science.house.gov/files/documents/HHRG-114-SY16-WState-GReisman-20150227.pdf&quot;&gt;Garret Reisman offered&lt;/a&gt;, on page 4, where he says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Designed in partnership with NASA and fabricated by SpaceX, Crew Dragon’s heat shield is made of PICA-X, a high-performance improvement on NASA’s original phenolic impregnated carbon ablator (PICA). PICA-X is designed to withstand heat rates from a lunar return mission, which far exceed the requirements for a low Earth orbit mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If he said it in testimony, it must be true, right? The proof as always is in the pudding, so lets see them do it, but they think they can.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="51" LastEditDate="2015-03-02T01:13:31.063" LastActivityDate="2015-03-02T01:13:31.063" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8317" PostTypeId="1" AcceptedAnswerId="8318" CreationDate="2015-03-02T00:19:33.110" Score="22" ViewCount="6870" Body="&lt;p&gt;I have noticed that interior of the ISS appears to be very disorganized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me clarify what I mean by that. I grew up watching Star Trek, and the Enterprise was &lt;em&gt;the&lt;/em&gt; reference of clean space-faring vehicle. Things were in drawers, hidden. Or structured in a clean way. Computers were built into larger panels. There were touch screens. Have you noticed they always needed to uncover a panel to access some occasionally accessed system? It was minimalistic design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I look at the ISS, everything is everywhere, and it seems like nothing is attached firmly to the main frame. Things are just loosely attached to the wall and there are so many cables, devices, objects, ... that one wonders  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;how do they not cut themselves all day with some sharp edge&lt;/li&gt;&#xA;&lt;li&gt;how do they not break or make a cable loose &lt;/li&gt;&#xA;&lt;li&gt;how come those objects, machines, devices did not break already by bumping into each other a long time ago&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;given there is no stabilizing force in a form of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another aspect is psychological. How can those astronauts feel comfortable and focus on the job, when there is constant visual noise in their peripheral area?&lt;/p&gt;&#xA;" OwnerUserId="8610" LastEditorUserId="8610" LastEditDate="2015-03-09T20:46:53.377" LastActivityDate="2022-02-04T22:25:21.807" Title="Why does the interior of the ISS appear so disorganized?" Tags="|iss|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8318" PostTypeId="2" ParentId="8317" CreationDate="2015-03-02T00:58:08.953" Score="50" Body="&lt;p&gt;In Star Trek there is artificial gravity, and lots of staff. On the ISS there is neither. The crew spends a lot of time maintaining the station. They barely get time for science operations, with all the time they spend on maintenance. Going to a crew of 7, which Commercial Crew vehicles will enable (bringing 4 at a time, acting as a lifeboat for 4, instead of Soyuz with only capacity for 3), is expected to greatly increase available time for science. Just by adding one more body, on top of all the time the current staff of 6, will more than double available science time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is not a lot of storage space in the station, all things considered, and when cargo comes up on a vehicle, it comes in bulk, all at once. They track where everything is, else it gets lost in the scrum. The Dragon and Cygnus vehicles are mostly volume limited, but the ATV and HTV bring up a fairly larger amount all at once. It all has to go somewhere, and you need to keep track of where everything is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a &lt;a href=&quot;https://www.youtube.com/watch?v=doN4t5NKW-k&quot;&gt;great video of Sunnita Williams&lt;/a&gt; hunting for something.  She looked up the location on the computer, went to the specified location, opened a cabinet, and pulled out three layers deep of boxes. The item she wanted was in the box at the back.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus there is a great deal of order, hiding behind the seeming disorder.  Looks can be deceiving. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The inner storage areas of the station are really well organized. The visible surfaces definitely look like a hodge-podge, but they are taking advantage of available surface area to store things they use a lot.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-03-02T01:41:04.050" LastActivityDate="2015-03-02T01:41:04.050" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8319" PostTypeId="1" CreationDate="2015-03-02T01:07:36.470" Score="5" ViewCount="526" Body="&lt;p&gt;Garrett Reisman, in testimony made this &lt;a href=&quot;http://science.house.gov/sites/republicans.science.house.gov/files/documents/HHRG-114-SY16-WState-GReisman-20150227.pdf&quot; rel=&quot;noreferrer&quot;&gt;comment on page 4:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Additionally, a movable ballast sled allows the angle of attack to be actively controlled during entry to further provide precision landing control.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The comment is made in terms of controlling the reentry of the Dragon V2 crewed vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do other capsules use a similar system, or is this approach unique to SpaceX?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-09T01:39:18.047" Title="Have any other capsules used a movable ballast system for control?" Tags="|spacex|dragon|dragon-v2|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8320" PostTypeId="1" CreationDate="2015-03-02T04:15:02.130" Score="4" ViewCount="103" Body="&lt;p&gt;Watching SpaceX webcasts, I've noticed this particular view during the Falcon 9's flights. No one I've asked has any idea what's shown here. (Though one person commented that it looked like a Minecraft Nether Portal!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JzYhy.jpg&quot; alt=&quot;Screencast&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What view of Falcon 9 is depicted here?&lt;/p&gt;&#xA;" OwnerUserId="4603" LastActivityDate="2015-03-02T04:15:02.130" Title="What view of the Falcon 9 does this screenshot show?" Tags="|spacex|falcon-9|" AnswerCount="0" CommentCount="3" ClosedDate="2015-03-02T04:40:39.583" ContentLicense="CC BY-SA 3.0" />
  <row Id="8321" PostTypeId="2" ParentId="8304" CreationDate="2015-03-02T06:08:04.907" Score="6" Body="&lt;p&gt;What we know about lunar bedrock is mostly indirect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo and Luna missions returned samples collected from the surface, and missions such as &lt;a href=&quot;http://lunar.gsfc.nasa.gov/science.html&quot; rel=&quot;noreferrer&quot;&gt;LRO&lt;/a&gt; use spectroscopy to analyze the lunar surface. The LRO &lt;a href=&quot;http://diviner.ucla.edu/instrument.html&quot; rel=&quot;noreferrer&quot;&gt;Diviner&lt;/a&gt; instrument&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;is a multi-channel solar reflectance and infrared radiometer ... It has nine spectral channels which transmit different portions of the visible and infrared spectrum ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Analysis of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Geology_of_the_Moon#Regolith&quot; rel=&quot;noreferrer&quot;&gt;lunar soil&lt;/a&gt; returned showed it to be a mixture (as might be expected) showing local variation with chunks from elsewhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2010-303&quot; rel=&quot;noreferrer&quot;&gt;JPL&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One thing not apparent in the [Diviner] data is evidence for pristine lunar mantle material, which previous studies have suggested may be exposed at some places on the lunar surface. Even in the South Pole Aitken basin, also known as SPA, the largest, oldest, and deepest impact crater on the moon -- deep enough to have penetrated through the crust and into the mantle -- there is no evidence of mantle material.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Analysis of the fines in the returned samples showed capture of solar wind particles, yielding H, C and N in the &lt;a href=&quot;http://www.uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources15.pdf&quot; rel=&quot;noreferrer&quot;&gt;10-100 ppm range&lt;/a&gt; (pdf). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall analysis of the returned samples gives ground-truth context to the &lt;a href=&quot;http://lunar.gsfc.nasa.gov/science.html&quot; rel=&quot;noreferrer&quot;&gt;LRO&lt;/a&gt; data.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The crust of the Moon is made up of two main types of rock, anorthosite and basalt. Anorthosite is light in color because it is made up of a light-colored mineral called plagioclase feldspar. Basalt is dark in color because it contains the iron-bearing minerals pyroxene, olivine and ilmenite, along with volcanic glass. The iron present in these minerals tend to absorb light, making them appear dark. The light and dark areas on the Moon are composed of these rock types. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Plagioclase&quot; rel=&quot;noreferrer&quot;&gt;Plagioclase&lt;/a&gt; has a chemical composition of $\require{mhchem}\ce{NaAlSi3O8$ – $CaAl2Si2O8}$ as a repeating unit. &lt;a href=&quot;http://en.wikipedia.org/wiki/Pyroxene&quot; rel=&quot;noreferrer&quot;&gt;Pyroxene&lt;/a&gt; is variable but (imprecisely speaking) contains either Si or Al and three Oxygen atoms. &lt;a href=&quot;http://en.wikipedia.org/wiki/Olivine&quot; rel=&quot;noreferrer&quot;&gt;Olivine&lt;/a&gt; has a composition of $\ce{(Mg+2, Fe+2)2SiO4}$. &lt;a href=&quot;http://en.wikipedia.org/wiki/Ilmenite&quot; rel=&quot;noreferrer&quot;&gt;Ilmenite&lt;/a&gt; has a composition of $\ce{FeTiO3}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has this summary of &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Prospector&quot; rel=&quot;noreferrer&quot;&gt;Lunar Prospector&lt;/a&gt; data &lt;img src=&quot;https://i.stack.imgur.com/hcWIV.png&quot; alt=&quot;Relative concentration of various elements on the lunar surface (in weight %)&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;and also this comparison of uplands, lowlands, and Earth: &lt;img src=&quot;https://i.stack.imgur.com/PjtIb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of this just shows that there is &lt;strong&gt;plenty of Oxygen&lt;/strong&gt; and other useful minerals available, but &lt;strong&gt;precious little of other volatiles&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerDisplayName="user8406" LastEditorUserId="49" LastEditDate="2015-03-03T04:01:48.160" LastActivityDate="2015-03-03T04:01:48.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8322" PostTypeId="1" AcceptedAnswerId="8325" CreationDate="2015-03-02T11:04:59.670" Score="5" ViewCount="1289" Body="&lt;p&gt;In &lt;a href=&quot;http://science.house.gov/sites/republicans.science.house.gov/files/documents/HHRG-114-SY16-WState-GReisman-20150227.pdf&quot; rel=&quot;noreferrer&quot;&gt;this document&lt;/a&gt; (Page 7, point 4.3), Garrett Reisman notes that an analysis by Aerospace Corporation showed the most common causes for launch failures between 1980 and 1999. This document shows the top 3 causes as:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Engine failure&lt;/li&gt;&#xA;&lt;li&gt;Stage-separation failure&lt;/li&gt;&#xA;&lt;li&gt;Avionics failure&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Given that there are several new players with different designs introduced recently, I'm curious if those were still major causes in 2000 - 2014. Additionally, I wonder how the Space Shuttle affected these statistics (Given that it is a significantly different design from most other launch vehicles). Finally, I would like to know if the launch success/failure rate has gone up or down since 2000 - 2014 as compared to 1980 - 1999?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="6312" LastEditDate="2015-03-02T11:35:30.153" LastActivityDate="2015-03-02T16:00:17.847" Title="Most common causes of 21st century launch failures and how often do launches fail?" Tags="|launch|spacex|failure|reliability|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8323" PostTypeId="1" AcceptedAnswerId="8324" CreationDate="2015-03-02T15:48:35.290" Score="8" ViewCount="711" Body="&lt;p&gt;My understanding is that the ISS provides 75-90 kWatts of power, so enough power for a 1 second Megawatt pulse for zapping debris in about 15 seconds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question is: is this right, have I over looked something? And if so, what is in the way of them deploying such a system?&lt;/p&gt;&#xA;" OwnerUserId="8504" LastActivityDate="2016-08-23T19:54:30.430" Title="Power requirements for zapping debris with lasers from ISS" Tags="|iss|debris|laser|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8324" PostTypeId="2" ParentId="8323" CreationDate="2015-03-02T15:55:03.017" Score="6" Body="&lt;p&gt;Obviously the reason they have that amount of power on station is to use it to run the station.  So the first obvious math hour is that you cannot have all the power for 15 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely the notion of using extra power that is unconsumed is valid. The Ad Astra guys with &lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket#Testing_on_the_International_Space_Station&quot;&gt;VASIMR&lt;/a&gt; plan to scavenge extra power to charge the batteries for their engine so they can run reasonable tests of 15 minutes of their 200kW engine.  So not out of the question in principle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, once VASIMR is on station, if it happens, then that spare power is spoken for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Better question, is a 1 Megawatt laser enough to matter to debris? Two approaches:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Small stuff vaporize it.&lt;/li&gt;&#xA;&lt;li&gt;Larger stuff, let the emitted burst of gases act to slow it down, drop orbit, hoping it is enough for it to reenter.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-03-02T15:59:43.127" LastActivityDate="2015-03-02T15:59:43.127" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8325" PostTypeId="2" ParentId="8322" CreationDate="2015-03-02T16:00:17.847" Score="4" Body="&lt;p&gt;The study by Futron is in 2002 is &lt;a href=&quot;http://www.futron.com/upload/wysiwyg/Resources/Whitepapers/Space_Transportation_Costs_Trends_0902.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As commenters have noticed the trends have differed quite a bit in the 2000s, from the preceding decades. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would argue that foreign object debris ingested by engines is the bigger cause of failure.  That and incorrect assembly. Both of which are root caused by poor quality control. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Amusingly, SpaceX's Merlin engine acceptance tests requires them to drop a 5/16th inch nut into the fuel line and have the engine survive and spit it out. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-03-02T16:00:17.847" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8326" PostTypeId="1" AcceptedAnswerId="10217" CreationDate="2015-03-02T16:05:13.940" Score="8" ViewCount="313" Body="&lt;p&gt;I know star trackers are used in most cases for attitude determination of space craft traveling within the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what methods are currently used for coordinate determination, i.e. the location of the craft within the solar system? If possible, please explain how they work.&lt;/p&gt;&#xA;" OwnerUserId="7053" LastEditorUserId="25" LastEditDate="2015-07-24T14:49:08.773" LastActivityDate="2015-07-24T14:49:08.773" Title="Navigation within solar system" Tags="|probe|navigation|solar-system|deep-space|" AnswerCount="1" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8327" PostTypeId="1" CreationDate="2015-03-02T17:02:30.077" Score="1" ViewCount="88" Body="&lt;p&gt;SpaceX's website says that a Falcon 9 launch costs 61 million while a Falcon Heavy launch 85 million. Considering that the Falcon Heavy is almost three times bigger than the Falcon 9, I don't understand why is the difference in price so tiny.&lt;/p&gt;&#xA;" OwnerUserId="8606" LastActivityDate="2015-03-02T17:02:30.077" Title="Why is there only a small difference between Falcon 9 and Falcon Heavy launch prices?" Tags="|spacex|falcon-9|cost|falcon-heavy|" AnswerCount="0" CommentCount="2" ClosedDate="2015-03-02T17:09:00.300" ContentLicense="CC BY-SA 3.0" />
  <row Id="8328" PostTypeId="1" AcceptedAnswerId="8331" CreationDate="2015-03-02T17:27:57.943" Score="14" ViewCount="2893" Body="&lt;p&gt;I read that the Boeing CST-100 could be reused up to 10 times, but the heat shield should be changed each time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are my questions:&#xA;How many times can the Dragon V2 be reused? &#xA;Will it be reused in the NASA missions?&#xA;Will the Dragon's heat shield be reusable without any maintanance?&lt;/p&gt;&#xA;" OwnerUserId="8606" LastEditorUserId="51" LastEditDate="2015-03-02T17:35:39.327" LastActivityDate="2015-03-02T20:48:12.710" Title="Dragon V2: How many times can the spacecraft be reused? Is the spacecraft's heat shield reusable?" Tags="|spacex|reuse|dragon|heat-shield|dragon-v2|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8329" PostTypeId="2" ParentId="8328" CreationDate="2015-03-02T17:34:58.407" Score="7" Body="&lt;p&gt;The CST-100 heat shield is replaced each flight because it is discarded as part of the landing process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The capsule has airbags, between the heat shield and bottom of the capsule.  As the parachutes slow it down for landing, it discards the heat shield, inflates the airbags, and lands on the ground. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus they can never reuse a heat shield, they throw it away on each flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon V2 on the other hand will start by landing under parachutes in the water, then under parachutes on land with minor SuperDraco firings, and finally fully powered on land with the other two approaches as backup modes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has strongly suggested that the heat shield will be reusable but of course they have yet to demonstrate that by reflying a used Dragon V1 capsule. They have spent the engineering resources to prove it, and they have 7 recovered heat shields to examine, so they likely have very good data on the issue. Thus they should be able to have a confidence level that it will work but the proof is in the pudding. Lets see one fly 2 or 3 times and how they stand up at that point. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-03-02T17:34:58.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8330" PostTypeId="1" AcceptedAnswerId="8461" CreationDate="2015-03-02T18:52:42.023" Score="28" ViewCount="31156" Body="&lt;p&gt;I don't know if this was already asked, but I'm very curious about the cost breakdown of a Falcon 9 launch.&lt;/p&gt;&#xA;" OwnerUserId="8606" LastEditorUserId="51" LastEditDate="2015-03-02T18:54:08.610" LastActivityDate="2016-01-31T12:10:36.130" Title="What is the cost breakdown for a Falcon 9 launch?" Tags="|spacex|falcon-9|cost|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8331" PostTypeId="2" ParentId="8328" CreationDate="2015-03-02T20:24:58.040" Score="14" Body="&lt;p&gt;Musk indicated that Dragon 2 could be reused &quot;with minimal rework and fueling&quot; about 10 times between overhauls. This is the target number but these are early days. The previous ablative shield was dumped in favor of a more advanced design. The goal then, for heat shield, engines and other major components would be at Least 10 flights, in theory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regards NASA, they will Not be reusing the capsule in the first few flights at least. (There will be 12 manned flights under the new contract). They will not likely reuse the capsule with people aboard until a large number of tests prove reusability is safe. Precisely what level of confidence Will be required has not yet been determined. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is likely that they may &quot;reuse&quot; Dragon 2s to deliver cargo to ISS as ISS cargoes are relatively cheap and non mission critical (they keep 4-6 months of critical stores on the ISS at all times). This will allow Dragon 2 to build up historical data on reuse. Note that NASA has already paid Spacex for 12 cargo flights - most will be used up by the end of 2015. By 2017, when Dragon2 flies, they might see the used capsules as a cheap way of resupplying the ISS.&lt;/p&gt;&#xA;" OwnerUserId="8616" LastEditorUserId="8616" LastEditDate="2015-03-02T20:48:12.710" LastActivityDate="2015-03-02T20:48:12.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8332" PostTypeId="1" AcceptedAnswerId="8333" CreationDate="2015-03-03T08:43:56.390" Score="6" ViewCount="1527" Body="&lt;p&gt;Looking at the launches and animations for the Falcon 9, you can clearly see some rockets firing at the top/side of the stage to maneuver it clear of the second stage and orient it correctly for return &amp;amp; landing. None of the documentation I've looked at ever mention those engines. I would think, given SpaceX's general preference to remain consistent, that they are Draco engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions are: What kind of engines are these, how many are there, how much fuel are required for them and where are they located?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="51" LastEditDate="2015-03-03T12:46:00.617" LastActivityDate="2015-03-03T12:46:00.617" Title="What auxiliary engines do the Falcon 9 first stage have?" Tags="|spacex|engines|falcon-9|attitude|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8333" PostTypeId="2" ParentId="8332" CreationDate="2015-03-03T12:45:33.523" Score="6" Body="&lt;p&gt;SpaceX controls the reentry of a Falcon 9 first stage with at least three different approaches.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Main engine gimballing&lt;/li&gt;&#xA;&lt;li&gt;Cold Gas Nitrogen thrusters&lt;/li&gt;&#xA;&lt;li&gt;Grid Fins on the interstage&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;After the main engine of a first stage is complete, the cold gas thrusters rotate the stage into the direction of flight which is quite a feat considering its size. But it mostly out of the atmosphere so it is possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three of the main engines re-ignite and decelerate the forward motion vector of the stages flight path. Depending on the mission profile, either enough to return to the launch site, or just enough to land on the ASDS barge, per-positioned downrange underneath it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cold gas thrusters then reorient the stage as needed so it is pointy end down (Engines pointing down) so that when it hits the thicker atmosphere it is most aerodynamic to survive the heating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They fire a single engine (center engine) again to slow down the interface with the heavier atmosphere so that the stage can survive it. The cold gas thrusters are used for attitude control throughout this process as needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then they start using the grid fins to fine tune the flight path to the designated landing location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally in the last few seconds of flight the center engine ignites again and tries to hover slam the landing. They have too much thrust for the remaining weight, so they cannot hover, rather they need to time the landing so that they hit 0 altitude with 0 velocity. Thus the term hover slam, which they practiced with the Grasshopper vehicles at the McGregor, TX test site.  (Also tested the grid fins on that vehicle.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whenever the main engines (Merlin 1D) are firing they can gimble and offer some level of control. But it is limited as was seen in the first ASDS landing attempt. They lost grid fins hydraulic fluid and came in off course and the main engine was desperately trying to correct on its own and failed.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-03-03T12:45:33.523" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8335" PostTypeId="1" AcceptedAnswerId="11978" CreationDate="2015-03-03T14:59:06.003" Score="16" ViewCount="6070" Body="&lt;p&gt;PICA-X is a SpaceX proprietary variant on the NASA Ames developed &lt;a href=&quot;http://www.nasa.gov/centers/ames/news/releases/2008/08_43AR.html&quot; rel=&quot;noreferrer&quot;&gt;PICA&lt;/a&gt; (Phenolic Impregnated Carbon Ablator) heat shield material. NASA &lt;a href=&quot;http://www.nasa.gov/exploration/commercial/cargo/spacex_heatshield.html&quot; rel=&quot;noreferrer&quot;&gt;quotes&lt;/a&gt; decades of rigorous research, testing and development that went into it, and on this photograph we see PICA tested with the use of NASA Ames' arcjet:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Y9N6p.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;NASA Ames' arcjets were integral to the testing of the PICA thermal protection material. Credit: NASA Ames&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX additionally worked on it with &lt;a href=&quot;http://www.nasa.gov/centers/ames/about/people/rasky.html&quot; rel=&quot;noreferrer&quot;&gt;Dan Rasky&lt;/a&gt;, one of the original developers of the PICA material, to develop the new PICA-X material. But how does PICA-X differ from its predecessor PICA, which, while it has some fantastic thermal properties, is really light and an improvement over FiberForm&lt;sup&gt;®&lt;/sup&gt;, is also &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015019.pdf&quot; rel=&quot;noreferrer&quot;&gt;structurally rather weak&lt;/a&gt; (PDF) and can still grow fractures in the matrix when subjected to thermal and mechanical loads during both launch and atmospheric entry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/xv5Dp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Matrix cracking in charred PICA samples. Source: &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015019.pdf&quot; rel=&quot;noreferrer&quot;&gt;Fracture in Phenolic Impregnated Carbon Ablator&lt;/a&gt;, NASA Ames Moffett Field, 2011&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To what degree has SpaceX managed to improve on PICA material characteristics with their proprietary PICA-X? Has it been tested for typical stress loads aboard Falcon 9 to LEO and reentry, and what were the results in terms of both comparing PICA and PICA-X and potential reuse cycle?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize that I'm asking about proprietary technology that not much might be known about yet, but I'm looking for stress test results of a material that will support human spaceflight in the near future and surely SpaceX will have much reassuring to do that their technology is safe and performs as expected also in real life. So I expect at least some research of PICA-X eventually also published. Please no speculations, I'm perfectly happy to wait for answers that can be supported by facts and references, once they become available.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-03-03T15:45:55.303" LastActivityDate="2015-09-12T14:25:46.067" Title="What improvements have SpaceX made to their proprietary Phenolic Impregnated Carbon Ablator (PICA-X) heat shield?" Tags="|spacex|technology|materials|dragon|heat-shield|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8336" PostTypeId="2" ParentId="8226" CreationDate="2015-03-03T16:05:55.507" Score="0" Body="&lt;p&gt;I would offer as an answer regarding the use of NASA research by commercial space firms, that the Merlin engines use a pintle type injector in the combustion chamber. The Apollo Lunar Excursion Module used a pintle injector in the descent engine. Pintle design is a lot different than plate injectors traditionally used. Such design allows for thrust modulation over a 10:1 ratio. This design was analyzed and tested extensively by NASA in a major effort for the Apollo program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems unlikely to me that SpaceX could have replicated or redone from scratch all the massive effort that went into the LEM Decent engine pintle injector design. It is more likely that SpaceX simply scaled the design based upon the NASA work efforts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LEM descent engine used storable propellants (A50/NTO) and Merlin uses LOX/hydrocarbon (RP1) so additional analyses and testing was necessary, but there was a massive amount of reports from NASA to be used as a starting point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems unlikely to me that SpaceX would have come up with the concept of a pintle injector approach at all without investigating the NASA Apollo efforts and having that data and analyses available.&lt;/p&gt;&#xA;" OwnerUserId="6501" LastEditorUserId="49" LastEditDate="2015-03-03T17:46:30.807" LastActivityDate="2015-03-03T17:46:30.807" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8337" PostTypeId="1" AcceptedAnswerId="8339" CreationDate="2015-03-03T18:23:36.110" Score="4" ViewCount="415" Body="&lt;p&gt;An &lt;a href=&quot;https://space.stackexchange.com/questions/845/without-renewable-resources-how-do-you-wipe-in-space/852#852&quot;&gt;answer&lt;/a&gt; and a &lt;a href=&quot;https://space.stackexchange.com/questions/845/without-renewable-resources-how-do-you-wipe-in-space#comment1412_845&quot;&gt;comment&lt;/a&gt; on a related question, got me wondering. What would happen if you tried to use a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bidet&quot; rel=&quot;nofollow noreferrer&quot;&gt;bidet&lt;/a&gt; to clean up after doing your business without gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water does not behave in space the same as it does on Earth. Could a bidet be used to clean up without making a mess in the ISS? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Watch Water Become Zero-Grav Jello on the International Space Station - ICTMN.com&#xA;&lt;img src=&quot;https://i.stack.imgur.com/0sIjZ.jpg&quot; alt=&quot;Water&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-03-04T03:33:56.933" Title="Could a bidet be used effectively in free fall?" Tags="|microgravity|water|astronaut-lifestyle|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8339" PostTypeId="2" ParentId="8337" CreationDate="2015-03-03T22:42:20.790" Score="7" Body="&lt;p&gt;A bidet can be used in micro-gravity, if you isolate the lower torso with a velcro-fitted cover and employ a vacuum hose to recover spent water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this is rather inefficient in terms of water and electricity consumption. All of the water would be considered wastewater/&quot;grey&quot; water and will have to go through a series of filters and tanks/canisters. Considering the (substantial) burden of managing toilets aboard the ISS, this would reduce crew time available for science. Wet wipes are more cost-effective right now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of engineering required, a functioning bidet will be akin to a working shower. A shower, a bidet, re-usable &lt;a href=&quot;https://space.stackexchange.com/questions/653/washing-clothes-in-space-are-there-any-technical-challenges&quot;&gt;washable&lt;/a&gt; garments and more comfortable ways of urinating and taking a dump are the basic elements of ECLSS of the future, where humans won't have to &lt;strong&gt;endure&lt;/strong&gt; space travel but to live and relish it.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-03-04T03:33:56.933" LastActivityDate="2015-03-04T03:33:56.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8341" PostTypeId="1" AcceptedAnswerId="8342" CreationDate="2015-03-03T23:36:12.677" Score="12" ViewCount="7357" Body="&lt;p&gt;If my understanding of the General Theory of Relativity is correct, according to the Equivalence Principle, forces due to gravity and acceleration are indistinguishable.&lt;/p&gt;&#xA;&lt;p&gt;If that's the case, then an accelerometer on Earth's surface would always read&#xA;&amp;quot;acceleration&amp;quot; of &lt;span class=&quot;math-container&quot;&gt;$9.8 \frac{\text{m}}{\text{s}^2}$&lt;/span&gt; in vertical direction due to Earth's gravity. Is my train of thought correct, or am I completely out to lunch?&lt;/p&gt;&#xA;&lt;p&gt;OK, moving on. Now, if I wanted to devise a very precise INS (inertial navigation system) for interplanetary travel, I would need to constantly account for &amp;quot;false acceleration&amp;quot; due to gravity of nearby (or all?) planets and their moons within the Solar System. Is that right?&lt;/p&gt;&#xA;&lt;p&gt;Could someone please explain?&lt;/p&gt;&#xA;&lt;p&gt;NB: I am aware of error accumulation inherent in INS, but this is not what this question is about.&lt;/p&gt;&#xA;" OwnerUserId="7053" LastEditorUserId="47149" LastEditDate="2022-06-05T05:14:04.083" LastActivityDate="2022-06-05T05:14:04.083" Title="Accelerometer in space" Tags="|gravity|interplanetary|navigation|physics|measurement|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8342" PostTypeId="2" ParentId="8341" CreationDate="2015-03-04T00:03:48.497" Score="16" Body="&lt;p&gt;Yes, a properly functioning accelerometer that is stationary relative to the surface of the Earth will read the acceleration due to gravity.  If it's a very good accelerometer, you could also see the factor of a few hundred smaller decrement in that acceleration due to the centripetal acceleration from the rotation of the Earth.  If it's a really &lt;em&gt;really&lt;/em&gt; good accelerometer, you could also see the tidal force due to the Moon, about seven orders of magnitude down from the surface acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An accelerometer in the spacecraft, at least if it's at or close enough to the center of gravity of the spacecraft, will read exactly zero if the spacecraft is in free fall.  I.e., not firing any thrusters.  The accelerometer is useful during rocket firings to get a measure of the actual acceleration during the burn.  (This is usually improved upon later with navigational tracking.)  But useless in free fall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will not detect the gravity of nearby objects with an accelerometer.  It will read zero the entire time you are moving through the vacuum.  With one caveat, which is again if it's a really &lt;em&gt;really&lt;/em&gt; good accelerometer, and you swing by very close to a large body, and you put the accelerometer as far from the center of gravity of the spacecraft as you can, then you &lt;em&gt;might&lt;/em&gt; be able to detect the tidal force from the body for a short time.  That force drops off as the cube of distance as opposed to the square, so it would be very difficult to detect.  In any case, you would not be able to navigate with that, since if you're already that close to the body then it's too late to correct your trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, you cannot navigate a spacecraft, e.g. correcting errors in your trajectory, without, effectively, looking out the window.  Either with optical or radio tracking data of some kind.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="49" LastEditDate="2015-03-04T02:02:02.073" LastActivityDate="2015-03-04T02:02:02.073" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8344" PostTypeId="1" CreationDate="2015-03-04T02:07:53.053" Score="7" ViewCount="1168" Body="&lt;p&gt;Why aren't orbital vehicles made of plastic? Some type of plastic have good characteristic such as airloys which have low thermal conductivity, good pressure resistance and very low density. Why don't we use those materials to build space vehicles? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Lets narrow down the question to airloys which I think it's considered a plastic material. &lt;/p&gt;&#xA;" OwnerUserId="8623" LastEditorUserId="2981" LastEditDate="2015-03-04T10:32:36.603" LastActivityDate="2015-03-06T14:45:34.693" Title="Why aren't orbital vehicles made of plastic?" Tags="|materials|construction|engineering|launch-vehicle|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8345" PostTypeId="2" ParentId="8341" CreationDate="2015-03-04T03:38:57.473" Score="3" Body="&lt;p&gt;There's already an answer, but to phrase it in a different way:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Accelerometers always measure acceleration with respect to a &quot;body&quot; frame, usually the body of the accelerometer.  On the ground, the accelerometer is virtually fixed in place, so any accelerations show up unchanged.  On a spacecraft, the frame is accelerating (because the spacecraft is in orbit/free fall), so the difference in acceleration is very small.&lt;/p&gt;&#xA;" OwnerUserId="3532" LastActivityDate="2015-03-04T03:38:57.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8346" PostTypeId="2" ParentId="8341" CreationDate="2015-03-04T05:17:07.407" Score="-3" Body="&lt;p&gt;I found this description on Wikipedia. An accelerometer is a device that measures proper acceleration (&quot;g-force&quot;). Proper acceleration is not the same as coordinate acceleration (rate of change of velocity). For example, an accelerometer at rest on the surface of the Earth will measure an acceleration g= 9.81 m/s2 straight upwards. By contrast, accelerometers in free fall orbiting and accelerating due to the gravity of Earth will measure zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was basically mentioned in the answers above. However, I would like to add that if we are talking about a single sensor accelerometer, and the accelerometer rests on the earth at an angle it will read less than 9.81m/s2. For example, if the accelerometer rests at a 45 deg. angle the reading will be 9.81 x cosine 45 which is roughly 9.81 x 0.7. Lesser angles will read less acceleration. And when the accelerometer rests parallel to the earth surface, it will read zero. I can see where accelerometers can present problems in certain situation, and corrections will have to be calculated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a rocket is going into an earth orbit, the effect of gravity on the accelerometer will lessen as the rocket is making it's turn from a vertical position to the horizontal position to the earth surface while still accelerating. A 3-axis accelerometer can overcome the directional problems, however gravity will still have an effect on the reading and must be taken into consideration.&lt;/p&gt;&#xA;" OwnerUserId="8558" LastEditorUserId="8558" LastEditDate="2015-03-07T05:13:37.397" LastActivityDate="2015-03-07T05:13:37.397" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8348" PostTypeId="1" AcceptedAnswerId="8359" CreationDate="2015-03-04T09:08:54.003" Score="2" ViewCount="127" Body="&lt;p&gt;ESA publishes quite a few images from the 67P comet, sent by Rosetta, from various distances away to collect as much as possible and as varied as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I write, Rosetta is 6 Km away from the comet, and can even take clear pictures of its own shadow on the comet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The comet does spin though, and dust may prevent Rosetta from getting too close or entering some cone formed by the comet's tail (there are related questions).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How close could Rosetta theoretically get to the comet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question may not be so easy to answer, as the comet is not spherical, and its spin may not be planar.&lt;/p&gt;&#xA;" OwnerUserId="8383" LastEditorUserId="8383" LastEditDate="2015-03-05T11:37:59.957" LastActivityDate="2015-03-05T12:38:41.487" Title="How close can Rosetta come to the 67P/Churyumov-Gerasimenko comet?" Tags="|rosetta|navigation|comet|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8349" PostTypeId="2" ParentId="8341" CreationDate="2015-03-04T09:59:57.143" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;If that's the case, then an accelerometer on Earth's surface would always read &quot;acceleration&quot; of 9.8m/s^2 in vertical direction due to Earth's gravity. Is my train of thought correct, or am I completely out to lunch?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You are correct, but only to two decimal places of accuracy. An accelerometer that gives only two decimal places of accuracy is an extremely lousy accelerometer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At sea level and at the equator, acceleration due to gravity is 9.8142 m/s$^2$, directed toward the center of the Earth. An accelerometer will register an acceleration of only 9.7803 m/s$^2$, directed upward. These differ in magnitude by 0.0339 m/s$^2$, or about 0.34%. (Note well: They differ in direction by 180 degrees.) Even a lousy accelerometer is good to three places of accuracy, so this variation of 0.34% is within the realm of detection by a mediocre accelerometer (but not an extremely lousy accelerometer).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a Newtonian perspective, an ideal accelerometer measures the acceleration due to the net sum of all real forces except gravity. What about fictitious forces such as the centrifugal and Coriolis forces? They don't count from a Newtonian perspective. They're fictitious forces. Ideal accelerometers don't sense those observer-dependent fictitious forces. What about gravity (which is a real force in Newtonian mechanics)? Accelerometers don't detect that, either. That's the only real force accelerometers don't detect from a Newtonian perspective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a relativistic perspective, an ideal accelerometer measures &lt;a href=&quot;http://en.wikipedia.org/wiki/Proper_acceleration&quot; rel=&quot;nofollow&quot;&gt;proper acceleration&lt;/a&gt;. Gravitation is a fictitious force in general relativity. A simple way to put this: An ideal accelerometer measures the acceleration due to the net sum of all real forces, period. Another way to put this: accelerometers measure all real forces except gravitation (the same explanation as the Newtonian explanation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some subtleties that the above ignores. @Rikki-Tikki-Tavi mentioned frame dragging in a comment. Unless you are orbiting a rotating black hole, frame dragging is exceptionally small. One would need an extremely sensitive accelerometer to sense frame dragging, and even then only after a whole lot of analysis. &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_Probe_B&quot; rel=&quot;nofollow&quot;&gt;Gravity Probe B&lt;/a&gt; was intended to measure those relativistic effects. This experiment was at best a partial success. The subtleties to which @Rikki-Tikki-Tavi alluded are very, very small for the Earth. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Now, if I wanted to devise a very precise INS (inertial navigation system) for interplanetary travel, I would need to constantly account for &quot;false acceleration&quot; due to gravity of nearby (or all?) planets and their moons within the solar system. Is that right?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Since accelerometers do not detect gravitation, deducing the evolution of a spacecrafts state (position and velocity) means that a spacecraft needs decent gravity models of the objects that affect the spacecraft gravitationally. This is called deduced reckoning, or &lt;a href=&quot;http://en.wikipedia.org/wiki/Dead_reckoning&quot; rel=&quot;nofollow&quot;&gt;dead reckoning&lt;/a&gt; for short.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a good reason for the change from ded(uced) to dead. A spacecraft that only relies on dead reckoning alone is soon to be dead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mentioned &quot;ideal accelerometers&quot; multiple times above. Real accelerometers differ from the ideal in a number of ways. A real accelerometer might report an acceleration of 1.001 m/s$^2$ when it should have reported an acceleration of 1 m/s$^2$, and an acceleration of 2.002 m/s$^2$ when it should have reported an acceleration of 2 m/s$^2$. This is a scale factor error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take an accelerometer apart and you'll see multiple devices that ideally measure acceleration in three orthogonal directions. In practice, they don't. Real accelerometers have a non-orthogonality error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scale factor and non-orthogonality are examples of the systematic errors associated with an accelerometer. Systematic errors can be addressed. A decent navigation system will try to estimate these systematic errors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What can't be addressed are random errors. Accelerometers are inherently noisy devices, ideally white noise. Integrated white noise results in a random walk. The deduced velocity vector takes a random walk from the true velocity vector. Flight software computes position by integrating velocity. This results in an integrated random walk error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over time, this can result in very bad estimates of position. That's why dead reckoning means you are dead. The only way to overcome this is to have an alternate measure of position. GPS does a very nice job for vehicles in low Earth orbit. Something else is needed for vehicles beyond LEO.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-03-04T09:59:57.143" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8350" PostTypeId="1" AcceptedAnswerId="8360" CreationDate="2015-03-04T11:08:49.367" Score="2" ViewCount="1605" Body="&lt;p&gt;Given a set of orbit elements (assume inclination = 0, arg perigee = 0, RAAN = 0, e = known, and a = known), combined with a radius position and a velocity in orbit I should be able to forward propagate the position of the satellite by a given time t.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My current method:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$n = \sqrt{\frac{\mu}{a^3}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$E_0 = \mathrm{acos}((1-\frac{R}{a})\cdot(\frac{1}{e}))$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$M_0 = E_0 - e \cdot \mathrm{sin}(E_0)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$M_1 = M_0 + n \cdot t$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$E_1 = M_1 + e \cdot \mathrm{sin}(E_1)$ &amp;lt;- for this I take an initial value of $E_1 = M_1$ and iterate&lt;/p&gt;&#xA;&#xA;&lt;p&gt;future positions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$R = a \cdot (1-e \cdot \mathrm{cos}(E_1))$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$TrueA = 2 \cdot \mathrm{atan}(\sqrt{\frac{1+e}{1-e}} \cdot \mathrm{tan}(\frac{E_1}{2}))$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V = \sqrt{\mu \cdot (2/R-1/a)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$fpa = \mathrm{atan}\frac{e \cdot \mathrm{sin}(TrueA)}{1+e \cdot \mathrm{cos}(TrueA)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm reasonably confident on every step except the finding the initial eccentric anomaly ($E_0$) and as it turns out this returns an error. The root of the error is trying to find the $\mathrm{acos}$ of a number less than -1 (first value I calculate is -1.0000000023241). I'm unsure of why this is occurring and any advice/help/alternative equations would be a great help.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="168" LastEditDate="2015-03-04T11:21:03.470" LastActivityDate="2015-03-05T09:39:36.997" Title="How to find eccentric anomaly for orbit propagation" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8351" PostTypeId="2" ParentId="5961" CreationDate="2015-03-04T13:15:36.920" Score="3" Body="&lt;p&gt;The people who build modern pads, know the power of the rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flame trenches are designed to take the engine exhaust, use water to protect it from heat and sound damage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are umbilicals that are much softer, and are often easier to replace than try to harden. SpaceX learned this the hard way, through trial and error on their first few launches.  They had to toughen up parts of the pad they thought would not need it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus a properly designed, modern launch facility will have most of its fixed structures survive with minimal work required, and possibly need some softer elements simply replaced.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-03-04T13:15:36.920" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8352" PostTypeId="2" ParentId="5410" CreationDate="2015-03-04T13:19:31.417" Score="6" Body="&lt;p&gt;It appears the answer is, they do not, at LC-40. LC-39A has much more room (Enough for 5 cores) and can process a Falcon Heavy (3 cores) and a Falcon 9 at the same time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, one core at a time is processed for the next flight.  The obvious downside is that a delay in one flight ripples down the schedule. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For payloads they have a clean room at the back where they can process a satellite or Dragon capsule for encapsulation in a shroud (if needed).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have access to the SPIF (Spacecraft Integration Facility) down the road as discussed with lots of pictures in this question and answer: &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/3288/what-is-the-alternate-hangar-spacex-has-available-at-the-cape&quot;&gt;What is the alternate hangar SpaceX has available at the Cape?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SPIF can be used to process a second payload, to be ready once a first/second stage core combo is available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They appear to be planning on leaving this mode running, and just move to 4 parallel launch sites.  LC-40 (Current), LC-39A (Manned flights), Boca Chica, TX (GEO flights), and Vandenburg SLC-4W (Polar orbits).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-11-06T18:26:20.947" LastActivityDate="2017-11-06T18:26:20.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8353" PostTypeId="2" ParentId="8344" CreationDate="2015-03-04T15:46:33.120" Score="0" Body="&lt;p&gt;Why should they use plastic? &#xA;It doesn't have tensile strength, It doesn't resist temperatures, it doesn't resist radiations, it is fragile. There is really no reason to use plastic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least you can use composite materials (and they are starting doing that), but it's not plastic. &lt;/p&gt;&#xA;" OwnerUserId="8628" LastActivityDate="2015-03-04T15:46:33.120" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8354" PostTypeId="2" ParentId="8344" CreationDate="2015-03-04T16:43:36.013" Score="9" Body="&lt;p&gt;The reason is quite simple. Plastics have a lower &lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_strength&quot; rel=&quot;nofollow&quot;&gt;specific strength&lt;/a&gt; (strength to weight ratio) than metal alloys, which in turn have a lower specific strength than various carbon composites. There are some other reasons too, such as resistance to temperature extremes, a spaceship is exposed both to high temperatures and extreme cold and metals will generally deal with this better than plastics. But just the strength to weight ratio is reason enough to prefer carbon composites or metal over plastic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving onto airloy/aerogels. They &lt;em&gt;are&lt;/em&gt; being used or developed by Nasa, as in &lt;a href=&quot;http://www.gizmag.com/polymer-aerogel-stronger-flexible-nasa/23955/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; or &lt;a href=&quot;http://www.nasa.gov/topics/technology/features/aerogels.html#.VPcVl3WUdNA&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, including for use as inflatable heat shields. The extremely high insulative value of aerogels would make them ideal for such a purpose, if made out of an appropriate substance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also some plastics already have very high applicability for use in space. One example is Kevlar which has &lt;a href=&quot;http://www.explainthatstuff.com/kevlar.html&quot; rel=&quot;nofollow&quot;&gt;many excellent properties&lt;/a&gt;, Kevlar has a specific tensile strength 8x greater than steel and is moderately resistant to both heat and cold. Because of it's great tensile strength Kevlar is often used in &lt;a href=&quot;http://en.wikipedia.org/wiki/Whipple_shield&quot; rel=&quot;nofollow&quot;&gt;whipple shields&lt;/a&gt;. Kevlar and other high-strength plastics are ideal materials to be used for future &lt;a href=&quot;http://en.wikipedia.org/wiki/Inflatable_space_habitat&quot; rel=&quot;nofollow&quot;&gt;inflatable space habitats&lt;/a&gt;. Kevlar would also be ideal for tethers. The weakness of kevlar is very low compressive strength and it is moderately vulnerable to degradation from UV radiation. Plastics are also more permeable than metal due to the long molecular chains. Composite materials can exploit the best properties of different materials, and most composites have a plastic component.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2015-03-06T12:26:34.723" LastActivityDate="2015-03-06T12:26:34.723" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8355" PostTypeId="1" AcceptedAnswerId="8357" CreationDate="2015-03-04T17:08:35.740" Score="11" ViewCount="2658" Body="&lt;p&gt;The Space Shuttle had several aerodynamic control surfaces to control it during reentry and the final phase of atmospheric flight to landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QgPeR.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems a waste to haul tons of control surface to orbit without making some use of them to control the vehicle during ascent. However, the Shuttle’s control surfaces were obviously optimised to be used in a glide, and with the centre of mass of the vehicle in a very different place compared to ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, were the shuttle’s aerodynamic control surfaces of used during launch and ascent? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was it even possible (or advisable) to use the control surfaces during launch?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-03-05T12:14:15.540" LastActivityDate="2015-03-05T12:14:15.540" Title="Were the aerodynamic control surfaces of the Space Shuttle used during ascent?" Tags="|launch|space-shuttle|guidance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8356" PostTypeId="2" ParentId="8350" CreationDate="2015-03-04T17:36:03.720" Score="0" Body="&lt;p&gt;At the furthest from the primary, $R=a(1+e)$, so the argument of your $\mathrm{acos}$ should be exactly $-1$.  It should never get larger. Your computation is $(1-(1+e))\cdot \frac 1e$, which has numeric cancellation if $e \ll 1$. Slight numeric inaccuracy between your values of $R,a,e$ can also be the problem.  As suggested, you might just use if statements to bring the value inside $[-1,1]$&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2015-03-04T17:36:03.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8357" PostTypeId="2" ParentId="8355" CreationDate="2015-03-04T19:42:40.017" Score="16" Body="&lt;p&gt;They &lt;em&gt;were&lt;/em&gt; used, but not to alter the flight path - rather, to reduce aerodynamic loads on the vehicle structure, particularly hinge moment loads on the elevons.  This is known as &lt;a href=&quot;https://books.google.com/books?id=shajBQAAQBAJ&amp;amp;lpg=PA124&amp;amp;ots=HhCTNavcLO&amp;amp;dq=space%20shuttle%20structural%20load%20relief%20ascent&amp;amp;pg=PA124#v=onepage&amp;amp;q=space%20shuttle%20structural%20load%20relief%20ascent&amp;amp;f=false&quot;&gt;load relief&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2015-03-04T19:42:40.017" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8358" PostTypeId="1" AcceptedAnswerId="8361" CreationDate="2015-03-05T03:22:09.300" Score="8" ViewCount="13816" Body="&lt;p&gt;There are plenty of photos of rocket tests and launches in atmosphere, and in these the exhaust plume tends to be a long thin flame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this true for when rocket engines operate in a vacuum? My guess would be that they would be more sort of 'bulb' shaped or similar as the gasses rapidly expand in a vacuum, however I couldn't find any images to prove/disprove this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do rocket engines operating in vacuum not produce as much light and are therefore harder to photograph?&lt;/p&gt;&#xA;" OwnerUserId="8629" LastEditorUserId="49" LastEditDate="2015-03-05T14:46:18.120" LastActivityDate="2021-05-24T21:47:20.940" Title="What does the exhaust plume of a rocket look like in vacuum?" Tags="|rockets|physics|vacuum|exhaust|plume|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8359" PostTypeId="2" ParentId="8348" CreationDate="2015-03-05T08:54:16.610" Score="3" Body="&lt;p&gt;The comet's gravitational force is so small that Rosetta often &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_swoops_in_for_a_close_encounter&quot; rel=&quot;nofollow noreferrer&quot;&gt;doesn't really orbit the comet&lt;/a&gt;. When Rosetta's distance from the comet gets too large (~100 km), it has to &lt;a href=&quot;https://space.stackexchange.com/questions/5126/is-rosetta-really-in-orbit-around-67p-or-just-conveniently-co-located?rq=1&quot;&gt;use its thrusters&lt;/a&gt; to adjust its trajectory every so often, and its path looks like a slightly curved triangle.&lt;br&gt;&#xA;ESA can choose pretty much any orbit they want. In practice, they'll try to make sure Rosetta is far enough away not to be damaged by the gas and dust outflow. They've got 20 years and a pile of money invested, they don't want to lose the spacecraft. The comet is expected to become more active as it's getting closer to the Sun, the recent pass at 6 km was a &quot;last chance&quot; to get this close. It's unlikely they'll get this close again before the end of the mission.&lt;br&gt;&#xA;I haven't been able to get a full list of Rosetta &quot;orbits&quot;, but &lt;a href=&quot;http://blogs.esa.int/rosetta/2015/02/13/mission-control-at-closest-approach/&quot; rel=&quot;nofollow noreferrer&quot;&gt;pages like these&lt;/a&gt; refer to the 6-km pass as Rosetta's closest approach, and indicate the remainder of 2015 will see longer-distance passes.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the rest of the current mission plan in 2015, in fact, Rosetta will always conduct flybys, and, based on predictions of increasing cometary activity, can no longer be manoeuvred so close to the comet as to be in a gravitationally bound orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-03-05T12:38:41.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8360" PostTypeId="2" ParentId="8350" CreationDate="2015-03-05T09:39:36.997" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;$E_0 = \mathrm{acos}((1-\frac{R}{a})\cdot(\frac{1}{e}))$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This has two problems. One problem is that using the inverse cosine only gives you half of the orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other is the problem you discovered. This expression is subject to precision loss. Suppose you want to start at apofocus, so you set $R=a(1+e)$. The expression $(1-\frac{R}{a})\cdot(\frac{1}{e})$ should be -1, but using double precision numbers on my computer, this expression evaluates to 0 with $e=1\times10^{-16}$ and -1.97372982 with $e=1.125\times10^{-16}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a result of precision loss. (My cherry-picked values represent 100% precision loss.) You are always going to suffer some amount of precision loss when you subtract two numbers close to one another. For nearly circular orbits, $R/a$ will always be close to one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both problems can be solved by using a different expression. If you know the eccentricity $e$ and the initial true anomaly $f_0$, the initial eccentric anomaly $E_0$ is given by $E_0 = 2\arctan \left(\sqrt{\frac{1-e}{1+e}} \tan \frac f 2\right)$.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-03-05T09:39:36.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8361" PostTypeId="2" ParentId="8358" CreationDate="2015-03-05T14:14:24.583" Score="11" Body="&lt;p&gt;Sorry, no &lt;em&gt;bulb-shaped&lt;/em&gt; exhaust plume, at least not in true vacuum. But you might see something similar to an incandescent light bulb shaped exhaust plume at high altitudes that first stages reach, up to about 135 km high (exact altitude depends on launch vehicle and its ascent profile) above sea-level where there is still some, albeit tenuous atmospheric pressure but the first stage is at separation still about three times lower than the orbital altitude of the International Space Station. And / or due to multiple engines geometry. SpaceX Falcon 9 first stage might be particularly good for such exhaust plume geometry due to its nine engines in octaweb layout with one central engine and remaining eight in a circle around it. Observe &lt;a href=&quot;https://www.youtube.com/watch?v=zkvw56LqUJ4&amp;amp;&amp;amp;t=197s&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; of &lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/&quot; rel=&quot;noreferrer&quot;&gt;DSCOVR&lt;/a&gt; (Deep Space Climate Observatory) launch atop Falcon 9 close to first stage separation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/zkvw56LqUJ4?start=197&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you're right, expansion rate of exhaust plume and with it its geometry are substantially different in atmosphere and in vacuum. Actually, even in atmosphere, with increase in altitude during first stage ascent and &lt;a href=&quot;http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html&quot; rel=&quot;noreferrer&quot;&gt;drop of atmospheric pressure&lt;/a&gt; this geometry will change;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During early ascent in high density atmosphere, exhaust plume that exits rocket engine nozzle is at lower pressure than its surrounding atmospheric pressure and it compresses it into a long and narrow jet. It also interacts heavily with surrounding atmosphere due to high exhaust velocity and temperature and forms a pronounced exhaust boundary layer where it ionizes surrounding air. This forms plume eddies that might, depending on nozzle ratio, form shock diamonds or shock disks where increased pressure plume layers intersect each other and a combination of higher and lower plume pressure and density gives it that distinct appearance. But initially, the plume will remain coherent and downstream of it, this also results in &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Contrail&quot; rel=&quot;noreferrer&quot;&gt;contrail&lt;/a&gt;&lt;/em&gt; or a vapor trail as the ionized air forming due to acoustic shock (supersonic exhaust velocity) and temperature difference draws in more atmospheric moisture that evaporates or even disassociates on contact with high temperature exhaust products. Rockets whose main engine exhaust product is water (such as STS Space Shuttle launch vehicle that used cryogenic liquid Hydrogen and Oxygen, or LH2/LOX for short) will of course produce much more vapor. So much vapor that it's been reported that &lt;a href=&quot;https://en.wikipedia.org/wiki/Noctilucent_cloud&quot; rel=&quot;noreferrer&quot;&gt;noctilucent clouds&lt;/a&gt; formed sometimes even &lt;a href=&quot;http://www.nasa.gov/centers/goddard/news/topstory/2003/0522shuttleshine.html&quot; rel=&quot;noreferrer&quot;&gt;several days after an STS launch&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soon after, as the vehicle builds up on speed and altitude, atmospheric pressure starts dropping and exhaust plume of first stage engines starts expanding. At a certain point, atmospheric pressure is exactly equal to exhaust pressure and exhaust plume is exactly the width of the rocket engine's nozzle. This is the so-called &lt;em&gt;optimum altitude&lt;/em&gt;, or where first stage engines reach highest highest thrust coefficient. As the rocket climbs higher, ambient pressure drops further still, doesn't match exhaust pressure and exhaust plume starts expanding over the nozzle ratio:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/cJ4ej.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Plume (a) over-expansion, (b) ideal-expansion, and (c) under-expansion in a bell nozzle during flight. Credit: Rocketdyne, 1999&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how much plume expands depends on the pressure difference between exhaust products and their environment. In vacuum however, where atmospheric pressure is nonexistent or near-nonexistent such as near-vacuum at LEO (Low Earth Orbit) altitudes where air density is so low it doesn't even behave as ideal gas any more, there's nothing in the environment to confine exhaust plume any more. It expands much more rapidly and loses density without any &lt;em&gt;pinch&lt;/em&gt; effect and the boundary layer doesn't form either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks a bit like a wide open reflector light shining through a slightly humid air in otherwise complete darkness. Depending on propellants used, heavier exhaust products might be somewhat more visible than lighter ones because they &lt;a href=&quot;https://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia&quot; rel=&quot;noreferrer&quot;&gt;retain higher temperature for longer&lt;/a&gt; without losing it to &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_radiation&quot; rel=&quot;noreferrer&quot;&gt;radiation&lt;/a&gt; and thus produce more light for longer through &lt;a href=&quot;https://en.wikipedia.org/wiki/Black-body_radiation&quot; rel=&quot;noreferrer&quot;&gt;black-body radiation&lt;/a&gt;. And there might be slight evaporation / ablation of nozzle materials creating more orange exhaust layer close to the nozzle for same reason. But since upper stages use as light exhaust products as possible (they achieve higher exhaust velocity which matters more than their mass due to the nature of &lt;a href=&quot;https://en.wikipedia.org/wiki/Kinetic_energy&quot; rel=&quot;noreferrer&quot;&gt;kinetic energy&lt;/a&gt; $\begin{smallmatrix} \frac{1}{2}mv^2 \end{smallmatrix}$ increasing with a square root of velocity), and higher rocket's velocity additionally reducing exhaust density, visibility of exhaust plumes of upper stages indeed won't be great.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For a bit more on exhaust plume physics, both at sea-level and high altitudes, I'd recommend Col. Michael Heil's article on &lt;a href=&quot;http://www.nimr.org/systems/rockets/72-001.htm&quot; rel=&quot;noreferrer&quot;&gt;Rocket Testing at AEDC&lt;/a&gt; (Arnold Engineering Development Complex) that includes additional images and much better explanations than mine.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-03-05T14:56:07.353" LastActivityDate="2015-03-05T14:56:07.353" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8362" PostTypeId="2" ParentId="770" CreationDate="2015-03-05T15:24:14.427" Score="3" Body="&lt;p&gt;The largest bodies we could safely exploit by hollowing it out to make interior living space (like a precursor to an O'Neill cyliner), would be a body whose gravity is too weak to differentiate the materials and sphericalize the body.  A spherical body implies the material strength of the asteroid is too weak to support the mass of material above it.  In general, all differentiated bodies are spherical but both criteria are necessary because differentiated bodies can be disrupted (like Vesta) into smaller differentiated pieces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting this another way, Pluto and Ceres are certainly too large.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vesta and Pallas may be too large.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to impose the additional constraint of wanting to spin whole body for artificial gravity, then the body you should be looking for must be primarily metallic (ceramics/rocks are weak in tension).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What this means, is you should be looking for the core of disrupted planetoids, bodies like &lt;a href=&quot;http://en.wikipedia.org/wiki/4_Vesta&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vesta&lt;/a&gt; but smaller would be ideal.  This table of measured &lt;a href=&quot;http://astrostatistics.psu.edu/datasets/asteroid_dens.dat&quot; rel=&quot;nofollow noreferrer&quot;&gt;asteroid densities&lt;/a&gt; indicates the following asteroids might be particularly good candidates:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;'# Name Density Error Notes&lt;br&gt;&#xA;4   Vesta      3.44 +/-0.12 Probably too large&lt;br&gt;&#xA;20  Massalia   3.26 +/-0.60 145 km mean diameter&lt;br&gt;&#xA;804 Hispania   4.90 +/-3.90 145 km mean diameter, Huge error bars on density  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Visual comparison of largest asteroids&#xA;&lt;img src=&quot;https://i.stack.imgur.com/xDwSq.jpg&quot; alt=&quot;Comparison of Largest Asteroids&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8630" LastEditorUserId="8630" LastEditDate="2015-03-05T17:29:56.747" LastActivityDate="2015-03-05T17:29:56.747" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8363" PostTypeId="2" ParentId="8358" CreationDate="2015-03-05T20:55:25.570" Score="4" Body="&lt;p&gt;Why, they look like this :)  Here's an image of 3 rocket engines firing simultaneously on the Orbiter. (up, right, and aft firing RCS jets)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nota bene: the sorta-rectangular object just above the right-firing jet is a reflection in the window through which the image is being taken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cNPSc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-03-05T20:55:25.570" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8364" PostTypeId="2" ParentId="7986" CreationDate="2015-03-05T22:53:18.803" Score="1" Body="&lt;p&gt;How much do you want to learn about orbital dynamics, orbital plane changes, and synergistic launch directions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Part 1: Launch&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes a change in velocity (here on called delta V) of ~7.8 km/s to go from stationary to orbital speed.  It takes another 1.5-2.0 km/s of delta V losses (gravitational and fluid dynamic drag) to actually achieve orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth's rotation can provide ~0.5 km/s for free to your velocity provided you launch directly East from the equator.  From anywhere else it works as 0.5km/s * cos(latitude).  But the key is to launch directly East.  From Cape Kennedy, this gives you ~0.43 km/s in free velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Part 2: Destination&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most launches require the satellite to get to a specific orbit.  That isn't just height, that includes a specific plane relative to the Earth, Sun, Moon, other planets, or Space Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the example of the Space Station, its orbit precesses about the Earth.  Launching directly East from Cape Kennedy doesn't always point you at where the Space Station will be.  So part of launching timing is picking a Space Station orbit that will allow for the most benefit from Earth's rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS ground track from Wikipedia&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/iS2c7.jpg&quot; alt=&quot;ISS ground track from Wikipedia&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly in cases of a direct launch to the destination, the launcher must time their launch such that it will reach the correct location at the same time as the target (e.g. a Shuttle trying to reach the Space Station).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Part 3: Orbital Plane Changes&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are extremely costly in fuel.  It is much more efficient to achieve the correct orbit from launch than to correct it after launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Part 4: Location, Location, Location&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't looked over the launch list but what I'm betting your initial bar chart is showing is the desirability of some orbits over others.  The timing must be based upon the objects destination orbital inclination and timing in the day.  This could suggest sun synchronous orbits (spy &amp;amp; weather satellites) but I doubt they can entirely explain the blip. &lt;/p&gt;&#xA;" OwnerUserId="8630" LastActivityDate="2015-03-05T22:53:18.803" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8365" PostTypeId="1" CreationDate="2015-03-06T04:23:50.853" Score="2" ViewCount="665" Body="&lt;p&gt;I have read &lt;a href=&quot;http://zeenews.india.com/news/space/mars-ocean-was-bigger-than-the-arctic-nasa_1557220.html&quot; rel=&quot;nofollow&quot;&gt;this news article&lt;/a&gt; and I wonder how Mars water is lost to space.  Mars has mass and hence it has also got gravity to pull objects towards it like the Earth does. So how can water be lost to space? Does not Mars pull the water body towards it?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="168" LastEditDate="2015-03-06T05:37:26.990" LastActivityDate="2019-08-22T16:01:35.740" Title="How is Martian water lost to space?" Tags="|mars|water|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8366" PostTypeId="2" ParentId="8365" CreationDate="2015-03-06T04:39:12.127" Score="5" Body="&lt;p&gt;Wikipedia in its &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_ocean_hypothesis&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Mars_ocean_hypothesis&lt;/a&gt; says (I added links to the full text of papers):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Fate of the ocean&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;As the Martian climate cooled, the surface of the ocean would have frozen.&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;One hypothesis states that part of the ocean remains in a frozen state buried beneath a thin layer of rock, debris, and dust on the flat northern plain Vastitas Borealis.[33 &lt;em&gt;Janhunen, P., 2002: &lt;a href=&quot;http://www.space.fmi.fi/~pjanhune/papers/Mars/paper.pdf&quot; rel=&quot;noreferrer&quot;&gt;Are the northern plains of Mars a frozen ocean?&lt;/a&gt;, Journal of Geophysical Research, 107, 5103.&lt;/em&gt;] &lt;/li&gt;&#xA;  &lt;li&gt;The water could have also been absorbed into the subsurface cryosphere[3 &lt;em&gt;Clifford, S. M. and T. J. Parker, 2001: &lt;a href=&quot;http://www.mars.asu.edu/christensen/advancedmarsclass/clifford_icarus_2001.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains&lt;/a&gt;, Icarus 154, 40-79, pii S0019103501966710&lt;/em&gt;.] or &lt;/li&gt;&#xA;  &lt;li&gt;been lost to the atmosphere (by sublimation) and eventually to space through atmospheric sputtering.[26 &lt;em&gt;Kass, D. M. and Y. L. Yung, 1995: &lt;a href=&quot;http://yly-mac.gps.caltech.edu/ReprintsYLY/N091Kass_1995.pdf&quot; rel=&quot;noreferrer&quot;&gt;Loss of atmosphere from Mars due to solar wind-induced sputtering&lt;/a&gt;, Science, 268, 697-699&lt;/em&gt;.]&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, water possibly migrated under the surface, or was &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Sputtering&quot; rel=&quot;noreferrer&quot;&gt;sputtered&lt;/a&gt;&quot; - particles in upper atmosphere hit by solar wind and get accelerated enough to start into space (reach escape velocity):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Atmospheric_escape&quot; rel=&quot;noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Atmospheric_escape&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://sz.ifsi-roma.inaf.it/hewg-serena2011/web/18-Rinaldi.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://sz.ifsi-roma.inaf.it/hewg-serena2011/web/18-Rinaldi.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to The Planetary Air Leak (SciAm 2009) &lt;a href=&quot;http://libserver.wlsh.tyc.edu.tw/sa/pdf.file/en/e088/e088p070.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://libserver.wlsh.tyc.edu.tw/sa/pdf.file/en/e088/e088p070.pdf&lt;/a&gt; (table on last page) Mars loses hydrogen by thermal methods, and Carbon, Oxygen, Nitrogen, Argon by nonthermal methods: photo-chemical and sputtering.&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2015-03-06T04:48:30.403" LastActivityDate="2015-03-06T04:48:30.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8367" PostTypeId="2" ParentId="8365" CreationDate="2015-03-06T04:48:54.100" Score="9" Body="&lt;p&gt;UV tends to crack H&lt;sub&gt;2&lt;/sub&gt;O into hydrogen and oxygen and the hydrogen is easily lost to space. The oxygen is more massive and leaks out to space more slowly. The Earth is also losing gas to space, albeit so slowly that our atmosphere will not change much due to the effect before the Sun heats up enough to give Earth a runaway greenhouse effect.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mars has mass and hence it has also got gravity to pull objects towards it like the Earth does. So how can water be lost to space?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At any specific temperature for a group of gas molecules, the state energy of any one gas molecule follows a &lt;a href=&quot;https://en.wikipedia.org/wiki/Boltzmann_distribution&quot; rel=&quot;nofollow&quot;&gt;Boltzmann distribution&lt;/a&gt;. Some are traveling very slow, most are at a median velocity, and some are traveling very fast. If the ones traveling very fast are traveling at greater than the escape velocity of the parent body, and don't run into something else that slows them down, they will likely be lost to space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, a grain of dust has a gravitational field, but you don't see any grains of dust with an atmosphere. &lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="238" LastEditDate="2015-05-05T23:55:14.957" LastActivityDate="2015-05-05T23:55:14.957" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8371" PostTypeId="1" AcceptedAnswerId="61120" CreationDate="2015-03-06T19:59:34.617" Score="3" ViewCount="351" Body="&lt;p&gt;Some discussion around the existing question &lt;a href=&quot;https://space.stackexchange.com/questions/770&quot;&gt;What is the largest naturally occurring body that could be hollowed and safely lived in?&lt;/a&gt; has lead me to post this question.  I think the solution revolves around the strength of water ice in a vacuum, but I may be mistaken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming 100% pure water ice is used to create a hollow structure could it be pressurized to support human life suitable for a space station or transportation device (spaceship)? If yes, what is the largest structure that could be safely constructed? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are of course many variables&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The structure is 100% pure ice, insulating materials inside the structure are anchored to the sphere/structure without providing structural integrity&lt;/li&gt;&#xA;&lt;li&gt;A naturally existing body may be hollowed&lt;/li&gt;&#xA;&lt;li&gt;A structure may be created from pure water&lt;/li&gt;&#xA;&lt;li&gt;A sphere is imagined but not required &lt;/li&gt;&#xA;&lt;li&gt;At least half the radius of the structure must be hollow (no building caves on Europa)&lt;/li&gt;&#xA;&lt;li&gt;Sublimation may be addressed without providing structural integrity &lt;/li&gt;&#xA;&lt;li&gt;Spin to create gravity is desirable but not required&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2022-11-29T04:20:39.490" Title="Could ice be used to create a habitat or space station?" Tags="|materials|space-station|water|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8372" PostTypeId="2" ParentId="7986" CreationDate="2015-03-06T20:29:33.783" Score="11" Body="&lt;p&gt;I don't have the reputation to comment, but it seems like you're using data that stretches back to the late 50's.  I'd say their concept of operations is so much different from now that you shouldn't be including that data in any discussion of modern-day practices.  What does your plot look like if you limit yourself to the last decade or so?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also suspect that orbital mechanics would play only a minor role in the larger statistics of when launches occur.  Orbital mechanics definitely determine your launch azimuth and launch timing for trying to rendezvous with the ISS or to target some interplanetary orbit or a formation / constellation flight...but across a wide array of missions, I don't see how that timing would possibly correlate to the local time of day.  Also, if you just want your satellite ground tracks to stay centered over your area of operations (which is the case for telecom satellites), you can do that launch at any time of day to make the orbit.  I suspect that any deviations from a uniform time-of-launch distribution are much more mundane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My personal intuition is that launches from the Cape or from Vandenberg should tend to be in the early morning or in the late evening.  This is because very large chunks of the national airspace have to be closed off from aircraft traffic and ships-at-sea must also be cleared out of the downrange areas.  Logistically speaking, this is a nightmare.  If you launch during the middle of the day, that's when most of the aircraft are in the air and so you will cause a very large disruption to the national airspace.  Without going into the specifics of proprietary data, the analyses that I've seen for historical launches to orbit from launch locations in Florida estimate that the aggregate cost to the airlines is around a few hundred thousand dollars per launch.  The cost comes from having many aircraft being rerouted around the launch danger zone, which translates into paying for more fuel burned and having to compensate your flight crew for working longer hours during that longer flight time.  &lt;strong&gt;There is a strong economic incentive from the airlines to have space vehicles operate at times that minimize the disruption to air traffic.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="8640" LastActivityDate="2015-03-06T20:29:33.783" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8373" PostTypeId="1" CreationDate="2015-03-06T23:41:52.793" Score="1" ViewCount="172" Body="&lt;p&gt;Number 8 on &lt;a href=&quot;http://www.cracked.com/photoplasty_1364_32-ridiculous-ways-modern-media-wouldve-covered-history_p5/&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; &lt;em&gt;Cracked&lt;/em&gt; PhotoPlasty article made me wonder - what would happen to an unprotected smartphone on the moon?&lt;/p&gt;&#xA;" OwnerUserId="8641" LastActivityDate="2015-03-07T01:53:21.583" Title="Selfies on the Moon" Tags="|vacuum|moonwalking|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8374" PostTypeId="2" ParentId="8373" CreationDate="2015-03-07T00:21:00.530" Score="1" Body="&lt;p&gt;Meaningful differences between Earth surface environment and Moon surface:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No atmosphere:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Heat cannot be dissipated by convection. The temperature due to the components will increase. The battery (lithium) may explode above 80°C.&lt;/li&gt;&#xA;&lt;li&gt;Screen may be difficult to read, because the light will be bright (directly exposed to the Sun).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;No radiation protection:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Chips are sensitive to radiation and may be &lt;a href=&quot;http://www.uio.no/studier/emner/matnat/fys/FYS4220/h11/undervisningsmateriale/forelesninger-vhdl/Radiation%20effects%20on%20space%20electronics.pdf&quot; rel=&quot;nofollow&quot;&gt;disturbed&lt;/a&gt;. This is because insulation/conduction state in semi-conductors will be disrupted by the energy from the ionized particles and electromagnetic radiation) Reboot may be required frequently. Chips may also be destructed. Normally chips used in space are &lt;a href=&quot;http://en.wikipedia.org/wiki/Radiation_hardening&quot; rel=&quot;nofollow&quot;&gt;hardened&lt;/a&gt; to sustain ionizing radiation.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Gravity 1/6th on the Moon:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Gravity and acceleration sensors will be inaccurate.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;No GPS, no wifi, no network:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Location cannot be determined.&lt;/li&gt;&#xA;&lt;li&gt;Communications cannot be established.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Devices used on the Moon will experience the same constraints that on board satellites and space probes.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-03-07T01:53:21.583" LastActivityDate="2015-03-07T01:53:21.583" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="8375" PostTypeId="1" CreationDate="2015-03-07T02:10:24.197" Score="14" ViewCount="1148" Body="&lt;p&gt;I was watching an old Stargate episode with big alien ships in orbit, and I thought 'in real life that would be spotted pretty quickly... um, wouldn't it?'&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not saying I'm on the search for alien ships (if they do exist I don't think they have any trouble staying a step ahead of us anyhow). It just made me wonder about how much of what is in the sky, even pretty close by, we are really aware of. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe there should be some parameters on this. Suppose a standard Federation star ship were to yet again accidentally go back in time to our era, due to their continuing disregard for basic tachyon safety procedures. The Enterprise is supposed to measure about 600 x 400 x 200 meters, weighs 4.5 million metric tons, and seems to have a metallic hull. How long would it take us to spot that in low Earth orbit? How about if it was in a high orbit?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-04-01T20:14:57.657" Title="If an unknown ship entered Earth orbit, would it be detected?" Tags="|observation|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8376" PostTypeId="2" ParentId="8375" CreationDate="2015-03-07T02:44:05.067" Score="18" Body="&lt;p&gt;An object of that size would be detected almost instantly on first pass by the &lt;a href=&quot;http://en.wikipedia.org/wiki/United_States_Strategic_Command&quot;&gt;United States Strategic Command&lt;/a&gt;, which sweeps Earth's Sphere Of Influence to track and catalog objects that are in Earth orbit. Radar picks up most objects which are close and reasonably large. Anything greater than a metre in diameter can be tracked with relative ease, in fact, objects down to &lt;a href=&quot;http://www.esa.int/esapub/bulletin/bullet109/chapter16_bul109.pdf&quot;&gt;1cm in size&lt;/a&gt; can be tracked too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All operational satellites in Earth orbit are in known, cataloged orbits. Its fair to say an object larger than that of the ISS would be very observable (from radar) from any orbit it would be placed in. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-03-07T02:44:05.067" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8378" PostTypeId="1" AcceptedAnswerId="8379" CreationDate="2015-03-07T12:18:18.097" Score="8" ViewCount="616" Body="&lt;p&gt;I've been reading here (SX) and SpaceX website and other sites and can see no reference to any attempt to try the barge landing of the first stage booster on the recent Falcon-9 launch on March 1st. Everything is totally Schtum, no confirmation &lt;em&gt;or&lt;/em&gt; denial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyone know what happened to the first stage of the &lt;a href=&quot;http://www.spacex.com/news/2015/03/01/falcon-9-launches-two-all-electric-communications-satellites&quot;&gt;1st March Launch&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2017-07-21T11:31:05.293" Title="Was there a barge landing attempt on Launch 16 (EUTELSAT 115 / ABS 3A) March 1st" Tags="|spacex|landing|falcon-9|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8379" PostTypeId="2" ParentId="8378" CreationDate="2015-03-07T12:31:15.540" Score="14" Body="&lt;p&gt;No.  &lt;a href=&quot;http://www.spaceflight101.com/spacex-falcon-9---eutelsat-115-west-b--abs-3a-launch-updates.html&quot;&gt;The booster did not even have landing legs.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To loft the two payloads with a total mass of 4,159 Kilograms into a Supersynchronous Transfer orbit required almost the entire performance of Falcon 9, not permitting the first stage to attempt a boost-back and propulsive landing because it had to burn its entire propellant load during the primary mission of boosting the second stage on its trajectory. With no option for a landing, the first stage was flown without landing legs and without grid fins.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-03-07T12:31:15.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8380" PostTypeId="1" CreationDate="2015-03-07T17:30:35.697" Score="9" ViewCount="631" Body="&lt;p&gt;By &quot;extraneous mass&quot; I am referring to any mass which had not as its primary purpose onboard or attached to the launch vehicle to contribute to the technical or scientific goals of each mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the Apollo &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot;&gt;Lunar Roving Vehicle&lt;/a&gt; carried by Apollo 15, 16 and 17 had as its primary purpose onboard to further the scientific goals of each of those missions. Reserve and abort propellant had as its purpose to help ensure the ability of the crew to safely carry out the mission. And so on. These do not count as &quot;extraneous&quot; for the purposes of this question, regardless of anyone being able to argue whether they in the end were &lt;em&gt;essential&lt;/em&gt; for each respective mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, &lt;a href=&quot;http://www.sworld.com.au/steven/space/apollo/apollo13.txt&quot;&gt;Apollo 13 did carry a regular tape recorder&lt;/a&gt; and presumably a set of tapes of music. While the &lt;a href=&quot;http://www.spaceaholic.com/index.php/Detail/Object/Show/object_id/27&quot;&gt;Apollo Data Storage Equipment&lt;/a&gt; could be said to further the technical and scientific goals of the mission, the same cannot be said for the tape recorder used by the crew to play music in the spacecraft cabin, so this small tape recorder counts as &quot;extraneous&quot; (even though I can easily imagine it being a significant morale booster after that cryo tank stir gone awry).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that definition, my question is simply: &lt;strong&gt;how much such extraneous mass did each of the Apollo missions carry?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For bonus points, &lt;strong&gt;for each mission, what were the extraneous items carried?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2015-03-07T17:44:21.010" LastActivityDate="2015-09-23T15:03:16.320" Title="How much extraneous mass did each of the Apollo missions carry?" Tags="|apollo-program|mass|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8381" PostTypeId="2" ParentId="8375" CreationDate="2015-03-07T17:57:05.430" Score="13" Body="&lt;p&gt;The United States &lt;a href=&quot;http://en.wikipedia.org/wiki/United_States_Space_Surveillance_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Surveillance Network&lt;/a&gt; has 20 sites around the world, and &lt;a href=&quot;https://www.ll.mit.edu/publications/journal/pdf/vol13_no2/13_2spacesurveillance.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;satellites&lt;/a&gt; as well, dedicated to detecting and monitoring objects near Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/m73Q2.jpg&quot; alt=&quot;map of space surveillance network sites worldwide&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/Space_Debris/Scanning_observing&quot; rel=&quot;nofollow noreferrer&quot;&gt;European Space Agency&lt;/a&gt; also operates installations that monitor near-Earth space, full-time or part-time, in Germany, Norway, and Spain. I couldn't find  public information on Russia's network. China could have installations too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The majority of the ground-based installations are radar, but there are some optical telescopes, notably ESA's installation in Spain. Surveillance satellites are all optical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Objects as small as a centimeter are detectable in low Earth orbit. In geosynchronous orbit objects of a meter are detectable by radar, the Spanish telescope can see things as small as 15 cm. So the Enterprise would really stick out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While parts of this system are designed to track small objects and have a small field of view, other parts are designed to watch large sections of the sky for anything passing by. When not actively tracking an object, the tracking units scan back and forth so that they also cover large parts of the sky. Both sorts of units are shown in the photo below, they are parts of &lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/Space_Situational_Awareness/Space_Surveillance_and_Tracking_-_SST_Segment&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA's network&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7pvy5.jpg&quot; alt=&quot;EISCAT radar station, Sweden&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Either kind of unit would be capable of seeing something as large as a ship. As the networks span the globe and operate continuously, even in a very high orbit a ship would be seen in a few minutes or less. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most inconspicuous place for a ship to loiter near Earth would be in the Earth-Moon L2 Lagrange point, always out of sight behind the Moon.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-03-07T17:57:05.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8382" PostTypeId="1" AcceptedAnswerId="8384" CreationDate="2015-03-08T01:15:46.867" Score="19" ViewCount="1248" Body="&lt;p&gt;I understand &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_%28spacecraft%29&quot;&gt;Dawn&lt;/a&gt; has a mission to undertake geologic (&quot;Cereologic&quot;?) mapping of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29&quot;&gt;Ceres&lt;/a&gt;, but its final orbit will be at 22,000&amp;nbsp;km / 49% disk illumination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely, there would be some value for adding (or finalizing) on a much closer orbit for some greater detail of specific features.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is Dawn not equipped for closer study? Is it because the reaction wheel failure, we only have enough energy for this view? Am I not understanding something about the physics of such an attempt? Are we just not interested, in this particular mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why will Dawn not go closer to Ceres?&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="286" LastEditDate="2015-03-08T12:46:27.137" LastActivityDate="2018-07-11T21:39:47.653" Title="Dawn Spacecraft: Why are the orbits so high?" Tags="|probe|dawn-mission|ceres|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8383" PostTypeId="2" ParentId="8382" CreationDate="2015-03-08T02:08:22.917" Score="8" Body="&lt;p&gt;Fear not.  If all goes well, &lt;a href=&quot;http://dawnblog.jpl.nasa.gov/2014/08/31/dawn-journal-august-31/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dawn will get down to 375 km&lt;/a&gt;.  You can read more details in the blog entry, but Dawn's lifetime will be limited by its use of hydrazine, which is in fact driven by the failure of two of the four reaction wheels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Update 3+ years later:&lt;/em&gt; Dawn is currently on it's way to a 35 km altitude orbit! (Not a typo.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2018-07-11T21:14:14.233" LastActivityDate="2018-07-11T21:14:14.233" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="8384" PostTypeId="2" ParentId="8382" CreationDate="2015-03-08T02:15:36.617" Score="27" Body="&lt;p&gt;Its final orbit will only be &lt;a href=&quot;http://en.wikipedia.org/wiki/Dawn_(spacecraft)#Ceres_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;375 km&lt;/a&gt; above Ceres, but you have to give it time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dawn is powered by &lt;a href=&quot;http://dawn.jpl.nasa.gov/technology/ion_prop.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;xenon ion engines&lt;/a&gt;, which are extremely efficient, but very weak. The usual comparison is that they are pushing the craft forwards about as much as a sheet of paper pushes down on your hand. Their advantage is that they can do this for a very long time. This is why Dawn has been able to visit &lt;a href=&quot;http://en.wikipedia.org/wiki/4_Vesta&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vesta&lt;/a&gt; &lt;em&gt;and&lt;/em&gt; &lt;a href=&quot;http://en.wikipedia.org/wiki/Ceres_(dwarf_planet)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ceres&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A probe with conventional chemical rocket engines can change its speed very quickly, but it only has fuel to fire a few times, briefly. What has been done in the past is that probes have been carefully aimed at their destinations, and then when they arrive, they swing in really close to the planet and fire their engines when grazing past at the closest point. This is the best way to slow down. By doing that over a few passes, it can be done with only brief firings and that saves on fuel. This is called an &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Oberth maneuver&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dawn doesn't have to fire engines at the closest approach to Ceres. Its engines just keep firing, and firing until finally it slows down enough to orbit much closer. In fact the word 'firing' doesn't really apply. The engines have been &lt;em&gt;running&lt;/em&gt;, continuously, for thousands of days. (Well, there's three, and they switch which is on periodically, and there were some periods of coasting.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Braking this way takes weeks. It has already been braking for weeks as it got closer to Ceres. Then at first it was captured only into a highly elliptical orbit whose far point was very far away from Ceres. Still it runs its engines over a large portion of its orbit rather than applying much greater force briefly at the orbit's lowest point, and thus the Oberth effect is much less important in its maneuvers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But because the engines are so very efficient, still less fuel is needed that if chemical rockets had been used. The luxury of having engines that run on and on may also have helped mitigate the problems of dealing with the failed reaction wheels, and also the (temporary) failure of an entire engine did not doom the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note - Probes like Cassini and &lt;a href=&quot;https://space.stackexchange.com/questions/6677/good-year-for-start-of-an-interstellar-mission-due-to-gravity-assist&quot;&gt;Voyager&lt;/a&gt; managed to visit several destinations by making extremely clever use of gravity assists as well, carefully entering into the gravity well of a planet or moon in just the right way so that they got swung around them and thrown out again in just the right direction to proceed on to the next destination. It is only because they were set up to do this that they managed to make it to several awesome destinations. But they wouldn't have been able to slingshot very much with these low gravity destinations.)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2018-07-11T21:39:47.653" LastActivityDate="2018-07-11T21:39:47.653" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="8388" PostTypeId="2" ParentId="2500" CreationDate="2015-03-08T05:46:40.487" Score="5" Body="&lt;p&gt;There were no manned flights of Apollo hardware prior to 7. The unmanned flights (of which there were several) apparently didn't earn much historical prominence as the manned flights from 7 onward. Apollo 1, sadly, earned its place in the history books due to the tragic fire.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each of the flights from 7 to about 15 was notable in some way, mostly for some manned space flight &quot;first&quot;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apollo 7: first manned flight of the Apollo crew vehicle&lt;/li&gt;&#xA;&lt;li&gt;Apollo 8: first time a crewed vehicle traveled beyond LEO... furthest any human had been from the Earth to that time, first time a human had circumnavigated the Moon.&lt;/li&gt;&#xA;&lt;li&gt;Apollo 9: first crewed, in-space operation of the LEM.&lt;/li&gt;&#xA;&lt;li&gt;Apollo 10: first flight of the LEM to the Moon, first descent (but not landing)&lt;/li&gt;&#xA;&lt;li&gt;Apollo 11: first landing on the Moon&lt;/li&gt;&#xA;&lt;li&gt;Apollo 12: first colour TV images from the Moon. Perhaps more significant was that it proved the accomplishment of Apollo 11 was repeatable.&lt;/li&gt;&#xA;&lt;li&gt;Apollo 13: first disaster in space. Lunar mission aborted, but crew returned safely.&lt;/li&gt;&#xA;&lt;li&gt;Apollo 14: successful mission following Apollo 13&lt;/li&gt;&#xA;&lt;li&gt;Apollo 15: first use of the lunar rover&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The last two missions (Apollo 16 and 17) accomplished a lot of science, and from a science perspective did new and different things, but from a press point of view, began to look like &quot;more of the same&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Against that backdrop, the early unmanned flights, important as they were to the Apollo program, and confusion in numbering aside, are not on quite the same level in terms of press and public perception.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what it's worth: footage from Apollo 4 and Apollo 6 turns up all over the place - especially one clip from a camera mounted on the bottom of the second stage in which the first stage is seen falling away, followed by the inter-stage ring.  &lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2015-03-08T15:04:51.870" LastActivityDate="2015-03-08T15:04:51.870" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8390" PostTypeId="1" CreationDate="2015-03-08T09:25:12.573" Score="5" ViewCount="67" Body="&lt;p&gt;On several devices on the Curiosity rover there are marks (see red ellipses in the image below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/a7jHB.png&quot; alt=&quot;Curiosity&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought that they are used in order for machine vision to determine the angle at which a particular part is turned. But some of them are placed on presumably static pats (e. g. at the end of the white shaft in the image below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is the purpose of these marks?&lt;/p&gt;&#xA;" OwnerDisplayName="user3049" LastEditorDisplayName="user3049" LastEditDate="2015-03-08T10:34:37.380" LastActivityDate="2015-03-08T10:34:37.380" Title="What is the purpose of these marks on Curiosity?" Tags="|mars|curiosity|" AnswerCount="0" CommentCount="0" ClosedDate="2015-03-08T11:13:36.413" ContentLicense="CC BY-SA 3.0" />
  <row Id="8391" PostTypeId="1" AcceptedAnswerId="8393" CreationDate="2015-03-08T18:10:27.827" Score="45" ViewCount="19683" Body="&lt;p&gt;No matter how much you fine-tune a satellite's orbit up front, it needs periodic adjustments. It's usually done through tiny rocket bursts. The system and fuel to do that costs money and weight, and, when it runs out of fuel, the satellite's life is near its end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/questions/5583/can-an-artificial-satellite-stay-in-orbit-forever&quot;&gt;Can an artificial satellite stay in orbit forever?&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By contrast, the Moon has been around forever, with no adjustments. It's getting further from Earth at about 4cm per year, which is negligible in the span of decades for most practical purposes. See &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Moon&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, why is the orbit stability seemingly inconsistent between artificial and natural satellites? Is it just an effect of the Moon's much larger mass, or is there something else going on? If it's just mass, how about Saturn's rings, made up from countless small rocks that have also been around forever?&lt;/p&gt;&#xA;" OwnerUserId="2765" LastEditorUserId="2765" LastEditDate="2017-06-22T19:10:04.220" LastActivityDate="2018-02-02T10:05:35.583" Title="Why do artificial satellites need orbit correction, but natural ones don't?" Tags="|artificial-satellite|orbit|moons|" AnswerCount="7" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="8392" PostTypeId="1" AcceptedAnswerId="8399" CreationDate="2015-03-08T18:55:52.300" Score="12" ViewCount="792" Body="&lt;p&gt;A &lt;a href=&quot;http://web.mit.edu/sydneydo/Public/Mars%20One%20Feasibility%20Analysis%20IAC14.pdf&quot;&gt;MIT Mars-One Study&lt;/a&gt; found that &quot;crop growth, if large enough to provide 100% of the settlement’s food, will produce unsafe oxygen levels in the habitat&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming this to be true, are there any effective ways for removing/decreasing oxygen in a habitat on Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MIT article came to the conclusion that venting the excess oxygen was necessary, which had the adverse effect of destabilizing the breathable atmosphere.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since oxygen is &quot;readily reactive with other elements&quot; (&lt;a href=&quot;http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/p-Block_Elements/Group_16%3A_The_Oxygen_Family/Chemistry_of_Oxygen&quot;&gt;Chemistry of Oxygen&lt;/a&gt;), it seems like there should be an alternative way to decrease/remove excess oxygen from a habitat's atmosphere.  What am I missing?&lt;/p&gt;&#xA;" OwnerUserId="8653" LastEditorUserId="8653" LastEditDate="2015-03-08T20:46:59.933" LastActivityDate="2022-01-28T13:22:18.710" Title="Are there any effective ways for removing/decreasing oxygen in a habitat on Mars?" Tags="|mars|colonization|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8393" PostTypeId="2" ParentId="8391" CreationDate="2015-03-08T19:04:20.897" Score="59" Body="&lt;p&gt;The instability in orbits of our artificial satellites come from a few basic causes:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Atmospheric drag and solar wind effects&lt;/li&gt;&#xA;&lt;li&gt;The Earth isn't a perfect uniform sphere but is slightly lumpy, which means its gravitational field isn't uniform&lt;/li&gt;&#xA;&lt;li&gt;Other massive objects in the solar system perturb their orbits with their gravity&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So let's consider them one by one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the atmospheric drag effect is substantial for low earth orbit satellites, but the moon is about 1000 times further than LEO, and the relative effect of atmospheric drag is less for larger objects, so for the moon this is completely negligible. Similarly, solar wind affects large objects less than small ones. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, the gravitational irregularity becomes less significant with distance as well; at the moon's distance, the Earth's gravity behaves very much like a perfect &quot;point mass&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, perturbation from other sources is a factor, but the planets and their natural satellites have had billions of years to settle into positions which are mutually stable; the planets are far enough from each other to interfere only slightly, and the moons are far enough away from their primaries and each other to accept a little bit of perturbation over astronomical timescales.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, fundamentally, artificial satellites in low Earth orbit need correction because they are easily &quot;tossed around&quot; and have so little margin for error -- if their altitude decreases by 50km, they're lost, whereas 50km of variation in the Moon's orbit wouldn't have any significant effect on the Earth or the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Artificial satellites in geosynch orbit need correction because we want them to be in very specific orbits (i.e. above particular positions on the Earth), which again isn't a concern for natural satellites -- it doesn't matter to anyone exactly where they go (with the possible exception of interplanetary mission planners). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of this also applies to the other natural satellites, even though our Moon is a bit of an outlier -- it's a lot larger than most, and it's the only massive body orbiting the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The individual bodies making up the rings of the outer planets are constantly changing their orbits due to collision and perturbation by the moons of those planets. The ring systems as a whole appear to be stabilized by the positions of the moons. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-06-22T19:52:30.880" LastActivityDate="2017-06-22T19:52:30.880" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8394" PostTypeId="2" ParentId="8391" CreationDate="2015-03-08T19:12:11.747" Score="13" Body="&lt;p&gt;The most general answer is this: The Solar System is 4.6 billion years old, anything that happens &quot;quickly&quot; has happened a very long time ago. For example, &lt;a href=&quot;http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System#Planetary_migration&quot;&gt;it is thought&lt;/a&gt; that in the early solar system the orbits of Neptune and Uranus where switched, with Neptune being further in than Uranus, but the gentle tug of Jupiter and Saturn eventually pushed them into their current orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would give a lot to know how close they must have gotten to colliding at one point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Andrew Thompson pointed out, man made objects are on orbits where no natural body is likely to be, because of the large orbital interference in lower orbits. We put them there, because that is were we get the most use out of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An earth surveillance satellite has to be as close to earth as possible to take usable pictures. We can't put it somewhere else, just because it would stay up indefinitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, nature doesn't care for any particular orbital arrangement, whereas, for example, TV satellites work only on a narrowly defined geostationary orbit. So we need to make sure they stay there. Putting an object on just any orbit is not as hard as putting it in a certain orbit and making sure it stays there. There are disused man-made objects like old probes and Apollo upper stages drifting around the solar system that may keep flying around for billions of years. &lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-03-08T19:12:11.747" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8395" PostTypeId="2" ParentId="8391" CreationDate="2015-03-08T19:13:23.990" Score="42" Body="&lt;p&gt;Essentially, this is a result of observational bias. A natural satellite will only orbit a parent for extended time periods &lt;em&gt;precisely&lt;/em&gt; because the orbit it is in is stable &lt;strong&gt;&lt;sup&gt;†&lt;/sup&gt;&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plain truth of the matter is that we are simply injecting satellites into unstable orbits. If you were to move natural satellites into the same orbits, they'd be unstable too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take for example, &lt;strong&gt;the moon&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons), beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies are significant enough to cause a lunar orbit to change significantly over the course of several days.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Orbit&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What this is saying is that there's no (or more correctly, very few) stable orbits around the moon due to its lumpy gravitational field. Why do we not see natural satellites orbiting the moon? Because they would have decayed due to the orbit being unstable! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another example, asteroid &lt;strong&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/3753_Cruithne&quot; rel=&quot;nofollow noreferrer&quot;&gt;3753 Cruithne&lt;/a&gt;&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;3753 Cruithne is an Aten asteroid in orbit around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. It is a minor planet in solar orbit that, relative to Earth, orbits in a bean-shaped orbit that ultimately effectively describes a horseshoe, and which can transition into a quasi-satellite orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Its orbit, is too, unstable. On the timescale of millions of years, it will transition out of its current arrangement too:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After many years, the Earth will have fallen so far behind that Cruithne will then actually be &quot;catching up&quot; on the Earth from &quot;behind&quot;. When it eventually does catch up, Cruithne will make a series of annual close approaches to the Earth and gravitationally exchange orbital energy with Earth; this will alter Cruithne's orbit by a little over half a million kilometres—while Earth's orbit is altered by about 1.3 centimetres (0.51 in)—so that its period of revolution around the Sun will then become slightly more than a year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But there's even natural examples of unstable orbits on human timescales. Look at &lt;a href=&quot;http://en.wikipedia.org/wiki/2006_RH120&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;2006 RH&lt;sub&gt;120&lt;/sub&gt;&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;2006 RH120 is a tiny near-Earth asteroid with a diameter of about 2–3 meters that ordinarily orbits the Sun but makes close approaches to the Earth–Moon system every twenty years or so, when it can temporarily enter Earth orbit through temporary satellite capture (TSC). Most recently it was in Earth orbit from September 2006 to June 2007.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But, we can also inject artifical satellites into stable orbits, as well. Now that Dawn is orbiting Ceres, it will stay there for many hundreds of years. You could easily consider that stable on human timescales.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To summarize, orbits don't care whether the bodies involved are artificial or natural. You're only likely to find natural satellites in stable orbits, because the chaotic nature of orbits takes place over geologic timescales. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;†From a physical perspective, no orbit is ever stable. Tidal effects and gravitational influences mean that most of the orbits we consider stable, on human timescale, are unstable in geologic timescales. Additionally, &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravitational_wave#Orbital_decay_from_gravitational_radiation&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational radiation&lt;/a&gt; results in orbital decay on a timespan longer than the scale of the universe. All orbits are merely a human approximation of an unstable, chaotic system.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-02-02T10:05:35.583" LastActivityDate="2018-02-02T10:05:35.583" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8396" PostTypeId="1" AcceptedAnswerId="8398" CreationDate="2015-03-08T20:56:02.633" Score="24" ViewCount="9286" Body="&lt;p&gt;Firstly, I understand the need to use hypergolics over cryogenics as part of Dragon 2's abort and landing sequence - they provide &amp;quot;instant on&amp;quot; thrust and are throttlable with very wide margins (SuperDraco itself can throttle down to 20%).&lt;/p&gt;&#xA;&lt;p&gt;But why use Nitrogen Tetroxide &amp;amp; Monomethyl Hydrazine specifically? Newer chemicals such as &lt;strong&gt;AF-M315E&lt;/strong&gt; (which will be tested as part of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Green_Propellant_Infusion_Mission&quot; rel=&quot;noreferrer&quot;&gt;Green Propellant Infusion Mission&lt;/a&gt;) offer some pretty cool advantages. Consider the numerous downsides Hydrazine possesses:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Highly toxic&lt;/strong&gt;. Hazmat suits are a requirement when handling Hydrazine. It's toxic in very low concentrations and also carcinogenic. This slows loading &amp;amp; unloading times and increases cost and complexity significantly - something that is diametrically opposite to SpaceX's goals.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Costly&lt;/strong&gt;. &lt;a href=&quot;http://www.astronautix.com/props/hydazine.htm&quot; rel=&quot;noreferrer&quot;&gt;In 1990, NASA was paying $17/kg for Hydrazine&lt;/a&gt;. I highly doubt it has become cheaper since then. Stricter environmental &amp;amp; handling regulations will only have increased the cost, and when Dragon 2 holds a few hundred kilograms of it, that cost stacks up.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Low specific impulse&lt;/strong&gt;. SuperDraco has an I&lt;sub&gt;sp&lt;/sub&gt; of 235 seconds, which is terrible. AF-M315E has &amp;quot;50% increased performance&amp;quot;. This could translate into a number of options for SpaceX. They could reduce the weight of Dragon 2 significantly, which could make recovery of Falcon 9 easier, alternatively, it could allow them to keep the same weight and pack more scientific instrumentation or cargo aboard. Another option is keeping the same amount of propellant and allowing for a longer, less aggressive landing burn or providing more maneuvering margin during abort situations.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Fiddly&lt;/strong&gt;. Hydrazine has to be kept warm too - which requires heaters which requires power.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h2&gt;Now, I realize...&lt;/h2&gt;&#xA;&lt;p&gt;that &lt;strong&gt;AF-M315E&lt;/strong&gt; is still relatively cutting edge, but so is Dragon 2. NASA is requiring that landings take place under parachutes for the time being, which leaves ample R&amp;amp;D time (something SpaceX are fond of) for Dragon integration with this newer chemical.&lt;/p&gt;&#xA;&lt;p&gt;Can anyone explain this decision to prefer Hydrazine? Is a greener, more performant propellant in the upgrade line?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-05-24T23:01:07.990" Title="Why did SpaceX choose to use Hydrazine over newer &quot;green&quot; propellants for Dragon 2?" Tags="|spacex|dragon|launch-escape-system|capsule|hydrazine|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8397" PostTypeId="2" ParentId="8392" CreationDate="2015-03-08T22:21:21.467" Score="1" Body="&lt;p&gt;Oxygen is quite easy to remove from air. An extreme measure would be to light a fire. But there are numerous other chemical reactions that will do it too. A lot of metals oxidize on contact with oxygen for example.&#xA;And of course there is breathing... A vigorous workout. Raising animals...&lt;/p&gt;&#xA;" OwnerUserId="8504" LastActivityDate="2015-03-08T22:21:21.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8398" PostTypeId="2" ParentId="8396" CreationDate="2015-03-09T00:02:28.427" Score="39" Body="&lt;p&gt;TL;DR:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The low technology readiness, the very, very low thrust, and the need for a catalyst bed means this was and still is the wrong technology for the intended purpose of a launch abort system and maneuvering in low Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Low technology readiness&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX began working on Dragon V2 over five years ago. At that point, AF-M315E had a rather low technology readiness level. The technology readiness had been raised to 5 by 2013, but that isn't that great and that was far too late.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When designing a new spacecraft, one needs to very carefully pick and choose which low TRL items truly need to be used in the spacecraft design. The spacecraft itself is, by definition, at a rather low TRL. Every low TRL item used in the spacecraft needs to be looked at with a rather skeptical point of view. In the case of Dragon V2, SpaceX had engineers and operations people who were already familiar with hazardous propellants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Very, very low thrust&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Green Propellant Infusion Mission referenced in the question is aiming at bringing 22 newton AF-M315E up to TRL 9. That's irrelevant with regard to the kinds of thrusters needed for a launch abort system for a human-rated spacecraft. Each Super Draco thruster provides 73,000 newtons of thrust. To provide the same level of thrust, the vehicle would need 26,500 of those 22 newton thrusters -- or it would need to increase the thrust by a factor of 3300. SpaceX took a huge risk in increasing the thrust of its Draco thrusters by a factor of 200. Proposing to increase thrust by a factor of 3300 is beyond risky. This is TRL 1 territory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Need for a catalyst bed&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Draco and Super Draco thrusters use nitrogen tetroxide and mono methyl hydrazine. Igniting this mixture is easy: Mix the two and they ignite. They are hypergolic. In contrast, the monopropellant proposed in the question needs a catalyst bed to trigger ignition. There's a big problem here: Catalysts beds don't scale up to the size of thruster needed by Dragon V2. Large engines have an annoying tendency of ingesting catalyst beds. Catalyst beds are by their very nature a bit flimsy. They have to be; gas needs to flow through them. They don't scale up that well, particularly so in rocket engines.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-03-09T00:02:28.427" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8399" PostTypeId="2" ParentId="8392" CreationDate="2015-03-09T05:41:45.953" Score="13" Body="&lt;p&gt;I had come across &lt;a href=&quot;http://selenianboondocks.com/2015/02/yhabft-mit-marsone-analysis-alternative-solutions-to-the-excess-o2-problem/&quot; rel=&quot;noreferrer&quot;&gt;a blog post&lt;/a&gt; on this subject soon after the report came out. That answers your question just as well as anyone else could.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My main take-away was that the problem is totally solvable. What the MIT report accomplished was that it forces anyone looking at the data to not accept the present Mars One architecture. Honestly, I found this confusing because I haven't seen an architecture specific enough to make such claims in the first place, and even the papers reference list doesn't clear this up easily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to solve it, the blog post proposed either a controlled burn or keeping animals. Either way, it's still a drain on the burnable matter, and possibly &lt;em&gt;food&lt;/em&gt;, available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the main problem (as far as I can tell) is Nitrogen. See the following graphs from the report, because this is critical:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/t9jxC.png&quot; alt=&quot;O2 fraction&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/R3WL5.png&quot; alt=&quot;N2 stores&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oxygen fraction goes up and down, but clearly it is actively maintained by the station's systems. However, if you look at the N2 stores, it's pretty darn obvious that one of those systems involves venting gas. Oxygen is continually produced by the plants, but the Nitrogen comes from a finite supply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The story becomes pretty clear with these two components. After you've vented all your Nitrogen out of the habitat, you will not longer be able to maintain both a) low O2 molar fraction and b) sufficient total pressure. To me, this seems pretty obvious from the point that we're venting atmosphere in the first place. You can't just throw away the gas which can only come from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CO2 is easy to get on Mars but in-situ Nitrogen production is vastly beyond the tech level that we're looking at for the Mars One scenarios. So venting it is fairly flagrantly unsustainable, and whether the colony lasts 60 days or 400 days seems like an unimportant distinction. You won't launch missions with this being a credible scenario.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the plant matter to burn couldn't be produced easily, it could still be a problem. To whatever extent that is true, then MIT has pointed out a very large planning problem for any mission of this general type.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you'll accept some solutions that involve heavier machinery and a greater mass penalty, I would tend to think that either gas separation or storage could address this. If you just compressed the air O2/N2 mix then it could hang out long enough for some heterotrophs to get there and draw down the O2 quantity. Alternatively, if you could concentrate the O2 and &lt;em&gt;then&lt;/em&gt; vent, that could be sustained as long as you need.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could be wrong, I hope I've understood the problem correctly. I would sooner call it a &quot;Nitrogen&quot; problem than an &quot;Oxygen&quot; problem.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2015-03-09T05:41:45.953" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8400" PostTypeId="2" ParentId="8309" CreationDate="2015-03-09T14:25:04.317" Score="2" Body="&lt;p&gt;The short answer is yes, changes in the Earth's magnetic field would cause a lot of troubles. But here's a longer answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently the Earth's magnetic field does a pretty good job of diverting particles from the Sun and protecting us from solar winds. The weakening of the Earth's magnetic field would change a lot of things:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all solar winds reaching the Earth could knock out our computers and communication systems, it's obvious that this will effect space exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second of all humans in space would be even less protected against radiation, thus making human space exploration even more dangerous and riskier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the weakening of the Earth's magnetic field our satellites would be also more exposed to solar winds. If the satellites aren't well protected against solar winds, we could loose them.&lt;/p&gt;&#xA;" OwnerUserId="8606" LastActivityDate="2015-03-09T14:25:04.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8401" PostTypeId="2" ParentId="8391" CreationDate="2015-03-09T15:18:08.250" Score="1" Body="&lt;p&gt;The important thing is that artificial satellites have a function that often requires the satellite to be in a specific orbit. Keeping it in that orbit requires correction.  Many artificial satellites, once they fail, run out of fuel, or are no longer useful, have the stationkeeping stopped.  If they are high enough, they stay in orbit just fine, but not any specific one.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2015-03-09T15:18:08.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8402" PostTypeId="2" ParentId="8391" CreationDate="2015-03-09T19:13:25.797" Score="8" Body="&lt;p&gt;The short answer is that they DON'T need a correction to stay in orbit. But, as Russell, Rikki, Ross, and others pointed out, they need correction for the &quot;stationary&quot; part of a &quot;stationary geo-synchronous orbit&quot; because they need to stay where our terrestrial dishes are pointing. &#xA;If they correct small errors they only require small amounts of energy (fuel). &#xA;If they are allowed to drift, then the correction takes more fuel, possibly an acceleration to get back to the correct location, and a deceleration to stay there and not over-shoot. Hence the need for tiny, but frequent, corrections for geo-stationary satellites. &lt;/p&gt;&#xA;" OwnerUserId="8665" LastEditorUserId="195" LastEditDate="2015-10-07T23:40:41.443" LastActivityDate="2015-10-07T23:40:41.443" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8403" PostTypeId="2" ParentId="8391" CreationDate="2015-03-09T19:44:26.490" Score="-3" Body="&lt;p&gt;Russell has given a very good response on sources of orbit perturbation, but I am with Rikki on this one!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main reason that &quot;heavenly bodies&quot; do not need orbit correction is that we do not have any means of correcting their orbits! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moon has been around the earth for a long time and will be there for a while!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know this is not the scientific response you were looking for or this answer may even receive some negative points, but believe me, if we were able to move Jupiter moons or even our moon, we would have found a scientific &lt;em&gt;need&lt;/em&gt; to do so!&lt;/p&gt;&#xA;" OwnerUserId="5982" LastEditorUserId="195" LastEditDate="2017-06-23T00:28:56.090" LastActivityDate="2017-06-23T00:28:56.090" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8404" PostTypeId="2" ParentId="8391" CreationDate="2015-03-09T20:03:33.030" Score="6" Body="&lt;p&gt;There is somewhat of a Darwinian nuance here, even though this is not a question of biology. If something has &quot;survived&quot; potentially billions of years of stable orbit, it is not one of a large number of things that either fell to earth or went away. If the question is, &quot;Why do artificial and natural satellites stay at orbit,&quot; the answer is that artificial satellites stay in motion because they are &quot;propped up&quot; to a very slight degree, and existing natural satellites are a tiny minority of satellites that have persistently remained in orbits (as opposed to a &lt;em&gt;silent majority&lt;/em&gt;, meaning dead, collection of immeasurably more particles that have either fallen to earth or escaped earth's orbit).&lt;/p&gt;&#xA;" OwnerUserId="8668" LastActivityDate="2015-03-09T20:03:33.030" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8406" PostTypeId="1" AcceptedAnswerId="8408" CreationDate="2015-03-09T21:47:05.147" Score="18" ViewCount="3351" Body="&lt;p&gt;For many years, &lt;a href=&quot;http://en.wikipedia.org/wiki/Juno_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Juno&lt;/a&gt; has been touted as NASA's &amp;quot;low cost&amp;quot; mission to the Jovian system. As a New Frontiers mission, it is cost capped at &lt;em&gt;only&lt;/em&gt; $1 billion.&lt;/p&gt;&#xA;&lt;p&gt;From its launch in August 2011 to its arrival at Jupiter in July 2016, it spends nearly 5 years in space. Yet, despite all this cost and time, it will only spend a single year orbiting and studying Jupiter.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Juno mission is set to conclude in October 2017, after completing 33 orbits around Jupiter, when the probe will be de-orbited to burn up in Jupiter's outer atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Why waste all that effort, money, and time, on a single year long mission? It does not appear to be due to the radiation environment around Jupiter:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Juno's planned polar orbit is highly ellipitical and takes it close to the poles—within 4,300 kilometers (2,672 mi)—but then far beyond even Callisto's orbit.&lt;/p&gt;&#xA;&lt;p&gt;This type of orbit helps the craft avoid any long-term contact with Jupiter's radiation belts, which can cause damage to spacecraft electronics and solar panels. The &amp;quot;Juno Radiation Vault&amp;quot;, with 1-centimeter-thick titanium walls, will also aid in protecting and shielding Juno's electronics.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZlwTK.png&quot; alt=&quot;Juno's Orbit&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Its orbit is optimized to reduce radiation and it has inherited prior design experience from Galileo, which spent &lt;strong&gt;8 years&lt;/strong&gt; orbiting Jupiter.&lt;/p&gt;&#xA;&lt;p&gt;Neither does it appear due to be mission funding. Juno will cost $1.1b, but that is predominantly dedicated to design, construction, and launch costs. Traditionally, operations has been a large amount of the mission budget, and NASA has experience with running missions on lean budgets now (see: Opportunity).&lt;/p&gt;&#xA;&lt;p&gt;Frankly, it seems like a very low science to dollar ratio.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;TL;DR: For such an incredible amount of resources that have been expended, why is Juno spending so little time at its target subject?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-05T05:56:13.913" Title="Why does NASA's Juno spacecraft only have a one year primary mission?" Tags="|nasa|spacecraft|jupiter|mission-design|juno|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8407" PostTypeId="2" ParentId="4003" CreationDate="2015-03-10T00:58:39.093" Score="6" Body="&lt;p&gt;As far as I'm aware, this distinction belongs to Pioneer 6, which was the first spacecraft launched from the set of &lt;a href=&quot;http://en.wikipedia.org/wiki/Pioneer_6,_7,_8,_and_9&quot;&gt;Pioneer 6, 7, 8, &amp;amp; 9 probes&lt;/a&gt; on &lt;strong&gt;December 16, 1965&lt;/strong&gt;. Whether it's still functional and contactable at this &lt;em&gt;very moment&lt;/em&gt;, I'm not sure. Pioneer 7 &amp;amp; 8 are supposedly still functional. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pioneer 6 was last contacted on &lt;strong&gt;December 8, 2000&lt;/strong&gt;. Nearly 15 years ago - and it exchanged communications with ground control for approximately 2 hours. You might be looking for something a bit more recent, however. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case, Wikipedia claims that &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_2&quot;&gt;Voyager 2&lt;/a&gt; is both the oldest functioning and operational spacecraft, which launched on August 20, 1977 - thus it has been operational continuously for over 37 years, and it is expected to remain operational until at least 2025. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-03-10T00:58:39.093" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8408" PostTypeId="2" ParentId="8406" CreationDate="2015-03-10T01:19:26.927" Score="23" Body="&lt;p&gt;While many missions have been able to continue beyond their design lifetimes (Cassini and the Mars Exploration Rovers being prominent examples), the type of mission and orbit Juno must undertake to accomplish its goals will subject the spacecraft to a truly massive radiation dose. In order to meet the mission’s science goals within the budget set by NASA, the Juno team designed the spacecraft to accomplish its science goals in about a year, after 30 science orbits around Jupiter. If they left the spacecraft in orbit, it would eventually become too damaged by radiation to continue operating. It also won’t have enough fuel to go anywhere else (carrying more fuel means a heavier spacecraft, which means a bigger, more costly launch vehicle). So once its mission is finished Juno will be commanded to dive into Jupiter, where it will burn up in the atmosphere like a meteor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They dispose of the spacecraft in this way in order to meet the planetary protection requirement set by NASA. In Juno’s case, planetary protection means that they do not want the spacecraft to potentially crash into one of Jupiter’s moons -- especially Europa, which might contain an ocean of liquid water suitable for living organisms. The mission team studied the odds of Juno ever impacting Europa if they left the spacecraft in orbit around Jupiter. They determined that the odds were very low, but it’s a risk NASA will not  take. So the de-orbit maneuver allows the mission to ensure a controlled end to the mission and avoids that risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the Galileo spacecraft survived for about 8 years at Jupiter, but its orbit kept the craft far away from the planet for much of the mission. Juno’s science requirements (in particular, high-resolution sensing of the gravity field and deep atmosphere) require a very close orbit. The only feasible way to accomplish this with current technology is to have a polar orbit that repeatedly dives very close to Jupiter while ducking below the most intense regions of radiation. Even with this special orbit, the spacecraft’s external surfaces will receive a radiation dose equivalent to 100 million dental x-rays. Inside the radiation vault that contains Juno’s most sensitive electronics, the dose will be a factor of 800 lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, these choices represent a carefully considered balance between cost, meeting mission requirements and managing risks to mission success.&lt;/p&gt;&#xA;" OwnerUserId="8671" LastActivityDate="2015-03-10T01:19:26.927" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8410" PostTypeId="1" CreationDate="2015-03-11T03:21:26.420" Score="0" ViewCount="1099" Body="&lt;p&gt;What was the temperature of space before the Big Bang event?&lt;/p&gt;&#xA;" OwnerUserId="8683" LastActivityDate="2016-04-07T22:37:58.793" Title="Before the big bang, what was the temperature?" Tags="|history|" AnswerCount="1" CommentCount="2" ClosedDate="2015-03-11T05:56:53.057" ContentLicense="CC BY-SA 3.0" />
  <row Id="8411" PostTypeId="2" ParentId="8410" CreationDate="2015-03-11T03:32:47.977" Score="3" Body="&lt;p&gt;The correct answer to this was &lt;strong&gt;there was no temperature because there was no &quot;BEFORE the big bang&quot;&lt;/strong&gt;.  The big bang wasn't just the creation of all matter in the universe, it was also the creation of space-time itself.  This is a difficult concept for people new to cosmology and relativistic space-time, but the book &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/A_Brief_History_of_Time&quot; rel=&quot;nofollow&quot;&gt;A Brief History of Time&lt;/a&gt;&quot; by Stephen Hawking gives a relatively (no pun intended) clear explanation of this.&lt;/p&gt;&#xA;" OwnerUserId="8682" LastEditorUserId="8682" LastEditDate="2016-04-07T22:37:58.793" LastActivityDate="2016-04-07T22:37:58.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8412" PostTypeId="1" CreationDate="2015-03-11T07:50:52.673" Score="5" ViewCount="1362" Body="&lt;p&gt;I've been working on this for a while with little success. I'm trying to model a continuous burn orbit maneuver for say 5 minutes. I've tried to split the burn into multiple sub-burns that are calculated instantaneously, but this doesn't seem to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My end game here is to model 2 continuous burns both around apogee (so first burn, orbit around to apogee again, second burn). The error I'm currently getting is that if I compare the results for a single burn of length N to the results of two burns of length N/2, I use more propellant to get to my target perigee altitude. I've found references to Gauss planetary equations, and even a book they are in:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Battin RH (1999) Introduction to the Mathematics and Methods of&#xA;  Astrodynamics AIAA Education Series. American Institute of Aeronautics&#xA;  and Astronautics, Reston, Virginia&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Apparently page 489 has the sorts of equations I'm looking for. But I can't get access to this book. Any online source of this book, or the appropriate method/equations would be a perfect answer!&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="8157" LastEditDate="2015-03-11T08:25:54.860" LastActivityDate="2016-01-06T02:24:47.487" Title="How to model a continous burn and recalculate orbit properties post burn" Tags="|orbital-mechanics|orbital-maneuver|reference-request|astrodynamics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8413" PostTypeId="1" CreationDate="2015-03-11T13:28:31.960" Score="-2" ViewCount="1464" Body="&lt;p&gt;I'd love to know the following two facts: how long is the &quot;flight time&quot; between rocket launch and arrival at, say, ISS? Secondly, would astronauts experience zero-g during that trip and for how long?&lt;/p&gt;&#xA;" OwnerUserId="8675" LastEditorUserId="335" LastEditDate="2015-03-11T15:33:23.977" LastActivityDate="2015-03-11T16:45:57.663" Title="How long is the &quot;flight time&quot; between rocket launch and arrival at the ISS?" Tags="|launch|" AnswerCount="1" CommentCount="4" ClosedDate="2015-03-11T20:01:41.993" ContentLicense="CC BY-SA 3.0" />
  <row Id="8414" PostTypeId="1" AcceptedAnswerId="8419" CreationDate="2015-03-11T13:47:57.023" Score="19" ViewCount="5760" Body="&lt;p&gt;The simplest solution would be a tetrahedron, but as the satellites all need to orbit in circles, it seems this configuration would only guarantee full coverage at one time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For simplicity, there's no minimum resolution requirement, the Earth is a smooth oblate ellipsoid, and the satellites must be in orbit around Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's the true minimum number, and what would be their configuration?&lt;/p&gt;&#xA;" OwnerUserId="61" LastEditorUserId="61" LastEditDate="2015-03-11T15:01:39.173" LastActivityDate="2015-03-12T10:07:58.690" Title="Minimum number of satellites to image the entirety of Earth's surface at all times" Tags="|artificial-satellite|earth-observation|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8415" PostTypeId="1" CreationDate="2015-03-11T13:55:27.053" Score="6" ViewCount="777" Body="&lt;p&gt;With the reveal of the Dragon V2 (Or is it Dragon 2 now?  Or Dragonrider?) we saw a new trunk design where the solar panels are wrapped around the trunk instead of being deployed like wings from the cargo version of the trunk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/94Ozf.png&quot; alt=&quot;Dragon V2 all around views&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, will the new trunk still carry unpressurized cargo to the station?  Or will the available space be used for different purposes?  (Batteries? Abort fuel tanks? Consumables?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, if it does, can the CanadArm reach around and into the trunk, when it is docked on the PMA-2 Node-2 forward?  How about PMA-3 Node - 2 Nadir (once it is moved).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-03-12T11:55:52.813" Title="Will Dragon V2 when crewed carry cargo in its trunk?" Tags="|spacex|dragon|dragon-v2|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8416" PostTypeId="2" ParentId="8413" CreationDate="2015-03-11T16:45:57.663" Score="2" Body="&lt;p&gt;The trip from the surface to low Earth orbit is a matter of about 10 minutes under thrust (as Hobbes commented); the crew on a Soyuz mission to ISS experiences varying accelerations up to about 4g during that time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in orbit, the astronauts are in zero g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's then, in the ISS case, a relatively long period of matching orbits with the station; it used to be done over the course of a couple of days; now they do it in 6 hours. The thrusters on the Soyuz are relatively low-powered; the accelerations felt during the intermittent maneuvering burns during orbit matching are about 1/18 g. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-03-11T16:45:57.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8417" PostTypeId="2" ParentId="8414" CreationDate="2015-03-11T17:52:23.727" Score="2" Body="&lt;p&gt;You just need two of them, placed in the same orbit, but at different (opposite) positions.&#xA;Wether they move relative to the surface or not (see &lt;a href=&quot;http://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;nofollow&quot;&gt;Geosynchronous orbit&lt;/a&gt; is irrelevant because they will move at the same speed, thus being always at the exact opposite to each other.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2015-03-11T17:52:23.727" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8418" PostTypeId="2" ParentId="8414" CreationDate="2015-03-11T18:53:17.497" Score="3" Body="&lt;p&gt;Five. 3 in polar orbits at 120º intervals, and two above the equator at opposite longitudes, at points perpendicular to the plane in which the polar satellites orbit. You'd have to synchronize the precession of the polar satellites with the equatorial satellites.&lt;br&gt;&#xA;Two will get you close, but not quite there: you'll miss a tiny section where (viewed from Earth) the satellite is too low above the horizon to see the surface.&lt;br&gt;&#xA;Here's a &lt;a href=&quot;http://www.neoprogrammics.com/spheres/visible_fraction_of_surface.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;calculator which shows what fraction of the Earth's surface is visible at each altitude&lt;/a&gt;. Even at 1.5 million km, coverage is below 50%.&lt;br&gt;&#xA;See also &lt;a href=&quot;https://space.stackexchange.com/questions/8140/single-shot-blue-marble-pictures?&quot;&gt;this related question.&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-03-11T19:39:38.167" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8419" PostTypeId="2" ParentId="8414" CreationDate="2015-03-11T19:42:53.470" Score="21" Body="&lt;p&gt;4 is actually the number, as is documented in &lt;a href=&quot;http://www.google.com/patents/US4854527&quot;&gt;Patent US4854527&lt;/a&gt;. It is a tetrahedral constellation using elliptical orbits. In one hemisphere two have their further point, while the other two have the opposite coverage. The orbital period is 27 hours.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-03-11T19:42:53.470" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8420" PostTypeId="1" AcceptedAnswerId="8421" CreationDate="2015-03-12T02:24:17.637" Score="10" ViewCount="4666" Body="&lt;p&gt;I've been told that for low thrust ion spirals, delta V would be the difference between speeds of departure and destination orbits. For example the delta V between a 7.7 km/s LEO and a 3.1 km/s GEO would be 4.6 km/s. Is this correct? How is that derived?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a ion drive accelerating a spacecraft 1 millimeter/sec^2, I get 11.6 days to accelerate 1 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To figure time it'd take to spiral from LEO to GEO I took 11.6 * 4.6 to get 54 days. Again, I don't know if this is correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an attempt to draw a LEO to GEO spiral with 1 mm/s^2 acceleration:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/oAJqk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was my line of thought: end orbit is about 24 hours, beginning orbit is 1.5 hours so average orbit is about 12.75 hours. !2.75 hours goes into 54 days about a 100 times so I made a logarithmic spiral that turns 100 times from LEO to GEO. (hot colored areas are Van Allen Belts)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I strongly suspect this is wrong. It seems to me an ion spiral would be wound tighter at LEO and gradually relax as the spacecraft ascends. But at this point I have no idea how to model an ion spiral in an Excel spreadsheet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In these questions I am using the specific example of LEO to GEO but I am hoping for guidelines for more general scenarios&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-10-09T16:47:30.583" Title="General guidelines for modeling a low thrust ion spiral?" Tags="|trajectory|ion-thruster|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8421" PostTypeId="2" ParentId="8420" CreationDate="2015-03-12T04:51:42.830" Score="14" Body="&lt;p&gt;The rule you have for the total $\Delta V$ of a low-thrust spiral is an upper limit arrived at as you let the thrust go to zero.  However that takes an infinite amount of time.  The total $\Delta V$ of a spiral with non-zero thrust is less, and the time is finite.  But it is a good rule of thumb for quick calculations when trying to establish feasibility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The derivation of the rule of thumb is quite simple.  Look at an infinitesimally small Hohmann transfer.  You will find that the $\Delta V$ total of the two infinitesimal burns at the initial orbit and at apoapsis of the transfer orbit is equal to the difference in the orbital velocities.  Then if you add those up for a finite raise in orbit, you get the difference in $\Delta V$ of the initial and final orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To find out the real total $\Delta V$ and to plot an actual trajectory that doesn't do an infinite number of orbits before it gets anywhere is best done using numerical integration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an example of a spiral from a circular orbit to escape ($C_3=0$):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WFOkk.png&quot; alt=&quot;spiral&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is normalized to the starting circular orbit, where the distances are in units of the initial orbit radius, and the acceleration is constant at $10^{-3}$ of the gravitational acceleration of the body at the initial orbit radius.  The total $\Delta V$ to escape is 0.856 of the initial orbit velocity, as compared to 1.0 for the rule of thumb.  The total time to escape is 136 initial orbit periods.  It goes around the body about 40 times before escaping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first several orbits are close enough that you can't make them out at the resolution shown.  This gets even worse for smaller accelerations.  $10^{-3}$ is actually pretty high.  I picked it so that you can see the spiral better.  That time from a low Earth orbit is about 8.5 days.  A typical spiral out might be more like months with accelerations of $10^{-4}$ of the initial gravitational acceleration, or less.  Attempts at plotting that show a solid disk until near the end where you see the spiral escape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an example of a spiral from LEO (400 km) to GEO with the same normalization and a normalized constant acceleration of $10^{-4}$.  It takes about two months over 945 orbits.  In this case the total $\Delta V$ is very close to the rule of thumb.  This is simplified, since the final flight path angle here is about half a degree.  So there is some time and $\Delta V$ remaining to circularize the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gCnYN.jpg&quot; alt=&quot;spiral to geo&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could approximate this plot by advancing one orbit at a time, using the orbit period times the acceleration as the $\Delta V$ and raising the orbit the corresponding amount, connecting each with a linearly increasing spiral.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-03-12T06:45:57.127" LastActivityDate="2015-03-12T06:45:57.127" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8423" PostTypeId="2" ParentId="8415" CreationDate="2015-03-12T11:55:52.813" Score="3" Body="&lt;p&gt;So far as I can tell, it seems that SpaceX hasn't fully figured out what will be in the trunk. There is frequent reference to &lt;a href=&quot;http://en.wikipedia.org/wiki/Dragon_V2#cite_note-sx20150227-33&quot; rel=&quot;nofollow&quot;&gt;&quot;Pressurized cargo&quot;&lt;/a&gt; that will be contained in the Dragon's pressurized cargo, but nothing said about the trunk. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, it seems as though the use of carrying cargo in the trunk was a bit of an afterthought in the Dragon v1. At the very least, it was not used to carry cargo until &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX_CRS-2&quot; rel=&quot;nofollow&quot;&gt;CRS-2&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What we do know about the trunk: it contains equipment including &quot;the solar arrays, heat-removal radiators, and will provide aerodynamic stability during emergency aborts&quot;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-03-12T11:55:52.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8424" PostTypeId="1" AcceptedAnswerId="8432" CreationDate="2015-03-12T12:01:08.767" Score="0" ViewCount="3229" Body="&lt;p&gt;Here's a hypothetical situation. There's an object, at rest, 500 km above the surface of Earth, with some force preventing it from accelerating downwards. The force is removed and the object immediately accelerates at 8.44 m/s^2 downwards. By the time it reaches the surface, it has an acceleration of 9.81 m/s^2. Assuming negligible air resistance, how would you calculate the final velocity of the object, at the instant before crashing?&lt;/p&gt;&#xA;" OwnerUserId="8694" LastActivityDate="2015-03-12T17:20:07.667" Title="How to calculate the final velocity of an object falling to Earth with variable acceleration?" Tags="|landing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8425" PostTypeId="2" ParentId="3068" CreationDate="2015-03-12T12:36:22.450" Score="1" Body="&lt;p&gt;One can see this for one's self playing Kerbal Space Program. They don't talk about angle of attack so much in KSP, but they do talk about the perigee of re-entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have a perigee of re-entry at, say, 40km in game, you will enter the atmosphere (At about 70km), start to slow down, and in the end, you will leave the atmosphere, going slower than you were to begin with, but will continue to do another orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your perigee is something like 30km, then you will most likely enter the atmosphere at a reasonable speed, keeping your spacecraft together and overall working quite well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you enter at a lower perigee, then you run the risk of hitting the air too fast, which could potentially damage your spacecraft. Note that KSP spacecraft are generally speaking more resistant to re-entry than corresponding real spacecraft, but the principal still exists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, for real spacecraft, you want to enter the atmosphere in such a way that you will stay in the high atmosphere for a while, and slow down your spacecraft more gradually. If you try and do this too conservatively, however, you will end up continuing in orbit, having not slowed down enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This shows a &lt;a href=&quot;https://www.youtube.com/watch?v=NoadICSrUR4&quot; rel=&quot;nofollow&quot;&gt;KSP too fast reentry&lt;/a&gt;. I can't find a good too slow one, but take a look a &lt;a href=&quot;https://youtu.be/lNDXRjN2piY?t=7m40s&quot; rel=&quot;nofollow&quot;&gt;this mission to Eve&lt;/a&gt;, the Venus equivalent KSP planet.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-03-12T12:36:22.450" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8426" PostTypeId="1" AcceptedAnswerId="8428" CreationDate="2015-03-12T12:39:04.060" Score="15" ViewCount="6910" Body="&lt;p&gt;I read somewhere that only 2/3 of the V2 rockets reached their target - but how good was their precision when they reached the target? ±100 m? ±1 km? ±10 km?&lt;/p&gt;&#xA;" OwnerUserId="8695" LastActivityDate="2021-05-08T05:56:46.463" Title="How good was the precision of German V2 rockets?" Tags="|rockets|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="8427" PostTypeId="1" CreationDate="2015-03-12T13:42:22.157" Score="11" ViewCount="1791" Body="&lt;p&gt;Apollo, Gemini, (Not Mercury), Dragon, CST-100, the set of American capsules all seem to use three parachutes for landing the capsules. This seems like it makes sense. They are usually designed for landing under two allowing some redundancy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However Soyuz has only a single large parachute. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the design considerations that led to that choice?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-03-12T13:58:25.040" LastActivityDate="2015-03-15T17:21:00.050" Title="Why does Soyuz use a single large parachute vs 3 on most American capsules?" Tags="|landing|soyuz-spacecraft|capsule|parachute|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8428" PostTypeId="2" ParentId="8426" CreationDate="2015-03-12T14:54:03.923" Score="15" Body="&lt;p&gt;From &lt;a href=&quot;http://www.astronautix.com/lvs/v2.htm&quot;&gt;Astronautix&lt;/a&gt;: &quot;Tests of prototype V-2's in 1943 indicated a 4.5 km CEP (circular error probable - the radius within which 50% of the shots impact). 100% of the shots fell within 18 km of the target. A radio beam guidance update system was introduced in December 1944, which in tests produced a 2 km CEP. In reality, in the campaign against Britain, 518 rockets were recorded as falling in the Greater London Air Defence Zone of 1225 fired, implying an average CEP of 12 km.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the &quot;implied CEP&quot; of 12km includes the effects of misinformation about where the rockets were landing (news of missiles hitting short of London was suppressed while news of missiles going long was reported, or fed to German intelligence via turned spies) as well as missiles which failed outright due to non-guidance problems, so the actual accuracy of the guidance system in the field was likely closer to the 2km-4.5km figures.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-03-12T20:09:42.620" LastActivityDate="2015-03-12T20:09:42.620" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8429" PostTypeId="1" AcceptedAnswerId="8433" CreationDate="2015-03-12T04:16:13.703" Score="10" ViewCount="367" Body="&lt;p&gt;I am working on a realistic space flight simulator as a hobby, but I am finding it difficult to find technical information about modern reaction control systems (RCS) like those used on the space shuttle. Wikipedia only has much more general information, and any books I have found do not include specific data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am looking for information about reaction control thrusters in space planes, rockets or pods, but especially the Space Shuttles. I am concerned with the mass of the mechanical system, mass of the fuel, force output and electrical requirements. I am also interested in any general information, such as typical burn times, maximum burn times or thermal energy generated.&lt;/p&gt;&#xA;" OwnerUserId="8708" OwnerDisplayName="Ric Magnuson" LastEditorUserId="4660" LastEditDate="2015-04-23T17:07:43.277" LastActivityDate="2015-04-23T17:07:43.277" Title="What are the technical specifications of modern reaction control systems?" Tags="|propulsion|space-shuttle|reference-request|specifications|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8430" PostTypeId="2" ParentId="8426" CreationDate="2015-03-12T15:44:10.170" Score="7" Body="&lt;p&gt;Adding to Russell's answer:&lt;/p&gt;&#xA;&lt;p&gt;A large hindrance to greater precision of the A-4 (technical name) was the lack of calibration data. At this late point in the war, the German intelligence network in the UK was compromised, giving the British secret service the ability to inject false information on the impact sites of the rockets. This lead the Germans to assume their rockets overshot London, so they re-targeted them to what was in fact the loosely populated area of Kent.&lt;/p&gt;&#xA;&lt;p&gt;Additionally, many of the missiles burst on the last few hundred meters and failed to detonate, because of the static air pressure. This air burst problem &lt;a href=&quot;https://en.wikipedia.org/w/index.php?title=V-2_rocket&amp;amp;oldid=1021976354#Air_burst_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;was alleviated&lt;/a&gt; in 30 August 1944 tests by adding 'sleeves'/'tin trousers' to the forward end of the rocket. When von Braun's team conducted further tests at the elevated (1300m) plateau of White Sands, New Mexico, no such failures occurred.&lt;/p&gt;&#xA;&lt;p&gt;Source: &amp;quot;Peenemunde to Canaveral&amp;quot; by Dieter Huzel&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="40574" LastEditDate="2021-05-08T05:56:46.463" LastActivityDate="2021-05-08T05:56:46.463" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="8431" PostTypeId="2" ParentId="8427" CreationDate="2015-03-12T16:24:19.053" Score="11" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.spaceflight101.com/soyuz-spacecraft-information.html&quot; rel=&quot;noreferrer&quot; title=&quot;spaceflight101&quot;&gt;spaceflight101&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[The Soyuz capsule] is outfitted with the spacecraft's fully redundant parachute system [...]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I would assume that the designers needed to keep a certain precision for the landing. If they deployed the main parachute and the backup simultaneously, the decent speed may be lowered to the point that they could no longer satisfy their landing area requirement, because a slower decent always means that there is a larger potential for the spacecraft to be moved by winds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.astronaut.ru/bookcase/books/popov/text/03.htm&quot; rel=&quot;noreferrer&quot;&gt;Popov's &quot;Surprises in orbit&quot;&lt;/a&gt; the backup parachute is smaller, with only 574m² (instead of 1000m²) area, leading to an impact speed of a whooping 8-11 m/s, which is far more than is comfortable. (Thank you, osgx)&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2015-03-15T17:21:00.050" LastActivityDate="2015-03-15T17:21:00.050" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8432" PostTypeId="2" ParentId="8424" CreationDate="2015-03-12T17:20:07.667" Score="4" Body="&lt;p&gt;Specific energy conservation. If it falls from r2 to r1...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ E = KE + GE = \frac{1}{2} v^2 - \frac{GM}{r} \\&#xA; E_1 = E_2 \\&#xA;-\frac{GM}{r_2} = \frac{1}{2} v^2 - \frac{ GM}{r_1} \\&#xA;v = \sqrt{ 2 GM \left( \frac{1}{r_1} - \frac{1}{r_2} \right) } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For small distances, Taylor approx of 1/r for r2 about r1...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \frac{1}{r_2} \approx \frac{1}{r_1}-\frac{1}{r_1^2} (r_2-r_1) \\&#xA;v \approx \sqrt{ 2 g (r_2 - r_1) } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You should recognize this as the usual form that comes from (1/2 m v^2 = m g h). This shows that the first expression for v is the proper generalization of the constant-gravity model which is taught first in physics classes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The specific energy conservation is one the first-principles from which the laws of orbital mechanics are derived.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2015-03-12T17:20:07.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8433" PostTypeId="2" ParentId="8429" CreationDate="2015-03-12T17:29:38.150" Score="6" Body="&lt;p&gt;The good old &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rcs.html#sts-rcs&quot;&gt;Space Shuttle News Reference Manual&lt;/a&gt; should get you started.  There is also good information in the &lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&quot;&gt;Crew Operations Manual&lt;/a&gt; (section 2.22).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know just linking is frowned on, but your question is so general that I'd just have to copy the entire sections and paste them in, which seems absurd.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="168" LastEditDate="2015-03-13T03:25:42.593" LastActivityDate="2015-03-13T03:25:42.593" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8434" PostTypeId="1" AcceptedAnswerId="8436" CreationDate="2015-03-12T23:22:20.103" Score="9" ViewCount="2230" Body="&lt;p&gt;Vanguard 1 became the first satellite to use solar panels as a power source in 1958, when it was launched into an orbit with the following parameters:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;654 by 3,969 kilometres (406 mi × 2,466 mi), 134.2 minute elliptical orbit inclined at 34.25 degrees on March 17, 1958&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Vanguard_1#Mission&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;, sadly without citation, from Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, its periapsis is within the tenuous grasp of the exosphere. It was estimated at the time of launch that its orbit would decay over 2000 years, but at the time, it was not known that the exosphere &quot;puffed up&quot; during periods of high solar activity:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[this] caused a significant decrease in its expected lifetime to only about 240 years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, if this is correct (and it will only be a rough estimate as orbital decay predictions are notoriously inaccurate), it has already spent nearly a quarter of its total orbital lifetime in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet, the satellite tracking website N2YO (which receives regular TLE updates from current orbital objects) &lt;a href=&quot;http://www.n2yo.com/satellite/?s=5&quot; rel=&quot;noreferrer&quot;&gt;has the satellite currently in a 653km x 3833.5km orbit&lt;/a&gt;, inclined 34.3 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Would we not expect to have a seen a greater decay than what has occurred so far?&lt;/strong&gt; If these details are correct, its apoapsis has only decayed by 136km since launch, and its periapsis has not shrunk at all, at least not appreciably. This does not appear to match its predicted orbital reentry timeframe of 240 years. What is going on here?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2020-01-25T22:42:32.880" Title="Why has Vanguard 1 not decayed significantly since its orbital injection in 1958?" Tags="|artificial-satellite|orbit|debris|orbital-decay|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8435" PostTypeId="2" ParentId="8375" CreationDate="2015-03-12T23:28:01.883" Score="0" Body="&lt;p&gt;A spaceship needs to keep it's occupants warm, and that means keeping it at a temperature easily detectable by IR telescopes (unless it's behind the moon). Firing the engines would be a dead giveaway.&lt;/p&gt;&#xA;" OwnerUserId="8697" LastActivityDate="2015-03-12T23:28:01.883" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8436" PostTypeId="2" ParentId="8434" CreationDate="2015-03-13T01:56:46.193" Score="21" Body="&lt;p&gt;Orbital decay due to aerobraking/drag typically acts to first circularise an orbit, then slowly spiral the satellite into reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is for one main reason:you get more drag at perigee. Where a force acts on an orbit collinear with the velocity vector the satellites current altitude remains unchanged, all other parts of the orbit are changed. So the natural progression of the satellite in question is:circularisation of orbit;slow reduction of semimajor axis (under practically constant 0 eccentricity);final rentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also to note, due to the exponential nature of the density of atmosphere the it takes alot longer to move from 500km to 400km than it would from 400km to reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're interested you can use DAS to calculate the expected deorbit time yourself using &lt;a href=&quot;https://orbitaldebris.jsc.nasa.gov/mitigation/das.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL's Orbital Debris tool&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorDisplayName="user20636" LastEditDate="2020-01-25T08:53:49.977" LastActivityDate="2020-01-25T08:53:49.977" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="8437" PostTypeId="2" ParentId="5686" CreationDate="2015-03-13T02:45:02.353" Score="4" Body="&lt;p&gt;Yes, &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_air_cycle_engine&quot; rel=&quot;nofollow&quot;&gt;Liquid Air Cycle Engine&lt;/a&gt;&quot; (LACE) is the proper nomenclature for this &quot;camel&quot; concept, and it has been studied before. Some incarnations have gone into early design by space agencies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;NASA GTX Reference&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently, a LACE had been seriously considered as something which would be the next design step after the X-33. I still find this a little confusing, but it would have been the 2nd out of 3 technology phases toward a 40% mass fraction (!!) SSTO craft. It appears to be differentiated from the 3rd phase by vertical takeoff and some initial stored Oxygen, whereas the 3rd phase would be a runway takeoff and have no Oxygen stored before takeoff. It is different from the X-33 obviously because the X-33 uses no propellant taken from the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the X-33 program was halted, it's not surprising this had little mention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://web.archive.org/web/20030827015859/http://www.grc.nasa.gov/WWW/RT2002/5000/5880trefny.html&quot; rel=&quot;nofollow&quot;&gt;http://web.archive.org/web/20030827015859/http://www.grc.nasa.gov/WWW/RT2002/5000/5880trefny.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Indian DRDO/ISRO's Avatar&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They had a concept for a SSTO where the Oxygen was liquified by some modifications to an airbreathing ramjet stage, which is closer to what I envisioning with Skylon as the baseline.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.webcitation.org/query?url=http://www.geocities.com/spacetransport/spacecraft-avatar.html&amp;amp;date=2009-10-25+22:15:24&quot; rel=&quot;nofollow&quot;&gt;http://www.webcitation.org/query?url=http://www.geocities.com/spacetransport/spacecraft-avatar.html&amp;amp;date=2009-10-25+22:15:24&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2015-03-13T02:45:02.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8439" PostTypeId="1" CreationDate="2015-03-13T13:47:57.683" Score="0" ViewCount="613" Body="&lt;p&gt;I was thinking how to have a situation on Earth of feeling weightlessness for an indefinite time, not for a few minutes like on a &lt;a href=&quot;http://en.wikipedia.org/wiki/Zero_Gravity_Corporation&quot; rel=&quot;nofollow&quot;&gt;diving plane&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not reduced gravity, weightlessness as I feel in a swimming pool.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to imitate a state of free fall inside the Earth's atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="8703" LastEditorUserId="4660" LastEditDate="2015-03-14T02:21:10.560" LastActivityDate="2015-03-14T02:21:10.560" Title="How can a body experience weightlessness or micro gravity on the ground?" Tags="|gravity|microgravity|simulation|" AnswerCount="2" CommentCount="8" ClosedDate="2015-03-14T05:53:51.643" ContentLicense="CC BY-SA 3.0" />
  <row Id="8440" PostTypeId="1" CreationDate="2015-03-13T14:57:17.960" Score="10" ViewCount="3502" Body="&lt;p&gt;If the &lt;a href=&quot;http://www.andersoninstitute.com/alcubierre-warp-drive.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alcubierre Warp Drive&lt;/a&gt; was used to travel to Alpha Centauri, how much time would pass on Earth? Per &lt;a href=&quot;http://earthsky.org/space/alpha-centauri-travel-time&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; article, it would take merely days to travel there. Say a ship travels there, spends a few months and then returns to Earth, how much time would have passed for the folks that did not travel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5il9V.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8705" LastEditorUserId="25" LastEditDate="2015-08-25T13:59:52.857" LastActivityDate="2020-07-14T22:19:03.187" Title="If the Alcubierre Warp Drive was used to travel to Alpha Centauri, how much time would pass on Earth?" Tags="|rockets|interstellar-travel|engines|time|travel|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8441" PostTypeId="2" ParentId="8439" CreationDate="2015-03-13T15:23:03.243" Score="4" Body="&lt;p&gt;The way I see it, you have 4 options:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Free fall. As pointed out, you won't do this for very long before hitting the ground.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Go in one direction really fast (Orbit). This will result in extreme air resistance if you don't hit anything else, causing intense heat and probable incineration. The energy required to achieve this kind of acceleration would also extraordinary, so it would just be easier to orbit in space.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Go to the center of the earth (Where gravity pulls equally in all directions, so net zero). If you do this, you will promptly be crushed by the immense weight of everything else around you.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Compress the air around you so that it has about the same density you do. This is why swimming feels weightless. We are talking several orders of magnitude compression here, so in this case, you will be crushed almost instantly and your &quot;weightless&quot; crushed body will float in the special tank you built.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Not sure if any of the options are very appealing, but as far as I can tell, there aren't any others&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-03-13T15:23:03.243" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8442" PostTypeId="1" CreationDate="2015-03-13T15:40:40.937" Score="1" ViewCount="287" Body="&lt;p&gt;I've noticed when I look this up online with a quick Google search this theme emerges: that without the moon, if it were to disappear, the days would speed up a lot to say, 9 hours long and you will get strong winds--greater variability in weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea seems to be, just as we have this causality of the early solar system fast forwarding to eventual Earth-moon tidal force lockstep with its effect of 'Earth rotation braking', then it self-evidently follows that there's an opposing angular momentum release unto Earth from moon's sudden absence and subsequent shortening of the day over time from this, as opposed to continuation on trend itself following from the same source justification of angular momentum. Earth's own angular momentum of our rotation slowing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The latter is more what I'm thinking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;24 hour days and sun tides, til over a very long time, 25, 26, 27 hour days et al.&lt;/p&gt;&#xA;" OwnerUserId="8704" LastActivityDate="2015-03-13T17:59:59.990" Title="What would happen if we took out the moon?" Tags="|orbital-mechanics|the-moon|earth|" AnswerCount="1" CommentCount="4" ClosedDate="2015-03-13T19:06:24.557" ContentLicense="CC BY-SA 3.0" />
  <row Id="8443" PostTypeId="2" ParentId="8439" CreationDate="2015-03-13T16:05:25.630" Score="1" Body="&lt;p&gt;I think a &lt;a href=&quot;http://en.wikipedia.org/wiki/Vertical_wind_tunnel&quot; rel=&quot;nofollow&quot;&gt;Vertical Wind Tunnel&lt;/a&gt; could do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you could build one which adapt its power to your current drag, you would be like in micro gravity, - minus a strong wind all around you.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2015-03-13T16:05:25.630" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8444" PostTypeId="1" CreationDate="2015-03-13T16:42:10.970" Score="1" ViewCount="68" Body="&lt;p&gt;The moon moves approximately 4cm away from Earth per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that it affects Earth's weather, will this increasing distance already have had an impact on the weather (regardless of how negligible this difference is)?&lt;/p&gt;&#xA;" OwnerUserId="8706" LastActivityDate="2015-03-13T16:42:10.970" Title="Has the weather been affected by the moons increasing distance from Earth?" Tags="|the-moon|weather|earth|" AnswerCount="0" CommentCount="1" ClosedDate="2015-03-13T17:38:55.083" ContentLicense="CC BY-SA 3.0" />
  <row Id="8445" PostTypeId="2" ParentId="8442" CreationDate="2015-03-13T17:59:59.990" Score="5" Body="&lt;p&gt;The moon saps rotational energy from Earth and converts it into an increasing distance from the Earth. Earth's day gets longer over time as a result.&#xA;Removing the moon altogether ends this. so the lengthening of Earth's day slows down.&lt;br&gt;&#xA;The moon also stabilizes Earth's axis tilt, IIRC. Without this, you'll get more variation in axis tilt which has major impact on climate.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-03-13T17:59:59.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8446" PostTypeId="1" AcceptedAnswerId="17121" CreationDate="2015-03-14T01:31:19.957" Score="4" ViewCount="1153" Body="&lt;p&gt;How was the Apollo LM ascent launch procedure controlled? Was it manual, semi-automatic or fully automatic? By &quot;launch procedure&quot; I am talking about the pitch control for the LM. I believe there is no thrust control for the LM ascent stage engine, hence correct orbit would only be achieved by pitch control? But again, how was this pitch control performed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, how much room for error was there? During the six successful missions, what was the largest error for lunar rendezvous that occurred?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm assuming that the CSM could use its SPS engine to perform any major translations that might be required, but was this ever needed? Beyond that, I'm assuming both the LM and CSM would use RCS fuel for finer granularity translation maneuvers?&lt;/p&gt;&#xA;" OwnerUserId="8707" LastActivityDate="2016-07-10T20:00:30.633" Title="Apollo LM ascent procedure, computer controlled or manual?" Tags="|the-moon|apollo-program|lunar-landing|launch-sequence|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8448" PostTypeId="1" CreationDate="2015-03-14T06:58:32.190" Score="18" ViewCount="2798" Body="&lt;p&gt;Could the space shuttles have been left in space and used as temporarily visited space stations? One could put a modified &lt;a href=&quot;http://www.astronautix.com/craft/spacehab.htm&quot; rel=&quot;noreferrer&quot;&gt;spacehab&lt;/a&gt; in the cargo bay, with solar panels and with which the Soyuz/Dragon could dock. Wouldn't it have been a cheap and capable alternative to the ISS? Could its systems for life support et cetera work long term? Could they have been put to better use than as museum pieces, with some planning and minor modifications, or is it completely crazy? (I realize that they are not launchable anymore).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or maybe more realistically, couldn't they simply have been left as modules docked to the ISS? They would've provided some extra working space for the crew, boosting capability for orbital station keeping, RTG power, cargo handling, independent life support and an emergency landing system. The whole point with landing them was to reuse them. Well, they weren't reused after the last landing, so they should never have been landed the last time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/m6dM2.jpg&quot; alt=&quot;Endeavours last visit at the ISS&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Endeavours last visit at the ISS, seen from Soyuz. Could it have stayed there?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-03-14T07:19:51.360" LastActivityDate="2021-12-20T05:09:43.750" Title="Could the space shuttle be permanently left in orbit as a space station?" Tags="|iss|space-shuttle|space-station|" AnswerCount="2" CommentCount="7" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8449" PostTypeId="1" AcceptedAnswerId="8454" CreationDate="2015-03-14T07:08:35.197" Score="6" ViewCount="1021" Body="&lt;p&gt;Is there a cost effective way to obtain Nitrogen/Argon on Mars for usage in a human breathable habitat?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is my understanding that a sustainable human breathable atmosphere is made of two major parts, oxygen and inert gasses.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The most common 2 inert gasses found on mars are Nitrogen and Argon, but an answer that includes any gas obtainable on Mars that is mixable with oxygen to produce a sustainable human breathable atmosphere would be accepted)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S.  Assume oxygen is available from an alternative solution. &lt;a href=&quot;https://space.stackexchange.com/questions/5094/how-do-you-make-oxygen-on-mars&quot;&gt;How do you make oxygen on mars&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8653" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2019-10-16T14:53:49.363" Title="Is there a cost effective way to obtain Nitrogen/Argon on Mars for usage in a human breathable habitat?" Tags="|mars|colonization|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8450" PostTypeId="2" ParentId="8448" CreationDate="2015-03-14T09:44:40.177" Score="16" Body="&lt;p&gt;The Shuttle was designed for missions of around 10 days. With the &lt;a href=&quot;https://en.wikipedia.org/wiki/Extended_Duration_Orbiter&quot;&gt;Extended Duration Orbiter&lt;/a&gt; system added, missions could be &lt;a href=&quot;https://books.google.nl/books?id=ZwD8uTpXet4C&amp;amp;pg=PA246&amp;amp;lpg=PA246&amp;amp;dq=%22space%20shuttle%22%20orbit%20lifetime&amp;amp;source=bl&amp;amp;ots=l-m8q_g5gw&amp;amp;sig=0Rl0oQvG__0zr09NZxNbBcJnnk4&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=2v0DVaq0LcKwPObegJAE&amp;amp;ved=0CGkQ6AEwDA#v=onepage&amp;amp;q=%22space%20shuttle%22%20orbit%20lifetime&amp;amp;f=false&quot;&gt;14-19 days long&lt;/a&gt;. &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-80&quot;&gt;STS-80&lt;/a&gt; was the longest actual mission at 17 days. Mission lifetime was limited by fuel for the fuel cells which power the Shuttle.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The EDO pallet weighed 3.2 tons fueled and was 4.5 m in diameter and (my estimate) 1.5-2 m long. NASA considered fitting a shuttle with 2 pallets for missions of up to 28 days. &#xA;Another limit is life support: CO2 removal is done by lithium hydroxide canisters which need replacement every 12 hours.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the Shuttle needs a lot of consumables to keep working. I suspect when the Shuttle was docked to the ISS, it relied on the ISS life support system (haven't been able to find references for this yet). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-03-14T09:44:40.177" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8451" PostTypeId="1" AcceptedAnswerId="13073" CreationDate="2015-03-14T13:07:25.883" Score="19" ViewCount="15459" Body="&lt;p&gt;We know that many people who take astronaut training and become astronauts may never get to fly to space at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How many astronauts (number or %, or both) have never flown to space in their astronaut career?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The percentage probably changed a lot through decades, so let's say my question targets present time. But historical figures are very welcome as well.&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="6750" LastEditDate="2015-03-14T13:58:44.987" LastActivityDate="2019-10-12T04:24:16.657" Title="How many astronauts never got to fly in space in their astronaut career?" Tags="|history|astronauts|crewed-spaceflight|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8452" PostTypeId="1" CreationDate="2015-03-14T20:09:19.607" Score="2" ViewCount="274" Body="&lt;p&gt;China and the US have reportedly both used kinetic kill vehicles to destroy satellites. This strikes me as rather inefficient, however.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's well known that micrometeoroids pose a risk to satellites, so satellites are generally hardened against such impacts. There's a limit to how well they can be hardened, though; it seems like an object with a diameter of 10 to 20 cm impacting the core of a satellite should be enough to destroy it. An artificial sphere of this size made out of steel would weigh about 30 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By contrast, a kinetic kill vehicle must have the same kinetic energy as the satellite it intends to destroy. If my understanding of orbital mechanics is correct, the speeds of the two objects is likely to be equal, so the kinetic kill vehicle must have a similar mass to the satellite it intends to destroy. In the case of the semi-recent Chinese launch, that's about 750 kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that kinetic kill vehicles are able to tear apart the satellites they impact, and a tiny object putting a hole through a satellite would at best disable it, and could not accomplish the stated goals of either the American or Chinese weapons. That being said, have such small weapons ever been seriously considered? It seems like the significantly lower mass of such a system would be significantly cheaper and easier to launch.&lt;/p&gt;&#xA;" OwnerUserId="6287" LastEditorUserId="12102" LastEditDate="2019-09-28T11:54:21.117" LastActivityDate="2019-09-28T11:54:21.117" Title="Could artificial micrometeoroids be a practical anti-satellite weapon?" Tags="|artificial-satellite|weapon|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8453" PostTypeId="1" AcceptedAnswerId="8457" CreationDate="2015-03-14T20:45:10.263" Score="14" ViewCount="3231" Body="&lt;p&gt;I found the following to be interesting. It appears that for the most part (Apollo 15, 16 and maybe parts of 17?) there were restrictions placed on the distance allowed for the LRV to travel. The astronauts should be able to walk back to the LM should the vehicle for any reason break down, which seems reasonable, it seems outrageous to bet your life that this car will not break down.&lt;/p&gt;&#xA;&lt;p&gt;From Wikipedia (the &amp;quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Roving Vehicle&lt;/a&gt;&amp;quot; article):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;An operational constraint on the use of the LRV was that the astronauts must be able to walk back to the LM if the LRV were to fail at any time during the EVA (called the &amp;quot;Walkback Limit&amp;quot;). Thus, the traverses were limited in the distance they could go at the start and at any time later in the EVA. Therefore, they went to the farthest point away from the LM and worked their way back to it so that, as the life support consumables were depleted, their remaining walk back distance was equally diminished. This constraint was relaxed during the longest traverse on Apollo 17, based on the demonstrated reliability of the LRV and spacesuits on previous missions. A paper by Burkhalter and Sharp provides details on usage.*&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;My question is, when the restriction was lifted, how far away was the furthest distance from the LM that was achieved? Not total distance driven on one EVA, but longest distance in a straight line from the rover to the LM.&lt;/p&gt;&#xA;&lt;p&gt;Is there a picture of this record? I'd love to see it.&lt;/p&gt;&#xA;&lt;p&gt;As an aside (but closely related): How reliable was really this vehicle? Is the Wikipedia statement accurate, that the vehicle was &amp;quot;so reliable&amp;quot; that they lifted this restriction?&lt;/p&gt;&#xA;&lt;p&gt;I have found that the LRV apparently has redundancy in its power system (two batteries) and redundancy in its steering system (front and back can be steered, and one system can be disconnected independently of the other).&lt;/p&gt;&#xA;&lt;p&gt;However this note from EVA-3 on Apollo 17 seems to suggest the LRV was not perfect:&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;169:18:27 Cernan: I'm reading 80 on the amps, 78 on the amps...Correction, that's amp-hours. Voltages, 62 and 65. Battery 1 (temperature) is 132, forget 0. Motor temps are 200 and 210 on the rear, 200 and 250 on the forward.&lt;/p&gt;&#xA;&lt;p&gt;[The battery 2 temperature gauge has malfunctioned, hence Gene's &amp;quot;forget 0&amp;quot;.]&lt;/p&gt;&#xA;&lt;p&gt;[Gene and Jack have driven a total of 36.0 kilometers and have used 57.2 amp-hours -plus another 14.8 amp-hours for LCRU operations - out of the 242 amp-hours available. Their average use rate of 1.58 amp-hours/km is similar to the 1.67 am-hours/km used on Apollo 15. The use rate on Apollo 16 was nearly twice as much due to an unexplained malfunction.]&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;*Burkhalter, Bettye B; Sharpe, Mitchell R (1995). &amp;quot;Lunar Roving Vehicle: Historical Origins, Development and Deployment&amp;quot;. Journal of the British Interplanetary Society 48 (5): 199–212.&lt;/p&gt;&#xA;" OwnerUserId="8707" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-14T13:22:18.057" Title="Apollo 17, Lunar Roving Vehicle walkback limit &quot;relaxed&quot;?" Tags="|the-moon|apollo-program|lunar-landing|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8454" PostTypeId="2" ParentId="8449" CreationDate="2015-03-14T20:51:42.763" Score="5" Body="&lt;p&gt;Both argon and nitrogen are usually produced on Earth by freezing a volume of air until it &lt;a href=&quot;http://en.wikipedia.org/wiki/Air_separation&quot; rel=&quot;nofollow noreferrer&quot;&gt;liquefies&lt;/a&gt;, and then separating its component chemicals by carefully adjusting the temperature so that each one boils off separately and is collected. This is very energy intensive. The lower temperatures on Mars would mitigate this to some extent, but on the other hand, Mars experiences big temperature swings between day and night. That would probably make temperatures much harder to control in the precise manner needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More recently, &lt;a href=&quot;http://www.hindawi.com/journals/isrn/2012/982934/&quot; rel=&quot;nofollow noreferrer&quot;&gt;pressure swing adsorption&lt;/a&gt; has begun to be used for producing &lt;a href=&quot;http://en.wikipedia.org/wiki/Nitrogen_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;nitrogen&lt;/a&gt;, and for sequestration of &lt;a href=&quot;http://www.rsc.org/chemistryworld/Issues/2007/October/HowToBuryTheProblem.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;carbon dioxide&lt;/a&gt;. This works by pumping air through a high-pressure chamber filled with adsorption material like activated carbon or &lt;a href=&quot;http://en.wikipedia.org/wiki/Zeolite&quot; rel=&quot;nofollow noreferrer&quot;&gt;zeolites&lt;/a&gt;. There are synthetic zeolites that selectively absorb nitrogen. The quote below is from &lt;a href=&quot;http://nzic.org.nz/ChemProcesses/production/1D.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this link&lt;/a&gt;, from the New Zealand Institute of Chemistry:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Zeolites are aluminosilicate minerals with complex crystal structures&#xA;  made up of interlocking rings of silicon, aluminium and oxygen ions.&#xA;  The chemical composition of the zeolite used for oxygen separation is&#xA;  Na12[(AlO2)12((SiO2)12].27H2O. It is the zeolite’s shape which&#xA;  provides most of the ability to selectively adsorb nitrogen. The&#xA;  zeolite used for oxygen production is shaped like a die with holes&#xA;  drilled on each face to form an internal cage. The corners of the die&#xA;  (providing the framework) are SiO2 and AlO2 units. Cations (either Na&#xA;  or Ca) are exposed throughout the crystal lattice. This zeolite is&#xA;  shown in Figure 1.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5h1jN.jpg&quot; alt=&quot;nitrogen adsorbing zeolite&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(It is sometimes used for oxygen production on Earth because removing atmospheric nitrogen leaves mostly oxygen.) The air that exits such a pressure chamber is stripped of some of its nitrogen. Once the zeolite has been fully charged in this way with nitrogen, the pressure in the chamber is reduced, and the nitrogen  naturally releases again as a gas, and can be collected. Over several passes, the concentration of nitrogen in the resulting gas is increased, it can in very refined systems be made of high purity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But for Mars, maybe something like 90% would be enough, in the interest of reducing system power needs, size, and mass that needs to be launched from Earth. The effectiveness of the system depends on how good the zeolites are. In some ways, this is a good thing. They wouldn't have any trouble surviving the rigors of launch and landing, and they work passively, meaning they don't need repairs and maintenance. They can be used over and over, just release the pressure, and they release their nitrogen, then they are ready to adsorb more. On the other hand, they are heavy. I can't find figures so far on how much nitrogen you can expect to get from each cycle from each kilogram of zeolite (density of about $0.7 gm/cm^3$), but they are like porous rock. I bet it isn't much. Perhaps the technique that has been proposed for other resources, of sending ahead an automated plant that will run on its own, storing up nitrogen while it awaits the arrival of the first crew, could work. The mechanical parts in the system are pretty simple - pumps and pressure chambers. It sounds like something that could reasonably be automated, and the hardware parts aren't difficult to repair or replace.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least the power needs of the system, compared to cryogenic separation, are much lower. Which is a whole other can of worms - how many nuclear thermal generators a crewed base would need, and whether a solar power system could be an effective substitute. At any rate, much of the mass you would have to ship in zeolite could be defended by counting up the mass of power generators, insulators, and heat exchangers you &lt;em&gt;wouldn't&lt;/em&gt; need to ship because a cryogenic system isn't being used. This table from the same article linked above, by NZIC, makes clear how much less power PSA requires:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CatOL.jpg&quot; alt=&quot;comparison cryogenic oxygen production vs pressure swing adsorption&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-03-15T19:10:47.030" LastActivityDate="2015-03-15T19:10:47.030" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8456" PostTypeId="1" CreationDate="2015-03-14T23:05:08.610" Score="7" ViewCount="503" Body="&lt;p&gt;I was reading &lt;a href=&quot;https://space.stackexchange.com/q/5690/4660&quot;&gt;Why does the ISS atmosphere contain nitrogen&lt;/a&gt;, and I wondered if some of the explanations there would also apply to a base on the Moon. There are considerations on the Moon that don't apply (so much) to the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lower pressure atmosphere is easier to contain. It doesn't require that as much stuff be shipped up from Earth - especially Nitrogen which is not available there, while at some point Oxygen can be obtained from lunar soil once the infrastructure exists. The larger a base gets, the more this becomes a factor. Also the larger it gets, the easier it is to install the proper airlocks so that people arriving and departing can transition between the atmosphere on the ships and that in the base. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't there wiggle room where the oxygen fraction can be raised, the nitrogen fraction can be lowered, and the pressure can be some substantial fraction of Earth sea-level so that transitions between base and ships aren't difficult? Can it be said what the best mix would likely be?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-03-21T02:35:18.323" Title="What would be the best recipe for a breathable atmosphere in a base on the Moon?" Tags="|the-moon|colonization|atmosphere|transport|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8457" PostTypeId="2" ParentId="8453" CreationDate="2015-03-15T00:17:23.523" Score="12" Body="&lt;p&gt;According to Google Earth's map of Apollo 17 on the Moon, 7.41 km, or 4.6 miles.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2NnHH.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;If you want to look at the map, open Google Earth, and in the menu bar at top click on View &amp;gt; Explore &amp;gt; Moon. All of the Apollo missions are indicated by a little astronaut icon on the place where they landed, holding a little flag with the number of the mission. Make sure the 'Moon Gallery' box is checked in the Layers section on the bottom left.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-03-15T00:17:23.523" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8461" PostTypeId="2" ParentId="8330" CreationDate="2015-03-15T19:44:09.283" Score="23" Body="&lt;p&gt;As Mike Wise elucidated in his comment, this is rather speculative, since SpaceX as a private company can (and do) choose to keep their cost structures and breakdowns secret, and the prices vary between launches. We can, however, use a few bits of information to get a coarse overview of the prices involved. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Customer-facing costs&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;SpaceX currently sells an &quot;off the shelf&quot; Falcon 9 launch for &lt;strong&gt;\$61.2 million USD&lt;/strong&gt; (&lt;a href=&quot;http://www.spacex.com/about/capabilities&quot;&gt;from 2016&lt;/a&gt;). This price has in the past been relatively negotiable. Both MDA Corporation and SES (owners and operators of the CASSIOPE and SES-8 satellites which flew on the first and second Falcon 9v1.1 launches, respectively) paid well under the market value for their flights (&lt;a href=&quot;http://spaceref.ca/missions-and-programs/canadian-space-agency/after-years-in-storage-cassiope-satellite-set-for-launch.html&quot;&gt;CASSIOPE&lt;/a&gt;, &lt;a href=&quot;http://spacenews.com/37312rocket-oversupply-or-not-satellite-operators-still-struggle-to-secure/&quot;&gt;SES-8&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This launch price is up on previous SpaceX rates for Falcon 9:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;\$35 million USD&lt;/strong&gt; in &lt;a href=&quot;https://web.archive.org/web/20050912050019/http://www.spacex.com/&quot;&gt;2005&lt;/a&gt;, 8700kg to LEO&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;\$36.75 million USD&lt;/strong&gt; in &lt;a href=&quot;https://web.archive.org/web/20090201094003/http://spacex.com/falcon9.php&quot;&gt;2009&lt;/a&gt;, with an increasing cost as mission demands increased.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;\$44 million USD&lt;/strong&gt; in &lt;a href=&quot;https://web.archive.org/web/20100102224858/http://spacex.com/falcon9.php&quot;&gt;early 2010&lt;/a&gt; before the first launch.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;\$49.9 million USD&lt;/strong&gt; in &lt;a href=&quot;https://web.archive.org/web/20101222155322/http://www.spacex.com/falcon9.php&quot;&gt;late 2010&lt;/a&gt; after the second launch&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;\$54 million USD&lt;/strong&gt; in &lt;a href=&quot;https://web.archive.org/web/20110416203343/http://www.spacex.com/falcon9.php&quot;&gt;2011&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;\$56.5 million USD&lt;/strong&gt; in &lt;a href=&quot;https://web.archive.org/web/20130924104819/http://www.spacex.com/falcon9&quot;&gt;2013-2014&lt;/a&gt; for Falcon 9v1.1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Prices has since stabilized a bit, NASA for example is &lt;a href=&quot;http://www.nasa.gov/press/2014/december/nasa-awards-launch-services-contract-for-transiting-exoplanet-survey-satellite/#.VQXZWOGc7vI&quot;&gt;paying&lt;/a&gt; &lt;strong&gt;\$87 million USD&lt;/strong&gt; for the launch of TESS aboard a Falcon 9 in 2017. This roughly agrees with &lt;a href=&quot;http://spaceflightnow.com/news/n1403/05spacexula/#.VQXaA-Gc7vI&quot;&gt;Musk's comments&lt;/a&gt; that U.S. government launches will cost approximately &lt;strong&gt;$90 million USD&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;SpaceX Internal costs&lt;/h1&gt;&#xA;&#xA;&lt;h3&gt;Propellant&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://shitelonsays.com/transcript/spacex-press-conference-at-the-national-press-club-2014-04-25&quot;&gt;Musk has famously stated&lt;/a&gt; at the National Press Club that the cost of propellant is only 0.3% the cost of the rocket, which yields about \$200,000 for a $60m launch. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;First stage&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://shitelonsays.com/transcript/spacex-press-conference-september-29-2013-2013-09-29&quot;&gt;We also know&lt;/a&gt; that the first stage is approximately &quot;less than three-quarters&quot; of the cost of the entire rocket. Now, what we can't be sure of is SpaceX's profit and margins on each launch, if the entirety of the rocket's manufacturing costs only \$40 million, then the first stage is approximately or just less than $30 million. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An employee I spoke to once concerning the SES-8 launch was that due to delays and the number of penalties that needed to be paid to SES, they barely broke even on that particular launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll try and improve this comment throughout the day as I find more information.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-03-15T19:44:09.283" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8462" PostTypeId="2" ParentId="8451" CreationDate="2015-03-15T22:54:50.673" Score="7" Body="&lt;p&gt;There are different definitions of the word &quot;astronaut.&quot; If you use NASA's definition (courtesy of &lt;a href=&quot;https://space.stackexchange.com/questions/8451/how-many-astronauts-never-got-to-fly-in-space-in-their-astronaut-career#comment22074_8451&quot;&gt;cpast&lt;/a&gt;), then you don't have to get off the ground to be considered an astronaut.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was going to go with a simple definition of an astronaut as a person who has entered outer space, but then I remembered that &quot;space&quot; is loosely defined! The &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot;&gt;Kármán line&lt;/a&gt; is, I think, the most commonly given definition, but it gets quite tricky to determine just where Earth's atmosphere ends and space begins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I decided to not aim for a comprehensive list, but to instead just give a few people.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Deke Slayton (sort of)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Deke_Slayton&quot;&gt;Deke Slayton&lt;/a&gt; may be the person who best embodied the early years of NASA. A former USAF pilot (and engineer by training), he was one of the seven applicants selected to be part of the Mercury 7 - the first seven American astronauts who would attempt to go to space. He was going to be the fourth to fly, but he was infamously grounded because of a heart condition. &lt;a href=&quot;https://en.wikipedia.org/wiki/Scott_Carpenter&quot;&gt;Scott Carpenter&lt;/a&gt; took over.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After Mercury, Slayton stayed on with NASA. He would never go into space, but he was an astronaut nonetheless. He became the unofficial leader of the astronauts, and continually worked with them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fortunately, Slayton &lt;em&gt;would&lt;/em&gt; get to space. He managed to get selected for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot;&gt;Apollo-Soyuz test project&lt;/a&gt; in 1975, and finally, after over a decade of waiting, reached outer space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, actually, he fails to make your list.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Michael J. Smith, Christa McAuliff, and Gregory Jarvis&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Michael_J._Smith_(astronaut)&quot;&gt;Smith&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Christa_McAuliffe&quot;&gt;McAuliff&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Gregory_Jarvis&quot;&gt;Jarvis&lt;/a&gt; were all rookies when they flew in the Space Shuttle &lt;em&gt;Challenger&lt;/em&gt; on January 28, 1986. Smith was the pilot, and McAuliff and Jarvis were payload specialists, with McAuliff to be the first teacher in space. 73 seconds into the flight, though, a catastrophic failure in one of the O-rings caused an explosion on one of the solid rocket boosters. All three underwent astronaut training, but none reached space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Barbara_Morgan&quot;&gt;Barbara Morgan&lt;/a&gt; was McAuliff's backup, but she eventually flew on the Space Shuttle later in life.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Some North America X-15 pilots&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/North_American_X-15&quot;&gt;North American X-15&lt;/a&gt; was an experimental rocket-propelled aircraft (dropped from a &quot;mothership&quot;, often a bomber such as a B-52). The X-15 was designed to go high and fast (though not always at the same time), with &lt;a href=&quot;https://en.wikipedia.org/wiki/X-15_Flight_91&quot;&gt;the highest flight&lt;/a&gt; being 67.0 miles, by &lt;a href=&quot;https://en.wikipedia.org/wiki/Joseph_A._Walker&quot;&gt;Joe Walker&lt;/a&gt;. While all those who flew above 50 miles were considered &quot;astronauts&quot; by NASA, only Walker made it past the Kármán line, which he did twice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you consider 50 miles to be the lower limit of space, some pilots did not reach it (&lt;a href=&quot;https://en.wikipedia.org/wiki/Neil_Armstrong&quot;&gt;Armstrong&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Albert_Scott_Crossfield&quot;&gt;Crossfield&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Forrest_S._Petersen&quot;&gt;Petersen&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Milton_Orville_Thompson&quot;&gt;Thompson&lt;/a&gt;), though Armstrong would go further than any of them and reach the Moon.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Part of NASA Group 3&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronaut_Group_3&quot;&gt;Astronaut Group 3&lt;/a&gt; were to be some of the members in the Gemini and Apollo programs. All but four flew in Apollo. &lt;a href=&quot;https://en.wikipedia.org/wiki/Roger_B._Chaffee&quot;&gt;Roger Chaffee&lt;/a&gt; died in the tragic fire during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_1&quot;&gt;&lt;em&gt;Apollo 1&lt;/em&gt;&lt;/a&gt; test along with Edward White and Virgil &quot;Gus&quot; Grissom; the latter two had previously been to space and were not in Astronaut Group 3. &lt;a href=&quot;https://en.wikipedia.org/wiki/Charles_Bassett&quot;&gt;Charles Bassett&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Theodore_Freeman&quot;&gt;Theodore Freeman&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Clifton_Williams&quot;&gt;Clifton Williams&lt;/a&gt; died in training accidents.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Part of NASA Group 4&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_4&quot;&gt;These astronauts&lt;/a&gt; were supposed to study science on the Moon. All but two made it to space, with the exceptions being &lt;a href=&quot;https://en.wikipedia.org/wiki/Duane_Graveline&quot;&gt;Duane Graveline&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Curt_Michel&quot;&gt;Curt Michel&lt;/a&gt;, both of whom resigned due to personal reasons.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Part of NASA Group 5&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_5&quot;&gt;these astronauts&lt;/a&gt; ended up going into space in one way or another. The two exceptions were &lt;a href=&quot;https://en.wikipedia.org/wiki/John_S._Bull&quot;&gt;John Bull&lt;/a&gt;, who did not fly after learning he had a medical condition, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Edward_Givens&quot;&gt;Edward Givens&lt;/a&gt;, who died in a car crash.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Part of NASA Group 6&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronauts in &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_6&quot;&gt;Astronaut Group 6&lt;/a&gt; were also tasked with performing science experiments. None flew in Apollo, but some flew in Skylab. Two did not: &lt;a href=&quot;https://en.wikipedia.org/wiki/Philip_K._Chapman&quot;&gt;Philip K Chapman&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Anthony_Llewellyn&quot;&gt;Anthony Llewllyn&lt;/a&gt;, both of whom resigned.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Almost all of those in the Manned Spaceflight Engineer Program&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Manned_Spaceflight_Engineer_Program&quot;&gt;Manned Spaceflight Engineer Program&lt;/a&gt; was the US military's attempt to put engineers on Space Shuttle flights. NASA, however, was not a fan of the idea, and only one (&lt;a href=&quot;https://en.wikipedia.org/wiki/Gary_Payton_(astronaut)&quot;&gt;Payton&lt;/a&gt;) reached space - on a military flight for the Department of Defense.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This has taken me way to long, and I'm only on those who trained with NASA! I'll try to add more to this, but for now, that's all I have. All of those listed were, by NASA's definition, &quot;astronauts&quot; (except for the few who did not complete training), but none of them reached space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should note that I avoided cancelled programs. If I included those, the list would be much, much longer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kudos to &lt;a href=&quot;https://space.stackexchange.com/questions/8451/how-many-astronauts-never-got-to-fly-in-space-in-their-astronaut-career#comment22070_8451&quot;&gt;Deer Hunter&lt;/a&gt; for providing &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_astronauts_by_year_of_selection&quot;&gt;this list&lt;/a&gt;. It helped me gather quite a few names that I did not already know.&lt;/p&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2015-03-15T22:54:50.673" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8463" PostTypeId="1" AcceptedAnswerId="8464" CreationDate="2015-03-16T09:55:46.120" Score="3" ViewCount="3503" Body="&lt;p&gt;I was told that if you theoretically jump down from a three-floor building (assume 10 m) on the Moon surface, you will equally break your bones as in if you were doing that on Earth. I do not know why would this be true: the acceleration due to free fall on Moon is significantly smaller, thus the final v shall be smaller than v on Earth. Would you be perfectly fine if you jumped down a 10 meter building on Moon?&lt;/p&gt;&#xA;" OwnerUserId="8715" LastActivityDate="2015-03-16T19:28:51.977" Title="Is it possible to jump down a 10 meter building on Moon and be perfectly fine?" Tags="|the-moon|gravity|" AnswerCount="2" CommentCount="3" ClosedDate="2015-03-16T19:34:23.410" ContentLicense="CC BY-SA 3.0" />
  <row Id="8464" PostTypeId="2" ParentId="8463" CreationDate="2015-03-16T10:29:56.330" Score="11" Body="&lt;p&gt;It is certainly not the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity on the Earth's surface is 9.81 m/s2. Assuming no upward or downward initial velocity, this means your velocity at ground level falling from 10 m is about &lt;strong&gt;14 m/s&lt;/strong&gt; (&lt;code&gt;sqrt(2 * 9.81 * 10)&lt;/code&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity on the Moon's surface is 1.625 m/s2. The same fall will result in final velocity of about &lt;strong&gt;5.7 m/s&lt;/strong&gt; (&lt;code&gt;sqrt(2 * 1.625 * 10&lt;/code&gt;). This is equivalent to a fall on Earth from about &lt;strong&gt;1.6 m&lt;/strong&gt; (&lt;code&gt;5.7 ^ 2 / 2 / 9.81&lt;/code&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming you don't land on your head, you should be okay.&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="8157" LastEditDate="2015-03-16T10:35:40.633" LastActivityDate="2015-03-16T10:35:40.633" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8465" PostTypeId="1" AcceptedAnswerId="14860" CreationDate="2015-03-16T10:54:30.960" Score="6" ViewCount="278" Body="&lt;p&gt;Is there any known web-based API services that provide downlink/uplink testing facilities to some Cubesat? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't want to have full hardware setup: antenna, sdr, auxiliary equipment. In same time, I want to test specific DSP algoritms.&lt;/p&gt;&#xA;" OwnerUserId="1217" LastEditorUserId="1217" LastEditDate="2015-03-16T13:11:31.973" LastActivityDate="2016-04-16T11:00:38.187" Title="Cubesat communication services" Tags="|communication|cubesat|communication-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8466" PostTypeId="2" ParentId="8463" CreationDate="2015-03-16T11:53:04.943" Score="3" Body="&lt;p&gt;If you come from Earth and make the jump, you'll be okay as the other answer says. But if you were to live full time on the moon or in space, your body would adapt to the weaker gravitational field, resulting in muscle mass loss and other effects - if these included changes to the bone structure, such a jump could be dangerous after six months or so. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be worth reading about the physiological changes to the MIR cosmonauts and ISS inhabitants to understand better what happens after a few months to a year - and how exercise programs mitigate the effects - but extending the lessons to multiple years or a lifetime in reduced gravity would probably be speculation.&lt;/p&gt;&#xA;" OwnerUserId="6864" LastActivityDate="2015-03-16T11:53:04.943" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8467" PostTypeId="1" AcceptedAnswerId="8468" CreationDate="2015-03-16T12:50:57.737" Score="3" ViewCount="348" Body="&lt;p&gt;I'm in the process of designing a spacecraft for a project, and I'm at the stage of choosing the propulsion system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm going to use a Boeing 702 ion thruster (requiring 4.5kW) for propulsion, to take a spacecraft from Earth to Mars. Hence I need to know ways of generating 4.5kW and the mass (mass should be minimised) of whatever is supplying this energy. &lt;/p&gt;&#xA;" OwnerUserId="8694" LastEditorUserId="6214" LastEditDate="2015-03-23T02:39:49.133" LastActivityDate="2015-03-23T02:39:49.133" Title="What are low-mass ways of supplying power for a 4.5kW ion thruster?" Tags="|propulsion|power|solar-power|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8468" PostTypeId="2" ParentId="8467" CreationDate="2015-03-16T17:18:29.253" Score="7" Body="&lt;p&gt;Batteries and fuel cells are probably out because of the long duration of an Earth-Mars mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some mass and power output numbers for &lt;a href=&quot;http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#RTG_for_interstellar_probes&quot; rel=&quot;nofollow&quot;&gt;radioisotope thermoelectric generators&lt;/a&gt; (RTGs); the most mass-efficient of these run about 5W/kg, so you're looking at about a ton of RTG to run your thruster full up. RTGs are attractive for outer planetary missions because solar energy falls off with the square of distance, but we're only going to Mars, so it's likely that solar is a better option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia, &lt;a href=&quot;http://en.wikipedia.org/wiki/Solar_panels_on_spacecraft#Power_available&quot; rel=&quot;nofollow&quot;&gt;solar arrays&lt;/a&gt; can produce as much as 300W/kg at Earth distance. It's not clear from that article how practical cells are at that power to mass ratio, or how much of the mass is structural/deployment mechanics versus actual cells, so your mileage may vary considerably if you're going beyond &quot;back of the envelope&quot; design of the spacecraft, but let's go with that number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At Mars orbit, 1.666 AU, you're only going to get about 35% of that, about 100W/kg -- still a lot better than RTGs! So your solar array is going to mass on the order of 50kg. You might want to take extra solar panels as insurance against degradation, or just run the ion thruster at lower power later in the mission if necessary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're very patient, there may be a sweet spot where a smaller solar array plus a large battery can run the thruster at full power for limited periods of time for less mass investment than a big solar array. This will mean that, e.g. orbital adjustments will have to be done in more, shorter burns than if you can sustain the thruster constantly.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-03-17T18:15:50.173" LastActivityDate="2015-03-17T18:15:50.173" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8470" PostTypeId="2" ParentId="986" CreationDate="2015-03-16T23:12:22.207" Score="0" Body="&lt;p&gt;Laura Seward Forczyk gives &lt;a href=&quot;http://laurasspaceonspace.blogspot.com/2015/03/mars-or-bust-mars-one-busted-and-other.html&quot; rel=&quot;nofollow&quot;&gt;one of the better summaries of potential manned Mars missions&lt;/a&gt; I've seen lately -- especially concerning Mars One.  In summary, Mars One is a gimmick and no one is going to Mars in the next 15 years and probably longer.  Most of what you hear otherwise is hype.  The best chance for someone to do it in our lifetime is probably Elon Musk -- and he's a long shot.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-03-17T20:31:00.153" LastActivityDate="2015-03-17T20:31:00.153" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8471" PostTypeId="1" CreationDate="2015-03-17T03:45:34.587" Score="4" ViewCount="638" Body="&lt;p&gt;I heard that &lt;a href=&quot;http://en.wikipedia.org/wiki/Kessler_syndrome&quot; rel=&quot;nofollow&quot;&gt;Kessler Syndrome&lt;/a&gt; would essentially end space exploration, but how expensive would it be to get past it if Earth really wanted to?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be more specific about the Syndrome, suppose all the satellites some time in the future, say in 2050, were destroyed.&lt;/p&gt;&#xA;" OwnerUserId="8722" LastEditorDisplayName="user8406" LastEditDate="2015-03-18T09:25:32.753" LastActivityDate="2015-03-18T12:05:19.327" Title="How expensive would it be to make a spacecraft that can survive leaving an Earth suffering from Kessler Syndrome?" Tags="|rockets|debris|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8472" PostTypeId="2" ParentId="8467" CreationDate="2015-03-17T06:25:49.787" Score="4" Body="&lt;p&gt;A quick bit of Googling finds that the Dawn mission's 2.3kW ion engine is powered by a 36.4 m^2 solar array that generates 10.3 kW at 1 AU, decreasing to about 4.6 kW at Mars.  So depending on how much power is needed for non-propulsion uses, you could probably get away with about 50-60 m^2.  Should be easy to find approximate mass of that size array.&lt;/p&gt;&#xA;" OwnerUserId="8445" LastActivityDate="2015-03-17T06:25:49.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8473" PostTypeId="2" ParentId="8471" CreationDate="2015-03-17T14:39:06.957" Score="-1" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Kessler_syndrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kessler Syndrome&lt;/a&gt; does not end space exploration. At worst it delays it. The number of satellites in LEO is self correcting in a few decades see &lt;a href=&quot;https://space.stackexchange.com/questions/999&quot;&gt;Space debris half life&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the amount of objects in LEO orbit gets high enough, they can become a fuel/reaction mass resource &lt;a href=&quot;https://space.stackexchange.com/questions/2297&quot;&gt;Can I get higher with Space Junk?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With sufficient technology it is not a concern, consider a star trek type shield, you would just turn it on and drive though same as debris in space. (I think we have a question/answer around this also, but not seeing it)&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-03-18T02:09:21.990" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8474" PostTypeId="1" AcceptedAnswerId="8475" CreationDate="2015-03-17T16:22:13.257" Score="3" ViewCount="593" Body="&lt;p&gt;The Net Positive Suction Head of a pump is defined as &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;NPSH=(pi-pv)/(g0*rho)&#xA;&#xA;  pi  : pressure at pump inlet [Pa]&#xA;  pv  : vapore pressure of a medium (propellant) [Pa]&#xA;  rho : density of a medium (propellant) [kg/m^3]&#xA;  g0  : gravitational constant [m/s^2]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Why is there g0 in this equation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It makes complete sense here on earth, but for a system in space shouldn't NPSH better be calculated with the acceleration of the vehicle?&lt;/p&gt;&#xA;" OwnerUserId="8724" LastEditorUserId="8724" LastEditDate="2015-03-17T16:39:11.273" LastActivityDate="2015-03-17T18:24:50.103" Title="Turbo Pumps and NPSH in Space" Tags="|propulsion|fuel|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8475" PostTypeId="2" ParentId="8474" CreationDate="2015-03-17T16:44:50.893" Score="3" Body="&lt;p&gt;NPSH is a quantity derived from water pumps. On earth tells you how high you can put the pump above the water line before the medium starts cavitating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Expressing the cavitation potential of a space-born turbopump in meters is an anachronism. Dividing by g0 is just a way of expressing a pressure as a length. It has nothing to do with the actual acceleration at the place.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-03-17T16:44:50.893" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8476" PostTypeId="2" ParentId="8474" CreationDate="2015-03-17T16:57:14.720" Score="1" Body="&lt;p&gt;NPSH is measured in meters (length).  It tells you how high a pump (assuming no friction losses) on Earth can be put above a fluid before you start damaging it due to cavitation.  If you were putting the pump on the moon, the height would be different.  You could use this on a rocket under static acceleration, but that would be a bit silly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is similar to describing acceleration in g's -- an arbitrary scaling factor.  All that matters in your pump design (generally) is that the number is greater than 0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NPSH is different than NPSP.  NPSP removes scaling and expresses the value in pressure (mass/(length*time^2)).  The pressures involved might increase or decrease due to the system's acceleration, but the value is still measured in pressure.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-03-17T18:24:50.103" LastActivityDate="2015-03-17T18:24:50.103" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8478" PostTypeId="2" ParentId="8471" CreationDate="2015-03-17T21:04:11.817" Score="5" Body="&lt;p&gt;If you are talking about going &lt;em&gt;through&lt;/em&gt; the debris, not staying in it (i.e. an escape trajectory or hohmann transfer to cleaner orbit) then the ship would only be in LEO for a few minutes to hours. Only the most severe Kessler Syndrome would cause a problem.&lt;/p&gt;&#xA;" OwnerUserId="8697" LastActivityDate="2015-03-17T21:04:11.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8479" PostTypeId="2" ParentId="8471" CreationDate="2015-03-18T08:55:01.473" Score="0" Body="&lt;p&gt;I'm guessing that it would be cheaper to clean up the space debris than to build and launch a super strong shield for everything you send into space. I'm also not sure that we have the technology to build a shield like that at the moment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As pointed out in another answer, orbits decay, so you can also just wait it out. Orbits tend to decay faster for objects in lower orbits, so it would probably be best to focus efforts on cleaning up higher orbit debris and larger debris items first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What follows is a back-of-the-envelope calculation of what it would cost using small ion thruster &quot;tug&quot; satellites to deorbit all debris larger than 5cm in earth orbit at the moment (Note that this still leaves an enormous amount of debris smaller than 5cm that can still easily inflict a lot of damage as well):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://images.spaceref.com/news/2009/ODMediaBriefing28Apr09-1.pdf&quot; rel=&quot;nofollow&quot;&gt;NASA estimates 14000 pieces in orbit as of 2009&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.extremetech.com/extreme/160630-new-kickstarter-ion-thruster-would-speed-space-exploration-using-nanosatellites&quot; rel=&quot;nofollow&quot;&gt;An ion thruster powered 3U cubesat tug could cost about \$65000&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;You can remove at least one debris item per tug using 3U cubesat tugs (No reference whatsoever)&lt;/li&gt;&#xA;&lt;li&gt;Assuming Geostationary Orbit for all objects (Worst case)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;nofollow&quot;&gt;SpaceX will launch 4850kg to GTO for \$61.2M&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;A 3U cubesat weighs 3kg (I think this is defined in the cubesat spec but it can vary)&lt;/li&gt;&#xA;&lt;li&gt;This means that you can deploy about 1600 tugs in one launch (4850 / 3 ... ignoring a lot of other relevant factors)&lt;/li&gt;&#xA;&lt;li&gt;Including launch, the cost of a tug is then about \$103000 (65000 + 61200000 / 1600)&lt;/li&gt;&#xA;&lt;li&gt;This brings the total cleanup cost to about \$1.4B (103000 * 14000)&lt;/li&gt;&#xA;&lt;li&gt;Projects like this require operations and engineering staff as well as admin, so you can double the number above to be more realistic&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The scenario outlined above is probably the least efficient way to do it, but it might give you an idea of the upper limit for cleanup operations.&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="8157" LastEditDate="2015-03-18T09:00:05.780" LastActivityDate="2015-03-18T09:00:05.780" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8480" PostTypeId="2" ParentId="8471" CreationDate="2015-03-18T09:41:14.237" Score="7" Body="&lt;p&gt;It is worth reading &lt;a href=&quot;http://webpages.charter.net/dkessler/files/KesSym.html&quot; rel=&quot;noreferrer&quot;&gt;http://webpages.charter.net/dkessler/files/KesSym.html&lt;/a&gt; and some of the linked articles in that article are also very informative reading, altough can be quite technical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first thing is that a cascade does not happen in a matter of days or even months. Instead what happens is increasing density of debris reduces the expected lifetime of a satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not all orbits are affected. The main problematic orbit is low earth orbit. Very low orbits, where atmospheric drag is significant, will not be badly affected because the atmosphere will quickly remove debris. Below around 800 km atmospheric drag will be reasonably effective as the orbits of debris will decay in the matter of decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Particularly problematic is low earth orbits in the range 800-2000km, at these altitudes it takes centuries for atmospheric drag to remove debris, the debris in those orbits stay. With nothing removing the debris, the density is free to build up from collisions. The low rate of orbital decay and the fact that they are below the van allen belts are factors which make these altitudes attractive for satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even in the worst case scenario, the low low earth orbit would remain reasonably hospitable because the atmosphere is quickly removing the debris and the very stability of the higher orbits means the debris flux from higher orbits would be reasonably low. During the worst of a kessler syndrome cascade there would be significant extra debris even at very low earth orbit, but it would improve in a matter of years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In higher orbits, the orbit doesn't become unusable per se. Instead what you have is probabilities. With a desired mission lifetime in mind, a level of shielding is required for a certain probability of achieving that lifetime. Above a certain projectile mass (~100g) shielding becomes impractical. However, the same debris which are too large to shield against, are also large enough to track effectively, so active collision avoidance could  be used. Both shielding and provision for active avoidance adds mass and complexity to the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Middle Earth Orbits will not be badly affected because the density of satellites is much lower. The density of satellites is much lower because these orbits are not as useful (also the van allen belts are found in MEO, and are problematic for satellites at certain altitudes). A satellite would still need to pass through the LEO debris belt, but it comes back down to probabilities, there is a probability of collision which damages the satellite, and you can add shielding to reduce the probability of damage to a level deemed acceptable. Also, a path can be plotted which avoids trackable debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary then, even in the case of a very bad kessler syndrome, very low earth orbits, where orbital decay measured on the matter of years, will remain quite usable. Collision avoidance and shielding can be used to reduce probability of satellite destruction to acceptable levels. It may be the case that certain altitude ranges become effectively unusable because the expense of making a satellite which can survive in the debris belt, would be greater than the expense of either having the satellite in a lower orbit (requiring frequent boosting) or a higher orbit (requiring a more powerful rocket and a dangerous pass through the worst of the debris). I think the impact of a kessler syndrome is sometimes exaggerated, although being deprived of the most useful altitudes and the destruction of nearly all existing LEO satellites would certainly be very harmful to humanity's ventures in space.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-03-18T09:41:14.237" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8481" PostTypeId="2" ParentId="8471" CreationDate="2015-03-18T11:21:14.830" Score="1" Body="&lt;p&gt;Kessler syndrome describes the cascade effect of debris to debris impacts. One catastrophic impact can cause many smaller object each capable of causing a further catastrophic impact, hence the cascade.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be clear, if in 2050 all satellites are destroyed this isn't strictly Kessler syndrome. It also depends on how much all the satellites are destroyed. A good measure for this is the median cross sectional area of the resulting cloud of material. If the cross sectional area is very small, then the mass is &lt;em&gt;even smaller&lt;/em&gt;! If you assume all objects are spheres then you can see through the area of a circle ($\pi r^2$) and the volume of a sphere ($\frac{4}{3} \pi r^3$) as you get 2 time smaller in radius you get 4 times smaller in area and 8 times smaller in volume (and hence mass). So very small particles actually deorbit pretty quickly because they have a much lower ballistic coefficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now to your question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the object are small say 1mm or smaller we can handle the impact of them easily with existing technologies such as wipple shields. Anything larger and you're in more trouble. Assuming you don't want to first remedy the debris situation you're left with a few choices:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;build a very thick hull, this would increase the size of impact you could withstand but it would increase your fuel costs.&lt;/li&gt;&#xA;&lt;li&gt;search for a gap in the cloud, the exploded debris objects are going to follow a similar path as they did when they were satellites, so if you're lucky you night be able to launch from the north pole for example and thrust directly upward through a nice gap in the cloud.&lt;/li&gt;&#xA;&lt;li&gt;nuclear pulse propulsion - NPP could be used to propel your spacecraft (it's never been tested, but more for social/political reasons and technological readiness) and since you already have the nukes on board throw a half yield nuke out ahead of your spacecraft to blast the debris fragments out of the way (half yield so you still get forward motion). We're really moving towards science fiction now but...&lt;/li&gt;&#xA;&lt;li&gt;you could use laser ablation to change the trajectory of any potential threat in real time (this would be extreme hard for pointing and tracking).&lt;/li&gt;&#xA;&lt;li&gt;an Alcubierre drive contracts space in front of the spacecraft and expands it behind the spacecraft (allowing faster than light speeds from an external reference frame) this sort of technology could be used to create a launch window by skipping some debris ahead of where it would be, but the TRL on this is about 1.&lt;/li&gt;&#xA;&lt;li&gt;wormholes. I guess this is a reasonably obvious metjod if we have the technology; although if you've watched enough stargate you'll at least know to avoid a wormhole that passes through a star that could emit a solar flare. No wait! Do go through those stars, and bring the technology back to today and we can stop the creation of the debris cloud. Although then there would be no need for you to bring it back - ok new rule: please take care of paradoxes before defining your flight plan.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So in terms of expense you're looking at increasing cost the further you go down the list. Whether is be launch costs (at say \$10,000/kg) for extra mass or development costs for wormhole technology (at say \$1 Bazillion). Putting a accurate number is going to be difficult without a narrower question (spatial density of objects, median object size etc.)&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="1567" LastEditDate="2015-03-18T12:05:19.327" LastActivityDate="2015-03-18T12:05:19.327" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8482" PostTypeId="1" AcceptedAnswerId="8490" CreationDate="2015-03-19T00:23:13.287" Score="0" ViewCount="518" Body="&lt;p&gt;To be more specific, if the relativity of time is governed by the mass and gravity of objects (I.E the satellites orbiting earth experience minor time dilation compared to us on the ground), how is it experienced in deep space, the space that separates the galaxies? Is time only relevant if gravity is present?&lt;/p&gt;&#xA;" OwnerUserId="8729" LastActivityDate="2015-03-19T08:26:57.793" Title="Considering the relativity of time, how is time experienced in deep space?" Tags="|gravity|time|mass|" AnswerCount="1" CommentCount="5" ClosedDate="2015-03-19T08:55:42.323" ContentLicense="CC BY-SA 3.0" />
  <row Id="8483" PostTypeId="1" CreationDate="2015-03-19T01:03:19.400" Score="6" ViewCount="641" Body="&lt;p&gt;We have all sorts of spacecraft orbiting just above a huge reservoir of nitrogen and oxygen. Isn't there some way one could be designed to reach down and pump that air into canisters, and store them until a ship came, picked them up, and left a new supply of empty canister?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps some type of &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_tether&quot; rel=&quot;noreferrer&quot;&gt;tether system&lt;/a&gt; could make that feasible - electrodynamic or momentum-exchange tethers. The tether could extend down from the main body of the spacecraft in LEO far enough for there to be enough air that it can be pumped into air cylinders. Cylinders and pumps could be lowered down the tether for filling and then drawn back up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This comes up due to fretting about volatiles on the Moon, which is becoming sort of a pastime. Could some kind of setup like this work? If it could, what hurdles would be involved in making it? Could it be cheaper than shipping from the surface of Earth (or Mars)?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-03-19T02:34:32.360" Title="Could air be collected by a tether system from orbit?" Tags="|technology|transport|space-tether|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8484" PostTypeId="2" ParentId="8483" CreationDate="2015-03-19T01:59:58.687" Score="6" Body="&lt;p&gt;There are some real hurdles here.  They include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Drag.  Some portion of the &quot;hose&quot; will experience drag which will have to be compensated for by some sort of propulsion.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Angular momentum.  Regardless of how you pump the gas up to a higher orbit, you will have to increase that mass' angular momentum.  More propellant...&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Rarefied gas.  In order to be able to scoop up any reasonable quantity of gas, you are going to have to go more than just a little ways into the atmosphere.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Thermal dynamics.  I suspect you will burn your tether up once you get deep enough to matter.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You might handle the first two with some fancy nuclear electric propulsion and a counterweight that would extend upwards.  But gosh...  So much complexity...  There are easier ways to get these gases in the inner solar system.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-03-19T01:59:58.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8485" PostTypeId="2" ParentId="8483" CreationDate="2015-03-19T02:34:32.360" Score="7" Body="&lt;p&gt;I'm going to say &quot;no&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought of a lot of objections but realized I was overlooking a fundamental one, namely:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;What's the most important difference between a kilogram of oxynitrogen in the upper atmosphere and that same kilogram in an air tank in low Earth orbit?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The difference is the speed that it is moving at.&lt;/strong&gt;  Eight km/sec!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No matter how your system works, it is going to have to accelerate that kg of gas to orbital speed, just like the kgs of aluminium-lithium in the tank that holds it had to be accelerated to orbital speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than diving your spacecraft through the atmosphere and ramming its tanks full, I can't think of a practical way to accelerate that gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Carrying it up in tanks seems like the best idea.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-03-19T02:34:32.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8486" PostTypeId="1" AcceptedAnswerId="8487" CreationDate="2015-03-19T04:05:15.453" Score="9" ViewCount="4652" Body="&lt;p&gt;&lt;strong&gt;Imaginary scenario&lt;/strong&gt;: The US, Russia, China, Europe, India, and all the private space companies in between combine resources for a sustained space exploration program beyond Earth. They can choose any location on Earth for their new program's launch site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question&lt;/strong&gt;: Energy-wise, what is the most optimal location on Earth for launching rockets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a launch pad on Mt. Chimborazo (  1° 28′ 9″ S, 78° 49′ 3″ W), be the most efficient launch pad on Earth in terms of delta-V? Or does distance from the center of the Earth not contribute much to the energy required to launch a payload into space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note&lt;/strong&gt;: I realize building a launch pad on a mountain and sending rockets up to the mountain isn't very efficient itself, but I'm just curious.&lt;/p&gt;&#xA;" OwnerUserId="8730" LastEditorUserId="8730" LastEditDate="2018-02-16T22:13:32.570" LastActivityDate="2018-02-16T22:13:32.570" Title="Where is the optimal location to launch payloads to outer space?" Tags="|orbital-mechanics|rockets|launch-site|delta-v|high-altitude-launch|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8487" PostTypeId="2" ParentId="8486" CreationDate="2015-03-19T04:51:43.047" Score="5" Body="&lt;p&gt;The equator enjoys an advantage as it's moving about .47 km/second. Other points on earth's surface are moving this speed * cos(latitude). So for example Kennedy Space Center at 28º degrees latitude moves .47 km/s * cos(28º) which is .42 km/s. So the equatorial &lt;a href=&quot;http://en.wikipedia.org/wiki/Guiana_Space_Centre&quot; rel=&quot;nofollow noreferrer&quot;&gt;French Guiana Space Centre&lt;/a&gt; enjoys a .05 km/s advantage over KSC.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for mountain tops like Chimborazo:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;6.4 kilometers is about 1/1000 of earth's radius. So if we choose earth radius as our unit of length, it's 1 vs 1.001.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity acceleration is $GM/r^2$. $1/1.001^2 = .998$. So gravity advantage is about .2%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So called centrifugal force is $\omega^2r$. $\omega^2*1.001$ gives a .1% advantage over $\omega^2*1$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are negligible advantages for a huge price tag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A little more interesting is the rarefied mountain top atmosphere -- about half that of sea level. Still doesn't justify the expense of building a launch pad atop a high mountain. You need to get up to around 100 km before the air's thin enough to do the major burn to achieve orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also good to have a launch pad on an eastern seaboard so the booster stage doesn't fall on a populated area. Unfortuneately there are people living downrange from Chimborazo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Somalia is on the equator and has an eastern seaboard. I've often thought this would be an excellent location for a space center. But I believe political instability is the show stopper in this case.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="8693" LastEditDate="2016-11-04T21:34:35.633" LastActivityDate="2016-11-04T21:34:35.633" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="8490" PostTypeId="2" ParentId="8482" CreationDate="2015-03-19T08:26:57.793" Score="1" Body="&lt;p&gt;Well, it's not my answer, but I came across a pretty good answer to this question.    &lt;a href=&quot;http://www.askamathematician.com/2015/02/q-since-the-earth-is-spinning-and-orbiting-and-whatnot-are-we-experiencing-time-wrong-because-of-time-dilation/&quot; rel=&quot;nofollow&quot;&gt;http://www.askamathematician.com/2015/02/q-since-the-earth-is-spinning-and-orbiting-and-whatnot-are-we-experiencing-time-wrong-because-of-time-dilation/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing they don't take into account in the link above is the speed of the Milky-Way, based on cosmic background radiation.   The site below says the Milky-Way is moving at 1.3 million miles per hour, which, in and of itself, if I've done my math right is 1 second every 6 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How fast are we moving?:  &lt;a href=&quot;http://astrosociety.org/edu/publications/tnl/71/howfast.html#6&quot; rel=&quot;nofollow&quot;&gt;http://astrosociety.org/edu/publications/tnl/71/howfast.html#6&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's another time dilation to consider, the dilation in relation to the gravitation of the entire universe - we are in the universe and so we are, by definition, in the gravitational field of the universe, so there would be a time dilation from the entire universe as well, but that calculation is above my pay grade.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to your final question, no, time is relevant without gravity, well, er, kind of.   That's sort of a trick question because there is nowhere in the universe without gravity.   You can experience no gravity - in orbit for example, you feel weightless, but you are in a gravitational field even if you don't &quot;feel it&quot; - so there's gravity everywhere and as a result, time is slowed down everywhere, at least, a little bit.    &lt;/p&gt;&#xA;" OwnerUserId="8711" LastActivityDate="2015-03-19T08:26:57.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8491" PostTypeId="1" AcceptedAnswerId="8495" CreationDate="2015-03-19T09:55:27.793" Score="-2" ViewCount="129" Body="&lt;p&gt;I have been researching how metals affect radio transmission but have not found anything conclusive to the specific scope about planet density if it is mostly metal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the posts that I found any info on was this page&#xA;&lt;a href=&quot;http://www.madsci.org/posts/archives/2002-03/1015162213.Eg.r.html&quot; rel=&quot;nofollow&quot;&gt;http://www.madsci.org/posts/archives/2002-03/1015162213.Eg.r.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have also found that gravity does affect wireless radio transmission. &#xA;&lt;a href=&quot;http://www.dslreports.com/forum/r13178764-Are-wireless-transmissions-affected-by-gravity-&quot; rel=&quot;nofollow&quot;&gt;http://www.dslreports.com/forum/r13178764-Are-wireless-transmissions-affected-by-gravity-&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what would it take to entirely prevent transmissions in our current radio systems used by our space travel agencies, and what modifications would need to be made to prevent such losses in transmission should we encounter such a situation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Background (what prompted me to ask)&lt;/strong&gt;&lt;br&gt;&#xA;I had a dream that i piloted a small craft to a comet to mine for metal. But its magnetism disrupted our radio communication. So I am wondering if this could really be the case. &lt;/p&gt;&#xA;" OwnerUserId="1401" LastEditorUserId="1401" LastEditDate="2015-03-19T20:53:53.330" LastActivityDate="2015-03-19T20:53:53.330" Title="Could a planet, moon, or comet having a high concentration of a certain metal disrupt radio transmission?" Tags="|communication|comet|mining|radio-communication|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8492" PostTypeId="1" AcceptedAnswerId="8496" CreationDate="2015-03-19T10:06:54.250" Score="0" ViewCount="859" Body="&lt;p&gt;When I researched the gravity on comets like on 67P/Churyumov-Gerasimenko's surface I found that it was&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;several hundred thousand times weaker than on Earth&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;source:&lt;/strong&gt; &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions&quot; rel=&quot;nofollow&quot;&gt;www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am asking because I had a dream that I landed on a comet. But its gravitational pull was so strong that we could barely move. What would be the case in which that is possible, perhaps very dense metal core?&lt;/p&gt;&#xA;" OwnerUserId="1401" LastActivityDate="2015-03-20T09:13:07.790" Title="How could the gravitational pull of a comet affect spacecraft and astronauts landing on it?" Tags="|gravity|comet|hypergravity|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8493" PostTypeId="2" ParentId="8492" CreationDate="2015-03-19T13:03:32.927" Score="2" Body="&lt;p&gt;A very simple answer is that gravitational force is defined by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F = \frac{G m_1 m_2}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here $G$ is the universal gravitational constant, $m_1$ is your mass, $m_2$ is the mass of the comet and $r$ is the distance between the two. So increasing either your mass or the mass of the comet or decreasing the distance between the two are the only ways to get a higher force. If you're on the surface then you're already as close as you can get. So for a comet to have a high gravitational force it would have to have a high mass. This can be either higher density or larger volume (or both, of course).&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="5963" LastEditDate="2015-03-19T22:42:06.757" LastActivityDate="2015-03-19T22:42:06.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8494" PostTypeId="1" AcceptedAnswerId="8586" CreationDate="2015-03-19T13:13:58.400" Score="22" ViewCount="4985" Body="&lt;p&gt;This question is &lt;em&gt;wholly seeking historical evidence&lt;/em&gt; and not about physics. It is a follow on from the Physics Stack Exchange question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/q/170962/26076&quot;&gt;Could we send a rocket to the Moon without knowledge of general relativity?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer is a definitive yes. A simple back of the envelope calculation with the Schwarzschild metric shows that the order of magnitude deviation between corresponding points Earth-Moon transfer trajectories calculated with Newtonian and GTR physics is of the order of 0.1 meters (you read that right- the length of your tallman finger). A neat approach to the problem is &lt;a href=&quot;https://physics.stackexchange.com/a/171008/26076&quot;&gt;John Rennie's answer here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This conclusion MUST have been reached by NASA (or even NACA) in the leadup to the Apollo landings. I should like to know any of the following &lt;em&gt;how&lt;/em&gt; was the conclusion reached, &lt;em&gt;who&lt;/em&gt; first raised the question and &lt;em&gt;when&lt;/em&gt;. A link to / citation of a report would be great. I suspect the question arose and was resolved in one of three ways:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The general relativity / gravitation literature holds this calculation done in the first half of the 1900s in the early days of &quot;dreaming&quot; about reaching the Moon (although I can't seem to find anything), and the relevant papers were known to orbital mechanical scientists;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The question was raised very early in the NASA programs, and quickly resolved by a back of the envelope calculation like John's. If so, I'd expect that there would be somewhere in the archives a short report of one or two pages comprising a calculation like John Rennie's with the endorsement of a prominent GR theorist of the day, like John Wheeler. This would be dated 1950s / early 1960s;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Empirically. Once Mariner / Ranger data became available, there would be no noticeable error between Newtonian theory and observations, so the question was never raised. GTR effects would be utterly swamped by others. In particular, I learnt today that the error Frank Borman was referring to when he said that Apollo 8's final position error after lunar orbit insertion was &quot;about a mile and a half from where we were supposed to be&quot; was indeed owing to five high density &quot;lumps&quot; on the Moon's surface &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&quot; rel=&quot;noreferrer&quot;&gt;see &quot;Bizzare Lunar Orbits&quot; here&lt;/a&gt; (Thanks to user David Hammen for this knowledge) and that the calculation error was thereafter reduced to a 120 meter difference between calculated and actual landing position for Apollo 12. Still three orders of magnitude bigger than the GTR effects.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So, in summary, references / answers to whom the question was raise by, when and how was it answered?&lt;/p&gt;&#xA;" OwnerUserId="6005" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2024-02-19T15:28:52.957" Title="How did NASA Conclude that the General Theory of Relativity was not Needed for Earth-Moon Flight Path Computation?" Tags="|orbital-mechanics|history|orbital-maneuver|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8495" PostTypeId="2" ParentId="8491" CreationDate="2015-03-19T13:15:07.883" Score="1" Body="&lt;p&gt;The magnetic field of the Earth isn't just based on the quantity of iron in the core. The metal has to be liquid and rotating to cause a magnetic field. This rotation is usually a product of the Coriolis effect, and I would suppose a very short day on the planet/comet would increase the magnetic field strength. It comes down to a comet just not being large enough, or more accurately not being massive enough, for the core to be molten.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also this is your second 'I dreamt... Would this work' question. If you have a different reason for asking more directed answer could br shared. For example if you're a writer, we could point you towards the worldbuilding stack exchange site.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-03-19T13:15:07.883" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8496" PostTypeId="2" ParentId="8492" CreationDate="2015-03-19T14:32:37.663" Score="1" Body="&lt;p&gt;ThePlanMan gives the basics in his answer. In addition: the densest material known to man is Osmium, at $\rho$ = 22 kg/dm$^3$. This compares to water at 1 kg/dm$^3$ and rock at 2-8 kg/dm$^3$. So the densest possible comet is only 22 times as dense as one that consists of water ice. That's not enough to give a gravity of anywhere near 1 G.&lt;br&gt;&#xA;Comet 67P/Churyumov-Gerasimenko has a mass of $10^{13}$ kg. To get 1 G, you need 10$^{24}$ kg, or the mass of Earth.&lt;br&gt;&#xA;A planet so heavy you can barely move, is either much larger than Earth or contains exotic matter. A neutron star, for instance, but then you're far beyond &quot;can barely move&quot; - a person would be instantly squashed flat.&lt;br&gt;&#xA;Could a smaller planet give a 1 G surface gravity? &#xA;We can calculate the mass of a planet:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ m_1 = 4/3 *{\pi r^3} * \rho $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I replace $m_1$ with this formula we get the gravitational force for a person on the surface of a planet:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F = \frac{G * 4/3 *{\pi r^3} * \rho * m_2}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which can be reduced to:   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F = {G * 4/3 *{\pi r} * \rho * m_2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravitational force scales linearly with radius and density. The smallest possible body (solid osmium) that has 1 G gravity is on the order of 1/20 the mass of Earth, i.e. it has a radius of 0.36 times Earth's radius. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-03-20T09:13:07.790" LastActivityDate="2015-03-20T09:13:07.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8497" PostTypeId="1" AcceptedAnswerId="8500" CreationDate="2015-03-19T16:00:34.360" Score="6" ViewCount="539" Body="&lt;p&gt;In this question: &lt;a href=&quot;https://space.stackexchange.com/questions/5410/how-does-spacex-manage-parallel-processing-at-the-lc-40-complex&quot;&gt;How does SpaceX manage parallel processing at the LC-40 Complex?&lt;/a&gt; I asked how SpaceX handles two cores at the HIF building at LC-40 at the same time.  Based on available data it looks like they just do not. One at a time.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I asked this question on the Space Show with David Livingston when Gwynne Shotwell was on and she misconstrued my question, instead answering that LC-39A, Vandenberg, and Boca Chica will allow more capacity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recently in March 2015, there was a scheduled launch of a GEO comm sat on a booster with no legs (due to lack of margin for landing attempts).  Followed by a CRS-6 mission about 20 someodd days later, per the schedule.  Due to undisclosed issues, they have swapped the order. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the question is now, how will they handle re-arranging the flight order.  Will they fly the CRS mission with the commsat booster, no legs. Then fly the commsat mission on the CRS booster, possibly removing the legs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will they retrofit the legs back onto the commsat booster for the CRS mission? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will they just move the commsat booster onto a truck and park it outside, off to the side, or somewhere at the Cape, and start work on the CRS booster? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The HIF building may be big enough to slide a booster over to the side, if only for a week or three. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it would be interesting to know how they plan on handling this changeup.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: It is possible I misunderstood the delays. This &lt;a href=&quot;http://spaceflightnow.com/2015/03/17/spacex-puts-off-next-falcon-9-launch/&quot; rel=&quot;nofollow noreferrer&quot;&gt;article at SpaceFlightNow&lt;/a&gt; suggests that they are just delaying the commsat launch, and possibly push back CRS mission after it, not reordering it.  I will continue looking for the original source I was working from. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: This &lt;a href=&quot;http://www.nasaspaceflight.com/2015/03/spaceport-america-spacex-reusability-testing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NasaSpaceFlight.com article&lt;/a&gt; suggests that the ordering has been swapped, with CRS-6 going before the comm sat.  Which to believe?  Who knows, but the question still stands. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-04-04T18:31:01.807" Title="How does SpaceX handle rearranging flight order?" Tags="|launch|spacex|falcon-9|launch-site|lc-40|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8498" PostTypeId="2" ParentId="8206" CreationDate="2015-03-19T16:16:29.727" Score="3" Body="&lt;p&gt;The EMU (Extravehicular Mobility Unit) has a drink bag which velcros to the front interior of the suit. The bag comes in two different sizes 21 oz and 32 oz and it is crew choice as to which bag they fly. The bag has a drink tube with a valve which prevents free flow of water into the suit. The valve is opened when the crew sucks on the tube for water. Their food was provided by a fruit bar wrapped in an edible rice paper. The stick was stowed in a sleeve which is mounted to the neck area of the EMU. To eat, the crew bent their neck down and take a bite of the bar and slide it up, which set it up for their next bite. This food bar is no longer flown, as most of the astronauts preferred&#xA; to eat prior to their spacewalk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, they do have something very similar to the pilot's tube food, I do not know if you could put anything other than water in the tube. I am not sure how much more research they have done on this. I guess it would be done at the point when astronauts opinions change (whereas right now they are wanting to eat prior to the EVA), but I think that with the current technology and the fact that they do have some tubed food on the ISS, that it could also expand to the EVA's. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check out this link: &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/faq/eva.html&quot; rel=&quot;nofollow&quot;&gt;http://spaceflight.nasa.gov/shuttle/reference/faq/eva.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/audience/forstudents/postsecondary/features/F_Food_for_Space_Flight.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/audience/forstudents/postsecondary/features/F_Food_for_Space_Flight.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8561" LastEditorUserId="8561" LastEditDate="2015-03-19T16:36:08.940" LastActivityDate="2015-03-19T16:36:08.940" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8499" PostTypeId="2" ParentId="4356" CreationDate="2015-03-19T16:44:55.710" Score="2" Body="&lt;p&gt;Google tries to use live images from the Slough Space Camera (and the Hubble as far as I know), so yes, I would assume that all of the geometrics are accurate or at least extremely close. You can actually use a feature called &quot;sky&quot; with Google Earth which enables you to: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Check out the new Slooh Space Camera layer to see live images of galaxies, objects and more.&#xA;View constellations and the movements of the planets.&#xA;Hear astronomy podcasts and read celestial research from expert sources.&#xA;Create and share your own imagery, placemarks and more.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check out this link, it would probably be the best place to start (and it's super cool because there is a video with a former astronaut!)  :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.google.com/earth/explore/showcase/sky.html&quot; rel=&quot;nofollow&quot;&gt;http://www.google.com/earth/explore/showcase/sky.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8561" LastActivityDate="2015-03-19T16:44:55.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8500" PostTypeId="2" ParentId="8497" CreationDate="2015-03-19T18:32:51.280" Score="3" Body="&lt;p&gt;SpaceX has one main processing facility at Cape Canaveral, but they have at least one building that is sufficiently large to store a second booster at the Cape. The second building can't do all of the pre-launch prep work, but can do some limited prep work. The storage facility is some ways away from the launch pad, it is my understanding it is about 5 miles away. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the missions which I am most familiar with at SpaceX, they have had the main rocket at the main integration facility, and the second rocket at the near site storage facility. I would presume that they simply switched the rockets, possibly the entire rocket, and almost certainly the lower stage.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-03-19T18:32:51.280" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8506" PostTypeId="2" ParentId="3512" CreationDate="2015-03-20T22:02:25.620" Score="3" Body="&lt;p&gt;If you refer to page 25* of this &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;nofollow&quot;&gt;Space Shuttle Main Engine orientation&lt;/a&gt; you will see that the LOX enters the engine at -297 deg F (-183 C) and the LH2 enters at -423 deg F (-253 C), whilst the main combustion chamber runs at a toasty 6000 deg F (3316 C).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You ask specifically about the tank temperatures; if I recall correctly, there was a small temperature rise from the tank to the engine inlet (the lines were all vacuum-jacketed to minimize this), but this pales in regard to the enormous temperature rise across the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the biggest total temperature rise; the figure also shows the temperatures at the preburner inlets and their combustion temperatures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*(page 25 of the .pdf, the page number is 19)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-03-20T22:07:58.147" LastActivityDate="2015-03-20T22:07:58.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8507" PostTypeId="2" ParentId="8456" CreationDate="2015-03-21T02:35:18.323" Score="5" Body="&lt;p&gt;Once you have a substantial refining operation on the moon you probably will go with an oxygen/argon atmosphere.  Argon is suitably inert and will be obtained when you melt lunar rock.  (It's from the decay of potassium, it's not primordial.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-03-21T02:35:18.323" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8508" PostTypeId="2" ParentId="1619" CreationDate="2015-03-21T02:50:34.543" Score="3" Body="&lt;p&gt;While the above answers are fine I think the question is prompted by a more fundamental misunderstanding of radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiation comes in 4 flavors:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alpha:  A helium nucleus.  Damaging &lt;strong&gt;if&lt;/strong&gt; it gets to your tissues but your own skin is a nearly perfect defense against it.  Alpha emitters are only of concern if you ingest them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beta:  An electron.  These are so light that they lack much penetrating power.  While they punch through better than alpha particles they're still stopped very easily--including by the container for your radioactive material.  Your skin does a reasonable job of stopping them, also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When dealing with such materials your only concern is to avoid ingesting them.  It doesn't matter how much is being emitted, they're not going to do anything to you.  Note that most radioactivity is of one of these forms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third form is x-ray &amp;amp; gamma radiation (the only distinction is the energy level, they are really the same thing.)  X-rays take a decent amount of mass to stop--such as that vest the dentist lays on you when shooting x-rays (and their machines are in shielded rooms.)  Gamma rays are simply too penetrating to stop by any portable shield--note the absence of the lead vests in the nuclear medicine labs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, there are the neutrons.  They are not as penetrating as gamma rays but they're still pretty nasty and they also have the extremely nasty property of sometimes inducing radioactivity in what they hit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you are making a nuclear battery you not only choose alpha or beta emitters for safety but because the penetrating nature of gamma and neutron energy means it's carried away from the battery--build your battery out of something like Cobalt-60 (a powerful gamma emitter) and it doesn't produce more than a trickle of power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only reason you would get any radioactivity outside a nuclear battery is because nothing is truly pure and sometimes they don't even try to remove the contaminants.  (Look at AlanSE's answer about plutonium batteries.  To separate P-238 from Pu-239 is 3x as hard as it is to separate U-235 from U-238--as is done in an enrichment plant.  Those are major facilities.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-03-21T02:50:34.543" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8509" PostTypeId="1" AcceptedAnswerId="8510" CreationDate="2015-03-21T04:12:16.607" Score="3" ViewCount="581" Body="&lt;p&gt;I assume that all space programs, manned and unmanned, have many &quot;what if&quot; plans for situations that might happen. For example, on the ISS they had fire alarm and evacuated to the Russian side. Are those &quot;what if&quot; plans publicly available? In specific, what if any of the Apollo missions that landed on the moon had had a catastrophic failure that would have prevented them from taking off from the moon?&lt;/p&gt;&#xA;" OwnerUserId="5968" LastEditorUserId="6214" LastEditDate="2015-03-22T02:13:24.840" LastActivityDate="2016-01-13T15:43:57.853" Title="Any &quot;what if&quot; plans for space programs?" Tags="|iss|apollo-program|future-missions|mission-design|planning|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8510" PostTypeId="2" ParentId="8509" CreationDate="2015-03-21T05:32:03.250" Score="7" Body="&lt;p&gt;Well, they prepared by writing a &lt;a href=&quot;http://www.archives.gov/presidential-libraries/events/centennials/nixon/images/exhibit/rn100-6-1-2.pdf&quot; rel=&quot;nofollow&quot;&gt;speech text&lt;/a&gt; for Nixon &lt;em&gt;&quot;In event of Moon disaster&quot;&lt;/em&gt;. The opportunities to &quot;fix a problem&quot; aren't that many in space. At least no one can hear them scream. And fewer than 20 out of 540+ have perished in space. The blackness is not as dangerous as it looks, when approached with care.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the space shuttle Columbia failed, NASA afterwards concluded that it just might have been possible to &lt;a href=&quot;http://arstechnica.com/science/2014/02/the-audacious-rescue-plan-that-might-have-saved-space-shuttle-columbia/&quot; rel=&quot;nofollow&quot;&gt;rescue them&lt;/a&gt; if they had known about the problem. And that was in the era of the space shuttle when such tasks were envisioned, by the elderly, to be &quot;routine&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo was designed to enable several abort points. Apollo 13 used one of them.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6214" LastEditDate="2015-03-22T02:13:29.947" LastActivityDate="2015-03-22T02:13:29.947" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8511" PostTypeId="1" AcceptedAnswerId="8515" CreationDate="2015-03-21T10:43:31.337" Score="9" ViewCount="1751" Body="&lt;p&gt;The Astrobotic team announced a while ago that they will launch their rovers on top of a Falcon 9. As I know the Saturn V rocket had a third-stage, which was used to redirect the spacecraft from Earth's orbit to the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How will the Falcon 9 accomplish this? Will SpaceX use a third-stage? Or will they have enough fuel to use the second-stage to get the rovers to the moon?&lt;/p&gt;&#xA;" OwnerUserId="8606" LastActivityDate="2015-03-21T20:36:10.510" Title="How will the Falcon 9 carry the lunar xprize rovers to the moon?" Tags="|spacex|falcon-9|rovers|lunar-landing|xprize|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8512" PostTypeId="2" ParentId="3217" CreationDate="2015-03-21T15:04:15.577" Score="0" Body="&lt;p&gt;On November 3, 1957, the second-ever orbiting spacecraft carried the first animal into orbit, the dog Laika. Laika, the first animal in space, did not die peacefully by poison after six days as initially reported, but died with in hours from overheating &amp;amp; stress. Russia kept it secret until 2002. &lt;a href=&quot;http://www.thestatworld.com/2015/01/Laika-the-Dog-The-sad-story-of-stray-dog.html&quot; rel=&quot;nofollow&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8745" LastActivityDate="2015-03-21T15:04:15.577" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8513" PostTypeId="1" AcceptedAnswerId="8519" CreationDate="2015-03-21T17:24:37.457" Score="6" ViewCount="4655" Body="&lt;p&gt;Do we (the humanity) have any satellites in the Sun-Earth $L_3$ point? If not, then what are the plans to put some ships into this point?&lt;/p&gt;&#xA;" OwnerUserId="5734" LastEditorUserId="168" LastEditDate="2015-03-22T09:34:20.520" LastActivityDate="2021-12-09T21:23:06.170" Title="Any satellites in Earth-Sun L3 point?" Tags="|lagrangian-points|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8514" PostTypeId="1" CreationDate="2015-03-21T18:12:38.293" Score="18" ViewCount="5889" Body="&lt;p&gt;I was reading about Russia's plans to detach their modules from the ISS before its deorbit and use them to begin a &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_Piloted_Assembly_and_Experiment_Complex&quot; rel=&quot;noreferrer&quot;&gt;new space station&lt;/a&gt;. The version I read said Zvezda (DOS-8) and Poisk (MRM-2) are planned for inclusion in this new station, and there are variants on that plan. The Nauka (FGB-2) would go, along with the hub module (Uzlovoy module) that will later dock to it, neither of which have yet launched. This can't be done with the other modules. The estimate is that sometime in the 2020s it will be necessary to do a &lt;a href=&quot;http://www.space.com/13071-international-space-station-reentry-plans.html&quot; rel=&quot;noreferrer&quot;&gt;controlled deorbit&lt;/a&gt; and these modules will be lost, even though some of them will still be quite functional at that time and could have continued service. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why was the choice made not to include mechanisms for disassembly in the design of these modules? They were designed to be modular so the ability to swap out old ones for new ones must have been discarded for specific reasons.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-03-25T16:28:05.170" LastActivityDate="2015-04-25T18:54:27.643" Title="Why is NASA planning to deorbit the ISS instead of reusing its newer modules, like the Russians will?" Tags="|iss|mission-design|end-of-life|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8515" PostTypeId="2" ParentId="8511" CreationDate="2015-03-21T20:36:10.510" Score="8" Body="&lt;p&gt;From &lt;a href=&quot;https://www.astrobotic.com/griffin&quot;&gt;Astrobotic's website&lt;/a&gt;, on the fifth slide, they state that Falcon 9 + Griffin can deliver 663kg to TLI (translunar injection), 515kg to Lunar orbit (althougth the specific operational parameters of this orbit are not stated), and 270kg to the Lunar surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, this is for payloads contracting the use of Astrobotic's Griffin platform as Falcon 9 can deliver far more to TLI than 663kg (should be a few thousand kg). This is confirmed in the &lt;a href=&quot;https://www.astrobotic.com/payload-user-guide&quot;&gt;Astrobotic Payload User's Guide&lt;/a&gt; (PDF warning), which goes into far more detail about the mission architecture:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Astrobotic’s 2015 mission explores a lunar skylight that may be an entrance to a subsurface cave network. A SPACEX Falcon 9, currently under contract, launches and performs trans-lunar injection. The spacecraft cruises for 4.5 days, orbits at 100KM, descends, and lands on the Moon 36 hours after local sunrise. A rover departs the lander to win the Google Lunar XPrize and explore the skylight, while the Griffin lander generates power and supports payload operations. During the night, Griffin and rover hibernate, then revive for several lunar days.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The situation appears similar to SpaceX's prior comsat launches. The second stage restarts to perform a burn which takes the payload into GTO (or in this case, TLI), and then the payload separates. From there, the payload operator is in control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, for what it's worth, the Griffin lander has the following specifications:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dry Mass: 535kg&lt;/li&gt;&#xA;&lt;li&gt;Fuel Mass: 1685kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This totals to 2220kg, which is in addition to the payloads flying on Griffin. This matches far more closely with F9's TLI payload specs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suffice to say, no, Falcon 9 does not, and cannot deliver the payloads directly to the lunar surface.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-03-21T20:36:10.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8516" PostTypeId="1" AcceptedAnswerId="8518" CreationDate="2015-03-21T22:46:42.330" Score="3" ViewCount="688" Body="&lt;p&gt;In fact, do we have any recorded evidence of spacecraft at Mars suffering from atmopsheric decay over time? It's well known that we commonly use &lt;strong&gt;aerobraking&lt;/strong&gt; (not be confused with aerocapture, which has never been attempted):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZyIw1.jpg&quot; alt=&quot;MRO undergoing aerobraking&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;to reduce our spacecraft's apoapsis' over a period of many months. In fact, MGS was the first spacecraft to ever attempt Martian aerobraking, which resulted in it damaging one of its solar arrays, which &lt;a href=&quot;http://mars.jpl.nasa.gov/mgs/sci/aerobrake/SFMech.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;got bent back&lt;/a&gt; by the forces generated. This aerobraking campaign took place at approximately 120km above the Martian surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What this indicates is that atmospheric decay is present around Mars, so it should therefore be expected that any spacecraft in similar low orbits will eventually suffer from orbital decay and reenter, but I can find no evidence that this has ever occurred.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally and specifically relating to MGS, I can find no accurate, authorative source of MGS' final orbit. &lt;a href=&quot;http://www.jpl.nasa.gov/news/fact_sheets/mgs.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This NASA JPL factsheet&lt;/a&gt; indicates it was in a circular orbit for sometime:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When Global Surveyor arrived at Mars in&#xA;  September 1997, it initially entered a looping, elliptical orbit around the planet that has been gradually circularized through aerobraking. Its winged solar panels&#xA;  — which feature a Kapton flap at the tip of each wing&#xA;  for added drag — supply most of the surface area that&#xA;  slowed the spacecraft by a total of more than 1,200&#xA;  meters per second (about 2,700 miles per hour) during&#xA;  the entire aerobraking phase. Since the start of aerobraking, Surveyor's orbit around Mars has shrunk from&#xA;  an initial elliptical orbit of 45 hours to the now nearly&#xA;  circular orbit taking less than two hours to complete.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Wikipedia also states:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Finally, from November 1998 to March 1999, aerobraking continued and shrank the high point of the orbit down to 450 km (280 mi). At this altitude, Surveyor circled Mars once every two hours. […] The spacecraft circled Mars once every 117.65 minutes at an average altitude of 378 km (235 mi). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Obviously, end of mission occurred in November 2006. It has been nearly 10 years since then.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, my question remains, given what we know about MGS' orbit, has it decayed, or is it in the process of decaying? If it is the latter, are there any models on when the decay might occur?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6214" LastEditDate="2015-03-22T02:09:49.950" LastActivityDate="2015-03-22T06:42:50.307" Title="Will, or when will, Mars Global Surveyor decay from orbit?" Tags="|mars|spacecraft|orbital-decay|mars-global-surveyor|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8517" PostTypeId="2" ParentId="8514" CreationDate="2015-03-22T00:46:37.563" Score="12" Body="&lt;p&gt;The difference between the US Operating Segment (USOS) and the Russian segment is one of self sufficiency. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first module launched, Zarya, was enough to base a station on.  Sure not a ton of living and working space but it had all the orbital control, solar, thermal radiators needed to be a space station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After that, everything grew off that initial kernel with larger more competent versions of those subsystems being added.  The obvious two are the huge solar arrays and the huge radiator fins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the Russian segment, has the solar, thermal, and orbital management it needs on its own at a small scale. But the US side relies on the bigger systems except it lacks the orbital management part.  I.e There are no engines on the US side to reboost the station. But there are on the Russian side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the USOS relies on the larger heat and power systems, that could not be provided on each node individually, it did not make sense to provide 100% self sufficiency on every module, since it is better to save mass/performance for the real purpose of the module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Nickloai notes in another answer, this is a difference between 1st, 2nd, 3rd, and 4th generation systems. ISS is a hybrid that started as a 4th gen station, and the USOS acts as add on modules to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if the US and international partners decided to go off on their own, they potentially could launch something like the ICM (Interim Control Module) and potentially move existing modules and components to a new base unit for a new station. They were not planned to do that from the outset, but if the new base unit provided the proper power, heat, and orbital control infrastructure they could unberth from one CBM port and berth to a second new one.  If done while all attached to the station, they could likely even use the CanadArm to do the work.  But the US, unlike the Russians, never seem to have planned for this.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said all that, your question is missing a subtle nuance. The Russian segment that would potentially separate and form a new Russian station post-ISS service life would be the newest modules, not yet delivered to the station (as of Mar 2015). They do not plan on taking Zarya or Zvezda with them.  If they did, the US has the Interim Control Module (ICM), developed in case Zarya failed on launch to consider refurbishing to replace them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Pirs module docked to Zvezda's earth facing port is scheduled to be removed by a Progress departing the station, and the Nauka module is due to dock there.  This is a similar module to Zarya. Then at the bottom of Nauka is the OM module, which is a 6 port docking sphere. It is ultimately the OM module that becomes the heart of the new Russian station, where modules come and go, but the OM remains. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two modules planned to dock to the OM that are not yet complete and may still change, as they have changed many times over the past decade.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some great images of the US and Russian segments in a &lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports?s=1%7C2.1048&quot;&gt;question about whether the ISS will need more docking ports.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-03-25T16:14:37.267" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8518" PostTypeId="2" ParentId="8516" CreationDate="2015-03-22T06:42:50.307" Score="5" Body="&lt;p&gt;Yes, MGS's orbit is in the process of decaying, but very slowly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can only provide a lower limit on the lifetime, since for planetary protection MGS was required to assure at least a 50-year orbital lifetime from when it entered orbit.  So no less than 32 years from now.  I'm sure that the actual lifetime is much more than that.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-03-22T06:42:50.307" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8519" PostTypeId="2" ParentId="8513" CreationDate="2015-03-22T09:26:34.947" Score="13" Body="&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt; No, there are no sats there today, and no declared plans from any of space agencies to do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's why:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An Earth-Sun $L_3$ point is an unfortunate place for a satellite to be in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, it is unstable (thus without constant station-keeping burns an object placed there will fall out and start roaming about the Solar System). An alternative would be a &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot; rel=&quot;noreferrer&quot;&gt;halo&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;noreferrer&quot;&gt;Lissajous&lt;/a&gt; orbit around $L_3$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, the satellite would face significant difficulties in communicating via DSN. It would be permanently in &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_conjunction&quot; rel=&quot;noreferrer&quot;&gt;solar conjunction&lt;/a&gt; with the Earth, and as the Sun is a very strong source of radio noise, the signal would drown in the noise. Of course, one can use satellites placed in Earth-Sun $L_4$ or $L_5$ points as relays, but it would make the whole $L_3$ program much more costly and unreliable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This leads us to the third point: what can be the mission &lt;strong&gt;requiring&lt;/strong&gt; a halo/Lissajous orbit around $L_3$?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Space telescope? Nah, there's nothing in the far space that can't be observed in the course of Earth's normal yearly travel around the Sun.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Solar weather observations? Nope, by definition we are interested in solar events going &lt;strong&gt;our&lt;/strong&gt; way, not in the opposite direction.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There's only one speculative mission that forces us to have something in $L_3$: &lt;strong&gt;watching out for an alien invasion&lt;/strong&gt;, 'cause an advanced spacefaring civilization would definitely want to reduce our reaction time and choose an approach obscured by the Sun. Even then, $L_4$ and $L_5$ points offer better link budgets and only slightly worse detection conditions. A smart alien invasion would seek to disrupt the early warning network by inducing a fault that could be attributed to natural causes.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To sum it up: an $L_3$ satellite is not worth it unless you believe in evil aliens wanting to invade the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Following andy256's &lt;a href=&quot;https://space.stackexchange.com/questions/8513/any-satellites-in-earth-sun-l3-point#comment22279_8519&quot;&gt;suggestion&lt;/a&gt;, I quote the para from Tantardini et al. (2010) paper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A space observatory placed at or around L3 could provide insight into local astrophysical phenomena such as the solar wind and the properties of the Sun’s magnetosphere; it could monitor the evolution of the sunspots, perform solar storm forecast and observe the space environment from a new perspective, hidden to the Earth, and in this way supplement and complete the&#xA;  information obtainable, e.g., from L1 or from Earth based observatories. A satellite at L3 could be even thought of as part of a circular or spherical constellation of s/c monitoring the Sun’s activity and the space environment at many different angles. Moreover, L3 may constitute a privileged site to perform relativity experiments, such as measuring the gravitational&#xA;  bending of light on behalf of the Sun, as a follow-up of the Cassini–Huygens radio science observations. Finally, some minor bodies such as NEOs and comets, hidden by the Sun as viewed from Earth, could be observed and even tracked from this more favorable location.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://dx.doi.org/10.1007/s10569-010-9299-x&quot; rel=&quot;noreferrer&quot;&gt;http://dx.doi.org/10.1007/s10569-010-9299-x&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that I'm not convinced in the slightest by their arguments.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-03-22T20:21:03.913" LastActivityDate="2015-03-22T20:21:03.913" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8522" PostTypeId="1" CreationDate="2015-03-22T20:11:19.293" Score="7" ViewCount="329" Body="&lt;p&gt;I was looking at the names of the crew for Soyuz TMA-16M (for the year long expedition) and I was thinking &quot;wow that's an experienced crew!&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2O04S.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I looked up their combined experience and it shows as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/bios/cosmonauts/english/padalka_gennadi.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gennady&#xA;Padalka&lt;/a&gt;&#xA;710.265&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/english/bio_ast.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scott Kelly&lt;/a&gt; 180.077&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/english/bio_cosm.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mikhail Kornienko&lt;/a&gt;&#xA;176.055&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Total 1121.0342 days&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I guess I could go through every spaceflight shown on &lt;a href=&quot;http://www.spacefacts.de/english/flights.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spacefacts.de/english/flights.htm&lt;/a&gt; and compute the total experience, but does anyone know if I'm right?&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="8162" LastEditDate="2015-03-30T09:43:56.610" LastActivityDate="2015-04-09T12:21:20.813" Title="Will Soyuz TMA-16M have the most experienced crew?" Tags="|iss|soyuz-spacecraft|future-missions|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8523" PostTypeId="1" AcceptedAnswerId="8524" CreationDate="2015-03-22T20:15:12.077" Score="15" ViewCount="794" Body="&lt;p&gt;At some point this year, the MESSENGER mission to Mercury will come to an end, as the spacecraft has exhausted its supply of maneuvering propellant necessary to keep it in orbit. It is currently using pressurized Helium from its fuel tanks as reaction mass in the interim. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I don't understand is &lt;em&gt;why&lt;/em&gt; its orbit is decaying so rapidly. Mercury has no appreciable atmosphere:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mariner 10's ultraviolet observations have established an upper bound on the exospheric surface density at about 10&lt;sup&gt;5&lt;/sup&gt; particles per cubic centimeter. This corresponds to a surface pressure of less than 10&lt;sup&gt;−14&lt;/sup&gt; bar (1 nPa).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So if it isn't the atmosphere causing the decay, what is it? Solar pressure? A lumpy gravitational field? (similar to the Moon), or a combination of both?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the craft is in a highly elliptical orbit with a periapsis close to the surface between Mercury and the Sun, and its apoapsis is far away from the planet, I can only assume with no atmospheric decay process, its orbit is 'translating' relative to the planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we additionally have any information on the specifics of this translation, and how fast it is happening?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-03-23T05:04:26.497" Title="Through what process does MESSENGER undergo orbital decay?" Tags="|orbit|mercury|orbital-decay|messenger|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8524" PostTypeId="2" ParentId="8523" CreationDate="2015-03-23T00:27:14.440" Score="17" Body="&lt;p&gt;Mercury's orbit isn't &quot;decaying&quot; so much as it is changing shape. The primary cause is perturbations from the Sun's gravity field. That &lt;a href=&quot;http://messenger.jhuapl.edu/&quot;&gt;MESSENGER&lt;/a&gt; is subject to a good deal of solar radiation pressure and that Mercury's gravity field is a bit lumpy add to this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary mechanism is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Kozai_mechanism&quot;&gt;Kozai effect&lt;/a&gt;. This effect applies to an object in a highly eccentric and highly inclined orbit about a secondary that in turn is orbiting a primary; the effect makes the orbit of the object undergo changes in eccentricity and inclination. The perturbative effects of the Sun's gravity are acting to make MESSENGER's orbit a bit less inclined. This makes MESSENGER's orbit more eccentric, and eventually, the periapsis will become equal to Mercury's radius. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;CRASH!!&lt;/em&gt;&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="288" LastEditDate="2015-03-23T05:04:26.497" LastActivityDate="2015-03-23T05:04:26.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8525" PostTypeId="2" ParentId="1854" CreationDate="2015-03-23T02:58:30.673" Score="5" Body="&lt;p&gt;The term &quot;feathering&quot; has different meanings, dependent on context. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In regard to vehicles such as SpaceShip Two, it refers to a high drag aerodynamic configuration which produces a slow, gentle, but steep descent. It only works for vehicles which begin their descent at relatively low airspeed and simply maintains it. Remember that SpaceShip Two ascends more-or-less vertically, and never attains a significant velocity in the horizontal. Use of the term in this context was probably inspired by the notion of a falling feather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would not have been practical for the Space Shuttle because the Shuttle re-entered from orbital velocity. It had to manage its drag profile very carefully to avoid excess g-forces and heat build-up. Also, the Shuttle has very high wing loading (mass per unit wing surface); it required high airspeed to maintain lift, practically the opposite of what SpaceShip Two's feathering approach is all about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, &quot;feathering&quot; has another meaning in aviation. It refers to a setting for a variable-pitch propeller. If an engine fails, the propeller can be &quot;feathered&quot; to minimize drag.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2019-07-23T21:56:19.070" LastActivityDate="2019-07-23T21:56:19.070" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="8526" PostTypeId="1" AcceptedAnswerId="8528" CreationDate="2015-03-23T03:03:44.807" Score="12" ViewCount="2958" Body="&lt;p&gt;I've seen the term &quot;reference mission&quot; used in connection with proposed missions to the Moon, Mars, etc..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What exactly is &quot;reference&quot; about a &quot;reference&quot; mission, as opposed to a proposal, plan, or whatever else? What is the objective behind the preparation of a &quot;reference&quot; mission? How does one shape the development of any actual missions? When does a &quot;reference&quot; mission evolve into something else?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-03-23T06:46:17.413" Title="What is a reference mission?" Tags="|terminology|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8527" PostTypeId="2" ParentId="8526" CreationDate="2015-03-23T03:14:25.453" Score="8" Body="&lt;p&gt;Well...you have to start somewhere. &lt;strong&gt;Reference Missions&lt;/strong&gt; (often, and more familiarly to me, &lt;strong&gt;Design Reference Missions&lt;/strong&gt;) are (hopefully) representative mission profiles which allow the initiation of studies and big picture designs.  They are not as formal as requirements although they may serve as the basis for requirements.  It is unlikely that any actual mission would exactly follow a Design Reference Mission profile, but they would probably be at least recognizable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a description from the intro to the &lt;a href=&quot;http://www.lpi.usra.edu/lunar/site_studies/tp_2003_210793.pdf&quot;&gt;Lunar Surface Reference Missions&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It should be noted that the Lunar Surface Reference Mission is a tool,&#xA;  not a prescription for a program. The selection of tasks to be&#xA;  accomplished will vary according to the emphases placed on the major&#xA;  themes of lunar exploration and development and the capabilities of&#xA;  specific missions. The technology for accomplishing the tasks will&#xA;  change with time, offering opportunities to improve the concepts&#xA;  presented here or replace them with other tasks. The Lunar Surface&#xA;  Reference Mission is intended as a tool for mission planners, to&#xA;  determine the general level of transportation and surface&#xA;  infrastructure support that must be provided. It can also be used by&#xA;  scientists and engineers who have ideas about what should be, or may&#xA;  be, accomplished on the lunar surface, to examine their ideas in the&#xA;  context of the resources expected to be available.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I couldn't find a full copy of JSC-07896 &quot;Space Shuttle Baseline Reference Missions&quot; online, but Jim Oberg has an extract posted &lt;a href=&quot;http://www.jamesoberg.com/sts-3A_B-DRM.PDF&quot;&gt;here&lt;/a&gt;.  It concerns the very interesting mission where the USAF would launch a shuttle out of Vandenberg on a one-orbit mission to grab a satellite and land.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mission 3B is defined as a payload retrieval and quick return mission.  The total mission duration is one revolution, or approximately 2 hours....The space shuttle, with no payload on board, is launched from WTR on a launch azimuth of 198.6....&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note that this document is from &lt;strong&gt;1973&lt;/strong&gt;, pretty early in the shuttle design process.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-03-23T03:33:04.117" LastActivityDate="2015-03-23T03:33:04.117" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8528" PostTypeId="2" ParentId="8526" CreationDate="2015-03-23T03:47:15.550" Score="8" Body="&lt;p&gt;You will often see &quot;Design Reference Mission&quot; or &quot;Design Reference Architecture&quot;.  It is a plan done to a sufficient level of detail (one hopes) to elucidate the content, risks, difficulties, technologies, scale, and (sometimes) cost of a possible future endeavor.  They are most often used to guide technology development efforts.  I count 46 design reference missions in a document used recently to guide NASA technology planning.  They are also used to guide early developments along a path to the endeavor, such as determining the size and capability of new large launch vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally the effort put into them is well short of what would be required to have a real plan that an agency could commit to.  But it is enough to get order-of-magnitude or maybe half-order-of-magnitude estimates, and to define the capabilities of required systems and the benefits of potential new technologies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the time is ripe, technologies are ready or promise to be ready, and funding appears to be on the horizon, then such concepts may turn into a funded endeavor.  However they will rarely look like what was originally conceived.  Even the basic objectives may have changed notably.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-03-23T06:46:17.413" LastActivityDate="2015-03-23T06:46:17.413" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8529" PostTypeId="2" ParentId="8509" CreationDate="2015-03-23T08:49:44.093" Score="4" Body="&lt;p&gt;NASA is renowned for their detailed mission plans, including contingency plans for everything that could go wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the ISS, there is a publically available &lt;a href=&quot;http://www.spaceref.com/iss/ops/iss.emergency.ops.pdf&quot; rel=&quot;nofollow&quot;&gt;emergency operations manual&lt;/a&gt; with instructions about what to do in case of what emergency. &quot;Fire&quot; is page 34 - 54. However, it is not that easy to understand for a layman due to the cryptic acronyms it uses to refer to equipment and station parts (many of them &lt;em&gt;in cyrillic&lt;/em&gt;) and the many cross-references to other operation manuals.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2015-03-23T09:13:25.540" LastActivityDate="2015-03-23T09:13:25.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8530" PostTypeId="1" CreationDate="2015-03-23T11:56:00.247" Score="16" ViewCount="1896" Body="&lt;p&gt;Reason I'm asking, it's seemed to me that the best way to create an atmosphere on Mars is by moving an icy planetoid/comet/Kuiper Belt object and basically smashing it into mars, though I think a low orbit, perhaps near the Roche Limit where it breaks apart pretty easily would be more practical than a crash landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roughly speaking, comets are mostly too small to be that interesting.   To create a 1 ATM atmosphere you'd need something in the range of 120 KM in radius - figuring some of it would be water, some of it would be dust and rock, some of it would be CO&lt;sub&gt;2&lt;/sub&gt; that you'd want to turn into plant life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of moving the object, you could do a gravity assist around Neptune and using Neptune, you could adjust the trajectory either towards mars or towards another planet for another gravity assist.   The object could be moved by large mirrors, melting some of the ices on the surface creating a propulsion or by explosions.   I think that would be the easiest way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And once in orbit, the process is rather simple, the sun would melt some of the lower freezing point ices, and in a near roche limit orbit, it could be broken apart fairly easily given the tidal forces already in play.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Figuring a Kuiper Belt object is made up of a combination of dust, frozen H&lt;sub&gt;2&lt;/sub&gt;O, CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt;, NH&lt;sub&gt;3&lt;/sub&gt; some frozen N&lt;sub&gt;2&lt;/sub&gt;, a bit of CO, maybe some NO, some O&lt;sub&gt;2&lt;/sub&gt;, a sprinkling of more complex molecules, a little Argon, Hydrogen and Helium, it's not something you'd want to breath, but it would create atmospheric pressure, perhaps similar to earth.   (120 KM radius, figure 1.5 density, mass would be about 10 billion billion KG - about twice the mass of the earths' atmosphere, but if half of it is rock, dust and water-ice - then you've got about what you need for 1 ATM on mars).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, while this would by no means be easy or quick, it certainly seems possible - given a thousand years or so, and we'd want to be careful not to miscalculate and send a 100 KM radius object into a path where it might hit earth - as that would be a bad day, but, with care, this should be doable - so you have an object in orbit around mars that's gradually breaking up and it gives the planet an atmosphere and water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a way to turn this quantity of NH&lt;sub&gt;3&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; as well as CO and NO and other things we maybe wouldn't want to breath, into N&lt;sub&gt;2&lt;/sub&gt; and CO&lt;sub&gt;2&lt;/sub&gt; and O&lt;sub&gt;2&lt;/sub&gt;? again in a relatively short (1,000 years or so) span of time, or would that be even more difficult and take a lot longer than getting the object where we wanted in the first place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, the question is, can icy space objects be used to create planetary atmosphere for terraforming?  It's always seemed so, to me, but I'm curious if my assumption is incorrect.&lt;/p&gt;&#xA;" OwnerUserId="8711" LastEditorUserId="238" LastEditDate="2015-03-23T14:30:41.290" LastActivityDate="2017-08-27T20:37:20.670" Title="How feasible would it be to move a Kuiper Belt object (a good sized one) into an orbit around Mars?" Tags="|terraforming|" AnswerCount="1" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="8531" PostTypeId="2" ParentId="8530" CreationDate="2015-03-23T15:12:14.803" Score="13" Body="&lt;p&gt;&lt;a href=&quot;http://www.users.globalnet.co.uk/~mfogg/zubrin.htm&quot;&gt;Here&lt;/a&gt; Robert Zubrin and Christopher McKay talk about terraforming Mars. Have your browser find &quot;Moving Ammonia Asteroids&quot; and it will take you to the relevant section about 3/4 down the page.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zubrin and McKay talk about using a gravity assist. But using Saturn's gravity to toss down a 2.8 kilometer Centaur, not Neptune's gravity to toss down a 240 kilometer KBO. They figure .3 km/s would suffice to nudge an ice-teroid close enough to Saturn to throw it Mars-ward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They also suggest vaporizing the asteroid's volatiles for the reaction mass. To accomplish this they propose a &quot;quartet of 5000 MW nuclear thermal rocket engines&quot;. That's 20 giga-watts. The Palo Verde power plant is the largest nuclear power plant in the U.S. It is about 3.3 giga-watts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So they're proposing sending the equivalent of 6 Palo Verde nuclear power plants out past Saturn. Not very plausible, in my opinion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You seem to be talking about an object about a million times more massive than Zubrin and McKay's pebble. Let's say we could send it to Mars by nudging it near Neptune. For around .3 km/s, like Z &amp;amp; M's scheme. You'd need 20 million giga-watts. Sunlight's power density out among the KBOs is about 1/1000 of what we enjoy. You would need a very large mirror.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then what happens at impact? An object falling from 30 A.U. would strike Mars at about 10 km/s (at least). It'd have a kinetic energy of about 50 mega-joules per kilogram. I believe most of the cometary volatiles would escape Mars. The impact would also probably blast off all Mars' CO2 atmosphere above the tangent plane at impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Terraforming Mars via volatile rich asteroids pre-supposes we could send massive power sources to the asteroids and establish extensive onsite infra-structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we had this ability, why not just use the asteroidal volatiles where they sit? Developing the small bodies would give us far more real estate and natural resources than the outer surface of Mars. I make this argument in more detail at &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2014/02/terraforming-mars-vs-orbital-habs.html&quot;&gt;Terraforming Mars vs Orbital Habs&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-03-23T23:13:45.803" LastActivityDate="2015-03-23T23:13:45.803" CommentCount="30" ContentLicense="CC BY-SA 3.0" />
  <row Id="8532" PostTypeId="2" ParentId="8513" CreationDate="2015-03-23T19:16:01.143" Score="0" Body="&lt;p&gt;Currently NASA has two satellites flying in Solar conjunction with Earth, &lt;em&gt;near&lt;/em&gt; the L3 point of Sun-Earth - STEREO A and STEREO B. They will not stop at L3, and the distance to L3 may be rather great. Current position (&quot;Where is STEREO Today?&quot;):&#xA;&lt;a href=&quot;http://stereo.gsfc.nasa.gov/where.shtml&quot; rel=&quot;nofollow&quot;&gt;http://stereo.gsfc.nasa.gov/where.shtml&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some information about mission: &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/STEREO&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STEREO is the mission to observe Sun from two additional angles. One satellite was started in 2006 to orbit, lower than Earth, and other - to higher orbit. One is &quot;ahead&quot; (A) of Earth and other is  &quot;behind&quot; (B) of Earth with the angle Satellite - Sun - Earth increasing at rate 21.650 degrees per year for A and decreasing at rate -22 degrees/year for B. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;they passed through Earth's Lagrangian points L4 and L5, in late 2009&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In 2015 both STEREO satellites are behind Sun - &lt;a href=&quot;http://www.nasa.gov/content/goddard/stereo-entering-new-stage-of-operations/&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/content/goddard/stereo-entering-new-stage-of-operations/&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;A&quot; - from March 24 to July 7, 2015&lt;/li&gt;&#xA;&lt;li&gt;&quot;B&quot; - from Jan. 22 to March 23, 2015&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In 2009 they where two artificial satellites made closest pass near L4 and L5 - &lt;a href=&quot;http://www.nasa.gov/mission_pages/stereo/news/gravity_parking.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/mission_pages/stereo/news/gravity_parking.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It will take several months for STEREO to pass through them, with&#xA;  STEREO A making its closest pass to L4 in September, and STEREO B&#xA;  making its closest pass to L5 in October.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="2843" LastEditDate="2015-03-23T19:25:10.107" LastActivityDate="2015-03-23T19:25:10.107" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8533" PostTypeId="2" ParentId="8509" CreationDate="2015-03-23T19:18:18.150" Score="5" Body="&lt;p&gt;They never built it, but the Apollo program had plans for what was essentially a chair with a rocket engine on it. It was an emergency device meant to get the astronauts from the surface into lunar orbit, where they could rendezvous with the command module. Its design philosophy favored simplicity over redundancy. So much so that it didn't even have life support, it relied on the astronaut's spacesuits to provide that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the relevant wikipedia article: &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Escape_Systems&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Lunar_Escape_Systems&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can you imagine riding that thing?!?!&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="1255" LastEditDate="2015-03-23T20:37:03.317" LastActivityDate="2015-03-23T20:37:03.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8534" PostTypeId="1" AcceptedAnswerId="8535" CreationDate="2015-03-24T03:57:42.373" Score="14" ViewCount="1448" Body="&lt;p&gt;Seeing as the Falcon 9R first stage is close to being reusable, could it be the basis for an SSTO rocket? In other words, does it have enough performance to make it into orbit, and then boost back and land, say with no payload (or second stage) at all, or just a very small one? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume not, but I haven't seen actual figures. How far away is it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And what about a Falcon Heavy (reusable of course)?&lt;/p&gt;&#xA;" OwnerUserId="8504" LastEditorUserId="51" LastEditDate="2015-07-09T02:31:06.697" LastActivityDate="2018-04-28T23:30:16.480" Title="Falcon 9R as SSTO" Tags="|spacex|reuse|falcon-9|falcon-heavy|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8535" PostTypeId="2" ParentId="8534" CreationDate="2015-03-24T04:52:34.493" Score="20" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;, the fueled mass of the first stage is 403 tons, and empty mass 18 tons. Its specific impulse ranges from 282 to 311 seconds going from sea level to vacuum, implying exhaust velocity of 2766-3050 m/s. Plugging those values into the &lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;noreferrer&quot;&gt;rocket equation&lt;/a&gt; gets you somewhere between 8600 and 9500 m/s of delta-v. I don't know the exact specific impulse curve through the atmosphere and can't be bothered with the calculus anyway, but I'd guess it's nearer the low end of the range since the rocket is spending more time moving relatively slowly in the lower, denser part of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delta-v to low earth orbit is about 9.3km/s to 10km/s depending on launch site and destination orbit, so F9R doesn't have quite enough oomph to get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's pretty close, to be sure. Could it be modified to do it? The F9 has already been &quot;stretched&quot; once (F9 1.0 to F9 1.1) and the engines improved a couple of times, and they're hoping to increase the performance another 15% (mainly by increasing the fuel density with further cooling). If they can achieve that, then they should be just barely able to orbit the first stage, with better than 9900 m/s of delta-v -- and &lt;em&gt;zero&lt;/em&gt; payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I've got the math right, in the Falcon Heavy crossfeed situation, again with no upper stage or payload, the boosters would provide another 2800 m/s of delta-v to the stack, for a total of about 12 km/s, so it can comfortably reach LEO. As far as the rocket equation is concerned, though, that's a two-stage rocket; it just has some engines in the wrong place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Re-entry and recovery, on the other hand, is probably impossible. The first stage is designed to withstand a much more modest aerodynamic situation than re-entry from orbital speed. Structurally and thermally, it's a completely different world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX would like to make the second stage reusable as well as the first, which would require them to solve orbital re-entry for a rocket stage but on a much smaller scale. Even if they don't accomplish that, a fully reusable first stage would offer them a huge competitive advantage. They have no need to do SSTO.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="18732" LastEditDate="2017-06-02T17:26:51.337" LastActivityDate="2017-06-02T17:26:51.337" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8536" PostTypeId="1" CreationDate="2015-03-24T10:17:42.650" Score="5" ViewCount="640" Body="&lt;p&gt;I was wondering if the microgroove &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_Golden_Record&quot; rel=&quot;nofollow&quot;&gt;golden records&lt;/a&gt; they put on the Voyager spacecraft, or the &lt;a href=&quot;https://en.wikipedia.org/wiki/Arecibo_message&quot; rel=&quot;nofollow&quot;&gt;Arecibo message&lt;/a&gt;, along with instruction on how to decode it, were first put to the test with the subjects most likely to decode them, i.e. humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would suppose they were, as Carl Sagan, who was part of the team who designed the messages, takes the complementary stance in Contact to have humans decode an alien message, but incidentally the alien message mentioned looks quite too close, both in methodology and content, to what humans sent a decade earlier, non-fictionally, on the Voyagers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, did they test whether or not if these messages are decodable, and was a 100% interpretation achieved?&lt;/p&gt;&#xA;" OwnerUserId="8755" LastEditorUserId="49" LastEditDate="2015-04-28T22:04:06.290" LastActivityDate="2015-04-30T20:01:06.620" Title="Were the Voyager Golden Record / Arecibo message tested for decodability" Tags="|communication|voyager|astrobiology|radio-communication|meti|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8537" PostTypeId="1" AcceptedAnswerId="8544" CreationDate="2015-03-24T10:41:52.697" Score="6" ViewCount="362" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/There_ain%27t_no_such_thing_as_a_free_lunch&quot; rel=&quot;nofollow noreferrer&quot;&gt;There ain't no such thing as a free lunch&lt;/a&gt; was often a plot point in the works of Robert Heinlein, particularly in regards to air. We know that &lt;a href=&quot;https://space.stackexchange.com/questions/5272/air-in-international-space-station&quot;&gt;Air in International Space Station&lt;/a&gt; comes at price, in Heinlein's work even the largest stations required regularly payments for oxygen, with penalties usually around loss of access to air. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth oxygen is made available by the biosphere (&lt;em&gt;plants&lt;/em&gt;). Nowhere else in our solar system is free (&lt;em&gt;unbound&lt;/em&gt;) oxygen readily available.  There are some processes that generate Oxygen such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Chemical_oxygen_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chemical oxygen generators.&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any place in the solar system where oxygen would be a waste product of industry, effectively making oxygen free (&lt;em&gt;without additional cost&lt;/em&gt;) to the inhabitants? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-03-25T00:59:13.643" Title="Where can I breathe for free?" Tags="|environment|oxygen|" AnswerCount="2" CommentCount="6" ClosedDate="2015-03-25T20:35:08.510" ContentLicense="CC BY-SA 3.0" />
  <row Id="8538" PostTypeId="2" ParentId="8509" CreationDate="2015-03-24T11:01:51.260" Score="5" Body="&lt;p&gt;Just a quick comment on getting Apollo off the lunar surface in an emergency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Schmitt and Cernan have described an emergency procedure to &quot;hot-wire the LM&quot;.  The ascent engine was very simple by design, and if the systems that fired it failed, it could be fired simply by passing a wire down through the hatch to manually open the valve that mixed the propellants!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would mean flying with suits pressurised, with the hatch open...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They mention it on this page: &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a17/a17.eva3post.html&quot;&gt;https://www.hq.nasa.gov/alsj/a17/a17.eva3post.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-03-24T11:01:51.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8539" PostTypeId="2" ParentId="8514" CreationDate="2015-03-24T14:17:01.297" Score="4" Body="&lt;p&gt;I'm speculating here, but I think that the broad 'reason' is that that's how they've designed the station. Let me expand on that: the first stations (early Salyut, Skylab), were launched all as one piece. All their maneuvering fuel, consumables, any spare parts, were launched in one go, and that was all you had. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These were the so-called &quot;first generation&quot; space stations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second generation begins with Salyut 6, which was the first station to have two docking ports. Now the station could be resupplied with fuel, consumables, and replacements for broken parts. This drastically increased the lifespan of the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third generation begins with Mir, and in this generation you have a main 'base block' of your station and then attach modules to it. The base block itself is one of the modules, however, and this will limit the lifespan of the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now comes the real speculation: I think that what Russia is doing with their 'node module' will be the fourth generation of space stations. As far as I can tell, the node module will be relatively simple, and so can be designed to have a very long lifespan. Modules would be able to dock to the node module, and undock when their lifespan or technology has run its course. The longer-living node module would remain in space and see many modules dock and undock. But I'm just speculating here, I have no idea if this is Russia's intent or if its how they plan to use the node module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer your question in this context, I see the ISS as a third generation space station. So it is modular, but a lot of the components are still interconnected, and as such the station itself had a very finite lifespan by design. Modules could still be disconnected (I think they've run a lot of cables between the modules, so they would have to disconnect all those first. Not impossible, but a lot of work I'd imagine), but with no 'core' or 'node' to reattach them to, the question then becomes what to do with them, and that's where geoffc's answer goes into more detail than mine does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately I think the 'real' answer to your question is a combination of my answer and geoffc's answer&lt;/p&gt;&#xA;" OwnerUserId="1255" LastEditorUserId="1255" LastEditDate="2015-03-24T20:57:51.257" LastActivityDate="2015-03-24T20:57:51.257" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8540" PostTypeId="1" AcceptedAnswerId="8542" CreationDate="2015-03-24T15:18:07.527" Score="10" ViewCount="398" Body="&lt;p&gt;I noticed these “nacelle” like protrusions on the first stage of &lt;a href=&quot;http://en.wikipedia.org/wiki/Black_Arrow&quot; rel=&quot;noreferrer&quot;&gt;Black Arrow&lt;/a&gt; while browsing Wikipedia. They look like they're spaced unequally but symmetrically in two sets of two around the first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/T7Mis.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I couldn't work out their purpose, they look like they could contain some sort of SRMs to aid with staging but they are facing the wrong direction. My next through was that they might be some sort of sensor (pressure, turbulence, etc) but I can’t find any definitive reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also noticed similar protrusions on the Cyclone-4 first stage from a &lt;a href=&quot;http://www.parabolicarc.com/2015/03/24/brazil-cyclone4-deal-ukraine/&quot; rel=&quot;noreferrer&quot;&gt;recent Parabolic Arc article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jQ5vr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do these protrusions share the same purpose, and what is that purpose?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="7062" LastEditDate="2021-10-23T10:30:07.563" LastActivityDate="2021-10-23T10:30:07.563" Title="What are the “nacelle” like protrusions on the first stage of Black Arrow?" Tags="|stages|black-arrow|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8541" PostTypeId="2" ParentId="6248" CreationDate="2015-03-24T16:19:33.783" Score="3" Body="&lt;p&gt;The Shuttle nails a vulture as it clears the tower&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/t9nUB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=zTCjx5DEPkE&quot; rel=&quot;nofollow noreferrer&quot;&gt;Video here&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-03-24T16:19:33.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8542" PostTypeId="2" ParentId="8540" CreationDate="2015-03-24T16:23:19.040" Score="13" Body="&lt;p&gt;You can see these in the diagram (from &lt;a href=&quot;http://www.spaceuk.org/ba/ba_dimensions.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/rxUNl.gif&quot; alt=&quot;Back Arrow Schematic Diagram&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which clearly indicates that these are the 4 &lt;a href=&quot;http://www.spaceuk.org/solids/solidsdata.htm&quot; rel=&quot;noreferrer&quot;&gt;'siskin' separation (and ullage)  motors&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2015-03-24T16:23:19.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8543" PostTypeId="2" ParentId="8537" CreationDate="2015-03-24T18:24:27.683" Score="4" Body="&lt;p&gt;A colony on one of Jupiter's moons, say Europa or Callisto, could have a rocket fuel extraction industry that yielded cheap oxygen as a byproduct. You'd mine ice, melt it, and electrolyze it into H₂ and O₂. The high-thrust rockets you'd want for manned travel back and forth to Jupiter would be LH₂/LOX chemical rockets or nuclear thermal rockets using LH₂ as reaction mass. LH₂/LOX rockets run best fuel-rich, so in either case you'd have surplus oxygen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know if that would make oxygen &lt;em&gt;free&lt;/em&gt;, though. You'd need a vast amount of power to run that industry (and solar doesn't work well 5 AUs out, so I think you want a big fission power plant), and the oxygen still has to be handled and directed where it needs to go -- but it seems like providing breathing air to colonists would be a reasonable, taxable function of the colony state (&lt;em&gt;pace&lt;/em&gt; Heinlein).&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-03-24T18:24:27.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8544" PostTypeId="2" ParentId="8537" CreationDate="2015-03-25T00:59:13.643" Score="3" Body="&lt;p&gt;Agriculture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to a &lt;a href=&quot;http://web.mit.edu/sydneydo/Public/Mars%20One%20Feasibility%20Analysis%20IAC14.pdf&quot; rel=&quot;nofollow&quot;&gt;MIT MarsOne paper&lt;/a&gt;, on the feasibility of the MarsOne program, they found that &quot;our habitation simulations revealed that crop growth, if large enough to provide 100% of the settlement’s food, will produce unsafe oxygen levels in the habitat&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AKA:  The industry of creating food for the people living on Mars, would produce excessive amounts of oxygen.  So it'd be &lt;em&gt;free&lt;/em&gt; to breath &lt;em&gt;oxygen&lt;/em&gt;, just not &lt;em&gt;free&lt;/em&gt; to eat.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Granted, growing plants on Mars definitely isn't &lt;em&gt;free&lt;/em&gt;, as they require water and fertilizer, neither of which are free on Mars.   (Mars has no shortage of CO2 at least, which is a very important part of photosynthesis) &lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2015-03-25T00:59:13.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8545" PostTypeId="1" CreationDate="2015-03-25T13:21:08.177" Score="15" ViewCount="1335" Body="&lt;p&gt;I wonder if there is any rule for how closely you can put engine nozzles to each other. Are there any vibration, heat, etc. constraints when you are assembling two or more engine bells together?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_I&quot; rel=&quot;noreferrer&quot;&gt;Saturn I&lt;/a&gt; had four central engines very close, with the external gimballing engines more distant. I know that the outboard ones were placed there, among other things, to allow better pitch, yaw, and roll authority. But are there any limits for clustering engine nozzles?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nY1tY.jpg&quot; alt=&quot;Saturn I engine bay. The engine nozzles are highlighted, and you can compare the distance between each nozzle&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8761" LastEditorUserId="1142" LastEditDate="2015-03-26T12:47:31.530" LastActivityDate="2015-03-26T12:47:31.530" Title="Is there any rule for rocket engine nozzle proximity from each other?" Tags="|launch|engine-design|nozzle|thrust|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8546" PostTypeId="2" ParentId="8545" CreationDate="2015-03-25T13:29:58.510" Score="12" Body="&lt;p&gt;Obviously there are differences between implementations that will make a general rule very hard to define.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, on the Space Shuttle, the 3 SSME engines were close together, and the two SRB's offset nearby.  But when they looked at 4 or 6 RS-68A engines between the two SRB's for one of the many SLS/Ares iterations they found the heat load was too high. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus every engine will have different requirements, different capabilities, different tolerances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a contrast the 5 F-1 engines on the Saturn V were clustered pretty close together.  The 9 Merlin 1C on the Falcon 1.0 in the tic tac toe pattern, then the change to the Merlin 1D on the Falcon 1.1 in the Octaweb pattern. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus I would argue that generally you can cluster them as close as they can handle.  But the parameters there depend on the engines specifications. Or to put it more simply &quot;It depends&quot;.  Odd how often that is the answer.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-03-25T14:10:20.223" LastActivityDate="2015-03-25T14:10:20.223" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8547" PostTypeId="1" CreationDate="2015-03-25T14:28:44.587" Score="2" ViewCount="262" Body="&lt;p&gt;I recall that I saw a diagram of Soyuz 16 (the full ASTP rehearsal performed by Filiptchenko and Rukavishinikov in late 1974) made by Schliadinsky in a magazine. It featured the docking ring simulator. I never saw another drawing, nor a actual flight hardware photo of Soyuz 16 and its docking ring simulator. Does anyone have something on this subject?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The photo is from Soyuz 19 in assembly shop and the rendering is my view of Soyuz 16.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5eim3.jpg&quot; alt=&quot;Soyuz 19 (ASTP) on MIK assembly hall&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cdtyL.jpg&quot; alt=&quot;My 3D rendering of Soyuz 16 &quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8761" LastEditorUserId="168" LastEditDate="2015-03-26T03:26:46.153" LastActivityDate="2015-04-25T15:04:50.727" Title="Soyuz 16 ASTP rehearsal for Apollo Soyuz - Diagram required" Tags="|soyuz-spacecraft|reference-request|docking-port|apollo-soyuz|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8548" PostTypeId="2" ParentId="8449" CreationDate="2015-03-25T14:40:34.847" Score="1" Body="&lt;p&gt;&lt;a href=&quot;http://www.msn.com/en-au/news/techandscience/mars-has-nitrogen-key-to-life-nasa/ar-AA9XTO7?ocid=mailsignout&quot; rel=&quot;nofollow&quot;&gt;Recently&lt;/a&gt;, the Curiosity rover on Mars discovered nitrogen in the form of nitrates. Depending on the location and size of the nitrate deposits it may be possible to mine the nitrates as a source of nitrogen.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-03-25T14:40:34.847" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8550" PostTypeId="2" ParentId="8522" CreationDate="2015-03-25T21:13:46.730" Score="6" Body="&lt;p&gt;I've crunched the numbers and can answer; my gut feeling was good, and in all 299 manned launches, no crew has a greater combined amount of time in space. My top 10 crew experience table is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&#xA;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-05m.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-05M&lt;/a&gt; - 723.009 days : 3 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tm31.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TM-31&lt;/a&gt; - 680.813 days : 3 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/sts-102.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-102&lt;/a&gt; - 651.844 days : 7 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-6.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-6&lt;/a&gt; - 646.175 days : 3 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/sts-134.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-134&lt;/a&gt; - 635.706 days : 6 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-01m.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-01M&lt;/a&gt; - 630.625 days : 3 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-3.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-3&lt;/a&gt; - 602.949 days : 3 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-04m.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-04M&lt;/a&gt; - 598.083 days : 3 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/sts-88.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-88&lt;/a&gt; - 572.844 days : 6 Crew&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tm33.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TM-33&lt;/a&gt; - 561.703 days : 3 crew&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The growth in spaceflight experience per launch can be illustrated on this graph:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vc4oi.png&quot; alt=&quot;Spaceflight hours versus time&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;@Nickolai raised an interesting point; what happens if you measure number of previous launches averaged by crew size, you get the following experience rankings: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-03.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-3&lt;/a&gt; - 9 launches : 3 Crew (3.00)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tm21.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TM-21&lt;/a&gt; - 8 launches : 3 crew (2.67)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tm31.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TM-31&lt;/a&gt; - 8 launches : 3 crew (2.67)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-11m.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-11M&lt;/a&gt; - 8 launches : 3 crew (2.67)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-16m.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-16M&lt;/a&gt; - 8 launches : 3 crew (2.67)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/sts-80.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-80&lt;/a&gt; - 13 launches : 5 crew (2.60)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/sts-103.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-103&lt;/a&gt; - 18 launches : 7 crew (2.57)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-t13.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz T-13&lt;/a&gt; - 5 launches : 2 crew (2.50)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/sts-79.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-79&lt;/a&gt; - 15 launches : 6 crew (2.50)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spacefacts.de/mission/english/sts-88.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-88&lt;/a&gt; - 15 launches : 6 crew (2.50) &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&#xA;&#xA;&lt;p&gt;The growth is launch experience per launch can be shown in the following graph:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vO2Gu.png&quot; alt=&quot;Launch experience versus time&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, we can conclude,  the TMA-16M crew are the most experienced by space experience but amongst the most experienced by launch counts.&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="8162" LastEditDate="2015-04-09T12:21:20.813" LastActivityDate="2015-04-09T12:21:20.813" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8556" PostTypeId="2" ParentId="8547" CreationDate="2015-03-26T14:49:17.187" Score="2" Body="&lt;p&gt;All of the authoratitive information about the apollo-soyuz project can be found in the official NASA &lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4209/toc.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;history documents&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these what you are looking for?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TakCE.jpg&quot; alt=&quot;Illustration of mockups of docking mechanism&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I do note that some of the schematic diagrams have been elided from that history document, but are probably still in the print version.)&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2015-03-26T14:49:17.187" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8557" PostTypeId="2" ParentId="3934" CreationDate="2015-03-26T16:23:11.710" Score="3" Body="&lt;p&gt;No, it does not seem that the 3 different CBC's are interchangeable. The Common Booster Core employs separate spherical bulkheads on the LH2 and LOX tanks. The tanks employ internal stringers for additional stability and the LOX tanks use anti-slosh baffles. An external wiring tunnel runs down the length of the entire booster whereas some of these components of the stap on common booster cores.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are subtle differences that could more than likely easily be altered to make them interchangeable although some components would have to be added (such as the external wiring, etc). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is some more info:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflight101.com/delta-iv-heavy.html&quot; rel=&quot;nofollow&quot;&gt;http://www.spaceflight101.com/delta-iv-heavy.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course some of these components they all share in common but the wiring I think is the big thing. And the need for booster separation.&lt;/p&gt;&#xA;" OwnerUserId="8561" LastEditorUserId="8561" LastEditDate="2015-03-26T18:27:59.687" LastActivityDate="2015-03-26T18:27:59.687" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8558" PostTypeId="1" CreationDate="2015-03-26T23:10:59.443" Score="11" ViewCount="208" Body="&lt;p&gt;Let's say my hyperjumpwarpgate McGuffin drive goes bing and I find myself about half a light year away from the sun. This is right in the heart of the outer Oort Cloud. Wikipedia says that there are a trillion objects there larger than 1km in size, and they're about 30 light seconds apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I want to find the closest one of these. How?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Out there it's very dark, so I'm unlikely to be able to pick one up by reflected sunlight.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Radar's possible, but it's going to be a very slow process scanning the complete sky with a narrow-angle high-power beam, particularly as I'm going to have to wait several minutes for each return ping. OTOH Venus has been mapped by radar from Earth, so it should at least work.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I've seen suggestions of using infrared to look for very distant planets; but I suspect these are looking for large bodies with a certain amount of their own heat, while the small bodies I'm looking for are going to be very cold.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Would it be feasible to look for shadows against the interstellar background? I'd have to photograph the entire sky at insane resolution, then move, photograph it again, and look for parallax. That would only work for objects at the right vector to my motion, and of course only where there was some sort of bright background --- but there's quite a lot of Milky Way.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What's the most practical real-world method of doing this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;b&gt;Edit:&lt;/b&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to calculate the apparent brightness of the sun from that distance: half a light year is $180*24*3600$ light seconds, which is $\frac{180*24*3600}{8*60} = 32400$ AU. That means that it should be $\frac{1}{32400^2} = \frac{1}{10^{10}}$ the brightness that it is here. That's quite dim.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that apparent magnitude is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_x - m_{x,0} = -2.5 log_{10} \frac {F_x}{F_{x,0}} $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That gives us, for the sun:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_x - -26.74 = -2.5 log_{10} \frac {1 / 10^{10}}{1} $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...or:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_x = -2.5 log_{10} (10^{-10}) - 26.74$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...so $m_x$ is -1.74, which makes the Sun about the same brightness as Sirius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I was totally not expecting the maths above to produce a meaningful result.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;b&gt;Edit edit:&lt;/b&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Yes, I'm bored at work.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angular diameter of an object is $ \tan \frac{\text {size}}{\text {distance}} $, which means that a 1km body at 30 light seconds is going to be $ \tan \frac{1000}{9 \times 10^9} $, which is about $5 \times 10^{-6} $ degrees. Or about 0.02 arcseconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has a &lt;a href=&quot;http://en.wikipedia.org/wiki/Angular_resolution#/media/File:Diffraction_limit_diameter_vs_angular_resolution.svg&quot; rel=&quot;nofollow&quot;&gt;handy table of diffraction limits vs telescopes&lt;/a&gt;. Turns out that the Hubble could resolve that 1km body, provided it was radiating in ultraviolet, which is probably unlikely. For high infrared I'd need a telescope with an aperture of several hundred metres, which is doable, but for low infrared I would need something 10km across, which is harder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may, however, be possible to detect the body &lt;em&gt;without&lt;/em&gt; having to resolve it (i.e. the same way we can see stars); but I don't know how that works.&lt;/p&gt;&#xA;" OwnerUserId="8192" LastEditorUserId="8192" LastEditDate="2015-08-08T19:59:39.740" LastActivityDate="2021-07-17T08:32:44.943" Title="Finding non-lit objects at light minute ranges" Tags="|asteroid|telescope|remote-sensing|oort-cloud|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8559" PostTypeId="1" AcceptedAnswerId="8560" CreationDate="2015-03-26T23:41:22.580" Score="6" ViewCount="1827" Body="&lt;p&gt;On Earth we call The Moon a moon, but if we were on The Moon what would we call the object we are orbiting around?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I'm not being very clear I'm looking for a word that would describe what Earth to The Moon, Pluto to Charon, and Dactyl to Ida.&lt;/p&gt;&#xA;" OwnerUserId="8779" LastActivityDate="2015-03-26T23:45:54.957" Title="What do you call an object that has a moon orbiting?" Tags="|terminology|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8560" PostTypeId="2" ParentId="8559" CreationDate="2015-03-26T23:45:54.957" Score="9" Body="&lt;p&gt;It's called its &lt;a href=&quot;http://en.wikipedia.org/wiki/Natural_satellite&quot;&gt;primary&lt;/a&gt;. Unless it is a double planet system where there is no primary.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-03-26T23:45:54.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8562" PostTypeId="1" AcceptedAnswerId="8568" CreationDate="2015-03-27T00:50:41.517" Score="3" ViewCount="128" Body="&lt;p&gt;It seems like to me all our analysis of stars and planets could be automated. A computer looks through the data for anomalies and then performs additional observations for that area and then outputs a presumed planet fact sheet based on all its calculations. If it's certainty is less than some threshold, it prompts a scientist to examine the data more closely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps this already happens--but from media headlines in the form of &quot;researchers discover another potential habitable planet!&quot; I got the impression that automated detecting wasn't a thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to know if we are automating, and if so, how it works (I am particularly interested in the search for Earth-like planets). Otherwise, if we are not automating, I would like to know why we are not.&lt;/p&gt;&#xA;" OwnerUserId="8780" LastEditorUserId="8780" LastEditDate="2015-03-27T01:12:51.897" LastActivityDate="2015-03-27T15:41:07.923" Title="Are we using software to analyze our telescope data?" Tags="|telescope|observation|raw-data|analysis|" AnswerCount="1" CommentCount="3" ClosedDate="2015-03-29T14:59:43.347" ContentLicense="CC BY-SA 3.0" />
  <row Id="8565" PostTypeId="1" AcceptedAnswerId="8587" CreationDate="2015-03-27T07:12:31.003" Score="11" ViewCount="1465" Body="&lt;p&gt;While browsing this site, I saw several references to &quot;Russian side of ISS&quot; and several questions arised in my head. The root question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there any other reason to split ISS to Russian and NASA side than pure logical?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Examples. In normal mission day on ISS (standard procedures, no emergency):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Can astronaut freely move wherever they want on ISS? (In their free time)&lt;/li&gt;&#xA;&lt;li&gt;Do Russian astronaut sleep in Russiann side, while NASA/ESA ones sleep in their side?&lt;/li&gt;&#xA;&lt;li&gt;Is &quot;sleepover&quot; allowed? (Russian astronaut sleeping in NASA side. Zero-G pillow fight involved)&lt;/li&gt;&#xA;&lt;li&gt;Are there separate Russian/NASA missions or is there only one &quot;common&quot; mission ivolving astronauts from all nationalities?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; By pure logical, I mean, that I am assuming answer: &quot;Russian side of ISS is called Russian simply because Russians did put it up there&quot;&lt;/p&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="6537" LastEditDate="2015-03-29T07:35:18.467" LastActivityDate="2015-03-29T07:35:18.467" Title="Is there any other reason to split ISS to Russian and NASA side than pure logical?" Tags="|iss|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8566" PostTypeId="1" AcceptedAnswerId="12229" CreationDate="2015-03-27T07:31:10.127" Score="17" ViewCount="34641" Body="&lt;p&gt;There are thousands of satellites orbitting our planet and everyone of them got there from a rocket launch. I know that when a launch fails it is practically always a catastrophic failure, otherwise its a minor or major setback and the launch is rescheduled. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the failure rate of space bound rocket launches, in general? I mean for all the nations that have tried.&lt;/p&gt;&#xA;" OwnerUserId="6392" LastEditorUserId="51" LastEditDate="2015-03-27T20:29:10.817" LastActivityDate="2016-08-11T09:43:17.300" Title="What is the success/failure ratio of space bound rockets?" Tags="|launch|rockets|failure|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8568" PostTypeId="2" ParentId="8562" CreationDate="2015-03-27T10:31:04.810" Score="4" Body="&lt;p&gt;This is done through APF, NEAT and LINEAR, as mins mentioned however an interesting project currently running is the &lt;a href=&quot;http://www.galaxyzoo.org/&quot; rel=&quot;nofollow&quot;&gt;Galaxy Zoo&lt;/a&gt;, which works on the principle that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Humans are much better at identifying certain types of pattern than computers&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So while computers trawl raw data for potential points of interest, these are then passed to humans for classification into type, interesting or not, anomalies etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Rikki-Tikki-Tavi commented, &lt;a href=&quot;http://www.planethunters.org/&quot; rel=&quot;nofollow&quot;&gt;planethunters&lt;/a&gt; does the same for planets.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2015-03-27T15:41:07.923" LastActivityDate="2015-03-27T15:41:07.923" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8569" PostTypeId="1" AcceptedAnswerId="8574" CreationDate="2015-03-27T10:31:36.813" Score="27" ViewCount="4221" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/questions/604/is-it-possible-to-get-pregnant-through-natural-means-in-space#comment22411_604&quot;&gt;comment by Organic Marble&lt;/a&gt; on the question &lt;a href=&quot;https://space.stackexchange.com/questions/604/is-it-possible-to-get-pregnant-through-natural-means-in-space&quot;&gt;Is it possible to get pregnant through natural means in space?&lt;/a&gt; includes a link to &lt;a href=&quot;http://www.spaceref.com/iss/medical/4041.pregnancy.pdf&quot; rel=&quot;noreferrer&quot;&gt;PREGNANCY TEST (ISS MED/3A - ALL/FIN)&lt;/a&gt; which seems to indicate there is a pregnancy test kit on the ISS.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As indicated in the original question mammals do not seem able to get (or stay) pregnant in the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is the test kit there? Has it ever been used? What is the procedure if the results where positive? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2023-11-09T00:15:47.957" Title="Why is there a pregnancy test kit on the ISS?" Tags="|iss|life|medical|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8570" PostTypeId="2" ParentId="8565" CreationDate="2015-03-27T11:01:38.313" Score="6" Body="&lt;p&gt;The Russian side has sleeping spots for three people in Zvevda and the US side has sleeping spots for four in Node 1 (Unity).  Usually the Russian crew sleep on the Russian side and the US crew on the US side.  (Europeans?  They flip coins?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a bathroom on the US side and one on the Russian side. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not think they do sleepovers, because the sleeping spaces are not really shared, rather they become the only really personal space for 'your stuff' on the station for each astronaut/cosmonaut. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian side has been described as loud and noisy. The US side, while many fans and stuff going on is apparently much quieter and nicer place to hang out.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-03-27T12:08:17.053" LastActivityDate="2015-03-27T12:08:17.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8571" PostTypeId="1" AcceptedAnswerId="8572" CreationDate="2015-03-27T11:37:53.077" Score="9" ViewCount="912" Body="&lt;p&gt;NASA &lt;a href=&quot;http://www.nasa-usa.de/content/one-year-mission/&quot;&gt;One Year Mission&lt;/a&gt; will send two people to ISS for one year to help us to get closer to Mars&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_spaceflight_records#Most_time_in_space&quot;&gt;Valeri Polyakov&lt;/a&gt; Is current record holder for longest space flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, there is one of most obvious questions: We have people who already have been in zero-g for year and even longer (record is 437.7 days). Why cannot we use medical data obtained from these missions?&lt;/p&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="25" LastEditDate="2015-03-27T12:45:44.790" LastActivityDate="2015-03-27T19:31:02.760" Title="One year Mission: Using data from Valeri Polyakov?" Tags="|iss|mars|future-missions|research|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8572" PostTypeId="2" ParentId="8571" CreationDate="2015-03-27T12:06:58.697" Score="10" Body="&lt;p&gt;The Russian medical monitoring of long term flight participants was not considered well done by the rest of the worlds medical establishment.  The ISS has much more equipment, resources, and crew time to monitor than did Mir (Crew of 6 vs 3). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a repeat with a larger (2 vs 1, yay!) sample set with better monitoring in place, using lessons learned from the past.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally this study has an incredibly fortuitous add on. The Kelly brothers are twins. Both are astronauts with very similar careers. Scott Kelly will be on the station, and his twin (Mark) will spend the year performing similar tasks on Earth.  Twin studies are great, since their genome is pretty darn close (compared to any other two random people) and helps remove genetic predispositions from the results.  Very fortuitous!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most importantly, repeatability and sample size are the keys in experimentation.  You should repeat it often to ensure you get the same results. Sample set of 1 is mostly meaningless. Or as commenter Federico notes, sample size of 1 is basically a Mythbusters episode.  (Credit to &lt;a href=&quot;http://www.phdcomics.com/comics/archive.php?comicid=1156&quot; rel=&quot;nofollow noreferrer&quot;&gt;PHD Comics&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IGY2A.gif&quot; alt=&quot;PHD Comics&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-03-27T19:31:02.760" LastActivityDate="2015-03-27T19:31:02.760" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8573" PostTypeId="2" ParentId="8571" CreationDate="2015-03-27T12:37:13.260" Score="6" Body="&lt;p&gt;The data from previous missions is largely valid and technically should be usable. They can act as a very good guide to what can be expected. However, there are always issues and it always comes down to details:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ownership of the Data&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Valeri Palyakov is a Russian, so Russia owns the data for his mission. Russia may or may not want to release all the data for Valeri's mission or reveal all of the conditions that Valeri had to deal with during his mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians can use their data to advance their programs in Space. America is a competitor to the Russians in Space. What does Russian gain by helping America advance its Space programs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The American would like to have their own data based on their personnel and their test conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mission Conditions &amp;amp; Operating Procedures&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The American mission may want to test for different conditions than those of the Russian mission that Valeri was on. The Americans may want to emphasise some things in their conditions that the Russians didn't in theirs. Likewise they may want to de-emphasise some things that the Russians did.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians and Americans have different cultures, different attitudes to authority, different attitudes to hardship and extremes and some of their operating procedures will be different. This may influence the results. Russians like to emphasise that no other people on Earth can tolerate cold conditions like they can. But is it really true? Similar statements are made by people who live in very hot environments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conducting such missions with your own personnel and with you own procedures gives you greater control over the results and how useful the results will be. You can also change the program as you go along, depending on circumstances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other thing is that in science results from experiments need to be tested, repeated and verified before they can be fully accepted. An American long duration test program will provide a second set of results that can be compared to those of Valeri's mission, if the Russians release all of the data for that mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Politics&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember, there once was a Space race in which the Russians and American competed against each other and there was a cold war between the two. A new cold war appears to be re-immerging as a result of what's happening in Ukraine. The attitude of sharing, between the two nations has changed and it is not as open as it has been in the recent past.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-03-27T12:37:13.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8574" PostTypeId="2" ParentId="8569" CreationDate="2015-03-27T13:11:03.197" Score="36" Body="&lt;p&gt;The test kit is there in case a female crewmember suspects that she might be pregnant.  Your second question will never be answered because of US medical privacy laws.  For the 3rd question, I have not found a documented answer, but I suspect a medical evacuation would be in order, due to the unknown developmental effects on the unborn child.  &lt;a href=&quot;http://www.quora.com/Does-the-International-Space-Station-ISS-have-an-emergency-escape-pod&quot;&gt;This would result in the return of one three person crew&lt;/a&gt; (two of whom, I suspect, would be grumpy).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-03-27T13:15:09.643" LastActivityDate="2015-03-27T13:15:09.643" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8575" PostTypeId="1" AcceptedAnswerId="8579" CreationDate="2015-03-27T15:07:38.280" Score="15" ViewCount="5153" Body="&lt;p&gt;This question comes from an answer at &lt;a href=&quot;https://space.stackexchange.com/questions/8569/why-is-there-a-pregnancy-test-kit-on-the-iss/8574#8574&quot;&gt;Why is there a pregnancy test kit on the ISS?&lt;/a&gt; that says &quot;US medical privacy laws&quot; apply on the ISS.  These US laws are for the most part addressed at &lt;a href=&quot;http://www.hhs.gov/ocr/privacy/&quot; rel=&quot;noreferrer&quot;&gt;http://www.hhs.gov/ocr/privacy/&lt;/a&gt; most US citizens are familiar with the term HIPAA as it applies to medical privacy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have a couple distantly related questions about law in space &lt;a href=&quot;https://space.stackexchange.com/questions/683&quot;&gt;Jurisdiction over crime in space&lt;/a&gt; &amp;amp; &lt;a href=&quot;https://space.stackexchange.com/questions/2372&quot;&gt;What would happen if a mutiny occurred on the International Space Station?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no doubt that a person on the ground in the US is subject to US law.  But what about in space or on the ISS; these two documents &lt;a href=&quot;http://download.esa.int/docs/ECSL/ISS_Crew_Code_of_Conduct.pdf&quot; rel=&quot;noreferrer&quot;&gt;ISS Crew Code of Conduct&lt;/a&gt; &amp;amp; &lt;a href=&quot;http://download.esa.int/docs/ECSL/ISS_Crew_Disciplinary_Policy.pdf&quot; rel=&quot;noreferrer&quot;&gt;ISS Crew Disciplinary Policy&lt;/a&gt; are military type command and reporting responsibilities. It seems like something similar to &lt;a href=&quot;http://en.wikipedia.org/wiki/Admiralty_law&quot; rel=&quot;noreferrer&quot;&gt;admiralty law or maritime law&lt;/a&gt; would apply, but I really don't know.  I am specifically excluding questions of a civil or liability nature, this is questions is about criminal activities which could result in jail time.  &lt;a href=&quot;http://www.ama-assn.org/ama/pub/physician-resources/solutions-managing-your-practice/coding-billing-insurance/hipaahealth-insurance-portability-accountability-act/hipaa-violations-enforcement.page?&quot; rel=&quot;noreferrer&quot;&gt;Violations of HIPAA can result &quot;imprisonment for up to ten years&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As person on the ISS are you subject to any criminal laws of the US?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; While the question is written around HIPAA I am interested in all criminal law.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="32182" LastEditDate="2021-07-26T23:12:31.720" LastActivityDate="2021-07-26T23:12:31.720" Title="Which, if any, US laws apply on the ISS?" Tags="|iss|law|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="8576" PostTypeId="1" AcceptedAnswerId="8577" CreationDate="2015-03-27T20:08:58.347" Score="4" ViewCount="176" Body="&lt;p&gt;Propulsion modules have been adapted to cope in earth's atmosphere. However what are some of the differences that would have to be taken into account if one was ever to fly in a thicker or thinner atmosphere. Would you need the same amount of energy? bigger engines? longer propellers (in case of a helicopter). etc..&lt;/p&gt;&#xA;" OwnerUserId="8789" LastEditorUserId="6312" LastEditDate="2015-03-27T20:24:07.763" LastActivityDate="2015-03-27T21:02:07.217" Title="What differences from conventional aerodynamics would result from a thinner atmosphere?" Tags="|propulsion|atmosphere|engineering|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8577" PostTypeId="2" ParentId="8576" CreationDate="2015-03-27T21:02:07.217" Score="4" Body="&lt;p&gt;Conventional Earthly jet aircraft already cope with a wide range of atmospheres -- from sea level pressure (&quot;1 atmosphere&quot;) to 20% atmosphere at 35,000 feet (a typical airliner cruise altitude) to 10% atmosphere for high-performance military jets at 60,000 feet. As the atmosphere gets thinner, your jet engines have less oxygen to burn, and your wings provide less lift, but the plane is contending with less atmospheric drag. To get much higher than that, you need very large wings or very high speeds to maintain lift in a winged plane (e.g. the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lockheed_U-2&quot;&gt;U-2&lt;/a&gt; or the &lt;a href=&quot;http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird&quot;&gt;SR-71&lt;/a&gt;), or you switch to high powered rocket engines and forego wings entirely, becoming a spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For thicker atmospheres, staying in the air is easy, but going fast is hard. If air pressure is higher, you can get the same amount of lift at lower speed or with smaller wings, but drag increases with the density of the air, so you'd be more efficient going very slowly. But, of course, if the air is that thick at the ground, you can always just go up! A plane designed for Earthly atmospheres would work just fine in a moderately denser atmosphere (assuming a reasonable amount of oxygen, and no unexpected pollutants) -- it would just have a higher flight ceiling.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-03-27T21:02:07.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8579" PostTypeId="2" ParentId="8575" CreationDate="2015-03-27T21:53:38.980" Score="24" Body="&lt;p&gt;The International Space Station Intergovernmental Agreement covers the ISS. Under &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/International_Space_Station/International_Space_Station_legal_framework&quot;&gt;the IGA&lt;/a&gt;, as it is often known, the laws of the nation (or region, in the case of the European Union) apply to its citizens and property :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This extension of national jurisdiction determines what laws are&#xA;  applicable for activities occurring on a Partner’s Space Station&#xA;  elements (e.g. European law in the European Columbus Laboratory). This&#xA;  legal regime recognises the jurisdiction of the Partner States’s&#xA;  courts and allows the application of national laws in such areas as&#xA;  criminal matters, liability issues, and protection of intellectual&#xA;  property rights. Any conflicts of jurisdiction between the Partners&#xA;  may be resolved through the application of other rules and procedures&#xA;  already developed nationally and internationally.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The basic rule is that 'each partner shall retain jurisdiction and&#xA;  control over the elements it registers and over personnel in or on the&#xA;  Space Station who are its nationals' (Article 5 of the&#xA;  Intergovernmental Agreement).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So if you are American, US law applies wherever you are in the station. If you are French, European Union law applies to you. If an American and a Frenchman get into a fight, during which Russian property is damaged, things get extremely complicated. To handle this, there is a &lt;a href=&quot;https://www.law.cornell.edu/cfr/text/48/1852.228-76&quot;&gt;cross-waiver of liability&lt;/a&gt;. Under it, liability for all damage is waived, including:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(i) Bodily injury to, or other impairment of health of, or death of,&#xA;  any person; &#xA;  (ii) Damage to, loss of, or loss of use of any property;&#xA;  (iii) Loss of revenue or profits; or &#xA;  (iv) Other direct, indirect, or&#xA;  consequential damage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So that covers everything serious, &lt;em&gt;as long as it happened as part of official duties&lt;/em&gt;. Obviously there is a big grey area there. In the above example, if there was some shouting and shoving while on duty and someone accidentally kicked a laptop monitor, perhaps it would be permitted to come under the waiver. If it is decided that an act was criminal, then&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a case involving misconduct on orbit that: (a) affects the life or&#xA;  safety of a national of another Partner State or (b) occurs in or on&#xA;  or causes damage to the flight element of another Partner State, the&#xA;  Partner State whose national is the alleged perpetrator shall, at the&#xA;  request of any affected Partner State, consult with such State&#xA;  concerning their respective prosecutorial interests. An affected&#xA;  Partner State may, following such consultation, exercise criminal&#xA;  jurisdiction over the alleged perpetrator provided that, within 90&#xA;  days of the date of such consultation or within such other period as&#xA;  may be mutually agreed, the Partner State whose national is the&#xA;  alleged perpetrator either:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(1) concurs in such exercise of criminal jurisdiction, or&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(2) fails to provide assurances that it will submit the case to its&#xA;  competent authorities for the purpose of prosecution.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That quote comes from the &lt;a href=&quot;ftp://ftp.hq.nasa.gov/pub/pao/reports/1998/IGA.html&quot;&gt;document itself&lt;/a&gt;. After that point it starts talking about extradition procedures. So it is a mess and let's hope it never happens on the ISS. If a citizen of Partner State A beat the hell out of a citizen of Partner State B, and Partner State B was not pleased by how  Partner State A was handling the matter, well then they could demand extradition and apply their own laws. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, everything up there depends on diplomacy.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-03-27T22:58:56.170" LastActivityDate="2015-03-27T22:58:56.170" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8581" PostTypeId="1" CreationDate="2015-03-28T15:39:56.377" Score="6" ViewCount="747" Body="&lt;p&gt;Would it orbit very slowly? It's kinda been stuck in my head lately...&lt;/p&gt;&#xA;" OwnerUserId="8796" LastEditorUserId="4660" LastEditDate="2015-03-28T15:45:18.980" LastActivityDate="2015-03-28T18:28:41.790" Title="If there was a probe orbiting at the edge of Earth's Sphere of influence, how slow would it orbit?" Tags="|orbital-mechanics|orbit|gravity|earth|soi|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8582" PostTypeId="2" ParentId="8581" CreationDate="2015-03-28T17:24:08.683" Score="5" Body="&lt;p&gt;The Earth's Hill Sphere extends to a radius of approx 1.5 million km.  The velocity of a circular orbit at such a distance is around 500 m/s - which is very slow compared to LEO, but very fast compared to most forms of transportation, so I suppose it depends on your perspective...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such an orbit has a period of approx 210 days, and is usually unstable.  Lunar and solar perturbations will increase the eccentricity until it (most likely) escapes into a heliocentric orbit.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2015-03-28T17:24:08.683" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8583" PostTypeId="2" ParentId="8581" CreationDate="2015-03-28T17:38:18.817" Score="7" Body="&lt;p&gt;I define &lt;a href=&quot;http://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)&quot; rel=&quot;noreferrer&quot;&gt;sphere of influence&lt;/a&gt; as:&#xA;$(M_1/M_0)^{2/5} * r$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this case $M_1$ is mass of the earth, $M_0$ is the mass of the sun, and $r$ is $149597871$ kilometers (distance from sun to earth).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to my &lt;a href=&quot;http://www.clowder.net/hop/railroad/Hohmann150328.xls&quot; rel=&quot;noreferrer&quot;&gt;Hohmann spreadsheet&lt;/a&gt;, altitude of Sphere of Influence (SOI) is $918149$ kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also according to my Hohmann spreadsheet a circular orbit at that altitude would move $.6565$ km/s and orbital period would be $102.4$ days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the spreadsheet is based on two body Newtonian mechanics. An actual orbit at that altitude might be distorted by the sun's influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CG1Gb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should you want to play with the spreadsheet here's an illustration. Typing the SOI altitude into both periapsis and apoapsis gives a circular orbit at that altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Underlined is the circular orbit speed at periapsis and period in days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's assumed the user has already typed Earth into the departure planet cell.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-03-28T18:28:41.790" LastActivityDate="2015-03-28T18:28:41.790" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8584" PostTypeId="1" AcceptedAnswerId="8585" CreationDate="2015-03-28T20:51:30.140" Score="4" ViewCount="885" Body="&lt;p&gt;I've heard (though I can't find the source) that sufficiently large interstellar space debris has the potential to destroy spacecraft, and that there is considerable uncertainty about how much of a risk it is. Is it plausible that the frequency of this debris, or perhaps the frequency of it outside the local bubble, is much higher than currently thought? Is it plausible that this space debris is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Great_Filter&quot; rel=&quot;nofollow&quot;&gt;Great Filter&lt;/a&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that this is partially opinion-based, but I hope it can be factually answered at least to some extent.&lt;/p&gt;&#xA;" OwnerUserId="8722" LastEditorUserId="49" LastEditDate="2015-04-21T14:52:33.997" LastActivityDate="2019-12-05T16:17:22.220" Title="Could unexpectedly high levels of interstellar space debris be the Great Filter?" Tags="|interstellar-travel|interstellar-medium|fermi-paradox|seti|drake-equation|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8585" PostTypeId="2" ParentId="8584" CreationDate="2015-03-28T22:17:09.293" Score="8" Body="&lt;p&gt;I don't see how it could possibly be the cause--it would make interstellar travel harder, it wouldn't make it impossible and it wouldn't destroy the species--once they found out about it they could beef up the defenses of their starships.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-03-28T22:17:09.293" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="8586" PostTypeId="2" ParentId="8494" CreationDate="2015-03-29T00:30:19.853" Score="26" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;This conclusion MUST have been reached by NASA (or even NACA) in the leadup to the Apollo landings.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Despite the name, getting people to the Moon is not rocket science. It's rocket engineering. Engineers know that effects that are orders of magnitude smaller than the sensitivity of their systems are essentially non-effects. General relativity is a non-effect. A memo to that effect was not needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much of the Apollo trajectory planning work was done in the late 1950s / early 1960s.That early trajectory work used the patched conic approximation of the N-body problem. Near the Earth, and on the way to the Moon, only the  gravitation from the Earth was considered. Once inside the Moon's sphere of influence, only the gravitation of the Moon was considered. Only one gravitating body is active at a time in the patched conic approximation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for using this low-level approximation was that they were solving a non-linear boundary value using extremely archaic computing machinery (by modern standards). Computers back then were slow. Very slow. The first Macintosh (1984) was ten times faster in terms of floating point operations per second. If you have a laptop bought within the last ten years, it dwarfs the supercomputers of the 1980s. The supercomputers of the early 1960s? They didn't exist. That's a mid-1960s development, and those first supercomputers weren't that super.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even using that archaic machinery, those pioneers of the space program saw they had a problem, and it wasn't that they weren't using general relativity. The problem was the sensitivity of the transfer orbit to initial conditions. Thrust errors over 10% were quite common back then. In fact, that more or less remains the case today. There have been some improvements, but not a lot. Rockets remain a bit chaotic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What used to surpass that challenge wasn't general relativity. It was the extended Kalman filter. NASA didn't need general relativity to send humans to the Moon, but they absolutely did need the Kalman filter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a really nice site: &lt;a href=&quot;http://www.ibiblio.org/apollo/index.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.ibiblio.org/apollo/index.html&lt;/a&gt; . There you can find the Apollo Guidance Computer (AGC) flight software, an AGC emulator, and lots of historic documentation. The AGC flight software modeled the following gravitational effects:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Earth gravity. Effects modeled were spherical gravity ($GM/r^2$) and the first four zonal harmonics ($J_2$, $J_3$, $J_4$, and $J_5$). No sectoral harmonics, no tesseral harmonics. Ignoring these (and ignoring higher order terms) is a much bigger breach of reality than ignoring general relativity. The general relativistic contribution to acceleration due to Earth's gravity is roughly equivalent to the 20th zonal harmonic.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Moon gravity. Effects modeled were spherical gravity, the first four zonal harmonics, and the first tesseral harmonic ($J_{22}$). This was not high enough fidelity to capture the effects of the lunar mascons. Then again, nobody knew about those mascons at the time the AGC flight software was written.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Optionally, Sun gravity. This apparently was disabled during lunar descent and ascent.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And that's it. Venus and Jupiter both have a greater perturbative impact on the trajectories of vehicles in the Earth-Moon system than does general relativity, and yet they weren't modeled. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-03-30T13:09:25.963" LastActivityDate="2015-03-30T13:09:25.963" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8587" PostTypeId="2" ParentId="8565" CreationDate="2015-03-29T04:23:11.287" Score="9" Body="&lt;p&gt;In order:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Astronauts and cosmonauts can and do move freely between the two segments. It's one crew and one station; it's not a Russian station docked to a US station.&lt;/li&gt;&#xA;&lt;li&gt;Crew can sleep in each others' segments. There are two crew quarters on the Russian segment and four on the US segment, and typically three Russian cosmonauts aboard, so there's normally at least one Russian cosmonaut sleeping in USOS. US astronauts used to sleep in Zvezda routinely back when there was a crew of 3 and only the two quarters in Zvezda, but now I think Zvezda is mostly Roscosmos (since the non-Russians can all use Harmony).&lt;/li&gt;&#xA;&lt;li&gt;As mentioned, crew can have quarters in each others' segments. I'm not sure what you mean by &quot;sleepovers;&quot; you won't see crew sharing quarters (they're a) not big and b) the only private area on the whole station), but crew could sleep anywhere on the station if needed (you just need a bag to keep yourself from drifting off). However, I don't imagine this is horribly common to do for fun, because you're already living with crewmates -- a sleepover doesn't really add much to that.&lt;/li&gt;&#xA;&lt;li&gt;While there are Russian missions, US missions, ESA missions, etc. (each partner can run its own scientific missions), the crew also does a number of joint experiments. If Roscosmos or NASA is running an experiment, it'll likely be their own crew doing it; joint experiments are done jointly. Station maintenance seems to be a joint activity.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Russian segment and USOS differ in a few ways. For one, there are engineering differences between the two; there are compatibility circuits installed. They generally have different docking/berthing connectors. They have different spacesuits. Different modules are controlled by different agencies. &lt;em&gt;Most&lt;/em&gt; of the Russian orbital segment belongs to Roscosmos (Zarya is NASA-owned). So there are differences between the two; it's a useful distinction to make for some purposes. But it's still fundamentally one station.&lt;/p&gt;&#xA;" OwnerUserId="3316" LastActivityDate="2015-03-29T04:23:11.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8588" PostTypeId="2" ParentId="8566" CreationDate="2015-03-29T05:46:11.270" Score="5" Body="&lt;p&gt;To qualify further Russell Borogove's comment:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;&lt;em&gt;94% success is over a long history of rocket development. More recently, launching agencies have refined their designs and processes to achieve really high reliabilities. Atlas II through Atlas V have had only one partial failure in 120 launches since 1991, for example.&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Let's take the one partial failure in 120 data and check whether this is statistically significantly lower than the $94\%$ success rate long term. One could apply the right same principles to the different launch vehicle categories on the &lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_orbital_launchers_families&quot; rel=&quot;noreferrer&quot;&gt;Comparison of Orbital Launchers Families&lt;/a&gt; Wiki page&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the true probability of partial failure were $p=6\%$, as in the figure quoted by Russell's comment, the probability of observing one partial failure or fewer in 120 launches is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\binom{120}{1} p^1\,(1-p)^{119} + \binom{120}{0}\,p^0\,(1-p)^{120}\approx 0.0051$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;i.e.&lt;/em&gt; highly statistically significant. So the Atlas data betoken statistically significantly better performance than the $94\%$ long term data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's estimate a lower bound on the true reliability data. Suppose we reject the null hypothesis at 99% confidence level, then the highest failure rate in keeping with the data at this confidence level is the solution to:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\binom{120}{1} p^1\,(1-p)^{119} + \binom{120}{0}\,p^0\,(1-p)^{120}\approx 0.01$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which comes out to about $p=5.4\%$. So this simple calculation shows that the true long failure rate of the Atlas family since 1991 is at most $5.4\%$ at 99 percent confidence.&lt;/p&gt;&#xA;" OwnerUserId="6005" LastEditorUserId="195" LastEditDate="2015-07-15T23:25:45.620" LastActivityDate="2015-07-15T23:25:45.620" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8590" PostTypeId="1" CreationDate="2015-03-29T09:30:31.167" Score="12" ViewCount="18971" Body="&lt;p&gt;As I know on Low Earth Orbit it takes around 90 minutes to orbit the Earth, but this changes with altitude. My question is at what altitude did the Apollo Command Module orbit the moon and how much time did it take?&lt;/p&gt;&#xA;" OwnerUserId="8606" LastActivityDate="2015-12-09T20:23:05.513" Title="How long did it take for the Apollo Command Module to orbit the moon?" Tags="|the-moon|orbit|apollo-program|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8591" PostTypeId="2" ParentId="8590" CreationDate="2015-03-29T10:53:04.057" Score="20" Body="&lt;p&gt;Please note these are average periods, since the CSM did its maneuvering (DOI, descent orbit initiation) to make it easier for the LM to land on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{matrix}&#xA;    \text{Mission}      &amp;amp; \text{Avg period, mm:ss} \\&#xA;    \text{Apollo 8}     &amp;amp; 121:01 \\&#xA;    \text{Apollo 10}    &amp;amp; 119:27 \\&#xA;    \text{Apollo 11}    &amp;amp; 119:00 \\&#xA;    \text{Apollo 12}    &amp;amp; 118:37 \\&#xA;    \text{Apollo 14}    &amp;amp; 117:31 \\&#xA;    \text{Apollo 15}    &amp;amp; 117:44 \\&#xA;    \text{Apollo 16}    &amp;amp; 117:57 \\&#xA;    \text{Apollo 17}    &amp;amp; 118:10&#xA;\end{matrix}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources: Apollo by the numbers. (&lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_18-01_General_Background.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_18-01_General_Background.htm&lt;/a&gt;) Individual parameters (exact inclination, for instance) for some missions can also be found (not always reliable) at &lt;a href=&quot;http://nssdc.gsfc.nasa.gov&quot;&gt;http://nssdc.gsfc.nasa.gov&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apollo 8 - &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_08g_Lunar_Orbit_Phase.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_08g_Lunar_Orbit_Phase.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo 10 - &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_10g_Lunar_Orbit_Phase.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_10g_Lunar_Orbit_Phase.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo 11 - &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_11g_Lunar_Orbit_Phase.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_11g_Lunar_Orbit_Phase.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo 12 - &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_12g_Lunar_Orbit_Phase.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_12g_Lunar_Orbit_Phase.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo 14 - &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_14g_Lunar_Orbit_Phase.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_14g_Lunar_Orbit_Phase.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo 15 - &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_15g_Lunar_Orbit_Phase.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_15g_Lunar_Orbit_Phase.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo 16 - &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_16g_Lunar_Orbit_Phase.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_16g_Lunar_Orbit_Phase.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo 17 - &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_17g_Lunar_Orbit_Phase.htm&quot;&gt;http://history.nasa.gov/SP-4029/Apollo_17g_Lunar_Orbit_Phase.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The orbit altitudes were on the order of 60 nmi, or 110 km.  See the links above for more details.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="265" LastEditDate="2015-03-29T21:39:52.757" LastActivityDate="2015-03-29T21:39:52.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8593" PostTypeId="1" AcceptedAnswerId="8594" CreationDate="2015-03-29T16:50:06.653" Score="5" ViewCount="268" Body="&lt;p&gt;A rocket launch generates a huge volume of gas which are then deflected in different directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/myqMf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;(source: &lt;a href=&quot;http://spacenews.com/sea-launch-exits-bankruptcy-under-russian-ownership/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space News&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ejected gas have the potential to move the deflecting panels, so the platform itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For &lt;a href=&quot;http://www.sea-launch.com/launch/11142&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sea Launch Odyssey&lt;/a&gt;, the platform is lowered into the water before the launch:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Both pontoons have three ballast pumps, allowing the Odyssey to&#xA;  partially submerge down 21.5 meters (70 feet) in advance of the start&#xA;  of launch operations to provide for a stable launch platform.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is that the main method used for the stability of the platform?&#xA;Does that limit the rocket engines thrust to some known maximum?&#xA;How does it compare to land-based sites?&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="6944" LastEditDate="2020-09-17T00:21:50.080" LastActivityDate="2020-09-17T00:21:50.080" Title="Sea Launch, and other ocean-based launch pads: Stability?" Tags="|launch|launch-site|thrust|sea-launch|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8594" PostTypeId="2" ParentId="8593" CreationDate="2015-03-29T18:49:32.990" Score="6" Body="&lt;p&gt;Sea Launch uses a Zenit 3SL, which was derived from the side boosters of the Energia booster.  Each one uses an RD-171 engine, which has 4 exhaust bells, but one turbopump assembly, thus counting as one engine.  It puts out about 1.7 million lbs of thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is as much or more than a single F-1 engine from the Saturn V.  That is twice what an Atlas V (using an RD-180, which is basically half an RD-171) produces. That is more than a Falcon 9 v1.1 and possibly even the new upgraded engine model. A Proton at 1.9 million lbs thrust is possibly higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only the Space Shuttle (7.5 million lbs total) and Ariane V (around 3.5 million lbs thrust) actually generate more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So right now, there is not really anything to limit the Zenit booster.  There is only one operational launch vehicle that generates more thrust at liftoff. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless an oil platform, which the Sea Launch platform is derived from is quite a bulky/massy object, designed to handle pretty heavy weights and high seas, thus a lot of momentum would need to be transferred to it to move it. Lowering the pontoons increases the stability a huge amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the basic answer is that yes, it is the main method of stability, it does not limit the thrust since there is really only one possible active booster that is significantly higher (Ariane V).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust is not exhausted on the surface of the launch platform at first, rather through a mostly open hole to the ocean below. Then as the rocket leaves the platform it of course spreads out, but this diffuses the affect over larger and larger areas, thus reducing it very quickly.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-03-29T18:49:32.990" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8595" PostTypeId="1" AcceptedAnswerId="8596" CreationDate="2015-03-29T19:51:33.297" Score="4" ViewCount="576" Body="&lt;p&gt;&lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&quot; rel=&quot;nofollow&quot;&gt;This article&lt;/a&gt; mentions the deployment of a couple of &quot;subsatellites&quot; into lunar orbit from the Apollo 15 and 16 spacecraft. I had never previously heard of any such action being taken. Where were these subsatellites stowed aboard the Apollo spacecraft, and how were they released from it? Did they have their own thrust capability for deployment? &lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-03-30T01:08:12.420" Title="Where were PFS-1 and PFS-2 stowed aboard Apollo 15 and 16, and how were they released?" Tags="|apollo-program|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8596" PostTypeId="2" ParentId="8595" CreationDate="2015-03-29T20:05:39.770" Score="5" Body="&lt;p&gt;They were deployed from the Scientific Instrument Module (SIM) bay in the Service Module. There's a long write-up &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1971-063D&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.  The launcher was spring-loaded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SIM bay was a volume in the Service Module which was loaded with scientific instruments.  The SIM bay cover was jettisoned around 75 hours into the mission according to &lt;a href=&quot;http://history.nasa.gov/afj/simbaycam/simbaycameras.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Apollo Scientific Instrument Module (SIM) Cameras website&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides the subsatellites, one of the major scientific payloads consisted of film cameras.  &lt;a href=&quot;http://apollo.sese.asu.edu/LIW/img/20071022_img5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;The CM pilot did an EVA to retrieve the film&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Artist's rendition from &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_15&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt; including the satellite (I just noticed this picture was in the article you started with, so I outlined the SIM bay):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BOM3b.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here's a real picture of the SIM bay:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nkUIf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a long history of spring-loaded small subsatellite launches throughout the US human spaceflight program.  The last picture ever taken of a Shuttle in space was taken from just such a subsatellite:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Dr0hD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-03-30T01:08:12.420" LastActivityDate="2015-03-30T01:08:12.420" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8597" PostTypeId="2" ParentId="8590" CreationDate="2015-03-29T21:59:54.263" Score="17" Body="&lt;p&gt;Interestingly the low circular orbit period about a body is independent of its size.  It depends only on the density of the object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T=2\pi\sqrt{a^3\over G m}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say that the low circular orbit altitude is 6% of the radius of a sphere with average density $\rho$.  Then:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T=2\pi\sqrt{3\left(1.06 r\right)^3\over{4\pi G\rho r^3}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T\approx{3.35\over\sqrt{G\rho}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So low orbits around rocky bodies will be on the order of two hours, regardless of the size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Earth, $\rho=5.51\,\mathrm{g/cm^3}$, which gives a period of 92 minutes.  For the Moon with $\rho=3.34\,\mathrm{g/cm^3}$, the period is 118 minutes.  Mars' $\rho=3.93\,\mathrm{g/cm^3}$, giving 109 minutes.  Tiny Ceres at $\rho=2.08\,\mathrm{g/cm^3}$ gives 150 minutes.  (Dawn won't get that low.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-03-29T22:12:16.777" LastActivityDate="2015-03-29T22:12:16.777" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8598" PostTypeId="1" AcceptedAnswerId="8615" CreationDate="2015-03-30T02:05:36.910" Score="10" ViewCount="453" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Low-Density_Supersonic_Decelerator&quot; rel=&quot;noreferrer&quot;&gt;NASA's LDSD project&lt;/a&gt;, which underwent a mostly successful test last year, has been indicated to improve the maximum payload deliverable to the surface of Mars from 1.5 metric tonnes to 2-3 metric tonnes (&lt;a href=&quot;http://www.nasa.gov/mission_pages/tdm/ldsd/ldsd_overview.html#.VRiq_PmUfRw&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will be undergoing another test shortly this year, in a larger 8m diameter variant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xKBdt.jpg&quot; alt=&quot;LDSD sample image&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any actual plans to use the technology developed by this project on operational missions, however? Mars Science Laboratory looks set to retain the title of largest payload delivered to the surface of Mars at 899kg for the time being, at least until the Mars 2020 rover, which may have a mass of 950kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Mars 2020 is based off the MSL architecture itself, and would not need the LDSD outright. At the other end of the timescale, manned Mars missions in the 2030's will require many-ton payloads to be delivered direct to the surface. &lt;a href=&quot;http://www.lpi.usra.edu/meetings/leag2007/presentations/20071001.drake.pdf&quot; rel=&quot;noreferrer&quot;&gt;DRM5.0&lt;/a&gt; calls for 30-50 tons, and Elon Musk has stated the design goal of the Mars Colonial Transporter is to deliver &quot;100 useful tons of payload&quot; to Mars surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The huge sizes of these payloads implies propulsive/powered landings near exclusively (although DRM5.0 does allow for an aerodynamic landing component), which would seemingly shut out LDSD from future missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in the interim, there may be a need for small multi-ton (1-5t) payloads to be landed, this might include a sample return or a supply depot for a future manned mission. &lt;strong&gt;My question is, does the use of LDSD appear in any current mission design, in any form, to Mars?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be totally clear, I'm not questioning LDSD' scientific value or usefulness, I actually think it's an incredibly interesting project and could provide many engineering advancements, well beyond use at Mars - I just want to know if any missions have called upon its immediate use. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="238" LastEditDate="2015-03-30T08:43:54.890" LastActivityDate="2015-04-03T16:55:00.670" Title="Are there any Mars missions currently in the design phase which could take advantage of NASA's Low Density Supersonic Decelerator?" Tags="|mars|future-missions|mission-design|reentry|ldsd|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8599" PostTypeId="1" AcceptedAnswerId="8601" CreationDate="2015-03-30T02:17:41.727" Score="21" ViewCount="13017" Body="&lt;p&gt;All ion thrusters are &lt;a href=&quot;https://space.stackexchange.com/a/8112/4660&quot;&gt;quite small&lt;/a&gt;. Having read through the more  approachable information about them, as they use physics pretty beyond me, I don't understand what it is about them that makes it necessary they be so small. Are they fundamentally limited in some way? Is there reason to hope what limits them now could be overcome at some point so their high Isp could be used for higher acceleration on larger spacecraft?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit (next day): The answers so far have helped me understand the basic issue that the power needs of these engines are huge. In some ways I think this deserves to be treated as a separate issue from the scalability of the engine itself - &lt;a href=&quot;http://eo.ucar.edu/staff/dward/sao/fit/electric.htm&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; makes the point that energy could be beamed to the craft, for instance, meaning you don't need to carry the mass of the power source with you. And the point has already been made that nuclear reactors could be used, which have the required power density, if the details of how to make use of that was figured out (see supercapacitor conversation in comments under EchoLogic's answer). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The delta V imparted for their weight is very high. &lt;a href=&quot;http://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness&quot; rel=&quot;noreferrer&quot;&gt;NSTAR&lt;/a&gt; engines weigh &lt;a href=&quot;http://www.researchgate.net/publication/3868954_In-flight_performance_of_the_NSTAR_ion_propulsion_system_on_theDeep_Space_One_mission&quot; rel=&quot;noreferrer&quot;&gt;48 kg&lt;/a&gt; including the thruster, power processing units, xenon feed system, and control interface. Three of those and 475 kg of xenon will impart delta V of over 10 km/s to the Dawn probe, itself weighing about 600 kg without those thrusters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if the power issue was handled, could you just make great big ones to push a large vessel fast, and take advantage of that efficiency?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2019-03-03T07:14:04.607" Title="Can ion thrusters be scaled up?" Tags="|propulsion|engine-design|ion-thruster|" AnswerCount="7" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8600" PostTypeId="2" ParentId="8599" CreationDate="2015-03-30T02:33:59.500" Score="13" Body="&lt;p&gt;While their ISP is insanely good, ion thrusters have miserable T/W ratio, less than 1/1000th of unity; you simply cannot take off from any large body with them, no matter how many of them you use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary limiting factor, I think, is power consumption. The thrusters used on Dawn and DS1 &lt;a href=&quot;https://space.stackexchange.com/a/8112/195&quot;&gt;require 24kW per &lt;em&gt;newton&lt;/em&gt;&lt;/a&gt; of thrust. You'd need 1.21 gigawatts to run a 50kN thruster. Ion thrusters currently in use are small just because you'd need ridiculously large solar arrays to make use of larger ones.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-03-30T02:33:59.500" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8601" PostTypeId="2" ParentId="8599" CreationDate="2015-03-30T02:39:01.810" Score="22" Body="&lt;p&gt;Assuming you mean &amp;quot;quite small&amp;quot; in terms of mass as well as thrust output. Fundamentally, current ion drives are limited by the amount of power available to them - it takes many, many kilowatts of input power to provide tiny amounts of thrust.&lt;/p&gt;&#xA;&lt;p&gt;As you know from the answer you linked, the Dawn spacecraft is powered by 3 NSTAR ion engines which generate a maximum thrust of 90mN, and each has a mass itself of ~25.5kg. What that answer failed to mention is the enormous amount of input power Dawn requires to operate its ion engines. It has an enormous 36.4m&lt;sup&gt;2&lt;/sup&gt; array of solar panels which when stretched, make the entire probe 19.7m across (&lt;a href=&quot;http://www.lpi.usra.edu/opag/nov_2007_meeting/presentations/solar_power.pdf&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vgedZ.jpg&quot; alt=&quot;Dawn undergoing testing&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;At 1AU, they generate a huge 10kW of power, but due to the inverse square law, this drops off to 1300W at 3AU&lt;/strong&gt;. This is a problem inherent to all solar-powered spacecraft - in fact, the 2007 NASA meeting which produced the above linked document on spacecraft solar power estimate it would take a &lt;strong&gt;250m&lt;sup&gt;2&lt;/sup&gt;&lt;/strong&gt; solar array to provide just 400W to a Uranus orbiter. This entire situation is acerbated when your propulsion source requires kilowatts of power to run - limiting ion engines' usefulness far from the sun.&lt;/p&gt;&#xA;&lt;p&gt;The NSTAR ion engine design reference calls for &lt;a href=&quot;http://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness&quot; rel=&quot;noreferrer&quot;&gt;2.3kW of power&lt;/a&gt; specifically. Much of this energy is needed to provide a voltage distance to accelerate the ions across (therefore producing the thrust).&lt;/p&gt;&#xA;&lt;p&gt;The rest of an ion-engine is mainly solid stage and digital, with few moving parts overall. This contributes to its low mass. This low mass isn't inherent to Ion engines either - the physical Hydrazine thrusters themselves commonly found on spacecraft don't weigh that much either, it's the mass of the fuel which is the problem.&lt;/p&gt;&#xA;&lt;h1&gt;Solutions&lt;/h1&gt;&#xA;&lt;p&gt;Most attempts to scale up ion engines inevitably either require large solar arrays or nuclear fission reactors (respectively named Solar Electric Propulsion &amp;amp; Nuclear Electric Propulsion). Neither power source is entirely ideal. The closest scale up attempt of ion-propulsion was a plan to attach a VASIMR engine to the ISS (which has since fallen though) to provide occasional reboosts to the station's altitude to compensate for its orbital decay. This would use &lt;strong&gt;200kW&lt;/strong&gt; of power to generate &lt;strong&gt;4N&lt;/strong&gt; of thrust - the problem is this is actually more than the ISS' entire power output. Their solution was to trickle charge a set of batteries allowing the engine to fire intermittently.&lt;/p&gt;&#xA;&lt;p&gt;Ad Astra, the company developing VASIMR, also proposed a mission architecture where one could reach Mars in &lt;a href=&quot;http://www.universetoday.com/42167/trips-to-mars-in-39-days/&quot; rel=&quot;noreferrer&quot;&gt;39 days&lt;/a&gt;, but such a mission would require a multi-megawatt nuclear reactor. The current technical and political climate renders this a non-starter.&lt;/p&gt;&#xA;&lt;h1&gt;In summary...&lt;/h1&gt;&#xA;&lt;p&gt;Fundamentally, ion engines are small and deliver small amounts of thrust because the size, mass, and power required to run them at higher thrusts is currently in-feasible.&lt;/p&gt;&#xA;&lt;p&gt;Until launch costs drop, and energy solutions can deliver more power to spacecraft, ion engines look set to be confined to low thrust applications.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-03-30T02:39:01.810" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8602" PostTypeId="1" AcceptedAnswerId="8603" CreationDate="2015-03-30T11:25:57.213" Score="5" ViewCount="666" Body="&lt;p&gt;Aside from providing pretty pictures for the media and public what purpose does the camera on the first stage of the Falcon 9 serve? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8F9V4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it actually used by controllers or engineers to gain information about the flight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so what sort of information is captured?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="51" LastEditDate="2015-03-30T11:34:49.847" LastActivityDate="2015-03-30T11:34:49.847" Title="Why does Falcon 9 carry a camera on the first stage?" Tags="|spacex|falcon-9|camera|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8603" PostTypeId="2" ParentId="8602" CreationDate="2015-03-30T11:33:39.847" Score="6" Body="&lt;p&gt;In a healthy flight, probably not that much info. Watching the plume expansion as altitude changes provides information that the engines were working properly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When something goes wrong, it might present an important clue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For landing, this view would be very useful to see the legs properly deploy, the angle of approach to landing, and whatever happens next.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-03-30T11:33:39.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8604" PostTypeId="2" ParentId="8599" CreationDate="2015-03-30T12:36:37.473" Score="12" Body="&lt;p&gt;Leaving aside the power consumption problems (which have been well discussed in other answers) and returning to the part of the question where you asked: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Are they [Ion Thrusters] fundamentally limited in some way? Is there&#xA;  reason to hope what limits them now could be overcome at some point so&#xA;  their high Isp could be used for higher acceleration on larger&#xA;  spacecraft?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There’s some &lt;a href=&quot;http://alfven.princeton.edu/projects/LiLFA.htm&quot;&gt;very interesting research&lt;/a&gt; being done at Princeton’s Electric Propulsion and Plasma Dynamics Lab (EPPDyL) into just that question using Lithium Lorenz force accelerators. The research is currently still in progress, but the results published so far look encouraging. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This graph from &lt;a href=&quot;http://alfven.princeton.edu/papers/JPC2010Lev.pdf&quot;&gt;Scaling of Effciency with Applied Magnetic Field in Magnetoplasmadynamic Thrusters&lt;/a&gt; (Page 5): &#xA;&lt;img src=&quot;https://i.stack.imgur.com/z94ml.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shows the thrust of their experimental thruster against it's current supply at different Li feed rates. The thrust to power ratio (if I'm reading the graph correctly) shows an efficiency of 1N/37.5kW at optimal configuration. About twice that quoted above for the VASIMIR device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there's definitely progress being made, but there's still a way to go before ion thrusters begin to replace chemical engines on larger craft.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-03-30T14:23:41.870" LastActivityDate="2015-03-30T14:23:41.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8605" PostTypeId="1" CreationDate="2015-03-30T13:20:56.067" Score="4" ViewCount="1406" Body="&lt;p&gt;Since 2013, several launches up to the ISS have taken a fast-launch. First initiated by Soyuz TMA-08M, the fast rendezvous takes the crew up in 6 hours where the previous time-to-dock was around 2 days. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously the human body is capable of surviving the acceleration/deceleration involved, and contemporary technology capable of the precision required in docking - cutting down time from 2D to 6H. Yet (&lt;strong&gt;please correct me if my assumption is misfounded&lt;/strong&gt;) &lt;a href=&quot;https://space.stackexchange.com/questions/4650/how-long-does-it-take-to-get-to-iss&quot;&gt;How long does it take to get to ISS?&lt;/a&gt; indicates the quantum of time to get to orbit itself is only a small fraction of the total time to rendezvous - and the time to orbit has not changed significantly over the years. From reading various reports available on the internet (e.g. STS reports) my take is time-to-orbit has consumed 30 to 40 minutes - and continues to do about the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Technology allows us to cut-down on time-to-rendezvous from 2D to 6H around 80% quicker. Say a mission does not need to get to ISS - it is only up-around-down; how much can the time to orbit from launch be reduced?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apart from the physiological constraints on acceleration, and hardware -how much quicker can we get to orbit from launch? Can the time to orbit at-least be halved from approximately 30m to 15m? &lt;/p&gt;&#xA;" OwnerUserId="288" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2019-07-23T15:27:20.270" Title="Is there a lower limit on time-to-orbit from launch?" Tags="|launch|orbit|launch-sequence|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8606" PostTypeId="2" ParentId="8605" CreationDate="2015-03-30T15:54:29.233" Score="7" Body="&lt;p&gt;I'm not sure where you're getting your 30-40 minute figure; initial orbital entry generally takes about 10. Getting into the exact orbit you want takes longer, of course, but getting from ground to a reasonably safe and stable orbit is pretty quick.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apollo 11: &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_11i_Timeline.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;12 minutes&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Soyuz: &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/structure/elements/soyuz/timeline_ascent.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;9 minutes&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Long March/Shenzhou: &lt;a href=&quot;http://www.spaceflightnow.com/china/shenzhou9/launchtimeline.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;10 minutes&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Getting to orbit substantially faster is possible, but rough on the passengers. Acceleration increases dramatically over the duration of a single ascent stage as fuel mass is expended, plus air resistance decreases and engine thrust increases as you gain altitude. Check out the the graphs near the &lt;a href=&quot;http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;bottom of this page&lt;/a&gt;. Saturn V limited acceleration to about 4g, but early American orbital flights on Atlas launchers got up around 8g: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury-Atlas-6: &lt;a href=&quot;https://www.nasa.gov/mission_pages/mercury/missions/friendship7.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;5 minutes 1 second&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-07-23T15:27:20.270" LastActivityDate="2019-07-23T15:27:20.270" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="8607" PostTypeId="1" AcceptedAnswerId="8620" CreationDate="2015-03-30T16:20:31.337" Score="8" ViewCount="696" Body="&lt;p&gt;Microgravity causes some health concerns in the long run, so it has been proposed that one could create what is often called &quot;artificial gravity&quot; by rotating the space station. This is not strictly physically correct and may spread misunderstandings about physics. So what should we call it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Artificial&quot; refers to something humans create. We can't create gravity, so the word artificial has sometimes been exchanged for &quot;simulated&quot; or &quot;emulated&quot;. But I think that the problem is that it isn't gravity, it is an acceleration which we do create artificially, so it is rather that word which needs replacement. However, &quot;artificial acceleration&quot; doesn't reveal that it is a rotating acceleration. How is this confusion clarified in a physically correct terminology which still can work as &quot;a term&quot; consisting of at most three or so words?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;-Is it gravity?&quot; &quot;-Nope!&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;-Is it centrifugal force?&quot; &quot;-Nope, that stuff doesn't even exist.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-08-27T22:18:08.560" Title="What would a physically correct term be for &quot;artificial gravity by rotation&quot;?" Tags="|terminology|artificial-gravity|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8608" PostTypeId="1" AcceptedAnswerId="8609" CreationDate="2015-03-30T17:28:50.980" Score="7" ViewCount="2808" Body="&lt;p&gt;On February 8, SpaceX had to &lt;a href=&quot;http://www.waaytv.com/space_alabama/spacex-discovr-launch-scrubbed/article_2a75d112-afeb-11e4-bd09-e7da992d6b35.html&quot;&gt;scrub a launch due to an Air Force technical problem&lt;/a&gt;. &lt;strong&gt;What happens to the cryogenic fuels when such a launch is delayed?&lt;/strong&gt; Are the tanks (LOx for the Falcon 9) drained? Are the cryogenics recovered or vented to atmosphere? How much does this cost the launch company, NASA, the insurance, and the launch customer?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am interested in the specific case of the Falcon 9, and also the general case for other rockets with cryogenic fuels.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2015-03-30T17:54:48.780" Title="If a rocket launch is delayed, what happens to the cryogenics?" Tags="|launch|spacex|falcon-9|abort|cryogenics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8609" PostTypeId="2" ParentId="8608" CreationDate="2015-03-30T17:34:55.433" Score="7" Body="&lt;p&gt;Typically they drain the cryogenics back, if they can. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now it depends on the cryogenic.  Oxygen can be drained and reused.  Hydrogen in liquid form is very hard to keep cool enough, and they will drain it, but they will lose lots of it to venting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Liquid Nitrogen that they use for pressurization they may just let vent, since the quantities are pretty small (compared to fuel or oxidizer loads) and LN2 is amazingly cheap (Less than milk).  Liquid Helium, even though it is very hard to keep cool enough is quite expensive, so even in small quantities they will likely try to recover it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wet dress rehearsals and hot fires are the perfect example of this. The vehicle is fully fueled in each case. In the first it is then fully drained.  In the second, the engines ignite for a few seconds (less than 3 in the case of Falcon 9, since they ignite at T-3 and stop at T-0).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the specific case of Falcon 9 and SpaceX, I was initially under the impression, that the tenth feed line in the Loxtopus image below was for cross feed (9 engines, 9 feed lines, plus one for drainage), but I was corrected, and it is in fact a drain line, for just such a case.&lt;img src=&quot;https://i.stack.imgur.com/zcw94.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-03-30T17:54:48.780" LastActivityDate="2015-03-30T17:54:48.780" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8610" PostTypeId="1" AcceptedAnswerId="8612" CreationDate="2015-03-30T20:00:52.440" Score="8" ViewCount="2366" Body="&lt;p&gt;In November/December 2013, during the Falcon 9 Flight 7 (SES-8) processing flow, SpaceX managed to get the vehicle onto the pad and attempted to launch a few times without success. Either a scrub was called or the flight computers decided to automatically terminate the countdown prior to liftoff.&lt;/p&gt;&#xA;&lt;p&gt;During the second launch attempt &amp;quot;a slower-than-expected increase in pressure&amp;quot; (&lt;a href=&quot;http://spacenews.com/38410engine-inspection-on-tap-after-spacex-scrubs-thanksgiving-launch/&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;) was seen on the first stage engines, and an abort was called by the flight computers at T-0.&lt;/p&gt;&#xA;&lt;p&gt;It's worth noting that this was &lt;strong&gt;later&lt;/strong&gt; found to be due to contamination of the pyrophoric TEA-TEB ignition fluid, with Musk stating:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;So there was some oxygen that inadvertently got in the TEA-TEB tank. Could have just been atmospheric oxygen that got in when the tank was refilled, or could have been a bit of oxygen that flowed back in from a previous engine test. That would then react with the TEA-TEB in the tank and reduce its concentration, so that the tank then contained a mix of TEA-TEB and combustion products.&lt;/p&gt;&#xA;&lt;p&gt;In other words, it was like they expected 180 proof TEA-TEB but only got 100 proof. The fire in the engines was monitored and was less than the launch computer expected, so it shut the engines down rather than proceeding to full thrust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(&lt;a href=&quot;http://www.wacotrib.com/blogs/joe_science/the-tea-teb-glitch-can-t-light-a-falcon-without/article_1b7c4ae6-5a16-11e3-afbb-0019bb2963f4.html&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;On the day of launch though&lt;/strong&gt;, Musk simply stated on Twitter:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;We called manual abort. Better to be paranoid and wrong. Bringing rocket down to borescope engines ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(&lt;a href=&quot;https://twitter.com/elonmusk/status/406209289494462464&quot; rel=&quot;noreferrer&quot;&gt;Twitter Post&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;What exactly does &amp;quot;borescoping an engine&amp;quot; involve though? How long would this process take? What would they be able to determine with this test that they couldn't by simply looking at flight computer data?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-02T20:50:30.407" Title="What would one learn by borescoping a rocket engine after an aborted launch?" Tags="|launch|spacex|abort|launch-sequence|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8611" PostTypeId="2" ParentId="8610" CreationDate="2015-03-30T20:06:59.170" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Borescope&quot;&gt;Borescoping&lt;/a&gt; is nothing other than optical inspection of the engines and plumbing from inside.  The purpose is usually to look for debris and/or damage in the fluid ducts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think &quot;how long&quot; can be answered because there are too many variables: what is the purpose, does it succeed, what is the configuration, what is the equipment.  On STS it was a non-trivial task. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The telemetry might suggest, for example, blockage due to debris, but boroscoping can confirm it.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-03-30T20:06:59.170" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8612" PostTypeId="2" ParentId="8610" CreationDate="2015-03-30T20:08:50.637" Score="10" Body="&lt;p&gt;I imagine that &lt;a href=&quot;http://en.wikipedia.org/wiki/Borescope&quot;&gt;borescoping&lt;/a&gt; allows them to get a look at the inside of the engine's combustion chamber without having to disassemble it. If the problem was physical damage or obstruction to the injectors, for example, this could be visually detected. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, it likely wouldn't have shown the contamination of the ignition fluid; it would rule out some possibilities pretty quickly.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-03-30T20:08:50.637" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8613" PostTypeId="2" ParentId="8610" CreationDate="2015-03-30T20:18:30.267" Score="22" Body="&lt;p&gt;Imagine a colonoscopy for a rocket engine. You go in with a flexible light pipe that has a camera on the end. (Engine is nicely sedated, usually). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks around, cuts out any polyps it sees, oh I mean obstructions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if a valve failed to open/close, they can see that without taking it all apart.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-03-30T20:18:30.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8614" PostTypeId="2" ParentId="6548" CreationDate="2015-03-30T20:58:38.520" Score="22" Body="&lt;p&gt;Nearly all balloons that have been constructed have been for flights from the surface to altitude. That requires a structure that can survive tethered at the surface in a range of wind speeds in high-density air, then expand to several times their original size in order to maintain lift as the air density decreases. The minimum density ends up being determined by the need to survive liftoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ATO is a two-stage system with the upper stage being constructed at 140k feet. Given that it will never be tethered to the ground, it will experience no wind at all, because the Dark Sky Station will be in exactly the same air the upper stage is in. That means the lower limit on its density ends up being determined by other factors such as differential air speeds at different parts of the structure. That could still be a killer, but &quot;the thinnest balloons&quot; (mentioned in Charles Pooley's answer, as being barely lighter than the 4 g/m&lt;sup&gt;3&lt;/sup&gt; of air at that altitude) is not relevant here, because the thinnest balloons anyone would have experienced must still be far denser than what you could build at 140k feet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lift is a function of drag. At any altitude where drag would prevent the upper stage from achieving orbital velocity for that altitude, the upper stage would continue to generate lift. This profile continues smoothly until the upper stage is close enough to orbital velocity that it no longer matters. It's possible that, at whatever altitudes you'd want to operate, the upper stage would need to generate thrust continuously, and anything you wanted in actual orbit would need an extra kick, but with solar-powered ion propulsion, that doesn't seem like much of an inconvenience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ATO will run into plenty of challenges, but I'm not aware of any that are obvious killers. There are problems like radiation and the logistics of construction in full gravity and extremely low pressure, but I imagine the DSS and upper stages will end up being constructed by robots. Even if it only ends up being used for cargo, a &quot;grown up&quot; spacefaring society should be sending humans and cargo up separately anyway, because they have different requirements for speed and safety. Humans can wait on the ground until the cargo they work with is safely in orbit.&lt;/p&gt;&#xA;" OwnerUserId="8816" LastEditorUserId="4660" LastEditDate="2017-04-05T19:55:38.130" LastActivityDate="2017-04-05T19:55:38.130" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8615" PostTypeId="2" ParentId="8598" CreationDate="2015-03-30T23:33:00.147" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Are there any actual plans to use the technology developed by this&#xA;  project on operational missions?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, not that I'm aware of.  And I would not expect there to be any before the technology has been proven.  If you are trying to sell a mission concept or keep a mission concept on the table, you would be well advised to not have it be critically dependent on a pending technology development that may or may not succeed.  The LDSD technologies are of the sort that, if you are using them, you are critically dependent on them for mission success.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-03-31T02:40:14.440" LastActivityDate="2015-03-31T02:40:14.440" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8616" PostTypeId="1" AcceptedAnswerId="8617" CreationDate="2015-03-31T01:12:33.003" Score="5" ViewCount="3836" Body="&lt;p&gt;Could water (steam) be used as an effective power source for a manned spaceship headed to Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic idea is simple...  Heat the water on the sunny side of the spacecraft, allow the steam to turn a turbine to produce power (to run computers/electrical equipment).   Cool the water on the dark side of the spacecraft and rinse/repeat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most spaceship designs/proposals utilize a form of solar power.   Is there a reason steam power gets overlooked?  (Is it feasible? Is the weight/complexity greater then a solar panel system?)   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the importance of water, there likely is already enough water on the spacecraft to run a steam powered generator.  The water is just sitting there, inert...  It seems more effective to use the water as a renewable power resource too.   Is there something obvious that I am overlooking?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS, while in the sun, can receive temperatures of 250F+ -- enough to boil water.&lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2001/ast21mar_1/&quot;&gt;ISS temperature differences&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water can be used as a form of radiation shielding. (AKA:  Assume the ship already has water on all sides of the ship)  &lt;a href=&quot;http://www.space.com/11230-water-powered-spaceship-mars-solar-system.html&quot;&gt;Water-Powered Spaceship&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Low&quot; temperature steam power generators, on earth, have a similar amount of power efficiency (&lt;a href=&quot;http://www.gizmag.com/solar-thermal-waste-heat-engine/10969/&quot;&gt;12%&lt;/a&gt;-&lt;a href=&quot;http://en.wikipedia.org/wiki/Steam_engine#Efficiency&quot;&gt;20%&lt;/a&gt;) as solar panels have (&lt;a href=&quot;http://pureenergies.com/us/how-solar-works/solar-panel-efficiency/&quot;&gt;11%-15%&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2023-04-13T21:56:02.543" Title="Could steam be used as a efficient power source for a space craft?" Tags="|mars|spacecraft|energy|crewed-spaceflight|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8617" PostTypeId="2" ParentId="8616" CreationDate="2015-03-31T01:24:33.440" Score="12" Body="&lt;p&gt;Very simple answer: You would need some kind of heating panels through which to let the water flow while it gets heated. Solar panels are thin slices of silicon. You will not be able to build the water heating panels lighter than the solar panels, and even if you somehow did, you would still have to carry a steam turbine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a commenter already pointed out, in your efficiency comparison you neglect the absorption efficiency of the panels themselves. Also, the value of 20% you quoted was for a high pressure turbine. If the panels have to withstand such a high pressure, that will make them even heavier.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2015-03-31T15:42:45.380" LastActivityDate="2015-03-31T15:42:45.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8619" PostTypeId="2" ParentId="8616" CreationDate="2015-03-31T05:47:41.607" Score="7" Body="&lt;p&gt;A 1971 Nasa &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710028154.pdf&quot;&gt;pdf&lt;/a&gt; says that &lt;em&gt;silicon solar cell arrays&lt;/em&gt; [meaning photovoltaic] &lt;em&gt;are established as the most reliable and economical generator of sustained power in space&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A key factor in reliability is that the cells themselves have no moving parts. However, the array must be mechanically deployed, and would typically be mounted on gimbals to face the sun. Failure in either of these systems are not uncommon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, a lot has changed since then - photovoltaic solar cells have become &lt;em&gt;much&lt;/em&gt; more efficient. &lt;a href=&quot;http://en.wikipedia.org/wiki/Multijunction_photovoltaic_cell&quot;&gt;Two layer&lt;/a&gt; &lt;em&gt;cells are widely available at 30% under one-sun illumination&lt;/em&gt; (meaning without concentrators).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 2013 &lt;a href=&quot;http://www.academia.edu/5253076/Solar_Cells_for_Space_Applications&quot;&gt;pdf&lt;/a&gt; gives comparable figures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, the answer is no. Photovoltaic solar cells are more efficient and more reliable for medium term (up to 10 year) uses.&lt;/p&gt;&#xA;" OwnerDisplayName="user8406" LastActivityDate="2015-03-31T05:47:41.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8620" PostTypeId="2" ParentId="8607" CreationDate="2015-03-31T08:10:03.657" Score="15" Body="&lt;p&gt;It is entirely correct to replace &quot;artificial&quot; with &quot;simulated&quot; or even &quot;emulated&quot; or &quot;imitated&quot;. Simulated can be defined as &lt;a href=&quot;https://www.google.com/search?q=define%20simulated&quot;&gt;&quot;imitating the conditions of something&quot;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that vain, simulated gravity would in no way imply that we are somehow creating actual gravity. It simply means that we are creating conditions similar to gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Replacing &quot;gravity&quot; with some other word loses the point of why we are doing it. Simulated gravity has the clear purpose of simulating gravity. Artificial acceleration could mean anything (Even firing rockets to move a spacecraft forward), so that is not a good term to use.&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-03-31T08:10:03.657" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8621" PostTypeId="2" ParentId="8616" CreationDate="2015-03-31T08:23:23.047" Score="10" Body="&lt;p&gt;Very simple answer. You are looking at the wrong metric for efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most expensive thing on a space station is not area, you don't pay per foot of &quot;land&quot; (and its associated sunlight) in space. The most expensive thing is mass, and getting it up there. A steam engine is heavy per unit power compared to a solar panel (especially when compared to the multi-junctioned solar panels that have ~30% efficiency that they use in space).&lt;/p&gt;&#xA;" OwnerUserId="6157" LastEditorUserId="6157" LastEditDate="2015-03-31T16:14:19.767" LastActivityDate="2015-03-31T16:14:19.767" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8622" PostTypeId="2" ParentId="8607" CreationDate="2015-03-31T08:25:38.307" Score="-3" Body="&lt;p&gt;Centripete is the correct term. Etymologically from the centripetal force. That is, the force that pulls you into the center of rotation. No, there is &lt;em&gt;no centrifuge&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6834" LastEditorUserId="6834" LastEditDate="2015-03-31T08:55:46.023" LastActivityDate="2015-03-31T08:55:46.023" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8623" PostTypeId="1" CreationDate="2015-03-31T14:25:59.123" Score="10" ViewCount="642" Body="&lt;p&gt;The atmosphere of Venus contains strong winds and is said to have a &amp;quot;super rotation&amp;quot; because the atmosphere (above a certain altitude) moves faster than what the ground does.&lt;/p&gt;&#xA;&lt;p&gt;No one can disagree with the fact that the atmosphere moves faster than the ground at most altitudes. However, does the wind pattern literally rotate around the planet as the common wording implies? Or... does the atmosphere actually zig-zag from the day side to the night side (nonetheless faster than the ground movement), making the common description of &amp;quot;super rotation&amp;quot; a popular misconception? This is one case where I think that pictures ask the question better than text:&lt;/p&gt;&#xA;&lt;h1&gt;True Rotation&lt;/h1&gt;&#xA;&lt;p&gt;This is just &lt;a href=&quot;http://global.jaxa.jp/article/interview/vol34/index_e.html&quot; rel=&quot;noreferrer&quot;&gt;one example&lt;/a&gt;. I think that the underlying notion is pretty obvious.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UDSBg.jpg&quot; alt=&quot;True Rotation&quot; /&gt;&lt;/p&gt;&#xA;&lt;h1&gt;Zig Zag&lt;/h1&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://commons.wikimedia.org/wiki/File:Venus_winds_lmb.png&quot; rel=&quot;noreferrer&quot;&gt;This illustration&lt;/a&gt; comes from the Wikimedia Commons. It is, in fact, the most detailed illustration of the wind pattern on Venus that Wikipedia has. In spite of my trying, I can not find any evidence that the image was ever depreciated and removed from the &lt;a href=&quot;http://en.wikipedia.org/w/index.php?title=Atmosphere_of_Venus&amp;amp;oldid=69204900&quot; rel=&quot;noreferrer&quot;&gt;relevant English page&lt;/a&gt; due to changing science, or the &lt;a href=&quot;http://en.wikipedia.org/w/index.php?title=Venus&amp;amp;oldid=15171832&quot; rel=&quot;noreferrer&quot;&gt;less relevant page&lt;/a&gt; for that matter. The image timestamp actually predates the corresponding article, which casts suspicion on the accuracy of those time stamps in general. Overall, the use history is very unclear to me. Also note, &lt;a href=&quot;http://airandspace.si.edu/exhibitions/exploring-the-planets/online/venus/global.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; is a site which might be the source and contains the diagram in English.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7eLKq.png&quot; alt=&quot;Venus wind currents&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Please do not mistake this illustration for &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/thumb/2/24/Venus_circulation.jpg/300px-Venus_circulation.jpg&quot; rel=&quot;noreferrer&quot;&gt;the one currently displayed on the Wikipedia page&lt;/a&gt;, since that one shows Meridional flow only. This question is strictly about zonal flow.&lt;/p&gt;&#xA;&lt;p&gt;Mechanistically, I can understand the Zig Zag a lot better since the sun's heat is the driving force. If the pattern was a true rotating, then where is the driving force coming from? There's not choked flow coming from the cold side or anything like that, so the driving force from the expansion of the gas due to heating can't exert a net-force along latitude lines. But it's difficult to track down the science, since I can't tell who is just incorrectly recounting something they heard from somewhere else, and who actually understands.&lt;/p&gt;&#xA;&lt;p&gt;So which is it? Does the wind pattern really rotate, or does it just zig zag?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-04T09:15:12.317" Title="Does the Venusian atmosphere's &quot;super rotation&quot; actually rotate?" Tags="|atmosphere|venus|weather|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8624" PostTypeId="2" ParentId="8599" CreationDate="2015-03-31T15:11:16.500" Score="5" Body="&lt;p&gt;What most of the other answers are failing to recognise is that ion thrusters are small BY DESIGN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is true that you COULD scale up your ion drive if you hook it up to a similarly scaled up power source. &lt;strong&gt;But you wouldn't.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason that Ion drives are used it because they have great ISP. The reason they have great ISP is that they have great exhaust velocity. The reason for that is that a HUGE amount of kinetic energy is put into a TINY amount of fuel.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;1/2 mv^2 = KE&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Scaling up the ion drive would simply increase the amount of fuel being dumped out the back. Which, yes, increases the thrust, but at the expense of ISP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact is scaling up a ion drive runs counter to the entire concept of an ion drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, there isn't so much as a &quot;hard&quot; limit to most ion drives on the ISP. However there are significant engineering problems. Lets take the classic linear accelerator style ion drive. At some point you need to accelerate ions towards an electrode. The more energy you put into the the ions, the heavier they slam into the electrode (should they hit). This causes damage to the electrode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This style of problem exists in all ion drives. Fundamentally you are trying to make the flimsiest possible engine, which produces the hottest flame. At some point your flimsy engine will leak the hot flame, and stop working efficiently.&lt;/p&gt;&#xA;" OwnerUserId="6157" LastEditorUserId="6157" LastEditDate="2015-03-31T17:15:39.833" LastActivityDate="2015-03-31T17:15:39.833" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8627" PostTypeId="2" ParentId="4085" CreationDate="2015-03-31T23:24:35.847" Score="14" Body="&lt;p&gt;The major burn is a 7.7 km/s acceleration in the horizontal direction. But the first part of the trip is usually a vertical climb:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/K7pWF.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo Ascent profile from &lt;a href=&quot;http://history.nasa.gov/ap08fj/01launch_ascent.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;a NASA history page&lt;/a&gt;. Rescaled to show how horizontal the flight was:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Df4sD.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Df4sD.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why the vertical ascent before the major horizontal burn?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice &lt;a href=&quot;http://en.wikipedia.org/wiki/Max_Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;max-Q&lt;/a&gt; is marked at the beginning of the trajectory. Max Q for spacecraft is often around 35 kilopascals. For scale, a severe hurricane is 3 kilo pascals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Achieving orbital velocity at sea level would subject the spacecraft to 36,000 kilo pascals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Temperature is another concern. Most meteors burn up in the mesosphere about 70 kilometers up. The air at sea level is about a thousand times as dense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a vertical ascent must be made before doing the horizontal burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ascent is often portrayed as trivial compared to achieving the needed horizontal velocity. The potential energy between sea level and a 120 km altitude is about 1.2 mega joules per kilogram. But the kinetic energy of low earth orbit is about 30 mega joules per kilogram. So the popular wisdom is that ascent is 1/25th as hard achieving orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But potential energy isn't the only cost of ascent. Gravity loss is incurred during a vertical climb:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lEiER.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity loss is 9.8 meters/second per second of vertical climb. A 102 second vertical climb costs 1 km/s delta V in gravity loss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A large &lt;a href=&quot;http://en.wikipedia.org/wiki/Thrust-to-weight_ratio#Rockets&quot; rel=&quot;nofollow noreferrer&quot;&gt;thrust to weight ratio&lt;/a&gt; (T/W) is desirable to minimize ascent time. The more oomph a booster has, the less ascent time and the less gravity loss. This is one reason a booster might have 9 rocket engines while the upper stage may have only one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ascent can cost an extra 1.5 km/s. Adding 1.5 km/s to a 7.7 km/s delta V budget boosts energy about 40%. Not the 4% increase that some would have you believe.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorDisplayName="user20636" LastEditDate="2020-06-07T09:42:05.503" LastActivityDate="2020-06-07T09:42:05.503" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="8628" PostTypeId="1" CreationDate="2015-03-31T23:37:28.670" Score="7" ViewCount="1575" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qiedH.png&quot; alt=&quot;the two equations&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering if it's appropriate to use the two equations in order to solve for m0/m1 to find the mass of propellant needed to perform a hohmann transfer. If it isn't, can someone suggest an alternative?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: I'm basically happy with any assumptions/simplifications (I don't have much knowledge on this subject so I'm open to any as long as they are explained)&lt;/p&gt;&#xA;" OwnerUserId="8825" LastEditorUserId="8825" LastEditDate="2015-04-01T22:45:35.663" LastActivityDate="2016-03-31T22:29:30.517" Title="Can I use Tsiolkovsky's rocket equation in combination with this equation for a Hohmann transfer to find what mass of propellant is needed?" Tags="|orbital-mechanics|orbital-maneuver|interplanetary|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8629" PostTypeId="1" CreationDate="2015-04-01T00:58:20.587" Score="3" ViewCount="370" Body="&lt;p&gt;I setup a simple rocket in openrocket with a body to fit the engine and a nose cone.  I did not add fins.  This puts the center of pressure up in the nose cone way ahead of the center of gravity.  What I am trying to do is setup a situation where you would use gimbaling to stabilize the rocket but in an environment where it may not be necessary so I turned off wind for the simulation.  The rocket still goes unstable and I'm curious why.  It seems to remain unstable until I make fins large enough to move the center of pressure behind the center of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears I can remove air pressure in which case the simulation does work, so I'm guessing there is some non-uniformity somewhere?&lt;/p&gt;&#xA;" OwnerUserId="8826" LastEditorUserId="8826" LastEditDate="2015-04-01T01:23:17.667" LastActivityDate="2015-04-01T02:38:59.747" Title="Simulation of a rocket with no fins" Tags="|launch|rockets|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8630" PostTypeId="2" ParentId="8629" CreationDate="2015-04-01T01:56:23.747" Score="7" Body="&lt;p&gt;If the center of pressure is ahead of the center of gravity, even the slightest difference will cause a divergence -- even a rounding difference or numerical representation in your simulation code.  It is your second simulation that &quot;works&quot; that is really not working.  You have what is known as an unstable equilibrium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/I3upH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-04-01T02:38:59.747" LastActivityDate="2015-04-01T02:38:59.747" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8632" PostTypeId="1" AcceptedAnswerId="8635" CreationDate="2015-04-01T09:18:02.610" Score="4" ViewCount="232" Body="&lt;p&gt;Almost all the articles I’ve read about BEAM say something like:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“At the end of BEAM's mission, it will be removed from the ISS and&#xA;  burn up during reentry”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://en.wikipedia.org/wiki/Bigelow_Expandable_Activity_Module#ISS_flight_unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WYu3j.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I can’t find any information on how BEAM will get from the ISS to atmospheric entry. It seems odd that NASA would attach a module with a limited mission life to the ISS with no firm plan as to how to dispose of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is NASA's plan to remove and dispose of BEAM?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-04-01T10:22:24.307" LastActivityDate="2015-04-01T22:30:27.643" Title="How will BEAM be deorbited?" Tags="|iss|reentry|bigelow|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8635" PostTypeId="2" ParentId="8632" CreationDate="2015-04-01T22:30:27.643" Score="5" Body="&lt;p&gt;Upon being released from the ISS, its orbit will decay and it will reenter by itself in about one year.  Its ballistic coefficient (mass divided by cross-sectional area, or colloquially a measure of how &quot;fluffy&quot; an object is) is about &lt;a href=&quot;http://en.wikipedia.org/wiki/Bigelow_Expandable_Activity_Module&quot; rel=&quot;noreferrer&quot;&gt;1360 kg&lt;/a&gt; / (4 m $ \times $ 3.2 m) $\approx$ 100 kg / m$^2$ - actually rather denser than I'd imagined, for an inflatable structure, and comparable to that of the ISS itself.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital lifetime for an object with ballistic coefficient around 100 kg/m$^2$ released from ISS altitude (approx 400 km) is in the range of 9 to 18 months, depending on solar activity.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2015-04-01T22:30:27.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8637" PostTypeId="1" AcceptedAnswerId="8639" CreationDate="2015-04-01T23:00:09.993" Score="8" ViewCount="9459" Body="&lt;p&gt;I know I can find the orbit radius of a satellite from the equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$r=\sqrt[3]{\frac{T^2GM}{4 \pi^2}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but what determines the orbit period $T$? If I assume a geosynchronous orbit, would that simply mean the orbit period is the same as how long the planet takes to turn?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is a safe orbit radius / period of a satellite that would, for example, send a lander to the planet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I ask is that I'm looking for a in this first equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V=\sqrt{\frac{\mu_s}{r_1}\left(\sqrt{\frac{2r_2}{r_1+r_2}}-1 \right)^2+\frac{2 \mu_1}{a_1}}-\sqrt{\frac{\mu_1}{a_1}}+\sqrt{\frac{\mu_s}{r_2}\left(\sqrt{\frac{2r_1}{r_1+r_2}}-1 \right)^2+\frac{2 \mu_2}{a_2}}-\sqrt{\frac{\mu_2}{a_2}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v=v \ln \frac{m_0}{m_1}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, what role does the mass of the satellite play in this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If someone could tell me how to calculate it that would be great, thanks&lt;/p&gt;&#xA;" OwnerUserId="8825" LastEditorUserId="5963" LastEditDate="2015-04-02T01:13:55.773" LastActivityDate="2015-04-04T19:04:40.623" Title="Given the orbital radius of a satellite, how is the orbital period calculated?" Tags="|orbit|low-earth-orbit|geostationary|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8639" PostTypeId="2" ParentId="8637" CreationDate="2015-04-02T00:16:26.643" Score="5" Body="&lt;p&gt;You could just solve your first equation for $T$....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simplistically:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a circular orbit orbital velocity is constant at $$\sqrt{(G/r)(M+m)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for orbital period: you know $r$ (also constant), so calculate the length of the orbit (circumference, assuming it's a circle) and period is just length / velocity&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice for a given primary the only thing that really matters is $r$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass of the satellite (assuming it's artificial) is kinda negligible ($M+m$) where $M$ is the primary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are right about the geosynchronous orbit...but the period is determined by the radius...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;props to HDE 22686 for adding the math graphics.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-04-02T01:12:36.140" LastActivityDate="2015-04-02T01:12:36.140" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8640" PostTypeId="2" ParentId="2216" CreationDate="2015-04-02T02:33:19.567" Score="9" Body="&lt;p&gt;Enthalpy released is about 165 kJ / mol of $\require{mhchem}\ce{CO2}$. 2,000 lb of water is about 50 kmol, requiring 25 kmol of $\ce{CO2}$.  Thus long-term average heat output is 165 kJ * 25000 / 1 year = 130 watts, which is trivial compared to the overall ISS heat profile.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sabatier equipment probably runs at a peak-to-average power ratio somewhat greater than 1, and there would be some additional heat generated by heating elements, pumps and electronics aside from that liberated by the reaction, but even so the ISS thermal rejection system would have to deal with &lt;strong&gt;under a kilowatt&lt;/strong&gt; from the Sabatier system.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is pretty small compared to e.g. the overall electrical load of the ISS, which is in the &lt;strong&gt;70 to 100 kW range&lt;/strong&gt; (and, of course, essentially all that electricity ends up as heat).&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2015-04-02T16:22:27.297" LastActivityDate="2015-04-02T16:22:27.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8643" PostTypeId="1" CreationDate="2015-04-02T11:05:03.033" Score="9" ViewCount="1163" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5075/when-is-the-next-outer-planet-lineup-voyager&quot;&gt;All 4 outer planets&lt;/a&gt; line up once every 175 years. 2 of them line up at least once every orbit Jupiter makes, about every 12 years. But how often do 3 of the outer planets line up?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.solarsystemscope.com/&quot; rel=&quot;nofollow&quot;&gt;Here's a simple orrery&lt;/a&gt; which I hope is reliable, that shows the planets' (angular) positions at a given date. But how do they have to line up to be useful for a flyby mission? For example, are Saturn and Uranus close enough in November 2020?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="897" LastEditDate="2015-04-23T11:31:17.427" LastActivityDate="2015-04-23T11:31:17.427" Title="When will 3 outer planets line up for a flyby mission next time?" Tags="|orbit|interplanetary|gravity-assist|flyby|conjunction|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8644" PostTypeId="1" AcceptedAnswerId="8652" CreationDate="2015-04-02T23:52:08.127" Score="19" ViewCount="10552" Body="&lt;p&gt;It is clear from the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/1397/do-iss-astronauts-have-to-be-conversant-in-both-english-and-russian&quot;&gt;this question&lt;/a&gt; that all astro- or cosmonauts need to study English or Russian, whichever is not their native tongue. But as a practical matter, what language prevails for daily operations when two people on the ISS have to communicate, and are not from the same country? I was especially struck by &lt;a href=&quot;https://www.youtube.com/watch?v=53TcxyN-gig&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; the two guys from the one-year crew are answering questions from a NASA interviewer, each in their native languages. &lt;/p&gt;&#xA;" OwnerUserId="2581" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-04-04T10:23:45.950" Title="What language is most commonly spoken on the ISS?" Tags="|iss|language|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8645" PostTypeId="2" ParentId="8637" CreationDate="2015-04-03T03:34:50.807" Score="4" Body="&lt;p&gt;The orbital period of a satellite is solely determined by the semi-major axis of its orbit and the body it’s orbiting, specifically: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T = 2\pi \sqrt{a^3/\mu}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $\mu$ is the gravitational constant of the body being orbited. For Earth, $\mu$ = 5.166 $km^3/hr^2$ (we neglect the mass of the satellite because the Earth weights about 1 &lt;a href=&quot;https://www.facebook.com/pages/The-Official-Petition-to-Establish-Hella-as-the-SI-Prefix-for-1027/277479937276&quot; rel=&quot;nofollow&quot;&gt;hellagram&lt;/a&gt;), and $a$ is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Semi-major_axis&quot; rel=&quot;nofollow&quot;&gt;semi-major axis&lt;/a&gt; of the orbit, which is related to the radius (they are equal for circular orbits).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you solve this equation with the orbital period $T$ equal to one &lt;a href=&quot;http://en.wikipedia.org/wiki/Sidereal_time&quot; rel=&quot;nofollow&quot;&gt;sidereal day&lt;/a&gt;, you can calculate the altitude of a geosynchronous orbit, which is at roughly 42,000 km.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2015-04-03T03:34:50.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8646" PostTypeId="2" ParentId="8637" CreationDate="2015-04-03T08:14:38.023" Score="3" Body="&lt;p&gt;Adam Wuerl has already given a good answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T = 2\pi \sqrt{a^3/\mu}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This equation can be made easier to work with by choice of units.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if we use years and astronomical units, $\mu$ becomes $4\pi^2AU^3/year^2$ which cancels the figure outside the square root sign.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we have&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T = \sqrt{a^3year^2/AU^3}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, suppose radius is 9 AU. square root of 9 is 3. 3 cubed is 27. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;9 AUs, 27 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;16 AUs, 64 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4 AUs, 8 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same trick can be used with other bodies. For example pick your unit of length as radius of a geosynchronous orbit. Use one sidereal day for the time unit. An earth orbit with 4 times geosynch radius would have a period of 8 sidereal days.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="127" LastEditDate="2015-04-04T19:04:40.623" LastActivityDate="2015-04-04T19:04:40.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8647" PostTypeId="2" ParentId="8644" CreationDate="2015-04-03T11:00:26.077" Score="3" Body="&lt;p&gt;I'll need to find a reference for the below (it could be urban legend).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When negotiating the cooperation between Russia and the USA &lt;em&gt;a lot&lt;/em&gt; of effort went into defining how everything would be handled. They agreed where training would happen, who would teach, what language they would teach in etc. The language of agreement for training was Russian and at the time the USA spent a reasonable sum of money employing translators to make sure nothing in the training on the Russian side was missed. However, the language in orbit was agreed to be English. That being said a lot of the language spoken between the crew is a combination of the two languages, it's really a 'whatever works' situation.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="7062" LastEditDate="2015-04-04T00:55:29.547" LastActivityDate="2015-04-04T00:55:29.547" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8648" PostTypeId="1" CreationDate="2015-04-03T16:37:39.277" Score="1" ViewCount="145" Body="&lt;p&gt;Hydrazine/tetroxide is already contaminating the outside of the ISS and clinging to astronaut space suits. It can create breathing problems. In addition, a monopropellant vastly simplifies maneuvering jets and in the case of NOFBX simplifies long term storage and handling both in space and on the ground. It can also self-pressurize, again reducing the complexity of systems using it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I understand, it can achieve ISP's in excess of 300. This would seem to qualify it for boosters. Also, it has a fairly high specific density, making it to have a dual advantage...less tankage and plumbing from being a monopropellant and less tankage from being dense. All tests so far indicate that it burns at lower temperatures, making it less demanding on nozzles...reusability anyone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For boost, I doubt that it could be self-pressurizing, but one pump instead of two seems desirable. Also, having a single source or at least single fuel type for both propulsion and maneuvering would seem to be a huge advantage. What am I missing here? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, you can refer to the wiki articles on NOFBX and mixed monopropellants and from the former to the company currently developing NOFBX. It should be noted that NASA has approved an engine using NOFBX for attachment to the ISS, meaning that it has already passed a rather high safety bar.&lt;/p&gt;&#xA;" OwnerUserId="4629" LastActivityDate="2015-04-03T16:37:39.277" Title="Considering hybrid monopropellants like NOFBX to replace current toxic bipropellants and for boost" Tags="|rockets|propulsion|" AnswerCount="0" CommentCount="3" ClosedDate="2015-05-30T00:19:59.120" ContentLicense="CC BY-SA 3.0" />
  <row Id="8652" PostTypeId="2" ParentId="8644" CreationDate="2015-04-04T03:42:24.737" Score="14" Body="&lt;p&gt;There is a film &lt;a href=&quot;http://www.youtube.com/watch?v=2yJYkC50JWg&quot;&gt;&quot;&lt;em&gt;Русский в космосе&lt;/em&gt;&quot;&lt;/a&gt; (&lt;em&gt;Russian language in space&lt;/em&gt;, 12.04.2014) by Russia24:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/2yJYkC50JWg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They say that Russian language is popular, and (18 minutes in) an unofficial mix of Russian and English is used - &lt;em&gt;рунглиш&lt;/em&gt; (Runglish) or &lt;em&gt;русам&lt;/em&gt; (RusAm). They often use native language of the person with whom they want to talk (this rule was introduced during the Apollo–Soyuz test docking). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example in &lt;a href=&quot;https://youtu.be/doN4t5NKW-k?t=1240&quot;&gt;this video&lt;/a&gt;, Sunita Williams says &quot;&lt;em&gt;извините.. это пробка&lt;/em&gt;&quot; in Russian (&lt;em&gt;&quot;sorry, this is traffic jam&quot;&lt;/em&gt;) to a Russian cosmonaut and he replies &lt;em&gt;&quot;no problem&quot;&lt;/em&gt; in English.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper &lt;a href=&quot;http://pubs.sciepub.com/education/2/9/20/&quot;&gt;Does Russian English Exist?&lt;/a&gt; (American Journal of Educational Research, 2014 2 (9), pp 832-839. DOI: 10.12691/education-2-9-20) says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Russian English (RE) has already attracted public attention. The term 'Runglish' was coined in 2000 for one of the languages aboard the International Space Station, a mixture of English and Russian [&lt;em&gt;Feuer, A. For the Thirsty Runglish Speaker: Try an Ized Ciawfeh. The N.Y. Times 14 June 2005.&lt;/em&gt;]. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And a &lt;a href=&quot;http://cjonline.com/stories/101000/new_runglish.shtml&quot;&gt;newspaper from 2000&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The crew said they will rely on a mixture of languages and on a mixed cuisine when they are aboard the international station.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;We say jokingly that we communicate in 'Runglish,' a mixture of Russian and English languages, so that when we are short of words in one language we can use the other, because all the crew members speak both languages well,&quot; Krikalyov said.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;The menu will be 'Runglish' too: partly American food and partly Russian,&quot; Shepherd added.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;English Wikipedia also has &lt;a href=&quot;https://en.wikipedia.org/wiki/Runglish&quot;&gt;a short article on Runglish&lt;/a&gt;. In Runglish, if you don't know some word in the current language, you can &lt;a href=&quot;http://themoscownews.com/extreme_english/20130911/191914522/Can-you-wrangle-with-Runglish.html&quot;&gt;say it in another language&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The term itself, in any case, is commonly traced to 2000, when the not-quite-bilingual Russian-American crew of the International Space Station coined it to describe their on-board speech: Lacking a word or phrase, they used what they knew and filled in around it (&quot;Давай маленький Phillips screwdriver, Костя&quot; -- &lt;em&gt;give the small Phillips screwdriver, Kostya&lt;/em&gt;).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From another &lt;a href=&quot;http://www.youtube.com/watch?v=Jt4BZalMbqs&amp;amp;feature=youtu.be&amp;amp;t=3m11s&quot;&gt;ISS crew interview from 2013&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Jt4BZalMbqs?start=191&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;А на каком языке вы будете общаться?&lt;/em&gt;&lt;br &gt;&#xA;  (What language are you going to communicate in?)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;Today we are going to use Runglish. This is our unofficial language of the ISS program. It is called Runglish, it is a blend of English and Russian.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="49" LastEditDate="2015-04-04T10:23:45.950" LastActivityDate="2015-04-04T10:23:45.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8653" PostTypeId="1" AcceptedAnswerId="8656" CreationDate="2015-04-04T19:26:47.067" Score="8" ViewCount="1189" Body="&lt;p&gt;On earth, there exists large amounts of water underground in aquifers.&#xA;Curiosity has proven that water used to exist on the surface.    If this is true, Mars still may have underground reservoirs of water...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any research/missions to search for underground water reservoirs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One technique used on earth to search for underground water reservoirs, without digging, is to use &lt;a href=&quot;http://terraplus.ca/products/pmr-proton-magnetic-resonance/numis_plus.aspx&quot;&gt;Proton Magnetic Resonance (PMR)&lt;/a&gt;.  Is there a reason such a technique has never been applied to Mars, to search for underground reservoirs up to 150 meters? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side note:   Can PMR also be used for discovery of caves/lava tubes too?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finding a cave, or underground water reservoir would be a place of high scientific interest and/or possibly a good place to colonize...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clarification -- My main question is in regards to looking for underground resources, such as water an/or caves, any technique is a welcomed answer.  PMR is simply one example that my research has found.&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2015-04-09T15:37:26.767" Title="How to find underground resources (water? caves?) on Mars?" Tags="|mars|water|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8654" PostTypeId="1" AcceptedAnswerId="8670" CreationDate="2015-04-05T01:00:23.803" Score="10" ViewCount="1312" Body="&lt;p&gt;As we all know, Ares I was to be the Crewed Exploration Vehicle (CEV) launcher of the Constellation program. It was based on a 5 segment SRB design, with a J-2X engine as an upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ohJfp.jpg&quot; alt=&quot;Ares I&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was to be complemented by the larger Ares V, which would be the cargo hauler of the program. Before the program was cancelled in 2010, it underwent a single flight test, called Ares I-X, but there was criticism that it was for &quot;show only&quot; due to the fact that it featured an inert upper stage, and a 4 segment SRB.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have only ever seen references to them rather than detailed documents, but I have heard that during the Ares I program, the vehicle had vibration issues, launch abort scenarios did not look survivable, and that the vehicle and capsule had mass problems; and that overall the system was over budget and could face delays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any truth to these claims? Would someone be able to give a brief bullet point analysis on the problems the vehicle was facing?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="49" LastEditDate="2015-07-20T18:40:24.877" LastActivityDate="2016-06-20T10:05:09.630" Title="What engineering problems was Ares I facing at the time of its cancellation?" Tags="|nasa|launch-vehicle|constellation-program|canceled-project|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8656" PostTypeId="2" ParentId="8653" CreationDate="2015-04-05T10:37:15.870" Score="10" Body="&lt;p&gt;A google search shows one scholarly paper, sponsored by NASA, which describes techniques for finding subsurface water to a depth of a kilometer. It is readable by those who have research library credentials:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1029/2002JE001882/pdf&quot;&gt;Grimm, R. E., A comparison of time domain electromagnetic and surface nuclear magnetic resonance sounding for&#xA;subsurface water on Mars, J. Geophys. Res., 108(E4), 8037, doi:10.1029/2002JE001882, 2003.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper discusses the combining of &lt;a href=&quot;https://en.wikipedia.org/wiki/TDEM&quot;&gt;time domain electromagnetic&lt;/a&gt; (TDEM) method with Surface &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_magnetic_resonance_logging&quot;&gt;nuclear magnetic resonance&lt;/a&gt; (SNMR), and also calculates the energy requirements of such instruments leading to a feasibility of performing such a survey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper includes the concluding remark:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The unambiguous detection of groundwater on Mars&#xA;  by SNMR is unlikely to be achieved without massive&#xA;  resources, perhaps those that must await human exploration.&#xA;  Classical low-frequency EM methods provide robotic&#xA;  reconnaissance missions the best trade of sensitivity to&#xA;  water (with some nonuniqueness) versus system size, mass,&#xA;  and power.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="8162" LastEditDate="2015-04-05T12:07:57.387" LastActivityDate="2015-04-05T12:07:57.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8658" PostTypeId="1" AcceptedAnswerId="8676" CreationDate="2015-04-05T17:07:43.547" Score="1" ViewCount="2285" Body="&lt;p&gt;Which among the Earth-Moon Lagrange points (EML1, EML2, EML3, EML4, EML5) is best for a permanent space settlement which requires materials from the moon for construction?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some say it's EML4 for offering stability, while others propose EML1 because any object can reach the settlement from the Moon within hours or in a day at most.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which Lagrange point supports good communication?&lt;br&gt;&#xA;Which Lagrange point can offer better movement to the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any other orbit that can be better than EML points for positioning the settlement, such as a lunar orbit? If so, can you provide specifications, please?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: The settlement will not depend on Earth resources.. but it will depend on lunar resources for construction only. After construction, it is independent... i.e. without depending on the Earth or lunar resources. By better, I mean... which is a more potential orbit to make sure things from the Moon can reach early and safely to the settlement and a stable orbit. &#xA;Thanks a lot!&lt;/p&gt;&#xA;" OwnerUserId="8849" LastEditorUserId="415" LastEditDate="2015-04-07T18:58:47.180" LastActivityDate="2015-04-08T11:49:15.197" Title="Which lagrange (earth-moon) point is better for a space settlement?" Tags="|communication|lagrangian-points|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8661" PostTypeId="1" AcceptedAnswerId="8662" CreationDate="2015-04-06T11:29:50.120" Score="14" ViewCount="2177" Body="&lt;p&gt;Space probes use radio waves to communicate back to Earth - I would imagine a probe sent many decades ago, using very little energy and from very far away would be sending an incredibly weak signal back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just wondering, wouldn't it be easy for anyone on Earth to broadcast a signal in the same frequency and jam the probe signal (whether unintentionally or with intent).&lt;/p&gt;&#xA;" OwnerUserId="8854" LastEditorUserId="25" LastEditDate="2015-04-06T15:12:59.003" LastActivityDate="2015-04-07T04:38:01.583" Title="What's stopping space probe communication from being jammed?" Tags="|probe|radio-communication|deep-space-network|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8662" PostTypeId="2" ParentId="8661" CreationDate="2015-04-06T12:05:24.543" Score="6" Body="&lt;p&gt;A broadcast from a random location won't work. The radio antennae used at the Deep Space Network sites to send data to and receive data from deep space probes are highly directional. To jam the signal, you would have to transmit from a location that is between the receiving Deep Space Network antenna and the deep space probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That would mean using a drone, and it would need to be a rather sophisticated drone. The airspace around those three DSN sites are highly restricted. Fly a drone into that restricted airspace and you will be in a boatload of trouble.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-04-06T12:05:24.543" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8663" PostTypeId="2" ParentId="8661" CreationDate="2015-04-06T12:46:05.853" Score="8" Body="&lt;p&gt;So, first of all, what does it take to send the signal in the first place? Each of the Deep Space Network sites has a 34m antenna that is typically used for inner solar system communications. The power is approximately 200- 400W, in a band where that much power is difficult to achieve. In order to jam the signal, you need to increase the noise significantly. Overall, it would be rather difficult to jam the signal going to the spacecraft. It could be done, but building dishes of the required size is a tricky matter, although I don't doubt that a dedicated source could make this happen, given the appropriate desire.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what about the return signal? Well, that one is a bit easier, as the signal is rather weak on the ground. I don't have the full rf budget of a mission, but it is safe to assume it is weak. However, this is going to have some serious difficulties. First of all, the antenna is very directional, very little RF will get in to it except in the direct path of the antenna beam. In fact, from &lt;a href=&quot;http://ipnpr.jpl.nasa.gov/progress_report2/42-20/20P.PDF&quot; rel=&quot;nofollow&quot;&gt;this paper&lt;/a&gt;, the feed horn, which is essentially the receiver, at 22 degrees off of the beam there is a 38 dB, and that is the best sidelobe! If the horn is properly shielded, as is likely being in the dish, then the side lobs drop down dramatically. Even if it is not properly shielded, while it would be possible to jam this signal if you can find the correct location to do it from, it  would have to get pretty close to the direction the antenna is pointed to take advantage of that. In addition, the bands of interest are essentially line of site only. You would have to get close to the antenna to have any success, and the signal would lead directly to the device that was transmitting. It could theoretically be done, but it would be difficult to achieve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the most likely scenario for jamming would leave the guilty party quickly found. All spacecraft I'm aware of have a method to re-transmit lost data. It would be painful for NASA, but of limited damage, and the person would end up in jail rather quickly, no doubt.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-04-06T18:12:32.857" LastActivityDate="2015-04-06T18:12:32.857" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8664" PostTypeId="2" ParentId="8661" CreationDate="2015-04-06T13:14:08.470" Score="-4" Body="&lt;p&gt;It is theoretically possible but would be tricky as NASA does not publish information such as the frequency in the public domain. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, it is not a major concern as there is really no motive for anyone to want to jam the signal.&lt;/p&gt;&#xA;" OwnerUserId="8854" LastActivityDate="2015-04-06T13:14:08.470" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8665" PostTypeId="2" ParentId="8661" CreationDate="2015-04-06T15:03:28.767" Score="3" Body="&lt;p&gt;Jamming the downlink signal is quite easy.  It is extremely low in power, so if you just set up a signal source in the vicinity of the receive antenna, there will probably be enough off-axis gain that yours will be the stronger signal.  &lt;a href=&quot;http://www.universetoday.com/81662/voyager-1-has-outdistanced-the-solar-wind/&quot; rel=&quot;nofollow&quot;&gt;This page&lt;/a&gt; gives the received signal level of Voyager 1 as $-150.7$ dBm.  Even if you generate a few Watts at the proper frequency  you will swamp this if the receive antenna has gain in your direction more than $-150$ dB or so.  But would you want to?&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2015-04-06T15:03:28.767" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8666" PostTypeId="2" ParentId="3772" CreationDate="2015-04-06T16:53:42.613" Score="21" Body="&lt;p&gt;It's Monday, so let me rain on this parade a little. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current magnetic shield designs are adequate to protect against ionizing radiation &lt;em&gt;from the sun&lt;/em&gt;. They aren't sufficient to protect against galactic cosmic radiation, which has a lot &lt;a href=&quot;https://books.google.com.mx/books?id=7Q49AAAAIAAJ&amp;amp;pg=PA47&amp;amp;lpg=PA47&amp;amp;dq=solar%20ionizing%20radiation%20flux&amp;amp;source=bl&amp;amp;ots=yDgBlmd-XX&amp;amp;sig=iGWmdnuZhSenREwMYsp5I7u-Euk&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=Z6giVbyNIsursAWDpoDwDQ&amp;amp;redir_esc=y#v=onepage&amp;amp;q=solar%20ionizing%20radiation%20flux&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;more energy&lt;/a&gt; in each particle. To effectively block that would take a shield with energy 100x greater. If Bamford's shield parameters from TildalWave's answer are used, then 500 kW of power would be needed. It can't be modulated according to current needs, it has to always be at that power level, because GCR fluctuates only very slowly, over the solar cycle, it isn't a matter of occasional storms like solar ionizing radiation. The mass and energy needs of such a system are prohibitive. The effect over time on human health of exposure to a magnetic field of such strength is unknown. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The health effect of cosmic rays at the levels in interplanetary space is not known. Exposure for a year or two may only increase the chances of cancer, or it could be debilitating, if not immediately then a few years down the road. Once we have much more data, it may turn out to be adequate to use a weaker magnetic field that diverts only particles with energies below 500 MeV, for instance - that would be the &lt;a href=&quot;http://www.srl.caltech.edu/ACE/ASC/DATA/bibliography/ICRC2005/usa-mewaldt-RA-abs1-sh35-oral.pdf&quot; rel=&quot;noreferrer&quot;&gt;majority&lt;/a&gt; of particles. Or it might not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://engineering.dartmouth.edu/~d76205x/research/Shielding/docs/Parker_06.pdf&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; on the topic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: To clarify the potential danger of GCR, here is a table taken from the document the 2nd link goes to, which is an excellent, up to date summary of the matter:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ALDSy.png&quot; alt=&quot;annual radiation doses from cosmic rays&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shielding postulated to calculate the last column is 3 g/cm&lt;sup&gt;2&lt;/sup&gt; of aluminum... plus the flesh of your body surrounding the important bits. Which is to say, they are assuming that if a 400 MeV proton hits a molecule in one of your muscles, that isn't really important, they are only considering the 'blood forming organs' (which doesn't include your brain). That alone, I think, communicates how much our knowledge of this is preliminary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also critical to bear in mind that this shielding is calculated to help &lt;strong&gt;because it is so thin&lt;/strong&gt;. Because that means that most particles will simply pass right through the whole ship practically as though it wasn't even there. Once one of those particles hits the nucleus of an atom in its path, that's when the real trouble begins. They smash new particles off the that nucleus, multiplying the problem. From &lt;a href=&quot;http://settlement.arc.nasa.gov/75SummerStudy/5appendE.html&quot; rel=&quot;noreferrer&quot;&gt;Appendix E&lt;/a&gt; of NASA's &quot;&lt;a href=&quot;http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html&quot; rel=&quot;noreferrer&quot;&gt;Space Settlements: A Design Study&lt;/a&gt;&quot; (which is an excellent introduction):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are three mechanisms that are important in mass shielding.&#xA;  First, a charged particle excites electrons for many hundreds of&#xA;  angstroms about its trajectory. This excitation extracts kinetic&#xA;  energy at a roughly constant rate for relativistic particles and acts&#xA;  as a braking mechanism. For relativistic protons in low-Z matter this&#xA;  &quot;linear energy transfer&quot; is $2 MeV/g*cm^-2$ of matter. If the thickness&#xA;  of the mass shield is great enough a particle of finite kinetic energy&#xA;  is stopped. This is the least effective shielding mechanism in matter&#xA;  for relativistic particles.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The second mechanism is nuclear attenuation. For silicon dioxide the&#xA;  average nuclear cross section is 0.4 &lt;a href=&quot;https://en.wikipedia.org/wiki/Barn_(unit)&quot; rel=&quot;noreferrer&quot;&gt;barn&lt;/a&gt; ($10^{-24}\ cm^2$). Thus if a&#xA;  charged particle traverses far enough in the shield (composed of&#xA;  silicon dioxide) it collides with a nucleus and loses energy by&#xA;  inelastic collisions with the nuclear matter. The measure of how far a&#xA;  particle must travel to have a substantial chance of nuclear collision&#xA;  is the mean free path, which for silicon dioxide is $106 g/cm^2$ . This&#xA;  mechanism is an exponential damper of primary beam particles.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Opposing the beam clearing tendency of nuclear attenuation is the&#xA;  creation of energetic secondary particles. For each nuclear collision&#xA;  there is beam loss from nuclear excitation, and beam enhancement&#xA;  (though with overall energy degradation through the increase of&#xA;  entropy) from the secondaries emitted by the excited nuclei. These&#xA;  secondary particles are, of course, attenuated themselves by further&#xA;  nuclear collisions with roughly the same mean free path as the primary&#xA;  particles&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, in all of the parts of the hypothetical ship where there is more stuff than the 3 g/cm&lt;sup&gt;2&lt;/sup&gt; of aluminum between you and space, there is a much higher chance that you will be struck by damaging particles from those directions, unless the solid stuff in between is at least a couple of meters thick. It might take 5 meters of stuff before the cascade of particles generated by such collisions peters out due to its kinetic energy having been dispersed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, maybe you could spend 10 years in deep space and get only the maximum allowable radiation for the career of an astronaut according to NASA, which means you don't have to worry about anything more than a higher chance of cancer, and cataracts. Or maybe that point of view is way too sunny. We really don't know. From the &lt;a href=&quot;https://engineering.dartmouth.edu/~d76205x/research/Shielding/docs/Parker_06.pdf&quot; rel=&quot;noreferrer&quot;&gt;Eugene Parker&lt;/a&gt; article above:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One estimate from NASA is that about one third of the DNA in an&#xA;  astronaut’s body would be cut by cosmic rays every year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-19T19:31:42.920" LastActivityDate="2015-04-19T19:31:42.920" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8667" PostTypeId="1" AcceptedAnswerId="8668" CreationDate="2015-04-06T23:04:39.653" Score="1" ViewCount="1393" Body="&lt;p&gt;Is Lagrange point L1 stable? If I were to place a space station in L1 will it remain in orbit without any difficulty? If there are any difficulties, please mention them here along with any suggestions to overcome the difficulties, if possible.&lt;/p&gt;&#xA;" OwnerUserId="8849" LastEditorUserId="2847" LastEditDate="2015-04-06T23:13:52.123" LastActivityDate="2015-04-07T16:39:21.063" Title="Is lagrange point L1 stable?" Tags="|lagrangian-points|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8668" PostTypeId="2" ParentId="8667" CreationDate="2015-04-06T23:09:40.870" Score="3" Body="&lt;p&gt;L1 is unstable, although you can establish a fairly stable halo orbit around L1 -- but even a halo orbit will require periodic correction.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-04-06T23:09:40.870" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8669" PostTypeId="2" ParentId="8661" CreationDate="2015-04-07T01:20:31.073" Score="3" Body="&lt;p&gt;Long time ago I studied in aerospace university of the former USSR and we briefly covered subject of space communications. The main reason was that those radios used a new technology of that time (~1985) called something like &quot;broad spectrum communications below noise level&quot;. This was combination of encryption and radio transmission, so what you receive would be a broad spectrum white noise of very low level that is indistinguishable from regular noise. However, if you know the modulating sequence then you could combine the sequence with the received white noise and extract the signal. Mind you that this technology wasn't digital and was based on analog computing. The main advantages were that it was impossible to detect, impossible to jam and encryption was given. As far as I know the similar technology was used by cell phone network operators for very same reasons.&lt;/p&gt;&#xA;" OwnerUserId="8856" LastEditorUserId="8856" LastEditDate="2015-04-07T04:38:01.583" LastActivityDate="2015-04-07T04:38:01.583" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8670" PostTypeId="2" ParentId="8654" CreationDate="2015-04-07T02:19:21.670" Score="4" Body="&lt;p&gt;Beyond vibration issues, which they were trying to address with a weight to detune the vibration mode, they had an abort issue. The possible blast radius of an SRB was greater than the abort range of Orion, and most likely the parachutes would come back down through the blast and likely melt. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-07T02:19:21.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8673" PostTypeId="2" ParentId="8658" CreationDate="2015-04-07T09:48:32.807" Score="2" Body="&lt;p&gt;Without much more details on exactly what the settlement will be used for, what materials / supplies / communication will be required from the Earth and the Moon and even how many and how frequently settlers will be ferried to / from the settlement, it is impossible to give any proper answer. I did a bit of research and can at least give a discussion on each point below:&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;L1&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;This would be an excellent point for communications between Earth and Moon. You would also have quick access to the Moon. This point would have lower exposure to solar wind and cosmic rays than the other Lagrangian points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This point is not fully stable, so you will need active stationkeeping.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;L2 and L3&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;This is at the far side of the moon and the opposite side of Earth from the Moon, so neither provide any real benefits to the settlement proposed.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;L4 and L5&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;These points are stable, so no need for active stationkeeping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are, however, less easy to access from the Moon and will have higher exposure to solar wind and cosmic rays than L1.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Lunar orbit&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;A settlement in lunar orbit should be fairly easy to access from the Moon. The Moon doesn't have enough of an atmosphere to cause any real orbital decay, however, due to the Moon's lumpy gravity, there are only a few orbital inclinations available for low lunar orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can design the orbit so that communications should not be a problem. And even if the materials you need to mine is on the far side of the Moon, it would be easy to set up the orbit so that the settlement can be accessed from the mine with moderate delta-v requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Relevant &lt;a href=&quot;http://en.wikipedia.org/wiki/Lagrange_point_colonization&quot; rel=&quot;nofollow&quot;&gt;wikipedia article&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-04-07T09:48:32.807" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8674" PostTypeId="1" AcceptedAnswerId="8677" CreationDate="2015-04-07T10:56:26.940" Score="16" ViewCount="2027" Body="&lt;p&gt;I know about the Iridum-33 and Kosmos-2251 collision, but has there been any collision between active spacecrafts? Any claim regarding the same?&lt;/p&gt;&#xA;" OwnerUserId="8860" LastActivityDate="2015-11-05T23:23:28.107" Title="Has there been any collision between two active spacecrafts?" Tags="|artificial-satellite|spacecraft|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8676" PostTypeId="2" ParentId="8658" CreationDate="2015-04-07T11:08:51.533" Score="2" Body="&lt;p&gt;L2 has the advantage of serving communication from the far side of the Moon. I think L2 will be an early starting point for cis-lunar infrastructure, and like cities grow around a village which started as a single farm, it might become the grain for a future space colony (if that stuff will work the same way). A spacecraft doesn't stay put in a lagrange point, it orbits it. While orbiting L2 you can always have line of sight to Earth and the Sun, the spacecraft is never strictly in line with Earth and the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You don't need a space station to directly improve transportation to the Moon. You simply launch from Earth surface to lunar orbit directly, like the Apollo missions did. Once over there you can supply yourself with fuel mined on the Moon, if that will become a reality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another option to consider, for low mass satellites at least, is using solar sails to create non-keplerian orbits, so called &lt;a href=&quot;https://en.wikipedia.org/wiki/Statite&quot; rel=&quot;nofollow&quot;&gt;statites&lt;/a&gt;. It could of course also be done with other propulsion technologies which require refueling. Using continuous low-thrust propulsion (especially sunlight/wind which is tax free) you can modify the geometry of effective geocentrical and other orbits as well as the lagrange points. But a &quot;settlement&quot; sounds too heavy for doing that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then there is &lt;a href=&quot;https://space.stackexchange.com/questions/5159/how-could-a-trip-to-mars-benefit-from-a-distant-lunar-retrograde-orbit-dro&quot;&gt;DRO, distant retrograde (lunar) orbits&lt;/a&gt;. A minor drawback with them is their availability. You can launch from Earth to the Moon or any lagrange point any day. But something in DRO might have much rarer launch windows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &quot;space station&quot; need not be stationary. When it needs materials from the Moon, it will enter some cis-lunar orbit. When it needs more energy it will go closer to the Sun. If an asteroid or the Jovian moons are of interest, it could go there. Massive structures change trajectory slowly and expensively, but they don't have any roots. Where ever you want a space settlement, it will likely be best to construct (most of) it in earth or lunar orbit, and then push it to its intended destination.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-04-07T11:13:51.793" LastActivityDate="2015-04-07T11:13:51.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8677" PostTypeId="2" ParentId="8674" CreationDate="2015-04-07T11:19:21.230" Score="28" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Progress_M-34&quot; rel=&quot;noreferrer&quot;&gt;Progress M-34&lt;/a&gt; collided with Mir in 1997.  Caused a leak in a module.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Iu3oy6_v73M?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="4660" LastEditDate="2015-11-04T22:28:00.930" LastActivityDate="2015-11-04T22:28:00.930" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8678" PostTypeId="1" CreationDate="2015-04-07T11:38:43.133" Score="7" ViewCount="1619" Body="&lt;p&gt;Rocket engines provide the most efficient velocity change per mass fuel used when in periapsis because of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot;&gt;Oberth effect&lt;/a&gt;. But is this an effect of a spacecraft losing mass (exhausting it through its rocket engine) in a gravity well so that it then can go up out of it lighter, or is it more general? Would a spacecraft without reaction engine, for example one which instead uses a solar sail or beamed microwaves from Earth, still enjoy the Oberth effect? Even if its mass was the same when entering the gravity well, as when leaving it.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-08-09T15:56:56.607" Title="Is the Oberth effect applicable to any speed change, or only to ejection of reaction mass?" Tags="|propulsion|oberth-maneuver|" AnswerCount="7" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8679" PostTypeId="2" ParentId="8678" CreationDate="2015-04-07T12:37:49.243" Score="3" Body="&lt;p&gt;The change in apoapsis is caused purely by the change in velocity applied - whether that velocity change is caused by rocket propulsion, or by any of the other methods you've mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(EDIT: I should add this - you also mentioned the craft's rise out of the gravity well - its path along the orbit.  This path is not affected by its mass, only by its velocity.  Much like a pendulum's frequency depends only on the length of the pendulum, or two objects on Earth fall at the same speed, provided there are no aerodynamic effects.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2015-04-07T12:43:55.000" LastActivityDate="2015-04-07T12:43:55.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8680" PostTypeId="2" ParentId="8678" CreationDate="2015-04-07T13:49:13.523" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Would a spacecraft without reaction engine, for example one which instead uses a solar sail or beamed microwaves from Earth, still enjoy the Oberth effect? Even if its mass was the same when entering the gravity well, as when leaving it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Mechanistically, there's actually no reason that these other means of propulsion would not be subject to the Oberth effect. In the two examples you gave, the propulsion is given by photons. These have an absurdly large specific impulse, and they are massless, but they follow similar rules. If the photons reflect off a mirror on the spacecraft while it makes a planetary flyby, then those photons could continue into space, but before they do so they will experience a slight redshift due to climbing the gravitational well - similar to how any other propellant would.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is pedantic because the energy change is tiny compared to the total energy, but this is only because of the very high specific impulse. In more practical terms, it would be extremely difficult to get a high Oberth effect because low-thrust engines (and solar sails) spend very little of their thrusting phase at perigee where the Oberth effect is great.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet another alternative might be electromagnetic acceleration as a craft does a flyby of a planet. In this case we can't use the same logic because the force is transmitted between one body and another. However, the argument still works by virtue of the craft's higher speed at perigee. The energy that a stationary accelerator imparts is the impulse times velocity (from the mechanical power=force x velocity relationship).&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2015-04-07T13:49:13.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8681" PostTypeId="2" ParentId="8667" CreationDate="2015-04-07T16:39:21.063" Score="5" Body="&lt;p&gt;Something parked at L1 is balanced between evenly matched tug-of-war teams.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On one side of the tug-of-war is the moon's gravity and inertia in a rotating frame (what we used to call centrifugal force). On the other side is earth's gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1Ljqo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Go a little bit to the left and Earth has the advantage. Move a little bit to the right and the moon and so called centrifugal force get stronger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So just a tiny nudge destroys this delicate balance. Something at the Earth Moon L1 (EML1) needs station keeping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want something high up that is long lasting and maintenance free, I would suggest a lunar Distant Retrograde Orbit (DRO). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An example of a moon in a DRO is Jupiter's &lt;a href=&quot;http://en.wikipedia.org/wiki/S/2003_J_2&quot; rel=&quot;noreferrer&quot;&gt;S/2003 J2&lt;/a&gt;. According to the Wikipedia article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Retrograde moons with axes up to 67% of the Hill radius are believed&#xA;  to be stable.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The moon's Hill radius is about 60,000 kilometers. So a lunar retrograde orbit at 40,000 kilometers would be pretty stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And in fact a lunar DRO is one of the suggested destinations for the proposed asteroid retrieval mission.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-04-07T16:39:21.063" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8682" PostTypeId="2" ParentId="8678" CreationDate="2015-04-07T17:25:13.450" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Would a spacecraft without reaction engine, for example one which instead uses a solar sail or beamed microwaves from Earth, still enjoy the Oberth effect?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose spacecraft A, powered by rockets, dives toward the Sun and fires all its thrusters at once near perihelion to get some desired delta-V. Next suppose spacecraft B, powered by solar sails, dives toward the Sun, reaches the same perihelion distance as spacecraft A, and turns its sails in just the right way near perihelion to get the same delta-V as spacecraft A.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both spacecraft will be subject to the same Oberth effect because the Oberth effect is solely a consequence of change in velocity and the velocity at which that delta-V occurs. The underlying physics that results in that delta-V doesn't matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both of these approaches have been proposed as mechanisms for escaping the solar system at high escape velocities. The idea of using solar sails to escape the solar system dates from the start of the space age (Tsu 1959). At a recent NASA Innovative Advanced Concepts symposium, Nosanov et al. analyzed using solar sails (with a final close approach to the Sun) as a means to escape the solar system, reaching the heliopause in 12 years or less and using existing materials. (Note that took Voyager 35 years to accomplish the same). I've only incorporated two references. There are many, many more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;T. C. Tsu, &quot;Interplanetary travel by solar sail,&quot; &lt;em&gt;Ars Journal&lt;/em&gt; 29.6 (1959): 422-427.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/740774main_NosanovSpringSymposium2013.pdf&quot; rel=&quot;nofollow&quot;&gt;J. Nosanov, D. Grebow, B. Trease, J. West, and H. Garrett, &quot;Solar System Escape Architecture for Revolutionary Science&quot;, &lt;em&gt;NASA NIAC Conference&lt;/em&gt;, 2013&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-04-08T14:23:59.350" LastActivityDate="2015-04-08T14:23:59.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8683" PostTypeId="1" CreationDate="2015-04-08T04:30:28.483" Score="12" ViewCount="1273" Body="&lt;p&gt;The Fermi paradox is basically that:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;There are billions of stars, life is probably abundant throughout the universe. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Even though it may take tens of thousands or millions of years for life on other planets to reach us, the universe is old enough that they should have reached us by now. (eg. a civilisation that developed space travel 100 million years ago could have explored the entire Milky Way.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;But is it possible that, yes aliens have reached earth, but they reached earth perhaps a million years ago, documented the life currently existing, and haven't bothered coming back? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's wrong with this explanation? &lt;/p&gt;&#xA;" OwnerUserId="6027" LastEditorUserId="49" LastEditDate="2015-04-21T14:56:01.863" LastActivityDate="2015-04-21T14:56:01.863" Title="Is a combination of 'space is big' and 'humans haven't been around long' a good solution to the Fermi paradox?" Tags="|fermi-paradox|seti|drake-equation|astrobiology|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8684" PostTypeId="2" ParentId="8683" CreationDate="2015-04-08T06:05:49.523" Score="4" Body="&lt;p&gt;If anyone anywhere anytime did space travel, they should have been here by now. The Milky Way is small compared to its age and travel times. The Sun has made about twenty orbits around it. The dinosaurs actually lived on Earth when it was on the other side of the galaxy. Either we are alone as a space faring society, or we have company very close by. The most unlikely scenario is that &quot;they&quot; exist but are far between. As if a part of the ocean had no fish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interstellar distances and the constant speed of light ensures that space faring civilizations, even colonies with the same origin, cannot be coordinated. When the communication time lag is thousands of years then technology, culture, policy will evolve completely differently. So if anything is out there, then everything is out there. If Stephen Hawkings is afraid of a dangerous type of aliens, he can rest assured that they, if anyone, do exist and that they are nearby. There are nice guys out there too, everyone is out there, they just cannot be similar to each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that logic has no support at all from observations. All life on Earth is related to each other and had the same origin. You and I have some genes in common with every living cell be it a dog, an insect, a fungus or a disease. We are all family since over three billion years. Nothing ever came here to change that. And the ancient surfaces of the Moon and Mars, where we now can find our own small spacecrafts from orbit, show no signs at all of any artificial manipulation. We are definitely alone as a space faring society in the solar system, and we always have been. This in spite of the fact that the Sun has been all around the galaxy many times and has had many different neighbors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And SETI has showed us that if they exist then they don't like to use radio very much. Nor do they rebuild galaxies or build Dyson spheres around stars. If we do have neighbors, then thankfully they aren't very noisy. Thinking about it, none of them ever was, which is a bit creepy. Like being all alone in a gigantic hotel, like in the movie The Shining. Your mind begins to make up ghosts as company.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-04-08T06:21:14.257" LastActivityDate="2015-04-08T06:21:14.257" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="8685" PostTypeId="2" ParentId="8683" CreationDate="2015-04-08T08:22:48.797" Score="10" Body="&lt;p&gt;A couple of issues with the Fermi paradox:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The Fermi paradox is closely tied to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Drake_equation&quot;&gt;Drake equation&lt;/a&gt;. This equation is the best method we currently have for thinking about the probability of intelligent life in our galaxy. The problem is that there are so many assumptions you have to make in that equation, that the results vary drastically. If you plug some of the more negative assumptions into the formula, you get as few as only 2 civilizations currently in our galaxy.&lt;/li&gt;&#xA;&lt;li&gt;To say that an advanced civilization would even bother to colonize the rest of the galaxy is a big assumption. Our forefathers colonized every place they could. Today, we have wildlife reserves where we specifically try to limit human interference. We take great care not to spread Earth life to other planets when we send science probes. Maybe a sufficiently advanced civilization will simply choose not to colonize everything.&lt;/li&gt;&#xA;&lt;li&gt;The more advanced a civilization becomes, the easier it is for them to wipe themselves out entirely. This is the view Carl Sagan took to explaining why we have not yet made contact with other intelligent life. If some rulers decided to wage war even 100 years ago, they could not wipe out all humanity even if they tried. If we detonate every nuclear bomb we have in stockpile today, the odds of even 1 human surviving is about zero. We are doing science experiments like trying to create black holes, creating catastrophic climate change and breeding ever more deadly viruses. The odds of us wiping ourselves out in the future seem to be increasing every year.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-04-08T08:22:48.797" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8686" PostTypeId="1" AcceptedAnswerId="8689" CreationDate="2015-04-08T10:44:15.660" Score="4" ViewCount="358" Body="&lt;p&gt;With the knowledge that space exploration is open to all, can a feasible and convenient attachment site be claimed and used only by a single country?&lt;/p&gt;&#xA;" OwnerUserId="8860" LastActivityDate="2015-04-08T13:33:51.650" Title="Can an attachment site on an asteroid by a spacecraft be privatized?" Tags="|docking|docking-port|site-selection|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8688" PostTypeId="2" ParentId="8658" CreationDate="2015-04-08T11:14:59.370" Score="5" Body="&lt;p&gt;In 1975 NASA did a &lt;a href=&quot;http://settlement.arc.nasa.gov/75SummerStudy/Design.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;design study&lt;/a&gt; that &lt;a href=&quot;http://settlement.arc.nasa.gov/75SummerStudy/Chapt4.html#Where&quot; rel=&quot;nofollow noreferrer&quot;&gt;selected L5&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;There remain the orbits about the five libration points. Three of these, L1, L2, and L3, are known to be unstable, and to maintain orbits around any of these three points for long periods of time requires appreciable expenditures of mass and energy for station keeping.&lt;/p&gt;&#xA;&lt;p&gt;There do exist, however, large orbits around both of the remaining [l]ibration points, L4 and L5. These have been shown to be stable (refs. 27, 28). A colony in either of these orbits would be reasonably accessible from both Earth and Moon. One of these libration points, L5, is chosen for the location of the first space colony. This choice is somewhat arbitrary for the differences between L4 and L5 are very slight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The study describes how the whole station could be built using lunar materials and technologies that were known at the time, or could be reasonably expected to become available.&lt;/p&gt;&#xA;&lt;p&gt;Both L4 and L5 allow direct communication lines to Earth-bound or Earth-orbit communication networks. Both points also allow direct communication with one hemisphere of the Moon.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;whole study&lt;/a&gt; was produced in the heady days before the NASA budget was cut. It is pretty amazing.&lt;/p&gt;&#xA;&lt;p&gt;You may also be interested in the NASA &lt;a href=&quot;http://settlement.arc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Settlements&lt;/a&gt; page.&lt;/p&gt;&#xA;" OwnerDisplayName="user8406" LastEditorUserId="-1" LastEditorDisplayName="user8406" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-04-08T11:49:15.197" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8689" PostTypeId="2" ParentId="8686" CreationDate="2015-04-08T12:42:12.953" Score="6" Body="&lt;p&gt;For the time being, the status of &lt;em&gt;regions of interest&lt;/em&gt; on celestial bodies remains &lt;em&gt;res nullius&lt;/em&gt;. I.e. there is no ownership rights, no state can claim exclusive access or otherwise prohibit other states, or state regulated parties from exploring such regions. This was drafted in the resolution of the United Nations General Assembly in &lt;em&gt;&lt;a href=&quot;http://www.unoosa.org/oosa/en/SpaceLaw/gares/html/gares_18_1962.html&quot;&gt;Declaration of Legal Principles Governing the Activities of States in the Exploration and Use of Outer Space&lt;/a&gt;&lt;/em&gt; more commonly known as &lt;em&gt;The Outer Space Treaty&lt;/em&gt; (OST), &lt;a href=&quot;http://www.unoosa.org/oosa/en/SpaceLaw/treatystatus/index.html&quot;&gt;ratified and signed&lt;/a&gt; by most countries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Namely, governing principles that carry the most weight with regard to your question are:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol start=&quot;2&quot;&gt;&#xA;  &lt;li&gt;Outer space and celestial bodies are free for exploration and use by all States on a basis of equality and in accordance with&#xA;  international law.&lt;/li&gt;&#xA;  &lt;li&gt;Outer space and celestial bodies are not subject to national appropriation by claim of sovereignty, by means of use or occupation,&#xA;  or by any other means.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As you can see, these two principles can oppose each other, and so far not much has been done to amend this, so is subject to open interpretation. This is not ideal, but while OST was also the basis for other &lt;a href=&quot;http://www.unoosa.org/oosa/en/SpaceLaw/treaties.html&quot;&gt;&lt;em&gt;space law&lt;/em&gt; treaties&lt;/a&gt;, among which &lt;em&gt;&lt;a href=&quot;http://www.unoosa.org/oosa/en/SpaceLaw/gares/html/gares_34_0068.html&quot;&gt;Agreement Governing the Activities of States on the Moon and Other Celestial Bodies&lt;/a&gt;&lt;/em&gt; aka &lt;em&gt;The Moon Treaty&lt;/em&gt; that remains &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon_Treaty&quot;&gt;mostly unratified, unsigned&lt;/a&gt; by most countries and is considered a failed agreement that was seeking to increase international cooperation (including e.g. sharing samples) and to reaffirm &lt;em&gt;common heritage&lt;/em&gt; status of celestial bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Through other treaties and agreements (e.g. &lt;em&gt;&lt;a href=&quot;http://www.unoosa.org/oosa/SpaceLaw/liability.html&quot;&gt;Convention on International Liability for Damage Caused by Space Objects&lt;/a&gt;&lt;/em&gt;, aka &lt;em&gt;The Liability Convention&lt;/em&gt; and &lt;em&gt;&lt;a href=&quot;http://www.unoosa.org/oosa/SpaceLaw/rescue.html&quot;&gt;Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space&lt;/a&gt;&lt;/em&gt;, aka &lt;em&gt;The Rescue Agreement&lt;/em&gt;) obligation of individual state parties to OST was emphasized and with it their legal reach. Some countries, including e.g. US, are trying to take this opportunity to draft unilateral laws that would regulate exploitation of outer space resources as they deem fit, as long as they broadly follow OST convention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One such try, or at least one of the latest ones, is the &lt;a href=&quot;https://www.congress.gov/bill/113th-congress/house-bill/5063/text&quot;&gt;ASTEROIDS act of 2014&lt;/a&gt; that the US House of Representatives had on their agenda, seeking to protect property rights and establish a legal framework for commercial exploitation of outer space resources, but so far didn't pass for being &lt;em&gt;&lt;a href=&quot;http://www.spacepolicyonline.com/news/gabrynowicz-warns-asteroids-act-needs-more-work&quot;&gt;in need of more work&lt;/a&gt;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in short, no. For the time being, there is no wherewithal legal framework upon which any signing / ratifying state party to the OST could claim ownership or exclusive access rights to any celestial bodies, or their individual regions of interest. So &lt;em&gt;caveat emptor&lt;/em&gt; when someone tries to sell you a deed to a property on the Moon, or on any other celestial body!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;There is a glaring loophole in all of this tho. Since, by &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Liability_Convention&quot;&gt;massively ratified&lt;/a&gt; frameworks like &lt;em&gt;The Liability Convention&lt;/em&gt;, launching countries bear responsibility for space objects that are launched within their territory, that also means the spacecraft and any supporting infrastructure they use are effectively that launching state's territory. Combined with &lt;em&gt;non-interference&lt;/em&gt; provisions of OST, this creates an &lt;em&gt;interesting&lt;/em&gt; (for lack of a better word) stage for &lt;em&gt;exclusive access by means of occupation&lt;/em&gt;. So if you can attach to any of these &lt;em&gt;regions of interest&lt;/em&gt; and stay attached, you're effectively prohibiting others access and via &lt;em&gt;non-interference&lt;/em&gt; they can't access it without your consent. You're supposed to enable them access, if so requested, but only under your terms which aren't internationally provisioned. A form of &lt;em&gt;outer space squatting&lt;/em&gt;, if you want.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-04-08T13:33:51.650" LastActivityDate="2015-04-08T13:33:51.650" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8691" PostTypeId="2" ParentId="8683" CreationDate="2015-04-08T16:41:57.890" Score="5" Body="&lt;p&gt;I think the answer to your question is &quot;yes&quot;.  The combination of &quot;space is big&quot; and &quot;humans haven't been around long&quot; is a valid (as opposed to good) explanation for the Fermi paradox.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I replaced &quot;good&quot; with &quot;valid&quot; because &quot;good&quot; is subjective and can be colored by one's own opinions, and desires, about the possibility of alien life.  But as logical explanations go, yes, I think this is perfectly valid reasoning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After all, we don't really know how hard it may be to break the FTL barrier.  Maybe it really is impossible for all practical reasons, and since space is so big, alien species don't come around very often, and certainly not for no good reason.  As far as colonization is concerned, you're going to pick the low-hanging fruit first - that is, the compatible systems that are closest will be the first to be colonized.  And, even if you are aware, through remote detection, that other compatible systems are out there, you're not going to need to go until you're pretty well established with the closer ones.&lt;/p&gt;&#xA;" OwnerUserId="1365" LastActivityDate="2015-04-08T16:41:57.890" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8692" PostTypeId="1" CreationDate="2015-04-08T17:01:43.900" Score="8" ViewCount="186" Body="&lt;p&gt;So you are in a high orbit, or Lunar orbit, or at an EML point, and a solar proton event is on its way. Does the Earth's magnetic field make it difficult to predict the direction of impact on your spacecraft? Is the magnetic field of the Moon a significant factor? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe I saw somewhere a diagram showing magnetic lines causing a solar storm to hit the Moon on its night side, but I can't find it now. Can that happen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I ask due to wondering if placing shielding against SPEs on only one part of a spacecraft, and then keeping that turned into the direction of the storm could work. I had also modeled a structure for a lunar base mock-up partly on the idea that if you are shaded from sunlight behind a barrier of sufficient thickness, then solar ionizing radiation is not a concern. But could the influence of magnetism cause dangerous ions to hit from other directions (in particular from above)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Relavant NASA new article: &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2010/21sep_zigzag/&quot; rel=&quot;nofollow&quot;&gt;Solar Storms can Change Direction&lt;/a&gt;, and&#xA;here is a very pretty video that is somewhat relevant, showing the magnetic field lines of Earth warping under the influence of a solar storm:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/EYdQG2YRQnc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-13T17:03:22.680" LastActivityDate="2015-08-08T04:06:37.507" Title="Can the direction of a solar storm be predicted near Earth? Will it come from one direction?" Tags="|mission-design|shielding|space-weather|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8693" PostTypeId="2" ParentId="8653" CreationDate="2015-04-08T18:26:19.663" Score="3" Body="&lt;p&gt;There is a method for measuring underground density using &lt;em&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_gradiometry&quot; rel=&quot;nofollow&quot;&gt;Gravity Gradiometry&lt;/a&gt;&lt;/em&gt; with wide range of applicability, including resource exploration (oil, precious metals, e.t.c.). It is based on measuring &lt;em&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_anomaly&quot; rel=&quot;nofollow&quot;&gt;local gravitational field anomaly&lt;/a&gt;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is &lt;em&gt;Spacecraft-Borne Gradiometer Mission Analysis&lt;/em&gt; conducted by NASA in 1976, summarized in paper by &lt;em&gt;P. Argentiero, R. Garza-Robles&lt;/em&gt; stating that: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;The spacecraft-borne gradiometer is an ideal instrument for globally mapping geopotential fine structure. Mean gravity anomalies exhibit&#xA;  considerable orthogonality in gradiometer data, and local blocks of&#xA;  gravity anomalies in local blocks of gradiometer data can therefore be&#xA;  successfully estimated.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the last decades there have been great increase in precision (exceeding all the rest of the methods) with &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atom_interferometer&quot; rel=&quot;nofollow&quot;&gt;Atom Interferometers&lt;/a&gt;&lt;/em&gt; used for gravitational measurements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on the above, my conclusion is that the most possible method they would use is Specraft-Borne (Atom) Gradiometry.    &lt;/p&gt;&#xA;" OwnerUserId="8845" LastEditorUserId="49" LastEditDate="2015-04-08T18:55:43.760" LastActivityDate="2015-04-08T18:55:43.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8694" PostTypeId="2" ParentId="8683" CreationDate="2015-04-08T19:14:22.333" Score="2" Body="&lt;p&gt;I think you have to extend that a bit, to &quot;(Technological) humans haven't been around long, and on present evidence aren't going to be around all that much longer&quot;.  If the average lifespan of a technological civilization, from the point at which it can produce radio waves (or other emissions) that could conceivably be detected from another star to the point at which it destroys itself through ecologial degradation, averages about two centuries, what's the probability that two will exist simultaneously, close enough for one to detect the other?&lt;/p&gt;&#xA;" OwnerUserId="8445" LastActivityDate="2015-04-08T19:14:22.333" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8695" PostTypeId="1" CreationDate="2015-04-09T01:05:17.020" Score="4" ViewCount="1261" Body="&lt;p&gt;So &lt;a href=&quot;http://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; says that the orbit period at 2000km (upper limit of LEO) is about 127 minutes, but when I calculate...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T=\sqrt{\frac{4 \pi^2r^3}{\mu}}=\sqrt{\frac{4 \pi^2(2000 \text{ km})^3}{398600 \text{ km}^3/\text{s}^2}}=890 \text{ s}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I get 14.8 minutes. Can someone tell me what I'm doing wrong?&lt;/p&gt;&#xA;" OwnerUserId="8825" LastEditorUserId="5963" LastEditDate="2015-04-09T01:12:01.070" LastActivityDate="2015-04-09T22:08:14.600" Title="Low Earth Orbit period calculation issues" Tags="|orbit|low-earth-orbit|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8696" PostTypeId="2" ParentId="8695" CreationDate="2015-04-09T01:12:36.433" Score="12" Body="&lt;p&gt;You're using the orbit's &lt;em&gt;altitude&lt;/em&gt; as the orbital radius. Add the radius of Earth to the orbit altitude to get the actual radius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that an Earth orbit at 2000km radius would be subject to significant lithospheric drag.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-04-09T22:08:14.600" LastActivityDate="2015-04-09T22:08:14.600" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8698" PostTypeId="1" AcceptedAnswerId="8700" CreationDate="2015-04-09T02:49:32.020" Score="22" ViewCount="16404" Body="&lt;p&gt;Ion engines are touted as a great source of propulsion for high deltaV missions that can afford an ample power supply and need low thrust maneuvers; and they are great. But I'd like to know how much they cost.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't seem to find any pricing details or indication on how much it costs to fill a spacecraft with Xenon, or even the pricing Xenon itself. A vague reference that doesn't seem right turns up &lt;a href=&quot;http://www.chemicool.com/elements/xenon.html&quot; rel=&quot;noreferrer&quot;&gt;\$120 for 100g&lt;/a&gt; of the rare noble gas. Alibaba is &lt;a href=&quot;http://www.alibaba.com/showroom/xenon-gas.html&quot; rel=&quot;noreferrer&quot;&gt;filled with prices&lt;/a&gt; for high purity quantities ranging from $15-300 per litre (and at STP, 5.98g/L makes that quite pricey!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I ask because, today, NASA's Michele Gates gave a presentation that shows how they want to evolve SEP for Mars Exploration, and one proposal utilizes &lt;strong&gt;16 metric tonnes&lt;/strong&gt; of Xenon:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Chart from Michele Gates’ presentation shows how NASA plans to evolve solar elec prop technology from ARM to Mars: &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9Lzov.png&quot; alt=&quot;Xenon you crazy&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;- &lt;a href=&quot;https://twitter.com/jeff_foust/status/585857929724559360/photo/1&quot; rel=&quot;noreferrer&quot;&gt;Jeff Foust&lt;/a&gt; on Twitter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which based on my calculations from Alibaba, would be between \$40m and \$815m; which seems just, &lt;em&gt;wrong&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dawn famously used 425kg of Xenon for its Asteroid belt exploration mission (&lt;a href=&quot;http://www.nasa.gov/mission_pages/dawn/spacecraft/&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;), do we know how much it cost to fill her up? Other possible missions to check are Deep Space 1 and GOCE. But I cannot even find out how much Xenon they had on-board. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fundamentally, I'd like to know how much it costs to fill a craft with Xenon, with evidence to support it.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2023-10-14T15:48:31.340" Title="How much does it cost to fill an ion thuster with Xenon for a spacecraft propulsion system?" Tags="|propulsion|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8700" PostTypeId="2" ParentId="8698" CreationDate="2015-04-09T10:19:56.557" Score="14" Body="&lt;p&gt;A June 2005 paper &lt;a href=&quot;http://www.me.mtu.edu/researchAreas/isp/Papers/1092-final_Kieckhafer.pdf&quot;&gt;Energetics of Propellant Options for High-Power Hall Thrusters&lt;/a&gt; (PDF) by Alex Kieckhafer and Lyon B. King, Michigan Technological University, published for the proceedings of the Space Nuclear Conference, San Diego, CA gives a cost of Xenon gas of \$US 850 / kg. That puts 16 metric tonnes of it at \$US 13.6 Million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that's free market price, as is evident from paper's:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Currently Xenon can be purchased for approximately \$5 per standard&#xA;  liter (\$850/kg).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://enu.kz/repository/2010/AIAA-2010-6798.pdf&quot;&gt;Capturing and Recycling of Xenon From A Cryopumped Vacuum Chamber&lt;/a&gt; (PDF) paper by Michael W. Swiatek, Sierra Lobo, Inc., published for the 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference in July 2010 in Nashville, TN (executive summary of it as it pertains to your question):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This gas [Xenon] is trapped by the cryopumps as part of the high&#xA;  vacuum pumping system and is normally discarded through venting after&#xA;  the testing  is completed.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The need for an efficient method of recovering and recycling  xenon &#xA;  became  evident when the cost per liter increased 10 fold in a &#xA;  matter of eight years.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;There have been previous attempts made at the NASA  Glenn Research&#xA;  Center to capture and recycle Xenon gas. The typical commercially&#xA;  available process utilizes a cylinder cooled in a liquid nitrogen bath&#xA;  that acts as a cryopumping surface to capture the Xenon by having it&#xA;  solidify on the tank wall. This process was installed and tested but&#xA;  found to be ineffective due to conductance losses.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Initial estimates of the cost of refining the recycled xenon would be a fourth of the cost of purchasing new.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So NASA could find substantial savings in price of Xenon if they extracted it from cyopumped surfaces on their own, and the paper claims that's what NASA Glenn is already doing. I wasn't able to find any figure, precise or otherwise, on how much saving NASA does make with it and how much Xenon gas costs them, but it could go as low as roughly \$US 250 / kg (high estimate, bluntly amortizing initial investment costs et al., quoted as &lt;em&gt;\$25,000 of hardware purchases to install [one of] the recovery system at GRC&lt;/em&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, in short, it could cost NASA as little as \$US 4 Million per 16 metric tonnes of Xenon gas because they have plenty of cryo-pumped surfaces on other systems where they could extract it from air on their own.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-04-09T10:19:56.557" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8701" PostTypeId="1" AcceptedAnswerId="8708" CreationDate="2015-04-09T11:57:28.700" Score="11" ViewCount="2444" Body="&lt;p&gt;I recently got into a twitter discussion over the ISS attitude. The point of the discussion is the attitude that the ISS mantains while orbiting around the Earth.&lt;/p&gt;&#xA;&lt;p&gt;From my reminescences of Oribital Mechanics, you need the highest inertia axis (usually the &lt;code&gt;y&lt;/code&gt;, for convention) to point towards the Earth, so that the gravitational differential pull will not affect too negatively the Attitude control (and as a neat consequence you can decouple the &lt;code&gt;x-z&lt;/code&gt; control loop from the &lt;code&gt;y&lt;/code&gt; one, plus you receive a side always pointing towards Earth for communication/observation) and I remember that it is somehow connected to the uncontrolled (and unexpected) spin of Explorer 1.&lt;/p&gt;&#xA;&lt;p&gt;The other side argued instead that having a side pointing towards Earth is required to have this communication/observation side (i.e., cause and effect are inverted).&lt;/p&gt;&#xA;&lt;p&gt;Which side is right? Why is it that the ISS always has the &amp;quot;cupola side&amp;quot; towards Earth?&lt;/p&gt;&#xA;" OwnerUserId="5862" LastEditorUserId="6214" LastEditDate="2021-10-09T00:10:55.710" LastActivityDate="2021-10-09T00:10:55.710" Title="Why does the ISS always point the same side towards the Earth?" Tags="|iss|attitude|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="8702" PostTypeId="2" ParentId="8701" CreationDate="2015-04-09T13:14:48.963" Score="7" Body="&lt;p&gt;In a nutshell, to minimise drag caused by the trace of atmosphere up there.  (The panels and radiators can be aligned as necessary in sun or shadow, but the best arrangement for the rest of the hull is to minimise area in the &quot;forward&quot; direction.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're quite right that having a long axis pointed to Earth is more stable, but it conflicts with the drag requirement.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-04-09T13:14:48.963" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8705" PostTypeId="2" ParentId="8653" CreationDate="2015-04-09T15:37:26.767" Score="6" Body="&lt;p&gt;Another recent publication in the media on this topic:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nbi.ku.dk/english/news/news15/mars-has-belts-of-glaciers-consisting-of-frozen-water/&quot; rel=&quot;noreferrer&quot;&gt;Niels Bohr Institute Press Release: &lt;strong&gt;Mars has belts of glaciers consisting of frozen water&lt;/strong&gt;&lt;/a&gt; which cites the &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/2015GL063219/abstract&quot; rel=&quot;noreferrer&quot;&gt;article from Geophysical Research Letters&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Summary:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Using radar measurements from the NASA satellite, Mars Reconnaissance Orbiter, researchers have been able to determine that is water ice. But how thick was the ice and do they resemble glaciers on Earth?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A group of researchers at the Niels Bohr Institute have now calculated this using radar observations combined with ice flow modelling. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also noted in &lt;a href=&quot;http://www.theregister.co.uk/2015/04/08/newlydiscovered_martian_glaciers_could_cover_entire_surface_in_water/&quot; rel=&quot;noreferrer&quot;&gt;this article in The Register: Drill, baby, drill: HIDDEN glaciers ON MARS hold 150bn cubic metres of precious frozen WATER&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2015-04-09T15:37:26.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8706" PostTypeId="2" ParentId="5713" CreationDate="2015-04-09T16:14:41.973" Score="4" Body="&lt;p&gt;When referring specifically to Felix Baumgartner's jump here are a few things taken from &lt;a href=&quot;http://www.theguardian.com/sport/shortcuts/2012/oct/15/felix-baumgartner-skydive-key-questions-answered&quot; rel=&quot;nofollow&quot;&gt;The Guardian's article &lt;em&gt;Felix Baumgartner skydive: the key questions answered&lt;/em&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;What was so special about the suit?&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Baumgartner's suit and helmet were described by his team as &quot;his&#xA;  personal life-support system&quot;. The suit was modelled on those worn by&#xA;  pilots of high-altitude reconnaissance aircraft, but it had never been&#xA;  tried in a free-fall before Baumgartner started testing it. It had&#xA;  four layers consisting of both &quot;breathable&quot; Gore-Tex, and heat- and&#xA;  flame-resistant Nomex. The internal layer was a &quot;comfort liner&quot;. Next&#xA;  was a &quot;gas membrane&quot; that helped to retain air pressure. A &quot;restraint&#xA;  layer&quot; then helped to maintain the suit's shape. Finally, the external&#xA;  layer was constructed of Nomex to protect against fire and temperature&#xA;  extremes. It was made by a US company called David Clark that has been&#xA;  making suits for astronauts and high-altitude aviators since 1941.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;How Much did it cost?&lt;/strong&gt; &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The non-reusable balloon used by Baumgartner cost $250,000, with the wider cost to the sponsor Red Bull of the&#xA;  whole record attempt estimated to have cost many millions of dollars.&#xA;  This will put off the vast majority of copycats, but the high-profile&#xA;  stunt is expected to provoke a surge in interest for parachuting.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Refer to &lt;a href=&quot;http://www.theguardian.com/sport/shortcuts/2012/oct/15/felix-baumgartner-skydive-key-questions-answered&quot; rel=&quot;nofollow&quot;&gt;the article&lt;/a&gt; for some more details about his jump. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, long story short, this was pretty much a stunt and world record attempt so it was extremely expensive. However, there are definitely cheaper ways of testing suits such as one comment mentioned above... vacuum chambers, wind tunnels, and maybe even in use with jet pilots of high altitude. &lt;/p&gt;&#xA;" OwnerUserId="8561" LastEditorUserId="49" LastEditDate="2015-04-09T16:20:00.080" LastActivityDate="2015-04-09T16:20:00.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8708" PostTypeId="2" ParentId="8701" CreationDate="2015-04-10T06:21:37.093" Score="12" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Which side is right?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Neither. Both of you are forgetting torque due to atmospheric drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Except for periods of high beta angle, the Space Station is typically maintained in a Torque Equilibrium Attitude. In this attitude, gravity gradient torque and atmospheric drag torque (ideally) cancel one another out. This means the disturbances that need to be controlled by the control moment gyros on the Station are rather small and are more or less periodic. That they are small means the CMGs can easily handle those disturbances without the need of assistance from thrusters. That they are more or less periodic means the CMGs can operate for a long time before needing to be desaturated.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-04-10T06:21:37.093" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8709" PostTypeId="1" CreationDate="2015-04-10T16:01:59.783" Score="13" ViewCount="3555" Body="&lt;p&gt;28 VDC is a &lt;a href=&quot;https://books.google.com/books?id=mTSSMhcmVbkC&amp;amp;pg=PA342&amp;amp;lpg=PA342&amp;amp;dq=28+v+spacecraft+bus&amp;amp;source=bl&amp;amp;ots=hmuY0tEv79&amp;amp;sig=mmvgTylQtI5gDsZuGlvdzBNRB3E&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=1vInVZuhJI3ZoASFn4CQAw&amp;amp;ved=0CCQQ6AEwAQ#v=onepage&amp;amp;q=28%20v%20spacecraft%20bus&amp;amp;f=false&quot;&gt;super-common spacecraft bus voltage&lt;/a&gt;, and the linked article claims (unsourced) this has aircraft heritage. So is this a true claim and if so why was 28 V an aircraft standard?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And regardless of the veracity of the aircraft claim, what are the sound engineering reasons 28 V has remained a standard. I mean humanity has been building spacecraft for over 50 years, so it must have some inherent advantages over legacy at this point. I’m assuming it’s a combination of solar cell and battery chemistry and the inherent efficiencies of power conversion circuits in electronics.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2015-04-11T00:20:36.333" Title="Why is 28 V a common nominal spacecraft bus voltage?" Tags="|spacecraft|electronics|avionics|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8710" PostTypeId="5" CreationDate="2015-04-10T16:06:31.857" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-04-10T16:06:31.857" LastActivityDate="2015-04-10T16:06:31.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8711" PostTypeId="4" CreationDate="2015-04-10T16:06:31.857" Score="0" Body="Avionics are the electronic systems used on aircraft, artificial satellites, and spacecraft." OwnerUserId="127" LastEditorUserId="127" LastEditDate="2015-04-10T16:38:42.727" LastActivityDate="2015-04-10T16:38:42.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8712" PostTypeId="2" ParentId="8709" CreationDate="2015-04-10T17:02:26.110" Score="9" Body="&lt;p&gt;From an forum titled &quot;The Wings of the Web&quot; on &lt;a href=&quot;http://www.airliners.net/&quot;&gt;Airliners.net&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a conversation asking the same thing and read this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Larger aircraft adopted 24/28 (battery/generator) reduce the weight of&#xA;  the wiring system and components. Since work relates to the Watts it&#xA;  consumes (electrical power) and Watts are a product of voltage and&#xA;  current, the higher the voltage you use the lower the current&#xA;  requirments and the smaller the components (to an extent) and the&#xA;  wiring.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Some aircraft (like the SAAB 340) have a 48 vdc starter that uses the&#xA;  2 on-board 24 volt batteries and connects them in series during a&#xA;  start. So even more power is available with relatively small starter wires.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Most larger aircraft use the battery for starting the APU only. So&#xA;  there is no need to go higher in voltage. But since AC power is&#xA;  plentiful, they use a battery charger that is powered from AC. It&#xA;  still only puts out 28 VDC though.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.airliners.net/aviation-forums/tech_ops/read.main/82465/&quot;&gt;Here&lt;/a&gt; is a link to the orginal forum post. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do realize that your question was directed towards spacecraft specifically, but since they are still in the world of aviation, there tend to be a lot of shared components. Because of this, I assume that the space industry's reasoning would be very similar to the aviation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope that this quote, helps, &lt;/p&gt;&#xA;" OwnerUserId="8561" LastActivityDate="2015-04-10T17:02:26.110" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8714" PostTypeId="2" ParentId="8446" CreationDate="2015-04-10T21:46:07.460" Score="1" Body="&lt;p&gt;&lt;a href=&quot;http://www.princeton.edu/~stengel/LM.pdf&quot; rel=&quot;nofollow&quot;&gt;This&lt;/a&gt; written by Robert F. Stengel for the Journal of Spacecraft and Rockets is a very well documented abstract that provides a great amount of detail about all of the attitude control of the Lunar Module. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is so much detail, that instead of quoting it I will let you read it. I think you will find a lot of answers in there!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that's not enough, there is even more great info &lt;a href=&quot;http://spaceref.com/missions-and-programs/nasa/apollo/apollo-lunar-landing-mission-symposium/apollo-lunar-module-landing-strategy.html&quot; rel=&quot;nofollow&quot;&gt;Here&lt;/a&gt; on spaceref.com in an article titled &quot;Apollo Lunar Module Landing Strategy&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope that this helps! Let me know if it doesn't and I can try to help hunt down answers to this since  it's an interesting question. &lt;/p&gt;&#xA;" OwnerUserId="8561" LastActivityDate="2015-04-10T21:46:07.460" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8715" PostTypeId="1" AcceptedAnswerId="19990" CreationDate="2015-04-10T23:01:32.803" Score="5" ViewCount="201" Body="&lt;p&gt;Can anyone provide a link to a detailed log of shuttle orbiter ferry flights?  I'd like to know dates of all overnight stops.  I can find beginning and end dates for each ferry, but info on the stops is sketchy.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="49" LastEditDate="2015-04-21T01:58:33.673" LastActivityDate="2017-02-01T00:09:17.590" Title="Seeking Shuttle Orbiter Ferry Flight log" Tags="|space-shuttle|history|reference-request|transport|shuttle-carrier-aircraft|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8716" PostTypeId="2" ParentId="8709" CreationDate="2015-04-11T00:20:36.333" Score="6" Body="&lt;p&gt;One reason a technical standard can persist is the availability of off-the-shelf components designed with a given standard in mind, and the economics of developing and manufacturing components around a new standard. Having a catalog of components with established reliability records can be a big time and cost saver over having to accumulate test experience with new designs based around work new standards. This is especially true where reliability matters - such as on a space probe a billion miles from Earth where there's no opportunity to swap out a failed part, or when a component failure can put human life in jeopardy.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-04-11T00:20:36.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8717" PostTypeId="2" ParentId="8446" CreationDate="2015-04-11T00:23:01.803" Score="1" Body="&lt;p&gt;In response to the comment above you can also check out one more article found on &lt;a href=&quot;http://spaceref.com&quot; rel=&quot;nofollow&quot;&gt;SpaceRef.com&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article is titled &quot;&#xA;Apollo Lunar Module Landing Strategy&quot; and can be found &lt;a href=&quot;http://spaceref.com/missions-and-programs/nasa/apollo/apollo-lunar-landing-mission-symposium/apollo-lunar-module-landing-strategy.html&quot; rel=&quot;nofollow&quot;&gt;Here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At staging, the control power of the ascent stage is about 35 deg/sec2&#xA;  for pitch and roll attitude maneuvers. Under emergency manual control&#xA;  where the pilot deflects his attitude hand controller hard-over, there&#xA;  is no attitude rate limitation. Normal manual control commands are&#xA;  limited to 20° /sec and automatic control limited to 10° /sec in pitch&#xA;  and 5° /sec in roll. These attitude rate limitations are important&#xA;  from the standpoint of determining how quickly the ascent stage&#xA;  attitude can be returned to the vertical in the event of an impending&#xA;  tipover.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is one small detail, but this article does talk about ascent in much greater detail and the control that the astronaut would have. I do not think that it mentions the margin of error. It is a very long article, so it might be in there somewhere that I over looked. So yes, they are able to control pitch and yaw, etc but this is the most detail that I can find at the moment so I hope that it explains the &quot;how&quot; a little bit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of the information about this refers to aborts, but the mechanics are mainly the same. &lt;/p&gt;&#xA;" OwnerUserId="8561" LastActivityDate="2015-04-11T00:23:01.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8718" PostTypeId="1" AcceptedAnswerId="8723" CreationDate="2015-04-11T00:37:31.300" Score="10" ViewCount="1908" Body="&lt;p&gt;Have there ever been any Satellites that were hacked by a rogue nation or hacker groups, if not is it possible to accomplish such a task? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hack a satellite and take control of it.&lt;/p&gt;&#xA;" OwnerUserId="6920" LastEditorUserId="168" LastEditDate="2015-04-11T17:59:47.167" LastActivityDate="2023-08-13T15:42:44.913" Title="Have there ever been any Satellites that were hacked, is that a possibility?" Tags="|artificial-satellite|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8719" PostTypeId="1" AcceptedAnswerId="8720" CreationDate="2015-04-11T03:21:15.313" Score="17" ViewCount="2757" Body="&lt;p&gt;There was, briefly, a discussion recently of the use of Mentos and Coke to get to space. It wasn't a serious question but pointed to a way of understanding the core of how rockets work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last year I asked a &lt;a href=&quot;https://space.stackexchange.com/q/5320/4660&quot;&gt;variant&lt;/a&gt; of this and had enough trouble understanding the answers that I decided to leave it until I had seriously brushed up my math and physics. There was a link in one of the answers to &lt;a href=&quot;http://en.wikipedia.org/wiki/Multistage_rocket#Delta-v&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With staging, the delta-v of each stage can be calculated via the&#xA;  rocket equation and summed:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$$\Delta v = \sum_{i=0}^{n-1} V_{\mathrm{e},i} \cdot \ln \left( \frac&#xA; {M_{\mathrm{initial},i}} {M_{\mathrm{final},i}}\right)$$ &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Where $V_\mathrm{e}$ is the effective exhaust velocity,&#xA;  $M_{\mathrm{initial}}$ the initial mass, and $M_{\mathrm{final}}$ the mass&#xA;  of the rocket at the point of burnout of each stage.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;When the $V_\mathrm{e}$ and mass ratios are the same for all the stages,&#xA;  this simplifies to:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$$\Delta v = n V_\mathrm{e} \cdot \ln (M_{\mathrm{ratio}})$$ &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;and it can be seen that the delta-v is limited only by the n, the&#xA;  number of stages.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That bends my mind. It seems to say if you have essentially infinite fuel and stages, you can get to space under any conditions - really high gravity, really poor fuel, really bad engines, gigantic payload, whatever. But that can't be right. There must be some definable limit to all of the different elements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here on Earth, taking the propellant part, what is the very minimum expansion rate - if that is a decent plain English equivalent to specific impulse - needed to get a rocket to orbit? &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-04-13T18:46:43.160" Title="How weak can a propellant be and still get a rocket to space, theoretically?" Tags="|launch|rockets|fuel|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8720" PostTypeId="2" ParentId="8719" CreationDate="2015-04-11T04:18:26.810" Score="27" Body="&lt;p&gt;A vertical &lt;a href=&quot;http://www.eepybird.com/featured-video/coke-and-mentos-featured-video/make-your-own-geysers/&quot; rel=&quot;noreferrer&quot;&gt;Mentos-coke geyser&lt;/a&gt; through a not particularly optimal nozzle reaches an altitude of roughly 6 meters (by my eyeballing), implying exit velocity of a little over 5 m/s, equivalent to an Isp on the order of ~0.5s. (I also see references to 40-foot/12-meter Mentos geysers, implying that figures closer to 8m/s or 0.8s ISP are possible). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The existence of Mentos-coke rocket videos indicates that such a rocket can briefly attain greater than 1:1 thrust-to-weight ratios. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume for a moment that we can create Mentos rockets of any scale with the same structure-to-propellant mass ratios as a two liter bottle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;2L plastic soda bottles mass about 50 grams empty, and hold about 2kg of fuel+payload. Say the payload is 20% (i.e. stage-to-stage mass ratio of 5:1) -- 400g payload and 1600g of propellant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you're looking at... 5 ln(2050/450) = a whopping 7.5m/s of delta V per stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you need &lt;em&gt;fourteen hundred&lt;/em&gt; stages of Mentos rocket to get to orbit. And each stage is 5 times bigger than the previous stage. The ratio of the first stage mass to the last stage mass is thus 5&lt;sup&gt;1400&lt;/sup&gt;, a number with 979 digits in it. Sooooo... yeah, not even theoretically possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(With a stage mass ratio of 20:1 you could get it down to more like 1000 stages. A stage mass ratio of 1000:1 gives you a whopping 35m/s delta V per stage, bringing you down to about 300 stages. Optimize the nozzle, improve the Mentos injector, maybe you can get the exhaust velocity up to 50m/s, now you're down to 30 stages -- but the final stage is still 1000&lt;sup&gt;30&lt;/sup&gt; times bigger, a 91 digit number.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a rather dramatic way to illustrate that &lt;em&gt;linear&lt;/em&gt; increases in delta-V require &lt;em&gt;exponential&lt;/em&gt; increases in rocket size. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you start with &quot;what's the biggest rocket that could plausibly be built&quot; you can work this logic backwards to find out what exhaust velocity you'd need to get a given payload to orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say we're building a 1 million ton rocket -- 300 times the mass of a Saturn V. To my mind, a rocket this size is neither &quot;obviously possible to build&quot; nor &quot;obviously impossible to build&quot;. Again let's go with a 5:1 stage mass ratio, and a 2.5 ton payload - a manned capsule a bit smaller than a Gemini. That's an 8-stage launcher (2.5t x 5&lt;sup&gt;8&lt;/sup&gt; = ~1x10&lt;sup&gt;6&lt;/sup&gt;t). Split the 10km/s orbital delta-V requirement evenly across the stages and you get 1.25km/s each. So:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$1250 = V_\mathrm{e} \cdot \ln 5$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which works out to ~777 m/s exhaust velocity, or ISP of 79s. Interestingly, that's right in line with black powder &lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_impulse#Model_rocketry&quot; rel=&quot;noreferrer&quot;&gt;model rocket engines&lt;/a&gt;! (I'm assuming that the XKCD &lt;a href=&quot;http://what-if.xkcd.com/24/&quot; rel=&quot;noreferrer&quot;&gt;What-If&lt;/a&gt; that Hobbes mentioned assumed ganging individual model rocket engines, rather than building a custom optimized rocket using comparable fuel, hence the different conclusion.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-04-13T18:46:43.160" LastActivityDate="2015-04-13T18:46:43.160" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8722" PostTypeId="2" ParentId="2812" CreationDate="2015-04-11T14:02:07.843" Score="10" Body="&lt;p&gt;I am not aware of any structural dynamics tests conducted on the ISS using crewmembers as 'excitation sources'.  However, the ISS does have a build-in &lt;a href=&quot;http://sem-proceedings.com/24i/sem.org-IMAC-XXIV-Conf-s20p03-Space-Station-Truss-Wireless-Instrumentation-Structural-Dynamics.pdf&quot;&gt;structural dynamics sensor system&lt;/a&gt; consisting of accelerometers and strain gauges.  &lt;a href=&quot;http://sem-proceedings.com/19i/sem.org-IMAC-XIX-190102-Structural-Dynamics-International-Space-Station-Using-Data-from-Laser.pdf&quot;&gt;This paper&lt;/a&gt; describes some early tests where a laser imaging system was also used.  Jet firings and spacecraft dockings probably provide more than enough excitation for the structural dynamics sensor system.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-04-11T14:02:07.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8723" PostTypeId="2" ParentId="8718" CreationDate="2015-04-11T16:59:21.933" Score="10" Body="&lt;p&gt;Yes, it has been done before. Check it out on &lt;a href=&quot;http://geek.com&quot;&gt;geek.com&lt;/a&gt; where you can read some details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article is titled &lt;a href=&quot;http://www.geek.com/geek-pick/chinese-hackers-took-control-of-nasa-satellite-for-11-minutes-1442605/&quot;&gt;Chinese Hackers Took Control Of NASA Satellites for 11 Minutes&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although in this case we aren't sure what they did with it. Nothing malicious seemed to come from it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;IT World talks about it &lt;a href=&quot;http://www.itworld.com/article/2734574/security/two-u-s--satellites-pwned-in--07---08--new-report-blames-china.html&quot;&gt;here&lt;/a&gt;  but the report seems to have been removed. &lt;/p&gt;&#xA;" OwnerUserId="8561" LastEditorUserId="8561" LastEditDate="2015-04-11T17:59:31.390" LastActivityDate="2015-04-11T17:59:31.390" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8724" PostTypeId="2" ParentId="6587" CreationDate="2015-04-11T23:05:50.790" Score="36" Body="&lt;p&gt;The American Petroleum Institute, in its standard 521, outlines limits for exposure of personnel to heat radiation from flares. As hydrocarbons and hydrogen are commonly flared, and also commonly used as rocket fuel, the data is relevant. This publication is used throughout the oil industry worldwide (and therefore is in far wider use than anything produced by any space agency.)&lt;/p&gt;&#xA;&lt;p&gt;Here are the limits from the 1997 edition (a bit easier to interpret for the purpose of this question than the &lt;a href=&quot;http://www.api.org/%7E/media/Files/Publications/Whats%20New/521%20e6%20PA.pdf&quot; rel=&quot;noreferrer&quot;&gt;latest edition&lt;/a&gt;.) The odd numbers are a result of conversion from round numbers of &lt;span class=&quot;math-container&quot;&gt;$\frac{BTU}{hft^2}$&lt;/span&gt;. For comparison, solar radiation is about 1 &lt;span class=&quot;math-container&quot;&gt;$\frac{kW}{m^2}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;9.45 &lt;span class=&quot;math-container&quot;&gt;$\frac{kW}{m^2}$&lt;/span&gt; - Exposure must be limited to a few (approx. six) seconds, sufficient for escape only. May consider tower or structure provide some degree of shielding.&lt;/p&gt;&#xA;&lt;p&gt;6.31 &lt;span class=&quot;math-container&quot;&gt;$\frac{kW}{m^2}$&lt;/span&gt; - Emergency actions lasting up to 1 minutes without shielding but with appropriate clothing.&lt;/p&gt;&#xA;&lt;p&gt;4.73 &lt;span class=&quot;math-container&quot;&gt;$\frac{kW}{m^2}$&lt;/span&gt; - Emergency actions lasting up to several minutes without shielding but with appropriate clothing.&lt;/p&gt;&#xA;&lt;p&gt;1.58 &lt;span class=&quot;math-container&quot;&gt;$\frac{kW}{m^2}$&lt;/span&gt; - Personnel with appropriate clothing can be continuously exposed&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The latest edition reduces the times for 4.73 and 6.31 to 2-3 minutes and 30 seconds respectively, and rather unhelpfully for the point of view of this question, does not specify any time for &lt;span class=&quot;math-container&quot;&gt;$9.45\frac{kW}{m^2}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Let's take an example with a popular engine. According to Wikipedia, a SpaceX Merlin 1-C engine has a thrust of &lt;span class=&quot;math-container&quot;&gt;$420000 N$&lt;/span&gt; and a nozzle velocity of &lt;span class=&quot;math-container&quot;&gt;$2600 \frac{m}{s}$&lt;/span&gt; at sea level, which means a propellant consumption of &lt;span class=&quot;math-container&quot;&gt;$\frac{420000}{2600}=161\frac{kg}{s}$&lt;/span&gt;, about two thirds (by mass) of which is oxygen. The rest (say &lt;span class=&quot;math-container&quot;&gt;$50\frac{kg}{s}$&lt;/span&gt;) is kerosene. The Lower Heating Value (i.e. not considering heat recoverable by condensation of water produced in combustion) of kerosene is about &lt;span class=&quot;math-container&quot;&gt;$43\frac{MJ}{kg}$&lt;/span&gt; so the power of a merlin 1-C is about &lt;span class=&quot;math-container&quot;&gt;$43 \times 50 = 2150\  MW$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$2150000\ kW$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Let's assume we want to at the &lt;span class=&quot;math-container&quot;&gt;$6.31 \frac{kW}{m^2}$&lt;/span&gt; distance and assume (as the API 521 standard does) that the radiation of a combustion source is identical in all directions. To keep the calculation simple, we will assume (for now) that the emissivity of the combustion source is 1: that is, perfect radiation.&lt;/p&gt;&#xA;&lt;p&gt;We now need to calculate the radius of a sphere such that &lt;span class=&quot;math-container&quot;&gt;$6.31 \frac{kW}{m^2}$&lt;/span&gt; radiation will be experienced from a point source of &lt;span class=&quot;math-container&quot;&gt;$2150000\ kW$&lt;/span&gt;. Such a sphere will have an area of &lt;span class=&quot;math-container&quot;&gt;$\frac{2150000}{6.31} = 340729\ m^2$&lt;/span&gt;. As the area of a sphere is &lt;span class=&quot;math-container&quot;&gt;$4\times \pi\times r^2$&lt;/span&gt;, this works out as a distance of 165 m.&lt;/p&gt;&#xA;&lt;p&gt;Two more things to consider: First, a Falcon 9 launch vehicle has 9 engines, not one. to factor this in, we need to multiply by &lt;span class=&quot;math-container&quot;&gt;$\sqrt{9}=3$&lt;/span&gt; so we need to be at &lt;span class=&quot;math-container&quot;&gt;$165 \times 3=495 m$&lt;/span&gt; distance. (Say, 500m.)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Secondly, the emissivity may be quite a bit less than 1&lt;/strong&gt; (values for combustion with oxygen are difficult to come by) but because of the square law it won't make much difference. Opaque smoke can make quite a difference to emissivity, but most rockets burn cleanly once they are clear of the launch pad. A low value for a smokeless flare burning heavy hydrocarbon would be 0.25 (&lt;span class=&quot;math-container&quot;&gt;$\frac{1}{4}$&lt;/span&gt;) so if this is was applicable to a rocket the distance would be halved to &lt;span class=&quot;math-container&quot;&gt;$250 m.$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;I reckon you would survive witnessing a Falcon 9 launch at a maximum radiation of &lt;span class=&quot;math-container&quot;&gt;$6.31 \frac{kW}{m^2}$&lt;/span&gt;, though quite possibly with significant burns. It's a fairly short time before the rocket is well clear of the Earth, but it would be hot and uncomfortable (painful) with 6.31 times the solar radiation in your face. I wouldn't be surprised if you turned and ran.&lt;/p&gt;&#xA;&lt;p&gt;Most propellants are not that toxic. Perhaps the worst exhaust fumes would be from the Space Shuttle solid rocket boosters, which produced aluminium oxide in a fine white powder form which would be very bad for your lungs. I'm pretty sure the heat radiation would still be the limiting factor though.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;EDIT 1&lt;/strong&gt;: The Soyuz launcher has five (quadruple nozzled) engines, of 813 kilonewton thrust and &lt;span class=&quot;math-container&quot;&gt;$2.4\frac{km}{s}$&lt;/span&gt; velocity, giving a total propellant consumption of &lt;span class=&quot;math-container&quot;&gt;$1694\frac{kg}{s}$&lt;/span&gt;. That is marginally more than the &lt;span class=&quot;math-container&quot;&gt;$9 \times 160 = 1440\frac{kg}{s}$&lt;/span&gt; used by the Falcon 9. Therefore I find the claim in the comments that the launch can be watched from &lt;span class=&quot;math-container&quot;&gt;$400 m$&lt;/span&gt; surprising, though it does not conflict with an emissivity of &lt;span class=&quot;math-container&quot;&gt;$0.25$&lt;/span&gt;. The emissivity is something of an unknown, and the cloud of debris and steam at the launch pad would shield the observer from  the heat radiation until the rocket gained some height. It's still closer than I would like to be to a launch.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;EDIT 2&lt;/strong&gt; I am receiving comments that my thermal calculations are an overestimate. I've checked the overall energy release and that at least is correct. So let's see what may be wrong:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The spherical radiation model is an oversimplification. In fact, most of the radiation will be downwards, so this would actually increase the thermal energy felt by an observer on the ground.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I took no separate account of the energy converted to thrust. Wikipedia indicates around 60% efficiency, leaving 40% energy available for emission. I checked this with my own expansion calculation:&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Chamber pressure &lt;span class=&quot;math-container&quot;&gt;$6.77\ MPa$&lt;/span&gt; (Merlin) &lt;span class=&quot;math-container&quot;&gt;$5.85\ MPa$&lt;/span&gt; (Soyuz): consider &lt;span class=&quot;math-container&quot;&gt;$60\ atm$&lt;/span&gt; (approx. &lt;span class=&quot;math-container&quot;&gt;$6\ MPa$&lt;/span&gt;) for convenience.&lt;/p&gt;&#xA;&lt;p&gt;Specific heat ratio: Both CO&lt;sub&gt;2&lt;/sub&gt; and H&lt;sub&gt;2&lt;/sub&gt;O are around 1.3.&lt;/p&gt;&#xA;&lt;p&gt;Heat not converted to thrust = &lt;span class=&quot;math-container&quot;&gt;$\frac{T2}{T1} = 60^{\frac{1-1.3}{1.3}} = 0.389$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;This is surprisingly close to the Wikipedia efficiency value.&lt;/p&gt;&#xA;&lt;p&gt;Given the general uncertainty of emissivity values, I do not consider a factor of 40% to be particularly significant.&lt;/p&gt;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;After some thought, it occurred to me that perhaps the most important difference between a flare (which as a combustion engineer in the oil industry I am very familiar with) and a rocket engine (which I am admittedly less familiar with) is the much greater turbulence with ambient air. This may lead to much greater mixing and a consequently lower emissivity.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;I'm reluctant to make another guess at emissivity, but if it was as low as &lt;span class=&quot;math-container&quot;&gt;$\frac{1}{25}$&lt;/span&gt; (that's just 4% of the heat released being converted to thermal radiation!) my estimate for the minimum non fatal distance from a Falcon 9 would be &lt;span class=&quot;math-container&quot;&gt;$\frac{500}{\sqrt{25}}=100\ m$&lt;/span&gt; (at which distance your hearing would be severely damaged.)&lt;/p&gt;&#xA;&lt;p&gt;It's notable that this is not much different from the radius of the cloud of dust and steam that forms at the launch pad. That debris cloud must be pretty hot (all that heat that doesn't get radiated has to go somewhere) so I think the risk of being killed by flying debris is irrelevant, as the heat would get you anyway.&lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="47149" LastEditDate="2022-05-24T08:55:12.863" LastActivityDate="2022-05-24T08:55:12.863" CommentCount="18" ContentLicense="CC BY-SA 4.0" />
  <row Id="8726" PostTypeId="1" CreationDate="2015-04-12T13:17:06.120" Score="4" ViewCount="3439" Body="&lt;p&gt;On April 10, 2015, Elon Musk tweeted 2 photos of the Dragon spacecraft being prepared for the CRS-6 mission:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z7MkF.jpg&quot; alt=&quot;Dragon Prep for CRS-6&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GxVc1.jpg&quot; alt=&quot;Dragon Prep for CRS-6&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He said &lt;a href=&quot;https://twitter.com/SpaceX/status/586635597940293632&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;points if you notice what's new&quot;&lt;/a&gt;. I'm wondering what is new about this Dragon?&lt;/p&gt;&#xA;" OwnerUserId="8895" LastEditorUserId="8162" LastEditDate="2015-04-12T17:28:34.147" LastActivityDate="2015-04-13T11:44:34.240" Title="SpaceX Dragon, what's new?" Tags="|spacex|dragon|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8727" PostTypeId="2" ParentId="8726" CreationDate="2015-04-12T18:38:32.963" Score="10" Body="&lt;p&gt;It's the Dragon logo! That's it! They changed it from this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/AbOTI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;to this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GWVll.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The logo was leaked to Reddit community /r/SpaceX a while back, and first appeared on the Dragon v2 test article. This is the first time it's making its appearance on a Dragon v1. My understanding for the change is that because Dragon v2 has no &quot;wings&quot; (extendable solar panels), they simplified the logo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, don't be surprised if this is only one of a dozen or more changes in those pictures. It'll be the only public-facing change they'll announce, but SpaceX iterate on nearly every vehicle and rocket they make. There'll be a lot of other changes too.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-04-12T18:38:32.963" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8728" PostTypeId="1" CreationDate="2015-04-13T00:09:45.800" Score="2" ViewCount="452" Body="&lt;p&gt;I am designing a LEO constellation (altitudes between 300-600 to be more specific) for a term project and i have some issues.&#xA;My goal is to design a satellite constellation which takes images of a location on earth as close as possible to 1 hour time interval.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently my setup is like this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;altitude 350 km, inclination 96.5 degrees to achieve sun synchronous orbit.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;6 orbital planes which are 30 degrees apart from each other.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;each plane has 3 satellites which are 120 degrees apart from each other&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;so i have 18 satellites in total.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used this setup on STK, pick a point on earth, used 7 day for calculation interval. I calculated elevation angles for that point on each passes left me only with a few results which are not even close to my goal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what i am asking is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;To achieve my goal, what are things i need to consider while determining orbital parameters.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Number of satellites is not limited to 18.&#xA;Only limiting factor is altitude and interval between passing from same spot by any of the satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not expecting a full solution but if you direct me to the right direction i will be very happy.&lt;/p&gt;&#xA;" OwnerUserId="8898" LastActivityDate="2015-04-13T00:09:45.800" Title="LEO constellation design" Tags="|design|orbital-elements|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8729" PostTypeId="1" AcceptedAnswerId="8730" CreationDate="2015-04-13T08:31:36.720" Score="1" ViewCount="130" Body="&lt;p&gt;I want to be able to &lt;strong&gt;model&lt;/strong&gt; the entire launch mission of the spacecraft &lt;strong&gt;GOES-O&lt;/strong&gt; and its Launcher, &lt;strong&gt;Delta 4,2 medium plus&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it in anyway possible to obtain:-&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;The burn profile&lt;/strong&gt;, (simplified ones are fine like constant burn rate, constant acceleration, etc.). &lt;/li&gt;&#xA;&lt;li&gt;The &lt;strong&gt;time&lt;/strong&gt; at which the &lt;strong&gt;gravity turn&lt;/strong&gt; starts, along with the information about how does the &lt;strong&gt;rate of change of angle&lt;/strong&gt; from vertical take place.&lt;/li&gt;&#xA;&lt;li&gt;The &lt;strong&gt;final velocity&lt;/strong&gt;, &lt;strong&gt;position&lt;/strong&gt; and &lt;strong&gt;angle&lt;/strong&gt; where the spacecraft is released, for eg. &lt;strong&gt;GOES-O&lt;/strong&gt;, in my case. So that I can calculate the further maneuvers on my own. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I have tried searching on the official NASA site, but couldn't get a clear information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I need to obtain the parameters so that I can broadly comment upon the &lt;strong&gt;Trajectory&lt;/strong&gt; followed by the rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be great if someone could provide me with the links or any such related information from their knowledge. I require it for my Project.  &lt;/p&gt;&#xA;" OwnerUserId="5918" LastEditorUserId="6944" LastEditDate="2015-04-13T12:12:03.680" LastActivityDate="2015-04-13T12:12:03.680" Title="How can I obtain the parameters related to launch of Delta 4 Medium?" Tags="|rockets|trajectory|orbital-maneuver|gravity-assist|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8730" PostTypeId="2" ParentId="8729" CreationDate="2015-04-13T08:44:00.200" Score="1" Body="&lt;p&gt;The operations manual is here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Page 5.5.2 (page 825 of the PDF!) has quite a bit of information on angles and timings...  but not enough information to fly the whole things manually, for example.  Bear in mind the pilots would largely be watching the automatic systems fly the machine at that stage, so this manual doesn't give the full information.  It should provide a good starting point though.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-04-13T08:44:00.200" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8731" PostTypeId="2" ParentId="8726" CreationDate="2015-04-13T11:44:34.240" Score="5" Body="&lt;p&gt;Hans Koenigsmann, SpaceX vice president of mission assurance, spoke during the NASA pre-flight press conference and stated that the logo was the only change to the Dragon spacecraft. &lt;/p&gt;&#xA;" OwnerUserId="8895" LastActivityDate="2015-04-13T11:44:34.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8733" PostTypeId="1" CreationDate="2015-04-13T21:01:40.437" Score="8" ViewCount="1875" Body="&lt;p&gt;ULA (United Launch Alliance) mentioned a new upper stage they are working on that will use IVF. Sounds like an internal combustion engine, running a generator, fueled on propellant and oxidizer that would otherwise be vented.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ok, clever way to generate power on an upper stage, since a IC engine can be small and efficient. Burning hydrogen is simple and known technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But they keep talking about how this makes it reusable? But really it sounds like it just provides power, removing the need for solar panels, batteries, hydrazine driven generators, etc.  A nice feature, but how is this a game changer? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure this means the second stage can stay alive much longer, but right now, that does not quite seem to be the major bottleneck? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main reuse suggested seems to be refueling in orbit. But again, that is not entirely what is usually meant by reuse in this context.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-13T23:00:01.247" LastActivityDate="2021-05-24T00:25:04.683" Title="What is the point of ULA's IVF? (Integrated Vehicle Fluids)" Tags="|launch|upper-stage|ula|vulcan|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8734" PostTypeId="5" CreationDate="2015-04-13T21:03:02.490" Score="0" Body="&lt;p&gt;United Launch Alliance booster that is LOX/CH4 powered, with optional SRB's initially using an RL-10 powered upper stage.  Later switching to a new upper stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the replacement for the Atlas and Delta family of boosters.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-13T21:03:02.490" LastActivityDate="2015-04-13T21:03:02.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8735" PostTypeId="4" CreationDate="2015-04-13T21:03:02.490" Score="0" Body="Next Generation Launch System (NGLS) got named Vulcan by ULA." OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-13T21:03:02.490" LastActivityDate="2015-04-13T21:03:02.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8736" PostTypeId="1" CreationDate="2015-04-13T21:05:19.123" Score="5" ViewCount="1077" Body="&lt;p&gt;Is there a current list of space launch attempts, and their outcomes? I'm not just looking for successful launches. I'm also looking to see scrubs and mission failures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The list should include, at minimum:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Information about the rocket type and configuration.&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Example: Delta IV Medium+ (5,4)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Information about the payload and mission.&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Example: USA-244 (WGS-6), Military communications satellite, GTO&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Date and time of planned T-0 or launch window.&lt;/li&gt;&#xA;&lt;li&gt;Launch site name &amp;amp; location.&lt;/li&gt;&#xA;&lt;li&gt;Outcome of the launch attempt.&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mission success&lt;/li&gt;&#xA;&lt;li&gt;Launch scrub&lt;/li&gt;&#xA;&lt;li&gt;Mission failure during launch&lt;/li&gt;&#xA;&lt;li&gt;Mission failure in space&lt;/li&gt;&#xA;&lt;li&gt;Mission failure on return (if return was intended and failed)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;If mission/launch was unsuccessful, reason for failure/scrub.&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Weather&lt;/li&gt;&#xA;&lt;li&gt;Rocket failure/malfunction&lt;/li&gt;&#xA;&lt;li&gt;Payload failure/malfunction&lt;/li&gt;&#xA;&lt;li&gt;Human error&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This idea initially came about out of an interest to compare the histories of different rockets, in terms of their technical reliability. That would require knowing all launch attempts, the rockets used in each, and whether any scrubs were caused by technical issues with the rocket as opposed to other factors. More information would also be interesting for other comparisons.&lt;/p&gt;&#xA;" OwnerUserId="126" LastEditorUserId="49" LastEditDate="2015-04-13T21:06:59.300" LastActivityDate="2015-11-18T23:11:33.317" Title="List of space launch attempts &amp; outcomes" Tags="|launch|history|reference-request|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8737" PostTypeId="2" ParentId="8736" CreationDate="2015-04-13T21:24:32.093" Score="3" Body="&lt;p&gt;I think &lt;a href=&quot;http://www.spacelaunchreport.com/&quot; rel=&quot;nofollow&quot;&gt;http://www.spacelaunchreport.com/&lt;/a&gt; has the info you are looking for, but it may not be in a single report. I.e., you may have to dig around a bit on the site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;note: I am fairly sure it does not list every scrub.  &lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-04-13T21:24:32.093" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8738" PostTypeId="1" CreationDate="2015-04-13T22:20:39.750" Score="3" ViewCount="195" Body="&lt;p&gt;I listened to the annoucement of the Vulcan NGLS booster from ULA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graphics released and show at SpaceFlightNow.com show a line.  &quot;350% more lift&quot; but I could not figure out where that is coming from.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Iropm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First issue is, 350% improvement is 3.5 times more than &quot;what&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CEO said they get about 30% more boost from core engine changes plus 20% from 6th booster. Where is the rest of the increase from? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 30% engine boost would likely be 2 X BE4's vs a RD-180. 1.1 Millions lbs thrust vs 800,000 lbs thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 20% increase from 6th booster makes sense.  If you had 5 and add a 6th that is about a 20% increase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the overall 350% compared to what?  And from what improvements?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will assume he is suggesting with the new upper stage, perhaps to BLEO or GTO/GSO orbits. But they never explain what this all really means.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-13T22:56:38.173" LastActivityDate="2015-04-13T22:56:38.173" Title="How does ULA propose to get &quot;350% more lift&quot; out of the Vulcan design?" Tags="|launch|ula|vulcan|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8739" PostTypeId="1" AcceptedAnswerId="8867" CreationDate="2015-04-14T00:14:22.687" Score="2" ViewCount="301" Body="&lt;p&gt;With the sad demise of the US Space Transportation System (Colloquially known as the &quot;Space Shuttle&quot;) return of payload from orbit is very limited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently the SpaceX Dragon can return the most payload, in the 2000 lbs range.  Soyuz is the only other current option, which has about 90-100 lbs of cargo it can return with the crew. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a Soyuz return empty, no crew, but all payload? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sort of a Soyuz used as a Progress, but mostly for the return payload.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="63" LastEditDate="2015-04-23T03:07:03.627" LastActivityDate="2015-04-23T03:07:03.627" Title="How much payload, could a Soyuz return, without crew?" Tags="|iss|spacex|soyuz-spacecraft|payload|return|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8740" PostTypeId="1" CreationDate="2015-04-14T00:17:41.377" Score="3" ViewCount="120" Body="&lt;p&gt;The first mission to the ISS by Dragonrider (and CST-100) is expected to be unmanned. That means when it returns, it will have ability to return a full cargo load. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, ISS payloads are often volume not mass limited, and a manned Dragon will not have the proper storage 'shelves' and 'fittings' for payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But does the return capacity of Dragon manned (Dragonrider) vs Dragon Cargo differ. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This could be different due to either parachute changes or else SuperDraco landings on land for Dragonrider. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-14T00:17:41.377" Title="How much payload could a manned Dragon return, if unmanned?" Tags="|iss|dragon|payload|dragon-v2|return|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8741" PostTypeId="1" AcceptedAnswerId="8742" CreationDate="2015-04-14T00:36:49.047" Score="26" ViewCount="7564" Body="&lt;p&gt;I just saw some footage of the ISS in which a dish antenna could be seen, constantly rotating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dish looked more-or-less parabolic, meaning that it is presumably sending/receiving along a fairly confined path. The central axis of the parabola (and what appeared to be a bit of terminal equipment at its focus) was inclined something like 45 degrees to its axis of rotation, and was rotating at something like 30RPM. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would seem to suggest that the antenna was essentially sweeping a hollow cone. If used for ranging or detection, it seemed as though it would not be covering a large volume inside the swept cone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that what was actually happening? For what purpose? &lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="741" LastEditDate="2018-04-04T04:48:04.003" LastActivityDate="2022-07-22T01:53:47.103" Title="What is the purpose of the rotating dish antenna on the ISS" Tags="|iss|antenna|identify-this-object|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8742" PostTypeId="2" ParentId="8741" CreationDate="2015-04-14T00:52:10.433" Score="31" Body="&lt;p&gt;You probably mean &lt;a href=&quot;http://winds.jpl.nasa.gov/missions/RapidScat/&quot; rel=&quot;noreferrer&quot;&gt;RapidScat&lt;/a&gt;.  It is a microwave scatterometer that measures near-surface wind speed and direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HswGr.png&quot; alt=&quot;rotating beam&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's RapidScat in action, installed on Columbus module's External Payloads Facility (CEPF), as seen from &lt;a href=&quot;http://www.ustream.tv/channel/live-iss-stream/theater&quot; rel=&quot;noreferrer&quot;&gt;one of ISS external cams&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/cdkOW.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="49" LastEditDate="2015-04-14T01:21:05.297" LastActivityDate="2015-04-14T01:21:05.297" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8743" PostTypeId="1" AcceptedAnswerId="8747" CreationDate="2015-04-14T05:41:58.497" Score="15" ViewCount="1681" Body="&lt;p&gt;From what I gather, the general consensus seems to be that if a satellite collision took place in LEO, then it would probably start a chain reaction, creating a debris cloud that could stop mankind's space travel for a long time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is that whilst this debris cloud will be moving at around eight kilometres a second &lt;em&gt;laterally&lt;/em&gt; with respect to the satellite's orbit, how much would the cloud spread &lt;em&gt;vertically&lt;/em&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, am I wrong in thinking that Kessler syndrome will only create a belt of debris? Is the collision sufficient enough to cause a significant deviation of debris from the original orbit of the satellite?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you in advance, and I note that I am probably wrong, I simply want a clarification. :)&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-04-14T11:25:50.990" LastActivityDate="2015-04-14T12:04:31.883" Title="How bad is Kessler Syndrome?" Tags="|low-earth-orbit|debris|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8745" PostTypeId="2" ParentId="8743" CreationDate="2015-04-14T08:12:24.533" Score="3" Body="&lt;p&gt;Earth (and a LEO orbit) is about 40 million meters around and there are about 30 million seconds per year. So two pieces of debris which move only at walking speed, 1 meter per second, &lt;em&gt;latitudinally&lt;/em&gt; relative to each other after a collision or explosion, could be on the opposite sides of Earth within a year. That part seems to be easy, debris from an explosion in LEO don't stay in an orbiting belt. But how bad it is to space flight is another question which I don't know the answer to.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-04-14T08:20:35.187" LastActivityDate="2015-04-14T08:20:35.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8747" PostTypeId="2" ParentId="8743" CreationDate="2015-04-14T12:04:31.883" Score="15" Body="&lt;p&gt;First of all, virtually all satellites have elliptical orbits. So if a cloud of debris is created around one of these satellites, then it will intersect with more than just those objects at the provided altitude. Some of the satellites are rather elliptical, and those few satellites would no doubt spread the damage to other planes as well. The most elliptical objects are often those with the most potential for damage, spend boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, the impact events are very energetic. The most well known satellite collision was the &lt;a href=&quot;http://en.wikipedia.org/wiki/2009_satellite_collision&quot; rel=&quot;noreferrer&quot;&gt;2009 impact&lt;/a&gt; between two satellites. Debris from that event has threatened the ISS. In fact, there's a nice graph that shows the altitude of the various space junk from &lt;a href=&quot;http://en.wikipedia.org/wiki/2009_satellite_collision#/media/File:Spacedebris_upd_2011.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;. It should be noted that this includes all debris, not just the debris from this event, although a significant portion of space debris came from this event. Note that the debris is spread out. This makes sense, as the two objects hit each other at a very high velocity, and thus can change their orbits quite dramatically (Although the point of impact must remain as an altitude to remain, unless some external force (Atmospheric drag, solar radiation, etc) changes it later). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eNCz4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, a single event seems unlikely to trigger Kessler Syndrome. It does seem quite likely though if the number of objects goes up, we could indeed have such an effect. Satellite collisions are very damaging, and steps should be taken to remove debris if possible, to prevent the problem from getting worse.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-04-14T12:04:31.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8748" PostTypeId="1" AcceptedAnswerId="8753" CreationDate="2015-04-14T14:55:55.383" Score="5" ViewCount="381" Body="&lt;p&gt;An article in today's BBC news &lt;a href=&quot;http://www.bbc.com/news/science-environment-32287609&quot; rel=&quot;noreferrer&quot;&gt;Evidence of liquid water found on Mars&lt;/a&gt; talks about brine (&lt;em&gt;salt&lt;/em&gt;) water forming and running on the surface of Mars.  It treats this as a new discover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have we really just made a discovery? Did we already know about this? If we did not already know (&lt;em&gt;and if it is true&lt;/em&gt;) how is that we are just making this discovery? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2015-04-14T20:40:17.653" Title="Running salt water on Mars, hype or not?" Tags="|mars|water|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8749" PostTypeId="2" ParentId="8733" CreationDate="2015-04-14T17:34:18.500" Score="5" Body="&lt;p&gt;The main advantage of IVF is &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Extended_Duration/Development_Status_of_an_Integrated_Propulsion_and_Power_System_for_Long_Duration_Cryogenic_Spaceflight_2012.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;described by ULA&lt;/a&gt; as weight-saving measure (~1 ton) for longer-duration missions, compared to batteries / photovoltaics / fuel cells:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An  existing  Centaur  goes  from  a  vehicle with nominal two main engine burns and 8 hour flight duration to one capable of 10+ burns and flight durations measured in  multiple  days.  Burnout  mass  is  reduced  by  10%  and  propellant  residuals  can  be  used  to  dispose  of  the  stage &#xA;  without mass or cost penalty.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's claimed to be a very efficient use of chemical energy, too. Because the combustion heat is reused for tank pressurization, and the exhaust stream is reused as reaction mass, almost none of the energy is wasted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/questions/8738/how-does-ula-propose-to-get-350-more-lift-out-of-the-vulcan-design&quot;&gt;your other question&lt;/a&gt;, one of the three claimed benefits of Vulcan is the &lt;strong&gt;10x greater &lt;em&gt;(mission)&lt;/em&gt; duration&lt;/strong&gt;, which seems to be provided entirely by the use of the IVF internal combustion engine:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Iropm.jpg&quot; alt=&quot;Vulcan infographic touting 350% more lift, 10x greater duration, and 100x more on-orbit operations&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="759" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-04-14T17:52:45.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8750" PostTypeId="1" CreationDate="2015-04-14T17:49:53.617" Score="1" ViewCount="111" Body="&lt;p&gt;is there a standard how NASA/ESA categorize hazardous propellant properties like flammability, toxicity, corrosiveness etc?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any links, papers, discussions would be greatly appreciated!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regards&#xA;Christian&lt;/p&gt;&#xA;" OwnerUserId="8724" LastActivityDate="2015-04-14T17:49:53.617" Title="Hazardous Propellant properties" Tags="|fuel|" AnswerCount="0" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="8751" PostTypeId="1" CreationDate="2015-04-14T17:56:36.433" Score="3" ViewCount="745" Body="&lt;p&gt;Is there a freely available and widely used, standard database on liquid rocket propellants with detailed information on the most important properties that is commonly used by NASA/associated aerospace companies?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found lots of references to &quot;Liquid Propellants &amp;amp; Fuels Database (LPFD) of JANNAF Interagency Propulsion Committee&quot;. But I can't find any way to log in (I believe access is restricted to US citizens, I'm German...)&lt;/p&gt;&#xA;" OwnerUserId="8724" LastEditorUserId="4660" LastEditDate="2015-04-15T14:13:01.670" LastActivityDate="2016-04-16T12:30:51.650" Title="Standard Database on Rocket Propellants" Tags="|fuel|reference-request|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8752" PostTypeId="2" ParentId="8741" CreationDate="2015-04-14T19:43:55.753" Score="8" Body="&lt;p&gt;The constantly rotating antenna is a good sign that the &lt;a href=&quot;http://2001.wikia.com/wiki/AE-35_Unit&quot; rel=&quot;noreferrer&quot;&gt;AE-35 unit&lt;/a&gt; is working correctly, and that the ISS astronauts should therefore avoid any unnecessary EVAs regardless of the advice of their onboard computer.&lt;/p&gt;&#xA;" OwnerUserId="8913" LastEditorUserId="8913" LastEditDate="2015-04-14T19:52:51.610" LastActivityDate="2015-04-14T19:52:51.610" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8753" PostTypeId="2" ParentId="8748" CreationDate="2015-04-14T20:40:17.653" Score="5" Body="&lt;p&gt;They haven't actually seen the water.  Curiosity has taken measurements that say the physics allows for small pools of water to form at night.  In such cases, anything that is not forbidden is mandatory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is speculation that inconstant streaks on the sides of craters seen from orbit is due to this phenomenon.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-04-14T20:40:17.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8754" PostTypeId="1" CreationDate="2015-04-14T22:08:27.570" Score="13" ViewCount="2540" Body="&lt;p&gt;I've seen a few Space Shuttle and rocket launches in-person, and it always seemed to me that the Shuttle would launch a bit slower than rockets. Today, I saw the CRS-6 launch of the Falcon 9 and its ascent seemed similarly sluggish. It wasn't until the press conference that I realized CRS-6 and Shuttle had a thing in common - live creatures on board.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've always assumed, but never verified, that the reason for the Shuttle's relative sluggishness was to keep G forces within human tolerance. Sure, some Shuttles would launch slower than others and I'd later realize they had a particularly hefty payload on board. But in general, nearly every unmanned rocket seemed like a Ferrari next to a golf cart in comparison.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the press conference, the SpaceX representative was asked about whether CRS-6 appeared unusually slow. The representative couldn't attest to the launch's appearance, since he was watching status screens rather than the rocket itself. But he did say that everything appeared nominal, and the payload wasn't especially large - so there's presumably no reason for this launch to have been slower than any other of its kind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are G force tolerances really taken into consideration when launches are planned with are living things on-board a spacecraft? Or is it just a simple matter of certain rockets having different performance capabilities, and different payloads affecting them? Or, are there other factors simply affecting the perception such as launch trajectories being more/less toward/away from the viewer?&lt;/p&gt;&#xA;" OwnerUserId="126" LastActivityDate="2015-04-15T16:19:42.363" Title="Do payloads with living creatures get launched slower to minimize G forces?" Tags="|launch|performance|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8756" PostTypeId="2" ParentId="8754" CreationDate="2015-04-14T23:27:48.463" Score="12" Body="&lt;p&gt;Launchers generally start at full throttle, so for the most part their immediate performance off the pad is determined solely by the type of launcher and mass of payload. Falcon 9's initial acceleration is quite modest, about 1.5 m/s&lt;sup&gt;2&lt;/sup&gt; vertically (thrust to weight ratio about 1.15, cancelling and exceeding 9.8m/s&lt;sup&gt;2&lt;/sup&gt; of surface gravity). Shuttle's initial TWR was higher, IIRC about 1.5:1 for initial acceleration on the order of 5 m/s&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't have great sources for this information; I generally look at the Wikipedia specs, do the math, and keep a grain of salt handy. First stage sea-level thrust in kN divided by total mass in metric tonnes = free acceleration in m/s&lt;sup&gt;2&lt;/sup&gt;. Either divide by Earth's surface gravity (&lt;em&gt;g&lt;/em&gt;, 9.81 m/s&lt;sup&gt;2&lt;/sup&gt; -- or just call it 10 for back-of-the-envelope estimates) to get TWR, or subtract &lt;em&gt;g&lt;/em&gt; to get acceleration-off-the-pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did note when watching Japan's Hayabusa 2 launch on an H-IIA rocket that the takeoff acceleration was quite dramatic, and according to Wikipedia's numbers the launch TWR would be about 1.6:1, for 5.6 m/s&lt;sup&gt;2&lt;/sup&gt; off the pad. An Atlas V 401 with 9 ton payload has TWR ~1.16 (1.6 m/s&lt;sup&gt;2&lt;/sup&gt;) at takeoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That initial acceleration, though, is always much lower than the peak acceleration of the launcher, because thrust remains more or less constant while fuel mass is thrown away. As noted in &lt;a href=&quot;https://space.stackexchange.com/questions/7829/launch-accelerations-values-history/7857#7857&quot;&gt;my answer to a related question&lt;/a&gt;, some launchers shut down one or more engines in a cluster while others throttle back in order to cap acceleration at a level that's comfortable for the payload. Saturn V goes from 5 engines to 4; Falcon 9 1.0 goes from 9 engines to 7 while Falcon 9 1.1 uses its deeper engine throttle capability; Shuttle drops the solid boosters and throttles back the SSMEs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the typical case, peak acceleration is reached at the instant before the first stage engines are cut off, when first stage propellants are depleted. So you need to know the thrust at that point (which is non-obvious, if the engines are throttleable or clustered), divided by the total launch mass of the rocket less the first stage fuel &amp;amp; oxidizer. That information can often be found on &lt;a href=&quot;http://www.spaceflight101.com/atlas-v-401.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight101&lt;/a&gt; - for Atlas V this would imply somewhere between 4.2g and 8.4g depending on throttle, most likely on the low end of that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that it can be difficult to judge the acceleration of a rocket visually; unless you're very consciously aware of their overall size, larger vehicles are going to seem slower-moving than smaller ones when seen from a distance.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-04-15T16:19:42.363" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8757" PostTypeId="2" ParentId="8754" CreationDate="2015-04-15T06:24:25.253" Score="5" Body="&lt;p&gt;They aren't launched slower but they are launched in different trajectories. This can be to control abort conditions and G-Loads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most efficient launch is to get as high as possible before going for speed. The STS however was shallower to allow for the abort modes and control the forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best example I can think of at the moment is the man rating of the Atlas V for commercial crew. The shallower trajectory is one of the things they need to deal with in order to get it approved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also important to point out that rocket launches can be quite deceptive. Rockets are massive and can look quite slow when far away. Even a change in cameras can change the perceived speed.&lt;/p&gt;&#xA;" OwnerUserId="1076" LastActivityDate="2015-04-15T06:24:25.253" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="8758" PostTypeId="2" ParentId="8623" CreationDate="2015-04-15T12:21:18.153" Score="5" Body="&lt;p&gt;I think this is the closest to a true answer I've seen:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ase.tufts.edu/cosmos/print_images.asp?id=7&quot; rel=&quot;noreferrer&quot;&gt;http://ase.tufts.edu/cosmos/print_images.asp?id=7&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/PtJOC.jpg&quot; alt=&quot;superrotation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both the image and the explanation seem to suggest a super-rotation that exists &lt;em&gt;above&lt;/em&gt; a lower layer of Hadley cells. Even the original image that I posted depicted both things on the same image, although it wasn't clear what the relative locations were. I also read somewhere (can not remember where now) that Hadley cells are what give momentum to sustain the super rotation. If I imagine some kind of asymmetry at the boundary between the two (not difficult to do), then it can make sense.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2015-04-15T12:21:18.153" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8759" PostTypeId="1" CreationDate="2015-04-15T16:12:16.520" Score="4" ViewCount="492" Body="&lt;p&gt;I was thinking that placing RCS on the top of the rocket could help manouver it at low speed where the fins are useless. Than i read somewhere that cool gas propellant are not enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we know that SpaceX is developing the SuperDraco engine. Do you believe that it would help?&lt;/p&gt;&#xA;" OwnerUserId="8925" LastEditorUserId="51" LastEditDate="2015-04-15T16:23:39.737" LastActivityDate="2016-01-29T20:02:04.637" Title="Can a SuperDraco engine help landing the SpaceX 1st stage?" Tags="|spacex|landing|design|draco|super-draco|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8760" PostTypeId="2" ParentId="8759" CreationDate="2015-04-15T16:21:51.243" Score="4" Body="&lt;p&gt;There was discussion on using the Super Draco design to land the second stage, but SpaceX has mostly given up on second stage recovery for Falcon. (NOT for the BFR booster that will be the vehicle to Mars). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Super Dracos do not gimble, so using them for directional control would not help that much.  Except perhaps differential throttling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The major advantage to a Super Draco is the lower thrust it delivers. The problem with landing a Falcon 9 V1.1 stage is that the Merlin 1D can only throttle down to about 70% and that still generates more thrust (about 100Klbs of thrust) than the empty stage weighs.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means they cannot hover nor slow down slowly. They have to come in hot, in what they call a hover-slam. This means decelerate down to zero m/s at zero height. Which looks really fast as they come in. This is why they spent so much practice time on it with the Grasshopper test vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The downside to a SuperDraco engine on the first stage is that it is more dead weight that is otherwise not needed if they could control it without the SuperDraco. On the Dragon capsule it is used on every mission. Either to abort, or to land. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-15T16:21:51.243" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8761" PostTypeId="1" CreationDate="2015-04-15T20:19:19.040" Score="3" ViewCount="621" Body="&lt;p&gt;How important is the mass penalty for shielding when using nuclear power on human space missions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to solar and cosmic radiation, living next to a nuclear reactor large enough to support a human mission and maybe also energy for propulsion, doesn't help. I've seen suggested solutions such as keeping it far off on a long truss on a spaceship (why not in a long cable?). Simply having enough shielding so one could sleep on top of it, doesn't seem to be a popular option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could it be enough with a smart architecture that puts already necessary mass such as heat radiators, water and supplies storage and the solar storm bunker between humans and the reactor?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-04-15T20:28:34.120" LastActivityDate="2015-06-02T22:45:17.427" Title="How heavy does radiation shielding have to be for a nuclear power plant on a crewed spaceship?" Tags="|radiation|humans|nuclear|shielding|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8762" PostTypeId="1" CreationDate="2015-04-15T20:47:21.310" Score="11" ViewCount="3964" Body="&lt;p&gt;Does the Falcon 9 first stage use an explosive FTS? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An FTS can operate in many different ways. The key goal is to terminate thrust, break up large chunks of burny stuff, and avoid killing people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A solid booster, you can just blow the cap off the front, and now you have terminated thrust (It blows out both ends, no net thrust).  But you might also want to split it down its length so as to reduce the size of burning chunks of propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A liquid fuel tank, you can run explosives down the length of it, to unzip the tank when needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A liquid fueled engine, if you trust the engines, you can kill the turbopumps, no more thrust. Or maybe you can quench the ignition, and run the turbo pumps to empty the tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was some discussion around the Falcon 9R-Dev1 loss that the FTS system was not explosive, rather it just ran the tanks dry, killing thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does the production Falcon 9 first stage use?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimate reason for asking is a short lived video was available that showed the failed landing on the CRS-6 mission, as the stage touched down, tipped over, it started to burn, then something clearly went kaboom, and things exploded outward.  I was wondering if that was the FTS going off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Watch the end of &lt;a href=&quot;https://pbs.twimg.com/tweet_video/CCqLieHVEAA6VzN.mp4&quot;&gt;this video&lt;/a&gt; to see the explosions. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-15T20:56:00.527" LastActivityDate="2015-04-16T11:26:04.633" Title="Does the Falcon 9 (v1.1) first stage have an explosive Flight Termination System?" Tags="|launch|spacex|falcon-9|flight-termination-system|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8763" PostTypeId="1" CreationDate="2015-04-15T20:52:03.490" Score="7" ViewCount="1030" Body="&lt;p&gt;At what point in the flight do the legs deploy on a Falcon 9R landing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Originally they were described as opening fairly early, to help avoid spinning the stage, (cause of first landing failure on Cassiopia launch). &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-07-12T14:00:14.477" LastActivityDate="2015-07-12T14:00:14.477" Title="When do the landing legs on a Falcon 9R first stage deploy in a landing attempt?" Tags="|launch|spacex|landing|falcon-9|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8764" PostTypeId="2" ParentId="8763" CreationDate="2015-04-15T20:55:06.280" Score="6" Body="&lt;p&gt;Looking at the videos posted of the landing, in the last 10 seconds or so of flight the legs extend at least halfway, or at least until they are parellel to the engine exhaust region.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only in the last second or three as it approaches the ground (or barge) do they fully extend. The extending of he legs can be shown in these two pictures, where you can see it change from partially open to fully open just before touch down:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/p9zo1.jpg&quot; alt=&quot;Falcon 9 Approaching the landing barge&quot;&gt;  &lt;img src=&quot;https://i.stack.imgur.com/idGRB.jpg&quot; alt=&quot;Falcon 9 above the barge&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likely this is done to protect them from the hot exhaust exposure any longer than needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it does seem like they need 4 valves (one per leg) to work twice instead of just once and adds a level of complexity and possible failures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also see that in this &lt;a href=&quot;https://pbs.twimg.com/tweet_video/CCqLieHVEAA6VzN.mp4&quot; rel=&quot;nofollow noreferrer&quot;&gt;sped up GIF&lt;/a&gt; of the landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk &lt;a href=&quot;https://twitter.com/elonmusk/status/536268250620125185&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweeted&lt;/a&gt; that by the 21st launch or so, they hope to make more use of the legs as aero surfaces. This implies earlier opening in later missions. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-06-14T20:04:41.573" LastActivityDate="2015-06-14T20:04:41.573" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8765" PostTypeId="1" AcceptedAnswerId="8767" CreationDate="2015-04-15T21:59:59.780" Score="2" ViewCount="1196" Body="&lt;p&gt;As I understand it, the Merlin 1D engine on the Falcon 9 that is used to land the first stage, is fairly limited in how low it can throttle.  This makes landing on the barge more difficult -- a suicide burn of sorts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How low can the engine throttle?&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-04-16T02:38:58.947" LastActivityDate="2015-04-16T02:38:58.947" Title="How low can the single Merlin engine on the Falcon 9 first stage throttle?" Tags="|spacex|reuse|falcon-9|merlin-1d|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8766" PostTypeId="1" CreationDate="2015-04-15T22:11:01.537" Score="6" ViewCount="416" Body="&lt;p&gt;Looking at this pretty awesome video of the CRS-6 first stage &lt;em&gt;almost&lt;/em&gt; making it back to the barge, it sort of seems that if something could have steadied it just a bit, it would have been okay. Attitude jets are visible firing at the upper end of the stage at 6 seconds of the video. Does the tip-over point to an issue with the attitude controls, or just a matter of refining them?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/BhMSzC1crr0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="51" LastEditDate="2015-04-16T02:15:39.543" LastActivityDate="2015-05-20T22:05:56.350" Title="Would it have taken much to steady SpaceX's CRS-6 returning stage so it landed alright?" Tags="|launch|spacex|reuse|falcon-9|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8767" PostTypeId="2" ParentId="8765" CreationDate="2015-04-15T23:00:14.783" Score="4" Body="&lt;p&gt;Wikipedia and Spaceflight101 both say 70%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D&lt;/a&gt; &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spaceflight101.com/falcon-9-v11.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the other specs given on the Spaceflight 101 page, this implies close to 2.5:1 TWR at touchdown (~15 m/s&lt;sup&gt;2&lt;/sup&gt; deceleration) on an empty tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://twitter.com/elonmusk/status/462104679116050432&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweet from Musk&lt;/a&gt; says &quot;~40%&quot; but if you read the responses to the tweet you'll see that's rather ambiguous. If it could actually reach 40% rated thrust that would be a much less dramatic deceleration in the hover-slam. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Someone with more free time than myself could count pixels and frames in the &lt;a href=&quot;https://space.stackexchange.com/questions/8766/would-it-have-taken-much-to-steady-spacexs-crs-6-returning-stage-so-it-landed-a&quot;&gt;CRS-6 landing video&lt;/a&gt; to settle the question (of deceleration rate; you'd need to know the final mass of the stage to get the thrust/throttle figures).&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-04-15T23:23:58.993" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8769" PostTypeId="2" ParentId="8766" CreationDate="2015-04-16T07:58:24.677" Score="5" Body="&lt;p&gt;If you watch the video again you'll see the main engine is gimballing - that is, changing the angle at which it points.  It goes through a few sweeps as it attempts to correct itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a well known problem in physics, the &quot;inverted pendulum&quot; problem, where a body can be held with its upper end vertical, simply by controlling the movement of its lower end.  (Try this at home by balancing a broom handle.)  This is being done here by sideways deflections of the main engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, there are the gas thrusters at the top which can help.  (I suspect these assist in crosswinds but I'm not sure.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What seems to be wrong here is the main engine - either its pointing angle went wrong or it didn't maintain the correct thrust while at an angle, causing the lower end to start over correcting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short answer: they need to do a bit more work on the software and on the understanding of the dynamic behaviour of the system (possibly including nonlinear behaviour in the rocket engine).  The rocket probably already has everything else it needs to work.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-04-16T07:58:24.677" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8770" PostTypeId="2" ParentId="8762" CreationDate="2015-04-16T08:11:37.930" Score="11" Body="&lt;p&gt;Yes, Falcon 9 does include an active FTS system, that is in control of the range safety officer. Falcon 9 specifically uses a detcord that runs down the length of the tank. When activated, it zips the stage open, causing the LOX &amp;amp; RP-1 to ignite instantaneously in midair. This is the best approach, the quicker you can combust all the reactants, the better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not everyone knows it, but the most famous use of Falcon 9's FTS is on F9R-Dev1 Flight 13, a.k.a., the last flight of Grasshopper's successor at McGregor, Texas:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HWzuq.jpg&quot; alt=&quot;F9R&amp;#39;s final moments.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/embed/lnqjnxfjgUk&quot; rel=&quot;noreferrer&quot;&gt;Watch the video here.&lt;/a&gt; What occurred was a sensor malfunction in F9R's engine(s) - this caused the rocket to stray outside of its predicted flight path, and for its Instantaneous Impact Point (IIP), the place it would land if on a parabolic trajectory at that moment, to be outside of SpaceX's predefined safe zone. The rocket was automatically terminated by the FTS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9v1.1 is a bit different. It doesn't have an automatic termination system, it's manually controlled, through C-band radio:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Rocket has a Flight Termination System consisting of two strings of transmitters, receivers and safe and arm devices. The FTS works with C-Band Communications and can be used to terminate the flight in case of any anomalies.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It has never been called upon operationally, and would not have helped CRS-5 or CRS-6. &lt;a href=&quot;https://youtu.be/csVpa25iqH0?t=23m13s&quot; rel=&quot;noreferrer&quot;&gt;Listen in to the CRS-6 webcast&lt;/a&gt;. At T+7:16, the controller reads out &quot;Stage 1 FTS has been safed&quot;. This occurs after the reentry burn and prior to the landing burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding is that the reasoning for this is that F9 has so little fuel left at this point it is not necessary to have the FTS armed, plus having a landed rocket with an active FTS system is more risky than just safing it. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="415" LastEditDate="2015-04-16T11:26:04.633" LastActivityDate="2015-04-16T11:26:04.633" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8771" PostTypeId="1" AcceptedAnswerId="8776" CreationDate="2015-04-16T12:06:50.687" Score="20" ViewCount="18795" Body="&lt;p&gt;Presumably, the booster would be a very light, very tall tube (Most of the fuel spent) standing on 4 fairly small legs on a ship in the middle of the ocean. It would also still contain some LOX and RP1 that would start boiling fairly soon after landing. The booster appears to be fairly prone to exploding based on the past 2 landing attempts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, what steps will be taken to secure the booster to the ship and drain the fuel and how likely is it to fall over or explode before this is done?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="12102" LastEditDate="2016-07-19T01:05:59.933" LastActivityDate="2016-07-19T01:05:59.933" Title="How stable would a Falcon 9 first stage be after it has landed on a drone ship?" Tags="|spacex|reuse|falcon-9|first-stage|" AnswerCount="7" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8772" PostTypeId="1" AcceptedAnswerId="8778" CreationDate="2015-04-16T12:42:25.653" Score="10" ViewCount="1944" Body="&lt;p&gt;My thinking is that:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The primary reason to try to land on a barge instead of land appears to be safety (&lt;a href=&quot;https://space.stackexchange.com/questions/7753/why-did-spacex-attempt-to-land-the-booster-on-a-barge-instead-of-somewhere-on-la&quot;&gt;Answers to this question&lt;/a&gt;). Ultimately, it looks like they want to return it to land.&lt;/li&gt;&#xA;&lt;li&gt;SpaceX have shown in the past 3/4 attempts that it can guide the booster to a precise location for landing.&lt;/li&gt;&#xA;&lt;li&gt;At least the second last attempt, they could not attempt recovery due to ocean swells that were too high.&lt;/li&gt;&#xA;&lt;li&gt;The ocean weather / salt can damage the booster on return to land.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Based on the above, why not just clear the immediate area of the landing pad and test landing on land from now on?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-12-29T23:57:12.950" Title="Could SpaceX start testing to return the Falcon 9 first stage to land instead of water?" Tags="|spacex|landing|reuse|falcon-9|first-stage|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8773" PostTypeId="2" ParentId="8772" CreationDate="2015-04-16T12:50:46.637" Score="15" Body="&lt;p&gt;SpaceX has leased LC-13 on the Florida coast, and renamed it Landing Complex 1. Then renamed to Landing Zone 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are building 5 landing pads there, but the Environmental assessment (the document you have to get to be able to start building and construction) says they will only land one at a time. (So Heavy's three cores not quite yet). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gwynne Shotwell in interviews post landing is suggesting that they should try for land on the next one.  As noted, they have proven they can hit the target.  Land is most stable so should be easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is all about assuaging the FAA's concerns for safety. Whatever the FAA says, goes in this instance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then December 21, 2015, on the Orbcomm OG2 mission, the first stage (F9-21) was landed successfully at LZ-1 in Florida.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-12-29T23:57:12.950" LastActivityDate="2015-12-29T23:57:12.950" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8774" PostTypeId="2" ParentId="8772" CreationDate="2015-04-16T13:00:42.113" Score="6" Body="&lt;p&gt;There are two reasons to attempt a landing on the sea vs on land:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;There are no safety requirements to land on sea, at least not beyond the obvious ones of clearing the immediate area. On land, one has to be subject to the laws of the country in which they are attempting the landing, so in other words, the United States FAA.&lt;/li&gt;&#xA;&lt;li&gt;Less fuel is required to land at sea. By the time the first stage is dropped, it has considerable horizontal velocity. Landing it at sea will allow one to take advantage of that velocity, while returning it back to where it came from requires one to fight the velocity.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Of course, landing on the sea has plenty of challenges as well, as have been mentioned. Still, no doubt the ability of SpaceX to hit the barge twice in a row is a good indicator that the system is safe enough to attempt and land on land, so long as all of the standard precautions are in effect (Range safety). The latest launch is the first time where it seems it would have been successful on land if that is where it had been attempted. I think they still need to prove out the system a bit more before they are willing to try on land. Remember, the previous attempt hit the mark, but the rocket immediately flew off after hitting the barge, at considerable speed. That would be potentially dangerous on land. I suspect a few more similar level of successes at sea would be sufficient for them to be able to try on land, but I believe it is not yet safe enough to try.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, it seems like it could take quite a bit more fuel. Granted, most of the horizontal velocity will take place in the first part of the flight, but there is a substantial velocity that will have to be negated. I'm sure, however, that they would prefer to land on land given the chance.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="8952" LastEditDate="2015-04-17T12:54:09.983" LastActivityDate="2015-04-17T12:54:09.983" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8775" PostTypeId="2" ParentId="8771" CreationDate="2015-04-16T13:34:31.243" Score="5" Body="&lt;p&gt;This &lt;a href=&quot;http://www.nasaspaceflight.com/2015/01/dscovr-recover-spacex-chase-cigar-falcon-9-mission/&quot;&gt;article&lt;/a&gt; says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Recent information notes that the rocket will be “tied down” via chains that extend from the engine section to the deck.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;but doesn't give any sources. It's not clear if the chains are already present on the F9, or are part of the structure of the barge platform.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since SpaceX refers to the barge as a drone, it's not clear how the chains would be secured.&lt;/p&gt;&#xA;" OwnerDisplayName="user8406" LastActivityDate="2015-04-16T13:34:31.243" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8776" PostTypeId="2" ParentId="8771" CreationDate="2015-04-16T13:34:44.950" Score="13" Body="&lt;p&gt;There's not much reliable data out there to estimate this ourselves, and SpaceX isn't exactly forthcoming with details. Or perhaps they would be, but media aren't asking the right questions during press conferences. So far, the only &lt;em&gt;somewhat&lt;/em&gt; reliable data point is the canceled recovery attempt during the DSCOVR launch on February 11, 2015;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.spacex.com/press/2015/02/11/dscovr-launch-update&quot; rel=&quot;noreferrer&quot;&gt;SpaceX press release&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The drone ship was designed to operate in all but the most extreme&#xA;  weather. We are experiencing just such weather in the Atlantic with&#xA;  waves reaching up to three stories in height crashing over the decks.&#xA;  Also, only three of the drone ship’s four engines are functioning,&#xA;  making station-keeping in the face of such wave action extremely&#xA;  difficult.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This would suggest that SpaceX are confident they could land and keep the first stage stable on the drone ship as long as it can provide a stable surface to land on and smooth out the swelling of the sea during strong lower atmosphere winds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for a bit of speculation on my part; Stability of the landed first stage, assuming weather permits it and all four of its landing legs are properly extended and on the platform, shouldn't be such a big deal even with occasional (not gale force) gusts of wind. Practically exhausted of propellants, with perhaps up to a few seconds of literally fumes of kerosene remaining (that's why it appears to explode so fast once the propellant tanks are breached on impact, or what Elon Musk called &lt;em&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/588490238936240129&quot; rel=&quot;noreferrer&quot;&gt;closer to a fast fire than an explosion&lt;/a&gt;&lt;/em&gt;), the first stage's center of gravity would be low. If position of the landing legs' mount points suggest where exactly, and those are quite low, it would have to be in the lower 1/3 of its length, possibly quite a bit lower, since lateral pressure on the truss increases with mount point's distance to CoG (law of the lever).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sadly, I can't estimate this analytically, because ideal pressure angle isn't known. For structures that can withstand pressure equally both in lateral and long axis direction, that would be 45°. But rocket stages can't be built like that, so this angle would be substantially higher, in favor of pressure along the long axis (vertically). If we knew that angle at legs' mount point ring (that's got to be reinforced for lateral forces, but how much?), and exact point of CoG, and we already know rough total weight of near dry landed stage (it was mentioned during the CRS-6 post-launch news conference), stage's height, how much legs extend and how high they suspend the stage (can be estimated through photographs), we could extend stress line from CoG, at that given angle, towards the landing surface and compare that to stage's total height. Then, with its know weight and surface area, we could calculate wind shear it could withstand laterally before it tips over. Sadly, as we lack crucial data, not even such estimates could be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, I wanted to write this so we identify data that we still need to (roughly) analyze landed first stage's stability, and why with too many unknowns, it can't be done. I'll trust that SpaceX can do, and did do, extended analysis on (to them) known structural properties of their first stages and will take their word for it - that if the barge can handle it, landed first stage should too. But I wouldn't rule out possible future use of mobile anchor shoes to clamp landing legs in place with their weight, or some other means of achieving this as the barge heads back to land with recovered stage on it, or prior to refueling and flying it to shore, as SpaceX indicated they eventually intend on doing. Of course, ultimate goal is still landing on terra firma, once they demonstrate to FAA and other authorities that it's safe. But that's a different story.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="25" LastEditDate="2016-03-11T17:34:49.287" LastActivityDate="2016-03-11T17:34:49.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8777" PostTypeId="2" ParentId="8771" CreationDate="2015-04-16T14:05:47.027" Score="8" Body="&lt;p&gt;In order for such an object to be stable, it has to have it's center of mass over the area of it's base. It's a bit tricky to know exactly where the center of mass is in such a rocket, but it must be fairly low, as there is little fuel left and the engine is one of the heavier parts of the rocket. Wind is also a factor in how stable it must be, as that will tend to tilt the rocket. My guess is it has to remain vertical with a few degrees of tilt allowed. From &lt;a href=&quot;http://www.wired.com/2015/04/analysis-falcon-9-crash-landing/&quot;&gt;Wired's analysis&lt;/a&gt;, it seemed to lose control when it hit more than 5 degrees of tilt, with a high degree of uncertainty. Still, I'd say that the rocket must remain within a few degrees, or else it will not remain steady. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for what can be done, there are a large number of things. The basic idea is, if you can fix the rocket to the barge, then it's the center of mass of the barge that makes a difference. If you can fix the rocket, then you could simply release the oxygen to keep the rocket from exploding. The kerosene is not as dangerous to keep around for a while, so presumably someone could help with that, or it could wait for the drainage of the oxygen.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-04-16T14:05:47.027" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8778" PostTypeId="2" ParentId="8772" CreationDate="2015-04-16T16:30:55.343" Score="10" Body="&lt;p&gt;&lt;a href=&quot;http://www.defensenews.com/story/defense/air-space/space/2015/04/15/spacex-ground-attempt-reusable-landing-sea/25827625/&quot;&gt;According to Defense News&lt;/a&gt;, SpaceX plans to make the next attempt on land:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;While not providing details of when or where that attempt would occur, Gwynne Shotwell, SpaceX President and COO, told Defense News on Wednesday that the company hopes its next attempted landing will take place on land, not at sea.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8938" LastActivityDate="2015-04-16T16:30:55.343" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8779" PostTypeId="1" CreationDate="2015-04-16T18:06:42.997" Score="25" ViewCount="6546" Body="&lt;p&gt;The descending Falcon 9 has got to hit (preferably gently) the tiny target of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot;&gt;Autonomous Spaceport Drone Ship (ASDS)&lt;/a&gt; in the middle of the ocean. How does the Falcon 9 know where the ASDS is?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radar seems the obvious solution, but on the Falcon or the ASDS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Updated: Now that a Falcon 9 first stage has successfully landed on land, there are two cases to consider. Are different systems used for landing on the ASDS and on land? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JzQOD.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JzQOD.png&quot; alt=&quot;Falcon 9 landing attempt on ASDS&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/hTCKg.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hTCKg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-12-25T20:52:15.347" LastActivityDate="2019-04-03T10:55:37.527" Title="How does the Falcon 9 know where the landing barge/pad is?" Tags="|landing|falcon-9|guidance|asds|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8780" PostTypeId="2" ParentId="8779" CreationDate="2015-04-16T18:54:04.527" Score="13" Body="&lt;p&gt;The issue is interesting, since the Bezos patent is specific about the barge and the rocket handshaking and talking directly to each other.  But SpaceX is contesting the patent (obviously, since they are actually doing it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Almost certainly the initial location is sent by Mission Control, refined the whole time. The ASDS is set to maintain a location, the rocket is aiming for that location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be interesting to know is if the barge tries talking to the booster. It appears that in fact the rocket does the targetting on its own, without Mission Control's help.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the Iridium-2 launch from Vandenberg Jun 2017, John Insprucker mentioned on the Iridium launch webcast that the ASDS does &lt;em&gt;not&lt;/em&gt; send any signals to the returning stage; it only receives telemetry from the stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This makes sense as you want a single system for RTLS and ASDS landings where possible.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-06-29T14:05:36.593" LastActivityDate="2017-06-29T14:05:36.593" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="8781" PostTypeId="1" CreationDate="2015-04-16T19:26:33.497" Score="5" ViewCount="628" Body="&lt;p&gt;In the question: &lt;a href=&quot;https://space.stackexchange.com/questions/8762/does-the-falcon-9-v1-1-first-stage-have-an-explosive-flight-termination-system&quot;&gt;Does the Falcon 9 (v1.1) first stage have an explosive Flight Termination System?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It became clear that the Falcon 9/9R first stage has an explosive flight termination system.  Consensus is, that it was NOT deliberately fired on landing after/before the stage crashed into the ASDS barge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you watch the video, it becomes clear that there is an initial fire when the stage hits the deck, likely the LOX tank splitting and hitting basically anything.  (LOX burns with almost anything. I am exaggerating, but you know what I mean).  Probably mixed with the Kerosene left over, but we know the volume was fairly low, in the 300 lbs of fuel range. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there is a secondary explosion, and then a very kinetic tertiary explosion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Possibly one of those was the residual TEA/TEB ignitors burning after the engines hit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But can the FTS system be triggered, by the heat of the fire otherwise. It is an explosive. You can have explosives that are very hard to trigger, but a LOX/RP1 fire can burn pretty hot. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not knowing the nature of the explosive chosen for the FTS, is it possible it ignited on its own due to the heat of the rest of the goings on?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The video coming in has been amazing!  From a chase plane, you can see the &lt;a href=&quot;https://www.youtube.com/watch?v=BhMSzC1crr0&quot; rel=&quot;nofollow noreferrer&quot;&gt;stage come right into the barge&lt;/a&gt;, land, cold gas system valiantly try to hold it upright, till it tips. This video shows the additional explosions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a great video from the barges perspective &lt;a href=&quot;https://vid.me/i6o5&quot; rel=&quot;nofollow noreferrer&quot;&gt;here.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-05-25T14:36:37.993" Title="When the CRS-6 Falcon 9 first stage hit the ASDS, did the Flight Termination System explode due to the heat of the fire?" Tags="|spacex|landing|falcon-9|flight-termination-system|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8782" PostTypeId="1" CreationDate="2015-04-16T20:02:15.903" Score="11" ViewCount="282" Body="&lt;p&gt;&quot;Medihelion&quot; is what I call the two points on a solar elliptical orbit where distance from the sun is the same as the ellipse's semi-major axis. These two points also correspond to the end-points of the ellipse's minor axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VnVvS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If &quot;medihelion&quot; is not kosher, is there a better name for these two points?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="49" LastEditDate="2015-08-20T11:10:42.573" LastActivityDate="2015-08-20T11:10:42.573" Title="Is &quot;medihelion&quot; a word?" Tags="|orbital-mechanics|orbit|terminology|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8784" PostTypeId="1" CreationDate="2015-04-16T20:55:18.827" Score="6" ViewCount="431" Body="&lt;p&gt;The &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/library/SatelliteFragHistory/TM-2008-214779.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;HISTORY OF ON-ORBIT SATELLITE FRAGMENTATIONS 14th Edition&lt;/a&gt; mentions that there were some design issues with Delta 2nd stages, and an Ariane 3rd stage, leading to in-space explosions/breakup of the spent stages. Some of the mechanisms are given in the answers to &lt;a href=&quot;https://space.stackexchange.com/q/2304/8942&quot;&gt;this question&lt;/a&gt;. I'm looking for specifics of the stages mentioned in the cited report, though, and I'm not talking about stranded upper stages, but spent stages that have successfully completed their primary objective at least.&lt;/p&gt;&#xA;" OwnerUserId="8942" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-04-16T21:46:06.203" Title="What are the causes of breakups of spent 2nd and 3rd stages, resulting in orbital debris?" Tags="|debris|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8785" PostTypeId="2" ParentId="8766" CreationDate="2015-04-16T21:06:48.293" Score="5" Body="&lt;p&gt;If you ignore the engine in the video and instead watch the F9's body, you'll see it go through more than a full cycle of oscillation (counter-clockwise, clockwise, counter-clockwise), and those oscillations appear to be increasing in amplitude as it approaches the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Motion like that is not typical of a controlled landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, in the unlikely event the F9 was successful at avoiding a tip-over, it would have been by sheer luck, because the trend was one of losing control, not regaining it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until there is more information released on what caused the loss of control, it is not possible to say what would have saved this particular landing attempt.&lt;/p&gt;&#xA;" OwnerUserId="3230" LastEditorUserId="3230" LastEditDate="2015-04-16T21:26:23.120" LastActivityDate="2015-04-16T21:26:23.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8786" PostTypeId="2" ParentId="8784" CreationDate="2015-04-16T21:46:06.203" Score="12" Body="&lt;p&gt;The Delta upper stages were a large cause of orbital debris growth up until the 1990's or so. After stages had completed their primary missions, Delta stages were left in derelict orbits with no further communication.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the 1970s Delta rockets left in orbit after delivering their payloads began blowing up. An investigation by McDonnell-Douglas, their manufacturer, showed that the bulkheads separating the leftover fuels were probably cracking as a result of the rocket's passing in and out of sunlight. NASA began recommending that leftover fuels be burned at the end of a flight, or that they be vented into space. Since then most public and private launchers have taken similar measures -- such steps are relatively inexpensive means of limiting debris. Still, every few months on average an old rocket or satellite explodes, flinging a cloud of debris into space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.theatlantic.com/past/issues/98jul/junk.htm&quot; rel=&quot;noreferrer&quot;&gt;Source: The Atlantic, July 1998&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/94cZh.jpg&quot; alt=&quot;Image of a spent Delta II upper stage taken from another satellite&quot;&gt;&#xA;&lt;sup&gt;Image of a spent Delta II upper stage taken from another satellite&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These days, almost all upper stages undergo intentional controlled deorbits, or if not possible passivation, or venting their propellants harmlessly into space to prevent internal pressures building up - which can produce some pretty effects for observers on the ground:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8Uv6b.jpg&quot; alt=&quot;Passivation of the slowly rotating Falcon 9 rocket over Australia caused this spiral pattern to appear.&quot;&gt;&#xA;&lt;sup&gt;Passivation of the slowly rotating Falcon 9 rocket over Australia caused this spiral pattern to appear.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above photo was taken in Australia in July 2010 after SpaceX launched its first Falcon 9 rocket into orbit. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;I heard people in Australia thought UFOs were visiting :),&quot; SpaceX's millionaire founder Elon Musk told SPACE.com in an e-mail. &quot;The venting of propellants, which is done to ensure that an overpressure event doesn't produce orbital debris, created a temporary halo caught the sun at just the right angle for a great view from Australia.  I thought the pictures looked really cool.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-04-16T21:46:06.203" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8787" PostTypeId="1" AcceptedAnswerId="8828" CreationDate="2015-04-16T22:32:13.403" Score="5" ViewCount="1002" Body="&lt;p&gt;I'm thinking, specifically, of the Falcon 9, where I read from their specs that the entire structure is coated with an ESD-conductive paint. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was also under the impression that the launch vehicle's (LV) electrical reference was its chassis.  Again, according to their specs, the paint surface is grounded to the vehicle's structure.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens, then, if surface current is induced on the vehicle's skin due to (indirect) lightning?&#xA;The chassis of the vehicle is closer to the internal electrical links than is the skin.&#xA;Why would you risk return current through your system's grounding?&lt;/p&gt;&#xA;" OwnerUserId="8944" LastEditorUserId="51" LastEditDate="2015-04-16T22:32:47.383" LastActivityDate="2015-04-21T09:58:00.630" Title="Is the entire skin of a launch vehicle its electrical reference?" Tags="|spacex|falcon-9|electronics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8788" PostTypeId="2" ParentId="1308" CreationDate="2015-04-17T00:29:41.140" Score="1" Body="&lt;p&gt;Microgravity is not harmful during shorter periods, like a few weeks. No space agency has any plans to send humans on a long term mission, such as a mission beyond the Moon. Nor any plans to let humans stay a long time in lunar gravity. So no one has any use for simulated gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS is a microgravity lab and deliberately exposes astronauts for it during 6-12 months at a time. It is a medical experiment with humans in order to find out exactly how the health of the astronauts is destroyed and if they can recover from it when back on Earth. Even though microgravity is the one health issue which does not plague people on Earth, the medical community hopes that this kind of experiments one day will discover something which is useful for human medicine in general. (The ISS only costs about as much as all cancer research ever undertaken, so it maybe is worth it in the hunt for a serendipitous discovery?)&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-04-17T00:42:35.393" LastActivityDate="2015-04-17T00:42:35.393" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8790" PostTypeId="2" ParentId="8607" CreationDate="2015-04-17T10:13:26.530" Score="11" Body="&lt;p&gt;The correct term is &lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifugal_force&quot; rel=&quot;noreferrer&quot;&gt;centrifugal force&lt;/a&gt;&lt;/strong&gt;.  It's correct that centrifugal forces does not exist in an inertial reference frame, but the inside of the station is not inertial.  Hence when you are using that as your reference frame you would see centrifugal forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are using an inertial reference frame the physically correct term for what is pulling you towards the floor is &lt;strong&gt;inertia&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="5934" LastEditorUserId="4660" LastEditDate="2015-08-27T14:20:50.820" LastActivityDate="2015-08-27T14:20:50.820" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8792" PostTypeId="1" CreationDate="2015-04-17T18:01:15.093" Score="3" ViewCount="184" Body="&lt;p&gt;I was curious to know what exactly are the changes done in any reusable launch vehicle to carry various different spacecrafts?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any standard procedure that is followed? Like do they change the fuel, or time of flight or burn rate to achieve the same injection velocity?&lt;/p&gt;&#xA;" OwnerUserId="5918" LastActivityDate="2019-02-16T09:31:28.377" Title="What are the modifications done to carry a different spacecraft with the same launch vehicle?" Tags="|launch|spacecraft|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8793" PostTypeId="1" CreationDate="2015-04-17T18:14:12.167" Score="3" ViewCount="59" Body="&lt;p&gt;I've followed press releases and news articles from Roscosmos for a number of years. Its particularly useful in tracking the Russian view of joint activities and comparing with NASA information on the same topic. As engines and launch capabilities flow between different space operators in the contemporary space entrepreneurial environment its important to stay informed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main RosCosmos News can be found at: &lt;a href=&quot;http://www.federalspace.ru/102/&quot; rel=&quot;nofollow&quot;&gt;http://www.federalspace.ru/102/&lt;/a&gt;, and it used to provide (official) english language translations concurrently. Then the translations went away for a long period and were replaced by a notice that &quot;English Language news will be provided again in the future&quot; (or similar).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My Russian skills are a bit rusty (last used in 1981) and rely on more accurate translations; so had to switch to using instant google translations. Despite the huge amount of US federal funds that went into Russian/English translations during the cold war, the google versions of RosCosmos press releases end up somewhat comic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The official English translations are now back again at &lt;a href=&quot;http://www.en.federalspace.ru/102/&quot; rel=&quot;nofollow&quot;&gt;http://www.en.federalspace.ru/102/&lt;/a&gt; but sometimes with a huge latency meaning that the google versions are still useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One can also fill in gaps from Anatoly Zak at &lt;a href=&quot;http://www.russianspaceweb.com/&quot; rel=&quot;nofollow&quot;&gt;http://www.russianspaceweb.com/&lt;/a&gt; but this is also is a delayed selection of prime items, rather than live.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know there is much interest on the Russian side for US information - take for example RT covering the ULA press conference recently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does someone have insight why English language news from RosCosmos is so retarded? Is it political, financial or bureaucratic?&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2015-04-17T18:14:12.167" Title="Why the time lag on RosCosmos News translations?" Tags="|roscosmos|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8794" PostTypeId="2" ParentId="8792" CreationDate="2015-04-17T21:10:24.957" Score="4" Body="&lt;p&gt;(For historical purposes)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Transportation System (aka Space Shuttle) once launched different spacecraft from a reusable launch vehicle.  For payloads that were not simply deployed (spring loaded or floated out of the payload bay, or removed with the manipulator arm), two classes of expendable upper stages were used to boost the spacecraft into their final orbits.  These were the Payload Assist Modules (PAMs) and the Inertial Upper Stages (IUSes).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/carriers.html&quot; rel=&quot;nofollow&quot;&gt;This document&lt;/a&gt; describes the different services, accomodations, flight sequences, and functions of the PAM / IUS / STS system.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-18T15:51:13.290" LastActivityDate="2015-09-18T15:51:13.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8795" PostTypeId="1" AcceptedAnswerId="8799" CreationDate="2015-04-17T21:15:57.560" Score="11" ViewCount="230" Body="&lt;p&gt;Is there an 'on-call' person dedicated for psychological therapy of people traveling in space?  Flights are getting longer and longer, and while isolation has been studied for space and certainly screening has been undertaken, this is a need that may become more important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Mars example, seeing the stronger curve of the horizon, smaller 'sun', coupled with isolation, routine, and communication lag for over a year may require a bit of support beyond just physiological.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there already, or are there plans for, on-call professional psychological support and monitoring after liftoff?&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="4660" LastEditDate="2015-12-05T15:37:25.110" LastActivityDate="2015-12-05T15:37:25.110" Title="What psychological therapy is available to space travelers" Tags="|crewed-spaceflight|health|safety|long-duration|psychology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8796" PostTypeId="2" ParentId="5186" CreationDate="2015-04-17T22:00:36.480" Score="2" Body="&lt;p&gt;SEI did a study on the Falcon 9: &lt;a href=&quot;http://www.sei.aero/eng/papers/uploads/archive/SpaceWorks%20VTVL%20Study%20-%20Release.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.sei.aero/eng/papers/uploads/archive/SpaceWorks%20VTVL%20Study%20-%20Release.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their simulations show that there's about a 20% payload hit for downrange landing and about a 40% hit for RTLS recovery; Elon's numbers are more optimistic at about 15%/30%.&lt;/p&gt;&#xA;" OwnerUserId="6423" LastActivityDate="2015-04-17T22:00:36.480" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8797" PostTypeId="1" AcceptedAnswerId="9208" CreationDate="2015-04-18T01:23:49.377" Score="7" ViewCount="367" Body="&lt;p&gt;Considering how bicycles use human power very efficiently for transport, would it be reasonable for astronauts on an outpost on the Moon or Mars to use easily replaceable human-powered equipment, like a 3D-printed light-rover powered by pedals or levers, or something like that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, considering a self-sustaining outpost, are humans a more efficient machine to indirectly transform solar energy into motion than the combination of solar panels, chargers, batteries and motors? If not, how inefficient they are in comparison?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: There are many issues with astronauts wearing a space suit in a near vacuum and low gravity riding on something like a tricycle or a bike, but I'm asking only about energy efficiency.&lt;/p&gt;&#xA;" OwnerUserId="5999" LastEditorUserId="12102" LastEditDate="2016-10-22T23:31:06.960" LastActivityDate="2016-10-22T23:31:06.960" Title="How efficient would it be for an outpost to use human-powered vehicles?" Tags="|colonization|energy|reduced-gravity-sports|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8799" PostTypeId="2" ParentId="8795" CreationDate="2015-04-18T06:38:09.813" Score="12" Body="&lt;p&gt;This is a very important subject.&lt;/p&gt;&#xA;&#xA;&#xA;&#xA;&lt;p&gt;Recommended reading: Nick Kanas, Dietrich Manzey. &lt;a href=&quot;http://www.springer.com/us/book/9781402067693&quot;&gt;Space Psychology and Psychiatry&lt;/a&gt;. Springer, 2008.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote (pp.193-194):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;From the very beginning of long-duration space flight, the provision of&#xA;  psychological in-flight support to crewmembers has been an important counter-&#xA;  measure in Russia [Grigoriev et al., 1987; Kanas, 1991]. For this purpose, &lt;strong&gt;a psychological support group was established&lt;/strong&gt; that coordinated different activities in order to counter feelings of monotony, isolation, and behavioral health issues like asthenia. Such activities have included surprise presents and favorite foods delivered via re-supply vehicles, increased on-board music and lighting, increased contact with people on Earth, and ground-crew counseling or psychotherapy [Kanas, 1991, 1998]. In addition, the arrival of visiting astronauts and cosmonauts has helped break the monotony and provided stimulation and assistance in performing mission activities.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Based on this experience, a similar system has been established by NASA&lt;/strong&gt; for its space station support activities [Flynn, 2005; Sipes and Vander Ark, 2005].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Minimum psychological support measures (after Kanas, Manzey/ISS MORD 2000):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Personal packages by re-supply flights (not applicable to Mars mission)&lt;/li&gt;&#xA;&lt;li&gt;Uplink of audio news once a week&lt;/li&gt;&#xA;&lt;li&gt;Uplink of written news summaries once every 2 days&lt;/li&gt;&#xA;&lt;li&gt;Uplink of sports/concerts/news videos&lt;/li&gt;&#xA;&lt;li&gt;Videos, books, music, computer games&lt;/li&gt;&#xA;&lt;li&gt;Onboard ham radio (not applicable to Mars mission)&lt;/li&gt;&#xA;&lt;li&gt;Daily uplink of e-mails from family and friends&lt;/li&gt;&#xA;&lt;li&gt;Private two-way videocalls contacts with family and friends (15 minutes or more for each crewmember per week)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Private two-way videocalls with members of the psychological support group (10 minutes or more for each crewmember once a fortnight)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Psychological intervention if necessary&lt;/li&gt;&#xA;&lt;li&gt;Family support during the mission as necessary&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;When all else fails, there are &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Risperidone&quot;&gt;risperidone&lt;/a&gt; and other anti-psychotics etc. in medkits.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-04-18T18:15:18.460" LastActivityDate="2015-04-18T18:15:18.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8800" PostTypeId="1" AcceptedAnswerId="8803" CreationDate="2015-04-18T14:39:23.837" Score="5" ViewCount="1113" Body="&lt;p&gt;The &lt;a href=&quot;http://www.bloodhoundssc.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bloodhound SSC&lt;/a&gt; (Supersonic Car) currently being constructed to break the 1000 mph barrier is powered by a &lt;a href=&quot;http://www.bloodhoundssc.com/project/facts-and-figures/vehicle-technical-specification/propulsion-rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hybrid Rocket&lt;/a&gt; (Solid fuel + HTP) and a &lt;a href=&quot;http://www.bloodhoundssc.com/project/facts-and-figures/vehicle-technical-specification/propulsion-jet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rolls-Royce EJ2000 Jet Engine&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nzIGB.png&quot; alt=&quot;Image of Bloodhound SSC&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the enormous thrust power, I did wonder, if it was mounted vertically could it be launched into orbit? Some of the outline data points necessary to compute the possibilities are shown on the web site:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.bloodhoundssc.com/project/facts-and-figures&quot; rel=&quot;nofollow noreferrer&quot;&gt;Total Thrust 212 kN&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.bloodhoundssc.com/project/facts-and-figures/vehicle-technical-specification/vehicle-mass-properties&quot; rel=&quot;nofollow noreferrer&quot;&gt;Total Mass 7786 kg&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Or perhaps, it might suffer the fate of the last British attempt to put a car in orbit (&lt;a href=&quot;https://www.youtube.com/watch?v=pJdrlWR-yFM&quot; rel=&quot;nofollow noreferrer&quot;&gt;A rocket-powered Reliant Robin on Top Gear&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=pJdrlWR-yFM&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4YwK3.jpg&quot; alt=&quot;Rocket Powered Reliant Robin&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="897" LastEditDate="2015-10-21T09:04:40.703" LastActivityDate="2015-10-21T09:04:40.703" Title="Could the Bloodhound Rocket Car be launched into orbit?" Tags="|launch|rockets|propulsion|engine-design|thrust|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8801" PostTypeId="1" CreationDate="2015-04-18T18:09:33.040" Score="14" ViewCount="1740" Body="&lt;p&gt;Why don't they let the Falcon 9 splash down IN water, and then recover it? Wouldn't this reduce both fuel use &amp;amp; potential damage?&lt;/p&gt;&#xA;" OwnerUserId="8959" LastEditorUserId="51" LastEditDate="2015-04-19T12:42:17.773" LastActivityDate="2018-02-01T00:23:57.807" Title="Could Falcon 9 splash down IN water?" Tags="|spacex|reuse|falcon-9|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8802" PostTypeId="2" ParentId="8801" CreationDate="2015-04-18T18:35:33.310" Score="20" Body="&lt;p&gt;The first several attempts to recover the Falcon 9 did include water landings:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5XKSw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2014/11/spacex-autonomous-spaceport-drone-ship/&quot; rel=&quot;noreferrer&quot;&gt;(Image source)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However there are many problems with water landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no way for the Falcon 9 to remain upright on water. To get an idea of the magnitude of the problem, try standing by a swimming pool and falling forward face first into the water. It's going to hurt. Now do it while being fifty meters tall and weighing 28 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Elon Musk this  can cause &lt;a href=&quot;https://twitter.com/elonmusk/status/488718649515986944&quot; rel=&quot;noreferrer&quot;&gt;kaboom&lt;/a&gt; (technical term).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All attempts to recover the rocket intact after water landings failed. Although one core survived the 90° fall it quickly broke up from wave action.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, even if the stage survived tipping into the sea there's the problem of salt water corrosion. Salt water is really nasty stuff if you're a rocket. After a dip in the sea actuators are going to need striping down and rebuilding, combustion chambers are going to need careful cleaning, the whole rocket will probably need rebuilding, and if even a drop of water gets into the computers you can kiss them goodbye for good.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The days of drone platform landings are probably numbered (even before they fully begin). SpaceX has stated that they want to start returning the cores to land in the near future. They've even begun planning and construction of the first &lt;a href=&quot;http://spacenews.com/spacex-leases-cape-canaveral-launch-pad-for-falcon-landings/&quot; rel=&quot;noreferrer&quot;&gt;landing facilities&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally (as pointed out by Luggage in the comments) the fuel required to land on a floating platform is almost identical to that required to perform a splash landing. There would be some extremely minor increase in fuel consumption (probably measurable in 10s of litres) for the platform landing. The Falcon would have to modify it's velocity slightly to aim for a small target, rather than just landing anywhere in the ocean with the most efficient &quot;go where the wind takes it&quot; trajectory.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-04-20T08:22:03.583" LastActivityDate="2015-04-20T08:22:03.583" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8803" PostTypeId="2" ParentId="8800" CreationDate="2015-04-18T20:11:41.800" Score="10" Body="&lt;p&gt;Obviously, it has the power.  It's not going to work for two reasons, though:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1)  Note that some of that power is coming from a jet engine.  That will cut out long before you reach orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2)  The big issue is that it takes about an 8 minute burn to reach orbit.  The car burns it's fuel in one minute despite being more than 1/5th fuel and not producing as much acceleration during that time.  Adding more fuel means you have to have even more power to push it needing even more fuel..  The rocket equation is brutal.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-04-18T20:11:41.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8804" PostTypeId="1" CreationDate="2015-04-18T20:15:06.850" Score="8" ViewCount="1886" Body="&lt;p&gt;In the video below we can briefly see the valiant attempt of a lone Nitrogen RCS thrust to keep the Falcon 9 upright during the failed landing attempt of CRS-6.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as the stage continues to tip more thrusters start firing [about the 10s mark in the video], but at 90° to the direct the stage is falling. They're obviously not helping keep the stage up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did those thrusters fire? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the Falcon conclude that its cause was hopeless and tried to move itself away from the ASDS to minimize damage?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/vmJK_v5wRZw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="51" LastEditDate="2015-07-09T02:31:18.387" LastActivityDate="2015-07-09T02:31:18.387" Title="Why did the RCS thrusters on Falcon 9 fire in the wrong direction?" Tags="|landing|falcon-9|asds|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8805" PostTypeId="2" ParentId="8800" CreationDate="2015-04-18T21:08:40.073" Score="6" Body="&lt;p&gt;Either the 90kN jet or the 120kN rocket alone does have enough power to lift the weight of the car. If you balanced it properly and pointed it upward, firing the jet first and then switching to the rocket when the jet ran out of usable air, it'd definitely go up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping it stable would be very difficult. The jet and rocket motors are vertically offset; you'd have to reorient them slightly to thrust through the vehicle's center of mass, and the center of mass shifts as it consumes fuel. For use as a supersonic car, its aerodynamics are going to be designed to produce down-force to hold it down on the track; you'd have to redesign it to go the direction it's pointed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Loren Pechtel's definitely correct that it would have nowhere near enough fuel to make orbit, though. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-04-18T22:02:02.133" LastActivityDate="2015-04-18T22:02:02.133" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8806" PostTypeId="1" AcceptedAnswerId="8807" CreationDate="2015-04-18T23:15:42.300" Score="21" ViewCount="24615" Body="&lt;p&gt;I know that there's a difference between SpaceX's Falcon 9 1st stage engines and the 2nd stage engine, since that stage is specifically tuned for vacuum. Wikipedia also says that the Merlin Vacuum Engine is larger than the standard Merlin 1D. That got me thinking... are there more differences?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the differences between the first stage Merlin engines and the second stage Merlin vacuum engine? (I'm especially interested in size comparison, efficiency, thrust, etc.)&lt;/p&gt;&#xA;" OwnerUserId="4603" LastActivityDate="2021-06-07T02:37:06.407" Title="What are the differences between a standard Merlin engine and the Merlin Vacuum engine?" Tags="|spacex|falcon-9|engines|merlin-1d|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8807" PostTypeId="2" ParentId="8806" CreationDate="2015-04-19T00:51:29.733" Score="27" Body="&lt;p&gt;The biggest difference is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket_engine_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;nozzle&lt;/a&gt;. For optimal performance in vacuum, you want a much larger one.&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;, the chamber pressure is the same, but the expansion ratio (throat area to end-of-nozzle area) is 7 times larger in the vacuum variant, which (if correct) implies about 2.7 times the nozzle diameter if the throat is unchanged.&lt;/p&gt;&#xA;&lt;p&gt;The Wikipedia description of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_Vacuum_.281C.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;1C-vacuum&lt;/a&gt; says the expansion nozzle length is 2.7 meters, while the overall length of the first-stage 1C is only 2.9 meters long - roughly half of that being nozzle. So the nozzle length is basically doubled. Presumably the relationship between the 1D and 1D vacuum is analogous.&lt;/p&gt;&#xA;&lt;p&gt;This pic is said to be, left to right: Falcon 1 Merlin 1C, Falcon 9 1C (different mounting), and Falcon 9 2nd stage 1C vacuum -- &lt;em&gt;without&lt;/em&gt; the extension nozzle, so it's a shorter, fatter nozzle than the others.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0A9SH.jpg&quot; alt=&quot;2 Merlin 1Cs and a 1C-vacuum&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;And here's what the extension nozzle looks like by itself:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/YC9rw.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Since the Falcon 9 second stage mounts a single engine, in the same diameter body as the first stage (with its cluster of 9), there's plenty of room for the large nozzle.&lt;/p&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.reddit.com/r/spacex/comments/2ww1ci/the_100th_merlin_1d_rocket_engine_helped_launch/&quot; rel=&quot;nofollow noreferrer&quot;&gt;reddit thread&lt;/a&gt; includes some inconclusive debate about how much different the vacuum engines actually are. There are certainly differences in mounting and layout (most obviously the gas generator exhaust nozzle is canted further out to avoid impinging on the nozzle extension), but the turbopumps etc. are apparently the same.&lt;/p&gt;&#xA;&lt;p&gt;According to the &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;October 2015 revision of the Falcon 9 user's guide&lt;/a&gt;, the 1D Vacuum has a much deeper throttling capability than the first-stage engine, down to about 40% of maximum thrust (360kN-934kN). It's unclear what the reason for the throttling difference is; it could be that the engine is susceptible to exhaust flow separation at low thrust settings into high ambient air pressure.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-06-07T02:37:06.407" LastActivityDate="2021-06-07T02:37:06.407" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="8808" PostTypeId="2" ParentId="5402" CreationDate="2015-04-19T05:36:24.280" Score="5" Body="&lt;p&gt;&lt;strong&gt;Almost, not quite:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Black Arrow - UK satellite launcher (1 success in 3 launches) had 8 &quot;Stentor&quot; engines in the first stage plus what appear to be two significant sized gas generator exhausts. The two exhausts would definitely have added small but measurable thrust. Whether you count them as 'engines' is up to you (but I suspect that you won't :-)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.b14643.de/Spacerockets_1/West_Europe/BlackArrow/Gallery/Arrow.htm&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Utterly superb Black Arrow page&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a hot end view of the 1st stage of the successful launcher, with engines.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Before:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/21fGj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very much larger and nicely detailed version &lt;a href=&quot;http://www.collectspace.com/review/blackarrow/BlackArrow_R3Stage03-lg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;here&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;After:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RSczh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very much larger and nicely detailed version &lt;a href=&quot;http://www.collectspace.com/review/blackarrow/BlackArrow_R3Stage01-lg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;here&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope to get to visit it sometime this lifetime, but may not manage. I've been within about 100 miles but &lt;a href=&quot;https://www.google.co.nz/maps/dir/Coober+Pedy,+South+Australia,+Australia/William+Creek+SA,+Australia/@-29.2954624,134.2499003,2375819m/data=!3m1!1e3!4m13!4m12!1m5!1m1!1s0x2acd47b70d421037:0x4033654628ed3b0!2m2!1d134.7544819!2d-29.0132437!1m5!1m1!1s0x6a9c92ad78c0b175:0x4033654628f2060!2m2!1d136.3332456!2d-28.9000841&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;the last 100 miles&lt;/strong&gt;&lt;/a&gt; is a no-no-no for rental camper vans (and my wife also had some thoughts on the subject). Next time.&lt;/p&gt;&#xA;" OwnerUserId="8362" LastActivityDate="2015-04-19T05:36:24.280" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8809" PostTypeId="1" AcceptedAnswerId="8814" CreationDate="2015-04-19T12:26:29.537" Score="16" ViewCount="1177" Body="&lt;p&gt;A recent &lt;a href=&quot;http://science.slashdot.org/story/15/04/18/1623207/rocket-lab-unveils-electric-rocket-engine&quot;&gt;article on Slashdot&lt;/a&gt; got me thinking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the article is about using electric turbopumps for moving fuel from tanks to engines during launch phase and is just a small, incremental advancement on current design, it got me thinking about using electric pumps in space flight phase in zone where solar energy is abundant - zones where we normally use ion engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can store extra energy in fuel by pressurizing it. The extra pressure converts to extra delta-V. Water-bottle rockets work that way trivially; ion propulsion uses highly pressurized xenon to give it initial boost as it enters the acceleration chamber. It's nothing new.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What puts a limit on pressure available is durability-&gt;weight of the container. In case of some fuels, like solid propellant, compression is pointless. In other cases like liquid hydrogen, the pressure isn't all that high because the volume and pressure if kept highly pressurized, would necessitate unduly heavy container.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I can picture a tiny solar-powered pump capable of creating very high pressures with minuscule throughput - possibly multi-stage piston pump with very small cross-section pistons, although choice of the exact pump design is an engineering detail, a subject of discussion and brainstorming later on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pump can create enormous pressure in a tiny volume of a small &quot;buffer tank&quot;. Keeping these things small means they don't weight all that much despite handling maybe gigapascals of pressure. Releasing that pressurized propellant through a nozzle would give it a very high initial velocity. It can then still be further accelerated by a ion drive or react with similarly pressurized oxidizer creating additional thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nevertheless, we're considerably increasing initial speed of the propellant, without changing the amount of propellant used. Unlike with ion engines we're not limited by available voltages and length of the acceleration chamber - by scaling the device down we can achieve staggering pressures in a small and lightweight package, and as result - exit energies,  without increasing fuel usage. And still we can use &quot;second-stage acceleration&quot; be it chemical, ion, or whatever we can think of, on the pressure-accelerated propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be a viable avenue of research or am I out on a limb here?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="51" LastEditDate="2015-04-19T15:57:27.930" LastActivityDate="2019-02-19T09:33:55.903" Title="Pumped propellant propulsion - is it viable?" Tags="|propulsion|engine-design|advanced-propulsion|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8811" PostTypeId="1" AcceptedAnswerId="8820" CreationDate="2015-04-19T16:05:44.450" Score="5" ViewCount="838" Body="&lt;p&gt;Lasting Lagrange points only exist where two bodies of mass dominate. But in the midst of for example the synchronous Jovian moons, is there a calendar and map for when a spacecraft can be near enough to one of the moons for a moment, that there in effect exists Lagrange points in relation to that moon and Jupiter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would that be useful for spaceflight? Such that when Io passes by, the spacecraft suddenly finds itself in a Lagrange like balance and could change its trajectory or orbit with less effort?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6816" LastEditDate="2015-04-20T21:03:48.740" LastActivityDate="2015-04-22T00:29:20.733" Title="Do Lagrange-like regions temporarily appear around planets with multiple moons?" Tags="|orbital-mechanics|lagrangian-points|moons|" AnswerCount="1" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="8812" PostTypeId="2" ParentId="8804" CreationDate="2015-04-19T22:31:22.293" Score="10" Body="&lt;p&gt;Looking at that video I'm not sure you're describing it accurately.  I don't think it's actually 90 degrees from the fall direction.  It's pretty much sideways but I'm not sure it's entirely sideways.  If there was even a small component in the needed direction I can see the computer firing it because it wasn't programmed to recognize impossible--it saw it was tipped and tried to straighten the rocket using everything it had.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-04-19T22:31:22.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8813" PostTypeId="1" CreationDate="2015-04-20T06:31:53.557" Score="5" ViewCount="2311" Body="&lt;p&gt;I am an engineer who is in charge of writing some software which makes use of those two elements. But I am a bit confused, both seem to be almost the same although there is a small difference, which one?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is the actual difference between mean anomaly and mean longitude?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you.&lt;/p&gt;&#xA;" OwnerUserId="8968" LastEditorUserId="7017" LastEditDate="2015-04-20T06:53:02.670" LastActivityDate="2015-04-21T11:29:25.623" Title="What is the difference between mean anomaly and mean longitude?" Tags="|planet|the-sun|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8814" PostTypeId="2" ParentId="8809" CreationDate="2015-04-20T07:33:01.417" Score="9" Body="&lt;p&gt;Edit: second attempt, my initial post was completely incorrect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My intuition tells me you'll run into a limit on the exhaust velocity with a system like this. My initial thought, &quot;The exhaust speed of a rocket is limited by the speed of sound&quot; is incorrect, so where could the limit be? So let's see where we end up if we use the highest pressure possible and a liquid propellant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://en.wikipedia.org/wiki/Orders_of_magnitude_%28pressure%29&quot; rel=&quot;noreferrer&quot;&gt;list of typical fluid or gas pressures&lt;/a&gt; found in various systems shows that &lt;a href=&quot;http://en.wikipedia.org/wiki/Water_jet_cutter&quot; rel=&quot;noreferrer&quot;&gt;water jet cutters&lt;/a&gt; use some of the highest pressures available outside a lab environment. &lt;a href=&quot;http://www.flowwaterjet.com/en/waterjet-cutting/uhp-pumps/hyperjet.aspx&quot; rel=&quot;noreferrer&quot;&gt;This example&lt;/a&gt; uses a pressure of 648 MPa (6480 bar) and its exhaust velocity is given as Mach 4 (or 1.3 km/s). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a comparison, the SSME has an exhaust velocity of 4.43 km/s, and ion engines have an exhaust velocity of &lt;a href=&quot;http://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;noreferrer&quot;&gt;20-50 km/s&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the efficiency of a rocket depends directly on its exhaust velocity. &#xA;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;noreferrer&quot;&gt;thrust&lt;/a&gt; of a rocket is Isp * M (i.e. the specific impulse, directly related to exhaust velocity times the mass flow). So you can swap exhaust velocity for mass: half the speed means double the mass.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At current launch prices, that's not a tradeoff people are willing to make. So a fluid pump doesn't seem a promising approach.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other ways to use electric power for propulsion: a &lt;a href=&quot;http://en.wikipedia.org/wiki/Mass_driver&quot; rel=&quot;noreferrer&quot;&gt;mass driver&lt;/a&gt; uses an electromagnetic rail system to throw inert mass at high speed. The only mass drivers in existence today are &lt;a href=&quot;http://en.wikipedia.org/wiki/Railgun&quot; rel=&quot;noreferrer&quot;&gt;rail gun&lt;/a&gt; prototypes, with an exhaust speed of 2.4 km/s (as of April 2015). Better than the water jet, but still not as good as a rocket engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for SF's suggestion to igniting a high-pressure jet: if you do this, you'll want to use the expansion of the combustion gases as much as possible, since that gives most of the thrust. When you enclose the jet in a tube, you can force all gases to expand in one direction. But now you're back to building a rocket engine, where the usable pressure is limited by the temperature in the combustion chamber. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: responding to SF's suggestion to add an ion drive to the high-pressure jet: there's little point in doing that. If the ion drive applies 20 km/s of delta-V to the exhaust stream, the pump only provides 5% of that. And the pump has to be very heavy to withstand 650 MPa of pressure. You can put that weight to better use improving the ion drive. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-04-20T19:02:57.487" LastActivityDate="2015-04-20T19:02:57.487" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8815" PostTypeId="2" ParentId="5449" CreationDate="2015-04-20T14:46:08.590" Score="4" Body="&lt;p&gt;I found this breakdown on &lt;a href=&quot;https://answers.yahoo.com/question/index?qid=20120830044334AAt9apf&quot; rel=&quot;nofollow&quot;&gt;another website&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.nasa.gov/pdf/345951main_4_Planetary_Science_FY%202010_UPDATED_final.pdf&quot; rel=&quot;nofollow&quot;&gt;NASA's FY2010 budget&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft \$ 943.3 million&lt;br&gt;&#xA;Payloads \$ 124.3 million&lt;br&gt;&#xA;Systems I&amp;amp;T \$ 92.0 million&lt;br&gt;&#xA;Launch Vehicle/Services \$ 215.1 million&lt;br&gt;&#xA;Ground Systems \$ 77.7 million&lt;br&gt;&#xA;Science/Technology \$ 16.9 million&lt;br&gt;&#xA;Other direct project cost \$ 161.7 million&lt;br&gt;&#xA;Total development cost $ 1.631 billion in 2009 dollars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't found a separate line item for software development, so that's part of the \$ 1067M for the R&amp;amp;D, design and construction of the spacecraft + payloads. &#xA;Even the &lt;a href=&quot;http://www.nasa.gov/news/budget/FY2010.html&quot; rel=&quot;nofollow&quot;&gt;600-page detailed budget&lt;/a&gt; doesn't list software development separately.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-20T14:46:08.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8816" PostTypeId="2" ParentId="5449" CreationDate="2015-04-20T16:24:30.063" Score="2" Body="&lt;p&gt;This &lt;a href=&quot;https://www.youtube.com/watch?v=9jVt5vb68xA&quot; rel=&quot;nofollow&quot;&gt;video on MSL flight software development&lt;/a&gt; has some information on the workforce for software development and software testing, from which you can derive some rough cost numbers.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-04-20T16:43:43.990" LastActivityDate="2015-04-20T16:43:43.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8817" PostTypeId="1" CreationDate="2015-04-20T17:30:08.193" Score="9" ViewCount="1157" Body="&lt;p&gt;Images of the second &lt;a href=&quot;https://youtu.be/44wZQUcuWUw&quot;&gt;attempt to land&lt;/a&gt; the first stage of the Falcon 9 on a barge, following its successful launch of CRS-6, show how it suddenly swung more than its cold gas engines could compensate for, as its rocket engine slowed it down. I would think that its tiny looking grid fins need high airspeed in order to provide stability. Near landing, airspeed suddenly gets slow and the fins become irrelevant. The stability of the rocket deteriorates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this the root cause for the catastrophic instability just before landing, and is it a conceptual flaw of the entire concept of hover-slam for F9R? Wind speed looked pretty harmless, I think, from how the waves and smoke moved.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="51" LastEditDate="2015-07-09T21:09:32.723" LastActivityDate="2015-07-09T21:09:32.723" Title="Did CRS-6 landing fail because the steering fins are ineffective at low speed?" Tags="|reuse|falcon-9|propulsive-deceleration|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8818" PostTypeId="1" CreationDate="2015-04-20T17:33:34.263" Score="6" ViewCount="814" Body="&lt;p&gt;Elon Musk has tweeted that &quot;Cause of hard rocket landing confirmed as due to slower than expected throttle valve response.&quot;  I am curious about what data SpaceX has available to support this kind of diagnosis.  Ideally you would put sensors on every single component of the rocket and transmit measurements continuously, but that would presumably involve a lot of additional cabling and electronics, which might or might not be a problem.  Or maybe the sensors don't add much complexity beyond what you need for the control systems anyway?  I also have no idea how much data one can reliably transmit.  Is any of this documented publicly?&lt;/p&gt;&#xA;" OwnerUserId="8971" LastEditorUserId="51" LastEditDate="2015-04-20T17:36:19.957" LastActivityDate="2015-04-23T19:15:28.377" Title="What telemetry is collected from Falcon 9?" Tags="|spacex|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8819" PostTypeId="2" ParentId="8817" CreationDate="2015-04-20T17:34:39.783" Score="12" Body="&lt;p&gt;The root cause was a &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-6&quot; rel=&quot;nofollow&quot;&gt;propellant valve&lt;/a&gt; for the Merlin engine that was slow to respond, so the engine had way too much power in the last few seconds. This results in the bottom of the stage having more lateral speed than it should, so it overshoots the vertical position.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tweet referenced &quot;biprop&quot; (bi-propellant) and the only engine on the stage that qualifies for that is the Merlin. The thrusters at the top of the stage are cold gas (nitrogen), so monopropellant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this is based on a tweet by Elon Musk that has since been deleted. I hope SpaceX will publish a more detailed account at some point.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit June 27, 2015&lt;/strong&gt;:&lt;br&gt;&#xA;SpaceX has published a &lt;a href=&quot;http://www.spacex.com/news/2015/06/24/why-and-how-landing-rockets?&quot; rel=&quot;nofollow&quot;&gt;more detailed account&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;That controlled descent was successful, but about 10 seconds before landing, a valve controlling the rocket’s engine power (thrust) temporarily stopped responding to commands as quickly as it should have. As a result, it throttled down a few seconds later than commanded, and—with the rocket weighing about 67,000 lbs and traveling nearly 200 mph at this point—a few seconds can be a very long time. With the throttle essentially stuck on “high” and the engine firing longer than it was supposed to, the vehicle temporarily lost control and was unable to recover in time for landing, eventually tipping over.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;... &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Post-launch analysis has confirmed the throttle valve as the sole cause of this hard landing. The team has made changes to help prevent, and be able to rapidly recover from, similar issues for the next attempt, which will be on our next launch—the eighth Falcon 9 and Dragon cargo mission to the space station, currently scheduled for this Sunday (28 June 2015 - Hobbes).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That page also includes a &lt;a href=&quot;https://www.youtube.com/watch?v=NcTOTeoaafU&quot; rel=&quot;nofollow&quot;&gt;longer video of the landing&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-06-27T08:24:39.760" LastActivityDate="2015-06-27T08:24:39.760" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8820" PostTypeId="2" ParentId="8811" CreationDate="2015-04-20T19:11:10.327" Score="5" Body="&lt;p&gt;An online 3-body text is &lt;a href=&quot;http://www.cds.caltech.edu/~marsden/volume/missiondesign/KoLoMaRo_DMissionBook_2011-04-25.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dynamical Systems, the Three Body Problem and Space Mission Design&lt;/a&gt; (big pdf).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trajectories they look at are those with a C3 close to zero (near parabolic) in the regions of the L1 or L2 necks. If a spacecraft is traveling just under or just over the moon's escape velocity in the moon's neighborhood, there may a big variety of trajectories emanating from the moon's L1 and L2 necks. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quantity to look at is $\mu$. No, not G * m. In 3 body mechanics $\mu$ tells you how much more massive the central body is than the orbiting one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mu$ = (mass orbiting body)/(mass orbiting body + mass central body)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have a big $\mu$, there's all sorts of different paths out of L1 or L2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If $\mu$ is tiny, nudging a payload from the L1 or L2 necks will result in a path nearly indistinguishable from the moon's orbit (Phobos and Deimos, I'm looking at you).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a screen capture from one of my spreadsheets. I tinted the cells for $\mu$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6whQc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's moon has a nice $\mu$. Pluto's moon Charon has a whopper $\mu$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter's big moons have more substantial $\mu$ s than the tiny Mars-Sun and Earth-Sun $\mu$ s &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/04/potholes-on-interplanetary-superhighway.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;you've seen me complain about&lt;/a&gt;. Are they big enough to allow travel from moon to moon via WSBs between L1 and L2 necks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This afternoon I had time to play with it. Here's a drawing of orbits of the 4 Galilean moons along with orbits payloads would follow if nudged from L1 or L2 necks:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/f6wjP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first glance it doesn't look like something nudged from Callisto's L1 would find it's way to Ganymede's L2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But repeated passes by a moon will alter the orbit. Here is a screen shot where pellets are nudged from Ganymede's L1:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ILbK5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the left side are pellets shortly after being nudged from an L1 where the sim has the same $\mu$ as Jupiter/Ganymede. On the right are the same pellets after the pellets orbit for a time. You can see repeated perturbations from Ganymede cause the pellets to wander over a larger territory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this time I believe payloads drifting out of one moon's Lagrange neck could possibly find their way to another moon's Lagrange neck via weak stability boundaries. And given these moons have periods on the order of days or weeks, using these paths might even be practical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Jovian moons are big enough that a moon swing by could drop a barely hyperbolic Jovian orbit into an elliptical Jovian capture orbit. I suspect the Jovian moons could play an interesting pinball game with such a captured object. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-04-22T00:29:20.733" LastActivityDate="2015-04-22T00:29:20.733" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8821" PostTypeId="1" AcceptedAnswerId="8822" CreationDate="2015-04-21T01:50:55.467" Score="16" ViewCount="2454" Body="&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Shuttle_Carrier_Aircraft#Design_and_development&quot;&gt;Wikipedia&lt;/a&gt; article mentions that NASA's Boeing 747 Shuttle Carrier Aircraft (SCA 747) have the upstairs first class seating, and the rest of the plane was stripped bare.  It doesn't mention anything going into the main fuselage, I'm guessing because nothing did go in.  But is there anything?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the limited fuel range of N911, I would wonder why the fuel tank wasn't expanded (too heavy, I presume).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus - why the heck is the plane escorted by unarmed FA18s when it has the orbiter on its back?  Can't it be escorted by anything else unarmed?  Or are they sometimes armed?&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="49" LastEditDate="2015-04-21T01:57:41.673" LastActivityDate="2019-06-19T17:31:34.583" Title="What's inside NASA N911 Shuttle Carrier Aircraft (SCA 747)?" Tags="|nasa|space-shuttle|transport|shuttle-carrier-aircraft|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8822" PostTypeId="2" ParentId="8821" CreationDate="2015-04-21T02:01:54.773" Score="16" Body="&lt;p&gt;Answer: not much.  Here is a photo I took looking aft inside of the 747.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Jwidr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is another just below the hard points.  Ignore the silly people.  ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yMq0z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-04-21T02:01:54.773" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8823" PostTypeId="2" ParentId="8821" CreationDate="2015-04-21T02:02:17.487" Score="7" Body="&lt;p&gt;As for your bonus question: NASA operates four &lt;a href=&quot;http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-006-DFRC.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Navy surplus F/A-18s&lt;/a&gt; as chase/observation planes for test flights. They're unarmed and demilitarized; if for some reason the SCA needed an armed escort, they'd get it from USAF or USN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to those F/A-18s, NASA has historically used &lt;a href=&quot;https://en.wikipedia.org/wiki/Northrop_T-38_Talon&quot; rel=&quot;nofollow noreferrer&quot;&gt;T-38&lt;/a&gt;s and &lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_F-104_Starfighter&quot; rel=&quot;nofollow noreferrer&quot;&gt;F-104&lt;/a&gt;s in this role -- they still have T-38s and still use them to maintain pilot proficiency training for astronauts, as they've done since 1962! For a chase plane that has to keep up with the sometimes unusual trajectories or performance envelopes of research aircraft (which NASA flies a lot of), you want something fairly fast and nimble -- a jet fighter or fighter-trainer. Two-seaters have obvious advantages there, and two of the '18s are 2-seat. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-06-19T17:31:34.583" LastActivityDate="2019-06-19T17:31:34.583" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="8824" PostTypeId="1" AcceptedAnswerId="8927" CreationDate="2015-04-21T08:42:32.393" Score="2" ViewCount="797" Body="&lt;p&gt;If you made a '2nd stage to orbit and return' vehicle capable of sitting atop the reusable &lt;a href=&quot;http://en.wikipedia.org/wiki/Falcon_Heavy&quot; rel=&quot;nofollow&quot;&gt;Falcon Heavy&lt;/a&gt; 1st stage, how heavy could it be, and to what height and speed could the 1st stage push it in a trajectory directly upwards&lt;sup&gt;1&lt;/sup&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk is quoted in the article linked above:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Falcon 9 will do satellites up to roughly 3.5 tonnes, with full reusability of the boost stage, and &lt;em&gt;Falcon Heavy&lt;/em&gt; will do satellites up to &lt;em&gt;7 tonnes&lt;/em&gt; with &lt;em&gt;full reusability&lt;/em&gt; of the all three boost stages &amp;#8230;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That suggests the mass it can push in fully reusable mode is '7 tonnes + the mass of the 2nd stage'.  Unfortunately I can't locate a mass for the 2nd stage!  As to the figures for altitude  &amp;amp; velocity at time of separation, I'm not having any luck.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Originally I was after a trajectory that would give the greatest time between separating from the first stage and the payload hitting a high before falling back to the atmosphere (with its associated drag) in order to allow the '2nd stage' enough time to reach orbital velocity (presuming it has a relatively low acceleration rate). But I'm thinking those figures are simply not available. So.. I'd happily accept the numbers for an actual mission profile instead.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="238" LastEditorUserId="238" LastEditDate="2015-04-27T04:15:46.047" LastActivityDate="2015-04-28T15:32:28.357" Title="Falcon Heavy first stage abilities" Tags="|spacex|stages|falcon-heavy|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8826" PostTypeId="1" CreationDate="2015-04-21T09:36:15.277" Score="5" ViewCount="267" Body="&lt;p&gt;I'm confronted to a problem of trajectory optimization. The problem consists in going from point A to point B in the Hill frame (relative navigation) subject to a dynamics described by Clohessy-Wiltshire equations. I want to find the fuel optimal trajectory with three $\Delta$v's: one at $t_0$ one at $t_f$ and one at an arbitrary (but set for the moment) time $t_k$. The evolution of the system is given by &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{x}(t+1) = A\mathbf{x}(t) + B\mathbf{u}(t)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mathbf{x}$ is the state vector and $\mathbf{u}$ is my input (i.e. the $\Delta$v's). Working on this relation one gets&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{x}_j = (A_{j-1}...A_1A_0)\mathbf{x}_0 + [A_{j-1}\cdot \cdot \cdot A_1B_0, A_{j-1}\cdot \cdot \cdot A_2B_1, ..., A_{j-1}B_{j-2}, B_{j-1}, \mathbf{0}][\mathbf{u}_0, ..., \mathbf{u}_{j-2}, \mathbf{u}_{j-1},...,\mathbf{u}_{N-1}]^T \equiv C_j\mathbf{x}_0 + D_j\mathbf{u} \quad (1)$&#xA;for $j = 1,...N$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my optimization problem is as follow: find min of $\sum_{i=1}^N ||\mathbf{u}_i||$ subject to (1) and $\mathbf{x}_0$ and $\mathbf{x}_N$ given (other constraints such as $||\mathbf{x}_i|| &amp;gt; d$ for all $i$ will later be implemented).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I'm correct, the decision variables are $\mathbf{z} = [\mathbf{x}_0,..., \mathbf{x}_N, \mathbf{u}_0,..., \mathbf{u}_N]$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure of how (and if it's possible) to formulate the constraints as $M\mathbf{z} \leq \mathbf{b}$ ? &#xA;How to implement it ? (for instance in matlab) Do I need an initial guess on the trajectory ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks&lt;/p&gt;&#xA;" OwnerUserId="8979" LastActivityDate="2015-04-21T09:36:15.277" Title="Trajectory optimization" Tags="|trajectory|astrodynamics|" AnswerCount="0" CommentCount="1" ClosedDate="2015-07-22T15:16:53.527" ContentLicense="CC BY-SA 3.0" />
  <row Id="8827" PostTypeId="1" CreationDate="2015-04-21T09:37:39.417" Score="2" ViewCount="349" Body="&lt;p&gt;Here's what I've got:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Universe size: &#xA;Milky way is 100,000 LY wide.&#xA;Nearest galaxy is 2.2m LY away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Number of stars:&#xA;about 300,000,000,000 in the Milky way.&#xA;100,000,000,000 galaxies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Age of universe&#xA;13.8 billion years.&#xA;Earth is 4.5 billion years old. Life on earth 3.5bn years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I was thinking about the Fermi paradox. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically two variables I can think of that determine whether other life could find us. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Probably that life is on another star.&lt;/li&gt;&#xA;&lt;li&gt;Time it takes them to get here.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So for example, say there's a 1/1,000,000 chance that there's life on another planet. (Life at least as intelligent as us).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves &lt;em&gt;only&lt;/em&gt; 300,000 other civilizations in our galaxy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now say they're 100,000 light years away. Then they have to be travelling at least 1/35,000 speed of light in order to have reached us in the last 3.5billion years. That's it's 30000km/hr, or ~Mach 25.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now of course using the same probability of life on other solar systems, you can extend it to other galaxies to vastly increase the number of other civilizations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that requires increasing the minimum speed they can travel. They'd need to travel ~200 times faster in order to get here, or 1/175 the speed of light, just to have reached us within the last 3.5 billion years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now obviously my maths is very rough (and please edit if I've made obvious major errors!). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it doesn't seem &lt;em&gt;that&lt;/em&gt; unlikely that if life is relatively rare, that other life simply hasn't had the time to find us yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question I have is, what's realistic for achievable interstellar/galactic travel speeds, and probabilities of life on other planets?&lt;/p&gt;&#xA;" OwnerUserId="6027" LastEditorUserId="6027" LastEditDate="2015-04-21T20:58:28.997" LastActivityDate="2015-07-02T15:50:48.447" Title="Is there good estimates/measures on the variables that make up the Fermi paradox?" Tags="|interstellar-travel|astrobiology|seti|fermi-paradox|drake-equation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8828" PostTypeId="2" ParentId="8787" CreationDate="2015-04-21T09:58:00.630" Score="3" Body="&lt;p&gt;Yes, the skin is usually (part of) the ground plane. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some examples of the construction of the Falcon 9. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the interstage:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/CAXJb.jpg&quot; alt=&quot;Falcon 9 interstage&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a section of the fuel tank:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/DpkHu.jpg&quot; alt=&quot;F9 tank&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In both cases the skin is a major structural element. There are some stringers in the tank, but these are shorter than the tank section, so they don't form a continuous path from the top to the bottom of the tank. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boeing has an &lt;a href=&quot;http://www.boeing.com/commercial/aeromagazine/articles/2012_q4/4/&quot; rel=&quot;nofollow noreferrer&quot;&gt;informative page&lt;/a&gt; on lightning protection for aircraft (which use similar construction methods).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the main strategy for lightning protection in spacecraft is avoidance: a launch will be postponed if thunderstorms are anywhere near the launch site. And the launch site has massive &lt;a href=&quot;https://space.stackexchange.com/questions/2958/what-are-the-towers-around-the-spacex-launch-pad-used-for&quot;&gt;lightning towers&lt;/a&gt; to protect the rocket before takeoff. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://paso.esa.int/5_training_materials/training_10_grounding%20aerospace%20hardw.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This document&lt;/a&gt; describes NASA's best practices for electrical grounding in rockets.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-04-21T09:58:00.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8829" PostTypeId="2" ParentId="8813" CreationDate="2015-04-21T11:29:25.623" Score="7" Body="&lt;p&gt;The mean longitude is the sum of the right ascension of the ascending node $\Omega$, the argument of periapsis $\omega$ and the mean anomaly $M$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$L = \Omega + M + \omega$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As $\Omega$ is measured in the equatorial plane and $\omega$ and $M$ in the orbital plane, $L$ is a broken angle and represents an angle if (and only if) the orbit is circular and equatorial (i.e. no inclination)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a satellite on a keplerian orbit, the mean anomaly defines the position along the orbit at a given epoch. It is for instance used in the two-line element set (TLE).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the mean longitude is a combination of measures in the equatorial plane and the orbital plane and the mean anomaly is a measure in the orbital plane only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope it will help.&lt;/p&gt;&#xA;" OwnerUserId="8979" LastActivityDate="2015-04-21T11:29:25.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8831" PostTypeId="1" AcceptedAnswerId="8836" CreationDate="2015-04-21T12:20:31.853" Score="2" ViewCount="691" Body="&lt;p&gt;With what sort of gravitation are Saturn's Rings held together? It is very spectacular and intriguing compared to the way Earth's gravitation acts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not have a basic or conceptual idea of what/how motion happens in the Rings. What makes water/ice matter  to be confined to the equatorial planes and yet be bound within so many individual Rings? Does inverse square law operate on the icy plate orbiting matter in a force field ? Is there a mathematical model of relative motion of individual glacier sort of ice-block stone masses orbiting inside of the flat planetary Rings?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT1:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has an ordinary differential equation been written for dynamic motion of particles in the Rings?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A purely conceptual model is written to include strong precession. Here each particle has radial velocity along with circumferential and is bound between  extreme radii. Wish to see such Ring orbits theoretically derived like Kepler/Newton planetary orbits of Earth satellites considering conservation of momentum, net forces acting as a function of radius and the Inverse squared law of gravitational attraction that operates universally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT2:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is a particle forever confined to a Ring ? Are there no Kepler type laws or rules for Saturn? Is there an elementary  method to verify calculate/ correlate observed and calculated time period of some Ring particles and some moons in most simple cases? ( For time being we leave out  the moon darting across some Rings and such advanced situations.) I hope to gain clarity about satellite particle fundamentals.Please point out references about same.&lt;/p&gt;&#xA;" OwnerUserId="8981" LastEditorUserId="8981" LastEditDate="2018-03-05T19:54:22.683" LastActivityDate="2018-03-05T19:54:22.683" Title="With what forces are Saturn's Rings cohesively held together?" Tags="|gravity|saturn|celestial-mechanics|rings|" AnswerCount="1" CommentCount="7" ClosedDate="2015-04-23T02:22:01.233" ContentLicense="CC BY-SA 3.0" />
  <row Id="8832" PostTypeId="1" AcceptedAnswerId="8838" CreationDate="2015-04-21T12:51:41.193" Score="4" ViewCount="1469" Body="&lt;p&gt;I wonder how practical it would be for a spacecraft to visit both moons of Mars on the same mission. Areostationary orbit lies between the two moons, and GEO is demanding to reach from LEO.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-05-25T00:28:23.867" Title="What delta-v is required to move between Phobos and Deimos?" Tags="|orbital-mechanics|delta-v|phobos|deimos|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8833" PostTypeId="1" CreationDate="2015-04-21T13:02:42.060" Score="5" ViewCount="2090" Body="&lt;p&gt;Animals in a &quot;free return&quot; trajectory around Mars and landed on Earth should give great data about the effects from microgravity and radiation. It would also test the life support systems. Wouldn't this be a very useful experience?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could mammals and other animals be kept alive for 500 days, without any human crew onboard? Have animals been kept that long on the ISS? Would it be in conflict with any policy NASA has for animal experiments?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-04-21T14:12:26.327" Title="Could NASA send animals to Mars and back as a rehearsal before a human mission?" Tags="|mars|animals|free-return|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8834" PostTypeId="2" ParentId="8833" CreationDate="2015-04-21T13:27:21.490" Score="2" Body="&lt;p&gt;It would certainly be possible.  However it would cost a great deal - less than a manned flight but still expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also wouldn't learn much - humans have already flown in space for over a year (Mir astronauts) in low earth orbit and we now know more about the radiation risks in the space between the planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However; if anyone ever runs an unmanned sample return mission, then perhaps a capsule with some live mice could be sent (as on Apollo 17 I think).  It would add a lot of complexity and weight though.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-04-21T13:27:21.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8835" PostTypeId="2" ParentId="8833" CreationDate="2015-04-21T14:12:26.327" Score="3" Body="&lt;p&gt;Animal maintenance over a long time period is next to impossible to be automated with current technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Humans need to be there to manage the animals. Short term, small animals, sure.  But problems crop up.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You know how they say &quot;Kids do the darnest things.&quot; Animals are worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A free return Mars mission is a 2 year mission as I recall (6 months there, 18 back?) but is no less that a year, which is too long for unattended animals.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-21T14:12:26.327" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8836" PostTypeId="2" ParentId="8831" CreationDate="2015-04-21T14:23:55.710" Score="4" Body="&lt;p&gt;The rings became flat over time as the trillions of particles in them collided over and over, slowly causing their vectors (direction of motion) to average out until they were all aligned in the same direction. See this short video from &lt;a href=&quot;https://www.youtube.com/watch?v=tmNXKqeUtJM&quot; rel=&quot;nofollow&quot;&gt;Minute Physics&lt;/a&gt;. A flat disk rotating in one direction is the only arrangement of these particles that is stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The divisions among the rings exist for two reasons. One, there are moonlets in the rings, too small to see from our distance (or for even the &lt;a href=&quot;http://saturn.jpl.nasa.gov/science/moons/&quot; rel=&quot;nofollow&quot;&gt;Cassini probe&lt;/a&gt; to catch). In fact many of these moonlets may be very short-lived, clumping together and then breaking apart due to collisions, &lt;a href=&quot;http://en.wikipedia.org/wiki/Roche_limit&quot; rel=&quot;nofollow&quot;&gt;tidal forces&lt;/a&gt; from Saturn, and gravitational interactions among all the many moons. These moonlets clear the particles in their orbit. Two, the gravitational fields of the moons collectively influence the rings in such a way that resonances clear the particles at certain distances from Saturn. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-04-21T14:23:55.710" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8837" PostTypeId="2" ParentId="8832" CreationDate="2015-04-21T14:41:51.723" Score="4" Body="&lt;p&gt;According to this &lt;a href=&quot;https://i.stack.imgur.com/nkeWa.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;delta-V map&lt;/a&gt; it requires at least ~1.5km/s to get from one to the other through an intermediate orbit, so as HopDavid notes, there's a shorter direct path. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's a substantial maneuvering investment, but less than getting a lander to the surface of Mars (let alone back up), and it could be done with a very lightweight lander. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-04-21T16:10:55.843" LastActivityDate="2015-04-21T16:10:55.843" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8838" PostTypeId="2" ParentId="8832" CreationDate="2015-04-21T14:58:59.997" Score="7" Body="&lt;p&gt;Assuming coplanar and circular mars orbits, going from 9378 km radius to a 23459 km radius orbit takes about .75 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tack on a little more since the moons aren't quite coplanar. Also tack on a little more for rendezvous delta V. So maybe .8 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Vis-viva_equation&quot;&gt;Vis Viva equation&lt;/a&gt; can be used to to determine circular orbit speeds as well as the periapsis and apoapsis speeds of the Hohmann transfer ellipse.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-04-21T14:58:59.997" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8839" PostTypeId="1" CreationDate="2015-04-21T15:15:25.803" Score="8" ViewCount="582" Body="&lt;p&gt;The ISS has had crew since the year 2000. Do the health issues with microgravity keep compounding, or do they stabilize after a few years and maybe even diminish? I suppose the animal needs to be a mammal, fairly big and long lived.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-04-23T10:28:54.967" Title="What is the longest time that a mammal has spent in microgravity?" Tags="|iss|microgravity|animals|mir|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8840" PostTypeId="2" ParentId="1567" CreationDate="2015-04-21T15:17:12.440" Score="1" Body="&lt;p&gt;Yes, unintentionally, all the time. Radio (incl. TV, etc.) propagate in a straight line from the transmitter. Some transmissions are reflected or absorbed by layers in the atmosphere (depending on frequency), but others leak into space. So there's a shell of radio transmissions around Earth with a radius of 127 light years, expanding at the rate of 1 ly/year.&lt;br&gt;&#xA;Most are not powerful enough to be intelligible by the time they arrive in another star system, though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-21T15:17:12.440" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8841" PostTypeId="2" ParentId="8827" CreationDate="2015-04-21T17:14:34.130" Score="6" Body="&lt;p&gt;It is actually the &lt;a href=&quot;http://en.wikipedia.org/wiki/Drake_equation&quot; rel=&quot;nofollow&quot;&gt;Drake Equation&lt;/a&gt; that applies best to this. The &lt;a href=&quot;http://en.wikipedia.org/wiki/Fermi_paradox#Drake_equation&quot; rel=&quot;nofollow&quot;&gt;Fermi Paradox&lt;/a&gt; framed the issue, the Drake Equation tries to wrestle it into an analyzable framework. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://io9.com/11-of-the-weirdest-solutions-to-the-fermi-paradox-456850746&quot; rel=&quot;nofollow&quot;&gt;io9 article&lt;/a&gt; gives an idea of what we are facing when we try to narrow down the possibilities here. It is an extremely compelling matter for which we have no data points. We don't even know enough to narrow down what a habitable planet is, or what modes of transportation exist. We can say nothing about the motivations of other possible intelligent beings.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Drake equation is:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c&#xA;\cdot L $ where:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ N $ = the number of civilizations in our galaxy with which&#xA;  radio-communication might be possible (i.e. which are on our current&#xA;  past light cone); and&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ R* $ = the average rate of star formation in our galaxy &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ fp $ = the fraction of those stars that have planets &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ ne $ = the average number of&#xA;  planets that can potentially support life per star that has planets &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ fl $ = the fraction of planets that could support life that actually&#xA;  develop life at some point &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ fi $ = the fraction of planets with life that&#xA;  actually go on to develop intelligent life (civilizations) &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ fc $ = the fraction of civilizations that develop a technology that&#xA;  releases&#xA;  detectable signs of their existence into space &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$ L $ = the length of time&#xA;   for which such civilizations release detectable signals into space&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Virtually nothing definite can be said about the last five factors of the equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Star formation rate is &lt;a href=&quot;http://www.ism.ucalgary.ca/Star_Formation/How_Often.html&quot; rel=&quot;nofollow&quot;&gt;estimated&lt;/a&gt; as something &lt;a href=&quot;http://www.astronomynow.com/news/n1005/24seti7a/&quot; rel=&quot;nofollow&quot;&gt;near 10&lt;/a&gt; per year, but there is still a lot of &lt;a href=&quot;http://journals.cambridge.org/download.php?file=%2FIAU%2FIAU7_S284%2FS1743921312009453a.pdf&amp;amp;code=88cf9180994cf98e3a840b894cced959&quot; rel=&quot;nofollow&quot;&gt;uncertainty&lt;/a&gt; about that, including whether the rate is constant or has bursts. We now think most stars have at least &lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_system#Planet-hosting_stars&quot; rel=&quot;nofollow&quot;&gt;one planet&lt;/a&gt; and roughly &lt;a href=&quot;http://arxiv.org/abs/1311.6806&quot; rel=&quot;nofollow&quot;&gt;one-fifth&lt;/a&gt; of Sun-like stars have Earth-like planets in their habitable zones. (Though 'Earth-like' and 'habitable' are stretched to include a rather broad range.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;N deals with civilizations that can communicate, not that can come see us in person, but even here it could be argued that they definition is too narrow. For all we know, if you are advanced enough, there is a way to get around the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So really, no, there aren't good estimates for the variables involved. We probably haven't even identified all the variables. But that is no reason not to give the matter a lot of serious thought. If you would like to continue looking at it, astrodigital has a &lt;a href=&quot;http://www.astrodigital.org/astronomy/drake_equation.html&quot; rel=&quot;nofollow&quot;&gt;spreadsheet&lt;/a&gt; you can download.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2015-07-02T15:50:48.447" LastActivityDate="2015-07-02T15:50:48.447" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8842" PostTypeId="1" CreationDate="2015-04-21T22:29:19.247" Score="13" ViewCount="1147" Body="&lt;p&gt;For having a Falcon 9 (almost) land on it, the SpaceX barge doesn't seem to have faired too badly:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Z0UvC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z0UvCl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Click for &lt;a href=&quot;https://i.stack.imgur.com/Z0UvC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;full version&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In June 2015 the first model barge was replaced, and here is an image of the deck of that barge:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/271uF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't look like it's coated with just asphalt, and it certainly isn't wood (although it almost seems like one can see wood grain on top). What material is the top of this barge coated with? &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="51" LastEditDate="2015-06-09T16:08:25.003" LastActivityDate="2015-06-09T16:08:25.003" Title="What is the SpaceX barge topped with?" Tags="|spacex|falcon-9|materials|protection|asds|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8843" PostTypeId="5" CreationDate="2015-04-21T22:46:44.137" Score="0" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow&quot;&gt;Autonomous Spaceport Drone Ship&lt;/a&gt; (ASDS) is the SpaceX mobile landing barge that serves as a landing platform for Falcon 9R first stage recovery after it has boosted the rocket's upper stage and spacecraft towards orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX named their first operational ASDS, built on a &lt;a href=&quot;http://www.mcdonoughmarine.com/ballast-water-capacity-docs.html&quot; rel=&quot;nofollow&quot;&gt;Marmac 300&lt;/a&gt; hull in service from November 2014 was named &quot;Just Read The Instructions&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In June 2015, the ships 'mind' was transferred from Marmac 300 to Marmac 304 with new enclosed structures for its wings. The wings from Marmac 300 were cut off, and loaded onto Marmac 303, to be named &quot;Of Course I Still Love You&quot; so it can fit through the Panama Canal and reassembled on the West Coast to catch stages launched from Vandenberg at SLC-4W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The names come from a science fiction series by Ian M Banks where the ships have funny names,  attached to the artificial intelligence that is installed in the ship. Thus the ability to transfer the 'mind' model. &lt;/p&gt;&#xA;" OwnerDisplayName="user12" LastEditorUserId="51" LastEditDate="2015-06-12T17:29:35.370" LastActivityDate="2015-06-12T17:29:35.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8844" PostTypeId="4" CreationDate="2015-04-21T22:46:44.137" Score="0" Body="Autonomous spaceport drone ship (ASDS) is SpaceX mobile landing barge that serves as a landing platform for Falcon 9R first stage recovery after they boosted rocket's upper stage and spacecraft towards orbit. SpaceX named their first operational ASDS,  in service since November 2014, &quot;Just Read the Instructions&quot;." OwnerDisplayName="user12" LastEditorUserId="51" LastEditDate="2015-06-12T17:29:35.370" LastActivityDate="2015-06-12T17:29:35.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8845" PostTypeId="5" CreationDate="2015-04-22T00:31:57.573" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-22T00:31:57.573" LastActivityDate="2015-04-22T00:31:57.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8846" PostTypeId="4" CreationDate="2015-04-22T00:31:57.573" Score="0" Body="A launch complex in Kazakhstan administered by Russia. Operational since the 1950s, it has 10 separate launch pad areas, its own railroad, and two dedicated airports. Soyuz, Progress, Dnepr, and Zenit rockets launch from here." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-22T00:31:57.573" LastActivityDate="2015-04-22T00:31:57.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8847" PostTypeId="2" ParentId="8842" CreationDate="2015-04-22T00:59:14.127" Score="8" Body="&lt;p&gt;The Marmac 300 barge, which was modified into ASDS JRTI is made out of steel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has been a lot of modelling and debate in the NasaSpaceflight.com forums trying to estimate how thick. Current thinking seems to come out around 25-35mm  (1 inch to a bit thicker) steel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heat load of a landing stage is not really that high. A single Merlin 1D at 70% thrust, for only a few seconds (2-3) on the surface is not enough to do a lot of damage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk tweeted that the fun stuff after the stage landed, being an  uncontained burn, was just a fast fire and not an explosion.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-22T00:59:14.127" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8848" PostTypeId="1" CreationDate="2015-04-22T01:43:13.603" Score="22" ViewCount="2589" Body="&lt;p&gt;SpaceX has &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/spacex/2015/04/21/spacex-targets-may-dragon-pad-abort-test/26131859/?from=global&amp;amp;sessionKey=&amp;amp;autologin=&quot;&gt;announced&lt;/a&gt; a pad abort test for the Dragon crew capsule on May 5th. They have a 4 hour launch window for the test. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no destination, so why would there be a launch window in this case?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="51" LastEditDate="2015-04-22T01:59:10.117" LastActivityDate="2015-04-22T02:07:28.557" Title="Why does SpaceX have a launch window for a pad abort test?" Tags="|launch|spacex|spacecraft-development|dragon|testing|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8849" PostTypeId="2" ParentId="8848" CreationDate="2015-04-22T01:48:11.533" Score="6" Body="&lt;p&gt;There are all kinds of time constraints on a range.  The obvious ones involve rendezvousing with another vehicle on orbit, but there are other, more mundane ones.  For example, work shifts and sunlight hours.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-04-22T01:48:11.533" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8850" PostTypeId="2" ParentId="8848" CreationDate="2015-04-22T01:58:52.783" Score="28" Body="&lt;p&gt;There are many different constraints that putting a time window around a launch attempt (even a pad abort) can help manage. Orbital constraints, facilities availability and many more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of a Pad Abort, consider the flight path. Ignite the Super Dracos, get high enough for the parachutes to be effective, and land in the ocean just off the coast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means a Notice to Mariners that they need to stay out of an exclusion area. Thus a launch window puts boundaries on the maritime exclusion zone.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-22T02:07:28.557" LastActivityDate="2015-04-22T02:07:28.557" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8851" PostTypeId="1" AcceptedAnswerId="8863" CreationDate="2015-04-22T03:27:20.117" Score="22" ViewCount="7383" Body="&lt;p&gt;As we know, from a surprising corollary to &lt;a href=&quot;https://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation&quot; rel=&quot;noreferrer&quot;&gt;Kirchhoff's law of thermal radiation&lt;/a&gt;, just as darker* objects absorb more light (and therefore energy), darker objects also &lt;em&gt;radiate&lt;/em&gt; more light (at lower temperatures, in the infrared) and therefore energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Observe the following photograph of the ISS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cpIBo.jpg&quot; alt=&quot;International Space Station photographed by an STS-134 crew member of the Space Shuttle Endeavour on May 29, 2011&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;International Space Station photographed by an STS-134 crew member of the Space Shuttle Endeavour on May 29, 2011. Image Credit: NASA&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I count ten radiators, all of them light-colored or metallic. The same sort of material appears on the Space Shuttle's radiators, which are on the inside of the payload doors:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Flgzb.jpg&quot; alt=&quot;The Space Shuttle Endeavour approaching the ISS during STS-118 on August 10, 2007&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;The Space Shuttle Endeavour approaching the ISS during STS-118 on August 10, 2007. Image Credit: NASA&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What this says to me is that these radiators are inefficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From research for this question, I found that the consensus of the internet for terrestrial radiators (e.g. for heating) is that a dark coating isn't worth it: just use a larger radiator and, in the presence of an atmosphere, most heat transfer is by convection anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in space, efficiency is paramount.  Someone needs to check up on all those panels, fix them up, repair damage when possible, &amp;amp;c.--to say nothing of lugging all those precious grams into orbit in the first place.  If you could, say, put a carbon nanotube layer on them and use one fewer, wouldn't you?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is this: why, with the particular examples of the ISS and Space Shuttle, aren't radiators jet black?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;* Note: darker here is not necessarily in the visible range.  If these panels have low albedo in the nonvisible range, for example, then that might be better than some blacker material with a higher albedo in the nonvisible range.  This would certainly explain these radiators.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="2707" LastEditorUserId="49" LastEditDate="2015-04-22T12:22:31.527" LastActivityDate="2016-11-07T00:00:09.417" Title="Why aren't the ISS's nor Space Shuttle's radiators black?" Tags="|iss|materials|physics|radiators|thermodynamics|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8853" PostTypeId="2" ParentId="6099" CreationDate="2015-04-22T05:16:38.177" Score="5" Body="&lt;p&gt;If you read ULA's &quot;Aft Bulkhead Carrier Auxiliary Payload User's Guide&quot; found here: &quot;&lt;a href=&quot;http://www.ulalaunch.com/Products_AtlasV.aspx&quot;&gt;http://www.ulalaunch.com/Products_AtlasV.aspx&lt;/a&gt;&quot; , you'll find details about the requirements for Auxiliary Payloads (AP).  Primarily, it says the Auxiliary Payload Coordinator (APC) is responsible for making sure the CubeSat meets ULA's requirements for Atlas V launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that the APC is going to come up with some plan with the CubeSat developer that ensures satisfactory payload capacity for the loads that it will be put through during transport and launch, the electromagnetic interference, and the thermal environment, in addition to making sure the CubeSat doesn't damage the Centaur with debris, outgassed materials or any other contamination. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what I gather from the user doc, I'll know better after a year is through, since my team's CubeSat (BYU PIC 1.0) has an offer to launch with the WorldView 3 satellite next year. &lt;/p&gt;&#xA;" OwnerUserId="6570" LastActivityDate="2015-04-22T05:16:38.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8854" PostTypeId="2" ParentId="3116" CreationDate="2015-04-22T11:12:36.287" Score="5" Body="&lt;p&gt;The failure you're referring to occurred in December 2013, Expedition 38. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the &lt;a href=&quot;http://www.spaceflight101.com/iss-expedition-38-us-eva-24-updates.html&quot; rel=&quot;noreferrer&quot;&gt;Mission Updates&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With the valve no longer closing, Loop A (of the External Thermal Control System) lost all of its thermal control capability. As a result, thermal loads on that channel had to be reconfigured to protect ISS systems. The Internal Cooling System that circulates water through a Low Temperature Loop and a Moderate Temperature Loop can be configured to operate in a single-loop mode for contingencies like this so that critical systems can remain powered-on.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Single-loop mode was configured for the US Lab, the Airlock and Node-3.&#xA;  The Harmony node does not have single-loop capability so that half of its systems had to be switched off (all those connected to the Moderate Temp Loop) - one of two Multiplexer/Demultiplexers and four of eight Direct Current Converter Units. With that, half the systems of the Japanese Experiment Module and the Columbus module were deactivated.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;All critical systems of ISS remain powered and in nominal condition, and ISS is in a stable thermal configuration using Loop B of the External Thermal Control System. However, operating on a single cooling loop leaves ISS in a non-fault tolerant configuration. Should Loop B fail for some reason, the situation would get much more serious for the crew members on board. That is why Mission Control is trying to restore full thermal control capability as quickly as possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the system is not fully redundant: some of the systems in the Harmony node had to be switched off until repairs were made. And with half the power supply from Harmony unavailable, 'half the systems of the Japanese Experiment Module and the Columbus module were deactivated'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have not been able to track down a list of affected experiments. There is a &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/experiments_by_expedition.html#3738&quot; rel=&quot;noreferrer&quot;&gt;list of all experiments in Expedition 38&lt;/a&gt;, and a &lt;a href=&quot;http://www.spaceflight101.com/iss-expedition-38-updates-december-2013.html&quot; rel=&quot;noreferrer&quot;&gt;day-by-day list of activity&lt;/a&gt; on the ISS including experiments. These don't specify the results of the experiments, and I suspect the day-by-day list omits longer-running experiments. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for how long this situation could be maintained, the sentence &quot;All critical systems of ISS remain powered and in nominal condition&quot; suggests the system could operate like this indefinitely. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-22T11:12:36.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8857" PostTypeId="2" ParentId="6099" CreationDate="2015-04-22T11:35:31.857" Score="2" Body="&lt;p&gt;You can also have a look at the European Cooperation on Space Standardization (ECSS) &lt;a href=&quot;http://www.ecss.nl/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. Plenty of documents are available describing what to test and how to do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance if you navigate to &quot;Standards (at the top)&quot; -&gt; &quot;1.ECSS Architecture&quot; -&gt; &quot;_2. PA branch&quot;, you'll see a list of all kind of material testing that should be made prior to launch. You can then access each document via &quot;Standards&quot; -&gt; &quot;2. Published Standards On-line&quot; (free registration required). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These standards are good practice but then each launcher will impose its own requirements.&lt;/p&gt;&#xA;" OwnerUserId="8979" LastActivityDate="2015-04-22T11:35:31.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8859" PostTypeId="2" ParentId="8643" CreationDate="2015-04-22T11:46:50.203" Score="6" Body="&lt;p&gt;I used &lt;a href=&quot;http://mgvez.github.io/jsorrery/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this solar system simulation&lt;/a&gt; to look for planet positions in an arc, more or less as shown in this image of the Grand Tour (provided by @userLTK).&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Bac8F.jpg&quot; alt=&quot;Grand Tour&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I entered Voyager 1's launch date in the simulation, I got this image: &#xA;&lt;img src=&quot;https://i.stack.imgur.com/KeP6w.png&quot; alt=&quot;1977&quot;&gt;&lt;br&gt;&#xA;Jupiter-Saturn-Uranus are spread around 135 degrees, Jupiter-Saturn-Neptune are spread around 180 degrees. I basically checked positions every few years, looking for those angles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2074, Jupiter, Saturn, and Uranus look to be nicely aligned:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cRuWJ.png&quot; alt=&quot;2074&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The window for a 3-planet flyby is probably several years wide (just like a &lt;a href=&quot;https://space.stackexchange.com/questions/5075/when-is-the-next-outer-planet-lineup-voyager&quot;&gt;Grand Tour&lt;/a&gt; window).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those 3 planets will be in conjunction in 2080. Saturn and Uranus align around 2036 as well, but Jupiter's too far away for a 3-planet tour. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And right now there's a favorable position for Jupiter, Saturn and Neptune:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ix0DE.png&quot; alt=&quot;2015&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Google's search results for '3-planet conjunction' and similar terms were useless, by the way; you end up knee-deep in astrology)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are more permutations for 3-planet tours than the two shown here, I didn't check all of them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, this is just a first-order approach. You'd have to use trajectory calculation software to get exact answers. I found some Matlab scripts, but I don't have access to Matlab...&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-04-23T06:07:58.823" LastActivityDate="2015-04-23T06:07:58.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8860" PostTypeId="2" ParentId="5062" CreationDate="2015-04-22T13:18:48.967" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://paso.esa.int/5_training_materials/training_10_grounding%20aerospace%20hardw.pdf&quot; rel=&quot;noreferrer&quot;&gt;This document&lt;/a&gt; describes NASA's best practices for electrical grounding and ESD handling in rockets.&lt;br&gt;&#xA;NASA also studied &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700004167.pdf&quot; rel=&quot;noreferrer&quot;&gt;electrostatic effects&lt;/a&gt; on board the Apollo spacecraft, its final design included several measures to reduce static charging, e.g:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;inserting a metal mesh in the couches to prevent a static charge from building up as astronauts moved around,&lt;/li&gt;&#xA;&lt;li&gt;adding discharge resistors to biomedical sensors&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-22T13:18:48.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8861" PostTypeId="2" ParentId="8839" CreationDate="2015-04-22T13:23:33.473" Score="14" Body="&lt;p&gt;437 days in a single stretch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mammal was a relatively standard specimen of Homo sapiens sapiens. Known affectionately as &lt;a href=&quot;http://en.wikipedia.org/wiki/Valeri_Polyakov&quot;&gt;Valeri Polyakov&lt;/a&gt; by his handlers, the subject was launched to orbit on-board &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TM-6&quot;&gt;Soyuz TM-6&lt;/a&gt; on August 29, 1988.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mammalian subject was allowed free movement in a spacious &lt;a href=&quot;http://en.wikipedia.org/wiki/Mir&quot;&gt;350 cubic meter enclosure&lt;/a&gt;, provided with a diet consisting of around 100 g of protein, 130 g of fat and 330 g of carbohydrates per day &lt;sup&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Mir#Food_and_drink&quot;&gt;source&lt;/a&gt;&lt;/sup&gt;, and allowed several &lt;a href=&quot;http://en.wikipedia.org/wiki/Mir_EO-3#Spacewalks&quot;&gt;trips outside&lt;/a&gt; during the course of his mission. &lt;br&gt;&#xA;The subject was returned to Earth on-board &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_TM-7&quot;&gt;Soyuz TM-7&lt;/a&gt; on April 27, 1989.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The record for the longest &lt;em&gt;total&lt;/em&gt; time any mammal has spent in microgravity is also held by a specimen of Homo sapiens sapiens; &lt;a href=&quot;http://en.wikipedia.org/wiki/Sergei_Krikalev&quot;&gt;Sergei Krikalev&lt;/a&gt;, with 803 days in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to the second part of your question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Do the health issues with microgravity keep compounding, or do they&#xA;  stabilize after a few years and maybe even diminish?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We simply don't know. Even Polyakov's record holding flight didn't break the 18 month mark. The ongoing &lt;a href=&quot;https://www.nasa.gov/content/one-year-mission/&quot;&gt;One Year Mission&lt;/a&gt; is a start towards the kind of long duration experiments that will eventually answer your question.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-04-23T10:28:54.967" LastActivityDate="2015-04-23T10:28:54.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8863" PostTypeId="2" ParentId="8851" CreationDate="2015-04-22T19:36:20.390" Score="21" Body="&lt;p&gt;Kirchhoff's law is only valid for objects in radiative equilibrium.  The emissivity and absorptivity of a material are the same for a given wavelength, but can vary dramatically for different wavelengths.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radiators on a spacecraft are not in radiative equilibrium, since they lose heat to radiation.  They emit heat in the longwave infrared spectrum, but receive heat energy (from the Sun) in the shortwave infrared, visible, and UV parts of the spectrum.  (They do also receive longwave IR from Earth, but the amount is only around 250&amp;nbsp;W/m&lt;sup&gt;2&lt;/sup&gt; vs 1300&amp;nbsp;W/m&lt;sup&gt;2&lt;/sup&gt; for sunlight.)  This means that the &lt;em&gt;effective&lt;/em&gt; emissivity and absorptivity can differ while the radiators are in operation.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The radiators on the ISS are a high-emissivity white paint, meaning that they are dark in the infrared spectrum where the heat is emitted.  They are white in the visible spectrum to reflect sunlight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radiators on the shuttle are have a two-layer coating: a silver reflective layer covered by a thin Teflon film.  The Teflon layer is opaque to infrared light, so the high emissivity of Teflon dominates.  Visible light passes through the Teflon layer and is reflected by the silver layer, so the solar absorbance is low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radiators on the Shuttle are exposed to more direct sunlight (the radiators on the ISS pivot so they are typically close to edge-on to the Sun), which is why they use the higher-performance but more expensive dual-layer design.&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="8996" LastEditDate="2016-11-07T00:00:09.417" LastActivityDate="2016-11-07T00:00:09.417" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8864" PostTypeId="5" CreationDate="2015-04-22T22:45:43.690" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-22T22:45:43.690" LastActivityDate="2015-04-22T22:45:43.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8865" PostTypeId="4" CreationDate="2015-04-22T22:45:43.690" Score="0" Body="equipment used to bring a spacecraft safely to a stop on the surface of a body (which isn't another spacecraft)." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-22T22:45:43.690" LastActivityDate="2015-04-22T22:45:43.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8866" PostTypeId="2" ParentId="1997" CreationDate="2015-04-22T23:22:12.763" Score="6" Body="&lt;p&gt;Europa's surface gravity is just a bit less than that of our moon (0.134 g versus 0.165 g), and as Mark Adler notes, the atmosphere is negligible. &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunokhod_1&quot; rel=&quot;noreferrer&quot;&gt;Russia&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Yutu_(rover)&quot; rel=&quot;noreferrer&quot;&gt;China&lt;/a&gt; have already put unmanned rovers on the moon via unmanned rocket-powered &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_3&quot; rel=&quot;noreferrer&quot;&gt;landers&lt;/a&gt;; both of these rovers are roughly in Opportunity's weight class (Yutu a bit lighter, and Lunokhod about 4x heavier). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While a Europa lander and rover would have to rely on RTGs rather than solar power, and would have to spend far more time on &quot;standby&quot; during the journey, and there would be countless other differences (particularly in thermal management), the overall approach to lander and rover design wouldn't have to be greatly different.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-04-22T23:22:12.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8867" PostTypeId="2" ParentId="8739" CreationDate="2015-04-23T02:59:22.327" Score="4" Body="&lt;p&gt;According to Anatoly Zak, &lt;a href=&quot;http://www.russianspaceweb.com/progress_gvk.html&quot; rel=&quot;nofollow&quot;&gt;550 kg returned, and 620 kg disposed of&lt;/a&gt;, if Progress M would include a Soyuz TMA reentry capsule, a concept configuration proposed by RKK Energia in 2006. Which, if I'm reading your question correctly, is exactly what you're describing. I.e., keep all that's needed for deorbit burn and landing, simplify life support system for pressurized cargo with possible live passengers (small animals, plants, etc), and throw out all the rest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're left with space for 550 kg of pressurized cargo in TMA that lands with a &lt;em&gt;bump-cushioning last second retrofire&lt;/em&gt; somewhere in Kazakh steppes, and 620 kg of refuse that burns on Progress reentry.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-04-23T02:59:22.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8868" PostTypeId="1" AcceptedAnswerId="8869" CreationDate="2015-04-23T05:16:11.787" Score="21" ViewCount="5806" Body="&lt;p&gt;The barge that SpaceX's Falcon 9 was trying to land on is creatively called &quot;Just Read The Instructions&quot;(hereafter known as JRTI). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/30zHo.jpg&quot; alt=&quot;Just read the Instructions&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been told that SpaceX is using the barge to avoid a land based landing (aka a &quot;landing&quot;) near a populated area - totally makes sense given the videos I've seen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After reviewing the &quot;landing&quot; video it seems like the Falcon 9 was trying to compensate for the tiny size of JRTI. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why isn't it larger? Sure it would be expensive, but Falcon 9s aren't cheap either...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: I've played Kerbal Space Program and know that landing accurately is difficult, even under power, but a suicide burn like this is... well suicide (rocketcide).&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="2974" LastEditDate="2015-04-24T00:45:49.187" LastActivityDate="2021-09-15T14:15:53.040" Title="Why isn’t the “JRTI” barge larger?" Tags="|spacex|falcon-9|landing|reuse|asds|" AnswerCount="8" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8869" PostTypeId="2" ParentId="8868" CreationDate="2015-04-23T06:21:47.817" Score="27" Body="&lt;p&gt;I bet that's every rookie US Navy pilot's thought too on their first carrier landing approach. Thing is though, that unlike aircraft, Falcon 9 first stage can't really maintain altitude before landing. Even single of its nine Merlin engines throttled all the way down is too powerful for hovering on nearly empty tanks and landing is really going to be more of a &lt;em&gt;&lt;a href=&quot;http://www.happylanding.com/articles/9%20ten_spot_landings.htm&quot;&gt;Slam Dunk&lt;/a&gt;&lt;/em&gt; (a combination of &lt;em&gt;Top Gun Arrival&lt;/em&gt; and &lt;em&gt;Let Gravity Do It&lt;/em&gt; spot landing techniques). There's only a small area where you're either getting it right, or you're not getting it right at all, regardless of the size of the landing area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you realize that, it's not difficult to come to the conclusion that a larger drone ship not only wouldn't solve anything, it would create additional problems. One of its main functions is to provide a stable surface to land on. On the sea, the bigger surface you have to stabilize, the more difficult it will get to do that. You'll need a lot more oomph in those stabilizers, more of them, and they'll fail more often. To &lt;em&gt;Just Read The instructions&lt;/em&gt; ASDS (Autonomous Spaceport Drone Ship), or &lt;em&gt;BargeX&lt;/em&gt; if you want, it already happened once before that one of her stabilizers didn't function correctly, albeit Elon Musk said that wouldn't have been the problem then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be also other problems with a larger drone ship, like being more costly to maintain, it would be heavier and probably slower and more difficult to maneuver, and so on. But those are not as important. What's important is that larger size, in this case, wouldn't solve anything.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-04-23T06:33:46.500" LastActivityDate="2015-04-23T06:33:46.500" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8870" PostTypeId="2" ParentId="8868" CreationDate="2015-04-23T06:43:35.407" Score="2" Body="&lt;p&gt;What SpaceX needs to do is to perfect its procedures for landings on a sea barge and then to practice, practice, … practice. That requires time and money, something that commercial operators are always short of because they are always thinking about return on investment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of all the Apollo lunar mission commanders only one was an ex air force pilot (David Scott, Apollo 15), all the others were ex navy pilots. Navy pilots are experienced at placing flying objects onto small targets at sea and all navies with aircraft carriers have perfected their procedures for this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX just needs expertise and lots of practice at landing on a barge.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="6537" LastEditDate="2017-12-18T23:42:02.570" LastActivityDate="2017-12-18T23:42:02.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8872" PostTypeId="2" ParentId="8868" CreationDate="2015-04-23T07:59:44.903" Score="5" Body="&lt;p&gt;SpaceX have already proved very accurate spot landings can be achieved with their Grasshopper.  Their next step is to prove it can be done with a vehicle that can't hover, and that sort of vertically timed landing doesn't really need a pad of a certain size; just good control over vertical speed (especially deceleration).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the size of the barge is pretty much irrelevant.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-04-23T07:59:44.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8873" PostTypeId="2" ParentId="8269" CreationDate="2015-04-23T08:18:07.103" Score="13" Body="&lt;p&gt;The wheel wells are not pressurized. As @Tildalwave says, pressurization wouldn't make much of a difference.&lt;br&gt;&#xA;The wheel wells are insulated to an extent: the wheel well doors are part of the Shuttle's thermal shield, and had to be well-sealed to prevent hot plasma from entering the wheel well.&lt;br&gt;&#xA;This meant the temperature swings were less extreme than they'd have been if the tires were exposed to space. The tires were tested for &lt;a href=&quot;http://www.aeronautics.nasa.gov/pdf/space_shuttle_tires_K_12_supplemental.pdf&quot;&gt;temperatures&lt;/a&gt; between –40 °C (–40 °F) and +55 °C (+130 °F). That sounds like a lot, but tires on Earth routinely reach these temperatures.&lt;br&gt;&#xA;Rubber can withstand much lower temperatures without problems: its &lt;a href=&quot;https://en.wikipedia.org/wiki/Glass_transition&quot;&gt;glass transition temperature&lt;/a&gt; is -70 °C.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-23T08:18:07.103" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8874" PostTypeId="2" ParentId="8868" CreationDate="2015-04-23T09:08:45.610" Score="7" Body="&lt;p&gt;Their eventual goal is to &quot;land&quot; on land anyway. For that, they need FAA clearance, and in order to get that, they don't need to prove that they can &quot;land&quot;, only that they can kinda-sorta crash into the landing site in such a way that the explosion doesn't harm anyone else than them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which they have already done with the current barge system.&lt;/p&gt;&#xA;" OwnerUserId="6156" LastActivityDate="2015-04-23T09:08:45.610" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8875" PostTypeId="1" CreationDate="2015-04-23T09:33:19.600" Score="6" ViewCount="124" Body="&lt;p&gt;A problem with using a traditional kinetic impactor in one piece, like the one used by Deep Impact, to deflect a hazardous NEA, is that its effects depend on the subsurface composition of the asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what if the impactor consisted of gravel which were released into a stream by a spacecraft in approaching trajectory? The same kinetic energy would be spread out over a larger surface on the asteroid and during a longer time. Shouldn't this prevent fragmentation? And as a bonus also eliminate the risk of hitting a steep topological feature obliquely so that the ejecta flies out sideways.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-06-05T13:31:20.540" Title="Would a kinetic impact &quot;cloud&quot; avoid fragmenting an asteroid to be deflected?" Tags="|impact|planetary-defense|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8877" PostTypeId="2" ParentId="6152" CreationDate="2015-04-23T10:18:19.937" Score="1" Body="&lt;p&gt;I have not been able to find mission proposals for dwarf planets in the &lt;a href=&quot;http://en.wikipedia.org/wiki/Kuiper_belt&quot; rel=&quot;nofollow&quot;&gt;Kuiper Belt&lt;/a&gt;. The first mission to take a look at any Kuiper Belt Object will be &lt;a href=&quot;http://www.nasa.gov/mission_pages/newhorizons/main/index.html&quot; rel=&quot;nofollow&quot;&gt;New Horizons&lt;/a&gt;. After its Pluto visit it will be sent to a small KBO; they have picked out some potential targets but the final choice &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow&quot;&gt;hasn't been made yet&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Because the flight path is determined by the Pluto flyby, with only minimal hydrazine remaining, objects needed to be found within a cone, extending from Pluto, of less than a degree's width, within 55 AU. Past 55 AU, the communications link will become too weak, and the RTG wattage will have decayed significantly enough to hinder observations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are also proposals for more missions to KBOs. &#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Argo_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Argo&lt;/a&gt; will combine a &lt;a href=&quot;http://www.spacepolicyonline.com/pages/images/stories/PSDS%20GP1%20Hansen_Argo_Neptune%20Mission%20Concept.pdf&quot; rel=&quot;nofollow&quot;&gt;Neptune flyby with a KBO flyby&lt;/a&gt;. This was a &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2008/1729.html&quot; rel=&quot;nofollow&quot;&gt;proposal&lt;/a&gt; for the &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Frontiers_program&quot; rel=&quot;nofollow&quot;&gt;New Frontiers 4&lt;/a&gt; mission, but has not made the shortlist.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At one point around 2004, a '&lt;a href=&quot;http://www.wired.com/2012/05/new-horizons-ii-2004-2005/&quot; rel=&quot;nofollow&quot;&gt;New Horizons II&lt;/a&gt;' mission was proposed as a contingency: it looked like there would be a shortage of Plutonium-238 for New Horizons, limiting its lifetime so it would only be able to operate until its Pluto flyby. The proposal was for a second, identical spacecraft to be sent (with a full load of Pu-238) a couple of years later for a KBO flyby. As it turned out, the Pu shortage didn't happen, and New Horizons II was dropped. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two organizations with the largest planetary science budgets (NASA and ESA) can only afford a few planetary science missions per decade. Which missions they choose, depends on a number of factors. Many of these factors are infavorable of a KBO mission.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The scientific community proposes missions, and NASA/ESA make choices between those proposals based on viability of the proposal, budget etc. For the well-known planets (e.g. Mars, Jupiter, Saturn) there's a large established scientific community that comes up with new mission proposals regularly. Kuiper Belt Objects are a new field of study and have fewer advocates so on that basis alone you get fewer mission proposals. If there's a huge stack of Mars missions and one KBO mission, statistics alone suggest a Mars mission will be chosen.  &lt;/li&gt;&#xA;&lt;li&gt;Outer planet missions must use a radiothermal generator as a power supply, and there's only enough plutonium-238 available to fuel one or two missions per decade. This may have been a factor for Argo: it needed an RTG, and none were available for New Frontiers 4.  &lt;/li&gt;&#xA;&lt;li&gt;Outer planet missions are long-term: New Horizons will take 9 years to get to Pluto, for instance. That's 9 years of spacecraft support, keeping the science team together etc. before you can start the actual science. In the past, this had to be included in the budget, making these missions more expensive and less likely to be chosen. NASA has &lt;a href=&quot;http://futureplanets.blogspot.nl/2015/03/four-ideas-to-bust-floor-on-outer.html&quot; rel=&quot;nofollow&quot;&gt;changed that recently&lt;/a&gt;, though. &lt;/li&gt;&#xA;&lt;li&gt;To reduce the transit time, New Horizons was launched at a very high speed. This makes it impossible for the spacecraft to enter orbit so you have a flyby where all your science has to fit in a few hours of closest approach. So there's less science return than e.g. a gas giant mission where you can enter orbit and do measurements for years. &lt;/li&gt;&#xA;&lt;li&gt;Outer planet missions need a big, expensive rocket to launch them at sufficient speed, again making the mission more expensive. &lt;/li&gt;&#xA;&lt;li&gt;Pluto is the nearest large KBO we know of. Others are several times further away. They're also very far apart. You could try to visit multiple KBOs, but you'd again have years of transit time in between. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-04-23T11:22:40.680" LastActivityDate="2015-04-23T11:22:40.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8878" PostTypeId="2" ParentId="8821" CreationDate="2015-04-23T12:17:46.703" Score="14" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Given the limited fuel range of N911, I would wonder why the fuel tank wasn't expanded&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;https://blogs.nasa.gov/shuttleferry/&quot;&gt;Shuttle Ferry Flight blog&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...the SCA has a very limited range when ferrying the Orbiter.  That range depends primarily on the weight of the Orbiter and the air temperature.  Winds are also a factor.  The heavier the Orbiter is, the less fuel we can load into the SCA, pure and simple. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This despite @RusselBorogrove's note that the Shuttle dry weight is below the the 747-100's payload capacity.&lt;br&gt;&#xA;The Shuttle was above its dry weight during (at least some) ferry flights. After landing, they &lt;a href=&quot;http://www.jsc.nasa.gov/history/oral_histories/STS-R/TaylorHT/TaylorHT_8-26-11.htm&quot;&gt;did not remove all remaining cargo&lt;/a&gt;, empty the fuel tanks etc. before it was ferried back.&lt;br&gt;&#xA;The SCA was also &lt;a href=&quot;http://en.wikipedia.org/wiki/Shuttle_Carrier_Aircraft&quot;&gt;modified&lt;/a&gt;: its fuselage was strengthened, increasing its weight and reducing its payload.&lt;br&gt;&#xA;They were close enough to the weight limit that adding more fuel wasn't an option. In fact, they couldn't even use all the standard fuel tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the words of &lt;a href=&quot;http://www.jsc.nasa.gov/history/oral_histories/STS-R/TaylorHT/TaylorHT_8-26-11.htm&quot;&gt;one of the flight engineers on the SCA&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are four main, one center wing, and two reserve tanks. We normally only use fuel out of the mains and reserves. We don’t use fuel out of center wing mainly because the Orbiter just takes up too much weight. The airplane can only weigh 710,000 pounds at takeoff. So with a 200-and-something-thousand-pound Orbiter we can’t put a full load of fuel on. So we never get fuel in the center because you make it too heavy.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Finally, places like Edwards are at higher altitudes which decreases performance. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-04-23T14:47:12.603" LastActivityDate="2015-04-23T14:47:12.603" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8880" PostTypeId="5" CreationDate="2015-04-23T16:25:09.727" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-23T16:25:09.727" LastActivityDate="2015-04-23T16:25:09.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8881" PostTypeId="4" CreationDate="2015-04-23T16:25:09.727" Score="0" Body="Relating to creating, testing, and refining technology used in space exploration." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-23T16:25:09.727" LastActivityDate="2015-04-23T16:25:09.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8882" PostTypeId="2" ParentId="8868" CreationDate="2015-04-23T17:03:31.390" Score="8" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;After reviewing the &quot;landing&quot; video it seems like the Falcon 9 was trying to compensate for the tiny size of JRTI.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This simply is not the case. SpaceX has stated that the rocket was trying to compensate for a slower-than-expected throttle response. They know what the problem is, and they very likely know how to address the problem. The target is plenty big enough, as evidenced by the fact that they can reliably hit it.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-04-23T17:03:31.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8883" PostTypeId="2" ParentId="8868" CreationDate="2015-04-23T19:08:24.320" Score="7" Body="&lt;p&gt;SpaceX has a number of problems they need to resolve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surviving re-entry with a first stage, finding a target to land on. Controlling it all the way down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last step is actually landing it. The ultimate goal is to do it on land, but they need to prove to the FAA that they are safe to come in over land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that they have hit the JRTI twice (literally) is pretty good proof that they can hit a small target from almost orbit. (Edit from 2017 - they have since landed over a dozen times, split between both ASDS barges and the LZ-1 pad on land).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they trash their own landing pad, that is kind of their problem. If they hit anything else on the way down, that is the FAA's problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus JRTI has served well. They still want to keep it for later landings for missions where the payload requirements mean not enough propellant margin for boostback to the landing site. Downrange landings. Especially for Falcon Heavy since the side boosters will usually have enough propellant margin to return to launch site, but the core booster wants to go higher and faster, so coming back is harder and more expensive in payload reduction. So landing downrange means recovering the stage vs not recovering it at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are building the &quot;Of Course I Still Love You&quot; ASDS for the West Coast launches for the same reason.  (Edit from 2017: In the end, JRTI's wings were removed, placed on top of a different Marmac barge, and taken through the Panama Canal to be the West Coast barge, and a new ASDS, OCISLY took over Atlantic duties).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They now just have to fix the issues with landing, which they think they have in hand.  So a bigger deck would not really help much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking back at this answer a couple of years, and 15 landings later, it seems like they have no trouble landing within the outer circle, let alone actually hitting the X. So turns out the size is just about correct.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-06-25T21:03:17.087" LastActivityDate="2017-06-25T21:03:17.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8884" PostTypeId="2" ParentId="8818" CreationDate="2015-04-23T19:15:28.377" Score="1" Body="&lt;p&gt;SpaceX has informally noted that they add more and more sensors as issues arise.  In sort of a horses out of the barn mode, after the fact. By this point they have a fair amount of sensors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They clearly feel confident enough from the data collected to be able to make assessments of the various failures in the past. (Landing failures, since they are doing pretty good on launches). &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-23T19:15:28.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8885" PostTypeId="2" ParentId="8801" CreationDate="2015-04-23T19:20:10.233" Score="3" Body="&lt;p&gt;The SRB and Falcon 9 first stage examples are very different. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRB is made of fairly heavy steel, considering that a burning SRB generates pressure throughout the body of the SRB. In fact to stop thrust, you pop the top off so that the thrust is equal in both directions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 first stage is an empty tin can, that is strong enough to hold itself up empty, but not much more. It gets pressurized somewhat during flight, to add strength to survive launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 140 foot tall, 12 foot wide pop can, falling over in the ocean is not survivable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRB also seems to enter heavy end first (engine) fairly quickly, submerges and bobs back up.  The F9 first stage is not strong enough to do that.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-23T19:20:10.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8886" PostTypeId="1" CreationDate="2015-04-23T19:53:01.003" Score="6" ViewCount="706" Body="&lt;p&gt;I heard that the Galileo spacecraft sent Galileo Probe into Jupiter, and it reported temperatures reaching 153 degrees Celsius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is, when you send a probe crashing into a planet that has an atmosphere (and gravity), wouldn't it heat up? So how would you actually measure the temperature of surrounding atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="8810" LastEditorUserId="7062" LastEditDate="2015-04-24T01:31:08.933" LastActivityDate="2018-03-29T21:42:34.470" Title="How does an atmospheric probe measure temperature during descent?" Tags="|probe|planetary-science|atmosphere|temperature|measurement|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8887" PostTypeId="2" ParentId="8868" CreationDate="2015-04-23T19:55:56.023" Score="5" Body="&lt;p&gt;The basic answer is, as others have said, JRtI doesn't need to be larger. SpaceX can find it, reach it, and put the stage engines down on it. The fact that their already highly-successful recovery tests have not yet &lt;em&gt;quite&lt;/em&gt; delivered an intact stage is not a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm answering mostly to respond to the idea that the technique chosen is a &quot;suicide burn&quot;. Not so. The approach speed chosen is the only one possible. As soon as the landing engine is running, acceleration is inevitably upwards, and only fine computer timing of the throttle setting allows the downward velocity to reach zero at the same moment that the stage reaches the landing height. If the engine continued running after the stage stopped descending, even at lowest possible throttle, the stage would be into accelerating ascent. Essentially throttle control is continuously adjusting the bottom of a near-parabolic curve with the aim that the minimum is at h=0 (while leaving throttle margin either side to cope with the unexpected).&lt;/p&gt;&#xA;" OwnerUserId="8392" LastActivityDate="2015-04-23T19:55:56.023" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8888" PostTypeId="1" AcceptedAnswerId="8891" CreationDate="2015-04-23T21:23:34.573" Score="18" ViewCount="4864" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;We’ll start by sending astronauts to an asteroid for the first time in history&quot;&lt;br&gt;&#xA;  (&lt;a href=&quot;http://www.nasa.gov/news/media/trans/obama_ksc_trans.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;President Barack Obama on Space Exploration in the 21st Century&lt;/a&gt;&quot;.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;With a very low gravity, an asteroid is not really friendly to the walking visitor. Any movement may send the walker back to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the schedule -- NASA could launch a mission as soon as 2025, -- a solution will need to be validated by less than 10 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of solution is foreseen by NASA to keep astronauts on the asteroid ground when collecting asteroid samples or performing other useful activities?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Related: &lt;a href=&quot;https://space.stackexchange.com/questions/5612/how-will-they-mine-asteroids?rq=1&quot;&gt;How will they mine asteroids?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Not a duplicate of &lt;a href=&quot;https://space.stackexchange.com/q/2741/7017&quot;&gt;Could a Human reach escape velocity by jumping from the surface of Ceres (a dwarf planet)?&lt;/a&gt;, I'm not asking if astronauts can reach escape velocity (which is possible), but what solutions are currently foreseen to perform activities in an efficient way for the missions planned by NASA &quot;as soon as 2025&quot;.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2018-09-09T11:27:22.650" LastActivityDate="2018-09-09T11:27:22.650" Title="Is it possible for a human to walk on an asteroid?" Tags="|gravity|asteroid|astronauts|eva|" AnswerCount="2" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="8889" PostTypeId="2" ParentId="8888" CreationDate="2015-04-23T22:27:38.220" Score="2" Body="&lt;p&gt;It might be preferably to be walking on asteroid than on Mars, given the length of the zero-gee time on the journey (if that's how they end up doing it). Hopefully the ISS experiments on avoiding muscle loss will have reached full fruition before then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that either walking poles, perhaps with retractable &quot;barbs&quot; of some sort, or equivalent footwear that has a temporary, reversible surface fixing action, would allow a reasonably cautious walker to make good time across the surface of an asteroid. And, yes, either safety lines or something like &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_Aid_for_EVA_Rescue&quot; rel=&quot;nofollow&quot;&gt;SAFER (link)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any activity that involves pushing down or into the surface - drilling, for example - would need extra caution.&lt;/p&gt;&#xA;" OwnerUserId="8392" LastActivityDate="2015-04-23T22:27:38.220" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8890" PostTypeId="2" ParentId="8886" CreationDate="2015-04-24T01:54:56.607" Score="3" Body="&lt;p&gt;After entering Jupiter's atmosphere at 170,700 kph (~47 Km/s) it used its ablative heat shield to decelerate. After 2 minutes, and reaching deceleration levels of 230g, it was traveling at only 430 kph. It then deployed its parachute, and descended 200 km during the next 57.6 minutes, at an average speed of 208 kph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several ways of measuring the temperature of a gas stream; I am still investigating the design of the temperature sensing system, and will update this section with the results. Although it can be said that when the gas stream is of low velocity, a viable method is to use a thermocouple. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Thermocouple&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thermocouple&lt;/a&gt;s have a wide range of capabilities. Of course any effective temperature gauge quickly reaches the temperature of its surrounds, otherwise it would not be very useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermocouples work by generating a small electrical voltage from the junction of two different metals, called a bimetal junction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As mentioned, updates to follow. Also references.&lt;/p&gt;&#xA;" OwnerDisplayName="user8406" LastEditorUserId="15906" LastEditDate="2018-03-26T09:47:09.557" LastActivityDate="2018-03-26T09:47:09.557" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8891" PostTypeId="2" ParentId="8888" CreationDate="2015-04-24T10:05:29.040" Score="12" Body="&lt;p&gt;It depends on the size of the asteroid. According to &lt;a href=&quot;http://www.spacedaily.com/reports/MIT_Tether_Could_Aid_Asteroid_Missions_999.html&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;, if the asteroid's diameter is more than 8 km, you can walk on it without the fist step sending you flying off.&lt;br&gt;&#xA;When the asteroid is smaller, the article proposes to loop a cable loosely around the asteroid. Astronauts can tether themselves to this cable and walk around. Of course this limits the range of the astronaut to within a few meters of the cable. And you'd need a small asteroid to avoid having to bring gigantic spools of cable.&lt;br&gt;&#xA;NASA is exploring this idea too, putting a &lt;a href=&quot;http://www.nasa.gov/exploration/multimedia/galleries/astronaut_tethered_prt.htm&quot; rel=&quot;noreferrer&quot;&gt;grid across the asteroid&lt;/a&gt;:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/YlTN0.jpg&quot; alt=&quot;grid tether&quot;&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This area of study is still young, we've never landed on a small asteroid yet so there are many questions regarding how solid the surface is etc. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-24T10:05:29.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8892" PostTypeId="2" ParentId="8771" CreationDate="2015-04-24T10:54:18.137" Score="6" Body="&lt;p&gt;The landing legs spread out pretty far. 60 feet or more in total around the core. The center of mass is fairly low due to the mass of the engines being right at the bottom and the rest of the cylinder is very uniform and light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus once it is down, SpaceX expects it to be very stable. Their bigger concern is slipping around.  Initially they said they would weld placeholders over the feet to secure it to the deck. Then people started reporting that they would use chains to secure it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the CRS-8 mission, Elon Musk (transcript not yet available) said they would be welding shoes over the feet. Upon reaching port a high resolution photo shows that they also chained it down to the deck via the launch mounts&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FYvrC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FYvrC.jpg&quot; alt=&quot;HIgh Res from Spaceflight now forums of CRS-8 first stage in port&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The deck has been shown in detailed photos to have lots of tie down points (like on an aircraft carrier) so both options are likely possible. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-04-12T20:29:35.643" LastActivityDate="2016-04-12T20:29:35.643" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8893" PostTypeId="2" ParentId="3924" CreationDate="2015-04-24T10:56:10.327" Score="11" Body="&lt;p&gt;With the release of fairly detailed landing video for the CRS-6 landing attempt it is very clear that the legs do not deploy until 20 seconds at most before landing, so they are clearly not used as aero control surfaces. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact it can be seen in the video that the legs deploy half way, then only in the last few seconds deploy all the way down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk &lt;a href=&quot;https://twitter.com/elonmusk/status/536268250620125185&quot; rel=&quot;noreferrer&quot;&gt;tweeted&lt;/a&gt; that by the 21st Falcon mission they will be using the legs more as aero surfaces, this means it will change the opening timeline.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-06-14T20:05:37.997" LastActivityDate="2015-06-14T20:05:37.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8894" PostTypeId="2" ParentId="3286" CreationDate="2015-04-24T11:01:57.563" Score="2" Body="&lt;p&gt;As more photos of the Vandenberg launch site become available, it becomes clear that the shoulders on the TEL were meant for the Falcon Heavy, even if the rest of the facility was not yet quite ready.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have added a base plate with hold downs and room for three cores, but it looks like the TEL has stayed much the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QCg5N.jpg&quot; alt=&quot;From NASASpaceflight.com forums&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a Reddit thread it was pointed out that the shoulders on the TEL's that widen at the base are not about holding up the two side cores in a Falcon Heavy, but rather to support attaching the wider base plate for a Falcon Heavy.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-02-05T14:33:16.510" LastActivityDate="2017-02-05T14:33:16.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8895" PostTypeId="2" ParentId="3285" CreationDate="2015-04-24T11:04:42.123" Score="1" Body="&lt;p&gt;To get to orbit there are two issues.  Height, gotta get high enough.  Speed, gotta get going fast enough. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the lower atmosphere is thick and a pain, so rockets generally launch almost straight up at first, to get out of the atmosphere as fast as possible, then tilt over to accelerate to orbital speeds. There is almost always an initial component of horizontal velocity, and the distance they travel is what is being called out as a downrange distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is important since once they clear particular boundaries, the exclusions needed to keep shipping, boaters, and airplanes out of that area can be relaxed or released.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-24T11:04:42.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8896" PostTypeId="2" ParentId="2910" CreationDate="2015-04-24T11:09:19.710" Score="10" Body="&lt;p&gt;The loss of the commercial market is clearly caused by a series of bad decisions starting in the 1980's and 1990's. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the advent of the Space Shuttle, it was US policy for a while to launch everything on the Shuttle.  This made some kind of sense at the time, since they still lived under the illusion that they would be able to meet a high flight rate on the Shuttle (Max was 9 in one year, so never happened) and thus needed a large number of flights waiting to fly to sustain that rate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to force this issue, the other commerical launchers were mostly shut down.  Atlas 2, Delta 2, etc.  (Titan was never really a commercial launcher). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then once the Challenger example forced a reexamination of this policy, the various boosters were shut down but they resurected the Atlas 2 at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It flew a fairly large number of flights post Challenger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But then with the advent of the Atlas 3/5 and Delta 4 in order to push usage of those boosters they pushed back on the Atlas 2, aiming for another shut down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each time they did this, prices went up and they priced themselves out of the market.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Europe at least kept the Ariane 4 running at affordable rates until the Ariane 5 was ready to take over. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is aiming to turn that around again.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-24T11:09:19.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8897" PostTypeId="2" ParentId="8291" CreationDate="2015-04-24T11:27:43.287" Score="2" Body="&lt;p&gt;There is a limit to the width of a fairing that can over hang the edge of a booster core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stability issues due to aerodynamics will quickly become an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an advantage a wide single core large booster like SLS has over a Falcon Heavy style booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS will be about the same width as the shuttle main tank so 30+ feet wide.  An overhang foe that width means a fairly large fairing is possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you look at a CST 100 on top of an Atlas V booster it has fairly large over hang as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But even with all that there has been very few payloads demanding bigger fairings.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-24T11:27:43.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8898" PostTypeId="1" AcceptedAnswerId="8900" CreationDate="2015-04-24T15:25:59.767" Score="8" ViewCount="943" Body="&lt;p&gt;As mentioned in the question &lt;a href=&quot;https://space.stackexchange.com/questions/5419&quot;&gt;Climbing at terminal velocity minimizes losses? But why? And &amp;quot;of what&amp;quot;?&lt;/a&gt; terminal velocity is important when launching space craft from Kerbal &lt;em&gt;(Game &lt;a href=&quot;https://kerbalspaceprogram.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kerbal Space Program&lt;/a&gt;)&lt;/em&gt;. The question has an answer suggesting &lt;a href=&quot;https://space.stackexchange.com/questions/5419/climbing-at-terminal-velocity-minimizes-losses-but-why-and-of-what/5486#5486&quot;&gt;&quot;Yes it does matter&quot;&lt;/a&gt; but the math is over my head and it does not reference any launches from Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do launches from Earth make any attempt to accelerate at or near terminal velocity?  What kind of real life impact is there for fuel consumption? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-04-24T19:43:31.000" Title="Does terminal velocity play a significant role when launching from Earth?" Tags="|launch|trajectory|guidance|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8899" PostTypeId="2" ParentId="8898" CreationDate="2015-04-24T15:37:43.727" Score="3" Body="&lt;p&gt;The most efficient trajectory (which includes a velocity profile) depends on a lot of factors.  Spending as little time as possible fighting &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_drag&quot; rel=&quot;nofollow&quot;&gt;gravity drag&lt;/a&gt; is generally a good thing, but there are limits.  Drag is one of those limits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drag increases with the square of velocity.  So at some point, going faster will be less efficient than going slower.  This depends on a lot of factors and is probably not terminal velocity.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-04-24T15:37:43.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8900" PostTypeId="2" ParentId="8898" CreationDate="2015-04-24T16:13:49.013" Score="10" Body="&lt;p&gt;No, launches don't specifically target terminal velocity.  They target an optimized trajectory that minimizes the total propellant required to counter both atmospheric drag and gravity losses on the way to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your only objective were to gain altitude, then you would go straight up and, while drag matters, try to track your terminal velocity at the current density.  However if you're trying to get to orbit, the altitude gain is a relatively small part of your problem.  A small rocket will get a given payload to orbital &lt;em&gt;altitude&lt;/em&gt; (and then fall back down).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/z2GfX.png&quot; alt=&quot;you will go to space today, and then you will quickly come back&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will need a much, much bigger rocket to get that same payload to orbital &lt;em&gt;velocity&lt;/em&gt; as well.  A launch vehicle will start to pitch over almost immediately after clearing the tower in order to start gaining horizontal velocity, at which point the terminal velocity optimization is irrelevant.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-04-24T19:43:31.000" LastActivityDate="2015-04-24T19:43:31.000" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8901" PostTypeId="2" ParentId="8514" CreationDate="2015-04-25T06:14:43.840" Score="-3" Body="&lt;p&gt;This is partially speculation, but I do know that the original plans for the ISS were much grander and more complex than the end result turned out to be, due to a number of reasons... Cost being the biggest one, but also the difficulties after the space shuttle crash that slowed down construction by years. &lt;/p&gt;&#xA;" OwnerUserId="9013" LastActivityDate="2015-04-25T06:14:43.840" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8903" PostTypeId="1" CreationDate="2015-04-25T11:56:17.267" Score="6" ViewCount="1293" Body="&lt;p&gt;I saw the &lt;a href=&quot;http://www.youtube.com/watch?v=3QAKvIugfzY&quot; rel=&quot;nofollow&quot;&gt;Ariane 6 animation&lt;/a&gt;, and I noticed, that the upper stage spins before deploying the satellites. Why are they doing this? Is it necessary or is it possible to successfully deploy a satellite to orbit without spinning it? Are the satellites spinning throughout their mission or they will stop spinning after reaching their desired orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/3QAKvIugfzY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="8606" LastEditorUserId="49" LastEditDate="2015-04-25T17:15:24.800" LastActivityDate="2015-04-25T17:15:24.800" Title="Why do upper stages spin before deploying satellites?" Tags="|artificial-satellite|orbital-maneuver|attitude|upper-stage|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8904" PostTypeId="2" ParentId="8903" CreationDate="2015-04-25T13:00:19.367" Score="7" Body="&lt;p&gt;It's a stabilization technique.  I'm not sure what you mean by &quot;is it necessary&quot;...they don't do things &quot;just because&quot; in rocketry.  It's an alternative to three-axis stabilization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellites are usually despun after the burn(s), often by the amusingly named &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Yo-yo_de-spin&quot;&gt;yo-yo de-spin&lt;/a&gt;&quot;.  (good animation at link)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-04-25T13:00:19.367" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8905" PostTypeId="1" AcceptedAnswerId="8907" CreationDate="2015-04-25T14:50:57.643" Score="5" ViewCount="366" Body="&lt;p&gt;A sling launcher (discussed in &lt;a href=&quot;http://home.earthlink.net/~geoffrey.landis/Lunar_Sling_Launcher.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this write-up&lt;/a&gt; by Landis) is a tower with a motor that spins a hub with two or more cables attached. Payload(s) are attached to the end of one or more cables and counterweights to others. Once the hub is spinning, the cables are reeled out a number of kilometers until the angular momentum at their tips is so high the payload reaches orbit after it is released. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BuYr1.jpg&quot; alt=&quot;schematic of sling launcher&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper linked to discusses its use on the Moon. I can see why drag makes the system unusable on Earth, but the atmosphere of Mars is only 1% that of Earth. There must be a density below which the atmospheric drag is manageable. Actually, considering the high efficiency of sling launchers as a launch system, and their relative simplicity, perhaps it could be worth dealing with quite a bit of drag, considering the savings on propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the atmosphere of Mars too thick to make sling launchers practical?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Sensibly, several people have mentioned that the cable for this launcher is impossible using current materials, including 2012rcampion's answer, which lays out the calculations that show that. The paper referenced based most calculations on carbon nanotube cables. This technology seems feasible if still distant, so let us assume it would be used in this case. From the paper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ultimate tensile strength of fullerene nanotubes is predicted by&#xA;  theory to be well over 100 GPa [9], with measured values on individual&#xA;  tubes approaching this value. Allowing an engineering factor of 5&#xA;  (including the added strands for cross-connections, discussed below),&#xA;  the material should allow a working stress of 20 GPa. A mass of one&#xA;  thousand kg at 11.5 gravities (110 m/sec2 ) results in a force on the&#xA;  cable of 112,000 N, as shown in table 1, so the required cross-section&#xA;  of the cable is 0.00389 cm2 (0.389 square millimeters) per ton of&#xA;  end-mass. (More likely, this will be in the form of a number of&#xA;  separate cables which sum to a total cross-sectional area .389 mm2 ).&#xA;  The density of fullerene nanotubes is 1.3 gr/cm3 . The mass of the 50&#xA;  km cable itself is then about 25 kg, and it is clear that neglecting&#xA;  the mass of the cable itself in the calculation was justifiable. The&#xA;  counterweight cable carries half the mass, but at half the&#xA;  acceleration, and half the length, so the counterweight cable has a&#xA;  total mass of about 13kg. An ultimate strength of 20 GPa may be&#xA;  optimistic for a realistic material. If the working stress is reduced&#xA;  to 10 GPa, the cable cross-section is doubled, and the mass increases&#xA;  to 51 kg and 15 kg for the launch cable and the counterweight&#xA;  respectively&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-25T23:56:37.733" LastActivityDate="2015-04-28T14:51:04.700" Title="Could a sling launcher be used on Mars?" Tags="|mars|atmospheric-drag|advanced-propulsion|feasibility|launch-system|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8906" PostTypeId="2" ParentId="8514" CreationDate="2015-04-25T18:48:51.863" Score="15" Body="&lt;p&gt;It will be several years before we get to see what actually happens here.  However the overall driver for the currently stated NASA plan is money.  (In contrast, the currently stated Russian plan seems disconnected from what one might assume about their budget realities.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the NASA budget remains flat, and there is all expectation that it will, indefinitely, then any plans to send humans to the moon, asteroids, or Mars will need the money currently being used to operate ISS to even get a running start at accomplishing those objectives.  ISS currently consumes over \$3B a year, growing to \$4B by 2020.  Plus civil servant salaries, which are not included in that.  (It's a little hard to track real costs at NASA.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many other excuses for ditching ISS, such as the control function mentioned, aging systems (e.g. hatch seals), etc.  But if there were some truly compelling reason to retain the ISS function, it seems likely that solutions could be developed.  So in the end it will come down to priorities and budget.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-04-25T18:54:27.643" LastActivityDate="2015-04-25T18:54:27.643" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8907" PostTypeId="2" ParentId="8905" CreationDate="2015-04-25T20:02:07.810" Score="9" Body="&lt;p&gt;We can compute the power required to maintain speed as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;P=\frac{C_D}2\rho A v^3&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the hypersonic drag coefficient is around $1$ and that the atmospheric density is $1\%$ of Earth's, we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac P A=\frac 1 2\times 0.01225~\text{kg}/\text{m}^3\times\left(5.0~\text{km}/\text{s}\right)^3 = 780~\text{MW}/\text{m}^2&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even on Mars the atmosphere is far too thick to achieve escape velocity inside the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The linked paper uses a power estimate of $100~\text{kW}$, meaning the largest payload you could launch would have a fairing diameter of around $1~\text{cm}$.  That's not even accounting for the drag from the cable itself!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The big attraction of this method is that it uses very little power, unlike electromagnetic launch systems, but even in a very thin atmosphere this benefit disappears.&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="8996" LastEditDate="2015-04-28T14:51:04.700" LastActivityDate="2015-04-28T14:51:04.700" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8908" PostTypeId="1" CreationDate="2015-04-25T22:40:39.617" Score="4" ViewCount="1171" Body="&lt;p&gt;What is the practical limit on the dimensions of a space station built following modern technology?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about various mechnical stresses, tensions, torsions, atmosphere drift, tide forces etc that would disintergrate the construction given currect module size and connections?&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="812" LastEditDate="2015-04-25T23:05:48.910" LastActivityDate="2015-04-26T16:54:36.993" Title="What is the practical limit on the space station dimensions given current technology?" Tags="|space-station|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8909" PostTypeId="1" AcceptedAnswerId="8910" CreationDate="2015-04-26T06:35:02.880" Score="7" ViewCount="1122" Body="&lt;p&gt;When launching from the east cost, I imagine all lower stages just crash/land in the ocean. However when launching the same direction from VAFB, the ocean is in the other direction. Do lower stages just crash into the ground, and if so, what's the risk?&lt;/p&gt;&#xA;" OwnerUserId="6505" LastEditorUserId="7062" LastEditDate="2015-04-26T07:18:59.817" LastActivityDate="2015-04-27T19:16:22.590" Title="When launching from the west coast of the USA (VAFB) where do lower stages land?" Tags="|launch|safety|booster|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8910" PostTypeId="2" ParentId="8909" CreationDate="2015-04-26T07:18:40.653" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Vandenberg_Air_Force_Base&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vandenberg Air Force Base&lt;/a&gt; is used for retrograde and polar or near-polar (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun-synchronous&lt;/a&gt; is slightly retrograde, near-polar) target orbits. That puts ground track of launched vehicles over the Pacific ocean. For example, polar orbits are launched due South from VAFB and first stage would fall into the ocean somewhere between Hawaii and West Coast US:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/S4n2g.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Ground track of Delta 2 rocket during &lt;a href=&quot;http://www.spaceflightnow.com/delta/d277/000325image.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's IMAGE satellite&lt;/a&gt; launch from VAFB on March 25, 2000. Credit: &lt;a href=&quot;http://www.spaceflightnow.com/delta/d277/000325orbittrace.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Boeing&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the image above, MECO (Main Engine Cut-Off) is closely followed by booster separation, at which point booster begins its ballistic fall into the ocean, in this case, more or less along the same ground track than the upper stage that continues ascending and accelerating towards its final speed at SECO (Second Stage Engine Cut-Off).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information, see also:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/3509/49&quot;&gt;Are there any benefits to launching from Vandenberg AFB instead of Cape Canaveral?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/4104/49&quot;&gt;Is Cape Canaveral the ideal launch location in the US?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-04-26T07:54:39.020" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8911" PostTypeId="1" AcceptedAnswerId="8915" CreationDate="2015-04-26T07:54:45.197" Score="32" ViewCount="35099" Body="&lt;p&gt;This might come of as a millionth-time award winner beginner question, but since I'm not familiar with either correct terms or designations, I'll put this question in my own language, since I'm not really sure what to search for in previously answered questions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am building a game where I simulate planetary orbits. These orbits are static and will not change, as expected. Up until now I have calculated these orbits with step-by-step iteration with gravity vectors, but the scale have grown beyond what is possible in real-time. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;My question is therefore how I calculate the planet's position at a given time $t$:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Known variables:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;planet position at time 0&lt;/li&gt;&#xA;&lt;li&gt;planet speed at time 0&lt;/li&gt;&#xA;&lt;li&gt;planet direction at time 0&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;What I wish to know:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;planet position at time $t$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I've probably seen some fancy answers to this questions out there, but they are not very intuitive for someone with limited knowledge in astrophysics like me.&#xA;Thank you for your patience with me!&lt;/p&gt;&#xA;" OwnerUserId="9021" LastEditorDisplayName="user12" LastEditDate="2015-04-30T02:06:06.370" LastActivityDate="2020-08-20T10:22:25.353" Title="Determining orbital position at a future point in time" Tags="|orbital-mechanics|orbit|time|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8912" PostTypeId="1" CreationDate="2015-04-26T14:03:55.277" Score="1" ViewCount="105" Body="&lt;p&gt;Hi everyone i am reading the Scifi classic Star&#xA;Maker by Olaf Stapledon.&#xA;The Second chapter starts with the following paragraph:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;WHILE I was thus contemplating my native planet, I continued to soar through space. The Earth was visibly shrinking into the distance, and as I raced eastwards, it seemed to be rotating beneath me. All its features swung westwards, till presently sunset and the Mid–Atlantic appeared upon its eastern limb, and then the night. Within a few minutes, as it seemed to me, the planet had become an immense half-moon. Soon it was a misty, dwindling crescent, beside the sharp and minute crescent of its satellite.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The author says that he heads eastwards while the Earths features swing Westword and the Atlantic appears from east ! Now considering the Earth rotates anti-clockwise relatively to Polaris, how is that possible? Shouldnt the Atlantic appear from West? How can the Earth swing Westwards if it rotates anti-clockwise? Please shed some light.. Thanks &lt;/p&gt;&#xA;" OwnerUserId="9023" LastActivityDate="2015-04-27T08:11:50.237" Title="Earth rotation confusion on Stapledon book" Tags="|earth|rotation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8913" PostTypeId="2" ParentId="8908" CreationDate="2015-04-26T14:29:14.147" Score="5" Body="&lt;p&gt;The problem with this question is that the technical, financial and political limitations are interlinked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, given enough money and political will, we &lt;em&gt;could&lt;/em&gt; probably build a space station of any size we wanted, up to something resembling &lt;a href=&quot;//en.wikipedia.org/wiki/Death_Star&quot; rel=&quot;noreferrer&quot;&gt;a small moon&lt;/a&gt;, just by throwing enough money and rockets at it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mechanical stresses are not an issue, at least not before the station becomes big enough to have a significant gravity field of its own (which, just in case it's not obvious, would be &lt;em&gt;absurdly&lt;/em&gt; huge) &amp;mdash; we'd just need to build the station strong enough to withstand them.  If we can build skyscrapers and giant cargo ships down here on the surface, where they have to withstand the Earth's gravity in addition to any other forces, we surely could build structures of similar (or much larger) size in orbit, if we could just get the necessary materials up there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure, we'd have to increase our current launch capacity by a factor of thousands or millions to complete such a station in any reasonable timeframe, but &lt;em&gt;in principle&lt;/em&gt;, that's perfectly doable.  And that's not even considering any crazy-but-technically-possible alternative launch technologies like &lt;a href=&quot;//en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29&quot; rel=&quot;noreferrer&quot;&gt;Project Orion&lt;/a&gt; that we could pull out of mothballs if we really wanted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the big assumption here is that diverting such a huge amount of resources to space launches wouldn't collapse the economy or trigger some kind of an environmental disaster, and that some opportunistic political / ideological / religious movement wouldn't exploit the resulting hardship and dissatisfaction to take over and, as a side effect, kill the space station project.  To make this work, we'd need a really good and really obvious reason for &lt;em&gt;everybody&lt;/em&gt; to commit to such a project &amp;mdash; something like an imminent attack by genocidal &lt;a href=&quot;//en.wikipedia.org/wiki/Iron_Sky&quot; rel=&quot;noreferrer&quot;&gt;space Nazis from the moon&lt;/a&gt; just &lt;em&gt;might&lt;/em&gt; be enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, the point I'm trying to make here is that the maximum size of a space station is &lt;em&gt;not&lt;/em&gt; limited by technology, except insofar as technology partly determines what is currently affordable.&lt;/p&gt;&#xA;" OwnerUserId="189" LastActivityDate="2015-04-26T14:29:14.147" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="8914" PostTypeId="1" AcceptedAnswerId="54542" CreationDate="2015-04-26T15:18:59.967" Score="13" ViewCount="1331" Body="&lt;p&gt;How practical is it to manufacture solar cells on the Moon? I understand there is a lot of silica there, and in theory if we could robotically make and deploy solar cells on the Moon, then we would (in time) have practically unbounded energy to use there.&lt;/p&gt;&#xA;" OwnerUserId="8504" LastEditorUserId="12102" LastEditDate="2019-07-18T07:51:31.077" LastActivityDate="2021-08-23T02:29:44.713" Title="Manufacturing solar cells on the Moon" Tags="|the-moon|solar-power|isru|natural-resources|reduced-gravity-manufacturing|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8915" PostTypeId="2" ParentId="8911" CreationDate="2015-04-26T15:20:25.223" Score="51" Body="&lt;h2&gt;Statement of the Problem&lt;/h2&gt;&#xA;&lt;p&gt;The problem you want to solve is called the &lt;strong&gt;Kepler problem&lt;/strong&gt;.  In your formulation of the problem, you're starting out with the &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_state_vectors&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Cartesian orbital state vectors&lt;/strong&gt;&lt;/a&gt; (also called &lt;em&gt;Cartesian elements&lt;/em&gt;): that is, the initial position and velocity.&lt;/p&gt;&#xA;&lt;p&gt;As you have discovered, the only way to propagate the Cartesian elements forward in time is by numerical integration.  This works OK, but it can be slow if you want high accuracy, and there are some numerical problems (errors caused by rounding[slowly accumulate, and many integrators cause &lt;a href=&quot;https://en.wikipedia.org/wiki/Energy_drift&quot; rel=&quot;noreferrer&quot;&gt;energy drift&lt;/a&gt;).  You can get around some of these problems by using a higher-order integrator (&lt;a href=&quot;https://en.wikipedia.org/wiki/Runge%E2%80%93Kutta_methods&quot; rel=&quot;noreferrer&quot;&gt;Runge-Kutta&lt;/a&gt; is a popular one) which allows you to take larger steps for the same level of accuracy, or get better accuracy for the same step size.  However, this is kind of overkill for simple simulation.&lt;/p&gt;&#xA;&lt;p&gt;If your simulation can be treated as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-body_problem&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;two-body problem&lt;/strong&gt;&lt;/a&gt;, then things simplify dramatically.  The two-body problem is a good simplification if the simulation objects are mainly affected by a single, large object.  For example, the Earth traveling around the Sun or a spacecraft traveling in a low Earth orbit are well-modeled as a two body problem; however, a spacecraft traveling from the Earth to the Moon is not (more on that later).&lt;/p&gt;&#xA;&lt;p&gt;Since you're trying to model the positions of the planets with medium accuracy, reduction to the two-body problem should work for you.&lt;/p&gt;&#xA;&lt;h2&gt;Definition of Terms&lt;/h2&gt;&#xA;&lt;p&gt;The traditional solution to the two-body problem involves a different way of representing the position of the orbiting body, called the &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_elements#Keplerian_elements&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Keplerian orbital elements&lt;/strong&gt;&lt;/a&gt; (also just called &lt;em&gt;orbital elements&lt;/em&gt;).  Instead of specifying position and speed, they specify six different parameters of the orbit (if you just want to get to the code, you can skip this part):&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-major_axis&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Semi-major axis, &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt;&lt;/strong&gt;&lt;/a&gt;: Half the maximum diameter of the ellipitical orbit, ( = cicle radius if the orbit is circular).  The energy and period of the orbit depend only on &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt;.  The &lt;a href=&quot;https://en.wikipedia.org/wiki/Conic_section#Conic_parameters&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;semi-latus rectum&lt;/em&gt; &lt;span class=&quot;math-container&quot;&gt;$\ell$&lt;/span&gt;&lt;/a&gt;, the &amp;quot;width&amp;quot; of the orbit, can be a better choice for orbits that are close to parabolic (like for asteroids) or that change from elliptical to hyperbolic (like interplanetary spacecraft).  The two are related by &lt;span class=&quot;math-container&quot;&gt;$\ell=a(1-e^2)$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_eccentricity&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Eccentricity, &lt;span class=&quot;math-container&quot;&gt;$e$&lt;/span&gt;&lt;/strong&gt;&lt;/a&gt;: The &amp;quot;pointiness* of the orbit.  Ranges from &lt;span class=&quot;math-container&quot;&gt;$e=0$&lt;/span&gt; for a perfectly circular orbit, to &lt;span class=&quot;math-container&quot;&gt;$e=1$&lt;/span&gt; for a parabolic orbit, to &lt;span class=&quot;math-container&quot;&gt;$e&amp;gt;1$&lt;/span&gt; for hyperbolic orbits.  Mercury is the most eccentric planet with &lt;span class=&quot;math-container&quot;&gt;$e \approx 0.2$&lt;/span&gt;.  Earth-orbiting spacecraft usually have &lt;span class=&quot;math-container&quot;&gt;$e&amp;lt;0.01$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Aside: From &lt;span class=&quot;math-container&quot;&gt;$e$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; we can determine the most far and the closest points in the orbit, the &lt;em&gt;apoapsis&lt;/em&gt; and &lt;em&gt;periapsis&lt;/em&gt; (together &lt;a href=&quot;https://en.wikipedia.org/wiki/Apsis&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;apsides&lt;/em&gt;&lt;/a&gt;):&#xA;&lt;span class=&quot;math-container&quot;&gt;$$ r_a = a(1+e) \\ r_p = a(1-e) $$&lt;/span&gt;&#xA;&lt;sub&gt;The naming of these points is a little funny: apoapsis and periapsis are the generic terms, but orbits around particular bodies have specific terms: a spacecraft around the Earth has an &lt;em&gt;apogee&lt;/em&gt; and &lt;em&gt;perigee&lt;/em&gt;, while the Earth (in orbit around the Sun) has an &lt;em&gt;aphelion&lt;/em&gt; and &lt;em&gt;perihelion&lt;/em&gt;.&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bmdtp.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bmdtp.png&quot; alt=&quot;Ellipse data&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/X42CV.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/X42CV.png&quot; alt=&quot;Orbit data&quot; /&gt;&lt;/a&gt;&#xA;The two parameters &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$e$&lt;/span&gt; are enough to determine the shape of the orbit.  The next three parameters define the orientation of the orbit relative to a coordinate system consisting of a reference plane, and a reference direction (parallel to the plane).&lt;/p&gt;&#xA;&lt;p&gt;For almost all orbits in the solar system, the coordinate system used is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic_coordinate_system&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;ecliptic coordinate system&lt;/em&gt;&lt;/a&gt;.  The reference plane is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;ecliptic plane&lt;/em&gt;&lt;/a&gt;, the plane of the Earth's orbit around the Sun.  The reference direction is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Equinox#Celestial_coordinate_systems&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;vernal equinox point&lt;/em&gt;&lt;/a&gt;, the direction from the Earth to the Sun at the moment of the spring equinox.  Since both these references drift slowly over time, we must specify a particular time at which these references are defined, called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Epoch_(astronomy)&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;epoch&lt;/strong&gt;&lt;/a&gt;.  The most common is &lt;a href=&quot;https://en.wikipedia.org/wiki/Epoch_(astronomy)#Julian_years_and_J2000&quot; rel=&quot;noreferrer&quot;&gt;J2000&lt;/a&gt;, noon on January 1, 2000 (UTC).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/viiRl.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/viiRl.png&quot; alt=&quot;Ecliptic coordinate system&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Earth-centered orbits often use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Equatorial_coordinate_system&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;equatorial coordinate system&lt;/em&gt;&lt;/a&gt;, whose reference plane is the equator of the Earth.  The situation with the epoch is a little complicated, so I won't get into it here.&lt;/p&gt;&#xA;&lt;p&gt;Following parameters locate the orbit w.r.t. Earth orbit:&lt;/p&gt;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Inclination, &lt;span class=&quot;math-container&quot;&gt;$i$&lt;/span&gt;&lt;/strong&gt;&lt;/a&gt;: the angle between the plane of the orbit and the reference plane.  Inclination between 90 and 180 degrees refers to a &lt;em&gt;retrograde&lt;/em&gt; orbit, one that orbits &amp;quot;backwards&amp;quot; from the usual direction.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Longitude_of_the_ascending_node&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Longitude of the ascending node, &lt;span class=&quot;math-container&quot;&gt;$\Omega$&lt;/span&gt;&lt;/strong&gt;&lt;/a&gt;: the ascending node is where the orbit crosses from below the reference plane to above it.  (It's at the intersection between the orbital plane and the reference plane)  &lt;span class=&quot;math-container&quot;&gt;$\Omega$&lt;/span&gt; is the angle between this point and the reference direction, measured counterclockwise.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Argument_of_periapsis&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Argument of periapsis, &lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt;&lt;/strong&gt;&lt;/a&gt;: the angle between the ascending node and periapsis (the lowest point in the orbit).  For orbits with very low inclination where the location of the ascending node is difficult to determine (since it's the intersection between two almost parallel planes), we instead use the &lt;em&gt;longitude of periapsis&lt;/em&gt; &lt;span class=&quot;math-container&quot;&gt;$\varpi = \Omega + \omega$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qJDNb.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qJDNb.png&quot; alt=&quot;Orbit-to-ecplitic paramaeters&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The sixth parameter defines the position of the object in its orbit.  There are a couple different choices, but the most common is:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mean_anomaly&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Mean anomaly, &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt;&lt;/strong&gt;&lt;/a&gt;: an &amp;quot;imaginary&amp;quot; angle that is zero at periapsis and increases at a constant rate of 360 degrees per orbit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The rate at which &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; changes is called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mean_motion&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;mean motion&lt;/em&gt;, &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt;&lt;/a&gt;, equal to &lt;span class=&quot;math-container&quot;&gt;$2\pi/T$&lt;/span&gt;.   Usually you have a measurement of &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; at a particular epoch &lt;span class=&quot;math-container&quot;&gt;$t_0$&lt;/span&gt;, called (unsurprisingly) the &lt;em&gt;mean anomaly at epoch&lt;/em&gt;, &lt;span class=&quot;math-container&quot;&gt;$M_0$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Just like the argument of periapsis, for low-inclination orbits we use a related value, the &lt;em&gt;mean longitude&lt;/em&gt;, &lt;span class=&quot;math-container&quot;&gt;$L=\varpi + M$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The actual angle between the orbiting body and periapsis is called the &lt;em&gt;true anomaly&lt;/em&gt;, &lt;span class=&quot;math-container&quot;&gt;$\nu$&lt;/span&gt;.  This is the angle we need to compute the position of the body.  Unfortunately there is no way to directly compute &lt;span class=&quot;math-container&quot;&gt;$\nu$&lt;/span&gt; from &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt;.  Instead we first solve for the &lt;em&gt;eccentric anomaly&lt;/em&gt; &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;M = E - e \sin E&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;This is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler%27s_equation&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Kepler's equation&lt;/strong&gt;&lt;/a&gt;, and it cannot be solved analytically.  Once we have &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt; though, there is a relatively simple expression for &lt;span class=&quot;math-container&quot;&gt;$\nu$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;h2&gt;Computing Position from Orbital Elements&lt;/h2&gt;&#xA;&lt;p&gt;We'll perform this computation in three steps: first, we'll solve Kepler's equation.  Second, we'll compute the 2d position of the body in the orbital plane.  Lastly, we'll rotate our 2d position into 3d coordinates.  I'll give some &amp;quot;pseudocode&amp;quot; in Javascript for most of these tasks.&lt;/p&gt;&#xA;&lt;p&gt;I'll assume that you're using a set of elements like &lt;a href=&quot;http://ssd.jpl.nasa.gov/?planet_pos&quot; rel=&quot;noreferrer&quot;&gt;these from JPL's web site&lt;/a&gt;.  These use &lt;span class=&quot;math-container&quot;&gt;$L$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\varpi$&lt;/span&gt; instead of &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt;.  The table gives two values for each of the elements; the second is the time derivative.  If you use the values in this table you should use the derivatives as well.&lt;/p&gt;&#xA;&lt;p&gt;Calculate the time &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt; in centuries from J2000:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;// month is zero-indexed, so 0 is January&#xA;var tMillisFromJ2000 = Date.now() - Date.UTC(2000, 0, 1, 12, 0, 0);&#xA;var tCenturiesFromJ2000 = tMillisFromJ2000 / (1000*60*60*24*365.25*100);&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Now we calculate the current values of each of the orbital parameters.  For example, the semimajor axis of Earth, using the values from Table 1 (valid from 1800–2500):&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;// a0 = 1.00000261; adot = 0.00000562&#xA;var a = a0 + adot * tCenturiesFromJ2000;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;(Note that the values are actually given for &amp;quot;EM &lt;a href=&quot;https://en.wikipedia.org/wiki/Barycenter&quot; rel=&quot;noreferrer&quot;&gt;Barycenter&lt;/a&gt;,&amp;quot; the center-of-mass of the Earth-Moon system.  The Earth is around 4600 kilometers from the barycenter in the opposite direction from the Moon.  If you want to correct this inaccuracy you'll need to simulate the motion of the Moon as well, but that's probably overkill.)&lt;/p&gt;&#xA;&lt;p&gt;Table 2a gives elements that are accurate from 3000 BC to 3000 AD; however, if you use the elements from table 2a, you &lt;em&gt;must&lt;/em&gt; supplement them with corrections to &lt;span class=&quot;math-container&quot;&gt;$L$&lt;/span&gt; from Table 2b!  For example, here is computing the longitude of Saturn:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;// L0 = 34.33479152; Ldot = 3034.90371757&#xA;// b = -0.00012452          &#xA;// c =  0.06064060&#xA;// s = -0.35635438&#xA;// f = 38.35125000&#xA;var L = L0 + Ldot * tCenturiesFromJ2000&#xA;           + b * Math.pow(tCenturiesFromJ2000, 2)&#xA;           + c * Math.cos(f * tCenturiesFromJ2000)&#xA;           + s * Math.sin(f * tCenturiesFromJ2000);&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;We don't need to explicitly compute the mean motion and add it to &lt;span class=&quot;math-container&quot;&gt;$L$&lt;/span&gt;, since both tables include it in &lt;span class=&quot;math-container&quot;&gt;$\dot L$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Now we're ready to compute &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt; (&lt;code&gt;w&lt;/code&gt;):&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;var M = L - p \\ p is the longitude of periapsis&#xA;var w = p - W \\ W is the longitude of the ascending node&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;On to step 2: we need to solve the Kepler equation:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;M = E - e \sin E&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;We can solve this numerically using &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_method&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Newton's method&lt;/strong&gt;&lt;/a&gt;.  Solving the Kepler equation is equivalent to finding the roots of &lt;span class=&quot;math-container&quot;&gt;$f(E) = E - e \sin E - M$&lt;/span&gt;.  Given &lt;span class=&quot;math-container&quot;&gt;$E_i$&lt;/span&gt;, an estimate of &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt;, we can use Newton's method to find a better estimate:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;E_{i+1} = E_i - f(E_i) / f'(E_i) \\&#xA;f'(E) = 1 - e \cos E&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Since the nonlinear part &lt;span class=&quot;math-container&quot;&gt;$e \sin E$&lt;/span&gt; is very small, we can start with the estimate &lt;span class=&quot;math-container&quot;&gt;$E=M$&lt;/span&gt;.  Our code looks something like this:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;E = M;&#xA;while(true) {&#xA;  var dE = (E - e * Math.sin(E) - M)/(1 - e * Math.cos(E));&#xA;  E -= dE;&#xA;  if( Math.abs(dE) &amp;lt; 1e-6 ) break;&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Now there are two ways to compute the position from the eccentric anomaly.  We can first compute the true anomaly and radius (the position of the object in polar coordinates), and then convert to rectangular coordinates; however, if we apply a bit of geometry we can instead compute the coordinates directly from &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt;:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;var P = a * (Math.cos(E) - e);&#xA;var Q = a * Math.sin(E) * Math.sqrt(1 - Math.pow(e, 2));&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;(&lt;code&gt;P&lt;/code&gt; and &lt;code&gt;Q&lt;/code&gt; form a 2d coordinate system in the plane of the orbit, with &lt;code&gt;+P&lt;/code&gt; pointing towards periapsis.)&lt;/p&gt;&#xA;&lt;p&gt;Finally, we can rotate these coordinates into the full 3d coordinate system:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;// rotate by argument of periapsis&#xA;var x = Math.cos(w) * P - Math.sin(w) * Q;&#xA;var y = Math.sin(w) * P + Math.cos(w) * Q;&#xA;// rotate by inclination&#xA;var z = Math.sin(i) * y;&#xA;    y = Math.cos(i) * y;&#xA;// rotate by longitude of ascending node&#xA;var xtemp = x;&#xA;x = Math.cos(W) * xtemp - Math.sin(W) * y;&#xA;y = Math.sin(W) * xtemp + Math.cos(W) * y;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;(&lt;code&gt;x&lt;/code&gt;, &lt;code&gt;y&lt;/code&gt;, and &lt;code&gt;z&lt;/code&gt; will be in units of AU.)&lt;/p&gt;&#xA;&lt;p&gt;And you're done!&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;A few tips:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;If you want to calculate the velocity as well, you can do it at the same time as you calculate &lt;span class=&quot;math-container&quot;&gt;$P$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$Q$&lt;/span&gt;, then rotate it in the same way.&#xA;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;   \dot M = n = \dot L \\&#xA;   \dot M = \dot E - e (\cos E) \dot E \\&#xA;   \dot E = \dot M / (1 - e \cos E) \\&#xA;   \dot P = -a (\sin E) \dot E \qquad \dot Q = a (\cos E) \dot E \sqrt{1 - e^2}&#xA;  $$&lt;/span&gt;&#xA;Note I don't include any of the derivatives (except &lt;span class=&quot;math-container&quot;&gt;$\dot L$&lt;/span&gt;) in this calculation, since they don't affect the outcome much.  You could code this as:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt; var vP = - a * Math.sin(E) * Ldot / (1 - e * Math.cos(E));&#xA; var vQ = a * Math.cos(E) * Math.sqrt(1 - e*e) * Ldot / (1 - e * Math.cos(E));&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Note that the velocities will be in AU per century.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If you are updating the positions very frequently, you could use the previous value of &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt; to seed Newton's method, and do a fixed number of iterations (probably just one would suffice).  Note however that you need to keep that value of &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt; local to each object!&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You can also just use a fixed number of iterations for the initial solution.  Even for &lt;span class=&quot;math-container&quot;&gt;$e=0.2$&lt;/span&gt;, after three iterations the error in &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt; is only about &lt;span class=&quot;math-container&quot;&gt;$10^{-13}$&lt;/span&gt;, and after four iterations the error is smaller than the rounding error of an IEEE double up to &lt;span class=&quot;math-container&quot;&gt;$e=0.42$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;If you want more information you can search online, but if you're really interested you should read an introductory text on orbital mechanics.  I personally recommend &lt;em&gt;Fundamentals of Astrodynamics&lt;/em&gt; by Bate, Mueller, and White &lt;a href=&quot;http://www.fgg.uni-lj.si/%7E/mkuhar/Zalozba/Fundamentals_of_Astrodynamics-Bate_Mueller&amp;amp;White-1971.pdf&quot; rel=&quot;noreferrer&quot;&gt;(pdf)&lt;/a&gt;.  My dad used this book back when he was in college, and I found it to be more readable than my college textbook.  You'd be interested in Chapter 4, &lt;em&gt;Position and Velocity as a Function of Time.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="3075" LastEditDate="2020-08-20T10:22:25.353" LastActivityDate="2020-08-20T10:22:25.353" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="8916" PostTypeId="2" ParentId="8908" CreationDate="2015-04-26T16:54:36.993" Score="1" Body="&lt;p&gt;While Ilmari Karonen's answer is fine I think there's a more fundamental issue here that you're missing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We built the ISS weak not because we couldn't do better but because there was no reason to do better.  Thus it is in no way an indication of what we can do.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-04-26T16:54:36.993" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8917" PostTypeId="2" ParentId="8911" CreationDate="2015-04-26T17:05:15.310" Score="10" Body="&lt;p&gt;Since its just a game, would you be happy with circular orbits and the planets' orbits only affected by the central body?  In that case, the propagation is quite simple.  In the plane of the orbit with the central body at (0,0), the position as a function of time is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$x(t)=a\cos\left({2\pi \left(t-t_0\right)\over T}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$y(t)=a\sin\left({2\pi \left(t-t_0\right)\over T}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $a$ is the semi-major axis, or really just the orbit radius in this case, $T$ is the orbit period, and $t_0$ determines the phasing of the orbit, where at $t=t_0$, the planet is on the x-axis on the positive side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make the orbits of the different planets consistent with each other, you just need to define the $GM$ of the central body, which we will call $\mu$.  Then for any orbit radius $a$, the orbit period is related to $a$ by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T=2\pi\sqrt{a^3\over\mu}$$&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-04-26T17:05:15.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8918" PostTypeId="2" ParentId="8912" CreationDate="2015-04-26T17:25:00.187" Score="4" Body="&lt;p&gt;I don't think he's saying Earth rotates to the West. He's on a trajectory eastwards that just makes it look as if Earth is rotating the other way. He's moving much faster than Earth's rotation speed. The lack of outside references makes it difficult to judge your trajectory in space. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-26T17:25:00.187" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8919" PostTypeId="2" ParentId="8909" CreationDate="2015-04-26T19:24:54.640" Score="1" Body="&lt;p&gt;The question of course meant discarded stages landing, since so far all first stages have been discarded.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course with the coming of SpaceX launches from Vandenberg Air Force Base's SLC-4E launch complex, they intend to land their first stages at SLC-4W. That would be an exception of where a lower stage may land, different than all other boosters launching out of VAFB.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-27T19:16:22.590" LastActivityDate="2015-04-27T19:16:22.590" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8920" PostTypeId="1" CreationDate="2015-04-27T02:26:17.740" Score="3" ViewCount="764" Body="&lt;p&gt;Is there any heavy enough gas that could be used to make an artificial atmosphere around Ceres without it escaping into space?&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="12102" LastEditDate="2019-06-07T13:09:11.287" LastActivityDate="2019-06-07T13:09:11.287" Title="Heavy gas artificial atmosphere on Ceres?" Tags="|atmosphere|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="8921" PostTypeId="2" ParentId="8912" CreationDate="2015-04-27T08:11:50.237" Score="0" Body="&lt;p&gt;His words &quot;seemed to be rotating beneath me&quot; indicates the apparent rotation of Earth, so he has &quot;launched&quot; from England at night time, moving east, and the ground appears to be moving in a westerly direction behind him.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;His mention of the mid-atlantic, then the night, indicates he's already made a full rotation around the Earth in this time, ascending in a spiral path.  (I don't interpret this as an orbit, as this is a work of fantasy.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-04-27T08:11:50.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8923" PostTypeId="2" ParentId="8920" CreationDate="2015-04-27T11:41:31.883" Score="11" Body="&lt;p&gt;Jeans escape is one of the principal mechanisms via which planets and moons loses their lighter atmospheric gases. Jeans escape would be a viable escape mechanism even for xenon on Ceres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Molecules of gas have a random velocity described by the Maxwell distribution. The most probable velocity is a function of temperature and atomic weight: $v_p = \sqrt{2kT/m}$. Escape velocity is independent of mass. The velocity distribution of a component of a celestial object's atmosphere needs to be well below escape velocity in order for a celestial object to hold on to that component of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll assume a modest atmosphere on Ceres of some high molecular mass gas. The Earth's exobase is about 600 km. I'll assume a much smaller value of 20 km for Ceres. The temperature in the Earth's atmosphere is complex. It drops linearly in the troposphere, rises in the stratosphere, falls again in the mesosphere, and rises in the thermosphere. It's the temperature at the top of the thermosphere that drives thermal escape. In the Earth's atmosphere, this can be well over 1000 kelvin. Ceres is further from the Sun; I'll assume a smaller value of 400 kelvin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Escape velocity from an altitude of 20 km on Ceres is about 500 m/s. The most probable velocity of xenon at 400 kelvin is about 225 m/s. This is far too large. The Maxwell distribution has a long tail. The loss rate is proportional to $v_\text{p} (\lambda+1) e^{-\lambda}$, where $\lambda = {v_\text{esc}}^2/{v_\text{p}}^2$. In the case of xenon at 400 kelvin and an escape velocity of 500 m/s, $\lambda$ is about 5. (Compare to helium escaping from the Earth's atmosphere, where $\lambda$ is almost 14.) Ceres wouldn't be able to hold on to a xenon atmosphere. It would escape thermally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about a complex molecule whose molecular mass is greater than that of xenon? There are two problems here. One is that Ceres is rather cold. Most large complex molecules aren't gases at 168 to 235 kelvin, Ceres' surface temperature. Uranium hexafluoride (molecular weight = 352) would be a nice candidate if Ceres was hot. Sulfur hexafluoride is close with a triple point of 223.5 kelvin at 2.3 bar, but its molecular mass of 146 is only slightly larger than that of xenon. Another problem is that the high energy radiation from the Sun would dissociate those complex molecules, and then it's game over because the products have smaller molecular masses and hence higher thermal velocities.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="6816" LastEditDate="2015-04-27T15:33:54.263" LastActivityDate="2015-04-27T15:33:54.263" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="8925" PostTypeId="1" AcceptedAnswerId="8926" CreationDate="2015-04-27T14:29:13.807" Score="4" ViewCount="913" Body="&lt;p&gt;Landing a first stage rocket (F9R) in the ocean has been stated that it is corrosive and will damage rocket components, thus making rapid reusability impossible or too expensive to repair each component. The other goals are to land on a barge or return to a launch pad on land. While we're still waiting for success, what is the affect of fresh water on a rocket? Would dropping a first stage rocket in a fresh water lake allow for rapid reusability? I don't know what the affect of fresh water, perch, walleye, and catfish is on a first stage rocket, but I imagine it is less than a fireball after smacking a barge or salty ocean water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One site I had in mind would be the Great Lakes, specifically, Lake Erie. In Cleveland it has rail connecting the docks at the Lake to a NASA flight center and international airport (8km from lake).&lt;/p&gt;&#xA;" OwnerUserId="8472" LastEditorUserId="51" LastEditDate="2015-04-27T15:11:33.220" LastActivityDate="2015-12-16T13:02:22.340" Title="Could a rocket first stage be rapidly reusable after landing in freshwater?" Tags="|spacex|landing|reuse|falcon-9|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8926" PostTypeId="2" ParentId="8925" CreationDate="2015-04-27T15:11:23.547" Score="6" Body="&lt;p&gt;Lake Superior is much much bigger, colder, and less populated on its rim.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the core issue with the F9R landing in the ocean has been survival of the impact in the water. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike an SRB, the Falcon 9 first stage is not very strong.  The SRB being a solid, requires that the entire case take the pressure of the burning engine. (To terminate an SRB, you pop the top off with explosives, or unzip it down the length so that there is no net thrust). But the SRB's are very strong, and STILL they would get bent out of shape on landing in the water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 first stage while not pressure stabilized like an original Atlas first stage (which could not support its own weight, unpressurized) it does rely on some pressurization for support during launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But not strong enough to hit the water and survive, as the attempts to land in the ocean without an ASDS barge to land on demonstrated. It can get to zero height, zero velocity, but the topple over into the water destroys it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-04-27T15:11:23.547" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8927" PostTypeId="2" ParentId="8824" CreationDate="2015-04-27T16:29:33.987" Score="2" Body="&lt;p&gt;If I understand your question correctly, what you actually want to know is, if you replaced the FH disposable second stage and payload with a combined get-into-low-orbit-and-return vehicle, what could the size of that vehicle be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that's just FH's second stage mass (~101 tons) plus LEO payload mass (~53 tons), for a total of ~154 tons. Quite a bit more than an STS orbiter!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging Spaceflight101's numbers for the Falcon Heavy into the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;rocket equation&lt;/a&gt;, and assuming it's using the planned fuel crossfeed system, the boosters provide 2387m/s ∆v, the core 3647m/s, and the second stage 3364m/s, for a total of 9398m/s. Orbital speed is about 7.7km/s, so subtracting out the second stage leaves you with 4.3km/s at core burnout. I'm not sure what sort of trajectory FH would normally fly, but you'd probably be above 175km at that point, effectively in vacuum for purposes of engine efficiency. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Lose about 1.1km/s (or cut the payload by 20 tons) if they wind up not doing fuel crossfeed.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for your orbit-and-return vehicle, you've got a mass budget of 154 tons, and you probably want 4000m/s of ∆v for orbit and deorbit -- more if you're thinking about a powered landing back on Earth. Put a NTR with an ISP of 800 seconds on it and you'd need about 40% propellant fraction, 62 tons of LH2. It'd be bulky as hell!  &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-04-28T15:32:28.357" LastActivityDate="2015-04-28T15:32:28.357" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="8928" PostTypeId="1" CreationDate="2015-04-27T19:40:21.330" Score="1" ViewCount="1031" Body="&lt;p&gt;I'm trying to calculate the required delta-v to travel from a 200 km Earth orbit to a 200 km Mars orbit using the last equation from &lt;a href=&quot;https://space.stackexchange.com/a/1444/49&quot;&gt;this answer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V=\sqrt{{\mu_S\over r_1}\left(\sqrt{\frac{2r_2}{r_1+r_2}}-1\right)^2+\frac{2\mu_1}{a_1}}-\sqrt{\mu_1\over a_1}+\sqrt{{\mu_S\over r_2}\left(\sqrt{\frac{2r_1}{r_1+r_2}}-1\right)^2+\frac{2\mu_2}{a_2}}-\sqrt{\mu_2\over a_2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe I have filled all the variables correctly, but I'm getting a delta-v value of 783 921 km/s. This is definitely not correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y8deF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here&lt;/a&gt; is what the filled in equation looks like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y8deF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y8deF.jpg&quot; alt=&quot;Hohmann transfer orbit calculation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know what I did wrong?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to use this equation in a space exploration game I'm working on.&lt;/p&gt;&#xA;" OwnerUserId="9032" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2018-02-23T00:09:31.013" Title="Need help calculating delta-v required for Earth to Mars low orbit Hohmann transfer" Tags="|mathematics|hohmann-transfer|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="8929" PostTypeId="5" CreationDate="2015-04-27T20:22:39.240" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T20:22:39.240" LastActivityDate="2015-04-27T20:22:39.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8930" PostTypeId="4" CreationDate="2015-04-27T20:22:39.240" Score="0" Body="Related to the fabrication of products to be used in spaceflight, or the techniques and logistics of production of spaceflight-related items." OwnerUserId="4660" LastEditorUserId="12102" LastEditDate="2019-07-18T08:02:06.863" LastActivityDate="2019-07-18T08:02:06.863" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="8931" PostTypeId="5" CreationDate="2015-04-27T20:25:56.903" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T20:25:56.903" LastActivityDate="2015-04-27T20:25:56.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8932" PostTypeId="4" CreationDate="2015-04-27T20:25:56.903" Score="0" Body="Any system that launches objects into space, by traditional rockets, reusable craft, or other means, such as mass drivers, cannons, tethers, launch loops, sling launchers, and so on." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T20:25:56.903" LastActivityDate="2015-04-27T20:25:56.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8933" PostTypeId="5" CreationDate="2015-04-27T20:44:52.163" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T20:44:52.163" LastActivityDate="2015-04-27T20:44:52.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8934" PostTypeId="4" CreationDate="2015-04-27T20:44:52.163" Score="0" Body="The technical measurements, parameters, or characteristics of spacecraft, and equipment used in space or in support of space missions. Examples: dry weight, gauge, thrust, materials, voltage, etc." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T20:44:52.163" LastActivityDate="2015-04-27T20:44:52.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8936" PostTypeId="5" CreationDate="2015-04-27T20:48:57.247" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T20:48:57.247" LastActivityDate="2015-04-27T20:48:57.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8937" PostTypeId="4" CreationDate="2015-04-27T20:48:57.247" Score="0" Body="A substance detected beyond Earth whose presence may indicate the existence of a living organism, in the present or past." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T20:48:57.247" LastActivityDate="2015-04-27T20:48:57.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8938" PostTypeId="5" CreationDate="2015-04-27T21:14:17.743" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T21:14:17.743" LastActivityDate="2015-04-27T21:14:17.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8939" PostTypeId="4" CreationDate="2015-04-27T21:14:17.743" Score="0" Body="Messaging to Extra-Terrestrial Intelligence - actively sending messages into space in an attempt to contact alien beings. Projects to date have sent messages to specific selected stars." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T21:14:17.743" LastActivityDate="2015-04-27T21:14:17.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8940" PostTypeId="5" CreationDate="2015-04-27T21:25:41.320" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-04-27T21:25:41.320" LastActivityDate="2015-04-27T21:25:41.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8941" PostTypeId="4" CreationDate="2015-04-27T21:25:41.320" Score="0" Body="Measurements that may indicate the presence of technology of non-human origin, typically in the telescope data on a planet or star. Examples are radiation emissions and spectrometry that don´t fit models for natural occurrence." OwnerUserId="4660" LastEditorUserId="8693" LastEditDate="2016-01-14T08:44:43.283" LastActivityDate="2016-01-14T08:44:43.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8942" PostTypeId="1" AcceptedAnswerId="9798" CreationDate="2015-04-27T22:30:06.727" Score="6" ViewCount="2602" Body="&lt;p&gt;I'm watching a launch sit on hold because the sky is overcast. The announced hold reason was &quot;thick cloud cover, anvil cloud&quot;. So they have a checklist. What is on that checklist? How do they assess that checklist - for instance, how far away does an anvil cloud have to be for them to consider conditions a go? In this launch, they are aiming for a gap in the conditions that they are calculating to arrive at a particular time, and adjusting the countdown accordingly. They must be looking at the radar and judging when the cloud cover and surrounding clouds will fall within acceptable parameters.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2015-04-27T22:56:16.047" LastActivityDate="2015-07-07T04:45:36.977" Title="How good does the weather have to be for a rocket launch?" Tags="|launch|rockets|launch-sequence|weather|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8943" PostTypeId="1" AcceptedAnswerId="8960" CreationDate="2015-04-27T23:53:16.510" Score="6" ViewCount="493" Body="&lt;p&gt;The only reference paper I have found on this subject is &lt;a href=&quot;https://www.researchgate.net/publication/267800402_Analysis_of_a_Lunar_Sling_Launcher&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt; by Landis, which is a great introduction to sling launchers - but the concept has been around for some time and there must be other treatments. At any rate, it is a simple device where a motor on a tower spins two cables - one bearing a payload and the other a counterweight - so fast that when released the payload flies into orbit or beyond. Modern motors are capable of doing this, and would even be highly efficient, if the cables used are very long - on the order of 50 km or more. The cables have to be incredibly strong, that's all.&lt;/p&gt;&#xA;&lt;p&gt;The paper looks at the use of modern materials for such a device, but only briefly because the cables would be huge and extremely costly. It moves on to future possibilities using carbon nanotube cables.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;If fullerene materials are not available, the concept could be&#xA;implemented with existing materials. This increases the tether mass,&#xA;and the sling launch becomes more difficult, but not impossible. The&#xA;highest strength-to-weight ration for a currently available tether&#xA;material are obtained with Poly(p-phenylene-2,6-benzobisoxazole), or&#xA;&amp;quot;PBO&amp;quot; fibers, or with gel-spun polyethylene fibers. PBO (sold under&#xA;the trade name &amp;quot;Zylon®&amp;quot;) has a with tensile strength of 5.8 GPA, and&#xA;density of 1.54 g/cm3 [12, 13]. High-strength polyethylene fiber (sold&#xA;under the trade name Spectra-2000) has an ultimate strength of 4.0 GPa&#xA;and a density of 0.97 g/cm3 [11]. Assuming an engineering factor of&#xA;2.5, the allowable load strength for the Spectra-2000 fiber is&#xA;1.6 GPa. For the example case of a launch to lunar orbit, 1.68 km/second, the required acceleration is&#xA;5.7 g (56 m/sec2 ). To carry a thousand kilogram payload, the force will be 56000 N. This will require a cable cross-section of 0.35&#xA;square centimeters at the tip. Since the cable must have  additional&#xA;cross-section to carry its own weight as well as the end mass, the&#xA;cable must now be made to increase in cross-section from the tip to a&#xA;wider cross-section toward the hub. This taper increases the cable&#xA;mass. The cable mass is now about 2500 kg, no longer less than the&#xA;mass of the launched object, but still a value which is feasible for&#xA;an engineering system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;How was this analysis of the cable done?&lt;/p&gt;&#xA;&lt;p&gt;The reference paper made this calculation for escape velocity from the Moon. Could these materials take much more load without snapping under their own weight in such a scenario?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2021-04-21T15:58:54.210" LastActivityDate="2021-04-21T15:58:54.210" Title="What payloads and launch speeds could a sling launcher get using modern materials on the Moon?" Tags="|the-moon|materials|analysis|performance|launch-system|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="8946" PostTypeId="1" CreationDate="2015-04-28T02:36:29.080" Score="9" ViewCount="304" Body="&lt;p&gt;Would it reduce the cost/time to go to Mars with people if the transit vehicle was accelerated using less expensive means (i.e. ion engine and/or gravity assists), and the people got took a small quick vehicle to catch up?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More detailed example:&#xA;Launch the transit vehicle (the space ship that will go to Mars) long before the people will actually start their voyage. Then use slow acceleration techniques (i.e. ion thruster and/or gravity assists) to speed up the transit vehicle. When the transit vehicle is going fast enough, send the people up in a small, quick craft to get on the transit vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the transit vehicle stayed orbiting Earth, presumably it would have to be in an elliptical orbit.&#xA;If the transit vehicle used gravity assists, presumably it could get much faster speeds than an elliptical orbit could produce and it would be harder to get the timing correct for the transit vehicle to pass Earth on its way to Mars.&lt;/p&gt;&#xA;" OwnerUserId="9034" LastActivityDate="2015-04-28T03:42:07.987" Title="Would it be cheaper to go to Mars if the empty transit vehicle is slowly accelerated before the passengers board?" Tags="|mars|propulsion|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="8947" PostTypeId="2" ParentId="8946" CreationDate="2015-04-28T03:42:07.987" Score="11" Body="&lt;p&gt;Yes.  This has &lt;a href=&quot;http://nicadd.niu.edu/~mwiesner/Landau_Strange.pdf&quot;&gt;already been proposed by Landau and Strange&lt;/a&gt;, using a high-Earth orbit as the staging point.  Our Moon provides a convenient and mostly free means of raising and lowering the periapsis of the orbit of the transit vehicle.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-04-28T03:42:07.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8954" PostTypeId="1" AcceptedAnswerId="8964" CreationDate="2015-04-28T08:48:02.813" Score="0" ViewCount="14790" Body="&lt;p&gt;With our current technology could we build a spaceship like the ones you see in Sci-Fi movies (&lt;a href=&quot;http://en.wikipedia.org/wiki/Starship_Enterprise&quot; rel=&quot;nofollow&quot;&gt;Enterprise&lt;/a&gt; in Star Trek or &lt;a href=&quot;http://en.wikipedia.org/wiki/Star_Destroyer&quot; rel=&quot;nofollow&quot;&gt;Star Destroyer&lt;/a&gt; in Star Wars)? Could self-gravitation be a problem?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would assume building it on earth would be a bad idea because of the immense power needed to bring it to orbit. But what about in orbit, is it possible with todays technology on such a big scale?&lt;/p&gt;&#xA;" OwnerUserId="9029" LastActivityDate="2017-01-31T15:55:09.540" Title="Could we build a spaceship like the ones you see in Sci-Fi movies with current technology?" Tags="|spacecraft|gravity|construction|" AnswerCount="1" CommentCount="5" ClosedDate="2015-04-28T15:17:13.313" ContentLicense="CC BY-SA 3.0" />
  <row Id="8956" PostTypeId="1" AcceptedAnswerId="8958" CreationDate="2015-04-28T09:23:40.717" Score="22" ViewCount="7433" Body="&lt;p&gt;I am watching loads of ISS related videos and there is one detail I recently noticed;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the time, when Space Shuttles were still in operation and visiting ISS, when astronauts returned home, they were able to walk off the orbiter as it landed to a full stop on the runway. See &lt;a href=&quot;https://www.youtube.com/watch?v=8akgPBjkL-Y&amp;amp;feature=youtu.be&amp;amp;t=16m45s&quot;&gt;related video&lt;/a&gt; (it will lead you to correct time) of Space Shuttle Atlantis:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/8akgPBjkL-Y?start=1005&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I compare it with returning home via Soyuz, the astronauts and cosmonauts have to be taken off by the rescue team and then carried on chairs where they would gradually adjust to Earth's gravity. &lt;a href=&quot;https://youtu.be/KxOp_Rd8_cA?t=5m4s&quot;&gt;Related video&lt;/a&gt; (again, at time of egress):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/KxOp_Rd8_cA?start=304&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I immediately assumed that it must be the design difference between Space Shuttles (USA made) and Soyuz craft (Russian). Am I assuming correctly? Or did I miss something?&lt;/p&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="49" LastEditDate="2015-04-28T09:53:16.097" LastActivityDate="2020-01-13T22:52:06.467" Title="Why were Space Shuttle astronauts able to walk off the orbiter?" Tags="|landing|space-shuttle|soyuz-spacecraft|design|procedure|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8957" PostTypeId="2" ParentId="8956" CreationDate="2015-04-28T09:47:49.087" Score="12" Body="&lt;p&gt;The shuttle post landing activities took a while to complete, and this would include the flight surgeon boarding to check the crew over before leaving.  This would enable the crew to stretch their legs and recover a bit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For interest, the &lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Crew Operations Manual&lt;/a&gt; has a list of post landing activities the crew go through (Section 5.5 - no timeline but this would have taken a while to get through).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's very rare for astronauts to admit to weaknesses of any sort, but &lt;a href=&quot;http://www.jsc.nasa.gov/history/oral_histories/FullertonCG/FullertonCG_5-6-02.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gordon Fullerton had some interesting comments&lt;/a&gt; (p. 36) on walking into door jambs and so on, for a day after shuttle landings.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="11721" LastEditDate="2020-01-13T22:52:06.467" LastActivityDate="2020-01-13T22:52:06.467" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="8958" PostTypeId="2" ParentId="8956" CreationDate="2015-04-28T10:40:09.517" Score="51" Body="&lt;p&gt;Look at the STS-122 video. How many astronauts do you see? I see six. Seven astronauts landed with STS-122. The six you see were the crew of STS-122 who spent twelve days in space. They could walk because twelve days in zero g isn't enough time to take a significant toll on musculature, bones, and blood.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The seventh returning astronaut, Daniel Tani, who you don't see in the video, spent three months in space. That is more than enough time to take a toll on musculature, even with a regular exercise regime on the Space Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Expedition 34/35 crew with Chris Hadfield, Thomas Marshburn and Roman Romanenko that you see landed on second video were on a long-duration mission that launched aboard Soyuz TMA-07M on 19 December 2012 and landed on 13 May 2013, for total duration in microgravity of 145 days and 14 hours. Spending almost five months in zero g takes a significant toll on the body.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-04-28T14:44:36.837" LastActivityDate="2015-04-28T14:44:36.837" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8959" PostTypeId="1" CreationDate="2015-04-28T11:57:59.303" Score="10" ViewCount="522" Body="&lt;p&gt;If I go for an Extra-Vehicular Activity on LEO and I drop a ball, I'm pretty sure nothing will happen in the short term. Both me and the ball are in a circular free fall around the earth CoG and dropping it has no incidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, what happens if I give it a gentle nudge down? Not on the order of magnitude of an end-of-mission deorbiting burn, just human powered, maybe with a tennis racket? &#xA;What happens if I push it up? If I push it back in my tail along our orbital path, or forward in front of me?&lt;/p&gt;&#xA;" OwnerUserId="8755" LastEditorUserId="7017" LastEditDate="2015-04-28T18:52:06.000" LastActivityDate="2015-06-27T16:22:15.697" Title="Pushing down a projectile from LEO" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8960" PostTypeId="2" ParentId="8943" CreationDate="2015-04-28T14:47:31.023" Score="7" Body="&lt;p&gt;The paper does not describe how the calculations for the tether are done, but I can make a guess.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We take a small piece of the tether with mass $\delta m$ and length $\delta r$, at distance $r$ from the center.  The tether is spinning at an angular rate $\omega$, and has an ultimate tensile strength $\sigma_\text{max}$ and density $\rho$.  The area $A$ is a function of $r$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can write a relationship between the tension $T(r)$ on both sides of the piece:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;T(r) - T(r+\delta r) = \delta m\ a_c = \delta m\ r \omega^2&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can substitute $\delta m=\rho A\delta r$, and recast this as a differential equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;-\frac{\partial T}{\partial r}\delta r = \delta r A(r) \rho r\omega^2 \\&#xA;\frac{\partial T}{\partial r} = -\rho\omega^2rA&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next we can substitute $T=\sigma A$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;T = \sigma A \\&#xA;\frac{\partial T}{\partial r} = \frac{\partial \sigma}{\partial r}A + \sigma\frac{\partial A}{\partial r} = -\rho\omega^2rA&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two types of solution we have to consider:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Cable with Constant Cross-section&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;When the cable has a constant area, the differential equation becomes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{\partial \sigma}{\partial r} = -\rho\omega^2r \\&#xA;\sigma = -\rho\frac{\left(r\omega\right)^2}2 + C&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tension on the end of the cable, where a payload of mass $M$ is supported, is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;T(R) = M\omega^2 R \\&#xA;\sigma(r)A = M\omega^2 R&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Setting $\sigma = M\omega^2 R/A$ at $r=R$ and $\sigma=\sigma_\text{max}$ at $r=0$, we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\sigma_\text{max} - \frac{(R\omega)^2}{2\rho} = \frac{M \omega^2 R}{A}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting in the edge velocity $V=\omega R$ and solving for $A$ we get a required cable area of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;A = \frac{MV^2}{R}\left(\sigma_\text{max} - \rho\frac{V^2}{2}\right)^{-1}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can rewrite this in terms of the critical velocity $v_\text{crit}=\sqrt{2\sigma_\text{max}/\rho}$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;A = \frac{MV^2}{R\sigma_\text{max}\left(1-(V/v_\text{crit})^2\right)}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for some examples.  I'll use the three speeds given in the paper:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lunar Orbit: $1.68~\text{km}/\text{s}$&lt;/li&gt;&#xA;&lt;li&gt;Lunar Escape Trajectory: $2.40~\text{km}/\text{s}$&lt;/li&gt;&#xA;&lt;li&gt;Mars Injection Orbit: $3.84~\text{km}/\text{s}$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And I'll assume a &lt;a href=&quot;http://www.honeywell-advancedfibersandcomposites.com/products/fibers/&quot; rel=&quot;noreferrer&quot;&gt;Spectra&lt;/a&gt; cable:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\sigma_\text{max}=3.6~\text{GPa}$&lt;/li&gt;&#xA;&lt;li&gt;$\rho = 0.97~\text{g}/\text{cm}^3$&lt;/li&gt;&#xA;&lt;li&gt;$v_\text{crit}=2.7~\text{km}/\text{s}$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/V3SbB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dashed lines represent the required area with a margin of $25\%$.  Note that the fastest speed doesn't even appear on the plot: since it's faster than $v_\text{crit}$ the cable would snap under its own weight.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Tapered Cable (Varying Cross-section)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;When the cable is operating at maximum performance, $\sigma=\sigma_\text{max}$ along its whole length:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{\partial A}{\partial r} = -\frac{\rho\omega^2}{\sigma_\text{max}}rA \\&#xA;A(r) = A(0) \exp\left\{-\frac{\rho \omega^2}{2\sigma_\text{max}}r^2\right\} \\&#xA;A(r) = A(0) \exp\left\{-\left(\frac V{v_\text{crit}}\right)^2\right\}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, to support a mass $M$ at the end of the cable ($r=R$) we have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\sigma_\text{max} A(r) = M\omega^2 R \\&#xA;\sigma_\text{max} A(0)\exp\left\{-\left(\frac V{v_\text{crit}}\right)^2\right\} = M\frac{V^2}{R} \\&#xA;A(0) = \frac{MV^2}{R\sigma_\text{max}}\exp\left\{\left(\frac V{v_\text{crit}}\right)^2\right\}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our plot now looks something like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/itNei.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can find the total mass of the cable by integrating $\delta m$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\int \delta m = \int \rho A\ \delta r\\&#xA;= M\sqrt{\pi}\left(\frac{V}{v_\text{crit}}\right)e^{\left(\frac{V}{v_\text{crit}}\right)^2}\text{erf}\left(\frac{V}{v_\text{crit}}\right)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ratio between cable mass and payload mass turns out to be purely a function of the ratio between tip velocity and the critical velocity.  It looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/P8UdW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tether works on the Moon, but we can see that on Mars or Earth where the escape speeds are two to five times higher, the tether system quickly becomes impractical with current materials.  Even carbon nanotubes, with $v_\text{crit}=9.6~\text{km}/\text{s}$, would have a cable-to-payload mass ratio of over 7 launching from Earth.&lt;/p&gt;&#xA;" OwnerUserId="8996" LastActivityDate="2015-04-28T14:47:31.023" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="8961" PostTypeId="2" ParentId="8959" CreationDate="2015-04-28T15:37:56.337" Score="9" Body="&lt;p&gt;This answer assumes that you start in a circular orbit of radius $r$ and speed $v_\text{circ}=\sqrt{\mu/r}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you push the ball along the direction of the orbit, it will go into an elliptical orbit of semimajor axis $a$ and eccentricity $e$, with perigee at the location of the push and a slightly increased apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/fIcfC.png&quot; alt=&quot;ball pushed forward, dv/vcirc = 0.05&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The periapsis radius will not change:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;r_\text{per} = (1-e)a=r&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the periapsis velocity will increase by a small amount $\Delta v$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;v_\text{per}=\sqrt{\frac{(1+e)\mu}{(1-e)a}}=\sqrt{\frac\mu r}+\Delta v&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solving these two equations we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;a = \frac r{1-\Delta v^2r/\mu-2\Delta v\sqrt{r/\mu}} \approx r\left(1+2\frac{\Delta v}{v_\text{circ}}\right) \\&#xA;e = \Delta v^2r/\mu+2\Delta v\sqrt{r/\mu} \approx 2\frac{\Delta v}{v_\text{circ}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The new apoapsis altitude is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;r_\text{ap} = (1+e)a \approx r\left(1+4\frac{\Delta v}{v_\text{circ}}\right)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you push the ball against your direction of motion, it will go into an elliptical orbit with apogee at the location of the push, and new perigee radius:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;r_\text{per} \approx r\left(1-4\frac{\Delta v}{v_\text{circ}}\right)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If you push the ball towards local nadir (down), the new orbit of the ball will be lower in the first half, cross the original orbit 180 degrees later, and then be higher in the second half.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BfPCb.png&quot; alt=&quot;ball pushed down: dv/vcirc = 0.25&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you push it towards zenith (up) the reverse happens: the orbit is higher in the first half and lower in the second half.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pushing the ball along the line between it and the planet's center doesn't change its orbital angular momentum, so that remains constant:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;h^2 = \mu r = \mu a(1-e^2)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the velocity increases slightly, increasing the amount of energy:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\epsilon = -\frac\mu{2a} = \frac{v_\text{circ}^2 + \Delta v^2}{2} - \frac\mu r = \frac{\Delta v^2}{2}-\frac\mu{2r}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solving for $a$ and $e$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;a = \frac{r}{1-\Delta v^2 r/\mu} \approx r\left(1+\left(\frac{\Delta v}{v_\text{circ}}\right)^2\right) \\&#xA;e = \Delta v\sqrt{\frac r\mu} = \frac{\Delta v}{v_\text{circ}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The new apoapsis and periapsis altitudes are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;r_\text{ap} \approx r\left(1+\frac{\Delta v}{v_\text{circ}}\right) \\&#xA;r_\text{per} \approx r\left(1-\frac{\Delta v}{v_\text{circ}}\right)&#xA;$$ &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If you push perpendicular to the orbital plane, the situation looks like pushing forward (raised apogee), but the orbital plane is also rotated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equations are almost the same as before, except that this time the orbital momentum changes as well.  The new speed $v$ is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;v = \sqrt{v_\text{circ}^2+\Delta v^2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the new orbital momentum equation looks like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;h^2 = (rv)^2 = r^2\left(v_\text{circ}^2+\Delta v^2\right) = \mu a(1-e^2)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy is the same as before:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;-\frac\mu{2a}= \frac{\Delta v^2}{2}-\frac\mu{2r}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solving for $a$ and $e$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;a = \frac{r}{1-\left(\Delta v/v_\text{circ}\right)^2} \\&#xA;e = \left(\frac{\Delta v}{v_\text{circ}}\right)^2&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ball will end up in an orbit with perigee equal to the original radius, but a slightly higher apogee:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;r_\text{ap} = r\left(\frac{1+\left(\Delta v/v_\text{circ}\right)^2}{1-\left(\Delta v/v_\text{circ}\right)^2}\right) \approx r\left(1+\left(\frac{\Delta v}{v_\text{circ}}\right)^2\right)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbital plane will also be rotated by an angle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta \phi = \arctan \frac{\Delta v}{v_\text{circ}} \approx \frac{\Delta v}{v_\text{circ}}&#xA;$$&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="8996" LastEditDate="2015-05-08T15:12:40.487" LastActivityDate="2015-05-08T15:12:40.487" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8962" PostTypeId="2" ParentId="8959" CreationDate="2015-04-28T16:08:14.177" Score="13" Body="&lt;p&gt;For a sense of what would happen just as a mental picture, what you do to the ball wouldn't translate clearly into a change in its orbit until you had both moved along on your paths through a fairly large angle. At the altitude of the ISS, of about 400 km, it takes about 90 minutes to complete an orbit. A whack on a tennis ball with a racket could impart a speed of 120 km/h in the direction it was struck. Things move in straight lines unless other forces act on them. Gravity is acting on the ball, but the direction it is pulling it in is changing over time as the the ball continues on its orbit. The speed you gave it of 120 km/h is small compared to the orbital speed it already had of around 8 km/s.  22.5 minutes after you hit the ball, you and it will have moved through 1/4 of your orbits, and the direction gravity is pulling on you both is now at a right angle from the direction when you hit it. In that time, the ball has moved about 40 km because of the impulse you gave it, in whatever direction you struck it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Hitting the ball towards zenith or nadir (directly towards or away from the center of the Earth):&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you hit the ball straight down towards Earth, it still has the orbital speed to be higher and it will return to that height - plus a little more. You added energy to it by hitting it and that will translate into a higher apogee. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;22.5 minutes after you strike it, the direction you pushed it is no longer straight towards Earth, it is horizontal to the Earth and aligned with its direction of orbit, because you have both covered a quarter orbit in that time. Over that time, the ball has been getting nearer to the surface of the Earth, and so the Earth's gravity has been pulling on it slightly more than before. That has added a little momentum to it. 90&lt;sup&gt;o&lt;/sup&gt; later, that extra speed, plus the speed you gave it, are both moving the ball faster in the direction of orbit. This point is where it will be closest to Earth (perigee) and moving fastest. While at first you would have seen the ball moving straight down, now you will see it moving faster and parallel to the ground, pulling out in front of you in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond that point, the higher orbital speed of the ball causes it to gain altitude. 180&lt;sup&gt;o&lt;/sup&gt; after hitting it, the ball crosses the altitude of its previous orbit and continues to rise. Through this quarter of its orbit, its orbital velocity was dropping. The orientation of the impulse you gave it is now in direct opposition to gravity, because the ball is now directly on the other side of the planet. But the greater speed gained as it fell towards Earth continues to push it away from the planet. Its orbital speed slows as it rises and you begin to catch up to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;270&lt;sup&gt;o&lt;/sup&gt; after hitting it, 3/4 of the way around the planet, it reaches apogee - its highest point above the planet, and also its slowest orbital speed. You are at a lower altitude and your greater orbital speed has caught you up to the ball. You pass under it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the ball's total energy, and its total angular momentum, is greater than yours due to the energy you gave it in striking it, it's orbital path is now longer than yours. The average height of the ball's orbit is now higher than yours, which is a larger ellipse with a slower average orbital speed. Once both of you have returned to the spot where the impulse happened, you are ahead of it, and the ball again passes the height of its previous orbit. From your point of view, the ball completes an asymmetric loop around you 90&lt;sup&gt;o&lt;/sup&gt; later. But because the ball will continue to fall further behind you will each orbit, it won't continue to loop around you. From your point of view, it will appear to continue to loop, but ever further behind you - until you eventually 'lap' it and pass it again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider the impulse as movement on a straight line along the direction you hit it, a line that doesn't change orientation, unlike the changing orientation of the pull of gravity. That line is longest 90&lt;sup&gt;o&lt;/sup&gt; later in your orbits, and the ball then reverses direction along it. 180&lt;sup&gt;o&lt;/sup&gt; after, the ball has returned along the line to the spot where it started and begins to extend it in the opposite direction. At 270&lt;sup&gt;o&lt;/sup&gt; it reaches the furthest point on the line in the opposite direction. Once back where the impulse occurred, it returns to the starting point on that line again. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hitting the ball towards the zenith, away from Earth, would cause a new orbit that is an exact mirror image of the one just described, with apogee 90&lt;sup&gt;o&lt;/sup&gt; later and perigee 270&lt;sup&gt;o&lt;/sup&gt; later. Consider the exact opposite action happening in each step along the way, and that is what results. But again it is a larger orbit, because energy has been added to the ball.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Hitting the ball forwards or backwards along your orbit (a prograde or retrograde impulse):&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the impulse here is aligned with the orbital path the ball already had, the dynamics are simpler. Impulses in these directions are what are usually used to raise and lower the orbits of spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hitting the ball forwards gives the ball a greater orbital speed, which causes it to pull further away from the planet. This time the arrow in the direction it was struck points away from the planet, at varying angles, through the first half of the following orbit, and towards the planet through the second half. Through that first half the ball gains height and slows, through the second half it loses height and speeds up. Again the added energy means the size of the orbital ellipse is larger and the orbital period is longer. In the first half, the ball would be in front of your position and higher, in the second half it would fall behind you and return to your altitude. Again it would appear to execute a loop from your perspective, and would continue to loop and fall further behind with each successive orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hitting the ball backwards robs the ball of orbital speed. It begins to lose altitude and reaches its lowest point half an orbit later. The dynamics are the opposite of the description above. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Energy was removed from the ball and its orbit is now smaller, and takes a shorter time to complete. That energy was transferred to you and your orbit changes in the opposite manner, but to a smaller degree because you weigh so much more. This is also true when you hit the ball in any other direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=_wPsTs2dt04&quot; rel=&quot;nofollow&quot;&gt;Coin funnels&lt;/a&gt; have a shape that imitates the dynamics of a gravity well.  The link is to a video that may help in visualization. The ball bearings move around the hole very much as objects orbit a central body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A great way to develop an instinct for this is to download even just the demo of &lt;a href=&quot;https://kerbalspaceprogram.com/en/&quot; rel=&quot;nofollow&quot;&gt;Kerbal Space Program&lt;/a&gt; and go through the tutorial. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-27T15:07:24.280" LastActivityDate="2015-06-27T15:07:24.280" CommentCount="7" CommunityOwnedDate="2015-05-09T16:38:44.093" ContentLicense="CC BY-SA 3.0" />
  <row Id="8963" PostTypeId="1" CreationDate="2015-04-28T16:09:13.370" Score="13" ViewCount="814" Body="&lt;p&gt;What existing life on Earth is best suited for the Martian environment? By best suited, I mean it could theoretically be placed on Mars and easily adapt or thrive within the existing environment the planet has to offer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know from my own research that &lt;a href=&quot;http://en.wikipedia.org/wiki/Extremophile&quot; rel=&quot;noreferrer&quot;&gt;extremophiles&lt;/a&gt; are potential candidates. There is a discussion about &lt;a href=&quot;https://space.stackexchange.com/questions/96/could-any-life-from-earth-survive-in-enceladus-oceans&quot;&gt;Could any life from Earth survive in Enceladus&amp;#39; oceans?&lt;/a&gt; which is relevant to my question, however this focuses on bacteria found in water only. There is a question on &lt;a href=&quot;https://space.stackexchange.com/questions/4089/is-it-possible-to-grow-plants-on-mars&quot;&gt;Is it possible to grow plants on Mars?&lt;/a&gt; which is related, however not exactly the same, and from the information that the answer contains leads me to believe that plants would not be good candidates for my question. There is also a discussion which relates to the search for life on Mars and how we could &lt;a href=&quot;https://space.stackexchange.com/questions/3109/how-would-the-search-for-life-on-mars-be-affected-by-having-boots-on-the-ground&quot;&gt;contaminate it with human boots on the ground&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="9040" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-05-04T10:18:53.510" Title="What existing life on Earth is best suited for the Martian environment?" Tags="|mars|life|astrobiology|feasibility|earth|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8964" PostTypeId="2" ParentId="8954" CreationDate="2015-04-28T16:24:16.923" Score="6" Body="&lt;p&gt;You are correct that it would be impractical to build such behemoths on a planet. Enormous projects like those require the necessary motivation, resources, and technology.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Motivation&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Building a starship the size of a city is a much greater challenge than building an earthly city. Building without gravity would come with some advantages. You wouldn't need cranes, and there would be no &quot;heavy lifting&quot; of the same sort when heaviness is no longer a thing, per se. But every advantage of space construction will be outweighed by numerous challenges, which I will go into more later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here on Earth, cities emerge for economic purpose. People gather in an area because there is a resource there, or something useful going on there. It's very rare for a city to be built from scratch at the direction of a wealthy person or entity. And even then, it is for some specific purpose. Building something the size of a city in space which can not only house hundreds or thousands of people, but also has the ability to accelerate to incredible speeds would need some extraordinary motive. In Star Wars, the motive is heavy-handed governance. There is no one to govern in space right now. In Star Trek, the motive is exploration. With current technology, we cannot realistically explore outside of our solar system. For the organizations dedicated to exploration within the solar system, like NASA, funding is a constant battle, and amounts to a fraction of a percent of the budget that would be required for an Enterprise class starship. The only motive I can imagine outside of the world of religion would be the opportunity to travel to other inhabited worlds.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Resources&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As you mentioned already, you don't want to build the thing here on the planet, because launching it up there would be too costly. Likewise, sending the resources to build the vehicle piecemeal would be too costly. So where do we get the materials that we would build a starship from? They need to be in space already, not in the gravity well of another planet, but just floating around out there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While some necessary trace elements could be mined on a planet and sent up, the bulk of the building material needs to come from asteroids or comets. So the primary thing we are missing right now, in terms of resources, is a space mining industry. There are &lt;a href=&quot;http://www.planetaryresources.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;people working&lt;/a&gt; on that already, but so far, no asteroid has been mined. And it's very likely a long way off. To support a project of this magnitude, the industry would need to be mature enough to provide the resources required on-demand at a price that is comparable to mining here at home. If this is not so, the total cost of the project would be even further out of reach.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Technology&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;We are already working on the technology to find other inhabited worlds. We have found nearly 4000 exoplanets so far, and NASA has predicted that we will find evidence of life out there within decades. If we do find life out there, can we send people there to explore it with current technology? Well, no. We have no hyperdrive. In fact, it appears most likely that travelling beyond the speed of light is not possible with any technology. It might be possible to send sleepers, or a &lt;a href=&quot;http://en.wikipedia.org/wiki/Generation_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;generation ship&lt;/a&gt;, which our ancesters might hear back from in many thousands or millions of years. But, it's a lot harder to get excited about sending away our people never to see them again. So again, who's going to pay for that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Engine technology is not the only thing we are missing. Assuming we have the raw resources we need, we still have to process them into the materials that we would need for this ship. So beyond space mining, we need to figure out space smelting, space manufacturing, processing, assembly, etc. There are a nearly unimaginable number of details to work out. We would essentially need to reimagine and rebuild civilization itself again. Consider, for example, the fact that most of these things require burning fuel when we do them here. All &quot;burning&quot; here on Earth, with few exceptions, requires the presence of a volatile chemical that is so abundant here that we tend to take it for granted: oxygen (O&lt;sub&gt;2&lt;/sub&gt;). In space, we either need to reinvent every process that involves oxygen, or come up with a whole oxygen production industry in space. Here on Earth, trees do that for us. But they are huge. And they like having gravity. So we might turn to algae or something. But again, my point is that whole industries and technologies have to be reinvented for space. Can we do it?! Yes we probably might be able to! But the technology does not currently exist. Not yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, a project like this would take a heck of a long time. Who is going to be up there doing all of that building? If thousands of people are going to be up there day after day, we need to develop an entire space farming industry to support them. We need whole cities to be already up there just to support the human needs of those construction workers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's pretty far out of reach right now. So what about robots? Artificial Intelligence is still nowhere near being able to provide an autonomous robot mind capable of executing a complex construction project, but what about remote control? If the whole project took place in orbit, we could communicate with a robot army with a reasonable time-delay, and control them completely from the ground. It would take just about forever to do almost all of the construction remotely. But, it's theoretically possible, I guess, given that this robot army exists up there somehow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So... I guess all we need is a robot army, full-blown space industry, technology that may or may not be possible, and a reason to want to build this thing. In the words of Lloyd Christmas, &quot;So you're telling me there's a chance!&quot;&lt;/p&gt;&#xA;" OwnerUserId="945" LastEditorUserId="945" LastEditDate="2017-01-31T15:55:09.540" LastActivityDate="2017-01-31T15:55:09.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8965" PostTypeId="2" ParentId="8963" CreationDate="2015-04-28T18:15:20.910" Score="15" Body="&lt;p&gt;There is a chamber at the German Aerospace Center designed to imitate the Martian environment. So far there are two published experiments showing that some organisms could survive in the chamber. An imitation of Martian regolith was used, and the organisms were exposed to an imitation of Martian atmosphere with the pressure, temperature range, moisture fluctuations, and solar radiation exposure of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;a href=&quot;http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-3409/#/gallery/5674&quot;&gt;first test&lt;/a&gt;, alpine and polar lichens managed to &lt;a href=&quot;http://www.astrobio.net/topic/origins/extreme-life/lichen-on-mars/&quot;&gt;increase&lt;/a&gt; levels of photosynthesis over the course of 34 days in this environment - when they had only a reduced radiation dose to deal with similar to that found in cracks and recesses in the regolith. Samples exposed to full radiation doses simply managed to survive. Some cyanobacteria also survived.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;a href=&quot;http://www.nature.com/srep/2014/140529/srep05114/pdf/srep05114.pdf&quot;&gt;second test&lt;/a&gt;, a species of black yeast recovered from an initial dive in metabolic activity after the first day of exposure, to normal levels after a full week. A second fungus, a microcolonial variety, survived but was struggling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: The test chamber doesn't imitate Martian gravity or the cosmic radiation that would be experienced on the Martian surface. Possibly that skews results.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-04T02:22:30.920" LastActivityDate="2015-05-04T02:22:30.920" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8967" PostTypeId="1" CreationDate="2015-03-22T21:29:48.443" Score="0" ViewCount="39" Body="&lt;p&gt;I was wondering if the &lt;a href=&quot;http://en.wikipedia.org/wiki/Voyager_Golden_Record&quot; rel=&quot;nofollow&quot;&gt;microgroove golden records&lt;/a&gt; they put on the Voyager spacecraft, along with instruction on how to decode it, were first put to the test with the subjects most likely to decode them, i.e. humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would suppose they were, as Carl Sagan takes the complementary stance in &lt;em&gt;Contact&lt;/em&gt; to have humans decode an alien message, but incidentally the alien message mentioned looks quite too close, both in methodology and content, to what humans sent a decade earlier, non-fictionally, on the Voyagers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, did they test whether or not they were decodable, and was a 100% interpretation achieved?&lt;/p&gt;&#xA;" OwnerUserId="8755" OwnerDisplayName="nmajoros" LastActivityDate="2015-04-28T21:50:29.353" Title="Were the Voyager Golden Records tested for being decoded?" Tags="|interstellar-travel|" AnswerCount="0" CommentCount="0" FavoriteCount="0" ClosedDate="2015-04-28T21:58:23.280" ContentLicense="CC BY-SA 3.0" />
  <row Id="8969" PostTypeId="2" ParentId="8452" CreationDate="2015-04-29T13:45:11.533" Score="7" Body="&lt;p&gt;Your question contains a couple of misconceptions: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a kinetic kill vehicle must have the same kinetic energy as the satellite it intends to destroy&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, it doesn't. It just needs enough kinetic energy to break critical parts inside the satellite. The same goes for micrometeoroids. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If my understanding of orbital mechanics is correct, the speeds of the two objects is likely to be equal, so the kinetic kill vehicle must have a similar mass to the satellite it intends to destroy. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That would be the least efficient way to destroy a satellite. You'd launch the kinetic kill vehicle on a trajectory that gives it the highest speed relative to the satellite. A head-on collision would be perfect: both objects traveling at the same speed but in opposite directions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the speeds involved, the KKV likely disintegrates on impact, creating a nice big cloud of debris tearing through the satellite. The &lt;a href=&quot;http://en.wikipedia.org/wiki/ASM-135_ASAT&quot; rel=&quot;noreferrer&quot;&gt;Vought ASM-135 ASAT&lt;/a&gt; used its third stage as its KKV. No need to have a separate impactor.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using micrometeoroids sounds nice and efficient, but you'd need to accelerate and guide all of them. Much easier to accelerate and guide one larger object. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-29T13:45:11.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8970" PostTypeId="1" AcceptedAnswerId="8972" CreationDate="2015-04-29T13:49:42.897" Score="5" ViewCount="553" Body="&lt;p&gt;For a university project I'm making thermal assumptions and introductionary design of a small satellite. &#xA;Satellite electronics use around 30W energy in total (inc. Camera, on board computer etc.) . Some of 30W of course, will not be used as work but they go waste as heat.&#xA;To make calculations about satellite, I need assumptions about how many watts will be dissipated as heat by electronics? What will be the waste heat?&#xA;Any help would be great.&lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="25" LastEditDate="2015-04-29T15:36:37.493" LastActivityDate="2015-04-29T15:36:37.493" Title="How can I make robust guesses about waste heat from satellites?" Tags="|artificial-satellite|thermal|thermal-control|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8971" PostTypeId="2" ParentId="8970" CreationDate="2015-04-29T14:19:12.377" Score="11" Body="&lt;p&gt;As a first approach, you can assume all electric power will be turned into heat. Some of the power will be used to do something first, but electrical and mechanical resistance will eventually turn all power into heat. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-04-29T14:19:12.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8972" PostTypeId="2" ParentId="8970" CreationDate="2015-04-29T14:35:10.493" Score="7" Body="&lt;p&gt;Almost all energy from a satellite will end up as heat. The exception is power that is transmitted away, which is primarily RF only. Sound and light are not options for transmission of power usually. So just assume that all electrical power turns in to heat, with possibly subtracting out the RF energy output from the transmitter.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-04-29T14:35:10.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8974" PostTypeId="1" AcceptedAnswerId="8975" CreationDate="2015-04-29T17:01:13.287" Score="4" ViewCount="184" Body="&lt;p&gt;I am an electronic engineer trying to dive in space engineering. &#xA;To develop an ADCS system, one of the most famous and easy algorithms is TRIAD. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does this algorithm need the sun and the magnetic field? What do those two elements provide us more than other elements to know the satellite's attitude (for example)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is (IMO) no need to dive into complex mathematics or physics. I would like to get a general understanding of the &quot;space-mechanics&quot; of the algorithm.&lt;/p&gt;&#xA;" OwnerUserId="8968" LastEditorDisplayName="user12" LastEditDate="2015-04-30T01:59:26.417" LastActivityDate="2015-04-30T01:59:26.417" Title="Why does TRIAD need sun vectors and magnetic field vector?" Tags="|magnetic-field|the-sun|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8975" PostTypeId="2" ParentId="8974" CreationDate="2015-04-29T19:39:37.640" Score="5" Body="&lt;p&gt;You can also use the triad method with other linearly independent reference vectors, like star trackers, earth/horizon sensors, or directional antennas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sun trackers are cheap, and useful for other tasks like solar panel and instrument positioning. Magnetometers are also pretty cheap.&lt;/p&gt;&#xA;" OwnerUserId="759" LastActivityDate="2015-04-29T19:39:37.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8976" PostTypeId="1" CreationDate="2015-04-29T19:02:05.747" Score="1" ViewCount="440" Body="&lt;p&gt;Suppose we want to do a lunar sample return mission. We can then launch a small rocket from the ISS that carries a small probe to the Moon. It will collect a sample and then return to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the entire mission happens in almost a perfect vacuum, it should be possible to do this using miniature rockets and a miniature probe. In principle everything can be scaled down to perhaps the molecular scale, but with the technology available today, there may be some limitations to how small one can make the probe. &lt;/p&gt;&#xA;" OwnerUserId="11234" OwnerDisplayName="Count Iblis" LastEditorUserId="4660" LastEditDate="2015-04-29T20:59:07.353" LastActivityDate="2015-12-30T15:02:50.540" Title="Smallest lunar probe that can be made using today's technology" Tags="|the-moon|probe|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8977" PostTypeId="2" ParentId="3631" CreationDate="2015-04-29T21:35:52.863" Score="9" Body="&lt;p&gt;In &lt;a href=&quot;https://kerbalspaceprogram.com/&quot; rel=&quot;noreferrer&quot;&gt;Kerbal Space Program&lt;/a&gt;, they are called &lt;strong&gt;Radial&lt;/strong&gt; and &lt;strong&gt;Anti-Radial&lt;/strong&gt; on the Nav-Ball.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Radial and anti-radial&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;These vectors are parallel to the orbital plane, and perpendicular to the prograde vector. The radial (or radial-in) vector points inside the orbit, towards the focus of the orbit, while the anti-radial (or radial-out) vector points outside the orbit, away from the body. Performing a radial burn will rotate the orbit around the craft like spinning a hula hoop with a stick. Radial burns are usually not an efficient way of adjusting one's path - it is generally more effective to use prograde and retrograde burns. The maximum change in angle is always less than 90°; beyond this point the orbit would pass through the center of mass of the orbited body and the ship would traverse a slow spiral in towards the orbited body.&lt;/p&gt;&#xA;&lt;p&gt;- &lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Maneuver_node#Directions&quot; rel=&quot;noreferrer&quot;&gt;KSP Wiki - Maneuver Node Directions&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/U5Eoq.png&quot; alt=&quot;KSP Symbol Gallery&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="6821" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-04-30T02:19:01.220" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="8978" PostTypeId="1" CreationDate="2015-04-30T05:48:15.623" Score="6" ViewCount="466" Body="&lt;p&gt;Recent ISS Cargo delivery via Progress ship was a failure, and it seems that Russian mission control lost the cargo ship completely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Progress ship is basically the same design as Soyuz:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the procedures for unlikely event when you lost communication with the ground? Is it better for astronauts and cosmonauts to try to reach ISS manually or go to abort and land?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume both variants of &quot;lost communication&quot;:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Communication via radio is OK (you can hear the crew), but you cannot upload any commands to on-board computer&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Communication is completely silent. Crew does not have any chance of contacting the mission control, but the ship functions &quot;normally&quot; otherwise.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="49" LastEditDate="2015-05-01T01:46:03.273" LastActivityDate="2015-05-01T01:46:03.273" Title="Emergency plan for Soyuz missions" Tags="|communication|soyuz-spacecraft|crewed-spaceflight|procedure|abort|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8979" PostTypeId="2" ParentId="8978" CreationDate="2015-04-30T12:19:21.890" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What are the procedures for unlikely event when you lost&#xA;  communication with the ground? Is it better for astronauts and&#xA;  cosmonauts to try to reach ISS manually or go to abort and land?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Back in the Apollo days, the astronauts would always have a &quot;PAD&quot; read up to them, hours or even days before any major manoeuvre.  For any burn, the ground would read up to them the angles to point the ship, the precise timing and thrust setting (if appropriate).  The astronauts would write this down on paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That way, if communication was lost, they would always be able to execute the velocity change well enough to carry on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For present flights to the ISS in low earth orbit, they would launch with all of the required burns known in advance, and I guess they would be able to make their own way if necessary provided the launch went as planned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note however, loss of communication is a different problem to the case of a vehicle spinning out of control, as is happening with the unmanned Progress vehicle.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: &quot;PAD&quot;, from &lt;a href=&quot;http://www.hq.nasa.gov/alsj/apollo.glossary.html&quot; rel=&quot;nofollow&quot;&gt;http://www.hq.nasa.gov/alsj/apollo.glossary.html&lt;/a&gt; :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Preliminary Advisory Data: the crew had pre-printed forms on which&#xA;  they could write lift-off times and other data they would need in the&#xA;  event that communications was lost with Houston. Before and after each&#xA;  rest period, the CapCom would read up a list of lift-off times&#xA;  covering the next 10 to 12 hours and, prior to launch, a longer list&#xA;  of data was read up.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-04-30T12:19:21.890" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8980" PostTypeId="1" AcceptedAnswerId="8981" CreationDate="2015-04-30T17:05:18.640" Score="6" ViewCount="884" Body="&lt;p&gt;Watching the &lt;a href=&quot;https://www.youtube.com/watch?v=rEdk-XNoZpA&amp;amp;t=47s&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt; of the first Blue Origin &lt;a href=&quot;http://en.wikipedia.org/wiki/Blue_Origin_New_Shepard&quot; rel=&quot;noreferrer&quot;&gt;New Shepard&lt;/a&gt; vehicle launch, it takes off from ground level, and does not seem to have an exhaust trench:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/rEdk-XNoZpA?start=47&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most launchers of reasonable size have a trench, so that the exhaust has somewhere to go, other than impinging back on the vehicle. On top of that, they often have a Niagara water system, where huge amounts of water are dumped on the pad, to vaporize (absorb heat energy) and absorb sound energy to protect the launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does the Blue Origin vehicle not need such systems?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-04-30T19:31:46.920" LastActivityDate="2015-05-03T01:09:32.217" Title="Why does the New Shepard vehicle not have an exhaust trench?" Tags="|launch|safety|launchpad|blue-origin|new-shepard|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="8981" PostTypeId="2" ParentId="8980" CreationDate="2015-04-30T17:08:16.420" Score="8" Body="&lt;p&gt;I may have my own answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like this test vehicle uses a single BE-3 engine, which only has about 110,000 lbs of thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have seen the SpaceX Grasshopper and F9R Dev1 vehicles take off from flat surfaces on a single engine as well, and the Merlin 1D engine produces 145,000 lbs of thrust (in 85% performance mode that apparently it has been running in for the first 15 or so F9 launches. SpaceX reports it plans to run it at 100% for GEO launches to provide margin from returning the first stage). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we have seen that 145,000 lbs of thrust does not require an exhaust trench, nor a Niagara water system. Thus a 110,000 lb thrust launcher is probably fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, we have seen the Saturn V (7.5 million lbs of thrust), the Space Shuttle (7 million lbs of thrust), and Falcon 9 (1.3 million lbs of thrust) do require both a trench and a Niagara system.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would be interesting to find how high you can go without those systems before issues arise.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-04-30T20:16:37.033" LastActivityDate="2015-04-30T20:16:37.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8982" PostTypeId="2" ParentId="8980" CreationDate="2015-04-30T17:11:47.797" Score="5" Body="&lt;p&gt;They are planning to take off and land vertically, so the vehicle has to be designed to tolerate engines firing close to the ground without a trench anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of thrust that an engine provides doesn't determine the need for a trench by itself.  You could, for example, raise the entire vehicle or provide protection on the engine itself instead of having a trench.  These tend to be less desirable approaches though -- especially for lower thrust engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This video shows some more details on the space below the engine on launch and landing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/-YJhymiZjqc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/-YJhymiZjqc&quot; rel=&quot;nofollow&quot;&gt;Blue Origin concept video&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-05-03T01:09:32.217" LastActivityDate="2015-05-03T01:09:32.217" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8983" PostTypeId="2" ParentId="4084" CreationDate="2015-04-30T19:12:02.653" Score="-1" Body="&lt;p&gt;It orbits same direction as Earth. You can expect all space stations always to be build to orbit in direction of earth. Saves fuel to reach them since you get extra boost while leaving Earth and less full again when slowing down when reaching the station. If it would rotate against earth the speed would be twice as night relative to you and you would need to spend more power to slow down. Or if you would chase it lose the extra boost leaving earth and waist there no fuel.&lt;/p&gt;&#xA;" OwnerUserId="9064" LastActivityDate="2015-04-30T19:12:02.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="8984" PostTypeId="2" ParentId="8536" CreationDate="2015-04-30T20:01:06.620" Score="5" Body="&lt;p&gt;Around pages 49ff., in chapter 2 of Carl Sagan's &quot;Murmurs of Earth&quot; (1978), Frank Drake describes how in 1960 he presented a test message encoded like the Arecibo message to a group of scientists called the Order of the Dolphin+, and was &quot;depressingly shocked&quot; that not a single receipient managed to decode it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;+The Order contained 11 scientists interested in SETI; among others: Carl Sagan, Nobel prize winner Melvin Calvin, dolphin expert John Lilly.&lt;/p&gt;&#xA;" OwnerUserId="8862" LastActivityDate="2015-04-30T20:01:06.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8985" PostTypeId="5" CreationDate="2015-04-30T20:24:36.793" Score="0" Body="&lt;p&gt;&lt;strong&gt;Blue Origin&lt;/strong&gt; is an American private aerospace manufacturer set up by Jeff Bezos, founder of Amazon. The company intends to provide commercial suborbital space tourism services on a reusable rocket, and is developing plans to expand service to orbital trips.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of April 30, 2015, the company has flown three test vehicles:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Charon&lt;/strong&gt;, which flew to an altitude of 316 feet on jet engines on March 5, 2005&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Goddard&lt;/strong&gt;, a nine-engine VTOL which flew to 285 feet and returned to its launch site on November 13, 2006&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;New Shepard&lt;/strong&gt;, a crew capsule as well as a liquid hydrogen-powered propulsion module. On March 27, 2015 the system completed its first suborbital test flight. The crew capsule reached 307,000 feet before parachuting back to Earth. The propulsion module reached 304,000 feet, and did not survive its first landing because hydraulic pressure was lost during the descent.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="759" LastEditorUserId="759" LastEditDate="2015-04-30T21:22:19.187" LastActivityDate="2015-04-30T21:22:19.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8986" PostTypeId="4" CreationDate="2015-04-30T20:24:36.793" Score="0" Body="Blue Origin is a Jeff Bezos-founded private American aerospace manufacturer which is attempting to commercialize space tourism." OwnerUserId="759" LastEditorUserId="759" LastEditDate="2015-04-30T21:21:47.600" LastActivityDate="2015-04-30T21:21:47.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8987" PostTypeId="2" ParentId="7956" CreationDate="2015-05-01T14:48:25.347" Score="6" Body="&lt;p&gt;In a NASA TV Press conference, before the first pad abort, Hans Konnigsman noted that the pad abort will not be using a full rocket underneath it.  (Makes sense, why potentially damage a $50 million vehicle that is not really needed).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;He also said that they would not be using the full Transport Erector either. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus these parts looks like they may be the truss and a mounting bracket for the pad abort. Or they may be development models that were later refined for the actual test.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as the Pad Abort date approaches, SpaceX released a picture of the Dragon V2 on the pad below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jSzRX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So unless the trunk like stage is the trunk in use here, it appears the truss segment is not being used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly the notion of launching from a structure the same height as a Dragon for launch would be is no longer happening, and it will be aborting right from ground level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceFlightNow (Stephen Clark) has an excellent photo of the test article on the stand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EoM5Z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see a support truss, (pretty thin) behind it. It looks nothing like the truss seen at McGregor. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-05-06T10:41:45.153" LastActivityDate="2015-05-06T10:41:45.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8988" PostTypeId="1" AcceptedAnswerId="8990" CreationDate="2015-05-01T21:11:26.483" Score="21" ViewCount="6097" Body="&lt;p&gt;From skimming &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot;&gt;the Wikipedia article on ion thrusters&lt;/a&gt;, I notice that xenon is frequently (though not exclusively) used as the reaction mass in systems that have actually been deployed - Deep Space 1, Hayabusa, SMART-1, and Dawn, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the properties of xenon that make it attractive for use in ion thrusters? I imagine that its inertness is certainly helpful, but in that case, why xenon as opposed to, say, krypton or some other noble gas?&lt;/p&gt;&#xA;" OwnerUserId="527" LastEditorUserId="846" LastEditDate="2015-05-02T21:58:45.480" LastActivityDate="2020-05-31T21:30:57.580" Title="Why do ion thrusters frequently use xenon as the reaction mass?" Tags="|ion-thruster|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8990" PostTypeId="2" ParentId="8988" CreationDate="2015-05-01T21:27:25.387" Score="29" Body="&lt;p&gt;Xenon is the heaviest non-radioactive elemental inert gas. The added mass allows for denser packing at less pressure. The mass is one of the limiting factors, so having a more dense gas helps tremendously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The limiting factor relates to the mass of the propellant. Essentially, a heavier mass allows for more momentum to come from the overall system. The mass will take longer to accelerate, allowing more momentum to be exerted on the particle. Radioactivity could cause all kinds of issues, as could something that would be reactive. Elemental is easier because it's easier to manipulate, and as you have to make the gas ionic, if it's not elemental it will have a much higher potential to react with something. Thus, it is more efficient to use a heavy elemental non-radioactive inert gas.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-22T14:58:52.857" LastActivityDate="2015-07-22T14:58:52.857" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="8991" PostTypeId="1" CreationDate="2015-05-01T21:34:31.810" Score="3" ViewCount="41" Body="&lt;p&gt;Why Messenger spacecraft fell upon Mercury? What made its orbit to decrease so to lead to the impact?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE. Seems a duplicate.&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="812" LastEditDate="2015-05-01T21:58:01.357" LastActivityDate="2015-05-01T21:58:01.357" Title="What made Messenger's orbit to decrease?" Tags="|messenger|" AnswerCount="0" CommentCount="6" ClosedDate="2015-05-01T21:53:20.090" ContentLicense="CC BY-SA 3.0" />
  <row Id="8992" PostTypeId="5" CreationDate="2015-05-01T23:28:26.903" Score="0" Body="&lt;p&gt;The Blue Origin New Shepard reusable launch system is a vertical-takeoff, vertical-landing (VTVL), suborbital manned rocket that is being developed by Blue Origin, a company owned by Amazon.com founder and businessman Jeff Bezos, as a commercial system for suborbital space tourism. The New Shepard name makes reference to the first United States astronaut in space, Alan Shepard.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/blue-origin&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;blue-origin&amp;#39;&quot; rel=&quot;tag&quot;&gt;blue-origin&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Blue_Origin_New_Shepard&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="897" LastEditDate="2015-07-22T07:15:19.807" LastActivityDate="2015-07-22T07:15:19.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8993" PostTypeId="4" CreationDate="2015-05-01T23:28:26.903" Score="0" Body="Vertical takeoff and landing suborbital rocket in development by Blue Origin." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:28:26.903" LastActivityDate="2015-05-01T23:28:26.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8994" PostTypeId="5" CreationDate="2015-05-01T23:33:48.277" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:33:48.277" LastActivityDate="2015-05-01T23:33:48.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8995" PostTypeId="4" CreationDate="2015-05-01T23:33:48.277" Score="0" Body="Machines making working copies of themselves using only raw materials found in their environment. The possible scope of missions in deep space, especially involving mining or construction, entirely change if such technology is developed." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:33:48.277" LastActivityDate="2015-05-01T23:33:48.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8996" PostTypeId="5" CreationDate="2015-05-01T23:39:25.590" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:39:25.590" LastActivityDate="2015-05-01T23:39:25.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8997" PostTypeId="4" CreationDate="2015-05-01T23:39:25.590" Score="0" Body="A formula with 7 terms used to estimate the number of civilizations in the Milky Way capable of radio communication with Earth. Most terms of the equation cannot be meaningfully defined. It functions principally to define what needs to be determined to find that number." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:39:25.590" LastActivityDate="2015-05-01T23:39:25.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8998" PostTypeId="5" CreationDate="2015-05-01T23:43:42.907" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:43:42.907" LastActivityDate="2015-05-01T23:43:42.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="8999" PostTypeId="4" CreationDate="2015-05-01T23:43:42.907" Score="0" Body="The study of non-human civilizations, currently confined to investigation of how to find such civilizations, if they exist, and to consideration of how to proceed if one is found." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:43:42.907" LastActivityDate="2015-05-01T23:43:42.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9000" PostTypeId="5" CreationDate="2015-05-01T23:52:43.017" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:52:43.017" LastActivityDate="2015-05-01T23:52:43.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9001" PostTypeId="4" CreationDate="2015-05-01T23:52:43.017" Score="0" Body="&quot;If extraterrestrial intelligence exists, why have we found no evidence?&quot;, first defined as a paradox by Enrique Fermi and colleagues, after estimating based on various facts that an advanced civilization has had time to colonize the entire galaxy many times over." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-01T23:52:43.017" LastActivityDate="2015-05-01T23:52:43.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9002" PostTypeId="5" CreationDate="2015-05-02T00:03:59.180" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-02T00:03:59.180" LastActivityDate="2015-05-02T00:03:59.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9003" PostTypeId="4" CreationDate="2015-05-02T00:03:59.180" Score="0" Body="Search for Extraterrestrial Intelligence, an organization that scans the sky for signs of non-human technology. Study has focused on analysis of the radio spectrum of other stars, but also involves other approaches." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-02T00:03:59.180" LastActivityDate="2015-05-02T00:03:59.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9004" PostTypeId="5" CreationDate="2015-05-02T00:32:14.077" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-02T00:32:14.077" LastActivityDate="2015-05-02T00:32:14.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9005" PostTypeId="4" CreationDate="2015-05-02T00:32:14.077" Score="0" Body="Modified 747 aircraft used to transport the space shuttles from landing sites to the Kennedy Space Center." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-02T00:32:14.077" LastActivityDate="2015-05-02T00:32:14.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9006" PostTypeId="1" CreationDate="2015-05-02T00:48:51.540" Score="3" ViewCount="226" Body="&lt;p&gt;Suppose you are sitting on a bench on Mars under open sky and under your feet there is a grate from which contantly there is an air flow. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under the bench there is a tube through which oxygen is pumped from a generating plant and released in the air just under the bench.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This creates higher pressure and oxygen content in the air locally so that you can breath even without a special mask or suite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder whether this setup is possible on Mars.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Under current Mars conditions&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In the case Mars is terraformed for greater pressure by melting its poles, but not for greater oxygen.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If the park is situated under a non-hermetic tent which, possibly, has a non-sealed opening as a door to the outside&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="812" LastActivityDate="2015-05-02T08:30:09.133" Title="Is it possible to make open-air parks on Mars by putting oxygen tubes here and there?" Tags="|colonization|terraforming|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9008" PostTypeId="2" ParentId="8988" CreationDate="2015-05-02T02:40:51.913" Score="7" Body="&lt;p&gt;According to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Ion_thruster&quot;&gt;wikipedia page on ion thrusters&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ionization energy represents a very large percentage of the energy&#xA;  needed to run ion drives. The ideal propellant for ion drives is thus&#xA;  a propellant molecule or atom that is easy to ionize, that has a high&#xA;  mass/ionization energy ratio. In addition, the propellant should not&#xA;  cause erosion of the thruster to any great degree to permit long life;&#xA;  and should not contaminate the vehicle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-05-02T17:28:44.707" LastActivityDate="2015-05-02T17:28:44.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9009" PostTypeId="2" ParentId="9006" CreationDate="2015-05-02T05:22:01.560" Score="7" Body="&lt;p&gt;No, due to the ambient pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmospheric pressure on Mars is less than 1% of earth's atmospheric pressure (see the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&quot;&gt;Nasa Mars Fact Sheet&lt;/a&gt;). As the Oxygen is released, if it is at a higher pressure than the surrounding atmosphere, it will rapidly expand until the pressures equalize. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even 100% oxygen is not enough to sustain human life at such low pressure; a person's blood would immediately bubble as gasses come out of solution and they would die of the bends. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An additional difficulty for the tourist is that the temperature would be way too low. The Nasa fact sheet tells us the the mean temperature is -63°C! Although on a balmy day, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Climate_of_Mars#Temperature&quot;&gt;temperature&lt;/a&gt; can exceed 0°C.&lt;/p&gt;&#xA;" OwnerDisplayName="user8406" LastEditorDisplayName="user8406" LastEditDate="2015-05-02T08:30:09.133" LastActivityDate="2015-05-02T08:30:09.133" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9012" PostTypeId="1" CreationDate="2015-05-02T11:41:35.103" Score="4" ViewCount="320" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Suitport&quot; rel=&quot;nofollow noreferrer&quot;&gt;Suitports&lt;/a&gt; seem like &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130013652.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;a brilliant idea&lt;/a&gt; that saves lots of mass and complexity compared to an airlock, which traditionally is a dumb pressurizable gas wasting tube where the astronaut suits up. The first EVA was made using a bag as airlock. Orion is said to be able to depressurize and repressurize once. Dragon2 and CST-100 can't even do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not put four or more suitports on the outside of the upper part of the spacecraft, and launch it with fairings through the atmosphere? The Orion has a side door already anyway, why not turn it into a suitport? Could be discarded before re-entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7huPG.jpg&quot; alt=&quot;suitport diagram&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9i50b.jpg&quot; alt=&quot;suitport separating&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1142" LastEditDate="2015-05-02T18:06:11.070" LastActivityDate="2015-05-02T18:06:11.070" Title="Could suitports on Orion/Dragon2/CST-100 make multiple EVA's available?" Tags="|eva|orion-spacecraft|airlock|suitport|dragon-v2|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9013" PostTypeId="2" ParentId="9012" CreationDate="2015-05-02T12:37:44.000" Score="3" Body="&lt;p&gt;Using EVA Suitports on spacefaring vessels has been thought of before. For example, &lt;a href=&quot;https://www.nasa.gov/pdf/458816main_FTD_InflatableModuleMission.pdf&quot; rel=&quot;nofollow&quot;&gt;Inflatable Mission Module&lt;/a&gt; (PDF) envisioned by Tony Sang and Gary Spexarth in 2010, discusses the use of an inflatable accommodation module that would include AR&amp;amp;D (Autonomous Rendezvous and Docking), ECLSS (Environmental Control and Life Support System), EVA suitport, and other interfaces. This proposal was never funded though, but it certainly appears achievable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some limitations to using EVA Suitports though. Spacesuits themselves would still have to be shielded when not in use to protect them from radiation damage, thermal cycling, micrometeorites and so on, and you'd require the ability to retract them back into the pressurized vehicle for pre-EVA and post-EVA inspection, be able to change at least gloves that wear out and suffer punctures the most. Otherwise yes, you could be able to save breathable air and nitrogen used for purging by having a Suitport installed on the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion isn't meant to be used on its own anyway, except for short-duration missions that don't include EVA at all. It will be used together with various service and habitation modules, so latter could, in theory, also include EVA Suitports. But they wouldn't be used on Orion crew module, or MPCV (Multi-Purpose Crew Vehicle). Since you'd require some manipulation space also inside the vehicle, MPCV is simply too small for that, and adding even more interfaces would further complicate its design.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-05-02T12:37:44.000" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9014" PostTypeId="1" AcceptedAnswerId="9017" CreationDate="2015-05-02T17:33:24.347" Score="1" ViewCount="2299" Body="&lt;p&gt;I am creating a space exploration game and I'm gathering formulas for determining how the player's spacecraft will act in space. One thing I need to know is how to calculate how far a spacecraft can be from a star when its orbital speed will be larger than the escape velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a formula for calculating this sphere of influence/heliosphere radius? Or should I check if the orbital speed is larger than the escape velocity?&lt;/p&gt;&#xA;" OwnerUserId="9032" LastEditorUserId="1142" LastEditDate="2015-05-02T17:49:41.497" LastActivityDate="2016-02-01T17:48:56.210" Title="How to calculate the sphere of influence radius of the sun and other stars?" Tags="|escape-velocity|soi|calculation|" AnswerCount="2" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9015" PostTypeId="2" ParentId="9014" CreationDate="2015-05-02T18:44:52.233" Score="2" Body="&lt;p&gt;If the spacecraft is in orbit around the star, its velocity is below escape velocity.  There is no radius above which this changes.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-05-02T18:44:52.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9017" PostTypeId="2" ParentId="9014" CreationDate="2015-05-03T13:55:50.383" Score="-1" Body="&lt;p&gt;If you want to determine if an object 'escapes' from velocity, yes, you should compare orbital velocity and escape velocity. For a two body problem, infinite radius is available for orbiting when you keep your spacecraft below escape velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, space is not a two body place; it is full of bodies and you should consider all objects. Ideally, you should use momentum and energy equations to make simulations in space. However, if you want a simpler approach which you always orbit the 'strongest' planet nearby &amp;mdash; this is what I understood from your question &amp;mdash; the best approach would be using Newton's law of gravitation.&lt;br&gt;&#xA;$F=\frac{GMm}{R^2}$ where $G$ is gravitational constant, $M$ is mass of the planet, $m$ is mass of your orbiter and $R$ is the distance between them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make maps of influences you can use $F=\frac{M}{R^2}$ for a planet because $G$ and $m$ are the same for all planets. You can make sphere-of-influence spheres with this formula. The active planet is the one which has strongest $F$ at given point, so spacecraft will orbit around it.&lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="6816" LastEditDate="2016-02-01T17:48:56.210" LastActivityDate="2016-02-01T17:48:56.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9018" PostTypeId="1" CreationDate="2015-05-03T16:20:22.737" Score="1" ViewCount="331" Body="&lt;p&gt;There has been rampant speculation relating to the EMdrive lately.    One such speculation talks about &quot;warp bubbles&quot;.   I'm not familiar with this concept.   Is there a scientific definition of what a warp bubble is?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any &quot;vague&quot; physics backing this up?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it purely uninformed media making things up to sound smart?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sciencetimes.com/articles/5966/20150430/nasa-accidentally-discovered-faster-light-travel.htm&quot; rel=&quot;nofollow&quot;&gt;NASA May Have Just Accidentally Discovered Faster than Light Travel&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I'm not asking how a device would create such a thing, simply what the scientific definition of such a concept is)&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2015-05-03T16:20:22.737" Title="What is a &quot;warp bubble&quot;?" Tags="|propulsion|physics|" AnswerCount="0" CommentCount="7" ClosedDate="2015-05-03T21:51:16.107" ContentLicense="CC BY-SA 3.0" />
  <row Id="9019" PostTypeId="1" AcceptedAnswerId="9021" CreationDate="2015-05-03T21:55:54.860" Score="11" ViewCount="919" Body="&lt;p&gt;Let's assume the flight rules don't prohibit you from going to the ISS in a malfunctioning craft and don't force you to land on next orbit, unlike the real life ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's also assume you have an Apollo era slide rule, sextant, watch, pen and paper, and the most recent orbital elements for the ISS. Can you still implement the propellant-efficient &lt;a href=&quot;https://dx.doi.org/10.1016/j.actaastro.2012.03.019&quot;&gt;fast &quot;Soyuz scheme&quot;&lt;/a&gt; rendezvous (&lt;em&gt;launch-to-docking in six hours&lt;/em&gt;) while doing all the calculations manually? For the sake of the argument, we can state that there's no other workload for all three crew members, and the transfer craft's initial orbital elements are fully known.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Will it be possible to calculate all burn times, angles and durations in time?&lt;/li&gt;&#xA;&lt;li&gt;What nomograms can drastically reduce calculation times?&lt;/li&gt;&#xA;&lt;li&gt;How will adding a navigation radar into the instrument mix ease up the calculation burden?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Notes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;sub&gt;I'm not talking about &lt;a href=&quot;http://dspace.mit.edu/handle/1721.1/12652&quot;&gt;Aldrin-type line-of-sight rendezvous scheme&lt;/a&gt;.&lt;/sub&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;sub&gt;The question does not dwell upon the final approach and docking phases.&lt;/sub&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Possibly useful: &lt;a href=&quot;https://web.archive.org/web/20120425122952/http://microcosminc.com/analysis/spie03.pdf&quot;&gt;Wertz, Bell (2003)&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-05-03T22:21:48.817" LastActivityDate="2016-04-03T14:20:42.160" Title="How can you rendezvous with the ISS via the &quot;Soyuz scheme&quot; without a computer and out of ground contact?" Tags="|orbital-mechanics|ballistics|rendezvous|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9021" PostTypeId="2" ParentId="9019" CreationDate="2015-05-04T01:55:11.607" Score="9" Body="&lt;p&gt;I'm going to hazard and say it's not very likely, more to human reaction time than anything. If one has to assume that the computers are not available for the quick launch sequence, one also has to accept that they are not available to control timing. Here's a blurb about the thruster capacity in the various nodes, from &lt;a href=&quot;http://www.spaceflight101.com/soyuz-spacecraft-information.html&quot; rel=&quot;nofollow&quot;&gt;Spaceflight 101&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SKD, the Soyuz Main Engine, provides a thrust of 2,942 Newtons. The&#xA;  entire Soyuz Attitude Control System is comprised of 28 DPO Thrusters.&#xA;  Two clusters of 14 DPO Thruster are mounted on the spacecraft with 12&#xA;  of these jets providing 26.5 Newtons of Thrust and the remaining 16&#xA;  providing 130 Newtons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Assuming a mass of around 7150 kg, as the same site indicates, that means that even with the smallest thruster, there's an acceleration of around 3.7mm/ second^2. Given a human reaction time of even a tenth of a second, that could result the in the delta-V being off by .3mm/s How much of a difference does that make? In 6 hours, that's only 6.5m. That doesn't sound so bad really, until you consider how long of a thrust is likely required. It's hard to know exactly, but I have found that to support a close orbit like this, a &lt;a href=&quot;http://www.nasaspaceflight.com/2012/08/progress-m-16m-launch-test-new-fast-rendezvous-iss/&quot; rel=&quot;nofollow&quot;&gt;30 degree phase&lt;/a&gt; angle or less is required. I don't have the exact delta v required, but I'm going to guess that it is around 10 m/s. For that length of time, one would have to do a burn with the smallest thruster around 45 minutes. I suspect the reaction time would be far worse for that length of time, but it could still theoretically be done. I'm sure the main engine is used more than the small ones, greatly reducing the thrust time required (By a factor of 100), but that would leave more uncertainty in the location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so let's say you get your angle off by 1 degree, which would be an easy thing to do without assistance. What does that do? That would given an error of around 1.6%. That means the planned 10m/s burn could be off by as much as 16 cm/s, which over 6 hours would mean an error of around 5km, fairly significant!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also keep in mind that the thrust requirements grow the longer the thrust takes to plan, and things get very complicated very quickly!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, unless you have access to some kind of a computer, it seems very unlikely that you could pull this off. There might be a person or two who could pull this off, but I doubt it. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-03T14:20:42.160" LastActivityDate="2016-04-03T14:20:42.160" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9022" PostTypeId="2" ParentId="8963" CreationDate="2015-05-04T08:55:32.990" Score="4" Body="&lt;p&gt;Humans. We have technologies to survive on Mars, and even inside Enceladus. Life doesn't only adapt itself to the environment, life also adapts the environment to its need. Spacesuits and constructions are just a part of what life does to stay alive. It is not only up to the nanoengineering of the microbes in our body, our brain is an emergent phenomena on top of that which helps the seemingly unstoppable process of creating survivability.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-05-04T08:55:32.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9026" PostTypeId="1" CreationDate="2015-05-05T13:01:20.063" Score="5" ViewCount="392" Body="&lt;p&gt;I cannot find references, but prior to the Pad Abort test, for years, it was stated or implied that the test would take place from a platform at LC-40 that would simulate the height of the first stage for realism.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But looking at pictures SpaceX sent out, of the Pad Abort unit at LC-40, it is clearly at ground level.  You can see the nozzles from the Niagara water deluge system right next to it, which are usually seen in images of the engines igniting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/h1d95.jpg&quot; alt=&quot;Dragon Pad Abort article at LC-40&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the pipe of the Niagara water deluge system, blocking our view of the engine in this photo to make the point of how close to ground level the Dragon is sitting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/I8vQQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;Hans Koenigsmann in the pre-test briefing suggested that there would be a truss structure to attach too, which I interpreted as something larger, and is most likely the upper right part of the image, with the umbilical connectors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what changed between the plans for using a platform high up, to a test down low. (A consequence noted in the press conference is that the lightning towers needed to be disassembled or maybe just the wiring removed to avoid a collision). &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-05-05T16:10:41.370" LastActivityDate="2015-06-09T07:14:06.207" Title="Why is the SpaceX Pad Abort test starting at ground level?" Tags="|spacex|testing|abort|dragon-v2|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9027" PostTypeId="1" CreationDate="2015-05-05T13:19:28.123" Score="2" ViewCount="203" Body="&lt;p&gt;I have been interested in trajectory optimization for a while now and I have read a few papers on that topic and bought the book &quot;Spacecraft trajectory optimization&quot; from Cambridge University Press and want to start programming with the goal to optimize a trajectory in a simplified solar system, not in real time but as close as possible, without the need of an exact solution but a preview of a good one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The books and papers propose different approaches like&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Analytical solutions with the primer vector&lt;/li&gt;&#xA;&lt;li&gt;Direct transcription + nonlinear programming&lt;/li&gt;&#xA;&lt;li&gt;particle swarm algorithms&lt;/li&gt;&#xA;&lt;li&gt;evolutionary/genetic algorithms&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;because I have little to no experience I would like to know what you guys would suggest because I could not find any comparisons with regard to speed of these algorithms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for your help!&lt;/p&gt;&#xA;" OwnerUserId="9091" LastEditorUserId="4660" LastEditDate="2015-05-05T13:27:37.670" LastActivityDate="2015-05-05T13:51:55.677" Title="Trajectory opimization: fast preview algorithm" Tags="|orbital-mechanics|spacecraft|trajectory|" AnswerCount="1" CommentCount="5" ClosedDate="2015-05-05T16:00:12.450" ContentLicense="CC BY-SA 3.0" />
  <row Id="9028" PostTypeId="1" CreationDate="2015-05-05T13:48:01.730" Score="7" ViewCount="887" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/press-release/statements-on-senate-confirmation-of-dava-newman-as-nasa-deputy-administrator&quot;&gt;A new deputy NASA administrator&lt;/a&gt;, Dava Newman, has been appointed. What is the actual function of a deputy administrator in NASA? What kind of decisions will she be responsible for? And is a deputy likely to become the next top boss?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-05-05T15:16:07.820" Title="What role does the NASA deputy administrator play?" Tags="|nasa|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9029" PostTypeId="2" ParentId="9027" CreationDate="2015-05-05T13:51:55.677" Score="3" Body="&lt;p&gt;Here are some quick pieces of information based on my experience coding / researching each of these areas:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Primer vector: The primer vector has a fairly elegant solution once you find the Hamiltonian and other required variables in the algorithm (e.g., the adjoints). Once you're able to set up the problem, the solution tends to fall out fairly well. Most of the applications I've seen with primer vector are part of the 'optimal transfer' problem.&lt;/li&gt;&#xA;&lt;li&gt;Nonlinear programming: NLP can be fairly cumbersome and difficult to code and you often end up coding a lot of constraints. BUT - of the four you mentioned, past literature reviews have shown NLP gives the best results. Conway gives some great examples that may or may not be in that text, but examine his pursuit/evasion works and the NLP generated trajectories.&lt;/li&gt;&#xA;&lt;li&gt;Particle swarm: PSO is one of my favorite algorithms to implement. Showalter and Black [1] recently used PSO to generate responsive theater maneuvers (This work actually includes a PSO within a PSO). With regard to orbital transfers, you'll find a lot of the time spent inside a PSO will be propagating your trajectory. You can simplify this using Keplerian motion, but full-up numerical integration with higher-order perturbations will increase your computation time. &lt;/li&gt;&#xA;&lt;li&gt;Genetic algorithms: GA's can be &lt;em&gt;very&lt;/em&gt; computationally expensive based on whether you are doing single-objective or multi-objective optimization, your design variables, your constraints, and so forth. Moreover, GA's also depend on your minimum number of generations generated. There are a lot of variants on the GA, but the computation time of a GA is, one can argue, user dependent.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Side notes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;These algorithms will also depend on how complex your cost function is, and your convergence criteria.&lt;/li&gt;&#xA;&lt;li&gt;Computation time will be a function of your model's fidelity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Finally, when you consider NLP as a higher-fidelity algorithm, the question becomes 'what is my initial guess?' to feed the NLP (since your convergence in a numerical algorithm is often based on how close you are to an answer). You can use PSO or primer vector or any number/combination of optimization routines as a warm-up / initial guess to NLP or higher fidelity model that will place you in the neighborhood of a, hopefully, good solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1: &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/1.A32989&quot; rel=&quot;nofollow&quot;&gt;http://arc.aiaa.org/doi/abs/10.2514/1.A32989&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1478" LastActivityDate="2015-05-05T13:51:55.677" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9030" PostTypeId="2" ParentId="9028" CreationDate="2015-05-05T14:36:25.320" Score="3" Body="&lt;p&gt;Dava Newman will, on the week of May 18 when she'll be sworn in, succeed Lori Garver, that was previous NASA Deputy Administrator till September 2013. So the answer to the latter part of your question, if &lt;em&gt;deputy is likely to become the next top boss&lt;/em&gt;, is no, not necessary, but it wouldn't be unheard of;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Garver is now the General Manager of the Air Line Pilots Association, and leads Obama Presidential Transition Agency Review Team for NASA. One previous Deputy Administrator however later became NASA Administrator - Dr. Thomas O. Paine (Deputy from March 25, 1968 to March 20, 1969, Acting Administrator from October 8, 1968 to March 21, 1969 and then Administrator from March 21, 1969 to September 15, 1970).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And for the first part, it would be easiest to appreciate Deputy Administrator's role if we analyzed Garver's work while she held that position (July 17, 2009 to September 6, 2013). According to NASA's &lt;a href=&quot;https://www.nasa.gov/about/highlights/garver_bio.html&quot; rel=&quot;nofollow&quot;&gt;Lori B. Garver, NASA Deputy Administrator&lt;/a&gt; page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As deputy administrator, Garver is NASA's second in command. She works&#xA;  closely with the administrator to provide overall leadership,&#xA;  planning, and policy direction for the agency. Together with the NASA&#xA;  administrator, Garver represents NASA to the Executive Office of the&#xA;  President, Congress, heads of government agencies, international&#xA;  organizations, and external organizations and communities. She also&#xA;  oversees the work of NASA's functional offices.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA Administrator Charles Bolden also urged US Senate during last few hearings to approve Newman as he needs help. So it's not merely a prestige position to climb the rungs from. Newman can expect a lot of work ahead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to compare which Deputy Administrators became NASA Administrators, here's &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Administrators_and_Deputy_Administrators_of_NASA&quot; rel=&quot;nofollow&quot;&gt;a list of all of them on Wikipedia&lt;/a&gt;, since NACA (National Advisory Committee for Aeronautics) became NASA (National Aeronautics and Space Administration).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-05-05T15:16:07.820" LastActivityDate="2015-05-05T15:16:07.820" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9031" PostTypeId="2" ParentId="9028" CreationDate="2015-05-05T14:58:17.147" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What is the actual function of a deputy administrator in NASA? What kind of decisions will she be responsible for?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That depends very much on the administrator and how he (they've all been &quot;he&quot; so far) delegates things. Deputy administrator is a political appointment. Sometimes the administrator doesn't like the person foisted on him, and the role is meaningless. Other times, the administrator delegates a lot to his deputy.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And is a deputy likely to become the next top boss?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The deputy has become the acting administrator six times. Acting administrator is a placeholder position. Someone needs to be in charge. As an acting administrator, what they do had better not be anything of consequence. A deputy who became the official administrator: That's happened but once.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The administrator and deputy administrator are the only two political appointments in NASA. When the party of the President changes, the incoming President typically accepts the resignation of the NASA administrator and deputy administrator. How those transitions work vary a lot from administration to administration.  Sometimes the President takes his time to get around to dealing with NASA; NASA is a rather small and unimportant government agency from the President's perspective.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-05-05T15:06:31.297" LastActivityDate="2015-05-05T15:06:31.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9032" PostTypeId="1" AcceptedAnswerId="9033" CreationDate="2015-05-05T16:01:51.087" Score="11" ViewCount="2086" Body="&lt;p&gt;I'm afraid this is a highly speculative question, but I wanted to put it out and see if anyone can provide more insight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say that at some time in the future, during a Falcon 9 launch, but before stage separation, an anomaly is detected with the second stage that is going to cause a loss of mission. &lt;em&gt;Theoretically&lt;/em&gt;, could the Falcon abort the assent and proceed to land with the second stage and payload still attached?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The extra weight might even help the landing. It would provide a more manageable T/W ratio and allow the F9 to avoid the tricky hover-slam landing. Of course the higher centre of mass isn't going to help things, and the landing software would need completely rewriting and new flight profile would need to be created. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;TL;DR: &lt;strong&gt;Are the any engineering problems that would completely rule out a Falcon 9 aborting an assent and landing with its payload?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="51" LastEditDate="2015-07-09T02:32:00.737" LastActivityDate="2015-07-09T02:32:00.737" Title="Could a Falcon 9 land carrying its second stage and payload?" Tags="|spacex|landing|falcon-9|abort|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9033" PostTypeId="2" ParentId="9032" CreationDate="2015-05-05T16:09:01.840" Score="14" Body="&lt;p&gt;Dry mass of the first stage from &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot;&gt;SpaceFlight101&lt;/a&gt; suggests 25,600 kilos. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dry mass of the second stage is 3,900 kilos, and 92,670 kilos fueled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring payload, that is one heck of a top heavy vehicle, an empty first stage, and a fully fueled second stage, almost 4 times the mass in the second stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current control facilities (Cold gas thrusters, Grid fins, Engine gimbal control) seem highly unlikely to be able to control such an unbalanced vehicle on landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core issue would be the mass of the fuel/oxidizer in the second stage, not the empty mass of the second stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Centaur (LOX/LH2 RL-10 powered upper stage, a variant still used by Atlas V and Delta IV and possible SLS Block I) was being considered for use in the Shuttle, but venting the fuel and oxidizer in case of an abort was too much work to figure out (And for the Shuttle, sometimes it seemed like money was not a problem most of the time).  It is a bit easier in the case of the Falcon 9, where there is no Orbiter walls to get the fuel dumped out through. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A venting system for the upper stage might make it possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondarily, a landing with a fully fueled upper stage that tipped over, for whatever reason, like the landing of the CRS-6 flight on the ASDS JRTI barge, has the potential for a VERY bad explosion.  Consider how much burning we saw with a mostly empty first stage on the CRS-6 almost landing, with how bad a fully fueled second stage might cause. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For that reason, the FAA might have a hard time accepting it as well.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-05-05T16:27:53.797" LastActivityDate="2015-05-05T16:27:53.797" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9034" PostTypeId="2" ParentId="9032" CreationDate="2015-05-05T17:33:22.410" Score="9" Body="&lt;p&gt;The aerodynamic model of the first stage return is defined by heavy engine machinery at the base, lightweight empty tanks above, and the grid fins at the top. That's a very stable configuration for downward flight, much like a throwing dart. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Loaded or dry, an attached second stage completely ruins all of that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't imagine this would be possible.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-05-05T17:33:22.410" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9035" PostTypeId="1" CreationDate="2015-05-05T17:54:55.160" Score="16" ViewCount="1384" Body="&lt;p&gt;I'm asking about &lt;a href=&quot;https://www.flickr.com/photos/spacexphotos/16661791299/&quot; rel=&quot;noreferrer&quot;&gt;this photograph&lt;/a&gt; that was published on &lt;a href=&quot;https://www.flickr.com/photos/spacexphotos/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Flickr photostream&lt;/a&gt;, which I also noticed in a few recent articles (e.g. &lt;a href=&quot;http://danspace77.com/2015/04/29/spacex-dragon-v2-prototype-ready-for-pad-abort-test/&quot; rel=&quot;noreferrer&quot;&gt;this one&lt;/a&gt;) about the upcoming Crew Dragon pad abort test:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.flickr.com/photos/spacexphotos/16661791299/&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xC8ww.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX wasn't too descriptive under it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Crew Dragon - Pad Abort Vehicle&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Crew Dragon, America’s next generation crewed spacecraft is almost&#xA;  ready for a test flight. Pad abort vehicle shipping to FL shortly.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But if I look around the Dragon, it appears to have been taken to some sort of an isolation room. Not any usual sound-proof room either. To me, it resembles more a &lt;em&gt;tempest shielded room&lt;/em&gt; of sorts (to prevent electromagnetic emanations, tho this one's purpose might be exactly the other way around and to prevent environmental EM noise). Especially if I zoom in on those walls. Those things are spiky with darker, what seems to be different material tips, so it can't merely be some soundproof foam.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There also appear to be some rather interesting instruments lying around that slightly resemble what you'd expect a rock band would use - a guitar amplifier with preamplifier on top of it at the far back right, guitar effects unit to the Dragon's right, some electric guitar cords all around, and an odd looking note stand on the left - but I very much doubt they were staging a rock concert or singing the Dragon lullabies. So to my questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;What exactly is the purpose of this room that the Dragon stands in on the photo?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Can you identify equipment lying around it and what would they be used for?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Did Buster* the dummy have anything to do with all of this?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Obviously, this is a bit of a fun question, but I'd still appreciate answers with references. My guess is that this room was used to test and calibrate all the sensors aboard the Crew Dragon test vehicle (if I remember correctly, roughly 270** of them in total will be used) in an electromagnetic interference free environment, but I wasn't able to confirm that. Nor did I find out how many sensors (if any) Buster carries, or if he indeed ever saw this room on the photo. Did he?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;*Buster is the name of a crash test dummy that SpaceX will use during the pad abort test.&lt;/sup&gt;&lt;br /&gt;&#xA;&lt;sup&gt;**I remembered correctly, here's &lt;a href=&quot;http://space.brevardtimes.com/2015/05/spacex-pad-abort-test-launch-set-for.html&quot; rel=&quot;noreferrer&quot;&gt;the article&lt;/a&gt; I read that in.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-05-05T18:01:02.393" LastActivityDate="2018-05-21T16:04:34.793" Title="Why was Dragon sent to the isolation room?" Tags="|spacex|crewed-spaceflight|testing|dragon-v2|launch-escape-system|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9036" PostTypeId="1" AcceptedAnswerId="9037" CreationDate="2015-05-05T19:58:03.010" Score="3" ViewCount="230" Body="&lt;p&gt;I'm looking at &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:NACA%27s_Special_Committee_on_Space_Technology.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;a photograph of a meeting&lt;/a&gt; of the Special Committee on Space Technology of the NACA (the precursor to NASA) from 1958, and there are some devices on the table that I can't figure out.  Considering the people present, and the year, I wouldn't be surprised if they're models or samples of something related to the early years of the Space Race, but I just can't figure out what they are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/oLAgQ.jpg&quot; alt=&quot;photograph of meeting&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2015-05-05T20:13:54.713" Title="What are these devices?" Tags="|history|" AnswerCount="1" CommentCount="8" ClosedDate="2015-05-06T08:23:45.310" ContentLicense="CC BY-SA 3.0" />
  <row Id="9037" PostTypeId="2" ParentId="9036" CreationDate="2015-05-05T20:13:54.713" Score="4" Body="&lt;p&gt;Hahaha!  They are ashtrays!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The metal 'hoop' that comes up has indents to place a cigar or cigarette in, if you need to put it down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The good old days, when Steely Eyed Missile Men ran NASA/NACA. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-05T20:13:54.713" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9038" PostTypeId="2" ParentId="9035" CreationDate="2015-05-05T23:15:14.167" Score="14" Body="&lt;p&gt;That is an &lt;a href=&quot;https://en.wikipedia.org/wiki/Anechoic_chamber&quot; rel=&quot;noreferrer&quot;&gt;anechoic chamber&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An anechoic chamber (&quot;an-echoic&quot; meaning non-reflective, non-echoing&#xA;  or echo-free) is a room designed to completely absorb reflections of&#xA;  either sound or electromagnetic waves. They are also insulated from&#xA;  exterior sources of noise. The combination of both aspects means they&#xA;  simulate a quiet open-space of infinite dimension, which is useful&#xA;  when exterior influences would otherwise give false results.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-05-05T23:15:14.167" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9039" PostTypeId="1" CreationDate="2015-05-06T07:38:07.513" Score="15" ViewCount="3658" Body="&lt;p&gt;Looking at this &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/images/falcon9/falcon9-render.png&quot; rel=&quot;noreferrer&quot;&gt;elevation view of the Falcon 9&lt;/a&gt;, the nozzle exit of the core engine in the cluster is in a different plane with respect to the nozzle exits of the surrounding eight engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Yo0K6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this? Is there an advantage to this layout?&lt;/p&gt;&#xA;" OwnerUserId="3082" LastEditorUserId="12102" LastEditDate="2021-06-07T04:33:31.983" LastActivityDate="2021-06-07T04:33:31.983" Title="Why is the center engine in the Falcon 9 (and Falcon Heavy) Octaweb layout at a different elevation with respect to the outer engines?" Tags="|spacex|falcon-9|design|octaweb|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9042" PostTypeId="1" AcceptedAnswerId="9043" CreationDate="2015-05-06T10:31:05.183" Score="14" ViewCount="3554" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_M-27M&quot;&gt;Progress 59&lt;/a&gt;'s orbit is decaying and it will soon reenter Earth's atmosphere. Will it be possible for observers on the ground to see it burn in the skies as it falls towards the Earth?&lt;/p&gt;&#xA;" OwnerUserId="10096" LastEditorUserId="49" LastEditDate="2015-05-06T12:42:45.173" LastActivityDate="2015-05-08T03:17:17.817" Title="Can we see the Russian Progress 59 spacecraft burn up on reentry?" Tags="|spacecraft|reentry|observation|progress|orbital-decay|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9043" PostTypeId="2" ParentId="9042" CreationDate="2015-05-06T10:59:36.130" Score="17" Body="&lt;p&gt;Progress-M 27M was (on date of writing this answer, see updates below)  &lt;a href=&quot;http://www.satflare.com/track.asp?q=40619,25544#TOP&quot; rel=&quot;nofollow noreferrer&quot;&gt;expected to decay around Thursday, May 7, 2015 at 23:01:00 &amp;pm;8 hours UTC&lt;/a&gt;. This predicted reentry time will get more precise as it experiences atmospheric decay, additional radar tracking measurements are taken and object's &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE&lt;/a&gt; are updated. Eventually, it should be possible to predict its final burn-up in the atmosphere to within one orbit (or about &amp;pm;43 minutes of an &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28110+km+%2B+6378.1+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%29+in+minutes&quot; rel=&quot;nofollow noreferrer&quot;&gt;87 minutes long&lt;/a&gt; final orbit at roughly 110 km altitude), but even then, due to &lt;a href=&quot;https://space.stackexchange.com/q/2753/49&quot;&gt;many uncertainties&lt;/a&gt;, it will be impossible to say exactly over which area that will happen, until it actually does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image below shows all the areas where it's possible it will decay, based on latest prediction data. Opaqueness of bands represents greater chance of it happening there, with median predicted decay happening on the vehicle's southward leg, just over New Zealand (that doesn't mean it will decay over it, it could equally likely decay over the other side of the world, say over France or Germany):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/uXyE2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Ground track bands displaying possible decay regions for Progress-M 27M.&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Image courtesy and made with &lt;a href=&quot;http://www.satflare.com/track.asp?q=40619,25544#TOP&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satflare reentry tracker&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming it reenters during night time, it should be fairly visible as it still carries all the supplies intended to be delivered to the International Space Station and most of its propellants, which could substantially increase visibility of its ionized trail. Some of its parts, especially the heavy and strong docking mechanism are also expected to impact on Earth and won't burn up in the atmosphere. For Progress spacecraft, this would normally have happened in a controlled fashion over large bodies of water, mostly East of Kamchatka peninsula in the Bering sea, and the vehicle would be more empty (basically its body, exhausted of propellants, with some waste produced on the station that's meant to burn on reentry) so there aren't any videos of it, and it's hard to compare how much more visible Progress-M 27M reentry could be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Educated guess, if it enters on the night side of its orbit, and over or close to populated areas where it would be expected to be seen by observers on the ground, it could look quite spectacular and perhaps similar to how some ATV reentries looked like (though note that Progress is a much smaller vehicle with roughly half the payload mass that &lt;a href=&quot;https://en.wikipedia.org/wiki/Automated_Transfer_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;ATV&lt;/a&gt; were capable of delivering to the station). Here's how ATV-1 (named Jules Verne) reentry looked like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/OhBw5yaR_SU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Progress-M 27M reentry happens during daylight, then it would obviously be less visible but might still produce visible trail of debris reflecting in sunlight, ionized trail and occasional explosions of its propellant and storage tanks and implosion of its hull. But most of this would happen still high in the atmosphere, with exception of a few heavier and stronger pieces of debris that might survive all the way to the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that with current decay time uncertainty of 16 hours, chances of this happening over land are fairly slim. With Progress-M 27M's orbital inclination of 51.635°, there's only roughly 30% chance any of its surviving remains would impact terra firma. Conversely, that means that it's about 70% chance that final decay will happen over oceans and, unless that happens within ~ 1,000 km radius to populated land areas (estimated visible horizon for bright, low altitude objects), nobody would be there to see it, even if it would otherwise produce bright enough streak:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/TmQcf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image credit: &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/newsletter/newsletter.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Orbital Debris Program Office's Orbital Debris Quarterly News&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update (May 7, 18:30 p.m. UTC)&lt;/strong&gt;: Refined prediction now puts expected decay time on May 8, 1.55:00 &amp;pm;2 hours UTC, with uncertainty of slightly less than three Progress-M 27M's orbital periods. Ground track bands for final decay location also become narrower as uncertainties are reduced:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/WLqq2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Image courtesy and made with &lt;a href=&quot;http://www.satflare.com/track.asp?q=40619,25544#TOP&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satflare reentry tracker&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This prediction puts median decay location in the middle of South Pacific ocean, on the southward leg of Progress-M 27M's orbit.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update (May 8, 3:00 a.m. UTC)&lt;/strong&gt;: Satflare prediction from previous update was pretty much spot on. Roscosmos &lt;a href=&quot;https://goo.gl/oPaszd&quot; rel=&quot;nofollow noreferrer&quot;&gt;issued a quick statement&lt;/a&gt; that Progress M-27M reentered the atmosphere on May 8 at 2:04 a.m. UTC (5:04 a.m. Moscow time) over the Pacific Ocean. By my calculations, decay time should place it in proximity of Kiribati and Jarvis island, 2:04 p.m. local time.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some sites are tracking this Progress-M 27M decay and you can read more about it there as events unfurl, for example &lt;a href=&quot;http://www.spaceflight101.com/progress-m-27m-re-entry.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101 publishes regular updates&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-05-08T03:17:17.817" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9044" PostTypeId="1" AcceptedAnswerId="9047" CreationDate="2015-05-06T12:53:38.743" Score="3" ViewCount="1538" Body="&lt;p&gt;I have listened to a number of launch countdowns, and especially as they poll for readiness, a large number of people (10-12 or more) need to respond when called on, but the sound quality is excellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I seem to live on conference calls for work, and people usually stink at muting/unmuting and background noise is almost always an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot imagine that Mission Control is a quiet place.  Yet the countdown net usually has excellent sound quality, everyone responds quickly and on time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What sort of headset/technology are they using to accomplish this?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-07T17:31:39.513" Title="How do the launch controller headsets work?" Tags="|launch|launch-sequence|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9045" PostTypeId="1" CreationDate="2015-05-06T12:57:32.330" Score="8" ViewCount="637" Body="&lt;p&gt;The day before the Pad Abort test attempt (May 5, 2015) SpaceX test fired the SuperDraco engines in a hot fire. Similar to how they test the Falcon 9 a day or three before launch, the engines spin up and fire, the booster/capsule is mechanically held down for the test.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But normally, SpaceX burns LOX/RP1 whose exhaust is carbon dioxide, water, and some sulfur by products. Basically car exhaust, just lots more of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Pad Abort vehicle, it is Nitrogen Tetroxide and MMDH which are astonishingly nasty nasty fluids. They are horribly corrosive and dangerous to humans. The products of full combustion are not very bad (water, ammonia, and other benign things) but you cannot guarentee 100% full combustion, and this stuff is just plain nasty.  (Potential additional question will be for capsules landing on land, how much work will be needed before they can leave the capsule safely. Shuttle had people in Hazmat suits cleaning up, before the crew were allowed to leave from its NTO/MMDH thrusters.). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much cleanup was required at the pad after the test?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the pictures, they did use the Niagara water deluge system.  Was that sufficient to wash away any issues?  (If so, do they process the deluge system water before letting it flow back into the Florida waterways?)&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-05-06T15:01:58.177" LastActivityDate="2015-05-28T13:56:36.737" Title="Dragon Pad Abort hot fire, how much cleanup work was needed?" Tags="|spacex|testing|abort|dragon-v2|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9046" PostTypeId="1" CreationDate="2015-05-06T13:09:53.777" Score="9" ViewCount="2275" Body="&lt;p&gt;For quite some time, I am reading about &lt;em&gt;space adhesives&lt;/em&gt;. Selecting the right adhesive for space applications is extremely important, mainly due to the high vacuum (around 10-8 Torr), and the variation in temperature. A key concern is the outgassing property of these adhesives. Any outgassing might lead to contamination of electronic or optical components, which can have a drastic impact on the optical properties of the system. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;What are the commonly used &lt;em&gt;space adhesives&lt;/em&gt; in satellites?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What are the standards of adhesives to qualify as space-worthy (other than Outgassing standard test ASTM E 595 / ESA PSS-014-702)?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Which would be the most important requirement or trade-off of a space adhesive (withstanding wide range of temperature / UV, gamma &amp;amp; other radiation / lower outgassing / resistance against free radical or atomic oxygen)?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10097" LastEditorUserId="10097" LastEditDate="2015-05-06T13:37:36.480" LastActivityDate="2021-07-20T12:17:53.857" Title="What are the common space application adhesives used in Earth orbiting satellites?" Tags="|artificial-satellite|materials|design|standards|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9047" PostTypeId="2" ParentId="9044" CreationDate="2015-05-06T13:42:20.923" Score="5" Body="&lt;p&gt;There are several factors at work here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: Noise canceling is not used in Mission Control. It &lt;a href=&quot;http://spinoff.nasa.gov/Spinoff2005/ch_9.html&quot; rel=&quot;nofollow&quot;&gt;is used in the astronauts' headsets&lt;/a&gt;, and has been since the 1960s. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Headset. Businesses typically use $20 headsets with lousy microphones and no noise cancelling. They have a non-adjustable microphone boom so the mic is always too far away, and tiny, low-quality microphone and speakers.&lt;br&gt;&#xA;NASA springs for the good stuff. For the Mercury program, Plantronics supplied headsets for the astronauts. They also supplied headsets for Mission Control, e.g. the &lt;a href=&quot;http://www.headsetexperts.com/Plantronics-MS30-1-StarSet-Commercial-Aviation-Headset-1-Plug.html&quot; rel=&quot;nofollow&quot;&gt;MS30&lt;/a&gt; (designed for aviation). &#xA;These headsets have fully adjustable microphones so the mic position is correct, and the mic itself is directional to reduce the pickup of ambient noise. These sets were designed to work in airplane cockpits, which are very noisy environments and where communications are a matter of life or death.  &lt;/li&gt;&#xA;&lt;li&gt;The headset mic isn't always on. Personnel has to use a Push-to-Talk button to trigger the microphone (either a switch somewhere along the mic cable, or a foot pedal). &lt;/li&gt;&#xA;&lt;li&gt;Discipline. Mission Control personnel is a lot better trained (&lt;a href=&quot;http://www.slate.com/blogs/quora/2014/05/25/how_do_nasa_s_mission_control_centers_operate.html&quot; rel=&quot;nofollow&quot;&gt;1-3 years&lt;/a&gt; before they are allowed to work on an actual mission) than the average office worker, including on the correct use of mute/PUsh-to-Talk and when to speak or not speak. &lt;/li&gt;&#xA;&lt;li&gt;I'd be very surprised if Mission Control were a noisy environment. This is a result of 2: nobody speaks until they absolutely have to. Also, the room is built to dampen all extraneous sounds, and people who do talk do it quietly. &lt;/li&gt;&#xA;&lt;li&gt;What you hear on a NASA launch broadcast is a broadcast commentator outside Mission control, sometimes with some Capcom messages mixed in. In Mission Control, every team has its own comm channel; only the team leader speaks to the launch director. E.g. before the readiness poll there will be an internal poll for each team, and the team leaders gives his Go to the launch director based on that internal poll). Only the team leader is in the Mission Control room, the remainder of his team is in a separate room nearby. There are dozens of comm channels (&lt;a href=&quot;http://www.baen.com/Burlison_Mission_Control.asp&quot; rel=&quot;nofollow&quot;&gt;this page&lt;/a&gt; has an image of one comm control station with 48 channels).&lt;br&gt;&#xA;People in Mission Control can select more than one channel to listen to (listening to two conversations at the same time is difficult, but it can be done). &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.baen.com/Burlison_Mission_Control.asp&quot; rel=&quot;nofollow&quot;&gt;A 'behind the scenes' look at Mission Control&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.slate.com/blogs/quora/2014/05/25/how_do_nasa_s_mission_control_centers_operate.html&quot; rel=&quot;nofollow&quot;&gt;And another&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://spinoff.nasa.gov/Spinoff2005/ch_9.html&quot; rel=&quot;nofollow&quot;&gt;The origins of the Plantronics headset, and its commercial spinoffs&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-05-07T17:31:39.513" LastActivityDate="2015-05-07T17:31:39.513" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9048" PostTypeId="1" AcceptedAnswerId="9049" CreationDate="2015-05-06T14:34:32.097" Score="5" ViewCount="2399" Body="&lt;p&gt;I'm working on a theoretical problem that I came up with; Given the extremely low atmosphere in low Earth orbit (300-500 km), would an arrow that had orbital velocity keep flying 'true' due to the center of mass being ahead of the center of pressure, &lt;em&gt;or&lt;/em&gt; would the gravity gradient forces take over so the arrow aligns with zenith/nadir?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My issue is that I can't find any simple method to calculate the gravity gradient forces.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="49" LastEditDate="2015-05-06T14:46:04.187" LastActivityDate="2021-09-07T18:58:58.857" Title="How can gravity gradient forces be calculated?" Tags="|orbital-mechanics|orbit|gravity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9049" PostTypeId="2" ParentId="9048" CreationDate="2015-05-06T16:56:03.570" Score="4" Body="&lt;p&gt;Assume the arrow is 1m long, and the reference altitude is 400km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration due to gravity is µ / r&lt;sup&gt;2&lt;/sup&gt;; Earth's µ (&lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow&quot;&gt;standard gravitational parameter&lt;/a&gt;) is 398,600.442 km&lt;sup&gt;3&lt;/sup&gt;/s&lt;sup&gt;2&lt;/sup&gt;, and radius is 6371 km. Figuring for altitude of 400km and 400.001km (these values added to the planet's radius), the difference in acceleration is about 0.000002568 m/s&lt;sup&gt;2&lt;/sup&gt; -- that's 2.6 &lt;em&gt;micro&lt;/em&gt;meters per second squared. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that's how fast the opposite ends of the arrow want to escape from each other when it's in the zenith-nadir orientation. At any LEO altitude the gradient is similar. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I suppose calculus can be applied to get the gradient more directly.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the acceleration due to drag force at 400km is more than that, it should stabilize in the &quot;airstream&quot;; if it's less than that, it should stay radial. I believe the drag equations will require you to estimate a mass for the arrow as well; the gravity gradient acceleration is independent of the mass, so I worked in acceleration units rather than force units.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, solar pressure and solar-heating-evaporation of surface layers would probably push the thing around, too; I don't know how to estimate those forces.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-05-06T18:10:58.447" LastActivityDate="2015-05-06T18:10:58.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9050" PostTypeId="1" CreationDate="2015-05-06T17:43:52.317" Score="8" ViewCount="1076" Body="&lt;p&gt;Based on satellite and robots research, is there an overview/inventory of mineral resources that has been discovered till now on the planets, moons, asteroids and comets of our solar system (e.g. list, database, scientific paper/article, book, map etc. of raw/crude materials)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mineral resouces - reserves, need and limitations (long-term economic impact):&#xA;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.labnol.org/home/how-long-will-metals-last/10648/&quot; rel=&quot;nofollow&quot;&gt;http://www.labnol.org/home/how-long-will-metals-last/10648/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://visual.ly/stock-check-how-long-will-earths-resources-last&quot; rel=&quot;nofollow&quot;&gt;https://visual.ly/stock-check-how-long-will-earths-resources-last&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mrdata.usgs.gov/mrds/&quot; rel=&quot;nofollow&quot;&gt;http://mrdata.usgs.gov/mrds/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Current initiatives looking for solutions (long-term economic potential):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.cnet.com/news/planetary-resources-seeks-to-mine-asteroids-riches/&quot; rel=&quot;nofollow&quot;&gt;http://www.cnet.com/news/planetary-resources-seeks-to-mine-asteroids-riches/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://web.mit.edu/12.000/www/m2016/finalwebsite/solutions/asteroids.html&quot; rel=&quot;nofollow&quot;&gt;http://web.mit.edu/12.000/www/m2016/finalwebsite/solutions/asteroids.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astronomysource.com/tag/rare-earth-metals-from-asteroids/&quot; rel=&quot;nofollow&quot;&gt;http://www.astronomysource.com/tag/rare-earth-metals-from-asteroids/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.reuters.com/article/2013/11/21/us-space-mining-asteroids-idUSBRE9AK0JF20131121&quot; rel=&quot;nofollow&quot;&gt;http://www.reuters.com/article/2013/11/21/us-space-mining-asteroids-idUSBRE9AK0JF20131121&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2147404/Found-The-single-asteroid-thats-worth-60-billion-years-financial-output-entire-WORLD.html&quot; rel=&quot;nofollow&quot;&gt;http://www.dailymail.co.uk/sciencetech/article-2147404/Found-The-single-asteroid-thats-worth-60-billion-years-financial-output-entire-WORLD.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.extremetech.com/extreme/148329-earth-skimming-da14-asteroid-contains-195-billion-of-minerals-if-we-can-catch-it&quot; rel=&quot;nofollow&quot;&gt;http://www.extremetech.com/extreme/148329-earth-skimming-da14-asteroid-contains-195-billion-of-minerals-if-we-can-catch-it&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science.howstuffworks.com/asteroid-mining1.htm&quot; rel=&quot;nofollow&quot;&gt;http://science.howstuffworks.com/asteroid-mining1.htm&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceX&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/SpaceX&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6204" LastEditorUserId="-1" LastEditDate="2015-06-02T13:43:59.793" LastActivityDate="2015-06-02T13:43:59.793" Title="How much do we know about minerals and raw materials that are in our solar system?" Tags="|solar-system|mining|planet|natural-resources|robotic-missions|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9051" PostTypeId="1" AcceptedAnswerId="9053" CreationDate="2015-05-05T15:30:14.437" Score="8" ViewCount="488" Body="&lt;p&gt;Why doesn't New Horizons fly by nearer to Pluto so the gravitational pull would slingshot the probe to one of the next &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_possible_dwarf_planets&quot; rel=&quot;noreferrer&quot;&gt;dwarf planets&lt;/a&gt; in the outer solar system (Eris, Haumea, Makemake,...)? I would think it could fly in sleep mode for another decade to visit another dwarf planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are The bigger KBOs too far away? What is the limiting resource to the probe. &lt;/p&gt;&#xA;" OwnerUserId="10100" OwnerDisplayName="rubo77" LastEditorUserId="10100" LastEditDate="2015-05-07T01:20:16.643" LastActivityDate="2015-05-07T01:20:16.643" Title="Use Pluto's gravitation to reach the next dwarf planet" Tags="|gravity|new-horizons|pluto|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9052" PostTypeId="2" ParentId="9051" CreationDate="2015-05-05T20:30:59.123" Score="3" Body="&lt;p&gt;That's pretty much what they're planning to do -- except that the planned target is a smaller Kuiper Belt Object, not a dwarf planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE :&lt;/strong&gt; I overestimated the course change possible by using Pluto's gravity. See &lt;a href=&quot;https://space.stackexchange.com/a/9053/574&quot;&gt;David Hammen's answer&lt;/a&gt; for the details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the Pluto flyby, the current plan is for New Horizons to continue on its path and fly by one or two Kuiper Belt objects (KBOs). Two such objects have been identified as possible targets. As far as I can tell a selection has not yet been made. One is estimated to be about 30-45 kilometers in diameter; the other is likely somewhat larger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The choice of target is tightly constrained by available fuel for course adjustments. Before October 2014, there were no known bodies beyond the Pluto system that New Horizons would be able to reach. A search was conducted using ground telescopes, and later the Hubble Space Telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once a target is chosen, the flight path will be adjusted during the Pluto flyby to ensure that it passes near the selected object. Pluto's gravity will have to be taken into account when planning the course correction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not clear that this would be a &quot;slingshot&quot;. The goal is not necessarily to increase the spacecraft's velocity, as was done by the Jupiter flyby in 2006; rather it will select whatever course is needed to reach the target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A flyby of a another dwarf planet after leaving the Pluto system is, unfortunately, not feasible. The choice of targets past Pluto is tightly constrained by fuel, distance, and time. The spacecraft has only a very limited amount of hydrazine propellant available for maneuvers, and it can't effectively return good science data past about 55 AU. This limits available targets to a cone less than 1 degree wide. There just aren't any dwarf planets past Pluto that New Horizons is able to reach. Only three objects past Pluto have been officially recognized as dwarf planets; a handful of others are likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This information is summarized from &lt;a href=&quot;http://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Wikipedia article&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="574" OwnerDisplayName="Keith Thompson" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-05-06T18:59:24.380" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9053" PostTypeId="2" ParentId="9051" CreationDate="2015-05-06T13:19:24.807" Score="15" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why doesn't New Horizons fly by nearer to Pluto so the gravitational pull would slingshot the probe to one of the next dwarf planets in the outer solar system (Eris, Haumea, Makemake,...)?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are two key reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It can't.&lt;/li&gt;&#xA;&lt;li&gt;Even if it could, it shouldn't.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The reason it can't is because the delta V would be rather small even if New Horizons was to flyby Pluto as close as possible. Pluto's mass and orbital velocity are rather small and New Horizons' velocity is rather large. A Pluto flyby at 100 km above the surface with a v&lt;sub&gt;∞￼&lt;/sub&gt; of 13 km/sec will turn New Horizons' trajectory by about 1.4 degrees. That's not much. Even a flyby that is just above the surface at closest approach is essentially a straight-line trajectory. There is very little gravity assist to be had in a Pluto flyby at New Horizons' high velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason it shouldn't is because that close flyby goes strongly against the grain of the primary mission, which is to observe Pluto. Suppose Pluto was orders of magnitude more massive than it is, thereby making a gravity assist feasible. Also suppose a large KBO was discovered after the New Horizons launch, and that this newly discovered KBO was reachable but only if the Pluto encounter was changed from its planned 10,000 km closest approach to a flyby that involves a very close approach. Should they do it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer is no. The Pluto encounter is the primary goal of the mission. That radical change in trajectory would mean failure to achieve that primary goal. New Horizons' sensor package was designed with that 10,000 km closest approach in mind. The sensor package cannot handle a close flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose New Horizons does achieve its primary goal. That alone is enough to qualify the mission as a complete success. If it fails to achieve its primary goal, that is enough to qualify the mission as a failure. The post-Pluto portion of the mission has always been viewed as an optional bonus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Details on &quot;it can't&quot;&lt;/strong&gt;&lt;br&gt;&#xA;One of the key parameters in determining the gravity assist the results from a flyby is the turn angle $\delta$. This is the angle by which the planet turns the satellite's hyperbolic trajectory (as viewed from a planet-centric inertial frame). This is given by&#xA;$$\delta = 2\arcsin\left( \frac 1 e \right)$$&#xA;where $e$ is the eccentricity of the hyperbolic trajectory. This can be computed from the planet's gravitational parameter $\mu = GM$, the satellite's periapsis distance $r_p = a(1-e^2)$, and the satellite's velocity at infinity $v_\infty = -\mu/a$:&#xA;$$e = \sqrt{1+\frac{r v_\infty}\mu}$$&#xA;The eccentricity is 227 in the case of a Pluto flyby with a v&lt;sub&gt;∞￼&lt;/sub&gt; of 13 km/sec and a closest approach of 10,000 km. This reduces to an eccentricity of 81 for a close-in flyby (100 km above the surface). Both of those eccentricities are too large to get a significant delta V from the flyby. The turn angle for the nominal mission is 0.5 degrees, 1.4 degrees for the close-in flyby.&lt;/p&gt;&#xA;" OwnerUserId="2752" OwnerDisplayName="David Hammen" LastActivityDate="2015-05-06T13:52:37.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9054" PostTypeId="1" AcceptedAnswerId="9055" CreationDate="2015-05-05T23:18:32.587" Score="25" ViewCount="2385" Body="&lt;p&gt;New Horizons is intended to pass within 10,000 km of Pluto on its flyby on July 14. This will give a maximum resolution on the surface of 50m per pixel. This seems to me a bit of a waste, when it could have passed much nearer to get a higher resolution of some areas of Pluto.&lt;/p&gt;&#xA;" OwnerUserId="10100" OwnerDisplayName="rubo77" LastEditorUserId="1142" LastEditDate="2015-05-06T22:41:36.767" LastActivityDate="2018-12-29T15:49:33.297" Title="Why doesn't the New Horizons probe fly any nearer than 10,000 km from Pluto?" Tags="|mission-design|new-horizons|pluto|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9055" PostTypeId="2" ParentId="9054" CreationDate="2015-05-06T14:28:06.707" Score="32" Body="&lt;p&gt;The long distance to the Sun mandates long exposure times. The New Horizons spacecraft needs to be relatively stable and its pointing accurate throughout these long exposure times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons does not have a scan platform. The cameras and other science instruments are fixed with respect to the vehicle. The satellite has to turn as a whole to keep the its scientific instruments pointed at Pluto. New Horizons also doesn't have control moment gyros or reaction wheels. All attitude control is via attitude thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vehicle has to rotate by 180° from well before closest approach to well after closest approach. With a somewhat remote flyby, this 180° turn is spread out a bit. With a close-in flyby, this 180° turn has to happen rather quickly, right at flyby. The constant on/off thrusting that would be needed for a very close approach would do significant damage to the quality of the close-in imagery. There's little value to a close-in flyby if all that one sees is fuzz. A close-in flyby would also require considerably more fuel than a more remote flyby.&lt;/p&gt;&#xA;" OwnerUserId="2752" OwnerDisplayName="David Hammen" LastEditorUserId="1142" LastEditDate="2015-05-06T22:44:09.803" LastActivityDate="2015-05-06T22:44:09.803" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9058" PostTypeId="1" AcceptedAnswerId="9059" CreationDate="2015-05-07T00:35:44.053" Score="9" ViewCount="1730" Body="&lt;p&gt;From what I understand, Martian dust is primarily composed of iron oxide, which is one ingredient for a &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermite&quot;&gt;thermite&lt;/a&gt; reaction. If heated to the proper temperature, would aluminum (possibly powdered) react with the Martian dust to produce molten iron?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, would it be dangerous to have aluminum structures/tools on Mars?&lt;/p&gt;&#xA;" OwnerUserId="4603" LastActivityDate="2021-09-09T15:55:56.103" Title="Could aluminum undergo a thermite reaction with Martian dust?" Tags="|mars|materials|chemistry|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9059" PostTypeId="2" ParentId="9058" CreationDate="2015-05-07T01:47:45.453" Score="16" Body="&lt;p&gt;Probably not any more likely than on Earth. Actually, &lt;a href=&quot;https://space.stackexchange.com/questions/3187/curiosity-why-did-they-reinvent-the-wheel-instead-of-using-pressure-tires&quot;&gt;Curiosity's (MSL) rover's wheels are made out of aluminum&lt;/a&gt; and they've been grinding through iron oxide rust that gives Martian regolith and the whole planet its color for quite a while now. And Spirit and Opportunity (MER) rovers also use several tools, among which even grinders, made out of aluminum. So why this isn't a problem?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For thermite welding, you require very fine powder mixture of metal oxide and a metal that will act as a reducing agent when both are heated to a sufficiently high temperature to initiate the exothermic chain reaction. Very fine, often nano-sized aluminum powders are used for that, as it's a very strong exothermic reducing agent. Actually, nano-aluminum powders are often also used in rocket propellant mixtures as catalysts, or even on their own. A solid aluminum construction, or a tool made out of it, won't do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even less likely as aluminum also slowly forms a thin oxidized layer naturally (there is a bit of oxygen in Martian atmosphere too), or is protected intentionally such to give it a bit more surface strength, reduce its triboelectric potential, paramagnetism, or simply to additionally shield it against wear and tear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens with thermite powders when sufficient heat is applied is, simplifying, that it excites individual granules to a point where metal oxide and reducing metal exothermically exchange oxygen atoms. This results in enough heat for surrounding, tightly packed granules that absorb this produced heat to continue the reaction on their own, setting off an exothermic chain reaction of sorts. For thermites with aluminum powder, this is called an &lt;a href=&quot;https://en.wikipedia.org/wiki/Aluminothermic_reaction&quot; rel=&quot;noreferrer&quot;&gt;aluminothermic reaction&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you require not only all the ingredients and the heat to set the reaction off, but also a rather large quantity of such fine powders, well mixed and packed tightly together, so the reaction can be sustained. So, unless you've set that up intentionally, I don't see it happening spontaneously on Mars any more likely than it would on Earth. And it doesn't.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-05-07T01:52:53.123" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9060" PostTypeId="2" ParentId="6688" CreationDate="2015-05-07T04:03:23.640" Score="-1" Body="&lt;p&gt;EMDrive + Solar sail = 80-100 years.   A little high, but seems in the realm of possibility.&lt;/p&gt;&#xA;&lt;p&gt;One &amp;quot;cutting edge&amp;quot;** technology being talked about is the EMDrive. Its basically a light powered ion engine that could perpetually accelerate, allowing for crazy high speeds.  (.1 times the speed of light)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Mr. Joosten and Dr. White stated that “a one-way, non-decelerating trip to Alpha Centauri under a constant one milli-g acceleration” from an EM drive would result in an arrival speed of 9.4 percent the speed of light and result in a total transit time from Earth to Alpha Centauri of just 92 years.&lt;/p&gt;&#xA;&lt;p&gt;However, if the intentions of such a mission were to perform in-situ observations and experiments in the Alpha Centauri system, then deceleration would be needed.&lt;/p&gt;&#xA;&lt;p&gt;This added component would result in a 130-year transit time from Earth to Alpha Centauri – which is still a significant improvement over the multi-thousand year timetable such a mission would take using current chemical propulsion technology.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2015/04/evaluating-nasas-futuristic-em-drive/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Evaluating NASA’s Futuristic EM Drive&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Sneak in a solar sail (to both power it, and use sunlight to give it that extra boost), and you can decrease the time by up to a factor of two.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;JAXA engineers used Doppler radar measurements of the Ikaros craft to determine that sunlight is pressing on the probe's solar sail with a force of about 1.12 millinewtons (0.0002 pounds of force).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/8748-solar-sail-passes-big-test-deep-space.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Sail Passes Big Test In Deep Space&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;**NOTE:  The credibility of the EMDrive is up for debate.&lt;/p&gt;&#xA;" OwnerUserId="8653" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-05-07T04:03:23.640" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9062" PostTypeId="1" AcceptedAnswerId="9063" CreationDate="2015-05-07T10:11:26.770" Score="20" ViewCount="2724" Body="&lt;p&gt;In the Dragon v2 abort test yesterday, it lifted off with its unpressurized trunk in tow, then ejected the trunk at apogee. Is this the real abort profile? Wouldn't it be easier to pull away without the trunk?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's the real profile, why spend the fuel mass to save the cargo?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, how would the (impressive) acceleration of the initial test compare with a realistic scenario, where the engines are fighting drag but pulling less mass?&lt;/p&gt;&#xA;" OwnerUserId="3054" LastEditorUserId="51" LastEditDate="2015-05-07T11:36:44.170" LastActivityDate="2018-10-14T03:53:19.060" Title="Why does Dragon 2 abort with the trunk attached?" Tags="|spacex|abort|dragon-v2|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9063" PostTypeId="2" ParentId="9062" CreationDate="2015-05-07T10:47:55.357" Score="30" Body="&lt;p&gt;It's an aerodynamics problem. SpaceX has designed both of their Dragon capsules to be inherently stable for reentry; should the capsule not be able to orient itself actively due to a Draco engine failure -  it will passively orient itself during reentry with the blunt heatshield pointing forward, into the velocity vector. This minimizes risk and ensures the heatshield is always facing into the atmosphere for descent. This is an inherent property of the &lt;a href=&quot;http://www.airspacemag.com/space/how-the-spaceship-got-its-shape-137293282/&quot; rel=&quot;noreferrer&quot;&gt;truncated cone&lt;/a&gt; reentry body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This design has one drawback however: if you have a scenario where Falcon 9 encounters a failure mode that risks the lives of the astronauts on board the Dragon, and you need to escape from the rocket, you want to do so as quickly as possible and encounter the least aerodynamic resistance as you can. This means you need to point &lt;em&gt;nosecone forward, and heatshield back&lt;/em&gt;, which is precisely opposite to the aerodynamic design of Dragon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solution SpaceX chose was to keep the trunk on during the abort. Note that the trunk itself does not keep the capsule pointing forwards, but rather the &lt;strong&gt;fins on the trunk&lt;/strong&gt; do. They sculpt the airflow around the capsule in such a way that it is stable nosecone forward into the velocity vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/aShOX.png&quot; alt=&quot;Dragon Pad Abort&quot;&gt;&#xA;&lt;sup&gt;&lt;strong&gt;Dragon immediately begins to rotate post-trunk deploy, prior to drogue chute jettison.&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see all of this during the Pad Abort video. As soon as the trunk deploys and separates from the vehicle, the capsule immediately begins to rotate, trying to stabilize with the heatshield forward. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, the trunk is incredibly lightweight. Dragon weighs in at around 8000-9000kg. The trunk is less than 1000kg. A minor drawback, sure, but who cares when you're saving the lives of the crew inside. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="16853" LastEditDate="2016-09-02T23:44:15.720" LastActivityDate="2016-09-02T23:44:15.720" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="9064" PostTypeId="1" AcceptedAnswerId="9065" CreationDate="2015-05-07T13:25:38.113" Score="16" ViewCount="2204" Body="&lt;p&gt;In light of the recent SpaceX Dragon test, I was wondering if any pad aborts have ever been performed during an actual launch? To my limited knowledge, none have occurred, despite the Soyuz and Apollo vehicles having the capability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only launch accident I know of is the Challenger disaster, which could not be saved with the abort modes available to the Shuttle.&lt;/p&gt;&#xA;" OwnerUserId="10107" LastEditorUserId="51" LastEditDate="2015-05-07T13:28:39.577" LastActivityDate="2015-05-11T14:29:29.550" Title="Has a pad abort ever been done on an actual launch?" Tags="|launch|abort|launch-escape-system|tractor-les|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9065" PostTypeId="2" ParentId="9064" CreationDate="2015-05-07T13:28:08.060" Score="15" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_7K-ST_No._16L&quot;&gt;Soyuz 10&lt;/a&gt; used the launch escape tower from the pad.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-07T13:28:08.060" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9066" PostTypeId="2" ParentId="9064" CreationDate="2015-05-07T15:42:13.683" Score="11" Body="&lt;p&gt;You'll have to define 'pad abort' as you are talking about both Apollo, Soyuz and the Space Shuttle. Apollo/Soyuz and Space Shuttle are vastly different vehicles, hence they have vastly different definitions of aborts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were five &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Redundant_Set_Launch_Sequencer_.28RSLS.29_abort&quot; rel=&quot;noreferrer&quot;&gt;RSLS&lt;/a&gt; (Redundant Set Launch Sequencer) &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Space_Shuttle_abort_history&quot; rel=&quot;noreferrer&quot;&gt;aborts during the STS programme&lt;/a&gt; (on STS-41-D, STS-51, STS-51-F, STS-55 and STS-68). Since these occur after main engine start but before booster ignition (after which the shuttle was committed to launch) I would technically qualify these as 'pad aborts', albeit a lot less spectacular than pad aborts for Apollo or Soyuz.&lt;/p&gt;&#xA;" OwnerUserId="8132" LastEditorUserId="415" LastEditDate="2015-05-11T14:29:29.550" LastActivityDate="2015-05-11T14:29:29.550" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9067" PostTypeId="1" CreationDate="2015-05-07T16:37:50.207" Score="5" ViewCount="699" Body="&lt;p&gt;With the SpaceX Pad Abort on peoples minds, I was wondering.  How does it compare to other abort systems in terms of G load on the passengers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX reports that their abort G load is around 4G's. That is uncomfortable, but not intolerable, and only for about 5 seconds at most.  (Though the twisting and turning seen in the Pad Abort after the trunk separates and chutes deploy is probably a great carnival ride, but not a good feeling if your rocket is exploding underneath you).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Soyuz_7K-ST_No._16L&quot;&gt;Soyuz 10 Abort&lt;/a&gt; is reported as being in the 14-17G range.  Again, short duration, but that has to hurt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What did Apollo, Gemini, Mercury's abort load work out too? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does the Boeing CST-100, and SNC Dream Chaser abort loads look like?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In comparison, how about a fighter jet ejection seat, since those are sadly significantly more common.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the answer leads to an interesting comparison on how the load of a pusher vs a tractor LES generates. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-08T17:51:31.083" Title="How do the G loads from various abort systems compare?" Tags="|abort|tractor-les|pusher-les|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9068" PostTypeId="2" ParentId="9067" CreationDate="2015-05-07T17:20:06.357" Score="5" Body="&lt;p&gt;I can't find any performance specifications for Dream Chaser; apparently it uses its orbital maneuvering rockets for powered abort, which I would not expect to be very high acceleration. Body lift would maybe help pull it out of the booster's flight line?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CST-100 is also hard to find specs for, but it looks like it's using 4 220kN &lt;a href=&quot;https://en.wikipedia.org/wiki/RS-88&quot; rel=&quot;nofollow&quot;&gt;RS-88 Bantam&lt;/a&gt; engines in pusher configuration against &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/cst-100.htm&quot; rel=&quot;nofollow&quot;&gt;13 tons of capsule&lt;/a&gt;, for about 7g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;6g for &lt;a href=&quot;https://twitter.com/elonmusk/status/596053122587365376&quot; rel=&quot;nofollow&quot;&gt;Dragon v2 per Musk&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;12-14g for &lt;a href=&quot;http://www.ejectionsite.com/ejectfaq.htm&quot; rel=&quot;nofollow&quot;&gt;modern fighter ejection seats&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Over 10g&quot; for &lt;a href=&quot;http://www.spaceflight101.com/orion-spacecraft-overview.html&quot; rel=&quot;nofollow&quot;&gt;Orion&lt;/a&gt; - another source said 0-800kph in 2 seconds, which is 11g &lt;em&gt;average&lt;/em&gt;, so I would expect peak above 12g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I did the math right, &lt;a href=&quot;http://en.citizendium.org/wiki/Project_Mercury&quot; rel=&quot;nofollow&quot;&gt;Mercury&lt;/a&gt; would be about 10g at ignition, increasing to maybe 13g at cutoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can't find good numbers for Apollo, but maybe around 10g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gemini and Vostok used ejection seats rather than an LES.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably you could make either a pusher or a tractor with any degree of thrust you felt was necessary. The advantage of a tractor LES is that it's easier to throw away later in the ascent, so you carry less weight to orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are probably some situations that are survivable with a 14g escape that aren't survivable with a 6g escape, but I would not think there were many such. At ~16g you're trading off guaranteed injury against very very very unlikely death.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-05-08T17:51:31.083" LastActivityDate="2015-05-08T17:51:31.083" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9069" PostTypeId="1" AcceptedAnswerId="9070" CreationDate="2015-05-07T17:22:10.607" Score="9" ViewCount="853" Body="&lt;p&gt;Is the structure on the bottom side of the X-37 in this photo a launch fairing or a transport jig?  In either case, what are the arrays of transparent-looking round things on it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Gs6xB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="12102" LastEditDate="2017-06-24T12:05:46.180" LastActivityDate="2017-06-24T12:05:46.180" Title="What are the features on X-37 fairing/transport jig" Tags="|x-37|identify-this-object|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9070" PostTypeId="2" ParentId="9069" CreationDate="2015-05-07T17:34:37.477" Score="17" Body="&lt;p&gt;That's the launch fairing (the other half is visible on the right). The silvery panels are used for noise reduction; they contain &lt;a href=&quot;http://www.arianespace.com/news-mission-update/2003/112.asp&quot; rel=&quot;noreferrer&quot;&gt;Helmholz absorbers&lt;/a&gt;, cavities tuned to &lt;a href=&quot;https://en.wikipedia.org/wiki/Helmholtz_resonance&quot; rel=&quot;noreferrer&quot;&gt;absorb a single frequency&lt;/a&gt;. Each round object contains the opening of a cavity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without protection, the noise level during launch is loud enough to damage the payload. The absorbers reduce the noise in the fairing just enough to make sure there's no damage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Helmholz resonator has a single resonance frequency. If sound at that exact frequency enters the resonator, you can get the cavity to absorb some of that energy (up to 60%) as heat. So a Helmholz absorber is a resonator &lt;a href=&quot;http://www.hunecke.de/en/knowledge/absorbers/helmholtz-resonators.html&quot; rel=&quot;noreferrer&quot;&gt;modified&lt;/a&gt; to create a high viscous resistance at the neck. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bbc.co.uk/rd/publications/rdreport_1949_29&quot; rel=&quot;noreferrer&quot;&gt;According to a&lt;/a&gt; BBC research paper on the subject: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...Under ideal conditions, (infinite rigidity of container etc. ),&#xA;  absorption of energy can be considered to take place entirely in the&#xA;  resistance at the neck. This resistance has two components, the&#xA;  viscous resistance of the air in the neck, and the radiation&#xA;  resistance, the latter being much the smaller component in the case of&#xA;  narrow necks. The viscous resistance is dependent on the dimensions&#xA;  of the neck and is influenced by the presence of resistive material.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://preserve.lehigh.edu/cgi/viewcontent.cgi?article=2015&amp;amp;context=etd&quot; rel=&quot;noreferrer&quot;&gt;Thesis on the subject (110 pages)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-09T09:28:11.273" LastActivityDate="2017-05-09T09:28:11.273" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9071" PostTypeId="2" ParentId="3078" CreationDate="2015-05-07T22:38:40.417" Score="6" Body="&lt;p&gt;Only one astronaut ejected from an &lt;a href=&quot;http://sploid.gizmodo.com/the-aircraft-accident-that-almost-killed-neil-armstrong-1589559600&quot; rel=&quot;noreferrer&quot;&gt;LLTV--Neil Armstrong, in May 1968&lt;/a&gt;; &lt;a href=&quot;http://en.wikipedia.org/wiki/Lunar_Landing_Research_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;the other two ejections were by training pilots&lt;/a&gt;. &#xA;Armstrong's ejection was caused by a roll to one side late in a flight, when the hydrogen peroxide levels were already low: after a certain degree of roll the pickup port in each saddle tank was now uncovered (low $\text{H}_2\text{O}_2$ level) meaning only high pressure helium came out the thruster nozzles for roll and translation control. He had no control at that point. Armstrong recognized the situation and had ejected before from a fighter so he knew what to do: pull the black-and-yellow strap and leave in a hurry.&lt;/p&gt;&#xA;" OwnerUserId="10113" LastEditorUserId="8162" LastEditDate="2015-05-09T11:53:36.443" LastActivityDate="2015-05-09T11:53:36.443" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9073" PostTypeId="1" CreationDate="2015-05-08T07:46:10.513" Score="0" ViewCount="150" Body="&lt;p&gt;Are there any laws regulating access to the International Space Station for an Indian company, for research such as microgravity tests?&lt;/p&gt;&#xA;" OwnerUserId="3542" LastEditorUserId="6407" LastEditDate="2015-05-08T12:27:43.890" LastActivityDate="2015-05-08T13:42:12.273" Title="What are the regulations about ISS access for research?" Tags="|iss|law|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9074" PostTypeId="2" ParentId="9073" CreationDate="2015-05-08T13:42:12.273" Score="2" Body="&lt;p&gt;Astronaut time, payload transfer capacity, instrument time and other resources on the ISS are divided among the partaking nations depending on their financial commitment to the project. Also, some of the partaking nations have modules which they can use exclusively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, all the partaking nations are allowed to sub-sell their time and resources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the ISRO or a private Indian company would like to place an experiment on the ISS, they could contract any of the ISS partners to perform the experiment for them using the resources allotted to the partner. The price is a matter of negotiation, but it will certainly not be cheap.&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2015-05-08T13:42:12.273" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9075" PostTypeId="1" AcceptedAnswerId="34066" CreationDate="2015-05-08T18:51:03.347" Score="6" ViewCount="630" Body="&lt;p&gt;There is a lot of science fiction and emerging science that move comet and asteroids as part of the main plot.  Pretty much everything in our solar system, is in orbit around the Sun, or in orbit around an object orbiting the Sun.  If you want something to be somewhere else, in essence you change it's solar orbit to match (or collide) with your desired location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have several man made bodies that we have placed in lots of different orbits. we have even caused &lt;a href=&quot;https://space.stackexchange.com/questions/2696&quot;&gt;some to leave the solar system&lt;/a&gt;. Given our current (2015) tools and knowledge what is the largest object we could meaningfully and accurately adjust the orbit of?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where &quot;meaningfully and accurately&quot; = bringing into a given orbit (specific and calculated) around any body that it is not currently orbiting, in a timely fashion (i.e. 5 years or less)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course the biggest challenge is getting your tools and knowledge away from Earth and to the body you want to adjust the orbit of, that is mater of economics.  Assuming you have the budget to get what you want off of Earth, what is the biggest thing you could move accurately? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="19335" LastEditDate="2019-09-24T17:49:13.537" LastActivityDate="2019-09-24T17:49:13.537" Title="What is the largest body in the solar system we could meaningfully and accurately adjust the orbit of?" Tags="|orbital-mechanics|propulsion|solar-system|asteroid-redirect-mission|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="9076" PostTypeId="2" ParentId="9039" CreationDate="2015-05-09T01:48:41.373" Score="6" Body="&lt;p&gt;I think ForgeMonkey is onto something. I just ran across this &lt;a href=&quot;http://www.reddit.com/r/spacex/comments/35b7rs/over_and_underexpanded_nozzles_pdf_how_are_merlin/&quot; rel=&quot;noreferrer&quot;&gt;article and thread&lt;/a&gt;, which includes speculation that the center engine gets some aerospike effect from the exhaust plumes of the surrounding engines, and maybe the vertical positioning of the nozzle is trying to optimize that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seems plausible, but I don't understand the fluid dynamics enough to say if that's really it. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-05-09T01:48:41.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9077" PostTypeId="2" ParentId="9075" CreationDate="2015-05-09T02:39:13.783" Score="4" Body="&lt;p&gt;The phrase &quot;2015 tools and knowledge&quot; combines two very different things.  If we are limited to today's tools, the best we can do is impact a body so the body absorbs the momentum.  The easiest way to satisfy &quot;meaningfully adjust the orbit of something&quot; is to make it not hit (or hit) the earth.  &lt;a href=&quot;http://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Rosetta&lt;/a&gt; is 2900 kg and my WAG for a closing speed, assuming you want it high even though everything goes around the sun, is $10\%$ of Mars orbital velocity or 2.4 km/s.  That gives you the delta v you can impart (though you can't necessarily do it in any chosen direction) by dividing by the mass of the object.  As far as I know, there are not any known objects heading this way.  If we found a comet in an extremely eccentric orbit that would hit the earth next pass (or the one after that) a small nudge would prevent the disaster.  Then you are into 2015 knowledge-how well can we measure the orbit.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2015-05-09T02:39:13.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9078" PostTypeId="2" ParentId="7978" CreationDate="2015-05-09T14:20:53.287" Score="20" Body="&lt;p&gt;OK, I cannot give you the answer for the ISS, MEO and GEO off the top of my head. However, if you do insist for any specific objects, please post their catalogue numbers (five numbers at the beginning of every &lt;a href=&quot;https://celestrak.org/columns/v04n03/&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE line&lt;/a&gt;, i.e. columns 03-07) in comments. But here we go.&lt;/p&gt;&#xA;&lt;p&gt;First of all, I suggest having a look at how we represent multidimensional uncertainty as a &lt;a href=&quot;http://en.wikipedia.org/wiki/Covariance_matrix&quot; rel=&quot;nofollow noreferrer&quot;&gt;covariance matrix&lt;/a&gt;. It's a way of representing a multivariate Gaussian distribution. The covariance matrix has standard deviations squared on the diagonal. You can think of uncertainty in this format as an ellipsoid, which looks like this:&lt;img src=&quot;https://i.stack.imgur.com/dLIWF.png&quot; alt=&quot;uncertaintyellipsoids&quot; /&gt; The small dots in the middle are the satellites and the ellipsoids are the regions where they might be. Representing uncertainties as covariance matrices assumes &amp;quot;gaussianity&amp;quot; of the uncertainty, which may not always be true. But it's, so far, a common assumption to make, so let's stick with it for the sake of clarity and simplicity.&lt;/p&gt;&#xA;&lt;p&gt;You should know that we can never know where a satellite is with absolute certainty. So even when we take some measurements, say with telescopes and radars, and fit an orbit to those it won't be perfect knowledge. Let's call this moment in time, when the orbit is fitted, epoch. So there is some uncertainty at the epoch already. When you propagate, this uncertainty grows over time.&lt;/p&gt;&#xA;&lt;p&gt;You can &lt;a href=&quot;https://ia802807.us.archive.org/7/items/DTIC_ADA446817/DTIC_ADA446817.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;estimate the accuracy of a TLE using previous TLEs for the same object (.PDF)&lt;/a&gt;. This may not seem very reliable but, interestingly, the resulting accuracies seem to be in the same order as ones obtained with &lt;a href=&quot;https://www.amostech.com/TechnicalPapers/2008/Orbital_Debris/Flohrer.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;much higher-fidelity algorithms (.PDF)&lt;/a&gt;. Since we're talking about the uncertainty of position as a matrix (the covariance matrix) we can measure this uncertainty by looking at eigenvalues of the matrix, which have units of standard deviations squared. For example, take a Delta 1 R/B (00862):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/NdzQ5.png&quot; alt=&quot;evolutionofeigenvalues&quot; /&gt;&#xA;You can see that the uncertainty is the greatest and keeps increasing in one direction, i.e. the in-track direction. That's generally how celestial mechanics work. You can see the magnitude of the uncertainty over the first couple orbits more clearly here (sorry, it's Saturday and I don't fancy reworking the Y-axes labels ;) ). &lt;img src=&quot;https://i.stack.imgur.com/l64jO.png&quot; alt=&quot;eigenvaluesshort&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;When you look at Envisat (27386, a circular low Earth orbit so much more similar to the ISS) you can see a similar pattern. &lt;img src=&quot;https://i.stack.imgur.com/dRpmJ.png&quot; alt=&quot;eigenvaluesEnvisat&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;You can see that the uncertainties of positions of both objects vary with frequency of the orbital period (or a multiple of it). So asking &amp;quot;what is the accuracy after one or ten orbits&amp;quot; is a lot more meaningful than 12 or 24 hours.&lt;/p&gt;&#xA;&lt;p&gt;Altitude is only one of the factors that impact the accuracy of the orbits. Another big one is eccentricity and where on the orbit a given object is located. This is to say that the uncertainty grows and shrinks around the orbit (you can see this on the eigenvalue plots before). We know where the object is around the apogee with much higher certainty than at the perigee. The following figure shows the magnitude of the largest eigenvalue of Delta 1's R/B covariance matrix.&lt;img src=&quot;https://i.stack.imgur.com/StJVS.png&quot; alt=&quot;00862eigenvalueMagnitude&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The orbit accuracy &amp;quot;at the epoch&amp;quot;, which is the orbit determination accuracy, is never going to be perfect. To see how well TLE accuracy at epoch compares to reality see &lt;a href=&quot;https://www.amostech.com/TechnicalPapers/2008/Orbital_Debris/Flohrer.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this reference (.PDF)&lt;/a&gt; from the AMOS conference I attached, but it's &amp;quot;a couple kilometres&amp;quot; as a rule of thumb. But the prediction accuracy using SGP4 is somewhat better: the propagated TLEs appear to be accurate to within several kilometres w.r.t. where the objects actually are (&lt;a href=&quot;https://amostech.com/TechnicalPapers/2014/Poster/OLTROGGE.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;source (.PDF)&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;Finally, it isn't about technology, propagators or anything - orbital mechanics on its own will increase the uncertainty regions quite quickly. If we increase the certainty of the orbit knowledge, or improve the propagators we can never remove this effect. So the uncertainty will always vary and grow as on the figures I've shown. Physics.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;On a side note:&lt;/strong&gt; with laser ranging, coherent Doppler ranging etc. you can improve the orbit determination accuracy to millimetres (important for satellites flying e.g. a radar).&lt;/p&gt;&#xA;" OwnerUserId="5749" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2018-08-13T20:51:31.813" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="9080" PostTypeId="1" CreationDate="2015-05-10T12:53:30.480" Score="6" ViewCount="333" Body="&lt;p&gt;Do different cargo carriers to the ISS generally have different profiles for their cargo composition, or is it basically just the number of tons that matter? Especially with respect to cargo which is expensive to replace, such as some spare parts and science experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many different spacecrafts have visited the space station: Proton, Soyuz, Shuttle, ATV (ESA), HTV (JAXA), Progress, Dragon, Cygnus. It was my impression that much more attention given to the experimental payload lost with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_CRS_Orb-3&quot; rel=&quot;noreferrer&quot;&gt;Antares/Cygnus Orb-3 failure&lt;/a&gt; in October 2014, than after the loss of &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_M-27M&quot; rel=&quot;noreferrer&quot;&gt;Progress 59 (or M-29M)&lt;/a&gt; in May 2015. Didn't the Progress carry as much value per ton? Or is it maybe a national issue? American Cygnus carrying American payloads which are reported in English speaking media, while Russian Progress carries mostly Russian payloads which are harder to learn about for non-Russian speakers? What factors play into this, technically and policy?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-05-10T14:17:16.990" LastActivityDate="2015-05-10T17:10:27.200" Title="Does the cargo type to ISS differ between different carrier spacecrafts such as Progress, Dragon, HTV, Cygnus?" Tags="|iss|progress|supply-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9081" PostTypeId="2" ParentId="9080" CreationDate="2015-05-10T17:10:27.200" Score="4" Body="&lt;p&gt;Progress is built around the same basic design as the Soyuz, three modules.  Propulsion is much the same.  The Orbital module at the other end, docks to the station and has cargo the crew removes.  The middle module on Progress (where the crew would normally be during launch/landing) is redone to carry fluids.  Water, fuel for the station, whatever. Thus Progress is optimized for fuel carriage. Dragon has recently been upgraded to allow carrying water to the station as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HTV has a large capacity (6000Kg total), and allows for a fairly large unpressurized cargo to be brought up (1500Kg).  Dragon has similar unpressurized cargo but in smaller size (Max of 3310Kg combined pressurized and unpressurized).  Thus Dragon and HTV have the better unpressurized external carriage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cygnus when fully expanded has pretty good internal volume and mass capability for payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why the disparity in concern on lost 'things' between Cygnus and Progress? Probably the incidental nature of the specific payloads. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Progress carries food, water, fuel, etc. Basics. Cygnus just happened to be carrying experiments of interest on that flight. As well as basic food, supplies, etc.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-10T17:10:27.200" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9082" PostTypeId="1" CreationDate="2015-05-10T17:31:16.153" Score="6" ViewCount="292" Body="&lt;p&gt;What are the proposed methods for transferring people from orbiting spacecraft to Mars surface specifically if multiple trips are required. Are there any that don't require a new heat shield for each descent?&lt;/p&gt;&#xA;" OwnerUserId="9046" LastEditorUserId="195" LastEditDate="2015-05-11T20:52:49.757" LastActivityDate="2015-05-11T20:52:49.757" Title="Transfer people from orbiting spacecraft to surface of Mars" Tags="|mars|reentry|landing|lander|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9083" PostTypeId="1" CreationDate="2015-05-10T18:24:44.643" Score="3" ViewCount="388" Body="&lt;p&gt;A power plant in space causes waste heat which has to be removed from the plant. The need for large heat sinks, and their mass, kind of bogs down some of the advantages of big nuclear power in space. But is that aspect any better for big solar power in space? Does the mass of heat sink per Watt produced differ between nuclear and photovoltaic (or fuel cells or discharging batteries or whatever) power source?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar panels are intrinsically large. Does this help solve their heat sink problem?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there some basic, or today practical, relationships between effect produced (continuously) and mass of heat sink needed to keep the plant at stable temperature?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-05-10T18:29:53.317" LastActivityDate="2015-05-12T15:14:48.143" Title="Is the waste heat (sink) system dependent on whether the energy source is nuclear or solar?" Tags="|spacecraft|power|solar-power|nuclear|heat|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9084" PostTypeId="2" ParentId="9082" CreationDate="2015-05-10T20:02:36.820" Score="3" Body="&lt;p&gt;The Shuttle didn't require a &lt;em&gt;new&lt;/em&gt; heat shield each time, just refurbishment. As far as I know, SpaceX are working with Mars in mind. Their Dragon seems to be built with such reusability in mind. I haven't found much on this but &lt;a href=&quot;http://www.spaceref.com/news/viewpr.html?pid=27612&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; seems to confirm this: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;All of SpaceX's initial production will be used for domestic in-house applications including the heat shields of the Dragon spacecraft, and the Falcon 9 second stage, which is designed to return from orbit for recovery and reuse.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Frankly, if there's anyone out there that does it I'd bet on SpaceX.&lt;/p&gt;&#xA;" OwnerUserId="5749" LastActivityDate="2015-05-10T20:02:36.820" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9085" PostTypeId="2" ParentId="9082" CreationDate="2015-05-11T00:43:16.840" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;https://linuxacademy.com/blog/space/comparing-heat-shields-mars-science-lab-vs-spacex-dragon/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt;, the PICA-X heat shield on SpaceX's Dragon ablates less than 15% on a single &lt;em&gt;Earth&lt;/em&gt; reentry. I assume that Mars's thinner atmosphere and lower gravity (-&gt; lower orbital speed) implies that there would be even less ablation in a Mars entry. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-05-11T00:43:16.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9086" PostTypeId="2" ParentId="9050" CreationDate="2015-05-11T05:47:02.620" Score="5" Body="&lt;p&gt;&lt;strong&gt;YES&lt;/strong&gt; and &lt;strong&gt;NO&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The Yes part&lt;/strong&gt;&lt;br&gt;&#xA;Many meteors have been traced back to their parent bodies.  We have done very detailed analysis of both mineral and elemental composition.  By comparing the spectrum of the bodies we have samples of (e.g. Vesta), we can determine what the composition of many other bodies possess.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based upon that we have a good idea of the total quantities of minerals and elements available for exploitation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The No part&lt;/strong&gt;&lt;br&gt;&#xA;The problem is, we only have a good idea of the overall composition of these bodies.  We do not know what sort of ores might be available for exploiting if we need to do something like harvest rare earth elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One problem is that on Earth elements/minerals are concentrated by a mix of processes only available on Earth (volcanic, hydraulic, and biological).  Since most bodies do not have these processes, we will likely only find lower grade ores for most elements we wish to exploit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are exceptions to this&lt;br&gt;&#xA; 1. metallic nodules in carbonaceous chondrites &amp;amp; stony asteroids&lt;br&gt;&#xA; 2. metallic meteors &amp;amp; asteroids  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plus most celestial bodies possess higher surface concentrations of several valuable elements than the Earth's surface does (it seems that when the Earth differentiated, these elements were pulled down into the core).&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;rare earth elements&lt;/li&gt;&#xA;&lt;li&gt;platinum group elements&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;But yeah, I've looked up the concentrations of these bodies and found reasonably accurate numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/C-type_asteroid&quot; rel=&quot;nofollow&quot;&gt;Type C Asteroid&lt;/a&gt; - carbonaceous, 75% of asteroids&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/S-type_asteroid&quot; rel=&quot;nofollow&quot;&gt;Type S Asteroid&lt;/a&gt; - stony, 17% of asteroids&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/M-type_asteroid&quot; rel=&quot;nofollow&quot;&gt;Type M Asteroid&lt;/a&gt; - metallic, 6-8% of asteroids  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;C-Type:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Asteroids of this class have spectra very similar to those of&#xA;  carbonaceous chondrite meteorites (types CI and CM). The latter are&#xA;  very close in chemical composition to the Sun and the primitive solar&#xA;  nebula, except for the absence of hydrogen, helium and other&#xA;  volatiles. Hydrated (water-containing) minerals are present&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=12&amp;amp;ved=0CGAQFjAL&amp;amp;url=http%3A%2F%2Fjoy.chara.gsu.edu%2F~white%2Fastr8100%2FLodders2003.pdf&amp;amp;ei=O0FQVeHvDIjloASO5oCQCA&amp;amp;usg=AFQjCNHq_MfrCZ4Z4zx77qXCZo7GtL8Ybg&amp;amp;sig2=YjzBfoe7EFPrU38A4Q1s6Q&quot; rel=&quot;nofollow&quot;&gt;Overall elemental abundances&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8630" LastEditorUserId="8630" LastEditDate="2015-05-14T04:23:03.957" LastActivityDate="2015-05-14T04:23:03.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9087" PostTypeId="1" AcceptedAnswerId="9088" CreationDate="2015-05-11T12:25:39.520" Score="48" ViewCount="39788" Body="&lt;p&gt;I know that ISS, being in low Earth orbit, requires regular boosts. But I can't seem to find information on how often does this happen. Is it done during each resupply mission, only during some of them, or are there missions which sole purpose is to boost the station's orbit (doesn't seem likely, but what do I know)? Are the boosts large, or small and periodical? How much does the orbital altitude oscillate over time. I don't mean between apsides; let's say how does the apogee oscillate? &lt;/p&gt;&#xA;" OwnerUserId="10124" LastEditorUserId="20987" LastEditDate="2020-01-13T20:50:16.770" LastActivityDate="2020-01-13T20:50:16.770" Title="How often does ISS require re-boosting to higher orbit?" Tags="|iss|orbital-mechanics|low-earth-orbit|orbital-maneuver|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="9088" PostTypeId="2" ParentId="9087" CreationDate="2015-05-11T13:00:27.277" Score="43" Body="&lt;p&gt;The easiest to see ISS orbital reboosts is by checking &lt;em&gt;&lt;a href=&quot;http://www.heavens-above.com/IssHeight.aspx&quot; rel=&quot;noreferrer&quot;&gt;Height of the ISS&lt;/a&gt;&lt;/em&gt; (where with &lt;em&gt;height&lt;/em&gt; they mean &lt;em&gt;orbital altitude above mean sea-level&lt;/em&gt;) over at Heavens Above. For example, for the last year, this is the graph:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/f8p3G.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;This plot shows the orbital height of the ISS over the last year.&#xA;  Clearly visible are the re-boosts which suddenly increase the height,&#xA;  and the gradual decay in between. The height is averaged over one&#xA;  orbit, and the gradual decrease is caused by atmospheric drag. As can&#xA;  be seen from the plot, the rate of descent is not constant and this&#xA;  variation is caused by changes in the density of the tenuous outer&#xA;  atmosphere due mainly to solar activity. (Source and credit: &lt;a href=&quot;http://www.heavens-above.com/IssHeight.aspx&quot; rel=&quot;noreferrer&quot;&gt;Heavens&#xA;  Above&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As you can see, reboosts (sudden peaks in altitude on graphs) are done on average about once per month, but there can be many consecutive months during which no adjustment in orbital altitude to the station is done. Which is nowadays done either by visiting Progress resupply vehicles, or the station's thrusters on Zarya and Zvezda orbital modules by using propellants delivered to the station by resupply vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS mean orbital altitude is however now higher than it used to be during the Space Shuttle visits when it was still being assembled, for easier access of the Space Shuttles and to increase their total payload to orbit capacity. Since &lt;em&gt;assembly complete&lt;/em&gt; in 2011, and retirement of the Space Shuttle fleet, its orbit has been raised from about 355 km to now roughly 415 km (average):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/0nNSt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Previously, orbital reboosts were also done to adjust station's phase angle so its orbit was accessible to the Space Shuttle fleet during convenient local times more often, but now these phase angle reboosts seem to be rarer but still performed for some visiting vehicle orbit rendezvous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more info on each specific reboost, refer to the &lt;a href=&quot;https://blogs.nasa.gov/stationreport/&quot; rel=&quot;noreferrer&quot;&gt;ISS On-Orbit Status Report&lt;/a&gt;. Last reboost (on date of writing this answer) was performed on May 6 by Progress M-26M, despite the loss of Progress M-27M vehicle:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;ISS Daily Summary Report – 05/06/15&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;ISS Reboost: This morning, the ISS performed a reboost using 58P&#xA;  thrusters to set up phasing requirements for 41S landing scheduled on&#xA;  May 13. Burn duration was 12 minutes, 17 seconds with a Delta-V of&#xA;  1.34 meters/second.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;During Space Shuttle years, small reboosts were also performed by the Shuttle Orbiters (&lt;a href=&quot;https://en.wikipedia.org/wiki/ISS_Propulsion_Module&quot; rel=&quot;noreferrer&quot;&gt;according to Wikipedia&lt;/a&gt;, they had 232 kg of fuel available for that), and in recent years also by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Automated_Transfer_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;ATV resupply vehicles&lt;/a&gt;, with the last of the five of them undocked from the station in February, 2015. Last ATV reboost was done by this &lt;a href=&quot;https://en.wikipedia.org/wiki/Georges_Lema%C3%AEtre_ATV&quot; rel=&quot;noreferrer&quot;&gt;Georges Lemaître ATV-5&lt;/a&gt; on &lt;a href=&quot;https://blogs.nasa.gov/stationreport/2014/10/08/iss-daily-summary-report-100814/&quot; rel=&quot;noreferrer&quot;&gt;August 10, 2014&lt;/a&gt;. In the future, the station might also receive a &lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket#Testing_on_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;testing version of VASIMR thruster&lt;/a&gt; that could perform such reboosts more efficiently in terms of reaction mass consumption.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-05-11T13:28:56.837" LastActivityDate="2015-05-11T13:28:56.837" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9091" PostTypeId="1" CreationDate="2015-05-11T17:59:14.373" Score="4" ViewCount="474" Body="&lt;p&gt;What is the scientific foundation for the hypothesis that human space exploration accidentally could spread Earth life to e.g. Mars and cause contamination which is indistinguishable from any indigenous Martian life?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The argument I know goes that if we later detect life on Mars, we wouldn't know if it is our own contamination or if it is indigenous Martian life which happens to be indistinguishable from Earth life. But that is obviously not a valid argument! Proteins are chains of hundreds to tens of thousands of 20 different amino acids. For any protein length, there are at between 20^100 to 20^30000 possible variants. So we know for sure that two similar proteins never occur independently anywhere in the visible universe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that &lt;strong&gt;&quot;convergent evolution&quot; doesn't even come in to play here&lt;/strong&gt; as a potential trend to uniformity, since variation and selection only works on actually existing proteins, not any of the possible but never existing proteins. Indigenous Martian life will very very easily be distinguishable from Earth life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any other arguments?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-05-13T01:23:38.793" LastActivityDate="2017-06-23T00:29:20.270" Title="What are the valid arguments for forward contamination, given the uniqueness of proteins?" Tags="|astrobiology|forward-contamination|" AnswerCount="3" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9093" PostTypeId="2" ParentId="9091" CreationDate="2015-05-11T18:45:08.757" Score="5" Body="&lt;p&gt;If life on Earth and on Mars have a &lt;em&gt;common&lt;/em&gt; origin (a microbial hitchhiker on &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_meteorite&quot;&gt;impact debris&lt;/a&gt;, for example), contamination could obscure that fact.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-05-11T18:45:08.757" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9094" PostTypeId="1" AcceptedAnswerId="9095" CreationDate="2015-05-11T19:45:48.720" Score="3" ViewCount="268" Body="&lt;p&gt;I have some old code for a simple interplanetary trajectory solver and thought it would be interesting to try and extend it to incorporate gravity assists as well. Does anyone know of any books/articles that explain the mathematics behind a simple gravity assist solver so that I can attempt to add it into my mission planner?&lt;/p&gt;&#xA;" OwnerUserId="10130" LastEditorUserId="49" LastEditDate="2015-05-11T19:51:27.213" LastActivityDate="2015-06-10T21:30:44.107" Title="Mathematics of Gravity Assists" Tags="|trajectory|interplanetary|mission-design|gravity-assist|reference-request|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9095" PostTypeId="2" ParentId="9094" CreationDate="2015-05-11T21:01:56.327" Score="2" Body="&lt;p&gt;Bate's &lt;a href=&quot;http://amzn.com/0486600610&quot; rel=&quot;nofollow&quot;&gt;Fundamentals of Astrodynamics&lt;/a&gt; is a classic in the field.  There are others, but I would start there.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-05-11T21:01:56.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9096" PostTypeId="2" ParentId="9083" CreationDate="2015-05-11T21:07:00.270" Score="3" Body="&lt;p&gt;This is a very difficult question to answer - as the answer depends much more on the missions and other design parameters than the actual design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First I wish to address the heat problem:&#xA;Actually solar cells create a lot of heat. For this reason for observation satellites typically the solar cells are far away from the camera. But so do nuclear power cells. Also for this question I consider only power cells which use natural nuclear decay (or Radioisotope Thermoelectric Generator, RTG), so not a nuclear fission reactor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nice thing about space is that any power that isn't used, becomes heat in some way. There is no losses through drag or anything. So this all has to be dissipated as heat.  &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Now waste heat comes from several different efficiencies:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Using the equipment (create torque/photographing/data transmission ..) - $\eta_{equip}$&lt;/li&gt;&#xA;&lt;li&gt;The electical equipment (batteries, wiring) $\eta_{e}$&lt;/li&gt;&#xA;&lt;li&gt;Efficiency of the power source $\eta_p$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now if we want to provide the same net power $W_{net}$ we can just compare the efficiencies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Equipment efficiency&lt;/strong&gt;&lt;br&gt;&#xA;Barring slight variations in geometrical constraints the efficiency for both nuclear and solar energy sourced satellites is near equal - so it can be ignored.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Electrical equipment efficiency&lt;/strong&gt;&lt;br&gt;&#xA;Where nuclear power will always work in a steady way. (Or at least almost, considering the half-life time of the plutonium/uranium is quite long). Solar energy is really not that easily generated, the solar radiation reduces with the inverse squared distance from the sun. (For example the Rosetta probe was &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/ESA_s_sleeping_beauty_wakes_up_from_deep_space_hibernation&quot; rel=&quot;nofollow&quot;&gt;put into hibernation&lt;/a&gt; for 31 months)&lt;br&gt;&#xA;And for celestial orbiting satellites, the time when it is in the shadow of the planet has to be taken into account. - On top of that solar panels are always quite large, requiring a lot of wiring to connect each separate cell. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\eta_{e,s} &amp;lt; \eta_{e,n}$&#xA;Though given modern day advancements in electrical power storage, this is quite small $\eta_{e} \approx 0.93-0.96$ - So only a percentage or so could be gained from using a nuclear power cell, but this depends entirely on the mission profile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Power efficiency&lt;/strong&gt;&lt;br&gt;&#xA;This is probably the biggest loss in spacecraft and will be the deciding factor:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In RTG engines typically the natural decay of the atoms heats another solid substance. This matter than uses electrostatic conversion to create power. Looking at the efficiency of the Curiosity rover $\eta_p = 125/2000 = 0.125$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is however a ready to implement proposal to use a Stirling engine instead of electrostatic conversion. The heated plates would then heat a  gas which then goes through the Stirling engine (called ASRG). The NASA predicted efficiency goes up to about 0.25.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar panels also lose a lot of power. This has mostly to do with the band-gap of the semi-conductor used. (Germanium, silicon). For simplicity consider that no radiation is reflected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically when a photon hits a solar cell 3 things may happen:&lt;br&gt;&#xA;The photon has less energy than required for the electrons to jump the gap -&gt; all energy of the photon is lost and converted to heat.&lt;br&gt;&#xA;The photon has exactly the right amount of energy -&gt; the whole energy of the photon is converted into electrical power&#xA;The photon has more energy than required to jump the gap -&gt; the part of energy that is required to jump is used, the rest is converted to heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see the efficiency is hence a function of the typical wavelength profile you expect - and also material used (and nowadays multijunction solar cells with different bandgaps are gaining popularity). Typical efficiency values of solar arrays lie in the order of &lt;a href=&quot;http://www.nrel.gov/ncpv/images/efficiency_chart.jpg&quot; rel=&quot;nofollow&quot;&gt;0.25&lt;/a&gt; (multijunciton cells are not yet used in real application for space a lot).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;hr&gt;&#xA;&lt;strong&gt;Conclusion&lt;/strong&gt;&lt;br&gt;&#xA;So concluding, which is more efficient (and hence creates less waste heat):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For RTGs we get around:&#xA;$$\eta_{t,n} = 0.98 \cdot 0.10 = 0.098$$&#xA;For solar we get around:&#xA;$$\eta_{t,s} = 0.94 \cdot 0.25 = 0.235$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, solar cells generate a lot less heat. And for heat management alone nuclear power cells are a bad idea. However Stirling engines could tip the balance. (Though similarly multijunction could also increase solar cells).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now as for the total mass - this is an even broader topic I will not touch here, one has to take into account that you also need a minimal power to operate, and having more solar panels also requires more mass / increasingly complex designs.&lt;/p&gt;&#xA;" OwnerUserId="10127" LastEditorUserId="4660" LastEditDate="2015-05-12T15:14:48.143" LastActivityDate="2015-05-12T15:14:48.143" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9097" PostTypeId="1" CreationDate="2015-05-11T22:39:48.977" Score="3" ViewCount="297" Body="&lt;p&gt;Generally first stage boosters are just tubes with tanks and engines attached to the later stages. However, &lt;a href=&quot;http://www.gizmag.com/jeff-bezos-blue-origin-flies-its-first-rocket/37274/pictures#3&quot; rel=&quot;nofollow noreferrer&quot;&gt;based on this photo&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4B5kQ.jpg&quot; alt=&quot;Tapered Ring&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Blue Origin has this tapered design of the crew capsule and a ring that attaches to stage I.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any benefit of this design as opposed to conventional?&lt;/p&gt;&#xA;" OwnerUserId="8903" LastEditorUserId="8903" LastEditDate="2015-05-12T18:24:24.743" LastActivityDate="2015-06-11T18:40:14.967" Title="Blue Origin's The New Shepard crew capsule's design" Tags="|blue-origin|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9098" PostTypeId="2" ParentId="9097" CreationDate="2015-05-11T23:13:59.450" Score="2" Body="&lt;p&gt;I believe the tapered ring allows an exhaust path for the &quot;full envelope&quot; abort system exhaust while not compromising the size of the capsule.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-05-11T23:13:59.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9100" PostTypeId="2" ParentId="9091" CreationDate="2015-05-12T10:17:40.330" Score="5" Body="&lt;p&gt;I can think of 3 reasons. The third is probably most important:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;1.&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The point made by @Russell Borogove about common origin is one reason. We still don't know that much about the origin of life on Earth, so we certainly cannot rule out that possibility.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;2.&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Another point is that we don't really know what alien life would look like. The only life we know of is Earth life. It could be possible that Earth-like life could be very likely to occur elsewhere as well and that there aren't that many lifeforms that are truly unlike Earth life. I wouldn't necessarily say that two similar proteins would never occur independently anywhere in the visible universe. &lt;a href=&quot;http://en.wikipedia.org/wiki/Convergent_evolution&quot; rel=&quot;nofollow noreferrer&quot;&gt;Convergent evolution&lt;/a&gt; show us that different lifeforms can evolve similarly given the same constraints. Proteins perform certain functions and would probably look similarly to other proteins that serve the same function.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;3.&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;A third point is that we are looking for &lt;a href=&quot;http://mars.nasa.gov/programmissions/science/goal1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;evidence of life&lt;/a&gt; rather than the actual DNA strings. This could be current or extinct (We don't have Dino DNA). Earth-life could invade Mars and change the landscape and chemistry to such an extent that we would not be able to determine if it was altered through Earth or Mars life.&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="195" LastEditDate="2017-06-23T00:29:20.270" LastActivityDate="2017-06-23T00:29:20.270" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9101" PostTypeId="1" CreationDate="2015-05-12T10:33:02.543" Score="4" ViewCount="2261" Body="&lt;p&gt;I suppose gravity assist on a flyby only works in the orbital plane of the assisting mass. But I wouldn't think that matters for orbit insertion. Is it as easy to enter into a polar orbit, as into an equatorial orbit around another planet? Could one simply aim at a certain latitude as first periapsis at arrival in order to get a capture orbit with any inclination one wants?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-05-12T16:56:56.997" Title="Is higher delta-V required to enter polar orbit than equatorial orbit around another planet?" Tags="|orbital-mechanics|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9103" PostTypeId="2" ParentId="9101" CreationDate="2015-05-12T13:33:43.617" Score="1" Body="&lt;p&gt;Assuming both bodies have no axis tilt and an initial orbital inclination of zero degrees, it will require more delta-V to enter a polar orbit around the second body.  However, since inclination changes are more efficient the farther away you are, this increase becomes very small at interplanetary distances.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-05-12T13:33:43.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9104" PostTypeId="2" ParentId="9091" CreationDate="2015-05-12T13:35:42.683" Score="5" Body="&lt;p&gt;You are correct that our current technology (e.g. polymerase chain reaction) allows us to easily distinguish and discriminate among life forms on Earth, and would allow us to readily identify Earth-derived life forms on Mars, should they survive.  Even if Earth and Mars life ultimately have a common origin, we could still distinguish them from each other as we distinguish archaea from bacteria on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, another potential impact of introducing Earth life would be to take over an ecosystem and cause the Mars life to become extinct before we had a chance to detect and analyze it.  I consider this scenario to be rather unlikely, since the putative Martian organisms have had the history of the planet to evolve to their environment.  Still, unlikely is not zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, if Earth and Mars life manage to coexist and they do not have a common origin, we could indeed detect the Earth life, but its presence would confound our ability to examine the Mars life in the same sample by adding a lot of noise on top of the signal.  It wouldn't be impossible, just much more difficult.  (Maybe we could develop an Earthicide that kills Earth life but not Mars life.  If we do though, we should be careful with that stuff.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-05-12T13:35:42.683" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9105" PostTypeId="2" ParentId="9101" CreationDate="2015-05-12T13:37:17.070" Score="1" Body="&lt;p&gt;This depends on the target body's axial tilt and its inclination to transfer orbit. For example, it would require less $\Delta v$ to inject into a polar orbit around Uranus with axial tilt of nearly 98° and orbital inclination of 0.8° to ecliptic, than it would into its equatorial orbit. You'd have to compute for exact injection epoch what this inclination would be to your transfer orbit at the time of orbital injection though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One other note, that gravity assisted $\Delta v$ changes still count towards total $\Delta v$ required to achieve a target orbit, some of it just might not be achieved propulsively. It also speaks nothing of propulsive efficiency, for example during &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;nofollow&quot;&gt;Oberth burns&lt;/a&gt;. So, it's good to know how much of it comes &lt;em&gt;free&lt;/em&gt; (can be achieved by exchanging momentum with gravity assisting bodies, by means of aerocapture,... say to reduce $v_\infty$ during injection or to slowly crank your orbital inclination) when reading $\Delta v$ charts, because that will define how much of propulsive $\Delta v$ your mission needs to be capable to achieve, but it still counts towards total $\Delta v$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, how &lt;em&gt;easy&lt;/em&gt; will be to achieve a specific target orbital inclination around some other planet (as per your question's title) depends on many factors, and would be specific to each planet. Take for example bodies with planetary rings (Jupiter, Saturn, Uranus, Neptune and possibly Pluto), where there might even be rings, moonlets,... in the way of achieving equatorial orbits of certain altitudes. Large portion of this problem will also be reducing your transfer trajectory's $v_\infty$, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Hyperbolic_trajectory#Hyperbolic_excess_velocity&quot; rel=&quot;nofollow&quot;&gt;hyperbolic excess velocity&lt;/a&gt;. Inclination change, once you've reduced $v_\infty$ and are captured in orbit, might be relatively easy. Perhaps, partially, by even slowly cranking your orbital inclination with propulsion methods that don't consume any reaction mass, like &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetorquer&quot; rel=&quot;nofollow&quot;&gt;magnetorquers&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-05-12T13:37:17.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9106" PostTypeId="2" ParentId="9101" CreationDate="2015-05-12T13:44:45.360" Score="8" Body="&lt;p&gt;With vanishingly small $\Delta V$ long before you arrive, you can select any orbital insertion point on a circle around the planet.  However your approach declination relative to the destination body equatorial plane limits what orbital inclinations you can get to, where that declination is defined by your interplanetary trajectory and is very expensive to change.  The orbital inclination after a single insertion burn cannot be &lt;em&gt;less&lt;/em&gt; than the approach declination.  So it will always be more expensive to do a plane change to an exactly equatorial orbit, unless your approach declination is exactly zero (which never happens).  It will always be free to go into a polar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plane change to get to an equatorial orbit does not have to be large, if you first insert into a very loose orbit.  Then the cost of the plane change at apoapsis will be small.  Once the orbital plane is set to what you want, you can continue with the lowering of the apoapsis to the desired destination orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-05-12T16:22:25.657" LastActivityDate="2015-05-12T16:22:25.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9108" PostTypeId="2" ParentId="779" CreationDate="2015-05-12T16:05:10.297" Score="3" Body="&lt;p&gt;Sometimes one picture, especially the one drawn by Randall, is worth a thousand words :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://xkcd.com/681/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://xkcd.com/681/&lt;/a&gt;&#xA;(xkcd classic &quot;Gravity Wells&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Citing the creator:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This chart shows the &quot;depth&quot; of various solar system gravity wells.&#xA;  Each well is scaled such that rising out of a physical well of that&#xA;  depth - in constant earth surface gravity - would take the same energy&#xA;  as escaping from that planet's gravity in reality&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/f00mR.png&quot; alt=&quot;fragment of &amp;quot;Gravity Wells&amp;quot; by Randall Munroe, under CC-BY-NC&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yes, it does not answer the question, but perhaps helps to visualize one of the concepts needed to understand what's going on. Maybe I should have added it as a comment...? &lt;/p&gt;&#xA;" OwnerUserId="6522" LastEditorUserId="6522" LastEditDate="2015-05-12T18:55:29.623" LastActivityDate="2015-05-12T18:55:29.623" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9109" PostTypeId="2" ParentId="9101" CreationDate="2015-05-12T16:56:56.997" Score="1" Body="&lt;p&gt;You can think of it this way - you can approach a planet from any direction and enter orbit in a plane intersecting with the poles. You are aligning yourself with two points, not a specific plane. But when you want to take up orbit aligned with the equator, you have to enter from a specific direction, or adjust inclination once in orbit. The more the planet's axial tilt deviates from the ecliptic, the more you will have to adjust to do that. The first goal only requires you aim at a certain point, the second requires that you also adjust the angle of the craft's motion. And of course, your craft is moving at high speed so changing direction takes a lot of energy.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-05-12T16:56:56.997" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9110" PostTypeId="1" AcceptedAnswerId="9115" CreationDate="2015-05-12T17:41:43.293" Score="24" ViewCount="2636" Body="&lt;p&gt;I work at a university that gets LN&lt;sub&gt;2&lt;/sub&gt;/LHe (liquid nitrogen and helium) delivered a couple times a week by a large tanker truck outside my office. It seems to work well enough to deliver a couple tons of cryogens per week, but a rocket (e.g. Falcon 9 or Delta 4 Heavy) could use 200-600 tons of cryogens. How are they delivered to the pad?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it just a fleet of trucks? Are they manufactured nearby (LOx, liquid oxygen, seems easy enough to produce given lots and lots of electricity) and piped to a holding tank?&lt;/p&gt;&#xA;" OwnerUserId="881" LastEditorUserId="881" LastEditDate="2015-05-13T01:34:12.333" LastActivityDate="2022-10-02T13:01:02.793" Title="How are cryogenic rocket propellants delivered to the launch pad?" Tags="|fuel|safety|cryogenics|logistics|storage|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9111" PostTypeId="2" ParentId="9110" CreationDate="2015-05-12T17:43:34.217" Score="6" Body="&lt;p&gt;Depends on the fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With Liquid Oxygen, for regular launches, it makes more sense to build a LOX plant nearby and pipe it to the pad/storage tank, than it is to buy it offsite.  You can see the SpaceX oxygen ball near the pad at LC-40. It seems that during the Saturn project they had such a plant at the Cape near LC-17 &lt;a href=&quot;http://afspacemuseum.org/ccafs/cemetery/&quot; rel=&quot;nofollow noreferrer&quot;&gt;but no longer.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nitrogen in large quantities is the same. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The others are usually trucked in or brought by rail car.  Kerosene/RP1 is most likely brought by train or truck for SpaceX. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider how much Kersone, LOX, and LH2 the Saturn V launches took?  Many times that of a Falcon 9 launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interesting side question is how SpaceX gets LOX at the McGregor test site where they test single Merlin engines almost daily, and often run tests on full up Falcon 9 first stages. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="23891" LastEditDate="2019-07-13T23:22:07.743" LastActivityDate="2019-07-13T23:22:07.743" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="9112" PostTypeId="2" ParentId="9046" CreationDate="2015-05-13T08:09:55.130" Score="7" Body="&lt;p&gt;If you've been reading &quot;for quite some time&quot;, you probably know much more than I do. However ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this general article&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://machinedesign.com/fasteners/epoxies-and-adhesives-fit-space&quot; rel=&quot;noreferrer&quot;&gt;http://machinedesign.com/fasteners/epoxies-and-adhesives-fit-space&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a mention of NASA's &quot;Vacuum stability requirements of polymeric material for spacecraft application&quot;. This standard doesn't appear to have been amended since the early 1980s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This doesn't really tell you the most commonly used space adhesives, but from the Machine Design article, it looks like lots of fairly generic epoxies pass muster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for tradeoffs, it probably depends on your application. If the glue won't be exposed to UV, and otherwise has nicer strength properties, it might mean you can use less of it, saving on spacecraft mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The book chapter here (&lt;em&gt;Handbook of Adhesives and Sealants, Volume 1: Basic Concepts and High Tech Bonding. Elsevier, 2005. ISBN 9780080445540&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://books.google.co.jp/books?id=9XFD8ZsZzAsC&amp;amp;lpg=PA334&amp;amp;ots=-oKiRMQAGm&amp;amp;dq=adhesives%20for%20satellite%20construction&amp;amp;pg=PA334#v=onepage&amp;amp;q=adhesives%20for%20satellite%20construction&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;https://books.google.co.jp/books?id=9XFD8ZsZzAsC&amp;amp;lpg=PA334&amp;amp;ots=-oKiRMQAGm&amp;amp;dq=adhesives%20for%20satellite%20construction&amp;amp;pg=PA334#v=onepage&amp;amp;q=adhesives%20for%20satellite%20construction&amp;amp;f=false&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;appears to go into the different tradeoffs for space applications generally, when considering adhesives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this helps.&lt;/p&gt;&#xA;" OwnerUserId="10144" LastEditorUserId="168" LastEditDate="2015-05-13T09:10:39.083" LastActivityDate="2015-05-13T09:10:39.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9113" PostTypeId="2" ParentId="9110" CreationDate="2015-05-13T13:01:46.690" Score="9" Body="&lt;p&gt;At Kourou, there's a &lt;a href=&quot;http://www.airliquide.com/file/otherelementcontent/pj/cr_ag_en_09%2036310.pdf&quot;&gt;production plant&lt;/a&gt; for liquid hydrogen and oxygen. The plant is run by &lt;a href=&quot;http://archives.interactions.airliquide.com/?lt=25&quot;&gt;Air Liquide&lt;/a&gt;, a French company that specializes in industrial gases.&lt;br&gt;&#xA;There's also a casting facility for solid rocket stages.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-05-13T13:01:46.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9114" PostTypeId="1" AcceptedAnswerId="9116" CreationDate="2015-05-13T13:48:47.220" Score="6" ViewCount="383" Body="&lt;p&gt;After the failed &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_M-27M&quot; rel=&quot;noreferrer&quot;&gt;Progress M-27M / Progress 59&lt;/a&gt; resupply mission in late April 2015, Roscosmos and NASA have been working on ISS schedule changes to accommodate loss of supplies to the station, while an investigation into Progress M-27M failure is still ongoing. Yesterday, May 12, 2015, the following schedule changes &lt;a href=&quot;http://spaceflightnow.com/2015/05/12/russia-delays-iss-crew-rotation-flights/&quot; rel=&quot;noreferrer&quot;&gt;were announced&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Return of Soyuz TMA-15M (Virts, Cristoforetti, Shkaplerov) in early June instead of on May 13,&lt;/li&gt;&#xA;&lt;li&gt;Progress M-28M launch in early July instead of August 6,&lt;/li&gt;&#xA;&lt;li&gt;Launch of Soyuz TMA-17M with Kjell Lindgren (NASA), Kimiya Yui (Japan Aerospace Exploration Agency) and Oleg Kononenko (Roscosmos) in late July instead of on May 26.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Not surprisingly, this includes changes to the crew rotation scheme. But it also delays Soyuz TMA-15M crew return for at least 18 days, possibly more. And that is somewhat surprising to me, considering that Progress M-27M supplies weren't delivered. While the station isn't running out of supplies, having a crew of six instead of a crew of three aboard for an extended period should thin them out faster. So to my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Given slightly extraordinary circumstances, is keeping a crew of six aboard the station for longer and maintaining tight crew rotation scheme better than extending the period during which the station only hosts a single Soyuz TMA crew of three?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, I'm asking why wouldn't the Soyuz TMA-15M crew return as previously scheduled, and then launch Soyuz TMA-17M later, keeping only the Soyuz TMA-16M crew of three on the station for a few weeks longer? Is Soyuz TMA-17M launch still considered too tentative, pending conclusion to the Progress M-27M failure investigation, or something else prompted Soyuz TMA-15M crew return delay?&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2018-05-29T10:45:56.177" Title="How does delaying Soyuz TMA-15M crew return help with ISS schedule slip due to failed Progress M-27M resupply vehicle?" Tags="|iss|soyuz-spacecraft|crewed-spaceflight|planning|progress|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9115" PostTypeId="2" ParentId="9110" CreationDate="2015-05-13T13:58:59.847" Score="20" Body="&lt;p&gt;Kennedy Space centre in Florida formerly had its own plant for the manufacture of cryogenic fuels and other liquids. &lt;a href=&quot;https://web.archive.org/web/20170226115543/http://afspacemuseum.org/ccafs/cemetery/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This is now closed&lt;/a&gt; as noted earlier.&#xA;The procurement for cryogenic fuel for NASA is done via the office of procurement at KSC. &lt;a href=&quot;https://www.fbo.gov/index?s=opportunity&amp;amp;mode=form&amp;amp;id=ad401dd8d445ff2ce5be2736841e8c3e&amp;amp;tab=core&amp;amp;_cview=0&quot; rel=&quot;nofollow noreferrer&quot;&gt;It was determined that the US has only one supplier cryogenics that can supply the cape with the necessary 30 trucks per day necessary to fuel a launch, and that is Praxair&lt;/a&gt;. &lt;a href=&quot;http://www.praxair.com/news/2015/praxair-awarded-nasa-contract&quot; rel=&quot;nofollow noreferrer&quot;&gt;They supply both oxygen and hydrogen to KSC&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/home/hqnews/2009/may/HQ_C09-026_KSC_gas_contracts.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Other NASA sites are also supplied by Praxair, and other contractors&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://plus.google.com/113416759749736985874/about&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vandenberg has an on site cryogenic plant operated by Air Liquide of America&lt;/a&gt;, but it is not clear what volume they supply for launches there - its probably classified.&lt;/p&gt;&#xA;&lt;p&gt;ESA's launch site at &lt;a href=&quot;http://www.airliquide.com/file/otherelementcontent/pj/cr_ag_en_09%2036310.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kourou has an on-site cryogenic plant operated by Air Liquide&lt;/a&gt;. &lt;a href=&quot;http://www.arianespace.com/news-feature-story/2010/07-30-2010-oxygen-nitrogen-factory.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;More information at the Kourou web site&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.russianspaceweb.com/baikonur_facilities.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Baikonur also has an onsite cryogenic plant&lt;/a&gt;, mainly because it is hundreds of miles from anywhere! &lt;a href=&quot;http://www.astronautix.com/sites/plesetsk.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;There is also a plant at Plesetsk&lt;/a&gt;, and &lt;a href=&quot;http://sen.com/news/russia-kicks-off-20-year-space-programme-by-building-new-space-centre&quot; rel=&quot;nofollow noreferrer&quot;&gt;one will be built at Vostochny&lt;/a&gt;. Most of the &lt;a href=&quot;http://cryogenmash.ru/en/catalog/specialized-systems/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Russian Cryogenic plants are now owned and operated by the Cryogenmash company&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;There is an extensive railway system at Baikonur and the fuels are moved from the storage tanks to the launch pad by rail cars. Here are some photos from Baikonur to illustrate that:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/B4EzK.jpg&quot; alt=&quot;Cryogenic rail cars&quot; /&gt; &lt;img src=&quot;https://i.stack.imgur.com/P4bo4.jpg&quot; alt=&quot;cryogenic tank farm&quot; /&gt; &lt;img src=&quot;https://i.stack.imgur.com/aa40a.jpg&quot; alt=&quot;Fuel Pipes&quot; /&gt; &lt;img src=&quot;https://i.stack.imgur.com/f8byw.jpg&quot; alt=&quot;Fuel Delivery&quot; /&gt; &lt;img src=&quot;https://i.stack.imgur.com/k4yBd.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="14637" LastEditDate="2022-10-02T13:01:02.793" LastActivityDate="2022-10-02T13:01:02.793" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="9116" PostTypeId="2" ParentId="9114" CreationDate="2015-05-13T14:14:19.567" Score="7" Body="&lt;p&gt;The reason for swapping the Progress and Soyuz flights is to ensure that the third stage (which is suspect at this time in the Progress failure) is working properly before launching a manned crew on the same basic booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By doing that, it means the manned launch is delayed and if TMA-15M crew returned, the station would be down to a crew of three. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with the ISS is that it is fairly maintenance intensive. The crew of three ends up doing very little science as between sleep, rest, and station maintenance there is very little time for science. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a crew of 6 they get more accomplished. The US believes that once they get the Commercial Crew vehicles flying 4 astronauts at a time, and thus a crew of 7, that they will effectively DOUBLE the science time available.  So with a crew of 6, they get the equivalent of a single persons work on Science each day.  Adding one more person is sufficient to double that amount which tells you how little science really gets done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The purpose of the ISS is to do science, and if you drop the crew down to three, you basically get zero science time, until July and the TMA-17M crew arriving,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of supplies, the station has reserves for many months. (It is complicated, since some consumables are in better supply than others).  I saw a table showing the various consumables and their state somewhere, but cannot find it to link in at the moment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a Dragon flight coming in June (CRS-7 with the IDA! Yay!  International Docking Adapter for Commercial Crew docking) with supplies. (Alas poor CRS-7 and IDA, we knew ye well. (CRS-7 was lost when the upper stage burst due to a strut failing, which dropped a COPV, which burst releasing high pressure helium, midflight)). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an HTV flight (which has a fairly hefty payload capacity compared to Dragon or Progress) due this summer as well.  So the supply situation is not close to critical, and there are two more strings of backup levels on the schedule. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the station managers are trading off the science value of a crew of 6 against possible failures in Dragon/HTV/Progress such that they run out of supplies in the near future. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-05-29T10:45:56.177" LastActivityDate="2018-05-29T10:45:56.177" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="9118" PostTypeId="2" ParentId="9054" CreationDate="2015-05-13T20:30:31.230" Score="12" Body="&lt;p&gt;The Pluto flyby is tricky to plan: after the launch of New Horizons, astronomers discovered the four smaller moons of Pluto, and mission planners began to worry that the system could be filled with even smaller moons and dusty particles. New Horizons will fly through the system at 14 km/s, so a particle the size of a grain of sand could destroy the spacecraft. See &lt;a href=&quot;http://guinan.space.swri.edu/nhepo/archive/2015/05/15&quot; rel=&quot;noreferrer&quot;&gt;http://guinan.space.swri.edu/nhepo/archive/2015/05/15&lt;/a&gt; (thanks @TildalWave).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I understand, the trajectory is planned to pass through space where Charon was a few days before, so that hopefully it has cleared out the dust in that region. From the slide it looks like they are also planning a trajectory where the spacecraft passes behind both Pluto and then Charon as seen from Earth (a set of 'occulations'), probably for radio measurements of the two bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: I saw a talk by James Green, director of NASA's Planetary Science division a few weeks ago &lt;a href=&quot;http://www.pppl.gov/events/colloquium-worlds-seen-first-time-ceres-and-pluto&quot; rel=&quot;noreferrer&quot;&gt;http://www.pppl.gov/events/colloquium-worlds-seen-first-time-ceres-and-pluto&lt;/a&gt; -- a really good presentation!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slides from a similar talk (showing a possible trajectory on page 29) are here: &lt;a href=&quot;http://astronautical.org/sites/default/files/goddard/2015/goddard_2015-03-11-1715_green.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://astronautical.org/sites/default/files/goddard/2015/goddard_2015-03-11-1715_green.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10146" LastEditorUserId="10146" LastEditDate="2015-05-17T18:51:44.283" LastActivityDate="2015-05-17T18:51:44.283" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9119" PostTypeId="1" AcceptedAnswerId="9120" CreationDate="2015-05-13T23:52:17.363" Score="14" ViewCount="4867" Body="&lt;p&gt;From what I understand, a lot of the difficulty in using conventional rockets to move payloads to other planets is that the object needs to lose its velocity one way or another so it doesn't slam into its destination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like this would be a difficult problem for solar sails, since they have no built-in propulsion to slow them down, and since the sun will always accelerate them in the same direction. A solar sail would require a tremendous amount of energy to slow itself down from its cruising speed. How could it be done?&lt;/p&gt;&#xA;" OwnerUserId="5748" LastEditorUserId="5748" LastEditDate="2015-05-14T16:38:51.060" LastActivityDate="2019-07-09T01:04:58.213" Title="How do you slow down a solar sail?" Tags="|energy|solar-sail|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9120" PostTypeId="2" ParentId="9119" CreationDate="2015-05-14T01:31:38.330" Score="16" Body="&lt;p&gt;If you're asking about interstellar travel, then the answer is pretty simple; For diffuse, unfocused sources of light like the one emitted by stars, photon flux density decreases with the inverse square of the distance to its source, and with it photon pressure (imparted momentum of absorbed or, better yet, reflected photons) on the sail. So you accelerate away from the Solar system on photon pressure of the Sun, and decelerate on photon pressure of the destination star as you get closer to it. That might involve rotating the whole sail the other way around at half point, but that's it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more local orbits though, things get a bit more interesting. One thing we need to consider is that wherever in the Solar system we are, we're already in orbit around something. We're always orbiting some &lt;em&gt;gravity well&lt;/em&gt;. And to move from one orbital altitude to another, say, decreasing heliocentric orbital altitude from one of the Earth (average of 1 AU) to one of Venus (about 0.7 AU), or increasing it to one of Mars (1.5 AU) and so on, we have to either increase angular momentum to move into a higher one, or decrease it to move into a lower one. So you don't really require the sail to be accelerating directly towards the Sun to decrease your orbital altitude with respect to it. It's enough to slow your orbital speed and with it decrease your angular momentum, and the orbit will progressively &lt;em&gt;shrink&lt;/em&gt; to a lower altitude one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, so how to do that with a solar sail? Well, simple enough. We need to set the solar sail's angle to the source so that the combined force vector of both incoming photon pressure and the reflected one (assuming 100% reflexivity at 0 angle to the source, total momentum imparted on the sail that results in forward motion is two times the force of photon flux*) points such that orbital speed is slowly reduced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saying it differently, and let's experiment a bit, if you take a mirror that will act as our solar sail, and point it to reflect some source of light at an angle on some wall, the combined force vector will point from between the source and the reflected light spot, and through the mirror away from that middle point. If you now imagine that you're circling that light source in some direction, if you pointed your mirror such that the combined force vector points more in the opposite direction of your &lt;em&gt;orbit&lt;/em&gt; than going with it, you're lowering your orbital speed, and with it angular momentum and altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/0dEfm.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this make sense? No? No problem. See &lt;a href=&quot;https://space.stackexchange.com/a/377/49&quot;&gt;PearsonArtPhoto's answer here&lt;/a&gt;, from where I'm borrowing the image above, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail#Changing_orbits&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Solar sail - changing orbits&lt;/a&gt;. Now, of course in reality it's a fair bit more difficult than that. For one, solar sails don't work as &lt;em&gt;momentum exchange devices&lt;/em&gt; only on photon pressure (or, if you want, radiation pressure) alone, but are also affected by particle pressure of solar wind. And that's already space weather, and as weather comes, it's a fair bit unpredictable and it can turn nasty. Our Sun is a relatively quiet, stable, invariable  star, but destination star might not be. And sails don't come with perfect 100% reflectance in all the electromagnetic spectrum, and some of the incident radiation is absorbed, converted into heat and radiated away (which again imparts momentum on the sail) in multiple directions that might, or might not follow the shape of the sail. And you'll be doing a lot of sail angle changing, which might be achieved with orbit-synchronized rotation on own axes, during recapture at some distant gravity well, say to stabilize (circularize, reduce precession,...) an orbit around Venus when you depart from Earth's orbit, or something like this. But the point is, that sailing on solar pressure isn't really a straight line point to point travel. A solar sail might make many orbits around the Sun (read: years) to lower its orbit into heliocentric orbital altitude of, say, Venus. This is sometimes referred to as &lt;em&gt;climbing up/down an orbit&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such sails could however also be used with beamed, focused, and optionally polarized sources of light, which can in theory (if we built a capable enough source) start resembling more a true point to point travel, with far more straight trajectories between the points. Say, if we had a 50 GW phased array laser source (think happy thoughts, because that would take about 10x10 kilometers array of solar cells if they were orbiting the Earth, which might take a while to materialize), and assuming spot diameter not being any larger than the size of the sail and the sail's reflexivity 100% (i.e. 100% beam efficiency), we could get 100 kg from Earth to Mars in about 3.4 days without decelerating in nearly a perfectly straight line. And in 4.9 days with deceleration at our destination. You can read a bit more about such concepts in &lt;a href=&quot;http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1035&amp;amp;context=stat_fac&quot; rel=&quot;noreferrer&quot;&gt;Relativistic Propulsion Using Directed Energy&lt;/a&gt; (PDF). Ideal trajectories for that (constant acceleration) also have a rather funny name - &lt;a href=&quot;https://en.wikipedia.org/wiki/Brachistochrone_curve&quot; rel=&quot;noreferrer&quot;&gt;branchistochrone curve&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Deceleration with a more focused source like that (Sun can't really be considered a focused point source, it's much too big for that in the inner Solar system and it emits light in all directions), could be done by deploying a secondary sail that detaches from the spacecraft, acts as a beam concentrator and reflects it from the opposite direction back onto spacecraft's remaining sail. This works again because of the double momentum exchange with 100% reflectance, half as the photons hit the sail, and half as they are reflected away from it. And they might look something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/8A7I5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Author and source of the image is Robert L. Forward's &lt;a href=&quot;http://www.lunarsail.com/LightSail/rit-1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Roundtrip Interstellar Travel Using Laser-Pushed Lightsails&lt;/a&gt; (PDF), where you'll find calculations and additional explanation of how it works. Or see this video of &lt;a href=&quot;https://www.youtube.com/watch?v=BrwK1wJ0V58&amp;amp;list=PLbbCsk7MUIGdscYLrtDFM4mpQeRPnrOyO&amp;amp;index=11&quot; rel=&quot;noreferrer&quot;&gt;James Benford's presentation on Sailships&lt;/a&gt; during the 2013 Starship Century Symposium:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/BrwK1wJ0V58?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, where solar sails, lightsails, or alike photon pressure sails wouldn't quite cut it, they could also be specially &lt;em&gt;tailored&lt;/em&gt; (is that the word for manufacturing a sail?) to use other properties of destination's environment, such as having good magnetic reflectance to act as a magnetic mirror and also decelerate with the help of destination's magnetic field. And so on. But that's already cheating, right?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;*It's also possible to polarize source of light and impart angular momentum of photons on the sail, which would result in sail's rotation. It does nothing for its forward motion, but it could be used to control sail's attitude from the light source, to send data to the craft, and/or to convert it into useful form of energy on the sail and use that to, say, generate own magnetic field.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-05-14T13:14:59.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9121" PostTypeId="2" ParentId="6391" CreationDate="2015-05-14T13:41:17.690" Score="1" Body="&lt;p&gt;I agree with all the costs listed here. Also, I would add the extra-operations loss: calculating the fuel remaining in the tanks is quite complex. Considering a good margin to add to the number, the operator is &quot;wasting&quot; lifetime thus revenues. Using dedicated systems for the decommissioning may help operators to &quot;passivate&quot; the tanks extending satellites' life while being able to dispose them at EOL. &#xA;For a GEO, let's say you need 16kg of propellant for the decommissioning. Thus about 3 to 5 months more time to spend in orbit making revenues. Even considering a lower lease price due to the old tech used at EOL, let's say $1M/y per beacon, and considering 24 to 60 beacons, the extra-revenues may be in the order of few million (6-15 million in case of 3 months extra-lifetime, 10-25 million in case of 5 months extra-lifetime). Not much considering the enormous amount of money generated so far, however, five times more than the cost of the independent decommissioning device. Plus the advantages of a short decommissioning - hence saving in operations time - much lower liability risk - difficult to calculate and being sure the sats are effectively removed even in case of failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course these are just guessed numbers. However I'm very interested on going deeper here, to put numbers on every pain related to debris mitigation impact for operators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cheers.&lt;/p&gt;&#xA;" OwnerUserId="10152" LastActivityDate="2015-05-14T13:41:17.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9122" PostTypeId="2" ParentId="6197" CreationDate="2015-05-14T14:01:03.570" Score="7" Body="&lt;p&gt;Oberth works both retrograde and prograde. The equivalent Oberth effect for retrograde is in fact the Oberth effect =) Pretend we are at periapsis and are about to get an infinitesimal but nonzero impulse from thrust. Let's look at the prograde impulse case first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Prograde, we have some orbital velocity in the same direction as thrust. The exhaust is pushed &lt;em&gt;retrograde&lt;/em&gt; and therefore into a lower orbit. It loses energy. Conservation of energy says it has to go somewhere - it goes into propelling the rocket itself, faster/into a higher orbit than it would get simply from the impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Retrograde, we have some orbital velocity in the &lt;em&gt;opposite&lt;/em&gt; direction as thrust. The exhaust is thus pushed into a &lt;em&gt;higher&lt;/em&gt; orbit because it is moving prograde. Conservation of energy says it had to get that energy from somewhere - it gets it from the rocket itself propelling the thrust, so that the rocket gets a slower/lower orbit than it would get simply from the impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because normal, antinormal, radial, and antiradial are all orthogonal to the velocity vector, no Oberth effect is obtained from these components of any impulse. Hope this helps!&lt;/p&gt;&#xA;" OwnerUserId="10153" LastActivityDate="2015-05-14T14:01:03.570" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9124" PostTypeId="1" CreationDate="2015-05-15T21:36:05.690" Score="14" ViewCount="6385" Body="&lt;p&gt;&lt;a href=&quot;http://scienceline.org/2008/03/ask-hamalainen-spacestuff/&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; talks about a 'Personal Preference Kit;, a bag measuring 5&quot; x 8&quot; x 2&quot;, that could weigh no more than 1.5 lbs. It was the sole space  an astronaut could use for personal items. But it turns out that was the case on &lt;a href=&quot;http://www.collectspace.com/resources/flown_nasa_ssmementos02.html&quot; rel=&quot;noreferrer&quot;&gt;Shuttle missions&lt;/a&gt;. On the ISS each astronaut has &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/167746main_FS_LivingandWorkinginSpace508c.pdf&quot; rel=&quot;noreferrer&quot;&gt;a locker&lt;/a&gt;, part of a private compartment where they sleep, that can hold a lot more stuff. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JHaY4.jpg&quot; alt=&quot;private crew compartment on the International Space Station&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the allotments for an ISS mission in terms of personal effects? What do people generally bring? How much of that allotment tends to get turned over to mementos and things crew bring with them on behalf of groups such as schools or clubs? What personal effects are supplied and thus don't have to be brought (for instance toiletries)?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-16T00:37:51.507" LastActivityDate="2018-09-05T11:02:18.353" Title="How does an astronaut pack for the ISS?" Tags="|iss|astronaut-lifestyle|psychology|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9125" PostTypeId="1" CreationDate="2015-05-16T00:29:13.457" Score="4" ViewCount="1255" Body="&lt;p&gt;This &lt;a href=&quot;https://www.quora.com/What-clothes-do-Astronauts-pack-for-space-missions-and-how-many&quot; rel=&quot;nofollow noreferrer&quot;&gt;post&lt;/a&gt; talked about the standardized clothing list each astronaut or cosmonaut has when on the ISS. It is specified carefully in consideration of the fact clothing can't be washed, so it is used and then discarded. This is the list:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;1 pair of shoes for the treadmill&lt;/li&gt;&#xA;  &lt;li&gt;1 pair of shoes for the bike&lt;/li&gt;&#xA;  &lt;li&gt;1 pair of exercise shorts for every 3 days of exercise&lt;/li&gt;&#xA;  &lt;li&gt;1 T-shirt for every 3 days of exercise&lt;/li&gt;&#xA;  &lt;li&gt;1 work shirt for every 10 days&lt;/li&gt;&#xA;  &lt;li&gt;1 T-shirt for under the workshirt for every 10 days&lt;/li&gt;&#xA;  &lt;li&gt;1 pair work pants/shorts for every 10 days&lt;/li&gt;&#xA;  &lt;li&gt;1 pair underwear for every 2 days&lt;/li&gt;&#xA;  &lt;li&gt;1 pair socks for every 2 days&lt;/li&gt;&#xA;  &lt;li&gt;2 sweaters&lt;/li&gt;&#xA;  &lt;li&gt;2 pairs Russian overalls (optional)&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Does the space agency of the crew member issue this clothing? Do they get to pick anything themselves for that wardrobe from their personal stuff? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there anything special about the clothing? Because in videos of the ISS, you see how much storage is used for clothing, both new and discarded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clothing stored in the docking tube joining the Russian and American sections of the ISS (video &lt;a href=&quot;https://youtu.be/afBm0Dpfj_k?t=24m24s&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1nZ0B.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-16T02:24:15.003" LastActivityDate="2015-05-18T21:42:26.893" Title="Where does the clothing for the crew of the International Space Station come from?" Tags="|iss|astronaut-lifestyle|psychology|" AnswerCount="1" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9126" PostTypeId="1" AcceptedAnswerId="12038" CreationDate="2015-05-16T00:47:10.457" Score="6" ViewCount="1308" Body="&lt;p&gt;Each crew member of the ISS gets care packages on cargo flights. In a video tour of the station, it was mentioned that each crew member has a stash of favorite food just for them. It is easy to see how such a comfort could be very useful on a tough day. My impression from the video is that this is actually officially mandated, that each person have a store of such foods.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I guess that is part of what goes in care packages. What other things are sent up? What is the size and mass allotment for such cargo? How much of care packages are sent by family and friends, and how much is assembled by the space agency of that crew member?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the &lt;a href=&quot;https://www.youtube.com/watch?v=afBm0Dpfj_k&amp;amp;t=19m55s&quot;&gt;part of the tour&lt;/a&gt; where personal 'bonus foods' are shown:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/afBm0Dpfj_k?start=1195&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2015-05-16T01:34:00.940" LastActivityDate="2015-09-18T16:07:45.463" Title="What is in the care packages sent to crew members of the International Space Station?" Tags="|iss|astronaut-lifestyle|psychology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9127" PostTypeId="1" AcceptedAnswerId="9129" CreationDate="2015-05-16T06:50:07.970" Score="5" ViewCount="825" Body="&lt;p&gt;It seems that some &quot;abnormal situation&quot; (нештатная ситуация) happened about a minute before Briz-M upper stage ignition, possibly during third stage separation of the International Launch Services' &lt;a href=&quot;http://www.ilslaunch.com/mission-control/mission-centenario&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centenario / Mexsat 1&lt;/a&gt; mission, launched atop Proton M rocket, and the live webcast was cut about 10 minutes earlier than scheduled without expected report on the third stage separation and Briz M upper stage ignition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For easier reference, since I likely got some of what I describe above wrong, here's the relevant part of Centenario mission's profile with timeline, ground track, and trajectory:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/GdVir.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GdVir.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far, not much else seems to be known about it, except what news Russian agencies hurried with minutes after webcast stopped, for example &lt;a href=&quot;https://translate.google.com/translate?sl=auto&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=en&amp;amp;ie=UTF-8&amp;amp;u=http%3A%2F%2Fria.ru%2Fspace%2F20150516%2F1064825008.html&amp;amp;edit-text=&quot; rel=&quot;nofollow noreferrer&quot;&gt;RIA Novosti&lt;/a&gt; or &lt;a href=&quot;https://translate.google.com/translate?hl=en&amp;amp;sl=auto&amp;amp;tl=en&amp;amp;u=http%3A%2F%2Ftass.ru%2Fkosmos%2F1972945&amp;amp;sandbox=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;TASS&lt;/a&gt;. What I'm interested here is whether this launch anomaly has been confirmed, and if that's the case, what are its consequences. In particular, regarding potential orbital debris that such late deployment anomaly could cause.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-05-16T07:18:44.137" LastActivityDate="2015-05-16T13:46:51.177" Title="What happened during ILS Proton M / Mexsat 1 launch?" Tags="|launch|debris|failure|proton-m|ils|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9128" PostTypeId="1" AcceptedAnswerId="9142" CreationDate="2015-05-16T09:37:24.003" Score="5" ViewCount="7356" Body="&lt;p&gt;Imagine you're, well, actually on the ISS so you're 395 km up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BVf5e.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've all seen many photos roughly like this... however this can be deceptive;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;They might be all fisheye lenses.&lt;/li&gt;&#xA;&lt;li&gt;A great many of these images are simulations or comps.&lt;/li&gt;&#xA;&lt;li&gt;It's hard to get the real view if you don't know what lenses are used.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In short, at that height, what angle does the Earth fill horizon to horizon?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;When you're standing on a (very flat) part of the Earth it's indeed roughly 180 degrees.&lt;/li&gt;&#xA;&lt;li&gt;When you're on the moon it's a degree or so.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What angle, actually does the Earth take when you're at 400km?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does it in fact &lt;strong&gt;feel like&lt;/strong&gt; the Earth is &lt;strong&gt;pretty flat&lt;/strong&gt; beneath you?&lt;/li&gt;&#xA;&lt;li&gt;Or does it really look more like a ball?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Perhaps someone has an astronaut's comments on this or?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again - Hollywood representations tend to fill our minds, but those could be basically quite wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would be very useful to know in making a VR where you are at that position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hiW73.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've found it difficult to get any real info.&lt;/p&gt;&#xA;" OwnerUserId="3289" LastEditorUserId="7017" LastEditDate="2015-05-20T18:20:11.760" LastActivityDate="2021-03-04T16:14:16.723" Title="Earth angular size looking from the ISS" Tags="|iss|observation|earth|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9129" PostTypeId="2" ParentId="9127" CreationDate="2015-05-16T10:25:13.590" Score="3" Body="&lt;p&gt;We now have (as of 10:25 UTC 16/05/15) a &lt;a href=&quot;http://www.ilslaunch.com/node/3480&quot; rel=&quot;nofollow&quot;&gt;statement from ILS on the anomaly.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The relevant quote: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Khrunichev and International Launch Services (ILS) regret to announce an anomaly during today's Proton mission with the Centenario satellite. The satellite was built for Mexican government’s Ministry of Communications and Transportation, the Secretaria de Comunicaciones y Transportes (SCT) by Boeing Satellite Systems International.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Proton Breeze M rocket lifted off at 11:47 local time from the Baikonur Cosmodrome, carrying the Centenario satellite.  Preliminary flight information indicates that the anomaly occurred during the operation of the third stage, approximately 490 seconds after liftoff.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A Russian State Commission has begun the process of determining the reasons for the anomaly.  ILS will release details when data becomes available. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://translate.google.co.uk/translate?sl=auto&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=en&amp;amp;ie=UTF-8&amp;amp;u=http%3A%2F%2Ftass.ru%2Fkosmos%2F1973065&amp;amp;edit-text=&amp;amp;act=url&quot; rel=&quot;nofollow&quot;&gt;TASS is also reporting&lt;/a&gt; a helicopter being dispatched to investigate reports of falling debris.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-05-16T13:46:51.177" LastActivityDate="2015-05-16T13:46:51.177" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9130" PostTypeId="1" AcceptedAnswerId="9132" CreationDate="2015-05-16T11:55:14.140" Score="9" ViewCount="2582" Body="&lt;p&gt;Is it scientifically possible to briefly walk on the moon bare footed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What possible side effects could there be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these side effects mild enough to make it worth it?&lt;/p&gt;&#xA;" OwnerUserId="10160" LastEditorUserId="4660" LastEditDate="2015-05-16T18:24:08.047" LastActivityDate="2015-05-18T04:40:53.783" Title="Is it scientifically possible to briefly walk on the moon bare footed?" Tags="|safety|moonwalking|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9132" PostTypeId="2" ParentId="9130" CreationDate="2015-05-16T14:10:35.453" Score="13" Body="&lt;p&gt;There are 3 main threats you'd have to account for: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Vacuum. This is discussed in the questions @Forgemonkey linked to. Conclusion: brief exposure of the feet only is survivable. &lt;/li&gt;&#xA;&lt;li&gt;Temperature. Surface temperatures on the Moon swing between + 120 and - 150 °C, so you'd have to pick your spot carefully to have a survivable temperature. &lt;/li&gt;&#xA;&lt;li&gt;Cuts and abrasion. Lunar &lt;a href=&quot;http://www.airspacemag.com/space/stronger-than-dirt-8944228/?no-ist&quot;&gt;regolith&lt;/a&gt; is &lt;a href=&quot;http://www.universetoday.com/96208/the-moon-is-toxic/&quot;&gt;very sharp&lt;/a&gt;. It'd be like walking across glass shards. On Earth, dust and sand are subject to erosion from wind and water, which tends to make everything smooth. On the Moon these forces are absent, so e.g. the debris from a meteorite impact all keeps its sharp edges indefinitely. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;All in all, not a pleasant experience, I'd think.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-05-16T16:09:37.217" LastActivityDate="2015-05-16T16:09:37.217" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="9133" PostTypeId="1" AcceptedAnswerId="9411" CreationDate="2015-05-16T18:25:00.073" Score="5" ViewCount="2101" Body="&lt;p&gt;For simplicity: Standard, 50,000km space elevator from equator to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_elevator#Counterweight&quot; rel=&quot;noreferrer&quot;&gt;counterweight&lt;/a&gt;.  Acceleration, followed by constant velocity, followed by deceleration, and a stop past geostationary orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What physical sensations do I experience&lt;/strong&gt; (e.g. - not specific numbers, but general sensations: &quot;.. then, you're pressed against the ceiling,&quot; and &quot;.. now 'up' is the Earth.&quot;)?  It has always been vague to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I'm not interested in emotional sensations.  Well, I am, but I can figure that out.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2015-08-24T17:43:19.967" Title="What sensations would I feel on a Space Elevator?" Tags="|gravity|microgravity|space-elevator|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9134" PostTypeId="1" CreationDate="2015-05-16T18:43:39.730" Score="9" ViewCount="311" Body="&lt;p&gt;I've been playing with lunar retrograde orbits in an orbital simulator. The retrograde lunar orbits with apolunes below 40,000 km altitude are pretty stable. They seem resistant to distortion from earth's gravity or the sun's gravity. So I've been thinking of the ceiling for stable LDROs as around 40,000 kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the floor for stable lunar orbits? I am told that if the orbits are too low, they'll be wrecked by mascons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By stable I mean an orbit that will last a few centuries without station keeping.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-11-14T15:31:17.380" Title="What's the floor for stable retrograde lunar orbits?" Tags="|the-moon|retrograde-orbit|mascon|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9135" PostTypeId="5" CreationDate="2015-05-16T18:57:51.550" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-16T18:57:51.550" LastActivityDate="2015-05-16T18:57:51.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9136" PostTypeId="4" CreationDate="2015-05-16T18:57:51.550" Score="0" Body="A mass concentration in a celestial body with higher gravitational pull than its surroundings, causing an uneven or distorted gravitational field. " OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-05-16T18:57:51.550" LastActivityDate="2015-05-16T18:57:51.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9137" PostTypeId="1" CreationDate="2015-05-16T18:58:49.407" Score="5" ViewCount="155" Body="&lt;p&gt;What is the threshold of detection for small orbital debris? How small can something get before it can no longer be reliably tracked (and therefore avoided)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that collision energy is dependent on the delta-V of the parties to a collision, and that some extreme cases may be unlikely or absurd e.g. geosynchronous vs &quot;retrograde geosynchronous(?)&quot;, how much collision energy could an untrackable debris item deliver to an unsuspecting space vehicle? In other words, how bad could the &quot;hidden hazard&quot; actually get?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-05-16T19:30:16.680" Title="What is the linear dimension/mass/possible energy state of the smallest trackable orbital debris item?" Tags="|debris|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9138" PostTypeId="2" ParentId="9137" CreationDate="2015-05-16T19:30:16.680" Score="1" Body="&lt;p&gt;The main method for detecting small objects is radar. An object's visibility to radar is expressed as its &lt;a href=&quot;https://en.wikipedia.org/wiki/Radar_cross-section&quot; rel=&quot;nofollow&quot;&gt;radar cross-section&lt;/a&gt; (RCS). This is a measure of how much radar energy is reflected by the object to the radar antenna.&lt;br&gt;&#xA;There is no direct relationship between size and RCS. A few examples: a highly reflective object (e.g. a &lt;a href=&quot;https://en.wikipedia.org/wiki/Corner_reflector&quot; rel=&quot;nofollow&quot;&gt;corner reflector&lt;/a&gt;) has a large RCS even if it's small, a stealth aircraft (like the B-2) has a small RCS despite its large size.&lt;br&gt;&#xA;There is one limit: a radar can't detect objects much smaller than its wavelength. The &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_Space_Surveillance_Network#Shemya_and_Diyarbakir_Radar_Sites&quot; rel=&quot;nofollow&quot;&gt;AN/FPS-80 Cobra Dane radar&lt;/a&gt; works in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Radar&quot; rel=&quot;nofollow&quot;&gt;L band&lt;/a&gt; with a wavelength of 15-30 cm. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-05-16T19:30:16.680" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9139" PostTypeId="1" AcceptedAnswerId="9140" CreationDate="2015-05-16T21:29:51.237" Score="10" ViewCount="21705" Body="&lt;p&gt;I found &lt;a href=&quot;http://en.wikipedia.org/wiki/Atmospheric_entry&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; that the entry speed of meteors reach 48 km/s. I mean just before hitting atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are reentry speeds for space vehicles like the lunar &lt;a href=&quot;http://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Command_Module_.28CM.29&quot; rel=&quot;noreferrer&quot;&gt;command module&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about soyuz spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="10167" LastEditorUserId="8157" LastEditDate="2015-05-18T07:47:17.127" LastActivityDate="2018-08-26T00:00:54.043" Title="What are reentry speeds of space vehicles?" Tags="|reentry|atmosphere|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9140" PostTypeId="2" ParentId="9139" CreationDate="2015-05-16T21:33:13.197" Score="12" Body="&lt;p&gt;That same &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article on Atmospheric entry&lt;/a&gt; that you link to in your question answers this later on:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... for entry from low Earth orbit where entry velocity is&#xA;  approximately 7.8 km/s. For lunar return entry of 11 km/s ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And also:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Stardust&lt;/a&gt; sample-return capsule was the fastest man-made object&#xA;  ever to reenter Earth's atmosphere (12.4 km/s or 28,000 mph at 135 km&#xA;  altitude).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More specifically, for Soyuz reentry see &lt;a href=&quot;https://space.stackexchange.com/q/6521/49&quot;&gt;What is the maximum velocity at which Soyuz TMA-M may transit through Earth&amp;#39; atmosphere at reentry without a heat-shield?&lt;/a&gt; And for Apollo missions see &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_18-40_Entry_Splashdown_and_Recovery.htm&quot; rel=&quot;noreferrer&quot;&gt;Apollo by the numbers - Entry, Splashdown, and Recovery&lt;/a&gt; (table of contents &lt;a href=&quot;http://www.history.nasa.gov/SP-4029/Apollo_00g_Table_of_Contents.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;). Apollo 10 had the fastest maximum entry velocity at 36,397 ft/s (11.094 km/s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Future Earth atmospheric reentry speeds might substantially increase for return missions to Mars, with entry speeds in the 15-21 km/s range, depending on trajectory and time of launch (source: &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660015097.pdf&quot; rel=&quot;noreferrer&quot;&gt;A simple atmosphere reentry guidance scheme for return from the manned Mars mission&lt;/a&gt;, Henry C. Lessing and Robert E. Coate, 1966, NASA Ames Research Center).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-05-16T21:59:35.067" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9142" PostTypeId="2" ParentId="9128" CreationDate="2015-05-17T01:38:35.293" Score="23" Body="&lt;p&gt;Horizon at ground level appears indeed as a plane taking up 180° of our field of view when looking at nadir. This &lt;a href=&quot;https://www.waypointamsterdam.com/Handy_stuf/Short_Guide_To_Astro_navigation.pdf#page=5&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;geoidal&lt;/em&gt; horizon&lt;/a&gt; (tangential to the geoid) hides half of the sky. &lt;a href=&quot;https://www.waypointamsterdam.com/Handy_stuf/Short_Guide_To_Astro_navigation.pdf#page=5&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Astronomical&lt;/em&gt; or &lt;em&gt;sensible&lt;/em&gt;&lt;/a&gt; horizon is nearly identical, this is the one from eyes level, and the reference to read the height of a body in the sky with a sextant.&lt;/p&gt;&#xA;&lt;p&gt;But as soon as the observer gains altitude the separation between Earth and sky moves below geoidal horizon to form a circle, known as the &lt;a href=&quot;https://physics.stackexchange.com/questions/39347/what-does-the-phrase-limb-of-the-earth-or-atmospheric-limb-mean&quot;&gt;limb&lt;/a&gt;, which determines the &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/7/7c/Horizons.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;true&lt;/em&gt;, &lt;em&gt;geometrical&lt;/em&gt; or &lt;em&gt;geographic&lt;/em&gt; horizon&lt;/a&gt;. The angular size of the limb is indeed less than 180°, and smaller as altitude increases.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/asf4m.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/asf4m.png&quot; alt=&quot;Geoidal, sensible and geometrical horizons&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sup&gt;Geoidal, sensible and geometrical horizons&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;The geometrical horizon is reduced by the skyline (buildings, trees, mountains...) and increased by &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_refraction&quot; rel=&quot;nofollow noreferrer&quot;&gt;atmospheric refraction&lt;/a&gt;. What we actually see is the &lt;em&gt;visible horizon&lt;/em&gt;, but for our calculation we will just use the geometrical horizon.&lt;/p&gt;&#xA;&lt;br&gt;&#xA;&lt;p&gt;&lt;strong&gt;Angular size of Earth limb for an observer in altitude.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The problem can be represented like in the image below, with distance OP being the sum of Earth radius (&lt;span class=&quot;math-container&quot;&gt;$OA$&lt;/span&gt;) and altitude of the observer.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/e8lel.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/e8lel.png&quot; alt=&quot;Limb apparent angle&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sup&gt;Limb apparent angle&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;The limb is the intersection of the sphere and the cone with vertex &lt;code&gt;P&lt;/code&gt; and tangential to the sphere at &lt;code&gt;A&lt;/code&gt; and &lt;code&gt;B&lt;/code&gt; on the figure (tangential to the limb actually).&lt;/p&gt;&#xA;&lt;p&gt;Let's assume the observer is at an altitude of 400 km:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$OA = 6371$&lt;/span&gt; km.&lt;/li&gt;&#xA;&lt;li&gt;&lt;span class=&quot;math-container&quot;&gt;$OP = 6371 + 400 = 6771$&lt;/span&gt; km.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;angular size of the limb: &lt;span class=&quot;math-container&quot;&gt;$\widehat {APB} = 2 \widehat {APO}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;We know: &lt;span class=&quot;math-container&quot;&gt;$\sin \widehat {APO} = \frac {OA} {OP} = \frac {6371} {6771}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;and we can calculate &lt;span class=&quot;math-container&quot;&gt;$\widehat {APO} \approx 70°$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;So &lt;span class=&quot;math-container&quot;&gt;$\widehat {APB} \approx 140°$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;It means for an observer aboard the ISS, Earth limb appears as a disk of apparent diameter of 140°. Of course, this apparent size will vary when ISS altitude varies. ISS orbit is not circular, Earth area swept by ISS is not a perfect sphere portion, and most significantly ISS is subject to &lt;a href=&quot;https://space.stackexchange.com/questions/1753/does-the-orbit-of-the-iss-decay&quot;&gt;orbital decay&lt;/a&gt; between &lt;a href=&quot;http://www.nasaspaceflight.com/2015/05/iss-enjoys-reboost-progress-m-26m-misbehaves/&quot; rel=&quot;nofollow noreferrer&quot;&gt;reboosts&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;At altitude 400 km the angular size of the &lt;em&gt;hidden&lt;/em&gt; equator is only 86°, two thirds of the limb.&lt;/p&gt;&#xA;&lt;p&gt;For comparison, The eyes binocular vision covers about 120° horizontally. While the horizontal field is much larger (&lt;a href=&quot;https://en.wikipedia.org/wiki/Field_of_view&quot; rel=&quot;nofollow noreferrer&quot;&gt;more than 200/220°&lt;/a&gt; without moving the eyes) and very sensitive to movements, details and colors are only perceived within 60 to 120°. In a theater, the screen width is about 90° for audience at the center of the first row (&lt;a href=&quot;https://www.unic-cinemas.org/fileadmin/CinemaAuditoriumDesign_v1.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;recommended maximum angle&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;br&gt;&#xA;&lt;p&gt;&lt;strong&gt;What does it look like from the ISS?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;ISS residents see this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/E9wDI.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;br /&gt;&#xA;&lt;sup&gt;&lt;a href=&quot;http://i.dailymail.co.uk/i/pix/2014/05/07/article-2622214-1DA2DCE000000578-349_634x327.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;or this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yoNZA.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&#xA;&lt;sup&gt;&lt;a href=&quot;https://c4.staticflickr.com/8/7315/12372503404_036eeff6dd_k.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;br&gt;&#xA;&lt;p&gt;&lt;strong&gt;Camera requirements to shoot the full Earth limb&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The lens required to get a full monolithic picture of the Earth must have a vertical &lt;a href=&quot;http://en.wikipedia.org/wiki/Angle_of_view&quot; rel=&quot;nofollow noreferrer&quot;&gt;angle of view&lt;/a&gt; of 140°. For a regular lens, angle of view, sensor size and focal length are linked by: &lt;span class=&quot;math-container&quot;&gt;$\widehat {AoV} = 2 \arctan \frac {size} {2 f}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;For a 35 mm sensor (&amp;quot;full frame&amp;quot;) the focal length to take in 140° is 4.2 mm. For a smaller sensor, the actual focal length would be even shorter. These values are approximate as the lens creates a circular image while the recorded image is generally rectangular, so the usable field of view is smaller).&lt;/p&gt;&#xA;&lt;p&gt;Such lens with a short focal length are difficult to locate close to the sensor (the image is focused at the &lt;a href=&quot;https://physics.stackexchange.com/questions/51522/put-a-sensor-at-the-focal-length-behind-or-in-front&quot;&gt;focal plane&lt;/a&gt; where the sensor surface must be located). After 110° we usually prefer a fisheye system which uses another type of projection. While a regular lens perform a rectilinear projection with a &lt;a href=&quot;http://en.wikipedia.org/wiki/Vanishing_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;single vanishing point&lt;/a&gt; on the sensor, the &lt;a href=&quot;http://en.wikipedia.org/wiki/Curvilinear_perspective&quot; rel=&quot;nofollow noreferrer&quot;&gt;curvilinear projection&lt;/a&gt; of a fisheye has five vanishing points.&lt;/p&gt;&#xA;&lt;p&gt;The fisheye lens has a longer focal for a given angle of view, and the latter can be very large (more than 180°, which is not possible for a linear lens). There are different &lt;a href=&quot;http://michel.thoby.free.fr/Fisheye_history_short/Projections/Fisheye%20projections-bis.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;types of fisheyes&lt;/a&gt;, but the most commercialized is one using an equisolid angle projection, where equal solid angles of the 3D space are projected as equal areas on the 2D sensor. This time the relation between AoV, size and focal length is: &lt;span class=&quot;math-container&quot;&gt;$\widehat {AoV} = 4 \arcsin \frac {size} {4 f}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;We can take in 140° with a lens up to 10.5 mm. This image is shot with a 8 mm, allowing to capture the limb, extra sky space and cupola panels...&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1VOhD.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;br /&gt;&#xA;&lt;sup&gt;&lt;a href=&quot;http://www.nasa.gov/sites/default/files/images/755812main_iss_view_full_full.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt; -- Focal length: 8 mm, on a full frame sensor&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;The appropriate lens is not the only condition required to see the whole Earth disk, we also need a window providing a good aperture...&lt;/p&gt;&#xA;&lt;br&gt;&#xA;&lt;p&gt;&lt;strong&gt;Is it possible to see the entire Earth disk through an ISS window?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The largest window ever used in space is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Nadir&quot; rel=&quot;nofollow noreferrer&quot;&gt;nadir&lt;/a&gt; one of the ISS cupola module. it is a circular glass panel with a diameter of 80 cm.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BgR4N.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;It allows a panoramic view from the Earth-facing side (nadir) of the ISS.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/id90s.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;br /&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Tih5O.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;br /&gt;&#xA;&lt;sup&gt;&lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/c/c2/Cupola_ISS_open_shutters_middle_crop.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source 1&lt;/a&gt; -- &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/thumb/1/1f/Exterior_of_Cupola_-_Exp28.jpg/220px-Exterior_of_Cupola_-_Exp28.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;source 2&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;Note how different the visible Earth portion is between the two pictures taken from the cupola. It's because on the first one the camera was closer to the center window. Curiously on the second image, while the camera has moved back, the visible cupola portion is smaller. It's because the angle of view of the lens is smaller in the second image (rectilinear lens) than in the first one (fisheye).&lt;/p&gt;&#xA;&lt;p&gt;The possibility to take in the full Earth limb is discussed here: &lt;a href=&quot;https://physics.stackexchange.com/questions/25509/how-far-into-space-does-one-have-to-travel-to-see-the-entire-sphere-of-earth&quot;&gt;How far into space does one have to travel to see the entire sphere of earth?&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The discussion is about another spacecraft, and another window size, it says that you need to be at about 525 km altitude to see the whole limb from a 70 cm window.&lt;/p&gt;&#xA;&lt;p&gt;Let's look at the ISS case at 400 km, with a 80 cm window, and Earth apparent size of 140°. How close from the glass the eye must be placed, so that the whole limb can be seen through the window?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ogbAQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ogbAQ.png&quot; alt=&quot;Earth limb visibility through cupola nadir window&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sup&gt;Earth limb visibility through cupola nadir window. Cupola drawing by &lt;a href=&quot;https://pixels.com/featured/cupola-iss-module-artwork-david-ducros.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;David Ducros&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;The values of &lt;span class=&quot;math-container&quot;&gt;$d$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$D$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\theta$&lt;/span&gt; (140°) are linked by the tangent formula:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\tan (\theta / 2) = \frac {D / 2} {d}$&lt;/span&gt;. then:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$d = \frac {D / 2} {\tan (\theta / 2)} = 14.5$&lt;/span&gt; cm&lt;/p&gt;&#xA;&lt;p&gt;The eye must be at a maximum distance of &lt;strong&gt;14.5 cm&lt;/strong&gt; of the &lt;strong&gt;external&lt;/strong&gt; panel.&lt;/p&gt;&#xA;&lt;p&gt;It happens the window is a bit thinner than 14.5 cm, So taking a picture of the full limb is possible. The horizon is visible, though difficult to see, as confirmed by astronaut &lt;a href=&quot;https://earthobservatory.nasa.gov/features/EdLu&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ed Lu&lt;/a&gt;: &amp;quot;&lt;em&gt;When I look out a window that faces straight down, it is actually pretty hard to see the horizon—you need to get your face very close to the window.&lt;/em&gt;&amp;quot;.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2021-03-04T16:14:16.723" LastActivityDate="2021-03-04T16:14:16.723" CommentCount="15" ContentLicense="CC BY-SA 4.0" />
  <row Id="9146" PostTypeId="2" ParentId="9130" CreationDate="2015-05-18T03:34:05.797" Score="3" Body="&lt;p&gt;For the record here's a direct quote, from the &lt;a href=&quot;http://www.universetoday.com/96208/the-moon-is-toxic/&quot; rel=&quot;nofollow&quot;&gt;same article&lt;/a&gt; Hobbes discovered:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“The dust was so abrasive that it actually wore through three layers&#xA;  of Kevlar-like material on Jack [Schmitt’s] boot.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;– Professor Larry Taylor, Director of the Planetary Geosciences Institute, University of Tennessee (2008)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I observe that the &quot;ground&quot; on Earth is soil, which is an organic substance and (even in the driest places) &lt;strong&gt;very wet and springy&lt;/strong&gt;. What we think of as &quot;very dry, hard&quot; earth (in the &quot;outback&quot;) is nothing compared to, say, imagine a try with an inch of tiny and large metal filings. I suggest the issue of what the lunar &quot;soil&quot; feels like is somewhat unknown.  We do know it wore-through three layers of kevlar, even in the low gravity, in about 20 hours.&lt;/p&gt;&#xA;" OwnerUserId="3289" LastEditorUserId="3289" LastEditDate="2015-05-18T04:40:53.783" LastActivityDate="2015-05-18T04:40:53.783" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9147" PostTypeId="1" AcceptedAnswerId="9150" CreationDate="2015-05-18T11:40:35.137" Score="4" ViewCount="372" Body="&lt;p&gt;From &lt;a href=&quot;http://spaceflightnow.com/2015/04/14/ula-chief-explains-reusability-and-innovation-of-new-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, I gather that ULA is trying to build reusability into their next generation of launch vehicles. Their plan is summed up nicely in the picture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zSxwB.jpg&quot; alt=&quot;ULA Reusability&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the article, they seem to do some really weird math:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“This will take up to 90 percent of the propulsion cost out of the&#xA;  booster. And this is just the beginning.”&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The engines represent a quarter of the first stage’s overall weight&#xA;  and 65 percent of the booster cost, according to ULA.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If the engines only represent 65 percent of the cost, how can they possibly save 90 percent of the cost when they only recover that part? Some other issues I have in this statement:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What about the cost of the recovery, such as the helicopter, boat, logistics personnel etc. ?&lt;/li&gt;&#xA;&lt;li&gt;What about the additional weight added to the system for the heat-shield, parachute, coupling between Engine and ATS etc. ?&lt;/li&gt;&#xA;&lt;li&gt;How many times can an engine be reused? If it can only be reused 10 times, the cost of the engines is divided by 10, not reduced to zero.&lt;/li&gt;&#xA;&lt;li&gt;How much would it cost to refurbish? Considering that the recovery system (Heat-shield and parachutes) would also need refurbishment&lt;/li&gt;&#xA;&lt;li&gt;There is also a very real risk of failure to recover based on this system which don't seem to be taken into consideration.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;My question is: What would a realistic estimate be of the cost savings involved?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-05-18T19:19:46.453" Title="What are the actual economics for the ULA reusability plan?" Tags="|reuse|economics|booster|ula|vulcan|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9148" PostTypeId="1" AcceptedAnswerId="9149" CreationDate="2015-05-18T11:46:48.977" Score="4" ViewCount="456" Body="&lt;p&gt;I wanted to know if China has a different approach or methodology for space exploration. China seems really fast in putting their first space station module in orbit compared to the US and Russia. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did China gather all data and technology from scratch? Has China used NASA's and USSR's data and technology for initial start-up?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does China share this information and technology with Public ?&lt;/p&gt;&#xA;" OwnerUserId="10096" LastEditorUserId="10096" LastEditDate="2015-05-19T05:41:11.783" LastActivityDate="2015-05-19T05:41:11.783" Title="Is China using same technology and approach as other countries in space exploration?" Tags="|iss|nasa|russia|china|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9149" PostTypeId="2" ParentId="9148" CreationDate="2015-05-18T15:37:21.210" Score="7" Body="&lt;p&gt;China did not develop a space program from scratch. Before the 1960 Sino-Soviet split, there was ample cooperation between the Soviet Union and China in rocket development. This can still be seen in the design of their rockets, for example the use of swirl injectors and UDMH as a fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of know-how also came from the western rocket development by von Brauns group (first Penemünde, then Canaveral), by way of one of the project's top scientists, Qian Xuesen. After a long career as a scientist and technician in the US, which included the questioning of von Brauns people after the war and co-founding JPL, he was denounced during the McCarthy era, for his Chinese birthplace, and went back there. He went on to head the Chinese launcher development.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-05-18T15:37:21.210" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9150" PostTypeId="2" ParentId="9147" CreationDate="2015-05-18T19:19:46.453" Score="2" Body="&lt;p&gt;I might give some answer to your five bullet points, although I don't have a number for the estimate cost (I doubt anyone has):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) &lt;em&gt;Cost of recovery:&lt;/em&gt; It should if anything be lower by capturing the engines in midair than having to soft land them on their own. Replacing the autonomous landing system and its fuel of the F9R with an ordinary helicopter sounds potentially very economical! (65% and 90% must be a misunderstanding somewhere).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) &lt;em&gt;Additional weight:&lt;/em&gt; It might turn out that SPX too will have to reuse only the engines of the F9R, but going through alot more trouble to do so than the new ULA idea, we'll see. The chopper is supposed to replace alot of the complexity and mass of the SPX concept for F9R and Dragon 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) &lt;em&gt;Reusability frequency:&lt;/em&gt; Soon no one will want to launch the same gadget more than 10 times because the vehicle will be ancient by then. And anyway 1 reuse gives 50% maximum potential production cost reduction (+50%!), 2 reuses gives 66% (max pot) cost reduction (+16%), 3 reuses gives 75% reduction (+9%), 4 reuses gives 80% reduction (+5%) and then no one cares anymore, almost all potential has already been won anyway. Going from a ninth to a tenth reuse only lowers costs per launch by 1%. Not counting refurbishment costs which likely increase by every reuse per rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) &lt;em&gt;Refurbishment costs:&lt;/em&gt; Less than for anyone else, thanks to the unpowered descent and soft parachute midair capture. ULA Vulcan engines will be as good as any used rocket engine could be. Heat shields are not needed when reusing the first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5) &quot;Failure&quot;? No, that is not an option anymore! Computer power, sensors, costs, materials, mass, batteries, ease of use and more have already gotten to where some PhD students prove that &lt;a href=&quot;https://www.youtube.com/watch?v=pp89tTDxXuI&quot; rel=&quot;nofollow&quot;&gt;total &lt;strong&gt;precision air capture is trivial&lt;/strong&gt; today&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ULA is today not in the position where they could hope to confront SPX frontally by trying to copy their concept. But by diversifying into some other mode of reusability, they might succeed if the concept of SPX fails. You might have heard of &lt;strong&gt;&quot;asymmetric strategy&quot;&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-05-18T19:19:46.453" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9151" PostTypeId="2" ParentId="9125" CreationDate="2015-05-18T21:42:26.893" Score="3" Body="&lt;p&gt;Two producers I am aware of:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NPP_Zvezda&quot; rel=&quot;nofollow&quot;&gt;NPP Zvezda&lt;/a&gt;  produces &lt;a href=&quot;http://www.zvezda-npp.ru/ru/node/115&quot; rel=&quot;nofollow&quot;&gt;ПК-14&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.federalspace.ru/20320/&quot; rel=&quot;nofollow&quot;&gt;Kentavr-Nauka&lt;/a&gt; produces &lt;a href=&quot;http://english.pravda.ru/news/science/04-11-2003/53604-0/&quot; rel=&quot;nofollow&quot;&gt;some other things&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Be prepared to use google translate.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastActivityDate="2015-05-18T21:42:26.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9152" PostTypeId="1" CreationDate="2015-05-18T23:13:17.373" Score="7" ViewCount="113" Body="&lt;p&gt;On launch date, when first reports of Progress M-27M mishap were received, it was reported (and seen in the released onboard video) that the ship was rotating around its barycenter at an approximate speed of 12 RPM. Later, in its final orbits before reentry, earth-based sightings reported variations in apparent magnitude with an approximate period of 1.8 seconds, which indicates a revolving speed of some 33 RPM. What could be the reason for this increment in rotational speed?&lt;/p&gt;&#xA;" OwnerUserId="10180" LastEditorUserId="25" LastEditDate="2015-07-24T14:43:11.113" LastActivityDate="2015-07-24T14:43:11.113" Title="Rotation speed of Progress M-27M" Tags="|orbital-mechanics|artificial-satellite|low-earth-orbit|attitude|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9153" PostTypeId="1" CreationDate="2015-05-19T00:01:32.113" Score="1" ViewCount="130" Body="&lt;p&gt;In Twitter the &lt;a href=&quot;https://twitter.com/NewHorizonsBot&quot; rel=&quot;nofollow noreferrer&quot;&gt;@NewHorizonsBot&lt;/a&gt; always announces whenever &#xA;#NewHorizons releases a new image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;#NewHorizons released a new image!&lt;br&gt;&#xA;  ⌚ 14 May 2015, 02:02:44 UTC.&lt;br&gt;&#xA;   73.1M km from #Pluto.&lt;br&gt;&#xA;   &lt;a href=&quot;http://pluto.jhuapl.edu/soc/Pluto-Encounter/view_obs.php?image=data/pluto/level2/lor/jpeg/029387/lor_0293874887_0x633_sci_2.jpg&amp;amp;utc_time=2015-05-14&amp;lt;br&amp;gt;02:02:44&amp;nbsp;UTC&amp;amp;description=OpNav+Campaign+3%2C+LORRI+4X4&amp;amp;target=PLUTO&amp;amp;range=73.1M&amp;nbsp;km&amp;amp;exposure=9967&amp;nbsp;msec&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://pluto.jhuapl.edu/soc/Pluto-Encou...&lt;/a&gt;&lt;br&gt;&#xA;  &lt;img src=&quot;https://i.stack.imgur.com/CvSDC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is there an easy way to watch these &lt;a href=&quot;http://pluto.jhuapl.edu/soc/Pluto-Encounter/lorri_about.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;LORRI images&lt;/a&gt; without those strong artifacts?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a place where those new images are being processed so they look like &lt;a href=&quot;http://www.space.com/29391-pluto-moons-family-photo-new-horizons.html#ooid=ltc3FzdDqOK0SRIhraerHos5ZuGKbDBa&quot; rel=&quot;nofollow noreferrer&quot;&gt;the video at the bottom here at space.com&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="10100" LastEditorUserId="49" LastEditDate="2015-05-19T01:35:11.677" LastActivityDate="2015-05-19T01:58:26.627" Title="Refurbish NewHorizonsBots images" Tags="|new-horizons|pluto|imaging|raw-data|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9154" PostTypeId="2" ParentId="9153" CreationDate="2015-05-19T01:34:35.603" Score="4" Body="&lt;p&gt;For complete catalogs of validated and calibrated images, not yet. Eventually, they will find their way into the &lt;a href=&quot;https://pds.jpl.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;Planetary Data System&lt;/a&gt; (PDS) and you'll be able to &lt;a href=&quot;http://pds-imaging.jpl.nasa.gov/search/&quot; rel=&quot;nofollow&quot;&gt;browse and search for images&lt;/a&gt;, too. From &lt;a href=&quot;http://pluto.jhuapl.edu/soc/Pluto-Encounter/index.php&quot; rel=&quot;nofollow&quot;&gt;LORRI Raw Image Browser&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Welcome to the New Horizons image site, where NASA and the New&#xA;  Horizons mission are happy to provide these JPEG images - displayed in&#xA;  raw form without special processing - for the public to use and enjoy.&#xA;  These JPEGs of images taken by the LOng Range Reconnaissance Imager&#xA;  (LORRI) are generally posted within 48 hours after receipt at the New&#xA;  Horizons Science Operations Center. The date/time listed in the image&#xA;  caption is when the picture was taken by the spacecraft, though&#xA;  receipt of the data on Earth could be many days later, depending on&#xA;  when the image is downloaded from New Horizons.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In addition, the fully validated and calibrated images will be made&#xA;  available at &lt;a href=&quot;http://pds.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;NASA's Planetary Data System&lt;/a&gt; within &lt;strong&gt;nine months to a year&lt;/strong&gt;&#xA;  after receipt of all images on the ground. In all cases, the images&#xA;  will be available to the public, but please refer to the official &lt;a href=&quot;http://pluto.jhuapl.edu/Multimedia/Images/Image-Use-Policy.php&quot; rel=&quot;nofollow&quot;&gt;New&#xA;  Horizons Image Use Policy&lt;/a&gt; for the proper use and referencing of these&#xA;  images.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So far though, no New Horizons data products appear in the list of &lt;a href=&quot;http://pds-imaging.jpl.nasa.gov/holdings/&quot; rel=&quot;nofollow&quot;&gt;PDS Imaging Node Data Holdings&lt;/a&gt; and we're left with what data is approved for early public release by New Horizons PI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of those (see releases &lt;a href=&quot;http://pluto.jhuapl.edu/Multimedia/index.php&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;) would come already calibrated and processed for easier &lt;em&gt;public consumption&lt;/em&gt;, and some the New Horizons team would deem are relevant to whatever they are trying to present to us in raw, or partially processed form. Note that for scientific data, verification and calibration can take substantial load on the resources the team operates with, their time, and often takes multiple frames or whole sessions to calibrate individual frames.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're impatient though, you could try finding enthusiast individuals and groups that try to process these raw images on their own. Those wouldn't be verified, but it's often possible to get good results with readily available image processing tools, and if you don't mind a bit of image processing (granted, that's often tedious work), you can try to &lt;em&gt;clean them up&lt;/em&gt; on your own too. For the specific image you attach to your question, artifacts affecting it are &lt;a href=&quot;http://pluto.jhuapl.edu/soc/Pluto-Encounter/view_obs.php?image=data/pluto/level2/lor/jpeg/029387/lor_0293874887_0x633_sci_3.jpg&quot; rel=&quot;nofollow&quot;&gt;described in the subsequent one&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This image contains one or more objects whose brightness exceeds the&#xA;  detector's saturation level. This sometimes produces a &quot;tail&quot; of&#xA;  bright and/or dark pixels to the right of the object. You may also&#xA;  notice a faint vertical white stripe passing through the saturated&#xA;  object; this is an artifact called &quot;frame transfer smear&quot; and is&#xA;  associated with the incomplete removal of signal produced when the&#xA;  image is transferred from the optically active region of the detector&#xA;  to the storage region of the detector. If the target is badly&#xA;  saturated, you may also notice a faint, X-shaped feature nearly&#xA;  centered on the object; these are optical diffraction spikes.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This image contains one or more streaks associated with cosmic rays&#xA;  passing through the detector. Nearly every LORRI image has at least&#xA;  one cosmic ray strike, but most are &quot;single pixel&quot; events (i.e., they&#xA;  only appear to be in single pixel and can easily be mistaken for&#xA;  stars). But sometimes a cosmic ray is energetic enough that it leaves&#xA;  a &quot;trail&quot; as it passes through the LORRI detector.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-05-19T01:58:26.627" LastActivityDate="2015-05-19T01:58:26.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9156" PostTypeId="1" AcceptedAnswerId="13019" CreationDate="2015-05-19T18:47:51.157" Score="13" ViewCount="3021" Body="&lt;p&gt;Wikipedia introduces the idea of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_sled_launch&quot;&gt;Rocket Sled Launch&lt;/a&gt; (also catapult launch, ramped launch, etc) as follows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With this concept the launch vehicle is supported by an eastward pointing rail or maglev track that goes up the side of a mountain while an externally applied force is used to accelerate the launch vehicle to a given velocity. Using an externally applied force for the initial acceleration reduces the propellant the launch vehicle needs to carry to reach orbit. This allows the launch vehicle to carry a larger payload and reduces the cost of getting to orbit. When the amount of velocity added to the launch vehicle by the ground accelerator becomes great enough, single-stage-to-orbit flight with a reusable launch vehicle becomes possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The idea has been used in fiction several times, but so far as I know, no one is even discussing building one.  In these days of SpaceX and &lt;em&gt;competitors&lt;/em&gt;, I'd expect that someone would be talking about it if it were feasible, so I assume that it's not currently.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if a direct launch to orbit from the sled isn't feasible, I'd think that this would be a efficient way to get a regular rocket into the upper atmosphere at significant velocity, instead of starting from a standing start on the ground.  The rocket would then be able to push the payload the rest of the way into orbit with far less fuel than a traditional one would need.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What obstacles would a system like this have to overcome to be considered?  Is it a matter of location?  Materials engineering? Simply financial?  Have there even been feasibility studies?&lt;/p&gt;&#xA;" OwnerUserId="898" LastEditorUserId="49" LastEditDate="2015-08-16T05:20:34.980" LastActivityDate="2017-08-31T07:39:24.350" Title="What would make a rocket sled launch feasible?" Tags="|launch|high-altitude-launch|rocket-sled|launch-assist|" AnswerCount="3" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="9157" PostTypeId="2" ParentId="9156" CreationDate="2015-05-19T20:11:34.320" Score="7" Body="&lt;p&gt;There are two limitations here, both of which are serious:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1)  As LocalFluff says, atmosphere. Any velocity component from any sort of launch track is going to be basically horizontal.  If you're in vacuum and the launch velocity is a good portion of orbital velocity this would be a good thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In atmosphere, though, there are three cases:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1a)  A fairly low launch velocity, say Mach 1.  The launch impulse is basically bled away in drag.  You gained nothing but lost a lot of energy while your rocket turned from horizontal to vertical.  (Something that rockets don't do well.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1b)  A medium launch velocity, say Mach 10.  Things get even worse--you need to beef up your rocket to handle this and you'll still bleed most of the velocity to drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1c)  A very high launch velocity, say Mach 40 (Yes, I realize this is above orbital velocity.  You'll need to be well above orbital velocity to make up for drag losses.)  This could actually be of some use.  It's only for payloads that can tolerate a lot of gs, both because of the launcher and the brutal deceleration that will happen when it hits the atmosphere.  You'll also need a beefy launch capsule to do this, but you get to orbit for only the cost of a circularization burn.  If you have enough such cargo it might make sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2)  You have a big problem with the strength of a rocket.  Rockets are normally built like eggshells--strong against the intended load, very weak against forces they aren't going to encounter.  You'll need to beef your rocket up substantially so it can survive sitting there on the launch cradle before someone pushes the button.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beefing a rocket up isn't actually that expensive construction but it's extra weight--and the rocket equation is brutal indeed.  Every extra pound holding your booster together is a pound that comes off your payload.  You only want to go to these costs if there is a substantial benefit to be gained.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2015-05-21T18:06:43.880" LastActivityDate="2015-05-21T18:06:43.880" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="9158" PostTypeId="1" AcceptedAnswerId="9159" CreationDate="2015-05-19T21:18:51.263" Score="9" ViewCount="7796" Body="&lt;p&gt;Consider a cloud-top colony on Venus. What materials needed for the upkeep and expanding of the colony can be taken from Venusian atmosphere, excluding any possibility of mining the surface?&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="49" LastEditDate="2015-05-20T02:30:26.033" LastActivityDate="2022-05-29T01:09:12.580" Title="What useful materials can be extracted from Venusian atmosphere?" Tags="|colonization|atmosphere|venus|natural-resources|isru|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9159" PostTypeId="2" ParentId="9158" CreationDate="2015-05-20T02:21:20.310" Score="13" Body="&lt;p&gt;This is a rather broad question, so I'll mostly try to point you in the right direction than directly answer it; First, &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venusian atmosphere&lt;/a&gt; is highly dynamic and diverse environment both in constitution as well as ambient pressure, temperature and even weather. It forms many distinct layers at different altitudes, and its troposphere extends to about 100 km (I presume there's a typo in Wikipedia article and they meant 65 miles, not 65 km). For easier reference, here's atmospheric pressure chart on Venus from already linked Wikipedia article:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UF2RN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus#/media/File:Atmosphere_of_venus.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;composition&lt;/a&gt; is mostly carbon dioxide ($\require{mhchem}\ce{CO2}$ 96.5%) and smaller amount of nitrogen ($\ce{N2}$ 3.5%), while other constituents are in trace amounts, but that doesn't really exclude them from the possibility of extracting them from the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/u7vYx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, in the cloud layers between ~ 50 and 70 km, large concentrations of sulfuric acid ($\ce{H2SO4}$) and smaller concentrations of hydrogen sulfide ($\ce{H2S}$) were detected and it would be possible to extract hydrogen, sulfur and oxygen from it by electrolysis and the use of reducing catalysts. Similar can be done with carbon dioxide to extract oxygen and carbon in a two-step &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5094/how-do-you-make-oxygen-on-mars/5096#5096&quot;&gt;electrolysis of the atmosphere&lt;/a&gt;&lt;/em&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Photosynthesis&quot; rel=&quot;nofollow noreferrer&quot;&gt;photosynthesis&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These constituent elements alone already give you any number of molecular compounds of interest, including for breathable air (nitrogen and carbon dioxide as they are, just in different concentrations, oxygen, and water to control ambient humidity), assuming reasonable deuterium to hydrogen concentrations also as potable water, otherwise deuterium and hydrogen atoms can be separated during electrolysis or &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Heavy_water&quot; rel=&quot;nofollow noreferrer&quot;&gt;heavy water&lt;/a&gt;&lt;/em&gt; ($\ce{D2O}$) from water ($\ce{H2O}$) with additional separation by molecular weight. Nitrogen and hydrogen give you &lt;a href=&quot;https://en.wikipedia.org/wiki/Ammonia&quot; rel=&quot;nofollow noreferrer&quot;&gt;ammonia&lt;/a&gt; ($\ce{NH3}$) that can be used as liquid coolant / refrigerant, fertilizer, fuel,...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Carbon and hydrogen also enable production of synthesized or naturally grown carbohydrates and polycarbohydrates as nutrients, carbohydrate chains and aromatic carbohydrates as &lt;a href=&quot;https://en.wikipedia.org/wiki/Plastic&quot; rel=&quot;nofollow noreferrer&quot;&gt;plastics&lt;/a&gt;, e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Vinyl&quot; rel=&quot;nofollow noreferrer&quot;&gt;vinyl&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Polyethylene&quot; rel=&quot;nofollow noreferrer&quot;&gt;polyethylene&lt;/a&gt; that you might recognize in many products by having that HDPE - high density polyethylene or &lt;a href=&quot;https://en.wikipedia.org/wiki/Polyethylene#/media/File:Polyethylene.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;PE recycle sign&lt;/a&gt; print or embedding and HDPE are even not corroded by sulfuric acid so they could be extensively used in Venusian atmosphere for construction. Additionally, provided you can extract chlorine that is also present just below the cloud layer, you could also make &lt;a href=&quot;https://en.wikipedia.org/wiki/Polyvinyl_chloride&quot; rel=&quot;nofollow noreferrer&quot;&gt;PVC&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.engineeringtoolbox.com/melting-temperature-metals-d_860.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Boiling points of some metals and metallic alloys&lt;/a&gt; are also below Venusian surface temperature of ~ 462 °C (e.g. cadmium, lead, some magnesium alloys, mercury, phosphorus, potassium, selenium,...) so they could form vapor that would be transported to higher altitudes by &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_wind&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermal winds&lt;/a&gt;, where they would condense or recombine into heavier molecules (the latter might be true for molecules containing phosphorus and potassium) with the drop of temperature, and form metallic precipitate deposits. It would be possible to collect some of these metals from lower atmosphere on condensing surface low altitude balloons, or even collect their &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus_snow&quot; rel=&quot;nofollow noreferrer&quot;&gt;snow-like precipitates&lt;/a&gt; from high mountain surfaces by cable cranes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, for any serious colony in Venusian clouds where pressure and temperature are Earth surface like, breathable air serves as lifting gas, and one that wouldn't have to import materials from elsewhere in the Solar system, you'd still require the ability to at least pluck rocks and boulders off the surface with cable cranes or alike, to also provide your colony with silicon, iron, copper, and other useful elements and compounds. E.g. to produce &lt;a href=&quot;https://en.wikipedia.org/wiki/Silicon_carbide&quot; rel=&quot;nofollow noreferrer&quot;&gt;silicon carbide&lt;/a&gt; ($\ce{SiC}$) low power electronics, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gallium_nitride&quot; rel=&quot;nofollow noreferrer&quot;&gt;gallium nitride&lt;/a&gt; ($\ce{GaN}$) &lt;a href=&quot;http://spectrum.ieee.org/tech-talk/semiconductors/design/gallium-nitride-transistors-priced-cheaper-than-silicon&quot; rel=&quot;nofollow noreferrer&quot;&gt;high power transistors&lt;/a&gt;, diodes or photovoltaics, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Gallium_phosphide&quot; rel=&quot;nofollow noreferrer&quot;&gt;gallium phosphide&lt;/a&gt; ($\ce{GaP}$) since all these materials have good heat and radiation tolerances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For further reference, I suggest &lt;a href=&quot;http://ntrs.nasa.gov/?N=0&amp;amp;Ntk=All&amp;amp;Ntt=landis%20venus&amp;amp;Ntx=mode%20matchallpartial&amp;amp;Nm=123%7CCollection%7CNASA%20STI%7C%7C17%7CCollection%7CNACA&quot; rel=&quot;nofollow noreferrer&quot;&gt;searching NASA Technical Report Server (NTRS) for Geoffrey A. Landis' works&lt;/a&gt;, perhaps staring with &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030022668.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Colonization of Venus&lt;/a&gt; (PDF) that was published for 2003 Conference on Human Space Exploration proceedings, and concisely presents possibility of constructing aerostat colonies in Venusian atmosphere and natural resources available to them.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-05-20T06:03:58.743" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9160" PostTypeId="1" CreationDate="2015-05-20T06:05:11.997" Score="9" ViewCount="1021" Body="&lt;p&gt;Suppose a cloud-top colony on Venus. Provided that it has no thrusting engines, where it will tend to go naturally with winds, to the equator, to the poles or elsewhere?&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="49" LastEditDate="2015-05-20T07:42:42.767" LastActivityDate="2016-02-14T11:57:52.363" Title="A cloud-top colony on Venus, will it drift to the poles?" Tags="|colonization|planetary-science|atmosphere|venus|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9162" PostTypeId="2" ParentId="9160" CreationDate="2015-05-20T08:02:39.743" Score="13" Body="&lt;p&gt;There's a lot to be said about circulation of Venusian troposphere first, but that's all nicely explained on &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;. But please read it as background, if that's required; For easier reference though, and from that same Wikipedia page, here's a cross-section diagram of the atmospheric circulation in the atmosphere of Venus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0hHCG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;An aerostat colony in Venusian atmosphere doesn't really require thrusters to control its latitude. It could simply do it the same way that &lt;a href=&quot;https://en.wikipedia.org/wiki/Hot_air_balloon&quot; rel=&quot;noreferrer&quot;&gt;hot air balloons&lt;/a&gt; in Earth's atmosphere do it - by changing the temperature of the air that provides them with buoyancy to move to altitudes where wind direction is suitable. Yes, such aerostat colonies would have to be flexible structures, but then you could do this with active heating and cooling of the buoyant air (or part of it) and increase you colony's volume and with it the mass of the displaced air, or more simply by passive means, such as increasing or reducing exposure to environmental heating of the buoyant mass. Perhaps as simply as synchronizing meridional drift with your synodic period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In simpler terms, if you would on average keep your aerostat colony at 50 km above ground and roughly over equator, in about 4 days that it would take it to circumnavigate Venus, you could spend 2* days at altitude of 55 km and let meridional flow drift you slowly towards higher latitudes, and the remaining two days when you're on the night side, lower air temperature within it, with it lower your altitude (to 45 km if I strictly follow my example), and let the return flow of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hadley_cell&quot; rel=&quot;noreferrer&quot;&gt;Hadley cell&lt;/a&gt; slowly drift you back towards the equator. Your colony's inhabitants might prefer a bit colder air during night than during day anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This diagram from Wikipedia on Hadley cell is for Earth, so the numbers and types of clouds depicted are off, but it should still help visualize how meridional drift could be controlled by lowering or raising aerostat's altitude:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/HEwQP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Otherwise, without any maneuvering whatsoever, while the chance of being &lt;em&gt;pulled&lt;/em&gt; through polar collars and into polar regions is rather small, it is not really excluded. Your colony's altitude would more likely &lt;em&gt;naturally&lt;/em&gt; decay into lower altitude one by entering night side and cooling down faster than getting close to &amp;pm;60° latitude (&lt;a href=&quot;https://en.wikipedia.org/wiki/Zonal_and_meridional&quot; rel=&quot;noreferrer&quot;&gt;zonal flow&lt;/a&gt; is faster than meridional), but you might get unlucky and catch a strong jet to &lt;em&gt;carry you over&lt;/em&gt; on &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertia&quot; rel=&quot;noreferrer&quot;&gt;inertia&lt;/a&gt; of a large mass colony.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;During the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_program#Balloon&quot; rel=&quot;noreferrer&quot;&gt;Vega program&lt;/a&gt;, while there was a great deal of problems establishing exact position of the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1984-125F&quot; rel=&quot;noreferrer&quot;&gt;two balloons&lt;/a&gt; released on June 11 and 15, 1985 into the Venusian atmosphere from Soviet VEGA 1 and VEGA 2 spacecraft, meridional drift doesn't seem to have been a problem at all while balloons lasted in the hostile environment (~ 46 hours, making about one-third of the way around Venus at 54 km altitude but with substantial vertical motion as they were deployed close to equator).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sadly, most of good references are paywalled, but here's a free access report on &lt;a href=&quot;http://ipnpr.jpl.nasa.gov/progress_report/42-85/85Q.PDF&quot; rel=&quot;noreferrer&quot;&gt;The Venus Balloon Project&lt;/a&gt; (PDF) that includes this diagram of the paths of the two balloons:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6F0Yd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;*Day and night time wouldn't be exactly half-half because of critical &lt;a href=&quot;https://en.wikipedia.org/wiki/Refractive_index&quot; rel=&quot;noreferrer&quot;&gt;refractive angle&lt;/a&gt; of Venusian atmosphere, but faster winds at higher altitudes (3 m/s per km) would offset this, so for our example where we navigate at higher altitude during day than during night, it should be close enough.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-14T11:57:52.363" LastActivityDate="2016-02-14T11:57:52.363" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9163" PostTypeId="1" CreationDate="2015-05-20T15:00:16.673" Score="7" ViewCount="573" Body="&lt;p&gt;For the booster stage of Falcon 9, test landings after a full flight profile are relatively frequent. SpaceX can try those on each CRS mission to the ISS, and in theory, any other launch that wouldn't require complete booster stage burnout to exhaustion, such as higher energy trajectories to GEO or beyond. And they were using &lt;a href=&quot;https://en.wikipedia.org/wiki/Grasshopper_%28rocket%29&quot;&gt;Grasshopper and F9R Dev1&lt;/a&gt; for development &lt;a href=&quot;https://en.wikipedia.org/wiki/VTVL&quot;&gt;VTVL&lt;/a&gt; test flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Dragon and Crew Dragon spacecraft powered landing however, while they can of course try short flight profile tests relatively easily, and they will have to complete at the minimum one more test landing of Crew Dragon during the in-flight abort for the CCDev contract with NASA, the latter will be a parachuted descent. And so far, same goes for CRS missions with Dragon splashing somewhere in the Pacific west of California.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So when will SpaceX be able to test full flight profile powered descent and landing of Dragon / Crew Dragon?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-05-20T15:58:13.220" Title="On which flights could SpaceX test Crew Dragon powered landing with complete flight profile?" Tags="|spacex|reuse|dragon|dragon-v2|vtvl|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9164" PostTypeId="2" ParentId="9163" CreationDate="2015-05-20T15:47:02.127" Score="3" Body="&lt;p&gt;As far as I know, NASA has specified Dragon v1 for the CRS missions, so no chance of landing tests within that contract.&lt;br&gt;&#xA;The pad abort test has shown that the Crew Dragon is far enough along for unmanned test flights. Well, sort of: it didn't include a full complement of attitude control thrusters, but those should be mostly identical to the Dragon v1 ones. So if they get the urge they could send one up pretty soon.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.nasaspaceflight.com/2014/08/dragon-v2-rely-parachutes-landing/&quot; rel=&quot;nofollow&quot;&gt;Initial Crew Dragon missions&lt;/a&gt; are planned to use parachutes as the main deceleration method, with a short burst of the SuperDraco thrusters to soften the landing, so there's some scope for testing there. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-05-20T15:47:02.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9165" PostTypeId="2" ParentId="9163" CreationDate="2015-05-20T15:58:13.220" Score="5" Body="&lt;p&gt;SpaceX decided to get certified and flying manned first with the simplest recovery system they had. That is parachute landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They will then use the DragonFly test vehicle in McGregor to test parachute to land, with SuperDraco to cushion the blow. Then test SuperDraco only landings. Then boosted hops and landings.  (Boosted hops saves on rental cost for helicopter). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Dragon Cargo vehicle uses the CBM style port (for berthing), not the LIDS/PMA adapter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon Crew will use only the LIDS/PMA adapter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The diameters and mechanisms of those two adapter modalities are very different, and each seems to have been designed around the docking port sizing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To test powered landing on a Dragon Crew would require a third configuration, a Dragon Crew, with SuperDracos et al, with a CBM port. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maintaining/developing/testing a third configuration may not be worth it, when DragonFly is coming, and NASA is comfortable with ocean recovery for the initial flights.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-20T15:58:13.220" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9166" PostTypeId="1" AcceptedAnswerId="11994" CreationDate="2015-05-20T17:05:01.027" Score="8" ViewCount="5251" Body="&lt;p&gt;This &lt;a href=&quot;https://youtu.be/hhkFh3tPSWI?t=2m6s&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; (a tour of the SpaceX factory by Elon Musk) suggests that the second stage of Falcon 9 has Draco thrusters as a Reaction Control System (RCS) (at 2:14), but I don't recall ever seeing them in photos:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/hhkFh3tPSWI?start=126&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, in the following photos I don't see obvious covers for nozzles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3syfa.jpg&quot; alt=&quot;F9 second stage&quot;&gt;&lt;br&gt;&#xA;This is an incomplete stage of course. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YSpQP.jpg&quot; alt=&quot;F9v1.1&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-08-15T07:25:53.243" LastActivityDate="2022-01-28T17:18:56.863" Title="Does the second stage of the Falcon 9 have RCS thrusters?" Tags="|spacex|falcon-9|attitude|design|engines|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9167" PostTypeId="2" ParentId="9166" CreationDate="2015-05-20T17:37:26.913" Score="4" Body="&lt;p&gt;On the sixth Falcon 9 (first in V1.1 configuration) launch for Cassiope there was a failed attempt to re-ignite the second stage again as a demonstration of required capability for GEO/GTO missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot find a link to this specific detail, but there was a report I recall that they were going to fire at 90 degrees to the flight path, since they had no where to go really, and just wanted to test reigniting while in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to rotate a stage as massive as the second stage (23,000 kg empty) you need either fairly large gyros or some kind of attitude control system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They do not have Draco engines on the second stage, but they do seem to have some kind of control system. Likely it is a cold gas nitrogen system, similar to what they have on the first stage and interstage. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-20T17:37:26.913" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9168" PostTypeId="1" CreationDate="2015-05-20T19:28:33.847" Score="-6" ViewCount="61775" Body="&lt;p&gt;My understanding is that there was supposed to be a delay between the communication from the moon to Earth. Why were the communication instantaneous?&lt;/p&gt;&#xA;" OwnerUserId="10192" LastEditorUserId="195" LastEditDate="2018-11-19T01:11:08.560" LastActivityDate="2020-11-22T08:12:48.303" Title="Why wasn't there a communication delay when Neil Armstrong landed on the moon?" Tags="|the-moon|apollo-program|communication|" AnswerCount="2" CommentCount="5" ClosedDate="2015-06-05T08:21:54.613" ContentLicense="CC BY-SA 4.0" />
  <row Id="9169" PostTypeId="2" ParentId="9168" CreationDate="2015-05-20T19:44:56.040" Score="13" Body="&lt;p&gt;Some modern presentations of the moon landing will edit out the gaps created by the communication delay to make conversation sound more natural, but if you look at the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11transcript_tec.html&quot;&gt;timestamped transcripts&lt;/a&gt;, you can verify the 2.5 second round trip delay. Note that the timestamps all refer to the time the Earth-side recording occurred, so the radio delay appears when one of the crew members (CMP, CDR, LMP in the transcript) is responding to the Capcom (CC). &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-05-20T19:51:34.550" LastActivityDate="2015-05-20T19:51:34.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9170" PostTypeId="2" ParentId="9168" CreationDate="2015-05-20T19:49:10.833" Score="14" Body="&lt;p&gt;The delay for Earth-Moon communications is about 1.25 second (distance is ~ 380,000 km, the speed of light is ~300,000 km/s). You'd notice this delay only in conversations: every time ground control says something, it takes 2 seconds for the astronauts to respond.&lt;/p&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/results?search_query=apollo%2011%20live%20ABC&quot; rel=&quot;nofollow noreferrer&quot;&gt;videos of the live broadcast&lt;/a&gt; (search for 'Apollo 11 live ABC') you can hear the delay: ground control says &amp;quot;go for depressurization&amp;quot;, then you hear the echo of that coming back from the Moon, and the astronauts' response.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2020-11-22T08:12:48.303" LastActivityDate="2020-11-22T08:12:48.303" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="9171" PostTypeId="2" ParentId="8781" CreationDate="2015-05-20T22:02:16.280" Score="2" Body="&lt;p&gt;It has been pointed out that the debris flying away after the first two explosions, is probably a COPV (Composite Overwrapped Pressure Vessel). These have been problematic, since they are very high pressure tanks, and when they fail, they fail almost explosively.  This was probably the COPV venting and throwing itself far away. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-20T22:02:16.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9172" PostTypeId="2" ParentId="8766" CreationDate="2015-05-20T22:05:56.350" Score="0" Body="&lt;p&gt;The reported issue is that the valves controlling the Merlin 1D had some internal friction (Stiction) that meant it was slow to respond to computer requests. It was ordered to correct one way, was slow, so over-corrected.  The computer ordered a correction to fix it, was slow, over-corrected the other way. Until it came in as you see. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-20T22:05:56.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9174" PostTypeId="1" CreationDate="2015-05-21T15:08:26.940" Score="3" ViewCount="177" Body="&lt;p&gt;Water is cheap and useful for many purposes in space (fuel, life support, radiation shielding and maybe even as the bulk part of construction material if covered from sublimation in vacuum). But getting it to orbit is expensive, and it is rare in the inner solar system. So I wonder if there have been any ideas around in this creative business, to &quot;launch&quot; water itself without packaging it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water leaves Earth's atmosphere naturally at some rate. Could water conceivable be &quot;steamed&quot; up (steam me up Scotty!;-) to orbit in a way so that at least a useful fraction of it could be collected in LEO? Maybe from a high altitude balloon. Maybe using microwave beams to excite and direct it.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-05-21T15:08:26.940" Title="Could it be possible to &quot;steam&quot; up water to orbit and collect it there?" Tags="|launch|water|advanced-propulsion|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9175" PostTypeId="1" AcceptedAnswerId="9177" CreationDate="2015-05-21T19:36:31.190" Score="15" ViewCount="2074" Body="&lt;p&gt;X-37 can fly in the same orbit as ISS but can they dock?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/huOV1lkK2io?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="8221" LastEditorUserId="51" LastEditDate="2015-05-21T19:39:37.930" LastActivityDate="2015-05-21T22:07:15.417" Title="Can X-37B dock with the ISS" Tags="|iss|docking|docking-port|berthing|x-37|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9176" PostTypeId="2" ParentId="9175" CreationDate="2015-05-21T19:39:16.667" Score="9" Body="&lt;p&gt;The ISS has 3 basic docking port types.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2 PMA's - used by Shuttle. With addition of LIDS adapter (coming on SpaceX CRS-7 in June 2015) the CST-100 and Dragon Crew will dock to it.&lt;/li&gt;&#xA;&lt;li&gt;2 CBM ports - HTV, Dragon Cargo, Cygnus, and ISS modules berth to these.&lt;/li&gt;&#xA;&lt;li&gt;4 APAS ports - Russian segment has 4 ports. ATV, Soyuz, and Progress use these.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You can see a description of these ports (US Segment on one answer, and Russian segment in the other) in &lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;Will the ISS need more docking ports?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stealing an image from there that sort of shows many examples. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZBnjd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;X-37B carries no ports compatible with these systems at this time. No doubt if there was serious demand something could be added.  The variety of available ports make that a greater possibility. (The Russians apparently will sell you a Kurs rendezvous system if you want, that is how ATV docks)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as of now, no.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-05-21T22:07:15.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9177" PostTypeId="2" ParentId="9175" CreationDate="2015-05-21T19:52:21.130" Score="21" Body="&lt;p&gt;Yes and no. The current X-37 as flown by the US Air Force is not equipped with any docking ports (PMA, CBM or APAS) as described by @geoffc, nor does it have an attachment for the SSRMS (aka Canadarm) to grab it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;However&lt;/em&gt;, Boeing, the manufacturer, have published designs that would permit it to be so equipped at some future time. These plans are described in detail in an &lt;a href=&quot;http://www.nasaspaceflight.com/2013/03/x-37b-expanded-capabilities-iss-missions/&quot; rel=&quot;noreferrer&quot;&gt;article on nasaspaceflight.com&lt;/a&gt;. This was part of their COTS proposals. You can get the sense of how this would work from these illustrations:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xj3Fb.jpg&quot; alt=&quot;X37 docking with ISS&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/G9C5j.jpg&quot; alt=&quot;X37 as crew transport&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2015-05-21T19:52:21.130" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9178" PostTypeId="1" AcceptedAnswerId="9181" CreationDate="2015-05-21T17:55:05.107" Score="23" ViewCount="9537" Body="&lt;p&gt;In 1997, NASA remotely fixed a bug that caused priority inversion on their Mars Pathfinder. How did they go about doing this? What kind of communication protocols are used? How do they update the source for an operating system, compile it, and run it from a remote location? This might be simpler than I thought, but to me this seems like quite the feat!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Story of the bugfix here:&#xA;&lt;a href=&quot;http://research.microsoft.com/en-us/um/people/mbj/mars_pathfinder/authoritative_account.html&quot;&gt;http://research.microsoft.com/en-us/um/people/mbj/mars_pathfinder/authoritative_account.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The author said to email him and he would provide details, but this was almost 20 years ago. Curious to see if anyone else knows how this worked.&lt;/p&gt;&#xA;" OwnerUserId="37893" OwnerDisplayName="kkopsa" LastEditorUserId="198" LastEditDate="2015-05-25T20:17:46.570" LastActivityDate="2023-11-09T17:26:44.000" Title="How did NASA remotely fix the code on the Mars Pathfinder?" Tags="|mars|nasa|software|guidance|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9179" PostTypeId="2" ParentId="7862" CreationDate="2015-05-21T22:16:35.487" Score="4" Body="&lt;p&gt;The video from the first and second barge landing attempts make it pretty clear that the stage comes in pretty fast. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the Merlin 1D engine can only throttle down to 70% and they land on a single (center) engine, so 70% of 165,000 lbs of thrust is about 115.5 klbs. From SpaceFlight101, it lists the mass of an empty first stage at around 23-25,000kg (lets call that 55,000 lbs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So they can only go down as low as 115,000 lbs of thrust, and they mass on the order of 55,000 lbs, so they have way to much thrust at landing time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They need to reach 0 m/s speed at 0 meters of altitude, in what SpaceX has called a &quot;Hover Slam&quot;. They practiced it on the Grasshopper test vehicle, which they apparently had ballasted to allow for hovering in initial tests of the control systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus it really is coming in pretty fast. It is also decelerating pretty darn quick. Which is by design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is why most speculative BFR projections assume 5-9 Raptor engines, as you want to be able to land close to T/W of 1 if possible. So you want a nice symmetric single engine to land on, and 5, 7, 9, and 13 seem to work geometrically for that. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-21T22:16:35.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9180" PostTypeId="2" ParentId="9178" CreationDate="2015-05-21T22:38:02.717" Score="5" Body="&lt;p&gt;Generally space probes have several copies of the operating system. The one being run, and one it can swap too, and often a third that is a safe mode/fallback that is simplest and allows for remote reboots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They would upload the new code to one of the backup memory locations, then swap over to run on it once it was complete.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-21T22:38:02.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9181" PostTypeId="2" ParentId="9178" CreationDate="2015-05-21T23:05:31.657" Score="16" Body="&lt;p&gt;Specifically, for the Mars Pathfinder priority inversion problem, this is explained in detail in &lt;a href=&quot;http://www.cse.chalmers.se/edu/course/EDA222/Documents/Misc/Report_MarsPathFinder.pdf&quot; rel=&quot;noreferrer&quot;&gt;Mars Pathfinder: Priority Inversion Problem&lt;/a&gt;, Report for the Seminar Series on Software Failures, Risat Mahmud Pathan, Chalmers University of Technology (PDF). I'm reproducing here an excerpt that's most relevant to your question, and I'd recommend reading the rest of it from the source:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h3&gt;2.2 How was it debugged?&lt;/h3&gt;&#xA;&lt;p&gt;The software on Mars Pathfinder had several debug features. One of&#xA;these tools is atrace/log facility. The feature remained in the&#xA;software in the final version of the design because the engineers at&#xA;JPL have the philosophy that &lt;em&gt;&amp;quot;test what you fly and fly what you&#xA;test&amp;quot;&lt;/em&gt;. So, they did not remove the debug facility; it was there in&#xA;Mars. After the problem occurred on Mars, they run the same set of&#xA;recorded activities (sent by Pathfinder before resetting) over and&#xA;over again in the lab and within three weeks they were able to&#xA;reproduce the error in the replica at JPL. The priority inversion&#xA;problem was obvious. The solution is to enable priority inheritance by&#xA;setting the mutex flag for the select() calls of ASI/MET to “on”.&#xA;However, the fix is not so obvious for several reasons:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Concern 1&lt;/strong&gt;: Setting the mutex flag is a global option and thus applicable to all mutex. Enabling it for ASI/MET would enable it for&#xA;other tasks. How would this change the behavior of the rest of the&#xA;system?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Concern 2:&lt;/strong&gt; The priority inheritance option was deliberately “off” by Wind River&lt;sup&gt;1&lt;/sup&gt; for optimum performance. How will performance degrade if&#xA;we turn priority inheritance “on”?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Concern 3&lt;/strong&gt;: Would the select() mechanism become incorrect if priority inheritance was enabled?&lt;/p&gt;&#xA;&lt;p&gt;Wind River concluded that the performance impact would be minimal and&#xA;that the behavior of select() would not change. The JPL engineers&#xA;tested, analyzed and concluded that changing the flag on a global&#xA;basis had no adverse impact. So, they decided to patch the software in&#xA;Mars by enabling the priority inheritance option.&lt;/p&gt;&#xA;&lt;h3&gt;2.3 How the patch was uploaded?&lt;/h3&gt;&#xA;&lt;p&gt;VxWorks contained a C language interpreter to execute statements on&#xA;the fly during debugging. The JPL engineers decided to launch the&#xA;spacecraft with this feature still enabled. A short C program was&#xA;uploaded to the spacecraft, which when interpreted, changed the values&#xA;of the mutex flag for priority inheritance from false to true. No more&#xA;system reset occurred!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Quoted source also lists several references worth reading, but for a broad overview of the problem and its solution I'd recommend &lt;a href=&quot;http://people.cis.ksu.edu/%7Ehatcliff/842/Docs/Course-Overview/pathfinder-robotmag.pdf&quot; rel=&quot;noreferrer&quot;&gt;Tom Durkin, What the Media Couldn’t Tell You About Mars Pathfinder&lt;/a&gt;, Robot Science and Technology Magazine, Issue 1, 1998 (PDF) that includes explanations by Glenn E. Reeves (JPL).&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/revisions/9181/1&quot;&gt;first revision of my answer&lt;/a&gt;, I answered by my mistake for Spirit (Mars Exploration Rover, or MER-A) rover's FLASH problem, so if you're interested in how such problems are fixed in more general sense, then that &lt;em&gt;sub-answer&lt;/em&gt; (is that the word?) should give you a clue that it really depends on each specific hardware and the nature of the problem. Pathfinder used COTS (Commercial, Off-The-Shelf) hardware with IBM's radiation hardened &lt;a href=&quot;https://en.wikipedia.org/wiki/IBM_RAD6000&quot; rel=&quot;noreferrer&quot;&gt;RAD6000 processor&lt;/a&gt; and Wind River vxWorks RTOS. Other Mars landers and rovers, unless they're of same series like Spirit MER-A and Opportunity MER-B, might use different hardware and OS combination. And while RAD6000 (Pathfinder, Mars Polar Lander, MER-A &amp;amp; MER-B, Phoenix Polar Lander) and vxWorks RTOS (Pathfinder, Spirit, Opportunity, Curiosity) seem to be a rather popular processor and OS combination on Mars, they're not the only ones either (Sojourner rover that Pathfinder deployed used a custom cyclic executive OS not too unlike today's Arduino platform, Curiosity uses &lt;a href=&quot;https://en.wikipedia.org/wiki/RAD750&quot; rel=&quot;noreferrer&quot;&gt;RAD750&lt;/a&gt; PowerPC,...).&lt;/p&gt;&#xA;&lt;p&gt;Communications channel to send these updates to hardware on Mars is NASA's &lt;a href=&quot;http://deepspace.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; (DSN) that uses Mars orbiters to relay communications to and from rovers, landers et al. on the ground of the red planet (you can observe its status at &lt;a href=&quot;https://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;noreferrer&quot;&gt;DSN Now&lt;/a&gt;). For distant hardware where communications delay could be dozens of minutes long, commands are usually transmitted in sessions for a whole Martian day (sol) or more in advance and, when necessary (like with changes to the FSW - &lt;em&gt;flight software&lt;/em&gt;), first uploaded and tested on replica that stays at home. Exact network protocols could be different, from radio frequency to packet size and structure (today, it's mostly in &lt;a href=&quot;https://en.wikipedia.org/wiki/X_band&quot; rel=&quot;noreferrer&quot;&gt;X-band&lt;/a&gt; DTN - &lt;a href=&quot;https://en.wikipedia.org/wiki/Delay-tolerant_networking&quot; rel=&quot;noreferrer&quot;&gt;Delay-Tolerant Networking&lt;/a&gt;), but all would be programmed to listen with their communication subsystem on specific frequencies for newly issued commands and schedule them according to priority flags within received data. Some hardware on Mars (like Mars Science Laboratory, better known as Curiosity) uses dual bus platform for redundancy and fault tolerance, but since that increases size, mass, power consumption, and so on, smaller ones don't.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; Wind River systems supplied the VxWorks Real-Time Operating Systems for Mars Pathfinder mission.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-05-22T01:43:38.083" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9183" PostTypeId="2" ParentId="4476" CreationDate="2015-05-22T09:23:02.647" Score="1" Body="&lt;p&gt;The risk with hyperbolic rendezvous could be alleviated by having a &quot;rescue cycler&quot; following e.g. one day after the &quot;station cycler&quot; in a very similar orbit. The rescue cycler would consist of a chemical rocket engine with a generous fuel tank. If the crewed taxi misses the rendezvous, the rescue cycler would set off to rendezvous with the taxi instead, and tug it to the station cycler. This would give a second chance and larger safety margin, without increasing the fuel mass of the chemically propelled crewed taxi. The rescue cycler could be relatively economically put in place with a slow ion thruster and would only need to be refueled if it has been used.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-05-22T09:28:31.327" LastActivityDate="2015-05-22T09:28:31.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9184" PostTypeId="2" ParentId="3509" CreationDate="2015-05-22T13:11:30.813" Score="11" Body="&lt;p&gt;Launches from Vandenberg AFB do not go over land, they either fly South or South-West for polar orbits or West for retrograde orbits (which is rarely ever done).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Air Force uses Cape Canaveral AFS for launches to the East to achieve prograde orbits, which is used for geosynchronous or geostationary orbits. North/South/West launches can't be done at Cape Canaveral AFS for the same reason North/East launches can't be done at Vandenberg AFB, landmass overflight restrictions.&lt;/p&gt;&#xA;" OwnerUserId="10206" LastActivityDate="2015-05-22T13:11:30.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9185" PostTypeId="1" AcceptedAnswerId="9705" CreationDate="2015-05-22T19:59:27.433" Score="14" ViewCount="1088" Body="&lt;p&gt;It's my understanding a geosynchronous satellite will put itself in a graveyard orbit just before it dies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many dead sats are in GEO or in neighboring graveyard orbits?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is their total mass?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="25" LastEditDate="2015-06-29T19:09:38.887" LastActivityDate="2015-06-29T19:22:22.737" Title="How many dead sats near GEO?" Tags="|artificial-satellite|geostationary|communication-satellite|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9186" PostTypeId="1" CreationDate="2015-05-22T22:21:27.127" Score="3" ViewCount="235" Body="&lt;p&gt;Do satellites sent into space have a physical &lt;strong&gt;intrusion detection system&lt;/strong&gt; on board to prevent tampering?&lt;/p&gt;&#xA;" OwnerUserId="10212" LastEditorUserId="168" LastEditDate="2015-06-05T06:51:42.687" LastActivityDate="2015-06-05T06:51:42.687" Title="Do space satellites have physical intrusion sensors?" Tags="|artificial-satellite|communication-satellite|" AnswerCount="0" CommentCount="4" ClosedDate="2015-09-22T21:30:45.650" ContentLicense="CC BY-SA 3.0" />
  <row Id="9187" PostTypeId="5" CreationDate="2015-05-23T00:17:18.527" Score="0" Body="&lt;p&gt;Orbital maneuver during which two spacecraft arrive at the same orbit and approach to a very close distance. Rendezvous requires a precise match of the orbital velocities and position vectors of the two spacecraft, and station-keeping. Rendezvous may or may not be followed by docking or berthing. &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_rendezvous&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-05-23T00:27:06.393" LastActivityDate="2015-05-23T00:27:06.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9188" PostTypeId="4" CreationDate="2015-05-23T00:17:18.527" Score="0" Body="Orbital maneuver during which two spacecraft arrive at the same orbit and approach to a very close distance. Rendezvous may or may not be followed by docking or berthing." OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-05-23T00:27:14.330" LastActivityDate="2015-05-23T00:27:14.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9189" PostTypeId="1" AcceptedAnswerId="9190" CreationDate="2015-05-23T00:29:13.617" Score="3" ViewCount="440" Body="&lt;p&gt;This recently released terrain of Mars as taken by the Curiosity Rover looks like it was done in black and white.  Is this really how it looks like to the naked eye?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q6hJu.jpg&quot; alt=&quot;Photo of Mars: Rover&amp;#39;s Reward for Climbing: Exposed Contact&quot;&gt;&#xA;From &lt;a href=&quot;http://mars.jpl.nasa.gov/msl/multimedia/images/?ImageID=7238&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="690" LastEditorUserId="49" LastEditDate="2015-06-03T20:15:05.313" LastActivityDate="2015-06-03T20:22:55.160" Title="Is this photo of Mars in color?" Tags="|mars|planetary-science|curiosity|navigation|imaging|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9190" PostTypeId="2" ParentId="9189" CreationDate="2015-05-23T00:45:42.333" Score="9" Body="&lt;p&gt;No, two pairs of MSL (Curiosity) &lt;em&gt;Navcams&lt;/em&gt; are black &amp;amp; white visible light stereoscopic (45 degree angle of view) cameras. They're mounted on a mast at a rough height of what a human would see if standing on the surface of Mars and the paired cameras (left and right pair) enable stereoscopic 3D view. Or, &lt;a href=&quot;https://d2cj35nmzi9erd.cloudfront.net/msl/mission/rover/eyesandother/&quot; rel=&quot;nofollow noreferrer&quot;&gt;straight from the horse's mouth:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Two Pairs of Engineering Navigation Cameras (Navcams):&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Mounted on the mast (the rover &quot;neck and head&quot;), these black-and-white&#xA;  cameras use visible light to gather panoramic, three-dimensional (3D)&#xA;  imagery. The navigation camera unit is a stereo pair of cameras, each&#xA;  with a 45-degree field of view that will support ground navigation&#xA;  planning by scientists and engineers. They work in cooperation with&#xA;  the hazard avoidance cameras by providing a complementary view of the&#xA;  terrain.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;These Navcam images are also contrast-enhanced so it's easier to discern terrain variation. The image you attach seems to be a subframe (crop) of &lt;a href=&quot;https://d2cj35nmzi9erd.cloudfront.net/msl/multimedia/raw/?rawid=NLB_485471647EDR_F0481146NCAM00270M_&amp;amp;s=991&quot; rel=&quot;nofollow noreferrer&quot;&gt;Navcam Left B frame taken on Sol 991 (2015-05-21 09:30:04 UTC)&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/mHW26.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This frame's timestamp should help you identify other &lt;a href=&quot;https://d2cj35nmzi9erd.cloudfront.net/msl/multimedia/raw/&quot; rel=&quot;nofollow noreferrer&quot;&gt;raw images from engineering and science cameras on MSL&lt;/a&gt; that would match MSL's point of view. Closest color images from Mastcam (Mast Camera) are &lt;a href=&quot;https://d2cj35nmzi9erd.cloudfront.net/msl/multimedia/raw/?s=991&amp;amp;camera=MAST_&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, for example, &lt;a href=&quot;https://d2cj35nmzi9erd.cloudfront.net/msl/multimedia/raw/?rawid=0991ML0043810030404526E01_DXXX&amp;amp;s=991&quot; rel=&quot;nofollow noreferrer&quot;&gt;this frame&lt;/a&gt; shows a part of the Navcam frame (but is narrower field of view):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/MkkH1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, the image you attach wouldn't be exactly what you'd see if you were at the same spot on Mars as MSL was on Sol 991 because it lacks colors, but it does have geometric fidelity of what you'd see in black &amp;amp; white with a single (left) eye. And it has a &lt;a href=&quot;https://d2cj35nmzi9erd.cloudfront.net/msl/multimedia/raw/?rawid=NRB_485471647EDR_F0481146NCAM00270M_&amp;amp;s=991&quot; rel=&quot;nofollow noreferrer&quot;&gt;right Navcam pair&lt;/a&gt; taken at exactly the same time from exactly the same spot so, in time, there should also be &lt;a href=&quot;https://d2cj35nmzi9erd.cloudfront.net/mars3d/&quot; rel=&quot;nofollow noreferrer&quot;&gt;anaglyph 3D images&lt;/a&gt; available from the same area. Or, if you're skilled in image editing software, you could make your own:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/cz9Lp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mrg6F.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So put on your anaglyph 3D (red &amp;amp; blue) glasses and click on the image above for full resolution. Made out of already mentioned left and right stereo pair frames.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-06-03T20:22:55.160" LastActivityDate="2015-06-03T20:22:55.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9191" PostTypeId="1" CreationDate="2015-05-23T03:39:07.707" Score="6" ViewCount="261" Body="&lt;p&gt;Are there any technical reasons for not simply continuing the space shuttle production, except for replacing the orbiter with a pretty simple cargo bay (a payload fairing)? Like the Russians planned in the 1980's with their &lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot; rel=&quot;noreferrer&quot;&gt;Energia&lt;/a&gt;. It would seem like a cheap and immediately applicable thing to do, compared to the SLS concept which requires the expensive and decade long redesign of each and every component in this old system, such as the main engines, the boosters, the fuel tank, the launch pad, the mobile launch tower, the barge, the welder, the test stand and on and on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would immediately seem much cheaper, safer, faster and simpler. Redesigning reusable engines to become non-reusable doesn't sound like the future to me, but I'm maybe missing something essential here?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-05-23T03:50:37.027" LastActivityDate="2015-05-23T03:50:37.027" Title="Couldn't the shuttle orbiter be replaced with a cargo bay, instead of creating the SLS?" Tags="|sls|hllv|shuttle-derived|ares-v|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9194" PostTypeId="1" AcceptedAnswerId="9198" CreationDate="2015-05-23T19:02:29.380" Score="0" ViewCount="1250" Body="&lt;p&gt;I am trying to understand the global workings of attitude determination, specifically in space, but I don't understand why two reference frames are needed to establish that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say you have a CubeSat and you put a light sensor on every side. By knowing which side gets the most light you already know, more or less, how it is positioned by its observational frame of reference. Why is an additional inertial reference frame required then?&lt;/p&gt;&#xA;" OwnerUserId="10215" LastEditorUserId="49" LastEditDate="2015-05-23T19:18:01.463" LastActivityDate="2015-05-24T18:26:11.227" Title="Why are two frames of reference needed for attitude determination?" Tags="|artificial-satellite|attitude|cubesat|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9195" PostTypeId="1" CreationDate="2015-05-23T21:25:06.480" Score="13" ViewCount="2280" Body="&lt;p&gt;OK, What is it?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://spaceflightnow.com/wp-content/uploads/2015/05/Screen-Shot-2015-05-20-at-11.24.08-AM-copy-940x400.jpg&quot; alt=&quot;Closeup of ULA Atlas V 501 launch, May 20, 2015&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have not noticed it on previous launches. It's also visible in AFSPC-5 launch videos, for example in &lt;a href=&quot;https://www.youtube.com/watch?v=AeQbY4rkQJ8&amp;amp;t=109s&quot; rel=&quot;noreferrer&quot;&gt;this one&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/AeQbY4rkQJ8?start=109&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And it also comes into view of the camera immediately following the vehicle's pitch, yaw &amp;amp; roll maneuver in this &lt;a href=&quot;https://www.youtube.com/watch?v=U5wQ-7WY-UY&amp;amp;t=27s&quot; rel=&quot;noreferrer&quot;&gt;mission profile video&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/U5wQ-7WY-UY?start=27&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6214" LastEditorUserId="6944" LastEditDate="2019-01-01T22:23:01.643" LastActivityDate="2019-01-01T22:23:01.643" Title="What is the object protruding from the left side of the payload fairing during ULA Atlas V 501 AFSPC-5 launch?" Tags="|identify-this-object|launch-vehicle|ula|atlas-v|x-37|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9196" PostTypeId="2" ParentId="9195" CreationDate="2015-05-23T22:41:11.497" Score="13" Body="&lt;p&gt;That's a hydrogen vent fin disposing of GH&lt;sub&gt;2&lt;/sub&gt; (gaseous hydrogen) that can outgas into the payload fairing from the Centaur upper stage's LH&lt;sub&gt;2&lt;/sub&gt; (liquid hydrogen) tank that is stored in a &lt;em&gt;balloon tank&lt;/em&gt; at the top of the upper stage. From &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Upper_Stages/HistoryoftheTitanCentaurLaunchVehicle.pdf&quot; rel=&quot;noreferrer&quot;&gt;History of the Titan Centaur Launch Vehicle&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/BgYUC.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This liquid hydrogen leakage caused explosion of the launch vehicle during the first Atlas-Centaur launch in May 8, 1962. According to &lt;a href=&quot;http://history.nasa.gov/SP-4230.pdf&quot; rel=&quot;noreferrer&quot;&gt;Taming Liquid Hydrogen: The Centaur Upper Stage Rocket 1958-2002&lt;/a&gt; (PDF), &lt;em&gt;The Failure of F-1, the First Atlas-Centaur Launch&lt;/em&gt; (starting on page 63):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Testing at Lewis not only provided independent verification of&#xA;  contractor performance, but also contributed new solutions to&#xA;  problems. The loss of F-1 had occurred within a tenth of a second&#xA;  after hydrogen gas had been vented from an opening at the top of the&#xA;  vehicle. A test program in the 8-by-6-foot supersonic wind tunnel at&#xA;  Lewis revealed that the hydrogen-venting system posed a fire hazard&#xA;  during flight. &lt;strong&gt;The solution was to design a vent fin or snout on the&#xA;  nose fairing that extended about 50 inches from the tank, just far&#xA;  enough away to keep the hydrogen gas from igniting along the hot&#xA;  surface of the vehicle.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This GH&lt;sub&gt;2&lt;/sub&gt; vent fin is not unique to the 5 meter diameter payload fairing used during AFSPC-5 launch, for example, here's a &lt;a href=&quot;https://www.youtube.com/watch?v=FUJ9Dtyiclw&amp;amp;t=130s&quot; rel=&quot;noreferrer&quot;&gt;video of SBIRS GEO-2 launch&lt;/a&gt; on Atlas V 401 with a 4.2 m Ø fairing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/FUJ9Dtyiclw?start=130&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is shown in &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/AtlasVUsersGuide2010.pdf&quot; rel=&quot;noreferrer&quot;&gt;Atlas V Launch Services User's Guide&lt;/a&gt; (PDF) in the schematic for the 4.2 m Ø fairing (page 267):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/QckA3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And identified in text for the 5 m Ø fairing (page 283):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/meT3v.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;http://www.universetoday.com/83713/us-military-x-37b-rolls-out-to-atlas-launch-pad-poised-for-march-4-launch-photo-album/&quot; rel=&quot;noreferrer&quot;&gt;this Universe Today article&lt;/a&gt;, on the first photograph (I can't reproduce it here because it's copyrighted), you can see that vent fin connected with umbilical to the launch tower, where it would be purged with nitrogen provided through the air conditioning (A/C) inlet (seen on photograph as the topmost duct) and vented out the vent fin at the bottom of the Centaur upper stage's forward adapter, just above the CFLR (Centaur Forward Load Reactor) deck.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-03T21:54:58.153" LastActivityDate="2015-07-03T21:54:58.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9198" PostTypeId="2" ParentId="9194" CreationDate="2015-05-24T18:26:11.227" Score="3" Body="&lt;p&gt;Starting with the most basic concept...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on which type of rotation sequence you use, you need three angles to describe orientation (attitude) in space. I'll use the 3-2-1 body sequence (yaw-pitch-roll) since it's the easiest to visualize. (You can also select a 1-2-1 sequence to describe precession, nutation, and spin.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each angle implies a change in reference frame. The first angle, yaw, describes the body's heading. The second angle, pitch, then describes its elevation, or how high the &quot;nose&quot; is pointing (using aircraft terminology). Finally, roll describes the rotation about that body about the &quot;nose&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Continuing with the aircraft as an example, the first frame is the inertial reference frame. It's not really inertial because it's fixed to the surface of the Earth, but the forces acting on an aircraft vary far more due to aerodynamics than to changes in gravity, so we can consider it inertial. In the case of spacecraft, the selection of the inertial frame requires greater precision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Important: In your question, you ask why an additional inertial reference frame is required. 1) It's simpler to consider a single reference frame &quot;inertial&quot;. 2) The only way to have multiple inertial reference frames is if they are motionless with respect to each other. That means a second, third, etc. inertial reference frame is NOT required. 3) I'm assuming you mean to ask why an additional reference frame is required - one that is not stationary with respect to the inertial frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that we have defined what is and is not &quot;inertial&quot;, and determined that three angles are required to describe orientation, let's count the reference frames. Start by naming them:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;n - inertial&lt;/p&gt;&#xA;&#xA;&lt;p&gt;e - intermediate&lt;/p&gt;&#xA;&#xA;&lt;p&gt;f - intermediate&lt;/p&gt;&#xA;&#xA;&lt;p&gt;b - body&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yaw angle/heading is the angle between n and e about the aircraft's z axis (pointing down relative to pilot)&#xA;Pitch is the angle between e and f about the aircraft's y axis (pointing right relative to pilot)&#xA;Roll is angle between f and b about the aircraft's x axis (pointing forward relative to pilot)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Note: Rotation about z-y-x is where &quot;3-2-1&quot; comes from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In total, there are four reference frames, only one of which is inertial, and just to be pedantic, you have to keep in mind that it's not really inertial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Translating this to a CubeSat...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're going to use Euler angles, satellite orientations are  typically defined in 1-2-1 or 1-3-1 precession-nutation-spin sequences since they're designed for single missions and need to maintain communication and observe a specific target. I won't get into that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some practical considerations...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose your airplane is pointing straight up and rolls to the right. Using the definitions above, is that a roll to the right or a yaw to the left? When using Euler angles, there are some orientations where one rotation sequence will not properly define the orientation of the aircraft/spacecraft and the orientation after that will be unknown as well. This is known as gimbal lock; mathematically, it's a singularity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Computationally, quaternions are the standard method of calculating orientation and rates. They're based on calculating an axis of rotation and the angle about that axis of rotation. They aren't at all easy to visualize, nor are they intuitive, but you can always translate them to Euler angles, even after gimbal lock (but not during), so if you need to analyze data, you still can.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way you're thinking of measuring orientation is also very imprecise. Basically useless. The amount of light hitting the spacecraft varies so much depending on the time of day and the orbit that you would never actually know where you're facing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to learn the math: &lt;a href=&quot;http://www.swarthmore.edu/NatSci/mzucker1/e27/diebel2006attitude.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.swarthmore.edu/NatSci/mzucker1/e27/diebel2006attitude.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10217" LastActivityDate="2015-05-24T18:26:11.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9199" PostTypeId="2" ParentId="8771" CreationDate="2015-05-25T04:19:02.620" Score="30" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1wLk0.png&quot; alt=&quot;CoG, estimation, CAD &quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is my estimation of CoG of nearly empty Falcon 9 booster. On the left is GoG with legs opened, on the right with legs retracted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/w03Q1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landed vehicle is pretty stable. It could withstand the winds of 50 m/sec (97 knots!), if sliding is prevented. There are relatively narrow limits on horizontal component of landing speed as well as on rotational velocity at touchdown which are not unusual for vertical landing craft.&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-05-26T15:07:39.957" LastActivityDate="2015-05-26T15:07:39.957" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9201" PostTypeId="2" ParentId="8817" CreationDate="2015-05-25T05:31:32.850" Score="-3" Body="&lt;p&gt;The root cause identified as a propellant valve lag  is not an official report only a twit – a clear PR. Elon Musk is ingenious communicator he makes understandable explanations of everything. Form the other perspective the faulty hardware appear  unusually frequently in experimental vehicles of SpaceX having in mind lack of such problems in highly successful Falcon 9 (as expendable LV). So the narrative about some valve lag should not be taken very serious. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer would be: Yes, CRS-6 landing failed because of 'a conceptual flaw'. The lost of efficiency aerodynamic controls at low speed even is not root cause but contributed to repetitive problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, at terminal descent the efficiency any aerodynamic control surface diminish dramatically because of low airspeed (under 60 m/s they are inadequate). While aerodynamic controls had lost their efficiency, the aerodynamic disturbances caused by surface proximity – turbulence and wind gradient remain significant due the large platform area of the booster and  legs deployed (ref picture). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/I7Kpz.png&quot; alt=&quot;platform area of legs deployed &quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graphs of angular movements tracked by &lt;a href=&quot;http://www.wired.com/2015/04/analysis-falcon-9-crash-landing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rhett Allain&lt;/a&gt; from 13 April crash video shows a classic pattern of dynamically unstable system – an oscillations with exponentially increasing amplitude. A very similar picture may be reconstructed from foggy night video of the first barge hard landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sfqQt.png&quot; alt=&quot;flightpath reconstructed from the video of first barge hard landing &quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Main suspect&lt;/strong&gt;&#xA;The primary cause of two very similarly failures should be different. Tha main suspect is single ended attitude control at final approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Well-tempered rocket&lt;/strong&gt; &#xA;The rocket flight control by single point vectoring the trust of its main engine have been developed prior WW II mainly by Goddard and Von Braun. One of problems of such arrangement is challenging requirement to maintain two variables (rotation and translation) by single effector (vectored trust). The metaphor of balancing a stick (in the middle of a wind storm) is in fact a physical model of the system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/E8pP0.png&quot; alt=&quot;typical angular/lateral stabilization [from 1990 Russian textbook]&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;typical angular/lateral stabilization &lt;a href=&quot;https://books.google.bg/books?id=-64fAQAACAAJ&amp;amp;dq=%D0%A0%D0%B0%D0%B7%D1%8B%D0%B3%D1%80%D0%B0%D0%B5%D0%B2&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=Ki9UVe2-NITjUcvjgfgH&amp;amp;redir_esc=y&quot; rel=&quot;nofollow noreferrer&quot;&gt;[from 1990 Russian textbook]&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceptable solution was refined during the development of intercontinental missiles by applying of emerging those days &lt;a href=&quot;https://www.youtube.com/watch?v=JwqArbHinCg&amp;amp;noredirect=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;automatic control theory&lt;/a&gt; . The basic on idea is to separate control channels by its &lt;a href=&quot;http://en.wikibooks.org/wiki/Signals_and_Systems/Frequency_Response&quot; rel=&quot;nofollow noreferrer&quot;&gt;frequency response&lt;/a&gt;. The attitude stabilization has priority and guidance system reacts faster to correct the angular position (high frequency). The lateral movements are corrected relatively slowly (low frequency) adjusting the flight path during the ascent progress. The modern flight computers apply adaptive and predictive algorithms to adjust guidance response during the changing flight conditions. Nevertheless, the basic principle remains the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A typical frequency responses of large ‘well-tuned’ rocket are shown in table below (flying normally upward). Note the slow reaction on center of mass movements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Center of mass movements    0.01 -:- 0.03 Hz&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Angular movements    0.1  -:- 0.3 Hz&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fuel slosh   0.5  -:- 1.5 Hz&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Body elastic modes  2-:- 15 Hz&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Deadly vacillation&lt;/strong&gt;&#xA;The try to use the same trick flying the large rocket backward has limited success to the moment. It works mostly on light vehicles, out of the dense atmosphere, or flying slowly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast, of orbital launch, the pinpoint landing imposed a strict limitations on lateral drift. Thus, the lateral maneuvering became an ultimate priority and forced a faster response. Because of close frequency the lateral feedback interfere with the pitch/yaw channel. Figuratively speaking, the guidance computer is in vacillation between contradictive  priorities: attitude stability and horizontal deviation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vbQoS.jpg&quot; alt=&quot;orbital launch margins and land margins&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flying backward the rocket performed divert maneuver in a little pervert way - by purposely destabilizing the booster tilting it in desired direction and then erecting it back. That tricky maneuver inserts large swing what is hardly to dump by rocket optimized for very different flight profile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Parallel parking problem&lt;/strong&gt;&#xA;Another problem of the vehicle trying landing back by same means using during the ascent is reversing the position of  gimbaled trust controls. The controls become in front regarding flight detection during the descent. Steering characteristics are changed dramatically this way. Even fully predictable movement of a car is les versatile (and more stable) driving forward with front steering wheels. A parallel parking driving forward is simple analogy of what vertical takeoff vertical landing rocket is trying to do. The top of tall cylindrical body lag the guidance intentions due the aerodynamic dumping. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Back to sixties&lt;/strong&gt; &#xA;Contemporary launch vehicles are strictly optimized for axial stress at vertical launch. Although any attempt to use a system developed and refined for specific purpose (vertical ascent) for completely different one (vertical descent) will certainly confront once more the basic problems already solved during primary use development. To enable for pinpoint vertical landing the rockets designed to fly against target or to launch a spacecrafts will need to rethink and possibly re-solve the basic problems of propulsive flight to meet the new emerged requirements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: 1.&#xA;An article on G-FOLD project. You can find how the single ended control works on practice.&#xA;&lt;a href=&quot;https://behcet.ae.utexas.edu/sites/default/files/gfold_aas_13.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;FLIGHT TESTING OF TRAJECTORIES COMPUTED BY G-FOLD:&#xA;FUEL OPTIMAL LARGE DIVERT GUIDANCE ALGORITHM FOR&#xA;PLANETARY LANDING&#xA;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-06-03T03:32:24.213" LastActivityDate="2015-06-03T03:32:24.213" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9202" PostTypeId="2" ParentId="7949" CreationDate="2015-05-25T05:59:11.230" Score="2" Body="&lt;p&gt;Rockets landing back vertically by series of retro burns are critical point in SpaceX reusability program. Even more, their human spaceflight program is based on concept pinpoint powered decent. A number of other companies work on propulsive atmospheric return. All they use for terminal descent the same controls used in ascent –vectored trust, usually by a gimbaled engine. This arrangement was very popular in the old Sci-Fi movies – the landing is simply ascent in reverse motion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The practical reason to follow such intuitive idea is the attractive opportunity to recover the existing rockets with minimum modifications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX leadership was confident that reliable recovering the rocket boosters is a step away after demonstration of controllable hovering and descent. In practice, it has proved more difficult. The test flights of the reusable booster in more realistic environment - reentering from suborbital trajectory disclosed a number of problems. Significant modifications of initial concept have been made but the success remains elusive. &#xA;&lt;img src=&quot;https://i.stack.imgur.com/RnneN.png&quot; alt=&quot;from Defa 1960 Sci-Fi move&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from Defa 1960 Sci-Fi move&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Contemporary launch vehicles are strictly optimized for axial stress at vertical launch. Although any attempt to use a system developed and refined for specific purpose (vertical ascent) for completely different one (vertical descent) will certainly confront once more the basic problems already solved during primary use development. To enable for pinpoint vertical landing the rockets designed to fly against target or to launch a spacecrafts will need to rethink and possibly re-solve the basic problems of propulsive flight to meet the new emerged requirements. &lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-05-25T06:30:11.153" LastActivityDate="2015-05-25T06:30:11.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9204" PostTypeId="2" ParentId="8804" CreationDate="2015-05-25T14:13:55.157" Score="-1" Body="&lt;p&gt;It is hard to judge what the rocket trying to do (to impute behavior to machine is may be to early). Anyway, the general  impression of majority of people commenting these videos is that flight computer is confused. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cold gas thrusters are used &lt;em&gt;only&lt;/em&gt; for roll control &lt;em&gt;at atmospheric decent phase&lt;/em&gt; so they should works in pairs. Cold gas thrusters (RCS in your question)are too week even to try to correct excess of angular velocity that killed the booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When gambled engine solely is used for attitude control the control forces are applied in single point. This way, the pitch/yawing control moments are coupled with uncompensated lateral force. Consequently, any attempt to compensate significant moments introduced by cross winds or wind shear inserts lateral force and deviation from target path. Imposed sideway velocities induce back aerodynamic pitch/yow moments. On the other hand, any divert move performed by single-ended attitude control is possible only by tricky tilt and set upright maneuver. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Evidently, on certain point, that highly nonlinear feedback becomes pathological and triggered a Stability Loss and hardover beyond recoverability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such deadly vacillation is caused of close frequency response of the lateral feedback interfere with the pitch/yaw channel. Figuratively speaking, the guidance computer is in vacillation between contradictive  priorities: attitude stability and horizontal deviation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EM9h8.jpg&quot; alt=&quot;cold gas nozzles&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust of cold gas thrusters may be estimated by their openings but even without calculations one may decide is it possible such two inches nitrogen nozzles may turn back 20 ton, 42 meter long vehicle in seconds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrusters was activated most likely by axial twist after violent contact of legs with platform.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CoMN0.png&quot; alt=&quot;roll thrusters&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-06-06T18:55:30.470" LastActivityDate="2015-06-06T18:55:30.470" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9205" PostTypeId="2" ParentId="8781" CreationDate="2015-05-25T14:36:37.993" Score="2" Body="&lt;p&gt;Tthe flight termination system is usually a liner relatively small explosive which rupture the propellant tanks. To explode it need to be initiated by detonators. What explodes in filed rockets are usually pressure vessels which includes pressurized propellant tanks.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, the explosive contained in FTS can not make spectacular fireball by its oun. &lt;/p&gt;&#xA;" OwnerUserId="10218" LastActivityDate="2015-05-25T14:36:37.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9206" PostTypeId="2" ParentId="7949" CreationDate="2015-05-25T15:45:50.697" Score="1" Body="&lt;p&gt;The argument between horizontal and vertical landing for reusable vehicles was conducted between noted space historian Henry Spencer and Mitchell Burnside Clapp who worked on both the DC-X and X-33 programs:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://yarchive.net/space/launchers/horizontal_vs_vertical_landing.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://yarchive.net/space/launchers/horizontal_vs_vertical_landing.html&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most likely Musk agrees with the advantages of vertical landing because of the minimal additional modifications to the rocket needed. &lt;/p&gt;&#xA;" OwnerDisplayName="user3544" LastEditorUserId="12778" LastEditDate="2018-12-27T05:21:43.603" LastActivityDate="2018-12-27T05:21:43.603" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="9208" PostTypeId="2" ParentId="8797" CreationDate="2015-05-26T16:09:53.503" Score="14" Body="&lt;p&gt;Human powered vehicles used within domed or cavern cities would seem to be extremely plausible. One way to look at it is a simple bicycle is less complex than a powered vehicle. The power source (aka a Human) is extremely complex and maintenance intensive, but given that we already have a functioning human and want to get that human to some other place, a bicycle or scooter is a fine way to do so at minimal cost. Of course, the city needs to be large enough to justify not simply walking.&#xA;Humans always need some exercise, and so within a biodome using a human powered vehicle can contribute to some of the exercise requirements of a human. In that sense, it is free.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside of an artificial biosphere, in the hard vacuum of space, or under a thin atmosphere we run into more problems. All things being equal, a life support system would sustain a human on a powered vehicle much longer than it would a human-powered vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had a little trouble finding exact numbers, but &lt;a href=&quot;http://www.arb.ca.gov/research/resnotes/notes/94-11.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; is one study which shows how much an adult human breathes per minute:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/lN6uw.gif&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A human simply sitting in a vehicle breathes 9-10L/minute, while doing somewhat strenuous exercise they will be breathing at least 30L/minute. Life support has to work at least 3 times harder - up to 6 times harder. What this means is that every minute spent riding a bike, would be a minute less work which could be done - or 2-5 minutes less work if the work mainly involves standing around. Or if travelling somewhere, the maximum range is halved or worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an obvious solution - upsize the life support system. This has an equally obvious problem - the human has to carry the life support system under their own power and thus work even harder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think we've established that humans become a lot harder to maintain once they are huffing and puffing, putting aside the oxygen problem, the question is, how does the efficiency compare with electric motors? Again it wasn't too easy to find clear answers to this, but one could look at this article: &lt;a href=&quot;http://physics.ucsd.edu/do-the-math/2011/11/mpg-of-a-human/&quot; rel=&quot;noreferrer&quot;&gt;http://physics.ucsd.edu/do-the-math/2011/11/mpg-of-a-human/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That article and other sources assert a human is around 25% efficient at converting food energy into kinetic energy (i.e. on a bicycle). This compares unfavorably with an internal combustion engine, at around 40% efficient, and very unfavorably with an electric motor, at around 90% efficiency. In this case, the lost energy becomes heat. A human outputting 250 watts of power, also generates 750 watts of heat. An electric motor would generate about 30 watts of heat. Unfortunately for the human, all that waste heat is something life support has to get rid of, through radiators (heavy) and/or active cooling (energy intensive).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now lets look at charging the battery. Solar panels are about 30% efficient at converting sunlight into electricity - it can be higher or lower, but 30% is a fair average assuming the environment is a near vacuum. Charging batteries can be extremely efficient - 80-90% for lithium batteries. So of the captured sunlight, 30% x 90% x 90% = 24% becomes mechanical energy driving the motor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And lets look at charging the human. The wikipedia article on &lt;a href=&quot;http://en.wikipedia.org/wiki/Photosynthetic_efficiency&quot; rel=&quot;noreferrer&quot;&gt;Photosynthetic efficiency&lt;/a&gt; is pretty good. To summarize, most crop plants are 0.25-0.5% efficient at creating food energy. That would give the human an abysmal 0.5% * 25% = 0.125% efficiency. Even with really efficient algae or something we'd still be struggling to break 5% efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's looking really bad for the human, but to add insult to injury the solar panels can be just plonked outside in hard vacuum. The plants need a whole life support system of their own, water, atmosphere, temperature, fertilizers - all these things need to be regulated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TL;DR A human powered vehicle is far less efficient than an electric vehicle: From sunlight to kinetic energy, only about 1% as efficient. Human powered vehicles might be justified within a biodome as humans need exercise anyway. But in a vacuum or hostile atmosphere the excessive life support requirements for a human doing strenuous exercise would make human powered vehicles much less practical than electric vehicles.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-05-26T16:09:53.503" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9209" PostTypeId="1" AcceptedAnswerId="9210" CreationDate="2015-05-27T02:49:26.910" Score="14" ViewCount="2561" Body="&lt;p&gt;I'm asking for historical scientific knowledge of our understanding of the outer planets, which seems hard to come by.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pioneer 10 was the first probe to an outer planet (Jupiter). Before then, &lt;strong&gt;did we believe that the outer planets were rocky with huge atmospheres?&lt;/strong&gt; This would seem to be the default position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or did we have good reason to believe they were truly gaseous planets? If so, why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: as an edition, can anyone find the best photos we had of Jupiter before Pioneer 10? Pre-probe photos of the other gas planets would be good too.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2015-05-28T01:13:39.880" LastActivityDate="2015-07-02T23:06:02.640" Title="Did we know the gas planets were truly gaseous before Pioneer 10?" Tags="|probe|planetary-science|history|gas-giant|" AnswerCount="3" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9210" PostTypeId="2" ParentId="9209" CreationDate="2015-05-27T03:44:36.500" Score="15" Body="&lt;p&gt;My 1965-vintage copy of &lt;em&gt;Sourcebook On The Space Sciences&lt;/em&gt; has this to say:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The model developed by DeMarcus in 1958 is regarded at present as the most acceptable for the interior of Jupiter. It postulates that the planet consists of 78% by weight of hydrogen molecules and the remainder helium ... calculations indicate that at a distance of about 0.8 times the planet's radius from the center, the pressure becomes so high ... that molecular hydrogen turns into a metallic form ... most of the internal volume (and mass) of Jupiter would thus appear to consist of this solid hydrogen ... it should be made clear that the model just described is used largely as a basis for calculation and is not meant to represent the exact condition of Jupiter's interior.&lt;/p&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;p&gt;Nothing is known about the nature of the surface of Jupiter, but it is probably a strange and complex system. Although much of the interior of the planet may be solid hydrogen (and helium), the pressures in the outer parts are too low and the temperatures probably too high to permit hydrogen to solidify. Hence, there may be some liquid hydrogen (and possibly methane) near the surface of Jupiter. Between this and the atmosphere, it is expected that there will be a transition layer containing solid or liquid ammonia (or both), methane, and possibly ice. The foregoing is admittedly speculation, since the temperature distributions in the atmosphere and interior of the planet are quite unknown.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This seems to agree to some extent with the modern picture of &lt;a href=&quot;http://en.wikipedia.org/wiki/Jupiter#Internal_structure&quot; rel=&quot;noreferrer&quot;&gt;Jupiter's structure&lt;/a&gt; as given by Wikipedia:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen ... The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths ... The presence of a core during at least part of Jupiter's history is suggested by models of planetary formation that require the formation of a rocky or icy core massive enough to collect its bulk of hydrogen and helium from the protosolar nebula ... A core may now be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely ... The core region is surrounded by dense metallic hydrogen, which extends outward to about 78% of the radius of the planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That is to say, both then and now, the belief is that Jupiter and the other giant planets have dense, non-gaseous interiors.&lt;/p&gt;&#xA;&lt;p&gt;Since the 1960s, the understanding of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Metallic_hydrogen&quot; rel=&quot;noreferrer&quot;&gt;behavior of hydrogen under extremely high pressure&lt;/a&gt; has changed, and it's believed that the bulk of Jupiter's and Saturn's volumes consist of metallic liquid hydrogen.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-05-27T16:52:34.557" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9211" PostTypeId="1" CreationDate="2015-05-27T03:55:22.120" Score="19" ViewCount="5739" Body="&lt;p&gt;So this may be a stupid question but I was wondering why we couldn't launch from space. I know that most of the fuel that we spend is for escaping Earth gravity. Yet, if we have already done this with the International Space Station, couldn't we bring parts up there. Then we could assemble a low power rocket there. Then we would only need a small unbalanced force to change the object's inertia. There would be no force that the rocket would have to overcome in space. This means that the object will continue in that direction, which means it would be so much easier to just get farther into space with less fuel. To bring the parts up there in the first place, they can  be slowly brought up there overtime, just like they did when they first made the space station. It seems like we could save so much fuel if we do this. Yet, I might be completely wrong. Please correct me if I am, since I'm only in the 7th grade, so I haven't taken any heavy physics or astronomy.&#xA;Thanks! &lt;/p&gt;&#xA;" OwnerUserId="10225" LastEditorUserId="49" LastEditDate="2015-05-27T06:11:23.357" LastActivityDate="2020-01-07T17:30:28.563" Title="Why can't we launch from space?" Tags="|launch|orbit|physics|mass|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9212" PostTypeId="2" ParentId="9211" CreationDate="2015-05-27T05:59:20.787" Score="24" Body="&lt;p&gt;We can &lt;em&gt;launch from space&lt;/em&gt; and in a sense, we already are. If you consider upper stages of orbital launch vehicles that might send spacecraft into higher Earth orbit or even escape Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_well&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity well&lt;/a&gt; altogether, we really ignite those when they're already in the vacuum of space. And depending on the launch vehicle's capability and intended trajectory, they might, or might not have already achieved orbital velocity. But your question, I guess, boils down to what we're usually referring to as &lt;em&gt;mass economy&lt;/em&gt;;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You see, gravitational acceleration at the orbital altitude that the International Space Station orbits the Earth (what we call Low Earth Orbit, or LEO for short) is still roughly &lt;a href=&quot;https://encrypted.google.com/#q=%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%28%286378.1+km+%2B+412.5+km%29%5E2&quot; rel=&quot;nofollow noreferrer&quot;&gt;8.64 m/s²&lt;/a&gt; (or ~ 88% of gravitational pull that keeps us on the ground at the surface), and for orbiting bodies of negligible mass compared to the parent body they orbit given by &lt;span class=&quot;math-container&quot;&gt;$\mu/r^2$&lt;/span&gt; where &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;standard gravitational parameter&lt;/a&gt; &lt;span class=&quot;math-container&quot;&gt;$\mu$&lt;/span&gt; is &lt;span class=&quot;math-container&quot;&gt;$G\cdot M$&lt;/span&gt;, or the product of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational constant&lt;/a&gt; &lt;span class=&quot;math-container&quot;&gt;$G$&lt;/span&gt; and mass &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; of the central body. To keep something from &lt;em&gt;falling back down to Earth&lt;/em&gt; once we launch it, we really have just two options:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Launch in a direct interplanetary trajectory with a highly capable launch vehicle that will at some altitude achieve Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;escape velocity&lt;/a&gt;, given by &lt;span class=&quot;math-container&quot;&gt;$\sqrt{2\mu/r}$&lt;/span&gt;. In simpler terms, we could accelerate the craft to a speed and beyond that would be sufficient to ensure that the craft is never coming back to Earth and escaping its gravity well, despite its gravitational pull still persisting high above it and decreasing with an &lt;a href=&quot;https://en.wikipedia.org/wiki/Inverse-square_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;inverse square&lt;/a&gt; of the distance to it. At Earth's surface, escape velocity is roughly &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%282*%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%286378.1+km%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;11.18 km/s&lt;/a&gt;, and at LEO altitudes still about &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%282*%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%286378.1+km+%2B+412.5+km%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;10.83 km/s&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Launch into an intermediary Earth orbit, where we accelerate the vehicle radially (around the Earth) at such horizontal velocity that it &lt;em&gt;perpetually misses the Earth&lt;/em&gt; in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Free_fall&quot; rel=&quot;nofollow noreferrer&quot;&gt;free fall&lt;/a&gt; around it, despite its gravitational pull. For orbital altitude of ISS, &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital speed&lt;/a&gt; is about &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%286378.1+km+%2B+412.5+km%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;7.66 km/s&lt;/a&gt; and given by &lt;span class=&quot;math-container&quot;&gt;$\sqrt{\mu/r}$&lt;/span&gt;, but in simple terms, with orbits, we need to reach the speed at which &lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifugal_force&quot; rel=&quot;nofollow noreferrer&quot;&gt;centrifugal force&lt;/a&gt; (away from the focus, or center, of the orbit) exactly equals &lt;a href=&quot;https://en.wikipedia.org/wiki/Centripetal_force&quot; rel=&quot;nofollow noreferrer&quot;&gt;centripetal force&lt;/a&gt; (towards the focus), where centripetal force is gravity at that altitude and I've already given you the number for LEO (and in the link the way to calculate it for any altitude). If you calculate centrifugal acceleration at ISS orbital period (~ &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28412.5+km+%2B+6378.1+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;1.55 hours&lt;/a&gt; or &lt;a href=&quot;https://encrypted.google.com/#q=1+minute+%2F+sqrt%28%28412.5+km+%2B+6378.1+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.0107729967 rpm&lt;/a&gt;), altitude (~ 412.5 km on average since &lt;em&gt;assembly complete&lt;/em&gt;) and radius (~ 6,790.6 km from its focus), perhaps with some online centrifugal force calculator like &lt;a href=&quot;http://www.calctool.org/CALC/phys/newtonian/centrifugal&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt;, you'll notice that it precisely matches 8.64 m/s² given before, but it would have an opposite sign (i.e. as a vector, it would point in the other direction) to the pull of gravity.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To reach these immense speeds needed to orbit or escape any &lt;em&gt;payload mass&lt;/em&gt;, we need to either gradually spend what we call &lt;em&gt;reaction mass&lt;/em&gt; (for chemical rockets that would be their propellants) to push the vehicle forward by accelerating its reaction mass backwards by the principle of &lt;em&gt;equal and opposite reaction&lt;/em&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's third law of motion&lt;/a&gt;, and/or accelerate the payload to sufficient speed already on the Earth's surface before it's released to space. But, the latter - i.e. &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_driver&quot; rel=&quot;nofollow noreferrer&quot;&gt;mass drivers&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_gun&quot; rel=&quot;nofollow noreferrer&quot;&gt;space guns&lt;/a&gt; - come with a huge problem that initially, they still need to move through dense Earth's atmosphere, which will introduce tremendous &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_%28physics%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;air resistance&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Friction&quot; rel=&quot;nofollow noreferrer&quot;&gt;friction heating&lt;/a&gt;, and form strong &lt;a href=&quot;https://en.wikipedia.org/wiki/Shock_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;shock waves&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Eddy_%28fluid_dynamics%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;eddies&lt;/a&gt; that could destabilize them and cause them to disintegrate. So you can see that the latter option, in atmospheres, is really difficult to achieve without having to spend most of the mass on the vehicle's or payload's shielding, and that there would be a point in trying to minimize that and maximizing &lt;em&gt;payload mass fraction&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, we can't really &lt;em&gt;launch from space&lt;/em&gt; without first having something to launch, and something to launch it with already there. And that's where that &lt;em&gt;mass economy&lt;/em&gt; term kicks in. To move mass from Earth to Earth's orbit, we have to spend mass to get there. A lot of it, and it's so frustrating that it's often referred to as &lt;em&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;the tyranny of the rocket equation&lt;/a&gt;&lt;/em&gt;, which refers to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt;, performance of high thrust capable propellants, and difficulties of lofting something towards space even with rocket staging, where rockets would in stages &lt;em&gt;shed dead weight&lt;/em&gt; of stages that are already exhausted of propellants as the rocket ascends.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if we had both something to launch and something to launch it with already in Earth's orbit, then yes, it would be tremendously easier. ISS actually does have two relatively simple &lt;em&gt;launch catapults&lt;/em&gt;, one installed on the Japanese &lt;a href=&quot;https://en.wikipedia.org/wiki/Kibo_%28ISS_module%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kibo module&lt;/a&gt; and called Small Satellite Orbital Deployer (SSOD), and another NanoRacks built &lt;a href=&quot;https://en.wikipedia.org/wiki/NanoRacks_CubeSat_Deployer&quot; rel=&quot;nofollow noreferrer&quot;&gt;CubeSat Deployer&lt;/a&gt;, that both can launch small and light &lt;a href=&quot;https://en.wikipedia.org/wiki/CubeSat&quot; rel=&quot;nofollow noreferrer&quot;&gt;CubeSat&lt;/a&gt; satellites into similar to ISS's orbits:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/lG4ui.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;A set of NanoRacks CubeSats deployed by the CubeSat Deployer at the end of the Japanese robotic arm. Image: NASA&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But these CubeSats already have to be delivered to the station, and they come on &lt;em&gt;conventional&lt;/em&gt; chemical rockets. And the acceleration these satellites get relative to the station from SSDO is tiny, so they can only be deployed to similar orbital altitude to station's own. For anything more, energy needed is simply too high, and payloads would still have to have their own means of propulsion, and with it increase in mass, to later circularize their orbital trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And Space Shuttle Orbiters also deployed and launched satellites, some with their own upper stages and would depart LEO and head in an interplanetary trajectory towards other planets, like for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo spacecraft&lt;/a&gt; that was carried into LEO and launched from Space Shuttle Atlantis &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-34&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-34&lt;/a&gt; flight in October 18, 1989:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/nThMx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;The Galileo spacecraft and its &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_Upper_Stage&quot; rel=&quot;nofollow noreferrer&quot;&gt;Inertial Upper Stage&lt;/a&gt; (IUS) being deployed from the Space Shuttle Atlantis. Image: NASA&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there's a lot more to be said about this, so I'll slowly try to wrap it up;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been many proposals to establish &lt;a href=&quot;https://en.wikipedia.org/wiki/Propellant_depot&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital propellant depots&lt;/a&gt; and perhaps even deliver reaction mass to them from &lt;em&gt;gravitationally less harsh mistresses&lt;/em&gt; like the Moon. Required change in velocity (delta-v) to launch from the Moon to LEO is &lt;em&gt;only&lt;/em&gt; &lt;a href=&quot;https://space.stackexchange.com/questions/4668/could-lunar-mining-be-used-to-service-low-earth-orbit/4669#4669&quot;&gt;about 1.87 km/s&lt;/a&gt; (without orbital circularization), and if we compare that with about &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space.E2.80.94high_thrust&quot; rel=&quot;nofollow noreferrer&quot;&gt;9.3 - 10 km/s&lt;/a&gt; needed to launch to LEO from Earth's surface, there really could be huge mass savings in it. But that comes with its own problems, and, perhaps more importantly, we currently don't have any infrastructure there to support this and it will take many &lt;em&gt;conventional launches&lt;/em&gt; to get to that point and we'll likely still build our satellites on Earth in the foreseeable future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, as a conclusion, it really boils down to where resources are and how much effort is needed to place them where we want them to be. Sadly, from the delta-v budget's perspective, strategically good staging areas in open space that would enable easy (or easier) reach of various destinations we'd want to go to, are in &lt;em&gt;the big void&lt;/em&gt; that's devoid of resources (there's roughly only 5 proton particles per cubic centimeter in the cislunar space) unless we place them there, so they need to be delivered there by some means. Or, can somehow do without additional resources needed and use what's already there (i.e. solar power and use that either directly with &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar sails&lt;/a&gt;, or convert it to electricity and use for already mentioned but this time orbital mass drivers and launch rings, &lt;a href=&quot;https://en.wikipedia.org/wiki/Beam-powered_propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;beam-powered propulsion&lt;/a&gt;, or all kinds of &lt;em&gt;fantastic&lt;/em&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Spacecraft_propulsion#Table_of_methods&quot; rel=&quot;nofollow noreferrer&quot;&gt;hypothetical propulsion methods&lt;/a&gt; that we've envisioned but are simply not there yet). So, for the time being, this means that resources needed to complete a launch in space, or also parts to assemble launch systems, are initially launched from the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But orbital rendezvous, docking and even assembly itself are certainly feasible and the ISS is one good example of that, though there were &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_space_stations&quot; rel=&quot;nofollow noreferrer&quot;&gt;earlier space stations&lt;/a&gt;, some of which were also assembled in orbit out of multiple prefabricated modules. As for actually going anywhere else than Earth's orbit by assembly in space, well, Apollo missions to the Moon used the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transposition%2C_docking%2C_and_extraction&quot; rel=&quot;nofollow noreferrer&quot;&gt;transposition, docking and extraction&lt;/a&gt; maneuvers and &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit_rendezvous&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar orbit rendezvous&lt;/a&gt;. And in the not too distant future, we expect the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;SLS&lt;/a&gt; (Space launch System) missions to do similar things, to launch crew and service modules on one launch vehicle, and additional advanced propulsion, habitat and landing modules on additional &lt;em&gt;cargo block&lt;/em&gt; launch vehicles and later dock in Earth's orbit and, hopefully, one day, also around Mars.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For further reference, since you mention you're still in your early studying years, I can't recommend warmly enough &lt;a href=&quot;https://solarsystem.nasa.gov/basics/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Basics of Space Flight&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;Basics of Space Flight&lt;/em&gt; is a tutorial designed primarily to help&#xA;  operations people identify the range of concepts associated with deep&#xA;  space missions, and grasp the relationships among them. It also enjoys&#xA;  popularity with college and high-school students and faculty, and&#xA;  people everywhere who are interested in interplanetary space flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="2741" LastEditDate="2020-01-07T01:56:43.593" LastActivityDate="2020-01-07T01:56:43.593" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="9213" PostTypeId="1" CreationDate="2015-05-27T08:45:06.630" Score="3" ViewCount="1170" Body="&lt;p&gt;There are &lt;a href=&quot;http://www.nasa.gov/centers/marshall/news/news/releases/2012/12-058_prt.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA reports&lt;/a&gt; on Falcon 9 runs in transonic in wind tunnel.&#xA;Tests were conducted at several orientations and speeds ranging from Mach numbers 0.3, or 228 miles per hour at sea level, to Mach 5, or 3,811 miles per hour at sea level, to gage how the first stage reacts during the descent phase of flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final retroburn began at ~250 m/s. A flight instability is observed at airspeed &amp;lt; 100 m/s. The legs opened possibly contributed to said instability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/9z9jA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;Does anybody have more information about low speed aerodynamic studies on F9R.&lt;/p&gt;&#xA;" OwnerUserId="10218" LastActivityDate="2015-05-27T12:09:36.013" Title="Does SpaceX performed low subsonic speed wind-tunnel runs of Falcon 9 booster in landing configuration and with legs eventually opened?" Tags="|spacex|landing|falcon-9|aerobraking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9214" PostTypeId="2" ParentId="9213" CreationDate="2015-05-27T08:52:18.087" Score="2" Body="&lt;p&gt;Yes, see the quote below. And to complement those wind tunnel tests, SpaceX also uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Grasshopper_%28rocket%29#Falcon_9_Reusable_Development_Vehicle_testing&quot; rel=&quot;nofollow&quot;&gt;F9R Dev&lt;/a&gt; (formerly known as Grasshopper) test vehicles to test their VTVL (vertical takeoff, vertical landing) technology in the subsonic regime. Here's &lt;a href=&quot;https://www.youtube.com/watch?v=ZwwS4YOTbbw&quot; rel=&quot;nofollow&quot;&gt;one video&lt;/a&gt; (out of many) how that looks like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ZwwS4YOTbbw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This test (&lt;a href=&quot;https://www.youtube.com/watch?v=DgLBIdVg3EM&quot; rel=&quot;nofollow&quot;&gt;F9R 1000m Fin Flight&lt;/a&gt;) included all aerodynamic control surfaces that a Falcon 9 first stage would use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, for deceleration to subsonic regime, which these F9R Dev test flights don't yet include, SpaceX is being helped by NASA and there have indeed been wind tunnel tests (176 runs) done in NASA's Marshall Space Flight Center. According to this article on &lt;a href=&quot;http://phys.org/news/2012-05-nasa-tunnel-aid-spacex-reusable.html&quot; rel=&quot;nofollow&quot;&gt;Physics.org&lt;/a&gt; from May 2012 :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Under a Reimbursable Space Act Agreement, Marshall conducted 176 runs&#xA;  in the wind tunnel test facility on the Falcon 9 first stage to&#xA;  provide SpaceX with test data that will be used to develop a re-entry&#xA;  database for the recovery of the Falcon 9 first stage. Tests were&#xA;  conducted at several orientations and speeds ranging from Mach numbers&#xA;  0.3, or 228 miles per hour at sea level, to Mach 5, or 3,811 miles per hour at sea level, to gauge how the first stage reacts during the&#xA;  descent phase of flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To &lt;em&gt;pay it forward&lt;/em&gt; a bit, NASA also does such test of VTVL and ALHAT (Autonomous Landing Hazard Avoidance Technology) technologies with test flights, recently for example with &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Morpheus&quot; rel=&quot;nofollow&quot;&gt;Project Morpheus&lt;/a&gt; that is interestingly explained by Jon Olansen (Manager, MORPHEUS Project, NASA JSC) in &lt;a href=&quot;http://livestream.com/viewnow/exploremars2015/videos/86362069&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt; from ExploreMars' 2015 Humans 2 Mars conference, roughly 23:30 into it. In that same video, about 21 minutes into it, Michelle Munk (Principal Investigator for EDL, NASA LaRC) also describes NASA-SpaceX cooperation on Supersonic Retropropulsion (SRP) and what NASA is getting out of it, not only the other way around. And about 54 minutes into it, Josh Brost (Account Manager, SpaceX) explains a bit more of what SpaceX is currently doing, too.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-05-27T12:09:36.013" LastActivityDate="2015-05-27T12:09:36.013" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9217" PostTypeId="1" CreationDate="2015-05-27T13:46:33.780" Score="6" ViewCount="509" Body="&lt;p&gt;Does someone knows why H-IIB rocket can stay fuelled on launchpad for many minutes, just venting excess gaseous H2 and O2, while Ariane 5 must be connected to umbilicals almost to the very last moment before lift-off?&lt;/p&gt;&#xA;" OwnerUserId="8761" LastActivityDate="2015-09-22T15:44:30.973" Title="Technology for LH2 &amp; LOX umbilicals for rockets" Tags="|launch|rockets|artificial-satellite|spacecraft|hydrogen|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9218" PostTypeId="2" ParentId="9217" CreationDate="2015-05-27T13:57:39.337" Score="2" Body="&lt;p&gt;Payload margins most likely. The reason to stay connected is to stay topped up to the last second, to have the most available fuel for the mission.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;HII possibly has sufficient margin for the flight even with venting.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-27T13:57:39.337" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9219" PostTypeId="1" CreationDate="2015-05-27T14:13:27.717" Score="3" ViewCount="399" Body="&lt;p&gt;I noticed that many rockets come out of the assembly building already with their fuelling umbilicals attached (I mean, with the entire umbilical tower - like Saturn 5, Ariane 5). Once in the launchpad, the fuelling of cryo propellants starts. My question is if there is feasible to attach manually these fuelling lines on the pad (ie, the rocket goes alone to launch table (like CZ-2FG, Soyuz, Zenit, etc) and only then the LH2 lines are attached. (CZ-2FG, Soyuz, Zenit doesnt uses LH2 - and LH2 fuelling is my main concern).&#xA;Is there any safety issues for ground crews to operate attaching procedures for the LH2 fuelling lines on the pad, often in open air?&lt;/p&gt;&#xA;" OwnerUserId="8761" LastActivityDate="2015-05-28T13:39:29.603" Title="Fuelling LH2/LOX conectors for Rockets - is it easy to manually attach to the intake nozzles?" Tags="|launch|rockets|spacecraft|crewed-spaceflight|hydrogen|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9220" PostTypeId="2" ParentId="9219" CreationDate="2015-05-27T17:12:04.613" Score="6" Body="&lt;p&gt;For the SpaceX Falcon 9 the fuel lines are &lt;a href=&quot;https://www.youtube.com/watch?feature=player_detailpage&amp;amp;v=TQ6tZtGrShg#t=505&quot;&gt;12.5 cm/5&quot;&lt;/a&gt; (link: youtube video, SpaceX factory tour by Elon Musk) in diameter, so those are pretty big hoses to attach manually. LH2 is under pressure, making for a heavier hose. So you probably need hoisting tools to get the hose into place.&lt;br&gt;&#xA;The attachment points for upper stages are fairly high up, meaning you'd need an access bridge at that point, and stairs or a lift to get up to them. These (and hoisting tools) are available in the assembly building. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you wanted to add these items to the launch tower, you'd have to protect them from the extreme heat of the launch. The Ariane launch tower for example is a simple column. Adding all this stuff would make it more complicated for no apparent benefit. Why would you want to leave attaching the umbilicals until the rocket is at the launch platform? &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-05-27T17:12:04.613" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9222" PostTypeId="2" ParentId="22" CreationDate="2015-05-28T06:18:01.717" Score="2" Body="&lt;p&gt;In the Bogota Declaration, several equatorial countries laid claim to geostationary orbit (which lies above the equator).  They carefully (if not carefully enough) called this a &lt;em&gt;resource&lt;/em&gt; rather than &lt;em&gt;territory&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.m.wikipedia.org/wiki/Bogota_Declaration#Declaration_of_the_First_Meeting_of_Equatorial_Countries&quot; rel=&quot;nofollow&quot;&gt;http://en.m.wikipedia.org/wiki/Bogota_Declaration#Declaration_of_the_First_Meeting_of_Equatorial_Countries&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This claim has been ignored by those nations and organizations that put satellites into geostationary orbit and by the international organization that allocates satellite positions within geostationary orbit.&lt;/p&gt;&#xA;" OwnerUserId="10238" LastActivityDate="2015-05-28T06:18:01.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9223" PostTypeId="1" AcceptedAnswerId="9225" CreationDate="2015-05-28T09:58:24.290" Score="7" ViewCount="1639" Body="&lt;p&gt;SpaceX has the Dragon 1 (Cargo) and Dragon 2 (Crew). It unveiled Dragon 2 about 1 year ago (30 May 2014). Dragon 2 is a radical change over Dragon 1 with many parts that are untested.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would think that the best way to test Dragon 2 for human flight would be to start using it for the cargo missions. There are a lot of aspects about Dragon 2 that could be tested in cargo missions that would be useful to ensure that the design is optimal. Some things I would think could be tested include:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Aerodynamics and heat shield for re-entry&lt;/li&gt;&#xA;&lt;li&gt;Parachute landings&lt;/li&gt;&#xA;&lt;li&gt;Electronics, software and other sub-systems&lt;/li&gt;&#xA;&lt;li&gt;Docking mechanism to ISS&lt;/li&gt;&#xA;&lt;li&gt;SuperDraco engines in space conditions&lt;/li&gt;&#xA;&lt;li&gt;Life support systems&lt;/li&gt;&#xA;&lt;li&gt;General reliability of all the components used&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;SpaceX seem to prefer testing more and using existing missions to test. Since they are already doing the resupply missions, why not test the Dragon 2 on those missions?&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-05-28T12:40:45.420" Title="Why does SpaceX not use the Crew Dragon for CRS missions to test it?" Tags="|spacex|dragon|dragon-v2|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9224" PostTypeId="1" CreationDate="2015-05-28T11:13:45.560" Score="4" ViewCount="881" Body="&lt;p&gt;On Friday May 6, A SpaceX Dragon capsule successfully completed a pad abort test at Kennedy Space Center. The abort test was performed by the same propulsion system that is envisioned to land Dragon 2 with precision in the near future. Dragon 2's capsule propulsion system differs from the experimental, reusable Falcon 9 booster, and the possibility to experience similar flight instability problems is minor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there is a lesson learned as a result the F9R experimental flights - the fact that the first stage failed to reduce significantly the range uncertainty solely by adjusting supersonic retro-burns. In science and technology, the negative results are sometime more significant than positive results, but this finding was obscured by advertising the &quot;invention&quot; of the ‘X-wings’. The grid fins, by using the lifting body properties of slender rocket body, have made the real breakthrough: reducing the landing point uncertainty from 10 miles to 10 meters. On the other side, experimental rocket landers like Morpheus and Xombe demonstrate less than a half mile lateral maneuvers at very high Delta V loss ( &gt; 500 m/s) even after hard optimization. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Dragon 2 does not appear to have any aerodynamic controls or significant lifting body abilities at transonic and subsonic speeds, how will it perform a pinpoint landing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX animation do not shows the flight phase between hypersonic deceleration and retroburn.&#xA;&lt;a href=&quot;https://www.youtube.com/watch?v=WotVw5FDVHY&quot; rel=&quot;nofollow&quot;&gt;SpaceX animation&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-05-29T03:11:25.350" LastActivityDate="2015-05-29T03:11:25.350" Title="How will the Dragon 2 perform a pinpoint landing without aerodynamic controls like the F9R did?" Tags="|spacex|falcon-9|dragon-v2|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9225" PostTypeId="2" ParentId="9223" CreationDate="2015-05-28T11:23:35.667" Score="6" Body="&lt;p&gt;I suspect the CRS contract specifies the use of Dragon 1, that alone would rule out use of Dragon 2.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one physical difference that would impact CRS: Dragon 2 has a smaller hatch than Dragon 1, so some cargo won't fit on Dragon 2.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX would have to do at least one flight outside the CRS program to qualify Dragon 2. The docking mechanism is new, and Dragon 2 is capable of automatic docking (Dragon 1 is berthed instead, i.e. it flies to a point near the station, and a robotic arm moves the Dragon to its docking port). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-05-28T11:23:35.667" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9226" PostTypeId="2" ParentId="9224" CreationDate="2015-05-28T12:29:42.397" Score="9" Body="&lt;p&gt;Dragon 2 does have aerodynamic controls. It contains a &lt;a href=&quot;http://science.house.gov/sites/republicans.science.house.gov/files/documents/HHRG-114-SY16-WState-GReisman-20150227.pdf&quot;&gt;movable ballast sled&lt;/a&gt; (written statement by Garrett Reisman for a congressional-hearing), which can be used to change the craft's attitude:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a movable ballast sled allows the angle of attack to be actively controlled during entry to further provide precision landing control.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-05-28T12:29:42.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9227" PostTypeId="2" ParentId="9224" CreationDate="2015-05-28T12:34:15.370" Score="1" Body="&lt;p&gt;A capsule can actually fly and control its path somewhat using just the aerodynamic lift while flying tilted. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has already demonstrated reasonably accurate landings under parachutes with the cargo Dragon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing under parachute (3 round ones, vs a more parawing form) is less accurate as the wind will blow you around during the fairly long parachute descent. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the movable ballast sled in the capsule will allow control of the angle of attack on the heat shield to adjust the path. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-28T12:34:15.370" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9228" PostTypeId="2" ParentId="9223" CreationDate="2015-05-28T12:40:45.420" Score="7" Body="&lt;p&gt;The ISS has 3 different types of docking ports. You can see most of them in this image from the question: &lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;Will the ISS need more docking ports?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZBnjd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The port the Shuttle is using in that image is the PMA, (Pressurized Mating Adapter) and it is getting an upgrade in June 2015 when the SpaceX Dragon CRS-7 mission delivers the LIDS adapter for the PMA. This is where the Commercial Crew vehicles will dock. (Third port type is on the Russian segment where Progress, Soyuz, and ATV dock).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Commercial Cargo vehicles use the CBM (Common Berthing Method) ports, that require capture by the CanadArm and manual intervention.  Crew will use PMA since it can dock on its own and leave on its own, quickly. A CBM berth will take too long to depart in the case of an emergency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CBM's are wider than PMA ports, so larger cargo can be moved in one piece, which is why this was preferred for cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the crew and cargo vehicles have a fairly large physical difference. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to test Crew features on a cargo flight a third configuration would be required, most of the crew features, but a different docking port.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-05-28T12:40:45.420" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9229" PostTypeId="2" ParentId="9224" CreationDate="2015-05-28T13:32:03.917" Score="4" Body="&lt;p&gt;Apollo capsules were designed with the center of gravity off center from the descent center of pressure.  That causes the capsule to have more lift on one side than the other.  The capsule was then slowly spun on descent to null out the lift to get a normal descent.  They could also stop the spin at a certain point, thus allowing the capsule lift to alter the trajectory.  I'm guessing the ballast will also allow the capsule to have it's c/g altered.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Dragon2 also has its 8 super-draco motors to further control the descent, something that is not on the F9R.  The F9R engines are not quite as adjustable.  To get the F9R to land, they originally added cold propellent thrusters to help stabilize the rocket.  These proved to be inadequate to fully adjust the descent so they also added the grid fins.&lt;/p&gt;&#xA;" OwnerUserId="10239" LastEditorUserId="-1" LastEditDate="2015-05-28T20:37:41.663" LastActivityDate="2015-05-28T20:37:41.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9230" PostTypeId="2" ParentId="9219" CreationDate="2015-05-28T13:39:29.603" Score="2" Body="&lt;p&gt;LOX and LH2 are super cold.  This makes the connection points much more complicated.  It also means a more complicated leak check.  By attaching them in the building, they can be connected under controlled conditions (no contamination or moisture to complicate things) and can be tested using sniffers.  Attaching them at the pad would be much more difficult.  Also remember, the F9R is assembled horizontal, not vertical like the Saturn and other rockets.  This makes it much easier to access areas in the barn.&lt;/p&gt;&#xA;" OwnerUserId="10239" LastActivityDate="2015-05-28T13:39:29.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9231" PostTypeId="1" CreationDate="2015-05-28T12:26:04.787" Score="3" ViewCount="1308" Body="&lt;p&gt;Man-made satellites have to use boosters to accelerate to maintain orbit around the Earth. Space debris and old satellites with depleted boosters should be either pulled by Earth's gravity, or slowed by our atmosphere (however thin it may be). So my question is, why is there still so much debris around the Earth?&lt;/p&gt;&#xA;" OwnerUserId="28760" OwnerDisplayName="user6760" LastEditorUserId="1028" LastEditDate="2015-05-29T07:15:22.360" LastActivityDate="2015-05-29T07:15:22.360" Title="Why doesn't all space debris burn up?" Tags="|orbit|gravity|earth|artificial-satellite|debris|" AnswerCount="1" CommentCount="2" ClosedDate="2015-05-29T18:19:50.930" ContentLicense="CC BY-SA 3.0" />
  <row Id="9232" PostTypeId="2" ParentId="9231" CreationDate="2015-05-28T13:14:06.253" Score="10" Body="&lt;p&gt;The NASA Orbital Debris &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/faqs.html&quot;&gt;web-page&lt;/a&gt; gives some interesting statistics :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;12). How long will orbital debris remain in Earth orbit?&#xA;  The higher the altitude, the longer the orbital debris will typically remain in Earth orbit. Debris left in orbits below 600 km normally fall back to Earth within several years. At altitudes of 800 km, the time for orbital decay is often measured in decades. Above 1,000 km, orbital debris will normally continue circling the Earth for a century or more.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So If we stopped sending things in orbit then eventually it would clear up. But we are probably sending more things up than are coming down.&lt;/p&gt;&#xA;" OwnerUserId="10240" OwnerDisplayName="Anthony Lethuillier" LastActivityDate="2015-05-28T13:14:06.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9233" PostTypeId="1" CreationDate="2015-05-28T09:05:20.580" Score="1" ViewCount="341" Body="&lt;p&gt;Firstly, has a man-made satellite, or indeed any spacecraft, ever collided with space debris and been destroyed? What are the chances of this happening? (I imagine very slim, but how slim?) And do we check that there is no space debris where we are going to send our spacecraft before we launch it?&lt;/p&gt;&#xA;" OwnerDisplayName="bnosnehpets" LastActivityDate="2015-05-28T13:52:57.817" Title="Chances of man-made satellite colliding with space debris" Tags="|artificial-satellite|debris|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9234" PostTypeId="2" ParentId="9233" CreationDate="2015-05-28T09:53:25.987" Score="4" Body="&lt;p&gt;The answer is yes, in 2009 an Iridium satellite was destroyed in a collision with a old Russian one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/news/orbital_debris.html&quot; rel=&quot;nofollow&quot;&gt;NASA page on the problem&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10998" OwnerDisplayName="Conrad Turner" LastActivityDate="2015-05-28T09:53:25.987" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9236" PostTypeId="2" ParentId="2911" CreationDate="2015-05-28T14:17:03.187" Score="3" Body="&lt;p&gt;Looks like these are the first (according to &lt;a href=&quot;http://www.newscientist.com/article/dn19239-whats-the-best-way-to-eject-astronauts-during-liftoff.html&quot; rel=&quot;nofollow&quot;&gt;http://www.newscientist.com/article/dn19239-whats-the-best-way-to-eject-astronauts-during-liftoff.html&lt;/a&gt;).  I believe Elon indicated it had never been done before in his D2 reveal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saying something has never been done before is a bit difficult to prove.&lt;/p&gt;&#xA;" OwnerUserId="10239" LastActivityDate="2015-05-28T14:17:03.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9237" PostTypeId="2" ParentId="5184" CreationDate="2015-05-28T14:41:09.633" Score="4" Body="&lt;p&gt;As time goes by, the technological backbone of any system becomes dated and outmoded.  For example, several of the space shuttle systems were still using items designed in the Apollo days.  These can be refit to use newer technology (i.e. using laptops to supplement shuttle computers, new glass cockpit technology, etc.), but there always comes a point where it's cheaper to start over.  That point depends on how fast the technology changes. Who knows what technology will be available by 2020. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many of the ISS systems are already outmoded, sometime by the very research being done on the ISS.  Things like full environment recycling (air, water, etc). are critical to a long duration stay in space.  Sounds like some of these systems, currently in use, are barely functional and need daily maintenance.  This leads to re-thinking and re-engineering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as robotic research, there is much that can be done by robots, but we're a long way from being able to do everything.  And some things can't be done, such as studying human reaction to space, which is one of the key goals of the ISS.  &lt;/p&gt;&#xA;" OwnerUserId="10239" LastActivityDate="2015-05-28T14:41:09.633" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9238" PostTypeId="1" AcceptedAnswerId="14861" CreationDate="2015-05-28T21:53:35.767" Score="5" ViewCount="10128" Body="&lt;p&gt;The obvious wikipedia articles...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&lt;/a&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Comparison_of_orbital_launchers_families&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Comparison_of_orbital_launchers_families&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...don't have this info. I'm not sure why; there must be reliable estimates somewhere, but not even tables at Astronautix have the flyaway costs: &lt;a href=&quot;http://www.astronautix.com/articles/costhing.htm&quot; rel=&quot;nofollow&quot;&gt;http://www.astronautix.com/articles/costhing.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only estimate I can think of right now is that Soyuz Rocket's flyaway cost must be something less than $50 million, because that's currently the widely reported price to take a ride on Soyuz to the ISS for a few days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Proton Rocket, I expect it to be higher not only because it launches 20 tons, but also because it uses those hypergolic propellants that are very dangerous and require a lot of extra safety on the ground, costing money.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone help us find reliable estimates for flyaway costs for these 2 rockets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be clear: Flyaway cost is the cost of manufacturing the rocket plus servicing and doing whatever else is necessary to launch it. It does NOT include payload costs, so don't add the cost of Soyuz spaceship to Soyuz rocket.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="4660" LastEditDate="2016-04-16T14:41:07.707" LastActivityDate="2016-04-16T14:41:07.707" Title="What is the flyaway cost of a Soyuz and Proton Rocket?" Tags="|rockets|russia|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9239" PostTypeId="1" CreationDate="2015-05-29T01:05:07.393" Score="12" ViewCount="3796" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/File:Expedition_28_Landing.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KP6rQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image shows a myriad of lines holding down the parachute. I can't even count them all. Is this really necessary?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All those strings must surely add a lot of weight to the Soyuz TMA. I would think that only 12, 6, or maybe even just 3 lines are all that's necessary. Surely we have strong enough strings whose tensile strength is enough to hold the weight of the capsule?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, don't all those lines pose a greater risk of tangling up the chute? The fewer lines, the more safety. Or is this incorrect somehow?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="49" LastEditDate="2015-05-29T02:49:53.130" LastActivityDate="2015-05-29T19:08:05.923" Title="Why does the Soyuz spacecraft's parachute have so many lines?" Tags="|landing|soyuz-spacecraft|design|crewed-spaceflight|parachute|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9240" PostTypeId="1" CreationDate="2015-05-29T01:34:42.577" Score="10" ViewCount="508" Body="&lt;p&gt;Observing a Space Shuttle launch, I noticed that the crew access gantry and its white room are too close to the SRB's flames during launch; the launch umbilical tower and its crew access device on Chinese CZ-2F (for Shenzhou) is also very close to the rocket's poisonous plumes during launch. My question: Is there a need for a post-launch procedure to clean the white room to avoid contamination by the engine's residues? I think the hydroxy-terminated poly-butadiene (HTPB) is very poisonous, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iE9AG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QkxFS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8761" LastEditorUserId="1419" LastEditDate="2016-05-30T13:46:14.813" LastActivityDate="2016-06-13T16:22:56.710" Title="Can a crew access room (white room) be contaminated by rocket propellant residues during launch?" Tags="|rockets|spacecraft|space-shuttle|crewed-spaceflight|solid-fuel|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9241" PostTypeId="2" ParentId="9239" CreationDate="2015-05-29T02:15:47.010" Score="12" Body="&lt;p&gt;All round parachutes have many suspension lines, and they're needed to distribute the load on the canopy, keep its shape, and their hooking (or even re-hooking) geometry during deployment can help with reefing and keeping its shape. It's not because we can't make suspension lines strong enough. Fewer suspension lines would require stronger canopy that would then likely result in even heavier parachute, and fewer sections would result in less controlled reefing and it would be more prone to collapsing and canopy tearing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz TMA main parachute is a rather large &lt;a href=&quot;https://en.wikipedia.org/wiki/Parachute#Annular_and_pull_down_apex&quot;&gt;annular pull down apex&lt;/a&gt; type that deploys after the spacecraft has already decelerated to subsonic speed with the help of two pilot and four drogue parachutes 15 minutes before landing&lt;sup&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/structure/elements/soyuz/landing.html&quot;&gt;&lt;code&gt;[1]&lt;/code&gt;&lt;/a&gt;&lt;/sup&gt; that reduce the rate of descent to ~ 80 m/s (288 km/h)&lt;sup&gt;&lt;a href=&quot;http://suzymchale.com/ruspace/soyland.html&quot;&gt;&lt;code&gt;[2]&lt;/code&gt;&lt;/a&gt;&lt;/sup&gt; before the main parachute deploys, and it has a small apex hole to increase stability and reduce oscillation. From that linked Wikipedia page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A variation on the round parachute is the pull down apex parachute.&#xA;  Invented by a Frenchman named Pierre-Marcel Lemoigne it is called a&#xA;  Para-Commander canopy in some circles, after the first model of the&#xA;  type. It is a round parachute, but with suspension lines to the canopy&#xA;  apex that apply load there and pull the apex closer to the load,&#xA;  distorting the round shape into a somewhat flattened or lenticular&#xA;  shape.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Some designs have the fabric removed from the apex to open a hole&#xA;  through which air can exit, giving the canopy an annular geometry.&#xA;  They also have decreased horizontal drag due to their flatter shape&#xA;  and, when combined with rear-facing vents, can have considerable&#xA;  forward speed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, while all those suspension lines do add to parachute's weight, and there's another point you missed in your question that a large parachute is even more difficult to fold properly (terribly important with parachutes!), if it used fewer suspension lines, it would have fewer sections that would somehow have to be even stronger then. And it would have problems keeping its shape once deployed because of larger sections, and due to this more prone to instabilities or tearing of the canopy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even if we could construct a chute with stronger suspension lines and canopy fabric, we likely wouldn't use any fewer suspension lines. As for entanglement of suspension lines, well there's always a chance you'll be having a really bad day, but that risk is reduced by using a single main parachute and pilot chutes that pull it out and help it reef. If lines entangle, there's still a good chance that the canopy will be able to keep its shape, lines will only stretch out (hook) from a bit higher up. Compare that with usually three main parachutes that most other spacecraft use, and you start appreciating that it's a tradeoff, and no solution is bulletproof.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using many main parachutes means they can get entangled, canopies can bump with each other and partially or fully collapse, and there's even more suspension lines, but you can still get home for dinner if only one of them fails. With a single one, if that one fails, you have to deploy the spare one&lt;sup&gt;&lt;a href=&quot;http://wsn.spaceflight.esa.int/docs/Factsheets/35%20Soyuz%20LR.pdf&quot;&gt;&lt;code&gt;[3]&lt;/code&gt;&lt;/a&gt;&lt;/sup&gt; and there might not be any time for that. It's a calculated risk and a design choice. But there isn't much point in reducing the number of suspension lines and parachute sections.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/structure/elements/soyuz/landing.html&quot;&gt;NASA - Soyuz Landing&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://suzymchale.com/ruspace/soyland.html&quot;&gt;RuSpace - Soyuz landing profile&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://wsn.spaceflight.esa.int/docs/Factsheets/35%20Soyuz%20LR.pdf&quot;&gt;ESA - Soyuz TMA&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-05-29T03:09:51.823" LastActivityDate="2015-05-29T03:09:51.823" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9242" PostTypeId="2" ParentId="9211" CreationDate="2015-05-29T05:54:54.233" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I know that most of the fuel that we spend is for escaping Earth gravity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That is not entirely true.  Worse, it places undue emphasis on a side issue.  A better way to understand it is that most of the fuel is expended to get the vehicle going fast enough to &lt;em&gt;not&lt;/em&gt; hit the Earth.  To simply send a rocket straight up 62 miles (100 km) doesn't take all that much fuel.  Keeping it up there is the tricky part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hard part is that near-Earth orbital speed is 8,000 m/s (8 km/s) which, as Randal Munroe points out in his &lt;a href=&quot;http://what-if.xkcd.com/58/&quot; rel=&quot;noreferrer&quot;&gt;What-If for Orbital Speed&lt;/a&gt; is ten times faster than a rifle bullet.  See that link for a well presented perspective.&lt;/p&gt;&#xA;" OwnerUserId="10247" LastActivityDate="2015-05-29T05:54:54.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9243" PostTypeId="1" AcceptedAnswerId="9245" CreationDate="2015-05-29T07:49:44.667" Score="167" ViewCount="268774" Body="&lt;p&gt;I don't know if this would be public, but what CPU / Microcontrollers are used and what software do they run on the Falcon 9? Do they use an OS or is it just bare metal programming?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found some model rocket flight computers that are available for purchase and just wanted to know what approach SpaceX took towards this.&lt;/p&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2020-06-07T02:58:29.180" Title="What computer and software is used by the Falcon 9?" Tags="|spacex|falcon-9|software|flight-computer|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9244" PostTypeId="1" CreationDate="2015-05-29T09:16:57.763" Score="12" ViewCount="267" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/content/funded-studies&quot; rel=&quot;nofollow noreferrer&quot;&gt;NIAC&lt;/a&gt; (&amp;quot;NASA Innovative Advanced Concepts&amp;quot;) has approved the following research application for 2015 funding:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/feature/cryogenic-selective-surfaces&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/feature/cryogenic-selective-surfaces&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h2&gt;Cryogenic Selective Surfaces&lt;/h2&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.techbriefs.com/component/content/article/24-ntb/features/whos-who/8917&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robert Youngquist&lt;/a&gt; (NASA Kennedy Space Center)&lt;/p&gt;&#xA;&lt;h3&gt;Description&lt;/h3&gt;&#xA;&lt;p&gt;Selective surfaces have wavelength dependent emissivity/absorption. These surfaces can be designed to reflect solar radiation, while maximizing infrared emittance, yielding a cooling effect even in sunlight. On earth cooling to -50 °C below ambient has been achieved, but in space, outside of the atmosphere, theory using ideal materials has predicted a maximum cooling to 40 K! If this result holds up for real world materials and conditions, then superconducting systems and cryogenic storage can be achieved in space without active cooling. Such a result would enable long term cryogenic storage in deep space and the use of large scale superconducting systems for such applications as galactic cosmic radiation (GCR) shielding and &lt;a href=&quot;https://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage&quot; rel=&quot;nofollow noreferrer&quot;&gt;large scale energy storage&lt;/a&gt;.  We propose, during this Phase I effort, to theoretically model the performance of real world selective surfaces to see if superconducting temperatures can be passively achieved in a deep space environment at 1 A.U. from the sun.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;A quick search gave me a &lt;a href=&quot;http://www.solec.org/wp-content/uploads/2014/02/SOLKOTEbrochure.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;sample material&lt;/a&gt; from Solec designed for the exactly opposite scenario.&lt;/p&gt;&#xA;&lt;p&gt;What are the candidate selective surface coatings (low on solar spectrum absorptivity, high on IR emissivity)?&lt;/p&gt;&#xA;&lt;p&gt;What are their technology readiness levels&lt;sup&gt;*&lt;/sup&gt;?&lt;/p&gt;&#xA;&lt;p&gt;Have any coatings been tested in vacuum for outgassing and its impact on performance?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;sup&gt;*&lt;/sup&gt; &lt;sub&gt;A white paper describing &lt;a href=&quot;https://www.hq.nasa.gov/office/codeq/trl/trl.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA TRLs&lt;/a&gt; (pdf)&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-11-27T19:42:04.137" Title="What cryogenic selective surface materials have high Technology Readiness Level?" Tags="|technology|materials|cryogenics|thermal-control|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9245" PostTypeId="2" ParentId="9243" CreationDate="2015-05-29T09:47:09.843" Score="116" Body="&lt;p&gt;In &lt;a href=&quot;http://www.reddit.com/comments/1853ap&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; AMA by the SpaceX software development team, they wrote:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;We've been getting a lot of questions about how C#/MVC/etc have to do&#xA;with rockets. They don't.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;About their development they said:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Flight Software team is about 35 people. We write all the code for&#xA;Falcon 9, Grasshopper, and Dragon applications; and do the core&#xA;platform work, also on those vehicles; we also write simulation&#xA;software; test the flight code; write the communications and analysis&#xA;software, deployed in our ground stations. We also work in Mission&#xA;Control to support active missions.&lt;/p&gt;&#xA;&lt;p&gt;The Ground Software team is about 9 people. We primarily code in&#xA;&lt;strong&gt;LabVIEW&lt;/strong&gt;. We develop the GUIs used in Mission and Launch control, for&#xA;engineers and operators to monitor vehicle telemetry and command the&#xA;rocket, spacecraft, and pad support equipment. We are pushing high&#xA;bandwidth data around a highly distributed system and implementing&#xA;complex user interfaces with strict requirements to ensure operators&#xA;can control and evaluate spacecraft in a timely manner.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;They also said:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Dragon and Falcon 9 use a version of &lt;strong&gt;Linux&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;They unfortunately replied to this question:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;What sort of computing power (CPU, RAM, Storage, etc.) do you have&#xA;access to on board the rockets and capsules themselves? What's&#xA;&amp;quot;smarter,&amp;quot; the Falcon 9 or the Dragon?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;with:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;100 Dragon sized Falcons or 1 Regular Sized Dragon&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So I'm guessing any further information is strictly classified :)&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-05-29T10:41:11.783" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9246" PostTypeId="1" CreationDate="2015-05-29T10:56:49.713" Score="19" ViewCount="2342" Body="&lt;p&gt;At Venusian temperatures and pressures, CO&lt;sub&gt;2&lt;/sub&gt; becomes supercritical. This means it behaves as a dense liquid yet has no clear upper bound and continuously becomes gas with height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can we thus say that Venus is covered with a global ocean of liquid CO&lt;sub&gt;2&lt;/sub&gt; rather than atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="5963" LastEditDate="2015-05-31T21:07:22.497" LastActivityDate="2015-05-31T21:07:22.497" Title="Can it be said that Venus is covered by an ocean rather than atmosphere?" Tags="|planetary-science|atmosphere|terminology|venus|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9247" PostTypeId="2" ParentId="9246" CreationDate="2015-05-29T11:18:37.660" Score="15" Body="&lt;p&gt;It's more a question of semantics, rather than a physical reality. An ocean would probably be thought to have a clearly defined surface. On the other hand, terminal velocity is only about 5m/s, which is a moderate running speed (Usain Bolt does 10 m/s). This gives you an idea about how thick the gas really is. In fact, one of the Pioneer Venus probes kept sending even after impacting the surface without a parachute or any other kind of landing equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would say the question is fair to ask, but if you consider the picture an average person has of an ocean, calling it an ocean would mostly cause confusion.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-05-29T11:18:37.660" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9248" PostTypeId="2" ParentId="5463" CreationDate="2015-05-29T14:03:39.537" Score="-1" Body="&lt;p&gt;I agree with Pavel, and also think this is probably a concept to look at:  The catapult has to be engineered as a pre-burning rocket phase. Meaning, we could use the catapult to launch a rocket, capice? So, we may engineer the catapult to accelerate at the maximum rate a human body can stand (let's say, 5g), so that it reaches considerable velocity (say, 600km/h), way bellow the limit that would make things burn in the lower atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this visionary mind experiment, we could have a rocket ignition, where the rocket has initial velocity of 600km/h. The rocket would keep accelerating at 5g after the catapult line is over; using it's rockets (as currently). The nice thing is that the burning-rocket would start its trip at 600Km/h instead of  0km/h . I guess the main impact would be on the necessary amount of initial fuel (someone could estimate that, please?). You can cut of all the fuel necessary to reach 600km/h in a non-catapult launch. The whole ship would be lighter! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering our catapult works at acceleration = 5g = 5x10 m^2/s . Going from V_0 = zero , to V = 600km/h , demands 3.3seconds . Lets round it to 3.5s .  Then, as next step we may think “who big should be the catapult line to allow this rocket launch?“ , And the answer is not bad at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Accelerating at a=5g , from X_0 = 0   and  V_0 = 0&lt;/p&gt;&#xA;&#xA;&lt;p&gt;X = X_0 + V_0 . t + (a . t^2)/2        ;  using t=3.5s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;X=300meters&lt;/p&gt;&#xA;&#xA;&lt;p&gt;…&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let’s say we would need something bigger, since the rocket itself could be 100m high… We are talking about a 500m rock-catapult lined up to the sky.  don’t think this sounds crazy to build. I don’t know how much would cost such catapult. But we have to take into account take the rocket would be much lighter , just because we need much less fuel to reach upper atmosphere .  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many mountains could meet the requirements for build this mind-catapult… &#xA;(someone could try extra more calculations considering Everest type launch … be careful to keep velocities lower than low-atmosphere burn! )  &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;About some drawbacks mentioned from previous comments...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Guys, a robe to stand 5g and catapult a light rocket is not a problem (is it?)... I mean, even if a robe to &quot;do the job&quot; is the biggest problem you can imagine, I think this could be engineered to work.... for example, we could look for currently robes-specifications for catapulting foo-fighters &lt;/p&gt;&#xA;&#xA;&lt;p&gt;About the &quot;problem&quot; of having rocket-motor failure during the &lt;em&gt;catapultagge&lt;/em&gt;... guys, come on... If a rocket-engine explodes, it will be a tragic problem from both ground and catapult launch. &#xA;If a rocket fail to burn during catapulting, I agree it will be hard to stop the rocket. Probably we may lose the rocket. But we already have technology to eject crew (or any valuable-payload) (see the Abort Pad tests from NASA or SpaceX)&lt;br&gt;&#xA;So, &quot;rocket failure&quot; should not be seem as a main strong argument against catapulting rockets (my view)... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, about extra structures needed to stand the catapulting... You know... maybe, we would not need to go to far to address this kind of problem. Today's launchers already stand &gt;5g acceleration ... So, concerning this point, the current rockets are fine. Also, lets think for a moment... Where would we connect the robe? Well, we can simply apply the force in the same mechanic structure that holds the currently burning rocket engine pull . I mean... the currently burning engines already apply force enough for a &gt;5g acceleration. So, we may apply the robe-force in the same place. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any extra structure needed for that can disconnect from the rocket at the catapult-end; no big extra-load have to be take up because of that. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;We could start a dedicated forum just to bird-eyes on this topic, in a more-realistic point of view... looking for materials and specifications. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See you!&lt;/p&gt;&#xA;" OwnerUserId="10250" LastEditorUserId="25" LastEditDate="2017-12-19T13:33:43.963" LastActivityDate="2017-12-19T13:33:43.963" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9249" PostTypeId="2" ParentId="9239" CreationDate="2015-05-29T16:02:51.657" Score="8" Body="&lt;p&gt;The number of ropes (suspension lines) is not arbitrary. Between every two gores (sectors) there is a load tape which transfers load from fabric canopy to lines. The width of lobed sectors (gores) is limited by strength of the fabric. Typically, the fabric is at least two times weaker than the fiber it is made of. The rope (unidirectional fiber structure) is the optimal way to transfer loads. Multiple ropes with small diameter ropes are stronger than an equivalent-section (and -weight) cable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The parachute of Soyuz has a single canopy with larger diameter than the three parachutes of Dragon for example. Dragon parachutes also have 'myriad' ropes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft parachutes are optimized by trade-off between number of gores, thickness of fabric etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wIfwB.jpg&quot; alt=&quot;parachute terminology&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="6816" LastEditDate="2015-05-29T19:08:05.923" LastActivityDate="2015-05-29T19:08:05.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9250" PostTypeId="2" ParentId="9246" CreationDate="2015-05-29T17:28:54.433" Score="9" Body="&lt;p&gt;Venusian lower atmosphere &lt;a href=&quot;https://en.wikipedia.org/wiki/Supercritical_fluid#Planetary_atmospheres&quot;&gt;qualifies&lt;/a&gt; as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Supercritical_fluid&quot;&gt;supercritical fluid&lt;/a&gt;, but there isn't any clear liquid-gas interface (a &lt;a href=&quot;https://en.wikipedia.org/wiki/Surface_layer&quot;&gt;surface layer&lt;/a&gt;) where Venusian &lt;em&gt;ocean&lt;/em&gt; and its atmosphere would come in contact with each other. Actually, this dense lower atmosphere that reaches supercriticality due to pressure and temperature only make this boundary layer less clearly defined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it's up to your definition of &lt;em&gt;an ocean&lt;/em&gt; if you still want to call the lower atmosphere of Venus like that, but I haven't noticed it called &lt;em&gt;an ocean&lt;/em&gt; in any planetary science literature, unless they discussed Venusian geologic evolution and potential that it could have once had large bodies of water. And the lack of clearly definable surface, in my view, qualifies as a good reason not to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that even if you still want to call it &lt;em&gt;an ocean&lt;/em&gt;, this lack of clearly defined boundary layer also means that you'd have problems establishing its volume.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-05-29T23:45:02.770" LastActivityDate="2015-05-29T23:45:02.770" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9251" PostTypeId="2" ParentId="8763" CreationDate="2015-05-29T21:29:20.350" Score="2" Body="&lt;p&gt;There is a big mess about landing gear (legs). In an &lt;a href=&quot;http://www.space.com/24800-spacex-falcon9-rocket-landing-legs-photo.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;early statement&lt;/a&gt; Elon Musk, say that legs may be used as stabilizers (fins):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;This particular stage was not equipped with landing gear, which could&#xA;  have helped stabilize the stage like fins would on an aircraft&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The role of legs as airbrakes has been also &lt;a href=&quot;https://twitter.com/elonmusk/status/536268250620125185&quot; rel=&quot;nofollow noreferrer&quot;&gt;conceptualized&lt;/a&gt;. However, air brakes in front of the vehicle will make it statically unstable. Opening the legs at higher velocity will possibly destroy the rocket. It is very unusual to place the air brakes in front of craft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although, the legs can’t contribute significantly to deceleration, they can contribute to loss of stability. Because of its specific deign the opening of legs is loosely controllable. A long pneumatic cylinders actuated by highly pressurized helium opened beautifully designed flaps used as landing gear by sliding a metal piston end down the groove in flaps. This arrangement makes opening speed to depend on drag of incident flow and friction in groove.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BFdif.jpg&quot; alt=&quot;landing legs general view&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As result, asymmetrically opened legs may be observed on &lt;a href=&quot;https://www.youtube.com/watch?v=O_9SEoEmANs&amp;amp;list=PLha68c7vAv466KOvFAE12_mMnSvXeyrtN&amp;amp;index=5&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt;. Along with their unusually large cross section and negative dihedral, that symmetry may contribute to loss of stability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Observed ‘double stack’ opening is caused most likely by specific kinematics of the opening mechanism. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=O_9SEoEmANs&amp;amp;list=PLha68c7vAv466KOvFAE12_mMnSvXeyrtN&amp;amp;index=5&quot; rel=&quot;nofollow noreferrer&quot;&gt;CRS-6 SpaceX landing attempt with additional data&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk tweeted that around Falcon flight 21 they will be changing to be more used as aero surfaces, implying early opening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/536268250620125185&quot; rel=&quot;nofollow noreferrer&quot;&gt;Musk Tweet&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="51" LastEditDate="2015-06-14T20:03:49.060" LastActivityDate="2015-06-14T20:03:49.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9252" PostTypeId="1" CreationDate="2015-05-29T21:53:39.223" Score="6" ViewCount="5985" Body="&lt;p&gt;I've come across this term a few times. IIRC, liquid hydrogen is &quot;self-pressurizing&quot;, but other fuels are not. Apparently, if a propellant is self-pressurizing, we do not need to build any complex plumbing to repressurize the tank as the propellant is used up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that, normally, a tank would undergo adiabatic cooling as its pressure drops (as propellant is used up by the engine), and thus the tank would eventually crack from the extreme temeperature differential. So it's important to keep the tank pressurized even as fuel is depleted. What I don't understand is how a certain propellant can be &quot;self-pressurizing&quot; even tho we are using up that propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What fuels and oxidizers are self-pressuring, and what ones aren't? And why and how?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. Chemistry and math are fine in your answer. I'm not afraid of either.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2015-05-30T01:05:57.113" LastActivityDate="2015-05-30T01:26:04.983" Title="What does &quot;self-pressurizing&quot; mean in regards to propellant tanks?" Tags="|rockets|fuel|chemistry|thermodynamics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9253" PostTypeId="2" ParentId="9252" CreationDate="2015-05-29T22:22:47.090" Score="5" Body="&lt;p&gt;Liquid hydrogen will boil off in the tank until the pressure reaches equilibrium.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-05-29T22:22:47.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9256" PostTypeId="2" ParentId="9252" CreationDate="2015-05-30T01:26:04.983" Score="5" Body="&lt;p&gt;From &lt;a href=&quot;http://spase.stanford.edu/Self-Pressurizing_Propellant_Dynamics.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; page, the defining feature of a self-pressurizing liquid (also referred to as &amp;quot;Vapor Pressurization&amp;quot;, or &amp;quot;VaPak&amp;quot;) is that it has a high vapor pressure.  Specifically for an oxidizer:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Nitrous oxide (N2O) is the most promising oxidizer that can self-pressurize because it is relatively energetic and its vapor pressure is approximately 750 psi at room temperature.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There's also an experimental setup and some charts at the above link that show how the pressure stabilizes as fuel drains out.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.holderaerospace.com/downloads/Technical_Papers/VaPak%20Systems%20Overview.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here&lt;/a&gt;'s another article that gives a much more in-depth explanation of the concept, benefits, and challenges of using a VaPak system (which are interesting but not really what you're asking for so I won't copy information out).  They did have some references to tests that mentioned fuels, which I list below.&lt;/p&gt;&#xA;&lt;p&gt;To answer your question, I've seen references to the following being self-pressurizing (linking to the reference).  I don't know what fuels have been actually used for flight as the links I found are either theoretical or from test firings.&lt;/p&gt;&#xA;&lt;h3&gt;Fuel&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.holderaerospace.com/downloads/Technical_Papers/VaPak%20Systems%20Overview.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Liquid Propane&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.holderaerospace.com/downloads/Technical_Papers/VaPak%20Systems%20Overview.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Methane&lt;/a&gt; (also &lt;a href=&quot;http://armadilloaerospace.com/n.x/Armadillo/Home/News?news_id=365&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Liquid Hydrogen (in your question)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h3&gt;Oxidizer&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.holderaerospace.com/downloads/Technical_Papers/VaPak%20Systems%20Overview.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Liquid Oxygen&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spase.stanford.edu/Self-Pressurizing_Propellant_Dynamics.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nitrous Oxide&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-05-30T01:26:04.983" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9258" PostTypeId="2" ParentId="2387" CreationDate="2015-05-30T11:36:45.253" Score="11" Body="&lt;p&gt;There are three major arguments against physical security on manned spacecraft: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A lock is a point of failure. Given the dire consequences of an astronaut being locked out after a spacewalk, you need to make really sure people can get in in an emergency. So adding a lock would make the hatch design a lot more complicated, for no added value (see point 2).&lt;/li&gt;&#xA;&lt;li&gt;Hijacking a manned spacecraft is really unlikely. There are only 3 nations on Earth capable of launching a manned mission. If one of them decides to hijack another nation's mission, they'd have an international crisis on their hand, if not a war. So the hijacker pays a huge price, for no benefit (see point 3).&lt;/li&gt;&#xA;&lt;li&gt;Why bother? Manned missions are largely peaceful scientific missions. If you want to take part, pay a few dozen million and you can send an astronaut up to the ISS. No need to kick in the door. Also, there's no point in hijacking a manned mission. Say you've taken over a spacecraft: what are you going to do with it? There's no benefit to hijacking.  &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;For unmanned satellites the situation can be more complicated, esp. in the case of military satellites. It's likely those have countermeasures against remote tampering (hacking) on board.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Physical security? Again, unlikely. Launching a space mission isn't covert: anyone can track your spaceship, so if you launched a mission to hijack an enemy's satellite, they'd know it's you. Their response would be to bomb your launch site, so you'd get to do this no more than once. Again high cost, little benefit. The only problem military satellites face is getting blown up by an antisatellite missile, and those are difficult to defend against. A ballistic missile defence system is huge and expensive, to add that to a satellite would add several tons of weight and cost a fortune.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-05-30T16:33:38.040" LastActivityDate="2015-05-30T16:33:38.040" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9259" PostTypeId="1" AcceptedAnswerId="9643" CreationDate="2015-05-30T13:25:18.920" Score="12" ViewCount="385" Body="&lt;p&gt;I've been wondering for a while who gets to decide where Curiosity goes. I'm not wondering so much about who gets to actually give it the commands (that seems fairly easy to find), but who decides its day-to-day destination, and how those decisions are made. For instance, is there a specific person in charge who ultimately decides where it goes, or is there a committee who decides? Can someone overrule them?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is the path chosen? Do they just plan day-by-day, or do they map out where they want it to be next week/month and then figure out specifically where it should be at the end of each day?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2015-06-24T20:25:11.307" Title="Who decides where Curiosity goes?" Tags="|mars|rovers|curiosity|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9260" PostTypeId="1" AcceptedAnswerId="9271" CreationDate="2015-05-30T21:44:52.907" Score="23" ViewCount="4660" Body="&lt;p&gt;It dawned on me that the tragic STS-107 disaster of (my favorite shuttle) Columbia was after more than 20 years of service.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Computers in 1981 were of course significantly inferior to what I had in my phone in 2003.  I'm picturing our old 1980s Tandy 1000 with 640k of memory still operating an incredible space craft in 2003.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what software was in the Shuttle Orbiters, but was it updated?  And were there rigorous tests to avoid software crashes?&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="6712" LastEditDate="2017-02-24T22:35:04.813" LastActivityDate="2017-02-24T22:35:04.813" Title="How often, if ever, was &quot;software&quot; updated in the shuttle orbiter?" Tags="|space-shuttle|software|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9261" PostTypeId="1" AcceptedAnswerId="9438" CreationDate="2015-05-30T22:04:13.663" Score="16" ViewCount="2569" Body="&lt;p&gt;I was reading the wikipedia article about the &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot;&gt;SRBs&lt;/a&gt;, which were retrieved, refurbished, updated and re-used.  I understand that if they were taken apart and pieces were replaced, that there was a 'new two' for each flight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, &lt;strong&gt;I'm looking for a rough idea of how many SRBs there were&lt;/strong&gt; - surely it wasn't the same two, and obviously the four that weren't recovered - so how many were made?  If they're famously recoverable, I would understand that they didn't make all 270 of them.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2020-02-28T02:19:29.647" Title="How many Solid Rocket Boosters were there in the Space Shuttle program?" Tags="|space-shuttle|srb|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9263" PostTypeId="1" CreationDate="2015-05-31T01:56:35.747" Score="3" ViewCount="3719" Body="&lt;p&gt;Ok, so sometimes you see pictures like this or this, where there is another planet super close on the horizon.  This morning, I was laying in bed thinking, &quot;How could we get two planets to orbit each other that close, but never crash?  That would be so cool to live on one, and commute to work at the other.&quot; etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, how close could the moon orbit before it got too close and would become unstable in its orbit?  What are the things that would bring it crashing down into the earth?&lt;/p&gt;&#xA;" OwnerUserId="10267" LastActivityDate="2015-05-31T09:09:12.027" Title="How close can the moon orbit before it becomes unstable in its path" Tags="|the-moon|orbit|earth|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9264" PostTypeId="2" ParentId="5463" CreationDate="2015-05-31T02:13:39.250" Score="4" Body="&lt;p&gt;While the answers are right lets look at it in a much simpler fashion:  Nothing about a catapult reduces the acceleration the craft being tossed experiences.  Watch a rocket launch--multiple gs for hundreds of miles as it flies off into space.  If you want to keep the acceleration down to the same amount you'll need as much space--your catapult will have to be hundreds of miles long.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-05-31T02:13:39.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9265" PostTypeId="2" ParentId="9263" CreationDate="2015-05-31T02:20:12.683" Score="7" Body="&lt;p&gt;I believe the orbit won't become unstable, but the structural strength of the Moon might fail due to tidal forces, see &lt;a href=&quot;http://en.wikipedia.org/wiki/Roche_limit&quot; rel=&quot;noreferrer&quot;&gt;Roche limit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbit itself might also be considered unstable if you are looking at very long time behavior, because if the Moon would orbit the Earth below geostationary altitude, tidal forces will slowly pull in the Moon. For example the closest moon of Mars, &lt;a href=&quot;http://en.wikipedia.org/wiki/Phobos_(moon)&quot; rel=&quot;noreferrer&quot;&gt;Phobos&lt;/a&gt;, is expected to crash into Mars in roughly 30 to 50 million years. &lt;/p&gt;&#xA;" OwnerUserId="1086" LastEditorUserId="1028" LastEditDate="2015-05-31T04:24:31.673" LastActivityDate="2015-05-31T04:24:31.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9267" PostTypeId="1" AcceptedAnswerId="9268" CreationDate="2015-05-31T05:04:32.680" Score="7" ViewCount="2537" Body="&lt;p&gt;Hohmann transfer to Jupiter is about 2 years 8 months. Juno spends 5 years getting there, almost twice as long. I note that it takes Juno 2 years 8 months to get to Jupiter after the Earth flyby, does this mean that it is in a Hohmann trajectory now?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is this gravity assist from Earth helpful? &#xA;Isn't an Atlas 551 enough to get a 3.6 ton launch mass spacecraft into Hohmann transfer to Jupiter? Was the flyby maybe done in order to help orbital insertion at Jupiter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0jP6j.jpg&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="51" LastEditDate="2016-07-08T13:37:36.877" LastActivityDate="2019-01-14T02:11:17.560" Title="Why does the Juno spacecraft flyby Earth on the way to Jupiter?" Tags="|orbital-mechanics|jupiter|flyby|juno|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9268" PostTypeId="2" ParentId="9267" CreationDate="2015-05-31T05:14:08.303" Score="13" Body="&lt;p&gt;No, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V#Versions&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas 551&lt;/a&gt; is &lt;em&gt;not&lt;/em&gt; powerful enough to send Juno to Jupiter. From &lt;a href=&quot;http://www.nasa.gov/jpl/juno/juno-earth-flyby-20131210.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; article on NASA's website:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Juno spacecraft was launched from Kennedy Space Center in Florida&#xA;  on August 5, 2011. Juno’s launch vehicle was capable of giving the&#xA;  spacecraft only enough energy to reach the asteroid belt, at which&#xA;  point the sun’s gravity pulled it back toward the inner solar system.&#xA;  Mission planners designed the swing by Earth as a gravity assist to&#xA;  increase the spacecraft’s speed relative to the sun, so that it could&#xA;  reach Jupiter. (The spacecraft’s speed relative to Earth before and&#xA;  after the flyby is unchanged.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The &lt;a href=&quot;http://d2mtuel436pcq3.cloudfront.net/Vault/VaultOutput/Juno-StoryMode_Chapt08_onlyVO_720-Nasa%20Reference.mov?VaultID=72&amp;amp;t=1425407166&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth flyby&lt;/a&gt; was a necessary gravitational slingshot to give Juno the final push to Jupiter.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Details (&lt;a href=&quot;http://www.spaceflight101.com/juno-mission-design-timeline-trajectories.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launch:&lt;/strong&gt; Juno launched aboard a &lt;a href=&quot;https://en.wikipedia.org/wiki/United_Launch_Alliance&quot; rel=&quot;nofollow noreferrer&quot;&gt;United Launch Alliance&lt;/a&gt; Atlas 551 rocket which is the most powerful Atlas V flown to date. Atlas V supplied a launch energy (C3) of 31.1 km²/s² leaving its payload in a heliocentric orbit with an approximately 2-year period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WSJkc.jpg&quot; width=&quot;200&quot; height=&quot;400&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Deep Space Manoeuvres (DSMs):&lt;/strong&gt; When coming close to aphelion, Juno performs two large deep space maneuvers. These events are critical mission events of the 2+ dV-EGA mission trajectory because they target Juno's path back to Earth. Theoretically, the DSMs could be performed with a single burn which would actually provide a better net result in terms of required delta-V, but the burn was cut in half since the Leros 1b main engine of the spacecraft was not qualified for 60-minute burns prior to Juno's launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KnPQn.jpg&quot; alt=&quot;DSM Simulation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth Flyby:&lt;/strong&gt; For the Flyby, more instruments will be turned on to acquire science data from Earth's environment, both, for calibrations and to support other studies. An additional benefit of the flyby is to give mission controllers the opportunity to practice operating the instruments and the spacecraft in general in preparation for the science mission. Earth flyby occurs on October 9, 2013 with the exact timing depending on the final trajectory maneuvers. Juno will be coming as close as 559 kilometers. Perigee occurs 200 kilometers off the southeastern coast of South Africa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Zd0mf.jpg&quot; alt=&quot;Flyby ground track&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The gravity assist flyby supplies Juno with a delta-V (velocity change) of 7.3 kilometers per second (26,280&amp;nbsp;kph, 16,330&amp;nbsp;mph). With its additional velocity, Juno's orbit around the Sun is modified in a way that takes the vehicle's aphelion outside Jupiter's orbit, crossing its orbit at the correct time to intercept the planet.&lt;/strong&gt; The gravity assist flyby will mark the only time on Juno's mission that the vehicle is in full eclipse as it passes through Earth's shadow. Eclipse time is about 20 minutes and Juno charges its batteries to 100% before the event so that there are no concerns regarding electrical power.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="286" LastEditDate="2019-01-14T02:11:17.560" LastActivityDate="2019-01-14T02:11:17.560" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="9269" PostTypeId="1" CreationDate="2015-05-31T06:35:14.323" Score="3" ViewCount="516" Body="&lt;p&gt;It would be nice to get an overview of what the launchers are capable of in terms of actual space missions. I understand that this depends on many factors, but the Hohmann trajectory seems to be a common and major factor for many missions. Could this Hohmann part of the puzzle be summarized in a table with payload mass for different launchers and destinations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most interesting would be to compare these three launcher classes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;SLS or Saturn V&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Falcon Heavy&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Atlas V551 or Ariane 5 or Proton&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The smaller Falcon 9, Soyuz, Indian PSLV would also be of interest since even little PSLV actually has launched an orbiter to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Latitude of the launch site should not make a difference big enough to matter for an overview purpose, one could assume the equator. The eccentricity and inclination of the destination object is much more important and variable over time. But except for Mars and Mercury, I would think that at least the planets have low enough eccentricity for an average to work well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity assists, Oberth effect, continuous ion thrusting of course totally distorts the Hohmann geometry and all spacecraft to outer planets have and surely will use Jupiter for gravity assist. So that kind of leaves only Jupiter and Venus and the Moon! But those are very likely mission targets. Each launcher should have a pretty good specification as an X tons to Venus or Y tons to Jupiter or Z tons to the Moon vehicle. For Mars and asteroids the specific launch date seems to be too important. However, Mars will make a close opposition in 2035 which could be used as a benchmark for a crewed mission.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-05-31T06:44:45.847" LastActivityDate="2015-05-31T07:36:46.237" Title="What payload mass can different launchers send in Hohmann trajectories to different destinations?" Tags="|interplanetary|launch-vehicle|hohmann-transfer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9270" PostTypeId="2" ParentId="9269" CreationDate="2015-05-31T07:36:46.237" Score="4" Body="&lt;h1&gt;To Mars&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The Delta-V required to transfer from Low Earth Orbit to Low Mars Orbit is around 7 km/s (utilising the Oberth effect). So the payload being sent up to LEO must have this ∆V budget. Additionally your launcher needs about 8km/s to get up to LEO in the first place. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/7/74/Delta-Vs_for_inner_Solar_System.svg&quot; width=&quot;400&quot; height=&quot;400&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your question is specifically about payload mass. You've given a multitude of different launchers and targets, and you can calculate each one's mass using the Tsiolkovsky Rocket Equation:&#xA;$$&#xA;\Delta V\ = v_e \ln \frac {m_0} {m_1}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have the ∆V needed. So now experiment with different payload masses (adjusting the total starting mass accordingly). You can thus get an idea of how large your launcher needs to be to send a reasonably sized manned payload to Mars. Rinse and repeat for any other target you have in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/2837/1028&quot;&gt;This answer&lt;/a&gt; performed the calculation for a manned mission to Mars and found: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I plugged the data provided by @PearsonArtPhoto into the rocket&#xA;  equation, with Isp=304 seconds (Merlin 1C in vacuum), and I get a&#xA;  &lt;strong&gt;launch mass of 262 tons to get a Dragon into Mars orbit&lt;/strong&gt;. A Falcon&#xA;  9v1.0 weighs 333 tons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h1&gt;Launcher Classes&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Use these values to get started.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Saturn V&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The Saturn V remains the tallest, heaviest, and most powerful rocket ever brought to operational status and still holds records for the heaviest payload launched and largest payload capacity to low Earth orbit (LEO) of 118,000 kilograms (260,000 lb). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launch Mass:&lt;/strong&gt; 2,970,000 kilograms&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Maximum Payload to orbit:&lt;/strong&gt; 118,000 kilograms&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Falcon Heavy&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Falcon Heavy (FH), is a spaceflight heavy lift launch vehicle being designed and manufactured by SpaceX. The Falcon Heavy is a variant of the Falcon 9 v1.1 launch vehicle and will consist of a standard Falcon 9 rocket core, with two additional strap-on boosters derived from the Falcon 9 first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launch Mass:&lt;/strong&gt; 1,462,836 kilograms&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Maximum Payload to orbit:&lt;/strong&gt; 53,000 kilograms&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Atlas V-551&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Atlas V is an active expendable launch system in the Atlas rocket family. Each Atlas V rocket uses an RD-180 engine burning kerosene and liquid oxygen to power its first stage and an American-built RL10 engine burning liquid hydrogen and liquid oxygen to power its Centaur upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launch Mass:&lt;/strong&gt; 334,500 kilograms&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Maximum Payload to orbit:&lt;/strong&gt; 18,814 kilograms&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-05-31T07:36:46.237" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9271" PostTypeId="2" ParentId="9260" CreationDate="2015-05-31T07:43:39.457" Score="38" Body="&lt;p&gt;&quot;Rigorous tests&quot; doesn't begin to describe the process used to make sure there are no bugs in the Shuttle software. &#xA;&lt;a href=&quot;https://www.fastcompany.com/28121/they-write-right-stuff&quot;&gt;This massive article details how the process works&lt;/a&gt;.&lt;br&gt;&#xA;A few salient points:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Shuttle software consists of ca. 420,000 lines. The total bug count hovers around 1. At one point around 1996, they built 11 versions of the code with a total of 17 bugs. Commercial programs of similar complexity would have thousands of bugs. &lt;/li&gt;&#xA;&lt;li&gt;The process of writing the software is beyond meticulous. They needed an update to the GPS software (6300 lines of code). Before any work was done on the code, they wrote a 2,500 page specification detailing the changes they needed.  &lt;/li&gt;&#xA;&lt;li&gt;Every change to the code is documented. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... the history of the code itself — with every line annotated, showing every time it was changed, why it was changed, when it was changed, what the purpose of the change was, what specifications documents detail the change. Everything that happens to the program is recorded in its master history. The genealogy of every line of code — the reason it is the way it is — is instantly available to everyone.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Every bug is analyzed exhaustively: &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For every one of those errors, the database records when the error was discovered; what set of commands revealed the error; who discovered it; what activity was going on when it was discovered — testing, training, or flight. It tracks how the error was introduced into the program; how the error managed to slip past the filters set up at every stage to catch errors — why wasn't it caught during design? during development inspections? during verification? Finally, the database records how the error was corrected, and whether similar errors might have slipped through the same holes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Everything the software group does is structured in processes, and every error is a reason to examine and improve these processes: &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How do they write the right stuff?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The answer is, it's the process. The group's most important creation is not the perfect software they write — it's the process they invented that writes the perfect software.&lt;br&gt;&#xA;  ...&lt;br&gt;&#xA;  Don't just fix the mistakes — fix whatever permitted the mistake in the first place.&#xA;  The process is so pervasive, it gets the blame for any error — if there is a flaw in the software, there must be something wrong with the way its being written, something that can be corrected. Any error not found at the planning stage has slipped through at least some checks. Why? Is there something wrong with the inspection process? Does a question need to be added to a checklist?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, Shuttle software is written to the highest standards in the world. And that's just one layer of the system NASA devised to prevent the control computers from causing trouble.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle is controlled by 5 &lt;a href=&quot;https://en.wikipedia.org/wiki/IBM_AP-101&quot;&gt;AP-101&lt;/a&gt; &lt;a href=&quot;https://www.nasa.gov/mission_pages/shuttle/flyout/flyfeature_shuttlecomputers.html&quot;&gt;General Purpose Computers&lt;/a&gt;. After an upgrade in 1991, they had 1 Mb of memory and ran at 1.4 MIPS. But they are built like a tank: each one weighs 64 pounds. It has been designed to run for years without crashing. Its memory circuits can detect memory cells becoming corrupted due to radiation, and will prevent the corrupt data from being used. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A software change typically goes through about nine months of in-house simulator testing and then another six months of testing in a unique NASA lab before it is accepted for flight. The results of the strenuous testing regimen? Well, it has been 24 years since the last time a software problem required an on-orbit fix during a mission. In the last 12 years, only three software errors have appeared during a flight. But perhaps the most meaningful statistic is that a software error has never endangered the crew, shuttle or a mission's success. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Each of the 5 GPCs can &lt;a href=&quot;https://www-03.ibm.com/ibm/history/exhibits/space/space_shuttle.html&quot;&gt;control the Shuttle on its own if necessary&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Five IBM computers — four of which were arranged in a redundant configuration, with a fifth computer acting as a backup unit — allowed early Shuttle missions to continue even if multiple failures were experienced. The computers cross-checked each other more than 500 times a second. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://history.nasa.gov/sts1/pages/computer.html&quot;&gt;4 redundant computers&lt;/a&gt; run the PASS (Primary Avionics Software System). The fifth computer runs an entirely separate program, the BFS (Backup Flight Control System). It has the same functions as the PASS, but is written and maintained by a different group. This adds another layer of security: a bug cannot affect all 5 computers at the same time. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the Shuttle, four identical AP-101Bs would function simultaneously as a quadruple-redundant set during critical mission phases such as ascent and reentry, processing the same information, derived from completely separate data buses, in precise synchronization. If a conflict arose among the four primary computers, the majority would rule, voting the conflicting unit out of the loop. None of the computers, singly or en masse, could turn off any other—that step was left to the crew.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;To ensure that the BFS was as independent as possible, NASA contracted with Rockwell to write it, and even different development environments and configuration management systems were specified. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The software is written in the &lt;a href=&quot;https://en.wikipedia.org/wiki/HAL/S&quot;&gt;HAL/S language&lt;/a&gt;, which was specifically designed for aerospace use.   &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The three key principles in designing the language were reliability, efficiency, and machine-independence. The language is designed to allow aerospace-related tasks (such as vector/matrix arithmetic) to be accomplished in a way that is easily understandable by people who have spaceflight knowledge, but may not necessarily have proficiency with computer programming.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;HAL/S was designed not to include some constructs that are thought to be the cause of errors. For instance, there is no support for dynamic memory allocation. The language provides special support for real-time execution environments.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Of course this doesn't come &lt;a href=&quot;http://history.nasa.gov/sts1/pages/computer.html&quot;&gt;cheap&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In an industry where the average line of code cost the government (at the time of the report) approximately \$50 (written, documented, and tested), the Primary Avionics System Software cost NASA slightly over \$1,000 per line. A total of \$500 million was paid to IBM for the initial development and support of PASS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you want to know more:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nap.edu/catalog/2222/an-assessment-of-space-shuttle-flight-software-development-processes&quot;&gt;An Assessment of Space Shuttle Flight Software Development Processes&lt;/a&gt; (free PDF downloadable after registration).  &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110014946_2011015696.pdf&quot;&gt;The Legacy of Space Shuttle Flight Software&lt;/a&gt; (PDF). This document lists 31 Operational Increment (OI) releases (i.e. major software changes) (page 22).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-11-29T17:02:50.377" LastActivityDate="2015-11-29T17:02:50.377" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="9272" PostTypeId="2" ParentId="9263" CreationDate="2015-05-31T09:09:12.027" Score="2" Body="&lt;p&gt;I believe all orbits are ultimately unstable, but they can still remain stable for a very long time, certainly billions perhaps trillions of years.   The most stable relatively close 2 body orbit is probably one where both planets are tidally locked with each other (Pluto and Charon are in such a formation and close enough that they exchange gas with each other).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Charon_%28moon%29&quot; rel=&quot;nofollow&quot;&gt;http://en.wikipedia.org/wiki/Charon_%28moon%29&lt;/a&gt;&#xA;&lt;a href=&quot;http://news.discovery.com/space/pluto-and-moon-charon-may-share-same-atmosphere-140609.htm&quot; rel=&quot;nofollow&quot;&gt;http://news.discovery.com/space/pluto-and-moon-charon-may-share-same-atmosphere-140609.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The earth's radius is 6,371 KM and the Moon's average distance 384,400 KM - about 60 earth radius from earth.   In terms of view, the moon, while it appears fairly large in the sky, it is only about 1/2 of 1 degree.   If the moon was the size of the earth and the same distance the &quot;dual earths&quot; would take up about 2% of the sky - almost 4 times the diameter of the moon and that would be a stable orbit, but the tides would be about 80 times as strong (earth's mass equals 80 moons), so, that means about 300 foot waves on the Atlantic and maybe 400 footers on the pacific, perhaps even bigger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mutual tidal locking solves the giant waves on the ocean problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Roche Limit - mentioned above is the hard limit to any planet to planet or planet to moon orbit, though to be safe, you'd want to be a fair distance above that.  (Roche limit is roughly 2.5 radius from the center, 1.5 radius from the surface - that's insanely close).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They think the moon, when it formed, was about 15 times closer than it is now.    &lt;a href=&quot;http://curious.astro.cornell.edu/our-solar-system/37-our-solar-system/the-moon/the-moon-and-the-earth/31-how-close-was-the-moon-to-the-earth-when-it-formed-intermediate&quot; rel=&quot;nofollow&quot;&gt;http://curious.astro.cornell.edu/our-solar-system/37-our-solar-system/the-moon/the-moon-and-the-earth/31-how-close-was-the-moon-to-the-earth-when-it-formed-intermediate&lt;/a&gt;  (taking up about 7 degrees in the sky - might not sound like much, but it would have appeared enormous), and we probobly had 1000 foot waves as a result - but the tidal forces spiraled the moon away from the earth fairly quickly.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto and Charon are an average of about 19,570 KM apart and Pluto's diameter is about 1,184 KM, so Charon orbits Pluto (or they orbit each other) at about 16.5 Pluto radius.  On the surface of Pluto, Charon would appear about 7 times as large as our moon does (it's a bit over 1/3rd the diameter of the moon, but some 17 times closer to Pluto than the moon is to the earth).   On the surface of Charon, Pluto would appear nearly 14 times larger than the moon or about 3.5 times larger than our earth appears from our moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's worth pointing out that such planet to planet orbits are probably unlikely, but a number of binary asteroids have been observed and many binary star systems, so binary planets could be possible . . . though probably rare, as the formation of planets during the formation of solar systems wouldn't lend itself to binaries, and it would probably require a capture, which would likely be rare or a &quot;just right&quot; collision like the one that formed our moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A curious fact, if the objects are small enough and orbiting each other, (say about the size of Mars Moon Phobos), it might be possible to throw a baseball from one to the other (the escape velocity of Phobos is 25 MPH - well within baseball throwing velocity)  &lt;a href=&quot;http://www.space.com/20346-phobos-moon.html&quot; rel=&quot;nofollow&quot;&gt;http://www.space.com/20346-phobos-moon.html&lt;/a&gt;  It might even be possible to play catch between a pair of binary asteroids.  Might take some practice though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, as mentioned above, to avoid tidal problems, you'd want the 2 planets to be mutually tidally locked, which means, they would always be in the same part of the sky all the time (not as much fun as orbiting each other I suppose), but that's the best way to avoid the giant waves problem - either that or move them quite far apart so that they'd appear about the size of the moon in the sky (also less fun).   If they are tidally locked with each other, then their distance would determine the rate of rotation (length of the day), so on Earth, if we had another earth, tidally locked, we'd want it at about a geosynchronous orbit (about 5.5 earth radius from the surface). to maintain a 24 hour day. - at least, I think that's about correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(too much?)&lt;/p&gt;&#xA;" OwnerUserId="8711" LastActivityDate="2015-05-31T09:09:12.027" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9273" PostTypeId="1" AcceptedAnswerId="9274" CreationDate="2015-05-31T10:21:25.773" Score="20" ViewCount="4405" Body="&lt;p&gt;Compared to most rockets I know of, Space X's Falcon 9 seems unusually tall and skinny. The v1.1 version is nearly 70m tall, yet only 3.6m thick.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, Atlas V is 58m tall and 3.8m thick, Proton-M is 58m tall and 7.4m thick, etc…&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what's the reason for Falcon 9's shape? Without any particular knowledge about rocketry, I can imagine a shorter, thicker rocket might be more stable and durable. What are the advantages (and disadvantages) of this approach?&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="51" LastEditDate="2015-05-31T11:30:14.130" LastActivityDate="2017-05-27T02:20:50.257" Title="Why is Falcon 9's shape so tall and skinny?" Tags="|launch|spacex|falcon-9|design|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9274" PostTypeId="2" ParentId="9273" CreationDate="2015-05-31T10:42:10.813" Score="30" Body="&lt;p&gt;SpaceX manufactures their booster in Hawthorne, CA. They then truck it on the highways to McGregor, TX for test firing with all 9 engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then it is back on a truck for the drive to Florida to LC-40 at the Air Force station for launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The diameter of the booster at 12 feet/3.6 meters is the largest they could go and still be road transportable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delta IV in comparison is built in Decatur, AL and shipped by barge to the Cape or Vandenberg for launch as it is too wide. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original Falcon 9, the 1.0 version was the appropriate size for its width in order to have enough fuel and oxidizer for a mission to orbit based on how long the Merlin-1C engines needed to fire for that mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 1.1 version changed the engine to the Merlin-1D a higher thrust engine, which means it also consumed more fuel and oxidizer so the tanks had to get bigger to hold the extra propellant. They could not go wider, and stay road transportable so they went longer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The legs are transported separate, and attached at the launch site as they make it just too tall to transport.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Amusingly it is height, not width that is the limiting factor. But for a round stage that distinction is without a difference. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-05-31T10:42:10.813" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="9275" PostTypeId="2" ParentId="9273" CreationDate="2015-05-31T10:45:15.190" Score="7" Body="&lt;p&gt;Your assumption is correct, a shorter and thicker rocket is obviously more stable on the ground. However the purpose of a rocket is not to stand strong on the launchpad forever. ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it comes to actually taking off into the stratosphere and beyond, a rocket needs to be &lt;strong&gt;aerodynamic&lt;/strong&gt;. Drag (air resistance) depends on the &lt;em&gt;cross-sectional area&lt;/em&gt; of a body, in this case the thickness of the rocket. Therefore by being a thinner rocket, the Falcon cuts through the atmosphere with less drag, wasting less fuel pushing against the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With reference to the comparisons you made; both Atlas V and Proton M were transported via rail, allowing for a thicker width. SpaceX moves it's booster from California to Texas, and then to Florida &lt;strong&gt;by road&lt;/strong&gt;. The thin design is small enough to be loaded onto a truck and driven around.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-05-31T10:45:15.190" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9277" PostTypeId="1" AcceptedAnswerId="9278" CreationDate="2015-05-31T13:55:33.100" Score="13" ViewCount="4160" Body="&lt;p&gt;Obviously, there is a 4.5 hour time lag so we can't expect live video, but will they have  a telecast of some sort showing images as they come in, along with the latest telemetry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Will there be a livestream by NASA during the Pluto Flyby by New Horizons on the 14th of June?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE:&lt;/strong&gt; There is an application called '&lt;a href=&quot;http://eyes.jpl.nasa.gov/eyes-on-pluto.html&quot; rel=&quot;nofollow&quot;&gt;Eyes by NASA'&lt;/a&gt; developed by JPL which shows a very detailed 3-D simulation of the New Horizons flyby, as well as details about the instruments and what they are focussed on. Watching Eyes on Pluto during the flyby, in real-time mode, &lt;em&gt;would be a similar experience to watching it live.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-07-11T20:40:04.340" LastActivityDate="2015-07-11T20:40:04.340" Title="Will the Pluto flyby by New Horizons be broadcast online?" Tags="|nasa|new-horizons|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9278" PostTypeId="2" ParentId="9277" CreationDate="2015-05-31T15:26:08.237" Score="27" Body="&lt;p&gt;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/03101637-pluto-image-expectations.html&quot; rel=&quot;noreferrer&quot;&gt;There's no point in having a live stream, because we won't receive encounter data in real time&lt;/a&gt;. On the 14th, New Horizons will be busy aiming its instruments at Pluto. Due to the design of the spacecraft, it can't do that and transmit information at the same time. The &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto/Mission-Timeline.php&quot; rel=&quot;noreferrer&quot;&gt;cameras and antenna&lt;/a&gt; are all fixed in place, so they have to rotate the entire spacecraft to aim them, and with the cameras pointed at Pluto the antenna doesn't point at Earth.&lt;br /&gt;&#xA;New Horizons will send one photo on July 13th, a few photos on July 15th and 16th, and more data will follow later. It will take until November 2016 to download everything.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Through July 12, New Horizons will take regular LORRI photos of Pluto, Charon, Nix, and Hydra, which will mostly be returned soon after acquisition because they are used for optical navigation.&lt;/p&gt;&#xA;&lt;p&gt;On Sunday, July 12, New Horizons will transmit the last of its optical navigation data. These images will have lower resolution than the images we have already received from Dawn at Ceres. Then, on Sunday and Monday, July 12 and 13, there will be a series of four &amp;quot;Fail Safe&amp;quot; downlinks. These are designed to return a minimum set of data from all instruments, just in case New Horizons does not survive the flyby. A last downlink ending overnight Monday July 13, called &amp;quot;E-Health 1,&amp;quot; will include one last pre-closest approach photo of Pluto.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-05-31T18:42:35.780" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9279" PostTypeId="2" ParentId="9277" CreationDate="2015-05-31T15:48:40.080" Score="15" Body="&lt;p&gt;No. NASA will release select low resolution images as shortly after the flyby as they can. They won't release any imagery during the flyby because the spacecraft won't be communicating with the Earth during the flyby. The focus of the mission during the flyby is observing Pluto. Sending data to the Earth runs directly against the grain of the primary mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two issues at hand. One is that New Horizons doesn't have a scan platform. This means that the high-gain antenna is fixed with respect to the spacecraft, as are all of the scientific payloads. This creates a conflict during the flyby: Should New Horizons point it's antenna Earthward, or should it point its sensors toward Pluto? It can't do both. The answer is obvious: New Horizons needs to orient itself so that its payloads are pointed toward Pluto during the flyby, and that means the antenna is not pointed toward the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other issue is data rate. The post-encounter downlink rate is expected to be 1 kbps (1000 bits per second). If you were around when the web was first created in the mid 1990s, you may remember how painstakingly long it took to receive even low resolution images from some web site. The typical mid 1990s modem was 16 times faster than the New Horizons post-encounter downlink. Several minutes will be needed to transmit a highly compressed, low resolution black and white image. A high resolution image will take hours. It will take a long, long time for New Horizons to downlink all of the data that it recorded before, during, and after the flyby.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-05-31T15:48:40.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9280" PostTypeId="1" CreationDate="2015-05-31T16:28:04.097" Score="9" ViewCount="676" Body="&lt;p&gt;An answer to &lt;a href=&quot;https://space.stackexchange.com/questions/9277/will-the-pluto-flyby-by-new-horizons-be-broadcast-online&quot;&gt;this question&lt;/a&gt; states that the expected downlink rate for New Horizons at Pluto encounter will be about 1,000 bits/sec. Understanding the issues of distant transmitters and data rates as posed in &lt;a href=&quot;https://space.stackexchange.com/questions/7776/why-does-data-transfer-rate-decrease-with-distance&quot;&gt;this question&lt;/a&gt;,&#xA;my question is about actual numbers. Are the values stated for New Horizons &quot;as good as it gets&quot;, or is it just the result of a limited mission budget? Could a modest increase in mission expenditure offer a meaningful increase in downlink data rate, or would even a modest increase in performance require a substantial increase in on-board equipment, and therefore mission cost?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also: data is still being downlinked from Voyager 1 despite its much greater distance from Earth. How does Voyager's power supply and transmitter differ from New Horizons in so far as it affects data rates?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-06-01T17:01:44.013" Title="What is the highest practical data rate for a downlink from a spacecraft at a distance such as Pluto to Earth?" Tags="|probe|radio-communication|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9282" PostTypeId="2" ParentId="9280" CreationDate="2015-06-01T10:14:44.507" Score="5" Body="&lt;p&gt;There are two ways to increase transmission speed. Both rely on increasing the power of the radio transmission.&lt;br&gt;&#xA;1. Diameter: &lt;a href=&quot;https://en.wikipedia.org/wiki/Antenna_gain&quot; rel=&quot;nofollow&quot;&gt;antenna gain&lt;/a&gt; increases with the square of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Parabolic_antenna#Gain&quot; rel=&quot;nofollow&quot;&gt;antenna diameter.&lt;/a&gt;&lt;br&gt;&#xA;2. Transmission power: RF power increases with the log of the amplifier input power.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you increase the antenna diameter, you run into limitations of the launcher: the fairing must be large enough to accommodate the antenna. Alternatively, you can build a folding antenna: the Russian radio telescope satellite &lt;a href=&quot;http://www.russianspaceweb.com/spektr_r_design.html&quot; rel=&quot;nofollow&quot;&gt;Spektr-R&lt;/a&gt; has an antenna diameter of 10 m. But this hasn't been done a lot. It makes the spacecraft a lot more complicated and adds failure points.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Increasing transmitter power also has issues. New Horizons and Voyager are powered by RTGs, which are very expensive. A &lt;a href=&quot;http://www.lpi.usra.edu/opag/nh2_final_report.pdf&quot; rel=&quot;nofollow&quot;&gt;2005 report&lt;/a&gt; estimated the cost of a New Horizons-class RTG at $60-90 million. The main driver is the cost of producing Plutonium-238. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So mission budget plays a large role. Antenna diameter feeds into the launch weight and fairing size, RTG power goes directly to cost. New Horizons used the biggest rocket available at the time apart from the Shuttle, Voyager benefited from multiple gravity assists. Increasing launch weight would have resulted in a slower launch and longer travel times, which also add to the cost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's compare our 2 data points: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;nofollow&quot;&gt;Voyager&lt;/a&gt;:&lt;br&gt;&#xA; - 470 W of power available at launch,&lt;br&gt;&#xA; - launch weight 721 kg,&lt;br&gt;&#xA; - 3.7 m main antenna.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow&quot;&gt;New Horizons&lt;/a&gt;:&lt;br&gt;&#xA; - 213 W of power available at launch,&lt;br&gt;&#xA; - launch weight 478 kg,&lt;br&gt;&#xA; - 2.1 m main antenna.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say you could increase transmission speed by a factor of 50 (go from a 2 m antenna to 10 m, and double the transmitter power). From Pluto, that's still only 25 kbit/s. This would shorten the download phase from 1.5 years to 2 weeks, but doesn't fundamentally change the equation: the encounter data still starts to arrive long after the encounter.&lt;br&gt;&#xA;Also your spacecraft now weighs 4 tons (WAG based on the weight of Spektr-R) and needs approximately a Saturn V to launch at sufficient speed. That money would easily pay for a year of DSN time.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-06-01T17:01:44.013" LastActivityDate="2015-06-01T17:01:44.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9283" PostTypeId="1" AcceptedAnswerId="9286" CreationDate="2015-06-01T11:36:53.707" Score="19" ViewCount="9841" Body="&lt;p&gt;What are the drag coefficients for a cylinder, a wedge, etc?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know there are other reasons for a rocket to be cylindrical that aren't related to aerodynamics such as efficiency when mixing the propellants etc&amp;hellip; or is that the exclusive reason why they are cylindrical?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is it more related to the logistics or both? If it is due to both things being true, then with 3D printing etc bringing costs of building other shapes down, is it likely we will see a wedge shaped rocket anytime soon?&lt;/p&gt;&#xA;" OwnerUserId="8984" LastEditorUserId="6816" LastEditDate="2015-06-01T21:27:39.677" LastActivityDate="2022-06-03T15:59:24.190" Title="Why are rockets cylindrical?" Tags="|launch|rockets|design|atmospheric-drag|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9285" PostTypeId="2" ParentId="9283" CreationDate="2015-06-01T11:52:25.507" Score="10" Body="&lt;p&gt;It's a combination of factors. Basically, cylinders are easy to make at large sizes, have low drag, and overall work well. The bulk of the drag is going to come from the top or bottom ends. The bottom end contains the engines, which have particular shape requirements. The top contains the payload typically, which also has particular requirements. The tank needs to have a shape that works with both. A cylinder is nearly optimal, and vastly easier to make than the air foil shape that would be optimal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for 3-d printing, rockets are large enough where it would be very difficult to make such a thing. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-01T11:52:25.507" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9286" PostTypeId="2" ParentId="9283" CreationDate="2015-06-01T11:53:00.023" Score="34" Body="&lt;p&gt;Rockets are cylindrical for the same reason maize silos are cylindrical: A circle has the largest area vs perimeter of any shape and also provides maximum strength from internal pressure. This means you can save on weight for the walls of a rocket when it is cylindrical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A cylinder is not the absolute best aerodynamic shape as &lt;a href=&quot;https://www.apogeerockets.com/Technical_Publication_1&quot;&gt;the Drag Coefficient section of this document&lt;/a&gt; shows (Also see the &lt;a href=&quot;http://en.wikipedia.org/wiki/Sears%E2%80%93Haack_body&quot;&gt;Sears-Haack Body&lt;/a&gt;), but aerodynamics are most certainly not the only consideration. On the other hand, the nose of a rocket is usually a cone purely for aerodynamic reasons.&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="8157" LastEditDate="2015-06-02T08:28:18.027" LastActivityDate="2015-06-02T08:28:18.027" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="9287" PostTypeId="2" ParentId="9283" CreationDate="2015-06-01T12:41:42.600" Score="8" Body="&lt;p&gt;Rockets fly mostly at supersonic speeds. Under those conditions, the Area Rule (constant cross-section area, or at least smoothly varying) is the most important determinant of drag. A cylinder trivially has a constant cross section, and the nose cone serves as a smooth transition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, a long rectangular box could also have a constant cross-section, but at that point neelsg's answer already explains why a cylinder is ideal. &lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2015-06-01T12:41:42.600" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9288" PostTypeId="1" AcceptedAnswerId="9289" CreationDate="2015-06-01T12:49:58.643" Score="11" ViewCount="901" Body="&lt;p&gt;In &lt;a href=&quot;http://photojournal.jpl.nasa.gov/jpegMod/PIA19057_modest.jpg&quot; rel=&quot;noreferrer&quot;&gt;this photograph&lt;/a&gt; of &lt;a href=&quot;http://en.wikipedia.org/wiki/Rhea_(moon)&quot; rel=&quot;noreferrer&quot;&gt;Rhea&lt;/a&gt;, taken by the Cassini orbiter, there are sections near the terminator that are very blurred, a stark contrast from the crisp sharpness of the rest of the image. The blurred sections look polygonal in shape, with defined edges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zgiRB.jpg&quot; alt=&quot;&amp;#39;Return to Rhea&amp;#39; (PIA19057). Zoomed in to the terminator, blurred portion highlighted. Courtesy NASA JPL&quot;&gt;&#xA;&lt;sup&gt;Return to Rhea (PIA19057). Zoomed in to the terminator, blurred portion highlighted. Courtesy &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA19057&quot; rel=&quot;noreferrer&quot;&gt;NASA JPL&lt;/a&gt;. &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why are these portions blurred?&lt;/strong&gt; My theory is that Cassini could not take photographs of those regions with its narrow-field camera, and hence used wide-field data instead. &lt;strong&gt;Am I right?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="49" LastEditDate="2015-06-02T20:06:25.223" LastActivityDate="2016-06-10T21:46:01.923" Title="Why are sections of Cassini photographs blurred?" Tags="|moons|imaging|cassini|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9289" PostTypeId="2" ParentId="9288" CreationDate="2015-06-01T13:12:21.520" Score="14" Body="&lt;p&gt;Your guess is correct.  To quote &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA19057&quot;&gt;NASA's page&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The mosaics each consist of multiple narrow-angle camera (NAC) images with data from the wide-angle camera used to fill in areas where NAC data was not available.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-06-01T13:12:21.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9292" PostTypeId="1" AcceptedAnswerId="12033" CreationDate="2015-06-01T21:33:23.750" Score="3" ViewCount="68" Body="&lt;p&gt;I recently realized that the state of the galaxy as we perceive it from Earth is entirely unique; because of relativity, celestial objects are all viewed with out of date information, but as we observe the sum of them, this information is unique to our position in the Cosmos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, would it be possible for a sufficiently advanced civilization (perhaps our own) to calculate the location of the source of observations that included the red/blue shift and spectral output of several celestial objects? I envision a probe that travels at less than relativistic speeds being captured long after its departure, but this might work even for other methods of information transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this would merely require that spectral emissions of bodies be sufficiently unique (don't know if that is true) and to be able to calculate the age of the data somehow (half-life? But I think this is usually normalized to the known age of a planet/source of the material that is decaying).&lt;/p&gt;&#xA;" OwnerUserId="10107" LastActivityDate="2015-09-18T09:10:15.330" Title="Galactic location from spectrum and red/blue shift" Tags="|celestial-mechanics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9293" PostTypeId="2" ParentId="8122" CreationDate="2015-06-01T22:29:44.080" Score="2" Body="&lt;p&gt;Thrust is thrust and its effects are the same no matter where it comes from.  In theory, yes electric propulsion such as hall effect thrusters, electrostatic ion thrusters and magnetoplasmadynamic (MPD) thrusters can be used to launch a satellite from the surface of the earth to orbit.  However, due to the very high exhaust velocity of electric propulsion systems they have a very high specific impulse, which is a nice way of saying that in theory they can produce a large change in spacecraft velocity for a small amount of propellant.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While a higher specific impulse may sound advantageous, the gravity and atmospheric drag at Earth's surface can counteract the effects of the high specific impulse.  By re-arranging the equations of motion and the equation for kinetic jet power we can see that as the specific impulse of an engine increases, the amount of power required to produce a pound of thrust also increases at a near logarithmic rate.  Because of this increase in the ratio of power to thrust, electric propulsion systems would require an enormous power source to produce enough thrust to accelerate against gravity and minimize drag losses.  Unfortunately, the specific mass of contemporary power systems (chemical, solar and nuclear) is too low to allow for this type of launch system.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in summary, if we could build power sources that could produce an enormous amount of power for a very low structure mass (~3 kg/kWe or less), that were also compact then yes, it is possible to use electric propulsion systems for launch vehicle applications.  However, this is beyond the current state of the art for power systems.&lt;/p&gt;&#xA;" OwnerUserId="5856" LastEditorUserId="5856" LastEditDate="2015-06-02T22:21:51.410" LastActivityDate="2015-06-02T22:21:51.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9295" PostTypeId="2" ParentId="6404" CreationDate="2015-06-02T00:20:39.643" Score="3" Body="&lt;p&gt;There used to be quite a lot of language problems right after the crews started to be international first on Mir and then ISS. But today, the cosmonauts are expected to speak several languages, whichever country they come from, as they are usually crazily well educated people anyway. As for the Russian parts of the ship, the the non russian austronauts selected for an ISS mission go through 2 months training to get familiar with the russian parts of the station and to get basic knowledge of Russian language.&lt;/p&gt;&#xA;" OwnerUserId="10292" LastActivityDate="2015-06-02T00:20:39.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9296" PostTypeId="1" CreationDate="2015-06-02T01:02:45.330" Score="8" ViewCount="8386" Body="&lt;p&gt;Looking at Gemini´s spacesuit G-4C I always wonder how did they make the neck joint able to swivel, allowing the user to look at both directions? Wouldnt be easier to make a slightly bigger helmet (i.e. SK-1 for Vostok) with a large faceplate visor?&lt;img src=&quot;https://i.stack.imgur.com/3yWto.png&quot; alt=&quot;Gemini spacesuit&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/UV6HX.jpg&quot; alt=&quot;Vostok SK-1 suit&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8761" LastActivityDate="2020-03-21T11:29:13.527" Title="How do the movable-neck space suit helmet works?" Tags="|rockets|spacecraft|crewed-spaceflight|spacecraft-development|spacesuits|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9297" PostTypeId="1" CreationDate="2015-06-02T01:29:14.643" Score="5" ViewCount="1141" Body="&lt;p&gt;Looking at Soyuz 11A511 type rocket (the one used for launch Soyuz T-M-and A-M models) you can notice that the payload fairing brake flaps are displaced from the ideal axis (they are not aligned with the middle space between side blocks B.V.G and D). The SAS escape tower nozzles are aligned, though. This make the fairing somewhat asymmetrical. Its clear that , if the rocket works for more than 45 years, the design is sound; But I wonder if there is a limit for putting external devices away from the axis of symmetry. As an example I mention the Soyuz and also the Titan 3-series solid motors, wich have big TVC-feeding-tanks on their sides.&lt;img src=&quot;https://i.stack.imgur.com/hIGdv.png&quot; alt=&quot;Soyuz front view&quot;&gt;  &lt;img src=&quot;https://i.stack.imgur.com/XQrv9.png&quot; alt=&quot;Titan SRBs TVC tanks&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8761" LastEditorUserId="168" LastEditDate="2015-11-06T18:34:32.773" LastActivityDate="2015-11-06T18:34:32.773" Title="What is the limit for asymmetrical design for a launch vehicle?" Tags="|rockets|spacecraft|space-shuttle|crewed-spaceflight|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9298" PostTypeId="2" ParentId="9296" CreationDate="2015-06-02T02:40:38.593" Score="7" Body="&lt;p&gt;Let's find a bigger version of the image that you attach and &lt;em&gt;blow it up&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xGPzQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, they used bearing with an arc rail to hold the umbilical in place. The outer ring of the bearing is attached to the collar of the suit, and the helmet slides into a locked position and attaches to the inner ring of the bearing. Similar bearings are still used on newer EVA suits in hips and gloves, and some newer designs to support future human exploration of Mars use even more of them at various angles (two in pairs at a canted angle to each other) to enable more natural joint movement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://www.dylpilot.org/photos.html&quot; rel=&quot;noreferrer&quot;&gt;image gallery&lt;/a&gt; at the source of above image I crop (basically I found it by drag-dropping your image into Google Image Search's search bar and &lt;a href=&quot;https://www.google.com/search?biw=1518&amp;amp;bih=949&amp;amp;q=gemini%20suit&amp;amp;tbm=isch&amp;amp;tbs=simg:CAQSJgnZWXpWpOcojBoSCxCwjKcIGgAMCxCOrv4IGgAMITArBRUVQEsL&amp;amp;sa=X&amp;amp;ei=JBZtVdGLEsz5UtqwgJAF&amp;amp;ved=0CBsQ2A4oAQ#imgrc=2Vl6VqTnKIwuZM%253A%3BMCsFFRVASwtqdM%3Bhttp%253A%252F%252Fwww.dylpilot.org%252Fuploads%252Fimages%252FGallery%252Fspacesuits%252F15%252520-%252520w2F4XKI.jpg%3Bhttp%253A%252F%252Fwww.dylpilot.org%252Fphotos.html%3B3189%3B4000&quot; rel=&quot;noreferrer&quot;&gt;cropped from the highest resolution it found&lt;/a&gt;), and there's some more information regarding bearings on NASA's &lt;em&gt;&lt;a href=&quot;https://www.nasa.gov/audience/foreducators/spacesuits/home/clickable_suit_nf.html&quot; rel=&quot;noreferrer&quot;&gt;Learn About Spacesuits&lt;/a&gt;&lt;/em&gt; page.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-06-02T02:40:38.593" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9299" PostTypeId="2" ParentId="9297" CreationDate="2015-06-02T03:22:25.633" Score="7" Body="&lt;p&gt;Discounting aerodynamic effects, a rocket is generally stable if its thrust axis passes through its center of mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since it's not possible to maintain this balance perfectly due to wind, shifting live payload, fuel slosh, etc., the rocket needs some way of correcting for small imbalances - thrust vectoring, fins, reaction wheels, attitude control thrusters, or some combination thereof. This can also compensate for any small structural mass asymmetry (though designers can simply add ballast on one side to minimize the need).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a symmetrical cylindrical rocket, the center of mass is always more or less along the vertical axis of the rocket even as the CoM shifts while fuel is consumed and stages are dropped; the net thrust vector is easy to keep pointed along the vertical centerline. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle is the most well-known asymmetrical case. The main engines are canted outward to keep the thrust axis pointing through the center of mass of the stack, and the engines are on gimbals that can shift the thrust axis by up to 10º in any direction; the solid rocket boosters also have thrust-vectoring nozzles, so at all points in the ascent to orbit, the net thrust can point through the shifting center of mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In principle, with properly positioned and vectorable thrust elements, even a wildly asymmetric structure could fly stably, but mass asymmetry usually comes with aerodynamic asymmetry that is harder to compensate for, so in the absence of other factors -- like an upper stage that needs to glide to a landing, like the shuttle -- simple cylindrical symmetry is the optimal layout, with simple radial symmetry the second choice if you need additional booster thrust.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-06-02T03:22:25.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9300" PostTypeId="2" ParentId="6345" CreationDate="2015-06-02T05:50:46.927" Score="4" Body="&lt;p&gt;Performing even single flight to Mars needs research capacity that single company of SpaceX size have not. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The absence of simple illustrated concept however beyond names is really strange. During the years, a number of concepts of Mars transportation and Martian base/colony have been published sometimes by small nonprofit groups. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the moment, Mars looks to be rather motivation mantra than specific goal for SpaceX.&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-06-02T10:32:26.050" LastActivityDate="2015-06-02T10:32:26.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9301" PostTypeId="1" CreationDate="2015-06-02T07:17:14.637" Score="23" ViewCount="26623" Body="&lt;p&gt;The air pressure on Mars is about 1% that of Earth's at sea level. But wind speeds on Mars can be higher, up to maybe 60 mph sustained (actually I have come across several different figures, some going very high indeed.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you hold your hand out in a 60 mph wind that is 1% the density of air, can you feel it? Would you feel the pressure against your back? Could it blow you over? Could it blow a landing vehicle over, as in Weir's book, The Martian? Thank you! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edit) This question appears to be the same as the other question (asking what it would &quot;feel like to be in a Martian dust storm&quot;), but in the full post it is clear that question is really whether or not the sand in Martian windstorms would be abrasive. I am asking, literally and exactly, whether or not you could feel the wind as pressure on your body; my question has nothing to do with sand. The wind speed is often higher than on Earth, but the atmospheric density is far lower, making this a worthy question, I feel. &lt;/p&gt;&#xA;" OwnerUserId="10293" LastEditorUserId="10293" LastEditDate="2015-06-06T05:26:22.237" LastActivityDate="2016-06-25T02:03:04.630" Title="Could you feel the wind on Mars?" Tags="|mars|atmosphere|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9302" PostTypeId="1" AcceptedAnswerId="9899" CreationDate="2015-06-02T14:09:29.610" Score="7" ViewCount="241" Body="&lt;p&gt;Originally it was suggested by Elon Musk that the Dragon shown at the unveiling was the test article, or at least large components of it, to be used in the &lt;a href=&quot;http://www.youtube.com/watch?v=1_FXVjf46T8&quot;&gt;Pad Abort (May 6, 2015)&lt;/a&gt; and then the Max drag abort test due later in 2015.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in forums, people are suggesting that in fact the Dragon used for Pad Abort was not the demoed Dragon and instead was a earlier Dragon model retrofitted with Super Dracos. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core evidence offered was the interior shots shown in some video I cannot find look very different from the demoed Dragon V2.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot find evidence either way at this time. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-14T05:37:55.383" Title="What Dragon was used in May 2015 Pad Abort test?" Tags="|spacex|dragon|testing|dragon-v2|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9304" PostTypeId="1" AcceptedAnswerId="10443" CreationDate="2015-06-02T18:42:04.843" Score="10" ViewCount="1230" Body="&lt;p&gt;I have a numerical interplanetary trajectory model and am trying to find a way to accurately insert a spacecraft into a rendezvous orbit with its target planet having a particular periapse radius (for example, I'd like to have a spacecraft travel from Earth to Mars, and when it reaches Mars its periapse radius must be 200km above Mars' surface). Currently I can simulate a spacecraft trajectory from Earth to Mars, but have to get my required periapse radius by trial and error which is very time consuming. I've looked at Fundamentals of Astrodynamics (Bate et al.) which gives a way of doing this using a fully patched-conic approach using phase angles (pages 372 to 377). However I require a mid-course correction using a Lambert solver in my simulation and was wondering how I could calculate the required delta-V so that I will get the required periapse radius upon target arrival from this mid-course correction point. Any help would be greatly appreciated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Having read some more articles it seems that a term called the impact parameter is used a lot in literature when calculating planetary rendezvous and is given by,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$b=r_{p}\sqrt{1+\frac{2 \mu}{v_{\infty}^{2}r_{p}}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if I know what I want my spacecraft's periapse radius, $r_{p}$, to be when it reaches Mars, I know Mars' gravitation parameter, $\mu = GM$, and I know the spacecraft's hyperbolic excess velocity relative to Mars, $v_{\infty}$, when it enters Mars' sphere of influence, then would the value of $b$ given to me from the above equation be a radial &quot;offset value&quot; that I can put into a Lambert solver to get into the required orbit upon Mars rendezvous? That is, would the rendezvous aiming point that I give to the Lambert solver (in Cartesian coordinates) now be $ (x_{mars}, y_{mars}) + (b_{x}, b_{y})$, where $x_{mars}$ and $y_{mars}$ are the original  heliocentric x and y coordinates of Mars upon intercept and $b_{x}$ and $b_{y}$ are the offset values that I'll add to the original Mars intercept point that will give me the required periapse radius upon Mars' rendezvous?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xCJQD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="10130" LastEditorUserId="1142" LastEditDate="2015-08-03T02:10:33.980" LastActivityDate="2015-08-04T16:14:23.660" Title="Finding accurate periapse rendezvous radius using interplanetary Lambert solver" Tags="|trajectory|interplanetary|rendezvous|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9305" PostTypeId="2" ParentId="8761" CreationDate="2015-06-02T22:45:17.427" Score="4" Body="&lt;p&gt;As little as three years ago you could go on to the OSTI website and freely download many studies on this topic produced by DOE and AEC laboratories.  Unfortunately a few years ago the DOE started removing many of the reports from that public website.  However, a good publicly available technical reference with respect to this topic is the book &quot;Space Nuclear Power&quot;, by Buden and Angelo.  Within the book, there is an entire chapter dedicated to state of the art shield designs as of the 1980's and not much has changed since then.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The preferred materials are typically intermittent layers of Lithium-Hydride (which protects against neutrons) and tungsten (to protect against x-rays and gamma rays).  The shield (typically called a shadow shield) is placed between the reactor and the crew at just the right geometry to provide a radiation shadow for the crew compartment.  If the crew compartment is placed a long ways away from the reactor, then the size of the shield can be minimized, because the required shadow angular limits is reduced and the effects of geometric attenuation also reduce the required thickness of the shield.  However, as the distance between crew and shielding increases, then the structural mass of the craft increases.  At some distance, there is a minimum spacecraft mass, but this distance is dependent on the reactor, shield and spacecraft design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I cannot provide the resources for this right now, I can promise you that to net absorbed dose from cosmic radiation will dwarf that received due to the reactor.  In fact for nuclear thermal propulsion and some nuclear electric propulsion designs, the use of nuclear propulsion reduces the time in space and reduces the expected radiation exposure compared to non-nuclear mission designs.  It is ironic, but nuclear power reduces the radiation dose in most mission studies.&lt;/p&gt;&#xA;" OwnerUserId="5856" LastActivityDate="2015-06-02T22:45:17.427" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9306" PostTypeId="2" ParentId="388" CreationDate="2015-06-02T23:04:24.867" Score="14" Body="&lt;p&gt;With respect to potential travel in our own solar system there are two general types of radiation that have our concern!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first type of radiation is solar radiation, which mostly consists of low- to intermediate-energy protons, electrons and x-rays from our own star.  We would shield against the protons with low molecular mass materials.  Typically hydrogen-bearing materials such as Lithium-Hydride are used for this because of how effective they are at stopping the protons as well as neutrons that might come from future reactors and because of how light they are.  The electrons and photons (x-rays) are best stopped with high-Z materials.  High-Z materials are comprised of elements that have many electrons per atom.  While high-Z materials are used to stop electrons and photons, they are also useful in stopping other charged particles to include assisting with protons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second type of radiation is Galactic Cosmic Rays (GCRs).  GCRs are typically very high energy massive particles such as carbon, and iron atoms.  Due to their energetic nature and how massive these particles are, they are very hard to stop.  Stopping GCRs requires thick layers of high-Z materials, which are typically dense and heavy.  Heavy shielding is expensive and difficult to get into space.  I will not go so far as to say that we cannot shield against GCRs, but I will say that the weight of contemporary shielding materials makes it seem as if current approaches to GCR shielding are not practical.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our star is a type-G main sequence star, which produces Helium through proton-proton fusion at its core.  Because of the dynamics of fusion in our star, ionized Helium nuclides are the primary product of this fusion.  However, some of the Helium produced from proton-proton fusion is itself fused, which produces carbon.  As stars become more massive they start fusing heavier elements, which can be ejected into space.  Iron-56 is the heaviest element that can be produced from traditional stars, with the heaviest elements being produced by much more energetic events such as a supernova.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy produced from the fusion of these isotopes ionizes gases near the edge of our star, producing copious quantities of protons and electrons, which are flung into space during coronal mass ejections.  Numerically speaking the majority of radiation from our star as well as other stars are in the form of protons, electrons and photons, with lesser quantities of heavy nuclides.  Statistically speaking, the heavier the nuclides, the rarer it is to find them streaming in space.  While I am mainly speaking about our star, the same is true of other stars, regardless of their mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other stars do indeed produce protons, electrons and photons that stream into our solar sphere of influence; however, these other stars eject radiation in all directions, with only a &lt;strong&gt;very&lt;/strong&gt; small fraction of them being ejected in the narrow cone angle to make it to our solar system.  Much of the charged radiation from other stars is also deflected by the sun's magnetic field.  As a result, the vast majority of protons and electrons in our solar system were ejected from our star and not other stars and the ones that are not are mostly of the same energy as the protons and electrons ejected from our own star.  Because of this we essentially neglect non-solar protons and electrons from our radiation exposure calculations because they are negligible in their effect on absorbed dose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the heavy elements ejected from super-energetic events such as supernova are traveling at near light speeds and as a result, have a profound effect on biological tissue and electronics that they encounter.  Even though they make up a very small fraction of the total particle count per unit volume in space, the effects that they can have on absorbed dose is not negligible.  Therefore when we talk about galactic cosmic rays, we generally are talking about the energetic heavy ions from extra-solar energetic events and not the protons and electrons from normal, everyday extrasolar sources.&lt;/p&gt;&#xA;" OwnerUserId="5856" LastEditorUserId="6816" LastEditDate="2015-10-08T19:50:22.213" LastActivityDate="2015-10-08T19:50:22.213" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9307" PostTypeId="1" AcceptedAnswerId="9310" CreationDate="2015-06-03T01:19:29.433" Score="16" ViewCount="1124" Body="&lt;p&gt;The New Horizon's team, in response to an announcement of a press conference from the Hubble team, posted &lt;a href=&quot;https://t.co/R0rDGOvSWq&quot;&gt;some speculations&lt;/a&gt; as to what the press conference might be about. On the list includes &quot;Small bodies (or boulders) around Pluto, located in the Sailboat Island of stability&quot;. What is the &quot;Sailboat Island of Stability&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wtkBv.png&quot; alt=&quot;Predictions of remarkable things&amp;#x0a;&amp;#x0a;- Finding N2 on Charon!&amp;#x0a;- Arcs instead of rings&amp;#x0a;- Significant frozen-in fossil bulges&amp;#x0a;- A retrograde satellite&amp;#x0a;- Small bodies (or boulders) around Pluto, located in the Sailboat Island of stability.&amp;#x0a;- Evidence fer liquid flows on Pluto or Charon or both on the past.&amp;#x0a;- Pluto will exhibit some terrain morphology unlike that seen on any other planetary body explored so far.&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="595" LastEditDate="2015-06-27T15:54:10.340" LastActivityDate="2015-06-27T15:54:10.340" Title="What is a &quot;Sailboat Island of Stability&quot;?" Tags="|orbital-mechanics|orbit|terminology|new-horizons|pluto|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9308" PostTypeId="1" CreationDate="2015-06-03T01:26:24.000" Score="5" ViewCount="248" Body="&lt;p&gt;The orbital phase of many probes around a planet or moon is frequently described as N km x M km, with N and M being two numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, China's Chang'e 5 lunar orbiter was &quot;placed in a 200 by 5,300-Kilometer orbit&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the numbers refer to the periapsis and apoapsis, or the number of orbits times the average altitude?&lt;/p&gt;&#xA;" OwnerUserId="10300" LastEditorUserId="49" LastEditDate="2015-06-03T03:22:55.257" LastActivityDate="2020-08-20T05:36:37.247" Title="How does orbital altitude nomenclature work?" Tags="|orbital-mechanics|orbit|terminology|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9309" PostTypeId="2" ParentId="9308" CreationDate="2015-06-03T01:32:57.693" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;placed in a 200 by 5,300-Kilometer [lunar] orbit&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The mean radius of the Moon, $R_☾ = 1,737.1 \text{ km}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$R_{perilune} = 200 \text{ km} + R_☾$ and $R_{apolune} = 5,300 \text{ km} + R_☾$ are the radii of an ellipse, perilune and apolune respectively, centered at the orbital focus, the Moon's center.&lt;/p&gt;&#xA;" OwnerUserId="8661" LastEditorUserId="49" LastEditDate="2015-06-03T03:39:49.427" LastActivityDate="2015-06-03T03:39:49.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9310" PostTypeId="2" ParentId="9307" CreationDate="2015-06-03T01:35:03.157" Score="21" Body="&lt;p&gt;The &lt;em&gt;“Sailboat Island”&lt;/em&gt; gets its name from a &lt;a href=&quot;https://en.wikipedia.org/wiki/Poincar%C3%A9_map&quot; rel=&quot;noreferrer&quot;&gt;Poincaré Surface-of-Section&lt;/a&gt; (SOS) plot of potentially stable S-type family of orbits in the Pluto-Charon system. From NASA Ames' blog post titled &lt;a href=&quot;https://blogs.nasa.gov/mission-ames/2013/07/25/playing-marbles-at-pluto-looking-at-the-dynamic-dust-environment-generators-sweepers-and-sweet-spots/&quot; rel=&quot;noreferrer&quot;&gt;Playing Marbles at Pluto. Looking at the Dynamic Dust Environment. Generators, Sweepers, and Sweet-Spots&lt;/a&gt; by Kimberly Ennico:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Othon Winter (UNESP Brazil) spoke about “On the Relevance of the&#xA;  Sailboat Island for the New Horizons Mission.” In investigating where&#xA;  particles would find stable orbits, their modeling predicted a region&#xA;  where there was a cluster of orbits characterized by high eccentricity&#xA;  (e= 0.2 to 0.8) and located around 0.6 Pluto-Charon semi-major axis&#xA;  (i.e. between Pluto and Charon). They nicknamed it “Sailboat Island’&#xA;  because on a eccentricity vs. distance from Pluto plot it looked like&#xA;  a sailboat. This population of “stable orbits” had not been predicted&#xA;  from previous work.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/opZ4l.jpg&quot; alt=&quot;S type Orbits&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The figure above is taken from Giuliatti Winter et al 2010 where they&#xA;  describe a family orbits called S-type that are stable. The plots are&#xA;  in d vs. e. where d, on the x axis is the Pluto-centric semi-major&#xA;  axis (how far from the Pluto barycenter) and e, on the y axis is the&#xA;  eccentricity. The “white” areas are orbit solution that were found to&#xA;  be stable. Area ‘1’ is the “Sailboat Island” described in the talk.&#xA;  Left are prograde (inclination=0) orbits, right are retrograde&#xA;  (inclination=180 degrees) orbits.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.abcm.org.br/anais/cobem/2013/PDF/2042.pdf&quot; rel=&quot;noreferrer&quot;&gt;A PECULIAR STABLE REGION AROUND PLUTO AND ITS ROLE ON THE NEW HORIZONS MISSION TRAJECTORY&lt;/a&gt; (PDF), whose list of authors, among others, includes already mentioned Othon C. and Silvia M. Giuliatti Winter, UNESP São Paulo State University, Brazil:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EYKBU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Figure 1&lt;/strong&gt;: Diagram of $a$ [&lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-major_axis&quot; rel=&quot;noreferrer&quot;&gt;semi-major axis&lt;/a&gt;] versus $e$&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_eccentricity&quot; rel=&quot;noreferrer&quot;&gt; [eccentricity]&lt;/a&gt;. The small stable region is shown&#xA;  in black. The nominal parameters of the particles are: $ω,= 0$  and $Ƭ&#xA;&amp;gt; = 0$, where $ω$ is the argument of pericentre and $Ƭ$ is the epoch of the pericentre.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GC1EC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Figure 2&lt;/strong&gt;: A sample of Poincaré surface of sections for six different&#xA;  values of the Jacobi Constant. In each case are shown only those&#xA;  points associated to the periodic and quasi-periodic orbits of the&#xA;  family “BD” (Broucke, 1968), which corresponds to the small region&#xA;  (Fig. 1).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KB371.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Figure 3&lt;/strong&gt;: The  set of periodic orbits, in the synodic frame, for the&#xA;  different values of $C_J$ as shown in Figure 2. The barycentre is&#xA;  located at 0.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;New Horizons' nominal trajectory passes near this region of “Sailboat Island” trajectories, the closest at about 1650 km. It is still unclear if New Horizons will have to adjust its trajectory to avoid any possible debris in its path, but &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150528&quot; rel=&quot;noreferrer&quot;&gt;with observations made so far&lt;/a&gt;, all seems clear. Alan Stern, New Horizons PI, describes possible alternative flyby trajectories in &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2013/20130520-new-horizons-encounter-planning-accelerates.html&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some additional reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.feg.unesp.br/~orbital/projeto/ANEXOS/Anexo-22.pdf&quot; rel=&quot;noreferrer&quot;&gt;Stable orbits: The New Horizons mission to Pluto&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.boulder.swri.edu/~throop/files/Icarus_2014_Throop.pdf&quot; rel=&quot;noreferrer&quot;&gt;Limits on Pluto’s ring system from the June 12 2006 stellar occultation&#xA;and implications for the New Horizons impact hazard&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.feg.unesp.br/~orbital/tematico2011/Anexo-36.pdf&quot; rel=&quot;noreferrer&quot;&gt;Small particles in Pluto’s environment: effects of the solar radiation&#xA;pressure&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www2.phy.ilstu.edu/~rfm/380s13/chapters/CH5.3_PoincSections.pdf&quot; rel=&quot;noreferrer&quot;&gt;Poincaré Surface-of-Section Plots&lt;/a&gt; (PDF)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-06-03T05:33:26.027" LastActivityDate="2015-06-03T05:33:26.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9311" PostTypeId="1" CreationDate="2015-06-03T02:55:01.767" Score="12" ViewCount="10227" Body="&lt;p&gt;This is something I just don't understand. The way I see it, propellant can be pressurized all on its own in the tank. Just pump a lot of the stuff in there and make sure the tank is strong enough. Any fluid can do this. There shouldn't be any need to pressurize it with something else, such as helium, which adds mass especially if you consider the helium tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also don't understand how you can mix helium with the propellant like that. How do you ensure that when you open the valve, the propellant comes out instead of a propellant-helium mix?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: im talking about small rockets like thrusters in an orbital satellite, or maybe a little bigger like the Apollo CSM engine (which was also pressure-fed). I'm &lt;strong&gt;not&lt;/strong&gt; talking about large rockets launched from the ground, which certainly can't be pressure-fed systems.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="9092" LastEditDate="2021-03-15T10:24:36.940" LastActivityDate="2021-03-15T10:24:36.940" Title="Why do pressure-fed systems have to be pressurized with helium or nitrogen?" Tags="|fuel|thrust|pressure|pressure-fed-engine|helium|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9312" PostTypeId="2" ParentId="9311" CreationDate="2015-06-03T03:49:03.600" Score="15" Body="&lt;p&gt;Interesting and non-trivial questions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most propellants are not self-pressurized because as soon as the engines turn on the pressure would drop precipitously as the tanks quickly emptied and the propellant was unable to vaporize fast enough to keep up. LOX, RP, and H2 are the most common launch vehicle liquid propellants and none vaporize fast enough to maintain pressure. (N.B. this is at launch vehicle scale. There are &lt;a href=&quot;http://www.xcor.com/engines/Reaction_control_RCS.html&quot;&gt;self-pressurized LOX/Methane thrusters for reaction control systems&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in practice pressurization is performed with another gas, typically nitrogen or helium (&lt;a href=&quot;http://xkcd.com/1133/&quot;&gt;kind of air that makes your voice funny&lt;/a&gt;)stored at very high pressure in a separate tank and fed into the ever-increasing ullage in the main tanks as they are emptied. Nitrogen is used because it is cheap and dense (even more dense if liquified, although I’m not aware of any such systems); helium is light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a bummer in practice as the pressurization system add significant complexity and mass to a rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for mixing of helium and the propellant, they tend not to mix too much because of the vastly different densities and the large accelerations during boost. That said, special care is given to design diffusers which keep the pressurization gas from being shot into the liquid propellants like the worlds most amazing soda stream. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for ensuring the liquid is near the outlet valve instead of the ullage gas, this is easy when the rocket is on the ground and under acceleration. It is a real problem for upper stage ignition after the main booster has shut off when the rocket is in free-fall. There are lots of solutions here, which is probably a topic for another question, but the short answer is tiny rockets to settle the propellants just before ignition, ensuring some small amount of acceleration remains to keep propellant settled, using screens or other features in the tank that take advantage of capillary action or surface tension to keep liquid near the outlet, etc.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2015-06-03T03:49:03.600" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9313" PostTypeId="2" ParentId="8817" CreationDate="2015-06-03T05:10:55.837" Score="2" Body="&lt;p&gt;The confidence that current concept feasible and reliable enough to provide reusability is based on single precedent. That is &lt;a href=&quot;https://www.youtube.com/watch?v=53hLiOWHByQ&amp;amp;feature=youtu.be&quot; rel=&quot;nofollow noreferrer&quot;&gt;Xombe&lt;/a&gt; experimental lander a vehicle with the same control arrangement but completely different size, dimensions and flight profile. Few key figures are common for two projects including Lars Blackmore is a SpaceX engineer involved in the very G-FOLD project.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/5tSID.jpg&quot; alt=&quot;http://www.skystalker.net/masten-tests-a-xombie/&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The only successful large rocket performed vertical landing is McDonnell Douglas (acquired by Boeing 1997) use different concept – differential throttling of four main engines. Please watch what difference 20 year ago!&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=wv9n9Casp1o&quot; rel=&quot;nofollow noreferrer&quot;&gt;DC-X Flight 8&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/wv9n9Casp1o?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-06-03T05:26:33.810" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9314" PostTypeId="1" CreationDate="2015-06-03T05:24:10.393" Score="2" ViewCount="259" Body="&lt;p&gt;It appears that the best way to produce oxygen on Mars is using reverse fuel cells to extract oxygen from the co2 in the atmosphere. However any machine we sent there would eventually wear out. Does Mars have the necessary elements to allow us to create the fuel cells in situ?&lt;/p&gt;&#xA;" OwnerUserId="10301" LastActivityDate="2015-06-03T14:56:00.580" Title="Can machines which manufacture oxygen using co2 conversion be created on Mars?" Tags="|oxygen|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9315" PostTypeId="2" ParentId="9301" CreationDate="2015-06-03T06:15:03.330" Score="19" Body="&lt;p&gt;Let's do some Maths!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pressure exerted by wind is &lt;a href=&quot;https://en.wikipedia.org/wiki/Dynamic_pressure&quot; rel=&quot;noreferrer&quot;&gt;dynamic pressure&lt;/a&gt;, or velocity pressure and it is given by the formula,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${q}$ ${=}$ ${0.5{\rho}v^2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where ${q}$ is the pressure, ${\rho}$ is the density of the atmosphere &amp;amp; ${v}$ is the velocity (speed) of the wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Mars Fact Sheet&lt;/a&gt;, the density of air on Mars is about ${0.020}$ ${kg/m^3}$ and from &lt;a href=&quot;https://en.wikipedia.org/wiki/Density_of_air&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; the density of air on Earth, at sea level is ${1.225}$ ${kg/m^3}$. A significant difference of densities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using your wind speed of 60 mph, this is 96.54 km/h or 26.817 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A wind on Earth with this speed would exert a pressure of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${q_E}$ ${=}$ ${0.5(1.225)(26.817)^2}$ ${=}$ ${440.480}$ ${Pa}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pressure from such a wind on Mars would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${q_M}$ ${=}$ ${0.5(0.02)(26.817)^2}$ ${=}$ ${7.192}$ ${Pa}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a wind on Earth to exert the same pressure as that on Mars, its speed would need to be lower. By manipulating the dynamic pressure equation, the speed would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${v}$ ${=}$ ${[2q/{\rho}]}$${^1}$${^/}$${^2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${v}$ ${=}$ ${[2(7.192)/1.225]}$${^1}$${^/}$${^2}$ ${=}$ ${3.427}$ ${m/s}$ ${=}$ ${7.667}$ ${mph}$, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wind on Earth, with a speed of ${7.7}$ ${mph}$, exerting a wind pressure of ${7.2}$ ${Pa}$ will not blow over a landing vehicle thus, a wind on Mars with a speed of ${60}$ ${mph}$, exerting the same pressure of ${7.2}$ ${Pa}$ would not blow over a landing vehicle.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="2981" LastEditDate="2015-10-06T09:24:53.920" LastActivityDate="2015-10-06T09:24:53.920" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="9316" PostTypeId="2" ParentId="9314" CreationDate="2015-06-03T06:52:49.213" Score="3" Body="&lt;p&gt;In short: yes, but you would have to wait inordinately long for that.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Sending parts to be assembled on Mars into replacement machines is a no-brainer, and requires either an advanced robot or (much more likely) a small human base. Doesn't save much mass over sending the entire replacement&#xA;electrochemical rig except one no longer has to engineer in extra stiffness to cope with end-of-launch maximum acceleration.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Sending raw materials and basic items (&lt;a href=&quot;https://en.wikipedia.org/wiki/Field-programmable_gate_array&quot; rel=&quot;nofollow&quot;&gt;FPGAs&lt;/a&gt; for electronics, metal powder and 3D-printer-ready plastics for everything else) to be manufactured into parts and then assembled. A great solution that gets rid of this nasty Earth-to-Mars delivery lag, and allows one to fabricate nearly anything that has malfunctioned. Still requires an unwieldy set of machines, and a fairly large Martian base for the investment to be economically sensible.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Producing raw materials &lt;em&gt;in situ&lt;/em&gt;. Not going to happen soon, since you'd need to replicate a goodly chunk of Earth-wise mining, metal-making and chemical industries in a hostile Martian environment. Very capital-intensive, not confined to one base (suitable deposits will most likely be spread across Mars), and hence energy-hungry (no cheap travel on Mars has been invented yet).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Note: on resources please also start reading linked articles from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ore_resources_on_Mars&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;. We simply don't know where the exploitable deposits are.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="4660" LastEditDate="2015-06-03T14:56:00.580" LastActivityDate="2015-06-03T14:56:00.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9317" PostTypeId="2" ParentId="9311" CreationDate="2015-06-03T07:47:08.590" Score="3" Body="&lt;p&gt;It's easy enough to have sufficient pressure when the tank is full. But with your method, you need to pump in enough gas to keep pressure up until the tank is empty.&lt;br&gt;&#xA;Say you'll use the tank until 5% of the fuel is left. In that case, your pressurant gas has expanded by a factor of 20, and has dropped pressure by a factor of 20. If the pressure at that point is 1 bar, you need to start at 20 bar. That means you need a very heavy tank: 20 times heavier than a tank that's good for 1 bar.&lt;br&gt;&#xA;So what you do instead, is build a small high-pressure tank, and connect it to the large tank via a pressure regulator. This way you always have enough pressure in the tank, without needing to build a heavy main tank.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ideal scenario is when you can use the propellant itself to provide pressure. Now, propellant always boils a bit. In fact before launch, tanks are usually vented to prevent the boiloff increasing the tank pressure too much.&lt;br&gt;&#xA;Bu when the stage is in use, as Adam said, propellant on its own doesn't boil off fast enough to keep the tank pressure up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an interesting new development in this area: ULA is working on a rocket stage (&lt;a href=&quot;http://aviationweek.com/space/ula-s-vulcan-rocket-embraces-reusability-new-upper-stage&quot; rel=&quot;nofollow&quot;&gt;ACES&lt;/a&gt;) that will use one or more small internal combustion engines to heat the propellant. They found they can generate enough boiloff this way to keep the second stage tanks pressurized.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-06-03T07:47:08.590" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9319" PostTypeId="2" ParentId="9311" CreationDate="2015-06-03T08:59:16.950" Score="7" Body="&lt;p&gt;Given that you are using a liquid propellant you cannot rely on its own compressibility to achieve the required pressure. Liquids do not compress very well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is especially true as you need a &lt;em&gt;high&lt;/em&gt; pressure for your thruster and to make it worse: usually you do not want this pressure to change through the time of engine usage. &#xA;Therefore, the common solution is to achieve a constant, high pressure of the propellant by inserting an inert (i.e. non-reactive) gas to the propellant tank. In addition, this allows to modify the propellant pressure during use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orientation of the liquid propellant is a difficult topic. As said in other comments, the commonly used techniques include:&#xA;- using a flexible bladder that separates the liquid from the gas and compresses around the liquid&#xA;- feeding the liquid droplets to the exhaust port by dapilar force, e.g. by small fibers or plates or a special wall-coating&#xA;- enforcing pre-acceleration of the tank (towards the exhaust port!) e.g. by rotation of the spacecraft or pre-acceleration into the desired direction through vernier engines (another use for the Nitrogene or Helium, by the way...)&#xA;- in general by avoiding 'sloshing' of the liquid in the tank by adding almost separated compartments in the tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During propellant budget computation you will also have to take into account that there always is going to be a residual amount of propellant in the tank that cannot be used for combustion in a planned, controlled operation.&lt;/p&gt;&#xA;" OwnerUserId="10302" LastActivityDate="2015-06-03T08:59:16.950" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9320" PostTypeId="1" AcceptedAnswerId="9321" CreationDate="2015-06-03T09:34:55.140" Score="9" ViewCount="1431" Body="&lt;p&gt;I know there are over 19000 objects tracked in Earth orbit. I'm wondering how much of it is:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Active man-made satellites that we currently use&lt;/li&gt;&#xA;&lt;li&gt;Junk that we discarded or resulted from collisions between satellites&lt;/li&gt;&#xA;&lt;li&gt;Meteoroids or other naturally occurring objects&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-06-03T16:05:33.033" Title="How much of the space debris around Earth is man-made?" Tags="|debris|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9321" PostTypeId="2" ParentId="9320" CreationDate="2015-06-03T12:13:18.607" Score="9" Body="&lt;p&gt;Every object space track is currently tracking can be attributed to a human launch. Therefore, there is only 1 natural object in Earth orbit, the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the number of active satellites, the current number of unclassified payloads is 4047, out of 17152 objects being tracked by Space-track. Most of these payload objects, however, are no longer functioning. The total number, according to the &lt;a href=&quot;http://www.ucsusa.org/nuclear_weapons_and_global_security/solutions/space-weapons/ucs-satellite-database.html&quot;&gt;UCS database&lt;/a&gt;, of functioning satellites is around 1265. The total number of debris is around 11000, many of which came from two incidents, the Chinese Anti-satellite test, and the Iridium-Cosmos collision, which took place around 2007.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-03T12:13:18.607" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9322" PostTypeId="2" ParentId="9301" CreationDate="2015-06-03T15:31:46.670" Score="4" Body="&lt;p&gt;Though I appreciate the thoroughness of Fred's answer, there is a simpler way to &quot;smell-test&quot; if the windo on Mars could be felt by a human observer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Mars_Exploration_Rover&quot; rel=&quot;nofollow&quot;&gt;MER Rovers&lt;/a&gt; landed on Mars in January 2004 for 90-day missions. Spirit ended her mission in 2010 and Opportunity is still running. One of the mission limiting factors was expected dust buildup on the horizontal solar panels. It turns out that &lt;a href=&quot;http://en.wikipedia.org/wiki/Cleaning_event&quot; rel=&quot;nofollow&quot;&gt;the Martian wind blows sand particles off the panels&lt;/a&gt;, keeping them clean. Now put some sand on a flat surface and check how gently you can blow on it to displace the sand. Put your finger there: can you feel that gust?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I therefore conclude, without math, that a human could feel the wind on Mars. K'Breel informs me that the rain is another matter, though.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2015-06-03T15:31:46.670" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9323" PostTypeId="2" ParentId="9320" CreationDate="2015-06-03T16:05:33.033" Score="1" Body="&lt;p&gt;By definition, all space debris is man-made.  From &lt;a href=&quot;http://en.wikipedia.org/wiki/Space_debris&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Space debris, also known as orbital debris, space junk and space waste, is the collection of defunct objects in orbit around Earth. This includes spent rocket stages, old satellites and fragments from disintegration, erosion and collisions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-06-03T16:05:33.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9324" PostTypeId="1" CreationDate="2015-06-03T16:49:19.413" Score="6" ViewCount="386" Body="&lt;p&gt;It seem that &lt;a href=&quot;https://en.wikipedia.org/wiki/Meteoroid&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Around 15,000 tonnes of meteoroids, micrometeoroids and different forms of space dust enter Earth's atmosphere each year&quot;&lt;/a&gt; some references say &lt;a href=&quot;https://en.wikipedia.org/wiki/Meteoroid#Meteor&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Millions of meteors occur in the Earth's atmosphere daily.&quot;&lt;/a&gt; it would seem with millions of chances per day, a one in a million occurrence should occur at least once per day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Answers at &lt;a href=&quot;https://space.stackexchange.com/questions/9320/&quot;&gt;How much of the space debris around Earth is man-made?&lt;/a&gt; are suggesting that everything in orbit around the Earth (other than the Moon) is man made.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring inaccuracies in get from Earth to Space, if a ship is in space how accurate would a launch from space need to be to achieve an stable orbit around another planet that would last at least 1 year (&lt;em&gt;longer is better&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Answer givers choice of destination planet&lt;/li&gt;&#xA;&lt;li&gt;Assume all fuel is expended at initial launch (&lt;em&gt;from space&lt;/em&gt;)&lt;/li&gt;&#xA;&lt;li&gt;No travel time constraints &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are &lt;a href=&quot;https://en.wikipedia.org/wiki/Natural_satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;173 known natural satellites&lt;/a&gt; in our solar system, some of which are presumed to be captured, so achieving a moderately stable orbit without adjustment or deceleration should be theoretically possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trying to clarify: Recent comments here and at the related question point out that the difficulties for a meteoroid to achieve orbit the Earth is related to the presence of the Moon. So using that example as part of my question complicated things unnecessarily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is my train of thought: on an interplanetary journey where the drive system is off for most of the journey, any number of things could go wrong that would prevent the engine from being available at the destination planet.  A solution is to have a predestined stable orbit on arrival.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the trajectory at main engine shut down when leaving Earth is such that the ship will enter a stable orbit on arrival, a main engine failure is not catastrophic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to set an initial trajectory that will result in a stable orbit on arrival? Would solar winds or minor gravitational attractions make a stable orbit a mater of luck (&lt;em&gt;or dependent on pre-calculations beyond our abilities&lt;/em&gt;)? Would the accuracy required at interplanetary distances be so tight that current technology would not allow for it?  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-06-05T00:35:37.867" Title="How much accuracy is required to achieve interplanetary orbit without adjustment or deceleration?" Tags="|orbital-mechanics|interplanetary|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9325" PostTypeId="1" AcceptedAnswerId="9337" CreationDate="2015-06-03T16:55:32.473" Score="6" ViewCount="242" Body="&lt;p&gt;The LDSD&lt;sup&gt;1&lt;/sup&gt; parachute is too large to test in any current wind tunnel, the only test possible on the ground was the &lt;a href=&quot;https://www.youtube.com/watch?v=3MW9fhkeO2k&quot; rel=&quot;nofollow&quot;&gt;rocket sled test&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this &lt;a href=&quot;https://www.youtube.com/watch?v=PnEl3e0ou-8&quot; rel=&quot;nofollow&quot;&gt;NASA Edge&lt;/a&gt; episode (starting at 15:12), Ian Clark talked about modelling the SIAD&lt;sup&gt;2&lt;/sup&gt;, saying it is a rigid body and so scale models can be tested. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why aren't scale models of the parachute useful for testing? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did software modelling help at all with the design? At 10:15 of the &lt;a href=&quot;https://www.youtube.com/watch?v=eJO-gTlhD2U&quot; rel=&quot;nofollow&quot;&gt;press conference&lt;/a&gt; for LDSD this week, Clark talked about how limited the data for supersonic parachutes was before the first LDSD test, and how much the data from it improved understandings of the dynamics. Are we at all close to being able to create a software simulation for that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because really, how do you design a supersonic parachute without such tools?&lt;sup&gt;3&lt;/sup&gt;&#xA;&lt;hr&gt;&#xA;&lt;sup&gt;1 Low Density Supersonic Decelerator&lt;/sup&gt;&lt;br&gt;&#xA;&lt;sup&gt;2 Supersonic Inflatable Aerodynamic Decelerator&lt;/sup&gt;&lt;br&gt;&#xA;&lt;sup&gt;3 That question is perhaps not really answerable briefly. But I just had to ask.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-03T18:05:19.663" LastActivityDate="2015-06-04T06:04:48.223" Title="Modelling for the LDSD parachute - what are the issues with scale models and software simulations?" Tags="|design|parachute|ldsd|supersonic|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9326" PostTypeId="1" AcceptedAnswerId="9336" CreationDate="2015-06-03T18:45:19.473" Score="6" ViewCount="282" Body="&lt;p&gt;In &lt;a href=&quot;https://www.nasa.gov/nasa-edge/0801-technology-demonstration-missions-part-2&quot; rel=&quot;nofollow noreferrer&quot;&gt;this interview&lt;/a&gt; on NASA Edge Ian Clark, principle investigator for the Low Density Supersonic Decelerator, talked about the environment the parachute part of the system has to operate in:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Parachutes are very fickle devices, particularly supersonic&#xA;  parachutes. When we have to use them like we do at Mars, it’s behind a&#xA;  very large, blunt vehicle.  That vehicle is screaming through the&#xA;  atmosphere.  It’s punching a hole in the atmosphere.  All the air is&#xA;  rushing in behind it to fill the vacuum that it’s creating. That&#xA;  creates a very turbulent, very unsteady environment for the parachute&#xA;  to live in.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The farther the parachute is from the vehicle, the less turbulence there is. So what limits how far away it can be deployed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iHfMY.jpg&quot; alt=&quot;LDSD supersonic decelerator parachute&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the above illustration shows the 6 m capsule with the SIAD deployed, it doesn't seem to be to scale - the parachute is 33.5 m. Is it of a different parachute and SIAD combination? Does it represent the proportions used in the real system accurately?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-03T20:39:13.623" LastActivityDate="2015-06-04T05:53:40.540" Title="How far can the supersonic parachute in the LDSD system for Mars be placed from the entry capsule?" Tags="|design|atmospheric-drag|lander|ldsd|reentry|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9327" PostTypeId="2" ParentId="9326" CreationDate="2015-06-03T20:08:15.160" Score="1" Body="&lt;p&gt;It all depends on the separation of the flow around the module; vortex formation is the most important contributor to turbulence, and if the flow has fully converged before it reaches the parachute, that's great! So it's not something that you can determine as a general rule, there's no rule of thumb other than &quot;farther is better&quot;: it depends on what the fluid dynamics of flow around the module look like.&lt;/p&gt;&#xA;" OwnerUserId="10174" LastActivityDate="2015-06-03T20:08:15.160" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9328" PostTypeId="1" CreationDate="2015-06-03T20:56:42.133" Score="2" ViewCount="2366" Body="&lt;p&gt;I was wondering if anyone had information on the availability of small liquid fuelled rocket motors in the US (or elsewhere)&lt;/p&gt;&#xA;" OwnerUserId="10307" LastActivityDate="2015-06-04T23:46:45.547" Title="What is the market for small liquid rocket motors (0.5 to 10 kN thrust)?" Tags="|rockets|commercial|liquid-fuel|" AnswerCount="2" CommentCount="2" ClosedDate="2015-06-05T00:07:20.907" ContentLicense="CC BY-SA 3.0" />
  <row Id="9329" PostTypeId="1" CreationDate="2015-06-03T21:40:09.910" Score="7" ViewCount="1585" Body="&lt;p&gt;For non-American  is hard to understand why the “New Space” companies are depicted as 'commercial'or 'privately operated' companies in contrast of old NASA contractors which should be commercial companies too. SpaceX for example is highly subsidized by NASA through multiple exclusive contracts. Air Force also backed the company from its foundation – one of satellites lost in Falcon 1 experimental launches was paid by Air Force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tipping point for SpaceX was &lt;a href=&quot;http://www.nasa.gov/offices/c3po/home/cots_project.html&quot;&gt;COTS contract&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This contract, designed by NASA to provide &quot;seed money&quot; for developing&#xA;  new boosters, paid SpaceX $278 million to develop the Falcon 9 launch&#xA;  vehicle, with incentive payments paid at milestones culminating in&#xA;  three demonstration launches&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Having this in mind 'privately funded' is little confusing.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX's achievements include the first privately funded,&#xA;  liquid-propellant rocket (Falcon 1) to reach orbit on 28 September&#xA;  2008;[5] the first privately funded company to successfully launch,&#xA;  orbit and recover a spacecraft (Dragon) on 9 December 2010; and the&#xA;  first private company to send a spacecraft (Dragon) to the ISS on 25&#xA;  May 2012&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="51" LastEditDate="2015-06-04T02:58:42.237" LastActivityDate="2016-01-30T13:41:04.587" Title="What is the fundamental difference between SpaceX from one side and Lockheed Martin from other?" Tags="|launch|spacex|commercial|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9330" PostTypeId="1" CreationDate="2015-06-04T01:51:07.173" Score="3" ViewCount="192" Body="&lt;p&gt;What is the minimum number of retransmission satellites that you would need to assist your LEO space station program - to provide a full time coverage to ensure a total communication with the space station, thus avoiding the use of ground-based antennas to communicate directly with the spacecraft? I guess that 2 GEO satellites, on oposite sides of Earth, would be enough. Any ideas? (green square - launch site; black dot - space station)&lt;img src=&quot;https://i.stack.imgur.com/r4y3Q.png&quot; alt=&quot;green- launch site. the two satellites are on each side&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8761" LastEditorUserId="8761" LastEditDate="2015-06-05T13:07:54.403" LastActivityDate="2015-06-05T13:07:54.403" Title="How is the minimum number of TDRS satellites needed for a space station program?" Tags="|orbital-mechanics|rockets|spacecraft|space-shuttle|crewed-spaceflight|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9331" PostTypeId="1" AcceptedAnswerId="9388" CreationDate="2015-06-04T02:47:01.963" Score="9" ViewCount="933" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_ray&quot; rel=&quot;noreferrer&quot;&gt;Galactic cosmic rays&lt;/a&gt; (GCRs) are &lt;a href=&quot;http://www.srl.caltech.edu/personnel/dick/cos_encyc.html&quot; rel=&quot;noreferrer&quot;&gt;composed mostly&lt;/a&gt; of single protons (90%), helium nuclei, otherwise known as alpha particles, which are two protons and two neutrons (9%), and heavier atomic nuclei like carbon and iron, known as high Z or HZE particles (1%). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the particles come in a range of energies that start at about 100 MeV, and most are less than 10 GeV. Their energy can on rare occasion be much higher, with particles having over 1 TeV of energy &lt;a href=&quot;https://en.wikipedia.org/wiki/Oh-My-God_particle&quot; rel=&quot;noreferrer&quot;&gt;on record&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yJkg8.jpg&quot; alt=&quot;cosmic ray particle incidence and energies&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graph above is titled &quot;Major components of the primary cosmic rays (from Simpson)&quot; and can be found in &lt;a href=&quot;http://www.int.washington.edu/PHYS554/2011/chapter9_11.pdf&quot; rel=&quot;noreferrer&quot;&gt;this online textbook&lt;/a&gt; made available by Washington University.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When these particles hit the nucleus of an atom in their path, they have enough energy to shatter it. The new particles thus created also have enough energy to shatter the nuclei of other atoms, and this process continues until the energy is dissipated. So what happens when a cosmic ray hits a spaceship is pretty complicated. Also it is quite possible, because of the amount of empty space between atoms, for a cosmic ray to pass through a whole spaceship without hitting anything at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have run across information here and there that it is the HZE particles that are the main concern. I don't understand this when they are such a small proportion of all cosmic rays. Sometimes it also seems like an article is saying that only particles below a certain energy are relevant. I've read (or at least skimmed) a bunch of articles and it is getting muddled for me. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does the damage done by cosmic rays of different energies and compositions compare? I think I need a way to picture this that I can't get from the graphs and charts in the papers I have found, which are pretty technical and dense. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-05T18:02:48.040" LastActivityDate="2017-04-01T03:15:29.490" Title="How do the effects of different cosmic rays compare?" Tags="|cosmic-radiation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9332" PostTypeId="2" ParentId="9329" CreationDate="2015-06-04T02:55:58.180" Score="13" Body="&lt;p&gt;In principle Lockheed Martin, Boeing, Orbital-ATK, and SpaceX are all commercial companies that build space related products.  (Boeing and Lockheed Martin of course build lots of other stuff, airplanes, fighter jets, etc).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference in the context of space is how they have contracted for products and how they get paid. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle was built mostly by Rockwell International (who was bought by someone I forget), with the SRBs made by ATK and so on. They did not build it, and sell services to NASA.  They worked with NASA who developed the specifications, built it to NASA requirements, and operated it, paid by NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same basic model is true for SLS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atlas V and Delta 4 as perhaps slightly different in that the companies developed the boosters on their own, with some large amounts of funding thrown in by NASA and the Defense Department. Originally Boeing and Lockheed Martin were competing for the contracts, but then they merged into the United Launch Alliance for both boosters. NASA, DoD, and other customers then pay for launches as needed. Except that they also pay a yearly 'maintenence' fee on the order of a billion dollars to keep the service available, irrespective of launch orders.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core difference to SpaceX and Orbital-ATK in the context of Commercial Space is that NASA said, we are offering a contract for 20,000 Kilos of cargo to the ISS. It needs to berth to a CBM port, and meet ISS safety standards, etc. They then funded a development program of a couple of hundred million dollars (Expanded a few times). NASA said, lets see what vehicles you come up with, your choice of launchers, and vehicle design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That ended with a contract for 20,000 Kilos of cargo over 4-5 years to each of Orbital-ATK and SpaceX. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next NASA said, lets do the same for manned flights. We will not hand you a design and say build it, rather you build a manned vehicle that meets our standards and we will buy flights.  Again they have funded a development program to help get it going and the first few flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then they will start buying flights, hopefully based on costs (but I doubt it) from Boeing (CST-100 vehicle) and SpaceX (Dragon V2). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core difference is &quot;Old Space&quot; was cost plus, where they made more money the more it cost, so incentives to do it efficiently were not there.  &quot;New Space&quot; is getting paid a fixed price, and they need to be able to deliver for less than that price to make a profit, so they are directly incentivized to be more efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way of looking at it might be that NASA owns the Space Shuttles and Orion vehicles. SpaceX owns the Dragons, NASA just uses them for delivery services. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-04T02:55:58.180" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9333" PostTypeId="1" AcceptedAnswerId="9334" CreationDate="2015-06-04T03:42:55.053" Score="6" ViewCount="2511" Body="&lt;p&gt;Why not have a craft go back and forth between inner and outer planets building up speed? Even if it took a hundred years, the final velocity would make traveling to another star time cost effective.&lt;/p&gt;&#xA;" OwnerUserId="10310" LastActivityDate="2015-06-04T05:10:03.797" Title="What is the maximum number of gravity assists that a spacecraft can use in our Solar System?" Tags="|gravity-assist|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9334" PostTypeId="2" ParentId="9333" CreationDate="2015-06-04T04:26:06.850" Score="5" Body="&lt;p&gt;This isn't a complete answer, but there's practical limits to this approach.   The planets likely wouldn't cooperate.    &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravity_assist&quot;&gt;http://en.wikipedia.org/wiki/Gravity_assist&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The main practical limit to the use of a gravity assist maneuver is&#xA;  that planets and other large masses are seldom in the right places to&#xA;  enable a voyage to a particular destination. For example the Voyager&#xA;  missions which started in the late 1970s were made possible by the&#xA;  &quot;Grand Tour&quot; alignment of Jupiter, Saturn, Uranus and Neptune. A&#xA;  similar alignment will not occur again until the middle of the 22nd&#xA;  century. That is an extreme case, but even for less ambitious missions&#xA;  there are years when the planets are scattered in unsuitable parts of&#xA;  their orbits.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Another limitation is the atmosphere, if any, of the available planet.&#xA;  The closer the spacecraft can approach, the more boost it gets,&#xA;  because gravity falls off with the square of distance from a planet's&#xA;  center. If a spacecraft gets too far into the atmosphere, the energy&#xA;  lost to drag can exceed that gained from the planet's gravity. On the&#xA;  other hand, the atmosphere can be used to accomplish aerobraking.&#xA;  There have also been theoretical proposals to use aerodynamic lift as&#xA;  the spacecraft flies through the atmosphere. This maneuver, called an&#xA;  aerogravity assist, could bend the trajectory through a larger angle&#xA;  than gravity alone, and hence increase the gain in energy.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In addition to this, the faster the spacecraft goes, the less of a push it gets from the planet and I'm not even sure back and forth gravity assists are even possible (well, maybe in a binary star system with 2 sets of orbiting planets).  &lt;/p&gt;&#xA;" OwnerUserId="8711" LastActivityDate="2015-06-04T04:26:06.850" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9335" PostTypeId="2" ParentId="9333" CreationDate="2015-06-04T05:10:03.797" Score="6" Body="&lt;p&gt;In addition to userLTKs answer pointing out the lack of availability of planets there's another even bigger problem:  Once you exceed solar escape velocity you're heading out no matter what.  You can grab one last helping at the gravity assist buffet if anything is in your path but you can't turn back for more.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-06-04T05:10:03.797" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9336" PostTypeId="2" ParentId="9326" CreationDate="2015-06-04T05:53:40.540" Score="8" Body="&lt;p&gt;The diagram may not be exactly to scale, but it's closer than you think.  The parachute is 30.5 meters (not 33.5), where that is the &lt;em&gt;constructed&lt;/em&gt; diameter, referred to as D0.  That is the diameter of, effectively, the parachute material laid flat on the ground.  The diameter of the skirt when flying is about 83% of the constructed diameter, but varies as the parachute breathes.  So it is not used as a specification for a parachute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We like to keep the parachute about ten forebody diameters away to limit its effects.  The further back you go, the more powerful the cannon has to be to shoot it out which has bad system impacts.  We'd rather live with the behavior of the parachute back there, which produces nearly the full drag it would with no forebody, than try to put it back farther.  But we need to understand that behavior in order to properly model it in simulations.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-04T05:53:40.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9337" PostTypeId="2" ParentId="9325" CreationDate="2015-06-04T06:04:48.223" Score="8" Body="&lt;p&gt;The scale model of a vehicle with a SIAD for use, for example, in a ballistic range is made out of steel.  This is ok, because the SIAD will not significantly change shape in flight.  The shape of a parachute on the other hand changes a great deal in flight, so a rigid model will not represent the behavior even close to correct.  Any attempt to make a small flexible model will fail, due to our inability to scale down fabric proportionally.  The fabric used in the full scale parachute is already not far from as thin as you can make it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for software simulations, no, we are nowhere near being close to making a realistic simulation of the reaction of a parachute to a supersonic flow and simultaneously the reaction of the supersonic flow to the parachute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Software modeling is used in a limited way, assuming for example that the parachute doesn't change shape and using the simulation to determine the loads on the fabric and seams for that particular shape.  Then we can do that with a different shape.  Then we apply factors on that for the actual materials and seams used, since we know that approach will underestimate the real loads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scale models were also used to compare different configurations for drag and stability, in our case at about 1/3 scale (so it would fit in that big wind tunnel).  However, we knew that we could only get comparisons that way, not absolute estimates for the full-scale parachute.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Because really, how do you design a supersonic parachute without such&#xA;  tools?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We do what we can, apply robust margins, and in the end it comes down to trial and error by test.  It is not possible today to design a high-performance, supersonic parachute on paper, and have any confidence that it will work the first time.  That is the unfortunate reality.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-04T06:04:48.223" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9339" PostTypeId="1" AcceptedAnswerId="9340" CreationDate="2015-06-04T10:21:06.243" Score="16" ViewCount="4752" Body="&lt;p&gt;I was reading about the Tunguska and Chelyabinsk meteors and I wonder&#xA;what would cause a meteor to explode in the air, instead of hitting the surface?&lt;/p&gt;&#xA;" OwnerUserId="10151" LastEditorUserId="10151" LastEditDate="2017-09-13T07:38:52.153" LastActivityDate="2019-02-12T21:23:28.427" Title="Why do some meteors explode in air?" Tags="|atmosphere|physics|reentry|meteors|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9340" PostTypeId="2" ParentId="9339" CreationDate="2015-06-04T11:02:49.900" Score="27" Body="&lt;p&gt;The Chelyabinsk meteor was travelling at over &lt;strong&gt;65,000 km/h&lt;/strong&gt; when it hit the brunt of the atmosphere &lt;strong&gt;23 km high&lt;/strong&gt; in the air. This is 60 times the speed on sound! NASA estimates that the meteor's mass at this point was &lt;strong&gt;10,000 tonnes&lt;/strong&gt;, and it had a diameter of &lt;strong&gt;20 meters&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At these incredible speeds, the body is placed under &lt;em&gt;immense stress&lt;/em&gt;. Colossal pressure is generated head-on, and the body disintegrates laterally in something called the &lt;em&gt;pancake effect&lt;/em&gt;. This creates an even larger surface area of debris which causes even more stress. Therefore, the meteor very quickly releases most of it's kinetic energy through this cascade of explosions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, the seismic shock generated by the air burst measured &lt;strong&gt;2.7 on the Richter scale&lt;/strong&gt; and caused injuries in a large radius. A report from the University of Western Ontario stated that the explosion caused by the meteor was equivalent to &lt;strong&gt;470 kilotons of TNT&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same logic applies to the Tunguska meteorite and indeed, all meteorites. The friction generated by Earth's atmosphere causes the outright disintegration of most &lt;em&gt;smaller&lt;/em&gt; foreign bodies. When a body is as large as the Chelyabinsk meteorite however, only a small portion is ablated away during re-entry. Therefore the rest of the meteor makes it to the dense portion of the atmosphere, causing a violent explosion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The opposite of this is if a meteor is &lt;em&gt;too large&lt;/em&gt;. In this case, the meteor will cross the atmosphere and collide with the ground before a pressure wave (traveling at the speed of sound in the solid) can even get from the front to the back of the object. There simply isn't time for pressure-induced fragmentation of the entire object to occur, meaning the kinetic energy isn't dissipated until the entire body slams into Earth.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/baYUE.jpg&quot; width=&quot;600&quot; height=&quot;750&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; In the name of pedantry, here is a chart (courtesy the &lt;a href=&quot;http://www.amsmeteors.org/fireballs/faqf/&quot; rel=&quot;noreferrer&quot;&gt;American Meteor Society&lt;/a&gt; via @TildalWave) details the naming convention for foreign bodies in and outside the Earth's atmosphere. &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/76045/why-do-meteors-explode&quot;&gt;https://physics.stackexchange.com/questions/76045/why-do-meteors-explode&lt;/a&gt; (heavily borrowed from)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further Reading: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://cams.seti.org/Popova2013-ms.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://cams.seti.org/Popova2013-ms.pdf&lt;/a&gt;&#xA;&lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/grl.50619/abstract&quot; rel=&quot;noreferrer&quot;&gt;http://onlinelibrary.wiley.com/doi/10.1002/grl.50619/abstract&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-06-05T06:50:17.423" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9341" PostTypeId="2" ParentId="9330" CreationDate="2015-06-04T12:40:56.590" Score="4" Body="&lt;p&gt;The configuration you've drawn is almost enough. Both satellites must be able to reach the ground station, so you need to move the ground station (or the satellites) a bit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-06-04T12:40:56.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9342" PostTypeId="1" CreationDate="2015-06-04T13:06:59.150" Score="3" ViewCount="305" Body="&lt;p&gt;The existence of extraterrestrial life or proto-life organic evolution in isolated environment of Mars is question of ontological magnitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is probably the only place in solar system where some living forms may exists. The extraterrestrial environments are protected by &lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_protection&quot; rel=&quot;nofollow noreferrer&quot;&gt;Outer Space Treaty&lt;/a&gt;. All &lt;a href=&quot;http://www.space.com/13783-nasa-msl-curiosity-mars-rover-planetary-protection.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA spacecraft&lt;/a&gt; sent to other planets must undergo meticulous procedures to make sure they don't carry biological contamination from Earth to their destinations. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;affirms that the search for extraterrestrial life is an important&#xA;  objective of space research, that the planet of Mars may offer the&#xA;  only feasible opportunity to conduct this search during the&#xA;  foreseeable future, that contamination of this planet would make such&#xA;  a search far more difficult and possibly even prevent for all time an&#xA;  unequivocal result, that all practical steps should be taken to ensure&#xA;  that Mars be not biologically contaminated until such time as this&#xA;  search can have been satisfactorily carried out, and that cooperation&#xA;  in proper scheduling of experiments and use of adequate spacecraft&#xA;  sterilization techniques is required on the part of all deep space&#xA;  probe launching authorities to avoid such contamination&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;May massive human presence avoid  massive contamination?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/po932.jpg&quot; alt=&quot;Antarctica pollution&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Antarctica pollution&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-06-04T13:25:57.007" LastActivityDate="2015-06-04T14:54:39.573" Title="How plans for Mars colonization comply with Planetary protection act?" Tags="|mars|environment|contamination|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9343" PostTypeId="2" ParentId="5716" CreationDate="2015-06-04T13:42:58.427" Score="2" Body="&lt;p&gt;Looks like SpaceX holds the record for now.  An interesting distinction is whether a vertical integration facility counts.  Which I do. In that case ULA has built a pad for Atlas and Delta on each coast, so 4.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-04T13:42:58.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9344" PostTypeId="1" CreationDate="2015-06-04T13:46:19.343" Score="16" ViewCount="1706" Body="&lt;p&gt;So one of the ways of calculating Isp is:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$I_{sp} = V_e /g_0 $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;You divide the effective exhaust velocity by acceleration at earth's surface. This gives you a number in seconds, and is generally considered helpful in determining an engine's efficiency.&lt;/p&gt;&#xA;&lt;p&gt;My question is why the g0 variable? Since Isp in seconds is used for all manner of spacecraft, many of which spend years in space (probes with ion thrusters), what's the point in that g0 variable.&lt;/p&gt;&#xA;&lt;p&gt;Would it not be more informative to simply list the effective exhaust velocity?&lt;/p&gt;&#xA;" OwnerUserId="8271" LastEditorUserId="6944" LastEditDate="2020-11-05T02:28:26.853" LastActivityDate="2020-11-05T02:28:26.853" Title="Why do we use the gravitational acceleration at the Earth's surface when calculating the specific impulse of an engine?" Tags="|rockets|propulsion|specific-impulse|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="9345" PostTypeId="2" ParentId="9344" CreationDate="2015-06-04T14:32:25.063" Score="6" Body="&lt;p&gt;Expressing specific impulse as a speed (Ve) is just as valid -- this effectively lets you use propellent mass alone vs. mass x g in the various equations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dividing by g0 is just a way of scaling the value and expressing specific impulse in seconds -- this effectively lets you use propellent weight in the various equations.  This can be easier for some applications.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-06-04T14:32:25.063" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9346" PostTypeId="1" CreationDate="2015-06-04T14:46:05.433" Score="14" ViewCount="1553" Body="&lt;p&gt;If you look on Google Maps, at the McGregor, TX test site, you can see the old test stand (Labelled A) where they test each Falcon 9 first stage for a 9 engine fire as an acceptance test before sending on to the launch facility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/r8MAY.jpg&quot; alt=&quot;GMaps of McGregor site&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see a nice view of a stage on that vertical test stand in this image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/h01nL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Labelled B in my first image is the new horizontal test stand.  You can see the very large 'trough' or exhaust vent they built. Initial speculation was that they would test Falcon Heavy for all in testing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But does it make any sense to test each Falcon Heavy, all three cores, at once, before launch? For Development, it makes sense.  For every launch, does it?  Test the individual core, and then assemble at the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional speculation suggests that since this is scaled for a 3+ million lb thrust vehicle, is that it would be good for Raptor engine testing, which floats around the 1 to 1.5 million pound thrust range.  (Seems to change regularly. Development program I guess). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still probably too small for a full up BFR test, which is likely to have an odd number of engines (3, 5, 7, 9, 13) to allow for a center engine for landing.  (Where does one test a 13 engine, 1.5 million lb thrust vehicle?  (almost 20 million lbs of thrust)). &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-07-19T17:17:37.083" LastActivityDate="2016-11-04T13:29:52.763" Title="What is the new horizontal test stand at SpaceX's McGregor, TX facility meant to test?" Tags="|spacex|falcon-9|testing|falcon-heavy|mcgregor-tx|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9347" PostTypeId="1" AcceptedAnswerId="9363" CreationDate="2015-06-04T14:49:24.787" Score="7" ViewCount="311" Body="&lt;p&gt;There are some planetary protection concerns about backward contamination of Earth by returning samples from Mars, a mission which may well happen around the Earth-Mars opposition of 2025. One of the concerns is that the return capsule could crash during atmospheric entry and landing and spread its content in the atmosphere/ocean/crater.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(How) Could a capsule containing a few kilograms or so samples from Mars be landed without any risk of leakage during the landing process?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A couple of wild suggestions to begin with:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The capsule could consist of an ablative heat shield or tungsten shell or something which would survive any atmospheric entry and hard landing scenario, given the return speed from Mars. The capsule would then be fished up or dug up intact. How heavy would such a capsule have to be?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Let the return capsule carry a nuclear bomb which can be detonated if the capsule is out of course. Even if in the atmosphere, it would only be about the 1,000th over-the-ground nuclear detonation made. But it of course assumes that the detonator does not fail, which is a challenging assumption in a scenario where something else already has failed. Probably not enough to calm those who worry about Mars bugs.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-06-04T14:55:58.383" LastActivityDate="2015-06-05T02:42:35.723" Title="Could a biohazard sample return capsule from Mars be safely hard landed on Earth?" Tags="|safety|life|contamination|sample-return|reentry|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9348" PostTypeId="2" ParentId="2741" CreationDate="2015-06-04T14:49:26.967" Score="3" Body="&lt;p&gt;If Ceres had an earth like atmosphere, you should be able to fly pretty easily with a pair of wings taped to your arms and you just might gain enough velocity with hard flapping to fly your way off the planet.     Probably not, but maybe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think manual powered flight would be one of the really fun aspects of low gravity living.&lt;/p&gt;&#xA;" OwnerUserId="8711" LastActivityDate="2015-06-04T14:49:26.967" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9349" PostTypeId="2" ParentId="9342" CreationDate="2015-06-04T14:54:39.573" Score="3" Body="&lt;p&gt;First of all, it's doubtful that decontamination procedures on US rovers landed thus far were even completely effective, and let's not talk about the Russians.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That aside, I think the answer to your question is most definitely: No. The human presence is in constant contact with its germ-spewing inhabitants and will most certainly contaminate the landing site by way of some slightly leaking seal or some such. The question is, if the germs have any way to hide from the sterilizing UV rays and make in into potentially life-bearing soil layers. The next question is if Earth-evolved bacteria could even compete with Mars-Lifeforms on their own turf.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astrobio.net/news-exclusive/the-overprotection-of-mars/#sthash.AdI6gYkT.dpuf&quot; rel=&quot;nofollow&quot;&gt;http://www.astrobio.net/news-exclusive/the-overprotection-of-mars/#sthash.AdI6gYkT.dpuf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-06-04T14:54:39.573" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9350" PostTypeId="2" ParentId="9347" CreationDate="2015-06-04T15:03:43.537" Score="5" Body="&lt;p&gt;The risk of back contamination is quite small, and in fact, most likely happens on a regular basis, via Martian meteorites (At least, according to Robert Zubrin). And in fact, Martian bacteria would be very unlikely to be able to thrive on Earth, making their possibility of disease spread even less likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_contamination#Back_contamination_issues&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; actually has several plausible scenarios on how to make the capsule safely. I suspect the following would happen:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The capsule should be made to survive intact without requiring parachutes. These could be fairly easy to design, terminal velocity on earth is usually around 100 miles/hour or less, which while difficult, could be done.&lt;/li&gt;&#xA;&lt;li&gt;There should be a vacuum between the sample and the outermost layer of protection, so that any contamination goes inward.&lt;/li&gt;&#xA;&lt;li&gt;The possibility of a space based experiment center has also been discussed, possibly at the ISS or similar lab. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;As for the nuclear option, that seems rather harsh. There are less damaging ways that could be done that should be considered in the event of bad control.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-04T15:03:43.537" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9351" PostTypeId="5" CreationDate="2015-06-04T15:58:19.343" Score="0" Body="&lt;p&gt;A meteor or &quot;shooting star&quot; is the passage of a meteoroid or micrometeoroid into the Earth's atmosphere, incandescent from air friction and shedding glowing material in its wake sufficiently to create a visible streak of light.Meteors typically occur in the mesosphere at altitudes between 76 to 100 km (47 to 62 mi). The root word meteor comes from the Greek meteōros, meaning &quot;high in the air.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A meteor or &quot;shooting star&quot; is the passage of a meteoroid or micrometeoroid into the Earth's atmosphere, incandescent from air friction and shedding glowing material in its wake sufficiently to create a visible streak of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Millions of meteors occur in the Earth's atmosphere daily. Most meteoroids that cause meteors are about the size of a grain of sand. Meteors may occur in showers, which arise when the Earth passes through a stream of debris left by a comet, or as &quot;random&quot; or &quot;sporadic&quot; meteors, not associated with a specific stream of space debris. A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the meteoroids producing the meteors have been calculated. All of the orbits passed through the asteroid belt. The atmospheric velocities of meteors result from the movement of Earth around the Sun at about 30 km/s (18 miles/second), the orbital speeds of meteoroids, and the gravity well of Earth.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-04T15:58:38.857" LastActivityDate="2015-06-04T15:58:38.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9352" PostTypeId="4" CreationDate="2015-06-04T15:58:19.343" Score="0" Body="A meteor or &quot;shooting star&quot; is the passage of a meteoroid or micrometeoroid into the Earth's atmosphere, incandescent from air friction and shedding glowing material in its wake sufficiently to create a visible streak of light." OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-04T15:58:52.850" LastActivityDate="2015-06-04T15:58:52.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9353" PostTypeId="5" CreationDate="2015-06-04T16:03:51.103" Score="0" Body="&lt;p&gt;Atmospheric entry is the movement of an object into and through the gases of a planet's atmosphere from outer space. There are two main types of atmospheric entry: uncontrolled entry, such as in the entry of astronomical objects, space debris or bolides; and controlled entry, such as the entry (or reentry) of technology capable of being navigated or following a predetermined course.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3XH4m.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Animated illustration of different phases as a meteoroid enters the Earth's atmosphere to become visible as a meteor and land as a meteorite. &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Atmospheric drag and aerodynamic heating can cause atmospheric breakup capable of completely disintegrating smaller objects. These forces may cause objects with lower compressive strength to explode.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-04T16:08:25.437" LastActivityDate="2015-06-04T16:08:25.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9354" PostTypeId="4" CreationDate="2015-06-04T16:03:51.103" Score="0" Body="Atmospheric entry is the movement of an object into and through the gases of a planet's atmosphere from outer space." OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-04T16:08:00.393" LastActivityDate="2015-06-04T16:08:00.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9355" PostTypeId="1" AcceptedAnswerId="9356" CreationDate="2014-05-30T21:29:11.973" Score="6" ViewCount="2633" Body="&lt;p&gt;I searched on the subject, thinking it could probably be cheaper to fly the spacecraft aboard a plane and then launch it from altitude. The only thing I found was &lt;a href=&quot;http://en.wikipedia.org/wiki/Air_launch_to_orbit&quot;&gt;this Wikipedia page&lt;/a&gt;, but it only enumerates advantages over ground launch without saying why it's not the traditional way of doing things. Knowing the &lt;a href=&quot;http://en.wikipedia.org/wiki/North_American_X-15&quot;&gt;X-15&lt;/a&gt; was tested in the 50s, if it was the better option, it would be known since that time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think one the reason may be limitations on the payload for the plane to takeoff and fly to a sufficient altitude, but it shouldn't be a problem when sending astronauts on the ISS, for example. Any idea on this?&lt;/p&gt;&#xA;" OwnerDisplayName="mxmcharbonneau" LastEditorUserId="49" LastEditDate="2015-06-04T19:34:48.370" LastActivityDate="2018-08-23T16:58:55.067" Title="Why are spacecraft not air-launched from airplanes" Tags="|launch|rockets|orbit|air-launch|" AnswerCount="3" CommentCount="3" ClosedDate="2015-06-04T20:12:02.460" ContentLicense="CC BY-SA 3.0" />
  <row Id="9356" PostTypeId="2" ParentId="9355" CreationDate="2014-05-31T00:12:03.977" Score="9" Body="&lt;p&gt;Because there is no plane that could do that. And even if there was one, you gain almost nothing from doing so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V rocket weighed almost 3 thousand tons before launch. The Space Shuttle weighed almost 2 thousand tons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Boeing 747 can &quot;only&quot; carry about 120 tons. That's something like 5% the weight of the space vehicles mentioned above. You'd need 20 Boeing planes to carry one fully fueled space vehicle.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;But how about the altitude gain? The 747 could carry a payload up to 14 km altitude, or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By comparison, the orbit of the International Space Station is at 420 km. The Hubble Space Telescope is at 560 km altitude. There are no orbits below 160 km (because they would decay rapidly due to drag), and most are above 200 km. Geosynchronous satellites are at 40 &lt;em&gt;thousand&lt;/em&gt; km above Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plane would only cover about 2...3% the orbital altitude, or even much less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, the numbers just don't add up.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;EDIT: The wiki page you mention refers to special designs, where the whole vehicle is made for takeoff as an airplane or balloon; the airplane is basically the first stage. But this is not a mainstream solution currently, for reasons indicated above. It may work for very small vehicles.&lt;/p&gt;&#xA;" OwnerDisplayName="Florin Andrei" LastActivityDate="2014-05-31T00:12:03.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9357" PostTypeId="2" ParentId="9355" CreationDate="2014-05-31T02:41:20.277" Score="4" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/17lHm.jpg&quot; alt=&quot;SpaceShipOne&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The picture above shows a picture of &lt;a href=&quot;http://en.wikipedia.org/wiki/SpaceShipOne&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Ship One&lt;/a&gt;. on the jet powered 'carrier aircraft'/'mother ship' &lt;a href=&quot;http://en.wikipedia.org/wiki/Scaled_Composites_White_Knight&quot; rel=&quot;nofollow noreferrer&quot;&gt;White Knight&lt;/a&gt; - that probably counts as an Aircraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I think these designs are only designed to reach low-earth orbit, not up high enough t carry a satellite, carry cargo to the ISS, etc&lt;/p&gt;&#xA;" OwnerUserId="14594" OwnerDisplayName="Wilf" LastEditorUserId="415" LastEditDate="2016-12-08T17:18:21.507" LastActivityDate="2016-12-08T17:18:21.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9358" PostTypeId="2" ParentId="9355" CreationDate="2015-06-04T18:59:52.957" Score="6" Body="&lt;p&gt;Lots of good stuff on this topic in Wikipedia.:&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Air_launch_to_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Air Launch to Orbit&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Air_launch&quot; rel=&quot;nofollow noreferrer&quot;&gt;Air Launch&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A typical rocket spends the first few seconds going straight up (almost) to get out of the atmosphere. After that it spends almost all of its time accelerating to orbital velocity. Thus getting out of the atmosphere, while hard (rocket is heaviest at this point) is really only a very small portion of the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass you would need to carry to a higher altitude is so large as to exceed the capacity of the largest aircraft ever built, let alone a balloon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider the case of StratoLaunch. They intend to build a carrier aircraft composed of parts from 2 Boeing 747 (in the top 3 of the biggest aircraft commercially available. (Update: The airplane named Roc has been completed and is undergoing runway taxi tests.)  A380, B747, and C5 Galaxy are probably the biggest).  It will be the largest airplane (in mass, wing length, etc) flying if it succeeds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even then, it can only carry a scaled down version of a Falcon (The original payload was to be a SpaceX Falcon 9 but with 5 engines instead of 9 and a concomitant reduction fuel/oxidizer load and thus mass).  So the biggest aircraft yet to be built could only carry a smaller version of a medium size booster.  Update: The Falcon 5 model was cancelled by SpaceX due to the large number of changes required. Orbital/Northrup Grumman's Pegasus will be the first booster, with larger models potentially to follow in 2022.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary benefit of air launch is not extra mass, but rather launch constraints.  If you launch from a fixed site, you have limited launch options to differing orbits, and inclinations. An aircraft can in theory fly to wherever is convenient to hit the right orbital parameters. (Assuming there is a big enough runway for something the size of Stratolaunch vehicle within flight range fully loaded).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, there is the example of Virgin Galactic's SpaceShip Two that uses the White Knight carrier vehicle, whose payload to actual orbit (LauncherOne if they actually build it) would be in the 100 kilogram range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pegasus, launched beneath a Lockheed L-1011 (3 engine airplane), maxs out around 500-1000 kilos to orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a European proposal to launch an orbital payload (in the 200Kilo range) from the top of an Airbus A300. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scaling up, that would be required just does not work. &lt;/p&gt;&#xA;" OwnerUserId="51" OwnerDisplayName="geoffc" LastEditorUserId="51" LastEditDate="2018-08-23T16:58:55.067" LastActivityDate="2018-08-23T16:58:55.067" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="9359" PostTypeId="2" ParentId="9328" CreationDate="2015-06-04T19:22:45.683" Score="1" Body="&lt;p&gt;This is a very strange question as far as strange questions come, so let me answer is straight away; I have not found any resource online that sells liquid fuelled rocket motors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now even if I had, the legality of a 500-10,000 Newton rocket motor is very shaky. According to the &lt;a href=&quot;http://www.ecfr.gov/cgi-bin/text-idx?rgn=div5&amp;amp;node=14:2.0.1.3.15#sp14.2.101.c&quot; rel=&quot;nofollow&quot;&gt;FAA Code of Federal Regulations&lt;/a&gt; (paraphrased):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;If the rocket &amp;amp; payload (including fuel) weighs more than 3.3 lbs...&#xA;&#xA;If the rocket has more than 125 grams of fuel...&#xA;&#xA;If a single motor rocket has more than 160 N-sec of impulse...&#xA;&#xA;If multiple stages add up to more than 320 N-sec of impulse...&#xA;&#xA;If the average thrust is more than 80 N...&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;... you will need a waiver from the FAA to launch the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any rocket exceeding these parameters is subject to approval on the FAA's discretion. There are a host of other complications like proximity to population and airports. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To conclude; there are &lt;a href=&quot;https://www.estesrockets.com&quot; rel=&quot;nofollow&quot;&gt;far better&lt;/a&gt; (safer) propulsion systems than liquid fuel motors.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-06-04T19:22:45.683" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9361" PostTypeId="2" ParentId="9328" CreationDate="2015-06-04T23:46:45.547" Score="2" Body="&lt;p&gt;Moog offers several such thrusters:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.moog.com/products/thrusters/apogee-upper-stage-thrusters/&quot; rel=&quot;nofollow&quot;&gt;http://www.moog.com/products/thrusters/apogee-upper-stage-thrusters/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocketdyne has an entire range of thrusters as well:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.rocket.com/propulsion-systems/bipropellant-rockets&quot; rel=&quot;nofollow&quot;&gt;https://www.rocket.com/propulsion-systems/bipropellant-rockets&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Europe, there is always airbus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/bipropellant-thrusters/index.html&quot; rel=&quot;nofollow&quot;&gt;http://cs.astrium.eads.net/sp/spacecraft-propulsion/bipropellant-thrusters/index.html&lt;/a&gt;&#xA;&lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/hydrazine-thrusters/index.html&quot; rel=&quot;nofollow&quot;&gt;http://cs.astrium.eads.net/sp/spacecraft-propulsion/hydrazine-thrusters/index.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-06-04T23:46:45.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9362" PostTypeId="2" ParentId="9324" CreationDate="2015-06-05T00:35:37.867" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is it possible to set an initial trajectory that will result in a stable orbit on arrival? Would solar winds or minor gravitational attractions make a stable orbit a mater of luck (or dependent on pre-calculations beyond our abilities)? Would the accuracy required at interplanetary distances be so tight that current technology would not allow for it?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In general, no.  Forget interplanetary, we could not launch a probe to another body in our solar system and hope to have it enter orbit without expending Delta-v.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Think of the situation in reverse.  How would you go from a stable orbit at your target and get back to earth without a burn?  You can't.  There's a huge energy deficit.  Unless you execute a burn or somehow involve an interaction from another body, you're staying there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On arrival, because you have excess energy rather than a deficit, and if your target has an atmosphere, you could possibly use aerobraking to reduce the Delta-v requirements, but you can't remove them all.  &lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2015-06-05T00:35:37.867" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9363" PostTypeId="2" ParentId="9347" CreationDate="2015-06-05T02:36:34.653" Score="7" Body="&lt;p&gt;We believe so, but a lot of work remains to test and qualify a design to assure containment.  &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20010078926.pdf&quot; rel=&quot;noreferrer&quot;&gt;These&lt;/a&gt; &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080023907.pdf&quot; rel=&quot;noreferrer&quot;&gt;papers&lt;/a&gt; outline the approach, which is a simple, extremely robust parachuteless entry vehicle design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FdGGC.jpg&quot; alt=&quot;Earth Entry Vehicle cross-section&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-06-05T02:42:35.723" LastActivityDate="2015-06-05T02:42:35.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9364" PostTypeId="2" ParentId="9344" CreationDate="2015-06-05T06:42:05.690" Score="10" Body="&lt;p&gt;One reason for this preference is that everyone agrees on seconds as a unit for measuring time, whereas we still have multiple conventional units in use for measuring speed (m/s, ft/s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By dividing by g&lt;sub&gt;0&lt;/sub&gt;, in whatever conventional units you're used to working with (9.8 m/s&lt;sup&gt;2&lt;/sup&gt; or 32 ft/s&lt;sup&gt;2&lt;/sup&gt;) you produce a value that anyone can work with, no matter which system they prefer.&lt;/p&gt;&#xA;" OwnerUserId="10320" LastEditorUserId="10320" LastEditDate="2015-06-05T07:07:57.523" LastActivityDate="2015-06-05T07:07:57.523" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9365" PostTypeId="1" AcceptedAnswerId="9367" CreationDate="2015-06-05T06:50:04.833" Score="130" ViewCount="34300" Body="&lt;p&gt;We often say that the planets orbit the Sun, which is usually a reasonable approximation. But in reality both Sun and the planets orbit the center-of-mass/center-of-gravity of the whole solar system, not the center of the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sun is by far the most massive body in the solar system, so it's plausible that the center-of-gravity lies within it. On the other hand Jupiter is rather massive as well and pretty far from the Sun. Thus the center-of-mass might not actually be inside the Sun, only near it. The influence of Saturn might not be negligible either.&lt;/p&gt;&#xA;" OwnerUserId="1244" LastEditorUserId="-1" LastEditDate="2015-06-10T18:33:41.257" LastActivityDate="2022-05-23T04:46:04.153" Title="Do the planets really orbit the Sun?" Tags="|gravity|solar-system|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9367" PostTypeId="2" ParentId="9365" CreationDate="2015-06-05T07:24:30.727" Score="157" Body="&lt;p&gt;You are correct, &lt;strong&gt;the centre of the Sun is not the Solar System's centre of gravity.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ud2KX.png&quot; alt=&quot;Solar system&amp;#39;s COG changing over time&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; A diagram (courtesy &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Solar_system_barycenter.svg&quot;&gt;Wikimedia Commons&lt;/a&gt;), showing how the barycentre of the Solar System has changed over time. &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sun is affected by the gravity of all planets in the Solar System, but you are right, it is most affected by the two most massive ones; Jupiter and Saturn. You can see in this animation (&lt;em&gt;a representation, not a simulation&lt;/em&gt;), how two bodies affect each other in a normal orbit like that between the Earth and the Sun:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/awzXQ.gif&quot; alt=&quot;Orbit diagram showing oscillation of the Sun.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Courtesy &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:Orbit4.gif&quot;&gt;Wikimedia&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apply this relation to every body in the Solar System (adjusting for mass and distance of course) and you get a rough idea of how the Sun is affected by the rest of the Solar System.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to this somewhat chaotic dance the centre of mass of the Solar System continuously moves around, sometimes under the Sun's surface and sometimes outside it. The further away from the centre of the Sun this barycentre is, the more the Sun appears to oscillate.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Another example of this phenomenon is the Pluto-Charon system:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/wIq8P.png&quot; alt=&quot;Pluto-Charon System&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Courtesy &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:Pluto-Charon_double_planet.png&quot;&gt;Wikimedia&lt;/a&gt;. &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Charon is about one-tenth the mass of Pluto (thank you for the correction @Hobbes), and yet exerts a significant gravitational pull on Pluto. Hence they both orbit around the centre-of-gravity of their system, well outside the surface of Pluto. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bonus:&lt;/strong&gt; We use this phenomenon to find planets outside the Solar System! If a distant star is observed to 'wobble' or oscillate about it's mean position, we can use that data to infer the presence of one or more exoplanets, and calculate their mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further Reading: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://homepages.wmich.edu/~korista/solarsystem_barycenter.pdf&quot;&gt;http://homepages.wmich.edu/~korista/solarsystem_barycenter.pdf&lt;/a&gt;&#xA;&lt;a href=&quot;http://spaceplace.nasa.gov/barycenter/en/&quot;&gt;http://spaceplace.nasa.gov/barycenter/en/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-08T07:49:56.153" LastActivityDate="2015-06-08T07:49:56.153" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9368" PostTypeId="2" ParentId="5531" CreationDate="2015-06-05T08:05:15.950" Score="1" Body="&lt;p&gt;Air Launch has its limitations listed in previous answers but also have a number of benefits. The delta V gain is not the most important. More important may be the air launch immunity to moderate weather related problems. You know how often the space launches delay because “shear winds” constrains. The other may be most important benefit is that the air launch is less problematic with range safety. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea simply to hang a large liquid propellant rocket under the super-heavy aircraft encountered unexpected difficulties. That scraped the joint project of Stratolaunch and SpaceX in the end of 2012.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vertically launched rockets dominate the market also by historic reasons. The modern launch vehicles are directly descended from Intercontinental Ballistic Missiles i.e.  Atlas, Titan 2 and R7. Some decommissioned missiles are used for satellite launches (Satan, MX). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The evolution of space transportation systems was slowed by multiple reasons:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/nwfSENkvJXY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-06-05T14:42:17.107" LastActivityDate="2015-06-05T14:42:17.107" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9369" PostTypeId="1" AcceptedAnswerId="9390" CreationDate="2015-06-05T08:15:58.597" Score="5" ViewCount="996" Body="&lt;p&gt;During the cold war we explored the use of nuclear weapons as a means of propulsion. Instead of trying it right away, we used conventional explosives to simulate them. Is it economically viable to use conventional explosives to propel a vehicle into and through space and if so why haven't we done that?&lt;/p&gt;&#xA;" OwnerUserId="10322" LastEditorUserId="1028" LastEditDate="2015-06-05T10:08:32.247" LastActivityDate="2015-07-25T23:59:36.817" Title="Are conventional explosives viable for pulsed propulsion?" Tags="|propulsion|spacecraft|explosives|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9370" PostTypeId="2" ParentId="9369" CreationDate="2015-06-05T08:34:25.147" Score="7" Body="&lt;p&gt;&lt;strong&gt;Traditional explosives are not viable for pulse propulsion&lt;/strong&gt; because they are not powerful enough.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Pulsed nuclear propulsion was explored during the Cold War as a part of &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;Project Orion.&lt;/strong&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/7/76/ProjectOrionConfiguration.png&quot; width=&quot;500&quot; height=&quot;200&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Diagram of what an Orion propulsion system might look like. [Courtesy &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:ProjectOrionConfiguration.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia] &lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Orion concept offered high thrust and high specific impulse, or propellant efficiency, at the same time. The unprecedented extreme power requirements for doing so would be met by nuclear explosions, of such power relative to the vehicle's mass as to be survived only by using external detonations without attempting to contain them in internal structures. &#xA;However the project was abandoned after the Limited Test Ban Treaty which prohibited all nuclear detonations above ground. The concept was also severely criticised for the risk it posed to ecology and human life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issue with using conventional explosives is that they simply aren't powerful enough. A pulse propulsion system relies on a massive specific impulse to the order of &lt;strong&gt;1 mega-newton·s/kg&lt;/strong&gt;. Nuclear bombs are the only explosive even vaguely capable of producing this force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even in later concepts of pulse propulsion, traditional explosives were never considered. Both &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Daedalus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Daedelus&lt;/a&gt; and &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Longshot&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Longshot&lt;/a&gt; discussed the use of inertial-confinement fusion reactors.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.663" LastActivityDate="2015-06-05T08:34:25.147" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9371" PostTypeId="5" CreationDate="2015-06-05T08:38:47.217" Score="0" Body="&lt;p&gt;&lt;strong&gt;An explosive is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Explosive materials may be categorized by the speed at which they expand. Materials that detonate (the front of the chemical reaction moves faster through the material than the speed of sound) are said to be &quot;high explosives&quot; and materials that deflagrate are said to be &quot;low explosives&quot;. Explosives may also be categorized by their sensitivity. Sensitive materials that can be initiated by a relatively small amount of heat or pressure are primary explosives and materials that are relatively insensitive are secondary or tertiary explosives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A wide variety of chemicals can explode; a smaller number are manufactured specifically for the purpose of being used as explosives. The remainder are too dangerous, sensitive, toxic, expensive, unstable, or decompose too quickly for common usage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast, some materials are merely combustible or flammable if they burn without exploding.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-05T11:40:35.047" LastActivityDate="2015-06-05T11:40:35.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9372" PostTypeId="4" CreationDate="2015-06-05T08:38:47.217" Score="0" Body="Explosives are chemical substances that can produce large quantities of gaseous products and heat by decomposition in very short periods of time through detonation.  Use [pyrotechnic fasteners] instead for devices that hold parts together, until their intended separation." OwnerUserId="-1" LastEditorUserId="26446" LastEditDate="2021-06-18T03:12:33.363" LastActivityDate="2021-06-18T03:12:33.363" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="9373" PostTypeId="5" CreationDate="2015-06-05T08:55:40.057" Score="0" Body="&lt;p&gt;The study of the origin, evolution, distribution, and future of life in the universe: extraterrestrial life and life on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astrobiology makes use of physics, chemistry, astronomy, biology, molecular biology, ecology, planetary science, geography, and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from the biosphere on Earth. The origin and early evolution of life is an inseparable part of the discipline of astrobiology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astrobiology concerns itself with interpretation of existing scientific data; given more detailed and reliable data from other parts of the universe, the roots of astrobiology itself—physics, chemistry and biology—may have their theoretical bases challenged. Although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="168" LastEditDate="2015-06-05T09:27:15.203" LastActivityDate="2015-06-05T09:27:15.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9374" PostTypeId="4" CreationDate="2015-06-05T08:55:40.057" Score="0" Body="The study of the origin, evolution, distribution, and future of life in the universe: extraterrestrial life and life on Earth." OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-05T11:40:44.557" LastActivityDate="2015-06-05T11:40:44.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9375" PostTypeId="2" ParentId="5463" CreationDate="2015-06-05T09:05:00.793" Score="-2" Body="&lt;p&gt;You ever see rocket city rednecks? These redneck rocket scientists (Father and Son) from Huntsville Alabama (and former and current NASA employees) came up with this idea and made it work on a small scale by taking it up a notch by attaching the a catapult to a weather balloon. They launched the rocket from the catapult that was suspended from the balloon in the air. It worked to.&lt;/p&gt;&#xA;" OwnerUserId="10324" LastActivityDate="2015-06-05T09:05:00.793" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9376" PostTypeId="2" ParentId="9365" CreationDate="2015-06-05T09:45:45.430" Score="57" Body="&lt;p&gt;Using the approximation&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\Delta \mathrm{center}=\frac{m_p}{m_\odot}\cdot\frac{\mathrm{dist}_p}{r_\odot}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;and data from &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_gravitationally_rounded_objects_of_the_Solar_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;List of gravitationally rounded objects of the Solar System - Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;div class=&quot;s-table-container&quot;&gt;&#xA;&lt;table class=&quot;s-table&quot;&gt;&#xA;&lt;thead&gt;&#xA;&lt;tr&gt;&#xA;&lt;th&gt;&lt;sub&gt;Name&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Distance from the Sun (km)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Mass (kg)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Mass (in solar masses)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Distance (in solar radii)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;ΔCenter&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/thead&gt;&#xA;&lt;tbody&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Sun&lt;/td&gt;&#xA;&lt;td&gt;0&lt;/td&gt;&#xA;&lt;td&gt;1.99 ⋅ 10&lt;sup&gt;30&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;1&lt;/td&gt;&#xA;&lt;td&gt;0&lt;/td&gt;&#xA;&lt;td&gt;0&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Mercury&lt;/td&gt;&#xA;&lt;td&gt;5.79 ⋅ 10&lt;sup&gt;7&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;3.30 ⋅ 10&lt;sup&gt;23&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.66 ⋅ 10&lt;sup&gt;-7&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;8.32 ⋅ 10&lt;sup&gt;1&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;0.00001&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venus&lt;/td&gt;&#xA;&lt;td&gt;1.08 ⋅ 10&lt;sup&gt;8&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;4.87 ⋅ 10&lt;sup&gt;24&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;2.45 ⋅ 10&lt;sup&gt;-6&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.55 ⋅ 10&lt;sup&gt;2&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;0.00038&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Earth&lt;/td&gt;&#xA;&lt;td&gt;1.50 ⋅ 10&lt;sup&gt;8&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;5.97 ⋅ 10&lt;sup&gt;24&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;3.00 ⋅ 10&lt;sup&gt;-6&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;2.15 ⋅ 10&lt;sup&gt;2&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;0.00065&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Mars&lt;/td&gt;&#xA;&lt;td&gt;2.28 ⋅ 10&lt;sup&gt;8&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;6.42 ⋅ 10&lt;sup&gt;23&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;3.23 ⋅ 10&lt;sup&gt;-7&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;3.27 ⋅ 10&lt;sup&gt;2&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;0.00011&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Jupiter&lt;/td&gt;&#xA;&lt;td&gt;7.78 ⋅ 10&lt;sup&gt;8&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.90 ⋅ 10&lt;sup&gt;27&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;9.55 ⋅ 10&lt;sup&gt;-4&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.12 ⋅ 10&lt;sup&gt;3&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.06735&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Saturn&lt;/td&gt;&#xA;&lt;td&gt;1.43 ⋅ 10&lt;sup&gt;9&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;5.69 ⋅ 10&lt;sup&gt;26&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;2.86 ⋅ 10&lt;sup&gt;-4&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;2.05 ⋅ 10&lt;sup&gt;3&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;0.58576&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Uranus&lt;/td&gt;&#xA;&lt;td&gt;2.87 ⋅ 10&lt;sup&gt;9&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;8.68 ⋅ 10&lt;sup&gt;25&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;4.37 ⋅ 10&lt;sup&gt;-5&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;4.12 ⋅ 10&lt;sup&gt;3&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;0.18007&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Neptune&lt;/td&gt;&#xA;&lt;td&gt;4.50 ⋅ 10&lt;sup&gt;9&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;1.02 ⋅ 10&lt;sup&gt;26&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;5.15 ⋅ 10&lt;sup&gt;-5&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;6.46 ⋅ 10&lt;sup&gt;3&lt;/sup&gt;&lt;/td&gt;&#xA;&lt;td&gt;0.33278&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/tbody&gt;&#xA;&lt;/table&gt;&#xA;&lt;/div&gt;&#xA;&lt;p&gt;Which sums to a shift of 2.17 Sun's radii if all planets were aligned. Jupiter clearly dominates (about half of the total effect) and would be enough to move the center-of-gravity outside the Sun. Only the heavy outer planets (Jupiter, Saturn, Uranus, Neptune) have a non negligible influence.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;C# source code for reproducibility:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;void Main()&#xA;{&#xA;    // http://en.wikipedia.org/wiki/List_of_gravitationally_rounded_objects_of_the_Solar_System&#xA;    Body[] planets={&#xA;        new Body{Name=&amp;quot;Mercury&amp;quot;,Distance=57909175,Mass=3.302E23},&#xA;        new Body{Name=&amp;quot;Venus&amp;quot;,Distance=108208930,Mass=4.8690E24},&#xA;        new Body{Name=&amp;quot;Earth&amp;quot;,Distance=149597890,Mass=5.9742E24},&#xA;        new Body{Name=&amp;quot;Mars&amp;quot;,Distance=227936640,Mass=6.4191E23},&#xA;        new Body{Name=&amp;quot;Jupiter&amp;quot;,Distance=778412010,Mass=1.8987E27},&#xA;        new Body{Name=&amp;quot;Saturn&amp;quot;,Distance=1426725400,Mass=5.6851E26},&#xA;        new Body{Name=&amp;quot;Uranus&amp;quot;,Distance=2870972200,Mass=8.6849E25},&#xA;        new Body{Name=&amp;quot;Neptune&amp;quot;,Distance=4498252900,Mass=1.0244E26},&#xA;    };&#xA;    var bodies=new[]{Sun}.Concat(planets);&#xA;    bodies.Select(planet=&amp;gt;new{&#xA;        Name=planet.Name,&#xA;        Distance=planet.Distance.ToString(&amp;quot;E2&amp;quot;),&#xA;        Mass=planet.Mass.ToString(&amp;quot;E2&amp;quot;),&#xA;        DistanceInSunRadii=planet.DistanceInSunRadii.ToString(&amp;quot;E2&amp;quot;),&#xA;        MassInSuns=planet.MassInSuns.ToString(&amp;quot;E2&amp;quot;),&#xA;        CenterOfGravityShift=planet.CenterOfGravityShift.ToString(&amp;quot;n5&amp;quot;)&#xA;    }).Dump();&#xA;}&#xA;&#xA;static Body Sun=new Body{Name=&amp;quot;Sun&amp;quot;,Distance=0,Mass=1.98855E30};&#xA;const double SunRadius=696342;&#xA;&#xA;class Body&#xA;{&#xA;    public string Name{get;set;}&#xA;    public double Distance{get;set;}// in km&#xA;    public double Mass{get;set;}// in kg&#xA;    &#xA;    public double MassInSuns{get{return Mass/Sun.Mass;}}&#xA;    public double DistanceInSunRadii{get{return Distance/SunRadius;}}&#xA;    public double CenterOfGravityShift{get{return MassInSuns*DistanceInSunRadii;}}&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="1244" LastEditorUserId="47149" LastEditDate="2022-05-23T04:46:04.153" LastActivityDate="2022-05-23T04:46:04.153" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="9377" PostTypeId="1" AcceptedAnswerId="9378" CreationDate="2015-06-05T10:35:56.960" Score="2" ViewCount="2201" Body="&lt;p&gt;I'm reading some rules for a Mars-rover competition, and one of the rules says you are not allowed to use &quot;Sensors that rely on the Earth’s magnetic field&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is - do accelerometers (like &lt;a href=&quot;https://www.sparkfun.com/products/9045&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; one) use the Earth's magnetic field?&lt;/p&gt;&#xA;" OwnerUserId="10326" LastEditorUserId="1028" LastEditDate="2015-06-05T11:11:14.813" LastActivityDate="2015-06-05T16:08:52.743" Title="Do accelerometers rely on magnetism?" Tags="|gravity|magnetic-field|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9378" PostTypeId="2" ParentId="9377" CreationDate="2015-06-05T11:16:10.367" Score="9" Body="&lt;p&gt;No, accelerometers do not rely on the magnetic field of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tmWpV.png&quot; alt=&quot;Inertia in an accelerometer&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Diagrammatic explanation of how inertia is used to measure acceleration. &lt;a href=&quot;http://www.starlino.com/imu_guide.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;(Source)&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As detailed &lt;a href=&quot;http://imtuoradea.ro/huromecha/platform/upload/print/Advanced%20Mechatronics%20Systems%20-%20laboratory%20-%20Mechatronics%20control%20systems%20-%201st%20ed.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (own highlighting):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Conceptually, an accelerometer behaves as a damped mass on a spring.&#xA;  When the accelerometer experiences an acceleration, the mass is&#xA;  displaced to the point that the spring is able to accelerate the mass&#xA;  at the same rate as the casing. The displacement is then measured to&#xA;  give the acceleration. &lt;strong&gt;Hence, they work on the principle of inertia,&#xA;  not magnetism.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-05T15:36:56.393" LastActivityDate="2015-06-05T15:36:56.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9379" PostTypeId="2" ParentId="9365" CreationDate="2015-06-05T13:39:00.067" Score="15" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;We often say that the planets orbit the Sun, which is usually a reasonable approximation. But in reality both Sun and the planets orbit the center-of-mass/center-of-gravity of the whole solar system, not the center of the sun.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That depends on what you mean by &quot;orbit&quot;. If you mean that the equations of motion take on their simplest form in a non-rotating frame centered at the solar system barycenter, that is correct. Ignoring perturbations from the galaxy and nearby stars, this barycentric frame is an Newtonian inertial frame of reference. (Taking those small perturbations into account means that a barycentric frame is only approximately an inertial frame.) A heliocentric (Sun-centered) frame is also very close to inertial, but less so than is a barycentric frame. You have two choices with a frame that is known to be rotating and/or accelerating: Account for that rotation/acceleration via fictitious forces, or ignore them. Accounting for them makes the equations of motion more complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If on the other hand you mean that each body in the solar system is attracted toward the solar system barycenter, that is incorrect.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-06-05T13:39:00.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9380" PostTypeId="1" AcceptedAnswerId="9392" CreationDate="2015-06-05T15:01:08.500" Score="7" ViewCount="1510" Body="&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; summarizes the Interplanetary Transport Network/Interplanetary Superhighway fairly succinctly, although it's pretty vague to someone who doesn't know what's going on: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Interplanetary Transport Network (ITN) is a collection of gravitationally determined pathways through the Solar System that require very little energy for an object to follow.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;ernestopheles&lt;/strong&gt; gives a better summary in &lt;a href=&quot;https://space.stackexchange.com/a/82/6971&quot;&gt;an answer&lt;/a&gt; to a related question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &quot;Interplanetary Transport Network&quot; may be a misleading term. When probes are sent into deep space, most of them make use of flybys or gravity assist manoeuvres. Virtually every celestial body can therefore be used for increasing the speed of a probe or decreasing it. The &quot;network&quot; refers to series of such manoeuvres.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have two parts to my question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What's the minimum amount of $\Delta v$ needed to transfer between say, Earth and Jupiter, using the ITN? Is it possible to transfer with near-zero $\Delta v$ from say, a Sun-Earth Lagrange point?&lt;/li&gt;&#xA;&lt;li&gt;If savings are large, why don't more missions use the ITN? Wikipedia's page on &lt;a href=&quot;http://en.wikipedia.org/wiki/Low-energy_transfer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low-energy transfers&lt;/a&gt; suggests to me that not a lot of missions even do that, let alone a more complicated maneuver through the ITN. Is it just the length of time needed for the transfer? Or maybe there are very few windows to execute such a thing?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(related: &lt;a href=&quot;https://space.stackexchange.com/questions/3895/is-it-possible-to-map-the-interplanetary-transport-network&quot;&gt;Is it possible to &amp;quot;map&amp;quot; the Interplanetary Transport Network?&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/72/how-much-of-the-interplanetary-transport-network-is-currently-known&quot;&gt;How much of the Interplanetary Transport Network is currently known?&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="6971" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-08-05T05:58:22.370" Title="How useful is the Interplanetary Transport Network?" Tags="|orbital-mechanics|trajectory|interplanetary|delta-v|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9382" PostTypeId="2" ParentId="9377" CreationDate="2015-06-05T15:35:11.387" Score="3" Body="&lt;p&gt;No, accelerometers do not use any magnetic fields to make measurements, they measure the deflection of a mass, which is proportional to the acceleration received (&lt;a href=&quot;https://www.sparkfun.com/datasheets/Components/SMD/adxl335.pdf&quot; rel=&quot;nofollow&quot;&gt;ADXL335 datasheet&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as you posted a link to the ADXL345 accelerometer, it is important to note that many commercial inertial measurement units which include accelerometers such as the ADXL345, may also include a magnetometer (e.g. &lt;a href=&quot;https://www.sparkfun.com/products/10736&quot; rel=&quot;nofollow&quot;&gt;the 9 Degrees of Freedom - Razor IMU&lt;/a&gt;) for use in tracking heading. These do not however rely on the Earth's magnetic field, they will estimate strength/direction of any magnetic field.&lt;/p&gt;&#xA;" OwnerUserId="10340" LastActivityDate="2015-06-05T15:35:11.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9383" PostTypeId="2" ParentId="9380" CreationDate="2015-06-05T15:49:18.600" Score="1" Body="&lt;p&gt;Regarding the second part of your question there are few windows where celestial bodies are lined up in such a way so that the gains outweigh the losses. The savings &lt;strong&gt;can&lt;/strong&gt; be large only if there's a gravitational field in the right place, right time, and heading in the right direction. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-06-05T15:49:18.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9384" PostTypeId="2" ParentId="1990" CreationDate="2015-06-05T16:03:58.693" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Can you buy land on the moon?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can buy.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Entirely from &lt;a href=&quot;http://www.moonestates.com/p1/One_acre_parcel_of_land_on_the_Moon/product_info.html?osCsid=7qhr2efibpa2t7rth11c2hsfv7&quot; rel=&quot;nofollow&quot;&gt;Moon Estates&lt;/a&gt; (they sell the Moon acre at £ 16.75):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;a href=&quot;http://www.unoosa.org/oosa/SpaceLaw/outerspt.html&quot; rel=&quot;nofollow&quot;&gt;Outer Space Treaty&lt;/a&gt; of 1967 explicitly forbids any government&#xA;  from claiming a celestial resource such as the Moon or a planet.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;What does this mean? Well it means that governments can not&#xA;  appropriate the Moon or other celestial bodies. Effectively,&#xA;  governments have signed to the fact that they have no rights to these&#xA;  bodies at all. What is actually important here is what the Outer Space&#xA;  Treaty does not say. It explicitly does not say whether commercial&#xA;  enterprises or private individuals can claim, exploit or appropriate&#xA;  the celestial bodies for profit. (Note that the Lunar Embassy is not a&#xA;  government body.)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The United Nations and all countries that signed the Outer Space&#xA;  Treaty became aware of this vital omission almost immediately after&#xA;  the treaty was ratified in 1967. In fact, the United Nations have&#xA;  expended a large amount of time trying to ratify an amendment to the&#xA;  treaty ever since, that would explicitly include corporations and&#xA;  individuals.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;All attempts at ratifying such an amendment failed&#xA;  because member states did not agree with it. So, in the end, all the&#xA;  ratification attempts were summarized into the famous &lt;a href=&quot;http://en.wikipedia.org/wiki/Moon_Treaty&quot; rel=&quot;nofollow&quot;&gt;Moon Treaty&lt;/a&gt; some&#xA;  15 years later. This information is a well documented fact today.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Summary:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You can buy portions of the Moon.&lt;/li&gt;&#xA;&lt;li&gt;You may sell portions of the Moon, though the law is not clear about that.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-06-05T16:11:59.267" LastActivityDate="2015-06-05T16:11:59.267" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9385" PostTypeId="2" ParentId="8733" CreationDate="2015-06-05T16:08:48.870" Score="6" Body="&lt;p&gt;&lt;em&gt;N.B.: this is based on my current understanding of the technology, and it may not be complete.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;I will focus here on your main question, which is &amp;quot;how [does IVF] make [a stage] reusable?&amp;quot;. I should first refine the question to &amp;quot;how does IVF make a stage re-fuelable?&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;The critical points are that IVF eliminates the need to carry pressurized helium, which is typically used in turn to maintain the pressure in the stage the LO&lt;sub&gt;2&lt;/sub&gt; and LH&lt;sub&gt;2&lt;/sub&gt; tanks as propellant is depleted. It also eliminates the need to carry hydrazine, a fuel that is hypergolic with LO&lt;sub&gt;2&lt;/sub&gt; and is typically used to power the thrusters used for e.g. attitude control, gimbaling, and the like. Instead, gaseous oxygen and gaseous hydrogen from the IVF system fill both of these roles.&lt;/p&gt;&#xA;&lt;p&gt;This means that the onboard consumable fluids are reduced to just liquid oxygen and liquid hydrogen – greatly simplifying an on-orbit re-fueling procedure.&lt;/p&gt;&#xA;&lt;p&gt;Here's a great source showing block diagrams of a typical configuration vs. IVF:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Supporting_Technologies/Space_Access_Society_2012.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ulalaunch.com/uploads/docs/Published_Papers/Supporting_Technologies/Space_Access_Society_2012.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10343" LastEditorUserId="35366" LastEditDate="2021-05-24T00:25:04.683" LastActivityDate="2021-05-24T00:25:04.683" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="9386" PostTypeId="2" ParentId="9377" CreationDate="2015-06-05T16:08:52.743" Score="3" Body="&lt;p&gt;An accelerometer measures the force on a mass that is caused by inertia or the pull of gravity.  The Earths magnetic field is not associated with inertia or gravity and is not used by an accelerometer.  However, for the benefit of others looking at this question, many accelerometers use magnetism.  But, they generate their own magnetic field and do not rely on the Earth.  The output of these accelerometers is equivalent to the magnetic force required to hold a weight in a neutral position.  A change in the motion of the accelerometer, or the pull of a gravity field, will try to move the weight.  The magnetic field holding the weight is increased to hold the weight in position.  The amount of energy required to generate the magnetic field is the output of the accelerometer.&lt;/p&gt;&#xA;" OwnerUserId="10344" LastActivityDate="2015-06-05T16:08:52.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9387" PostTypeId="1" AcceptedAnswerId="14855" CreationDate="2015-06-05T16:11:07.730" Score="8" ViewCount="316" Body="&lt;p&gt;The United Arab Emirates (UAE) recently announced plans to send a probe, named al-Amal (Hope), into Mars orbit. A new group called the Emirates Mars Mission was created to head this effort. The Emirates has insisted that they want this to be more than a technology demonstration mission. As such, they have stated certain science goals:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once there, its work will include analysis of the Martian atmosphere in hopes of finding answers to ongoing conundrums involving Mars’ long term water loss via atmospheric photo-dissociation. That is, the reactive chemical breakdown of water (H2O) brought on by the Sun’s incoming photons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Its mission aims include observing clouds and dust storms; changes in atmospheric layers and for the first time how the atmosphere interacts with Mars’ varied topography, from the large shield volcano Olympus Mons to the deep canyons of Mars’ Valles Marineris.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;after being inserted into an elliptical 55-hour orbit in the first quarter of 2021, Hope will carry out its nominal two-year science mission at altitudes ranging between 22,000 to 44,000 kilometers. From there, the mission will investigate how the lower and upper levels of the Martian atmosphere are connected. One goal is to create the first global picture of how the Martian atmosphere changes throughout the day and between seasons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Science Lead for the mission is Sarah Amiri, who seems to have experience in technology, but not in planetary science.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xIt2G.jpg&quot; alt=&quot;Sarah Amiri&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will any planetary scientists be advising the Emirates Mars Mission team? If so, who?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;All quotes and picture from the following article:&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;http://www.forbes.com/sites/brucedorminey/2015/05/13/uae-space-agency-finalizes-first-arab-mars-probe/&quot; rel=&quot;noreferrer&quot;&gt;Arab Mars Probe May Resolve Red Planet's Atmospheric Puzzles - Forbes&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2020-07-21T14:36:46.443" LastActivityDate="2020-07-21T14:36:46.443" Title="Will there be a team of planetary scientists advising the Emirates Mars Mission team?" Tags="|mars|planetary-science|al-amal-emm|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9388" PostTypeId="2" ParentId="9331" CreationDate="2015-06-05T17:22:27.800" Score="3" Body="&lt;p&gt;This is actually a very complex question as the effect of radiation on materials and biological tissue is affected by the particle energy, the molecular mass of the materials being impacted, the electron density of the material being impacted, and the charge to mass ratio of the particle.  Also if the particle is above a certain energy threshold it can cause activation that leads to residual radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To start with, radiation effects materials and tissue in two primary ways.  First of all a particle can collide with a nucleus and eject it from its lattice position which leads to material defects that make materials more brittle and changes the mechanical and physical properties.  Secondly, and more important to electronics and people, radiation can knock electrons out of atomic shells, which prevents materials from chemically bonding.  When this happens to DNA and the proteins are not properly bonded, they will not reproduce (mitosis) correctly and when this happens, it can lead to cancer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Directly ionizing radiation, typically refers to charged particles.  As a charged ion streams through space, the electric field caused by its charge can interact with electrons in an atoms electric shell and rip them from the shell.  It can do all of this without ever actually touching the atom that it is ionizing.  Indirectly ionizing radiation, typically refers to neutral particles like photons and neutrons that must collide with electrons to remove them from a shell, which is statistically less likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all think about throwing a ping pong ball at a bowling ball.  If you did this, the ping pong ball would bounce off the bowling ball at essentially the same speed/energy it impacted with because there is a large mass difference between the particle and the collision medium.  It would not transfer much energy in the collision process.  However, if you throw a ping-pong ball at another ping-pong ball, the one you threw might transfer all of its energy to the impacted ball and come to rest, causing the other pin-pong ball to go flying in some direction, this is similar to radiation interactions.  If a neutron slams into an iron nucleus in a steel slab, it may do little damage because the iron nucleus is like the bowling ball and the much less massive neutron is like the ping-pong ball and there is little energy transfer.  However, if the neutron slammed into hydrogen in a lithium-hydride shield, the hydrogen atom and neutron have nearly the same mass and there can be much more energy transfer and therefore more damage.  This is how the mass of the radiated particle and the molecular mass of the bombarded material affect radiation damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Charged particles and photons also interact much more with materials that have a high electron density (i.e. metals) than they do with materials containing fewer electrons per atom.  As a result, protons can be stopped much more easily with a steel shield than in water (although the density of the materials also has an effect).  However, even particles with a high charge (such as iron ions) can be hard to stop in high electron density materials if they have a lot of mass, as iron does.  If someone throws a baseball at you at 20 mph, you can catch it in a glove easily, but if someone throws a bowling ball at you at the same velocity, it has a lot of inertia, which means it is not so easy to stop it.  The same holds true for subatomic particles.  So you end up viewing ions by their charge to mass ratio, which is a ratio of its ability to be stopped by electric field interactions vs. its ability to resist stopping through inertia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other stars emit the typical types of radiation as our star (electrons, protons, photons), but since that is day to day stuff and exists at very small quantities in our star system and at the same energy level as radiation from our star, it is negligible in its effects on people and satellites.  However, GCRs are typically referring to heavy ions ejected at VERY large fractions of the speed of light from very high energy events like super-nova.  They have a relatively to very high charge and are traveling at a very high speed, with a high atomic mass.  This means that they have a charge that allows them to interact with the electric fields of other atoms.  This process causes damage as it flies by the various atoms in a material ripping electrons out of their shells.  But because they have an enormous velocity/energy and also have a high mass, they have allot of inertia and are not easily stopped.  A shield or material may sustain as much damage from a few GCR collisions as it does from the flux of 10^9 solar protons collisions.  The potential for radiation damage from GCRs increases as the mass and charge of the particle increases, so protons are the least damaging, hydrogen and helium nuclides are more damaging, carbon even a bit more with iron being amongst the most damaging.&lt;/p&gt;&#xA;" OwnerUserId="5856" LastEditorUserId="4660" LastEditDate="2017-04-01T03:15:29.490" LastActivityDate="2017-04-01T03:15:29.490" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9389" PostTypeId="1" CreationDate="2015-06-05T18:24:43.047" Score="10" ViewCount="1347" Body="&lt;p&gt;Let's say we have a monopropellant, pressure-fed rocket engine. We don't want the combustion in the chamber to propagate back up the fuel line and into the fuel tank or we will have a big kaboom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Blow-back is always theoretically possible with monopropellants, because they burn on their own. It's not like bipropellant kerosene and oxygen; in this case, combustion can't propagate back up either one because a given propellant line has only the fuel or oxidizer, not both.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So how is this prevented in monopropellant pressure-fed engines?&lt;/strong&gt; Do we have to install a bunch of super-strong check valves everywhere in the fuel lines? Especially right before injection into the combustion chamber? And if so, wouldn't that imply that the engine can only fire in pulse mode (non-continuous)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing I can think of is that the pressure is so much greater than the pressure in the combustion chamber, such that the liquid (or vaporized liquid after injection) flows through the chamber so fast (speed of sound maybe, which might prevent some thermodynamic things from flowing backwards). Is this right?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2015-06-05T23:29:45.790" LastActivityDate="2015-06-05T23:29:45.790" Title="How is blow-back prevented in monopropellant chemical rockets?" Tags="|fuel|engines|combustion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9390" PostTypeId="2" ParentId="9369" CreationDate="2015-06-05T19:58:21.100" Score="1" Body="&lt;p&gt;Let's examine your idea: you want to drop bomblets behind the rocket, and utilize product gases for propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, here's hurdle No. 1: the time between pulses is wasted - nothing propels the rocket when there's no explosion. The obvious solution is to make pauses between pulses shorter. In the limit you'd &quot;feed&quot; a line of explosive as fast as it is &quot;consumed&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here we come to hurdle No.2: if you simply &lt;strong&gt;detonate&lt;/strong&gt; the stick the process will be uncontrollable. That's a runaway reaction, can't be stopped and cannot be throttled. The speed of detonation is around 10 km/s, give or take, and that gives you an appreciation of the time constants involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a high order explosion the pressure pulse and the heat load will vary greatly depending on the propagation of the detonation wave. To tolerate uncertainties in both, you will have to convert the thin skin of a rocket into thick armor which is self-defeating - you are simply carrying along all the stuff you don't need. And thickness only takes care of the pressure, not the heat load which you'll have to shed by other means.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's why you don't want high order detonation, low order detonation/deflagration. You want &lt;strong&gt;controlled burning&lt;/strong&gt;. And that's where solid and liquid rocket propellants come into play (although sometimes they don't obey your rules and force &quot;rapid disassembly&quot; of the engine).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We're back at square one. Leave chemical explosives for demolition work and propellants for, well, propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Please note that General Atomics did test a one-meter flying model working off plastic explosives (the Hot Rod) in 1959. See pictures below (taken from George Dyson's book &quot;The True Story of Project Orion&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cSCUX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cSCUX.png&quot; alt=&quot;Pic 1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QnBEh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QnBEh.png&quot; alt=&quot;Hot Rod test&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-07-25T23:59:36.817" LastActivityDate="2015-07-25T23:59:36.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9391" PostTypeId="2" ParentId="9389" CreationDate="2015-06-05T20:30:08.863" Score="6" Body="&lt;p&gt;Sometimes there are solenoid start/shutoff valves after pressurant tanks, and at the injector (although in the simplest cases only one control valve is used before the injector). Check valves are also present - mostly to guard against the case of propellant tank overpressurization and relief valves being triggered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/IucDC.png&quot; alt=&quot;Typical N2H4 catalyst bed&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.mtc.edu.eg/ASAT13/pdf/PP22.pdf&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that the monoprop decomposes in the presence of a catalyst (say, Shell 405 for hydrazine monoprop), thus there's no much hazard of decomposition wave going backwards. It may be a problem for high-test peroxide monoprops and in fact there are known cases of HTP causing &lt;a href=&quot;https://en.wikipedia.org/wiki/K-141&quot; rel=&quot;noreferrer&quot;&gt;deadly accidents&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding the pulse vs continuous firing (the wording of your question isn't clear), there are two major constraints: a) avoidance of overheating, b) total thrust required. If you need to have large thrust delivered, gas-pressurized monoprop is decidedly not your choice - you'd do better by switching to a bi-propellant engine, and, at the higher end of the scale, by adding a turbopump. Longer burns also lead to higher gravity losses. At such a cost, you may consider installing a higher specific impulse ion engine instead.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-06-05T20:51:33.550" LastActivityDate="2015-06-05T20:51:33.550" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9392" PostTypeId="2" ParentId="9380" CreationDate="2015-06-05T20:33:58.043" Score="9" Body="&lt;p&gt;ITN usually refers to ballistic captures/throws via Weak Stability Boundaries (WSBs) to/from L1 and L2 necks. I don't like Ernestopheles' answer. Conflating this with swing by gravity assists muddies the waters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shane Ross and friends are credited with inventing the term Interplanetary Superhighway. From &lt;a href=&quot;http://www2.esm.vt.edu/~sdross/superhighway/description.html&quot; rel=&quot;noreferrer&quot;&gt;Shane Ross' page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Impractical for Interplanetary Transfers Due to Long Transfer Time:&#xA;  Due to the long time needed to achieve the low energy transfers&#xA;  between planets, the Interplanetary Superhighway is impractical for&#xA;  transfers such as from Earth to Mars at present.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;My terms:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SEL1 Sun Earth Lagrange 1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SEL2 Sun Earth Lagrange 2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SML1 Sun Mars Lagrange 1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SML2 Sun Mars Lagrange 2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My convention is to put the initial for central body first and orbiting body second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sun Earth L1 and L2 points are about 1.5 million kilometers from earth. This is only 1% of an A.U.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Payloads nudged from SEL1 or SEL2 would follow paths like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/spK9K.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ellipse from SEL2 has a 1.07 A.U. aphelion. This doesn't come close to the ~1.52 A.U. aphelion needed to reach Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An ITN defender might reply &quot;But repeated gravity assists from earth might boost aphelion.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 1.01 x 1.07 A.U. ellipse has a period of of 1.063 years. It's synodic period with regard to earth is 16.87 years. (think of two runners running almost the same speed -- it takes the slightly faster runner a long time to lap the other)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean a nice gravity assist every 17 years or so? Not quite. For a good gravity assist, it would need to pass by earth during perihelion. A flyby during perihelion would only occur once every 8th fly by. So gravity assists every 136 years or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as the earth comes up from behind the payload, it pulls it backward. Then after it passes, it is pulling the payload forward. So usually the net effect of a fly by is nearly zip. It would take millennia to get an aphelion up to 1.52 A.U.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A key quantity in 3 body mechanics is (mass orbiting body)/(mass orbiting body + mass central body) often denoted μ&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some μ numbers for various orbiting bodies:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth Moon .012&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sun Jupiter .00095&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sun Saturn .00029&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sun Neptune .000052&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter Ganymede .000078&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter Europa 000025&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sun Earth .00000304&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can comets ride WSBs from one gas giant to another? Sure. Could a Jupiter orbiter ride WSBs between Galilean moons? Sure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But a WSB from Earth to Mars? That is a bridge too far. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I go into more detail at my blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/04/potholes-on-interplanetary-superhighway.html&quot; rel=&quot;noreferrer&quot;&gt;Potholes on the Interplanetary Superhighway&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice the earth moon has a hefty μ. Using WSBs for travel about the earth moon neighborhood is much more interesting. I write about this at my blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;noreferrer&quot;&gt;EML2&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in response to &quot;How useful is the Interplanetary Transport Network&quot; I'd reply &quot;Not very&quot;.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-06-05T20:39:52.920" LastActivityDate="2015-06-05T20:39:52.920" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9393" PostTypeId="2" ParentId="9365" CreationDate="2015-06-05T21:11:02.910" Score="10" Body="&lt;p&gt;Mach's principle (which is somewhat controversial, but has led to some inspired thinking - &lt;a href=&quot;http://en.wikipedia.org/wiki/Mach&amp;#39;s_principle&quot;&gt;see Wikipedia&lt;/a&gt;) can be interpreted as saying there is no center really, it is the overall distribution of mass that determines motion. And that is where Einstein's General Relativity takes over and says things just follow their local geodesics which are determined by the overall mass distribution. &#xA;In that view some (well, many) configurations look like one object is orbiting another, but really every object is just doing its thing oblivious to all other objects. For a great book about this with wonderful illustrations (but lots of math) see &lt;a href=&quot;http://en.wikipedia.org/wiki/Gravitation_(book)&quot;&gt;Gravitation by Misner, Thorn and Wheeler&lt;/a&gt; and &lt;a href=&quot;http://www.scribd.com/doc/84484334/Misner-Gravitation-Freeman-1973#scribd&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think this is always the simplest and most useful way to look at things, but it certainly has its uses (more so in complex configurations) and I just wanted to put it out there because no one else has in this thread.&lt;/p&gt;&#xA;" OwnerUserId="8504" LastEditorUserId="8362" LastEditDate="2015-06-05T23:04:25.343" LastActivityDate="2015-06-05T23:04:25.343" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9394" PostTypeId="1" CreationDate="2015-06-05T21:11:23.127" Score="6" ViewCount="883" Body="&lt;p&gt;The Apollo Skylab CSM had painted white surfaces as well as polished aluminum ones. Was this paint applyed over the aluminum, or was there any surfacing ( a primer? ) to ensure that the white paint would adhere to the metal? Is there any study on the performance of this kind of paint during the longest Skylab mission (89 days)? Is this a paint, in the strict sense of the word?&lt;img src=&quot;https://i.stack.imgur.com/dur8f.jpg&quot; alt=&quot;Apollo white pattern&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8761" LastEditorUserId="25" LastEditDate="2015-10-25T18:22:22.210" LastActivityDate="2019-06-27T17:32:02.593" Title="What were the characteristics of the Apollo Skylab CSM white paint?" Tags="|apollo-program|spacecraft|materials|skylab|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9395" PostTypeId="1" AcceptedAnswerId="9417" CreationDate="2015-06-05T23:01:20.117" Score="7" ViewCount="434" Body="&lt;p&gt;Yesterday was quite a good night to watch the sky using my 8 inch dobsonian (until moonrise at least). I was lucky enough to catch some satellites flying into my sight as I was searching for some deep sky objects. One of those satellites was a little different though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I followed it a bit and noticed a period oscillation in its brightness. When I first saw it, it was at its brightnesses peak point, going fainter from there until it eventually vanished into darkness. I thought it might have set over the horizon, without any more sunlight for it to reflect, but a few seconds later it became visible again. I've seen three full cycles of it going from invisible, to visible, to invisible again, before I lost it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The object went overhead from east-northeast to west-southwest and according to my sky tracking app it appeared to be a Cosmos 1400 rocket stage. Approximate position was Frankfurt/Germany, at about 11pm-12am local time. My question is, how come the satellite's brightness is oscillating so periodically?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My initial thought was that the satellite (rocket stage) is rotating and only reflective on one of its &quot;sides&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I right with my initial assumption? If not, how else could it be explained?&lt;/p&gt;&#xA;" OwnerUserId="3176" LastEditorUserId="3176" LastEditDate="2015-06-05T23:20:02.750" LastActivityDate="2015-06-06T17:01:05.647" Title="Orbiting object with oscillating brightness" Tags="|artificial-satellite|observation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9398" PostTypeId="5" CreationDate="2015-06-06T03:21:53.013" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:21:53.013" LastActivityDate="2015-06-06T03:21:53.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9399" PostTypeId="4" CreationDate="2015-06-06T03:21:53.013" Score="0" Body="Moving at greater than the speed of sound, which is Mach 1. Relevant to movement through an atmosphere, but 'Mach x' may also simply indicate a speed. At above Mach 5 or 6, the term 'hypersonic' is used." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:21:53.013" LastActivityDate="2015-06-06T03:21:53.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9400" PostTypeId="5" CreationDate="2015-06-06T03:26:04.827" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:26:04.827" LastActivityDate="2015-06-06T03:26:04.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9401" PostTypeId="4" CreationDate="2015-06-06T03:26:04.827" Score="0" Body="The burning process, especially of propellants, or as it otherwise relates to spacecraft or missions in space." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:26:04.827" LastActivityDate="2015-06-06T03:26:04.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9402" PostTypeId="5" CreationDate="2015-06-06T03:29:19.357" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:29:19.357" LastActivityDate="2015-06-06T03:29:19.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9403" PostTypeId="4" CreationDate="2015-06-06T03:29:19.357" Score="0" Body="The release of a rocket or spacecraft from another vessel or platform within the atmosphere, after which it propels itself into space. " OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:29:19.357" LastActivityDate="2015-06-06T03:29:19.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9404" PostTypeId="5" CreationDate="2015-06-06T03:34:29.403" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:34:29.403" LastActivityDate="2015-06-06T03:34:29.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9405" PostTypeId="4" CreationDate="2015-06-06T03:34:29.403" Score="0" Body="A sled propelled by a chemical rocket along a fixed track, used to test vehicles and equipment under high acceleration. " OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:34:29.403" LastActivityDate="2015-06-06T03:34:29.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9406" PostTypeId="5" CreationDate="2015-06-06T03:43:18.163" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:43:18.163" LastActivityDate="2015-06-06T03:43:18.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9407" PostTypeId="4" CreationDate="2015-06-06T03:43:18.163" Score="0" Body="A rocket designed for the Constellation program, which was cancelled in 2010. It was a LH2/LOX system with 2 SRBs, with a planned capacity of 188 metric tons to LEO." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T03:43:18.163" LastActivityDate="2015-06-06T03:43:18.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9408" PostTypeId="1" CreationDate="2015-06-06T05:15:39.637" Score="7" ViewCount="438" Body="&lt;p&gt;Impactors or explosive missiles have the advantage of not having to match velocity with an Earth-bound asteroid, so they are cheaper and quicker to launch (or so I understand), and detection need not be as early. But they run the risk of fragmenting the asteroid and making the problem harder to deal with. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why not send a missile that releases a cloud of dense gas, such as xenon or even one of those hexaflouride gases, into the path of the incoming asteroid? The cloud would not be very dense, but it would be huge, and the relative velocities could be enormous. The cloud would not slow the asteroid with kinetic impact, but with aerodynamic drag. I suppose the tricky part would be timing the release so that the gas does not dissipate too much. Could it deflect the asteroid appreciably?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scenario I envision is this: we spot an incoming asteroid. We launch a rocket on a trajectory to pass closely in front of the asteroid. When the rocket reaches that point, it releases a cloud of gas so that the asteroid then has to pass through it. This could be done several times in sequence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not envision a cloud surrounding the Earth (we already have one) or hanging in space for days or weeks. I envision a high-speed probe aimed to intersect an incoming asteroid's path, then exploding perhaps a millisecond before it crosses the asteroid's path. In that millisecond, the cloud of gas would expand to some tremendous size, perhaps kilometers wide, that the asteroid must pass through. Several probes could create a series of expanding clouds, one after the other. While the gas cloud will expand rapidly, reducing its density, a proper calculation would take that into account, by integrating over time. Concepts from fluid dynamics such as drag or shock obviously apply, because they apply to mediums as rarified as the solar wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the question really is, how much aerodynamic drag could a cloud of gas expanding into vacuum exert on a largish asteroid? Could it significantly alter the asteroid's course or speed? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(At least two other people, one the famous astronomer Eugene Shoemaker, have suggested similar ideas, &lt;a href=&quot;http://en.wikipedia.org/wiki/Asteroid_impact_avoidance#Other_proposals&quot; rel=&quot;noreferrer&quot;&gt;according to Wikipedia&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Another question asks whether a cloud of gravel could deflect an asteroid with kinetic impact without breaking it apart. This is a different suggestion. Other questions about the behavior of liquids or gas are also different.) &lt;/p&gt;&#xA;" OwnerUserId="10293" LastEditorUserId="6816" LastEditDate="2016-04-13T20:07:12.743" LastActivityDate="2016-04-14T04:38:54.243" Title="Could a cloud of dense gas, such as xenon, deflect a hazardous asteroid?" Tags="|asteroid|asteroid-redirect-mission|planetary-defense|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="9409" PostTypeId="1" CreationDate="2015-06-06T06:09:14.703" Score="1" ViewCount="123" Body="&lt;p&gt;There is little hope that chemical propulsion will have alternative orbital launch in near future. Using the same (even optimized) engines and fuel for inter-orbital operations will increase start-to-payload mass ratio exponentially. Roughly, inter-orbital operations and propulsive landing will need the same delta-V as orbital launch one-way only. By hypothetical reduction of launching cost by two orders of magnitude(the maximum whoever could promise), the Mars delivery will cost nearly the same as ISS resupply. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some projects for permanent human presence in Mars are based on large reusable rockets. May this type of logistic travel support Martian colony for less than $1M per person?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;So, sorry, you're asking about, what have we done in respect to&#xA;  thinking about Mars, sort of, colonial systems? The question is, how&#xA;  did I come up with half a million dollar price tag to move to Mars.&#xA;  Well, umm, I sorta started back from the half a million dollar point.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://shitelonsays.com/transcript/elon-musk-lecture-at-the-royal-aeronautical-society-2012-11-16&quot; rel=&quot;nofollow&quot;&gt;Transcript - Elon Musk lecture at the Royal Aeronautical Society&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-06-06T16:06:51.007" LastActivityDate="2015-06-06T16:06:51.007" Title="May the chemical propulsion (only) provide sustainable transportation to Mars" Tags="|mars|orbit|propulsion|" AnswerCount="0" CommentCount="10" ClosedDate="2015-06-07T03:11:44.693" ContentLicense="CC BY-SA 3.0" />
  <row Id="9410" PostTypeId="2" ParentId="9408" CreationDate="2015-06-06T06:23:43.573" Score="3" Body="&lt;p&gt;Gas can act as a brake for moving objects in space, in fact NASA has used the atmosphere of (Planets) as a brake on four different occasions with spacecraft. (edited, correction pointed out in comments).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it's not very practical to create a cloud of gas in space. Gas requires gravity to maintain it's cohesion. In space, a cloud of gas would become very diffuse and spread out very quickly &amp;mdash; even heavy gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A cloud of gravel would be better than gas, but neither provides any advantage to a rocket. There's no benefit to leaving a random collection of objects to act as deflection in space for two reasons.  One, space is very large, so the amount of material needed would be enormous (and it wouldn't stay in the right place either), and two, a ball of gas or gravel in space would be just as likely to deflect something towards earth as away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real trick is to see precisely where something is headed and if it's headed towards the earth, to give it a small push well before it reaches earth.&lt;/p&gt;&#xA;" OwnerUserId="8711" LastEditorUserId="8711" LastEditDate="2015-06-08T04:55:54.147" LastActivityDate="2015-06-08T04:55:54.147" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9411" PostTypeId="2" ParentId="9133" CreationDate="2015-06-06T06:55:55.973" Score="1" Body="&lt;p&gt;I will try to answer without using math, because I don't know how enter math notation in these answers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's break it down. The first force to consider is gravity, and for this, let's assume you are stationary at whatever altitude. At sea level, you experience one g of gravity of course (I will use &quot;force&quot; when the more correct term would be &quot;acceleration&quot;, but with a constant mass, the two are equivalent enough), and that force drops off as the inverse square of the distance from the Earth's center. So at one Earth radius (i.e., the surface), you feel 1 g, at 2 radii or 4,000 miles up, you feel 1/4 g, and at 3 radii or 8,000 miles up, you feel 1/9 g, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that is just gravity. There is also centripetal force, since you aren't just hovering motionless as the Earth spins below you, you are being slung around like a rock on a string. That force goes up linearly with your distance from the center of the Earth, so at 4,000 miles altitude, your centripetal force has doubled, and at 8,000 miles, it has tripled, etc. It is also in the opposite direction as the gravitational force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you rise from the surface, the gravitational force starts out much stronger than centripetal force (otherwise people on the ground would be slung out into space), but falls off rapidly, while the centripetal force increases steadily. The place where the two are exactly balanced is the geostationary altitude. For the Earth it happens to be about 22,200 miles. Past that, the centripetal force pulling outward exceeds the force of gravity pulling inward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you were moving at a steady speed up the elevator, you would feel yourself get lighter rather rapidly. At 22,200 miles up, you would feel weightless, and then past that, you would feel a tug &quot;upwards,&quot; towards the ceiling. You would settle your feet on what used to be the ceiling, and you could look &quot;up&quot; and see the Earth. That pull would increase until you reach the end of the tether, and it could be less than the 1 g you feel on the surface, or much more. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is still another force acting on you, and that is simple linear acceleration, from getting up to speed. Travel up and down the elevator would have to be pretty zippy, maybe a few times the speed of sound, so that the trip doesn't take weeks or months. The problem is if you accelerate straight up from the ground, the Earth's gravity will add its force to that acceleration force. (Fighter pilots know that their planes and their bodies can take more acceleration downwards than upwards.) If I were a clever designer, I would build long acceleration tunnels running along the ground that gradually curve upwards to meet the elevator trunk, so that some or most of the acceleration could take place without adding gravity. That would take some strain off the passengers' bodies. (And now that I think about it, those tunnels should be east of the main shaft.) I would also try to avoid much acceleration during the early parts of the vertical climb, where Earth's gravity is strongest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And braking at the far end would also add to the centripetal force outward (which is increasing as you go, remember.) A clever designer would call for early braking early to spare the passengers some g forces. In fact, I imagine few people would actually travel to the far end, because braking would be take so long, and require so much energy (not to mention the return trip.) Most traffic to the far end would be outbound spacecraft, and for them there would be no braking at all. The craft would be slung off like David's stone that slew Goliath. The passengers in that craft would feel some side force as their lateral velocity (i.e., perpendicular to the radial velocity) increases. But that depends on the rigidity and design of the elevator trunk. (Imagine slinging a lead ball with a pipe. A rigid pipe works pretty well. A flexible pipe does not.) The instant they leave the elevator, they would be in free fall. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a lot depends on the design choices made in the elevator. If the payload were allowed to accelerate all the way from 22,200 miles out, that would maximize the muzzle velocity (so to speak), but would place some demands on the bending strength of the elevator. On the other hand, if the payload came to a dead stop at the far end, and then was released, that would put no bending force on the tip of the elevator, but would waste some velocity. A compromise would be an elevator with a bent tip, to reduce bending stress on the tip, while still giving the departing spacecraft plenty of push. (See Larry Niven's &quot;Integral Trees&quot; for an analogous configuration.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I can imagine this. A passenger gets on a train car perhaps 50 miles from the actual elevator. It accelerates briskly, at perhaps 2 g's. The track gradually curves upward to meet the main trunk. Acceleration slows or stops, to maintain the 2 g's on the passengers. Then, as the car rises and gravity weakens, it begins to accelerate again, maintaining the net 2 g's. (How long could passengers tolerate 2 g's? Hours? Days? That might be a limiting factor.) At a certain point, braking begins, and at another, possibly different point, there is free-fall, and then ceiling and floor flip. There are more hours of 2 g, this time from deceleration plus centripetal force (minus the weakening gravity). The car comes to a stop somewhere above 22,200, where there is enough centripetal force to make things like walking and eating sort of ordinary, without requiring training. Then they stop and watch the actual spacecraft scream past them on the way to the other planets. :-)  &lt;/p&gt;&#xA;" OwnerUserId="10293" LastActivityDate="2015-06-06T06:55:55.973" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9412" PostTypeId="1" AcceptedAnswerId="9415" CreationDate="2015-06-06T07:54:59.973" Score="7" ViewCount="1158" Body="&lt;p&gt;In the novel &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/The_Mars_Project&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Mars Project&lt;/a&gt;&quot; by Wernher von Braun humans fly in convoy to Mars. Is it being considered seriously at all today, to send several spacecrafts together?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most proposed missions to Mars require pre-deployment of some payloads on Mars before humans arrive. If instead sent together with the crewed spacecraft, I can imagine some potential benefits. In an emergency the crew might evacuate to the surface habitat in flight. They could sacrifice one of the spacecrafts if they have better need for the fuel meant to land it. At Mars, they could teleoperate the landing of the spacecrafts, or maybe land in formation to make sure they all land within reach for the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drawbacks might be the risk of collision, difficulties of docking spacecrafts to transfer fuel, crew or cargo. And it doesn't seem possible to reliably make several simultaneous launches from the ground (although the military has that ability with solid launchers) so the convoy would have to get together in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5edtP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5edtP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-09-30T22:38:27.783" LastActivityDate="2016-09-30T22:38:27.783" Title="Space travel in convoy, what are the pro's and con's?" Tags="|mars|spacecraft|crewed-spaceflight|conceptual|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9414" PostTypeId="1" CreationDate="2015-06-06T10:31:36.020" Score="1" ViewCount="986" Body="&lt;p&gt;I want to build a simple personal project, a software that allows users to choose between 5 or 6 satellites, then draw the chosen satellite's path on a google map. But I really don't know where to start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought about targeting communication satellites only -due to their fixed locations-, but I cannot figure out the ground tracking. is there an equation or anything similar to concluded and predict where the satellite would be with respect to earth's rotation, and thus draw its full path ?&lt;/p&gt;&#xA;" OwnerUserId="10359" LastActivityDate="2015-06-06T10:31:36.020" Title="Simple software to plot satellites' orbits?" Tags="|artificial-satellite|software|" AnswerCount="0" CommentCount="7" ClosedDate="2015-06-06T15:33:16.737" ContentLicense="CC BY-SA 3.0" />
  <row Id="9415" PostTypeId="2" ParentId="9412" CreationDate="2015-06-06T13:06:31.780" Score="7" Body="&lt;p&gt;That's an interesting idea. There are several considerations to bear in mind here:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Ability to launch several rockets into the same plane in quick succession. Very expensive: you have to double &lt;strong&gt;everything&lt;/strong&gt; and more than double quality control since you are working with tighter margins and in the atmosphere of let's say undue haste. You'll have to pay severance packages after the surge which strains the budget.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Trans-Martian orbit Insertion accuracy: a second's worth of delay would mean transit modules won't be anywhere near each other at engine cut-off. Travel between the modules won't be easy.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Ballistics nightmare. Instead of aiming solely for the entry point, all midcourse corrections will have to keep the modules together along the route.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Entry, Descent and Landing difficulties - as you said, collision is a real possibility if you want to land the modules within a certain distance (20-30 meters) from each other.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Lack of any clear requirement to keep modules together. Transit is the least risky phase (galactic cosmic rays and solar flares notwithstanding), and life support systems are more efficient with larger crews. If one module fails in flight, there's no spare capacity in other modules. You definitely want to keep all crewmembers in one craft. You also don't want to hold cocktail parties for guests from other modules.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Pre-positioned cargos can be launched on more efficient trajectories, lowering total mission cost.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;TL;DR - formation transit raises costs and doesn't lower risks.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="415" LastEditDate="2015-06-08T11:32:46.443" LastActivityDate="2015-06-08T11:32:46.443" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9416" PostTypeId="2" ParentId="9412" CreationDate="2015-06-06T16:04:02.683" Score="2" Body="&lt;p&gt;Travelling through space in a string of vehicles relatively close together, in the same trajectory, is expensive and unnecessary, as Deer Hunter points out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But travelling to Mars, say, in a series of ships that are spread apart, maybe very widely, might make sense. One vehicle could carry people, and the others could carry supplies or fuel. They could be launched from several different pads around the world, which could be one way to spread out the cost. It might also let us spread out the cost over time. It might allow us to use existing rocket boosters like Delta or Atlas instead of having to develop new, more powerful ones (like the planned SLS.) The supply craft could be launched months or even years in advance, and take slow, cheap routes, while the manned ship might take the fastest trip possible, to minimize the crew's exposure to radiation. I believe that is actually one scenario for a manned trip to Mars. Robots might arrive first to build the habitat, probably underground, so that people could arrive and move in quickly. Robots would also start producing fuel, water, oxygen, maybe even food, so that it doesn't all have to be carried there from Earth. (On the other hand, why send people at all?)&lt;/p&gt;&#xA;" OwnerUserId="10293" LastActivityDate="2015-06-06T16:04:02.683" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9417" PostTypeId="2" ParentId="9395" CreationDate="2015-06-06T17:01:05.647" Score="6" Body="&lt;p&gt;There are actually a couple things that can account for changes in brightness of a satellite, but for an oscillation like that (especially for a rocket body) it's almost certain that the object is tumbling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.satobs.org/tumble/tumbleintro.html&quot; rel=&quot;noreferrer&quot;&gt;This site&lt;/a&gt; offers pictures and analysis (some of the pictures/charts are even rocket bodies from Cosmos satellites).  To quote their explanation of periodic flasing:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Many satellites do not have a constant brightness, they give off flashes at (usually) regular times. This flashing behavior is caused by the rotation of the satellite around its rotation axis. The satellite's metallic surfaces act as mirrors for the sun (specular reflection). Objects with a diffusely reflecting surface will also show varying brightness since the observer will see a changing amount of light reflecting area of the rocket as it tumbles about in its orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'd say that pretty well covers tumbling objects, but as for &quot;how else could it be explained&quot; (taken to be asking &quot;what else causes flashes from satellites&quot;) &lt;a href=&quot;http://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;noreferrer&quot;&gt;Iridium Flares&lt;/a&gt; come to mind.  Iridium satellites have large, flat antennas that are reflective, and active iridium satellites are stabilized to ensure the antennas point in a useful direction.  The combination of known orbits and orientation means that flares are predictable, and &lt;a href=&quot;https://www.heavens-above.com/IridiumFlares.aspx?lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&quot; rel=&quot;noreferrer&quot;&gt;this site&lt;/a&gt; even has a tool to tell you when the next flares will be (enter a lat/lon in the url).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same concept applies to other satellites (orientation + reflectivity results in a bright flash when the sun and observation point are lined up properly), but iridium is especially easy to see due to how many satellites are in the iridium constellation and how bright the reflection from them is.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-06-06T17:01:05.647" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9418" PostTypeId="2" ParentId="9344" CreationDate="2015-06-06T17:33:49.000" Score="3" Body="&lt;p&gt;Specific impulse is simply ratio of thrust to propellant consumption (mass flow) as have been mentioned. Traditionally, the force is measured by &lt;em&gt;weight&lt;/em&gt; of unit mass (pound, kilogram). Related unit often are referred as &lt;em&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Pound_%28force%29&quot; rel=&quot;nofollow&quot;&gt;pound force&lt;/a&gt;&lt;/em&gt;  (&lt;strong&gt;kpf&lt;/strong&gt;) or &lt;em&gt;kilogram force&lt;/em&gt; (&lt;strong&gt;kgf&lt;/strong&gt;). If the trust is measured by, kpf and consumption in &lt;strong&gt;lb&lt;/strong&gt;/&lt;strong&gt;second&lt;/strong&gt; dimension of specific impulse will be in &lt;strong&gt;seconds&lt;/strong&gt;. Therefore, the answer would be &lt;strong&gt;&lt;em&gt;g&lt;/em&gt;&lt;/strong&gt; came from definition of force as weight of unit mass. In SI Isp is measured in &lt;strong&gt;m/sec&lt;/strong&gt; i.e. theoretic exhaust velocity. The reason of sec to be preferred over m/sec or ft/sec is answered in previous posts. &lt;/p&gt;&#xA;" OwnerUserId="10218" LastActivityDate="2015-06-06T17:33:49.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9419" PostTypeId="2" ParentId="9408" CreationDate="2015-06-06T19:34:35.440" Score="2" Body="&lt;p&gt;It may be helpful to have a sense of the scales involved to see why gas would be ineffective. A stony asteroid with a roughly spherical shape 50 m across is probably the minimum size that might be worth mounting a mission to deflect (at least arguably, especially once we have greatly expanded into space and such things aren't so hard any more). The estimated mass of such an object is around &lt;a href=&quot;https://en.wikipedia.org/wiki/Impact_event#Frequency_and_risk&quot; rel=&quot;nofollow&quot;&gt;170,000 metric tons&lt;/a&gt;. Its speed would likely be around &lt;a href=&quot;http://www.spacedaily.com/reports/Protecting_Earth_Against_Asteroids_999.html&quot; rel=&quot;nofollow&quot;&gt;25 km/s&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow&quot;&gt;SLS&lt;/a&gt; was in operation and we could send a mission to deflect it while it was in Earth's neighborhood (on a pass across our orbit one to a few times before the pass that could result in impact), the maximum payload of gas we could get to it is maybe 10 metric tons&lt;sup&gt;1&lt;/sup&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you manage to place that 10 tons of gas right in front of the asteroid in a cloud, it will be immediately dispersing as the asteroid passes through it. It is a collection of particles surrounded by a vacuum, those particles are moving in random directions at high speed as they collide with each other, and they will expand into the vacuum extremely quickly. In this situation drag doesn't apply. The asteroid isn't an object moving through a sea of fluid which exerts pressure on it from all sides, it is larger and far heavier than the cloud of gas it hits, which it disperses with the dynamics of one object hitting a number of tiny objects. The gas cloud might display some behavior analogous to a fluid if it has remained dense enough, but it would be more like the way there are some phenomena like fluids when you run through a pile of leaves. The impact moves the particles out of the way and to the sides, it isn't really like drag. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider that all pumps work by creating partial vacuums, which fluids immediately fill. Releasing a gas into a near perfect vacuum is like surrounding it with a perfect pump, sucking it outwards in all directions at once. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The payload size deliverable will of course increase with time, but it would never be more effective than other options. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;hr&gt;&#xA;&lt;sup&gt;1 the Orion MPCV has a dry mass of 21 metric tons, but isn't for missions with the kind of delta V this would take. 10 tons is a total loose guesstimate of what might be left for payload once the delta V capacity had been satisfied&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-06T19:42:41.323" LastActivityDate="2015-06-06T19:42:41.323" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9420" PostTypeId="1" CreationDate="2015-06-06T20:16:36.303" Score="7" ViewCount="575" Body="&lt;p&gt;sideFrom the &lt;a href=&quot;https://en.wikipedia.org/wiki/Range_safety#Launch_corridor&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article on range safety&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rockets are usually launched into a space above the launch range&#xA;  called the launch corridor. If rocket engines fail while the rocket&#xA;  flies inside the corridor, the rocket falls in an uninhabited area.&#xA;  Engine failure outside the launch corridor may cause the rocket to&#xA;  fall on people or property. Therefore, if the rocket is about to exit&#xA;  the launch corridor, the RSO will terminate powered flight to ensure&#xA;  that no debris falls outside the launch corridor.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The launch corridor or hazard zone is usually a corridor in the direction of launch. What will be the shape of hazard zone for a rocket flying back?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/51rh7.gif&quot; alt=&quot;Flight Corridor for Guided Suborbital Vehicle &quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flight Corridor for Guided Suborbital Vehicle &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/voLdr.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;Keep Out Areas SpaceX requests for each launch.&#xA;Eventually, the red zone will cover one of the most  dense populated areas in US if Falcon 9 try landing on 'Landing Site 1'.&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="51" LastEditDate="2015-08-07T23:18:37.700" LastActivityDate="2015-08-30T02:30:07.447" Title="What would be the range safety for vertically landing over land rockets?" Tags="|launch|rockets|falcon-9|range-safety|vtvl|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9421" PostTypeId="1" AcceptedAnswerId="9679" CreationDate="2015-06-07T04:28:32.167" Score="15" ViewCount="5605" Body="&lt;p&gt;LMP-103S is the name of a new monopropellant, tested on the PRISMA mission in 2010. It's main ingredient is ADN (Ammonium Dinitramide), a strange chemical invented in the 70's in USSR and independently invented in the West in the early 90's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Swedish Company that owns the intellectual rights to this fuel and operated it on PRISMA have reported &lt;strong&gt;6% better specific impulse and 30% better density impulse than Hydrazine.&lt;/strong&gt; LMP-103S is also a much &lt;strong&gt;safer&lt;/strong&gt; liquid: stable, not sensitive to shock, air, or moisture, not very toxic or corrosive, and has good temperature ranges for storability and use. It also uses a catalyst to &quot;ignite&quot; and &quot;burn&quot; the liquid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The chemical makeup is reported as 60-65% ADN, 15-20% methanol, 3-6% Ammonia, and the remainder is water. These percentages are by mass, not volume. &lt;strong&gt;The exact percentages are still a trade secret.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now the questions begin:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;What's taking so long? Why hasn't it replaced nasty Hydrazine? It's been 5 years now, with not much more heard. Is LMP-103S too good to be true?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Why use methanol instead of ethanol? Ethanol has about 33% more energy per kg and is otherwise very similar in properties such as miscibility.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What is the temperature in the combustion chamber? For the record, for Hydrazine its usually around 800 C, so as long as LMP-103S is not greater than this, it can't be a disadvantage. Nevertheless, in all my searching, I have never been able to find what the temperature of &quot;combustion&quot; is in the combustion chamber.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Does it leave behind any solid residue from the burn? Such as salts? This could be a major disadvantage. Or does it corrode metals or other materials?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Also, in all my searching, I did find one disadvantage but don't know what it means. LMP-103S is not &quot;cold-startable&quot;, but Hydrazine is. What does that mean?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2018-01-23T19:05:14.707" Title="Is LMP-103S truly a viable replacement for Hydrazine?" Tags="|engines|monopropellant|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9422" PostTypeId="1" CreationDate="2015-06-07T05:07:51.960" Score="3" ViewCount="899" Body="&lt;p&gt;There is an idea to use shadow &lt;a href=&quot;http://encyclopedia2.thefreedictionary.com/Radiation+Shielding&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiation shielding&lt;/a&gt; for deep space craft to protect the crew in case of a solar flare. &lt;strong&gt;Will it work, and will it save mass?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8QNHs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-06-27T03:19:36.577" LastActivityDate="2015-06-27T09:47:12.470" Title="Can a column of water pointed at the sun be used as a shield from solar flares?" Tags="|radiation|deep-space|shielding|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9423" PostTypeId="1" CreationDate="2015-06-07T06:15:04.257" Score="9" ViewCount="1118" Body="&lt;p&gt;Russia has two types of hypergolic liquid fuel ICBMs deployed, &lt;a href=&quot;https://en.wikipedia.org/wiki/UR-100N&quot;&gt;UR-100N&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/R-36_(missile)&quot;&gt;R-36M2 Vojewoda&lt;/a&gt; and they are working on the big &lt;a href=&quot;https://en.wikipedia.org/wiki/Sarmat&quot;&gt;Sarmat&lt;/a&gt;. There's in this category also the Chinese DF-A5. (A list of ICBMs &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/c/cb/Icbm-hist-en.png&quot;&gt;here&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the main reasons for the US going for solid fuel ICBMs while Russia and China develop hypergolic liquid fuel ones? Why is hypergolic fuel prefered for ICBMs but not used for any new orbital launcher (except for India's GSLV)? Are there economic motives in the form of significant synergies between for example the UDMH/NTO Sarmat and the H+LOX Angara? &lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-06-07T06:20:33.090" LastActivityDate="2022-04-04T02:47:52.223" Title="Are ICBMs and orbital launchers similar enough to be co-developed today?" Tags="|economics|solid-fuel|launch-vehicle|liquid-fuel|icbm|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9424" PostTypeId="1" CreationDate="2015-06-07T06:30:05.893" Score="3" ViewCount="1814" Body="&lt;p&gt;Reading about aerogels again, they seem to be some of the greatest thermal insulators ever made, which is weird because they are so light. When I read about a very lightweight material with very high efficiency in something, I immediately think spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here it is. Can aerogels be used for ablative heatshields? It could be re-entry heatshields. It could be ablative material placed inside an engine chamber, throat, or nozzle, to help keep the walls cool. Is any of this possible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an important starting point tho, what is normally the material used for ablative heatshielding inside rocket engines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not a materials scientist and don't know how to figure out all the properties needed to &quot;know&quot; something. Maybe someone here can lend a hand.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="168" LastEditDate="2015-06-08T07:47:23.407" LastActivityDate="2015-06-08T07:47:23.407" Title="Can aerogel be used as an ablative material for heatshields?" Tags="|heat-shield|" AnswerCount="1" CommentCount="5" ClosedDate="2015-06-08T18:27:02.960" ContentLicense="CC BY-SA 3.0" />
  <row Id="9425" PostTypeId="2" ParentId="9423" CreationDate="2015-06-07T06:32:57.767" Score="13" Body="&lt;p&gt;It's not hypergolics per se that are super-desirable for ICBMs, but room-temperature-storable fuels. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ICBMs have to stay ready for long periods of time and be launched on short notice, so that means they have to stay fueled up more or less constantly. In practice, that means solid fuels or something in the UDMH/NTO family. A cryogenic fuel ICBM would need hours of lead time before a launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid fuels are less dangerous to store (and possibly less toxic to launch?) than hypergolic liquids; this may be why the US prefers solids, even though liquid fuels yield higher specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think ICBM vs orbital launcher development is generally going to tend to be divergent. ICBMs have an upper limit of practical required payload (like a handful of warheads under a ton each) and their payloads don't need to reach orbital velocity, so the design constraints are very different.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-06-07T06:56:44.567" LastActivityDate="2015-06-07T06:56:44.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9426" PostTypeId="1" CreationDate="2015-06-07T06:42:11.130" Score="13" ViewCount="2083" Body="&lt;p&gt;If suddenly put in a low gravity as on the Moon or on Mars, humans should feel very strong, easily breaking some olympic records for example. Apollo astronauts have said that they did not get tired from walking, carrying and working all day on the Moon. They just got tired in their fingers because of the bending resistance of the pressurized gloves (the excitement might have affected their perception, it is anecdotal afaik).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the muscle volume adapt to the lower gravity within a few months, making humans as relatively weak as we are on Earth? Are there reasons to believe that weight lifting exercises in low gravity can be efficient enough to retain the human Earth adapted strength?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-06-07T09:48:55.243" LastActivityDate="2020-03-10T20:09:58.430" Title="Would human &quot;superpowers&quot; in low gravity fade away through reduced muscle mass?" Tags="|gravity|microgravity|health|humans|medical|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9429" PostTypeId="1" CreationDate="2015-06-07T14:47:30.253" Score="5" ViewCount="213" Body="&lt;p&gt;If a human space mission or even a very valuable space telescope, gets into trouble, an extra launch from Earth might save the mission. But launchers today have to be prepared for months in advance. How much of the launch cost would it typically require to keep a rescue launcher ready? It would only be consumed if needed, but there are preparation costs in any case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the payload and the launcher aren't actually launched, but only prepared, how expensive would that be? Maybe there's useful data about costs for preparing today's large launchers from the occasional mishaps when they are refitted and refueled for another day. Does the preparation and readiness of a crewed rescue mission need to be much more expensive?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-08-28T15:00:48.843" Title="How expensive is a prepared launch in reserve, compared to an actual launch?" Tags="|launch|cost|rescue-missions|backup|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9430" PostTypeId="2" ParentId="9423" CreationDate="2015-06-07T15:12:04.250" Score="6" Body="&lt;p&gt;LOX/LH2 motors can achieve a higher specific impulse than hypergolic motors. According to &lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_rocket_propellant&quot; rel=&quot;nofollow&quot;&gt;the liquid rocket propellant&lt;/a&gt; page on wikipedia, theoretically LOX/LH2 has 111 second lead over UDMH/NTO. This might not seem like much, but changes in the specific impulse affect the rocket's delta-v significantly according to the &lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;rocket equation&lt;/a&gt;, so any gain is a big deal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, LOX/LH2 is not used in ICBMs because cryogenic fuels are difficult to store and must be loaded directly before launch. Since ICBMs do not need to achieve orbital velocity and carry a fixed payload, efficiency can be sacrificed for robustness and storability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For heavy orbital launchers, efficiency matters a lot. So designers will tend towards more efficient propulsion systems. This means that orbital launcher designs will diverge from ICBMs.&lt;/p&gt;&#xA;" OwnerUserId="10291" LastEditorUserId="10291" LastEditDate="2015-06-07T18:19:57.453" LastActivityDate="2015-06-07T18:19:57.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9431" PostTypeId="2" ParentId="9423" CreationDate="2015-06-07T16:48:42.080" Score="9" Body="&lt;p&gt;In addition to what the previous answers are already saying, I would like to point out that recent developments have almost all been solid, even on the Russian and Chinese side. If you look at the DF-31, the Topol-M or Yars, you will see why military leaders like them: They are doomsday devices in the truest sense of the word. They can be deployed anywhere, even through rough terrain, kept there for years, and then fired within 10 minutes at the push of a button.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not only is the propellant no immediate danger to the surroundings, they also don't need elaborate tubing to route the propellants, that eliminates problems that are known for hydrazine-based solutions, like the tendency of gasket materials to soak up hydrazine (or derivatives) and thereby gain volume and lose tensile strength. This could cause a rocket to suddently start leaking deadly propellant after quietly sitting somewhere for years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for synergy, I would say you're right on the money. France has long fought to replace the main engine of the Ariane 6 Launch vehicle with a solid rocket motor. This was in hopes that they could cut one of the two engine development programs that they finance. You see, for much the same reasons as the ICBMs discussed above France needs the solid fuels for it's &lt;a href=&quot;http://en.wikipedia.org/wiki/M51_%28missile%29&quot;&gt;SLBM&lt;/a&gt; arsenal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For first stages this kind of makes sense. The specific impulse problem that jxexk talked about mostly rears its head in the final stage of a rocket. In the lower stages generating a lot of thrust is key, and solids are good at that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, they are expensive to make, and satellite providers are nervous about the additional vibrations that they introduce harming their precious devices.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-06-07T16:48:42.080" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9433" PostTypeId="2" ParentId="9424" CreationDate="2015-06-08T04:52:05.217" Score="3" Body="&lt;p&gt;No.  The glass would just melt and disappear in very short order if it were the first thing presented to the flow.  I'd love to see the arc jet test of that.  I bet that the survival time of the aerogel would be measured in milliseconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe it could be an insulator between a usable heat shield and the structure to reduce the temperature the structure sees, but there would need to be passthroughs to carry the load since the aerogel would just crush under the load.  The passthroughs would be thermal shorts that might negate the benefit of the aerogel as an insulator.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-08T04:52:05.217" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9436" PostTypeId="1" AcceptedAnswerId="9437" CreationDate="2015-06-08T11:29:11.863" Score="6" ViewCount="4312" Body="&lt;p&gt;I found this terrific photo of an STS orbiter cockpit online, however it does not mention which orbiter:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eCFl5.jpg&quot; alt=&quot;Shuttle cockpit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How might one determine which orbiter this is&lt;/strong&gt;? The wear on the consoles seem to indicate that they were brushed often in a fashion that seems consistent with floating in, not walking in, so I don't think that this is a simulator. Also, the photographs of simulators don't seem to have the velcro stickers everywhere. Here is the &lt;a href=&quot;https://i.stack.imgur.com/eCFl5.jpg&quot; rel=&quot;noreferrer&quot;&gt;high res photograph&lt;/a&gt; for detail.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2015-08-04T11:47:06.420" Title="How to identify which Space Shuttle, from a high-res cockpit photograph?" Tags="|spacecraft|space-shuttle|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9437" PostTypeId="2" ParentId="9436" CreationDate="2015-06-08T11:38:49.890" Score="7" Body="&lt;p&gt;It's Space Shuttle Columbia, as I learned from a Google image search of the image itself (find similar), which came up with this result among others:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://airandspace.si.edu/explore-and-learn/multimedia/detail.cfm?id=1654&quot; rel=&quot;nofollow&quot;&gt;http://airandspace.si.edu/explore-and-learn/multimedia/detail.cfm?id=1654&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would be very interested to learn if there are ways to tell the cockpits apart from just the image, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the comments:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It's a whole lot easier to tell from looking at the aft cockpit or the middeck. In fact, after MEDS was installed, I don't know if one can tell by looking at the forward panels - Organic Marble&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2015-06-09T14:04:22.077" LastActivityDate="2015-06-09T14:04:22.077" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9438" PostTypeId="2" ParentId="9261" CreationDate="2015-06-08T13:00:10.607" Score="18" Body="&lt;p&gt;The answer to the question is an ancient philosophical question: &lt;a href=&quot;http://en.wikipedia.org/wiki/Ship_of_Theseus&quot; rel=&quot;noreferrer&quot;&gt;Ship of Theseus&lt;/a&gt;. As Organic Marble mentioned in his comment which could have been an answer:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The SRBs were made up of segments, and the reuse was at the segment level. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The obvious lower bound for number of &lt;strong&gt;SRB segments&lt;/strong&gt; produced would be eight: four SRB segments per side per launch. We can double that to 16 as &lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/5/53/Space_shuttles_Atlantis_%28STS-125%29_and_Endeavour_%28STS-400%29_on_launch_pads.jpg&quot; rel=&quot;noreferrer&quot;&gt;this photo&lt;/a&gt; shows two STS stacks on the pad together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each &lt;a href=&quot;https://www.flickr.com/photos/steve-umms/422691784&quot; rel=&quot;noreferrer&quot;&gt;SRB segment&lt;/a&gt; was rated for 20 launches. &lt;a href=&quot;http://pbs.twimg.com/media/B_yV_STU0AATjBO.png&quot; rel=&quot;noreferrer&quot;&gt;This document&lt;/a&gt; for the SLS Qualification Motor shows booster segment serial numbers in use on that test:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;O32, O89, O43now, L82, L89n, L109, L98n, L92, L95n, O59&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;I'm sure that I've misread or misinterpreted that slightly, but for purposes of estimating population the numbers will do.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we apply the &lt;a href=&quot;http://en.wikipedia.org/wiki/German_tank_problem&quot; rel=&quot;noreferrer&quot;&gt;German tank problem&lt;/a&gt; to that data, then we can &lt;strong&gt;estimate 120-130&lt;/strong&gt; total segments that were given serial numbers, as the higher serial numbers are better represented. Divide 120 by 4 segments per SRB, and I would say that enough SRB segments were produced to build &lt;strong&gt;about 30 SRBs&lt;/strong&gt;, given enough nose caps, nozzles, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#Components&quot; rel=&quot;noreferrer&quot;&gt;other components&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="472" LastEditDate="2015-06-09T09:34:19.750" LastActivityDate="2015-06-09T09:34:19.750" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9441" PostTypeId="1" AcceptedAnswerId="9442" CreationDate="2015-06-08T15:29:41.227" Score="4" ViewCount="1720" Body="&lt;p&gt;Yeah, I know, &quot;launch into target orbit&quot;. But let's assume we need to perform the maneuver. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A satellite in LEO equatorial orbit would need excessive amounts of delta-V to enter polar orbit. The amount needed in a very high orbit, on the other hand, is very low. Thing is getting from low to high orbit is costly, so the answer &lt;em&gt;probably&lt;/em&gt; lies somewhere in the middle. So - what would be the optimal maneuver of turning LEO 90 degrees?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-06-08T16:51:17.837" Title="Optimal turning from equatorial to polar orbit" Tags="|orbital-mechanics|low-earth-orbit|orbital-maneuver|polar-orbit|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9442" PostTypeId="2" ParentId="9441" CreationDate="2015-06-08T16:07:24.437" Score="4" Body="&lt;p&gt;Let's take a look at this. The fuel usage, from &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_inclination_change#Calculation&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, is &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta{v_i}= {2\sin(\frac{\Delta{i}}{2})\sqrt{1-e^2}\cos(w+f)na \over {(1+e\cos(f))}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a 0 e orbit to begin with, this basically becomes $sqrt(2)*v$, where v is the orbital velocity. Thus, for a LEO velocity of around 7.8 km/s, that adds up to be quite a lot of fuel!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the best strategy would be to do a nearly escape elliptical orbit, set so the $A_n$ or $D_n$ is at the peak of that elliptical orbit. When you reach the appoasis, your velocity will be very small, you can easily do the inclination switch there for minimal delta v. Then when you come back you give yourself the correct eccentricity. This will cost a significant amount of fuel still, but still reasonable. The escape velocity is about $sqrt(2)-1$ , so doubling that will give you an effective fuel budged of something less than your orbital velocity. The trick is to make the inclination change when your orbit is almost not moving, and that works best at extreme orbital heights on highly elliptical orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, I'm pretty confident you could do the same thing by a very clever Lunar flyby, much quicker. The delta v to do a lunar flyby from LEO is about 4.1 km/s, each way. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-06-08T16:51:17.837" LastActivityDate="2015-06-08T16:51:17.837" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9444" PostTypeId="2" ParentId="131" CreationDate="2015-06-08T20:59:21.733" Score="-4" Body="&lt;p&gt;Yes, you would become a superhero. It's nowhere stated that heavy planets should be gas giants, there are heavy outer planets with hard surface. And like effects of micro gravity are measurable (muscle atrophy), so would be effects of hyper gravity (muscle hypertrophy and consequentially higher bone density). Every move would become exercise and there is little overall Earth training that could be compared to that (except maybe swimming). If carried incrementally it could work like in Dragon Balls.&lt;/p&gt;&#xA;" OwnerUserId="10396" LastActivityDate="2015-06-08T20:59:21.733" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9445" PostTypeId="2" ParentId="9346" CreationDate="2015-06-09T00:05:43.680" Score="14" Body="&lt;p&gt;The new test stand is for the full Falcon Heavy. Every Merlin is tested on the stand before being combined into a full F9 stage. It is reasonable to extrapolate this out to the three core FHeavy. The benefits of a 3 core test are unclear. The Falcon Heavy was planned to have cross feed capability, where fuel would flow from the two external boosters to the center core to increase efficiency and allow the center core to be mostly full at stage separation. However, it appears that crossfeed has been put on hold since no customer needs such extremely large payload capacity from SpaceX. This test stand would have allowed for crossfeed testing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No test stand at McGregor is designed to handle Raptor. Raptor is currently undergoing component testing at a NASA test stand at the Stennis Space Center. There are various reasons for this, one mainly being that the Raptor is a Methalox engine instead of the Kerolox design of the Merlin. The McGregor test stands are plumbed to support kerosene and liquid oxygen. Also while the tripod may be able to support the Raptor, the existing engine test stands are not rated for the estimated thrust of Raptor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;br&gt;&#xA;&lt;a href=&quot;http://blog.gulflive.com/mississippi-press-news/2014/04/stennis_space_center_cuts_ribb.html&quot; rel=&quot;nofollow&quot;&gt;Regulatory Approval to Test FHeavy at McGregor&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=32202.msg1225930#msg1225930&quot; rel=&quot;nofollow&quot;&gt;NasaSpaceFlight Forum Discussion with Pictures of Flame Trench&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="6816" LastEditDate="2015-09-26T02:36:27.230" LastActivityDate="2015-09-26T02:36:27.230" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9446" PostTypeId="2" ParentId="9243" CreationDate="2015-06-09T00:17:00.230" Score="193" Body="&lt;p&gt;SpaceX uses an Actor-Judge system to provide triple redundancy to its rockets and spacecraft. The Falcon 9 has 3 dual core x86 processors running an instance of linux on each core. The flight software is written in C/C++ and runs in the x86 environment. For each calculation/decision, the &quot;flight string&quot; compares the results from both cores. If there is a inconsistency, the string is bad and doesn't send any commands. If both cores return the same response, the string sends the command to the various microcontrollers on the rocket that control things like the engines and grid fins. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The microcontrollers, running on PowerPC processors, received three commands from the three flight strings. They act as a judge to choose the correct course of actions. If all three strings are in agreement the microcontroller executes the command, but if 1 of the 3 is bad, it will go with the strings that have previously been correct. The Falcon 9 can successfully complete its mission with a single flight string.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The triple redundancy gives the system radiation tolerance without the need for expensive rad hardened components. SpaceX tests all flight software on what can be called a table rocket. They lay out all the computers and flight controllers on the Falcon 9 on a table and connect them like they would be on the actual rocket. They then run a complete simulated flight on the components, monitoring performance and potential failures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX engineers perform what they call &quot;Cutting the strings&quot; where they randomly shut off a flight computer mid simulation, to see how it responds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon uses a similar triple redundant system for its flight computers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strike&gt;Bonus: Dragon V2 control panels are modified 17&quot; Tesla Model S touchscreens and include the same Nvidia Tegra SoCs. They are not part of the flight hardware and are only used to display the GUI.&lt;/strike&gt;&#xA;After talking with the Dragon team at GDC2016, it appears Tesla hardware is not used. They do use some interesting software on Dragon 2. They use Chromium and JavaScript for the Dragon 2 flight interface. The actual flight computers still run on C++.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strike&gt;Source: Discussion with various SpaceX engineers at GDC 2015/2016&lt;/strike&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: After four years, SpaceX software engineers have confirmed Chromium and JS powered Crew Dragon's displays on-orbit during a &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/gxb7j1/we_are_the_spacex_software_team_ask_us_anything/&quot; rel=&quot;noreferrer&quot;&gt;Reddit AMA.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="10398" LastEditDate="2020-06-07T02:58:29.180" LastActivityDate="2020-06-07T02:58:29.180" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="9447" PostTypeId="2" ParentId="9026" CreationDate="2015-06-09T00:42:22.697" Score="7" Body="&lt;p&gt;There is relatively no advantage to constructing a truss system and there is actually a small advantage to launching from pad level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Pad Abort Test took place on LC-40 which is an active launch pad and the only pad SpaceX currently can use to launch Geostationary Satellites from. Building a 100+ foot steel truss on the pad and tearing it down would have taken time, time in which LC-40 couldn't be used for launching rockets with paying payloads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal of the Pad Abort Test is to validate that the Launch Escape system activates quickly enough to pull the capsule away from the launch vehicle before it is destroyed and get it to an altitude in which the parachutes can safely deploy. The Falcon 9 is 68.4m tall. The capsule will reach a max altitude of &gt;1500 meters. The extra height doesn't significantly impact the test and achieving the desired height with a ~70m deficit actually proves the LES more capable than required.&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="415" LastEditDate="2015-06-09T07:14:06.207" LastActivityDate="2015-06-09T07:14:06.207" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9448" PostTypeId="2" ParentId="8319" CreationDate="2015-06-09T01:39:18.047" Score="5" Body="&lt;p&gt;To first answer your question I think it is prudent to identify why SpaceX is moving the Center of Mass(CoM) in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space capsules since the Mercury days have needed heat shields for reentry. The high speeds and relatively flat shapes cause compressive heating which leads to exceedingly high temperatures. While heat shield materials like AvCoat and PICA-X are marvelous materials that can withstand high temperature, extended exposure to reentry plasma can cause the structure to fail. Engineers identified that a ballistic reentry from Lunar Return velocities(11km/s) would create unmanageable heating effects. Their solution was to change the ballistic trajectory into a semi-controlled descent. The Apollo Command Module had a CoM offset from its axis of symmetry. That created an aerodynamically stable &quot;edge&quot; of the capsule that could be controlled by rotating the craft. This allowed the capsule to generate lift and slow the acceleration from reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo used its Reaction Control System(RCS) to change the orientation of its CoM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Atmospheric_entry#Sphere_or_spherical_section&quot; rel=&quot;noreferrer&quot;&gt;Atmospheric Reentry&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://www.youtube.com/watch?v=aW5ozq4Tqew&quot; rel=&quot;noreferrer&quot;&gt;Apollo EDL Plan&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastActivityDate="2015-06-09T01:39:18.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9449" PostTypeId="2" ParentId="2911" CreationDate="2015-06-09T01:46:12.147" Score="6" Body="&lt;p&gt;No. Dragon V2 will likely be the first capsule with a built in &quot;pusher&quot; LES, an array of &lt;a href=&quot;https://en.wikipedia.org/wiki/SuperDraco&quot; rel=&quot;nofollow noreferrer&quot;&gt;SuperDraco&lt;/a&gt; engines. The reason it has never been done is because the LES is usually considered dead weight once the craft has reached orbit. Jettisoning the LES as early as possible saves payload mass. The reason SpaceX and Dragon keep it is twofold. Falcon 9 v1.1 is overpowered for sending cargo Dragon to the ISS. Payload capacity is volume limited on CRS missions. There is no reason to save weight by jettisoning the LES during flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, the SuperDracos can be useful on EDL by providing a controlled, accurate landing on solid ground. SpaceX has always planned Dragon to be reusable; however, salt water ocean landings are terrible to the spacecraft. Being able to gently land on solid ground saves the spacecraft systems from wear and tear and greatly eases recovery.&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="6816" LastEditDate="2019-02-27T22:35:25.640" LastActivityDate="2019-02-27T22:35:25.640" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="9451" PostTypeId="1" AcceptedAnswerId="9459" CreationDate="2015-06-09T14:51:51.073" Score="10" ViewCount="847" Body="&lt;p&gt;Watching the daily ISS live broadcast from NASA the PRO mentioned the specific mass of the ISS complex (today) and what it would be when the Soyuz leaves shortly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One can find out the mass of each upward flight, but the mass of the departing trash-loaded craft is sometimes not so clear. Its obviously important for repositioning burns to know the mass of ISS any time. Is this made available on a continuous basis (via NASA, ESA, Roscosmos etc)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can find Mass estimates, such as &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html&quot;&gt;here&lt;/a&gt;: 419,455 kilograms, but I can't find recent information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering, if the mass was plotted graphically (over time) what would it be like? Does more stuff go up than down, for example?&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2018-06-23T08:16:22.437" Title="Does any site track the mass of the ISS?" Tags="|iss|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9452" PostTypeId="2" ParentId="5290" CreationDate="2015-06-09T18:10:50.897" Score="6" Body="&lt;p&gt;The OctaWeb design of Falcon 9 v1.1 actually forms a crude approximation of an aerospike nozzle when all engines are firing. To understand why the exhaust plume looks like that in the photo, its important to understand how rocket engine bells works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dG7ZV.png&quot; alt=&quot;Nozzle Expansion Description&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image shows how the nozzle expands gases to produce thrust. Rocket combustion chambers combine fuel and oxidizer to produce a high pressure gas. This gas is actually moving relatively slowly in the combustion chamber (below mach 1). The expansion nozzle takes the high pressure gas and expands it to a low pressure, high velocity gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The four figures shows the same nozzle with the same expansion ratio in four different operating environments (different external pressures).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first nozzle is underexpanded, where the exhaust gas pressure &gt; ambient air pressure. This means that when the exhaust gases leave the nozzle, some of the gas expands perpendicular from the thrust vector and thus does no useful work on the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second figure shows the ideal expansion nozzle, where gas is expanded so that exhaust pressure=atmospheric pressure and all of its kinetic energy is heading out directly away from the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third figure shows an overexpanded nozzle where exhaust pressure &amp;lt; ambient pressure. The exhaust is compressed by the ambient air and loses some efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fourth figure shows a grossly overexpanded nozzle where exhaust pressure &amp;lt;&amp;lt; ambient pressure. This can lead to instability as the exhaust is separated from the walls of the nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ideal scenario is exhaust pressure==ambient pressure, however this ideal scenario does not happen often. Rockets are used to go to space, and as they rise in altitude the ambient air pressure decreases. Therefore a nozzle that is 'perfect' at sea level will be underexpanded as the rocket flies. Most rocket manufacturers design their first stage engines to be slightly overexpanded so that they will be in the ideal range as they gain altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to combat this however is to use a variable geometry nozzle to stay in the ideal range for as long as possible. An aerospike is a form of variable geometry nozzle and can be considered an inverted rocket nozzle. Fuel is injected along a cone (toroidal) or a wedge (linear). Ambient pressure and the walls of the spike create a virtual nozzle to expand the gases. This engine type will be more efficient because the expansion ratio changes with ambient pressure since the ambient pressure is what is expanding the gases in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/D2z6U.jpg&quot; alt=&quot;Aerospike Expansion Photo&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now back to the Falcon 9 v1.1. The Merlin 1D-Sea Level is slightly overexpanded at sea level. As the Falcon rises, it reaches ideal expansion then becomes underexpanded. The 9 engines in the OctaWeb create an aerospike-like effect where the 8 engines on the outside ring push the air away from the center engine and create a region of low pressure. At takeoff, the middle Merlin is actually producing more thrust than the rest of the Merlin's due to this effect. The plume in the photo is the underexpansion of all engines combined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket_engine_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Engine Expansion&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://en.wikipedia.org/wiki/Aerospike_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aerospikes&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="6816" LastEditDate="2015-06-10T19:28:49.457" LastActivityDate="2015-06-10T19:28:49.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9454" PostTypeId="1" CreationDate="2015-06-10T00:49:39.050" Score="10" ViewCount="2791" Body="&lt;p&gt;More than a month has passed since Dawn's arrival to Ceres but we have not seen a single colour image of it in natural colors. Why is this so? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Clarification.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not interested in the details on how space imaging is working in general. I am interested to know why Dawn does not produce color images of Ceres so far unlike other spacecrafts (for instance, Cassini, Messenger, Voyagers, Pioneer). Even New Horizons &lt;a href=&quot;http://www.nasa.gov/content/first-pluto-charon-color-image-from-new-horizons&quot; rel=&quot;nofollow&quot;&gt;already took a color image of Pluto&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="4660" LastEditDate="2015-06-19T22:02:06.643" LastActivityDate="2015-07-10T19:36:56.733" Title="Why doesn't Dawn create and release colour images of Ceres?" Tags="|imaging|dawn-mission|" AnswerCount="4" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="9455" PostTypeId="2" ParentId="9454" CreationDate="2015-06-10T05:07:15.680" Score="5" Body="&lt;p&gt;There are some color pictures.   The first article below indicates the early pictures were black and white but they now have a color map of the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cnet.com/news/dwarf-planet-ceres-reveals-surface-secrets-in-nasa-color-map/&quot;&gt;http://www.cnet.com/news/dwarf-planet-ceres-reveals-surface-secrets-in-nasa-color-map/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and from the NASA website:   &lt;a href=&quot;http://www.nasa.gov/jpl/pia19063/dawns-first-color-map-of-ceres/#.VXfE9kYTDW4&quot;&gt;http://www.nasa.gov/jpl/pia19063/dawns-first-color-map-of-ceres/#.VXfE9kYTDW4&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: as pointed out in the other answers, I should mention that the color in the photos in these articles is enhanced.  Ceres isn't that colorful.&lt;/p&gt;&#xA;" OwnerUserId="8711" LastEditorUserId="8711" LastEditDate="2015-06-10T07:48:59.843" LastActivityDate="2015-06-10T07:48:59.843" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9457" PostTypeId="2" ParentId="9454" CreationDate="2015-06-10T06:06:48.813" Score="6" Body="&lt;p&gt;The sensory equipment aboard deep-space satellites tend to be very different from commonly encountered analogs. In the case of Dawn, it has &lt;a href=&quot;http://www.nasa.gov/mission_pages/dawn/spacecraft/instruments.html&quot;&gt;2 framing cameras capable of capturing visible and near-infrared light using 7 color filters&lt;/a&gt;. The goal is to combine the color filtered images with data from the mapping spectrometer to analyze surface mineral composition. The storage capacity of each instrument and transmission bandwidth to Earth are both limited, and researchers are working under significant time constraints, so producing photorealistic images is not a high priority task.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, as userLTK's article pointed out, &lt;a href=&quot;http://www.nasa.gov/jpl/pia19063/dawns-first-color-map-of-ceres/#.VXfTBM8zhHx&quot;&gt;the color differences on the surface are subtle&lt;/a&gt;, so map rendering is mostly done to make identifying differences in surface composition easier.&lt;/p&gt;&#xA;" OwnerUserId="10427" LastActivityDate="2015-06-10T06:06:48.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9458" PostTypeId="2" ParentId="9454" CreationDate="2015-06-10T06:07:47.020" Score="31" Body="&lt;p&gt;The Dawn spacecraft is carrying 3 electromagnetic instruments; the Framing Camera (FC), the Gamma Ray and Neutron Detector (GRND), and the Visible and Infrared Mapping Spectrometer (VIMS). Your question is presumably about colour images in the visual spectrum and not false-color images of wavelengths not visible to humans, so let's focus on the &lt;strong&gt;Framing Camera.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.nasa.gov/sites/default/files/thumbnails/image/pia19310a.jpg&quot; width=&quot;500&quot; height=&quot;275&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Ceres as seen by the Dawn orbiter (Courtesy &lt;a href=&quot;https://www.nasa.gov/mission_pages/dawn/images/index.html?id=342069&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/dawn/spacecraft/instruments.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;According to NASA:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Framing Camera is designed to acquire detailed optical images for&#xA;  scientific purposes as well as for navigation in the vicinities of&#xA;  Vesta and Ceres. Dawn carries two identical and physically separate&#xA;  cameras for redundancy, each with its own optics, electronics and&#xA;  structure. Each camera is equipped with an f/7.9 refractive optical&#xA;  system with a focal length of 150mm and can use 7 color filters.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The camera on the Dawn orbiter, and most cameras in space, do not have the kind of image sensor a regular digital camera has. In &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2011/3239.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this fascinating article&lt;/a&gt; on the Planetary Society website, &lt;a href=&quot;http://www.planetary.org/about/staff/emily-lakdawalla.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Emily Lakdawalla&lt;/a&gt; writes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Space cameras usually have detectors that can detect light across&#xA;  slightly more of the electromagnetic spectrum than human eyes can see,&#xA;  from near-ultraviolet to near-infrared wavelengths. More importantly,&#xA;  while the pictures that they take are often pretty, they are not&#xA;  intended to be for snapshots; they are sophisticated, precise&#xA;  scientific measuring devices.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The FC is more of a 'photon measuring device' than a conventional camera. It measures the numbers of photons that land on each pixel of the sensor, not their wavelengths. However, it can perceive photons of different colours using &lt;strong&gt;filters.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A filter is like a gel on a theater light. It turns white light into&#xA;  colored light. It does this by limiting the wavelengths that can pass&#xA;  through the filter. Dawn's Framing Camera has eight different filters&#xA;  fairly evenly spaced across the part of the spectrum that its CCD is&#xA;  capable of detecting.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://planetary.s3.amazonaws.com/assets/images/z_changeover/Dawn_FC_filter_bandpasses.png&quot; width=&quot;400&quot; height=&quot;300&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; A graph showing the different wavelengths of visual light each filter (numbered F1-F8) allows through. (Courtesy &lt;a href=&quot;http://indico.cern.ch/event/43007/material/6/0?contribId=19&quot; rel=&quot;nofollow noreferrer&quot;&gt;CERN&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://spinoff.nasa.gov/Spinoff2010/cg_3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;image sensors&lt;/a&gt; in ordinary consumer cameras here on Earth actually also record light &lt;a href=&quot;https://en.wikipedia.org/wiki/Image_sensor#Color_separation&quot; rel=&quot;nofollow noreferrer&quot;&gt;according to its intensity in three separate colours&lt;/a&gt; and then process that information to create a full-colour image. Different parts of the sensor are designed to respond only to certain wavelengths of light - some in red, some in green, some in blue. There are no sensors that can detect what wavelength of light is hitting them, they only count how many photons hit. One way or another, all cameras combine information gathered separately about light intensity at different wavelengths, and then combine that into a colour image. In fact, &lt;a href=&quot;https://en.wikipedia.org/wiki/Photoreceptor_cell&quot; rel=&quot;nofollow noreferrer&quot;&gt;your eyes&lt;/a&gt; do this too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, Dawn takes 'black-and-white' photographs at a very high resolution &lt;em&gt;with each filter&lt;/em&gt;. Hence around eight images are created, each showing the raw photon-measurement in that respective section of the visual spectrum. These images can then be used to pretty accurately colourise the surface of Ceres and produce 'colour images'. However these would still be classified as 'false colour' pictures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why haven't any colour pictures of Ceres been released by Dawn? The answer is that they simply haven't been made yet. You can access the latest raw images from the Framing Camera &lt;a href=&quot;http://photojournal.jpl.nasa.gov/mission/Dawn?subselect=Target:Ceres:&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. NASA will presumably create colour pictures from the raw data in the future, and &lt;a href=&quot;https://twitter.com/vedantchan/status/605001607269343232&quot; rel=&quot;nofollow noreferrer&quot;&gt;so could I&lt;/a&gt;, and &lt;a href=&quot;https://twitter.com/TildalWave/status/603081056774975488&quot; rel=&quot;nofollow noreferrer&quot;&gt;so does @TildalWave&lt;/a&gt;. The colour these sorts of images are created on Earth, by people, &lt;strong&gt;not&lt;/strong&gt; by the probe itself. There's no real reason to colourise an image other than beauty, the scientific data is gathered from the RAW file itself. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Further Reading:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://indico.cern.ch/event/43007/material/6/0?contribId=19&quot; rel=&quot;nofollow noreferrer&quot;&gt;CERN document on the Framing Camera.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2011/3239.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planetary Society article on the Framing Camera.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/marc-rayman/0529-dawn-journal-spiralling-closer-to-ceres.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planetary Society article on the Dawn mission as a whole.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.680" LastActivityDate="2015-06-11T15:34:36.193" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9459" PostTypeId="2" ParentId="9451" CreationDate="2015-06-10T06:21:12.593" Score="15" Body="&lt;p&gt;Space Station live has a very useful tool to track all the current parameters of the ISS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://isslive.com/displays/adcoDisplay1.html&quot; rel=&quot;noreferrer&quot;&gt;http://isslive.com/displays/adcoDisplay1.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of now, the mass reads as &lt;strong&gt;390,377.5 kg.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Update, 22-June-2018:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New link: &lt;a href=&quot;https://isslive.com/displays/topoDisplay1.html&quot; rel=&quot;noreferrer&quot;&gt;https://isslive.com/displays/topoDisplay1.html&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;and a new mass! &lt;strong&gt;417,501.56 kg.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="12102" LastEditDate="2018-06-22T14:25:04.980" LastActivityDate="2018-06-22T14:25:04.980" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="9460" PostTypeId="1" AcceptedAnswerId="9481" CreationDate="2015-06-10T12:05:10.970" Score="22" ViewCount="3885" Body="&lt;p&gt;I understand that all manned landings on the Moon were on the near side, so technically they could be visible from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was it possible to observe landings and/or extravehicular activities (EVAs) with Earth-based telescopes? If not, why?&lt;/p&gt;&#xA;" OwnerUserId="2891" LastEditorUserId="15685" LastEditDate="2023-07-12T12:46:35.210" LastActivityDate="2023-07-12T12:46:35.210" Title="Were the Apollo lunar activities observed from Earth?" Tags="|the-moon|apollo-program|lunar-landing|observation|telescope|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9461" PostTypeId="2" ParentId="9460" CreationDate="2015-06-10T12:14:50.297" Score="21" Body="&lt;p&gt;No they were not.  Telescopes, even today cannot resolve that small a detail from the distance. LCROSS, orbiting the moon was able to barely resolve the lunar modules left behind. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More good details in this similar question and answer on the Astronomy site:&#xA;&lt;a href=&quot;https://astronomy.stackexchange.com/questions/6026/visibility-of-apollo-11-module&quot;&gt;Visibility of the Apollo-11 Module&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2015-06-10T12:14:50.297" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9462" PostTypeId="1" CreationDate="2015-06-10T18:49:38.227" Score="7" ViewCount="468" Body="&lt;p&gt;The original ASDS (&lt;a href=&quot;http://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;Autonomous Spaceport Drone Ship&lt;/a&gt; known as &quot;Just Read The Instructions&quot; (JRTI) was based on the &lt;a href=&quot;http://www.mcdonoughmarine.com/ballast-water-capacity-docs.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Marmac 300&lt;/a&gt; hull. It has been replaced with the Marmac 304 hull for use in the CRS-7 launch attempt of a first stage landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Marmac 303 has the wings from Marmac 300 and presumably will become the West Coast ASDS after a trip through the Panama Canal with the wings removed. Per Elon Musk, the West Coast ASDS will be named &quot;Of Course I Still Love You&quot; (OCISLY). Edit: In hindsight a few years later, OCISLY remained the East Coast/Atlantic barge, and JRTI moved over to the West Coast/Pacific.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What changes were made to the East Coast ASDS in the switch from Marmac 300 hull to the Marmac 304 hull?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-06-25T21:06:21.967" LastActivityDate="2017-06-25T21:06:21.967" Title="What did SpaceX change in the upgrade of the ASDS?" Tags="|spacex|landing|falcon-9|asds|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9463" PostTypeId="2" ParentId="9462" CreationDate="2015-06-10T18:51:30.237" Score="8" Body="&lt;p&gt;Based on a &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=36326.msg1387245#msg1387245&quot; rel=&quot;nofollow noreferrer&quot;&gt;rumour&lt;/a&gt;, who so far has been accurate the following is reported:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The wings have been boxed in for added strength on both the original and new wing sets. They are attached by welding only; no hull penetrations needed.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Only penetrations are through the barge decks, for fresh water ballast, which helps stabilize in towing. Ballast water is also used for flooding the decks during landing, which, along with heat resistant paint, protects the deck.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Barge is unmanned during operations/landings, with no provisions for people, which helps keep USCG compliance as &quot;unmanned vessels.&quot;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;No equipment located below decks.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Thrusters are intended only for stationkeeping, not long-distance autonomous self-transport. They could do a full trip but would require larger fuel cells.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Front wall is a breakwater, back wall is a blast shield, both to protect the containers, etc.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In late 2016/early 2017 one of the cargo boxes was raised on stilts, and a garage door installed on the blast shield, as a garage for the &quot;SpaceX Roomba&quot; like robot designed to hold onto stages in bad weather. No official word on its name.  (I like Xoomba, other like Attitude Adjuster).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-06-25T21:05:23.240" LastActivityDate="2017-06-25T21:05:23.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9464" PostTypeId="1" CreationDate="2015-06-10T19:04:01.830" Score="3" ViewCount="237" Body="&lt;p&gt;How can one determine the required altitude/velocity/time from release required for a small spacecraft (1 to 10 kg) in a suborbital trajectory to ensure that it burns up as it re-enters the atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="10438" LastEditorUserId="1028" LastEditDate="2015-06-11T07:11:37.207" LastActivityDate="2015-06-11T07:11:37.207" Title="What are the requirements of a suborbital trajectory, to ensure that a small payload burns up?" Tags="|heat|reentry|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9465" PostTypeId="2" ParentId="6590" CreationDate="2015-06-10T19:42:33.407" Score="3" Body="&lt;p&gt;For the first set of missions, the ASDS JRTI was towed by two barges, &lt;a href=&quot;http://www.smithmaritime.us/vessel-elsbeth3.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elsbeth III&lt;/a&gt; (you can see where &lt;a href=&quot;https://www.vesselfinder.com/?mmsi=367017460&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elsbeth III is on VesselFinder&lt;/a&gt;) and &lt;a href=&quot;https://www.vesselfinder.com/vessels/GO-QUEST-IMO-8987876&quot; rel=&quot;nofollow noreferrer&quot;&gt;Go Quest&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The tug Rhea towed the Marmac 303 based ASDS (OCISLY probably) through the Panama Canal to the West Coast.&lt;/p&gt;&#xA;&lt;p&gt;The Thrustmaster units on board (one on each corner) are designed for position keeping only. They would not be efficient to travel any kind of distance, and there is not enough fuel for them for any appreciable distance of travel.&lt;/p&gt;&#xA;&lt;p&gt;The Thrustmaster units are powered from the deck, with hydraulic fluid running down to turn the propellers which can rotate in plane, and the arm of the Thrustmaster can be lifted up as the water gets shallower.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-06-10T19:42:33.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9467" PostTypeId="2" ParentId="940" CreationDate="2015-06-10T20:47:05.123" Score="18" Body="&lt;p&gt;&lt;strong&gt;Yes, we know of a least one CubeSat that has flown (and is currently flying) with an active propulsion system.&lt;/strong&gt; Specifically a steam powered warp drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/dYsVq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.surrey.ac.uk/ssc/research/onboarddata/strand/&quot; rel=&quot;noreferrer&quot;&gt;STRaND-1&lt;/a&gt; (Surrey Training, Research and Nanosatellite Demonstrator 1) was launched aboard &lt;a href=&quot;http://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;PSLV-C20&lt;/a&gt; on February 25, 2013. Developed jointly by the &lt;a href=&quot;http://www.surrey.ac.uk/ssc/&quot; rel=&quot;noreferrer&quot;&gt;Surrey Space Laboratory&lt;/a&gt; and &lt;a href=&quot;http://www.sstl.co.uk/&quot; rel=&quot;noreferrer&quot;&gt;Surrey Satellite Technology Ltd&lt;/a&gt;, STRaND-1 and was the first CubeSat developed in the UK.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apart from being the first smartphone based satellite in space it also carried a system of pulsed plasma thrusters for attitude control and a &lt;a href=&quot;http://www.sstl.co.uk/Blog/January-2013/WARP-speed-ahead&quot; rel=&quot;noreferrer&quot;&gt;Water Alchohol Resistojet Propulsion system&lt;/a&gt; (WARP DRiVE, their name, not mine) for orbital maintenance and decommissioning.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Resistojet_rocket&quot; rel=&quot;noreferrer&quot;&gt;Resistojets&lt;/a&gt; work by electrically heating a fluid (in this case a water-alcohol mix) to create high pressure gas (in this case mostly steam) that is then vented through a conventional nozzle to provide thrust. Hence: Steam powered WARP DRiVE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the results, &lt;a href=&quot;http://www.n2yo.com/?s=39090&quot; rel=&quot;noreferrer&quot;&gt;see for yourself&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether it was the &lt;em&gt;first&lt;/em&gt; CubeSat propulsion system to fly I'm not sure, but it certainly has the best name of any I've heard of.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.surrey.ac.uk/mediacentre/press/2013/98519_worlds_first_phonesat_smartphone_strand1_successfully_launched_into_orbit.htm&quot; rel=&quot;noreferrer&quot;&gt;Launch press release.&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://epubs.surrey.ac.uk/26828/2/STRaND-1%20IAC%20Paper.pdf&quot; rel=&quot;noreferrer&quot;&gt;STRAND-1: USE OF A $500 SMARTPHONE AS THE CENTRAL AVIONICS OF A&#xA;NANOSATELLITETE. S Kenyon, C Bridges, et al.&lt;/a&gt; &#xA;(Note that this was written in 2011, and states that the fuel for the resistojet was planned to be Methane. As the later documentation shows this was changed to water-alcohol sometime later in development.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-06-11T08:42:54.323" LastActivityDate="2015-06-11T08:42:54.323" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9468" PostTypeId="2" ParentId="9156" CreationDate="2015-06-10T21:13:27.087" Score="2" Body="&lt;p&gt;A special case of a launch sled would be to incline it up the side of a mountain. At which point, the lower end would be in the atmosphere, the top end thinner due to altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would probably want to enclose it and evacuate the tube so you could accelerate through a vacuum or close to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you now have a tube, you might want to use magnetic induction to propel it.  But if not, you might want to ignite something behind it, and you have a launch gun.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-10T21:13:27.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9469" PostTypeId="1" AcceptedAnswerId="9471" CreationDate="2015-06-10T21:46:05.750" Score="2" ViewCount="154" Body="&lt;p&gt;I found a table of historically largest telescopes &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_largest_optical_telescopes_historically&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, so I'm guessing it's the Hale Telescope of the Palomar Observatory on Mount Wilson. However, that Wikipedia article warns that size is not everything when it comes to light-gathering-power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I want to know: &lt;strong&gt;In the 1960's, what was the best optical telescope on Earth, in terms of apparent magnitude or light-gathering-power? And what is the faintest apparent magnitude it could see?&lt;/strong&gt; I'm hoping it will be able to observe things down to an apparent magnitude of at least 15.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="5963" LastEditDate="2015-06-10T22:53:07.490" LastActivityDate="2015-06-10T23:24:53.837" Title="What observatory had the best optical telescope in the 1960's, based on faintest apparent magnitude it could image?" Tags="|telescope|" AnswerCount="1" CommentCount="1" ClosedDate="2015-06-11T04:45:58.187" ContentLicense="CC BY-SA 3.0" />
  <row Id="9470" PostTypeId="2" ParentId="9460" CreationDate="2015-06-10T21:46:43.020" Score="28" Body="&lt;p&gt;As several nations' governments and plenty of amateurs pointed radio frequency antennas at the sites and received signals, one might conclude that they were observed.  Observation does not have to imply visible light observation.&#xA;See this &lt;a href=&quot;http://www.arrl.org/eavesdropping-on-apollo-11&quot;&gt;http://www.arrl.org/eavesdropping-on-apollo-11&lt;/a&gt;&#xA;My recollection from the time is that plenty of amateurs tuned in to the transmissions coming from the moon.&lt;/p&gt;&#xA;" OwnerUserId="10441" LastActivityDate="2015-06-10T21:46:43.020" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9471" PostTypeId="2" ParentId="9469" CreationDate="2015-06-10T23:09:45.150" Score="5" Body="&lt;p&gt;This related &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_largest_optical_telescopes_in_the_20th_century&quot; rel=&quot;noreferrer&quot;&gt;list&lt;/a&gt; shows nothing even close to the size of Hale through the 1960s. Size is not everything but it is certainly the dominant factor in limiting magnitude; I'd be extremely surprised if improvements in optical quality between 1948 and 1969 could compensate for the sheer size of that scope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Magnitude 15 would be no problem, achievable even in modestly sized telescopes; &lt;a href=&quot;http://www.twcac.org/Tutorials/limiting_magnitude_table.htm&quot; rel=&quot;noreferrer&quot;&gt;this table&lt;/a&gt; suggests the limit would be beyond magnitude 20. Long photographic exposures extend the limit; Palomar's &lt;a href=&quot;http://en.wikipedia.org/wiki/Palomar_Observatory#POSS-I&quot; rel=&quot;noreferrer&quot;&gt;1948-1958 survey&lt;/a&gt; on the much smaller &lt;a href=&quot;http://en.wikipedia.org/wiki/Samuel_Oschin_telescope&quot; rel=&quot;noreferrer&quot;&gt;48&quot; Samuel Oschin&lt;/a&gt; telescope was sensitive to magnitude 22.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-06-10T23:24:53.837" LastActivityDate="2015-06-10T23:24:53.837" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9472" PostTypeId="5" CreationDate="2015-06-11T02:52:04.993" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-11T02:52:04.993" LastActivityDate="2015-06-11T02:52:04.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9473" PostTypeId="4" CreationDate="2015-06-11T02:52:04.993" Score="0" Body="International Launch Services, which launches Proton rockets from Baikonur in Kazakhstan. A joint venture created in 1995 between Lockheed Martin, Energia, and Khrunichev. Peviously they also launched Atlas rockets, but the last such launch was in 2006." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-11T02:52:04.993" LastActivityDate="2015-06-11T02:52:04.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9474" PostTypeId="5" CreationDate="2015-06-11T03:02:34.770" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-11T03:02:34.770" LastActivityDate="2015-06-11T03:02:34.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9475" PostTypeId="4" CreationDate="2015-06-11T03:02:34.770" Score="0" Body="An orbit that passes over or near the poles of a celestial body. Each orbit passes over different longitudes of the orbited body. Earth satellites use this orbit for tasks such as mapping and observation." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-11T03:02:34.770" LastActivityDate="2015-06-11T03:02:34.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9476" PostTypeId="5" CreationDate="2015-06-11T03:17:05.583" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-11T03:17:05.583" LastActivityDate="2015-06-11T03:17:05.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9477" PostTypeId="4" CreationDate="2015-06-11T03:17:05.583" Score="0" Body="A propellant that does not require an oxidizer, it can be used alone. An exothermic reaction is started with a catalyst or by heating. Hydrazine is the most common such fuel. New ones are in development." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-06-11T03:17:05.583" LastActivityDate="2015-06-11T03:17:05.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9478" PostTypeId="2" ParentId="9464" CreationDate="2015-06-11T07:08:46.317" Score="2" Body="&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/43768/after-what-speed-air-friction-starts-to-heat-up-an-object/43795#43795&quot;&gt;This question&lt;/a&gt;&lt;/strong&gt; from Physics.SE is very relevant, I'll condense it's main features here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The change in temperature of a body in the Earth's atmosphere is calculated using:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{T_2}{T_1} = \left( 1+\frac{1}{2} \rho v^2 / P1 \right)^{2/7}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming: Air temperature is $293 K$, and air density is $1.3 kg/m^3$, and pressure is $1 atm$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using this equation, you can plug in the required change in temperature to burn up your payload, and hence calculate the velocity required. With the velocity, you can reverse-engineer one out of infinite possible suborbital trajectories.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This particular question assumed a sphere of radius 1 meter, and asked about what speed would be required to increase it's temperature by $200º C$. Approximately, the answer was $2000 mph$ or around $900 m/s$. De-orbit speed from LEO is about &lt;em&gt;7 times that&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My (uneducated) guess is that &lt;strong&gt;&lt;em&gt;any&lt;/em&gt; payload under $10 kg$ will burn up in the atmosphere if it re-enters from LEO at $7 km/s$&lt;/strong&gt;. It depends on the material and shape of the craft, as well as the size and density, but I still don't think any debris will survive the fall.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Sources and Further Reading:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/43768/after-what-speed-air-friction-starts-to-heat-up-an-object/43795#43795&quot;&gt;Physics.SE Question&lt;/a&gt; (heavily referenced)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section%20III.4.1.7%20Returning%20from%20Space.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;FAA 4.1.7 on Atmospheric Re-entry&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://personal.ee.surrey.ac.uk/Personal/A.Pechev/lnlvp/AP-lec4.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;'Re-entry Dynamics' : Dr. A. Pechev, University of Surrey.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-06-11T07:08:46.317" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9479" PostTypeId="1" AcceptedAnswerId="9480" CreationDate="2015-06-11T07:58:10.510" Score="0" ViewCount="1108" Body="&lt;p&gt;You often hear scientists talk about it: we have found traces of molecules that are necessary for life on Mars. But we all know that Mars is not habitable right now. It might have been, but we cannot live there now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am making a space exploration video game, and I'm wondering what properties a planet needs in order for Earth-like life to exist there. I know about the habitable zone around a star, temperature, atmospheric density, gravity, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to get a more complete list of every parameter on planets that makes unaided life possible.&lt;/p&gt;&#xA;" OwnerUserId="9032" LastEditorUserId="4660" LastEditDate="2015-06-11T16:55:06.240" LastActivityDate="2015-06-17T23:26:14.330" Title="What are the required parameters for modern Earth-like life on a planet?" Tags="|life|goldilocks-zone|" AnswerCount="2" CommentCount="5" ClosedDate="2015-06-15T12:06:03.287" ContentLicense="CC BY-SA 3.0" />
  <row Id="9480" PostTypeId="2" ParentId="9479" CreationDate="2015-06-11T08:51:36.773" Score="0" Body="&lt;p&gt;&lt;strong&gt;Planetary Habitability&lt;/strong&gt; depends on a whole host of parameters, I will attempt to summarise some of them. These represent the absolute basic characterises a planet should have to sustain life. I leave the advanced mechanisms of the actual creation of life to your creative freedom.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vxppy.jpg&quot; alt=&quot;The blue planet&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Our Earth, the only planet in our infinitely large universe known to harbour life. (Courtesy &lt;a href=&quot;http://en.wikipedia.org/wiki/File:The_Earth_seen_from_Apollo_17.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;)  &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Mass&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Geologically, a small planet will have a high surface-to-volume ratio. As a result, most of the energy acquired during formation is lost, and plate tectonics do not tend to occur. Current research suggests that tectonic movement is vital for evolution, and of course, geothermal heat. Do note that exceptions to this rule exist, such as Io, which maintains it's tectonic system due to gravitational stress imparted by Jupiter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planets with a very low mass tend not to have the necessary gravitational pull to retain an atmosphere. For example Mars is less massive than the Earth, and has a much thinner atmosphere. A thin atmosphere means less heat insulation, less protection from radiation, and less protection from meteors. Additionally a minimum atmospheric pressure of $0.006 atm$ is required for water to exist in it's liquid form. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand if a planet is too large, then it's surface gravity starts becoming an issue. Any life on that planet would have to evolve to withstand the high gravity. However presuming such evolution can take place, larger planets are more suited to harbour life. They have a tendency to create magnetic fields from their iron cores, protecting the planet from radiation. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Orbital Characteristics&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The Earth has an orbital eccentricity (very roughly, deviation from a circular orbit) of 0.02. This means the the distance between the Earth and the Sun stays more or less the same over the period of a year. Hence, heating is uniform year-round. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a planet has a high orbital eccentricity, then it will go through massive swings in global climate over the course of it's 'year'. These fluctuations could be too severe for any life to evolve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rotation and tilt are two important factors, being responsible for seasons. Rotation should be fast enough to maintain a reasonable day-night cycle. If it is too slow, then the large difference in temperature each day would cause issues. Rotation also influences the magnetic field of the planet by inducing a magnetic dynamo in it's iron core. Finally, there is significant scientific evidence that the presence of the Moon is a crucial factor in stabilising the Earth's tilt.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Geochemistry&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Assuming that extra-terrestrial life is similar in composition to life on Earth, four fundamental elements are required; Oxygen, Carbon, Hydrogen, and Nitrogen. However since this is a video game you are developing, I presume you have the artistic freedom to create new, different mechanisms for life. Thus, I shall not elaborate more here.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Others&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Once you have your planetary characteristics sorted out, it's up to you to define further parameters for life to exist on that planet. What type of star is it orbiting? Are solar winds frequent, and how do they affect the planet? Does the planet's solar system have an Oort Cloud, hurling massive asteroids through space, causing mass extinctions after a fixed time period? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Life is fascinating because it is the single most complex phenomenon we have observed so far. Only your imagination limits the world you can create. Do some research on your own as well, read up on evolution and biochemistry. I wish you the best for your game!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://en.wikipedia.org/wiki/Planetary_habitability&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://en.wikipedia.org/wiki/Planetary_habitability&lt;/a&gt; (Structured around this)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://adsabs.harvard.edu/abs/2007AsBio...7...66R&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://adsabs.harvard.edu/abs/2007AsBio...7...66R&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Further Reading:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900013148.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900013148.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://web.archive.org/web/20110117011137/http://astrobiology.arc.nasa.gov/roadmap/g1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://web.archive.org/web/20110117011137/http://astrobiology.arc.nasa.gov/roadmap/g1.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.cfa.harvard.edu/news/2008-02&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.cfa.harvard.edu/news/2008-02&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-17T23:26:14.330" LastActivityDate="2015-06-17T23:26:14.330" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9481" PostTypeId="2" ParentId="9460" CreationDate="2015-06-11T08:52:18.487" Score="36" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;Was it possible to observe landings and/or extravehicular activities&#xA;(EVAs) with Earth-based telescopes?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Short answer: Not possible in 1969&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;In 1969, we had not the faintest possibility to see a man on the Moon from Earth. The angular resolution of the existing telescopes was by far insufficient to see an object the size of the LM.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Why?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Diffraction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Light diffraction&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Airy_disk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Airy pattern&lt;/a&gt; associated with optical instruments prevent us to see smallest details even if we can &amp;quot;zoom&amp;quot; at the required level. To overcome this limit, the size of the aperture (mirror) must be increased. So knowing the visibility threshold just requires knowing the size of the telescope apertures.&lt;/p&gt;&#xA;&lt;p&gt;In 1969 the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hale_telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hale telescope&lt;/a&gt; of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Palomar_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;Palomar observatory&lt;/a&gt; was the largest available. With its 5m mirror it couldn't resolve details smaller than five times the LM height. (See how the limit is determined at the end of the answer).&lt;/p&gt;&#xA;&lt;p&gt;But what size are we talking about exactly?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Angular size of the LM&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;When looking at objects, the actual linear size itself is not a meaningful information. What counts is their &lt;em&gt;apparent&lt;/em&gt; size which depends on the actual size &lt;em&gt;and&lt;/em&gt; the distance. This apparent size is expressed as an arc length: The &lt;a href=&quot;http://www.1728.org/angsize.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;angular size&lt;/a&gt;. In astronomy, the arc is often expressed in milliarcsecond (&lt;a href=&quot;https://en.wiktionary.org/wiki/milliarcsecond&quot; rel=&quot;nofollow noreferrer&quot;&gt;mas&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/x7p28.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/x7p28.png&quot; alt=&quot;Angular size&quot; /&gt;&lt;/a&gt;&#xA;&lt;sup&gt;Angular/apparent size (objects images from &lt;a href=&quot;https://www.hiclipart.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;HiClipart&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;Seen from Earth surface, the Moon lander has the same &amp;quot;apparent height&amp;quot; than a fly at the distance of the ISS: 5.4 mas. Seeing the LM on the Moon requires the capability to see a fly at 400 km, which is not an easy task.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Telescope angular resolution in 1969&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_resolution&quot; rel=&quot;nofollow noreferrer&quot;&gt;The angular resolution&lt;/a&gt;, is the smallest angular size that an instrument can see, anything smaller is blurry due to light diffraction. For example, the cosmic microwave background (CMB) seen with instruments of different angular resolution (best resolution on the left):&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/l1b3c.png&quot; alt=&quot;CMB maps (10° wide) obtained by Planck (5' resolution), WMAP (12') and COBE (5°)&quot; /&gt;&lt;br /&gt;&#xA;&lt;sup&gt;CMB maps (10° wide) obtained by Planck (5' resolution), WMAP (12') and COBE (5°). &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_microwave_background&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Hale_telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;5m-Hale&lt;/a&gt; telescope of Palomar observatory had a theoretical resolution of 25 mas, far from the 5.4 mas that would have been required to see the LM. Whatever the power of the telescope, a group of 5 rows of 5 LM would have been seen as a blurry dot.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Barely better today, no improvement expected in the short term&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The first system which will be able to achieve 5 mas alone will be the &lt;a href=&quot;https://en.wikipedia.org/wiki/Extremely_Large_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;European Extremely Large Telescope&lt;/a&gt; (E-ELT) planned for 2025 (&lt;a href=&quot;https://mapper.acme.com/?ll=-24.58930,-70.19160&amp;amp;z=17&amp;amp;t=SL&quot; rel=&quot;nofollow noreferrer&quot;&gt;site&lt;/a&gt;). Used with the &lt;a href=&quot;https://www2.physics.ox.ac.uk/research/visible-and-infrared-instruments/harmoni&quot; rel=&quot;nofollow noreferrer&quot;&gt;Harmoni imager&lt;/a&gt;, the resolution should be 4 mas per &lt;a href=&quot;https://pdfs.semanticscholar.org/2f59/7dd5a588417664cac45e240a8090b7ff93e9.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaxel&lt;/a&gt;. So a single pixel LM. Other optical telescopes under construction:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Thirty_Meter_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thirty Meter Telescope&lt;/a&gt; expected for 2027, 10 mas.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;http://www.osa-opn.org/home/articles/volume_20/issue_7/features/optics_for_the_giant_magellan_telescope/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Giant Magellan Telescope&lt;/a&gt; expected for 2029, 10 mas.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CJlVU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CJlVU.png&quot; alt=&quot;Comparison of existing and planned telescope sizes&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sup&gt;Comparison of existing and planned telescope sizes, adapted from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Comparison_optical_telescope_primary_mirrors.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;original by Cmglee on Wikipedia&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;A 40m mirror (E-ELT) is probably close to the maximum reasonable to build, and further developments actually follow the interferometer solution combining two or more instruments to get a resolution better than the one of the individual instruments.&lt;/p&gt;&#xA;&lt;p&gt;Images from distant telescopes are merged in a correlator. From a resolution standpoint, such assembly works as if there was a single mirror of the diameter equal to the distance between mirrors, regardless of the actual size of the mirrors (but the luminosity still depends on the actual size). The technique is also known as a &lt;a href=&quot;http://www.dunlap.utoronto.ca/wp-content/uploads/2016/08/Radio-Interferometry-v2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;synthetic aperture&lt;/a&gt;, a designation first found in the radar domain.&lt;/p&gt;&#xA;&lt;p&gt;The different light beams from each source must be correlated into a single image, which means their trip from the different sources (which would be e.g. 100 m long) must be maintained constant with a precision of several nanometers, a very tough task as distance is increased.&lt;/p&gt;&#xA;&lt;p&gt;Today's most advanced fixed optical telescope is the &lt;a href=&quot;https://www.eso.org/public/teles-instr/paranal-observatory/vlt/&quot; rel=&quot;nofollow noreferrer&quot;&gt;VLT&lt;/a&gt; made of 8 instruments (4 unit telescopes and 4 auxiliary telescopes) which can be coupled. From &lt;a href=&quot;https://www.eso.org/public/about-eso/faq/faq-vlt-paranal/#2&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESO site&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;When two or more telescopes are combined in interferometric mode, the&#xA;spatial resolution is determined by the maximum distance between them.&#xA;The VLTI, operating with two 8.2-metre Unit Telescopes, reaches a&#xA;spatial resolution equivalent to a single 130-metre giant telescope,&#xA;which is about 2 milliarcseconds. This is equivalent to distinguishing&#xA;two points separated by the size of a sesame seed on the International&#xA;Space Station as seen from the ground.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There is no better solution expected in the short term. Longer baseline interferometry is needed, but as we mentioned there are great challenges to overcome for maintaining the distance in the correlator.&lt;/p&gt;&#xA;&lt;p&gt;In 2020 we can resolve details down to a couple of mas. This means the 5 mas LM would appear as a 3x3 pixel image.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Will it be ever possible to look at the landing site from Earth?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;To get a 50x50 pixel image of the LM, we need to increase the resolution and reach 100 µas in the visible spectrum. This requires a technology equivalent to a telescope with a mirror diameter of at least 1km, likely an interferometer with a 1km baseline.&lt;/p&gt;&#xA;&lt;p&gt;Optical interferometers with kilometric baselines on the ground may be feasible,  but not in the short term, though some variants, like the &lt;a href=&quot;https://www.sciencedirect.com/science/article/pii/S0927650512001314&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cherenkov Telescope Array&lt;/a&gt;, are promising.&lt;/p&gt;&#xA;&lt;p&gt;One last possibility: Being able to &lt;a href=&quot;https://www.nature.com/articles/nphys1358&quot; rel=&quot;nofollow noreferrer&quot;&gt;undo diffraction&lt;/a&gt; as it seems possible to &lt;a href=&quot;https://physics.aps.org/articles/v10/24&quot; rel=&quot;nofollow noreferrer&quot;&gt;undo light scattering&lt;/a&gt;. In that case, no need to build large instruments.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Why is there a resolution limit in the first place?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;When light rays from a very remote light source are focused by an optical instrument after passing through an aperture (pupil), and the mirror/lens is far enough from the aperture (far field) contrary to what is expected by geometric optic approximation, light is not condensed into a single point but into a small spot with rings around. Most of the light (83.8%) is concentrated in the central spot. This pattern is due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Fraunhofer_diffraction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fraunhofer refraction&lt;/a&gt; by the edge of the aperture, and &lt;a href=&quot;https://opentextbc.ca/universityphysicsv3openstax/chapter/single-slit-diffraction/&quot; rel=&quot;nofollow noreferrer&quot;&gt;subsequent interference&lt;/a&gt; between the direct wave and the refracted wave:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lX3eZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lX3eZ.png&quot; alt=&quot;Refraction ring&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sup&gt;Refraction spot and rings due to Fraunhofer refraction&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;The angle of refraction can take any value between zero and a maximum depending on the wavelength and the aperture size: &lt;span class=&quot;math-container&quot;&gt;$$\sin \theta \lt 1.22 \frac {\lambda}{2a}$$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;So the greater the aperture, the smaller the cone where the diffracted rays can be found. The distribution of the light after diffraction is called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Airy_disk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Airy pattern&lt;/a&gt; after the name of the person who studied it. This is not a continuous spot because fringes are created due to the rays traveling along different distances (so the phase at the focal point is also different).&lt;/p&gt;&#xA;&lt;p&gt;A consequence is we can never, even with a perfect instrument, focus a light point into a single point, the central spot is the best we can have and we need to deal with it. Another consequence is if two light sources are observed within the same aperture, the Airy patterns will interfere and create artifacts. These pictures extracted from &lt;a href=&quot;https://www.moleculardevices.com/en/assets/tutorials-videos/dd/img/resolving-molecular-organization-and-dynamics-using-localization-based-super-resolution-microscopy#gref&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; show how:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2ZHsg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2ZHsg.png&quot; alt=&quot;Rayleigh criterion and angular resolution&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sup&gt;Rayleigh criterion and angular resolution&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;On the left there is a single light source, below is the corresponding intensity graph where the rings are visible.&lt;/p&gt;&#xA;&lt;p&gt;The middle image show a case with two light sources close enough, actually at an angular distance equal to the angular resolution of the optical instrument (maybe a telescope or a camera or an eye, it doesn't matter). Instead of having two superimposed images of spots and rings, some high and low intensity areas have appeared.&lt;/p&gt;&#xA;&lt;p&gt;Because the intensity curves actually represent light waves that are not in phase, some portions of the curves add, some subtract, this is the principle of interferences. The result is shown in the red dotted line.&lt;/p&gt;&#xA;&lt;p&gt;On the right the sources are moved closer than the resolution limit, interferences are more visible and completely change the picture. There is a bright area at the center, where intensities have added, and around darker areas where they are subtracted. These darker rings don't correspond to something real in the sources.&lt;/p&gt;&#xA;&lt;p&gt;These interferences now prevent to distinguish the two sources which are blurred into this visual artifact created by diffraction at the entrance of the instrument. We mostly see the central bright area.&lt;/p&gt;&#xA;&lt;p&gt;Interferences create the Airy rings, which in turn hide the details, but by increasing the aperture size, the ring area is reduced and more details are preserved.&lt;/p&gt;&#xA;&lt;p&gt;This is the very reason high resolution telescopes have a large diameter. This is true for any optical instrument and small binoculars or camera with small photographic lens (smartphones) can't have a high resolution whatever their magnification/zoom capability.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Practical determination of the resolution&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Angular resolution vs diameter of the mirror.&lt;br /&gt;&#xA;The larger the mirror, or the lens, the better the angular resolution. Similarly for a given optic diameter, the shorter the wave used for the sensor, the better the resolution. The best (theoretical) resolution that can be obtained from an instrument if given by the &lt;a href=&quot;http://cnx.org/contents/f403618a-c8e1-4b20-8737-6164df697aea@5/Limits_of_Resolution:_The_Rayl&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rayleigh limit&lt;/a&gt; formula which for a resolution in mas is:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\theta = 2.52 \cdot 10^5 \cdot {\lambda}/{D}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;where:&#xA;- θ is in arc-seconds&#xA;- λ is the wavelength considered&#xA;- D is the diameter of the mirror / lens&#xA;- λ and D are in the same unit&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;The minimum object size which can be resolved is:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$s = \tan ({\theta}/{3600}) \cdot d$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;where:&#xA;- s is the minimum object size    &#xA;- θ is the angular resolution in arc-seconds  &#xA;- d is the distance to the object  &#xA;- s and d are in the same unit&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Application for 5m-Hale telescope at 500 nm (green):&lt;/p&gt;&#xA;&lt;p&gt;Angular resolution&lt;br&gt;&#xA;θ = 2.52 x 10&lt;sup&gt;5&lt;/sup&gt; x λ/D&lt;br&gt;&#xA;θ = 2.52 x 10&lt;sup&gt;5&lt;/sup&gt; x 500&lt;sup&gt;-9&lt;/sup&gt; / 5&lt;br&gt;&#xA;θ = 25.2 mas&lt;/p&gt;&#xA;&lt;p&gt;Linear resolution at Moon distance&lt;br&gt;&#xA;s = tan (θ / 3600) x d&lt;br&gt;&#xA;s = tan (0.0252 / 3600) x 380,000,000&lt;br&gt;&#xA;s = 46.4 m&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2020-06-25T23:39:13.663" LastActivityDate="2020-06-25T23:39:13.663" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="9482" PostTypeId="1" AcceptedAnswerId="9512" CreationDate="2015-06-11T09:52:15.427" Score="24" ViewCount="19662" Body="&lt;p&gt;Apparently ISS is so close to Earth's atmosphere that it &lt;a href=&quot;https://space.stackexchange.com/questions/9087/how-often-does-iss-require-re-boosting-to-higher-orbit&quot;&gt;requires periodical boosts to stay in orbit&lt;/a&gt;. If the station were completely abandoned and it wouldn't get the boosts anymore, how long would it take until it crashed on Earth? Would it burn completely in the atmosphere or would falling debris pose any danger if it fell on populated areas?&lt;/p&gt;&#xA;" OwnerUserId="10431" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2018-12-19T13:10:45.420" Title="How long would ISS stay in orbit if it didn't get reboosts?" Tags="|iss|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9483" PostTypeId="1" AcceptedAnswerId="9486" CreationDate="2015-06-11T10:25:15.350" Score="21" ViewCount="4591" Body="&lt;p&gt;Neither the USSR nor Russia has ever sent a cosmonaut beyond low Earth orbit. In the 1960's it would've been very challenging for them to land a cosmonaut on the Moon, and their attempts were aborted. But didn't they have the ability to send a cosmonaut around the Moon before NASA? Would it have been technologically doable for the Russians to do an Apollo 8 kind of mission before NASA did it in 1968?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6816" LastEditDate="2015-06-12T19:43:50.747" LastActivityDate="2015-06-12T19:43:50.747" Title="Could the Soviets have rounded the Moon before Apollo? Why didn't they?" Tags="|apollo-program|history|crewed-spaceflight|russia|space-race|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9484" PostTypeId="2" ParentId="9482" CreationDate="2015-06-11T10:39:47.070" Score="11" Body="&lt;p&gt;According to &lt;a href=&quot;http://suzymchale.com/ruspace/issorbit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; article, the ISS has an orbital decay of &lt;strong&gt;100 metres every 24 hours&lt;/strong&gt;, which means around a kilometre every ten days. In it's stable orbit, there is a difference of about ten kilometres between the highest and lowest points in it's orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see, the decay is not hugely significant, and this is why boosts are usually performed months apart. However if boosts were to stop altogether, the ISS would fall lower faster as time went on, because the atmosphere would thicken as it fell. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the second part of your question, we already have information about station de-orbiting from the Mir station. Indeed, debris &lt;strong&gt;did survive&lt;/strong&gt; the reentry, and splashed down in the Pacific Ocean. This is why the eventual de-orbiting of the ISS will be planned and controlled so that the debris field can be far away from populated areas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Further Reading:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1753/does-the-orbit-of-the-iss-decay&quot;&gt;Does the orbit of the ISS decay?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/9087/how-often-does-iss-require-re-boosting-to-higher-orbit&quot;&gt;How often does ISS require re-boosting to higher orbit?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-06-11T10:39:47.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9486" PostTypeId="2" ParentId="9483" CreationDate="2015-06-11T11:04:29.307" Score="32" Body="&lt;p&gt;The Soviets had designed the &lt;strong&gt;Soyuz 7K-L1&lt;/strong&gt; as a part of the &lt;strong&gt;Zond&lt;/strong&gt; program, with the aim of a manned flyby of the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DcCl2.jpg&quot; alt=&quot;Soyuz 7K-L1&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Artists rendition of the Soyuz 7K-L1 en-route to the Moon (Courtesy &lt;a href=&quot;http://en.wikipedia.org/wiki/File:ZOND.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Developed in the late 1960's, after political disputes regarding the expulsion of &lt;a href=&quot;http://en.wikipedia.org/wiki/Nikita_Khrushchev&quot; rel=&quot;noreferrer&quot;&gt;Nikita Khrushchev&lt;/a&gt;, it was originally designed to be assembled in-orbit. The final design was however of a traditional manned capsule, to be launched by the new &lt;strong&gt;Proton 7K-L1&lt;/strong&gt; launcher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pCasa.jpg&quot; alt=&quot;Proton Launcher&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Proton 7K-L1, Soviet analogue to the Saturn V. (Courtesy &lt;a href=&quot;http://en.wikipedia.org/wiki/File:Proton-Zond2.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After a year of proper development in 1967, and several critical failures resulting in three deaths, two unmanned versions of the Soyuz made two successful flybys of the Moon under the mission names Zond 5 and Zond 6. A final, manned launch was attempted on January 20 1969, which resulted in a launch failure with no casualties, since the launch escape systems worked nominally. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By this time, the Soviets had already been beaten by the Americans, with Apollo 8's launch in December 1968. Finally in August 1969 the Soviets succeeded in sending Zond 7 and Zond 8 to the Moon (still unmanned), flying past it and returning using the then-uncommon skip reentry method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, that leg of the Space Race had already been lost, and the Zond program was defunded and cancelled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Further Reading:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/craft/soyz7kl1.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/craft/soyz7kl1.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1969-067A&quot; rel=&quot;noreferrer&quot;&gt;http://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1969-067A&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mentallandscape.com/C_CatalogMoon.htm&quot; rel=&quot;noreferrer&quot;&gt;http://mentallandscape.com/C_CatalogMoon.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/ap08fj/pdf/a08-missionreport.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://history.nasa.gov/ap08fj/pdf/a08-missionreport.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-11T11:09:49.220" LastActivityDate="2015-06-11T11:09:49.220" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9487" PostTypeId="1" AcceptedAnswerId="40084" CreationDate="2015-06-11T12:29:45.610" Score="18" ViewCount="4741" Body="&lt;p&gt;The Sun throws lots of high speed protons around. Especially during its sudden and short intensive &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_particle_event&quot;&gt;&quot;events&quot;&lt;/a&gt;. Much of that is blocked by Earth's magnetosphere and atmosphere. But astronauts work outside some of those natural shields. Could a Sun event actually have disabled and killed an Apollo crew during flight? Or did their spacecrafts have sufficient radiation protection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not interested in shortened lifetime or other statistical cancer incidents here. But instead of whether there was a real risk of acute radiation sickness disabling the crew and becoming a serious risk to the mission. If they'd had bad luck with the space weather.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2019-11-25T12:15:46.177" Title="Could an Apollo crew have been killed by Solar radiation?" Tags="|apollo-program|radiation|the-sun|shielding|" AnswerCount="6" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9488" PostTypeId="1" AcceptedAnswerId="9490" CreationDate="2015-06-11T16:10:12.033" Score="6" ViewCount="8271" Body="&lt;p&gt;There seems to be a consistent &lt;em&gt;mist&lt;/em&gt; effect around the Saturn V when launched. It looks like rapidly evaporating water.&lt;br&gt;&#xA;What is the material?&lt;br&gt;&#xA;What is the purpose?&lt;br&gt;&#xA;Did it come from the rocket itself, or from something at launch (water from the ground or air?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mMjfQ.jpg&quot; alt=&quot;Saturn V launches&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; All Saturn V launches, 1967–1973 (Courtesy &lt;a href=&quot;http://en.wikipedia.org/wiki/Saturn_V#/media/File:Saturn_V_launches.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: At the linked question ( &lt;a href=&quot;https://space.stackexchange.com/questions/1649/what-is-the-vapour-smoke-that-comes-from-the-rocket-boosters&quot;&gt;What is the vapour/smoke that comes from the rocket boosters?&lt;/a&gt;) it is said to be water vapor from cryogenic boosters.  However a comment by &lt;a href=&quot;https://space.stackexchange.com/users/3127/phil-perry&quot;&gt;Phil Perry&lt;/a&gt; also says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the Saturn V, if memory serves, the first stage LOX tank was uninsulated, and accumulated a large load of ice, which started shedding when the engines fired up. The second and third stages' LH2 and LOX tanks were insulated, and experienced little external ice buildup.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the case of the images, was this from ice buildup or LOX pressure bleedoff?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-07-21T15:46:11.440" Title="What is the mist on Saturn V during launch?" Tags="|launch|apollo-program|saturn-v|" AnswerCount="2" CommentCount="2" ClosedDate="2015-06-11T18:25:11.673" ContentLicense="CC BY-SA 3.0" />
  <row Id="9489" PostTypeId="2" ParentId="9488" CreationDate="2015-06-11T16:13:24.093" Score="3" Body="&lt;p&gt;That mist started out as water vapor in the air. The Saturn V used liquid oxygen and (in the upper stages) liquid hydrogen, both of which are really cold. So the water vapor would condense and freeze onto the outside of the oxygen and hydrogen tanks. During launch, the vibration from the engines would break the ice and it would fall off the vehicle.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-06-11T16:13:24.093" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9490" PostTypeId="2" ParentId="9488" CreationDate="2015-06-11T16:13:52.707" Score="1" Body="&lt;p&gt;Ice, mostly water ice on cold LH2 (liquid hydrogen) and LOX (liquid oxygen) tank surfaces with temperatures being low enough to condense, liquefy and freeze solid atmospheric water vapor on contact. Both these propellants are cryogenic to increase their storage density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the main stage ignites during liftoff, they're mostly shaken off the launcher due to the vibration of the sonic shock (basically just like we intentionally trigger avalanches with a loud noise, usually with explosives, but this time main engine ignition expands surrounding air so fast it produces a &lt;em&gt;sonic boom&lt;/em&gt;, emphasized as it reflects off the ground and blast deflectors), producing that &lt;em&gt;mist&lt;/em&gt; that you observe. And some still clings onto the launcher and boils-off during ascent due to aerodynamic heating, with most of it usually gone by the time the launch vehicle reaches maximum dynamic pressure (max q), but you can still observe smaller pieces falling off during staging from the interstage space where ice can form due to trapped bled-off propellants (tho some launch vehicles use an open interstage / aft skirt to help purge those bled-off propellants during ascent and prevent their rapid expansion due to aerodynamic heating, e.g. Soyuz).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the Saturn V first stage used RP-1 (essentially a rocket grade kerosene) as fuel and LOX as oxidizer, and other stages used LH2 and LOX. How much and where the launch vehicle is covered by this condensation ice depends on where on the launch vehicle these tanks are placed, temperature at which cryogenic propellants are stored (LH2 is usually much colder than LOX), tank insulation, ullage and interstage volume, even how much time the launch vehicle sat fueled on the launchpad prior to launch, paint, and of course weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Part of the &lt;em&gt;mist&lt;/em&gt; could also be due to purging of bled off propellants with gaseous nitrogen just prior to ignition, as can be seen in &lt;a href=&quot;https://www.youtube.com/watch?v=lwovkumqIsw&quot; rel=&quot;nofollow&quot;&gt;this slow motion video of Saturn V launch&lt;/a&gt;, but most of it would be condensation ice falling off cryogenic surfaces (also seen in the video after main engine ignition):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/lwovkumqIsw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-21T15:46:11.440" LastActivityDate="2015-07-21T15:46:11.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9491" PostTypeId="2" ParentId="9487" CreationDate="2015-06-11T16:59:10.290" Score="3" Body="&lt;p&gt;CME or Coronal Mass Ejections were a huge question, still are and NASA was very concerned about them as well as our first trip out side the Van Allen belt which protect the earth from CME. We still are concerned about CME and actually sent the crew of the ISS to safe areas in the last year because SOHO picked up on a huge one. &lt;/p&gt;&#xA;" OwnerUserId="10456" LastActivityDate="2015-06-11T16:59:10.290" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9492" PostTypeId="2" ParentId="9479" CreationDate="2015-06-11T17:38:06.303" Score="-1" Body="&lt;p&gt;Earth like life ... lots of luck. the proteins and water seem to come from the cold outer areas of a solar system. if in the goldilocks zone and given enough time I would submit that the same thing that happened here would repeat itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;remember that there was once many zillions of life forms on this planet. with many different mechanisms of biology. then it came to pass that one evolved and created this poisonous gas, killing off most of the life. that gas, oxygen produced by algae, separated the life forms into two basic types aerobic and anaerobic. some of the latter persist to this day. viruses were part of that old pool of life too. &lt;/p&gt;&#xA;" OwnerUserId="10456" LastActivityDate="2015-06-11T17:38:06.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9494" PostTypeId="2" ParentId="9487" CreationDate="2015-06-11T18:28:35.233" Score="6" Body="&lt;p&gt;Part of the plan was to time the missions for a time period when the sun was not expected to be active. Secondary plan was to point the Service Module, getting as much mass between the astronauts and the Sun as possible. But likely would have died. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion is being designed to have an area where they can hang out, in such a case, where the mass of the vehicle best protects them.  (That is, not an area that will protect them, but best protects them. As best can be done.  Best is to avoid it entirely).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-11T18:28:35.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9495" PostTypeId="1" CreationDate="2015-06-11T18:53:07.480" Score="11" ViewCount="745" Body="&lt;p&gt;I know that there is medication to prevent the menstrual cycle, but what side-effects does this entail? &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Does regularly supressing the menstrual cycle pose any health risks to female astronauts? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If the female astronaut's period was allowed to continue, how would zero-Gee conditions effect it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a wider question, is it necessary for a woman to menustrate at all (a question I have asked myself &lt;em&gt;many&lt;/em&gt; times!)?&lt;/p&gt;&#xA;" OwnerUserId="10459" LastEditorUserId="1028" LastEditDate="2015-06-11T18:56:48.250" LastActivityDate="2015-06-11T21:37:15.617" Title="What do female astronauts do on their menstrual cycle?" Tags="|medical|astronaut-lifestyle|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9496" PostTypeId="1" CreationDate="2015-06-11T18:59:53.150" Score="1" ViewCount="172" Body="&lt;p&gt;I am just curious, in theory about what the potential impacts would be to science if we were to have a space station that was outside of Low Earth Orbit. Of course, it would take longer to get to and would possibly have a harder time maneuvering around debris (and let's not even discuss budget issues and not having a shuttle to carry and assemble parts for a new space station). But I was also curious if the lower gravity and or increased distance from Earth would open any scientific experimental options that we have not looked into so far. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just think that it would be good to start adventuring into different spaces to master new techniques. I think we have LEO down pat and it might be good to venture outside of our comfort zone. &lt;/p&gt;&#xA;" OwnerUserId="8561" LastActivityDate="2015-06-11T19:44:54.890" Title="Has the U.S. looked at the option of having a space station outside of LEO? If so, why or why not?" Tags="|low-earth-orbit|altitude|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9497" PostTypeId="2" ParentId="9422" CreationDate="2015-06-11T19:12:55.033" Score="3" Body="&lt;p&gt;The typical space environment will consist of electron, proton and photon radiation.  Each of these three particle types interact differently with different materials.  Because of this, a spacecraft shield should consist of different materials in a layer-cake fashion to best shield against all particles.  The water will due a pretty good job of stopping protons, because of the similarity in mass between the protons and the hydrogen atoms in the water.  However, water does not do a very good job at stopping high energy photons (x-rays and gamma rays), nor does it do a good job stopping electrons.  The electrons will bremsstrahl, creating a build up flux of photons, and the solar photons will undergo Compton scattering, which in turn causes a build up flux of electrons that will also bremsstrahl into lower energy photons.  The water is great for protons, but you will probably want another shield layer or two of some material with a higher electron density to stop the photons and electrons.&lt;/p&gt;&#xA;" OwnerUserId="5856" LastActivityDate="2015-06-11T19:12:55.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9498" PostTypeId="2" ParentId="9496" CreationDate="2015-06-11T19:44:54.890" Score="4" Body="&lt;p&gt;Initial obvious locations are various Lagrange points or lunar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many people have looked, thought about it, but nothing has been seriously funded for reasons of logistics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until recently, access to orbit was very expensive, and large scale orbital construction even more so. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle, the primary launch vehicle for the United States for the last 30 years was limited to LEO. So anything you wanted outside LEO needed an upper stage and you lost the use of the Shuttle's unique facilities. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is not a show stopper, but when all you have is a Shuttle, most things look like nails that a Shuttle would make a great hammer to hit it with. If you were to try to build the ISS with today's stable of launchers it would probably look very different. (Much more like Mir probably). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;New launch entrants, are dropping the cost of access to space already.  (SpaceX has disrupted the market so much, that ULA and Ariane have both announced plans for partial re-usability (Vulcan and Adeline respectively) which they had been saying for years would not be cost effective. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;No more Space Shuttle, so that is no longer an option nor a limit.  What can be done now?  Well any payload you want to send to L1 or lunar orbit, you need an upper stage that can propel it. This goes for manned vehicles as well. So everything will cost more than LEO (say at the ISS), and we have already seen how prohibitively expensive, business as usual at the ISS can be. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now everything you wish to send as resupply, crew, etc will be more expensive, since you have to launch the upper stage to get there and back, as well as the actual payload.  But say the Falcon Heavy is fully reusable, and as cheap as SpaceX seems to imply, that may become less of an issue.  Launch crew on a Falcon Heavy, instead of a regular Falcon 9. If it is fully reusable, assume it might also be affordable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS in LEO is protected by the Van Allen belts, something that would be lacking at L1 or Lunar orbit, and thus more mass would be required and of course, more cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right now, a Soyuz is always on hand, with a seat for everyone on the ISS available. If there is a medical emergency, and air leak, whatever, they can be back on Earth in a matter of hours.  At L1 or lunar orbit, it could be potentially 3 days to get back. This is not a show stopper, but something that needs to be considered (and solved for Martian missions of 2 years duration). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next issue is why?  What value does a station at L1 add that the ISS does not already provide? In reality the ISS's mission is tenous enough at best. The cost would be high, the value unclear, so not a lot of desire for such a station at this time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is a quick summary of possible issues, there are more, but they are all solvable.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-11T19:44:54.890" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9499" PostTypeId="2" ParentId="9495" CreationDate="2015-06-11T21:20:07.323" Score="7" Body="&lt;p&gt;Women astronauts &lt;a href=&quot;http://elitedaily.com/news/world/nasa-engineers-asked-sally-ride-if-100-tampons-were-enough-for-a-one-week-mission/641385/&quot; rel=&quot;noreferrer&quot;&gt;have plenty of tampons available&lt;/a&gt; (presumably pads as well). As with other zero-G hygiene issues, I would guess that patience and &lt;a href=&quot;http://airandspace.si.edu/collections/artifact.cfm?object=nasm_A19850477007&quot; rel=&quot;noreferrer&quot;&gt;wet wipes&lt;/a&gt; are indispensable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hormonal contraceptives can (somewhat unreliably) suppress menstruation entirely -- a woman who normally uses typical oral contraceptives could probably just skip her placebo week to delay her period on a short space mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unclear what the long term health effects of use of e.g. &lt;a href=&quot;http://en.wikipedia.org/wiki/Depot_medroxyprogesterone_acetate&quot; rel=&quot;noreferrer&quot;&gt;Depo-Provera&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/Norplant&quot; rel=&quot;noreferrer&quot;&gt;Norplant&lt;/a&gt; are; Depo usually stops menstruation for three months or longer at a stretch.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-06-11T21:37:15.617" LastActivityDate="2015-06-11T21:37:15.617" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9500" PostTypeId="1" AcceptedAnswerId="9508" CreationDate="2015-06-12T01:21:55.383" Score="10" ViewCount="619" Body="&lt;p&gt;I've been reading about the Hipparcos Satellite recently and it seems it was the first big telescope with precision-measuring devices to accurately pinpoint a star's location in the sky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know Hipparcos was launched in 1989, and Hubble didn't come till 1990 (and its optics weren't corrected till 1993), but I thought Hubble was planned a lot earlier and could do such things as star mapping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I wrong? Can't Hubble accurately measure star positions? And it seems to me that Hubble's large mirror could do it better than Hipparcos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only things I can think of:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Maybe Hubble was too busy booked by other scientists wanting to study other things like the planets, or searching for asteroids. Maybe they auction time on it and other interests have been willing to pay more?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Maybe Hipparcos was designed differently to see stars better. I know Hubble can only view the visible and slightly ultraviolet light, and maybe some near infrared too. But that seems enough to view the stars and map their location. Did Hipparcos need to view other parts of the spectrum for some reason?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;EDIT: I'm asking this question not because I want to sour-grapes NASA or ESA or anything. I'm trying to understand why different telescopes are apparently needed for similar things. Just looking at Hipparcos compared to Hubble, they look very different. But I could have sworn Hubble is capable of doing the same kind of star mapping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT 2: My understanding is that Hubble has Control Moment Wheels so it can very finely adjust and maintain its attitude (direction). This, plus Hubble's apparently much bigger light gathering power, seem to imply that it can do the same job as Hipparcos.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2015-06-12T06:51:39.117" LastActivityDate="2015-06-12T10:19:02.463" Title="Was Hipparcos Satellite really necessary? Can't Hubble do everything that Hipparcos did?" Tags="|artificial-satellite|astronomy|hubble|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9503" PostTypeId="2" ParentId="9500" CreationDate="2015-06-12T06:08:07.920" Score="12" Body="&lt;p&gt;The Hipparcos satellite was solely devoted to &lt;strong&gt;astrometry&lt;/strong&gt;. It did not take 'regular' images, it used an eccentric Schmidt telescope to overlay images onto a grid. It finally resulted in a catalogue of stars, not an archive of images. &lt;strong&gt;It's purpose was to detect and find the position of stars in the sky.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/05vdo.gif&quot; alt=&quot;Hipparcos&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; An 'image' taken by Hipparcos. As you can see, it's more of a measurement than an image. (Courtesy &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:Hipparcos-grid.gif&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt; &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hubble space telescope was exactly what it says on the box; a telescope. It consisted of five scientific cameras and a Ritchey–Chrétien telescope. &lt;strong&gt;It's function is to take advanced images of distant worlds in incredible detail.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/G4JBt.jpg&quot; alt=&quot;Hubble&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Image of the Butterfly Nebula, taken by Hubble. (Courtesy &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:NGC_6302_Hubble_2009.full.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, their functions were completely different. Hipparcos was far more accurate when it came to astrometry, and Hubble could not 'replace' it in that aspect.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Update&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Whilst it is true that even Hubble can perform astrometric measurements, &lt;em&gt;this is not it's main function.&lt;/em&gt; Hipparcos was &lt;em&gt;specifically designed&lt;/em&gt; to measure the sky, and Hubble was &lt;em&gt;specifically designed&lt;/em&gt; to image it. Hubble's FGS system was installed primarily to aim it with high precision, not to perform astrometry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As such, Hubble's astrometry has a median accuracy of $±0.0003 arcseconds$, whilst Hipparcos has a median accuracy of $±0.0001 arcseconds$. &lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="1028" LastEditDate="2015-06-12T07:41:53.327" LastActivityDate="2015-06-12T07:41:53.327" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9504" PostTypeId="1" AcceptedAnswerId="9505" CreationDate="2015-06-12T07:58:31.597" Score="29" ViewCount="6515" Body="&lt;p&gt;For example: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How should a spacecraft pass the Moon to reach escape velocity from  Earth?&lt;/li&gt;&#xA;&lt;li&gt;How should it pass Venus for a slingshot towards Jupiter orbit?&lt;/li&gt;&#xA;&lt;li&gt;Conversely, to descend Venus to Mercury?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there some intuitive way to explain what angle a probe needs to enter and leave a body's gravity well to perform a gravity assist to speed-up or slow-down?&lt;/strong&gt; &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="4660" LastEditDate="2015-06-12T13:58:51.757" LastActivityDate="2017-02-06T12:37:42.587" Title="How can I intuitively understand gravity assists?" Tags="|orbital-mechanics|orbital-maneuver|gravity-assist|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9505" PostTypeId="2" ParentId="9504" CreationDate="2015-06-12T08:39:58.377" Score="40" Body="&lt;h2&gt;Understanding the Principle&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Let's start by understanding the mechanism of a &lt;strong&gt;gravity assist&lt;/strong&gt;. As a spacecraft approaches a planetary body, it gets affected by the planets gravitational pull. Getting nearer, the pull increases, and eventually when the spacecraft passes the planet, the pull decreases. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think about a stationary planet as an absolute frame of reference, picture throwing a body near it. The body will curve towards it, accelerating, then pass it, and then carry on, &lt;em&gt;eventually retaining its original speed&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However as you know, planets are not stationary but rather, they orbit around the Sun at speeds to the order of tens of kilometres per second. &lt;strong&gt;Hence any spacecraft affected by the planets gravity, is affected by its orbital speed too.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's use a simple analogy.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Imagine a girl on a merry-go-round. She is sitting on the edge, with&#xA;  her hand stretched out, and the the merry-go-round is rotating at a&#xA;  certain speed counter-clockwise. You throw a ball at her hand. She&#xA;  catches it for but a second, and then lets go. The ball is observed to&#xA;  &lt;em&gt;accelerate&lt;/em&gt; during this process.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In this analogy, the girl was a planet, the merry-go-round was her orbit, and her hand was the gravitational pull of that 'planet'. This is &lt;strong&gt;highly simplified&lt;/strong&gt; explanation that serves only to explain the directions involved. &lt;strong&gt;If you throw a ball from behind her as she moves away from you, the ball will accelerate.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's very easy to now imagine the inverse of this analogy; &lt;em&gt;what if you throw the ball to the girl head on&lt;/em&gt;, as she comes closer to you. You can intuitively tell that the ball will &lt;strong&gt;decelerate&lt;/strong&gt; during this process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many other analogies, like a ping-pong ball hitting a ceiling fan, or a skateboarder grabbing a car on a round-about. The spacecraft is basically &lt;strong&gt;stealing the angular momentum of the planet.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you have these basics down, the rest of gravitational mechanics are easy to comprehend. The following diagram will show you the change in direction observed in an &lt;em&gt;accelerating&lt;/em&gt; assist.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/cgRcv.png&quot; alt=&quot;Gravitational Slingshot&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; A more complex representation of what a gravity assist looks like when the planet is approached from &lt;em&gt;behind&lt;/em&gt;. (&lt;a href=&quot;https://commons.wikimedia.org&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;On a (slightly) Larger Scale&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;For a real-world example, take &lt;strong&gt;Cassini-Huygens&lt;/strong&gt; (my favorite spacecraft). Cassini's mission plan looked like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lGUBB.png&quot; alt=&quot;Cassini Mission&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Orbital diagram of the path Cassini took to eventually reach Saturn (&lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/thumb/b/bd/Cassini_interplanet_trajectory.svg/640px-Cassini_interplanet_trajectory.svg.png&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cassini made a total of &lt;em&gt;four flybys&lt;/em&gt;, all to &lt;strong&gt;gain speed&lt;/strong&gt;. From what we've learnt earlier, acceleration occurs when we approach a planet from &lt;em&gt;behind&lt;/em&gt;. Take a closer look at the diagram above and you can see that all the assists approached their respective planets from &lt;em&gt;behind them in their orbits&lt;/em&gt;. This is the fundamental principle of gravity assists, observed in the real world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/LHefk.png&quot; alt=&quot;Cassini Assists&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Cassini's speed plotted over time, with the gravity assists labelled. Using multiple assists shaved off as much as 5 km/s ∆V for the mission, when compared to a simple Hohmann transfer. (&lt;a href=&quot;http://upload.wikimedia.org/wikipedia/commons/8/80/Cassini%27s_speed_related_to_Sun.png&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I hope this clarifies the concept of a gravitational assist for you. The first two examples you gave -- Moon to escape Earth, Venus to reach Jupiter -- an &lt;strong&gt;accelerating&lt;/strong&gt; assist is required and you should approach the planet &lt;strong&gt;from behind&lt;/strong&gt;. For the last example -- from Venus down to Mercury -- you need to &lt;strong&gt;decelerate&lt;/strong&gt; the spacecraft, and Venus should be approached &lt;strong&gt;head on&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Update&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;A couple of diagrams to show you in the clearest way possible, how gravity assists work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KU5ov.gif&quot; alt=&quot;Assist&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; The simplest form of an &lt;strong&gt;accelerating assist&lt;/strong&gt;. (Courtesy &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html&quot; rel=&quot;noreferrer&quot;&gt;Planetary Society&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eHT3i.gif&quot; alt=&quot;Assist 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Some of the possible forms of a gravity assist. (Courtesy &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html&quot; rel=&quot;noreferrer&quot;&gt;Planetary Society&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Further Reading: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www2.jpl.nasa.gov/basics/grav/primer.php&quot; rel=&quot;noreferrer&quot;&gt;http://www2.jpl.nasa.gov/basics/grav/primer.php&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/missiongravityassistprimer/&quot; rel=&quot;noreferrer&quot;&gt;http://saturn.jpl.nasa.gov/mission/missiongravityassistprimer/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="415" LastEditDate="2017-02-06T12:37:42.587" LastActivityDate="2017-02-06T12:37:42.587" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9506" PostTypeId="2" ParentId="9504" CreationDate="2015-06-12T08:48:48.777" Score="5" Body="&lt;p&gt;An analogy I use to explain gravity assist to a true layman is a skateboarder and buses (the buses are old style with steel bodywork). The skateboarder has a powerful a magnet and an engine with very little fuel. The skateboarder can change direction by using her engine and modest amount of fuel, or she can use magnetic attraction on the buses going on the different routes around the area. The skateboarder can use the magnet to attract the board to the a bus that's going in a direction she wants to get pulled in. If she wants to speed up she gets behind a bus that's going the right direction and gets dragged up to the bus' speed. As she gets closer to the bus the magnetic forces get more powerful, so she steers a course that will keep her close enough to get a boost but far enough away that she doesn't get pulled into the bus. At the closest point of approach to the bus she fires her engine a little bit and propels herself away from the bus in the direction she wants to go. She's now faster than the bus.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even with a small amount of fuel the skateboarder can get around pretty well. She can change direction or slow down as long as there are buses going the right way at the right time, that magnet will drag her towards any bus that she gets near. If there's no bus going her way though she's in for a long, slow ride though. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2015-06-12T09:11:12.503" LastActivityDate="2015-06-12T09:11:12.503" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9507" PostTypeId="1" CreationDate="2015-06-12T10:09:20.783" Score="3" ViewCount="407" Body="&lt;p&gt;For the coming CRS-7 mission to the ISS, where exactly will SpaceX position the ASDS for the Falcon 9R first stage recovery attempt?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously it will be in the Atlantic Ocean.&lt;/p&gt;&#xA;" OwnerUserId="10465" LastEditorUserId="51" LastEditDate="2015-06-12T13:51:44.613" LastActivityDate="2015-06-15T23:37:59.250" Title="ASDS Position during Falcon 9R first stage recovery" Tags="|spacex|landing|falcon-9|asds|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9508" PostTypeId="2" ParentId="9500" CreationDate="2015-06-12T10:19:02.463" Score="12" Body="&lt;p&gt;As @Vedant Chandra says, &lt;a href=&quot;https://en.wikipedia.org/wiki/Hipparcos&quot;&gt;Hipparcos&lt;/a&gt; was dedicated to astrometry. To do this properly, you need to make sure your measurements are accurate. The pointing accuracy of your satellite is a nice specification, but a system that does not rely on this can achieve more accurate measurements.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To do this, Hipparcos looks in two directions at once. The telescope combines two images from sky regions 58 degrees apart, and measures star positions relative to each other. This eliminates the satellite itself as a source of errors.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble was made to be pointed at a tiny region of sky for a long time, to observe faint objects. Hipparcos, on the other hand, spins continuously and measures stars as they move across its field of view. It also looks at a much larger area of the sky than Hubble.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, yes, Hipparcos was necessary and its measurements could not have been made by Hubble. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-06-12T10:19:02.463" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9509" PostTypeId="2" ParentId="9504" CreationDate="2015-06-12T11:23:25.957" Score="11" Body="&lt;p&gt;First - to answer your questions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How should a spacecraft pass the Moon to reach escape velocity from&#xA;  Earth?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Well, for the most part, by the time you've reached the moon, you're likely past escape velocity from earth.   The moon only moves at about 1 KM/Second, which isn't all that fast.  Spacecraft usually travel in a couple to a few 10s of KMs per second.  If the spacecraft is moving slower than that, it could use the moon as a gravity assist, but the moon isn't great in most cases.   The 2 factors of a gravity assist are how fast the object's orbital speed is and how strong the objects gravity is.   The orbital speed is what the spacecraft borrows and the planets gravity is what causes the spacecraft's angle to change.   You need both for the assist to work.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How should it pass Venus for a slingshot towards Jupiter orbit?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is a bit trickier, but basically, the spacecraft has to approach Venus from behind it's orbit and as it flies around Venus from behind (but close), it picks up speed (and Venus, correspondingly slows down, but much less cause it's far more massive - the combined orbital momentum is conserved, but like the ball bouncing off the train, the train speeds up the ball (a lot) and the ball slows down the train (a little).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Conversely, to descend Venus to Mercury?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This one, the ship flies ahead of Venus, and it slows down as it passes Venus and Venus speeds up (a little), but be careful with this one. If you slow down too much you'll fly into the sun, so this kind of slow down around Venus - as I understand it, needed to be done more gradually.   That's why Messenger did 2 slow-downs around Venus and 2 around Mercury before it settled into orbit around the tiny-hot planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, rather than think about it, play with the simulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/Slingshot.htm&quot;&gt;http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/Slingshot.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) hit reset&#xA;2) Set the velocity and angle of approach.&#xA;3) hit start (to start Jupiter moving)&#xA;4) hit shoot - to shoot the space-ship.&#xA;and don't have your speakers at full volume - in case you crash.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Watch the space ship's velocity as it goes around Jupiter.   In a nutshell with this simulation, if the ship flies past Jupiter from behind it gains velocity, if it flies past Jupiter from ahead, it loses velocity.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also like this one (which is in the link that Vedant Chandra provided), but the picture is helpful.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/HqXCh.jpg&quot; alt=&quot;http://www2.jpl.nasa.gov/basics/grav/cartoon.jpg&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the angle - bouncing off the train from perfect 90 degrees (from straight ahead) is the biggest gain, as the angle the ball hits the train  the velocity gain decreases because that's how vector addition works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source (also in Vedant's link):  &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/grav/primer.php&quot;&gt;http://www2.jpl.nasa.gov/basics/grav/primer.php&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also found this one - the website isn't in English but it might help a little, but not with angles.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jHqSM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and this explanation might be worth a look:   &lt;a href=&quot;http://www.schoolphysics.co.uk/age14-16/Astronomy/text/Slingshot_/index.html&quot;&gt;http://www.schoolphysics.co.uk/age14-16/Astronomy/text/Slingshot_/index.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and if all else fails, try this one:   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zmZiE.jpg&quot; alt=&quot;http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/SISEE3a.jpg&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just kidding.   :-)&lt;/p&gt;&#xA;" OwnerUserId="8711" LastActivityDate="2015-06-12T11:23:25.957" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9510" PostTypeId="2" ParentId="9507" CreationDate="2015-06-12T13:51:18.417" Score="1" Body="&lt;p&gt;The ASDS JRTI (Based on Marmac 304 hull, so not the original ASDS which was based on the Marmac 300 hull) will be downrange of the launch, appropriate to catch the returning first stage booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can follow the track of the ISS for Jun 26th, and work backwards from there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX will issue a Notice To Mariners that they need to stay clear of the area where they will attempt landing.  That will provide sufficient details to narrow it down to a couple of square kilometers.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact location is probably unknowable to us, at this time. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-12T13:51:18.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9511" PostTypeId="1" AcceptedAnswerId="9694" CreationDate="2015-06-12T14:24:11.710" Score="12" ViewCount="1280" Body="&lt;p&gt;I was browsing through Wikipedia when I came across &lt;a href=&quot;https://en.wikipedia.org/wiki/File:F-1_rocket_engine.jpg&quot; rel=&quot;noreferrer&quot;&gt;this infographic&lt;/a&gt; on the Rocketdyne F-1. It gives the engine dry weight as 18,500 pounds, and the engine &lt;em&gt;burnout&lt;/em&gt; weight as 20,180 pounds, meaning the engine somehow gains 1,680 pounds in-flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VysKY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone confirm what causes the weight discrepancy? 1,680 pounds of soot stuck to the engine bell seems excessive.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="51" LastEditDate="2015-06-12T14:25:47.627" LastActivityDate="2015-06-29T04:42:34.313" Title="Did the Apollo F-1 engines gain weight during flight?" Tags="|launch|apollo-program|engine-design|saturn-v|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9512" PostTypeId="2" ParentId="9482" CreationDate="2015-06-12T18:34:40.077" Score="28" Body="&lt;p&gt;Looking at this recent data, which conveniently has a long period with no substantial reboosts, we see about a 3 km/month drop at around 410 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qQYuT.png&quot; alt=&quot;ISS decay&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using a standard atmosphere model, I backed out a ballistic coefficient of $165\,\mathrm{kg/m^2}$.  Running the current altitude to decay, I get that we would have a deep sea space station in about 15 months.  I think they recently raised the average altitude.  It used to more like 350 km.  From there, the lifetime would be more like six months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a plot of the decay:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UIJ1z.png&quot; alt=&quot;altitude vs. time&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However &lt;em&gt;your mileage may vary&lt;/em&gt;.  This assumes that ISS maintains its roughly edge-on attitude which gives that high ballistic coefficient.  If ISS is in a different attitude, or if it's tumbling, its ballistic coefficient could be much lower, and so it would come down much faster.  Also, the atmosphere is never &quot;standard&quot;.  We could have solar activity that heats and blooms the upper atmosphere, increasing the density and again bringing down the station much faster.  Furthermore, the atmosphere in December and January was probably not &quot;standard&quot;, so my backed-out ballistic coefficient is only approximate.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-06-17T04:14:16.350" LastActivityDate="2015-06-17T04:14:16.350" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9513" PostTypeId="1" AcceptedAnswerId="9532" CreationDate="2015-06-12T22:48:14.080" Score="4" ViewCount="456" Body="&lt;p&gt;Thanks to answers to my question about &lt;a href=&quot;https://space.stackexchange.com/q/9504/4660&quot;&gt;gravity assists&lt;/a&gt;, I now pretty much understand how they work on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic#Plane_of_the_Solar_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;ecliptic&lt;/a&gt;, or near such. For that plane, the &quot;null net speed change&quot; path goes on a crash course right into the middle of the planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what if we add the third dimension - about orbits of the sun that are 90&lt;sup&gt;o&lt;/sup&gt; to the plane of the ecliptic or highly inclined? Which close fly-by would affect the orbital plane and direction, but not the speed? Polar? (wouldn't an approach from behind and passing the planet through the front serve as a fully-featured speed increase?)&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-06-16T23:51:56.120" Title="What near flyby wouldn't modify the speed of the vessel, just direction?" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9514" PostTypeId="1" CreationDate="2015-06-13T01:28:25.720" Score="6" ViewCount="1959" Body="&lt;p&gt;I recently observed someone being worried about small space debris (e.g. discarded gloves (?), paint chips) hitting them on the ground.  (For these cases, the worry is of course unwarranted, since the object would disintegrate into tiny pieces, and eventually dust or gas, leaving no solid impactor).  The object must be large, and if not designed to survive, larger still.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It then occurred to me, &lt;strong&gt;what would be the smallest possible object to survive a re-entry, &lt;em&gt;alone?&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, you get &lt;em&gt;one&lt;/em&gt; object in a circular LEO (eventually; it will re-enter due to weak air resistance).  At least part of it must survive the re-entry in a macroscopic form traveling at dangerous speed.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How small (volume) can you make this object?&lt;/li&gt;&#xA;&lt;li&gt;How small (mass) can you make this object?&lt;/li&gt;&#xA;&lt;li&gt;In both cases, what is it made of?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;EDIT: &lt;a href=&quot;http://www.space.com/16518-space-junk.html&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; puts the limits much lower than I would have thought:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Bits and pieces of trash constantly &lt;a href=&quot;http://www.space.com/13053-falling-nasa-satellite-space-junk-orbital-debris-interview.html&quot; rel=&quot;nofollow&quot;&gt;fall from the sky&lt;/a&gt;, but nearly everything larger than 4 inches (10 cm) survives in some form, likely in smaller fragments.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To salvage this question, let's focus on the danger aspect.  The smaller objects reported are probably harmless dust specks by the time they reach the ground, or maybe pebble-sized nodules.  So, how big (by mass, by volume) would an object have to &lt;em&gt;begin&lt;/em&gt; to injure someone on the ground (culminating in, say, maybe a golf-ball-sized object falling at terminal velocity)?&lt;/p&gt;&#xA;" OwnerUserId="2707" LastEditorUserId="25" LastEditDate="2015-07-21T20:41:13.470" LastActivityDate="2015-07-21T21:14:56.573" Title="What is the smallest object that would survive a reentry from LEO? What would it be made of?" Tags="|low-earth-orbit|reentry|debris|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9515" PostTypeId="1" AcceptedAnswerId="9517" CreationDate="2015-06-13T01:34:38.170" Score="5" ViewCount="346" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Life#Extraterrestrial_life&quot; rel=&quot;nofollow noreferrer&quot;&gt;In this section of a wiki article&lt;/a&gt; it talks about scientist finding amino acids in outer space, or at least complex molecules and such.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also &lt;a href=&quot;https://www.youtube.com/watch?v=ppK1Ilii7NU&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; says how there is alcohol in Sagitarius B2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How are these detected? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since I know we haven't sent any probles there, the only logical answer is spectroscopy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what's happening is they are looking through a telescope, have the computer do some analysis, then they see this&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rgCAT.jpg&quot; alt=&quot;spectro 1&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or this&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nTPYz.jpg&quot; alt=&quot;spectro 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and they say &quot;hey, it's an amino acid, for sure!!&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that really all it takes and are these results that came from millions of kilometers away really accurate, unique, free of distortions, and reliable enough to say &quot;it's an amino acid, for sure!&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd have to imagine there might be interference and inaccuracies. I mean detecting the structure of something so tiny as a molecule from such large distances...? What's the magic they use or is it really just an educated guess and we're only like 53.7% sure that's what it is?&lt;/p&gt;&#xA;" OwnerUserId="3481" LastActivityDate="2015-06-13T11:41:33.947" Title="How do scientists detect amino acids in space?" Tags="|technology|research|science|scientific-data|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9517" PostTypeId="2" ParentId="9515" CreationDate="2015-06-13T11:41:33.947" Score="2" Body="&lt;p&gt;The Wikipedia article you link to links to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Spectral_signature&quot; rel=&quot;nofollow&quot;&gt;Spectral signature&lt;/a&gt; page.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Spectral signatures are the specific combination of emitted, reflected or absorbed electromagnetic radiation (EM) at varying wavelengths which can uniquely identify an object. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As with any science, I'd expect that such reports are at least at the 95% &lt;a href=&quot;https://en.wikipedia.org/wiki/Confidence_interval&quot; rel=&quot;nofollow&quot;&gt;confidence level&lt;/a&gt;, or at the 99% level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For conclusions regarding interstellar molecules, the observers need to spectroscopically identify the radiation the samples receive, then determine what molecules produce the signature that is detected. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As already mentioned in the quote, some of the radiation could be absorbed (filtered out), some could be reflected (toward us), and after absorption some could be radiated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To reach a high confidence level, a sample of the gas (in the lab) would have to be irradiated with the detected stellar radiation spectrum and the resultant spectrum compared to the spectrum measured via the telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lab-derived result would have to be adjusted in the red or blue directions account for possible velocities in the remote gas cloud.&lt;/p&gt;&#xA;" OwnerDisplayName="user8406" LastActivityDate="2015-06-13T11:41:33.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9518" PostTypeId="2" ParentId="9513" CreationDate="2015-06-13T17:50:23.770" Score="6" Body="&lt;p&gt;Suppose we do a flyby of a planet. Then the spacecraft has constant energy in the reference frame of the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the reference frame of the planet, let the spacecraft's initial and final velocity be $\mathbf{v}_i$ and $\mathbf{v}_f$. In the reference frame of the sun, call the initial and final velocities $\mathbf{v}_i'$ and $\mathbf{v}_f'$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kinematically, we see that&#xA;$$\mathbf{v}_i' = \mathbf{v}_i + \mathbf{v}_p$$&#xA;$$\mathbf{v}_f' = \mathbf{v}_f + \mathbf{v}_p$$&#xA;where $\mathbf{v}_p$ is the velocity of the planet in the Sun's frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know that $v_i = v_f$ and you desire $v_i' = v_f'$, where non-bolded quantities denote magnitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using&#xA;$$\mathbf{v}_i' \cdot \mathbf{v}_i' = \mathbf{v}_i \cdot \mathbf{v}_i + 2\mathbf{v}_i\cdot \mathbf{v}_p + \mathbf{v}_p \cdot \mathbf{v}_p$$&#xA;and likewise for $\mathbf{v}_f'$, we obtain&#xA;$$v_i'^2 = v_f'^2 - 2(\mathbf{v}_f - \mathbf{v}_i)\cdot\mathbf{v}_p$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus you are asking for orbits where the change in velocity, from the planet's reference frame, is perpendicular to the planet's velocity from the Sun's reference frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft's orbit around the planet is a hyperbola in the planet's frame. The spacecraft's change in velocity is along the symmetry axis of the hyperbola. The spacecraft can take any orbit whose symmetry axis is perpendicular to the planet's motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An approach at 90 degrees to the ecliptic in the sun's reference frame is not quite right. In that sort of orbit, the spacecraft, as seen from the planet, has motion in the ecliptic of $-\mathbf{v}_p$, and thus its change in velocity will have some component pointing along the direction of motion of the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a three-dimensional space of allowed orbits for the spacecraft. The requirement that the symmetry axis be perpendicular to the planet's motion removes only a single dimension from the space of all approaches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note, also, that your original assertion that all orbits along the ecliptic change the spacecraft's speed is incorrect.&lt;/p&gt;&#xA;" OwnerUserId="10473" LastActivityDate="2015-06-13T17:50:23.770" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9519" PostTypeId="1" CreationDate="2015-06-14T01:42:06.560" Score="8" ViewCount="823" Body="&lt;p&gt;From a similar post: &lt;a href=&quot;https://space.stackexchange.com/questions/19/how-much-of-a-problem-is-space-junk-and-how-can-we-clean-it-up#comment25904_19&quot;&gt;How much of a problem is space junk, and how can we clean it up?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I'm wondering why space junk seems to be ever-present. Most of it seems to be in low Earth orbit in the first place, and most of it seems to be very small fragments of metal, paint, or other stuff. So it seems to me that its orbit would decay very quickly (a few days) because it's so light-'weight'.&lt;/p&gt;&#xA;&lt;p&gt;Are we really depositing junk in orbit at such a high rate? Are we really that dirty? Can final stage separation really blow off that much debris into orbit? And correct me if I'm wrong, but we are not launching orbital payloads at a rate of one every three days... or are we? So I don't see what's replenishing this stream of space junk.&lt;/p&gt;&#xA;&lt;p&gt;Note: I know about those two satellites blown up by USA and China, along with that collision between Iridium and some Russian sputnik. Hopefully, these kind of of things are exceptions rather than the primary 'supply' of space junk.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="47149" LastEditDate="2022-05-25T11:11:58.517" LastActivityDate="2022-05-25T11:11:58.517" Title="Why is space junk so persistent in LEO?" Tags="|orbital-mechanics|spacecraft|low-earth-orbit|debris|orbital-decay|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="9521" PostTypeId="1" AcceptedAnswerId="9522" CreationDate="2015-06-14T04:11:07.460" Score="20" ViewCount="4569" Body="&lt;p&gt;What influenced the end-of-life planning for the Apollo lunar lander ascent stage after it ascended and the crew returned to the command module?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was looking at Wikipedia for Apollo Lunar Module. It says,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Having completed its job, the LM was separated and sent into solar orbit or to crash into the Moon&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Most of them were crashed. Apollo 11 and 16 ended up in lunar orbit (both decayed) and Apollo 10 ended up in solar orbit (I wonder if anyone tracks it, if it is even possible?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the Apollo missions were so meticulously laboured over, but the Wikipedia page makes it seems like a reasonably arbitrary point. What was their decision-making process? &lt;/p&gt;&#xA;" OwnerUserId="10477" LastEditorUserId="10477" LastEditDate="2015-06-14T22:14:42.407" LastActivityDate="2019-06-27T09:13:52.647" Title="Why choose to crash the Apollo lunar lander ascent stage after it ascended?" Tags="|apollo-program|debris|crewed-spaceflight|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9522" PostTypeId="2" ParentId="9521" CreationDate="2015-06-14T06:36:46.023" Score="37" Body="&lt;p&gt;First off, &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects&quot; rel=&quot;noreferrer&quot;&gt;lunar orbits tend to be unstable&lt;/a&gt; due to mascons (&lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)&quot; rel=&quot;noreferrer&quot;&gt;mass concentrations&lt;/a&gt;, which are places of higher density produced by impacts with the moon that cause it to have a non-uniform gravity field).  That means that anything not placed in very specific orbits which weren't discovered until 2001 (frozen orbits as referenced by the first link above) will shift over time, possibly resulting in impact with the lunar surface.&lt;/p&gt;&#xA;&lt;p&gt;Because the reason they're decoupling from it before burning to return to Earth is so they don't have to spend the fuel to bring back the no longer needed ascent module they really don't have many options on what to do with it.  They have some fuel remaining (because everything was planned with some margin for error), but the options boil down to:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Use remaining fuel to intentionally impact the surface.&lt;/li&gt;&#xA;&lt;li&gt;Use remaining fuel to maybe get it out of lunar orbit (depending on amount of fuel left).&lt;/li&gt;&#xA;&lt;li&gt;Not use the remaining fuel, which eventually results in orbital decay due to pertubation effects.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;As for specific flights:&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_10#Hardware_disposition&quot; rel=&quot;noreferrer&quot;&gt;Apollo 10&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;After being jettisoned, Snoopy's ascent stage engine was fired to fuel depletion, sending it on a trajectory past the Moon into a heliocentric orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I have no real source saying so, but &amp;quot;fired to fuel depletion&amp;quot; sounds like an engineering test to me, especially with the further comment that they tracked it until 1969 meaning they could have calculated how much delta-V they got out of the engine/fuel. For what it's worth, &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/000269.html&quot; rel=&quot;noreferrer&quot;&gt;this forum thread&lt;/a&gt; agrees it was &amp;quot;for data purposes&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;The wikipedia link also mentions that&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In 2011, a group of amateur astronomers in the UK started a project to search for it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Unfortunately solar orbit is an awfully big place and the ascent module is small and no longer transmitting.  I admittedly didn't search for long but I didn't find any suggestion that it's been found/identified.&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11#Lunar_ascent_and_return&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;p&gt;This one was left in lunar orbit as you noted and was estimated to have &amp;quot;decayed within months&amp;quot;.  Even the &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_11a_Summary.htm&quot; rel=&quot;noreferrer&quot;&gt;mission summary&lt;/a&gt; just says that the &amp;quot;ascent stage would remain in lunar orbit for an indefinite period&amp;quot;.  Without the geophones/seismometers planted on the other landing missions there was no specific need to create an impact event so it seems it was simply abandoned because there was no useful science to be produced by further maneuvering.&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package&quot; rel=&quot;noreferrer&quot;&gt;ALSEP on Apollo 12, 14, 15, 16, 17&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;p&gt;These flights left seismometers on the surface for the purpose of &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_seismology&quot; rel=&quot;noreferrer&quot;&gt;lunar seismology&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Part of the deployed package was the &amp;quot;Active Seismic Experiment&amp;quot;, or ASE, which was had a mortar to launch explosive charges for the purpose of creating shockwaves that would be picked up by geophones to analyze the subsurface structure of the moon.  Crashing things into the moon once the geophones were in place was for science to measure how the shock waves from the impact propagate through the moon.  As noted &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_12#Return&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; for Apollo 12:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The seismometers the astronauts had left on the lunar surface registered the vibrations for more than an hour.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;Apollo 13&lt;/h2&gt;&#xA;&lt;p&gt;As mentioned in comments and from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package&quot; rel=&quot;noreferrer&quot;&gt;ALSEP page&lt;/a&gt;, despite the mission abort and focus on saving the crew the &lt;a href=&quot;https://en.wikipedia.org/wiki/S-IVB&quot; rel=&quot;noreferrer&quot;&gt;S-IVB&lt;/a&gt; stage from Apollo 13 &amp;quot;was deliberately crashed on the Moon to provide a signal for the Apollo 12 PSE&amp;quot; (Passive Seismic Experiment).&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://history.nasa.gov/afj/ap16fj/a16summary.html&quot; rel=&quot;noreferrer&quot;&gt;Apollo 16&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;p&gt;In the &amp;quot;And Return&amp;quot; section:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This may have been partly responsible for the likely failure to leave one of the switches in Orion in the been correct position when the crew carried out their final checks. Consequently, when Orion was jettisoned from Casper, it began tumbling and did not fire its RCS thrusters in preparation for the engine burn to remove it from orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It sounds like they intended to use it as an impactor but were unable to.  It did eventually decay nearly a year later but that wasn't the original plan.&lt;/p&gt;&#xA;&lt;p&gt;I'd guess they intended to use it as an impactor but were unable to.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-06-27T09:13:52.647" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="9523" PostTypeId="2" ParentId="9519" CreationDate="2015-06-14T10:26:52.137" Score="6" Body="&lt;p&gt;The way I see it, the problem is two-fold:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, regulations aim to limit the amount of time debris / satellites after the end of their useful lifetimes spend in orbit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(e) The orbital lifetime of objects passing through LEO (lower than 2,000 km) shall be shorter than 25 years after the end of operation. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;(Source: ISO 24113 Debris mitigation requirements and compliance)&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why exactly Inter-Agency Space Debris Coordination Committee (IADC) wants to keep the satellites in orbit after the end of the mission (after all, you would think that it made more sense to take out the trash when the bin gets full, and not after 25 years?) is outlined a bit better by &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Engineering_Technology/Clean_Space/ESA_invites_ideas_to_cut_space_debris_creation&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A growing number of countries have introduced regulations to limit the production of fresh debris within the protected low orbits, typically intending satellites to be brought down or boosted out of here within 25 years of the end of their working lives while reducing the risk to people on the ground to less than 1 in 10 000.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;They also need to be ‘passivated’, removing leftover propellant and powering down batteries to avoid explosions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the debris is &lt;strong&gt;purposefully kept (for the moment; still deorbited within the planned 25 years)&lt;/strong&gt; in orbit to try and decommission the satellite to reduce the chance of damage to humans. Also, if you read between the lines, this is done so that there would not be a hail of exploding metal on Earth, which would severely affect both the reputation and funding of space agencies. (An example of this would be to look at how strong opinions the population has towards nuclear power, whilst stating that they don't know it good enough; some might describe nuclear power as &quot;a gift from gods, stomped on out of fear&quot;.)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The second issue with prolonged presence of debris is the almost complete lack of opposing forces. In the frame of reference of debris, practically the only constant opposing force is the drag provided by Earth's atmosphere. The atmospherical drag affects everything within about 500 kilometres (about 310 miles or about 1.6 million feet) from the Earth's surface. &lt;a href=&quot;http://gps.mae.cornell.edu/AIAA-2010-8258_hinks&amp;amp;psiaki_asc2010.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;A document by Cornell University&lt;/a&gt; gives the drag mathematically:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For many satellites in low earth orbit (LEO), the largest dynamic model uncertainty stems from atmospheric drag. Acceleration due to atmospheric drag $\boldsymbol{a}_D$ is related to atmospheric density $\rho$ by the equation:&#xA;  $$&#xA;\boldsymbol{a}_D = −\frac{1}{2} \left( C_D \frac{A_v \left( t \right) }{m_s} \right) \rho v_r^2 \boldsymbol{e}_v&#xA;$$&#xA;  where $C_D$ is a drag coefficient, $A_v \left( t \right)$ is the cross-sectional area of the satellite in the direction of travel, $m_s$ is the total spacecraft mass, $v_r$ is the velocity magnitude relative to the ambient atmosphere, and $\boldsymbol{e}_v$ is a unit vector in the relative velocity direction.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From the above formula, we can see that indeed a lower mass object would have a greater atmospherical drag like you suggested. What also plays a major role here is the cross-sectional area $A_v \left( t \right)$ of the debris. If you think about trying to throwing a beachball and a small rock of about equal masses, you know that you can throw the small rock way faster than the beachball, and this is due to the different cross-sectional area $A_v \left( t \right)$ of the two objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, let's consider entire satellites vs smaller pieces of debris: the smaller piece is probably not hollow, but just a chunk of metal or some composite, where as the satellite will be relatively empty and hollow from the inside. In this case, &lt;strong&gt;the ratio between the cross-sectional area $A_v \left( t \right)$ and the total spacecraft mass $m_s$&lt;/strong&gt; would be greater for the satellite than it would be for the smaller piece of debris, i.e. &lt;em&gt;the satellite would have a greater drag and deorbit sooner than the smaller piece of debris&lt;/em&gt; (assuming the constants and coefficients would stay the same for the two). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This brings into question why did USA and China try blowing up old satellites, creating tens of thousands of new, smaller, longer lasting pieces of debris, when it does not make sense physically?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Long story short, the rate is, sadly, not that high. There are efforts to combat it (see the ESA link above), but so far it is looking pretty horrible on short-term. And to top that, anything above the 500km threshold effectively feels no drag due to the atmosphere. For example, satellites in the geosynchronous orbit (GEO, around 36,000 km / 22,000 mi out) have to be positioned into a graveyard orbit at the end of their lifetime. From Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A graveyard orbit, also called a junk orbit or disposal orbit, is a supersynchronous orbit that lies significantly above synchronous orbit, where spacecraft are intentionally placed at the end of their operational life. It is a measure performed in order to lower the probability of collisions with operational spacecraft and of the generation of additional space debris.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To answer some of your shorter questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;This picture from Wikipedia gives some idea how dirty we are: &lt;img src=&quot;https://i.stack.imgur.com/rjQnH.jpg&quot; alt=&quot;Space debris seen from outside geosynchronous orbit (GEO). The two main debris fields are the ring of objects in GEO and the cloud of objects in low Earth orbit (LEO).&quot;&gt;&lt;/li&gt;&#xA;&lt;li&gt;Final stage separation is usually designed not to blow off a lot of debris into space, and agencies / launch companies are increasingly trying to bring earlier stages back to Earth (intact or not). Otherwise you may have a piece or two of debris that already has a downwards trajectory on stage separation (Newton's Third Law of Motion: the &quot;extra push&quot; on stage separation affects the earlier stage equally, so while the payload is pushed further away from Earth, the earlier stage is pushed towards Earth.)&lt;/li&gt;&#xA;&lt;li&gt;There are about 70-90 launches per year these days, so no, we are not launching orbital payloads at a rate of 1 every 3 days... Merely at a rate of about 1 every 4 days. ;) This does not equate to the &lt;strong&gt;number&lt;/strong&gt; of satellites deployed however: for example, recent developments of CubeSats (tiny (10 cm by 10 cm by 10 cm), cheap satellites) have led to numerous smaller satellites being launched at once along with bigger payload. In late 2013, USAF launched a Minotaur I rocket loaded with its main payload, the Air Force's Space Test Program Satellite-3, and along with it 28 CubeSats, so a single launch deployed an impressive 29 satellites.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So, after all this mumbling, the primary source of space junk really is us. And not really us launching more and more stuff into space, but our lack of progress in effectively removing our past, present, and future orbiters from space after their missions have ended. I hope this helped.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(An obstacle I did not mention is political: governments do not really want to fund missions to remove the space junk. Politicians are much more interested in missions that can &quot;be seen&quot; and credited to their time in government, whereas removing debris would be quite opposite of this (nobody ever thinks highly of the garbageman, despite their infrastructurally important job).)&lt;/p&gt;&#xA;" OwnerUserId="8880" LastEditorUserId="8880" LastEditDate="2015-06-14T19:17:12.920" LastActivityDate="2015-06-14T19:17:12.920" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9524" PostTypeId="1" AcceptedAnswerId="9720" CreationDate="2015-06-14T13:26:46.927" Score="6" ViewCount="258" Body="&lt;p&gt;Now that &lt;a href=&quot;http://blogs.esa.int/rosetta/2015/06/14/rosettas-lander-philae-wakes-up-from-hibernation/&quot; rel=&quot;nofollow&quot;&gt;Philae has woken&lt;/a&gt;, is more known about its exact location?  In other words: does &lt;em&gt;Philae&lt;/em&gt; know where Philae is?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Navigation on Churi sounds challenging.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="4660" LastEditDate="2015-06-14T14:36:40.660" LastActivityDate="2015-06-30T13:29:29.383" Title="Does Philae's waking enable scientists to determine its location?" Tags="|rosetta|navigation|philae|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9525" PostTypeId="2" ParentId="9524" CreationDate="2015-06-14T18:50:15.557" Score="4" Body="&lt;p&gt;As of June 11th, it is still unknown according to &lt;a href=&quot;http://blogs.esa.int/rosetta/2015/06/11/the-quest-to-find-philae-2/&quot; rel=&quot;nofollow&quot;&gt;this ESA blog post&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rosetta and Philae teams continue to search for the current location of the lander, piecing together clues from its unexpected flight over the surface of Comet 67P/Churyumov-Gerasimenko after its initial landing on 12 November.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, I understand that it is looking likelier that Philae will be found now that it is transmitting back data again. At least the fact that its solar panels are generating power means that Philae is in the sun (periodically) and thus casts a shadow of itself, which should make it stand out more clearly in the surface pictures taken by Rosetta.&lt;/p&gt;&#xA;" OwnerUserId="8880" LastActivityDate="2015-06-14T18:50:15.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9527" PostTypeId="1" AcceptedAnswerId="9529" CreationDate="2015-06-15T12:59:32.213" Score="34" ViewCount="6719" Body="&lt;p&gt;I know that capsules typically require heat shields to survive reentry from orbit. I'm wondering how an object's size, density and aerodynamic profile affects this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more specificity:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The feather would be the tail feather of an adult pigeon&lt;/li&gt;&#xA;&lt;li&gt;The orbit would be that of the ISS at +-7.6 km/s&lt;/li&gt;&#xA;&lt;li&gt;By &quot;drop&quot;, I mean throw downwards toward earth at 5 m/s so that it de-orbits in a reasonable timeframe&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8157" LastActivityDate="2015-06-16T05:07:30.063" Title="If I drop a feather from orbit, would it burn up or &quot;hit&quot; the ground?" Tags="|reentry|physics|" AnswerCount="1" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="9528" PostTypeId="2" ParentId="9524" CreationDate="2015-06-15T13:27:17.417" Score="2" Body="&lt;p&gt;Doppler shifting between Rosetta and Philae's transmitters could in theory be used for mutual tracking... however Rosetta's orbital velocity is very low, so I think the answer is a firm &quot;No&quot;, unless perhaps Rosetta can be risked making a few more low passes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only real chance I think is to get some pictures back from Philae, of its surroundings, and try to eliminate all possible locations by comparison with Rosetta's pictures.  A few possible locations are described in this post:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://blogs.esa.int/rosetta/2015/06/11/the-quest-to-find-philae-2/&quot; rel=&quot;nofollow&quot;&gt;http://blogs.esa.int/rosetta/2015/06/11/the-quest-to-find-philae-2/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-06-15T13:27:17.417" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9529" PostTypeId="2" ParentId="9527" CreationDate="2015-06-15T13:54:45.443" Score="44" Body="&lt;p&gt;Throwing it down at 5 m/s will do basically nothing.  That will simply cause it to advance in its orbit a bit.  To deorbit, you need to throw it backwards, not down.  However in this case, since the feather has a such a low ballistic coefficient, it will promptly deorbit from ISS altitude on its own, without you having to do anything at all.  Just wait a bit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given a 10 cm feather with a mass of 0.05 g, I looked at two cases intended to bound the possibilities.  The first case is that the feather trims to a face-on attitude, with the lowest possible ballistic coefficient.  It reenters from the ISS altitude in less than three hours.  The maximum deceleration is about 10 G's at 100 km altitude.  My heating estimate is highly suspect (I am applying a blunt body formula with a nose radius based on feather feature sizes), but my rough estimate is $30\,\mathrm{W/cm^2}$, which is rather a lot for organic material.  The feather would not look like a feather anymore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However it is not clear how it would maintain that attitude.  More likely would be a trim with the heavier part of the root of the feather forward.  For that I used about 1/40th of the previous $C_D A$, measured from a Vulture feather.  (Albeit at completely the wrong Reynolds numbers, but hey, this is just for fun.)  Then I get that it decays from the ISS orbit in less than four days, with a maximum deceleration of 8 G's at about 80 km altitude.  The heating is much worse, at $200\,\mathrm{W/cm^2}$.  That is a typical Mars entry heat rate.  The feather would be gone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anything, I suspect that my choice of approach and parameters underestimates the heating.  So my conclusion is that, alas, the feather will burn up.  And I had such high hopes for the feather.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-06-16T05:07:30.063" LastActivityDate="2015-06-16T05:07:30.063" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9530" PostTypeId="1" AcceptedAnswerId="9531" CreationDate="2015-06-15T14:49:07.067" Score="10" ViewCount="1049" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Teledesic&quot; rel=&quot;noreferrer&quot;&gt;Teledesic&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1994&lt;/li&gt;&#xA;&lt;li&gt;backed by Microsoft&lt;/li&gt;&#xA;&lt;li&gt;Inclination: 98.2 degrees (sun-synchronous)&lt;/li&gt;&#xA;&lt;li&gt;Altitude: 700 km&lt;/li&gt;&#xA;&lt;li&gt;840 satellites&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/OneWeb&quot; rel=&quot;noreferrer&quot;&gt;OneWeb&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2014&lt;/li&gt;&#xA;&lt;li&gt;backed (?) by Google&lt;/li&gt;&#xA;&lt;li&gt;Altitudes: 800 or 950 km&lt;/li&gt;&#xA;&lt;li&gt;650 satellites&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is a mission design question, with concomitant economic issues popping up. Barring large gov't subsidies (e.g. for installing Uncle Sam's payloads), how can this breathtaking new venture succeed where the others failed (or, in the case of Iridium, had to resort to a DoD bailout)? What are the technological and market differences that make this scheme more feasible today?&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-06-18T07:04:30.613" LastActivityDate="2018-01-10T17:36:26.697" Title="How can OneWeb constellation succeed where Teledesic failed?" Tags="|artificial-satellite|mission-design|communication-satellite|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9531" PostTypeId="2" ParentId="9530" CreationDate="2015-06-15T15:23:43.687" Score="6" Body="&lt;p&gt;First off it is not clear this will succeed, it is early days still.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your lining up issues was well done, since it highlights a number of important points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect most of the issues are economic and not so much technical.  There are technical issues of course, but if the money is not there, it can't happen.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Launch Costs&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Launch costs are way down, by virtue of new entrants in the market dropping costs.  (SpaceX, but they are forcing their competitors to drop prices, new designs (ULA Vulcan, Ariane ADELINE, etc). The need to launch 600+ satellites also helps feed the launch market, and in fact last time around, the collapse of Teledesic knocked out some marginal launch providers who were counting on the launch rate to be profitable. With SpaceX planning Starlink with 4400 satellites, and possibly a VLEO 7000+ constellation, prices MUST go down. SpaceX is pushing their BFR for all launches, and that is designed for severly low launch costs.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Financial Backing&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Microsoft at the time was one of the larger companies in the world, and very profitable. Today, Google dwarfs Microsoft of those days. Google has immense cash reserves and if they are willing to back this through thick and thin, have the money to do it.  Microsoft in theory did back them, but the cost was a much larger proportion of their income compared to Google.  (I don't have numbers, but Google has cash reserves I had heard on the order of 200 Billion.  Microsoft never had that much.)&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Core Market&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The Internet in 1994 vs 2014 is very different. Google requires Internet to operate. Microsoft still sold software with or without the Internet access.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The belief at Google seems to be, mobile is the next market to win, and there are billions without access.  If Google can provide that access, it gives them a leg up on getting those billions as new customers. (Be that in Android and hardware on the device side.  Or Search, Voice, Mail, everything else to drive ad revenues on the software side). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google has considered long duration solar powered drones, or balloons as relay stations for Internet access. Every aspect of their core business requires connected people. They want to connect everyone, and sometimes it seems like they take a 'no matter the cost' perspective.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Other Competitors&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;For fun, lets also consider the SpaceX constellation, now named Starlink. On the one hand, SpaceX do not have the financial backing Google can provide OneWeb (Even the $1 billion reported to be committed is a pittance on a constellation this size). On the other hand, SpaceX has the launch capabilities, and needs the launch rate to increase to get costs down for other customers, so is incentivized to get a constellation launched. Also, they own all excess capability on launches, and can resell it or use it for their own satellites. Thus the cost of starting Starlink for SpaceX is severely reduced. (Musk is reported to want to launch 2 test satellites to test design early (Scheduled for a Jan 2018 launch as a secondary payload on a paying customer.)  But SpaceX's core market (Nor Tesla, nor Solar City, the trifecta of Musk's interests) is NOT Internet related per se, so it will be interesting to see if they can succeed. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-01-10T17:36:26.697" LastActivityDate="2018-01-10T17:36:26.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9532" PostTypeId="2" ParentId="9513" CreationDate="2015-06-15T15:43:35.607" Score="3" Body="&lt;p&gt;SF had asked how to preserve speed while changing direction. I'll try to answer that question. But I'll also talk about changing direction without changing speed. My goal is to give a feel for hyperbolas and swing bys.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Preserving direction wrt sun but changing speed&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For my examples I'll use earth as the body doing the gravity assist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From earth's point of view, the space ship's path is a hyperbola. Incoming Vinfinity and outgoing Vinfinity have the same speed but different directions. I attempt to illustrate the notion of Vinf on page 36 of &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1936037106&quot; rel=&quot;nofollow noreferrer&quot;&gt;my orbital mechanics coloring book&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/yKozF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since incoming and outgoing Vinfinity are the same speed, the velocity vectors are two sides of an isosceles triangle. Delta V is the base of the triangle. Top angle δ is the hyperbola's turning angle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RNJVe.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From earth's point of view, direction is changed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it's possible to preserve direction from the sun's point of view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's path with to the sun is nearly horizontal, a flight path of 0º. It's speed is about 30 km/s. Let's imagine a space ship that has flight path 9º and speed 28 km/s when it crosses earth path:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xoJyh.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, incoming Vinf wrt earth is about 3 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To preserve direction, delta V vector must be parallel or anti-parallel to the space ship's vector:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rzMcU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this to occur, the incoming Vinf and outgoing Vinf need to form symmetrical angles. The center angle is the hyperbola's turning angle, δ. So the angle between spaceship's vector and the incoming Vinf vector must be 90º-δ/2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this example the space ship would maintain direction with regard to the sun but it's speed would be boosted from 28 km/s to 29.6 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's change the scenario to a space ship in a circular 1 A.U. polar orbit. Speed is 30 km/s, flight path is zero but inclination is 90 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ssgUG.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case incoming Vinf wrt earth would be 42 km/s. δ would be 90º. Delta V would be 60 km/s. This would flip the space ship to a 270º inclination orbit and the space ship's speed would remain 30 km/s but in the opposite direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This sort of gravity assist isn't possible unless we drastically reduced earth's diameter. Turning angle increases as hyperbola's perigee gets closer to earth. Closest perigee we can get without lithobraking is an altitude of 0 kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With 0 km altitude perigee and a vinf of 42 km/s, turning angle is only 4º.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Preserving speed wrt sun but changing direction&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the ship's new velocity vector has the same speed as the old, the vectors are sides of an isosceles triangle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YXP0V.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The delta V vector is the base of this isosceles triangle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to preserve speed, the angle Vinf forms with earth's velocity vector needs to be half of the hyperbola's turning angle.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-06-16T23:51:56.120" LastActivityDate="2015-06-16T23:51:56.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9534" PostTypeId="2" ParentId="9507" CreationDate="2015-06-15T23:37:59.250" Score="6" Body="&lt;p&gt;SpaceX has not yet published the specific locations for landing for CRS-7, however we can make a reasonable guess by looking at the previous landing attempt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the Hazard Map of the previous landing attempt&#xA;&lt;img src=&quot;https://i.stack.imgur.com/cglVa.png&quot; alt=&quot;Hazard Map&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It outlines the Keep Out Areas SpaceX requests for each launch. The orange area is for early termination safety. The red area is the expected splashdown zone of the first stage. The ASDS is positioned towards the close end of this region because the Falcon 9 performs a boostback manuever to get closer to shore. The far right end of the region contains the splashpoint for the first stage on a ballistic trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ASDS is approximately 225 miles from Cape Canaveral. The exact position of the barge for CRS-7 will be available once the FCC Application has been filed for that mission&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;br&gt;&#xA;/u/Darga69 on Reddit for making the maps&lt;br&gt;&#xA;&lt;a href=&quot;https://apps.fcc.gov/oetcf/els/reports/STA_Print.cfm?mode=initial&amp;amp;application_seq=64682&amp;amp;RequestTimeout=1000&quot; rel=&quot;nofollow noreferrer&quot;&gt;FCC Application for Barge Landing&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastActivityDate="2015-06-15T23:37:59.250" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9535" PostTypeId="1" CreationDate="2015-06-16T02:37:57.273" Score="6" ViewCount="1402" Body="&lt;p&gt;A lot of the costs of launching space probes to other moons and planets must be the staggering research and engineering costs to design the vehicle as well as the costs of planning a mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, there's often limited windows of time required as the planets all need to be aligned in a certain way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So with that in mind, why only launch 1 or 2 probes at a time? Why not launch 100 (obviously, just a figure off the top of my head) all at once? Obviously, the cost of launching 100 space probes will be more than 1, but surely, it's not going to be 100x more expensive. You've already paid for the fixed costs - you just have to pay the variable costs like the fuel and parts for each ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Introducing redundancy also minimises risks of the mission failing. These missions often require some pinpoint accuracy and a failure at any stage may end the mission.&lt;/p&gt;&#xA;" OwnerUserId="10504" LastEditorUserId="4660" LastEditDate="2015-06-16T20:17:03.940" LastActivityDate="2015-08-07T20:04:25.293" Title="Why aren't space probes launched in batches?" Tags="|launch|probe|cost|" AnswerCount="6" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9537" PostTypeId="2" ParentId="9535" CreationDate="2015-06-16T07:28:50.703" Score="3" Body="&lt;p&gt;According to rocket equation, you will need fuel in proportion to your payload mass and exponentially in relation with mass exhaust velocity. Indeed, maintenance costs will be less in batch launches, however, you will have to use more fuel which has its own consequences on launch preparation. In addition, today's launchers are not designed to be chained in arbitrary configurations, i.e, you cannot stack five upper stages over thirty core stages.&lt;/p&gt;&#xA;" OwnerUserId="6834" LastEditorUserId="6834" LastEditDate="2015-06-16T10:01:14.020" LastActivityDate="2015-06-16T10:01:14.020" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9538" PostTypeId="2" ParentId="9535" CreationDate="2015-06-16T07:38:19.760" Score="5" Body="&lt;p&gt;A batch of probes all going to the same destination wouldn't be economic at the moment.  Each probe would need its own propulsion, communications (unless there's one &quot;parent&quot; with a long range antenna), guidance and other electronics.  This would add weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also for general observations, a single probe can just take all of the measurements/pictures and send them back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However some groups have been looking at a swarm of micro probes.  There might be a use for flying them to their destination on one lander then releasing them on the surface for exploration, for example.  Another example that's been given is for multiple entry probes to examine Jupiter's atmosphere...&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-06-16T07:38:19.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9539" PostTypeId="2" ParentId="9535" CreationDate="2015-06-16T12:17:32.997" Score="11" Body="&lt;p&gt;The cost savings from mass production are not linear. You will see savings in some areas, but none in others. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Space probes, and even commercial satellites (which are standardized to an extent) are hand-built. So if you build 2 (or 20) the labor cost rises more or less linearly. Only when you build much larger numbers (thousands), it becomes interesting to invest in production tooling to exchange for lower labor cost. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The same goes for the components you'll use. 100 probes isn't close to the scale you need to profit from mass production. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Capability does not rise linearly with size. If you have a choice between 1 large probe and 100 small probes, the large probe can contain much more advanced science instruments than 100 small ones. Outer planet missions need an RTG, and these are rare enough that building 100 of them isn't feasible at the moment. A small probe won't be able to transmit directly from e.g. Mars to Earth, it doesn't have the transmitter power budget.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There are cases where you'd want to have multiple measurements at different locations. Conveniently, an orbiting spacecraft will visit every location on the planet eventually (depending on the chosen orbit). NASA is &lt;a href=&quot;http://www.space.com/29306-cubesats-deep-space-exploration.html&quot;&gt;planning several missions&lt;/a&gt; that will contain a few cubesats in addition to the main spacecraft, to do measurements in several places &lt;em&gt;at once&lt;/em&gt;. The main craft will act as a relay. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Deep space launches are hard. For outer planet missions, the current probes are near the launch weight limit of the rockets we have. So sending 100 Voyagers would have required 100 launches. Sending 100 spacecraft in 1 launch would have meant building cubesats - something that's only recently become feasible at all. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I guess my thinking is that if a company spent all their resources designing an innovative new product, they're not just going to want to create one. They're going to want to create many - because each unit they produce will lower the overall costs per unit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For now, deep space probes are generally designed and built by space agencies and/or universities, not commercial companies. They don't care about mass production.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-06-16T12:17:32.997" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9542" PostTypeId="2" ParentId="9535" CreationDate="2015-06-16T18:01:06.443" Score="4" Body="&lt;p&gt;Besides the arguments about the efficiencies, probes are specially-designed objects with lots of discussion about the particular payloads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a serial-launch scenario, you get to incorporate your understanding of the object from early probes into the design of the later probes.  With a mass-launch scenario, you have to decide on all the payloads with your current knowledge of the target.  It would be like playing &quot;20 questions&quot;, but you have to submit all your questions at the beginning of the game rather than develop them over time.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2015-06-16T18:01:06.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9543" PostTypeId="1" CreationDate="2015-06-17T02:58:22.683" Score="4" ViewCount="1571" Body="&lt;p&gt;How much thrust is necessary to launch a regulation size FIFA soccer ball into orbit around the Earth? Let us assume a size 5 which weighs between 420g and 450g. Remember, we will need thrust to ensure the ball will continue until the minimum altitude and orbital velocity have been attained otherwise it will fall back to the Earth. Minimum altitude is rarely desirable, therefore thrust must continue to be generated to gain additional orbital altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, would the ball explode from the air pressure?&lt;/p&gt;&#xA;" OwnerUserId="10513" LastEditorUserId="14137" LastEditDate="2016-04-17T09:29:58.193" LastActivityDate="2016-04-17T16:08:36.453" Title="How much thrust to launch a regulation size FIFA soccerball into orbit" Tags="|escape-velocity|thrust|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9544" PostTypeId="2" ParentId="9543" CreationDate="2015-06-17T03:59:09.533" Score="15" Body="&lt;p&gt;What's your engineering budget?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a hideously impractical undertaking. Most of the mass of an orbital rocket is fuel and the tanks to hold it; even though your payload is tiny, all the rest of that stuff is big. The smaller a rocket is, the harder it is to design it with the high &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot;&gt;fuel-mass-to-dry-mass ratio that is required&lt;/a&gt; to attain high speeds, because some elements (like electronics) don't scale with the size of the rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on what design assumptions you make, you will get wildly different results. Some arbitrary examples:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scenario 1: two stage solid rocket, 90% propellant fraction. &#xA;Stage 1: 700kN thrust, 240s sea level Isp, 40 tons propellant. Contributes 3886 m/s ∆v.&#xA;Stage 2: 80kN thrust, 280s vacuum Isp, 5 tons propellant. Contributes 6581 m/s ∆v. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scenario 2: single stage kerosene/LOX rocket using the SpaceX &lt;a href=&quot;https://en.wikipedia.org/wiki/Kestrel_(rocket_engine)&quot;&gt;Kestrel&lt;/a&gt; engine, fuel tanks whittled by hand from carbon fiber by a team of elves on Adderall, total dry mass 100kg, of which the rocket motor is &lt;em&gt;half&lt;/em&gt;. 96% propellant fraction, 2.4 tons of propellant, 31kN thrust, 317s vac Isp, total ~9600 m/s ∆v. (At the end of the burn for the Kestrel option, the rocket would be accelerating at better than 30g, so I hope those elves know what they're doing!) I don't think this is actually plausible, but it's theoretically possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And a real world example. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scenario 3: &lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_(rocket)&quot;&gt;Vanguard 1/TV-4&lt;/a&gt; was a 3-stage rocket that put a satellite into orbit that weighed only about 3 soccer balls. First stage thrust 135kN. From my brief research, Vanguard and the Japanese 4-stage solid-rocket &lt;a href=&quot;https://en.wikipedia.org/wiki/Lambda_4S&quot;&gt;Lambda 4S&lt;/a&gt; seem to be the two smallest orbital launchers in history. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So depending on how you tweak the other variables, the needed thrust could be anywhere from 31kN to 700kN. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the ball, FIFA regulations say the air pressure in a soccer ball needs to be between 8.5 psi and 15.6 psi (above ambient), so presumably it can handle somewhat more pressure than that without exploding. Sea level ambient air pressure is about 15 psi, so if you deflated the ball before launch (to ambient air pressure), then took it into vacuum, it would then be at 15 psi overpressure - the high end of regulation inflation, so a very taut ball, but not exploded. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-06-17T16:52:08.323" LastActivityDate="2015-06-17T16:52:08.323" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9545" PostTypeId="1" AcceptedAnswerId="9549" CreationDate="2015-06-17T05:24:17.763" Score="10" ViewCount="57023" Body="&lt;p&gt;I was just watching a video about a &lt;a href=&quot;https://www.youtube.com/watch?v=uqhnxAjK7qY&quot;&gt;competition to put rovers on the Moon&lt;/a&gt;. Near the beginning of the video, it mentioned a handful of the objects that landed there over the decades. Then at about the 5 minute mark, it begins talking about satellites in orbit around the Moon and it made me wonder what's up there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found &lt;a href=&quot;https://en.wikipedia.org/wiki/Category:Artificial_satellites_orbiting_the_Moon&quot;&gt;this list on Wikipedia&lt;/a&gt; of artificial satellites orbiting the Moon, but based on what the video said, I don't think it's up to date. From what I can tell this list only includes satellites sent by NASA and the Soviet Union, but the video mentioned NASA, ESA, China, India, and Japan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many satellites are orbiting the Moon, and what are their names (or other identifiers)?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2015-06-17T09:03:46.903" Title="How many artificial satellites orbit the Moon?" Tags="|the-moon|artificial-satellite|" AnswerCount="1" CommentCount="4" ClosedDate="2015-06-17T18:59:39.340" ContentLicense="CC BY-SA 3.0" />
  <row Id="9546" PostTypeId="1" AcceptedAnswerId="9547" CreationDate="2015-06-17T07:01:10.150" Score="1" ViewCount="1427" Body="&lt;p&gt;I was wondering about the following situation: First, you send up a balloon with a satellite with some thrusters built-in. As soon as the balloon reaches the geostationary height, it deploys from the balloon. After some adjustments in speed and position, it stays up there, bright and breezy! Is that achievable?&lt;/p&gt;&#xA;" OwnerDisplayName="user10514" LastEditorUserId="49" LastEditDate="2015-06-17T20:08:35.687" LastActivityDate="2015-06-17T20:08:35.687" Title="Is it possible to put a satellite into geostationary orbit using a weather balloon?" Tags="|artificial-satellite|geostationary|feasibility|balloons|launch-system|" AnswerCount="1" CommentCount="2" ClosedDate="2015-06-17T17:45:29.483" ContentLicense="CC BY-SA 3.0" />
  <row Id="9547" PostTypeId="2" ParentId="9546" CreationDate="2015-06-17T07:22:58.803" Score="10" Body="&lt;p&gt;No, altitude is not the hardest thing about getting to space, it's getting enough lateral velocity so that you literally miss the earth as you fall. A weather balloon would get you to 23 miles (37 kilometres). Geostationary orbit is 22,000 miles (35,000 km), almost &lt;em&gt;one thousand times that&lt;/em&gt;. In fact no balloon could get you that high since the earth's atmosphere ends far below that altitude and it is the density differential between the gas of the balloon and the ambient air that creates the lift that causes balloons to rise &lt;/p&gt;&#xA;&#xA;&lt;p&gt;XKCD explains it quite well in this &lt;a href=&quot;https://what-if.xkcd.com/58/&quot;&gt;What-If&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="10398" LastEditDate="2015-06-17T16:50:16.317" LastActivityDate="2015-06-17T16:50:16.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9548" PostTypeId="1" CreationDate="2015-06-17T08:04:12.180" Score="6" ViewCount="221" Body="&lt;p&gt;Cassini at Saturn has sampled geysers although it wasn't designed to do it. The orbiter NEAR Shoemaker was soft landed on its asteroid. Two examples of how interplanetary probes have been partly repurposed while in flight, there are certainly many more subtle ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What could be done to make for example the proposed Europa clipper so flexible that it could reconfigure itself in order to follow up on unexpected discoveries instead of having to wait for a completely new probe? Do we still need to keep it simple in order to achieve reliability?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about reusable orbiter platforms? Somewhat like a space station or a Mars cycler. A platform which once put in orbit then is gradually upgraded and redesigned during many decades. &lt;em&gt;The simplest might be to just launch a radiation shielded platform to Jupiter orbit once and for all, to which several probes could dock one after the other during many decades.&lt;/em&gt; The extreme would be to have a probe factory in orbit of an outer planet which only needs the low-weight high tech instrument components and fuel sent to it from Earth, in order to be redesigned to whatever kind of probe is desired next.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-06-17T13:59:48.637" LastActivityDate="2015-06-18T20:33:10.400" Title="Could interplanetary orbiters and rovers be designed as reusable upgradeable platforms?" Tags="|probe|mission-design|construction|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9549" PostTypeId="2" ParentId="9545" CreationDate="2015-06-17T09:03:46.903" Score="7" Body="&lt;p&gt;To the best of my knowledge, there are &lt;strong&gt;four&lt;/strong&gt; active satellites orbiting the Moon.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;The Lunar Reconnaissance Orbiter&lt;/a&gt;: Polar mapping orbit, mission by NASA.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/THEMIS&quot; rel=&quot;noreferrer&quot;&gt;ARTEMIS&lt;/a&gt; (two spacecraft): &quot;Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun&quot;. ARTEMIS P1 and P2 orbit the Moon right now. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JkNkD.jpg&quot; alt=&quot;Artemis&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Visualisation of ARTEMIS near the Moon. (&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/0/09/ARTEMIS_mission.jpg/320px-ARTEMIS_mission.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_5-T1&quot; rel=&quot;noreferrer&quot;&gt;Chang'e 5-T1&lt;/a&gt;: The command module of this Chinese spacecraft has been in a stable orbit around Luna since January 2015.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-06-17T09:03:46.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9550" PostTypeId="2" ParentId="9548" CreationDate="2015-06-17T10:19:34.407" Score="5" Body="&lt;p&gt;There is a big difference between making the most of the equipment you have on board, and fabricating new sensors on demand. The first has already been done.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second is much more complex. &quot;Following up on unexpected discoveries&quot; means you'd have to create new sensors. These instruments take a fully-equipped lab (plus its massive infrastructure in the shape of parts suppliers) on Earth months, if not years to build. We're many years away from being able to build such a factory within the space constraints of a space probe.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the comments, you've suggested an in-between solution: send new instruments as a package to rendezvous with the original probe, link the two together and use them as one platform. That could have some interesting uses. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your package is going to need engines, a power system and a radio for the rendezvous at Jupiter, so you're already halfway to building a full probe. But there could be some benefit to reusing things like the high-gain antenna (which is large) and the RTG (which is expensive).   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the most expensive parts of a probe are the instruments. The rest (apart from the RTG) is a piece of cake by comparison, so I doubt you'd save much money in the end. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-06-17T12:05:08.247" LastActivityDate="2015-06-17T12:05:08.247" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9552" PostTypeId="2" ParentId="9548" CreationDate="2015-06-17T15:12:07.277" Score="6" Body="&lt;p&gt;I see this as really unlikely. There has only really been one un-manned spacecraft that this was successful, and that was the Hubble Space Telescope. Even as such, it required a team of astronauts to upgrade the telescope. Why was it cheaper to upgrade Hubble?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The mirror was a very large expense, along with the pointing, power, and communications.&lt;/li&gt;&#xA;&lt;li&gt;The structure was unchanged, so the instruments just had to be included in the box.&lt;/li&gt;&#xA;&lt;li&gt;The satellite is in low earth orbit, easy to reach by astronauts.&lt;/li&gt;&#xA;&lt;li&gt;The position is really well known, because it is so close to Earth.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;How is that different from, say, a Jupiter orbiter?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The position of Cassini isn't that well known. I don't have a great estimate, but I'm sure the position can't be know to more accurate than the few mile range, and likely isn't even that well know. This makes docking difficult, you have to do some kind of active updates on site to find the spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;The distance makes it so the time required to send out an upgrade package takes a significant amount of time. There would be no guarantee that the far spacecraft would still work after such a long period of time.&lt;/li&gt;&#xA;&lt;li&gt;Not being in LEO, it would not allow for an astronaut to install the new instruments. The most technology would really allow for right now is a docking maneuver, sharing power, data, and pointing, but not things like a mirror.&lt;/li&gt;&#xA;&lt;li&gt;An unmanned mission would require constant Earth communication. The same system could presumably send data from the instrument.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, there doesn't seem to be a whole lot of gain to make with this kind of an upgrade, at least not with technology anticipated in the near term future, except for perhaps another instrument like Hubble.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-17T15:12:07.277" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9553" PostTypeId="2" ParentId="9548" CreationDate="2015-06-17T15:37:45.817" Score="3" Body="&lt;p&gt;An orbiter platform, minus its instruments, would consist mainly of power, attitude control, and communication systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A secondary mission to deliver new instruments to that orbiter would &lt;em&gt;also&lt;/em&gt; require power, attitude control, and communication systems, in order to reach the destination. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there's no real savings from re-using parts of the first platform, so there's no incentive to tackle the complexity of a modular upgradable system. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-06-17T15:37:45.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9554" PostTypeId="1" AcceptedAnswerId="9570" CreationDate="2015-06-17T17:08:43.350" Score="1" ViewCount="325" Body="&lt;p&gt;If a vessel is to enter an orbit around some body (approaching from the &quot;outside&quot; fast), and can choose between a very high circular orbit and a very elongated elliptical orbit of the same apoapsis, which will consume less fuel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Intuitively, when reversing the situation, entering the wide orbit from low orbit around the body would require more fuel: first increase the apoapsis (entering the elliptical orbit) and then circularize it. So, in direction of outside the system -&gt; low orbit, if the trend is reversed, the orbit with lower average altitude would consume more fuel. On the other hand, Oberth effect is anything but intuitive, and a maneuver performed at a high speed, close to the body's surface is supposed to be more efficient than one performed at a high orbit. So I can't decide which is the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Please answer urgently, I don't have much fuel and Minmus is drawing near.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="49" LastEditDate="2015-06-17T20:14:32.033" LastActivityDate="2015-06-19T16:24:35.687" Title="Descending into any orbit: Which requires less delta-v, a wide one, or a narrow-elliptic one?" Tags="|orbital-mechanics|orbital-maneuver|delta-v|oberth-maneuver|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9555" PostTypeId="1" CreationDate="2015-06-17T17:19:33.527" Score="5" ViewCount="331" Body="&lt;p&gt;&lt;a href=&quot;http://hamsterworks.co.nz/mediawiki/index.php/Field_Programmable_Gate_Arrays&quot; rel=&quot;nofollow&quot;&gt;FPGAs&lt;/a&gt; are microprocessors that can be reconfigured on the fly, through programming, to do different things. They can essentially &lt;a href=&quot;https://www.youtube.com/watch?v=L2wsockKwPQ&quot; rel=&quot;nofollow&quot;&gt;be rewired&lt;/a&gt;, to change what the chip does, in ways that once would have required making a new chip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This changes how probes can be reprogrammed in response to discoveries when they reach their destinations, or to work around damage to a probe, or to assign new missions once the primary missions are over, since probes often survive beyond the mission lifetime. ESA has some papers examining these possibilities listed &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Engineering_Technology/Microelectronics/The_use_of_reprogrammable_FPGAs_in_space&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have FPGAs already been used in this way? What possibilities do they open up for the future? What needs to be borne in mind?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="12102" LastEditDate="2017-06-03T09:54:07.317" LastActivityDate="2017-06-03T09:54:07.317" Title="How do Field Programmable Gate Arrays affect the capabilities of probes?" Tags="|probe|mission-design|computing|data-systems|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9556" PostTypeId="1" AcceptedAnswerId="10108" CreationDate="2015-06-17T18:23:05.573" Score="5" ViewCount="1734" Body="&lt;p&gt;I used &lt;a href=&quot;http://eyes.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;NASA's eyes&lt;/a&gt; tool to see what's going at the time of New Horizon's closest approach to Pluto.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realized that in the programmed sequence, New Horizons is rotated on its axis by more than 90° in just a few seconds, and then immediately maintains nadir fixed attitude with respect to Pluto to make the next observation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is such precise attitude control done mechanically?&lt;/p&gt;&#xA;" OwnerUserId="139" LastEditorUserId="49" LastEditDate="2015-06-17T20:22:38.463" LastActivityDate="2015-07-22T12:13:45.920" Title="How does New Horizons control its attitude?" Tags="|spacecraft|attitude|observation|new-horizons|rotation|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9557" PostTypeId="2" ParentId="9554" CreationDate="2015-06-17T18:24:00.603" Score="1" Body="&lt;p&gt;Well lets think about this one logically. &#xA;Say you're sitting in a spaceship out in the oort cloud, orbiting the sun at a very leisurely pace of about 3m/s, which is equivalent to jogging speed. It doesn't take much delta-v at all to stop dead, and fall directly towards the sun. With such a long fall, a slight adjustment will let us pick what planet we want to intercept. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are now coming in from the outside, fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly, it is MUCH easier to enter a narrow elliptical orbit. In order to enter a circular orbit you'd have to wait until you're at that orbital distance from the target planet, and then fire your rockets to overcome all the velocity gained falling from the oort cloud.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aiming closer to the planet (MUCH closer), we can areobrake in the planet's atmosphere and bleed off a lot of the velocity that way. We aren't actually in a stable orbit, we'll either fly off into space again (if still travelling at escape velocity relative to the planet), or we'll subsequently crash into the planet without a correcting burn (but we can always do a correcting burn).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know the Oberth effect isn't exactly intuitive, but think about it this way and I'm sure it will make sense; when we are coming in fast, the planet doesn't have much time to pull on us, so it can't increase our velocity much. So we zoom in really fast, and don't gain much velocity. When close to the planet, we fire our thrusters to slow down as much as we can. We now leave the planet's influence more slowly than we entered it, and the planet's gravity has more time to slow us down, so we lose more velocity leaving the planet, than we gained approaching the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if while still quite far away from the planet, we'd fired our thusters, then because we have slowed our approach, the planet's gravity has more time to pull on us and increase our velocity. As we leave the planet's influence, the planet has exactly the same amount of time to decrease our velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Oberth affect is essentially about minimizing or maximizing the amount of time being 'pulled' strongly by the planet. This is because in accordance with basic Newtonian calculus, Velocity is Acceleration times Time, the more time we are being accelerated, the more velocity we gain. The less time we are being accelerated, the less velocity we gain. Rockets burn their fuel as close to Earth as they can, so they minimize the time in the strongest gravity nearest the Earth. Gravity is NOT like drag, it doesn't pull on you more the faster you're travelling - it pulls on you the same amount (at a given distance) however fast you're travelling. That is why Oberth effect works, and also why it may not be intuitive, because we often think of gravity as a &quot;drag&quot; on rockets, but it's not a drag like that caused by air or water, it's something readily modeled by simple calculus.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-06-17T18:24:00.603" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9558" PostTypeId="2" ParentId="9556" CreationDate="2015-06-17T18:33:10.927" Score="6" Body="&lt;p&gt;According to the NASA &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Spacecraft/Systems-and-Components.php&quot;&gt;New Horizon's site&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;New Horizons has operated mostly in a spin-stabilized mode while cruising between planets, and also in a three-axis “pointing” mode that allows for pointing or scanning instruments during calibrations and planetary encounters (like the Jupiter flyby and, of course, at Pluto). There are no reaction wheels on the spacecraft; small thrusters in the propulsion system handle pointing, spinning and course corrections.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-17T18:33:10.927" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9559" PostTypeId="1" CreationDate="2015-06-17T19:00:13.610" Score="8" ViewCount="1576" Body="&lt;p&gt;The second stage of Proton (8K-82K) rocket has two types of main engines (RD-0210 and RD-0211). Does someone knows why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JcJ1P.jpg&quot; alt=&quot;Proton rockets - the first version, 1965, and the recent M version.&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8761" LastEditorUserId="49" LastEditDate="2015-06-17T20:05:57.103" LastActivityDate="2015-06-17T20:05:57.103" Title="Why do Proton rockets use two types of main engines in its second stage?" Tags="|launch|spacecraft|engines|stages|proton-m|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9560" PostTypeId="2" ParentId="9559" CreationDate="2015-06-17T19:07:22.393" Score="8" Body="&lt;p&gt;They are the same basic engine, just the RD-0211 has a heat exchanger, that is lacking on the RD-0210.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The long history of the Proton went through three or more major revisions, and the engines were renumbered in each revision, as a pair.  Each time, one with heat exchanger, one without.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is much more detail on Wikipedia about the &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-0210&quot;&gt;RD-210 Engine&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-17T19:07:22.393" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9561" PostTypeId="2" ParentId="9555" CreationDate="2015-06-17T21:35:41.627" Score="7" Body="&lt;p&gt;Here is a list of some space probes that used FPGAs and how they were used.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Mars Rover and Lander&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;main brain&quot; of the rover vehicle, controlling the motors for the wheels, steering, arms, cameras and various instrumentation&lt;/li&gt;&#xA;&lt;li&gt;control the crucial pyrotechnic operations during the multi-phase descent and landing procedure&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Optus C1 &#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apply signal processing algorithms to communications data coming from Earth&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Grace&lt;br&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Sensor&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;FedSat&#xA;&lt;ul&gt;&#xA;&lt;li&gt;As a critical component of the High Performance Computing (HPC-I) payload, the reconfigurable nature of Xilinx FPGAs enable satellites to be rewired without having to be retrieved,&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Rosetta&lt;br&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Wide range of lander and orbiter instrument applications including the main computer, power system, mass memory controller and the antenna point control&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;NEAR Shoemaker&lt;br&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Navigation, command, telemetry and scientific data collection applications&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;International Space Station&lt;br&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mission critical apps&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Mars Pathfinder&lt;br&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Camera controls and communications&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Mars Rovers&#xA;&lt;ul&gt;&#xA;&lt;li&gt;various mission-critical digital logic functions, including power management, attitude and orbit control, command and data handling, and instrumentation and telemetry&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;from: &lt;a href=&quot;http://esl.eng.ohio-state.edu/~rstheory/iip/RadHardFPGA.doc&quot;&gt;http://esl.eng.ohio-state.edu/~rstheory/iip/RadHardFPGA.doc&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16" LastActivityDate="2015-06-17T21:35:41.627" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9563" PostTypeId="1" AcceptedAnswerId="9603" CreationDate="2015-06-18T09:04:38.063" Score="10" ViewCount="1610" Body="&lt;p&gt;Let's say we have an elliptical orbit that goes from LEO periapsis to apoapsis near Earth's Hill Sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can merely nudge your satellite at the apoapsis a bit higher (a tiny burn at periapsis), and it becomes Sun's satellite, leaving Earth for a long, long time, moving really slow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or you can spend lots of fuel (and energy) to circularize the wide orbit. Then, at any point you can speed up just a little (again, just enough to breach the Hill sphere) and you'll escape the planet - choosing the point and speed right, just at the same speed and in the same direction as in the prior case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where did all the energy used to circularize the orbit vanish to?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-06-20T19:00:24.037" Title="Oberth is confusing me. Apparent violation of conservation of energy" Tags="|orbital-mechanics|oberth-maneuver|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9564" PostTypeId="1" CreationDate="2015-06-18T09:05:01.923" Score="3" ViewCount="200" Body="&lt;p&gt;I've read a bit about space-based solar power. It would be sent to earth by either laser or microwave. Could it not also be reversed and have energy sent from earth to satelites in LEO, where they use that power to transfer to GSO and other places in the solar system. It seems to me to be a hell of a lot cheaper than carrying all that fuel with them&lt;/p&gt;&#xA;" OwnerUserId="10524" LastEditorUserId="-1" LastEditDate="2015-06-19T05:05:16.600" LastActivityDate="2015-06-19T05:05:16.600" Title="Energy in space" Tags="|propulsion|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9565" PostTypeId="2" ParentId="9564" CreationDate="2015-06-18T09:19:56.887" Score="10" Body="&lt;p&gt;It's not enough to just have power, you also need reaction mass to move a spacecraft (see, for instance, &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot;&gt;ion engine&lt;/a&gt;). The spacecraft would still need to carry the gas (or other propellant with it); the only exception is if you use a solar sail and shine a laser from Earth onto it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beaming power to a spacecraft &lt;em&gt;could&lt;/em&gt; work (the &lt;a href=&quot;https://en.wikipedia.org/?title=Beam-powered_propulsion&quot;&gt;Wikipedia article&lt;/a&gt; goes into some detail), but there are a number of practical problems which remain unsolved. Among them:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Earth is permanently rotating; you would need a number of &quot;beaming&quot; stations around the planet to constantly power a spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;The machinery on board the spacecraft to convert the beamed power into electricity weighs; the question is, does it weigh less than solar panels? &lt;/li&gt;&#xA;&lt;li&gt;Beamed power attenuates fairly strongly through the atmosphere, and diffuses with distance. So you lose a fair chunk of power by beaming it through the atmosphere, and the further the spacecraft is away, the less power it will receive. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So: in theory it's possible, but there are a number of obstacles to be overcome first, and given our current level of technology it's not worth doing. &lt;/p&gt;&#xA;" OwnerUserId="3063" LastActivityDate="2015-06-18T09:19:56.887" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9567" PostTypeId="2" ParentId="9563" CreationDate="2015-06-18T12:36:50.003" Score="1" Body="&lt;p&gt;The effect of the Oberth largely comes from reducing something known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_drag&quot; rel=&quot;nofollow&quot;&gt;Gravity Drag&lt;/a&gt;. Let's ignore air resistance. If we launch a rocket directly into space, for every second we are thrusting, we are also dragged down. This effect can be overcome in large part by moving faster, as we are in the gravity well for a smaller amount of time. Thus, gravity can't have as much of an effect on us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the elliptical case, we accelerate slightly when at the peak velocity. Thus, the drag of gravity on the spacecraft is less as we are moving faster. In effect, the energy comes because gravity has less of a drag on the spacecraft. Basically, the pull of the gravity well has accelerated the object to a high speed, and moving away faster gives the gravity source less time to slow it down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the highly circular orbit, there is not a large amount of speed, in fact the orbital velocity is quite low. That velocity will only increase by the rocket power directly, and will not have the additional effect of the gravity drag.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-06-18T15:05:56.187" LastActivityDate="2015-06-18T15:05:56.187" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9568" PostTypeId="2" ParentId="9563" CreationDate="2015-06-18T15:08:00.247" Score="5" Body="&lt;p&gt;Remember, work done is force times distance.  At periapsis, one second of thrust is performed over a larger distance than one second of thrust at apoapsis.  Thus, more work is done by firing at periapsis.  This is the Oberth effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The work done (energy used) to circularize the orbit at apoapsis went into increasing the energy (potential + kinetic) of the object at all points in its orbit.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-06-18T15:18:02.410" LastActivityDate="2015-06-18T15:18:02.410" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9569" PostTypeId="1" AcceptedAnswerId="9836" CreationDate="2015-06-18T15:28:11.370" Score="5" ViewCount="374" Body="&lt;p&gt;I have a Lambert solver that is used to calculate the required delta-V for interplanetary transfer trajectories, but I recently ran into a problem. It works well for &quot;long&quot; time-of-flight trajectories (for example, a 250 day trajectory from Earth to Mars), but if I try a &quot;short&quot; time-of-flight trajectory (for example, a 45 day trajectory from Earth to Mars), the solver seems to break, by having its calculations not converge. I realise that a 45 day TOF is rather unrealistic, but I usually perform a mid-course correction halfway through a trajectory, so if I have a fast 90 day TOF, then I'll perform a MCC 45 days into the flight (which is when the solver breaks).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following is the Lambert solver which uses a Universal Variables formulation (coded in the Mathematica language) and a simple iterative bisection solver to calculate the required delta-V given a require time-of-flight:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I've since fixed the code and the Lambert solver below seems to work quite well:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;G = 6.672*10^-11; (*Gravitational Constant*)&#xA;m[0] = 1.988544*10^30; (*Mass of Sun*)&#xA;TOF = (45) (86400); (*Time-of-flight in seconds*)&#xA;R[1] = {-1.1751563715176448`*^11, 8.982523733108002`*^10} (*Heliocentric position of Earth*)&#xA;R[2] = {-5.256112524631399`*^10, -2.1604439066188406`*^11} (*Heliocentric position of Mars*)&#xA;R1 = Sqrt[R[1].R[1]]&#xA;R2 = Sqrt[R[2].R[2]]&#xA;\[CapitalDelta]\[Nu] = ArcCos[R[1].R[2]/(R1 R2)] (*Change in true anomaly, in radians*)&#xA;A = Sqrt[R1 R2 (1 + Cos[\[CapitalDelta]\[Nu] ])];&#xA;&#xA;iterationCount = 0;&#xA;z = 0;&#xA;zhi = 4 \[Pi]^2;&#xA;zlow = -4 \[Pi];&#xA;c[z_] := If[z &amp;gt; 0, (1 - Cos[Sqrt[z]])/z, &#xA;  If[z &amp;lt; 0, (1 - Cosh[Sqrt[-z]])/z, 1/2]]&#xA;S[z_] := If[z &amp;gt; 0, (Sqrt[z] - Sin[Sqrt[z]])/Sqrt[z^3], &#xA;  If[z &amp;lt; 0, (Sinh[Sqrt[-z]] - Sqrt[-z])/Sqrt[(-z)^3], 1/6]]&#xA;Y[z_] := R1 + R2 - (A (1 - S[z] z))/Sqrt[c[z]];&#xA;X[z_] := Sqrt[Y[z]/c[z]];&#xA;t[z_] := (X[z]^3 S[z] + A Sqrt[Y[z]])/Sqrt[G m[0]];&#xA;&#xA;t[z] = t[z]; (*Initial value for t[z]*)&#xA;&#xA;(*Iterative Bisection Solver*)&#xA;While[Norm[t[z] - \[CapitalDelta]t] &amp;gt; 1*10^-6 &amp;amp;&amp;amp; iterationCount &amp;lt; 100,&#xA; c[z_] := &#xA;  If[z &amp;gt; 0, (1 - Cos[Sqrt[z]])/z, &#xA;   If[z &amp;lt; 0, (1 - Cosh[Sqrt[-z]])/z, 1/2]];&#xA; S[z_] := &#xA;  If[z &amp;gt; 0, (Sqrt[z] - Sin[Sqrt[z]])/Sqrt[z^3], &#xA;   If[z &amp;lt; 0, (Sinh[Sqrt[-z]] - Sqrt[-z])/Sqrt[(-z)^3], 1/6]];&#xA; Y[z_] := R1 + R2 + (A (S[z] z - 1))/Sqrt[c[z]];&#xA; (*Making sure Y&amp;gt;0*)&#xA; While[A &amp;gt; 0 &amp;amp;&amp;amp; Y[z] &amp;lt; 0,&#xA;  zlow = zlow + 0.01;&#xA;  z = (zhi + zlow)/2;&#xA;  c[z_] := &#xA;   If[z &amp;gt; 0, (1 - Cos[Sqrt[z]])/z, &#xA;    If[z &amp;lt; 0, (1 - Cosh[Sqrt[-z]])/z, 1/2]];&#xA;  S[z_] := &#xA;   If[z &amp;gt; 0, (Sqrt[z] - Sin[Sqrt[z]])/Sqrt[z^3], &#xA;    If[z &amp;lt; 0, (Sinh[Sqrt[-z]] - Sqrt[-z])/Sqrt[(-z)^3], 1/6]];&#xA;  Y[z_] := R1 + R2 + (A (S[z] z - 1))/Sqrt[c[z]];&#xA;  ];&#xA; X[z_] := Sqrt[Y[z]/c[z]];&#xA; t[z_] := (X[z]^3 S[z] + A Sqrt[Y[z]])/Sqrt[G m[0]];&#xA; If[t[z] &amp;lt;= \[CapitalDelta]t, zlow = z, zhi = z];&#xA; z = (zhi + zlow)/&#xA;  2; (*Re-calculating z using bisection root finding method*)&#xA; Print[Norm[t[z] - \[CapitalDelta]t]];&#xA; iterationCount++;];&#xA;&#xA;    f = 1 - Y[z]/R1;&#xA;    g = A Sqrt[Y[z]/(G m[0])];&#xA;    gdot = 1 - Y[z]/R2;&#xA;    vLambert[1] = (R[2] - f R[1])/g; (*Required velocity at start of trajectory*)&#xA;    vLambert[2] = (gdot R[2] - R[1])/g; (*Velocity at target arrival*)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;As can be seen, there are checks in place to make sure Y always stays positive, so I'm not too sure what the problem could be. Searching the internet for any academic articles or book chapters has unfortunately not given me a solution to this problem either. Any help would be greatly appreciated, thanks very much.&lt;/p&gt;&#xA;" OwnerUserId="10130" LastEditorUserId="6494" LastEditDate="2017-09-22T10:46:27.293" LastActivityDate="2017-09-22T10:46:27.293" Title="Low Time-of-Flight Lambert Solver Trajectory Errors" Tags="|trajectory|interplanetary|time-of-flight|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9570" PostTypeId="2" ParentId="9554" CreationDate="2015-06-18T19:12:31.363" Score="3" Body="&lt;p&gt;There are two different scenarios that I'll consider:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Entering the circular orbit from high altitude.&lt;/li&gt;&#xA;&lt;li&gt;Entering the elliptical orbit from low altitude.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Entering the circular orbit from low altitude will always take more fuel than entering the elliptical orbit, since you have to enter an elliptical transfer orbit of the same size (using the same $\Delta v$), and then make an &lt;em&gt;additional&lt;/em&gt; burn at apoapsis to circularize your orbit.  (Using aerobraking to replace the initial burn will change this.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, entering the elliptical orbit from high altitude will always take more fuel than entering the circular orbit, since the elliptical orbit will have a slower speed at apoapsis, requiring a greater $\Delta v$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll make the assumption that the hyperbolic excess velocity ($v_\infty$) upon entry is the same for both cases, that all the orbital intersections are parallel (no plane changes), and that all burns are instantaneous.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;A circular orbit of radius $r_a$ requires a speed of $v_\text{circ}=\sqrt{\mu/r_a}$.  Solving for the entry velocity using conservation of energy:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{v_\infty^2}{2} = \frac{v_\text{entry}^2}{2} - \frac{\mu}{r_a} \\&#xA;v_\text{entry} = \sqrt{v_\infty^2 + \frac{2\mu}{r_a}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $\Delta v$ required is simply the difference between the two:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta v_\text{circ} = \sqrt{v_\infty^2 + \frac{2\mu}{r_a}}-\sqrt{\frac\mu{r_a}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In the second scenario, we have a periapsis velocity of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;v_p = \sqrt{\frac{2\mu}{r_a+r_p}\frac{r_a}{r_p}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The entry velocity is almost the same, with $r_p$ in place of $r_a$, giving us:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta v_\text{ellip} = \sqrt{v_\infty^2 + \frac{2\mu}{r_p}}-\sqrt{\frac{2\mu}{r_a+r_p}\frac{r_a}{r_p}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Now, let's find under what conditions $\Delta v_\text{ellip} &amp;lt; \Delta v_\text{circ}$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta v_\text{ellip} &amp;lt; \Delta v_\text{circ} \\&#xA;\sqrt{v_\infty^2 + \frac{2\mu}{r_p}}-\sqrt{\frac{2\mu}{r_a+r_p}\frac{r_a}{r_p}} &amp;lt; \sqrt{v_\infty^2 + \frac{2\mu}{r_a}}-\sqrt{\frac\mu{r_a}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll make the substitutions $v_\infty^2\to \alpha\mu/r_a$ and $r_p\to\beta r_a$ to help us out a little.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\sqrt{\frac{\alpha\mu}{r_a} + \frac{2\mu}{\beta r_a}}-\sqrt{\frac{2\mu}{r_a+\beta r_a}\frac{r_a}{\beta r_a}} &amp;lt; \sqrt{\frac{\alpha\mu}{r_a} + \frac{2\mu}{r_a}}-\sqrt{\frac\mu{r_a}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's take out a factor of $\sqrt{\mu/r_a}$ (which is positive, so it doesn't affect the inequality):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\sqrt{\alpha + \frac{2}{\beta}}-\sqrt{\frac{2}{\beta\left(1+\beta\right)}} &amp;lt; \sqrt{\alpha + 2}-1&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Treating the left side as a function of $\beta$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;f(\beta) = \sqrt{\alpha + \frac{2}{\beta}}-\sqrt{\frac{2}{\beta\left(1+\beta\right)}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;f(1) = \sqrt{\alpha + 2} - \sqrt{\frac{2}{1\cdot 2}} = \sqrt{\alpha + 2} - 1&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting into our inequality, we now must only prove:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;f(\beta) &amp;lt; f(1)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that since $0&amp;lt;r_p&amp;lt;r_a$, we know that $\beta\in(0,1)$.  Looking at the derivative of $f$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;f'(\beta) = -\frac{1}{\beta^2\sqrt{\alpha+2/\beta}}+\frac{\beta+(1+\beta)}{\beta^2(1+\beta)^2}\sqrt{\frac{\beta(1+\beta)}{2}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can check that $f'(\beta)&amp;gt;0$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{1}{\beta^2\sqrt{\alpha+2/\beta}} &amp;lt; \frac{\beta+(1+\beta)}{\beta^2(1+\beta)^2}\sqrt{\frac{\beta(1+\beta)}{2}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since both sides are positive, we can square both sides without affecting the inequality:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{1}{\beta^4(\alpha+2/\beta)} &amp;lt; \frac{\left(\beta+(1+\beta)\right)^2\beta(1+\beta)}{2\beta^4(1+\beta)^4} \\&#xA;\frac{1}{(\alpha+2/\beta)} &amp;lt; \frac{(1+2\beta)^2\beta}{2(1+\beta)^3} \\&#xA;2(1+\beta)^3 &amp;lt; (\alpha\beta+2)(1+2\beta)^2 \\&#xA;2\beta^3+6\beta^2+6\beta+2 &amp;lt; 4\alpha\beta^3 + (4\alpha+8)\beta^2 + (\alpha+8)\beta + 2 \\&#xA;0 &amp;lt; (4\alpha-2)\beta^3 + (4\alpha+2)\beta^2 + (\alpha+2)\beta \\&#xA;0 &amp;lt; 4\alpha\beta^2(1+\beta) + 2\beta^2(1-\beta) + (\alpha+2)\beta&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All three terms are positive when $\alpha&amp;gt;0$ and $\beta\in(0,1)$, so $f'(\beta)$ is always positive.  This means that its maximum must occur on the right endpoint, $f(1)$, so that $f(\beta)&amp;lt;f(1)$ for all values under consideration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, &lt;strong&gt;the elliptical orbit is always more efficient&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The greatest difference between the two orbits is when $\alpha=2$ (that is, $v_\infty=v_\text{esc}$ at apoapsis), where we have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{\Delta v_\text{circ}}{\Delta v_\text{ellip}} = \frac 1{\sqrt{2}}\sqrt{1+\frac 1\beta}&#xA;$$&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="8996" LastEditDate="2015-06-19T16:24:35.687" LastActivityDate="2015-06-19T16:24:35.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9571" PostTypeId="2" ParentId="4771" CreationDate="2015-06-18T19:30:26.587" Score="48" Body="&lt;p&gt;I actually worked on the team that built HDEV (came across this question while googling).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;TildalWave's answer is mostly correct, though we leave the cameras on during black (ISS night) to help with dead pixel analysis.  The aperture and focus settings are fixed for sunlight viewing to better characterize any radiation caused issues with the sensors (the primary purpose of HDEV).  If something is extremely bright and close-ish to the station you might see it, but doubtful.  I remember watching it once in a severe thunderstorm system over Brazil and could just &lt;em&gt;barely&lt;/em&gt; see tiny specs of lightning, though even then it wasn't in complete night but only partial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We sometimes change the cycle between cameras/durations to try and capture specific events either on the surface or vehicles docking to the station within view.  Nominal ops though is just cycling through the cameras every ten minutes or so.  If the payload has to be reset, it will default back to sixty seconds per camera.&lt;/p&gt;&#xA;" OwnerUserId="10528" LastEditorUserId="10528" LastEditDate="2015-06-22T01:13:46.160" LastActivityDate="2015-06-22T01:13:46.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9572" PostTypeId="2" ParentId="9555" CreationDate="2015-06-18T20:25:25.470" Score="5" Body="&lt;p&gt;From a standpoint of &quot;reprogrammability&quot; FPGAs don't hold any advantage over any other processor.  You can update/modify software on any type of system whether it's x86 or FPGA or any other architecture.  The advantage of FPGAs is their parallel nature, the ability to do many things at once, and act as a state machine rather than a linear sequence of instruction crunching.  For that reason, in the space industry (which I work in), FPGA programmers are very highly desired.&lt;/p&gt;&#xA;" OwnerUserId="10528" LastActivityDate="2015-06-18T20:25:25.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9574" PostTypeId="2" ParentId="2046" CreationDate="2015-06-18T23:49:32.030" Score="2" Body="&lt;p&gt;Most of these delta V maps are based on Hohmann orbits and patched conics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll use the example of an earth to Mars Hohmann:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3DuYW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's moving 30 km/s, Transfer orbit at perihelion is moving 33 km/s so 3 km/s to match velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Transfer at aphelion is 21.5 km/s and Mars is 24 km/s. So 2.5 km/s to match velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So where do the 33, 30, 21.5 and 24 come from?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis viva equation&lt;/a&gt;.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$V=\sqrt{\mu_{sun}(2/r - 1/a)}$&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mu_{sun}$ = &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravitational Constant&lt;/a&gt; * Mass of Sun&lt;/p&gt;&#xA;&#xA;&lt;p&gt;a = semi major axis of ellipse&lt;/p&gt;&#xA;&#xA;&lt;p&gt;r = distance from sun&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of a circular orbit, circular orbit's radius is the same as semi-major axis so r = a. Substituting r for a reduces the vis viva equation to:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V=\sqrt{\mu_{sun}/r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speed of a circular orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For earth orbit r = ~150,000,000 kilometers.&#xA;For Mars orbit r = ~225,000,000 kilometers&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the transfer orbit a = (150,000,000 + 225,000,000)/2 kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At perihelion of transfer orbit r = 150,000,000&#xA;At aphelion of transfer orbit r = 225,000,000&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging those numbers into the vis viva should give you numbers in the neighborhood of 33 km/s, 30 km/s, 21.5 km/s and 24 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've rounded a lot for simplicity and also earth and Mars orbits aren't perfectly circular and coplanar. The circular, coplanar model isn't exact but it can give you ball park numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once inside of a planet's sphere of influence, the paths are no longer well modeled as ellipses about the sun but as hyperbolas about a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;A hyperbola's speed:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$V=\sqrt{V_{inf}^2+V_{esc}^2}$&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doesn't that remind you of the Pythagorean theorem? I use this memory device to remember a hyperbola's speed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3b8vA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's $V_{inf}$? In the case of earth departure it's 3 km/s. (Remember the transfer's perihelion velocity of 33 km/s and earth's 30 km/s?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's $V_{esc}$? Escape velocity in the earth's neighborhood is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{esc} = \sqrt{2\mu_{earth}/r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to think of a parabolic escape orbit is as an ellipse with infinite semi-major axis. In which case 1/a would be zero. So the vis viva equation would reduce to the Vesc equation above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mu_{earth}$ = Gravitational constant * mass of earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might notice that escape velocity is circular velocity times square of two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For our example, let's set r to be 300 km above earth's surface. r = 6678 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging that r into the equation for escape velocity above you should get about 10.9 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the hyperbola for Trans Mars Insertion (TMI) is $\sqrt{10.9^2+3^2}$ which comes to 11.3 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Setting r = 6678 km, a low, circular earth orbit has speed 7.7 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;11.3 - 7.7 = 3.6. So 3.6 km/s for TMI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On arrival at Mars you'd use a similar process. But $\mu_{Mars}$ would be gravitational constant times mass of Mras. Vinf would be 2.5 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've made &lt;a href=&quot;http://clowder.net/hop/railroad/Hohmann.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;a Hohmann spreadsheet&lt;/a&gt; that uses the equations I've given. It can give you delta Vs for various scenarios. It interesting to change the apoapsis and periapsis of parking orbits about a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used this same spreadsheet to calculate most the numbers is &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/04/cartoon-delta-v-map.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;my own delta V map&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using Google or Wikipedia it's easy to find planetary radii, orbital radii and masses for various bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you remember the vis viva equation as well as the pythagorean memory device for hyperbolic orbits, it's not hard to patch conics.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-06-18T23:57:38.777" LastActivityDate="2015-06-18T23:57:38.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9575" PostTypeId="1" AcceptedAnswerId="9596" CreationDate="2015-06-19T00:08:37.200" Score="11" ViewCount="2020" Body="&lt;p&gt;When utilizing &quot;artificial gravity&quot; generated through a rotating structure using centripetal force, i.e. a centrifuge, is there a minimum diameter or some other attribute that causes or helps alleviate any kind of nausea or motion sickness in humans?&lt;/p&gt;&#xA;" OwnerUserId="10529" LastEditorDisplayName="user20636" LastEditDate="2018-06-04T05:49:54.260" LastActivityDate="2018-06-04T05:49:54.260" Title="Mitigating nausea when generating artificial gravity by centrifuge" Tags="|health|humans|artificial-gravity|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="9578" PostTypeId="1" AcceptedAnswerId="9606" CreationDate="2015-06-19T08:56:20.567" Score="8" ViewCount="1153" Body="&lt;p&gt;...because I have a budding sense it's not quite spherical. Or at least the center isn't equal to the body's center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For one thing, at the distant end (say, in case of Moon's Hill Sphere, the point opposite from Earth) Earth's gravity is weaker than on the Earth side (due to the distance), meaning Moon's sphere of influence should reach farther. Or in case of certain elliptic polar orbits (similar to Sun-synchronous Earth orbit) they could reach much farther as the distance from &quot;the big body&quot; doesn't change by much, and the smaller body's gravity should be able to pull the satellite back from a quite a distance &quot;above&quot; or &quot;below&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what's the shape of the Hill Sphere? And what magnitude of eccentricity/irregularity in its shape can be expected?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2019-05-26T21:54:56.863" Title="How spherical is the Hill Sphere?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9580" PostTypeId="1" AcceptedAnswerId="10758" CreationDate="2015-06-19T13:59:04.170" Score="5" ViewCount="247" Body="&lt;p&gt;How does efficiency of a burn of given duration change with altitude of periapsis?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I right, that for Earth (equatorial radius 6,371km), for orbits with 100 and 200km periapsis altitude respectively, it would be something along the lines of ${6472^{\ 2}}\over{6572^{\ 2}}$ or only about 3% ? Or less, or more?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="9060" LastEditDate="2015-08-25T15:04:04.887" LastActivityDate="2015-08-25T16:56:48.740" Title="Efficiency of Oberth maneuver depending on altitude - is getting the last few kilometers worth it?" Tags="|orbital-mechanics|orbital-maneuver|oberth-maneuver|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9581" PostTypeId="1" AcceptedAnswerId="9582" CreationDate="2015-06-19T14:14:29.860" Score="2" ViewCount="2418" Body="&lt;p&gt;I recently read multiple sources that indicate human-occupied vehicles (Space Shuttle, ISS, Soyuz, etc) are all pressurized to 1 atmosphere of pressure (approx 100 kPa).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since I believe that people function just fine to an altitude of 8,000 ft, or about 70 kPa, why aren't vehicles pressurized to less than one atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(At 70kPa, I believe the atmosphere would be approximately .15 partial pressure of O2 + .56 partial N2 because &lt;strong&gt;0.70 * 0.21 == 0.15&lt;/strong&gt; )&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ratio of N2 / O2 would be the same, but there'd just be a lot less gas around, which leads to various benefits.  There would be less O2/N2 gas to launch into orbit, or existing supplies would last longer.&lt;/p&gt;&#xA;" OwnerUserId="10533" LastEditorUserId="10533" LastEditDate="2015-06-19T14:50:20.120" LastActivityDate="2015-06-19T14:50:20.120" Title="Why are space vehicles at 1 Atmospheric Pressure?" Tags="|iss|physical-activity|" AnswerCount="1" CommentCount="6" ClosedDate="2015-06-19T14:42:00.353" ContentLicense="CC BY-SA 3.0" />
  <row Id="9582" PostTypeId="2" ParentId="9581" CreationDate="2015-06-19T14:24:21.657" Score="4" Body="&lt;ol&gt;&#xA;&lt;li&gt;Safety.&lt;/li&gt;&#xA;&lt;li&gt;Comfort.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Humans can quite well survive in about 30% of Earth atmospheric pressure providing there's enough oxygen. The lower limit is somewhere between when 100% oxygen is not enough for breathing and where human blood starts boiling at body temperature - I'd have to find the exact number, but that's not far below that 30%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The prolonged health effects are hard to predict, generally, while survivable, that's not a natural environment. But that's a secondary concern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary concern is that fire only cares about percentage of oxygen, not the pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo vehicles operated like that, and - as you remember - that resulted in a bad fire. Everything that isn't inherently fire-proof becomes extremely flammable. Plastic, polymer clothes, aluminium, things that would be very difficult to burn in air, and would extinguish themselves as soon as removed from flame, burn quite well in oxygen-rich atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And it degrades materials that oxidate over time. Rubber seals, plastic foils, even small amounts of moisture makes metals rust. The atmosphere is not good for long-term missions. But primarily, a small spark or a tiny overheat may cause a very rapid and difficult to extinguish fire.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-06-19T14:24:21.657" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9583" PostTypeId="2" ParentId="9563" CreationDate="2015-06-19T14:34:30.890" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Where did all the energy used to circularize the orbit vanish to?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Not much of fuel/energy is needed to circularize at the edge of the SoI. So the difference is not that big.&lt;/li&gt;&#xA;&lt;li&gt;If the fuel is burned and left at periapsis, it has much less energy, then burned fuel left at the apoapsis. The energy spent during circularization goes mostly to both kinetic and potential energy of the used propelant.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10534" LastActivityDate="2015-06-19T14:34:30.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9584" PostTypeId="1" AcceptedAnswerId="9587" CreationDate="2015-06-19T15:18:01.797" Score="7" ViewCount="1205" Body="&lt;p&gt;In reading a question over on Physics.SE (I can't quite remember what it was about, something to do with rocket stages and launching over water) I came across a link to this picture:- &lt;img src=&quot;https://i.stack.imgur.com/lihP3.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over on the west coast there is a shaded area dubbed 'ET Disposal Problem'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first guess I would say it's an area in which a jettisoned external tank would have a higher chance, than &quot;usual&quot;, of hitting a populated area/land in general.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I correct? If so is there anything else notable about it?&lt;/p&gt;&#xA;" OwnerUserId="3384" LastActivityDate="2015-06-19T16:38:05.017" Title="What is the 'ET Disposal Problem'?" Tags="|launch|launch-site|external-tank|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9585" PostTypeId="2" ParentId="9584" CreationDate="2015-06-19T16:07:42.540" Score="2" Body="&lt;p&gt;I believe you are right, it is the External Tank. Why does it have problems? The external tanks were deposited in either the Indian or Pacific Oceans, clear across the world. So, what would that do for a low southward inclination launch from Vandenburg? It would cut across South America, then over Africa, the Middle East, and Russia, with only a brief stay over the Atlantic and Pacific oceans. Basically, there wouldn't be any significant gaps at all in the land mass, and so de-orbiting it would be difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A higher inclination allows for dropping it over the Antarctic region, or the Indian Ocean&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-19T16:07:42.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9587" PostTypeId="2" ParentId="9584" CreationDate="2015-06-19T16:38:05.017" Score="10" Body="&lt;p&gt;This was when planning safe disposal of the External Tank on certain types of launch abort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015564.pdf&quot;&gt;Space Shuttle Abort Evolution&lt;/a&gt; page 9-10&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A major design activity was conducted preparing for Shuttle launches on the West coast from Vandenberg AEB. Though not flown due to new program directions following the Challenger accident, a lot of design work&#xA;  and analysis was done. The launch abort environments were significantly different than those for the East coast launches and required a lot of changes for the aborts. Therefore the launch aborts for the initial WTR launch were designed to be benign with considerable margin. Instead of returning to the runway at Vandenberg AFB, the RTLS was designed to return to Edwards AFB where the lakebed offered multiple runway choices for surprises. This introduced a problem with an unacceptable ET disposal on land following MECO. A dogleg trajectory was inserted to steer the ET out in the Pacific Ocean prior to MECO to avoid disposal on a populated&#xA;  area. Then bank the Orbiter to glide in to EAFB.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In other words, if an engine failed early the plan for an &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Return_To_Launch_Site_.28RTLS.29&quot;&gt;RTLS abort&lt;/a&gt; was to turn around after SRB burnout, use the remaining fuel to aim the shuttle at a suitable landing field, and drop the External Tank.  For Vandenburg launches, this additional step of targeting the tank safely was done.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-06-19T16:38:05.017" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9589" PostTypeId="1" CreationDate="2015-06-17T23:41:13.200" Score="2" ViewCount="244" Body="&lt;p&gt;I know this is a weird question. I can't find a radiation hardened microcontroller (μC) that does &lt;a href=&quot;https://en.wikipedia.org/wiki/Bluetooth_low_energy&quot; rel=&quot;nofollow&quot;&gt;Bluetooth Low Energy&lt;/a&gt; or even WiFi. Is there any reason this would not work on Mars, if a radiation hardened μC pops up?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize there is good chance no one would take me seriously, but a Mars settlement with local WiFi, talking through a gateway to devices scattered outside. The devices would measure things like temperature or methane concentration. The idea is they could be scattered outside and form a network of sorts.&lt;/p&gt;&#xA;" OwnerUserId="10546" OwnerDisplayName="BSEE" LastEditorUserId="49" LastEditDate="2015-06-19T20:53:06.690" LastActivityDate="2015-06-19T23:48:55.723" Title="Is it possible to make an Internet of Things device that works locally on Mars?" Tags="|mars|communication|radiation|feasibility|electronics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9590" PostTypeId="2" ParentId="9589" CreationDate="2015-06-18T05:28:21.220" Score="3" Body="&lt;p&gt;Commercial off the shelf radios have already been used on  Mars, to connect a small rover to its lander. From &lt;a href=&quot;http://mars.nasa.gov/MPF/rovercom/radio.html&quot; rel=&quot;nofollow&quot;&gt;this site&lt;/a&gt; about the Mars pathfinder mission :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The radios that are used in the Microrover telecommunications system were purchased from Motorola's Paging Products Division. Several components that were designed and used in these radios were made by a company named DataRadio. These are off-the-shelf commercial radio modem's (modulator+demodulator) that were modified to meet the communication needs of the Microrover mission. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So for local comms, not critical to life, you might just use regular wifi chips. The challenge will be the cold, not the radiation.&lt;/p&gt;&#xA;" OwnerUserId="35829" OwnerDisplayName="tomnexus" LastActivityDate="2015-06-18T05:28:21.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9591" PostTypeId="2" ParentId="9589" CreationDate="2015-06-19T22:08:09.197" Score="4" Body="&lt;p&gt;Sure, as we have seen from the other answer and from documents like &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130011281.pdf&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt;, we can see that not only are wireless devices in high-radiation environments feasible, but they offer quite a long list of benefits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you really think about it, you will realize that any device humans have sent into space whether the device is an orbiter, satellite, rover, etc, they are all equipped with an antenna and an RF transceiver so they are capable of receiving and transmitting data. We already have our very own &quot;IoT&quot; consisting of these devices.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is, however, an issue. IoT, or &quot;Internet of Things&quot; is a term that is commonly misused. An IoT device is a piece of embedded hardware that provides an interface to access its data over the internet. What you seem to be referring to is what is called a &quot;mesh&quot; network. This is a very important distinction to make.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many topologies and various technologies that can be used to create a mesh network. 802.11 (aka WiFi) is a good example, but there are other networks such as 802.15.4 (commonly known as Zigbee). In fact, the paper that I linked talks about using 802.11 devices in a mesh network on the surface of Mars in real-time while in orbit around Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The distinction between a mesh network and &quot;the internet&quot; is very important here because the designers of the mesh network in the aforementioned paper are not required to make that network accessible by someone on the Earth's internet. Doing something like this on Earth would be fairly trivial, but from Mars it would be a completely different story.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hardware necessary to make each device accessible from the Earth would be large, costly, and consume gobs of power. Also, consider the possibility for vulnerabilities in the code that runs on each device, you wouldn't want some miscreant to connect to it and cause some untold havoc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, consider this: the latency from Mars to the Earth is bottlenecked by the speed of light. Depending on the distance between Mars and the Earth, you could be looking at a latency of 14 minutes (this is an average value, but you could see times between 4 and 22 minutes one-way, double that for round-trip time - both direct line of sight when Mars is not in solar conjunction with the Earth which is just about to happen this year, roughly between June 7 to June 21, 2015). This means that if you sat down and opened your laptop in the Mars settlement and tried connecting to &lt;code&gt;google.com&lt;/code&gt;, it would take several &lt;em&gt;minutes&lt;/em&gt; just to connect.&lt;/p&gt;&#xA;" OwnerUserId="630" LastEditorUserId="49" LastEditDate="2015-06-19T23:48:55.723" LastActivityDate="2015-06-19T23:48:55.723" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9593" PostTypeId="2" ParentId="9454" CreationDate="2015-06-19T22:52:06.667" Score="2" Body="&lt;p&gt;Combining the data from the different filters into a color image is complicated by the fact that because the filter has to be changed before each image is taken, the probe moves enough while that happens that the perspective in the next image isn't quite the same. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get the color image, several frames, from filters for different wavelengths of light, have to be aligned and then combined. This is tricky because the perspective has changed, so some manipulation of the image has to occur to try to compensate for that as best as possible. It takes time, and if the image that would come from it is of no scientific interest, and wouldn't be pretty enough to be interesting to the public, it is just a low priority.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-06-19T22:52:06.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9594" PostTypeId="2" ParentId="9575" CreationDate="2015-06-19T23:45:58.370" Score="7" Body="&lt;p&gt;&lt;em&gt;Is&lt;/em&gt; there a minimum diameter?  Yes.  Every study I've come across agrees on that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is that minimum diameter?  As I found &lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/1445/what-minimal-radius-is-needed-for-rotation-to-simulate-gravitation-without-avers/2462#2462&quot;&gt;while answering this question over on WorldBuilding.SE&lt;/a&gt;, there's quite a bit of disagreement there.  Allowable minimum diameters vary from as high as &lt;a href=&quot;http://www.spacefuture.com/archive/inhabiting_artificial_gravity.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;180 km&lt;/a&gt; to as low as &lt;a href=&quot;http://adsabs.harvard.edu/full/2002ESASP.501..151H&quot; rel=&quot;nofollow noreferrer&quot;&gt;8 meters&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:23.630" LastActivityDate="2015-06-19T23:45:58.370" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9596" PostTypeId="2" ParentId="9575" CreationDate="2015-06-20T04:58:26.603" Score="10" Body="&lt;p&gt;The problem is about a so called 'comfort zone'. In fact it is not radius but a ratio of radius and angular velocity. Nausea or motion sickness is caused mostly by Coriolis acceleration which makes 'artificial gravity' different form nearly homogeneous earth gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a real expert on topic &lt;a href=&quot;http://www.twhall.com/&quot; rel=&quot;noreferrer&quot;&gt;Theodore W. Hall&lt;/a&gt;. &#xA;Among a lot of staff about artificial gravity he published an &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.artificial-gravity.com/sw/SpinCalc/SpinCalc.htm&quot; rel=&quot;noreferrer&quot;&gt;online calculator&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which may provides the best answer of the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vSv7z.png&quot; alt=&quot;comfort zone of artificial gravity&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Direct answer if your question is R&amp;lt;12 meters or &amp;lt;60 m for 1 g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, it is what the quality Sci-Fi movies lake 'Odyssey 2001' depicted. Arthur Clark and Stanley Kubrick have had some info on topic.&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="10218" LastEditDate="2015-06-21T18:59:14.733" LastActivityDate="2015-06-21T18:59:14.733" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9598" PostTypeId="1" AcceptedAnswerId="9600" CreationDate="2015-06-20T10:51:41.290" Score="2" ViewCount="5673" Body="&lt;p&gt;I'm creating a space exploration game in Unity 3D, written in C# and I'm currently working on a system where I plot the new position of a celestial object as a function of time. This way, I don't have to constantly calculate the position of an object, but I can use the time that has passed since the player has started playing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have an adequate understanding of Kepler's three laws of planetary motion and the six orbital elements. I can also calculate or randomly generate these elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is true anomaly, as it does not change with a constant value like in a circular orbit. I need to calculate the true anomaly for my time problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand the main equations behind this element and why it is neccesary to use the mean anomaly and the eccentric anomaly. But here is where my limited knowledge of math leaves me stranded. I can not figure out how to calculate the mean anomaly and use the eccentric anomaly to reach the true anomaly. I've used many sources: &lt;a href=&quot;http://www.braeunig.us/space/orbmech.htm#position&quot; rel=&quot;nofollow&quot;&gt;Basics of orbital mechanics&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/watch?v=cf9Jh44kL20&quot; rel=&quot;nofollow&quot;&gt;Youtube&lt;/a&gt; etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The part where I get stuck is where I need to calculate the eccentric anomaly from the mean anomaly. I can not find any sources online that explain how to do this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone explain how this works?&lt;/p&gt;&#xA;" OwnerUserId="9032" LastEditorUserId="9032" LastEditDate="2015-06-20T14:22:57.823" LastActivityDate="2015-06-20T16:09:21.690" Title="Calculating true anomaly of a celestial object" Tags="|orbital-mechanics|orbit|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9600" PostTypeId="2" ParentId="9598" CreationDate="2015-06-20T15:33:46.940" Score="6" Body="&lt;p&gt;The reason for using mean anomaly is that the time behavior of mean anomaly is very simple: It's linear. In other words, $M(t) = M(t_0) + n(t-t_0)$ where $n$ is the &quot;mean motion&quot; In units of radians per time unit, mean motion is given by $n = \sqrt{G(m_1+m_2)/a^3}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kepler's equation, $M=E-e\sin E$, tells how to calculate the mean anomaly $M$ given the eccentricity $e$ and the eccentric anomaly $E$. This is essentially a function definition, $M(E,e)$. You need to invert this to calculate the eccentric anomaly given the mean anomaly and the eccentricity. This, too, is a function, $E(M,e)$. While $M(E,e)$ is readily expressed in terms of the elementary functions, $E(M,e)$ is not. In fact, it is provably not invertible in terms of the elementary functions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solving for $E$ given $M$ and $e$ is sometimes called Kepler's problem. The standard approach nowadays is to use a Newton-Raphson iterator. Start by placing the mean anomaly calculated via $M=M_0+n(t-t_0)$ to a value between 0 and $2\pi$. Next, make an initial guess regarding the eccentric anomaly. If the eccentricity is high, the initial guess should be $E_0=\pi$. Otherwise, start with $E_0=M$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now start iterating using a Newton-Raphson iterator. Given a guess $E_{i}$ to the eccentric anomaly, the next guess is given by a the Newton-Raphson iterator&#xA;$$E_{i+1} = E_i - \frac{E_i-(M+e\sin E_i)}{1-e\cos E_i}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The iteration is done when the correction term (the second term on the right hand side) becomes small.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-06-20T16:09:21.690" LastActivityDate="2015-06-20T16:09:21.690" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9601" PostTypeId="1" AcceptedAnswerId="9609" CreationDate="2015-06-20T18:23:38.753" Score="19" ViewCount="1989" Body="&lt;p&gt;Soyuz descent module re-entry flight is piloted (one of two modes AUS/computer or RUS/manual), using 8 thrusters, toward the Kazakhstan landing site. The ballistic re-entry occurs only when the other modes are inoperative (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-11#Ballistic_reentry&quot; rel=&quot;noreferrer&quot;&gt;TMA-11&lt;/a&gt;) or in case of an urgent departure from the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a controlled descent, the actual landing location should be equally distributed around the target point, but it seems most of the landings occur N or NE of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GlL2E.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;(&lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=19141.0;all&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typical flight path of the re-entering spacecraft:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/BOd7V.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;(TMA-21, &lt;a href=&quot;http://www.spaceflight101.com/soyuz-tma-21-landing1.html&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As pointed out by &lt;strong&gt;TildalWave&lt;/strong&gt;, the winds may cause the module to drift after the parachute is deployed. From the re-entry profile described at &lt;a href=&quot;http://www.spaceflight101.com/soyuz-spacecraft-information.html&quot; rel=&quot;noreferrer&quot;&gt;SpaceFlight-101&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At an altitude of about 9 Kilometers, the Pilot Chute opens and&#xA;  deploys the Drogue Chute that slows the vehicle from 240 meters per&#xA;  second to 90m/s. At an altitude of 7.5 Kilometers, the Main Chute is&#xA;  opened and slows the vehicle down to 6 meters per second. While flying&#xA;  under the main chute, the Soyuz transitions from a nearly horizontal&#xA;  flight to a vertical descent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This gives about 8 kms of trajectory at low vertical speed (12 kts / 22 km/h) and high drag configuration where the module may drift. About the wind, as researched by &lt;strong&gt;Rikki-Tikki-Tavi&lt;/strong&gt;, the prevailing wind blows to NE in Kazakhstan. The wind blows most of the time to NE and E, according to &lt;a href=&quot;https://weatherspark.com/averages/33762/Kyzylorda-Kyzylorda-Province-Kazakhstan&quot; rel=&quot;noreferrer&quot;&gt;WeatherSpark&lt;/a&gt; for Kyzylorda Airport WX station:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7AZmS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first sight, a correlation is possible. Buy why would the Russian Federal Space Agency ignore wind in the flight planning? Maybe this is their reasonning:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The aircraft departure form the ISS is possible only when the wind is within given acceptable criteria which guarantee a drift cannot push the spacecraft outside a predefined safe area.&lt;/li&gt;&#xA;&lt;li&gt;A return schedule and flight path is computed without wind and published.&lt;/li&gt;&#xA;&lt;li&gt;Soyuz is deorbited according to this schedule.&lt;/li&gt;&#xA;&lt;li&gt;Some drift occurs due to the actual wind, the rescue team follows.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are other possible explanations, e.g. this one:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A delay occurs in the re-entry schedule.&lt;/li&gt;&#xA;&lt;li&gt;The predicted landing location is adjusted, but not shared with the public.&lt;/li&gt;&#xA;&lt;li&gt;The landing actually occurs at the adjusted location, but we don't know.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Neither of these scenario is really satisfactory. &lt;em&gt;Is it possible to shed light on the way the re-entry path is managed, and whether the wind is taken into account?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-06-23T22:02:22.720" LastActivityDate="2020-06-26T14:15:48.340" Title="Why do Soyuz landings seem to be overshot?" Tags="|landing|soyuz-spacecraft|crewed-spaceflight|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9602" PostTypeId="1" CreationDate="2015-06-20T18:47:39.350" Score="7" ViewCount="3699" Body="&lt;p&gt;&lt;a href=&quot;http://www.boeing.com/assets/pdf/defense-space/space/spacestation/systems/docs/ISS%20Electric%20Power%20System.pdf&quot; rel=&quot;noreferrer&quot;&gt;This document&lt;/a&gt; does a pretty decent job of detailing the ISS solar array. There's one part, however, that is sort of ambiguous and critical to an analysis I'm performing, and so I thought I might ask about it here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On page 11, paragraph 1, the Integrated Equipment Assembly (IEA) for the photovoltaic modules is described. The first part of the paragraph says that there are four of these IEAs, located at P6, P4, S4, and S6 on the main truss. It then goes on to describe all of the subassemblies which compose the IEA. However, the ambiguity appears when describing the dimensions of the IEAs:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The IEA measures 16 feet (4.9 meters) by 16 feet (4.9 meters) by 16 feet ((4.9 meters), weighs nearly 17,000 pounds a (7,711.1 kilograms) and is designed to condition and store the electrical power collected by the photovoltaic arrays for use on board the Station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So is EACH IEA a cube 5 meters on a side weighing 7.7 metric tons, or are ALL FOUR of these IEAs, in total, these dimensions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total mass of the ISS is ~390 metric tons, and the radiators are huge, so the total mass of these four IEAs COULD weigh ~30 metric tons in total. &lt;/p&gt;&#xA;" OwnerUserId="8962" LastEditorUserId="8962" LastEditDate="2015-06-21T21:50:03.300" LastActivityDate="2021-04-05T07:57:32.047" Title="Total Mass of the ISS Solar Array" Tags="|iss|power|mass|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9603" PostTypeId="2" ParentId="9563" CreationDate="2015-06-20T19:00:24.037" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Where did all the energy used to circularize the orbit vanish to?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose a spacecraft with mass $m$ performs an instantaneous (either impulsive or infinitesimal) $\Delta v$ at some instant along its orbit by ejecting some quantity of matter $\Delta m$ at some velocity $\vec u$ relative to the spacecraft. The quantity of ejected matter is finite in the case of an impulsive burn, infinitesimal in the case of an infinitesimal burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of an infinitesimal burn, the change in velocity of the spacecraft is frame-independent, given by $\Delta \vec v = -\frac{\Delta m} m \vec u$. This ultimately leads to the ideal rocket equation. The change in energy of the spacecraft+exhaust cloud system is also frame-independent, $\Delta E_\text{sys} = \frac 1 2 m\,\Delta m \, u^2$. The quantity of chemical potential energy released by burning that fuel (less thermodynamic inefficiencies) dictates the change in mechanical energy of the spacecraft+exhaust cloud system. Note: The above applies to infinitesimal burns. In the case of an impulsive burn, the change in velocity and in system energy are also frame-independent, just a bit messier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How this change in the total mechanical energy of the vehicle plus exhaust cloud system is partitioned between the vehicle and the exhaust cloud is frame-dependent. Comparing identical burns at periapsis and apoapsis, more energy gets transferred to the vehicle when the vehicle is at periapsis than when it is at apoapsis. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-06-20T19:00:24.037" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9604" PostTypeId="2" ParentId="9601" CreationDate="2015-06-20T19:08:02.373" Score="14" Body="&lt;p&gt;They are not targeting what they think they are targeting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure that there is a logical explanation (I don't think it's chance).  However they apparently don't know the explanation.  If they did, they would include it in their simulations and they would then be actually targeting what they think they're targeting, and the errors would be distributed more evenly around the target.  I don't know the explanation either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though much smaller in number, I will note that our Mars landings tend to be long as well.  That may be chance.  If it isn't chance, then there is a systematic error there as well.  Probably not the same systematic error as Soyuz.  (All Mars entries except for MSL were ballistic.  MSL was guided, like Soyuz.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From there, all you can do is speculate.  I'm sure that they do a very thorough job simulating all of the obvious things, so most of our speculations would be wrong.  If we can think of it, chances are it's in their simulations with appropriate dispersions.  Atmosphere density variations, wind, gravity field, aerodynamic coefficients of the entry vehicle, entry flight path angle, etc.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-20T19:08:02.373" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9606" PostTypeId="2" ParentId="9578" CreationDate="2015-06-20T19:57:52.273" Score="4" Body="&lt;p&gt;tl;dr answer, and my math might be a little off, but here goes:&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hill Sphere&lt;/a&gt; is an approximation which takes just 4 things into account:&lt;/p&gt;&#xA;&lt;h3&gt;&lt;span class=&quot;math-container&quot;&gt;$$r \approx a (1-e) \cdot \sqrt[3\ \ \ ]{\frac{m}{3 M}}$$&lt;/span&gt;&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the &lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-major_axis&quot; rel=&quot;nofollow noreferrer&quot;&gt;Semi Major Axis&lt;/a&gt; &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;li&gt;the eccentricity of the orbit &lt;span class=&quot;math-container&quot;&gt;$e$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;li&gt;and the mass of the 2 objects, in this case the Earth and the Moon.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The Hill Sphere formula doesn't, by the way, take into account gravitational imperfections due to varied density, so it doesn't rise and fall with every mountain on Earth and it approximates a radius, which by definition, represents a perfect sphere, so, the Hill Sphere is a sphere, calculated by a formula. By definition, as David Hammen said.&lt;/p&gt;&#xA;&lt;p&gt;So ...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;...because I have a budding sense it's not quite spherical. Or at&#xA;least the center isn't equal to the body's center.   For one thing, at&#xA;the distant end (say, in case of Moon's Hill Sphere, the point&#xA;opposite from Earth) Earth's gravity is weaker than on the Earth side&#xA;(due to the distance), meaning Moon's sphere of influence should reach&#xA;farther.  Or in case of certain elliptic polar orbits (similar to&#xA;Sun-synchronous Earth orbit) they could reach much farther as the&#xA;distance from &amp;quot;the big body&amp;quot; doesn't change by much, and the smaller&#xA;body's gravity should be able to pull the satellite back from a quite&#xA;a distance &amp;quot;above&amp;quot; or &amp;quot;below&amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;What you're describing works exactly like that with planetary magnetic fields. It doesn't work with gravitational orbits hardly at all because orbits don't switch from being stable to unstable as they go from the Earth side of the Moon to the far side of the Moon. The entire orbit is considered one or the other, though in a certain sense, all orbits are ultimately unstable given enough time, so there's some fuzziness to those definitions as well.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;So, what's the shape of the Hill Sphere? And what magnitude of&#xA;eccentricity/irregularity in its shape can be expected?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Well, we know the Hill Sphere is a perfect sphere, and what we might call a region of orbital stability, generally considered 1/3rd to 1/2th the Hill Sphere's radius (also in the Wikipedia article), that's pretty much a perfect sphere too, though it's also not definable with precision.&lt;/p&gt;&#xA;&lt;p&gt;But, is there any form of gravitational influence that is stretched on the far side and compacted on the near side. Maybe. Any objects gravitational influence pretty much extends to any distance at the speed of light, so gravitational influence doesn't work either, but lets try this;&lt;/p&gt;&#xA;&lt;p&gt;Lets make up a term and call it gravitational capture zone, or capture zone for short, which I'll define by a region where a planet can capture an object, usually an asteroid or comet that flies close to it, and lets define this as a situation where the object has to complete at least 1 full, 360 degree orbit. One is enough. Now, obviously, capture also depends on the speed of the asteroid or comet, so this is kind of rough, but the Earth does have, in a sense, a capture zone - so does Mars, in fact, Mars' 2 moons are believed to be captured asteroids. In fact, all the outer planets have captured asteroids orbiting them as moons.&lt;/p&gt;&#xA;&lt;p&gt;Another way of looking at this, is, an asteroid that flies at a set speed, 1,000 km above the Moon's surface will be influenced by the Moon's gravity more than the Earth's, at least, when it flies that close (before it gets that close, it's more affected by the Earths - but the math gets complicated).&lt;/p&gt;&#xA;&lt;p&gt;So, an asteroid flying 1,000 km above the Moon on the far side of the Earth would bend more than an asteroid flying 1,000 km above the Moon but in between the Moon and the Earth, where, in that position, the Moon's and Earth's gravity on the asteroid would be pulling in opposite directions - much less bend, and if we take that in mind, the influence where the Moon might capture an asteroid is greater on the away side of Earth than the near side of the Earth.&lt;/p&gt;&#xA;&lt;p&gt;So, lets return to Hill sphere, and I'm going to simplify the math by ignoring eccentricity and assuming circular orbits - from the same Wikipedia link:&lt;/p&gt;&#xA;&lt;h3&gt;&lt;span class=&quot;math-container&quot;&gt;$$r \approx a \cdot \sqrt[3\ \ \ ]{\frac{m}{3M}}$$&lt;/span&gt;&lt;/h3&gt;&#xA;&lt;p&gt;Lets do the Earth-Sun first;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; (semi major axis, or, lets just call it distance), and I hate math so we'll call &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; 1, for 1 AU, and mass, lets call little &lt;span class=&quot;math-container&quot;&gt;$m$&lt;/span&gt; 1 and big &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; 333,000 because the Sun is 333,000 times more massive than the Earth:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$1 AU \cdot \sqrt{1/333,000\cdot3\ }$&lt;/span&gt; is just about 1/100th of an AU, so from the near end of the Earth's Hill Sphere to the Sun and the far end away from the Sun, the gravitational effect of the Sun at 0.99 AU to 1.01 AU (we're basically just talking tidal effects at this point), but, gravity decreases by the square of the distance, so this is about a 4% change in the Sun's gravitation from the far end to the near end of the Earth's Hill sphere.&lt;/p&gt;&#xA;&lt;p&gt;The sphere is still a sphere, but the Earth should be slightly more able to capture an asteroid on it's far side from the Sun than the near side. In that sense, the term I made up, capture zone, would be stretched by a little bit. 4% might be upper limit of the stretching and it's probably less than that, because a gravitational capture zone would be smaller than the Hill Sphere. (if somebody wants to correct that, feel free, but this is ballpark.)&lt;/p&gt;&#xA;&lt;p&gt;The Earth actually does this, by the way - capture asteroids. The Moon probably doesn't, because it's gravity is too weak and comparatively, asteroids move too fast. Asteroids captured by the Earth usually don't stick around very long though:&lt;/p&gt;&#xA;&lt;p&gt;Source:  &lt;a href=&quot;http://gizmodo.com/5869840/earth-has-a-second-moon-astronomers-say&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://gizmodo.com/5869840/earth-has-a-second-moon-astronomers-say&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Now, lets look at the Earth-Moon system. The Earth weighs approximately 81 times as much as the Moon, so the Hill Sphere of the Moon is &lt;span class=&quot;math-container&quot;&gt;$1/\sqrt{81*3}$&lt;/span&gt;, or 16% of the distance between the Earth and the Moon. The Moon's theoretical gravitational capture zone would be significantly more elongated around it's far side than it's Earth side. The elongation of a capture zone depends on the ratio of size between the 2 objects, so the Sun being comparatively so large, makes Earth's &amp;quot;capture zone&amp;quot; much closer to a sphere, not much more stretched out as you suggest.&lt;/p&gt;&#xA;&lt;p&gt;Now, in reality, the Moon's Hill sphere is all but irrelevant, because there is no stable orbit around the Moon due to tidal effects from the Earth and to a lesser extent, orbital perturbation from the Sun. But if the Moon was to capture an asteroid - that would need to just about exactly match the Moon's speed, in that hypothetical scenario, I think the Moon would be more likely to capture an asteroid on it's far side than it's near side - so there is in a sense, an elongation for temporary capture of a passing object.&lt;/p&gt;&#xA;&lt;p&gt;But this is all quite vague, because anything that orbits the Moon also orbits the Earth, and any elongation of this theoretical capture zone is pretty irrelevant in terms of stable orbits, because a stable orbit has to account for all parts of the orbit, not just the part where the tidal effect is the lowest. So, I don't think it's a good way of looking at orbital mechanics, but I think you're correct in thinking there's something about a planet's gravitational sphere of influence not being quite spherical.&lt;/p&gt;&#xA;&lt;p&gt;Loosely related on Moon's capturing asteroids:  &lt;a href=&quot;http://www.universetoday.com/109666/can-moons-have-moons/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.universetoday.com/109666/can-moons-have-moons/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8711" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-05-26T21:54:56.863" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="9607" PostTypeId="1" CreationDate="2015-06-20T21:49:41.980" Score="5" ViewCount="703" Body="&lt;p&gt;When we look at larger objects (the Moon, Mars, Earth), they are pulled into spherical shapes by their own gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question I have relates to smaller objects... those close to the threshold at which gravity pulls them into spherical shapes. Early in their evolution, they are irregular shapes. As more material accretes, something happens - perhaps somewhere between the size of &lt;a href=&quot;https://en.wikipedia.org/wiki/4_Vesta&quot; rel=&quot;noreferrer&quot;&gt;Vesta&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)&quot; rel=&quot;noreferrer&quot;&gt;Ceres&lt;/a&gt; - even while the surface gravity is tiny compared to Earth - the accretion process appears to produce a spherical body. Why doesn't a body need to get much bigger - say Moon sized, for that to happen? What is going on that is already taking hold at that size?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-08-26T11:50:08.693" Title="Why are smaller celestial bodies like Ceres spherical?" Tags="|gravity|planetary-science|asteroid|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9609" PostTypeId="2" ParentId="9601" CreationDate="2015-06-20T23:53:06.023" Score="10" Body="&lt;p&gt;Prevailing winds in Kazakhstan seem to be in a north-eastern direction. One factor may be that they are under-compensated in the targeting. Why, I cannot say. I think it is possible, though that the Russians are following a &quot;never change a running system&quot; approach here. Meaning they don't want to spend the money and manpower on fixing a problem that isn't more than a minor inconvenience to them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iD0Jy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2015-06-20T23:53:06.023" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9610" PostTypeId="1" CreationDate="2015-06-21T13:41:23.220" Score="8" ViewCount="941" Body="&lt;p&gt;The Hohmann Transfer is clear enough; Accelerate on your (circular) orbit at the right time, raising the apoapsis until it meets the target orbit. Once there, burn again to raise the periapsis - matching the target orbit, and as result speed of your target. If you picked the moment right, you're really close to the target, and ready for final approach and proximity operations. The only really tricky part is finding the right moment to start, so that the moment you're at your new periapsis the target is there too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, how is the orbital transfer performed if raising the apoapsis can't be easily done in one short, neat burn? Say, you use a ion engine that will need two hours to create the delta-v necessary for the Hohmann transfer orbit, and your whole orbital period currently is forty minutes?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="49" LastEditDate="2015-06-21T14:15:38.613" LastActivityDate="2015-06-21T15:15:09.840" Title="Orbital rendezvous with low-thrust engines?" Tags="|orbital-mechanics|orbital-maneuver|rendezvous|hohmann-transfer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9611" PostTypeId="2" ParentId="9610" CreationDate="2015-06-21T15:15:09.840" Score="8" Body="&lt;p&gt;You can change orbits by slowly spiraling out, but then it isn't a Hohmann transfer anymore. In the extreme case of very low thrust, this will mean your orbit is almost circular at all times. You'll also burn more delta-v than with the Hohmann transfer, because on average you add orbital energy while your speed is lower.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Hohmann_transfer_versus_low_thrust_orbits&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; has this to say:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It can be shown that going from one circular orbit to another by gradually changing the radius costs a delta-v of simply the absolute value of the difference between the two speeds....&lt;/p&gt;&#xA;&lt;p&gt;Such a low-thrust maneuver requires more delta-v than a 2-burn Hohmann transfer maneuver, requiring more fuel for a given engine design. However, if only low-thrust maneuvers are required on a mission, then continuously firing a low-thrust, but very high-efficiency (high effective exhaust velocity) engine might generate this higher delta-v using less propellant mass than a high-thrust engine using an otherwise more efficient Hohmann transfer maneuver.&lt;/p&gt;&#xA;&lt;p&gt;Transfer orbit using Electrical Propulsion or Low Thrust enginees optimize the transfer time to reach the final orbit and not the delta-v as in the Hohmann transfer orbit. For geostationary orbit, the initial orbit is set to be supersynchronous and by thrusting continuously in the direction of the velocity at Apogee, the transfer orbit transforms to a circular geosynchronous one. This method however takes much longer to achieve due to the low thrust injected into the orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10547" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-06-21T15:15:09.840" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9612" PostTypeId="1" AcceptedAnswerId="9613" CreationDate="2015-06-21T20:22:03.160" Score="8" ViewCount="1574" Body="&lt;p&gt;Mars has only &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Mars&quot; rel=&quot;noreferrer&quot;&gt;0.6%&lt;/a&gt; the (surface) atmospheric pressure of Earth. Does this have a significant impact on the difficulty (heat shield parameters) of atmospheric entry? Is it basically the same as for Earth because most of the kinetic energy must be dissipated in the uppermost low pressure regions of any atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="716" LastEditorUserId="49" LastEditDate="2015-06-22T00:26:33.633" LastActivityDate="2015-06-22T00:26:33.633" Title="Difference in atmospheric entry for Earth and Mars" Tags="|mars|heat-shield|reentry|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9613" PostTypeId="2" ParentId="9612" CreationDate="2015-06-21T23:42:44.307" Score="14" Body="&lt;p&gt;If you're only referring to surviving the phase of peak heating, then Mars is easier than Earth.  The entry velocity is more like 6 to 7 km/s at Mars vs. 11 to 12 km/s at Earth, due to their respective gravity and typical approach velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everything else about landing on Mars is harder, due to the low density of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The density can be a problem even as early as the peak-heating phase.  While the heat shielding is easier, you still have to make sure that that the peak-heating phase takes place &lt;em&gt;above&lt;/em&gt; the surface.  If the ballistic coefficient of the vehicle is high enough, you may have a problem with that.  You may need, for example, a lifting body to keep it from experiencing a short-lived lithobraking phase.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-21T23:42:44.307" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9614" PostTypeId="1" AcceptedAnswerId="9634" CreationDate="2015-06-22T01:05:31.363" Score="5" ViewCount="355" Body="&lt;p&gt;Suppose I want to climb the tallest mountain on each of the four Galilean satellites. Is it known what they are?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Io has one of the most spectacularly tall mountains in the solar system, Boösaule Montes, at about 18 km from base to peak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But googling turns up almost nothing specific on Europa, Ganymede, and Callisto. I'm having a hard time figuring out whether this is just because they don't have as much surface relief, so they aren't as much fun to discuss, or if there is actually no data on their topography. How do we know, for example, how tall Boösaule Montes is? Since we've never inserted anything into orbit around these bodies, I assume there is no radar data. Is our knowledge from estimates based on the lengths of shadows?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guesses were that since Callisto and Ganymede are heavily pocked with craters, their highest mountains would be the ramparts of crater rims. Maybe on Callisto it would be one of the concentric rims of the Valhalla system. Europa is thought to be extremely smooth, which might make it difficult to determine much about its topography.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2023-01-24T20:49:55.813" Title="Tallest mountains on the Galilean satellites?" Tags="|jupiter|moons|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9615" PostTypeId="1" CreationDate="2015-06-22T07:53:33.277" Score="3" ViewCount="93" Body="&lt;p&gt;A &quot;clipper&quot;, as I understand it, is orbiting a planet to repeatedly flyby one of its moons. Mariner 10 orbited the Sun to fly by Mercury 3 times. Should clippers in general use retrograde orbits in order to get more frequent flybys (or &quot;clips&quot;)? Or are instead slower flybys prefered?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-06-22T10:18:44.723" Title="Should a Europa clipper use a retrograde orbit?" Tags="|probe|moons|flyby|retrograde-orbit|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9616" PostTypeId="2" ParentId="9615" CreationDate="2015-06-22T09:01:40.443" Score="2" Body="&lt;p&gt;The purpose of most missions are to study planetary bodies in detail, taking images, using radar or lasers for ranging and mapping, using gravimetric instruments to measure composition, etc. All these types of activities benefit from slower relative movement between the spacecraft the the body being studied. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be some mission profiles where more frequent flybys would be better than longer lasting flybys, although I can't think of any at the moment. Of course it may be that fuel limitations would mean that only a retrograde orbit is possible, in which case you take what you can get. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2015-06-22T10:18:44.723" LastActivityDate="2015-06-22T10:18:44.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9618" PostTypeId="1" CreationDate="2015-06-22T13:21:44.140" Score="2" ViewCount="1766" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Clohessy-Wiltshire_equations&quot; rel=&quot;nofollow&quot;&gt;Clohessy-Wiltshire equations&lt;/a&gt; are a simplified (linearized) model that is applicable under few assumptions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;close range&lt;/li&gt;&#xA;&lt;li&gt;circular target orbit&lt;/li&gt;&#xA;&lt;li&gt;no disturbances&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;My question: Up to which boundaries are these assumptions valid? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In detail:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Up to which relative distance can CW equations be used?&lt;/li&gt;&#xA;&lt;li&gt;Up to which eccentricity?&lt;/li&gt;&#xA;&lt;li&gt;For which maximum time span is a prediction based on CW reasonable?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I guess the answers might not be absolute numbers, but maybe one can say something like: at a relative distance of 10 km the position error is about 10%. Or maybe there is an approximation or formula.&lt;/p&gt;&#xA;" OwnerUserId="10552" LastEditorUserId="49" LastEditDate="2015-06-22T13:54:08.840" LastActivityDate="2015-06-22T13:54:08.840" Title="Limitations of Clohessy-Wiltshire equations" Tags="|orbital-mechanics|rendezvous|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9619" PostTypeId="1" AcceptedAnswerId="9620" CreationDate="2015-06-22T13:23:10.637" Score="1" ViewCount="3200" Body="&lt;p&gt;I'm curious, does the Sun actually move? The concept behind the question is, if you wear a shirt and you move, does the shirt actually move? Like the Solar system orbits around the Milky Way, but does the Sun move?&lt;/p&gt;&#xA;" OwnerUserId="10554" LastEditorUserId="49" LastEditDate="2015-06-22T14:31:31.163" LastActivityDate="2015-06-22T15:56:21.410" Title="Does the Sun actually move on its own, or does it move with the Solar system?" Tags="|physics|the-sun|astronomy|celestial-mechanics|" AnswerCount="1" CommentCount="2" ClosedDate="2015-06-29T03:10:17.510" ContentLicense="CC BY-SA 3.0" />
  <row Id="9620" PostTypeId="2" ParentId="9619" CreationDate="2015-06-22T13:59:44.803" Score="8" Body="&lt;p&gt;This is to do with &lt;a href=&quot;https://www.youtube.com/watch?v=pyBNImQkRuk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Frames of Reference&lt;/a&gt;. The video below gets across nicely that all motion is relative, your perception of it depends on your point of view:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/pyBNImQkRuk?start=33&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Frames of Reference - Hume and Ivey, 1960, Educational Services Inc.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we chose the Sun as the center point of the Universe, we could say everything moves around it - but that doesn't work so well because the movement of stars in our galaxy is much more easily understood if the galaxy's center of gravity is used as the fixed point. Also, stars are massive enough that the ones near each other influence each other's movement a lot and they tend to sort of weave around one another - the study of this is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Stellar_dynamics&quot; rel=&quot;nofollow noreferrer&quot;&gt;stellar dynamics&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Equally, the movement of planets in the Solar system was extremely confusing until we accepted the Sun as the center of motion, and saw it as the &lt;em&gt;local&lt;/em&gt; fixed point, for the Solar system. (Actually that isn't entirely accurate either - the center of gravity of the Solar system is the point everything moves around, it's common barycenter - see &lt;a href=&quot;https://space.stackexchange.com/q/9365/4660&quot;&gt;this question&lt;/a&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-06-22T15:56:21.410" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9621" PostTypeId="1" CreationDate="2015-06-22T15:10:34.670" Score="2" ViewCount="663" Body="&lt;p&gt;When the stability of dual spin satellite with rotor along z axis an additional equation for the relative motion between the rotor and satellite is added to Euler's equation. Similarly on introducing a reaction wheel a relative motion equation is added to the system. If I introduce a reaction wheel to dual spin satellite, does my dynamics equation remain the same for a dual spin or does now my satellite becomes three axis spin stabilized?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: eg. If I have a rotor along $Z$ then I use modified Euler equations with relative dynamics of rotor given by $M_r = I_r \cdot \dot \omega$, and the moment equation given by $I_i \cdot \dot \omega_i + S_{ij} + I_r \cdot \omega_{rk} = M$, where $i,j,k$ represents the corresponding axes, and $S_{ij}$ is the difference of inertia.&#xA;At this point I want to introduce 3 reaction/inertia wheels aligned with each axes. &#xA;The required torque is computed using the standard control equations, which goes into the above mentioned dynamics equation. But at this point does my dynamics equation changes with the introduction of new rotors along each axes or does it stays the same as before.&lt;/p&gt;&#xA;" OwnerUserId="10558" LastEditorUserId="9060" LastEditDate="2015-06-22T17:36:30.887" LastActivityDate="2015-07-02T16:24:27.377" Title="Dual Spin satellite control using momentum exchange" Tags="|artificial-satellite|attitude|mission-control|reaction-wheel|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9623" PostTypeId="1" AcceptedAnswerId="9624" CreationDate="2015-06-22T18:20:02.043" Score="2" ViewCount="511" Body="&lt;p&gt;I already asked here about how to calculate orbits of satellites programmatically then plot them on 2d plane, but my question was too broad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm still on my journey to understand &amp;amp; pull such software out ; I learned that Orbital state vectors are the way to start plotting a certain celestial object's orbit, but I don't quite understand the parameters of the vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Needing R (position) &amp;amp; V (velocity) &amp;amp; T (time), &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since V can be fetched from dividing the differentiation of R/T.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But R is composed of (x,y,z), how can I calculate it with respect to the frame of reference (earth-centered equatorial system) ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I moving on the right track ?&lt;/p&gt;&#xA;" OwnerUserId="10359" LastActivityDate="2015-06-22T18:30:24.533" Title="Orbital state vectors : elaboration needed on the position vector" Tags="|artificial-satellite|software|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9624" PostTypeId="2" ParentId="9623" CreationDate="2015-06-22T18:30:24.533" Score="3" Body="&lt;p&gt;For plotting, it is easiest to compute the positions &lt;em&gt;and&lt;/em&gt; the time as a function of the eccentric anomaly, as opposed to trying to compute position as a function of time.  You can compute the velocity as a function of the eccentric anomaly as well.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-22T18:30:24.533" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9627" PostTypeId="1" CreationDate="2015-06-23T03:00:53.103" Score="14" ViewCount="824" Body="&lt;p&gt;Today's launch of Arianespace Vega rocket deploying Sentinel 2A Earth observation satellite for ESA &amp;amp; European Commission went ahead as planned, lifting off on June 23, 2015 at exactly 01:51:58 UTC from Kourou, French Guiana. Here's a &lt;a href=&quot;https://www.youtube.com/watch?v=AZkVBGCI27c&quot; rel=&quot;noreferrer&quot;&gt;video of it&lt;/a&gt;, in case you missed it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That they proceeded with the launch is however somewhat surprising to me, considering that the launch time coincided with a brewing geomagnetic storm that reached category G4 (&lt;em&gt;severe&lt;/em&gt;) just hours before, hasn't entirely subsided by launch time, and next predicted index for hours ahead was G3 (&lt;em&gt;strong&lt;/em&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4lkUy.gif&quot; alt=&quot;Space Weather Overview&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.swpc.noaa.gov&quot; rel=&quot;noreferrer&quot;&gt;NOAA's Space Weather Prediction Center&lt;/a&gt;, where I acquired the top graphs from, these are descriptions for the &lt;em&gt;24-Hour Observed Maximums&lt;/em&gt; for radio blackouts, solar storm and geomagnetic storm indexes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;G4 (Severe) Geomagnetic Storm Impacts&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Power systems&lt;/strong&gt;: Possible widespread voltage control problems and some protective systems will mistakenly trip out key assets from the&#xA;  grid.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Spacecraft operations&lt;/strong&gt;: May experience surface charging and tracking problems, corrections may be needed for orientation problems.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Other systems&lt;/strong&gt;: Induced pipeline currents affect preventive measures, HF radio propagation sporadic, satellite navigation degraded&#xA;  for hours, low-frequency radio navigation disrupted, and aurora has&#xA;  been seen as low as Alabama and northern California (typically 45°&#xA;  geomagnetic lat.).&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;S3 (Strong) Solar Radiation Storm Impacts&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Biological&lt;/strong&gt;: Radiation hazard avoidance recommended for astronauts on EVA; passengers and crew in high-flying aircraft at high&#xA;  latitudes may be exposed to radiation risk.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Satellite operations&lt;/strong&gt;: Single-event upsets, noise in imaging systems, and slight reduction of efficiency in solar panel are likely.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Other systems&lt;/strong&gt;: Degraded HF radio propagation through the polar regions and navigation position errors likely.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;R2 (Moderate) Radio Blackout Impacts&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;strong&gt;HF Radio&lt;/strong&gt;: Limited blackout of HF radio communication on sunlit side, loss of radio contact for tens of minutes.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Navigation&lt;/strong&gt;: Degradation of low-frequency navigation signals for tens of minutes.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And here's a few more plots from that same source for the relevant date and time (descriptions at &lt;a href=&quot;http://www.swpc.noaa.gov/communities/space-weather-enthusiasts&quot; rel=&quot;noreferrer&quot;&gt;the source&lt;/a&gt;, click for higher resolution images):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MI1ej.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MI1ejm.gif&quot; alt=&quot;3-Day Satellite Environment&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/q9n2H.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/q9n2Hm.png&quot; alt=&quot;D Region Absorption Prediction&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/PEwQ7.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PEwQ7m.gif&quot; alt=&quot;ACE Mag and SWEPAM&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/Rgtet.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Rgtetm.gif&quot; alt=&quot;USAF Wing KP Prediction Activity Index&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next predicted geomagnetic storm index for hours following the launch and satellites injection into Sun-synchronous orbit (SSO) was G3, then upgraded again to G4 (actual):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;G3 (Strong) Geomagnetic Storm Impacts&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Power systems&lt;/strong&gt;: Voltage corrections may be required, false alarms triggered on some protection devices.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Spacecraft operations&lt;/strong&gt;: Surface charging may occur on satellite components, drag may increase on low-Earth-orbit satellites, and&#xA;  corrections may be needed for orientation problems.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Other systems&lt;/strong&gt;: Intermittent satellite navigation and low-frequency radio navigation problems may occur, HF radio may be&#xA;  intermittent, and aurora has been seen as low as Illinois and Oregon&#xA;  (typically 50° geomagnetic lat.).&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's quite a few scary descriptions there. So to my question;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How real are all these possible effects listed on NOAA page when it comes to final preparations for a launch and launch into a near-polar SSO while the geomagnetic and solar radiation storms last?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, is there something in particular about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;Vega launch vehicle&lt;/a&gt; and its today's payload (Sentinel-2A) that Arianespace has decided to completely ignore space weather and proceed with the launch? I know that some past launches of smaller launch vehicles, like e.g. Antares, &lt;a href=&quot;http://christianready.com/2014/01/08/antares-launch-postponed-due-to-weather-in-space/&quot; rel=&quot;noreferrer&quot;&gt;have been delayed&lt;/a&gt; because of &lt;em&gt;space weather&lt;/em&gt;, even just threatening sunspots moving at close to solar equator just in front of Earth's orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VU2dx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And these were &lt;a href=&quot;http://christianready.com/2014/01/08/antares-launch-postponed-due-to-weather-in-space/&quot; rel=&quot;noreferrer&quot;&gt;described by some&lt;/a&gt; as &lt;em&gt;Holy mother of all sunspots, Batman, those are HUGE!&lt;/em&gt; Well, today the Sun actually doesn't look much better. HMI Intensitygram from &lt;a href=&quot;http://sdo.gsfc.nasa.gov/data/&quot; rel=&quot;noreferrer&quot;&gt;SDO/AIA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/UCI04.jpg&quot; alt=&quot;HMI Intensitygram&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I do apologize for a media heavy question, but I think these images tell the story. If these sunspots aren't the &lt;em&gt;Holy Batman&lt;/em&gt;, they're at the very least the &lt;em&gt;Holy Robin&lt;/em&gt; class, I'd say.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what is different between Vega and other launch vehicles whose launches were scrubbed and delayed due to space weather or even just for the threat of it? Surely it cannot be that Vega launched from near equatorial latitudes and those that were scrubbed for space weather launched from US? Because Vega still passed polar regions with its upper stage as it deployed Sentinel 2A into a near-polar (retrograde inclination of 98.5°) Sun-synchronous orbit, which then passes over any of the two polar regions about every 50 minutes in its 786 km altitude, &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28786+km+%2B+6378.1+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%29+in+minutes&quot; rel=&quot;noreferrer&quot;&gt;100.6 minute&lt;/a&gt; orbit.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-06-23T12:46:58.350" LastActivityDate="2022-02-09T17:33:04.217" Title="How risky is launching a rocket during a geomagnetic and solar radiation storm?" Tags="|launch|low-earth-orbit|space-weather|risk-management|sun-synchronous|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9628" PostTypeId="2" ParentId="8396" CreationDate="2015-06-23T07:18:24.433" Score="3" Body="&lt;ul&gt;&#xA;&lt;li&gt;The success of SpaceX is based mostly on using  the best of&#xA;well-proofed technologies, not on bold innovations. There are&#xA;innovations in production process and in business model, but very few&#xA;in rocket design.&lt;/li&gt;&#xA;&lt;li&gt;Highly energetic monopropellants are very dangerous. Hydroxylammonium nitrate (AF-M315E) is rated as  an explosive. It is probably unacceptable for human rated propulsion, and generally is not tested.&lt;/li&gt;&#xA;&lt;li&gt;Monomethylhydrazine,  dimethylhydrazine, and N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; are nasty chemicals but there is a solid experience with maintenance of these hazardous liquids. In fact, they work perfectly as propellants.&lt;/li&gt;&#xA;&lt;li&gt;Development of new thruster on MMH/N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; is backed by enormous data base and experts' knowledge, as hundreds of such thrusters have been designed and are in use. Development of a new engine based on a new propellant may take long time, maybe decades. For example, methane based propellants have been under investigation for many years. There is no operational methane rocket, despite of benefits of the methane as rocket fuel.&lt;/li&gt;&#xA;&lt;li&gt;There are other nontoxic propellant combinations as have be mentioned in comments. All they are bi-propellant.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="47149" LastEditDate="2022-05-24T23:01:07.990" LastActivityDate="2022-05-24T23:01:07.990" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="9629" PostTypeId="1" CreationDate="2015-06-23T11:07:58.873" Score="0" ViewCount="184" Body="&lt;p&gt;One of the main factors limiting manned space exploration is the effects of living in zero-gravity on a long term basis. Aside from planting a big lump of rock beneath a starship or using a centrifuge (Like Interstellar), what other ways could a permanent crew get around the effects of microgravity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My first thought was that (Assuming we could do it), some kind of cryogenics would serve well to stop nonessential crew (Ex. Soldiers, scientists, colonists) from experiencing bone warping and muscle deterioration (Though this isn't verified at all, but my own thoughts)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without creating or creating the effects of gravity, how can we get around the problems of microgravity  for if/ when we start exploring in manned craft? &lt;/p&gt;&#xA;" OwnerUserId="10125" LastActivityDate="2015-06-24T00:54:42.743" Title="What ways can we get around the effects of microgravity?" Tags="|gravity|microgravity|" AnswerCount="1" CommentCount="5" ClosedDate="2015-06-24T08:58:53.867" ContentLicense="CC BY-SA 3.0" />
  <row Id="9630" PostTypeId="2" ParentId="9629" CreationDate="2015-06-24T00:44:24.217" Score="1" Body="&lt;p&gt;The &quot;easiest&quot; solution is to develop a space drive capable of continuous 1g acceleration. Accelerate for half the trip, flip the drive around and decelerate for the second half.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Easier said then done.  There would be two obvious problems:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Acceleration and deceleration at that level would require literally astronomical amounts of energy especially as you reached the speed of light.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The speed of light remains a barrier. Acceleration at 10 m/s^2 would reach light speed within a year.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="4676" LastEditorUserId="4676" LastEditDate="2015-06-24T00:54:42.743" LastActivityDate="2015-06-24T00:54:42.743" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9631" PostTypeId="1" CreationDate="2015-06-24T11:03:39.407" Score="10" ViewCount="1720" Body="&lt;p&gt;According to NASA &lt;a href=&quot;https://www.nasa.gov/mission_pages/newhorizons/overview/index.html&quot;&gt;&quot;New Horizons launched on Jan. 19, 2006&quot;&lt;/a&gt; according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto#Classification&quot;&gt;Wikipedia&lt;/a&gt; this is about the same time Pluto started on the path to loss of planet status. Is it just coincidence that both of these happened at the same time or is there a relationship? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2015-06-25T12:24:09.320" Title="What is the relationship between New Horizons launch and Pluto's loss of Planet status?" Tags="|planet|new-horizons|pluto|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9632" PostTypeId="2" ParentId="9631" CreationDate="2015-06-24T11:17:32.217" Score="17" Body="&lt;p&gt;It's just a coincidence, and officially, the dates aren't &lt;em&gt;that&lt;/em&gt; close. The IAU (International Astronomical Union) General Assembly in Prague, Czech Republic where the &lt;a href=&quot;https://en.wikinews.org/wiki/Pluto_loses_planet_status&quot;&gt;new definition of &lt;em&gt;a planet&lt;/em&gt; was endorsed&lt;/a&gt; and with it Pluto losing its planetary status, happened in late August, 2006. Final draft that was voted on states: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;A planet is a celestial body that&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;&lt;em&gt;is in orbit around the Sun,&lt;/em&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;em&gt;has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)&#xA;  shape,&lt;/em&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;em&gt;has cleared the neighbourhood around its orbit.&lt;/em&gt;&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;A dwarf planet is a celestial body that&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;&lt;em&gt;is in orbit around the Sun,&lt;/em&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;em&gt;has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round)&#xA;  shape&lt;/em&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;em&gt;has not cleared the neighbourhood around its orbit,&lt;/em&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;em&gt;is not a satellite.&lt;/em&gt;&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;All other objects except satellites orbiting the Sun shall be referred to collectively as Small Solar-System Bodies.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot;&gt;New Horizons&lt;/a&gt; launch was more than seven months earlier, in mid January that same year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How this IAU process happened is explained in &lt;a href=&quot;https://www.youtube.com/watch?v=2RNGSuFqmro&quot;&gt;this Harvard-Smithsonian's Center for Astrophysics (CfA) lecture&lt;/a&gt;, presented by David A. Aguilar:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/2RNGSuFqmro?start=70&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unclear how many scientists that voted had vested interest in New Horizons mission, but apparently not enough, if that would have skewed the vote at all. I'm not saying that it would or that it should, but New Horizons Principal Investigator (PI) &lt;a href=&quot;http://news.bbc.co.uk/2/hi/science/nature/5283956.stm&quot;&gt;Alan Stern made his stance clear&lt;/a&gt; on several occasions that he stands behind the notion that Pluto is and should be considered a planet: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;It's an awful definition; it's sloppy science and it would never pass peer review – for two reasons. Firstly, it is impossible and contrived&#xA;  to put a dividing line between dwarf planets and planets. It's as if&#xA;  we declared people not people for some arbitrary reason, like 'they&#xA;  tend to live in groups'. Secondly, the actual definition is even&#xA;  worse, because it's inconsistent.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But he didn't get to vote in Prague:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;I was not allowed to vote because I was not in a room in Prague on&#xA;  Thursday 24th. Of 10,000 astronomers, 4% were in that room - you can't&#xA;  even claim consensus.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-06-24T12:04:44.593" LastActivityDate="2015-06-24T12:04:44.593" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9633" PostTypeId="2" ParentId="9631" CreationDate="2015-06-24T11:59:32.763" Score="17" Body="&lt;p&gt;The official trigger for what caused Pluto's demotion as a planet was the discovery of Eris, in October 2005. For a number of years, starting with the discovery of non-Pluto Kuiper Belt Objects (KBOs), and later with the discovery of other large objects in the Kuiper Belt. When Eris was discovered, the IAU decided that it needed to decide what a planet was. New Horizons had absolutely nothing to do with the team that ultimately discovered Eris (Although they probably have some interest as the post-Pluto mission to visit a KBO). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The discussion had been happening for some time before, but ultimately New Horizons had nothing to do with the vote that demoted Pluto as a planet, and had far more to do with the discovery of an object larger than Pluto in the Kuiper belt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might be worth mentioning that essentially the same thing happened with the Asteroid belt. The first asteroids discovered were considered planets, and after a large number of them had been found, they were re-classified. Ceres was the first, discovered in 1801. By 1851, they were classified as Asteroids. See &lt;a href=&quot;https://en.wikipedia.org/wiki/Timeline_of_discovery_of_Solar_System_planets_and_their_moons#18th_century&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; for a bit more history.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-06-25T12:24:09.320" LastActivityDate="2015-06-25T12:24:09.320" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9634" PostTypeId="2" ParentId="9614" CreationDate="2015-06-24T12:47:44.047" Score="8" Body="&lt;p&gt;Mountains on Europa, Callisto, and Ganymede aren't particularly well known. The moons are made of ice, which tends to be flat, and haven't had a mission that could really measure their terrain. This is partially a function of &lt;a href=&quot;https://en.wikipedia.org/wiki/Timeline_of_Galileo_Jupiter_orbiter_tour&quot;&gt;relatively few flybys&lt;/a&gt;, 5 of Ganymede, 8 of Callisto, 12 of Europa, and 6 of Io. Io was easy to identify features, but there simply isn't enough surface features to use the data from the other moons to identify terrain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for Io, it has a much more diverse altitude, not being covered in ice, and having volcanic activity. It has high spots that are measured by stereo images and shadows. Neither of these methods is particularly accurate, but they do work to getting an estimate of the height compared to the surrounding terrain. In fact, they have given a wide range for the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_mountains_on_Io&quot;&gt;mountains of Io&lt;/a&gt;. Boösaule Montes, for instance, has an uncertainty of 700m to it's height (17.5-18.2 km)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the best altitude information from orbit comes from a laser altimeter (MOLA for Mars), and the second best from RADAR, neither of which has been tested at Jupiter. Next best is a specific stereo image, but that will only measure relative heights, and not absolute heights.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-06-24T13:02:44.657" LastActivityDate="2015-06-24T13:02:44.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9635" PostTypeId="2" ParentId="2848" CreationDate="2015-06-24T15:13:41.573" Score="5" Body="&lt;p&gt;The best I can find on this is &lt;a href=&quot;http://energy.gov/sites/prod/files/2014/03/f9/compressed_hydrogen2011_11_chato.pdf&quot; rel=&quot;nofollow&quot;&gt;a presentation from 2011&lt;/a&gt; about the (discontinued) Altair Lunar Lander's needs, written in conjuction with the Department of Energy, in which the state of the art about cryogenics was discussed. This specifically focuses on Hydrogen, the most challenging cryogenic used in space flight. During the &quot;Power Reactant Storage Hydrogen Tank&quot; test, fuel was stored for 21 days in a state where it could be used, with a boil off of about 2% per day. It also mentions the Titan-Centaur 5, which stored fuel for about 9 hours, with a drop off of 21%/day for extended coasting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find a quote anywhere, but I believe the desire of NASA with the Altair program was to extend this storage to about 6 months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Service_Module_.28SM.29&quot; rel=&quot;nofollow&quot;&gt;Apollo's fuel&lt;/a&gt; beyond the Saturn 5 launch was not cryogenic, in fact it was Aerozine 50 as fuel and nitrogen tetroxide, neither of which is cryogenic. The Apollo Service module, as well as the Space Shuttle, used hydrogen and oxygen as a fuel cell to make electricity. The Space shuttle had a small liquid hydrogen tank it kept on orbit, for using as a fuel cell fuel and for water generation. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_orbiter#Electrical_power&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, &quot;The hydrogen and oxygen for the fuel cells was kept in pairs of cryogenic storage tanks in the mid-fuselage underneath the payload bay liner, and a variable number of such tanks could be installed (up to five) depending on the requirements of the mission.&quot;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-06-25T12:13:35.743" LastActivityDate="2015-06-25T12:13:35.743" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9636" PostTypeId="1" AcceptedAnswerId="9638" CreationDate="2015-06-24T18:21:55.557" Score="10" ViewCount="3782" Body="&lt;p&gt;As the center of gravity between Pluto and Charon is outside Pluto does Pluto's satellites other than Charon such as Nix, Hydra, Kerberos or Styx orbits around Pluto or the COG?&lt;/p&gt;&#xA;" OwnerUserId="10579" LastEditorUserId="25" LastEditDate="2015-07-16T13:17:18.457" LastActivityDate="2015-07-16T13:17:18.457" Title="Pluto's satellites other than Charon orbits around Pluto or center of gravity between Pluto and Charon?" Tags="|gravity|pluto|moons|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9637" PostTypeId="2" ParentId="9636" CreationDate="2015-06-24T18:36:04.010" Score="4" Body="&lt;p&gt;They orbit the barycenter of Pluto/ Charon, as there is no other place for them to orbit. Their orbit extends beyond the Charon, so thus the only stable orbit would go around both objects. See this image from &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Pluto_moon_P5_discovery_with_moons%27_orbits.jpg/470px-Pluto_moon_P5_discovery_with_moons%27_orbits.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JpL3T.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-06-24T18:53:23.440" LastActivityDate="2015-06-24T18:53:23.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9638" PostTypeId="2" ParentId="9636" CreationDate="2015-06-24T18:37:34.290" Score="6" Body="&lt;p&gt;The other moons orbit around both Pluto and Charon, so in a way it is accurate to say that they orbit the Pluto-Charon system rather than just Pluto, since Pluto and Charon orbit a center of gravity outside of Pluto. For now, the IAU has not pursued classifying Pluto-Charon as a double planet, so the moons are all said to be satellites of Pluto.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/da2LbamN7DQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;NASA.gov Video&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;For more information:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Moons_of_Pluto&quot; rel=&quot;noreferrer&quot;&gt;Moons of Pluto - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-06-24T19:43:41.397" LastActivityDate="2015-06-24T19:43:41.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9639" PostTypeId="2" ParentId="9636" CreationDate="2015-06-24T18:37:41.257" Score="2" Body="&lt;p&gt;&lt;strong&gt;The four moons (Nix, Hydra, Kerberos and Styx) all orbit around the centre-of-gravity of the Pluto-Charon binary system.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JpL3T.jpg&quot; alt=&quot;Pluto System&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Image from the Hubble Space Telescope highlighting the orbits of the four moons. You can clearly see that they are orbiting the barycentre of the system. (Courtesy &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Pluto_moon_P5_discovery_with_moons%27_orbits.jpg/470px-Pluto_moon_P5_discovery_with_moons%27_orbits.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More details about the exact orbits of the moons will come out soon, as the &lt;strong&gt;New Horizons&lt;/strong&gt; probe sends back data. &lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-06-24T18:37:41.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9640" PostTypeId="2" ParentId="9636" CreationDate="2015-06-24T18:53:27.353" Score="5" Body="&lt;p&gt;There are potentially stable Pluto-centric* orbits in the Pluto-Charon system, but so far &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150528&quot; rel=&quot;nofollow noreferrer&quot;&gt;nothing has been detected&lt;/a&gt; in the &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/9307/what-is-a-sailboat-island-of-stability&quot;&gt;Sailboat Island of Stability&lt;/a&gt;&lt;/em&gt; or anywhere else along the New Horizon's flyby trajectory. Otherwise, like the other three answers before me said; All of them, Pluto, Charon, Nix, Hydra, Styx and Kerberos orbit a common center of gravity, the barycenter, that's slightly outside of Pluto's body:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/CjCYp.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Animation source: &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150429&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizon's News Center&lt;/a&gt; (see both Pluto-centric and Barycentric animations there)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;*Even any other Pluto-centric orbits around Pluto but not also around Charon or any other satellites in the system would technically orbit around the system's common barycenter, but on top of that also be periodically (resonant orbits) or chaotically perturbed by gravitational influence of Charon and other satellites, so their orbital focus wouldn't be exactly at the common barycenter exactly all of the time. But on average, over time, and assuming they're not perturbed out of the system or hitting any other body, yes. Even multiple focal points of S-type orbits would converge at the barycenter of the Pluto-Charon system.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-06-25T13:01:01.083" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9641" PostTypeId="1" CreationDate="2015-06-24T19:40:27.500" Score="3" ViewCount="3884" Body="&lt;p&gt;I have been to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Artificial_gravity&quot; rel=&quot;nofollow&quot;&gt;Wikipedia pages on alternatives to gravity&lt;/a&gt;, and found the usual alternative forces, with the potential for magnets and the specious Russian claims, but I'm wondering why not just use some kind of magnetic system that, say, if a person wore a special suit or boots, etc., they could walk around and feel the same weight or attraction?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the energy produced from a fusion reactor, surely enough energy could be produced for diamagnets, no? Or, if this concept is simply totally flawed, then do you know of some other way that so convenient a force as magnetism could be used?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If diamagnetism can levitate and therefore cancel out Earth's gravity for small objects or even a mouse, maybe it could also produce gravity like effects for ships that won't be designed to travel far, but to have excellent attitude control and articulation?&lt;/p&gt;&#xA;" OwnerUserId="10420" LastEditorUserId="10420" LastEditDate="2015-06-27T19:39:13.127" LastActivityDate="2020-09-26T10:15:43.047" Title="Could magnetic &quot;boots&quot; be used to simulate the effects of gravity for asteroid ships?" Tags="|gravity|magnetic-field|artificial-gravity|feasibility|habitat|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9642" PostTypeId="2" ParentId="9641" CreationDate="2015-06-24T20:04:27.277" Score="7" Body="&lt;p&gt;The strength of a magnetic field is proportional to $1/r^3$, so if you build magnets into the floor, there's a huge difference in field strength between your head and feet. So using a suit to provide an evenly distributed force on your body is almost impossible. &#xA;Even if you only use boots with a metal plate in them, you get strange effects. When you lift your boots off the floor, it'll be difficult at first, but will rapidly get easier as the distance increases. This makes it difficult to walk naturally.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting the magnet in the boots is a better proposition, you can build an active system to compensate for the changes in field strength: measure the distance between the boot and the floor, adjust the electromagnetic field strength so that the force attracting the boot to the floor remains constant.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need a really strong EM field for diamagnetism to have any effect. This would cause problems: any ferromagnetic materials in the field would be hugely affected by the field. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-06-25T09:00:14.687" LastActivityDate="2015-06-25T09:00:14.687" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9643" PostTypeId="2" ParentId="9259" CreationDate="2015-06-24T20:25:11.307" Score="11" Body="&lt;p&gt;The science team.  If there is not consensus, the Project Scientist will decide, currently Ashwin Vasavada.  There are higher authorities, but they would only very rarely intervene.  I believe there was some pressure on the science team from above at one point to head for the hills.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are daily &lt;a href=&quot;http://msl-scicorner.jpl.nasa.gov/scienceoperations/&quot;&gt;Science Operations Working Group&lt;/a&gt; meetings for tactical decisions and long range planning meetings for setting the direction.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-24T20:25:11.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9644" PostTypeId="1" CreationDate="2015-06-24T21:18:45.943" Score="9" ViewCount="323" Body="&lt;p&gt;So we all know that there are lots of engineering processes that are made 100 times better by negligible gravity, and that certain engineering process need to be performed in a vacuum (electron beam welding comes to mind for some reason).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, are there any processes that &lt;em&gt;require&lt;/em&gt; an atmosphere, or at least significantly benefit from one? I'm only talking about vacuum v non-vacuum here, so micro-gravity is fine. Also I'm really only interested in processes the specifically relate to satellite/space vehicle construction. This is a pretty general question (which is bad I know! :/) so I'll try and pin it down a little more:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What processes are required to produce satellites/space vehicles that would directly benefit from a low gravity/microgravity environment but are significantly hindered by a vacuum?&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="12102" LastEditDate="2019-07-18T07:49:24.817" LastActivityDate="2019-07-18T13:46:20.297" Title="What engineering processes (satellite/space vehicle related) require an atmosphere" Tags="|artificial-satellite|engineering|vacuum|manufacturing|reduced-gravity-manufacturing|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9645" PostTypeId="2" ParentId="9641" CreationDate="2015-06-24T23:11:51.853" Score="6" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_levitation#Diamagnetic_levitation&quot; rel=&quot;noreferrer&quot;&gt;Diamagnetic gravity&lt;/a&gt; would require absurdly strong magnetic fields.  The field strength required is proportional to the product of the field strength and the rate of field strength change; in order to get a reasonably uniform force across a two-meter-tall human, the rate of change needs to be small.  A back-of-the-envelope calculation says you're looking for a field on the order of 100-1000 Teslas to provide simulated Earth gravity, stronger than that produced by the largest laboratory magnets -- and you need to provide that field throughout the entire habitable volume, rather than just the few thousand cubic millimeters at the core of a laboratory magnet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, such a strong field means the entire spacecraft needs to be made of non-ferromagnetic materials; even strongly diamagnetic materials (such as graphite) are questionable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that because diamagnetism is a repulsive force, the magnets providing the gravity would need to be in the ceiling, not the floor.)&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2015-06-24T23:11:51.853" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9647" PostTypeId="2" ParentId="9644" CreationDate="2015-06-25T07:42:54.343" Score="4" Body="&lt;p&gt;Applying adhesives or laying up composite materials such as carbon fibre could both be tricky. I suspect outgassing/boiling of the liquid compounds would occur causing separation in both cases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These manufacturing processes don't depend on low gravity, unless you're working on huge structures.  But the hindrance of vacuum seems clear to me.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="6816" LastEditDate="2015-06-25T17:29:44.590" LastActivityDate="2015-06-25T17:29:44.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9648" PostTypeId="2" ParentId="9636" CreationDate="2015-06-25T08:54:24.323" Score="3" Body="&lt;p&gt;TLDR: &lt;em&gt;If we feel we have to mention Charon every time we speak of the Plutonian system, then we should give the same privilege to all the other moons as well, as even they contribute to the COG; even if, admitedly, not to the extent of being able to move the COG to inside or outside of Pluto itself. So, we would have to say that Nix orbits the COG of the Pluto-Charon-Styx-Nix-Kerberos-Hydra system, and that sounds clumsy.  So we usually say that Nix orbits Pluto.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not a physicist, but rather a logician and linguist by education.  So let me try a logical analysis of the question to complement the existing great answers.  The question sounds to me like &quot;Does Earth orbit the Sun, or the COG of the Sun-Jupiter system?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether the center of gravity is inside or outside of the heaviest body in the system has little bearing on the answer.  The center of the solar system is frequently outside of the Sun, depending on how the planets are distributed at the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether we speak of the whole Plutonian system as a planet with moons, dwarf planet with moons, or a double dwarf planet with moons, is an almost arbitrary convention and we (non-physicists) have IAU to tell us what's the standard terminology of the day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even the &quot;Plutonian system&quot; is just a figure of speech, a simplification.  It was discovered the way it was because Uranus and Neptune are paying occasional visits to the club, too, presumably perturbing the moons unevenly in the process.&lt;/p&gt;&#xA;" OwnerUserId="10585" LastEditorUserId="10585" LastEditDate="2015-06-25T09:02:30.337" LastActivityDate="2015-06-25T09:02:30.337" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9649" PostTypeId="1" AcceptedAnswerId="9650" CreationDate="2015-06-25T14:26:48.060" Score="5" ViewCount="5233" Body="&lt;p&gt;On circular orbit burning prograde/retrograde makes the orbit elliptical with raising or lowering the orbit on the opposite side of the body, making the burn position the new periapsis/apoapsis respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, burning radial or antiradial makes the orbit an ellipse as well, but by changing simultaneously the apoapsis and periapsis. In some cases the new shape of the orbit may be desirable (e.g. setting up a bielliptical transfer). In other (in reverse) it could help circularize an elliptical orbit to a given altitude if its periapsis and apoapsis are on two sides of that altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet I almost never read about this being done. The bielliptical transfer is described as pure prograde/retrograde burns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a specific reason? Is it considerably less efficient than a combination of prograde/retrograde burns setting up the same orbit, or something like that?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-06-25T15:12:59.687" Title="Prograde/retrograde vs radial/antiradial burn - why are the former so much more common?" Tags="|orbital-mechanics|orbital-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9650" PostTypeId="2" ParentId="9649" CreationDate="2015-06-25T14:39:12.827" Score="8" Body="&lt;p&gt;Burning prograde at periapsis to start the bi-elliptic transfer takes advantage of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;noreferrer&quot;&gt;Oberth effect&lt;/a&gt;, so it's generally advantageous to arrange that the orbit is near-circular or that the pre-burn orbital periapsis is at the same angle as the transfer orbit periapsis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, if you're inserting into orbit at the other end of the bi-elliptic (as opposed to doing a flyby), it's most efficient to do that by burning retrograde at the target's periapsis (in the target's frame of reference, of course, rather than the origin's).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the desired final orbit isn't circular, of course, the insertion burn may have a substantial radial/antiradial component, but the retrograde portion of the ∆v vector is generally dominant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, the only time you'd want to burn radial/antiradial is if you're in an elliptical orbit and need to change the orientation of the ellipse.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-06-25T15:12:59.687" LastActivityDate="2015-06-25T15:12:59.687" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9651" PostTypeId="1" AcceptedAnswerId="9656" CreationDate="2015-06-25T14:57:26.223" Score="3" ViewCount="323" Body="&lt;p&gt;I imagine that measuring real positions and orbital elements of satellites orbiting other planets after they perform braking maneuver to get to their final orbit must be rather difficult. Especially with precision required for further maneuvering. What methods are used to measure position of probes orbiting other planets? &lt;/p&gt;&#xA;" OwnerUserId="6213" LastEditorUserId="49" LastEditDate="2015-06-25T15:25:03.890" LastActivityDate="2015-06-25T22:36:14.263" Title="How is real position of probes orbiting other planets measured?" Tags="|probe|interplanetary|navigation|measurement|orbit-determination|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9652" PostTypeId="1" AcceptedAnswerId="9666" CreationDate="2015-06-25T15:12:38.827" Score="5" ViewCount="1052" Body="&lt;p&gt;According to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;nofollow&quot;&gt;Keplerian system of orbital elements&lt;/a&gt;, a reference direction is given; the longitude of the ascending node is an angle with respect to that direction, and the argument of periapsis, specifying the orientation of the axes of the ellipse, is given relative to the LAN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a low-inclination, high-eccentricity orbit, the longitude of ascending node may be very uncertain (becoming undefined in the purely theoretical zero-inclination case) even though the orientation of the ellipse as projected to the reference plane is clear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is the AOP not taken as the angle made with the reference direction when the periapsis is projected to the reference plane? This would make its measurement independent of the LAN, and the angle would be uncertain only in a very small region above the poles -- a much rarer situation than a near-zero-inclination orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see there's an element called &lt;a href=&quot;https://en.wikipedia.org/wiki/Longitude_of_the_periapsis&quot; rel=&quot;nofollow&quot;&gt;longitude of periapsis (ϖ)&lt;/a&gt;, which if I understand correctly is almost (but not quite) the same thing as what I'm proposing, that's occasionally used for orbital elements of the planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is AOP defined this way?&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-06-26T15:10:48.550" LastActivityDate="2015-06-26T23:07:20.083" Title="Why is argument of periapsis defined in terms of longitude of ascending node?" Tags="|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9653" PostTypeId="2" ParentId="9277" CreationDate="2015-06-25T16:09:49.313" Score="6" Body="&lt;p&gt;Just as support to already existing answers, here's to the question the most relevant excerpt from &lt;em&gt;&lt;a href=&quot;https://www.nasa.gov/press-release/nasa-announces-television-coverage-media-activities-for-pluto-flyby&quot; rel=&quot;nofollow&quot;&gt;NASA Announces Television Coverage, Media Activities for Pluto Flyby&lt;/a&gt;&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 14&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;7:30 a.m.&lt;/strong&gt; – Media Briefing: Arrival at Pluto, Inside the Pluto System and New Horizons’ Perilous Path (live on NASA TV)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;At 7:49 a.m., the New Horizons spacecraft will make history as flies&#xA;  past Pluto, after a journey of more than nine years and 3 billion&#xA;  miles. For much of the day the New Horizons spacecraft will be out of&#xA;  communication with mission control as it gathers data on Pluto and its&#xA;  moons.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The moment of closest approach will be marked with a live NASA TV&#xA;  broadcast that includes a countdown, a discussion of images and data&#xA;  received thus far, and what’s expected next as New Horizons makes its&#xA;  way past Pluto and potentially dangerous debris. Follow the path of&#xA;  the spacecraft in real time with a visualization of the actual&#xA;  trajectory data, using NASA’s &lt;a href=&quot;http://eyes.nasa.gov/pluto&quot; rel=&quot;nofollow&quot;&gt;Eyes on Pluto&lt;/a&gt;.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;9 a.m. – noon&lt;/strong&gt; -- Interview Opportunities (no NASA TV coverage)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Informal group briefings and availability for one-on-one interviews.&#xA;  An updated schedule will be posted in the New Horizons Media Center.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Noon – 3 p.m.&lt;/strong&gt; – Panel Discussions (no NASA TV coverage)&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;New Horizons mission overview and history&lt;/li&gt;&#xA;  &lt;li&gt;Pluto system discoveries on approach&lt;/li&gt;&#xA;  &lt;li&gt;Mariner 4 and Pluto: 50 years to the day&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;8 – 9:15 p.m.&lt;/strong&gt; -- NASA TV program, Phone Home, broadcast from APL Mission Control&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA TV will share the suspenseful moments of this historic event with&#xA;  the public and museums around the world. The New Horizons spacecraft&#xA;  will send a preprogrammed signal after the close approach. The mission&#xA;  team on Earth should receive the signal at about 9:02 p.m. When New&#xA;  Horizons “phones home,” there will be a celebration of its success and&#xA;  the anticipation of data to come over the days and months ahead.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;9:15 – 10 p.m.&lt;/strong&gt; -- Media Briefing: New Horizons Health and Mission&#xA;  Status (live on NASA TV)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All times are in &lt;a href=&quot;https://www.timeanddate.com/time/zones/edt&quot; rel=&quot;nofollow&quot;&gt;EDT&lt;/a&gt; (Eastern Daylight Time) and this schedule is subject to change. For mentioned NASA activities that won't be covered on &lt;a href=&quot;https://www.nasa.gov/multimedia/nasatv/&quot; rel=&quot;nofollow&quot;&gt;NASA TV&lt;/a&gt;, and also other New Horizons related activities organized by other organizations, please follow updates to our &lt;a href=&quot;http://chat.stackexchange.com/rooms/info/9682/the-pod-bay?tab=schedule&quot;&gt;Chat Room Schedule&lt;/a&gt; and join us in &lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/the-pod-bay&quot;&gt;our main chat room&lt;/a&gt; as events begin, where we discuss them and share links to where they can be accessed from live and/or as recordings. Invitation extends also to non-related to New Horizons events that we schedule there. Welcome!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;And here are New Horizons' scheduled transmission dates, from &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto/NH_Obs_Playbook_LORRI-MVIC.pdf&quot; rel=&quot;nofollow&quot;&gt;June 20 to July 23 Observation Playbook&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Approach Hi Speed Playback – Before July 12th, 2015 (Before DOY 193)&lt;/li&gt;&#xA;  &lt;li&gt;Fail Safe – July 12-13th, 2015 (DOY 193-194)&lt;/li&gt;&#xA;  &lt;li&gt;E_HEALTH_1 &amp;amp;  Phone Home – July 14th, 2015 (DOY 195)&lt;/li&gt;&#xA;  &lt;li&gt;1st Look Playback – July 15-16th, 2015 (NYTimes datasets)  (DOY 196-197)&lt;/li&gt;&#xA;  &lt;li&gt;Early High Priority – July 17-20th, 2015  (DOY 198-201)&lt;/li&gt;&#xA;  &lt;li&gt;Browse Playback (lossy) – all high speed data sent back Sept. 14th - Nov. 18th, 2015 (DOY 257-332)&lt;/li&gt;&#xA;  &lt;li&gt;Full Playback (lossless) – all data sent back between Nov. 18th, 2015 and Oct. 2016&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;Note: EDT runs March 8th – Nov. 1st, 2015 (DOY 67–305 – switches half&#xA;  way through Browse)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-09T23:43:35.377" LastActivityDate="2015-07-09T23:43:35.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9655" PostTypeId="2" ParentId="9644" CreationDate="2015-06-25T17:44:28.287" Score="4" Body="&lt;p&gt;Anything that involves liquids, is easier with an atmosphere. What are some of these items?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lubricant on reaction wheels, and other joints.&lt;/li&gt;&#xA;&lt;li&gt;Glass making.&lt;/li&gt;&#xA;&lt;li&gt;Applying films on surfaces like lenses&lt;/li&gt;&#xA;&lt;li&gt;Working with &lt;a href=&quot;https://en.wikipedia.org/wiki/Materials_for_use_in_vacuum&quot; rel=&quot;nofollow&quot;&gt;certain types of materials&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Of these, the most significant one is probably applying films on lenses. Lenses in general could be made much better in space than on Earth, but the difficulty in applying films would make it tricky. Ironically, many of &lt;a href=&quot;https://en.wikipedia.org/wiki/Thin_film&quot; rel=&quot;nofollow&quot;&gt;these films&lt;/a&gt; are applied in vacuum chambers on Earth, so in theory, the same thing could be done by creating a sufficiently large sealed chamber with a vacuum in both ends. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-25T17:44:28.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9656" PostTypeId="2" ParentId="9651" CreationDate="2015-06-25T22:36:14.263" Score="6" Body="&lt;p&gt;Here is a &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf13-1.php&quot;&gt;good tutorial&lt;/a&gt; on the navigation of deep space vehicles.  The two main data types used are two-way Doppler (using an atomic clock reference at the DSN station, with the frequency locked to and sent back to Earth by the spacecraft), which gives the velocity component along the Earth-spacecraft line to better than 0.1 mm/s, and ranging (sending a pseudo-noise signal which is immediately reflected back by the spacecraft radio), which gives the distance from the DSN station to the spacecraft to better than one meter.  Though this only gives you one component of position and one component of velocity, you can use many of these high-accuracy measurements over time combined with the constraints of motion in the gravity field of the Sun and planets to solve for the other components of position and velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is another data type, used less frequently, which is to use two widely separated DSN stations (i.e. at two different complexes on Earth) and a quasar of known position to determine the apparent position of the spacecraft with interferometry.  This can determine the angular position of the spacecraft on the projection of the baseline between the two stations to an accuracy of a few nanoradians, letting you solve immediately for one more component.  It is more expensive, and so is generally used when higher precision is required over a shorter time, e.g. on approach to Mars to accurately target a landing site.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-25T22:36:14.263" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9657" PostTypeId="1" CreationDate="2015-06-26T00:53:13.293" Score="8" ViewCount="405" Body="&lt;p&gt;Now that very high resolution imagery of the Moon's whole north, above 60 °N, can be viewed on one giant awesome &lt;a href=&quot;http://lroc.sese.asu.edu/images/gigapan&quot; rel=&quot;nofollow noreferrer&quot;&gt;zoomable map&lt;/a&gt;, I've been taking a close look at a number of craters. Anaxagoras sticks out to me as odd. Here is a photo of the &lt;a href=&quot;http://bit.ly/1KewZIk&quot; rel=&quot;nofollow noreferrer&quot;&gt;width of the crater&lt;/a&gt;, roughly 45 km across, from the gigapan site linked above:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JeGme.png&quot; alt=&quot;floor of Anaxagoras crater showing mountains, ridges, and collapsing walls&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;And here are images of a couple of details, the top one taking in an area about 5 km wide, the bottom one about 12 km:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2TTUZ.png&quot; alt=&quot;sinuous gullies in crater wall&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rb2FW.png&quot; alt=&quot;impact melt field full of cracks and ripples, ejecta on hills&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;And a topographic image, where the red areas are about 5 km higher than the pink areas:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pU5SF.jpg&quot; alt=&quot;topographic map of Anaxagoras crater&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;From the sharp lines of the crater wall, I've gathered that this crater is young. Also because rays of ejecta extend from it, though not of bright material like Tycho. Those rays are a bit more visible in the &lt;a href=&quot;http://pub.lmmp.nasa.gov/LMMPUI/LMMP_CLIENT/LMMP.html#&quot; rel=&quot;nofollow noreferrer&quot;&gt;LMMP&lt;/a&gt; version of this same map.&lt;/p&gt;&#xA;&lt;p&gt;These are my confusions:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Why are there mountains over the entire floor of the crater, not just in the center like usual? Associated with that, how can there be such large ridges on the south-west wall of the crater?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How can all the hills and mountains be covered by gigantic boulders (one pixel is about 2 m so a lot of those boulders have got to be 30 m across), while the melt plains have almost none? Wasn't it the impact that produced both the boulders and the melt?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr&gt;&#xA;Notes: Because this crater is so close to the north pole, the sun here is casting long shadows that fall to the north of everything, making it look lit from below.&#xA;&lt;p&gt;All data is from the Lunar Reconnaissance Orbiter, which can also be explored at the &lt;a href=&quot;http://ode.rsl.wustl.edu/moon/indexMapSearch.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Orbital Data Explorer&lt;/a&gt; site, as well as at the sites linked above.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2021-10-31T18:23:00.473" LastActivityDate="2021-10-31T18:23:00.473" Title="Why does the Moon's Anaxagoras crater seem sort of jumbled?" Tags="|the-moon|planetary-science|impact|crater|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="9658" PostTypeId="2" ParentId="9644" CreationDate="2015-06-26T01:34:44.007" Score="0" Body="&lt;p&gt;Many common chemical reactions require an atmosphere to function.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gasoline combustion, for example:&#xA;2C8H18 + 25O2 ~&gt; 16CO2 + 18H2O &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without atmospheric oxygen, a car would need stored oxidizer.&lt;/p&gt;&#xA;" OwnerDisplayName="user9077" LastActivityDate="2015-06-26T01:34:44.007" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9659" PostTypeId="1" AcceptedAnswerId="9660" CreationDate="2015-06-26T08:32:52.943" Score="3" ViewCount="474" Body="&lt;p&gt;I've read about a Michielsen chart, but cannot currently find Kaplan's &lt;a href=&quot;https://www.worldcat.org/title/modern-spacecraft-dynamics-control/oclc/2317997&quot; rel=&quot;nofollow&quot;&gt;1976 book&lt;/a&gt;. What is this chart? Is it outdated nowadays?&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="49" LastEditDate="2015-06-26T13:45:27.270" LastActivityDate="2015-06-26T13:46:25.720" Title="What is a Michielsen chart and how should one use it?" Tags="|orbital-mechanics|history|terminology|reference-request|ballistics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9660" PostTypeId="2" ParentId="9659" CreationDate="2015-06-26T12:41:23.980" Score="5" Body="&lt;p&gt;From &lt;em&gt;Gerald R. Hintz, &lt;a href=&quot;https://books.google.com/books?id=bewXBgAAQBAJ&amp;amp;pg=PA261&amp;amp;lpg=PA261&amp;amp;dq=space%20Michielsen%20chart&amp;amp;source=bl&amp;amp;ots=X61F7a7YMs&amp;amp;sig=p2lrZfjervsayQTloujdpcJ7CWo&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ei=HzqNVZCOC8Lc-QGCqoGADw&amp;amp;redir_esc=y#v=onepage&amp;amp;q=space%20Michielsen%20chart&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Mechanics and Astrodynamics: Techniques and Tools for Space Missions&lt;/a&gt;&lt;/em&gt;, 2015:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Michielsen devised a graphical display for all lunar transfer&#xA;information, including passage effects, on a single plot. This chart&#xA;is presented in Fig. 7.9 [missing from preview, see below] with an&#xA;application to the Apollo 11 Mission. The &lt;span class=&quot;math-container&quot;&gt;$v_r$&lt;/span&gt; axis is calibrated in&#xA;km/s, but is directly related to transfer time. Thus, each value of&#xA;&lt;span class=&quot;math-container&quot;&gt;$v_r$&lt;/span&gt; associated with the transfer orbit corresponds to a unique value&#xA;of transfer time. The two vertical lines marking the constant values&#xA;of &lt;span class=&quot;math-container&quot;&gt;$v_\theta = \pm 0.19\text{ km/s}$&lt;/span&gt; at &lt;span class=&quot;math-container&quot;&gt;$r_\text{M}$&lt;/span&gt; are calibrated in&#xA;days to reach &lt;span class=&quot;math-container&quot;&gt;$r_\text{M}$&lt;/span&gt;, i.e., the transfer time from Earth to the&#xA;Moon. Transfers which have &lt;span class=&quot;math-container&quot;&gt;$v_\theta = + 0.19\text{ km/s}$&lt;/span&gt; are direct&#xA;and those with &lt;span class=&quot;math-container&quot;&gt;$v_\theta = - 0.19\text{ km/s}$&lt;/span&gt; are retrograde.&lt;/p&gt;&#xA;&lt;p&gt;The Earth intercept zone connotes return to the Earth without any&#xA;thrusting maneuvers.&lt;/p&gt;&#xA;&lt;p&gt;Note the Moon's velocity vector &lt;span class=&quot;math-container&quot;&gt;$v_M$&lt;/span&gt;. The magnitude&lt;/p&gt;&#xA;&lt;h3&gt;&lt;span class=&quot;math-container&quot;&gt;$$v_{\theta_M} = v_c = \sqrt{\frac{\mu_⊕}{r_M}} = 1.02\text{ km/s}$$&lt;/span&gt;&lt;/h3&gt;&#xA;&lt;p&gt;since the Moon is assumed to be in a circular orbit about the Earth.&#xA;So the point &lt;span class=&quot;math-container&quot;&gt;$v_\theta \cong 1\text{ km/s}$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$v_r \cong 0$&lt;/span&gt; is the&#xA;orbital velocity of the Moon with respect to the Earth.&lt;/p&gt;&#xA;&lt;p&gt;A probe's approach &lt;span class=&quot;math-container&quot;&gt;$(v_\theta, v_r)$&lt;/span&gt; is specified by the transfer&#xA;trajectory. Hyperbolic passage of the Moon is handled in an analogous&#xA;manner to that for planetary passage.&lt;/p&gt;&#xA;&lt;p&gt;As the spacecraft reaches &lt;span class=&quot;math-container&quot;&gt;$r_M$&lt;/span&gt;, the Earth's gravitational attraction&#xA;is &amp;quot;turned off&amp;quot; (in the model). The deflection angle &lt;span class=&quot;math-container&quot;&gt;$\delta$&lt;/span&gt; measures&#xA;the turning of the spacecraft's velocity in the Moon-relative&#xA;hyperbolic trajectory. A typical velocity vector diagram for lunar&#xA;passage is shown in Fig. 7.10. Since the spacecraft passes in front of&#xA;the Moon, the turn angle &lt;span class=&quot;math-container&quot;&gt;$\delta$&lt;/span&gt; is taken clockwise and geocentric&#xA;energy is decreased via a gravity assist (see Fig. 3.18 [missing from&#xA;preview]). These vector triangles predict the probe's departure&#xA;velocity &lt;span class=&quot;math-container&quot;&gt;$v^+$&lt;/span&gt; from the Moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Instead of the missing Fig. 7.9 from the quote source, here's &lt;em&gt;Michielsen Chart for Lunar Encounter&lt;/em&gt; from the &lt;em&gt;Marshall H. Kaplan, Modern Spacecraft Dynamics and Control&lt;/em&gt; that you mention, taken from Robert Stengel's (Princeton University) &lt;a href=&quot;http://www.princeton.edu/%7Estengel/MAE342Lecture9.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;untitled lecture slides&lt;/a&gt; (PDF, approved for educational use), page 7. Sharpened a bit for clarity but I do apologize for its quality, it's the best I could find:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Michielsen Chart for Lunar Encounter&lt;/strong&gt;&lt;br /&gt;&#xA;&lt;em&gt;(Kaplan, Modern Spacecraft Dynamics and Control, 1976)&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JxufV.png&quot; alt=&quot;Michielsen Chart for Lunar Encounter&quot; /&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Orbital velocity of the Moon (wrt Earth), &lt;span class=&quot;math-container&quot;&gt;$v_\theta \sim 1\text{ km/s}$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$v_r \sim 0$&lt;/span&gt;&lt;/li&gt;&#xA;&lt;li&gt;Probe's approach &lt;span class=&quot;math-container&quot;&gt;$v_\theta$&lt;/span&gt;, and &lt;span class=&quot;math-container&quot;&gt;$v_r$&lt;/span&gt;, specified by transfer trajectory&lt;/li&gt;&#xA;&lt;li&gt;Probe's approach deflection angle, &lt;span class=&quot;math-container&quot;&gt;$\delta$&lt;/span&gt;, specified by Moon-relative hyperbolic trajectory&lt;/li&gt;&#xA;&lt;li&gt;Vector triangles predict probe's departure velocity vector&lt;/li&gt;&#xA;&lt;li&gt;Transfer time to Moon shown&lt;/li&gt;&#xA;&lt;li&gt;“Earth intercept zone” connotes return to Earth without thrusting maneuver&lt;/li&gt;&#xA;&lt;li&gt;Earth escape also possible&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-06-26T13:46:25.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9661" PostTypeId="1" AcceptedAnswerId="22025" CreationDate="2015-06-26T16:16:16.170" Score="20" ViewCount="3655" Body="&lt;p&gt;I was looking at the &lt;a href=&quot;http://www.nasa.gov/image-feature/watching-meteors-from-the-space-station&quot;&gt;NASA image of the day&lt;/a&gt; today, and it made me wonder about the windows on the ISS. I often see pictures of Earth taken from the ISS, but I was wondering if they had any windows that looked &quot;up&quot;, towards zenith, opposite to &lt;a href=&quot;https://en.wikipedia.org/wiki/Cupola_%28ISS_module%29&quot;&gt;Cupola&lt;/a&gt; with windows facing &quot;down&quot;, towards nadir.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think I've ever seen any pictures taken of space or the Moon (except when they captured the &lt;a href=&quot;http://media.mnn.com/assets/images/2011/06/gall_moonaglow.jpg.638x0_q80_crop-smart.jpg&quot;&gt;Moon&lt;/a&gt; near the &lt;a href=&quot;http://www.armaghplanet.com/blog/wp-content/uploads/2013/02/Image-of-Moon-from-ISS.jpg&quot;&gt;horizon of Earth&lt;/a&gt;) from the ISS. Do they have anything looking in that direction?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="6944" LastEditDate="2020-09-09T12:13:54.800" LastActivityDate="2020-11-22T02:39:06.867" Title="Does the ISS have zenith-facing windows?" Tags="|iss|design|observation|spacecraft-window|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9662" PostTypeId="2" ParentId="9661" CreationDate="2015-06-26T16:51:17.373" Score="10" Body="&lt;p&gt;There are many windows on the ISS.  Destiny (The US Lab, between Node 1 and Node 2) has a 20 inch window facing Earthwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian segment has several, and they have covers to protect them when not in use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot;&gt;Zvezda&lt;/a&gt; is the key module for windows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Zvezda has 14 windows— There are two 9-inch-diameter (230 mm) windows, one in each of the two crew sleep compartments (windows No. 1 and 2). Six 9-inch-diameter (230 mm) windows (No. 3, 4, 5, 6, 7 and 8) on the forward Transfer Compartment earth facing floor. As well a 16-inch-diameter (410 mm) window in the main Working Compartment (No. 9) and one 3-inch-diameter (76 mm) window in the aft transfer compartment (No. 10). There are a further three 9-inch-diameter (230 mm) windows in the forward end of the forward transfer compartment (No. 12, 13 and 14), for observing approaching craft. Note: Window No. 11 is unaccounted for in all available sources.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The US segment has some windows, but the primary viewing location in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cupola_%28ISS_module%29&quot;&gt;Cupola&lt;/a&gt;, which is attached to Node 3, which faces off the main line of the station.  The Cupola is facing down, (nadir) towards the Earth, but it has a main window facing out, and a series of windows around the edge.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus you could look over the limb of the Earth to see space.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-06-26T17:31:55.523" LastActivityDate="2015-06-26T17:31:55.523" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9663" PostTypeId="1" CreationDate="2015-06-26T17:12:41.380" Score="0" ViewCount="177" Body="&lt;p&gt;I realize this may belong over in SciFi &amp;amp; Fantasy, but it's Friday and maybe the experts here will humor me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine if you will that the Cold War never happened. Instead, after WWII the governments of the US and Soviet Union come to the mutual agreement that it is in humanity's best interest to make space exploration a top priority. The first satellite launch is a joint effort. The first space capsule to orbit the Earth carries an American and a Soviet. Inspired by these achievements, China avoids a cultural revolution and embarks on the &quot;Glorious Path to the Stars&quot;, rapidly modernizing their space program and joining the burgeoning global space initiative. In 1969, the whole world celebrates when astronauts from six different nations step out of a shared lunar lander and walk on the Moon. From here on, billions of dollars that once supported war budgets are instead devoted to R&amp;amp;D for space exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With that as the backdrop, here are my questions. What could be possible outcomes of this unified effort to explore space? What would mankind's presence in space be in 2015 if organizations like NASA had the full support of their governments? Would we have already colonized Mars and be moving to planets beyond, or is mankind's ingenuity hampered by technological constraints that make the outcome of an idealized alternate history like this not so much different from the current status of space exploration?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you for your thoughts and time. &lt;/p&gt;&#xA;" OwnerUserId="10591" LastActivityDate="2015-06-26T17:34:45.677" Title="Given hypothetical support post WWII, where would space exploration be today?" Tags="|colonization|history|interstellar-travel|" AnswerCount="1" CommentCount="7" ClosedDate="2015-06-26T17:40:23.163" ContentLicense="CC BY-SA 3.0" />
  <row Id="9664" PostTypeId="2" ParentId="9663" CreationDate="2015-06-26T17:34:45.677" Score="2" Body="&lt;p&gt;There is a great example, of how the change from competition to international cooperation worked out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fusion research, when a competition between countries was competitively pursued, and progress was made.  However when the agreement to work together on a very large single project was made, progress slowed down, as the large single project sucked all funding and researchers into it. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-26T17:34:45.677" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9665" PostTypeId="2" ParentId="9661" CreationDate="2015-06-26T19:12:20.480" Score="14" Body="&lt;p&gt;The RSC Energia has recently published &lt;a href=&quot;http://www.energia.ru/ktt/archive/2015/01-2015/01-05.pdf&quot; rel=&quot;noreferrer&quot;&gt;a study on the effect of high-velocity impacts of meteoroids and space debris on glass windows of the ISS&lt;/a&gt; in their &quot;Kosmicheskaya Tekhnika i Tekhnologia&quot; quarterly journal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At page 55 they have a nice three view scheme of Zvezda windows locations with the sizes, directions and angles specified:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4pQNr.png&quot; alt=&quot;Location of Zvezda service module windows&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Figure 1. Service module windows (location, Type):&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;1 — TSK.316.02.000 (∅228); 2 — TSK.316.02.000; 3 — TSK.316.02.000; 4 — TSK.316.03.000;&lt;br /&gt;5 — TSK.316.02-1.000; 6 — TSK.316.02-1.000; 7 — TSK.316.02-1.000; 8 — TSK.316.02-1.000;&lt;br /&gt;9 — TSK.316.01.000 (∅426, uncoated); 12 — TSK.316.02.000; 13 — TSK.316.02.000;&lt;br /&gt;14 — TSK.316.02.000; 26. — TSK.316.03.000 (∅80);&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2015-06-28T19:13:53.523" LastActivityDate="2015-06-28T19:13:53.523" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9666" PostTypeId="2" ParentId="9652" CreationDate="2015-06-26T23:07:20.083" Score="6" Body="&lt;p&gt;The longitude of the ascending node $\Omega$, the inclination $i$, and the argument of periapsis $\omega$ are simply the classic &lt;a href=&quot;http://mathworld.wolfram.com/EulerAngles.html&quot; rel=&quot;noreferrer&quot;&gt;Euler angles&lt;/a&gt; $\phi$, $\theta$, and $\psi$, in that order.  Then the classic Euler transform applied to an orbit in the X-Y plane with the periapsis on the +X-axis gives you the actual orbit in X-Y-Z.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is probably no reason other than the use of the conventional transform to specify the rotation of a rigid 3-D object.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-26T23:07:20.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9667" PostTypeId="1" CreationDate="2015-06-27T02:10:25.763" Score="4" ViewCount="390" Body="&lt;p&gt;Here it is very simply:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A hypothetical spacecraft has total mass of 1000 kg and a main engine that has an effective exhaust velocity of 3 km/s (or 305.915 seconds if you want Specific Impulse in seconds).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now burn 1 kg of propellant. So speed = 3000*ln(1000/999) = 3.0015 m/s. Kinetic energy = &lt;strong&gt;$E_k = \frac12 mv^2$ = 4,499.997 joules.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Later, burn another 1 kg of propellant. So speed = 3.0015 + 3000*ln(999/998) = 6.0060 m/s. &lt;strong&gt;Kinetic energy should now be 17,999.988 joules.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...What in the Universe just happened? It spent &lt;strong&gt;the same chemical energy both times&lt;/strong&gt; (1 kg)...but got a much bigger increase in kinetic energy the second time? Where did this much bigger increase in energy come from?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If burning 1 kg of propellant yields 4.5 kilojoules of kinetic energy, then doing the same thing again should yeild another 4.5 kJ, for a total of 9.0 kJ. &lt;strong&gt;But no, we get about 18 kJ instead. Where in the world did this extra energy come from?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dare I ask, are spacecraft perpetual motion machines? Or free energy machines? I don't see how you can put in the same chemical energy and get out very different kinetic energies. Have I done something wrong in my math?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2016-07-01T17:26:44.340" Title="How can a spacecraft gain more energy from burning the same amount of fuel, but at different times?" Tags="|propulsion|physics|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9670" PostTypeId="2" ParentId="9667" CreationDate="2015-06-27T04:08:18.783" Score="7" Body="&lt;p&gt;You forgot about the exhaust kinetic energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the first case, it would be 1kg at speed of nearly 3km/s (3000 m/s for the first molecule exhausted and 2997 m/s for the last one).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the second burn the speed is slower (approx. by 3 m/s), because the exhaust and the spacecraft move in the opposite directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exhaust energy is about 4.5 MJ in both cases and the difference in its kinetic energy compensates the difference in the spacecraft energy.&lt;/p&gt;&#xA;" OwnerUserId="10534" LastActivityDate="2015-06-27T04:08:18.783" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9672" PostTypeId="2" ParentId="9422" CreationDate="2015-06-27T09:47:12.470" Score="0" Body="&lt;p&gt;This is an interesting concept. The water shield can both absorb and scatter particles and photons. If the water shield is shaped into a concave lens it can additionally refract photons, although high energy photons such as x rays and gamma rays are very difficult to refract, so the useful thing is likely to be scattering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the shield is very close to the spacecraft, then this scattering wont be very useful as the particles/photons, even scattered, will probably still hit the habitat, but if the shield is some distance away then incoming particles which are refracted or scattered are likely to miss the habitat entirely, the water column would thus effectively provide a shadow of reduced radiation independently of its ability to completely stop incoming radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I can see, the concept would work, and it would be more effective than a closer shield. One obvious thing, is that the closer the Sun is to being a point light source in the sky, the more effective it would be. In order to be effective, the shield should completely block the disc of the Sun as seen from the spacecraft (as in an eclipse). This would be very easy at Pluto, and very difficult at Mercury. So I would say the degree to which it could save mass, would depend largely on how far from the Sun the spacecraft is. One could say the concept is most useful where it is least needed.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-06-27T09:47:12.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9673" PostTypeId="1" CreationDate="2015-06-27T13:25:30.063" Score="4" ViewCount="827" Body="&lt;p&gt;I've been googling this question because I haven't seen the answer anywhere, yet I know it must be calculated with a high degree of precision. Anyone know if it will be faster or slower than its 13.8 km/s approach speed? I would assume faster, but by how much?&lt;/p&gt;&#xA;" OwnerUserId="10599" LastEditorUserId="49" LastEditDate="2015-06-27T15:58:41.077" LastActivityDate="2015-06-27T15:58:41.077" Title="When New Horizons is closest to Pluto, how fast will it be going (km/s)?" Tags="|trajectory|reference-request|new-horizons|pluto|flyby|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9674" PostTypeId="2" ParentId="9673" CreationDate="2015-06-27T13:45:04.297" Score="6" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Pluto_flyby&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on New Horizons&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;New Horizons is intended to pass within 10,000 km (6,200 mi) of Pluto,&#xA;with this closest approach date estimated to occur on July 14, 2015 at&#xA;11:50 UTC. New Horizons will have a relative velocity of &lt;strong&gt;13.78 km/s&lt;/strong&gt;&#xA;(49,600 km/h; 30,800 mph) at its closest approach, and will come as&#xA;close as 27,000 km (17,000 mi) to Charon, although these parameters&#xA;may be changed during flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;For changes to New Horizons flyby trajectory, check &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Archives.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons News Center&lt;/a&gt;. That same portal (&lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Where-is-New-Horizons/index.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons at Johns Hopkins Applied Physics Laboratory&lt;/a&gt;) also has a nice overview of &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Where-is-New-Horizons/index.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons current position and velocities&lt;/a&gt; relative to Pluto and the Sun (Heliocentric velocity), which is currently &lt;strong&gt;14.54 km/s&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h3&gt;Full Trajectory: Side View&lt;/h3&gt;&#xA;&lt;p&gt; &lt;img src=&quot;https://i.stack.imgur.com/WNlI0.png&quot; alt=&quot;New Horizons Full Trajectory: Side View&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;This image shows New Horizons' current position along its full planned&#xA;trajectory. The green segment of the line shows where New Horizons has&#xA;traveled since launch; the red indicates the spacecraft's future path.&#xA;Positions of stars with magnitude 12 or brighter are shown from this&#xA;perspective, which is slightly above the orbital plane of the planets.&lt;/p&gt;&#xA;&lt;h3&gt;Full Trajectory: Overhead View&lt;/h3&gt;&#xA;&lt;p&gt;             &lt;img src=&quot;https://i.stack.imgur.com/IKoJU.jpg&quot; alt=&quot;New Horizons Full Trajectory: Overhead View&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;This image shows New Horizons' current position along its full planned&#xA;trajectory. The green segment of the line shows where New Horizons has&#xA;traveled since launch; the red indicates the spacecraft's future path.&#xA;Positions of stars with magnitude 12 or brighter are shown from this&#xA;perspective, which is above the Sun and &amp;quot;north&amp;quot; of Earth's orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;These velocities won't change by much by the time New Horizons comes the closest to Pluto during its flyby, but you can simulate relative velocity change yourself with &lt;a href=&quot;http://eyes.nasa.gov/pluto&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Eyes on Pluto&lt;/a&gt;. Exact (to within meters per second) relative velocities are yet unknown, since it might be required for New Horizons to perform trajectory corrective maneuvers (last such maneuver, as of the time of writing this answer, was &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150615&quot; rel=&quot;nofollow noreferrer&quot;&gt;performed on June 13 UTC&lt;/a&gt;) and it's still not too late to decide on a different to nominal flyby trajectory (called &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2013/20130520-new-horizons-encounter-planning-accelerates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SHBOT - Safe Haven By Other Trajectory&lt;/a&gt;), shall New Horizons detect any debris along its current trajectory that crosses only about 1650 km away from so-called &lt;a href=&quot;https://space.stackexchange.com/a/9310/49&quot;&gt;Sailboat Island&lt;/a&gt; where it's been calculated that debris could linger in relatively stable S-type orbits between Pluto and Charon. But so far, so good.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-06-27T15:35:03.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9676" PostTypeId="1" AcceptedAnswerId="9677" CreationDate="2015-06-27T16:45:33.897" Score="3" ViewCount="914" Body="&lt;p&gt;Hubble takes very long exposures for its images so that it can collect enough light to see very distant objects. These exposures can be several hours, or even several days, long. I often read that the &lt;a href=&quot;http://apod.nasa.gov/apod/image/0403/hudf_hst_big.jpg&quot; rel=&quot;nofollow&quot;&gt;Hubble Ultra Deep Field&lt;/a&gt; image was taken with a 1,000,000 second exposure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes Hubble about 97 minutes to orbit Earth once, so while it's orbiting Earth how do they manage to keep it focused on a single point? When Hubble goes behind Earth on its orbit, wouldn't Earth get in the way of the image?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="49" LastEditDate="2015-06-28T13:11:12.547" LastActivityDate="2015-06-28T13:11:12.547" Title="Does Earth get in Hubble's way?" Tags="|orbital-mechanics|observation|imaging|hubble|space-telescope|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9677" PostTypeId="2" ParentId="9676" CreationDate="2015-06-27T16:52:32.933" Score="5" Body="&lt;p&gt;This is explained in detail in &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field#Planning&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Hubble Ultra-Deep Field: Planning&lt;/a&gt;, with perhaps the simplest way to imagine it being this illustration:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qtg5w.jpg&quot; alt=&quot;Location of the Hubble Ultra Deep Field in the space&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Location of the Hubble Ultra Deep Field in the space. Image source:&#xA;  &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Hudf-illustration.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While the location of HUDF, like explained in the Wikipedia article, isn't within the Hubble's Continuous Viewing Zone (CVZ), Hubble Space Telescope's (HST) orbit during those roughly 11.5 days (a bit over 174 orbits) effective, allowed for long continuous observations of the area in multiple sessions, without the Earth getting in the way:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The observations were done in two sessions, from September 23 to&#xA;  October 28, 2003, and December 4, 2003, to January 15, 2004. The total&#xA;  exposure time is just under 1 million seconds, from 400 orbits, with a&#xA;  typical exposure time of 1200 seconds. In total, 800 ACS exposures&#xA;  were taken over the course of 11.3 days, 2 every orbit, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Near_Infrared_Camera_and_Multi-Object_Spectrometer&quot; rel=&quot;nofollow noreferrer&quot;&gt;NICMOS&lt;/a&gt;&#xA;  observed for 4.5 days. All the individual ACS exposures were processed&#xA;  and combined by &lt;a href=&quot;http://www.stsci.edu/~koekemoer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anton Koekemoer&lt;/a&gt; into a single set of scientifically&#xA;  useful images, each with a total exposure time ranging from 134,900&#xA;  seconds to 347,100 seconds. To observe the whole sky to the same&#xA;  sensitivity, the HST would need to observe continuously for a million&#xA;  years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More on this in &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Ultra-Deep_Field#Observations&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Hubble Ultra-Deep Field:&lt;/a&gt; Observations.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-06-28T13:09:13.807" LastActivityDate="2015-06-28T13:09:13.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9678" PostTypeId="1" CreationDate="2015-06-27T16:54:31.467" Score="2" ViewCount="141" Body="&lt;p&gt;Consider the hypothetical case that we encounter alien life and it is based on the same chemical foundation as terrestrial life (amino acids, nucleic acids, etc.). Would it be reasonable to expect the same chirality as terrestrial, a 50/50 chance of it being the same, or some intermediate probability?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, is chirality truly random (50/50), are the forces which influence it favouring one choice while not excluding the other, or is one choice sufficiently favoured that the other will be inevitably extinguished?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, is this a question which can only be answered by actually finding and cataloging life of non-terrestrial origin?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-06-27T23:33:21.603" Title="Is chirality random, or is a specific chirality favoured?" Tags="|astrobiology|" AnswerCount="1" CommentCount="2" ClosedDate="2015-06-29T03:09:57.207" ContentLicense="CC BY-SA 3.0" />
  <row Id="9679" PostTypeId="2" ParentId="9421" CreationDate="2015-06-27T18:05:52.093" Score="22" Body="&lt;p&gt;LMP-103S and hydrazine differ in more ways than that hydrazine unlike LMP-103S is highly toxic and carcinogenic and that LMP-103S has higher performance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;LMP-103S requires more catalyst heating to burn and the burning temperature (about 1600 C) is considerably higher than for hydrazine (about 800 C). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The propulsion system and thus also the on-board software must take into account and ensure that the thruster-catalyst used is really hot (about 350 C), i.e. if the preheating due to a technical failure has failed and thrusting has been started, the whole mission could be lost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A hydrazine thruster start burning without preheating of the catalyst, i.e. there is no absolute need to be properly functioning and the power required for normal heating is significantly less than that of LMP-103S. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, hydrazine and LMP-103S have different properties as liquids and all valves that previously was qualified for hydrazine must first be space-qualified for LMP-103S to be safely used in space, i.e. in weightlessness &amp;amp; vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Se also two new US patent applications:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.faqs.org/patents/app/20150121843&quot; rel=&quot;noreferrer&quot;&gt;http://www.faqs.org/patents/app/20150121843&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.faqs.org/patents/app/20150128563&quot; rel=&quot;noreferrer&quot;&gt;http://www.faqs.org/patents/app/20150128563&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The space qualification of LMP-103S is finished. I give here a short update of passed milestones.&lt;/strong&gt; (updated 2015-09-17)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;September 2011 (after about 15 months in space) &lt;a href=&quot;http://www.lsespace.com/latest-news.aspx&quot; rel=&quot;noreferrer&quot;&gt;http://www.lsespace.com/latest-news.aspx&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Finally, all planned firings with the HPGP propulsion system on Prisma have been successfully been completed. &#xA;On Friday night, September 9th, a 75 sec continuous firing was performed; the longest yet with a HPGP thruster in space, and the accumulated number of pulses exceeded 50,000. &#xA;&lt;strong&gt;363 sequences have been fired and 63% of the propellant has been consumed.&lt;/strong&gt; &#xA;2 kg of the HPGP propellant remains which will be used for providing ΔV for new mission objectives during the remainder of the mission before decommissioning. &#xA;After 15 months in space, with no indications of degradation the flight demonstration is evolving into a space flight qualification.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From: &lt;a href=&quot;http://www.sffmt2013.org/PPAbstract/4088p.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.sffmt2013.org/PPAbstract/4088p.pdf&lt;/a&gt; &#xA;Dated 21:th of May 2013&#xA;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Table 2-1: The PRISMA mission phases&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mission phase........Start Date......Duration.......Operator&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nominal mission....2010-06-15.....273 days......OHB-SE (SSC)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Extended mission..2011-03-15.....160 days......DLR/GSOC&lt;/p&gt;&#xA;&#xA;&lt;p&gt;External parties.....2011-08-22......588 days......OHB-SE&#xA;mission&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Final mission.........2013-04-01......Ongoing…...OHB-SE&lt;/p&gt;&#xA;&#xA;&lt;p&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Table 2-2: External parties participants&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Organization....Experiment.............Start Date.......Duration&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space-SI........Interferometry,.........2011-09-19......7 days&lt;/p&gt;&#xA;&#xA;&lt;p&gt;......................Space debris,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;......................Distributed instrument,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;......................Formation flying&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GMV...............HARVD.................2011-10-03.......5 days&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CNES.............FFRF cont. ............2011-10-10......23 days&lt;/p&gt;&#xA;&#xA;&lt;p&gt;DLR............... ARGON..................2012-04-16...... 5 days&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CNES.............µNEAT....................2012-09-19.......3 days&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;ECAPS...........HPGP6...................2013-02-15.......1 day&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From: &lt;a href=&quot;http://www.dlr.de/ra/Portaldata/55/Resources/dokumente/rheform/replacement-of-hydrazine.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.dlr.de/ra/Portaldata/55/Resources/dokumente/rheform/replacement-of-hydrazine.pdf&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;From EU Horizon 2020 Project Rheform - A most important document&lt;/strong&gt;&#xA;Dated 15.th of June 2015&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;More than 344 sequences&lt;/strong&gt; comprising over 50,525 pulses have been performed in continuous, pulse or off-modulation mode. &#xA;Performance mapping has been performed by executing firing sequences with pulse durations from 50 ms up to 100 s and pulse mode firings have been performed with duty cycles ranging from 0.1 % to 99 %. &#xA;Pulse trains lasting up to 90 min have been executed. &#xA;Pulse mode and single impulse bit predictability has been demonstrated to be very accurate for the HPGP&#xA;system. &#xA;The accumulated burn time is more than 3.5 h to date and &lt;strong&gt;76 % of the propellant being consumed&lt;/strong&gt;. &#xA;Theremaining propellant will be used to provide ?V for extended mission objectives before eventual decommissioning.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/p/prisma-prototype#mission-status&quot; rel=&quot;noreferrer&quot;&gt;https://directory.eoportal.org/web/eoportal/satellite-missions/p/prisma-prototype#mission-status&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;During the rendezvous phase in August 2014, there was a problem with one of the propulsion system on PRISMA and the PRISMA project could not catch the opportunity to execute the inspection before the fly-by of Picard.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SNSB (&lt;strong&gt;S&lt;/strong&gt;wedish &lt;strong&gt;N&lt;/strong&gt;ational &lt;strong&gt;S&lt;/strong&gt;pace &lt;strong&gt;B&lt;/strong&gt;oard) January 27:th 2015&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;We're just discussing how to best use the fuel remaining in the HPGP system and if de-orbit can be combined with something else.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SNSB February 23:th 2015&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;It is, as said hydrazine which is at the end, not the LMP-103S. The HPGP system works just fine and will be used for the maneuvers remaining. Both systems have been used in parallel until the hydrazine ended.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a comment to: &#xA;&lt;a href=&quot;http://www.popularmechanics.com/space/rockets/a17162/nasa-green-rocket-fuel/&quot; rel=&quot;noreferrer&quot;&gt;http://www.popularmechanics.com/space/rockets/a17162/nasa-green-rocket-fuel/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mike Lara · Works at Orbital ATK&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;The ECAPS LMP-103S based HPGP systems has performed extremely well on-orbit for five years in support of the PRISMA satellite.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SNSB September 8:th 2015&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Mango has in 2015 been given the task to circularize / lowering its orbit to reduce the risk of collision. In connection with this, the HPGP system is used and has thus been able to demonstrate 5 years of operational capability in space.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="10602" LastEditorUserId="10602" LastEditDate="2015-09-17T10:59:37.470" LastActivityDate="2015-09-17T10:59:37.470" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9681" PostTypeId="2" ParentId="9678" CreationDate="2015-06-27T23:33:21.603" Score="2" Body="&lt;p&gt;Who knows?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like it would be random.  In that case there would be a 25% chance to have the same chirality choices, since both DNA and proteins each have their own chosen direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nature.com/news/force-of-nature-gave-life-its-asymmetry-1.15995&quot; rel=&quot;nofollow&quot;&gt;There is a theory&lt;/a&gt; that weak nuclear decays, which have a preferred handedness, could have preferentially destroyed organic molecules of a particular handedness, resulting in the choices we see today.  It is a very small effect, but accumulated over time might have been what tipped the hand, so to speak.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-06-27T23:33:21.603" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9682" PostTypeId="1" AcceptedAnswerId="9687" CreationDate="2015-06-28T15:44:52.193" Score="33" ViewCount="17280" Body="&lt;p&gt;I don't know much about Rockets, but all that I have seen, from the Saturn V to SpaceX's Falcon 9 have the engine at the bottom. Doesn't this make the Rocket really unstable, like balancing a pencil on your finger and trying to lift it up? Why do we not pump the exhaust gases up to the top of the rockets before expelling them? Surely that'd give a more stable rocket with less chance of faliure?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, what reason is there for having the engine at the bottom, rather than at the top like a firework rocket?&lt;/p&gt;&#xA;" OwnerUserId="10604" LastEditorUserId="51" LastEditDate="2015-07-09T21:09:09.413" LastActivityDate="2020-12-18T16:54:46.733" Title="Why are Rocket engines at the base of the rocket?" Tags="|engine-design|design|engines|" AnswerCount="7" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9684" PostTypeId="1" CreationDate="2015-06-28T16:05:11.553" Score="7" ViewCount="505" Body="&lt;p&gt;I'm looking for a way to (very) roughly estimate the convective heat transfer onto a rocket hull during atmospheric ascent (ideally through different planetary atmospheres). While I'm comfortable with most of the necessary variables, the heat transfer coefficient &lt;em&gt;h&lt;/em&gt; seems to be a bit of a problem since it depends on a range of different parameters (Prandtl, Reynolds,...) that continuously change during the ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I'm aiming for a simple and highly idealized approach, I fear that too much estimation adds up to a worthless solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm probably not the first person to think about that topic. Do you know of any links, papers, etc. to get me started?&lt;/p&gt;&#xA;" OwnerUserId="8724" LastEditorUserId="1142" LastEditDate="2015-06-28T16:34:47.537" LastActivityDate="2019-12-12T22:37:07.223" Title="Convective heat transfer on rocket during ascent" Tags="|rockets|atmosphere|heat|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9685" PostTypeId="2" ParentId="9682" CreationDate="2015-06-28T16:10:03.037" Score="3" Body="&lt;p&gt;When you exhaust hot gases at the top of the rocket, you have to shield the rocket body from those gases. You don't want the body to heat up and boil the propellant and oxygen.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those shields would have to be thick and heavy, reducing the rocket's performance.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, thousands of rocket launches (and millions of flight hours by airplanes with jet engine exhausts at the tail) have shown that stability isn't an issue worth worrying about. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-06-28T16:10:03.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9686" PostTypeId="1" AcceptedAnswerId="9707" CreationDate="2015-06-28T16:13:13.850" Score="11" ViewCount="1610" Body="&lt;p&gt;Today's CRS-7 launch from Cape Canaveral (unfortunately) ended rather spectacularly, with an explosion about a minute after reaching Max-Q (maximum dynamic pressure), and with the rocket apparently disintegrating afterwards. The mission blurb said that an 'anomaly' had been observed, and engineers were looking into it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SkmWr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;From the live SpaceX feed, a few seconds after the explosion. &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What exactly went wrong with the launch? Whose fault (if at all) is it? What corrective measures will they take to secure future launches?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note: I understand that more details will come soon, I'm looking for an answer compiling the best information to answer my three questions.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="49" LastEditDate="2015-06-28T17:36:24.993" LastActivityDate="2016-05-29T23:21:47.240" Title="What went wrong with the CRS-7 launch?" Tags="|launch|spacex|failure|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9687" PostTypeId="2" ParentId="9682" CreationDate="2015-06-28T16:17:55.193" Score="45" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;..what reason is there for having the engine at the bottom, ..&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For the fundamental logical flaw in the desire to move the engines, please see &lt;a href=&quot;https://space.stackexchange.com/a/9688/238&quot;&gt;this answer&lt;/a&gt; of Russell Borogove - it comes down to The Pendulum Fallacy (the exact same fallacy I made when considering &lt;em&gt;other&lt;/em&gt; reasons for 'engines at bottom of stack').  For more on the The Pendulum Fallacy see links at the bottom of this answer and in Russell's answer.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a nutshell the fallacy is based on the assumption that a 'top engine' layout is more stable than a 'bottom engine' layout.  In fact, they are exactly equally as (un)stable and therefore other factors determine the decision behind engine placement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This list below is some of the &lt;em&gt;other&lt;/em&gt; reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Heat.&lt;/strong&gt;  Rocket exhaust is very hot, therefore before the rocket is safe from them, the exhaust would have to come out of the engine and &lt;em&gt;past the long fuel/oxidizer tanks&lt;/em&gt; (which often contain fuel or oxidizer at 'cryonic temperatures').&lt;sup&gt;1&lt;/sup&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Control.&lt;/strong&gt;  Rockets already need sophisticated control even to get where they are intended to go.  That ability automatically ensures the rocket 'does not fall over' even under full acceleration.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Fuel flow.&lt;/strong&gt;  It is harder to pump fuel up to a rocket engine (against both gravity and the acceleration of the rocket), than to allow the fuel to flow down to the engine.  &lt;em&gt;This point has been disputed in comments below, along the lines that it is not a significant difference.  We are hoping for more data to determine how much difference it makes.&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;Hagen von Eitzen mentioned in a comment below:-  &quot;Many &lt;strong&gt;materials and structures&lt;/strong&gt; are more stable under pressure than under pull; in fact things you push in principle need not even be fixed to one another.&quot;&lt;/li&gt;&#xA;&lt;li&gt;Hobbes also points out in a comment:- &quot;It does &lt;strong&gt;complicate the engine layout:&lt;/strong&gt; you'd need a turbopump at the bottom of the stage, and pressurized lines all the way to the top. This would make using a regenerative engine (which dumps the turbopump exhaust into the combustion chamber) much harder.&quot;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_escape_system&quot; rel=&quot;noreferrer&quot;&gt;'emergency abort' rockets&lt;/a&gt; of particular American (at least) launchers put a small rocket engine &lt;em&gt;above&lt;/em&gt; the capsule, but this was only expected to fire for a very short time (and might have used solid propellants that don't need to be pumped anywhere).  Also note how the exhausts of the rockets are angled outwards in efforts to keep the capsule cool, but in doing so waste some of the thrust of the rocket engines.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ErXxF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a little ironic that this 'real life example' of engines above payload is also shown on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy&quot; rel=&quot;noreferrer&quot;&gt;Pendulum Rocket Fallacy&lt;/a&gt; page discussed in the answer of Russell Borogove/comment of Philipp.  In fact, that small sideways jet near the top of the rocket seems to be a way of changing the direction of the combined rocket/capsule rig.  I.E. further evidence that a top engine rocket is not self stabilizing, and requires active guidance.&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-06-30T19:09:56.303" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9688" PostTypeId="2" ParentId="9682" CreationDate="2015-06-28T17:12:52.637" Score="45" Body="&lt;p&gt;Putting the rocket nozzles nearer the top wouldn't make the rocket any more stable; this is the well-known &lt;a href=&quot;https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy&quot;&gt;pendulum rocket fallacy&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, some rockets have used a tractor (engine on top) configuration (Goddard's first liquid-fueled rocket in 1926, for example), but the advantages to the pusher configuration as outlined in the other answers here are dominant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be clear, the point of the pendulum fallacy isn't &quot;tractor rockets are unstable&quot;; it's &quot;tractor rockets are not self-stabilizing and are not inherently more stable than pusher rockets given the same aerodynamics and guidance and control systems&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-06-28T23:45:42.857" LastActivityDate="2015-06-28T23:45:42.857" CommentCount="22" ContentLicense="CC BY-SA 3.0" />
  <row Id="9689" PostTypeId="1" AcceptedAnswerId="9693" CreationDate="2015-06-28T18:12:30.957" Score="16" ViewCount="1607" Body="&lt;p&gt;At June 28th 2015, the SpaceX flight CRS-7 failed when the Falcon 9 v1.1 rocket exploded minutes after liftoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shotwell said twice during the press conference that Dragon capsule was transmitting after the event. This &lt;a href=&quot;https://www.youtube.com/watch?v=fTom8xVzFdo&quot;&gt;slow motion video&lt;/a&gt; of today's LOV anomaly seems to show the Dragon capsule drifting away before the final conflagration. The &lt;a href=&quot;https://twitter.com/search?q=%23SpaceX%20debris&amp;amp;src=typd&quot;&gt;debris field&lt;/a&gt; seems established already. The Rapid Unplanned Disassembly event of Falcon today occurred at about T+2:18 into its flight, well past maxQ, around 44.5 km up.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the Dragon did not sustain major damage: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How much could have survived reentry?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How much could have survived oceanic impact?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2015-08-24T13:07:43.470" LastActivityDate="2015-08-24T13:07:43.470" Title="How much of Dragon might have survived the explosion of CRS-7 and for how long?" Tags="|spacex|reentry|dragon|failure|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9690" PostTypeId="2" ParentId="9682" CreationDate="2015-06-28T19:23:50.763" Score="11" Body="&lt;p&gt;Besides the heat factor brought up by Andrew Thompson, I think another big issue is decoupling spent stages. If the stage is below you during the decouple, there is no way it could physically come back up and hit you again (unless you decouple while the thruster is still powered). If the stage is above you, then you need some kind of trick to avoid the rest of the rocket ramming into the now-empty decoupled stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another related point is the decouple itself: By its nature, the decoupler applies downward force on the element below and upward force on the element above. If the element below is the discarded stage, great! Now the junk is falling faster and the rocket got a slight boost. But if it's the other way around then the junk gets a boost and the rest of the rocket slows down, wasting Delta-V.&lt;/p&gt;&#xA;" OwnerUserId="6733" LastActivityDate="2015-06-28T19:23:50.763" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9691" PostTypeId="1" CreationDate="2015-06-28T19:28:02.860" Score="2" ViewCount="127" Body="&lt;p&gt;If a constant acceleration starship started at rest, then accelerated at 25G (245m/s^2) how long would it take to fly an AU (149 597 871 kilometers)? I am a writer, so I have no experience with kinematics, but I do know that T= Distance/Velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm having a problem with the ridiculously high numbers and unit conversion (m/s into 149 million km). &lt;/p&gt;&#xA;" OwnerUserId="10607" LastActivityDate="2015-06-28T19:43:47.667" Title="Kinematics help" Tags="|spacecraft|interplanetary|physics|astronomy|travel|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9692" PostTypeId="2" ParentId="9691" CreationDate="2015-06-28T19:38:44.120" Score="11" Body="&lt;p&gt;Wolfram Alpha can do things like this easily, handling the units for you.  Just entering &lt;a href=&quot;http://www.wolframalpha.com/input/?i=time%20to%20travel%201%20AU%20at%20an%20acceleration%20of%2025%20g&quot;&gt;&quot;time to travel 1 AU at an acceleration of 25 g&quot;&lt;/a&gt; gives the answer.  The result is 9.7 hours.  You can click on &quot;Show formula&quot; to see the formula used.  It is one step beyond T=Distance/Velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That gets you up to &lt;a href=&quot;http://www.wolframalpha.com/input/?i=speed+after+9.704+hours+at+an+acceleration+of+25+g&quot;&gt;0.03 c&lt;/a&gt; (click &quot;More units&quot;), so relativistic effects can be mostly ignored.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-06-28T19:43:47.667" LastActivityDate="2015-06-28T19:43:47.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9693" PostTypeId="2" ParentId="9689" CreationDate="2015-06-28T20:54:34.610" Score="14" Body="&lt;p&gt;As the RUD event occurred during stage 1 flight the Dragon wouldn't having been travelling fast enough to need a heatsheild to survive reentry. (And from 44.5 km it hadn't really gone high enough to be said to be 'reentering', it never really left). The biggest danger to the dragon during the fall would be if it started tumbling. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The conic structure of Dragon is designed to take aerodynamic pressure from the nose on launch, and the base on descent, but a mix of forces in a tumble might have been too much for it. Whether it could have survived a tumbling separation and decent depends on how wide an engineering margin SpaceX used, and given how little we know it's difficult to speculate with any accuracy at this point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to your second question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How much could have survived oceanic impact?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All of it. But it would be in lots of little pieces.&#xA;It's often said that when you're going at several hundred km/h hitting water is almost the same as hitting concrete, and it's true. Dragon is a beautiful spaceship, but there's no way she could remain intact after that kind of impact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course it probably would have been possible (assuming minimal/no damage) for Dragon to open its parachutes as normal and survive completely intact. I doubt very much that this happened. A failure of this nature, with Falcon destroyed but Dragon intact, was probably never envisaged during the construction of Dragon's software.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best we can realistically hope for is maybe some hull-panels, a Draco combustion chamber or two, and possibly a battered flight computer.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2015-06-28T20:54:34.610" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9694" PostTypeId="2" ParentId="9511" CreationDate="2015-06-28T23:52:54.180" Score="9" Body="&lt;p&gt;The difference in the engine weights is due to trapped propellant.  Referring to the &lt;a href=&quot;http://history.nasa.gov/ap08fj/pdf/sa503-flightmanual.pdf&quot; rel=&quot;noreferrer&quot;&gt;Saturn V Flight Manual for SA-503&lt;/a&gt;, Figure 2-21, we can see that the residual propellant in the 5 engines is 2160 lbs LOX and 6585 lbs RP-1.  Dividing the sum of these numbers (8745) by 5 we get a per-engine trapped prop mass of 1749 which is probably close enough to the 1680 lbs/engine shown in the graphic above.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have reproduced the appropriate table for your convenience below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3ybCy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-06-28T23:52:54.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9695" PostTypeId="2" ParentId="9511" CreationDate="2015-06-29T00:15:55.880" Score="1" Body="&lt;p&gt;The weight difference is due to residual propellants in the engine. The engine cannot run dry, mainly for two reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first reason is the bearings in the turbopumps. They need to be constantly cooled by the pumped fluid. Because this is a LOX-Kerosene Engine, the cooling is done with Kerosene, as can be seen on page 11 (p.25 of the pdf) of this document:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750012398.pdf&quot; rel=&quot;nofollow&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750012398.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The barrier seal is clearly marked and located on the oxygen side of the bearings. In Kerosene, an overheating bearing can cause all sorts of headaches, such as thermal stress and as a result little spalls that could cog something downstream.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In hydrogen-oxygen engines on the other hand, the oxygen and hydrogen pumps are on separate shafts, because the lower density of hydrogen means that a higher rotational speed is needed. In that case the LOx pump bearings are cooled by liquid oxygen and running them dry is disastrous. The pump will ignite very quickly. This happened on a test stand for the SSME bearings once. Notice the scary font on page 25 of this pdf:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100023061.pdf&quot; rel=&quot;nofollow&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100023061.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second big reason that comes to my mind is the regenerative cooling. Whatever propellants and pressure you use, the combustion temperature is by far larger than the survivable temperature of your chamber wall. In the F1, the chamber is cooled by the fuel and the nozzle extension is cooled by a film of dumped turbine exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the engine is shut down by closing valves directly downstream of the pumps, the fuel is left stagnant in the cooling tubes because there is nothing to push it onwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible to purge the channels using an inert gas, but then the tubes would be filled with gas, while the engine still burns, possibly causing an unclean shutdown. Probably not an explosion, since the cooling channels are filled with inert gas now, so them burning through will not be so much of a problem. However, the additional complexity is most likely not worth it, especially for a first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the nozzle extension, the cooling requirement is not quite as high here, and it is cooled by gas to begin with. When the main valves close, the gas generator is quickly drained of propellants and the cooling of the nozzle extension stops. But since the combustion chamber is extinguished a the same time, this is not likely to cause any trouble.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="1028" LastEditDate="2015-06-29T04:42:34.313" LastActivityDate="2015-06-29T04:42:34.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9698" PostTypeId="1" AcceptedAnswerId="9699" CreationDate="2015-06-29T06:46:51.420" Score="4" ViewCount="296" Body="&lt;p&gt;I've been searching all over the site and the internet, but I can't find something about it. Is there really no way for a plane, helicopter or quad-copter to fly on the moon Europa? &#xA;If it is possible, please specify which kind of thing it would be (even if it's not a real model but a theoretical one)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the answer is no, is there any other rocky planets or satellites (besides Mars) where you could achieve atmospheric flight of any kind?&lt;/p&gt;&#xA;" OwnerUserId="10617" LastEditorUserId="8693" LastEditDate="2016-03-17T22:40:04.520" LastActivityDate="2016-03-17T22:40:04.520" Title="Is there a way to achieve any kind of atmospheric flight on the moon Europa?" Tags="|atmosphere|europa|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9699" PostTypeId="2" ParentId="9698" CreationDate="2015-06-29T07:14:23.187" Score="4" Body="&lt;p&gt;In our solar system the rocky planets/satellites which have atmospheres dense enough for flying are Venus, Earth, Mars, and Titan.  I believe that after Mars the next thickest atmosphere on a rocky world would be on Triton and that is much thinner than that of Mars. It has detectable winds but probably is way too thin for flying.  Of course no guarantee how pleasant the flight will be is given for Earth or any other of those worlds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe that the moon Europa has basically a vacuum instead of an atmosphere.  Rockets would have to be used instead of airplanes or balloons, or people would have to use ground transportaton.&lt;/p&gt;&#xA;" OwnerUserId="10618" LastActivityDate="2015-06-29T07:14:23.187" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9700" PostTypeId="2" ParentId="9209" CreationDate="2015-06-29T08:01:03.080" Score="7" Body="&lt;p&gt;As I remember, Saturn was often described as being less dense than water and thus able to float in a giant ocean, long before the probes.  The orbiting  natural satellites were used to measure the  total masses and thus densities of the gas planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;James Blish coined the term &quot;gas giant&quot; to describe the outer planets about 1952 in a science fiction story, so by then it was accepted that they had low density and very deep dense atmospheres above any &quot;surfaces&quot; they might have.&lt;/p&gt;&#xA;" OwnerUserId="10618" LastEditorUserId="10618" LastEditDate="2015-06-30T04:51:30.250" LastActivityDate="2015-06-30T04:51:30.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9701" PostTypeId="2" ParentId="5299" CreationDate="2015-06-29T12:01:19.040" Score="16" Body="&lt;p&gt;While I don't have a complete answer I thought I'd share what I've found in the hope that someone can come up with something better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can find nothing online about exactly how and why the visiting vehicle schedule is constructed (Although this could simply be my poor google-fu skills). One thing that &lt;em&gt;is&lt;/em&gt; clear is just how tightly packed the schedule is. &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/structure/resupply.html#.VZEdiRtVhBc&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; is an ISS vehicle schedule from NASA covering expeditions 10-41 (10/16/04-09/25/14). And &lt;a href=&quot;http://spider.seds.org/shuttle/iss-sche.html&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; is a schedule until mid 2020 complied from various sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also came across a good graphic that shows the visiting vehicle schedule from the beginning of the station to mid 2013. &#xA; &lt;img src=&quot;https://i.stack.imgur.com/Xuygm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130013168.pdf&quot; rel=&quot;noreferrer&quot;&gt;Source.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been numerous instances where a launch has had to be delayed because a vehicle lost its slot in the schedule. perhaps &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/International_Space_Station/Traffic_jam_in_space_ATV_docking_under_tight_schedule&quot; rel=&quot;noreferrer&quot;&gt;the most interesting&lt;/a&gt; is when ATV Jules Verne had to wait on-orbit for STS-123 to leave the station before docking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably the most useful document I found is &quot;&lt;a href=&quot;http://www.esa.int/esapub/bulletin/bullet94/LUETTGENS.pdf&quot; rel=&quot;noreferrer&quot;&gt;Operations Planning for the International Space Station&lt;/a&gt;&quot; by the ESA Directorate for Technical and Operational Support. It details how operational plans are constructed, but doesn't contain enough detail to directly answer the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally there's &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/NACRS_ISSResourceOverview_022014T.pdf&quot; rel=&quot;noreferrer&quot;&gt;this document&lt;/a&gt; that, while it doesn't help answer the question, does include a huge amount of detail about the resouces available on ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In conclusion&lt;/strong&gt; I think it is very unlikely that these missions were targeted for a time when the station was half-crewed, I think it's far more likely that it's just a product of coincidence and a full schedule. That said I can't find a definitive statement confirming this.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2015-06-29T12:01:19.040" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9702" PostTypeId="1" AcceptedAnswerId="9703" CreationDate="2015-06-29T12:20:43.133" Score="8" ViewCount="2315" Body="&lt;p&gt;There are 4 major spacecraft that can supply the ISS. Progress, Dragon, ATV, and Cygnus. Of these 4 spacecraft, only the ATV has not had an issue in the last 8 months. ATV has stated they will not launch any further vehicles. Bottom line is, every spacecraft that resupplies the ISS has had an issue such that it seems like they just can't launch at this moment. Will this cause problems getting supplies to the ISS?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-29T20:42:41.420" Title="Who can now resupply the ISS?" Tags="|iss|dragon|progress|cygnus|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9703" PostTypeId="2" ParentId="9702" CreationDate="2015-06-29T12:28:10.137" Score="14" Body="&lt;p&gt;You forgot HTV, the Japanese vehicle that has a fairly large payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has a planned flight Aug 15, 2015. There are several more HTV flights planned on the manifest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Progress is due to fly again with cargo in early July 2015. Assuming the Russian space agency is able to resolve its issues that will help with the supply situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cygnus flying on Atlas V is due to fly in December of 2015. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming SpaceX can resolve their flight issue they had flights booked for September and December 2015. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are issues, but they should be able to work out. With 4 providers (Cygnus, Dragon, HTV, Progress) it should be assumed at least one will succeed.  Now would be a great time for ESA to spin back up ATV production and come in as the white knight and save the day!  (I know how hard it would be, long lead items, etc but they are putatively developing the the Service Module for Orion based on the ATV already anyway.)&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-06-29T14:51:42.513" LastActivityDate="2015-06-29T14:51:42.513" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9704" PostTypeId="2" ParentId="9702" CreationDate="2015-06-29T12:40:28.977" Score="2" Body="&lt;p&gt;The Chinese might come to help with their Long-March launcher and Shenzhou spacecraft, which is very similar to the Soyuz spacecraft so it should be compatible. Only Russia and China launch humans to orbit these days, so that is the obvious short-term option I think.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-06-29T12:40:28.977" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9705" PostTypeId="2" ParentId="9185" CreationDate="2015-06-29T19:02:00.027" Score="9" Body="&lt;p&gt;This is a difficult question to answer, so I'm going to take a stab based off of a few assumptions.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;GEO satellites are ones with a period between 1430-1450 minutes (Standard definition from NORAD)&lt;/li&gt;&#xA;&lt;li&gt;I have no knowledge of classified satellites, those will not be included.&lt;/li&gt;&#xA;&lt;li&gt;I'm going to assume you mean GSO satellites (Geostationary). &lt;/li&gt;&#xA;&lt;li&gt;Active GEO satellites have a near 0 inclination, dead ones will have a non-zero inclination. Why? &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot;&gt;GSO satellites&lt;/a&gt; drift in inclination over time.&lt;/li&gt;&#xA;&lt;li&gt;GSO satellites will have an eccentricity near zero, dead or living. This doesn't vary as much as the inclination.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Okay, so given those, what are we looking at? Norad lists 804 objects in that orbital period range in orbit. Of these, here are some statistics:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;797 have a perigee of at least 35000 km (Low eccentricity)&lt;/li&gt;&#xA;&lt;li&gt;696 are considered payloads.&lt;/li&gt;&#xA;&lt;li&gt;18 of the payloads have very large inclinations (Greater than 16). These were probably not launched into GSO&lt;/li&gt;&#xA;&lt;li&gt;307 have inclinations less than 0.5 degrees. These are probably the active GSO satellites.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'm going to remove the large inclination satellites from consideration. That leaves 371 payload objects near GSO that are not being maintained in inclination, and thus are probably dead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the mass of these, assuming an average of 1800 kg, which is the average dry mass of a satellite, that means there is about 668,000 kg of satellites in GEO that aren't being properly maintained.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-06-29T19:22:22.737" LastActivityDate="2015-06-29T19:22:22.737" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9706" PostTypeId="1" CreationDate="2015-06-29T19:56:46.777" Score="4" ViewCount="484" Body="&lt;p&gt;Robert Zubrin makes a compelling case for &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Direct&quot; rel=&quot;nofollow&quot;&gt;using Mars&lt;/a&gt; as a site to make methane fuel for the return trip back to Earth. However, maybe there is an additional refinement to this model;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the landers and habitats to be shipped to Mars, I would like to suggest that a 'tanker' vehicle be left in Mars orbit. As each batch of Methane is completed on Martian surface, it is shuttled up and transferred to the 'tanker' vehicle. This tanker vehicle should be large enough to contain enough fuel not only for the return trip to Earth, but also for the return trip to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth can then restrict its activities to providing the Hydrogen feed stock for further Methane production (back on Mars) and for the supply of manufactured goods. Does all this make any sense?  &lt;/p&gt;&#xA;" OwnerUserId="10627" LastEditorUserId="49" LastEditDate="2015-06-30T15:56:30.147" LastActivityDate="2015-07-02T09:41:57.073" Title="Should we use Mars as the main source of interplanetary fuel?" Tags="|mars|colonization|feasibility|isru|fuel-depots|" AnswerCount="2" CommentCount="3" ClosedDate="2015-07-02T13:22:57.980" ContentLicense="CC BY-SA 3.0" />
  <row Id="9707" PostTypeId="2" ParentId="9686" CreationDate="2015-06-29T20:06:23.240" Score="24" Body="&lt;p&gt;&lt;strong&gt;Update 7/20/2015 via Elon Musk's Conference Call, compiled by /r/spacex and twitter sources&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Preliminary conclusion is that a &lt;strong&gt;COPV (helium container) strut&lt;/strong&gt; in the CRS-7 second stage failed at 3.2Gs.&#xA;  We analysed a lot of data, took 0.893 seconds before first sign of trouble and end of data. Preliminary failure arose from a strut in the second stage liquid oxygen tanks that was holding down one composite helium bottle used to pressurize the stage. High pressure helium bottles are pressurized at 5500 psi, stored inside in LOX tank. Several helium bottles in upper stage. At ~3.2 g, one of those struts snapped and broke free inside the tank. Released lots of He into LOX tank. buoyancy increases in accordance w/ G-load. Data shows a drop in the helium pressure, then a rise in the helium pressure system. Quite confusing. As helium bottle broke free and pinched off manifold, restored the pressure but released enough helium to cause the tank to fail. it was a really odd failure mode. Data indicates helium tank did not burst.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What Happened?&lt;/strong&gt;&lt;br&gt;&#xA;At T+139 seconds after launch, the second stage Liquid Oxygen Tank(LOX) experienced an overpressure event. The Falcon 9 flight computer determined a failure had occurred and activated its Flight Termination System. Dragon decoupled from the rocket and made a hard impact in the Atlantic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why did this happen?&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We will not know for sure until SpaceX releases more information which will not occur until after their investigation is closed. However we can do some basic detective work based on the information that's been released so far.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There was an overpressure event in the upper stage liquid oxygen tank.&#xA;  Data suggests counterintuitive cause.&lt;/p&gt;&amp;mdash; Elon Musk (@elonmusk)&#xA;  &lt;a href=&quot;https://twitter.com/elonmusk/status/615185076813459456&quot; rel=&quot;nofollow&quot;&gt;June&#xA;  28, 2015&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Just before Main Engine CutOff(MECO), the second stage disintegrated. Elon Musk said that the LOX tank experienced an overpressure event. Basically, the tank ruptured from the inside out. The tanks of a rocket form its main structure. Once the tank failed, the aerodynamic pressure from the leading edge destroyed the second stage and Dragon detached and fell off. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Did the landing attempt cause this?&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both the Dragon capsule and Falcon 9 first stage were performing nominally and were sending telemetry back to ground after this event occurred. In the &lt;a href=&quot;https://youtu.be/OAX7UFd70M8?t=2m35s&quot; rel=&quot;nofollow&quot;&gt;video&lt;/a&gt;, you can see that the first stage engines keep firing for a few seconds after the initial explosion. None of the landing modifications(fins, legs, etc) were the cause of this incident.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If the the problem was with the second stage, why did the whole rocket explode?&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every Falcon 9(and almost all rockets in general) have a Flight Termination System(FTS). This is usually a line of plastic explosive(C4) that runs down the body of the rocket. There are many reasons why this system is in place. It is better to blow up a rocket once it starts going out of control than to let it fly in an uncontrolled direction and crash. Also rockets contain an immense amount of chemical energy in their fuel. Crashing those tanks can cause the fuel to compress and ignite all at once creating a massive explosion. The FTS 'unzips' the tanks, allowing it to burn in the air instead of exploding. The &lt;a href=&quot;https://youtu.be/BSr4hUcROwo?t=3m59s&quot; rel=&quot;nofollow&quot;&gt;Antares Failure&lt;/a&gt; resulted in a large fireball, but the FTS was actually activated before it hit the ground, preventing a lot of serious damage to the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since SLC-40 is an Air Force base, the Air Force has the responsibility to maintain Range Safety. They have a metaphorical big red button that will instantly destroy the rocket if it deviates from the flightpath. However the Air Force didn't blow up the rocket, the Falcon 9 did it itself. Once the rocket clears the range, it is now SpaceX's responsibility. Usually they would have someone also holding a big red button. However SpaceX implemented an automatic FTS based on GPS as well as the rocket's own sensors. Once the vehicle detected the failure it automatically &quot;safed&quot; itself.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why didn't SpaceX try to land the first stage?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing a 150 foot tall rocket stage is already an improbably hard task. The second stage disintegrated close to Max-Q, where the rocket is under the most air pressure during its flight. It's highly unlikely the stage could have maneuvered sufficiently to attempt to land. Also the incident occurred close to shore(~13km) while the landing barge was &lt;a href=&quot;https://www.google.com/maps/d/u/0/viewer?mid=zXmjsQgIEPtk.kSyT9f2SlzlM&quot; rel=&quot;nofollow&quot;&gt;318km from shore.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Whose fault (if at all) is it?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a tricky area. SpaceX's rocket exploded. Something went wrong in either the manufacturing or engineering of the vehicle causing it to fail. However, the contract awarded to SpaceX by NASA, like most launch contracts, is based on launch attempts not successes. Since SpaceX successfully got the payload off of the pad, they will still get paid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What corrective measures will they take to secure future launches?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first measure, unfortunately, is that Falcon 9 is grounded until the problem is resolved. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Cause still unknown after several thousand engineering-hours of&#xA;  review. Now parsing data with a hex editor to recover final&#xA;  milliseconds.&lt;/p&gt;&amp;mdash; Elon Musk (@elonmusk) June 29,&#xA;  2015&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The next step is to gather as much data as possible on the incident. That includes all pictures, any debris they can recover, as well as the telemetry the rocket broadcast during flight. There were over 3000 telemetry sensors on CRS-7 &lt;a href=&quot;http://www.reddit.com/r/spacex/comments/3beu7w/rspacex_crs7_official_postlaunch_conference_thread/&quot; rel=&quot;nofollow&quot;&gt;according to the post-incident briefing&lt;/a&gt;. Then what is called a Fault Tree Analysis will begin. SpaceX engineers will work back from the smallest changes in the rocket to find what went wrong in the second stage LOX tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the problem is identified, they will need to fix it. If it is a manufacturing error, a change of processes or stricter quality control may be enough to fix it. If it is an engineering error, the design must be fixed to prevent it from happening again.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Upgrades in the works to allow landing for geo missions: thrust +15%,&#xA;  deep cryo oxygen, upper stage tank vol +10%&lt;/p&gt;&amp;mdash; Elon Musk&#xA;  (@elonmusk) March 2,&#xA;  2015&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is some hope in that the SES-9 launch is/was supposed to be the introduction of the Falcon 9v1.2 &quot;FullThrust&quot; which uses its engines at full throttle, uses densified LOX, and has an improved/lengthened second stage. If the problem has already been fixed on v1.2 then we could see Falcon flying relatively quickly.&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="195" LastEditDate="2016-05-29T23:21:47.240" LastActivityDate="2016-05-29T23:21:47.240" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9709" PostTypeId="1" AcceptedAnswerId="9710" CreationDate="2015-06-29T22:48:37.930" Score="9" ViewCount="668" Body="&lt;p&gt;Solar Impulse is on target to &lt;a href=&quot;http://www.bbc.com/news/science-environment-33244912&quot;&gt;circle the world.&lt;/a&gt; Presumably Solar Impulse has the most current and efficient solar cells available, while those on the ISS range between &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/structure/elements/solar_arrays.html#.VZHKYEZEJmg&quot;&gt;5 and 10 years old&lt;/a&gt;.  How do the two different solar panel systems compare? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="51" LastEditDate="2015-07-01T11:58:18.583" LastActivityDate="2015-07-01T11:58:18.583" Title="What is the difference in solar panel efficiency between Solar Impulse &amp; ISS?" Tags="|iss|technology|solar-power|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9710" PostTypeId="2" ParentId="9709" CreationDate="2015-06-30T00:19:19.027" Score="8" Body="&lt;p&gt;The efficiency of the panels on Solar Impulse is &lt;a href=&quot;http://info.solarimpulse.com/en/our-adventure/building-a-solar-airplane/&quot; rel=&quot;noreferrer&quot;&gt;listed as 23%&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The panels on the ISS are much older than 5 and 10 years.  The first array (Z-) was added in 2000, which implies a manufacture date of at least 1-2 years earlier. The remaining three sets were added in 2006, 2007, and 2009. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS Panels get about &lt;a href=&quot;http://blog.urthecast.com/space/international-space-stations-solar-panels-have-a-no-fail-mission/&quot; rel=&quot;noreferrer&quot;&gt;14% efficiency&lt;/a&gt;, which was pretty good when they were manufactured. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good chart listing solar cell efficiency's as available for home installation, is available &lt;a href=&quot;http://sroeco.com/solar/most-efficient-solar-panels&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.  A screenshot of the table is below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VQv7Z.png&quot; alt=&quot;Home Solar Cell Efficiency&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus Solar Impulse seems like it has much better arrays. Of course they are not space rated, which is a very different design (exposure to atomic oxygen, weight, etc).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian Mir space station had an array on Kvant-1 with &lt;a href=&quot;http://www.americaspace.com/?p=28747&quot; rel=&quot;noreferrer&quot;&gt;12% efficiency&lt;/a&gt; but dropped to 5% at the end of life. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-06-30T12:25:57.227" LastActivityDate="2015-06-30T12:25:57.227" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9711" PostTypeId="1" CreationDate="2015-06-30T02:02:20.497" Score="6" ViewCount="1818" Body="&lt;p&gt;If a ship enters orbit around the Moon's equator, any place on the equator is equally accessible and doesn't take long to get to. The ship passes over the same strip of land with each orbit. Equally, if a ship orbits over the poles, with each orbit it will pass over different strips of land underneath it as the Moon spins while the ship's orbit stays aligned in the same direction, but it will always pass over the poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reading I've seen it mentioned that the location of a lunar base should consider accessibility from orbit - how easy it is to land there. I can't picture why that is an issue. Can't you adjust where you will enter lunar orbit so far in advance that the delta v required to get any inclination you want is minimal?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon spins pretty slowly, it takes it a whole month to turn once. If you are aiming for a spot other than the poles or the equator, isn't how much the Moon turns over the course of one orbit around it not very much? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Supposing it was necessary to orbit for some time before landing, couldn't an orbit with a high apoapsis be set up so that a small burn would once again align the ship with the landing site?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-06-30T23:31:43.537" Title="How much harder is it to land on the Moon away from the equator and the poles?" Tags="|orbital-mechanics|the-moon|orbital-maneuver|delta-v|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9712" PostTypeId="2" ParentId="9711" CreationDate="2015-06-30T07:01:08.653" Score="8" Body="&lt;p&gt;Equatorial (or near equatorial) sites would be preferred, because the flight from Earth to the Moon can be achieved on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Free_return_trajectory&quot;&gt;free return trajectory&lt;/a&gt;.  (Meaning, if the engine fails during the long coast then the spacecraft will loop around the moon and coast back to Earth.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other locations can be reached, for example the later moon landings went to landing sites further from the preferred area.  If these had suffered a  failure in flight they would have had to arrange an engine firing somehow to get back to Earth.  (This was done in &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13&quot;&gt;Apollo 13&lt;/a&gt; for example.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-06-30T07:01:08.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9713" PostTypeId="2" ParentId="9711" CreationDate="2015-06-30T07:06:27.553" Score="4" Body="&lt;p&gt;First off, I don't have the background to calculate actual values on how much harder other landing sites are to reach, but I'm happy to add my thoughts about it.&lt;/p&gt;&#xA;&lt;p&gt;There are a few different mission profile options to consider (adapting names from &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program#Choosing_a_mission_mode&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo mission modes&lt;/a&gt;):&lt;/p&gt;&#xA;&lt;h2&gt;Direct Ascent&lt;/h2&gt;&#xA;&lt;p&gt;Your entire ship lands, leaving nothing in orbit, then on return all or part of your ship leaves the moon to return directly to Earth (or some other destination), again leaving nothing in orbit.&lt;/p&gt;&#xA;&lt;p&gt;In this mode your lunar orbits don't particularly matter: because you're not leaving anything in orbit the landing and ascent orbits don't need to align.  You just need an insertion orbit that goes over the site (easy to set up while approaching the moon without requiring plane changes when you get there), then on departure you just need to take off in whatever direction is most efficient for where you want to go (and again do course corrections while in transit instead of plane changes while in orbit).&lt;/p&gt;&#xA;&lt;p&gt;Note that this assumes you have no issues that could delay landing such that your orbit and landing site no longer line up (either making landing at the planned site impossible or costing more fuel to correct, and how much fuel buffer did you have to include specifically for this reason that you could have left out given a polar/equatorial site?).  Having the ability to make fallback plans that still result in a successful mission is generally a plus.&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit_rendezvous&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Orbit Rendezvous&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;p&gt;To save the fuel cost of landing and later launching your return stage you leave it in orbit, taking only a lighter-weight lander to the surface.&lt;/p&gt;&#xA;&lt;p&gt;Given that you eventually need to rendezvous with your return stage you have a few options (drawing from what you suggested):&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Plan the duration of your stay on the surface arrange your orbit to align with the landing site for both descent and ascent.  You don't have to wait for a full revolution, you just have to wait for your landing site to be in the plane of your orbiter (though you do save some fuel if you can take advantage of the rotation of the moon by taking off in a prograde direction which would require waiting the full ~29.5 days).  I can best explain this with a picture (pardon my lack of artistic abilities):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/CC73U.png&quot; alt=&quot;Lunar Orbits&quot; /&gt;&#xA;You're not landing at the pole so you have orbital inclinations anywhere between your latitude (aligns once every 29.5 days) and polar orbit (aligns once every 14.75 days alternating ascending and descending passes).  The closer your orbital inclination is to your landing latitude the closer the time on the surface will be to 0 or 29.5 days (land on an ascending pass for a descending pass to be overhead soon, or land on a descending pass and wait most of a month for the ascending pass to come around).  For instance, if you moved the landing site to the other part of the blue line that crosses that latitude it would take 20 days for the next orbit that aligns with the landing site instead of the 10 days that I represented in the picture.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Also, &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/A15CSMRev1-14.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here's a picture&lt;/a&gt; of orbit traces from Apollo 15 illustrating how much the moon shifted underneath the orbit of the command module in just 14 orbits.&lt;/p&gt;&#xA;&lt;p&gt;You're saving fuel by not needing any plane changes, but your schedule is tightly constrained by the orbit of your orbiting stage.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Have a high apoapsis so plane changes are relatively cheap.  This means you'll have more velocity to burn off on descent and more velocity to make up on ascent.  I suspect the overall result would be cheaper in terms of fuel than plane changes to align with a circular orbit, but I don't know the math to check this offhand and it's still not going to be cheap.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Plan your landing site and orbit such that &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects&quot; rel=&quot;nofollow noreferrer&quot;&gt;lunar gravitational anomalies&lt;/a&gt; take care of the plane changes of your orbital stage for you.  I don't know how good a map of the lunar gravitational field we have but I think you'd have to be crazy to plan on this.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h2&gt;Conclusion&lt;/h2&gt;&#xA;&lt;p&gt;Landing on the moon is all about tradeoffs:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;You can land your entire ship to gain convenience (not needing a lunar rendezvous) and maybe eliminate complexity to reduce risk, but it'll cost you more fuel to land the additional mass.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You can leave part of your ship in orbit (reduce lander mass) to save fuel on landing, but now you have to align orbits and rendezvous with your orbiter when you leave so you have to consider the latitude of your landing site.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;I'd say a less accessible landing site could certainly be worked with, but you have to decide if that landing site adds enough value over a site that's more convenient to access for it to be worth the extra trouble.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-06-30T07:06:27.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9714" PostTypeId="1" AcceptedAnswerId="9739" CreationDate="2015-06-30T07:13:38.070" Score="11" ViewCount="8003" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/9709/what-is-the-difference-in-solar-panel-efficiency-between-solar-impulse-iss&quot;&gt;this question&lt;/a&gt;, Geoffc's answer stated that the ISS solar arrays are about 14% efficient. Higher efficiency panels were available when these arrays were manufactured. Given the high cost per pound of launching stuff to orbit, why were more efficient panels not used? &lt;a href=&quot;https://space.stackexchange.com/questions/9602/total-mass-of-the-iss-solar-array&quot;&gt;This question&lt;/a&gt; estimates the mass of the arrays at 30 tons. Double the efficiency (as seems to be possible with triple-junction panels), and you'd be able to halve that.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2021-06-20T19:06:14.447" Title="Why does the ISS not use the most efficient solar panels available?" Tags="|iss|solar-power|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9715" PostTypeId="2" ParentId="9711" CreationDate="2015-06-30T07:57:59.427" Score="9" Body="&lt;p&gt;For a polar orbit, the spaceship can enter the moon's sphere of influence at a place where the velocity vector with regard to the moon is coplanar with a great circle passing through the lunar poles&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ItrDw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At apogee the ship's moving about .19 km/s with regard to earth and the moon is moving about 1.02 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GMZ8p.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/fSRVh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The .19 and 1.02 km/s vectors are at angles to each other which increases delta V a little but not much. Vinf wrt Moon is about .86 km/s which is nearly the same as the Vinf when entering an equatorial orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The extra delta V expense for reaching the poles is minor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more important consideration are opportunities for earth return. For a low latitude lunar orbit, a launch opportunity occurs each orbit. You can head back to earth most any time. With a polar lunar orbit, launch opportunities open every two weeks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another consideration is lighting. Landing near a pole means landing near the terminator. Shadows are very long, so it's harder to discern lay of the land at the launch site.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-06-30T23:31:43.537" LastActivityDate="2015-06-30T23:31:43.537" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9716" PostTypeId="2" ParentId="5617" CreationDate="2015-06-30T08:21:16.547" Score="3" Body="&lt;p&gt;There have been &lt;a href=&quot;http://www.science.gov/topicpages/a/asteroid+impact+hazard.html&quot; rel=&quot;nofollow&quot;&gt;lots of studies&lt;/a&gt; into breaking up an asteroid. &lt;a href=&quot;http://adsabs.harvard.edu/abs/2013AGUFMNH23D1556W&quot; rel=&quot;nofollow&quot;&gt;This one&lt;/a&gt; and &lt;a href=&quot;http://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-10-04495&quot; rel=&quot;nofollow&quot;&gt;this one&lt;/a&gt; suggest that detonating a nuclear bomb inside an asteroid would be effective, breaking up the asteroid in many fragments and giving them enough delta-V that (if you do it early enough) many fragments will miss Earth altogether.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-06-30T08:21:16.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9717" PostTypeId="2" ParentId="9682" CreationDate="2015-06-30T09:36:11.437" Score="2" Body="&lt;p&gt;The other answers have covered why you don't put the rocket motor at the top but not covered this point of the question.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why do we not pump the exhaust gases up to the top of the rockets before expelling them?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you're &lt;em&gt;pumping&lt;/em&gt; the exhaust gasses to the top of the rocket, the thrust of your rocket is exactly the power of the pump, minus losses due to inefficiency. What's powering the pump? Wouldn't it be better to use that power source to drive the rocket directly, instead of using it to move around the waste products of some messy chemical reaction?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So lets assume you just mean diverting the exhaust from a burner at the bottom of the rocket to nozzles at the nose.  The power of a rocket comes from carefully shaping the combustion chamber and exhaust nozzle. Only a part of the thrust comes from the pressure of the exhaust gasses causing them to squirt out of the combustion chamber. Most of the thrust comes from the shaping of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Propelling_nozzle&quot; rel=&quot;nofollow&quot;&gt;nozzle&lt;/a&gt;, which lowers the pressure of the exhaust gasses and accelerates them to very high speeds. Fitting a pipe from the bottom of the rocket to the top (which would have to contain at least one 180&amp;deg; turn) would completely ruin the geometry of the nozzle, almost certainly to the point of making it not work at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd also need to contain the extremely hot exhaust gasses, which would be trying very hard to melt your pipe, losing energy all the way.&lt;/p&gt;&#xA;" OwnerUserId="2784" LastActivityDate="2015-06-30T09:36:11.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9718" PostTypeId="2" ParentId="8914" CreationDate="2015-06-30T11:06:34.757" Score="5" Body="&lt;p&gt;There are parts that would be easy, and parts that would be quite difficult. Here is a video that shows you how solar cells are made on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/fZ1SC-vUe_I?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a few things you need:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A very high efficiency clean room.&lt;/li&gt;&#xA;&lt;li&gt;Wafer manufacturing capabilities&lt;/li&gt;&#xA;&lt;li&gt;Etching equipment&lt;/li&gt;&#xA;&lt;li&gt;Diffusing equipment&lt;/li&gt;&#xA;&lt;li&gt;Metal inlaying&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, this is very large and difficult equipment. It would probably not work very well. What are some alternatives? The best alternative would be to make mirrors, which in turn heat a substance, like water, and runs like any turbine engine runs. &lt;a href=&quot;https://en.wikipedia.org/wiki/Concentrated_solar_power#/media/File:PS10_solar_power_tower.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; has a nice photo of one of these on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lWdF1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mirrors, especially non-precision mirrors, would be relatively easy to manufacture on the moon. Aluminum and silicon are quite plentiful, the two key elements to making mirrors. The turbines might be difficult to manufacture on the Moon (I really don't know about this one), but they could be brought from Earth, reducing the weight significantly vs the solar panels that would be required to make this system work via direct transport from Earth.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-30T11:06:34.757" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9719" PostTypeId="2" ParentId="2114" CreationDate="2015-06-30T11:35:27.807" Score="2" Body="&lt;p&gt;I don't know the answer to this one for sure. I've searched the entire internet, with little luck. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can tell you that every satellite system I am aware of uses some form of MPPT type algorithm, in which the power coming from the solar arrays is maximized by modifying the resistance of the cell to maximize output. I'm confident that NASA would use a similar system for JUNO. There no doubt will be some tweaks to how it performs, but I'd be shocked if they don't use a process such as MPPT to maximize the power.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-30T11:35:27.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9720" PostTypeId="2" ParentId="9524" CreationDate="2015-06-30T13:08:21.627" Score="8" Body="&lt;p&gt;Yes, the science/engineering teams will be able to use the radio communications between Rosetta and Philae to better pinpoint the location of the lander. However, by far the best data for finding the lander will be the CONSERT instrument, which has a part on the lander and a part on the orbiter. CONSERT &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/11/21/homing-in-on-philaes-final-landing-site/&quot;&gt;works by transmitting radio signals from the orbiter to the lander&lt;/a&gt; in order to study the comet interior, but can also be used to help identify the location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm quite new to this site, so I can't comment on the answer posted by Hobbes, but there is no Star Tracker onboard Philae. The wikipedia section he was referring to seems to have been a copy/paste error by somebody and was refering to Rosetta orbiter instruments, not the Philae lander. Star trackers are used by spacecraft to determine their attitude, for purposes of navigation. Since Philae has no navigation ability, it has no need for such equipment. The surfaces of Philae are optimised for its mission, i.e. they are covered in solar panels and two passive thermal absorber foils, with the exception of the backpanel (&quot;balcony&quot;) which is dedicated to instrument units.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that a star tracker wouldn't work on the surface of the comet anyway. Rosetta has had problems with its star trackers &lt;a href=&quot;http://spaceflightnow.com/2015/04/05/rosetta-probe-disoriented-by-comet-dust/&quot;&gt;getting confused by dust&lt;/a&gt;, and the environment at the surface would be so much worse they would be unusable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Philae does have a panoramic camera system (CIVA) but the images this has provided have been far from conclusive when it comes to identifying the location of the lander.&lt;/p&gt;&#xA;" OwnerUserId="10635" LastActivityDate="2015-06-30T13:08:21.627" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9721" PostTypeId="2" ParentId="9706" CreationDate="2015-06-30T13:27:06.347" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Does all this make any sense?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Part of it does. The idea of leaving the &lt;em&gt;tanker&lt;/em&gt; as an orbital fuel depot in Martian orbit is OK (if you can reuse a spent upper stage for that), but it would be better to leave propellants themselves on the surface of Mars until you actually need them in orbit, to prevent propellants boil-off and risk of explosion due to thermal cycling (thermal expansion during one complete orbit as the vehicle enters and exits Martian shadow, and rotation of the vehicle itself to cycle surface area exposure to the Sun and other sources of radiation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methane itself isn't such a big deal, it's a fairly big molecule compared to some other relatively good performance fuels so boil-off rate, assuming your orbital fuel depot doesn't spring a leak, would/should be fairly slow. It does have a rather big isobaric coefficient (expansion due to temperature and pressure), so it would have to be actively cooled. Bigger problem is the oxidizer, which would in this case be LOX (cryogenic liquid oxygen) that is more difficult to store long-term in orbit and it would require active cryogenic cooling which is a fair bit trickier to achieve since it's highly corrosive, doesn't react well in contact with most surfaces and with increased pressure (e.g. if your cryocoolers fail) becomes more metallic, i.e. any exposure to electric wiring or even vehicle's surface charging through &lt;em&gt;solar weather&lt;/em&gt; and discharging through the insulation and tank walls would result in loss of vehicle. And if you depended on it, also loss of mission or worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Otherwise, and I'm not entirely sure if that's what you meant with your question, Mars isn't really suitable as an intermediary stop to interplanetary exploration of other targets than Mars, its two small moons Phobos and Deimos, and perhaps &lt;em&gt;some&lt;/em&gt; asteroids in the Main Belt, because it's rarely suitably aligned with outer planets, if those would be your final destination. Refueling at Mars would also mean that you'd first have to inject into its orbit and rendezvous with the orbital fuel depot, instead of using it for gravity assist during flyby, so you're really losing a lot of delta-v, propellants, and time and with it exposure to radiation and so on. It would be more of a detour than a shortcut, unless you meant with &lt;em&gt;interplanetary&lt;/em&gt; specifically to support / facilitate Earth-Mars transports.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And no, such setup wouldn't depend on delivery of hydrogen from Earth. Hydrogen is locked in Martian water and can be extracted from it fairly easily with electrolysis. It's unclear how much of that hydrogen is deuterium though (atmospheric escape is slightly faster for &lt;em&gt;normal&lt;/em&gt;, lighter hydrogen isotopes than slightly heavier deuterium), so you'd perhaps also have to mass-separate it before use first. Besides, delivering hydrogen from Earth, with trip times of roughly 9 months (depends on orbital positions of Earth and Mars), would be even more difficult than already described long-time storage of oxygen. Typical LH2 (cryogenic liquid hydrogen) boil-off rates, depending on tank design, are too high to store it for so long in the vacuum of space, and by the time you'd reach Mars, it would be either all gone and lost to space, or you'd have to insulate and thicken tank walls so much that it becomes impractical from mass economy perspective.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-06-30T13:27:06.347" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9722" PostTypeId="1" CreationDate="2015-06-30T14:04:40.570" Score="16" ViewCount="2813" Body="&lt;p&gt;Now that we know the results of the latest spaceX launch (ka-boom), who (directly, and economically) pays for that rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA and spaceX have a contract for a fixed number of launches, does it include any stipulations regarding failure?   (Aka, did NASA pay for the rocket that exploded, or does spaceX owe them another one?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets are expensive.   Are they commonly insured?   (Aka:  is it some random insurance company that is paying for this, or does this come directly out of NASA/spaceX pocket?)&lt;/p&gt;&#xA;" OwnerUserId="8653" LastEditorUserId="6214" LastEditDate="2015-07-01T14:12:37.700" LastActivityDate="2015-07-01T14:12:37.700" Title="Who pays when a mission to the ISS fails?" Tags="|launch|rockets|spacex|economics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9723" PostTypeId="2" ParentId="9722" CreationDate="2015-06-30T14:12:01.757" Score="11" Body="&lt;p&gt;The CRS contract pays upon delivery. Cygnus and Dragon both failed to deliver on their last missions. They do not get paid for those flights. These are no longer development contracts, they are contracts for delivery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then they get to negotiate with NASA as to how to proceed. Do they repeat the mission? Do they consolidate into fewer heavier launches (Which is what Orbital with Cygnus on Atlas V will do)? Do they do fewer launches and not get paid for the missed flight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for insurance, that is up to the vendor. Does Russia insure their launches?  Did SpaceX? Did Cygnus?  Do they self insure?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who pays for the lost IDA and other experiments?  All good questions. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-30T14:12:01.757" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9724" PostTypeId="1" CreationDate="2015-06-30T14:13:03.667" Score="2" ViewCount="2152" Body="&lt;p&gt;How can I compute the attitude of a satellite given its yaw, pitch, roll, and velocity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yaw, Pitch, and Roll are given with respect to the velocity vector (A vector whose origin is the satellite's center towards to the direction of motion).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity is given in ECEF.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By attitude I mean the direction of the satellite's body after the rotations along different axes caused by yaw/pitch/roll steering.&lt;/p&gt;&#xA;" OwnerUserId="10638" LastEditorUserId="-1" LastEditDate="2015-06-30T20:45:51.293" LastActivityDate="2015-07-02T15:02:23.233" Title="How can I compute the attitude of a satellite given its yaw, pitch, roll, and velocity?" Tags="|attitude|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9725" PostTypeId="2" ParentId="9722" CreationDate="2015-06-30T14:37:04.717" Score="7" Body="&lt;p&gt;The company that launched the rocket pays for the cost of the rocket, as geoffc said. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the payload, it seems likely that those who provided the items have the choice of what to do. They can insure it, otherwise they are held accountable. This is why NASA does in fact have a say about the launch of their rockets.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-06-30T14:37:04.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9726" PostTypeId="1" AcceptedAnswerId="9737" CreationDate="2015-06-30T15:41:37.560" Score="4" ViewCount="188" Body="&lt;p&gt;The solar arrays used on the ISS were launched in 4 different launches. Starting in 2000 for the first array, used on the Z-0 truss, to 2007, 2008, and 2009 for the final three. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are all 4 of them identical in design?  Therefore the question, were they manufactured as a set?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or did the first array go up, and based on its experience in orbit and new solar cell technology contribute to a modernized design for the next three?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-01T03:44:18.750" Title="Were the ISS Solar Arrays manufactured as a set?" Tags="|iss|design|solar-power|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9727" PostTypeId="1" AcceptedAnswerId="9728" CreationDate="2015-06-30T16:27:33.153" Score="6" ViewCount="981" Body="&lt;p&gt;Commercial Resupply Services (CRS) are contracts awarded by NASA for delivery of cargo and supplies to the International Space Station (ISS) on commercially operated spacecraft. SpaceX signed a contract for 15 launches, the first of which was named &lt;strong&gt;CRS-1&lt;/strong&gt;, and launched on 7th October, 2012.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Subsequent launches were named using step-iteration. However on June 28, 2015, &lt;strong&gt;CRS-7&lt;/strong&gt; experienced a &lt;a href=&quot;http://spaceref.biz/company/spacex-falcon-9-mishap-more-details-emerge.html&quot;&gt;launch failure&lt;/a&gt;, resulting in a 'loss-of-vehicle'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Presuming SpaceX gets FAA clearance to re-fly the mission, what will the launch be called? What is the protocol followed for naming launches in the events of A) &lt;em&gt;failure with loss-of-vehicle&lt;/em&gt; or B) &lt;em&gt;failure with an undamaged vehicle&lt;/em&gt;.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In the event that the launch is transferred to another corporation (eg. Orbital Sciences), are the old naming conventions retained?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-07-01T00:57:21.937" Title="What will the next ISS resupply launch be named?" Tags="|launch|spacex|failure|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9728" PostTypeId="2" ParentId="9727" CreationDate="2015-06-30T16:29:58.907" Score="15" Body="&lt;p&gt;The next Orbital Cygnus flight, on Atlas V is called Orb-4 (per SpaceflightNow.com's launch schedule). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the lost flight was Orb-3, then we can assume that SpaceX will follow a similar pattern and the next flight will be CRS-8. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next Progress is named 60, after Progress 59 failed. So looks like the pattern (2 data points makes a line, one data point tells you everything when on the Internet) is to just increment the count and move on. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-06-30T16:29:58.907" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9729" PostTypeId="2" ParentId="9727" CreationDate="2015-06-30T20:56:26.407" Score="4" Body="&lt;p&gt;Mission names are never reused.  Even if the mission never launches or fails to reach orbit, reusing the name would cause confusion in documenting separate missions.&lt;/p&gt;&#xA;" OwnerUserId="10641" LastActivityDate="2015-06-30T20:56:26.407" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9730" PostTypeId="1" CreationDate="2015-06-30T22:05:55.160" Score="7" ViewCount="1774" Body="&lt;p&gt;I understand the rocket constantly transmits the telemetry information to an engineering facility on the surface. I also understand it is normal for a transmission channel to create errors in the transmitted data. I know there are redundancy-based data error correction algorithms in place. My question is, what happens if an error is so big that it cannot be corrected by the algorithm. Does the failed packet gets re-requested from the rocket and transmitted again? Or is it considered irrevocably lost?&lt;/p&gt;&#xA;" OwnerUserId="10643" LastEditorUserId="51" LastEditDate="2015-07-02T20:41:00.327" LastActivityDate="2015-07-02T20:41:00.327" Title="How are Falcon-9 telemetry data transmission errors handled?" Tags="|launch|rockets|spacex|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9731" PostTypeId="2" ParentId="9730" CreationDate="2015-06-30T22:16:01.030" Score="12" Body="&lt;p&gt;This is likely asked in reference to a &lt;a href=&quot;https://twitter.com/elonmusk/status/615431934345216001&quot; rel=&quot;noreferrer&quot;&gt;Tweet by Elon Musk&lt;/a&gt; about the CRS-7 failure. Text of the Tweet is:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Cause still unknown after several thousand engineering-hours of review. Now parsing data with a hex editor to recover final milliseconds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;On &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3bhmpw/elon_musk_on_twitter_cause_still_unknown_after/&quot; rel=&quot;noreferrer&quot;&gt;Reddit&lt;/a&gt;, someone posted this very excellent explanation. Not a great source, but best explanation I have seen to date. Specifically, &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3bhmpw/elon_musk_on_twitter_cause_still_unknown_after/csmfvpi&quot; rel=&quot;noreferrer&quot;&gt;this node&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;That's not how this stuff works at all. It's all transmitted CCSDS, typically. Each packet of 1024 bytes typically has it's own error correction code added, in addition to all of the other required features (frame sync markers, virtual channel encoding, etc).&lt;/p&gt;&#xA;&lt;p&gt;Having had to do this same analysis myself many, many times, I'm guessing that the data is corrupted, particularly the frame sync markers (0x1ACFFC1D... when you do this for a living, yes that's right off the top of your head). When that happens, the Reed-Solomon correction can't really be applied because the frame could have bit slipped and that would make things worse.&lt;/p&gt;&#xA;&lt;p&gt;So they have to go through each part of the stream in a hex editor manually offsetting things one bit at a time looking for reasonable packet contents (which is almost impossible, as the payload is PN encoded...), and then if they see that, they can then pull of the RS correction symbols and do an error correction, then decode the corrected version again. Then on to the next Kb of data... by hand. You can imagine why this takes a while.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;On a Nasaspaceflight.com forum there was an &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=37476.msg1398608#msg1398608&quot; rel=&quot;noreferrer&quot;&gt;interesting comment&lt;/a&gt; on telemetry that is worth pointing at:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Since we're talking about telemetry, I'll throw a bit of my knowledge into the ring.&lt;/p&gt;&#xA;&lt;p&gt;I build systems that process ground and flight telemetry for spacecraft and instrument avionics (not launch vehicles).&lt;/p&gt;&#xA;&lt;p&gt;I know nothing about SpaceX' implementation, so I won't even try to give anything specific about their vehicles.&lt;/p&gt;&#xA;&lt;p&gt;Also, some of what I describe below for spacecraft doesn't apply to launch vehicles as there isn't any time to send commands to a launch vehicle as the launch sequence is highly dynamic and relies heavily on pre-programmed event sequences.  As far as I know, there are no commands sent to a launch vehicle during launch after liftoff, aside from an FTS destruct command.&lt;/p&gt;&#xA;&lt;p&gt;There are standards at work for telemetry, e.g. CCSDS.  There are also as many different implementations of the 'standard' as there are manufacturers of avionics.&lt;/p&gt;&#xA;&lt;p&gt;In  everything I work on, flight software does in fact format telemetry, packaging frames inside of headers.  There are multiple layers of framing at work, and there can be many different types of telemetry frames coming down at various rates.&lt;/p&gt;&#xA;&lt;p&gt;Some things happen on a deterministic schedule, some telemetry can be inserted out-of-order if the priority of the data is high enough, and some things may only come down if there is available bandwidth to send it without disrupting the flow of high-priority telemetry.&lt;/p&gt;&#xA;&lt;p&gt;Depending on the operating modes of the avionics, there can be many different rates and maps.  For example, if a spacecraft has entered safe mode (or if an emergency mode controller has taken over the operation of a spacecraft), the data rate and telemetry map can switch to fairly low bit rates (better for retrieving data on the ground if there is a pointing or power problem with the transmitters), and the maps can prioritize only critical engineering telemetry.&lt;/p&gt;&#xA;&lt;p&gt;Many avionics systems can and do acquire data more rapidly than they send telemetry, in which case the telemetry maps define how often and what kinds of data to send.  It is not unusual to have instrumentation for developmental hardware sending telemetry and higher than normal rates.&lt;/p&gt;&#xA;&lt;p&gt;Likewise, any anomalous condition will often times be prioritized to send that status back as a high-priority telemetry frame.&lt;/p&gt;&#xA;&lt;p&gt;On all the flight avionics that I work on, there are multiple boxes intercommunicating to flow data that ends up send down in telemetry.  A high-energy event can (and indeed is very likely to) cause telemetry to stop being transmitted even if the avionics and flight software are continuing to acquire data.&lt;/p&gt;&#xA;&lt;p&gt;Unlike launch vehicles, the rates and maps on most spacecraft can be commanded as well as switched autonomously by flight software.&lt;/p&gt;&#xA;&lt;p&gt;Almost everything I work on is capable of sending unencrypted telemetry, however in flight use everything I deal with defaults to using encryption.  Aside from encryption, there can be randomization also applied.&lt;/p&gt;&#xA;&lt;p&gt;There are no analog telemetry signals on anything I work on, and as far as I know, modern launch vehicles to not transmit any analog telemetry- it's all digitally encoded, framed, and transmitted by flight software within the avionics.&lt;/p&gt;&#xA;&lt;p&gt;The other Jim above mentioned that this type of event could spell the end of store-and-forward telemetry, and I have my own take on this.  I don't believe that it is appropriate to &lt;em&gt;ONLY&lt;/em&gt; log telemetry on a launch vehicle, and that engineering telemetry during the early phases of launch should be transmitted live.&lt;/p&gt;&#xA;&lt;p&gt;I do see the value of a rugged data logger which is essentially digital storage packaged as robustly as it can be, with the intent of building a package that can survive a mishap and be recovered.  This is a very difficult challenge on a launch vehicle as they operate right on the edge of having sufficient margin to even survive as the physics of launch are very challenging.&lt;/p&gt;&#xA;&lt;p&gt;In the scenario above where a high-energy event kills the communication between avionics, a high-rate log might be able to provide some critical insight into sensor data in a mishap, were it to be recovered.&lt;/p&gt;&#xA;&lt;p&gt;I to expect, however, with the proliferation of sensors generating engineering and operational telemetry that there will be some lower-priority data which will always be stored and made available for downlink if conditions permit.&lt;/p&gt;&#xA;&lt;p&gt;It is always better to get all critical sensor data down in near real time than count on retrieval after a mishap.  Picking what sensors are critical is a challenging activity.  In the words of Mr. Musk, some failures can be counter intuitive and a sensor that you thought wasn't critical can, in retrospect, be the most critical sensor needed to decipher an event chain that leads to catastrophic failure of the vehicle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-01T19:07:25.080" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9735" PostTypeId="2" ParentId="9682" CreationDate="2015-07-01T01:27:46.487" Score="7" Body="&lt;p&gt;The thought of placing the engine on top of the rocket never crossed my mind. One of the reasons against it I see is as follows. In case of the traditional layout (engine at the bottom), the thrust vector of ALL engines is exactly in the direction of the intended movement of the rocket. While in case of the engine-at-the-top layout, we cannot direct the exhaust along the line of the movement, simply because underneath the engine there's the rest of the rocket. That means we have to use several smaller engines, each throwing its exhaust at an angle to the rocket body (like in the picture in the accepted answer). And the thrust vectors of all engines should sum up to create the main thrust vector in the direction of intended movement. Now, the more the angle the less power of each engine will be applied to move the rocket. When the angle is 90 degrees, there will be no thrust, with engines completely compensating each other's thrust. In other words, placing engine at the top makes the whole setup less effective energy-wise, i.e. more fuel will be required for the same performance.&lt;/p&gt;&#xA;" OwnerUserId="10643" LastActivityDate="2015-07-01T01:27:46.487" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9737" PostTypeId="2" ParentId="9726" CreationDate="2015-07-01T03:44:18.750" Score="5" Body="&lt;p&gt;Took a bit of digging but I found &lt;a href=&quot;http://www.spaceref.com/news/viewpr.html?pid=27742&quot;&gt;a press release&lt;/a&gt; from 2009 that indicates that they were all identical:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The truss segment, a carbon copy of the Port 6 element already on orbit...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The 2009 launch was the fourth (last) set of solar arrays added to the station, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Integrated_Truss_Structure#Solar_arrays&quot;&gt;wikipedia&lt;/a&gt; confirms that the port 6 segment was the first set of solar arrays:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first pair of arrays are attached to the P6 truss segment, which was launched and installed on top of Z1 in late 2000 during STS-97.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While the press release doesn't discuss the manufacture of all the solar panels this quote strongly suggests they were all manufactured one after another:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The truss has had the longest time on the ground of any single ISS element -- it arrived at KSC on Dec. 17, 2002 -- with a Boeing team regularly maintaining it, cleaning it and inspecting it for corrosion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-07-01T03:44:18.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9739" PostTypeId="2" ParentId="9714" CreationDate="2015-07-01T11:19:36.887" Score="11" Body="&lt;p&gt;As 2012rcampion noted you are using the wrong metric of cost in your efficiency figures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Triple junction solar panels are extremely inefficient wrt weight when compared to single junction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To understand why this is you need to understand what a triple junction PV is. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To do that I need to explain the major inefficiency of standard single junction PV is due to quantum effiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each single junction PV has a semi conductor junction with an associated voltage or colour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That PV junction will only give you energy for photons of that colour/voltage or bluer. So if you have a PV junction that is tuned to green, that panel would give no power under a red light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely higher energy photons (bluer) would only give off an electron with that same voltage (the rest of the energy lost as heat).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Triple junction solar panels literally stack a red, green and blue solar panel on top of each other to maximise both the number of photons turned into electricity and the electrical energy gained per photon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if you are stacking 3 panels you could just build 3 panels instead and place them side by side so they don't block each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a naive approach triple junction is 3 times heavier, but not 3 times as powerful.&lt;/p&gt;&#xA;" OwnerUserId="6157" LastActivityDate="2015-07-01T11:19:36.887" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9740" PostTypeId="2" ParentId="9706" CreationDate="2015-07-01T11:46:35.700" Score="1" Body="&lt;p&gt;The first thing to note, is that the fuel generation gear would be perfectly sized to create the amount of fuel required to bring the astronauts back to Earth. It is not like it has surplus capacity. So we really need to consider bringing fuel back from Mars, somewhat separately to the plan of bringing the astronauts back. It is going to require bringing a lot of extra equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the intention is to bring something to be used as fuel back to Earth, the logical thing to bring would actually be plain water either in liquid or ice form. Water can be turned into H2 and O2 in a perfect ratio for fuel using electrolysis. However, in the H2O form it is highly stable and easily transported, while in the H2/O2 form it is highly volatile - with both forms having exactly the same mass. Instead of bringing extra electrolysis gear, extra power generation and extra tanks all the way to Mars, it'd make more sense to put all that extra gear in Earth Orbit, bring water back from Mars, and perform the electrolysis in Earth Orbit, where the fuel would be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would also be possible to bring CO2 back from Mars (probably as dry ice) and perform the Sabatier reaction and electrolysis in Earth Orbit, with Hydrogen brought only from Earth to Earth Orbit instead of all the way to Mars. CO2 has the advantage of being easily extracted from the Martian atmosphere, while water has to be mined from the ground or extracted from the atmosphere at considerably greater effort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that these options result in just as much usable fuel in Earth Orbit, with &lt;em&gt;much less mass needing to be sent to Mars&lt;/em&gt;. Of course, it would still be necessary to generate a lot more methane and lox to bring the H2O or CO2 back to Earth - the whole point of in situ generation is that carbon and oxygen are heavy elements (compared with hydrogen) and not desirable things to be delivering between planets, so the plan of bringing carbon or oxygen from Mars to Earth somewhat defeats the purpose of in situ generation - which is to save on mass. Yet it could still be cheaper to bring them from Mars to Earth Orbit, than from Earth, at least in terms of delta-V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are potentially cheaper places to get water (and maybe carbon) from, it would actually be a lot cheaper in terms of delta-V to bring fuel from the moons of Mars than Mars itself. This comes down to gravity, Mars has a relatively deep gravity well while the moons have very shallow gravity wells. In fact, various factors conspire to making Phobos and Deimos even closer in terms of Delta-V than the Moon - but only for very restricted windows when the Earth and Mars are properly aligned. At the moment we don't really know what resources are available on Phobos and Deimos, it is only speculated that there are exploitable quantities of volatiles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other good candidate to bring water from is the Moon, even though Phobos and Deimos are occasionally slightly closer in terms of Delta-V, the Moon is far more accessible in every other way, most importantly the trip time is mere days or weeks instead of 8 months. The moon likely has water in the form of ice in permanently shaded craters, which could be extracted and processed into fuel in-situ, or delivered to Earth Orbit for processing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the much greater accessibility of the Moon, I believe it does not make sense at this time to bring extra equipment to Mars with the purpose of bringing fuel back to Earth Orbit. In situ generation of fuel on Mars only makes sense if that fuel is to be used on Mars.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-07-01T11:46:35.700" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9741" PostTypeId="1" AcceptedAnswerId="10844" CreationDate="2015-07-01T17:08:07.183" Score="11" ViewCount="5768" Body="&lt;p&gt;Assuming a simple rocket engine, with a combustion chamber and de laval nozzle, fueled by liquid methane and liquid oxygen, what temperature must the combustion chamber, choke and nozzle withstand?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a &lt;a href=&quot;http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=4824&amp;amp;context=ujmm&quot; rel=&quot;nofollow noreferrer&quot;&gt;university article&lt;/a&gt;, the adiabatic temperature of a perfect mix would be about 5000C:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Knuv.png&quot; alt=&quot;adiabatic temperature&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when combustion products expand they cool, so this question could be rephrased as what pressures exist in the three main parts of the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Really I am asking &quot;how tough does the inside of a rocket engine need to be&quot;? What is the max temperature and pressure the materials must be able to withstand?&lt;/p&gt;&#xA;" OwnerUserId="10584" LastEditorUserId="25" LastEditDate="2015-08-31T00:46:11.687" LastActivityDate="2015-08-31T02:03:59.677" Title="What is the temperature inside a methane/oxygen rocket engine?" Tags="|rockets|engine-design|fuel|engines|temperature|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9746" PostTypeId="2" ParentId="9724" CreationDate="2015-07-02T15:02:23.233" Score="3" Body="&lt;p&gt;You can combine the 3 successive rotations to obtain the rotation matrix from the initial to the final attitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's name $\vartheta$, $\varphi$ and $\psi$ the angles respectively about roll, pitch and yaw. And suppose we want to combine the rotations in exactly this order. The rotation matrices related to each rotation are:&#xA;$$&#xA;A_{\rm r} = \begin{bmatrix}&#xA;    1       &amp;amp; 0 &amp;amp; 0 \\&#xA;    0       &amp;amp; c_\vartheta &amp;amp; s_\vartheta \\    &#xA;    0       &amp;amp; -s_\vartheta &amp;amp; c_\vartheta&#xA;\end{bmatrix}, \;&#xA;A_{\rm p} = \begin{bmatrix}&#xA;    c_\varphi       &amp;amp; 0 &amp;amp; -s_\varphi \\&#xA;    0       &amp;amp; 1 &amp;amp; 0 \\    &#xA;    s_\varphi       &amp;amp; 0 &amp;amp; c_\varphi&#xA;\end{bmatrix},  \;&#xA;A_{\rm y} = \begin{bmatrix}&#xA;    c_\psi       &amp;amp; s_\psi &amp;amp; 0 \\&#xA;    -s_\psi       &amp;amp; c_\psi &amp;amp; 0 \\    &#xA;    0       &amp;amp; 0 &amp;amp; 1&#xA;\end{bmatrix}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then just perform the product of the 3 matrices in the reverse order (successive rotations rule):&#xA;$$&#xA;A(\vartheta, \varphi, \phi) = A_{\rm y}A_{\rm p}A_{\rm r}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A couple of notes&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If you change the order, the final attitude will be different&lt;/li&gt;&#xA;&lt;li&gt;I suppose that RPY reference frame rotates while the satellite moves along the orbit (i.e. the roll axis is always aligned with the velocity vector). This means that the attitude matrix you compute describes actually the orientation of a body fixed reference frame (initially coincident with RPY) wrt RPY related to a particular position of the satellite.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Hope this helps&lt;/p&gt;&#xA;" OwnerUserId="8528" LastActivityDate="2015-07-02T15:02:23.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9747" PostTypeId="2" ParentId="9621" CreationDate="2015-07-02T16:18:45.390" Score="1" Body="&lt;p&gt;Given that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Euler equations are general, there exist no special or modified forms. Simply some terms are equal to zero if some conditions are satisfied&lt;/li&gt;&#xA;&lt;li&gt;I do not quite understand what $S_{ij}$ is in your expression &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Anyway, if the centre of mass of the body coincides with the origin of the body frame, the total angular momentum of the satellite can be written as&#xA;$$&#xA;\underline{H} = J\underline{\omega} + \underline{h}&#xA;$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $J$ is the inertia tensor, $\underline{\omega}$ is the angular velocity vector, $\underline{h}$ is the total angular momentum of &lt;strong&gt;all&lt;/strong&gt; the rotating internal equipments. The second law of motion states that the derivative of the total angular momentum equals the sum of the external torques&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{D\underline{H}}{Dt}=\underline{\dot H}+\underline{\omega}\times\underline{H}=\underline{M}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neglecting the variation of the inertia tensor, the previous can be written as&#xA;$$&#xA;J\underline{\dot \omega}+\underline\omega\times J\underline\omega + \underline {\dot{h}} +\underline\omega\times\underline h=\underline M&#xA;$$&#xA;If you have 3 rotors each aligned with one of the satellite axes, then $\underline h$ is&#xA;$$&#xA;\underline h = &#xA;\begin{bmatrix}&#xA;J_{r_1}\omega_{r_1} &amp;amp; J_{r_2}\omega_{r_2} &amp;amp; J_{r_3}\omega_{r_3}&#xA;\end{bmatrix}^\rm T&#xA;$$&#xA;You can name the vector of control torques (coming from some control technique as you mentioned) as&#xA;$$&#xA;\underline{M}_c = -\underline {\dot{h}} -\underline\omega\times\underline h&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and compute the needed torque each wheel needs to provide&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\underline {\dot{h}} = -\underline{M}_c -\underline\omega\times\underline h&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this helps&lt;/p&gt;&#xA;" OwnerUserId="8528" LastEditorUserId="8528" LastEditDate="2015-07-02T16:24:27.377" LastActivityDate="2015-07-02T16:24:27.377" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9748" PostTypeId="2" ParentId="4944" CreationDate="2015-07-02T17:56:19.040" Score="1" Body="&lt;p&gt;I don't think I'd completely eliminate the Earth facing board, but I would reduce the shielding. The shielding should be stronger for the side pointing away from Earth, and be reduced as one gets to a side closer to Earth. Basically, this is a geometry problem. You can still hit the spacecraft from the sides of the Earth, even though cosmic rays will not be transmitted through the Earth itself. Thus, for any position not on the surface of a body, there is still a chance of some radiation coming through. The closer to Earth, the less radiation would spill through.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it's worth noting that Earth's magnetic field does a pretty reasonable job at removing such radiation. They also move the direction somewhat differently. In some cases, they might even come in from below (Although I'd have to actually do the math, which I am not at the moment). Still, I suspect more shielding would be needed on the sides, and less the side pointed directly to Earth and directly away from Earth, due to the magnetic field.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-02T17:56:19.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9749" PostTypeId="2" ParentId="9209" CreationDate="2015-07-02T23:06:02.640" Score="3" Body="&lt;p&gt;The orbital period of a satellite tells you the mass of the object it's orbiting.  Thus we could weigh the gas giants and know that they were so light they had to be almost all gas.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-07-02T23:06:02.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9750" PostTypeId="2" ParentId="1184" CreationDate="2015-07-03T00:49:20.587" Score="3" Body="&lt;p&gt;The question really has two questions, the first is well answered, about go straight up.  Second question is about taking off from a runway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main benefit of a runway takeoff is to use aerodynamics to generate lift, requiring less thrust (thus less fuel) to get airborne and to a certain height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that you run out of usable atmosphere for aerodynamic flight before you have gotten high enough to be useful. After that you need to become a rocket again.  While you were in the atmosphere, you could capture oxygen to burn the fuel in, but once you get high enough you need to carry your own oxidizer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The math just does not work out, to get a useful payload. The vehicle starts to be so big to carry all the fuel and oxidzer that the wheels, wings, other paraphernalia eat up any savings you might have gotten. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One idea that is a twist on this is to launch mostly empty, saving on takeoff mass, and refuel from a tanker, twice. Once for fuel, once for oxidizer then continue on.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-03T00:49:20.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9751" PostTypeId="2" ParentId="123" CreationDate="2015-07-03T00:53:09.433" Score="7" Body="&lt;p&gt;Throwing away the equipment used in every launch definitely makes launches more expensive. (SpaceX plans to recover the first stage. This effort has pushed ULA to suggest Vulcan as a way to partially recover the engine block, and ArianeSpace to suggest ADELINE to recover the engine block).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel itself is expensive to a homeowner, but is only a tiny fraction of the launch cost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vehicles themselves are expensive to develop, dangerous, hard to test completely, and turns out, really are rocket science. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-03T00:53:09.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9752" PostTypeId="2" ParentId="1325" CreationDate="2015-07-03T00:59:59.447" Score="3" Body="&lt;p&gt;The Shuttle just did not have good abort options for most of the flight window. One possible change that would have required being designed in from the beginning would have been an abort system where the entire crew compartment would separate like on an F-111. But it would have added cost and complexity and was not actually designed in.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-03T00:59:59.447" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9753" PostTypeId="2" ParentId="1184" CreationDate="2015-07-03T05:10:00.733" Score="5" Body="&lt;p&gt;Horizontal launch would be a great idea if you didn't have to deal with an atmosphere and had some altitude above the terrain as a starting point.  This would allow you to burn perpendicular to the gravity vector for your entire ascent -- thus eliminating gravity drag completely.  This is effectively what proposed mass drivers do for, say, lunar launches.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2015-07-03T05:10:00.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9754" PostTypeId="1" AcceptedAnswerId="9755" CreationDate="2015-07-03T07:24:52.593" Score="4" ViewCount="451" Body="&lt;p&gt;According to &lt;a href=&quot;https://youtu.be/PbabP9ttrZc?t=4m33s&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt; human-rated launch vehicles can have six guidance computers while cargo-rated launch vehicles seldom have more than two. If these computers function independently for greater redundancy, why don't supply-rated launch vehicles have more than two such computers? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How large are such guidance computers? Is their size/mass or cost a prohibiting factor from installing more?&lt;/p&gt;&#xA;" OwnerUserId="733" LastActivityDate="2015-07-03T15:12:33.713" Title="Why aren't there more guidance computers on cargo rated launch vehicles?" Tags="|failure|flight-computer|launch-vehicle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9755" PostTypeId="2" ParentId="9754" CreationDate="2015-07-03T12:47:46.990" Score="14" Body="&lt;p&gt;Don't take random youtube videos as fact. That part of that video was incorrect. The unmanned SpaceX Dragon has three pairs of computers used for flight control (so six flight control computers), and has 54 processors in all. The Falcon 9 launch vehicle has another 30+ processors. See &lt;a href=&quot;http://aviationweek.com/blog/dragons-radiation-tolerant-design&quot;&gt;http://aviationweek.com/blog/dragons-radiation-tolerant-design&lt;/a&gt;. Orbital's Cygnus also uses more than two computers for flight control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle had four primary flight control computers, plus one more as a completely independent backup flight system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Visiting vehicles to the Space Station, whether crewed or uncrewed, must protect against any two failures creating a catastrophic hazard on the Space Station. You aren't going to get two fault tolerance with only two components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Crewed vehicles add the additional requirement of being two fail safe (no one on the vehicle dies even if two failures happen). &lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-07-03T15:12:33.713" LastActivityDate="2015-07-03T15:12:33.713" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9757" PostTypeId="1" AcceptedAnswerId="9764" CreationDate="2015-07-03T20:52:50.580" Score="6" ViewCount="967" Body="&lt;p&gt;In Progress 59 mission, the service module was damaged and venting pressured gas, which caused the spacecraft to tumble.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What could be done if similar situation occurs on a Soyuz?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or generally, What abort modes does Soyuz have in space?&lt;/p&gt;&#xA;" OwnerUserId="340" LastActivityDate="2015-07-05T05:48:07.340" Title="What abort modes does Soyuz have in space?" Tags="|soyuz-spacecraft|abort|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9758" PostTypeId="2" ParentId="9602" CreationDate="2015-07-03T21:06:15.587" Score="10" Body="&lt;p&gt;I am going to answer your specific question, to wit:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;So is EACH IEA a cube 5 meters on a side weighing 7.7 metric tons, or&#xA;are ALL FOUR of these IEAs, in total, these dimensions?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This question was ridiculously difficult to answer, because NASA seems to go out of their way to obfuscate simple info like this.  But here goes.&lt;/p&gt;&#xA;&lt;p&gt;First of all, the dimensions given in that document are &lt;strong&gt;clearly&lt;/strong&gt; wrong.  Why is that?  Well, it says the IEA is a 16 foot cube.  All you have to do is recollect that the shuttle payload bay was a &lt;a href=&quot;https://pds.jpl.nasa.gov/planets/captions/shuttle/sts03.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;60 x 15 foot cylindrical volume&lt;/a&gt; and you will immediately realize that it would not fit.  You can not fit a 16 foot wide IEA in a 15 foot wide payload bay, especially since you have to have clearance to deploy it, room for the trunnions and payload attach system mechanisms, etc.&lt;/p&gt;&#xA;&lt;p&gt;Also, even in that document you linked, &lt;strong&gt;it clearly isn't a cube&lt;/strong&gt;. It's more like a cube sawn in half.  It was also ridiculously difficult to find a decent picture of the IEA in space, in fact I gave up, but on this one you can at least tell that it isn't a cube. (The letters 2B are right on the IEA in this picture)  &lt;img src=&quot;https://i.stack.imgur.com/MBsoB.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;So if we know the dimensions of the payload bay, and we can find a picture of the element in the payload bay, we can do some high tech photogrammetry. In other words, count some pixels.  I used this one: &lt;img src=&quot;https://i.stack.imgur.com/OCPC1.jpg&quot; alt=&quot;enter image description here&quot; /&gt;.&lt;br /&gt;&#xA;The IEA is at the aft end of the truss segment.  You can see the battery boxes under the folded up solar array.  From this we can figure out that the IEA is about 16 feet long by 14 feet wide (so at least the 16 foot number was valid for something). You can figure that the IEA is about 7.5 feet thick max because it does not stick up much above the sill of the payload bay so it's about half the payload bay dimension.&lt;/p&gt;&#xA;&lt;p&gt;So if you accept my highly technical analysis, the IEA is roughly 16 x 14 x 7.5 feet and the document linked is pretty incorrect about the size.  Now how about the mass?&lt;/p&gt;&#xA;&lt;p&gt;Again NASA makes it ridiculously difficult to find this information.  But we can reason it out to a first approximation.  That document says the IEA weighs 17,000 lb.  The &lt;a href=&quot;https://ntrs.nasa.gov/api/citations/20110001406/downloads/20110001406.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle Missions Summary document&lt;/a&gt;, which I do trust, says the P6 had a payload weight of ~36,000 lbs.  Since the IEA is about half of the truss segment, I'm willing to buy that 17K number.  Also, if 17K lbs was for all 4 IEAs, each one would only weigh 4250 lbs, and that does not make sense in light of the credible 36klb weight of the P6.&lt;/p&gt;&#xA;&lt;p&gt;So, the tl;dr version of the answer:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The document is incorrect on the dimensions of a single IEA.  It's more like 16 x 14 x 8 than a 16 ft cube.&lt;/li&gt;&#xA;&lt;li&gt;The document's figure for the mass of a single IEA of 17klb is reasonable.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-03-15T14:48:40.230" LastActivityDate="2021-03-15T14:48:40.230" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="9759" PostTypeId="1" AcceptedAnswerId="9765" CreationDate="2015-07-04T00:05:01.943" Score="5" ViewCount="2245" Body="&lt;p&gt;Given the position $(p,q)$ and velocity $(v_p,v_q)$ of a satellite in perifocal coordinates $(\hat{p},\hat{q})$ where $\hat{p}$ is pointing toward periapsis, I can easily calculate the specific angular momentum $h$ with:&#xA;\begin{equation}&#xA;    h = (p \times v_q) - (q \times v_p)&#xA;\end{equation}&#xA;And I can get the eccentricity $e$ with the orbit equation naturally:&#xA;\begin{equation}&#xA;    e = \frac{\left(\frac{h^2}{μ r} - 1\right) }{\cos(\theta)}&#xA;\end{equation}&#xA;where $\mu$ is the graviational parameter of the body being orbited and the radius $r$ and true anomaly $\theta$ was calculated with:&#xA;\begin{equation}&#xA;    r = \sqrt{p^2 + q^2}, \mathrm{and}&#xA;\end{equation}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{equation}&#xA;    \theta = \arccos\left(\frac{p}{r}\right).&#xA;\end{equation}&#xA;However, I am having trouble calculating the eccentricity directly using the speed $v$ instead of the specific angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using these equations:&#xA;\begin{eqnarray}&#xA; h^2 = \mu r (1 + e \cos(\theta)), \\&#xA; h = v_\text{perp} r, \\&#xA; v_\text{radi} = \frac{\mu}{h} e \sin(\theta), \\&#xA; v^2 = v_\text{perp}^2 + v_\text{radi}^2&#xA;\end{eqnarray}&#xA;where $v_\text{perp}$ and $v_\text{radi}$ are the perpendicular and radial speed relative to the position vector from the orbited body, I derived an equation to solve for the eccentricity:&#xA;\begin{equation}&#xA;    \theta = \frac{\mu}{r} e^2 + \left[\left(\frac{2\mu}{r} - v^2\right) \cos(\theta)\right] e + \left(\frac{\mu}{r} - v^2\right).&#xA;\end{equation}&#xA;This is just a quadratic and the solution looks like this:&#xA;\begin{equation}&#xA;    e = \frac{- \left[\left(\frac{2\mu}{r} - v^2\right) \cos(\theta)\right] \pm \sqrt{\left[\left(\frac{2\mu}{r} - v^2\right) \cos(\theta)\right]^2 - \frac{4\mu}{r}\left(\frac{\mu}{r} - v^2\right)}}{\frac{2\mu}{r}}&#xA;\end{equation}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This all looked OK to me, but when I tried to compare the first equation (for $h$) with this last equation (for $e$), I find inconsistent results. For example, consider a satellite with these parameters:&#xA;\begin{eqnarray}&#xA;    (p,q) = (7000, 9000), \\&#xA;    (v_p,v_q) = (-5, 7).&#xA;\end{eqnarray}&#xA;Using the first equation to find $h$ gives:&#xA;\begin{equation}&#xA;    h = 94000&#xA;\end{equation}&#xA;Now, here I try to calculate $h$ by first calculating $e$ using $v$, $r$ and $\theta$ (in these units, I'll say $\mu = 398600$):&#xA;\begin{equation}&#xA;    v = \sqrt{v_p^2 + v_q^2} = 8.602,&#xA;\end{equation}&#xA;\begin{equation}&#xA;    r = \sqrt{p^2 + q^2} = 11401,&#xA;\end{equation}&#xA;\begin{equation}&#xA;    \theta = \arccos\left(\frac{p}{r}\right) = 0.90975.&#xA;\end{equation}&#xA;So we have (taking the positive solution to the quadratic equation above):&#xA;\begin{equation}&#xA;    e = 1.0932,&#xA;\end{equation}&#xA;and working back through the orbit equation, I get $h$ again:&#xA;\begin{equation}&#xA;    h = \sqrt{\mu r (1 + e \cos(\theta))} = 87149.&#xA;\end{equation}&#xA;But this is inconsistent with my previously calculated value for $h$ of 94000. I have checked my math several times and feel I must be making some fundamental error though I don't see it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, I'm trying to reconcile two examples (2.12 and 3.6) found in Curtis' book &quot;&lt;a href=&quot;http://disciplinas.stoa.usp.br/pluginfile.php/66104/mod_resource/content/1/OrbitalMechanicsForEngineeringStudents-AerospaceEngineering.pdf&quot; rel=&quot;nofollow&quot;&gt;Orbital Mechanics for Engineers&lt;/a&gt;,&quot; 3rd ed.&lt;/p&gt;&#xA;" OwnerUserId="10661" LastEditorUserId="1142" LastEditDate="2016-01-31T16:16:56.370" LastActivityDate="2016-01-31T16:16:56.370" Title="Perifocal coordinates and the orbit equation" Tags="|orbital-mechanics|orbital-elements|calculation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9760" PostTypeId="1" AcceptedAnswerId="9762" CreationDate="2015-07-04T01:27:52.813" Score="5" ViewCount="430" Body="&lt;p&gt;From &lt;a href=&quot;http://www.cbc.ca/player/News/Technology/ID/2671077175/&quot;&gt;this video&lt;/a&gt;, it appears that material is being flung outward from the limb of the Sun at pretty high speed. Considering that the Sun is some 1.4 million km in diameter, and at least one band seems to cover the distance of perhaps a quarter of the Sun's diameter in about a second, that's on the order of 300,000 km/sec or the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that right? Is the video sped up? If sped up, by how much? If shown at actual speed, is the expanding band actually material moving, or is it just electromagnetic field lines?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="49" LastEditDate="2015-07-04T01:35:02.413" LastActivityDate="2015-07-04T03:24:13.043" Title="Speed of solar coronal mass ejection" Tags="|observation|the-sun|cme|solar-dynamic-observatory|space-weather|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9762" PostTypeId="2" ParentId="9760" CreationDate="2015-07-04T02:29:07.190" Score="13" Body="&lt;p&gt;The video is significantly sped up (videos of the Sun generally are sped up). CMEs move at an average speed closer to 500 km/s. That's why when we see the activity we generally have a warning of up to a few days before the CME gets to where we are. If the CME was moving at the speed of light, it would hit us as we saw it happen (a little more than eight minutes after it actually leaves the Sun).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The band that you're seeing is called a prominence (or filament*), and it is material moving through an area of the magnetic field. When they burst, such as in the image below (Thank you, TildalWave), you can get a CME. This image has a timestamp so you can see exactly how long it took in this one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vf0qD.gif&quot; alt=&quot;CME&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;* Whether it's called a prominence or filament depends on your perspective. A prominence is when you see it from the angle in your video. It would be called a filament if you were looking at it straight on and couldn't see the actual arch. It would appear as a dark line on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an example of a filament:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/czcSt.jpg&quot; alt=&quot;Sun filament&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Edit: &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2005/10jun_newstorm/&quot; rel=&quot;noreferrer&quot;&gt;An article&lt;/a&gt; at NASA.gov (Thank you, briligg) talks about a solar storm event in 2005 that reached the Earth after only about thirty minutes following the flare. This wasn't exactly a CME, but a different kind of solar storm. &lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-04T03:24:13.043" LastActivityDate="2015-07-04T03:24:13.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9764" PostTypeId="2" ParentId="9757" CreationDate="2015-07-04T05:40:49.417" Score="7" Body="&lt;p&gt;I'll start with a general overview of the abort modes available to Soyuz (including before getting to space for completeness).&lt;/p&gt;&#xA;&lt;h2&gt;Abort Modes&lt;/h2&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.spaceflight101.com/soyuz-spacecraft-information.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In the early portion of the mission and while sitting on the Launch Pad, the crew can be evacuated by the use of the SAS Launch Escape System. On top of the Soyuz Rocket is a Launch Escape Tower that is used for Launch Aborts prior to liftoff and during the first 157 seconds of the flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;At T+157 seconds, the SAS and Payload Fairing is jettisoned. After that, a variety of abort modes are available for the Soyuz Spacecraft. Depending on the abort scenario, different mechanisms are used to separate the Soyuz Capsule from the Rocket. After separation from the Launcher, the Soyuz conducts a commanded Module Separation and the Entry Module uses its attitude control system to place the vehicle in the correct entry attitude.&lt;/p&gt;&#xA;&lt;p&gt;The abort trajectory that takes the Soyuz to a landing point somewhere downrange the ground track, largely depends on the timing of the failure. The later an abort occurs in the ascent, the higher the G load the crew and vehicle experiences. For an abort 400 seconds in the flight, up to 21Gs occur on the way back to Earth. After Entry, the Soyuz performs nominal landing operations to bring the crew back to Earth. &lt;strong&gt;All abort scenarios are considered to be survivable by the crew.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Emphasis added by me.  Also, for any Kerbal Space Program fans who were wondering, SAS is the transliteration of the Cyrillic САС abbreviation of Система Аварийного Спасения, which means Emergency rescue system.&lt;/p&gt;&#xA;&lt;p&gt;I'm having a rather difficult time finding details on the &amp;quot;different mechanisms used to separate the Soyuz Capsule from the Rocket&amp;quot;, but &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-T_No.39&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz 18a&lt;/a&gt; actually did this:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;At T+295 seconds, the deviation was detected by the Soyuz's guidance system, which activated an automatic abort program. As the escape tower was long gone by this point, the abort had to be performed with the Soyuz's own engines. This separated the spacecraft from the third-stage booster and then separated the orbital and service modules of the Soyuz from the re-entry capsule.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://books.google.com/books?id=dbGchpi1HP8C&amp;amp;pg=PA70#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;This book (on page 70)&lt;/a&gt; references another document and indicates that before T+522 (which is shortly before third stage burnout) the orbital and descent modules should separate from the rocket and go into a controlled descent, while after T+522 the entire Soyuz vehicle should separate and go into a ballistic reentry.  Unfortunately it doesn't describe how the separation from the service module would be accomplished and the book goes on to comment that Soyuz 18a aborted before T+522 but still had the entire vehicle separate.&lt;/p&gt;&#xA;&lt;p&gt;I suspect I'd have better luck finding more information if I were searching in Russian, but my Russian isn't that good.  In the absence of additional sources I'm going to speculate that splitting the Soyuz vehicle in an abort before T+522 depends on the third stage booster being shut off, leaving the descent module in a suborbital trajectory, while Soyuz 18a was forced to use the Soyuz engine to escape from the third stage booster.  Furthermore, after T+522 the trajectory is either orbital or doesn't reenter in a convenient location so the Soyuz engine has to be used to ensure a proper reentry.&lt;/p&gt;&#xA;&lt;h2&gt;Response to Progress 59-type Problem&lt;/h2&gt;&#xA;&lt;p&gt;What we know about &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_M-27M&quot; rel=&quot;nofollow noreferrer&quot;&gt;Progress 59&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The vehicle was left spinning (at one revolution every five seconds according to a different source).&lt;/li&gt;&#xA;&lt;li&gt;There were communication issues before vehicle separation from the rocket.  The vehicle did separate but telemetry was intermittent and control of the vehicle couldn't be established.&lt;/li&gt;&#xA;&lt;li&gt;It reentered due to orbital decay almost ten days after launch.&lt;/li&gt;&#xA;&lt;li&gt;There was additional debris tracked near the vehicle and it's spent rocket body, indicating a breakup of some sort in either the vehicle or the spent booster.&lt;/li&gt;&#xA;&lt;li&gt;The vehicle was written off as a loss &amp;quot;with multiple systems suffering from failure and the main engine's fuel lines depressurized&amp;quot;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The spin wasn't as dramatic as the one revolution per second spin of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_8#Emergency&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gemini 8&lt;/a&gt;, which was manually corrected for by the astronauts on board.  In the absence of a system failure causing the RCS system to not work I'd say the occupants of a Soyuz vehicle could have countered the spin without a problem.&lt;/p&gt;&#xA;&lt;p&gt;Loss of contact with the ground isn't as critical with a manned vehicle as with an unmanned one - the astronauts/cosmonauts on board could initiate the abort sequence where the automated ship has to be commanded remotely when something goes wrong.&lt;/p&gt;&#xA;&lt;p&gt;Assuming that wasn't damaged, Soyuz has life support for 30 man-days (so 3 people, 10 days).  That would have been enough time to last until the vehicle decayed.&lt;/p&gt;&#xA;&lt;p&gt;The breakup (probably from an explosion) and multiple system failure would have been the biggest issue.&lt;/p&gt;&#xA;&lt;p&gt;If the engine was damaged or unable to fire (as is suggested by loss of pressure in the fuel lines) then the ability to use the vehicle to initiate deorbit is in question.  Without the main engine perhaps RCS thrusters could be used to help decay faster, but I don't know how much delta-v they have available or how much they would need.&lt;/p&gt;&#xA;&lt;p&gt;If the explosion suggested by the breakup event damaged the service module beyond just the engine that could impact life support or prevent the onboard controls from working.  All we know is that multiple systems failed, the specifics of which ones would impact what could be done to abort successfully.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-05T05:48:07.340" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9765" PostTypeId="2" ParentId="9759" CreationDate="2015-07-04T06:47:27.583" Score="7" Body="&lt;p&gt;The example initial conditions do not result in an orbit.  The energy is positive and the eccentricity is greater than one.  It is a hyperbola.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also you over-specified the problem by claiming it is in perifocal coordinates, but then providing initial conditions that result in a periapsis that is not on the p-axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The computed eccentricity is close, but not correct.  $e\approx 1.10768$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-04T06:47:27.583" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9766" PostTypeId="5" CreationDate="2015-07-04T15:17:57.980" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-07-04T15:17:57.980" LastActivityDate="2015-07-04T15:17:57.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9767" PostTypeId="4" CreationDate="2015-07-04T15:17:57.980" Score="0" Body="the effect on a spacecraft of moving through atmosphere, how its density, pressure, temperature, flow velocity, and viscosity affect a craft, and how lift and drag can be used to modify that." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-07-04T15:17:57.980" LastActivityDate="2015-07-04T15:17:57.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9768" PostTypeId="1" AcceptedAnswerId="54821" CreationDate="2015-07-04T17:14:50.080" Score="5" ViewCount="498" Body="&lt;p&gt;Wikipedia has this to say about &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_particle_event&quot; rel=&quot;nofollow&quot;&gt;solar particle events&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A solar particle event (SPE), or &quot;proton storm,&quot; occurs when particles&#xA;  (mostly protons) emitted by the Sun become accelerated to very high&#xA;  energies either close to the Sun during a solar flare or in&#xA;  interplanetary space by the shocks associated with coronal mass&#xA;  ejections.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Okay, so they seem to have to do with both solar flares and CMEs. And &lt;a href=&quot;http://helios.gsfc.nasa.gov/cme.html&quot; rel=&quot;nofollow&quot;&gt;this page&lt;/a&gt; on part of the Goddard Space Flight Center website says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The super-heated electrons from CMEs move along the magnetic field&#xA;  lines faster than the solar wind can flow. Rearrangement of the&#xA;  magnetic field, and solar flares may result in the formation of a&#xA;  shock that accelerates particles ahead of the CME loop.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So is that what a proton storm is, a piece of a coronal mass ejection that was accelerated beyond the rest of the CME by interaction with a solar flare? How does that work?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used to think that SPEs were CMEs hitting Earth, but that doesn't seem to be the case, or at least only sometimes. After all, why have two terms if they are the same thing? I'm especially interested in it because of the highly variable time it takes for an SPE to reach the Earth. Under the right conditions, they move almost at the speed of light - there was one like that documented &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2005/10jun_newstorm/&quot; rel=&quot;nofollow&quot;&gt;in 2005&lt;/a&gt;. So the way storms develop, and how that affects their speed, makes a big difference to how people and spacecraft need to be protected against them. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-07-05T21:21:39.440" LastActivityDate="2021-09-02T14:27:29.350" Title="How do solar flares and coronal mass ejections create proton storms?" Tags="|the-sun|space-weather|cme|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9769" PostTypeId="1" AcceptedAnswerId="9770" CreationDate="2015-07-04T17:18:53.827" Score="-3" ViewCount="495" Body="&lt;p&gt;My question is about Mars-one's plan to send people to Mars in 2025. I want to know why it is a one-way trip and not a round-trip. People already landed on Moon in 1969 and successfully came back to earth. also NASA sent New Horizon in 2006. so the problem can't be about fuel technology. there's also no problem in escaping Earth's gravity, going to another planet, and landing successfully on Earth. so why they can't leave people on Mars and go back in 2 or 3 years later to pick them up? is the one-way trip a part of their plan or they're simply unable to make a round-trip between Earth and Mars? if so then why?&lt;/p&gt;&#xA;" OwnerDisplayName="user10667" LastEditorUserId="1028" LastEditDate="2015-07-05T07:09:42.120" LastActivityDate="2015-07-05T07:09:42.120" Title="About Mars One's journey to Mars" Tags="|mars|new-horizons|earth|mars-one|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9770" PostTypeId="2" ParentId="9769" CreationDate="2015-07-04T17:47:58.870" Score="6" Body="&lt;p&gt;To fully answer your question requires giving you an enormous amount of background information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best explanation I have seen of this for the layman is astronaut Dr. Stan Love's presentation on &lt;a href=&quot;https://www.youtube.com/watch?v=fturU0u5KJo&quot; rel=&quot;noreferrer&quot;&gt;why it is hard to go to Mars&lt;/a&gt;.  Watch, be entertained, and learn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some highlights (these are not my points, they are his, so it is fruitless to argue them with me): &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Delta V for lunar mission = 42K mph, for Mars mission 48K mph.  DeltaV is not the problem&lt;/li&gt;&#xA;&lt;li&gt;It took the Saturn V to launch 6 tons on a roundtrip lunar mission. &#xA;A Mars mission would probably take 100 tons, or 25 Saturn Vs.&lt;/li&gt;&#xA;&lt;li&gt;We don't know how to keep the crew alive, healthy, and sane for the&#xA;duration of the mission.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-07-04T17:58:19.353" LastActivityDate="2015-07-04T17:58:19.353" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9771" PostTypeId="5" CreationDate="2015-07-04T18:49:24.943" Score="0" Body="&lt;p&gt;Crewed spaceflight (also referred to as human spaceflight or manned spaceflight) is space travel with a crew aboard the spacecraft. When a spacecraft is crewed, it can be operated directly, as opposed to being remotely operated or autonomous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This tag is most appropriate for questions directly relating to the manned aspect of spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Related Tags&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/uncrewed-spaceflight&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;uncrewed-spaceflight&amp;#39;&quot; rel=&quot;tag&quot;&gt;uncrewed-spaceflight&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Links&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Human_spaceflight&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:32:01.733" LastActivityDate="2015-07-26T20:32:01.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9772" PostTypeId="4" CreationDate="2015-07-04T18:49:24.943" Score="0" Body="Crewed spaceflight (also referred to as human spaceflight or manned spaceflight) is space travel with a crew aboard the spacecraft. When a spacecraft is crewed, it can be operated directly, as opposed to being remotely operated or autonomous." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-04T19:15:35.473" LastActivityDate="2015-07-04T19:15:35.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9773" PostTypeId="2" ParentId="8044" CreationDate="2015-07-05T12:21:16.680" Score="5" Body="&lt;p&gt;It is not possible to do this with just one communication satellite orbiting around the Moon, due to the reasons described by @HopDavid.&#xA;However, it can be done with just one comsat in Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine a satellite in polar orbit of &lt;strong&gt;Earth&lt;/strong&gt;, with a high apogee and a low perigee. If the apogee is over the north pole, the comsat will be over the equatorial plane in &lt;strong&gt;more than half of the time&lt;/strong&gt;, with free line of sight to the Moon's pole. The short periods when the satellite is under the equatorial plane is covered by direct line of sight between the Moon and Earth.&#xA;Such a high eccentricity orbit is somewhat analogue to a &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Molnya orbit&lt;/a&gt;, an orbit that is often used for Russian communication satellites to cover the high latitude regions of Russia most of the time of their orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iB6g3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iB6g3.png&quot; alt=&quot;configuration&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbit has to be high tough, with one revolution a month, but the  stability for this kind of orbit is good. See for instance the planned orbit for TESS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/mpViVEO-ymc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&#xA;(at the end of the video)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-28T22:45:00.037" LastActivityDate="2016-01-28T22:45:00.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9774" PostTypeId="2" ParentId="8044" CreationDate="2015-07-05T20:37:10.310" Score="2" Body="&lt;p&gt;Yes, if you'll accept a sufficiently loose definition of &quot;orbit&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Put the satellite above Earth's north pole, deploy a solar sail to keep it there.  Beware that it is going to be more than a million miles out, your lag time will be 10x what it is for a direct path transmission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Robert L. Forward's patent on this (although he was intending it for reaching the polar regions of Earth, I would think his patent applies for reaching the moon) has run out, you're free to do it.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-07-05T20:37:10.310" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9776" PostTypeId="1" CreationDate="2015-07-05T21:49:15.490" Score="55" ViewCount="21104" Body="&lt;p&gt;My question is very close to the question answered in the link below and is raised by the answers that question received:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4646/is-a-moon-base-inherently-more-dangerous-than-a-space-station&quot;&gt;Is a Moon-base inherently more dangerous than a Space Station?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why are we struggling to get to Mars prior to building something substantial on our own moon?&lt;/p&gt;&#xA;" OwnerUserId="10674" LastEditorUserId="6816" LastEditDate="2019-03-26T20:41:26.050" LastActivityDate="2019-03-26T20:41:26.050" Title="Why are we trying to build a base on Mars before the Moon?" Tags="|mars|the-moon|crewed-spaceflight|colonization|lunar-base|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9777" PostTypeId="1" AcceptedAnswerId="9806" CreationDate="2015-07-05T23:26:27.177" Score="13" ViewCount="704" Body="&lt;p&gt;Recently the New Horizon's satellite has given us some lovely color images of Pluto.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lTjIV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When did we first discover that Pluto is red? Secondly, did we suspect its color was red before we were able to verify it?&lt;/p&gt;&#xA;" OwnerUserId="4603" LastActivityDate="2015-07-15T16:05:32.097" Title="How long have we known Pluto is red?" Tags="|planetary-science|new-horizons|pluto|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9778" PostTypeId="2" ParentId="9776" CreationDate="2015-07-06T00:32:21.800" Score="84" Body="&lt;p&gt;At the moment, there are a few groups trying to reach Mars, and a few groups who are trying to reach the Moon. &lt;a href=&quot;http://www.mars-one.com/&quot;&gt;Mars One&lt;/a&gt; has grabbed headlines lately, &lt;a href=&quot;http://www.spacex.com/about&quot;&gt;SpaceX&lt;/a&gt; states Mars colonization as its long term goal, and there are numerous smaller groups. For the Moon, there is the Google &lt;a href=&quot;http://lunar.xprize.org/&quot;&gt;Lunar X prize&lt;/a&gt;, &lt;a href=&quot;http://www.shackletonenergy.com/overview/#goingbacktothemoon&quot;&gt;Shackleton Energy&lt;/a&gt; Corp, and &lt;a href=&quot;http://openluna.org/&quot;&gt;OpenLuna&lt;/a&gt;, and again numerous other groups. Different space agencies have gone back and forth favoring one destination or the other at different times over history, and that is a matter of politics as much as science and technology.  The arguments for each destination are many and varied, but can be grouped and outlined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Material Resources&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars has a carbon dioxide atmosphere. The technology and resources needed to &lt;a href=&quot;https://en.wikipedia.org/wiki/In-situ_resource_utilization#Mars&quot;&gt;process that&lt;/a&gt; into oxygen and methane for fuel are not that demanding (page 11 &lt;a href=&quot;http://wp10988215.server-he.de/MSE/wp-content/uploads/2011/10/Mars-Direct-Scenario_Zubrin_19911.pdf&quot;&gt;here&lt;/a&gt;). At first hydrogen for the process would need to be brought from Earth, later the frozen water that exists in large quantities beneath its surface could be used. &lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120001775.pdf&quot;&gt;frozen water&lt;/a&gt; on Mars would need to be dug up, melted, and separated from the soil it is mixed with. That takes heavy infrastructure, but nothing complicated. The frozen water is distributed over a large area too.&lt;/li&gt;&#xA;&lt;li&gt;The Moon has frozen deposits of &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2014/1117-lunar-polar-volatile-puzzle.html?referrer=https://www.google.com.mx/&quot;&gt;volatiles&lt;/a&gt;&lt;sup&gt;1&lt;/sup&gt; in craters in permanent shade at its poles, and beneath the surface near the poles.  How big these reserves are is still a matter of investigation, but there is a fair bit of evidence that they are quite &lt;a href=&quot;https://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html&quot;&gt;substantial&lt;/a&gt;. They seem to be a mixture of chemicals, mostly water, some carbon dioxide, and others, and may be mixed with a lot of soil. How difficult it would be to extract them and process them is not yet clear. Because they are only in a few places, that are deep, and extremely cold, it could be complicated.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Access&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars is really far away. With current technology it takes &lt;a href=&quot;http://www.braeunig.us/space/interpl.htm&quot;&gt;6 to 10 months&lt;/a&gt; (depending on alignment of the planets and how much money is poured  into the rocket) to get there, or get back, and launch windows occur every 2 years. &lt;/li&gt;&#xA;&lt;li&gt;The Moon is &lt;a href=&quot;http://www.universetoday.com/13562/how-long-does-it-take-to-get-to-the-moon/&quot;&gt;3 days away&lt;/a&gt; at any time, there or back. &lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_atmospheric_entry&quot;&gt;atmosphere of Mars&lt;/a&gt; makes it necessary to have heat shields on any lander, and to come in at the right angle and with the right orientation. But the atmosphere is not thick enough to slow a vehicle much as it comes in, just enough to heat it up. If rockets are used to slow the ship down, then they must handle firing into a wind whose dynamics constantly change as speed lowers from hypersonic to subsonic. If parachutes are used, they must handle supersonic velocities unless deployed only near the end. These are huge technical challenges. &lt;/li&gt;&#xA;&lt;li&gt;On the Moon shields aren't necessary, and there is never any complication with using rockets. A ship landing on the Moon can come in at any angle with any orientation. No &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobraking&quot;&gt;aerobraking&lt;/a&gt; can be done to save fuel by using friction with an atmosphere to slow a ship down enough to enter orbit. Thus the &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/images/mission/solarSubway.jpg&quot;&gt;delta V&lt;/a&gt; to get to either destination is similar. However, the technology needed to land is known, and can be scaled. &lt;/li&gt;&#xA;&lt;li&gt;Because of the time it takes to reach Mars, a ship needs to shield its occupants from radiation on the way. It may be possible to use the supplies of food and water already needed for the trip and the time on the surface for that, or that may not be enough. This is still not known. At any rate the required supplies make the ship much heavier, which adds considerable expense.&lt;/li&gt;&#xA;&lt;li&gt;Radio waves take between 3 and 20 minutes to make the trip between Earth and Mars, depending on how far apart they are. They take 1.3 seconds to go between Earth and the Moon. One interesting aspect of this is that it makes it possible to operate equipment on the Moon remotely from Earth, but that wouldn't work for Mars.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Impact&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars can be &lt;a href=&quot;https://en.wikipedia.org/wiki/Terraforming_of_Mars&quot;&gt;terraformed&lt;/a&gt;. At least, that seems to be the case, although it would take a vast, vast effort over centuries. There is much we don't know about this, but it seems like a reasonable, if ambitious, conjecture. There is enough water for shallow seas, and may be enough frozen carbon dioxide&lt;sup&gt;2&lt;/sup&gt; for an atmosphere about 30% the density of ours, very roughly, and plants could process that into oxygen. &lt;/li&gt;&#xA;&lt;li&gt;The Moon would open up the solar system if colonized. Because it has so little gravity and no atmosphere, and it is right next door, all the heavy stuff could be made there and launched from there, saving tremendously on the resources needed to send missions far and near. Some say space stations could be used instead, and resources from asteroids, and the Moon could be skipped. That puts a 3rd horse in this race. The factors in that one are distance weighing against, and low gravity weighing in favor.&lt;/li&gt;&#xA;&lt;li&gt;Mars may &lt;a href=&quot;https://en.wikipedia.org/wiki/Life_on_Mars&quot;&gt;harbor life&lt;/a&gt;. 'Nuff said. Then again, that also complicates things, see the next list.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Problems&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Moon is covered in very &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_soil#Properties&quot;&gt;fine sharp dust&lt;/a&gt; that sticks to everything because it has a &lt;a href=&quot;http://www.space.com/3080-lunar-explorers-face-moon-dust-dilemma.html&quot;&gt;static charge&lt;/a&gt;. Breathing it in would be a &lt;a href=&quot;https://en.wikipedia.org/wiki/Adverse_health_effects_from_lunar_dust_exposure&quot;&gt;health hazard&lt;/a&gt; because the fine particles would lodge in the lungs and could cause silicosis. Moving mechanical parts outdoors will need protection or things will seize. Seals will fail if not protected. &lt;/li&gt;&#xA;&lt;li&gt;Mars is also covered in &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_soil&quot;&gt;fine dust&lt;/a&gt; that is not so sharp because it is weathered. But the fine particles remain suspended in the atmosphere and that makes it harder to keep them out of the indoors. Silicosis again is &lt;a href=&quot;http://www.space.com/4051-future-mars-explorers-face-dusty-challenges.html&quot;&gt;a hazard&lt;/a&gt;, and there are other toxic constituents as well. And the dust also has a static charge that makes it stick to everything.&lt;/li&gt;&#xA;&lt;li&gt;Mars has toxic chemicals in its soil. &lt;a href=&quot;http://www.newscientist.com/article/dn23505-toxic-mars-dust-could-hamper-planned-human-missions.html#.VZnEA3UVhBc&quot;&gt;Perchlorates&lt;/a&gt; have been identified and may be widespread. They are toxic to people. &lt;/li&gt;&#xA;&lt;li&gt;Mars may harbor life. That life must &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_protection&quot;&gt;be protected&lt;/a&gt;. Colonizing a planet while protecting its native life may be very difficult and would certainly require special precautions that add complexity and cost to such an endeavor. &lt;/li&gt;&#xA;&lt;li&gt;The Moon has 1/6 the gravity of Earth. Living long-term in such an environment may cause health problems, and muscles and bones could change so much that returning to Earth would be difficult or impossible. Mars has 2/5 the gravity of Earth. There is a better chance this can be adapted to. However, much is not known about this. The right exercise program may eliminate this problem. In other respects, different gravity levels each have a suite of advantages and disadvantages.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So. Depending on how people weigh the advantages and disadvantages of each place, some feel we should go to Mars first, some feel we should go to the Moon first. One extra spin on the argument is that if the Moon is colonized first, then sending a lot of stuff to Mars is much easier. But you have to build all the infrastructure on the Moon to do that, so maybe that should be a long term consideration, but first a base should be established on Mars. Once you get into all the possible combinations of missions, the question becomes even more complex than has been briefly conveyed here. There is more discussion of the possibilities &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_colonization&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And we haven't even gotten into the more subjective qualities of each place. Does it matter more that Mars has a day the same length as Earth's and has an atmosphere that creates a sky a lot like we are used to here, or does it matter more that the Earth hangs in the sky on the Moon, big and beautiful?&#xA;&lt;hr&gt;&#xA;&lt;sup&gt;1 - 'Volatiles' refers to any chemical that is lost to space when subjected to the barrage of radiation and particles from the Sun, on bodies that are too small to hold them by gravity or with the help of a magnetic field.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;2 - The real carbon dioxide reserves on Mars are unknown. Models predict frozen CO2 mixed into the frozen water of the poles and in the ground. In the early history of Mars when the atmosphere was much thicker, carbon dioxide may have been sequestered in carbonate minerals such as calcium carbonate. See &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/0019103585901162&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;http://www.sciencemag.org/content/170/3957/531.short&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbonates_on_Mars&quot;&gt;here&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-01-02T16:35:56.127" LastActivityDate="2016-01-02T16:35:56.127" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="9779" PostTypeId="1" AcceptedAnswerId="9780" CreationDate="2015-07-06T02:05:50.990" Score="3" ViewCount="160" Body="&lt;h1&gt;Background&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;In airports and shopping malls multiple, parallel moving &quot;&lt;a href=&quot;http://connection.ebscohost.com/c/articles/10855844/two-speed-moving-walkway&quot; rel=&quot;nofollow noreferrer&quot;&gt;slidewalks&lt;/a&gt;&quot; allow people to increase their velocity from a low speed to a high speed. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Overview&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Consider the following image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/q6p0O.png&quot; alt=&quot;Multi-stage acceleration&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image shows a multi-stage acceleration trajectory. The idea is similar to people stepping onto ever-faster-moving parallel slidewalks. Instead of slidewalks, we use orbiting vessels. Instead of people, we use a transport shuttle (that carries people).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the image, the orange concentric circles (1-4) represent the orbital stages of spacecraft traveling at different speeds, with an increase of speed proportional to its distance from Earth. The X's mark transfer points for the human-toting transport shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each spacecraft in an orange orbit gains its velocity by swapping time for speed, such as an ion drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the transport shuttle reaches stage 4 (or higher, if need be), it is at a sufficient velocity that a trip to Mars would only take a few weeks.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Questions&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;I am wondering:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is this a viable acceleration technique to minimise human exposure to space?&lt;/li&gt;&#xA;&lt;li&gt;What is this graduated acceleration technique called?&lt;/li&gt;&#xA;&lt;li&gt;What are the disadvantages (cost, fuel, time, etc.) compared with current Mars visitation endeavours?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Related&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/8946/would-it-be-cheaper-to-go-to-mars-if-the-empty-transit-vehicle-is-slowly-acceler&quot;&gt;Would it be cheaper to go to Mars if the empty transit vehicle is slowly accelerated before the passengers board?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10671" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-07-06T02:51:47.370" Title="Gradulated Accelerated Transfer" Tags="|mars|propulsion|trajectory|transport|low-energy-transfer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9780" PostTypeId="2" ParentId="9779" CreationDate="2015-07-06T02:51:47.370" Score="2" Body="&lt;p&gt;There's two ways to view your proposal;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can do impulse burns but not constant acceleration burns, like your mention of multi-stage accelerations suggests, then what you describe is a series of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Hohmann transfers&lt;/a&gt; into higher orbital altitude, but instead of doing an orbital circularization burn, you continue with another apogee rising burn as the previous transfer reaches maximum altitude. This is not optimal as you don't take the most advantage of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;nofollow&quot;&gt;Oberth effect&lt;/a&gt; and gradually increasing apogee with more energy efficient perigee burns, stretching into an ever more eccentric ellipse until you reach escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what you describe would work for non-impulse, constant acceleration propulsion, that is currently limited to low thrust but high &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;nofollow&quot;&gt;specific impulse&lt;/a&gt; propulsion like ion thrusters, beamed propulsion, solar sails and alike.Those would be &lt;a href=&quot;https://en.wikipedia.org/wiki/Low-energy_transfer&quot; rel=&quot;nofollow&quot;&gt;low energy transfer&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_maneuver#Constant_Thrust_Trajectory&quot; rel=&quot;nofollow&quot;&gt;constant thrust trajectories&lt;/a&gt;. But it's also often referred to simply as &lt;em&gt;&quot;orbit climbing&quot;&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-07-06T02:51:47.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9781" PostTypeId="2" ParentId="9759" CreationDate="2015-07-06T03:01:06.663" Score="2" Body="&lt;p&gt;First of all the eccentricity is a constant, so it should not change with the true anomaly. You could use it as an temporary variable, bit it is not required to calculate $e$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you use different definitions for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_relative_angular_momentum&quot; rel=&quot;nofollow&quot;&gt;angular momentum&lt;/a&gt; you can find the eccentricity using only the position and velocity at one given time,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;h=r v_{perp} = \sqrt{\mu a (1-e^2)}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The semi-major axis can be found using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_orbital_energy&quot; rel=&quot;nofollow&quot;&gt;specific orbital energy&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;a = \frac{\mu r}{2\mu - r v^2}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combining these two equations yields,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;e = \sqrt{1 + \frac{r v_{perp}^2}{\mu} \left(\frac{r v^2}{\mu} - 2\right)}.&#xA;$$&lt;/p&gt;&#xA;" OwnerUserId="1086" LastActivityDate="2015-07-06T03:01:06.663" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9782" PostTypeId="1" CreationDate="2015-07-06T12:14:56.747" Score="5" ViewCount="172" Body="&lt;p&gt;At the end of the Jupiter flyby, &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/PI-Perspectives.php?page=piPerspective_3_26_2007&quot;&gt;New Horizons entered safe mode&lt;/a&gt;. I remember at that time a question being asked &quot;What would happen if this happened during the Pluto flyby&quot;, and the answer was that safe mode was going to be disabled for a few weeks around the time of the flyby. I can't remember very precisely the time period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flash forward, New Horizons entered safe mode less than 2 weeks before the flyby. I can't find the original source of my knowledge. Deos anyone out there know what the plan was 8 years ago to disable safe mode around the time of the Pluto encounter?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-06T14:42:19.350" Title="New Horizon's disabling safe mode near flyby (in 2007)" Tags="|new-horizons|pluto|safe-mode|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9783" PostTypeId="2" ParentId="9782" CreationDate="2015-07-06T14:42:19.350" Score="5" Body="&lt;p&gt;You remember &lt;a href=&quot;http://news.discovery.com/space/private-spaceflight/glitch-halts-new-horizons-operations-as-it-nears-flyby-150705.htm&quot; rel=&quot;noreferrer&quot;&gt;correctly&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On Sunday, ground controllers planned to relay a final batch of instructions to New Horizons to prepare for its July 14 flyby of Pluto, the only major body in the solar system that has not yet been visited by a robotic spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;The “encounter program” includes software to prohibit the very type of automated safe mode that New Horizons executed Saturday afternoon.&lt;/p&gt;&#xA;&lt;p&gt;“Encounter mode short-circuits the on board intelligent autopilot so that if something goes wrong, instead of calling home for help, which is what most spacecraft do and what New Horizons does during cruise flight, it will just stay on the timeline. It will try to fix the problem, but it will rejoin the timeline because if it ‘went fetal,’ as we say, if it just called home for help, it could miss the flyby,” New Horizons lead scientist Alan Stern told Discovery News before Saturday’s problem.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-06T14:42:19.350" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9784" PostTypeId="1" AcceptedAnswerId="9786" CreationDate="2015-07-06T14:42:27.570" Score="16" ViewCount="3259" Body="&lt;p&gt;xkcd today posted &lt;a href=&quot;http://www.xkcd.com/1547/&quot; rel=&quot;noreferrer&quot;&gt;the following graphic&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/KWXGo.png&quot; alt=&quot;xkcd: Solar System Questions&quot; title=&quot;My country&amp;#39;s World Cup win was exciting and all, but c&amp;#39;mon, what if the players wore nylon wings and COULD LITERALLY FLY?&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of these, the only one that I really don't understand is &quot;What pushes a spacecraft during a flyby?&quot; I don't understand why this is an issue. How energy is passed during a flyby is well known, however, this question implies there is a mystery. Is there a mystery surrounding some phantom force during flybys, or is this just a reference to how flybys seem mysterious?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="10701" LastEditDate="2015-07-07T09:23:48.273" LastActivityDate="2015-07-16T23:58:59.397" Title="What pushes a spacecraft during a flyby?" Tags="|orbital-mechanics|gravity-assist|flyby|xkcd|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9785" PostTypeId="2" ParentId="9784" CreationDate="2015-07-06T15:43:43.960" Score="2" Body="&lt;p&gt;I think your assumption is correct, and &lt;em&gt;this is just a reference to how flybys seem mysterious&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MOKuM.gif&quot; alt=&quot;Gravity assist&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Example of a gravity assist around Jupiter, wherein velocity is &lt;em&gt;added&lt;/em&gt;. (Courtesy &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/grav/bsf16-23a.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA JPL&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To anyone without a reasonable knowledge of orbital mechanics, upon hearing about spacecraft accelerating during a gravity assist, will presume that it was 'pushed' by some force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We do of course know that it is in fact the gravitational &lt;strong&gt;pull&lt;/strong&gt; of a planet that imparts the force. However to the layperson, a &lt;strong&gt;push&lt;/strong&gt; comes close enough to describe the phenomenon. &lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-07-06T15:43:43.960" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9786" PostTypeId="2" ParentId="9784" CreationDate="2015-07-06T16:25:16.810" Score="21" Body="&lt;p&gt;Whilst the main factors involved in transferring energy to space craft during a flyby are well known, I believe this is a reference to &lt;a href=&quot;http://www.space.com/5037-nasa-baffled-unexplained-force-acting-space-probes.html&quot;&gt;several observed anomalies&lt;/a&gt; that have occurred to various space probes, covering everything from the early Pioneer probes to the much more recent Rosetta probe.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The researchers looked at five deep-space probes — Galileo to Jupiter,&#xA;  the NEAR mission to the asteroid Eros, the Rosetta probe to a comet,&#xA;  Cassini to Saturn, and the MESSENGER craft to Mercury. Each spacecraft&#xA;  flew past the our planet to either gain or lose orbital energy in&#xA;  their quests to reach their eventual targets. (Galileo made two&#xA;  flybys.)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In five of the six flybys, the scientists have confirmed anomalies.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10686" LastActivityDate="2015-07-06T16:25:16.810" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9787" PostTypeId="1" AcceptedAnswerId="9789" CreationDate="2015-07-06T17:51:35.790" Score="6" ViewCount="4310" Body="&lt;p&gt;I've been developing a planetary simulator in Unity 3D (C#) and I need some help, however I feel this would be a better place for it. I'm starting my planet from a position vector. On initialisation I apply a direction vector, which puts it into orbit around a larger star. The orbits aren't circular, and I'd like for them to be elliptical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've read up on foci, the semi-major axis and semi-minor axis, on several laws by Newton and Kepler, I understand the apoapsis and periapsis, and yet I just can't figure out what I need to do. What I've done is leave a trail from the planets, and the orbits can be seen. (note: I know the layout of planets aren't correct!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/uv5aX.png&quot; alt=&quot;planets&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks nice, but it's not what I wanted. I'd like to be able to predict the orbit. I've found equations so I can calculate the sphere of influence, which require the semi-major axis. I could calculate the semi-major axis if I had the orbital time, and I could calculate the orbital time if I had the semi-major axis. However I only have an initial position, a vector force I apply to it, the distance between the planets, and also the masses of each. I'm at a dead end here, I'm not sure what I'm looking for. Could anybody please give me some pointers (or formulae) in order to do what I'd like to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd really appreciate any help, thanks in advance!&lt;/p&gt;&#xA;" OwnerUserId="10688" LastActivityDate="2015-09-16T00:03:55.170" Title="How can you predict an orbit, based on an initial position and an initial direction vector?" Tags="|orbital-mechanics|orbit|simulation|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9789" PostTypeId="2" ParentId="9787" CreationDate="2015-07-06T18:15:37.333" Score="10" Body="&lt;p&gt;In three dimensions you need six numbers.  What you are asking about is how to convert from one set of six numbers, three position coordinates and three velocity components (note that simply direction is not sufficient), to another set of six numbers, the six orbital elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not going to give you the equations, but I'll get you started.  You can google the equations.  First compute the energy, a scalar.  That will tell you if it even is in orbit (negative energy) or hyperbolic (positive energy).  From the energy you can readily compute the semi-major axis.  Then compute the angular momentum vector.  The direction of that vector will tell you the plane of the trajectory and the direction in that plane (that vector is perpendicular to the plane of the trajectory).  Then compute the eccentricity vector.  That vector will lie in the plane, and will point to the periapsis or closest approach for a hyperbola.  It's magnitude is, of course, the eccentricity of the orbit or hyperbola.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you'll have all you need to generate the orbital elements.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-06T18:15:37.333" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9790" PostTypeId="1" CreationDate="2015-07-06T19:45:25.797" Score="5" ViewCount="371" Body="&lt;p&gt;Recently, the New Horizons spacecraft on its way to Pluto suffered a &lt;a href=&quot;http://gizmodo.com/everyone-relax-nasa-solved-the-problem-with-its-pluto-1715910558&quot;&gt;&quot;hard-to-detect timing flaw in the spacecraft command sequence.&quot;&lt;/a&gt; Does that mean that it was a multi-threading error or something like that? What kind of data would be sent back to Earth to help the NASA diagnose the issue? Would they receive a stack trace? I imagine that the enormous distance would cause some unreliability in the transmission besides the 4.5 hour transmission time, so I would guess that the data that gets sent back and forth needs to be rather minimal.&lt;/p&gt;&#xA;" OwnerUserId="10690" LastEditorUserId="49" LastEditDate="2015-07-06T20:09:26.723" LastActivityDate="2015-07-10T21:23:12.117" Title="What type of data did New Horizons send to Earth in order to diagnose the switch to Safe Mode issue?" Tags="|communication|deep-space|new-horizons|data-transmission|recovery|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9792" PostTypeId="2" ParentId="9787" CreationDate="2015-07-06T21:27:47.130" Score="1" Body="&lt;p&gt;Forgive my lack of a concrete answer, but it really depends on how accurate you need to be. I once wrote a simulator that used an n-body Newtonian approximation. It was nice, but not particularly well-implemented and could only support a couple dozen bodies before it slowed to a crawl.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;KSP uses a simpler, more scaleable approach; a spaceship is only attracted to the body who's sphere of influence it inhabits, and planets and moons are on rails. This is a close enough approximation for a video game, and it doesn't grow out of control as you add more objects to the simulation. The tradeoff is that it means you're only ever attracted to one object at a time, so you don't see Lagrange points or other interesting gravitational effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even a well-implemented Newtonian n-body model would be far more computationally expensive than KSP's system, and still wouldn't tell the whole story, as Newtonian physics doesn't take relativistic effects into account. (If you think relativistic effects don't matter to orbital mechanics, read up on the planet Vulcan: &lt;a href=&quot;https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might be worth asking this question on Game Development stack exchange, as deciding how to best approximate an orbit has as much to do with the kind of game-play you want and the practical limitations of available computing hardware as it does with the physics of orbital mechanics. &lt;/p&gt;&#xA;" OwnerUserId="10693" LastActivityDate="2015-07-06T21:27:47.130" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9793" PostTypeId="5" CreationDate="2015-07-07T02:46:55.377" Score="0" Body="&lt;p&gt;Use this tag for questions about or relating to xkcd comics.  The associated &quot;What If?&quot; questions are also an appropriate use for this tag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;xkcd is a webcomic created by Randall Munroe.  The comic's tagline describes it as a &quot;A webcomic of romance, sarcasm, math, and language&quot; (formerly a &quot;Stick-figure strip featuring humour about technology, science, mathematics and relationships&quot;).  Munroe states on the comic's website that the name of the comic is not an acronym but &quot;just a word with no phonetic pronunciation&quot;.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Sample Space Exploration-Related Pages&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://xkcd.com/681/&quot; rel=&quot;nofollow&quot;&gt;Gravity Wells&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://xkcd.com/695/&quot; rel=&quot;nofollow&quot;&gt;Spirit&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://xkcd.com/1133/&quot; rel=&quot;nofollow&quot;&gt;Up Goer Five&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://what-if.xkcd.com/24/&quot; rel=&quot;nofollow&quot;&gt;Model Rockets (What if?)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Useful links&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://xkcd.com/&quot; rel=&quot;nofollow&quot;&gt;xkcd.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://what-if.xkcd.com/&quot; rel=&quot;nofollow&quot;&gt;What If?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.explainxkcd.com&quot; rel=&quot;nofollow&quot;&gt;explainxkcd.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-07T02:53:57.073" LastActivityDate="2015-07-07T02:53:57.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9794" PostTypeId="4" CreationDate="2015-07-07T02:46:55.377" Score="0" Body="A webcomic created by Randall Munroe, tagline: &quot;A webcomic of romance, sarcasm, math, and language.&quot;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-07T02:54:05.870" LastActivityDate="2015-07-07T02:54:05.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9796" PostTypeId="2" ParentId="3251" CreationDate="2015-07-07T03:09:11.263" Score="7" Body="&lt;p&gt;As posted in a comment, the most likely contribution of a motel/restaurant to a manned space program is a discount on rooms or food.  To be more specific, this specific plaque is almost certainly a reference to food/lodging related to the April 3/May 2, 2012 drop tests of &lt;a href=&quot;https://en.wikipedia.org/wiki/CST-100&quot; rel=&quot;noreferrer&quot;&gt;Boeing's CST-100 crew capsule&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;h2&gt;Evidence:&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The picture in the background of the plaque is the same as this picture (found attached to &lt;a href=&quot;http://www.nasa.gov/exploration/commercial/crew/boeingdroptest.html&quot; rel=&quot;noreferrer&quot;&gt;this NASA post&lt;/a&gt;, image credit: Boeing) with the ground cropped off: &lt;img src=&quot;https://i.stack.imgur.com/3y01T.jpg&quot; alt=&quot;May 2 Parachute test of Boeing's CST-100 crew capsule&quot; /&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The article and original caption of that image referenced Delamar Dry Lake, which is only &lt;a href=&quot;https://www.google.com/maps/dir/Delamar+Dry+Lake/Windmill+Ridge,+2111+Windmill+Circle,+Alamo,+NV+89001/@37.3049872,-115.0762557,12z/data=!4m13!4m12!1m5!1m1!1s0x80b7d486d5df8f6b:0x3cf48221276a8931!2m2!1d-114.9486194!2d37.3205194!1m5!1m1!1s0x80b7c4cdbe140d39:0x90cdffc5393e8324!2m2!1d-115.1658152!2d37.3945978&quot; rel=&quot;noreferrer&quot;&gt;about 20 miles from Windmill Ridge by road&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Parachute tests happened on April 3 and May 2 of 2012 (both at Delamar Dry Lake Bed) and the plaque is dated July 2012.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-07T03:09:11.263" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9797" PostTypeId="2" ParentId="3251" CreationDate="2015-07-07T04:22:23.710" Score="7" Body="&lt;p&gt;It's not even a discount. Sometimes it's just moral support.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every NASA space center that involves dangerous activities has a select few restaurants and bars that everyone knows by name. The same goes for Navy and Air Force bases that involve dangerous activities. For example, there is no Building Zero on the JSC campus (building numbers start at one, which is the main administrative building). Nonetheless, if you tell people &quot;we're getting together at Building Zero after work&quot;, nobody needs directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before this bar, astronauts and engineers hung out at the now closed Outpost Tavern, formerly known as The Ultimate Joint. It's the astronaut bar in &lt;em&gt;Apollo 13&lt;/em&gt;. Edwards AFB had its Happy Bottom Riding Club (watch &lt;em&gt;The Right Stuff&lt;/em&gt;). The only centers that don't are the ones that don't do dangerous stuff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a bit difficult to give a bar a &quot;Certificate of Appreciation&quot;. A restaurant? That's not a problem. I've been to multiple NASA centers and multiple Air Force and Navy bases. Each one has restaurants that have pictures on the wall. Here's one in the JSC area:&#xA;&lt;img src=&quot;https://ww4.hdnux.com/photos/17/72/45/4172123/9/920x920.jpg&quot; alt=&quot;restaurant wall with pictures of astronauts&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are multiple restaurants in the neighborhood of NASA JSC, NASA KSC, NASA ARC, JPL, Edwards AFB, Fort George Meade, and China Lake that have certificates exactly like this; I've been to many of them. I've never been to the Delamar Dry Lake  area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a place such as that (aka &quot;the middle of nowhere&quot;), it's not that hard to get such a certificate. All you have to do is serve food that is better than cafeteria food. Note well: From the online reviews of the Windmill Ridge, the food there is downright good, much better than cafeteria food.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="10273" LastEditDate="2019-01-13T10:54:16.260" LastActivityDate="2019-01-13T10:54:16.260" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="9798" PostTypeId="2" ParentId="8942" CreationDate="2015-07-07T04:45:36.977" Score="6" Body="&lt;p&gt;As discussed in the related questions TildalWave posted in a comment it really depends on the rocket (because I have it &lt;a href=&quot;http://www.universetoday.com/90939/soyuz-launches-to-station-amid-swirling-snowy-spectacular/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;'s an article about a Soyuz launch in a snowstorm) and what the range they're launching from supports.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering you asked on April 27th you were probably watching the SpaceX TurkmenÄlem52E/MonacoSat (Thales) launch.  For that specific launch I found this screenshot of the launch commit criteria:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/rfaGv.jpg&quot; alt=&quot;TurkmenÄlem52E launch commit criteria&quot;&gt;&#xA;That's sourced from &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/33xqcj/rspacex_turkmen%C3%A4lem52emonacosat_thales_official/cqqgbz3&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; reddit post.  I don't know where he got the screenshot but it's an interesting post all around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the checklist breaks it down into separate groups for the range and the Falcon 9, &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_commit_criteria#Falcon_9&quot; rel=&quot;noreferrer&quot;&gt;this wikipedia page&lt;/a&gt; provides more detail but doesn't give any such separation.  On the other hand, the wikipedia page is sourced from &lt;a href=&quot;http://www.nasa.gov/pdf/649911main_051612_falcon9_weather_criteria.pdf&quot; rel=&quot;noreferrer&quot;&gt;this NASA document&lt;/a&gt; which only includes the Liftoff Winds rule from the Falcon 9 specific criteria and otherwise appears to match the range criteria.  Also, if I remember correctly the first stage landing criteria was a nice to have, not a requirement to launch (maybe why it's marked as N/A).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote wikipedia (since it's better formatted for copying text out of than the NASA pdf), the Falcon 9 cannot be launched under the following conditions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;sustained wind at the 162 feet (49 m) foot level of the launch pad in excess of 30 knots (56 km/h; 35 mph)&lt;/li&gt;&#xA;&lt;li&gt;upper-level conditions containing wind shear[quantify] that could lead to control problems for the launch vehicle.&lt;/li&gt;&#xA;&lt;li&gt;launch through a cloud layer greater than 4,500 feet (1,400 m) thick that extends into freezing temperatures&lt;/li&gt;&#xA;&lt;li&gt;launch within 19 kilometres (10 nmi) of cumulus clouds with tops that extend into freezing temperatures,&lt;/li&gt;&#xA;&lt;li&gt;within 19 kilometres (10 nmi) of the edge of a thunderstorm that is producing lightning within 30 minutes after the last lightning is observed.&lt;/li&gt;&#xA;&lt;li&gt;within 19 kilometres (10 nmi) of an attached thunderstorm anvil cloud&lt;/li&gt;&#xA;&lt;li&gt;within 9.3 kilometres (5 nmi) of disturbed weather clouds that extend into freezing temperatures&lt;/li&gt;&#xA;&lt;li&gt;within 5.6 kilometres (3 nmi) of a thunderstorm debris cloud,&lt;/li&gt;&#xA;&lt;li&gt;through cumulus clouds formed as the result of or directly attached to a smoke plume,&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The following should delay launch:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;delay launch for 15 minutes if field mill instrument readings within 9.3 kilometres (5 nmi) of the launch pad exceed +/- 1,500 volts per meter, or +/- 1,000 volts per meter&lt;/li&gt;&#xA;&lt;li&gt;delay launch for 30 minutes after lightning is observed within 10 nautical miles (19 km; 12 mi) of the launch pad or the flight path&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is a hard question to answer generally (without just resorting to &quot;it depends), but if you're interested in other rockets the wikipedia link above also includes criteria for Atlas V and the Space Shuttle.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-07-07T04:45:36.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9799" PostTypeId="1" AcceptedAnswerId="9804" CreationDate="2015-07-07T12:58:48.963" Score="16" ViewCount="5006" Body="&lt;p&gt;There are a lot of variables with the speed of sound, a sonic boom is created when all of those variables meet.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the speed of sound, a critical speed known as Mach 1, and is approximately 1,225 km/h (761 mph) at sea level and 20 °C (68 °F).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sonic_boom&quot;&gt;Sonic boom&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During reentry the Space Shuttle is moving at several times the speed of sound&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The orbiter's maximum glide ratio/lift-to-drag ratio varies considerably with speed, ranging from 1:1 at hypersonic speeds, 2:1 at supersonic speeds and reaching 4.5:1 at subsonic speeds during approach and landing&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle#Re-entry_and_landing&quot;&gt;Re-entry and landing&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no sound in space, so presumably there has to be a point when speed and air density combine to create a sonic boom. When does an aerobraking space craft create a sonic boom?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="4660" LastEditDate="2015-07-11T14:38:59.707" LastActivityDate="2020-05-18T08:53:16.240" Title="When does an aerobraking space craft create a sonic boom?" Tags="|reentry|atmosphere|aerobraking|sound|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9800" PostTypeId="1" AcceptedAnswerId="10102" CreationDate="2015-07-07T12:59:40.023" Score="8" ViewCount="385" Body="&lt;p&gt;We have some estimats on the density of the bodies in the main belt. E.g. &lt;a href=&quot;https://space.stackexchange.com/questions/231/whats-the-particle-density-of-the-asteroid-belt&quot;&gt;What&amp;#39;s the (particle) density of the asteroid belt?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much of this is discovered? And how strongly does discovery correlate with the size of the body?&lt;/p&gt;&#xA;" OwnerUserId="5934" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-07-21T20:05:08.257" Title="How much of the asteroid belt is discovered?" Tags="|asteroid|asteroid-belt|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9801" PostTypeId="2" ParentId="9799" CreationDate="2015-07-07T15:05:57.683" Score="1" Body="&lt;p&gt;I found an answer here: &lt;a href=&quot;http://www.mach25media.com/history1.html&quot; rel=&quot;nofollow&quot;&gt;http://www.mach25media.com/history1.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thus Mach 25 is the velocity that a spacecraft is flying at the&#xA;  moment it reenters the atmosphere of Earth (entry interface). This&#xA;  interface occurs at approximately &lt;strong&gt;75 miles (400,000 feet)&lt;/strong&gt; when a&#xA;  vehicle such as the Space Shuttle orbiter has a speed of about &lt;strong&gt;17,200&#xA;  miles per hour&lt;/strong&gt;. At that altitude, the speed of sound is generally&#xA;  around &lt;strong&gt;688 miles per hour&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's &lt;strong&gt;121 920m&lt;/strong&gt; high and  &lt;strong&gt;27680km/h&lt;/strong&gt;, little bit under &lt;strong&gt;mach 25&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boom.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2015-07-07T15:05:57.683" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9803" PostTypeId="1" AcceptedAnswerId="9805" CreationDate="2015-07-07T16:35:01.730" Score="21" ViewCount="1970" Body="&lt;p&gt;I guess that everyone is familiar with the problem of aligning a TV satellite dish: some degrees off and the signal you desire is gone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How sensitive is communication with deep space probes to this problem? Is it addressed with special solutions or the radio emissions are not focused enough for this to be a problem?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand also that &lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot;&gt;sometimes&lt;/a&gt; the same antenna is used for 2-way communication with a spacecraft. Is this possible for the above-mentioned reason?&lt;/p&gt;&#xA;" OwnerUserId="5862" LastActivityDate="2015-07-10T11:33:03.663" Title="How precise must be the antenna orientation to communicate with a deep space probe?" Tags="|communication|deep-space|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9804" PostTypeId="2" ParentId="9799" CreationDate="2015-07-07T17:20:44.273" Score="9" Body="&lt;p&gt;How many grains of sand does it take to form a heap?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An orbiting spacecraft is flying many times faster than the speed of sound. It's starting in atmosphere too thin to sustain an audible shock wave. As it descends, it's going to be producing a shock cone continuously, but in the very thin atmosphere high up, the amplitude of the shock wave is too faint to hear. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some point in the descent, the air would be thick enough for you to barely hear it if it were close by -- the tiniest sonic boom. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some lower point, it would be audible even at a distance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low enough, the shock wave is occurring in air dense enough that it's loud enough it could rattle windows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the speed of sound is lower at high altitude -- and doesn't change all that much along the way -- the sonic boom would be continuous all the way down, until the craft went subsonic at relatively low altitude.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-07-07T17:37:12.063" LastActivityDate="2015-07-07T17:37:12.063" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9805" PostTypeId="2" ParentId="9803" CreationDate="2015-07-07T17:59:06.360" Score="14" Body="&lt;p&gt;You can calculate this using the diffraction limit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\sin\theta\approx{\lambda\over D}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $\lambda$ is the wavelength being used for the radio communication, $D$ is the diameter of the dish (either on the spacecraft or on the ground), and $\theta$ is the beam width.  You need to point the antenna accurately enough to keep the target in the beam.  The ratio is usually very small, so you can just drop the $\sin$, where $\theta\approx{\lambda\over D}$ is then in radians.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-07T17:59:06.360" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9806" PostTypeId="2" ParentId="9777" CreationDate="2015-07-07T21:27:18.257" Score="10" Body="&lt;p&gt;It's important to note that Pluto's colour is seasonal. Here's a 2010 article at &lt;a href=&quot;http://www.universetoday.com/54114/new-hubble-images-show-pluto-is-changing/&quot; rel=&quot;noreferrer&quot;&gt;Universe Today&lt;/a&gt; discussing Hubble Space Telescope (HST) observations:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But the images do show Pluto is significantly redder than it had been for the past several decades. Astronomers use the word “red” to mean it reflects more red light than blue or green light. To the human eye, Pluto has a yellowish-orange color, and is about 20% redder than it used to be. “It’s not as red as the surface of Mars, but more red than Io,” Buie said.&lt;/p&gt;&#xA;&lt;p&gt;Red is usually associated with carbon. The astronomers said there is also methane, which is not usually stable in an environment like Pluto’s.&lt;/p&gt;&#xA;&lt;p&gt;“This business about the color change had me scared for awhile,” Buie confessed. “I got the result years ago, but it was so hard to understand and believe. I’m still nervous about it. It could be that I completely screwed this up, but I can tell you Charon is on the same images, and Charon has the same color throughout but Pluto changed. I don’t’ know how the camera system on HST could have given me the wrong colors on Pluto.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Scientists believe that Pluto’s coloration is likely caused by hydrocarbon molecules called tholins that are formed when cosmic rays and solar ultraviolet light interact with methane in Pluto’s atmosphere and on its surface.&lt;/p&gt;&#xA;&lt;p&gt;It's been known since the late 80s that Charon is significantly darker (has a lower albedo) than Pluto. On the the &lt;a href=&quot;http://hubblesite.org/newscenter/archive/releases/solar%20system/pluto/1994/17/image/a/&quot; rel=&quot;noreferrer&quot;&gt;HST description page&lt;/a&gt; for the clearest view (at the time – 1994) of the Pluto and Charon system it was stated (emphasis mine):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Hubble's corrected optics show the two objects as clearly separate and sharp disks. This now allows astronomers to measure directly (to within about 1 percent) Pluto's diameter of 1440 miles (2320 kilometers) and Charon's diameter of 790 miles (1270 kilometers). &lt;strong&gt;The Hubble observations show that Charon is bluer than Pluto.&lt;/strong&gt; This means that both worlds have different surface composition and structure. A bright highlight on Pluto suggests it has a smoothly reflecting surface layer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Triton was observed by Voyager 2 in 1989 and shown to be reddish in colour. I believe even then that scientists hypothesized that the composition of Triton and Pluto were similar, but I haven't yet found any references describing the colour of Pluto as &amp;quot;reddish&amp;quot; before the 2010 Hubble Space Telescope observations.&lt;/p&gt;&#xA;&lt;h3&gt;Update&lt;/h3&gt;&#xA;&lt;p&gt;Phil Plait just &lt;a href=&quot;http://www.slate.com/blogs/bad_astronomy/2015/07/15/art_pluto_s_features_predicted_in_1979.html&quot; rel=&quot;noreferrer&quot;&gt;posted on his Bad Astronomy blog&lt;/a&gt; a painting of Pluto made in 1979 which is remarkably similar to recent imagery from New Horizons. The artist, Don Dixon, &lt;a href=&quot;http://www.cosmographica.com/spaceart/pluto-predicted.html&quot; rel=&quot;noreferrer&quot;&gt;wrote the following when explaining his painting:&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Every so often, an astronomical artist gets lucky. As the New Horizons spaceprobe closes on minor planet Pluto after its nine-year voyage, I'm astounded by how close I came to accurately depicting  Pluto in 1979.&lt;/p&gt;&#xA;&lt;p&gt;I'd like to claim prophetic powers, but the painting was guided by the reasonable assumption that Pluto likely has a periodically active atmosphere that distributes powdery exotic frosts into lowland areas. The reddish color of the higher features is caused by tholins – hydrocarbons common in the outer solar system. The partial circular arcs would be caused by flooding of craters by slushy exotic ices.  Pluto is apparently more orange than I painted it, however; I assumed the exotic ices would push colors more into the whites and grays.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So there you have it, at least one space artist in 1979 reasonably guessed that Pluto's colour would be reddish.&lt;/p&gt;&#xA;" OwnerUserId="373" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-15T16:05:32.097" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9808" PostTypeId="1" CreationDate="2015-07-08T04:37:03.483" Score="9" ViewCount="1303" Body="&lt;p&gt;If a cubesat is launched as secondary payload to LEO, what ways are there to put it in a higher orbit, for example outside of the van Allen belt to make a radiation experiment? The ISS launches cubesats and larger minisats. Can it help a sat leave LEO?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="9092" LastEditDate="2021-03-26T14:57:44.303" LastActivityDate="2021-03-26T18:44:00.580" Title="By what methods can cubesats move from LEO to higher orbits?" Tags="|iss|orbit|low-earth-orbit|cubesat|secondary-payload|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9809" PostTypeId="1" CreationDate="2015-07-08T04:43:34.963" Score="6" ViewCount="697" Body="&lt;p&gt;I assume that the crew in ISS can manipulate the minisats manually before they are launched, so they must be launched through some kind of airlock. Was this airlock designed for this purpose, or do they make use of an ordinary airlock for astronauts?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-07-08T18:49:15.583" Title="How was the airlock for the ISS' minisat launchers installed?" Tags="|iss|artificial-satellite|cubesat|airlock|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9810" PostTypeId="2" ParentId="9809" CreationDate="2015-07-08T05:45:47.057" Score="8" Body="&lt;p&gt;There are two CubeSat deployers on the ISS, the JEM (Japanese Experiment Module) Small Satellite Orbital Deployer built by JAXA, and NanoRacks CubeSat Deployer;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/908.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;J-SSOD&lt;/a&gt; is installed on the Multi-Purpose Experiment Platform (MPEP) of the extended slide table in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kibo_%28ISS_module%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kibō module's&lt;/a&gt; airlock where two its deployment slots are loaded with up to three CubeSat units (10 cubic centimeters) each, the slide table is then extended out of the airlock and launches CubeSats:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/73O39.jpg&quot; alt=&quot;JEM Small Satellite Orbital Deployer&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Japan Aerospace Exploration Agency astronaut Aki Hoshide, Expedition 33 flight engineer, works near the airlock in the Kibo laboratory of the International Space Station. The Small Satellite Orbital Deployer (SSOD) previously installed on the Multi-Purpose Experiment Platform (MPEP) is visible in the airlock.&lt;/sup&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://nanoracks.com/products/smallsat-deployment/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NRSCD&lt;/a&gt;, capable of deploying up to 48 units (8 slots with up to 6U each), is also accessible via the Kibō airlock but attaches to the Remote Manipulator System (JEMRMS) robotic arm which then grabs it and CubeSats are deployed from it. &lt;a href=&quot;https://www.youtube.com/watch?v=JQy9EwMrILI&quot; rel=&quot;nofollow noreferrer&quot;&gt;This video&lt;/a&gt; is made by NanoRacks and it is promotional in nature, so terribly sorry for that (full disclosure: I have nothing to do with NanoRacks), but it's the best I could find and showing deployment of CubeSat satellites from NRCSD attached at the end of JEMRMS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/JQy9EwMrILI?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So both small satellite deployers currently available on the station are accessible via the &lt;a href=&quot;http://iss.jaxa.jp/iss/kibo/al_e.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kibō airlock&lt;/a&gt;, which itself is too small to be used for egress and ingress of astronauts during EVA (Extra-Vehicular Activity), and is exclusively used to access externally, on Exposed Facility (EF) mounted experiments, deploy SmallSats (with the two deployers mentioned), and similar, where MPEP and JEMRMS serve as means of getting hardware and experiments into the airlock and back out again. Its dimensions are 576x830x800 mm, it has less than 600 W of peak power available to it, and it is attached to the Kibō's Pressurized Module (PM).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, Kibō airlock was always attached to the Kibō PM, since it was &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts124/main/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;launched to the station on May 31, 2008 aboard STS-124&lt;/a&gt;. Actually, even before, as mentioned on &lt;a href=&quot;http://iss.jaxa.jp/iss/kibo/develop_status_07_e.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this page&lt;/a&gt; that's describing airlock's functional test (test was already conducted inside the flight model of Kibō's PM, in August 1999).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image explains its use (from &lt;a href=&quot;http://iss.jaxa.jp/iss/kibo/al_e.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JAXA's page on Kibō Structure - Airlock&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/J8SHn.png&quot; alt=&quot;Kibō airlock - Using the airlock&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-08T18:16:57.423" LastActivityDate="2015-07-08T18:16:57.423" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9811" PostTypeId="2" ParentId="9808" CreationDate="2015-07-08T06:57:29.163" Score="5" Body="&lt;p&gt;As long as there are ways to change its velocity, the CubeSat can move to a higher orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the CubeSat deployers onboard ISS are most likely spring-driven and will not provide much delta-V for the job. If they equip ISS with a railgun and somehow find a way to deal with the reaction, it will certainly launch a CubeSat to orbit higher than LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using ion thrusters on a CubeSat will probably be more easier than the a railgun method.&lt;/p&gt;&#xA;" OwnerUserId="340" LastEditorUserId="340" LastEditDate="2015-07-08T09:19:06.673" LastActivityDate="2015-07-08T09:19:06.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9813" PostTypeId="2" ParentId="9808" CreationDate="2015-07-08T09:56:43.750" Score="12" Body="&lt;p&gt;Engines are under development for &lt;a href=&quot;https://en.wikipedia.org/wiki/CubeSat&quot;&gt;CubeSats&lt;/a&gt;.  For example there's the &lt;a href=&quot;https://en.wikipedia.org/wiki/Colloid_thruster&quot;&gt;Electrospray (colloid) thruster&lt;/a&gt; which is effectively a low-thrust ion drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An alternative is to launch from the ISS with a catapult (eg railgun/coilgun) or a small single stage solid rocket.  This would give an elliptical orbit, so only part of the time would be spent at the target altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that at the moment the &lt;a href=&quot;http://www.nasa.gov/pdf/627972main_LSP-REQ-317_01A.pdf&quot;&gt;CubeSat Requirements Document&lt;/a&gt; prohibits propulsion systems on CubeSats though. Presumably this would include ion drive as well as the more obviously hazardous solid rockets.  So launching self propelled cubes directly from the ISS is really only a theoretical possibility at the moment.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2015-07-08T13:10:13.300" LastActivityDate="2015-07-08T13:10:13.300" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9814" PostTypeId="2" ParentId="9809" CreationDate="2015-07-08T18:49:15.583" Score="4" Body="&lt;p&gt;This isn't really an answer but offered up as a supplement to Tidal Wave's answer - I can't post an image as a comment. This picture was taken as the payload bay doors were being closed on May 10, 2008 in prep for the STS-124 launch, and shows the airlock being present between the JEMRMS (robotic arm) and the four-pronged connector that would hold the exposed payload facility in the future.  Feel free to take the image and incorporate it into the other answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image also shows the enormous size of this payload.&#xA; &lt;img src=&quot;https://i.stack.imgur.com/uFTkR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-07-08T18:49:15.583" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9815" PostTypeId="2" ParentId="1940" CreationDate="2015-07-08T19:45:14.640" Score="19" Body="&lt;p&gt;I am infocat13 who wrote that unmannedspaceflight.com article, I base my opinion based on a personal communication with Dr. Stone, the PI of the Voyager project and many AIAA astrodynamics journal articles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mr. Lasher, a JPL astrodynamics specialist, informed me of the Pioneer 11 missing its B plane Jupiter aimpoint before TCM-1, so that solid motor is most likely not in a solar system escape orbit but more likely in a orbit like the Ulysses spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dr. Stone was kind enough in 1994 at my request to have someone run the state of the spacecraft before TCM-1 (before the spacecraft did it own first maneuver) to establish for certain the locations of Voyagers 1 and 2 third stages. The result was that one of the third stages might have entered the Jupiter B plane impact radius! This would have been for the Voyager that had the traveling wave tube malfunction, so I am unsure if we have a true vector for that Voyager. I believe that the wrong data was used and so both Voyager solid stages, like Pioneer 10, are in solar system escape orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have spent several decades now trying to convince PI's to (that would be you Dr. Stern!) preserve the historical knowledge of the process of getting human artifacts in deep space and their final disposition. Dr. Stern, like Dr. Stone before him, does a wonderful job of trying to answer questions on places like unmannedspaceflight.com.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT&#xA;I found an image per my personal communication with Dr. Lasher (retired JPL)&lt;img src=&quot;https://i.stack.imgur.com/kkh4C.jpg&quot; alt=&quot;Pioneer 11 Jupiter initial injection aimpoints&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note the off-nominal Pioneer 11/ TE-364-4 (Star motor)  injection aim point&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit 30 Aug 2015&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750013263.pdf&quot; rel=&quot;noreferrer&quot;&gt;Pioneer 11 Initial injection aimpoint page 101&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730011461.pdf&quot; rel=&quot;noreferrer&quot;&gt;Pioneer 10 initial injection aimpoint page 173&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hyrJ5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hyrJ5.png&quot; alt=&quot;Visual depiction of B-plane&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0SMc3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0SMc3.png&quot; alt=&quot;describes what In and out V is&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://solarsystem.nasa.gov/basics/bsf4-1.php&quot; rel=&quot;noreferrer&quot;&gt;Describes what &quot;Departure from Jupiter Heliocentric Radial Component&quot; is  &lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3297" LastEditorUserId="3297" LastEditDate="2016-03-19T16:54:19.943" LastActivityDate="2016-03-19T16:54:19.943" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9817" PostTypeId="2" ParentId="829" CreationDate="2015-07-08T22:30:40.523" Score="5" Body="&lt;p&gt;The answer is no, because the Sun and planets are all moving though the galaxy at the same speed. However, you could do an Oberth maneuver (sometimes called a powered gravity assist) but which is different from gravity assist. If you want to climb out to say Neptune and then fall back you could get there free (not counting leaving Earth's gravity) by using several gravity assists, Venus/Earth/Jupiter/Saturn for example, (unpowered) then use Neptune to send you back and do an Oberth maneuver using the Sun. The gravity assist at Jupiter would put you past breakeven for leaving the solar system but if you want more speed you  could do the Oberth maneuver using the Sun.&lt;/p&gt;&#xA;" OwnerUserId="10652" LastActivityDate="2015-07-08T22:30:40.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9818" PostTypeId="2" ParentId="4743" CreationDate="2015-07-09T07:35:30.510" Score="15" Body="&lt;p&gt;The disks on Voyager 1 and Voyager 2 (they are the same) are copper disks, plated in gold. The disks are encased in an aluminum housing to protect them from the &quot;elements&quot; of space. Barring any impacts with space rocks or other events like that, the records will last and be playable pretty much forever. The two space craft will not make a near approach to any stars in the foreseeable future--V1 will get about 1.4ly from another star in about 40,000 years. If anyone's out there to play the records, surely they will have found them and/or made friends with us way before then--unless they think we are delicious.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/goldenrec.html&quot;&gt;http://voyager.jpl.nasa.gov/spacecraft/goldenrec.html&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="10723" LastActivityDate="2015-07-09T07:35:30.510" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9819" PostTypeId="1" AcceptedAnswerId="9820" CreationDate="2015-07-09T08:30:19.153" Score="20" ViewCount="6598" Body="&lt;p&gt;I've been doing some searching and I can find some technical details about the flight computer of the New Horizons spacecraft. What I've found is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has a Mongoose-V 32-bit CPU (a radiation hardened MIPS R3000) running at 12MHz.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to find out what operating system is running New Horizons and any other details around this. I am guessing VxWorks but I haven't found any sources to validate this.&lt;/p&gt;&#xA;" OwnerUserId="10724" LastEditorUserId="49" LastEditDate="2015-07-09T10:34:57.027" LastActivityDate="2015-07-10T02:43:42.380" Title="Operating system of New Horizons spacecraft" Tags="|software|new-horizons|pluto|electronics|flight-computer|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9820" PostTypeId="2" ParentId="9819" CreationDate="2015-07-09T10:15:16.913" Score="20" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I am guessing VxWorks but I haven't found any sources to validate&#xA;  this.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Close, but no cigar. If New Horizons flight software was built by Jet Propulsion Laboratory (JPL), then I guess Wind River's &lt;a href=&quot;https://en.wikipedia.org/wiki/VxWorks&quot;&gt;VxWorks RTOS&lt;/a&gt; would have been their first choice (it's also a &lt;a href=&quot;https://en.wikipedia.org/wiki/VxWorks#Aerospace_and_defense&quot;&gt;popular choice for hardware on Mars&lt;/a&gt; - Pathfinder, Sojourner, Phoenix lander, Spirit, Opportunity, Curiosity,... all use it). But New Horizons flight software was built by Johns Hopkins University's Applied Physics Laboratory (APL) and they decided to build it atop &lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Nucleus_RTOS&quot;&gt;Nucleus RTOS&lt;/a&gt; by Mentor Graphics&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Refer to &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1520&amp;amp;context=smallsat&quot;&gt;Linux and the Spacecraft Flight Software Environment&lt;/a&gt;, E. Birrane et al., JHU/APL (PDF) for more info. Mind that despite document's title, neither VxWorks nor Nucleus are Linuxes. I selected this document to link to as it's a good source of background information on both Nucleus and VxWorks, both commercial RTOS'es used on spacecraft, but it discusses possible evolutionary path of flight software and Linux' role in it (there are real-time Linux microkernels available, such as &lt;a href=&quot;https://en.wikipedia.org/wiki/RTLinux&quot;&gt;RTLinux&lt;/a&gt;), thus the title.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An overview (but oddly no mention of OS) is also available in &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Spacecraft/Systems-and-Components.php&quot;&gt;Spacecraft Systems and Components&lt;/a&gt; page over at &lt;a href=&quot;http://pluto.jhuapl.edu/index.php&quot;&gt;New Horizons at APL&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-10T02:43:42.380" LastActivityDate="2015-07-10T02:43:42.380" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9821" PostTypeId="1" CreationDate="2015-07-09T14:36:31.990" Score="5" ViewCount="2592" Body="&lt;p&gt;I have my position and velocity vectors, and also the mass and position of the sun. I need to convert my vectors to the Keplerian Orbit elements, but I can't get a definite number when I do. The orbit isn't changing, yet the 'constant' values I'm calculating are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been reading &lt;a href=&quot;https://downloads.rene-schwarz.com/download/M002-Cartesian_State_Vectors_to_Keplerian_Orbit_Elements.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; and others such as &lt;a href=&quot;https://space.stackexchange.com/questions/1904/how-to-programmatically-calculate-orbital-elements-using-position-velocity-vecto&quot;&gt;this&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I first calculate the angular momentum with $\vec{h} = \vec{r} \times \vec{v}$. This value changes as the planet goes around the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then I calculate the eccentricity with $$\vec{e} = \frac{(v^2-\mu /r)\vec{r}-(\vec{r} \cdot \vec{v})\vec{v}}{\mu}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this changes between 0.9 and 1.1 as the planet orbits, which as far as I'm aware, shouldn't happen. What is going wrong? Perhaps I'm missing something, but surely the calculations I have say that the orbit changes from an ellipse to a hyperbola, and back again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any help would be appreciated, thanks.&lt;/p&gt;&#xA;" OwnerUserId="10688" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2022-12-31T05:56:09.193" Title="Converting Cartesian vectors to Keplerian Orbital elements - having problems" Tags="|orbital-mechanics|orbit|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9822" PostTypeId="5" CreationDate="2015-07-09T16:35:11.650" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-07-09T16:35:11.650" LastActivityDate="2015-07-09T16:35:11.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9823" PostTypeId="4" CreationDate="2015-07-09T16:35:11.650" Score="0" Body="SpaceX's second version of the Dragon spacecraft, also called &quot;Crew Dragon&quot;. It is designed to carry humans." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-09T17:40:10.373" LastActivityDate="2015-07-09T17:40:10.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9824" PostTypeId="1" AcceptedAnswerId="9825" CreationDate="2015-07-09T21:05:39.263" Score="18" ViewCount="12289" Body="&lt;p&gt;I recently looked up how weak the Voyager signals are when they get to Earth from the 22 W transmitters (as of September 2013, it was roughly -245 dBm). But that got me wondering how many watts of power the Deep Space Network (DSN) needs to send signals the other way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any attempts at Google searching always resulted in how much Voyager throws our way....&lt;/p&gt;&#xA;" OwnerUserId="8728" LastEditorUserId="49" LastEditDate="2015-07-09T22:27:20.283" LastActivityDate="2016-11-04T23:57:12.947" Title="How much RF transmit power does DSN need to send commands to Voyager?" Tags="|nasa|voyager|power|radio-communication|deep-space-network|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9825" PostTypeId="2" ParentId="9824" CreationDate="2015-07-09T21:27:51.013" Score="20" Body="&lt;p&gt;Check the &lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;noreferrer&quot;&gt;DSN Now&lt;/a&gt; page when it will show any of its stations communicating with Voyager 1 (code VGR1) or Voyager 2 (VGR2), select that dish and then expand the side column on the right to show all the data. It will show transmit power under &lt;em&gt;up signal&lt;/em&gt; section. I'll update this answer as soon as I see that happen (see below for updates), but it would be in the 20 kW range and more, depending on transmitter used to communicate with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I.e. pretty much at the peak power available to it and the transmitter can handle. For example, farthest range probe currently communicated with is New Horizons (NHPC) at distance of 31.86 AU from the Earth (round-trip light time of 8 hours, 49 minutes and 52 seconds), via the Madrid station 63 (their largest dish), transmitting at 20.63 kW:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/t5Rsb.png&quot; alt=&quot;DSN Now Madrid station 63, NHPC New Horizons&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Still a temporary fix while I update with exactly what you asked for, but since I Googled for images a bit, I also found &lt;a href=&quot;http://orbiter-forum.com/showthread.php?t=33474&quot; rel=&quot;noreferrer&quot;&gt;this screen grab&lt;/a&gt; showing Canberra station 34 communicating with Voyager 2. It's not as far as Voyager 1 (see e.g. &lt;a href=&quot;http://voyager.jpl.nasa.gov/where/index.html&quot; rel=&quot;noreferrer&quot;&gt;Where are the Voyagers&lt;/a&gt; at &lt;a href=&quot;http://voyager.jpl.nasa.gov/index.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Voyager&lt;/a&gt;), and the screen grab seems to have been made on March 21, 2014, so over a year ago, but it was then still over three times the New Horizons' current distance away from the Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qesZl.jpg&quot; alt=&quot;DSN Now Canberra station 34 VGR2 Voyager 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The station here transmits at the power of 19.08 kW. To compare, this same station is currently communicating with &lt;a href=&quot;http://sohowww.nascom.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;SOHO&lt;/a&gt; (Solar and Heliospheric Observatory) that is stationed at L1 &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Lagrange point&lt;/a&gt; between the Earth and the Sun (SEL1), about 1.5 million kilometers away from the Earth or ~ 1% the distance towards the Sun, and is transmitting towards SOHO with the power of 1.82 kW at frequency of 2.1 GHz.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that transmit power, unless you're already giving it all you can, does vary and depends on many parameters, including frequency, distance to target and angle to horizon, i.e. how thick of an atmosphere the transmitted signal has to go through, even wind, temperature and humidity in the lower atmosphere as those transmitters do get terribly hot when operating at near their peak power. And the lower the angle, the stronger the Earth's atmospheric effects, including diffraction, refraction and attenuation in general.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Thanks to OP's tracking of the &lt;a href=&quot;http://voyager.jpl.nasa.gov/mission/soe-sfos/sfos2015pdf/15_07_02-15_08_03.sfos.pdf&quot; rel=&quot;noreferrer&quot;&gt;Voyager's Space Flight Operations Schedule&lt;/a&gt; (PDF), we've now managed to capture Madrid station 63 (70 m dish, not the 30 m smaller ones) transmitting towards the Voyager 1 at the distance of over 131 AU away (Roundtrip Light Time from Earth of over 36 hours and 24 minutes). Temperature around Madrid, Spain was at the time of transmission over 31 °C at 11:36 p.m. local time (CEST), and nearly no wind (picking up from stall to only ~ 5 km/h), so it shouldn't be too surprising that its transmit power was under 20 kW (19.08 kW) to keep thermal noise down and the transciever below its peak TDP (Thermal Design Power):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/M3Oej.png&quot; alt=&quot;DSN Deep Space Network Madrid station 63 70 m dish&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Transmit power was slowly increasing as the environmental temperature dropped somewhat and the wind picked up on speed a bit. Dish's elevation is also increasing as it tracks Voyager 1's position, which will also lower atmosphere's attenuation of the radio signal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For another comparison, here's Goldstone's same size dish giving it all it can (21.44 kW on image, going up to 21.6 kW) during transmission to New Horizons, but the temperature there was at the time of capture 15 °C and a nice breeze reaching 27 km/h:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/I59CC.png&quot; alt=&quot;DSN Deep Space Network Goldstone station 14 70 m dish&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For additional screen grabs see links in the comments below.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-16T07:42:11.777" LastActivityDate="2015-07-16T07:42:11.777" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="9826" PostTypeId="2" ParentId="9824" CreationDate="2015-07-09T22:51:19.367" Score="6" Body="&lt;p&gt;I don't know how much they &lt;em&gt;need&lt;/em&gt;, but they &lt;em&gt;use&lt;/em&gt; a 20 kW S-band transmitter on a 70 m antenna at 16 bits per second for uplink.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-09T22:51:19.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9827" PostTypeId="1" AcceptedAnswerId="9828" CreationDate="2015-07-09T23:58:21.553" Score="19" ViewCount="3690" Body="&lt;p&gt;The space shuttle computers use triple-redundancy, and if one computer contradicted the other two, it would be ignored. I read that if all three gave contradictory output, one computer would be randomly chosen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would it be randomly chosen? Presumably random number generator, but I am curious.&lt;/p&gt;&#xA;" OwnerUserId="10735" LastActivityDate="2015-07-10T01:25:12.357" Title="If the space shuttle computers all output contradictory commands, how is it chosen?" Tags="|nasa|space-shuttle|flight-computer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9828" PostTypeId="2" ParentId="9827" CreationDate="2015-07-10T00:39:01.707" Score="26" Body="&lt;p&gt;It was not random.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Background: The shuttle data processing system was quad+ redundant, not triple. There were four Primary Avionics Software System (PASS) computers and a completely independent Backup Flight System (BFS) computer with totally different software.  Each PASS computer controlled a &quot;string&quot; of avionics equipment (some really critical avionics such as main engine controllers were connected to all 4 strings and performed voting at the avionics box level).  The BFS just listened to what was going on and did its own calculations in case of need.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally the PASS computers check each other and a bad one (defined as not agreeing) is voted out of the redundant set. Voting results were displayed on the CAM matrix on the cockpit panel in front of and above the commander's head.  This matrix of lights displays graphically how each computer can cast fail votes against any or all of the others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sdC4j.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A failed computer's string can be assigned to a different computer if necessary.  If all four of the PASS computers disagreed simultaneously, each computer would vote the others out resulting in a loss of flight control. (Each would still be controlling its own string based on what it thought was the right thing to do.)  The crew would then engage the BFS by pressing a button on the rotational hand controller, and the BFS would take over all 4 strings all by itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other engage scenarios other than a total set breakup. For example, if 2 sets of 2 PASS computers agree with each other, but not the other 2, you would be in a &quot;2+2 set split&quot; and might have to engage the BFS if it was not quickly apparent which pair was wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a brief summary.  For more detailed information, refer to the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&quot; rel=&quot;noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt; pages 6.8-8, 6.9-1, and the DPS section 2.6.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-07-10T01:25:12.357" LastActivityDate="2015-07-10T01:25:12.357" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9829" PostTypeId="1" CreationDate="2015-07-10T04:04:29.547" Score="18" ViewCount="2959" Body="&lt;p&gt;First of all, the title is just an assumption based on a very small sample size of the Voyager spacecrafts and New Horizons. We have orbiters around the moon and Mars because we had/have plans to land humans there. Is that the only reason we would spend the money to establish long term observation satellites?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Naturally I expect the answer to be that the extra cost of fuel necessary to achieve orbit is unjustified, but is that the only factor? I would imagine that adding the capability to remain in orbit around a planet (specifically Jupiter and beyond) with the potential of accumulating decades of data is far more sensible than trying to maneuver a single probe to take quick shots of multiple planets on a one-way trip out of the solar system.&lt;/p&gt;&#xA;" OwnerUserId="10736" LastEditorUserId="10736" LastEditDate="2015-07-10T04:25:40.253" LastActivityDate="2018-06-11T21:52:31.140" Title="Why do space agencies invest more in flyby probes rather than orbiting satellites?" Tags="|artificial-satellite|probe|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9830" PostTypeId="1" AcceptedAnswerId="9839" CreationDate="2015-07-10T05:30:04.640" Score="4" ViewCount="317" Body="&lt;p&gt;When space probes like New Horizon's are traveling towards their target &amp;amp; they take photographs of their target en-route, does the incoming light need to be adjusted for the Doppler effect because of the speed of the probe?&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="10234" LastEditDate="2015-07-10T19:35:38.277" LastActivityDate="2015-07-10T19:35:38.277" Title="The Doppler effect on light when space probes take pictures" Tags="|probe|photography|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9831" PostTypeId="2" ParentId="9803" CreationDate="2015-07-10T05:59:12.217" Score="11" Body="&lt;p&gt;It really depends on the &lt;em&gt;gain&lt;/em&gt; of the antenna used. In antenna design you can either go for gain (directing all the energy into a narrow cone) or wide response (sensitivity in all directions).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The further away the space craft you want to communicate with, the more important gain becomes. The gain of an antenna is &quot;almost free&quot; - up to a limit it does not add a lot of noise. Once you get to the electronic amplifier, any amplification includes amplification of he (input) noise of the amplifier. So you want to collect as much as possible of the weak signal. That means high gain, large dish, and precise alignment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course astronomers are used to this kind of thing - they know exactly where their spacecraft is, and where their antenna is pointing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the Mars Orbiter has a 3 m dish that is used in the X band (8-12 GHz) - a wavelength of approximately 3 cm. that makes the dish 100 wavelengths across. This means that it has to be pointed to a small fraction of a degree - for which it is equipped with a precision gimbal. The antenna on earth is much larger - 34 m across - for even greater gain (and requiring pointing to better than a milliradian).  See for example &lt;a href=&quot;http://mars.nasa.gov/mer/mission/comm_size.html&quot;&gt;http://mars.nasa.gov/mer/mission/comm_size.html&lt;/a&gt; &lt;a href=&quot;http://marsmobile.jpl.nasa.gov/mro/mission/spacecraft/parts/antennas/&quot;&gt;http://marsmobile.jpl.nasa.gov/mro/mission/spacecraft/parts/antennas/&lt;/a&gt; and other links on the NASA site. In fact the angular position is determined to tens of nano radians- meaning that knowing where to point the antenna is trivial by comparison.&lt;/p&gt;&#xA;" OwnerUserId="10738" LastEditorUserId="10738" LastEditDate="2015-07-10T11:33:03.663" LastActivityDate="2015-07-10T11:33:03.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9832" PostTypeId="2" ParentId="9829" CreationDate="2015-07-10T06:55:08.377" Score="26" Body="&lt;p&gt;NASA likes to do things gradually, building knowledge as it goes along. Each step provides information used in building the next probe.&lt;br /&gt;&#xA;The evolution goes something like this:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Flyby. Initial reconnaissance of a planet. Gather information on e.g. the local radiation environment, and other potential obstacles for an orbiter mission.&lt;/li&gt;&#xA;&lt;li&gt;Orbiter. Detailed surface mapping to help plan a lander mission.&lt;/li&gt;&#xA;&lt;li&gt;Lander. More details on surface composition to help plan a rover mission.&lt;/li&gt;&#xA;&lt;li&gt;Rover.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Each step costs more than the previous one. So only the most interesting targets have rovers sent to them.&lt;/p&gt;&#xA;&lt;p&gt;There are other factors. Sending an orbiter to Pluto wasn't possible in the timeframe of the New Horizons launch: an orbiter would need a much heavier rocket than was available at the time. NH was hurried to launch because scientists had discovered Pluto has an atmosphere at the moment, and predictions were this atmosphere would collapse somewhere between 2014 and 2020, the probe had to arrive before this happened.&lt;/p&gt;&#xA;&lt;p&gt;Because Pluto's so small and doesn't have a dense atmosphere, all braking must be done using rocket fuel. So either:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;you launch at a low speed to reduce the delta-V for getting into Pluto orbit. This means the orbiter takes very long to get there (wild guess: at least 30 years instead of 9). Keeping a science team together for that long is difficult.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;or you launch a probe with a huge fuel tank and engine, and you need &lt;a href=&quot;http://science.slashdot.org/comments.pl?sid=7672499&amp;amp;threshold=-1&amp;amp;commentsort=0&amp;amp;mode=thread&amp;amp;pid=50081739&quot; rel=&quot;noreferrer&quot;&gt;two SLS or Saturn V&lt;/a&gt; to get that off the ground. This means 20 times the launch cost of New Horizons.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;New Horizons goes through the Pluto system at a relative velocity of 11 km/s.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://science.slashdot.org/comments.pl?sid=7672499&amp;amp;threshold=-1&amp;amp;commentsort=0&amp;amp;mode=thread&amp;amp;pid=50081739&quot; rel=&quot;noreferrer&quot;&gt;This Slashdot post&lt;/a&gt; contains an interesting calculation of launch weight for an orbiter:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Space Shuttle Main Engines, one of our most efficient rocket engines, has an Isp of 4.436 km/s. By the rocket equation [this means that, to change velocity by 11 km/s using this engine, a spacecraft would need a ratio of wet mass to dry mass of exp(11/4.436) = 11.9. In other words, to stop the New Horizons probe at Pluto, we'd need to have sent along an extra 10.9 times its mass in fuel. And that's ignoring the mass of the engine and tankage, which makes things worse.&lt;/p&gt;&#xA;&lt;p&gt;Fuel boil-off... is an additional problem: it means we couldn't use the liquid-hydrogen/liquid-oxygen propellant used by the SSMEs, but some more stable (and less efficient) propellant, which further increase the required fuel mass.&lt;/p&gt;&#xA;&lt;p&gt;...New Horizons was launched on an Atlas V 551, which has a capacity of 19t to LEO. To send the probe plus 10.9 times its mass in fuel would therefore take an equivalent capacity of ~11.9*19 = 226t to LEO. The Saturn V, the most powerful launcher ever made, had a capacity of 118t to LEO. So you'd need two Saturn V launches, rendesvousing in orbit, to get a spacecraft with enough fuel to fly to Pluto and stop there. (Probably 3-4 launches, when you consider the other problems described above.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The Pioneer and Voyager missions had similar constraints. No knowledge at all of space beyond Mars' orbit, budget limits because of the Apollo project.&lt;/p&gt;&#xA;&lt;p&gt;Your initial premise is also off. We've had some high-profile flyby missions, but much more money has been spent on orbiters (Cassini, various Mars orbiters, Rosetta), landers (Viking) and rovers (Curiosity is several times more expensive than New Horizons).&lt;/p&gt;&#xA;&lt;p&gt;Voyager and New Horizons are just the tip of the iceberg. &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_interplanetary_voyages&quot; rel=&quot;noreferrer&quot;&gt;47 orbiters and landers, 43 flyby missions&lt;/a&gt;, with some overlap in that list (Rosetta is counted as both a flyby and an orbiter, for instance).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-10T17:55:09.660" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9833" PostTypeId="1" AcceptedAnswerId="10541" CreationDate="2015-07-10T13:05:35.443" Score="9" ViewCount="599" Body="&lt;p&gt;&lt;a href=&quot;http://www.universetoday.com/54114/new-hubble-images-show-pluto-is-changing/&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Pluto] is about 20% redder than it used to be.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Have there been any further articles/Hypotheses on both the cause of the color, and more importantly, the color change?  Or do we have to wait for &lt;em&gt;New Horizons&lt;/em&gt; to gather more data?&lt;/p&gt;&#xA;" OwnerUserId="10740" LastEditorUserId="25" LastEditDate="2015-07-21T20:37:20.467" LastActivityDate="2015-08-08T04:18:03.357" Title="Why does Pluto change color?" Tags="|planetary-science|new-horizons|pluto|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9834" PostTypeId="1" AcceptedAnswerId="9873" CreationDate="2015-07-10T13:52:25.077" Score="11" ViewCount="557" Body="&lt;p&gt;An answer to &lt;a href=&quot;https://space.stackexchange.com/questions/9829/why-do-space-agencies-invest-more-in-flyby-probes-rather-than-orbiting-satellite&quot;&gt;this question&lt;/a&gt; sparked my curiosity. Pluto's atmosphere seems to come and go depending on its orbital position and distance from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the heat up from coming closer to the Sun causes methane to be released and an atmosphere to form, what then causes it to recede? Is it from solar wind making it disappear? Some interaction with Charon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it assumed that this atmosphere is cyclical based on the orbit, or has it been slowly bleeding off over a long period of time and several orbits?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this was a mission drive on getting the New Horizon probe out in time, what does the flyby hope to learn about the atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-07-12T15:07:55.240" Title="What causes Pluto's atmosphere to recede/collapse?" Tags="|planetary-science|atmosphere|new-horizons|pluto|flyby|" AnswerCount="1" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9835" PostTypeId="2" ParentId="9829" CreationDate="2015-07-10T14:29:19.750" Score="12" Body="&lt;p&gt;There certainly were concepts developed for a Pluto orbiter.  Some argued as you do that even though such a flagship mission would be much more expensive than a flyby, it would provide more science return per dollar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However those concepts were never funded, so they never returned any science at all.  Pluto was not a high enough science priority to demand that much of a small pie.  So what made it there now is a flyby mission.  A flyby provides a &lt;em&gt;huge&lt;/em&gt; advancement over what we can get from Hubble (or eventually Webb), so it was deemed worth the cost.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-10T14:29:19.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9836" PostTypeId="2" ParentId="9569" CreationDate="2015-07-10T14:32:53.820" Score="4" Body="&lt;p&gt;I don't have experience with Mathematica, but at least the line&#xA;&lt;code&gt;While[A &amp;gt; 0 &amp;amp;&amp;amp; Y[z] &amp;lt; 0, zlow = zlow + 1];&lt;/code&gt; seems to be strange. Increasing the lower bound for &lt;code&gt;z&lt;/code&gt; without updating your criteria will result in an endless loop once it is started! &lt;code&gt;A&lt;/code&gt; is a constant of the specific problem and &lt;code&gt;Y&lt;/code&gt; depends on &lt;code&gt;z&lt;/code&gt;, not on &lt;code&gt;zlow&lt;/code&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10741" LastEditorUserId="49" LastEditDate="2015-07-10T15:04:44.453" LastActivityDate="2015-07-10T15:04:44.453" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9837" PostTypeId="1" AcceptedAnswerId="9858" CreationDate="2015-07-10T16:20:34.053" Score="11" ViewCount="429" Body="&lt;p&gt;I've heard a lot of people talk about sending a probe to Enceladus (one of Saturn's natural satellites). It would drill (or melt ice) to burrow through the thick layer of ice and get to the water below in an attempt to find life. If the thickness of Enceladus' ice shell could be hundreds of meters thick, how will the probe communicate its findings back to us? Will it be able to effectively communicate through the ice?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2018-01-13T08:02:30.760" Title="How will Enceladus' ice-burrowing probe communicate?" Tags="|probe|communication|saturn|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9838" PostTypeId="2" ParentId="9829" CreationDate="2015-07-10T17:10:10.710" Score="4" Body="&lt;p&gt;In addition to the fuel concerns, radiation becomes a much larger concern when orbiting certain bodies. Radiation impact can be minimized with fly-by's but is hard to avoid in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a real-world example, NASA's &lt;a href=&quot;http://www.nasa.gov/press-release/all-systems-go-for-nasas-mission-to-jupiter-moon-europa&quot; rel=&quot;nofollow&quot;&gt;upcoming mission&lt;/a&gt; to Europa will actually orbit Jupiter to avoid flying through Jupiter's powerful radiation belts.&lt;/p&gt;&#xA;" OwnerUserId="3278" LastActivityDate="2015-07-10T17:10:10.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9839" PostTypeId="2" ParentId="9830" CreationDate="2015-07-10T18:17:17.970" Score="6" Body="&lt;p&gt;You can calculate the wavelength shift between the transmitted light and that observed by the receiver from the doppler equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\lambda_r = \frac{\lambda c}{(c - v_r)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Helios probes reached a speed of $70km/s$ as they approached the sun.  What effect would that have on them observing something with a wavelength of $600nm$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\lambda_r = \frac{(600nm)(3.0\times 10^8 m/s)}{(3.0\times 10^8 m/s) - (7.0\times 10^4 m/s)}$$&#xA;$$\lambda_r = 600.14nm$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unlikely that any sort of external sensor observations are going to be affected by shifts of that magnitude.  However communications (which rely on very precise timing) can be affected.  This was potentially a problem with the &lt;a href=&quot;http://www.thespacereview.com/article/306/1&quot;&gt;Huygens-Cassini mission&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2015-07-10T18:17:17.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9841" PostTypeId="2" ParentId="9829" CreationDate="2015-07-10T19:41:11.090" Score="1" Body="&lt;p&gt;Pluto satellite would cost much more because it would need a lot of fuel for decelerating and orbit insertion. It is just the question of capabilities, building a flyby probe is much easier and cheaper.&lt;/p&gt;&#xA;" OwnerUserId="812" LastActivityDate="2015-07-10T19:41:11.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9842" PostTypeId="1" AcceptedAnswerId="9843" CreationDate="2015-07-10T20:37:34.567" Score="9" ViewCount="3104" Body="&lt;p&gt;Do International Space Station astronauts have to perform spacewalks only while its not facing the Sun directly? If so, I'm curious about the reason behind it. Is it only because they would be shielded from solar winds and radiation? Can't the spacesuits protect them completely? Also, what is the average speed of charged particles during solar proton events when they reach the station?&lt;/p&gt;&#xA;" OwnerUserId="8794" OwnerDisplayName="PEIN" LastEditorUserId="49" LastEditDate="2015-07-10T21:45:41.363" LastActivityDate="2015-07-10T21:49:45.170" Title="Do astronauts only perform spacewalks while the ISS is on the night side of the Earth?" Tags="|iss|low-earth-orbit|radiation|eva|procedure|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9843" PostTypeId="2" ParentId="9842" CreationDate="2015-07-10T20:52:02.483" Score="18" Body="&lt;p&gt;No. Most International Space Station (ISS) spacewalks last in the order of several hours, often up to 8 hours during more difficult and time-consuming repairs and installations, and some nearly 9 hours*. And the station, in its &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28412.5+km+%2B+6378.1+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%29+in+minutes&quot; rel=&quot;nofollow noreferrer&quot;&gt;roughly 93 minutes orbit&lt;/a&gt; is never in Earth's shadow for that long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are both advantages as well as disadvantages to spacewalking in either sunlight or complete darkness, but that's already discussed by two answers in &lt;a href=&quot;https://space.stackexchange.com/q/8296/49&quot;&gt;How are EVA's in LEO affected by being at the night side of Earth?&lt;/a&gt; You'll also find answers to your other questions on our site, if you search for them.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;As for your last question, while that should really be a new question, because it's rather difficult to answer and we prefer that standalone questions are asked separately, here's a few interesting tidbits that might perhaps help you frame it; Most of solar proton events are harmless in LEO except the most violent ones, since ISS orbits below &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Van Allen belts&lt;/a&gt;. But its orbit does cross &lt;a href=&quot;https://en.wikipedia.org/wiki/South_Atlantic_Anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;South Atlantic Anomaly&lt;/a&gt; during some orbits for a short period and it's less protected there. And while highest velocity that these particles reach is in polar regions due to acceleration of Sun's and Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_reconnection&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetic reconnection&lt;/a&gt;, ISS' orbital inclination &lt;a href=&quot;https://space.stackexchange.com/q/5297/49&quot;&gt;doesn't take it over such high latitudes&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;*Refer to &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spacewalks_and_moonwalks_1965%E2%80%931999&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia's List of spacewalks and moonwalks 1965–1999&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spacewalks_2000%E2%80%942014&quot; rel=&quot;nofollow noreferrer&quot;&gt;List of spacewalks 2000—2014&lt;/a&gt; that both include their duration.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-07-10T21:43:08.350" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9844" PostTypeId="2" ParentId="9790" CreationDate="2015-07-10T20:56:37.877" Score="6" Body="&lt;p&gt;The first clue was in which frequency the spacecraft switched to, so they learned a great deal just from reestablishing communications lock. Then, it appears that the necessary data was in the normal housekeeping data/logfiles, as no special data collection is mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Multithreading was not the issue and in fact, the software never crashed. It was simply asked to do two computationally intensive things at once: (1) compress science data and (2) copy a command load to non-volatile memory. Doing so used more CPU percentage than the system expected, and the fault management systems decided that the processor was misbehaving, and forced a switch to the backup computer. A stack trace would not have been helpful in this case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.washingtonpost.com/national/health-science/the-inside-story-of-new-horizons-apollo-13-moment-on-its-way-to-pluto/2015/07/10/fb361248-25ad-11e5-b72c-2b7d516e1e0e_story.html&quot; rel=&quot;noreferrer&quot;&gt;Washington Post&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="917" LastEditorUserId="917" LastEditDate="2015-07-10T21:23:12.117" LastActivityDate="2015-07-10T21:23:12.117" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9845" PostTypeId="2" ParentId="9790" CreationDate="2015-07-10T21:21:08.937" Score="4" Body="&lt;p&gt;New Horizons carries two computer systems. As the New Horizons team &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150704&quot; rel=&quot;nofollow noreferrer&quot;&gt;posted on July 4&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The mission operations center at the Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, lost contact with the unmanned spacecraft -- now 10 days from arrival at Pluto -- at 1:54 p.m. EDT, and regained communications with New Horizons at 3:15 p.m. EDT, through NASA’s Deep Space Network.&lt;/p&gt;&#xA;&lt;p&gt;During that time the autonomous autopilot on board the spacecraft recognized a problem and – as it’s programmed to do in such a situation – switched from the main to the backup computer. The autopilot placed the spacecraft in “safe mode,” and commanded the backup computer to reinitiate communication with Earth. New Horizons then began to transmit telemetry to help engineers diagnose the problem.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;On July 5, they &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150705&quot; rel=&quot;nofollow noreferrer&quot;&gt;posted this&lt;/a&gt;, and explained the problem:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The investigation into the anomaly that caused New Horizons to enter &amp;quot;safe mode&amp;quot; on July 4 has concluded that no hardware or software fault occurred on the spacecraft. The underlying cause of the incident was a hard-to-detect timing flaw in the spacecraft command sequence that occurred during an operation to prepare for the close flyby. No similar operations are planned for the remainder of the Pluto encounter.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;I'm pleased that our mission team quickly identified the problem and assured the health of the spacecraft,&amp;quot; said Jim Green, NASA's Director of Planetary Science. &amp;quot;Now - with Pluto in our sights - we're on the verge of returning to normal operations and going for the gold.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;They mentioned nothing about needing to download logs, just &amp;quot;telemetry&amp;quot;, and given the communications lag and rapidity that they diagnosed the issue, I doubt they downloaded much data. I know little about the technical details, but I would be absolutely shocked if they didn't have a simulator or duplicate MIPS R3000 CPU on Earth.&lt;/p&gt;&#xA;&lt;p&gt;I presume they simply replicated the series of commands they sent to the probe to their &amp;quot;virtual New Horizons&amp;quot; computer on Earth, and realized the error of their timing glitch. The error, while they say was hard to detect, is perhaps (very simplified) something along the lines of:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Command A executed.&lt;/li&gt;&#xA;&lt;li&gt;Probe requires X seconds to reorient.&lt;/li&gt;&#xA;&lt;li&gt;Command B executed Y seconds later.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Y &amp;lt; X. Command B Fails. Computer detects failure, switches to backup, phones home.&lt;/p&gt;&#xA;" OwnerUserId="373" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-10T21:21:08.937" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9846" PostTypeId="2" ParentId="9842" CreationDate="2015-07-10T21:49:45.170" Score="7" Body="&lt;p&gt;The ISS orbit is 93 minutes, and space walks last for hours. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the difficulty in space suit design. It needs to handle being in direct sunlight for 45 or so minutes, then almost immediatley into deep darkness. (Though the reflection off Earth helps a lot in low earth orbit. Which makes a difference for space suits used Beyond LEO). &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-10T21:49:45.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9847" PostTypeId="1" AcceptedAnswerId="9848" CreationDate="2015-07-10T22:17:15.440" Score="7" ViewCount="2717" Body="&lt;p&gt;Following the most recent Falcon 9 Rapid Unscheduled Disassembly, Elon Musk tweeted that &quot;There was an overpressure event in the upper stage liquid oxygen tank. Data suggests counterintuitive cause.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasaspaceflight.com/2015/07/spacex-working-falcon-9-diagnosis-treatment/&quot; rel=&quot;nofollow&quot;&gt;This article&lt;/a&gt; mentions &quot;a potential pointer towards the helium pressurization system’s bottles in the Second Stage&quot; or &quot;&lt;em&gt;Composite Overwrap Pressure Vessels&lt;/em&gt;&quot; (COPV). Where are these on the upper stage? How likely is it that this could have played a role in the RUD?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="49" LastEditDate="2015-07-10T22:52:51.740" LastActivityDate="2021-02-20T20:27:31.477" Title="Where are the COPVs on Falcon 9's second stage?" Tags="|spacex|falcon-9|failure|supply-mission|rud|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9848" PostTypeId="2" ParentId="9847" CreationDate="2015-07-10T22:27:12.933" Score="7" Body="&lt;p&gt;On the Falcon 9 v1.0 that was used for the first 5 launches, they were down around the engine area, and required insulation and protection from the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the move to Falcon 9 v1.1 and the Merlin 1D engine they moved them into the LOX tanks on both stages. This de-cluttered the engine area a great deal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;COPV's are notoriously difficult to build, since they hold immense pressure (6000 psi in rocket use cases) and if they leak/burst/fail then they go off like bombs. With shrapnel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX used to buy the COPV's from an external vendor but decided to bring manufacture in house. During development there was a failure of one of theirs, and it looked like minor bomb damage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have had flight delays due to issues with the Helium pressurization system on previous flights. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current thinking floating around is not necessarily that one of the COPV failed (as in exploded very very violently as a 6000 psi vessel releases), but instead some component of them failed and leaked the Helium at a somewhat slower rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I was cheering for the IDA breaking lose as the root cause but it looks like that has been ruled out.  If it was the payload not the booster). &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-07-10T22:39:16.980" LastActivityDate="2015-07-10T22:39:16.980" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9849" PostTypeId="5" CreationDate="2015-07-10T22:28:14.123" Score="0" Body="&lt;p&gt;Also sometimes &quot;rapid unplanned disassembly&quot; or &quot;rapid unexpected disassembly,&quot; the term was in use by Naval personnel by at least by the 70s for the explosion of an improperly handled gun. Anecdotal evidence from this site suggests its use in the aerospace industry as far back as the 80s. Often improperly attributed to Elon Musk, who has popularized the phrase.&lt;/p&gt;&#xA;" OwnerUserId="-1" LastEditorUserId="1142" LastEditDate="2016-06-16T01:39:19.210" LastActivityDate="2016-06-16T01:39:19.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9850" PostTypeId="4" CreationDate="2015-07-10T22:28:14.123" Score="0" Body="&quot;Rapid Unscheduled Disassembly&quot; - a whimsical term, popularized by rocket scientists, for an explosion." OwnerUserId="10234" LastEditorUserId="1142" LastEditDate="2016-06-16T01:10:51.563" LastActivityDate="2016-06-16T01:10:51.563" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9851" PostTypeId="1" AcceptedAnswerId="9854" CreationDate="2015-07-11T01:06:09.130" Score="34" ViewCount="4924" Body="&lt;p&gt;I have seen &lt;a href=&quot;http://reddit.com/r/askscience/comments/3culek&quot;&gt;this question&lt;/a&gt; posed often recently, without a satisfactory&#xA;answer in my opinion:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why won’t New Horizons orbit Pluto?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The answers I read are always along this line:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;too much fuel&lt;/li&gt;&#xA;&lt;li&gt;too expensive&lt;/li&gt;&#xA;&lt;li&gt;probe too large&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I would like to know the approximate scale of these factors, for example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;how much more fuel would it take?&lt;/li&gt;&#xA;&lt;li&gt;how much more would it cost?&lt;/li&gt;&#xA;&lt;li&gt;how much larger would the probe need to be?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="5724" LastEditorUserId="49" LastEditDate="2015-07-11T05:21:16.393" LastActivityDate="2021-06-28T01:53:22.243" Title="Requirements to orbit Pluto" Tags="|orbit|probe|mission-design|new-horizons|pluto|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9852" PostTypeId="2" ParentId="9851" CreationDate="2015-07-11T02:52:25.837" Score="23" Body="&lt;p&gt;This is probably the easiest to answer if we take the same mission profile and just track from the Pluto flyby backwards. I'll make some rather broad assumptions and first order approximations, like that NASA had their &lt;a href=&quot;https://en.wikipedia.org/wiki/NEXT_%28ion_thruster%29&quot;&gt;NEXT ion thruster&lt;/a&gt; developed to the highest technology readiness level (they did exist when New Horizons launched, but not at TRL required to actually launch on a &lt;em&gt;deep space voyage of discovery&lt;/em&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll also spare you with pesky details and just assume that if we can bring New Horizons close to a dead stop somewhere at Pluto and relative to it from ~ &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Where-is-New-Horizons/index.php&quot;&gt;13.79 km/s&lt;/a&gt;, that would do. Delta-v to insert into Pluto's orbit from then on won't be that much compared to everything else we need to change, part of the job would simply be falling towards it on its own gravity, then circularize at some desired altitude. I'll also assume that the orbital space is free of debris. Brace yourself, it's going to be a dirty, &lt;em&gt;Mos Eisley Spaceport&lt;/em&gt; type of a job;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Hew Horizons launch mass: ~ 478 kg&lt;/li&gt;&#xA;&lt;li&gt;Mass of a single NEXT thruster: ~ 100 kg (guesstimating)&lt;/li&gt;&#xA;&lt;li&gt;NEXT thruster power envelope: ~ 6.9 kW&lt;/li&gt;&#xA;&lt;li&gt;6.9 kW @ 16 years operation (~ 7.9 kW BoF) &lt;a href=&quot;https://en.wikipedia.org/wiki/GPHS-RTG&quot;&gt;GPHS-RTG&lt;/a&gt;'s fuel mass: &lt;a href=&quot;https://encrypted.google.com/#q=7.9128+kW+%2F+300+W+*+57+kg&quot;&gt;1,503.4 kg&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.google.com/webhp?hl=en#hl=en&amp;amp;q=%287%2C913+%2F+300%29+*+4%2C400+W+-+7.9+kW&quot;&gt;108 kW&lt;/a&gt; RTG's excess thermal power radiator mass: ~ 12,000 kg (est.)&lt;/li&gt;&#xA;&lt;li&gt;Additional truss / structure mass: ~ 4,000 kg (est.)&lt;/li&gt;&#xA;&lt;li&gt;Total dry mass: ~ 18,080 kg&lt;/li&gt;&#xA;&lt;li&gt;NEXT thruster xenon reaction mass for Δv of 13.79 km/s @ &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot;&gt;Isp&lt;/a&gt; of 4,190 s: ~ 8,220 kg&lt;/li&gt;&#xA;&lt;li&gt;Total wet mass: ~ 26,300 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So we got with propulsion that's great for the job but wasn't really available at the time, with &lt;a href=&quot;https://www.google.com/webhp?hl=en#hl=en&amp;amp;q=%287%2C913+%2F+300%29+*+7.8+kg&quot;&gt;205.7 kg&lt;/a&gt; of plutonium for our RTG that nobody really had available at such quantity, and a few rather generously small estimates for the mass of additional systems and xenon fuel needed &lt;strong&gt;from probe's launch mass of 478 kg to 26,300 kg&lt;/strong&gt;. And that's just the beginning of the story of our makeshift Pluto orbiter that now launches $(22,190\text{ kg} \times 13.79\text{ km/s} / 236\text{ mN})^{(1/2)}$ or about 6 years and 4 months sooner to get to Pluto at the same time as New Horizons will, assuming nothing breaks and our NEXT thruster's performance doesn't degrade during this additional time of continuous 236 mN thrust and consuming 6.9 kW of our RTG generated power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, this is all generously underestimating the problem and throwing at it technology that wasn't yet ready, requiring parts and consumables that weren't available and would have to operate for over six years longer without degradation in performance (but I did account for Pu-238 decay rate with half-life of 87.7 years), i.e. if New Horizons was made in  &lt;em&gt;Mos Eisley Spaceport&lt;/em&gt; into an orbiter, it would be 55 times its launch mass as a flyby probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure how or on what you could hurl such mass off the Earth and give it such a kick to still get there in time, but that's a different matter. Note that over six years longer mission also means that the Earth and Pluto don't align as nice and that comes with even more problems. But you now have an orbiter, and with enough power for the upcoming centuries to even transmit science and telemetry data back at a much faster rate. Oh, you might want a better transceiver antenna too, bigger and perhaps one that can gimbal independently of the orbiter itself, unlike the flyby version of New Horizons, so you can receive and transmit towards the Earth without having to take your eyes off the target of your observations. I hope Pluto is worth it, chances are that it you made it into its orbit, you'll be there a long, long time. ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Others will give you different estimates, depending on their choice of propulsion and power systems, but that's my view on the matter. I didn't read suggestions made in your link, because I wanted to see how that plays out on my own. This is now an updated edit with recalculation for required thrust time (not sure where I got those numbers I plugged in before, but they were suspiciously enough off to warrant a second look). Hope this helps.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-11T03:47:48.573" LastActivityDate="2015-07-11T03:47:48.573" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="9853" PostTypeId="2" ParentId="9851" CreationDate="2015-07-11T03:41:48.880" Score="9" Body="&lt;p&gt;To bottom-line TildalWave's answer: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Matching speed with Pluto at the intercept requires a very large amount of ∆v, necessitating an ion thruster rather than a chemical rocket;&lt;/li&gt;&#xA;&lt;li&gt;Ion thrusters draw a lot of electrical power;&lt;/li&gt;&#xA;&lt;li&gt;Solar power is very hard to come by that far from the sun, necessitating a very heavy radioisotope-thermal generator rather than solar panels. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-07-11T03:41:48.880" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9854" PostTypeId="2" ParentId="9851" CreationDate="2015-07-11T03:41:57.227" Score="37" Body="&lt;p&gt;&lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Pluto.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This mission study&lt;/a&gt; came up with a 900 kg nuclear-electric-propulsion spacecraft launched on an Ariane V with a C3 of 100 km&lt;sup&gt;2&lt;/sup&gt;/s&lt;sup&gt;2&lt;/sup&gt; and a Jupiter gravity assist along the way.  1.05 kW electrical power at Pluto from RTGs is required.  That would be four &amp;quot;classic&amp;quot; NASA RTGs, or about nine MMRTGs.  It has a 20 kg science payload.  (New Horizons has ~30 kg of instruments, one classic RTG, with a launch mass of 478 kg.)&lt;/p&gt;&#xA;&lt;p&gt;There are some optimistic assumptions (only 20% dry mass margin on most components?!), but this gives a general idea of the scale of such a mission.  It is not out of the question, but it would definitely be in the flagship cost class, comparable to Cassini.  Cassini has three classic RTGs, but uses chemical propulsion.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="12102" LastEditDate="2021-06-28T01:53:22.243" LastActivityDate="2021-06-28T01:53:22.243" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="9855" PostTypeId="2" ParentId="9799" CreationDate="2015-07-11T06:51:06.433" Score="-3" Body="&lt;p&gt;As I understand it, a reentering spacecraft wouldn't make a sonic boom in the traditional sense. It is decelerating. A fighter jet, for example, only makes a sonic boom as it accelerates past mach 1. A decelerating spacecraft would create a lot of noise and turbulence, but no boom.&lt;/p&gt;&#xA;" OwnerUserId="10744" LastActivityDate="2015-07-11T06:51:06.433" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9856" PostTypeId="1" AcceptedAnswerId="9864" CreationDate="2015-07-11T07:29:56.773" Score="6" ViewCount="1033" Body="&lt;p&gt;I wonder how is it possible to direct the probe's antenna from enormous distances (like Pluto) toward the Earth. Because if even an extremely small inclination in antenna direction emerge, it will make a huge miss (of target Earth), having in mind such long distance. How is this problem solved, or maybe I misunderstood something ? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, does for example, Mars orbiter have to change antenna position over time in order to retain direct visibility to Earth ? &lt;/p&gt;&#xA;" OwnerUserId="10754" LastEditorUserId="4660" LastEditDate="2015-07-11T14:26:39.610" LastActivityDate="2015-07-11T16:13:39.827" Title="How is a space probe's antenna directed toward Earth?" Tags="|probe|communication|radio-communication|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9857" PostTypeId="2" ParentId="3185" CreationDate="2015-07-11T07:31:56.347" Score="8" Body="&lt;h3&gt;What are radar calibration spheres&lt;/h3&gt;&#xA;&lt;p&gt;I couldn't find any specific information about the SKRL calibration spheres, but based on descriptions (&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2013-078A&quot; rel=&quot;nofollow noreferrer&quot;&gt;passive, spherical satellites&lt;/a&gt;) and information about other radar calibration satellites (&lt;a href=&quot;https://space.skyrocket.de/doc_sdat/lcs-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;LCS&lt;/a&gt;) I'd agree with your guess that they're simply hollow metal targets.&lt;/p&gt;&#xA;&lt;p&gt;Radar calibration requires known data to compare with the output of the radar to allow tuning of the radar such that the radar output matches the expected results.  Calibration spheres are intended to have a known, constant Radar Cross Section (RCS) value when viewed from any angle, so they're most useful for calibrating RCS (though if the satellites can be tracked with a more precise tracking tool, like lasers, the same satellites could potentially be used to calibrate radar position data).&lt;/p&gt;&#xA;&lt;p&gt;Fundamentally, the calibration satellites can be used by any radar that has the range to reach space that the satellites will fly over.  Considering the inclination of the satellites is 82.42° only the poles won't get direct overflights, but with the ~600km altitude of the satellites should still be visible above the horizon at the poles.  In terms of what radars would actually be capable of using satellites for calibration the radars of the &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_Space_Surveillance_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Surveillance Network&lt;/a&gt;, though when you look at some of the radars that make up that network a lot of the &amp;quot;Contributing Sensors&amp;quot; are &lt;a href=&quot;https://en.wikipedia.org/wiki/Ballistic_Missile_Early_Warning_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ballistic Missile Early Warning System&lt;/a&gt; radars.&lt;/p&gt;&#xA;&lt;p&gt;Between that note and the fact that RCS is used to characterize objects (if you confidently know how big an object is in terms of its radar reflection you can make a guess at what you're looking at) I'd guess that the primary reason for launching the calibration spheres is to calibrate Russian ballistic missile early warning radars (though if the expected RCS of the spheres was published anyone else could use them for calibration too) so they're best able to determine what they're tracking based on the RCS detected.  I know basically nothing about Russian ground-based radars so I can't speculate on specific sensors or target precision.&lt;/p&gt;&#xA;&lt;h3&gt;Where they are&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://web.archive.org/web/20150925203104/http://www.agi.com/media-center/launch-notifications/view-launch-notification.aspx?id=3223&quot; rel=&quot;nofollow noreferrer&quot;&gt;This page&lt;/a&gt; tags the calibration spheres as having NORAD identifiers 39490 and 39491.  The current (July 2015) orbital parameters you asked for (follow the links for full TLE/kepler elements/state vector) are:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.calsky.com/observer//tle.cgi?satid=13078A&amp;amp;tdt=2457214&quot; rel=&quot;nofollow noreferrer&quot;&gt;SKRL 756 1&lt;/a&gt;: 82.42° inclination, 96.8 min period, 592.0 km - 615.1 km altitude&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.calsky.com/observer//tle.cgi?satid=13078B&amp;amp;tdt=2457214&quot; rel=&quot;nofollow noreferrer&quot;&gt;SKRL 756 2&lt;/a&gt;: 82.42° inclination, 96.7 min period, 584.4 km - 614.6 km altitude&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/news/debris_faq.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;This NASA page&lt;/a&gt; claims that &amp;quot;Debris left in orbits below 370 miles (600 km) normally fall back to Earth within several years.&amp;quot; though going back to a much earlier (December 31, 2013) &lt;a href=&quot;http://www.calsky.com/observer//tle.cgi?satid=13078B&amp;amp;tdt=2456658&quot; rel=&quot;nofollow noreferrer&quot;&gt;element set&lt;/a&gt; for SKRL 756 B indicates an altitude of 592.0 - 622.5 km orbit.  In a year and a half we've seen measurable but minor decay, but that will speed up as the perigee drops lower and encounters more atmosphere.  I don't have the tools to really calculate a timeline for decay but I'd estimate it will be more than several years but probably not more than a few decades.&lt;/p&gt;&#xA;&lt;h3&gt;Followup&lt;/h3&gt;&#xA;&lt;p&gt;To answer the comment: &amp;quot;is aluminum or another material used?&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Again, I don't have details for SKRL, but I do have reference material for &lt;a href=&quot;https://ieeexplore.ieee.org/document/1446249&quot; rel=&quot;nofollow noreferrer&quot;&gt;LCS&lt;/a&gt; (Lincoln Calibration Sphere):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;...a precisely-machined and carefully-polished rigid aluminum sphere of unit square meter optical cross section. ... The sphere was fabricated from standard certified 0.125 inch 60-61 aluminum stock.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Also, the first hit on Google for &amp;quot;radar calibration sphere&amp;quot; is &lt;a href=&quot;https://web.archive.org/web/20160326100156/http://www.centurymetalspinning.com/products/radar_calibration_spheres.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;this company&lt;/a&gt; that's advertising/selling &amp;quot;Hollow Aluminum Calibration Spheres&amp;quot;.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10273" LastEditDate="2022-04-21T17:14:20.250" LastActivityDate="2022-04-21T17:14:20.250" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="9858" PostTypeId="2" ParentId="9837" CreationDate="2015-07-11T08:03:58.340" Score="5" Body="&lt;p&gt;I don't know about multiple mission proposals, but I was able to find a fairly well spelled-out plan so I'll answer based on that.  The proposal is based on the &lt;a href=&quot;https://en.wikipedia.org/wiki/IceMole&quot; rel=&quot;noreferrer&quot;&gt;IceMole&lt;/a&gt; melting probe but I don't know if there's a name for the proposal itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best source of data I've found is &lt;a href=&quot;http://www.igsoc.org:8080/annals/55/65/t65A004.pdf&quot; rel=&quot;noreferrer&quot;&gt;this PDF from 2014&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It indicates that at least for this proposed mission they wouldn't be building a full-on submarine that would drop into the sub-ice ocean, but instead they'd burrow down to the side of a geyser to sample the water from it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Analysis of the plume material strongly suggests that it originates from a body of liquid salt-water or even a global ocean below the icy crust. The unique chemistry found in the plumes has fueled speculation that Enceladus may&#xA;  harbor life. A lander mission equipped with a subsurface ice-melting probe may help to resolve this question. Landing on the rough terrain close to the tiger-stripe cracks from which the plumes emanate, however, is considered highly risky (see Fig. 2), so a probe that lands at a safe distance from a&#xA;  crack and melts its way to the inner wall to analyze the plume material in situ would be desirable.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(In-line references removed for quotability, see paper for more details)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That simplifies things in terms of communication: only burrowing a short distance and not leaving the tunnel make a tether entirely reasonable.  In this proposal the tether would carry both communications from the probe and power to the probe, which also simplifies the design by only requiring one power source instead of a power source for the base station and a separate power source for the probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This may be in reference to the use of the probe in the antarctic, but it indicates that they've made plans for reasonably long distances (though not to the scale required to go completely through &lt;a href=&quot;https://en.wikipedia.org/wiki/Enceladus#Internal_water_ocean&quot; rel=&quot;noreferrer&quot;&gt;30-40 km of ice shelf&lt;/a&gt; over the expected liquid water):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For greater depths of several hundred meters, the cable can be packed into separate containers (each containing several tens of meters of cable) that are added to the IceMole.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-07-11T08:03:58.340" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9859" PostTypeId="2" ParentId="9851" CreationDate="2015-07-11T08:24:10.937" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://science.slashdot.org/comments.pl?sid=7672499&amp;amp;threshold=-1&amp;amp;commentsort=0&amp;amp;mode=thread&amp;amp;pid=50081739&quot; rel=&quot;nofollow noreferrer&quot;&gt;This Slashdot post&lt;/a&gt; contains a calculation of launch weight for an orbiter, using these constraints:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the same probe as New Horizons&lt;/li&gt;&#xA;&lt;li&gt;the same flight time as New Horizons, transiting at the same speed as NH (11 km/s)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Space Shuttle Main Engines, one of our most efficient rocket engines, has an Isp of 4.436 km/s. By the rocket equation [this means that, to change velocity by 11 km/s using this engine, a spacecraft would need a ratio of wet mass to dry mass of exp(11/4.436) = 11.9. In other words, to stop the New Horizons probe at Pluto, we'd need to have sent along an extra 10.9 times its mass in fuel. And that's ignoring the mass of the engine and tankage, which makes things worse.&lt;/p&gt;&#xA;&lt;p&gt;Fuel boil-off... is an additional problem: it means we couldn't use the liquid-hydrogen/liquid-oxygen propellant used by the SSMEs, but some more stable (and less efficient) propellant, which further increase the required fuel mass.&lt;/p&gt;&#xA;&lt;p&gt;...New Horizons was launched on an Atlas V 551, which has a capacity of 19t to LEO. To send the probe plus 10.9 times its mass in fuel would therefore take an equivalent capacity of ~11.9*19 = 226t to LEO. The Saturn V, the most powerful launcher ever made, had a capacity of 118t to LEO. So you'd need two Saturn V launches, rendesvousing in orbit, to get a spacecraft with enough fuel to fly to Pluto and stop there. (Probably 3-4 launches, when you consider the other problems described above.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-11T08:24:10.937" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9861" PostTypeId="2" ParentId="9799" CreationDate="2015-07-11T11:03:42.167" Score="5" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-016-DFRC.html&quot; rel=&quot;noreferrer&quot;&gt;this page,&lt;/a&gt; an SR-71 flying supersonic at 80,000 ft (24.3 km) generates an audible sonic boom, so that gives us a lower bound.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.vibrationdata.com/space/sonic.htm&quot; rel=&quot;noreferrer&quot;&gt;This page&lt;/a&gt; explains why you hear &lt;a href=&quot;https://www.youtube.com/watch?v=f6r8wU2tDrc&quot; rel=&quot;noreferrer&quot;&gt;two separate booms&lt;/a&gt; (video) when the Shuttle comes in for a landing.&lt;br&gt;&#xA;And this article states the sonic boom &lt;a href=&quot;http://articles.orlandosentinel.com/2011-07-19/news/os-space-shuttle-atlantis-landing-pat20110719_1_sonic-boom-sts-135-shuttle-landing&quot; rel=&quot;noreferrer&quot;&gt;is audible&lt;/a&gt; for the last 10 minutes of a Shuttle flight. Coupling that with the Shuttle's landing profile should give us a good answer, but I haven't found that yet. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-11T11:03:42.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9864" PostTypeId="2" ParentId="9856" CreationDate="2015-07-11T15:52:44.297" Score="7" Body="&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/a/9805/265&quot;&gt;this answer&lt;/a&gt; for how accurately the antenna must be pointed.  That accuracy is not a function of the distance.  It is only a function of the size of the antenna and the wavelength of the transmission.  The antenna on New Horizons has the same required pointing accuracy at Pluto as it did at Jupiter.  What changes is the received power at Earth, and therefore the maximum data rate.  That goes down with the square of the distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The size of the antenna and the wavelength determine the tightest possible angle that the beam can be focused to.  Once you have that angle, you just need to keep Earth somewhere in there.  The angular size of the Earth at the distance of the spacecraft is generally much, much smaller than the beam width of the antenna.  So only a tiny fraction of the power from the antenna falls on the Earth.  An even tinier fraction falls on the receiving antenna on the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Antennas are pointed using star trackers and an on-board model of the position of Earth relative to the spacecraft on its trajectory.  Star trackers are cameras that accurately determine the attitude of the spacecraft by matching star field patterns.  Just a few bright stars in the field of view will do the trick.  Typical accuracies are a few arcseconds.  Ball has a 0.1 arcsecond tracker.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, Mars Orbiters and all spacecraft have to change the pointing of their antennas over time to track the Earth.  Both the Earth and the spacecraft are moving in inertial space (i.e. relative to the stars the star tracker is tracking), so the direction to point the antenna is always changing.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-07-11T16:13:39.827" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9866" PostTypeId="1" AcceptedAnswerId="9867" CreationDate="2015-07-11T20:45:30.713" Score="8" ViewCount="995" Body="&lt;p&gt;I'm trying to find the rate of precession of a binary system consists of two similar mass stars A and B, so I need to find the value of the argument of periapsis $\omega$ first, with velocity and position of both stars are given. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have done the calculation of the value of $\omega$ for Mercury and the method used can be seen here &lt;a href=&quot;https://space.stackexchange.com/questions/1904/how-to-programmatically-calculate-orbital-elements-using-position-velocity-vecto&quot;&gt;How to programmatically calculate orbital elements using position/velocity vectors?&lt;/a&gt;, or as the person in that post suggested, you can refer to the book &lt;em&gt;Fundamentals of Astrodynamics and Applications&lt;/em&gt;, by Vallado, 2007. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, for the Sun-Mercury case, Sun is much heavier and basically Mercury just rotates around and the Sun almost stays still, so it makes sense to just put the Sun(which is also the center of mass for this system) as the origin and the computation just follows, but for the A-B binary system which has similar mass and the center of mass is approximately half way between A and B, what should I put as the origin for my computation(and what is the mass of this origin that A and B rotate around)? &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Update: So now I'm using center of mass as my origin and use the effective mass $\frac{m_{B}^{3}}{(m_{A}+m_{B})^{2}}$ as the mass. I plotted the $\omega$(of star A) vs. $t$ but it doesn't make sense to me because the $\omega$ is not zero which it should be since there are no other planets to perturb and I didn't include GR term, any suggestions?&lt;img src=&quot;https://i.stack.imgur.com/VdBYh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(The mass of the two stars mA = 0.29097 Solar mass, mB = 0.356 Solar mass. This system is PSR J0737-3039 if details needed.)&lt;/p&gt;&#xA;" OwnerUserId="10368" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-07-20T13:39:06.797" Title="How to find orbital elements of a binary system" Tags="|astronomy|astrodynamics|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9867" PostTypeId="2" ParentId="9866" CreationDate="2015-07-12T01:35:16.690" Score="6" Body="&lt;p&gt;Make the stationary center of your coordinate system the stationary center of mass of the two bodies (called the &quot;barycenter&quot;).  Then by conservation of momentum, the total momentum of the system will always be zero.  If the net momentum of the two bodies is not zero, then first subtract the velocity of their center of mass from both to make it stationary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you normally see as the orbit solution is in reality the vector difference between the positions of the two bodies.  If one of the bodies is effectively not moving because it is so much larger than the other, then you simply have that solution as the motion of the smaller body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the general case of the two-body problem, you solve for the vector difference of the two positions using the sum of the two masses as the central mass, getting the usual Kepler solution for that vector difference.  Then to get the motions of the two actual bodies, you solve for two equations in two unknowns, where the difference of the two positions is your solution, and the sum of the momentums is zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Try this simple example in two dimensions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{r_1}=\left(2,0\right),\,\,\vec{r_2}=\left(-1,0\right)$$&#xA;$$\vec{v_1}=\left(0,2\right),\,\,\vec{v_2}=\left(0,-1\right)$$&#xA;$$\mu_1=6,\,\,\mu_2=12$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mu$ is the notation for the mass times Newton's gravitational constant $G$.  Verify that the CG position and velocity are at the origin.  Then determine the orbit for:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{r}=\vec{r_1}-\vec{r_2}$$&#xA;$$\vec{v}=\vec{v_1}-\vec{v_2}$$&#xA;$$\mu=\mu_1+\mu_2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\vec{r}$ and $\vec{v}$ are the initial position and velocity of a massless object orbiting a central body with mass times $G$ of $\mu$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You get $a=6$, $e=1/2$, $\omega=0$, and $\nu=0$.  We then have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{r}=\left(6\left(-{1\over 2}+\cos\tau\right),6{\sqrt{3}\over 2}\sin\tau\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\tau$ is the eccentric anomaly.  The time equation is the same for both the zero mass and general two-body cases:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$t=2\sqrt{3}\left(\tau-{1\over 2}\sin\tau\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solving the simultaneous equations, we get in general:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{r_1}={\mu_2\over\mu}\vec{r},\,\,\vec{r_2}=-{\mu_1\over\mu}\vec{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That gives:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{r_1}=\left(-2+4\cos\tau,\,\,2\sqrt{3}\sin\tau\right)$$&#xA;$$\vec{r_2}=\left(1-2\cos\tau,\,\,-\sqrt{3}\sin\tau\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we can plot it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/GIexj.gif&quot; alt=&quot;animation of two co-orbiting bodies&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Update for updated question:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the same Newtonian solution for PSR J0737-3039, where &lt;a href=&quot;http://aspen13.phys.wvu.edu/aspen_talks/ferdman_aspen_double_pulsar.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this presentation&lt;/a&gt; provides the masses, period, and eccentricity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/tlWJd.gif&quot; alt=&quot;PSR J0737-3039 animation&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scale on the plot is km, and the period is 2.45 hours (the animated gif runs a little faster than that).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The eccentricity is small, 0.0878, and the masses are close to equal, where A is 1.338 and B is 1.249 solar masses.  As a result, they are in close to a common circular orbit about their center of mass.  (A is blue and B is orange.)  The positions in km and time in seconds are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{r_A}=\left(424000\,(-0.0878+\cos\tau),\,\,422000\sin\tau\right)$$&#xA;$$\vec{r_B}=\left(-454000\,(-0.0878+\cos\tau),\,\,-452000\sin\tau\right)$$&#xA;$$t=1404\,(\tau-0.0878\sin\tau)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\tau$ goes from $0$ to $2\pi$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their speed in their orbits is an impressive 0.1% the speed of light.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-07-20T13:39:06.797" LastActivityDate="2015-07-20T13:39:06.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9868" PostTypeId="1" CreationDate="2015-07-12T04:54:24.903" Score="17" ViewCount="1834" Body="&lt;p&gt;The two Kennedy Space Center &lt;a href=&quot;https://en.wikipedia.org/wiki/Crawler-transporter&quot; rel=&quot;noreferrer&quot;&gt;crawler-transporters&lt;/a&gt;, used to transport Saturn V, Shuttle and SLS rockets to the launchpad, move on caterpillar treads:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/LH33p.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's &lt;a href=&quot;https://www.youtube.com/watch?v=RV6i_VhBy3A&quot; rel=&quot;noreferrer&quot;&gt;a SpaceFlight Insider video clip&lt;/a&gt; about current work on modifying the crawlers for SLS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/RV6i_VhBy3A?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The much lighter Soyuz is instead transported to the launchpad on tracks, a standard Russian gauge railway, it seems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rail tracks seem to me an obvious choice for moving super-heavy cargo a short distance between few fixed points. Caterpillar treads are more useful to traverse rough terrain, and is mostly used on construction and military vehicles. Why do the crawler-transporters use caterpillar treads instead of running on rail tracks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Added: It seems as if &lt;a href=&quot;https://books.google.se/books?id=VRb1yAGVWNsC&amp;amp;pg=PA200&amp;amp;lpg=PA200&amp;amp;dq=buran+transporter&amp;amp;source=bl&amp;amp;ots=UyU9oAmhKE&amp;amp;sig=LN5rkJ3AeEunIiozC_ME85OI_E0&amp;amp;hl=sv&amp;amp;sa=X&amp;amp;ei=82GiVaeyCcnmywOgpb4Y&amp;amp;ved=0CF0Q6AEwCw#v=onepage&amp;amp;q=buran%20transporter&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Buran&lt;/a&gt; and the N1 launcher also had a crawler transporter, the same one, but towed by locomotives on rail tracks (or do I misunderstand that?) Hmm, the book linked to says it was towed by four 100 horsepower locomotive. Hardly.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-07-12T12:59:39.870" LastActivityDate="2019-07-24T21:08:07.167" Title="Why do KSC crawler-transporters use caterpillar treads instead of rail tracks?" Tags="|launch-site|engineering|launchpad|operations|kennedy-space-center|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9869" PostTypeId="1" AcceptedAnswerId="9870" CreationDate="2015-07-12T05:09:35.700" Score="8" ViewCount="461" Body="&lt;p&gt;JAXA's Akatsuki was supposed to enter orbit around Venus in 2010, but the insertion failed. Its orbital maneuvering engine was supposed to burn for 12 minutes to get it into position, but it stopped after about three minutes. Following the failure, JAXA put it into orbit around the Sun, just inside the orbit of Venus. In December 2015 they want to try to put it into orbit around Venus again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The confusion I have with this is that when the first attempt failed, part of the combustion system was destroyed and the oxidizer ended up being dumped. How are they going to get Akatsuki into orbit around Venus without its engine?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2015-07-18T15:55:32.853" Title="How will Akatsuki get into orbit around Venus?" Tags="|orbital-mechanics|venus|jaxa|akatsuki|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9870" PostTypeId="2" ParentId="9869" CreationDate="2015-07-12T08:21:11.727" Score="9" Body="&lt;p&gt;They will use the &lt;a href=&quot;http://global.jaxa.jp/projects/sat/planet_c/topics.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;attitude control engines&lt;/a&gt; instead of the main engine. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;JAXA decided to carry out orbit control of the AKATSUKI using its liquid-fuel thrusters for altitude control (or the reaction control subsystem, RCS.) Based on this decision, almost all of the unnecessary oxidizer was discarded in Oct. 2011. As a result, the satellite became lighter, and remaining fuel can be more efficiently used for orbit control.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://global.jaxa.jp/projects/sat/planet_c/topics.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Akatsuki's orbit&lt;/a&gt; seems to have been a fortunate accident. At least, I could not find a statement that this was planned beforehand. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On its way to Venus, Akatsuki was in a heliocentric orbit that intersects Venus'orbit. And with limited delta-V available for insertion, this orbit must have been reasonably similar to Venus' own orbit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When orbital insertion failed, it was inevitable that Akatsuki would cross Venus' orbit again. JAXA calculated it would be possible to aim Akatsuki to meet up with Venus again in 2015. To achieve this, Akatsuki performed several orbital maneuvers in 2011 with a delta-V of &lt;a href=&quot;https://en.wikipedia.org/wiki/Akatsuki_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;243.8 m/s&lt;/a&gt; that would lead to it meeting Venus in 2015.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By december 2015, Akatsuki will have made 9 orbits around the Sun, while Venus has made 8 orbits in the same time. JAXA got lucky in the sense that Akatsuki's orbit was close enough to an 8:9 resonance that they could achieve 8:9 exactly with the limited delta-V available to them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/XuYyU.gif&quot; alt=&quot;Akatsuki&amp;#39;s orbit&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-18T15:55:32.853" LastActivityDate="2015-07-18T15:55:32.853" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9871" PostTypeId="2" ParentId="9868" CreationDate="2015-07-12T10:41:51.190" Score="13" Body="&lt;p&gt;Weight distribution would be be the main reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle stack (or Saturn V stack) empty, weighed an immense amount.  Shuttle more so, since the SRB's were full during movement. (Can't fuel a solid rocket on the pad). The SRB's weighed 1.3 million pounds each ready for flight. That is really an immense amount, over a small area. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tracks of the crawlers are huge, which better distributes the mass over a larger area of the ground. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was talk that for SLS with its 5 segment SRB's the ramp up to the pad will have to be rebuilt/reinforced to handle the extra weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rail line can distribute the weight over more wheels, but they are very narrow. While a mile long train might weigh as much as the Shuttle stack, remember it is distributed over a mile of track. The crawler is fairly compact in foot print. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Soyuz or Proton which are taken to the launch pad by rail are much much smaller in comparison.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-12T10:41:51.190" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9872" PostTypeId="2" ParentId="9866" CreationDate="2015-07-12T12:56:51.550" Score="2" Body="&lt;p&gt;When solving the Kepler problem is often solved for the case in which the central body is stationary, such that the body orbiting it has a negligible mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a two body problem this solution can still be used. For this the center of mass of the two bodies, also called the barycenter, can be chosen as inertial reference frame, if there are not external forces acting on this system. In this reference frame the following will then always be true,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mu_1 \vec{r}_1 = -\mu_2 \vec{r}_2, \tag{1}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mu_i$ and $\vec{r}_i$ respectively are the gravitational parameter and position vector of body with index $i$. Equation $(1)$ can be used to simplify the acceleration experienced by body with index $i$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\ddot{\vec{r}}_i = \frac{\mu_j}{\|\vec{r}_j - \vec{r}_i\|^3} (\vec{r}_j - \vec{r}_i) = \frac{-\mu_j^3}{(\mu_i+\mu_j)^2 \|\vec{r}_i\|^3} \vec{r}_i. \tag{2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final expression of equation $(2)$ looks similar to that of the case in which the central body is so massive that it can be assumed to be stationary. The only difference is that the constant/gravitational parameter is different. This different constant can be used as the effective gravitational parameter, $\mu_{eff}$, for a fictional body positioned at the barycenter for the body with index $i$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mu_{eff,i} = \frac{\mu_j^3}{(\mu_i+\mu_j)^2}, \tag{3}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;such that this fictional body be assumed to be so massive, relative to the body with index $i$, that it can be assumed to be stationary. Finding the orbital elements will now be the same as when the central body is so massive that it can be assumed to be stationary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It can also be noted that you only have to find the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;nofollow&quot;&gt;orbital elements&lt;/a&gt; of one of the orbits, because the orbital elements of the second orbit can be derived from the orbital elements of the first orbit. Starting with both orbital planes are the same, thus the longitude of ascending node, $\Omega$, and inclination, $i$, have to be the same. From equation $(1)$ one can be derive that: the eccentricities, $e$, have to be the same; the semi-major axes have to satisfy $\mu_1a_1=\mu_2a_2$; the arguments of periapsis, $\omega$, are separated by 180° (or $\pi$ radians); the true anomalies, $\nu$, have to be the same.&lt;/p&gt;&#xA;" OwnerUserId="1086" LastEditorUserId="1086" LastEditDate="2015-07-20T09:58:26.673" LastActivityDate="2015-07-20T09:58:26.673" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9873" PostTypeId="2" ParentId="9834" CreationDate="2015-07-12T15:07:55.240" Score="6" Body="&lt;p&gt;Keep in mind that until New Horizon gets there we don't know a great deal about the atmosphere of Pluto.  There seem to be a number of theories (the article I'm using as a source doesn't match the paper linked in a comment) but our only way to observe the atmosphere so far has been during infrequent occultation events when Pluto moves between an observatory on Earth and a relatively bright star allowing the star to be viewed through the atmosphere.  That can tell us a good bit, but the first observed occultation was in 1988 and Pluto has an orbital period of about 248 Earth years, so we've only been viewing the atmosphere near perihelion (which happened in 1989).  That in turn means we've never actually viewed the atmosphere at apohelion so we don't actually know what it does there.  We can model it, but we know little enough pre-New Horizon that I'd expect the models to change as more data comes in.&lt;/p&gt;&#xA;&lt;p&gt;That said, I'm using this article: &lt;a href=&quot;http://www.space.com/29885-pluto-atmosphere-to-be-revealed-by-nasa-new-horizons-spacecraft.html&quot; rel=&quot;noreferrer&quot;&gt;Lifting the Veil on Pluto's Atmosphere&lt;/a&gt;, which presents a broad overview of what we know from observation, what we suspect from modeling, and what we hope to discover from New Horizon.&lt;/p&gt;&#xA;&lt;p&gt;Pluto has an eccentric orbit: it goes from a perihelion of 30 AU to an aphelion of 50 AU with temperature swings from -220 degrees Celsius to -240 degrees Celsius.  The low temperature is cold enough for the gasses that make up the atmosphere to freeze and &amp;quot;snow&amp;quot; to the ground in solid form.  It's unclear this actually happens because we haven't observed Pluto at aphelion.  From the article I linked above:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Scientists originally thought that as Pluto recedes from the sun, and the temperature decreases to about minus 240 C (minus 400 F), the vapors freeze and fall back down to the dwarf planet's surface. However, observations made as recently as 2013 and coordinated by the Portable High-Speed OccultationTelescope group from multiple sites including the 0.9 m astrograph at Cerro Tololo Inter-American Observatory (CTIO) and the 1 m Liverpool Telescope on the Canary Islands, indicate that Pluto's atmosphere is not collapsing, but rather thickening.&lt;/p&gt;&#xA;&lt;p&gt;So, although Pluto's atmosphere gets thicker and thinner through its orbit of the sun, it may never completely freeze out and &amp;quot;collapse.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Michael Summers, New Horizons co-investigator and member of the atmospheres science theme team, said it's too early to tell whether Pluto's atmosphere freezes out or persists through its orbit. This makes sense, as Pluto has made only one-tenth of an orbit around the sun since the discovery of its atmosphere in 1988.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;To answer your question directly: As Pluto warms at perihelion the atmosphere grows because the gasses are warming and expanding (or melting from solid form).  At aphelion the opposite occurs as the gasses drop in temperature and either freeze or just become more dense.&lt;/p&gt;&#xA;&lt;p&gt;For fun (and to demonstrate a gas reducing volume due to cold), here's a video of dipping a balloon in liquid nitrogen: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Z0F3q1ZCwf4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-12T15:07:55.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9874" PostTypeId="1" AcceptedAnswerId="9877" CreationDate="2015-07-13T00:28:48.237" Score="5" ViewCount="563" Body="&lt;p&gt;In December of 2010, Akatsuki was meant to enter orbit around Venus, but the insertion failed. They now have &lt;a href=&quot;https://space.stackexchange.com/q/9869/10234&quot;&gt;a new plan&lt;/a&gt; to put Akatsuki into orbit in December 2015, but the orbit will be different. Originally the elliptical orbit was meant to range from 300 km to 80,000 km. Akatsuki's purpose in this mission is to image and investigate the clouds and surface of Venus for the purpose of studying the planet's meteorology, and attempt to confirm the existence of lightning and volcanoes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the new plan, the numbers that I can find for its orbit will be from 5,000 km to 300,000 km. With such a different orbit, how much of the planned science for the original mission will be possible?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-07-20T10:12:37.333" Title="How much of Akatsuki's original mission will be possible?" Tags="|artificial-satellite|orbit|venus|science|akatsuki|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9875" PostTypeId="5" CreationDate="2015-07-13T00:50:08.417" Score="0" Body="&lt;p&gt;Akatsuki is a Japanese space probe that was launched in March 2010 for the purpose of exploring Venus' meteorology, and to attempt to verify the existence of lightning and volcanoes. Its original orbital insertion failed and it was put into a heliocentric orbit until December 2015, when it will attempt to orbit Venus again.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-13T01:48:52.610" LastActivityDate="2015-07-13T01:48:52.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9876" PostTypeId="4" CreationDate="2015-07-13T00:50:08.417" Score="0" Body="A Japanese space probe intended to explore Venus for the purpose of examining its meteorology." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-13T01:48:31.380" LastActivityDate="2015-07-13T01:48:31.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9877" PostTypeId="2" ParentId="9874" CreationDate="2015-07-13T01:42:03.560" Score="4" Body="&lt;p&gt;Without its main thruster, orbit maneuver engine (OME), that's been completely dismissed as possible to restart (they did try but only got a small fraction of thrust out of it, about 10%) and have even dumped oxidizer (Mixed Oxides of Nitrogen, or MON) to save on spacecraft's mass as reaction control system (RCS) thrusters don't need it, extremely low (300 km) periapsis injection into elliptical Venusian orbit isn't an option any more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RCS thrusters, having much smaller thrust and also smaller specific impulse, will require longer burn time at periapsis to sufficiently trim down apoapsis (with the help of &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Oberth effect&lt;/a&gt;) so the orbiter stays in Venusian orbit and isn't ejected once again into heliocentric orbit, like it happened on the first try. And aerobraking isn't an option for this craft, as it doesn't pack a heat shield. So the only way to do that with RCS thrusters is to increase periapsis and with it extend the time at it. &lt;a href=&quot;https://en.wikipedia.org/wiki/Akatsuki_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; mentions that periapsis will increase from 300 km to 5,000 km to facilitate that, but that is marked with &lt;em&gt;citation needed&lt;/em&gt;, and I wasn't able to confirm that number. Still, it does make sense, so I'll just assume it's about right. Later lowering periapsis from initial insertion altitude is likely also not on the table, because that requires highly inefficient in delta-v apoapsis burns, and the craft wouldn't have enough of hydrazine to complete more taxing burns it was never designed for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Takeshi Imamura mentions in his &lt;a href=&quot;http://www.lpi.usra.edu/vexag/meetings/archive/vexag_12/presentations/05_Imamura.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Toward Venus orbit insertion of Akatsuki&lt;/a&gt; (PDF) presentation to the NASA's &lt;a href=&quot;http://www.lpi.usra.edu/vexag/meetings/archive/&quot; rel=&quot;nofollow noreferrer&quot;&gt;VEXAG&lt;/a&gt; (Venus Exploration Analysis Group) in April 9, 2015 that Akatsuki will try to insert into a highly elliptical 8-16 days orbit instead of the originally planned 30 hours one, but information there doesn't say much about periapsis or apoapsis, only its semi-major axis which would then be somewhere between 150,000 and 250,000 km instead of &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%2840000+km+%2B+6%2C051.8+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+4.8676%C3%9710%5E24+kg%29%29+in+hours&quot; rel=&quot;nofollow noreferrer&quot;&gt;40,000 km&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, assuming a periapsis of 5,000 km instead of 300 km above Venus, all of its instruments will lose quite a lot of their resolving power, and that likely means that its Longwave Infrared Camera (LIR) thermal emissions camera measuring cloud-top temperature differential won't be able to detect finer details in cloud formations. It has spatial resolution of 0.05° &lt;em&gt;which corresponds to ~70 km on the Venus &#xA;surface when viewed from the apoapsis and ~26 km from the distance of 5 Rv&lt;/em&gt; (&lt;em&gt;Rv&lt;/em&gt; is radius of Venus or 6,051.8 ±1.0 km). Likewise, its high-speed Lightning and Airglow Camera (LAC) that was supposed to detect lightnings and atmospheric glow on the night side of Venus won't be able to detect smaller discharges, if any at all. It's still reasonable to assume that it should be able to detect their afterglow together with LIR and the brightest flashes in ionization of their surroundings. Its sensitivity is &lt;em&gt;1/100 of standard lightning on the Earth when viewed from 1000 km altitude&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/XLbvX.gif&quot; alt=&quot;JAXA Akatsuki Outline of the Lightning and Airglow Camera&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Outline of the Lightning and Airglow Camera. Image source: &lt;a href=&quot;http://global.jaxa.jp/projects/sat/planet_c/topics.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JAXA Venus Climate Orbiter &quot;AKATSUKI&quot; (PLANET-C)&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other three of its five imagers, on top of already mentioned also 1-μm (IR1) and 2-μm (IR2) cameras, and Ultraviolet Imager (UVI), should still provide fairly good science return. Albeit at smaller resolution, and they might miss smaller volcanic events, they will also benefit from slower periapsis passes relative to Venusian surface and the direction of its super-rotating atmosphere. Of course, all of this is but a theory so far, since nothing really guarantees that its second injection will work out OK. And it should be noted that all of the Akatsuki's science instruments have been switched off for more than three years due to thermal constraints and will only be turned on right before VOI (Venus Orbit Injection). Its batteries would also have degraded capacity by now and it's questionable if they will be able to support power requirements of all of the instruments on the Venusian night side. Fingers crossed tho.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ERVnk.png&quot; alt=&quot;JAXA Akatsuki 3-D observation of the atmosphere from Venus&amp;#39;s orbit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Akatsuki's 3-D observation of the atmosphere from Venus's orbit. Image source: &lt;a href=&quot;http://www.lpi.usra.edu/vexag/meetings/archive/vexag_12/presentations/05_Imamura.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;JAXA / Takeshi Imamura&lt;/a&gt; (PDF)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Refer to &lt;a href=&quot;http://www.stp.isas.jaxa.jp/venus/docs/vco.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;PLANET-C: Venus Climate Orbiter mission of Japan&lt;/a&gt; (PDF), Takeshi Imamura et al., for description of spacecraft's instruments and their design and limitations, and &lt;a href=&quot;http://www.lpi.usra.edu/vexag/meetings/archive/vexag_12/presentations/05_Imamura.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Toward Venus orbit insertion of Akatsuki&lt;/a&gt; (PDF), Takeshi Imamura for VEXAG, for some fairly recent (April 9, 2015) details on orbiter's state after failed Venus Orbit Insertion in 2010, what Venus aeronomy science it has done so far, and plans for contingency VOI five years later.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-20T10:12:37.333" LastActivityDate="2015-07-20T10:12:37.333" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9878" PostTypeId="5" CreationDate="2015-07-13T03:11:05.277" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-07-13T03:11:05.277" LastActivityDate="2015-07-13T03:11:05.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9879" PostTypeId="4" CreationDate="2015-07-13T03:11:05.277" Score="0" Body="Rocket engines which consist of a solid propellant and a liquid or gaseous oxidizer." OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2015-07-13T03:13:28.213" LastActivityDate="2015-07-13T03:13:28.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9880" PostTypeId="2" ParentId="5887" CreationDate="2015-07-13T03:20:16.627" Score="7" Body="&lt;p&gt;It turns out that scientists thought the same thing very recently. According to this &lt;a href=&quot;http://rosetta.jpl.nasa.gov/science/comet-primer/anatomy-comet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anatomy of a Comet&lt;/a&gt; article at NASA Jet Propulsion Laboratory's website, related to the Rosetta mission, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;Scientists used to think that it was solid and firm, but NASA’s Deep Impact mission (2005), in which Rosetta participated, surprised them.&#xA;  They found that the nucleus of comet Tempel-1 was more silicate or&#xA;  dusty than they expected.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, it was only about a decade ago that scientists were able to see that the surface of a comet was dusty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Following the Deep Impact mission, JAXA also had an asteroid-related mission in 2005 called &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa&lt;/a&gt;. Hayabusa landed on the surface of the asteroid, which disturbed some of the dust on the surface so that it could be collected and returned to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When comets are farther from the Sun, they tend to be completely frozen. When the comet moves in closer, the heat can cause nucleus to start shooting streams of gas and dust from within the comet, creating the coma and eventually the dust tail. It makes sense that some of the particles that aren't lost to space and remain in the coma would eventually succumb to the comet's gravity and settle back down on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a somewhat related note, ESA are interested in the dust on/around the comet. They included a device on Rosetta called &lt;a href=&quot;http://blogs.esa.int/rosetta/2015/04/09/giada-investigates-comets-fluffy-dust-grains/&quot; rel=&quot;nofollow noreferrer&quot;&gt;GIADA&lt;/a&gt; (Grain Impact Analyser and Dust Accumulator) to collect and analyze dust from 67P's coma.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/L3gfF.jpg&quot; alt=&quot;GIADA&quot;&gt;&#xA;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;GIADA on the Rosetta probe. Image source: &lt;a href=&quot;http://blogs.esa.int/rosetta/2015/04/09/giada-investigates-comets-fluffy-dust-grains/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta Blog: GIADA Investigates Comet's “Fluffy” Dust Grains&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T18:16:18.590" LastActivityDate="2015-07-20T18:16:18.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9881" PostTypeId="2" ParentId="7776" CreationDate="2015-07-13T04:23:08.540" Score="18" Body="&lt;p&gt;TL;DR: Data rate is directly related to effective noise floor, which is related to bandwidth and received power (and lack of interference).  Lower received power means increasing transmit power or reducing bandwidth (speed).  APL had to choose the latter due to constraints.  Read on for a much more detailed and long-winded explanation (sorry if too long winded!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a bunch of inter-related reasons for this that boils down to Link Budget, which is basically how much signal you have above the noise floor to work with in your receiver.  Thanks to the Inverse-Square Law mentioned above, the incredible distance to Pluto (at 32 Astronomical Units distance--1 AU is the Sun-Earth distance) means a very, very weak signal to work with regardless of transmit power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons transmits TWELVE (12) MEASLY WATTS from each of its TWTA amps, which can be used primary-secondary or in a cross-polarized mode for 2 separate data streams.  The full-size 2.1m/7ft high-gain dish antenna has a 42dBi gain, about the same as an SNG satellite truck's--only, instead of having to shoot to GEO at 35,000km or 22k miles, you're trying to shoot SOOOOOOO much further at around a tenth (-10dB) of the transmit power.  On the plus side, they will be using the largest Deep Space Network dishes to receive and talk to it, which are 70 meters in diameter, but that only makes up so much for all this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lack of power is due to using an RTG (Radioisotope Thermal Generator) which uses the heat from decaying Plutonium-238 (one neutron less than the Pu-239 used in nuclear fission) to supply a thermocouple setup.  The documents state just under 4kW thermal output and around 245 watts electric at launch, dropping to 200-220 watts at the Pluto encounter.  That little bit of electricity has to be able to supply EVERYTHING on the spacecraft, and it decreases with time, which is why the Voyagers can no longer send images or much else that requires significant power; NH will end up the same way eventually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To come full circle, the massive distance and low power means that there won't be much signal left by the time it arrives at either destination.  The effective noise floor is directly affected by the bandwidth of the signal, which is why automated transmission systems--like the modems in your cell phones, land lines, etc--have both fast rates for good-quality links and slower rates for weaker links: this is especially true with LTE.  Ham radio operators (which I'm one) have use CW and Morse Code for talking using moon or aurora bounce because the distortion is too bad to use any kind of voice mode, and now there are new data modes that optimize for VERY SLOOOOOOW data rates (one bit per second or less) that exceed CW-by-ear for reliability by sacrificing bandwidth and data rate, but this obviously would be useless for this type of science mission other than health telemetry.&lt;/p&gt;&#xA;" OwnerUserId="10771" LastActivityDate="2015-07-13T04:23:08.540" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9882" PostTypeId="1" CreationDate="2015-07-13T05:01:07.997" Score="0" ViewCount="72" Body="&lt;p&gt;I realize that the Earth has an atmosphere that protects us from solar radiation, but what if we go beyond the atmosphere? I researched that the atmosphere is 300 miles thick and the moon is 238,400 miles away from earth. So would the shuttle, and, if need be, the suits, have lead within them? Also, I know that the atmosphere isn't spherical from the radiation from the sun. So the atmosphere varies in thickness around the earth, but my logic tells me that it doesn't reach all the way to the moon..... Thank you for your time. &lt;/p&gt;&#xA;" OwnerUserId="10772" LastActivityDate="2015-07-13T05:01:07.997" Title="How are spacecraft shielded from solar radiation?" Tags="|spacecraft|radiation|protection|" AnswerCount="0" CommentCount="2" ClosedDate="2015-07-13T18:32:10.717" ContentLicense="CC BY-SA 3.0" />
  <row Id="9884" PostTypeId="1" AcceptedAnswerId="9885" CreationDate="2015-07-13T13:51:10.163" Score="14" ViewCount="1319" Body="&lt;p&gt;I noticed in the New Horizon's release time schedule by &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/06240556-what-to-expect-new-horizons-pluto.html&quot;&gt;Emily Lakdawalla/ Planetary Society&lt;/a&gt; that after July 20th, there won't be any data returns from New Horizons until September. Why is there a 2 month period of no data being returned?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-13T15:10:39.903" Title="Why will there not be any images returned after July 20th until September?" Tags="|new-horizons|pluto|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9885" PostTypeId="2" ParentId="9884" CreationDate="2015-07-13T15:10:39.903" Score="13" Body="&lt;p&gt;From the same blog entry you linked to:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The transmission of the High Priority data set will be complete on July 20, and then image transmission will pause. For nearly two months, until September 14, New Horizons will switch to near-real-time downlinking of data from other, so-called &quot;low-speed&quot; instruments while it transmits just housekeeping information for all of the rest of the data. No new images will arrive on the ground during this time. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I read somewhere else that the &quot;housekeeping information&quot; includes e.g. the exact locations where photos were taken. This is seen as vital information (it helps put the data we already received in context), so it was given priority over the full image data. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-13T15:10:39.903" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9886" PostTypeId="2" ParentId="2355" CreationDate="2015-07-13T17:49:12.990" Score="-1" Body="&lt;p&gt;you can use (PreviSat) you can enter your house coordinates with the help of google earth and predict when you can see the international space station,iridium flairs or track any satellite you want the program is very accurate it predict the path of the ISS that will path exactly in top of my house i could see the ISS paths directly over my house &lt;/p&gt;&#xA;" OwnerUserId="10786" LastActivityDate="2015-07-13T17:49:12.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9887" PostTypeId="1" AcceptedAnswerId="9888" CreationDate="2015-07-13T20:31:52.173" Score="6" ViewCount="286" Body="&lt;p&gt;A team of Boeing employees (&lt;a href=&quot;http://www.bloomberg.com/research/stocks/private/person.asp?personId=6410684&amp;amp;privcapId=48715049&quot; rel=&quot;noreferrer&quot;&gt;Budica&lt;/a&gt;/&lt;a href=&quot;http://www.patentbuddy.com/Inventor/Herzberg-James-S/7994801&quot; rel=&quot;noreferrer&quot;&gt;Herzberg&lt;/a&gt;/&lt;a href=&quot;http://www.patentbuddy.com/Inventor/Chandler-Frank-O/305739&quot; rel=&quot;noreferrer&quot;&gt;Chandler&lt;/a&gt; &lt;sup&gt;1&lt;/sup&gt;) have &lt;a href=&quot;http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&amp;amp;Sect2=HITOFF&amp;amp;d=PALL&amp;amp;p=1&amp;amp;u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&amp;amp;r=1&amp;amp;f=G&amp;amp;l=50&amp;amp;s1=9068562.PN.&amp;amp;OS=PN/9068562&amp;amp;RS=PN/9068562&quot; rel=&quot;noreferrer&quot;&gt;patented&lt;/a&gt; an inertial confinement fusion engine (the basic principle going back to C.D.Orth's &lt;a href=&quot;http://www.osti.gov/scitech/servlets/purl/15015945&quot; rel=&quot;noreferrer&quot;&gt;VISTA from 1999&lt;/a&gt; and Rod Hyde's 1983 ICF proposal, but with a few quirks of their own):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DoYfn.png&quot; alt=&quot;A picture is better than a thousand words&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not your ordinary Orion, since it:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;uses much smaller &quot;pulse units&quot; (hence avoids the whole recoil problem);&lt;/li&gt;&#xA;&lt;li&gt;works on D-T fusion, not (as a primary mechanism) fission;&lt;/li&gt;&#xA;&lt;li&gt;doesn't achieve thermonuclear breakeven (pumps more energy through lasers than obtains from fusion), and potentially cannot be considered to violate the &quot;no nukes in space yadda yadda&quot; treaties;&lt;/li&gt;&#xA;&lt;li&gt;uses fast neutron flux from fusion to fission depleted uranium in a reactor that powers the lasers;&lt;/li&gt;&#xA;&lt;li&gt;and, in the traditional NERVA/NTR fashion, uses liquid hydrogen to a) cool the reactor, b) cool the chamber, c) provide extra thrust.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;My question is about the nuclear reactor part. How does one guarantee the reactor's stability if there are, say, power excursions - a pellet is too large, and corresponding neutron flux higher than expected? What are the negative feedback mechanisms that keep the reactor under control?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; &lt;sub&gt;Director of Advanced Vehicle Subsystems and Technologies at Boeing's Phantom Works.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-07-13T20:52:05.553" LastActivityDate="2015-07-13T20:52:05.553" Title="Boeing US pat. 9,068,562: inertial confinement fusion engine (D-T pellet gun)" Tags="|laser|nuclear-pulse-propulsion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9888" PostTypeId="2" ParentId="9887" CreationDate="2015-07-13T20:49:53.313" Score="2" Body="&lt;p&gt;Depleted uranium cannot sustain fission on its own, so there is no runaway possible and no negative feedback is needed to control it.  As soon as the neutrons from the fusion stop, the fission stops.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However once you start this thing, you will have highly radioactive fission products mixed in with your depleted uranium, so it is probably not something you want to operate until you have no chance of crashing back on Earth.  E.g. you wouldn't start it up until you were in a highish low-Earth-orbit, around 1000 km.  I'm not seeing this used on a launch vehicle, at least not if under the auspices of a democratic government.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-13T20:49:53.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9889" PostTypeId="2" ParentId="2355" CreationDate="2015-07-13T23:13:59.770" Score="1" Body="&lt;p&gt;I find software on a tablet or phone to be more convenient for identifying satellites in the field; &lt;a href=&quot;http://www.fifthstarlabs.com/#sky-guide&quot; rel=&quot;nofollow&quot;&gt;Sky Guide&lt;/a&gt; from Fifth Star on iOS is a treat; it identifies whatever you point the phone at and can tell you what the brightest upcoming satellite passes are. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(There's an unrelated app with the same name on Android.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-07-13T23:13:59.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9891" PostTypeId="1" AcceptedAnswerId="9892" CreationDate="2015-07-13T23:51:48.383" Score="19" ViewCount="7400" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Valentina_Tereshkova&quot;&gt;Valentina Tereshkova&lt;/a&gt; was the first woman in space. I have been reading the book called &quot;The Astronaut Wives club&quot;. In it, they discuss women flying in space, and mention that Valentina was the first woman in space. They also state her being pregnant. I can't believe that anyone would allow the first woman in space to be pregnant when she launched. I've never heard of this before. Was this actually true, that she was pregnant when she orbited the Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The quote is in &lt;a href=&quot;https://books.google.com/books?id=Sj_M2jWuXvYC&amp;amp;lpg=PT87&amp;amp;ots=0FCwt8iNW3&amp;amp;dq=Valentina%20Tereshkova%20astronaut%27s%20wives%20club&amp;amp;pg=PT87#v=onepage&amp;amp;q=Valentina%20Tereshkova&amp;amp;f=false&quot;&gt;chapter 8&lt;/a&gt;, as below (Emphasized as included). Note that the preceding sentence mentioned she orbited the Earth 48 times.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... comrade Valentina (Tershokova) was &lt;em&gt;pregnant&lt;/em&gt; at the time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-14T16:56:28.050" LastActivityDate="2018-02-02T16:44:29.293" Title="Was Valentina Tereshkova pregnant while in space?" Tags="|history|crewed-spaceflight|astronauts|health|myth-busting|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9892" PostTypeId="2" ParentId="9891" CreationDate="2015-07-14T00:14:58.140" Score="32" Body="&lt;p&gt;If Wikipedia page you link to can be trusted with dates, then she gave birth to her and &lt;a href=&quot;https://en.wikipedia.org/wiki/Andriyan_Nikolayev&quot;&gt;Andriyan Nikolayev&lt;/a&gt;'s daughter on 8 June 1964. &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_6&quot;&gt;Vostok 6&lt;/a&gt; launched on 19 June 1963, and was in orbit a bit under 3 days. So unless she was pregnant for a bit under a year (352 days) which is impossible even for prodigious pregnancy considering early-term exposure of the fetus to microgravity, stress and increased levels of radiation, or she terminated her pregnancy early and got pregnant again both within under 3 months which is highly unlikely, then no, she couldn't have been pregnant in space.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-14T02:49:45.707" LastActivityDate="2015-07-14T02:49:45.707" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9893" PostTypeId="1" AcceptedAnswerId="9907" CreationDate="2015-07-14T00:21:07.813" Score="8" ViewCount="1247" Body="&lt;p&gt;In 2003, JAXA sent a probe, &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa&lt;/a&gt;, to an asteroid named Itokawa. On the way to the asteroid, there was a large solar flare which damaged the probe's solar cells. I've been searching through the site here, and I've found a couple of similar questions related to &lt;a href=&quot;https://space.stackexchange.com/q/1034/10234&quot;&gt;protecting astronauts from solar flares&lt;/a&gt; and even the impact of &lt;a href=&quot;https://space.stackexchange.com/q/5329/10234&quot;&gt;dust on solar panels&lt;/a&gt;, but I haven't found anything about the damaging effects of the Sun on solar panels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine there will be different methods in different circumstances (for instance, low-earth orbit vs geostationary orbit vs out of Earth's orbit), but what are some of the ways that the Sun can damage solar panels, and what do space agencies do to try to protect them?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2018-11-18T17:34:00.967" Title="How are solar panels protected from the Sun?" Tags="|artificial-satellite|solar-power|the-sun|space-weather|jaxa|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9894" PostTypeId="2" ParentId="9891" CreationDate="2015-07-14T02:05:16.377" Score="18" Body="&lt;p&gt;It seems as though Valentina became pregnant shortly after her flight, as @TildalWave. She was worried about how her pregnancy might have been affected by her recent space flight, and it was that fact that is the truth behind this error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some &lt;a href=&quot;http://starchild.gsfc.nasa.gov/docs/StarChild/whos_who_level2/tereshkova.html&quot;&gt;documented reports&lt;/a&gt; of Valentina being particularly concerned with respect to her pregnancy. Both her and her husband were Cosmonauts, the sometimes having Elena be known as the first Star Child, with both of her parents having traveled to space. That likely is the source of this confusion, although it seems she was not pregnant in space. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-14T02:05:16.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9895" PostTypeId="2" ParentId="1441" CreationDate="2015-07-14T02:53:36.903" Score="12" Body="&lt;p&gt;They have absolutely used these, in various mission badges:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was the logo for the Mercury project:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://history.nasa.gov/project_mercury.jpg&quot; width=&quot;150&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a patch from the Viking program:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fi1mO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fi1mO.jpg&quot; width=&quot;200&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;http://rammb.cira.colostate.edu/dev/hillger/Viking_mission-patch.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;colostate.edu&lt;/a&gt;)&lt;/sub&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And one from Mariner 2, to Venus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.spacecollection.info/patches/mariner_2_patch.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://www.spacecollection.info/patches/mariner_2_patch.jpg&quot; width=&quot;200&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of these are for any sort of technical reason; I'm sure the symbols aren't used for anything important, but as a little bit of astronomical history, they're not completely forgotten, either.&lt;/p&gt;&#xA;" OwnerUserId="10793" LastEditorUserId="10273" LastEditDate="2019-03-09T23:02:38.943" LastActivityDate="2019-03-09T23:02:38.943" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="9896" PostTypeId="2" ParentId="7776" CreationDate="2015-07-14T04:08:03.277" Score="6" Body="&lt;p&gt;The probability of correct detection of a data bit is a function of the energy (not the power) in the received pulse. When the received power diminishes due to increasing distance, we can still meet the required energy by making the bit last longer (E = P x T). Longer pulses mean reduced data rate.&lt;/p&gt;&#xA;" OwnerUserId="10795" LastActivityDate="2015-07-14T04:08:03.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9897" PostTypeId="2" ParentId="7910" CreationDate="2015-07-14T05:07:15.190" Score="3" Body="&lt;p&gt;You asked this 8 days before NASA published a description of on how they're testing the heating: &lt;a href=&quot;http://www.nasa.gov/sls/base-heating-tests.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nasa.gov/sls/base-heating-tests.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently the answer is to build a 2% scale model of the entire rocket including engines and boosters and fire it in short duration bursts of 50-150 milliseconds at a time in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Expansion_tunnel#Shock_Tunnel&quot; rel=&quot;nofollow noreferrer&quot;&gt;shock tunnel&lt;/a&gt; (CUBRC is working with NASA, so probably &lt;a href=&quot;http://www.cubrc.org/index.php?option=com_content&amp;amp;view=article&amp;amp;id=48:aero-thermalaero-optic-evaluation-center&amp;amp;catid=26:aeronautics&amp;amp;Itemid=64&quot; rel=&quot;nofollow noreferrer&quot;&gt;these tunnels&lt;/a&gt;) that's producing flight conditions for various stages of the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/LKyHk.jpg&quot; alt=&quot;SLS Core shock tunnel model&quot;&gt;&#xA;(Image from NASA in the article linked above)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The short duration of testing is is mandated by what the model can handle without melting and very short operation time of a shock tunnel (Wikipedia quotes 20 milliseconds, though that isn't for a CUBRC tunnel).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Testing of the mini models will provide data on the convective heating environments the base of the rocket will experience upon ascent for both planned and unplanned flight conditions. Data from the test series will be used to verify flight hardware design environments and set specifications for the design of the rocket's base thermal protection system. The system keeps major hardware, wiring and crews on future missions safe from the extreme heat the boosters and engines create while burning during ascent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note that this isn't testing the actual shielding, it's just providing the data required to determine how much shielding is required.  Given knowledge of the heating it has to withstand the shielding can be tested separately to prove it handles the heat load before stacking all the rockets together.  I wouldn't be surprised if the first time they test the shielding with an actual SRB on one side and a block of SSMEs on the other is the first time they launch it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Experimental data is collected by:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The full-stack configuration has 200 heat flux and pressure sensors within the aft section of the rocket to collect data on the base flow environment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The test program uses new technology that wasn't available during past human spaceflight programs, such as high-speed visible light and infrared cameras, laser diagnostics and new designs of the model propulsion systems that more adequately simulate the SLS full-scale systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The first article I linked mentioned that the testing should be done by early summer of 2015.  I haven't found any recent references to the testing, but I did find &lt;a href=&quot;http://www.nasa.gov/exploration/systems/sls/sls-mini-rocket-models.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this older NASA post&lt;/a&gt; about the scale rockets before they were attached to a model of the rocket for shock tunnel testing.  That included a video of a ~65ms long hot-fire test of the core:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/IOFxrzOR7q4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No video of firing with the boosters attached but there was this still image (with an old configuration where the core was turned 45 degrees with respect to the boosters):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/7RkLu.jpg&quot; alt=&quot;SLS Model hot fire&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-14T18:01:16.630" LastActivityDate="2015-07-14T18:01:16.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9899" PostTypeId="2" ParentId="9302" CreationDate="2015-07-14T05:37:55.383" Score="8" Body="&lt;p&gt;The best I've been able to come up with is interior shots from the unveiling and of the test article (complete with test dummy), but I'd say the visual difference is a pretty compelling argument that at least the hull of the test vehicle isn't the one used in the unveiling:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A still from the video of the unveiling, Elon climbing in:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/qBRzH.jpg&quot; alt=&quot;Dragon V2 unveiling interior&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Test vehicle with test dummy and mass simulators for the other seats (from &lt;a href=&quot;http://www.spacex.com/news/2015/05/04/5-things-know-about-spacexs-pad-abort-test&quot; rel=&quot;noreferrer&quot;&gt;spacex.com&lt;/a&gt;):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/676bV.jpg&quot; alt=&quot;Dragon V2 test article interior&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note the differences in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Isogrid&quot; rel=&quot;noreferrer&quot;&gt;isogrid&lt;/a&gt; pattern, that's not something I would imagine you could readily replace.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, from &lt;a href=&quot;http://www.nasaspaceflight.com/2015/05/dragon-2-pad-abort-leap-key-spacex-test/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; describing the test vehicle: &quot;There are no actual windows in the capsule, with gold mirrors mimicking the outer windows of the operational Dragon 2.&quot; while you can clearly see windows (and see through them to show they're not painted on) in the unveiling picture.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-07-14T05:37:55.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9900" PostTypeId="5" CreationDate="2015-07-14T06:37:04.620" Score="0" Body="&lt;p&gt;There are multiple ways to define an orbit:&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Keplerian Elements&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Also known as classical orbital elements.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Eccentricity:&lt;/strong&gt; shape of the ellipse&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Semimajor axis:&lt;/strong&gt; size of the orbit&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Inclination:&lt;/strong&gt; angle from the reference plane&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Longitude of the ascending node:&lt;/strong&gt; anchors the rotation of the plane with respect to the reference body&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Argument of periapsis:&lt;/strong&gt; defines the orientation of the ellipse in the orbital plane&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Mean anomaly at epoch:&lt;/strong&gt; where the orbiting object is at a specific time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;State Vector&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The frame of reference is typically an inertial reference frame centered on the body being orbited.  In addition to the six vector elements (three each for position and velocity) a time at which both vectors are valid must be specified.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Position vector:&lt;/strong&gt; where the object is at a reference time&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Velocity vector:&lt;/strong&gt; how fast/what direction the object is moving at the reference time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Useful Links&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia - Orbital Elements&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_state_vectors&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia - Orbital State Vectors&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10273" LastEditDate="2018-10-03T23:05:27.527" LastActivityDate="2018-10-03T23:05:27.527" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="9901" PostTypeId="4" CreationDate="2015-07-14T06:37:04.620" Score="0" Body="Orbital elements are a set of parameters that uniquely identify a specific orbit." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-14T06:48:39.107" LastActivityDate="2015-07-14T06:48:39.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9902" PostTypeId="1" AcceptedAnswerId="9903" CreationDate="2015-07-14T07:00:49.717" Score="18" ViewCount="12450" Body="&lt;p&gt;What does New Horizons use to keep itself traveling (so far), while keeping all of its monitoring and communication systems active?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did read that it leveraged force from a gravity sling shot from Jupiter, but what else contributes? Solar?&lt;/p&gt;&#xA;" OwnerUserId="10712" LastEditorUserId="25" LastEditDate="2015-07-14T16:54:06.150" LastActivityDate="2015-07-15T14:32:44.290" Title="What powers New Horizons?" Tags="|propulsion|probe|power|mission-design|new-horizons|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9903" PostTypeId="2" ParentId="9902" CreationDate="2015-07-14T07:17:07.350" Score="28" Body="&lt;p&gt;Space is basically a vacuum, so there's no air resistance. A probe that's been launched will travel at the same speed indefinitely. Because New Horizons is moving away from the Sun, it loses some speed to overcome the Sun's gravity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons was launched on the fastest rocket they could get. Then it used a gravity assist from Jupiter to gain some more speed. Since then, it's been coasting. New Horizons does carry small thrusters which are used for small course and attitude changes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To power its systems, New Horizons uses a &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;Radioisotope thermoelectric generator&lt;/a&gt;. Plutonium pellets generate heat from atomic decay, this heat is converted to electricity. &#xA;At the start of the mission the RTG produced 4500 W of heat, which was converted to 290 W of electricity. During the Pluto encounter, ca. 200 W was available (due to plutonium decay and deterioration of the thermocouples).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflight101.com/new-horizons-spacecraft.html&quot; rel=&quot;noreferrer&quot;&gt;Most of the heat&lt;/a&gt; from the RTG seems to be radiated out through the radiator panels on the outside of the RTG. Some of the heat is conducted through the spacecraft frame to the propellant tank to keep it from freezing. The spacecraft is well-insulated. When the electronics aboard draw more than 150W the heat they produce is enough to keep the spacecraft at its operating temperature (+10 to +30 °C). When the spacecraft is in hibernation, electric heaters are used.&lt;br&gt;&#xA;NH contains a series of louvres that can be opened to radiate away excess heat, these were designed to be used mostly during the initial phases of the flight (nearer the Sun).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kun9u.jpg&quot; alt=&quot;NH block diagram&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-15T07:40:36.857" LastActivityDate="2015-07-15T07:40:36.857" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9904" PostTypeId="2" ParentId="9902" CreationDate="2015-07-14T07:53:02.313" Score="22" Body="&lt;h3&gt;Propulsion&lt;/h3&gt;&#xA;&lt;p&gt;Until someone solves the &lt;a href=&quot;https://en.wikipedia.org/wiki/N-body_problem&quot; rel=&quot;noreferrer&quot;&gt;N-body problem&lt;/a&gt; every spacecraft needs some kind of propulsion to correct its course during the mission. New Horizons uses a &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrazine&quot; rel=&quot;noreferrer&quot;&gt;Hydrazine&lt;/a&gt; based &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Propulsion_and_attitude_control&quot; rel=&quot;noreferrer&quot;&gt;propulsion system&lt;/a&gt; including four 4.4 N main thrusters and twelve 0.9 N attitude control thrusters. Its 77 kg fuel tank allows a total  post launch delta-v of somewhere over 290 m/s (650 mph).&lt;/p&gt;&#xA;&lt;p&gt;New Horizons has carried out &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Trajectory_corrections&quot; rel=&quot;noreferrer&quot;&gt;several course corrections&lt;/a&gt; since its launch. The last at the time of writing was &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150630&quot; rel=&quot;noreferrer&quot;&gt;only a few weeks ago&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;h3&gt;Electrical Power&lt;/h3&gt;&#xA;&lt;p&gt;Because of the vast distances from the sun New Horizons will operate at Solar power is impractical. At the elliptical orbit of Pluto the strength of the sun varies from &lt;a href=&quot;http://blogs.discovermagazine.com/badastronomy/2012/03/15/bafact-math-how-bright-is-the-sun-from-pluto/#.VaS2iflVhBc&quot; rel=&quot;noreferrer&quot;&gt;0.0012 to 0.0004&lt;/a&gt; times what we get on Earth.&lt;/p&gt;&#xA;&lt;p&gt;Instead of trying to collect the tiny amount of sunlight available New Horizons uses a &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;Radioisotope Thermal Generator&lt;/a&gt; (RTG). Specifically, a General Purpose Heat Source — Radioisotope Thermoelectric Generator (GPHS-RTG) manufactured by General Electric Space Division (now part of Lockheed Martin).&#xA;&lt;img src=&quot;https://i.stack.imgur.com/9xbCX.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The GPHS-RTG that New Horizons carries is actually a spare from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;noreferrer&quot;&gt;Cassini mission&lt;/a&gt;. It provided about 250W at launch, and the decay of the Plutonium causes the power output to drop by approximately 5% every four years, meaning that by now the RTG should be providing about 200W of power.&lt;/p&gt;&#xA;&lt;p&gt;This graphic shows the position of the RTG on New Horizons.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pZB6h.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;For some more reading on New Horizons power systems see &lt;a href=&quot;http://www.boulder.swri.edu/%7Etcase/Ottman-Hersman_IECEC_paper.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Pluto-New Horizons RTG and Power System Early&#xA;Mission Performance&lt;/a&gt;, Geffrey K. Ottman and Christopher B. Hersman. (source of the above graphic)&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-14T13:28:17.273" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="9905" PostTypeId="1" CreationDate="2015-07-14T10:25:32.283" Score="1" ViewCount="84" Body="&lt;p&gt;My understanding is that New Horizons has an RTG on board, which generates electricity for it's systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How and when was the RTG manufactured, and how does it work? How exactly does New Horizons distribute power around the spacecraft?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-07-14T10:25:32.283" Title="How is New Horizons powered?" Tags="|power|new-horizons|" AnswerCount="0" CommentCount="2" ClosedDate="2015-07-14T13:24:40.093" ContentLicense="CC BY-SA 3.0" />
  <row Id="9906" PostTypeId="1" AcceptedAnswerId="9929" CreationDate="2015-07-14T10:29:02.560" Score="9" ViewCount="963" Body="&lt;p&gt;&lt;a href=&quot;http://eyes.jpl.nasa.gov/eyes-on-pluto.html&quot; rel=&quot;nofollow&quot;&gt;Watching&lt;/a&gt; the New Horizons flyby, I notice that the probe stops probing and points back at Earth for about an hour at a time, right around the closest approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This happens on one occasion 6 hours before closest approach, lasting for 30 minutes, and again just 24 minutes after closest approach, lasting for about 2.5 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the reason for this? Does it entail reduced scientific results?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems impossible that additional commands could make the round trip in time. And the data returned in just a half hour can't put much of a dent in its storage.&lt;/p&gt;&#xA;" OwnerUserId="3054" LastEditorUserId="3054" LastEditDate="2015-07-15T05:01:24.927" LastActivityDate="2015-07-15T11:32:41.250" Title="Why does New Horizons point at Earth during its flyby" Tags="|new-horizons|flyby|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9907" PostTypeId="2" ParentId="9893" CreationDate="2015-07-14T11:42:17.147" Score="10" Body="&lt;p&gt;Radiation from the Sun does indeed cause solar panels to deteriorate over time, and powerful single events can lead to a sudden reduction in the efficiency of a solar panel. In 2003, a &lt;a href=&quot;http://l-zone.info/2015/03/dscovr-and-the-impacts-of-space-weather/&quot;&gt;large solar storm led to an average 3% drop in power&lt;/a&gt; for all of EUMETSAT's geostationary spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We don't really have a good way to &quot;protect&quot; panels from such effects, and indeed degradation rates are of key interest on the ground as well as in orbit. The National Renewable Energy Laboratory published an in-depth &lt;a href=&quot;http://www.nrel.gov/docs/fy12osti/51664.pdf&quot;&gt;analytical review&lt;/a&gt; of the issue a few years ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we cannot prevent the radiation from striking the solar panels, panels have to be built in such a way that they are resilient to &lt;a href=&quot;https://www.spenvis.oma.be/help/background/soldam/soldam.html&quot;&gt;the effects of radiation damage&lt;/a&gt;. That last link is far more comprehensive than I can be here, so I'll just let you go read that if you want to go deep into the science :)&lt;/p&gt;&#xA;" OwnerUserId="10635" LastActivityDate="2015-07-14T11:42:17.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9908" PostTypeId="2" ParentId="9906" CreationDate="2015-07-14T11:50:23.953" Score="12" Body="&lt;p&gt;This &lt;a href=&quot;http://www.astronomy.com/year-of-pluto/2015/07/phoning-home-from-pluto-what-to-expect-from-new-horizons-this-week&quot; rel=&quot;nofollow&quot;&gt;last downlink transmission, ending ~24 hours before the flyby&lt;/a&gt; is a contingency plan. If New Horizons has a catastrophic collision as it passes through the Pluto system, we'll at least have some data. This downlink is called &quot;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/06240556-what-to-expect-new-horizons-pluto.html&quot; rel=&quot;nofollow&quot;&gt;Fail Safe D&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NH Closest approach: July 14, 2015 11:49:57 UTC&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/06240556-what-to-expect-new-horizons-pluto.html&quot; rel=&quot;nofollow&quot;&gt;Emily Lakdawalla's list&lt;/a&gt;, all times are Earth time (i.e. the time signals are received here, 4.5 hours after they were sent):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Monday, July 13 at 16:24 UT / 12:24 ET / 09:24 ET: 3.5hr downlink: Fail Safe D&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Tuesday, July 14 at 03:15 UT / Monday, July 13 at 23:15 ET / 20:15 PT: 0.9hr downlink: E-Health 1&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Flyby occurs at &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/The-Flyby.php&quot; rel=&quot;nofollow&quot;&gt;13:49:57 UT&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Wednesday, July 15 at 01:09 UT / Tuesday, July 14 at 21:09 ET / 18:09 PT: 0.3hr downlink: Phone home. No science data, just a brief burst of telemetry confirming that the spacecraft survived the flyby. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Your &quot;6 hours before closest approach&quot; would be July 14 around 10:20 which doesn't line up with any of the downlinks in this schedule, so somebody's off. I haven't found an original NASA downlink schedule yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://utprosim.com/newhorizons/&quot; rel=&quot;nofollow&quot;&gt;Live view of events&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pluto.jhuapl.edu/Mission/The-Flyby.php&quot; rel=&quot;nofollow&quot;&gt;Full list of probe activities&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="3054" LastEditDate="2015-07-15T11:32:41.250" LastActivityDate="2015-07-15T11:32:41.250" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9909" PostTypeId="2" ParentId="9847" CreationDate="2015-07-14T12:02:31.270" Score="4" Body="&lt;p&gt;While @geoffc explained where they are, I'll explain why they are there. These tanks contain very high pressure helium. Helium is a very difficult gas to store, and as any gas, storing works best at low temperatures. Storing them inside a tank with already cold LOX makes it much easier to manage than storing it outside of the tank would manage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Careful followers will note that there was an issue with the helium tanks in the preparation for the &lt;a href=&quot;http://www.nasaspaceflight.com/2014/05/spacex-targets-june-11-falcon-9-orbcomm/&quot; rel=&quot;nofollow&quot;&gt;ORBCOMM launch&lt;/a&gt; the summer of 2014. Immediately before the static fire, there was a gas seen leaving the first stage, which caused a hold of the static fire test. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted that the failure in this case was quite a bit different than the ORBCOMM pre-launch failure, this seems to be related to the struts not working as designed, which the ORBCOMM failure seemed to be an issue with the tank itself.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-22T14:44:29.287" LastActivityDate="2015-07-22T14:44:29.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9911" PostTypeId="5" CreationDate="2015-07-14T16:42:32.153" Score="0" Body="&lt;p&gt;This tag is frequently used for questions about objects in orbit around Earth, especially those that are in orbit for the purpose of studying Earth. It can also be used for questions related to the impact of space exploration on Earth, the effects of Earth features (gravity, magnetism, etc) on space exploration, and anything else that specifically relates to Earth.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Information:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://solarsystem.nasa.gov/planets/profile.cfm?Object=Earth&quot; rel=&quot;nofollow&quot;&gt;NASA - Earth: Overview&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-14T16:54:53.240" LastActivityDate="2015-07-14T16:54:53.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9912" PostTypeId="4" CreationDate="2015-07-14T16:42:32.153" Score="0" Body="Questions of the science and space exploration related to Earth." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-14T16:54:43.450" LastActivityDate="2015-07-14T16:54:43.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9913" PostTypeId="1" CreationDate="2015-07-14T16:58:05.133" Score="8" ViewCount="517" Body="&lt;p&gt;As much as I know of integrated circuits is that military components have lowest temperature working limit - I think -40 deg Celsius. Having in mind that temperatures in open space are less than -200, I come to conclusion that electronics inside spacecraft need heating. Although spacecraft devices spent much time in sleep mode, some electronic circuits must be alive (at least real time clock and few devices that should wake up the rest when needed), by my opinion. &#xA;I wonder if this way of thinking is OK or maybe there exist some special kind of components regarding severe temperature conditions ? If heating is needed, does it mean that entire interior of spacecraft should be heated (when not sleep) or just few it's separated regions ? In that case I'm really surprised how generator's energy is conserved, having in mind heating and external temperature values, but maybe I missed some important issues.&lt;/p&gt;&#xA;" OwnerUserId="10754" LastEditorUserId="25" LastEditDate="2015-07-14T17:16:45.297" LastActivityDate="2018-02-25T22:57:10.053" Title="Spacecraft electronics heating" Tags="|artificial-satellite|spacecraft|technology|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9914" PostTypeId="2" ParentId="9913" CreationDate="2015-07-14T17:11:45.070" Score="6" Body="&lt;p&gt;Space is really cold, that is true. However, how does that temperature get to the spacecraft? There are 3 means of transferring heat, namely:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Conduction (Direct contact)&lt;/li&gt;&#xA;&lt;li&gt;Convection (Contact through air, water, or some other liquid medium)&lt;/li&gt;&#xA;&lt;li&gt;Radiation (Radio waves emitted based on the temperature. See &lt;a href=&quot;https://en.wikipedia.org/wiki/Planck%27s_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planck's Law&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Of these, in space the only one that matters is the latter. The spacecraft heats up by the rays of the sun, and cools off by large areas that aren't seeing sunlight. Okay, so what does that mean for a spacecraft?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most spacecraft have electronics that generate heat. Some items do very well with little heat, others are more specialized. For instance, most radios have an Oven Controlled Oscillator (OCXO) that is heated to a precise temperature. Batteries usually have a modest range where they work best. Other electronics might be more forgiving. Propellant is often very sensitive as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the highest powered devices is usually the radio. These will heat up quite quickly, while other components require heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For spacecraft further from the Sun, there is less heating from the sun. A combination of insulation, helps, although the biggest advantage is using the heat from the RTG, which will also help heat the spacecraft, even passively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, most spacecraft do have a few heaters on specific item, but only heat what needs to be heated, and don't heat what doesn't need to be heated. It's actually quite possible to get a spacecraft that doesn't need any heating around Earth, if it can be maintained in the same environment (Same eclipse periods, for instance) Usually the heat is designed to work best in the hottest environment, and the spacecraft warms itself up otherwise, because it's easier to heat something up than cool it down.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="15906" LastEditDate="2018-02-25T22:57:10.053" LastActivityDate="2018-02-25T22:57:10.053" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9915" PostTypeId="1" CreationDate="2015-07-14T20:53:26.010" Score="4" ViewCount="627" Body="&lt;p&gt;Isn't it possible to use a laser as an &quot;antenna&quot; that transmits data? This would save quite a lot of mass since you don't need a large dish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only problem I can think of is how would you modulate the signal to encode bits? But lasers can modulate their signal somehow, can't they? I'm pretty sure that you could use amplitude modulation, or at the very least, on/off key.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Another problem is that lasers won't be able to receive any signal, so a dish is necessary for the probe to receive signals with a reasonable gain.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2015-07-14T23:40:02.380" LastActivityDate="2015-07-15T06:25:31.820" Title="Can't we use laser transmitters instead of large radio dishes on deep space probes?" Tags="|probe|laser|antenna|dish|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9916" PostTypeId="1" AcceptedAnswerId="9918" CreationDate="2015-07-14T21:01:46.300" Score="12" ViewCount="23117" Body="&lt;p&gt;I'd like to understand how does New Horizons space craft send its data back to Earth, billions miles away from it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I read in a &lt;a href=&quot;http://time.com/3944157/new-horizons-pluto/&quot; rel=&quot;noreferrer&quot;&gt;Time article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;Also, at the distance of Pluto, we can only send data back at a rate that’s comparable with an old 1990s modem. Because of that, during the encounter, we’ll be taking many, many pictures, but those pictures will all be stored on the solid state memory and radioed back to the Earth months after the encounter.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the required delay is really in months, then those images which we are all over Internet now must be about a month old?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of details has been skipped from the news article, I'd like to understand how this is done, in simpler terms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks&lt;/p&gt;&#xA;" OwnerUserId="10814" LastEditorUserId="10814" LastEditDate="2016-10-31T18:31:02.307" LastActivityDate="2018-12-19T00:10:02.547" Title="How are New Horizons images sent back to Earth?" Tags="|nasa|new-horizons|pluto|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9917" PostTypeId="2" ParentId="9915" CreationDate="2015-07-14T21:15:19.760" Score="1" Body="&lt;p&gt;You modulate the signal for a comms laser the same way you do for radio antenna (effectively) so this isn't the issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problems are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Powerful lasers are heavy &lt;/li&gt;&#xA;&lt;li&gt;They require a fair bit of power&lt;/li&gt;&#xA;&lt;li&gt;They are not very robust, compared to a dish antenna, which can take a fair amount of damage from fast moving particles and just suffer incremental loss of signal strength&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="38" LastActivityDate="2015-07-14T21:15:19.760" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="9918" PostTypeId="2" ParentId="9916" CreationDate="2015-07-14T22:15:23.530" Score="18" Body="&lt;h3&gt;Short answer&lt;/h3&gt;&#xA;&lt;p&gt;Yes, what you see in the news are recent images. It takes about 1 hour to the probe to send one image bit by bit, and about 4 hours for the bits to travel from the probe to Earth.&lt;/p&gt;&#xA;&lt;p&gt;If New Horizons could do the scientific work, take images, and send them at the same time, this would be great, but that's not the case. While the probe had more time available for transmitting images in the previous days, now that the probe is close to Pluto and its moons the high-priority task is to collect scientific data (including images) and store them aboard the probe.&lt;/p&gt;&#xA;&lt;p&gt;Data will be downloaded to Earth as other activities permit. Many more images are coming, some in the following days, but at only 1 image per hour, this will take months, so an image shot today might be transmitted to Earth only in several months.&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;All the details in the following full (and long) answer!&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jGPhZ.jpg&quot; alt=&quot;Labeled diagram of New Horizons&quot; /&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/Resources/Press-Kits/NHPlutoFlybyPressKitJuly2015.pdf&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;How slow is the communication between New Horizons and Earth?&lt;/h3&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Communication with the spacecraft is via X band. The craft had a&#xA;communication rate of 38 kbit/s at Jupiter; at Pluto's distance, a&#xA;rate of approximately &lt;strong&gt;1 kbit/s&lt;/strong&gt; is expected. Besides the low&#xA;bandwidth, Pluto's distance also causes a latency of about &lt;strong&gt;4.5&#xA;hours&lt;/strong&gt; (one-way). The 70 m (230 ft) NASA Deep Space Network (DSN)&#xA;dishes are used to relay commands once it is beyond Jupiter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It says that the traffic between the probe and the Earth is possible at about 1 kb/s. So to transmit an image it takes time: 1 kilobit image will take more or less 8 seconds, 1 megabit will take 1,000 seconds or 17 mn, etc.&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;(That's not completely accurate because additional control and correction bits are added to the pixel bits. But this is the principle.)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;How long does it take to send an image file?&lt;/h3&gt;&#xA;&lt;p&gt;A &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Long-Range_Reconnaissance_Imager_.28LORRI.29&quot; rel=&quot;noreferrer&quot;&gt;LORRI&lt;/a&gt; image can be compressed aboard New Horizons, down to the size of 2.5 Mbits (depending on the content of the image). It takes about 42 mn to send this from the probe. But the signal has yet to travel to Earth. . . .&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;How long does it take to receive the image file?&lt;/h3&gt;&#xA;&lt;p&gt;Pluto and New Horizons are now at a distance of about 4.6 billion km. The radio waves travel at about 300,000 km/s. So it takes 4h20 to get to Earth.&lt;/p&gt;&#xA;&lt;p&gt;To sum up:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;T0: New Horizons sends the first bit of an image and continues sending bits during 42 mn&lt;/li&gt;&#xA;&lt;li&gt;T0 + 42 mn: New Horizons sends the next image, and at T0+84 mn another one, etc.&lt;/li&gt;&#xA;&lt;li&gt;T1 = T0 + 4h20: We receive the first bit of the first image on Earth.&lt;/li&gt;&#xA;&lt;li&gt;T1 + 42 mn: We get the last bit of the first image, and then the first bit of the next image, etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;How many images can be received in a month?&lt;/h3&gt;&#xA;&lt;p&gt;It has taken 5h02 for the first image. The next image can be received, and will be complete 42 mn later.&lt;/p&gt;&#xA;&lt;p&gt;At this rate we are capable of receiving 34 images per day. In a month we can get 1045 images.&lt;/p&gt;&#xA;&lt;p&gt;But actually this will be done in two phases: A browse catalog will be sent first to Earth in medium compressed format like explained so far, then the larger full resolution images (lossless compression). This is the current schedule:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TjT56.png&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Are images currently sent?&lt;/h3&gt;&#xA;&lt;p&gt;I you look at the downlink schedule provided by @TildalWave (&lt;a href=&quot;http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto/NH_Obs_Playbook_LORRI-MVIC.pdf&quot; rel=&quot;noreferrer&quot;&gt;Observation Playbook&lt;/a&gt;), you can see that images are currently sent:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/iR7xC.png&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Today and tomorrow, there are medium compressed image sets named &lt;em&gt;NY Times&lt;/em&gt; to be downloaded.&lt;/li&gt;&#xA;&lt;li&gt;High-resolution images will be sent planned 17-20th July.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Be ready to see them in the news!&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;This is very slow! Why?&lt;/h3&gt;&#xA;&lt;p&gt;New Horizons is very far, so we receive a very faint signal from it. To be able to decide if we are currently receiving a 0 or a 1 we need to accumulate received energy. To do that we need to listen the signal of a bit a longer time. So they must be sent at a slower rate.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Images cannot be sent continuously&lt;/h3&gt;&#xA;&lt;p&gt;New Horizons cannot send the images 24x7 to Earth:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Sometimes the Earth is not visible&lt;/li&gt;&#xA;&lt;li&gt;Sometimes the antenna cannot be oriented in the direction of Earth (because the scientific instruments may need the probe to be oriented differently to work).&lt;/li&gt;&#xA;&lt;li&gt;Sometimes data other than images must be transmitted.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The most important parts of the mission in this current Pluto fly-by phase is:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;To collect data about the atmosphere. Pluto is moving away from the Sun, and so is cooling down, and its frozen particles atmosphere is going to change.&lt;/li&gt;&#xA;&lt;li&gt;To take pictures of Pluto and its moons. New Horizons is now very close Pluto, and can shoot very detailed images, at a level never equaled in the past.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;If you look at the spacecraft at the beginning of this post, you can see that the antenna cannot be oriented (to Earth) without changing other instruments and sensors orientation too, and that the LORRI imager and the dish are 90° apart, which limits the direction for imaging while transmitting to Earth.&lt;/p&gt;&#xA;&lt;p&gt;So the high-priority data will be collected (images and other sensors output), and stored in memory. Later, at appropriate times, the probe will be oriented for a &amp;quot;call home&amp;quot; activity, so that stored data can be downloaded to Earth.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Where do images land?&lt;/h3&gt;&#xA;&lt;p&gt;The images are received by the &lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; (NASA) with antennas at 3 locations around the world. At this time this is Madrid which is looking to New Horizons with its largest antenna (diameter 70 m). But as you see there is no signal, the communication is not active.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TZqU6.png&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-12-19T00:10:02.547" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="9920" PostTypeId="1" AcceptedAnswerId="9926" CreationDate="2015-07-14T23:30:57.760" Score="2" ViewCount="209" Body="&lt;p&gt;Say we put a satellite in orbit around Neptune or Uranus that simply stores and sends data back to earth? Like as a big range extender for missions like New Horizons that work so far out that communications are an issue?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know there would be issues with orbits lining up and data storage, but is that sort of thing feasible?&lt;/p&gt;&#xA;" OwnerUserId="10813" LastEditorUserId="49" LastEditDate="2015-07-14T23:38:16.407" LastActivityDate="2015-07-15T12:04:54.527" Title="Is it possible to have a satellite that bounces data back from sattelites farther out in the solar system?" Tags="|communication|interplanetary|deep-space|feasibility|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9922" PostTypeId="2" ParentId="9920" CreationDate="2015-07-15T00:07:37.063" Score="4" Body="&lt;p&gt;Because the planets don't line up, there's not much point in putting relays in orbit around a particular planet. For a general solution, you'd need to put a bunch of them out there in circular orbits at various distances from the sun, at great expense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could also send one or more relays after each specific probe mission you launched, at progressively slower speeds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But why bother? The only problem with New Horizons' faint signal is that the transmitted data rate is very slow; when it takes nine and a half years to get to your destination, what does it matter if it takes a day or a week or a month to get the pictures back?&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-07-15T00:07:37.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9924" PostTypeId="2" ParentId="7776" CreationDate="2015-07-15T00:45:46.967" Score="22" Body="&lt;p&gt;New Horizons also uses very strong &lt;a href=&quot;https://en.wikipedia.org/wiki/Forward_error_correction&quot;&gt;error correction coding&lt;/a&gt;. Specifically, rate 1/6 &lt;a href=&quot;https://en.wikipedia.org/wiki/Turbo_code&quot;&gt;turbo coding&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Phase-shift_keying#Binary_phase-shift_keying_.28BPSK.29&quot;&gt;BPSK&lt;/a&gt; (binary phase shift keying). This expands the signal bandwidth by a factor of six, but it still reduces the required energy per user data bit. This allows a higher data rate for a given transmitter power. The minimum &lt;a href=&quot;https://en.wikipedia.org/wiki/Eb/N0&quot;&gt;Eb/N0&lt;/a&gt; for this code is about 0 decibels (dB), about the lowest that's ever been achieved. &lt;a href=&quot;https://en.wikipedia.org/wiki/Claude_Shannon&quot;&gt;Shannon's&lt;/a&gt; famous &lt;a href=&quot;https://en.wikipedia.org/wiki/Channel_capacity&quot;&gt;channel capacity&lt;/a&gt; theorem says it's impossible to achieve a low error rate with an Eb/N0 less than ln(2) = -1.6 dB even with infinite bandwidth, so we're pretty close to the theoretical limit. Turbo coding wasn't even discovered until the early 1990s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is Eb/N0? It's the ratio of the signal energy per bit to the noise power spectral density. Bit energy is measured in joules and power spectral density is in watts/Hz, which is equivalent to joules so the ratio Eb/N0 is dimensionless. Eb/N0 is a fundamental figure of merit in all digital modulation and coding schemes.&lt;/p&gt;&#xA;" OwnerUserId="10789" LastEditorUserId="49" LastEditDate="2015-07-15T01:28:12.973" LastActivityDate="2015-07-15T01:28:12.973" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9925" PostTypeId="2" ParentId="9906" CreationDate="2015-07-15T02:39:11.550" Score="3" Body="&lt;p&gt;Let's take a look &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/The-Flyby.php&quot; rel=&quot;nofollow&quot;&gt;the timeline&lt;/a&gt; when the signal was sent.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;15:52:57 New Horizons is taking radio uplink data with REX. New Horizons is 31.9 AU from Earth.&lt;/li&gt;&#xA;&lt;li&gt;15:53:12 New Horizons is taking radio uplink data with REX. New Horizons is 31.9 AU from Earth.&lt;/li&gt;&#xA;&lt;li&gt;16:27:00 Signal Has Left New Horizons that it crossed through the Pluto System&lt;/li&gt;&#xA;&lt;li&gt;17:02:32 New Horizons is taking 3 images of Pluto with LORRI from 456694.82 km away at est. resolution 2.3 km/pix.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I believe that during this time Pluto and Charon are essentially in complete eclipse, making for limited observation capabilities during this time. The antenna also happens to be pointed at Earth, to do the data collection with REX. Also, the team would really like to know the status of things so they can have a party, before they have to start working analyzing the data. Bottom line, it was a lucky happenstance that they are taking advantage of.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-15T02:39:11.550" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9926" PostTypeId="2" ParentId="9920" CreationDate="2015-07-15T02:47:44.390" Score="6" Body="&lt;p&gt;No, this is a bad idea for several reasons. It might be a good idea if there ever becomes a lot of spacecraft out there, but otherwise...&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The relative positions changes over time. One would need a number of these to work reliably, otherwise there's a good chance it just wouldn't work.&lt;/li&gt;&#xA;&lt;li&gt;Solar power doesn't work that far our. The current method to power spacecraft is radioactive decay. It looses efficiency over time, so the mission would be limited.&lt;/li&gt;&#xA;&lt;li&gt;Orbiting that far out is really difficult, and hasn't been done yet.&lt;/li&gt;&#xA;&lt;li&gt;The pointing of the antenna correctly would be more difficult. Basically pointing at the Sun points at the Earth when you are that far out, pointing at the nearest satellite is more difficult.&lt;/li&gt;&#xA;&lt;li&gt;If one of the satellites failed, you'd potentially lose the mission. Not really an option.&lt;/li&gt;&#xA;&lt;li&gt;If you are really far out there, it doesn't matter that much. Voyager's are both over 100 AU. Uranus is only 19. Hardly a scratch. Most spacecraft going out that far for the foreseeable future aren't going to stop, they will be at a solar escape velocity, and thus keep moving further with time.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Basically I think it'd only work if there was a lot of spacecraft out there. Maybe around 100, but no fewer than 30. That means you aren't quite as reliant, which should improve things somewhat. I can't see that happening for a long time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where this does work is if you have a number of spacecraft close together. Where does that happen? Mars. A Mars communication satellite has been discussed for a number of years, and in fact most Martian satellites double as communication relays after their primary mission is over, and to a lesser extent before their primary mission is over. It complicates the operations somewhat, but it does work.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-15T12:04:54.527" LastActivityDate="2015-07-15T12:04:54.527" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9927" PostTypeId="1" CreationDate="2015-07-15T03:15:02.107" Score="2" ViewCount="86" Body="&lt;p&gt;The New Horizon's data transfer rate is only about 2kbps right now. At its peak the rate was 38kbps. Why is this transfer rate so slow? Is this due to using radio waves, limitations in the data relay, Doppler effect, etc?&lt;/p&gt;&#xA;" OwnerUserId="10827" LastEditorUserId="168" LastEditDate="2015-07-15T07:21:29.137" LastActivityDate="2015-07-15T07:21:29.137" Title="Why are space probes / satellites data rates so low?" Tags="|communication|new-horizons|deep-space|data-transmission|radio-communication|" AnswerCount="0" CommentCount="3" ClosedDate="2015-07-15T08:53:37.220" ContentLicense="CC BY-SA 3.0" />
  <row Id="9929" PostTypeId="2" ParentId="9906" CreationDate="2015-07-15T04:17:32.730" Score="2" Body="&lt;p&gt;According to the &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/The-Flyby.php&quot; rel=&quot;nofollow&quot;&gt;full activity timeline&lt;/a&gt;, the two communication sessions before and after the closest approach are &lt;em&gt;uplinks&lt;/em&gt;, not downlinks. Presumably these are used to provide refined targeting based on the preview observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also DSN tracking sessions after the uplinks, which don't correspond to any returned results. I guess that either&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the REX transmitter can transmit a carrier wave during an uplink, but can't encode data on it, or&lt;/li&gt;&#xA;&lt;li&gt;the DSN antennas are &quot;illuminating&quot; the spacecraft with sideband signals during the uplink, and then looking for a passive reflection after the round trip.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The first session:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;July 13 21:00:00 EDT&lt;/strong&gt;&#xA;  New Horizons soon to be communicating with Madrid 70 m (DSS-63)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 13 23:30:00 EDT&lt;/strong&gt;&#xA;  New Horizons' track ending with Madrid 70 m (DSS-63)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 14 01:57:59 EDT&lt;/strong&gt;&#xA;  New Horizons is taking radio uplink data with REX. New Horizons is 31.8 AU from Earth.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 14 02:05:00 EDT&lt;/strong&gt;&#xA;  New Horizons soon to be communicating with Goldstone 70 m (DSS-14)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 14 06:40:00 EDT&lt;/strong&gt;&#xA;  New Horizons' track ending with Goldstone 70 m (DSS-14)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The second session, overlapping the first:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;July 14 03:15:00 EDT&lt;/strong&gt;&#xA;  New Horizons soon to be communicating with Canberra 70 m (DSS-43) &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 14 06:40:00 EDT&lt;/strong&gt;&#xA;  New Horizons' track ending with Canberra 70 m (DSS-43)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 14 09:28:02 EDT&lt;/strong&gt;&#xA;  New Horizons is taking radio uplink data with REX. New Horizons is 31.9 AU from Earth.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 14 10:55:00 EDT&lt;/strong&gt;&#xA;  New Horizons soon to be communicating with Canberra 70 m (DSS-43) &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;July 14 12:00:00 EDT&lt;/strong&gt;&#xA;  New Horizons' track ending with Canberra 70 m (DSS-43)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't see any REX activity corresponding to the last Canberra tracking, so I guess that indicates they're looking either for reflection of signals from the earlier part of the preceding track, 9 hours earlier, or simply looking for it without any illumination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the simulator's depiction of a 2.5 hour radio session immediately following closest approach disagrees with that timeline. Perhaps such a session was scheduled to conservatively return results ASAP, before passing through the entire system, but then they decided to perform more observations instead and delay the First Look returns.&lt;/p&gt;&#xA;" OwnerUserId="3054" LastEditorUserId="3054" LastEditDate="2015-07-15T04:27:48.783" LastActivityDate="2015-07-15T04:27:48.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9930" PostTypeId="1" AcceptedAnswerId="9965" CreationDate="2015-07-15T06:14:23.163" Score="24" ViewCount="3397" Body="&lt;p&gt;I've been reading about Russia's (former Soviet Union) Venera missions, and one thing about them that has impressed me is the incremental improvements the landers made in their lifespans before succumbing to Venus' heat. The survival of their landers on the surface are as follows:&lt;/p&gt;&#xA;&lt;div class=&quot;s-table-container&quot;&gt;&#xA;&lt;table class=&quot;s-table&quot;&gt;&#xA;&lt;thead&gt;&#xA;&lt;tr&gt;&#xA;&lt;th&gt;Mission&lt;/th&gt;&#xA;&lt;th&gt;Survival time&lt;/th&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/thead&gt;&#xA;&lt;tbody&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venera 7&lt;/td&gt;&#xA;&lt;td&gt;23 minutes&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venera 8&lt;/td&gt;&#xA;&lt;td&gt;50 minutes&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venera 9&lt;/td&gt;&#xA;&lt;td&gt;53 minutes&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venera 10&lt;/td&gt;&#xA;&lt;td&gt;65 minutes&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venera 11&lt;/td&gt;&#xA;&lt;td&gt;95 minutes&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venera 12&lt;/td&gt;&#xA;&lt;td&gt;110 minutes&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venera 13&lt;/td&gt;&#xA;&lt;td&gt;127 minutes&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venera 14&lt;/td&gt;&#xA;&lt;td&gt;57 minutes&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/tbody&gt;&#xA;&lt;/table&gt;&#xA;&lt;/div&gt;&#xA;&lt;p&gt;&lt;sub&gt;(Only went backwards once. I guess even scientists and engineers&#xA;aren't perfect)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Wikipedia articles for each of the missions are not very descriptive in the improvements of the cooling methods. In fact, a couple of them say exactly the same thing:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A system of circulating fluid was used to distribute the heat load.&#xA;This system, plus pre-cooling prior to entry, permitted operation of&#xA;the lander for [xx] minutes after landing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Others don't mention cooling at all, and I can't really find any other information about the cooling systems.&lt;/p&gt;&#xA;&lt;p&gt;The ones that do mention cooling seem to have similar methods. However, seeing increased lifespans with every lander (except the last), they must have made cooling improvements in some ways. Were different methods used in later missions (maybe a different type of cooling fluid)? Was it just blind luck that the landers survived longer every mission, or was it something different?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Related Question:&#xA;&lt;a href=&quot;https://space.stackexchange.com/q/175/10234&quot;&gt;What material properties would be necessary to shield a lander from the environment of the Venusian surface?&lt;/a&gt; - talking about the materials used for longevity against the heat and sulfuric acid, but not the cooling systems used in the Venera missions.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="47149" LastEditDate="2022-05-23T04:44:59.957" LastActivityDate="2022-05-23T04:44:59.957" Title="What cooling improvements did the Venera missions make?" Tags="|venus|lander|thermal-control|heat|venera|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="9931" PostTypeId="2" ParentId="9915" CreationDate="2015-07-15T06:25:31.820" Score="11" Body="&lt;p&gt;We're &lt;a href=&quot;https://gcd.larc.nasa.gov/projects/deep-space-optical-communications/&quot;&gt;working on it&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You do need a little telescope on the spacecraft to serve as the antenna.  You &lt;em&gt;can&lt;/em&gt; have two-way laser communications.  It may in fact be essential to achieve the required pointing accuracy.  There are many modulation schemes -- that's not an issue.  Pulse position modulation is a front runner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upside is much higher data rates with the same power.  E.g. two orders of magnitude.  The downside is that you need much more accurate pointing.  You need large telescopes on the ground to play the role of the large DSN antennas, but in the optical range.  You are much more vulnerable to weather conditions at the Earth sites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the high pointing accuracy, you will likely want to have a backup radio communication system to debug problems pointing the laser accurately enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you like, you can read &lt;a href=&quot;http://descanso.jpl.nasa.gov/monograph/series7/Descanso_7_Full_Version+.pdf&quot;&gt;the book on the subject&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-15T06:25:31.820" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9932" PostTypeId="5" CreationDate="2015-07-15T06:38:33.710" Score="0" Body="&lt;p&gt;Russia (former Soviet Union) sent several space probe missions to Venus between 1961 and 1984 named Venera (transliteration of Венера, which is Russian for Venus). The missions consisted of several flyby missions, atmpospheric probes, orbiters, and landers.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Primary Accomplishments&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_4&quot; rel=&quot;nofollow&quot;&gt;Venera 4&lt;/a&gt; was the first spacecraft to ever enter another planet's atmosphere and return data.&lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_7&quot; rel=&quot;nofollow&quot;&gt;Venera 7&lt;/a&gt; mission was the first time a spacecraft successfully landed on another planet's surface, and sent the first broadcast from another planet's surface.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_9&quot; rel=&quot;nofollow&quot;&gt;Venera 9&lt;/a&gt; sent back the first images of Venus' surface.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_12&quot; rel=&quot;nofollow&quot;&gt;Venera 12&lt;/a&gt; recorded potential evidence of lightning&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_13&quot; rel=&quot;nofollow&quot;&gt;Venera 13&lt;/a&gt; sent back the first color images of Venus' surface&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Russia is also planning a potential Venera missions in 2024 named Venera-D.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h1&gt;References:&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera&quot; rel=&quot;nofollow&quot;&gt;Venera - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mentallandscape.com/V_Venus.htm&quot; rel=&quot;nofollow&quot;&gt;Don P. Mitchell's Venus pages&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10132" LastEditDate="2015-07-20T04:11:56.213" LastActivityDate="2015-07-20T04:11:56.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9933" PostTypeId="4" CreationDate="2015-07-15T06:38:33.710" Score="0" Body="Venera was a series of space probe missions the Soviet Union sent to Venus." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T07:51:03.663" LastActivityDate="2015-07-15T07:51:03.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9934" PostTypeId="2" ParentId="9920" CreationDate="2015-07-15T07:19:17.657" Score="4" Body="&lt;p&gt;In addition to the points already mentioned: on Earth, we can build dish antennas with a diameter of 70 m. Even the 300 m Arecibo antenna has been used on special occasions. In space, the largest antenna ever deployed is 10 m (the &lt;a href=&quot;http://www.russianspaceweb.com/spektr_r_mission.html&quot; rel=&quot;nofollow&quot;&gt;Spektr-R&lt;/a&gt; radio astronomy mission). Weight goes up with the square of the antenna radius, so a large antenna gets very expensive to launch.&lt;br&gt;&#xA;You also run into space limitations: the largest single piece that can be launched is in the region of 20 m long and 5 m wide, so a large antenna must be folded and/or assembled in space. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-15T07:19:17.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9935" PostTypeId="2" ParentId="9916" CreationDate="2015-07-15T07:37:59.667" Score="4" Body="&lt;p&gt;To answer this part of the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;then those images which we are all over internet now, are actually a month only or something like that?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the last few days before the flyby, New Horizons sent us a few images. This is just a small part of the data that were collected (images and other instrument readings).&lt;br&gt;&#xA;New Horizons can't take photos and send data to Earth simultaneously. So they planned the mission to have as much time as possible available for observations. To make sure we had at least some data in case of a catastrophic problem, some time was reserved for sending a few images to Earth just before the encounter. The rest of the data collected is stored aboard the spacecraft and will be sent when the observation phase of the mission has ended. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image was taken on July 13th: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/JrveO.png&quot; alt=&quot;Pluto&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-15T07:37:59.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9936" PostTypeId="1" CreationDate="2015-07-15T08:17:07.847" Score="1" ViewCount="63" Body="&lt;p&gt;I understand that New Horizons has &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Future_mission_objectives&quot; rel=&quot;nofollow noreferrer&quot;&gt;further objectives&lt;/a&gt; after Pluto, but they seem to be very iffy, whether they will produce anything valuable.  &lt;a href=&quot;https://space.stackexchange.com/questions/4634/can-new-horizons-be-turned-into-an-orbiter-for-the-pluto-system&quot;&gt;This question&lt;/a&gt; is related, but I'm more interested in &lt;em&gt;why&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the design of the mission, was the reason to leave Pluto due to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Concrete and specific objectives to continue outward,&lt;/li&gt;&#xA;&lt;li&gt;Previous lack of know-how to put New Horizons into close orbit of Pluto, or&lt;/li&gt;&#xA;&lt;li&gt;Is the function and nature of New Horizons not compatible with close-up imagery, data collection and communication with Earth?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why aren't we staying with Pluto, and gathering more information?&lt;/strong&gt;  Given that it's possible nothing will be found of interest in the Kuiper Belt.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-07-15T08:17:07.847" Title="Would it have been possible to instead keep New Horizons with Pluto?" Tags="|probe|new-horizons|pluto|" AnswerCount="0" CommentCount="2" ClosedDate="2015-07-15T08:27:51.857" ContentLicense="CC BY-SA 3.0" />
  <row Id="9937" PostTypeId="1" AcceptedAnswerId="9940" CreationDate="2015-07-15T10:29:22.643" Score="9" ViewCount="1891" Body="&lt;p&gt;The Voyager probes have a scan platform, which allowed them to point their cameras independently of the orientation of the spacecraft. New Horizons doesn't have a scan platform, so the entire spacecraft must be rotated to point the cameras in the right direction. As a result, the mission planners had to choose between observations and downlinking data at several points during the Pluto flyby. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why wasn't a scan platform included in New Horizons?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS: The reason I've asked this question: a couple of recent questions here are about Pluto encounter planning. The answers usually refer to NH's lack of a scan platform, but we haven't tackled the underlying reason yet. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-15T11:49:19.207" LastActivityDate="2015-07-20T18:06:44.917" Title="Why can't New Horizons send data and do observations at the same time?" Tags="|voyager|new-horizons|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9939" PostTypeId="1" CreationDate="2015-07-15T12:19:25.863" Score="1" ViewCount="735" Body="&lt;p&gt;I would like to know what is the modulation technique used on the New Horizons Spacecraft? Is it BPSK or GMSK?&#xA;Also what is the coding technique-turbo-codes or LDPC codes?&#xA;And do you have a link budget available?&lt;/p&gt;&#xA;" OwnerUserId="10839" LastEditorUserId="25" LastEditDate="2015-07-15T12:47:01.123" LastActivityDate="2015-07-19T22:56:21.987" Title="What is the modulation technique and what is the coding technique used to send information from the New Horizons spacecraft?" Tags="|communication|new-horizons|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9940" PostTypeId="2" ParentId="9937" CreationDate="2015-07-15T12:35:10.580" Score="15" Body="&lt;p&gt;Mass, reliability and power. Adding the ability to rotate a device adds to the mass and power requirements of the system. It also gives more points that could go wrong. New Horizons is one of the spacecraft that has taken the longest to reach its intended target. Moving parts would have made it more complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember, the New Horizons only weights 478 kg. To add more mass would be very difficult. Hard drive space is much cheaper and better now than it was when Voyager was made. With Voyager, it was more important to send data quickly than it is with New Horizons. Voyager was also doing quite a bit longer encounter than New Horizons, due to the larger systems it visited. As a result, it was easier to store it, allowing for more science to be accomplished.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2015-07-15T20:25:52.807" LastActivityDate="2015-07-15T20:25:52.807" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="9941" PostTypeId="1" AcceptedAnswerId="9944" CreationDate="2015-07-15T12:44:38.743" Score="6" ViewCount="351" Body="&lt;p&gt;New Horizons just passed by Pluto, but was unable to see the southern hemisphere, as a result of it being in perpetual eclipse. When will the southern half be lit, so that the rest of the planet could be seen?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-15T13:45:09.960" Title="When could we see Pluto's Southern hemisphere?" Tags="|planetary-science|new-horizons|pluto|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9942" PostTypeId="2" ParentId="9937" CreationDate="2015-07-15T12:58:02.690" Score="9" Body="&lt;p&gt;(beginnings of a long answer)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were several reasons for that decision:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Cost. &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/PI-Perspectives.php?page=piPerspective_06_23_2014&quot; rel=&quot;nofollow&quot;&gt;According to Alan Stern&lt;/a&gt;, New Horizons' Principal Investigator:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager also had a scan platform that allowed its remote sensing instruments to be trained on their targets even while the spacecraft was transmitting data to Earth- a high-cost technology that the New Horizons team chose to forego. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://llis.nasa.gov/lesson/394&quot; rel=&quot;nofollow&quot;&gt;Reliability&lt;/a&gt;.&#xA;The &lt;a href=&quot;http://www.nasaspaceflight.com/2011/08/thirty-four-years-voyager-2-continues-explore/&quot; rel=&quot;nofollow&quot;&gt;Voyagers used a scan platform&lt;/a&gt;. It's difficult to make sure mechanical components continue to work reliably in space. Lubricants evaporate in the vacuum, temperature swings, cold welding.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;shortly after concluding operations at Saturn, Voyager 2’s science scan platform seized, and its gear and shaft were damaged because of the apparent migration of lubricant away from the gear-shaft interface due to overuse in a short amount of time.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Chris Hersman, Mission systems engineer for NH:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;We chose to avoid mechanisms, because mechanisms have a tendency to fail&quot;&#xA;  (source: 'Mission Pluto', National Geographic Channel)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://usuaris.tinet.org/klunn/new-horizons.html&quot; rel=&quot;nofollow&quot;&gt;Secondary source for reliability&lt;/a&gt; as a driver for this design:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;New Horizons’ elimination of excess mechanisms was implemented to save weight, shorten the schedule, and improve reliability to achieve a lifetime of at least 15 years. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Voyager incident also provided proof that operation without a scan platform was possible: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Between Voyager 2’s encounter with Saturn in 1981 and Uranus in 1986, controllers developed a technique called “image motion compensation.” This involved moving the scan platform at slow rates, which was found to be possible despite the damage, in conjunction with thruster firings to rotate the entire spacecraft at a rate that would allow a target to be tracked long enough for imaging.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-20T18:06:44.917" LastActivityDate="2015-07-20T18:06:44.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9944" PostTypeId="2" ParentId="9941" CreationDate="2015-07-15T13:45:09.960" Score="6" Body="&lt;p&gt;The seasons of Pluto correspond to the closest approach to the Sun. The best time to visit Pluto would have been in 1990, the time of the closest approach to the Sun, which also would have allowed both hemispheres to receive equal light. While 25 years is only a fraction of Pluto's orbit, due to Pluto's large tilt, it has considerable parts in darkness even at this time. A nice chart of this can be seen on &lt;a href=&quot;http://www.planetary.org/multimedia/space-images/charts/plutos-seasons.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planetary.org&lt;/a&gt;, which Emily has graciously allowed for posting elsewhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/EyPdF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Deer Hunter mentioned, part of the southern hemisphere can be seen using reflected light from Charon, however, there is a portion of the Southern hemisphere in complete darkness for some time to come. The next opportunity won't be until Pluto next reaches aphelion, in 2124.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-15T13:45:09.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9946" PostTypeId="1" AcceptedAnswerId="9949" CreationDate="2015-07-15T16:54:16.327" Score="11" ViewCount="2762" Body="&lt;p&gt;New Horizions just reached Pluto after almost a decade in space. That means it was launched with technology that is well over a decade old (when you factor in planning and construction time). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we had a new probe with modern technology on the launch pad today, how long would it take to get to Pluto? Would it be significantly faster? Would it depend heavily on the alignment of the planets (ie the Jupiter slingshot maneuver)?&lt;/p&gt;&#xA;" OwnerUserId="3141" LastEditorUserId="3141" LastEditDate="2015-07-15T17:01:31.163" LastActivityDate="2015-07-15T22:12:03.483" Title="How much faster (or slower) would it be for a new probe to reach Pluto?" Tags="|new-horizons|pluto|time|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="9947" PostTypeId="1" AcceptedAnswerId="9948" CreationDate="2015-07-15T17:28:51.777" Score="5" ViewCount="820" Body="&lt;p&gt;Part of New Horizon's path took it near Mars. Why didn't they use Mars for a gravity speed boost like they did Jupiter? I'm guessing it had to do with the relatively small size of Mars being unable to pull New Horizon due to the probe's speed.&lt;/p&gt;&#xA;" OwnerUserId="3141" LastEditorUserId="25" LastEditDate="2015-07-15T18:29:40.173" LastActivityDate="2015-07-16T14:56:54.100" Title="Why didn't New Horizon use Mars as a gravity assist like it did Jupiter?" Tags="|mars|new-horizons|gravity-assist|pluto|jupiter|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9948" PostTypeId="2" ParentId="9947" CreationDate="2015-07-15T17:40:03.213" Score="8" Body="&lt;p&gt;It really wasn't that close, and going out of it's way would have been more trouble than it was worth. See &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Where-is-New-Horizons/Passing-Planets/Passing-Planets-Mars.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;this map from APL&lt;/a&gt; of the planets position when New Horizons crossed Mars's orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mpvIo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the list of objects that New Horizons crossed it's orbit, the only one that even was close enough to consider was the Moon, and as that's never been done, I don't think it was really considered. My educated guess shows it could have added a few hundred m/s to the calculation, but it would have significantly limited the launch window, making it unlikely to be of much use.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-16T14:56:54.100" LastActivityDate="2015-07-16T14:56:54.100" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9949" PostTypeId="2" ParentId="9946" CreationDate="2015-07-15T18:13:44.113" Score="22" Body="&lt;p&gt;Modern technology doesn't change the major constraints of rocket propulsion significantly.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the absence of a gravity assist, the most fuel-efficient route from Earth to Pluto is approximately a 45-year journey. Bringing that down to 9 years already represents a large investment in propulsion fuel; &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot;&gt;New Horizons&lt;/a&gt; set the record for the highest launch speed of a human-made object from Earth (~16 km/s); the Jupiter flyby increased its speed by another ~4 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If NH had an ion thruster capability similar to that of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot;&gt;Dawn&lt;/a&gt; spacecraft, with enough fuel for an additional 4km/s of maneuvering, it could have managed roughly the same travel time without the Jupiter assist, or, with both the ion drive and the gravity assist, managed more like a 7-8 year trip. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That kind of capability would come at the cost of something else; NH's very high launch speed was only possible because the spacecraft was so small (about half a ton), and the fuel needed for such an ion thruster would be about a third the mass of the craft. Either a much larger launcher would be needed, or the science package on the probe would have to be much reduced. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-07-15T18:13:44.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9950" PostTypeId="2" ParentId="9946" CreationDate="2015-07-15T18:35:50.030" Score="11" Body="&lt;p&gt;For a launch to Pluto, a Jupiter gravitational slingshot opportunity occurs once every 12 years or so (one Jupiter orbit + 12/248, to account for Pluto's movement in those 12 years). The next one is ~2-3 years away.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's the rocket. You want the fastest launch possible. The Atlas V 551 used by New Horizons can put 18 tons into LEO. As a first approximation, more weight to LEO = faster launch of a New Horizons-class probe on its way to Pluto. So going through &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot;&gt;this comparison list&lt;/a&gt;, the Proton M does a little better at 21 tons, but the real improvements are still in development, notably the SpaceX Falcon Heavy (53 tons) and the SLS Block II (130 tons). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those rockets also give the freedom to use an ion engine, and to design better upper stages than the Centaur and STAR-48. I'll leave the number crunching on those to others. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-15T18:55:03.697" LastActivityDate="2015-07-15T18:55:03.697" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9951" PostTypeId="5" CreationDate="2015-07-15T19:04:47.013" Score="0" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobraking&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Aerobraking is a spaceflight maneuver that reduces the high point of&#xA;  an elliptical orbit (apoapsis) by flying the vehicle through the&#xA;  atmosphere at the low point of the orbit (periapsis). The resulting&#xA;  drag slows the spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All questions related to aerobraking should use this tag. For example, this tag has been used for questions related to methods, complications, and advantages of aerobraking.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional resources:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.qrg.northwestern.edu/projects/vss/docs/navigation/3-what-is-areobraking-or-retroburn.html&quot; rel=&quot;nofollow&quot;&gt;What is aerobraking?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T19:07:21.047" LastActivityDate="2015-07-15T19:07:21.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9952" PostTypeId="4" CreationDate="2015-07-15T19:04:47.013" Score="0" Body="Questions related to causing a spacecraft to slow down by flying through a planet's atmosphere to produce aerodynamic drag." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T19:07:17.773" LastActivityDate="2015-07-15T19:07:17.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9953" PostTypeId="5" CreationDate="2015-07-15T19:18:42.220" Score="0" Body="&lt;p&gt;Apollo 11 was the first manned mission to the lunar surface. The mission was launched from Kennedy Space Center in Florida on July 16, 1969. Neil Armstrong and Buzz Aldrin stepped on the lunar surface on July 21, 1969, while Michael Collins remained in lunar orbit inside the command module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The astronauts returned to Earth on July 24, 1969. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Information:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11&quot; rel=&quot;nofollow&quot;&gt;Apollo 11 - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/alsj/a11/A11_MissionReport.pdf&quot; rel=&quot;nofollow&quot;&gt;Apollo 11 Mission Report - NASA&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T19:23:38.150" LastActivityDate="2015-07-15T19:23:38.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9954" PostTypeId="4" CreationDate="2015-07-15T19:18:42.220" Score="0" Body="Apollo 11 landed the first humans, Neil Armstrong and Buzz Aldrin, on the surface of the Moon on July 20, 1969." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T19:23:35.363" LastActivityDate="2015-07-15T19:23:35.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9955" PostTypeId="5" CreationDate="2015-07-15T19:36:09.557" Score="0" Body="&lt;p&gt;Cassini is an unmanned spacecraft that was launched on October 15, 1997, and has been in Saturn's orbit since June 30, 2004. On its way to Saturn, Cassini made flyby maneuvers at Venus, Earth, and the Jupiter system. It has studied Saturn and its natural satellites extensively since 2004 and is still performing its science mission there. The planned end of the mission will be September 15, 2017, when it will enter Saturn's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Questions related to Cassini, including its launch, flyby maneuvers, science mission, discoveries, and technology should use this tag.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional resources:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;nofollow&quot;&gt;Cassini-Huygens - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/overview/&quot; rel=&quot;nofollow&quot;&gt;Cassini Overview - NASA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://saturn.jpl.nasa.gov/interactive/missiontimeline/&quot; rel=&quot;nofollow&quot;&gt;Cassini Mission Timeline - NASA&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T19:40:07.297" LastActivityDate="2015-07-15T19:40:07.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9956" PostTypeId="4" CreationDate="2015-07-15T19:36:09.557" Score="0" Body="Questions related to the Cassini–Huygens spacecraft sent to Saturn." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T19:40:04.567" LastActivityDate="2015-07-15T19:40:04.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9957" PostTypeId="5" CreationDate="2015-07-15T20:08:07.837" Score="0" Body="&lt;p&gt;The Chinese Lunar Exploration Program (CLEP), or Chang'e Program, is China's mission program for their unmanned Lunar spacecrafts and rovers. &#xA;Some important dates:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The first mission in the program was launched October 24, 2007&lt;/li&gt;&#xA;&lt;li&gt;The first Chang'e lander was launched on December 2, 2013&lt;/li&gt;&#xA;&lt;li&gt;The first Chang'e landing (and first Lunar soft landing since 1976) on December 14, 2013. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All questions related to China's Chang'e Lunar missions should use this tag.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Resources:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Chinese_Lunar_Exploration_Program&quot; rel=&quot;nofollow&quot;&gt;Chinese Lunar Exploration Program - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/27670-china-moon-missions-explained-infographic.html&quot; rel=&quot;nofollow&quot;&gt;China's Moon Missions Explained (Infographic) - Space.com&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T20:27:57.120" LastActivityDate="2015-07-15T20:27:57.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9958" PostTypeId="4" CreationDate="2015-07-15T20:08:07.837" Score="0" Body="Questions related to China's unmanned Moon exploration missions" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-15T20:28:00.283" LastActivityDate="2015-07-15T20:28:00.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9959" PostTypeId="1" CreationDate="2015-07-15T21:24:10.200" Score="9" ViewCount="301" Body="&lt;p&gt;How does the New Horizons probe know it's close to Pluto and turn its science instruments towards Pluto and other targets of its observations during the flyby?&lt;/p&gt;&#xA;" OwnerUserId="10851" LastEditorUserId="25" LastEditDate="2015-07-16T11:52:44.283" LastActivityDate="2015-07-16T11:52:44.283" Title="New Horizons - Orientation in space" Tags="|probe|new-horizons|flyby|attitude|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9960" PostTypeId="2" ParentId="9946" CreationDate="2015-07-15T22:08:56.307" Score="7" Body="&lt;p&gt;Hobbes and Russell Borogove gave good answers. I'd like to add another factor: distance from the sun. Pluto's perihelion is 29.64 AU. Aphelion is 49.12 AU. Last perihelion was in 1989. Pluto's orbital period is about 246 years. So next perihelion is in the year 2,235. During our lifetimes Pluto is only going to get more distant.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="195" LastEditDate="2015-07-15T22:12:03.483" LastActivityDate="2015-07-15T22:12:03.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9961" PostTypeId="2" ParentId="9959" CreationDate="2015-07-16T02:03:29.967" Score="9" Body="&lt;p&gt;What this is called primarily is called Attitude Determination, and is a part of the attitude control system in most spacecraft. &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot;&gt;Wikipedia&lt;/a&gt; has a great bit on that, which states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Two star cameras are used to measure the spacecraft attitude. They are mounted on the face of the spacecraft and provide attitude information while in spin-stabilized or 3-axis mode. In between the time of star camera readings, spacecraft orientation is provided by dual redundant miniature inertial measurement units. Each unit contains three solid-state gyroscopes and three accelerometers. Two Adcole Sun sensors provide attitude determination. One detects the angle to the Sun, whereas the other measures spin rate and clocking.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Okay, so what does all of that mean? There are two main ways that the spacecraft lines up it's attitude, then it measures the differences by measuring how much attitude change there is. Okay, so what are the systems?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Star camera- Points to known bright stars, which gives a fixed orientation in space. Giving two points will allow you to know exactly how you are pointed.&lt;/li&gt;&#xA;&lt;li&gt;Sun Sensor- Similar to a Star camera, but really just finds the Sun. I'm rather impressed that it works as far as Pluto, this might not work as well the further it gets out. These are generally simpler than a star camera.&lt;/li&gt;&#xA;&lt;li&gt;Gyroscope- Used to detect rotation. These can find how the spacecraft has rotated from a known configuration.&lt;/li&gt;&#xA;&lt;li&gt;Accelerometer- Will measure acceleration in a particular direction. These also assist in finding rotation.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Put all of this together, and you know how the spacecraft is pointed. According to &lt;a href=&quot;http://www.spaceflight101.com/new-horizons-spacecraft.html&quot;&gt;Spaceflight101&lt;/a&gt;, the accuracy is 471 urads, or about 0.025 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so that's how it knows where it is pointed, now how does it know where to point? These come from the ground, where a sequence of commands was provided some time ago, and the commands began execution. They most likely were commanded to start at a certain time, which there is a clock on board that provides the time. This was determined using some kind of ground software based on the travel path of New Horizon, and the position of Pluto. Some spacecraft have the ability to move slightly to photograph a target, but this is usually reserved for things like Comets, which have some uncertainty about them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, another good question is, how do we know exactly where Pluto is? And even a small difference makes a huge effect, when you are only a few miles away. This was detailed in a &lt;a href=&quot;http://www.universetoday.com/113705/where-exactly-is-pluto-pinpoint-precision-needed-for-new-horizons-mission/&quot;&gt;Universe Today article&lt;/a&gt; some time ago. Prior to the launch, Pluto's position was known to several thousand km. That seems like a lot, but is miniscule when compared to its large distance away, only a millionth off. But for New Horizon, that could mean missing the planet entirely. Through dedicated measurements, and even using New Horizon's instruments, we were better able to identify its position. Basically, the spacecraft was able to image Pluto, and depending where it showed up on the image determined exactly where it was. This allowed for it to be more accurately determined, especially relatively to New Horizon's trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of this put together allowed New Horizon to point it's camera exactly at the right spots to get some cool science done.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-16T02:10:18.430" LastActivityDate="2015-07-16T02:10:18.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9962" PostTypeId="2" ParentId="1621" CreationDate="2015-07-16T02:08:17.693" Score="6" Body="&lt;p&gt;Here is a helpful image I received from Dr Lasher from one of his AIAA astrodynamics papers from the 1970's&#xA;it reflects the Jupiter aim points for the Star 37E after separation from the Centaur upper stage for the pioneer spacecraft.&#xA; &lt;img src=&quot;https://i.stack.imgur.com/HAx5S.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The B plane of a planet is always changing do to spacecraft incoming speed and line of flight(geometry) and epoch(time) notice the upper stage did not do such a good job with Pioneer 11 so Pioneer 11 had to do a TCM-1 to move the spacecraft over to the Hyperbolic circle on the right.so in this case the pioneer 11 Star 37E solid upper stage is in a heliocentric orbit while Pioneer 11 went to a Saturn flyby into interstellor space.   &lt;/p&gt;&#xA;" OwnerUserId="3297" LastActivityDate="2015-07-16T02:08:17.693" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9963" PostTypeId="1" CreationDate="2015-07-16T04:49:47.877" Score="8" ViewCount="1192" Body="&lt;p&gt;Is the Asteroid Belt in the same plane as Earth and Jupiter or they are in different planes?&lt;/p&gt;&#xA;" OwnerUserId="10857" LastEditorUserId="897" LastEditDate="2015-07-16T07:02:07.547" LastActivityDate="2019-03-19T23:06:41.180" Title="Is the Asteroid Belt in a plane that intersects Earth and Jupiter?" Tags="|asteroid-belt|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9964" PostTypeId="2" ParentId="9963" CreationDate="2015-07-16T05:22:25.350" Score="11" Body="&lt;p&gt;This question is similar to: &lt;a href=&quot;https://astronomy.stackexchange.com/questions/10062/do-the-planets-asteroid-belt-kuiper-belt-and-scattered-disc-lie-on-the-same-p&quot;&gt;Do the planets, asteroid belt, kuiper belt, and scattered disk lie on the same plane?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth&lt;/a&gt; is on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic&quot; rel=&quot;nofollow noreferrer&quot;&gt;ecliptic plane&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter&lt;/a&gt; is 1.305° inclined relative to the ecliptic, so not quite coplanar but pretty close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_belt&quot; rel=&quot;nofollow noreferrer&quot;&gt;asteroid belt&lt;/a&gt;, being made up of a diverse set of rocks, is all over the place (from &lt;a href=&quot;http://burro.astr.cwru.edu/stu/advanced/asteroid.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/p0xF5.jpg&quot; alt=&quot;Asteroid inclination&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The description of that image:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The orbital inclination histogram [above], which is binned in intervals of 0.1°, shows that asteroids are another exception to this rule. The average orbital inclination is about 8.2°.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Between planets (especially Jupiter) perturbing orbits of asteroids and asteroids occasionally hitting each other and breaking apart the asteroid belt isn't stable enough to gather into a planet or even a single plane or type of orbit.  To present the orbits differently, &lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_belt#/media/File:Asteroid_proper_elements_i_vs_e.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;this image&lt;/a&gt; from Wikipedia shows the grouping of asteroids in similar orbits:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/guc8D.png&quot; alt=&quot;Asteroid proper elements, inclination vs eccentricity&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$i_p(^\circ)$&lt;/span&gt; is inclination from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic&quot; rel=&quot;nofollow noreferrer&quot;&gt;ecliptic&lt;/a&gt; in degrees&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$e_p$&lt;/span&gt; is orbital &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_eccentricity&quot; rel=&quot;nofollow noreferrer&quot;&gt;eccentricity&lt;/a&gt; from 0=circular to 1=a &lt;a href=&quot;https://www.mathopenref.com/ellipseeccentricity.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;maximally elongated&lt;/a&gt; ellipse&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;and the subscript &lt;span class=&quot;math-container&quot;&gt;$p$&lt;/span&gt; stands for &lt;a href=&quot;https://en.wikipedia.org/wiki/Proper_orbital_elements&quot; rel=&quot;nofollow noreferrer&quot;&gt;proper orbital elements&lt;/a&gt; (a kind of average over time) which are distinct from current or &lt;a href=&quot;https://en.wikipedia.org/wiki/Osculating_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;osculating&lt;/a&gt; orbital elements. See &lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_family#Description&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Asteroid_family#Description&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="12102" LastEditDate="2019-03-19T23:06:41.180" LastActivityDate="2019-03-19T23:06:41.180" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="9965" PostTypeId="2" ParentId="9930" CreationDate="2015-07-16T07:50:26.643" Score="25" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Observations_and_explorations_of_Venus#Observation_by_spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; launch windows to Venus occur every 19 months.  In some cases multiple successful probes were launched with the same design at the same time so I'll group those together (the Soviets launched two similar/identical missions per launch window they actually used to ensure mission success through redundancy).  I'll label each mission with the launch year.&lt;/p&gt;&#xA;&lt;p&gt;Un-annotated statements are from the link in the title of the section, references used for more than one probe will be at the end.  My primary source is &lt;a href=&quot;http://mentallandscape.com/V_Venus.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site&lt;/a&gt; by Don Mitchell, individual pages are referenced below and his site includes a bibliography page.  I'm extracting mainly the comments related to cooling and mission duration, there's a lot more information available there.&lt;/p&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;http://mentallandscape.com/V_Lavochkin1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 7&lt;/a&gt; (1970)&lt;/h1&gt;&#xA;&lt;p&gt;The primary focus for this probe was on surviving to the surface (withstand higher pressure, temperature, and the shock of landing)&lt;sup&gt;4&lt;/sup&gt;- previous attempts had been crushed by the atmospheric pressure before getting there.  To this end the pressure vessel was designed to withstand 180 atmospheres or pressure (massively overbuilt, had little room for scientific payload) and the parachute was reefed to descend quickly (it was supposed to fully open when the heat melted the reefing cord).&lt;/p&gt;&#xA;&lt;p&gt;The only mention I can find of cooling is that the spacecraft bus cooled the descent capsule to -8° C before separation.  I'm taking the lack of room for payload and fast descent profile to mean that they were trying to hit the ground before the sealed pressure vessel got too hot.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The capsule antenna was extended, and signals return commenced. Six minutes later the parachute ripped, and then collapsed, leaving the probe to fall towards the surface for another 29 minutes. The probe impacted on the Venus surface at 05:34:10 UT at about 17 meters/sec and the signals weakened, reached full strength for about one second, and then seemingly ceased. Later analysis of the recorded radio signals revealed that the probe had survived the impact and continued transmitting a weak signal for another 23 minutes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Entertainingly, &amp;quot;The fact that the capsule survived impact at 17 m/sec was used to estimate the bearing strength of the soil.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Considering the conditions of landing I'm not sure how much can be read into the survival time in terms of resistance to the environment/temperature.  Having survived to the surface with the ability to report temperature the design of later probes could be refined instead of being as overbuilt.&lt;/p&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;http://mentallandscape.com/V_Lavochkin1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 8&lt;/a&gt; (1972)&lt;/h1&gt;&#xA;&lt;p&gt;This time they cooled the descent capsule to -15° C and only designed for 105 atmospheres of pressure&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Plumbing the Atmosphere of Venus (footnote &lt;sup&gt;1&lt;/sup&gt; and the title link for this section) has an excellent diagram for this probe:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/vLE8s.jpg&quot; alt=&quot;Layout of Venera-8 descent module&quot; /&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&#xA;&lt;ol start=&quot;5&quot;&gt;&#xA;&lt;li&gt;heat exchanger&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ol start=&quot;6&quot;&gt;&#xA;&lt;li&gt;heat accumulator&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ol start=&quot;7&quot;&gt;&#xA;&lt;li&gt;internal thermal insulation&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ol start=&quot;9&quot;&gt;&#xA;&lt;li&gt;heat accumulator&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ol start=&quot;11&quot;&gt;&#xA;&lt;li&gt;external thermal insulation&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ol start=&quot;15&quot;&gt;&#xA;&lt;li&gt;circulation fan&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ol start=&quot;16&quot;&gt;&#xA;&lt;li&gt;cooling conduit from bus&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;(other numbers listed at the source linked in footnotes)&lt;/p&gt;&#xA;&lt;p&gt;Also:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;One new idea, for additional thermal protection, was the addition of phase-change material. Lithium nitrate trihydrate melts at 30° C, absorbing a large amount of heat, due to its high latent heat of fusion. Besides large &amp;quot;heat accumulators&amp;quot; of this material in the capsule, sensors that extended outside were jacketed with it. Insulation of a honeycomb composite material was developed, to insulate under high pressures and temperatures.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;http://mentallandscape.com/V_Lavochkin2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 9 and 10&lt;/a&gt; (1975)&lt;/h1&gt;&#xA;&lt;p&gt;(launched 6 days apart, same instruments/experiments)&lt;/p&gt;&#xA;&lt;p&gt;This was a complete redesign based on lessons learned from Venera 7 and 8.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The core of the descent vehicle was a spherical titanium hull about 80 cm in diameter. It was formed in several sections, bolted and sealed with gold-wire gaskets. That was covered in a 12 cm layer of thermal insulation (a composite honeycomb material) and a thin outer skin of titanium. The pressure hull was lined inside with insulation, possibly layers of fiberglass and metal foil. A large thermal accumulator of lithium nitrate trihydrate and a circulating fan distributed and absorbed excess heat.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It was cooled to -10° C before separating from the bus, and the interior temperature rose to 60° C after an hour on the surface. Mission lifetime was limited by loss of radio contact, not thermal failure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The orbiter was in an eccentric orbit that would remain over the landing site for a long period of time, but for both Venera-9 and Venera-10 the data stream ended when the range to the orbiter was too great for the probe transmitter to reach.&lt;/p&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;http://mentallandscape.com/V_Venera11.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 11 and 12&lt;/a&gt; (1978)&lt;/h1&gt;&#xA;&lt;p&gt;(launched 5 days apart, same instruments)&lt;/p&gt;&#xA;&lt;p&gt;The only reference on temperature management is:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The outer insulation consisted of two materials: KG-25 an high-temperature polyurethane foam, and PTKV-260 an high-temperature thermal shield of unknown composition (probably a honeycomb composite material).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Apparently the system worked as well as the Venera 9 and 10 because transmission was lost due to radio range again:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Venera-11 landed at 14° S 299° E. Conditions there were 92.6 atmospheres of pressure and a temperature of 452° C (846° F). It remained in radio contact for 95 minutes, until the bus vehicle moved out of range. Venera-12 landed about 850 km from there, at 7° S 294°, finding 93.6 atm and 468° C. It transmitted data to Earth for 110 minutes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;http://mentallandscape.com/V_Venera11.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 13 and 14&lt;/a&gt; (1981)&lt;/h1&gt;&#xA;&lt;p&gt;(launched 5 days apart, identical craft &lt;sup&gt;3&lt;/sup&gt;)&lt;/p&gt;&#xA;&lt;p&gt;For these probes they were in part building the science payload to withstand the conditions without having to be inside the cooled pressure hull:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;By this time, Soviet engineers had developed new heat-resistant materials and electronics that were comfortable in this working environment. Exotic lubricants, based on molybdenum disulphide and microscopic metal flakes, were designed to function at high pressures and temperatures up to 1000° C.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Similar to Venera 11 and 12 the spacecraft bus flew by Venus instead of entering orbit.  I can't find anything that explicitly states why the missions ended (though &lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_13&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; claims that design life was only 32 minutes) but I'm guessing it was once again loss of signal due to range, not equipment failure.&lt;/p&gt;&#xA;&lt;h1&gt;Conclusions&lt;/h1&gt;&#xA;&lt;p&gt;After the first attempts the probes were built to withstand the temperature and pressure conditions of Venus for as long as the orbital parameters of the spacecraft bus kept it within range of the descent module on the surface.  Especially considering that several of the probes were identical pairs the trend of increasing mission duration seems to be either coincidence or intentional changes to how the decent and bus orbit/flyby were aligned, not related to increasing thermal tolerance.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;References (not already linked):&lt;/h3&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mentallandscape.com/V_Lavochkin1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Plumbing the Atmosphere of Venus&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Chron&amp;amp;MCode=Venera_08&amp;amp;StartYear=1970&amp;amp;EndYear=1979&amp;amp;Display=ReadMore&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 8, solarsystem.nasa.gov&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_14&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 14, Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1970-060A&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 7, nasa.gov&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2021-03-10T10:47:23.467" LastActivityDate="2021-03-10T10:47:23.467" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="9966" PostTypeId="1" CreationDate="2015-07-16T11:09:59.093" Score="5" ViewCount="174" Body="&lt;p&gt;As far as I know, LORRI is panchromatic (black-and-white) and Ralph has red, blue and two infrared filters but no green filter. As such, I wonder whether New Horizons is capable of making natural color images.&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="49" LastEditDate="2015-07-16T11:23:34.417" LastActivityDate="2015-07-16T17:43:06.037" Title="Is New Horizons capable of making natural color photos?" Tags="|new-horizons|payload|camera|instrument|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9967" PostTypeId="1" CreationDate="2015-07-16T11:52:13.370" Score="1" ViewCount="531" Body="&lt;p&gt;After its mission, New Horizons will be destined to travel perpetually forever. I wonder though how far it will be when the sun explodes. Will it be far enough away that when this happens, billions of years from now, it will not be destroyed? If this is the case, what will happen to New Horizons? Is it likely to literally just travel forever without ever hitting anything?&lt;/p&gt;&#xA;" OwnerUserId="10863" LastEditorUserId="25" LastEditDate="2015-07-16T13:34:23.017" LastActivityDate="2015-07-16T13:34:23.017" Title="Will New Horizons, Voyager or any other space craft survive the death of the sun?" Tags="|new-horizons|the-sun|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="9969" PostTypeId="2" ParentId="9967" CreationDate="2015-07-16T12:04:07.850" Score="3" Body="&lt;p&gt;Absolutely. The &lt;a href=&quot;http://www.space.com/23154-death-of-sun-will-destroy-earth-infographic.html&quot; rel=&quot;nofollow&quot;&gt;death of the Sun&lt;/a&gt; will likely swallow an area of about Mars's radius, most likely a bit further. Thus, any spacecraft in the outer solar system or further will survive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, it's likely that any spacecraft orbiting Earth will have a good chance of survival, as the Earth's orbit will be pushed back further. Mars is almost certainly going to survive somewhat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to all of this, New Horizons, and the Voyagers, etc, are all moving quite fast. Voyager 1 is moving at a rate of 3.6 au/year. The Sun won't explode for billions of years. Using 4 billion as a conservative value for that time, Voyager will be 14.4 billion au away from the sun, or basically the other side of the Milky Way (I show 227,000 light years away).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the Sun's destruction will have no bearing on the lifetime of any satellite in current solar escape, and probably won't have any affect on anything in the outer solar system. Other events might have an effect, however.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-16T12:04:07.850" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9970" PostTypeId="1" AcceptedAnswerId="9976" CreationDate="2015-07-16T12:45:47.707" Score="35" ViewCount="12892" Body="&lt;p&gt;Why did NASA send a probe so close to Pluto instead of using a high-tech telescope to capture images of its surface? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/krcFo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;The Icy Mountains of Pluto. Image credit: &lt;a href=&quot;http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?gallery_id=2&amp;amp;image_id=229&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons / Johns Hopkins Applied Physics Laboratory&lt;/a&gt;, July 15, 2015&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I always imagined its possible to take at least low-quality pictures of tiny little things from far far away using a space telescope. But to be honest, I got confused seeing latest pictures of Pluto taken by New Horizons, and I actually don't understand why we couldn't take such low-quality images without sending an entire probe close to Pluto?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that it's impossible to take such picture from this angle from Earth or telescopes close to Earth, but the image quality is poor comparing to Hubble images (the ones taken from deep space) or other similar images of other planets (like Mars, Saturn or Moon or even planets from other solar systems).&lt;/p&gt;&#xA;" OwnerUserId="10865" LastEditorUserId="10865" LastEditDate="2017-01-07T17:35:29.777" LastActivityDate="2017-01-07T17:35:29.777" Title="Why is it hard to take a high quality image of far away objects (like Pluto) using a high-tech space telescope?" Tags="|nasa|probe|new-horizons|pluto|space-telescope|" AnswerCount="5" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9971" PostTypeId="2" ParentId="9970" CreationDate="2015-07-16T12:56:57.817" Score="22" Body="&lt;p&gt;For getting really high resolution stuff, you need to get outside of the atmosphere. The best instrument to do this is the Hubble Space Telescope. The resolution of Hubble is about 0.05 arc seconds. Pluto is currently around 3.5 billion miles. That gives a resolution of 850 miles or so. This is limited by the size of the mirror that Hubble has, which is about 2m wide. This is all very rough, mind you. Pluto is a rather small object, of 1470 miles, give or take. So Pluto is only a few pixels from Hubble. This was further explained by &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/02141014-hubble-galaxy-pluto.html&quot; rel=&quot;noreferrer&quot;&gt;the Planetary Society&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The absolute best image that came before New Horizons was done by watching Pluto for a long time, and doing some very complex stuff, to come up with &lt;a href=&quot;http://hubblesite.org/newscenter/archive/releases/2010/06/image/a/format/web/&quot; rel=&quot;noreferrer&quot;&gt;the following&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/J24x6.jpg&quot; alt=&quot;Pluto from Hubble&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all, I heard it cited that until 9 days from the closest approach, Hubble could do a better job of imaging Pluto than New Horizons. (Although that is with the super resolution image, as &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/0218-new-horizons-pluto-better-hubble.html&quot; rel=&quot;noreferrer&quot;&gt;Emily Lakdawalla&lt;/a&gt; points out, it actually started near the beginning of this year. The super resolution with Hubble is easier because we have more data from it, and more accurate data.) And most of the images we've seen so far have been from before that period of time. Even then, while some of the features from the Hubble composite are visible (In particular, the Heart), the detail has been better for a number of days for New Horizons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The images that were captured, and will be sent to Earth soon, are of much higher resolution than what has been seen so far. It will take as long as 18 months to get all of the data from the flyby to the ground. Some of the high resolution images are already showing up, more of them will come.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The summary is that, yes, we have amazing cameras near Earth, but the distance is astronomical to Pluto, requiring sending a spacecraft close to get a good image.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="168" LastEditDate="2015-07-16T22:24:25.030" LastActivityDate="2015-07-16T22:24:25.030" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9972" PostTypeId="2" ParentId="9970" CreationDate="2015-07-16T12:57:36.513" Score="40" Body="&lt;p&gt;Distance. Size of the target. Its poor albedo at such distance to its only source of illumination, the Sun, compared to closer celestial bodies. And movement of the target and the vantage point in their orbits preventing advanced image interpolation techniques combining multiple observations of same side of Pluto at same lighting conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto is currently 31.9 AU (astronomical units, 1 AU is average distance between the Earth and the Sun) away from Earth, where currently highest spatial resolution orbital optical telescope, the Hubble Space Telescope (HST) orbits around it. In other words, Pluto is nearly 32 times farther away from us, and the Hubble, than the Sun is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/Gi1fq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Distance to Pluto to scale. Image source: &lt;a href=&quot;http://www.solarsystemscope.com/&quot; rel=&quot;noreferrer&quot;&gt;Solar System Scope&lt;/a&gt;, using realistic (non-orrery) model and sizes to scale&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HST's highest resolution imager, ESA's Faint Object Camera has spatial resolution of 0.0072 arcsecond pixels and can produce roughly 16x16 pixel images of Pluto from such a distance:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/T2BEe.gif&quot; alt=&quot;ESA Faint Object Camera&amp;#39;s view of Pluto&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;ESA Faint Object Camera's views of Pluto and computed interpolations  (Credit: ESA)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, even those 16x16 pixels are badly stricken by &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_resolution#Explanation&quot; rel=&quot;noreferrer&quot;&gt;Rayleigh limit&lt;/a&gt; of HST's &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassegrain_reflector&quot; rel=&quot;noreferrer&quot;&gt;Cassegrain&lt;/a&gt; type optics, but I'll spare you with technical details. Point is, that for anything better than what HST can produce, we'd either require an even larger telescope than the Hubble, or what we've done with New Horizons, move smaller ones much, much closer to its target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, is the image you attach to your question really of poor quality? It resolves features that are smaller than a mile (1.6 km) across. And it also took other images resolving detail up to 70 meters across. We'll just have to wait for them. With its transmission rate of ~ 2 kb/s, it will take nearly 1 and a half years for it to send all the data it took during the flyby back to Earth.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-16T13:41:32.580" LastActivityDate="2015-07-16T13:41:32.580" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9973" PostTypeId="2" ParentId="9966" CreationDate="2015-07-16T14:17:21.873" Score="7" Body="&lt;p&gt;Not really, but by being really careful one can get a reasonably close approximation. There are a number of cameras on board, with different wavelengths. The ones that cover visible or near visible wavelenths are (&lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Spacecraft/Payload-Technical-Specifications.php&quot;&gt;Source&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;RALPH &quot;Blue&quot; channel 400-550 nm.&lt;/li&gt;&#xA;&lt;li&gt;RALPH &quot;RED&quot; channel- 540-700 nm.&lt;/li&gt;&#xA;&lt;li&gt;RALPH &quot;Pan&quot; (Clear filter)- 450-1000 nm&lt;/li&gt;&#xA;&lt;li&gt;LORRI- 350-850 nm&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, so what qualifies as &quot;Natural Color&quot;? I'm going to say that is best done close to the human eye response. &lt;a href=&quot;https://en.wikipedia.org/wiki/Color_vision&quot;&gt;Wikipedia&lt;/a&gt; has a table with these response values:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;S 400–500 nm &lt;/li&gt;&#xA;&lt;li&gt;M 450–630 nm&lt;/li&gt;&#xA;&lt;li&gt;L 500–700 nm &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note that there is considerable overlap between the S and M. These are basically blue/green. Given all of this, RALPH can produce a good 2 color approximation in human wavelength, but not really all 3 colors. Some good approximations can be made, however, if one really takes the time to get it close. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not natural color? The colors that were selected were chosen to give the most interesting scientific results. Near infrared gives some very interesting information that can't be picked out in visible. Also, the blue overlaps so much with the green that there is little new information by including both bands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, even most color imagers don't quite get the color right. I won't post the image (Not sure about the copyright), but &lt;a href=&quot;https://www.maxmax.com/spectral_response.htm&quot;&gt;this site&lt;/a&gt; contains a the spectral response of the Canon 40d camera, a moderately high end color digital camera. It sees far beyond human vision cutoff (700nm), although in a camera this is typically filtered out at the sensor level with an IR filter. Still, few people would doubt that this camera produces near true color images.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, New Horizons doesn't have a &quot;True Color&quot; image, but it can make some pretty close approximations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://starizona.com/acb/ccd/advtheorycolor.aspx&quot;&gt;Starizona's Guide&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user12" LastEditDate="2015-07-16T17:43:06.037" LastActivityDate="2015-07-16T17:43:06.037" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="9974" PostTypeId="1" AcceptedAnswerId="9981" CreationDate="2015-07-16T14:39:57.887" Score="122" ViewCount="8848" Body="&lt;p&gt;For the purposes of this question, we will consider a &quot;lander&quot; any probe which was sent to the surface of an extraterrestrial body or entered the atmosphere to a point which exceeds an air pressure of 5 x 10&lt;sup&gt;-7&lt;/sup&gt; kPa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question simply seeks the identity of the extraterrestrial bodies which have hosted such &quot;landers&quot;, but the number and identity of the &quot;landers&quot; for each body would make for a better answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus points for anyone who includes an example picture &lt;em&gt;taken by&lt;/em&gt; a &quot;lander&quot; for each body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Extra bonus points for anyone who includes an example picture &lt;em&gt;taken by&lt;/em&gt; each &quot;lander&quot; for each body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surface pictures are preferred.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-07-16T15:57:51.207" LastActivityDate="2021-03-11T04:19:11.283" Title="How many extraterrestrial bodies have hosted a &quot;lander&quot; which returned images?" Tags="|probe|lander|photography|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9975" PostTypeId="2" ParentId="9970" CreationDate="2015-07-16T15:48:19.017" Score="5" Body="&lt;p&gt;For over forty years, there have been man made objects sitting on the moon that are not exactly tiny and it is only quarter of a million miles away, but even our best telescopes have been unable to get a decent photo of them. Pluto is between two and a half, and four and a half &lt;em&gt;billion&lt;/em&gt; miles away. &lt;/p&gt;&#xA;" OwnerUserId="10877" LastActivityDate="2015-07-16T15:48:19.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9976" PostTypeId="2" ParentId="9970" CreationDate="2015-07-16T17:16:13.137" Score="62" Body="&lt;p&gt;I think a lot of folks see these gorgeous photos of distant galaxies, with fine detail on dust lanes and spiral arms and assume that since they’re so far away, seeing Pluto would be easy.  But while these galaxies are far away, they’re also &lt;em&gt;huge&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The (relative) detail that can be seen in any given telescope will be found by the object’s size divided by the distance from the telescope.  In other words, the angular size in the sky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\theta = \frac{\mathrm{size}}{\mathrm{distance}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let’s compare the angular size of a distant galaxy with Pluto.  A nice spiral galaxy (like ours) is about 100,000 light years in extent.  We’ll make it old and put it at 10 billion light years away.  How big is it in a telescope?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\theta = \frac{10^5\thinspace\mathrm{ly}}{10^9\thinspace\mathrm{ly}}$$&#xA;$$\theta = 10^{-5}\thinspace\mathrm{radians}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, how much can we see Pluto?  It has a diameter of ~2400km, and when farthest from Earth is around 48AU in distance.  So it has an angular size of&#xA;$$\theta = \frac{2.4\times 10^6\thinspace\mathrm{m}}{7.2\times 10^{12}\thinspace\mathrm{m}}$$&#xA;$$\theta = 3.3\times 10^{-7}\thinspace\mathrm{radians}$$&#xA;$$\text{linear ratio} = 30:1$$&#xA;$$\text{area (square) ratio} = 900:1$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that compared to a fuzzy distant galaxy, Pluto covers about 3 orders of magnitude less angular area in the sky.  No wonder that it’s more difficult to image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, at these angular scales you run into the &lt;a href=&quot;http://astro.cornell.edu/academics/courses/astro201/diff_limit.htm&quot; rel=&quot;noreferrer&quot;&gt;diffraction limit&lt;/a&gt;.  For visible light and for an aperture the size of Hubble’s, you can’t expect to resolve features that are significantly smaller than about $2.5 \times 10^{-7}$ radians across, which is comparable to the angular size of Pluto itself.  To do better from Earth orbit, you'd need to get a much larger mirror.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, if you look at this image from NASA showing the Hubble extreme deep field:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/3Oa4V.jpg&quot; alt=&quot;Hubble XDF&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;at full size (976 pixels wide), then by my calculations the most zoomed in portion in the upper right is about $2.5 \times 10^{-7}\thinspace\mathrm{radians/pixel}$.  In such a picture, Pluto would be just a bit bigger than 1 pixel.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastEditorUserId="268" LastEditDate="2015-07-17T21:00:29.910" LastActivityDate="2015-07-17T21:00:29.910" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="9977" PostTypeId="2" ParentId="9970" CreationDate="2015-07-16T17:25:27.423" Score="13" Body="&lt;p&gt;To add to some already great answers, I'd like to toss in the basic physics of photography that you can experience here on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you take a picture of something, you're collecting information in the form of light that has been reflected off of it. In a bright, well-lit environment where you're taking pictures up-close or have a huge lens, you can collect enough light to make the detail of the image apparent in a fraction of a second. When you shoot in low-light, the aperture of the camera has to stay open longer to collect enough light to make the detail discernible (for an easy example, think of how it takes time for your eyes to adjust to low light enough for you to see well in it - imagine a camera shutter having to stay open that long to collect that much light to see clearly as well), so the object you're shooting has to stay perfectly still during that time or the picture will end up blurred. The same effect applies when you're shooting something very far away, even if it's well-lit, because a smaller amount of reflected light is making its way to your lens (the light isn't all reflecting directly to you, it's going in many directions and spreading out, weakening).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With Pluto we're combining both of those problems, extremely low light and extremely long distance. We receive such a tiny amount of reflected light from Pluto here on Earth that it's not even visible without excellent optics, and then virtually only as a speck of light. In order to get a &quot;higher-resolution&quot; image from our vantage point, we'd have to make a single exposure of Pluto last for weeks or longer (compared to the fraction of a second required to take pictures here), but over the course of those weeks, the Earth is orbiting the sun, Pluto is orbiting the sun, Pluto is rotating, etc. so that basically &lt;em&gt;nothing is holding still for your shot&lt;/em&gt;. We can do some amount of correction for this but it's still limited in its effectiveness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are only two solutions. Build a bigger telescope to collect more reflected light in less exposure time, or get closer. The size of telescope required to take higher-resolution images would be &lt;em&gt;enormous&lt;/em&gt; and vastly less cost-effective than simply bolting a camera on a metal box and slinging it at Pluto as fast as humanly possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apologies to the New Horizons team for the astonishing over-simplification of their mission... I'm very much in awe of their work.&lt;/p&gt;&#xA;" OwnerUserId="10557" LastEditorUserId="10557" LastEditDate="2015-10-15T14:34:39.723" LastActivityDate="2015-10-15T14:34:39.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9978" PostTypeId="1" CreationDate="2015-07-16T17:51:02.463" Score="5" ViewCount="774" Body="&lt;p&gt;Why has no one launched a probe that goes into a highly elliptical orbit around the sun ( on a path that goes beyond Pluto without escaping the sun's gravitational field) and capture images as it tours the deep space and deliver all the data when it's on it's approach back to Sun? &lt;/p&gt;&#xA;" OwnerUserId="10851" LastEditorUserId="25" LastEditDate="2015-07-16T18:50:10.097" LastActivityDate="2015-07-16T20:00:50.347" Title="Has any probe ever been launched into a highly elliptical orbit with say 20-30 year to orbit the sun?" Tags="|probe|deep-space|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9979" PostTypeId="2" ParentId="9978" CreationDate="2015-07-16T18:11:06.483" Score="5" Body="&lt;p&gt;I'm guessing it's because 'delivering the data when it returns' isn't as good as it sounds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We're receiving data just fine from New Horizons (albeit slowly), and I think this will always outweigh the benefits of a spacecraft physically coming closer. Light travels very, very fast, and having the point of transmission closer doesn't make a &lt;em&gt;huge&lt;/em&gt; difference to the data downlink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main point, however, is that &lt;strong&gt;the spacecraft will eventually become useless&lt;/strong&gt; in it's heliocentric orbit, because Pluto would have moved on when the probe returns to aphelion on it's second orbit. A flyby probe like New Horizons can go on and collect data from other Kuiper Belt Objects, and later outside the &#xA;heliosphere. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/FTDpH.jpg&quot; alt=&quot;New Horizons&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; The trajectory of New Horizons, on a path to escape the Solar System after flying past Pluto, and hopefully exploring new worlds beyond. (Courtesy &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Where-is-New-Horizons/index.php&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;) &lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-07-16T18:11:06.483" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9980" PostTypeId="2" ParentId="9978" CreationDate="2015-07-16T18:35:53.773" Score="14" Body="&lt;p&gt;The closest to this design was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ulysses_(spacecraft)&quot;&gt;Ulysses probe&lt;/a&gt;, which ended up in a heliocentric orbit with a semi-major axis of 3.37 AU and an eccentricity of 0.603, and an orbital period of 6.2 years.  (&lt;a href=&quot;http://www.cosmos.esa.int/web/ulysses/orbital-elements&quot;&gt;Orbital elements available here.&lt;/a&gt;)  It was launched in 1990, and operated until about 2008.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the point of this probe wasn't to take pictures from deep space, but rather to get pictures of the Sun—specifically, the sun's poles.  To get a spacecraft into a polar orbit, you would have to get rid of a large amount of momentum in the plane of the Solar System and add a bunch of momentum perpendicular to the plane;  and as we all know, the best way to get a big delta-v is to head out to Jupiter.  So the craft went out to Jupiter, and used its gravity to slingshot itself out of the plane of the Solar System.  (If you do the math, the perihelion of Ulysses's orbit is at about 5.4 AU, which you might recognize as the orbital radius of Jupiter.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ulysses did some science when it swung by Jupiter the first time, but most of its subsequent data collection was confined to the times (over three orbits) when it was over the Sun's poles.  When it got back to Jupiter after its first orbit, of course, Jupiter had gone on around its orbit, and there wasn't much there to see.  (It did apparently do some &quot;distant observations&quot; of Jupiter at the time, but it wasn't a close flyby—&lt;a href=&quot;http://www.cosmos.esa.int/web/ulysses/mission-status-reports#200401&quot;&gt;about 0.8 AU away.&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="10884" LastEditorUserId="25" LastEditDate="2015-07-16T18:47:28.290" LastActivityDate="2015-07-16T18:47:28.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9981" PostTypeId="2" ParentId="9974" CreationDate="2015-07-16T18:38:15.860" Score="221" Body="&lt;p&gt;To be honest, I don't know how to measure atmospheric pressure, so I cannot discriminate on that basis. Initially my answer only included unmanned missions, but based on suggestions I've decided to add the Apollo missions as well.&lt;/p&gt;&#xA;&lt;h2&gt;Moon&lt;/h2&gt;&#xA;&lt;p&gt;The first landers performed intentionally hard landings and did not get pictures from the surface. I want to include them because they were the first man-made objects to land on The Moon:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Luna 2 (Soviet Union - impacted 13 September 1959)&lt;/li&gt;&#xA;&lt;li&gt;Ranger 4 (USA - impacted 26 April 1962)&lt;/li&gt;&#xA;&lt;li&gt;Ranger 6 (USA - impacted 2 February 1964)&lt;/li&gt;&#xA;&lt;li&gt;Ranger 7 (USA - impacted 31 July 1964)&lt;/li&gt;&#xA;&lt;li&gt;Ranger 8 (USA - impacted 20 February 1965)&lt;/li&gt;&#xA;&lt;li&gt;Ranger 9 (USA - impacted 24 March 1965)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The first attempts at soft landings failed, but they still did land (crashed) on the surface:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Luna 5 (Soviet Union - impacted 12 May 1965)&lt;/li&gt;&#xA;&lt;li&gt;Luna 7 (Soviet Union - impacted 7 October 1965)&lt;/li&gt;&#xA;&lt;li&gt;Luna 8 (Soviet Union - impacted 6 December 1965)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Unmanned Missions&lt;/h3&gt;&#xA;&lt;p&gt;The first successful soft landing was made by the Soviet Union's &lt;strong&gt;Luna 9&lt;/strong&gt; mission. It landed on the Lunar surface on 3 February 1966 and lasted for six days:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vBJMc.jpg&quot; alt=&quot;Luna 9&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://mentallandscape.com/C_CatalogMoon.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;USA had its first successful soft landing shortly after with its &lt;strong&gt;Surveyor 1&lt;/strong&gt; mission. It landed on 2 June 1966 and survived for 65 hours:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/61OuL.jpg&quot; alt=&quot;Surveyor 1&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_1&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Luna 13&lt;/strong&gt; was the next lander to make it to the Lunar surface. The Soviet lander landed on 24 December 1966 and survived six days.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Ye3R0.jpg&quot; alt=&quot;Luna 13&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://mentallandscape.com/C_CatalogMoon.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Surveyor 3&lt;/strong&gt; was sent by USA and landed on 20 April 1967. It was also the only lander (so far, as of May 7, 2018) to be later visited by astronauts. Its mission lasted about 14 days.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/m7cjj.jpg&quot; alt=&quot;Surveyor 3&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_3&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Surveyor 5&lt;/strong&gt; landed on the Moon on 11 September 1967. Its mission lasted 65 hours.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/7EF0R.jpg&quot; alt=&quot;Surveyor 5&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_5&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Surveyor 6&lt;/strong&gt; landed on the Moon on 10 November 1967. According to &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1967-112A&quot; rel=&quot;noreferrer&quot;&gt;NASA's website&lt;/a&gt;, Surveyor 6 was put in hibernation mode on 26 November 1967. It was awakened on 14 December 1967, but no useful data was received, and communication was permanently lost that day.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/jvIet.jpg&quot; alt=&quot;Surveyor 6&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_6&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Surveyor 7&lt;/strong&gt; landed on the Moon on 10 January 1968. It performed its mission collecting images and scientific data until the mission was put on hold on 26 January 1968 because the lunar day had ended where the lander was. The mission resumed on the following lunar day on 12 February 1968 until the mission ended on 21 February 1968.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/gnBKp.jpg&quot; alt=&quot;Surveyor 7&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_7&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Luna 16&lt;/strong&gt; was the next successful (unmanned) mission to the Moon. It landed on 24 September 1970 and survived for 12 days. This was the first successful robotic mission that returned a sample from the Lunar surface. &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1970-072A&quot; rel=&quot;noreferrer&quot;&gt;NASA's website&lt;/a&gt; says that according to an observatory in Germany, good quality pictures were sent back from Luna 16, but those images were never made available.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;strong&gt;Luna 17/Lunokhod 1&lt;/strong&gt; mission was the first rover mission. It landed on 17 November 1970, and the mission continued for 322 days.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/sAj9X.jpg&quot; alt=&quot;Luna 17/Lunokhod 1&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://mentallandscape.com/C_CatalogMoon.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Luna 20&lt;/strong&gt; was another sample return mission. It landed on 21 February 1972 and returned to Earth on 25 February 1972.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/TIWLP.jpg&quot; alt=&quot;Luna 20&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://mentallandscape.com/C_CatalogMoon.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;The &lt;strong&gt;Luna 21/Lunokhod 2&lt;/strong&gt; mission sent another rover to the Moon. It landed on 8 January 1973 and survived 146 days.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Esqyw.jpg&quot; alt=&quot;Luna 21/Lunokhod 2&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://mentallandscape.com/C_CatalogMoon.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Luna 24&lt;/strong&gt; was the final mission sent in the Luna Program. It landed on 18 August 1976 and returned a sample to Earth on 22 August 1976. I haven't yet found an image taken by Luna 24 on the surface, and from what I've been able to gather so far it doesn't seem as though it was equipped to take pictures.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Chang'e 3&lt;/strong&gt; is the first Chinese lander to make it to the Moon, and the first soft landing since 1976. It landed on 14 December 2013. The lander survived for about a year, but the rover only survived for about three months. This picture was taken of the lander by the rover:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/UTqtL.jpg&quot; alt=&quot;Chang'e 3&quot; /&gt;&lt;br /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_3&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&#xA;&lt;br /&gt;&lt;br /&gt;&lt;br /&gt;&lt;/p&gt;&#xA;&lt;h3&gt;Apollo Missions&lt;/h3&gt;&#xA;&lt;p&gt;&lt;strong&gt;Apollo 11&lt;/strong&gt; was the famous first manned landing on the lunar surface. Neil Armstrong and Buzz Aldrin, on 20 July 1969, were the first two people to step foot on the Moon. Michael Collins was the other astronaut in this mission.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/e9MJU.gif&quot; alt=&quot;Apollo 11&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo11info.html&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Apollo 12&lt;/strong&gt; landed on the Moon on 19 November 1969. Charles Conrad Jr, Richard F. Gordon, and Alan L. Bean were the three astronauts on this mission. Conrad and Bean were the two astronauts to walk on the surface during this mission.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/ftMqy.gif&quot; alt=&quot;Apollo 12&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo12info.html&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Apollo 14&lt;/strong&gt; landed on 5 February 1971. Alan B. Shepard Jr, Stuart A. Roosa, and Edgar D. Mitchell were the participants in this mission. Shepard and Mitchell were the two astronauts who walked on the surface.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/RnvbU.jpg&quot; alt=&quot;Apollo 14&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_14&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Apollo 15&lt;/strong&gt; landed on 30 July 1971. David R. Scott, Alfred M. Worden, and James B. Irwin participated in this mission. Scott and Irwin walked on the lunar surface, and they were the first to drive a NASA lunar rover.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/xDJUA.jpg&quot; alt=&quot;Apollo 15&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Apollo 16&lt;/strong&gt; landed on 21 April 1972. John W. Young, Thomas K. Mattingly II, and Charles M. Duke Jr. were the three astronauts on this mission. Young and Duke were the two to walk on the surface in this mission, and they drove the second NASA lunar rover.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/ctxdN.jpg&quot; alt=&quot;Apollo 16&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_16&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Apollo 17&lt;/strong&gt; landed on 11 December 1972, and was the last manned mission to land on the lunar surface. Eugene A. Cernan, Ronald E. Evans, and Harrison H. Schmitt were the astronauts on this mission. Cernan and Schmitt were the two to walk on the surface for this mission, and they drove the third NASA lunar rover.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/QpEAq.jpg&quot; alt=&quot;Apollo 17&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_17&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Mercury&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;Messenger&lt;/strong&gt; did technically land on Mercury, although it was a hard landing. I want to include it because it is the only thing we've sent to the surface of Mercury, and although it didn't take pictures &lt;em&gt;from&lt;/em&gt; the surface, it did take pictures &lt;em&gt;of&lt;/em&gt; the surface.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/neVrD.jpg&quot; alt=&quot;Messenger&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/MESSENGER&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Venus&lt;/h2&gt;&#xA;&lt;p&gt;The Soviets are the only ones who ever successfully performed a soft landing on Venus, and they were successful in several Venera and Vega missions. NASA also sent two orbiters to Venus as part of the Pioneer project. The second orbiter, &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/pioneer_venus.html&quot; rel=&quot;noreferrer&quot;&gt;Pioneer Venus Multiprobe&lt;/a&gt;, included four probes (one large, and three small) that were deployed to analyze the atmosphere. The large probe was deployed on 16 November 1978, and the three small probes were deployed on 20 November, 1978. While these probes were not meant to be landers, one of the small probes did survive the impact with the surface and continued to transmit information for more than an hour before it perished. They were not equipped with cameras, so they took no pictures.&lt;/p&gt;&#xA;&lt;p&gt;On 15 December 1970, The Soviet Union had its first successful landing on the surface of Venus with the &lt;strong&gt;Venera 7&lt;/strong&gt; mission. It was the first successful soft landing on another planet. &lt;strong&gt;Venera 8&lt;/strong&gt; landed on the surface of Venus on 22 July 1972. Neither of these missions returned any images.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Venera 9&lt;/strong&gt; landed on the surface on 22 October 1975, and returned the first image from Venus. This was the first image ever sent back from the surface of another planet.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/1p2SU.jpg&quot; alt=&quot;Venera 9&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://www.strykfoto.org/venera.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Venera 10&lt;/strong&gt; landed on 25 October 1975, and also returned images from the surface of Venus.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/D3nhg.jpg&quot; alt=&quot;Venera 10&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://www.strykfoto.org/venera.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Venera 11&lt;/strong&gt; and &lt;strong&gt;Venera 12&lt;/strong&gt; landed on Venus on 21 December 1978 and 25 December 1978. They were equipped to take images, but their lens covers did not separate, so images were not taken.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Venera 13&lt;/strong&gt; landed on Venus on 1 March 1982. It was the first mission to return color images from the surface of Venus.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/LnoLy.jpg&quot; alt=&quot;Venera 13&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://www.strykfoto.org/venera.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Venera 14&lt;/strong&gt; landed on 5 March 1982. It also returned color images.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/Z5oJf.jpg&quot; alt=&quot;Venera 14&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://www.strykfoto.org/venera.htm&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Vega 1&lt;/strong&gt; and &lt;strong&gt;Vega 2&lt;/strong&gt; missions sent landers to Venus on 11 June 1985 and 15 June 1985, but there were no cameras on board so no images were returned.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Mars&lt;/h2&gt;&#xA;&lt;p&gt;The Soviet Union sent the first three missions that landed on the surface, but because of complications none of them returned complete images &lt;em&gt;(Mars 3 did attempt to return &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Mars_3_Image.png/800px-Mars_3_Image.png&quot; rel=&quot;noreferrer&quot;&gt;an image&lt;/a&gt; (Wikipedia) that some consider to be of the horizon, but it was unintelligible. On the subject, Wikipedia says &amp;quot;According to the Soviet Academy of Sciences there is nothing, horizon or otherwise, identifiable in the photograph&amp;quot;)&lt;/em&gt;. These missions were &lt;strong&gt;Mars 2&lt;/strong&gt; (27 November 1971), &lt;strong&gt;Mars 3&lt;/strong&gt; (2 December 1971), and &lt;strong&gt;Mars 6&lt;/strong&gt; (12 March 1974).&lt;/p&gt;&#xA;&lt;p&gt;The &lt;strong&gt;Viking 1&lt;/strong&gt; lander was the first to send back images from the surface of Mars. It landed on 20 July 1976 and survived for 1824 days.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/6obJy.png&quot; alt=&quot;Viking 1&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_1&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Viking 2&lt;/strong&gt; landed on the surface on 3 September 1976. It operated for 1316 days.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/I3P11.png&quot; alt=&quot;Viking 2&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_2&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Mars Pathfinder/Sojourner&lt;/strong&gt; mission landed on 4 July 1997. Mars Pathfinder was the name of the lander, and Sojourner was the rover. This was the first rover on Mars.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/zXsjb.jpg&quot; alt=&quot;Mars Pathfinder/Sojourner&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Pathfinder&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;Part of ESA's &lt;strong&gt;Mars Express&lt;/strong&gt; mission was to deploy a British lander named &lt;strong&gt;Beagle 2&lt;/strong&gt;. It was deployed from Mars Express on 19 December 2003 and was scheduled to land on 25 December 2003, but communication with Beagle 2 was never established. The most current explanation is that the solar panels did not deploy properly. Because the communication equipment was housed below the solar panels, Beagle 2 was unable to communicate.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Spirit&lt;/strong&gt; landed on Mars on 3 January 2004 and survived 2269 days.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/ca7B2.jpg&quot; alt=&quot;Spirit&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Spirit_(rover)&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Opportunity&lt;/strong&gt; landed on Mars on 25 January 2004, three weeks after its sister, Spirit. Contact was lost after 10 June 2018.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/ZlfW3.jpg&quot; alt=&quot;Opportunity&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_(rover)&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Phoenix&lt;/strong&gt; landed near the north pole of Mars on 25 May 2008. Contact was lost on 2 November 2008.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/BZYRp.jpg&quot; alt=&quot;Phoenix&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Phoenix_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Curiosity&lt;/strong&gt; landed on 6 August 2012. It is still operational.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/5ZmVn.jpg&quot; alt=&quot;Curiosity&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_(rover)&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;InSight&lt;/strong&gt; landed on 26 November 2018. It is still operational.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/raNiI.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/raNiI.jpg&quot; alt=&quot;InSight&quot; /&gt;&lt;/a&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://mars.nasa.gov/insight/multimedia/raw-images/?order=sol%20desc%2Cdate_taken%20desc&amp;amp;per_page=50&amp;amp;page=0&amp;amp;mission=insight&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Perseverance&lt;/strong&gt; landed on 18 February 2021. It is still operational.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/A3whf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/A3whf.jpg&quot; alt=&quot;Perseverance&quot; /&gt;&lt;/a&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;https://mars.nasa.gov/resources/25699/perseverance-hazcam-first-drive/&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Jupiter&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;Galileo&lt;/strong&gt; was a probe that studied several bodies in the solar system, especially Jupiter and its moons. On 7 December 1995 it sent an atmospheric probe into Jupiter, and on 21 September 2003 it was sent into Jupiter as well. While these technically weren't landings, it's about as close as you can get on Jupiter (Check out &lt;a href=&quot;https://space.stackexchange.com/a/5041/10234&quot;&gt;TildalWave's answer&lt;/a&gt; on a question about standing on the &amp;quot;surface&amp;quot; of Jupiter).&#xA;&lt;img src=&quot;https://i.stack.imgur.com/wI7F5.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Top image is in visible light, bottom is in near-infrared.&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Titan&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;Huygens&lt;/strong&gt; was a probe sent to Saturn's Moon called Titan as part of the Cassini mission. It landed on the surface on 14 January 2005. It survived for 90 minutes following the landing. According to &lt;a href=&quot;http://www.nature.com/nature/journal/v438/n7069/full/nature04347.html&quot; rel=&quot;noreferrer&quot;&gt;Nature&lt;/a&gt;, it is thought that Huygens operated until the batteries were drained.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/q739J.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Huygens_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;433 Eros&lt;/h2&gt;&#xA;&lt;p&gt;A mission called &lt;strong&gt;NEAR Shoemaker&lt;/strong&gt; landed on this asteroid on 12 February 2001. What is interesting about this one is that it wasn't even intended to be a lander, but it was the first soft landing on an asteroid. The decision to attempt the landing was made during its nearly yearlong orbit around the asteroid. It survived the landing and continued to operate for about two more weeks. Communication was lost on 28 February 2001.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SqC7p.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NEAR_Shoemaker&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;25143 Itokawa&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;Hayabusa&lt;/strong&gt; was designed to make a very short landing on the surface of the asteroid so that it could return a sample. It made two very short landings on 19 November 2005 and on 25 November 2005. Hayabusa collected dust and returned it to Earth.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/w2oLU.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;http://www.space.com/28-hayabusa-japan-asteroid-mission.html&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;162173 Ryugu&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;Hayabusa2&lt;/strong&gt; (the later mission to Hayabusa) touched down on the asteroid 162173 Ryugu on October 25, 2018. The second touchdown happened on May 30, 2019. There are also 4 rovers on-board Hayabusa2 (HIBOU, OWL, MASCOT, Rover-2). The samples from 162713 Ryugu are planned to return to Earth on December 2020.&lt;/p&gt;&#xA;&lt;p&gt;Here's Hayabusa2's second touchdown on Ryugu.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J05Gw.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J05Gw.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;https://www.sciencenews.org/article/hayabusa2-touchdown-asteroid-ryugu-second-sample-subsurface&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here's one of the rovers (HIBOU) taking a picture of Ryugu during one of its hops (it didn't use wheels because the surface gravity is too low).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SvYyF.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SvYyF.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa2&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;More pictures of Ryugu from Hayabusa 2:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wrw4s.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wrw4s.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;https://astronomynow.com/2018/09/24/japans-hayabusa2-lands-to-rovers-on-asteroid/&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Tempel 1&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;Deep Impact&lt;/strong&gt; was sent to impact the comet Tempel 1 on 4 July 2005 to study its interior. Because of how it impacted, it wasn't able to take pictures &lt;em&gt;from&lt;/em&gt; the surface, but did take pictures of the surface just before the impact.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/x6O8L.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tempel_1&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Comet 67P/Churyumov–Gerasimenko&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;Philae&lt;/strong&gt; is a lander deployed by the Rosetta satellite, and was the first to make a soft landing on a comet. Its landing was on 12 November 2014.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/TOltP.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://blogs.discovermagazine.com/imageo/2014/11/11/4-billion-mile-journey-just-half-marathon-go-philae/#.Vaf47PlViko&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2021-03-11T04:19:11.283" LastActivityDate="2021-03-11T04:19:11.283" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="9982" PostTypeId="2" ParentId="9978" CreationDate="2015-07-16T18:55:54.370" Score="6" Body="&lt;p&gt;There is not a spacecraft that does exactly as you suggest. The closest was probably Cassini, which was in an elliptical orbit to achieve orbit around Saturn, or Ulysses, which was used to orbit the Sun's poles, as @MichaelSeifert explained. Why not? There are quite a few reasons. Let's look at a few.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The way that you have an elliptical orbit instead of an escape trajectory is a result of speed. An escape trajectory is a fast trajectory. For New Horizons to just meet Pluto and return home, it would take about a hundred years (About half of it's orbital period, which is the general rule of thumb for such). That's a really long time, and people wouldn't want to wait so long for their data.&lt;/li&gt;&#xA;&lt;li&gt;While sending data back quicker is nice, it is not essential. New Horizons will be able to send it's data back over 18 months. Even if it could somehow bounce and return home at the same speed that it went there, it wouldn't be able to send the data back much quicker, as it would still be far away during that time.&lt;/li&gt;&#xA;&lt;li&gt;There are other potentially useful missions for New Horizons to accomplish afterward.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it's faster, cheaper, and better to just let the spacecraft keep going it's way. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-16T18:55:54.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9983" PostTypeId="2" ParentId="9978" CreationDate="2015-07-16T20:00:50.347" Score="8" Body="&lt;p&gt;A 30-year mission is difficult to engineer for. The Voyagers are still alive (more or less) after 30 years, but that's as much luck as skill.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The probe would also have to be large:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it needs enough fuel to maneuver close to anything interesting (e.g. a KBO or comet)  .&lt;/li&gt;&#xA;&lt;li&gt;it needs a radar to find those objects. Radars are large and use lots of power (for the distances you need, dozens of kilowatts if not megawatts) so you need a nuclear reactor.&lt;/li&gt;&#xA;&lt;li&gt;sample return (the biggest reason for designing a probe to return) is complicated and we've only done it a few times using unmanned probes. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Voyager is just about the largest object we can send to the Kuiper Belt at the moment, this mission would need to be many times heavier.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we're talking about a massively expensive mission, with a mission objective that's really vague and has a low probability of success.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the realities of planetary exploration (only a few missions per year, tight budget constraints), only the missions that are most likely to succeed will be funded. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-16T20:00:50.347" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9984" PostTypeId="1" AcceptedAnswerId="9985" CreationDate="2015-07-16T20:22:02.640" Score="6" ViewCount="991" Body="&lt;p&gt;The power that deep space probes have allows an RF communication capacity of a few kbps from a range like Pluto-Earth. If we go into optics, the range will be better. But, of course, we want to send scouts (small probes) to far away galaxies. This would potentially require having a chain of communication relays (or repeaters, let's say).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can we send these repeaters, and at what periods, so that they always keep moving like a train, and so that we keep communications with the probe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(irrelevant, but, the repeaters would probably be smaller, but they should be able to travel the same speeds as the probe, and should have one beam towards the probe (or the forward Repeater), and another beam towards the behind repeater)&lt;/p&gt;&#xA;" OwnerUserId="10897" LastActivityDate="2015-07-19T17:43:09.477" Title="How can we form a data-communication relay chain for deep space exploration?" Tags="|communication|deep-space|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="9985" PostTypeId="2" ParentId="9984" CreationDate="2015-07-16T20:53:07.227" Score="5" Body="&lt;p&gt;If you want to send a probe to another &lt;em&gt;star&lt;/em&gt; with the kind of technology we can think about (if not design and build) today, you're talking about a mission lasting hundreds of years; the nearest stars are more than 8000 times farther away than Pluto. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We don't have the slightest idea how to send a probe to the next galaxy over, let alone &quot;far away galaxies.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in the general case, by simply sending relays on a following course at fixed intervals behind the prime probe, I believe you can turn the inverse square law problem into an approximately inverse linear one -- 100 relays each with transmission power &lt;em&gt;x&lt;/em&gt; can transmit a signal 10 times as far as a single probe of power 100 &lt;em&gt;x&lt;/em&gt;. For an interplanetary mission conducted in a reasonable amount of time, the delta-v provided by gravity assist from a Jupiter flyby is a relatively small component; perhaps the primary probe with a heavy scientific payload could use Jupiter to match the speed of the lighter relays which would be launched following it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On an interstellar mission, solar power would be out of the question (unless you consider an absurdly vast number of really low power relay stations), so lifetime of the relays' power sources would be a huge constraint. RTGs are the power supply of choice for our outer system probes, and their performance degrades over a few decades. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-07-17T18:27:58.537" LastActivityDate="2015-07-17T18:27:58.537" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9986" PostTypeId="2" ParentId="9984" CreationDate="2015-07-16T21:39:50.333" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://maccone.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Claudio Maccone&lt;/a&gt; has been pushing for cleverly exploiting physics of radiowave and light propagation, namely, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_lensing&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational lensing&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main idea (attributed to Von Eshleman from Stanford - 1979) is to send a probe (or several) to the nearest star AND to send a large space radiotelescope in an opposite direction relatively to the Sun, 550 to 740 AU away. The Sun would focus the transmission from the probe, thus gaining around 57.5 dB at the 21 cm wavelength - this gain cannot be obtained by DSN dishes on the Earth. Certainly, economy-wise it is better to send one radiotelescope and one probe instead of a thousand or a hundred probes spaced in time (for the same order of expected scientific results).&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;How lensing works&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2QBqo.png&quot; alt=&quot;FOCAL mission drawing 1&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;How to organize the interstellar comms network&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Lhpah.png&quot; alt=&quot;Not to scale&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Other technical tricks and tips to chirp (no pun intended) away at the problem&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Raise &lt;a href=&quot;https://en.wikipedia.org/wiki/EIRP&quot; rel=&quot;nofollow noreferrer&quot;&gt;EIRP&lt;/a&gt;. Instead of an RTG with decaying fuel, install a nuclear reactor with (say) sodium coolant, keep it turned off until flyby at the target star's planetary system, turn on and transmit all you want to transmit.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Make the communications asymmetrical (a laser downlink and microwave uplink), see the paper by Boone et al. (2002).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Another exotic line of thought relies on neutrinos. Sending a reactor will kinda help in generating &lt;strong&gt;some&lt;/strong&gt; neutrinos, but we have no way to generate enough neutrinos to cover the horrendously large distances, and no way to focus them. Sci-fi at its best, and the &quot;sci&quot; part isn't realistic at all. Of course, you can provide antimatter propulsion, and communicate via modulated thrust - but please bear in mind antimatter is prohibitively expensive.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Please also consider the amount of data you want to transmit. At interstellar distances your probe is essentially travelling in a straight line, and there's little if any opportunity to deflect the trajectory to aim precisely at the other star or any of its planets, if any unexpected anomaly pops up. Thus the flyby of the other star's system will be quite unlike the New Horizons - you'll be sending back mostly multispectral data from far away - a few dots here, a few dots there, spectral analysis of the atmosphere, summary of intercepted radio transmissions :) That's more or less it. No full color spy sat pictures, no live feeds. Thus, the need in high-speed comms is not as acute as you'd thought.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;References&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens Claudio Maccone. Springer, 2009.&lt;/li&gt;&#xA;&lt;li&gt;Von R. Eshleman. Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances. Science, 205 (1979). Pp.1133-1135. DOI &lt;a href=&quot;https://dx.doi.org/10.1126/science.205.4411.1133&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://dx.doi.org/10.1126/science.205.4411.1133&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Bradley G. Boone; Robert S. Bokulic; G. B. Andrews; Ralph L. McNutt, Jr. and Nicholas G. Dagalakis. Optical and microwave communications system conceptual design for a realistic interstellar probe, Proc. SPIE 4821, Free-Space Laser Communication and Laser Imaging II, 225 (December 1, 2002) DOI &lt;a href=&quot;https://dx.doi.org/10.1117/12.451807&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://dx.doi.org/10.1117/12.451807&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-07-19T17:43:09.477" LastActivityDate="2015-07-19T17:43:09.477" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="9987" PostTypeId="1" AcceptedAnswerId="9992" CreationDate="2015-07-16T22:40:29.703" Score="11" ViewCount="779" Body="&lt;p&gt;It's a pretty well known fact among the regulars of this site that New Horizons will take up to 18 months to completely upload the data from the Pluto encounter. When it was at Jupiter, it was much closer than it is today, and thus I would expect it would be able to upload the data faster. I expect that Jupiter had at least as much, if not more, data collected by New Horizons than at Pluto. How long did it take to download all of the data off of New Horizons after its Jupiter encounter?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="18732" LastEditDate="2019-01-08T05:31:10.657" LastActivityDate="2019-01-08T08:25:59.223" Title="How long did New Horizons take to upload all of its data from the Jupiter encounter?" Tags="|communication|new-horizons|jupiter|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="9988" PostTypeId="2" ParentId="9784" CreationDate="2015-07-16T23:58:59.397" Score="3" Body="&lt;p&gt;Michael Minovitch worked out in the early 1960s that a spacecraft which flies past a planet orbiting the Sun will gain or lose speed. It gains speed if it flies behind the planet and it loses speed if it flies in front. Gravity is responsible, combined with the planet's momentum as it moves around the Sun. The planet loses momentum, slows down, as the spacecraft speeds up. The planet gains momentum, speeds up, as the spacecraft slows down. Thus nature balances its books. It's not really complicated, though folks have done their best to make it that way.&lt;/p&gt;&#xA;" OwnerUserId="10905" LastActivityDate="2015-07-16T23:58:59.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="9989" PostTypeId="1" AcceptedAnswerId="10127" CreationDate="2015-07-17T02:09:39.150" Score="4" ViewCount="704" Body="&lt;p&gt;The New Horizons team &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-to-release-new-pluto-images-science-findings-at-july-17-nasa-tv-briefing&quot; rel=&quot;nofollow noreferrer&quot;&gt;released&lt;/a&gt; the following photo of Charon today&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Fzf2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This choice of image seems rather odd to me for a number of reasons. First of all, according to &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/06240556-what-to-expect-new-horizons-pluto.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Emily&lt;/a&gt;'s schedule, there were 3 images slated to come down from Charon. These 3 images were chosen months ago, with some slight tweaks based off of the timing. LORRI is a square imaging camera. The fact that the image was rectangular seems odd to me. I've confirmed with the provided data that the height of this image is 1024 pixels, as expected from LORRI. However, the width appears to be about half that. What this tells me is that the right hand part of the image must have been deep space. Considering that every image released now is a contingency, it seems odd that the very edge of the moon is the area of focus, and even more that the moon was missed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've come up with a number of theories as to why this image is cut off, along with some thought behind them.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The images could have been looking for dust or at the shadowed region. This seems unlikely because the exposure for such an image would have been different, and I doubt this would be a high priority image.&lt;/li&gt;&#xA;&lt;li&gt;The image was deliberately set for the edge of the moon, and it missed the mark slightly (About 1 field of view). This seems likely, but I'm not sure why the edge of the moon would be such a high priority image.&lt;/li&gt;&#xA;&lt;li&gt;The image was intended to be near the center of the moon, but missed the mark by quite a bit, barely capturing the edge of the moon. This is the one that has me the most fearful, hinting there is a chance that many of the best images could be missed because of some kind of a positioning error, which could be very bad.&lt;/li&gt;&#xA;&lt;li&gt;The 3 images were chosen of different parts of the moon (Center, middle, somewhere between?), and this was the one of the 3 deemed most interesting, again with a likely slight targeting miss.&lt;/li&gt;&#xA;&lt;li&gt;The predicted uncertainty was high. As a result, the 3 images were spaced well apart, to ensure the Moon was there. 3 images were downlinked, spaced in the uncertainty range such that there was a good chance Charon would be in the field of view. This one just happened to be on the edge, by coincidence.&lt;/li&gt;&#xA;&lt;li&gt;Part of the image was removed because the science team didn't want to include it, for some interesting science it holds. This seems to me to be very unlikely.&lt;/li&gt;&#xA;&lt;li&gt;Part of the image was cropped due to some significant artifacts that resulted from image compression/ dropped data. This one has some merit, as least as high as any other suggestion on here.&lt;/li&gt;&#xA;&lt;li&gt;Only part of the image was downloaded. This seems really unlikely, as all LORRI images we have seen so far are squares.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It's entirely possible none of these is correct as well. Bottom line, why was this image selected as one of the top 3 high resolution Charon images (As determined without knowing at all what Charon looked like), and why did so little of it come through in the image?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-18T16:30:55.683" LastActivityDate="2015-07-22T12:46:46.427" Title="Why is part of this New Horizon's Charon high resolution image missing?" Tags="|planetary-science|new-horizons|pluto|imaging|flyby|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="9991" PostTypeId="2" ParentId="4809" CreationDate="2015-07-17T05:54:28.467" Score="6" Body="&lt;p&gt;Looking at historical missions with the constraints you gave:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Docking in low earth orbit is a precursor to leaving low earth orbit.&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;This rules out space station mission, Apollo-Soyuz, shuttle missions that captured a satellite (like the Hubble repair flights) and the like because they didn't leave LEO.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Craft from multiple launches are docked.&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;This eliminates the docking of the Apollo command and lunar modules after the trans-lunar injection burn (which admittedly means it was already leaving LEO).&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Leaving LEO is to go do something useful.&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Your comments imply you mean to exclude &lt;a href=&quot;http://science.ksc.nasa.gov/history/gemini/gemini-xi/gemini-xi.html&quot;&gt;Gemini 11&lt;/a&gt; because it didn't go very far (only 1368.9 km at apogee).  &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_11#Flight&quot;&gt;Wikipedia&lt;/a&gt; claims this is still the record for apogee (because Apollo wasn't in the sphere of influence of Earth at its farthest point) and in terms of absolute manned distance it's only beaten by the Apollo moon missions (which already don't count for the purposes of the question).&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Considering Gemini 11 is still the highest Earth orbit and Apollo is the only manned program to leave Earth orbit your constraints have eliminated all historical manned missions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To take the question a step farther I'll relax the manned requirement: Are there any missions launched in two+ launches that docked and went to do something useful?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer appears to still be a solid &quot;no&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacecraft#Docking_of_unmanned_spacecraft&quot;&gt;Wikipedia&lt;/a&gt; lists the very few docking mission types that weren't between a crew vehicle and a habitat (or other crew vehicle) but suggests that two commercial companies have proposed satellite servicing vehicles to refuel satellites in the geosynchronous belt or &lt;em&gt;offer high-delta V orbital maneuvers&lt;/em&gt;.  Unfortunately, the two proposed vehicles were &lt;a href=&quot;http://www.vivisat.com/?page_id=10&quot;&gt;Mission Extension Vehicle&lt;/a&gt; (last news post from August 2014 suggesting they have a government contract but no mention of concrete plans for development or launch) and &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Infrastructure_Servicing&quot;&gt;Space Infrastructure Servicing&lt;/a&gt; (last news I could find was from &lt;a href=&quot;http://www.space-airbusds.com/en/press_centre/-kx9.html&quot;&gt;September 2012&lt;/a&gt; with news of a contract but no indication they've flown anything yet).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was actually a test flight for autonomous &lt;a href=&quot;https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacecraft#Non-cooperative_docking&quot;&gt;non-cooperative docking&lt;/a&gt; in 2007, but that doesn't lend itself to missions where docking is planned and doesn't seem to have progressed past the single flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall it appears that even with probe missions there have been no attempts to launch parts separately and dock them in orbit for the purpose of going to do something useful.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Back to the question in the title.  To state the obvious: they'll only be first if they actually fly the mission including an EOR as planned and nobody else beats them to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, considering &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_proposed_missions_to_the_Moon#Manned_missions&quot;&gt;this list&lt;/a&gt; (taking moon exploration to be the soonest thing that might include EOR for manned flight) they appear to have a very good shot at being first if they can even come within a few years of meeting their schedule of flying by 2018.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are proposed &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29#Future_missions&quot;&gt;Orion&lt;/a&gt; missions that I believe count on EOR, but considering Orion isn't even scheduled for manned flight until &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_Mission_2&quot;&gt;EM-2&lt;/a&gt; in 2021 (which is a single launch) and anything more bold than that is only proposed (and subject to politics) I wouldn't be concerned with Orion actually managing EOR and going somewhere any time soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that the &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Exploration/The_European_Space_Exploration_Programme_Aurora&quot;&gt;ESA Aurora Program&lt;/a&gt; seems to have no concrete plans that leaves China with their &lt;a href=&quot;http://www.universetoday.com/107716/china-considers-manned-moon-landing-following-breakthrough-change-3-mission-success/&quot;&gt;possible manned moon landing by the mid 2020s&lt;/a&gt;.  No mission details to suggest if it would be using EOR, but it's also distant enough that Space Adventures could suffer years of schedule slip and still be first.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-07-17T05:54:28.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9992" PostTypeId="2" ParentId="9987" CreationDate="2015-07-17T10:19:00.343" Score="10" Body="&lt;p&gt;According to NASA's &lt;a href=&quot;http://www.nasa.gov/pdf/168024main_011607_JupiterPressKit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NH Jupiter press kit&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Closest approach was on February 28, 2007&lt;/li&gt;&#xA;&lt;li&gt;Download began on March 14&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;After the Jupiter encounter, the &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Jupiter_encounter&quot; rel=&quot;nofollow noreferrer&quot;&gt;first hibernation mode cycle&lt;/a&gt; started on June 28, 2007. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On May 1, &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2007/01may_fantasticflyby/&quot; rel=&quot;nofollow noreferrer&quot;&gt;70% of the data&lt;/a&gt; was downloaded. At that rate, it'd take 8.5 weeks to download the full data set. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;About 70 percent of the expected 34 gigabits of data has come back so far&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The data rate at Jupiter was 38 kbit/s, at Pluto it's 1-2 kbit/s. &#xA;If they transmit at 38 kbit/s continuously, they could have transferred the data in 10 days. There are several factors that account for the longer transmission time.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;NH won't have a DSN antenna available full-time. If they have 8h/day, that adds a factor of 3. &lt;/li&gt;&#xA;&lt;li&gt;Because of 1. you lose time setting up each communications session, so not all 8 hours are spent downloading data.&lt;/li&gt;&#xA;&lt;li&gt;34 Gbit is the net figure, probably. You need to add a lot more for error correction data, retries when a transmission is garbled etc. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The Jupiter encounter (34 Gbit) &lt;a href=&quot;https://arxiv.org/pdf/1806.08393&quot; rel=&quot;nofollow noreferrer&quot;&gt;did not produce more data&lt;/a&gt; than the Pluto encounter (55 Gbit) or the MU69 encounter (50 Gbit). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2019-01-08T08:25:59.223" LastActivityDate="2019-01-08T08:25:59.223" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="9993" PostTypeId="1" CreationDate="2015-07-17T14:24:08.360" Score="10" ViewCount="2705" Body="&lt;p&gt;Is there a &lt;em&gt;single&lt;/em&gt; term that completes this sentence: &quot;at its _____, Pluto is about 27AU from Earth.&quot;  &lt;em&gt;peri-&lt;/em&gt; something...?&lt;/p&gt;&#xA;" OwnerUserId="10934" LastEditorUserId="10934" LastEditDate="2015-07-17T14:43:09.693" LastActivityDate="2018-05-22T05:42:27.587" Title="What term can we use to describe a planet's or dwarf planet's nearest distance to Earth?" Tags="|terminology|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9994" PostTypeId="2" ParentId="9993" CreationDate="2015-07-17T14:29:36.627" Score="15" Body="&lt;p&gt;Usually the term '&lt;a href=&quot;https://en.wiktionary.org/wiki/closest_point_of_approach&quot; rel=&quot;noreferrer&quot;&gt;closest approach&lt;/a&gt;' is used. It applies to any two bodies in motion. Perigee is a term occasionally used, but the dictionaries say it only applies to things orbiting Earth. Use your discretion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the planets that orbit farther from the sun than us their closest approaches occurs when they are on the opposite side of Earth that the sun is, which is known as an opposition. For Venus and Mercury, it happens when they align with the sun from our viewpoint, and that is known as an inferior conjunction. From Wikipedia - &lt;a href=&quot;https://en.wikipedia.org/wiki/Opposition_(planets)&quot; rel=&quot;noreferrer&quot;&gt;Opposition (planets&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QLTDq.png&quot; alt=&quot;diagram of opposition, conjunction, elongation, and quadrature&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html&quot; rel=&quot;noreferrer&quot;&gt;Pluto&lt;/a&gt; has an inclination of 17&lt;sup&gt;o&lt;/sup&gt; to the ecliptic, and an orbital eccentricity of 0.25. Either one of those things is enough to mean that when Pluto is closest to the sun, that brings it much closer to us, and the change in distance between when we are in front of or behind the sun in relation to it each year is a much smaller factor by comparison. For instance, right now it is about 33 AU from the sun, while on average it is 39 AU. When it is in opposition, that puts us about 32 AU away, and 34 AU when it is in conjunction. However, the orbit of Pluto is 248 years, so for all practical purposes the point each year when it is in opposition is what matters, unless you are willing to wait for as much as that time for it to get closer. The same holds true for all the possible dwarf planets in the Kuiper Belt.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-07-17T18:29:43.983" LastActivityDate="2015-07-17T18:29:43.983" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="9995" PostTypeId="2" ParentId="9993" CreationDate="2015-07-17T14:42:23.363" Score="3" Body="&lt;p&gt;Perigee would be the closest, but that may be abusing the term. Perigee means &quot;closest to Earth&quot;, but generally it's used for objects that are in &lt;a href=&quot;http://www.brighthub.com/science/space/articles/115750.aspx&quot; rel=&quot;nofollow&quot;&gt;Earth's orbit&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-17T14:42:23.363" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9996" PostTypeId="1" AcceptedAnswerId="9999" CreationDate="2015-07-17T14:48:35.393" Score="14" ViewCount="1837" Body="&lt;p&gt;When using a camera and and not being stable in respect to the target, picture smear occurs. Pluto is 2370 km wide and flyby occurred at speeds ~14 km/s and from distance of ~10,000 km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put in perspective, let's scale that down to Earth terms: 14 km/s ~= 50,000 km/h =&gt; 60 km/h is a scale of ~840. Scaling everything down linearly:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Speed: 14 km/s =&gt; 60 km/h&lt;/li&gt;&#xA;&lt;li&gt;Pluto size: 2370 km =&gt; 2.8 km&lt;/li&gt;&#xA;&lt;li&gt;Distance: 10,000 km =&gt; 12 km&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I could not find the zoom factor applied, but given that some images are zoomed to ~240 km, I would assume it is at least 10x.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So New Horizon's Pluto flyby is like going 60 km/h in a car and taking a picture of something of the size of ~2.8 km from the distance of ~12 km with a 10x zoom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is it that the pictures are so clear without any visible smear? Looks to me I have some concepts, data or calcs wrong - some clarification would be great.&lt;/p&gt;&#xA;" OwnerUserId="10931" LastEditorUserId="415" LastEditDate="2015-07-17T17:50:03.937" LastActivityDate="2018-09-09T08:13:00.820" Title="Why do the pictures returned from the New Horizons flyby of Pluto lack motion blur?" Tags="|new-horizons|pluto|imaging|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="9997" PostTypeId="2" ParentId="9993" CreationDate="2015-07-17T15:01:13.297" Score="4" Body="&lt;p&gt;Not quite what you want, but very closely related to what you want are the terms &quot;opposition&quot; (for the outer planets) and &quot;inferior conjunction&quot; (for Venus and Mercury). &lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-07-17T15:01:13.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="9999" PostTypeId="2" ParentId="9996" CreationDate="2015-07-17T15:06:34.490" Score="25" Body="&lt;p&gt;New Horizons pans during shots to eliminate motion smear. See &lt;a href=&quot;https://space.stackexchange.com/questions/9937/why-cant-new-horizons-send-data-and-do-observations-at-the-same-time&quot;&gt;this related question&lt;/a&gt;. &#xA;The &lt;a href=&quot;http://usuaris.tinet.org/klunn/new-horizons.html&quot; rel=&quot;noreferrer&quot;&gt;Voyager mission proved&lt;/a&gt; that this was possible:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Between Voyager 2’s encounter with Saturn in 1981 and Uranus in 1986, controllers developed a technique called “image motion compensation.” This involved moving the scan platform at slow rates, which was found to be possible despite the damage, in conjunction with thruster firings to rotate the entire spacecraft at a rate that would allow a target to be tracked long enough for imaging.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Because New Horizons doesn't have a scan platform, they rotate the whole spacecraft to achieve this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons &lt;a href=&quot;https://mattcbergman.com/2015/07/21/new-horizons-propulsion-system/&quot; rel=&quot;noreferrer&quot;&gt;uses these thrusters&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;12 Aerojet MR-103H 0.8 N thrusters (those were used on Voyager too)&lt;/li&gt;&#xA;&lt;li&gt;4 &lt;a href=&quot;http://www.rocket.com/propulsion-systems/monopropellant-rockets&quot; rel=&quot;noreferrer&quot;&gt;Aerojet MR-111C&lt;/a&gt; 4.4 N thrusters&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are all hydrazine monopropellant thrusters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MR103-H can be run in very short pulses, down to 4 ms. This gives a minimum impulse bit of 0.0026 N-s at 100 psia. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The requirement: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In particular, the RALPH instrument requires that the observatory be capable of setting up and maintaining a scan rate of +/- 34 μrad/s about any given axis.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://seitzman.gatech.edu/classes/ae6450/MR-103H_Hydrazine_Thruster_New_Horizons.pdf&quot; rel=&quot;noreferrer&quot;&gt;Paper on NH's thruster system&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-09-09T08:13:00.820" LastActivityDate="2018-09-09T08:13:00.820" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="10001" PostTypeId="2" ParentId="9993" CreationDate="2015-07-17T16:03:58.617" Score="3" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Minimum_orbit_intersection_distance&quot; rel=&quot;nofollow&quot;&gt;MOID&lt;/a&gt;. Minimum Orbit Intersection Distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually used for Near Earth Objects when assessing risk of impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But Wikipedia writes &quot;It is defined as the distance between the closest points of the osculating orbits of two bodies. &quot; So perhaps MOID could have a more general use than just the minimum distance between an NEA and earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general the minimum distance to an outer body would occur at opposition and when the outer body is at perihelion. Minimum distance to an inner body would occur at inferior conjunction when inner body is at aphelion. But even these general rules could have exceptions if the orbits are inclined to one another and/or resonant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example a Hilda asteroid might have a 5.19 A.U. aphelion and Jupiter's orbit is 5.2 A.U.. Does this mean that a Hilda could get within .01 of Jupiter? No. Because of a Hilda's resonance with jupiter, aphelions will only occur near Jupiter's L4, L5 and L3 regions. Finding a Hilda Jupiter MOID is an interesting geometry problem. I suspect it doesn't occur at opposition or inferior conjunction. If I have time and energy to play with this puzzle, I'll may add to my answer&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-07-17T16:10:32.240" LastActivityDate="2015-07-17T16:10:32.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10002" PostTypeId="1" CreationDate="2015-07-17T16:17:53.493" Score="0" ViewCount="60" Body="&lt;p&gt;&lt;em&gt;Question 1&lt;/em&gt;  -  &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_decay&quot; rel=&quot;nofollow&quot;&gt;orbital decay&lt;/a&gt; - does it apply to &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_debris&quot; rel=&quot;nofollow&quot;&gt;space debris&lt;/a&gt;? Why so many debris continue to circle earth while ISS needs to be boosted to stay flying?&#xA;Is it because most debris are higher where orbital decay does not apply? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, than &lt;em&gt;question 2&lt;/em&gt; - at what height orbital decay stops being a factor?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="10167" LastActivityDate="2015-07-17T16:17:53.493" Title="Orbital decay - does it apply to space debris?" Tags="|low-earth-orbit|debris|" AnswerCount="0" CommentCount="2" ClosedDate="2015-07-17T17:40:24.357" ContentLicense="CC BY-SA 3.0" />
  <row Id="10003" PostTypeId="1" AcceptedAnswerId="10006" CreationDate="2015-07-17T17:29:55.280" Score="12" ViewCount="3597" Body="&lt;p&gt;This came up while discussing the recent Falcon 9 failure with a friend.  She posed the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How long does it take SpaceX to construct a new Falcon 9?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For the sake of the question, presume:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;No new engineering designs/equipment to be developed&lt;/li&gt;&#xA;&lt;li&gt;No new launch facilities&lt;/li&gt;&#xA;&lt;li&gt;Time from request for creation to ready to launch&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If possible, please also give clarification on time subtracted for Salvaged/Backstocked parts (ie: recovered stages, engines made en mass previously, etc) and how long those parts would take to construct.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="25" LastEditDate="2017-12-19T13:24:30.437" LastActivityDate="2017-12-19T13:24:30.437" Title="How long does it take to build a Falcon 9 Rocket?" Tags="|spacex|falcon-9|construction|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10004" PostTypeId="1" AcceptedAnswerId="10012" CreationDate="2015-07-17T17:48:14.803" Score="7" ViewCount="805" Body="&lt;p&gt;I've noticed that different launches of the Soyuz launcher have a variety of logos depicted on the side of the fairing and boosters.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What do the logos in the different positions signify?&lt;/li&gt;&#xA;&lt;li&gt;Who decides what goes where?&lt;/li&gt;&#xA;&lt;li&gt;Is there any informative log of the various logos for different flights?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/oLi6w.jpg&quot; alt=&quot;Soyuz TMA-3 at launch&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the above example, there is what looks like the Flag of Spain, red text (Союз - Cyrillic Soyuz) and another icon to the left of that text.  There's also a square blue/white logo on the bottom near one of the boosters.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/P71UM.jpg&quot; alt=&quot;universetoday-rus.com&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the above there is what looks like the logo of the Russian Federal Space Agency.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="195" LastEditDate="2020-09-23T04:17:00.607" LastActivityDate="2020-09-23T04:17:00.607" Title="What do the various logos and text on the Soyuz launcher signify?" Tags="|mission-design|soyuz-launcher|insignia|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="10005" PostTypeId="1" CreationDate="2015-07-17T18:14:47.973" Score="4" ViewCount="479" Body="&lt;p&gt;I was pondering a recent comment by Henning Makholm to &lt;a href=&quot;https://space.stackexchange.com/a/9994/4660&quot;&gt;this answer&lt;/a&gt;, he was addressing whether the distance from us of objects beyond Uranus are more affected by our orbit, or their orbits, depending on their&#xA;orbital eccentricity:   &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Assuming that our hypothetical object orbits (very) roughly in the&#xA;  ecliptic plane, I don't think eccentricity matters... if the object&#xA;  moved fast enough to get away from Earth faster than Earth goes around&#xA;  half its orbit, it would have more than solar escape velocity!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I had to chew on that for a while. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the relationship between an object's distance from its primary, and how eccentric its orbit can be before it will escape the primary? Does this depend on how deep the gravity well is? Is there a term for this, and a formula?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-07-17T21:31:28.993" Title="How does possible orbital eccentricity vary with distance from the Sun?" Tags="|orbital-mechanics|terminology|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10006" PostTypeId="2" ParentId="10003" CreationDate="2015-07-17T18:45:38.293" Score="5" Body="&lt;p&gt;Publicly SpaceX officials have verbally said that they are aiming to build at a rate of 40 cores (First Stages) a year.  As of just before the CRS-7 mission they said they were at a 20/year rate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;52/40= 1.3 weeks/core for their desired optimum rate.&#xA;52/20= 2.6 weeks/core for their current state.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were aiming for 12-13 launches (one being a Falcon Heavy, so two more cores) thus 14-15 cores to be used in 2015. This will not likely happen due the CRS-7 loss of mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They build the upper stage at a somewhat slower rate, since their stated desire is a mix of 10 Falcon 9 and 10 Falcon Heavy boosters a year to get to 40 cores, which only requires 20 upper stages. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-17T18:45:38.293" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10007" PostTypeId="1" AcceptedAnswerId="20467" CreationDate="2015-07-17T18:51:55.780" Score="5" ViewCount="1187" Body="&lt;p&gt;As New Horizon's signal is so weak, probability exists of errors in detected signal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What error detection and correction codes are used in the signal? Is it known, what is the error rate in received signal?&lt;/p&gt;&#xA;" OwnerUserId="10942" LastEditorUserId="1142" LastEditDate="2015-08-30T23:31:45.950" LastActivityDate="2017-03-02T21:22:55.030" Title="Error detection and correction techniques on New Horizons data" Tags="|communication|new-horizons|deep-space|software|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10009" PostTypeId="2" ParentId="10005" CreationDate="2015-07-17T21:31:28.993" Score="1" Body="&lt;p&gt;A periodic orbit always has eccentricity in the interval $[0,1)$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An eccentricity of $1$ denotes a parabolic orbit, in which the object always has exactly the solar escape velocity for its distance from the sun. Such an orbit has no aphelion; after perihelion the object will escape to infinity, in an ultimate direction in direct opposition to its perihelion, while its speed drops off to $0$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An eccentricity &lt;em&gt;greater than&lt;/em&gt; $1$ is a hyperbolic orbit, corresponding to an interstellar trajectory that is deflected to some degree by the sun. Here the object also escapes to infinity, but instead of converging to $0$ its speed (relative to the sun) will drop towards a finite limit as it proceeds ever farther away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(All of this is unless the object meets another planet or star that pertubes its path away from the ideal Keplerian trajectories we're talking about here).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, &lt;em&gt;by definition&lt;/em&gt; (more or less), an eccentricity of $1$ marks the boundary between periodic and escaping orbits. This is independent of how close to the sun the object comes; the eccentricity measures the &lt;em&gt;shape&lt;/em&gt; of the orbit rather than its &lt;em&gt;size&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10165" LastActivityDate="2015-07-17T21:31:28.993" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10010" PostTypeId="1" AcceptedAnswerId="10013" CreationDate="2015-07-17T21:58:49.490" Score="4" ViewCount="235" Body="&lt;p&gt;As I recall, the main cause of the high temperatures inside e.g. the Earth is thermal heating due to the decay of radioisotopes.  The thinking is that:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Although small, the heat has nowhere to go, so it piles up.&lt;/li&gt;&#xA;&lt;li&gt;Similarly, thermal flux from solar heating at the surface has a hard time propagating down.&lt;/li&gt;&#xA;&lt;li&gt;Residual leftover heating from the solar system's formation would be expected to be only a minor component by now.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I was thinking about this, and realized that it implies a somewhat interesting conclusion: if the rocky planets/bodies are of similar composition, then their core temperatures should be functions of their radius, with their distance to the sun being basically irrelevant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, I might expect that Ganymede (radius 2634km) has a warmer core temperature than Mercury (radius 2440km), even though Mercury is 13.4 times closer to the sun.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;My question: about how accurate is this conclusion?  Certainly, composition varies somewhat, but as a heuristic &lt;em&gt;in general&lt;/em&gt;, is it fair to say that larger rocky bodies have warmer interiors?&lt;/p&gt;&#xA;" OwnerUserId="2707" LastEditorUserId="4660" LastEditDate="2015-07-18T17:23:29.227" LastActivityDate="2020-10-28T22:26:21.827" Title="Do the size of rocky planets determine how hot they are inside?" Tags="|planetary-science|heat|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10011" PostTypeId="2" ParentId="9996" CreationDate="2015-07-17T22:18:35.740" Score="4" Body="&lt;p&gt;Keep scaling linearly if you like:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Speed: 14 km/s =&amp;gt; 60 km/h  =&amp;gt; 60m/h&#xA;Pluto size: 2370 km =&amp;gt; 2.8 km =&amp;gt; 2.8m&#xA;Distance: 10,000 km =&amp;gt; 12 km =&amp;gt; 12m&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;That's like a car sized object 4 car lengths away. With that scale in mind 60 meters per hour doesn't seem very fast at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's some additional information I have gathered from an informative article on the LORRI imager I found &lt;a href=&quot;http://www.boulder.swri.edu/pkb/ssr/ssr-lorri.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combined with the fact that the imager is combined with a scope of focal length 2630mm, there is some significant chance of blur.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To summarize those specs:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;FOV: 0.29° × 0.29°&lt;/li&gt;&#xA;&lt;li&gt;Nominal exposure time: 50-200ms&lt;/li&gt;&#xA;&lt;li&gt;Focal length: 2630mm&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The fov of LORRI is 0.29° which is around 5060μrads. At 1024 pixels that's about 4.94μrads per pixel. That number is confirmed in the linked PDF on page 5. The nominal exposure times are 50-200ms. Using the numbers from above (14km/s at 10000km) I get 280μrads of radial distance moved at the longest exposure. That's 56 pixels or about 5% of the FOV. That IS large enough to cause blur except for the fact that the vehicle does maneuver to keep the target in place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article explains how they deal with another phenomenon referred to as &quot;readout smear&quot; which is caused by the way the electronic data is flushed from the CCD.&lt;/p&gt;&#xA;" OwnerUserId="6392" LastEditorUserId="6392" LastEditDate="2015-07-19T05:02:41.937" LastActivityDate="2015-07-19T05:02:41.937" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10012" PostTypeId="2" ParentId="10004" CreationDate="2015-07-18T00:24:50.310" Score="8" Body="&lt;p&gt;The Spanish flag is the key to identify which of the Soyuz launches that was. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the first pictrue we seem to have &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-3&quot; rel=&quot;noreferrer&quot;&gt;Soyuz TMA-3&lt;/a&gt; (Союз ТМА-3), launched on October 18, 2003. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's review its logos:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The Spanish flag is for &lt;a href=&quot;https://en.wikipedia.org/wiki/Pedro_Duque&quot; rel=&quot;noreferrer&quot;&gt;Pedro Duque&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Below the spanish flag we have Pedro's &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Cervantes_Mission&quot; rel=&quot;noreferrer&quot;&gt;&quot;Cervantes&quot; mission&lt;/a&gt; nice looking &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Cervantes_mission_logo.png&quot; rel=&quot;noreferrer&quot;&gt;patch&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://cs11352.vk.me/g35144182/a_fb45e5d2.jpg&quot; rel=&quot;noreferrer&quot;&gt;The logo on the booster&lt;/a&gt; is for Soyuz rocket producer &lt;a href=&quot;http://en.samspace.ru/&quot; rel=&quot;noreferrer&quot;&gt;TsSKB Progress&lt;/a&gt; (It says &quot;ЦСКБ Прогресс&quot;).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Now lets rotate the rocket a little. This is the same rocket but from a different angle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eSdLB.jpg&quot; alt=&quot;Soyuz TMA-3&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;4&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;The big logo in the middle of the central block is the same TsSKB Progress one, as the one on the booster.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Under the Russian flag there is &lt;a href=&quot;http://epizodsspace.airbase.ru/bibl/nk/1999/5/obl2-2.jpg&quot; rel=&quot;noreferrer&quot;&gt;an old logo&lt;/a&gt; of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Russian_Federal_Space_Agency&quot; rel=&quot;noreferrer&quot;&gt;Russian Space Agency&lt;/a&gt;. It says &quot;РКА&quot; for Российское Космическое Агентство.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;And below there is &lt;a href=&quot;http://www.fakel-russia.com/partners_image/10/image?thumbnail=y&amp;amp;maxwidth=200&amp;amp;maxheight=100&quot; rel=&quot;noreferrer&quot;&gt;an older version&lt;/a&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/S.P._Korolev_Rocket_and_Space_Corporation_Energia&quot; rel=&quot;noreferrer&quot;&gt;RSC Energia&lt;/a&gt; logo (it says &quot;РКК Энергия&quot;). The producer of the Soyuz spacecraft.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Now, I could not find a picture of the same rocket from yet another angle, but I assume there should be their &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Soyuz_TMA-3_Patch.png/200px-Soyuz_TMA-3_Patch.png&quot; rel=&quot;noreferrer&quot;&gt;Soyuz TMA-3 patch&lt;/a&gt; too somewhere, as well as USA flag for &lt;a href=&quot;https://en.wikipedia.org/wiki/Michael_Foale&quot; rel=&quot;noreferrer&quot;&gt;Michael Foale&lt;/a&gt;. Plus, I wouldn't be surprised by their &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_8#/media/File:Expedition_8_insignia_%28iss_patch%29.png&quot; rel=&quot;noreferrer&quot;&gt;ISS-8 mission patch&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all seems quite logical.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="897" LastEditDate="2015-07-18T08:02:13.013" LastActivityDate="2015-07-18T08:02:13.013" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10013" PostTypeId="2" ParentId="10010" CreationDate="2015-07-18T02:20:20.743" Score="3" Body="&lt;p&gt;You idea is supported by &lt;a href=&quot;https://en.wikipedia.org/wiki/Internal_heating&quot; rel=&quot;nofollow noreferrer&quot;&gt;this source&lt;/a&gt;, which states:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Of the terrestrial planets in our solar system, Earth has the most internal heating because it is the most massive.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It is speculated that Venus also has a high internal temperature due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Volcanology_of_Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;volcanism&lt;/a&gt;. An interesting fact that supports this concept is that Venus is very similar in size to Earth, being slightly smaller. &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size&quot; rel=&quot;nofollow noreferrer&quot;&gt;This source&lt;/a&gt; lists the objects in the solar system by size.&lt;/p&gt;&#xA;&lt;p&gt;As you mention, composition is also very important. A rocky mantle and crust are better able to withstand the temperature of a hot core than water ice. Additionally, rock acts as a good insulator of heat and the thickness of the mantle and crust increases the insulating affect. The other critical part of composition and heat generation is enough longer lived radioactive isotopes such as &amp;quot;potassium-40, thorium-232, uranium-235 and uranium-238&amp;quot;.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="1235" LastEditDate="2020-10-28T22:26:21.827" LastActivityDate="2020-10-28T22:26:21.827" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="10014" PostTypeId="1" AcceptedAnswerId="10018" CreationDate="2015-07-18T03:06:46.843" Score="7" ViewCount="852" Body="&lt;p&gt;I found &lt;a href=&quot;http://phys.org/news/2014-09-baseline-array-radio-image-voyager.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://science.howstuffworks.com/question431.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; that Voyager transmits its main radio signal at 23 watts. In other words, the strength of the signal coming from its radio dish is 23 watts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, what I want to know is &lt;strong&gt;how much electrical power does that dish draw?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No system is 100% efficient, so it has to draw more than 23 watts. Maybe it's 24, 25, 50, 100, or 23.001. I don't know what the typical efficiency of a dish is when measured by electric power in versus signal power out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, if I'm not mistaken, this will be the same for Voyager 1 and 2.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="26446" LastEditDate="2021-04-06T05:36:21.663" LastActivityDate="2021-04-06T05:36:21.663" Title="How much power does the Voyager radio dish draw?" Tags="|radio-communication|voyager|power|dish|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10015" PostTypeId="1" CreationDate="2015-07-18T07:11:04.913" Score="0" ViewCount="39" Body="&lt;p&gt;In the movie &quot;Gravity&quot;, the main character, Dr. Ryan Stone is portrayed as being knowledgeable about some field of science, but not especially good at managing a spacecraft. The movie has her as a &lt;a href=&quot;http://gravitymovie.wikia.com/wiki/Mission_Specialist&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mission Specialist&lt;/a&gt;, though it's speculated that a Payload Specialist would more accurately describe her role.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In real life, are mission specialists and payload specialists less capable of handling spacecrafts than other members of the crew? What standards are expected of the different roles? For example, if nothing had gone wrong except the death of all of their crew-mates (a less severe scenario than that in &quot;Gravity&quot;), would a payload specialist be expected to safely return to earth without a rescue mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/2405/625&quot;&gt;Related question, but less specific&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="625" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-07-18T10:04:26.143" Title="Are mission/payload specialists less capable of managing a spacecraft?" Tags="|training|" AnswerCount="0" CommentCount="2" ClosedDate="2015-07-18T11:22:53.433" ContentLicense="CC BY-SA 3.0" />
  <row Id="10016" PostTypeId="1" CreationDate="2015-07-18T09:38:58.473" Score="7" ViewCount="253" Body="&lt;p&gt;Dawn orbits Ceres at thousands of kilometers periapsis altitude, and doesn't seem to plan to go lower than &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)#Ceres_orbit&quot;&gt;375 km&lt;/a&gt;. In contrast, Rosetta has been down to &lt;a href=&quot;http://blogs.esa.int/rosetta/2014/09/10/down-down-we-go-to-29-km-or-lower/&quot;&gt;about 30 km altitude&lt;/a&gt;, and will finally even &lt;a href=&quot;http://spaceref.com/rosetta/rosetta-mission-extended.html&quot;&gt;try to soft-land&lt;/a&gt;. And that's at a comet with a coma that might already have caused Rosetta to enter safe mode as its &lt;a href=&quot;http://blogs.esa.int/rosetta/2015/04/01/rosetta-status-report-close-flyby-navigation-issues/&quot;&gt;star tracker confused debris around it for stars&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it maybe because of gravity, propulsion, instrumentation or spacecraft design? Isn't closer better?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2015-07-18T11:36:07.427" LastActivityDate="2015-07-18T16:11:07.640" Title="Why is Dawn in a much higher observation orbit than Rosetta?" Tags="|orbital-mechanics|orbit|rosetta|altitude|dawn-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10017" PostTypeId="2" ParentId="10016" CreationDate="2015-07-18T10:34:45.970" Score="6" Body="&lt;p&gt;Much, much higher &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_orbital_energy&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific orbital energy&lt;/a&gt; ($\epsilon$) and Dawn's lack of high impulse thrusters. OK, technically Dawn has 12 x off-axis 0.9 N RCS thrusters, but it only launched with 46 kg of hydrazine for them, so it pretty much depends on its three 90 mN ion thrusters alone. See &lt;a href=&quot;https://space.stackexchange.com/a/6665/49&quot;&gt;this answer&lt;/a&gt; for more detailed explanation of its propulsive systems and limitations of low thrust propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for $\epsilon$, rough calculation for Rosetta's 30 km orbit around 67P/Churyumov–Gerasimenko gives me ~ &lt;a href=&quot;https://encrypted.google.com/#q=-%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+10%5E13+kg%29%2F%282*%282.65+km+%2B+30+km%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;$-0.01\mathrm{\ m^2s^{-2}}$&lt;/a&gt; (note that 67P is a highly irregular body) and for Dawn in 375 km orbit around Ceres &lt;a href=&quot;https://encrypted.google.com/#q=-%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+9.39%C3%9710%5E20+kg%29%2F%282*%28463.5+km+%2B+375+km%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;$-37,364\mathrm{\ m^2s^{-2}}$&lt;/a&gt;. That's a huge difference of a few orders of magnitude and where the amplitude of required energy to do corrective orbital maneuvers comes from (multiplied with the mass of the orbiter to get required energy in Joules, but Rosetta and Dawn have roughly the same mass by now), should those be necessary, say due to mascons (mass concentrations) and otherwise uneven gravity field or even presence of surface outgassed (or ejected in Ceres' case since it's suspected to be a geothermally active body with cryogeysers of salt water as one possible explanation for those infamous white spots) materials increasing density of the medium it has to orbit through (and thus influencing orbital decay).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rosetta doesn't have these problems, because it will end its (primary) mission by the time it runs out of propellants and it requires a great deal less of them and much shorter bursts to maintain its quasi-orbit around 67P. &lt;em&gt;Quasi&lt;/em&gt; because it's constantly maintained with impulse burns and would not stay in even close to desired one by the time it takes it to complete one single orbit around the comet without them. I can't find a good info on Rosetta's current orbital period, but it should be &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%282.65+km+%2B+30+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+10%5E13+kg%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;roughly 16 days&lt;/a&gt;, &amp;pm; quite a bit due to all the uncertainties. Luckily, from observational and Philae lander's perspective, the comet rotates much faster on its own axis (12.4 hours). Still, perhaps a better term would be that Rosetta is station-keeping around 67P than orbiting it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that a 375 km orbit around Ceres takes ~ &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28463.5+km+%2B+375+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+9.39%C3%9710%5E20+kg%29%29+in+minutes&quot; rel=&quot;nofollow noreferrer&quot;&gt;320 minutes&lt;/a&gt;, while a 30 km one less than half that at ~ &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28463.5+km+%2B+30+km%29%5E3*4*pi%5E2%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+9.39%C3%9710%5E20+kg%29%29+in+minutes&quot; rel=&quot;nofollow noreferrer&quot;&gt;145 minutes&lt;/a&gt;. Perturbations, both by &lt;a href=&quot;https://en.wikipedia.org/wiki/Nodal_precession&quot; rel=&quot;nofollow noreferrer&quot;&gt;nodal precession&lt;/a&gt; since Ceres isn't exactly a spherical body either, and possible mascons would mean that it would have (bigger at least) problems maintaining nadir-fixed attitude for observations, and orbital speed would increase from about &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+9.39%C3%9710%5E20+kg%29%2F%28463.5+km+%2B+375+km%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;273 m/s&lt;/a&gt; to roughly &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+9.39%C3%9710%5E20+kg%29%2F%28463.5+km+%2B+30+km%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;356 m/s&lt;/a&gt;. I'll let you calculate relative velocity to the ground, but as you can see, you might have a lot less time to point towards a single area in a single pass and it would reduce in size too, since angular resolution of your instruments remains fixed. This is especially important for observations of Ceres on its night side, with required long exposure times.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;There's another point to keeping Dawn's final orbit relatively high. I can't provide a reference because I don't remember where I heard this (could have been during some lecture, press conference, or &lt;a href=&quot;http://dawnblog.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dawn's blog&lt;/a&gt; - I will update when I find it), but unless I remember it wrong, plan for Dawn after completing its primary mission and once it runs out of propellants / reaction mass is to simply leave in orbit around Ceres, and the word was (wherever I remember this from) that is should stay there and be able to return science data for quite a while longer. Of course, if you placed it in an even lower final LAMO (low-altitude mapping orbit) before burnout to dry, its orbit would precess much faster to its final lithobraking demise as a small, fan-like debris field on Ceres' surface. Maybe that's what those bright spots are? Or graveyards of disco balls from the 60-80's? Joking aside, since it's solar powered, there's no reason why most of its instrumentation couldn't operate for many more years to come. In its environment, assuming no astronomical event, it should only lose about 1% of its power per year. And while at least one of its flywheels still functions, it should be able to maintain periodic contact with Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely, while that is true also for Rosetta, the comet chasing probe can't orbit 67P for very long before its orbit is perturbed too much by comet's uneven gravity field and gravitational attraction of other Solar system bodies during perihelion and close approaches at not too big of an inclination to the invariable plane (~ 7°), and either crashed into the comet ungracefully or nudged far away from it never to return anywhere close.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-07-18T16:11:07.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10018" PostTypeId="2" ParentId="10014" CreationDate="2015-07-18T12:37:47.370" Score="12" Body="&lt;p&gt;The &quot;radio frequency&quot; system &lt;a href=&quot;http://stickings90.webspace.virginmedia.com/voyager.pdf&quot;&gt;draws up to 111.7 W&lt;/a&gt; (PDF on Voyager design, table 2 on page 10). This is purely the power amplifier, modulation/demodulation is listed separately at another 11 W.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this has nothing to do with the dish. Those 23 W is what comes out of the antenna as radio waves. The dish reflects pretty much 100% of this. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-18T12:37:47.370" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10019" PostTypeId="1" AcceptedAnswerId="10020" CreationDate="2015-07-18T15:15:09.643" Score="14" ViewCount="1015" Body="&lt;p&gt;At the &lt;a href=&quot;http://solarsystem.nasa.gov/news/display.cfm?News_ID=49506&quot; rel=&quot;noreferrer&quot; title=&quot;NASA web site&quot;&gt;NASA web site&lt;/a&gt; is a story about how the first images returned from Mariner 4 were rendered by hand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jxQWd.jpg&quot; alt=&quot;Hand rendered image from Mariner 4&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do these images still exist, for example in a museum?&lt;/p&gt;&#xA;" OwnerUserId="10953" LastEditorUserId="12102" LastEditDate="2021-05-12T01:51:14.360" LastActivityDate="2021-05-12T01:51:14.360" Title="Do Mariner 4 hand rendered images still exist?" Tags="|mars|imaging|flyby|museum-exhibits|mariner|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10020" PostTypeId="2" ParentId="10019" CreationDate="2015-07-18T15:28:52.590" Score="14" Body="&lt;p&gt;It was displayed publicly yesterday! At a &lt;a href=&quot;http://www.jpl.nasa.gov/events/lectures_archive.php?year=2015&amp;amp;month=7&quot; rel=&quot;noreferrer&quot;&gt;Karman lecture&lt;/a&gt; held by Mr. Blaine Baggett, Director, Office of Communication and Education, JPL. It might not be archived and available to download right now, but soon it will. And I'm sorry for being a spoiler, but it is in the last few minutes of his show. Even the crayons used to draw it are preserved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I might add that I have no idea how this looks like anything on Mars. Does anyone know what region this could be?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT addition: Was this really the best map of Mars 50 years ago? According to the &lt;a href=&quot;http://www.directedplay.com/first-tv-image-of-mars&quot; rel=&quot;noreferrer&quot;&gt;link&lt;/a&gt; TidalWave provided in the a comment below. With the old canali and everything. Some progress in space exploration has been made indeed. Methink I see triagonal structures on the early images from Pluto, but they might be like Lowell's imaginations.&#xA;&lt;img src=&quot;https://i.stack.imgur.com/wiT6b.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-07-18T17:01:41.320" LastActivityDate="2015-07-18T17:01:41.320" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10021" PostTypeId="1" AcceptedAnswerId="10023" CreationDate="2015-07-18T16:40:47.770" Score="13" ViewCount="2744" Body="&lt;p&gt;So there are some very complex gravity assists out there. The Cassini mission, for instance, used a VVEJ (Venus Venus Earth Jupiter) gravity assist to make it to Saturn. Similar flybys have been done for other missions. While I understand how to make a simple Hohmann transfer orbit very well, I don't quite see how to even set up a relatively simple gravity assist. I would like to write a program that helps me to set this up, but I'm struggling even to understand the principle I should be aiming for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus question: Even more complex to me is ion drive missions such as Rosetta, which have ion drives AND use gravity assists. This seems even more complex to me, and I would like to better understand this. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="265" LastEditDate="2015-07-18T18:39:50.123" LastActivityDate="2015-07-18T19:58:17.197" Title="How are gravity assists conceived?" Tags="|orbital-mechanics|gravity-assist|mathematics|planning|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10022" PostTypeId="1" AcceptedAnswerId="10219" CreationDate="2015-07-18T17:35:43.893" Score="70" ViewCount="85253" Body="&lt;p&gt;Usually simply abbreviated as a &lt;a href=&quot;https://en.wiktionary.org/wiki/rapid_unplanned_disassembly&quot;&gt;RUD&lt;/a&gt;, and also sometimes expanded as Rapid Unplanned Disassembly, and being a way of understating that a rocket exploded. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I saw it attributed recently to Elon Musk, but although he has popularized the phrase among the public, when he mentioned it once in a speech I saw he simply said it is how the engineers on the SpaceX team like to refer to such an event. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is the history of this phrase, and can it be traced back to the person who came up with it?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="1142" LastEditDate="2016-06-15T15:23:50.780" LastActivityDate="2023-05-11T17:54:46.453" Title="Who coined the phrase 'Rapid Unscheduled Disassembly'?" Tags="|rockets|history|terminology|rud|" AnswerCount="7" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="10023" PostTypeId="2" ParentId="10021" CreationDate="2015-07-18T19:16:33.220" Score="12" Body="&lt;p&gt;First off, you need to know where all the bodies in the solar system are as a function of time.  I would start by getting the &lt;a href=&quot;http://naif.jpl.nasa.gov/naif/toolkit.html&quot; rel=&quot;noreferrer&quot;&gt;SPICE toolkit&lt;/a&gt; from JPL, supported in C, Fortran, IDL, and MATLAB.  You will also need the &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/de430.bsp&quot; rel=&quot;noreferrer&quot;&gt;planetary and lunar ephemerides&lt;/a&gt; and the latest &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/generic_kernels/lsk/naif0011.tls&quot; rel=&quot;noreferrer&quot;&gt;leap-second kernel&lt;/a&gt;.  (The planetary kernel linked is good from 1550 AD to 2650 AD, and the leap second kernel linked includes the leap second added a few weeks ago.  The planetary kernel includes the Sun, Pluto, and Earth's moon.  There is more recent kernel with an updated orbit for Pluto, but the available one only spans 1950 to 2050.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, you need to be able to do patched conic trajectories for rapid evaluation of gravity assist trajectories.  You simply assume that there is only the Sun when traversing between planets to get, say, the arrival $\vec{V}_{\infty}$ at the planet (subtracting the heliocentric velocity vector from the planet velocity vector), and then assume that there is only the planet to use a &lt;a href=&quot;https://space.stackexchange.com/a/6510/265&quot;&gt;simple equation&lt;/a&gt; to convert that to a departure $\vec{V}_{\infty}$ leaving the planet, adding the heliocentric velocity of the planet back in to propagate from there.  As part of this, you will be converting between orbital elements and position/velocity, and back for every flyby and every propulsive maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then play.  Look at where the planets are and try things.  Keep looking at where they are as time progresses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Try going backwards -- Venus is especially effective due to its high mass and depth in the Sun's gravity well (Oberth effect), even though you have to go the &quot;wrong way&quot; when you are trying to get to the outer solar system.  (This was an &quot;a ha!&quot; moment for Roger Diehl of JPL who discovered this approach in trying to figure out how to get Galileo to Jupiter without a Shuttle-Centaur after Challenger blew up.  Venus was then used twice by Cassini.)  You may want to add propulsive maneuvers to get you back to planets.  Cassini had to do a large deep-space maneuver to lower its perihelion so it could get back to Venus a second time.  It is interesting that both of Cassini's big propulsive maneuvers on the way to Saturn (launch and the DSM) &lt;em&gt;reduced&lt;/em&gt; its heliocentric energy, even though the objective was to significantly &lt;em&gt;increase&lt;/em&gt; its heliocentric energy in order to reach Saturn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Important note: after developing a trajectory, verify that the flybys are &lt;em&gt;above&lt;/em&gt; the surface and atmosphere of the bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will be good enough to discover trajectories.  Try rediscovering the Grand Tour of the late 1970's as a first exercise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you need to really fly one of these trajectories, then you need to:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, be able to accurately numerically integrate trajectories in the gravity fields of the Sun and all of the planets, and integrate the approach/flyby/departure accurately instead of using the patched conic approximation.  Then you can verify that you really have what you thought you had, and know with some confidence the $C_3$, the $\Delta V$'s, and the times required.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-07-18T19:23:52.787" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10024" PostTypeId="2" ParentId="10021" CreationDate="2015-07-18T19:58:17.197" Score="7" Body="&lt;h1&gt;Background&lt;/h1&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/9504/how-can-i-intuitively-understand-gravity-assists&quot;&gt;How can I intuitively understand gravity assists?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;From the frame of reference of the planet you're gaining the assist from you leave the sphere of influence (SOI) of the planet at the same speed as you entered it, just in a different direction (from an external frame of reference you've changed both speed and direction).  In a simple case (no thrusting during the assist) this limits the planning parameters to your vector relative to the planet when entering the SOI: knowing your input vector allows you to compute the exit vector and slightly changing the angle you enter produces larger changes to the resulting angle when leaving the SOI.&lt;/p&gt;&#xA;&lt;p&gt;Aim closer to the planet for a lower pass for a greater change in direction (and velocity from an external frame of reference), farther from the planet for a smaller change; aim ahead of the planet for a slower resulting external velocity, behind for a higher velocity:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/VvKxP.png&quot; alt=&quot;gravity assist guide&quot; /&gt;&#xA;(image from &lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Tutorial:_Gravity_Assist&quot; rel=&quot;nofollow noreferrer&quot;&gt;this KSP tutorial&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;Also note that to gain velocity from a gravity assist you have to change direction.&lt;/p&gt;&#xA;&lt;h1&gt;Planning Methods&lt;/h1&gt;&#xA;&lt;p&gt;If you're willing to freehand the assists (say &lt;a href=&quot;https://www.youtube.com/watch?v=lIORB7svhaA&quot; rel=&quot;nofollow noreferrer&quot;&gt;you're playing KSP and are trying to get somewhere on the cheap&lt;/a&gt;) you can simply line up an encounter with a planet and tweak the parameters of the pass until you have an encounter with another planet lined up, then repeat until you get where you want to go.  That requires a good interactive simulator and may or may not result in an acceptable mission duration, delta-v budget, etc, but it should get you there eventually.&lt;/p&gt;&#xA;&lt;p&gt;If you want a more optimal solution you can teach a computer to do the above and have it just try all reasonable paths you have delta-v budget for in your target time frame.  Actually, it seems that implementing a &lt;a href=&quot;http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1011&amp;amp;context=aerosp&quot; rel=&quot;nofollow noreferrer&quot;&gt;brute force algorithm for optimizing gravity assisted trajectories&lt;/a&gt; is a good start on a master's thesis at California Polytechnic State University.  To quote the paper:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In order to plan out multiple gravity assisted trajectories, complex and robust computer simulations are required to filter through the continuum of possibilities and select trajectories that optimally satisfy the mission&#xA;requirements. This paper discusses one such computer simulation which seeks to minimize the propulsive delta v requirements of a spacecraft for a trajectory between two specified planets utilizing a specified number of planetary flybys along the way.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;And farther down:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The approach  outlined  in  this  report  is  referred  to  as  a  patched  Lamberts  problem and  utilizes  a brute force tactic. The underlying principle of this strategy involves breaking down a trajectory into separate segments or legs and analyzing each leg one at time with respect to the leg prior to it. Each leg is defined as an orbital trajectory between planet A and planet B, where planets A and  B  can  be  either  the  same  planet  or  different  ones.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From skimming the paper it sounds like you start with a range of launch times (determines what planets are in alignment for providing useful assists), then simulate the range of possible planetary encounters to determine how much energy you can gain/lose at each, then from the time of encounter figure out what time frame you'd be leaving the planet and repeat until you've found an encounter with your target planet.  That actually sounds very much like automating my original suggestion of just playing with it manually in a simulator until it gets you there.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;In terms of existing planning tools &lt;a href=&quot;http://forum.kerbalspaceprogram.com/threads/36476-WIN-KSP-Trajectory-Optimization-Tool-v0-10-Now-with-Rendezvous-Planner!&quot; rel=&quot;nofollow noreferrer&quot;&gt;KSP Trajectory Optimization Tool&lt;/a&gt; claims to support gravity assists and you can change the planets by modifying a config file so in theory you can configure it for the real solar system values.  I didn't try it (doesn't advertise linux support or I'd have been very tempted) but the KSP community is the only group I know of that does this type of mission planning for fun and provides tools and documentation online so I don't know where else you'd find a decent tool.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-18T19:58:17.197" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10025" PostTypeId="1" CreationDate="2015-07-18T21:04:12.043" Score="7" ViewCount="1578" Body="&lt;p&gt;My question is about operating costs of propellants, from production, handling, fueling, and anything else needed for the props. I am &lt;strong&gt;not&lt;/strong&gt; just asking about how expensive it is to produce them. Operating costs must be taken into account, from production to transportation to loading and launching the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My meager understanding of these radically different propellants is giving me conflicting information:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;LH2/LOX is extremely cryogenic. It's hard to store and requires frequent replenishment. It's also more expensive to insulate everything from the tanks to the plumbing.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;UDMH/N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; is extremely acidic, toxic, sensitive to air, and apparently has just about every other terrible chemical property imaginable. It's hard to handle (transport and fuel up), requiring a lot more safety precautions (expenses).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So which is actually more expensive to use? LH2/LOX or UDMH/N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure there are some specific cases where one will be cheaper than the other, but in other specific cases it will be the other way around. Therefore, I want to be specific and list a concrete design problem:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Let's design a final rocket stage capable of providing a 3 km/s delta-v to a 10 ton payload. Would it be cheaper to use LH2/LOX or UDMH/N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;?&lt;/strong&gt; In other words, what is the &lt;strong&gt;cost to build and launch&lt;/strong&gt; this stage using LH2/LOX, and then what is the &lt;strong&gt;cost to build and launch&lt;/strong&gt; this stage using UDMH/N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And it doesn't need to be precise costs. &lt;strong&gt;I'm looking for which one will be relatively cheaper and why.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: since it seems that different nations will have different regulations affecting cost of these propellants, I will be clear and ask for the cost &lt;strong&gt;in the USA.&lt;/strong&gt; However, if someone wants to do the same calculations/comparisons for Russia, that would be very useful too and certainly wouldn't go amiss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is admittedly not an easy question, but I believe it's answerable if you can find the right engineering data and/or experience (or white paper). I haven't been able to, however.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="49" LastEditDate="2016-02-01T09:21:56.010" LastActivityDate="2016-06-17T18:36:10.010" Title="Which is overall more expensive to operate for a final rocket stage? LH2/LOX or hypergolics like UDMH/N2O4?" Tags="|rockets|fuel|cost|upper-stage|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="10026" PostTypeId="2" ParentId="9989" CreationDate="2015-07-18T22:38:48.630" Score="2" Body="&lt;p&gt;LORRI images &lt;a href=&quot;http://pluto.jhuapl.edu/soc/Pluto-Encounter/lorri_about.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;may be cropped&lt;/a&gt; to save bandwidth.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This figure is an example of a LORRI 1x1 image that is missing some data. In this case, the data are missing by design to reduce downlink data volume using a process called &quot;windowing&quot;. The region of interest, also called the &quot;window&quot;, is captured from the LORRI detector (the location of Pluto is marked), and the rest of the image is not even downlinked.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm not sure whether the &quot;slice&quot; is necessarily down the middle, or if they saved the parts that weren't downlinked. It does seem pretty certain that the terminator was the most interesting part, so that's all they scheduled for immediate downlink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Uvrlm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="3054" LastActivityDate="2015-07-18T22:38:48.630" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10027" PostTypeId="2" ParentId="8782" CreationDate="2015-07-18T23:06:44.370" Score="2" Body="&lt;p&gt;No, it's not a word currently in use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I can't find an existing name for the ends of the minor axis of an orbit (or even an ellipse).  As you noted there doesn't seem to be much use for the point, which would be why it's not named.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As TildalWave pointed out in a comment the general word would be mediapsis (-helion being the -apsis suffix for orbits around the sun), and that doesn't actually make sense when considering the modern use of -apsis (as in &quot;apoapsis&quot; and &quot;periapsis&quot;).  To quote the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apsis&quot; rel=&quot;nofollow&quot;&gt;wikipedia page for &quot;Apsis&quot;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An apsis (Greek ἁψίς, gen. ἁψίδος), plural apsides (/ˈæpsɨdiːz/; Greek: ἁψίδες), is a point of least or greatest distance of a body in an elliptic orbit about a larger body.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On the other hand, in a comment Mark pointed out that the &lt;a href=&quot;http://www.newworldencyclopedia.org/entry/Apsis&quot; rel=&quot;nofollow&quot;&gt;etymology of apsis&lt;/a&gt; indicates that other usage besides peri- and apo- would be valid:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The term apsis is derived from the Latin word apsis, meaning &quot;arch&quot; or &quot;vault,&quot; which can be traced back to the Greek word hapsis, meaning &quot;loop&quot; or &quot;arch,&quot;...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The lack of an existing name means if you want a name you'll have to make it up and &quot;medihelion&quot; at least sounds like part of an orbit (and arguably works as a use of the -apsis suffix).  I'd recommend providing (or at least implying) a definition to mitigate the risk of people trying to look it up and being frustrated when they can't find it.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-19T14:54:10.727" LastActivityDate="2015-07-19T14:54:10.727" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10028" PostTypeId="1" CreationDate="2015-07-19T00:03:54.053" Score="5" ViewCount="441" Body="&lt;p&gt;Why does the New Horizons spacecraft have so little memory (&lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Spacecraft/Systems-and-Components.php&quot; rel=&quot;noreferrer&quot;&gt;8 Gigabytes for a single buffer&lt;/a&gt;)? I'd have donated a few memory sticks if I could.&lt;/p&gt;&#xA;" OwnerUserId="10972" LastEditorUserId="168" LastEditDate="2015-07-19T01:21:31.807" LastActivityDate="2015-07-19T03:21:22.080" Title="What drives the choice of onboard memory size for the New Horizons probe?" Tags="|new-horizons|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10029" PostTypeId="1" AcceptedAnswerId="10039" CreationDate="2015-07-19T00:16:37.060" Score="30" ViewCount="3767" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt; recently hit &lt;a href=&quot;http://hubble25th.org/&quot; rel=&quot;noreferrer&quot;&gt;25 years in space&lt;/a&gt;.  It orbits at ~550 km altitude (limited by the altitude the shuttle could reach to service it) and has sustained minor/trivial exterior damage from debris strikes (see image below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that the main mirror is down the tube from the opening that lets light in, but as I understand it there's nothing inside to protect it from debris that enters directly on the axis of the telescope.  &lt;strong&gt;What strategies were used in the design and operation to minimize damage to the Hubble main mirror such that it's still able to perform after 25 years in orbit?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At only 15 years the radiator section (rear end) of the Wide Field Planetary Camera II (WFPC2) had sustained these debris strikes (note that the holes are from coring out the impact points, not the debris itself which would have left much smaller marks):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://airandspace.si.edu/explore-and-learn/multimedia/detail.cfm?id=6572&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/25vbc.jpg&quot; alt=&quot;Wide Field Planetary Camera II radiator&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;This image shows the radiator section (rear end) of the Wide&#xA;  Field Planetary Camera II (WFPC2), installed on the Hubble Space&#xA;  Telescope. During the various Hubble servicing missions, astronauts&#xA;  noticed tiny dimples and dents in the radiator – the result of space&#xA;  debris. After more than15 years of exposure to space, this surface&#xA;  became a record of the accumulation of such debris in low Earth orbit.&#xA;  Naturally, NASA wanted to evaluate the amount and nature of this&#xA;  debris, and so after the camera was returned to Earth the impact sites&#xA;  were analyzed. The largest core samples left holes about 30 mm in&#xA;  diameter, but the debris particles were less than a mm in size.&lt;/sup&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;sup&gt;Image credit: Eric Long, National Air and Space Museum. Source:&#xA;  &lt;a href=&quot;http://airandspace.si.edu/explore-and-learn/multimedia/detail.cfm?id=6572&quot; rel=&quot;noreferrer&quot;&gt;Smithsonian National Air and Space Museum&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Unlike the WFPC2 the main mirror is not serviceable on-orbit so it's been up there exposed to space for the entire 25 years (or at least all the time the telescope is actually observing/has the lens cap open).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe I overestimate the amount of debris in orbit (my professional introduction to stuff in LEO was in the context of building/improving a system to track space debris), but I would think by now there would have been enough (un)lucky debris strikes to have meaningfully damaged the mirror.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Note that this question is similar but asks about the effects of damage, not what was done to prevent it: &lt;a href=&quot;https://space.stackexchange.com/questions/4703/accumulated-environmental-damage-to-hubble-main-mirror&quot;&gt;Accumulated environmental damage to Hubble main mirror&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2019-03-28T15:39:38.903" Title="How is the Hubble main mirror protected from debris?" Tags="|low-earth-orbit|debris|hubble|protection|meteoroid|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10030" PostTypeId="1" CreationDate="2015-07-19T01:07:00.870" Score="8" ViewCount="1368" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/questions/9209/did-we-know-the-gas-planets-were-truly-gaseous-before-pioneer-10&quot;&gt;my related question&lt;/a&gt;, I'm still in search of historical knowledge of the outer planets. That is, how our understanding of them developed over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So right now I'm hunting for the highest quality photos of Uranus and Neptune that we had before Voyager flew by them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've looked &lt;a href=&quot;http://www.anamorfose.be/night-photography/solar-system-and-comets/mount-wilson-palomar-observatories-jupiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; since it has a good one of Jupiter, but couldn't find any for Neptune or Uranus. I'm starting to wonder if those last 2 planets would even show up as resolvable disks, given the huge distances and relatively small telescopes of the era. I think the Hale Telescope at the Palomar Observatory was just 5 m wide in its primary mirror.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After googling this for so long, I've come to the conclusion that the best bet would be old astronomy books, which sadly, I do not have. Can anyone help me out?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2023-08-24T06:39:27.323" Title="What were the highest quality photos and knowledge of Uranus and Neptune before Voyager?" Tags="|history|neptune|uranus|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10031" PostTypeId="2" ParentId="10028" CreationDate="2015-07-19T02:33:06.923" Score="2" Body="&lt;p&gt;Space rated memory is much more expensive, and much less dense than commercially available RAM. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Single Event Upsets are common, even in space rated hardware, and commercial stuff would be riddled with errors due to high energy particles that the Earth's atmosphere normally protects us from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Error detection and correction is critical in a way that commercial RAM does not care about, even as compared to high end servers. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-19T02:33:06.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10032" PostTypeId="1" CreationDate="2015-07-19T03:06:37.093" Score="3" ViewCount="389" Body="&lt;p&gt;I found &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1270&amp;amp;context=smallsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; PDF while searching for any and all info related to LMP-103S, which you can read more about in &lt;a href=&quot;https://space.stackexchange.com/questions/9421/is-lmp-103s-truly-a-viable-replacement-for-hydrazine&quot;&gt;my related question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the PDF, if you scroll down to page 4, you will find a chart listing various performances of several thrusters, starting from 0.5 N to 220 N. If you look at that last one, it actually lists the fuel as &lt;em&gt;&quot;LMP-103&quot;&lt;/em&gt;, not &lt;em&gt;&quot;LMP-103&lt;strong&gt;S&lt;/strong&gt;&quot;&lt;/em&gt;, and the exit velocity listed is 2.8 km/s for a specific impulse of 285 seconds!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this a typo? Or is there some new derivative fuel lurking out there that has this amazing performance? I hope so, and tend to think so because that listed Isp is a rather big jump from the others. I really hope it is accurate.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-02T17:04:54.283" Title="Is there a derivative of LMP-103S, called LMP-103, that has even higher performance?" Tags="|fuel|monopropellant|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10033" PostTypeId="2" ParentId="10028" CreationDate="2015-07-19T03:21:22.080" Score="9" Body="&lt;p&gt;Two reasons why the memory is so small:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;New Horizons was designed around the year 2000.  Solid-state memory sizes were much smaller back then: looking at the Internet Archive, the typical CompactFlash memory card was a 64 MB card costing around $50.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The memory needs to be space-rated.  That $50 card I mentioned?  Outside the Earth's atmosphere, the memory will be scrambled in a matter of days due to thermal effects, solar radiation, or cosmic rays, and the card will likely be permanently destroyed over the course of a year or so.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The 8GB of solid-state memory aboard New Horizons probably cost half a million to a million dollars.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2015-07-19T03:21:22.080" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10034" PostTypeId="5" CreationDate="2015-07-19T04:02:36.190" Score="0" Body="" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-19T04:02:36.190" LastActivityDate="2015-07-19T04:02:36.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10035" PostTypeId="4" CreationDate="2015-07-19T04:02:36.190" Score="0" Body="Questions about meteoroids and micrometeoroids, small solid extraterrestrial bodies traveling through interplanetary space. Meteoroids that enter the Earth's atmosphere are called meteors, and remains of those that don't entirely burn or explode in the atmosphere and hit the ground are called meteorites. " OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-19T04:02:36.190" LastActivityDate="2015-07-19T04:02:36.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10036" PostTypeId="1" AcceptedAnswerId="10037" CreationDate="2015-07-19T04:43:56.620" Score="5" ViewCount="3073" Body="&lt;p&gt;As everyone knows, New Horizons, which was sent some decades ago, reached Pluto on July 14th, 2015 and it stayed there for only 24 hours and after that it is going into the Kuiper Belt and thereafter into interstellar space like Pioneer and Voyager probes did before it. My question is, why was New Horizons not allowed to stay in Pluto's orbit to collect information on the dwarf planet, like MOM (Mars orbiter Mission) is doing around Mars? Is this due to scientists being less interested in searching for life in the Pluto system, or are there any other reasons?&lt;/p&gt;&#xA;" OwnerUserId="1062" LastEditorUserId="49" LastEditDate="2015-07-19T06:27:50.507" LastActivityDate="2015-07-19T06:27:50.507" Title="Why didn't New Horizons stay in Pluto's orbit?" Tags="|orbit|probe|new-horizons|pluto|mission-design|" AnswerCount="1" CommentCount="2" ClosedDate="2015-07-19T11:03:05.567" ContentLicense="CC BY-SA 3.0" />
  <row Id="10037" PostTypeId="2" ParentId="10036" CreationDate="2015-07-19T06:17:00.563" Score="4" Body="&lt;p&gt;New Horizons was launched roughly 9 and a half years ago, on January 19, 2006. So not &lt;em&gt;decades ago&lt;/em&gt;. &lt;a href=&quot;http://www.joshworth.com/dev/pixelspace/pixelspace_solarsystem.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pluto is far, far away&lt;/a&gt;. In fact, so far away, that both the Sun and the Earth, together with the rest of the inner Solar system planets (Mercury, Venus and Mars) that are closer to us, appear in that small inner ellipse on this image of the New Horizons flyby trajectory:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/olWai.png&quot; alt=&quot;New Horizons flyby trajectory&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And while we managed to launch New Horizons at record setting speed of 16.26 kilometers per second (58,536 km/h or 36,373 miles per hour) on its way to Pluto and setting it free from the Earth's gravitational grip that keeps you well seated on your chair, all of its way there it was still pulled by the immense Sun's gravity towards it and was slowed down by it on its way to Pluto, so it also used a Jupiter &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity assist&lt;/a&gt; to gain an extra boost, trading some of Jupiter's &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_momentum&quot; rel=&quot;nofollow noreferrer&quot;&gt;angular momentum&lt;/a&gt; in its orbit around the Sun for higher exit velocity towards Pluto. A tiny amount of this momentum for the super-massive gas giant Jupiter, but a big push forward for in comparison tiny New Horizons (nearly 1/4 of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Quadrillion&quot; rel=&quot;nofollow noreferrer&quot;&gt;quadrillionth&lt;/a&gt;, or nearly 4,000,000,000,000,000,000,000,000 of New Horizons would equal the mass of Jupiter). And its velocity gain with respect to the Sun looked like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/UERtc.png&quot; alt=&quot;Heliocentric Velocity of the New Horizons Spacecraft over the flight from Earth to Pluto&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now that the New Horizons probe is going &lt;em&gt;real fast, so vastly hugely mind-boggling fast that your trip to the chemist seems just peanuts in comparison&lt;/em&gt;, it managed to get to Pluto at a &lt;a href=&quot;https://space.stackexchange.com/q/9673/49&quot;&gt;relative velocity of 13.78 km/s&lt;/a&gt; (49,600 km/h or 30,800 miles per hour). And even for a &lt;em&gt;measly&lt;/em&gt; 478 kg probe, halting it to a stop would require a tremendous amount of energy and some form of reaction mass to accelerate in the opposite direction, and quite a bit of it. And that's something that New Horizons doesn't have available to it, and if it did, it would increase its mass, it would then be launched at much slower speed towards Pluto, and really take decades to get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in other words, New Horizons was going too fast to have halted to a near stop and remained at Pluto for longer. It wasn't a bus trip with stops along the way, but a speeding bullet (actually about 50-60 times faster than that) moving through the vast nothingness of the vacuum of space. And unless it hits something along its way, it will continue to do so for many millennia to come and exit our Solar system entirely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information, see &lt;a href=&quot;https://space.stackexchange.com/q/9851/49&quot;&gt;Requirements to orbit Pluto&lt;/a&gt; and other answered questions tagged as &lt;a href=&quot;/questions/tagged/new-horizons&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;new-horizons&amp;#39;&quot; rel=&quot;tag&quot;&gt;new-horizons&lt;/a&gt;. All images in my answer are from &lt;a href=&quot;http://www.boulder.swri.edu/pkb/ssr/ssr-mission-design.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons Mission Design&lt;/a&gt;, Yanping Guo and Robert W. Farquhar, Johns Hopkins University Applied Physics Laboratory (PDF).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-07-19T06:22:04.137" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10038" PostTypeId="1" CreationDate="2015-07-19T07:17:29.940" Score="9" ViewCount="1270" Body="&lt;p&gt;OK, so chemistry and physics are not my strong suit, so this idea is just cobbled together from stuff I've heard can be done. But I'd be curious if it is at all plausible, and what the most likely pitfalls would be.&lt;/p&gt;&#xA;&lt;h2&gt;Procedure:&lt;/h2&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Set up a hydrogen mining platform on Jupiter.&lt;/li&gt;&#xA;&lt;li&gt;Harvest iron from as near as possible (not sure if there is enough iron dust around Jupiter, or if it would have to come from further afield. I suppose there is no practical way to get iron and nickel from the core with the crazy pressures down there).&lt;/li&gt;&#xA;&lt;li&gt;Get the iron/iron ore to the &lt;span class=&quot;math-container&quot;&gt;$\require{mhchem}\ce{H2}$&lt;/span&gt; mining platform.&lt;/li&gt;&#xA;&lt;li&gt;Fabricate &lt;span class=&quot;math-container&quot;&gt;$\ce{H2}$&lt;/span&gt; filled iron spheres.&lt;/li&gt;&#xA;&lt;li&gt;Use a rail gun to shoot them out of Jupiter's gravity, on a trajectory to intersect with Venus.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;h2&gt;Expected results:&lt;/h2&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The iron spheres would burn up or react with &lt;span class=&quot;math-container&quot;&gt;$\ce{H2SO4}$&lt;/span&gt; in the Venusian atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;The temperature will be high enough for the &lt;span class=&quot;math-container&quot;&gt;$\ce{H2}$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\ce{CO2}$&lt;/span&gt; to react, and the disintegrating iron from the sphere would catalyze a Bosch reaction, producing &lt;span class=&quot;math-container&quot;&gt;$\ce{H2O}$&lt;/span&gt; and graphite.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;h2&gt;Reasons why this might be a good idea:&lt;/h2&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Scalable. Experimental &lt;span class=&quot;math-container&quot;&gt;$\ce{H2 - Fe}$&lt;/span&gt; spheres could be made near Earth, shot at Venus, and the effect measured, so the reaction could be tested and optimized.&lt;/li&gt;&#xA;&lt;li&gt;Swarm based. The fate of any individual sphere matters little, making the system more resilient to space debris.&lt;/li&gt;&#xA;&lt;li&gt;The magnetic properties of the spheres could be harnessed to minimize the cost of propelling the spheres to Venus.&lt;/li&gt;&#xA;&lt;li&gt;Could probably be fully automated.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;h2&gt;Reasons why this might be a bad idea.&lt;/h2&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The magnetic launch of the spheres might not practical. This plan depends on it being feasible and cost effective to hurl the spheres at Venus such that they do not need propulsion systems of their own. I imagine a hose &lt;span class=&quot;math-container&quot;&gt;$\ce{H_2 - Fe}$&lt;/span&gt; bubbles squirting Venus from across the Solar System, but it is late at night and my imagination may be getting the better of me.&lt;/li&gt;&#xA;&lt;li&gt;It might be too hard to target the spheres, even if they could be shot across the Solar System at low cost. Too many might miss. What would happen if some rained on Earth?&lt;/li&gt;&#xA;&lt;li&gt;The &lt;span class=&quot;math-container&quot;&gt;$\ce{Fe}$&lt;/span&gt; could be eaten up by the &lt;span class=&quot;math-container&quot;&gt;$\ce{H2SO4}$&lt;/span&gt; in the upper atmosphere of Venus, preventing it from acting as a catalyst.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;h2&gt;Added questions for clarity:&lt;/h2&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How much energy would it take to launch H2 filled iron spheres from Jupiter's orbit across the solar system to Venus? Assuming they are on the order of a few grams to a few kilograms each.&lt;/li&gt;&#xA;&lt;li&gt;How fast would they have to be shot in order that their path not be&#xA;overly disrupted by the gravitational pull of other objects on their&#xA;way their (say, to get 50% of the spheres to fall on Venus.&lt;/li&gt;&#xA;&lt;li&gt;What would be the optimal ratio of &lt;span class=&quot;math-container&quot;&gt;$\ce{H2}$&lt;/span&gt; pressure inside to thickness of the iron sphere?&lt;/li&gt;&#xA;&lt;li&gt;Would an &lt;span class=&quot;math-container&quot;&gt;$\ce{Fe}$&lt;/span&gt; sphere filled with &lt;span class=&quot;math-container&quot;&gt;$\ce{H2}$&lt;/span&gt; falling into the&#xA;Venusian atmosphere as a meteor be expected to produce a Bosch&#xA;reaction?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10979" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-19T16:27:16.723" Title="Terraforming Venus with the Bosch reaction, using hydrogen from Jupiter" Tags="|venus|mining|terraforming|chemistry|rail-gun|" AnswerCount="0" CommentCount="8" ClosedDate="2015-07-23T22:41:37.767" ContentLicense="CC BY-SA 3.0" />
  <row Id="10039" PostTypeId="2" ParentId="10029" CreationDate="2015-07-19T09:43:47.623" Score="17" Body="&lt;p&gt;There was a &lt;a href=&quot;http://ston.jsc.nasa.gov/collections/trs/_techrep/TP-2003-210788.pdf&quot;&gt;design requirement&lt;/a&gt; (PDF, page 30) for a &quot;probability of no penetration&quot; of 95% for 2 years. &#xA;The &lt;a href=&quot;http://www.hq.nasa.gov/office/codeq/doctree/NHBK871914.pdf&quot;&gt;Space Debris Handbook&lt;/a&gt; (PDF, page 137) indicates the main risk was considered to be a light leak in the aft shroud. &#xA;I haven't been able to find any specifics on construction details that ensure this.&lt;br&gt;&#xA;Specifically, I haven't found &lt;a href=&quot;http://hubblesite.org/the_telescope/nuts_.and._bolts/spacecraft_systems/#housing&quot;&gt;any mention of Whipple shielding&lt;/a&gt; (which seems to be the default method). The outer layer of insulation on most of the HST is &lt;a href=&quot;http://asd.gsfc.nasa.gov/archive/sm3a/downloads/sm3a_media_guide/HST-systems.pdf&quot;&gt;MLI&lt;/a&gt; (PDF, page 17). The aft shroud has a &lt;a href=&quot;https://www.spacetelescope.org/images/s82e5242/&quot;&gt;rigid metallic outer layer&lt;/a&gt;, but I haven't been able to confirm if this has any shielding function. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-19T09:43:47.623" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10040" PostTypeId="2" ParentId="10030" CreationDate="2015-07-19T11:51:20.350" Score="5" Body="&lt;p&gt;I believe it's so hard to find a good picture because there wasn't one. The best telescope in the world in the 1980s was Hale telescope at Palomar Observatory. It was a 5m telescope, but subject to atmospheric effects. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Uranus has a resolution of about 18 urads from Earth, or about 3.7 arc seconds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pre-adaptive optics, the best one could hope for in terms of angular resolution was around 1 arc second. Given that, it seems that Uranus would be little more than a blur, resolvable, but with no real detail. Neptune would be even worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that for the most part, all of the fancy tricks that can be used today to observe planets weren't around. There was no adaptive optics, no digital cameras recording hundreds of frames to find a &quot;lucky&quot; frame, and other similar things. This was a serious challenge of the day, to get good pictures of items like planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also keep in mind that Neptune and Uranus are less colorful than Saturn and Neptune, making photos of them less interesting than would otherwise be obtained.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-19T17:46:44.070" LastActivityDate="2015-07-19T17:46:44.070" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10041" PostTypeId="2" ParentId="10030" CreationDate="2015-07-19T14:30:27.293" Score="7" Body="&lt;p&gt;My 1965 &lt;em&gt;Sourcebook On The Space Sciences&lt;/em&gt; indicates that they are resolvable:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The polar flattening of [Uranus] is clear from its telescopic image and the ellipticity determined from the motion of the nearest satellite is 1/18. The result derived from direct measurement of the equatorial and polar diameters is about 1/12, but since the maximum angular diameter of Uranus as seen from Earth is only 4 sec of arc, it is evident that this value of the ellipticity cannot be very accurate...&lt;/p&gt;&#xA;&lt;p&gt;In the telescope, Uranus is seen as a somewhat flattened spherical body with a greenish color... at certain times a whitish equatorial zone and other bands, similar to but fainter than those of Jupiter and Saturn, can be observed.&lt;/p&gt;&#xA;&lt;p&gt;Like Uranus... Neptune has a greenish color... Observers have occasionally reported seeing faint irregular markings and a bright equatorial zone on Neptune, but the situation is uncertain.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;However, there are no photos of either in the book, unlike Jupiter and Saturn, which both have fairly detailed mid-century photos from Hale-Palomar.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The 1964 English translation edition of &lt;em&gt;Flammarion Book Of Astronomy&lt;/em&gt; has photos of Uranus from &lt;em&gt;1936&lt;/em&gt;! A very small disc is visible, with no evident features. No photos of Neptune are in the book, possibly because it would show merely a point.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/X8Hfg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/X8Hfg.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Interestingly, the book has a variety of pictures of Jupiter and Saturn from the 1910s through the 1950s, from different observatories (Flammarion, Lowell). Remarkably, they all show comparable levels of detail in Jupiter, with the main difference, as far as I can tell, being the quality of the photographic film grain.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-11T04:03:04.750" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10042" PostTypeId="2" ParentId="8782" CreationDate="2015-07-19T15:38:42.173" Score="8" Body="&lt;p&gt;There is a name in geometry for the two points on the ellipse / conic section that you describe - they're called co-vertices:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sgbMw.png&quot; alt=&quot;vertex and co-vertex&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image source: &lt;a href=&quot;http://www.ck12.org/book/CK-12-Algebra-II-with-Trigonometry-Concepts/r3/section/10.5/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CK-12, Conic Sections - Ellipses Centered at the Origin&lt;/a&gt;, Image licensed under &lt;a href=&quot;http://www.ck12.org/about/terms-of-use/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CC BY-NC&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wasn't able to find any mention of &lt;em&gt;medihelion&lt;/em&gt; or &lt;em&gt;mediapsis&lt;/em&gt; in literature, neither does it return any signs of life in &lt;a href=&quot;https://books.google.com/ngrams/graph?content=medihelion%2Cmediapsis&amp;amp;case_insensitive=on&amp;amp;year_start=1800&amp;amp;year_end=2008&amp;amp;corpus=15&amp;amp;smoothing=3&amp;amp;share=&amp;amp;direct_url=&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google's Ngram Viewer&lt;/a&gt; searching through large corpus of books from 1800 - 2008. So it looks like you could claim authorship of the new term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for your comment, if it &lt;em&gt;wouldn't be bad form to slap on a Latin prefix to modify a Greek word&lt;/em&gt; – &lt;a href=&quot;https://space.stackexchange.com/a/6641/49&quot;&gt;it wouldn't be a first&lt;/a&gt;. We also slap Latin prefices to English words all the time (e.g. &lt;em&gt;&lt;a href=&quot;http://dictionary.reference.com/browse/pre-&quot; rel=&quot;nofollow noreferrer&quot;&gt;preschool; prewar; prepay&lt;/a&gt;&lt;/em&gt;...) and majority doesn't seem to have any issues with it, even though there is a perfectly good Old English equivalent &lt;em&gt;&lt;a href=&quot;http://dictionary.reference.com/browse/fore-&quot; rel=&quot;nofollow noreferrer&quot;&gt;fore-&lt;/a&gt;&lt;/em&gt; for it.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-07-19T21:13:37.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10043" PostTypeId="1" AcceptedAnswerId="10045" CreationDate="2015-07-19T16:03:09.840" Score="3" ViewCount="6584" Body="&lt;p&gt;Is there a relation between the mass of a satellite and its orbital altitude? I mean, will a heavy satellite orbit at a higher or lower altitude than a lighter one, considering everything else being equal?&lt;/p&gt;&#xA;" OwnerUserId="9062" LastEditorUserId="49" LastEditDate="2015-07-19T17:44:20.597" LastActivityDate="2017-11-21T23:04:35.420" Title="Relationship between satellite's mass and orbital altitude" Tags="|artificial-satellite|orbit|low-earth-orbit|gravity|altitude|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10044" PostTypeId="2" ParentId="10043" CreationDate="2015-07-19T17:12:23.787" Score="4" Body="&lt;p&gt;Yes and no. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;No: The equations that govern &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion&quot; rel=&quot;nofollow&quot;&gt;the orbit of a satellite&lt;/a&gt; don't say anything about weight, so if you launch a heavy satellite and a light one to the same orbit, they'll stay together. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes: In the equations that govern &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;the launch of a satellite&lt;/a&gt;, weight is a variable. When you launch two identical rockets, one with a heavy payload and one with a light payload, the light payload will end up in a higher orbit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-19T17:12:23.787" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10045" PostTypeId="2" ParentId="10043" CreationDate="2015-07-19T18:59:21.010" Score="8" Body="&lt;p&gt;While this question betrays a serious lack of understanding of physics, it's also a common lack of understanding, so I think it is in a way a good question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are used to helium balloons floating up into the air, wood floating on the surface of water, and cannon balls sinking to the bottom of the sea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Intuitively, when we think of things &quot;floating&quot; in space we try to do it with the analogy of buoyancy because that's what we are used to, but the physics of buoyancy simply do not apply in a vacuum where there is no air or water pressure to &quot;bouy&quot; an object up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When an object is in space, it is actually in perpetual freefall, &lt;em&gt;nothing is holding it up against the force of gravity and it is constantly falling towards Earth with no resistance whatsoever&lt;/em&gt;, the force of gravity is unopposed. Because there is no resistance, the material properties of the object simply do not matter, explanations of this often involve &lt;a href=&quot;http://www.physicsclassroom.com/mmedia/newtlaws/efff.cfm&quot; rel=&quot;noreferrer&quot;&gt;a feather compared with something heavy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet how can something be constantly falling but never reach the ground? The reason is because it is also travelling sideways at very high speeds, this causes it to take a curved trajectory that &lt;em&gt;misses the earth&lt;/em&gt;. See &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_cannonball&quot; rel=&quot;noreferrer&quot;&gt;Newton's cannonball&lt;/a&gt; for a visual example of how this kind of trajectory works.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-07-19T18:59:21.010" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10046" PostTypeId="1" AcceptedAnswerId="10047" CreationDate="2015-07-19T19:55:56.590" Score="1" ViewCount="882" Body="&lt;p&gt;I'm using an API from NASA on this URL: &lt;a href=&quot;http://api.open-notify.org/iss-now.json&quot; rel=&quot;nofollow&quot;&gt;http://api.open-notify.org/iss-now.json&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It gives me the location of the ISS as a longitude and latitude. &lt;strong&gt;How can I find it's position in space?&lt;/strong&gt; &lt;/p&gt;&#xA;" OwnerUserId="10986" LastEditorUserId="1028" LastEditDate="2015-07-20T14:13:40.473" LastActivityDate="2015-07-20T14:13:40.473" Title="Why does the ISS use longitude and latitude to specify its position?" Tags="|iss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10047" PostTypeId="2" ParentId="10046" CreationDate="2015-07-19T20:02:53.653" Score="2" Body="&lt;p&gt;The location of the ISS, when represented as a longitude and latitude, signifies the &lt;strong&gt;location on the surface of the Earth directly below the ISS at a given time.&lt;/strong&gt; The location of spacecraft orbiting the Earth are usually represented in ground-terms because they're far easier to comprehend than the actual orbital parameters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to visualise the ISS in space, picture the longitude and latitude. Then imagine the station about 400 kilometers above that point traveling at ~7 kilometers per second. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Here is a useful resource where you can find the location of the ISS at the current time. &lt;a href=&quot;http://iss.astroviewer.net/&quot; rel=&quot;nofollow&quot;&gt;http://iss.astroviewer.net/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-07-19T20:02:53.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10048" PostTypeId="1" AcceptedAnswerId="10053" CreationDate="2015-07-19T22:28:30.883" Score="19" ViewCount="6284" Body="&lt;p&gt;Today an asteroid, &lt;a href=&quot;http://news.sky.com/story/1521488/mining-firm-aiming-for-platinum-loaded-asteroid&quot;&gt;2011 UW158&lt;/a&gt;, is passing within 2.4 million km of Earth. There are a lot of reports of this asteroid containing $5.4 trillion of platinum in its core. People seem to want to mine it, but how do they know there is so much platinum within it? They seem to have a fairly specific number.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2017-06-08T00:07:06.257" Title="How is it known that asteroid 2011 UW158 has so much platinum?" Tags="|asteroid|mining|natural-resources|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10049" PostTypeId="2" ParentId="9939" CreationDate="2015-07-19T22:56:21.987" Score="3" Body="&lt;p&gt;Phil has explained it nicely. Yes, it is BPSK with R=1/6 Turbo coding, that is used in New Horizons. Some aspects of the X band digital receiver used is discussed &lt;a href=&quot;http://www.boulder.swri.edu/~tcase/NH%20Uplink%20Card%20Paper_Haskins.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. For the coding enthusiasts, I can add that the Blocklength is k=1784 bits (even though it is programmable, for low data rate support, this minimum block length is used). The information code length (post coding) n =(k+4)/R=10728 bits. &lt;/p&gt;&#xA;" OwnerUserId="10991" LastActivityDate="2015-07-19T22:56:21.987" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10050" PostTypeId="1" AcceptedAnswerId="13076" CreationDate="2015-07-20T00:13:05.440" Score="5" ViewCount="1071" Body="&lt;p&gt;I asked this over at &lt;a href=&quot;https://physics.stackexchange.com/questions/192884/what-factors-are-influencing-a-woosh-rocket?noredirect=1#comment410904_192884&quot;&gt;Physics&lt;/a&gt;, but no one seemed to know, so I'll try here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I fill a plastic bottle with a small amount of rubbing alcohol, shake it around and hold a match to the bottle neck, I'll get a flame wooshing out of the opening. This can easily be used to propel the bottle upwards, like a rocket. I tried it already, there are lots of youtube videos about this, and NASA has instructions for the experiment as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When trying to go for maximum speed or height of the rocket, there are a few factors influencing this experiment. Here are the ones that I can think of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Some related to the bottle:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;volume, shape, aerodynamics, weight, size of opening, possibly shape of the bottle (long and thin or short and fat) When the bottle neck is smaller it creates a stronger thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Others related to the propellant:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How fast does it burn, how dense is it (especially compared to air), how heavily does it vaporize. If the vapor is denser then air it all floats out of the opening of my bottle before I can light it and I don't have as much fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you might be able to see, I don't know a lot about the propellants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here's my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the factors related to the propellant, that influence the launch height and speed of the woosh rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What's the best propellant to choose for a woosh rocket, and why?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="3481" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2019-08-15T22:37:50.527" Title="What factors are influencing a woosh rocket?" Tags="|rockets|fuel|experiment|model-rocketry|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10051" PostTypeId="5" CreationDate="2015-07-20T00:27:15.453" Score="0" Body="&lt;p&gt;The Hubble Space Telescope (HST) is a space telescope that was launched into low Earth orbit in 1990, and remains in operation. With a 2.4-meter (7.9 ft) mirror, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared spectra. The telescope is named after the astronomer Edwin Hubble.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble's orbit outside the distortion of Earth's atmosphere allows it to take extremely high-resolution images with negligible background light. Hubble has recorded some of the most detailed visible-light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When launched the main mirror of Hubble was ground incorrectly and was therefore unable to focus light properly.  The telescope was designed to be serviced by Space Shuttle missions, so in 1993 the first servicing mission installed the Corrective Optics Space Telescope Axial Replacement (COSTAR) to act as &quot;spectacles&quot; to correct the spherical aberration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When asking questions relating to Hubble observations keep in mind that this is the &quot;Space Exploration&quot; stack exchange.  While we will likely try to answer you questions the &lt;a href=&quot;http://astronomy.stackexchange.com/&quot;&gt;Astronomy&lt;/a&gt; stack exchange may provide better answers depending on the subject of your question.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://asd.gsfc.nasa.gov/archive/hubble/missions/intro.html&quot; rel=&quot;nofollow&quot;&gt;NASA Hubble Mission Archive&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Mirror Articles&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://people.tamu.edu/~v-buenger/658/Hubble.pdf&quot; rel=&quot;nofollow&quot;&gt;Hubble Error: Time, Money and Millionths of an inch&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.terrydunkle.com/glass.php&quot; rel=&quot;nofollow&quot;&gt;The Big Glass&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-20T01:47:17.267" LastActivityDate="2015-07-20T01:47:17.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10052" PostTypeId="4" CreationDate="2015-07-20T00:27:15.453" Score="0" Body="The Hubble Space Telescope (HST) is a space telescope in low Earth orbit with a 2.4 meter main mirror capable of observing near ultraviolet, visible, and near infrared light.  It was launched in 1990 and remains in operation (following five servicing missions) as of 2015." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-20T01:46:47.293" LastActivityDate="2015-07-20T01:46:47.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10053" PostTypeId="2" ParentId="10048" CreationDate="2015-07-20T01:57:14.087" Score="20" Body="&lt;p&gt;With all the hype in the news this was surprisingly hard to research for specifically 2011 UW158.  I never did find a source for the claim on the value, though the &lt;a href=&quot;http://main.slooh.com/event/5-trillion-dollar-asteroid-makes-close-approach-to-earth/&quot;&gt;Slooh Community Observatory&lt;/a&gt; put the claim at &quot;\$300 billion to as much as \$5.4 trillion dollars worth of precious metals and minerals&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer basically seems to come down to: we've analyzed the composition of meteorites, meteorites come from asteroids, we can make a guess at which asteroids would match the composition of meteorites we've analyzed based on spectroscopy and visible characteristics (such as 2011 UW158 is spinning fast enough that if it were rocks simply held together by gravity it would have flown apart, indicating it must be melted together).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Research results:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more general search turned up &lt;a href=&quot;http://www.uapress.arizona.edu/onlinebks/ResourcesNearEarthSpace/resources19.pdf&quot;&gt;Review of Asteroid Compositions&lt;/a&gt;.  To quote the summary at the top of the paper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Most of our information on asteroid surface compositions comes from telescopic reflectance spectroscopy, and most of our information on asteroid bulk compositions has been determined from the study of meteorites.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;With respect to platinum specifically I found this paper: &lt;a href=&quot;http://www.nss.org/settlement/asteroids/RoleOfNearEarthAsteroidsInLongTermPlatinumSupply.pdf&quot;&gt;The Role of Near-Earth Asteroids in Long-Term Platinum Supply&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While a large part of that paper discusses the uses and sources of platinum on earth page 7 starts discussing planetary formation and asteroids.  To summarize, the gravity of large bodies pulled platinum-group metals (PGMs) (and iron and nickel) into the core of the planets (which is why platinum is rare in the Earth's crust) but asteroids weren't big enough for gravity to separate their composition before they cooled.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Meteorite samples are the primary source of detailed data for asteroid chemical composition, especially trace metals.  Platinum, rhodium, iridium, rhenium, osmium, ruthenium, palladium, germanium and gold are found in significant concentrations (ranging from 1.1-30.7 grams per ton for each &#xA;  metal) across a variety of meteorite samples attributed to the LL Chondrites (see Table 3).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And lower down:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Strong statistical continuity exists for the Chondrites examined by the planetary science community, providing a basis for the expectation that certain large asteroids match the chemical abundance of the meteorite samples &#xA;  (it is hypothesized that these asteroids are the source of the meteorites).  However, without a sample returned from an asteroid, the evidence remains circumstantial.  Detailed asteroid reconnaissance by spacecraft is has &#xA;  dramatically improved geologic models, but has only been carried out recently for a handful of asteroids.  Spectral analysis of data from telescopic observation can sometimes be used to infer general geological characteristics such as bulk composition for newer asteroid discoveries.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Just for fun, here's a website that's dedicated to listing asteroids along with the estimated profit of mining them: &lt;a href=&quot;http://www.asterank.com/&quot;&gt;asterank&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-20T02:08:59.987" LastActivityDate="2015-07-20T02:08:59.987" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10054" PostTypeId="1" AcceptedAnswerId="10056" CreationDate="2015-07-20T04:00:29.193" Score="6" ViewCount="751" Body="&lt;p&gt;The planet Venus has hostile conditions: a surface temperature of 462 °C, a corrosive atmosphere and a surface atmospheric pressure 92 times that of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the Soviet era, the Russians managed to land probes on the surface of Venus, some being soft landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this experience and the short life of the probes, any mission to the surface of Venus to take a significant rock sample (up to 10 kg) and bring to Earth would need to get the job done quickly to get away from the heat and the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the high surface temperature on Venus create a major problem for the rocket engines and the fuel of a sample extraction probe?&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="49" LastEditDate="2015-07-20T08:54:20.697" LastActivityDate="2015-07-20T15:58:46.080" Title="The practicalities of a rock sampling mission to Venus" Tags="|rockets|probe|venus|fuel|sample-return|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10055" PostTypeId="1" CreationDate="2015-07-20T04:21:57.267" Score="2" ViewCount="502" Body="&lt;p&gt;The transmitting speed of the New Horizon probe is $4$ kb/s. Thus, NASA states that it will take around 16 months to download all of the data that the probe gathered in its flyby. Why is this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. Should I ask this question in this forum or in the Engineer forum? Thank!&lt;/p&gt;&#xA;" OwnerUserId="10832" OwnerDisplayName="TBBT" LastEditorUserId="168" LastEditDate="2015-07-20T06:29:12.183" LastActivityDate="2015-07-20T07:41:20.643" Title="Why is the file transfer speed of New Horizon so slow?" Tags="|communication|new-horizons|" AnswerCount="1" CommentCount="0" ClosedDate="2015-07-20T08:33:20.543" ContentLicense="CC BY-SA 3.0" />
  <row Id="10056" PostTypeId="2" ParentId="10054" CreationDate="2015-07-20T06:13:34.843" Score="6" Body="&lt;h1&gt;Challenges&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;High surface temperature would be a problem, but far from the only problem:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Atmospheric pressure:&lt;/strong&gt; the atmospheric pressure on the surface Venus is 92 times greater than on earth.  That has several implications:&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Rocket nozzle expansion: Rocket engine bells work best at the pressure they're designed for, with bell expansion being much larger at vacuum than at 1 atm.  92 atm to 0 atm is such a wide range you're probably going to be forced into using an &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerospike_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerospike&lt;/a&gt;, which is perfect for this application but a large aerospike has never been flown so you'll need to develop this.&lt;/li&gt;&#xA;&lt;li&gt;Tank pressure: Launching from 1 atm fuel tanks have to be pressurized to maintain tank integrity as the fuel is pumped out.  On Venus you'll have to maintain the pressure high enough that the 92 atm of ambient pressure doesn't crush your tank as you lift off.  This adds extra weight and complexity to your rocket.&lt;/li&gt;&#xA;&lt;li&gt;Drag: The atmosphere is thick enough that the &quot;soft landing&quot; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_13&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera 13&lt;/a&gt; involved releasing the parachute and just falling at terminal velocity for the last 50 km (with an airbrake, but still).  Your rocket needs enough thrust and delta-v to push its way back through that.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Max_Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;Maximum dynamic pressure&lt;/a&gt;: 92 atm pressure at the surface doesn't tell the full story.  During ascent the rocket engine is pushing up and drag is pushing down so your rocket has to be structurally strong enough to not be crushed.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Gravity:&lt;/strong&gt; Acceleration due to gravity on Venus is 0.904 g (90.4% of Earth's gravity).  That means that if the atmosphere were similar you'd need to take a fueled rocket capable of escaping Earth with your desired sample return mass and land it safely on Venus.  As noted above, the atmosphere on Venus makes for a much more challenging environment than Earth so you need a bigger rocket on Venus.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Temperature:&lt;/strong&gt; Electronics don't like heat.  The &lt;a href=&quot;https://space.stackexchange.com/questions/9930/what-cooling-improvements-did-the-venera-missions-make&quot;&gt;Venera missions&lt;/a&gt; found ways to overcome that for a short mission with a heavily insulated chamber, or maybe you can build electronics with high heat tolerance.  Any gasses in your rocket are going to want to expand as they heat up, and you should make sure your rocket fuel doesn't boil below 500°C.  Having hot fuel is going to impact the &lt;a href=&quot;https://en.wikipedia.org/wiki/Regenerative_cooling_%28rocket%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;regenerative cooling&lt;/a&gt; of your rocket engine too, so design extra temperature tolerance or another solution into it.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Composition&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atmospheric composition&lt;/a&gt;:&lt;/strong&gt; There are sulfuric acid clouds but the atmosphere in general is 96.5% carbon dioxide.  Fortunately, &lt;a href=&quot;http://www.twi-global.com/capabilities/materials-and-corrosion-management/corrosion/co2-corrosion/&quot; rel=&quot;nofollow noreferrer&quot;&gt;dry CO2 isn't corrosive&lt;/a&gt; so this might be the least of your worries.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wind&lt;/a&gt;:&lt;/strong&gt; You're starting off with winds of 0.3 to 1.0 m/s at the surface (&quot;but due to the high density of the atmosphere at the surface, this is still enough to transport dust and small stones across the surface, much like a slow-moving current of water&quot;) and getting to winds of 100 ± 10 m/s in the upper troposphere.  So long as the winds pick up gradually without any sheer breaks this is probably manageable.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Most or all of the above factors will add complexity to your mission, and any additional insulation, structural supports, shielding, etc will add mass.  Added mass means you need a bigger rocket to leave Venus.  A bigger rocket to leave Venus means you need to get that rocket fueled and on a transfer orbit from Earth to Venus, meaning you need something capable of launching your Venus rocket from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something else to keep in mind: As far as I've been able to find the only things we've launched separately and assembled in orbit have been related to manned flight: space stations, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo-Soyuz&lt;/a&gt;, some flights of &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Gemini&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Gemini&lt;/a&gt;, Hubble repair missions, etc, never multiple unmanned pieces of a probe to be assembled in orbit.  That's not to say it can't be done, just the added complexity is significant enough nobody has tried it for a probe so far.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Alternatives&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Everything in the challenges section assumes dropping a lander all the way to the surface and using rockets to get back into orbit.  Building off the responses to comments:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Use an &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerostat&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerostat&lt;/a&gt; to keep from dropping all the way to the surface and lower a cable.  This probably has a lot of promise for something like a colony that has more infrastructure in place (the context of the paper that suggested it), but I feel like a 50km cable (which keeps you at 1 atm pressure) is a bit much for a sample return mission.  Also, the winds &quot;have a strong vertical gradient&quot; &lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;&lt;/sup&gt; so without a heavy weight on the end you'll end up trailing the cable more horizontally than vertically due to drag on the cable.&lt;/li&gt;&#xA;&lt;li&gt;Use a &lt;a href=&quot;https://en.wikipedia.org/wiki/Rockoon&quot; rel=&quot;nofollow noreferrer&quot;&gt;rockoon&lt;/a&gt; type design: drop to the surface, collect your sample, then use a balloon to lift you out of the more dense atmosphere before firing a rocket when the balloon stops rising.  You should be able to get a large amount of buoyancy from a balloon considering the density of the atmosphere but I don't know what challenges you'd face trying to fill the balloon and keep it from popping on ascent.  For reference, a weather balloon (from 1 atm to &gt;0 atm) &lt;a href=&quot;http://www.stratostar.net/faq.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;expands by 83 times by volume&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If you're familiar with Kerbal Space Program look up some of the monstrosities that people have built to return from the surface of Eve.  That has some similarities with Venus, but doesn't have to deal with temperature or wind and uses a simplified model in many areas.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I know I missed stuff.  I know there could be more numbers included to give a better sense of the scale of the challenge.  Any edit for the sake of adding data I didn't include is welcome, or add a comment and I'll try to incorporate the data for completeness.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-07-20T15:58:46.080" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10057" PostTypeId="1" AcceptedAnswerId="10475" CreationDate="2015-07-20T06:51:55.483" Score="12" ViewCount="1233" Body="&lt;p&gt;What level of circuitry is built into the US EVA spacesuit (&lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit&quot; rel=&quot;noreferrer&quot;&gt;EMU&lt;/a&gt;)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the sophistication of control algorithms used? (PID, discrete logic, $H_\infty$?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rationale: re-reading Andy Weir's &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Martian_(Weir_novel)&quot; rel=&quot;noreferrer&quot;&gt;book&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-08-05T09:25:18.000" LastActivityDate="2020-02-20T20:34:25.643" Title="How smart is an EMU (...spacesuit)?" Tags="|eva|spacesuits|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10058" PostTypeId="1" CreationDate="2015-07-20T07:28:43.593" Score="10" ViewCount="1265" Body="&lt;p&gt;Here's the idea: A large spherical LH2 tank is placed inside a spherical LOX tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I thought of this is so the inner tank doesn't need to be insulated. It doesn't need to be a double-walled, or &quot;dewar-flask&quot; tank. The outer tank full of LOX keeps it insulated. Therefore, the LH2 tank would reduce its weight by 50%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or at least, that's the idea. Would this actually be good insulation for the inner tank? If so, would it actually cut the LH2 tank weight in half? And even then, are there insurmountable complications with this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In other words, is the tank within a tank a sound engineering concept for rocket stages?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some possible complications I can think:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Both tanks might have to be filled at the same time.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Plumbing from the LH2 tank to the engine would have to go through the LOX tank too, at first.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The LOX might be cooled even more than normal, changing its density and maybe other properties important in the pumps that feed it to the engine.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;AFAIK these are not bad/expensive enough to be impossible. But I want to hear what you think.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Yes I'm talking about large cryo tanks for large rocket stages that carry payloads to orbit. It could be any stage, although I'm not aware of any sole first stage that uses LH2/LOX (with no other parallel boosters), so probably it would be the 2nd or 3rd stage. These can still be pretty large for, say, Saturn 1B or Saturn V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: It looks like LOX would not be a great insulator, because the temperature difference between LOX and LH2 is very great. I thought that, with both of them exchanging heat through conduction, the LOX would also go down or near to LH2's temperature.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="25" LastEditDate="2015-07-21T20:47:27.847" LastActivityDate="2022-08-03T13:13:18.300" Title="A cryo tank within another cryo tank...is it a sound engineering concept?" Tags="|rockets|stages|cryogenics|hydrogen|lox|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10059" PostTypeId="2" ParentId="10055" CreationDate="2015-07-20T07:41:20.643" Score="2" Body="&lt;p&gt;The bit rate of a channel depends on the signal to noise ratio (and their statistics but I will assume these are fixed). As the received signal for a given system declines with range the bit rate of the channel falls also (conditioned on a fixed noise power). The &lt;a href=&quot;https://en.wikipedia.org/wiki/Channel_capacity#Channel_capacity_in_wireless_communications&quot; rel=&quot;nofollow&quot;&gt;Shannon-Hartley theorem&lt;/a&gt; quantifies this for a channel subject to Additive Gaussian White Noise: &#xA;$$&#xA;C=B\log_2\left(1+\frac{S}{N} \right)&#xA;$$&#xA;where $C$ is the bit rate (bits/s), $B$ the bandwith of the channel in Hz, and $S/N$ is the signal to noise power ratio.&lt;/p&gt;&#xA;" OwnerUserId="10998" LastActivityDate="2015-07-20T07:41:20.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10060" PostTypeId="1" CreationDate="2015-07-20T08:09:57.460" Score="15" ViewCount="740" Body="&lt;p&gt;Solar Probe Plus will flyby Venus 7 times(!) during 6 years in order to enter its &quot;low&quot; Solar orbit. Will it carry any instrument useful for studying Venus, for example seasonal changes? Would its trajectory be useful for secondary payload to study Venus during the first flyby? I see that SPP will be launched on a Delta IV Heavy, the most capable rocket in operation today, does that speak for allowing some secondary minisat payload?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="12102" LastEditDate="2021-08-25T02:22:54.427" LastActivityDate="2021-08-25T02:22:54.427" Title="Will or could the Parker Solar Probe do science at Venus?" Tags="|mission-design|venus|the-sun|parker-solar-probe|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10061" PostTypeId="2" ParentId="7776" CreationDate="2015-07-20T09:36:37.447" Score="6" Body="&lt;p&gt;&lt;strong&gt;In short&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The capacity of the downlink between New Horizons and a &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSN&lt;/a&gt; station, decreases when the distance between them increases, according approximately to &lt;em&gt;C=log&lt;sub&gt;2&lt;/sub&gt;(1/distance)&lt;/em&gt;. The actual bit rate is lower than this limit, depending on the type of modulation, and the error correction code. The decrease of this maximal bit rate is explained with two elements:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The strength of a radio signal decreases with the distance from the emitter.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shannon–Hartley theorem&lt;/a&gt;:&lt;br&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/YY0vr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;says that:&lt;br&gt;&#xA;&lt;br&gt;&#xA;The maximum capacity of a communication channel is limited by a value, known as the &lt;em&gt;signal-to-noise ratio&lt;/em&gt; (SNR), which is the ratio between the strength of the signal at the receiving end of the channel and the amount of noise introduced by the receiver and the rest of the link.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In July 2015, New Horizons is now far from us, by about 40 times the distance Earth-Sun. The radio signal is very faint while the noise is still the same. With a small SNR, the maximum capacity of the radio channel is very limited.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Details&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This formula links 4 quantities:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;C&lt;/strong&gt;, which is the &lt;em&gt;channel maximal capacity&lt;/em&gt;. It's expressed in bits * second. This is what you ask for in your question. &lt;br&gt;&lt;br&gt;Note that this is what non-engineers may call the &lt;em&gt;bandwidth&lt;/em&gt; (this is inaccurate, and it especially matters in this formula): &quot;My Internet ADSL connection has a 10 Mbps &lt;em&gt;bandwidth&lt;/em&gt;&quot;, &lt;em&gt;capacity&lt;/em&gt; should have been used.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;B&lt;/strong&gt; is the &lt;em&gt;bandwidth&lt;/em&gt; used for the communication. &lt;br&gt;&lt;br&gt; The bandwidth is the space taken up by the channel in the frequency spectrum. A communication usually happens around a &lt;em&gt;carrier&lt;/em&gt; frequency, and the information create sidebands around this frequency, by using some more or less clever principle (New Horizons uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Phase-shift_keying#Binary_phase-shift_keying_.28BPSK.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;binary PSK&lt;/a&gt;). The larger the amount of information to transmit per unit of time, the larger the frequency slot required. But there are two problems in extending the bandwidth: The peak power decreases, and the noise increases (see below). &lt;br&gt;&lt;br&gt;Note: The bandwidth is expressed in Hz, and mustn't be confused with C as explained earlier.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;S&lt;/strong&gt; is the &lt;em&gt;strength&lt;/em&gt; of the signal at the receiver side. &lt;br&gt;&lt;br&gt;S depends mostly on the emitter power, the gain of both antennas, and the distance between the emitter and the receiver. For a given power, and given antennas, S is essentially dependent on the distance. Each time the distance is doubled, S is divided by 4, this is a case of &lt;a href=&quot;https://en.wikipedia.org/wiki/Inverse-square_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;inverse square law&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;N&lt;/strong&gt; is the channel &lt;em&gt;noise&lt;/em&gt;, that is random false signals collected or created by the receiver. The most important is created by the receiver, due to the limits of electronic components (&lt;a href=&quot;https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermal noise&lt;/a&gt;). &lt;a href=&quot;https://en.wikipedia.org/wiki/Low-noise_amplifier&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low noise receiver&lt;/a&gt; are difficult to build. N is quite constant for a given system (antenna and amplifier) at a given temperature. &lt;br&gt;&lt;br&gt;S and R are usually expressed together as S/N, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Signal-to-noise_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;signal-to-noise ratio&lt;/a&gt; (or SNR), and the dimensionless value is expressed in dB, e.g. a 18 dB SNR.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;With all approximations made earlier: For a given receiving system (antennas and amplifiers), and when the carrier frequency and the bandwidth are constant, then &lt;strong&gt;C varies according to log&lt;sub&gt;2&lt;/sub&gt;(1/distance)&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To deal with that and to regain the previous S/N ratio, we need to either increase S or decrease N. This means larger antennas (larger gain), more accurate antenna alignment, and lower noise amplifiers. None of these possibilities is really possible for New Horizons downlink.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-07-20T21:08:49.613" LastActivityDate="2015-07-20T21:08:49.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10063" PostTypeId="2" ParentId="10060" CreationDate="2015-07-20T12:06:49.317" Score="6" Body="&lt;p&gt;Let's take a look at the instruments, as seen at &lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/instruments.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;JHU APL's site&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Solar Wind Electrons Alphas and Protons Investigation- Measures solar wind particles. Might do a bit of science at Venus with how the solar wind reacts to Venus. I personally don't know much about the science value of this, but as @gerrit mentioned, it is currently being done by Mars Express.&lt;/li&gt;&#xA;&lt;li&gt;The Wide-field Imager, This will allow for images to be taken. I suspect this will give some science, but not a whole lot, as Venus requires specific cameras to make anything of interest.&lt;/li&gt;&#xA;&lt;li&gt;The Fields Experiment, measuring electric and magnetic fields. I think this has a real chance of making some interesting studies at Venus, to determine more about any magnetic fields it might have.&lt;/li&gt;&#xA;&lt;li&gt;The Integrated Science Investigation of the Sun- This might help to determine things like atmospheric leakage at Venus. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Why not have something specific to Venus? Take a look at this image below, copied from the &lt;a href=&quot;http://solarprobe.jhuapl.edu/common/content/SolarProbePlusFactSheet.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;fact sheet&lt;/a&gt;. Note that even the first approach will go beyond Mercury towards the Sun. This is with a flyby of Venus, but still, it no doubt needs a significant amount of delta v to achieve this. I suspect there just isn't much room for any mass improvements, even with the large rocket that it will be launched on, much as New Horizons was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/LvnLY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-21T12:19:24.207" LastActivityDate="2015-07-21T12:19:24.207" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10064" PostTypeId="2" ParentId="10058" CreationDate="2015-07-20T12:16:04.537" Score="4" Body="&lt;p&gt;Yes it is, and in fact, this is being done, to some extent, already. It does have a number of risks, however, and is typically only used for small items as a result of that risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 is the rocket that I am aware of that does this. There are a number of helium tanks, known as &lt;a href=&quot;https://space.stackexchange.com/q/9847/25&quot;&gt;COPVs&lt;/a&gt;. These would need to be insulated more if they were not in the LOX tank, as the linked question hints.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why has this not been done for a larger tank? These pressure vehicles are already quite difficult to manage. It becomes even more difficult with additional size. You have to be able to fuel them, and run the fuel. Also, having the fuel and oxidizer in the same tank means even a very small leak is virtually guaranteed to explode, a very bad thing. It also would mean that you would need a hole large enough to put the smaller tank inside the larger one, potentially causing more issues to maintain pressure. The helium tanks work because they are small, and inert, any issue with the tank will just result in an overpressure event. The Falcon 9 has has at least one issue where a tank ruptured on the ground, which delayed the flight by a few months, but caused no serious damage. I personally suspect this was the cause of the previous failure. If it was a non-inert gas, then there would have been an explosion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall, I think it sounds like a good idea, however, there are some serious risks. If these risks could be overcome, it might save a bit of weight. Still, I don't expect this to be significantly changing things anytime soon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-07-20T12:16:04.537" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10065" PostTypeId="2" ParentId="5861" CreationDate="2015-07-20T13:30:52.477" Score="14" Body="&lt;p&gt;There's a lot of uncertainty at all levels, mainly because we've never had a manned structure in space for this long. Mir set the previous record at 15 years. The ISS reached that milestone in 2013. We've already found that the station lasts longer than its &lt;a href=&quot;https://oig.nasa.gov/audits/reports/FY14/IG-14-031.pdf&quot;&gt;initial design life&lt;/a&gt; (PDF, page 3): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In November 2013, the International Space Station (ISS or Station) completed 15 years of continuous operation in low Earth orbit, marking a significant achievement in the history of human spaceflight. Two months later, the Administration announced its intent to extend Station operations until 2024. Originally designed and tested for a 15-year life span, the ISS may now operate for 26 years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At one point, NASA was working on certifying the station for use until 2028: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hardware Assessment and Certification. NASA and its international partners designed and tested the ISS for a 15-year life span – a benchmark the oldest segments of the Station surpassed in November 2013. The NASA Authorization Act of 2010 extended the life of the Station until 2020, and NASA and its Canadian, Japanese, and European partners have certified that the ISS structure and hardware are sufficient to continue operations until that date.16 In January 2014, the President announced his support for extending Station operations until at least 2024.17 Prior to this announcement, NASA had initiated a feasibility assessment for continuing operations until 2028. (NASA defines an assessment as the method for performing verification or certification. For ISS certification, the focus is primarily on analytical assessments using the performance data from the 15 years of ISS operation. NASA expects to complete its 2028 assessment in 2018.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Investigations into extending ISS lifespan to 2028 started in &lt;a href=&quot;http://www.space.com/8034-international-space-station-fly-2028-nasa-partners.html&quot;&gt;2010&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are &lt;a href=&quot;http://www.space.com/22105-international-space-station-lifespan-2020.html&quot;&gt;several components&lt;/a&gt; that are seen as 'most likely to cause trouble':&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the hardware that attaches the truss to the pressurized modules.&lt;/li&gt;&#xA;&lt;li&gt;high-pressure oxygen lines&lt;/li&gt;&#xA;&lt;li&gt;solar panels (power levels are dropping faster than expected)&lt;/li&gt;&#xA;&lt;li&gt;EPROMs that lose their stored data &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Then there are things like the seals between modules that would be difficult to replace if they failed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So at some point, there are components you need to replace (in some cases, in a very expensive operation) before they fail. Then prolonging the life of the station becomes a political and budgetary issue: you're weighing risks to the crew against the cost of refurbishment. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-20T13:30:52.477" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10066" PostTypeId="1" AcceptedAnswerId="10068" CreationDate="2015-07-20T13:50:20.537" Score="25" ViewCount="5776" Body="&lt;p&gt;The following &lt;a href=&quot;http://www.nasa.gov/sites/default/files/739459main_acd97-0036-2.jpg&quot; rel=&quot;noreferrer&quot;&gt;image&lt;/a&gt; from NASA shows the flight paths of Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2, as well as the positions of the outer planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/J8X6R.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that with a little tweaking of the flight paths, it would have been possible for Pioneer 11 to visit Uranus and Neptune, and for Voyager 1 to visit Pluto.  So why didn't that happen?&lt;/p&gt;&#xA;" OwnerUserId="10966" LastActivityDate="2019-05-05T00:51:37.277" Title="Why didn't Pioneer 11 visit Uranus/Neptune, and why didn't Voyager 1 visit Pluto?" Tags="|voyager|pluto|pioneer|neptune|uranus|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10068" PostTypeId="2" ParentId="10066" CreationDate="2015-07-20T14:23:05.047" Score="26" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_program&quot; rel=&quot;noreferrer&quot;&gt;initial plan was&lt;/a&gt; to visit all of the outer planets:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Planetary Grand Tour was to send several pairs of probes to fly by all the outer planets (and Pluto) along various trajectories, including Jupiter-Saturn-Pluto and Jupiter-Uranus-Neptune. Limited funding ended the Grand Tour program, but elements were incorporated into the Voyager Program, which fulfilled many of the flyby objectives of the Grand Tour except a visit to Pluto.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;For Voyager 1, scientists had to &lt;a href=&quot;http://astronomy.com/year-of-pluto/2015/06/why-didnt-voyager-visit-pluto&quot; rel=&quot;noreferrer&quot;&gt;make a choice&lt;/a&gt;: either visit Saturn's interesting moon Titan, or visit Pluto. A visit to Titan would bend Voyager's path and make a visit to Pluto impossible.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Astronomers decided that in order to optimize their science at Saturn, they’d need an orbit that brought Voyager 1 up close with Titan. But that flyby also would put Pluto out of reach after the spacecraft lifted out of our solar system’s ecliptic plane.&lt;/p&gt;&#xA;&lt;p&gt;“It was a pretty straightforward decision for them because they thought there was going to be a third Voyager mission that could come along and go to Pluto,” Stern says.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(see also the &lt;a href=&quot;https://web.archive.org/web/20150720064642/http://voyager.jpl.nasa.gov/faq.html&quot; rel=&quot;noreferrer&quot;&gt;Voyager FAQ&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;Back then, they chose Titan, and then NASA cancelled the third Voyager mission.&lt;/p&gt;&#xA;&lt;p&gt;There's an alternate point of view in &lt;a href=&quot;http://science.slashdot.org/comments.pl?sid=7613123&amp;amp;cid=50003607&quot; rel=&quot;noreferrer&quot;&gt;this Slashdot comment&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;I had the opportunity to ask Ed Stone, the JPL Director &amp;amp; Voyager scientist, this question. His rather glib answer was, &amp;quot;well, Titan was 3 hours away, and Pluto was 3 years away - and I had to make payroll.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I haven't found a reason why Pioneer 11 didn't visit Uranus or Neptune, or if those visits were even seriously considered.&#xA;Pioneer 11 was launched &lt;a href=&quot;https://space.stackexchange.com/questions/3609/is-there-any-way-to-communicate-with-pioneer-11-through-voyager-1-or-2&quot;&gt;out of the ecliptic plane&lt;/a&gt; by its Saturn encounter, that made it impossible to visit Uranus or Neptune. But it's possible a different trajectory would have given the opportunity to visit Uranus or Neptune.&lt;br /&gt;&#xA;Budget pressures may have played a role in not scheduling more flybys. &lt;a href=&quot;https://books.google.nl/books?id=RoJMadct4TQC&amp;amp;pg=PA289&amp;amp;lpg=PA289&amp;amp;dq=%22voyager%203%22%20nasa&amp;amp;source=bl&amp;amp;ots=ZaRNE_-Y4B&amp;amp;sig=1NynnV7_ccwaK6REpb3k1mqpqwo&amp;amp;hl=en&amp;amp;sa=X&amp;amp;redir_esc=y#v=onepage&amp;amp;q=%22voyager%203%22%20nasa&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Voyager 3 was cancelled in 1975&lt;/a&gt;. At that time, NASA needed money to fund development of the Space Shuttle.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-05-05T00:51:37.277" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="10072" PostTypeId="2" ParentId="10058" CreationDate="2015-07-20T16:36:15.837" Score="15" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;...is the tank within a tank a sound engineering concept for rocket&#xA;  stages?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I take this to mean that you are not talking about pressurant bottles or other small devices submerged in the propellant tanks. Instead you mean the large primary propellant tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, &lt;em&gt;No, this is not a good idea&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;It doesn't need to be a double-walled, or &quot;dewar-flask&quot; tank.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Boosters don't have double-walled tanks, so there is no need for a design to replace this.  Boosters can't afford the weight of double walled-tanks.  This problem is mitigated by fueling the booster as late as possible in the launch count.  The cost of boiling off a little prop is way less than the penalty you'd pay by having an vacuum-insulated tank.  Plus..don't forget...&lt;strong&gt;for upper stages, really soon you are going to be in a vacuum anyway&lt;/strong&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;The outer tank full of LOX keeps it insulated.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Insulating LH2 by wrapping it in LOX is like insulating an ice cube by putting it in a hot frying pan.  The temperature difference between LOX and LH2 is tremendous.  In fact, Centaur-G, which had a common bulkhead between the LOX and LH2 tanks, had a special insulation scheme specifically to prevent heat transfer between the LH2 and LOX.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;A large spherical LH2 tank is placed inside a spherical LOX tank.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Booster propellant tanks are not spherical.  They are cylindrical.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6816" LastEditDate="2018-11-27T23:44:58.673" LastActivityDate="2018-11-27T23:44:58.673" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="10075" PostTypeId="1" AcceptedAnswerId="10076" CreationDate="2015-07-20T17:45:27.530" Score="23" ViewCount="20872" Body="&lt;p&gt;In researching the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/10066/why-didnt-pioneer-11-visit-uranus-neptune-and-why-didnt-voyager-1-visit-pluto&quot;&gt;this recent question,&lt;/a&gt; I came across some information that was new to me. &lt;a href=&quot;https://books.google.nl/books?id=RoJMadct4TQC&amp;amp;pg=PA289&amp;amp;lpg=PA289&amp;amp;dq=%22voyager%203%22%20nasa&amp;amp;source=bl&amp;amp;ots=ZaRNE_-Y4B&amp;amp;sig=1NynnV7_ccwaK6REpb3k1mqpqwo&amp;amp;hl=en&amp;amp;sa=X&amp;amp;redir_esc=y#v=onepage&amp;amp;q=%22voyager%203%22%20nasa&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;A third Voyager mission&lt;/a&gt; was planned, and then canceled. Apparently, &lt;a href=&quot;http://www.thespacerace.com/forum/index.php?topic=3060.0&quot; rel=&quot;noreferrer&quot;&gt;Voyager 3 was cannibalized&lt;/a&gt; during construction:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I am currently reading the book &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/B010EWQY4O&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Voyager: Seeking Newer Worlds In The Third Great Age Of Discovery&lt;/em&gt;&lt;/a&gt; by Stephen J. Pyne.&#xA;  On the second chapter, it is listed that there were three Voyager spacecraft.  The second Voyager, VGR 77-2 had flaws and it was used for spare parts for Voyager 1 (VGR 77-1) and Voyager 2 (VGR 77-3).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At one point, NASA had a &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_Grand_Tour&quot; rel=&quot;noreferrer&quot;&gt;Planetary Grand Tour&lt;/a&gt; plan that consisted of 4 missions (Mariner 11-14). Was Voyager 3 one of these spacecraft?  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2024-03-08T19:11:57.080" Title="Voyager 3: what would have been its mission, and why was it canceled?" Tags="|nasa|probe|voyager|canceled-project|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10076" PostTypeId="2" ParentId="10075" CreationDate="2015-07-20T18:23:48.370" Score="23" Body="&lt;p&gt;Voyager 3 would have been Mariner 13, before the name of the mission was changed. It's actually fairly common when multiple spacecraft are designed to have one with flaws that is used primarily for spare parts. It is also common practice to build a spare spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, when there were 4 spacecraft there would have been two Jupiter-Saturn-Pluto missions, and 2 Jupiter-Uranus-Neptune missions. As these two were redundant, they canceled one of each. My guess is that at that time, they had the parts to build 3, and decided to build all 3 of the spacecraft. One of these had issues, and thus was never launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for what the original purpose was, it is necessary to realize the primary purpose of the Voyager probes was to study Saturn and Jupiter. Voyager 2 received an extension after Voyager 1 made a close pass of Titan, and not before. Thus, there are 4 possibilities:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Jupiter/ Saturn/ Pluto&lt;/li&gt;&#xA;&lt;li&gt;Jupiter/ Uranus/ Neptune&lt;/li&gt;&#xA;&lt;li&gt;Jupiter/Saturn (Only)&lt;/li&gt;&#xA;&lt;li&gt;A spare/ EDU system (Engineering Design Unit, used on the ground to verify commands and troubleshoot problems&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If Voyager 3 had launched, I predict it would have followed the Jupiter/ Saturn/ Pluto approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to why it was canceled, that is certainly part of the Planetary Grand Tour budget crisis that happened post-moon launches.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-20T18:30:58.637" LastActivityDate="2015-07-20T18:30:58.637" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10077" PostTypeId="1" AcceptedAnswerId="10093" CreationDate="2015-07-20T18:45:53.813" Score="18" ViewCount="1305" Body="&lt;p&gt;I was reading about the Venera missions to Venus, and saw this map on &lt;a href=&quot;https://en.wikipedia.org/wiki/Venera&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/rFnfw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venera 7 is missing from the map for some reason, but it landed at 5° S, 351° E, so it would have been very close to Venera 8.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the Mars landing sites are here:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/VyoVl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://www.planetary.org/multimedia/space-images/charts/mars_landing_site_map_lakdawalla.html?referrer=https://www.google.com/&quot; rel=&quot;noreferrer&quot;&gt;The Planetary Society - &quot;Map of all Mars landing sites, failed and successful&quot;&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here are the landing sites on the Moon:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/k6LDr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br /&gt;&#xA;&lt;sub&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo.html&quot; rel=&quot;noreferrer&quot;&gt;NASA.gov - Lunar Landing Site Map&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Mars and the Moon the landing spots are very spread out across the surface, but on Venus the Venera mission landers were clumped together. What was the reason for this?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="49" LastEditDate="2015-07-20T19:25:44.973" LastActivityDate="2021-06-11T19:39:00.367" Title="Why did the Venera missions land so close together?" Tags="|probe|venus|lander|site-selection|venera|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10078" PostTypeId="5" CreationDate="2015-07-20T20:30:57.647" Score="0" Body="&lt;p&gt;Dawn was launched on 27 September 2007 to study to Asteroid Belt, specifically Vesta and Ceres. Its mission is to study what would have been the early conditions of the solar system. By analyzing Vesta and Ceres, scientists hope to learn what conditions lead to protoplanets becoming planets, and how important size and water were in the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dawn entered orbit around Vesta on 16 July 2011, and left its orbit on 5 September 2012. Dawn then entered orbit around Ceres on 6 March 2015.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Resources:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Dawn&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://dawn.jpl.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;NASA - Dawn&lt;/a&gt;&lt;br/&gt;&#xA;&lt;a href=&quot;http://dawn.jpl.nasa.gov/mission/goal_index.asp&quot; rel=&quot;nofollow&quot;&gt;NASA - Dawn Mission Objectives&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T21:03:45.300" LastActivityDate="2015-07-20T21:03:45.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10079" PostTypeId="4" CreationDate="2015-07-20T20:30:57.647" Score="0" Body="Dawn is a spacecraft that was launched in 2007 to study the Asteroid Belt, specifically Vesta and Ceres." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T21:03:51.403" LastActivityDate="2015-07-20T21:03:51.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10080" PostTypeId="5" CreationDate="2015-07-20T20:56:37.347" Score="0" Body="&lt;h3&gt;Asteroid Belt&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;The region between the orbits of Mars and Jupiter contains potentially millions of asteroids made up of rock, metal, and ice. However, most of the asteroids in the main belt are probably smaller than 1 km in diameter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The largest body in this region is Ceres, a dwarf planet that is about 940 km across. It makes up about 1/3 of the mass of the Asteroid Belt. Ceres was also the first object in the Asteroid Belt to be discovered. Giuseppe Piazzi found it on 1 January 1801. At the time it was considered a planet, until more objects in the region were discovered. They were then called &quot;asteroids&quot; by William Herschel in 1802.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Resources&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_belt&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Asteroid Belt&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.space.com/16105-asteroid-belt.html&quot; rel=&quot;nofollow&quot;&gt;Space.com - Asteroid Belt: Facts &amp;amp; Formation&lt;/a&gt;&lt;br /&gt;&#xA;See also - &lt;a href=&quot;/questions/tagged/asteroid&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;asteroid&amp;#39;&quot; rel=&quot;tag&quot;&gt;asteroid&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T21:24:53.583" LastActivityDate="2015-07-20T21:24:53.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10081" PostTypeId="4" CreationDate="2015-07-20T20:56:37.347" Score="0" Body="The Asteroid Belt is a region of space between the orbits of Mars and Jupiter where potentially millions of asteroids orbit." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T21:03:48.553" LastActivityDate="2015-07-20T21:03:48.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10082" PostTypeId="5" CreationDate="2015-07-20T22:29:16.257" Score="0" Body="&lt;p&gt;The Deep Space Network is a series of large antennas located in Madrid, Spain; Canberra, Australia; and California, USA. The three locations are arranged in such a way that once a spacecraft gets about 30,000 km away from Earth, it should always be in view of at least one of the locations (unless that view is blocked by the Sun, or some other planetary body).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The DSN network keeps space agencies in contact with the deep space spacecraft that have been launched, including New Horizons, Rosetta, Mars Reconnaissance Orbiter, and others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On NASA's DSN NOW website, each antenna shows which craft, if any, it is communicating with.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Resources:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow&quot;&gt;NASA Deep Space Network NOW&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Deep Space Network&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T22:56:26.010" LastActivityDate="2015-07-20T22:56:26.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10083" PostTypeId="4" CreationDate="2015-07-20T22:29:16.257" Score="0" Body="Deep Space Network is a series of communication antennas around the world used for communication with deep space spacecraft." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T22:56:36.503" LastActivityDate="2015-07-20T22:56:36.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10084" PostTypeId="5" CreationDate="2015-07-20T22:39:22.620" Score="0" Body="&lt;p&gt;Indian Space Research Organisation launched Mars Orbiter Mission to Mars on 5 November 2013. It reached Mars' orbit on 24 September 2014. It was launched primarily as a technology demonstration for ISRO. It has scientific secondary goals while in Mars orbit, including studying the Mars surface and atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Questions related to the craft itself, its launch, technology, science goals, future potential, and its impact on the understanding of Mars are all appropriate for this tag.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Resources:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Mars Orbiter Mission&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.isro.gov.in/pslv-c25-mars-orbiter-mission&quot; rel=&quot;nofollow&quot;&gt;ISRO.gov.in - Mars Orbiter Mission&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T22:56:03.100" LastActivityDate="2015-07-20T22:56:03.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10085" PostTypeId="4" CreationDate="2015-07-20T22:39:22.620" Score="0" Body="The Mars Orbiter Mission is a satellite sent by Indian Space Research Organisation to Mars as a technology demonstration. " OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T22:56:30.493" LastActivityDate="2015-07-20T22:56:30.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10086" PostTypeId="1" AcceptedAnswerId="10510" CreationDate="2015-07-20T22:41:47.083" Score="18" ViewCount="1666" Body="&lt;p&gt;Cryo tanks are so cold, they tend to condense moisture out of the air, creating ice covering a lot of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen many videos where this ice starts shedding at launch, but I still have to ask, isn't this a (temporary) weight penalty? Is it a considerable penalty for a large rocket with large tanks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: hopefully someone can post a numerical answer in the form of additional weight to nominal weight, or thickness and density of ice plus surface area for a given rocket (stage).&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2015-08-06T18:04:34.893" LastActivityDate="2015-08-10T07:05:01.157" Title="How big a weight problem is ice sticking to a rocket?" Tags="|rockets|cryogenics|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10087" PostTypeId="1" AcceptedAnswerId="10091" CreationDate="2015-07-20T22:44:13.313" Score="16" ViewCount="1452" Body="&lt;p&gt;Last week New Horizons passed within 14,000 km of the center of Pluto.  I assume this will have put New Horizons within the gravitational influence of Pluto.  Pluto's average orbital speed is 4.7 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will New Horizons have gained (or lost) any significant speed due to gravity-assist effects as it passed though Pluto's gravitational influence?  If so, how much?&lt;/p&gt;&#xA;" OwnerUserId="758" LastActivityDate="2015-07-24T17:27:46.487" Title="Did the Pluto flyby give New Horizons any significant gravitational boost?" Tags="|new-horizons|gravity-assist|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10088" PostTypeId="5" CreationDate="2015-07-20T22:50:22.833" Score="0" Body="&lt;p&gt;The National Oceanic and Atmospheric Administration is a science agency in the United States focused primarily on the study of the oceans and atmosphere of Earth, but they also watch and try to predict space weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Questions here that would use the NOAA tag would generally be related to NOAA's space weather website or its DSCOVR space weather satellite. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Additional Resources:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/National_Oceanic_and_Atmospheric_Administration&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - NOAA&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.noaa.gov/index.html&quot; rel=&quot;nofollow&quot;&gt;NOAA.gov&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;http://www.swpc.noaa.gov/&quot; rel=&quot;nofollow&quot;&gt;NOAA - Space Weather Prediction Center&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T23:54:01.707" LastActivityDate="2015-07-20T23:54:01.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10089" PostTypeId="4" CreationDate="2015-07-20T22:50:22.833" Score="0" Body="Questions related to the National Oceanic and Atmospheric Administration, of the United States." OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-20T23:53:32.967" LastActivityDate="2015-07-20T23:53:32.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10090" PostTypeId="1" CreationDate="2015-07-20T23:12:55.043" Score="11" ViewCount="320" Body="&lt;p&gt;Last month, on June 8, 2015, NASA ran the &lt;a href=&quot;https://blogs.nasa.gov/ldsd/2015/06/08/ldsd-flight-complete-nasa-to-hold-briefing-to-discuss-status-of-flying-saucer-test/&quot; rel=&quot;noreferrer&quot;&gt;second Low-Density Supersonic Decelerator (LDSD) flight test&lt;/a&gt; off the island of Kauai, Hawaii. The inflatable decelerator portion of the test was successful, but the supersonic parachute shredded as it opened, similarly as it did during the first flight test.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they know specifically why it shredded when it opened, and what are they doing to improve it for the next year's third LDSD flight test?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="49" LastEditDate="2015-07-21T04:26:25.743" LastActivityDate="2019-07-12T20:49:12.400" Title="What is NASA doing to improve the supersonic parachute for the third LDSD test?" Tags="|nasa|materials|testing|parachute|ldsd|" AnswerCount="1" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10091" PostTypeId="2" ParentId="10087" CreationDate="2015-07-21T00:58:50.940" Score="14" Body="&lt;p&gt;I've been trying to figure out where I read about this, and then realized that I didn't read it. I saw it in a video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NASA New Horizons channel on Youtube made some short videos called &quot;Pluto in a Minute&quot;. In the video, &lt;a href=&quot;https://www.youtube.com/watch?v=Hm6ga-g9ACU&quot; rel=&quot;nofollow&quot;&gt;How Did Pluto Accelerate New Horizons?&lt;/a&gt;, the speaker explains how a planetary body's sphere of influence works, and then mentions that as New Horizons flew past Pluto, it gained about 13 mph (approximately 21 km/h). Very insignificant, indeed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Hm6ga-g9ACU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2015-07-24T17:27:46.487" LastActivityDate="2015-07-24T17:27:46.487" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10093" PostTypeId="2" ParentId="10077" CreationDate="2015-07-21T05:59:17.200" Score="21" Body="&lt;p&gt;The simple answer is that taken as a whole the Venera missions didn't land particularly close together.  Starting with Venera 4:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/T4TYt.jpg&quot; alt=&quot;Venera Landing Map&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;The overall spread of Venera landing sites (when you include the probes that didn't survive to the surface) is nearly 110° of longitude.  For comparison, the spread of Luna landing sides on the map you provided of the moon is about 130°.&lt;/p&gt;&#xA;&lt;p&gt;The last Venera landings did end up spread across only 60° longitude so I'll try to answer what drove the choices of individual landing sites.&lt;/p&gt;&#xA;&lt;h1&gt;Overview&lt;/h1&gt;&#xA;&lt;p&gt;To start with, the launch/landing dates of the Venera lander missions were:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Venera 7: 17 August 1970/15 December 1970&lt;/li&gt;&#xA;&lt;li&gt;Venera 8: 17 March 1972/22 July 1972&lt;/li&gt;&#xA;&lt;li&gt;Venera 9: 8 June 1975/22 October 1975&lt;/li&gt;&#xA;&lt;li&gt;Venera 10: 14 June 1975/25 October 1975&lt;/li&gt;&#xA;&lt;li&gt;Venera 11: 9 September 1978/25 December 1978&lt;/li&gt;&#xA;&lt;li&gt;Venera 12: 14 September 1978/21 December 1978&lt;/li&gt;&#xA;&lt;li&gt;Venera 13: 30 October 1981/1 March 1982&lt;/li&gt;&#xA;&lt;li&gt;Venera 14: 4 November 1981/5 March 1982&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Note that Venera 9-14 come in pairs - for each launch window they used the Soviets built two identical or very similar spacecraft and launched them both for redundancy (Venera 7 and 8 actually had sister ships that failed to leave Earth orbit&lt;sup&gt;1&lt;/sup&gt;).  When I saw the question I honestly expected to just see the landers paired off by launch window and was surprised that wasn't the whole answer (though each pair did end up fairly close together in terms of longitude).&lt;/p&gt;&#xA;&lt;h1&gt;Mission Constraints&lt;/h1&gt;&#xA;&lt;h2&gt;External Factors&lt;/h2&gt;&#xA;&lt;p&gt;The first radar mapping missions were Pioneer Venus 1 (arrived at Venus in December 1978) and Venera 15 and 16 (arrived at Venus in October 1983)&lt;sup&gt;2&lt;/sup&gt;.  The map you posted with the landing sites marked is from Pioneer Venus 1 and has a date on it of November 1981, but I was able to find a similar map dated June 1980 on the &lt;a href=&quot;http://ails.arc.nasa.gov/ails/printPreview.php?rid=7953&quot; rel=&quot;noreferrer&quot;&gt;Ames Imaging Library System&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;That means that the only Venera landers that were launched when there was mapping data available were Venera 13 and 14.&lt;/p&gt;&#xA;&lt;h2&gt;Venusian Environment&lt;/h2&gt;&#xA;&lt;p&gt;In terms of general environment, Venus has an orbital period of 224.65 Earth days and a retrograde rotation slow enough that a full rotation takes 243 Earth days.  That means that probes landing mere days apart have almost exactly the same landing options in terms of ballistics because the planet has barely moved relative to the orbital path of the probes.&lt;/p&gt;&#xA;&lt;p&gt;Taken together the rotation and orbital period form a resonance with Earth: &lt;strong&gt;whenever Venus and Earth are at their closest approach the same side of Venus is facing Earth&lt;/strong&gt; &lt;sup&gt;&lt;a href=&quot;https://www.newscientist.com/article/dn18660-are-venus-and-earth-in-a-long-distance-relationship/&quot; rel=&quot;noreferrer&quot;&gt;newscientist&lt;/a&gt;&lt;/sup&gt;.  That means that missions with similar constraints are likely to end up in similar places because the starting alignment of planets is the same every launch window.&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_7&quot; rel=&quot;noreferrer&quot;&gt;Venera 7&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;Landed on the night side of the planet in line-of-sight of Earth for radio contact reasons&lt;sup&gt;1&lt;/sup&gt;.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Venera 7 actually entered with its interplanetary bus still attached to keep the cooling system active as long as possible.  That means that it didn't have the option of using the bus as a relay.&lt;/p&gt;&#xA;&lt;p&gt;This was actually the first time the surface was reached with the probe intact and no mapping/imaging of the surface had been done.  For that matter, this probe didn't have a camera and landed at night for optimal position with respect to radio contact with Earth &lt;sup&gt;1&lt;/sup&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Besides the temperature readings returned (pressure wasn't transmitted) the carrier signal was monitored for Doppler shifts to measure the wind during descent and rotation of the planet after landing.  It seems that this is the first indication that the high wind speeds at high altitude weren't matched by the rotation speed of the planet&lt;sup&gt;1&lt;/sup&gt;.  Until this mission there was basically no data on the surface so there was nothing to aim for in terms of surface features.&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_8&quot; rel=&quot;noreferrer&quot;&gt;Venera 8&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To allow the V-72 missions to measure sunlight, they were designed to land within the illuminated crescent seen from Earth, still in radio line-of-sight.&lt;sup&gt;1&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This was the first attempt at measuring light low in the atmosphere, but they used a &lt;a href=&quot;https://en.wikipedia.org/wiki/Photometer&quot; rel=&quot;noreferrer&quot;&gt;photometer&lt;/a&gt; so only light intensity was returned, no images/mapping.  That said, the light intensity readings near the morning terminator with the sun only 5° above the horizon provided the first evidence that the cloud layer had a floor and that there may be light at the surface to take pictures &lt;sup&gt;1&lt;/sup&gt;.&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_9&quot; rel=&quot;noreferrer&quot;&gt;Venera 9&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_10&quot; rel=&quot;noreferrer&quot;&gt;10&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;p&gt;9: &amp;quot;landing was made with the Sun near zenith at 05:13 UTC on October 22&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;10: &amp;quot;landing was made with the sun near zenith, at 0517 UT, on October 25&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;A major component of the mission was to take the first pictures of the surface of Venus.  Not only did they land near local noon for the best possible natural lighting but they included halogen lamps (that went unused thanks to sunlight)&lt;sup&gt;3&lt;/sup&gt;.  The interplanetary bus of each craft was used as a communication relay to get around the line-of-sight to Earth limitations of previous landers.&lt;/p&gt;&#xA;&lt;p&gt;Recall that Venus rotates extremely slowly so the three day separation between landings didn't mean that zenith would have moved far.  On the map it's clear that these two landers ended up at different latitudes but nearly the same longitude (291.64° E vs 291.51° E).&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_11&quot; rel=&quot;noreferrer&quot;&gt;Venera 11&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_12&quot; rel=&quot;noreferrer&quot;&gt;12&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This launch window was a poor opportunity to orbit Venus, because the vehicles would arrive with three times the kinetic energy that Venera-9 and 10 did. Instead, hyperbolic trajectories and precise timing permitted sufficiently long radio links after landing. &lt;sup&gt;3&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The bus had to remain in sight to act as a relay and these were the first probes to include color cameras so good lighting would have again been desirable (though due to a design flaw all 4 of the identical cameras failed to eject their lens caps)&lt;sup&gt;3&lt;/sup&gt;.&lt;/p&gt;&#xA;&lt;p&gt;I did find a reference claiming that &amp;quot;the targeting was based purely on ballistic considerations&amp;quot;&lt;sup&gt;4&lt;/sup&gt;, but that seems at odds with the goal of returning pictures which drove them to land on the day side of the planet.  Still, ballistic considerations would have limited their options without forcing them into a specific landing area.&lt;/p&gt;&#xA;&lt;h2&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_13&quot; rel=&quot;noreferrer&quot;&gt;Venera 13&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera_14&quot; rel=&quot;noreferrer&quot;&gt;14&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;p&gt;This was the first mission for which there was mapping data available, though the wording of this report makes it sound like they used the mapping to better understand the areas they landed on instead of using it to plan a landing site:&lt;/p&gt;&#xA;&lt;p&gt;13 &lt;sup&gt;3&lt;/sup&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;From high resolution radar imaging of the landing site, Russian geologists A.M. Abdrakhimov and A.T. Bazilevskii concluded that Venera-13 probably landed on the most common type of Venusian terrain, plains with wrinkle ridges.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;14 &lt;sup&gt;3&lt;/sup&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Radar imaging of the site reveals a younger volcanic plain with lobate flows of lava, probably formed a few million years ago. The rock, which was first considered to be solidified lava, is actually layered and crunchy (as determined by impact deceleration analysis).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That seems to be somewhat at odds with &lt;sup&gt;4&lt;/sup&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Starting with the Venera 13 mission, maps of the radar properties of Venus&#xA;surface, acquired by the Pioneer Venus orbiter, became available, so within the ballistically achievable zones, areas with low surface roughness were selected and used for targeting.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;On the other hand, they were using spherical entry capsules with no ability to steer during entry then parachutes in the higher altitude region of the atmosphere (where the wind speeds reach 100 m/s&lt;sup&gt;5&lt;/sup&gt;) so it's possible they vaguely aimed at easy to hit (based on ballistic constraints) apparently flat regions then examined the radar maps closer for the areas where they actually landed.&lt;/p&gt;&#xA;&lt;h1&gt;Conclusion/Summary&lt;/h1&gt;&#xA;&lt;p&gt;Venera 4-7 were simply dropped into the atmosphere in line-of-sight of Earth for radio contact.&lt;/p&gt;&#xA;&lt;p&gt;Venera 8 had to be in line-of-sight of Earth but also in sunlight, forcing it to land in the narrow band of Venus that met both constraints.&lt;/p&gt;&#xA;&lt;p&gt;Venera 9/10 wanted to land with the sun directly overhead for imaging and did so because they didn't have to be in line-of-sight of Earth.&lt;/p&gt;&#xA;&lt;p&gt;Venera 11/12 also wanted sunlight but had constraints based on the energetic transfer and geometry for using the fast-moving interplanetary bus for a relay to Earth.&lt;/p&gt;&#xA;&lt;p&gt;Venera 13/14 were roughly targeted at flat areas based on available radar mapping data but subject to the same constraints as 11/12.&lt;/p&gt;&#xA;&lt;p&gt;The extremely slow rotation of Venus kept mission pairs (shared constraints) within 10° longitude of each other.  The resonance of planetary alignment mentioned in the environment section above drove all the landers with cameras (9-14) to land within a 20° longitude band (the same place on the planet had optimal lighting every launch window).&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mentallandscape.com/V_Lavochkin1.htm&quot; rel=&quot;noreferrer&quot;&gt;Plumbing the Atmosphere of Venus&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.planetary.org/explore/space-topics/space-missions/missions-to-venus-mercury.html&quot; rel=&quot;noreferrer&quot;&gt;Missions to Venus and Mercury&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mentallandscape.com/V_Lavochkin2.htm&quot; rel=&quot;noreferrer&quot;&gt;First Pictures of the Surface of Venus&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.planetary.brown.edu/pdfs/3437.pdf&quot; rel=&quot;noreferrer&quot;&gt;Landing on Venus: Past and Future&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus&quot; rel=&quot;noreferrer&quot;&gt;Atmosphere of Venus&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-22T13:03:42.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10094" PostTypeId="2" ParentId="9689" CreationDate="2015-07-21T06:55:27.907" Score="4" Body="&lt;p&gt;SpaceX received &lt;a href=&quot;http://www.spacex.com/news/2015/07/20/crs-7-investigation-update&quot; rel=&quot;nofollow&quot;&gt;Dragon telemetry until it disappeared below the horizon&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In addition, the Dragon spacecraft not only survived the second stage event, but also continued to communicate until the vehicle dropped below the horizon and out of range.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-21T17:04:39.060" LastActivityDate="2015-07-21T17:04:39.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10096" PostTypeId="2" ParentId="5402" CreationDate="2015-07-21T07:55:18.420" Score="5" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/OTRAG&quot; rel=&quot;noreferrer&quot;&gt;OTRAG&lt;/a&gt; rockets, with von Braun involved until his death, apparently fired off at least one rocket with 12 engines, and had plans using the same basic hardware to build a 1 metric ton to orbit rocket with 64 engines, 48 in the 1st stage, 12 in the 2nd, and 4 in the 3rd. All were mounted in parallel with each stage peeling off after firing. &lt;/p&gt;&#xA;" OwnerUserId="11011" LastEditorUserId="168" LastEditDate="2015-07-21T08:03:35.493" LastActivityDate="2015-07-21T08:03:35.493" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10098" PostTypeId="1" AcceptedAnswerId="10126" CreationDate="2015-07-21T10:49:07.000" Score="13" ViewCount="7454" Body="&lt;p&gt;As we all know Jupiter is a gaseous planet so what would happen if an Earth-sized object happened to encounter Jupiter?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Would Jupiter engulf the object and make it stay at Jupiter's core?&lt;/li&gt;&#xA;&lt;li&gt;Would the object get fragmented as when two solid objects hit each&#xA;other?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;or what would be the effect?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="25" LastEditDate="2015-07-21T15:59:36.473" LastActivityDate="2016-04-23T15:49:26.633" Title="What would happen if an Earth-sized body encountered Jupiter?" Tags="|planetary-science|jupiter|impact|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10099" PostTypeId="2" ParentId="10098" CreationDate="2015-07-21T11:10:50.933" Score="16" Body="&lt;p&gt;If you saw the video of the Shoemaker-Levy collision you will see that although that was a collection of smaller objects, there is very much a collision, rather than a gas engulfing a solid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the object gets within Jupiter's Roche limit it will be torn to pieces, so you will have a hail of objects falling in and burning up through the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Roche limit (pronounced /ʁoʃ/ in IPA, similar to the sound of rosh), sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Roche_limit&quot;&gt;Roche limit - Wikipedia&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This distance for Earth would be about 16.35 Earth radii, or 104,200 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Large enough chunks may survive and 'float' at a depth that matches their density - this may not be at the core...&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="58" LastEditDate="2015-07-21T13:34:56.153" LastActivityDate="2015-07-21T13:34:56.153" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="10100" PostTypeId="2" ParentId="2438" CreationDate="2015-07-21T12:29:02.830" Score="2" Body="&lt;p&gt;From the page you linked to: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The new Integrated Resistance and Aerobic Training Study (Sprint) protocol uses an evidence based approach to develop a higher intensity, lower duration exercise program utilizing ISS exercise hardware: the Advanced Resistance Exercise Device (ARED) and a second generation treadmill, Combined Operational Load Bearing External Resistance Treadmill (COLBERT). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100039331.pdf&quot; rel=&quot;nofollow&quot;&gt;This presentation&lt;/a&gt; (PDF, page 9-12) contains details on the exercise regimen.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Resistance training: 35-60 min every odd day  &lt;/li&gt;&#xA;&lt;li&gt;Aerobic interval training: varying amounts every even day  &lt;/li&gt;&#xA;&lt;li&gt;Aerobic continuous training: 30 min every odd day  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;More details in the PDF.&#xA;Note that SPRINT is an &lt;a href=&quot;https://blogs.nasa.gov/stationreport/2015/06/12/iss-daily-summary-report-061215/&quot; rel=&quot;nofollow&quot;&gt;ongoing&lt;/a&gt; study, not an operational training regimen. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-09-07T19:01:37.420" LastActivityDate="2015-09-07T19:01:37.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10102" PostTypeId="2" ParentId="9800" CreationDate="2015-07-21T13:19:32.960" Score="4" Body="&lt;p&gt;The total mass of the asteroid belt &lt;a href=&quot;http://adsabs.harvard.edu/abs/2002Icar..158...98K&quot; rel=&quot;nofollow&quot;&gt;can be estimated&lt;/a&gt; &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103502968375&quot; rel=&quot;nofollow&quot;&gt;from the effect&lt;/a&gt; the belt has on the major planets. The total mass &lt;a href=&quot;http://adsabs.harvard.edu/abs/2005SoSyR..39..176P&quot; rel=&quot;nofollow&quot;&gt;is estimated&lt;/a&gt; to be $2.8×10^{21}$ to $3.2×10^{21}$ kilograms. About 1/3 of this is &lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_belt#Characteristics&quot; rel=&quot;nofollow&quot;&gt;concentrated in Ceres alone&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103502968375&quot; rel=&quot;nofollow&quot;&gt;Hidden Mass in the Asteroid Belt&lt;/a&gt; (2002): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These results make it possible to predict the total number of minor planets in any unit interval of absolute magnitude H. Such predictions are compared with the observed distribution; the comparison shows that at present only about 10% of the asteroids with absolute magnitude H&amp;lt;14 have been discovered (according to the derived distribution, about 130,000 such asteroids are expected to exist).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How much of it is discovered by number of objects is a more complex question. For one, new discoveries are made daily. See &lt;a href=&quot;https://www.youtube.com/watch?v=S_d-gs0WoUw&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt; (highly recommended) that charts the number of discovered asteroids from 1980-2010.&lt;br&gt;&#xA;In 1980, about 8000 asteroids were known. By 2010, around 530,000 asteroids were discovered. Now (July 2015), the number's over 680,000. Estimates suggest that there are about a billion asteroids larger than 100 m.&lt;br&gt;&#xA;&lt;a href=&quot;ftp://ftp.lowell.edu/pub/elgb/astorb.html&quot; rel=&quot;nofollow&quot;&gt;Page pointing to a list of currently known asteroids (updated daily).&lt;/a&gt;&lt;br&gt;&#xA;(note: the link is an html page hosted on an FTP server. When you open this page, you get a local copy of the page, but the links to the data seem to work okay. The full list is 50 Mb)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Discovering an asteroid is one thing, determining its mass is more complex. Of the discovered asteroids (&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/datasetDisplay.do?id=PSSB-00051&quot; rel=&quot;nofollow&quot;&gt;lists here&lt;/a&gt;), we only have &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_asteroid_physical_characteristics#Mass&quot; rel=&quot;nofollow&quot;&gt;decent estimates for a few&lt;/a&gt;. For many, we've no idea how big they are. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some correlation between asteroid size and discovery, but other factors come into play as well. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;albedo (in visible light, radar or IR, in which many recent discoveries were made)&lt;/li&gt;&#xA;&lt;li&gt;location (see the video, with discoveries concentrated in regions that are visible from asteroid location programs on Earth by night)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In summary, we have an idea of how much mass is present in the asteroid belt, but into how many bodies that mass is divided remains a big question. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-21T20:05:08.257" LastActivityDate="2015-07-21T20:05:08.257" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10103" PostTypeId="2" ParentId="9556" CreationDate="2015-07-21T13:38:24.237" Score="2" Body="&lt;p&gt;Attitude measurement is done by an &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Spacecraft/Systems-and-Components.php&quot; rel=&quot;nofollow&quot;&gt;inertial navigation unit&lt;/a&gt; (gyroscopes) and by &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576505002535&quot; rel=&quot;nofollow&quot;&gt;modified Galileo Avionica A-STR&lt;/a&gt; &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2006-6383&quot; rel=&quot;nofollow&quot;&gt;star trackers&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Pluto's position is not measured by the spacecraft&lt;/strong&gt;. During the encounter, the spacecraft is preprogrammed to point in direction X at time Y and they have to hope Pluto is in frame. (source: the National Geographic documentary &quot;Mission Pluto&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a recent &lt;a href=&quot;http://www.washingtonpost.com/blogs/achenblog/wp/2015/07/20/the-heroes-of-pluto/&quot; rel=&quot;nofollow&quot;&gt;Washington Post article&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I asked Project Manager Glen Fountain if it was possible the spacecraft would turn in the wrong direction and take photographs of empty space, and he said no — not unless the &lt;em&gt;ephemeris data&lt;/em&gt; was way off.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Emphasis mine: they use ephemeris data collected on Earth, plus navigation images made by NH's main cameras which are interpreted on Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For attitude control, NH uses &lt;a href=&quot;http://soliton.ae.gatech.edu/people/jseitzma/classes/ae6450/MR-103H_Hydrazine_Thruster_New_Horizons.pdf&quot; rel=&quot;nofollow&quot;&gt;12 Aerojet MR-103H&lt;/a&gt; hydrazine thrusters, which were chosen because they can be run for intervals as short as 4 ms. A single solenoid valve controls the amount of fuel injected. Because this is a monopropellant, there's no uncertainty about mixing propellant and oxidiser, and thrust is predictable even at these short bursts. The result is that the craft can be pointed accurately enough using thrusters alone. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-22T12:13:45.920" LastActivityDate="2015-07-22T12:13:45.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10104" PostTypeId="2" ParentId="10098" CreationDate="2015-07-21T13:55:14.540" Score="8" Body="&lt;p&gt;Depending on what you mean by encounter:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Close to Jupiter's Gravity Influence&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;If the planet comes into Jupiter's gravity influence the trajectory will be changed significantly.  If it no longer has the necessary &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot;&gt;velocity to escape&lt;/a&gt; then it may be captured and become a new moon of Jupiter.  On a smaller scale, this is what likely happened around Mars with &lt;a href=&quot;https://en.wikipedia.org/wiki/Moons_of_Mars&quot;&gt;Phobos and Deimos&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the Earth-sized planet would encounter Jupiter within it's &lt;a href=&quot;https://en.wikipedia.org/wiki/Roche_limit&quot;&gt;Roche Limit&lt;/a&gt; it would be torn apart.  It's possible then that the torn apart debris would settle into a more vibrant ring system than &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Jupiter&quot;&gt;Jupiter's current&lt;/a&gt;.  This is &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Saturn#Formation_of_main_rings&quot;&gt;possibly what happened&lt;/a&gt; to one of Saturn's moons to form it's current ring system.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Direct Impact Into Jupiter&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;If the Earth-sized planet is to directly hit Jupiter, it would follow the same as above to begin; the planet would reach an altitude below the Roche Limit and be broken into smaller pieces.  These pieces would be further broken down as the rubble moved closer, until the final pieces impacted with Jupiter.  The pressure of the pieces moving through the Jovian atmosphere would likely cause a heatup and explosion, similar to objects entering &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry&quot;&gt;Earth's atmosphere&lt;/a&gt;.  The majority, if not the entirety of the objects would be vaporized as a result.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastActivityDate="2015-07-21T13:55:14.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10105" PostTypeId="2" ParentId="10048" CreationDate="2015-07-21T14:09:23.927" Score="11" Body="&lt;p&gt;Sorry to spoil everyone’s excitement, but one claim, credited to Mining.com, that the kilometer-wide asteroid might contain “up to 90 million metric tons of platinum and other precious metals” is wrong and is orders of magnitude too high.  Assuming the asteroid to be roughly spherical and 3280 feet (1 kilometer) across and being the more uncommon “nickel-iron” type (having a specific gravity of about 7.5) as opposed to the more common “stony” type, it would have a total mass of about 3.9 billion metric tons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nickel-iron asteroids, based on similar compositions to recovered nickel-iron meteorites, are expected to have more than 95% of their weight in just the metals iron, nickel and cobalt.  One good reference (&lt;a href=&quot;http://authors.library.caltech.edu/51511/1/jgr4710.pdf&quot;&gt;http://authors.library.caltech.edu/51511/1/jgr4710.pdf&lt;/a&gt; ) states that the precious metals Ru, Rh, Pd, Ir, Pt are individually present in such meteorites in 1-10 ppm concentrations.  For convenience let’s optimistically assume these five precious metals total 50 ppm, and then let’s double that to get 100 ppm to account for all the other trace elements that might be considered “precious metals”.&#xA;The above considerations result in a net “precious metals” estimate of only 390,000 metric tons for the asteroid, a factor of about 230 lower than claimed by Mining.com.&#xA;Of course, this assumes that nickel and cobalt are not viewed as precious metals, which I believe is true.&lt;/p&gt;&#xA;" OwnerUserId="11018" LastEditorUserId="7062" LastEditDate="2015-07-21T14:58:12.017" LastActivityDate="2015-07-21T14:58:12.017" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10107" PostTypeId="1" CreationDate="2015-07-21T14:28:17.823" Score="6" ViewCount="707" Body="&lt;p&gt;What would happen if a large non-gaseous body, equal to or greater than that of Jupiter hits with Jupiter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My assumption is the larger body (name it as &lt;strong&gt;A&lt;/strong&gt;) will have a large gravity and a large mass even if they are of same size, because Jupiter mainly consists of hydrogen and helium so the average mass will be less. &lt;strong&gt;A&lt;/strong&gt; will cut through Jupiter and because &lt;strong&gt;A&lt;/strong&gt; will have a high gravity an atmosphere of hydrogen and helium will be formed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I don't know to what percentage my assumption is correct. So what could be the answer?&lt;/p&gt;&#xA;" OwnerUserId="1006" LastEditorUserId="25" LastEditDate="2015-07-21T15:57:52.587" LastActivityDate="2021-08-09T09:39:39.113" Title="What would happen if an object larger than Jupiter hit Jupiter?" Tags="|jupiter|impact|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10108" PostTypeId="2" ParentId="9556" CreationDate="2015-07-21T14:48:05.217" Score="7" Body="&lt;p&gt;I will try to answer your general question with application to NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons&lt;/a&gt; spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The AOCS/GNC (Attitude and Orbital Control System / Guidance Navigation and Control) subsystem takes care that a spacecraft points to a specific point in space, determines the s/c attitude, and does trajectory corrections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in simple terms, an AOCS system is built up by: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;sensors (star-trackers, Earth sensor, magnetometer IMU, etc.)&lt;/li&gt;&#xA;&lt;li&gt;actuators (the different thrusters, reaction wheels, magnetorquers)&lt;/li&gt;&#xA;&lt;li&gt;control algorithm (for example C code).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now, to put this into perspective, you need a basic diagram to understand the very basics of control engineering:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Feedback_loop_with_descriptions.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NvHq1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;A typical, single-input, single-output feedback loop with descriptions for its various parts. Image: &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Feedback_loop_with_descriptions.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I won't get into too much detail. In essence: you are trying to minimize the error based on sensory feedback via controlling the s/c actuators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Johns Hopkins Applied Physics Laboratory's (APL's) mission description, &lt;a href=&quot;http://pluto.jhuapl.edu/Mission/Spacecraft/Systems-and-Components.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacecraft Systems and Components page&lt;/a&gt;, New Horizons has the following GNC system:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Attitude determination – knowing which direction New Horizons is&#xA;  facing – is performed using star-tracking cameras, Inertial&#xA;  Measurement Units (containing sophisticated gyroscopes and&#xA;  accelerometers that measure rotation and horizontal/vertical motion),&#xA;  and digital Sun sensors. Attitude control for the spacecraft – whether&#xA;  in a steady, three-axis pointing mode or in a spin-stabilized mode –&#xA;  is accomplished using thrusters.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The IMUs and star trackers provide constant positional information to&#xA;  the spacecraft’s Guidance and Control processor, which like the&#xA;  Command and Data Handling processor is a 12-MHz Mongoose V. New&#xA;  Horizons carries two copies of each of these units for redundancy. The&#xA;  star-tracking cameras store a map of about 3,000 stars; 10 times per&#xA;  second one of the cameras snaps a wide-angle picture of space,&#xA;  compares the locations of the stars to its onboard map, and calculates&#xA;  the spacecraft’s orientation. The IMU feeds motion information 100&#xA;  times a second. If data shows New Horizons is outside a predetermined&#xA;  position, small hydrazine thrusters will fire to re-orient the&#xA;  spacecraft. The Sun sensors back up the star trackers; they would find&#xA;  and point New Horizons toward the Sun (with Earth nearby) if the other&#xA;  sensors couldn’t find home in an emergency.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Operators use thrusters to maneuver the spacecraft, which has no&#xA;  internal reaction wheels. Its smaller thrusters are used for fine&#xA;  pointing; thrusters that are approximately five times more powerful&#xA;  are used during the trajectory course maneuvers that guide New&#xA;  Horizons toward its targets. New Horizons spins – typically at 5&#xA;  revolutions per minute (RPM) – during trajectory-correction maneuvers&#xA;  and long radio contacts with Earth, and while it “hibernated” during&#xA;  long cruise periods. Operators steady and point the spacecraft during&#xA;  science observations and instrument-system checkouts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="11016" LastEditorUserId="49" LastEditDate="2015-07-22T12:05:23.777" LastActivityDate="2015-07-22T12:05:23.777" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10109" PostTypeId="2" ParentId="9689" CreationDate="2015-07-21T15:00:21.910" Score="7" Body="&lt;p&gt;This was too long to post as a comment:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The investigation also revealed that if the Dragon had deployed its parachutes before falling into the ocean, the spacecraft would have survived. The software in this cargo version of Dragon (Dragon 1), Musk explained, is inert on ascent and was not programmed to release the parachute in the event of a failure.  Software in the version of Dragon under development for taking people into space (Dragon 2 or Crew Dragon) is programmed to do just that.  Musk said they would be working on software fixes to ensure that the Dragon 1 cargo spacecraft can do what it needs to survive. “We could have saved Dragon if we had the right software there,” he said.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacepolicyonline.com/news/failed-strut-likely-cause-of-falcon-9-failure-but-investigation-continues&quot;&gt;Failed Strut Likely Cause of Falcon 9 Failure, But Investigation Continues&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2015-07-21T15:00:21.910" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10111" PostTypeId="1" AcceptedAnswerId="10119" CreationDate="2015-07-21T15:51:08.403" Score="8" ViewCount="522" Body="&lt;p&gt;This was sparked by an interesting conversation in the chatroom. We were discussing how Jupiter, being made of gas, has a lot of color. Looking into it, I found this statement, from &lt;a href=&quot;http://www.universetoday.com/15152/color-of-jupiter/&quot;&gt;Universe Today&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Most of Jupiter is hydrogen and helium, but the top of its clouds are composed of ammonia crystals, with trace amounts of water ice and droplets, and possibly ammonium hydrosulfide.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, there are a number of questions which result from this. The primary question is why does Jupiter have color? Secondly, if it is based off of ammonia crystals, as is indicated, why are these materials on the top of the atmosphere? It seems they should sink.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="12102" LastEditDate="2021-08-09T01:34:06.947" LastActivityDate="2021-08-09T01:34:06.947" Title="Why does Jupiter have the colors it does?" Tags="|planetary-science|jupiter|weather|ammonia|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10112" PostTypeId="2" ParentId="10107" CreationDate="2015-07-21T16:17:43.577" Score="6" Body="&lt;p&gt;I'm working on the assumption that you mean equal in mass, and by &amp;quot;non-gaseous&amp;quot; you mean composed of elements heavier than hydrogen and helium (technically, Jupiter is mostly liquid or solid due to the extreme pressures). According to our current theories such a body could not evolve naturally, but let's ignore that little fact.&lt;/p&gt;&#xA;&lt;p&gt;Let's say that the density of the impacting body is 8 times higher than Jupiter, this would make it denser than iron, I don't know if that is realistic at that level of compression by gravity. This would give the impactor a radius half that of Jupiter (If the density is 8 times higher, the volume must by 1/8th, and because volume is proportional to radius cubed, the radius would thus be the cube root of 1/8th, or 1/2).&lt;/p&gt;&#xA;&lt;p&gt;According to Newton's Impact Depth theory, an impactor will penetrate to approximately its own length times relative density before running out of momentum and this is true regardless of velocity.&lt;/p&gt;&#xA;&lt;p&gt;Making the simplifying assumption that both bodies have uniform density, the impactor could thus punch 8 times its own diameter into Jupiter, as Jupiter only has twice the diameter of the impactor, it could punch through 4 Jupiters before running out of momentum.&lt;/p&gt;&#xA;&lt;p&gt;In reality, a disproportionate amount of Jupiter's mass will be in a dense rocky core. If it hits the core then more of its momentum will be absorbed and Jupiter's core will end up being punched out the far side of Jupiter in a great fiery bulge.&lt;/p&gt;&#xA;&lt;p&gt;So far we haven't considered the energy of the impact. Since the body falls in from outside the solar system, it will be traveling at, at the very least, 70 km/s; this is from a combination of Heliocentric escape velocity at Jupiter's orbit, and Jupiter escape velocity (the two giant masses will pull on each other very strongly and so come together at a great velocity).&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.wolframalpha.com/input/?i=kinetic+energy+of+jupiter+at+70km%2Fs&quot; rel=&quot;nofollow noreferrer&quot;&gt;kinetic energy of a Jupiter mass at 70 km/s&lt;/a&gt; is 4.65 × 10&lt;sup&gt;36&lt;/sup&gt; J, while the &lt;a href=&quot;https://www.wolframalpha.com/input/?i=gravitational+binding+energy+of+jupiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational binding energy of Jupiter&lt;/a&gt; is 2.086 × 10&lt;sup&gt;36&lt;/sup&gt; J, so the impact could potentially yield a blast powerful enough to overcome the gravitational binding energy of both planets, although only just so - a lot of the mass could coalesce into a new planet.&lt;/p&gt;&#xA;&lt;p&gt;That amount of energy is equal to the energy released by the sun in 390 years. Jupiter is four times further from Earth than the Sun is, and thanks to the inverse square rule, we only get 1/16th of that, so we receive a blast equal to 24 years of sunlight—all in the matter of hours—now a lot of that energy will be absorbed in the process of blasting Jupiter to pieces but there would be plenty leftover to irradiate Earth (the blast would also be somewhat directional, if the site of impact is aimed towards Earth we are goners as soon as the radiation arrives, but even if the initial radiation doesn't get us, the blast wave of super heated Jupiter atmosphere will)&lt;/p&gt;&#xA;&lt;p&gt;So the answer to your question, is that Jupiter is blown to pieces and we all die.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="7062" LastEditDate="2021-08-09T09:39:39.113" LastActivityDate="2021-08-09T09:39:39.113" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="10113" PostTypeId="1" AcceptedAnswerId="10115" CreationDate="2015-07-21T16:20:37.703" Score="18" ViewCount="2719" Body="&lt;p&gt;The answers to &lt;a href=&quot;https://space.stackexchange.com/questions/10098/what-would-happen-if-an-earth-sized-body-encountered-jupiter&quot;&gt;this question about encounters with Jupiter&lt;/a&gt; keep mentioning the Roche limit, inside which solid objects break up due to tidal gravity gradients. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over what time scale does this effect actually occur? The Roche limit is usually discussed in the context of near-circular satellite orbits, implying that the time scale could be quite long - millions of years even.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a collision trajectory such as discussed in the linked question, the time from entering the Roche limit to contact might be measured in minutes or hours. Would an Earth-sized body approaching Jupiter have time to break up between entering the Roche limit and hitting Jupiter? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Roche limit in this case is about 53,000km (assuming Earth density for the intruder). If a collision's going to happen it will happen within no more than 35 minutes from crossing the Roche limit.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-07-21T19:50:06.717" Title="How fast does Roche limit disintegration proceed?" Tags="|orbital-mechanics|planetary-science|tides|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10114" PostTypeId="2" ParentId="4634" CreationDate="2015-07-21T16:22:44.423" Score="0" Body="&lt;p&gt;It is my understanding that New Horizons does not have sufficient reactive control energy to establish an orbit about Pluto.&lt;/p&gt;&#xA;" OwnerUserId="11022" LastActivityDate="2015-07-21T16:22:44.423" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10115" PostTypeId="2" ParentId="10113" CreationDate="2015-07-21T16:58:17.957" Score="14" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Roche_limit&quot;&gt;Roche limit&lt;/a&gt; does not cause an object to instantly degrade, it has to orbit within the Roche limit to degrade. Phobos, larger and closer moon of Mars, is believed to orbit inside the Roche limit, and has not as of yet broken apart. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, it isn't fair to say that an object intersecting Jupiter would break apart as a result of the Roche limit. However, once you cross the Roche limit, for a weakly held together object like a comet, it is increasingly likely it will break up. And the closer it is, the more likely that becomes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comet &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9&quot;&gt;Shoemaker-Levy&lt;/a&gt; was in fact a loose collection of rock, and broke apart when it reached deep within the Roche limit of Jupiter, about 2 years prior to its impact on Jupiter. But if something is more strongly held together (Earth, for instance), it probably wouldn't break up, but it would cause strong atmospheric tides due to tidal stress and might lose it to accretion disk connecting with Jupiter. Otherwise, any object headed on a direct impact course would probably remain almost intact until impact due to stronger cohesive forces and own gravity than the amplitude of tidal stress within the Roche radius.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-21T17:16:57.970" LastActivityDate="2015-07-21T17:16:57.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10116" PostTypeId="2" ParentId="4590" CreationDate="2015-07-21T17:25:29.327" Score="3" Body="&lt;p&gt;There are no serious plans at this time to build a liquid space telescope. All references I can find involve construction, as the planned size is ~100m. The plan was to use &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2008/09oct_liquidmirror/&quot; rel=&quot;nofollow&quot;&gt;Ares 5&lt;/a&gt; to help build a telescope on the moon, or similar. I'm sure this idea will come when we can get to the Moon again, and form a base, but I don't think any serious studies can be done that don't involve a human base on the Moon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-21T17:25:29.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10117" PostTypeId="2" ParentId="10113" CreationDate="2015-07-21T17:38:50.143" Score="14" Body="&lt;p&gt;The Roche limit is just the distance from Jupiter at which the &quot;stretching&quot; of the object by Jupiter's tidal force exceeds the internal gravitational force of the object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So certainly in some sense yes, 35 minutes is long enough for the Earth to &quot;break up&quot;. Anything loose on the surface, such as dust and litter and the atmosphere and pedestrians and the hydrosphere, which in astronomical terms is all &quot;part of the Earth&quot;, will start to fall away from the surface of the Earth, starting at the Roche limit. Note that this happens on both sides, with things &quot;falling&quot; either towards or away from Jupiter, for much the same reason that there are two high tides per day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the Earth has some tensile strength, which means that anything &quot;nailed down&quot; will need to be closer to Jupiter before it comes free of the surface. Over the course of 35 minutes, Earth probably isn't very ductile (or anyway its surface isn't), so it's not really a question of time, so much as the magnitude of the force. Rock won't slowly give over 35 minutes, like stretching gum with a constant force until it thins out and comes into two pieces. It will reach breaking point and shatter catastrophically or it won't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tidal force goes as the inverse cube of distance, so it will ramp up quite quickly over the course of that 35 minutes to impact. However, the limit you state is little more than 1/3 the diameter of Jupiter, so I suspect that most of the Earth will still be in one piece when it &quot;impacts&quot; (or &quot;enters&quot; if you prefer) the surface of Jupiter. A geologist or an architect might come along and contradict me, though, since rock doesn't have a whole &lt;em&gt;lot&lt;/em&gt; of tensile strength compared with its weight. So it might only require that the tidal force reaches a small proportion above Earth-weight before great chunks of rock come loose (or, you know, continents).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, I reckon that to anyone standing on the surface, it will feel like the planet is breaking up before it actually does. Any tidal deformation of the core will break up the less-fluid surface, even before it detaches.&lt;/p&gt;&#xA;" OwnerUserId="6946" LastEditorUserId="6946" LastEditDate="2015-07-21T19:50:06.717" LastActivityDate="2015-07-21T19:50:06.717" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="10118" PostTypeId="2" ParentId="9627" CreationDate="2015-07-21T17:43:44.243" Score="7" Body="&lt;p&gt;It isn't that risky, if the proper precautions are taken. The key item, as it is in most cases with spaceflight, is redundancy. There is a larger chance of single event upsets during a geomagnetic storm. This can be mitigated by hardware redundancies. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, geomagnetic storms only really become an issue for most spacecraft when they are beyond LEO. As most rockets don't fire beyond LEO, it isn't much of a concern. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some rocket companies choose to not invest as much in testing and redundancy, and thus when a geomagnetic storm happens, there is more to be concerned with. Antares was one you specifically mentioned. The &lt;a href=&quot;https://www.nasa.gov/offices/c3po/home/cots_project.html&quot; rel=&quot;noreferrer&quot;&gt;COTS&lt;/a&gt; (Commercial Orbital Transportation Services) ISS payload system has been considered relatively low priority, thus, if the mission has to be delayed a day to ensure no issues, it is of little impact. More commonly used rockets need to have higher safeguards, which not only protect against solar storms, but also will give the rocket an overall higher reliability, thus they are more protected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think about it, many satellites operate through solar storms all the time, thus, it can easily be done, if you invest the time and money to get it right ahead of time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2015-07-21T18:50:35.387" LastActivityDate="2015-07-21T18:50:35.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10119" PostTypeId="2" ParentId="10111" CreationDate="2015-07-21T18:16:37.960" Score="6" Body="&lt;p&gt;According to wikipedia: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The exact nature of chemicals that make Jovian zones and bands so colorful is not known, but they may include complicated compounds of sulfur, phosphorus and carbon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Liquid ammonium hydrosulfide, for instance, is yellow-orange in color; besides direct coloration, even colorless gases scatter different colors of light differentially.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the second question, small solid crystals and droplets fall very slowly in a gaseous medium (and are lifted by updrafts); terrestrial clouds are made of ice crystals and water droplets. I imagine that ammonia crystals would be forming and precipitating in a simultaneous cycle.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-07-21T19:16:34.287" LastActivityDate="2015-07-21T19:16:34.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10120" PostTypeId="2" ParentId="6382" CreationDate="2015-07-21T19:48:10.467" Score="3" Body="&lt;p&gt;Building a Lunar orbiting station would be really hard. Maintaining it in orbit would require constant care, far more than is required in LEO. Getting supplies there would be difficult. The radiation environment is much worse than at Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there's the issue of what one would do with the station. I'm not convinced that there is any utility in a lunar station, aside from tourism and as a stopover point between lunar landing missions. Any science that can be done from lunar orbit could be either better done by an unmanned satellite, or done in Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm convinced that the time delay would not be a significant issue, nor would eclipses. Neither is much longer than on Earth. The ISS could be moved to lunar orbit if required, although I suspect the ISS would be better off remaining where it is, and just building a new station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the more credible science fiction books, lunar stations aren't used, but stations at the L4/L5 Earth/Lunar points are used. I think that would be a better option overall.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-21T19:48:10.467" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10121" PostTypeId="2" ParentId="9514" CreationDate="2015-07-21T20:40:42.607" Score="2" Body="&lt;p&gt;The most likely thing to survive that I'm aware of is a Tungsten block. I'm aware of a space mission that used tungsten blocks weighing a few kg, and there was serious concern that one of them could survive reentry in the event of an issue, to the point where it would cause damage. Tungsten is dense and heat resistant, and would probably survive with a relatively small block (A few inches on each side.) Your golf ball of tungsten would probably survive reentry and potentially hurt someone or something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually materials are selected to not survive re-entry. Aluminum is a great choice for such a material. With proper material selection, this is not usually a problem.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-21T20:40:42.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10122" PostTypeId="2" ParentId="9514" CreationDate="2015-07-21T20:54:17.030" Score="7" Body="&lt;p&gt;We know that tiny dust particles can survive even hyperbolic entry.  This is a piece of interplanetary dust collected by an aircraft in the stratosphere:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/G1zmG.jpg&quot; alt=&quot;several micron dust particle&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It probably looks pretty much like it did before entry.  It experienced very high acceleration with relatively little heating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cosmic rays survive as well, and they are much, much smaller.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for dangerous things, it doesn't really matter if it came from space.  All that matters is the mass and its terminal velocity in the atmosphere.  Any and all size and mass objects can result from a satellite reentry and breakup.  The question really comes down to what kinds of falling objects can kill you if they hit you in the head.  That's not a space question.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-07-21T21:14:56.573" LastActivityDate="2015-07-21T21:14:56.573" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10123" PostTypeId="2" ParentId="6382" CreationDate="2015-07-21T21:01:22.940" Score="4" Body="&lt;p&gt;&lt;strong&gt;Pros for LOS&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible lunar orbital infrastructure could make entering and leaving our earth moon neighborhood less difficult. It could also make access to the lunar surface less difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The notions I'm tossing out are more or less science fiction but not prohibited but the laws of physics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth Moon Lagragne 2 a.k.a. &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;EML2&lt;/a&gt; is a very interesting location. It is quite close to C3=0 yet close to the earth in terms of delta V as well as travel time. If getting to Mars or a near asteroid is the goal line, EML2 is on the 10 yard line, energetically speaking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EML2 is also about 2.5 km/s from the moon. They may be rich volatile deposits at the lunar poles. If so, the lunar cold traps might supply EML2 with propellent as well as life support consumables.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But EML2 is only quasi stable. A staging platform at EML2 would be like a ball balanced on top of a hill. A slight nudge in any direction could send it rolling. So there is a station keeping expense to keep a station parked at EML2. EML2 station keeping wouldn't take much delta V, on the order of tens of meters per second. But even this small delta V can be expensive if your station is massive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Retrograde lunar orbits can be stable, especially if the orbit is within 40,000 km of the lunar center. Get too close to the moon and the orbit can be destabilized by &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mascons&lt;/a&gt;. If we parked a &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbonaceous_chondrite#CI_group&quot; rel=&quot;nofollow noreferrer&quot;&gt;carbonaceous ivuna&lt;/a&gt; asteroid at a 40,000 km retrograde lunar orbit, it would be about .4 km/s from EML2. Such asteroids are thought to be rich in water and organic compounds. Another potential source of propellent and life support consumables.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Asteroidal mass could also be used for radiation shielding. An important consideration since lunar orbit doesn't enjoy protection from earth's magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An asteroid parked in a high retrograde lunar orbit could also be a momentum bank for a tether:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0feWI.png&quot; alt=&quot;enter image description here&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this illustration the asteroid is parked at a balance point 24,200 from the moon's center. Tether below is pulled taught by gravity that exceeds centrifugal force. Tether above the balance point is held aloft by centrifugal force. Releasing a payload from this tether top at the correct time could send a payload to a perigee just above geosynchronous orbit. Releasing a payload from this tether foot would send a payload to the lunar surface. On reaching the surface, a payload would need to shed 2.23 km/s for a soft landing. So this tether could substantially cut the delta V for parking in lunar orbit or reaching the lunar surface. I talk about this as well as other stations/tethers at my blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/orbital-momentum-as-commodity.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Momentum as Commodity&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Answers to some of the your other questions&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The I.S.S. isn't designed to be moved big distances. So I don't think hauling I.S.S. modules is an option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parking a rock in lunar orbit ARM style takes a large power source, ion thrusters, and software for the rock fetcher to rendezvous with an inanimate object. Doable in my opinion, but a &lt;a href=&quot;http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Keck&lt;/a&gt; style vehicle doesn't presently exist. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of delta V, time and distance, lunar orbit is more distant from the earth's surface. Getting stuff to LEO takes about 9 km/s. Getting stuff to lunar orbit from earth takes about 13 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So building a LOS would be harder than the ISS.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-07-21T21:01:22.940" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10125" PostTypeId="1" CreationDate="2015-07-21T22:52:43.280" Score="4" ViewCount="368" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/questions/10058/a-cryo-tank-within-another-cryo-tank-is-it-a-sound-engineering-concept#10058&quot;&gt;my related question&lt;/a&gt;, it turns out cryo tanks are &lt;strong&gt;not&lt;/strong&gt; double-walled for insulation purposes. So now I will continue along a similar line and ask, aren't cryo tanks heavier than normal tanks on a per surface area basis?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A normal tank would be one for kerosene or hypergolics---room temperature normal pressure things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My thinking is that the extremely cold conditions of the tank would make them more brittle or structurally weaker somehow, so they'd hafta be thicker to be stronger. OR they could have some other insulation besides double-walls that weighs them down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can pick either LOX or LH2 tanks. If there's some major structural difference between them, well then I'm actually looking forward to learn about it.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-07-23T14:16:48.420" Title="Are cryo tanks heavier than normal tanks (on a per surface area basis)?" Tags="|cryogenics|hydrogen|lox|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10126" PostTypeId="2" ParentId="10098" CreationDate="2015-07-21T23:56:59.080" Score="8" Body="&lt;p&gt;I like the answers, but I'm going to give a brief intuitive answer in addition.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would Jupiter engulf the object and make it stay at Jupiter's core?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;reading the other question, which I'm sure you read as well, &lt;a href=&quot;https://space.stackexchange.com/questions/10107/what-would-happen-if-an-object-larger-than-jupiter-hit-jupiter&quot;&gt;here&lt;/a&gt; I think the safe answer is yes, Earth would be engulfed in Jupiter, though the impact spash would be impressive.   I wouldn't want to guess as to exactly what would happen, but it would be, impressive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if it was more of a glancing blow, large chunks of the Earth like planet could pass through and escape, but Jupiter is 10 times the diameter of Earth and quite solid with very high pressure inside.  It's hard to see anything short of a Neutron Star passing through that with relative ease.   The Jupiter mass rocky planet scenario wouldn't be so much a pass through as a blow up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As pointed out, the Earth like planet simply approaching Jupiter would begin to break apart. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would the object get fragmented as when two solid objects hit each&#xA;  other?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Fragmentation is slowed by gravity.   When Earth was hit by &lt;a href=&quot;https://en.wikipedia.org/wiki/Theia_%28planet%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Theia&lt;/a&gt; - a mars sized planet, at, not sure the velocity but lets say 15-25 KM per second, as it was likely a Trojan orbit, similar to Earth's orbit so the velocity was largely in line, and mostly driven by Earth's and Theia's combined gravitational pull towards each other.    If you imagine 2 rocks smashing into each other at 20 times the speed of a bullet, the rocks would pulverize each other, and that's basically what happened with Earth and Theia, you got pulverized planets, a whole lot of heat and the Earth was sent a spinning, but you don't always get debris everywhere cause gravity is a big factor and that can keep a planet together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an Earth-Theia simulation, a glancing blow (a direct hit and more mass would likely have been lost).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/N8Vct.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is probobly a pretty good estimate for an Earth like planet hitting Jupiter.   Most of the debris would fall back to jupiter, a share, would have a good chance of forming either a ring (inside the Roche limit) or a moon (outside Jupiter's Roche limit) and a fair bit of matter would fly off and escape Jupiter's gravity given an impact that large.   But as a general rule, the larger the planet being hit, the harder it is to blow material into space.   Still, a collision this big, I think it would happen.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;or what would be the effect?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Earth, being 1/300th the mass of Jupiter, the crash would change Jupiter's orbit slightly but measurably, by a few tens of MPH and perhaps having a similar smallish effect on it's rotational velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The collision could also be hot enough to make Jupiter glow, probobly enough to be clearly visible to the naked eye.  How much - too hard for me to say, but perhaps it would look like a tiny sun but far outshining Venus, perhaps even, approaching the brightness of the full moon for a few hours.   The glow might also change with Jupiter's rotation, which takes 10 hours, you could see it grow brighter and dimmer over a 10 hour period for a few days.   It would be quite the show through a telescope, assuming it wouldn't be too bright, but you could watch the debris cloud take shape over several hours and perhaps form into a new moon in a few days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, another effect would likely be debris, even if it's mostly hydrogen, scattered around the solar system.  If Jupiter was rocky, it would be far worse for us, as you'd likely get some enormous meteor showers in a few years and perhaps a few major meteor impacts (for example, if this happened on Mars, it would be quite bad for us), but Jupiter's more gaseous composition, I don't think that would be the case.  The new dust clouds would be more curiosities than things to worry about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's worth pointing out that an earth sized object simply passing through the solar system could be quite problematic, not affecting the planets too much, but stirring up Kuiper belt objects or asteroids in the asteroid belt and sending them every which way, perhaps a few towards Earth - not a good situation for us, but that kind of new object passing through happens very rarely.&lt;/p&gt;&#xA;" OwnerUserId="8711" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-07-22T07:37:55.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10127" PostTypeId="2" ParentId="9989" CreationDate="2015-07-22T00:01:40.243" Score="7" Body="&lt;p&gt;In fact, the New Horizons team is &lt;a href=&quot;http://www.washingtonpost.com/blogs/achenblog/wp/2015/07/20/the-heroes-of-pluto/&quot; rel=&quot;nofollow noreferrer&quot;&gt;now admitting&lt;/a&gt; that they missed the Pluto image, and likely Charon as well. Here's a few quotes from the Washington Post.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If you look at the first big close-up photo of Pluto, you’ll notice the shadow line, the terminator. The spacecraft wasn’t supposed to aim right there&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And later on:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The New Horizons team was aiming for a rectangular box in space 90 kilometers by 60 kilometers. They hit their box, but came in a little bit high, which translated to arriving about 72 seconds early and 40 miles closer to Pluto’s surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It seems like they were aiming for the first pictures to be in shadowed regions, but not at the edge. In Pluto's case, they missed a bit and had the terminator, in Charon's case it missed a bit more. But this was actually planned, it just resulted in less pleasing pictures to begin with. In fact, this can be seen from the recently released &lt;a href=&quot;http://pluto.jhuapl.edu/soc/Pluto-Encounter/view_obs.php?image=data/pluto/level2/lor/jpeg/029917/lor_0299175721_0x632_sci_3.jpg&amp;amp;utc_time=2015-07-14%3Cbr%3E10:30:02%20UTC&amp;amp;description=[not%20yet%20coded]&amp;amp;target=CHARON&amp;amp;range=0.1M%20km&amp;amp;exposure=60%20msec&quot; rel=&quot;nofollow noreferrer&quot;&gt;raw image&lt;/a&gt; of Charon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TrDFS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TrDFS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, the first picture wasn't in the exact optimal place, but it was in fact within the expected error box. I'm confident that when we see the raw images Friday, there will only be the small sliver we saw of Charon, with a whole lot of blank space, but that's okay. By the time all of the images come across, the entire visible portion of Pluto will have been photographed in high resolution.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-22T12:46:46.427" LastActivityDate="2015-07-22T12:46:46.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10128" PostTypeId="2" ParentId="10125" CreationDate="2015-07-22T01:25:22.517" Score="-1" Body="&lt;p&gt;Generally speaking, yes. This is because the tank must act as an insulator to prevent these cryogenic propellants from reaching their boiling point. Additionally, they must be thicker due to the thermal stress from the temperature differential between the inside and outside of the tanks.&lt;/p&gt;&#xA;" OwnerUserId="11008" LastActivityDate="2015-07-22T01:25:22.517" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10129" PostTypeId="5" CreationDate="2015-07-22T05:07:27.927" Score="0" Body="&lt;p&gt;Octaweb refers to the arrangement of 8 engines in an octagon surrounding a central engine, which is used by the &lt;a href=&quot;/questions/tagged/spacex&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;spacex&amp;#39;&quot; rel=&quot;tag&quot;&gt;spacex&lt;/a&gt; &lt;a href=&quot;/questions/tagged/falcon-9&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;falcon-9&amp;#39;&quot; rel=&quot;tag&quot;&gt;falcon-9&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This arrangement replaces the 3x3 grid used in the original version of the falcon 9 and has the benefit of all the outer engines being aligned with the outer skin of the fuel tanks (as opposed to the corners sticking out like in the previous arrangement).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8jHOh.jpg&quot; alt=&quot;Octoweb engine arrangement&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image credit: &lt;a href=&quot;http://www.spacex.com/news/2006/01/01/octaweb&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-22T07:11:53.300" LastActivityDate="2015-07-22T07:11:53.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10130" PostTypeId="4" CreationDate="2015-07-22T05:07:27.927" Score="0" Body="The pattern of 8 engines in an octagon surrounding one central engine used on the falcon 9 rocket." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-22T07:11:30.020" LastActivityDate="2015-07-22T07:11:30.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10131" PostTypeId="5" CreationDate="2015-07-22T05:44:58.420" Score="0" Body="&lt;p&gt;The Low-Density Supersonic Decelerator or LDSD is a reentry vehicle designed to test techniques for atmospheric entry on Mars. The disc-shaped LDSD uses an inflatable structure called the Supersonic Inflatable Aerodynamic Decelerator (SIAD), which is essentially a donut-shaped balloon, to create atmospheric drag in order to decelerate the vehicle before deploying a large supersonic parachute. The goal of the project is to develop a reentry system capable of landing 2- to 3-ton payloads on Mars, as opposed to the 1-ton limit of the currently used systems.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/tdm/ldsd/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Mission Page&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Low-Density_Supersonic_Decelerator&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10273" LastEditDate="2018-11-06T20:50:15.333" LastActivityDate="2018-11-06T20:50:15.333" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="10132" PostTypeId="4" CreationDate="2015-07-22T05:44:58.420" Score="0" Body="The Low-Density Supersonic Decelerator or LDSD is a reentry vehicle designed to test techniques for atmospheric entry on Mars." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-22T07:12:25.870" LastActivityDate="2015-07-22T07:12:25.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10133" PostTypeId="5" CreationDate="2015-07-22T05:59:19.767" Score="0" Body="&lt;p&gt;Performing astronomy from Earth's surface is limited by the filtering and distortion of electromagnetic radiation (scintillation or twinkling) due to the atmosphere. Some terrestrial telescopes (such as the Very Large Telescope) can reduce atmospheric effects with adaptive optics. A telescope orbiting Earth outside the atmosphere is subject neither to twinkling nor to light pollution from artificial light sources on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space-based astronomy is even more important for frequency ranges which are outside the optical window and the radio window, the only two wavelength ranges of the electromagnetic spectrum that are not severely attenuated by the atmosphere. For example, X-ray astronomy is nearly impossible when done from Earth, and has reached its current importance in astronomy only due to orbiting X-ray telescopes such as the Chandra observatory and the XMM-Newton observatory. Infrared and ultraviolet are also greatly blocked.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/jwst&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;jwst&amp;#39;&quot; rel=&quot;tag&quot;&gt;jwst&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/hubble&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;hubble&amp;#39;&quot; rel=&quot;tag&quot;&gt;hubble&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_space_telescopes&quot; rel=&quot;nofollow&quot;&gt;List of Space Telescopes&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Spitzer_Space_Telescope&quot; rel=&quot;nofollow&quot;&gt;Spitzer Space Telescope&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;nofollow&quot;&gt;Hubble Space Telescope&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope&quot; rel=&quot;nofollow&quot;&gt;James Web Space Telescope&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-22T07:13:57.767" LastActivityDate="2015-07-22T07:13:57.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10134" PostTypeId="4" CreationDate="2015-07-22T05:59:19.767" Score="0" Body="A telescope placed in space, typically for the purpose of avoiding atmospheric distortion, background light, and attenuation of light by the atmosphere." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-22T07:15:27.897" LastActivityDate="2015-07-22T07:15:27.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10136" PostTypeId="2" ParentId="942" CreationDate="2015-07-22T09:15:34.853" Score="4" Body="&lt;p&gt;This is currently an unsolved problem. In addition to the X-ray pulsars described by Deer Hunter, NASA funded a study into using &lt;a href=&quot;http://sbir.gsfc.nasa.gov/SBIR/abstracts/11/sbir/phase1/SBIR-11-1-O4.02-9385.html&quot; rel=&quot;nofollow&quot;&gt;exoplanets as a source for navigation data&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This proposal puts forth an innovative star tracker hardware sensor that allows for autonomous calculation of a spacecraft's orbit by employing Doppler Spectroscopy and Astrometric techniques. The proposed advanced star tracker provides onboard deep space orbit self determination capabilities through the use of specialized reference stars that have exoplanet companions. The motion of exoplanets around a reference star's barycenter provides a stable, highly predictable natural signal pattern. An advanced exoplanet star tracker enhances mission capabilities for future manned and unmanned space vehicles as well as reducing Deep Space Network (DSN) tracking requirements and resources. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is far from ready, though: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Estimated &lt;a href=&quot;https://en.wikipedia.org/wiki/Technology_readiness_level&quot; rel=&quot;nofollow&quot;&gt;Technology Readiness Level&lt;/a&gt; (TRL) at beginning and end of contract:&lt;br&gt;&#xA;  Begin: 2&lt;br&gt;&#xA;  End: 3 (i.e. it's in the 'research to prove feasbility' phase)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-22T09:15:34.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10137" PostTypeId="2" ParentId="806" CreationDate="2015-07-22T12:35:36.197" Score="2" Body="&lt;p&gt;In fact, this is something that is an area of exploration. There are a number of studies that simulate weightlessness by forcing the person to lay on a bed 24 hours a day, with the bed at a slight angle. The angle is such that the head is downward, simulating the increase of blood when in microgravity in the upper body. See &lt;a href=&quot;http://www.vice.com/read/nasa-patient-8179-100&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; from a journalist who participated in the experiment, as well as &lt;a href=&quot;http://www.forbes.com/sites/crossingborders/2013/09/18/nasa-will-pay-18000-to-watch-you-rest-in-bed-for-real/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; requesting participants.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-22T12:35:36.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10139" PostTypeId="2" ParentId="5537" CreationDate="2015-07-22T14:22:32.827" Score="3" Body="&lt;p&gt;The noise around Jupiter is quite significant. It is primarily around 40 MHz, according to &lt;a href=&quot;http://www.radiosky.com/rjcentral.html&quot; rel=&quot;nofollow&quot;&gt;this site&lt;/a&gt;, which teaches one to listen to Jupiter. This is not an issue for satellites around Jupiter/Saturn, as they use a much higher frequency (In the GHz range) to communicate with Earth, thus the radio noise doesn't affect them much. It releases these radio waves because of it's large magnetosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saturn doesn't have such a large magnetosphere, and so while it might emit some noise, it isn't considered to be a huge amount of noise. Jupiter, Earth, and the Sun are the largest producers of radio waves in the Solar System (And Earth isn't just because of humans).&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-22T14:22:32.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10140" PostTypeId="1" CreationDate="2015-07-22T15:19:52.370" Score="2" ViewCount="211" Body="&lt;p&gt;The solid boosters of Shuttle/SLS fly in pairs. Could a pair of them fly without SLS, carrying a smaller second stage and payload to orbit? What capacity would it have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rescue launcher needs to launch on short notice, hence solid first stage. The second stage still has to be fueled. The prepared payload could be a space tug that for example could prevent a predicted debris collision in LEO from happening.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-07-22T15:26:48.053" LastActivityDate="2015-07-24T16:06:56.553" Title="Could the solid boosters of SLS be paired together to a rescue launcher?" Tags="|launch|rockets|solid-fuel|rescue-missions|shuttle-derived|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10141" PostTypeId="2" ParentId="5846" CreationDate="2015-07-22T15:22:36.047" Score="6" Body="&lt;p&gt;Solid rockets are cheaper, and easier to maintain. They also have quicker turnaround time, and are often used for missiles as such. They are considered safer, and overall have a lot of advantages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you stated, they have some significant disadvantages, namely poor ISP and less flexibility. I believe you could make some rockets that would work, but almost all serious rocket launches are liquid, because of the increased performance. Solids are saved for some specialty circumstances, or to give a slight boost at takeoff, where they can greatly assist in the launch of a vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, Solid rockets are worth studying to see if they would still give the required performance, but liquids overall are more likely to give you the performance you want.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-22T15:22:36.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10142" PostTypeId="1" AcceptedAnswerId="10150" CreationDate="2015-07-22T15:24:47.827" Score="12" ViewCount="3140" Body="&lt;p&gt;I know the upper stage of the Falcon 9 can be restarted once, thus allowing an additional thrust for some circumstances that might require it. However, I don't really understand the mechanism that limits the number of restarts. What limits the number of restarts that a liquid fueled rocket can perform?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-23T02:18:22.947" LastActivityDate="2016-02-28T22:41:56.927" Title="What limits the number of times that a liquid fueled rocket can restart?" Tags="|launch|rockets|spacex|falcon-9|restartable-engine|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10144" PostTypeId="1" AcceptedAnswerId="10146" CreationDate="2015-07-22T15:29:58.520" Score="7" ViewCount="1488" Body="&lt;p&gt;In many online resources, like &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, five Lagrange points are specified. From my understanding, a Lagrange point is a point where the gravitational attractions of the two bodies (Sun and Earth, for example) cancel themselves. This can be easy to see in the case of L4 and L5, where the distances to Earth and Sun are equal. Considering this, why can't Lagrange be points outside of the elliptic plane, or on the line connecting L4 and L5?&lt;/p&gt;&#xA;" OwnerUserId="11033" LastEditorUserId="7062" LastEditDate="2015-07-23T07:42:06.990" LastActivityDate="2015-07-23T07:42:06.990" Title="Are there really just 5 Lagrange points?" Tags="|orbital-mechanics|lagrangian-points|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10145" PostTypeId="1" AcceptedAnswerId="10149" CreationDate="2015-07-22T15:48:19.763" Score="32" ViewCount="8576" Body="&lt;p&gt;Before the Apollo program committed to the lunar orbit rendezvous mode, one of the Earth orbit rendezvous proposals involved launching two Saturn Vs, one with the Apollo spacecraft and large lunar descent stage, with little or no oxidizer in the S-IVB translunar stage, and the second carrying a large LOX tank. The two would rendezvous in Earth orbit and transfer LOX before TLI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As it happened, the much cheaper but slightly riskier single-launch LOR plan won out and there was no in-flight fuel transfer in Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has any space mission done a rendezvous specifically for propellant transfer? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume in most cases it's far cheaper to launch on one larger rocket than on two smaller ones -- if you have a larger one.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="23891" LastEditDate="2019-06-08T21:26:26.130" LastActivityDate="2019-06-08T21:26:26.130" Title="Has in-space refueling been done?" Tags="|apollo-program|propulsion|docking|rendezvous|refueling|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10146" PostTypeId="2" ParentId="10144" CreationDate="2015-07-22T15:51:23.847" Score="8" Body="&lt;p&gt;The points can't be out of the plane of the orbit because then the combined gravity at that point would be pulling toward the plane instead of being balanced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This diagram shows the contours of the gravitational attraction for a two-body system:&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#/media/File:Lagrange_points2.svg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gcKrY.jpg&quot; alt=&quot;lagrange points&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The points where forces are balanced are at the saddles and peaks (reading it like a topographical map).  Between L4 and L5 would be more attracted to the sun or the earth (except for L1 where those forces are balanced).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should add that none of the Lagrange points is completely stable; the best you can get is stable in one dimension at L1, L2, and L3 (indicated by the red arrows in the diagram).  That means that a slight offset in the direction of a blue arrow will cause you to start drifting that direction away from the Lagrange point you started at, so you still have to do some station-keeping to maintain your position.  The closeness of the contours indicates how much station keeping you would need to do to maintain the orbit relative to the sun/Earth.  While there are only the 5 Lagrange points the gentle slope around L4 and L5 means that there are large regions there where not much station keeping is required to maintain relative position (the points are the absolute minimum).&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-22T23:43:48.307" LastActivityDate="2015-07-22T23:43:48.307" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10147" PostTypeId="1" CreationDate="2015-07-22T16:18:43.437" Score="6" ViewCount="204" Body="&lt;ul&gt;&#xA;&lt;li&gt;Thousands of NEAs have been observed out of estimated &lt;a href=&quot;http://www.iau.org/public/themes/neo/nea/&quot; rel=&quot;nofollow&quot;&gt;285,000&lt;/a&gt; larger than 40 meters. A &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_trojan&quot; rel=&quot;nofollow&quot;&gt;handful&lt;/a&gt; of Martian trojans have been found, but should there be more or fewer NMAs than NEAs?&lt;/li&gt;&#xA;&lt;li&gt;NEAs can become useful resources for spaceflight thanks to their low delta-v relative to Earth. Could NMAs be useful for Mars exploration? For example, perhaps they have more water ice out there, so that fuel for the return trip to Earth could be manufactured in orbit, fuelwise near LMO (Low Mars Orbit), instead of on the surface?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6816" LastEditDate="2015-08-07T22:54:15.007" LastActivityDate="2015-08-07T22:54:15.007" Title="Are NMAs (Near Mars Asteroids) common and useful for exploration?" Tags="|mars|asteroid|isru|near-earth-object|mars-orbiters|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10148" PostTypeId="2" ParentId="10145" CreationDate="2015-07-22T16:28:34.180" Score="13" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The ISS is periodically &lt;a href=&quot;http://www.universetoday.com/84546/first-ever-in-flight-refueling-for-iss-set-for-mid-may/&quot;&gt;refueled&lt;/a&gt; by its cargo supply ships. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Robotic_Refueling_Mission&quot;&gt;Robotic Refueling Mission&lt;/a&gt; has experimented with refueling satellites.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;DARPA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_Express&quot;&gt;Orbital Express&lt;/a&gt; demonstrated hydrazine refueling (among other autonomous orbital maintenance operations) between two satellites in 2007.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-07-22T16:35:31.413" LastActivityDate="2015-07-22T16:35:31.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10149" PostTypeId="2" ParentId="10145" CreationDate="2015-07-22T16:29:34.550" Score="44" Body="&lt;p&gt;Short answer: Space stations have been refueled on orbit, as well as some small demonstration missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is only a limited number of objects that this is even an option. There are 3 types of docking which have generally happened. Those involving manned spacecraft but not a space station, those involving a space station, and those with unmanned spacecraft. In the first category there are the following missions that I'm aware of.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Gemini 8/ Agena- No fuel transfer attempted.&lt;/li&gt;&#xA;&lt;li&gt;Soyuz/ Soyuz- No fuel transfer attempted. This type was tried early on.&lt;/li&gt;&#xA;&lt;li&gt;Apollo/LEM- No fuel transfer attempted.&lt;/li&gt;&#xA;&lt;li&gt;Apollo/ Soyuz- No fuel transfer attempted.&lt;/li&gt;&#xA;&lt;li&gt;Hubble Space Telescope- Has no propulsion, thus no refueling was possible.&lt;/li&gt;&#xA;&lt;li&gt;Other Shuttle satellite servicing missions- I have found a few other missions, but none with fuel transfers.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As for space station dockings, here's a list of all of the space stations that have existed: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Salyut stations- Contained an engine. The first 4 had no means of refueling, however, &lt;a href=&quot;https://en.wikipedia.org/wiki/Salyut_6&quot; rel=&quot;noreferrer&quot;&gt;Salyut 6&lt;/a&gt; and 7 was refueled on orbit.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot; rel=&quot;noreferrer&quot;&gt;Skylab&lt;/a&gt;- Would have been possible to refuel via an EVA, but this doesn't seem to have been done. &lt;/li&gt;&#xA;&lt;li&gt;Mir- &lt;a href=&quot;http://history.nasa.gov/SP-4225/mir/mir.htm&quot; rel=&quot;noreferrer&quot;&gt;Frequently refueled&lt;/a&gt; by Progress spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;International Space Station (ISS)- Frequently refueled&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In addition, a few tests have been done with unmanned spacecraft to demonstrate refueling. One of these was done on the ISS (&lt;a href=&quot;http://ssco.gsfc.nasa.gov/robotic_refueling_mission.html&quot; rel=&quot;noreferrer&quot;&gt;Robotic Refueling Mission&lt;/a&gt;), which demonstrates how to fuel with a spacecraft not designed to be refueled. The other is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_Express&quot; rel=&quot;noreferrer&quot;&gt;Orbital Express&lt;/a&gt; mission, sponsored by the US government to demonstrate such refueling capacity, which refueled using purposely build spacecraft on orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are plans for two additional spacecraft to assist with refueling other spacecraft, namely:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Infrastructure_Servicing&quot; rel=&quot;noreferrer&quot;&gt;Space Infrastructure Servicing&lt;/a&gt; (SIS)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mission_Extension_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;Mission Extension Vehicle&lt;/a&gt; (MEV)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6642" LastEditDate="2016-09-30T05:40:52.170" LastActivityDate="2016-09-30T05:40:52.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10150" PostTypeId="2" ParentId="10142" CreationDate="2015-07-22T16:48:47.243" Score="10" Body="&lt;p&gt;Per this &lt;a href=&quot;http://yarchive.net/space/rocket/restart.html&quot; rel=&quot;noreferrer&quot;&gt;old usenet thread&lt;/a&gt;, there are a lot of design choices, but the  main problems are in getting the fuel to the engines while in free-fall, and igniting non-hypergolic propellants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get the fuel into the pumps / engines, you can use:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;bladder pressurization (which is a materials problem with cryo fuels)&lt;/li&gt;&#xA;&lt;li&gt;piston pressurization (difficult to do on a large scale)&lt;/li&gt;&#xA;&lt;li&gt;small solid-fuel &lt;a href=&quot;https://en.wikipedia.org/wiki/Ullage_motor&quot; rel=&quot;noreferrer&quot;&gt;ullage rockets&lt;/a&gt; (which you'd have a limited number of)&lt;/li&gt;&#xA;&lt;li&gt;small liquid-fuel ullage rockets (which has the same fuel supply issues, but on a smaller and more easily solvable scale)&lt;/li&gt;&#xA;&lt;li&gt;boiloff venting of cryo propellants for ullage!&lt;/li&gt;&#xA;&lt;li&gt;a mesh or membrane system to hold sufficient amounts of fuel at the pump inlet via capillary action&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To ignite non-hypergols, you can use:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;hypergolic chemical igniters (which you'd have a limited number of)&lt;/li&gt;&#xA;&lt;li&gt;electric spark igniters (which might dirty up or wear down)&lt;/li&gt;&#xA;&lt;li&gt;solid fuel pyrotechnics (which you'd have a limited number of, and in turn require electric igniters)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-02-28T22:41:56.927" LastActivityDate="2016-02-28T22:41:56.927" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10151" PostTypeId="1" CreationDate="2015-07-22T16:50:53.720" Score="2" ViewCount="175" Body="&lt;p&gt;The solar system is like a big rotating disk, so it has an axis. Where do the two ends of the axis point in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put it another way, where in the sky are the two furthest points from the ecliptic?&lt;/p&gt;&#xA;" OwnerUserId="11037" LastEditorUserId="7017" LastEditDate="2015-07-23T20:25:26.600" LastActivityDate="2015-07-23T20:25:26.600" Title="Where do the poles of the Solar System point?" Tags="|navigation|astronomy|" AnswerCount="1" CommentCount="4" ClosedDate="2015-07-23T11:29:15.320" ContentLicense="CC BY-SA 3.0" />
  <row Id="10152" PostTypeId="2" ParentId="9487" CreationDate="2015-07-22T19:47:21.033" Score="10" Body="&lt;p&gt;Every Apollo mission sent to the moon was a roll on the dice praying like hell that a major solar flare wouldn't be sending an ejection  Earthside.  This is why NASA  didn't raise that much of a fuss when the last 3 missions were canceled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As demonstrated in the movie &quot;Space&quot;  the Lunar Module would have provided no protection whatsoever.  The Command Module would have been able to put a good deal of mass in the way, but anyone outside or on the lunar surface would been killed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long term missions that venture outside the Van Allen Belts will have to deal with this hazard.&lt;/p&gt;&#xA;" OwnerUserId="11038" LastActivityDate="2015-07-22T19:47:21.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10153" PostTypeId="2" ParentId="10140" CreationDate="2015-07-22T20:49:59.533" Score="5" Body="&lt;p&gt;By themselves, a pair of SRBs with no payload would produce about 5500 m/s of ∆v, which gets them something more than half-way to orbit. They have a fairly poor ISP and propellant mass fraction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you added a 100-ton second stage using storable hydrazine-family propellants, you could get about 5 tons of payload to LEO if that's your goal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0sKlX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0sKlX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="6944" LastEditDate="2015-07-24T16:06:56.553" LastActivityDate="2015-07-24T16:06:56.553" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10154" PostTypeId="2" ParentId="131" CreationDate="2015-07-22T21:10:19.310" Score="2" Body="&lt;p&gt;It might be a bit of a surprise, but only two of our outer planets actually have higher surface gravity than Earth - that is Jupiter and Neptune, and Neptune is only 12% higher. Saturn and Uranus, although much heavier than Earth, actually have lower surface gravity because of their very low density, this puts the surface much further from the center of mass than in the case of Earth, and is sufficient to result in lower surface gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that although they have lower surface gravity, the escape velocity is still much higher than Earth's. A way to look at this is that the gravity well is &quot;deeper&quot; but isn't &quot;steeper&quot;. Also even though the gravity is weaker, the required velocity for orbit is still higher - the required orbital velocity to offset the centripetal force of gravity is proportional to the square root of the orbital radius, Uranus has 4 times the radius of Earth, so at &quot;LUO&quot; just above Uranus's surface orbital velocity is twice that of Earth LEO, and so getting into orbit requires 4 times the energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So surprisingly enough, at least for two of the heavy outer planets, it's actually an easier workout for a weight lifter - but it's still much more expensive to lift things off the planet.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2015-07-23T14:23:05.330" LastActivityDate="2015-07-23T14:23:05.330" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10155" PostTypeId="2" ParentId="10147" CreationDate="2015-07-23T01:33:25.807" Score="3" Body="&lt;p&gt;These objects aren't of particular interest at the moment, as we would have a hard enough time reaching Near Earth asteroids. However, they offer some potential benefits in the future, namely:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;They could provide orbital resources for Mars for fuel and such.&lt;/li&gt;&#xA;&lt;li&gt;They could assist in transforming Mars (Impacts could provide useful resources, for instance.)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;However, they are much more difficult to observe than rocks nearer to Earth, and as a result, would be much more difficult to intercept. They might some day be useful, but I doubt that day happens in my lifetime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect they are actually more common than NEA, due to the proximity to the Asteroid Belt, however, they are harder to see from Earth, so we don't know about as many.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-23T02:26:55.173" LastActivityDate="2015-07-23T02:26:55.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10156" PostTypeId="2" ParentId="10151" CreationDate="2015-07-23T02:11:20.267" Score="3" Body="&lt;p&gt;The common reference in the Solar System isn't the sun's rotation, but the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic_pole&quot; rel=&quot;nofollow noreferrer&quot;&gt;ecliptic plane&lt;/a&gt;, or the &lt;a href=&quot;https://en.wikipedia.org/wiki/Invariable_plane&quot; rel=&quot;nofollow noreferrer&quot;&gt;invariant plane&lt;/a&gt;. The ecliptic plane is nearly constant, on the order of thousands of years, and is the plane around which the Earth revolves around the Sun, and the invariant plane involves the momentum of the objects in the Solar System, and is thus dominated by Jupiter. The difference between the two is less than 2 degrees, and is thus sufficient for our purposes. Here is a view of the north Ecliptic Plane, followed by the south, grabbed from Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FkFb1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FkFb1.png&quot; alt=&quot;North Ecliptic Pole&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oHELq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oHELq.png&quot; alt=&quot;South Ecliptic Pole&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the Sun is close to that pole, only 6 degrees off, although I'm not sure about declination.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-23T13:08:39.603" LastActivityDate="2015-07-23T13:08:39.603" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10157" PostTypeId="5" CreationDate="2015-07-23T03:37:58.267" Score="0" Body="&lt;p&gt;A spaceplane is an aerospace vehicle that operates as an aircraft in Earth's atmosphere, as well as a spacecraft when it is in space. It combines features of an aircraft and a spacecraft, which can be thought of as an aircraft that can endure and maneuver in the vacuum of space or likewise a spacecraft that can fly like an airplane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of 2015 only five spaceplanes have successfully flown, having reentered Earth's atmosphere, returned to Earth, and safely landed: the North American X-15, Space Shuttle, Buran, SpaceShipOne, and Boeing X-37.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/space-shuttle&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;space-shuttle&amp;#39;&quot; rel=&quot;tag&quot;&gt;space-shuttle&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/x-37&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;x-37&amp;#39;&quot; rel=&quot;tag&quot;&gt;x-37&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/buran&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;buran&amp;#39;&quot; rel=&quot;tag&quot;&gt;buran&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Spaceplane&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T07:44:08.200" LastActivityDate="2015-07-23T07:44:08.200" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10158" PostTypeId="4" CreationDate="2015-07-23T03:37:58.267" Score="0" Body="A spaceplane is an aerospace vehicle that operates as an aircraft in Earth's atmosphere, as well as a spacecraft when it is in space." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T07:43:01.917" LastActivityDate="2015-07-23T07:43:01.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10159" PostTypeId="5" CreationDate="2015-07-23T04:26:53.527" Score="0" Body="&lt;p&gt;A ring system is a disk or ring orbiting an astronomical object that is composed of solid material such as dust, moonlets, and is a common component of satellite systems around giant planets. A ring system around a planet is also known as a planetary ring system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In our solar system the gas giants (&lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Jupiter&quot; rel=&quot;nofollow&quot;&gt;Jupiter&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Saturn&quot; rel=&quot;nofollow&quot;&gt;Saturn&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Uranus&quot; rel=&quot;nofollow&quot;&gt;Uranus&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Neptune&quot; rel=&quot;nofollow&quot;&gt;Neptune&lt;/a&gt;) all have rings, but the rings around Saturn are the most extensive.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ring_system_%28astronomy%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T07:43:26.100" LastActivityDate="2015-07-23T07:43:26.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10160" PostTypeId="4" CreationDate="2015-07-23T04:26:53.527" Score="0" Body="A ring system is a disk or ring orbiting an astronomical object that is composed of solid material such as dust, moonlets, and is a common component of satellite systems around giant planets." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T07:42:44.393" LastActivityDate="2015-07-23T07:42:44.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10161" PostTypeId="1" AcceptedAnswerId="10168" CreationDate="2015-07-23T05:03:47.177" Score="7" ViewCount="2334" Body="&lt;p&gt;The escape system is that thin pointy part at the very top of the rocket. It has powerful solid rockets that basically act as an &quot;ejection seat&quot; for the entire crew capsule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/b/b7/Soyuz_tm-31_transported_to_launch_pad.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/b/b7/Soyuz_tm-31_transported_to_launch_pad.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So when we talk about &quot;human-rating&quot; a rocket, doesn't this mean the same thing as the escape system? It's supposed to pull you away really fast (15 g's in Soyuz case) so it can outrun an exploding rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To me that seems like the main thing. If the escape system is reliable, it will save you no matter what the rocket does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to sum up, is human-rating the escape system the only thing needed to human-rate the rocket?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="195" LastEditDate="2018-04-23T20:37:39.400" LastActivityDate="2018-04-23T20:37:39.400" Title="What exactly does it mean to human-rate a rocket? As opposed to the escape system?" Tags="|rockets|safety|launch-escape-system|reliability|human-rating|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10162" PostTypeId="5" CreationDate="2015-07-23T05:06:48.673" Score="0" Body="&lt;p&gt;A space capsule is an often manned spacecraft which has a simple shape for the main section, without any wings or other features to create lift during atmospheric reentry. Capsules have been used in most of the manned space programs to date, including the world's first manned spacecraft Vostok and Mercury, as well as in later Soviet Voskhod, Soyuz, Zond/L1, L3, TKS, US Gemini, Apollo Command Module, Chinese Shenzhou and US, Russian and Indian manned spacecraft currently being developed. A capsule is the specified form for the Orion Multi-Purpose Crew Vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A manned space capsule must have everything necessary for everyday life, including air, water and food. The space capsule must also protect astronauts from the cold and radiation of space. A capsule must be well insulated and have a system that controls the inside temperature and environment. It also must have a way that the astronauts would not be knocked around during launch or reentry. Additionally, since the inside will be weightless, there must be a way for the astronauts to stay in their seats during the flight. For this each seat has a system of straps and buckles. One of the most important things that a space capsule must have is a way to communicate with people back on Earth, or mission control.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/orion&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;orion&amp;#39;&quot; rel=&quot;tag&quot;&gt;orion&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/cst-100&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;cst-100&amp;#39;&quot; rel=&quot;tag&quot;&gt;cst-100&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/apollo&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;apollo&amp;#39;&quot; rel=&quot;tag&quot;&gt;apollo&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/soyuz&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;soyuz&amp;#39;&quot; rel=&quot;tag&quot;&gt;soyuz&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_capsule&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T09:25:03.177" LastActivityDate="2015-07-23T09:25:03.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10163" PostTypeId="4" CreationDate="2015-07-23T05:06:48.673" Score="0" Body="A space capsule is an often manned spacecraft which has a simple shape for the main section, without any wings or other features to create lift during atmospheric reentry." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T09:25:14.160" LastActivityDate="2015-07-23T09:25:14.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10164" PostTypeId="5" CreationDate="2015-07-23T05:11:05.097" Score="0" Body="&lt;p&gt;The Orion Multi-Purpose Crew Vehicle (Orion MPCV) is a spacecraft intended to carry a crew of up to four astronauts to destinations at or beyond low Earth orbit (LEO). Currently under development by NASA for launch on the Space Launch System, Orion is intended to facilitate human exploration of asteroids and of Mars, as well as to provide a means of delivering or retrieving crew or supplies from the ISS if needed.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/nasa&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;nasa&amp;#39;&quot; rel=&quot;tag&quot;&gt;nasa&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/sls&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;sls&amp;#39;&quot; rel=&quot;tag&quot;&gt;sls&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/exploration/systems/orion/&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T09:24:16.807" LastActivityDate="2015-07-23T09:24:16.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10165" PostTypeId="4" CreationDate="2015-07-23T05:11:05.097" Score="0" Body="The Orion Multi-Purpose Crew Vehicle (Orion MPCV) is a spacecraft intended to carry a crew of up to four astronauts to destinations at or beyond low Earth orbit (LEO). Orion is intended to facilitate human exploration of asteroids and of Mars, as well as to provide a means of delivering or retrieving crew or supplies from the ISS if needed." OwnerUserId="10132" LastEditorUserId="49" LastEditDate="2015-07-23T09:25:34.017" LastActivityDate="2015-07-23T09:25:34.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10166" PostTypeId="5" CreationDate="2015-07-23T05:16:33.010" Score="0" Body="&lt;p&gt;In spaceflight, a human-rating certification is the assurance that the space system accommodates human needs, effectively utilizes human capabilities, controls hazards with sufficient certainty to be considered safe for human operations, and provides, to the maximum extent practical, the capability to safely recover the crew from hazardous situations. In the United States, the National Aeronautics and Space Administration (NASA) has published NASA Procedural Requirement NPR 8705.2B - Human Rating Requirements for Space Systems, defining the certification process and a set of technical requirements to be applied to its crewed space systems in addition to the standards and requirements that are mandatory for all of NASA's space flight programs.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Human-rating_certification&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8705_002B_&quot; rel=&quot;nofollow&quot;&gt;NASA: Human-Rating Requirements for Space Systems&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T09:23:45.500" LastActivityDate="2015-07-23T09:23:45.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10167" PostTypeId="4" CreationDate="2015-07-23T05:16:33.010" Score="0" Body="Human-rated or man-rated are terms used to describe the certification of a spacecraft or launch vehicle as worthy of transporting humans." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T09:25:46.580" LastActivityDate="2015-07-23T09:25:46.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10168" PostTypeId="2" ParentId="10161" CreationDate="2015-07-23T05:46:28.373" Score="20" Body="&lt;p&gt;While the launch escape system (LES) is important for getting the crew away on ascent there are other thing required for human-rating a craft/launch system (and you do need to pay attention to the entire system and mission, not just the rocket during ascent).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically for Atlas V they needed to: &lt;sup&gt;&lt;a href=&quot;http://www.space.com/15170-private-space-taxis-atlas5-rocket.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;space.com&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;upgrade the emergency detection system (your LES does no good if it doesn't get enough warning to get away in time)&lt;/li&gt;&#xA;&lt;li&gt;figure out how to get astronauts onto the rocket, including options for emergency evacuation&lt;/li&gt;&#xA;&lt;li&gt;develop a different upper stage (no explanation provided on this one)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;NASA actually has a document available online that details what all goes into human-rating a spacecraft by their standards: &lt;a href=&quot;https://nodis3.gsfc.nasa.gov/displayDir.cfm?t=NPR&amp;amp;c=8705&amp;amp;s=2C&quot; rel=&quot;nofollow noreferrer&quot;&gt;Human-Rating Requirements for Space Systems&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It starts off with the very basics:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The space system shall provide the capability to sustain a safe, habitable environment for the crew&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sets expectations for chance of failure:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The space system shall meet probabilistic safety criteria derived from the Agency-level safety goals and safety thresholds with a specified degree of certainty&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Insists allowances be made for the fact that humans are operating the craft:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The space system shall be designed to tolerate inadvertent operator action (minimum of one inadvertent action), as identified by the human error analysis, without causing a catastrophic event&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's also an entire section on requirements for the launch escape system (LES).  First, there has to be one:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The space system shall provide abort capability from the launch pad until Earth-orbit insertion to protect for the following ascent failure scenarios: ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And beyond that, it has to be activatable by the crew, the ground control, and the rocket itself (only quoting the last):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The crewed space system shall monitor the Earth ascent launch vehicle performance and automatically initiate an abort when an impending catastrophic failure is detected&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And at end of mission:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The space system shall provide recovery forces with the location of the spacecraft after return to Earth&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt; The launch escape system is a key requirement but it's intended to be a last-resort, not the only option for crew protection.  You also need provisions for supporting a crew on the ground and in space.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="18663" LastEditDate="2018-04-23T19:35:59.187" LastActivityDate="2018-04-23T19:35:59.187" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10169" PostTypeId="1" AcceptedAnswerId="10172" CreationDate="2015-07-23T06:20:06.557" Score="3" ViewCount="778" Body="&lt;p&gt;What is meant with &quot;near-equatorial&quot; when it comes to orbits? Specifically, how many kilometers off the equatorial plane would a satellite in a near equatorial orbit cover? e.g. +/- __ km from equator.&lt;/p&gt;&#xA;" OwnerUserId="11043" LastEditorUserId="49" LastEditDate="2015-07-23T11:42:03.630" LastActivityDate="2016-01-06T02:37:48.033" Title="What orbits are referred to as &quot;near-equatorial&quot;?" Tags="|orbit|terminology|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10170" PostTypeId="1" CreationDate="2015-07-23T08:36:36.440" Score="10" ViewCount="975" Body="&lt;p&gt;The Earth has 7 Lagrange points nearby since SEL1 and SEL2 (Sun-Earth Lagrange points 1 and 2, respectively) are only between 3 and 5 LD (&lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_distance_%28astronomy%29&quot; rel=&quot;noreferrer&quot;&gt;lunar distances&lt;/a&gt;) away from the five EML (Earth-Moon Lagrange) points. Can it be useful for spacecraft to move in a sequence between SEL's and EML's in order to lower their missions' delta-v requirements? Has it been done already?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would think that the monthly and substantial variation in their relative distances and angles should help lower delta-v for a wide range of maneuvers in this wider cislunar region. The SEL1-to-EML2 distance varies between 1,100,000 km and 1,900,000 km over a fortnight, while the EML points always have stationary locations relative to each other. But my impression is that SEL1-EML connections are rarely discussed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rZJWL.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rZJWL.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-07-23T21:34:35.223" LastActivityDate="2022-08-10T19:52:06.183" Title="How could transfers between SEL1&amp;2 and EML Lagrange points be utilized?" Tags="|orbital-mechanics|orbital-maneuver|lagrangian-points|cislunar|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10171" PostTypeId="2" ParentId="10170" CreationDate="2015-07-23T11:45:35.220" Score="10" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;Can it be useful for spacecraft to move in a sequence between SEL's and EML's in order to lower their missions' delta-v requirements? Has it been done already?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Yes and yes. You are asking about weak stability boundary trajectories. These offer a significantly reduced delta V trajectory from the Earth to the Moon compared to a direct transfer. Two downsides compared to direct transfer are much longer transit times and rather narrow launch windows.&lt;/p&gt;&#xA;&lt;p&gt;That is exactly how Japan's HITEN and NASA's GRAIL satellites went to the Moon. Those satellites used a weak stability boundary trajectory to significantly reduce delta V requirements. The GRAIL satellites went indirectly from the Earth to lunar orbit via the Sun-Earth L1 point.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.skyandtelescope.com/wp-content/uploads/GRAIL_trajectory.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://www.skyandtelescope.com/wp-content/uploads/GRAIL_trajectory.jpg&quot; alt=&quot;Image of the trajectory of GRAIL A and B in the Sun Earth synodic frame. The vehicles were sent close to the Sun-Earth L1 point before traveling to the Moon, with each undergoing trajectory correction maneuvers prior to and after passing near the L1 point.&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-03-11T08:59:22.267" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="10172" PostTypeId="2" ParentId="10169" CreationDate="2015-07-23T12:18:08.057" Score="2" Body="&lt;p&gt;I'm not aware of any single interpretation of the &quot;near-equatorial&quot; term, and if you search through the literature, we're left with several options. One thing is clear though, we're referring with it to orbits that have low inclination angle to the equatorial plane. &lt;a href=&quot;https://en.wikipedia.org/wiki/Near-equatorial_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Near-equatorial orbit&lt;/a&gt; is rather vague:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A near-equatorial orbit is an orbit that lies close to the equatorial&#xA;  plane of the object orbited. This orbit allows for rapid revisit times&#xA;  (for a single orbiting spacecraft) of near-equatorial ground sites.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And marks that with &lt;em&gt;citation needed&lt;/em&gt;. Indeed. Luckily, we also have a more specific definition of an equatorial orbit to work with. Equatorial orbits are considered &lt;a href=&quot;https://en.wikipedia.org/wiki/Non-inclined_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;non-inclined&lt;/a&gt; to the equatorial plane, so orbits with either inclination of 0° for prograde or 180° for retrograde orbits:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A non-inclined orbit is an orbit which is contained in the plane of&#xA;  reference. The inclination is 0 for prograde orbits, and π (180°) for&#xA;  retrograde orbits. If the plane of reference is the equator, these&#xA;  orbits are called equatorial; if the plane of reference is ecliptic,&#xA;  they are called ecliptic. As these orbits lack nodes, the ascending&#xA;  node is usually taken to lie in the reference direction (usually the&#xA;  vernal equinox), and thus the longitude of the ascending node is taken&#xA;  to be zero. Also, the argument of periapsis is undefined.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So we know this much, that &quot;non-equatorial&quot; cannot refer to &quot;equatorial&quot;, or non-inclined. So inclined orbits then, but by how much? This is where it gets argumentative, and you'll get different definitions by different people;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One possible interpretation is that the term refers to those orbits whose &lt;a href=&quot;https://en.wikipedia.org/wiki/Ground_track&quot; rel=&quot;nofollow noreferrer&quot;&gt;ground track&lt;/a&gt; (projection of the movement of the satellite on celestial sphere from the orbiting body towards the orbital focus, i.e. nadir projection from the satellite's point of view, or zenith projection from ground observer's point of view) stays within the &lt;a href=&quot;http://encyclopedia2.thefreedictionary.com/Equatorial+belt&quot; rel=&quot;nofollow noreferrer&quot;&gt;equatorial belt&lt;/a&gt;, so up to &amp;pm;4° inclined orbits for Earth, if you consider the equatorial region as the area within &lt;em&gt;5°–8° Northern and 4°–11° Southern latitude&lt;/em&gt; of the equator. Of course, this will then depend on which body the orbit is at, and definitions of the stretch of their equatorial belt will vary significantly depending on who you ask. At planetary bodies with super-rotating atmospheres, it would make sense to describe the belt as the region of equatorial doldrums (e.g. Venus). And at bodies with no atmosphere, it would simply be some arbitrary value at a small degree inclination to its equatorial plane. And if even where equatorial plane is isn't clear and the body &lt;em&gt;wobbles&lt;/em&gt;, it rotates on more than one axis, then you'd also need to adopt some definition of &lt;a href=&quot;https://space.stackexchange.com/q/5600/49&quot;&gt;how prime meridian is selected&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this above described option of how &quot;near-equatorial&quot; &lt;em&gt;could&lt;/em&gt; be defined, while it makes perfect sense to me, also seems a bit ambiguous, or at least difficult to work with and non-intuitive, if inclinations of orbits around other bodies than the Earth are described with it. And that's perhaps why there isn't any strict definition to follow, and we usually mean with &quot;near-equatorial&quot; those orbits with low inclination to the equatorial plane, i.e. &amp;pm;&lt;em&gt;n&lt;/em&gt;° where &lt;em&gt;n&lt;/em&gt; is, erm, small. And what &lt;em&gt;low&lt;/em&gt; means to you, might mean something different to me or anyone else.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternative definition could be that they're orbits with large margin of error regarding their &lt;a href=&quot;https://en.wikipedia.org/wiki/Longitude_of_the_ascending_node&quot; rel=&quot;nofollow noreferrer&quot;&gt;longitude of the ascending node&lt;/a&gt;, which is then given as for equatorial orbits to be zero, despite the orbital inclination not being exactly zero or 180°. You'd be looking at orbits with inclination well below 1° to the equatorial plane at Earth then, but this inclination might be larger around bodies where orbital perturbations (mass concentrations, secondary bodies, own orbital eccentricity,...) are, over time, more significantly affecting satellite's ascending node.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, there isn't any single definition from an absolute authority source to adopt. What I describe is just one option, and my own opinion on the matter. It does somewhat follow the general adoption of the &quot;near-&quot; prefix when it comes to orbits (e.g. &quot;near-polar&quot;, &quot;near-circular&quot;,...) and it constrains inclination to a predefined region by ground track, if someone wants you to be more specific than &lt;em&gt;low inclination orbits&lt;/em&gt;. But &lt;em&gt;caveat emptor&lt;/em&gt;, my definition isn't authoritative in any way whatsoever. Don't use this as an answer to an exam question, or anything like that. Use whatever definition you were told to use by the one asking the question.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-07-23T12:40:33.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10173" PostTypeId="1" AcceptedAnswerId="10175" CreationDate="2015-07-23T12:30:24.887" Score="6" ViewCount="247" Body="&lt;p&gt;One of the original ideas behind what became the NOAA &lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/mission.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Climate Observatory (DSCOVR)&lt;/a&gt; was to to provide a live feed of the Earth from L-1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aLcfe.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aLcfe.jpg&quot; alt=&quot;DSCOVR first photo of the Americas&quot;&gt;&lt;/a&gt;&#xA;&lt;br /&gt;&lt;a href=&quot;http://www.nasa.gov/press-release/nasa-satellite-camera-provides-epic-view-of-earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSCOVR-EPIC photo released 20 July&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.wmo-sat.info/oscar/instruments/view/793&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth Polychromatic Imaging Camera (EPIC)&lt;/a&gt; views the sunlit part of Earth in UV, visible, and near infrared.  Among its products are natural-colour images such as the one released on 20 July.  How frequently are those images produced and what is the timeliness of their release to the general public?  &lt;a href=&quot;http://www.wmo-sat.info/oscar/satellites/view/525#inst-793&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSCAR states data are released in near real time&lt;/a&gt;.  It would be awesome to have a live or nearly live feed.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2015-07-23T12:37:06.520" LastActivityDate="2015-10-20T15:56:00.390" Title="What is the timeliness of DSCOVR-EPIC image acquisition and public release?" Tags="|dscovr|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10174" PostTypeId="2" ParentId="10142" CreationDate="2015-07-23T13:00:35.207" Score="1" Body="&lt;p&gt;There is no &lt;strong&gt;SENSIBLE&lt;/strong&gt; upper limit in terms of the likely requirements in the next few decades. Increasing restart capability adds complexity  and a certain amount of mass but you can soon get to a system which is essentially reliability limited. Tens to hundreds of restarts would readily be achievable with existing technology. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As one only example &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can use cold gas thrusters to achieve ullage location of main tank fuel and if desired replenish the gas reservoirs from main fuel or oxidiser once the contents are stable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anywhere outside a permanent shadow  (which are rather hard to find usually) you can use photovoltaic panels to recharge electrical system energy stores. Existing battery technologies have calendar and cycle lifetimes well in excess of what is sensibly needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lack of hypergolicity is a challenge but, if the need exists, small igniters running on main fuel mixes heated to combustion point electrically are an entirely realistic solution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's likely that dozens of restarts could be achieved by such systems, and unlikely that more than that number would be required in any likely application short term. Extension to say hundreds of restarts is &quot;just a matter of engineering&quot; with increased need for maintaining reliability as restarts increased. &lt;/p&gt;&#xA;" OwnerUserId="8362" LastActivityDate="2015-07-23T13:00:35.207" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10175" PostTypeId="2" ParentId="10173" CreationDate="2015-07-23T13:45:02.360" Score="6" Body="&lt;p&gt;Part of the &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-satellite-camera-provides-epic-view-of-earth&quot; rel=&quot;nofollow&quot;&gt;news release you link to&lt;/a&gt; (under the image) has the information you're looking for:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once the instrument begins regular data acquisition, new images will&#xA;  be available every day, 12 to 36 hours after they are acquired by&#xA;  EPIC. These images will be posted to a dedicated web page by&#xA;  September [2015].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'll update with links and more information once that's made available.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update (Oct. 20, 2015)&lt;/strong&gt;: EPIC images are now finally available to the public since Monday, Oct. 19, 2015 at &lt;a href=&quot;http://epic.gsfc.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;epic.gsfc.nasa.gov&lt;/a&gt;. The page is still a bit rough around the edges, with search function not working and date picker for some reason returns one day before than selected, but the rest seems to be there. Regarding timeliness, 12 to 36 hours old images are available as taken by NASA's Earth Polychromatic Imaging Camera's (EPIC) four megapixel CCD and telescope, with new image taken roughly once every hour. More info is available on &lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/index.html&quot; rel=&quot;nofollow&quot;&gt;DSCOVR mission pages&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking forward to the next solar eclipse then. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-20T15:56:00.390" LastActivityDate="2015-10-20T15:56:00.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10176" PostTypeId="2" ParentId="10125" CreationDate="2015-07-23T14:00:50.123" Score="2" Body="&lt;p&gt;Not necessarily.  For a counterexample, we can look at one of IMHO the coolest boosters ever, the Atlas 1.  This is as close as the US has ever come to an operational SSTO vehicle and one of the reasons is its amazing construction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This vehicle utilized the famous &lt;a href=&quot;https://en.wikipedia.org/wiki/Balloon_tank&quot; rel=&quot;nofollow noreferrer&quot;&gt;balloon tank&lt;/a&gt; design in which the tank walls were so thin that they could not even support their own weight without being pressurized.  The propellants were RP-1 (room temperature hydrocarbon) and LOX.  Tankage for the entire vehicle was built from &quot;a special cold-rolled austenitic steel (A1S1 grade 301) which was produced by the Washington Steel Corporation.&quot;  (All quotes are from &quot;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/B0000CM2IP&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacecraft and Boosters&lt;/a&gt;&quot; by Gatland - basically a collection of articles from Flight International magazine in the Space Age, and a superb book).  Pages 219-225 give a detailed description of the construction and state that &quot;several of the 27 (ring) sections are thinner than the walls of a milk carton. The heaviest skin gauge is less than 40 mil.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for this missile at least, there was no significant difference in the tank construction for the RP-1 and LOX tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a poor-quality scan of one of the great schematics from this book.  It shows how the tankage &lt;em&gt;is&lt;/em&gt; the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6xQbe.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6xQbe.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-07-23T14:16:48.420" LastActivityDate="2015-07-23T14:16:48.420" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10177" PostTypeId="1" CreationDate="2015-07-23T14:05:57.307" Score="5" ViewCount="215" Body="&lt;p&gt;In case of collision warning occurring to the ISS, if ISS can't be moved the procedure is to go to a Soyuz and wait.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone gives details about the procedure? &#xA;Is the Soyuz hermetically closed from the ISS? &#xA;Do the crew have to put on the space suits? &#xA;Is the Soyuz powered on and ready to go back to Earth? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="25" LastEditDate="2015-07-23T14:19:57.497" LastActivityDate="2015-07-23T14:29:47.107" Title="ISS: Procedure in case of collision warning" Tags="|iss|debris|soyuz-spacecraft|astronauts|conjunction|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10178" PostTypeId="2" ParentId="10177" CreationDate="2015-07-23T14:29:47.107" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://www.astrowatch.net/2015/07/space-station-crew-hides-from-space-junk.html&quot; rel=&quot;nofollow&quot;&gt;Astrowatch&lt;/a&gt; has the answer, which is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Per Flight Rule B4-101, a “late conjunction” call results in the crew being ordered to close the hatches between Station modules and enter the Soyuz vehicles – which serve as lifeboats during their docked stay at the ISS – before TCA (Time of Closest Approach) breaches the 10 minute mark.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is my understanding that the hatchs are closed (From above), and the spacecraft is ready to depart on a moments notice using an emergency departure system. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-23T14:29:47.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10179" PostTypeId="2" ParentId="131" CreationDate="2015-07-23T14:38:13.907" Score="3" Body="&lt;p&gt;There is a limit to how fast humans can build up muscles. Body builders choose weights and exercises to put their muscles under the ideal strain for muscle growth. The art of body-building is to find that ideal level for each muscle which neither under- nor overstrains it. A different gravity would mean that &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;the weight of the athlets body itself increases&lt;/li&gt;&#xA;&lt;li&gt;the weight of any weights increases&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;That means that in a higher gravity, exercises will be more straining than in earth environment. That means that their usual training plan would be far too intense and they would just injure their muscles. The athlete needs to decrease the intensity of their training plan to get the strain back to the ideal level. That means a body builder could not achieve significantly more progress in a high-gravity environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when you would put a normal person in a high-gravity environment who does not consciously exercise, they would likely still build up muscles. The reason is that every-day tasks put more strain on their muscles than on Earth. Eating a meal with a fork could be equivalent to a workout with a barbell and a leisure walk to another building would be like running up a mountain. But the effect is unlikely to be better than with a well-planned intensive training program on Earth.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2015-07-23T15:30:21.207" LastActivityDate="2015-07-23T15:30:21.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10180" PostTypeId="1" CreationDate="2015-07-23T15:22:05.920" Score="1" ViewCount="157" Body="&lt;p&gt;Consider the following idea for inserting a human on the surface of the earth into orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In orbit, attach a long (~80km) rigid shaft to a large weight (presumably an asteroid) and set it spinning.  Alter its trajectory and angular velocity on a collision course with the surface, such that the shaft will briefly touch the person's harness on the surface with zero instantaneous relative velocity, and become attached.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the apparatus continues spinning and falling, further rotation of the shaft carries our human up and out of the atmosphere.  By detaching at apogee, our traveler achieves orbit, leaving the the apparatus to destroy itself in a collision with the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do plausible future materials allow it, and do the angular momenta work out?&lt;/p&gt;&#xA;" OwnerUserId="11048" LastEditorUserId="11048" LastEditDate="2015-07-23T16:15:02.487" LastActivityDate="2015-07-25T02:44:41.367" Title="Orbital insertion by rotating shaft" Tags="|orbital-mechanics|atmosphere|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10181" PostTypeId="1" AcceptedAnswerId="14858" CreationDate="2015-07-23T15:45:54.967" Score="11" ViewCount="1311" Body="&lt;p&gt;The American STS Space Shuttle had the engines that provided a large portion of its thrust mounted directly on the shuttle itself.  The &lt;code&gt;Space Shuttle Main Engines&lt;/code&gt; (SSME) were designed to be reusable and were put on the orbiter to be recovered.  Because the shuttle was side-mounted on the External Fuel Tank, the SSMEs had to gimbal as much as 10 degrees to accommodate for the offset center of mass.  If this wasn't done, the stack would tip over and likely spin catastrophically out of control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Buran mounted on the Energia had a different design from the STS stack.  The Buran Orbiter itself didn't provide the main thrust to get itself to space (I believe it only provided a bit of thrust at the end to adjust and reach its orbit.)  The main engines were instead that of the Energia rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did the Energia rocket compensate for the offset center of mass?  Via gimbaling of the engines on the Energia, or some offsetting design in the rocket itself?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="168" LastEditDate="2015-07-23T16:15:38.857" LastActivityDate="2019-06-25T02:59:07.877" Title="How did Energia Buran handle center of mass during flight?" Tags="|design|buran|energia|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10182" PostTypeId="1" CreationDate="2015-07-23T17:39:25.567" Score="2" ViewCount="1590" Body="&lt;p&gt;So if we have a satellite in a non circular orbit, can't state a time independent altitude (we can only state the altitude at a given time). We can however state a constant value for the semi-major axis (I'm ignoring orbit perturbations here). The semi-major axis is the in essence;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(perigee altitude + apogee altitude + 2 x radius of Earth)/2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So is there a term specifically for;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;semi-major axis - radius of Earth&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-07-23T17:39:25.567" Title="Is there an equivalent term for average altitude like semi-major axis?" Tags="|orbit|terminology|altitude|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10183" PostTypeId="2" ParentId="131" CreationDate="2015-07-23T18:54:52.043" Score="2" Body="&lt;p&gt;Ultimately, human strength is not limited by the amount of muscle mass you can build, but by things that don't grow as much (tendons, joints). So a human who's born on Earth and goes to a high-G planet to train for a few years will not become stronger than his peers who've been training on Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I suspect would make a difference is living on a high-G planet for a number of generations. The body will adapt to changing circumstances, so eventually you'll see humans born on a high-G planet becoming stronger on average, and being able to reach a higher peak strength. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There'll be some interesting side effects to training on a high-G planet. The heart will have more trouble pumping blood 'uphill' to the brain, for instance.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-23T18:54:52.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10184" PostTypeId="5" CreationDate="2015-07-23T21:49:46.400" Score="0" Body="&lt;p&gt;In physics, escape velocity is the speed at which the sum of an object's kinetic energy and its gravitational potential energy is equal to zero. It is the speed needed to &quot;break free&quot; from the gravitational attraction of a massive body, without further propulsion, i.e., without spending more propellant.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;References&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T23:49:37.583" LastActivityDate="2015-07-23T23:49:37.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10185" PostTypeId="4" CreationDate="2015-07-23T21:49:46.400" Score="0" Body="The speed needed to &quot;break free&quot; of the gravitational attraction of a body without spending more propellant." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T22:35:37.817" LastActivityDate="2015-07-23T22:35:37.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10186" PostTypeId="5" CreationDate="2015-07-23T22:01:10.930" Score="0" Body="&lt;p&gt;In combination with an oxidizer such as liquid oxygen, liquid hydrogen yields the highest specific impulse, or efficiency in relation to the amount of propellant consumed, of any known rocket propellant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although liquid hydrogen gives a high Isp, its low density is a significant disadvantage: hydrogen occupies about 7x more volume per kilogram than dense fuels such as kerosene. This not only penalises the tankage, but also the pipes and fuel pumps leading from the tank, which need to be 7x bigger and heavier. (The oxidiser side of the engine and tankage is of course unaffected.) This makes the vehicle's dry mass much higher, so the use of liquid hydrogen is not as advantageous as might be expected. Indeed, some dense hydrocarbon/LOX propellant combinations have higher performance when the dry mass penalties are included.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/topics/technology/hydrogen/hydrogen_fuel_of_choice.html&quot; rel=&quot;nofollow&quot;&gt;Liquid Hydrogen--the Fuel of Choice for Space Exploration&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_propellant&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Rocket Propellant&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrogen&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Hydrogen&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T23:49:28.570" LastActivityDate="2015-07-23T23:49:28.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10187" PostTypeId="4" CreationDate="2015-07-23T22:01:10.930" Score="0" Body="Questions about the element hydrogen, typically related to its use as rocket fuel." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T22:35:57.430" LastActivityDate="2015-07-23T22:35:57.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10188" PostTypeId="5" CreationDate="2015-07-23T22:12:45.323" Score="0" Body="&lt;p&gt;The Pioneer program is a series of United States unmanned space missions that were designed for planetary exploration. There were a number of such missions in the program, but the most notable were Pioneer 10 and Pioneer 11, which explored the outer planets and left the solar system. Pioneer 10 and Pioneer 11 carry a golden plaque, depicting a man and a woman and information about the origin and the creators of the probes, should any extraterrestrials find them someday.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_program&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T23:49:15.993" LastActivityDate="2015-07-23T23:49:15.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10189" PostTypeId="4" CreationDate="2015-07-23T22:12:45.323" Score="0" Body="The Pioneer program was a series of United States unmanned planetary exploration missions from 1958 to 1978." OwnerUserId="10132" LastEditorUserId="168" LastEditDate="2015-07-23T22:42:05.300" LastActivityDate="2015-07-23T22:42:05.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10190" PostTypeId="5" CreationDate="2015-07-23T22:25:41.647" Score="0" Body="&lt;p&gt;Retrograde motion is motion in the direction opposite to the movement of something else and the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such as precession or nutation of the axis. In reference to celestial systems, retrograde motion usually means motion which is contrary to the rotation of the primary, that is, the object which forms the system's hub.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Solar System, all of the planets and most of the other objects that orbit the Sun, with the exception of many comets, do so in the &quot;prograde&quot; direction, i.e. the same sense as the rotation of the Sun. Also the rotations of most planets are prograde, with the exceptions of Venus and Uranus, which have retrograde rotations. Most satellites of planets revolve around their planets in the prograde sense. (In the case of the satellites of Uranus, this means they revolve in the same sense as Uranus's rotation, which is retrograde relative to the Sun.) There are some satellites which orbit in the retrograde sense, but these are generally small and distant from their planets, except for Neptune's satellite Triton, which is large and close. It is thought that these retrograde satellites, including Triton, are bodies which have been captured into orbit around their planets, having been formed elsewhere.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Retrograde_and_prograde_motion&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-23T23:49:50.250" LastActivityDate="2015-07-23T23:49:50.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10191" PostTypeId="4" CreationDate="2015-07-23T22:25:41.647" Score="0" Body="An orbit with an equatorial inclination between 90° and 180°, that is, an orbit in the opposite direction of the rotation of the body being orbited." OwnerUserId="10132" LastEditorUserId="168" LastEditDate="2015-07-23T22:37:17.943" LastActivityDate="2015-07-23T22:37:17.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10192" PostTypeId="1" AcceptedAnswerId="10193" CreationDate="2015-07-24T00:34:30.003" Score="26" ViewCount="72766" Body="&lt;p&gt;&lt;a href=&quot;http://www.answers.com/Q/What_is_the_fastest_space_rocket_in_the_world&quot;&gt;Answers.com&lt;/a&gt; lists the New Horizons to have the fastest rocket. Is this info still valid? 7 light hours in 11 years at 50,000 km/h?&lt;/p&gt;&#xA;" OwnerUserId="5968" LastEditorUserId="10897" LastEditDate="2015-07-24T20:20:16.353" LastActivityDate="2017-02-23T11:31:35.207" Title="What is the fastest speed ever reached in space travel as measured from the point in space from which it was launched to its current/final position?" Tags="|spacecraft|interstellar-travel|pluto|travel|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10193" PostTypeId="2" ParentId="10192" CreationDate="2015-07-24T01:30:07.563" Score="43" Body="&lt;p&gt;Velocity relative to what? There's no central universal point to measure velocity at, so your answer is likely going to change based on your frame of reference. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons did indeed have the fasted launch velocity of any spacecraft that has left Earth (relative to Earth): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After three years of construction, and several delays at the launch site, New Horizons was launched on January 19, 2006, from Cape Canaveral, directly into an Earth-and-solar-escape trajectory with an Earth-relative speed of about 16.26 kilometers per second (58,536 km/h; 36,373 mph); it set the record for the highest launch speed of a human-made object from Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;- &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is equivalent to a C&lt;sub&gt;3&lt;/sub&gt; (Characteristic Energy) of approximately 164 km&lt;sup&gt;2&lt;/sup&gt;/s&lt;sup&gt;2&lt;/sup&gt; (&lt;a href=&quot;http://www.boulder.swri.edu/pkb/ssr/ssr-mission-design.pdf&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, Voyager 1 is leaving the solar system faster (despite launching at a slower velocity), due to the dual gravity assists it recieved from both Jupiter and Saturn. As of January 2015, it was travelling at 17.027 km/s relative to the sun (&lt;a href=&quot;http://voyager.jpl.nasa.gov/mission/weekly-reports/&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So despite leaving Earth both slower than escape velocity and New Horizons, Voyager 1 maintains the record. These graphs demonstrate this:&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;New Horizons Heliocentric Velocity&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vUdwE.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Voyager 2 (twin to Voyager 1) Heliocentric Velocity&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YrEKT.png&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, we aren't done here. The solar-studying &lt;a href=&quot;https://en.wikipedia.org/wiki/Helios_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Helios Probes&lt;/a&gt;, achieved a maximum velocity of 252,792km/h (70.22km/s) (&lt;a href=&quot;https://books.google.co.nz/books?id=Ud2icgujz0wC&amp;amp;pg=PA37&amp;amp;hl=en#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;) as they passed perihelion in their solar orbits. So do you consider this the record?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How about future missions? The title holder here is arguably &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_Probe_Plus&quot; rel=&quot;noreferrer&quot;&gt;Solar Probe Plus&lt;/a&gt;, scheduled for launch in 2018:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As the probe passes around the Sun, it will achieve a velocity of up to 200 km/s (120 mi/s) at that time making it the fastest manmade object ever, almost three times faster than the current record holder, Helios II.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The point I'm trying to make it's all relative man! I hope this answers your question.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="265" LastEditDate="2015-11-17T03:44:53.510" LastActivityDate="2015-11-17T03:44:53.510" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10194" PostTypeId="1" AcceptedAnswerId="10198" CreationDate="2015-07-24T02:00:14.100" Score="6" ViewCount="5026" Body="&lt;p&gt;Scientists recently discovered an exoplanet named Kepler-452b which they state is &quot;&lt;a href=&quot;https://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth&quot;&gt;is 6 billion years old&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did they determine this age?  How could we possibly predict the age of any exoplanet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can understand estimates of size and orbit shape but age seems like a shot in the dark, errr, space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote Jon Jenkins from NASA's Ames Research Center:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;It’s awe-inspiring to consider that this planet has spent 6 billion&#xA;  years in the habitable zone of its star; longer than Earth. That’s&#xA;  substantial opportunity for life to arise, should all the necessary&#xA;  ingredients and conditions for life exist on this planet.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="690" LastActivityDate="2015-08-14T10:46:59.413" Title="How do we determine and estimate the age of exoplanets like Kepler-452b?" Tags="|planet|exoplanet|kepler-telescope|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10195" PostTypeId="1" AcceptedAnswerId="10254" CreationDate="2015-07-24T03:41:39.353" Score="54" ViewCount="7774" Body="&lt;p&gt;I was actually answering &lt;a href=&quot;https://space.stackexchange.com/questions/10192/what-is-the-fastest-speed-ever-reached-in-space-travel/10193#10193&quot;&gt;this&lt;/a&gt; question today regarding velocities of spacecraft, and it occurred to me I've never really looked at this graph in much detail before:&lt;/p&gt;&#xA;&lt;h3&gt;Heliocentric velocity of Voyager 2 plotted against distance from the sun&lt;/h3&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qiWvW.png&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;As you can see, after leaving Earth, the craft rapidly slowed down (an example of the inverse square law), and it remained below escape velocity until its encounter with Jupiter raised its velocity by over 10 km/s. It received another ~8-9 km/s slingshot from Saturn and another 1 km/s assist from Uranus.&lt;/p&gt;&#xA;&lt;p&gt;However, the interesting event occured at Neptune: It appears to have had a gravitational slowdown of approximately ~2 km/s, as opposed to an assist. Why was this?&lt;/p&gt;&#xA;&lt;p&gt;This indicates to me that it took the opposite trajectory of this graph:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/gSun7.jpg&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Instead of approaching the planet from behind its velocity vector, it flew in from the front and out the back.&lt;/p&gt;&#xA;&lt;p&gt;I am aware that gravitational slowdowns can have many interesting uses, &lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Pluto.pdf&quot; rel=&quot;noreferrer&quot;&gt;ESA's mission proposal for a Pluto orbiter&lt;/a&gt; require a gravitational slowdown at Jupiter to reduce its velocity for Pluto orbit insertion, so I suspect this was either a result of a science or engineering constraint.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Is there any explanation for why this was done?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="7062" LastEditDate="2023-07-13T21:23:50.420" LastActivityDate="2024-01-11T19:10:43.677" Title="Why did Voyager 2 receive a gravitational slowdown (as opposed to a slingshot) at Neptune?" Tags="|voyager|gravity-assist|neptune|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="10197" PostTypeId="2" ParentId="10195" CreationDate="2015-07-24T03:53:01.700" Score="44" Body="&lt;p&gt;The mission was to fly by the outer planets.  Once it got to Neptune that mission was complete.  From &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2#Encounter_with_Neptune&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Because this was the last planet of the Solar System that Voyager 2 could visit, the Chief Project Scientist, his staff members, and the flight controllers decided to also perform a close fly-by of Triton, the larger of Neptune's two originally known moons, so as to gather as much information on Neptune and Triton as possible, regardless of Voyager 2's departure angle from the planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The probe was going into interstellar space no matter what by the time it got to Neptune (barring a collision with Neptune or a moon) and speed of getting there wasn't a concern, so they set up the pass to gain as much knowledge as they could.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-07-24T03:53:01.700" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10198" PostTypeId="2" ParentId="10194" CreationDate="2015-07-24T04:40:52.013" Score="9" Body="&lt;p&gt;From &lt;a href=&quot;http://www.universetoday.com/76495/the-hunt-for-young-exoplanets/&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The age of the exoplanet is not independently derived, but instead, taken from the age of the host star.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So how do they calculate the age of the host star?  &lt;a href=&quot;https://en.wikipedia.org/wiki/Stellar_age_estimation&quot;&gt;Wikipedia&lt;/a&gt; has a number of methods:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;As stars grow older, their luminosity increases at an appreciable rate. Given the mass of the star, one can use this rate of increase in luminosity in order to determine the age of the star.&lt;/li&gt;&#xA;&lt;li&gt;Various properties of stars can also be used to determine their age. For example, the Eta Carinae system is emitting large quantities of gas and dust. These enormous outbursts can be used to infer that the star system is nearing the end of its life, and will explode as a supernova within a relatively short period of astronomical time.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gyrochronology&quot;&gt;Gyro-chronology&lt;/a&gt; is a method used to determine the age of field stars by measuring their rotation rate, and then comparing this rate with the rotation rate of the Sun, which serves as a precalibrated clock for this measurement.&lt;/li&gt;&#xA;&lt;li&gt;Membership in a star cluster or star system permits an assignment of rough ages to a large number of stars present within. When one can determine the age of stars through other methods, such as the ones listed above, one can identify the age of all of the bodies in a system.&lt;/li&gt;&#xA;&lt;li&gt;Along with other factors, the presence of a protoplanetary disk sets a maximum limit on the age of stars. Stars with protoplanetary disks are typically young, having moved onto the main sequence only a relatively short time ago. This likely isn't applicable for your question considering you're asking about planets that would be formed from the disk.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-07-24T04:40:52.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10200" PostTypeId="5" CreationDate="2015-07-24T04:50:47.777" Score="0" Body="&lt;h1&gt;Mission Objectives&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Determine the abundance of terrestrial and larger planets in or near the habitable zone of a wide variety of stars;&lt;/li&gt;&#xA;&lt;li&gt;Determine the distribution of sizes and shapes of the orbits of these planets;&lt;/li&gt;&#xA;&lt;li&gt;Estimate how many planets there are in multiple-star systems;&lt;/li&gt;&#xA;&lt;li&gt;Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets;&lt;/li&gt;&#xA;&lt;li&gt;Identify additional members of each discovered planetary system using other techniques; and&lt;/li&gt;&#xA;&lt;li&gt;Determine the properties of those stars that harbor planetary systems. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/space-telescope&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;space-telescope&amp;#39;&quot; rel=&quot;tag&quot;&gt;space-telescope&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/telescope&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;telescope&amp;#39;&quot; rel=&quot;tag&quot;&gt;telescope&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/exoplanet&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;exoplanet&amp;#39;&quot; rel=&quot;tag&quot;&gt;exoplanet&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://kepler.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;NASA Kepler homepage&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/kepler/main/index.html&quot; rel=&quot;nofollow&quot;&gt;NASA Kepler mission pages&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-24T05:17:59.270" LastActivityDate="2015-07-24T05:17:59.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10201" PostTypeId="4" CreationDate="2015-07-24T04:50:47.777" Score="0" Body="Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-24T05:18:02.227" LastActivityDate="2015-07-24T05:18:02.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10202" PostTypeId="1" AcceptedAnswerId="10213" CreationDate="2015-07-24T05:20:09.973" Score="1" ViewCount="1318" Body="&lt;p&gt;Say you've got a mothership that is on its way to a star. Could you launch a smaller ship from it while it's in either the coasting or braking phase of the journey? A scout ship, for instance. Or would the scout ship lose momentum too quickly and turn everyone inside into borscht?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm assuming a maximum speed of about .5c and a three phase journey: acceleration, coasting, and deceleration approaching target system.&lt;/p&gt;&#xA;" OwnerUserId="11049" LastEditorUserId="11049" LastEditDate="2015-07-24T15:05:39.393" LastActivityDate="2015-07-24T15:05:39.393" Title="Could a ship be launched from a mothership while in interstellar space?" Tags="|spacecraft|interstellar-travel|physics|" AnswerCount="4" CommentCount="7" ClosedDate="2015-07-24T13:53:50.060" ContentLicense="CC BY-SA 3.0" />
  <row Id="10203" PostTypeId="2" ParentId="10202" CreationDate="2015-07-24T06:27:11.940" Score="12" Body="&lt;p&gt;In the vacuum of the interstellar space, there is nothing to lose momentum to (unless you hit something, which is unlikely), so no, a &lt;em&gt;scout ship&lt;/em&gt; wouldn't lose momentum too quickly and turn everyone inside into borscht. It would &lt;em&gt;cruise&lt;/em&gt; on &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertia&quot; rel=&quot;nofollow noreferrer&quot;&gt;inertia&lt;/a&gt; just like its mothership, and move alongside it, unless some external force acts on it differently than the mothership (e.g. entry into an atmosphere, with it providing drag on the scout ship), or the scout ship exchanges momentum with some other matter (e.g. reaction mass of own propulsion system, gravity assist, magnetic or solar sail, and so on), or the mothership itself (some sort of repulsions, like magnetism, photon pressure,...).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is actually a big problem for proposed methods of interstellar travel, with perhaps most feasible one that is also capable of traversing such immense distances in reasonable time being &lt;a href=&quot;https://en.wikipedia.org/wiki/Beam-powered_propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;beam-powered propulsion&lt;/a&gt;, where the source of momentum stays at the departure point and the ship is propelled on the power of deflected photons ($\mathrm{F} = 2\times \mathrm{P} / c$, in Newtons, for 100% sail reflectance, where $\mathrm{P}$ is total reflected power from the source in Watts, and $c$ the speed of light. Divide that with craft's mass in kilograms and you have acceleration that it would get from it. E.g. a 100 kg sail reflecting 50 GW at 100% reflectance, would accelerate at 3.36 m/s², so about 1/3 the gravitational acceleration on the surface of the Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That comes with its own problems, of course, and I won't go into too much detail about it here, but most such proposed designs also call for a &lt;a href=&quot;https://space.stackexchange.com/a/9120/49&quot;&gt;decelerating sail&lt;/a&gt; detaching from the mothership and acting as a reflector to reverse direction of the beam and decelerate the mothership in the opposite vector relative to its motion (sometimes called a &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Brachistochrone_curve&quot; rel=&quot;nofollow noreferrer&quot;&gt;brachistochrone&lt;/a&gt; turn&lt;/em&gt;, since it would be a constant acceleration system, but it is a bit of a mouthful and, in earnest, I've still not mastered pronunciation of this term). Of course, the deceleration sail is still being propelled in the direction of the beam while it reflects it, so the distance between the mothership and the deceleration stage would constantly increase. And this is where exchanged momentum goes to decelerate the mothership in a system like that. It's just an example though, but I've picked it because it's directly applicable to your question and perhaps not as intuitive how momentum is exchanged in it, while also demonstrating the need for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Point is, that all &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's laws of motion&lt;/a&gt; of course apply also in the interstellar space:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;First law&lt;/strong&gt;: When viewed in an inertial reference frame, an object&#xA;  either remains at rest or continues to move at a constant velocity,&#xA;  unless acted upon by an external force.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Second law&lt;/strong&gt;: The vector sum of the external forces $\mathrm{F}$ on an object is&#xA;  equal to the mass $m$ of that object multiplied by the acceleration&#xA;  vector $\mathrm{a}$ of the object: $\mathrm{F} = m\mathrm{a}$.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Third law&lt;/strong&gt;: When one body exerts a force on a second body, the second&#xA;  body simultaneously exerts a force equal in magnitude and opposite in&#xA;  direction on the first body.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Consequence of that is that a smaller mass body like a scout ship detached from the mothership will require less force to decelerate, or conversely it would decelerate faster at same force applied in the vector opposite to its direction of movement than a more massive body, but unless it's acted upon by some external force, this won't happen and will just cruise alongside its mothership until something changes for it with respect to the mothership.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They would likely already have different mass, so you could then apply same source of decelerating force to each differently (e.g. with surface per mass for passive sails, burn rate per mass for own propulsion,...) and they will start moving away from each other. But the force of this decelerating force acting on its inhabitants would likely be small relative to its net momentum, so acceleration felt by its inhabitants would also be small. Again, unless you hit a solid body at largely different relative velocity. Then, they would &lt;em&gt;turn into borscht&lt;/em&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kinetic_energy&quot; rel=&quot;nofollow noreferrer&quot;&gt;energy released&lt;/a&gt; upon impact equals $E_\text{k} =\tfrac{1}{2} mv^2$, so as you can see, relative velocity matters a whole lot more than the mass of the object, tho it's still relevant, too. But that's a different matter (no pun intended).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-07-24T06:32:51.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10204" PostTypeId="2" ParentId="10202" CreationDate="2015-07-24T06:29:00.823" Score="8" Body="&lt;p&gt;First off, space is mostly empty, and simply dropping a probe will only lead to the mothership and the probe flying in formation (during coasting phase). During braking phase the probe will fly forward happily without mechanical complications (just remember to launch it from the side of the ship and maintain lateral separation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, there are possible physical complications and they do actually depend on the speed of the mothership. A mothership going at an appreciably large speed relative to $c$, the speed of light, hits interstellar matter which gives rise to nasty &lt;a href=&quot;https://en.wikipedia.org/wiki/Bremstrahlung&quot;&gt;Bremsstrahlung&lt;/a&gt; radiation. Protecting manned scout ships is costly in terms of mass. Of course, you don't want to launch a scout ship if it cannot decelerate to achieve orbit within the target solar system or around one of the planets. It may have to use either aerobraking in a planetary atmosphere or solar braking (if it is light enough) around the target star, in addition to whatever propellant it has on its own.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here's the rub: aerobraking needs exact knowledge of the atmosphere's density and chemical properties profile otherwise the probe will either skip out and fail to decelerate to orbital velocity, never to return, or burn if the probe dips too low. The ability to aerobrake is furthermore limited by thermal shielding, maximum allowed internal temperatures, structural strength, and heat absorption/rejection capacity. One may envision aerobraking from 13 km/s in the near future, and perhaps a bit more later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, if you are really going there in a huge mothership, it makes sense to decelerate below the star's escape velocity, or even get into orbit around one of the planets and only then launch probes - first unmanned to scout particular planets, then a shuttle (most likely multistaged) to get exploration crew on the ground for some photo sessions and flag planting stuff.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2015-07-24T06:29:00.823" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10205" PostTypeId="2" ParentId="1484" CreationDate="2015-07-24T08:20:03.163" Score="-1" Body="&lt;p&gt;It's July 2015. NASA Claims that they have discovered Earth 2.0 , Approx 1400 Light years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler-452b&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Kepler-452b&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11058" LastActivityDate="2015-07-24T08:20:03.163" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10206" PostTypeId="2" ParentId="806" CreationDate="2015-07-24T09:13:13.387" Score="5" Body="&lt;p&gt;There is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_medicine#Effects_of_space-travel&quot;&gt;very long list&lt;/a&gt; of symptoms experienced by astronauts, some of which are related to the microgravity environment and others that are caused by something else. Determining exactly which causes what isn't always straightforward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In microgravity, blood pools more in the head and upper body than it does on Earth, where gravity pulls it down to your feet. Similarly, the internal organs move upwards towards (or into) the chest cavity. Lying down also removes this gravitational pull towards your feet, so you'd expect the same thing to happen .. except...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lying down isn't the same as being in microgravity. Imagine a pea in a glass tube. In microgravity, it will float, and a gentle breeze through the tube will cause it to move along the tube. If you lie that tube down on its side here on Earth, however, friction with the tube walls would prevent it from moving.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, lying down IS a good analogy for being in microgravity for a lot of conditions, even if it isn't perfect. In 2014, NASA conducted a 70-day bed-rest study, called &lt;a href=&quot;https://bedreststudy.jsc.nasa.gov/cft.aspx&quot;&gt;CFT70&lt;/a&gt;, to &quot;test the effectiveness of exercise on loss of muscle, bone and cardiovascular function&quot;. As @PearsonArtPhoto has already commented, you can find a first-person write-up of that experiment &lt;a href=&quot;http://www.vice.com/read/nasa-patient-8179-100&quot;&gt;over at Vice&lt;/a&gt;. This wasn't the first such experiment, and it probably won't be the last, either. Some of the results from such studies might surprise you, such as &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130000800.pdf&quot;&gt;the effects of lying down on your senses of taste and smell&lt;/a&gt; or even &lt;a href=&quot;http://www.ajnr.org/content/early/2015/07/16/ajnr.A4406.abstract&quot;&gt;structural changes in the brain&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="10635" LastActivityDate="2015-07-24T09:13:13.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10207" PostTypeId="2" ParentId="10202" CreationDate="2015-07-24T10:05:31.080" Score="4" Body="&lt;p&gt;Other answers adequately explain the problems with a manned scout ship, but it actually makes loads of sense to launch a small &lt;strong&gt;unmanned&lt;/strong&gt; probe before the deceleration phase. It will speed through the system at interstellar velocities, but can collect various kinds of data on the system and transmit it back to the mothership, allowing last minute course corrections and so on, or even just giving the opportunity to explore some of the non-target bodies in the system, in terms of delta-v it's going to be much cheaper to launch a (fly-by) probe while still in interstellar space - you already have lots of velocity for getting places quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the chance of a high speed probe being obliterated by debris is non-negligible, you would probably actually launch a swarm of small probes - they don't need to be very big, just various instruments a transmitter and limited attitude control.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-07-24T10:05:31.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10208" PostTypeId="1" CreationDate="2015-07-24T10:29:04.997" Score="7" ViewCount="734" Body="&lt;p&gt;There are some concerns about Lunar dust attaching to the space suits during the Apollo missions. So &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130013652.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Suitports&lt;/a&gt; are being developed. On Earth, cars get pretty dirty too, but a car wash solves that. Would a similar washing method be feasible in vacuum or in thin Martian atmosphere, using some mixture of cleaning fluids or gasses? Or if I may add, cleaning by rubbing some powder or fluff nap to it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uK5CA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uK5CA.jpg&quot; alt=&quot;At least this dirty boy (Cernan?) cleaned the flag on his shoulder before this photo opportunity after having played in the big sand box&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="7062" LastEditDate="2021-09-13T12:05:16.017" LastActivityDate="2021-09-13T12:05:16.017" Title="Could you &quot;hose wash&quot; a space suit on the Moon or Mars?" Tags="|crewed-spaceflight|spacesuits|vacuum|dust|moonwalking|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="10211" PostTypeId="2" ParentId="10208" CreationDate="2015-07-24T10:59:30.560" Score="6" Body="&lt;p&gt;You could, though gaseous nitrogen and a pressure hose are usually used for such purging of contaminants. Such purging systems were also &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-eclss-airlock.html#sts_eclss-rtg&quot; rel=&quot;nofollow noreferrer&quot;&gt;installed on Discovery and Atlantis Space Shuttle Orbiters&lt;/a&gt; for the purpose of purging payloads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem with water is that it's heavier for the task, it tends to freeze in and around the nozzle without constant source of heat and then block it, and, as a liquid, it also doesn't have the best cleaning properties due to its relatively low viscosity yet high surface tension, so it tends to clump and &lt;em&gt;miss spots&lt;/em&gt;, so to say. It also gets things wet which can then freeze, or freezes in hard to reach places, expands as it does that and can cause damage due to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/q/1897/49&quot;&gt;Why was the Hubble Space Telescope purged with nitrogen gas?&lt;/a&gt; for description why nitrogen is a better for purging, and since you mention Apollo, also check &lt;a href=&quot;https://space.stackexchange.com/q/1483/49&quot;&gt;How was dust-mitigation addressed during the Apollo program?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-07-24T14:53:57.197" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10213" PostTypeId="2" ParentId="10202" CreationDate="2015-07-24T11:18:45.483" Score="-2" Body="&lt;p&gt;I registered here to help the OP because these long answers don't seem to answer a major &lt;strong&gt;subtlety&lt;/strong&gt; of the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As said you can leave in the &lt;strong&gt;coasting phase&lt;/strong&gt; fine - &lt;strong&gt;inertia is the same&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in the &lt;strong&gt;braking phase&lt;/strong&gt; - no, you could not assume it is possible to leave the mothership in a scout ship if the mothership was accelerating or decelerating at the time, it would immediately have a different relative velocity, because the scout ship's inertia would only be the same as that of the mothership at the moment of departure, and immediately the mothership acceleration or deceleration would become apparent, as soon as launch occurred. Whatever the rate acceleration or deceleration was at the time of it's leaving the mothership would appear to accelerate at that speed away from the scout ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However if the rate of acceleration or deceleration wasn't large it may be possible to leave and return literally &quot;catching up&quot; the mothership i.e. the only way for the scout ship to counter this is to apply the same (or greater) acceleration or deceleration to the scout ship which would take fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally if the mothership was sufficiently massive it could drag the scout ship in it's gravitational &quot;wake&quot; to some degree mitigating the relative velocity issues.&lt;/p&gt;&#xA;" OwnerUserId="11061" LastEditorUserId="168" LastEditDate="2015-07-24T11:51:48.943" LastActivityDate="2015-07-24T11:51:48.943" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10216" PostTypeId="2" ParentId="9152" CreationDate="2015-07-24T14:42:55.193" Score="2" Body="&lt;p&gt;Short answer: Atmospheric Drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer: A spacecraft which is in the high atmosphere can start to spin, and the speed of that spin can increase. It is essentially like a windmill. This really only affects spacecraft in the realm of 200-300 km or so, that aren't build to withstand such changes, and thus aren't in full control. It is entirely possible that this was purely a result of atmospheric drag.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-24T14:42:55.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10217" PostTypeId="2" ParentId="8326" CreationDate="2015-07-24T14:48:48.717" Score="3" Body="&lt;p&gt;There is really only one method to obtain the location of the spacecraft, and this is via carefully tracking the spacecraft from the ground. Usually they determine the range by a process called, well, ranging, basically they send a ping to the spacecraft, which responds as soon as it can. The time it takes for the probe to respond determines it's exact range. If you have its range for a few time samples, and you know the gravitational constants of everything, you can determine what trajectory the spacecraft is on. This method is very good, and works well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other methods are usually only used if the location of the target body isn't very well known. These can include images, radar, and similar systems from the spacecraft to determine where the object is. This is because knowing the exact position of some objects can be difficult, and more accurate measurements can be made from close. However, this will only tell you where the object of interest is relative to the spacecraft. The spacecraft is determined by the ranging operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very close objects can also be found via a ground based radar system.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-24T14:48:48.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10218" PostTypeId="2" ParentId="5872" CreationDate="2015-07-24T14:57:36.320" Score="6" Body="&lt;p&gt;The European Space Agency (ESA) is subject to the &lt;em&gt;Regulations of the European Space Agency&lt;/em&gt;. One of these documents is ESA/REG/008 - &lt;em&gt;Rules on Information, Data and Intellectual Property&lt;/em&gt;, the goal of which is stated as follows:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Rules on Information, Data and Intellectual Property, as adopted by Council on 19 December 2001 under the reference ESA/C/CLV/Rules 5 (Final), are designed to promote access to information and data and to cover the use of Intellectual Property resulting from Agency's activities, while fully taking into account the provisions of the Convention and the interests of the Member States and of persons and bodies under their jurisdiction.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Chapter II, Section II, subpoint 1a states:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Access and use to be reserved by the Agency&lt;/p&gt;&#xA;&lt;p&gt;Information, Data and Intellectual Property resulting from work performed under an Agency contract shall be available to the Agency and Participating States, including Persons and Bodies under their Jurisdiction, free of charge, for the Agency’s Own Requirements in the field of space research and technology and their space applications.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The 24 page document goes on to further detail the exact regulation of release of information, so I would highly recommend reading it if you wish to know more specific rules.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Source&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://esamultimedia.esa.int/docs/LEX-L/Contracts/ESA-REG-008-EN.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA/REG/008 - &lt;em&gt;Rules on Information, Data and Intellectual Property&lt;/em&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-24T15:06:38.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10219" PostTypeId="2" ParentId="10022" CreationDate="2015-07-24T15:26:21.613" Score="65" Body="&lt;p&gt;So far as I can tell, it was first a saying used by military personnel as the phrase &quot;&lt;a href=&quot;https://books.google.com/books?id=86zpYQO5yYcC&amp;amp;q=Rapid%20unintentional%20Disassembly&amp;amp;dq=Rapid%20unintentional%20Disassembly&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CC0Q6AEwAjgKahUKEwisiKjUh_TGAhWIGx4KHYQCC-o&quot;&gt;Rapid Unintentional Disassembly&lt;/a&gt;&quot;, for a phrase when a gun broke apart if you misused it. This was used by a book for Navy Personel in 1970, so I suspect it was in use for a while before then. This seems to have evolved from that phrase somewhat over the years. The earliest I can find it in rocketry specifically was the book entitled &quot;&lt;a href=&quot;https://books.google.com/books?id=Weg1WPmzupkC&amp;amp;pg=PA4&amp;amp;dq=Rapid%20Disassembly%20rocket&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CCIQ6AEwAWoVChMIm-b8nIb0xgIVyZoeCh2-mAJi&quot;&gt;Rocket Religion&lt;/a&gt;&quot;, copyright 2002, again with the same phrasing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to have exploded in popularity around 2011. I can't tell the exact source, but it was either Elon Musk or Kerbal Space Program, most likely.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-06-15T15:23:39.487" LastActivityDate="2016-06-15T15:23:39.487" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10222" PostTypeId="1" AcceptedAnswerId="10305" CreationDate="2015-07-24T16:54:25.540" Score="3" ViewCount="1163" Body="&lt;p&gt;I noticed from &lt;a href=&quot;https://space.stackexchange.com/a/1488/25&quot;&gt;this answer&lt;/a&gt; the phrase:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;by sweeping all the dust we could find&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How did they sweep dust in the LM? And where did they sweep it to?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-08-26T18:42:33.737" Title="How did they sweep dust on the moon from inside the Apollo Lunar Lander?" Tags="|the-moon|apollo-program|dust|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10223" PostTypeId="1" AcceptedAnswerId="10229" CreationDate="2015-07-24T17:10:34.163" Score="24" ViewCount="2192" Body="&lt;p&gt;I have heard many times that the designers of the Voyager spacecraft neglected to account for the angular momentum of the tape recorders (on which data was stored).  To compensate for it after launch, they were forced to use external thrusters every time they wanted to spin up or stop the tapes.  I have been able to find a number of sources which re-tell this story (such as &lt;a href=&quot;http://www.education.com/science-fair/article/caused-voyager-point-wrong-direction/&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://www.tutorvista.com/content/physics/physics-iii/rigid-body/angular-momentum-laws.php&quot;&gt;here&lt;/a&gt;) but none which are very credible.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much truth is there to the story?  Are any more details known?  If true, how did mission control compensate for the effect?&lt;/p&gt;&#xA;" OwnerUserId="2699" LastEditorUserId="49" LastEditDate="2015-07-24T18:56:11.277" LastActivityDate="2015-07-26T18:04:11.813" Title="Did the designers of Voyager neglect the angular momentum of the tape recorders?" Tags="|nasa|voyager|attitude|myth-busting|failure|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10224" PostTypeId="1" CreationDate="2015-07-24T17:15:44.080" Score="1" ViewCount="1485" Body="&lt;p&gt;If they found the those planets in the habitable zone, are they planning ahead for human settlement of them?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jvU1f.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CgRay.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kh5sY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11075" LastEditorUserId="49" LastEditDate="2015-07-24T19:28:13.043" LastActivityDate="2020-06-14T11:32:17.940" Title="Are exoplanets in the habitable zone suitable for human colonization?" Tags="|colonization|interstellar-travel|humans|habitat|exoplanet|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10225" PostTypeId="2" ParentId="352" CreationDate="2015-07-24T18:28:49.847" Score="20" Body="&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/ytxFQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not currently possible to get the details of a planet from a distance like a light year or more. Furthermore the projects mentioned below do not aim to get good images of the surface, but only to &lt;em&gt;detect&lt;/em&gt; exoplanets, and do basic measurements. The reason is that getting &lt;em&gt;detailed&lt;/em&gt; pictures of the surface is beyond current technology capability and research.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble, the space telescope, has a better performance than any equivalent on the ground, due to the absence of atmospheric perturbations. An interferometer in space would benefit as well from this air-free environment. This led to several concepts:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8Iy4x.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;Source: &lt;a href=&quot;http://www.sciencepresse.qc.ca/origine/planetes5.html&quot; rel=&quot;noreferrer&quot;&gt;Agence Science-Presse&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Darwin_overview&quot; rel=&quot;noreferrer&quot;&gt;Darwin&lt;/a&gt; canceled in 2007&lt;/li&gt;&#xA;&lt;li&gt;Space Interferometry Mission (&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Interferometry_Mission&quot; rel=&quot;noreferrer&quot;&gt;SIM&lt;/a&gt;), canceled in 2010.&lt;/li&gt;&#xA;&lt;li&gt;Terrestrial Planet Finder (&lt;a href=&quot;https://en.wikipedia.org/wiki/Terrestrial_Planet_Finder&quot; rel=&quot;noreferrer&quot;&gt;TPF&lt;/a&gt;), canceled in 2011.&lt;/li&gt;&#xA;&lt;li&gt;Labeyrie's &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_interferometer#Labeyrie.27s_hypertelescope&quot; rel=&quot;noreferrer&quot;&gt;Hypertelescope&lt;/a&gt;, not funded.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Darwin_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Darwin article&lt;/a&gt; on Wikipedia summarizes the technological difficulty:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To produce an image, the telescopes would have&#xA;  had to operate in formation with distances between the telescopes&#xA;  controlled to within a few micrometres, and the distance between the&#xA;  telescopes and receiver controlled to within about one nanometre.&#xA;  Several more detailed studies would have been needed to determine&#xA;  whether technology capable of such precision is actually feasible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Objects with a small apparent size are better observed using astronomical interferometry, but the current technology allows to get only a rough image of a few large and ultra-bright objects.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Example of resolved object: &lt;a href=&quot;https://en.wikipedia.org/wiki/Epsilon_Aurigae&quot; rel=&quot;noreferrer&quot;&gt;ε Aurigae&lt;/a&gt;, a supergiant star with a strange orbiting dark disk. Instrument: &lt;a href=&quot;http://wii09.obs.ujf-grenoble.fr/TALKS/May28-1400-Monnier-2009-Goutelas.pdf&quot; rel=&quot;noreferrer&quot;&gt;MIRC&lt;/a&gt; interferometer on the &lt;a href=&quot;http://www.chara.gsu.edu/&quot; rel=&quot;noreferrer&quot;&gt;CHARA&lt;/a&gt; array (&lt;a href=&quot;https://en.wikipedia.org/wiki/Mount_Wilson_Observatory&quot; rel=&quot;noreferrer&quot;&gt;Mount Wilson&lt;/a&gt;):&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/YQ1rl.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YQ1rl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;(source: &lt;a href=&quot;http://nsf.gov/news/mmg/mmg_disp.jsp?med_id=66617&quot; rel=&quot;noreferrer&quot;&gt;NSF&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Earth-like planet at a distance of one light year has an apparent size similar to ε Aurigae, but the faintness of exoplanets currently prevent to see details on their surface: Increasing the exposure allows to overcome the low light conditions but blurs the image due to the apparent motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The alternative to send probes and take photos is not currently possible either, Voyager 1 and 2, launched in 1977 are just at the border of our own Solar system, 10,000 th of the distance to travel to nearest exoplanet.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bh8QU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the thousand of exoplanets &lt;a href=&quot;https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets&quot; rel=&quot;noreferrer&quot;&gt;already discovered&lt;/a&gt; have been detected using indirect methods, like the &lt;a href=&quot;http://astrobites.org/2011/02/03/dip-detection-in-the-kepler-data/&quot; rel=&quot;noreferrer&quot;&gt;brightness dip&lt;/a&gt; of the central star during the transit of the orbiting planet. The question refers to an exceptional case, a &lt;a href=&quot;https://en.wikipedia.org/wiki/HD_95086_b&quot; rel=&quot;noreferrer&quot;&gt;direct observation&lt;/a&gt; of a massive planet in the IR spectrum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two determining elements when observing an object:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The object's apparent size, or &lt;em&gt;angular size&lt;/em&gt;. &lt;/li&gt;&#xA;&lt;li&gt;The object's apparent brightness&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Apparent size&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this image, the three objects have the same angular size, and will be seen similarly:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/1Q53O.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this formula:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;θ = 2 • arctan (½ • d / D)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the angular size of a planet with the diameter d of the Earth, at a&#xA;  distance D of 1 l.y., is &lt;strong&gt;0.3 milliarcsecond&lt;/strong&gt; (mas)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To see this planet as one pixel, the worst possible level of detail, the telescope needs to resolve 0.3 mas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Angular resolution using a single telescope&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://cnx.org/contents/f403618a-c8e1-4b20-8737-6164df697aea@5/Limits-of-Resolution:-The-Rayl&quot; rel=&quot;noreferrer&quot;&gt;Rayleigh's limit&lt;/a&gt;, the angular size θ a telescope with a diameter d mirror can resolve at λ wavelength is:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;θ° = 70 * (λ / d)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To resolve 0.3 mas in the middle of the visible spectrum, the&#xA;  telescope mirror must have a diameter of 500 m.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The result would be like this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;img src=&quot;https://i.stack.imgur.com/ybbmZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pale_Blue_Dot&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;. The &lt;em&gt;pale blue dot&lt;/em&gt; on this image is actually the Earth seen from Voyager 1, &quot;only&quot; 5 light-hours away, with an imager associated to a &lt;a href=&quot;http://pds-rings.seti.org/voyager/iss/inst_cat_na1.html#inst_opt&quot; rel=&quot;noreferrer&quot;&gt;18 cm diameter mirror&lt;/a&gt;. But the result would be the same with a 500 m telescope located at a distance of 1 l.y.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If the telescope had a diameter of 2 km, then the number of pixels for the planet would still only be 4x4. This means scientists are far from being able to build a telescope to show the details of a planet at a few light years. Also this one light year distance is purely for discussion, since the &lt;a href=&quot;https://en.wikipedia.org/wiki/Proxima_Centauri&quot; rel=&quot;noreferrer&quot;&gt;closest star&lt;/a&gt; is already distant of 4.2 l.y.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Angular resolution using synthesis aperture and interferometry&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If two 1 m diameter instruments are moved away by 10 m, and their images are combined so that they can interfere, the resulting resolution power will be the one of a 10 m instrument. The distance between the instruments is named the &lt;em&gt;baseline&lt;/em&gt;. As regard to resolution power, the system behaves like a single instrument the size of the baseline.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_interferometer&quot; rel=&quot;noreferrer&quot;&gt;first interferometer&lt;/a&gt; was used for astronomical purposes in 1920.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interferences are created by phase difference between the images, and the precision required for the baseline value is a fraction of a wavelength. Long baselines are easier to build for radio-telescopes than for optical telescopes. Optical interferometry was not actually effective since recently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare the size of &lt;a href=&quot;https://en.wikipedia.org/wiki/Karl_G._Jansky_Very_Large_Array&quot; rel=&quot;noreferrer&quot;&gt;VLA&lt;/a&gt; (radio-telescope) and &lt;a href=&quot;http://www.eso.org/paranal/telescopes/vlti/&quot; rel=&quot;noreferrer&quot;&gt;VLTI&lt;/a&gt; (optical telescope):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/VrTpW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/4D67X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On one the best resolution in optical astronomy is obtained with the &lt;a href=&quot;http://wii09.obs.ujf-grenoble.fr/TALKS/May28-1400-Monnier-2009-Goutelas.pdf&quot; rel=&quot;noreferrer&quot;&gt;MIRC&lt;/a&gt; interferometer on the &lt;a href=&quot;http://www.chara.gsu.edu/&quot; rel=&quot;noreferrer&quot;&gt;CHARA&lt;/a&gt; array at &lt;a href=&quot;https://en.wikipedia.org/wiki/Mount_Wilson_Observatory&quot; rel=&quot;noreferrer&quot;&gt;Mount Wilson Observatory&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See image of ε Aurigae in the &lt;em&gt;short answer&lt;/em&gt; section, and more on &lt;a href=&quot;http://spiff.rit.edu/classes/phys301/lectures/size/size.html&quot; rel=&quot;noreferrer&quot;&gt;astronomical interferometry&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Interferometry in space&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble, the space telescope, has a better performance than any equivalent on the ground, due to the absence of atmospheric perturbations. An interferometer in space would benefit as well from this air-free environment. ESA studied &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Darwin_overview&quot; rel=&quot;noreferrer&quot;&gt;Darwin&lt;/a&gt; project in the perspective of exoplanet search:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/8Iy4x.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;Source: &lt;a href=&quot;http://www.sciencepresse.qc.ca/origine/planetes5.html&quot; rel=&quot;noreferrer&quot;&gt;Agence Science-Presse&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the project has been stopped in 2007. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Darwin_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To produce an image, the telescopes would have&#xA;  had to operate in formation with distances between the telescopes&#xA;  controlled to within a few micrometres, and the distance between the&#xA;  telescopes and receiver controlled to within about one nanometre.&#xA;  Several more detailed studies would have been needed to determine&#xA;  whether technology capable of such precision is actually feasible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Similar projects:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Terrestrial Planet Finder (&lt;a href=&quot;https://en.wikipedia.org/wiki/Terrestrial_Planet_Finder&quot; rel=&quot;noreferrer&quot;&gt;TPF&lt;/a&gt;), canceled in 2011.&lt;/li&gt;&#xA;&lt;li&gt;Space Interferometry Mission (&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Interferometry_Mission&quot; rel=&quot;noreferrer&quot;&gt;SIM&lt;/a&gt;), canceled in 2010.&lt;/li&gt;&#xA;&lt;li&gt;Labeyrie's &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_interferometer#Labeyrie.27s_hypertelescope&quot; rel=&quot;noreferrer&quot;&gt;hypertelescope&lt;/a&gt;, not funded.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Apparent brightness&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A planet does not create light, it only reflects the light from its sun, at some degree.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The quantity of light reflected by the planet is proportional to the luminosity of its sun, its albedo (reflectivity) and its radius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/L2hGM.png&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/qIJuQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.as.utexas.edu/astronomy/education/sum11/endl/secure/AST_s309_ss11_9.pdf&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As visible on the pictures above, the orbit inclination and the phase also determine the quantity of light reflected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually the brightness of an exoplanet is only thousandths of its sun, and well below the level of sensibility of the best sensors. Only very long exposure times can detect the faint light beam after accumulation, but the details are blurred due to the relative motion of the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only the brightest stars send enough photons to have some details visible. Details of an exoplanet with the same angular size cannot be seen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the resolution power is improvement by interferometry techniques, this improvement doesn't apply to the quantity of photons collected. The actual aperture of the individual telescopes is the only one which determine the quantity of light collected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difficulty for direct imaging of exoplanets also includes the high contrast between the star and the planet. To improve detection, some telescopes use a &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronagraph&quot; rel=&quot;noreferrer&quot;&gt;coronagraph&lt;/a&gt; which hides the star to the imager.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="7017" LastEditDate="2015-07-26T13:50:15.530" LastActivityDate="2015-07-26T13:50:15.530" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="10226" PostTypeId="2" ParentId="10224" CreationDate="2015-07-24T18:44:18.943" Score="6" Body="&lt;p&gt;There's a few different definitions of &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Circumstellar_habitable_zone&quot; rel=&quot;noreferrer&quot;&gt;circumstellar habitable zone&lt;/a&gt;&lt;/em&gt; (CHZ), and it goes by a few names, but in a nutshell, no, planets orbiting within this zone are not necessarily habitable for humans (Mars isn't, and it's within the CHZ), thus no &lt;em&gt;&quot;planning&quot;&lt;/em&gt; to even visit, let alone colonize any specific ones yet either;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What we mean with it is that these planets orbit within the &lt;em&gt;Goldilocks zone&lt;/em&gt; (another term for it, referring to the Goldilocks' &lt;em&gt;&quot;just right&quot;&lt;/em&gt; from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Goldilocks_and_the_Three_Bears&quot; rel=&quot;noreferrer&quot;&gt;Goldilocks and the Three Bears&lt;/a&gt;), so the zone that permits liquid surface water and is &lt;em&gt;&quot;just the right distance&quot;&lt;/em&gt; from the star the planet orbits that it &lt;em&gt;seems it could be&lt;/em&gt; far enough from it that its surface isn't too hot, and close enough that its surface isn't too cold. Additionally, &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets&quot; rel=&quot;noreferrer&quot;&gt;potentially habitable exoplanets&lt;/a&gt;&lt;/em&gt; within CHZ are also suspected to be rocky bodies and not too massive (it's easier to establish their mass, even their radius than their composition), so their surface gravity would be &lt;em&gt;Earth-like&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, more often than not, we don't know nearly enough about them to say if they even have any water at all, or a breathable atmosphere, or any other conditions for life as we know it. We don't even know for sure their surface temperature, since we'd require direct imaging to establish that to any precision, and the bodies in question could be geothermally active, have an atmosphere in a runaway greenhouse effect like Venus does, which is just barely outside of most definitions of CHZ, or other surface activity completely changing reality from our estimates based on limited data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So definitions for circumstellar habitable zone, whichever you apply, also aren't the best indication of exoplanet's habitability. For example, on top of not guaranteeing any water, breathable atmosphere, even suitable temperature, and so on, it also rules out as &lt;em&gt;potentially habitable&lt;/em&gt; other bodies where conditions for life might be suitable below the planet's surface (there's a few candidates in our own Solar system, like Europa, Ganymede, Enceladus, even Ceres, all outside of CHZ), planet's possible moons that might be habitable even if the planet itself isn't (same examples as before, except Ceres they're all moons to rather inhospitable planets), or have habitable regions, e.g. at its poles, but the most of the planet otherwise isn't suitable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, so being within the CHZ isn't the best indication of potential habitability on its own, so we had to create additional habitability indexes. And, like you might have suspected by now, there's really &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets#Key&quot; rel=&quot;noreferrer&quot;&gt;many of those&lt;/a&gt; too:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Earth Similarity Index (ESI)&lt;/strong&gt;—Similarity to Earth on a scale from 0 to 1, with 1 being the most Earth-like. ESI depends on the&#xA;  planet's radius, density, escape velocity, and surface temperature.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Standard Primary Habitability (SPH)&lt;/strong&gt;—Suitability for vegetation on a scale from 0 to 1, with 1 being best-suited for growth. SPH&#xA;  depends on surface temperature (and relative humidity if known).&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Habitable Zone Distance (HZD)&lt;/strong&gt;—Distance from the center of the star's habitable zone, scaled so that −1 represents the inner edge of&#xA;  the zone, and +1 represents the outer edge. HZD depends on the star's&#xA;  luminosity and temperature and the size of the planet's orbit. Note&#xA;  that even though many planets have an HZD value similar to Venus&#xA;  (−0.93), including Kepler-438b, the HZD is not used to rule on whether&#xA;  a planet has suffered a runaway greenhouse effect or not, and&#xA;  therefore, Kepler-438b is currently assumed to be a mesoplanet rather&#xA;  than a hyperthermoplanet.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Habitable Zone Composition (HZC)&lt;/strong&gt;—Measure of bulk composition, where values close to zero are likely iron–rock–water mixtures. Values&#xA;  below −1 represent bodies likely composed mainly of iron, and values&#xA;  greater than +1 represent bodies likely composed mainly of gas. HZC&#xA;  depends on the planet's mass and radius.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Habitable Zone Atmosphere (HZA)&lt;/strong&gt;—Potential for the planet to hold a habitable atmosphere, where values below −1 represent bodies&#xA;  likely with little or no atmosphere, and values above +1 represent&#xA;  bodies likely with thick hydrogen atmospheres (e.g. gas giants).&#xA;  Values between −1 and +1 are more likely to have atmospheres suitable&#xA;  for life, though zero is not necessarily ideal. HZA depends on the&#xA;  planet's mass, radius, orbit size, and the star's luminosity.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Planetary Class (pClass)&lt;/strong&gt;—Classifies objects based on thermal zone (hot, warm, or cold, where warm is in the habitable zone) and mass&#xA;  (asteroidan, mercurian, subterran, terran, superterran, neptunian, and&#xA;  jovian).&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Habitable Class (hClass)&lt;/strong&gt;—Classifies habitable planets based on temperature: hypopsychroplanets (hP) = very cold (&amp;lt; −50 °C);&#xA;  psychroplanets (P) = cold; mesoplanets (M) = medium-temperature (0–50&#xA;  °C; not to be confused with the other definition of mesoplanets);&#xA;  thermoplanets (T) = hot; hyperthermoplanets (hT) = very hot (&gt; 100&#xA;  °C). Mesoplanets would be ideal for complex life, whereas class hP or&#xA;  hT would only support extremophilic life. Non-habitable planets are&#xA;  simply given the class NH.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, it appears we're really that complicated beings, requiring that complicated set of conditions to thrive. And there's more, if you look at the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets#Table&quot; rel=&quot;noreferrer&quot;&gt;list of potentially habitable exoplanets&lt;/a&gt;, you'll be hard-pressed to find any for which you could say with any degree of certainty that they fit our requirements. Uncertainty of data we work with is simply too great to depend with your life on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, nobody would be planning human settlement of any specific exoplanets that we lack crucial data on, despite that we might refer to them as &lt;em&gt;potentially habitable&lt;/em&gt;. It's simply not good enough to send a &lt;a href=&quot;https://en.wikipedia.org/wiki/100_Year_Starship&quot; rel=&quot;noreferrer&quot;&gt;hundred year starship&lt;/a&gt; or anything similar towards it, then later realize there's nothing to colonize.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest one we currently know of, &lt;a href=&quot;https://en.wikipedia.org/wiki/Tau_Ceti_e&quot; rel=&quot;noreferrer&quot;&gt;Tau Ceti e&lt;/a&gt;, is 11.9 light years (ly) away. To put that in perspective, that's 5,701 times the distance traversed by Voyager 1 (132 AU) in nearly 38 years, and it's not even close to being out of the Solar system's &lt;a href=&quot;https://en.wikipedia.org/wiki/Sphere_of_influence_%28astrodynamics%29&quot; rel=&quot;noreferrer&quot;&gt;sphere of influence&lt;/a&gt;, so it's still being decelerated by its collective gravity and it will be for the next 30,000 years or so when it will be gravitationally closer to &lt;a href=&quot;https://en.wikipedia.org/wiki/Gliese_445&quot; rel=&quot;noreferrer&quot;&gt;Gliese 445&lt;/a&gt; that's a star of between 0.15 - 0.3 Solar masses and about 17.6 ly away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, not wanting to leave you with a bitter taste for interstellar colonization, some scientists and groups are trying to prepare for that time early. And when you think about it, it makes a lot of sense even though we don't yet have a single suitable target we're certain of, or indeed means of getting there in reasonable time, since it's not exactly as simple as planning a trip to Venice and it will take centuries to do it, and many decades of developing better remote surveying technologies to decide where to go. If you're interested in finding out more, and I haven't lost you by now, see for example videos of the &lt;a href=&quot;http://www.starshipcentury.com/blog/videos-of-starship-century-symposium/&quot; rel=&quot;noreferrer&quot;&gt;2013 Starship Century Symposium&lt;/a&gt;, and also &lt;a href=&quot;http://www.icarusinterstellar.org/congress-livestream/&quot; rel=&quot;noreferrer&quot;&gt;2013 Starship Congress&lt;/a&gt;. Also check our other threads tagged as &lt;a href=&quot;/questions/tagged/interstellar-travel&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;interstellar-travel&amp;#39;&quot; rel=&quot;tag&quot;&gt;interstellar-travel&lt;/a&gt; and &lt;a href=&quot;/questions/tagged/colonization&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;colonization&amp;#39;&quot; rel=&quot;tag&quot;&gt;colonization&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-24T21:34:42.880" LastActivityDate="2015-07-24T21:34:42.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10227" PostTypeId="2" ParentId="10224" CreationDate="2015-07-24T18:48:48.033" Score="3" Body="&lt;h2&gt;Are the planets in the &quot;habitable zone&quot; actually habitable for humans?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Unknown. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We did discover exoplanets in the goldilock zone around some planets, but we know almost nothing about these planets. We know their approximate mass and orbit and for some we know some very unreliable data about the upper atmosphere from their transmission spectrum (subtle color change of the light of the star they orbit when they pass in front of it). But we don't even know if they are solid. They could even be very small gas giants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do not let those pretty pictures fool you. They are &quot;artist impressions&quot;, i.e. how artists with lots of creativity and not much scientific background imagines how those planets &lt;em&gt;could&lt;/em&gt; look.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;If they found the those planets in the habitable zone are they planning ahead for human settlement of them?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With our current knowledge of physics and the resources available to space exploration, there is just no feasible method to bring humans to another solar system. There aren't even serious plans for unmanned interstellar missions, except in form of a couple thought experiments which never went further than the whiteboarding stage. Interstellar travel is still very, very far in the realm of science fiction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next big milestone for manned space exploration is Mars. And the plans for Mars are already very vague.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2020-06-14T11:32:17.940" LastActivityDate="2020-06-14T11:32:17.940" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="10228" PostTypeId="1" CreationDate="2015-07-24T19:43:59.027" Score="3" ViewCount="61" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_Lake_In-situ_Sampling_Propelled_Explorer&quot; rel=&quot;nofollow noreferrer&quot;&gt;TALISE&lt;/a&gt; (Titan Lake In-situ Sampling Propelled Explorer) is a proposed automated boat that would float in a lake called Ligeia Mare on Saturn's moon Titan. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I saw &lt;a href=&quot;http://www.space.com/17829-titan-lakes-boat-talise-mission.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;an article&lt;/a&gt; that said once it touches down in the lake, TALISE would begin working its way toward the shore of the lake. There are some people who think &lt;a href=&quot;https://en.wikipedia.org/wiki/Life_on_Titan&quot; rel=&quot;nofollow noreferrer&quot;&gt;life could be found on Titan&lt;/a&gt;, which is one of the reasons people want to explore it (but also, how cool would it be to send a boat through space?), do they think that life might be easier to find closer to shore? Is there another reason they want to move toward shore?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FpRkt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FpRkt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sup&gt;From a &lt;a href=&quot;http://www.space.com/17829-titan-lakes-boat-talise-mission.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space.com&lt;/a&gt; article&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2015-07-24T19:43:59.027" Title="Why would TALISE be directed to the shore of Ligeia Mare?" Tags="|probe|life|lander|titan|talise|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10229" PostTypeId="2" ParentId="10223" CreationDate="2015-07-24T19:46:28.740" Score="19" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: Yes, momentum of the tape recorder (DTR) was an issue. However, I don't think it was an issue that was overlooked in the design phase. &#xA;The Voyager team knew it would become an issue for the Uranus and Neptune flybys, and had a fix in place before the Uranus flyby. I've found no indication of a &quot;last-minute fix&quot;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1973, NASA published a report &quot;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19740009051&amp;amp;hterms=motion%20compensation%20voyager%20tape%20recorder&amp;amp;qs=Ntx%3Dmode%2520matchall%7Cmode%2520matchall%26Ntk%3DAll%7CAll%26N%3D0%26No%3D60%26Ntt%3Dvoyager%2520tape%2520recorder%7C%2522motion%2520compensation%2522&quot;&gt;The applicability of frame imaging from a spinning spacecraft&lt;/a&gt;&quot;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Another source of image motion is random vibrations of the sensor mounting platform. These are often due in part to the limit cycles in the IMC servo system and spacecraft attitude control.&#xA;  In addition, any moving part on the spacecraft will cause some jitter at the sensor.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;..&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The spin motion of the spacecraft is the only significant component of image motion, and the only component that will cause any appreciable smear.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(note: calculations in this report were made for a hypothetical spin-stabilized spacecraft at Jupiter)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as far back as 1973, NASA was aware of moving parts on the spacecraft being a potential source of image smear. In my opinion, it's unlikely they subsequently neglected to account for the DTR. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19900004096&amp;amp;hterms=voyager%20DTR&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3Dvoyager%2520DTR%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA&quot;&gt;The Voyager Neptune travel guide&lt;/a&gt; (PDF page 140) (document prepared by the Voyager mission planning office) has a good overview of the problem and the fix:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As Voyager cruises along in a zero-gravity environment, the start-stop motion of its tape recorder can add more jiggle to the spacecraft's natural limit cycle motion. To reduce these types of disturbances, the Voyager Flight Team has devised new software that fires the spacecraft thrusters to offset the tape recorder speed change whenever the tape recorder starts or stops. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This was more of an issue on the Neptune encounter than earlier in the flight, because of the longer exposure times required.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's an &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19860063178&quot;&gt;AIAA conference paper&lt;/a&gt; that may contain more information on this topic in relation to the Uranus encounter, but I don't have access to that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://books.google.nl/books?id=DulLuzqJLw8C&amp;amp;pg=PA213&amp;amp;lpg=PA213&amp;amp;dq=voyager%20smear%20uranus&amp;amp;source=bl&amp;amp;ots=a3bwBsSpDY&amp;amp;sig=L8oxi_L1N4PHXCwTapHhdPsq09k&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CGoQ6AEwDWoVChMIx8Lims72xgIVpqDbCh07jQDx#v=onepage&amp;amp;q=voyager%20smear%20uranus&amp;amp;f=false&quot;&gt;New Cosmic Horizons: Space Astronomy from the V2 to the Hubble Space Telescope&lt;/a&gt; and &lt;a href=&quot;https://books.google.nl/books?id=rTCr0H6sRSoC&amp;amp;pg=PT101&amp;amp;lpg=PT101&amp;amp;dq=voyager%20smear%20uranus&amp;amp;source=bl&amp;amp;ots=DD2oIoRCSN&amp;amp;sig=KC08QNL-uwJunUv-7UlSyGogdJQ&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CHYQ6AEwEGoVChMIx8Lims72xgIVpqDbCh07jQDx#v=onepage&amp;amp;q=voyager%20smear%20uranus&amp;amp;f=false&quot;&gt;Pale Blue Dot: A Vision of the Human Future in Space&lt;/a&gt; (by Carl Sagan, who was closely involved with the Voyager program), this motion compensation technique was used at Uranus as well as Neptune. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Saturn encounter, they used image motion compensation on the Rhea flyby. In this case, they flew by Rhea so close they had to rotate the spacecraft to keep Rhea centered and prevent smearing. The Saturn press briefing &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19810008442&amp;amp;hterms=Voyager%20motion%20compensate%20jupiter&amp;amp;qs=Ntx%3Dmode%2520matchall%26Ntk%3DAll%26N%3D0%26No%3D10%26Ntt%3DVoyager%2520motion%2520compensate%2520jupiter&quot;&gt;doesn't talk about compensating&lt;/a&gt; for the motion of the DTR.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-26T18:04:11.813" LastActivityDate="2015-07-26T18:04:11.813" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10230" PostTypeId="1" CreationDate="2015-07-24T21:13:43.133" Score="19" ViewCount="8347" Body="&lt;p&gt;In the old days, most rocket engines weren't throttleable. I think the first one was actually the Apollo Lunar Module descent engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I'm interested in large, turbopump-fed engines using RP-1/LOX or LH2/LOX. Why is it so hard to makes these throttleable? The way I see it, the preburner running the turbopump should be throttleable just by varying the intake flow with some valves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I could have a mistaken impression, of course. Maybe it's not all that hard, but they just never needed this for rocket stages and thus didn't want to add unnecessary complexity?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I don't need a huge range like 1% to 100% of thrust. More like 50% to 100%, or even 75% to 100% would probably be fine.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2018-06-25T22:57:17.950" Title="What is so hard about making a throttleable large rocket engine?" Tags="|engines|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10231" PostTypeId="2" ParentId="10192" CreationDate="2015-07-24T21:14:14.583" Score="3" Body="&lt;p&gt;EchoLogic's answer is great, and I like the chart of Voyager 2 because it shows escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there's a few different approaches (maybe I should say, points of view, from a layman like me) that can be used, for example, the Earth orbits the sun at 30 km/s (67,000 miles per hour), so any object leaving the Earth's orbit, even if it leaves the Earth ever so slowly, like let's say, it climbs a space elevator and then is given a push, just enough to escape Earth's gravity - that object is already traveling at 67,000 miles per hour compared to the Sun (and it's a good thing, if it wasn't, it would fall towards the Sun).   So, 67,000 and 0 can be used for the same ship;  which do you use as a starting point?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fastest velocity relative to the Sun can be reached by ships flying towards the inner planets because the Sun's gravity adds to the ship's velocity.&#xA;Messenger, which orbited Mercury had to do a few slow down maneuvers to get slow enough to orbit Mercury, or it had to slow down to about 106,000 MPH or there abouts to get into a good orbit.   &lt;a href=&quot;http://www.space.com/11147-nasa-mercury-spacecraft-surprising-facts-messenger.html&quot; rel=&quot;nofollow&quot;&gt;http://www.space.com/11147-nasa-mercury-spacecraft-surprising-facts-messenger.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fastest velocity relative to Earth can be a kind of cool indication of launch power and improved engine design and all that, but velocity relative to Earth isn't that interesting once an object is outside the Earth's orbit.   Then velocity relative to the Sun is usually used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think a better, truer but less traditional measurement of velocity would be ship velocity minus orbital velocity at that distance from the sun.   Ship velocity minus escape velocity also works as a measurement of the ship's final speed as it gets far from our solar-system, but the cool thing about ship's velocity minus orbital velocity is that it tells us how fast the ship is moving relative to what it's close to.   For example, for the recent Pluto fly-by, the ship was traveling far too fast to do anything other than a fly-by, but on the Messenger journey to Mercury, Messenger had to match orbital speed around Mercury, so, relative to orbital speed around the Sun, it was traveling quite slowly for a ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's all in how you look at it, but I personally like velocity relative to the Sun minus orbital velocity at that distance from the Sun for solar-system travel and I like velocity minus escape velocity for travel outside the solar system, which in a practical sense, hasn't really been done yet.&lt;/p&gt;&#xA;" OwnerUserId="8711" LastEditorUserId="8711" LastEditDate="2015-07-26T06:06:58.380" LastActivityDate="2015-07-26T06:06:58.380" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10233" PostTypeId="1" AcceptedAnswerId="10317" CreationDate="2015-07-24T21:34:37.360" Score="4" ViewCount="196" Body="&lt;p&gt;I'm designing a world, and I really like the idea of a recurring moon. If a planet were in the proper orbital resonance with another planet, is it possible that moons might occasionally traverse between the two? How might that occur, and what restrictions would there need to be?&lt;/p&gt;&#xA;" OwnerUserId="1537" LastActivityDate="2015-07-28T23:35:25.013" Title="How feasible is a natural satellite (moon) that moves between planets?" Tags="|moons|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10234" PostTypeId="2" ParentId="5939" CreationDate="2015-07-24T21:38:51.780" Score="4" Body="&lt;p&gt;I'm going to go out on a limb and suggest that the technology to refine the ore works, as well as the ability to make the rocket. Here's a few things for thought:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Assuming an ISP of, say, 200, a proper mixture of 1/3rd mass, 2/3rd fuel would be sufficient to achieve lunar orbit. For a 4/5 fuel ratio, you could return directly to Earth.&lt;/li&gt;&#xA;&lt;li&gt;Producing the LOX at the Moon would be a fairly difficult problem, as it would need to be kept cool the entire time.&lt;/li&gt;&#xA;&lt;li&gt;Solids have never really been used in rockets before (Aside from solid rockets, which is a different mechanism) This would be a hybrid type rocket, pumping solids, which seems challenging. Also, you'd have to have the aluminum as a dust, which might be abrasive. &lt;/li&gt;&#xA;&lt;li&gt;This seems like a fairly risky thing to do, based primarily on #3, for a manned mission. You gain a relatively small weight boom, but with a much more risky rocket.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I could see this as a return stage for lunar mining mission, but not for a manned mission. ISRU just isn't as important for a Lunar mission as it would be for a Mars mission.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-24T21:38:51.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10236" PostTypeId="2" ParentId="10224" CreationDate="2015-07-25T00:12:13.087" Score="-4" Body="&lt;p&gt;No, humans cannot survive on any other planet than Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everything we encounter physically in our everyday lives is biological or of biological origin. The ground, the things, the air, everything. Earth surface and atmosphere is like a cell membrane which &quot;we&quot; have both created and are dependent on. And only in the configuration which &quot;we&quot; have given it for the time being. &quot;We&quot; is everything you get in contact with. We are all relatives. Even if all humans die now, 99.5% of our genes are out there anyway. We are our own environment, and that makes us alone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pure math of combinatorics proves that there can never exist a world in the visible universe where we can survive, other than the one we have created here on Earth. Just take the number of amino acids to the power of the protein length times the number of proteins, and you'll see that there aren't enough numbers in the visible universe to describe it. We only live once.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-07-25T00:12:13.087" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10237" PostTypeId="2" ParentId="1484" CreationDate="2015-07-25T00:19:39.300" Score="0" Body="&lt;p&gt;We are in eternal pursuit of a solution to a vast equation on the likelihood of life elsewhere in the universe. With the extremophiles that we have found here on Earth, it is extremely unlikely that life itself is missing or even rare out there. That presumed, the next question is about intelligence. The factor that seems most important for us is the geological history of Earth with the various extinctions timed such that successor forms of life become dominant. It is certainly possible that at least the dinosaurs could have evolved to significant intelligence and sufficient manipulation capability to make and use tools. The timing of extinction events and the ecosystems of the intervals certainly could determine the overall outcome. With basic intelligence achieved, can intelligence and concomitant technology occur with sufficient speed for the intelligent species to survive a major extinction event and move on to a level detectible across space to us. There are so many ways for intelligence to wipe itself out or to be destroyed by external factors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without building truly massive long base line instruments, we may never know except by random chance. That is a very unsatisfying answer, but I fear that it is the only one that we shall ever have. Every day, I awake with the hope that I am wrong. &lt;/p&gt;&#xA;" OwnerUserId="11011" LastActivityDate="2015-07-25T00:19:39.300" CommentCount="0" CommunityOwnedDate="2015-07-25T00:19:39.300" ContentLicense="CC BY-SA 3.0" />
  <row Id="10238" PostTypeId="2" ParentId="8317" CreationDate="2015-07-25T00:21:13.913" Score="20" Body="&lt;p&gt;At 4,000 to 14,000 dollar per kg in launch costs, pretty panels are a huge waste of money. It needs to be fixed solidly enough to withstand a crewmember bumping into it, and easily detachable enough to be replaced when needed. Beyond that, the crewmembers are professionals who watch where they're going and don't need a kiddieproofed environment.&lt;/p&gt;&#xA;" OwnerUserId="11080" LastActivityDate="2015-07-25T00:21:13.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10239" PostTypeId="1" CreationDate="2015-07-25T00:45:04.373" Score="11" ViewCount="1082" Body="&lt;p&gt;Rotating a spacecraft to create a simulated 1 g should cure most medical problems with microgravity. But can one play ball? A rotating spacecraft with several 10s or 100 meter to the center of rotation, seems to cause funny effects for items thrown &quot;upwards&quot; (inwards to the hub). How big are these effects and would they make for games which for once were interesting to watch? Would the players adapt quickly, or would they remain utterly frustrated? &lt;a href=&quot;http://spirit.as.utexas.edu/~fiso/telecon10-12/Hall_11-17-10/&quot; rel=&quot;noreferrer&quot;&gt;Illustration&lt;/a&gt; below from Theodore W. Hall&#xA;&lt;a href=&quot;https://i.stack.imgur.com/8coSo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8coSo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="12102" LastEditDate="2017-05-06T01:06:41.860" LastActivityDate="2017-05-06T01:06:41.860" Title="Can humans play basketball in simulated gravity?" Tags="|artificial-gravity|reduced-gravity-sports|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10241" PostTypeId="2" ParentId="8607" CreationDate="2015-07-25T02:15:28.280" Score="2" Body="&lt;p&gt;The proper phrase is &quot;centripetal force&quot;, because that is only created in a rotating system where release is inhibited. Linear force upward would be simply acceleration, and it would lack the vertical force differential from top to bottom and Coriolis forces applied on axial movement.&lt;/p&gt;&#xA;" OwnerUserId="11011" LastActivityDate="2015-07-25T02:15:28.280" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10242" PostTypeId="2" ParentId="10180" CreationDate="2015-07-25T02:44:41.367" Score="2" Body="&lt;p&gt;In the reading that I have done, it was indicated that, for the person to survive, they would need an initial velocity of Mach 10 or better and an altitude of well over 100k feet. Otherwise, the only thing that would reach orbit would be well cooked protoplasm. For many reasons beyond the marginal physical capabilities of materials, an Earth elevator or tether is highly unlikely in this century. However, the Moon is a candidate for an elevator with existing materials, but requires an unreasonably length for anything but cargo. Mars is a perfect candidate for both tethers and elevators, with the top of Olympus Mons being the perfect site for both. The atmosphere there is effectively zero, and the Areosychronous orbit is quite low, restricting the length of both the elevator and the counterbalance. &lt;/p&gt;&#xA;" OwnerUserId="11011" LastActivityDate="2015-07-25T02:44:41.367" CommentCount="0" CommunityOwnedDate="2015-07-25T02:44:41.367" ContentLicense="CC BY-SA 3.0" />
  <row Id="10245" PostTypeId="1" CreationDate="2015-07-25T09:13:56.200" Score="3" ViewCount="488" Body="&lt;p&gt;Orion is designed to support only one EVA. There's no airlock so the entire spacecraft would be evacuated of air during the space walk and then repressurized from gas flasks, with a storage enough for doing it only once. This might be sufficient for current ambitions for HSF, but could it be upgraded to do several EVA's? And what about Dragon v2 and CST-100, are they designed for any EVA capability?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that the service module could be extended to allow for gas storage for multiple pressurizing events. But couldn't one also attach an inflatable airlock so that the entire spacecraft does not have to be evacuated of air? That's actually how the first space walk ever was done, by &lt;a href=&quot;http://spacerockethistory.com/?p=1011&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alexey Leonov in 1964&lt;/a&gt; as &lt;a href=&quot;http://www.space.com/28868-how-first-spacewalk-worked-infographic.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;illustrated&lt;/a&gt; below. Today maybe a &lt;a href=&quot;http://human.space.edu/projects/suitport.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;suitport&lt;/a&gt; would be a better idea because Alexey had quite some problems getting back into that rubber bag. (Maybe a couple of suitports should be put on the ISS just because it's simpler than airlocks?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gGfuM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gGfuM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-07-25T09:19:16.557" LastActivityDate="2015-07-25T12:31:59.770" Title="Can Orion (or Dragon or CST-100) be upgraded to do multiple EVA's (space walks)?" Tags="|eva|orion-spacecraft|dragon-v2|cst-100|expandable|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10246" PostTypeId="1" CreationDate="2015-07-25T09:34:06.343" Score="5" ViewCount="906" Body="&lt;p&gt;Solid rockets have some advantages such as high thrust (good for using the Oberth effect), storability and reliability. The main problem is that they fire all their fuel once in one set way. But couldn't a spacecraft utilize an arbitrary orbital correction by simply jettisoning the solid rocket before it has finished? Depleted solid boosters have been jettisoned from launchers hundreds of times. Could it be done in space, for example as an emergency system to deorbit a crewed mission in LEO?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-07-26T01:49:07.110" LastActivityDate="2016-07-01T15:16:09.683" Title="Can solid rockets be used for flexible burn time by jettisoning them?" Tags="|rockets|propulsion|orbital-maneuver|solid-fuel|oberth-maneuver|" AnswerCount="6" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="10247" PostTypeId="2" ParentId="10246" CreationDate="2015-07-25T10:03:19.573" Score="5" Body="&lt;p&gt;If you jettison a booster before depletion, it will accelerate ahead of the rocket/spaceship, and you get blasted by the booster's exhaust gases. You'd have to use powerful motors to get the booster out of the way quickly, and if one of those fails, the booster will ram the rocket/spaceship. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-25T10:03:19.573" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10248" PostTypeId="2" ParentId="10246" CreationDate="2015-07-25T10:42:14.737" Score="10" Body="&lt;p&gt;Problem with separating SRB during burn, be it to use SRB to accelerate or decelerate for reentry, is that you'd have big problems designing their mounts so they detach while they still accelerate in the direction relative to the main vehicle for which they would be designed to have the strongest grip to it. Once they're depleted, it's the vehicle itself that accelerates in some vector opposite to what before it SRB were also meant to help with, or they're inertial to each other, so it's easier to simply slide them off to detach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it's not impossible, it would just complicate design somewhat, which makes everything heavier and potentially less reliable, so it's avoided. SRB near depletion don't have that much thrust any more due to less than ideal expansion of solid propellant's grain geometry, so if you do it late enough, it ought not create additional problems with its exhaust plume to the vehicle that's designed to either go supersonic on ascent, or hypersonic on descent, even if they were pointed for a short period close towards it. Of course, you could always use gimballed nozzles to avoid that entirely. Or eject their core, blow open the top, muffle the nozzle, tear apart SRB's sides lengthwise with small charges, or use some flame retarding system, whichever makes more sense depending on exact implementation. And you might need to do some of this anyway, to avoid collision with SRB body, which &lt;em&gt;would&lt;/em&gt; have catastrophic consequences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, use a tractor or a puller design. You'll lose some thrust with canted nozzles (cosine of the angle to acceleration vector, so it's not so bad for smaller angles), but it's designs we already use for launch abort systems and alike. And more often than not, they rely on solid propellants for such tasks. E.g. Saturn V ullage motors used during stage separation, or Orbital ATK designed Orion MPVC's launch abort use solid motors. Those would be relatively safe to jettison before depletion, but you might wanna make sure they don't lose attitude during separation.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-25T12:24:00.610" LastActivityDate="2015-07-25T12:24:00.610" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10249" PostTypeId="2" ParentId="10245" CreationDate="2015-07-25T12:31:59.770" Score="5" Body="&lt;p&gt;Orion was designed to be able to support travel beyond LEO but instead of trying to add everything asked for to the base module (think space shuttle) they kept the capsule itself small and focused &lt;sup&gt;&lt;a href=&quot;http://human.space.edu/files/ADAM_ICES_Final.pdf&quot; rel=&quot;noreferrer&quot;&gt;&lt;code&gt;[1]&lt;/code&gt;&lt;/a&gt;&lt;/sup&gt;.  Instead of adding features to the capsule there are multiple proposals to dock with an additional module in space to extend capability:&lt;/p&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;http://human.space.edu/files/ADAM_ICES_Final.pdf&quot; rel=&quot;noreferrer&quot;&gt;Proposed Androgynous Docking Airlock/Utility Module&lt;/a&gt; (2009)&lt;/h1&gt;&#xA;&lt;p&gt;This proposal suggests using a commercial launch provider to launch an inflatable module into orbit that Orion would dock with to provide an airlock, storage space for tools/suits, and some equipment to enhance the life support system.&lt;/p&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/03-Walz_Cygnus_Beyond_Low-Earth_Orbit.pdf&quot; rel=&quot;noreferrer&quot;&gt;Cygnus Beyond Low-Earth Orbit – Logistics and Habitation in  Cis-Lunar Space (2014, page 12)&lt;/a&gt;&lt;/h1&gt;&#xA;&lt;p&gt;While I found this referenced as a NASA presentation, this appears to be an &lt;a href=&quot;https://www.orbitalatk.com/&quot; rel=&quot;noreferrer&quot;&gt;Orbital&lt;/a&gt; proposal to extend their &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Cygnus&lt;/a&gt; craft to provide living space, logistics for extended missions, and a few options including a toilet and an airlock.  The idea would be to launch a Cygnus-based craft designed for Orion support, dock it and Orion together in orbit, then go do the mission with the two modules connected.  One news article reporting on the presentation (to which I've lost the link) suggested Cygnus could be left in orbit for re-use, though I'm not seeing that mentioned in the presentation slides.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-25T12:31:59.770" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10250" PostTypeId="2" ParentId="10246" CreationDate="2015-07-25T14:23:29.417" Score="4" Body="&lt;p&gt;What is usually better is to eject the fuel, and not the rocket. Alternatively you can simply expose the combustion chamber, without an ability to maintain pressure the rocket will stop firing. Ejecting the rocket can be quite dangerous, as others have mentioned. But these techniques can effectively leave the rocket inert.  They aren't really precise, but they do work. See, for instance, &lt;a href=&quot;http://www.google.com/patents/US6631610&quot; rel=&quot;nofollow&quot;&gt;this patent&lt;/a&gt;, among other concepts&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is commonly used in military rockets on Earth, which have to be ready to go at any time, and require precise maneuvers. The launch any time leads to solid rockets, the precision requires cutting off the thrust.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-26T01:54:24.597" LastActivityDate="2015-07-26T01:54:24.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10251" PostTypeId="2" ParentId="10230" CreationDate="2015-07-25T17:49:09.100" Score="20" Body="&lt;p&gt;For a large bipropellant rocket engine to fire safely and stably, the fuel and oxidizer have to mix very thoroughly at high flow rate and pressure before they ignite. Otherwise, there will be sputtering and popping, which is very bad at that scale. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, in many engines, a &quot;film&quot; of unburned propellant flowing along the walls of the nozzle is critical for cooling. You need that film to be even or you'll get burn-through. When exhausting into atmosphere at low power levels, the exhaust flow tends to separate one side of the nozzle and adhere to the other, causing off-center thrust and undesirable spot heating, instead of moving freely down the middle. To avoid this flow separation you either need a very short nozzle (reducing specific impulse) or an &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerospike_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerospike&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Expansion_deflection_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;expansion-deflection&lt;/a&gt;, or other compensating nozzle; those generally increase weight and haven't been thoroughly researched and developed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, the design of the fuel injection system is critical, and it's very sensitive to flow rates. The common injector designs for large liquid engines are referred to as &quot;showerhead&quot; injectors and like common showerheads, if the propellant valves are partially shut, you'll get an unsteady dribble out of it instead of a fine, predictable spray. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So early engines in particular were designed for stability in a narrow regime around full throttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pintle injectors like that used in the SpaceX Merlin are apparently less sensitive than showerhead injectors, and the state of the art in fluid dynamic simulation is way better today than it was in the 60s, so the Merlin is more throttlable than some, and can throttle over a wide range. The &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;October 2015 revision of the Falcon 9 user's guide&lt;/a&gt; claims 70%-100% throttling for the first-stage Merlins, but 39%-100% for the second stage Merlin Vacuum; this suggests that nozzle flow issues (vastly reduced out of atmosphere) are the limiting factor rather than chamber stability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Henry Spencer's usenet posts, as always, have some good information:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://yarchive.net/space/rocket/deep_throttling.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://yarchive.net/space/rocket/deep_throttling.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://yarchive.net/space/rocket/lunar_module_throttling.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://yarchive.net/space/rocket/lunar_module_throttling.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://yarchive.net/space/rocket/injectors.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://yarchive.net/space/rocket/injectors.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These indicate that 70%-100% throttling is relatively easy to &#xA;accomplish, with the challenge being to get much below that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100033271.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA study on deep throttling&lt;/a&gt; (2010?) is more optimistic, suggesting that 4:1 ranges (i.e. 25%-100%) are straightforward to achieve with small modifications to fixed-thrust engines. It mentions in particular:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The RL-10-derived &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10#Common_Extensible_Cryogenic_Engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;CECE&lt;/a&gt; with better than 10:1 range (i.e. &amp;lt;10%-100%)&lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;SSME&lt;/a&gt;, designed to throttle down to 65%, but stable combustion demonstrated as low as 17% of rated power (this is somewhat misleading, because the pumps, and therefore the engine as a whole, have various problems at low power).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A bunch of modern launchers use engines with substantial throttling capability, used to reduce aerodynamic stress around max-Q and/or excessive g-force at the end of the first-stage run when fuel tanks are near-empty:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;http://spaceflight101.com/spacerockets/atlas-v-401/&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-180 used on the Atlas V&lt;/a&gt; throttles down to 47%-50%;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceflight101.com/spacerockets/delta-iv-medium-42/&quot; rel=&quot;nofollow noreferrer&quot;&gt;RS-68 on Delta IV&lt;/a&gt; throttles down to 57.5%; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceflight101.com/spacerockets/long-march-6/&quot; rel=&quot;nofollow noreferrer&quot;&gt;YF-100 on Long March 6&lt;/a&gt; throttles down to 65%;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceflight101.com/spacerockets/zenit-3slb/&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-171 on Zenit&lt;/a&gt; throttles down to 56%.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-06-25T22:57:17.950" LastActivityDate="2018-06-25T22:57:17.950" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="10252" PostTypeId="2" ParentId="4441" CreationDate="2015-07-25T18:28:39.817" Score="10" Body="&lt;p&gt;A satellite can naturally remain aligned to the local vertical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In orbit are two forces to consider: force of gravity and centrifugal force. Centrifugal force is actually inertia in a rotating frame. But if you happen to be on the merry-go-round it feels like a force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Centrifugal force is $\omega^2r$ and gravity is $GM/r^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To portray these up and down tugs I'll use balloons and passengers being carried by balloons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lHZGX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lHZGX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This picture portrays a balance of gravity and centrifugal force. Net force is zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens if we double r, the distance from body center?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1ntpA.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1ntpA.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doubling radius doubles upward tug. Downward tug is cut to 1/4. Net acceleration is up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if we cut radius in half…&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CAyMm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CAyMm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Upward tug is cut in half while downward tug is quadrupled. Net acceleration is down&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tie these three together and you get a tether that remains aligned to the local vertical:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nd46s.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nd46s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are satellites that use &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity-gradient_stabilization&quot; rel=&quot;noreferrer&quot;&gt;gravity gradient stabilization&lt;/a&gt; to remain aligned. This also what keeps a lot of moons tidelocked. If we ever have vertical tethers or space elevators, this is what would hold them vertical.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-07-25T18:28:39.817" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10253" PostTypeId="2" ParentId="10246" CreationDate="2015-07-25T18:28:45.550" Score="7" Body="&lt;p&gt;The same problem was encountered by the designers of the Minuteman missile. They used a system whereby ports were opened in the nozzle, and the extremely fast drop in pressure would snuff out the flame. &lt;a href=&quot;https://en.wikipedia.org/wiki/LGM-30_Minuteman#Edward_Hall_and_solid_fuels&quot; rel=&quot;noreferrer&quot;&gt;Link&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8697" LastActivityDate="2015-07-25T18:28:45.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10254" PostTypeId="2" ParentId="10195" CreationDate="2015-07-25T18:34:18.880" Score="40" Body="&lt;p&gt;They wanted a close flyby of Triton. Triton's orbit is at a large angle to the ecliptic plane, and Triton was below Neptune at the time of Voyager's flyby, so they needed a course change that pointed &amp;quot;down&amp;quot; from Neptune.&lt;/p&gt;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19900004096&amp;amp;hterms=voyager%20DTR&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3Dvoyager%2520DTR%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA&quot; rel=&quot;noreferrer&quot;&gt;Voyager Neptune travel guide&lt;/a&gt; (large PDF), page 118-119 of the PDF (page number 107-108 indicated on the page):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;However, Voyager 1 passed (slightly) the leading hemisphere of Saturn, and Voyager 2 will pass (slightly) the leading hemisphere of Neptune. In these two cases, the spacecraft slowed down and the planets speeded up.&lt;/p&gt;&#xA;&lt;p&gt;Neptune is Voyager 2's last planet. There being no next planet to seek&#xA;(Pluto is not reachable; refer to Figure 6-2), Voyager 2 is not limited to passing Neptune through any particular gravity-assist corridor, and can instead concentrate on Neptune's large moon, Triton. Triton is as interesting to many planetary scientists as Neptune is. Triton is large enough to have an atmosphere. Its surface temperature and pressure are close to the triple point of nitrogen, raising the possibility of nitrogen clouds, frozen nitrogen pools, and snow/ice on the surface.&#xA;In 1980, Andrey Sergeyevsky discovered that there was indeed a way to pass closely by both Neptune and Triton, thereby maximizing the scientific return from each. The means was a final application of the gravity-deflection concept. The spacecraft would pass very close to Neptune (within 4850 kilometers of the cloud tops) in order to bend its path by about 45 degrees to pass close by Triton 5.2 hours later (see Figure 6-1.) The close passage of Neptune occurs near its North Pole, and is just barely on the leading hemisphere. Voyager 2 will slow down slightly (and Neptune will speed up even more slightly) as a result of this final gravity assist.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qEFKd.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qEFKd.png&quot; alt=&quot;Voyager trajectory at Neptune&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image from &lt;a href=&quot;http://planetary.s3.amazonaws.com/voyager_msb/VoyagerMSB91.PDF&quot; rel=&quot;noreferrer&quot;&gt;Voyager mission status report 91&lt;/a&gt;, via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Voyager_2_Neptune_Encounter_Trajectory.png&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-26T10:23:42.297" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10257" PostTypeId="2" ParentId="9369" CreationDate="2015-07-25T22:40:01.687" Score="2" Body="&lt;p&gt;Chemical explosives are nowhere near as energetic per pound as rocket fuel.  That's going to be brutal when it comes to the ISP of such a propulsion system.  The rocket equation is brutal once you need energies that involve a vehicle that is mostly fuel (As all orbital launches are.)  Brutal * brutal = total insanity.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-07-25T22:40:01.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10258" PostTypeId="1" AcceptedAnswerId="10260" CreationDate="2015-07-26T00:26:16.327" Score="3" ViewCount="3177" Body="&lt;p&gt;We recently found out that Pluto has a molten core. Since the Moon is bigger than Pluto should the Moon have had a molten core at some point in time?&lt;/p&gt;&#xA;" OwnerUserId="11087" LastEditorUserId="7062" LastEditDate="2015-07-26T01:39:52.463" LastActivityDate="2021-01-19T21:39:29.810" Title="Why does Pluto have a molten core and the Moon does not?" Tags="|the-moon|pluto|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10260" PostTypeId="2" ParentId="10258" CreationDate="2015-07-26T01:35:21.757" Score="10" Body="&lt;p&gt;About one-third of the near side of the Moon is covered by basaltic plains, called Mare. These plains are evidence that molten rock once flowed on those areas of the Moon and that in its early history the Moon experienced &lt;a href=&quot;http://volcano.oregonstate.edu/oldroot/volcanoes/planet_volcano/lunar/Overview.html&quot;&gt;volcanism&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/connect/chat/moon_core_chat.html&quot;&gt;NASA analysis of Apollo era seismic data&lt;/a&gt; indicates the Moon has a core similar to Earth: a solid central core within a molten outer core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The molten Lunar core is small compared to the size of the Moon and the Moon's mantle is so thick that molten rock from the core can no longer reach the surface of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From my review of NASA's &lt;a href=&quot;http://www.nasa.gov/mission_pages/newhorizons/main/index.html&quot;&gt;New Horizon's website&lt;/a&gt;, there is no statement about the composition of Pluto's core. Two other sources, &lt;a href=&quot;http://www.news.com.au/technology/science/pluto-fly-by-puts-you-aboard-the-new-horizons-mission-of-discovery/story-fnjwlcze-1227448662603&quot;&gt;News.com&lt;/a&gt; and &lt;a href=&quot;http://www.space.com/43-pluto-the-ninth-planet-that-was-a-dwarf.html/&quot;&gt;Space.com&lt;/a&gt; make different statements about Pluto's core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;News.com states,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Essentially, it looks as though Pluto has a molten radioactive core and tectonic plates.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Whereas, Space.com states,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The dwarf planet probably has a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane, carbon monoxide and nitrogen ice coating the surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;These comments appear to be conjecture and are not official NASA releases based on NASA acquired data from the New Horizons probe that flew past Pluto.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="7062" LastEditDate="2015-07-26T01:52:06.733" LastActivityDate="2015-07-26T01:52:06.733" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10261" PostTypeId="1" CreationDate="2015-07-26T08:04:58.000" Score="1" ViewCount="465" Body="&lt;p&gt;SEL1 and SEL2 are about 0.01 AU away, 10 seconds light travel time back and forth, 4 times Lunar distance, 45 times the distance to GEO. From those two regions combined, all surface on Earth and the Moon and all of cislunar space are always in line of sight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SEL1&amp;amp;2 are already used for scientific applications. But wouldn't they be useful also for commercial and military &quot;inward looking&quot; applications? Isn't it great real estate for communication and surveillance of military spacecrafts and of debris and impacting asteroids? Or are they too far away?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-07-26T10:48:09.443" Title="Are Sun-Earth Lagrange points SEL1 and SEL2 useful for communication and space surveillance?" Tags="|lagrangian-points|communication-satellite|space-surveillance|cislunar|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10262" PostTypeId="2" ParentId="10261" CreationDate="2015-07-26T10:48:09.443" Score="1" Body="&lt;p&gt;For voice and video communications, a 1-second delay is annoying. 10 seconds is way too much to be useful. Current data networks don't tolerate 10-second delays either so you'd need new communications technology. You'd also need large dishes on the ground instead of the small rod antenna now used in satphones.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Military surveillance of space can be done from the ground for now. It's much easier to build a megawatt radar on Earth than to cram the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cobra_Dane&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cobra Dane&lt;/a&gt; installation into a satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TF6zL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TF6zL.jpg&quot; alt=&quot;Cobra Dane&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If militarization of space starts in earnest, that's early enough to start thinking about monitoring locations that are hidden from Earth's view. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-26T10:48:09.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10263" PostTypeId="2" ParentId="10223" CreationDate="2015-07-26T16:38:35.960" Score="7" Body="&lt;p&gt;This seems to be very similar to the myth of NASA spending millions of dollars to develop a space pen. One thing is certain: This did not happen (quoting from &lt;a href=&quot;http://www.tutorvista.com/content/physics/physics-iii/rigid-body/angular-momentum-laws.php&quot; rel=&quot;noreferrer&quot;&gt;tutorvista.com&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager 2, in 1986, as it flew past the planet Uranus, was set into unwanted rotation by this flywheel effect, every time its tape recorder was turned on at high speed. The ground staff at the Jet Propulsion Laboratory had to program the on-board computer to turn on counteracting thruster jets, every time the tape recorder was turned on or off.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Even worse, (from &lt;a href=&quot;http://www.education.com/science-fair/article/caused-voyager-point-wrong-direction&quot; rel=&quot;noreferrer&quot;&gt;education.com&lt;/a&gt;),&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What Caused Voyager to Point in the Wrong Direction?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While the second statement in that tutorvista.com link is true (see Hobbes' answer), the first is not. The title of the page at education.com is even worse; it's completely bogus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;At the time of the Uranus encounter, it took 2.5 hours for information sent from Voyager 2 to reach the Earth. It would have taken an additional 2.5 hours for any corrections to have been sent back to Voyager 2 -- and that's assuming the engineers could have instantaneously recognized the problem developed a plan to compensation for that undesired rotation. The Uranus encounter was of a very short duration, much like the recent fly-by of Pluto by the New Horizons spacecraft. The bulk of the encounter would have been already been over with that (minimum) five hours time between problem recognition and problem resolution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, it would have taken much longer than five hours. Propellant was the one of the most precious resource on the Voyager spacecraft. A plan that involved expending propellant would have had to have gone to the highest level for approval. The time frame needed just for this approval would have involved weeks, perhaps months. It would also have taken a good amount of time to develop the compensation algorithm. This was not a seat-of-the-pants decision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's another problem with the entire premise. Quoting Donald Rumsfeld, &quot;There are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don't know we don't know.&quot; Spacecraft attitude control systems need to deal with the known knowns, the known unknowns, and the unknown unknowns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Had they been uncompensated, the attitude rate errors that would have resulted from starting and stopping of the tape drive would have fallen into the category of &quot;unknown unknowns&quot; (&quot;process noise&quot; in terms of Kalman filters). The attitude control system would eventually have addressed these errors, but only after the fact. The result would have been fuzzy images rather than pointing in the wrong direction. Addressing these knowable consequences of starting and stopping the tape drive as an integral part of the attitude control system moved this from the &quot;unknown unknown&quot; category to the &quot;known known&quot; category.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps this was foreseen during the design stage, perhaps not. It's hard to tell; I searched and searched. At the very minimum, someone at JPL foresaw this months in advance of the Uranian system encounter.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-07-26T16:38:35.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10265" PostTypeId="1" CreationDate="2015-07-26T17:44:34.147" Score="18" ViewCount="1434" Body="&lt;p&gt;I'm looking for a comprehensive pros and cons of the two most commonly used station-keeping types of orbits used at libration points, &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot;&gt;Lissajous&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot;&gt;halo orbits&lt;/a&gt;. When would one select one over the other and why? Do any of the criteria change depending on which Lagrange point (L1-L5) and libration points of which two bodies one would like to place an artificial satellite at? Or is this more mission specific, and depends more on what these satellites are doing there and how wide of an orbital box they require (say, to avoid transits through Earth's umbra or penumbra)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally, I'd like to see some mission design for which both types of station-keeping orbits were considered, and one chosen over the other based on some documented criteria. Perhaps &lt;a href=&quot;https://www.nasa.gov/mission_pages/themis/news/artemis-orbit.html&quot;&gt;ARTEMIS&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Cometary_Explorer&quot;&gt;ISEE-3&lt;/a&gt; produced some detailed documentation about this? But if that isn't available, let's, for the sake of argument, pretend that I can't decide between the two for &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope&quot;&gt;JWST&lt;/a&gt;. Why would I have picked a halo orbit for it, and not a Lissajous one?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-30T12:08:52.740" LastActivityDate="2015-09-23T22:45:39.207" Title="Halo vs Lissajous orbit: Which station-keeping strategy to select and when?" Tags="|orbit|lagrangian-points|mission-design|reference-request|station-keeping|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10266" PostTypeId="1" CreationDate="2015-07-26T18:13:30.967" Score="9" ViewCount="354" Body="&lt;p&gt;Russians have been doing &lt;em&gt;expedited&lt;/em&gt; four-orbit rendezvous with the International Space Station (ISS) for quite a while now, and when it works, its benefits are apparent; You get there before you'd even have to &lt;em&gt;relieve bladder&lt;/em&gt; again, which, in a crowded Soyuz spacecraft, should be a lot more important than anyone riding it is ready to publicly admit. But there are other benefits, too. Including for live cargo on Progress, any other time-sensitive or urgently needed equipment and goods, less time to fret any risk management nightmare or otherwise things going wrong on orbit, and it doesn't hurt that mission controllers get to go home sooner either, have everyone rested and avoid human error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the meantime, all the past and current US cargo and human crew providers, including NASA's Space Shuttle Orbiter, SpaceX's Dragon, Orbital ATK's Orion,... did/do the &lt;em&gt;standard&lt;/em&gt;, about two and a half days, or roughly 40 orbits long rendezvous, just like when Soyuz or Progress spacecraft fail to insert into their ISS chasing orbit precisely enough, or something else needed to accomplish it fails (like failing to deploy KURS antennas or alike). So to my question;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Does NASA require in any version of their COTS (&lt;a href=&quot;https://en.wikipedia.org/wiki/Commercial_Orbital_Transportation_Services&quot;&gt;Commercial Orbital Transportation Services&lt;/a&gt;) contracts that some, or all providers have to reach capability of &lt;em&gt;expedited rendezvous&lt;/em&gt;? Or, if that isn't the case, did any of the current providers, or future contract competitors announce such capability?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize that doing expedited rendezvous might require phase corrections of ISS' orbit, but that's what Russians have been doing so far without any problems that I'm aware of. Could US do the same?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-26T18:44:25.357" LastActivityDate="2015-07-28T16:34:07.213" Title="Will any of the COTS providers be able to perform expedited rendezvous with the ISS?" Tags="|iss|nasa|low-earth-orbit|commercial|rendezvous|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10267" PostTypeId="5" CreationDate="2015-07-26T18:56:49.967" Score="0" Body="&lt;p&gt;Titan Lake In-situ Sampling Propelled Explorer (TALISE) is a proposed space probe which would land in Ligeia Mare, the second largest lake on Saturn's moon Titan. TALISE would navigate across the lake for six months to one year. If this mission is approved, it will analyze liquid and take scientific measurements while it navigates to the coast in the northern region of Titan. It is also planned in the mission to study the surrounding terrain of Ligeia Mare.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A comparable mission concept was NASA's Titan Mare Explorer, which, unlike TALISE, would not have had its own propulsion system.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/titan&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;titan&amp;#39;&quot; rel=&quot;tag&quot;&gt;titan&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_Lake_In-situ_Sampling_Propelled_Explorer&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://meetingorganizer.copernicus.org/EPSC2012/EPSC2012-64.pdf&quot; rel=&quot;nofollow&quot;&gt;Mission Abstract&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:01:29.083" LastActivityDate="2015-07-26T19:01:29.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10268" PostTypeId="4" CreationDate="2015-07-26T18:56:49.967" Score="0" Body="Titan Lake In-situ Sampling Propelled Explorer (TALISE) is a proposed space probe which would land in Ligeia Mare, the second largest lake on Saturn's moon Titan." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:01:22.680" LastActivityDate="2015-07-26T19:01:22.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10269" PostTypeId="1" CreationDate="2015-07-26T19:02:42.160" Score="9" ViewCount="207" Body="&lt;p&gt;Is this primarily done analyzing the Voigt profiles absorption bands on background of stellar spectra or on re-emission bands of atmospheric compounds?&#xA;Assuming Pluto's atmosphere behaves like an ideal gas, one is dealing with three unknowns - pressure, particle density, and temperature. &#xA;Looking at the Lorentzian portion of curve, one can get the product of particle density and relative velocity. While looking at the Gaussian portion of the curve, one can get the temperature. &#xA;Just curious how this is actually done.&lt;/p&gt;&#xA;" OwnerUserId="11095" LastEditorUserId="49" LastEditDate="2015-07-26T19:18:42.917" LastActivityDate="2018-03-25T23:48:52.133" Title="Measuring the pressure and temperature of Pluto's atmosphere using stellar occultations" Tags="|planetary-science|atmosphere|pluto|measurement|remote-sensing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10270" PostTypeId="5" CreationDate="2015-07-26T19:04:46.120" Score="0" Body="&lt;p&gt;Energia (Russian: Энергия, Energiya, &quot;Energy&quot;) was a Soviet rocket that was designed by NPO Energia to serve as a heavy-lift expendable launch system as well as a booster for the Buran spacecraft. Control system main developer enterprise was the NPO &quot;Electropribor&quot;. The Energia used four strap-on boosters each powered by a four-chamber RD-170 engine burning kerosene/LOX, and a central core stage with 4 one-chamber RD-0120 (11D122) engines fueled by liquid hydrogen/LOX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch system had two functionally different operational variants: Energia-Polyus, the initial test configuration, in which the Polyus system was used as a final stage to put the payload into orbit, and Energia-Buran, in which the Buran spacecraft was the payload and the source of the orbit insertion impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket had the capacity to place about 100 tonnes in Low Earth orbit, up to 20 tonnes to geostationary orbit and up to 32 tonnes to a translunar trajectory.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/buran&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;buran&amp;#39;&quot; rel=&quot;tag&quot;&gt;buran&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:18:54.350" LastActivityDate="2015-07-26T19:18:54.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10271" PostTypeId="4" CreationDate="2015-07-26T19:04:46.120" Score="0" Body="Energia was a Soviet rocket that was designed by NPO Energia to serve as a heavy-lift expendable launch system as well as a booster for the Buran spacecraft." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:18:57.243" LastActivityDate="2015-07-26T19:18:57.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10272" PostTypeId="5" CreationDate="2015-07-26T19:21:13.493" Score="0" Body="&lt;p&gt;Atlas V is an expendable launch system in the Atlas rocket family. Each Atlas V rocket uses a Russian-built RD-180 engine burning kerosene and liquid oxygen to power its first stage and an American-built RL10 engine burning liquid hydrogen and liquid oxygen to power its Centaur upper stage. The RD-180 engines are provided by RD AMROSS, while Aerojet Rocketdyne provides both the RL10 engines and the strap-on boosters used in some configurations. The standard payload fairing sizes are 4 or 5 meters in diameter and of various lengths, and are made by RUAG Space. Fairings sizes as large as 7.2 m in diameter and up to 32.3 m in length have been considered.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:26:02.427" LastActivityDate="2015-07-26T19:26:02.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10273" PostTypeId="4" CreationDate="2015-07-26T19:21:13.493" Score="0" Body="Atlas V is an expendable launch system in the Atlas rocket family." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:02:52.810" LastActivityDate="2015-07-26T20:02:52.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10274" PostTypeId="5" CreationDate="2015-07-26T19:25:08.750" Score="0" Body="&lt;p&gt;Phobos is the larger and inner of the two natural satellites of Mars. Phobos is a small, irregularly shaped object with a mean radius of 11 km, and is seven times more massive than Deimos, Mars's outer moon.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/mars&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;mars&amp;#39;&quot; rel=&quot;tag&quot;&gt;mars&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/deimos&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;deimos&amp;#39;&quot; rel=&quot;tag&quot;&gt;deimos&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Phobos_%28moon%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:25:56.160" LastActivityDate="2015-07-26T19:25:56.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10275" PostTypeId="4" CreationDate="2015-07-26T19:25:08.750" Score="0" Body="Phobos is the larger and inner of the two natural satellites of Mars." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:25:59.473" LastActivityDate="2015-07-26T19:25:59.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10276" PostTypeId="5" CreationDate="2015-07-26T19:25:29.417" Score="0" Body="&lt;p&gt;Deimos is the smaller and outer of the two natural satellites of the planet Mars with a mean radius of 6.2 km, the other being Phobos. Deimos takes 30.3 hours to orbit Mars.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/mars&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;mars&amp;#39;&quot; rel=&quot;tag&quot;&gt;mars&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/phobos&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;phobos&amp;#39;&quot; rel=&quot;tag&quot;&gt;phobos&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Deimos_%28moon%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:26:08.563" LastActivityDate="2015-07-26T19:26:08.563" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10277" PostTypeId="4" CreationDate="2015-07-26T19:25:29.417" Score="0" Body="Deimos is the smaller and outer of the two natural satellites of the planet Mars." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T19:26:05.737" LastActivityDate="2015-07-26T19:26:05.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10278" PostTypeId="5" CreationDate="2015-07-26T19:56:11.137" Score="0" Body="&lt;p&gt;The Angara rocket family is a family of space-launch vehicles being developed by the Moscow-based Khrunichev State Research and Production Space Center. The rockets are to put between 3,800 and 24,500 kg into low Earth orbit and are intended, along with Soyuz-2 variants, to replace several existing launch vehicles.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Angara_%28rocket_family%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.russianspaceweb.com/angara.html&quot; rel=&quot;nofollow&quot;&gt;russianspaceweb.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:02:39.403" LastActivityDate="2015-07-26T20:02:39.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10279" PostTypeId="4" CreationDate="2015-07-26T19:56:11.137" Score="0" Body="The Angara rocket family is a family of space-launch vehicles in development by Russia." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:02:49.510" LastActivityDate="2015-07-26T20:02:49.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10280" PostTypeId="5" CreationDate="2015-07-26T20:00:29.567" Score="0" Body="&lt;p&gt;Hydrazine is also used as a low-power monopropellant for the maneuvering thrusters of spacecraft, and was used to power the Space Shuttle's auxiliary power units (APUs). In addition, monopropellant hydrazine-fueled rocket engines are often used in terminal descent of spacecraft. Such engines were used on the Viking program landers in the 1970s as well as the Phoenix lander and Curiosity rover which landed on Mars in May 2008 and August 2012, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In all hydrazine monopropellant engines, the hydrazine is passed by a catalyst such as iridium metal supported by high-surface-area alumina (aluminium oxide) or carbon nanofibers, or more recently molybdenum nitride on alumina, which causes it to decompose into ammonia, nitrogen gas, and hydrogen gas.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/monopropellant&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;monopropellant&amp;#39;&quot; rel=&quot;tag&quot;&gt;monopropellant&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrazine#Rocket_fuel&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:02:42.090" LastActivityDate="2015-07-26T20:02:42.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10281" PostTypeId="4" CreationDate="2015-07-26T20:00:29.567" Score="0" Body="Hydrazine is an inorganic compound used as a monopropellant." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:02:45.870" LastActivityDate="2015-07-26T20:02:45.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10282" PostTypeId="5" CreationDate="2015-07-26T20:11:17.177" Score="0" Body="&lt;p&gt;The habitable zone is the region around a star within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The habitable zone is also called the Goldilocks zone, a metaphor of the children's fairy tale of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is &quot;just right&quot;.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Circumstellar_habitable_zone&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:11:34.623" LastActivityDate="2015-07-26T20:11:34.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10283" PostTypeId="4" CreationDate="2015-07-26T20:11:17.177" Score="0" Body="The region around a star within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:11:39.087" LastActivityDate="2015-07-26T20:11:39.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10284" PostTypeId="1" AcceptedAnswerId="10424" CreationDate="2015-07-26T20:19:23.663" Score="6" ViewCount="1317" Body="&lt;p&gt;I've played a lot of Kerbal Space Program and always wondered: When in orbit around a body without an atmosphere, which trajectory does a lander take to approach and land using minimum delta-v, without wasting too much time, and landing at the exact right place that has been chosen during mission planning?&lt;/p&gt;&#xA;" OwnerUserId="10836" LastEditorUserId="10836" LastEditDate="2015-07-26T21:54:29.270" LastActivityDate="2015-08-01T17:04:29.660" Title="How does a lander get from orbit to the ground?" Tags="|landing|trajectory|delta-v|lander|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="10285" PostTypeId="5" CreationDate="2015-07-26T20:29:32.587" Score="0" Body="&lt;p&gt;A robotic spacecraft is an unmanned spacecraft, usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to lower cost and lower risk factors. In addition, some planetary destinations such as Venus or the vicinity of Jupiter are too hostile for human survival, given current technology. Outer planets such as Saturn, Uranus, and Neptune are too distant to reach with current crewed spaceflight technology, so telerobotic probes are the only way to explore them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This tag is most appropriate for questions directly about the technologies used for or limitations of not having a crew on board.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/crewed-spaceflight&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;crewed-spaceflight&amp;#39;&quot; rel=&quot;tag&quot;&gt;crewed-spaceflight&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Robotic_spacecraft&quot; rel=&quot;nofollow&quot;&gt;Wikipedia - Robotic Spacecraft&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:31:58.797" LastActivityDate="2015-07-26T20:31:58.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10286" PostTypeId="4" CreationDate="2015-07-26T20:29:32.587" Score="0" Body="Questions about space flight activities without humans onboard." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T20:32:05.120" LastActivityDate="2015-07-26T20:32:05.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10287" PostTypeId="5" CreationDate="2015-07-26T21:07:20.497" Score="0" Body="&lt;p&gt;Lockheed Martin is an American global aerospace, defense, security and advanced technologies company with worldwide interests. It stands as one of the world's premier companies in the aerospace, defense, security, and technologies industry based on revenue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With regard to space exploration, Lockheed Martin is currently developing the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Orion&lt;/a&gt; capsule, worked on the cancelled &lt;a href=&quot;https://en.wikipedia.org/wiki/VentureStar&quot; rel=&quot;nofollow&quot;&gt;VentureStar&lt;/a&gt; spaceplane, built the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_external_tank&quot; rel=&quot;nofollow&quot;&gt;Space Shuttle external tank&lt;/a&gt; (as Martin Marietta), and has developed/built a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_Martin_Space_Systems#Sensing_.26_Exploration_Systems&quot; rel=&quot;nofollow&quot;&gt;wide range of probes for NASA&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/orion&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;orion&amp;#39;&quot; rel=&quot;tag&quot;&gt;orion&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/venturestar&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;venturestar&amp;#39;&quot; rel=&quot;tag&quot;&gt;venturestar&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.lockheedmartin.com/&quot; rel=&quot;nofollow&quot;&gt;lockheedmartin.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_Martin&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T21:15:24.817" LastActivityDate="2015-07-26T21:15:24.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10288" PostTypeId="4" CreationDate="2015-07-26T21:07:20.497" Score="0" Body="A defense contractor that builds rockets, satellites, and probes." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-26T21:15:21.963" LastActivityDate="2015-07-26T21:15:21.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10289" PostTypeId="2" ParentId="10284" CreationDate="2015-07-26T22:25:29.930" Score="14" Body="&lt;p&gt;The minimum $\Delta V$ is effectively a Hohmann transfer.  You would de-orbit just enough to barely touch the surface at periapsis, and right at the surface you would do an instantaneous impulsive burn (also known as a &quot;&lt;a href=&quot;https://space.stackexchange.com/questions/10307/what-is-a-suicide-burn&quot;&gt;suicide burn&lt;/a&gt;&quot;) to exactly cancel your velocity relative to the surface.  Done!  A perfect landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the real world however you don't have instantaneous burns.  So you need to start sometime before you get to the surface and do a gravity turn to minimize the gravity loss during the finite burn.  The higher your thrust-to-weight ratio, the shorter the burn and the less gravity loss that you'll have.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then the time it takes will be the time it takes.  If your objective is to minimize $\Delta V$, then you can neither &quot;waste&quot; time nor be more efficient with the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the accuracy, you want to have the landing site in the plane of the orbit.  Then its just a matter of timing your de-orbit properly.  The residual error can be taken out with a smidge more $\Delta V$ and some means of sensing your location relative to the intended landing point, ideally before you start your landing burn.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-07-28T00:58:24.417" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10291" PostTypeId="5" CreationDate="2015-07-27T05:23:22.320" Score="0" Body="&lt;p&gt;The Mariner program was a program conducted by the American space agency NASA in conjunction with Jet Propulsion Laboratory (JPL) that launched a series of robotic interplanetary probes designed to investigate Mars, Venus, and Mercury from 1962 to 1973. The program included a number of firsts, including the first planetary flyby, the first pictures from another planet, the first planetary orbiter, and the first gravity assist maneuver.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mariner_program&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-27T07:05:13.900" LastActivityDate="2015-07-27T07:05:13.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10292" PostTypeId="4" CreationDate="2015-07-27T05:23:22.320" Score="0" Body="The Mariner program was a program conducted by the American space agency NASA in conjunction with Jet Propulsion Laboratory (JPL) that launched a series of robotic interplanetary probes designed to investigate Mars, Venus, and Mercury from 1962 to 1973." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-27T07:05:10.773" LastActivityDate="2015-07-27T07:05:10.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10293" PostTypeId="5" CreationDate="2015-07-27T05:26:43.857" Score="0" Body="&lt;p&gt;Mars Global Surveyor (MGS) was a US robotic spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. Mars Global Surveyor was a global mapping mission that examined the entire planet, from the ionosphere down through the atmosphere to the surface. As part of the larger Mars Exploration Program, Mars Global Surveyor performed atmospheric monitoring for sister orbiters during aerobraking, and it helped Mars rovers and lander missions by identifying potential landing sites and relaying surface telemetry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It completed its primary mission in January 2001 and was in its third extended mission phase when, on 2 November 2006, the spacecraft failed to respond to messages and commands. A faint signal was detected three days later which indicated that it had gone into safe mode. Attempts to recontact the spacecraft and resolve the problem failed, and NASA officially ended the mission in January 2007.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Global_Surveyor&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science.nasa.gov/missions/mars-global-surveyor/&quot; rel=&quot;nofollow&quot;&gt;NASA Science Missions&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mars.nasa.gov/mgs/&quot; rel=&quot;nofollow&quot;&gt;NASA Mars Global Surveyor&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-27T07:05:16.740" LastActivityDate="2015-07-27T07:05:16.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10294" PostTypeId="4" CreationDate="2015-07-27T05:26:43.857" Score="0" Body="Mars Global Surveyor was a US robotic spacecraft developed by NASA's Jet Propulsion Laboratory and launched November 1996. It was a global mapping mission that examined the entire planet, from the ionosphere down through the atmosphere to the surface." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-27T07:05:19.660" LastActivityDate="2015-07-27T07:05:19.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10295" PostTypeId="5" CreationDate="2015-07-27T05:42:29.177" Score="0" Body="&lt;p&gt;SuperDraco is a hypergolic propellant liquid rocket engine designed and built by SpaceX. It is part of SpaceX's Draco family of rocket engines. A redundant array of eight SuperDraco engines provides fault-tolerant propulsion for use as a launch escape system and also propulsive-landing thrust for the Dragon V2 passenger-carrying space capsule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SuperDraco rocket engines utilize a pressure fed storable (non-cryogenic) propellant which allows the engines to be fired many months after fueling and launch. They are rated at 73k newtons (16.4k lbf) and are highly throttleable.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/SuperDraco&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="1142" LastEditDate="2015-08-24T06:59:44.677" LastActivityDate="2015-08-24T06:59:44.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10296" PostTypeId="4" CreationDate="2015-07-27T05:42:29.177" Score="0" Body="SuperDraco is a hypergolic propellant liquid rocket engine designed and built by SpaceX for use on the Dragon V2 capsule." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-27T07:05:07.280" LastActivityDate="2015-07-27T07:05:07.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10298" PostTypeId="1" CreationDate="2015-07-27T07:46:38.037" Score="0" ViewCount="162" Body="&lt;p&gt;This is difficult for me to explain, but hopefully I can get my question across...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say there is a planet called Planet X one million light years from Earth (there probably is). Let's also say there is a craft named Speedy 1, which could travel at the speed of light. If we hopped into Speedy 1 and travelled toward Planet X, we would arrive in one million years. If we could view Planet X from Earth, we would be seeing events from one million years ago. (Please stop me here if I'm wrong by the way.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But traveling in Speedy 1 as a passenger, you are shortening the time delay between Planet X and yourself. You are now witnessing events from a &lt;em&gt;variable-amount-of-time&lt;/em&gt; ago. Does anyone know how this would look to the passenger? While traveling, would we witness events unfolding at a faster rate than they actually are happening?&lt;/p&gt;&#xA;" OwnerUserId="10712" LastActivityDate="2015-07-27T09:12:58.853" Title="What would happen to perceived time in this scenario?" Tags="|time|spacetime|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10299" PostTypeId="1" AcceptedAnswerId="10301" CreationDate="2015-07-27T08:50:14.040" Score="4" ViewCount="2135" Body="&lt;p&gt;I am an amateur in space, and I usually read here to better my understanding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was camping Saturday night (July 25, 2015), and while I was looking at the sky I noticed a bright object moving really fast (maybe it looked like a comet), I saw several of them and they were so beautiful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it normal to see several fast objects moving at night? (I saw that 6 times in 2 hours. I don't know what to call them, but they had a tail.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More info:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were extremely fast, vanished in 1 or 2 seconds maximum. They were as bright as the other stars and planets. As for the angle, they were somehow at top of me, and went somehow straight. It was at night around 2 a.m.. What made them interesting to me is that I saw several of them, about six of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no specific direction, they appeared from different directions. I mean, if they appeared from north they would go in a straight line to south or east. They got brighter, though I noticed something in one of them--its direction wasn't straight line. It had a really small diversion. I don't know how to explain that. It covered maximum 2 seconds. They didn't reach the horizon. They covered small parts of the sky. There was no change in colors, just bright white. I did notice a dust trail for seconds after they disappeared. I was on a mountain and it was a clear sky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was in the Middle East, specifically Lebanon.&lt;/p&gt;&#xA;" OwnerUserId="11002" LastEditorUserId="58" LastEditDate="2015-07-28T12:47:12.657" LastActivityDate="2015-07-28T13:54:30.287" Title="What are the fast-moving, bright things I saw July 25, 2015 at night?" Tags="|atmosphere|observation|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10300" PostTypeId="2" ParentId="10298" CreationDate="2015-07-27T09:12:58.853" Score="3" Body="&lt;p&gt;The only way you could travel at the speed of light, not assuming highly theoretical &lt;a href=&quot;https://en.wikipedia.org/wiki/Faster-than-light&quot; rel=&quot;nofollow&quot;&gt;faster than light&lt;/a&gt; (FTL) travel which actually bends &lt;a href=&quot;https://en.wikipedia.org/wiki/Spacetime&quot; rel=&quot;nofollow&quot;&gt;spacetime&lt;/a&gt; to shorten the distance between two points (highly simplifying here), is if you &lt;a href=&quot;https://en.wikipedia.org/wiki/Mind_uploading&quot; rel=&quot;nofollow&quot;&gt;transfer consciousness&lt;/a&gt; as information about it into a distant neural network or any other physical medium later supporting its function. So in that sense, you wouldn't &lt;em&gt;see&lt;/em&gt; anything (or otherwise perceive information) about the environment that the information about your consciousness would have traveled through.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With FTL, and bending or warping of spacetime, it would be more like taking a shortcut. You'd &lt;em&gt;see&lt;/em&gt; the shortcut at the (likely non-relativistic) speed you're traversing it as your point of origin behind you and destination point in front of you, but you wouldn't see the &lt;em&gt;long way around&lt;/em&gt; path or anything from it, because you're not on it. If you can traverse this &lt;em&gt;shortcut&lt;/em&gt; at relativistic speeds approaching some significant portion of the speed of light (&lt;em&gt;c&lt;/em&gt;), then you'd also see objects behind you &lt;a href=&quot;https://en.wikipedia.org/wiki/Redshift&quot; rel=&quot;nofollow&quot;&gt;redshifting&lt;/a&gt; and objects in front of you &lt;a href=&quot;https://en.wikipedia.org/wiki/Blueshift&quot; rel=&quot;nofollow&quot;&gt;blueshifting&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Length_contraction&quot; rel=&quot;nofollow&quot;&gt;length-contracted&lt;/a&gt; space between two points at your sides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're of mass though, be it &lt;a href=&quot;https://en.wikipedia.org/wiki/Baryon#Baryonic_matter&quot; rel=&quot;nofollow&quot;&gt;baryonic&lt;/a&gt; or any other &lt;a href=&quot;https://en.wikipedia.org/wiki/Exotic_matter&quot; rel=&quot;nofollow&quot;&gt;exotic matter&lt;/a&gt; that we have yet to directly detect, define, and work with, you simply can't travel at 1 &lt;em&gt;c&lt;/em&gt; throught spacetime. Energy required for it is infinite, and you'd be of infinite mass to your environment, attracting spacetime towards you at the speed of light. You would have been, literally, the end of everything. You wouldn't &lt;em&gt;see&lt;/em&gt; anything, even if you were some Marvel's superhero that can withstand the lack of reality, as time, from your vantage point, would have &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Time_dilation&quot; rel=&quot;nofollow&quot;&gt;dilated&lt;/a&gt;&lt;/em&gt; to 1/∞. Form your point of view, the whole Universe would have collapsed onto you before you'd be able to realize you're in a bit of a pickle.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-07-27T09:12:58.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10301" PostTypeId="2" ParentId="10299" CreationDate="2015-07-27T09:22:55.660" Score="5" Body="&lt;p&gt;You most likely saw &lt;a href=&quot;https://en.wikipedia.org/wiki/Meteoroid&quot; rel=&quot;nofollow noreferrer&quot;&gt;meteors&lt;/a&gt;. Around this time, it could be the &lt;a href=&quot;http://earthsky.org/astronomy-essentials/earthskys-meteor-shower-guide#delta-aquarids&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta Aquarids&lt;/a&gt; meteor swarm. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rj20I.jpg&quot; alt=&quot;Meteor&quot;&gt;&lt;br&gt;&#xA;(from the &lt;a href=&quot;http://www.bbc.com/news/science-environment-23675321&quot; rel=&quot;nofollow noreferrer&quot;&gt;BBC site&lt;/a&gt;)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fast-moving objects without a tail are usually satellites. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-27T09:22:55.660" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10302" PostTypeId="2" ParentId="5117" CreationDate="2015-07-27T12:47:15.857" Score="1" Body="&lt;p&gt;In addition to the other answers: the comet's gravity is so weak that when the probe is &lt;a href=&quot;http://rosetta.jpl.nasa.gov/news/how-rosetta-arrives-comet&quot; rel=&quot;nofollow&quot;&gt;more than about 30 km&lt;/a&gt; away from the comet, &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/02101026-rosetta-flyby.html&quot; rel=&quot;nofollow&quot;&gt;gravity isn't enough to hold it&lt;/a&gt; in &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;orbit&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for higher &quot;orbits&quot;, the spacecraft flies in formation with the comet, more or less sharing the comet's heliocentric orbit. The spacecraft flies at just above its escape velocity from 67P, so a tiny thruster burn is enough to change course to the next leg of the triangle. If something happens to Rosetta (e.g. a debris collision), the spacecraft will continue to slowly move away from the comet, giving the team time to reestablish contact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do those high orbits have a triangle shape: it's the shape with the fewest &quot;corners&quot;: you only need 3 course corrections per orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And why did ESA use higher orbits: directly after arrival, and now that the comet is approaching perihelion, they want to make sure the spacecraft doesn't get blasted by debris coming off the comet. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-27T12:47:15.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10304" PostTypeId="2" ParentId="2387" CreationDate="2015-07-27T16:27:08.667" Score="23" Body="&lt;p&gt;Locks aren't needed, &lt;em&gt;because it is physically impossible to open the hatch from outside unless the airlock is depressurized&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All airlock hatches are inward opening (the STS Orbiter side hatch was not an airlock hatch) and the air pressure inside amounts to tons of force keeping the hatch closed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, check out this picture of the ISS US airlock.  That white thing flipped up is just a thermal cover.  You can see the actual outer hatch hinges behind Cassidy and the hatch is down behind his backpack.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/f0bzG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f0bzG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-07-27T16:27:08.667" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10305" PostTypeId="2" ParentId="10222" CreationDate="2015-07-27T17:16:04.070" Score="3" Body="&lt;p&gt;According to the &lt;a href=&quot;http://www.hq.nasa.gov/alsj/TP-2006-213726.pdf&quot; rel=&quot;nofollow&quot;&gt;Constellation report&lt;/a&gt; referenced earlier, there was a sampling brush that could be used, and some spare cloth that seems to have been attempted. The dust problem was expected, especially in later missions, and they had a dust brush to brush off the dust from the lunar suits. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-27T17:40:08.593" LastActivityDate="2015-07-27T17:40:08.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10307" PostTypeId="1" AcceptedAnswerId="10308" CreationDate="2015-07-27T19:59:30.650" Score="41" ViewCount="41901" Body="&lt;p&gt;The term &lt;em&gt;suicide burn&lt;/em&gt; occasionally comes up in answers and comments, usually without any real explanation or definition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is meant by the term &quot;suicide burn&quot;?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="6816" LastEditDate="2017-05-02T19:44:33.697" LastActivityDate="2017-05-02T19:44:33.697" Title="What is a &quot;suicide burn&quot;?" Tags="|terminology|entry-descent-landing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10308" PostTypeId="2" ParentId="10307" CreationDate="2015-07-27T20:09:51.177" Score="59" Body="&lt;p&gt;For a powered descent to the surface of a massive body like the Moon, it turns out to be most fuel efficient to do all your deceleration at the very end of the trajectory, right before impact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(This is because if you decelerate sooner than that, you will be in flight longer; the longer you're up, the more fuel you need to spend counteracting gravity accelerating you toward the surface.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the fuel-optimum landing involves falling faster and faster toward the surface before doing your deceleration &lt;em&gt;burn&lt;/em&gt;, which looks &lt;em&gt;suicidal&lt;/em&gt; (and will be, if you don't leave sufficient margin for error). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence: suicide burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX's first-stage recovery is another example. In that case, suicide burn is a requirement, because even at minimum stable throttle, the engine produces more thrust than the weight of the stage, so it can't hold a constant low downward speed for any length of time; it has to time the burn so that it hits altitude zero just before it reaches velocity zero. SpaceX calls this maneuver &quot;hover-slam&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure of the origins of the term &quot;suicide burn&quot;; it's extremely popular in the Kerbal Space Program community. It doesn't appear in the Google books nGram viewer at all. It comes up in some SpaceX discussions but a lot of that looks like bleed from KSP. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-07-27T22:51:28.190" LastActivityDate="2015-07-27T22:51:28.190" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10309" PostTypeId="1" AcceptedAnswerId="10311" CreationDate="2015-07-27T21:47:25.270" Score="13" ViewCount="737" Body="&lt;p&gt;Given the short amount of time that New Horizons focused on Pluto, did it capture enough of its surface to create a global map of it?  &lt;/p&gt;&#xA;" OwnerUserId="11104" LastEditorUserId="49" LastEditDate="2015-07-27T22:36:17.527" LastActivityDate="2022-09-28T22:28:23.607" Title="Did New Horizons capture enough of Pluto's surface to create its global map?" Tags="|probe|new-horizons|pluto|imaging|mapping|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10310" PostTypeId="2" ParentId="10266" CreationDate="2015-07-27T22:24:43.083" Score="5" Body="&lt;p&gt;The reason current cargo providers have not been doing expedited rendezvous is because cargo can take the wait. The rapid rendezvous trajectory requires the ISS to be in a precise point in its orbit and requires exact timing on launch. It also requires precisely timed burns by the craft in orbit which leave little room for delays. All of this restricts the number and frequency of launch windows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason Russia and Soyuz started doing expedited rendezvous is because the Soyuz is particularly cramped for 3 people, and getting the astronauts to the ISS faster keeps them happy and makes the adjustment to zero-g easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As EchoLogic has mentioned Dragon 2 will perform the fast rendezvous with crew like the Soyuz.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: Hans Koenigsmann, Director of Mission Assurance, said that short rendezvous profiles are possible for current Dragon 1 and future Dragon 2 missions&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source:&lt;br&gt;&#xA;&lt;a href=&quot;https://www.youtube.com/watch?v=9TUQ52907ew&amp;amp;t=2109s&quot; rel=&quot;nofollow&quot;&gt;CRS-4 Mission Brief, Relevant part at 35:20&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="10398" LastEditDate="2015-07-28T16:34:07.213" LastActivityDate="2015-07-28T16:34:07.213" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10311" PostTypeId="2" ParentId="10309" CreationDate="2015-07-27T22:31:42.350" Score="15" Body="&lt;p&gt;If you mean if New Horizons' data return could produce a &lt;em&gt;global high-resolution map of Pluto's surface&lt;/em&gt;, then no, and here's why:&lt;/p&gt;&#xA;&lt;h3&gt;Pluto at New Horizons approach:&lt;/h3&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/mVto5.png&quot; alt=&quot;Pluto at New Horizons approach&quot; title=&quot;Pluto at New Horizons approach&quot; /&gt;&lt;/p&gt;&#xA;&lt;h3&gt;New Horizons Ground Track on Pluto:&lt;/h3&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/5pFYA.png&quot; alt=&quot;New Horizons Ground Track on Pluto&quot; title=&quot;New Horizons Ground Track on Pluto&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;Source of both images is Alan Stern's (New Horizons PI) &lt;a href=&quot;http://www.lpi.usra.edu/opag/meetings/july2014/presentations/Day_1/6-OPAG_2014July-Stern.pdf&quot; rel=&quot;noreferrer&quot;&gt;presentation&lt;/a&gt; (PDF) to &lt;a href=&quot;http://www.lpi.usra.edu/opag/meetings/july2014/presentations/&quot; rel=&quot;noreferrer&quot;&gt;OPAG Meeting in July 23, 2014&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;As you can see, on New Horizons' close approach, Pluto was sunlit on its Southern hemisphere, with its North polar cap in shadow. Solar phase angle on approach was 15°, with New Horizons approaching from Southern hemisphere. Mapping of all the longitudes of the complete Southern hemisphere and up to 15° North was possible due to Pluto's 6.4 Earth days long rotation, and it produced &lt;a href=&quot;http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?gallery_id=2&amp;amp;image_id=204&quot; rel=&quot;noreferrer&quot;&gt;this much of its global map&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;h3&gt;The Whale and the Donut:&lt;/h3&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/lO9st.jpg&quot; alt=&quot;The Whale and the Donut&quot; title=&quot;The Whale and the Donut&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;This map of Pluto, created from images&#xA;taken from June 27-July 3, 2015, by the Long Range Reconnaissance&#xA;Imager (LORRI) on New Horizons, was combined with lower-resolution&#xA;color data from the spacecraft's Ralph instrument. The center of the&#xA;map corresponds to the side of Pluto that will be seen close-up during&#xA;New Horizons' July 14 flyby.&lt;/p&gt;&#xA;&lt;p&gt;This map gives mission scientists an important tool to decipher the&#xA;complex and intriguing pattern of bright and dark markings on Pluto's&#xA;surface. Features from all sides of Pluto can now be seen at a glance&#xA;and from a consistent perspective, making it much easier to compare&#xA;their shapes and sizes.&lt;/p&gt;&#xA;&lt;p&gt;The elongated dark area informally known as &amp;quot;the whale,&amp;quot; along the&#xA;equator on the left side of the map, is one of the darkest regions&#xA;visible to New Horizons. It measures some 1,860 miles (3,000&#xA;kilometers) in length. Directly to the right of the whale’s &amp;quot;snout&amp;quot; is&#xA;the brightest region visible on the planet, which is roughly 990 miles&#xA;(1,600 kilometers) across. This may be a region where relatively fresh&#xA;deposits of frost—perhaps including frozen methane, nitrogen and/or&#xA;carbon monoxide—form a bright coating.&lt;/p&gt;&#xA;&lt;p&gt;Credit: NASA/Johns Hopkins University Applied Physics&#xA;Laboratory/Southwest Research Institute&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Since the flyby, Pluto is showing New Horizons its &lt;em&gt;dark side&lt;/em&gt;, with only a thin sliver, or a crescent, illuminated by the Sun and showing its atmosphere, and the rest of it is in the shadow. Which is bad for high resolution mapping, but also good for a number of science experiments the probe will do now, including e.g. occultation radio science measurements of Pluto's atmosphere, science requiring long exposure without bright background burn-out, and so on. But it got a good deal of Pluto's surface topography with its instruments, too. For a single flyby, a pretty good return, and expect many more high-resolution images focused on smaller areas, showing a lot more detail than this global map, as it transmits the rest of its flyby data in the following year and a half or so.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;You can view the partial global map of Pluto also in Google Earth, if you &lt;a href=&quot;http://pluto.jhuapl.edu/Multimedia/Google-Map/&quot; rel=&quot;noreferrer&quot;&gt;follow instructions here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;: &lt;a href=&quot;http://pluto.jhuapl.edu/Multimedia/Science-Photos/image.php?gallery_id=2&amp;amp;image_id=254&quot; rel=&quot;noreferrer&quot;&gt;Higher resolution partial global map&lt;/a&gt; was just released today:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h3&gt;Global Map of Pluto (with grid):&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3McNz.jpg&quot; rel=&quot;noreferrer&quot; title=&quot;Global Map of Pluto (with grid)&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3McNz.jpg&quot; alt=&quot;Global Map of Pluto (with grid)&quot; title=&quot;Global Map of Pluto (with grid)&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The science team of NASA’s New Horizons mission has produced an&#xA;updated global map of the dwarf planet Pluto. The map includes all&#xA;resolved images of the surface acquired between July 7-14, 2015, at&#xA;pixel resolutions ranging from 40 kilometers (24 miles) on the&#xA;Charon-facing hemisphere (left and right sides of the map) to 400&#xA;meters (1,250 feet) on the anti-Charon facing hemisphere (map center).&#xA;Many additional images are expected in fall of 2015 and these will be&#xA;used to complete the global map.&lt;/p&gt;&#xA;&lt;p&gt;The New Horizons spacecraft flew past Pluto and its moons on July 14.&#xA;The Johns Hopkins University Applied Physics Laboratory in Laurel,&#xA;Maryland, designed, built, and operates the New Horizons spacecraft,&#xA;and manages the mission for NASA's Science Mission Directorate. SwRI,&#xA;based in San Antonio, leads the science team, payload operations and&#xA;encounter science planning. New Horizons is part of the New Frontiers&#xA;Program managed by NASA's Marshall Space Flight Center in Huntsville,&#xA;Alabama.&lt;/p&gt;&#xA;&lt;p&gt;Credit: NASA/Johns Hopkins University Applied Physics&#xA;Laboratory/Southwest Research Institute&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Grid here is apparently drawn at every 30°, but isn't labeled. Also obvious is that the map was created with early &lt;em&gt;failsafe&lt;/em&gt; New horizons science return using many highly compressed JPeG images, so this will get better over time as complete datasets are transmitted.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;sup&gt;&lt;strong&gt;Commentary&lt;/strong&gt;: There's an obvious confusion over North and South hemispheres between released &lt;em&gt;The Whale and the Donut&lt;/em&gt; map of Pluto and Alan Stern's presentation. I'm not exactly sure who's in error here, but something got mislabeled. Clarification by APL would be appreciated, but the point stands, we won't get a complete high resolution map of Pluto with this single flyby, despite APL release descriptions being somewhat ambiguous about that. And with all the raw data they're preparing for release on short notice, often requiring a lot of processing, such errors will happen, but should be corrected with higher level releases later on.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-28T11:32:17.353" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10312" PostTypeId="1" AcceptedAnswerId="10313" CreationDate="2015-07-28T05:46:53.813" Score="10" ViewCount="5645" Body="&lt;p&gt;Bulk information regarding anything in this world is available in a single touch through Internet. My question is whether it is possible to replace this Internet with a constellation of Cubesats in low Earth orbit for transferring data all over the world more securely than over Internet? &lt;/p&gt;&#xA;" OwnerUserId="3542" LastEditorUserId="168" LastEditDate="2015-07-28T10:14:08.203" LastActivityDate="2020-05-25T23:42:44.090" Title="Is it possible to replace Internet with LEO satellites?" Tags="|communication-satellite|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10313" PostTypeId="2" ParentId="10312" CreationDate="2015-07-28T05:54:15.187" Score="25" Body="&lt;p&gt;You can't replace the Internet like that since Internet is also a collection of protocols for routing and addressing, and transmitting users' data. You can try swapping fiber optic links for LEO sat comm ones, but throughput will invariably suffer and connection will be less reliable.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Satellites' transponders are the bottleneck - instead of an hierarchy of ISPs and various telecom companies with widening link capacities as you go up the &quot;food chain&quot; you have to put maximum imaginable downlink/uplink capacity into each satellite - while now a satellite may be servicing Boise, Idaho, an orbit later it will fly over Los Angeles and will be swamped with traffic.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Expanding capacity is much more costly and risky. You have to plan a year or two in advance, book launches, pay hefty sums to sat manufacturers, launch providers, insurance companies, sweat about some doofus  &lt;a href=&quot;https://space.stackexchange.com/questions/1783/was-the-noaa-n-prime-satellite-really-dropped-on-the-floor&quot;&gt;dropping your sat from a table&lt;/a&gt;. That doesn't mean land links are cheap - rights-of-way, and actual construction cost money, but much less than satellites. One must say a large scale satellite constellation has its advantages, especially if you can lower launch costs and create reliable satellite-to-satellite laser interconnections. If a huge company like Google goes into a price war with ISPs to win over part of the mobile user base, it may well establish a beachhead, drive some ISPs to bankruptcy and lock in many users. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Comms will be patchy - heavy rain and magnetic storms will bite into your link budget and sometimes block communications over whole continents. While sat-to-sat optical links will be unaffected, customers will suffer. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Alaska, Canada, Norway, Sweden, Greenland, Iceland, Finland, Russia, and the Antarctic penguins may be worse off after the switch - first because high-latitude connections need high-inclination satellites in a separate constellation (which is not profitable), and second because of aurora borealis/magnetic storms. (Please bear in mind this point is about small and mid-sized townships, not nomadic settlements/geological parties). &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&#xA;&#xA;&lt;p&gt;Security will be actually &lt;strong&gt;much&lt;/strong&gt; worse - fiber optic cables are intercepted by advanced teams and state-level agencies, while satcomms are much more easily intercepted, jammed, and triangulated. Much like the mobile vs. fixed phones.&lt;/p&gt;&#xA;&#xA;&lt;hr/&gt;&#xA;&#xA;&lt;p&gt;This said, here are some afterthoughts/nice tricks (that haven't been covered elsewhere) to improve link budgets and make a LEO satellite comms constellation more competitive:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;There are usually many smartphones in a given urban locality&lt;/strong&gt;. One can &lt;a href=&quot;http://spectrum.ieee.org/tech-talk/telecom/wireless/5g-service-on-your-4g-phone&quot; rel=&quot;noreferrer&quot;&gt;exploit that&lt;/a&gt; to do some heavy preprocessing (stealth bomber-style) to correct for multipath propagation and raise downlink speeds. The number crunching will have to be done in the satellite or, less likely, in the ground control center.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Again, many nearby emitters (smart phones) may kind of be made to coordinate with each other to act as a distributed phased array to get higher uplink speeds. This would make use of the fact that statistically not everyone is uploading a Youtube video (in normal situations).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr/&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;I'm specifically not discussing &lt;strong&gt;latency&lt;/strong&gt; - a variable well covered in other answers and comments. Nor do I touch upon the mass/size/energy budget of satellites in a global constellation.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-07-29T02:05:12.737" LastActivityDate="2015-07-29T02:05:12.737" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10314" PostTypeId="2" ParentId="10312" CreationDate="2015-07-28T07:22:14.337" Score="9" Body="&lt;p&gt;Cubesats won't work. Because they're so small, you need powerful transmitters and receivers on the ground, plus large and complicated tracking dish antennas. If you want to use a small antenna on the ground, you need a large satellite.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites have far less bandwidth (=how much data can it send per second) than a fiber cable. And you can upgrade a fiber cable to have more bandwidth easily (just replace the equipment at both ends, no new cable necessary). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-07-28T07:22:14.337" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10315" PostTypeId="2" ParentId="10312" CreationDate="2015-07-28T08:48:21.380" Score="4" Body="&lt;p&gt;A significant drawback to satellite based internet connectivity has nothing to do with the satellites but connection hardware, and the restrictions you'd have using the technology. Satellite communications requires line of sight, so you'd only be able to use a service like this if you had access to unobstructed sky. High-speed satellite communications requires a dish, and significant power requirements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any mobile devices using a satellite based internet service would be very limited in access and bandwidth - you'd have to be in clear space outside to use it and as you'd be stuck with directionless antennas your access speed would be very, very slow.&lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-07-28T08:48:21.380" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10316" PostTypeId="1" AcceptedAnswerId="10318" CreationDate="2015-07-28T12:04:55.307" Score="10" ViewCount="436" Body="&lt;p&gt;There has now been a &lt;a href=&quot;http://www.sciencealert.com/independent-scientists-confirm-that-the-impossible-em-drive-produces-thrust&quot; rel=&quot;noreferrer&quot;&gt;third positive study&lt;/a&gt; of the EM drive, and while I'm still skeptical, I'm starting to wonder if this is really possible. As such, I'm wondering what would be required to make this happen, specifically the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mass&lt;/li&gt;&#xA;&lt;li&gt;Size&lt;/li&gt;&#xA;&lt;li&gt;Power&lt;/li&gt;&#xA;&lt;li&gt;Thrust&lt;/li&gt;&#xA;&lt;li&gt;Expense (Expensive materials, etc?)&lt;/li&gt;&#xA;&lt;li&gt;Anything else worthy of notice?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-29T16:42:27.863" LastActivityDate="2018-05-22T03:32:23.707" Title="EM drive requirements" Tags="|propulsion|emdrive|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10317" PostTypeId="2" ParentId="10233" CreationDate="2015-07-28T12:30:08.797" Score="4" Body="&lt;p&gt;It is simply not possible without actively adjusting orbits&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an asteroid to get captured as a moon is actually very tricky, it is most easily achieved through a collision, or a momentum exchange with a 3rd body via gravity. &#xA;The reason why capture is difficult, is that if an object falls &quot;from infinity&quot; towards a planet it has escape velocity and so it will indeed escape, following a hyperbolic orbit. Capture is very nearly a fluke, and often involves an interaction with a 3rd body such as a collision to lose some of that velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once captured, escape is even more difficult, and will only happen through an interaction with a 3rd body - most likely the gravity of a massive passing object pulls it out of orbit. Of course, the massive passing object will also seriously perturb the orbit of the planet - and that is also why the other planet cannot be that massive body - if the two planets pass so close together as to steal each other's moons, they will shortly collide or have a violent gravitational interaction which flings them into very different orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a moon to fall between two different planets being captured and then lost by them in turn, would require a series of flukes. Under the environment the transfers could happen, the orbits of the planets themselves would be highly unstable, and some of the bodies would have to collide or be flung out of the system before the system could adopt a stable state. In other words, even if it could be set it up to happen once or twice (and no doubt it could, in an evolving solar system), it wouldn't be stable in any cosmic time scale.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2015-07-28T12:37:51.737" LastActivityDate="2015-07-28T12:37:51.737" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10318" PostTypeId="2" ParentId="10316" CreationDate="2015-07-28T13:49:47.640" Score="7" Body="&lt;p&gt;There is a great article by &lt;a href=&quot;http://www.gizmag.com/cannae-reactionless-drive-space-propulsion/33210/&quot; rel=&quot;noreferrer&quot;&gt;gizmodo&lt;/a&gt;, which gives an example of what would be required:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Power- 700W&lt;/li&gt;&#xA;&lt;li&gt;Thrust- 88 uN&lt;/li&gt;&#xA;&lt;li&gt;System Mass- 9 kG&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To get 700W of power would require about 2 square meters of high efficiency solar panels, which is probably beyond most small satellites, but could still potentially be done. Where this could fairly easily be done is the International Space Station, however, the small thrust generated from this system would be difficult to distinguish from atmospheric drag. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, in order to make this work in space today, one would need a &quot;mini&quot; class satellite, probably one with ~100 kg. Those aren't exactly cheap, so I believe testing this on orbit would not be practical at this time, until further tests indicate that this does indeed work.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-28T21:18:13.963" LastActivityDate="2015-07-28T21:18:13.963" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10319" PostTypeId="2" ParentId="10299" CreationDate="2015-07-28T13:54:30.287" Score="3" Body="&lt;p&gt;If you saw a tail on the objects, it was very likely a meteor.  The trail is caused by the object impacting with the Earth's atmosphere, and the pressure causing it to vaporize.  There are several meteor shower events each year where you can see dozens of these occuring over the span of a few hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oddly enough, I also went camping on Saturday.  When I reached the location I spent a good 30 minutes outside and noticed many fast moving objects in the sky too.  However, these objects did not have a tail on them, and were much more dim than what you report to have seen.  These were almost surely satellites, which in dark skys can be picked out with little difficulty.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastActivityDate="2015-07-28T13:54:30.287" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10320" PostTypeId="1" CreationDate="2015-07-28T14:19:14.493" Score="10" ViewCount="360" Body="&lt;p&gt;While the US has claimed only sending one nuclear reactor (&lt;a href=&quot;http://gizmodo.com/for-50-years-now-the-u-s-has-had-a-nuclear-reactor-or-1695601513&quot;&gt;SNAP 10-A&lt;/a&gt;) into space, &lt;a href=&quot;http://www.eoearth.org/view/article/154970/&quot;&gt;Russia has sent many&lt;/a&gt;.  Here on earth we've adopted a &lt;a href=&quot;https://www.iaea.org/ns/tutorials/regcontrol/assess/assess3213.htm&quot;&gt;Defense in Depth&lt;/a&gt; strategy to design for safety controls.  Though, everything in space is different...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;What special design considerations are needed for nuclear reactors in space?&lt;/em&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;What have we learned from the number of nuclear reactors already in orbit?&lt;/em&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;What are the dangers of these reactors in orbit and can existing reactors be safely sequestered?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="11104" LastEditorUserId="1142" LastEditDate="2015-08-03T00:08:13.647" LastActivityDate="2015-08-06T21:15:48.560" Title="What are the unique design considerations for space bound nuclear reactors (TOPAZ, SNAP 10-A), and their respective dangers?" Tags="|launch|orbit|mission-design|energy|nuclear|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10321" PostTypeId="1" CreationDate="2015-07-28T14:19:15.553" Score="1" ViewCount="5614" Body="&lt;p&gt;I know that, if I wanted to map the circular orbit of an object by an iteration of time, I know that I can use the relationship between centripetal force and gravitational force...this is how we can find our orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I also know that the second we move away from a circular orbit an into an elliptical one, I can't use the relationship between centripetal and gravitational forces to map out my path.  Why not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the force vectors look like when broken down into x and y vectors, where y is a line that runs through the object and planet (i.e. always perpendicular), and x is consistently tangent to the planet?&lt;/p&gt;&#xA;" OwnerUserId="11102" LastActivityDate="2015-07-28T16:30:16.043" Title="What are the forces in an elliptical orbit, broken down into x and y factors?" Tags="|orbital-mechanics|artificial-satellite|orbit|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10322" PostTypeId="2" ParentId="10321" CreationDate="2015-07-28T14:23:44.367" Score="1" Body="&lt;p&gt;The truth is, there is only 1 force acting on a satellite at any time, and that is the gravitational force. The gravitational force pulls the object straight towards the body around which it is orbiting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What ends up happening is you have a movement in one direction, and a velocity in a different direction. For a circular orbit, and certain parts of an elliptical orbit, the pull is 90 degrees from the velocity direction. In this case, the direction of the velocity will change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in the case of a highly elliptical orbit, sometimes the velocity vector will not be perpendicular to the gravity vector. In fact, for a very elliptical orbit, they can almost be in the same direction! I think that is the mistake that you are making in your analysis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a great image found at &lt;a href=&quot;http://www.gregegan.net/SCIENCE/ConicSectionOrbits/ConicSectionOrbits.html&quot; rel=&quot;nofollow&quot;&gt;this site&lt;/a&gt;, that I can't find the copyright to use here, unfortunately. It shows how these vectors are not at 90 degrees for an elliptical orbit.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-28T14:23:44.367" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10323" PostTypeId="2" ParentId="10321" CreationDate="2015-07-28T15:03:33.113" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What would the force vectors look like when broken down into x and y&#xA;  vectors, where y is a line that runs through the object and planet&#xA;  (i.e. always perpendicular), and x is consistently tangent to the&#xA;  planet?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Exactly the same as in the circular case.  The force in the tangential direction is zero, and the force in the normal direction is $GMm\over r^2$ directed towards the center of the planet.  $M$ is the mass of the planet, $m$ is the mass of the object, and $r$ is the distance between the center of the object and the center of the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's different from a circular orbit is that $r$ varies in time in the elliptical orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-28T15:03:33.113" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10324" PostTypeId="1" AcceptedAnswerId="10327" CreationDate="2015-07-28T16:08:31.373" Score="21" ViewCount="5740" Body="&lt;p&gt;Does our moon have any captured objects orbiting it that have been discovered, such as asteroids or other debris?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it doesn't, is it possible that it could capture them in the future, given the right circumstances? If so, what would be the maximum size of such an object?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="415" LastEditDate="2015-07-29T18:34:11.403" LastActivityDate="2023-04-12T22:21:23.513" Title="Does the Moon have any captured objects orbiting it?" Tags="|the-moon|orbit|asteroid|space-dust|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10325" PostTypeId="2" ParentId="10321" CreationDate="2015-07-28T16:20:44.000" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What would the force vectors look like when broken down into x and y vectors, where y is a line that runs through the object and planet (i.e. always perpendicular), and x is consistently tangent to the planet?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A better name for those vectors are $\boldsymbol e_r$ and $\boldsymbol e_\theta$ (or $\hat r$ and $\hat \theta$) in place of your y and x (in that order; it's standard to represent $r$ first and $\theta$ second). These are the unit vectors for polar curvilinear coordinates, which is what you are using.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The displacement vector from the center of the planet to the orbiting object is given by $\boldsymbol r = r \boldsymbol e_r$. Differentiating this with respect to time from the perspective of a non-rotating planet-centered frame yields the velocity vector, $\boldsymbol v = \dot r \boldsymbol e_r + r \dot {\boldsymbol e}_r = \dot r \boldsymbol e_r + r \dot \theta \boldsymbol e_\theta$. Note that the velocity vector is orthogonal to the position vector only in the case of circular motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about acceleration? Gravitation is a central force (note well: not centripetal force). In central forces, the forces are directed along or against the line connecting the two interacting bodies. The gravitational acceleration vector in the two body point mass problem is directed against the displacement vector: $\boldsymbol a = - \frac {GM}{r^2} \boldsymbol e_r$. Since the velocity vector in general is not orthogonal to the position vector, it in general also is not orthogonal to the acceleration vector. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-07-28T16:30:16.043" LastActivityDate="2015-07-28T16:30:16.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10326" PostTypeId="2" ParentId="10312" CreationDate="2015-07-28T16:49:52.133" Score="0" Body="&lt;p&gt;Wikipedia defines LEO as 100-1200 miles above the earth's surface.  Let's assume 1,000 miles, since it's a nice round number, and we want to stay well clear of the atmosphere for fuel minimization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your minimum round trip distance is now 2,000 miles. If you previously had a fiber connection to a server 100 miles away, your latency has just degraded by an order of magnitude!  And this assumes assumes a satellite directly overhead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is common for major websites to be hosted at multiple physical servers to cut down on latency - when I check BBC News from the East Coast of the US, I am not waiting for my packets to travel to Britain and back.  The BBC has set up a mirror in the States - probably within a few hundred miles of my location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, industrial grade routing equipment is big.  A Google search for &quot;industrial router&quot; turned up this site:&#xA;&lt;a href=&quot;http://www.juniper.net/us/en/products-services/routing/ptx-series/ptx5000/&quot; rel=&quot;nofollow&quot;&gt;http://www.juniper.net/us/en/products-services/routing/ptx-series/ptx5000/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It lists the router as over five feet tall and weighing over 1,200 lbs - and it is plugged into grid power, and kept in a climate controlled room.  If you start adding solar panels, altitude management, thermal control, RF comms equipment, radiation hardening - this is a multi-ton comms sat, not a cube sat.  The market for routers this size is large enough to support several companies, each with several different models offered.  There are likely many hundreds to thousands of routers this size currently in use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could leave the router on the ground, and turn the satellite into a &quot;dumb&quot; repeater - passing the packets on to other satellites and finally a ground station in a pre-set path, determined by the router on the ground.  This exacerbates the latency issues.  Now, any time a bit error or lost packet occurs, the satellite must send a message back through the chain all the way to the ground site to ask for a re-transmission - round trip minimum is now 4,000 miles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't have stats to address power requirements, but I wouldn't be surprised if RF transmissions were an order of magnitude (or more) more energy expensive than fiber. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, I think you've massively underestimated the amount of traffic that the Internet carries, the size of the hardware required to handle it, and the speed at which packets are required to move.  &lt;/p&gt;&#xA;" OwnerUserId="11115" LastActivityDate="2015-07-28T16:49:52.133" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="10327" PostTypeId="2" ParentId="10324" CreationDate="2015-07-28T18:04:11.573" Score="22" Body="&lt;p&gt;It does not have any known objects. It seems unlikely that it would happen, although it could theoretically happen for a short period of time. There are few stable orbits around the Moon, and even fewer that are likely to remain stable for any length of time. Even Lunar orbiting missions don't remain stable, and they have a better chance than a random object would of orbiting the Moon, as they are deliberately placed away from the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Size would not be an issue, the maximum size limit would be rather large. There are no known Earth crossing objects that are that large.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An object would have to have a roughly circular, but high, polar retrograde orbit to remain in Lunar orbit. That seems very unlikely to happen by pure chance.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-28T18:04:11.573" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10328" PostTypeId="2" ParentId="10324" CreationDate="2015-07-28T18:11:29.827" Score="4" Body="&lt;p&gt;At the moment the Earth's moon currently does not have any natural satellites.&#xA;Although it is possible for the moon to capture an object, the proximity of the moon to Earth makes it unlikely that an orbit would be stable. Perhaps this is the reason no natural satellites currently orbiting the moon.&lt;/p&gt;&#xA;" OwnerUserId="11101" LastActivityDate="2015-07-28T18:11:29.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10329" PostTypeId="1" AcceptedAnswerId="10473" CreationDate="2015-07-28T19:56:40.170" Score="14" ViewCount="7913" Body="&lt;p&gt;Somewhat apropos of the question of whether &lt;a href=&quot;https://space.stackexchange.com/questions/634/would-terraforming-mars-be-possible&quot;&gt;Mars could be terraformed&lt;/a&gt; (which really seems a non-runner due to lack of molten core) the interesting consideration of biodomes was thrown up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By using biodomes an atmosphere (albeit small) could be created, and radiation levels made safe. Is the surface gravity of Mars sufficient to make such structures viable for permanent habitation? Besides the difficulty and expense of creating such structures in the first place, are there other considerations which could make them infeasible as long-term habitats?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MWVXs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MWVXs.jpg&quot; alt=&quot;eden project&quot;&gt;&lt;/a&gt;&#xA;Image of &lt;a href=&quot;https://en.wikipedia.org/wiki/Eden_Project&quot; rel=&quot;nofollow noreferrer&quot;&gt;Eden Project&lt;/a&gt; included for point of reference.&lt;/p&gt;&#xA;" OwnerUserId="11122" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2018-12-14T00:31:32.523" Title="Could permanent self-sustaining biodomes be built on Mars?" Tags="|mars|environment|habitat|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10330" PostTypeId="1" AcceptedAnswerId="10336" CreationDate="2015-07-28T20:24:57.943" Score="12" ViewCount="6344" Body="&lt;p&gt;What was the correct procedure to stop the descent engine on the Apollo 11 LEM?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the LEM had a contact light on Apollo 11, there was some terminology spoken that I'm assuming is important (and which I find quite interesting regardless).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://apollo11.spacelog.org/page/04:06:45:04/&quot; rel=&quot;noreferrer&quot;&gt;http://apollo11.spacelog.org/page/04:06:45:04/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, what of this was actually required to stop the engine? As an aside, I think it is highly relevant to ask for a clarification for the following terminology without making a separate question for each phrase. In particular the meaning of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;ENGINE STOP&quot;: Does this actually stop the engine, or is it simply a part of the steps that must be taken? If not, then what is the meaning of the other phrases spoken?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;ACA -- out of detent&quot;: What is this? A mechanical fuel pump that is stopped? What is ACA? Attitude Control Assembly (&lt;a href=&quot;https://www.hq.nasa.gov/alsj/apollo.glossary.html&quot; rel=&quot;noreferrer&quot;&gt;https://www.hq.nasa.gov/alsj/apollo.glossary.html&lt;/a&gt;)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;MODE CONTROL—both AUTO&quot;: I don't know what this is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;DESCENT ENGINE COMMAND OVERRIDE—OFF&quot;: I'm assuming this is to prevent the abort stage from firing automatically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;ENGINE ARM—OFF&quot;: Perhaps this is the actual step of stopping the engine, and the &quot;ENGINE STOP&quot; is merely announcing that they now start the procedure of stopping the engine, and this is the final step?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the LEM descent engine uses hypergolic fuel with a pintle injector, it doesn't seem like a mechanical pump is required.&lt;/p&gt;&#xA;" OwnerUserId="8707" LastEditorUserId="7062" LastEditDate="2020-03-15T05:55:42.863" LastActivityDate="2022-02-20T10:02:41.553" Title="What was the procedure to stop the descent engine on the Apollo 11 LEM?" Tags="|engine-design|terminology|lunar-landing|apollo-11|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="10332" PostTypeId="1" AcceptedAnswerId="10334" CreationDate="2015-07-28T20:47:17.257" Score="34" ViewCount="5451" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_1&quot; rel=&quot;noreferrer&quot;&gt;Sputnik's&lt;/a&gt; ball shape is instantly recognizable to most of us:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/aI6it.jpg&quot; alt=&quot;Sputnik 1&quot; title=&quot;Sputnik 1&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was looking at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_1&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia page for Luna 1&lt;/a&gt;, and saw it was also a ball-shaped spacecraft:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qcnfI.jpg&quot; alt=&quot;Luna 1&quot; title=&quot;Luna 1&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do these spacecraft have a spherical shape? Possible partial reasons (without full explanation) that I can come up with are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The sphere minimizes the surface area to volume ratio. I don't know why this would be a specific advantage though.&lt;/li&gt;&#xA;&lt;li&gt;The sphere is the strongest shape for a pressurized vessel. According to the Sputnik Wikipedia page, it was pressurized with nitrogen. However a spherical shape is not a necessity for a pressure vessel, as demonstrated by other spacecraft.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="758" LastEditorUserId="49" LastEditDate="2015-07-28T20:59:32.117" LastActivityDate="2015-07-28T21:17:50.783" Title="Why were early Soviet spacecraft spherical?" Tags="|artificial-satellite|spacecraft|soviet-union|sputnik|luna|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10333" PostTypeId="1" CreationDate="2015-07-28T21:16:30.573" Score="20" ViewCount="1291" Body="&lt;p&gt;The lunar astronauts universally described the &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/30jan_smellofmoondust/&quot; rel=&quot;nofollow noreferrer&quot;&gt;smell of moon dust&lt;/a&gt; as similar to spent gunpowder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exhaust gases from the LM's descent propulsion system (which combusts Aerozine-50 with N2O4) didn't instantly vanish from the landing site; it's estimated that &lt;a href=&quot;https://www.wired.com/2012/04/artificial-lunar-atmosphere-1974/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the mass of the moon's atmosphere was briefly doubled by LM exhaust&lt;/a&gt; during each landing. (The RCS thrusters on the LM use the same propellant combination.) In addition, the remaining contents of the descent stage oxidizer tanks were vented shortly after landing (&lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11.landing.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;102:47 in this transcript&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moon dust doesn't have a lot of nitrogen in it. Hydrazines, N2O4, and nitrocellulose/nitroglycerin gunpowders all do. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;N2O4 has &quot;&lt;a href=&quot;https://cameochemicals.noaa.gov/chemical/4075&quot; rel=&quot;nofollow noreferrer&quot;&gt;a sharp, unpleasant chemical odor&lt;/a&gt;&quot;; it vaporizes rapidly to NO2 which has &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrogen_dioxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;a characteristic sharp, biting odor&lt;/a&gt;&quot;. Combined with moisture in the astronauts' lungs and nasal passages, it could convert to nitric acid, with an &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Nitric_acid&quot; rel=&quot;nofollow noreferrer&quot;&gt;acrid, suffocating odor&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible the Apollo astronauts were smelling traces of their LM rocket exhaust, or the vented oxidizer? Call it the &quot;who smelt it, dealt it&quot; hypothesis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that to an Apollo astronaut -- i.e. a 20th-century American with military experience -- &quot;gunpowder&quot; would mean &lt;a href=&quot;https://en.wikipedia.org/wiki/Smokeless_powder&quot; rel=&quot;nofollow noreferrer&quot;&gt;smokeless powder&lt;/a&gt;, not &lt;a href=&quot;https://en.wikipedia.org/wiki/Gunpowder&quot; rel=&quot;nofollow noreferrer&quot;&gt;black powder&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-07-29T14:57:18.877" LastActivityDate="2019-07-29T14:57:18.877" Title="What does burnt Aerozine-50/N2O4 smell like?" Tags="|the-moon|apollo-program|lunar-landing|chemistry|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="10334" PostTypeId="2" ParentId="10332" CreationDate="2015-07-28T21:17:50.783" Score="39" Body="&lt;p&gt;From &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980003950.pdf&quot;&gt;this pdf&lt;/a&gt; (Reconsidering Sputnik: Forty Years Since the Soviet Satellite) at NASA.gov referring to &lt;a href=&quot;https://en.wikipedia.org/wiki/Sergei_Korolev&quot;&gt;Sergei Korolev&lt;/a&gt;, the lead rocket engineer for the Sputnik project:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There were many debates on the shape of the first satellite, with most senior OKB-1 designers preferring a conical form since it fit well with the nose cone of the rocket. At a meeting early in the year, Korolev had a change-of-heart and suggested a metal sphere at least one meter in diameter. There were six major guidelines followed in the construction of PS-l:&lt;br /&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;the satellite would have to be of maximum simplicity and reliability while keeping in mind that methods used for the spacecraft&#xA;  would be used in future projects;&lt;/li&gt;&#xA;  &lt;li&gt;the body of the satellite was to be spherical in order to determine atmospheric density in its path;&lt;/li&gt;&#xA;  &lt;li&gt;the satellite was to be equipped with radio equipment working on at least two wavelengths of sufficient power to be tracked by amateurs&#xA;  and to obtain data on the propagation of radio waves through the&#xA;  atmosphere;&lt;/li&gt;&#xA;  &lt;li&gt;the antennae were to be designed so as to not affect the intensity of the radio signals due to spinning;&lt;/li&gt;&#xA;  &lt;li&gt;the power sources were to be onboard batteries ensuring work for two to three weeks; and&lt;/li&gt;&#xA;  &lt;li&gt;the attachment of the satellite to the core stage would be such that there would be no failure to separate.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also (interestingly, in my opinion at least), according to the contents of that same pdf, apparently it was polished in an attempt to help prevent it from overheating:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Korolev came over to the shop and insisted that both halves of the&#xA;  sputnik's metallic sphere be polished until they shone, that they be&#xA;  spotlessly clean,&quot; recalled Konstantin Feoktistov, who would be the&#xA;  first engineer-cosmonaut to go into orbit in the three-man Voskhod 1&#xA;  seven years later. &quot;The people who developed the radio equipment were&#xA;  actually the ones demanding this. They were afraid of the system&#xA;  overheating, and they wanted the orbiting sphere to reflect as many&#xA;  rays of the Sun as possible.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2015-07-28T21:17:50.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10335" PostTypeId="1" AcceptedAnswerId="10353" CreationDate="2015-07-28T21:50:32.487" Score="14" ViewCount="585" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/DRAKON&quot; rel=&quot;nofollow&quot;&gt;DRAKON&lt;/a&gt; is a Russian language standard programming interface which was created for the Soviet space program.  It had the added benefit of being a visual language, and possibly the first for the Soviet space program.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wanted to find out if there are any space programs still using the language today?  Either new, or old equipment still functioning.&lt;/p&gt;&#xA;" OwnerUserId="11104" LastEditorUserId="10132" LastEditDate="2015-07-30T01:16:47.147" LastActivityDate="2015-07-30T01:16:47.147" Title="Is DRAKON Still in use Today?" Tags="|spacecraft|operations|language|soviet-union|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10336" PostTypeId="2" ParentId="10330" CreationDate="2015-07-28T21:54:57.240" Score="13" Body="&lt;p&gt;The &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a11/a11.landing.html#1024540&quot; rel=&quot;nofollow noreferrer&quot;&gt;annotated transcripts&lt;/a&gt; are great for this stuff. Also &lt;a href=&quot;https://books.google.com/books?id=3fKzL0HfJp4C&amp;amp;pg=PA285&amp;amp;lpg=PA285&amp;amp;dq=LM%20engine%20stop%20button&amp;amp;source=bl&amp;amp;ots=cG3V4jqXjJ&amp;amp;sig=6Nr6GXRjY0L-MZGxvhNsKOrjv3E&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CC4Q6AEwBWoVChMIo5KJ0en-xgIVTDOICh3X1g4l#v=onepage&amp;amp;q=LM%20engine%20stop%20button&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;O'Brien's AGC book&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Engine Stop&lt;/strong&gt;: The engine stop button is pressed, which I believe would set throttle immediately to zero. In this picture, the stop button assembly is visible between the yellow handle and the start button, pointed leftward. Presumably the placement of it is chosen to avoid accidental shutdown.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/obIBM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/obIBM.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Once the contact light came on (about 1.6m above the surface), the commander would hit the button, the engine would stop, and landing would be inevitable.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://books.google.com/books?id=x-taL4N0sjIC&amp;amp;pg=PA307&amp;amp;lpg=PA307&amp;amp;dq=aca%20detent%20apollo&amp;amp;source=bl&amp;amp;ots=v84zJVoPOT&amp;amp;sig=3tCSSxkG3pWlG6RXeK9ckFdi1Sk&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CE0Q6AEwB2oVChMImZzq8Ob-xgIVwZWICh3rjwDv#v=onepage&amp;amp;q=aca%20detent%20apollo&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;ACA out of detent&lt;/a&gt;&lt;/strong&gt;:&#xA;The control system was in &amp;quot;attitude hold&amp;quot; mode; as long as the commander wasn't actively trying to rotate the ship, the system would try to maintain the LM in fixed orientation. At touchdown, the attitude control thrusters would be trying to hold the last commanded attitude. By pushing the Attitude Controller Assembly just barely out of its neutral (detented) position, the system is instructed that the current attitude is the desired attitude, stopping the RCS rockets from firing.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Mode control both auto&lt;/strong&gt;: I assume this is related to the guidance mode control -- automatic landing (LM commander designates a landing spot and the LM computer does the rest) versus attitude hold (LM computer holds the LM's orientation stable and the commander does the rest). I imagine you'd want it back in automatic mode in case you needed to do an emergency ascent. Not sure what &amp;quot;both&amp;quot; refers to; best guess would be that the PGNS (primary guidance) and AGS (abort guidance) each had mode controls.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Descent engine command override&lt;/strong&gt;: Again from the annotated transcript:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Command Override is a shorthand for 'Descent Engine Command Override', which will allow either pilot to assume control of the descent engine's thrust using the Thrust/Translational Hand Controllers, if required.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Engine arm off&lt;/strong&gt;: I think this is just a master switch for the descent engine, independent of the throttle. In the transcript, Aldrin arms the descent engine at 102:32:34, but ignition occurs some 30 seconds later.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2022-02-18T14:46:41.960" LastActivityDate="2022-02-18T14:46:41.960" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="10337" PostTypeId="2" ParentId="10333" CreationDate="2015-07-28T22:06:06.023" Score="12" Body="&lt;p&gt;No, I don't think so.  Combusted hydrazine smells like ammonia, because it &lt;em&gt;is&lt;/em&gt; ammonia, plus nitrogen and hydrogen.  Unburnt hydrazine also smells like ammonia, but has a sharp sting to it that, if you smell it, you should leave that building.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-07-28T22:06:06.023" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10338" PostTypeId="5" CreationDate="2015-07-28T22:16:12.763" Score="0" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_1&quot; rel=&quot;nofollow&quot;&gt;Sputnik 1&lt;/a&gt; was the first artificial earth satellite.&#xA;Sputnik may also refer to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_2&quot; rel=&quot;nofollow&quot;&gt;Sputnik 2&lt;/a&gt; - the first spacecraft to carry a living animal; the dog Laika&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_3&quot; rel=&quot;nofollow&quot;&gt;Sputnik 3&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_(rocket)&quot; rel=&quot;nofollow&quot;&gt;Sputnik Rocket&lt;/a&gt; - Launch vehicle for the above.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="758" LastEditorUserId="758" LastEditDate="2015-07-28T22:42:42.850" LastActivityDate="2015-07-28T22:42:42.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10339" PostTypeId="4" CreationDate="2015-07-28T22:16:12.763" Score="0" Body="The Sputnik series of spacecraft achieved several firsts in the late 1950s, most notably Sputnik 1, which was the first artificial earth satellite." OwnerUserId="758" LastEditorUserId="758" LastEditDate="2015-07-28T22:42:27.637" LastActivityDate="2015-07-28T22:42:27.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10340" PostTypeId="5" CreationDate="2015-07-28T22:29:33.493" Score="0" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_programme&quot; rel=&quot;nofollow&quot;&gt;Luna programme&lt;/a&gt; was a series of robotic spacecraft missions sent to the Moon by the Soviet Union between 1959 and 1976.&lt;/p&gt;&#xA;" OwnerUserId="758" LastEditorUserId="758" LastEditDate="2015-07-28T22:42:38.260" LastActivityDate="2015-07-28T22:42:38.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10341" PostTypeId="4" CreationDate="2015-07-28T22:29:33.493" Score="0" Body="The Luna programme was a series of robotic spacecraft missions sent to the Moon by the Soviet Union between 1959 and 1976." OwnerUserId="758" LastEditorUserId="758" LastEditDate="2015-07-28T22:42:48.003" LastActivityDate="2015-07-28T22:42:48.003" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10342" PostTypeId="1" AcceptedAnswerId="10343" CreationDate="2015-07-28T22:46:08.323" Score="23" ViewCount="3551" Body="&lt;p&gt;I just learned today about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_15&quot;&gt;Luna 15 mission&lt;/a&gt; which was a Soviet attempt to land an unmanned probe on the moon which coincided with the successful Apollo 11 mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were the Apollo 11 astronauts aware in any way of the the Luna 15 mission?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.telegraph.co.uk/news/science/space/5737854/Russian-spacecraft-landed-on-moon-hours-before-Americans.html&quot;&gt;According to this article&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Jodrell_Bank_Observatory&quot;&gt;Jodrell Bank&lt;/a&gt; was tracking Luna 15.  Was this information relayed to NASA, and was it given to the Apollo 11 astronauts during their mission?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Were the Apollo 11 astronauts able to intercept radio transmissions from Luna 15?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Were the Apollo 11 astronauts able to see the Luna 15 spacecraft or its rocket trail as it orbited the moon?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="758" LastEditorUserId="758" LastEditDate="2015-07-29T04:26:02.460" LastActivityDate="2015-12-16T21:29:09.183" Title="Were the Apollo 11 astronauts aware of any parts of the Luna 15 mission?" Tags="|the-moon|apollo-program|space-race|luna|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10343" PostTypeId="2" ParentId="10342" CreationDate="2015-07-28T22:58:47.897" Score="29" Body="&lt;p&gt;Yes, they were aware. According to the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11transcript_tec.html&quot;&gt;Apollo transcript&lt;/a&gt;, Luna 15 was prominent in the daily news items that mission control shared with the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the transcript, timestamps are day/hour/minute/second; speakers are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;CC = CAPCOM&lt;/li&gt;&#xA;&lt;li&gt;CDR = Commander (Armstrong)&lt;/li&gt;&#xA;&lt;li&gt;LMP = Lunar Module Pilot (Aldrin)&lt;/li&gt;&#xA;&lt;li&gt;CMP = Command Module Pilot (Collins)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;23 hours into the mission (i.e. on July 17), we have the first mention of Luna 15:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;00 23 14 09 CC 11, this is Houston. If you are interested in the&#xA;  morning news, I have a summary here from PAO. Over. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;00 23 14 20 CDR Okay. We're all listening. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;00 23 14 23 CC Okay. From Jodrell Bank, England, via AP: Britain's big&#xA;  Jodrell Bank radio telescope stopped receiving signals from the Soviet&#xA;  Union's unmanned Moon shot at 5:49 EDT today. A spokesman said that it&#xA;  appeared the Luna 15 space ship &quot;had gone beyond the Moon.&quot; Another&#xA;  Quote: &quot;We don't think it has landed,&quot; said a spokesman for Sir&#xA;  Bernard Lovell, Director of the Observatory.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Buzz Aldrin was probably the most interested in &lt;a href=&quot;http://dspace.mit.edu/handle/1721.1/12652&quot;&gt;spacecraft trajectories&lt;/a&gt; among the crew, so small surprise that he was the one to inquire the next day:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;01 08 53 58 LMP Roger. What's the latest on Luna 15? &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;01 08 54 02 CC Stand by. I'll get the straight story for you. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;01 09 37 44 CC Hello, Apollo 11. Houston. Over. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;01 09 37 48 CMP Go ahead, Charlie. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;01 09 37 50 CC Roger. Latest on Luna 15 - TASS reported this morning&#xA;  that the spacecraft was placed in orbit close to the lunar surface,&#xA;  and everything seems to be functioning normally on the vehicle. Sir&#xA;  Bernard Lovell said the craft appears to be in an orbit of about 62&#xA;  nautical miles. Over.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Three days in, before the landing, another news item:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;03 00 31 11 CC In Moscow, space engineer Anatol Koritsky was quoted by&#xA;  TASS as saying that Luna 15 could accomplish everything that has been&#xA;  done by earlier Luna spacecraft. This was taken by the press to mean&#xA;  Luna 15 could investigate the gravitational fields, photograph the&#xA;  Moon, and go down to the surface to scoop up a bit for analysis.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Much later, after leaving the moon, another daily news update from CAPCOM:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;06 04 23 13 CC Luna 15 is believed to have crashed into the Sea of&#xA;  Crises yesterday after orbiting the Moon 52 times. The Soviet News&#xA;  Agency TASS reported that &quot;scientific research in near Moon space was&#xA;  carried out.&quot; Sir Bernard Lovell at Jodrell Bank Observatory said that&#xA;  Luna 15 hit the surface of the Moon at a speed of about 300 miles per&#xA;  hour.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's no mention of Luna 15 in the transcript outside of the world news context; it's very unlikely they would have seen it. Space is big, exhaust plumes are small, rocket burns are short, and spacecraft windows are surprisingly dirty. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-08-25T14:07:02.510" LastActivityDate="2015-08-25T14:07:02.510" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10344" PostTypeId="2" ParentId="10233" CreationDate="2015-07-28T23:35:25.013" Score="0" Body="&lt;p&gt;Occasionally traverse, no--Blake Walsh addressed this case fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it's not theoretically impossible to have a &quot;moon&quot; that orbits neither planet but rather goes back and forth between them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice you'll never find this because it's an unstable path and soon would wander off.  Put a guidance system and some engines on it and you could have a traveling &quot;moon&quot;, though.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-07-28T23:35:25.013" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10346" PostTypeId="1" AcceptedAnswerId="10371" CreationDate="2015-07-29T07:52:02.210" Score="29" ViewCount="6688" Body="&lt;p&gt;As per &lt;a href=&quot;https://space.stackexchange.com/questions/3520/when-will-new-horizons-overtake-voyager-1-in-distance-from-the-sun/3522#comment27123_3522&quot;&gt;@kasperd's comment&lt;/a&gt; on another question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the graph below, it appears that Voyager 2 started at just over escape velocity of the solar system. Just before the Jupiter flyby, it was travelling much slower than escape velocity. Presumably, travelling to Jupiter the only thing that would have changed the probe's velocity is the Sun's gravity. As the probe went further away from the Sun, you would expect it to slow down, but why would it drop below escape velocity if it started above it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/F8DlO.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/F8DlO.png&quot; alt=&quot;Graph on Voyager 2 velocity&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8157" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2023-07-13T18:51:28.720" Title="Why did Voyager 2's velocity drop far below escape velocity before the first gravity assist?" Tags="|gravity|voyager|delta-v|escape-velocity|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10347" PostTypeId="2" ParentId="10346" CreationDate="2015-07-29T08:31:35.223" Score="19" Body="&lt;p&gt;The image (&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Voyager_2_velocity_vs_distance_from_sun.svg&quot; rel=&quot;noreferrer&quot;&gt;original at Wikimedia Commons&lt;/a&gt;) is only an approximation, as evidenced by the noticeable change in shape of the solar system escape velocity line at 14AU. The line is only defined with three points, and my guess would be that the creator of the graph tried to shape the curve manually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity#List_of_escape_velocities&quot; rel=&quot;noreferrer&quot;&gt;According to Wikipedia&lt;/a&gt; the solar system escape velocity at the earth's orbit (1 AU) is 42.1km/s, considerably more than the ~35.5km/s of this graph. The graph's author notes that it is based on &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf16-22.gif&quot; rel=&quot;noreferrer&quot;&gt;this image&lt;/a&gt;, which despite being from NASA, seems to share the same inaccuracies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_artificial_objects_escaping_from_the_Solar_System&quot; rel=&quot;noreferrer&quot;&gt;another Wikipedia page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The only objects to date to be launched directly into a solar escape trajectory were the New Horizons spacecraft, its third stage and its two small de-spin masses.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8657" LastEditorUserId="415" LastEditDate="2015-07-31T09:33:43.753" LastActivityDate="2015-07-31T09:33:43.753" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10349" PostTypeId="2" ParentId="7792" CreationDate="2015-07-29T12:28:45.017" Score="5" Body="&lt;p&gt;The closest to a &quot;mandate&quot; in recent times was George Bush's &lt;a href=&quot;http://history.nasa.gov/Bush%20SEP.htm&quot; rel=&quot;noreferrer&quot;&gt;&quot;New Vision&quot;&lt;/a&gt; in 2004. We would not be satisfied with a mere lunar lander today, with a few walks, we would want instead to have a longer base, perhaps more people, etc. The New Vision program was to develop 2 new spacecraft, the Ares I and Ares V/ Altair. Ares I later evolved into the COTS program/ , it was intended for LEO orbit. One part of that still remains, the Orion program. Ares V was intended to launch a much larger payload to space, with the Moon being the primary target. The goal was to have Ares I in 10 years, and Ares V/ Altair about 5 years after that. This of course all changed after George Bush left office as president. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.nasa.gov/pdf/289914main_fs_altair_lunar_lander.pdf&quot; rel=&quot;noreferrer&quot;&gt;Altair&lt;/a&gt; mission was to send humans to the Moon for up to 7 days initially at a time, turning the Moon into a research base similar to the Antarctic research base. It would land 4 people on the Moon, with an orbiting Orion vehicle for Earth return. Orion would remain unmanned for its duration around the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that if the mandate were given today, it would be very similar to the Altair mission, in that it would have an Earth rendezvous, unmanned capsule remaining around the Moon, and a longer mission on the Moon with more people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it became really important to make this happen quickly, it could be done, at more cost and risk. The US public isn't as willing to spend huge sums of money on the Moon as it once was, nor take significant risks. The end result is that additional time would be required to make this happen, at a lower cost and risk.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-29T12:28:45.017" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10350" PostTypeId="1" AcceptedAnswerId="10352" CreationDate="2015-07-29T13:29:02.737" Score="33" ViewCount="11576" Body="&lt;p&gt;My understanding is the shuttle was designed as &lt;strong&gt;cost-effective&lt;/strong&gt; workhorse, a space-van to ferry cargo up to the ISS, conduct zero-G experiments and put satellites into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why didn't NASA take money from the private sector to do these later two on the behalf of corporations? Or even restock the ISS on the behalf of other government organizations such as ESA and RFSA? I don't feel the public would have objected, if all the money had all gone on future space exploration. Maybe NASA could even have become a self-sustaining government organization, shutting up all nay sayers who think space exploration is a waste of taxes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or was the space shuttle program a huge financial failure, considering Russia scrapped their shuttles, the ISS gets restocked by capsules launched by traditional staged rockets (Soyuz and Falcon 9?) and private companies launch their new satellites with staged rockets?&lt;/p&gt;&#xA;" OwnerUserId="11135" LastEditorUserId="22203" LastEditDate="2023-12-14T15:13:00.150" LastActivityDate="2023-12-14T15:13:00.150" Title="Why didn't NASA use the shuttle to make a profit?" Tags="|launch|nasa|space-shuttle|economics|cost-to-orbit|" AnswerCount="7" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10351" PostTypeId="2" ParentId="10350" CreationDate="2015-07-29T13:40:00.130" Score="41" Body="&lt;p&gt;The Space Shuttle was designed to be inexpensive, but in the first launch, it was actually found to be quite a bit more expensive than it was thought. The shuttle was supposed to take only a week or two to turn around, in which case it would have been a very effective launch vehicle. Instead, the tiles ended up needing replacing after every flight, which ended up taking months, instead of a week. If you can't launch the Shuttle very often, then the cost goes up enormously. As it stands, the cost to launch the Shuttle was about $450 million per mission, far higher than even the most expensive rocket today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Post-Challenger, there were additional restrictions on what items the Shuttle could carry into space. This became an issue for the Galileo mission, for instance, that originally intended to have another stage on board, that was removed because of potential issues. There are many material limitations on what the Shuttle would carry, due to it's requirement to protect the humans on board. Also, the cost of life of an Astronaut had to be considered any time the Shuttle was launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the zero g experiments, NASA was willing to host some of those, with the request that the science all be returned to the public. Feel free to take a look at the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/experiments_by_partners.html&quot;&gt;partners&lt;/a&gt; that NASA had with the shuttle experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early days of the Shuttle, as @OrganicMarble mentioned, NASA did launch some commercial satellites. Post-Challenger, it was deemed too risky to do so, and instead focused on scientific and military payloads. Afterward, NASA did occasionally launch some satellites, but mostly only for schools as a means to inspire people to the future. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA might some day be able to take funds, but it isn't set up right now. And I don't think anyone would actually pay NASA to launch anything, it's costs are far higher than any rocket launch provider.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-29T14:15:26.967" LastActivityDate="2015-07-29T14:15:26.967" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="10352" PostTypeId="2" ParentId="10350" CreationDate="2015-07-29T13:49:00.970" Score="77" Body="&lt;p&gt;Your question is based on a false assumption namely:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;So why didn't NASA take money from the private sector to do these&#xA;later two on the behalf of corporations?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;NASA &lt;em&gt;did&lt;/em&gt; take money from the private sector to do these things.  The STS-5, STS-7, STS-8, STS-41-B, STS-41-D, STS-51-A, STS-51-D, STS-51-G, STS-51-I, STS-61-B, and STS-61-C Space Shuttle missions all launched comsats for paying customers.  The original plan, ridiculous as it was in hindsight, was for STS to be the sole US launcher of all payloads.&lt;/p&gt;&#xA;&lt;p&gt;However, after the loss of the Orbiter Challenger only seconds into mission STS-51-L, US policy was changed to only allow the Shuttle to launch scientific and military payloads that required human presence.  This destroyed any economic rationale for making STS self supporting or a revenue generator.&lt;/p&gt;&#xA;&lt;p&gt;The first 4 missions were test flights, so omitting them, you can see that 11 of the 21 &amp;quot;operational&amp;quot; missions before the policy change carried commercial satellite payloads, often multiple, up to 3 comsats per mission. A total of 18 commercial satellites were deployed from the Shuttle. Commercial payloads were very important.  Coke and Pepsi even paid to fly their products on STS for promotional purposes.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ExhKK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ExhKK.jpg&quot; alt=&quot;photo of an experimental Coke machine flown on the shuttle&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Now, did the planned economics make sense? Almost certainly never.  Cost projections were based on unrealistic flight rates so the efficiencies of scale were never going to happen.&lt;/p&gt;&#xA;&lt;p&gt;Data on early comsat launches culled from &lt;a href=&quot;https://ntrs.nasa.gov/api/citations/20110001406/downloads/20110001406.pdf&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Missions Summary&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Shuttle commercial launch pricing policy, circa 1985:&lt;/p&gt;&#xA;&lt;p&gt;(Text on page before scanned page:)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The price charged to a customer is based on the percentage of the&#xA;Orbiter's weight or length capability that the customer's payload&#xA;uses. To determine these load factors, the total weight (including all&#xA;support equipment) and the total length (including 6 inches for&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VK9PE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VK9PE.jpg&quot; alt=&quot;page from the cited document showing that payload pricing was a function of the &amp;quot;load factor&amp;quot; - the fraction of available payload bay size or throw weight the payload consumed&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Source: &lt;em&gt;&lt;a href=&quot;https://archive.org/download/jsc-21000-hbk-sts-customer-accomodations-a-handbook-for-space-shuttle-users/JSC-21000-HBK_STS_Customer_Accomodations_A_Handbook_for_Space_Shuttle_Users.pdf&quot; rel=&quot;noreferrer&quot;&gt;STS Customer Accomodations JSC-21000-HBK&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-11-09T17:59:52.790" LastActivityDate="2021-11-09T17:59:52.790" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="10353" PostTypeId="2" ParentId="10335" CreationDate="2015-07-29T14:35:37.463" Score="9" Body="&lt;h1&gt;Is it still in use today?&lt;/h1&gt;&#xA;&lt;p&gt;Considering the &lt;a href=&quot;http://sourceforge.net/projects/drakon-editor/&quot; rel=&quot;noreferrer&quot;&gt;sourceforge site for a DRAKON editor&lt;/a&gt; shows over 100 downloads this week and it was last updated June 1st, 2015 I'd say that project is active, which strongly suggests the language is still in use.&lt;/p&gt;&#xA;&lt;h1&gt;Are there any space programs still using the language today?&lt;/h1&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/DRAKON&quot; rel=&quot;noreferrer&quot;&gt;wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The work was finished in 1996 (3 years after the Buran project was officially closed), when an automated CASE programming system called &amp;quot;Grafit-Floks&amp;quot; was developed.&lt;/p&gt;&#xA;&lt;p&gt;This CASE is used since 1996 in many major space programs: an international project Sea Launch, Russian orbit insertion upper stage Fregat (Russian: Фрегат, frigate), upgraded heavy launch vehicle (carrier rocket) Proton-M, etc.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(CASE standing for &lt;a href=&quot;https://en.wikipedia.org/wiki/Computer-aided_software_engineering&quot; rel=&quot;noreferrer&quot;&gt;Computer Aided Software Engineering&lt;/a&gt;, basically the editor for developing in DRAKON)&lt;/p&gt;&#xA;&lt;p&gt;Now the question becomes: do Fregat or Proton-M still fly?&lt;/p&gt;&#xA;&lt;p&gt;As of 2012 Fregat was in &lt;a href=&quot;http://www.russianspaceweb.com/fregat.html&quot; rel=&quot;noreferrer&quot;&gt;active production&lt;/a&gt; for flight with 10+ stages produced per year.  I haven't found any reference to it no longer being used, and it appears to still be on offer on the &lt;a href=&quot;http://www.arianespace.com/launch-services-soyuz/soyuz-introduction.asp&quot; rel=&quot;noreferrer&quot;&gt;Arianespace Soyuz Launch Services&lt;/a&gt; page as of July 2015.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Proton_launches_%282010%E2%80%93present%29&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; lists 3 Proton-M launches so far in 2015, though the last one failed.&lt;/p&gt;&#xA;&lt;p&gt;That gives at least two ongoing programs that use DRAKON.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-30T01:14:00.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10354" PostTypeId="1" AcceptedAnswerId="10356" CreationDate="2015-07-29T14:48:34.860" Score="9" ViewCount="1538" Body="&lt;p&gt;On 29 July 2015, NASA &lt;a href=&quot;http://www.nasa.gov/image-feature/africa-and-europe-from-a-million-miles-away/&quot; rel=&quot;noreferrer&quot;&gt;released&lt;/a&gt; a second image from the Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate Observatory (DSCOVR) satellite:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MdmzW.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MdmzW.jpg&quot; alt=&quot;Second released EPIC image&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you zoom in on the full image, you will find a purple dot in the middle of the Egyptian desert:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TLl9X.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TLl9X.png&quot; alt=&quot;Purple dot&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This does not look physical, and it appears to be exactly in the middle.  I don't find it in the &lt;a href=&quot;http://www.nasa.gov/press-release/nasa-satellite-camera-provides-epic-view-of-earth&quot; rel=&quot;noreferrer&quot;&gt;first released EPIC image&lt;/a&gt;.  It looks like some instrumental artefact.  Why is it there?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="25" LastEditDate="2015-07-29T15:04:35.760" LastActivityDate="2015-07-29T16:05:00.060" Title="Why is there a purple dot in the middle of this EPIC image?" Tags="|imaging|camera|instrument|dscovr|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10355" PostTypeId="1" AcceptedAnswerId="10360" CreationDate="2015-07-29T15:09:22.993" Score="34" ViewCount="9099" Body="&lt;p&gt;The L-2 point of the Sun-Earth system is away from the Earth on the night-side of the Earth; i.e. it's always local midnight at the sub-satellite point.  This is an attractive property for &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#Spacecraft_at_Sun.E2.80.93Earth_L2&quot; rel=&quot;noreferrer&quot;&gt;some spacecraft&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/et8iF.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/et8iF.png&quot; alt=&quot;Sun-Earth Lagrange points&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Lagrange_points_simple.svg&quot; rel=&quot;noreferrer&quot;&gt;Illustration from Wikimedia Commons&lt;/a&gt;.  Not to scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fortunately for solar-powered spacecraft, &lt;a href=&quot;https://space.stackexchange.com/a/3573/33&quot;&gt;the Sun is still visible&lt;/a&gt;, or those satellites couldn't be solar powered.  But the disk of the Earth must be a sizeable fraction of the disk of the Sun (this should affect energy budgets too...).  What does the Sun-Earth-Moon system look like from the Earth-Sun L-2 point?  It should be extra interesting when Earth-based observers see a lunar eclipse.  Have there been any photos of Earth taken from this point?&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2022-02-07T18:23:13.187" Title="What does the Sun-Earth-Moon system look like from the Sun-Earth L-2 point?" Tags="|orbit|lagrangian-points|earth|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10356" PostTypeId="2" ParentId="10354" CreationDate="2015-07-29T15:21:04.713" Score="16" Body="&lt;p&gt;&lt;a href=&quot;http://www.nesdis.noaa.gov/DSCOVR/pdf/DSCOVR%20-%20EPIC%20Instrument%20Info%20Sheet.pdf&quot; rel=&quot;noreferrer&quot;&gt;EPIC&lt;/a&gt; (PDF) is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassegrain_reflector&quot; rel=&quot;noreferrer&quot;&gt;Cassegrain type reflector&lt;/a&gt; telescope so there's the fixed hyperbolic secondary mirror in the middle of the telescopes light path / focal plane. While that could be removed during post-processing and combining multiple exposures focusing at slightly different angles of the telescope itself (or shifting of the sensor on the focal plane, depends on its exact design), it is also the region that would be most susceptible to even the slightest of deformations, or slight misalignment, possibly due to thermal cycling, of both the mirrors that are fixed and were &lt;a href=&quot;https://en.wikipedia.org/wiki/Collimated_light&quot; rel=&quot;noreferrer&quot;&gt;collimated&lt;/a&gt; before flight:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cassegrain_reflector#/media/File:Cassegrain.en.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/60EPP.png&quot; alt=&quot;Light path in a Cassegrain reflecting telescope&quot; title=&quot;Light path in a Cassegrain reflecting telescope&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, due to DSCOVR's vantage point from Sun-Earth &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Lagrangian point&lt;/a&gt; 1 (SEL1), the telescope would focus on nearly 100% (99.6% of its &lt;a href=&quot;https://en.wikipedia.org/wiki/Great_circle&quot; rel=&quot;noreferrer&quot;&gt;great circle&lt;/a&gt;, to be precise) illuminated Earth with the Sun behind it. That ought to create a central region of &lt;a href=&quot;https://en.wikipedia.org/wiki/Opposition_surge#Coherent_backscatter&quot; rel=&quot;noreferrer&quot;&gt;coherent backscatter&lt;/a&gt; which would be difficult to remove even with multiple exposures, since it would be always present regardless of the telescope's slightly changing angle. So it would appear as an overexposed region, that might appear as pretty much anything after post-processing, including the &lt;a href=&quot;https://en.wikipedia.org/wiki/Moir%C3%A9_pattern&quot; rel=&quot;noreferrer&quot;&gt;Moiré fringed&lt;/a&gt; dot with typical read and blue channel interference pattern around it on the image that you noticed, once the RGB sources, or slightly different angles, or both, are interpolated. Notice that there's also a bit more of color bleeding around the central dot, than elsewhere on the image, with odd yellow-green hue patches for somewhere in the middle of the Saharan desert.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-29T16:05:00.060" LastActivityDate="2015-07-29T16:05:00.060" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10357" PostTypeId="2" ParentId="10355" CreationDate="2015-07-29T15:30:36.270" Score="25" Body="&lt;p&gt;The spacecraft that orbit at L2 usually don't just stay at the L2 point, but do what's called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Halo_orbit&quot;&gt;Halo Orbit&lt;/a&gt;, or related &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot;&gt;Lissajous Orbit&lt;/a&gt;. Essentially they orbit the L2 point, instead of right at it. As a result, they actually see the Earth and Moon as distinct from the Sun. And they usually orbit the point in such a way that they will not have a direct line from Earth to Sun, which makes communication much easier (The Sun produces a significant amount of RF noise)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the view, if at the exact L2 point, the Earth would show up as a disc about 0.48 degrees, and would thus actually be in partial eclipse perpetually, as the sun is a disc about 0.53 degrees from the L2 point. Thus, most (~82%) of the Sun would be covered all of the time by the Earth, with slightly more if the Moon was also covering the Sun. The atmosphere of Earth would also cover some, perhaps another few percent. This would be a significant annular eclipse. A typical annular eclipse on Earth of the Moon is over 90%.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-07-29T21:51:47.403" LastActivityDate="2015-07-29T21:51:47.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10360" PostTypeId="2" ParentId="10355" CreationDate="2015-07-29T19:40:52.653" Score="56" Body="&lt;p&gt;I treated this as a problem of geometry and came up with this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GxCzR.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GxCzR.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The sun is the large yellow disk.&lt;/li&gt;&#xA;&lt;li&gt;The earth is the largest black disk, obscuring most of the sun&lt;/li&gt;&#xA;&lt;li&gt;The left-hand dark-grey disk is the moon as it transits across the near-side of the earth, with respect to L2.  In reality in this position, the disk of the moon would appear completely black.  I made it dark-gray for illustrative purposes.&lt;/li&gt;&#xA;&lt;li&gt;The right-hand dark-grey disk is the moon as it transits behind the far-side of the earth, with respect to L2.  In this position, the moon might not appear as a completely black disk - its possible it may be illuminated somwwhat by light reflected off the back of the earth.  However this effect may well be minimal given the overwhelming power of direct light from the annulus of the sun.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;My numbers look a little different to @PearsonArtPhoto's, so here they are:&lt;/p&gt;&#xA;&lt;p&gt;Averages from wikipedia/google:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Sun-Earth distance: 150 million Km&lt;/li&gt;&#xA;&lt;li&gt;Earth-Moon distance: 384,000 Km&lt;/li&gt;&#xA;&lt;li&gt;Earth-L2 distance: 1.5 million Km&lt;/li&gt;&#xA;&lt;li&gt;Sun radius: 696,000 Km&lt;/li&gt;&#xA;&lt;li&gt;Earth radius: 6370 Km&lt;/li&gt;&#xA;&lt;li&gt;Moon radius: 1740 Km&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;From these I calculated:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Sun-L2 distance: 151.5 million Km&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Sun radius angle: 4590 μrad&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Earth radius angle: 4250 μrad&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Near moon-L2 distance: 1.12 million Km&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Near moon-L2 radius angle: 1560 μrad&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Far moon-L2 distance: 1.88 million Km&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Far moon-L2 radius angle: 924 μrad&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Notes - my terminology may be inaccurate.  Here are some clarifications:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;In all the above, I am using &lt;code&gt;L2&lt;/code&gt; to refer to the sun-earth L2, i.e. the location of the observer.&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;radius angle&lt;/code&gt; is the angle subtended from the center of the given object to the edge of its disk&lt;/li&gt;&#xA;&lt;li&gt;Since angles are small, I used the approximation &lt;code&gt;x ≈ tan(x)&lt;/code&gt;&lt;/li&gt;&#xA;&lt;li&gt;The sizes of the objects I have drawn are directly proportional to their calculated radius angles.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Update:&lt;/h3&gt;&#xA;&lt;p&gt;I just saw &lt;a href=&quot;https://www.nasa.gov/feature/goddard/from-a-million-miles-away-nasa-camera-shows-moon-crossing-face-of-earth&quot; rel=&quot;noreferrer&quot;&gt;this from NASA today&lt;/a&gt;.  While this is taken from L1, and this question is about L2, these two Lagrange points are similar distances from the Earth (in opposite directions), so we can see that the proportions, at least of the Earth and moon, are more or less correct in my simulation.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Earth from L1 in photo&lt;/strong&gt; (similar distance to L2, obviously lit differently though)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/iTSBX.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iTSBX.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="758" LastEditorUserId="758" LastEditDate="2022-02-07T18:23:13.187" LastActivityDate="2022-02-07T18:23:13.187" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="10361" PostTypeId="1" AcceptedAnswerId="10363" CreationDate="2015-07-29T20:48:51.523" Score="11" ViewCount="2373" Body="&lt;p&gt;There is much variation in the number of engines used in the liftoff stage of various launchers.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Saturn V had &lt;strong&gt;5&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Space shuttles had &lt;strong&gt;5&lt;/strong&gt; (3 main + 2 SRB)&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 has &lt;strong&gt;9&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Soyuz has &lt;strong&gt;20&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;N1 had &lt;strong&gt;30&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Delta Heavy has &lt;strong&gt;3&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Long March 3B appears to have &lt;strong&gt;4&lt;/strong&gt; main + &lt;strong&gt;2&lt;/strong&gt; or &lt;strong&gt;4&lt;/strong&gt; boosters&lt;/li&gt;&#xA;&lt;li&gt;Ariane 5 had &lt;strong&gt;1&lt;/strong&gt; main + &lt;strong&gt;2&lt;/strong&gt; boosters&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;While the size and performance of these launchers varies, I can't see any straightforward correlation between those factors and the number of engines.  The only correlation I can see is that Soviet/Russian launchers have a whole lot more and smaller engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the design tradeoffs when deciding to use many small engines vs few large engines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does the Soviet/Russian calculus seem to be so different to everyone elses in this regard?&lt;/p&gt;&#xA;" OwnerUserId="758" LastEditorUserId="195" LastEditDate="2015-07-30T17:17:53.463" LastActivityDate="2017-05-28T05:38:33.457" Title="Why is there so much variation in the number of engines on launchers?" Tags="|launch|rockets|design|engines|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10362" PostTypeId="2" ParentId="10361" CreationDate="2015-07-29T22:33:28.643" Score="12" Body="&lt;p&gt;Everything's a tradeoff. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Designing and building small, low-thrust engines is generally easier than designing and building large, high-thrust engines, but the larger engines may be more efficient. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The more engines you have, the more likely one or more are to fail in any given launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you have more than 5 or so, you may be able to continue on if one fails (depending on which engine, and at what point in the ascent it happens). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The more engines you have, the harder it is to predict how they may interact; this is likely part of what contributed to the &lt;a href=&quot;https://en.wikipedia.org/wiki/N1_(rocket)#Development_problems&quot; rel=&quot;nofollow noreferrer&quot;&gt;failure of the N1.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that N1 never flew successfully, and Soyuz is really a 4 booster + 1 sustainer first stage, the &quot;Russians use more engines&quot; assertion is misleading. Energia was 4+4, but flew only twice; Zenit uses a single 4-chamber engine; Proton uses 6 single-chamber engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Almost every engine and launcher in production has a complex development history; sometimes your requirements change and you wind up building a much larger launcher than you had expected to. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Engine development generally takes longer than the rest of launcher development (tankage, avionics, airframe, etc.), so you often don't have time to develop the optimal engine for a new launcher. During the height of the Cold War/Space Race, if your government made a request for proposals for a new launcher, you'd look around at the engines you had already developed or had experience with, divide the needed thrust at launch by the thrust of one of those engines, and that's how many engines you'd design for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX wanted to use a common engine design for Falcon 1, Falcon 5 and Falcon 9, and use the same engine in the second stages of the 5 and 9. The upper stage naturally needs less thrust, so they designed the Merlin to drive a single-engine second stage, and payload goals set the number of engines in the first stage accordingly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn program was originally going to be a larger family of launchers. The Saturn C-3 was to use 2 of the enormous &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;F-1&lt;/a&gt; engines, the C-4 would use 4, and the C-5 became the Saturn V with 5 of them. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-28T05:38:33.457" LastActivityDate="2017-05-28T05:38:33.457" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10363" PostTypeId="2" ParentId="10361" CreationDate="2015-07-30T00:40:12.423" Score="12" Body="&lt;p&gt;The number of engines and strapons are a series of tradeoffs made based on available technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Americans, planning a series of monstrous rockets, started on the huge F-1 engine. (1.5 to 1.8 million lbs of thrust).  The only things the Russians ever had that came close was the RD-170 engine, with similar thrust, but needs 4 thrust chambers instead of the F-1's one.  The RD-170 came several decades after the F-1 as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combustion instability in large scale rockets like this is very hard to solve, and the Americans were possibly lucky that they succeeded. The Russians were not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So when the time came to build a large booster, the Americans were able to get by with just 5 F-1s (Which is an astonishing amount of thrust, 7.5 million lbs of thrust is truly awe inspiring) whereas the Russians with the NK-15/NK-33 in the 380,000 lbs of thrust needed to use 30 of them to get the needed thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a pragmatic explanation for each of those examples. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Strapon solids are usually a sign that the design grew during development. Usually the main engine size is set early in the design, and the vehicle grows over time. The way to mitigate this growth, since usually the main engine design is hard to scale up, is to add strap ons.  Solid strapons are often available from the makers of ICBM's. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Strapons are great for being high in thrust, usually with pretty low ISP, so that they are mostly used to get the vehicle moving off the pad, and drop off early. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of Soyuz, that is a bit of a misnomer, since the engines have multiple thrust chambers/bells, but count as a single engine.  So the side boosters look like they have 4 bells, but is really a single engine each. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-30T00:40:12.423" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10364" PostTypeId="5" CreationDate="2015-07-30T03:22:28.407" Score="0" Body="&lt;p&gt;The Union of Soviet Socialist Republics abbreviated to USSR or shortened to the Soviet Union, was a Marxist–Leninist state on the Eurasian continent that existed between 1922 and 1991. It was governed as a single-party state by the Communist Party with Moscow as its capital. A union of multiple subnational Soviet republics, its government and economy were highly centralized.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soviet_Union&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-30T05:36:33.840" LastActivityDate="2015-07-30T05:36:33.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10365" PostTypeId="4" CreationDate="2015-07-30T03:22:28.407" Score="0" Body="The Soviet Union was a world power and major player in the space race. It dissolved in 1991 into individual states, the largest of which is the Russian Federation." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-30T05:36:14.870" LastActivityDate="2015-07-30T05:36:14.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10366" PostTypeId="2" ParentId="10346" CreationDate="2015-07-30T07:14:38.377" Score="4" Body="&lt;p&gt;At first glance, both &lt;a href=&quot;https://en.wikipedia.org/wiki/Hyperbolic_trajectory&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital velocity&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;escape velocity&lt;/a&gt; are proportional to $\sqrt {\frac1{distance to the sun}}$, so I expected the curves to run parallel, with an offset for the launch velocity of the probe, plus further offsets for each gravity assist. Mark's answer shows it's more complicated than this.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Escape velocity&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;For a spherically symmetric massive body&lt;/a&gt; such as a (non-rotating) star or planet, the escape velocity at a given distance is calculated by the formula:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_{Escape} = \sqrt {\frac{2GM}{r}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where G is the universal gravitational constant ($G = 6.67×10^{−11} m^3 kg^{−1} s^{−2}$), M the mass of the body, and r the distance from the point in space to its center of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging this into Excel gets me this graph for escape velocity vs. distance: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7aMsL.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7aMsL.png&quot; alt=&quot;Escape velocity&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm still working on orbital velocity data, to be continued.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-31T07:29:26.153" LastActivityDate="2015-07-31T07:29:26.153" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10367" PostTypeId="1" AcceptedAnswerId="10370" CreationDate="2015-07-30T11:07:01.390" Score="2" ViewCount="1216" Body="&lt;p&gt;Pretty much like the title says. Beyond a simple (one or more) retro burn into unpowered plummeting into the atmosphere, is &lt;strong&gt;some form of powered descent a viable method of reentry from orbit&lt;/strong&gt; on celestial bodies &lt;em&gt;that have an atmosphere,&lt;/em&gt; such as Earth, Mars or the moons of the gas giants? What are the limitations of such an approach?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm primarily thinking about starting altitudes corresponding to orbits similar to low Earth orbit and higher, including transfer orbits from elsewhere. I am &lt;em&gt;not&lt;/em&gt; asking about landing (or simply unpowered, partial-ballistic-trajectory jettison) starting from altitudes where there still is an appreciable atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the purposes of this question, consider reasonably modern technology and missions profiles; missions launched perhaps around year 2000 or so and later. A historical perspective is nice, but not the main focus of this question.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="25" LastEditDate="2015-07-30T12:16:45.617" LastActivityDate="2015-07-30T18:31:47.240" Title="Is powered descent from orbit a viable method of reentry on bodies with an atmosphere?" Tags="|propulsion|landing|atmosphere|reentry|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10368" PostTypeId="2" ParentId="10367" CreationDate="2015-07-30T12:15:22.013" Score="5" Body="&lt;p&gt;There are a number of systems that have been used that essentially use a heat shield and rockets to slow down, no parachutes or similar systems. The heat shield acts as a protection during the reentry phase. Entering the atmosphere will slow you down, no matter what you do. Then rockets are used to finish the job, so to speak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;http://www.spacex.com/news/2014/05/30/dragon-v2-spacexs-next-generation-manned-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;Dragon V2&lt;/a&gt; plans to use this form of re-entry. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Mars&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Viking 1 and 2 both employed similar systems&lt;/li&gt;&#xA;&lt;li&gt;Human missions to Mars have been proposed using this technique, as it is the only known method to land large amounts of payload on Mars.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-07-30T12:15:22.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10369" PostTypeId="2" ParentId="10367" CreationDate="2015-07-30T12:46:23.033" Score="12" Body="&lt;p&gt;Assuming you are asking whether you could use a powered re-entry instead of a thermal protection system (aka heat shield) in re-entry, then it's certainly theoretically possible, however it's totally impractical. What you are talking about is essentially a reverse launch, slowing a rocket down from orbital speeds down to relative zero on the surface of the earth. It takes a huge amount of energy to put something into orbit and it will take a huge amount of energy to land it, so a landing rocket would have to be big. It would be incredibly expensive to put a payload like that into orbit in the first place. If earth had no atmosphere it would be much harder to have manned space flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat shields and parachutes are an extremely efficient way to re-enter, you get so much effect for very little weight. This is why they are still in use, there's simply nothing better. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-07-30T12:46:23.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10370" PostTypeId="2" ParentId="10367" CreationDate="2015-07-30T17:28:19.570" Score="5" Body="&lt;p&gt;Aerodynamic descent (chutes, balloons, wings, lifting bodies, etc.) is generally much more weight-efficient than powered descent, so you'll nearly always use it for the bulk of your deceleration to surface in an atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are two big factors that make powered descent attractive as a supplement to aerodynamic systems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Precision landings. Once a chute is out, you have limited control over where you land; wings can take you where you want to go but incur significant weight penalties. Using a rocket for the very last portion of the descent can give you more precise control of the landing site, as with the Falcon 9R. &lt;/li&gt;&#xA;&lt;li&gt;Thin atmospheres. Mars doesn't have enough atmosphere to make a purely aerodynamic descent practical for large landers; Curiosity needed a rocket-powered skycrane to get down safely. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-07-30T17:28:19.570" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10371" PostTypeId="2" ParentId="10346" CreationDate="2015-07-30T18:26:44.817" Score="35" Body="&lt;p&gt;You are correct that Voyager did not change from above escape velocity to below escape velocity shortly after launch.  The plot is misleading in that it is just not very accurate right there at 1 AU.  The plot lines are kind of thick and a smidge off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that I look at it more closely, the escape velocity line in that plot is wrong in other places as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a better plot:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NLM2W.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NLM2W.png&quot; alt=&quot;Voyager 2 and solar escape velocities&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-07-30T23:47:49.777" LastActivityDate="2015-07-30T23:47:49.777" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10372" PostTypeId="2" ParentId="10367" CreationDate="2015-07-30T18:31:47.240" Score="2" Body="&lt;p&gt;A heat shield and a parachute are attempts to use the atmosphere to get the vehicle to its terminal velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth that suffices for something like Dragon V2 to need only fairly small amounts of fuel and reasonably sized engines (SuperDraco) to slow down the final amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The utility of a parachute/heatshield depend on what terminal velocity the vehicle will get due to shape and atmospheric density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Otherwise, you need an inordinate amount of fuel for that final step to slow down. Eventually that becomes untenable. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-07-30T18:31:47.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10373" PostTypeId="2" ParentId="10350" CreationDate="2015-07-30T19:11:09.073" Score="0" Body="&lt;p&gt;The shuttle was &lt;strong&gt;conceived&lt;/strong&gt; as a cost-effective launch vehicle, but by the time they actually had built one they could launch, it had been designed to do so many other things that it was no longer actually cost-effective.  They stayed true to their original plans to sell satellite launch capability, however their costs were so high that this was an enormous money-loser for NASA. Worse, by charging low prices, they put would-be cheaper companies (such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Starstruck_(company)&quot; rel=&quot;nofollow&quot;&gt;Starstruck&lt;/a&gt;) out of business.&lt;/p&gt;&#xA;" OwnerUserId="11159" LastActivityDate="2015-07-30T19:11:09.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10375" PostTypeId="5" CreationDate="2015-07-31T03:09:39.600" Score="0" Body="&lt;p&gt;In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits of different radii in the same plane. The orbital maneuver to perform the Hohmann transfer uses two engine impulses: one to move a spacecraft onto the transfer orbit and a second to move off it.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf4-1.php&quot; rel=&quot;nofollow&quot;&gt;Basics of Space Flight: Interplanetary Trajectories&lt;/a&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T03:33:18.353" LastActivityDate="2015-07-31T03:33:18.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10376" PostTypeId="4" CreationDate="2015-07-31T03:09:39.600" Score="0" Body="An orbital maneuver to transfer between two circular orbits consisting of one burn to enter the transfer orbit and one to circularize in the final orbit." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T03:33:30.603" LastActivityDate="2015-07-31T03:33:30.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10377" PostTypeId="5" CreationDate="2015-07-31T03:21:10.287" Score="0" Body="&lt;p&gt;RF resonant cavity thrusters are a type of electromagnetic thruster that is being researched. Unlike conventional electric thrusters, they do not use any form of reaction mass, nor are they designed to emit any directional radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few designs for such thrusters have been proposed. aerospace engineer Roger Shawyer designed the EmDrive in 2001, and has persistently promoted the idea since then through his company, Satellite Propulsion Research. Chemical engineer Guido Fetta designed the Cannae Drive, operating on similar principles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few independent teams of physicists, notably a team at Xi'an's Northwestern Polytechnical University (NWPU) and one at NASA's Eagleworks Laboratories, have built prototypes of these designs, and have tentatively observed a small amount of net thrust from those prototypes. None of this experimental work has been published in independent peer reviewed journals, in part because observed thrust has been small, there are no compelling explanations for the apparent violation of momentum conservation, and claims of such a surprising result would need especially thorough efforts to rule out possible sources of error.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://emdrive.com/&quot; rel=&quot;nofollow&quot;&gt;emdrive.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T03:33:25.420" LastActivityDate="2015-07-31T03:33:25.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10378" PostTypeId="4" CreationDate="2015-07-31T03:21:10.287" Score="0" Body="The EMDrive is a type of electromagnetic thruster that is being researched. Unlike conventional electric thrusters, it does not use any form of reaction mass." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T03:33:05.467" LastActivityDate="2015-07-31T03:33:05.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10379" PostTypeId="1" AcceptedAnswerId="10389" CreationDate="2015-07-31T03:22:09.643" Score="21" ViewCount="14270" Body="&lt;p&gt;I've been wondering about the internet connection on the ISS, and how it's used. We were talking in &lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/the-pod-bay&quot;&gt;chat&lt;/a&gt; about astronauts personal belongings and free time, and it made me wonder about how they use the internet. This made me wonder about their internet connection and what kind of restrictions it might have. For instance, can astronauts get Netflix? Is there anything technologically or in terms of policy that would prevent them for having it or anything else like it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, are there technology difficulties or policies that dictate what the astronauts can and cannot do online during their free time aboard the ISS?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2016-10-04T07:19:49.203" Title="Do astronauts get Netflix on ISS?" Tags="|iss|astronauts|astronaut-lifestyle|data-transmission|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10380" PostTypeId="5" CreationDate="2015-07-31T03:23:39.463" Score="0" Body="&lt;p&gt;Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth-largest planet by diameter and the third-largest by mass. Among the giant planets in the Solar System, Neptune is the most dense. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth, and not as dense as Neptune. Neptune orbits the Sun at an average distance of 30.1 astronomical units (4.50×109 km).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Neptune&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T03:33:10.983" LastActivityDate="2015-07-31T03:33:10.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10381" PostTypeId="4" CreationDate="2015-07-31T03:23:39.463" Score="0" Body="Neptune is the eighth and farthest planet from the Sun in the Solar System." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T03:33:28.203" LastActivityDate="2015-07-31T03:33:28.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10382" PostTypeId="5" CreationDate="2015-07-31T03:25:24.147" Score="0" Body="&lt;p&gt;Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have different bulk chemical composition from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as &quot;ice giants&quot; to distinguish them from the gas giants. Uranus's atmosphere, although similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, contains more &quot;ices&quot;, such as water, ammonia, and methane, along with traces of other hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224.2 °C), and has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane the uppermost layer of clouds. The interior of Uranus is mainly composed of ices and rock.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Uranus&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T08:25:35.473" LastActivityDate="2015-07-31T08:25:35.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10383" PostTypeId="4" CreationDate="2015-07-31T03:25:24.147" Score="0" Body="Uranus is the seventh planet from the Sun." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T08:26:13.310" LastActivityDate="2015-07-31T08:26:13.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10384" PostTypeId="5" CreationDate="2015-07-31T03:27:37.690" Score="0" Body="&lt;p&gt;The solar wind is a stream of plasma released from the upper atmosphere of the Sun. It consists of mostly electrons, protons and alpha particles with energies usually between 1.5 and 10 keV. The stream of particles varies in density, temperature, and speed over time and over solar longitude. These particles can escape the Sun's gravity because of their high energy, from the high temperature of the corona and magnetic, electrical and electromagnetic phenomena in it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar wind flows outward supersonically to great distances, filling a region known as the heliosphere, an enormous bubble-like volume surrounded by the interstellar medium. Other related phenomena include the aurora (northern and southern lights), the plasma tails of comets that always point away from the Sun, and geomagnetic storms that can change the direction of magnetic field lines and create strong currents in power grids on Earth.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T08:26:31.347" LastActivityDate="2015-07-31T08:26:31.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10385" PostTypeId="4" CreationDate="2015-07-31T03:27:37.690" Score="0" Body=" The solar wind is a stream of plasma released from the upper atmosphere of the Sun. It consists of mostly electrons, protons and alpha particles with energies usually between 1.5 and 10 keV." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T08:26:35.017" LastActivityDate="2015-07-31T08:26:35.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10386" PostTypeId="5" CreationDate="2015-07-31T03:29:30.930" Score="0" Body="&lt;p&gt;The Boeing X-37, also known as the Orbital Test Vehicle (OTV), is a reusable unmanned spacecraft. It is boosted into space by a launch vehicle, then re-enters Earth's atmosphere and lands as a spaceplane. The X-37 is operated by the United States Air Force for orbital spaceflight missions intended to demonstrate reusable space technologies. It is a 120%-scaled derivative of the earlier Boeing X-40.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The X-37 began as a NASA project in 1999, before being transferred to the U.S. Department of Defense in 2004. It conducted its first flight as a drop test on 7 April 2006, at Edwards Air Force Base, California. The spaceplane's first orbital mission, USA-212, was launched on 22 April 2010 using an Atlas V rocket. Its successful return to Earth on 3 December 2010 was the first test of the vehicle's heat shield and hypersonic aerodynamic handling. A second X-37 was launched on 5 March 2011, with the mission designation USA-226; it returned to Earth on 16 June 2012. A third X-37 mission, USA-240, launched on 11 December 2012 and landed at Vandenberg AFB on 17 October 2014.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_X-37&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T08:25:54.310" LastActivityDate="2015-07-31T08:25:54.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10387" PostTypeId="4" CreationDate="2015-07-31T03:29:30.930" Score="0" Body="The Boeing X-37, also known as the Orbital Test Vehicle (OTV), is a reusable unmanned spacecraft. It is boosted into space by a launch vehicle, then re-enters Earth's atmosphere and lands as a spaceplane." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T07:25:39.770" LastActivityDate="2015-07-31T07:25:39.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10388" PostTypeId="2" ParentId="10350" CreationDate="2015-07-31T03:55:19.210" Score="14" Body="&lt;p&gt;One perspective from &amp;quot;Stonekettle Station: &lt;em&gt;Pie in the Sky&lt;/em&gt;&amp;quot; (via the glorious &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/macguffinite.php&quot; rel=&quot;noreferrer&quot;&gt;Project Rho&lt;/a&gt;), edits and emphasis mine:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;The objectives of our space program are many and varied, but none of those objectives will ever lead to the kind of self-sustaining commercial ventures visualized in the popular speculation of the Golden Age.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Shuttle is a perfect example. Government cannot build a spaceship—at least not a very efficient one. &lt;strong&gt;The Shuttle as first designed was supposed to make access to space simple and cheap.&lt;/strong&gt; Getting out of Earth’s gravity well and into LEO is the hardest part of space travel. That first step is a doozy, but once you’re in orbit, you’re halfway to anywhere in the Solar System. The Shuttle was supposed to do that for us. And even with 1970’s technology, the Shuttle could have made access to space relatively cheap and easy and a whole lot safer.&lt;/p&gt;&#xA;&lt;p&gt;Instead we got just exactly the opposite.&lt;/p&gt;&#xA;&lt;p&gt;Why? &lt;strong&gt;Because NASA engineers didn’t build the shuttle, Congress did.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;And the lawmakers on Capitol Hill don’t give a fart in a spacesuit about exploration. To them, the Shuttle meant, and still means, jobs and pork and votes. By the time Congress got done redesigning the Shuttle it was astounding that the damned thing could even clear the pad. Gone were the safety features like air-breathing engines that would have let the ship abort a landing and make a once around on final approach, gone was the piloted reusable main booster, gone was the simplicity that would have gotten rid of much of the previous Apollo infrastructure. &lt;strong&gt;Gone too was the once-a-week turnaround time from recovery to relaunch that would have made efficient use the economies-of-scale and reduced ground-to-orbit costs to dollars a pound instead of tens of thousands of dollars per pound.&lt;/strong&gt; To this day parts are manufactured all across the country, many as far from the launch and assembly site as it is possible to get and still be on the same continent, because Senators and Representatives from powerful states like California insisted that it be so. The ship does nothing well, it’s too complicated and it requires far too much infrastructure, it’s a poor lift vehicle, it’s a poor science platform, it’s a poor crew vehicle and it falls short of the original design and concept in almost every way. Everybody got a piece of the Shuttle and as a result it shudders into orbit like Frankenstein’s Monster and the fact that it’s only blown up twice in 30 years is a minor miracle in itself.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2707" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-31T03:55:19.210" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="10389" PostTypeId="2" ParentId="10379" CreationDate="2015-07-31T03:55:46.953" Score="19" Body="&lt;p&gt;Yes, they can and do watch TV shows on the ISS.  From an &lt;a href=&quot;http://www.cnbc.com/2015/07/24/astronaut-space-internet-as-bad-as-dial-up.html&quot; rel=&quot;noreferrer&quot;&gt;interview with Scott Kelly&lt;/a&gt; aboard the ISS:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Apart from posting pictures to Twitter and Instagram, Kelly also said he spends some of his limited downtime watching television.&lt;/p&gt;&#xA;&lt;p&gt;What exactly does a highly trained astronaut watch while orbiting the planet and contemplating the mysteries of life? Kelly said his two indulgences are &amp;quot;Game of Thrones&amp;quot; and &amp;quot;Better Call Saul.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That said, the article does make clear that connection quality isn't necessarily great because it's dependent on ground stations being within sight:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;You have to be patient sometimes and be flexible and access it when we have the comm coverage, because we don't always have it,&amp;quot; he said. &amp;quot;Probably about 45 to 50 minutes an hour…we have the capability to get on the Internet on the ground.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;NASA says the spotty internet service is the result of the space station's proximity to certain ground-based sites.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In terms of restrictions on use the article mentioned using it for social media, online banking, the above quote about watching TV shows, but did make it sound like it's locked down enough to impact the speed:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The agency said it's &amp;quot;somewhat slower because there are additional security measures in place to ensure the safety and integrity of the internet data that is viewable aboard ISS.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-31T06:00:22.867" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10390" PostTypeId="1" AcceptedAnswerId="10392" CreationDate="2015-07-31T03:59:40.780" Score="16" ViewCount="3032" Body="&lt;p&gt;Consider how valuable ancient artifacts are to us.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Paleolithic_flutes&quot;&gt;look at this fancy stick!&lt;/a&gt; In as little as a few hundred years debris from skylab would be way more valuable.  I'm baffled why &lt;em&gt;the agency&lt;/em&gt; didn't want to preserve it as a relic.  Don't &lt;em&gt;they&lt;/em&gt; have any sense of posterity?  Don't they want to leave a lasting legacy for future generations to marvel upon?  I'd think that skylab would be &lt;em&gt;invaluable&lt;/em&gt; as an exhibit for the first orbital museum, or as a field trip for students of the early space age.  Instead they just let it crash and burn like useless scrap (which it could have been, but comeon!).  I know it would have cost a lot to haul it into a MEO or however far it needs to be to be stable.  But wouldn't any price be worth it or am I just being oversensitive?&lt;/p&gt;&#xA;" OwnerUserId="11162" LastEditorUserId="415" LastEditDate="2015-07-31T09:22:45.733" LastActivityDate="2015-07-31T09:22:45.733" Title="Why was Skylab allowed to fall, instead of being pushed into a higher orbit and preserved for the future?" Tags="|history|space-station|skylab|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10391" PostTypeId="1" AcceptedAnswerId="10395" CreationDate="2015-07-31T04:25:44.513" Score="23" ViewCount="4238" Body="&lt;p&gt;Update 08 April 2017: &lt;a href=&quot;https://twitter.com/elonmusk/status/850453029987917824&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk tweeted&lt;/a&gt; an indication that they're revisiting the possibility of second stage recovery, so this question is again relevant, and the details of the new solution may be different from the old.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Fairing is ~$5M, but that should be reusable this year. Am fairly confident we can reuse upper stage too by late next year to get to 100%.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;SpaceX wants to make the Falcon 9 completely reusable. They're making definite, if slow, progress toward recovery of the first stage via powered landing, which requires several features:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Cold gas thrusters to turn the stage tail-first after second stage separation&lt;/li&gt;&#xA;&lt;li&gt;Aerodynamic control surfaces at the top of the stage for trajectory control during the landing approach&lt;/li&gt;&#xA;&lt;li&gt;Deep throttling a single main engine to get a low enough TWR for a controlled landing&lt;/li&gt;&#xA;&lt;li&gt;Landing legs&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The upper stage will be coming in a much faster trajectory, so they have to &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9#Reusability&quot; rel=&quot;noreferrer&quot;&gt;add a full heat shield&lt;/a&gt; at the front of the stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first glance, the much shorter second stage would seem to need shorter legs because its center of gravity would be lower, but the upper stage version of the Merlin engine has a very long nozzle, which is radiatively cooled -- which means exposed, not shrouded, so the legs will have to mount quite high, and be fairly long (and thus heavy). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest problem, however, would seem to be the engine throttling. The empty upper stage weighs something like a quarter of what the first stage does, but will be firing almost the same thrust level. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are they going to try to throttle deeper? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are they going to do the same hover-slam trick as the first stage, only on a much shorter timeframe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are they going to add 2 or 3 &lt;a href=&quot;https://en.wikipedia.org/wiki/Kestrel_(rocket_engine)&quot; rel=&quot;noreferrer&quot;&gt;Kestrels&lt;/a&gt; alongside the Merlin on the second stage and land gently on those?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parachutes? Airbags? Really big pogo sticks?&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="1391" LastEditDate="2017-04-09T01:58:33.513" LastActivityDate="2017-04-12T03:08:02.477" Title="How does SpaceX plan to achieve reusability of the Falcon 9 *second* stage?" Tags="|falcon-9|reuse|asds|merlin-1d|kestrel|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10392" PostTypeId="2" ParentId="10390" CreationDate="2015-07-31T05:37:25.123" Score="32" Body="&lt;p&gt;Short answer is that there were multiple contributors: bad predictions of how long the orbit would last, schedule delays on the Space Shuttle, and cost/benefit analysis of a last minute rescue mission for an aged space station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things to keep in mind: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#Re-entry&quot;&gt;Skylab reentered and burned up in July 1979&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;the last Apollo capsule to fly was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot;&gt;Apollo-Soyuz Test Project&lt;/a&gt; in 1975 (and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab_4&quot;&gt;Skylab 4 mission&lt;/a&gt; ended in February 1974)&lt;/li&gt;&#xA;&lt;li&gt;the first Space Shuttle mission was &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-1&quot;&gt;STS-1&lt;/a&gt; in 1981&lt;/li&gt;&#xA;&lt;li&gt;Skylab was never designed for automated docking or servicing&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The final Skylab mission boosted the station into a higher orbit, where the original estimates of orbit duration thought that Skylab would last until the 1980s &lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#After_departure&quot;&gt;Wikipedia&lt;/a&gt;&lt;/sup&gt;.  At the time, the Space Shuttle was expected to &lt;a href=&quot;https://en.wikipedia.org/wiki/Canceled_Space_Shuttle_missions#STS-2A_.28Columbia.29&quot;&gt;launch in 1979&lt;/a&gt; before Skylab decayed, and there were &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#Shuttle_mission_plans&quot;&gt;plans&lt;/a&gt; for boosting its orbit that got so far as to award a contract for developing the rocket module and at least plan training for astronauts to use it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the models used for the original prediction &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#Solar_activity&quot;&gt;failed to take into account solar activity&lt;/a&gt; (which heats the atmosphere, causing it to expand and increases drag at higher altitudes) and in late 1977 decay was predicted for 1979.  Up until December 1978 there were still plans (&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#Solar_activity&quot;&gt;and nearly completed hardware&lt;/a&gt;) for a shuttle mission to save Skylab, but the shuttle simply wasn't going to be ready in time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the time it was clear that the shuttle wouldn't be ready before Skylab decayed there weren't many options.  The rocket designed to boost it was almost ready, but Skylab wasn't designed for automated docking and a somewhat rushed mission to launch it on existing rockets was deemed too expensive when the money was needed for the Space Shuttle budget.  Also, neither NASA nor congress (who controlled the budget) were particularly interested in the station (despite that &quot;microbiologists were excited at the prospect of studying microbes that had been reproducing in the trash for hundreds of generations in a spacecraft&quot;). &lt;sup&gt;&lt;a href=&quot;http://www.astronautix.com/articles/skyyfate.htm&quot;&gt;Skylab's Untimely Fate&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen no indication that there was ever a concern for preserving the station as an artifact. There's a big difference between donating artifacts to museums (who will then foot the bill for preserving them), &lt;a href=&quot;http://www.space.com/13346-nasa-guidelines-protect-apollo-moon-landing-sites.html&quot;&gt;publishing guidelines telling everyone to stay away from your historic sites&lt;/a&gt;, and spending at least several hundred million dollars of taxpayer money developing and launching a rescue mission for a station that you don't have the ability to fly astronauts to (especially when &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#Skylab_B&quot;&gt;you've built a backup but can't afford to launch it because you need the money for developing your next manned spacecraft&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astronautix.com/articles/skyyfate.htm&quot;&gt;Skylab's Untimely Fate&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2015-07-31T05:37:25.123" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10393" PostTypeId="5" CreationDate="2015-07-31T05:40:55.947" Score="0" Body="&lt;p&gt;Skylab was a space station launched and operated by NASA and was the United States' first space station. Skylab orbited the Earth from 1973 to 1979, and included a workshop, a solar observatory, and other systems. It was launched unmanned by a modified Saturn V rocket, with a weight of 169,950 pounds (77 t). Three manned missions to the station, conducted between 1973 and 1974 using the Apollo Command/Service Module (CSM) atop the smaller Saturn IB, each delivered a three-astronaut crew.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/space-station&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;space-station&amp;#39;&quot; rel=&quot;tag&quot;&gt;space-station&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astronautix.com/articles/skyyfate.htm&quot; rel=&quot;nofollow&quot;&gt;Skylab's Untimely Fate&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T10:30:23.010" LastActivityDate="2015-07-31T10:30:23.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10394" PostTypeId="4" CreationDate="2015-07-31T05:40:55.947" Score="0" Body="Skylab was a space station launched and operated by NASA and was the United States' first space station." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-07-31T10:30:25.733" LastActivityDate="2015-07-31T10:30:25.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10395" PostTypeId="2" ParentId="10391" CreationDate="2015-07-31T06:37:51.653" Score="18" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2vFWW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2vFWW.png&quot; alt=&quot;Old/Original Proposal for Second Stage Reuse&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Old/Original Proposal for Second Stage Reuse&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second stage reuse for Falcon 9 was a planned feature that has been discontinued.&#xA;In &lt;a href=&quot;https://www.youtube.com/watch?v=sSF81yjVbJE&quot; rel=&quot;noreferrer&quot;&gt;this old render&lt;/a&gt;(Shows a Falcon 9v1.0 with square engine block) you can see a reuse plan very similar to what you described. The second stage nozzle would retract into the tank structure, small legs would deploy, and smaller engines(kestrel, draco?) would perform the actual landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, second stage reuse is hard. The first stage is a massive beast that does most of the heavy lifting into orbit. However, the second stage provides most of the speed. Once the second stage inserts Dragon into orbit, it would need to have enough fuel to deorbit/intersect the atmosphere then use a heavy heatshield to reduce its velocity on reentry. All of these things add weight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the nature of staging and the rocket equation, roughly every 4 pounds of dead weight on the first stage takes away 1 pound of payload. On the second stage it is a 1:1 conversion. All the extra weight for reuse(heat shield, legs, extra fuel, machinery for nozzle retraction) take away from the total payload mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX already has an inefficient upper stage relative to most launch providers. Their use of Kerolox on both stages simplifies design and tanking but means that the second stage Isp is much worse than a cryogenic Hydrogen stage. &#xA;Now why did SpaceX change their mind? &#xA;Elon Musk explained this at the &lt;a href=&quot;https://youtu.be/y13jbl7ASxY?t=14m20s&quot; rel=&quot;noreferrer&quot;&gt;MIT Aero/Astro Centennial&lt;/a&gt;. When asked about second stage reuse&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;strong&gt;next generation vehicles&lt;/strong&gt; after the Falcon architecture will be designed for full reusability. I don't expect the Falcon 9 to have a reusable upper stage, just because the - with a kerosene-based system, the &lt;strong&gt;specific impulse&lt;/strong&gt; isn't really high enough to do that, and a lot of the missions we do for commercial satellite deployment are geostationary missions. So, we're really going very far out. These are &lt;strong&gt;high delta-velocity&lt;/strong&gt; missions, so to try to get something back from that is really difficult. But, with the next generation of vehicles, which is going to be a sub-cooled methane/oxygen system where the propellants are cooled close to their freezing temperature to increase the density, we could definitely do full reusability - and that system is intended to be a fully reusable Mars transportation system. So, not merely to low Earth orbit but all the way to Mars and back, with full reusability. [Within 3 years?] Ha. I am an optimistic person, but - I think we could expect to see some test flights in the five or six year time frame. But, we're talking about a much bigger vehicle, and we're also going to be upgrading to a new generation - a harder engine cycle, which is a full-flow staged combustion. What we have right now is an open cycle engine. Right now, I'd say, engines are our weakest point at SpaceX, but they will become as strong as the structures and avionics in the next generation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OrHNe.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OrHNe.jpg&quot; alt=&quot;Falcon 9 won&amp;#39;t be able to pull this off but the BFR/MCT will&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Falcon 9 won't be able to pull this off but the BFR/MCT will&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastActivityDate="2015-07-31T06:37:51.653" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10396" PostTypeId="1" CreationDate="2015-07-31T07:07:31.607" Score="15" ViewCount="2628" Body="&lt;p&gt;Just out of curiosity, and because we hopefully are at the dawn of reusable launchers, what items have been sent to orbit from Earth, then landed on Earth and sent back to orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space shuttles of course, but did they bring any satellites back and up again? And same for X-37. Are or have there been more spacecrafts like that? Soviet &lt;a href=&quot;http://www.astronautix.com/craft/zenit.htm&quot;&gt;Zenit spy satellites&lt;/a&gt; returned capsules with film, has any such system led to relaunch of material? Manned flight, like those to the ISS today, also offer opportunities for reuse of payload, but is it done, and for what? I suppose it was a unique event that the RTG from the crashed &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/nimbus-3.htm&quot;&gt;Nimbus B&lt;/a&gt; weather satellite was salvaged and reused on Nimbus 3. And has anything been sent to the Moon twice?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-07-31T07:12:36.533" LastActivityDate="2015-07-31T15:45:47.477" Title="What objects have been sent to space twice?" Tags="|reuse|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10397" PostTypeId="2" ParentId="10396" CreationDate="2015-07-31T07:47:06.023" Score="15" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Spacelab&quot;&gt;Spacelab&lt;/a&gt; was flown on 22 Shuttle missions. Two pressurized modules were built, each of these flew multiple times (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Spacelab#Spacelab_missions&quot;&gt;the list of missions&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/structure/elements/spacehab.html&quot;&gt;The SPACEHAB Logistics Double Module&lt;/a&gt; was flown on 3 Shuttle missions. There were more flights with Spacehab modules, I haven't tracked down reuse in all of them. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-07-31T10:34:45.617" LastActivityDate="2015-07-31T10:34:45.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10398" PostTypeId="2" ParentId="10396" CreationDate="2015-07-31T08:27:09.830" Score="14" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_2&quot; rel=&quot;noreferrer&quot;&gt;Gemini 2&lt;/a&gt; capsule was refurbished and relaunched as a test article for the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Manned_Orbiting_Laboratory&quot; rel=&quot;noreferrer&quot;&gt;Manned Orbiting Laboratory&lt;/a&gt; military space station program.  This may have been the first reusable spacecraft, although it was unmanned in both its flights.&lt;a href=&quot;https://i.stack.imgur.com/US32a.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/US32a.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle mission &lt;a href=&quot;https://en.m.wikipedia.org/wiki/STS-51-A&quot; rel=&quot;noreferrer&quot;&gt;STS-51A&lt;/a&gt; retrieved 2 comsats that had been launched on an earlier shuttle mission and returned them to earth. They were refurbished and relaunched.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/UbOMQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UbOMQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on how fine-grained of a list you want, there are hundreds of items that were reused on shuttle mission after mission: EMUs, tools, etc.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-07-31T12:19:23.433" LastActivityDate="2015-07-31T12:19:23.433" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10399" PostTypeId="1" CreationDate="2015-07-31T09:12:54.263" Score="7" ViewCount="194" Body="&lt;p&gt;Are there libration points in a restricted 4-body problem (system of three orbiting bodies of significant mass, plus the libration point orbiting body)? If so, how many of them exist and where are they located?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jovian system must have its periodic or quasi-periodic libration points. Would two or more of its Galilean moons in orbital resonance with Jupiter play a role despite their low masses relative to the primary, or is an orbiting system of 3 bodies of significant mass required for such libration points to remain stable? How does such a system's effective potential change and could any of these libration points be periodic?&lt;/p&gt;&#xA;" OwnerUserId="11164" LastEditorUserId="49" LastEditDate="2015-07-31T11:19:20.950" LastActivityDate="2015-12-16T05:36:11.347" Title="What, if any, libration points exist in systems of multiple orbiting bodies?" Tags="|orbital-mechanics|n-body-problem|lagrangian-points|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10400" PostTypeId="2" ParentId="10396" CreationDate="2015-07-31T10:37:11.187" Score="5" Body="&lt;p&gt;As I know, many internal parts of Soyuz reentry capsules are reused in next flights after testing and refurbishing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the same happened in Apollo program, then we may have objects that visited the Moon twice.&lt;/p&gt;&#xA;" OwnerUserId="5796" LastEditorUserId="5796" LastEditDate="2015-07-31T10:55:44.750" LastActivityDate="2015-07-31T10:55:44.750" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10402" PostTypeId="2" ParentId="5062" CreationDate="2015-07-31T13:12:37.277" Score="7" Body="&lt;p&gt;Regarding grounding on the ISS,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;blockquote&gt;&#xA;    &lt;blockquote&gt;&#xA;      &lt;p&gt;The space station solar arrays operate at 160 VDC. When the arrays are producing power, the station structure will also tend to float to a voltage close to the array voltage. Under these conditions, the space station could be subjected to problems like arcing from its surface to the surrounding environment, or arcing to an astronaut. To avoid these problems, the structure has been grounded with a Plasma Contactor Unit (PCU). To protect the astronauts from shock hazards, the PCU is operated during all spacewalks.&lt;/p&gt;&#xA;      &#xA;      &lt;p&gt;The PCU acts as an electrical ground rod to connect the space station structure to the local environment and harmlessly dissipate the structure charges. Glenn [NASA Glenn Research Centre - ed.] engineers designed, manufactured, tested and installed the hollow cathode assembly, which is the critical component of the PCU. The Hollow Cathode Assembly performs this function by converting a small supply of gas into ions and electrons and discharging this stream to space. The stream carries with it the excess &#xA;      electrons that created the surface charge.&lt;/p&gt;&#xA;    &lt;/blockquote&gt;&#xA;  &lt;/blockquote&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.nasa.gov/centers/glenn/pdf/590338main_PS-00537_ISS_tag508.pdf&quot; rel=&quot;nofollow&quot;&gt;NASA Factsheet PS-00537-0811, &quot;Powering the Future&quot;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that it would also be operated during docking procedures as well, to minimise ESD and ground potential difference between the ISS and the docking spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also have a look at &quot;Principles of Space Instrument Design&quot;, Cruise, A.M et al., Cambridge Uni Press, ISBN 0521451647. There is a chapter on electronics. (I haven't read it yet - only just ordered it today.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Within manned spacecraft atmospheres, ESD mitigation would most probably be a function of environmental control via humidifiers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE 31 Jan 2016:&lt;br&gt;&#xA;See Chapter 3, specifically &lt;strong&gt;section 3.2 Design Guidelines&lt;/strong&gt;, of the definitive NASA JPL Space Science &amp;amp; Technology Series &lt;a href=&quot;http://descanso.jpl.nasa.gov/SciTechBook/series3/ChgingBook--110629-RibbonC.pdf&quot; rel=&quot;nofollow&quot;&gt;book&lt;/a&gt;: &lt;strong&gt;Guide to Mitigating Spacecraft Charging Effects&lt;/strong&gt;, Henry B. Garrett, Albert C. Whittlesey, June 2011, 244pp.&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2016-01-30T19:10:32.093" LastActivityDate="2016-01-30T19:10:32.093" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10403" PostTypeId="1" AcceptedAnswerId="10616" CreationDate="2015-07-31T14:12:46.970" Score="11" ViewCount="617" Body="&lt;p&gt;The document &lt;strong&gt;JSC-07896 'Space Shuttle Baseline Reference Missions'&lt;/strong&gt; would be an invaluable reference for anyone seeking to understand why the US Space Transportation System was designed the way it was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, other than &lt;a href=&quot;http://www.jamesoberg.com/sts-3A_B-DRM.PDF&quot; rel=&quot;noreferrer&quot;&gt;one excerpt&lt;/a&gt; found on www.jamesoberg.com, and a couple of cryptic references in &lt;a href=&quot;http://www.sti.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;NTRS&lt;/a&gt;, I have not been able to locate a copy of the document.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know if a publicly accessible copy exists?&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="49" LastEditDate="2015-07-31T16:02:47.390" LastActivityDate="2021-08-03T18:18:16.133" Title="Is the document &quot;Space Shuttle Baseline Reference Missions&quot; available anywhere?" Tags="|space-shuttle|design|mission-design|reference-request|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10404" PostTypeId="1" AcceptedAnswerId="10406" CreationDate="2015-07-31T14:20:27.933" Score="8" ViewCount="323" Body="&lt;p&gt;A &lt;a href=&quot;http://www.bbc.com/news/science-environment-33720951&quot; rel=&quot;nofollow noreferrer&quot;&gt;recent news article&lt;/a&gt; says that the lander Philae on comet Churyumov-Gerasimenko 67P, has discovered a &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;rich array of organics&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and it quotes one leading scientist as saying it is a&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a frozen primordial soup&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While &quot;organics&quot; don't necessarily mean life, (see related question &lt;a href=&quot;https://space.stackexchange.com/questions/6021&quot;&gt;What is meant by &amp;#39;Organics&amp;#39; when discussing space exploration?&lt;/a&gt; ) it does offer the chance. On the other hand, DNA without life seems unlikely (&lt;em&gt;there are some varied opinions on the matter&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is Philae or any other space craft able to identify DNA? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scope: past or current, spacecraft or lander. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-08-01T09:42:42.047" Title="If there was DNA in the &quot;organics&quot; of a comet could we tell?" Tags="|spacecraft|life|lander|philae|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10405" PostTypeId="2" ParentId="10396" CreationDate="2015-07-31T14:33:09.507" Score="9" Body="&lt;p&gt;The Russian KURS docking computer/radar system was reused from some Progress vehicles in the European ATV. It was returned by the Shuttle for reuse in future ATV missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MPLM modules built by Alenia in Italy were used as logistics modules on several Shuttle flights to carry pressurized cargo to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KCA7E.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KCA7E.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US KH-9 Hexagon spy satellite had film canisters that reentered, caught by a helicopter in mid-air and were reused on future flights. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6944" LastEditDate="2015-07-31T15:45:47.477" LastActivityDate="2015-07-31T15:45:47.477" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10406" PostTypeId="2" ParentId="10404" CreationDate="2015-07-31T14:40:01.260" Score="6" Body="&lt;p&gt;The surest currently known way to detect DNA and analyze it to rule out sample contamination with genetic material from the Earth hinges on rapidly multiplying the DNA molecules via PCR (&lt;a href=&quot;https://en.wikipedia.org/wiki/Polymerase_chain_reaction&quot; rel=&quot;nofollow noreferrer&quot;&gt;polymerase chain reaction&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No PCR kits have been deployed to space so far. However, this is going to change: Boeing is considering sending Amplyus' miniPCR platform to the ISS and starting &lt;a href=&quot;http://genesinspace.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Genes in Space&lt;/a&gt; program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.genesinspace.org/blog/congratulations-anna-sophia-boguraev-genes-in-space-2015-winner/&quot; rel=&quot;nofollow noreferrer&quot;&gt;winner&lt;/a&gt; of the 2015 experiment contest is Anna-Sophia Boguraev (below (right), with Cady Coleman(left)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SqSs3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SqSs3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.minipcr.com/wp-content/uploads/PCR-in-Space-_-GenomeWeb-_-Genes-in-Space-20150409.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.minipcr.com/wp-content/uploads/PCR-in-Space-&lt;em&gt;-GenomeWeb-&lt;/em&gt;-Genes-in-Space-20150409.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Automating the analysis onboard an unmanned probe/rover is possible but not compatible with the scientific requirement of reproducibility. You certainly want the DNA to be aliquotted to different laboratories to avoid any allegations of fraud and reduce probability of error. &lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-08-01T09:42:42.047" LastActivityDate="2015-08-01T09:42:42.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10407" PostTypeId="1" CreationDate="2015-07-31T16:06:24.957" Score="7" ViewCount="2757" Body="&lt;p&gt;Perhaps this is a strange question but I've always wondered how astronauts, particularly those aboard the ISS, sleep in zero G. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they just hang out in &quot;space&quot; or do they have beds they strap themselves into? I'd imagine if you were free floating whilst asleep you might bump into something, but maybe not?  &lt;/p&gt;&#xA;" OwnerUserId="11171" LastEditorUserId="49" LastEditDate="2015-07-31T17:30:16.840" LastActivityDate="2015-07-31T19:31:39.757" Title="How do astronauts sleep in zero G?" Tags="|iss|microgravity|astronauts|astronaut-lifestyle|sleep|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10408" PostTypeId="2" ParentId="10407" CreationDate="2015-07-31T16:17:22.247" Score="9" Body="&lt;p&gt;See &lt;a href=&quot;http://www.nasa.gov/audience/forstudents/5-8/features/F_Sleeping_in_Space.html&quot; rel=&quot;noreferrer&quot;&gt;Sleeping in Space&lt;/a&gt;..&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;…  Space Station crews usually sleep in sleeping bags. On the Space Station there are two &lt;strong&gt;small crew cabins.&lt;/strong&gt; Each one is just big enough for one person. Inside both crew cabins is a &lt;strong&gt;sleeping bag&lt;/strong&gt; and a large window to look out in space. Currently, Space Station crews have three astronauts living and working in space for months at a time. Where does the third astronaut sleep? If it's okay with the commander, the astronaut can sleep anywhere in the Space Station so long as they &lt;strong&gt;attach themselves to something.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Astronaut Susan Helms slept in the huge Destiny Laboratory Module by herself while she was living aboard the International Space Station. This is on the opposite side of the station from the Service Module where her crewmates slept. …&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;Update&lt;/h2&gt;&#xA;&lt;p&gt;That is the basics of &lt;strong&gt;how&lt;/strong&gt; the crew sleep.  Here is a more detailed discussion of &lt;strong&gt;where&lt;/strong&gt; they sleep as well as more details on the cabins etc.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;@geoffc  This info is pre-ISS full build. Node 2 has 4 sleeping quarters, and there are 3 sleeping berths in the Russian side.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;More from &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Crew_activities&quot; rel=&quot;noreferrer&quot;&gt;International Space Station: Crew activities&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;… The station provides crew quarters for each member of the expedition's crew, with two 'sleep stations' in the &lt;em&gt;Zvezda&lt;/em&gt; and four more installed in &lt;em&gt;Harmony&lt;/em&gt;. The American quarters are private, approximately person-sized soundproof booths. The Russian crew quarters include a small window, but do not provide the same amount of ventilation or block the same amount of noise as their American counterparts. A crewmember can sleep in a crew quarter in a tethered sleeping bag, listen to music, use a laptop, and store personal items in a large drawer or in nets attached to the module's walls. The module also provides a reading lamp, a shelf and a desktop. Visiting crews have no allocated sleep module, and attach a sleeping bag to an available space on a wall — it is possible to sleep floating freely through the station, but this is generally avoided because of the possibility of bumping into sensitive equipment. It is important that crew accommodations be well ventilated; otherwise, astronauts can wake up oxygen-deprived and gasping for air, because a bubble of their own exhaled carbon dioxide has formed around their heads.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ICG7e.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ICG7e.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-07-31T19:31:39.757" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10409" PostTypeId="2" ParentId="10407" CreationDate="2015-07-31T16:17:30.483" Score="7" Body="&lt;p&gt;You would want to watch &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/main/suni_iss_tour.html&quot;&gt;Suni Williams' tour of the station videos&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/XkM_04Ch76E?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's the easiest to understand it. But in essence, they have &lt;em&gt;sleeping stations&lt;/em&gt; like for example in the Node 2 (&lt;a href=&quot;https://en.wikipedia.org/wiki/Harmony_%28ISS_module%29&quot;&gt;Harmony&lt;/a&gt;) module shown in video, with sleeping bags inside. If the video gets a bit confusing where some direction is, note that there's labels with orientations around the bulkhead behind Suni (OVHD for overhead, PORT,...).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might wonder where the need to have those sleeping bags tied down comes from, if they're sleeping in microgravity. It's essentially to prevent them from unwittingly pushing themselves around during sleep, but there are slight external forces acting on them too, mainly air ventilation system preventing pockets of carbon dioxide, and slight gravity gradient that still exists in orbit and where the term &lt;em&gt;microgravity&lt;/em&gt; comes from, describing its amplitude relative to 1 g. The bags are needed because the temperature at the station is kept slightly on the cold side of things if you're used to warmer climates (&lt;a href=&quot;http://isslive.com/displays/ethosDisplay1.html&quot;&gt;currently 23.20 °C in the Destiny module&lt;/a&gt; but &lt;a href=&quot;http://isslive.com/timeline/index.html&quot;&gt;the crew is up now with three out of six doing exercises&lt;/a&gt;) for various reasons, including the air itself serving as a coolant for various equipment, for hygienic reasons to prevent body odor, sweating, slow down growth of fungi and yeasts,... and due to air ventilation. And the doors are of course there to ensure some privacy.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-07-31T17:27:28.797" LastActivityDate="2015-07-31T17:27:28.797" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10411" PostTypeId="2" ParentId="10464" CreationDate="2015-07-31T20:33:45.460" Score="2" Body="&lt;p&gt;This question will likely result in a range of answers because terminology ranges from country to county, institution to institution and person to person. However the etymology of the two phrases could add some knowledge:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astrodynamics; Astro - star, dynamic - study of bodies in motion&#xA;Space dynamics;  space - area outside the atmosphere of planetary bodies&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the definition of astrodynamics is frequently used as a coverall for the dynamics of orbiting bodies (at least my my experience). So I would suggest the following definition:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astrodynamics: The study of interactions between bodies at motion in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space dynamics: The study of bodies at motion in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above would suggest astrodynamics is a subset of space dynamics. &lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-07-31T20:33:45.460" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10413" PostTypeId="1" CreationDate="2015-08-01T00:33:15.823" Score="8" ViewCount="556" Body="&lt;p&gt;What CAD software (Autocad, CATIA, Solid Edge etc.) is the ISS designed using? Or has this changed over time / modules? With such careful mating of components needed in very constrained environments with little margin for error I assume they do very detailed mating &amp;amp; interference studies on the components?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these done virtually within CAD, or do they actually have a parallel mock up built on earth with identical components &amp;amp; modules to ensure everything fits?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, did the Russians &amp;amp; Americans use the same CAD tools? If not how do they ensure interoperability of CAD formats? I'm alluding to the massive problems Airbus faced when different teams used merely different versions of CATIA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen to-scale sketches &amp;amp; drawings of the ISS released by various agencies but is anyone releasing the actual CAD drawings or are they confidential? &lt;/p&gt;&#xA;" OwnerUserId="11180" LastActivityDate="2015-09-23T21:42:02.390" Title="What CAD system do the ISS designers use?" Tags="|iss|design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10414" PostTypeId="2" ParentId="5278" CreationDate="2015-08-01T02:29:40.157" Score="4" Body="&lt;p&gt;The main reason for that delay from early rockets to orbit-capable rockets is known as the rocket equation:&#xA;    &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In layman's terms, the faster you want a rocket to go, the more fuel it requires.  The fuel increases the weight, requiring larger engines, and yet more fuel.  To achieve orbital speeds, you need a rocket where the vast majority of the rocket's mass is fuel.  For the Space Shuttle for example, approximately 80% if the vehicle's takeoff weight was fuel, 15% was the mass of the vehicle, and only 5% was the weight of the payload.  Building such a craft means developing very strong and very lightweight materials.  Those materials, and the means of working with them, all had to be invented.  The inertial guidance systems to accurately steer the rockets had to be invented.  The advanced digital and analog telemetry had to be invented.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upside to this is that the vast majority of advanced technology that we take for granted today traces its roots directly to those inventions necessary for space exploration.&lt;/p&gt;&#xA;" OwnerUserId="10641" LastActivityDate="2015-08-01T02:29:40.157" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10415" PostTypeId="2" ParentId="10413" CreationDate="2015-08-01T02:50:19.667" Score="10" Body="&lt;p&gt;The ISS was designed a while ago.  The primary CAD/CAM tool was called &lt;a href=&quot;http://en.wikipedia.org/wiki/Siemens_NX&quot; rel=&quot;nofollow&quot;&gt;Unigraphics&lt;/a&gt; (UG) -- which is now (not then) a Siemens product I believe.  UG was developed by McDonnell Douglas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interfaces are/were governed by ICDs (Interface Control Documents).  Data exchange is/was largely lowest (=greatest really) common denominator -- IGES files.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2015-09-23T21:42:02.390" LastActivityDate="2015-09-23T21:42:02.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10416" PostTypeId="5" CreationDate="2015-08-01T06:18:03.913" Score="0" Body="&lt;p&gt;The N1 (Russian: Н1) was a heavy lift rocket intended to deliver payloads beyond low Earth orbit, acting as the Soviet counterpart to the NASA Saturn V rocket. This heavy lift booster had the capability of lifting very heavy loads into orbit, designed with crewed extra-orbital travel in mind. Development work started on the N1 in 1959. Its first stage is the most powerful rocket stage ever built.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/N1_%28rocket%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-01T06:35:58.213" LastActivityDate="2015-08-01T06:35:58.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10417" PostTypeId="4" CreationDate="2015-08-01T06:18:03.913" Score="0" Body="The N1 was a heavy lift rocket intended to deliver payloads beyond low Earth orbit, acting as the Soviet counterpart to the NASA Saturn V rocket." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-01T06:34:48.807" LastActivityDate="2015-08-01T06:34:48.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10418" PostTypeId="5" CreationDate="2015-08-01T06:20:11.113" Score="0" Body="&lt;p&gt;MESSENGER (a backronym of MErcury Surface, Space ENvironment, GEochemistry, and Ranging, and a reference to the Roman mythological messenger, Mercury) was a NASA robotic spacecraft which orbited the planet Mercury between 2011 and 2015. The spacecraft was launched aboard a Delta II rocket in August 2004 to study Mercury's chemical composition, geology, and magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MESSENGER entered orbit around Mercury on March 18, 2011, becoming the first spacecraft to do so. It successfully completed its primary mission in 2012. Following two mission extensions, the MESSENGER spacecraft used the last of its maneuvering propellant and deorbited as planned, impacting the surface of Mercury on April 30, 2015.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/MESSENGER&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-01T07:33:01.560" LastActivityDate="2015-08-01T07:33:01.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10419" PostTypeId="4" CreationDate="2015-08-01T06:20:11.113" Score="0" Body="MESSENGER was a NASA robotic spacecraft which orbited the planet Mercury between 2011 and 2015." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-01T07:33:20.053" LastActivityDate="2015-08-01T07:33:20.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10420" PostTypeId="5" CreationDate="2015-08-01T06:26:38.430" Score="0" Body="&lt;p&gt;A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight. The rockets are used to carry instruments from 50 to 1,500 kilometres (31 to 932 mi) above the surface of the Earth, the altitude generally between weather balloons and satellites (the maximum altitude for balloons is about 40 kilometres (25 mi) and the minimum for satellites is approximately 120 kilometres (75 mi)).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sounding_rocket&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-01T08:58:15.120" LastActivityDate="2015-08-01T08:58:15.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10421" PostTypeId="4" CreationDate="2015-08-01T06:26:38.430" Score="0" Body="A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-01T07:33:09.483" LastActivityDate="2015-08-01T07:33:09.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10422" PostTypeId="1" CreationDate="2015-08-01T08:30:21.400" Score="3" ViewCount="159" Body="&lt;p&gt;SLS Block I will have about three and Block II five times more capacity in terms of payload mass to LEO, than any launcher which has been used for an interplanetary mission to date.  But the mission doesn't seem more ambitious than Cassini. Is that a fair statement and if so why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-s-europa-mission-begins-with-selection-of-science-instruments&quot; rel=&quot;nofollow&quot;&gt;instruments selected&lt;/a&gt; do not seem to make use of extra mass. There's no 2 meter mirror, not huge PV panels for a powerful radar, no landing probe or impactor. And not enough shielding to orbit Europa directly instead of Jupiter. Shouldn't they do something conceptually new with all the extra mass? Do they want to keep it within the capability of other launchers? Is it likely that two or more missions to the outer planets will be launched at the same time? 3 missions to Mars can do a conservative sample return, wouldn't it be possible with 3 simultaneous missions to Europa too? &lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-08-01T08:30:21.400" Title="Why isn't the Europa mission more ambitious given the capacity of SLS?" Tags="|mission-design|future-missions|sls|robotic-missions|europa|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10423" PostTypeId="1" CreationDate="2015-08-01T11:11:22.387" Score="3" ViewCount="402" Body="&lt;p&gt;Let's say that the big goal for HSF is decided to be to land humans on the surface of Mars. Is it certain that the total mission/campaign risk decreases if one does crewed precursor missions?&lt;/p&gt;&#xA;&lt;p&gt;For example, if a series of precursor missions to LEO, to the Moon and to Phobos each have a 5% probability of failure, then they better lower the risk to land on Mars by more than 15%. Is this a valid way to reason?&lt;/p&gt;&#xA;&lt;p&gt;Is risk per mission more important than overall campaign risk?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="12102" LastEditDate="2021-08-12T23:03:41.440" LastActivityDate="2021-08-12T23:03:41.440" Title="Couldn't precursor missions increase the risk of losing a crew?" Tags="|mars|crewed-spaceflight|experiment|risk-management|multi-launch|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="10424" PostTypeId="2" ParentId="10284" CreationDate="2015-08-01T12:56:58.243" Score="6" Body="&lt;p&gt;If the body on which you want to land has no atmosphere, you need to reduce the spacecraft velocity using the onboard propulsion system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you perform an Hohmann-like manoeuvre you end up having a touch down which is not vertical and the resulting $\Delta$v is actually an underestimation of the real value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ctVir.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ctVir.jpg&quot; alt=&quot;Hohmann-like manoeuvre&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, if you try to perfom a landing like the one shown in the second figure, you will have a vertical terminal trajectory but the $\Delta$v required for the break makes this manoeuvre unfeasible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/c16qW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/c16qW.png&quot; alt=&quot;second type of manoeuvre&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A better estimation of the actual $\Delta$v needed can be found applying an optimal control theory, which will lead to a trajectory more like the one in the third picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s94oV.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s94oV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: The optimal control theory approach&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like any optimization problem, you need to set a goal (what you want to optimize, in this case could be the amount of fuel used for the manoeuvre), then choose a model for the process and parameterize it (to have a finite number of parameters to be tuned). Finally tou have to define a cost function and some constraints.&#xA;The third case I have presented represents a 2-phases landing:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;coasting phase, which is ballistic&lt;/li&gt;&#xA;&lt;li&gt;powered descent phase&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;For this case, you need also to make some assumptions about the panet gravity field (which is assumed to be perfectly spherical) and also you have to assume the the manoeuvre is 2D and that during the power descent the magnitude of the thrust is constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you have set all these things you are ready for the numerical optimization which will give you the resulting trajectory which will minimize the amount of fuel needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All what I have told you was part of a lecture I had last year. The lecturer had developed all this during his master thesis and you can download it here: &#xA;&lt;a href=&quot;http://hdl.handle.net/10589/79781&quot; rel=&quot;noreferrer&quot;&gt;http://hdl.handle.net/10589/79781&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6608" LastEditorUserId="6608" LastEditDate="2015-08-01T17:04:29.660" LastActivityDate="2015-08-01T17:04:29.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10425" PostTypeId="2" ParentId="10423" CreationDate="2015-08-01T13:39:47.257" Score="10" Body="&lt;p&gt;If that were your only criteria, then you would lower the risk even further by not going at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal is for each mission to have an acceptable risk and acceptable reward, to build crewed experience in deep space (i.e. to not have 30 years of uncrewed missions and one big bang at the end of that), and to have the overall developments fit within an achievable budget profile.  That leads to a series of precursor missions of increasing capability as systems are developed within a roughly flat budget profile.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-01T13:39:47.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10426" PostTypeId="2" ParentId="5689" CreationDate="2015-08-01T16:15:25.280" Score="10" Body="&lt;h1&gt;Early Rocket Use&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Starting from the &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=31053.0&quot; rel=&quot;noreferrer&quot;&gt;nasaspaceflight thread&lt;/a&gt; mentioned in comments:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The first gimbaled rocket engines were flown on &lt;a href=&quot;https://en.wikipedia.org/wiki/RTV-A-2_Hiroc&quot; rel=&quot;noreferrer&quot;&gt;MX-774&lt;/a&gt; on July 14, 1948. It had four combustion chambers that could each swivel on one axis:&#xA;&lt;a href=&quot;http://jpcolliat.free.fr/x11/x11-2.htm&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uwB1I.jpg&quot; alt=&quot;MX-774 swiveled engines&quot;&gt;&lt;/a&gt;&#xA;There were &lt;a href=&quot;http://www.astronautix.com/lvs/mx774.htm&quot; rel=&quot;noreferrer&quot;&gt;three failed test flights&lt;/a&gt; where thrust cut off early, the first two don't give a reason but the third blames vibration for closing a valve early.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The second instance of a gimbaled engine (this time a single engine gimbaled on two axes) was &lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;Viking&lt;/a&gt;, first flown on May 3, 1949:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Viking pioneered important innovations over the V-2. One of the most significant for rocketry was the use of a gimbaled thrust chamber which could be swiveled from side to side on two axes for pitch and yaw control, dispensing with the inefficient and somewhat fragile graphite vanes in the engine exhaust used by the V-2.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_%28rocket%29#Flight_history&quot; rel=&quot;noreferrer&quot;&gt;flight history&lt;/a&gt; references a number of successful and failing flight tests, but none blamed specifically on the gimbaling of the engine.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Was this technology first used by jet fighters, or was it first invented for rocket engines?&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Actually, if you're just looking for instances of angling the engine for control purposes &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust_vectoring#Aircraft&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thrust vectoring is also used as a control mechanism for airships. An early application was the British Army airship Delta, which first flew in 1912.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and references &lt;a href=&quot;https://en.wikipedia.org/wiki/Blimp&quot; rel=&quot;noreferrer&quot;&gt;modern non-rigid airships&lt;/a&gt; use of vectorable ducted fans as being similar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at jet engines I'm not finding anywhere that gimballing the engine is used: they just use &lt;a href=&quot;https://en.wikipedia.org/wiki/Vectoring_nozzle&quot; rel=&quot;noreferrer&quot;&gt;vectoring nozzles&lt;/a&gt; to redirect the airflow.  If that's accepted as gimballing then the graphite vanes of the &lt;a href=&quot;https://en.wikipedia.org/wiki/V-2_rocket&quot; rel=&quot;noreferrer&quot;&gt;V-2&lt;/a&gt; would arguably be applicable.  Also, the first reference I found to a jet using thrust vectoring (referenced from &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust_vectoring#Aircraft&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;) is &lt;a href=&quot;http://www.diomedia.com/public/en/18094603/imageDetails.html&quot; rel=&quot;noreferrer&quot;&gt;this concept diagram from 1949&lt;/a&gt;, though the first flight appears to be October 21, 1960 on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hawker_Siddeley_P.1127#P.1127&quot; rel=&quot;noreferrer&quot;&gt;P.1127 prototype&lt;/a&gt; for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hawker_Siddeley_Harrier&quot; rel=&quot;noreferrer&quot;&gt;Harrier&lt;/a&gt;, meaning rockets got there well before jets.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;What were the early successes and failures?&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;I linked to the descriptions of the early test flights for the rockets above.  The rockets had their share of engine problems, but there were no mentions of the engine gimbal failing in flight. I don't have any data on ground testing, which I would expect is where they would have proven it to be capable of withstanding the force of the rocket thrust.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-01T16:21:23.110" LastActivityDate="2015-08-01T16:21:23.110" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10427" PostTypeId="2" ParentId="10324" CreationDate="2015-08-01T21:21:46.230" Score="14" Body="&lt;p&gt;The main problem is that the Moon has many mascons, not a uniform sphere. Close orbits around it tend to get perturbed. They keep the same semi-major axis but become more and more elliptical until they hit the Moon. Early satellites sometimes hit the moon within weeks, while others lasted for months or longer, and it took them a while to figure out why. Contrary to what some articles say on the web, its Hill sphere is easily large enough to permit moonlets to orbit the moon, the problem is, the mascons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from the NASA page: &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit/&quot;&gt;Bizarre Lunar Orbits&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;There are actually a number of 'frozen orbits' where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27º, 50º, 76º, and 86º&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's especially so for surface skimming orbits, or ones that get quite clos. For more distant orbits - the most stable orbits are retrograde. A retrograde orbit around the moon, looked at another way is an elliptical prograde orbit around the Earth, with the same orbital period as the Moon. When closer to the Earth then it orbits it more quickly than the moon and when further away, then it orbits more slowly than the moon, so combined effect is that when seen from the moon it orbits it in a retrograde direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can get retrograde orbits both inside and outside of the lunar L1 and L2 positions - i.e. either inside both of them, or outside both of them, relative to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These orbits are stable over time periods of about a century or so. But not necessarily over longer time periods. With the NASA asteroid capture mission, one of the proposals is to return it to a lunar retrograde orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far though there are no known natural objects in any lunar orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However we wouldn't be able to see really small objects of just a few meters in diameter if they were orbiting the moon. So that leads to a natural question - could it have temporary satellites that we don't know about, which eventually either hit it, or escape to interplanetary space via the Sun Earth L2 (by coincidence the Sun Earth L2 is accessible to either of the lunar L1 or L2 points by a low energy transfer with almost no delta v).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You do get temporary extra moons of the Earth, which orbit it for a few months and then leave it again. Such as &lt;a href=&quot;https://en.wikipedia.org/wiki/2006_RH120&quot;&gt;2006 RH120&lt;/a&gt;, 2-3 meters across which was a satellite of the Earth for nearly a year: September 2006 to June 2007.  &lt;a href=&quot;http://www.newscientist.com/article/mg21328464.600-hundreds-of-tiny-moons-may-be-orbiting-earth.html#.VUzsvvlVhBc&quot;&gt;It's estimated that at any particular time there is probably at least one temporary moon of the Earth one meter in diameter, and perhaps 700 or so tiny moons at least 10 cm in diameter&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also there may be &lt;a href=&quot;https://en.wikipedia.org/wiki/Kordylewski_cloud&quot;&gt;dust in the L4 and L5 positions in the Kordylewski cloud&lt;/a&gt; if it exists - temporary because those are unstable in the Earth Moon system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So given that population of tiny moonlets of the Earth, and these L4 and L5 transient continually replenished and dissipating clouds (if they exist), could there be some moonlets at least 10 cm in diameter in 28 day elliptical orbits around Earth that make them temporary retrograde moonlets of the moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I don't know of any similar result for the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Indeed there are, so far, no known moons anywhere in the solar system that themselves have moons or rings.  At one point Rhea, moon of Saturn was thought to possibly have rings. It still remains a possibility, though the evidence is indirect and inconclusive and a search for a ring hasn't turned up anything. A ring is basically a large number of very small moonlets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We don't yet know of any moon in our solar system with moonlets or rings orbiting it, or indeed, don't know of any &quot;double moon&quot; although double asteroids are quite common.The nearest to this are &lt;a href=&quot;https://en.wikipedia.org/wiki/File:PIA08170_Epimetheus_and_Janus.jpg&quot;&gt;Saturn's two co-orbital moons Epimetheus and Janus&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be interested in my article on this topic, which I wrote as New Horizons was approaching Pluto hence the title - to make it topical - but of course New Horizons didn't find any new moons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.science20.com/robert_inventor/can_moons_have_moonlets_or_rings_moonlets_of_plutos_moons-154917&quot;&gt;Can Moons Have Moonlets? Or Rings? Moonlets Of Pluto's Moons?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="3038" LastEditDate="2015-08-01T21:29:12.940" LastActivityDate="2015-08-01T21:29:12.940" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10428" PostTypeId="2" ParentId="10423" CreationDate="2015-08-02T02:19:25.820" Score="4" Body="&lt;p&gt;To some extend, these are very helpful. For instance, Mercury taught the United States a lot of valuable lessons about spaceflight, and in general each mission was safer than the previous one. Each Apollo mission learned from the lessons of the previous missions. This incremental ability allowed the mission to succeed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Personally, I don't think a manned mission to Mars's moons is ideal for early Mars missions. The only thing that the Mars mission really does more risky than Deimos/ Phobos is it's easier to leave. On the other hand, it has less science and less appeal, although it does still leave some new science. There are several items which are more risky on one of Mars's moons. The possibility of micro-meteor impacts is higher. Such low gravity can cause issues. It seems unlikely that a Phobos mission would be productive for the entire time that a Mars mission would be. The atmosphere of Mars could produce some items (In-Situ Resource Utilization) that would be helpful in the required length of the mission. Overall, I think more would be gained from a direct to Mars mission. But I do think that the entire concept should be tested unmanned at least once, maybe twice, prior to sending humans to Mars.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-02T02:19:25.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10429" PostTypeId="1" AcceptedAnswerId="10435" CreationDate="2015-08-02T08:29:28.853" Score="4" ViewCount="250" Body="&lt;p&gt;Sometimes orbital insertion fails which could be fixed by a space tug. I wonder if a tug could be placed in a long term orbit such that it sometimes matches the intended orbit of a launch from Earth. If the orbital insertion fails, then the tug would not be far away. Application could be for GTO, deep space and for human spaceflight to LEO. It would be available for many launches, those who choose to match their launch window, but would only be consumed when it is needed. Are such rendezvous windows too rare to wait for?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-08-02T18:27:01.747" Title="Could a rescue spacecraft be in an orbit with rendezvous windows for ordinary LEO, GTO or deep space launches?" Tags="|orbital-mechanics|rendezvous|rescue-missions|space-tug|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10430" PostTypeId="2" ParentId="10429" CreationDate="2015-08-02T11:24:28.737" Score="0" Body="&lt;p&gt;This is probably not worthwhile. Essentially, a mission would have to remain in orbit to rescue a mission that made it most of the way there, but not completely. These types of mission are fairly rare. Making the rendezvous would be difficult, the fuel from the tug would have to work long term, the objects would somehow have to dock, and the launch window would have to match pretty closely. All of these put together are pretty unlikely to happen, and even if they did, one is still launching a satellite into orbit to make it happen, which could probably do better if it had a real launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What might work better is to have a secondary launch to fix a spacecraft that had too low of an insertion, launched shortly after the issue with the primary launch. That should reduce the cost of the mission, and make it more likely to succeed.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-02T11:24:28.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10432" PostTypeId="1" CreationDate="2015-08-02T16:05:03.223" Score="11" ViewCount="532" Body="&lt;p&gt;Changing gravity level can be a problem for flows of fluids, gasses, heat and for structural loads. Much equipment designed for Earth gravity fail in microgravity (even light bulbs and ballpoint pens).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how is it the other way around? Do microgravity life support systems in general work also at 1g, maybe simply because it is practical for manufacturing and testing them? Is for example the ISS reliant on life support systems which would not work if we built a copy of the ISS on Earth' surface? And the same for the space shuttle, could a crew have spent a week inside it in 1g? (Consider only difference in gravity, no other environmental factors).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ADDITION:&#xA;Would the coriolis effect and gradient of acceleration in a centrifuge or rotating spacecraft cause significant problems to mechanical equipment, such as flows of heat and fluids and moving parts?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-08-05T02:59:03.853" LastActivityDate="2017-05-15T15:44:08.543" Title="Would the life support systems on the ISS and STS work well also in 1g?" Tags="|space-shuttle|microgravity|space-station|life-support|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10433" PostTypeId="2" ParentId="10432" CreationDate="2015-08-02T16:17:07.867" Score="8" Body="&lt;p&gt;All space shuttle life support systems worked in 1G (in fact, in 3G) because the shuttle experienced those environments for extended periods of time.  The ASCII &quot;graphic&quot; on page 23 of the &lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/492871main_D-O_G_Q_5.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deorbit Prep Checklist&lt;/a&gt; shows the ECLS configuration for all mission phases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least some of the ISS life support systems wouldn't work &quot;out of the box&quot; in 1G - I am thinking specifically about &lt;a href=&quot;http://www.quora.com/How-is-urine-processed-in-the-International-Space-Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;a device that distills waste water&lt;/a&gt;, which has to include &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030066933.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;a rotating centrifuge&lt;/a&gt; to get the distillation process to work.  However, a device to do this in 1G would actually be simpler.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-05-15T15:44:08.543" LastActivityDate="2017-05-15T15:44:08.543" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10434" PostTypeId="2" ParentId="10350" CreationDate="2015-08-02T17:48:33.020" Score="2" Body="&lt;p&gt;NASA isn't in business; it's a branch of government. Governments don't exist for the purpose of turning a profit like most corporations do. The primary goal of the Space Shuttle was cheap and reliable access to space. Unfortunately, for various reasons as covered in other answers, it failed to deliver on that promise. While the Shuttle was in service, other launchers were delivering commercial payloads to space because they were more cost-effective, capable, or available for the mission required. In other words, the Shuttle would have been incapable of delivering a profit from a commercial launch business because it would have been out-competed on price or unable to perform the mission required. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the Shuttle did offer some unique capabilities, which made it the vehicle of choice for certain missions. For one thing, only the Shuttle could recover an object from orbit and bring it safely down to Earth - such as a faulty satellite or experiment package not designed/equipped to re-enter/touchdown/splashdown on its own. When you're the only option for a niche service where the job must get done, cost takes a back seat. But, with no charter to deliver a profit, you just charge out at cost anyway.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-08-02T17:48:33.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10435" PostTypeId="2" ParentId="10429" CreationDate="2015-08-02T18:27:01.747" Score="7" Body="&lt;p&gt;An on-orbit rescue tug isn't likely to be viable in the near future.&lt;/p&gt;&#xA;&lt;h1&gt;General&lt;/h1&gt;&#xA;&lt;p&gt;Automated docking is typically only done at the ISS and requires extra hardware, both the docking port and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kurs_%28docking_navigation_system%29&quot; rel=&quot;noreferrer&quot;&gt;radar systems&lt;/a&gt; to align vessels.  This is a non-trivial amount of additional mass and power required just to allow a space tug to have a chance of rescuing your satellite in the unlikely case of failure.&lt;/p&gt;&#xA;&lt;p&gt;Any thrust you apply must be through the center of mass of the combined tug + satellite or you'll get rotation.  That adds an additional design constraint in limiting where the docking attachment point is on anything that would want to take advantage of your space tug. Also, the satellite is going to need to support docking in-line with the direction that it can tolerate thrust.  Unfortunately, that's the perfect place to put an engine so anything self-propelled is likely to have an engine right where you want to dock.&lt;/p&gt;&#xA;&lt;p&gt;Many satellites deploy mission-related hardware after they're inserted into their final orbit (extend solar panels, antennas, sensors, etc).  I understand they typically rely on internal battery power through launch/insertion.  They won't be able to extend solar panels when there is still thrust to be applied (or the panels would bend/snap off) and batteries don't last forever, so your window of rescue is likely to be very small to fetch the satellite from the failed insertion and place it where it needs to be.&lt;/p&gt;&#xA;&lt;h1&gt;LEO&lt;/h1&gt;&#xA;&lt;p&gt;In LEO if orbital insertion outright fails then the satellite is reentering the atmosphere within the next couple of hours.  That doesn't give much time for rendez-vous.  It would be possible (given that the desired orbit lines up) to launch specifically to meet up with the tug, but that puts additional constraints on the launch window, just so that if something that usually works happens to fail you could recover from it.&lt;/p&gt;&#xA;&lt;p&gt;Also, &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination_change&quot; rel=&quot;noreferrer&quot;&gt;plane changes are expensive&lt;/a&gt; to the point that mission planners avoid them if at all possible.  There are a wide variety of LEO missions in different inclinations:&#xA;&lt;a href=&quot;http://www.wmo.int/pages/prog/sat/globalplanning_en.php&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Fj1PK.jpg&quot; alt=&quot;types of LEO orbits&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Even if your inclination matches that of the launched satellite you also have to align your &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;noreferrer&quot;&gt;longitude of ascending node&lt;/a&gt; to be in the same plane, and again, different missions have different desired planes so they are very unlikely to be launching into the plane your tug is parked in.&lt;/p&gt;&#xA;&lt;p&gt;If your orbital insertion mostly works (you're in orbit, just not where you wanted to be) then you may have time to catch the satellite but the manner in which it failed becomes important.  If the satellite is left tumbling rapidly like the Progress flight that failed a while back then you won't be able to dock with it anyway.&lt;/p&gt;&#xA;&lt;h1&gt;GEO&lt;/h1&gt;&#xA;&lt;p&gt;In terms of time before decay and alignment boosting a satellite from an equatorial LEO parking orbit to GEO is more reasonable: your tug could already be in a LEO equatorial orbit and would just need to rendez-vous with the satellite.  On the other hand, according to &lt;a href=&quot;http://www.lr.tudelft.nl/en/organisation/departments/space-engineering/space-systems-engineering/expertise-areas/space-propulsion/design-data/typical-delta-v-value/&quot; rel=&quot;noreferrer&quot;&gt;this chart&lt;/a&gt; the delta-v required to get from LEO to GEO is nearly 4 km/s (or ~2.5 km/s if the satellite will cover the transfer orbit to GEO burn).  That's nearly half the delta-v required to just get to orbit (9.5 km/s), meaning your tug is going to have to be fairly large to carry enough fuel to even go one way and if you want to bring it back to LEO to reuse at least double that.&lt;/p&gt;&#xA;&lt;p&gt;In this case you've also got to deal with an upper stage that failed to (re)ignite to get from the parking orbit to transfer orbit.  Even assuming correct separation of the stage from the satellite to allow your tug to attach the stage will be nearby with the potential for collision (depending on how quickly you get the tug there and why it failed).&lt;/p&gt;&#xA;&lt;h1&gt;Interplanetary&lt;/h1&gt;&#xA;&lt;p&gt;Rescue of interplanetary missions stuck in LEO would probably have the biggest pay-off because launch windows come so infrequently.  On the other hand, providing the delta-v to get a mission on an interplanetary trajectory would require a large tug and to complete the mission it's likely the tug would have to go along for the entire mission.  That means you're not getting it back for reuse, and it also has to have enough power to send a signal back to Earth from wherever it ends up.&lt;/p&gt;&#xA;&lt;p&gt;Also, if the propulsion for the mission is built into the spacecraft (rather than being a detachable stage) you now have to haul all the fuel that would have been used with you, adding mass to what the tug has to move.&lt;/p&gt;&#xA;&lt;h1&gt;Conclusion&lt;/h1&gt;&#xA;&lt;p&gt;A lot has to go right for a tug to have a chance at working, and mission planning/spacecraft design would have to support the rescue tug (docking ports, launch into a plane with the tug).  Also, all planning to support the tug is planning specifically for propulsion to fail in specific places (but not catastrophically) and if that were so compelling a reason I would think we'd see backup systems to recover (I know Falcon 9 has engine-out capability on the first stage, I can't find any indication that upper stages have backup systems).&lt;/p&gt;&#xA;&lt;p&gt;Considering how rare failure in a situation where a tug could help out I can't see the support for a tug being built into satellites as a backup plan.  On the other hand, if you could build a space tug that were efficient and cheap to operate that may have potential for being incorporated in mission planning as the first option.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-02T18:27:01.747" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10436" PostTypeId="5" CreationDate="2015-08-03T02:46:49.760" Score="0" Body="&lt;p&gt;The Kestrel engine is an LOX/RP-1 pressure-fed rocket engine. The Kestrel engine was developed by SpaceX for upper stage use on the Falcon 1 rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kestrel was built around the same pintle architecture as the Space X Merlin engine but does not have a turbo-pump and is fed only by tank pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kestrel is ablatively cooled in the chamber and throat and radiatively cooled in the nozzle, which is fabricated from a high strength niobium alloy. As a metal, niobium is highly resistant to cracking compared to carbon-carbon. According to SpaceX, an impact from orbital debris or during stage separation might dent the metal but have no meaningful effect on engine performance. Helium pressurant efficiency is substantially increased via a titanium heat exchanger on the ablative/niobium boundary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thrust vector control is provided by electro-mechanical actuators on the engine dome for pitch and yaw. Roll control (and attitude control during coast phases) is provided by helium cold gas thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A TEA-TEB pyrophoric system is used to provide multiple restart capability on the upper stage. In a multi-manifested mission, this allows for drop off at different altitudes and inclinations.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Kestrel_%28rocket_engine%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-03T03:06:02.340" LastActivityDate="2015-08-03T03:06:02.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10437" PostTypeId="4" CreationDate="2015-08-03T02:46:49.760" Score="0" Body="The Kestrel engine is an LOX/RP-1 pressure-fed rocket engine. The Kestrel engine was developed by SpaceX for upper stage use on the Falcon 1 rocket." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-03T03:16:23.290" LastActivityDate="2015-08-03T03:16:23.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10438" PostTypeId="5" CreationDate="2015-08-03T02:50:56.447" Score="0" Body="&lt;p&gt;In astronautics, a powered flyby, or Oberth maneuver, describes a maneuver in which a rocket falls into a gravitational well, and then accelerates when its fall reaches maximum speed. The resulting maneuver is actually a more efficient way to gain kinetic energy than applying the same impulse outside of a gravitational well. The gain in efficiency is explained by the Oberth effect, a phenomenon that allows employing kinetic energy to generate greater mechanical energy. In practical terms, this means that the most efficient time for a spacecraft to burn its engine is at orbital periapse, when its orbital velocity (and so, its kinetic energy) is greatest. In some cases, it is even worth spending fuel on slowing the rocket into a gravity well to take advantage of the efficiencies of the Oberth effect. The maneuver and effect are named after Hermann Oberth, the Austro-Hungarian-born German physicist and a founder of modern rocketry, who first described them in 1927.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Oberth effect is strongest at a point in orbit known as the periapse, where the gravitational potential is lowest, and the speed is highest. This is because firing a rocket engine at high speed causes a greater change in kinetic energy than when fired at lower speed. Because the vehicle remains near periapse only for a short time, for the Oberth maneuver to be most effective, the vehicle must be able to generate as much impulse as possible in as short a time as possible. Thus, the Oberth effect is much more useful for high thrust rockets like liquid-propellant rockets, and less useful for low-thrust reaction engines such as ion drives, which take a long time to gain speed. The Oberth effect also can be used to understand the behavior of multi-stage rockets: the upper stage can generate much more usable kinetic energy than the total chemical energy of the propellants it carries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The effect does not violate the law of conservation of energy. The Oberth effect occurs because the propellant has more usable energy due to its kinetic energy on top of its chemical potential energy. The vehicle is able to employ this kinetic energy to generate more mechanical power.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/orbital-mechanics&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;orbital-mechanics&amp;#39;&quot; rel=&quot;tag&quot;&gt;orbital-mechanics&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-03T03:05:23.777" LastActivityDate="2015-08-03T03:05:23.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10439" PostTypeId="4" CreationDate="2015-08-03T02:50:56.447" Score="0" Body="A powered flyby, or Oberth maneuver, describes a maneuver in which a rocket falls into a gravitational well, and then accelerates when its fall reaches maximum speed. The resulting maneuver is actually a more efficient way to gain kinetic energy than applying the same impulse outside of a gravitational well." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-03T03:16:14.920" LastActivityDate="2015-08-03T03:16:14.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10440" PostTypeId="5" CreationDate="2015-08-03T03:53:14.147" Score="0" Body="&lt;p&gt;Gaia is a space observatory of the European Space Agency (ESA) designed for astrometry. The mission aims to construct a 3D space catalog of approximately 1 billion astronomical objects, mainly stars (approximately 1% of the Milky Way population) brighter than 20 G magnitudes, where G is the Gaia magnitude passband between about 400 and 1000 nanometres light wavelengths. Additionally Gaia is expected to detect thousands to tens of thousands of Jupiter-sized planets beyond the Solar System, 500,000 quasars and tens of thousands of new asteroids and comets within the Solar System. The spacecraft will monitor each of its target stars about 70 times over a period of five years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Successor to the Hipparcos mission, the telescope is part of ESA's Horizon 2000+ long-term scientific program. Gaia was launched on 19 December 2013 by Arianespace using a Soyuz ST-B/Fregat-MT rocket flying from Kourou in French Guiana. The spacecraft currently operates in a Lissajous orbit around the Sun–Earth L2 Lagrangian point.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Related Tags&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/space-telescope&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;space-telescope&amp;#39;&quot; rel=&quot;tag&quot;&gt;space-telescope&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;/questions/tagged/esa&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;esa&amp;#39;&quot; rel=&quot;tag&quot;&gt;esa&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Resources&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sci.esa.int/gaia/&quot; rel=&quot;nofollow&quot;&gt;ESA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gaia_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-03T05:07:21.743" LastActivityDate="2015-08-03T05:07:21.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10441" PostTypeId="4" CreationDate="2015-08-03T03:53:14.147" Score="0" Body="Gaia is a space observatory of the European Space Agency (ESA) designed for astrometry. The mission aims to construct a 3D space catalog of approximately 1 billion astronomical objects." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-03T05:07:30.587" LastActivityDate="2015-08-03T05:07:30.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10442" PostTypeId="1" AcceptedAnswerId="10445" CreationDate="2015-08-03T04:51:09.213" Score="6" ViewCount="314" Body="&lt;p&gt;With the recent images from New Horizons, my question is simple. How do we know &lt;a href=&quot;https://www.nasa.gov/image-feature/the-icy-mountains-of-pluto&quot;&gt;these mountains&lt;/a&gt; are water ice? The linked article doesn't have much detail on how water ice could make 10,000m+ mountains.&lt;/p&gt;&#xA;" OwnerUserId="7052" LastActivityDate="2015-08-03T09:19:59.140" Title="Ice Mountains on Pluto" Tags="|new-horizons|pluto|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10443" PostTypeId="2" ParentId="9304" CreationDate="2015-08-03T04:53:05.187" Score="6" Body="&lt;p&gt;You pretty much answered your own question.  You target a point on what is called the B-plane.  Imagine for a moment that the planet is just a point in space (where the center of the planet is) with no mass.  The B-plane goes through that point and is perpendicular to your trajectory through that point.  To target particular flyby conditions, you target a point on that B-plane.  The distance from the center of the B-plane determines your closest approach radius, per the formula in your question ($b$).  That gives you a circle of targets to pick from that determines either your flyby $\Delta V$ direction or your orbit insertion plane and retrograde or prograde.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-08-03T14:02:41.993" LastActivityDate="2015-08-03T14:02:41.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10444" PostTypeId="2" ParentId="10442" CreationDate="2015-08-03T09:10:27.983" Score="5" Body="&lt;p&gt;From the article you linked to:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The mountains are &lt;em&gt;probably&lt;/em&gt; composed of Pluto’s water-ice “bedrock.”&lt;/p&gt;&#xA;&lt;p&gt;Although methane and nitrogen ice covers much of the surface of Pluto, these materials are not strong enough to build the mountains. Instead, a stiffer material, most likely water-ice, created the peaks. “At Pluto’s temperatures, water-ice behaves more like rock,” said deputy GGI lead Bill McKinnon of Washington University, St. Louis.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;emphasis mine. For now, water ice is our best guess based on the elements we know exist op Pluto and their properties. Confirmation should come from &lt;a href=&quot;http://www.nasa.gov/mission_pages/newhorizons/multimedia/Ralph.html&quot; rel=&quot;noreferrer&quot;&gt;RALPH&lt;/a&gt; spectroscopic measurements of the region.&lt;/p&gt;&#xA;&lt;p&gt;Planetary geologists have studied the behavior of &lt;a href=&quot;https://en.wikipedia.org/wiki/Geology_of_Pluto&quot; rel=&quot;noreferrer&quot;&gt;various compounds at the temperatures found on Pluto&lt;/a&gt;, and:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Water ice is the only ice detected on Pluto that is strong enough at Plutonian temperatures to support such heights.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-03T09:10:27.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10445" PostTypeId="2" ParentId="10442" CreationDate="2015-08-03T09:19:59.140" Score="4" Body="&lt;p&gt;To quote from the source directly, &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto&quot; rel=&quot;nofollow&quot;&gt;Pluto&lt;/a&gt; is a dwarf planet in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kuiper_belt&quot; rel=&quot;nofollow&quot;&gt;Kuiper belt&lt;/a&gt;&quot;. The Kuiper belt is similar to the asteroid belt but wider and located beyond Neptune.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. Although many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed &quot;ices&quot;), such as methane, ammonia and water. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://scientiflix.wordpress.com/2015/07/21/pluto-in-a-minute-ice-mountains-were-a-real-surprise-for-new/&quot; rel=&quot;nofollow&quot;&gt;The average surface temperature on Pluto&lt;/a&gt; is $40$ $K$, $-233$ $^oC$.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At temperatures that cold, ice behaves like a rock, and can actually hold on to the topography and texture that we see on rocks on Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;During the &lt;a href=&quot;https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System&quot; rel=&quot;nofollow&quot;&gt;formation of the solar system&lt;/a&gt;, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The inner Solar System, the region of the Solar System inside 4 AU, was too warm for volatile molecules like water and methane to condense, so the planetesimals that formed there could only form from compounds with high melting points, such as metals (like iron, nickel, and aluminium) and rocky silicates. These rocky bodies would become the terrestrial planets (Mercury, Venus, Earth, and Mars). &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid. The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the giant planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Being beyond the orbit of Neptune, Pluto and the Kuiper belt are also well beyond the frost line. This is how scientists know Pluto is largely composed of frozen volatiles and that its mountains are made of water ice.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-08-03T09:19:59.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10447" PostTypeId="1" CreationDate="2015-08-03T12:02:24.810" Score="1" ViewCount="840" Body="&lt;p&gt;If a rocket carrying a GPS device ascended &lt;em&gt;above&lt;/em&gt; the height of the orbit of the GPS satellites (about 20,000km), what height would the GPS device report?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it keep reporting the correct height as it passed through the orbit height, or would it start reporting a reducing height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In simple math, if GPS satellite height is &lt;code&gt;h&lt;/code&gt;, at height &lt;code&gt;h + x&lt;/code&gt;, would a GPS device report its height as &lt;code&gt;h + x&lt;/code&gt; or &lt;code&gt;h - x&lt;/code&gt;, or something else?&lt;/p&gt;&#xA;" OwnerUserId="10584" LastActivityDate="2015-08-03T12:02:24.810" Title="Would GPS work above the GPS satellites?" Tags="|gps|" AnswerCount="0" CommentCount="2" ClosedDate="2015-08-03T14:24:19.953" ContentLicense="CC BY-SA 3.0" />
  <row Id="10448" PostTypeId="1" CreationDate="2015-08-03T15:54:26.503" Score="8" ViewCount="2073" Body="&lt;p&gt;I recollect, that after each of the early shuttle launches, 100 or so thermal control tiles were found missing.  These had to be replaced before each subsequent flight.  I remember wondering why the lost tiles were there in the first place if they apparently weren't necessary for thermal protection in the area of the shuttle where they were placed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I am wondering whether it was just dumb luck that more shuttles weren't lost during re-entry. Was it just luck that these tiles just happened to come off in areas where their thermal control was not necessary?  Perhaps they came off after the major portion of the re-entry was completed and the thermal loads were not very high?  Was the fly-off of these tiles a potential source of vehicle impact damage during re-entry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, as this problem seemed to go away, how were fixes made?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyone have any detailed knowledge (damage reports, analysis reports) of the impact these lost tiles could have had to the vehicle?&lt;/p&gt;&#xA;" OwnerUserId="6501" LastEditorUserId="25" LastEditDate="2015-08-03T16:42:44.797" LastActivityDate="2015-08-04T04:58:55.557" Title="Was Shuttle tile loss an accident waiting to happen?" Tags="|nasa|reentry|space-shuttle|safety|" AnswerCount="3" CommentCount="6" ClosedDate="2015-08-12T05:40:33.397" ContentLicense="CC BY-SA 3.0" />
  <row Id="10449" PostTypeId="2" ParentId="10448" CreationDate="2015-08-03T16:41:48.207" Score="3" Body="&lt;p&gt;Yes, in fact. &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-27&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-27&lt;/a&gt; was impacted at the solid booster release, leaving a large hole. In fact, the aluminum structure holding the shuttle was damaged, as is show in the below photo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H2APX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H2APX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-03T16:41:48.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10451" PostTypeId="1" AcceptedAnswerId="10646" CreationDate="2015-08-03T18:12:09.670" Score="9" ViewCount="429" Body="&lt;p&gt;As Voyager 1 continues its trek across interstellar space it has been reporting increased amounts of &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot; rel=&quot;nofollow noreferrer&quot;&gt;CME Plasma&lt;/a&gt; and Interstellar Plasma.  When these two types of plasma interact &lt;a href=&quot;http://www.jpl.nasa.gov/video/details.php?id=1347&quot; rel=&quot;nofollow noreferrer&quot;&gt;a shock wave is generated&lt;/a&gt; with the last going on for more than a year.  As these shock waves are &lt;a href=&quot;http://voyager.jpl.nasa.gov/news/tsunami_wave.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;expected to propagate far out into space&lt;/a&gt;, how does this affect future probes and astronauts?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Do spacecraft require additional shielding based on what we've learned so far?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Is the shock wave generated by the plasma's meeting each other dangerous to life or manned space craft?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XHYRk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XHYRk.png&quot; alt=&quot;Tsunami Waves&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11104" LastActivityDate="2015-08-14T14:00:12.273" Title="Is interstellar plasma a danger for future probes and astronauts?" Tags="|interstellar-travel|voyager|cosmic-radiation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10452" PostTypeId="2" ParentId="10448" CreationDate="2015-08-03T21:37:25.847" Score="9" Body="&lt;p&gt;This question is too broad IMHO but if you want to learn about the history of tile problems and the organizational response, there is no better starting place than Chapter 6 of &lt;a href=&quot;https://www.nasa.gov/columbia/caib/html/report.html&quot; rel=&quot;noreferrer&quot;&gt;the CAIB report&lt;/a&gt;, &quot;Decision Making at NASA&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a very pertinent graph to pique your interest.  It shows a count of tile damage sites of greater than one inch diameter per mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9E14m.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9E14m.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: I can address a couple more of your questions.  There is information about all these points in the CAIB report.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Perhaps they came off after the major portion of the re-entry was&#xA;  completed and the thermal loads were not very high?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This seems to be incorrect.  From the reference above, most of the known tile damage was caused by impacts during ascent. This is determinable by looking at the resulting thermal damage. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Also, as this problem seemed to go away, how were fixes made?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you are referring to the decrease in damage after the 2nd return to flight, an enormous amount of effort was expended to reduce foam shedding from the External Tank, a primary source of tile-damaging debris during ascent.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-08-04T00:00:24.580" LastActivityDate="2015-08-04T00:00:24.580" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10453" PostTypeId="2" ParentId="10432" CreationDate="2015-08-04T00:07:02.633" Score="8" Body="&lt;p&gt;A strange corollary that demonstrates unexpected issues in low-G, is the water filtration systems, that process urine on the ISS.  They were getting clogged in orbit, because the astronauts were losing much more calcium than when under a full G, and it was excreted in their urine, and was calcifying the filtration system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it could work in 1G as well, and if fact would work better, since less calcium to cause a problem.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6944" LastEditDate="2015-08-04T00:10:05.373" LastActivityDate="2015-08-04T00:10:05.373" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10454" PostTypeId="2" ParentId="10448" CreationDate="2015-08-04T02:24:23.470" Score="5" Body="&lt;p&gt;Lets break this down to specific questions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;after each of the early shuttle launches, 100 or so thermal control&#xA;  tiles were found missing. These had to be replaced before each&#xA;  subsequent flight. Why [were] the lost tiles were there in the first&#xA;  place if they apparently weren't necessary for thermal protection in&#xA;  the area of the shuttle where they were placed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are several assumptions here, which are not supported in fact. There were many spots of tile damage greater than 1 inch (25mm) in diameter. That's not the same as a missing tile, and the design of the tiles may have included increased thickness to allow for expected abrasions, dents, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would be surprised if there were hundreds of missing tiles. Hundreds of spots of damage greater than 1 inch in diameter is quite a different thing. Each tile was several inches thick and its likely they could meet specification even with some amount of damage on the exposed side. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are multiple books about the detailed design of the shuttle and the operating regimes they were expected to face, and actually faced. Perhaps the first step is identifying kinds of tile damage (&amp;lt;1% of depth affected, &gt;1 inch skin face affected &amp;lt;10% of depth affected, tile missing, tile and adjacent tile missing.) you care about, or that someone has already studies. Then you need to get the title damage survey data. Chart damage kind or instance by launch date.&#xA;Columbia, the Shuttle lost from surface damage, was not destroyed by tile damage. It had its carbon-carbon leading edge damaged. Different material, common damage mechanism (foam falling off the booster tank).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;was just dumb luck that more shuttles weren't lost during re-entry?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A very large question, but almost certainly luck, aka capable design, competent manufacturing, responsive operators, redundant systems, conservative rules, occasional reasonable management, played a role.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Was it just luck that these tiles just happened to come off in areas&#xA;  where their thermal control was not necessary?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Objection. Presumes hundreds of tiles damaged == hundreds missing. Missing is different and far more dangerous. If you can get tile damage and tile missing data, a composite plot of the locations which lost tiles, which suffered damage &gt; 1 inch diameter, &gt; 1% of tile thickness, &gt; 10% tile thickness, and lost tiles AND adjacent tiles would be worthy reductions of fact, on the way to information.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Perhaps they came off after the major portion of the re-entry was&#xA;  completed and the thermal loads were not very high?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Earlier response indicates majority lost during ascent, which makes sense because: Fast in dense air, tank shedding insulation not present on re-entry, &#xA;water, ice, bird exhaust, etc, more likely from Earth rather than orbit.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Was the fly-off of these tiles a potential source of vehicle impact&#xA;  damage during re-entry?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Another respondant says, &quot;No&quot;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;this problem seemed to go away, how were fixes made?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;meaning tiles, not including leading edges? As noted, big efforts to keep foam from shedding from booster.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Anyone have any detailed knowledge (damage reports, analysis reports)&#xA;  of the impact these lost tiles could have had to the vehicle?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You might adjust this question once you know more of what actually happened.&lt;/p&gt;&#xA;" OwnerUserId="11196" LastEditorUserId="-1" LastEditDate="2015-08-04T04:58:55.557" LastActivityDate="2015-08-04T04:58:55.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10457" PostTypeId="2" ParentId="940" CreationDate="2015-08-04T05:45:16.560" Score="7" Body="&lt;p&gt;The Aerospace Corporation flew a cold gas (xenon) thruster on the MEPSI mission in 2006.  It thrusted as expected.   MEPSI was a 4&quot; x 4&quot; 5&quot; spacecraft, so it wasn't a quite a CubeSat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerospace will fly a steam thruster on their &lt;a href=&quot;https://eoportal.org/web/eoportal/satellite-missions/a/aerocube-ocsd&quot;&gt;AeroCube OSSD-B&amp;amp;C&lt;/a&gt; missions early next year.&lt;/p&gt;&#xA;" OwnerUserId="11200" LastEditorUserId="168" LastEditDate="2015-08-04T06:29:31.923" LastActivityDate="2015-08-04T06:29:31.923" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10459" PostTypeId="1" AcceptedAnswerId="10472" CreationDate="2015-08-04T09:19:31.213" Score="4" ViewCount="370" Body="&lt;p&gt;The accepted answer to &lt;a href=&quot;https://space.stackexchange.com/questions/9757/what-abort-modes-does-soyuz-have-in-space&quot;&gt;this question&lt;/a&gt; states that the SAS Launch Escape System of a Soyuz spacecraft is used for aborts early in the ascend phase and it has a variety of abort modes after the SAS is jettisoned. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the SuperDracos of the Dragon v2 are to my understanding the equivalent to the SAS, what is used as an abort system after the point where Soyuz's SAS were to be jettisoned(after 157 seconds into the flight)? Are the SuperDracos the only abort system or does Dragon v2 have a variety of abort modes like the Soyuz does after the SAS is jettisoned?&lt;/p&gt;&#xA;" OwnerUserId="9029" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-08-04T17:16:48.733" Title="Does Dragon v2 only have Superdracos as an abort system?" Tags="|spacex|abort|dragon-v2|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10460" PostTypeId="1" AcceptedAnswerId="10463" CreationDate="2015-08-04T11:10:28.027" Score="16" ViewCount="4995" Body="&lt;p&gt;Despite having very little geological activity, without any active tectonic plates, the moon is prone to shaking shifts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I know, moonquakes can be caused by the Sun, thermal quakes, or by meteorites but moonquakes of up to 5.5 in the Richter scale, lasting 10 minutes, have been reported. I doubt either the Sun or a meteorite could cause them. In the first case, thermal quakes would have a low magnitude and, in the case of meteorites, the moonquake wouldn't last 10 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do these long and high magnitude occur and what causes them?&lt;/p&gt;&#xA;" OwnerUserId="11164" LastEditorUserId="25" LastEditDate="2015-08-04T13:30:09.840" LastActivityDate="2015-08-04T13:30:09.840" Title="What causes moonquakes?" Tags="|the-moon|planetary-science|geology|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10463" PostTypeId="2" ParentId="10460" CreationDate="2015-08-04T13:28:39.150" Score="16" Body="&lt;p&gt;You are correct about some of the sources of moon quakes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/15mar_moonquakes/&quot;&gt;NASA states&lt;/a&gt; there are are least four types of moon quakes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are at least four different kinds of moonquakes: (1) deep moonquakes about 700 km below the surface, probably caused by tides; (2) vibrations from the impact of meteorites; (3) thermal quakes caused by the expansion of the frigid crust when first illuminated by the morning sun after two weeks of deep-freeze lunar night; and (4) shallow moonquakes only 20 or 30 kilometers below the surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The first three were generally mild and harmless. Shallow moonquakes on the other hand are the problematic ones. They are the ones that can last for more than 10 minutes.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The reason has to do with chemical weathering, Neal explains: &quot;&lt;strong&gt;Water weakens stone, expanding the structure of different minerals. When energy propagates across such a compressible structure, it acts like a foam sponge—it deadens the vibrations.&quot; Even the biggest earthquakes stop shaking in less than 2 minutes&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The moon, however, is dry, cool and mostly rigid, like a chunk of stone or iron. So moonquakes set it vibrating like a tuning fork&lt;/strong&gt;. Even if a moonquake isn't intense, &quot;it just keeps going and going,&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for the shallow moon quakes and where they occur is unknown.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The Apollo seismometers were all in one relatively small region on the front side of the moon, so we can't pinpoint [the exact locations of these quakes].&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It has been suggested that one possible cause may be the rims of large relatively young crater rims occasionally slumping.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-08-04T13:28:39.150" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10464" PostTypeId="1" AcceptedAnswerId="10465" CreationDate="2015-07-31T18:05:24.257" Score="8" ViewCount="3808" Body="&lt;p&gt;Within the context of Aeronautics, there are seemingly different meanings corresponding to &lt;em&gt;Space Dynamics&lt;/em&gt; and &lt;em&gt;Astrodynamics&lt;/em&gt;. What is the difference between them in an engineering perspective, such as spacecraft dynamics and control?&lt;/p&gt;&#xA;" OwnerDisplayName="user53147" LastEditorUserId="18728" LastEditDate="2017-03-05T21:28:12.040" LastActivityDate="2018-10-31T08:59:42.227" Title="What is the difference between Space Dynamics &amp; Astrodynamics in the engineering perspective?" Tags="|engineering|astrodynamics|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10465" PostTypeId="2" ParentId="10464" CreationDate="2015-08-04T14:01:31.057" Score="6" Body="&lt;p&gt;Vallado begins [&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1881883140&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;] by borrowing Griffin and French's definition of astrodynamics as &quot;...the study of the motion of man-made objects in space, subject to both natural and artificially induced forces.&quot; Vallado uses this definition because other related subjects - e.g., orbital dynamics, attitude dynamics - don't solely encapsulate what astrodynamics is. In the context of your question, one could claim that space dynamics is covered under the umbrella term of astrodynamics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most literature equates space dynamics with the kinematics of a problem, while astrodynamics covers both the kinetics and the kinematics. From an engineering perspective, think of this as space dynamics dealing with the right-hand-side of $F = ma$, while astrodynamics develops the left-hand-side and (attempts to) finds a solution to the overall system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1: &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1881883140&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vallado, David A. Fundamentals of Astrodynamics and Applications, 3rd Ed. 2007&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1478" LastEditorUserId="1142" LastEditDate="2017-03-06T01:19:17.747" LastActivityDate="2017-03-06T01:19:17.747" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10466" PostTypeId="1" AcceptedAnswerId="10474" CreationDate="2015-08-04T14:46:54.473" Score="11" ViewCount="14969" Body="&lt;p&gt;The Moon has less gravity than the Earth and its own orbital speed around the Earth removes some of the velocity required to leave the Earth-Moon space, right? We could send probes or spacecraft much faster than if we do it from the Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why don't we send material to the Moon, maybe along with a human crew (just for a few days), to use the Moon as a launch site for interplanetary spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="11205" LastEditorUserId="49" LastEditDate="2015-08-05T12:59:11.060" LastActivityDate="2023-12-10T12:35:12.357" Title="Why don't we launch spacecraft from the Moon?" Tags="|launch|the-moon|probe|interplanetary|crewed-spaceflight|" AnswerCount="4" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="10467" PostTypeId="2" ParentId="10466" CreationDate="2015-08-04T15:03:47.020" Score="7" Body="&lt;p&gt;It doesn't work as well as you'd think. Here's a few problems:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;If resources are launched to the Moon, then one is taking away from the possible use of fuel. The only advantage gained by launching from the Moon is using resources from the Moon, otherwise it's cheaper to just launch the rocket from Earth.&lt;/li&gt;&#xA;&lt;li&gt;Most launches need to get to GEO or LEO orbit for satellites. These are about as difficult to launch from the Moon as from Earth, assuming no atmospheric braking. &lt;/li&gt;&#xA;&lt;li&gt;There are no satellite manufacturing stations on the Moon. Few parts used in satellites could be manufactured there easily. The most useful I believe would be the fuel used in satellites, but most of the others would require extensive work.&lt;/li&gt;&#xA;&lt;li&gt;For interplanetary missions, it's rather difficult to launch directly from the moon. What is actually best is to do a 2 stage system, using a flyby of the Earth. Again, some gain would be present in the delta v, but not as much as you'd think.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, I think this could be done given a base on the Moon, but I don't think it's particularly useful at this time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6599" LastEditDate="2017-03-16T11:41:59.130" LastActivityDate="2017-03-16T11:41:59.130" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10468" PostTypeId="1" AcceptedAnswerId="10469" CreationDate="2015-08-04T15:31:50.213" Score="13" ViewCount="3809" Body="&lt;p&gt;In NASA spacecraft development, subsequent versions of a design are referred to as &quot;Block 1&quot;, &quot;Block 2&quot;, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the origin of this naming convention?  Why not just call them &quot;Version 1&quot;, &quot;Version 2&quot; (or &quot;Model 1&quot;, &quot;Model 2&quot;)?&lt;/p&gt;&#xA;" OwnerUserId="11208" LastEditorUserId="25" LastEditDate="2015-08-04T18:39:19.157" LastActivityDate="2020-11-29T03:57:11.723" Title="Origin of term &quot;Block I&quot;, &quot;Block II&quot;, etc" Tags="|nasa|design|terminology|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10469" PostTypeId="2" ParentId="10468" CreationDate="2015-08-04T16:07:58.257" Score="16" Body="&lt;p&gt;I am not sure, but I think it comes from US military procurement, where it refers to the actual items (vehicles, weapons systems, etc.) being produced and delivered. For example, if you want 10 ships of a single class built over 4 years, but the latest and greatest radar system won't be available right away, the contract might refer to two &quot;blocks&quot; of ships, 4 ships in Block I with the older radar, and 6 ships in Block II with the new radar. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To me, it connotes something more concrete than &quot;model&quot; or &quot;version&quot; -- the block is the actual set of items going to a customer, as opposed to the picture in the catalog or the set of design drawings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know how long it's been in use, or where it started. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The earliest military use I've seen is in reference to the &lt;a href=&quot;https://en.wikipedia.org/wiki/FIM-43_Redeye#Development&quot;&gt;Redeye&lt;/a&gt; surface-to-air missile, which had a &quot;Block I&quot; as early as 1963. The first set of &lt;a href=&quot;https://en.wikipedia.org/wiki/GPS_(satellite)#Block_I_satellites&quot;&gt;GPS satellites&lt;/a&gt; contracted in 1974 and launched in 1978 was referred to as &quot;Block I&quot;, and the first version of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Phalanx_CIWS&quot;&gt;Phalanx CIWS&lt;/a&gt; that went into production in 1978 is &quot;Block 0&quot;, but that name may well have been retroactively applied when the Block 1 upgrade became a thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ranger_program&quot;&gt;Ranger&lt;/a&gt;'s three development blocks predate Redeye, though:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ranger was originally designed, beginning in 1959, in three distinct phases, called &quot;blocks&quot;. Each block had different mission objectives and progressively more advanced system design. The JPL mission designers planned multiple launches in each block, to maximize the engineering experience and scientific value of the mission and to assure at least one successful flight. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;USN also uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Arleigh_Burke-class_destroyer#Development&quot;&gt;&quot;flight&quot;&lt;/a&gt; in exactly the same sense as &quot;block&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-08-04T16:43:20.277" LastActivityDate="2015-08-04T16:43:20.277" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10471" PostTypeId="2" ParentId="10459" CreationDate="2015-08-04T16:52:29.640" Score="3" Body="&lt;p&gt;The Super Draco pods provide launch abort capability all the way to orbit. They are not jettisoned since they are integrated into the frame of the spacecraft. This is less efficient but is negated by the large payload capacity of Falcon 9 and the opportunity to attempt propulsive landings which are critical to reuse. Since the super Draco is high thrust and and able to be throttled, it can provide a safe abort at any point in the flight to orbit. Elon Musk pointed this out in the Dragon 2 reveal. &lt;/p&gt;&#xA;" OwnerUserId="10398" LastActivityDate="2015-08-04T16:52:29.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10472" PostTypeId="2" ParentId="10459" CreationDate="2015-08-04T17:16:48.733" Score="2" Body="&lt;p&gt;The scenario for an abort with the Dragon V2 requires the use of the Super Draco pods to get far enough away from the rocket to prevent damage, following the use of parachutes to safely land. The parachutes are only used in an abort (Or otherwise atypical landing), as it takes longer to re-certify the parachutes after a landing than the rockets. The rockets are either used as an abort mechanism, or the primary landing system if there is no abort that occurs, thus they last all the way to the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that if you achieve orbit, there are additional abort mechanisms, such as land without docking with the ISS.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-04T17:16:48.733" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10473" PostTypeId="2" ParentId="10329" CreationDate="2015-08-04T17:22:48.170" Score="23" Body="&lt;p&gt;There are no technological barriers, provided one is not too ideological about biospheres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a comment on the question it is remarked that self-sustaining biodomes have not yet been successful on Earth. This is somewhat besides the point. These exercises on Earth have generally been about creating a self-contained biosphere within a dome, independent of Earth's atmosphere and other resources. In my opinion it's a pipe dream. Being self-contained is unnecessary and more difficult than the task of creating a self-sustaining biodome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When creating a Mars biodome, there is no reason to not mine both the atmosphere and regolith to help sustain the dome. Waste gasses can be vented to the atmosphere, rubbish can be dumped outside, and fresh resources extracted from the atmosphere and regolith. Solar panels can also be placed outside the dome for extra electricity - and possibly wind turbines (which would only work during storms - otherwise the wind speed is too low considering the very low air pressure).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point is, there is little reason to have a closed system on a world bursting with exploitable resources, the important thing is only that the population within the dome is sustained in an environment with a semblance of Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shortlist of important resources for biological processes which it might be desirable to extract from outside the dome rather than trying to fully recycle are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Water (from atmosphere, regolith and possibly aquifers or ice)&lt;/li&gt;&#xA;&lt;li&gt;Nitrogen gas (from atmosphere) and nitrogen compounds (from regolith)&lt;/li&gt;&#xA;&lt;li&gt;Carbon Dioxide (atmosphere - it is 95% pure CO2)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I would anticipate that it would be easier to not try and perfectly balance the CO2, Nitrogen and Water cycles within a biosphere using only biological processes, and instead augment the cycles with machinery, using the outside atmosphere as a buffer and to provide a margin of error.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Economical Biodome Design&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;In terms of safety and security, I have thought about domes before, and it would make sense to place a biodome inside an outer dome which is pressurized but inhospitable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jxLIc.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jxLIc.png&quot; alt=&quot;An inner dome contained in an outer dome&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biodome would be filled with breathable air. The outer dome would be filled with compressed martian atmosphere, to a pressure slightly less than that of the inner dome so that the inner dome can support it's own weight, with positive pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All things being equal, breathable air mix is expensive. Nitrogen is scarce on Mars - it might improve with regolith mining to extract nitrates, but extracting nitrogen gas from the atmosphere is expensive, also the breathable air mix has to be carefully regulated - probably by machines. Many of the purposes for which we need air &lt;em&gt;other&lt;/em&gt; than breathing, can be served equally well by CO2 and the outer dome could be maintained by nothing more than some pressure sensors and an air compressor. It could even be slightly leaky as long as the air compressor can compensate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The outer dome would serve several purposes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Providing additional air mass for radiation shielding and insulation (greenhouse effect) - you would want at least 1km of air to help protect against cosmic radiation&lt;sup&gt;1&lt;/sup&gt;. An outer dome could also be made of a layer of water/ice sandwiched between two sheets partitioned like bubble wrap and supported by air pressure &lt;sup&gt;2&lt;/sup&gt;, which would give supreme radiation protection while still permitting some light through.&lt;/li&gt;&#xA;&lt;li&gt;Mitigating leaks. If the inner dome is punctured, the air inside will only slowly diffuse out into the outer dome, because of the relatively equal pressures. If the outer dome is punctured, the inner dome continues to contain the breathable atmosphere - this also goes for incidental releases of gas such as when opening an airlock to the outside.&lt;/li&gt;&#xA;&lt;li&gt;Providing a more comfortable working environment. Because the pressure in the outer dome is close to that of the inner dome, humans can work and travel within the outer dome with only breathing apparatus, there is no need for a space suit protecting against near-vacuum, cold or radiation. This would provide a convenient zone for industry, staging of machines to be sent outside, and so on.&lt;/li&gt;&#xA;&lt;li&gt;The outer dome can encompass multiple inner domes and structures, with differing requirements. For example, plants present significant atmosphere balancing challenges such as excessive oxygen production. Plants do not require a human breathable atmosphere nor as much radiation protection and separate greenhouse tents could be used for the bulk of agricultural food production, separating the requirements of managing a human-breathable atmosphere, with an atmosphere adequate for plant growth, and isolating dangers such as out of control fires.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Closing Considerations&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The above kind of biosphere is heavily machine based, at least at first, probably forever if Earth is any guide, although in time if the nitrogen scarcity is overcome breathable air could be used more extensively and larger enclosed air volumes would provide a more robust buffer against fluctuations in the water/nitrogen/CO2 cycles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initially a Martian colony would be dependent on Earth for nearly everything. However breathable air, fuel and food can all be created on Mars from local resources using existing technologies. The real and final dependency on Earth would be the machinery enabling this production.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Independence from Earth, would not come from creating self-sustaining biospheres on Mars, but instead, when Martian industry can build and repair all the machines responsible for sustaining their biospheres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This might seem like a cop-out on the idea of a biosphere, but really it's not. Just think about how many machines are involved in each morsel of food you eat - from irrigation, combine harvesters, factories, delivery vans to refrigerators and so many more. On Earth we are now utterly dependent on machines to keep the population alive, and on Mars it will be the same only moreso.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;[1]: See &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_protection#Shielding&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Radiation_protection#Shielding&lt;/a&gt;, 150m of air halves gamma radiation intensity. 1km of air would reduce intensity to less than 1%.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;[2]: Using the logic in &lt;a href=&quot;https://space.stackexchange.com/a/773/6599&quot;&gt;this answer on moon caves&lt;/a&gt; 1 atm of pressure, could support 26m of water on Mars, even 2m of water would reduce radiation by 99.96%, and could be supported by an outer dome pressurized to less than 0.1atm.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-08-04T17:22:48.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10474" PostTypeId="2" ParentId="10466" CreationDate="2015-08-04T18:46:45.200" Score="41" Body="&lt;p&gt;First a few terms:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Low Earth Orbit&lt;/strong&gt; (LEO) All spacecraft must first achieve low Earth orbit. This is true whether you're sending stuff to the Moon or Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Trans Mars Insertion&lt;/strong&gt; (TMI) The burn needed to send something on its way to Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Delta-v&lt;/strong&gt; Change in velocity needed. Usually measured in kilometers/second. An important metric for space missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth Moon Lagrange 1&lt;/strong&gt; (EML1) A region between the Earth and the Moon where the Moon's gravity and centrifugal force balance the Earth's gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth Moon Lagrange 2&lt;/strong&gt; (EML2) A region beyond the far side of the Moon where centrifugal force balances the Earth's and the Moon's gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1pfcE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1pfcE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delta-v to get from LEO to the Moon's surface is about 6 km/s. To get out of the Moon's gravity well is around 2.5 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From LEO, TMI is about 3.6 km/s. &lt;strong&gt;&lt;em&gt;From LEO it take less delta-v to send something on its way to Mars than it does to send a payload to EML1.&lt;/em&gt;&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if all propellent and materials come from the Earth, we gain nothing from launching from the Moon's surface. It is better to launch from LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there may be &lt;a href=&quot;http://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html&quot; rel=&quot;noreferrer&quot;&gt;water ice deposits at the lunar poles&lt;/a&gt;. If so, it might make interplanetary flight easier, if we exported lunar propellent and life support consumables to EML1 or EML2. A Mars-bound vehicle could stop at EML1 or EML2 and stock up on propellent, water, and air before departing from Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2Qr2Y.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Qr2Y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the delta-v map indicates, the Lagrange regions are close to other destinations of interest besides Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it remains an open question, if there are rich volatile deposits in the lunar cold traps.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-08-05T14:14:21.903" LastActivityDate="2015-08-05T14:14:21.903" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10475" PostTypeId="2" ParentId="10057" CreationDate="2015-08-04T18:47:11.610" Score="14" Body="&lt;p&gt;The EMU is not smart at all.  It was designed in the late 70s.  There is no automated control of the suit.  The astronaut controls the suit through the use of switches and rotary knobs on the Display and Control Module mounted on the suit's chest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only computer utilized in the suit is for the Caution and Warning system.  This simply monitors various suit parameters, formats them for display on the LCD unit, and sets off alarms if they go out of limits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following description of the suit controls is taken from &lt;a href=&quot;https://www.nasa.gov/pdf/188963main_Extravehicular_Mobility_Unit.pdf&quot; rel=&quot;noreferrer&quot;&gt;Suited for Space&lt;/a&gt;, which although intended for young people, is the best description of the suit I've been able to find in the open literature.  (Emphasis mine)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A small, irregularly-shaped box, the Displays and Control Module&#xA;  (DCM), houses a variety of switches, valves, and displays.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Along the DCM top are four switches for power, feedwater, communications&#xA;  mode selection, and caution and warning.  A suit-pressure purge valve&#xA;  projects from the top at the left.  It is used for depressurizing the&#xA;  suit at the end of an EVA and can be used in an emergency to remove&#xA;  heat and humidity when oxygen is flowing from both the primary and&#xA;  secondary oxygen systems. Near the front on the top is an&#xA;  alpha-numeric display.  &lt;strong&gt;A microprocessor inside the PLSS permits&#xA;  astronauts to monitor the condition of the various suit circuits by&#xA;  reading the data on the display.&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Stepped down from the top of the DCM, on a small platform to the&#xA;  astronaut’s right, is a ventilation-fan switch and a push-to-talk&#xA;  switch. The astronaut has the  option  of  having  the  radio&#xA;  channel open at all times or only when needed.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;On a second platform, to the left, is an illuminated mechanical-suit&#xA;  pressure gauge. At the bottom, on the front of the DCM, are additional&#xA;  controls for communications volume, display lighting&#xA;  intensity, temperature control, and a four position selector for&#xA;  controlling suit pressure in different EVA operating modes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D7NSv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D7NSv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The text is reversed, because the astronaut can only view the controls by using a wrist mirror.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some information about the EMU in section 3.4 of the &lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/Suit_Water_Intrusion_Mishap_Investigation_Report.pdf&quot; rel=&quot;noreferrer&quot;&gt;Suit Water Intrusion Mishap Report&lt;/a&gt;, but the technical information is heavily redacted, and what is left is actually less informative than the children's writeup.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this answer was originally written, an &lt;a href=&quot;https://archive.org/details/EMUSystemsWorkbookJSC19450B/mode/2up&quot; rel=&quot;noreferrer&quot;&gt;EMU Systems Training Workbook&lt;/a&gt;  has appeared on the Internet Archive. It contains fairly detailed information on the suit's workings. The Display and Control Module and the Caution and Warning subsystem are described starting on page 3-37. Unfortunately, the manual assumes that the reader has access to the Space Shuttle Systems Handbook for schematics; that document is not publicly available. A partially labeled schematic can be found at NTRS &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120009158.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, I have reproduced it below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XTSOt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XTSOt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-02-20T20:34:25.643" LastActivityDate="2020-02-20T20:34:25.643" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="10476" PostTypeId="1" AcceptedAnswerId="10477" CreationDate="2015-08-04T19:56:49.427" Score="9" ViewCount="385" Body="&lt;p&gt;My current favorite lunar imagery tool is Arizona State's &lt;a href=&quot;http://wms.lroc.asu.edu/lroc/&quot; rel=&quot;noreferrer&quot;&gt;Lunar Reconnaissance Orbiter&lt;/a&gt; site. (Sorry &lt;a href=&quot;http://pub.lmmp.nasa.gov/LMMPUI/LMMP_CLIENT/LMMP.html#&quot; rel=&quot;noreferrer&quot;&gt;LMMP&lt;/a&gt;.) It has a section where you can search for imagery by filling out a form, which is great because little did I know most of the Moon has been imaged many times now by various instruments, and sorting through them on their map is difficult, no matter how great the interface is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JhI3p.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JhI3p.png&quot; alt=&quot;&amp;#39;this search returned 3174848 results&amp;#39;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But alas, it is still pretty confusing to a lay person. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VxZA2.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VxZA2.png&quot; alt=&quot;LROC image search form&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What do the different angles mean? I think what I want is photos with a high slew angle, looking more towards the horizon and less downwards, but I'm not sure. Wouldn't that be the same as incidence angle? I also don't understand sub-solar azimuth or latitude. Or orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have sorted out that observation type is 'narrow angle' for the ones that start with NA, but don't get the difference between CL and CR. WA is wide angle, and is color, monochrome, ultra-violet, and visible - but wouldn't visible be the same as monochrome? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If people haven't looked at this imagery, I wanted to include a little sample of how detailed it can be now. This is a small bit of the central peak of Tycho Crater:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lXmkY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lXmkY.png&quot; alt=&quot;boulders and strata on peak of tycho crater&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="25" LastEditDate="2015-08-04T20:42:46.650" LastActivityDate="2015-08-04T20:42:46.650" Title="How do I properly use the LROC Image Search tool?" Tags="|the-moon|planetary-science|imaging|scientific-data|lro|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10477" PostTypeId="2" ParentId="10476" CreationDate="2015-08-04T20:42:09.973" Score="4" Body="&lt;p&gt;These are standard terms used by the Planetary Database System. &lt;a href=&quot;http://www.msss.com/moc_gallery/m13_m18/explain.html&quot; rel=&quot;nofollow&quot;&gt;MSSS&lt;/a&gt; provides some explanations of these terms.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Product ID- Each product (image) has a unique id, if you know the exact image, put this one in.&lt;/li&gt;&#xA;&lt;li&gt;Coordinate Range- Range to the coordinates from the spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;Incidence Angle- This is the angle that the sun is to the surface. A low angle means the sun is straight overhead.&lt;/li&gt;&#xA;&lt;li&gt;Emission Angle- This is the angle of the point on an image to the perpendicular of the spacecraft. Straight down is 0 degrees, higher means off-angle imaging.&lt;/li&gt;&#xA;&lt;li&gt;Slew Angle- Similar to Emission Angle, but the Slew is the off angle pointing for the spacecraft, while Emissions is the off angle at a particular point. Most of the time these will be very close to each other.&lt;/li&gt;&#xA;&lt;li&gt;Resolution- Resolution of image&lt;/li&gt;&#xA;&lt;li&gt;Exposure- How long the exposure was of the image (How long did it take to make the image)&lt;/li&gt;&#xA;&lt;li&gt;Sub-Solar Azimuth - This is the angle of the sunshine on the image, measured from 0 at the right hand size of the image. If 0, the sun is coming from the right hand size, etc.&lt;/li&gt;&#xA;&lt;li&gt;Sub-Solar Latitude- This is the latitude on the object where the sun is shining directly overhead. On Earth this would vary between +- 23, for instance, depending on the time of year, other objects have different ranges.&lt;/li&gt;&#xA;&lt;li&gt;Orbit - What orbit number the image was taken&lt;/li&gt;&#xA;&lt;li&gt;Observation Date- Date of the observation&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In your case, you probably care the most about the Slew Angle and the Incidence Angle.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-04T20:42:09.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10479" PostTypeId="2" ParentId="9380" CreationDate="2015-08-05T05:58:22.370" Score="1" Body="&lt;p&gt;I'd recommend also looking at Vallado's book Ch12 three-body mission design, as well as Jeff Parker's work. Low Energy Transfer, also topics on manifold. Problem with low energy transfer is that it will take a long time to fly which make it less useful/practical. Although, if possible one would pre-plan a mission by using these low energy trajectory, for example, if you have a optimal trajectory to Mars (for human) say in the next 20 years, you can potentially calculate for one of these low energy trajectory, and send any essential stuff for the Mars stay, and the Mars return part of the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another use of these low energy trajectory is a cislunar mission redirect when spacecraft is low on fuel or damaged. As you have probably already read from the wiki on Japan's Hiten.&lt;/p&gt;&#xA;" OwnerUserId="11221" LastActivityDate="2015-08-05T05:58:22.370" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10480" PostTypeId="1" CreationDate="2015-08-05T08:33:26.473" Score="6" ViewCount="1765" Body="&lt;p&gt;Sun-Jupiter L1 is 1/3 of an AU from Jupiter. Could it be used to slow down or redirect a spacecraft approaching Jupiter? And the same for Sun-Jupiter L4 and L5 for a spacecraft on the way to e.g. Saturn?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2015-08-05T14:12:01.847" LastActivityDate="2015-08-06T01:00:56.250" Title="How can Sun-Jupiter Lagrangian points be used by flyby probes?" Tags="|trajectory|lagrangian-points|gravity-assist|jupiter|flyby|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10481" PostTypeId="1" CreationDate="2015-08-05T09:06:43.090" Score="1" ViewCount="40" Body="&lt;p&gt;From what I know, to communicate with Earth, a space probe locates signals sent from Earth and then sends data (in that direction). For probes that are several light hours away (Voyagers, New Horizons), however, the Earth will rotate (and with it, the locations of the DSN arrays will change).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the spot size of these communication signals large enough to compensate for this movement? Or does the probe need to calculate where the next available array will be, and make small adjustments?&lt;/p&gt;&#xA;" OwnerUserId="10107" LastActivityDate="2015-08-05T09:06:43.090" Title="Spot size of interplanetary communications" Tags="|probe|communication|deep-space-network|" AnswerCount="0" CommentCount="2" ClosedDate="2015-08-05T19:54:35.920" ContentLicense="CC BY-SA 3.0" />
  <row Id="10482" PostTypeId="1" CreationDate="2015-08-05T09:38:39.107" Score="8" ViewCount="152" Body="&lt;p&gt;This is an offshoot of &lt;a href=&quot;https://space.stackexchange.com/questions/10057/how-smart-is-an-emu-spacesuit&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a new generation of space suits coming up (&lt;a href=&quot;https://en.wikipedia.org/wiki/Z_series_%28space_suits%29&quot;&gt;Z-series&lt;/a&gt;, anything that the &lt;a href=&quot;https://en.wikipedia.org/wiki/NPP_Zvezda&quot;&gt;Zvezda NPP&lt;/a&gt; has up its sleeves), and I wonder at the level of sophistication of control algorithms and circuitry used there. Have there been any articles in space engineering journals on using $H_\infty$ methods to get controllers with guaranteed performance? Logic to work around unexpected failures?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Browsing through the list of spacewalks on Wikipedia, I gather quite a few cases where suits' subsystems failed or underperformed (cooling failures, water/humidity control glitches etc.) which had material impact on EVA length and achieved results. While the root causes might have been in hardware, software/controller algorithms could be improved as well.&lt;/p&gt;&#xA;&#xA;&#xA;" OwnerUserId="168" LastActivityDate="2015-08-05T09:38:39.107" Title="Smart suits in development" Tags="|eva|spacesuits|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10483" PostTypeId="1" AcceptedAnswerId="10484" CreationDate="2015-08-05T09:58:08.557" Score="2" ViewCount="171" Body="&lt;p&gt;Our Solar system poses a significant problem for colonisation insofar that the rocky planets are predominantly on the... small scale; at least as far as mass is concerned (Earth is the exception rather than the rule). However, there are many rocky planets, dwarf planets, moons, and asteroids scattered throughout the system. Excluding the severe logistical difficulties in generating such giant impact events in the first place, are there theoretical means by which the masses of different bodies can be combined, rather than their collisions simply shearing vast amounts of debris off the larger body?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back in the early stages of the Solar system when the planets where hot and less solid, such combinations &lt;a href=&quot;http://www.space.com/7116-worlds-collide-deep-space.html&quot; rel=&quot;nofollow&quot;&gt;seem to have been a real possibility&lt;/a&gt;. Is it still hypothetically possible in a mature system?&lt;/p&gt;&#xA;" OwnerUserId="11122" LastEditorUserId="49" LastEditDate="2015-08-05T20:09:15.070" LastActivityDate="2015-08-08T12:12:10.623" Title="Planet building: collision without fragmentation?" Tags="|planetary-science|asteroid|planet|impact|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10484" PostTypeId="2" ParentId="10483" CreationDate="2015-08-05T11:18:22.880" Score="4" Body="&lt;p&gt;Absolutely, in fact, many of these objects have been discovered. This is what is known as a &quot;Contact Binary&quot;. It happens when two objects collide very slowly. In fact, the comet that Rosetta is orbiting, 67P, is in fact believed to be such an object. It is expected that 10-15% of all larger (200m+) asteroids will be contact binaries. &lt;a href=&quot;https://en.wikipedia.org/wiki/Contact_binary_%28small_Solar_System_body%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; has a list of all known contact binaries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OwV2M.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OwV2M.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-05T11:18:22.880" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10485" PostTypeId="1" CreationDate="2015-08-05T14:05:21.323" Score="14" ViewCount="1222" Body="&lt;h1&gt;Background&lt;/h1&gt;&#xA;&lt;p&gt;The second Soviet &lt;a href=&quot;https://en.wikipedia.org/wiki/Salyut_3&quot; rel=&quot;noreferrer&quot;&gt;Almaz military space station&lt;/a&gt; both had and tested an externally-mounted &lt;a href=&quot;https://en.wikipedia.org/wiki/Salyut_3#On-board_gun&quot; rel=&quot;noreferrer&quot;&gt;autocannon&lt;/a&gt;, specifically a &lt;a href=&quot;https://en.wikipedia.org/wiki/Rikhter_R-23&quot; rel=&quot;noreferrer&quot;&gt;Rikhter R-23&lt;/a&gt; (according to some sources).  It had 32 rounds of &lt;a href=&quot;https://en.wikipedia.org/wiki/Rikhter_R-23#Ammunition_specifications&quot; rel=&quot;noreferrer&quot;&gt;ammunition&lt;/a&gt;. The gun was fixed to the station such that the only way to aim was to rotate the entire station.&lt;/p&gt;&#xA;&lt;p&gt;The International Space Station is designed to withstand impact from orbital debris, largely through the use of &lt;a href=&quot;https://en.wikipedia.org/wiki/Whipple_shield&quot; rel=&quot;noreferrer&quot;&gt;Whipple shielding&lt;/a&gt; on at least the pressurized modules. &lt;a href=&quot;http://iss.jaxa.jp/iss/kibo/develop_status_09_e.html&quot; rel=&quot;noreferrer&quot;&gt;Here's an article&lt;/a&gt; describing the testing of a Whipple shield as on the Kibo module: it maxes out at an 11 mm aluminum ball fired at 6 km/s which failed to penetrate the inner wall.&lt;/p&gt;&#xA;&lt;p&gt;On the other hand, &lt;a href=&quot;http://www.nap.edu/openbook.php?record_id=5532&amp;amp;page=27&quot; rel=&quot;noreferrer&quot;&gt;this book&lt;/a&gt; describes some limitations of Whipple shielding:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Whipple bumper is most effective at high impact velocities, where the disruption and dispersion of the impacting projectile can be maximized. At lower velocities, the collision with the bumper may not break up or liquefy the impactor; thus it may still be intact when it strikes the catcher.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Also:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Other more effective shields are placed in forward-facing areas where most impacts are expected. Less capable shields are located in aft and nadir-facing areas that are expected to be hit less frequently.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h1&gt;Question&lt;/h1&gt;&#xA;&lt;p&gt;Given 32 rounds of ammunition and attempting to aim by pointing the entire station could the Almaz gun be expected to puncture the pressure hull of the ISS?  Where would the best chance of success be?  How much damage could the gun do (and would depressurizing the station be the worst case)?&lt;/p&gt;&#xA;&lt;p&gt;Assume that Almaz 2 and the ISS are nearby in the same orbit (so no orbital-velocity speed boosts to the bullets).  Also, try to answer using the ammunition the Rikhter R-23 fired: 175 gram &lt;a href=&quot;https://en.wikipedia.org/wiki/High_explosive_incendiary&quot; rel=&quot;noreferrer&quot;&gt;HEI&lt;/a&gt; projectiles (though I'm also curious about the damage generic chunk of lead could do).&lt;/p&gt;&#xA;&lt;p&gt;To put it another way: The ISS is shielded to protect against random strikes by high-velocity orbital debris; does that shielding protect well against a limited number of lower-velocity but targeted small projectiles?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Thoughts&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Whipple shielding seems like it's only effective for multiple strikes if the impacts aren't all in the same small area, but I don't know how well the Almaz station could maintain aim or how accurate the gun would be.&lt;/p&gt;&#xA;&lt;p&gt;I get the impression a variety of different shields are used on the ISS, but I didn't find a comprehensive comparison of them.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-06T04:27:50.003" Title="Space Station Battle: Could the Almaz gun depressurize the ISS?" Tags="|iss|history|debris|shielding|almaz|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10486" PostTypeId="2" ParentId="10480" CreationDate="2015-08-05T17:59:45.517" Score="4" Body="&lt;p&gt;The Sun-Jupiter L1 and L2 (SJL1 and SJL2) are lot more interesting than the Sun-Earth L1 and L2 (SEL1 and SEL2). The Sun-Earth &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/06/mass-parameter-and-itn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;mass parameter&lt;/a&gt; is only 3.04e-6 while the Sun-Jupiter mass parameter is 9.54e-4. The Weak Stability Boundaries (WSBs) emanating from SJL1 and SJL2 are a lot more extensive than the WSBs from SEL1 and SEL2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comet Oterma enjoys some fame due to it's occasional transits through SJL1 and SJL2. Sometimes Oterma orbits beyond Jupiter's 5.2 A.U. orbit. Then it falls through SJL2 and leaves Jupiters' realm via the SJL1 neck. Here's a screen capture from &lt;a href=&quot;http://www.cds.caltech.edu/~koon/papers/comet_paper.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;a pdf by Koon, Lo, Marsden &amp;amp; Ross&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZMryp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZMryp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's interesting that in a rotating frame, Oterma's inside orbit looks a lot like a &lt;a href=&quot;https://en.wikipedia.org/wiki/Hilda_family&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hilda Comet&lt;/a&gt; but rotated 60º. The Hildas have a 3:2 resonance but their aphelions take them to the L4, L5 and L3 regions far from Jupiter's influence. Oterma's third aphelion brings it to SJL1, close enough that Jupiter is a big influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer your question, is the SJL1 useful for fly-by probes? In my opinion, no. The arrival Vinf for an earth to Jupiter ellipse is around 5.6 km/s. Too high for ballistic capture. SJL1 orbits a little slower than Jupiter, but this delta V savings is minor compared to the substantial Oberth benefit Jupiter's deep gravity well can offer.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-08-06T01:00:56.250" LastActivityDate="2015-08-06T01:00:56.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10487" PostTypeId="1" CreationDate="2015-08-05T18:02:42.323" Score="9" ViewCount="1209" Body="&lt;p&gt;What are the two or three most accurate technologies used to determine interplanetary spacecraft distance (e.g. Doppler / radiometric tracking...), and what accuracy is currently obtained from such methods? Do they rely on long integration time or on Earth signals?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Links appreciated.&lt;/p&gt;&#xA;" OwnerUserId="7017" LastActivityDate="2015-08-05T23:17:46.237" Title="Interplanetary ranging accuracy?" Tags="|navigation|deep-space|radar|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10488" PostTypeId="2" ParentId="10487" CreationDate="2015-08-05T18:41:03.703" Score="10" Body="&lt;p&gt;OK, here's &lt;a href=&quot;http://www2.jpl.nasa.gov/basics/bsf13-1.php&quot;&gt;the Link&lt;/a&gt; from NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Spacecraft velocity&lt;/strong&gt; is determined by measuring the Doppler shift of a coherent downlink to determine its earth-relative velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Spacecraft distance&lt;/strong&gt; is measured by sending a uniquely coded ranging pulse to the spacecraft (including the transmission time of the pulse) and then measuring the time it takes for the spacecraft to return this pulse. The time it takes for the spacecraft to process the transmission is calibrated and measured before launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Spacecraft position&lt;/strong&gt; in the sky can be very accurately measured by observing the spacecraft's signal and doppler shift from two different locations widely separated on the Earth. The signals are synchronized using Quasar measurements. More details are on the link I referenced above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft that are equipped with imaging instruments also use them to observe its destination planet or other body against the background starfield.&lt;/p&gt;&#xA;" OwnerUserId="373" LastActivityDate="2015-08-05T18:41:03.703" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10489" PostTypeId="2" ParentId="10485" CreationDate="2015-08-05T19:18:25.270" Score="7" Body="&lt;p&gt;Extrapolation is always dangerous but here's my shot at it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assumed the bullet was a chunk of lead.  I have no way of incorporating the effects of any kind of explosive charge in the bullet.  Presumably they would increase the damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used a chart from Page 60 of &lt;a href=&quot;http://ston.jsc.nasa.gov/collections/trs/_techrep/TP-2003-210788.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5mZce.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5mZce.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The chart is for aluminum...here is where some shaky extrapolation gets in...is the mass or size important? For a 175 gram lead bullet with lead at 11.34 gm/cm^3, I got an ~ 3 cm diameter sphere.  For a 175 gram aluminum bullet with Al at 2.7 gm/cm^3, I got an ~ 5 cm diameter sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All these sizes are way bigger than what's on the chart.  More extrapolation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if we enter the chart with a 5 cm diameter sphere at .85 km/sec, we get....probably it penetrates. Depending on exactly where that line would go after it leaves the chart, and where your ruler tells you 0.85 km/second is.  Obviously it can't be linear or you would get some large mass that penetrates at zero velocity....   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other, similar charts in the document for different kinds of shields.  None go up big enough though...but feel free to pick a different chart!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The document shows diagrams of what kind of shielding is used on different parts of the station.  It's kind of old though, 2003, and ISTR shuttle mission(s) carrying up enhanced shielding for Russian modules.  But for this kind of calculations, probably does not matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/t6SAC.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t6SAC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-08-05T19:26:21.290" LastActivityDate="2015-08-05T19:26:21.290" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10490" PostTypeId="1" AcceptedAnswerId="10504" CreationDate="2015-08-05T20:43:12.293" Score="14" ViewCount="1733" Body="&lt;p&gt;I want to extract the Chebyshev coefficients for the positions of the Moon and Sun relative to the Earth from a SPICE kernel (mainly because DE430 is too large for me to include in a software package, and I don't need all the other ephemerides, but also because I don't want to include SPICE code in my software release directly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any thoughts on what SPICE method I might use to produce these coefficients? My mind is a little boggled by all of the information in these packages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is there a better way that I'm not considering currently?&lt;/p&gt;&#xA;" OwnerUserId="869" LastActivityDate="2015-08-15T18:57:23.270" Title="Is there a way to extract the Chebyshev coefficients for a body from a SPICE kernel?" Tags="|ephemeris|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10492" PostTypeId="2" ParentId="10490" CreationDate="2015-08-05T23:05:02.483" Score="9" Body="&lt;p&gt;What's wrong with including some of the SPICE code?  It is a library, so it would only link in what you need.  SPICE can be used freely in any products, including commercial, for-profit use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally you would still use the SPICE code to get states from the kernel.  Then you can just edit down the kernel to include only the bodies and time spans of interest.  &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/utilities/MacIntel_OSX_64bit/spkmerge.ug&quot; rel=&quot;nofollow&quot;&gt;SPKMERGE&lt;/a&gt; will do that for you.  With this input to SPKMERGE:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;LEAPSECONDS_KERNEL = naif0010.tls&#xA;SPK_KERNEL = pared-down.bsp&#xA;   SOURCE_SPK_KERNEL = de430.bsp&#xA;      BODIES = 10 301 399&#xA;      BEGIN_TIME = 2001-01-01&#xA;      END_TIME = 2051-01-01&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;the resulting pared-down.bsp with only the Moon, Earth, and the Sun over the specified date range, is 3.2 MB, as compared to de430.bsp which is 114 MB.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a program that extracts the Chebyshev coefficients from an SPK file into a memory structure that is then used to compute positions and velocities without use of the SPICE library.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;/* cheby.c - extract and use Chebyshev position-only coefficients from an SPK&#xA;   file.  This illustrates how the coefficients may be extracted once using the&#xA;   SPICE library, and then used after that without the SPICE library.&#xA;&#xA;   Mark Adler   August 15, 2015   placed in the public domain&#xA; */&#xA;&#xA;#include &amp;lt;stdio.h&amp;gt;&#xA;#include &amp;lt;stdlib.h&amp;gt;&#xA;#include &amp;lt;string.h&amp;gt;&#xA;#include &amp;lt;stdarg.h&amp;gt;&#xA;#include &amp;lt;math.h&amp;gt;&#xA;#include &quot;SpiceUsr.h&quot;&#xA;&#xA;/*&#xA;    DAF/SPK file format notes:&#xA;&#xA;    ic[0] - target code&#xA;    ic[1] - center code&#xA;    ic[2] - frame code&#xA;    ic[3] - representation code (2 == Chebyshev position only)&#xA;    ic[4] - initial address of array&#xA;    ic[5] - final address of array&#xA;&#xA;    len = ic[5] - ic[4] + 1&#xA;&#xA;    dc[0] - initial epoch of data (seconds relative to J2000)&#xA;    dc[1] - final epoch of data (seconds relative to J2000)&#xA;&#xA;    seg[len-4] - initial epoch of the first record (seconds relative to J2000)&#xA;    seg[len-3] - interval length of each record (seconds)&#xA;    seg[len-2] - elements in each record&#xA;    seg[len-1] - number of records&#xA;&#xA;    seg[len-1] * seg[len-2] + 4 == len&#xA;    n = seg[len-2]&#xA;    num = (n - 2) / 3&#xA;&#xA;    rec[0] - midpoint of interval covered by record (seconds relative to J2000)&#xA;    rec[1] - radius of interval (seconds)&#xA;    rec[2..num+1] - X coefficients (constant term first)&#xA;    rec[num+2..2*num+1] - Y coefficients&#xA;    rec[2*num+2..n-1] - Z coefficients&#xA;&#xA;    For t, evaluate the Chebyshev polynomials T_n at (t - rec[0]) / rec[1],&#xA;    multiply by the coefficients, and sum.  The derivatives of the polynomials&#xA;    can be used to compute the velocity.  See cheby_eval() here.  The results&#xA;    are in km and km/s.  Note that all times are ephemeris times, and so do not&#xA;    take into account leap seconds.&#xA;&#xA;    A SpiceDouble is simply a C double.  A SpiceInt is an integer type whose&#xA;    size is half that of double, so that two SpiceInt's fit in a SpiceDouble.&#xA; */&#xA;&#xA;/* Evaluate the given Chebyshev polynomial at x, returning both the evaluated&#xA;   polynomial in *f, and the evaluated derivative of the polymonial in *df. The&#xA;   number of coefficients is num (the degree of the polynomial is num - 1), and&#xA;   the coefficients are coeff[0..num-1].  The first coefficient coeff[0] is the&#xA;   constant term.  The scaling of x is provided by the midpoint scale[0] and&#xA;   the radius scale[1].  x must fall in the range scale[0] - scale[1] to&#xA;   scale[0] + scale[1].  Outside of that range, the polynomial is not valid. */&#xA;void cheby_eval(double x, double *scale, double *coeff, long num,&#xA;                double *f, double *df)&#xA;{&#xA;    double x2, w0 = 0., w1 = 0., dw0 = 0., dw1 = 0., tmp;&#xA;&#xA;    x = (x - scale[0]) / scale[1];&#xA;    x2 = x * 2.;&#xA;    while (--num) {&#xA;        tmp = dw1;&#xA;        dw1 = dw0;&#xA;        dw0 = w0 * 2. + dw0 * x2 - tmp;&#xA;        tmp = w1;&#xA;        w1 = w0;&#xA;        w0 = coeff[num] + (x2 * w0 - tmp);&#xA;    }&#xA;    *f = coeff[0] + (x * w0 - w1);&#xA;    *df = (w0 + x * dw0 - dw1) / scale[1];&#xA;}&#xA;&#xA;/* Find the appropriate SPK record for time t and compute the position and&#xA;   velocity for that time.  Returns 0 on success, 1 if the time is not covered&#xA;   by the segment. */&#xA;int cheby_posvel(double t, double *seg, long len, double pos[3], double vel[3])&#xA;{&#xA;    long k, num;&#xA;&#xA;    k = (long)floor((t - seg[len - 4]) /    // seg[len-4] is initial epoch&#xA;                    seg[len - 3]);          // seg[len-3] is record span&#xA;    if (k &amp;lt; 0 || k &amp;gt;= (long)seg[len - 1])   // seg[len-1] is number of records&#xA;        return 1;&#xA;    num = (long)seg[len - 2];               // seg[len-2] is size of record&#xA;    seg += k * num;                         // point seg to the record for t&#xA;    num = (num - 2) / 3;                    // number of coefficients&#xA;    cheby_eval(t, seg, seg + 2, num, pos, vel);&#xA;    cheby_eval(t, seg, seg + 2 + num, num, pos + 1, vel + 1);&#xA;    cheby_eval(t, seg, seg + 2 + 2 * num, num, pos + 2, vel + 2);&#xA;    return 0;&#xA;}&#xA;&#xA;/* Verify that the provided segment meets the constraints of a uniform set of&#xA;   coefficient records.  Return 0 on success or 1 if the segment is invalid.&#xA;   This should be done before using the segment in order to avoid segfaults on&#xA;   invalid data. */&#xA;int cheby_verify(double *seg, long len)&#xA;{&#xA;    double&#xA;        recs = seg[len - 1],    // number of records&#xA;        elts = seg[len - 2],    // elements (doubles) in each record&#xA;        span = seg[len - 3],    // time span of each record in seconds&#xA;        init = seg[len - 4];    // initial epoch in seconds relative to J2000&#xA;    long n, k;&#xA;    double *p, *q;&#xA;&#xA;    if (recs != (long)recs ||                       // recs is an integer&#xA;        elts != (long)elts ||                       // elts is an integer&#xA;        (long)recs * (long)elts + 4 != len ||       // total length is correct&#xA;        3 * (((long)elts - 2) / 3) + 2 != elts ||   // integer number of coeffs&#xA;        seg[0] - seg[1] != init ||                  // 1st start is init&#xA;        span != 2 * seg[1])                         // 1st radius matches span&#xA;        return 1;&#xA;    n = (long)recs;&#xA;    k = (long)elts;&#xA;    p = seg;&#xA;    while (--n) {&#xA;        q = p + k;                                  // scan all q following p&#xA;        if (q[1] != p[1] ||                         // all radii the same&#xA;            q[0] - q[1] != p[0] + p[1])             // next start is last end&#xA;            return 1;&#xA;        p = q;&#xA;    }&#xA;    return 0;&#xA;}&#xA;&#xA;/* Print an error message. */&#xA;void err(char *msg, ...)&#xA;{&#xA;    fputs(&quot;cheby error: &quot;, stderr);&#xA;    va_list ap;&#xA;    va_start(ap, msg);&#xA;    vfprintf(stderr, msg, ap);&#xA;    va_end(ap);&#xA;    putc('\n', stderr);&#xA;}&#xA;&#xA;/* SPK segment descriptor. */&#xA;typedef struct {&#xA;    long target;        // target body code&#xA;    long center;        // center body code&#xA;    long frame;         // frame of reference code&#xA;    long len;           // length of segment in doubles&#xA;    double *seg;        // allocated segment&#xA;} segment_t;&#xA;&#xA;/* Load one segment of an SPK file, which covers one target over a range of&#xA;   epochs.  Save the target code, reference location code for the target&#xA;   position, and the reference frame code.  Load the segment and verify its&#xA;   structure.  On success return 0.  If there is an error, return 1 and set&#xA;   s-&amp;gt;seg to NULL. */&#xA;int cheby_segment(SpiceInt daf, SpiceDouble *dc, SpiceInt *ic, segment_t *s)&#xA;{&#xA;    SpiceDouble *last;&#xA;&#xA;    // save segment codes&#xA;    s-&amp;gt;target = ic[0];&#xA;    s-&amp;gt;center = ic[1];&#xA;    s-&amp;gt;frame = ic[2];&#xA;&#xA;    // allocate memory for the segment and read it in&#xA;    s-&amp;gt;len = ic[5] - ic[4] + 1;                 // number of doubles in segment&#xA;    s-&amp;gt;seg = malloc(s-&amp;gt;len * sizeof(SpiceDouble));&#xA;    if (s-&amp;gt;seg == NULL) {&#xA;        err(&quot;out of memory&quot;);&#xA;        return 1;&#xA;    }&#xA;    dafgda_c(daf, ic[4], ic[5], s-&amp;gt;seg);        // load segment&#xA;    if (failed_c()) {&#xA;        reset_c();&#xA;        free(s-&amp;gt;seg);&#xA;        s-&amp;gt;seg = NULL;&#xA;        err(&quot;could not read SPK segment from file&quot;);&#xA;        return 1;&#xA;    }&#xA;&#xA;    // verify the integrity of the segment&#xA;    last = s-&amp;gt;seg + s-&amp;gt;len - 4 - (long)(s-&amp;gt;seg[s-&amp;gt;len - 2]);&#xA;    if (cheby_verify(s-&amp;gt;seg, s-&amp;gt;len) ||         // segment structure ok&#xA;        dc[0] != s-&amp;gt;seg[s-&amp;gt;len - 4] ||          // start epoch matches&#xA;        dc[1] != last[0] + last[1]) {           // end epoch matches&#xA;        free(s-&amp;gt;seg);&#xA;        s-&amp;gt;seg = NULL;&#xA;        err(&quot;SPK segment format is invalid&quot;);&#xA;        return 1;&#xA;    }&#xA;&#xA;    // return loaded segment&#xA;    return 0;&#xA;}&#xA;&#xA;/* Scan through the SPK file path and extract all of the Chebyshev&#xA;   position-only segments, saving them in an allocated array of segment_t,&#xA;   which is returned.  If there is an error, NULL is returned.  *segs is set to&#xA;   the number of segments in the array.  Once this is done, this array can be&#xA;   used by cheby_verify() and cheby_posvel() above, with no dependency on or&#xA;   reference to the SPICE library. */&#xA;segment_t *spk_extract(char *path, long *segs)&#xA;{&#xA;    SpiceInt daf;&#xA;    SpiceBoolean found;&#xA;    union {&#xA;        SpiceDouble d[128];&#xA;        SpiceChar c[1024];&#xA;    } sum;&#xA;    const SpiceInt nd = 2, ni = 6;&#xA;    SpiceDouble dc[nd];&#xA;    SpiceInt ic[ni];&#xA;    segment_t *spk, *mem;&#xA;&#xA;    // turn off error reporting and aborts for SPICE functions&#xA;    errprt_c(&quot;set&quot;, 0, &quot;none&quot;);&#xA;    erract_c(&quot;set&quot;, 0, &quot;return&quot;);&#xA;&#xA;    // open the file and verifiy that it is a DAF SPK file&#xA;    dafopr_c(path, &amp;amp;daf);                       // open SPK file for reading&#xA;    if (failed_c()) {&#xA;        reset_c();&#xA;        err(&quot;could not open %s as a DAF&quot;, path);&#xA;        return NULL;&#xA;    }&#xA;    dafgsr_c(daf, 1, 1, 128, sum.d, &amp;amp;found);    // read first record&#xA;    if (failed_c() || !found || memcmp(sum.c, &quot;DAF/SPK &quot;, 8)) {&#xA;        reset_c();&#xA;        dafcls_c(daf);&#xA;        err(&quot;%s is not an SPK file&quot;, path);&#xA;        return NULL;&#xA;    }&#xA;&#xA;    // count the number of Chebyshev position-only segments in the DAF file&#xA;    *segs = 0;&#xA;    dafbfs_c(daf);                              // begin forward search&#xA;    while (daffna_c(&amp;amp;found), found) {           // find the next array&#xA;        dafgs_c(sum.d);                         // get array summary&#xA;        dafus_c(sum.d, nd, ni, dc, ic);         // unpack the array summary&#xA;        if (failed_c())&#xA;            break;&#xA;        if (ic[3] == 2)                         // Chebyshev position only&#xA;            (*segs)++;                          // count segment&#xA;    }&#xA;    if (failed_c() || *segs == 0) {&#xA;        reset_c();&#xA;        dafcls_c(daf);&#xA;        err(&quot;file error or Chebyshev position-only segments in %s&quot;, path);&#xA;        return NULL;&#xA;    }&#xA;&#xA;    // allocate table of segment descriptors&#xA;    spk = malloc(*segs * sizeof(segment_t));&#xA;    if (spk == NULL) {&#xA;        dafcls_c(daf);&#xA;        err(&quot;out of memory&quot;);&#xA;        return NULL;&#xA;    }&#xA;&#xA;    // read and save the Chebyshev position-only segments&#xA;    *segs = 0;&#xA;    dafbfs_c(daf);                              // begin forward search&#xA;    while (daffna_c(&amp;amp;found), found) {           // find the next array&#xA;        dafgs_c(sum.d);                         // get array summary&#xA;        dafus_c(sum.d, nd, ni, dc, ic);         // unpack the array summary&#xA;        if (failed_c())&#xA;            break;&#xA;        if (ic[3] == 2 &amp;amp;&amp;amp; !cheby_segment(daf, dc, ic, spk + *segs))&#xA;            (*segs)++;&#xA;    }&#xA;    if (failed_c() || *segs == 0) {&#xA;        reset_c();&#xA;        dafcls_c(daf);&#xA;        free(segs);&#xA;        err(&quot;no valid Chebyshev position-only segments in %s&quot;, path);&#xA;        return NULL;&#xA;    }&#xA;&#xA;    // close the DAF file and return segment table&#xA;    dafcls_c(daf);&#xA;    errprt_c(&quot;set&quot;, 0, &quot;short&quot;);&#xA;    erract_c(&quot;set&quot;, 0, &quot;abort&quot;);&#xA;    mem = realloc(spk, *segs * sizeof(segment_t));&#xA;    if (mem != NULL)&#xA;        spk = mem;&#xA;    return spk;&#xA;}&#xA;&#xA;/* Free the resources of an SPK structure created by spk_extract(). */&#xA;void spk_free(segment_t *s, long n)&#xA;{&#xA;    long i;&#xA;&#xA;    for (i = 0; i &amp;lt; n; i++)&#xA;        free(s[i].seg);&#xA;    free(s);&#xA;}&#xA;&#xA;/* Load the SPK files on the command line and verify the position and velocity&#xA;   at J2000 + 0 seconds for each Chebyshev position-only segment against the&#xA;   same result from the SPICE library. */&#xA;int main(int argc, char **argv)&#xA;{&#xA;    segment_t *s;&#xA;    long n, i;&#xA;    double pos[3], vel[3];&#xA;    SpiceInt eph, frame, center;&#xA;    SpiceDouble desc[5], pv[6];&#xA;    SpiceBoolean found;&#xA;    SpiceChar id[41];&#xA;&#xA;    while (++argv, --argc) {&#xA;        s = spk_extract(*argv, &amp;amp;n);&#xA;        if (s == NULL) {&#xA;            err(&quot;could not load %s as an SPK file&quot;, *argv);&#xA;            continue;&#xA;        }&#xA;        furnsh_c(*argv);&#xA;        for (i = 0; i &amp;lt; n; i++) {&#xA;            // show segment info and position and velocity at J2000 + 0&#xA;            printf(&quot;target = %ld, center = %ld, frame = %ld\n&quot;,&#xA;                   s[i].target, s[i].center, s[i].frame);&#xA;            if (s[i].seg == NULL || cheby_verify(s-&amp;gt;seg, s-&amp;gt;len)) {&#xA;                err(&quot;bad segment&quot;);&#xA;                putchar('\n');&#xA;                continue;&#xA;            }&#xA;            if (cheby_posvel(0, s[i].seg, s[i].len, pos, vel)) {&#xA;                err(&quot;J2000 + 0 out of range (!)&quot;);&#xA;                putchar('\n');&#xA;                continue;&#xA;            }&#xA;            printf(&quot;pos(0) = (%g, %g, %g)\n&quot;, pos[0], pos[1], pos[2]);&#xA;            printf(&quot;vel(0) = (%g, %g, %g)\n&quot;, vel[0], vel[1], vel[2]);&#xA;&#xA;            // check position and velocity against SPICE library access&#xA;            spksfs_c(s[i].target, 0, sizeof(id), &amp;amp;eph, desc, id, &amp;amp;found);&#xA;            if (!found) {&#xA;                err(&quot;target %d not found!&quot;, s[i].target);&#xA;                putchar('\n');&#xA;                continue;&#xA;            }&#xA;            spkpvn_c(eph, desc, 0, &amp;amp;frame, pv, &amp;amp;center);&#xA;            if (s[i].frame != frame || s[i].center != center)&#xA;                err(&quot;codes mismatch&quot;);&#xA;            if (pos[0] != pv[0] || pos[1] != pv[1] || pos[2] != pv[2])&#xA;                err(&quot;position mismatch&quot;);&#xA;            if (vel[0] != pv[3] || vel[1] != pv[4] || vel[2] != pv[5])&#xA;                err(&quot;velocity mismatch&quot;);&#xA;            putchar('\n');&#xA;        }&#xA;        unload_c(*argv);&#xA;        spk_free(s, n);&#xA;    }&#xA;    return 0;&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-08-15T18:57:23.270" LastActivityDate="2015-08-15T18:57:23.270" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10493" PostTypeId="2" ParentId="10487" CreationDate="2015-08-05T23:17:46.237" Score="8" Body="&lt;p&gt;I might go look for some links, but just off hand Doppler velocity is good to less than 1 mm/s (in the line-of-sight dimension), can be as good as 0.1 mm/s, ranging is good to better than a meter (also in line-of-sight), and delta-DOR position in one dimension, in the plane connecting the two DSN complexes and the spacecraft, is good to about 1 nanoradian.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first two are used all the time.  delta-DOR less so, since it requires engaging two stations at different complexes at the same time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, they rely on long integration times, if minutes to hours is long.  Doppler requires a very accurate clock on Earth sending a frequency to the spacecraft, which is turned around and sent back.  Similarly, ranging requires a pseudo-noise signal from Earth which is turned around on the spacecraft with a well-characterized latency.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-05T23:17:46.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10494" PostTypeId="1" AcceptedAnswerId="10511" CreationDate="2015-08-05T23:37:28.260" Score="9" ViewCount="3171" Body="&lt;p&gt;I see two threats to the ISS as its altitude goes up:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Cosmic/solar radiation, which would endanger the crew and the electronics aboard.&lt;/li&gt;&#xA;&lt;li&gt;Orbital debris, which would have higher relative speed and might increase danger.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Disregarding the fact that it would be impractical because of higher service cost, how high could we put the ISS while maintaining it safe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, how high could we push it with current technology? (Perhaps using a better launch site like Kourou)&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="25" LastEditDate="2015-08-06T11:18:11.630" LastActivityDate="2016-08-17T10:10:17.963" Title="What is the ISS maximum altitude?" Tags="|iss|nasa|low-earth-orbit|soyuz-spacecraft|altitude|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10495" PostTypeId="2" ParentId="10145" CreationDate="2015-08-05T23:40:15.150" Score="9" Body="&lt;p&gt;I didn't see this referenced anywhere so I'm including it for completeness.  Feel free to merge it in with another answer.&lt;/p&gt;&#xA;&lt;p&gt;STS-41G (launched October 5 1984) included a secondary payload called the Orbital Refueling System (ORS).  Here's a description from the &lt;a href=&quot;http://www.jsc.nasa.gov/history/shuttle_pk/pk/Flight_013_STS-41G_Press_Kit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Press Kit&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h2&gt;ORBITAL REFUELING SYSTEM (ORS)&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Orbital Refueling System experiment is a demonstration of Shuttle-human-tended capabilities to refuel already  orbiting satellites once their self-contained thruster systems  have depleted fuel reserves. This demonstration is a precursor to actual Shuttle refueling missions for satellites.&lt;/p&gt;&#xA;&lt;p&gt;The ORS has been designed to deliver about 550 lb. of fuel  to a retrieved satellite. The system, developed by the  Engineering Directorate at Johnson Space Center, will be tested on 41-G  in various stages leading to a full  hydrazine transfer between two different tanks on the ORS support structure.&lt;/p&gt;&#xA;&lt;p&gt;For the final fuel transfer stage, mission specialists David Leestma and Kathryn Sullivan will  don their spacesuits  and proceed to the aft end of the payload bay where the ORS equipment is mounted on an MPESS (Mission  Peculiar Experiment Support Structure) along with the Large Format Camera. There the crewmembers will open the  tool kit and remove the hydrazine servicing tool which will already be  hooked up to the fuel supply tank. The crewmembers will connect it to the ground  fill panel of a simulated satellite panel, thus completing the fuel supply link. After pressure checking the hookup, the crewmembers will return to the cabin.&lt;/p&gt;&#xA;&lt;p&gt;The actual transfer of the hydrazine, which is a very toxic and corrosive material, will be controlled from the aft flight deck experiment control panels. The ORS is equipped with sensors which provide pressure and temperature  values and switch and valve positions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;ISTR this project got cancelled as too hazardous after the STS-51L failure. I always found that name &lt;em&gt;Mission Peculiar Experiment Support Structure&lt;/em&gt; amusing.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-01-09T21:10:57.650" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="10497" PostTypeId="1" CreationDate="2015-08-05T23:49:20.887" Score="16" ViewCount="1373" Body="&lt;p&gt;There are a lot of questions here about looking for life on Mars, potentially finding life on Mars, how to deal with life on Mars as far as contamination one way or the other, even whether or not sending people could help in the search for life. But I wonder how finding life would affect any plans to send people.&lt;br /&gt;&#xA;If a rover were to find life on Mars, would that prompt space agencies to accelerate their goals to get humans to Mars, slow down any potential plans, or would it have no effect? &lt;strong&gt;For instance, let's say life is found tomorrow, does NASA have anything that says &quot;any talks of plans to send people to Mars are suspended&quot;?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="1142" LastEditDate="2015-08-06T13:38:05.367" LastActivityDate="2015-08-06T17:18:59.100" Title="How would finding life on Mars affect the timeline to send people?" Tags="|mars|nasa|life|law|planning|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10498" PostTypeId="2" ParentId="10494" CreationDate="2015-08-06T01:03:37.037" Score="7" Body="&lt;p&gt;The driving factor for the ISS altitude is cheaper maintenance, and orbital debris mitigation. There is an optimal altitude for maintenance, which would actually be a bit higher than it is, because the ISS has to actively maintain its altitude by thrusting on a regular basis, while if it was higher that wouldn't be the case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital debris is the primary limiting factor to raising the altitude higher. See the below chart for the altitude of most orbital debris, from &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_debris#/media/File:Spacedebris_upd_2011.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;. Note that increasing the altitude only slightly dramatically increases the amount of debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so looking at the chart below, above 1600 would also be a good area. Let's take away the fact that this is harder to maintain, and would create more space debris. This, however brings one to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;noreferrer&quot;&gt;Van Allen belts&lt;/a&gt;, which start around 1000 km. The ISS doesn't really have the radiation shielding to manage that environment well, although it could be added.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, even if cost wasn't a factor, it still makes sense to have the ISS at or near the altitude it is currently orbiting, until the space debris can be removed, or better radiation shielding added.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VDJLR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VDJLR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-06T01:03:37.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10499" PostTypeId="2" ParentId="10497" CreationDate="2015-08-06T03:32:46.627" Score="10" Body="&lt;p&gt;If life is found on Mars, NASA does not have anything that would suspend travel to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, NASA does have a &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PD_8020_007G_&amp;amp;page_name=main&amp;amp;search_term=8020.7&quot;&gt;NPD: Biological Contamination Control for Outbound and Inbound Planetary Spacecraft &lt;/a&gt;, and would also have to comply with the &lt;a href=&quot;http://www.nasa.gov/content/nepa-frequently-asked-questions&quot;&gt;NEPA process&lt;/a&gt;.   (They have to do these either way, but knowing there was life would ensure increased scrutiny) &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Section 102 requires NASA to incorporate environmental considerations&#xA;  in their planning and decision-making through a systematic&#xA;  interdisciplinary approach leading to a detailed assessment, called an&#xA;  environmental impact statement (EIS), of the environmental impact of&#xA;  and alternatives to NASA actions significantly affecting the&#xA;  environment. Section 102 also requires NASA to lend appropriate&#xA;  support to initiatives and programs designed to anticipate and prevent&#xA;  a decline in the quality of mankind's world environment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Discovery of life on Mars, would likely result in an accelerated timeline, of the missions already proposed, eventually leading to humans on Mars, in a slightly shorter time frame.  (At a minimum, it would result in a higher likelihood, the existing strategy is not derailed.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First and foremost, it would lead to greater public support for additional flights to Mars.   This support would likely lead to increased funding, and more focused political will.&#xA;Additional funding, and additional political will, would result in shorter time spans for existing martian missions, most notably the &lt;a href=&quot;http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Alpha&amp;amp;Letter=M&amp;amp;Alias=Mars%20Sample%20Return&quot;&gt;Mars sample return&lt;/a&gt; mission.  The sooner humanity can practice and prove safe harbor of potential contaminates from another planet (without humans), the sooner the necessary approvals for allowing humans to return to earth would be completed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this process would be long, even after such an amazing discovery, I believe, it would be shorter then the &lt;a href=&quot;https://www.nasa.gov/content/nasas-journey-to-mars&quot;&gt;current plan&lt;/a&gt;, which is still decades away.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA is developing the capabilities needed to send humans to an&#xA;  asteroid by 2025 and Mars in the 2030s&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8653" LastEditorUserId="8653" LastEditDate="2015-08-06T04:21:50.053" LastActivityDate="2015-08-06T04:21:50.053" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10501" PostTypeId="2" ParentId="10497" CreationDate="2015-08-06T06:20:26.003" Score="13" Body="&lt;p&gt;The &lt;a href=&quot;http://www.state.gov/www/global/arms/treaties/space1.html&quot;&gt;Outer Space Treaty&lt;/a&gt; (signed by the US in 1967) specifies that (Article IX):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If a State Party to the Treaty has reason to believe that an activity or experiment planned by it or its nationals in outer space, including the Moon and other celestial bodies, would cause potentially harmful interference with activities of other States Parties in the peaceful exploration and use of outer space, including the Moon and other celestial bodies, it shall undertake appropriate international consultations before proceeding with any such activity or experiment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note the &quot;activity or experiment planned by it or its nationals in outer space&quot;: that means SpaceX or other private companies can't just ignore the treaty because they aren't a state.  It does leave a lot open (restricts &quot;harmful contamination&quot;, not all contamination, etc), but the second snippet appears to be the basis for &lt;a href=&quot;https://en.wikipedia.org/wiki/COSPAR&quot;&gt;COSPAR&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it's not binding (they &quot;develop recommendations&quot;) COSPAR has a list of &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_protection#COSPAR_recommendations_and_categories&quot;&gt;guidelines for avoiding interplanetary contamination&lt;/a&gt;.  Of particular interest for Mars:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Category IVa.&lt;/em&gt; Landers that do not search for Martian life - uses the Viking lander pre-sterilization requirements, 300,000 spores per spacecraft and 300 spores per square meter.&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Category IVb.&lt;/em&gt; Landers that search for Martian life. Adds stringent extra requirements to prevent contamination of samples.&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Category IVc.&lt;/em&gt; Any component that accesses a Martian special region must be sterilized to at least to the Viking post-sterilization biological burden levels (30 spores total per spacecraft).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I would interpret that to mean that if life were detected, especially in a &quot;Martian special region&quot; &lt;strong&gt;there are no guidelines that would prevent a manned mission&lt;/strong&gt; from landing outside the special regions, especially if the intent of the manned mission wasn't to search for life.  That leaves NASA without a rule that would mean &quot;any talks of plans to send people to Mars are suspended&quot;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;On the other hand, the possibility of life would most likely focus research money (and political will/public opinion) on attempting confirmation, and depending on the approach taken it could either help or hinder manned activities:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Focus on a sample return mission would almost certainly rule out a manned lander: sterilizing the spacecraft to &quot;30 spores total per spacecraft&quot; (per Category IVc from above) doesn't sound friendly to any astronauts aboard.  Sample return would certainly have timeline issues (from &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_contamination#Precautions_suggested_for_sample_return&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/National_Environmental_Policy_Act&quot;&gt;National Environmental Policy Act (NEPA)&lt;/a&gt; would likely require a formal environment impact statement, including public hearings during which all the issues would be aired openly.  Wikipedia suggests that the process could take years to complete.  This is in sharp contrast to the Apollo 11 quarantine regulations, which were announced the day of launch, preventing any public discussion from taking place.&lt;/li&gt;&#xA;&lt;li&gt;NASA has proposed a &lt;a href=&quot;http://www.lpi.usra.edu/pss/presentations/200803/04-Atlas-PPSonMSR.pdf&quot;&gt;Mars Sample Return Receiving facility (MSRRF)&lt;/a&gt;, but the timeline speculation on the wikipedia page estimates 7-10 years to build it and an additional 2 years to get the staff accustomed to the facility to reduce risk of accidents.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In-situ research could be aided by &lt;a href=&quot;http://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4214.pdf&quot;&gt;astronauts in orbit controlling rovers via telepresence&lt;/a&gt;.  The astronauts wouldn't be going to the surface and the rovers could be sterilized and launched separately to minimize the risk of contamination, but having astronauts orbiting Mars would reduce the latency such that they could directly control rovers doing analysis or even collecting material to return to orbit for further analysis.  This would allow volatiles to be analyzed without the delay of return to Earth allowing them to break down.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It's impossible to say for sure at this point (and my two options didn't even touch on in-situ research not aided by astronauts, which is what all the existing rovers/landers have done), but given a fresh (and abundant) source of funding NASA might go with option 2 for the purpose of being first with manned flight to Mars (which is already planned, just not this exact mission).&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-06T16:58:51.483" LastActivityDate="2015-08-06T16:58:51.483" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10502" PostTypeId="5" CreationDate="2015-08-06T06:44:12.140" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-08-06T06:44:12.140" LastActivityDate="2015-08-06T06:44:12.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10503" PostTypeId="4" CreationDate="2015-08-06T06:44:12.140" Score="0" Body="The Almaz program was a highly secretive Soviet military space station program, begun in the early 1960s." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-08-06T11:40:10.300" LastActivityDate="2015-08-06T11:40:10.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10504" PostTypeId="2" ParentId="10490" CreationDate="2015-08-06T07:17:14.650" Score="15" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I want to extract the Chebyshev coefficients for the positions of the Moon and Sun relative to the Earth from a SPICE kernel (mainly because DE430 is too large ...)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You've described almost half (44.8%) of DE430 with just those three items, the positions of the Moon and Sun relative to the Earth. This is not why DE430 is so large. The primary reason DE430 is so large is the large time it covers, 1550 AD to 2650 AD.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a huge amount of programming work, you can cut the size down by a factor of eleven if you're only interested in 1950 AD to 2050 AD. With a bit more programming work you can cut the size by another factor of two by eliminating everything but the position of the Earth-Moon barycenter, the Moon, and the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you go to &lt;a href=&quot;ftp://ssd.jpl.nasa.gov/pub//eph/planets/ascii/de430&quot;&gt;ftp://ssd.jpl.nasa.gov/pub//eph/planets/ascii/de430&lt;/a&gt; , you'll see a bunch of ASCII files. There are eleven files named ascp[yyyy].430 and two files named header.430_[xxx]. Suppose you're only interested in the period 1950 AD to 2050 AD. You need the Chebychev coefficients in ascp1950.430, plus the stuff in header.430_229 that helps you read those coefficients. If you look at that header file, you'll see that it starts with two coefficients &lt;code&gt;KSIZE= 2036 NCOEFF= 1018&lt;/code&gt; followed by data organized in groups. That &lt;code&gt;NCOEFF= 1018&lt;/code&gt; is important. It indicates the number of Chebychev coefficients per block (more on this later). With regard to the GROUPs,&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;GROUP 1030 contains&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the start time (as a TDB Julian date),&lt;/li&gt;&#xA;&lt;li&gt;the end time (as a TDB Julian date), and&lt;/li&gt;&#xA;&lt;li&gt;the number of days in a block.  &lt;/li&gt;&#xA;&lt;li&gt;In the case of DE430, it starts on JD 2287184.5, ends on JD 2688976.5, and is organized in blocks of 32 days each. &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;GROUP 1040 contains the names of 229 parameters.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;GROUP 1041 contains the values of those 229 parameters. You'll want the value of EMRAT, the ratio of the Earth's mass to that of the Moon. I'll leave it up to you to determine what to do with that, but you truly do want it.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;GROUP 1050 contains 3 lines of 13 integers each.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The first line contains the initial word (Fortran numbering) of the Chebychev coefficients in a block of coefficients (more on this later) for&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Mercury,&lt;/li&gt;&#xA;&lt;li&gt;Venus,&lt;/li&gt;&#xA;&lt;li&gt;the Earth-Moon barycenter,&lt;/li&gt;&#xA;&lt;li&gt;Mars,&lt;/li&gt;&#xA;&lt;li&gt;Jupiter,&lt;/li&gt;&#xA;&lt;li&gt;Saturn,&lt;/li&gt;&#xA;&lt;li&gt;Uranus,&lt;/li&gt;&#xA;&lt;li&gt;Neptune,&lt;/li&gt;&#xA;&lt;li&gt;Pluto,&lt;/li&gt;&#xA;&lt;li&gt;the Moon,&lt;/li&gt;&#xA;&lt;li&gt;the Sun,&lt;/li&gt;&#xA;&lt;li&gt;the librations of the Earth, and&lt;/li&gt;&#xA;&lt;li&gt;the mutations of the Moon.&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The second line contains the number of Chebychev polynomial coefficients (one plus the polynomial order) for each those objects.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;The third line contains the number of sets of Chebychev polynomial coefficients per 32-day block for each of those objects.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For example, the parameters for the Earth-Moon barycenter are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Initial word = 231&lt;/li&gt;&#xA;&lt;li&gt;Number of Chebychev coefficients = 13&lt;/li&gt;&#xA;&lt;li&gt;Number of sets of coefficients per block = 2&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Every item except for #12 (Earth librations) represents a set of three values. In that special case of Earth librations, there are only two values.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at (for example) ascp1950.430, you'll see that it starts with the line &quot; 1  1018&quot; followed by 1017 numbers. There's a missing value; there should be 1018 numbers. (Oops! Somebody at JPL didn't test their software. That's just one of a number of wonderful features.) You'll need to fix that missing value bug (just add a 0.0 to complete the set). Then you'll need to interpret the values per the GROUP 1050 spec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the Earth-Moon barycenter comprises two sets of 12th order Chebychev polynomial coefficients for each of x, y, and z, starting at word 231. The first thirteen values are the values for the first set of coefficients for x, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first two words in each block of 1018 values are very important. These are the start and end times (in TDB Julian days) of the block. In general, you never, ever want to extrapolate when dealing with Chebychev polynomials. The DE4xx comprises coefficients for sets of piecewise continuous Chebychev polynomials. You absolutely need to find the right set. Those first two words of each block, coupled with the object-specific number of sets of polynomial coefficients per block, will target you toward the correct set of coefficients for the point in time and object of interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To use these coefficients, you need to&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Transform your time (presumably something like UTC, TAI, or TT) to TDB.&lt;/li&gt;&#xA;&lt;li&gt;Find the block (and the sub-block in the case of objects such as the Earth-Moon barycenter) that contains the coefficients pertinent to that object and that point in time,&lt;/li&gt;&#xA;&lt;li&gt;Retrieve the Chebychev coefficients for that time interval,&lt;/li&gt;&#xA;&lt;li&gt;Offset and scale the point in time to a value between -1 (start of interval) to +1 (end of interval), and finally,&lt;/li&gt;&#xA;&lt;li&gt;Compute the values of the x,y, and z Chebychev polynomials given that scaled/offset time value.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Alternatively, you could just use the SPICE libraries and SPICE kernels. I went through this mess over a decade ago. The effort to do so represents a few months of my life that I want back.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-08-06T07:22:35.587" LastActivityDate="2015-08-06T07:22:35.587" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10506" PostTypeId="1" AcceptedAnswerId="10508" CreationDate="2015-08-06T11:32:05.300" Score="4" ViewCount="2114" Body="&lt;p&gt;Consider a probe approaching a planet for a hyperbolic flyby manoeuvre; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The eccentricity of the hyperbolic trajectory can be calculated using the following formula:&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;$$e = 1 + \frac{r_pv_\infty^2}{\mu_1}$$&lt;/h3&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$v_\infty$ refers to the probe's &lt;a href=&quot;https://en.wikipedia.org/wiki/Hyperbolic_trajectory#Hyperbolic_excess_velocity&quot; rel=&quot;nofollow&quot;&gt;hyperbolic excess velocity&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;$\mu_1$ refers to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow&quot;&gt;standard gravitational parameter&lt;/a&gt; $GM$ of the primary.&lt;/li&gt;&#xA;&lt;li&gt;$r_p$ refers to the periapsis radius. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In the event that eccentricity and periapsis radius are both unknown, however, this formula is insufficient. Does there exist an alternative formula that would allow the eccentricity and the periapsis radius to be worked out separately, or by use of simultaneous equations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume for now that all other necessary values are known, excluding $e$ and $r_p$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect it could be possible using a known aiming radius $∆$. Where:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta= r_p\sqrt{1+\frac{2\mu_2}{r_pv_\infty^{2} }},&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I unfortunately can't quite get my head around the maths.&lt;/p&gt;&#xA;" OwnerUserId="11209" LastEditorUserId="11209" LastEditDate="2015-08-06T13:52:26.553" LastActivityDate="2015-08-06T16:55:35.967" Title="Calculating the periapsis radius of a flyby manoeuvre without knowledge of the eccentricity of hyperbolic trajectory" Tags="|orbital-mechanics|trajectory|flyby|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10508" PostTypeId="2" ParentId="10506" CreationDate="2015-08-06T16:55:35.967" Score="4" Body="&lt;p&gt;You have answered your own question.  It appears that you know $\mu$ and $v_\infty$.  Solve your $\Delta$ equation for $r_p$.  It's just a quadratic (pick the positive solution).  Then from $r_p$, you can get $e$ from your other formula.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-06T16:55:35.967" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10510" PostTypeId="2" ParentId="10086" CreationDate="2015-08-06T17:58:29.347" Score="12" Body="&lt;p&gt;For the &lt;a href=&quot;http://www.collectspace.com/ubb/Forum3/HTML/004515-3.html&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11 launch&lt;/a&gt;, at S-IC ignition the frost weights were:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;S-IC: 1400 lb/635 kg&#xA;S-II: 450 lb/204 kg&#xA;S-IVB: 300 lb/136 kg&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;But by holddown arm release:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;S-IC: 650 lb/295 kg&#xA;S-II: 450 lb/204 kg&#xA;S-IVB: 200 lb/91 kg&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So about half the ice was shed (due to vibrations) before the stack left the ground. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19730025115&amp;amp;hterms=SA-506%20ice%20weight&amp;amp;qs=Ntx%3Dmode%2520matchallpartial%26Ntk%3DAll%26N%3D0%26No%3D10%26Ntt%3D%2522SA-506%2522%2520ice%2520weight&quot; rel=&quot;noreferrer&quot;&gt;Saturn V Skylab flight evaluation report&lt;/a&gt; has more data (see the tables on page 187). &#xA;At first stage ignition 1400 lbs of ice was present on the first stage. By S-IC cutoff, 750 lbs remained. By S-II ignition, all ice on the S-II stage was gone.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Space Shuttle external tank, foam insulation was used to reduce icing. That didn't eliminate the problem completely. The insulation foam was porous, so ice would build up inside the foam. According to &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130010710.pdf&quot; rel=&quot;noreferrer&quot;&gt;this test report&lt;/a&gt;, on an average launch about 1100 kg of water ice had accumulated in the ET insulation (depending on the age of the insulation, older tanks would accumulate more).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most rockets don't use insulation AFAIK, so this is a bit of an outlier. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA307683&quot; rel=&quot;noreferrer&quot;&gt;This study&lt;/a&gt; suggests that on LH tanks, ice is a problem because of thermal transfer from the water/ice to the LH, warming up the LH and increasing the boiloff. The study doesn't mention weight of the ice as a problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19970009949&amp;amp;hterms=SA-506%20frost%20weight&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3DSA-506%2520frost%2520weight%26Ntx%3Dmode%2520matchallpartial&quot; rel=&quot;noreferrer&quot;&gt;Stages to Saturn&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Development of the J-2 engine turned up the inevitable gaggle of problems to perplex project designers, engineers, and workers. In using cryogenic propellants, it was obvious that great care was needed to ensure installation of very efficient insulation at critical points to control thermal losses. In the case of most early rocket technology using LOX as the oxidizer, the problem was not immediate. Designers simply took advan- tage of the fact that LOX components had a tendency to frost over. The frosty coating worked surprisingly well as natural insulation-so well that many components were designed without insulation from the start. The super-cold liquid hydrogen permitted no such easy design shortcuts. When air touched the extremely cold LH2 surfaces, it did not frost, but actually liquified. As a result, streaming liquid air not only became an annoyance, but also created a serious heat leak.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-08-10T07:05:01.157" LastActivityDate="2015-08-10T07:05:01.157" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10511" PostTypeId="2" ParentId="10494" CreationDate="2015-08-06T18:57:04.003" Score="11" Body="&lt;p&gt;The official NASA &lt;a href=&quot;http://www.spaceref.com/iss/ops/iss.familiarization.pdf&quot; rel=&quot;noreferrer&quot;&gt;ISS Familiarization Manual&lt;/a&gt; (used in crew training) states on page 1-3: (emphasis mine)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;All of these IGAs, MOUs, SPIP volumes, and control centers are&#xA;required to support a vehicle that, at Assembly Complete, is to be the&#xA;largest man-made object ever to orbit the Earth. The ISS is to have&lt;/p&gt;&#xA;&lt;p&gt;• A pressurized volume of 1200 cubic meters&lt;/p&gt;&#xA;&lt;p&gt;• Mass of 419,000 kilograms&lt;/p&gt;&#xA;&lt;p&gt;• Maximum power output of 110 kilowatts (kW), with a payload average&#xA;power allocation of 30 kW&lt;/p&gt;&#xA;&lt;p&gt;• A structure that measures 108.4 meters (truss length) by 74 meters (module length)&lt;/p&gt;&#xA;&lt;p&gt;• &lt;strong&gt;An orbital altitude of 370-460 km&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;• An orbital inclination of 51.6°&lt;/p&gt;&#xA;&lt;p&gt;• A crew of six (three until Assembly Complete)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Rationale for these limits is given in section 1-5:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The orbit is bounded on the “high” end by equipment design limits and&#xA;the ability of the ETOVs to reach the ISS; it is bounded on the “low”&#xA;end by the requirement that the ISS must be able to miss a propellant&#xA;resupply/reboost mission and still be above a defined safe minimum&#xA;altitude by the next resupply/reboost mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(ETOV stands for &lt;strong&gt;E&lt;/strong&gt;arth &lt;strong&gt;t&lt;/strong&gt;o &lt;strong&gt;O&lt;/strong&gt;rbit &lt;strong&gt;V&lt;/strong&gt;ehicle)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-06T18:57:04.003" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10512" PostTypeId="2" ParentId="10320" CreationDate="2015-08-06T21:15:48.560" Score="1" Body="&lt;p&gt;There is no universally accepted design criteria for fission reactors launched into orbit, mostly because until recently only the U.S. and the Soviet Union had the capability and both countries employed vastly different design and operation philosophies.  Most of the existing standards in the west are likely to be based on the U.S. DOE policy for the use of radioisotope systems which looks at statistical failure models and their effects on radioisotope dispersal and their effects on the health of affected populations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fission reactors have some aspects that make launch inherently safer, and in some aspects more dangerous.  When launched, fission reactors have yet to fission a single atom and therefore have no radioactive fission products built up, making dispersal of fuel a lesser health concern.  However, there is a concern that enriched fuel materials could land in countries attempting to acquire nuclear weapons.  Also, the majority of earth is covered by water, which is a great neutron reflector and also softens the neutron spectrum to a  region where an unintended criticality event becomes more likely.  Finally, to date there has been little work to define a minimum safe orbit for fission operations.  Obviously, the primary concern is that radiation could have negative effects on manned space operations, but to be honest, any gamma, neutron or beta radiation, would be negligible to what already exists due to our own star and other stars.  The bigger concern, is how it could affect scientific instruments in orbit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this is a lengthy response, the quick answer is that there are few existing guidelines, because at present nobody is launching nuclear reactors.  However, there are a few areas of concern that have to be addressed when procedures are put in place.&lt;/p&gt;&#xA;" OwnerUserId="5856" LastActivityDate="2015-08-06T21:15:48.560" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10514" PostTypeId="1" AcceptedAnswerId="10516" CreationDate="2015-08-07T12:22:25.743" Score="24" ViewCount="2048" Body="&lt;p&gt;The Dragon v2 has 7 seats whereas the Soyuz only has 3. But the Soyuz uses all 3 seats for astronauts/cosmonauts to bring to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that Dragon v2 will bring 3-4 people to the ISS in one go but would it ever be seated with 7 people and brought up to the station? If yes, what would be the reason? And wouldn't the ISS then be too crowded?&lt;/p&gt;&#xA;" OwnerUserId="9029" LastEditorUserId="168" LastEditDate="2015-08-07T15:05:16.803" LastActivityDate="2015-08-08T15:10:11.717" Title="Will a Dragon v2 ever bring 7 astronauts to the ISS?" Tags="|iss|spacex|astronauts|dragon-v2|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="10515" PostTypeId="2" ParentId="10514" CreationDate="2015-08-07T13:04:01.697" Score="15" Body="&lt;p&gt;When the space shuttle was in operation, it could carry as many as seven extra people, bringing the total population of the ISS up to ten. Three crew members is the standard &lt;em&gt;permanent&lt;/em&gt; capacity of the space station, but the ISS can actually hold more. Ten is the working limit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, the maximum number of people the shuttle carried was eight, and that was on a return trip from Mir.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that the shuttle only once carried greater than seven crew members, the seven person capacity of the Dragon v2 makes sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/missions/highlights/webcasts/shuttle/sts112/iss-qa.html&quot;&gt;STS-112 International Space Station - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Space_Shuttle_missions&quot;&gt;List of Space Shuttle missions - Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-08-07T13:04:01.697" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10516" PostTypeId="2" ParentId="10514" CreationDate="2015-08-07T13:14:02.183" Score="21" Body="&lt;p&gt;NASA has only contracted for flights with 4 astronauts, since the complement of the station is a maximum of 7 in the long term. The limit of 7 has many reasons, but some are for sleeping accommodations and life support systems capacity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz can carry 3.  Thus the current complement of ISS is limited to 6 (two Soyuz craft) at the moment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bringing 7 to the station would work in one flight of Dragon (or CST-100), if there were no Soyuz on station, but that is not a planned configuration.  Could it happen?  Sure. Anything could happen, but that is not the plan as currently set.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One possible case would be the direct hand-off. Lately, Soyuz flights have been doing indirect hand-off between missions. That is, a crew of 6 sends three home in a Soyuz, a week or three later another Soyuz launches with 3 more crew.  On Mir, they would sometimes do it that way, but sometimes do it as a direct hand-off, where the new crew would launch on a Soyuz, the previous crew of 3 would be there, they would share the station for a few days to a week, then the old crew would depart. Sometimes they would change out the Soyuz, and the new Soyuz crew would leave on the old Soyuz to keep the on-station Sotyuz within its usage date. (Soyuz's in-space lifespan is 280-odd days.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a consequence, when shifting only 2 of the 3 crew members, there was room for a tourist. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With commercial crew (Dragon V2 or CST-100) there are more options. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the current plan of 2 long-term stay (1 year) astronauts there are seats available for tourists on rotation flights.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="4703" LastEditDate="2015-08-08T15:10:11.717" LastActivityDate="2015-08-08T15:10:11.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10517" PostTypeId="1" AcceptedAnswerId="10518" CreationDate="2015-08-07T16:10:45.513" Score="27" ViewCount="7402" Body="&lt;p&gt;Correct me if I'm wrong but a probe to Mars will take months, during which time it's exposed to a vacuum and irradiated by the sun. It sounds to me like this would kill any micro-organisms on the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why then do we irradiate and sterilize our Mars probes? The journey would kill all micro-organisms anyway, right?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6816" LastEditDate="2017-04-29T17:46:19.117" LastActivityDate="2017-04-29T17:46:19.117" Title="Why do deep space probes have to be sterilized?" Tags="|probe|deep-space|astrobiology|planetary-protection|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10518" PostTypeId="2" ParentId="10517" CreationDate="2015-08-07T16:27:50.010" Score="57" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tardigrade&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tardigrades&lt;/a&gt; can survive vacuum, low temperatures, and moderate radiation for quite a while. They're &lt;em&gt;multicelled&lt;/em&gt; organisms. How tough is the toughest hypothetically viable &lt;em&gt;single-celled&lt;/em&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Extremophile&quot; rel=&quot;nofollow noreferrer&quot;&gt;extremophile&lt;/a&gt;? How sure are you that we've found all of them on Earth already?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a matter of caution. Contamination of another planet (or moon) is likely to be irreversible, &lt;a href=&quot;https://space.stackexchange.com/questions/9091/what-are-the-valid-arguments-for-forward-contamination-given-the-uniqueness-of&quot;&gt;may very easily make it difficult or impossible to determine the origin of any lifeforms there&lt;/a&gt;, or might outright take over and drive hypothetical native organisms to extinction (either before or after discovery), so all precautions are taken to avoid that.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-08-07T21:30:33.333" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10519" PostTypeId="2" ParentId="10517" CreationDate="2015-08-07T16:28:30.863" Score="12" Body="&lt;p&gt;In fact, this is &lt;a href=&quot;http://motherboard.vice.com/blog/lets-stop-wasting-money-sterilizing-our-spacecraft&quot;&gt;a subject of much debate&lt;/a&gt;. The bottom line is, bacteria wouldn't survive, but their spores might. A few of these spores will make it to Mars, and even a few could potentially spawn life on Mars from Earth. This has been proven from samples sent to the International Space Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are a lot of arguments against such protection. There have almost certainly been rocks which have survived from Earth to Mars that contain bacteria, so almost certainly life has already had a chance to reach Mars from Earth. Thus, it isn't particularly likely that a spacecraft will contaminate Mars with Earth life any more than has already been done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current thinking is to build Mars landers in a cleanroom, but not require strict contamination protection. The spacecraft surfaces are exposed to harsh light during the transit, and overall almost everything on them will die. The only things that are really subject to sterilization are those designed to detect life or organic chemicals, which it does still make sense to protect against. There is a lesser concern that such measures are still required for anything that digs into a surface, as it is more likely to reach fertile locations under the surface. Curiosity did not have as strict as measures regarding Planetary Protection, as one can see from this article.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-07T16:28:30.863" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="10520" PostTypeId="1" AcceptedAnswerId="10523" CreationDate="2015-08-07T17:10:25.003" Score="13" ViewCount="1080" Body="&lt;p&gt;What systems are used for range, speed and alignment when docking in orbit? Whether automatic, human controlled arm, or Apollo style human controlled ship to ship?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Playing Kerbal Space Program, I have my speed, range, and direction in game. Granted docking is still somewhat difficult, keeping all of these things in range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering, when docking to the ISS, or even earlier stations, what systems were used to enable docking? When trying to dock in game, I need to know, at the very least, my speed and my range. Lately they have been using Dragon (SpaceX?) docking capsules to resupply the ISS. What systems are used remotely or robotically to dock these modules?&lt;/p&gt;&#xA;" OwnerUserId="7052" LastEditorUserId="8383" LastEditDate="2015-08-08T04:46:40.927" LastActivityDate="2015-08-08T04:46:40.927" Title="Docking in orbit" Tags="|orbital-mechanics|docking|rendezvous|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10521" PostTypeId="2" ParentId="10520" CreationDate="2015-08-07T17:45:40.073" Score="2" Body="&lt;p&gt;The most commonly used system is either Radar or LIDAR, both of which will get the range and velocity of the differing target very accurately. LIDAR is a bit better, and will give you a larget image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alignment is a bit trickier, but again, the same systems, LIDAR and Radar, are the primary systems used.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-07T17:45:40.073" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10522" PostTypeId="2" ParentId="10517" CreationDate="2015-08-07T18:30:35.353" Score="27" Body="&lt;p&gt;So back in the summer of 1935, some folks down in Australia were having problems with a beetle's larvae that were nomming on the sugar cane roots and harming crops.  Since traditional methods of getting rid of the pest failed, they decided a good approach would be to introduce a few cane toads to go eat the beetles... a hundred or so of them in a couple of places in the north east.  Seemed like a good solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fast forward to today, there's now hundreds of millions of the things happily munching their way across the country, even evolving traits that help them move faster.  They are spreading at a rate of 60km/year, trashing the ecosystem of everything but the dratted beetles they were introduced to combat.  See the problem is that they're toxic and none of the native biosphere had evolved in the presence of that toxin, thus they had no natural predators and any potential predator deciding to eat one of these toads is likely to eat little else afterwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does this have to do with space?  Two things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, this is an example of an introduced invasive species... in the case of the toads it was intentional, rabbits are another species that has had similar disastrous results in Australia's ecosystem.  There are also examples of unintentionally introduced species, with much the same result.  If space probes successfully transferred viable micro-organisms to another planet we have the risk that any micro-organisms already existing there will be overrun by our earth bred critters.  That would be bad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we can mess up a relatively similar ecosystem with something as large and predictable as a rabbit, how much of a mess could we cause by introducing microscopic and unpredictable lifeforms to an ecosystem we know nothing about?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, the purpose of a lot of our planetary probes is to locate signs of life.  Ideally currently live life, but signs that life once existed will also do in a pinch.  This is kind of difficult if we're sending probes that are covered in lifeforms.  It's equivalent to making a radiation detector, putting it in a uranium housing, and then releasing a study on the elevated background radiation in your location.  You can't reliably detect something if your detection equipment is contaminated with the very thing you're trying to detect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And even if the probe that is doing the detecting is clean, if another probe isn't then the life you detected might have piggybacked on that.  If you want to say for certain &quot;this life evolved on Mars&quot; you need to ensure that it definitely didn't arrive on a probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Worst case, you introduce some lifeform to the planet and it promptly starts eradicating the life you're looking for.  Red squirrels are the native species in the UK but with the way the introduced grey squirrel has been pushing them out for decades, if you were looking for signs of squirrels here then it's possible you might never encounter evidence of red squirrels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is that it's unlikely that such a contamination could happen but not nearly as unlikely as you'd think.  If you send millions and millions of viable lifeforms or spores in to space, some of them are going to be able to survive, some of them are going to be in a spot that the solar radiation isn't hitting, some of them are going to be inside a shielded bit, some of them ... you get the idea.  Accidental sterilisation isn't in the slightest bit reliable enough and the potential problems posed by the two reasons above are sufficient to justify the effort spent on trying to reduce the risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The argument that life could travel from Earth to Mars via meteorite is valid, but actually demonstrates why planetary protection is so important:  If you find Earth micro-organisms on Mars, how do you know if they got there via meteorite and not via a probe?  Knowing if life &lt;em&gt;could&lt;/em&gt; make that trip would be very useful, in theories such as those involving the inverse, life being introduced to Earth via meteorite.&lt;/p&gt;&#xA;" OwnerUserId="11250" LastActivityDate="2015-08-07T18:30:35.353" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10523" PostTypeId="2" ParentId="10520" CreationDate="2015-08-07T19:43:03.813" Score="11" Body="&lt;p&gt;Shuttle, for rendezvous sensors, used the Ku-band in radar mode and an optical sight (the Crewman Optical Alignment Sight or COAS). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ggEYh.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ggEYh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Closer in there was a laser radar mounted on the docking system called the Trajectory Control System (TCS) that used reflectors mounted on the target vehicle to get range and range rate.  (The highlighted box is a different rendezvous sensor being tested, the TCS is the white box with a dark rectangular window to its left). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U53SS.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U53SS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;This was later supplemented by a hand-held LIDAR to give range and range rate (basically a modified police laser speed detector).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uysWZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uysWZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Early on this was also done by triangulating CCTV cameras from the payload bay and using an overlay on the TV monitors...but I believe this was considered a backup method by late in the program, as I don't recall much training on it being done, as opposed to the TCS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, for the last few feet, there was a optical docking target, viewed through a CCTV camera looking up through the airlock hatch window. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/W5JaQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/W5JaQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good narrative description &lt;a href=&quot;http://www.baen.com/rendezvous-part2.asp&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And just for coolness, the rendezvous flip maneuver (sped up)&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/BqcWnKkEOLs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-08-07T21:37:48.823" LastActivityDate="2015-08-07T21:37:48.823" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10524" PostTypeId="2" ParentId="9535" CreationDate="2015-08-07T19:49:24.973" Score="5" Body="&lt;p&gt;I think the general principle you're thinking about is only partially valid. Here is how I summarize it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mass production cannot happen without a factory. Factories can cost billions. But we do reuse the design of probes, especially the early probes in our history.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Research, design, and development have costs all their own. So I think there's a valid part of your principle that says, why design a new probe? We could just make multiple copies and send them everywhere they need to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, this is true &lt;strong&gt;for early space probes.&lt;/strong&gt; A probe designed to flyby Jupiter would gather the same data from Saturn, Uranus, and Neptune.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is indeed how it was done at first. Voyager 1 &amp;amp; 2 were virtually identical, and I'm pretty sure the same is true of Pioneer 10 &amp;amp; 11 and the Mariner Probes. In addition, a lot of the really early probes to the Moon, Mars, and Venus were copies of each other. The Ranger series, for example, goes out to at least Ranger 8.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That does &lt;strong&gt;not&lt;/strong&gt; mean we mass produce them. 8 Ranger probes is no justification for spending billions on a factory when they can just be assembled by hand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But there comes a point&lt;/strong&gt; when the same probe just won't gather any new data, because we've already done it! Time to design a new probe that gathers new data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may also be wondering about worlds unvisited, or recently visited, using entirely new probes. &lt;strong&gt;Why build New Horizons to go to Pluto when we can just reuse the Voyager design, for example?&lt;/strong&gt; Why spend money on new development (and risk new failures) when we already have a valid design proven to work? The problem here is that the launch budget wasn't allocated for a 720 kg probe (Voyager's mass). Instead, we wanted a 480 kg probe (New Horizons' mass) so we could hurl it out to Pluto's distance really fast. Also, these days, designing a new space probe is NOT as hard as it used to be (thankfully).&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2015-08-07T19:49:24.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10525" PostTypeId="2" ParentId="9535" CreationDate="2015-08-07T20:04:25.293" Score="3" Body="&lt;p&gt;There are definite cost savings when launching multiple satellites. A company I am familiar with launched many communication satellites. Using a common design saved engineering costs, some labor (Things got faster with time, and less down time for experts), software, and even testing (Some satellites were more heavily tested, reducing the cost). You are absolutely right, more probes would be less expensive. However, there are some drawbacks.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The amount of science gained from a new probe goes down with the number of spacecraft. For instance, MRO has a camera that has photographed the entire surface of Mars at fairly high resolution. Right now it can only take pictures of the surface to see things changing. It's practical science value has dropped dramatically.&lt;/li&gt;&#xA;&lt;li&gt;Communication with the group of satellites becomes difficult. One of the most difficult aspects of managing many space missions is when they are all in the same part of the sky at the same time. This reduces the availability of communications windows, making it harder overall to hear from these probes. So many at one location would be very difficult.&lt;/li&gt;&#xA;&lt;li&gt;So many missions at once would greatly reduce the effective budget of NASA, making it difficult to launch new missions in the future. This would end up with less science, as new better instruments are available all the time.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Making a duplicate of an instrument can be useful. For instance, the two Voyager probes are clones, as were the two Viking landers. This level of redundancy can be helpful, and doesn't cost significantly more. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be interested to know there is a mission in the planning that sounds very much like yours. It is the expedition satellites planned by Planetary Resources to identify satellites to mine for minerals. It will take essentially a duplicate satellite and send it to many different potential targets. This makes perfect sense, to use low cost copies of probes to study many targets around the Solar System, learning as much as one can.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-07T20:04:25.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10526" PostTypeId="1" CreationDate="2015-08-07T21:24:44.927" Score="2" ViewCount="113" Body="&lt;p&gt;I am currently reading &lt;a href=&quot;http://www.goodreads.com/book/show/19815802-the-heritage-trilogy?from_search=true&amp;amp;search_version=service&quot; rel=&quot;nofollow&quot;&gt;the heritage trilogy&lt;/a&gt; by William H. Keith, Jr, and one piece of technology got my attention. In these books, the advanced nations of the mid-21st century use thermal anti-matter drives for interplanetary travel. It is explicitly mentioned that certain kinds of ships can travel from earth to mars using a steady acceleration of 1g. At the same time, the ships are quite small, only some 100m in length and it is well known that AM annihilation produces some radiation (both primary and secondary). Hence I wondered, whether an AM ship that size would actually be viable (assumed one does not want to kill the crew in the process):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the reduction in fuel weight outweigh the required shielding?&#xA;Is the radiation less than thermal fission or fusion drives?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2015-08-07T21:24:44.927" Title="Shielding requirements for Antimatter drive" Tags="|rockets|antimatter|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10527" PostTypeId="1" AcceptedAnswerId="13458" CreationDate="2015-08-07T21:37:30.947" Score="9" ViewCount="383" Body="&lt;p&gt;Until recently space travel has been something governmental entities take on.  With commercial companies entering the foray, what liabilities are they accountable for in regards to the crap they leave behind?  Does the EPA's influence reach out into space?  Who regulates &quot;Space Safety&quot;?&lt;/p&gt;&#xA;" OwnerUserId="11104" LastActivityDate="2016-01-10T18:37:30.470" Title="What liability do commercial space firms have for abandoned space junk?" Tags="|debris|commercial|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10528" PostTypeId="2" ParentId="10086" CreationDate="2015-08-07T22:22:37.610" Score="1" Body="&lt;p&gt;Icing on rocket tanks is not a significant problem. Only cryogenic fuels will cause icing so the vast majority of military rockets are unaffected. However some space launch vehicles use cryogenic oxygen or hydrogen which will cause ice build up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets are filled up with cryogenic fuels several hours before launch. This combined with the humid air found at some launch locations(Cape Canaveral/French Guiyana) will lead to a non-negligible amount of ice building up on the side of tank. Cryogenic fuels are pumped constantly into the tanks until right before lift off to minimize boil off of the propellants. The Space Shuttle External Tank and Delta IV CBC had foam insulation since the large surface area of hydrogen tanks leads to large amounts of thermal transfer. Rockets like Falcon 9 and Atlas V that only have cryogenic oxygen tend not to have heavy insulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One reason for the lack of insulation on these rockets is that the ice forms a layer of natural insulation preventing more ice build up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the weight of the ice is not negligible however most of it falls off during launch due to extreme vibrations and air resistance.&lt;/p&gt;&#xA;" OwnerUserId="10398" LastActivityDate="2015-08-07T22:22:37.610" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10529" PostTypeId="1" AcceptedAnswerId="10530" CreationDate="2015-08-07T22:54:07.417" Score="3" ViewCount="189" Body="&lt;p&gt;There seems to be talk (and even some action done by DARPA) about engineering organisms to terraform Mars, however, current space organizations like NASA clearly do not want to get anything that could survive in space (other than humans) to stay in space for research purposes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While sending organisms to do terraforming is something that can't be done yet, why is it that major space organizations seem not to care, some even embracing the idea? From their current behavior, I'd think they'd be extremely against it if anything.&lt;/p&gt;&#xA;" OwnerUserId="11258" LastEditorUserId="6816" LastEditDate="2015-08-08T19:06:55.380" LastActivityDate="2015-08-08T19:06:55.380" Title="How are organisms allowed be engineered to terraform Mars when NASA and others go through such measures to sterlize their probes?" Tags="|probe|life|terraforming|sterilization|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10530" PostTypeId="2" ParentId="10529" CreationDate="2015-08-07T23:16:26.350" Score="4" Body="&lt;p&gt;Terraforming Mars, if it turns out to be viable at all, will be a very long term project, and will start very far in the future. (We'll probably have to learn how to terraform the Earth first; we have messed up our own environment pretty badly.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probe sterilization efforts are intended to avoid interfering with the search for current or fossil life on Mars, which is a much nearer term goal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once we have determined the extent of life on Mars, being humans, we'll modify the Martian ecosystem to our own purposes.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-08-07T23:16:26.350" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10531" PostTypeId="2" ParentId="10517" CreationDate="2015-08-07T23:22:23.797" Score="5" Body="&lt;p&gt;Leaving aside the very good reasons why you don't want any organisms on a probe trying to detect Martian organisms... we &lt;em&gt;already&lt;/em&gt; have fairly good evidence that life can survive irradiation and vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo 12 crew landed near an early unmanned lander, Surveyor 3, and recovered parts of it for examination. Bacteria were successfully cultivated after samples were taken from the lander's camera, which had spent two or three years on the lunar surface. Whether these bacterial samples had actually survived that long, or whether they were contamination in the lab, is debatable; the results are contested, and we don't have firm evidence to prove or disprove reliable sterile procedure, but they're certainly seen as &lt;em&gt;plausible&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More recently, and with substantially more rigorous sterile procedures, &lt;a href=&quot;http://dx.doi.org/10.1007/s11084-012-9282-5&quot; rel=&quot;noreferrer&quot;&gt;bacterial spores&lt;/a&gt; and &lt;a href=&quot;http://dx.doi.org/10.1017/S1473550414000214&quot; rel=&quot;noreferrer&quot;&gt;lichen&lt;/a&gt; have been regrown after spending a year and a half on an exposed platform on the ISS - albeit they seem to have had some rock as protection. (The main problem for microbial hitchhikers in long-duration spaceflight seems to be UV, rather than vacuum.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most planetary probes don't haul along bits of sandstone... but they do have sheltered corners that might easily escape UV throughout the flight. &lt;/p&gt;&#xA;" OwnerUserId="11259" LastActivityDate="2015-08-07T23:22:23.797" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10532" PostTypeId="2" ParentId="7948" CreationDate="2015-08-08T00:14:57.250" Score="3" Body="&lt;p&gt;There's a fair amount written on the guidance system for the Apollo lunar landers; the same principles apply for any powered landing, although you have to add atmospheric drag into your model for Earth landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equations shown under &quot;Powered Descent Guidance Theory&quot; on &lt;a href=&quot;http://dodlithr.blogspot.com/2014/08/lm-descent-to-moon-part-1-theory-and.html&quot; rel=&quot;nofollow&quot;&gt;this page&lt;/a&gt; are the key to getting position and velocity deltas to zero at the same time (which is what you need for a precise soft landing). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Robert Braeunig's notes on his &lt;a href=&quot;http://www.braeunig.us/apollo/LM-descent.htm&quot; rel=&quot;nofollow&quot;&gt;LM simulation&lt;/a&gt; are also potentially useful. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-08-10T01:40:29.543" LastActivityDate="2016-08-10T01:40:29.543" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10533" PostTypeId="1" AcceptedAnswerId="10535" CreationDate="2015-08-08T02:05:52.080" Score="1" ViewCount="2371" Body="&lt;p&gt;I know it was to dock with the Lunar Module, but why could the parts not be connected from the start to avoid this additional maneuver?&lt;/p&gt;&#xA;" OwnerUserId="11263" LastEditorUserId="6816" LastEditDate="2015-08-08T02:22:32.467" LastActivityDate="2015-08-08T02:43:05.950" Title="Why did the CSM have to perform a 180 degree turn in the Apollo missions after stage separation?" Tags="|apollo-program|docking|" AnswerCount="1" CommentCount="1" ClosedDate="2015-08-08T02:38:42.760" ContentLicense="CC BY-SA 3.0" />
  <row Id="10534" PostTypeId="2" ParentId="10520" CreationDate="2015-08-08T02:10:18.040" Score="7" Body="&lt;p&gt;My favorite reference for this is JSC 63400 &quot;History of Space Shuttle Rendezvous,&quot; available &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023479.pdf&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023479.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some highlights (page numbers for Rev 3):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;TACAN transmitters and radar transponders were considered for cooperative targets during the Shuttle program (pg 83)&lt;/li&gt;&#xA;&lt;li&gt;Pg 89 shows several of the (mostly text) computer displays used by shuttle during rendezvous and prox ops (RPO).  Very Mechjeb.&lt;/li&gt;&#xA;&lt;li&gt;Pg 108 introduces PLBay, which became RPOP, a situational awareness program I now work on.  The program shows where the spacecraft has been, where it will go...useful for relative mechanics, which aren't always intuitive.  Pictures of what RPOP looks like are on Pgs 134, 238 (baaaarely), 239 (much more obvious in-use view illustration), 241 (closer in), 243, 244, 249-252.&lt;/li&gt;&#xA;&lt;li&gt;Pg 225 starts a VERY in-depth look at a pretty typical Shuttle-to-ISS mission.  Long story short is that RPOD (rendezvous, prox ops and docking) starts with pre-launch math to identify favorable docking opportunities, and then a lot of exacting measurement using all the things Organic Marble mentioned, plus a lot of supporting instruments to transform that data into things that the crew can understand and fly.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2015-08-08T02:10:18.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10535" PostTypeId="2" ParentId="10533" CreationDate="2015-08-08T02:22:44.373" Score="2" Body="&lt;p&gt;This maneuver was called &lt;a href=&quot;https://en.wikipedia.org/wiki/Transposition%2C_docking%2C_and_extraction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transposition, docking, and extraction&lt;/a&gt;, or TD&amp;amp;E for short, and the whole reason behind it was so the Saturn V's &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_escape_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;Launch Escape System&lt;/a&gt; (LES) didn't have to also provide thrust for the whole configuration, but only for the Command Module (CM) at the top of the stack:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MXQ1C.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MXQ1C.jpg&quot; alt=&quot;Apollo LES pad abort test with boilerplate crew module&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Since this is already described in sufficient detail in &lt;a href=&quot;https://space.stackexchange.com/questions/3995/reasons-behind-the-transposition-docking-and-extraction-maneuver&quot;&gt;Reasons behind the “Transposition, docking and extraction” maneuver&lt;/a&gt;, I'll just post one more video of how &lt;a href=&quot;https://www.youtube.com/watch?v=AqeJzItldSQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;in-flight abort of Apollo&lt;/a&gt; looked like during the Apollo Launch Abort System Test:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/AqeJzItldSQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Unmanned Apollo 'boilerplate' Command Module #22 was launched aboard a&#xA;  Little Joe II booster to test of the dynamics of an in-flight abort.&#xA;  Although the booster unintentionally went out of control and&#xA;  disintegrated - in a spectacular scene eerily reminescent of the&#xA;  Challenger disaster - the Launch Escape System functioned well, and&#xA;  the CM was recovered undamaged.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-08-08T02:43:05.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10537" PostTypeId="2" ParentId="8558" CreationDate="2015-08-08T03:30:14.983" Score="2" Body="&lt;p&gt;First, everything in the Oort Cloud is really, really far apart. You could spend years looking and never see anything because of the reasons you gave. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for your ideas&amp;hellip;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shadows idea is a great one, and might be your best bet, depending on the size of the objects and the equipment you have with you. You'd probably have to look in a lot of directions at once (i.e. a wide-field telescope) to catch anything happening. If the object was big enough, it could totally block out a star in the same way as the Moon blocks out the Sun's light during a solar eclipse. If the object was smaller (but still big enough to notice), it would block just a tiny fraction of the star's light. This almost certainly would not be enough to notice with your eye (unless it was a pretty big object!), but a sensitive camera and sensitive software could definitely pick up the slight dimming of the star. (This is how many extrasolar planets are discovered!) It would be an almost useless observation, however, because you would have great difficulty seeing that object pass in front of another star again. You'd have to have a pretty good idea &lt;em&gt;where&lt;/em&gt; and &lt;em&gt;when&lt;/em&gt; to look in order to have any chance of spotting it a second time. Using this method, you would have a pretty good idea of the object's angular size. Unfortunately, you would not know its absolute size or how far away it was. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radar is a definite possibility, assuming you have a powerful enough radar transmitter and a sensitive enough receiver. The objects would need to have a pretty big size for this to work. The upside is that if you successfully found an object using this technique, you'd know how far away it was, and you'd probably have more information about it as well (e.g. size, shape, albedo/reflectivity).&lt;/p&gt;&#xA;" OwnerUserId="11264" LastEditorUserId="6816" LastEditDate="2015-08-08T03:54:08.997" LastActivityDate="2015-08-08T03:54:08.997" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10538" PostTypeId="2" ParentId="10527" CreationDate="2015-08-08T03:37:37.353" Score="4" Body="&lt;p&gt;Nobody regulates space debris yet. It's a new frontier for all of us, and as such, nobody has claimed any &quot;rights&quot; to the airspace or established binding rules regarding its use. Besides, if one government passed regulations regarding space junk, other governments wouldn't necessarily be bound to follow those regulations. As such, the EPA could set the strictest rules for US space vehicles, but a Russian satellite company wouldn't be bound by those rules at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, NASA has a program (the &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;Orbital Debris Program&lt;/a&gt;) dedicated to &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/measure/measurement.html&quot; rel=&quot;nofollow&quot;&gt;monitoring space debris&lt;/a&gt; and has established &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/mitigate/safetystandard.html&quot; rel=&quot;nofollow&quot;&gt;its own standards&lt;/a&gt; to help minimize space debris generation. As far as I know, this is the world's premiere space debris program. :-) &lt;/p&gt;&#xA;" OwnerUserId="11264" LastActivityDate="2015-08-08T03:37:37.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10539" PostTypeId="2" ParentId="9124" CreationDate="2015-08-08T03:45:07.607" Score="1" Body="&lt;p&gt;Robert Frost, an engineer and instructor who has trained many astronauts for the International Space Station, gave the following list on &lt;em&gt;Quora&lt;/em&gt;: (&lt;a href=&quot;http://www.quora.com/What-clothes-do-Astronauts-pack-for-space-missions-and-how-many&quot; rel=&quot;nofollow&quot;&gt;link&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1 pair of shoes for the treadmill&lt;/li&gt;&#xA;&lt;li&gt;1 pair of shoes for the bike&lt;/li&gt;&#xA;&lt;li&gt;1 pair of exercise shorts for every 3 days of exercise&lt;/li&gt;&#xA;&lt;li&gt;1 T-shirt for every 3 days of exercise&lt;/li&gt;&#xA;&lt;li&gt;1 work shirt for every 10 days&lt;/li&gt;&#xA;&lt;li&gt;1 T-shirt for under the workshirt for every 10 days&lt;/li&gt;&#xA;&lt;li&gt;1 pair work pants/shorts for every 10 days&lt;/li&gt;&#xA;&lt;li&gt;1 pair underwear for every 2 days&lt;/li&gt;&#xA;&lt;li&gt;1 pair socks for every 2 days&lt;/li&gt;&#xA;&lt;li&gt;2 sweaters&lt;/li&gt;&#xA;&lt;li&gt;2 pairs Russian overalls (optional)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11264" LastActivityDate="2015-08-08T03:45:07.607" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10540" PostTypeId="2" ParentId="8692" CreationDate="2015-08-08T04:06:37.507" Score="3" Body="&lt;p&gt;To first order, everything will come from the Sun. This is more true further from the Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some secondary effects might pop up, however....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One possibility is that increased radiation could further ionize parts of the atmosphere, and this plasma could be damaging if it hit the spacecraft in a different direction than the shields were protecting against. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_field_of_the_Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetic field of the Moon&lt;/a&gt; is significantly less than that of the Earth, so I would not expect it to play any significant role (other than providing some degree of shielding if the Moon is almost directly between the Earth and the Sun). A big reason for why it's so weak is because there's no central dynamo generating a permanent dipole magnetic field. The Moon's core &quot;froze out&quot;, so the magnetic field that we can detect originates from crustal rocks that have &lt;a href=&quot;https://en.wikipedia.org/wiki/Paleomagnetism&quot; rel=&quot;nofollow noreferrer&quot;&gt;fossil magnetization&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth's magnetic field certainly will play a role, however, but how it interacts with the solar storm depends on the nature of the storm! Charged particles will be caught up in the Van Allen radiation belts via a phenomenon known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_mirror&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetic mirroring&lt;/a&gt;, causing them to spiral as they travel back and forth between the magnetic poles. Energetic photons will travel unfazed through the magnetic field. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Just for coolness's sake, here's a schematic showing some different effects:&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QNLmF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QNLmF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11264" LastActivityDate="2015-08-08T04:06:37.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10541" PostTypeId="2" ParentId="9833" CreationDate="2015-08-08T04:18:03.357" Score="1" Body="&lt;p&gt;The jury is still out! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the community is buzzing with theories, most of which are suggesting that there may be transport processes (i.e. winds, rivers, glaciers, etc.) happening on Pluto, or that the surface and/or atmospheric chemistry is a little different because it recently hit perihelion (its point of closest approach to the Sun), so it was slightly warmer than it would have been. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For examples, check out these articles: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103502000684&quot; rel=&quot;nofollow&quot;&gt;Photometry of pluto in the last decade and before: evidence for volatile transport?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://iopscience.iop.org/1538-3881/139/3/1117&quot; rel=&quot;nofollow&quot;&gt;PLUTO AND CHARON WITH THE HUBBLE SPACE TELESCOPE. I. MONITORING GLOBAL CHANGE AND IMPROVED SURFACE PROPERTIES FROM LIGHT CURVES&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://iopscience.iop.org/2041-8205/804/1/L6&quot; rel=&quot;nofollow&quot;&gt;PHOTOMETRY OF PLUTO 2008–2014: EVIDENCE OF ONGOING SEASONAL VOLATILE TRANSPORT AND ACTIVITY&#xA;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11264" LastActivityDate="2015-08-08T04:18:03.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10542" PostTypeId="1" AcceptedAnswerId="10544" CreationDate="2015-08-08T07:20:32.247" Score="4" ViewCount="196" Body="&lt;p&gt;Is there any information available about, If first stage of Falcon 9 v1.1 will land as intended:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How many times it can be used again?&lt;/li&gt;&#xA;&lt;li&gt;After which time it will be ready for launch?&lt;/li&gt;&#xA;&lt;li&gt;What need to be done to prepare first stage for second launch?&lt;/li&gt;&#xA;&lt;li&gt;How many financial economy will give the reuse of first stage?&lt;/li&gt;&#xA;&lt;li&gt;Also is there any other details about reuse of first stage?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I understand that, all above cannot be answered for sure before several first stage landings, but I think there should be some estimations about them.&lt;/p&gt;&#xA;" OwnerUserId="8137" LastEditorUserId="10234" LastEditDate="2015-08-08T18:12:02.780" LastActivityDate="2015-08-09T01:56:59.317" Title="Is there any information about the second and further use of first stage of Falcon 9 v1.1 if it will be landed as intended?" Tags="|rockets|spacex|landing|falcon-9|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10543" PostTypeId="2" ParentId="10529" CreationDate="2015-08-08T11:08:26.160" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;While sending organisms to do terraforming is something that can't be done yet, why is it that major space organization seems to not care, some even embracing the idea?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Not everyone is of the same mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current planetary protection rules exist for two reasons, one practical, the other, moral. The practical reason is that should we discover life on another planet, we need to be able to know that that life is not a consequence of contamination by the very probes sent to look for that life. The moral reason is that committing planetocide is a crime that makes committing genocide look petty in comparison.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Others see things differently. If there is life on Mars (and that remains a very big &quot;if&quot;), it's obviously extremely primitive life. To this group of people, killing off hypothetical primitive microbes is not a crime. To them, terraforming Mars would not be a crime against the universe. It would instead be a &quot;good thing&quot;.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-08-08T11:08:26.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10544" PostTypeId="2" ParentId="10542" CreationDate="2015-08-08T11:10:41.187" Score="3" Body="&lt;p&gt;Most of these questions can only really be answered after the first time a successful first stage is landed. I suspect that at first, each first stage will be given a complete inspection, to ensure it is ready to go for the next launch. I imagine this process will last at least a few months. If the inspection passes, I don't believe anything is required other than a refuel, although I'm sure they will put it through it's complete testing regime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the target number of reuses for each first stage is around 10, although that number is subject to change. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eventually, if the process is really nailed down, there will probably be minimal inspection, some kind of a test firing, and perhaps nothing more than that. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-08T11:10:41.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10545" PostTypeId="2" ParentId="10483" CreationDate="2015-08-08T12:12:10.623" Score="1" Body="&lt;p&gt;If I'm reading your question right, this is a crazy idea and I love crazy ideas.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Our Solar system poses a significant problem for colonisation insofar&#xA;  that the rocky planets are predominantly on the... small scale; at&#xA;  least as far as mass is concerned (Earth is the exception rather than&#xA;  the rule).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Size is important, cause you want a magnetic field, requires a solid/liquid inner core (less likely if the planet is too small), you want the planet to retain it's atmosphere (again, size helps - gravity being stronger), and you probably want plate tectonics (heat in the core is important there too) and you need the right temperature, but presumably if we can move enough mass to build a 2nd earth, we can move that mass as well.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, there are many rocky planets, dwarf planets, moons, and&#xA;  asteroids scattered throughout the system. Excluding the severe&#xA;  logistical difficulties in generating such giant impact events in the&#xA;  first place, are there theoretical means by which the masses of&#xA;  different bodies can be combined, rather than their collisions simply&#xA;  shearing vast amounts of debris off the larger body?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;When you're talking about moving millions, billions and even trillions of tons, on objects that orbit the sun at many thousands of miles per hour, that's no easy trick, add to that, most of these objects are very far apart, so, as you say, logistical difficulties do exist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By creating an atmosphere around the object, that will buffer to some extent any debris fly off.   Glancing blows vs direct hits can help too and lower velocity impact, such as starting from mostly stable Trojan points might help.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Back in the early stages of the Solar system when the planets where&#xA;  hot and less solid, such combinations seem to have been a real&#xA;  possibility. Is it still hypothetically possible in a mature system?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Another problem that perhaps you haven't though of is the mass of all the stuff out there might not be as much as you think.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total mass of the entire asteroid belt, including dwarf planet Ceres is about 4% of the mass of our moon.   &lt;a href=&quot;http://www.sciencedaily.com/terms/asteroid_belt.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;(Source)&lt;/a&gt;.  The mass of Jupiter's Trojans might be around that much as well (I read that somewhere).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass of all Kuiper belt objects might be significantly larger than that, but those objects are more icy, less rocky, not as good for building a rocky planet and enormously spread out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even the mass of all moons wouldn't be enough.  Ganymede is larger than Mercury, but less than half it's mass, all the moons together would give you a bit more than the mass of Mars, and many of the larger moons are far out in the solar system and have more water, less dense elements.   Pull too many moons together and you'll have a planet covered with oceans, which might not be all bad, but it's probobly not what you're going for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ej4eH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ej4eH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source:  &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if you really want to build another Earth like planet with objects in our solar system, you'd probobly have to combine Venus, Mars and Mercury (and maybe Europa for water). Those 3 planets combined would have a mass almost equal to the Earth.   There's probobly enough Iron in there to create the desired Iron core, once the planet after the heat of smashing together cooled enough to have a solid metal center and a liquid outer metal core.   Mars has a lower Iron ratio but Mercury has a lot of Iron.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once they smash into each other, the newly formed planet would need a long time to cool, likely tens of millions of years or so.  But you might be able to build an earth that way.   I don't believe you can build an earth out of dwarf planets, moons and asteroids though.  There's simply not enough material.&lt;/p&gt;&#xA;" OwnerUserId="8711" LastActivityDate="2015-08-08T12:12:10.623" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10546" PostTypeId="1" AcceptedAnswerId="10547" CreationDate="2015-08-08T18:45:33.670" Score="52" ViewCount="7159" Body="&lt;p&gt;When the Columbia Shuttle broke apart in 2003, it was known after the launch that a piece of foam had fallen and hit the Shuttle. NASA apparently chose not to &lt;a href=&quot;http://www.space.com/19436-columbia-disaster.html&quot;&gt;investigate&lt;/a&gt; it as well as they could have. If they had known (or even suspected) the damage to Columbia would have resulted in catastrophe upon re-entry, what process would have taken place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could they have performed an impromptu spacewalk to assess and possibly repair the damage? Could another Shuttle have been launched to retrieve the crew? Could the ISS have been used in some way to help their situation? (I think these questions are more for my curiosity of what &lt;em&gt;could&lt;/em&gt; have been done and they were just part of my thought process. They don't necessarily need to be part of the answer. I'm more interested in what NASA's process &lt;em&gt;would&lt;/em&gt; have been).&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="29286" LastEditDate="2020-08-31T18:04:56.623" LastActivityDate="2022-05-29T14:53:15.823" Title="What would NASA have done if they knew Columbia was catastrophically damaged?" Tags="|nasa|space-shuttle|failure|procedure|shuttle-columbia|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10547" PostTypeId="2" ParentId="10546" CreationDate="2015-08-08T18:50:13.243" Score="56" Body="&lt;p&gt;The answers to all your questions are described at length in &lt;a href=&quot;http://history.nasa.gov/columbia/Troxell/Columbia%20Web%20Site/CAIB/CAIB%20Website/CAIB%20Report/Volume%201/Part%202/chapter6.pdf&quot; rel=&quot;noreferrer&quot;&gt;section 6.4 (page 173)&lt;/a&gt; &quot;Possibility of Rescue or Repair&quot; of the &lt;a href=&quot;http://history.nasa.gov/columbia/CAIB_reportindex.html&quot; rel=&quot;noreferrer&quot;&gt;CAIB Report&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/columbia/Troxell/Columbia%20Web%20Site/CAIB/CAIB%20Website/CAIB%20Report/Volume%202/part13.pdf&quot; rel=&quot;noreferrer&quot;&gt;Appendix D-13&lt;/a&gt; &quot;STS-107 In-flight Options Assessment&quot; is a very detailed description of the process that was utilized to come up with the self-repair and rescue options.  Major elements of the process:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Assumptions were made about the type and extent of the damage&#xA;(although ground testing and analysis after the fact&#xA;identified an extremely likely candidate for the damage, this was&#xA;never and can never be completely verified).&lt;/li&gt;&#xA;&lt;li&gt;Analysis was done to determine how long &lt;em&gt;Columbia&lt;/em&gt; would have been&#xA;able to remain on orbit to await rescue or attempt self-repair.&lt;/li&gt;&#xA;&lt;li&gt;The feasibility of accelerating the launch preparation of &lt;em&gt;Atlantis&lt;/em&gt;&#xA;was studied.&lt;/li&gt;&#xA;&lt;li&gt;The required modifications to &lt;em&gt;Atlantis's&lt;/em&gt; flight software loads were&#xA;scoped.&lt;/li&gt;&#xA;&lt;li&gt;The rescue EVA was planned.&lt;/li&gt;&#xA;&lt;li&gt;Disposal of the abandoned &lt;em&gt;Columbia&lt;/em&gt; was studied.&lt;/li&gt;&#xA;&lt;li&gt;On-board materials were evaluated for their possible utility in a&#xA;self-repair.&lt;/li&gt;&#xA;&lt;li&gt;Analysis of possible entry trajectory changes to reduce the heat load&#xA;(including payload jettison) was performed.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;TL;DR: It might just have been possible to mount a rescue mission using a different shuttle being prepped for the next mission.  However, the debate over whether this would have been wise, given that the foam shedding problem would not have been fixed, would have been intense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r5M6h.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r5M6h.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Self-repair options using EVA tools and techniques are also described in the report.  Basically they consist of accessing the breach and filling it with materials which might help to slow the rate of damage increase.  The chance of any of this working is mostly unknown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZLdZe.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZLdZe.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS would have played no role in any rescue and could have done nothing to help due to its different orbital inclination from that of the STS-107 mission.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-05-03T13:31:04.017" LastActivityDate="2017-05-03T13:31:04.017" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="10548" PostTypeId="1" CreationDate="2015-08-06T18:26:30.897" Score="14" ViewCount="3334" Body="&lt;p&gt;I've dreamed about it ever since I was six, and where I live people laugh at me when I tell them about this because for one, no one's ever been an astronaut in my country and two, I'm a girl. But I'll do anything to make it real! I don't care, it's all I've ever wanted!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please I know this is childish, but I don't have many sources of information except the internet, so please it would be very kind if you could share your experience or any kind of detail that could help me pursue my dream. I would be very thankful. &lt;/p&gt;&#xA;" OwnerDisplayName="maya" LastEditorUserId="58" LastEditDate="2015-08-09T01:44:58.043" LastActivityDate="2018-08-20T08:40:36.150" Title="Does NASA hire foreign nationals as astronauts?" Tags="|nasa|astronauts|career|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10549" PostTypeId="2" ParentId="10542" CreationDate="2015-08-09T01:56:59.317" Score="2" Body="&lt;p&gt;The current expectation is that the first landed F-9 First Stage will be taken to McGregor, TX for tear down and inspection.  Depending on its state, it or the second landed stage will be taken to New Mexico, to Spaceport America in Mojave and used as the F9R-Dev2 style vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;F9R-Dev1 was lost when due to control issues the vehicle left the allowed flight box and terminated in flight in Mcgregor, TX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;F9R-Dev2 was built but repurposed to be used for the Max Drag Abort at Vandenberg Air Force Base, California.  It is now delayed to post the first unmanned Dragon V2 flight, since the current Dragon V2 vehicle they intended to use is no longer considered flight representative. So they will refly the first unmanned mission as the Max Drag Abort mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first landed stage will thus be used to test and develop the edges of the landing scenario further.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-08-09T01:56:59.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10550" PostTypeId="1" AcceptedAnswerId="10694" CreationDate="2015-08-09T02:12:47.867" Score="12" ViewCount="658" Body="&lt;p&gt;A shootoff from &lt;a href=&quot;https://space.stackexchange.com/questions/10517/why-do-deep-space-probes-have-to-be-sterilized/10531?noredirect=1#comment29542_10531&quot;&gt;my other question&lt;/a&gt;, has there ever been a deep space probe that was &lt;strong&gt;not&lt;/strong&gt; sterilized to prevent micro-organism contamination of the celestial body?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm thinking early space age history, starting with the Lunas and Pioneers attempting to hit the Moon in 1958. I consider myself well-read on Soviet space efforts (and American efforts too), but have never heard a peep about sterilization for either side.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-08-18T21:49:12.763" Title="Was there ever a non-sterilized probe sent to a celestial body?" Tags="|probe|history|deep-space|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10551" PostTypeId="2" ParentId="10548" CreationDate="2015-08-09T02:29:56.097" Score="16" Body="&lt;p&gt;NASA astronauts must be US citizens: see &lt;a href=&quot;http://www.nasa.gov/audience/forstudents/postsecondary/features/F_Astronaut_Requirements.html&quot;&gt;this document&lt;/a&gt;. In the past this resulted in &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/htmlbios/patrick.html&quot;&gt;some&lt;/a&gt; &lt;a href=&quot;http://www.space.com/17056-kalpana-chawla-biography.html&quot;&gt;individuals&lt;/a&gt; changing citizenship to qualify.  However, the international collaborative nature of spaceflight projects since Apollo has meant that many foreign nationals were trained by NASA and flown on NASA vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early days of the Shuttle program, nations that launched comsats from the Shuttle could fly a crewmember as a Payload Specialist...&lt;a href=&quot;http://www.jsc.nasa.gov/Bios/htmlbios/al-saud.html&quot;&gt;Saudia Arabia&lt;/a&gt; and &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/htmlbios/nerivela-r.html&quot;&gt;Mexican&lt;/a&gt; nationals flew under this program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a entire &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-61-A&quot;&gt;Shuttle Spacelab mission&lt;/a&gt; funded by West Germany; three West German nationals flew as Payload Specialists on this flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The deep involvement of Canada in the Shuttle and Station programs, through that nation's extensive contributions in robotics, lead to &lt;a href=&quot;https://en.wikipedia.org/wiki/Category:Canadian_astronauts&quot;&gt;several Canadian nationals&lt;/a&gt; flying multiple missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_astronauts_by_year_of_selection&quot;&gt;In the 90s&lt;/a&gt; many European and Japanese individuals were trained by NASA to fly as mission specialists, especially in Groups 13 and 14.  This was all handled by intergovernmental arrangements and the individuals were astronauts of their respective space agencies, not NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/astrobio_international.html&quot;&gt;complete list&lt;/a&gt; of international astronauts, defined by NASA as &quot;those individuals from international space agencies who have trained or served with NASA Astronauts.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's the history.  In pursuit of your dream, you should know that unfortunately the number of astronauts needed by NASA is going to be very small for a long time, due to the long duration of missions flown to the ISS and the long delay until any other kind of NASA human spaceflight missions begin.  But the bright spot is the possible growth in commercial spaceflight in the near future.  Keep yourself in top physical condition, study medicine, engineering, or science and excel at it, obtain a pilot's licence if possible, and follow your dream!  I salute your ambition!&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-08-09T03:48:00.643" LastActivityDate="2015-08-09T03:48:00.643" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10552" PostTypeId="2" ParentId="3197" CreationDate="2015-08-09T03:36:58.380" Score="1" Body="&lt;p&gt;You are looking for the &lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Propellant_mass_fraction&quot; rel=&quot;nofollow&quot;&gt;Propellant mass fraction&lt;/a&gt;&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course it depends on the components and will vary from one engine to another like you suggest, but for a quick answer, on average:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Propellant mass fractions are typically around 0.8 to 0.9.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="690" LastActivityDate="2015-08-09T03:36:58.380" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10555" PostTypeId="1" CreationDate="2015-08-09T08:34:45.530" Score="2" ViewCount="184" Body="&lt;p&gt;How close could a probe fly past a planet before forces such as air resistance make it impossible for the probe to escape the pull of its gravity? Obviously there will be different answers for different planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I ask this is that a project called &quot;InspirationMars&quot; claims that they will send a manned probe to slingshot around Mars only 100 miles from its surface. Is it not probable that Mars' exosphere will slow the probe significantly? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.inspirationmars.org/&quot; rel=&quot;nofollow&quot;&gt;http://www.inspirationmars.org/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11209" LastActivityDate="2015-08-09T14:30:45.157" Title="How close to a planet can a flyby manoeuvre realistically take place before air resistance becomes a problem?" Tags="|mars|probe|flyby|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10556" PostTypeId="2" ParentId="10555" CreationDate="2015-08-09T09:48:17.850" Score="2" Body="&lt;p&gt;The point where the atmosphere of the Earth 'ends', and anything further can be called 'space', is the &lt;strong&gt;Kármán Line&lt;/strong&gt;. On the Earth, this line is just over $100 kilometres$ from the surface. However as we know, the ISS orbits at $400 kilometres$, and still faces air resistance and drag that requires correction. Hence the Kármán line alone does not define where air resistance itself stops.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/3101/1028&quot;&gt;this&lt;/a&gt; interesting answer, Mark Adler calculates the Mars-analog of the Kármán line to be &lt;strong&gt;$88 Km$&lt;/strong&gt;. Therefore if you are higher than $88 km$ on Mars, you are technically in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;However&lt;/em&gt;, he goes on to say that the actual safe altitude, below which a spacecraft is considered to be 're-entering', is &lt;strong&gt;$125 Km.$&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At Earth for Apollo it &lt;em&gt;[altitude below which atmosphere is significant]&lt;/em&gt; was defined as 400,000 feet, or about 122 km. At Mars is has been defined at 125 km. That is when the $n$ minutes of terror begins.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is very interesting, because I'm sure $n$ minutes of terror is not an ideal scientific flyby of Mars. My own personal guess is that the webpage you linked to does not reflect the exact altitude of the flyby, but rather an approximation for public understanding. Another possibility is that they want to intentionally aerobrake, but that's inconsistent with a free-return mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta&lt;/a&gt; flew by Mars in February 2007, making it's closest approach at &lt;strong&gt;$250 Km$&lt;/strong&gt;. Leaving room for error, based on Mark Adler's answer, I would say that a flyby of &lt;strong&gt;$150 Km$&lt;/strong&gt; should be safe. &lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-08-09T09:48:17.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10557" PostTypeId="2" ParentId="10546" CreationDate="2015-08-09T11:35:16.780" Score="21" Body="&lt;p&gt;A few additions to Organic Marble's answer.&lt;/p&gt;&#xA;&lt;p&gt;The plan detailed in the CAIB report &lt;a href=&quot;http://arstechnica.com/science/2014/02/the-audacious-rescue-plan-that-might-have-saved-space-shuttle-columbia/&quot; rel=&quot;noreferrer&quot;&gt;shows how difficult it would have been&lt;/a&gt; to mount a rescue mission.&#xA;Even though Atlantis was being prepared for the next mission, they would have had to cut mission preparation time in half. No time for tests and checks. Everyone working frantically under gigantic pressure with no room for mistakes.&lt;br /&gt;&#xA;Meanwhile Columbia's crew would need to extend the mission far beyond the designed on-orbit lifetime of the Shuttle, with air (actually carbon dioxide scrubber canisters) being the most critical limitation.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Accelerate&amp;quot; is a prosaic word for the herculean effort that would have been needed. Activities that normally take place across weeks or months would have to happen in hours or days. Civil servants and contractors at KSC would have to begin 24/7 shift work, keeping the lights on and the process running every hour of every day, for a minimum of 21 days, to power Atlantis through checkout and make it ready to launch.&lt;/p&gt;&#xA;&lt;p&gt;Three unceasing, brutal weeks of 24/7 shift work—and that's with absolutely no margin factored in for errors or failures. The Orbital Processing Facility team, the Vehicle Assembly Building team, and the Launch Complex 39 pad team would have had to get every one of the millions of steps right, and every component of Atlantis would have had to function perfectly the very first time, or it would all be wasted.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From the CAIB:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It should be noted that although each of the individual elements could be completed in a best-case scenario to allow a rescue mission to be attempted, the total risk of shortening training and preparation time is higher than the individual elements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;A spacewalk to assess Columbia's damage would have been difficult. Columbia didn't have SAFER jet packs on board, so its crew would have had to free-climb across the outside of the shuttle (a precarious affair without handholds) to get to the wing.&lt;/p&gt;&#xA;&lt;p&gt;From the Ars Technica article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The error-free rescue of Columbia would have depended not just on the flawless execution of teams at all of the NASA centers but also on an unknown number of events that happened days, weeks, months, or even years in the past leading up to the mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-09T11:35:16.780" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10558" PostTypeId="1" CreationDate="2015-08-09T13:34:43.270" Score="1" ViewCount="174" Body="&lt;p&gt;The atmosphere of Mars protects it's surface from micrometeorites. How significantly does the Martian atmosphere reduce radiation in comparison with, for example, the Moon's surface or open space?&lt;/p&gt;&#xA;" OwnerUserId="10218" LastEditorUserId="1028" LastEditDate="2015-08-09T14:12:48.833" LastActivityDate="2015-08-09T14:55:28.897" Title="Martian atmosphere's effect on radiation?" Tags="|mars|atmosphere|radiation|protection|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10559" PostTypeId="2" ParentId="10555" CreationDate="2015-08-09T14:30:45.157" Score="6" Body="&lt;p&gt;100 miles (160 km) at Mars is plenty far enough.  Negligible heating and drag will be seen at that altitude for a single pass.  (That would not be a good orbit altitude though, since the very small amount of drag there would accumulate over time and decay the orbit quickly from there.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typical aerobraking altitudes at Mars are around 100 km, which result in a few m/s $\Delta V$ for each pass and some non-negligible heating.  So you wouldn't want to fly by any lower than that.  The atmosphere density at those altitudes can &lt;a href=&quot;http://www.universetoday.com/677/aerobraking-mars-orbiter-surprised-scientists/&quot;&gt;vary by a factor of three or more&lt;/a&gt; from your prediction, so it's a bit of a crap shoot when you go that low.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-09T14:30:45.157" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10560" PostTypeId="2" ParentId="10558" CreationDate="2015-08-09T14:55:28.897" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Colonization_of_Mars#Radiation&quot; rel=&quot;nofollow&quot;&gt;Radiation on Mars&lt;/a&gt; is an issue,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mars has no global magnetic field comparable to Earth's geomagnetic field. Combined with a thin atmosphere, this permits a significant amount of ionizing radiation to reach the Martian surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sci-news.com/space/science-mars-radiation-measurements-surface-01629.html&quot; rel=&quot;nofollow&quot;&gt;Readings by the Curiosity rover&lt;/a&gt; during its first 300 days on Mars were between 200 and 250 micro-Gray per day.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-08-09T14:55:28.897" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10562" PostTypeId="1" AcceptedAnswerId="10563" CreationDate="2015-08-09T21:16:57.467" Score="22" ViewCount="5884" Body="&lt;p&gt;In other words, does the &lt;em&gt;centrifugal force&lt;/em&gt; inside a centrifuge only work to simulate gravity when an object is &quot;attached&quot; to the reference frame inside the centrifuge?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1G at sea level is normally about 9.8 m/s^2, correct?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However Centrifugal Force is different than Gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were to jump straight up in a centrifuge, would you move laterally when you landed? Would you land at all, or continue flying straight up until you hit the other side of the centrifuge?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(Spoiler:)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another example, the broken window inside the giant centrifuge at the end of &lt;strong&gt;Interstellar&lt;/strong&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/K2HUy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/K2HUy.jpg&quot; alt=&quot;Interstellar-Centrifuge-Baseball-clip1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(a thrown baseball should be affected by the same forces as a bouncy ball bounced against the floor, correct? Either continuing flight path with 9.8 m/s^2 downward acceleration, or not)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TGydt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TGydt.jpg&quot; alt=&quot;Interstellar-Centrifuge-Baseball-clip2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a window, suspended above the floor of a centrifuge, were broken, would the pieces fall to the floor of the centrifuge, as with gravity, or continue on a relative velocity to the centrifuge?&lt;/p&gt;&#xA;" OwnerUserId="192" LastEditorUserId="192" LastEditDate="2015-08-09T22:53:00.903" LastActivityDate="2017-11-27T03:27:11.613" Title="Would a ball bounced in a centrifuge return to the floor?" Tags="|artificial-gravity|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10563" PostTypeId="2" ParentId="10562" CreationDate="2015-08-09T22:44:23.167" Score="13" Body="&lt;p&gt;Your intuition is correct. So-called &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifugal_force&quot; rel=&quot;noreferrer&quot;&gt;centrifugal force&lt;/a&gt;&lt;/em&gt; is a product of mass and acceleration in a rotating frame. If you're in a centrifuge, and you let go of the rotating frame, you're no longer accelerated radially, but &lt;em&gt;merely&lt;/em&gt; keep the momentum imparted onto you by it and your own force as you let go of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this centrifuge was setup in zero gravity, like e.g. &lt;em&gt;space wheels&lt;/em&gt;, if you jumped, then your &lt;em&gt;trajectory&lt;/em&gt; would be a product of both the direction of your jump (your vertical component), as well as direction of the frame's rotation at the point you let go of it (your horizontal component). But, unless something acts on you while &lt;em&gt;in air&lt;/em&gt; so the product of both these velocities you're now a subject to change its magnitude (either accelerate or decelerate you, like, say, air within the &lt;em&gt;space wheel&lt;/em&gt; co-rotating with it), you're now in constant motion, i.e. no longer accelerating, so no &lt;em&gt;simulated gravity&lt;/em&gt; by constant acceleration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's illustrate this a bit; If we say that you jumped at the same speed that the wheel rotates and towards its center of rotation, you'd jump at a 45° angle to your normal and the same angle towards the center of rotation. Since the rotating frame moved while you were &lt;em&gt;in the middle of the air&lt;/em&gt;, you might land at the same spot you jumped from, which would seem a bit spooky, but that's the fun of rotating frames. This perceived effect on your motion once you &lt;em&gt;let go of the rotating frame&lt;/em&gt; even has a name: &lt;a href=&quot;https://en.wikipedia.org/wiki/Coriolis_effect&quot; rel=&quot;noreferrer&quot;&gt;Coriolis effect&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://en.wikipedia.org/wiki/Coriolis_effect#/media/File:Corioliskraftanimation.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bUCmk.gif&quot; alt=&quot;Coriolis effect&quot; title=&quot;Coriolis effect&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image is from Wikipedia and might not be the best to illustrate the point since it shows an object falling in a rotating frame away from the center of rotation, so here's a couple of animations from a film taken during cosmonaut training in Star City centrifuge system in the 60's (to my knowledge still the largest centrifuge that actually hosted people during experiments, even though Americans built some larger ones that were never really used):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/MC9eg.gif&quot; alt=&quot;enter image description here&quot;&gt; &lt;img src=&quot;https://i.stack.imgur.com/aIOkL.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Better example, but tells only half of the story of jumping from the rotating frame towards the center of rotation, since it doesn't also include return path back to the rotating frame. And this centrifuge was built on the Earth with additional vertical component of Earth's gravity, so it wouldn't be the best example anyway. I think I'll rather let you imagine how this would look like in zero gravity, but try to first guess in which direction the centrifuge on the last two animations rotates (hint: not the one you'd think).&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-08-09T22:44:23.167" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10564" PostTypeId="1" AcceptedAnswerId="10566" CreationDate="2015-08-09T22:53:45.843" Score="7" ViewCount="1795" Body="&lt;p&gt;I have been pondering the relative advantages of placing a base near the Moon's equator, or at the poles. The poles have been defended as having a superior business case because of deposits of &lt;a href=&quot;http://www.space.com/27388-nasa-moon-mining-missions-water.html&quot; rel=&quot;noreferrer&quot;&gt;volatiles&lt;/a&gt;, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Peak_of_eternal_light&quot; rel=&quot;noreferrer&quot;&gt;peaks of (mostly) eternal light&lt;/a&gt;. I lean more towards the equator because:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;you can place it between a mare and a highland and thus have access to the resources of both&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;you might &lt;a href=&quot;https://www.google.com.mx/search?q=moon%20lava%20tubes&amp;amp;espv=2&amp;amp;biw=1538&amp;amp;bih=862&amp;amp;tbm=isch&amp;amp;tbo=u&amp;amp;source=univ&amp;amp;sa=X&amp;amp;ved=0CCcQsARqFQoTCMCgro2LnccCFQj2gAodOowAMQ&quot; rel=&quot;noreferrer&quot;&gt;find&lt;/a&gt; a mighty handy &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_lava_tube&quot; rel=&quot;noreferrer&quot;&gt;lava tube&lt;/a&gt; around there&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;and if several fueled stages or fuel depots (let's say fuel depot, shall we?) have been parked around the equator for handy rendezvous, there would be launch opportunities several times a day. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But how many times a day, supposing that the base is right on the equator? Which is to say, how long would it take for the fuel depot to make a full orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And how long would the gaps be when the depot's orbital inclination means it isn't passing overhead for a while, if a base was, say, 20°N, where the Apollo 17 mission landed, which seems like a nice place to put a base according to the parameters mentioned above? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many stages or depots would be needed to be in orbit in order for one to be sufficiently well aligned for rendezvous, once it passes overhead, at any time?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2015-08-10T02:29:05.903" LastActivityDate="2024-03-22T21:23:31.983" Title="How long does one orbit around the Moon take in a typical parking orbit for a stage a lander is going to meet?" Tags="|orbital-mechanics|the-moon|rendezvous|ground-track|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10565" PostTypeId="2" ParentId="10562" CreationDate="2015-08-09T23:03:45.037" Score="8" Body="&lt;p&gt;The short answer is that when you throw a ball in a centrifuge in space, it travels in a straight line until it hits something. It experiences no pseudo gravity and effectively is not experiencing any acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is simplified somewhat by evacuating the air so it's a vacuum. Lets say the surface of the centrifuge is moving at 50km/h, you're standing on the surface, and you can throw a ball at 50km/h. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First you drop the ball - simply releasing it from your hand. The ball will fly forward in a straight line at 50km/h, you are also moving at 50km/h, but will be pushed up by the surface of the centrifuge. This will create the visual illusion the ball is falling even though it's actually you being pushed up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next you throw it backwards at 50km/h so it has zero net velocity. What happens? The ball stays right where it is when you threw it, hanging in space with no force acting on it. If you stay standing where you are on the surface of the centrifuge, next revolution you will smack into the ball at 50km/h. From your perspective, it will look like the ball flies away from you at 50km/h, flies around the centrifuge, then smacks you in the head - even though it was actually just hanging in space. This is where a centrifuge deviates most absurdly from actual gravity, in a rotating cylinder, the illusion of gravity is caused by the object moving in a straight line until it intercepts the floor (which is curving up), an object with zero net velocity will simply hang there as the floor of the cylinder spins under it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you throw the ball at 50km/h in the direction of motion, then it will travel in a straight line at 100km/h and smack into the surface of the centrifuge due to the curvature of the cylinder. The trajectory is actually no different to throwing the ball at 100km/h in a non-rotating cylinder, in both cases the ball will travel in a straight line, getting closer and closer to the floor, until it hits it. The apparent trajectory is not a parabola, as it would be with gravity, but instead a segment of a circle, which can however be a good approximation for a parabola for small angles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you add air, you get air resistance which will act to generally slow the ball, and air will be swept along with the surface of the centrifuge, effectively making a wind that will carry things with it. Air pressure will tend to push less buoyant objects to the surface of the centrifuge, and lighter than air objects to the center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exactly whether you can throw an object across the centrifuge to the other side (or &quot;around the world&quot;), depends on being able to overcome the velocity of the surface of the centrifuge. If the surface was moving at 100km/h, then a ball thrown at 50km/h will always fly ahead in a straight line and smack into the floor. The larger and faster the centrifuge, the more closely it will emulate gravity.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2015-08-10T17:22:36.740" LastActivityDate="2015-08-10T17:22:36.740" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="10566" PostTypeId="2" ParentId="10564" CreationDate="2015-08-10T00:22:08.177" Score="10" Body="&lt;p&gt;For the first part of your question, and assuming an equatorial orbit, it depends on the orbital altitude, but maths are simple enough to start with:&lt;/p&gt;&#xA;&lt;h3&gt;&lt;span class=&quot;math-container&quot;&gt;$$T = 2\pi\sqrt{\frac{a^3}{\mu}}$$&lt;/span&gt;&lt;/h3&gt;&#xA;&lt;p&gt;Where &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; is semi-major axis (in our case Moon's equatorial radius of 1,738.14 km, plus mean orbital altitude above it), and &lt;span class=&quot;math-container&quot;&gt;$\mu$&lt;/span&gt; is &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;noreferrer&quot;&gt;standard gravitational parameter&lt;/a&gt; of the central body (in our case 4,903.86 km³/s², someone mistyped something in that Wikipedia page) that is a product of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;noreferrer&quot;&gt;gravitational constant&lt;/a&gt; &lt;span class=&quot;math-container&quot;&gt;$G$&lt;/span&gt; (6.674×10&lt;sup&gt;−&lt;/sup&gt;¹¹ N⋅m²/kg²) and central body's mass &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; (7.3477×10²² kg).&lt;/p&gt;&#xA;&lt;p&gt;So for an orbital altitude of 200 km above the Moon, our orbital period &lt;span class=&quot;math-container&quot;&gt;$T$&lt;/span&gt; is &lt;a href=&quot;https://encrypted.google.com/#q=2pi*sqrt%28%28200+km+%2B+1%2C738.14+km%29%5E3%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+7.3477%C3%9710%5E22+kg%29%29&quot; rel=&quot;noreferrer&quot;&gt;127 minutes and 34 seconds&lt;/a&gt;. But, during this time, the Moon also &lt;a href=&quot;https://encrypted.google.com/#q=2pi*sqrt%28%28200+km+%2B+1%2C738.14+km%29%5E3%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+7.3477%C3%9710%5E22+kg%29%29+%2F+29.530589+days+*+360&quot; rel=&quot;noreferrer&quot;&gt;rotates 1.08°&lt;/a&gt;, so we have to do this a bit differently. We have to find our satellite's &lt;a href=&quot;https://en.wikipedia.org/wiki/Sidereal_time&quot; rel=&quot;noreferrer&quot;&gt;sidereal time&lt;/a&gt;. Since we know it's not off by much to  our satellite's orbital time &lt;span class=&quot;math-container&quot;&gt;$T$&lt;/span&gt;, we can simplify this and say that the Moon's rotation is &lt;em&gt;increasing our sidereal time&lt;/em&gt; for prograde orbits, and &lt;em&gt;decreasing&lt;/em&gt; it for retrograde orbits (we know this because it rotated on its axis for less than 180° during one of our satellite's orbits). So we can say that:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Our satellite's &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;noreferrer&quot;&gt;orbital speed&lt;/a&gt; (&lt;span class=&quot;math-container&quot;&gt;$\sqrt{\mu/r}$&lt;/span&gt;) is &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+7.3477%C3%9710%5E22+kg%29%2F%28200+km+%2B+1%2C738.14+km%29%29&quot; rel=&quot;noreferrer&quot;&gt;1,590.53671 m/s&lt;/a&gt;, so its &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_velocity&quot; rel=&quot;noreferrer&quot;&gt;angular velocity&lt;/a&gt; &lt;span class=&quot;math-container&quot;&gt;$\omega = \Delta\phi/\Delta t$&lt;/span&gt; is &lt;a href=&quot;https://encrypted.google.com/#q=360+deg%2F%282pi*sqrt%28%28200+km+%2B+1%2C738.14+km%29%5E3%2F%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+7.3477%C3%9710%5E22+kg%29%29%29&quot; rel=&quot;noreferrer&quot;&gt;0.000820651095 rad/s&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Moon's orbital period is 29.530589 days, or in &lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt;, &lt;a href=&quot;https://encrypted.google.com/#q=360+deg%2F29.530589+days&quot; rel=&quot;noreferrer&quot;&gt;2.4626008 × 10&lt;sup&gt;-6&lt;/sup&gt; rad/s&lt;/a&gt; and we'll add it to our satellite's orbital speed for retrograde orbit, and subtract it from it for prograde orbit&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;So, to complete one sidereal time of a 200 km orbit around the Moon, and for an equatorial station where this sidereal time is observed, and assuming circular orbit, we have:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;For prograde orbit: &lt;a href=&quot;https://encrypted.google.com/#q=%28360+%2F+57.2957795%29+%2F+%280.000820651095+-+2.4626008+%C3%97+10%5E-6%29+%2F+60&quot; rel=&quot;noreferrer&quot;&gt;127.99 minutes&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;For retrograde orbit: &lt;a href=&quot;https://encrypted.google.com/#q=%28360+%2F+57.2957795%29+%2F+%280.000820651095+%2B+2.4626008+%C3%97+10%5E-6%29+%2F+60&quot; rel=&quot;noreferrer&quot;&gt;127.22 minutes&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;In case you were wondering so far why I did it the long way around, I think you started appreciating it. Or, you better have, because it gets even more complicated for inclined and/or elliptical orbits. I won't do exact maths for it, because we also have to consider &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_node&quot; rel=&quot;noreferrer&quot;&gt;orbital node&lt;/a&gt; (or more specifically its &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_phasing&quot; rel=&quot;noreferrer&quot;&gt;phase&lt;/a&gt;, i.e. where in the orbital position is the satellite when its ground track aligns with our off-equatorial longitude) and time of insertion into orbit (our epoch), as well as decide on what range is sufficiently close for our case (likely different margins for longitudinal drift and phase shift), but assuming no orbital precession (&lt;a href=&quot;https://en.wikipedia.org/wiki/Nodal_precession&quot; rel=&quot;noreferrer&quot;&gt;nodal&lt;/a&gt; due to non-oblateness of the central body, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apsidal_precession&quot; rel=&quot;noreferrer&quot;&gt;apsidal&lt;/a&gt; due to eccentricity of non-circular orbits, and so on), equally spread constellation, and circular &lt;em&gt;horizon&lt;/em&gt; within which we still count an overshot as our conjunction, you should be able to &lt;em&gt;relatively simply™&lt;/em&gt; integrate it (you'd probably want to use &lt;em&gt;&lt;a href=&quot;http://mathworld.wolfram.com/LagrangeInterpolatingPolynomial.html&quot; rel=&quot;noreferrer&quot;&gt;Lagrange interpolating polynomial&lt;/a&gt;&lt;/em&gt;). Someone else might be &lt;em&gt;more merciful&lt;/em&gt; and post a wall of formulae for you, but I won't be so cruel now (also on myself, because it's been a while since &lt;em&gt;they made me do it&lt;/em&gt;). Have fun!&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-10T02:34:27.400" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10568" PostTypeId="1" CreationDate="2015-08-10T07:32:02.477" Score="5" ViewCount="161" Body="&lt;p&gt;${CO_2}$ is plentiful in Martian atmosphere. $H_2$ can be produced on Mars using sunlight and can be reacted with $CO_2$ to form $H_2O+C$, or $CH_4+H_2O$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it a feasible chemical reaction?&lt;/p&gt;&#xA;" OwnerUserId="11290" LastEditorUserId="25" LastEditDate="2015-08-10T11:25:38.483" LastActivityDate="2015-08-10T11:25:38.483" Title="Is production of methane possible on Mars?" Tags="|mars|rockets|isru|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10569" PostTypeId="1" AcceptedAnswerId="10573" CreationDate="2015-08-10T10:28:56.630" Score="0" ViewCount="1234" Body="&lt;p&gt;I wonder if is there any databse about space objects, giving information about types and may be relative positions of objects in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm asking this because my company aims to develop a software which will show space images, and I wonder if we can have another source but space photographs about modelling space. When images are not enough we could simulate space by having an information like &quot;There is a star, this type, so it can be shown like that&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope I could explain my question. Thank you. &lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="25" LastEditDate="2015-08-10T12:21:31.487" LastActivityDate="2015-08-10T12:21:31.487" Title="Is there any space objects database?" Tags="|asteroid|technology|interplanetary|deep-space|stars|" AnswerCount="1" CommentCount="1" ClosedDate="2016-05-05T14:08:15.267" ContentLicense="CC BY-SA 3.0" />
  <row Id="10571" PostTypeId="2" ParentId="10568" CreationDate="2015-08-10T11:06:03.767" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is production of methane possible on Mars?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, via the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sabatier_reaction&quot;&gt;Sabatier process&lt;/a&gt;. This has been proposed multiple times as a type of in-situ resource utilization on Mars. All that's needed is energy (e.g., sunlight), CO&lt;sub&gt;2&lt;/sub&gt;, and hydrogen. The first two are easily obtainable. Except near the poles, the last is a bit of a challenge.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-08-10T11:06:03.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10572" PostTypeId="1" AcceptedAnswerId="10575" CreationDate="2015-08-10T11:15:22.833" Score="8" ViewCount="938" Body="&lt;p&gt;To refill the ISS, many spacecraft have been or are used&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The shuttle (retired for obvious reasons)&lt;/li&gt;&#xA;&lt;li&gt;ATV (retired)&lt;/li&gt;&#xA;&lt;li&gt;HTV (will be retired)&lt;/li&gt;&#xA;&lt;li&gt;Progress &lt;/li&gt;&#xA;&lt;li&gt;Soyuz&lt;/li&gt;&#xA;&lt;li&gt;Dragon&lt;/li&gt;&#xA;&lt;li&gt;Cygnus&lt;/li&gt;&#xA;&lt;li&gt;CST-100 (future)&lt;/li&gt;&#xA;&lt;li&gt;Dream Chaser (largely developed)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I assume the R&amp;amp;D for developing each spacecraft is VERY expensive, and some (HTV and ATV) will have had a limited lifespan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand the need for different craft for cargo and crew transport, but why did we choose to make that many different cargo craft ?&#xA;Is there any cost rational behind this or is it just a matter of national pride ? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="1457" LastEditDate="2015-08-10T11:33:43.453" LastActivityDate="2015-08-12T08:29:44.363" Title="Why do we have so many different spaceships to refill ISS?" Tags="|iss|cost|supply-mission|international-politics|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10573" PostTypeId="2" ParentId="10569" CreationDate="2015-08-10T11:23:28.647" Score="2" Body="&lt;p&gt;There are a number, usually broken down into 3 parts. I've included the 3 parts below, and a database where one might find information about these objects.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Extra-solar objects- &lt;a href=&quot;http://simbad.u-strasbg.fr/simbad/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SIMBAD&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Solar System Objects- &lt;a href=&quot;http://ssd.jpl.nasa.gov/?ephemerides&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Ephemerides&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Satellites- Either &lt;a href=&quot;https://www.space-track.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;JSPOC&lt;/a&gt; or &lt;a href=&quot;https://celestrak.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2015-08-10T11:23:28.647" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10574" PostTypeId="1" CreationDate="2015-08-10T11:25:31.273" Score="7" ViewCount="631" Body="&lt;p&gt;ULA with their Atlas V seems to be the leading launch service provider for secondary payloads. This &lt;a href=&quot;http://spirit.as.utexas.edu/~fiso/telecon/Szatkowski_11-20-13/&quot; rel=&quot;nofollow noreferrer&quot;&gt;FISO presentation&lt;/a&gt; is convincing anyway. And good for them since miniaturization of satellites certainly seems to be happening for real. How do other launch providers, commercial and foreign, compare on this market? And how do launch costs per kilogram differ from that of a heavy primary payload?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kbNNc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kbNNc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="9092" LastEditDate="2021-03-26T14:28:09.303" LastActivityDate="2021-03-26T14:28:09.303" Title="How do the secondary payload abilities of different launchers compare?" Tags="|launch|payload-deployment|atlas-v|ula|secondary-payload|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10575" PostTypeId="2" ParentId="10572" CreationDate="2015-08-10T11:32:05.673" Score="10" Body="&lt;p&gt;Essentially each country who has agreed to send such spacecraft wants to build their own spacecraft. This is most noted in US/ Russian, but is to a lesser extend to European/ Japanese partners. Breaking them down by country, we have the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;United States&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Shuttle&lt;/li&gt;&#xA;&lt;li&gt;Cygnus&lt;/li&gt;&#xA;&lt;li&gt;Dragon&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Russia&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Progress&lt;/li&gt;&#xA;&lt;li&gt;Soyuz&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;ESA&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ATV&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Japan&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;HTV&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, so there are 4 parties who have build spacecraft. Why did each of these build their own? Mostly, they want to build them as both a matter of national pride, and a means to develop new technology. This development of man rated technology allows each of them to enhance their space capabilities, and generally speaking is good for the economy of the affected partners. There are two that have developed more than one, let me explain those.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;United States- The Shuttle had to be discontinued, and a replacement was needed. Two were provided for cargo replacement, as a means of redundancy, which allows for the United States to continue to send spacecraft in the event of a problem with one.&lt;/li&gt;&#xA;&lt;li&gt;Russia- The Progress/ Soyuz have both been around for a long time. One is manned, the other unmanned.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, basically it's a chance to build technology in one's own nation (Or EU), and the pride and technological advancement that comes from such. In addition, valuable redundancy is gained from such.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-10T11:32:05.673" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10576" PostTypeId="1" AcceptedAnswerId="10587" CreationDate="2015-08-10T12:44:06.107" Score="13" ViewCount="2969" Body="&lt;p&gt;Dragon is the only spacecraft I know of currently in use that doesn't need a fairing for launch on top of a rocket. I know there are others in development or in testing like Dragon v2 and CST-100(not sure about this one).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that building the spacecraft to withstand the aerodynamic flow would weight less than a spacecraft plus fairing to protect it. So the spacecraft that doesn't need a fairing can bring slightly more payload to orbit on the same rocket.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Why was Dragon the first spacecraft, to my knowledge that is, that doesn't need a fairing for for launch on top of a rocket? Didn't anyone before Elon think about a spacecraft that doesn't need a fairing? And why was Dragon designed like that in the first place, was it because it could deliver more payload?&lt;br&gt;&#xA;I'm aware that Dragon has fairings to protect the solar panels, I'm asking about fairings to protect the spacecraft itself. The space shuttle and buran don't count since they were attached at the side and too big for a fairing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;This fairing is what I mean&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/wtA12.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wtA12.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;See also step 4 here&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ZdYmJ.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZdYmJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pictures of Soyuz TMA-16M taken from &lt;a href=&quot;http://www.spaceflight101.com/soyuz-tma-16m-mission-updates.html&quot;&gt;spacefilght101&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="9029" LastEditorUserId="9029" LastEditDate="2015-08-10T13:22:28.583" LastActivityDate="2016-04-15T19:35:08.387" Title="Why is Dragon the only spacecraft that is currently in use to not need a fairing for launch?" Tags="|launch|spacex|spacecraft|dragon|aerodynamics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10577" PostTypeId="2" ParentId="10576" CreationDate="2015-08-10T13:02:20.133" Score="4" Body="&lt;p&gt;The Dragon doesn't need a fairing because it needs to re-enter the Earth's atmosphere, and thus must be made rigorously to begin with. The same things that allow it to launch from Earth safely allow it to re-enter without burning up. There are a number of vehicles that don't need fairings, including every manned spacecraft I'm familiar with. It seems though that the key is re-entry, every vehicle I'm aware of that doesn't need a fairing can re-enter the Earth's atmosphere safely.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-08-10T13:08:28.540" LastActivityDate="2015-08-10T13:08:28.540" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10578" PostTypeId="2" ParentId="10562" CreationDate="2015-08-10T13:03:22.897" Score="6" Body="&lt;p&gt;The centrifugal force is a product of your frame of reference.  Not a product of what you are or are not attached to.  So it would not disappear simply because you no longer are in contact with the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you are in a reference frame with centrifugal forces you are also in a reference frame with coriolis forces.  This pseudo force is proportional with your velocity relative to the frame of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The consequences of this is that when you leave the floor where and if you return depends on your velocity. - However you don't actually have to leave the floor for this force to be noticable.  If you start travelling against the spin of the cylinder the coriolis force would start to counteract the centrifugal force.  When you travel at the same speed as the cylinders rotation. This means that you would be moving at the same veocity as the center of the cylinder from the perspective of an inertial reference frame.  At this point the coriolis force and the centrifugal force presisely counteracts, and you would be left floating along the surface of the cylinder.&lt;/p&gt;&#xA;" OwnerUserId="5934" LastActivityDate="2015-08-10T13:03:22.897" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10579" PostTypeId="2" ParentId="10572" CreationDate="2015-08-10T14:32:19.307" Score="4" Body="&lt;p&gt;Diversity is good, as recent history has shown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the failure of Cygnus, Progress, and Dragon, there is still an HTV due to launch with cargo. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is national prestige involved, and while manned spaceflight is very hard and expensive, cargo is significantly easier and cheaper. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS consortium agreement does a lot of horse trading and barter for services, so the ATV while probably not cost effective, was used to barter for services for the ESA.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-08-10T14:32:19.307" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10580" PostTypeId="1" AcceptedAnswerId="10581" CreationDate="2015-08-10T14:55:44.977" Score="6" ViewCount="853" Body="&lt;p&gt;I'm not quite familiar with space exploration vocabulary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Apollo missions, the trajectory was figure-8-shaped. Thus, the direction of rotation of the Earth orbit was opposite to the one around the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When speaking about a mission to Mars, I imagine the spacecraft first describes a few Earth orbits, then performs trans-Mars trajectory injection, and then describes a few Mars orbits before beginning its mission.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;for the initial orbits around earth, finding the direction of the orbit is quite easy as rockets are usually launched toward the East.&lt;/li&gt;&#xA;&lt;li&gt;for the Mars orbit, if the orbit goes in the same direction, the trans-Mars trajectory put the spacecraft slightly further; if the direction is the opposite, the trans-Mars trajectory looks like the Apollo trans-lunar trajectory.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;When launching a spacecraft to Mars, what is the direction of the Mars orbit?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="6816" LastEditDate="2015-08-10T15:20:45.350" LastActivityDate="2015-08-10T18:30:52.380" Title="After a trans-Mars transfer, what is the direction of the rotation of the orbit around Mars?" Tags="|mars|orbital-maneuver|mars-orbiters|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10581" PostTypeId="2" ParentId="10580" CreationDate="2015-08-10T15:37:31.893" Score="16" Body="&lt;p&gt;Either prograde or retrograde entry is easy to achieve; the choice of trans-Mars trajectory can put the spacecraft slightly ahead of Mars' position at arrival (yielding retrograde orbit) or slightly behind (prograde). A midcourse correction maneuver of about 1 m/s ∆v can make the difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Apollo, the retrograde entry was, I think, needed for the free return option. Prograde approach would have sped the spacecraft up on flyby and made for a very long trip home.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, unlike the moon, Mars has significant rotation speed. Beginning  descent from a prograde orbit makes the relative surface speed for landing slightly lower, and if a returning Mars lander is to rendezvous in orbit like Apollo did, taking advantage of the free horizontal speed provided by Mars' rotation likewise calls for eastward ascent toward a mothership's prograde orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @MarkAdler notes, Viking 1 and 2 each consisted of an orbiter which released a lander after achieving orbit. Viking 1's orbit was prograde, but inclined at 39.3˚ (inclination of 0˚ means west-to-east, equatorial, prograde; inclination of 180˚ means east-to-west, equatorial, retrograde). Viking 2's orbit was more steeply inclined, at 55˚, and after the departure of the lander, it was moved into progressively more inclined orbits, 75˚ and 80˚, that is, orbiting almost north-south.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-08-10T18:30:52.380" LastActivityDate="2015-08-10T18:30:52.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10583" PostTypeId="1" AcceptedAnswerId="10614" CreationDate="2015-08-10T17:46:58.987" Score="13" ViewCount="3910" Body="&lt;p&gt;Obviously the pics aren't floating around, but how can I explain this to a 10 year old?&lt;/p&gt;&#xA;" OwnerUserId="11302" LastEditorUserId="25" LastEditDate="2015-08-11T15:06:03.043" LastActivityDate="2017-02-02T20:29:36.333" Title="Where are the pictures during the time the pictures are sent from space to home?" Tags="|communication|imaging|photography|data-transmission|" AnswerCount="7" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10584" PostTypeId="2" ParentId="10583" CreationDate="2015-08-10T17:51:47.700" Score="19" Body="&lt;p&gt;(10 year old version)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pictures are sent by radio, and the radio signals travel at the speed of light.  Although the speed of light is very fast, it is not unlimited, and space probes can be very far away.  So the time that elapses is simply the time it takes for the radio waves to travel at the speed of light over the very long distances between the probe and Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(there is also a low data rate factor, but I am not sure if this what you are asking about)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-08-11T11:53:41.900" LastActivityDate="2015-08-11T11:53:41.900" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10585" PostTypeId="2" ParentId="10583" CreationDate="2015-08-10T18:05:56.830" Score="26" Body="&lt;p&gt;Spacecraft that take pictures take them similar to a digital camera. However, the camera is very far away. Similar to downloading a movie off of a website so you can watch it on your local device, it takes time to transmit those images. However, because the distance is so far away, it can take a lot of time to download the images.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The images reside on a memory card, similar to the memory card that a digital camera has, and stay on there until deleted. They are deleted only when someone from Earth confirms the image was downloaded correctly.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-10T18:05:56.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10586" PostTypeId="2" ParentId="10583" CreationDate="2015-08-10T20:39:49.250" Score="10" Body="&lt;p&gt;I really can't add much to the already excellent answers but as seen as this is for a 10 year old the following picture from wikipedia may help explain the concept. It shows a pulse of light travelling at the finite speed mentioned in the other answers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YeKnb.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YeKnb.gif&quot; alt=&quot;Diagram of a light pulse travelling from the earth to the moon&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11305" LastActivityDate="2015-08-10T20:39:49.250" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10587" PostTypeId="2" ParentId="10576" CreationDate="2015-08-10T22:24:34.777" Score="17" Body="&lt;p&gt;Dragon is not the first manned spacecraft to launch without a fairing&#xA;&lt;a href=&quot;https://i.stack.imgur.com/wcPx1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wcPx1.jpg&quot; alt=&quot;Mercury-Atlas Launch&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;Mercury had a Launch Escape Tower bolted to the top.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Je7HA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Je7HA.jpg&quot; alt=&quot;Gemini-Titan Launch&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;Gemini had ejection seats so no tower was needed&#xA;&lt;a href=&quot;https://i.stack.imgur.com/713ox.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/713ox.jpg&quot; alt=&quot;Apollo Launch&quot;&gt;&lt;/a&gt;&#xA;Starting with Apollo, a small fairing cover was included as part of the LES. This cover detached with the LES during flight to reduce weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With Soyuz all capsules have had a fairing during launch.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/tw2hJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tw2hJ.jpg&quot; alt=&quot;R-7 Launch Family&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the question is why do some capsules have fairings and some do not? One theory is that capsules designed for reentry do not need aerodynamic protection from a fairing. That would explain why Soyuz has a fairing, since only 1 part of the craft is designed for reentry. However, the Apollo Command Module was designed for reentry, but yet had a fairing. Another, more likely theory is that it is merely a result of LES design. The LES escape motors on Apollo were much more powerful than Mercury's because Apollo was a more massive capsule. Therefore there needs to be a protective fairing to protect the capsule from this rocket exhaust. Orion, being even more massive, requires a full capsule fairing to protect it from its LES.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both Dragon and CST-100 have pusher LES systems, therefore the exhaust is directed downwards away from the capsule and there is no need for additional protections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/916zH.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/916zH.gif&quot; alt=&quot;Apollo LES Breakdown&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;According to this image of the Apollo LES, the bottom fairing is a &quot;Boost Protective Cover&quot;, presumably to protect the Apollo capsule from the Launch Escape Motor exhaust.&lt;/p&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="10398" LastEditDate="2015-08-11T07:15:13.873" LastActivityDate="2015-08-11T07:15:13.873" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10588" PostTypeId="2" ParentId="1564" CreationDate="2015-08-11T05:29:18.403" Score="18" Body="&lt;p&gt;The oldest still operational communication satellite in use is the low budget amateur radio satellite &lt;a href=&quot;https://en.wikipedia.org/wiki/AMSAT-OSCAR_7&quot;&gt;AMSAT-OSCAR 7&lt;/a&gt; made by radio amateurs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was launched on 15th of November 1974 from Vandenberg Air Force Base with a &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_2000&quot;&gt;Delta 2000&lt;/a&gt; rocket. OSCAR 7 has operational HF/VHF/UHF transponders, which allows communication over distances of up to 9,000 km with a relatively simple ground station equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite is in a 1,500 km retrograde polar orbit. The batteries are dead, although the satellite's solar cells works fine. The satellite is used daily during times when the solar cells are sunlit.&lt;/p&gt;&#xA;" OwnerUserId="11307" LastEditorUserId="49" LastEditDate="2015-08-11T06:12:47.743" LastActivityDate="2015-08-11T06:12:47.743" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10589" PostTypeId="1" CreationDate="2015-08-11T09:21:58.707" Score="5" ViewCount="346" Body="&lt;p&gt;My assumptions around building up infrastructure for dealing with space launches from Venus assumes colonization based on the principles outlined by Geoffrey A. Landis of NASA in paper &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030022668.pdf&quot; rel=&quot;nofollow&quot;&gt;Colonization of Venus&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To summarize. This assumes we DON'T build bases on the Venus surface which is too hot and with too high pressure but rather 50 km above surface where temperatures are 0-50 Celsius and pressure is the same as on earth. The thick atmosphere gives a lot of buoyancy even for balloons with regular air. So it is feasible to build large habitats hanging in the air or existing inside balloons with breathable air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think these ideas are quite interesting, however staying in the atmosphere deprives a colony of many resources, so I am interested in finding out about the economics of utilizing asteroids from Venus. Accessing the asteroids could happen in many ways. Either by going from Venus straight to the Asteroid belt, mine them and return the resources. Alternatively pull one or more asteroids from the asteroid belt and deposit them in orbit around Venus for later processing. E.g. one could potentially maintain processing facilities in orbit as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This suggests to me that one depends on avoiding to have to build spaceships on Venus or doing complicated repairs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I imagined one could enter Venus and &quot;land&quot; similar to how it is shown in this &lt;a href=&quot;http://www.extremetech.com/extreme/196114-nasa-is-developing-a-plan-to-explore-venus-in-a-manned-solar-powered-ariship&quot; rel=&quot;nofollow&quot;&gt;article on HAVOC&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Inflate balloons rapidly on entry to keep space ship floating in Venus atmosphere instead of hitting the surface.&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Move with Zeppelin like vehicle to &quot;landed&quot; space ship with e.g. materials from asteroid and refuel spaceship. If it is multi stage, collect the pieces and assemble it first.&lt;/li&gt;&#xA;&lt;li&gt;Start rockets and fly up again. I assume start angle doesn't matter that much as you can probably adjust later.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;On the earth one would need to hit hard soil at very accurate angle and not hard enough to destroy the space ship (launch vehicle), or alternatively land in the sea which based on what I've read about the shuttle program causes lots of maintenance problems. On Venus of course you avoid this as you don't need to hit at any specific angle or altitude on re-entry.&lt;/p&gt;&#xA;" OwnerUserId="11308" LastEditorUserId="11308" LastEditDate="2015-08-16T22:11:13.607" LastActivityDate="2015-08-16T22:11:13.607" Title="Are there any benefits on Venus compared to Earth with respect to reusing launch vehicles?" Tags="|rockets|reuse|venus|first-stage|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10590" PostTypeId="2" ParentId="5846" CreationDate="2015-08-11T12:07:15.163" Score="4" Body="&lt;p&gt;A few more reasons against solid rockets:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;They're difficult to control once ignited. To my knowledge, it's very difficult-to-impossible to throttle, shut down or restart them. While ability to throttle might not matter for a first stage, the fact that you can't just stop the turbo-pumps to shut down the engine might pose a safety risk. (&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster&quot; rel=&quot;nofollow&quot;&gt;Challenger disaster&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;SpaceX wants to develop a rapid reusability capability on Falcon 9's first stage. Here, liquid fuel seems to be superior. Although Space Shuttle's SRBs demonstrated some reusability (by parachuting the booster to the ocean and then refurbishing it), it was a long, painstaking process. By contrast, re-use of a liquid fuel rocket could, in principle, be as simple as filling the tanks with new fuel.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10274" LastActivityDate="2015-08-11T12:07:15.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10592" PostTypeId="2" ParentId="10583" CreationDate="2015-08-11T15:23:10.813" Score="1" Body="&lt;p&gt;I am sure your 10 year old can &quot;see&quot; &amp;amp; &quot;experience&quot; how Cell Phones &amp;amp; Remote Controls work. Not see the invisible waves, but the can see the effect of 'doing' X and seeing a response on another device. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My sisters 2 year old uses remote controls with full awareness to turn on A/Cs, TV etc. They know it works, just not sure what's the underlying mechanism. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Easiest illustration would be to send take a few wireless devices and send pictures between them e.g.  Take a Picture/ Photos from one Phone/ Tablet to another in front of your kid. Take some pictures and send again. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ask him whether he knows how it works? Explain how invisible special waves act as a link to send it across. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To establish further how a link is formed: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Use 2 wireless devices for a voice transmit - Cordless Phones, Cell Phones, Baby Monitor&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;To show a visible link you can do the 2 Matchboxes + 1 taut string thingy we've done as kids and see how sounds can transmit over a visible medium. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These illustrations will establish foundation for how communication and transfer of information over visible as well as wireless &amp;amp; invisible links. So even though not visible it works. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can choose the order of the above illustrations that work best for you &amp;amp; your kid. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then use a, a visual depiction like the above answer would be much easier than explaining verbally. The right picture, animation or video would be worth more than all the words in the above answers; albeit if you can find a visual way to represent the above answer - you're good to go. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google Search: Satellite (Communication OR Transmission)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/search?biw=1350&amp;amp;bih=861&amp;amp;tbm=isch&amp;amp;sa=1&amp;amp;q=satellite+%28communication+OR+transmission%29&amp;amp;oq=satellite+%28communication+OR+transmission&quot; rel=&quot;nofollow&quot;&gt;https://www.google.com/search?biw=1350&amp;amp;bih=861&amp;amp;tbm=isch&amp;amp;sa=1&amp;amp;q=satellite+%28communication+OR+transmission%29&amp;amp;oq=satellite+%28communication+OR+transmission&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Examples of search results vary from simple diagrams to complex ones. Choose the ones you feel can be most clearly representative.  &lt;/p&gt;&#xA;" OwnerUserId="3415" LastActivityDate="2015-08-11T15:23:10.813" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10593" PostTypeId="1" CreationDate="2015-08-11T15:31:09.457" Score="12" ViewCount="834" Body="&lt;p&gt;I can't find info on NASA site, but saw a picture of romaine on ISS and am wondering how it maintains its shape in zero-gravity-  why doesn't it spread out, what makes it grow up...?&lt;/p&gt;&#xA;" OwnerUserId="11296" LastEditorUserId="10234" LastEditDate="2015-08-11T15:38:54.843" LastActivityDate="2015-08-12T17:18:00.283" Title="Growing food in space" Tags="|iss|nasa|microgravity|food|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10594" PostTypeId="2" ParentId="10583" CreationDate="2015-08-11T15:59:04.713" Score="12" Body="&lt;p&gt;Radio waves are such an abstract concept that it might be useful to explain to a child using an analogy of concrete objects. In a real way, picture information contained in a radio transmission could just as well be a string of baseballs thrown at high velocity, where the patterns or aiming of the throws enable reconstructing the picture at the other end. If the baseballs have to transit a very long distance, all of them in flight together &quot;are&quot; the picture--representing it faithfully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further analogy to aid comprehension is possible. For example, explain how the signal being digital enables clear transmission even if any baseball gets slightly off course and arrives early or late or slightly to the side. Each baseball has a box in which it should arrive (both in space and time), which  means that you can still tell what its target was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the child still doesn't understand, show them a bean bag toss tic tac toe game:  &lt;a href=&quot;https://i.stack.imgur.com/Mxtl1.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mxtl1.jpg&quot; alt=&quot;Bean bag toss tic tac toe&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This could help them conceptualize how flying objects can result in assembling a picture at the other end.&lt;/p&gt;&#xA;" OwnerUserId="2793" LastEditorUserId="2793" LastEditDate="2015-08-11T16:06:52.540" LastActivityDate="2015-08-11T16:06:52.540" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10595" PostTypeId="2" ParentId="10593" CreationDate="2015-08-11T16:00:43.787" Score="11" Body="&lt;p&gt;The structure of lettuce is determined by a combination of genetics and environmental effects. What the growth of the lettuce on the ISS indicates is that gravity is negligible in terms of its effect on lettuce growth. Atmospheric pressure would have a more noticeable impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The direction in which a plant grows (shoots up, roots down) is determined by light (for leaves) and water source (for roots). The process by which the plant shoot moves toward light is called phototropism, and gravity has negligible impact here. With the roots gravity does have some impact (geotropism), but the ISS project was able to overcome this by using stakes to guide the roots to the water. This water-seeking quality of the roots is called hydrotropism.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where gravity would have an impact is in the ultimate height of a plant. Trees are ultimately limited in height by gravity, but if somehow they evolved in a microgravity environment they could potentially grow a lot taller.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Further Reference:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://biology.stackexchange.com/a/8101&quot;&gt;Top answer to &quot;What chemicals and structures control the &lt;em&gt;direction&lt;/em&gt; of plant growth in leaves, stems and roots?&quot;&lt;/a&gt; on Biology Stack Exchange&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/a/1077/58&quot;&gt;My answer to &quot;Can plants grow in microgravity?&quot;&lt;/a&gt; on Space Exploration Stack Exchange&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-08-12T17:18:00.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10596" PostTypeId="2" ParentId="5674" CreationDate="2015-08-11T18:19:01.100" Score="3" Body="&lt;p&gt;&lt;strong&gt;[Update: This information has been superseded by more recent info, which challenges the operational value of the sensor data. See &lt;a href=&quot;https://space.stackexchange.com/a/33612/58&quot;&gt;the other answer&lt;/a&gt;.]&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of March 2015, the Mars Orbiter Mission (MOM) methane sensor has been working, but has not detected any methane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISRO has released some data on the operations so far. Emily Lakdawalla explains:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The map shows the reflectance of the surface of Mars in the methane sensor's reference channel, a wavelength in which atmospheric methane is transparent to infrared radiation. The data that underlie this map will eventually be compared to the methane sensor's other channel, a wavelength in which methane absorbs light, to attempt to map methane at the part-per-billion level in Mars' atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8ruy3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8ruy3.jpg&quot; alt=&quot;Mars surface reflectance map&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/03040850-mars-orbiter-mission-methane.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Orbiter Mission Methane Sensor for Mars is at work - Emily Lakdawalla at The Planetary Society&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2019-07-20T21:07:28.970" LastActivityDate="2019-07-20T21:07:28.970" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="10597" PostTypeId="2" ParentId="10572" CreationDate="2015-08-11T20:13:30.253" Score="3" Body="&lt;p&gt;ESA and Japan are partners in the ISS venture. As such, they are supposed to &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/ATV/ATV_history&quot; rel=&quot;nofollow&quot;&gt;contribute to the operating cost of the station&lt;/a&gt;. They have chosen to do this in the form of resupply flights. This gave them the opportunity to bolster their space industry by developing new technology (and by producing a pile of hardware). It also meant they could spend the several billion $ that their contribution would cost, in their own country/union instead of e.g. paying NASA or the Russians.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For both, independent access to space is a political mandate. Not an unwise one, as the recent developments around NASA and Russia have shown.&lt;br&gt;&#xA;For Europe, &lt;a href=&quot;http://www.spacebooks-online.com/product_info.php?cPath=101&amp;amp;products_id=17094&quot; rel=&quot;nofollow&quot;&gt;this policy has its origins in the 1970s&lt;/a&gt;. At the time, the USA was trying to establish a monopoly on commercial satellites. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-08-12T08:29:44.363" LastActivityDate="2015-08-12T08:29:44.363" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10598" PostTypeId="1" AcceptedAnswerId="10603" CreationDate="2015-08-11T20:41:52.770" Score="30" ViewCount="9104" Body="&lt;p&gt;It was once theorized that the moon might have deep oceans of dust, potentially a problem for any craft landing on its surface.  I've heard that NASA was concerned about this possibility in the early days of the Space Race.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there any evidence that deep lunar dust was an actual concern at NASA?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;sub&gt;Apparently the depth of the lunar dust has become a topic of discussion for young-Earth creationists, making it incredibly hard to search for useful information about the topic.&lt;/sub&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="552" LastEditDate="2015-08-11T21:09:04.350" LastActivityDate="2019-08-22T00:22:39.940" Title="Is there any evidence that NASA was worried about deep lunar dust?" Tags="|the-moon|nasa|history|dust|regolith|" AnswerCount="5" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10599" PostTypeId="2" ParentId="10598" CreationDate="2015-08-11T21:11:27.350" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/SP-350/ch-5-3.html&quot; rel=&quot;noreferrer&quot;&gt;NASA SP-530 &quot;Apollo Expeditions to the Moon&quot;&lt;/a&gt;, edited by Edgar M. Cortright, which is available on the NASA History Program Office website, states that prior to the landing of Surveyor I on the moon:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Some scientists had theorized that astronauts could be engulfed in dangerously deep dust layers&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It does not detail whether these claims were taken seriously by NASA, but it was at least worthy of mention when compiling this record.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-08-11T21:11:27.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10600" PostTypeId="1" CreationDate="2015-08-11T21:23:04.227" Score="13" ViewCount="2810" Body="&lt;p&gt;In the question &lt;a href=&quot;https://space.stackexchange.com/questions/1325/why-didnt-the-space-shuttle-have-a-launch-escape-system&quot;&gt;Why didn&amp;#39;t the Space Shuttle have a launch escape system?&lt;/a&gt; there was discussion in the commentary about why the Space Shuttle could not safely abort through much of its envelope. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why could the Space Shuttle not abort?  What problems were there?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2024-01-10T19:50:36.693" Title="What is the problem with aborting a Space Shuttle launch?" Tags="|launch|space-shuttle|abort|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10601" PostTypeId="2" ParentId="10600" CreationDate="2015-08-11T21:27:36.230" Score="7" Body="&lt;p&gt;There were many abort modes on the Space Shuttle during launch, and the options changed depending on the specifics of each portion of the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main issue is that separating from two burning SRB's (Solid Rocket Boosters) producting 2.8 million pounds of thrust each is just plain not going to work. That is, the tiniest asymmetry in the disconnect and you are likely dead. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the Shuttle was stuck for the first 2 minutes and change until the SRB's burned out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the hypersonic airflow, the slightest deviation from flight could destroy the Shuttle airframe, which is part of why abort was so difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the SRB's were dropped, there was an RTLS (Return to Launch Site) which had been described by astronauts as better than nothing, but not with great confidence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would require the Shuttle to retain the External Tank for fuel, gimbal the engines to get high enough, and turn around and fly back towards the launch site (once out of the atmosphere to protect from the airflow) and then drop the tank once it had the energy to glide back to Kennedy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were Abort to Orbit modes (Used once I believe).  Abort Once Around, and more. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But for much of the flight, with SRB's burning the options are hugely limited, since they cannot be turned off. They can be flight terminated by blowing a linear charge along the length of it, but that would also take out the tank, and thus probably the Orbiter if still attached. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-08-12T06:45:45.377" LastActivityDate="2015-08-12T06:45:45.377" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10602" PostTypeId="2" ParentId="3023" CreationDate="2015-08-11T21:37:14.807" Score="2" Body="&lt;p&gt;Right up front, the government administrators listed a number of experimental technologies that they wanted incorporated in the VentureStar program. One of these technologies was a composite cryogenic fuel tank. The &lt;em&gt;engineers&lt;/em&gt; stated immediately that this wasn't possible &amp;mdash; the reason is obvious to anyone examining the facts: the composite material has a honeycomb-like layer inside and the cryogenic liquid would create condensation both on the outside of the tank and inside the cells of the honeycomb layer. Because of the extreme cold, that condensation would freeze, rupturing the honeycomb cells and weakening and/or destroying the strength of the tank.  The administrators insisted, but the test tanks failed on numerous occasions. Meanwhile, the engineers decided to manufacture a tank from aluminum/lithium metal &amp;mdash; though the main structure was heavier, the connections between the three lobes of the tank were much lighter in the Al/Li tanks vs the composite tank &amp;hellip; problem solved!  But a NASA scientist went before Congress and stated, basically, &quot;Stick with the 'more advanced' composite design &amp;mdash; we must use all the advanced technology or nothing!&quot;  And Congress canceled the program.&lt;/p&gt;&#xA;" OwnerUserId="11313" LastEditorUserId="6816" LastEditDate="2015-08-11T22:20:23.760" LastActivityDate="2015-08-11T22:20:23.760" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10603" PostTypeId="2" ParentId="10598" CreationDate="2015-08-11T21:50:20.197" Score="34" Body="&lt;p&gt;Yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fear that lunar dust was fine, deep, and motile enough that a lander could sink under its surface was mostly backed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Thomas_Gold#Relationship_with_NASA&quot; rel=&quot;noreferrer&quot;&gt;Thomas Gold&lt;/a&gt;, a noted astrophysicist who was a consultant to NASA in the 1950s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note especially in the image below of a &lt;a href=&quot;https://books.google.com.mx/books?id=PuMDAAAAMBAJ&amp;amp;pg=PA87&amp;amp;lpg=PA87&amp;amp;dq=proponent%20lunar%20deep%20surface%20dust&amp;amp;source=bl&amp;amp;ots=qadnp5muJx&amp;amp;sig=Pt_bUfppcmfpM23tFEk7grlVLXk&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CCsQ6AEwAmoVChMIyq-2lvyhxwIVRpMNCh1T1wCi#v=onepage&amp;amp;q=proponent%20lunar%20deep%20surface%20dust&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Popular Mechanics&lt;/a&gt; article from 1964, the quote from his article in Science magazine on the subject: &#xA;&quot;Without any clear signs [in Ranger images]&#xA;of firm rock, the pictures must lead to more concern about sinkage of  impact.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p7DsX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p7DsX.png&quot; alt=&quot;popular mechanics article on ranger mission and moon dust&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He became in the small minority in this thinking over time and isolated in his position, but he stuck to his guns. There were &lt;a href=&quot;https://books.google.com.mx/books?id=IoHnPR92Aw0C&amp;amp;pg=PA71&amp;amp;lpg=PA71&amp;amp;dq=lunar%20lander%20will%20sink%20in%20dust&amp;amp;source=bl&amp;amp;ots=P3NPFCL0vr&amp;amp;sig=3A1w_LF3wXbG7qE7jIcDxTlDsNs&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CDYQ6AEwBjgKahUKEwjjwOaB-6HHAhWGjA0KHdS4Auc#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;some&lt;/a&gt; at the time who felt no more needed to be known about the lunar surface before attention could turn to sending people. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this makes him the first person to recognize that the surface of the Moon has been primarily shaped by impacts. The entire surface of the Moon is covered with dust that is tens of meters deep (mixed with larger particles) entirely produced by meteor impacts pulverizing it and throwing it around over billions of years. (You don't &lt;em&gt;sink&lt;/em&gt; in it, but it's &lt;em&gt;there&lt;/em&gt;.) This understanding led to the realization that all solar system bodies experienced high levels of collisions for large parts of its history, and research later established that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Late_Heavy_Bombardment&quot; rel=&quot;noreferrer&quot;&gt;Late Heavy Bombardment&lt;/a&gt; occurred, and that impacts were a major mechanism of the solar system's evolution. In turn, this realization probably influenced the development of the theory that the Moon was in fact &lt;a href=&quot;https://en.wikipedia.org/wiki/Giant_impact_hypothesis&quot; rel=&quot;noreferrer&quot;&gt;created&lt;/a&gt; by an impact event between early Earth and an object about the size of Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first Surveyor mission was thus in large part meant to determine whether the dust on the surface would support a spacecraft. From Wikipedia on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_program&quot; rel=&quot;noreferrer&quot;&gt;Surveyor program&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Several Surveyor spacecraft had robotic shovels designed to test lunar&#xA;  soil mechanics. Before the Soviet Luna 9 mission (landing four months&#xA;  before Surveyor 1) and the Surveyor project, it was unknown how deep&#xA;  the dust on the Moon was. If the dust was too deep, then no astronaut&#xA;  could land. The Surveyor program proved that landings were possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-08-12T17:41:07.037" LastActivityDate="2015-08-12T17:41:07.037" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10604" PostTypeId="2" ParentId="10598" CreationDate="2015-08-11T22:36:11.527" Score="17" Body="&lt;p&gt;The best physical evidence is the fact the foot pads of the lunar lander were dish shaped pads and not something smaller.  This is not evidence of DEEP dust, but of at least the belief there was SOME dust likely to sink into.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NiUl5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NiUl5.jpg&quot; alt=&quot;From a Lunar Landing denier site, but hey&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-08-11T22:36:11.527" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10605" PostTypeId="2" ParentId="10600" CreationDate="2015-08-11T23:11:18.093" Score="20" Body="&lt;p&gt;There were four types of intact aborts designed for Shuttle (not counting pad aborts).  An intact abort is defined as one that returns the Orbiter intact to a runway.&lt;/p&gt;&#xA;&lt;p&gt;In order of least to most desirable:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Return to Launch Site (RTLS)&lt;/li&gt;&#xA;&lt;li&gt;Trans-Atlantic Abort (TAL)&lt;/li&gt;&#xA;&lt;li&gt;Abort Once Around (AOA)&lt;/li&gt;&#xA;&lt;li&gt;Abort to Orbit (ATO)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Only the ATO was ever performed in the Shuttle program.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eD4cJ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eD4cJ.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In addition, contingency aborts were designed.  A contingency abort is defined as one that allows the crew to bail out, but does not allow for safe return of the Orbiter.&lt;/p&gt;&#xA;&lt;p&gt;There was a type of contingency abort called East Coast Abort Landing (ECAL) that could have allowed the orbiter to land...but since it was not a certified abort mode, it was still considered contingency.&lt;/p&gt;&#xA;&lt;p&gt;Aborts could be performed either for performance reasons (failure of, or underperformance of one or more main engines) or systems reasons (cabin leak, loss of cooling, etc).&lt;/p&gt;&#xA;&lt;p&gt;The aborts overlapped to some degree.  Abort selection was based largely on vehicle state at the time of failure and performance.  For example, the last possible time to select RTLS ocurred when the vehicle did not have enough propellant remaining to burn its velocity down to zero and then back up high enough to make it to Kennedy Space Center.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015564.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This paper gives good detailed information about all I have said.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The reason that the SRBs couldn't be &amp;quot;jettisoned&amp;quot; at any desired time is because the separation system as designed was not powerful enough to safely jettison the boosters if they were delivering a significant amount of thrust.  The separation system simply severed the bolts holding the boosters to the External Tank and fired booster-mounted motors to provide clearance between them and the accelerating Orbiter/ET stack.  If significant thrust loads had been present, then significant forces and moments would have been applied to the Orbiter/ET stack.  It is possible that a more robust system could have been designed, but it wasn't incorporated in the system we had.&lt;/p&gt;&#xA;&lt;p&gt;Similarly, the ET separation system simply severed the bolts holding the ET to the Orbiter.  No separation motors were provided because the system was supposed to be in a low dynamic pressure environment at separation - the Orbiter manoeuvred away using its Reaction Control System jets.  In a high dynamic pressure environment with significant propellant sloshing around in the tank, the potential for recontact with the Orbiter was high.&lt;/p&gt;&#xA;&lt;p&gt;There was some discussion in the other question about the ejection seats used in early Columbia missions.  These had limited utility - at a point in ascent the plume expanded out so much that the seat trajectory would pass through it and a &amp;quot;Negative seats&amp;quot; call was made.  Similarly, on entry, the vehicle had to be going slow enough that a relatively unshielded ejecting crewmember could slow to a safe velocity.&lt;/p&gt;&#xA;&lt;p&gt;I deeply wish that a better abort/escape system had been designed in, but that was not the reality of STS.&lt;/p&gt;&#xA;&lt;p&gt;Edit: I just noticed that the crew training handbooks for intact and contingency aborts are available on Ken Hollis's &lt;em&gt;Rocket Scientists&lt;/em&gt; page.  These are wonderful references for STS aborts.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://gandalfddi.z19.web.core.windows.net/Shuttle/USA007151%20Rev%20A%20-%20Intact%20Ascent%20Aborts%20Workbook%2020061010.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Intact abort&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://gandalfddi.z19.web.core.windows.net/Shuttle/USA005671%20Rev%20D%20-%20Contingency%20Aborts%2021007%2031007%2020070214.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Contingency aborts&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://gandalfddi.z19.web.core.windows.net/Shuttle/USA009026%20-%20Crew%20Escape%20Workbook%2020050171.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Crew escape systems&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2024-01-10T19:50:36.693" LastActivityDate="2024-01-10T19:50:36.693" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="10606" PostTypeId="1" CreationDate="2015-08-12T01:27:49.737" Score="3" ViewCount="571" Body="&lt;p&gt;This is a follow up question my earlier &lt;a href=&quot;https://space.stackexchange.com/questions/6359/what-is-the-least-energetic-trajectory-design-to-send-uncrewed-supplies-to-mars&quot;&gt;question about sending supplies to Mars.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's 2250 AD (or some such future date). Say you have a young man with a dream that inherits a 500 billion widget fortune. He decides to plow his windfall into a space freighter business. For his inaugural loads, he plans to send as many 10,000 kg blocks of ice to Mars at the end of the next 24 month cycle, where he'll fetch a decent price from the Martian colonists.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What's the cheapest practical flight plan to send 10,000 kg of water ice to Mars from LEO?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume no breakthrough physics, just perhaps incremental increases in current technology. Let's also say he already can get his payload into orbit at a low-enough cost, and the Martians can retrieve it by whatever means from low Mars orbit (LMO). He wants to ship from LEO to LMO at the lowest cost per kilogram.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-08-13T02:57:59.220" Title="What's the cheapest way to send 10,000 kg to Mars?" Tags="|mars|trajectory|mission-design|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10607" PostTypeId="2" ParentId="10606" CreationDate="2015-08-12T02:30:52.043" Score="1" Body="&lt;p&gt;This answer will almost certainly prove to be wrong (but you'll have to wait until 2250 to know)!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we operate under the assumption that we don't do anything &lt;em&gt;new&lt;/em&gt; from the point of view of physics, then we can just look at the limits of some current technologies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conventional momentum exchange rockets: by this I mean solid, liquid, electric etc. anything that carries it's fuel with it.  These aren't going to win out in the long run simply because the rocket equation can not be worked around. You need to carry the fuel with you, and that makes launch and transfers expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space elevator: I'm going to assume we manage this at some point in the next few hundred years. A space elevator would significantly reduce the energy cost of launch - we don't need to carry the fuel for launch with us! I'm not suggesting our entrepreneur builds his own elevator, but use of one would be the equivalent of an extrememly high energy bill!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar sails: if you can build a big enough surface the solar sails are a great way to get where your going without carrying any fuel. The mass of the sail is typically reasonably small but the support mass can often be significant. However in a world where we have a space elevator we can probably build a very large sail for low mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now when you get to Mars 10,000 kg of anything is going to reenter with a lot of momentum, but who's to say we don't have a Martian space elevator as well. In fact a Martian space elevator is much more achieveable since Mars has a lower gravity.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-08-12T02:30:52.043" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10609" PostTypeId="2" ParentId="10583" CreationDate="2015-08-12T09:43:51.207" Score="1" Body="&lt;p&gt;The best way to explain it:&#xA;1. Understand it completely yourself&#xA;2. Give short and precise answers.&#xA;3. Encourage your kid to ask follow up questions to clarify and expand the answers.&#xA;4. If you get stuck, sit down with him on the computer and google it. And then break down the answers for him if they are too technical/complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a kid asks a question, you are not really supposed to prepare a whole lecture that covers everything. Just talk to him and see what he already knows and what is missing to get the whole picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If he askes questions like that, he is also probably smarter than you expect in some regards. If you lecture him, he will just get bored if you are simplifying too much.&lt;/p&gt;&#xA;" OwnerUserId="11324" LastActivityDate="2015-08-12T09:43:51.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10610" PostTypeId="1" AcceptedAnswerId="10612" CreationDate="2015-08-12T12:40:35.207" Score="10" ViewCount="657" Body="&lt;p&gt;Everytime I look at a space probe I see a whole different design and this puzzles me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do understand that each mission is different and requires different science instruments. I also do understand that for each launch, the amount of available mass is different and thus probe designers might need to use radical designs each time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I struggle to understand why there isn't a common design or a standardized structure that space agencies can reuse trying to save costs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it because in such a mission-specific project the costs savings related to using a similar layout are almost none, since most of the efforts are focused on designing specific science instruments rather than the probe chassis?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am also thinking about the nanosat example, where most of the components (as well as the chassis) are made with standard pieces.&lt;/p&gt;&#xA;" OwnerUserId="8925" LastEditorUserId="1142" LastEditDate="2016-05-24T02:09:34.297" LastActivityDate="2020-05-26T06:44:00.633" Title="Why space probes are so different from each other?" Tags="|spacecraft|probe|design|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10611" PostTypeId="1" AcceptedAnswerId="10615" CreationDate="2015-08-12T13:30:56.060" Score="6" ViewCount="661" Body="&lt;p&gt;ESA provides commercial satellite launches. But is this actually a profitable business? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you would subtract the cost of any purely scientific activities and only look at the commercial launches, would ESA generate a profit or at least be able to operate without requiring subsidiaries from the member countries? The financial statements in the &lt;a href=&quot;http://www.esa.int/About_Us/ESA_Publications/ESA_Publications_Annual_Report&quot; rel=&quot;nofollow&quot;&gt;ESA Annual reports&lt;/a&gt; accumulate the costs of both commercial and scientific operations, so it is hard for a layman to tell how much of the expenses are which.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="25" LastEditDate="2015-08-12T14:52:17.607" LastActivityDate="2015-08-12T19:02:16.640" Title="Are commercial rocket launches by ESA profitable?" Tags="|launch|commercial|esa|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10612" PostTypeId="2" ParentId="10610" CreationDate="2015-08-12T14:28:23.520" Score="9" Body="&lt;p&gt;Rather than establishing a common design for space probe construction, the aerospace industry establishes common standards for &lt;em&gt;managing&lt;/em&gt; construction. This makes more sense with the scope and life cycle of these types of projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When probes are rare and have very specific destinations and purposes--and need to be constructed in a cost-managed, timely fashion--you don't want to constrain the design where you don't have to. You want to give the designers the freedom to maximize success for the probe, because often these are one-shot missions--if you fail, you may not get funding for a retry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That isn't to say that there aren't situations in which it is appropriate to reuse designs. For example, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020&quot;&gt;Mars 2020 rover&lt;/a&gt; is planned to use a similar design to the Curiosity rover with different science instruments. Curiosity's design has proven to be effective in navigating the Martian terrain, so it makes sense to reuse parts of the design for another rover. Notice that NASA sent four rovers to more or less the same terrain before they decided to reuse the design. There were a lot of lessons to learn before reusing design made sense.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-08-12T14:28:23.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10613" PostTypeId="2" ParentId="10611" CreationDate="2015-08-12T15:24:45.187" Score="4" Body="&lt;p&gt;According to FAA estimates, European commercial launch revenue was about $920 million in 2014, which was a 30% increase from 2013. This was the second highest of the regions compared after the USA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VNX2x.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VNX2x.png&quot; alt=&quot;commercial launch revenues&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The FAA states the following method was used to derive the estimate:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These revenues are assessed based on commercial launch price estimates for each launch vehicle using publically available information.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The ESA's total revenues for 2013 were about $4300 million. If we assume a similar amount for 2014, then commercial launch revenue would be about 20% of total revenues. This seems to be a reasonable amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, we can't compare directly to expenses since the ESA doesn't seem to distinguish commercial and non-commercial costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA's entire Commercial Spaceflight budget for 2014 is $821 million. If the ESA has comparable expenses, then commercial launches are certainly profitable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.faa.gov/about/office_org/headquarters_offices/ast/media/FAA_YIR_2014_02-25-2015.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Commercial Space Transportation: 2014 Year in Review - FAA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/740427main_NASAFY2014SummaryBriefFinal.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;FY 2014 Budget Estimates - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-08-12T16:25:01.547" LastActivityDate="2015-08-12T16:25:01.547" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10614" PostTypeId="2" ParentId="10583" CreationDate="2015-08-12T18:08:13.677" Score="11" Body="&lt;p&gt;A correct analogy would be:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;There is an image at some location.&lt;/li&gt;&#xA;&lt;li&gt;Someone is here and talks to someone else at a distance.&lt;/li&gt;&#xA;&lt;li&gt;The first person announces the color of each pixel, one by one.&lt;/li&gt;&#xA;&lt;li&gt;The second person, who hears the first one, draws each pixel according to the announcements.&lt;/li&gt;&#xA;&lt;li&gt;The sound of the voice takes time to travel, the further, the longer (this can be demonstrated easily by clapping hands at 100 yards).&lt;/li&gt;&#xA;&lt;li&gt;At the end there are two exact copies, albeit no image has traveled.&lt;/li&gt;&#xA;&lt;li&gt;For the spacecraft, the two persons and the voice are replaced by modulation and demodulation of a radio wave, which really allows to transmit information at very long distances.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;You may also explain it takes time to announce all pixels; the two persons must know beforehand how to organize the pixels (image size); the names for all colors must be defined; etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may also experiment Morse code with a torch light to simulate how information can be shared at distance. You can replace light by dots on a ball to simulate slow travel of the carrier... this is endless when the analogy is correct (no image is moving).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To understand the details of how an image is transmitted in practical, how long it takes and why images are not systematically transmitted in real time, you may read this other answer: &lt;a href=&quot;https://space.stackexchange.com/questions/9916/how-are-new-horizons-images-sent-back-to-earth&quot;&gt;How are New Horizons images sent back to Earth?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="7017" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-02-02T20:29:36.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10615" PostTypeId="2" ParentId="10611" CreationDate="2015-08-12T18:13:13.203" Score="6" Body="&lt;p&gt;It might be more meaningful to look at &lt;a href=&quot;http://www.arianespace.com/about-us/service-solutions.asp&quot; rel=&quot;nofollow&quot;&gt;Arianespace&lt;/a&gt; rather than ESA - Arianespace is the commercial operation (owned ~1/3  by CNES and ~2/3 by various European aerospace firms) which actually operates the launcher. ESA currently gives it a fixed cash payment every year (~100m euro in 2013 &amp;amp; 2014, 145m in 2012, possibly less in 2011).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In recent years (&lt;em&gt;after&lt;/em&gt; taking account of the ESA subsidy) it's about broken even - in 2014 it posted &lt;a href=&quot;http://seradata.com/SSI/2015/05/arianespace-revenues-up-41-and-it-even-makes-a-profit-after-its-usual-cash-injection/&quot; rel=&quot;nofollow&quot;&gt;a slight profit&lt;/a&gt; &amp;amp; &lt;a href=&quot;http://www.parabolicarc.com/2012/04/25/arianespace-makes-profit-with-large-esa-subsidy/&quot; rel=&quot;nofollow&quot;&gt;similarly in 2012&lt;/a&gt;; I can't find a precise figure for 2013 but it was apparently a small loss. So it's not making a profit, but nor is it falling apart or in a declining spiral; ESA are presumably happy to pay a (relatively!) small regular subsidy to ensure Arianespace stays in operation. Note that subsidies have &lt;a href=&quot;http://aviationweek.com/space/arianespace-esa-we-need-help&quot; rel=&quot;nofollow&quot;&gt;declined substantially&lt;/a&gt; since 2005.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Strictly speaking this will include &quot;non-commercial&quot; Ariane launches purchased by ESA (presumably on a cash basis), but it's probably still useful.&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="11259" LastEditDate="2015-08-12T19:02:16.640" LastActivityDate="2015-08-12T19:02:16.640" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10616" PostTypeId="2" ParentId="10403" CreationDate="2015-08-12T20:42:58.660" Score="4" Body="&lt;p&gt;The mention of the three mission profiles in Jenkins' &lt;em&gt;The Space Shuttle&lt;/em&gt; (p. 160) cites &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19730022148&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA-CR-132282&lt;/a&gt;, a 1973 contractor report on the heat shield, which is public and does helpfully have a brief outline of missions 1, 2, and 3. (3 had not been split into 3A and 3B yet?). Not immensely helpful, but it does at least give the basic parameters.&lt;/p&gt;&#xA;&lt;p&gt;There's a little more detail on the various reference missions in &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023479.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;History of Space Shuttle Rendezvous&lt;/em&gt;&lt;/a&gt;, 2011 (ch. 9) - but again not really a substitute for the full thing. It confirms planning for both 3A and 3B was abandoned in late 1975.&lt;/p&gt;&#xA;&lt;p&gt;Finally &lt;a href=&quot;https://arc.aiaa.org/doi/pdf/10.2514/6.1973-608&quot; rel=&quot;nofollow noreferrer&quot;&gt;this conference paper&lt;/a&gt; from 1973 gives an outline of the four &amp;quot;representative missions&amp;quot; being considered at the time. I have not yet tracked down a full copy but the first page is still helpful.&lt;/p&gt;&#xA;&lt;p&gt;Combining the three we get:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mission 1 - 185km circular orbit (presumably 28.5 degrees) to deploy a satellite into geostationary orbit, and return another, with 29 tonnes payload, 290 m/s OMS and 37 m/s RCS dV. Launched from KSC. This used a space tug as a third stage to transfer the satellite to geostationary orbit.&lt;/li&gt;&#xA;&lt;li&gt;Mission 2 - 500km circular orbit (55 degrees) to resupply a space station or perform maintenance on an orbiting satellite, 11 tonnes payload, 430 m/s OMS and 37 m/s RCS dV. Launched from KSC.&lt;/li&gt;&#xA;&lt;li&gt;Mission 3 - a &lt;em&gt;single orbit&lt;/em&gt; mission to a 185km circular polar orbit to deploy or retrieve a satellite, with 18 tonnes payload, 150 m/s OMS and 46 m/s RCS dV. Launched from Vandenberg.&lt;/li&gt;&#xA;&lt;li&gt;Mission 3A - single orbit deployment variant&lt;/li&gt;&#xA;&lt;li&gt;Mission 3B - single orbit recovery variant&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;There is also mention of a Mission 4 in that last paper, which seems to have been a general class for &amp;quot;long duration flight with a particular scientific payload/objective&amp;quot;, of up to 30 days.&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="11259" LastEditDate="2021-08-03T18:18:16.133" LastActivityDate="2021-08-03T18:18:16.133" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="10617" PostTypeId="1" AcceptedAnswerId="10897" CreationDate="2015-08-12T22:22:15.377" Score="2" ViewCount="591" Body="&lt;p&gt;In space vehicle control and fault recovery, how it works is that a big brother copy is on the ground and being fed with most recent data from spacecraft, the little brother. This ground one is an exact model plus extensive instrumentation that cannot be put into the spacecraft. Thus, the spacecraft does not need to run diagnostics and always sends them back to the big brother. Sending the most recent data has actually the &lt;strong&gt;minimum&lt;/strong&gt; communication overhead because it's needed anyways. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to know how they make the communication low-overhead, and how such protocols work. Is there any high-level documentation or scientific paper for that? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE&lt;/strong&gt;: Right- we know the data rate is so low, and so the probe should make the best use of this low bandwidth. Now, my question is &quot;how&quot; do they maximum gain out of this low bitrate? What is their transmission protocol, their naming, etc? (at application level)&lt;/p&gt;&#xA;" OwnerUserId="11331" LastEditorUserId="897" LastEditDate="2015-08-28T17:03:29.747" LastActivityDate="2015-09-02T23:14:50.893" Title="Efficiency of communications between spacecraft and control center" Tags="|nasa|spacecraft|communication|voyager|data-transmission|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10618" PostTypeId="1" AcceptedAnswerId="10619" CreationDate="2015-08-13T00:52:40.300" Score="8" ViewCount="2984" Body="&lt;p&gt;Why was the decision made to have 2 boosters burning solid fuel alongside the liquid fueled SSME?  Other designs of the time were pushing for all liquid fueled rockets (Russian Soyuz, Chinese Long March). Why didn't NASA take the same SRBs and make them have LOX/LH2 engines? They offer a higher ISP not to mention are completely environmentally friendly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In googling for an answer, I have come up with a few arguments-&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;So they could be easily recovered and reused.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Boosters with liquid fuel tanks could be recovered as well.  In fact SpaceX's self-recovering 1st stage is probably the best argument for an all liquid fueled design.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Classic rocket engines are only efficient at a certain altitude. SSMEs are ignited but throttled down at launch to be throttled back up to full power after SRB separation, where they are more efficient.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Engineer the rocket engine to be efficient at sea level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other things to consider:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SRBs are difficult to control once ignited&lt;/li&gt;&#xA;&lt;li&gt;Challenger disaster occurred due to o-ring failure in the SRBs.&lt;/li&gt;&#xA;&lt;li&gt;According to wikipedia astronauts who have flown on multiple spacecraft report that Shuttle delivers a rougher ride than Apollo or Soyuz. [Due to SRBs].&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11333" LastEditorUserId="195" LastEditDate="2015-08-13T12:10:58.800" LastActivityDate="2015-08-13T12:10:58.800" Title="Why wasn't the space shuttle fully Liquid fueled?" Tags="|space-shuttle|srb|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10619" PostTypeId="2" ParentId="10618" CreationDate="2015-08-13T01:05:16.240" Score="12" Body="&lt;p&gt;Thrust and cost. Each &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;noreferrer&quot;&gt;SRB&lt;/a&gt; produced as much thrust as 6.5 of the shuttle orbiter's &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;noreferrer&quot;&gt;main (hydrogen fueled) engines (SSMEs)&lt;/a&gt; with a much simpler design. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A high powered liquid fuel engine would not come through a hard water landing like the SRB in reusable or repairable condition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.nss.org/resources/library/shuttledecision/chapter08.htm&quot; rel=&quot;noreferrer&quot;&gt;winged reusable liquid fueled booster&lt;/a&gt; strategy &lt;em&gt;was&lt;/em&gt; considered, but I suspect it would have probably doubled development costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An incremental strategy of developing a powered booster return, like SpaceX's, wouldn't have been considered; reusability was a hard requirement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I've done my math right, a pair of liquid-fueled boosters using either 5 SSMEs or 2 &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot; rel=&quot;noreferrer&quot;&gt;F-1s&lt;/a&gt; each could have worked as a replacement for the SRBs. The F-1 based booster -- a shorter version of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_C-3#Pyrios&quot; rel=&quot;noreferrer&quot;&gt;Pyrios&lt;/a&gt; booster being proposed for &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;noreferrer&quot;&gt;SLS&lt;/a&gt; -- might even have had enough performance headroom to do a Falcon 9-style powered descent, but a lot of very expensive engines would have been dropped in the Atlantic before that was accomplished.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-08-13T03:50:52.333" LastActivityDate="2015-08-13T03:50:52.333" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="10620" PostTypeId="2" ParentId="10606" CreationDate="2015-08-13T02:57:59.220" Score="2" Body="&lt;p&gt;Cheapest:  Orion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting your hands on the bombs and getting permission to launch are quite another matter....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, perhaps he can find an icy comet and use those bombs to send it to Mars.  If it's not too big it might be possible to hard land it in an unoccupied part of Mars--although the result would more go into the Martian atmosphere than be readily collected.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-08-13T02:57:59.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10621" PostTypeId="1" AcceptedAnswerId="10624" CreationDate="2015-08-13T04:36:11.163" Score="5" ViewCount="1073" Body="&lt;p&gt;On a rocket nozzle, are the stresses (namely &lt;a href=&quot;https://en.wikipedia.org/wiki/Cylinder_stress&quot; rel=&quot;noreferrer&quot;&gt;hoop stresses&lt;/a&gt;) from pressure differentials compressive or tensile? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, in a combustion chamber, the pressure inside is greater than outside. As the exhaust velocity increases, however, &lt;a href=&quot;https://en.wikipedia.org/wiki/Bernoulli%27s_principle#Derivations_of_Bernoulli_equation&quot; rel=&quot;noreferrer&quot;&gt;Bernoulli's theorem&lt;/a&gt; would state that the exhaust pressure decreases:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a flow where no energy is being added or taken away, the sum of its&#xA;  various energies is a constant: consequently where the velocity&#xA;  increases the pressure decreases and vice versa.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is there a specific point at which the pressure and hoop stress switch directions? Is this something that nozzle design must account for? Are altitude based changes in atmospheric pressure factored into this?&lt;/p&gt;&#xA;" OwnerUserId="10856" LastEditorUserId="49" LastEditDate="2015-08-13T14:07:41.327" LastActivityDate="2015-08-13T14:07:41.327" Title="What's the nature of hoop stresses on a rocket nozzle?" Tags="|design|engine-design|physics|nozzle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10622" PostTypeId="1" AcceptedAnswerId="12058" CreationDate="2015-08-13T06:44:50.933" Score="16" ViewCount="8370" Body="&lt;p&gt;Most (ok, all) of my space exploration experience has been from Kerbal Space Program which has reaction control wheels with greatly exaggerated performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm aware of real life missions (notably Apollo and STS) that used hypergolic orbital maneuvering systems which leads me to believe that RCW are either science fiction or relatively useless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Kerbal Space Program, even the smallest single man(kerbal) capsule (700kg) has 3kn of reaction wheel torque and can rotate 10s (if not 100s) of degrees per second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How powerful are reaction control wheels in real life? &lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="723" LastEditDate="2015-08-18T06:24:57.110" LastActivityDate="2015-09-23T04:06:58.367" Title="How powerful are reaction control wheels?" Tags="|attitude|power|reaction-wheel|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10624" PostTypeId="2" ParentId="10621" CreationDate="2015-08-13T13:50:14.053" Score="3" Body="&lt;p&gt;At low altitudes, engine nozzles are overexpanded. The pressure inside the nozzle rim is lower than the ambient pressure, so there's a compressive force on the nozzle.&lt;br&gt;&#xA;At high altitudes, ambient pressure has dropped to the point where it's lower than the pressure inside the nozzle rim: the nozzle is now underexpanded, and the force on the rim is outward.&lt;br&gt;&#xA;The altitude at which the pressures are in equilibrium depends on the nozzle design (the expansion ratio).&lt;br&gt;&#xA;The hoop stresses are bound to be a factor in nozzle design, giving a lower bound for material thickness. I can't put a number on how important this issue is though. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OaTKP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OaTKP.jpg&quot; alt=&quot;Nozzles&quot;&gt;&lt;/a&gt;&#xA;(image from &lt;a href=&quot;http://www.aerospaceweb.org/question/propulsion/q0220.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aerospaceweb&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-08-13T13:50:14.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10625" PostTypeId="1" CreationDate="2015-08-13T14:09:29.840" Score="9" ViewCount="668" Body="&lt;p&gt;Have there been any designs for a Mars habitat that could be built out of the rocks that are on Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe by having a rover find and scan rocks to build up a catalogue of 3D scans of rocks and then calculating how they will all fit together best and then stacking them together to construct a building. Then you could still have a pressurized and airtight habitat inside the rock structure but it wouldn't need to provide the structural strength.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That would seem like the best way to keep the weight of materials needed to bring to Mars down to me, and if you could build a rover that keeps going as long as the current rovers. You could easily have spare building being built after the main habitat has been constructed as well.&lt;/p&gt;&#xA;" OwnerUserId="10094" LastEditorUserId="49" LastEditDate="2015-08-13T15:02:39.787" LastActivityDate="2016-02-02T15:46:04.703" Title="Are there any designs for a Mars Habitat made out of stone?" Tags="|mars|colonization|construction|habitat|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10626" PostTypeId="2" ParentId="10622" CreationDate="2015-08-13T14:42:37.027" Score="3" Body="&lt;p&gt;They are neither science fiction, nor are they relatively useless.  But the areas where they might be used instead of alternatives are fairly limited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because they consume power but not mass, they might be preferred for long-duration missions.  Apollo and STS were of limited length.  Thrusters are more powerful, lighter, and of reduced complexity.  For a short-duration mission with limited mass budget, thrusters are a win.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides reaction wheels, you can go for a full control moment gyroscope.  CMGs will deliver more torque for the same power, but are also more complex.  If your vehicle is large, reaction wheels just don't cut it.  They're simply impractical above a certain size.  You'll want to use CMGs.  But for smaller vehicles, the simplicity of a reaction wheel could make it preferable over a CMG.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1080&amp;amp;context=smallsat&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; has some discussion of uses of reaction wheels and control moment gyros and has some figures about each.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2015-08-13T14:42:37.027" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10627" PostTypeId="1" CreationDate="2015-08-13T15:16:46.460" Score="5" ViewCount="312" Body="&lt;p&gt;I'm all in favour of space travel. But &lt;em&gt;purely from a scientific standpoint&lt;/em&gt;, what can be done by sending people to Mars that can't be done by sending probes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: To clarify (as in the comments), I meant what scientific tests/etc. do we want to do now that we can't with current probe/rover technology, but that we could feasibly do if we sent a person there. I'm not entirely sure if this is too broad or not.&lt;/p&gt;&#xA;" OwnerUserId="11343" LastEditorUserId="11343" LastEditDate="2015-08-13T15:36:09.597" LastActivityDate="2016-12-18T13:54:32.977" Title="What (if any) science can be done only by sending people to Mars?" Tags="|mars|crewed-spaceflight|" AnswerCount="1" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="10628" PostTypeId="1" AcceptedAnswerId="10647" CreationDate="2015-08-13T17:53:05.387" Score="2" ViewCount="1186" Body="&lt;p&gt;This question focus on space missions docking with the ISS.&#xA;On this &lt;a href=&quot;https://www.youtube.com/watch?v=qFjw6Lc6J2g&quot; rel=&quot;nofollow&quot;&gt;youtube video&lt;/a&gt;, the process to rendez-vous with the ISS is described. If I understand correctly, it consists in taking of, place the spacecraft into a stable but lower orbit, wait there for the good timing, and then perform a Hohmann transfer to the orbit the ISS is flying. The good timing is the one such that once on the ISS' orbit, the spacecraft is just few miles in front of the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although the last step seems clear to me, the first ones don't. What happen between the spacecraft launch and the stable lower orbit from which the last Hohmann transfer will be performed? How is this orbit chosen? How to get there?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short: What are the steps between the spacecraft launch and the spacecraft being in position to dock to the ISS?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastActivityDate="2015-08-14T14:02:32.027" Title="What are the steps between rocket launch and docking to the ISS?" Tags="|iss|orbital-maneuver|rendezvous|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10629" PostTypeId="1" AcceptedAnswerId="10631" CreationDate="2015-08-13T18:48:35.143" Score="11" ViewCount="592" Body="&lt;p&gt;The Space Shuttle made several servicing mission for satellites, most famous ones would be to service Hubble.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5ZUoa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5ZUoa.jpg&quot; alt=&quot;Hubble serviced by the shuttle&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would theses kinds of operations be feasible with a Soyuz spacecraft ?&#xA;Would it be practical or make sense (maybe todays technology make it more practical to just send a new satellite to replace the old one) ? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2015-08-13T23:43:00.340" Title="Can Soyuz be used to perform maintenance on satellites" Tags="|soyuz-spacecraft|maintenance|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10630" PostTypeId="2" ParentId="10598" CreationDate="2015-08-13T18:50:22.960" Score="7" Body="&lt;p&gt;There is an excellent series called &lt;a href=&quot;http://www.imdb.com/title/tt1203167/?ref_=fn_al_tt_1&quot;&gt;&quot;Moon Machines&quot;&lt;/a&gt; that is often shown on the Science Channel and is also available on DVD. The episode &lt;a href=&quot;http://www.imdb.com/title/tt1610832/?ref_=ttep_ep4&quot;&gt;&quot;The Lunar Module&quot;&lt;/a&gt; discusses the fact that NASA was concerned that lunar dust might be deep enough for the lunar lander to sink into it. In my opinion, this series is an excellent documentary of the Apollo Program well worth the time spent to watch it.&lt;/p&gt;&#xA;" OwnerUserId="11344" LastActivityDate="2015-08-13T18:50:22.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10631" PostTypeId="2" ParentId="10629" CreationDate="2015-08-13T19:22:51.383" Score="8" Body="&lt;p&gt;In short, no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soyuz could probably rendezvous with a satellite, assuming it was in the right orbital range, but would not be able to dock to it (unless the satellite happened to have been built with this in mind) or grapple it - the Hubble here was grappled by the manipulator arm, which the Soyuz does not have, and then berthed inside the payload bay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After this, you'd have to have someone go outside to do the work. A Soyuz could theoretically support an EVA (using the orbital module as an airlock) - but it would have to be modified for it. The last time an EVA took place directly from a Soyuz was in early 1969. Even if they did get outside, there would be very little space for handholds and so forth compared to the much large Shuttle - certainly no spacious payload bay with attachment points - and of course no manipulator arm to help them reach the satellite and move around safely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even if they did get outside (from a &lt;em&gt;heavily&lt;/em&gt; modified Soyuz) they'd be more or less stuck hanging on and waving at the satellite...&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="11259" LastEditDate="2015-08-13T19:57:48.200" LastActivityDate="2015-08-13T19:57:48.200" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10632" PostTypeId="2" ParentId="10598" CreationDate="2015-08-13T20:02:59.780" Score="2" Body="&lt;p&gt;Whether covered in dust, or cheese, obviously they had to consider how to get a craft &lt;em&gt;off&lt;/em&gt; the surface to fulfill the requirement of landing a man on the moon and &lt;strong&gt;&lt;em&gt;&quot;returning him safely to the earth.&quot;&lt;/em&gt;&lt;/strong&gt; The last thing you want is an unsteady footing in the lander such that the return vehicle would not operate as intended.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The unorthodox wheels and 12 inch ground clearance on the lunar rover are probably the strongest unspoken evidence that they had no idea what the consistency of the surface was and needed to cover all bases. They did not even deploy the rover until missions had already touched down and returned and dust was still a concern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/ZavBX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZavBX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11345" LastActivityDate="2015-08-13T20:02:59.780" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10633" PostTypeId="2" ParentId="10625" CreationDate="2015-08-13T22:11:42.217" Score="4" Body="&lt;p&gt;There are really no truly serious studies on Mars bases, since none have really been funded, thus most are in the nature of stories, concepts, or ideas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One example from a lunar base of a proposed concept would be 3-D printing the base &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Engineering_Technology/Building_a_lunar_base_with_3D_printing&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://sservi.nasa.gov/articles/building-a-lunar-base-with-3d-printing/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic notion is making cement of some kind from lunar resources with minimal resources brought from Earth. This one has some real demo work behind it as there is a device built to use a cement analogue to extrude buildings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lots of fun science fiction on the topic.  The &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0553560735&quot; rel=&quot;nofollow&quot;&gt;Red Mars&lt;/a&gt; series by Kim Stanley Robinson spans a sufficient time period to discuss almost every known base and terra forming idea ever suggested. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-08-13T22:11:42.217" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10635" PostTypeId="1" AcceptedAnswerId="10639" CreationDate="2015-08-13T23:39:08.497" Score="13" ViewCount="3996" Body="&lt;p&gt;I have read that so far the LRO has detected about &lt;a href=&quot;https://solarsystem.nasa.gov/news/display.cfm?News_ID=47903&quot; rel=&quot;noreferrer&quot;&gt;200 skylights&lt;/a&gt; - they appear to be holes into larger spaces below them, which could be lava tubes or domes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4WQ3L.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4WQ3L.jpg&quot; alt=&quot;10 skylights on the Moon&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would really like to know where all of them are. ASU's &lt;a href=&quot;http://lroc.sese.asu.edu/&quot; rel=&quot;noreferrer&quot;&gt;LRO site&lt;/a&gt; has all the images and multiple ways to search through them, but I don't know how to search for this. There is the &lt;a href=&quot;http://wms.lroc.asu.edu/lroc/rdr_product_select#_ui-id-1&quot; rel=&quot;noreferrer&quot;&gt;RDR product search&lt;/a&gt; page... I don't know what that means but it looks like it might be the place if I knew how to use it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But also, these are being actively searched for and there has to be a list out there of all of them, even a map. Does anyone know where?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/iQLWIuaNg68?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-08-14T02:13:38.620" LastActivityDate="2015-08-16T13:16:54.957" Title="Is there a map or list of all the lava tube skylights the LRO has detected on the Moon?" Tags="|the-moon|planetary-science|mapping|lro|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10636" PostTypeId="2" ParentId="10629" CreationDate="2015-08-13T23:43:00.340" Score="2" Body="&lt;p&gt;The other thing the Shuttle had that was unique was a lot of space in the payload bay. For Hubble servicing missions, the telescope was captured by the CanadArm and docked to a turntable in the payload bay. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus they had a stable platform to work from. But they also had places where equipment (New components) and tools could be stored easy to access. There was lots of room to store all the tools, and parts they needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz is severely limited in this respect. You could not really mount things on the outside and survive launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you could build a platform that Soyuz could dock too, that would carry all this with Soyuz behaving as the launch and return vehicle for the fragile humans. But everything else on the other vehicle. \&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Orion was to be seriously considered for such a task it would probably follow such an approach of some kind of logistics or service module to carry most of the stuff needed and Orion just being the human transport component of it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-08-13T23:43:00.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10637" PostTypeId="2" ParentId="10628" CreationDate="2015-08-13T23:53:06.617" Score="2" Body="&lt;p&gt;All space launches take a rocket on the earth and give it some altitude and a lot of sideways speed until it reaches orbit.  The ways to do this are all pretty well known and not specific to Soyuz.  You &quot;just&quot; have to plan your orbit and feed in the parameters to the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an ISS docking, the initial orbit selected will be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;In plane with the ISS orbit.  Plane changes are terribly expensive for fuel, so the first requirement is to match that.  This is done by letting the Earth turn until the launch site is brought underneath the orbital plane.  Launch at that time in the right direction and the plane is selected.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;High enough that you have a few orbits to check out your equipment, make sure stuff is working, and start getting closer.  Much of the equipment necessary to do this could not be tested during the launch.  Now is your time to make sure it's working.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;Low enough that the craft will re-enter on its own.  As a contingency against certain failures, a nominal launch orbit will decay in a few days, allowing the crew to return without the need for a re-entry burn.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That constrains the initial parking orbit.  If this were earlier in the program, you'd now wait until phasing was correct.  The Soyuz and ISS would be in the same plane, but could be at any distance along the orbit relative to the other craft.  The Soyuz at a lower altitude would orbit faster.  This would allow it to &quot;catch up&quot; and change the phase angle. Rendezvous burns would begin when phasing was correct.  This wait meant that launch to dock took up to 3 days.  Not too bad on a shuttle which had a lot of room, but quite annoying packed into a Soyuz.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the last few years, an updated launch profile has been used for crewed launches.  The ISS orbit is modified to place it at the correct phasing point at the predicted launch time.  This takes a lot of planning and uses fuel on the ISS, so it is not done for cargo flights.  When it works, the phasing step is complete at launch, and the total process goes from a few days to 6 hours.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest is just playing golf.  You know (basically) where you are and (basically) where you want to be, but not with such precision that you can expect to swing and get it there in one shot.  When the Soyuz was developed, it didn't have enough smarts to do orbital rendezvous by itself.  Instead it would have to pass over ground stations to analyze the status and communicate new commands.  So rather than something like flying a plane or driving a car where you have constant control, you're sending commands to do actions and then waiting to see how well you have done.  The multiple approach burns get things nice and close in a way that reduces the chance of colliding with the station and doesn't require impossible precision in each burn.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2015-08-13T23:53:06.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10638" PostTypeId="2" ParentId="10625" CreationDate="2015-08-14T00:28:56.233" Score="4" Body="&lt;p&gt;This idea is essentially pointless on Mars. It would be valuable to pile at least 1m of regolith on top of a habitat for radiation shielding and I think most plans for extraterrestrial colonies propose this, but there is little point in using rocks, or trying to interlock them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a start it doesn't require much structural strength for a cylinder to hold up a large mass of soil. I've seen a corrugated iron culvert under at least 10m of soil and rocks, with heavy trucks driving over it. It looked way too flimsy, but a cylindrical void is intrinsically quite stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore on Mars the habitat could support a large mass of soil from air pressure alone, 1atm of air pressure would support about 5m of Martian regolith. Piling a bunch of regolith on top might even improve stability by equalizing pressure on both sides of the shell. See &lt;a href=&quot;https://space.stackexchange.com/a/773/6599&quot;&gt;this answer&lt;/a&gt; to learn more about how much rock air pressure can support.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So unfortunately for the idea of an interlocking rock structure, unless it's at least 5m thick, it would actually get exploded by air pressure. The problem on Mars is primarily tensile strength to keep the air in. If the habitat's membrane/shell has sufficient tensile strength, then once pressurized it also has ample structural strength to support a great weight of loose rock/regolith - up to 5x more than required for radiation shielding. And once regolith has been piled on top and compacted it'll be intrinsically fairly stable so even if the habitat depressurizes it probably wont collapse.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-08-14T00:28:56.233" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10639" PostTypeId="2" ParentId="10635" CreationDate="2015-08-14T01:37:20.613" Score="18" Body="&lt;p&gt;There is a map of lunar pits, created by R. V. Wagner and M. S. Robinson of the School of Earth and Space Exploration, Arizona State University, in 2014. From &lt;a href=&quot;http://nesf2014.arc.nasa.gov/poster-uploader/files/Wagner_Robert_Lunar_Pits.pdf&quot; rel=&quot;noreferrer&quot;&gt;Distribution, Age, and Formation Mechanisms of Lunar Pits&lt;/a&gt; (PDF) by mentioned authors:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PpYAt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YyVyO.png&quot; alt=&quot;Map of lunar pits&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Map of the locations of all currently-known pits. Orange stars&#xA;  indicate mare or highland pits, and blue dots indicate craters with&#xA;  impact melt pits. 50° lines mark PitScan's search range.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Click on the image for a higher resolution version. Complete list with locations should be available in the &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103514001857&quot; rel=&quot;noreferrer&quot;&gt;Distribution, formation mechanisms, and significance of lunar pits&lt;/a&gt; Icarus volume by same authors, but is paywalled. From its highlights:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;We developed an algorithm to search for collapse pits in LROC NAC images.&lt;/li&gt;&#xA;  &lt;li&gt;We found ten pits in maria and highlands, typically 40–100 m in diameter and depth.&lt;/li&gt;&#xA;  &lt;li&gt;We found over 200 pits in impact melt, which average 16 m in diameter and 7 m in depth.&lt;/li&gt;&#xA;  &lt;li&gt;Pits are often significantly younger than their host materials.&lt;/li&gt;&#xA;  &lt;li&gt;Some pits have confirmed overhangs, and may connect to larger sublunarean voids.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Authors have searched for lunar pits with own PitScan, a semi-automated search algorithm that scanned LROC NAC images for pit candidates, and 200x200 pixel crops were created for human confirmation. The algorithm went through a three-step process:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;Determine a cut-off value for what is “shadowed” in the image.&lt;/li&gt;&#xA;  &lt;li&gt;Locate all blocks of &quot;shadowed&quot; pixels more than 10 pixels across.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;For each such block of pixels, determine if the up-Sun or down-Sun side  of the shadow is brighter. If the up-Sun side is brighter, then&#xA;  it is probably a rock, and should be ignored (see figure). Otherwise,&#xA;  save a small image of the shadow for classification by a human.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/jOBOv.png&quot; alt=&quot;PitScan classification process&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Comparison of PitScan output for a pit (left) and a rock (right). Note the relative brightnesses of the up-Sun and down-Sun regions of the DN profiles (bottom panels). &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;About that &lt;em&gt;RDR&lt;/em&gt;, it stands for &lt;em&gt;Reduced Data Record&lt;/em&gt; and is described &lt;a href=&quot;http://www.lroc.asu.edu/archive&quot; rel=&quot;noreferrer&quot;&gt;in this LROC page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;LROC utilizes more than one million images we've colleted in a&#xA;  variety of data products. An EDR (Engineering Data Record) is an image&#xA;  and its associated metadata. A CDR is and EDR that has been&#xA;  decompanded and calibrated. An RDR (Reduced Data Record) can be made&#xA;  from as few as one image or as many as thousands that are processed&#xA;  and reduced for the purpose of, for example, making a mosaic that&#xA;  combines multiple images of a particular area or feature, or a high&#xA;  resolution global digital elevation model created with stereo&#xA;  observations.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;There are many way to search through LROC's Data products:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://wms.lroc.asu.edu/lroc&quot; rel=&quot;noreferrer&quot;&gt;Map Interface&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://wms.lroc.asu.edu/lroc/thumbnails&quot; rel=&quot;noreferrer&quot;&gt;Browse EDRs and CDRs&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://wms.lroc.asu.edu/lroc/search&quot; rel=&quot;noreferrer&quot;&gt;Search by metadata for EDRs and CDRs&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;a href=&quot;http://wms.lroc.asu.edu/lroc/rdr_product_select&quot; rel=&quot;noreferrer&quot;&gt;Search for RDRs&lt;/a&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;More info about lunar pits from Lunar and Planetary Science Conferences (this was bugging me because I couldn't remember where I saw talks on the topic):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2011/pdf/2771.pdf&quot; rel=&quot;noreferrer&quot;&gt;Lunar Pits: Sublunarean Voids And The Nature Of Mare Emplacement&lt;/a&gt; (A. K. Boyd et al., 42nd LPSC 2012, PDF). Includes a LROC image table of some of them:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;&lt;pre&gt;&lt;code&gt;Location             Image        Depth  Shadow  Diameter   Slew  Incidence  &#xA;                                    (m)  length  Max  Min  angle      angle  &#xA;                                            (m)  (m)  (m)    (°)        (°)&#xA;▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬&#xA;Mare Tranquilitatis  M106662246R    102      60   97   86   0.00      30.52&#xA;                     M126710873R    106      53   95   85   0.00      26.56&#xA;                     M137332905R     92      70   98   81   7.05      37.37&#xA;                     M144395745L     NM      NM  100   NM -50.46      47.92        &#xA;———————————————————————————————————————————————————————————————————————————&#xA;Marius Hills         M114328462R    &amp;gt;32*     NM   62   48   6.80      61.38&#xA;                     M122584310L     36      19   57   46   0.00      28.08&#xA;                     M133207316L     &amp;gt;8*     NM   76   61  29.13      82.84&#xA;                     M137929856R     NM      NM   NM   NM  42.86      33.79&#xA;———————————————————————————————————————————————————————————————————————————&#xA;Mare Ingenii         M115225180L    &amp;gt;24      NM  146  107   0.00      74.52&#xA;                     M121124338L     68      87  125   97   0.00      52.00&#xA;                     M123485893R     47      38  104   68  -5.45      39.05&#xA;                     M128202846L     76     105  125   68   0.00      54.25&#xA;                     M136465172L    &amp;gt;47*     NM  110   73  15.51      55.47&#xA;                     M138819477R     52      42  101   73   0.00      38.91&#xA;▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬&#xA;* depth estimate less certain due to high slew angles&#xA;NM not measureable&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can load images with &lt;a href=&quot;http://wms.lroc.asu.edu/lroc/search&quot; rel=&quot;noreferrer&quot;&gt;LROC Image Search&lt;/a&gt; if you copy the image name in the Product ID field, but sadly this source doesn't provide exact coordinates and images cover large areas and features discussed are tiny in comparison.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2013/pdf/1761.pdf&quot; rel=&quot;noreferrer&quot;&gt;Electromagnetic Simulations of Ground-Penetrating Radar Propagation near Lunar Pits and Lava&lt;/a&gt; (M. I. Zimmerman et al., 44th LPSC 2014, PDF). Just an interesting read. Carry on. :)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.hou.usra.edu/meetings/lpsc2014/pdf/2901.pdf&quot; rel=&quot;noreferrer&quot;&gt;A Reevaluation of Lunar Impact Pit Craters&lt;/a&gt; (P. T. Malinski et al., 45th LPSC 2015, PDF). Includes a map of surveyed pits using &lt;a href=&quot;http://lunar.gsfc.nasa.gov/lola/&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbiter Laser Altimeter&lt;/a&gt; (LOLA) data, which is another way to detect them, as we discussed in the chat:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6ihAH.png&quot; alt=&quot;LOLA generated lunar surface with surveyed pits&quot;&gt;&#xA;        LOLA generated lunar surface with surveyed pits (dots). The lines represent borders and the area between the lines represent the&#xA;  area survey for the lunar highlands.  Maria that fell within the&#xA;  highland study area were evaluated based upon their natural borders.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-08-16T13:16:54.957" LastActivityDate="2015-08-16T13:16:54.957" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10642" PostTypeId="2" ParentId="8271" CreationDate="2015-08-14T10:31:57.183" Score="6" Body="&lt;p&gt;The main reason was cost and reliability and efficiency. The Russian engines won out in all areas. As simple as that. The USA did nothing out of good will as the first reply states. It came down to economics. Also at the time Russia was able to easily build the new core segment for the proposed ISS based on there Mir Station. No other nation had any real experience with Space Stations at the time. It simply was far cheaper and saved years of time to use the Russian core module. Probably billions. &#xA;Overall Russian engines simply were the best and probably still are. It was recently reported to congress it would take the USA until at least 2019 to develop and produce rocket engines to replace the present Russian engines used for Military and civilian launches in the USA even if the developers had all the money they needed. You only have to consider how long it is taking all the commercial operators to get there new rockets reliable. Swapping to a European engine also would take years and require a considerable amount of redesign of say the Atlas. I am no expert in that area. &lt;/p&gt;&#xA;" OwnerUserId="11353" LastActivityDate="2015-08-14T10:31:57.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10643" PostTypeId="2" ParentId="10194" CreationDate="2015-08-14T10:40:54.583" Score="-1" Body="&lt;p&gt;How does one determine the age of anything?&#xA;Generally humans use experience of known items to guess the age of something which they deem similar to what they have encountered before. Or else they consult an eye-witness to the event of birth to verify age as in using a birth certificate.&#xA;In the case of a planet out in space, scientists would use the age of what is known, i.e. our own solar system as some kind of calibration clock. &#xA;The assumptions would be that the object under scrutiny has undergone a similar kind of history as our own planet(s) or star.&#xA;However, therein lies the big rub. The accepted theory of star birth and planetary accretion has zero credibility when it comes to actual real, documented observed evidence to support it. Current theory cannot explain how the first stars got their existence. Nor can planets grow by rocks smashing into each other - physics 101 shows clearly that they simply bounce off each other. If someone proposes a cloud of hot uranium sludge as the substrate they have to explain where that came from in the first place. And so the shortcomings go on and on.&#xA;So even though scientists have used various means to guesstimate the age of our system[e.g. radioactivity of elements in meteorites etc.], they lack real life confirmation that that age is in fact correct, i.e. they've never observed how a star or planet gets born. &#xA;In human terms, there were no eye-witnesses to confirm on a birth certificate that the birthdate is correct. This is the only true way to determine the age of anything - some record of birth plus the history that led up to the current state.&#xA;To answer the question - they cannot in any way or fashion supply a true age for extra solar planets.&lt;/p&gt;&#xA;" OwnerUserId="11354" LastEditorUserId="11354" LastEditDate="2015-08-14T10:46:59.413" LastActivityDate="2015-08-14T10:46:59.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10644" PostTypeId="2" ParentId="10627" CreationDate="2015-08-14T11:21:30.613" Score="4" Body="&lt;p&gt;I think you probably could do the same science with rovers and robots for a lot of the things. But by sending humans to Mars you also get to study the humans, and how their bodies cope on Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also an engineering challenge. Science deals with data and a lot of the theoretical side and the new discoveries. But the engineering that goes into a mission like taking people to Mars is also going to make a lot of discoveries about what really works and about how to keep people alive in long-duration space flight, and landing and taking off from another planet. I think even if there is no scientific value this probably would be a worthwhile mission just for the discoveries you could make by doing a manned mission to Mars as an engineering challenge.&lt;/p&gt;&#xA;" OwnerUserId="10094" LastEditorUserId="415" LastEditDate="2016-12-18T13:54:32.977" LastActivityDate="2016-12-18T13:54:32.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10646" PostTypeId="2" ParentId="10451" CreationDate="2015-08-14T14:00:12.273" Score="4" Body="&lt;p&gt;I asked a colleague (R. Jokipii, a retired expert in this field) about this, and here's my summary of what he told me. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the spacecraft separates the plasma outside from the people inside, any people inside would require no additional shielding from the plasma—it's only a significant threat through contact. He emphasized that plasma itself is so tenuous, even shocks would not present significant dangers to the spacecraft: there's simply not enough of it there to be a significant threat. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;He added: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Even with the significantly lower intensity of galactic cosmic rays at Earth's orbit, astronauts would exceed current lifetime limits for astronauts within about 6 months when outside of the Earth's atmosphere and magnetic field. At present, there is no practical method to shield them from the galactic cosmic rays. Of course, the situation would even be worse at [location of] the Voyagers.&quot; &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, it's the &lt;em&gt;radiation&lt;/em&gt; that would be the danger for astronauts on manned probes—not the plasma.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;He pointed me to a paper by Eugene Parker, an even more well-known plasma physicist who studied these interactions (&lt;a href=&quot;http://www.nature.com/scientificamerican/journal/v294/n3/full/scientificamerican0306-40.html&quot; rel=&quot;nofollow&quot;&gt;&quot;Shielding Space Travelers&quot;, &lt;em&gt;Nature&lt;/em&gt;, 2006&lt;/a&gt;). The point of the paper is that cosmic rays pose the greatest threat to space travel, so shielding against these is critically important for any manned space exploration. &lt;/p&gt;&#xA;" OwnerUserId="11264" LastActivityDate="2015-08-14T14:00:12.273" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10647" PostTypeId="2" ParentId="10628" CreationDate="2015-08-14T14:02:32.027" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What are the steps between the spacecraft launch and the spacecraft being in position to dock to the ISS?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This varies from vehicle to vehicle. Some vehicles dock to the ISS, others berth with the ISS. Some vehicles do it very quickly, others take days. Amongst those that dock, there are a number of docking ports on the ISS. The timing of the operation, whether the vehicle docks or berths, and location and orientation of the docking port or berthing box make for widely varying sets of steps. I'll look at things from the perspective of how the Japanese HTV berths with the ISS. Orbital's Cygnus and SpaceX's Dragon follow a very similar pattern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first phase of flight is launch. Typically the vehicle that will eventually dock or berth with the ISS is a passive payload during this initial phase; it's the launch vehicle that is in charge. This phase ends with orbit insertion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The insertion orbit may be rather low, so low that the vehicle would reenter in short order without boosting to a higher orbit. This higher orbit is a phasing orbit, one whose altitude is lower than that of the ISS. This lower altitude means the orbital period is faster than than of the ISS. How much time is spent in this phasing orbit (or phasing orbits; some vehicles use more than one) depends on how far the ISS (the target vehicle) is ahead of the vehicle in question (the chaser vehicle). Some vehicles (e.g., the European ATV) might spend multiple weeks in their phasing orbit(s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the timing is right, the chaser will raise its altitude so that it is ideally a pre-planned distance behind the ISS. This is not a Hohmann transfer. It is instead a sequence of burns that target a specific point in space at a specific point in time. One approach to doing this is to use Lambert targeting, or some modification thereof. The start of this sequence of burns marks the start of the far-field rendezvous phase of flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the vehicle reaches the aim point (typically hundreds of kilometers behind the ISS), it performs another burn that makes its perigee a bit smaller than that of the ISS. With time, this moves the vehicle toward the ISS. At some point, the vehicle comes within communications range of the ISS. This marks the transition from far-field to near-field rendezvous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During far-field ops, the HTV (and Cygnus and Dragon) uses GPS to correct its navigated state. The ISS also uses GPS, but the distance between the two vehicles means that the ISS and the chaser may be using different sets of GPS satellites to estimate their state. Once the HTV (or Cygnus or Dragon) comes within in comm range, it switches to using relative GPS. This significantly increases the accuracy of the estimate of the relative state between the two vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the chaser comes within a prescribed distance of the ISS, it switches from near-field rendezvous to approach. In the case of HTV (and Cygnus and Dragon), the vehicle switches at this time from using relative GPS to visual navigation. GPS on the Earth doesn't work as well in cities as it does out in the country because of signals bouncing off of tall buildings (the multipath problem). Relative GPS doesn't work very well in the close vicinity of the ISS for the same reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the approach phase, the chaser must follow a prescribed corridor that eventually bring it to the docking port or berthing box. The approach phase has a number of hold points where the chaser is to come to rest with respect to the ISS. Deviations outside of the corridor or failure to stop at a hold point are signs that the vehicle needs to transition to yet another mode, abort or retreat. There are multiple decision points along the way where the chaser (or the human controllers on the ground) will decide that the best thing to do is either abort or retreat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The very last phase is of course the actual docking or berthing.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-08-14T14:02:32.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10648" PostTypeId="1" AcceptedAnswerId="10851" CreationDate="2015-08-14T14:17:47.670" Score="9" ViewCount="499" Body="&lt;p&gt;Dr. Jim Green, Director, &lt;a href=&quot;https://science.jpl.nasa.gov/PlanetaryScience/&quot; rel=&quot;noreferrer&quot;&gt;NASA Planetary Science Division&lt;/a&gt;, mentioned during his &lt;em&gt;&quot;The Martian&quot;: Science Fiction &amp;amp; Science Fact&lt;/em&gt; talk for the &lt;a href=&quot;http://www.marssociety.org/a/marssociety.org/www/conventions/18th-annual-international-mars-society-convention&quot; rel=&quot;noreferrer&quot;&gt;18th Annual International Mars Society Convention&lt;/a&gt; (you can &lt;a href=&quot;http://www.ustream.tv/channel/catholic-university-live-1/theater&quot; rel=&quot;noreferrer&quot;&gt;watch it live here&lt;/a&gt; while it lasts, August 13-16, 2015, and they will be posted on &lt;a href=&quot;https://www.youtube.com/user/TheMarsSociety&quot; rel=&quot;noreferrer&quot;&gt;The Mars Society YouTube channel&lt;/a&gt; later), that future Mars orbiters will include laser communications capability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is kind of old as well as new information to me at the same time;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Old, because there's already been plans since the early noughties to include &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Laser_Communication_Demonstration&quot; rel=&quot;noreferrer&quot;&gt;Mars Laser Communication Demonstration&lt;/a&gt; (MLCD) on then planned &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;Mars Telecommunications Orbiter&lt;/a&gt;, with a planned launch date in 2009, but was subsequently canceled in 2005. New, because to my knowledge, so far none of the latter (since early plans for MLCD) launched Mars orbiters that are now already orbiting the red planet include any such laser communications hardware. Not the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;MRO&lt;/a&gt;, not &lt;a href=&quot;https://en.wikipedia.org/wiki/MAVEN&quot; rel=&quot;noreferrer&quot;&gt;MAVEN&lt;/a&gt;, and neither do ESA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Express&quot; rel=&quot;noreferrer&quot;&gt;Mars Express&lt;/a&gt; or ISRO's &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;noreferrer&quot;&gt;Mars Orbiter Mission&lt;/a&gt;. Neither do I know of any specific mission that will include it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA did however test such technology closer to home, using &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Atmosphere_and_Dust_Environment_Explorer&quot; rel=&quot;noreferrer&quot;&gt;LADEE&lt;/a&gt; with &lt;a href=&quot;http://esc.gsfc.nasa.gov/267/271.html&quot; rel=&quot;noreferrer&quot;&gt;Lunar Laser Communications Demonstration&lt;/a&gt; (LLCD) from roughly one Lunar distance orbiting the Moon, and also &lt;a href=&quot;https://en.wikipedia.org/wiki/OPALS&quot; rel=&quot;noreferrer&quot;&gt;Optical Payload for Lasercomm Science&lt;/a&gt; (OPALS) on the ISS from Low Earth Orbit. So to my question(s):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Which specific future Mars orbiters was Dr. Jim Green alluding to when mentioning plans to include laser communications payloads on them? How far have these plans progressed so far and is there any technical documentation available?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, if it's possible to also answer this without making it too broad (otherwise I'll post this as a new question), how did LLCD and OPALS demonstrators help mature the technology that will be included on future Mars orbiters?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-08-14T18:43:56.297" LastActivityDate="2015-08-31T11:44:21.070" Title="What's the current timeline for NASA's Mars orbiter laser communications?" Tags="|mars|nasa|communication|reference-request|laser|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10649" PostTypeId="1" AcceptedAnswerId="10664" CreationDate="2015-08-14T15:19:32.177" Score="17" ViewCount="5230" Body="&lt;p&gt;Inside a rocket, tanks are put one above the other. This make sense as it may be a good compromise between tanks shape and the aerodynamicity of the whole rocket. In some rocket stage, the oxygen tank is above the fuel tank, in other it is the opposite. Here are some illustrative examples:&amp;nbsp;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;oxygen tank above the fuel tank: &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/b/b2/Saturn_v_schematic.jpg&quot; rel=&quot;noreferrer&quot;&gt;Saturn V first stage&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Sts_et_cutaway.jpg&quot; rel=&quot;noreferrer&quot;&gt;american space shuttle external tank&lt;/a&gt;, &lt;a href=&quot;https://airandspace.si.edu/exhibitions/space-race/online/sec500img/531l10il10.jpg&quot; rel=&quot;noreferrer&quot;&gt;soyuz first stage&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;fuel tank above the oxygen tank: &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/b/b2/Saturn_v_schematic.jpg&quot; rel=&quot;noreferrer&quot;&gt;Saturn V second and third stages&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Aggregat4-Schnitt-engl.jpg&quot; rel=&quot;noreferrer&quot;&gt;V2&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What are the criteria taken into account to decide which tank is put above the other when designing a rocket stage?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="8162" LastEditDate="2015-08-15T18:58:22.413" LastActivityDate="2020-06-10T05:24:52.227" Title="What are the criteria to put the oxygen tank above or below the fuel tank for a given stage of a rocket?" Tags="|rockets|design|" AnswerCount="6" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10650" PostTypeId="2" ParentId="10649" CreationDate="2015-08-14T15:43:53.203" Score="10" Body="&lt;p&gt;One factor is the relative density of the propellants; putting the denser one higher in the stack gives a higher center of gravity, which is advantageous for aerodynamic stability (think of an arrow or throwing dart with its heavy head). LOX is denser than kerosene or liquid hydrogen, so you want it on top in general.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the Saturn V S-II and S-IVB stages, unlike your other examples, there's a single vessel with a common bulkhead separating the LOX and LH tankage. There might be a structural load reason for putting the lighter hydrogen propellant on top, or a temperature management reason for putting the colder one on top, further from the rocket exhaust.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-08-14T16:45:09.913" LastActivityDate="2015-08-14T16:45:09.913" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10651" PostTypeId="2" ParentId="10649" CreationDate="2015-08-14T16:00:32.540" Score="14" Body="&lt;p&gt;From Sutton, &quot;Rocket Propulsion Elements&quot;, 1976 Edition:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Tanks can be arranged in a variety of ways and the tank design can be&#xA;  used to exercise some control over the change in the location of the&#xA;  center of gravity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/m8iiR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/m8iiR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As in all aerospace development, the booster tank arrangement is a design tradeoff.  Minimization of structural weight and overall system cost would be &#xA;traded off. I seriously doubt that thermal considerations play a major role in the arrangement of the tanks; the booster is only fueled for a short time and the flight time is even shorter.  More important considerations are what happens to the c.g. during propellant drain and the effects on the overall system stability. The overall size of the stage and its ability to be used with existing or planned facilities is also a factor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, there is no simple answer, and the different configurations found in existing boosters show the results of these tradeoffs.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-08-14T16:08:02.627" LastActivityDate="2015-08-14T16:08:02.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10652" PostTypeId="1" CreationDate="2015-08-14T16:10:00.507" Score="6" ViewCount="105" Body="&lt;p&gt;Do you know of any reliable statistics on the successrate of (aero)space project development?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many projects suffer major delay and/or significant over budgets? How many projects get cancelled completly during development?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm mainly interested in official projects at the taxpayers expense, I'm not so much interested in private studies of companies or universities.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess that excludes also very early projects in Pre-Phase A (NASA)/ Phase 0 (ESA), since investments up to this point are quite small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(For clarification: I'm working on a thesis about preliminary design methods and some numbers would come in handy to support my point that an approvement of conventional tools and methods would affect the successrate of big aerospace projects)&lt;/p&gt;&#xA;" OwnerUserId="8724" LastEditorUserId="8724" LastEditDate="2015-08-14T19:48:28.960" LastActivityDate="2015-08-14T19:48:28.960" Title="Statistic on cancelled space projects" Tags="|space-program|canceled-project|" AnswerCount="0" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="10653" PostTypeId="1" AcceptedAnswerId="10666" CreationDate="2015-08-14T17:13:21.273" Score="14" ViewCount="1233" Body="&lt;p&gt;Today I saw a video in which ESA astronaut Samantha Cristoforetti &lt;a href=&quot;https://www.youtube.com/watch?v=4exaXdPKS3Y&quot; rel=&quot;nofollow noreferrer&quot;&gt;teaches us how to make pitas in space, and invites us to try to make them at home&lt;/a&gt;. I understand that much of the food on the ISS is freezed dried. My sister and I enjoy freeze-dried mangos and would like to send some to the ISS crew to taste. &lt;strong&gt;Who should I contact to propose to send the astronauts our favorite snack?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I feel that this question is related to space exploration as one of the purposes of the ISS is to &lt;strong&gt;establish a human presence off-planet&lt;/strong&gt;, or to practice doing so. All permanent human dwellings have mail service, even if it is irregular or not periodic. Therefore, to normalize the ISS to a normal human dwelling it should be expected that people would be able to send packages there, even under scrutiny. &lt;strong&gt;Realizing this would be a terrific step forward in man's conquest of space.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note that this question was translated into English by my father, who is the one typing it. I'm eight years old and &lt;a href=&quot;https://aviation.meta.stackexchange.com/questions/1693/move-a-question-to-a-new-user&quot;&gt;I don't use SE unsupervised&lt;/a&gt;.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="11357" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:28.293" LastActivityDate="2015-08-16T01:38:53.727" Title="How would civilians send snacks to the ISS crew?" Tags="|iss|crewed-spaceflight|procedure|food|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10654" PostTypeId="1" CreationDate="2015-08-14T23:25:11.117" Score="2" ViewCount="115" Body="&lt;p&gt;My understanding of the idea of a space tether is that currently you wouldn't be able to build one because even just having it support it's own weight we don't have strong enough material, and with the material we do have the thickness you would need for the cable makes it impractical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I am wondering if there have been any designs for a space tether that have tried to break the problem down into smaller stages or parts to try and over come some of these problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example if you use a high point like a mountain to start from.&#xA;If you then built a tall building.&#xA;If you then tethered balloons to get up higher.&#xA;If you then tethered electric multi-rotor drones and powered them with power coming up the tether. &#xA;Maybe there are other stages you could have that I haven't thought of and maybe things like a tall building as a starting point don't make much of a difference.&#xA;I am just wondering if anyone has ever done the math on doing a space-tether in multiple stages?&lt;/p&gt;&#xA;" OwnerUserId="10094" LastActivityDate="2020-11-29T00:17:54.487" Title="Have there been any designs for multi staged space tethers?" Tags="|space-tether|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10655" PostTypeId="1" AcceptedAnswerId="10656" CreationDate="2015-08-15T01:32:17.110" Score="20" ViewCount="1270" Body="&lt;p&gt;As I write this, the latest image in the &lt;a href=&quot;https://www.nasa.gov/mission_pages/newhorizons/images/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's New Horizon's image gallery&lt;/a&gt; is this spectacular photo showing Pluto's atmosphere:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/image-feature/pluto-s-breathtaking-farewell-to-new-horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mar8l.jpg&quot; alt=&quot;Pluto’s Breathtaking Farewell to New Horizons&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The full web page for the image is &lt;a href=&quot;https://www.nasa.gov/image-feature/pluto-s-breathtaking-farewell-to-new-horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That image was posted several weeks ago. Why have there been no new ones since then?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that the data rate is vvveeerrryyy ssslllooowww, and that we won't get all the flyby data until some time in 2016. But just after the flyby, we were getting a few new images once a week or so. Why haven't we been seeing updates at the same rate since then? Or is it just that web page that hasn't been updated lately?&lt;/p&gt;&#xA;" OwnerUserId="574" LastEditorUserId="49" LastEditDate="2015-08-15T02:22:21.607" LastActivityDate="2015-08-15T02:22:21.607" Title="Why have there been no new New Horizons images in the last few weeks?" Tags="|probe|new-horizons|pluto|imaging|data-transmission|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10656" PostTypeId="2" ParentId="10655" CreationDate="2015-08-15T02:09:07.750" Score="26" Body="&lt;p&gt;Organic Marble is correct in the comment, New Horizons is now busy with Departure Phase (&lt;a href=&quot;https://blogs.nasa.gov/mission-ames/2013/07/22/the-architecture-of-the-pluto-fly-by-sequence/&quot; rel=&quot;noreferrer&quot;&gt;DP2 from Aug 5-Oct 22&lt;/a&gt;) science and transmitting plasma and dust data, and &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/06240556-what-to-expect-new-horizons-pluto.html&quot; rel=&quot;noreferrer&quot;&gt;no additional images will be transmitted until September 14&lt;/a&gt; when Science Data Playback phase starts. All images from &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/Press-Conferences/April-14-2015.php&quot; rel=&quot;noreferrer&quot;&gt;New Horizons Press Conferences Materials&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/t7V4N.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t7V4N.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qurvo.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qurvo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HRyov.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HRyov.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image sources: (1): Alan Stern, New Horizons PI, (2 &amp;amp; 3): Cathy Olkin, New Horizons Deputy project scientist. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-08-15T02:09:07.750" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10657" PostTypeId="1" CreationDate="2015-08-15T04:14:49.103" Score="-1" ViewCount="1164" Body="&lt;p&gt;Question relates to this topic &lt;a href=&quot;https://space.stackexchange.com/questions/1184/horizontal-space-launch&quot;&gt;Horizontal space launch&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question is rewritten, so some details are probably lost.&lt;br&gt;&#xA;Here I make a sketch of the launch principle and I want to know, if it could improve the whole launch process regarding its safety in first place. In other words, I suppose that this method should reduce the probability of a fault by a launch.&lt;br&gt;&#xA;This is a possible carrier, which should be reusable in ideal case. The rocket is placed on the top and splits off at certain height: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/o8YSX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/o8YSX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;A ramp is ca 30 degrees on its end. This looks realistic, since more inclination means more construction complexity and thus safety. After liftoff it should smoothly correct it's trajectory to more vertical position, but that is not a must, it depends on how much fuel I can save due to total air resistance difference:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/3oc6v.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3oc6v.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;Whole trajectory could be like that:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/YFSek.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YFSek.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;Rocket detachement happens when it is time to make the trajectory more horizontal again, lets say at 10 km altitude.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/bQCkY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bQCkY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Mass of the vehicle I suppose should be also optimised, it should not be too big. The carrier should land after split off, how exactly, I don't know, it is subject to a separate question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So main questions: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;This seems a much more intuitive and natural way to launch a heavy&#xA;thing to the orbit. So I wonder, is vertical start believed to be safer or just some economical aspects?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What can be a strong argument against my scheme, besides fuel consumption?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;What could go wrong by rocket detachement on high speed? Like for&#xA;example a strong trajectory declination due to turbulence?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Please note, this all is to a known grade just a speculation, since I am not expert and do not know specific parameters. Also it is &lt;strong&gt;not&lt;/strong&gt; focused on fuel consumption aspects in first place.&lt;br&gt;&#xA;Main benefit as I see it, we don't need a huge engine on first stage, which I believe is unnecessarily powerful.&lt;br&gt;&#xA;Also the vehicle by vertical start does not seem to be much controllable, if at all. Small crack on the nozzle edge - and it will loose stability. Mechanical vibrations in engine area, overheating, it all adds scepticism. Looks so freaking dangerous even compared to jet fighter take off from an aircraft carrier. &lt;/p&gt;&#xA;" OwnerUserId="11359" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-08-17T03:02:22.453" Title="Horizontal space launch from a ramp" Tags="|launch|" AnswerCount="2" CommentCount="10" ClosedDate="2015-08-16T21:14:06.983" ContentLicense="CC BY-SA 3.0" />
  <row Id="10658" PostTypeId="2" ParentId="10657" CreationDate="2015-08-15T05:51:58.763" Score="2" Body="&lt;p&gt;Any space launch system should be capable of taking the payload (satellite) to a certain altitude. The amount of work done in this lifting is the same whatever may be the method/trajectory used to do this lifting (because gravitational potential energy depends only on height).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the best launch design has to reduce the amount of time spent in the atmosphere to reduce drag and heating. Even if a horizontal launch system is used, the escape velocity is the same and the drag would be the same. However, the amount of time spent in the atmosphere and the required fuel goes up significantly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the forces acting on the rocket, the aerodynamic forces will be the same and the only force that can be reduced thrust so that the acceleration is very small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this increases the time spent in atmosphere and associated drag. It is important to note that all horizontal launch systems which have been developed use air breathing engines and are aimed at reducing the requirement of oxidizer to be carried.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-08-15T05:51:58.763" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10659" PostTypeId="1" AcceptedAnswerId="11921" CreationDate="2015-08-15T07:41:39.737" Score="8" ViewCount="3719" Body="&lt;p&gt;Payload to LEO is commonly used as the bottom line of what a launcher can accomplish. But it must depend on factors which are not launcher specific, such as launch location and orbital characteristics.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How is payload to LEO calculated?&lt;/li&gt;&#xA;&lt;li&gt;What error bars should one put on such estimates?&lt;/li&gt;&#xA;&lt;li&gt;Would heaviest or average payload &lt;strong&gt;actually launched&lt;/strong&gt; be better&#xA;measures, and is such statistics easily available to the public?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Example: Saturn V's payload to LEO is confused online. &lt;a href=&quot;http://www.astronautix.com/lvs/saturnv.htm&quot;&gt;Encyclopedia Astronautica&lt;/a&gt; says 127 tons. &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot;&gt;Wikipedia&lt;/a&gt; says 140 tons. 118 tons is another common figure. The difference of 22 tons is about equal to today's heaviest launcher's capacity. Does this variation depend on unspoken choices of assumptions? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia's source is &lt;a href=&quot;https://www.cbo.gov/sites/default/files/109th-congress-2005-2006/reports/10-09-spacelaunch.pdf&quot;&gt;US Congressional Budget Office&lt;/a&gt; which mentions 140 mt to LEO at least six times, so this metric might have policy implications. That source says in a footnote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The payload of 140 metric tons is derived from weight data provided&#xA;  in Richard W. Orloff, &lt;strong&gt;Apollo by the Numbers&lt;/strong&gt;: A Statistical Reference,&#xA;  NASA SP-2000-4029 (National Aeronautics and Space Administration,&#xA;  updated September 27, 2005), available at &lt;a href=&quot;http://history.nasa.gov/SP-4029/SP-4029.htm&quot;&gt;http://history.nasa.gov/SP-4029/SP-4029.htm&lt;/a&gt;. In that reference, 140 mt is the&#xA;  weight of the Apollo 17 command-and-service modules, the lunar module,&#xA;  the spacecraft/lunar module adapter, the instrument unit, and the&#xA;  S-IVB stage (the third stage of the Saturn V), including the fuel&#xA;  remaining in that stage needed to propel the Apollo&#xA;  command-and-service modules and lunar module from low earth orbit to&#xA;  the moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here is &lt;a href=&quot;http://georgetyson.com/files/apollostatistics.pdf&quot;&gt;Apollo by the Numbers&lt;/a&gt; available.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2015-08-15T13:10:08.050" LastActivityDate="2015-12-24T02:48:58.423" Title="How is &quot;payload to LEO&quot; estimated for a launcher?" Tags="|launch|rockets|low-earth-orbit|delta-v|mass-fraction|" AnswerCount="3" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10660" PostTypeId="2" ParentId="10659" CreationDate="2015-08-15T11:42:14.610" Score="1" Body="&lt;p&gt;The &lt;a href=&quot;https://www.grc.nasa.gov/www/k-12/rocket/rktwtp.html&quot; rel=&quot;nofollow&quot;&gt;payload fraction&lt;/a&gt; of a launcher, $\lambda$ is usually given by,&#xA;$\lambda = \frac{m_{d}}{m_{p} + m_{s}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $m_{d}$ is the payload mass, $m_{p}$ is the propellant mass and $m_{s}$ is the launcher structural mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, the payload fractions of the launchers are very small, less than 1$\%$ or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as space launches are considered, each one is unique for all purposes and it is difficult to compare them. For this reason, it is better to compare the $\mathit{actual}$ $\mathit{average}$ payload over the missions rather than any theoretical values. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is because the launchers are being improved in every flight and other conditions are also different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://georgetyson.com/files/apollostatistics.pdf&quot; rel=&quot;nofollow&quot;&gt;document&lt;/a&gt; in the question on Saturn V indicates that the rated thrust of all the three stages increased over the launches. Also, the payload of 140 tonnes is the $\mathit{maximum}$ achieved over the launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, it is better to take only the actual values from th mission for any comparison.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastEditorUserId="11361" LastEditDate="2015-08-15T12:48:53.680" LastActivityDate="2015-08-15T12:48:53.680" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10661" PostTypeId="1" AcceptedAnswerId="10759" CreationDate="2015-08-15T13:10:39.077" Score="4" ViewCount="3542" Body="&lt;p&gt;GPS satellites are right in the outer Van Allen radiation belt (a little above 20,000 km). Communications satellites in GEO are higher, but still in it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I recall correctly, both the inner and outer Van Allen belts have increased levels of radioactivity and can damage satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why are there so many satellites in an area which is such a bad place for them to be orbiting in? Is there some major benefit making it all worth it?&lt;/p&gt;&#xA;" OwnerUserId="8983" LastEditorUserId="25" LastEditDate="2015-08-25T15:29:42.177" LastActivityDate="2015-08-25T15:29:42.177" Title="Does orbiting within the outer Van Allen radiation belt increase or reduce environmental radiation levels?" Tags="|artificial-satellite|orbit|radiation|mission-design|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10662" PostTypeId="2" ParentId="10661" CreationDate="2015-08-15T14:08:39.660" Score="2" Body="&lt;p&gt;The main reason for satellites to be in the Van Allen belts is because the synchronous orbits are located there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GPS satellites orbit at around 20,200 km altitude above MSL because the orbital period at this altitude is 12 hours and the satellite returns to the same place on earth twice a day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the other end is the geosynchronous orbit at 35,786 km above MSL, where the satellites have an orbital period of 24 hours, thus returning to the same place once a day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An important thing about the synchronous satellites is that they trace the same ground track every day (since their rotation matches with that of the earth's) and this makes tracking easy. For example, the geostationary satellites make satellite broadcasting to individual households possible.&#xA;This also means that they can used to cover the same areas everyday.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though it is possible to locate the satellites in an active geosynchronous orbit, it requires continuous expenditure of some form of energy. Since lifting payload to space cost astronomical sums of money, it is better to safeguard the satellites against any radiation rather than getting more fuel there.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-08-15T14:08:39.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10663" PostTypeId="2" ParentId="5116" CreationDate="2015-08-15T17:19:30.967" Score="6" Body="&lt;p&gt;Yes, at least in some Centaur applications, the LOX pump shaft is splined, and in fact has a gear drive attached to it.  This drive provides mechanical power to a hydraulic pump which in turn supplies hydraulic fluid to the Centaur's Thrust Vector Control (TVC) actuators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source is &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Upper_Stages/TheCentaurUpperStageVehicleHistory.pdf&quot; rel=&quot;noreferrer&quot;&gt;this document&lt;/a&gt;, I quote the relevant paragraph (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During steady state, the vehicle Propellant Utilization (PU) system&#xA;  provides commands to the OFCV to modulate mixture ratio.  The chamber&#xA;  pressure is held nearly constant when the mixture ratio is increased&#xA;  or decreased by the TCV.  &lt;strong&gt;The LO2 pump accessory drive pad supplies&#xA;  power to the vehicle hydraulic  power unit pump which provides 1100&#xA;  psia to vehicle thrust vector control servo actuators.&lt;/strong&gt;  The solenoid&#xA;  valves are closed at shutdown, closing both inlet valves and the&#xA;  MFSOV.  The cooldown valves are opened, bleeding off system pressure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here's the schematic referenced in the question:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/AqmD8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AqmD8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here is a picture of a very similar RL10 from the museum at MSFC.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/MXlFf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MXlFf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't see the drive gear in the above picture, but the accompanying label shows it:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/yam5B.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yam5B.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MSFC museum pictures taken from &lt;a href=&quot;https://www.reddit.com/r/ThingsCutInHalfPorn/comments/1omxm7/rl10_rocket_engine_turbopump_by_pratt_whitney/&quot; rel=&quot;noreferrer&quot;&gt;this reddit thread&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-08-15T17:19:30.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10664" PostTypeId="2" ParentId="10649" CreationDate="2015-08-15T22:38:40.170" Score="18" Body="&lt;p&gt;&quot;Stages to Saturn, A Technological History of the Apollo/Saturn Launch Vehicles&quot; by Roger E Bilstein (also NASA SP-4206, available &lt;a href=&quot;http://history.nasa.gov/SP-4206/contents.htm&quot;&gt;http://history.nasa.gov/SP-4206/contents.htm&lt;/a&gt; and elsewhere) is a great reference on design decisions in the Saturn V rockets.  One of the things it notes multiple times is that the cryogenic propellants cause freezing problems with the other propellants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stage I of the Saturn V is LOX and RP1.  From Chapter 7 (pg 191 in the book):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The special problem of the LOX tank involved the feed lines leading to&#xA;  the thirsty engines about 15 meters below the fuel tanks. To do the&#xA;  job, the S-IC used five LOX suction lines, which carried oxidizer to&#xA;  the engines at 7300 liters (2000 gallons) per second. To achieve such&#xA;  high rates of flow, the lines could not be bent around the outside of&#xA;  the fuel tank; therefore, designers ran them right through the heart&#xA;  of the fuel tank. This in turn caused considerable fabrication&#xA;  problems, because it meant five extra holes in both the top and bottom&#xA;  of the fuel tank and presented the difficulty of avoiding frozen fuel&#xA;  around the super-cold LOX lines. The engineering fix on this included&#xA;  a system of tunnels, each one enclosing a LOX line, especially&#xA;  designed to carry an effective blanket of insulating air. Even so, the&#xA;  warmer fuel surrounding lines created some thermal difficulties in&#xA;  keeping the LOX lines properly cool. So the S-IC used some of its&#xA;  ground-supplied helium to bubble up through the LOX lines, and kept&#xA;  the liquid mixed at a sufficiently low temperature to avoid&#xA;  destructive boiling and geysering, or the creation of equally&#xA;  destructive cavities in the LOX pumps.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So on the S-I (Saturn V Stage 1), you want to avoid turning the RP-1 into a slushie while still delivering the LOX to the engines as liquid.  The S-II (Saturn V Stage 2) actually has similar thermal design in its way, but the colder element (now LH2) in the stage is again stacked atop and flowed past the warmer element (now the LOX).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could swear there's phrasing to this effect that I'm just not finding in this read, but the basic idea is that if you freeze the stuff in the lines, the rocket is done for, but if you're flowing cold lines past a warmer tank the balance of the heat transfer is likely to keep your cold lines slightly warm, and your warm tank slightly cold, and everything keeps on working.&lt;/p&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2015-08-15T22:38:40.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10666" PostTypeId="2" ParentId="10653" CreationDate="2015-08-16T01:38:53.727" Score="7" Body="&lt;p&gt;Ultimately, you would want to contact &lt;em&gt;NASA&lt;/em&gt; (or another one of the participating space agencies) to request sending anything to or from the International Space Station. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could try contacting a former astronaut via social media (e.g. Cmdr. Chris Hadfield, who has a &lt;a href=&quot;https://twitter.com/cmdr_hadfield&quot;&gt;lively Twitter account&lt;/a&gt;). Even though an astronaut (probably) wouldn't have the authority to make this kind of decision, they would very likely know who to contact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could try contacting the Student Spaceflight Experiments Program (&lt;a href=&quot;http://ssep.ncesse.org/&quot;&gt;link&lt;/a&gt;). While your idea isn't an &lt;em&gt;experiment&lt;/em&gt;, this is an open channel for collecting students' ISS-related ideas and putting them into action! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My &lt;em&gt;guess&lt;/em&gt; is that the officials managing the ISS crew would want to send food that is professionally prepared for the ISS crew...but who knows!  :-)&lt;/p&gt;&#xA;" OwnerUserId="11264" LastActivityDate="2015-08-16T01:38:53.727" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10669" PostTypeId="1" AcceptedAnswerId="10675" CreationDate="2015-08-16T10:03:22.470" Score="3" ViewCount="1250" Body="&lt;p&gt;If I understand correctly, a liquid propellant rocket engine combustion chamber is fed with fuel and oxygen from a turbopump. Those turbopumps are driven by a gas generator. The gas generator is fed with fuel and oxygen; but I cannot find if fuel is burnt in the gas generator, or if it is just decompressed and transformed from liquid to gas. My reflection says it would be useless to mix both fuel and oxygen if it is not to burn the fuel, but I fail to see any ignition mechanism in the (simple) &#xA;&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Rocket_liquid_fuel_engine.svg&quot; rel=&quot;nofollow&quot;&gt;drawing&lt;/a&gt; I see on wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is fuel burnt in the gas generator? If so, how is it ignited?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="-1" LastEditDate="2016-03-28T12:06:49.887" LastActivityDate="2016-03-28T15:45:36.493" Title="Is fuel and oxygen burnt in a gas generator?" Tags="|engines|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10670" PostTypeId="1" CreationDate="2015-08-16T10:05:09.460" Score="14" ViewCount="2791" Body="&lt;p&gt;What explains the relation between how much a launcher weights on the launch pad, and how much mass it can lift to orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had expected that more payload requires more fuel to be launched and that a launcher is built around that fuel mass. I'm surprised to see that these two measures are just weakly correlated. The data I use here is indeed sloppy but it can't be that much off, &lt;strong&gt;or can it?&lt;/strong&gt; (I'm getting suspicious). I can understand that Falcon 9 v1.1 is much heavier than v1.0 because it is supposed to be more robust and have fuel enough to be reusable. I note that the (older style) Russian and Chinese launchers are the least efficient according to this simple ratio, but with Ariane 5 a bit worse than Proton. I'm surprised to realize that Atlas V 551 and Ariane 5 both lift about the same mass to orbit, but that Ariane 5 is well more than twice as heavy on the launch pad! Is it because of the large solid fuel boosters? What other factors explain this lack of general relation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below are figures I took from Wikipedia about eleven different launchers. I've picked values for the maximum LEO capacity configuration. The four columns are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mass of the launcher on the launch pad (tons).&lt;/li&gt;&#xA;&lt;li&gt;Mass payload the launcher can put in low Earth orbit (tons).&lt;/li&gt;&#xA;&lt;li&gt;The ratio between the two above (tons on pad / tons in LEO).&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The difference between that ratio and the average ratio in this sample, which is 33 tons on the launch pad per ton payload to LEO, ranging hugely from 21 (Saturn V) to 55 (Long March 2F).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pad, &lt;strong&gt;LEO&lt;/strong&gt;, &lt;em&gt;Ratio&lt;/em&gt;, Deviation from average ratio [tons]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;240 &amp;#8199;&amp;#8199;&lt;strong&gt;6.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;40&lt;/em&gt; &amp;nbsp;&amp;#8199;&amp;#8199;7 &amp;nbsp;Antares (not in the chart)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;308 &amp;#8199;&amp;#8199;&lt;strong&gt;6.5&lt;/strong&gt; &amp;nbsp;&lt;em&gt;47&lt;/em&gt; &amp;nbsp;&amp;#8199;15 &amp;nbsp;Soyuz&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;333 &amp;#8199;&lt;strong&gt;13.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;26&lt;/em&gt; &amp;nbsp;&amp;#8199;&amp;minus;7 &amp;nbsp;Falcon 9 v1.0&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;334 &amp;#8199;&lt;strong&gt;19.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;18&lt;/em&gt; &amp;nbsp;&amp;minus;15 &amp;nbsp;Atlas V&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;464 &amp;#8199;&amp;#8199;&lt;strong&gt;8.5&lt;/strong&gt; &amp;nbsp;&lt;em&gt;55&lt;/em&gt; &amp;nbsp;&amp;#8199;22 &amp;nbsp;Long March 2F&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;506 &amp;#8199;&lt;strong&gt;13.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;39&lt;/em&gt; &amp;nbsp;&amp;#8199;&amp;#8199;6 &amp;nbsp;Falcon 9 v1.1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;531 &amp;#8199;&lt;strong&gt;19.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;28&lt;/em&gt; &amp;nbsp;&amp;#8199;&amp;minus;5 &amp;nbsp;H-IIB, Japan&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;694 &amp;#8199;&lt;strong&gt;21.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;33&lt;/em&gt; &amp;nbsp;&amp;#8199;&amp;#8199;0 &amp;nbsp;Proton&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;733 &amp;#8199;&lt;strong&gt;29.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;25&lt;/em&gt; &amp;nbsp;&amp;#8199;&amp;minus;8 &amp;nbsp;Delta IV Heavy&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;#8199;777 &amp;#8199;&lt;strong&gt;21.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;37&lt;/em&gt; &amp;nbsp;&amp;#8199;&amp;#8199;4 &amp;nbsp;Ariane 5&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3000 &lt;strong&gt;140.0&lt;/strong&gt; &amp;nbsp;&lt;em&gt;21&lt;/em&gt; &amp;nbsp;&amp;minus;11 &amp;nbsp;Saturn V (not in the chart)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9pw78.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9pw78.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Chart: Tons payload to LEO versus tons of rocket on the launch pad.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="11371" LastEditDate="2015-08-17T00:35:41.787" LastActivityDate="2015-08-18T15:21:28.853" Title="Why are rocket mass on the launch pad and payload mass to LEO not strongly correlated?" Tags="|rockets|low-earth-orbit|launchpad|mass|mass-fraction|" AnswerCount="4" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="10672" PostTypeId="2" ParentId="10657" CreationDate="2015-08-16T10:42:01.247" Score="3" Body="&lt;p&gt;Your premise, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;More safety, since we don't have such mechanical stress on start, which is essential for the rocket itself and its cargo. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;is incorrect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a vertical launch, mechanical stress due to acceleration is lowest at liftoff, and increases gradually as you go up: as fuel is used up, the rocket gets lighter while thrust stays the same. The point where acceleration stress is highest is somewhere near the end of the first-stage burn.&lt;br&gt;&#xA;Also, mechanical stress is all in one direction: vertical. This means you can build a rocket that's strong in the vertical direction, but weak in the horizontal directions (because the forces in those directions are much smaller). Some rockets take this to such extremes that you can't transport them horizontally unless their tanks are pressurized. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second source of stress is aerodynamic drag. The point where this is highest is called max-Q. This is a function of altitude (higher altitude=thinner air and less drag) versus speed (more speed = more drag). In a vertical launch, max-Q is at high altitude and often near the end of the first stage burn.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's look at a horizontal launch. The acceleration force now has two components: you've added a lateral force throughout the bend in your ramp. You need to make the rocket stronger (=heavier) to compensate for this. &#xA;If you want to decrease acceleration, you need to make the ramp longer. Pretty soon you're building a structure that's capable of supporting several hundred tons and has to be 10 km long and 1 km high. This is not cheap by any means. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because you do some of the acceleration while horizontal, the rocket will be going faster at low altitudes. This means more drag, and a higher max-Q. Again, you need to make the rocket stronger (=heavier) to compensate for this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for safety: in a vertical launch, you can ignite the engines and make sure they're working well before the rocket leaves the ground. In a ramp launch, you have to start the engines while the rocket is almost at the end of the ramp. Any delays or engine problems, and your rocket will fall off the end of the ramp and be destroyed (or alternatively, you'll have to make the ramp twice as long to provide a safe runout area).&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-08-16T10:42:01.247" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10673" PostTypeId="2" ParentId="10669" CreationDate="2015-08-16T10:51:04.123" Score="4" Body="&lt;p&gt;In the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gas-generator_cycle&quot; rel=&quot;nofollow noreferrer&quot;&gt;gas generator&lt;/a&gt; cycle, some of the fuel is burnt in the gas generator. The hot gasses are used to run the turbines which power the pumps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not able to make out anything from the given drawing (its in Russian, for one thing). Hope the diagram below makes things clear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ixNiS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ixNiS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source:Wikimedia Commons&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the ignition system a &lt;a href=&quot;http://www.appbv.nl/uploads/SP2010_1841750.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;number of methods&lt;/a&gt; can be used like a pyrogen, laser, catalyst etc. For example, the &lt;a href=&quot;https://en.wikipedia.org/wiki/J-2X&quot; rel=&quot;nofollow noreferrer&quot;&gt;J-2X&lt;/a&gt; uses a pyrotechnic ignition system.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastEditorUserId="6944" LastEditDate="2015-08-16T11:11:15.167" LastActivityDate="2015-08-16T11:11:15.167" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10674" PostTypeId="2" ParentId="10670" CreationDate="2015-08-16T15:01:41.587" Score="15" Body="&lt;p&gt;It typically takes a total expenditure of 9400-10000 meters per second of delta-v to reach LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per the rocket equation, delta-v is proportional to the log of the propellant mass ratio, but also proportional to the exhaust velocity of the rocket engines or their specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid rocket boosters have relatively low specific impulse: 275 sec for Atlas V's SRBs. Liquid hydrogen engines have high specific impulse: 449 sec for the Atlas Centaur upper stage. Kerosene engines fall in between. &#xA;So depending on how heavily a launcher relies on (cheap but inefficient) solids, you can see that the mass efficiency of the launcher as a whole is going to be very different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also a lot of variation in how rocket structures are built, leading to a lot of variation in weight. The tankage can be separate vessels inside the stage fuselage, or the tank walls can serve as the stage; the structure can be cheap, durable, and heavy, or expensive, light, and fragile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, mass at launch -- particularly first stage mass -- is less important than launch cost, so a heavier but simpler to construct structure may be preferred.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-08-17T20:21:07.720" LastActivityDate="2015-08-17T20:21:07.720" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10675" PostTypeId="2" ParentId="10669" CreationDate="2015-08-16T15:55:16.090" Score="4" Body="&lt;p&gt;I've attached the image the Original Questioner referred to below.  I can't read the Cyrillic labels, but I can tell you that in this schematic&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;yellow = fuel&lt;/li&gt;&#xA;&lt;li&gt;blue = oxidizer&lt;/li&gt;&#xA;&lt;li&gt;pink/red = hot gas&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The object labeled 6 is a gas generator, and one can see that fuel and oxidizer combine / burn there to produce hot gas, which spins the turbine to drive the pumps.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/tCkT6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tCkT6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Aeroalias mentions, a number of ignition schemes are possible.  He didn't list one important one, though: electrical - as seen in this slide from a &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Main Engine orientation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cfzXE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cfzXE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-08-16T15:55:16.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10676" PostTypeId="1" AcceptedAnswerId="10680" CreationDate="2015-08-16T23:13:41.213" Score="6" ViewCount="1618" Body="&lt;p&gt;Quote from &lt;a href=&quot;https://space.stackexchange.com/a/10672/4660&quot;&gt;recent answer&lt;/a&gt;: &quot;Some rockets [can't be transported] horizontally unless their tanks are pressurized&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oh? How is transportation handled then, and which rockets are like this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets are made only just strong enough to withstand the forces they are subjected to, otherwise the 1% to 3% or so that is available for payload vanishes to nothing. This I understand. I went looking for examples of a few different rocket stages being transported:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZXCe6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZXCe6.jpg&quot; alt=&quot;delta 4 stage on trailer bed&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Delta 4 stage, courtesy &lt;a href=&quot;http://spacenews.com/timing-of-russian-engine-ban-puts-ula-air-force-in-a-bind/&quot; rel=&quot;nofollow noreferrer&quot;&gt;spacenews&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZV88f.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZV88f.jpg&quot; alt=&quot;Proton M stage on railroad car&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;Proton M stage, courtesy &lt;a href=&quot;http://www.americaspace.com/?p=44200&quot; rel=&quot;nofollow noreferrer&quot;&gt;americaspace&lt;/a&gt; and Roscosmos&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5RjMS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5RjMS.jpg&quot; alt=&quot;Falcon 9 stage on trailer bed&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;Falcon 9, from &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/2sfg3z/falcon_9_core_dscovr_on_the_move/&quot; rel=&quot;nofollow noreferrer&quot;&gt;reddit&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9s, Delta 4s, and also Atlas stages are supported the same way - at two points, near the top of the stage, and just above the engines. I'm not sure what is happening with the Proton. Maybe that belongs in a separate question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, those are pretty long unsupported spans. Are they cases where the tanks are pressurized,thus adding rigidity? What do they put in the tanks? How are they placed on the trailer beds, and taken off them?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-08-17T11:34:23.560" Title="How are fragile rocket stages, built to withstand vertical stress, transported horizontally?" Tags="|rockets|transport|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10678" PostTypeId="1" CreationDate="2015-08-17T04:47:06.877" Score="5" ViewCount="757" Body="&lt;p&gt;What is the most recent and comprehensive lunar craters catalog which contains information about their depths as well as diameters?&lt;/p&gt;&#xA;" OwnerUserId="11373" LastEditorUserId="49" LastEditDate="2015-08-17T13:03:39.493" LastActivityDate="2015-08-17T13:03:39.493" Title="Catalog of lunar crater depths and diameters" Tags="|the-moon|planetary-science|reference-request|crater|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10680" PostTypeId="2" ParentId="10676" CreationDate="2015-08-17T11:34:23.560" Score="5" Body="&lt;p&gt;It is true that some of the rockets, especially the initial Atlas family of launchers, are to be pressurized even during storage or they will collapse under their own weight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, these are simply &lt;a href=&quot;https://en.wikipedia.org/wiki/Balloon_tank&quot; rel=&quot;nofollow noreferrer&quot;&gt;steel balloons&lt;/a&gt; that will retain their shape and rigidity only under constant pressurization. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This 'steel balloon' design was essentially adopted from the ICBMs on which the first launch vehicles were developed. The first thermonuclear weapons were heavy and had to reach Soviet Union when launched from continental US. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rozsF.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rozsF.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photo Courtesy: www.alternatewars.com&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This meant that the launch vehicle should have a number of stages or be exceptionally light. The designers used a balloon tank to achieve this in &lt;a href=&quot;https://en.wikipedia.org/wiki/SM-65_Atlas&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas ICBM&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case of the ICBMs, they were constantly in a pressurized state, using Nitrogen. In case of Atlas ICBM (and launchers), the tanks should have Nitrogen at 34 kPa for rigidity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, with advances in materials and manufacturing techniques, the tanks no longer need pressurization (&lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_%28rocket_stage%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centaur&lt;/a&gt; stage is an exception).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During storage and transport, these tanks were filled with Nitrogen. However, present day tanks can be stored and transported without pressurization, and most of them use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Isogrid&quot; rel=&quot;nofollow noreferrer&quot;&gt;isogrid&lt;/a&gt; design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gJJ73.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gJJ73.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photo Courtesy: SpaceX&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some launchers somewhere in between these two cases. For example,  Falcon 1 first stage can be transported without pressurization; however, it gains additional strength when pressurized for flight.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-08-17T11:34:23.560" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10681" PostTypeId="2" ParentId="10678" CreationDate="2015-08-17T11:43:37.550" Score="3" Body="&lt;p&gt;Lunar crater catalogs usually offer the &lt;a href=&quot;http://www.planetary.brown.edu/html_pages/LOLAcraters.html&quot; rel=&quot;nofollow&quot;&gt;latitude, longitude and diameter&lt;/a&gt; of the craters as only these can be measured directly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the topography has been used to &lt;a href=&quot;http://astrogeology.usgs.gov/search/details/Moon/Research/Craters/GoranSalamuniccar_MoonCraters/&quot; rel=&quot;nofollow&quot;&gt;determine the depth&lt;/a&gt; in some cases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://the-moon.wikispaces.com/Catalogs+of+Lunar+Craters&quot; rel=&quot;nofollow&quot;&gt;This&lt;/a&gt; wiki has a history and list of lunar craters.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-08-17T11:43:37.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10685" PostTypeId="1" CreationDate="2015-08-17T15:52:37.937" Score="3" ViewCount="564" Body="&lt;p&gt;I have a simple question: Why isn't space junk orbiting Earth used for gravity boosts to launch small cubesats into higher orbits or to other planets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know the current gravity boost and gravity braking math is built from numerical solutions to the 3 and 4 body problem in astrodynamics, but couldn't something similar be done in Low Earth Orbit between space junk and a small satellite (maybe a cubesat or kicksat)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From my understanding, if the cubesat is launched into orbit opposing the space junk, if it makes a close enough pass to the space junk, the cubesat can get a massive velocity boost (almost 8 km/s) as its orbit is changed by the space junk. Am I missing something here? (maybe overestimating the gravitational influence of the space junk?)&lt;/p&gt;&#xA;" OwnerUserId="11376" LastEditorUserId="25" LastEditDate="2015-08-17T16:07:29.830" LastActivityDate="2015-09-16T20:49:05.587" Title="Why can't cubesats get gravity assists from space junk in Low Earth Orbit?" Tags="|orbital-mechanics|debris|gravity-assist|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10686" PostTypeId="2" ParentId="10685" CreationDate="2015-08-17T16:06:59.623" Score="3" Body="&lt;p&gt;It could theoretically be done, but would be very difficult to line up correctly, with very high consequences for failure (Collision of the two objects), and wouldn't do much if anything to change the speed. The mass of space junk is so small that it really wouldn't have much of an effect on the orbital speed of said space junk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, for this to work it would require a much denser satellite than is typically found, one that likely would survive reentry. This satellite would then have to be perfectly lined up with the receiving satellite. As there is almost no gravity from such a small object, any force impact would be minimal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you had some massively dense object, to line the two objects up you'd have to use fuel. Even the smallest thrust to accomplish this would almost certainly be more than you'd gain from the maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity assists only work with really massive objects. Planets are the usually required size to have it work. Large moons will also work, but only slightly. Space junk, well, any effect would be extremely minimal.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-16T20:49:05.587" LastActivityDate="2015-09-16T20:49:05.587" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10687" PostTypeId="1" AcceptedAnswerId="13713" CreationDate="2015-08-17T17:05:11.107" Score="9" ViewCount="325" Body="&lt;p&gt;Near and on the surface of the Moon, there is not only radiation emanating from the Sun (solar wind, CME, SPE), but also deep space (cosmic) radiation.&#xA;The cosmic radiation also interacts with the nuclei of the elements comprising the lunar surface minerals and causes &lt;em&gt;secondary radiation&lt;/em&gt;, like gamma and neutron radiation. This would affect the radiation dose calculation for astronauts while walking on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have there been any measurements on neutron radiation performed by the Surveyor (or later) missions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to derive an upper bound on neutron radiation energies and flux from models of interaction of nuclei with cosmic radiation?&lt;/p&gt;&#xA;" OwnerUserId="716" LastEditorUserId="49" LastEditDate="2015-08-17T17:13:00.243" LastActivityDate="2016-01-26T15:08:31.623" Title="Did the Surveyor missions measure secondary radiation?" Tags="|the-moon|apollo-program|radiation|cosmic-radiation|surveyor-program|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10688" PostTypeId="1" CreationDate="2015-08-17T18:27:14.810" Score="2" ViewCount="244" Body="&lt;p&gt;My understanding is that it's easier to send something out of the solar system than into the sun due to the need to overcome the speed at which the earth moves around the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding of planetary flybys is that the spacecraft is pulled forward when headed towards the planet and is pulled backwards upon leaving. Net gain: 0. However let's imagine the solar system as a flat disc and we're observing from the top (read north pole of earth is facing us). The spacecraft approaches Jupiter and passes it on its right side. The difference in speed in the direction it was going is still 0, however Jupiter has pulled it &quot;left&quot; on both approach and retreat which means the spacecraft is headed &quot;left&quot; in addition to its original speed. The new vector is faster than the old (though in a different direction).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First question:&#xA;Can't following a planet in its orbital path produce a speed boost? The planet is moving away from you so it seems like you would 'fall' farther than if the planet was stationary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second question:&#xA;If you fly by the planet on the 'left' side (again viewed from the top) the planet is moving towards you. You have angular momentum with respect towards the sun from when your rocket left earth. But if you fly by Mars on the left side, this momentum is lessened. Continue on and go by any other planets you can on the same side. When you hit the last planet, orbit around and head back in the direction you came again passing planets on the same side. If you pass enough of them wouldn't you end up removing all momentum with respect to the sun and eventually be able to hit it?&lt;/p&gt;&#xA;" OwnerUserId="6000" LastEditorUserId="6816" LastEditDate="2015-08-17T18:50:22.170" LastActivityDate="2015-08-17T18:50:22.170" Title="Limitations of gravitational assist flybys" Tags="|gravity-assist|flyby|escape-velocity|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10692" PostTypeId="1" AcceptedAnswerId="10708" CreationDate="2015-08-18T01:25:24.460" Score="10" ViewCount="914" Body="&lt;p&gt;It is well known there is a lot a space junk orbiting the Earth. From previous questions posted on this site it appears that if it were possible to establish a space elevator cable for Earth it would have to be very long. Also, it would need to be placed through the space junk zone orbiting the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In such a situation could impact by space junk/debris be serious enough to make a space elevator impractical?&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2017-10-22T22:30:25.000" Title="Impact of space junk on Earth based space elevators" Tags="|debris|space-elevator|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10693" PostTypeId="2" ParentId="10670" CreationDate="2015-08-18T02:23:26.320" Score="4" Body="&lt;p&gt;I believe Russell Borogove's answer is correct but maybe could be stated a little more directly:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason your 2 factors of payload mass and liftoff mass don't correlate is that you are ignoring the other factor in the equation.  The 3 main factors in the rocket equation are &lt;strong&gt;mass ratio&lt;/strong&gt;, &lt;strong&gt;delta-v&lt;/strong&gt;, and &lt;strong&gt;specific impulse&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since your delta-v is basically fixed (to LEO) you are trying to correlate mass ratio without using specific impulse.  If you can calculate a back of the envelope value of system specific impulse for the launchers in your list, I think your data will make a lot more sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a simple thought experiment- take 2 boosters that deliver the exact same payload weight to LEO.  One has a system specific impulse of 300 and the other has a system specific impulse of 450.  The one with bad engines is going to be enormously bigger at liftoff and yet it delivers the exact same payload, so on your chart these data points would look uncorrelated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd expect to see a sheaf of lines on your chart - for launchers with similar system specific impulse, the payload mass and liftoff mass would be somewhat correlated.  Launchers with a different system specific impulse would follow a different curve.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="195" LastEditDate="2015-08-18T15:21:28.853" LastActivityDate="2015-08-18T15:21:28.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10694" PostTypeId="2" ParentId="10550" CreationDate="2015-08-18T05:17:25.070" Score="15" Body="&lt;p&gt;The &lt;em&gt;only&lt;/em&gt; spacecraft that I'm aware of that were fully sterilized were the Viking 1 and Viking 2 landers.  They went through &lt;a href=&quot;http://planetaryprotection.nasa.gov/methods&quot;&gt;dry heat microbial reduction&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That page notes that some parts of the Beagle 2 lander were sterilized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MSL (Curiosity) drill bits and other components were sterilized, though the drill bits were later exposed to a clean-room environment without being re-sterilized.  That &lt;a href=&quot;http://www.space.com/13783-nasa-msl-curiosity-mars-rover-planetary-protection.html&quot;&gt;turned out to be ok though&lt;/a&gt;, due to the nature of the selected landing site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other US Mars landers were simply cleaned and assayed to verify the bioburden limits (which are not zero).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/SP-4902/ch6.htm&quot;&gt;Parts of the early Rangers (1-5) were sterilized and had gaseous treatments&lt;/a&gt;.  They all failed, blamed in part on those processes.  The direction was changed in 1962 to not apply either dry heat or gaseous sterilization to lunar probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is difficult to know what exactly the Soviets did to Moon and Mars vehicles.  &lt;a href=&quot;http://www.nasa.gov/pdf/607072main_WhenBiospheresCollide-ebook.pdf&quot;&gt;This book&lt;/a&gt; describes some attempts to find out.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Gause assured Sagan that mission staff had sterilized Luna II’s&#xA;  instrument package and its third stage carrier rocket, both of which&#xA;  had impacted the Moon. But when pressed for details, Gause said some&#xA;  curious things, claiming “that his knowledge of the sterilization did&#xA;  not exceed that published in Izvestia.&quot; He said that the sterilization&#xA;  methods were those known to every graduate student in microbiology at&#xA;  the University of Chicago, and to every manufacturer of canned food,&#xA;  but he would not be more specific than that.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For Soviet Mars spacecraft:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Vashkov claimed that individual parts of landers were sterilized by&#xA;  heat (although he did not give the temperature or duration of the&#xA;  procedure) or by a radiation dose of&#xA;  2.5 millirads, depending on the characteristics of the part. The landers were assembled in cleanrooms, and individual parts were&#xA;  carefully cleaned, probably with hydrogen peroxide. UV light was&#xA;  applied during assembly as an additional sterilant. Finally, before&#xA;  launch, Soviet mission personnel exposed the entire lander to a methyl&#xA;  bromide–ethylene oxide gas mixture for 6 hours at 50°C.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-08-18T21:49:12.763" LastActivityDate="2015-08-18T21:49:12.763" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10695" PostTypeId="2" ParentId="10670" CreationDate="2015-08-18T08:34:12.827" Score="4" Body="&lt;p&gt;In addition to the other answers, larger rockets are more efficient:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Larger tanks have a better volume-to-surface ratio, so there's less structural weight per kg of content. &lt;/li&gt;&#xA;&lt;li&gt;Some parts on a rocket don't scale up when the rocket gets larger. For instance the guidance system on a Saturn V isn't 14 times larger than that of a Falcon 9. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And speaking of the Falcon 9: the v1.1 in your chart has its payload listed for the 'reuse the first stage' flight mode. In expendable mode, you can expect the payload ration to be slightly better than the Falcon 9 v1.0. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-08-18T11:00:51.233" LastActivityDate="2015-08-18T11:00:51.233" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10696" PostTypeId="1" CreationDate="2015-08-18T08:36:35.420" Score="6" ViewCount="641" Body="&lt;p&gt;What would rockets look like if the Earth had no atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much easier would it be to launch satellites, and how much harder will it  be to launch returnable manned missions?&lt;/p&gt;&#xA;" OwnerUserId="11384" LastEditorUserId="168" LastEditDate="2015-08-18T12:15:36.557" LastActivityDate="2015-08-19T02:21:37.977" Title="Shapes of rockets on the Earth with no atmosphere" Tags="|rockets|crewed-spaceflight|atmospheric-drag|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10698" PostTypeId="2" ParentId="10670" CreationDate="2015-08-18T10:12:16.517" Score="2" Body="&lt;p&gt;I don't know if that adds anything useful, but lets see:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta v = c_e* ln(m_0/m_b) = c_e* ln(\sigma/\mu_L)&#xA;$$&#xA;with&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v$ : characteristic velocity (constant for fixed orbits)&lt;br&gt;&#xA;$c_e$ : velocity of the rocket exhaust&lt;br&gt;&#xA;$m_0$ : initial total mass&lt;br&gt;&#xA;$m_b$ : burnout mass&lt;br&gt;&#xA;$\sigma$  : structural mass ratio [σ=(mM+mS)/m0]&lt;br&gt;&#xA;$m_M$ : mass of rocket motor&lt;br&gt;&#xA;$m_S$ : mass of rocket structure&lt;br&gt;&#xA;$\mu_L$ : payload ratio [μL=mp/m0]&lt;br&gt;&#xA;$m_P$ : payload mass  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second part of the equation is the only practical correlation between total mass and payload mass that I am aware of.  &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;It's my first answer here at Space Exploration and I'm still a student, so please go easy on me :) &lt;/p&gt;&#xA;" OwnerUserId="8724" LastEditorUserId="8724" LastEditDate="2015-08-18T12:27:13.187" LastActivityDate="2015-08-18T12:27:13.187" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10699" PostTypeId="1" CreationDate="2015-08-18T11:26:04.673" Score="2" ViewCount="3840" Body="&lt;p&gt;Molniya orbits are always associated with Russian satellites. What advantage do they have over other orbits?&lt;/p&gt;&#xA;" OwnerUserId="11386" LastEditorUserId="168" LastEditDate="2015-08-18T12:17:38.923" LastActivityDate="2015-11-17T21:29:25.780" Title="Why are Molniya orbits peculiar?" Tags="|artificial-satellite|orbit|russia|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10700" PostTypeId="2" ParentId="10699" CreationDate="2015-08-18T11:54:28.190" Score="10" Body="&lt;p&gt;Most countries use geostationary satellites for communications. They have the advantage that the satellite appears to be stationary in the sky, so you can use a simple fixed dish to communicate with it.&lt;br&gt;&#xA;Russia/the former USSR has a lot of land at high latitudes. A geostationary satellite would need a lot of transmitter power to be usable there. A &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot;&gt;Molniya orbit&lt;/a&gt; fixes that by having the satellite at a high inclination so it has a good view of a most of Russian territory at the same time. The orbit is also highly elliptical so it spends most of its time in a good position. &#xA;The drawbacks are that you need a tracking dish for communications, and you need 3 satellites to have continuous coverage. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-08-18T11:54:28.190" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10701" PostTypeId="2" ParentId="10699" CreationDate="2015-08-18T12:23:16.110" Score="4" Body="&lt;p&gt;This orbit allowed you to carry 1600 kg of payload, instead of the 100 kg into geostationary orbit possible under the same launch conditions, i.e. Baikonur in the 1960s with rocket 8К78.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://geektimes.ru/post/260316/&quot; rel=&quot;nofollow&quot;&gt;Russian source&lt;/a&gt; (&lt;a href=&quot;https://translate.google.com/translate?sl=auto&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;u=http%3A%2F%2Fgeektimes.ru%2Fpost%2F260316%2F&quot; rel=&quot;nofollow&quot;&gt;Google Translate this URL&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="11384" LastEditorUserId="6816" LastEditDate="2015-08-19T05:19:51.563" LastActivityDate="2015-08-19T05:19:51.563" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10702" PostTypeId="2" ParentId="10696" CreationDate="2015-08-18T13:27:17.497" Score="1" Body="&lt;p&gt;Fundamentally, the atmosphere of the earth affects the launch vehicle in two ways:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The drag it causes, and&lt;/li&gt;&#xA;&lt;li&gt;The friction caused by the air molecules, which heats the launcher.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;A simple rocket equation shows how the thrust produced by the rocket is used to overcome the forces acting against it. The thrust, &lt;span class=&quot;math-container&quot;&gt;$T$&lt;/span&gt; can be given as,&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$T = ma + mg + F_D$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where m is the mass of the rocket(and payload), a i the acceleration, g is the acceleration due to gravity and &lt;span class=&quot;math-container&quot;&gt;$F_D$&lt;/span&gt; is the drag force acting on it.&lt;/p&gt;&#xA;&lt;p&gt;If there is no atmosphere, the Drag term will not be there and the excess thrust can be used to increase the acceleration. Also, because there is no atmosphere, there is no need for a heat shield and this further reduces mass of the rocket. So, basically, we can  reach higher altitudes with lesser fuel or use less fuel for reaching the same altitude.&lt;/p&gt;&#xA;&lt;p&gt;As for landing, it is instructive to see how NASA landed in MARS, which has about 1&lt;span class=&quot;math-container&quot;&gt;$\%$&lt;/span&gt; of earth's atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;For landing the &lt;a href=&quot;https://en.wikipedia.org/wiki/Phoenix_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phoenix&lt;/a&gt; in MARS, NASA had to use a two stage system- a parachute first slowed the spacecraft down and then retro rockets were used to land it.&lt;/p&gt;&#xA;&lt;p&gt;If there is no atmosphere, it is useful in one way because there is no reentry heating of the spacecraft; however, the only way to land will be using retro rocket thrusters.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YMtIa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YMtIa.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Photo Credit: NASA/JPL/Corby Waste&lt;/p&gt;&#xA;&lt;p&gt;This will make the system much more complicated. Note that I'm comparing only the usual single-use systems, not reusable space systems like the shuttle.&lt;/p&gt;&#xA;&lt;p&gt;So, if we were to consider only the launch/recovery, if there is no atmosphere, it will be comparatively much easier to launch the rockets into space while it will be as much if not more difficult in the return leg.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-18T13:27:17.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10703" PostTypeId="2" ParentId="10696" CreationDate="2015-08-18T14:21:42.170" Score="10" Body="&lt;p&gt;Atmospheric drag and gravity losses account for about 1.5-2km/s of the normal 9.4-10km/s launch-to-LEO budget. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the trajectories we use, the atmospheric drag accounts for only a small portion of that, but we'd use more efficient trajectories and lower orbital altitudes if we didn't have to worry about the atmosphere! So at a guess, we'd need 8.5km/s instead of 10km/s of ∆v, which because of the nonlinear nature of the rocket equation, suggests you'd need only about 55-60% as much rocket for a given payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the design of Earth-launched rockets is heavily constrained by the atmosphere. They're made tall and skinny to minimize drag, and have a streamlined skin around their tankage and payload, all of which adds weight. A rocket made to launch from a vacuum world would be made mostly of big spherical tanks attached to a minimal frame. The more compact shape would lead to a lot of miscellaneous weight savings -- shorter cable runs, propellant lines, etc -- which, while individually small, would add up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launchers could use efficient, light-weight, low-pressure, liquid hydrogen fueled engines from the first stage, as well; system specific impulse gets better, rockets get lighter, everyone's happy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Return missions would be just brutal, though. In our manned missions, atmospheric braking provides essentially all the deceleration from orbital speed. Without the atmosphere, if you needed 8.5km/s to get up, you need 8.5km/s to get back down in one piece. To land a one man capsule like Mercury you might need to get something like 50 tons of payload into orbit!&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-08-18T14:21:42.170" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10704" PostTypeId="1" AcceptedAnswerId="10705" CreationDate="2015-08-18T14:30:06.020" Score="5" ViewCount="341" Body="&lt;p&gt;If I understand correctly, the altitude of low Earth orbit is as low as possible and yet high enough to avoid air drag for few orbits (numbers of orbits depending of the orbit's purpose, a parking orbit being quite low compare to a space station's orbit)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For planets or other celestial objects without atmosphere (e.g. Mercury, Moon), air drag is out of concerns. Thus how is the altitude for low orbit chosen?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastActivityDate="2015-08-19T00:07:32.380" Title="What is the altitude of low planetary orbit for celestial object without atmosphere?" Tags="|orbital-mechanics|orbit|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10705" PostTypeId="2" ParentId="10704" CreationDate="2015-08-18T14:49:31.817" Score="6" Body="&lt;p&gt;It depends on the goals of the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For photographic missions, for example, there's a tradeoff between coverage area and image resolution. The lower you are, the better the pictures, but the more pictures you need to map the entire body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For very low orbits, note that celestial bodies aren't perfect spheres; besides the obvious concern about hitting a mountain, the irregular distribution of mass within the body perturbs the orbit; higher orbits minimize this effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbital period is often a significant factor. A low orbit around the moon takes about two hours; if you don't want to have your probe go in and out of contact every two hours, use a higher orbit. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-08-18T14:49:31.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10706" PostTypeId="2" ParentId="10704" CreationDate="2015-08-19T00:07:32.380" Score="4" Body="&lt;p&gt;In the particular case of the Moon, there is a disturbance that is not an atmosphere that will take you out of orbit and crash you into the body just the same.  That is the irregular gravity field of the Moon.  There is a tradeoff between orbital lifetime, fuel required to stay in orbit, and orbit altitude.  The lower you go, the less variation you can tolerate in your orbit in order to not intersect the surface, and so the more fuel you need per month to reduce those variations to an acceptable level.  From what I've seen in typical lunar orbit mission designs, that resulting effective low Lunar orbit altitude is about 50 km.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-19T00:07:32.380" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10707" PostTypeId="2" ParentId="10696" CreationDate="2015-08-19T02:21:37.977" Score="8" Body="&lt;p&gt;If Earth had no atmosphere I think you would see a far more radical change to spaceflight that the rocket redesigns other posters are talking about.  While rockets might have been used at the dawn of the space age I think we would be using something akin to maglev trains for launch.  Take a big &lt;strong&gt;flat&lt;/strong&gt; area and put a very, very long track on it.  Unlike a train it would boost at rocket-type accelerations.  You boost your craft until it reaches the velocity of an orbit with perigee at 0 and apogee at the desired altitude.  The craft heads off into space, the train slows and is recovered.  When it reaches apogee it does a circularization burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your accuracy is good enough you can even use a similar system to land--although I would do so with cables to grapple the returning spacecraft rather than attempting to actually dock with the train.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-08-19T02:21:37.977" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="10708" PostTypeId="2" ParentId="10692" CreationDate="2015-08-19T14:23:33.420" Score="7" Body="&lt;p&gt;The space elevator basically consists of a base station that acts as an anchor, a long cable for movement of 'climbers' and a counterweight to keep the cable in place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The length of the cable is determined by the intended purpose. Below the altitude of geostationary orbit (~ 35,786 km), the gravity is more than the centrifugal force and above that the centrifugal force is more than the gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j9ogd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j9ogd.jpg&quot; alt=&quot;Space Elevator&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: Wikimedia Commons: User:Booyabazooka&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A space elevator constructed as shown above will pass through the atmosphere, the earth satellite orbits and the Van Allen radiation belts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The debris in the low earth orbit is but one of the hazards faced by the space elevator. Aircrafts, meteorites and orbiting satellites all can potentially snap the cable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible to design the satellites so that they avoid the cable(of course you would have to replace every one out there). The orbital debris is another thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are about 6000 tonnes of space junk in orbit and most of them are in the LEO orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZihcD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZihcD.jpg&quot; alt=&quot;Orbital debris in Low Earth Orbit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image Source: NASA&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are more than &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/faqs.html&quot; rel=&quot;noreferrer&quot;&gt;21,000&lt;/a&gt; orbital debris of diameter 10 cm or more in orbit, which can cause serious damage to the cable as they usually travel in speeds excess of 10 km per second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the fact that the elevator cable will be practically stationary and the very high number of debris in orbit, it is certain that one of them will hit the elevator cable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case, the cable will snap and more importantly, the counterweight will simply fly out of the earth orbit. Considering the probable impact height (~1000 km) and the total cable height (&gt;36000 km), the entire system will have to be rebuilt again. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a serious safety issue for the vehicles using the space elevators during that time as well as they most probably will not have a propulsions stem of their own (this being the most important advantage of space elevator)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In conclusion, the safety of the elevator cable from impacts is one of the most important challenges that has to be solved if the system is to become practical.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-08-19T14:23:33.420" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10709" PostTypeId="2" ParentId="1827" CreationDate="2015-08-19T17:03:24.627" Score="-3" Body="&lt;p&gt;Think of pressure and temperature as relating to an overall density.  The more pressure the more stuff you can get to the place where temperature expansion kicks into Newton's third law.  Perhaps it helps to think of unburned fuel as being the same thing as already burned fuel which had been compressed back into the same space - now wouldn't that be a lot of pressure, and if you can multiply that before ignition (the initial pressure) then, well, you've got lift off!&lt;/p&gt;&#xA;" OwnerUserId="11395" LastActivityDate="2015-08-19T17:03:24.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10710" PostTypeId="2" ParentId="10692" CreationDate="2015-08-19T22:37:44.520" Score="1" Body="&lt;p&gt;I've been thinking about this very thing for some reason.  I think, in terms of engineering and feasibility, cleaning up all that debris would be &lt;em&gt;simpler&lt;/em&gt; than building a space elevator.  So I don't think this is really all that big of a deal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, supposing for whatever reason we do not clean up LEO (or to protect it against any case) I'd think large sections of the elevator would have to be radiation shielded, especially where it passes through the van allen.  We're also most assuredly needing electricity to course through the thing, so I suppose we could 'kill two stoned birds' by embedding or paralleling the elevator with superconducting material and sending a charge through it.  I just don't know how much we'd need produce a magnetic field big enough to, not only shield from rads, but deflect incoming debris as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the gundam plots was centered around vast photo-voltaic power plants in orbit, connected to the earth by such elevators.  This could also necessitate the superconducting core.  My point is, I'm sure if we ever got around to building the damn thing, a solution will be generated almost implicitly.&lt;/p&gt;&#xA;" OwnerUserId="11162" LastActivityDate="2015-08-19T22:37:44.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10711" PostTypeId="1" AcceptedAnswerId="10744" CreationDate="2015-08-20T05:49:25.570" Score="3" ViewCount="433" Body="&lt;p&gt;We have &lt;em&gt;intergalactic&lt;/em&gt; to describe the area between galaxies. And &lt;em&gt;interstellar&lt;/em&gt; space is between star systems. And &lt;em&gt;interplanetary&lt;/em&gt; is between planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is the term &quot;interlunar&quot; applicable to the space between moons other than Luna?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a satellite was to transfer between Galilean moons, would it perform &lt;em&gt;interlunar&lt;/em&gt; burns?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I ask because we don't say intermilkyway or intersol or interearth. So why do we say interlunar?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="49" LastEditDate="2015-08-20T07:56:40.880" LastActivityDate="2015-08-24T08:44:58.590" Title="Is &quot;interlunar&quot; applicable to the moons of other planets?" Tags="|terminology|moons|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10713" PostTypeId="1" CreationDate="2015-08-20T07:38:21.967" Score="7" ViewCount="822" Body="&lt;p&gt;When a launcher's upper stage fails to put a commercial communication satellite in geostationary orbit, it cannot be used as intended. But could it be used in satellite-to-satellite communication instead even if orbits at much lower altitude? For example to enhance the capacity of the Tracking and Data Relay satellite network which is used to communicate with the space station among other things. Or is it impractical to change its pointings multiple times per orbit or do they lose much of their solar power or get damaged from passing through the van Allen Belt?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-08-20T14:46:06.013" LastActivityDate="2015-08-21T15:52:54.617" Title="Could a communication satellite that fails to reach GEO be repurposed?" Tags="|artificial-satellite|orbit|failure|geostationary|communication-satellite|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10714" PostTypeId="2" ParentId="10711" CreationDate="2015-08-20T07:50:20.200" Score="5" Body="&lt;p&gt;&lt;em&gt;Interjovian&lt;/em&gt; (alt. &lt;em&gt;interjove&lt;/em&gt;) is most commonly used term for maneuvers within the orbital space of the Jovian system (&lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2002/sturtevant/&quot; rel=&quot;noreferrer&quot;&gt;example use&lt;/a&gt;). &lt;em&gt;&lt;a href=&quot;http://dictionary.reference.com/browse/cis-&quot; rel=&quot;noreferrer&quot;&gt;Cis-&lt;/a&gt;&lt;/em&gt; prefix followed by an adjective for a specific &lt;a href=&quot;https://en.wikipedia.org/wiki/Galilean_moons&quot; rel=&quot;noreferrer&quot;&gt;Gelilean moon&lt;/a&gt; within orbital space of which such maneuvers are performed could also work. E.g. &lt;em&gt;cis-ionian&lt;/em&gt;, &lt;em&gt;cis-europan&lt;/em&gt;, &lt;em&gt;cis-ganymedean&lt;/em&gt;, &lt;em&gt;cis-callistonian&lt;/em&gt;. If you're unsure about possible adjectives to use, Wikipedia is quite handy for that and listing most common ones in the right column of (nearly) every specific celestial body. Mind that unlike proper names, their adjectives are not necessarily capitalized, and the use of the hyphen seems to be optional, too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Interlunar&lt;/em&gt; doesn't quite work, since it's not specific to natural satellites of a single celestial body, so it could be interpreted as equivalent to &lt;em&gt;interplanetary&lt;/em&gt;. Similar could be said for suggested &lt;em&gt;intersatellite&lt;/em&gt;. Perhaps a better, more specific term could be &lt;em&gt;intergalilean&lt;/em&gt;, but I've not come across it in literature yet.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-08-20T07:50:20.200" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10715" PostTypeId="2" ParentId="10713" CreationDate="2015-08-20T14:01:57.313" Score="7" Body="&lt;p&gt;Possibly, but not likely. Geo satellites are made to work at that constant orbit, and getting it to work without being at the GEO orbit can cause a lot of problems. The radiation isn't the concern, but the various sensors on the spacecraft are meant to work at GEO altitude. Thermal issues are also of concern, as are the batteries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the tasks could be done, and in fact have been attempted in the past. The most famous example of this was &lt;a href=&quot;https://en.wikipedia.org/wiki/PAS-22&quot;&gt;PAS-22&lt;/a&gt;, which failed to obtain the correct orbit, was sold to a company by the insurance company, which created a way to swing by the moon to obtain the correct orbit, allowing it to have some service. Other ideas were considered as well, but that made the most sense. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-20T14:01:57.313" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10716" PostTypeId="2" ParentId="10713" CreationDate="2015-08-20T14:06:30.757" Score="6" Body="&lt;p&gt;The main reason for sending communication satellites to GEO is that their position is stationary to ground observers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case the satellite fails to reach orbit, the ground control usually tries to  raise the orbit using on-board fuel, though the life of the satellite may be reduced. For example when PSLV C1 failed to place IRS-ID on orbit, the satellites on-board fuel was used to raise orbit (this was in LEO, though the principle is same).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if this is not possible, the mission is usually classified a failure and satellite, in all probability becomes a space junk due to various reasons.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Satellite to Satellite communication is not possible for practical reasons. It is difficult to change the orientation of the satellite continuously as it uses up on-board fuel, resulting in orbital decay.&lt;/li&gt;&#xA;&lt;li&gt;Also, unless the satellite is placed in another stable orbit, tracking is very difficult and communicating becomes very difficult.&lt;/li&gt;&#xA;&lt;li&gt;Another issue is that the communications method of the two satellites (or space station for that matter) should be compatible. Satellites are usually designed for communicating with the ground station, not other satellites.&lt;/li&gt;&#xA;&lt;li&gt;Even the ground communication is difficult for obvious reasons. A satellite in GEO is expected to be stationary and the ground station is designed with this in mind. In case of Polar satellites, they pass over a set of ground stations at fixed times per day. However, for an orbit somewhere in between, it is difficult to arrange for a set of ground tracking stations to track and communicate with the satellite.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Reduction in orbit altitude doesn't change the solar power available by much. Also, the satellites are in Van Allen belts anyway even inside GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main problem with failed launches is the failure to get inserted into a stable orbit and ground communication.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-08-20T14:06:30.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10717" PostTypeId="1" AcceptedAnswerId="10718" CreationDate="2015-08-20T20:31:57.367" Score="8" ViewCount="624" Body="&lt;p&gt;Autonomous navigation started in early phases of deep space exploration as a provision to achieve accuracy for planet/natural satellite targeting and approach, where radio tracking accuracies were low, as well as ephemeris data was not quite up to accuracy marks. AutoNav and OpNav were the initial systems used by DS1, New Millenium and Cassini among others. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These autonomous techniques employed a camera which would scan primary body w.r.t an inertial background of fixed stars. Basically, to find the position of the spacecraft. Since then, various autonomous systems have been developed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, for orbit determination, total state (velocity and position vector in a reference frame) information is a prime necessity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, as far as I can tell, each of autonomous techniques, if employed for a deep space mission would require the support of Deep Space Networks for velocity determination. Is it so? &#xA;Are there any other provisions(Methods, Geometrical or others) to determine velocity or any component of velocity autonomously on-board? &lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2020-05-26T05:37:08.423" Title="Autonomous Navigation for deep space missions" Tags="|spacecraft|navigation|deep-space|tracking|deep-space-network|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10718" PostTypeId="2" ParentId="10717" CreationDate="2015-08-20T23:37:49.210" Score="11" Body="&lt;p&gt;Positions over time can be used to determine velocity, not just in the obvious way but more accurately than that using a model of the gravity field the spacecraft is moving in, dominated by the Sun for deep space vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible to develop a system to navigate a deep space vehicle fully autonomously, using only a camera for data and designing its own maneuvers or its own ion thrust profiles on-board to achieve an objective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I figure we will see common use of this capability when we have hundreds of deep space vehicles out there.  When we have something closer to a dozen, like we do now, it is not onerous to use (super accurate) radiometric tracking and design maneuvers on the ground.  So that's what we do now, which gives greater peace of mind.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-20T23:37:49.210" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10719" PostTypeId="1" CreationDate="2015-08-21T04:23:51.897" Score="7" ViewCount="178" Body="&lt;p&gt;Recently ISRO released 3D terrain models of Mars' Ophir Chasma terrain &lt;a href=&quot;http://www.isro.gov.in/pslv-c25-mars-orbiter-mission/3d-portrayals-of-ophir-chasma-terrain&quot; rel=&quot;nofollow&quot;&gt;as shown here&lt;/a&gt;. LiDAR is one instrument which helps in generating such 3D models. ISRO's MARS orbiter sensor payload does not have a LiDAR. Is it using any other instrument or is it calculating the depth information from just aerial images?&lt;/p&gt;&#xA;" OwnerUserId="11402" LastEditorUserId="25" LastEditDate="2015-08-21T11:56:29.590" LastActivityDate="2015-08-21T12:00:31.543" Title="How are these 3D terrain models of Mars generated?" Tags="|mars|isro|imaging|mapping|mars-orbiter-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10720" PostTypeId="1" AcceptedAnswerId="10724" CreationDate="2015-08-21T06:25:36.200" Score="16" ViewCount="1853" Body="&lt;p&gt;Has there ever been a successful satellite launch done by amateurs? I mean both building the satellite and a rocket to carry it.&lt;/p&gt;&#xA;" OwnerUserId="11396" LastEditorUserId="25" LastEditDate="2015-08-21T12:01:32.640" LastActivityDate="2023-09-02T04:49:32.490" Title="Amateur satellite launch" Tags="|launch|rockets|artificial-satellite|orbit|amateur-rocketry|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10721" PostTypeId="1" CreationDate="2015-08-21T07:03:35.717" Score="14" ViewCount="1971" Body="&lt;p&gt;Is there for example some L1 like libration point where the Hill spheres of the Sun and of the Alpha+Beta Centauri meet? And are Lagrange points between stars inside of a binary system, like Alpha and Beta Centauri, more or less stable because they are much more massive than planets?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="415" LastEditDate="2015-08-21T08:05:00.857" LastActivityDate="2021-09-05T12:13:41.420" Title="Are there interstellar Lagrange points?" Tags="|interstellar-travel|lagrangian-points|mass|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10722" PostTypeId="2" ParentId="10721" CreationDate="2015-08-21T09:48:18.893" Score="0" Body="&lt;p&gt;Yes, there are Lagrange points between starts, however, most are not stable enough to orbit around them because they are too far away from the stars.&lt;/p&gt;&#xA;" OwnerUserId="6834" LastActivityDate="2015-08-21T09:48:18.893" CommentCount="0" CommunityOwnedDate="2015-08-21T09:48:18.893" ContentLicense="CC BY-SA 3.0" />
  <row Id="10723" PostTypeId="2" ParentId="10721" CreationDate="2015-08-21T10:25:23.247" Score="15" Body="&lt;p&gt;Lagrange points do exist between stars. In case of single stars, they are too far away from the stars to have any practical effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in case of the binary stars, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Roche_lobe&quot; rel=&quot;noreferrer&quot;&gt;Roche Lobe&lt;/a&gt; has its apex located at L1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KTJ9v.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KTJ9v.png&quot; alt=&quot;Roche Lobe&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:RochePotential_color.PNG#/media/File:RochePotential_color.PNG&quot; rel=&quot;noreferrer&quot;&gt;RochePotential color&lt;/a&gt;&quot; by SamuelHon - Own work. Licensed under CC BY-SA 4.0 via &lt;a href=&quot;https://commons.wikimedia.org/wiki/&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case a star's surface extends beyond the Roche Lobe, it will lose the material  outside the Roche Lobe to its companion star through the first Lagrangian point.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastEditorUserId="11361" LastEditDate="2015-08-21T10:37:42.890" LastActivityDate="2015-08-21T10:37:42.890" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10724" PostTypeId="2" ParentId="10720" CreationDate="2015-08-21T11:38:55.810" Score="11" Body="&lt;p&gt;No, in fact, it's very difficult to even reach the threshold of space (50 miles or 100 km) by amateurs, they've never even gone close to the required velocity to achieve orbit. See the &lt;a href=&quot;https://en.wikipedia.org/wiki/Amateur_rocketry&quot;&gt;Amateur Rocket Wikipedia&lt;/a&gt; page. Basically, it's really really hard to build a rocket, taking a company like SpaceX years with many engineers to achieve success, and it just seems outside the range of amateurs at this time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As was mentioned in comments, Amateur satellites are always secondary payloads on larger missions. There have been a number of these missions launched.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-21T11:38:55.810" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10725" PostTypeId="2" ParentId="10719" CreationDate="2015-08-21T12:00:31.543" Score="3" Body="&lt;p&gt;There are a couple of ways these can come. In the case of the image you provided, I believe they are using known altitude that comes from &lt;a href=&quot;http://mola.gsfc.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;MOLA&lt;/a&gt; data which accurately found the elevation of the entire Martian surface, and using that to transform the image as it was. They are likely using some terrain features from the image to determine it's altitude as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two other ways to determine this information. The first is to use shadows to find the relative height of each surface, and appropriately manipulate that to get the right height. The second is to take two images, and use them much as our eyes do to see 3-d. There seems to only be a single image used to create these images, and the shadowing, while I suspect it might be used in part, likely isn't the only thing done with this image.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-21T12:00:31.543" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10726" PostTypeId="1" AcceptedAnswerId="10727" CreationDate="2015-08-21T13:30:21.027" Score="1" ViewCount="2456" Body="&lt;p&gt;I have seen this picture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HQ7ae.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HQ7ae.jpg&quot; alt=&quot;image.slidesharecdn.com/englishastronomie21-111216014333-phpapp01/95/hubble-telescope-photos-34-728.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://www.slideshare.net/lcurry12/english-astronomie21-10611120&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Hubble telescope Photos&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and was wondering how they managed to get both the moon and the earth in the same frame for a shot (a relative position/orbit circles would be nice) - or has this picture been altered or taken by something other than the Hubble telescope?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. - a quick &lt;a href=&quot;https://www.google.com/search?q=hubble+orbit+height&amp;amp;ie=utf-8&amp;amp;oe=utf-8&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google revealed an orbit height of 347 miles&lt;/a&gt; for the Hubble and &lt;a href=&quot;https://www.google.com/search?hl=en&amp;amp;source=hp&amp;amp;q=how%20far%20away%20is%20the%20moon&amp;amp;cts=&amp;amp;aq=f&amp;amp;oq=&amp;amp;aqi=&amp;amp;gws_rd=ssl&quot; rel=&quot;nofollow noreferrer&quot;&gt;a distance of 239 thousand miles&lt;/a&gt; for the Moon&lt;/p&gt;&#xA;" OwnerUserId="6137" LastEditorUserId="25" LastEditDate="2015-08-21T13:38:36.580" LastActivityDate="2016-07-09T11:24:03.447" Title="Is the Hubble's orbit larger than that of the moon?" Tags="|the-moon|orbit|low-earth-orbit|photography|hubble|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10727" PostTypeId="2" ParentId="10726" CreationDate="2015-08-21T13:33:40.027" Score="20" Body="&lt;p&gt;No, Hubble is in low Earth orbit, much lower than the Moon. The shuttle delivered it to orbit, and the Shuttle can't get near the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image you reference is very similar to one that came from &lt;a href=&quot;https://www.nasa.gov/feature/goddard/from-a-million-miles-away-nasa-camera-shows-moon-crossing-face-of-earth&quot;&gt;DSCOVR&lt;/a&gt;, which is at a point about a million miles from Earth between the Earth and Sun, which always sees the sunlit portion of the Earth. However, there are some differences that just don't quite make sense. It seems more likely that someone simply inserted a picture of the Moon over the Earth. One of the key ways you can tell this is that the Moon is brighter than Earth, while in reality, the moon is rather dark. In fact, the brightness of the Earth and Moon should be reversed, the Earth bright, and the Moon dim.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2015-08-21T17:51:53.570" LastActivityDate="2015-08-21T17:51:53.570" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10728" PostTypeId="2" ParentId="10713" CreationDate="2015-08-21T15:52:54.617" Score="2" Body="&lt;p&gt;In principle a part failed satellite could be repurposed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The technical obstacles already given are all quite plausible though none are necessarily &quot;game-over&quot; (though recall that the frequency band used by the failed satellite's payload would have to be compatible with the new use which for the satellite-satelite case which could well be the end of the matter). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A further non-technical difficulty is establishing a market for the services against the budget needed for continuing operations. Again, its not an insurmountable challenge. &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-08-21T15:52:54.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10729" PostTypeId="1" AcceptedAnswerId="10730" CreationDate="2015-08-21T16:09:29.803" Score="7" ViewCount="1230" Body="&lt;p&gt;Polar orbit is quite common for &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth_observation_satellite&quot; rel=&quot;noreferrer&quot;&gt;observation satellites&lt;/a&gt;. It seems logic to put probes whose goal is to observe Mars (pictures, cartography) on a polar orbit around Mars. But traveling from Earth to any Mars orbit is difficult enough to avoid targeting any orbit whose inclination is far away from the ecliptic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have we sent any observation satellites or any other kind of probe designed to orbit over Mars on a polar orbit?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="25" LastEditDate="2015-08-21T16:17:59.080" LastActivityDate="2015-08-21T17:54:57.087" Title="Do any spacecraft orbit Mars on a polar orbit?" Tags="|orbital-mechanics|mars|artificial-satellite|orbit|polar-orbit|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10730" PostTypeId="2" ParentId="10729" CreationDate="2015-08-21T16:13:08.713" Score="14" Body="&lt;p&gt;Targeting an inclination for a satellite is much easier than targeting a landing spot, and both are routinely done. The trick is to know exactly what you are aiming for, and take frequent small corrective steps to ensure it will happen. Here's a list of the objects I know orbit Mars in polar (Which I define as between 80-100 inclination) orbits:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars Reconnaissance Orbiter&lt;/li&gt;&#xA;&lt;li&gt;Mars Odyssey&lt;/li&gt;&#xA;&lt;li&gt;Mars Global Surveyor (Inactive)&lt;/li&gt;&#xA;&lt;li&gt;Mars Express&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In addition, MAVEN is in a near polar inclination (75), although not quite what I consider polar.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;From NASA's May 2015 story &lt;a href=&quot;https://www.nasa.gov/jpl/mars/traffic-around-mars-gets-busy&quot; rel=&quot;noreferrer&quot;&gt;Traffic Around Mars Gets Busy&lt;/a&gt;, here's a graphic depicting the relative shapes and distances from Mars for five active orbiter missions plus the planet's two natural satellites (Mars Global Surveyor is inactive and likely in safe mode since 2006):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Qqscd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qqscd.jpg&quot; alt=&quot;Active orbiters around Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2015-08-21T17:54:57.087" LastActivityDate="2015-08-21T17:54:57.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10731" PostTypeId="1" CreationDate="2015-08-21T20:07:31.637" Score="12" ViewCount="1402" Body="&lt;p&gt;Practical aspects of a total low energy transfer to the Moon have been seen in missions like GENESIS, which uses Weak stability Boundary legs of Earth and Sun to reach ESL-2. This four body model transfers justifies that: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is possible to use the unstable manifolds of the planar Lyapunov periodic orbits about the Sun-Earth L2 point to provide a low energy transfer from the Earth to the stable manifolds of planar Lyapunov periodic orbits around the Earth-Moon L2 point. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Thus transferring the spacecraft around EML2 which&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;act as separatrices in the energy manifold of the flow through the equilibrium point, provide the dynamical channels in phase space that enable ballistic captures of the spacecraft by the Moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Such Transfers have theoretically been proved to have 25-40% delta-v savings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, instead of going 1.5 million miles away, can we use just the Earth-Moon system? Dynamical nature of CRTBP in Earth-Moon system suggests that certain states in phase space if attained, can lead spacecraft to asymptotically approach L-points into periodic/quasiperiodic orbits (In case of Earth-Moon system, lets just have Jacobi Energy constant, just enough to open up zero-velocity surface at both EML-1 and EML-2) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, Homo/Heteroclinic connection between Lyapunov orbits between two L-points, like utilized in ARTEMIS allows us to traverse space in a way shown here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qO7sw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qO7sw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can we in intermediate stage of such trajectory execute a decay manoeuvrer to get captured around the Moon in some way (because I'm guessing, we cannot get ballistically captured by the Moon from EML-1) ?&#xA;What margin of delta-v would be needed in such case?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, is there a possibility of ballistic capture from a periodic orbit around EML-2, spacecraft being transferred to EML-2 from EML-1 as in the figure ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quoted Text reference: &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.307.2362&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low Energy Transfer to the Moon, W.S.Koon&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorDisplayName="user20636" LastEditDate="2018-08-18T16:18:40.680" LastActivityDate="2018-08-19T03:01:28.960" Title="Low Energy Transfer within Earth-Moon system" Tags="|orbital-mechanics|the-moon|lagrangian-points|low-energy-transfer|three-body|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="10732" PostTypeId="2" ParentId="2339" CreationDate="2015-08-21T21:32:20.730" Score="1" Body="&lt;p&gt;I was just looking at a clip from The Martian (2015) movie, and their space station looking spacecraft got me thinking about this very thing. I had an idea that I think has been covered in two separate ways but not together, and it doesn't involve any low leak seals to the rotating/stationary section because there wouldn't be any seals. The whole vacuum proof area should rotate, as it has been said, and then everything that needs be stationary would rotate like the solar panels and the heat dissipation radiators and antenna/dish, etcetera could easily rotate outside the vacuum safe section. Two or three rings on the outside of round sections of the hull that counter-rotate as necessary would do the trick. For the electronics, a metal disc rolling along a metal strip. if the engines are in a circular pattern it doesn't matter if they rotate. &lt;/p&gt;&#xA;" OwnerUserId="11408" LastActivityDate="2015-08-21T21:32:20.730" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10733" PostTypeId="1" AcceptedAnswerId="10734" CreationDate="2015-08-21T22:51:56.973" Score="5" ViewCount="249" Body="&lt;p&gt;From what I understand you can't use aerobraking on Mars for landing as the atmosphere is too thin. From what I understand Mars is quite problematic because the atmosphere is too thin to benefit from parachutes, but not thin enough like the moon where you can enter in whatever angle you want.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I wondered if there are any significant reductions in required energy to enter Venus compared to Mars. And also how it compares to the Earth. E.g. would the heat-shield need to be thicker or lighter. Will it be easier or harder to enter Venus than Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume parachutes would be more effective on Venus. &lt;/p&gt;&#xA;" OwnerUserId="11308" LastActivityDate="2015-08-22T02:31:25.747" Title="What are the difference in energy costs for entering Venus vs Mars vs the Earth?" Tags="|venus|aerobraking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10734" PostTypeId="2" ParentId="10733" CreationDate="2015-08-22T02:22:03.663" Score="12" Body="&lt;p&gt;Aerobraking is most definitely used at Mars.  Going from about 6000 m/s down to 100 m/s is done entirely with aerobraking, first with a heat shield, and then with a parachute.  The last bit from 100 m/s to about 1 m/s is done with rockets.  (There seems to be a lot of excitement about the bit at the end for some reason, but by then nearly all the work has already been done.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus entry velocities are higher, so it is much rougher on the heat shield.  Once you're past that part, the atmosphere is much thicker than Mars' and far enough down, much thicker than Earth's.  The atmosphere is so thick that a small metal structure can serve as a sufficient parachute for a slow descent to the surface.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-08-22T02:31:25.747" LastActivityDate="2015-08-22T02:31:25.747" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10737" PostTypeId="1" AcceptedAnswerId="10739" CreationDate="2015-08-22T11:48:55.813" Score="8" ViewCount="494" Body="&lt;p&gt;Often people will use &amp;quot;aerobraking&amp;quot; in the context of landing a space ship or probe on a planet with atmosphere. This appears to be a casual and technically incorrect usage, for example from &lt;a href=&quot;https://en.wiktionary.org/wiki/Appendix:Glossary_of_atmospheric_reentry&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wiktionary.org/wiki/Appendix:Glossary_of_atmospheric_reentry&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;aerobraking&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;when the free molecular gas of a planet's upper atmosphere is used to reshape the orbit of a spacecraft. Heating due to aerobraking is normally insignificant thus requiring no special thermal protection. The term aerobraking is often used incorrectly by people outside the aerospace industry.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;And also of relevance:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;aerocapture&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;when the continuum gas of a planet's atmosphere is used to dissipate the kinetic energy of a spacecraft entering from a heliocentric hyperbolic trajectory and which then skips out of the atmosphere into an elliptical orbit centered around the capturing planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I haven't found any contradiction to &lt;em&gt;aerobraking&lt;/em&gt; referring specifically to gentle orbital reshaping. But what then is the correct way to refer to using a planet's atmosphere as a brake during entry and descent?&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-08-22T20:15:04.580" Title="What is the word for using atmosphere to dissipate kinetic energy during reentry?" Tags="|terminology|atmospheric-drag|aerobraking|reentry|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10739" PostTypeId="2" ParentId="10737" CreationDate="2015-08-22T20:15:04.580" Score="18" Body="&lt;p&gt;I can't say that the terminology is consistent across all users, but where I work (at JPL) we use aerobraking to refer to many light dips to lower an orbit, aerocapture to refer to a single deep dip to bring a hyperbolic approach to an elliptical orbit, aeroentry or simply entry to refer to an entry into an atmosphere with no exit, and aeroassist as a general term for the use of an atmosphere to effect a change in trajectory.  Aerocapture has also been used to describe going from high-elliptical to nearly circular in a single pass, e.g. a lunar return to LEO.  There are other kinds of aeroassists, such as a hyperbolic to hyperbolic aerogravity assist (usually making use of a lifting body flying upside down).  There is also skip entry, which is a hyperbolic or high-elliptical (a la lunar return) approach with an entry and an exit, followed by a final entry.  Both Apollo and Shuttle were qualified for skip entry, in order to increase cross range, but neither used it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Entry is sometimes called reentry, humorously even when that is incorrect, such as for a vehicle entering at a planet it hasn't been to before.  (I suppose a Mars ascent could appropriately be called a re-exit.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have seen aerobraking used elsewhere as the generic term, instead of aeroassist.  I would not call that use incorrect.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-22T20:15:04.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10740" PostTypeId="1" CreationDate="2015-08-23T00:00:03.830" Score="3" ViewCount="209" Body="&lt;p&gt;This is about planets on which life as we know it could arise, and how different they might seem to Earth. By &quot;life as we know it&quot;, I mean life sharing the same chemical basis as our own - requiring liquid water and employing the same amino acid / nucleic acid chemistry, etc..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, there is the matter of a &quot;Goldilocks zone&quot; within which water can exist in liquid form on a planet's surface. But that's not a specific distance (or distance range) from the host star... it depends on how brightly the star is shining. So, some &quot;Earth-like&quot; planets might have a very different year length than Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there is the matter of a planet's gravity... enough to hold an atmosphere - also to allow liquid water to exist on its surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also the composition of the planet and its effect on density and therefore gravity, not to mention availability of the required elements for life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably some variables could be traded off against others - a hotter star might have its Goldilocks zone further away than a cooler one. A planet with higher gravity could contain an atmosphere and host liquid water at a higher temperature than one with lower gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point of my question is: how much can the various parameters like type of host star, distance from host star, composition of the planet itself, size/density, rotational period (length of day), etc. vary and be traded off one for another and still leave an environment suitable for the origin and evolution of life as we know it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example: can a star emit too much radiation at the wrong place in the spectrum e.g. too much UV or perhaps X-rays at a distance close enough to keep adequately warm? How long or short could a year be on a planet that can host life? How strong or weak might its surface gravity be? What could it be made of besides iron, nickel, and silicate rocks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Disregarding the local flora/fauna, how unfamiliar could the surface conditions of such a place seem to us?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2015-08-23T00:06:26.803" LastActivityDate="2015-08-23T10:36:52.330" Title="How Earth-like is &quot;Earth-like&quot;?" Tags="|planetary-science|" AnswerCount="1" CommentCount="1" ClosedDate="2015-08-26T14:22:03.560" ContentLicense="CC BY-SA 3.0" />
  <row Id="10741" PostTypeId="2" ParentId="10740" CreationDate="2015-08-23T10:36:52.330" Score="4" Body="&lt;p&gt;The term lacks, as far as I've read, an IAU definition, so its use has been vague.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, it's being applied to extrasolar rocky bodies under about 4 Earth masses, or with diameters under about 1.5 earth diameters, that are in or near the habitable zone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has referred to a number of bodies as &quot;Earth-Like&quot;... the most recent is &lt;a href=&quot;https://www.nasa.gov/ames/kepler/nasas-kepler-discovers-first-earth-size-planet-in-the-habitable-zone-of-another-star&quot; rel=&quot;nofollow&quot;&gt;Kepler 186F&lt;/a&gt;. It's estimated to be 1.11 earth diameters, and at the outer edge of the habitable zone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But with &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth&quot; rel=&quot;nofollow&quot;&gt;Kepler 452B&lt;/a&gt;, the more cautious term &quot;cousin to Earth&quot; is used... despite being 1.6 Earth diameters across, in the habitable zone, and having a 385 day year, and orbits a sun-like star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA, on their &lt;a href=&quot;http://planetquest.jpl.nasa.gov/news/207&quot; rel=&quot;nofollow&quot;&gt;Planetquest site&lt;/a&gt;, defines it thusly:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;About a dozen habitable zone planets in the Earth-size ballpark have been discovered so far -- that is, 10 to 15 planets between one-half and twice the diameter of Earth, depending on how the habitable zone is defined and allowing for uncertainties about some of the planetary sizes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It also mentions (in a long-winded way) that to be Earth-like requires it to be in the &quot;Goldilocks Zone&quot; (a.k.a. the habitable zone).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in general, a firm definition is lacking, but the weak one is 0.5 to 1.5 Earth diameters, presumed to be a rocky body, and in the habitable zone of the parent star. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/ames/kepler/nasas-kepler-discovers-first-earth-size-planet-in-the-habitable-zone-of-another-star&quot; rel=&quot;nofollow&quot;&gt;https://www.nasa.gov/ames/kepler/nasas-kepler-discovers-first-earth-size-planet-in-the-habitable-zone-of-another-star&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth&quot; rel=&quot;nofollow&quot;&gt;https://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://planetquest.jpl.nasa.gov/news/207&quot; rel=&quot;nofollow&quot;&gt;http://planetquest.jpl.nasa.gov/news/207&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="158" LastActivityDate="2015-08-23T10:36:52.330" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10742" PostTypeId="1" CreationDate="2015-08-23T21:46:53.663" Score="14" ViewCount="873" Body="&lt;p&gt;Researching small asteroids up close by use of robotic probes seems like an appealing near-future mission. One option is a sample return mission that goes to one large body, another is a mission where a number of smaller bodies are visited over time. In either case, the thing is to land, then launch again, and ultimately return to Earth with samples, perhaps even &lt;a href=&quot;http://www.honeybeerobotics.com/services/space/planetary-exploration/&quot;&gt;drilled&lt;/a&gt; core samples. Maybe a little digging could be done, or seismic tests, to see what it is really like to &lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_mining&quot;&gt;work on&lt;/a&gt; the surface of an asteroid. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot;&gt;Dawn&lt;/a&gt; visited both Vesta and Ceres by using ion thrusters. How large a body could a probe similar to Dawn - with 3 &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness&quot;&gt;NSTAR&lt;/a&gt; ion engines, the same amount of xenon propellant, and of similar mass - land on and return from in the asteroid belt? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, how many smaller bodies could such a craft visit, probe the surface of, and then return from? (I suppose this depends greatly on how the bodies are spaced out, and their relative velocities and orbits, but perhaps it is possible to make a fair estimate of what those would probably be.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Partly I am curious about the kinds of applications ion thrusters can be used for, partly each time I think through examples of orbital mechanics and propulsion in space, I understand it a little better. (Because so far life has prevented me from taking the time to learn how to work through the formulas myself.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note: &lt;a href=&quot;http://www.asteroidmission.org/&quot;&gt;OSIRIS-REx&lt;/a&gt; is going to use hydrazine thrusters to reach the Near Earth Asteroid Bennu, and will return a sample to Earth. It is the first instance of a mission returning a sizable sample, so perhaps deserves mention. &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa#Scientific_study_of_samples&quot;&gt;Hayabusa&lt;/a&gt; earlier returned a milligram or so from Itokawa)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-08-23T22:25:44.310" LastActivityDate="2015-08-23T22:31:10.720" Title="How large a body could a probe with ion engines land on and launch from?" Tags="|orbital-mechanics|mission-design|ion-thruster|asteroid-belt|sample-return|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10743" PostTypeId="2" ParentId="10742" CreationDate="2015-08-23T22:31:10.720" Score="18" Body="&lt;p&gt;Here's how you can work it out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, thrust in kilo-Newtons (kN) divided by mass in metric tons yields acceleration in meters-per-second-per-second. Divide by 10 to get acceleration in approximate Earth surface gravities (9.81 is the real factor). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dawn uses its thrusters only one at a time (they aren't pointed the same direction), and a single NSTAR thruster yields 90 &lt;em&gt;milli&lt;/em&gt;-Newtons of thrust, i.e. 0.00009 kN. Dawn massed 1.24 tons fully loaded; 0.425 tons of that is Xenon fuel, so when it's almost empty it masses more like 0.8 tons. 0.00009 divided by 0.8 yields about 0.00011 m/s&lt;sup&gt;2&lt;/sup&gt; acceleration, which is not very much at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When landing, you need somewhat more acceleration from thrusters than the gravity you're fighting, otherwise you won't slow down at all. A factor of 1.5 is reasonably sufficient; the more thrust margin you have, the less fuel you'll spend on the way down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ceres has substantial surface gravity - 0.29 m/s&lt;sup&gt;2&lt;/sup&gt; worth - so landing there is obviously a no-go by a factor of about 4000.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even comet 67P/Churyumov–Gerasimenko, of Rosetta/Philae fame, has a surface gravity around 0.001 m/s&lt;sup&gt;2&lt;/sup&gt;, 15 times too much for a safe landing (although using collapsible landing gear or similar lithobraking techniques, it could be done).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For bodies of similar density, surface gravity is proportional to radius, so we want a body about 1/15 the size of 67P, or a couple hundred meters across.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dawn also has chemical thrusters about ten times as powerful as the ion thrusters, though, which it could use to land; with those, a body about one km across would probably be landable. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-08-23T22:31:10.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10744" PostTypeId="2" ParentId="10711" CreationDate="2015-08-24T08:44:58.590" Score="0" Body="&lt;p&gt;In the absence of an acceptable alternative I'm inventing a new word - &lt;strong&gt;selenial&lt;/strong&gt; (also interselenial).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Selene&quot; rel=&quot;nofollow&quot;&gt;Selene&lt;/a&gt;, of course is the Greek equivalent of Luna. However, while Luna is used to explicitly describe Earth's moon, selenial should be used to refer to any moon (including but not limited to Luna).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The usage of selenial and lunar is a parallelism of stellar system (any star system) and solar system (our specific star system - Sol).&lt;/p&gt;&#xA;" OwnerUserId="723" LastActivityDate="2015-08-24T08:44:58.590" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10746" PostTypeId="1" AcceptedAnswerId="10748" CreationDate="2015-08-24T20:55:18.813" Score="3" ViewCount="584" Body="&lt;p&gt;So, as I've recently learned, reaction wheels rotate a craft with Newton's Third Law, remain spinning while the craft should rotate, then stop, resulting in no rotational speed, but after having rotated the craft, yes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what's to stop you from tying a brick to your space craft, and throwing it, as to send you forward and the brick back relatively, until the brick pulls on the rope, and nulls to change. Would this work? If not, why not? Could this be used for maneuvering position wise rather than velocity wise? Could you create a massive object with an incredibly long/strong rope to say, go to the moon(but have no change in velocity other than gravity and other expected things).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I figure that in order to pull the brick back, and not end up in the same position you started at, you'd have to anchor to something to increase your mass, say the moon.&lt;/p&gt;&#xA;" OwnerUserId="11428" LastActivityDate="2015-08-24T22:48:22.023" Title="Linear motion reaction wheels?" Tags="|propulsion|physics|reaction-wheel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10747" PostTypeId="2" ParentId="10726" CreationDate="2015-08-24T21:26:28.790" Score="6" Body="&lt;p&gt;No way it's from Hubble.  Being in Low Earth orbit, Hubble will only see roughly the same side of the Moon we see on Earth.  For reference, here's some images of the Moon from Hubble:  &lt;a href=&quot;http://hubblesite.org/newscenter/archive/releases/1999/14&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://hubblesite.org/newscenter/archive/releases/1999/14&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The DSCOVR Satellite at at the L1 Earth-Sun Lagrangian Point recently captured a similar image, but with the Moon much closer (meaning the image would have to be from something even further).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q6y34.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q6y34.gif&quot; alt=&quot;Earth and Moon&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon is fully illuminated in the above image sequence, but still much darker than the one from the site claiming to be from Hubble.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whatever captured that particular image would have had to tune its exposure specifically for the Moon then overlay it with a capture of the Earth.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="6471" LastEditDate="2016-07-09T11:24:03.447" LastActivityDate="2016-07-09T11:24:03.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10748" PostTypeId="2" ParentId="10746" CreationDate="2015-08-24T22:48:22.023" Score="4" Body="&lt;p&gt;In principle, yes, you can use this technique to change your position in such a way that you return to your initial velocity at the end of a maneuver. In practice, it doesn't scale well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an Earth-to-moon trip, you need a 770,000km rope if the brick is the same mass as your spaceship, and if you want to get there in a matter of days, you need to throw that brick away at 6km/s (or 3km/s for the brick and -3km/s for the spaceship, if you prefer to look at it that way). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an Earth-to-moon trip, going back to your starting velocity at the moon end of the trip is definitely &lt;em&gt;not&lt;/em&gt; what you want. Earth orbital speed in LEO is about 7.8 kilometers per second; orbital speed at the moon's distance is about 1 kilometer per second. So for the brick trick, you have to harpoon the moon at a relative speed of no less than 6.8 km/s. Good luck with that! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without the rope in the way, using the usual rocket techniques, your moon-relative speed on the approach is (I think) more like 700 m/s. The harpoon technique is still not recommended, but using a rocket to get into orbit is very practical. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-08-24T22:48:22.023" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10749" PostTypeId="1" CreationDate="2015-08-25T00:28:27.993" Score="29" ViewCount="4850" Body="&lt;p&gt;I was looking through an old copy of &lt;a href=&quot;http://www.nss.org/settlement/moon/library/mmm.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moon Miner's Manifesto&lt;/a&gt; a while ago and found the concept of a liquid airlock in an early issue.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/q9ifE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/q9ifE.png&quot; alt=&quot;schematic of air lock with a column of liquid&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;It is on page 32 of &lt;a href=&quot;http://www.nss.org/settlement/moon/library/mmm1988.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this issue&lt;/a&gt; from 1988. It posits this as an option in places with gravity, or simulated gravity on rotating space stations, and suggested 3 possible liquids that could be used: mercury, gallium, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Sodium%E2%80%93potassium_alloy&quot; rel=&quot;nofollow noreferrer&quot;&gt;a sodium-potassium alloy known as NaK&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;At first glance it seems like it could be really useful in certain situations. I wonder what the likely problems and limitations would be.&lt;/p&gt;&#xA;&lt;p&gt;In a pinch it even seems like it would be a faster way to get to safety - if your space suit could deal with immersion in liquid. I suppose it would be a good idea to put it inside an alcove or shed, and maybe have a lid on it for when it isn't being used. What else needs to be considered?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="47149" LastEditDate="2022-05-17T21:49:41.630" LastActivityDate="2022-05-18T01:50:49.643" Title="Could liquid airlocks work?" Tags="|the-moon|colonization|technology|habitat|airlock|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="10750" PostTypeId="2" ParentId="10749" CreationDate="2015-08-25T01:49:34.290" Score="28" Body="&lt;p&gt;The diagram you show would work only for specific instances. A few things that it relies on:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Gravity is required for this to work, or else all of the liquid will escape. The diagram you show works because of pressure differential, basically the pressure due to gravity counteracts the pressure from the air. Without gravity, the air would simply push the liquid completely out.&lt;/li&gt;&#xA;&lt;li&gt;The &amp;quot;Vacuum&amp;quot; must be an enclosed surface, or else the liquid will all boil away. The enclosed surface will then become composed of a thin (at best) atmosphere of the liquid. In a pure vacuum, such as space, the pressure won't be high enough to keep the liquid in its liquid state. See this question on &lt;a href=&quot;https://physics.stackexchange.com/q/132341/871&quot;&gt;Physics.SE&lt;/a&gt;. One could theoretically keep the a lid on the liquid, which would be sufficient to keep it from boiling, until one actually wishes to use the airlock. This would allow for an airlock of sorts.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Thinking about this, the only places I could really see this potentially working is on Mars or Titan, both of which have a thin atmosphere. Titan in particular might be of interest as its atmosphere could be explosive if combined with oxygen, and this would be an effective way to keep the oxygen away from Titan's atmosphere when performing an EVA.&lt;/p&gt;&#xA;&lt;p&gt;There is an option that could work. Gallium is a relatively toxic free material. Its melting point is near room temperature, so it would be reasonable to survive going through it, although a human could very well have parts solidified to one's suit after passing through it, as they would freeze on it (Unless the suit was pre-warmed).  It is a fairly expensive material, and the amount required to use for the space application would be quite large.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="47149" LastEditDate="2022-05-17T21:49:31.847" LastActivityDate="2022-05-17T21:49:31.847" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="10751" PostTypeId="2" ParentId="10749" CreationDate="2015-08-25T09:06:53.007" Score="8" Body="&lt;p&gt;The only reason to have an airlock is to move through it: if you're not going to move through it, you'd just replace the whole thing with a steel plate.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NaK&quot; rel=&quot;nofollow noreferrer&quot;&gt;NaK&lt;/a&gt; is a total non-starter for this. It is &lt;em&gt;extremely&lt;/em&gt; reactive so anything passing through it would need to be extremely well protected.  It tends to catch fire when exposed to air, so the surfacing pool needs to be carefully designed to avoid this. If you're going to pass through another liquid first, you need to make sure there are no air bubbles.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mercury is a non-starter because of its extremely high density: you'd have to transport a huge mass of it to the Moon and moving things through the mercury would require you to deal with huge buoyancy forces.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Gallium seems like a non-starter because of its rarity.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Gallium#Production&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; quotes annual world production at a few hundred tons a year and you're going to need tens of cubic metres of the stuff, at a density of 5.9 tons/m&lt;sup&gt;3&lt;/sup&gt; and a price of a few hundred dollars a kilogram in today's market, but a whole lot more than that if NASA decides to buy up half the world's production. It would also need to be kept warm: its melting point is just under 30 °C.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;That seems to rule out all the options that are currently on the table.&lt;/p&gt;&#xA;" OwnerUserId="2784" LastEditorUserId="47149" LastEditDate="2022-05-18T01:50:42.517" LastActivityDate="2022-05-18T01:50:42.517" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="10752" PostTypeId="1" AcceptedAnswerId="10788" CreationDate="2015-08-25T10:46:42.287" Score="2" ViewCount="257" Body="&lt;p&gt;Radiation causes genetic mutation and every (human) cell in our body have the same genes. But on Earth we don't get cancer as often in the foot as in the genitals or in the brain. Some parts of our physiology are more complex untested and sensitive than others.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could an astronaut be well protected by wearing only a (perfectly&#xA;shielded) helmet and a hauberk, or even a backpack to cover half of her&#xA;upper body?&lt;/li&gt;&#xA;&lt;li&gt;What happens to me if, say, my left foot was exposed to off the scale&#xA;super radiation? Would I get acute radiation sickness? Would it give me brain cancer later on?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The idea here is that maybe we could do an 80/20 by shielding only the &quot;upper&quot; half of a spaceship. And of course by wearing funny clothes.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-08-25T11:21:03.373" LastActivityDate="2019-07-11T01:24:33.397" Title="Is exposure to cosmic radiation more dangerous on some body parts than on other?" Tags="|astronauts|health|shielding|cosmic-radiation|protection|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10753" PostTypeId="2" ParentId="2110" CreationDate="2015-08-25T12:42:17.203" Score="3" Body="&lt;p&gt;There surely isn't anything that meets the two criteria, namely:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Can launch with a very small crew.&lt;/li&gt;&#xA;&lt;li&gt;Can survive for 30 minutes.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The only vehicles which can make it to space on short notice are ballistic missiles. Aside from the fact that they are in very secure locations, they have no capacity to take a payload other than the default one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your best bet would be to sneak into an ISS resupply mission somehow, before it launches. That might be possible, although it might throw off the expected mass and thus rocket performance slightly. You won't launch yourself without any kind of guidance, but it would work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, this is a problem because most orbital spacecraft are liquid fueled, and virtually all of those are fueled right before launch. That would require a lot of attention to get it right.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-25T12:42:17.203" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10754" PostTypeId="2" ParentId="9580" CreationDate="2015-08-25T12:53:22.537" Score="2" Body="&lt;p&gt;There is an optimal height for the maximum safe oberth maneuver. This will depend on a few factors, namely the atmosphere of the object, the mountains and gravitational anomalies on said object, uncertainty of the aimpoint, etc. Bottom line, to get the maximal speed boost, aim for the lowest point that won't cause you to have significant atmospheric drag, and won't put you in any danger of hitting the surface.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-25T12:53:22.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10755" PostTypeId="2" ParentId="10749" CreationDate="2015-08-25T12:57:25.500" Score="8" Body="&lt;p&gt;In order for a liquid airlock to work, some points must be considered; they have been mentioned in some of the other answers, but I will try to combine them and suggest some new implementations.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Gravity&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;p&gt;As stated, there must be some gravity (or constant acceleration) present for this to work in principle. The required column height (see sketch in the question) of the pressure head is &lt;span class=&quot;math-container&quot;&gt;$h = p/(\rho \cdot g)$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;To compensate for a 1 atm habitat pressure on the Moon, a water column would need to be about 60 m high, and a mercury column 4.5 m. On Earth, 1 atm corresponds to only 760 mmHg (millimeters of mercury).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Vapour pressure of the liquid vs. pressure of the atmosphere (if any)&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;p&gt;The rate of evaporation depends on the external pressure as well as the intrinsic vapour pressure of the liquid.&lt;/p&gt;&#xA;&lt;p&gt;Liquid metals have low vapour pressures, which is why they were suggested in the original article. Water, on the other hand, has a much higher vapour pressure and will evaporate / freeze and sublimate very quickly in space or on a body without a significant atmosphere, e.g. the Moon.&lt;/p&gt;&#xA;&lt;p&gt;There are some other possibilities for liquids from this stand point; silicone oils, for example, are used in vacuum applications because they can have low vapour pressure. Quite recently (compared to the article's publication date) room temperature ionic liquids have been studied which have very low vapour pressures and have also been used in vacuum applications, e.g. in &lt;a href=&quot;http://cdn.intechopen.com/pdfs-wm/40002.pdf&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Ionic liquids in a vacuum chamber&quot;&gt;this article&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;In any case there will always be a small rate of evaporation on a body without any atmosphere, which means you should top up your air lock once in a while.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Practicalities of passing through the liquid airlock, buoyancy forces and reactivity of the liquid&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;p&gt;Liquid metals like mercury and gallium tend to be very reactive and will form solutions with most other metals. NaK is very corrosive as well. This could presumably be solved by coating exposed portions of cargo with inert polymers like PTFE. Silicone oils, on the other hand, are very inert and should pose no problem at all.&lt;/p&gt;&#xA;&lt;p&gt;David Richerby &lt;a href=&quot;https://space.stackexchange.com/a/10751/49&quot;&gt;mentioned&lt;/a&gt; that buoyancy can also be a problem, especially with the high density of metals. Buoyancy is proportional to gravitational acceleration (see &lt;a href=&quot;https://what-if.xkcd.com/124/&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Lunar swimming&quot;&gt;xkcd's What If&lt;/a&gt;) but diving through roughly 5 m of mercury in your non-metallic space suit on the Moon would be next to impossible unless you had big tungsten or platinum ankle weights).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;So, to sum up, it would be pretty difficult to make as well as use a liquid airlock. The volumes and masses of the liquid alone would be way too great to carry even to the Moon. Maybe you could extract some of the resources in-situ, however, to me it seems this type of an airlock is totally impractical, unless you already have a well developed industry on another planet. Although, it is a nice physics and chemistry problem to think about.&lt;/p&gt;&#xA;" OwnerUserId="3372" LastEditorUserId="47149" LastEditDate="2022-05-18T01:50:49.643" LastActivityDate="2022-05-18T01:50:49.643" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="10756" PostTypeId="2" ParentId="8440" CreationDate="2015-08-25T14:02:35.893" Score="3" Body="&lt;p&gt;I believe it would not take any more time than a normal ship. The bending of space-time will negate the relativistic effects, so in essence the ship, while moving quite fast, doesn't have the relativity effects based on the bending of space-time. Bottom line, ship time and Earth time should be approximately the same. There might be some differences at most at the hours range (Or more likely minutes or seconds), depending on things like correcting for the speed of Alpha Centauri relative to Earth, but these would be minor.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-25T14:02:35.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10757" PostTypeId="2" ParentId="9607" CreationDate="2015-08-25T14:21:43.343" Score="3" Body="&lt;p&gt;The size required to do this is a function of the materials that make up the object. There are a &lt;a href=&quot;http://clarkplanetarium.org/why-are-planets-spherical/&quot; rel=&quot;nofollow&quot;&gt;number of articles&lt;/a&gt; on the internet that explain what it takes. The bottom line is, the stronger the material, the more of it it takes to make the object spherical. Imagine if there was a mountain 500 miles high on Earth. It would require tremendous strength to stand up to the pressure from the rest of the planet trying to pull it back down, and in fact, this wouldn't work for very long. Basically, the mountain will flatten itself out, spreading it's pressure over a larger area. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Owing to the above, every planet (Or asteroid, etc) has a maximum height for mountains. &lt;a href=&quot;http://www.hk-phy.org/articles/mount_high/mount_high_e.html&quot; rel=&quot;nofollow&quot;&gt;Further physics&lt;/a&gt; has an article that explains this, for a purely vertical block. For a triangular shaped block, the distance is about twice as much. Mount Everest is close to the maximum theoretical height for a mountain on Earth, although slightly higher is possible. Smaller planets allow for higher mountains. Eventually, one reaches a point where it doesn't matter how high a mountain is, it won't make a difference. That is the point where an object is considered near spherical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It turns out that the size where one can't have mountains larger than the object turns out to be size at which an object will be spherical. Ceres, for instance, could theoretically have a mountain 177 km high, which is well below its size. Vesta, on the other hand, could support a mountain 234 km high, which is basically the same size as its radius of 260 km. Thus, Vesta is nearly spherical, but can have some significant deviations, as can be observed in the images taken by Dawn.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="415" LastEditDate="2015-08-26T11:50:08.693" LastActivityDate="2015-08-26T11:50:08.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10758" PostTypeId="2" ParentId="9580" CreationDate="2015-08-25T14:42:27.150" Score="6" Body="&lt;p&gt;The proportional change in $\Delta V$ is about $\Delta r\over 2 r$.  So going from 6472 km to 6572 km is a proportional increase of about 0.8%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can vary some depending on your starting and ending specific energies.  Also at these small differences you need to be careful to make sure you compare apples to apples.  You would need a particular example, to take into account the fact that you are ending up in or starting from an orbit with higher or lower energy to begin with.  You should construct examples with identical starting and ending trajectories, which would mean more than one impulsive burn, and then comparing doing the burns in different places.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, for a single pass at 100 km altitude at Earth, the atmospheric density is not a factor.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-08-25T16:56:48.740" LastActivityDate="2015-08-25T16:56:48.740" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10759" PostTypeId="2" ParentId="10661" CreationDate="2015-08-25T15:28:10.373" Score="3" Body="&lt;p&gt;The Van Allen belts, while damaging to satellites, aren't fatal to them. Basically, if there's a justifiable reason to orbit at a particular location, then satellites will orbit at that location, even if additional damage could result. The satellites will simply be hardened to take advantage of that. There is a whole slew of factors involved in orbit determination, including some of the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Coverage&lt;/li&gt;&#xA;&lt;li&gt;Launch availability&lt;/li&gt;&#xA;&lt;li&gt;Time of day on Earth (Earth observing)&lt;/li&gt;&#xA;&lt;li&gt;Customer locations&lt;/li&gt;&#xA;&lt;li&gt;Lifetime of satellite.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The bottom line is though, it might add a small amount to protect against additional radiation, but the cost of the satellite won't be drastically more for orbiting in the Van Allen belts.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-25T15:28:10.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10760" PostTypeId="1" CreationDate="2015-08-25T16:01:59.220" Score="6" ViewCount="347" Body="&lt;p&gt;Next year NASA is launching its &lt;a href=&quot;http://www.jpl.nasa.gov/missions/insight/&quot; rel=&quot;noreferrer&quot;&gt;InSight&lt;/a&gt; lander to Mars to study the rock deep below the surface, so they can try to understand how the planet formed. Where on the Martian surface will InSight land, and how/why was that site chosen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ibHUL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ibHUL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2015-08-25T16:17:12.323" Title="Where will NASA's InSight land on Mars?" Tags="|mars|lander|site-selection|insight|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10761" PostTypeId="2" ParentId="10752" CreationDate="2015-08-25T16:06:03.367" Score="2" Body="&lt;p&gt;To my knowledge the only in-space study was that on Apollo 17, when pocket mice were flown in a closed container in the command module. (Note this means that the mice never made it to the moon.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Zlb4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Zlb4.png&quot; alt=&quot;Apollo mice container&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Four of the five mice survived.  One cage was left empty.  On their return, the courageous Mousetronauts were killed, dissected and examined for after effects. The results were inconclusive though - brain and eye lesions were found but there wasn't enough data to statistically point to any particular part of the body that suffered more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Apollo 17 all manned flights have been in low Earth orbit, so I don't think we'll find such a study that's been done since.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="26446" LastEditDate="2019-07-11T01:24:33.397" LastActivityDate="2019-07-11T01:24:33.397" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="10762" PostTypeId="2" ParentId="10760" CreationDate="2015-08-25T16:11:22.407" Score="8" Body="&lt;p&gt;All four candidate landing sites were identified in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Elysium_Planitia&quot; rel=&quot;noreferrer&quot;&gt;Elysium Planitia&lt;/a&gt; and the most likely landing site (pending final evaluation) will be this one, for providing the smoothest terrain in the landing ellipse:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RRkFv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RRkFv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is the context map:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wbCcM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wbCcM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources and further reading:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2013-269&quot; rel=&quot;noreferrer&quot;&gt;NASA Evaluates Four Candidate Sites for 2016 Mars Mission&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://insight.jpl.nasa.gov/newsdisplay.cfm?Subsite_News_ID=37592&quot; rel=&quot;noreferrer&quot;&gt;Single Site on Mars Advanced for 2016 NASA Lander&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-08-25T16:17:12.323" LastActivityDate="2015-08-25T16:17:12.323" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10763" PostTypeId="2" ParentId="10749" CreationDate="2015-08-25T21:25:12.633" Score="4" Body="&lt;p&gt;In terms of protecting some kind of habitat viable for humans you can eliminate NaK and Hg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is incorrect to say NaK is pyrophoric or reacts violently with air (I have worked with NaK).  But it does react very violently with water.  It also would consume the oxygen of the habitat over time (and you would form salts from these reactions which would not be fluid in nature).  Basically, you'd have a zero humidity habitat on the other side of the airlock and over time the oxygen would be consumed as well.  NaK is basically a substance that only behaves nicely in very controlled and isolated environments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A human habitat sealed in by liquid mercury would also be a habitat where everyone lost their hair, fingernails, went insane, and died prematurely...  Mercury has a low vapor pressure but that vapor pressure is vastly greater than what is required to slow poison most mammals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know what the health factors with Gallium are, but out of the three it would be the only one that you could maintain a viable habitat on the other side with.&lt;/p&gt;&#xA;" OwnerUserId="11434" LastActivityDate="2015-08-25T21:25:12.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10764" PostTypeId="2" ParentId="10749" CreationDate="2015-08-25T23:34:23.647" Score="0" Body="&lt;p&gt;A potential solution would be an heterogeneous solution.  A denser solution (e.g. water) covered with a sufficiently deep layer of a less dense solution.  Under a gravity field such as the Moon's this would separate out into two dissimilar layers (assuming there was no significant solubility of one solution in the other).  The lighter layer would have to have a low vapour pressure.  An oil such as (honestly) olive oil or kerosene would have much lower vapour pressures than water and as such would evaporate significantly more slowly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact when there were believed to be canals on Mars several authors suggested that the canals would be protected from the smaller atmospheric pressure (assumed due to the lower mass of Mars) causing excessive water loss by floating oils on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whilst this would be insufficient in itself to stop vapour loss, a thermal condenser could be placed above the solution which would cool vapour losses and condense them back to liquid which would dribble back into the original lock.  This could be powered on the moon by simply keeping it in the shade &amp;mdash; no Maxwell's Demon would be required!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual exit in the low pressure region would be effected by opening the pressure equilibrated cap, leaving, and closing it.  Why a cap?   Because pressure equilibration could occur through a small diameter tube which would allow a small thermal condenser.&lt;/p&gt;&#xA;" OwnerUserId="11435" LastEditorUserId="6816" LastEditDate="2015-08-25T23:49:25.423" LastActivityDate="2015-08-25T23:49:25.423" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10766" PostTypeId="1" CreationDate="2015-08-26T02:49:08.280" Score="5" ViewCount="896" Body="&lt;p&gt;I was doing some back of the envelope calculations to see improvements SpaceX would have to make to reach a cost target of $100/kg into orbit. After doing some basic calculations, it seems that SpaceX would need to increase the cycle life (number of ignitions or stress cycles) that the engines go through. I tried back calculating what the current cycle life of the SpaceX Merlin 1D engine is, but couldn't find the relevant data (a weight breakdown of the engine components) to calculate it. Does anybody know what it is or where to find it?&lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="6312" LastEditDate="2015-08-26T07:19:55.287" LastActivityDate="2016-08-12T14:29:04.597" Title="What is the cycle life of the Merlin 1D?" Tags="|spacex|falcon-9|merlin-1d|cost-to-orbit|lifetime|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10768" PostTypeId="1" AcceptedAnswerId="10769" CreationDate="2015-08-26T06:27:56.017" Score="9" ViewCount="1234" Body="&lt;p&gt;Excluding our own, which bodies have we directly (or indirectly) gathered samples from? I'm aware of these:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Moon: moon rocks from Apollo&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mars: Martian meteorites found on Earth&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What else?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="6772" LastEditDate="2015-08-26T13:51:05.483" LastActivityDate="2015-08-26T14:05:57.057" Title="From which celestial bodies do we have samples?" Tags="|planet|sample-return|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10769" PostTypeId="2" ParentId="10768" CreationDate="2015-08-26T09:06:16.587" Score="9" Body="&lt;p&gt;There have been several &lt;a href=&quot;https://en.wikipedia.org/wiki/Sample_return_mission&quot;&gt;sample return missions&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;NASA mission &lt;a href=&quot;https://en.wikipedia.org/wiki/Genesis_(spacecraft)&quot;&gt;Genesis&lt;/a&gt; collected samples of the solar wind&lt;/li&gt;&#xA;&lt;li&gt;comet Wild 2 was visited by NASA mission &lt;a href=&quot;https://en.wikipedia.org/wiki/Wild_2&quot;&gt;Stardust&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;asteroid 25143 Itokawa was visited by JAXA spacecraft &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot;&gt;Hayabusa&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;2 more asteroid sample return missions are currently active: Hayabusa 2 was launched in 2014, Osiris-REX is being built.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plans for more missions exist, including sample return from Mars moon Phobos and from Mars itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to Mars, we've identified samples from Vesta in the form of HED meteorites. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Angrite&quot;&gt;Angrites&lt;/a&gt; group of meteorites are possibly from Asteroids 289 Nenetta or 3819 Robinson, or Mercury. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-08-26T12:27:00.847" LastActivityDate="2015-08-26T12:27:00.847" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10770" PostTypeId="1" CreationDate="2015-08-26T10:28:37.607" Score="5" ViewCount="989" Body="&lt;p&gt;Gloves on space suits are hard to use because of their high gas pressure relative to the surrounding vacuum. It's simply strenuous to bend the fingers and astronauts are even said to lose their nails after having worked with them. This is not good enough for spending a year on Mars or Phobos with daily EVA's. Currently, the research and development seems focused on bringing many different technologies together, like &lt;em&gt;&quot;&lt;a href=&quot;https://gcd.larc.nasa.gov/wp-content/uploads/2014/09/FS_EVAglove_factsheet_140916.pdf&quot; rel=&quot;noreferrer&quot;&gt;smart&lt;/a&gt;&quot; textiles, aerogels, moisture control strategy, dust tolerant lightweight bearings and low torque finger and thumb joint designs&quot;&lt;/em&gt; and even casting the hands of the individual astronauts for custom made designs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a smarter way to bypass all that cumbersome and questionable work? Tools like shovels and drills could maybe be hooked onto the suit and used without need to bend any finger. A glove box, much smaller than the stationary one imaged below, could maybe be strapped around the glove when teleoperation with a joystick or a cyber glove is needed. If such a box (why not make it an inflatable &quot;glove bag&quot;) has the same pressure as the suit, there would be no resistance due to pressure differences. An exoskeleton, maybe inside of the glove, could add motion power to light finger movements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qs46V.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qs46V.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2022-07-29T23:55:33.687" Title="How can space suit gloves be improved?" Tags="|eva|spacesuits|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10771" PostTypeId="1" CreationDate="2015-08-26T13:17:17.730" Score="24" ViewCount="8134" Body="&lt;p&gt;I'm wondering why artificial gravity is not included on the ISS, or on the currently planned &lt;a href=&quot;http://en.wikipedia.org/wiki/Orbital_Piloted_Assembly_and_Experiment_Complex&quot;&gt;OPSEK&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are long-term health effects of weightlessness, which would be avoided, using artificial gravity based on centripetal force.&lt;/p&gt;&#xA;" OwnerUserId="8873" LastEditorUserId="1142" LastEditDate="2015-08-26T14:33:19.937" LastActivityDate="2015-09-01T15:11:14.607" Title="Why is artificial gravity not included on space stations?" Tags="|iss|gravity|microgravity|space-station|artificial-gravity|" AnswerCount="0" CommentCount="0" FavoriteCount="0" ClosedDate="2016-07-02T14:31:55.187" ContentLicense="CC BY-SA 3.0" />
  <row Id="10772" PostTypeId="2" ParentId="1308" CreationDate="2015-08-26T13:28:56.247" Score="32" Body="&lt;p&gt;It is worse than that. In order to really plan for future human habitation elsewhere, you do not want just 1G fields, you want 1/6th to mimic the Moon, and 1/3 to mimic Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need to be able to run long term studies comparing similar samples at 0G, and other G levels. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was planned to be a &lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module&quot;&gt;Centrifuge Accomodations Module&lt;/a&gt;, on the sky facing port of Kibo (JAXA Science module) but it was stripped due to funding. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondarily, a centrifuge large enough for people would need to be very large, and that is basically out of the question for something as small as ISS. A centrifuge for running experiments on mice, or small items introduces another problem. Vibration. This can affect other experiments on the ISS relying on 0G. The ISS is not a great 0G facility, too many darn humans moving around and bumping the walls all the time, and all the altitude adjustments.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://ssi.org/&quot;&gt;Space Sciences Institute&lt;/a&gt; headed by Gary Hudson (Of Roton fame) is pushing for funding for a space based centrifuge facility. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short answer to why? Money. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-08-26T13:28:56.247" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10773" PostTypeId="2" ParentId="5394" CreationDate="2015-08-26T13:29:46.010" Score="1" Body="&lt;p&gt;I've done an examination of the literature on the subject, and I can't find any mention of any volatiles except for water ice, but there are indicators that there should be others. Let's examine what characteristics of volatiles should be on the Lunar surface.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It would have to be solid in a vacuum at low temperatures, &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1029/JB084iB10p05659/epdf&quot; rel=&quot;nofollow&quot;&gt;about 100K&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The material should be abundant, rare materials are unlikely to have this performance.&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A volatile will vaporize at the temperature of the moon in the daylight. Let's call that about room temperature, 300K. It could also vaporize by the presence of sunlight.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Water is an obvious candidate for such, as it is available throughout the solar system, and meets the temperature requirements. &lt;/li&gt;&#xA;&lt;li&gt;Nitrogen wouldn't remain a solid, and thus wouldn't solidify. &lt;/li&gt;&#xA;&lt;li&gt;Carbon Dioxide would probably remain frozen, it will remain solid in the realm of 100 degrees. &lt;/li&gt;&#xA;&lt;li&gt;Carbon Monoxide probably won't remain stable, it doesn't seem to remain solid with an extremely low temperature.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line, there likely won't be many others, but I suspect Carbon Dioxide would also be at the surface in permanently shadowed craters on the Moon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-26T13:29:46.010" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="10774" PostTypeId="2" ParentId="10766" CreationDate="2015-08-26T13:35:05.270" Score="3" Body="&lt;p&gt;No Merlin 1D engines have been reflown, per se, to date. I qualify with 'per se' since every Merlin engine does qualification tests at the McGregor, Tx test stand singly, then as a group of 9.  However many exact firings that is, is not entirely clear. (2? More?) Tom Mueller has said that as part of acceptance testing of a Merlin engine (in the 1C days) that they drop in a 5/16th nut that has to be injested and survived. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then on a typical reuse profile, there is:&#xA;Hot Fire 2-3 days before, only 1-2 seconds, but full ignition cycle&#xA;Launch - 3 minutes of flight (not all at full power, 2 engines are shut down, and the rest throttled later in flight to control G loads)&#xA;Recovery - 2 or 3 burns - First with three engines, second and third with center engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus before a single reuse attempt is made, a Merlin 1D in the center position is likely to have fired (ignition to full power, regardless of duration) at least 7 times. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much beyond that is proprietary information of SpaceX. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How will this change with the full thrust Merlin 1D to be used on all future launches (Post Jason-3 as of this writing.  After return to flight, there is one older Falcon 9 left, everything after that is Full Thrust Falcon (or Falcon v1.2, but they are avoiding the name due to certification issues)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Falcon Full Thrust recovery missions, they have experimented with 3 engine firings during the reentry and landing burns in order to minimize fuel usage fighting gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The JCSAT-14 core after landing was retested in the summer of 2016 at least 3 times in McGregor by the end of July. Gwynne Shotwell at the Smallsat 2016 conference in Logan, UT said their aim is to run that stage 4-10 more times until they find the lifespan of a stage. She suggested they would slip the JCSAT-14 booster onto the test stand, every time it is vacated and not needed by a production booster test. The interesting question this raises is how busy the test stand is, how long it takes to mount/dismount a stage, and then run a test.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-08-12T14:29:04.597" LastActivityDate="2016-08-12T14:29:04.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10775" PostTypeId="2" ParentId="10768" CreationDate="2015-08-26T14:05:57.057" Score="1" Body="&lt;p&gt;Stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Almost all elements on Earth were formed in the heart of a star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We're made from stardust.&lt;/p&gt;&#xA;" OwnerUserId="11438" LastActivityDate="2015-08-26T14:05:57.057" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10776" PostTypeId="2" ParentId="10720" CreationDate="2015-08-26T14:33:51.300" Score="3" Body="&lt;p&gt;Actually they were not always done as secondary payload, in the early day of CubeSat (check out CalPoly CP CubeSats, CalPoly P-POD). Those were launched on modified ICBM specifically for the CubeSats (not in US). The practice was picked up and modified by NASA to put them as secondary payload mostly for educational purpose. But back to your question, it is very difficult for amateur to get to space, hence most academic focuses mostly on building CubeSats or instruments.&lt;/p&gt;&#xA;" OwnerUserId="11221" LastActivityDate="2015-08-26T14:33:51.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10777" PostTypeId="1" CreationDate="2015-08-26T14:40:32.597" Score="15" ViewCount="769" Body="&lt;p&gt;We were discussing this in the chat room yesterday, and I thought it might make an interesting question. While missions are lasting longer today than they perhaps ever have, it seems like there are relatively few launches compared to the recent past. We could only come up with one mission in the window of SpaceFlightNow that is planned, March 4th InSight. I'm sure there are others, but it seems strange that NASA only has one interplanetary mission for the next many months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is, is the rate of launch of missions over a short (Let's say 2 years) lower than the standard rate of launch of missions over, say, the last 10 years, and if so, why?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-05T09:22:12.350" Title="Is NASA launching fewer interplanetary missions?" Tags="|launch|nasa|planetary-science|interplanetary|cost|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10778" PostTypeId="2" ParentId="1308" CreationDate="2015-08-26T14:48:16.510" Score="7" Body="&lt;p&gt;Because adding centripetal force makes everything more complicated and more expensive, and it's already so difficult and expensive to put anything, and especially people, in space that it's just been a non-starter.  Since people can survive a relatively short stint in zero-gee with few permanent health effects, it's just easier to rotate out the crew than worrying about a trillion-dollar space station breaking in half because some bearing in the artificial gravity system seizes up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember reading about a very few experiments along these lines.  In &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Case_for_Mars&quot;&gt;&lt;em&gt;The Case for Mars&lt;/em&gt;&lt;/a&gt;, I think the author suggested that we could attach the ship to a counterweight with a long tether, and spin that assembly for gravity.  I think he cited some previous mission where they'd tried that as an experiment, and the cable melted or something, possibly because they were sending too much electricity across it.&lt;/p&gt;&#xA;" OwnerUserId="11439" LastEditorUserId="11439" LastEditDate="2015-08-26T14:57:16.560" LastActivityDate="2015-08-26T14:57:16.560" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10779" PostTypeId="2" ParentId="10617" CreationDate="2015-08-26T14:48:53.347" Score="6" Body="&lt;p&gt;Efficiency in communications affects all aspects of the communication system design.&lt;br&gt;&#xA;It's useful to look at this in terms of a model like the &lt;a href=&quot;https://en.wikipedia.org/wiki/OSI_model&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSI model&lt;/a&gt;. Now that's aimed at computer networks, but the general principle is similar: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;at the lowest level, you have the hardware: transmitter, receiver, antennas. Anything you do here influences the total system throughput: a bigger antenna or more powerful transmitter = more data can be sent.&lt;/li&gt;&#xA;&lt;li&gt;at higher levels, you have communications protocols like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Turbo_code&quot; rel=&quot;nofollow noreferrer&quot;&gt;turbo coding&lt;/a&gt; used by New Horizons. This makes sure you can send your message in as few bits as possible.&lt;/li&gt;&#xA;&lt;li&gt;the top level is the application: the actual science data and diagnostics/housekeeping information you're interested in. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;At the application level, a lot of research as been done into &lt;a href=&quot;https://en.wikipedia.org/wiki/Data_compression&quot; rel=&quot;nofollow noreferrer&quot;&gt;data compression&lt;/a&gt;. Typically, you want to send all data from the instruments to Earth (no compression), or you want to send enough that the original data can be reconstructed (lossless compression).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In recent missions, we've seen lossy compression being used: the spacecraft decides which parts of its data are irrelevant and can be thrown away. New Horizons has done this in two ways: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;by cropping images. For instance, this image of a region of Charon is a rectangular cropped region. The remainder of the image (which would have shown empty space so it wasn't very interesting) was not sent at all. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/4Fzf2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Fzf2.jpg&quot; alt=&quot;Charon&quot;&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;by compressing the image data using a lossy algorithm (&lt;a href=&quot;https://en.wikipedia.org/wiki/JPEG&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPEG&lt;/a&gt;, in this case). JPEG throws away data based on what the human eye can see: tiny differences are rounded off because they'd all look the same anyway. There is a risk in this: by throwing away data you reduce the scientific usefulness of the image. So New Horizons sends us compressed images now so we have at least something to look at, and will send the full, uncompressed data set over the next year and a half. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Data compression will become more common. Some of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gaia_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;new space telescopes&lt;/a&gt; generate way more data than they can ever send to Earth, so the spacecraft has to contain powerful algorithms that help decide which images are interesting enough to send to Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: this answer focuses mostly on the science data as that will be bulk of the data volume. I haven't found good information on how housekeeping data is handled. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-08-27T09:55:17.063" LastActivityDate="2015-08-27T09:55:17.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10780" PostTypeId="2" ParentId="10777" CreationDate="2015-08-26T15:57:50.953" Score="14" Body="&lt;p&gt;Yes.  And even worse, there are &lt;em&gt;no&lt;/em&gt; planetary launches planned in 2017, 2018, or 2019!  Zero.  Zippo.  Nada.  As you can see from the Planetary Society plot, this is due to not having such missions in planning in several preceding years.  Here is the &lt;a href=&quot;http://planetary.org/multimedia/space-images/charts/historical-levels-of-planetary-exploration-funding-fy2003-fy2019.html&quot; rel=&quot;noreferrer&quot;&gt;source of the graphic&lt;/a&gt; with some more information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hVYxq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hVYxq.jpg&quot; alt=&quot;planetary launches 2004 - 2020&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="25" LastEditDate="2015-08-26T16:03:47.753" LastActivityDate="2015-08-26T16:03:47.753" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="10781" PostTypeId="2" ParentId="8607" CreationDate="2015-08-26T17:20:15.357" Score="4" Body="&lt;p&gt;I'm amazed no one else has suggested it by now, but one term I've heard used for this is &quot;&lt;strong&gt;pseudogravity&lt;/strong&gt;&quot;.  The prefix &quot;pseudo&quot; is defined by &lt;a href=&quot;http://www.merriam-webster.com/dictionary/pseudo&quot; rel=&quot;nofollow&quot;&gt;Merriam-Webster&lt;/a&gt; as &quot;being apparently rather than actually as stated&quot;, which acts as a built-in caveat that it's not &lt;em&gt;really&lt;/em&gt; gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The term occurs mostly in science fiction, admittedly, but then again, so does the concept it names.  It's also used in passing in a couple of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_stations_and_habitats_in_fiction#Space_stations_rotating_for_pseudogravity&quot; rel=&quot;nofollow&quot;&gt;Wikipedia articles&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="11439" LastActivityDate="2015-08-26T17:20:15.357" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10783" PostTypeId="1" CreationDate="2015-08-26T19:30:43.830" Score="4" ViewCount="354" Body="&lt;p&gt;Earth and Mars have conjunctions every 15/7 of a year, or every 26 month or every 2 years. And it varies substantially from conjunction to conjunction because of orbital precessions and eccentricities. Is there an astronomical term for the period between two conjunctions? Like the word &quot;olympiad&quot; which refers to the four year period between the games.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-08-26T19:42:38.557" LastActivityDate="2015-08-26T19:42:38.557" Title="What is the word for the period between two Mars conjunctions?" Tags="|history|terminology|conjunction|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10784" PostTypeId="2" ParentId="10783" CreationDate="2015-08-26T19:40:51.460" Score="5" Body="&lt;p&gt;That is called the synodic period.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-26T19:40:51.460" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10785" PostTypeId="1" CreationDate="2015-08-26T19:54:08.007" Score="12" ViewCount="8333" Body="&lt;p&gt;It's (relatively) common knowledge that Mars is known as the &quot;Red Planet&quot; because of an abundance of iron oxide, AKA &lt;em&gt;rust&lt;/em&gt;, on its surface.  At first glance, that looks like &quot;iron, iron, everywhere.&quot;  But of course that's just the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How common is iron in general on Mars?  For example, if colonists wanted to produce iron, how long would it be before they'd have to start specifically digging mines to get at more? How easy would it be to refine iron from known surface substances on Mars, and how much iron you could reasonably expect to obtain thereby?&lt;/p&gt;&#xA;" OwnerUserId="10171" LastEditorUserId="1142" LastEditDate="2015-08-28T13:04:05.783" LastActivityDate="2022-06-09T01:31:46.073" Title="How much iron is on the surface of Mars?" Tags="|mars|mining|natural-resources|isru|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10787" PostTypeId="2" ParentId="9429" CreationDate="2015-08-26T22:49:17.363" Score="3" Body="&lt;p&gt;There are a few variations on this, so let me talk about some of the limitations. I don't have a good grasp on the exact numbers, but hopefully this will give you an idea. I'm going to start with things that would allow you to re-use a mission, and what are some of the factors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first step is to have a rocket in assembly that can have it's mission changed. This would allow you to do a turn around a rocket as quickly as you make them, and doesn't add a lot of up front cost. It would cost quite a bit if you excersized the option, but if you need it, then it's available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next would be to have a rocket on the launch site. This would require a location to store the rocket. They are usually stored in a clean room of sorts. The expense is in building a second room, basically it would double the operating costs of the facilities at the launch location. This could still prove useful as it would allow you to do almost twice as many launches, processing two rockets at the same time. If you have one waiting in continual readiness, it reduces this effectiveness, but would allow you to launch a rocket even sooner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond this, you could have a mission on the pad, but not fueled. This would probably allow for a launch within a few hours. This could not be kept up for a long time, as the rocket would rust eventually, or have to be taken in due to weather constraints. If one of these two items happened, it would add considerably to the cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, you could have a fully fueled rocket on the launch pad, ready to go. This would cost quite a bit extra. For the Space Shuttle, a scrub costs &lt;a href=&quot;http://www.space.com/6991-5-shuttle-launch-scrubs-cost-millions.html&quot; rel=&quot;nofollow&quot;&gt;$1.2 million&lt;/a&gt;. Other vehicles are less, but it is still expensive. Thermal cycles add stress to the equipment, eventually leading to the potentials of failures. Also, keep in mind that this assumes a launch over a short period of time. If one were to leave the rocket exposed ready to launch for a longer period of time, the cost would go up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, it all just depends how much you want to pay. I suspect that the readiness for the scenario you described wouldn't require anything more than having a backup rocket at the launch site. This allows for fairly quick turnaround of a launch, but not that quick. It doubles the cost at the pad, and might have some more subtle effects in increased maintenance for the rocket, but overall wouldn't break the bank.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-08-28T15:00:48.843" LastActivityDate="2015-08-28T15:00:48.843" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10788" PostTypeId="2" ParentId="10752" CreationDate="2015-08-27T02:31:23.897" Score="4" Body="&lt;p&gt;Is radiation more dangerous to some parts of the body than to others?  Yes.  Health physics uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Effective_dose_(radiation)#Use_of_tissue_weighting_factor_WT&quot; rel=&quot;nofollow&quot;&gt;tissue weighting factors&lt;/a&gt; as a way of factoring in the relative sensitivity of different tissue types.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is partial shielding practical?  For the most part, no.  Some of the highly sensitive tissues are the bone marrow, lungs, and intestines, meaning an effective shield would need to cover most of the body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's one situation where partial shielding is practical, though: since the hands are relatively insensitive to radiation, you don't need to shield them.  A radiation-armored spacesuit would have heavy armor covering most of it, while the gauntlets would be thin to give the user reasonable dexterity.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2015-08-27T02:31:23.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10789" PostTypeId="1" AcceptedAnswerId="10791" CreationDate="2015-08-27T06:03:39.583" Score="2" ViewCount="414" Body="&lt;p&gt;I am curious as to what does the crater of Olympus Mons contain? Does NASA have any mission to send a space probe to land on it? If not, what about other parts in Olympus Mons?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="25" LastEditDate="2015-08-27T12:11:55.610" LastActivityDate="2015-08-27T12:12:45.490" Title="Are there plans for a space probe to land on the crater of Olympus Mons?" Tags="|mars|probe|lander|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10790" PostTypeId="2" ParentId="1308" CreationDate="2015-08-27T06:52:14.460" Score="27" Body="&lt;p&gt;I feel like the other answers are missing one fundamental design driver of the ISS. Unlike space stations like the rotating von Braun station or the Stanford Torus, the ISS was primary designed as a &lt;em&gt;scientific space station&lt;/em&gt; and not as a leisure outpost in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Microgravity (in fact there are not absolute zero-G on the ISS, but a very small force differential - see MrPaulch comment) is the big unique characteristic, which differentiates the ISS from every other human laboratory. No microgravity would more or less render the ISS unnecessary from a scientific point of view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore it was never intended to build a whole rotating space station. Of course money is a second reason (it always is a reason), but if money is a real concern, the ISS would've never been built. Building an international space station was not an economic or scientific decision, it was mainly a political decision.&lt;/p&gt;&#xA;" OwnerUserId="11391" LastEditorUserId="11391" LastEditDate="2015-08-28T06:41:21.500" LastActivityDate="2015-08-28T06:41:21.500" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="10791" PostTypeId="2" ParentId="10789" CreationDate="2015-08-27T11:58:08.627" Score="3" Body="&lt;p&gt;No, landing on any of the really high points on Mars is generally considered less interesting, and it's much more difficult to do, than any of the rest of Mars. Here's a few reasons why:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Landing that high up, you can't really use parachutes. It's pretty common for missions to go to the lowest points on Mars (See map below, blue represents lower areas on Mars)&lt;/li&gt;&#xA;&lt;li&gt;The 4 calderas have relatively little interesting about them. For the most part, they appear very featureless (See photo from &lt;a href=&quot;http://www.uahirise.org/PSP_004821_1985&quot; rel=&quot;nofollow noreferrer&quot;&gt;HiRISE&lt;/a&gt; of Olympus Mons)&lt;/li&gt;&#xA;&lt;li&gt;There are other areas of Mars that show more interesting chemical compounds, etc, that just don't show up on the mountain. Thus, they are higher priority.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I suggest if you are curious then look up HiRISE images from Olympus Mons. There are quite a few of them, and they give a pretty good indicator as to what it's like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nLcOI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nLcOI.jpg&quot; alt=&quot;Lander Location with elevation map background&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Ep88R.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ep88R.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-08-27T12:12:45.490" LastActivityDate="2015-08-27T12:12:45.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10792" PostTypeId="2" ParentId="4908" CreationDate="2015-08-27T12:19:26.343" Score="4" Body="&lt;p&gt;It would really help you to represent this as a 3-d vector. It makes the math much simpler, and it allows for some interesting effects. I would translate this in game to the simplier representation of the maximum altitude, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given a position and velocity vector, one could calculate the TLE elements from this. You simply find the position and velocity at the time, add the new velocity component, and then re-calculate. We already have an &lt;a href=&quot;https://space.stackexchange.com/a/1919/25&quot;&gt;excellent answer&lt;/a&gt; that shows how to convert position and velocity to the orbital elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to simplify this, you could assume a 0 inclination orbit, which allows you to use a 2-d vector, and assume that all propulsion is either prograde or retrograde, adding or taking away from the velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-08-27T12:19:26.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10793" PostTypeId="2" ParentId="6185" CreationDate="2015-08-27T12:24:10.937" Score="6" Body="&lt;p&gt;From my experience, most of the time the elements don't jump more than a few km at most over a 24 hour time span. There are some rare instances where there are dramatic jumps for no reason, but things in space just don't move around that much without a reason. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the meter accuracy, that seems a bit high. There are a few things that might be why that is the case:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The ISS is really large, and close. Between the two, it is much easier to determine it's exact location, which helps in making more accurate predictions.&lt;/li&gt;&#xA;&lt;li&gt;The ISS being manned is of more interest than most objects, it might have more updates as a result.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I would test something like a booster or dead satellite, and see if you get the same result. But it does seem possible that the result you have is reasonable.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-27T12:24:10.937" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10794" PostTypeId="2" ParentId="9821" CreationDate="2015-08-27T12:27:21.113" Score="1" Body="&lt;p&gt;I suspect your problem is that you are using the altitude of the object, and not the distance from it's center. Make sure you are using the distance to the center of the object, and I believe your problem will resolve itself.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-27T12:27:21.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10797" PostTypeId="1" AcceptedAnswerId="10799" CreationDate="2015-08-27T15:07:26.130" Score="4" ViewCount="380" Body="&lt;p&gt;I was doing a mission analysis on returning a nuclear payload (a spent reactor, nuclear battery, or something slightly radioactive) from orbit and was seeing the potential ways of bringing it back. The only way that seemed possible (from a regulatory point of view) was if the payload landed in the open ocean without having the potential of redirecting to a populated place, making the following tracks most viable: Reentering due south towards Antarctica in the Indian Ocean, Reentering due south or north in the middle of the Atlantic Ocean, and Reentering on any track into the Pacific Ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is this: Would the operator or owner of the payload need to get a reentry license from the FAA to fly these trajectories? Or are these trajectories outside the FAA's jurisdiction since they end in international waters?&lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="25" LastEditDate="2015-08-27T15:11:57.580" LastActivityDate="2015-08-27T15:26:59.657" Title="Can you reenter a satellite into the Earth's atmosphere without permission if it lands in the open ocean?" Tags="|reentry|law|faa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10798" PostTypeId="1" CreationDate="2015-08-27T15:16:39.910" Score="7" ViewCount="218" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/q/10777/25&quot;&gt;this question&lt;/a&gt;, we have learned that there is in fact fewer interplanetary missions being planned now than there has a decade ago. One of the items discussed as to why this might be the case is that there are more active missions, requiring more money, allowing less for new missions. Is that the case?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically I'm looking for if there is more operating interplanetary missions now than there has been in the last, say, 10 years, and (bonus) what the funding required to keep those missions alive is compared to the overall budget. Ultimately, I'm trying to figure out if NASA's decrease in funds is responsible for fewer missions, or if it's that the current missions cost more (Or something else, as of yet to be determined).&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2021-09-02T14:29:38.663" Title="Are there more operating interplanetary missions today than in the past?" Tags="|nasa|interplanetary|cost|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10799" PostTypeId="2" ParentId="10797" CreationDate="2015-08-27T15:26:59.657" Score="5" Body="&lt;p&gt;The jurisdiction to enforce these rules is based on where 'you' the responsible party are. If you are a US citizen the FAA and others will assert their authority over you, wherever you reside. Some of this is because the US is responsible for launches (and reentries) by its citizens. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing a nuclear device without prior permission anywhere on the planet is very likely to get you in a lot of trouble, no matter where you reside. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-08-27T15:26:59.657" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10800" PostTypeId="2" ParentId="10749" CreationDate="2015-08-27T15:38:37.713" Score="4" Body="&lt;p&gt;There are several answers now that examine different aspects of this. I wanted to add the matter of waterglass, (&lt;a href=&quot;https://en.wikipedia.org/wiki/Sodium_silicate#Liquid_phase&quot; rel=&quot;nofollow noreferrer&quot;&gt;sodium silicate&lt;/a&gt; Na&lt;sub&gt;2&lt;/sub&gt;SiO&lt;sub&gt;3&lt;/sub&gt;, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Potassium_silicate&quot; rel=&quot;nofollow noreferrer&quot;&gt;potassium silicate&lt;/a&gt; K&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;Si) as a potential liquid for use in such a design. A liquid solution is roughly half water. Though water is volatile, the vapor pressure (page 10 &lt;a href=&quot;http://www.heraproject.com/files/14-f-05-ra%20risk%20assessment%20of%20soluble%20silicates%20final%20draft.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;) of waterglass is very low. With a cover over the exit of the airlock, there would be no vapor loss issue other than a scum developing on the surface as the water there evaporated. It is non-toxic, non-reactive, and the elements for it are all found on the Moon. (Sodium is much more widely available and plentiful than potassium, oxygen and silicon bound in minerals make up about 2/3 of the Moon's surface.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is a clear, somewhat viscous liquid with a density of about 1.3 g/cm&lt;sup&gt;3&lt;/sup&gt; in a half-water mix. Thus a person in a space suit would have little trouble moving through it. The column needed to balance a full atmosphere of pressure would be about 8 m. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-08-27T15:38:37.713" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10801" PostTypeId="2" ParentId="1308" CreationDate="2015-08-27T15:46:03.543" Score="16" Body="&lt;p&gt;What hasn't been mentioned so far are the &lt;strong&gt;extra problems caused by gravity&lt;/strong&gt;: it's not just that gravity is an expensive and inconvenient luxury, it also brings problems. (In this answer I will just say &quot;gravity&quot; to refer to a constant force along one axis, whether that's real or simulated.)&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Accessibility&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;In zero-g, you can get anywhere on the ship by pushing/pulling/drifting there.  In gravity, you can't, unless it's all so compact that you can reach anything by hanging off a ladder or just standing and reaching up. Even if you can reach everything, it might be the case that you can't reach it from quite the right angle to use it most easily. In zero-g you can cover every surface with &quot;stuff&quot; and not worry about what angle people are at when they use it, since they can always just rotate themselves round.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Accidents&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;All of a sudden you can fall down a ladder, or drop something on another astronaut. This is not a problem in zero-g.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Consistency&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Most space stations are sort of long cylinders, maybe with a few branching cylinders. Where would you have the gravity in this situation? Would it be parallel to the long axis of the ship, so you climb up and down to traverse the ship? Or, parallel to the side axis, so that you stand on the side of a cylinder, and walk around it as if it was all laying flat on the earth?  Either way, it's hard to achieve a consistent centrifugal (or is it centripetal? I can never get that) force over the whole space station. The further you get from the axis of rotation, the more it's going to feel like you're falling towards the walls.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Docking&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Docking with spinning things is so much harder. The docking point can only work if it is in the middle of the axis of rotation, which brings in some extra constraints to the design and layout of the station. Also, the need to match rotation also adds extra complexity to the docking, which is probably dangerous enough as it is.&lt;/p&gt;&#xA;" OwnerUserId="11449" LastEditorUserId="6816" LastEditDate="2017-08-26T15:01:59.450" LastActivityDate="2017-08-26T15:01:59.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10802" PostTypeId="2" ParentId="10798" CreationDate="2015-08-27T16:44:48.093" Score="7" Body="&lt;p&gt;The space program is busy right now, but Dawn and Rosetta and New Horizons and Cassini are 10 year old missions. Some asteroid and a Solar mission are planned. And something will be sent to Mars at every conjunction from now on. But the Moon, Venus and the outer planets are completely left without any plans for exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Emily Lakdawalla at the Planetary Society makes a &lt;em&gt;&quot;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/08101604-whats-up-in-solar-system.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;What's up in solar system&lt;/a&gt;&quot;&lt;/em&gt; summary once or twice a year. Maybe you could search them and find the trend? Her illustration below. She explicitly does not care at all about &quot;planned&quot; missions, only realized ones. At least space telescopes are looking good. Nobel Prize winning cosmologists might have more political influence than planetary scientists. (I think Emily misses Gaia space telescope there, the historical final nail in the coffin of astrometry, now we'll know where everything is, everything. Well, alot of it anyway).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I saw a talk of an astronomer who entertained with telling that their university arranges cross-discipline gatherings over lunch. He was upset because the budget for a space telescope had been slashed down to only 450 million dollar when he had asked for almost a billion. So he had lunch with a psychologist who was so happy because she had been granted 100,000 dollar for her project (figuring out what makes people happy or something useless like that). I wish more was invested in science, but among scientists, astronomers are not discriminated against. They go ask the politicians for billions of dollars to drill down a telescope in the Antarctic or to send something strange far away to study who knows what. And the politicians just nod and pay. It could've been worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tglej.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tglej.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="265" LastEditDate="2015-12-04T01:41:46.120" LastActivityDate="2015-12-04T01:41:46.120" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10803" PostTypeId="1" AcceptedAnswerId="10804" CreationDate="2015-08-27T17:45:40.473" Score="28" ViewCount="67626" Body="&lt;p&gt;The first stage would be a high altitude balloon, which could reach 30km. Then start a rocket-candy from that high.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without cargo, is it possible to reach the edge of space (Kármán line - 100km), or even Low Earth Orbit (160km)?&lt;/p&gt;&#xA;" OwnerUserId="8873" LastEditorUserId="25" LastEditDate="2015-08-27T18:00:08.720" LastActivityDate="2017-10-16T16:54:39.157" Title="Is it possible to reach space using home-made rocket?" Tags="|rockets|low-earth-orbit|balloons|amateur-rocketry|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10804" PostTypeId="2" ParentId="10803" CreationDate="2015-08-27T17:57:51.440" Score="28" Body="&lt;p&gt;First of all, LEO isn't just reaching 160 km, it's reaching there and moving at a very fast speed. In fact, 160 km would be a really poor orbit, you really need something more like 350 km to get anything practical done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to your question, yes, it is theoretically possible. In fact, there have been a few amateur made rockets that have reached the Kármán line. The first happened &lt;a href=&quot;http://www.civilianspace.com/about/default.asp&quot;&gt;May 17, 2004&lt;/a&gt;, by the  Civilian Space eXploration Team (CSXT). This is the only known amateur rocket to make it past 100 km. For a more complete list, see &lt;a href=&quot;http://www.hobbyspace.com/Rocketry/Advanced/records.html&quot;&gt;HobbySpace&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that any team that has managed to make it that close usually has corporate sponsorships, and often has failures. It's really hard to get a rocket that high up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to making it to orbit, well, I don't think that's really possible given today's technology. It takes a lot of really smart guys working really hard to get a rocket to orbit, I don't think that's the kind of thing an amateur is going to do any time soon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-08-28T12:48:38.937" LastActivityDate="2015-08-28T12:48:38.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10805" PostTypeId="1" CreationDate="2015-08-27T18:30:33.880" Score="9" ViewCount="369" Body="&lt;p&gt;Assuming we collect sulfuric acid from Venus clouds using some sort of airship, what would be the best way of turning this into hydrogen and oxygen? I came across something called the hybrid sulfur cycle which goes like this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\require{mhchem}\ce{H2SO4(aq) → H2O(g) + SO2(g) + \frac{1}{2} O2(g)}\ \ \ \ \text{(thermochemical, T &amp;gt; 800 °C)}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\ce{SO2(aq) + 2 H2O(l) → H2SO4(aq) + H2(g)}\ \ \ \ \text{(electrochemical, T = 80-120 °C)}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Are there better ways than this? I assume you can also somehow exploit the fact that it is quite hot on Venus to get the temperature required for the reaction. Like go down to a level with, say, 200 degrees Celsius, and use a heat-pump to increase the temperature further. I have no idea if that works for high temperatures.&lt;/p&gt;&#xA;&lt;p&gt;The assumption here is that both hydrogen and oxygen are useful chemical for long term survival. I will assume the electrochemical energy will likely come from solar cells, as Venus gets quite a lot of sunlight. But I was wondering if the process could be made more efficient by utilizing the high temperature already existing on Venus. Planetary surface is probably unlikely as the pressure is too high, so it would be at a higher altitude.&lt;/p&gt;&#xA;" OwnerUserId="11308" LastEditorUserId="47149" LastEditDate="2022-05-28T22:56:47.143" LastActivityDate="2022-05-28T22:56:47.143" Title="Is the hybrid sulfur cycle a good way of making hydrogen and oxygen on Venus?" Tags="|venus|chemistry|oxygen|isru|hydrogen|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="10806" PostTypeId="1" CreationDate="2015-08-27T18:56:26.033" Score="5" ViewCount="368" Body="&lt;p&gt;I assume utilizing habitats floating high in the Venus atmosphere as described by Geoffrey Landis. So weight, size etc is a constraint, and we will not be operating at the surface with the crushing pressure and heat. More like around 50 km above surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given say 1 ton of material. We could use that to make solar cells and batteries to store it's electricity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My alternative idea was to produce methane and feed this to a methane fuel cell like the &lt;a href=&quot;http://www.redoxpowersystems.com/products&quot; rel=&quot;nofollow&quot;&gt;Redox fuel cell&lt;/a&gt; which weights 450 kilo and generates 25kW of electricity. Assume one gets hydrogen from the sulfuric acid clouds and suck CO2 straight out of the air to generate methane using the Sabatier reaction. Hydrogen could be made with say the Hybrid sulfur cycle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These chemical reactions will require high temperature and I assuming we can get most of that heat directly from the Venus atmosphere. I am speculating that one can use heat pumps to take in say CO2 at 200 degrees and turn it into 800 degrees celsius required for the Hybrid sulfur cycle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The details don't need to be like this. It is just to give an example of how I speculate one could generate electricity and at the same time have a way of storing power in the form of methane rather than using batteries which are heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I am no chemical engineer so I have no idea how feasible this is or if this would just be very inefficient compared to plain old solar cells with batteries.&lt;/p&gt;&#xA;" OwnerUserId="11308" LastEditorUserId="25" LastEditDate="2015-08-28T01:22:23.317" LastActivityDate="2015-08-31T11:44:20.697" Title="Is utilizing the high temperature for chemical reactions on Venus a feasible way of generating electricity?" Tags="|venus|power|solar-power|temperature|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="10807" PostTypeId="1" AcceptedAnswerId="26448" CreationDate="2015-08-27T19:08:31.477" Score="8" ViewCount="206" Body="&lt;p&gt;This is relevant for Venus colonization since that considers floating cities. Since one can't use cranes, I was thinking perhaps helicopters or airships could be used to move construction pieces around. One inspiration is the &lt;a href=&quot;http://www.skylifter.eu&quot;&gt;SkyLifter&lt;/a&gt; airship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So imagine one had multiple of these airships, could they then coordinate the assembly of multiple parts in the air, or would this just be way too complicated? I assume one could hoist humans down in a tether to connect the parts once two airships got the parts close to each other and managed to hold them close for long enough time for humans to connect them.&lt;/p&gt;&#xA;" OwnerUserId="11308" LastActivityDate="2018-04-04T19:57:21.453" Title="Could airships be used to construct large structures in the air?" Tags="|venus|construction|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="10813" PostTypeId="2" ParentId="10785" CreationDate="2015-08-28T15:27:21.960" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Composition_of_Mars#Elemental_composition&quot; rel=&quot;nofollow noreferrer&quot;&gt;Martian meteorites&lt;/a&gt; suggest that the Martian crust has about twice as much iron as the Earth's crust. Of course Mars is smaller than Earth, but most of Earth is covered in water. As a result, Mars has more available iron.&lt;/p&gt;&#xA;&lt;p&gt;Concentration is another matter, as Earth's water tends to collect iron in deposits. Still, Mars has hematite and olivine ores - it had water when it was young. And you might even vacuum up all that red dust. Even more convenient, the &lt;em&gt;mafic&lt;/em&gt; (iron rich) dust has a &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_soil&quot; rel=&quot;nofollow noreferrer&quot;&gt;smaller grain size&lt;/a&gt; than the &lt;em&gt;felsic&lt;/em&gt; (iron poor) dust. Magnets and a sieve are enough to get iron-enriched dust.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastEditorUserId="7062" LastEditDate="2022-06-09T01:31:46.073" LastActivityDate="2022-06-09T01:31:46.073" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="10814" PostTypeId="2" ParentId="7745" CreationDate="2015-08-28T19:47:38.310" Score="2" Body="&lt;p&gt;I think we can assume the problem of landing is solved by having bases there in the first place. The NASA &lt;a href=&quot;http://www.space.com/28081-nasa-vision-for-venus-havoc-airships-video.html&quot; rel=&quot;nofollow&quot;&gt;HAVOC&lt;/a&gt; suggestion has ships inflate balloons quickly after entering atmosphere, thus they float at desired altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next problem is how to do this cheaply and efficiently, if anything like that can be said about space. Asteroids apparently are very &lt;a href=&quot;http://clowder.net/hop/railroad/Venus.html&quot; rel=&quot;nofollow&quot;&gt;preferable to put in Venus orbit&lt;/a&gt;, so one would I assume get supplies from stuff mined from asteroids orbiting Venus. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Aerobraking&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;When something is in orbit it goes at high speed so getting it down requires reducing that speed. Using rockets is expensive but since Venus has a thick atmosphere aerobraking is a cheaper way to do it. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Heat shields&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Heat shields can be made from &lt;a href=&quot;http://www.permanent.com/space-industry-refractories.html&quot; rel=&quot;nofollow&quot;&gt;readily accessible resources in space&lt;/a&gt;. I've read of one approach of using disposable heat shields one just wraps around a package and it burns up eventually by the time the package is delivered.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Regenerative Aerobraking&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Another cool approach is utilizing the aerobraking for &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.77.8000&quot; rel=&quot;nofollow&quot;&gt;energy generation&lt;/a&gt;. An enormous amount of energy is released when lowering orbit, which could be utilized. A &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.77.8000&quot; rel=&quot;nofollow&quot;&gt;paper&lt;/a&gt; by Robert W. Moses discuss this.&lt;/p&gt;&#xA;" OwnerUserId="11308" LastEditorUserId="4660" LastEditDate="2015-08-28T20:07:49.857" LastActivityDate="2015-08-28T20:07:49.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10815" PostTypeId="1" AcceptedAnswerId="10816" CreationDate="2015-08-28T20:25:16.467" Score="19" ViewCount="722" Body="&lt;p&gt;I noticed to my surprise today that the very early Soviet flights were to a very high inclination - all the Vostok flights were between 64.9° and 65°, and the Voskhod flights were at 64.7° and 64.8°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After this, Soyuz 1 was at 50.8°, and subsequent flights seem to have been at or around a standard 51.6°. The only exception appears to be Soyuz 22, which went to 64.75°. On the American side, all orbital Mercury flights were at 32.5°; early Gemini flights were the same before dropping to 28.8°. Apollo lunar flights used a parking orbit around 32.5°, and Skylab was at 50°. Several Shuttle flights went to 57°, and STS-36 made a rather complex flight to 62°. Vandenberg launches would have gone to nearly 90°, but they were cancelled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally speaking, the high-inclination flights seem to have been aiming for earth observation opportunities (or in the case of STS-36, to deploy an earth observation satellite). Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_22&quot;&gt;explicitly mentions&lt;/a&gt; this reason for Soyuz 22.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it's not clear why it was used for the earliest Vostok/Voskhod flights - was it to support earth observation objectives? Was there a political justification, to ensure it could be observed as widely as possible? Was it a quirk of the launch system which made this inclination easy to achieve?&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="25" LastEditDate="2015-08-28T20:41:16.713" LastActivityDate="2015-08-30T00:54:35.487" Title="Early high-inclination crewed flights" Tags="|launch|low-earth-orbit|history|crewed-spaceflight|russia|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10816" PostTypeId="2" ParentId="10815" CreationDate="2015-08-28T20:39:51.460" Score="11" Body="&lt;p&gt;I have a few suspicions, but I can't find a really good source that outlines exactly what. Here's a few bits of information:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The Vostok rocket was used to launch spy satellites, which would be a higher inclination.&lt;/li&gt;&#xA;&lt;li&gt;Being only 1 orbit, the inclination could have been determined to allow for the uncertainty ellipse of landing to happen over Russia in a key part of the country, where it would be unlikely to damage anything when it landed, yet remain accessible. &lt;/li&gt;&#xA;&lt;li&gt;I suspect the ground track was made to avoid certain places, not fly deliberately over them.&lt;/li&gt;&#xA;&lt;li&gt;The communication network might have only been set up to support a higher inclination launch, likely due to #1.&lt;/li&gt;&#xA;&lt;li&gt;It could be to give really wide berth to China. I'm sure that the launches weren't as accurate as today, but I find it hard to believe they had a 10 degree inclination uncertainty. Still, it is possible...&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Here's an image of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_1#/media/File:Vostok_1_orbit.png&quot; rel=&quot;noreferrer&quot;&gt;ground track&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/h2uVn.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h2uVn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the higher inclination puts it clearly over Russia for most of it's early flight, and has it over the main part of Russia during re-entry. They probably wanted to avoid the Black Sea as a potential landing spot, as the vehicle could not land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I suspect the main reason was that most launches to test the Vostok were spy satellites, which benefit from a higher inclination, and the communication network was set up to support such. Given the secrecy of the early Soviet space program, the exact answer might not be known to the public...&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-08-29T01:58:52.617" LastActivityDate="2015-08-29T01:58:52.617" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10820" PostTypeId="1" AcceptedAnswerId="10824" CreationDate="2015-08-29T03:59:58.210" Score="4" ViewCount="399" Body="&lt;p&gt;The locations of the collinear Lagrange points L1, L2, L3 are mass weighted, so for example Sun-Earth-L1 is only 1% of the distance to the Sun from Earth. But L4 and L5 are one AU from both the Sun and from Earth. Shouldn't Earth's gravity be negligible that far away? Are L4 and L5 another kind of phenomena than L1, L2, L3?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6944" LastEditDate="2015-08-29T15:03:02.380" LastActivityDate="2015-12-23T17:56:57.630" Title="Why are the Trojan libration points equidistant and not mass weighted?" Tags="|gravity|lagrangian-points|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10822" PostTypeId="1" AcceptedAnswerId="10880" CreationDate="2015-08-29T08:07:12.667" Score="12" ViewCount="1018" Body="&lt;p&gt;New Horizons will fly past the Kuiper Belt object &lt;a href=&quot;https://en.wikipedia.org/wiki/2014_MU69&quot; rel=&quot;nofollow noreferrer&quot;&gt;2014 MU&lt;sub&gt;69&lt;/sub&gt;&lt;/a&gt; in January 2019. This will use 56 m/s of the remaining ∆V budget of 130 m/s &lt;a href=&quot;http://www.hou.usra.edu/meetings/lpsc2015/pdf/1301.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;(1)&lt;/a&gt;. The encounter will occur near the inner edge of the belt:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i2.wp.com/www.universetoday.com/wp-content/uploads/2015/08/150828-ring.png&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks perhaps feasible to find another target, but only if they look for one. Is it, and will they?&lt;/p&gt;&#xA;" OwnerUserId="3054" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.717" LastActivityDate="2019-01-02T10:53:58.387" Title="Will New Horizons seek a third target?" Tags="|new-horizons|navigation|flyby|kuiper-belt|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="10823" PostTypeId="2" ParentId="10822" CreationDate="2015-08-29T10:03:18.440" Score="5" Body="&lt;p&gt;I'm confident that if New Horizon's can reach another target, they will attempt to visit it. One of the reasons PT1 was selected as the target is that it was the closest object they could reach, leaving a reasonable amount of delta v, about half of what remained after flying by Pluto. I'm also confident they will try to find such potential targets. However, I suspect it will be difficult to find another candidate at all that could be reached. Even the selected candidate now was difficult to find, and I suspect it would be even more challenging to find another suitable target. Still, no doubt an attempt to find a candidate will be attempted, if it looks like there is remaining fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that likely the fuel will be further used to refine the trajectory to be at a very optimal location, which will allow better science of the one target, but further reduce the likelihood of another flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note from &lt;a href=&quot;http://spaceflightnow.com/2015/08/31/distant-ice-ball-to-become-farthest-object-ever-visited/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; the following quote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It also keeps open a possibility for New Horizons to continue on a second extended mission after the flyby, potentially making astronomical observations and probing the boundary between the solar system and interstellar space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That seems to be the current plan, although I would still be willing to bet that an attempt will be made to visit another object, given the opportunity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Another target will be sought, possibly using Hubble but also using LORRI to try and find a potential target. It seems unlikely another target will be found, but they will still make the attempt.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-01-02T10:53:58.387" LastActivityDate="2019-01-02T10:53:58.387" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="10824" PostTypeId="2" ParentId="10820" CreationDate="2015-08-29T14:54:06.493" Score="4" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; (emphasis mine)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason these points are in balance is that, at L4 and L5, the distances to the two masses are equal. Accordingly, &lt;strong&gt;the gravitational forces from the two massive bodies are in the same ratio as the masses of the two bodies, and so the resultant force acts through the barycenter of the system&lt;/strong&gt;; additionally, the geometry of the triangle ensures that the resultant acceleration is to the distance from the barycenter in the same ratio as for the two massive bodies. The barycenter being both the center of mass and center of rotation of the three-body system, this resultant force is exactly that required to keep the smaller body at the Lagrange point in orbital equilibrium with the other two larger bodies of system. (Indeed, the third body need not have negligible mass.) The general triangular configuration was discovered by Lagrange in work on the three-body problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also from the article:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ycwqg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ycwqg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-08-29T14:59:21.173" LastActivityDate="2015-08-29T14:59:21.173" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="10826" PostTypeId="2" ParentId="10820" CreationDate="2015-08-29T22:32:42.657" Score="3" Body="&lt;p&gt;The brief answer is that the L1 and L2 points are different in the sense that the gravity of Earth exerts a far more significant force on objects at L1 and L2. Earth's gravity is about 10,000 times stronger at L1/L2 than L4/L5 and 40,000 times stronger than at L3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Objects at L3, L4 and L5 are orbiting the sun at very nearly but not quite exactly 1AU. They naturally orbit in about 365.25 days, would continue to do so if the Earth was zapped out of existence. Earth's gravity isn't exerting much force on them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast, the heliocentric distance of L1 is about 0.99AU and L2 about 1.01AU. Normally an object at 0.99AU would orbit the sun in about 363.4 days and an object at 1.01AU would orbit the sun in about 367.1 days. But at L1 Earth's gravity partially counteracts the Sun's gravity - by about 3% - sufficient to allow an object to orbit in 365.25 days despite being closer to the Sun. And at L2 the Earth's gravity adds to the Sun's - again about 3% - sufficient to allow an object to orbit in 365.25 days despite being further from the Sun.&#xA;&lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/What_are_Lagrange_points&quot; rel=&quot;nofollow&quot;&gt;Here's a neat webpage on the ESA site with animations and detailed descriptions&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay so what's the difference? Any object at very nearly 1AU and not too close to Earth will maintain its position relative to Earth relatively well for a time span measured in decades, although over time the cumulative effects of Earth's gravity will perturb the orbit. L3 is more stable and L4 and L5 are stable infinitely for idealized 3 body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast any object with a semi-major axis around the Sun of 0.99AU or 1.01AU will normally very quickly change its position relative to Earth, being at least 2 days ahead or behind the Earth within a single year. The &lt;em&gt;exception&lt;/em&gt; is L1 and L2 where the gravity of the Sun and Earth combine such that an object with a suitable velocity and minor station-keeping can follow an orbit synchronized with Earth's. In that sense, the L1 and L2 points are pretty special and Earth's gravity plays a much larger role in their stability.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2015-12-22T10:49:42.877" LastActivityDate="2015-12-22T10:49:42.877" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="10827" PostTypeId="2" ParentId="10815" CreationDate="2015-08-30T00:54:35.487" Score="0" Body="&lt;p&gt;Baikonur is at 46 deg. latitude.  You can lift the most payload by going into an inclination that equals your launch latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS ended up at 57 deg., so Shuttle flights needed to launch close to that to be able to rendezvous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A consideration is where debris might go for a launch failure, so you don't have low inclination launches from Vandenberg.  For the Shuttle, a consideration was what could be reached in case of an abort.  Easter Island was the plan if SLC-6 at Vandenberg had ever been used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Early Soviet launch vehicles had limited or no capability to gimbal the engines, so would set azimuth by rotating the entire stack on the launch pad.&lt;/p&gt;&#xA;" OwnerUserId="11468" LastActivityDate="2015-08-30T00:54:35.487" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10828" PostTypeId="2" ParentId="10803" CreationDate="2015-08-30T01:07:46.323" Score="1" Body="&lt;p&gt;Your first question is a short description of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Da_Vinci_Project&quot; rel=&quot;nofollow&quot;&gt;da Vinci Project&lt;/a&gt; &#xA;that was an entrant in the X Prize contest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reaching orbit means getting your vehicle up to 9.3 kilometers per second.  Without cargo, this still leaves you pushing the weight of the empty fuel tank and whatever guidance equipment you have.&lt;/p&gt;&#xA;" OwnerUserId="11468" LastEditorUserId="1142" LastEditDate="2015-08-30T16:46:50.193" LastActivityDate="2015-08-30T16:46:50.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10829" PostTypeId="1" CreationDate="2015-08-30T01:18:52.480" Score="17" ViewCount="4079" Body="&lt;p&gt;If I understand correctly, solid booster rockets never expend literally all their fuel before jetissoning, because the burn rate slows way down in the end and it would take too long to spend it all, plus the burn rate becomes chaotic near the end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So they separate before that happens. What I want to know is how much fuel is leftover for the average Space Shuttle mission. Either a percentage of the full fuel, or absolute value, are fine.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6944" LastEditDate="2019-07-02T23:25:52.023" LastActivityDate="2021-01-27T20:23:57.230" Title="For a typical Shuttle mission, how much solid fuel is leftover at SRB separation?" Tags="|space-shuttle|solid-fuel|stages|booster|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10833" PostTypeId="2" ParentId="7994" CreationDate="2015-08-30T01:49:14.533" Score="2" Body="&lt;p&gt;Your inner ears will complain, not your eyes, if you rotate at more than ~1 rpm.  The &lt;a href=&quot;http://en.wikipedia.org/wiki/Stanford_torus&quot; rel=&quot;nofollow&quot;&gt;Stanford Torus&lt;/a&gt; study, which was the source of your picture, was just under 1000 m as a radius.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sunlight was bounced off of a large mirror, a small ring mirror, and several other surfaces to get high energy particles absorbed before shining on the crops and habitated area.&lt;/p&gt;&#xA;" OwnerUserId="11468" LastEditorUserId="1142" LastEditDate="2015-08-30T16:43:17.297" LastActivityDate="2015-08-30T16:43:17.297" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10837" PostTypeId="1" AcceptedAnswerId="10840" CreationDate="2015-08-30T06:14:09.363" Score="43" ViewCount="27580" Body="&lt;p&gt;Most satellites are in Low Earth Orbit.  Some other satellites are in Geostationary Orbit because their function requires it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GPS (and other GNSS, e.g. GLONASS) satellites are in a much higher MEO orbit (sub-GEO):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WCS9M.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WCS9M.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why to they need to be in such a high orbit?  Clearly the GPS design does not require that they need to be in GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System&quot; rel=&quot;noreferrer&quot;&gt;GPS wikipedia page&lt;/a&gt; mentions that with this orbit, the satellites have an orbital period of about 12 hours, and thus follow the same track over the earth - this was useful for debugging when the system was first being set up.  But surely a similar effect could have been achieved with an 8 or 6 hour orbital period (or some other divisor of 24) for much less expense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Possible, though unconfirmed reasons I can think of for the high orbit:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Initially (and still) a military project, having the satellites in such a high orbit makes them harder for &lt;em&gt;the enemy&lt;/em&gt; to shoot down.&lt;/li&gt;&#xA;&lt;li&gt;Being higher up means more satellites are in line-of-sight to any given point on the surface of the earth.  I don't know how many satellites would be required for the same level of service if they were at the 8 or 6 hour period orbit, though I'd be interested to see how the costs compare to put more satellites in lower orbits.&lt;/li&gt;&#xA;&lt;li&gt;LEO satellites are more affected by atmospheric drag, so will need to perform more regular station-keeping maneuvers.  Presumably they need to be temporarily taken out of GPS service when performing these maneuvers - perhaps this is unacceptable within the GPS design.  Also more fuel is required for station-keeping, or there will be shorter service lives which perhaps offsets the extra expense of the higher orbit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, why are GPS satellites in such high orbits?&lt;/p&gt;&#xA;" OwnerUserId="758" LastEditorUserId="38" LastEditDate="2018-01-02T17:36:09.880" LastActivityDate="2022-01-02T16:00:50.133" Title="Why are the GPS constellation satellites in such a high orbit?" Tags="|artificial-satellite|orbit|gps|gnss|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10838" PostTypeId="1" AcceptedAnswerId="10843" CreationDate="2015-08-30T06:29:16.727" Score="8" ViewCount="839" Body="&lt;p&gt;The Tardigrade (or Water Bear) is a remarkable creature:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jXUyx.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jXUyx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is able to survive in all sorts of extreme environments (taken from wikipedia):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;temperatures from just above absolute zero to well above the boiling point of water&lt;/li&gt;&#xA;&lt;li&gt;pressures about six times greater than those found in the deepest ocean trenches&lt;/li&gt;&#xA;&lt;li&gt;ionizing radiation at doses hundreds of times higher than the lethal dose for a human&lt;/li&gt;&#xA;&lt;li&gt;they can go without food or water for more than 10 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce&lt;/li&gt;&#xA;&lt;li&gt;and last but not least, they can survive the vacuum of outer space&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Are there any other life forms that are known to survive the vacuum of space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in any forms of life, be it plant, animal, prokaryote or some other form (I'm not a biologist).  &lt;/p&gt;&#xA;" OwnerUserId="758" LastEditorUserId="758" LastEditDate="2015-08-30T06:36:39.110" LastActivityDate="2015-08-31T13:39:22.590" Title="Which earth-based lifeforms can survive the vacuum of space?" Tags="|life|astrobiology|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10839" PostTypeId="2" ParentId="10837" CreationDate="2015-08-30T07:55:15.567" Score="25" Body="&lt;p&gt;GPS / GNSS satellites are orbiting at an altitude where their orbital period is half the Earth's mean sidereal day (23 hours, 56 minutes, 4.0916 seconds) so their &lt;a href=&quot;https://en.wikipedia.org/wiki/Nodal_precession&quot;&gt;nodal precession&lt;/a&gt; rate is both small (roughly 4 minutes, or &amp;pm;222 km East-West drift along the Earth's equator per day) and fairly constant, or perhaps better said stable, over longer periods of time. This keeps their &lt;a href=&quot;https://en.wikipedia.org/wiki/Longitude_of_the_ascending_node&quot;&gt;longitude of the ascending node&lt;/a&gt; to within &amp;pm;2 degrees off nominal and enables ground track repeatability for the constellation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/CLhtv.png&quot; alt=&quot;Daily time shift of GPS satellite ground track repeat relative to 24 hours based on broadcast ephemeris data&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Daily time shift of GPS satellite ground track repeat relative to 24 hours based on broadcast ephemeris data. Source: &lt;a href=&quot;http://www.insidegnss.com/&quot;&gt;InsideGNSS.com&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This ground track repeatability was important in the early days of GPS, so that sufficient ground coverage was assured (in sessions, not really all day long) with a much smaller number of constellation satellites. Lower orbits would have been subject to stronger orbital perturbations, especially the already mentioned nodal precession due to Earth's shape being an &lt;a href=&quot;https://en.wikipedia.org/wiki/Figure_of_the_Earth&quot;&gt;oblate spheroid&lt;/a&gt; and not a perfect sphere, so satellites' East-West drift rate would have been higher, while not completely eliminating other perturbing effects (such as the Sun's and the Moon's gravity, solar radiation pressure, ...) or would have been higher still (atmospheric drag) and causing higher orbital decay rate or otherwise require more frequent orbit corrective burns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is explained in more detail in &lt;a href=&quot;http://www.insidegnss.com/auto/JulAug06GNSSSolutions_secured.pdf&quot;&gt;June/July 2006 issue of Inside GNSS&lt;/a&gt;, in the &lt;em&gt;GNSS Solutions: Orbital precession, optimal dual-frequency techniques, and Galileo receivers&lt;/em&gt; article by Penina Axelrad and Kristine M. Larson.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2015-08-30T22:56:36.210" LastActivityDate="2015-08-30T22:56:36.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10840" PostTypeId="2" ParentId="10837" CreationDate="2015-08-30T10:57:38.257" Score="30" Body="&lt;p&gt;The main reason they are in such a high orbit is to allow for more of the Earth to be visible at any one time. In order to have a reasonable amount of the Earth visible, you have to be high up. A lower altitude could in theory work as well, but the chosen altitude seems to be a far enough distance to be useful, but not so far as to have communication link issues, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost to get a GPS satellite to its orbit isn't substantially different than if it were at a, say, 6 hour orbit. The link budget would improve somewhat, allowing for a slightly cheaper satellite to be built. The big problem, however, is that you would need more satellites to ensure that the complete coverage had been met. GPS is fundamentally a military system, and it is required not to have gaps on the ground. It should be noted, here's the percentage of Earth visible from various altitudes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;12 hour orbit- 38%&lt;/li&gt;&#xA;&lt;li&gt;8 hour orbit - 34.3%&lt;/li&gt;&#xA;&lt;li&gt;6 hour orbit - 31%&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It should be noted that every other &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_navigation#Global_navigation_systems&quot; rel=&quot;noreferrer&quot;&gt;GNSS system&lt;/a&gt; that has been launched uses a similar orbit to GPS. GLONASS is 8/17 of a day, BeiDou 9/17, and Galileo is 10/17. India is working on a system using purely GEO satellites. These chose a similar band because GPS proved it worked well at those altitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;s&gt;Another factor is the orbital velocity. The orbital speed at a 6 hour orbit is about 5 km/s. At GPS, it is 3.8 km/s. This slower speed allows for a narrower bandwidth (since the Doppler frequency shifts are smaller), using less spectrum and allowing more channels to be in use.&lt;/s&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other reasons as well, involving the accuracy of the GPS. That particular altitude works well to provide sufficient accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, the altitude that GPS is at works quite well for it, there are few other spacecraft using such orbits making them more stable overall, and it seems like a good idea to continue using GPS satellites in the 12 hour orbits they are being placed in.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="12102" LastEditDate="2017-01-21T17:31:58.477" LastActivityDate="2017-01-21T17:31:58.477" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="10841" PostTypeId="2" ParentId="10803" CreationDate="2015-08-30T20:51:17.547" Score="12" Body="&lt;p&gt;While it's theoretically possible to home-build something capable of attaining the needed 9.3+ km/s, it's not something that the average person can pull off. You'd need to be exceptionally talented and extremely wealthy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd also still need multiple flying stages, and a rather large rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's implausible, but not impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that balloon launching of a solid fuel rocket to suborbital is directly comparable to the SpaceShip 1 and White Knight combo - Attaining orbital insertion would require a higher thrust to drag ratio - which can be attained by upscaling the rocket or adding some (parachute recoverable) SRB's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipOne&quot; rel=&quot;noreferrer&quot;&gt;Space Ship One&lt;/a&gt; is within the realm of what can be home-built. It's a carbon fiber shell, with a solid rocket/liquid oxidizer motor.... The project, including multiple aircraft designs, ran 25 million. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Converting to a balloon launch, you're looking at (potentially) about twice the ceiling of &lt;a href=&quot;https://en.wikipedia.org/wiki/Scaled_Composites_White_Knight&quot; rel=&quot;noreferrer&quot;&gt;White Knight&lt;/a&gt; (which is about 16000 m). Assuming a safe drop of 1000m at launch, then climbout, there should be just barely enough delta-V to get it into a technical orbit... but you'd want some external thrust (parachute recovered) to put it into a useful orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=teOq5yIX5vs&amp;amp;t=3328s&quot; rel=&quot;noreferrer&quot;&gt;OII∞&lt;/a&gt; is a low cost launch system (in development) that uses a high altitude balloon launch of up to 0.2 tonnes payload. It's also a multi-stage design - &quot;2 donuts and a muffin&quot; hung from a balloon and then launching from high altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Refs:&lt;br&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipOne&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia Space Ship One&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Scaled_Composites_White_Knight&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia: Scaled Composites White Knight &lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://www.youtube.com/watch?v=teOq5yIX5vs&amp;amp;t=3328s&quot; rel=&quot;noreferrer&quot;&gt;TMRO: Toroids are doughnuts with OIIOO - Orbit 10.11 (Youtube)&lt;/a&gt;&lt;br&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="158" LastEditDate="2017-03-19T19:51:52.840" LastActivityDate="2017-03-19T19:51:52.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10842" PostTypeId="1" CreationDate="2015-08-30T22:52:25.847" Score="7" ViewCount="903" Body="&lt;p&gt;I'm surprised wikipedia doesn't have a history section for its &lt;a href=&quot;https://en.wikipedia.org/wiki/Ammonium_perchlorate_composite_propellant&quot; rel=&quot;noreferrer&quot;&gt;APCP&lt;/a&gt; article. If I'm not mistaken, early solid rockets used something other than APCP, and really really early solid rockets used black powder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what was the first large rocket to use APCP in a stage? I thought it might be the Polaris missile (1960), but neither wikipedia nor astronautix describes exactly what kind of solid fuel it uses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm hoping the answer will lead me to the developmental history of this fuel as well.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="12102" LastEditDate="2021-08-23T01:11:27.440" LastActivityDate="2021-08-23T01:11:27.440" Title="What was the first large rocket to use APCP solid fuel?" Tags="|rockets|fuel|solid-fuel|stages|spaceflight-firsts|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10843" PostTypeId="2" ParentId="10838" CreationDate="2015-08-30T23:17:41.913" Score="8" Body="&lt;p&gt;There's a list of micro-organisms tested in outer space here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_microorganisms_tested_in_outer_space&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/List_of_microorganisms_tested_in_outer_space&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it doesn't say the results of the tests.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lichens certainly shown to survive:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.newscientist.com/article/dn8297-hardy-lichen-shown-to-survive-in-space/&quot; rel=&quot;nofollow&quot;&gt;https://www.newscientist.com/article/dn8297-hardy-lichen-shown-to-survive-in-space/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it comes to multicellular animals - then apart from the tardigrades, other ones of interest are Bdelloid rotifers &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bdelloid rotifers are radiation resistant animals&#xA;&lt;a href=&quot;http://blogs.discovermagazine.com/notrocketscience/2008/03/24/bdelloid-rotifers-the-worlds-most-radiation-resistant-animals/#.VeOOlflViko&quot; rel=&quot;nofollow&quot;&gt;http://blogs.discovermagazine.com/notrocketscience/2008/03/24/bdelloid-rotifers-the-worlds-most-radiation-resistant-animals/#.VeOOlflViko&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two especially resistant species of bdelloid rotifers Adineta vaga and Philodina roseola survived 1000 grays of radiation and still had 10% of the population able to reproduce&#xA;&lt;a href=&quot;http://www.astrobio.net/topic/origins/extreme-life/radiation-resistant-rotifers/&quot; rel=&quot;nofollow&quot;&gt;http://www.astrobio.net/topic/origins/extreme-life/radiation-resistant-rotifers/&lt;/a&gt;&#xA;For the original paper with the figures, see&#xA;&lt;a href=&quot;http://www.pnas.org/content/105/13/5139.long&quot; rel=&quot;nofollow&quot;&gt;http://www.pnas.org/content/105/13/5139.long&lt;/a&gt;&#xA;They also survive 82 days storage in vacuum (followed by 30 minutes exposure to 100C)&#xA;&lt;a href=&quot;http://plankt.oxfordjournals.org/content/early/2013/04/07/plankt.fbt032.full&quot; rel=&quot;nofollow&quot;&gt;http://plankt.oxfordjournals.org/content/early/2013/04/07/plankt.fbt032.full&lt;/a&gt;&#xA;But I can't yet find a record of them flown to space. Do say if anyone knows of one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On micro-organisms, green algae (cyanobacteria), haloarchaea, radiodurans, are a few of the examples that are strongly UV / radiation resistance and that could survive the near vacuum conditions of the Mars surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some of the candidate lifeforms for Mars - that's a near vacuum, for the habitats exposed to the atmosphere, but not all of these could survive the vacuum of space. This is a draft article I did for Wikipedia which turned out to be too specialist for them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Draft:Present_day_habitability_of_Mars#Candidate_lifeforms_for_Mars&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Draft:Present_day_habitability_of_Mars#Candidate_lifeforms_for_Mars&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the Expose R microbes survived both Mars and vacuum conditions, but I can't remember which now (when writing that I was more interested in whether they could survive on Mars).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll see if I can find a more complete list of the ones that can survive vacuum conditions.&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="3038" LastEditDate="2015-08-31T13:39:22.590" LastActivityDate="2015-08-31T13:39:22.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10844" PostTypeId="2" ParentId="9741" CreationDate="2015-08-31T00:37:44.897" Score="6" Body="&lt;p&gt;I found a paper by &lt;a href=&quot;http://www.dlr.de/Portaldata/55/Resources/dokumente/sart/0095-0212prop.pdf&quot; rel=&quot;noreferrer&quot;&gt;4 authors&lt;/a&gt; that compares Kerosene engines to Methane engines. It states the chamber temperature of a Methane rocket is 3533K, comparable to the RP-1 3676K chamber temperature. The optimal temperature in a vacuum is 3563K, compared to 3701K. So it seems the temperature constraints are very similar to RP-1 rockets. See the paper for other comparisons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isn't the highest temperature, but results in the highest ISP, as determined parametrically.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-08-31T02:03:59.677" LastActivityDate="2015-08-31T02:03:59.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10845" PostTypeId="1" AcceptedAnswerId="10852" CreationDate="2015-08-31T01:33:08.230" Score="11" ViewCount="4082" Body="&lt;p&gt;I am trying to find out of there is a term that can be used when referring to both a planet and any accompanying moons (if present) as a whole collection.  For example if you were to refer to Mars and its moons as a whole, is there a term for that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT to clarify: I am trying to find a generic term if one exists.&lt;/p&gt;&#xA;" OwnerUserId="11474" LastEditorUserId="11474" LastEditDate="2015-08-31T01:42:17.490" LastActivityDate="2019-06-16T21:03:37.370" Title="Is there a term to refer to a planet and its accompanying moons as a whole collection?" Tags="|terminology|planet|moons|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10846" PostTypeId="1" CreationDate="2015-08-31T05:15:10.903" Score="14" ViewCount="14036" Body="&lt;p&gt;I've seen multiple images taken from outside the Space Station like this one &lt;a href=&quot;https://i.stack.imgur.com/qACBb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qACBb.jpg&quot; alt=&quot;ISS image from https://upload.wikimedia.org/wikipedia/commons/thumb/8/81/STS132_undocking_iss2.jpg/640px-STS132_undocking_iss2.jpg&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;or this one &lt;a href=&quot;https://i.stack.imgur.com/PPIu9.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PPIu9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &#xA;and even timelapses from far outside showing the solar panels moving over time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a satellite that follows the ISS that they could take pictures from that distance? I can't imagine they have an astronaut out that far away from the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know the exact way they take these type of pictures?&lt;/p&gt;&#xA;" OwnerUserId="11475" LastEditorUserId="5979" LastEditDate="2018-11-17T20:54:33.743" LastActivityDate="2018-11-17T20:56:22.910" Title="How do astronauts take pictures from outside the ISS?" Tags="|iss|photography|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="10847" PostTypeId="2" ParentId="5394" CreationDate="2015-08-31T05:54:27.803" Score="5" Body="&lt;p&gt;Edit prompted by DougSpace on December 30, 2015. LCROSS numbers may have been over estimated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a screen capture from the &lt;a href=&quot;https://www.facebook.com/groups/6457556893/search/?query=LCROSS&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moon Society's Facebook page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KgZlO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KgZlO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hate to break this bad news but it seems the numbers I reported below are optimistic :(.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't have access to the 23 Sept. 2011 edition of Science. If anyone can access it and report back, I'd be grateful.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The Oct. 2010 Science Journal looked at Lcross ejecta. Here are some of the percentages:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;N 6.6000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CO 5.7000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;H2O 5.5000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zn 3.1000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;V 2.4000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ca 1.6000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Au 1.6000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mn 1.3000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hg 1.2000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Co 1.0000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;H2S 0.9213%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NH3 0.3317%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cl 0.2000%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SO2 0.1755%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;C2H4 0.1716%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CO2 0.1194%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;C 0.0900%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sc 0.0900%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CH3OH 0.0853%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CH4 0.0366%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;O 0.0200%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OH 0.0017%&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-12-31T01:04:38.390" LastActivityDate="2015-12-31T01:04:38.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10848" PostTypeId="2" ParentId="10846" CreationDate="2015-08-31T08:16:03.317" Score="9" Body="&lt;p&gt;Like Andrew Thompson comments, these, let's call them &lt;em&gt;medium-range&lt;/em&gt; photographs, are taken from the visiting spacecraft, either during approach or during departure. There are several approach and departure modes in use, mainly v-bar (along the target's velocity vector) and r-bar (in the target's radial vector), but sometimes a z-bar (from the side to the velocity vector) approach is also used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes, spacecraft would also be repositioned from one docking location to another, so they make room for additional visiting spacecraft or as risk management procedures, something that is done with its complete crew (in case of problems they could simply deorbit and land), so there would be many different angles that the station could be photographed from at these &lt;em&gt;medium-ranges&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I always wanted to tack this &lt;a href=&quot;https://www.youtube.com/watch?v=pRaZgSVnsNs&quot;&gt;video of the One-Year Crew Docking to the International Space Station&lt;/a&gt; aboard &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-16M&quot;&gt;Soyuz TMA-16M&lt;/a&gt; somewhere here, because it's really cool and this &lt;a href=&quot;https://www.nasa.gov/content/one-year-crew&quot;&gt;One-Year Crew&lt;/a&gt; (Scott Kelly and Mikhail Kornienko) is currently on the station. It only shows one rendezvous approach (r-bar docking to the zenith-facing port on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Poisk_%28ISS_module%29&quot;&gt;Poisk&lt;/a&gt; module) out of the three described, but should give you an overview of what opportunities to take photographs of the station the visiting crew would have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/pRaZgSVnsNs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Closer to the station, photographs can also be taken by astronauts during EVA (Extravehicular Activity), using various point &amp;amp; shoot cameras in protective enclosures and strapped to their suit, but still keeping them relatively small so they don't get in the way of astronaut's work. There wouldn't be any satellites this close to the station because orbital speeds are &lt;em&gt;ludicrous&lt;/em&gt; and the station would also perform debris avoidance maneuvers when needed, not to come too close and avoid collision with those that conjunction analysis has shown their orbits cross at some future point in time. But the station does also serve as an orbital launch platform for small CubeSat satellites (in units of 10 cubic centimeters), so it's entirely possible that some future nanosatellite launched from it does take a snap or two of the station and send those to its ground control. To my knowledge tho, that hasn't happened yet.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-08-31T08:16:03.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10849" PostTypeId="2" ParentId="10846" CreationDate="2015-08-31T08:18:38.053" Score="18" Body="&lt;p&gt;Because it's not easy to get good images of many portions of the structure, the standard departure for many of the later shuttle flights included a &quot;flyaround&quot; of the structure.  This allowed pictures of significant portions and is how the first image was taken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The standard location for this was at 400 to 600 feet in distance.  The procedure for STS-117 was mentioned in this &lt;a href=&quot;http://www.nasaspaceflight.com/2007/06/atlantis-undocks-from-iss-stunning-flyaround/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nasaspaceflight.com article&lt;/a&gt;, and the video for that flight is on &lt;a href=&quot;https://www.youtube.com/watch?v=cWJWb1MnA7c&quot; rel=&quot;nofollow noreferrer&quot;&gt;youtube&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second photo is taken from further away and shows a shuttle in it, so was not taken the same way.  This one was taken by Paolo Nespoli on a departing Soyuz craft as he and the crew returned to earth. &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/multimedia/gallery/iss027e036700.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/mission_pages/station/multimedia/gallery/iss027e036700.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Random craft cannot take such pictures.  Unless it's actually there to dock or depart from the station, nothing controllable would be allowed to be so close.  No other craft are kept near the ISS orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No astronaut can take an image similar to that.  Astronauts on the ISS are tethered and on the structure at all times.  Tethers are not long enough to reach such distances.  Some free-flights were taken during the shuttle program, but the shuttle was maneuverable enough to retrieve a free-flyer in distress.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastEditorUserId="5979" LastEditDate="2018-11-17T20:56:22.910" LastActivityDate="2018-11-17T20:56:22.910" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="10850" PostTypeId="2" ParentId="10806" CreationDate="2015-08-31T11:15:40.433" Score="1" Body="&lt;p&gt;My hunch here is that there is something wrong with the heatpump idea.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Solar powered heat pump&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Heat pumps usually have a maximum coefficient of performance of about 4 - for every 1J of electricity, they can transfer 4J of heat. Solar panels usually have an efficiency of about 25%, for every 1J of sunlight, they can generate 0.25J of electricity. So for solar powered heat pumps, for every 1J of sunlight, you could get 1J of heat. I'm sure the problem is obvious - you could skip the solar panels and heat pump and just use concentrated sunlight to generate the heat.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Nuclear powered heat pump&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The other viable energy source is nuclear, but a nuclear reactor already generates ample waste heat. I believe nuclear reactors are usually somewhere between 33% and 50% efficiency, so for every 1J of electricity, you get 1-2J of waste heat. That is low grade heat, and we want high grade heat here, so extracting high grade heat from the reactor will lower it's electrical output, but reduced efficiency is also what would happen if you take the reactor into to lower, hotter altitudes, the problem is &lt;a href=&quot;http://www.world-nuclear.org/info/Current-and-Future-Generation/Cooling-Power-Plants/&quot; rel=&quot;nofollow&quot;&gt;cooling the reactor&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The bigger the temperature difference between the internal heat source and the external environment where the surplus heat is dumped, the more efficient is the process in achieving mechanical work – in this case, turning a generator. Hence the desirability of having a high temperature internally and a low temperature in the external environment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The take home is you would be much better off designing the reactor to run at 50km, and extracting high grade heat directly from the reactor, this would dramatically simplify the setup, and probably still result in higher efficiency.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Other problems&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;I think there are also other grounds on which this idea doesn't work, for one, the vertical altitude difference is about 10-12km, it is going to take energy to make that trip, for example it might involve compressing a lifting gas to cause the airship to descend. The problem is, that while heat pumps become more efficient with a hotter heat source, it wouldn't be clear that the energy expended in descending and then ascending 10-12km, wouldn't have been better spent just running a heat pump at an altitude of 50km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The clincher is that if you are above the sulfuric cloud layer you get ample solar power during the day but if you are below the clouds you need to rely on either a nuclear reactor (already covered) or stored energy. But at least half the point of generating methane is as a means of energy storage, using stored energy to create more stored energy does not sound terribly efficient.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;TL;DR&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;You're either going to have a nuclear reactor, which already provides ample high grade heat, or you're going to be using solar power and will be wanting to perform energy storage where energy is abundant above the cloud layer, which incidentally also provides an abundant source of high grade heat in the form of concentrated sunlight.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2015-08-31T11:44:20.697" LastActivityDate="2015-08-31T11:44:20.697" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10851" PostTypeId="2" ParentId="10648" CreationDate="2015-08-31T11:44:21.070" Score="5" Body="&lt;p&gt;The Mars 2022 orbiter is expected to have this capacity. From the  &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Telecommunications_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;Mars Telecommunications Orbiter&lt;/a&gt; wiki page, one can see the entire history, including the latest plans. See also &lt;a href=&quot;http://spaceflightnow.com/2015/03/03/nasa-eyes-ion-engines-for-mars-orbiter-launching-in-2022/&quot; rel=&quot;noreferrer&quot;&gt;SpaceFlightNow&lt;/a&gt;. This mission will be similar to MRO, in that it has a science mission, but doubles as a high speed data relay, although it will be a higher speed relay than MRO, as it will have laser communications. I don't believe it has an official name yet, but I'm sure there will be one as it gets to be closer to launch.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-08-31T11:44:21.070" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10852" PostTypeId="2" ParentId="10845" CreationDate="2015-08-31T11:49:49.473" Score="8" Body="&lt;p&gt;The common word, as was mentioned in the comments, is the use of the word System. I have seen this in reference to the &lt;a href=&quot;http://solarsystem.nasa.gov/galileo/&quot; rel=&quot;nofollow&quot;&gt;Jovian&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Moons_of_Saturn&quot; rel=&quot;nofollow&quot;&gt;Saturnian&lt;/a&gt;, and &lt;a href=&quot;http://www.space.com/29559-pluto-moons-weird-orbit-chaos.html&quot; rel=&quot;nofollow&quot;&gt;Plutonian&lt;/a&gt; systems, referring to the planet and collection of moons. If one is referring to just the moons, without the planet, usually the word Satellite is injected (Saturnian Satellite System). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems this isn't commonly used with Mars, likely as a result of it only having two moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generically, it's called a System. There doesn't seem to be a good word for the system for just the moons. Planetary System refers to the system of planets surrounding a star. I would call it Planetary Satellite System, or something like that, generically.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="2986" LastEditDate="2015-08-31T22:53:45.413" LastActivityDate="2015-08-31T22:53:45.413" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10853" PostTypeId="2" ParentId="10846" CreationDate="2015-08-31T12:53:33.473" Score="4" Body="&lt;p&gt;In addition to the other answers, some images and time lapses which may &lt;em&gt;seem&lt;/em&gt; to be taken from outside were actually taken from inside. Firstly they tend to be taken with wide angle lenses, which make objects (such as solar panels) seem further away than they are. And secondly they may have been taken from inside the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cupola_%28ISS_module%29&quot; rel=&quot;nofollow&quot;&gt;cupola&lt;/a&gt;, which significantly sticks out and provides a panoramic view of the outside as well as much of the space station itself.&lt;/p&gt;&#xA;" OwnerUserId="2647" LastActivityDate="2015-08-31T12:53:33.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10854" PostTypeId="1" CreationDate="2015-08-31T16:11:41.340" Score="10" ViewCount="913" Body="&lt;p&gt;It has been proposed on Earth to use airborne wind turbines, anchored to the ground and flying at high altitudes where wind is strong and consistent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Venus, I would immediately say that anchoring anything to the ground is a no-go. However assuming a floating cloud colony, a wind turbine would be attached to a several km long cable and dangled below. This would exploit the differential in wind speeds between the colony and turbine to drag the turbine through the air and generate electricity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary reason to consider such an option is that it would provide an energy source independent from sunlight, generating energy continuously including at night and under dense cloud cover.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-10-01T14:22:15.217" Title="Would it be feasible to use wind turbines on Venus?" Tags="|colonization|venus|energy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10855" PostTypeId="2" ParentId="10846" CreationDate="2015-08-31T16:16:12.853" Score="7" Body="&lt;p&gt;One other source of steadily improving ISS shots:  patient photographers on the ground!&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ljW91.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ljW91.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://www.universetoday.com/63461/stunning-look-at-iss-and-docked-disovery-from-the-ground/&quot; rel=&quot;noreferrer&quot;&gt;http://www.universetoday.com/63461/stunning-look-at-iss-and-docked-disovery-from-the-ground/&lt;/a&gt;&lt;a href=&quot;https://i.stack.imgur.com/wXdkg.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wXdkg.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;from &lt;a href=&quot;http://blogs.discovermagazine.com/badastronomy/2011/02/28/ridiculously-awesome-pic-of-discovery-and-the-iss-taken-from-the-ground/&quot; rel=&quot;noreferrer&quot;&gt;http://blogs.discovermagazine.com/badastronomy/2011/02/28/ridiculously-awesome-pic-of-discovery-and-the-iss-taken-from-the-ground/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2015-08-31T16:16:12.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10859" PostTypeId="2" ParentId="4000" CreationDate="2015-08-31T20:53:15.933" Score="9" Body="&lt;p&gt;I would swing by Jupiter, dive towards the Sun, and do a quick chemical rocket burn at perihelion.  With a reasonable-sounding 2.2 km/s burn at a 0.3 AU perihelion, I can get to Sedna in 50 years.  (5.5 years to get to the Sun, 44.5 years to Sedna.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 0.3 AU, I need to put up with eleven times the solar intensity seen at Earth.  Solar Probe is being designed to tolerate greater than 600 times the intensity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe I should go closer.  I can reduce the burn to 1.3 km/s with a 0.1 AU perihelion.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-08-31T20:53:15.933" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10860" PostTypeId="1" AcceptedAnswerId="10867" CreationDate="2015-08-31T21:16:05.830" Score="30" ViewCount="23237" Body="&lt;p&gt;When people talk about geosynchronous orbit--an orbit in which the satellite continuously remains &quot;directly overhead&quot; for the same ground position on Earth--they talk about it being at a specific altitude, approximately 22,000 miles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Intuitively, this doesn't seem to make any sense.  You would think that a geosynchronous orbit would be attainable at any altitude, by flying exactly fast enough that the satellite keeps pace with the rotation of the earth beneath it, and therefore the required speed would be greater the higher up you go.  What's so special about the magic number 22,000 that makes it possible to do a geosynchronous orbit at that altitude but not at any arbitrary altitude?&lt;/p&gt;&#xA;" OwnerUserId="10171" LastActivityDate="2020-07-25T12:49:55.667" Title="Why is geosynchronous orbit an altitude, rather than a velocity?" Tags="|orbital-mechanics|geosynchronous|" AnswerCount="7" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10861" PostTypeId="2" ParentId="10860" CreationDate="2015-08-31T21:19:49.773" Score="24" Body="&lt;p&gt;Simply put, for a circular orbit and a given central body, the orbital period is solely a function of the radius.  A geosynchronous orbit is just the orbital radius at which the corresponding period is equal to the rotational period of the Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could fly around the Earth in 24 hours at any altitude, but not without propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/questions/8637/given-the-orbital-radius-of-a-satellite-how-is-the-orbital-period-calculated/8639#8639&quot;&gt;this question&lt;/a&gt; for the math.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-08-31T21:19:49.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10864" PostTypeId="1" AcceptedAnswerId="10868" CreationDate="2015-08-31T22:27:38.393" Score="9" ViewCount="7666" Body="&lt;p&gt;I &lt;em&gt;used&lt;/em&gt; to know how to do this, but for the life of me I can't remember the source of the equations. This is clearly a question of geometry rather that any specific space science however I still feel this is the correct place to ask the question. For more detail:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Orbit Altitude: 500km&#xA;Inclination: 0&#xA;Eccentricity: 0&#xA;Orbital Period: 5677s&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="25" LastEditDate="2016-07-19T17:28:53.657" LastActivityDate="2018-03-25T10:11:10.127" Title="How long will a 500 km altitude satellite spend in Earth - shade per orbit" Tags="|orbital-mechanics|artificial-satellite|low-earth-orbit|sunlight|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10865" PostTypeId="2" ParentId="10864" CreationDate="2015-08-31T22:45:34.640" Score="3" Body="&lt;p&gt;We can roughly approximate the answer by considering how wide a swath of shade the Earth casts, relative to the total length of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 500km altitude + 6371 km Earth radius (= 6871km), the satellite's circular orbit describes a circle $2 π r$ in circumference, or 43,172 km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume rays of sunlight at Earth orbit to be essentially parallel, so the width of the shadow cast on the orbit is the width of Earth, 12,742 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the sat will spend about 12742/43172 of its time in shadow, or 1676 seconds (27.93 minutes) on each orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's a severe underestimate, though. For more accurate computation you'd need to take into account that the shadow width is a chord across the orbit rather than a circular arc -- and since it's a large fraction of the orbit, that difference is pretty significant. (If the orbital altitude were much higher, the chord would be much closer to the arc, and this method would be more accurate.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For that, you can do a little bit of trig; if &lt;em&gt;c&lt;/em&gt; is the shadow width (Earth diameter) and &lt;em&gt;r&lt;/em&gt; is the orbital radius (Earth radius plus orbital altitude), then the angular width of the shadowed arc is $$2 \arcsin \frac c {2r} $$ or 136 degrees; 0.378 of the circle, or 2145 seconds (35.74 minutes). See HopDavid's answer for the excellent diagram corresponding to this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For still more accurate computation you'd need to take into account the sun's distance and radius, and make decision about umbra vs. penumbra.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-08-31T23:08:33.273" LastActivityDate="2015-08-31T23:08:33.273" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10866" PostTypeId="2" ParentId="10864" CreationDate="2015-08-31T22:50:38.493" Score="5" Body="&lt;p&gt;It will vary dependent on the time of the year. According to &lt;a href=&quot;http://www.agi.com/products/stk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;STK&lt;/a&gt;, the exact time is about 2123 s. The time won't vary that much over the course of the year, I don't expect it would be much more than a 100 s variation. The low inclination and LEO altitude results in the low variation.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-03-25T10:11:10.127" LastActivityDate="2018-03-25T10:11:10.127" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10867" PostTypeId="2" ParentId="10860" CreationDate="2015-08-31T22:51:33.707" Score="73" Body="&lt;p&gt;I quite agree that it is not intuitive. However, orbital mechanics are frequently not intuitive, probably because we don't get to experience an orbital environment on a regular basis (if ever).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's just assume we're talking about circular orbits for the remainder of my post, since you are a beginner in orbital mechanics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is only one speed that a given circular orbit of a certain altitude can go. Keep in mind that stable orbits do not require any force from an engine to keep going as they have been. Basically, in a circular orbit, the falling-toward-the-planet motion is matched exactly by the moving-forward motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sir Issac Newton figured this out, and exemplified it with a thought experiment called &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_cannonball&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Newton's Cannonball&lt;/em&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that if the orbital speed is too slow for that altitude, the cannonball crashed into the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Dc0TW.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Dc0TW.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if the orbital speed is too high for the altitude, the orbit will be an ellipse, rather than circular, or the cannonball may even escape Earth altogether!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/8/84/Newtonsmountainv%3D8000.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/8/84/Newtonsmountainv%3D8000.gif&quot; alt=&quot;enter&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, if the cannonball is launched at the 'correct' orbital speed to be in a circular orbit at that altitude, it will neither crash, nor fly away, but will remain stable, traveling around earth at that particular velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/1/12/Newtonsmountainv%3D7300.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/1/12/Newtonsmountainv%3D7300.gif&quot; alt=&quot;enter&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At different altitudes, this Goldilocks velocity is different. If the orbit is closer to the planet, the effect of gravity is higher, so the orbiting object must be moving faster to counteract the falling. When the orbiting object is further away, there is less falling force due to gravity (because gravitational force is based on distance), and so the object does not need to be moving as fast to counteract the falling force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Geocentric_orbit&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia's Geocentric Orbit article&lt;/a&gt;, we know that Low Earth Orbit could be, for example, an altitude of 160km. At this altitude, the Goldilocks velocity to keep a circular orbit is about 8000 m/s, and takes about 90 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now what happens if we look at a slightly higher altitude? Well the velocity is lower, and the path the orbiting object travels gets bigger (the circle is bigger), so both of those factors make the orbit take longer. A slightly higher orbit might take 100 minutes instead of 90.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a geosynchronous orbit, the orbit has to take 24 hours instead of 90 minutes, because the earth takes 24 hours to spin. This happens when the circle is expanded to an altitude of about 35000 km. The Goldilocks velocity at this altitude is about 3000 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is all somewhat simplified, but the broad strokes are all there. As Organic Marble pointed out, you could try to force a craft to orbit at a different altitude in a 24 hour period, but it would not be a stable orbit, you would need engines to keep it going.&lt;/p&gt;&#xA;" OwnerUserId="11076" LastEditorUserId="195" LastEditDate="2017-10-15T21:28:57.610" LastActivityDate="2017-10-15T21:28:57.610" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10868" PostTypeId="2" ParentId="10864" CreationDate="2015-08-31T23:11:00.737" Score="20" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V1pl1.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V1pl1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this pic the angle $\phi$ is asin(6378/6878) or about 68º. It stays in the shade over 2 * 68º or about 136º. That's about 38% of the period. That's 2145 seconds or about 36 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what the orbit would be during the fall or spring equinox. Other times of the year it could be shorter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this pic I assume the sun's ray's are parallel. A more accurate drawing would have the shadow boundary about half a degree from horizontal, but it gives an approximation.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-09-01T22:59:42.750" LastActivityDate="2015-09-01T22:59:42.750" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="10869" PostTypeId="2" ParentId="10860" CreationDate="2015-09-01T02:26:37.160" Score="8" Body="&lt;p&gt;Think of it this way. A circular orbit is characterized by the fact that the fictitious centrifugal force is exactly canceled out by the (centripetal) force of gravity. If that wasn't the case, if gravity was stronger, the satellite would begin to sink; if gravity was weaker, it would begin to rise. In either case, it would no longer be in a circular orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A geostationary orbit is characterized by its angular velocity (specifically, $2\pi$ radians per day). The centrifugal force for circular motion at constant angular velocity is proportional to the radius. The gravitational force is proportional to the inverse square of the radius. So you have an equation in the (generic) form, $Ar = B/r^2$ where $A$ and $B$ are some numbers. This equation is not valid for arbitrary $r$; rather, you can calculate the value of $r$ by solving the equation for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you plug in the numbers, this is exactly what happens. The centrifugal force for a mass $m$ is given by $F_c=mv^2/r = m\omega ^2r$ where $\omega$ is the angular velocity. The gravitational force for a mass $m$ is $F_g = GMm/r^2$ where $G$ is Newton's constant of gravity and $M$ is the Earth's mass. When these two are equal, you have $m\omega^2 r = GMm/r^2$ or $r = \sqrt[3]{GM/\omega^2}$. When you plug in the numbers, you get $r \simeq 4.23\times 10^7$ meters, or after subtracting the radius of the Earth, an altitude of approximately 36,000 km. This is the only value for which the two forces cancel at an angular velocity of one full revolution per day, so this is the geostationary altitude.&lt;/p&gt;&#xA;" OwnerUserId="11487" LastActivityDate="2015-09-01T02:26:37.160" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10871" PostTypeId="2" ParentId="8140" CreationDate="2015-09-01T05:51:18.850" Score="4" Body="&lt;p&gt;First off I'll note that the linked article doesn't actually claim that &lt;s&gt;Triana&lt;/s&gt; DSCOVR is the &lt;em&gt;first&lt;/em&gt; spacecraft since Apollo to see the whole daylit side of Earth.  The relevant quote (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;DSCOVR’s two Earth-observing sensors will provide views of Earth that &lt;strong&gt;have only rarely been available before&lt;/strong&gt;. [EPIC] will view the entire sunlit side of Earth at once, providing images reminiscent of the iconic Blue Marble photograph taken by Apollo 17 astronauts in 1972.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;David has already pointed out that L&lt;sub&gt;1&lt;/sub&gt; is far beyond the Moon's orbit, and that the geometry of the &quot;blue marble&quot; photograph is much closer to that of images taken from geostationary orbit, like this recent image from &lt;a href=&quot;https://en.wikipedia.org/wiki/GOES_13&quot; rel=&quot;nofollow noreferrer&quot;&gt;GOES-EAST&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lp3wl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lp3wl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GOES program traces back to the &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/goes-a_sms.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;SMS series&lt;/a&gt; of satellites, meaning that we have had nearly continuous full-disk coverage of the Earth since 1974, only two years after the &quot;blue marble&quot; photograph was taken.  There are two slight caveats: first, the GOES/SMS instruments have always focused more on the infrared spectrum (the current block has only one visible-wavelength band, and the next-generation GOES will have &lt;a href=&quot;http://www.goes-r.gov/spacesegment/ABI-tech-summary.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;only two&lt;/a&gt;) so the images are not in true color.  Second, they only produce fully-illuminated images around the equinoxes, when the intersection of their orbital plane (the equatorial plane) and the ecliptic plane lines up with the Sun.  (This is in fact the definition of the equinoxes.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;However&lt;/em&gt;, there is an additional major hiccup: it turns out that geostationary satellites don't capture &quot;the entire Earth in a single frame!&quot;  The current generation (which are 3-axis stabilized) use a &lt;a href=&quot;http://goes.gsfc.nasa.gov/text/GOES-N_Databook/section03.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;two-axis steerable mirror&lt;/a&gt; to scan across the entire disk, while the early spin-stabilized GOES satellites used a one-axis steerable mirror to scan North-South and the rotation of the satellite itself to scan East-West. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;It turns out that it is not unusual for departing spacecraft to take pictures of the Earth, simply because humans like looking at their planet!  (Or because it makes a convenient calibration target.)  However, most spacecraft leave Earth roughly parallel to Earth's orbit, so they end up seeing a partially-illuminated disk.  &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/6/67/MESSENGERearth.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;MESSENGER&lt;/a&gt; and &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/0/0b/Galileo_Earth_-_PIA00114.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo&lt;/a&gt; came close, but were still far off from the correct viewing angle.  You are correct that we usually don't send spacecraft on orbits towards the Sun, and those that we have, have all been solar-observing satellites that were simply not equipped to take pictures of the comparatively dim Earth.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here is &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;LRO&lt;/a&gt; taking a &lt;a href=&quot;http://lroc.sese.asu.edu/posts/513&quot; rel=&quot;nofollow noreferrer&quot;&gt;picture of Earth during a solar eclipse&lt;/a&gt; in 2012!  This picture is black-and-white, but it is as close to fully-illuminated as you can get (except for the shadow of the Moon!) and it was taken in a single frame.  (LRO has captured other, shadow-free views of the Earth, but I wanted to share this one in particular.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0Ujv8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0Ujv8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In conclusion, I think that the article's claim that the DSCOVR's view of earth is unique is mostly &quot;marketing,&quot; &lt;em&gt;unless&lt;/em&gt; we disregard mosaic images as you suggest.&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="8996" LastEditDate="2015-09-01T15:25:52.160" LastActivityDate="2015-09-01T15:25:52.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10872" PostTypeId="2" ParentId="10854" CreationDate="2015-09-01T07:13:36.230" Score="7" Body="&lt;p&gt;In general, the wind gradient is such that as the height above the ground increases, the wind speed also increases. It is true for Venus also. However, the gradient slope is heavy there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lLfDC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lLfDC.jpg&quot; alt=&quot;Wind Speeds in Venus&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: ase.tufts.edu&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case of Venus, any floating cloud colony would have to be above the sulphuric acid clouds if it has to have any chance of survival. So, basically the colony would experience as much or more winds compared to the dangling turbine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, as one goes down the atmosphere, the pressure and density increases much more than earth. This would, in turn require the turbine to be smaller (for less drag) and sturdier (to overcome the pressure, to say nothing about the sulphuric acid). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the main advantages of having floating turbines in earth is that they can be of large sizes and are exposed to high speed winds at altitude. However, both these advantages are lost in case of Venus. So, it will be highly impracticable for using airborne wind turbines on Venus.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastEditorUserId="415" LastEditDate="2015-10-01T14:22:15.217" LastActivityDate="2015-10-01T14:22:15.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10875" PostTypeId="1" AcceptedAnswerId="12951" CreationDate="2015-09-01T13:20:03.367" Score="8" ViewCount="473" Body="&lt;p&gt;What is the actual calendar of observed global dust storms on Mars? And if available, any data about regional dust storms' dates, duration and coverage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I find online information about &lt;em&gt;global&lt;/em&gt; Martian dust storms having been observed since the 1950s, and one probably having destroyed Soviet Mars 3 lander in 1971 and two more in the 1970s during the Viking missions, one in 2001, one in 2007. But what is the complete schedule registered during the last 50 years or so?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(Shouldn't there be some kind of Mars weather site around by now, like for the Solar weather?)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="4660" LastEditDate="2015-09-01T13:55:44.783" LastActivityDate="2015-12-12T17:07:40.113" Title="What are the known historic global and regional dust storms on Mars?" Tags="|mars|observation|dust|weather|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10877" PostTypeId="1" AcceptedAnswerId="10914" CreationDate="2015-09-01T17:46:47.123" Score="8" ViewCount="1915" Body="&lt;p&gt;I want to find out how long before predictions for a satellites location made using the SGP4 algorithm break down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example if I use TLE data from a satellite which was gathered today, how far into the future can I make accurate predictions for that satellites location using that TLE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I am asking is that TLE data for satellite's are updated irregularly, so I want to have some idea of how long they can be used for predicting the location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example if I want to know if any earth observing satellites line of sight passes over my house at exactly 5pm in 7 days time, could I use the SGP4 algorithm to give me a correct result? Or would I need to use a more accurate propagator?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the wikipedia page, the error of the SGP4 model is ~1km at epoch and grows at ~1-3km a day.&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_perturbations_models&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Simplified_perturbations_models&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I'm wondering what this means for making accurate predictions using this model, is an error of 5-10km much when talking about the area covered by the satellites point of view? (can't remember the name for the area of ground a satellite covers along its path during orbit)&lt;/p&gt;&#xA;" OwnerUserId="8410" LastEditorUserId="8410" LastEditDate="2015-09-02T16:47:01.620" LastActivityDate="2015-11-10T13:45:55.343" Title="How far in the future can SGP4 make accurate predictions?" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10879" PostTypeId="1" AcceptedAnswerId="12965" CreationDate="2015-09-01T19:34:50.800" Score="30" ViewCount="764" Body="&lt;p&gt;Assume that despite our current level of space exploration and technology, we have somehow missed the existence of the other planets in our solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have developed &lt;a href=&quot;https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets&quot;&gt;various methods of detecting exoplanets&lt;/a&gt; orbiting other stars.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using &lt;a href=&quot;https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Direct_imaging&quot;&gt;direct observation&lt;/a&gt; of the sun's position or possibly &lt;a href=&quot;https://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets#Radial_velocity&quot;&gt;radial velocity&lt;/a&gt; observations from current earth- or space-based solar observatories, which of the planets would we be able to detect?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would we be able to detect anything smaller than a planet too?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory&quot;&gt;SOHO&lt;/a&gt; was launched about 20 years ago, so lets assume 20 years worth of observations.  &lt;strong&gt;Extra credit:&lt;/strong&gt; I'd also be interested to see what effect it would have to hypothetically have 100 or 1000 years worth of similar observations.&lt;/p&gt;&#xA;" OwnerUserId="758" LastEditorUserId="8693" LastEditDate="2016-01-14T08:20:08.140" LastActivityDate="2019-05-21T11:29:34.443" Title="Which of the planets could we detect today from just the movements of the sun?" Tags="|planet|observation|the-sun|exoplanet|celestial-mechanics|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10880" PostTypeId="2" ParentId="10822" CreationDate="2015-09-01T23:51:42.493" Score="5" Body="&lt;p&gt;It's unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per one of the scientists working navigation on the New Horizons mission, &lt;a href=&quot;https://twitter.com/AscendingNode/status/637348166438772737&quot; rel=&quot;noreferrer&quot;&gt;Simon Porter&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Lack of known objects prevents [targeting an additional flyby]; PT1 is almost on the outer edge of the [cold classical Kuiper Belt].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/PI-Perspectives.php?page=piPerspective_10_23_2014&quot; rel=&quot;noreferrer&quot;&gt;Many expensive hours of Hubble time&lt;/a&gt; and months of calendar time were dedicated to finding the three candidate objects.  There is not sufficient delta V to visit more than one of the detected objects, so any possible third encounter would need to be an as-yet-undetected object significantly further (and fainter) than PT1.  It's probably safe to assume that anything detectable by Hubble would already have been found, and we will have no assets that outperform Hubble operational prior to the PT1 encounter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no particular reason why a new target has to be found before the PT1 encounter - in particular PT1 is far too small to provide a useful gravity assist, so a maneuver could be made some time after that to target a yet-to-be-discovered deeper object.  Of course, the longer you wait, the smaller the region of reachable targets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If JWST launches on time (haha), it is slated to complete commissioning a few months after the PT1 encounter.  Its limiting magnitude &lt;a href=&quot;http://www.stsci.edu/jwst/science/sensitivity&quot; rel=&quot;noreferrer&quot;&gt;will be&lt;/a&gt; approximately 1 magnitude fainter than Hubble for a comparable length of exposure.  1 magnitude is not a tremendous increase in sensitivity, and the corresponding probability of finding an object that both wasn't already detected by Hubble and is reachable with NH's dwindling delta V reserve after the PT1 flyby is small.  It will probably not be considered worthwhile to spend the JWST time on that search.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2015-09-01T23:51:42.493" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10882" PostTypeId="1" AcceptedAnswerId="10884" CreationDate="2015-09-02T11:56:36.593" Score="21" ViewCount="2822" Body="&lt;p&gt;Typically, a Soyuz crew vehicle takes 4 orbits to reach the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS&lt;/a&gt;, but the &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-18M&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz TMA-18M&lt;/a&gt; took 34 orbits or 2 days to reach it. Why is there so much variation from typical missions?&lt;/p&gt;&#xA;" OwnerUserId="6057" LastEditorUserId="12102" LastEditDate="2017-07-06T10:34:45.250" LastActivityDate="2017-07-06T10:34:45.250" Title="Why did Soyuz TMA-18M take two days to reach the ISS?" Tags="|iss|crewed-spaceflight|spacecraft|soyuz-spacecraft|transfer-time|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10883" PostTypeId="2" ParentId="10882" CreationDate="2015-09-02T12:04:01.317" Score="8" Body="&lt;p&gt;The fast approach mission is relatively rare, and has only been done recently. According to this &lt;a href=&quot;http://www.nasaspaceflight.com/2015/09/soyuz-tma-18m-one-expedition-crewmember-iss/&quot;&gt;news article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Soyuz TMA-18M was to be tasked with achieving a fast-track, six-hour rendezvous with the ISS. However, that approach was cancelled due to the current altitude of the Station.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;To achieve such a fast-track docking, Soyuz TMA-18M would have had to perform the first two pre-programmed engine burns during its very first orbit of Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-02T12:04:01.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10884" PostTypeId="2" ParentId="10882" CreationDate="2015-09-02T12:06:20.207" Score="26" Body="&lt;p&gt;Actually, a roughly 2-day rendezvous is &lt;em&gt;standard*&lt;/em&gt;, and the 4-orbit &lt;em&gt;expedited rendezvous&lt;/em&gt;, while being practiced more in the recent years, also involves phasing the International Space Station's orbit so it precisely coincides with the launch site at precise time (thus the &lt;em&gt;instantaneous launch window&lt;/em&gt;), which sometimes isn't possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This Soyuz TMA-18M rendezvous was one of those latter cases, because the station had to do &lt;a href=&quot;https://blogs.nasa.gov/stationreport/2015/07/27/&quot;&gt;Pre-determined Debris Avoidance Maneuvers in July 2015&lt;/a&gt;, and also slight phasing corrections for the next week's Soyuz TMA-16M landing, which set it out of phase for the &lt;em&gt;expedited rendezvous&lt;/em&gt; with Soyuz TMA-18M. Since it was already out of phase, it also performed an &lt;a href=&quot;https://blogs.nasa.gov/stationreport/2015/08/31/iss-daily-summary-report-083115/&quot;&gt;orbital reboost on August 31&lt;/a&gt;, just two days before the TMA-18M launch. An additional reboost will be performed on September 7, to finalize setup for TMA-16M landing and next uncrewed Progress 61P launch to the station on October 1.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;*I said in the beginning that the 2-day rendezvous is &lt;em&gt;standard&lt;/em&gt;, so I should also mention that failing to perform &lt;em&gt;expedited rendezvous&lt;/em&gt; for any reason, one of the fallback procedures is the switch to 2-day rendezvous. So 2-day rendezvous mode is always an option with crewed Soyuz launches, while the &lt;em&gt;expedited rendezvous&lt;/em&gt; not necessarily. Thus &lt;em&gt;standard&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="286" LastEditDate="2015-09-02T20:15:33.067" LastActivityDate="2015-09-02T20:15:33.067" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10885" PostTypeId="2" ParentId="10860" CreationDate="2015-09-02T16:39:19.023" Score="0" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What's so special about the magic number 22,000 that makes it possible to do a geosynchronous orbit at that altitude but not at any arbitrary altitude?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Lift an object to an orbital altitude of 1 meter. Let it go. What happens? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Splat&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The centrifugal force of a geosynchronous orbit of 1 meter cannot support an object against gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then assume that Pluto is in a geosynchronous orbit... that is to say the dwarf planet needs to revolve around Earth in 24 hours. The speed it would need for that is approximately light speed. What happens?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;WHOOOSH&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto will disappear out into the big black yonder, because Earth's gravity cannot possibly contain an object in a geosynchronous orbit of 7.5 billion kilometers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Somewhere in between these two extremes is the altitude where gravity and the centrifugal force of a 24 hour orbit are equal and balance each other out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That - special - altitude is 22 000 miles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Move higher up and the centrifugal force of a 24 hours orbit is too strong... it will overcome gravity and result in an elliptic orbit, or make the object break away from Earth all-together. Move lower, and the centrifugal force is too weak to balance out gravity and the object will begin to lose altitude, again resulting in an eccentric orbit, or possibly even crash into the atmosphere. &lt;/p&gt;&#xA;" OwnerUserId="10493" LastEditorUserId="10493" LastEditDate="2015-09-02T20:41:12.310" LastActivityDate="2015-09-02T20:41:12.310" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10886" PostTypeId="1" CreationDate="2015-09-02T16:44:49.480" Score="9" ViewCount="345" Body="&lt;p&gt;I remember being really interested in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Robonaut&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robonaut&lt;/a&gt; project when it was first announced, and thinking how cool it was when it was finally launched to the ISS.  Since then, I've heard little about the project. I happened across TildalWave's &lt;a href=&quot;https://space.stackexchange.com/a/1530/10129&quot;&gt;excellent answer here&lt;/a&gt; today and was surprised to learn that it has been &lt;em&gt;four years&lt;/em&gt; since it was first powered up on the station and over a year since it was fitted with &quot;legs&quot;.  Yet besides that update, there seems to be little information available online as to what Robonaut actually &lt;strong&gt;does&lt;/strong&gt;.  There's a lot of talk about what it &lt;em&gt;can&lt;/em&gt; do, but from what I can tell, its accomplishments seem to be:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Looking cool &lt;/li&gt;&#xA;&lt;li&gt;Staying boxed up &lt;/li&gt;&#xA;&lt;li&gt;Serving as something to be upgraded in the future for vague purposes&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Surely I've missed something.  Is there an actual &lt;em&gt;plan&lt;/em&gt; for Robonaut 2?  I'd welcome a knowledge bomb to be dropped on my cynicism. &lt;/p&gt;&#xA;" OwnerUserId="10129" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-12-18T19:36:08.573" Title="What is/was the point of Robonaut 2? Has it accomplished anything?" Tags="|iss|robotics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10887" PostTypeId="2" ParentId="10886" CreationDate="2015-09-02T17:34:42.587" Score="3" Body="&lt;p&gt;The best way I know of to keep up with R2 is by searching the (usually updated daily) &lt;a href=&quot;https://blogs.nasa.gov/stationreport/?s=robonaut&quot; rel=&quot;nofollow&quot;&gt;ISS On-orbit Status Reports for &quot;robonaut&quot;&lt;/a&gt;.  The actual &lt;a href=&quot;http://robonaut.jsc.nasa.gov/default.asp&quot; rel=&quot;nofollow&quot;&gt;Robonaut web pages&lt;/a&gt;, like many NASA sites, are not updated very often.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I just performed this search, it sounds like some major problems have been encountered in July and August of this year, and may not yet be resolved. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speculation: some of the lack of news may be because a private company (GM) is involved in funding the project.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-02T18:36:51.093" LastActivityDate="2015-09-02T18:36:51.093" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10888" PostTypeId="1" AcceptedAnswerId="10889" CreationDate="2015-09-02T18:27:21.690" Score="1" ViewCount="1349" Body="&lt;p&gt;I've seen countless photos of rocket launches that show the path taken by a rocket, and they always seem to arc rather than go in a straight line. Like the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dTRxp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dTRxp.jpg&quot; alt=&quot;photo of gentle launch arc&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The adage &quot;the shortest distance between two points is a straight line&quot; comes to mind when I see this, so I wonder: am I overlooking something because reality is in three dimensions or is there another reason to launch a rocket in an arc?&lt;/p&gt;&#xA;" OwnerUserId="11513" LastEditorUserId="6816" LastEditDate="2015-09-02T18:57:39.933" LastActivityDate="2015-09-02T21:28:20.007" Title="Why don't some rockets launch in a straight line?" Tags="|launch|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ClosedDate="2015-09-02T19:57:04.873" ContentLicense="CC BY-SA 3.0" />
  <row Id="10889" PostTypeId="2" ParentId="10888" CreationDate="2015-09-02T18:49:09.340" Score="5" Body="&lt;p&gt;Drag is highest at low altitudes, and drag can cause problems with heating, pressure strain on various parts, and of course slowing acceleration. So the rocket starts by climbing above most of that, then converting most of its upward velocity into sideways and accelerating a whole lot more to get up to orbital speed (&lt;a href=&quot;http://www.spaceacademy.net.au/watch/track/leopars.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;nearly 8 km/s&lt;/a&gt;, or about an order of magnitude faster than most bullets; slower, higher orbits require even more energy). This acceleration takes the vast majority of the rocket's fuel, at least &lt;a href=&quot;https://space.stackexchange.com/a/5539&quot;&gt;90%&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-09-02T21:28:20.007" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10890" PostTypeId="1" AcceptedAnswerId="10894" CreationDate="2015-09-02T19:22:54.180" Score="41" ViewCount="5005" Body="&lt;p&gt;The ISS has thrusters to avoid debris and to combat orbital decay. Do the astronauts feel the station accelerating? Do they have to hold onto something and is it the same when they rotate the station using reaction wheels for incoming spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="11514" LastEditorUserId="32757" LastEditDate="2019-11-01T13:46:15.043" LastActivityDate="2019-11-02T15:56:11.653" Title="When the ISS accelerates, do the astronauts feel it?" Tags="|iss|propulsion|orbital-maneuver|astronauts|reaction-wheel|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="10891" PostTypeId="1" AcceptedAnswerId="10903" CreationDate="2015-09-02T19:24:51.640" Score="5" ViewCount="394" Body="&lt;p&gt;Follow up to this question: &lt;a href=&quot;https://space.stackexchange.com/q/10882/758&quot;&gt;Why will Soyuz TMA-18M take two days to reach the ISS?&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly the expedited rendezvous is cheaper in terms of time.  However I was wondering how fuel usage compares.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the fuel usage:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;about the same (after all the same orbital parameters are being targeted either way)?&lt;/li&gt;&#xA;&lt;li&gt;more for the expedited rendezvous (perhaps to get there faster, more fuel is needed)?&lt;/li&gt;&#xA;&lt;li&gt;less for the expedited rendezvous (perhaps shorter time to rendezvous means less fuel spent)?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="758" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-09-09T12:51:10.083" Title="Soyuz TMA-18M ISS rendezvous: expedited 4-orbit vs regular 34-orbit - How does fuel usage compare?" Tags="|iss|orbital-maneuver|soyuz-spacecraft|crewed-spaceflight|rendezvous|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10893" PostTypeId="2" ParentId="10890" CreationDate="2015-09-02T19:39:34.047" Score="12" Body="&lt;p&gt;The rate of change is pretty low with pretty low acceleration.  (Consider the mass of the ISS(370,000 kilos) and the output of the Progress booster engines at just under 3000N of thrust).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One interesting experiment to consider is something you can try on a train that has open gangways. I.e. The cars are connected and it is one long tube.  Stand at either the front or back of the train.  When the train starts up from a station, pay attention to the breeze. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At rest, the air inside is at rest. (or is moving at the same speed as the earth rotates). As the train starts, the air stays still at first but the train moves around it. Then it piles up on the front of the train and bounces back towards the tail of the train.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Opposite when you stop.  Toronto on the Yonge University line has these trains, and it is a pretty cool feeling once you realize what is happening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same happens in the ISS, but acceleration is much slower in comparison. The Toronto TR-1 subways accelerate at 0.9 m/s^2.  There is no way the ISS accelerates that much. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-09-02T19:39:34.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10894" PostTypeId="2" ParentId="10890" CreationDate="2015-09-02T19:42:09.737" Score="42" Body="&lt;p&gt;If you watch these videos:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=cmHamp0IIyE&quot;&gt;ATV boost&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=8MR3daaWLXI&quot;&gt;Zvezda boost&lt;/a&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;...you can see that the acceleration is quite gentle, but definite. The astronauts do need to hang on to something if they don't want to drift to the back of whatever room they're in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first video was a reboost performed with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Automated_Transfer_Vehicle&quot;&gt;ATV&lt;/a&gt; service ship, as described in &lt;a href=&quot;http://www.universetoday.com/13889/atv-jules-verne-boosts-space-station-to-higher-orbit-video/&quot;&gt;this article&lt;/a&gt;. Depending on what numbers you look at, acceleration during an ATV boost is something like 0.0035-0.005 m/s&lt;sup&gt;2&lt;/sup&gt; -- about 1/3000 or 1/2000 of a g. So an ounce or two of force (~30 grams-force) is enough to keep an astronaut in place -- easily done with a pinky finger. You can see in the first video that the astronauts can throw themselves to the opposite end of a compartment against the acceleration with little effort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second video is a reboost performed with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot;&gt;Zvezda&lt;/a&gt; module's two built-in thrusters, which are more powerful than ATV's. You can see that the acceleration is a little more rapid in that video; they say it's 0.0185 m/s&lt;sup&gt;2&lt;/sup&gt; -- still only 1/600 g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ATV's thrusters yield about ~2kN of thrust. Progress's thrusters, also used for reboost, yield ~3kN; the Zvezda module's built-in thrusters about 6kN. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unclear to me how fast the station is turned by its &lt;a href=&quot;https://en.wikipedia.org/wiki/Control_moment_gyroscope&quot;&gt;control moment gyros&lt;/a&gt;, but I think it's usually less than 0.1 degree per second. They've done 90-degree and 180-degree rotations as tests, and the 180 (not done all at once) took 2 hours 45 minutes to complete. I think that would be harder to notice for a person near the center of the station.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-08-12T18:37:49.323" LastActivityDate="2016-08-12T18:37:49.323" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10895" PostTypeId="1" AcceptedAnswerId="10901" CreationDate="2015-09-02T21:40:32.760" Score="23" ViewCount="20089" Body="&lt;p&gt;Given a similar mix of gases to those in our Earth's atmosphere, what is the upper limit of survivable atmospheric pressure for a human?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a human survive higher pressures with a gas mix unlike those on Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about mixes such as Titan or Venus, assuming one didn't instantly die from any of the other ways those planets will kill you?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="49" LastEditDate="2015-09-02T21:47:12.923" LastActivityDate="2018-08-20T05:23:13.840" Title="Maximum survivable atmospheric pressure" Tags="|atmosphere|venus|medical|titan|high-pressure-environment|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="10896" PostTypeId="1" CreationDate="2015-09-02T23:03:41.317" Score="6" ViewCount="329" Body="&lt;p&gt;I have heard than under some conditions a user of the Orlan EVA Suit can retract his/her arm inside the suit torso, then re-insert arm into suit sleeve and glove (under suit pressure). Can you confirm this? And where can I get more info on this, with specific instances?&lt;/p&gt;&#xA;" OwnerUserId="11519" LastActivityDate="2015-10-14T15:45:24.440" Title="Orlan suit inside arm/hand access" Tags="|eva|spacesuits|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10897" PostTypeId="2" ParentId="10617" CreationDate="2015-09-02T23:14:50.893" Score="3" Body="&lt;p&gt;Protocols for space communication are specified by the CCSDS: &lt;a href=&quot;https://en.wikipedia.org/wiki/CCSDS&quot; rel=&quot;nofollow&quot;&gt;Consultative Committee for Space Data Systems&lt;/a&gt;. Founded in 1982, they &quot;discuss and develop standards for space data and information systems&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In brief they provide protocols for six areas: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Space Internetworking Services &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/CCSDS_Mission_Operations&quot; rel=&quot;nofollow&quot;&gt;Mission Ops&lt;/a&gt; And Information Management Services &lt;/li&gt;&#xA;&lt;li&gt;Spacecraft Onboard Interface Services &lt;/li&gt;&#xA;&lt;li&gt;System Engineering&lt;/li&gt;&#xA;&lt;li&gt;Cross Support Services&lt;/li&gt;&#xA;&lt;li&gt;Space Link Services&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Your question cannot be answered in brief because getting into all of this will cover page after page here. I will simply say that probably everything you need to know can be found on their website, right &lt;a href=&quot;http://public.ccsds.org/publications/default.aspx&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10493" LastActivityDate="2015-09-02T23:14:50.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10898" PostTypeId="1" CreationDate="2015-09-03T00:08:57.333" Score="10" ViewCount="2024" Body="&lt;p&gt;Recall that geostationary satellites are placed into an orbit which is designed to stay above a fixed point on the Earth's surface. If the Earth and the satellite were both spherical cows in an otherwise-empty universe, the satellite, once placed in orbit at geostationary altitude over the equator, would presumably co-rotate with the Earth for the rest of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, of course, there are no spherical cows. Real satellite orbits are presumably perturbed by various factors, like gravity from other large bodies (especially the moon?); collisions with &quot;stuff&quot; (dust, etc.); and the non-constant motion of the Earth itself (tidal deceleration, earthquakes altering the Earth's mass distribution, etc.). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This brings me to my two questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How much does a typical geostationary satellite wobble away from its designated position over the Earth before being station-kept back into place? &lt;/li&gt;&#xA;&lt;li&gt;Which factors contribute the most to this wobble?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="527" LastEditorUserId="527" LastEditDate="2015-09-03T18:14:45.430" LastActivityDate="2015-09-03T18:14:45.430" Title="How much &quot;wobble&quot; does a typical geostationary satellite experience?" Tags="|orbital-mechanics|artificial-satellite|orbit|geostationary|station-keeping|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10899" PostTypeId="2" ParentId="10890" CreationDate="2015-09-03T00:18:10.207" Score="5" Body="&lt;p&gt;They can definitely feel it when, like in January 2009, the Russians mess up and send crazy command data, resulting in too high of an acceleration and scary oscillations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Video of the ISS rock and roll dance &lt;a href=&quot;http://www.nbcnews.com/id/28998876/ns/technology_and_science-space/t/shaking-space-station-rattles-nasa/&quot;&gt;here&lt;/a&gt; - the burn is in progress when the video starts, and cuts off about halfway through.  What I wouldn't give to have an exterior camera view of the solar arrays.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-09-03T00:18:10.207" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10900" PostTypeId="2" ParentId="10898" CreationDate="2015-09-03T01:10:15.467" Score="11" Body="&lt;p&gt;Most geostationary satellites are kept within a very small margin, about 0.5 degrees. Wobbling happens due to a number of factors, including the Sun, Moon, and flattening of Earth at it's poles, causes some precession to occur (&lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot;&gt;Wikipedia&lt;/a&gt;). It requires about 50 m/s of delta v to remain in a geostationary orbit per year, due to this effect. In addition, solar wind, sunlight and the unevenness of the Earth's equator all contribute to instabilities.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-03T01:10:15.467" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10901" PostTypeId="2" ParentId="10895" CreationDate="2015-09-03T02:28:54.937" Score="24" Body="&lt;p&gt;Based on saturation diving operations, it looks like the limits are as follows:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Compressed air: &lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrogen_narcosis&quot;&gt;Nitrogen narcosis&lt;/a&gt; limits you to around four times Earth's atmospheric pressure.&lt;/li&gt;&#xA;&lt;li&gt;Any gas mix: &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydreliox&quot;&gt;Hydreliox&lt;/a&gt; was used for the current depth record; &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturation_diving#Extreme_depth_effects&quot;&gt;insomnia and fatigue issues&lt;/a&gt; appear to limit you to around 65 times Earth's pressure regardless of gas mix.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Neither Titan nor Venus has any oxygen in their atmosphere.  If you supplemented the &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus#Atmosphere_and_climate&quot;&gt;atmospheric gas of Venus&lt;/a&gt; with oxygen, you would die in short order from &lt;a href=&quot;https://en.wikipedia.org/wiki/Hypercapnia&quot;&gt;carbon dioxide poisoning&lt;/a&gt;, long before any pressure-related effects from the 92-bar atmosphere had time to set in.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_(moon)#Atmosphere&quot;&gt;Titan has a low-pressure atmosphere&lt;/a&gt; (only 1.5 times Earth's) that is mostly nitrogen and methane; assuming that none of the other hydrocarbons are present in toxic concentrations, you could breathe it if you had supplemental oxygen.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2015-09-03T02:28:54.937" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10902" PostTypeId="2" ParentId="10379" CreationDate="2015-09-03T02:31:31.417" Score="13" Body="&lt;p&gt;I didn't &lt;a href=&quot;http://www.prweb.com/releases/2015/03/prweb12562316.htm&quot; rel=&quot;noreferrer&quot;&gt;work on the server end of things&lt;/a&gt;, but I recall hearing at work that normal Internet streaming doesn't work because the general assumptions made about connections are violated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The latency is long, and connection pathways change often and by extreme amounts.  A normal web-app will detect a problem and reset the connection.  The software I worked on needed the client end &quot;adjusted&quot; to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The official NASA communications uses a special network whose name I don't recall (a bunch of letters). Internet access uses a totally different &quot;be nice if...&quot; system plugged into consumer Internet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The computers inside the Space Network cannot access Internet servers. They can use an airgapped proxy in Houston (connected by HDMI cable only so the Internet PC seems to be a camera on the Space PC), so they could in principle browse anything and use the “special” remote desktop video (explained above) to view it; but that requires a helper in Houston to do the actual typing for you, and for viewing streaming video it would be pretty lousy if it wasn’t aware of the final leg in a seamless system.  Watch a video on a remote desktop over a horrible laggy connection to get the feel for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what do &lt;em&gt;you&lt;/em&gt; do about watching video with a bad connection?  Download the file first and watch (eventually) when you know you got it all; or use lots of buffering.  It seems that most &lt;em&gt;watching&lt;/em&gt; apps are doing away with expected buffering; e.g. pausing for a while doesn't make it buffer the whole movie: and is unfriendly to poor or intermittent connections. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Game of Thrones&lt;/em&gt; is on DVD, so maybe he's not streaming at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: see &lt;a href=&quot;https://vsee.com/blog/nasa-chooses-vsee-telemedicine-video-conferencing-iss/&quot; rel=&quot;noreferrer&quot;&gt;this press release&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6471" LastEditorUserId="6471" LastEditDate="2016-10-04T07:19:49.203" LastActivityDate="2016-10-04T07:19:49.203" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10903" PostTypeId="2" ParentId="10891" CreationDate="2015-09-03T05:08:25.503" Score="4" Body="&lt;p&gt;My initial thought was that it should be about the same.  My understanding is that most of the difference between the profiles is getting the phasing correct prior to launch.  But fuel isn't consumed to correct phasing in a standard approach, just loiter time at a lower altitude.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing I found was this &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576512000914&quot; rel=&quot;nofollow&quot;&gt;Murtazin/Petrov paper&lt;/a&gt; (linked to from another question).  I don't have access to the paper itself, but from the notes in the abstract (&quot;test in 2012&quot;) it may be describing the current &quot;short profile&quot; scheme.  One of the highlights it mentions is &quot;The fuel consumption for short profile corresponds to the 2-day rendezvous mission&quot; &lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2015-09-03T05:08:25.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10904" PostTypeId="1" AcceptedAnswerId="11936" CreationDate="2015-09-03T05:19:53.257" Score="7" ViewCount="2801" Body="&lt;p&gt;I've been thinking about space solar power for quite some time, as to how it could be more profitable than ground based solar power. From all the concepts that I have seen the main problem I always see is the conversion of majority of the solar power to microwave energy to beam back to the ground, for re-conversion into electrical energy, and having its only benefit being above the atmosphere to get more consistent solar energy. So, here is the thought I had. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Why aren't the solar panels placed closer to the sun where the solar flux is MUCH higher? (at 2 million kilometers from the Sun, the solar flux is nearly 10,000X as high as the flux experienced on Earth, meaning large amounts of power can be extracted with smaller panels) Space offers the advantage of being able to place collectors much closer to the energy source, is the main reason for not doing this the issues with redirecting this energy to Earth? Or being able to build solar panels able to handle the massive amount of power?&lt;/li&gt;&#xA;&lt;li&gt;Why isn't the solar energy captured from satellites in solar power satellites transferred to the ground optically? (essentially reflecting sunlight to the ground instead of converting it into microwaves) I would imagine the energy would get lost through scattering, but wouldn't the efficiency of sending optical energy down be higher than converting to microwaves? (since its only one conversion process being used) &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;From the calculations I've done, the main problems seem to be transmitting the power from the satellite close to the sun to the satellite near Earth and ensuring a constant link between the two. I was thinking an optical link (using the satellite close to the Sun to serve as a lens to focus the sunlight on the Earth orbiting satellite) could be used, but I don't know how practical that is since laser communications for deep space still haven't been built. Still, since the solar flux is a lot higher, the initial network could be setup using cubesats launched from the ISS using the Nanoracks system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has this been considered by other people? And if it was scrapped why was it scrapped?&lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="238" LastEditDate="2015-09-03T05:40:35.120" LastActivityDate="2020-06-22T23:34:59.550" Title="Why aren't solar collectors in space solar power concepts placed closer to the sun?" Tags="|power|solar-power|cubesat|" AnswerCount="4" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="10905" PostTypeId="1" AcceptedAnswerId="10906" CreationDate="2015-09-03T07:54:56.433" Score="16" ViewCount="2631" Body="&lt;p&gt;Today Roscosmos &lt;a href=&quot;http://ria.ru/space/20150903/1226652410.html&quot;&gt;reported&lt;/a&gt; (in Russian, here's a &lt;a href=&quot;https://translate.google.com/translate?sl=ru&amp;amp;tl=en&amp;amp;hl=en&amp;amp;u=http%3A%2F%2Fria.ru%2Fspace%2F20150903%2F1226652410.html&quot;&gt;machine translation into English&lt;/a&gt;) that the Soyuz vehicle on its way to the ISS avoided collision with a piece of Japanese rocket body, allegedly launched back in 1989.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does Soyuz detect such objects? Does it have some sort of radar like aircraft have, or receives information about it from ground-based debris tracking stations (e. g. the NORAD observing the objects flying in the orbit)?&lt;/p&gt;&#xA;" OwnerDisplayName="user3049" LastEditorUserId="6407" LastEditDate="2015-09-03T18:42:00.947" LastActivityDate="2015-09-03T18:42:00.947" Title="How does a spacecraft such as Soyuz detect when it's on collision course with an object?" Tags="|low-earth-orbit|debris|tracking|collision-avoidance|space-track|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10906" PostTypeId="2" ParentId="10905" CreationDate="2015-09-03T08:44:18.507" Score="23" Body="&lt;p&gt;Spacecraft rely on information from earth to avoid space debris, they don't have instruments for scanning and detecting debris. There's a few reasons for this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Power: most spacecraft don't have enough electrical power to run a radar powerful enough to detect small debris far enough away to matter&lt;/li&gt;&#xA;&lt;li&gt;Weight: in order to be useful for detecting debris the radar would have to have a very wide coverage as debris could be from any direction, this would be a heavy thing to haul around&lt;/li&gt;&#xA;&lt;li&gt;Cost: not only would the radar be expensive in itself, the weight of launching it would make the required rocket much bigger and expensive&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Even if you did put a radar on a spacecraft for detecting debris it's utility would be limited because orbital debris is moving so fast. Even if a radar was able to detect debris at 40 miles away there would be mere seconds to react. First a radar would need to detect an object, then track it long enough to project its course with the accuracy needed to determine it's a threat, and by that time it's probably either past or your spacecraft is in tiny little pieces. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to maneuver a spacecraft out of the way of debris you have to know about it while it's still over the horizon, preferably several orbits away, this is why debris detection is ground based. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-09-03T08:44:18.507" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10907" PostTypeId="2" ParentId="10904" CreationDate="2015-09-03T11:36:51.160" Score="6" Body="&lt;p&gt;This actually is a fairly popular concept in Science Fiction, although they usually don't go quite so close to the Sun. They usually conveniently ignore the elephant in the room, which is how to get said power back to Earth. You are correct, the only way to reasonably do this is to have it be transmitted via some kind of a laser. It is theoretically possible to have a very focused laser, but it would have to be very large, much larger than anything we have now. I've seen that for a Mars to Earth communication link, which is about the same distance as Mercury to Earth, for instance, the size of the laser beam would be on the order of a US Region, such as the Southwest Region, or perhaps the size of European countries (The larger ones). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, I suspect that this could eventually be considered, but isn't really practical in the short term. Microwave transmission might be possible from Earth orbit, which is why that concept is discussed far more than a system very close to the Sun.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-03T11:36:51.160" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="10908" PostTypeId="2" ParentId="10905" CreationDate="2015-09-03T12:23:49.537" Score="13" Body="&lt;p&gt;Let me tell you how a typical satellite avoids debris, which I believe will answer your question about how Soyuz does it.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;All space objects are tracked by a US military organization called JSPOC. They also release data to those operating satellites via &lt;a href=&quot;https://www.space-track.org/&quot;&gt;https://www.space-track.org/&lt;/a&gt; . Note that they will not release information about US classified payloads via this site. Also note that the Russians may have their own method of determining the orbit of such objects.&lt;/li&gt;&#xA;&lt;li&gt;JSPOC determines the orbit of these objects by a worldwide array of sensors, which ping the objects and find where they are. The position and velocity of each object is determined, and updates are continually made.&lt;/li&gt;&#xA;&lt;li&gt;JSPOC periodically determines if any two objects are going to be very close to each other. This requires knowing precise orbits of all of the potential candidates.&lt;/li&gt;&#xA;&lt;li&gt;When a close approach is determined to be likely, JSPOC issues a notice to the team responsible for managing said satellites (If the satellites are alive).&lt;/li&gt;&#xA;&lt;li&gt;The team uses the information from JSPOC to determine if they are going to attempt a collision avoidance maneuver. If they do, they typically want to do this as early as possible, to use little fuel. 24 hours in advance is typically the timeframe for such decisions, if possible, otherwise they do it as quickly as they can.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;A few other interesting tidbits:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The close approach velocity of two objects can have speeds above Orbital Velocity. It's not uncommon for the velocity to be 10 km/s!&lt;/li&gt;&#xA;&lt;li&gt;It is very difficult to determine if these objects will be a threat.&lt;/li&gt;&#xA;&lt;li&gt;Even if you could detect objects in your area, the typical thrust of a satellite is so small that a few seconds wouldn't be enough to get it out of the way.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-03T12:23:49.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="10910" PostTypeId="1" CreationDate="2015-09-03T17:42:43.830" Score="40" ViewCount="6506" Body="&lt;p&gt;Pluto was recently re-classified, losing its official status as the ninth planet, because a bunch of other dwarf planets were found in its orbital region that were too numerous and insignificant to count as &quot;real&quot; planets, and it quickly became apparent that Pluto wasn't really any different from them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yet, one unambiguous fact remains: Pluto was discovered in 1930, and the rest of the Kuiper belt dwarf planets didn't even begin to be discovered until the 1990s. This raises a somewhat obvious question: if Pluto is no more exceptional than other Kuiper belt dwarf planets, why did it take more than half a century of technological advances before we were able to discover the rest of them?&lt;/p&gt;&#xA;" OwnerUserId="10171" LastEditorUserId="49" LastEditDate="2015-09-07T08:03:25.093" LastActivityDate="2019-03-22T01:06:22.513" Title="If there's nothing special about Pluto, why was it discovered so early?" Tags="|planetary-science|history|pluto|astronomy|kuiper-belt|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="10911" PostTypeId="2" ParentId="5676" CreationDate="2015-09-03T17:54:23.293" Score="0" Body="&lt;p&gt;According to this 2014 &lt;a href=&quot;http://www.esep.pro/sites/esep/IMG/pdf/nanosat_upmc_de_c2014_marcel_guyot.pdf&quot; rel=&quot;nofollow&quot;&gt;study&lt;/a&gt; &lt;em&gt;(pdf, french language)&lt;/em&gt;, nothing is readily available (looks like they didn't searched very well).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the same paper paper, some projects working on it:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;μLPPT (poland)&lt;/li&gt;&#xA;&lt;li&gt;arc-jet (Japan, Germany)&lt;/li&gt;&#xA;&lt;li&gt;PPT (Austria)&lt;/li&gt;&#xA;&lt;li&gt;ionic liquid (Swiss, Netherland, USA)&lt;/li&gt;&#xA;&lt;li&gt;ion thruster (US)&lt;/li&gt;&#xA;&lt;li&gt;The french project from the paper &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some pictures and figures are available on the article.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2015-09-03T17:54:23.293" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="10912" PostTypeId="2" ParentId="10910" CreationDate="2015-09-03T17:58:55.373" Score="37" Body="&lt;p&gt;There are a few special things about Pluto, as compared to the other dwarf planets in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kuiper_belt&quot; rel=&quot;noreferrer&quot;&gt;Kuiper belt&lt;/a&gt;. These include:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It is the largest dwarf planet known, by diameter. (Note, this was only determined after the &lt;a href=&quot;http://news.sciencemag.org/space/2015/07/pluto-confirmed-largest-object-kuiper-belt&quot; rel=&quot;noreferrer&quot;&gt;New Horizon's flyby&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;It orbits relatively close to the Sun, at times it is even closer than &lt;a href=&quot;https://en.wikipedia.org/wiki/Neptune&quot; rel=&quot;noreferrer&quot;&gt;Neptune&lt;/a&gt;!&lt;/li&gt;&#xA;&lt;li&gt;It has the largest satellite system of any non-gas giant in the solar system.&lt;/li&gt;&#xA;&lt;li&gt;It has an atmosphere. It seems unlikely that any other dwarf planet has an atmosphere that has been discovered thus far.&lt;/li&gt;&#xA;&lt;li&gt;It is the only &quot;double planet&quot;, where the planetary satellite system's &lt;a href=&quot;https://en.wikipedia.org/wiki/Barycenter&quot; rel=&quot;noreferrer&quot;&gt;barycenter&lt;/a&gt; outside of the system's main planet!&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Okay, so why else was it discovered early? Here's a few reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_largest_optical_telescopes_historically&quot; rel=&quot;noreferrer&quot;&gt;Telescopes&lt;/a&gt; didn't grow much in size for some time around the discovery of Pluto.&lt;/li&gt;&#xA;&lt;li&gt;Discovering new objects so far out takes a lot of time. Someone really has to be looking for such objects, or else have a dedicated computer program to do so. In fact, often objects are discovered to be in pictures from long ago, such as &lt;a href=&quot;http://phys.org/news/2015-08-dwarf-planet-orcus.html&quot; rel=&quot;noreferrer&quot;&gt;Orcus&lt;/a&gt;, discovered in 2004, but pictures were taken of it in 1951.&lt;/li&gt;&#xA;&lt;li&gt;Computers didn't come to be popular for such purposes until the 1990s, which is why the number of such objects discovered increased dramatically at that time.&lt;/li&gt;&#xA;&lt;li&gt;Pluto is &lt;a href=&quot;https://en.wikipedia.org/wiki/Apparent_magnitude&quot; rel=&quot;noreferrer&quot;&gt;quite a bit brighter&lt;/a&gt; than the other dwarf planets, &lt;a href=&quot;https://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29&quot; rel=&quot;noreferrer&quot;&gt;Ceres&lt;/a&gt; excluded.&lt;/li&gt;&#xA;&lt;li&gt;Astronomers were looking for a new planet in the outer Solar System when Pluto was discovered. Pluto was not the planet they were looking for, still, it was there, largely by coincidence.&lt;/li&gt;&#xA;&lt;li&gt;The 1990s saw a dramatic improvement in telescopes, namely in that the mirror could be moved in a way to correct for atmospheric issues, called &lt;a href=&quot;https://en.wikipedia.org/wiki/Adaptive_optics&quot; rel=&quot;noreferrer&quot;&gt;adaptive optics&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="690" LastEditDate="2015-09-08T21:37:41.337" LastActivityDate="2015-09-08T21:37:41.337" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="10913" PostTypeId="2" ParentId="10910" CreationDate="2015-09-03T18:04:45.913" Score="30" Body="&lt;p&gt;Pluto was discovered by a manual search of the sky using a blink comparator. This is an extremely laborious process. For Pluto, it made sense to go to all this trouble, because there were indications that a ninth planet had to be out there: Neptune's orbit was perturbed by the gravity of another planet, it seemed.&lt;br&gt;&#xA;Later, it turned out that &lt;a href=&quot;https://en.wikipedia.org/wiki/Kuiper_belt#Hypotheses&quot; rel=&quot;nofollow&quot;&gt;that wasn't the case&lt;/a&gt;, but by then Pluto had been found.&lt;br&gt;&#xA;Now we know that Pluto is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Trans-Neptunian_object&quot; rel=&quot;nofollow&quot;&gt;brightest object&lt;/a&gt; in the Kuiper Belt. Everything else is a lot dimmer and harder to find. Because of that, we only started finding KBOs in 1992, when the process could be automated.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before that, &lt;a href=&quot;https://en.wikipedia.org/wiki/Charles_Kowal&quot; rel=&quot;nofollow&quot;&gt;Charles Kowal&lt;/a&gt; did an extensive search using a blink comparator:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Between December 1976 and February 1985, Kowal searched 6400 square&#xA;  degrees of sky in the ecliptic plane for distant, slow-moving Solar&#xA;  System objects. Only one object was found beyond Jupiter: 2060&#xA;  Chiron...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Chiron orbits between Saturn and Uranus. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-09-07T10:32:10.770" LastActivityDate="2015-09-07T10:32:10.770" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="10914" PostTypeId="2" ParentId="10877" CreationDate="2015-09-03T21:38:48.863" Score="-1" Body="&lt;p&gt;If the satellite is at 150 miles altitude, it can see a spot of radius $\sqrt{4150^2-4000^2}\approx 1105$ miles-draw a right triangle from the center of the earth to the limb of the earth to the satellite.  $5-10$ km is trivial compared to that.  Even if the satellite insists on being at a $30^\circ$ elevation the radius is $285$ miles. The field of view of the sensor may be much smaller, however.  That will depend on the particular satellite.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2015-09-03T21:38:48.863" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="10915" PostTypeId="1" CreationDate="2015-09-04T00:19:10.880" Score="4" ViewCount="482" Body="&lt;p&gt;Before a human crew goes to Mars' moon Phobos or to even lower mass asteroids, could a long stay at such an object be simulated gravitationally in the space station? I've heard that the ISS people don't like movements which disturb their microgravity experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But (when they are finished with that :) ) could the ISS at least for a few months spin enough to simulate the tiny surface gravity of Phobos? 0.0057 m/² I think. Wouldn't it be interesting to learn how dust fall and how humans orient in such an environment? If stuff eventually sail downwards, and if people prefer to sleep horizontally, then maybe that says something about what such miniscule gravity does to us. And maybe it would be good even for the ISS to swing a bit.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6944" LastEditDate="2015-09-04T02:38:56.323" LastActivityDate="2015-09-04T07:57:06.527" Title="Could the ISS be spun to simulate Phobos' gravity?" Tags="|iss|microgravity|artificial-gravity|phobos|deimos|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11915" PostTypeId="2" ParentId="10915" CreationDate="2015-09-04T01:43:52.547" Score="8" Body="&lt;p&gt;First off, there are good reasons &lt;a href=&quot;https://space.stackexchange.com/questions/10771/why-is-artificial-gravity-not-included-on-space-stations&quot;&gt;here&lt;/a&gt; for why they didn't design for artificial gravity on the ISS.  Also, it's not that there are some experiments on the ISS that require microgravity; rather, that migrogravity is largely the point of doing experiments on the ISS since most other conditions can be practically replicated on Earth.  That aside...&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The components of the ISS were designed with microgravity in mind, not a constant centripetal force.  Any constant rotation would produce extra stress, possibly beyond designed limits, on things that stick out like the solar panel arrays.  Also, all pumping of fluids, air, etc would suddenly have the resistance of pumping uphill when going inward radially relative to the axis of rotation (and any valves/fitting/pipes on the outer edges would have to deal with additional pressure).  Power (solar panels) and heat management (which require pumping ammonia through radiators) are some of the things you really don't want to get damaged and neither of them were set up with spinning in mind.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was also a question a while back (&lt;a href=&quot;https://space.stackexchange.com/questions/10432/would-the-life-support-systems-on-the-iss-and-sts-work-well-also-in-1g&quot;&gt;here&lt;/a&gt;, thanks Organic Marble) about what components on the ISS would be affected by the presence of gravity.  One of the responses pointed out how the &lt;a href=&quot;http://www.quora.com/How-is-urine-processed-in-the-International-Space-Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Water Recovery System&lt;/a&gt; is highly optimized for microgravity and would be negatively affected by the presence of gravity (it would most likely still work but possibly at reduced efficiency with Phobos-equivalent gravity).  That's just one (highly critical) component: pretty much everything else on the ISS is also designed and optimized for use without gravity.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The farther you are from the center of rotation the more acceleration you feel.  Because the ISS is largely linear you would have to rotate end over end so there would be differing gravity forces between the outside ends and the center of rotation.  Where would you target the gravity of Phobos: at the very ends?  How much time to astronauts spend at the ends of the station, and for that matter do you get the data you want if the astronauts are constantly moving between the target gravity and much less as they move around the station?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Disregarding the force produced by spinning, nothing on-board the ISS is designed to cope with the spinning itself.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The solar arrays are rotated for maximum sunlight collection; with the station spinning the arrays would have to constantly rotate counter to that, producing additional wear on the motors.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The center of rotation is defined by the center of mass, and since the station wasn't designed to spin that's not an intentionally placed location.  That means that you're unlikely to have a docking port that's at the center so docking either gets very complicated or you have to despin for docking and departure.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-09-04T07:57:06.527" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="11916" PostTypeId="1" AcceptedAnswerId="11917" CreationDate="2015-09-04T13:57:45.057" Score="14" ViewCount="1722" Body="&lt;p&gt;Currently (September 4, 2015) there is a complement of 9 people aboard the ISS.  This will change a week from now, when 3 of those people return to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When was the last time the ISS had 9 people on board, if ever?  It seems over the last few years it has been much more common for 3 or 6 people to be on board.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And why are there 9 people on board now?  Based on the recently crew history of the ISS, this seems a bit exceptional.  My guess is that it is related to the year-long mission of Scott Kelly and Mikhail Kornienko, whereas typical ISS missions last about 6 months.&lt;/p&gt;&#xA;" OwnerUserId="8350" LastActivityDate="2015-09-04T16:17:13.200" Title="When was the last time 9 people were on the ISS?" Tags="|iss|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11917" PostTypeId="2" ParentId="11916" CreationDate="2015-09-04T15:12:54.877" Score="11" Body="&lt;p&gt;Last time the station briefly hosted 9 astronauts before today's (September 4 2015) &lt;em&gt;&lt;a href=&quot;http://www.spaceflight101.com/soyuz-tma-18m-mission-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;One-Week Handover&lt;/a&gt;&lt;/em&gt;, was on November 2013 (for 3 days, Nov. 7 - Nov. 10) before the 2014 Sochi Winter Games when the &lt;a href=&quot;https://blogs.nasa.gov/stationreport/2013/11/07/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Olympic Torch was in space for the first time in history&lt;/a&gt;. Before that, without involving publicity stunts, the final flight of the Space Shuttle, &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-135&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-135&lt;/a&gt; (Atlantis) carried a crew of four, which joined on 10 July 2011 the &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_28&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expedition 28&lt;/a&gt; crew of 6 on station for a total of 10.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During final assembly of the station and before Space Shuttle fleet retirement in 2011, the station often hosted 9 or more crew at the same time. For example, the final mission of the Space Shuttle Endeavour, flight &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-134&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-134&lt;/a&gt;, delivered a crew of 6 joining on 18 May 2011 first the complete &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_27&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expedition 27&lt;/a&gt; crew of six for the total of 12 crew members on the station, before the Soyuz TMA-20 departed on May 23 with thee Expedition 27/28 crew members, for a total on station of 9.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That was dubbed in the media as &lt;em&gt;&lt;a href=&quot;http://www.space.com/11753-astronaut-sleep-patterns-space-station-traffic.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;space traffic jam&lt;/a&gt;&lt;/em&gt; and it had some interesting side effects, like not all of the crew on the station being able to also sleep on it. But it actually wasn't the first. I think that the record was 13 with &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-131&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-131&lt;/a&gt; (Discovery) crew of 7 joining another crew of 6 on 7 April 2010 for a total of 13 (as shown in this picture):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/cU6Le.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cU6Le.jpg&quot; alt=&quot;TS-131 and Expedition 23 crew members gather for a group portrait in the Kibo laboratory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;STS-131 and Expedition 23 crew members gather for a group portrait in the Kibo laboratory of the International Space Station while space shuttle Discovery remains docked with the station. Image credit: &lt;a href=&quot;https://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts131/multimedia/fd10/Image_Gallery_Collection_archive_1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-09-04T16:12:25.300" LastActivityDate="2015-09-04T16:12:25.300" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="11918" PostTypeId="2" ParentId="11916" CreationDate="2015-09-04T15:46:19.253" Score="7" Body="&lt;p&gt;The last time nine people were on board the ISS was in November 2013. &lt;a href=&quot;https://www.flickr.com/photos/nasa2explore/10876146696/&quot;&gt;Here's a photo of the crew of nine on the official flickr feed&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://blogs.nasa.gov/spacestation/2015/09/04/international-crew-of-nine-in-space-working-on-the-orbital-laboratory/&quot;&gt;NASA states explicitly in a recent blog&lt;/a&gt; that the last time nine people were on board the ISS was in 2013, and I subsequently looked at the individual mission descriptions to infer it was in November 2013, when a brief overlap between expedition 37 and expedition 38 (unofficially dubbed &quot;expedition 37.5&quot;) &lt;a href=&quot;http://www.space.com/23546-olympic-torch-spacewalk-special-patch.html&quot;&gt;brought the Olympic torch into space for the 2014 Olympic Winter Games&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I couldn't find a simple &quot;ISS occupancy timeline&quot; online but found a great &lt;a href=&quot;http://english.tachyonbeam.com/2015/09/04/three-new-astronauts-reached-the-international-space-station-one-of-them-completes-the-expedition-45-crew/&quot;&gt;blog with details&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your question made me curious about what the process looks like when the crew goes from six to nine and I found an interesting &lt;a href=&quot;http://www.universetoday.com/106266/can-the-space-station-fit-bigger-astronaut-crews/&quot;&gt;interview&lt;/a&gt; with Luca Parmitano during the last nine person stay on the ISS detailing how the logistics worked out in hat particular case (spoiler alert: the old crew called dibs on the sleeping stations and the new guys were left to make do with temporary arrangements...)&lt;/p&gt;&#xA;" OwnerUserId="11397" LastEditorUserId="11397" LastEditDate="2015-09-04T16:17:13.200" LastActivityDate="2015-09-04T16:17:13.200" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11919" PostTypeId="1" AcceptedAnswerId="12072" CreationDate="2015-09-04T15:49:45.040" Score="11" ViewCount="793" Body="&lt;p&gt;This morning I was watching another video of Soyuz docking to the ISS (&lt;a href=&quot;https://www.youtube.com/watch?v=ukcZodM9j8E&quot; rel=&quot;noreferrer&quot;&gt;Expedition 45/Visiting Crew Docks to the Space Station&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lF6cp.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lF6cp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The commentator noted that the two quantities $\rho$ and $\dot\rho$ in the lower-left are the range and range-rate, respectively.  Retrospectively this is pretty obvious, but I had never thought about it before.  This got me a little curious and I tried to look up what all the other numbers mean, with no luck.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What are the quantities displayed on the Soyuz docking camera?&lt;/strong&gt;  I'm particularly interested in the right-hand side, which looks like something to do with the orientation of the spacecraft or station.&lt;/p&gt;&#xA;" OwnerUserId="8996" LastActivityDate="2015-09-23T20:03:24.443" Title="What are the numbers on the Soyuz docking cam?" Tags="|soyuz-spacecraft|attitude|docking|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="11920" PostTypeId="2" ParentId="4576" CreationDate="2015-09-04T17:38:44.517" Score="2" Body="&lt;p&gt;It seems like there isn't any serious work on the table relating to it. I was able to find a &lt;a href=&quot;http://physics.anu.edu.au/cpf/sp3/ds4g/&quot; rel=&quot;nofollow&quot;&gt;recent article&lt;/a&gt; talking about it, but it doesn't seem to contain any new information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The great limitation of this system is that it requires a lot of energy. It was developed in 2005, which was an interesting time in the development of high power spacecraft, namely the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Orbiter&quot; rel=&quot;nofollow&quot;&gt;Jupiter Icy Moons Orbiter&lt;/a&gt;, a nuclear powered spacecraft that was a concept from 2004-2005, when it was cancelled. Although JIMO had it's own ion propulsion system &lt;a href=&quot;https://en.wikipedia.org/wiki/High_Power_Electric_Propulsion&quot; rel=&quot;nofollow&quot;&gt;HiPEP&lt;/a&gt;, I suspect this engine was developed as an alternative. Currently, no spacecraft in planning has anywhere near the power to even consider using DS4, and so it doesn't make much sense to continue to develop it. 250 kW is a lot of power, far more than any spacecraft in existence currently uses.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-04T17:38:44.517" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11921" PostTypeId="2" ParentId="10659" CreationDate="2015-09-04T17:54:07.623" Score="7" Body="&lt;ul&gt;&#xA;&lt;li&gt;How is payload to LEO calculated?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These days, they run simulations on everything. The payload to LEO is either an analytical assessment of the payload using worst case assumptions, or else some threshold of simulation. Basically, the rocket is designed to meet a minimum LEO threshold, using a nominal orbit. These values are broken down into what every piece on the rocket needs to accomplish in order to meet said requirements. Then once everything is designed, they again test using the actual limits as found.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the things that might be varied, for instance, is the temperature of the rocket fuel, the exact flow of fuel of each type into the rocket (Resulting in differing thrust and ISP), wind, weather, other kinds of part efficiency, etc. All of this put together will give a worse case to LEO, which is what they advertise. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What error bars should one put on such estimates?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The error bars are already assumed in the publicly disclosed mass to LEO. If one wanted to launch an item slightly above the maximum, the company could theoretically do a study to see how likely there would be an issue. If there was likely to be an issue, they could inform the customer, and act accordingly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted that the uncertainty in part performance will go down with time, which will usually demonstrate the rocket is better performing than first thought (Unless the rocket truly was off-nominal)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Would heaviest or average payload actually launched be better measures, and is such statistics easily available to the public?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Not really. The heaviest payload will under almost all conditions be less than the LEO max mass. The statistics usually aren't available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give you an idea, every rocket I've seen launch does something like this plot after the launch. In this case, the two lines represent the two &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#/media/File:Srbthrust2.svg&quot; rel=&quot;noreferrer&quot;&gt;SRBs in a Space Shuttle launch&lt;/a&gt;. The ones I've seen compare the expected performance, max/min, with the actual performance, to see if there are any serious deviations from the expected performance, and get an idea as to what direction from ideal the performance leans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hsiBE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hsiBE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-04T17:54:07.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11922" PostTypeId="1" CreationDate="2015-09-04T19:04:58.183" Score="1" ViewCount="1006" Body="&lt;p&gt;I think that if we could build a powerful enough laser/maser to heat the core of Mars up and melt it, it could generate a magnetic field, which would allow for easier living on Mars. Could this be done?&lt;/p&gt;&#xA;" OwnerUserId="11548" LastEditorUserId="49" LastEditDate="2015-09-04T19:21:52.600" LastActivityDate="2015-09-18T12:22:25.823" Title="Could a powerful enough laser or maser heat the core of Mars?" Tags="|mars|colonization|magnetic-field|feasibility|terraforming|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11923" PostTypeId="2" ParentId="11922" CreationDate="2015-09-04T19:10:12.057" Score="2" Body="&lt;p&gt;Not without also heating the surface. And such a beam would have to be REALLY powerful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All electromagnetic waves are either absorbed or reflected by Mars. Thus, any heat that was applied via the beam would also heat the surface. The only heating of the core would come via indirect methods.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think anything short of somehow drilling to Mars's core would allow for us to heat it any.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-04T19:10:12.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11924" PostTypeId="1" AcceptedAnswerId="12154" CreationDate="2015-09-04T20:13:20.420" Score="8" ViewCount="1210" Body="&lt;p&gt;I'm wondering where one can find more or less up-to-date data on the inertia matrix of the ISS. A full historical dataset would be great, of course. A discussion of error margins is desirable.&lt;/p&gt;&#xA;" OwnerUserId="168" LastActivityDate="2015-09-30T11:53:24.237" Title="What are the ISS moments of inertia around design axes?" Tags="|iss|attitude|reference-request|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="11925" PostTypeId="2" ParentId="11922" CreationDate="2015-09-04T21:19:34.310" Score="10" Body="&lt;p&gt;This question assumes Mars doesn't have a magnetic field because Mars' core is frozen solid. It's not. Mars has a partially liquid core, just as does the Earth. (The Earth has a liquid outer core and a solid inner core.) Whether Mars has a solid inner core is unknown, but it certainly does have a liquid outer core (and possibly a fully liquid core). (See C.F. Yoder, et al. &quot;Fluid core size of Mars from detection of the solar tide.&quot; Science 300.5617 (2003): 299-303.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three (or more) ingredients are needed for a planet to have a substantial magnetic field:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;A conductive fluid. Mars has that. It has, at a minimum, a molten outer core.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Planetary rotation. Mas has that as well.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A significant heat flux across the core-mantle boundary. Mars does not have that. (Nor does Venus.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Mars has a stagnant lid geography. (So does Venus.) This keeps the heat bottled up in the core. A planet with a stagnant lid doesn't have enough heat flux out of the core to maintain a magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want Mars to have a magnetic field, you'll have to unloose the heat that is bottled up in Mars' core. That means either rampant vulcanism (the best way a terrestrial planet can release heat into the universe) or plate tectonics (the second best approach).&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-09-04T21:19:34.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11926" PostTypeId="1" CreationDate="2015-09-04T22:05:47.810" Score="4" ViewCount="570" Body="&lt;p&gt;I'm a controls guy and I would like to &quot;emulate&quot; a given rocket engine's response behavior to different thrust signals inside a computer simulation. Thus I'm looking for a book that explains well the different equations that govern a rocket engine's response as a function of propellant, nozzle shape, configuration, etc. You can see that this does not concern the deep fluid dynamics and thermodynamics of it all... I'm just looking for the &quot;high level dynamics&quot; if you see what I mean. Does such a book or resource exist? Thanks a lot for your help!&lt;/p&gt;&#xA;" OwnerUserId="11551" LastActivityDate="2015-09-05T02:19:50.587" Title="Books on rocket engine *dynamics*?" Tags="|propulsion|advanced-propulsion|simulation|" AnswerCount="1" CommentCount="1" ClosedDate="2015-09-05T08:32:34.307" ContentLicense="CC BY-SA 3.0" />
  <row Id="11927" PostTypeId="2" ParentId="11926" CreationDate="2015-09-04T22:25:23.633" Score="6" Body="&lt;p&gt;Rocket Propulsion Elements by Sutton is a good general reference. Chapter 7, Liquid Propellant Rocket Fundamentals, contains the basic equations that should get you started.  Figure 7-7 is particularly good, showing a schematic gas generator engine with the equations that govern the performance of each component.  You could probably gen up a decent simulation just starting from this diagram and learning about the equations it contains.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/spHtR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/spHtR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Zsguw.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Zsguw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(All my references are to my nearly worn out 1976 printing of the 4th edition)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also have the 1984 Oates &quot;Aerothermodynamics of Gas Turbine and Rocket Propulsion&quot; and it's quite good but more difficult. Oates had useful information about computing gas temps in the combustion chamber (it is an iterative relationship).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-05T02:19:50.587" LastActivityDate="2015-09-05T02:19:50.587" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="11930" PostTypeId="1" CreationDate="2015-09-05T06:51:01.057" Score="1" ViewCount="637" Body="&lt;p&gt;Are there any statistics on satellite failures and their causes? What would be the most frequent causes for satellite to become defunct (either before or after design lifespan): ran out of propellant, valve failure, reaction wheel failure, electric failure, GNC failure, anything else?&lt;/p&gt;&#xA;" OwnerUserId="11538" LastEditorUserId="168" LastEditDate="2015-09-05T09:30:16.587" LastActivityDate="2015-09-05T09:30:16.587" Title="Statistics on satellite failures" Tags="|artificial-satellite|" AnswerCount="0" CommentCount="3" ClosedDate="2015-09-05T18:07:02.403" ContentLicense="CC BY-SA 3.0" />
  <row Id="11931" PostTypeId="1" AcceptedAnswerId="11939" CreationDate="2015-09-05T16:20:33.063" Score="6" ViewCount="1464" Body="&lt;p&gt;I was doing some thinking about approaches for reducing the launch cost of a rocket, and discovered that one of the main drivers for cost (especially for expendable rockets and somewhat reusable rockets) is manufacturing touch labor, I.E. the number of manhours needed to finish the rocket. I've asked around and I've gotten pretty bad answers on this question, but how does one estimate how long a part will take to be made? Is it purely by analogy (this part is similar to something we made earlier, therefore it should take about the same amount of time) or is it from fundamentals? (this part has x number of welds, faces to cut, etc.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know the manufacturing method will greatly influence this, as that will determine how much the fabrication can be automated. From first glance, it would seem like 3D printing is a godsend for a low volume production product like a rocket, since 3D printing can theoretically automate production of every part of the rocket. Why hasn't this been done yet? Is it because the materials aren't strong enough or the materials are too expensive? &lt;/p&gt;&#xA;" OwnerUserId="11377" LastActivityDate="2015-09-15T12:10:37.677" Title="How is manufacturing cost for a rocket engine design estimated based on drawings alone?" Tags="|rockets|cost|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="11932" PostTypeId="1" AcceptedAnswerId="13859" CreationDate="2015-09-05T16:20:54.623" Score="6" ViewCount="172" Body="&lt;p&gt;Is &lt;a href=&quot;https://en.wikipedia.org/wiki/Pogo_oscillation&quot;&gt;pogo oscillation&lt;/a&gt; accounted for in design of &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_engine&quot;&gt;hybrid motors&lt;/a&gt;? I'm thinking particularly about hybrids that use Nitrous Oxide as the oxidizer because the &lt;a href=&quot;http://www.bu.edu/rocket/files/2010/02/The-physics-of-nitrous-oxide.pdf&quot;&gt;weird triple point effects&lt;/a&gt; that one experiences with nitrous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, do hybrid designers ever have to consider pogo? Why or why not?&lt;/p&gt;&#xA;" OwnerUserId="10856" LastActivityDate="2016-02-02T19:59:54.553" Title="Is pogo oscillation a risk in hybrid motors?" Tags="|propulsion|engine-design|hybrid-rocket|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11935" PostTypeId="1" AcceptedAnswerId="11987" CreationDate="2015-09-05T17:34:13.067" Score="6" ViewCount="800" Body="&lt;p&gt;I'd be very interested to hear what exactly do ISS crew members spend their time on — the split between different major activities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yesterday I saw someone on this site mention that close to 30% of the time is spent preparing for and handling incoming cargo. Also, I recall hearing that Commercial Crew adding one crew member would double the time spent on science — suggesting that less than 20% of available time is spent on that. (I can't find the sources)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in addition to those two things, what are astronauts aboard ISS doing with their days? Obviously there's sleep, eating, exercise, and free time…  what about work time? What's the split, more or less, there? I'm guessing maintenance could be a major activity — but what's the number there?&lt;/p&gt;&#xA;" OwnerUserId="10274" LastActivityDate="2016-07-19T20:12:06.347" Title="How is time split on different activities on ISS?" Tags="|iss|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="11936" PostTypeId="2" ParentId="10904" CreationDate="2015-09-05T18:12:44.600" Score="6" Body="&lt;p&gt;This is really two questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Where is the best place to put solar power collection on the line between the Sun and the Earth?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What is the best wavelength to use for power transmission to Earth?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;For #2, microwaves go through clouds.  Visible light does not.  That is why you will almost always see microwave transmission used in space solar power concepts.  Recall that the rationale to consider space solar power at all, which seems odd given that our atmosphere is transparent to most of the energy from the Sun, is that you don't get solar power at night, and it can be reduced dramatically by clouds.  If you use visible light to transmit the power, then you have lost the second advantage (as well as part of the first advantage on cloudy nights).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other advantages to microwaves, such as the fact that the ground collectors can be much lower cost (wires and diodes vs. solar panels), and the relatively long wavelength allows a mostly transparent wire mesh antenna for the ground collector, permitting crops under the collector, or even solar panels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For #1, if you would like more concentrated solar power on your collector, then you can do that with mirrors without having to get closer to the Sun.  There are indeed concepts that do that, since photovoltaic cells can be more efficient at higher insolation, and solar thermal systems require high concentration for any kind of decent efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now on either side we're left with the diffraction limit for how big the reflectors need to be to collect the solar energy on the input side and to focus the energy to a collector on the ground on the output side.  Yes, your concentrator/collector can get smaller as you get closer to the Sun, but now your transmission antenna needs to get larger to be able to focus on the same size collector on the Earth.  If you are converting light to microwaves, then you want to be closer to Earth since the microwave antennas have to be bigger at the same angular limit than for light, due to the longer wavelength.  If you are converting light to light (so you don't need to do energy conversion at all -- just use mirrors), then it's a wash, so you put it near the Earth since its easier to get there.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2018-06-10T02:40:22.030" LastActivityDate="2018-06-10T02:40:22.030" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="11937" PostTypeId="1" CreationDate="2015-09-05T19:19:45.267" Score="8" ViewCount="3566" Body="&lt;p&gt;I have been given the following problem:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/rHJQN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rHJQN.png&quot; alt=&quot;Problem&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To solve it I used the formulas:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_e = I_{sp}.g_0$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$M_{propellant} = (e^{\Delta_v/v_e} - 1).M_{final}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I assumed that the apogee kick manoeuvre was executed first. So I started by calculating the propellant required for the second manoeuvre:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_e = 200\times9.81 = 1962m/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$M_{p2} = (e^{1050/1962} - 1).3000 = 2123.21Kg$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then for the first manoeuvre I got:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_e = 320\times9.81 = 3139.21m/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$M_{p2} = (e^{2300/3139.2} - 1).(3000+2123.21) = 5536.29Kg$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Giving a total propellant mass of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$M_p = 7659.5Kg$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this answer differs from the correct answer. Furthermore, if I consider the manoeuvres being performed in the reverse order, I get different masses for each propellant, but somehow, the total mass is the same, which seems strange, since the hint suggests that the order matters in the total amount of fuel required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could you please point out where did I make a mistake?&#xA;Thank you.&lt;/p&gt;&#xA;" OwnerUserId="11560" LastEditorUserId="6944" LastEditDate="2015-09-05T20:53:40.617" LastActivityDate="2023-01-23T17:58:50.780" Title="Propellant mass calculation" Tags="|rockets|fuel|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="11938" PostTypeId="2" ParentId="10904" CreationDate="2015-09-05T22:15:55.140" Score="3" Body="&lt;p&gt;&lt;strong&gt;Money&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...that is the reason. That and the fact that you will need to put more energy into getting this stuff into orbit than you will ever get back from it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting 1 kg to Low Earth Orbit costs between \$3 800 and \$13 000. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting 1 kg to Geostationary Transfer Orbit costs between \$12 500 and \$25 000. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting anything to a close orbit around the Sun will be &lt;strong&gt;much&lt;/strong&gt; more expensive than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy needed to get this supposed energy collector into that orbit will be a lot more than it can ever gather. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then - as people have pointed out - we cannot get the energy it collects back to us because no matter what kind of radiation you use, it will spread out and become very faint.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end it is a lot more efficient to just use all that money to build the energy collectors down here on Earth. &lt;/p&gt;&#xA;" OwnerUserId="10493" LastActivityDate="2015-09-05T22:15:55.140" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="11939" PostTypeId="2" ParentId="11931" CreationDate="2015-09-05T23:05:41.923" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How does one estimate how long a part will take to be made? Is it purely by analogy (this part is similar to something we made earlier, therefore it should take about the same amount of time) or is it from fundamentals? (this part has x number of welds, faces to cut, etc.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Both.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used to work-with/design high complexity, low volume, high performance biomedical devices that made use of materials and processes similar to rocket components, so the following is based on my personal experience (your mileage may vary, more experienced engineers may point and laugh) over several years and projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You start by analyzing the part and your chosen production method. How many cuts/welds/coats/folds/etc does the part have? How are they orientated? Do you need to use more that one machine? How many machines? How can you move the part from one machine to the next? How many different tasks are involved in assembly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You should end up with a highly detailed list of every task required to go from metal stock and components to finished part. Next you add a time to each step. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the part is going to use subtractive manufacturing you hand your CAD files to a CNC programmer who computes tool paths based on the geometry of the part and comes back with the total time the part will need on the machine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the time drawn from fundamentals you stated in the question; based purely on cuts/processes defining the part. Substitute the CNC machine for a 3D printer or any other large production machine for our particular part, the process is the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is harder to estimate accurately is how much time outside the machines will be required. What may look a perfectly straightforward task often turns out to be unexpectedly complicated. You look at a sprung valve assembly and assume that it would take mere minutes to slot together. Then you actually try and it's impossible to hold both o-rings compressing the spring while holding the enclosure open with your other hand, and your hand better not slip or the spring will launch an o-ring across the lab, which you will then spend the next five minutes searching for on hands and knees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Experience helps a lot (and sure would have helped me), but for novel devices/parts there's no substitute for actually &lt;em&gt;doing it&lt;/em&gt;. Walking through the process step by step, literally walking the route the part would take across the factory floor, stopping at every station and performing (or simulating) every task, assembly, and process with your pre-production prototype (You do have one of those, right?). Even then your estimate will only be accurate if the production line is staffed with clones of yourself. For really accurate timings you'll need to try it with real manufacturing technicians, that's usually called &quot;production&quot; and you better hope you got the estimates right before that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most important is talking to the people who are actually going to make the part, manufacturing technicians and production engineers have invaluable experience which can allow them to make very accurate estimates on most parts of the production process.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(At some point you'll probably want to draw up &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_time_(manufacturing)&quot; rel=&quot;nofollow&quot;&gt;standard times&lt;/a&gt; for the part and/or production processes, sometimes this is part of the pre-production time estimations, other times it's used during production to analyse and optimize processes and resource flow)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm no expert, and there's a lot more to read on the subject if you're interested:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S2212827114003394&quot; rel=&quot;nofollow&quot;&gt;Knowledge-based Estimation of Manufacturing Lead Time for Complex Engineered-to-order Products&lt;/a&gt; For more advanced methods.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.werc.org/assets/1/workflow_staging/Publications/660.PDF&quot; rel=&quot;nofollow&quot;&gt;Implementing Time Studies And The Development Of Standard Times.&lt;/a&gt; For more reading on Standard Times&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/offices/ooe/CAD/nasa-cost-estimating-handbook-ceh/#.VetrmxFVhBc&quot; rel=&quot;nofollow&quot;&gt;NASA Cost Estimating Handbook&lt;/a&gt; For a Nasa/Spaceflight focused in depth cost  guide, also includes a little on time estimations.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2015-09-15T12:10:37.677" LastActivityDate="2015-09-15T12:10:37.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11940" PostTypeId="2" ParentId="10910" CreationDate="2015-09-06T03:53:43.740" Score="3" Body="&lt;p&gt;One reason is its brightness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto passes near and far and the atmosphere freezes and thaws.  So while most bodies are pretty dingy, Pluto is covered with fresh snow.&lt;/p&gt;&#xA;" OwnerUserId="6471" LastActivityDate="2015-09-06T03:53:43.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11941" PostTypeId="1" CreationDate="2015-09-06T10:39:21.227" Score="5" ViewCount="1593" Body="&lt;p&gt;It has been proposed that a human mission to Mars could park the transfer habitat in high Martian orbit while only a small capsule lands on the surface or on a moon. This in order to minimize the mass entering the deep gravity well of Mars, saving fuel. And aerobraking could help a lot to get the spacecraft captured in Mars orbit. But do those two ideas work together? Aerobraking requires very low periapsis. Can aerobraking itself be used to raise periapsis, or would it need a costly maneuver terms of fuel?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-09-06T11:06:05.027" LastActivityDate="2015-09-06T21:10:20.427" Title="How can aerobraking be used to enter high orbit without landing?" Tags="|mars|orbital-maneuver|aerobraking|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11942" PostTypeId="2" ParentId="11941" CreationDate="2015-09-06T11:04:20.237" Score="11" Body="&lt;p&gt;The idea with aerobraking is usually to do something like the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Make your orbital insertion burn as usual, but instead of slowing all the way down into your final orbit, slow down just enough to get captured by the planet.  (This burn is executed close to the planet, using the Oberth effect to reduce the amount of fuel required.)  You'll end up in a highly elliptical orbit with periapsis (lowest-altitude point) close to the planet.&lt;/li&gt;&#xA;&lt;li&gt;On every succeeding orbit make a small correction burn at apoasis (highest-altitude point).  The ground team calculates how deep into the atmosphere the spacecraft can safely fly, and computes the correction burn to adjust the periapsis altitude accordingly.  On each pass through the atmosphere the drag slows you down slightly, reducing your apoapsis altitude.&lt;/li&gt;&#xA;&lt;li&gt;When you have the correct apoasis altitude, make a burn at apoasis to bring the periapsis back up out of the atmosphere.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;That gets the spacecraft into a low Martian orbit with substantially less fuel than would be required to enter into that orbit directly.  Note that step 1 is optional: you can actually use the atmosphere to capture into orbit, but this is a rather advanced version of aerobraking called aerocapture.  It hasn't been attempted yet because of both the precision required and the stress on the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://mars.nasa.gov/mro/mission/timeline/mtmoi/&quot; rel=&quot;noreferrer&quot;&gt;Mars Reconnaissance Orbiter&lt;/a&gt; (MRO) was an example of a spacecraft that used aerobraking at Mars.  Its initial orbit was 300 km x 45,000 km (periapsis altitude x apoapsis altitude).  It then followed a &lt;a href=&quot;http://mars.nasa.gov/mro/mission/timeline/mtaerobraking/&quot; rel=&quot;noreferrer&quot;&gt;long timeline of aerobraking maneuvers&lt;/a&gt; over the course of about six months lower its orbit to 250 km x 316 km.  Note that as we get better knowledge of Mars's atmosphere, we can do lower dips, making things quicker.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a rough depiction of how MRO's orbit changed during the aerobraking procedure:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pMPR9.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerobraking doesn't raise the periapsis by itself, but the periapsis-raising maneuver isn't that costly—especially considering how much fuel is saved overall. Generally speaking, Aerobraking can only take energy away from a spacecraft, making it impossible to do something like raising an orbit (which requires an increase in energy).&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="8996" LastEditDate="2015-09-06T21:10:20.427" LastActivityDate="2015-09-06T21:10:20.427" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="11944" PostTypeId="2" ParentId="11941" CreationDate="2015-09-06T16:13:47.483" Score="6" Body="&lt;p&gt;The &quot;high Mars orbit&quot; being referred to is an elliptical orbit with a low periapsis and a high apoapsis.  I usually see something like a one-day period orbit.  The idea is to reduce the amount of fuel needed to depart Mars and return to Earth, which however puts more of a burden on the Mars Ascent Vehicle, which now has to reach this highly elliptical orbit.  It is an overall mass trade that tends to favor elliptical orbit rendezvous for chemical systems, especially when the components should have similar Earth departure masses.  (I don't think it will happen this way though, since we will very likely use electric propulsion systems, which will favor low Mars orbit rendezvous.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The aerobraking to help capture into Mars orbit from a hyperbolic approach is referred to as aerocapture.  Yes, an aerocapture works quite nicely to capture into an elliptical orbit.  Though you don't want it to be so elliptical that your aerocapture uncertainty could result in not be being captured at all and flying by Mars.  That would be bad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the aerocapture event, a propulsive maneuver must be performed at the apoapsis to raise the periapsis and prevent a second atmospheric entry.  (Or at least adjust it, in the case of a high aerobraking pass to fine tune the orbit period.)  That maneuver is small.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-09-06T16:13:47.483" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="11949" PostTypeId="1" CreationDate="2015-09-07T17:22:01.503" Score="7" ViewCount="916" Body="&lt;p&gt;In architecture and engineering obviously wind and wind shear are important issues for tall buildings.  I understand that weather is an important condition for the launch of rockets or the hundreds of shuttle launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Was there ever a near miss when an unanticipated wind/weather event nearly jeopardized a rocket or shuttle launch?  And what was the response?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is for a (historically accurate) storyline, but I figured this is the most relevant SE section to ask this.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2015-09-08T02:01:20.333" Title="Were there any issues with wind on rockets / shuttle?" Tags="|launch|rockets|weather|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="11950" PostTypeId="2" ParentId="11949" CreationDate="2015-09-07T18:18:23.443" Score="9" Body="&lt;p&gt;&lt;a href=&quot;http://www.thespacereview.com/article/1662/1&quot;&gt;The Space Review&lt;/a&gt; has an entire article devoted to such failures. I've also found a few other. Missions where wind was, or was almost, a failure, include the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Atlas Centaur AC-67 FLTSATCOM 6- Weather was go until a last minute weather balloon was launched, not according to plan. The balloon found the winds were unacceptable for launch a mere 15 minutes before launch, likely saving the mission.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astronautix.com/lvs/thogenab.htm&quot;&gt;Thor DM-21 Agena-D&lt;/a&gt;- Upper atmosphere winds were too high, resulting in a failed rocket launch, which was self-destructed when it went off course.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-07T18:34:49.883" LastActivityDate="2015-09-07T18:34:49.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11951" PostTypeId="1" CreationDate="2015-09-07T19:16:12.610" Score="-2" ViewCount="302" Body="&lt;p&gt;It is somewhat hard to fill in the table without knowing the exact details beside what I can gather up from wiki that the incident was mainly attributed to&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;damage to lift wing of shuttle going undetected during lift-off and&#xA;  even before point of re-entry. The reason for being so is due to&#xA;  change in design specifications of thermal protection system which&#xA;  didn't cater up for uncommon occurrence of impact damage to it during&#xA;  flight i.e damage during launch sequence.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/m2yJa.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/m2yJa.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ones in red I believe are some of the area which were missed or inadequately addressed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why the Zachman framework? This is the kind of framework that would be used to see where NASA is successful as an institution in putting a man on the moon. But failing to add a check on sensors as basic as one which would alert when significant damage is done to its critical structure, i.e. the left wing?&lt;/p&gt;&#xA;" OwnerUserId="11571" LastEditorUserId="6816" LastEditDate="2015-09-07T19:30:16.383" LastActivityDate="2015-09-07T20:25:49.923" Title="Can Zachman framework be used to find out reason of Columbia space shuttle disaster?" Tags="|space-shuttle|risk-management|failure|analysis|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="11952" PostTypeId="1" AcceptedAnswerId="50723" CreationDate="2015-09-07T19:27:08.900" Score="12" ViewCount="565" Body="&lt;p&gt;The surface of the Moon looks very different in orbiter images taken at local noon than it does in images from other times. Here are two examples of the same places in Lalande Crater. In each image, the top section was taken near local sunset (sun incidence angle 68&lt;sup&gt;o&lt;/sup&gt;) and the bottom section taken near local noon (sun incidence angle 12&lt;sup&gt;o&lt;/sup&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wLKbC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wLKbC.jpg&quot; alt=&quot;fresh impact crater in central peak of Lalande Crater&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above are images &lt;a href=&quot;http://wms.lroc.asu.edu/lroc/view_lroc/LRO-L-LROC-3-CDR-V1.0/M1096551351LC&quot; rel=&quot;noreferrer&quot;&gt;M1096551351LC&lt;/a&gt; and &lt;a href=&quot;http://wms.lroc.asu.edu/lroc/view_lroc/LRO-L-LROC-3-CDR-V1.0/M1103624254LC&quot; rel=&quot;noreferrer&quot;&gt;M1103624254LC&lt;/a&gt; (which is flipped horizontally in the online display). The crater shown is about 80 m across. Coordinates about 4.47&lt;sup&gt;o&lt;/sup&gt;S by 8.6&lt;sup&gt;o&lt;/sup&gt;W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ELzbp.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ELzbp.jpg&quot; alt=&quot;large chunks of debris on south rim of Lalande Crater&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above are images &lt;a href=&quot;http://wms.lroc.asu.edu/lroc/view_lroc/LRO-L-LROC-3-CDR-V1.0/M1154271821RC&quot; rel=&quot;noreferrer&quot;&gt;M1154271821RC&lt;/a&gt; (which is upside down in the online display), and again M1103624254LC. The white boulder debris field is about 120 m across. It is at coordinates about 4.85&lt;sup&gt;o&lt;/sup&gt;S by 8.6&lt;sup&gt;o&lt;/sup&gt;W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of these images were taken with the narrow-angle camera of the Lunar Reconnaissance Orbiter, so they are of the visible-light spectrum only (but recorded in black and white). I have been trying to understand how the big changes in apparent tone work. This &lt;a href=&quot;http://blog.moonzoo.org/2010/07/12/fresh-white-craters-the-brightest-things-on-the-moon/&quot; rel=&quot;noreferrer&quot;&gt;blog post&lt;/a&gt; from the Moon Zoo project says that fresh impacts and their ejecta are bright because &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;their newly exposed and broken surfaces are clean and shiny and have a&#xA;  relatively high albedo in comparison to the mature, darker mare&#xA;  material they lie on top of&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Does this basically mean that particle surfaces are smoother at the microscopic level? Is there a chemical difference too? I have read about the &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2006/03jan_moonshadows/&quot; rel=&quot;noreferrer&quot;&gt;Opposition Effect&lt;/a&gt;, but that doesn't seem relevant. And none of this explains the &lt;em&gt;black&lt;/em&gt; stuff - what is that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are a couple of other images of the area, the first also from the LRO, showing normalized surface temperature variations, and the second from Clementine, showing optical maturity, imaged in the UV spectrum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qrRcF.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qrRcF.png&quot; alt=&quot;Average surface temperature from Diviner instrument&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/dOR8R.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dOR8R.png&quot; alt=&quot;Optical maturity from Clementine&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-09-08T21:01:20.773" LastActivityDate="2023-10-17T17:34:36.243" Title="What makes materials on the Moon look so different when the sun is high?" Tags="|the-moon|planetary-science|imaging|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="11953" PostTypeId="2" ParentId="11949" CreationDate="2015-09-07T20:07:22.567" Score="3" Body="&lt;p&gt;Weather-related holds and scrubs are extremely common. There are strict rules about wind speed and cloud cover limits for launches, and often there's a window of an hour or less on a given day for a launch; if the weather doesn't cooperate through the launch window, they detank the rocket and try again the next day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A recent example is the &lt;a href=&quot;http://www.spaceflight101.com/delta-iv-wgs-7-launch-updates.html&quot; rel=&quot;nofollow&quot;&gt;Delta IV/WGS-7&lt;/a&gt; launch, postponed for a day in July 2015. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-09-07T20:07:22.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11954" PostTypeId="2" ParentId="11951" CreationDate="2015-09-07T20:25:49.923" Score="2" Body="&lt;p&gt;From a cursory read of the Wikipedia article, it seems to me that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Zachman_Framework&quot; rel=&quot;nofollow&quot;&gt;Zachman framework&lt;/a&gt; is intended to be a top-level overview of a project. I suspect the errors that led to the loss of Columbia would not even show up at this level.&lt;br&gt;&#xA;The primary source for data on the Columbia accident is the &lt;a href=&quot;http://history.nasa.gov/columbia/CAIB_reportindex.html&quot; rel=&quot;nofollow&quot;&gt;Accident Investigation Report&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-07T20:25:49.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11955" PostTypeId="2" ParentId="11949" CreationDate="2015-09-07T20:30:32.190" Score="7" Body="&lt;p&gt;In addition to the incidents listed in @PearsonArtPhoto's  answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the STS-115 launch countdown a wind direction change occurred at ~15,000 feet (15 ft/s tailwind to 10 ft/s headwind) over a period of roughly an hour.  Since the first stage trajectory had been designed using the original winds, this resulted in a reduction in dynamic pressure margin.  A special assessment was done using the L-1:10 winds measurement and dynamic pressure was predicted to be at ~100.2% of the limit where 100.5% would have been a &quot;no-go&quot; call. None of the &quot;wind change&quot; rules had been violated.  This analysis took place between T-39 and T-28 minutes - per the normal ascent preparation timeline, the final assessment of this regime had already been done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No limits were violated and none would have been if the issue hadn't been detected.  Also, there was some amount of conservatism built into the system.  But, we were ~0.3% away from launching into a possible structural loads exceedance.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-08T02:01:20.333" LastActivityDate="2015-09-08T02:01:20.333" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="11956" PostTypeId="1" CreationDate="2015-09-08T10:44:58.583" Score="26" ViewCount="6990" Body="&lt;p&gt;Here's the situation: The ISS suffered from some kind of catastrophic failure. The crew is unharmed, but the nature of the failure includes:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The destruction of any present vehicles capable of returning safely to Earth.&lt;/li&gt;&#xA;&lt;li&gt;The crew has to be extracted immediately. The nature of the damage does not mean that any unusual procedures have to be performed once a spaceship gets to the ISS to get the crew out and back home safe.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;So in order to get the crew out, a spaceship has to be sent up to rescue them. How quickly could one be ready to launch to retrieve them? They'd need to prepare a ship, brief the crew, wrangle the press monkeys and all the myriad things that come with launching a ship. Luckily all the world's space agencies are willing to contribute to this heroic mission (though likely it's because of the goodwill it would curry them if they succeeded), so it's not just up to the ones running the ISS.&lt;/p&gt;&#xA;&lt;p&gt;Thus I am wondering:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;What is the smallest possible time frame we could get a spaceship to the ISS to retrieve the crew without seriously endangering the crew of either the ISS or the rescue vessel?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="11575" LastEditorUserId="8922" LastEditDate="2023-06-05T21:03:27.777" LastActivityDate="2023-06-05T21:03:27.777" Title="How quickly could a space agency get to the ISS in case of an emergency?" Tags="|launch|iss|mission-design|rescue-missions|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="11958" PostTypeId="2" ParentId="11956" CreationDate="2015-09-08T11:40:38.870" Score="21" Body="&lt;p&gt;Not very quickly, for a number of reasons. Here's a list of some of the reasons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Soyuz requires 2 astronauts just to fly it (Under nominal operations) It can only take down 3. Thus, 6 Soyuz launches would be required. (Note, this might be a reference to manned flight, Soyuz does have remote capabilities). Some work could probably be done to reduce that to 1 person, thus only requiring 3 launches. This is probably the quickest pathway to rescue. In theory, Soyuz can be flown completely unmanned, but this hasn't been done for an ISS mission (Although it is similar to Progress). &lt;/li&gt;&#xA;&lt;li&gt;If there was a true emergency, I would imaging a Dragon capsule could have restraints placed inside, allowing it to land. Most of the upgrades to man rate Dragon require changes for going up, not coming back down. I think there's a Dragon that's ready to go.&lt;/li&gt;&#xA;&lt;li&gt;China's ability to launch people into space is rather limited. They might be able to do one of the rescue missions, but certainly not all of them.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The only 3 vehicles currently capable of reentry and contain pressurized compartments are the ones listed. Anything else would take a huge amount of time. I suspect the quickest that any such launches could be attempted would be on the order of weeks to months, especially considering the large number of launches that would be required to make this happen.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-08T17:03:49.290" LastActivityDate="2015-09-08T17:03:49.290" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="11961" PostTypeId="1" CreationDate="2015-09-08T17:51:55.597" Score="10" ViewCount="1128" Body="&lt;p&gt;The ISS space station orbits at an inclination of about 50 degrees. That's in between polar orbit, which would be useful for Earth observations, and the ecliptic which would be useful for synchronous or interplanetary (Lunar) missions. Paying fuel for 40 or so degrees inclination change from the LEO orbit of the ISS sounds fuel expensive .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But minisats are anyway shipped to the ISS and launched from there, with the help of several systems now installed there. How helpful is that for the satellites compared to being launched directly as secondary payloads?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-09-08T19:30:32.643" LastActivityDate="2015-09-11T04:20:34.167" Title="How is the ISS helpful for launching cubesats and minisats?" Tags="|launch|iss|artificial-satellite|low-earth-orbit|cubesat|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11962" PostTypeId="2" ParentId="11961" CreationDate="2015-09-08T17:59:56.527" Score="8" Body="&lt;p&gt;The reason that Cubesats are launched from the ISS has more to do with launch availability. It's rather difficult to launch as such a small secondary payload. Furthermore, for small satellites, one has to be close to Earth (To reenter in a reasonable amount of time, and it assists with the communication budget), which is even more difficult to get. Virtually all of the launches that are low enough to have the FCC required 25 year reentry period of time are either to the ISS, or spy satellites. Spy satellites don't like having secondary payloads (Although there have recently been a few, see &lt;a href=&quot;http://spacenews.com/38695nro-satellite-cubesats-launched-by-atlas-5-rocket/&quot; rel=&quot;nofollow&quot;&gt;this link&lt;/a&gt;), thus the ISS is one of the best, if not the best, options for a satellite orbit. Note that if one has the required fuel, one could launch as a secondary launch to most Earth observing satellites, which there are a large number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so if you are going to be a secondary payload heading to the ISS, why actually launch from the ISS? The reason is that the space around the ISS is carefully controlled. By launching from the ISS, they can do so at a time and in a manner that will ensure that they don't re-collide with such objects. They might choose to do so right before a thruster firing, for instance. It also makes it simpler to launch, as no rocket control is required to separate the payload, and the satellites can remain as any other cargo. Launch systems are complex devices, launching directly to the ISS avoids having one of these systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cubesats are missions of opportunity, and can't afford to choose their launch trajectories very carefully. Bottom line is, they do what they can, and virtually all of them I'm familiar with don't care that much about what their orbit is (Just that they have a high enough inclination for them to control from their ground station). All cubesat missions I'm familiar with have waited years on the ground to launch. Mostly they just want to make it to orbit, they don't care about the inclination of the orbit very much.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-08T19:52:30.143" LastActivityDate="2015-09-08T19:52:30.143" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="11963" PostTypeId="2" ParentId="11956" CreationDate="2015-09-08T21:59:19.047" Score="22" Body="&lt;p&gt;There is a Soyuz capsule (source&lt;a href=&quot;https://en.m.wikipedia.org/wiki/Crew_Return_Vehicle&quot;&gt;1&lt;/a&gt;) docked to the ISS at all times, allowing for immediate evacuation in an emergency.  This limits the ISS to 3 people when it is the only capsule available.  For Expidition 20, they docked 2 capsules, allowing for six astronauts at a time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your scenario involves the destruction of these vessels and simultaneously jeopardizing the safety of the astronauts sufficiently to demand an emergency return.  Given the redundancy of the systems, this would be an impressive set of failures.  At such a point, evac is no longer a simple issue.  Questions like &quot;can we even dock another craft&quot; begin to come into play due to the unknown-but-catastrophic damage.  Accordingly, plans for such an evacuation will be like those of combat plans.  Then again, I fully expect the planning to have been done, if nothing else along Winston Churchil's line &quot;Plans are useless, planning is essential.&quot;&lt;/p&gt;&#xA;" OwnerUserId="3532" LastActivityDate="2015-09-08T21:59:19.047" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="11964" PostTypeId="2" ParentId="6263" CreationDate="2015-09-09T02:08:47.277" Score="1" Body="&lt;p&gt;There is a fuel source: the lunar regolith. There is power to crack it: solar panels. But &lt;em&gt;where&lt;/em&gt; are you going to get a manufacturing plant and &lt;em&gt;where&lt;/em&gt; are you going to get enough food and water (let alone oxygen) to wait for that trickle of power to ever produce enough fuel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No indeed. You cannot get off. Most of what you need is present, but you will die of biological needs long before you can make of it what you need to get off the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you would want to communicate. If we presume the most favorable conditions, that you are within walking distance of enough serviceable parts, you might be able to get a message to earth. But it took us nine years to get to the moon last time, and so much of the solution is lost. We don't have a big enough booster as ready-to-build plans right now. To get a lander + lifter + mercury style pod would be a multi-year and multi-billion dollar effort. It would be reasonable to assume most of that nine years again. Same problem. Biological needs kill you long before they pick you up.&lt;/p&gt;&#xA;" OwnerUserId="10442" LastEditorUserId="10442" LastEditDate="2015-09-09T02:34:27.817" LastActivityDate="2015-09-09T02:34:27.817" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="11965" PostTypeId="2" ParentId="11961" CreationDate="2015-09-09T06:44:06.787" Score="4" Body="&lt;p&gt;CubeSats are always secondary payloads and very rarely have a propulsion system. This means that you have to take the orbit you can get. It is not uncommon that the launch opportunity, which was planned years ahead during the design phase, changes shortly before launch. Therefore CubeSats are not designed for missions which require a specific orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you have the opportunity to launch from the ISS, there are at least two big advantage to every other launcher: Firstly, you know the inclination and the rough altitude (350 to 450 km) of the orbit. Secondly, you are guaranteed to reenter the atmosphere when launched from the ISS, which can not be guaranteed by launching with a standard earth observation satellite. These satellites orbit at above 600 km, while 600 km is approximately the maximum altitude if you need to reenter during 25 years after end of life without carrying a propulsion or drag system to speeden up your orbital decay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example Germany's space agency (DLR), is not funding CubeSats that are not guaranteed to reenter the atmosphere within the 25 years, because they are strongly following the European Code of Conduct for Space Debris Mitigation (which is good!). This leaves the ISS as one of the best launch possibilities. &lt;/p&gt;&#xA;" OwnerUserId="11391" LastActivityDate="2015-09-09T06:44:06.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11966" PostTypeId="2" ParentId="10891" CreationDate="2015-09-09T12:51:10.083" Score="2" Body="&lt;p&gt;Theoretically, the fuel usage should be the same. In practice, here are a few things that might change the fuel usage each way:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;More Fuel for quick&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;More orbital uncertainty might result in an incorrect thrust.&lt;/li&gt;&#xA;&lt;li&gt;Higher thrust is required from early on. There is little time to correct any errors. If a small error happens, it will have to be corrected. As the time of approach is only a few orbits, the corrections will require substantially more thrust than others. &lt;/li&gt;&#xA;&lt;li&gt;Some burns might have to happen at non-optimal times. This should be a small effect for LEO, but isn't negligible.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;More Fuel for 2 day&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A certain amount of fuel is required for station keeping. This fuel usage will increase.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Overall, I believe it would be close, but slightly higher to do the quick burn, unless the errors are brought down to something negligible. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-09T12:51:10.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11967" PostTypeId="1" AcceptedAnswerId="12030" CreationDate="2015-09-09T15:11:05.870" Score="10" ViewCount="284" Body="&lt;p&gt;MIT Professor Sadoway believes we can make oxygen anywhere on the moon with Molten Oxide Electrolysis (see &lt;a href=&quot;http://mpc-www.mit.edu/mpc/item/479-making-oxygen-on-the-moon&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://sadoway.mit.edu/wordpress/wp-content/uploads/2011/06/132.pdf&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume that you need to make enough oxygen to sustain an astronaut for 24 hours. You need to make this amount of oxygen in about 8 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this can't be done, what is the minimum amount of time it would take to produce that much oxygen and how much equipment would this require?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you believe there is another method besides Molten Oxide Electrolysis that can accomplish this, you are free to suggest it.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-09-18T14:37:08.767" Title="What is the minimum amount of equipment you would need to make a reasonable amount of oxygen on the Moon with Molten Oxide Electrolysis?" Tags="|the-moon|life-support|isru|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="11969" PostTypeId="1" CreationDate="2015-09-10T20:01:38.357" Score="10" ViewCount="469" Body="&lt;p&gt;Assume a hab in outer space with 1 atmosphere pressure and 23ºC (comfortable room temperature),&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To simulate gravity, the hab is spun. Acceleration is $\omega^2r$ where $\omega$ is angular velocity in radians/time and r is radius of hab. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously the lower the &quot;gravity&quot;, the smaller the hab can be. High rpm's make people sick and less gravity reduces the need for high rpm's. Also the structure doesn't need to be as strong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am wondering at what point gravity overcomes surface tension and water flows downhill. Thus the astronauts could enjoy showers, flush toilets, and their sinuses could drain.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-11-10T09:24:25.867" Title="How much acceleration does it take for water to flow downhill?" Tags="|artificial-gravity|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="11970" PostTypeId="2" ParentId="11961" CreationDate="2015-09-11T04:20:34.167" Score="1" Body="&lt;p&gt;You are correct that a plane change from the ISS inclination of 51.6° to a polar or equatorial orbit is ludicrously infeasible. For example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination_change#Circular_orbit_inclination_change&quot; rel=&quot;nofollow noreferrer&quot;&gt;assuming circular orbits&lt;/a&gt; the required ΔV is given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QOE1f.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QOE1f.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a 50° plane change costs ~6 km/sec — that's almost as much as the original launch vehicle! (This assumes 7 km/sec orbital velocity, close enough to make the point.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cubesats deployed from the ISS because it’s available, and many cubesats are used to demonstrate new technologies, or as training for universities or start-ups. They don't care about &lt;strong&gt;the&lt;/strong&gt; orbit; they care about orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, useful Earth observation can be done from an ISS inclination, as most of the Earth's population lives within ±51° latitude. &lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2015-09-11T04:20:34.167" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="11972" PostTypeId="1" AcceptedAnswerId="11973" CreationDate="2015-09-11T22:26:21.893" Score="8" ViewCount="8893" Body="&lt;p&gt;I would like to know what is the envelope of thrust to weight ratios that is available to the Falcon 9 first stage during landing. In other words, what's specifically the lower (and hopefully, if you know, the upper) bound on the thrust to weight ratio?&lt;/p&gt;&#xA;" OwnerUserId="11551" LastEditorUserId="25" LastEditDate="2015-09-11T22:51:48.503" LastActivityDate="2016-09-04T10:46:26.343" Title="What's the thrust to weight ratio of Falcon 9 at landing?" Tags="|launch|spacex|propulsion|falcon-9|landing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11973" PostTypeId="2" ParentId="11972" CreationDate="2015-09-11T22:48:40.677" Score="8" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9&quot;&gt;Wikipedia&lt;/a&gt;, we gain the following value at launch (Pounds deliberately chosen because both are units of force, instead of kg, which is a unit of mass)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1,530,000 lbf thrust&lt;/li&gt;&#xA;&lt;li&gt;1,194,000 lb weight&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Thus, the TWR at launch is 1.28. &lt;a href=&quot;http://www.spacex.com/news/2012/10/08/spacex-crs-1-mission-update&quot;&gt;The maximum is 5&lt;/a&gt;, because Falcon 9 will start shutting engines down if the TWR is ever over 5. This is what it is during the flight portion of the launch. Okay, so what would it be during landing? Well, the highest it could be is if the fuel is completely spent, and it is only using a single engine. SpaceX doesn't actually release that, but &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot;&gt;it is estimated&lt;/a&gt; to be between 23- 26 metric tons. Okay, so what does that entail? Well, with only 1 engine, it is thrusting 1/9th of the maximum thrust. There is some information that the engine can throttle down as well, &lt;a href=&quot;https://twitter.com/elonmusk/status/462104679116050432&quot;&gt;by 40%&lt;/a&gt;. Most likely this means 40% lower, per Space Shuttle performance, but it could eb interpreted in other ways. The weight is between 50-57 thousand pounds. So, what is that TWR? About 1.8-2.05. The first stage is usually returned almost empty, so this is probably a good estimate of the TWR at landing.  Bottom line is, the following values are estimated TWR for the Falcon 9:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Launch- 1.28&lt;/li&gt;&#xA;&lt;li&gt;Maximum in flight- 5&lt;/li&gt;&#xA;&lt;li&gt;Landing- 1.8-2.05 (Estimated maximum, actual could be lower if there is remaining fuel)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-11T23:08:43.713" LastActivityDate="2015-09-11T23:08:43.713" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="11974" PostTypeId="1" CreationDate="2015-09-12T02:06:14.590" Score="2" ViewCount="236" Body="&lt;p&gt;I was looking through the historical record and found that over time the money input into rocket development has yielded more performance benefits. I imagine that the development cost has reduced because of productivity enhancements (like computers and robotics) and better understanding of structural and combustion physics. That said, the development cost of a reasonable sized rocket (such as a cubesat launch vehicle) is still very high, locking out everyone except for the best funded companies. I was seeing how this cost could be brought down and the best two approaches I found was the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Standardizing mechanical/pneumatic/hydraulic/electrical connections between &#xA;sections to allow reuse of components developed for prior rockets on new&#xA;rocket (making rocket LEGOs to let designers focus on configuration design &#xA;instead of part design and reduce fabrication costs) &lt;/li&gt;&#xA;&lt;li&gt;Develop a reusable rocket and find a way of flight testing it incrementally &#xA;so a new vehicle isn't required every new flight (the aircraft development&#xA;approach)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;However, this is just what I came up with, I was wondering what approaches other people might come up with. How would you reduce the cost of developing a new rocket?&lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="11377" LastEditDate="2015-09-12T15:37:27.843" LastActivityDate="2015-09-13T10:42:11.813" Title="How can new LEO manned launch vehicle development costs be reduced?" Tags="|rockets|cost|development|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="11975" PostTypeId="1" CreationDate="2015-09-12T09:05:43.513" Score="20" ViewCount="3876" Body="&lt;p&gt;Here is the Cargo Dragon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bK7Ha.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bK7Ha.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here is the Orion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oy3PC.pnghttps://i.stack.imgur.com/YJlOf.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oy3PC.pnghttps://i.stack.imgur.com/YJlOf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;cone&quot; that orion makes has a large opening angle - perhaps about 70 degrees. Meanwhile, the Dragon is almost cylindrical - the opening angle is about 20 degrees I would say.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the reason for these vastly different &quot;cones&quot;? I would have thought an optimal &quot;cone&quot; exists for minimal aerodynamic drag, or minimum thermal heating on reentry for example. What would a chain of decisions be like that would produce the very different geometries (as in the above example of Dragon vs. Orion).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you for your insight!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YJlOf.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YJlOf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11551" LastEditorUserId="1457" LastEditDate="2015-09-12T10:03:55.677" LastActivityDate="2017-05-01T18:06:56.690" Title="Reason for different &quot;cone angles&quot; of different space capsules?" Tags="|spacex|spacecraft|dragon|orion-spacecraft|aerodynamics|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11976" PostTypeId="1" CreationDate="2015-09-12T09:51:31.623" Score="17" ViewCount="2867" Body="&lt;p&gt;Already in the 1960s rocket engines seem to have achieved near maximum chemical efficiency. What are the reasons for new designs being in demand?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much better could a new engine design be than the Shuttle's main engine and Ariane 5's Vulcain for hydrogen, or than Soyuz' RD107 and Falcon's Merlin for kerosene? New designs might be cheaper to manufacture, but does that make up for the costs and risks with development? I can maybe imagine a need for methane engines under development by SpaceX and Blue Origin, but everyone in the business seem to develop new engines with old fuel with similar performance as existing engines.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-09-12T11:59:25.403" LastActivityDate="2015-09-12T12:00:51.223" Title="Why design new rocket engines instead of using the existing types?" Tags="|rockets|engine-design|engineering|cost|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11977" PostTypeId="2" ParentId="11976" CreationDate="2015-09-12T10:37:52.097" Score="29" Body="&lt;p&gt;Ars Technica has a &lt;a href=&quot;http://arstechnica.com/science/2013/04/how-nasa-brought-the-monstrous-f-1-moon-rocket-back-to-life/&quot; rel=&quot;noreferrer&quot;&gt;pair&lt;/a&gt; of &lt;a href=&quot;http://arstechnica.com/science/2013/04/new-f-1b-rocket-engine-upgrades-apollo-era-deisgn-with-1-8m-lbs-of-thrust/1/&quot; rel=&quot;noreferrer&quot;&gt;articles&lt;/a&gt; that give some insight in why it's desirable to use new designs: NASA has been working on an updated version of the F-1 (the first stage engine for the Saturn V).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/okFf7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/okFf7.jpg&quot; alt=&quot;F-1B overview&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Some of the major differences:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Another clear difference is the construction of the exhaust nozzle itself. The F-1's nozzle was made up of two parts: the first portion was actually an extremely complex series of tubes brazed together and bound by hoops, like staves in a barrel. Kerosene fuel was circulated through the tubes to absorb heat and cool the exhaust. The tubes stretched down to the distinctive turbopump exhaust manifold, and then looped back up. Below the manifold, which wrapped around the engine like a pair of fingers, was a removable nozzle extension that focused the engine's combustion and helped the engine deliver additional thrust.&lt;/p&gt;&#xA;&lt;p&gt;Advances in manufacturing techniques will allow the F-1B to dispense with the complicated upper nozzle tubing; as it's currently envisioned, the new rocket will feature a much simpler thrust chamber and nozzle made of steel—according to Andy Crocker of Dynetics, the nozzle will consist of an inner liner and outer jacket, brazed together, with cooling provided by fuel flowing through simple slots in the inner liner. This is far easier and less expensive to build than the labor intensive &amp;quot;barrel hoop&amp;quot; tube wall design of the original F-1.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The control system will be based on software rather than an analog, hydraulic computer:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Another significant difference would be the incorporation of modern electronics into the engine's ignition and firing sequence. The F-1 employed an almost Rube Goldbergian system of valves and pressure checking,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The F-1 had some kludges built in because they couldn't analyze the engine's behavior fully. The combustion instability problem, for example, was 'fixed' by trial and error. These days you'd be able to run simulations on a CAD model and find an optimal solution.&lt;br /&gt;&#xA;CAD also allows you to optimize the design for minimal weight: the F-1 was over-engineered (i.e. heavier than necessary).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;although the principles behind the F-1 are well known, some aspects of its operation simply weren't fully understood at the time. The thrust instability problem is a perfect example. As the F-1 was being built, early examples tended to explode on the test stand. Repeated testing revealed that the problem was caused by the burning plume of propellent rotating as it combusted in the nozzle. These rotations would increase in speed until they were happening thousands of times per second, causing violent oscillations in the thrust that eventually blew the engine apart. ...&#xA;engineers eventually used a set of stubby barriers (baffles) sticking up from the big hole-riddled plate that sprayed fuel and liquid oxygen into the combustion chamber (the &amp;quot;injector plate&amp;quot;). These baffles damped down the oscillation to acceptable levels, but no one knew if the exact layout was optimal.&lt;br /&gt;&#xA;The baffle arrangement &amp;quot;was just a trial and error thing,&amp;quot; explained Senior Propulsion Engineer R.H. Coates. &amp;quot;But we'd like to model that and say, well, what if you took one of those baffles out?&amp;quot; Because the baffles are mounted directly to the injector plate, they take up surface area that would otherwise be occupied by more injector holes spraying more fuel and oxidizer; therefore, they rob the engine of power. &amp;quot;So if you want to up the performance on this thing, we can evaluate that with modern analytical techniques and see what that does to your combustion stability.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Modern manufacturing techniques would drastically reduce the part count:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;One thing I notice when I look back at older engines,&amp;quot; commented Coates, the senior engineer, &amp;quot;was just like Nick and Erin were alluding to: the complexity of the welds. You didn't have the kind of advanced manufacturing we had today, so quite honestly, these were hand-made machines. They were sewn together with arc welders, and it's pretty amazing to see how smooth and elegant it came out. Today, you'd look at doing precision casting, not these thousands of welds.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;...F-1B engines (will be) built with techniques that more resemble 3D printing than traditional casting or milling. The main combustion chamber and nozzle in particular will undergo tremendous simplification and consolidating; the parts count for those two assemblies together will be reduced from 5,600 manufactured elements in the original F-1 down to just 40.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So they reduced the part count by a factor of ~ 100. That's not a small saving. Along with the part count, the amount of manual labor drops by a factor of more than 100 (fewer parts, and parts that can be fabricated automatically instead of requiring manual steps).&lt;/p&gt;&#xA;&lt;p&gt;There are also non-technical reasons to go for a new design. Many rocket engines are designed by commercial companies who won't share their designs with others. In other cases, nations want to build an independent space industry, so they'll accept that their industry will have to re-invent the wheel to some extent.&lt;/p&gt;&#xA;&lt;p&gt;Finally, size and complexity play a role. You need engines that are suitable for the amount of payload you want to launch. You could build a rocket similar to the Falcon 9 using 3 SSME, but then you'd lose the Falcon 9's engine-out capability.&lt;br /&gt;&#xA;The SSME is also really expensive, in part because it was designed to be reused. A manufacturer of expendable rockets would want something simpler and cheaper.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-09-12T12:00:51.223" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="11978" PostTypeId="2" ParentId="8335" CreationDate="2015-09-12T14:25:46.067" Score="7" Body="&lt;p&gt;SpaceX made improvements in several areas:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Production cost. PICA-X is &lt;a href=&quot;http://www.airspacemag.com/space/is-spacex-changing-the-rocket-equation-132285884/?all&quot; rel=&quot;noreferrer&quot;&gt;10 times less expensive&lt;/a&gt; than the original.  &lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Resilience. As you noted, PICA is structurally weak. Reportedly, PICA-X is &lt;a href=&quot;https://www.reddit.com/r/space/comments/3dq02v/what_are_the_main_differences_between_spacex/&quot; rel=&quot;noreferrer&quot;&gt;better in this regard&lt;/a&gt;: it's more resisant to cracking, so it can be manufactured in large tiles (notice the tile size in this image):&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/JEsGL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JEsGL.jpg&quot; alt=&quot;PICA-X&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.airspacemag.com/space/is-spacex-changing-the-rocket-equation-132285884/?all&quot; rel=&quot;noreferrer&quot;&gt;Heat resistance&lt;/a&gt;. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;SpaceX also wanted to be &lt;a href=&quot;http://www.jsc.nasa.gov/history/oral_histories/C3PO/ShotwellGE/ShotwellGE_1-15-13.htm&quot; rel=&quot;noreferrer&quot;&gt;able to manufacture the material in-house&lt;/a&gt; in order not to be dependent on a single supplier. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;PICA-X has been &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140000085&amp;amp;hterms=PICA-X&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3DPICA-X%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA%7C%7C125%7CCollection%7CNIX&quot; rel=&quot;noreferrer&quot;&gt;tested by NASA&lt;/a&gt; before its first use, but the test results haven't been published as of 2015. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, I haven't been able to find detailed studies on PICA-X's performance, cost or composition. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-12T14:25:46.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11979" PostTypeId="2" ParentId="11975" CreationDate="2015-09-12T17:40:53.207" Score="12" Body="&lt;p&gt;As usual in this sort of thing, there's no single optimal answer because the capsule has to do a few different things, so the shape has to balance different requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A narrow cone angle gives less drag on ascent. A large broad base is required for deceleration on re-entry. A squat cylinder gives more convenient usable interior space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the stubby cone of Apollo/Orion/CST provides good passive aerodynamic stability in the reentry; Dragon is closer to the &quot;biconic&quot; shape, which might offer more lift on reentry (thus potentially a shallower entry with lower G-load for the crew). Wikipedia has some info on &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry#Entry_vehicle_shapes&quot;&gt;shapes for re-entry vehicles.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-09-13T00:41:35.003" LastActivityDate="2015-09-13T00:41:35.003" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="11980" PostTypeId="2" ParentId="11956" CreationDate="2015-09-12T18:48:11.900" Score="4" Body="&lt;p&gt;It depends: if there's a mission already being prepared that you could use, you'd be in luck. Otherwise, you'd have to wait for a rocket and capsule to come off the production line. Your best chance of that happening soon is with a rocket that has a high rate of production. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2014, &lt;a href=&quot;http://www.spacelaunchreport.com/log2014.html&quot; rel=&quot;nofollow&quot;&gt;23 R-7 rockets were launched&lt;/a&gt;. 4 of these were Soyuz manned capsules, and 4 were Progress. The others were satellite launches. This is one launch every 2.2 weeks.&lt;br&gt;&#xA;If they had a Soyuz capsule lying around, it might be possible to swap the planned payload for the Soyuz. So that gives a ballpark figure of 2 weeks. Maybe a little less by cutting corners and/or going to a 24/7 schedule.&lt;br&gt;&#xA;The main constraint would be the production rate of the capsules. A rate of 4 per year means on average, the next one will be delivered in 3 months.&lt;br&gt;&#xA;Other countries have fewer launches, so it'd tend to take longer for them to mount a rescue mission.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-09-14T19:24:05.843" LastActivityDate="2015-09-14T19:24:05.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11981" PostTypeId="1" AcceptedAnswerId="11982" CreationDate="2015-09-12T21:18:48.023" Score="6" ViewCount="763" Body="&lt;p&gt;Fluorine is a bit more efficient chemically than oxygen. It has never been used as a launcher oxidizer because it and its exhaust products are so toxic. That's as much I've heard about it anyway. But, would it be useful for engines used only in space where toxicity doesn't matter, as in a third stage or for satellite station-keeping in small engines? Or is it too corrosive even for that?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2015-09-12T21:43:48.693" LastActivityDate="2015-09-14T03:21:40.630" Title="Is a fluorine/hydrogen engine feasible for use in space?" Tags="|rockets|engine-design|liquid-fuel|oxidizer|" AnswerCount="1" CommentCount="7" ClosedDate="2015-09-14T03:07:01.927" ContentLicense="CC BY-SA 3.0" />
  <row Id="11982" PostTypeId="2" ParentId="11981" CreationDate="2015-09-12T21:58:15.547" Score="7" Body="&lt;p&gt;The safety-related costs of developing, testing, and fueling fluorine-oxidized rockets outweigh the costs of building slightly bigger and less efficient rockets using hypergolics or LOX. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In launcher stages, hydrolox is only 5% less efficient by Isp than hydrogen-fluorine; the higher density of fluorine makes for more compact tankage, though, which means less structural mass overall. Your rocket will wind up bigger using lox, but it will also be cheaper, safer, and less toxic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrogen-fluorine also &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_rocket_propellant#Propellant_table&quot; rel=&quot;nofollow&quot;&gt;burns about 900 degrees hotter than hydrolox&lt;/a&gt;; keeping the chamber and nozzle robust against hot corrosive gases is going to add weight one way or another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fluorine is cryogenic, liquifying at close to the same temperature as oxygen, so it's impractical to store for long-term missions. For long-term satellite stationkeeping, the &lt;a href=&quot;http://www.americaspace.com/wp-content/uploads/2012/08/Aerojet-EP-Satellites-rev-3-08.jpg&quot; rel=&quot;nofollow&quot;&gt;go-to solution nowadays is an electric propulsion system&lt;/a&gt; such as ion thrusters; way more efficient than H/F and again, much safer.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-09-14T03:21:40.630" LastActivityDate="2015-09-14T03:21:40.630" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="11983" PostTypeId="2" ParentId="11974" CreationDate="2015-09-13T10:42:11.813" Score="3" Body="&lt;p&gt;One possible approach has been shown by SpaceX over the past few years:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Use a common engine for all stages, instead of having different engine designs for each stage.  &lt;/li&gt;&#xA;&lt;li&gt;Use a common tank design for all stages. This means you only need one set of jigs and tools for all the bulkheads and only one welding installation.  &lt;/li&gt;&#xA;&lt;li&gt;Design for low cost instead of maximum performance. For instance, use a simple open-cycle engine design.  &lt;/li&gt;&#xA;&lt;li&gt;Make the rocket easy to handle instead of making it as light as possible: no balloon tanks, but a more rugged design (this also helps with reusability).  &lt;/li&gt;&#xA;&lt;li&gt;Design for manufacturability. For instance, make the tank walls from thick-enough plate, instead of a thin skin with stringers (which would be lighter, but take a lot longer to build).  &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-13T10:42:11.813" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11985" PostTypeId="2" ParentId="11975" CreationDate="2015-09-14T06:50:59.567" Score="9" Body="&lt;p&gt;This is such an expansive question that it is difficult to answer without writing an entire essay. The answer comes down to the &lt;strong&gt;systems engineering process and the mission requirements&lt;/strong&gt;. As in all spaceflight missions, the payload needs to be protected, especially when a &lt;strong&gt;human&lt;/strong&gt; is the payload. The shape will largely be derived from payload, aside from its passengers, the materials, and entry trajectory (including velocity, etc.). Once you know the 'ballpark' for your entry corridor as determined by your mission, then you can modify your trajectory or even your vehicle design. Depending on your mission you might have to make major changes to the design and in some cases you may need to change your flight plan. There is a very specific re-entry corridor for each craft. Too little drag results in overshoot and too much causes your spacecraft to undershoot.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now that we know the process of arriving at the vehicle shape (very broadly), we can talk about why they are shaped differently. This all comes down to the vehicles ballistic coefficient (BC), which determines the amount of lift. A blunted cone vehicle will &lt;em&gt;generally&lt;/em&gt; have a lower BC than a regular cone. The BC will also affect heat, shockwave, rate of deceleration, and a few other aspects of re-entry. Shape will also affect accuracy. The less time spent up in the air, means the less chance that your vehicle will be off target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no single, magical solution to the shape of a spacecraft. The &quot;cone angles&quot; are a direct result of the mission and its requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are interested in reading a more detailed explanation there is a book that lightly covers many aspects of space exploration. It is titled &lt;em&gt;&lt;a href=&quot;http://www.directtextbook.com/isbn/9780072943641&quot; rel=&quot;noreferrer&quot;&gt;Understanding Space, 4th edition&lt;/a&gt;&lt;/em&gt;, by Sellers, Astore, Giffen, and Larson.&lt;/p&gt;&#xA;" OwnerUserId="11610" LastEditorUserId="1142" LastEditDate="2015-10-17T14:06:21.567" LastActivityDate="2015-10-17T14:06:21.567" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11986" PostTypeId="1" AcceptedAnswerId="11989" CreationDate="2015-09-14T15:25:23.280" Score="10" ViewCount="3112" Body="&lt;p&gt;Let's say a rocket launches from Iceland at 65&lt;sup&gt;o&lt;/sup&gt;N headed for LEO, but instead of using an inclination that works from that latitude, it heads due East. It attains orbital altitude and acceleration, and cuts its engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If its path continued due East it would be defying gravity because the orbit wouldn't be around the Earth's center of mass, it would be off to one side of it so it would essentially be constantly climbing. So the pull of gravity has to torque the orbit to a different inclination. How would that work? If it had used the kind of acceleration profile that would be used to launch a rocket from the equator due East, would the pull of gravity just drain its speed until it falls back to Earth? If that had been calculated for and extra acceleration had been done to compensate for it, what would its path be while it attains orbit and how would it change the energy needed? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/THUnO.png&quot; alt=&quot;orbit trajectories from Iceland&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Red trajectory can't work, the path would be something like the green?&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-09-14T17:03:06.573" LastActivityDate="2015-09-14T20:44:25.190" Title="If a rocket launched due East from a high latitude, what happens to its path?" Tags="|launch|orbital-mechanics|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11987" PostTypeId="2" ParentId="11935" CreationDate="2015-09-14T15:31:22.840" Score="4" Body="&lt;p&gt;This is a difficult question to answer. There's no single schedule that's adhered to for months at a time. As far as I can tell, every day has a unique schedule assigned to it, based on the amount of crew on board, the experiments that need to be run and the station operational details (cargo vessels docked or not, reboosts, repairs necessary etc.).&#xA;You can review these daily schedules (called the timeline) &lt;a href=&quot;https://www.nasa.gov/content/2014-international-space-station-timelines/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. There's also a live &lt;a href=&quot;http://isslive.com/timeline/timelineAstronaut.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;timeline&lt;/a&gt; where you can see what astronauts are doing at the moment.&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://www.quora.com/What-do-astronauts-do-while-on-the-ISS&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robert Frost&lt;/a&gt; (he trained ISS astronauts to operate the motion control system):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The seven day week consists of five and half days schedule for working nominal tasks and a contiguous 1.5 days off.  That doesn't necessarily mean the crew do no work during that 1.5 days, just that they aren't scheduled for tasks other than mandatory things like exercise.&lt;/p&gt;&#xA;&lt;p&gt;A 24 hour day is composed of 8.5 hours allotted for sleep. 6.5 hours allotted to scheduled work tasks, 2.5 hours schedule for required exercise, 1 hour scheduled for lunch.  There is also time allotted for daily planning conferences with the ground, work preparation time (time to read procedures and gather tools), and plan familiarization time (time to review the day's schedule).  And then pre-sleep and post-sleep time during which hygiene activities, dinner, and breakfast are completed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The daily schedule looks &lt;a href=&quot;https://www.quora.com/Whats-the-typical-daily-schedule-in-the-international-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;something like this&lt;/a&gt;:&lt;br /&gt;&#xA;06.00 wake up, breakfast&lt;br /&gt;&#xA;07.30 planning conference&lt;br /&gt;&#xA;08.15 start of the work day: science and maintenance activities plus workouts.&lt;br /&gt;&#xA;13.00 lunch&lt;br /&gt;&#xA;14.00 resume work&lt;br /&gt;&#xA;18.15 work preparation for the next day, another planning conference&lt;br /&gt;&#xA;19.30 dinner, then free time&lt;br /&gt;&#xA;21.30 sleep&lt;/p&gt;&#xA;&lt;p&gt;So work from 7.30 to 13.00 is 5.5 h, 14.00 to 19.30 is another 5.5 h, for a total of 11 h including 2.5 h of exercise and 1.5 h for planning conferences and preparation.&lt;/p&gt;&#xA;&lt;p&gt;In the 2003 report '&lt;a href=&quot;https://books.google.nl/books?id=_zRswqKTUkUC&amp;amp;pg=PA9&amp;amp;lpg=PA9&amp;amp;dq=iss%20amount%20time%20maintenance&amp;amp;source=bl&amp;amp;ots=uW31LuQL0J&amp;amp;sig=LTMPwRWTl5YRkeyP2vEaPfMzb3U&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CCYQ6AEwAWoVChMIy9OHzuX2xwIVRNUUCh3rUw-6#v=onepage&amp;amp;q=iss%20amount%20time%20maintenance&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Factors Affecting the Utilization of the International Space Station for research in the biological and physical sciences&lt;/a&gt;' (download available &lt;a href=&quot;http://www.nap.edu/catalog/10614/factors-affecting-the-utilization-of-the-international-space-station-for-research-in-the-biological-and-physical-sciences&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;), NASA estimates that 2.5 persons are needed for maintenance, on average. I.e. when 3 people are aboard the station, they spend 100 hours/wk on maintenance and get to do science for no more than 20 hours/week. When the crew is larger, science time goes up.&lt;/p&gt;&#xA;&lt;p&gt;In 2011, the goal was &lt;a href=&quot;http://www.nasaspaceflight.com/2011/08/iss-managers-expedition-29-frr-prepare-station-post-shuttle-operations/&quot; rel=&quot;nofollow noreferrer&quot;&gt;40h/wk of science time&lt;/a&gt;, but it still varied wildly, with 9h/wk when only 3 crew were aboard, and a maximum of 70h/wk.&lt;br /&gt;&#xA;This article contains an interesting graph of planned science time vs. accomplished time, but it's reproduced too small to be readable:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0J57i.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0J57i.jpg&quot; alt=&quot;OOS&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the 2006 study &amp;quot;&lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/1.16135?journalCode=jsr&quot; rel=&quot;nofollow noreferrer&quot;&gt;Applying Analysis of International Space Station Crew-Time Utilization to Mission Design&lt;/a&gt;&amp;quot;, it was found that crewmembers spent an average of 1.9 h per work day and 1.8h per rest day on station maintenance. This doesn't add up to the figures I listed earlier, unfortunately I don't have access to the entire study.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: I recently had the chance to ask André Kuipers (who spent 6 months on the ISS during Expedition 30/31) how much time he was able to spend on science. His estimate was 40%.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-19T20:12:06.347" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11988" PostTypeId="2" ParentId="11986" CreationDate="2015-09-14T16:05:14.473" Score="6" Body="&lt;p&gt;First of all, keep in mind that most of the acceleration happen essentially in a single point. The actual trajectory will be somewhat of a typical satellite pass, although it will be slightly irregular at the very beginning. The green line you have drawn is close, but not quite, it will look more sinusoidal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think about it, for any positive inclination rocket, there will exist a point in it's orbit where it is moving due East, at the top and bottom of the orbit. That point is the point where the orbit was achieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The inclination will be approximately the same as the latitude for a due East launch. So in the case you provided, the inclination will be about 65 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think about this, the satellite moving straight East will be pulled towards the center of the Earth. As the center of the Earth isn't straight East, then it will start moving to the south, and continue until it passes the equator, in which case the southward motion will slow down.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-14T17:30:14.840" LastActivityDate="2015-09-14T17:30:14.840" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11989" PostTypeId="2" ParentId="11986" CreationDate="2015-09-14T17:16:20.560" Score="6" Body="&lt;p&gt;The gravity pulling the rocket toward the center of the Earth doesn't wait for the rocket to achieve orbit, so if the rocket is pointing due East for the entire ascent, the course will be somewhat south of East from the beginning, and at the moment of orbital insertion/engine cutoff, the rocket will be somewhere on the southbound leg of the inclined orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If instead the rocket points in whatever direction is required to keep the net velocity vector of the rocket Eastward, it will be pointed slightly northerly, and at insertion it will be at the peak latitude of the inclined orbit and just about to start the south leg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not certain how much additional fuel each of these trajectories would require.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-09-14T20:44:25.190" LastActivityDate="2015-09-14T20:44:25.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11990" PostTypeId="2" ParentId="11986" CreationDate="2015-09-14T19:40:36.573" Score="22" Body="&lt;p&gt;For the sake of simplicity, I'll assume the rocket starts off due east, and quickly climbs and accelerates into orbit. It then turns off it's engines and coasts along it's orbit. We'll ignore that first bit, and think about what happens once it's 150km above Iceland and heading due east.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first thing to realise is that, as far as something in orbit is concerned, there is nothing special about Iceland. Or the north pole, or the equator or anything else.  The only thing that makes these points special is that the earth rotates around it's axis, but once the ship is in orbit it doesn't matter how the planet rotates below it. So lets forget about the rotation of the earth for a minute.  Now all points on the surface are the same, and you can launch from any one, in any direction, and you'll end up in an orbit around the planet. If you launch east from Iceland, you'll end up in this orbit (viewed edge on):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FEYDg.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FEYDg.png&quot; alt=&quot;A picture of the earth, centred on Iceland, with a red line drawn across it from east to west&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Image adapted from www.webglearth.com&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we turn the globe to look straight at Iceland, you can see how an orbit can pass through Iceland going east and still be centred on Earth's centre of mass.  If we take that same orbit and draw it on a map of the Earth, it would have the familiar sinusoidal shape we associate with inclined orbits.  That's just what happens when you draw a circle on a globe, then unwrap it in the (actually slightly odd) way we traditionally unwrap a sphere to make a flat map.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we were ignoring the planet's rotation there. What happens if we put that back in? Very little changes.  Because the planet is rotating, Iceland is moving eastwards fairly fast, we can take advantage of that to make the rocket launch easier. Let's assume that we launch in the same direction, achieve the same speed, but use a little less fuel doing so. The second change is that once the ship is up in orbit, the planet rotates underneath it. If you look back at the picture above, you can imagine the planet rotating about it's axis, and the orbit staying put. When we flatten out the globe to make a map, this means the sinusoidal shape seems to move across the earth.&lt;/p&gt;&#xA;" OwnerUserId="11611" LastActivityDate="2015-09-14T19:40:36.573" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11992" PostTypeId="2" ParentId="11956" CreationDate="2015-09-15T01:09:39.790" Score="10" Body="&lt;h2&gt;Types of Emergency&lt;/h2&gt;&#xA;&lt;p&gt;The most probable causes of an emergency are Fire aboard, leak aboard, or life threatening illness aboard. A failure of Life Support is also a dire emergency...&lt;/p&gt;&#xA;&lt;p&gt;Fire and leak are the two most emergent of these.&lt;/p&gt;&#xA;&lt;p&gt;In order to render the need for a rescue flight, you would need to damage a Soyuz, possibly two.&lt;/p&gt;&#xA;&lt;h2&gt;Self Rescue via Soyuz&lt;/h2&gt;&#xA;&lt;p&gt;At any point, the station is to have no more persons aboard than it has return capsules for.&lt;/p&gt;&#xA;&lt;p&gt;This means, most of the time, there are two soyuz capsules docked.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.spaceref.com/iss/ops/iss.emergency.ops.pdf&quot; rel=&quot;noreferrer&quot;&gt;Emergency Ops Plan&lt;/a&gt; is pretty clear that plan 1 in an emergency is &amp;quot;Tell NASA&amp;quot;, priority 2 is Stay alive, and priorty 3 is &amp;quot;Fix it if you can, come home if you can't fix it.&amp;quot; Most of the outcomes on the flow chart on page 7-1 involve leaving the station in a Soyuz capsule.&lt;/p&gt;&#xA;&lt;h2&gt;Self-Rescue via cargo ship.&lt;/h2&gt;&#xA;&lt;p&gt;At times, there is also a cargo capsule docked - either the Dragon, a Cygnus, or Progress capsule. In a pinch, any of these three could be used as a lifeboat, as well... but more drastic, because it would require (1) travel in a space suit, and (2) inadequate cushioning of note for the astronauts. The cargo units are pressurized, but don't have the same atmospheric processing as manned capsules. They have several person-hours of breathable air, presuming, of course, that the air aboard isn't contaminated by the emergency.&lt;/p&gt;&#xA;&lt;h2&gt;SpaceX to the Rescue&lt;/h2&gt;&#xA;&lt;p&gt;At KSC, SpaceX has rented a hangar. At KSC, SpaceX is testing the Dragon Manned Capsule.&lt;/p&gt;&#xA;&lt;p&gt;At any given point, SpaceX seems to have a Falcon rocket nearly ready to launch. By the time they roll one out, the next one is already in assembly. In a pinch, a dragon cargo capsule can be fitted with straps and foam, loaded with &lt;a href=&quot;https://www.hq.nasa.gov/alsj/LM15_Portable_Life_Support_System_ppP1-5.pdf&quot; rel=&quot;noreferrer&quot;&gt;PLSS&lt;/a&gt; packs, and on the pad in about 2 days (It takes a day to move the crawler each way). Finishing up the rocket could be a week or two. Current production is a Falcon 9 every month... (&lt;a href=&quot;http://spacexstats.com/upcoming.php&quot; rel=&quot;noreferrer&quot;&gt;cite&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;h2&gt;Rescue by Russians or Chinese&lt;/h2&gt;&#xA;&lt;p&gt;The Soyuz program launches about once every two months, with the capability to push to one every 3 weeks, as I recall. So, a Progress with seats installed could be sent up for emergency rescue in probably a month, along with extra PLSS packs and suits.&lt;/p&gt;&#xA;&lt;p&gt;The Chinese are much slower - about 1 per quarter is what I've seen guessed for them. In an emergency on the ISS, they are quite likely to sent a ship within a window of 3 weeks, if they have a launcher far enough along.&lt;/p&gt;&#xA;&lt;h2&gt;Conclusions&lt;/h2&gt;&#xA;&lt;p&gt;Since the scenario includes no soyuz available at station, and immediate extraction the failure mode is &amp;quot;death.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;The best possibility would be if a CRS mission was waiting to launch. in which case, unloading time, and charging time for the needed PLSS packs. Which would mean, assuming they simply write off the cargo, about 6 hours, leaving the cargo in the launch gantry.&lt;/p&gt;&#xA;&lt;p&gt;If a Cargo mission or a personnel swap isn't scheduled, then &amp;quot;immediate&amp;quot; would be at least 2 days, if not a week or two, for the next available launcher.&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-09-15T01:09:39.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11994" PostTypeId="2" ParentId="9166" CreationDate="2015-09-15T14:30:44.650" Score="7" Body="&lt;p&gt;According to the &lt;a href=&quot;http://www.spaceflightnow.com/falcon9/001/f9guide.pdf&quot; rel=&quot;noreferrer&quot;&gt;2009 Falcon 9 User Guide&lt;/a&gt;, the second stage does have 4 Draco thrusters. I suspect this was done on the Falcon v1.0 only. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the &lt;a href=&quot;http://www.spaceflightnow.com/falcon9/001/f9guide.pdf&quot; rel=&quot;noreferrer&quot;&gt;Falcon 9 User Guide v2.0&lt;/a&gt;, the second stage has nitrogen gas thrusters (page 11-12) for pitch, yaw and roll control. They can also gimbal the main engine. I still haven't found photos of either configuration though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-03-28T09:44:59.790" LastActivityDate="2016-03-28T09:44:59.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11995" PostTypeId="1" AcceptedAnswerId="11997" CreationDate="2015-09-15T15:17:39.473" Score="8" ViewCount="2417" Body="&lt;p&gt;I wonder why the space agencies chose only 2 males for the Year In Space ISS Mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think it would have made more sense to have at least 1 male and 1 female astronaut, or more preferably 3 male and 3 female astronauts, for the following 2 reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Compare any effects between males and females in long duration space missions; &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;To have a minimal statistical comparison between differences in physiologies, 3 would be a minimum sample to establish any &lt;em&gt;hint&lt;/em&gt; of any trends.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I am aware of a previous related question &lt;a href=&quot;https://space.stackexchange.com/questions/8571/one-year-mission-using-data-from-valeri-polyakov&quot;&gt;One year Mission: Using data from Valeri Polyakov?&lt;/a&gt; but I believe this to be a different question.&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-09-15T16:51:52.577" Title="Why only 2 male astronauts for the Year In Space ISS Mission?" Tags="|iss|mars|humans|research|long-duration|" AnswerCount="2" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="11996" PostTypeId="2" ParentId="11995" CreationDate="2015-09-15T15:37:52.343" Score="1" Body="&lt;p&gt;They have several more years to do more of these experiments before they go to Mars.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-09-15T15:37:52.343" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11997" PostTypeId="2" ParentId="11995" CreationDate="2015-09-15T16:51:52.577" Score="16" Body="&lt;p&gt;The test had to start somewhere. There are a number of reasons why they couldn't simply do a 6 person stay, that make it somewhat difficult at least. These include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;There is a natural cycle to bring down astronauts from the ISS, in that a Soyuz needs to be replaced every 6 months. While in theory a crew could bring a Soyuz, and bring the old one down, that hasn't been done in some time.&lt;/li&gt;&#xA;&lt;li&gt;NASA is required to follow OSHA standards, although they have a partial exemption for one particular aspect, namely that of radiation. &lt;a href=&quot;https://space.stackexchange.com/a/25/25&quot;&gt;My answer&lt;/a&gt; to this question gives the requirements, which are different for men and women. Women are more sensitive to radiation, because breast cancer is a bigger risk in women, and all other cancers are roughly equivalent between the two sexes. This means that the ideal candidate from NASA for a long duration test is an older male (But not too old).&lt;/li&gt;&#xA;&lt;li&gt;There are a lot of astronauts who want to go up to space, but very few who currently have that opportunity. Reducing the opportunity to get to space makes astronauts unhappy.&lt;/li&gt;&#xA;&lt;li&gt;Certain experiments that are running on the ISS require special training. There are more than just a long duration space flight test required. Thus, a new flow of astronauts is required.&lt;/li&gt;&#xA;&lt;li&gt;There is one NASA and one Russian doing this test. That seems required to balance the ISS workload.&lt;/li&gt;&#xA;&lt;li&gt;NASA chose a person with an identical twin, to give more exact comparisons.&lt;/li&gt;&#xA;&lt;li&gt;No doubt there will be extra tests done on the two astronauts, which would take away from required astronaut duties, if not handled in limited doses.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line, they could have done this as you proposed, but they want to start slowly. This will give them valuable data, without causing risks. I'm sure if they learn anything interesting from this, they will continue doing similar tests. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-09-15T16:51:52.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="11998" PostTypeId="2" ParentId="6448" CreationDate="2015-09-15T17:29:00.880" Score="3" Body="&lt;p&gt;As you stated, there will be 3 ports on the American side. It seems very likely (Although I can't find it written anywhere) that the plan will be to alternate ISS resupply missions at one dedicated port, alternate crewed missions at the other port, and keep the 3rd port as a backup in case there are problems. Thus, there will be a crew port, a cargo port, and a backup port (Although the ports might be changed from time to time). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does seem like the American side is definitely short on ports, but I haven't heard any plans to bring new ones to the station. It seems like they will be able to manage (Although barely) with the moving around of ports as is currently planned.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-15T17:29:00.880" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="11999" PostTypeId="2" ParentId="9787" CreationDate="2015-09-15T19:37:28.930" Score="2" Body="&lt;p&gt;This &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/spicelib/oscelt.html&quot; rel=&quot;nofollow&quot;&gt;JPL/NAIF SPICE toolkit routine OSCELT&lt;/a&gt; answers what appears to be a query about the two-body problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;N.B. I assume your &quot;direction vector&quot; is actually a &quot;velocity vector;&quot; the STATE input to OSCELT is a six-element vector that is the the concatenation of the position and velocity vectors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;N.B. as noted above you do need a seventh number:  MU, the GM value for the central body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Technically you also need an eighth number, the time of the initial conditions, to have a starting point to calculate future (and past) ephemerides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That link is for the FORTRAN version of the SPICE toolkit because that is what Google returned first.  There are also C, IDL, Matlab, Java and even informal Python interfaces for SPICE; making a DLL of the C-version that can be called from C# should be possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I won't make this any longer than it has to be:  poke around the SPICE pages; SPICE has a steep learning curve but is worth the effort.  The documentation is about the best you will find anywhere, or contact me for more details.&lt;/p&gt;&#xA;" OwnerUserId="11619" LastEditorUserId="11619" LastEditDate="2015-09-16T00:03:55.170" LastActivityDate="2015-09-16T00:03:55.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12000" PostTypeId="1" AcceptedAnswerId="12001" CreationDate="2015-09-16T01:15:04.327" Score="3" ViewCount="606" Body="&lt;p&gt;The Apollo spacecraft, launched by Saturn V, the most powerful rocket ever build, could only take 3 man to the Moon. But how can SpaceX's Falcon Heavy, second to Saturn, take seven crew to Mars on a single Dragon V2?&lt;/p&gt;&#xA;" OwnerUserId="11622" LastEditorUserId="1054" LastEditDate="2019-02-20T17:25:39.230" LastActivityDate="2019-02-20T17:25:39.230" Title="Second powerful Falcon Heavy for Mars, most powerful Saturn V for Moon?" Tags="|mars|spacex|dragon-v2|falcon-heavy|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="12001" PostTypeId="2" ParentId="12000" CreationDate="2015-09-16T02:03:49.813" Score="7" Body="&lt;p&gt;The Falcon Heavy cannot send humans to Mars in a single launch and probably won't ever send humans to Mars in any mission scenario. I believe the source of your confusion stems from the recent postings of Elon Musk on &lt;a href=&quot;https://instagram.com/elonmusk/&quot; rel=&quot;noreferrer&quot;&gt;instagram&lt;/a&gt;.&#xA;These 5 images depict the EDL of a Crew Dragon in a mission known as &lt;a href=&quot;http://www.space.com/24984-spacex-mars-mission-red-dragon.html&quot; rel=&quot;noreferrer&quot;&gt;Red Dragon&lt;/a&gt;. Red Dragon is a mission architecture developed by NASA(not SpaceX) in which a Dragon capsule is sent to Mars as a large lander. Since Crew Dragon has 8 SuperDraco thrusters and a robust heatshield designed for Mars, it could land on Mars with a sizable payload if the fuel supply was increased.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyways your main question is why can Crew Dragon plus Falcon 9/Falcon Heavy seat 7 when Apollo plus Saturn V only sat 3. The answer is that the capsules were designed for different roles, different destinations, and in two different time periods. Crew Dragon is a LEO Taxi. It is designed to be fully automated and take people to and from the ISS. It has the internal cargo to seat 7 but will most likely fly with 4 astronauts and cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo is the crew module for a lunar mission. Its service module contains the consumables and propulsion equipment necessary to go to the Moon. The mission plan called for 3 astronauts so it was designed with 3 seats. There was actually a plan to have a 5 person Apollo capsule in order to rescue a stranded Skylab crew.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/pF13O.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pF13O.jpg&quot; alt=&quot;Apollo Rescue Capsule&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10398" LastActivityDate="2015-09-16T02:03:49.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12002" PostTypeId="1" CreationDate="2015-09-16T02:39:58.837" Score="7" ViewCount="701" Body="&lt;p&gt;This is a question I've been pondering for the last few weeks after I back-calculated the details of the Merlin 1D engine. Just as a thought exercise, I wanted to see what upgrades the Merlin 1D would need to reach aircraft gas turbine engine time between overhauls. After backing out some dimensions from the published data about the design, I came up with a series of upgrades. I made the following assumptions to get the dimensions:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Sensor ports, ignition fluid injection ports, and tight bends in piping (such as on the turbine exhaust or propellant exit) generate a stress concentration factor of 3.&lt;/li&gt;&#xA;&lt;li&gt;In places where the material is not published, aluminum alloy 2021-T6 is assumed on low temperature components, and Rene 88 is assumed on high temperature components. (High temperature is anything above 400 K).&lt;/li&gt;&#xA;&lt;li&gt;The mixture ratio is stoichiometric for the propellants.&lt;/li&gt;&#xA;&lt;li&gt;The fuel is assumed to be made up of &lt;span class=&quot;math-container&quot;&gt;$C_{12}H_{26}$&lt;/span&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The parts are assumed to be crack free.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;From those assumptions, the following: static pressure/static temperature/wall temperature/passage dimensions/wall thickness/average stress/maximum stress/cycle lives were calculated using the published performance specifications (wikipedia and spacenews.com).&lt;/p&gt;&#xA;&lt;h3&gt;Combustion Chamber&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pressure: 6.77 MPa&lt;/li&gt;&#xA;&lt;li&gt;Static Temperature: 3006 K&lt;/li&gt;&#xA;&lt;li&gt;Wall Temperature: 566 K&lt;/li&gt;&#xA;&lt;li&gt;Diameter: 0.609 meters&lt;/li&gt;&#xA;&lt;li&gt;Wall Thickness: 0.004 meters&lt;/li&gt;&#xA;&lt;li&gt;Average Stress: 233 MPa&lt;/li&gt;&#xA;&lt;li&gt;Maximum Stress: 700 MPa/1,000,000 cycles or 54,000 hours of engine time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h3&gt;Throat&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Static Pressure: 3.9 MPa&lt;/li&gt;&#xA;&lt;li&gt;Static Temperature: 2613 K&lt;/li&gt;&#xA;&lt;li&gt;Wall Temperature: 529 K&lt;/li&gt;&#xA;&lt;li&gt;Diameter: 0.223 meters&lt;/li&gt;&#xA;&lt;li&gt;Wall Thickness: 0.004 meters&lt;/li&gt;&#xA;&lt;li&gt;Average Stress: 105 MPa&lt;/li&gt;&#xA;&lt;li&gt;Maximum Stress: 316 MPa/Infinite cycles, below endurance limit of 600 MPa&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h3&gt;Exhaust&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Static Pressure: 101 kPa&lt;/li&gt;&#xA;&lt;li&gt;Static Temperature: 1697 K&lt;/li&gt;&#xA;&lt;li&gt;Wall Temperature: 388 K&lt;/li&gt;&#xA;&lt;li&gt;Diameter: 0.6077 meters&lt;/li&gt;&#xA;&lt;li&gt;Wall Thickness: 0.004 meters&lt;/li&gt;&#xA;&lt;li&gt;Average Stress: 7 MPa&lt;/li&gt;&#xA;&lt;li&gt;Maximum Stress: 10 MPa/ Infinite cycles, below endurance limit of 600 MPa&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h3&gt;Oxidizer Pump&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Exit Pressure: 10.6 MPa&lt;/li&gt;&#xA;&lt;li&gt;Casing Diameter: 0.116 meters&lt;/li&gt;&#xA;&lt;li&gt;Wall Thickness: 0.005 meters&lt;/li&gt;&#xA;&lt;li&gt;Average Stress: 123 MPa&lt;/li&gt;&#xA;&lt;li&gt;Maximum Stress: 320 MPa10,000 cycles or 544 hours of engine time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h3&gt;Fuel Pump&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Exit Pressure: 16.1 MPa&lt;/li&gt;&#xA;&lt;li&gt;Casing Diameter: 0.173 meters&lt;/li&gt;&#xA;&lt;li&gt;Wall Thickness: 0.011 meters&lt;/li&gt;&#xA;&lt;li&gt;Average Stress: 128 MPa&lt;/li&gt;&#xA;&lt;li&gt;Maximum Stress: 383 MPa/8,000 cycles or 435 hours of engine time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Based on these results, the weak point in terms of static endurance is the turbopump, specifically the fuel turbopump casing. To reach 1000 hours between engine overhauls, the average stress on the casing needs to come down. Nesting the fuel pump inside the oxidizer pump would reduce the net stress experienced by the fuel pump by using the pressure developed by the oxidizer pump to compress the fuel pump casing. And the oxidizer pump increases its wall thickness to 0.009 meters to reach 1000 hours between overhauls.&lt;/p&gt;&#xA;&lt;p&gt;However, this is based on the data that I could find out. In my opinion, the real weak points are the impellers, shafts, and bearings in the turbopump. Since they spin at 20,000 rpm, they can quickly go through fatigue lives if they have an imbalance occur due to cavitation, debris, or external vibration. I'm sure SpaceX and Barbor-Nichols (the folks who designed the turbopumps) are aware of this and maybe have some solutions. Outside of increasing the damping inside the pump by using a ferrofluid/magnet and having magnetic bearings, I don't know how one would increase the overhaul time.&lt;/p&gt;&#xA;&lt;p&gt;Thus, I ask the community: How would you do it? What would you change in the engine design to reach 1000 hours between engine overhauls?&lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="33864" LastEditDate="2020-12-31T15:35:03.223" LastActivityDate="2021-01-03T00:52:22.080" Title="What upgrades does the Merlin 1D need to increase its time between overhauls to 1000 hours?" Tags="|spacex|propulsion|falcon-9|engine-design|merlin-1d|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="12003" PostTypeId="1" AcceptedAnswerId="12004" CreationDate="2015-09-16T05:55:17.490" Score="9" ViewCount="1676" Body="&lt;p&gt;Is it possible to get the data on Mars terrain somewhere? Something like &lt;a href=&quot;http://www2.jpl.nasa.gov/srtm/&quot; rel=&quot;noreferrer&quot;&gt;SRTM&lt;/a&gt; data for Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally in a format like this:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;X,Y,E&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;where X and Y are coordinates of the region and E is the elevation (altitude) in that region.&lt;/p&gt;&#xA;" OwnerDisplayName="user3049" LastEditorUserId="168" LastEditDate="2015-09-16T17:22:07.393" LastActivityDate="2015-09-17T06:52:03.780" Title="Are the Mars elevation data publicly available?" Tags="|mars|reference-request|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12004" PostTypeId="2" ParentId="12003" CreationDate="2015-09-16T08:34:35.637" Score="9" Body="&lt;p&gt;There are two sources: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Laser_Altimeter&quot; rel=&quot;nofollow&quot;&gt;MOLA&lt;/a&gt; laser altimetry (&lt;a href=&quot;http://tharsis.gsfc.nasa.gov/mapping_data.html&quot; rel=&quot;nofollow&quot;&gt;http://tharsis.gsfc.nasa.gov/mapping_data.html&lt;/a&gt;) with the actual data available from MEGDR (Mission Experiment Gridded Data Records) at &lt;a href=&quot;http://pds-geosciences.wustl.edu/missions/mgs/megdr.html&quot; rel=&quot;nofollow&quot;&gt;http://pds-geosciences.wustl.edu/missions/mgs/megdr.html&lt;/a&gt; (the closest analog to SRTM you can get, but at a higher fidelity level).&lt;/li&gt;&#xA;&lt;li&gt;and DTM (digital terrain models from &lt;a href=&quot;https://en.wikipedia.org/wiki/Photogrammetry&quot; rel=&quot;nofollow&quot;&gt;stereo pairs&lt;/a&gt;) (&lt;a href=&quot;http://www.uahirise.org/dtm/&quot; rel=&quot;nofollow&quot;&gt;http://www.uahirise.org/dtm/&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;DTM coverage is small, but the resulting topo models are awesomely detailed - they are good for assessing landing sites and building hi-fi virtual terrains at great locations (Marineris, Olympus etc.) One such example is the terrain model for the fictional Ares 3 landing site from Andy Weir's &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Martian_(Weir_novel)&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;The Martian&lt;/em&gt;&lt;/a&gt; available &lt;a href=&quot;http://www.uahirise.org/dtm/dtm.php?ID=ESP_041277_2115&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that lava tubes/subsurface features aren't generally resolvable from either MOLA or DTMs and one has to go through the list of known tubes to guesstimate how far the &lt;strike&gt;li'l green man&lt;/strike&gt; rabbit hole goes.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-09-17T06:52:03.780" LastActivityDate="2015-09-17T06:52:03.780" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12005" PostTypeId="1" AcceptedAnswerId="12009" CreationDate="2015-09-16T10:05:28.357" Score="10" ViewCount="1168" Body="&lt;p&gt;Was the roll made using the SSME gimbal control?&#xA;I would imagine the SRB does not have this capability.&#xA;Is there any use of flight control surfaces as well?&lt;/p&gt;&#xA;" OwnerUserId="11220" LastActivityDate="2015-09-16T13:19:03.573" Title="How was the Space Shuttles rollover maneuver achieved?" Tags="|space-shuttle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12006" PostTypeId="1" AcceptedAnswerId="12010" CreationDate="2015-09-16T10:13:40.063" Score="16" ViewCount="5708" Body="&lt;p&gt;I want to understand the reasons why the shuttle did the rollover manoeuvre after launch.&#xA;I would think better to do it earlier after launch while the air flow pressure is low.&#xA;Why not just adjust attitude when you reach orbit?&#xA;Is the roll the same each time or set to where you want to end up?&lt;/p&gt;&#xA;" OwnerUserId="11220" LastEditorUserId="8162" LastEditDate="2016-08-29T17:33:38.430" LastActivityDate="2017-03-05T16:22:23.113" Title="Why did the Space Shuttles perform a rollover maneuver during launch?" Tags="|launch|space-shuttle|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12008" PostTypeId="1" CreationDate="2015-09-16T11:36:07.257" Score="16" ViewCount="641" Body="&lt;p&gt;The relationship is not intuitively clear to me, would like an answer with references and/or graphs. The question arose while reading about Space Shuttle's SRBs.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="12102" LastEditDate="2021-06-16T12:42:25.660" LastActivityDate="2021-06-16T13:52:30.143" Title="How does solid propellant mean bulk temperature influence solid rocket thrust and specific impulse?" Tags="|temperature|solid-fuel|" AnswerCount="1" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12009" PostTypeId="2" ParentId="12005" CreationDate="2015-09-16T12:49:05.410" Score="12" Body="&lt;p&gt;Attitude control during first stage was achieved by &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/srb/thrust.html&quot;&gt;SRB thrust vector control&lt;/a&gt;.  The large distance between the nozzles and the high thrust levels meant that the SRB TVC had enormous control authority.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line: The roll program was performed by gimballing the SRB nozzles.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-16T13:19:03.573" LastActivityDate="2015-09-16T13:19:03.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12010" PostTypeId="2" ParentId="12006" CreationDate="2015-09-16T13:00:05.973" Score="27" Body="&lt;p&gt;Towards the end of the program, the Shuttle performed two roll maneuvers during powered flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first, performed throughout the Shuttle program, and known as the &quot;roll program&quot; or &quot;Single Axis Rotation&quot;, was actually a multi axis maneuver that mainly served to set the launch azimuth.  Since the shuttle stack sat on the pad in a fixed orientation, it is clear that some sort of rotation must be done to point the vehicle in the desired direction.  In the ISS era, most of the flights followed the same azimuth to that destination's orbital inclination.  You state &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I would think better to do it earlier after launch while the air flow pressure is low.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and in fact the roll program occurred as soon as practical after tower clear. In fact, the initiation of this roll program was a cue to the crew that guidance and control was working properly, and hence the call to the ground to acknowledge this &quot;Roll Program, Houston.&quot;  If the roll program did not occur on time, the crew would have engaged the Backup Flight System redundant control scheme.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also important to achieve the proper launch azimuth early in the trajectory because orbital plane changes are very costly in terms of propellant usage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the Orbiter wings developed lift at zero angle of attack, the high dynamic pressure portion of ascent had to be flown at a negative angle of attack, close to the zero lift angle.  This could have been done heads-up or down, but the early crew members expressed a preference for having a horizon view.  This led to the heads-down attitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other roll maneuver, which was added later in the program, and known as the &quot;roll to heads up&quot;, took place in second stage when dynamic pressure was low.  This roll was driven by budget reasons - the desire to close a ground tracking and communication site.  Rolling to heads up allowed the Orbiter to communicate through a TDRSS satellite instead of the ground station formerly used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A point of interest here was that since the stack was rolling from almost exactly heads down to exactly heads up, it was not predictable in advance whether the vehicle would roll clockwise or counterclockwise.  Small variances in attitude would decide this.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-16T13:17:01.210" LastActivityDate="2015-09-16T13:17:01.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12011" PostTypeId="1" AcceptedAnswerId="12016" CreationDate="2015-09-16T13:29:41.227" Score="35" ViewCount="15146" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle#Re-entry_and_landing&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia claims&lt;/a&gt; (although with no citation) that in order to make the space shuttle land, an initial powered delta-v of 322 km/h was applied in orbit, retrograde to the shuttle's orbit. 322 km/h is equal to 89.4 m/s. This caused the orbit to be lowered into the atmosphere, ultimately causing the shuttle to come to a full stop intact on the ground (or that was the general idea; as we know, it didn't work out perfectly every time).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/users/6944/organic-marble&quot;&gt;Organic Marble&lt;/a&gt; points out the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&quot; rel=&quot;noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt;, which states that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The deorbit burn usually decreases the vehicle's orbital velocity anywhere from 200 to 550 fps, depending on orbital altitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;where 200 fps is about 61 m/s, and 550 fps is about 168 m/s. Given this data and the space shuttle's operational range, 90 m/s seems a reasonable figure to use as a median-mission deorbit-burn delta-v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I fail to understand is how this relatively small (about 1% velocity change: in the case of Wikipedia's figure, 90 m/s out of the on the order of 7 km/s orbital velocity in a low Earth orbit) could be enough to sufficiently lower the orbit to commit the orbiter to landing, rather than being just a small change in the shuttle's orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why was such a small delta-v applied under power in orbit sufficient to &lt;a href=&quot;https://space.stackexchange.com/a/5415/415&quot;&gt;commit&lt;/a&gt; the orbiter to landing?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I expect that good answers will draw on orbital mechanics and atmospheric gas density (aerobraking) to show why the small change was sufficient.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-09-19T16:23:37.383" Title="How could a 90 m/s delta-v be enough to commit the space shuttle to landing?" Tags="|reentry|space-shuttle|landing|orbital-maneuver|delta-v|" AnswerCount="7" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12012" PostTypeId="2" ParentId="6161" CreationDate="2015-09-16T13:46:26.560" Score="6" Body="&lt;p&gt;None: It is practically impossible to image all of a sphere in a single shot.  As mentioned in another answer there a lot of photos; BUT all of those only show half the globe. It is possible to patch photos together to give an image of the whole Earth but there are none that have been taken in a single exposure. &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2015-09-16T13:46:26.560" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12013" PostTypeId="1" AcceptedAnswerId="12021" CreationDate="2015-09-16T13:52:00.603" Score="4" ViewCount="65" Body="&lt;p&gt;I was browsing through the old ISDC archives, and I noticed several topics and speakers that I might be interested in seeing. Are videos of past ISDCs made available somewhere? All I could find was a couple of keynote speeches and some clips from the 2013 and 2015 conferences. Would I need to make a request from the &lt;a href=&quot;http://www.nss.org/&quot; rel=&quot;nofollow&quot;&gt;National Space Society (NSS)&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2019-04-23T16:07:23.340" LastActivityDate="2019-04-23T16:07:23.340" Title="Are there accessible videos of past International Space Development Conferences?" Tags="|video|nss|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12014" PostTypeId="2" ParentId="12011" CreationDate="2015-09-16T13:53:00.067" Score="35" Body="&lt;p&gt;Page 33&lt;sup&gt;1&lt;/sup&gt; in the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&quot; rel=&quot;noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt;, an official NASA astronaut training document, confirms that &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The deorbit burn usually decreases the vehicle's orbital velocity&#xA;  anywhere from 200 to 550 fps, depending on orbital altitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The deorbit burn was not intended to reduce the Orbiter's velocity to a small value, but rather to change its orbital parameters, so that its orbit intersected the sensible atmosphere.  Specifically, it significantly lowered the orbital perigee.  This example from &lt;a href=&quot;http://quest.arc.nasa.gov/space/ask/landing/Shuttle_Orbit.txt&quot; rel=&quot;noreferrer&quot;&gt;the old NASA Quest site&lt;/a&gt; states that on STS-82, the deorbit burn changed the orbit from 333x312 nautical miles, to 333x28.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerodynamic drag then performed the majority of the velocity reduction.  This drag, by converting the kinetic energy of the Orbiter to heat, led to the high temperatures experienced on entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; Page 33 in the pdf, not the internal document page numbering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: since your question may really boil down to &quot;how can a small burn change the perigee so much?&quot;, here's a handy-dandy guide to orbital adjustment burns and their effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LsNY7.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LsNY7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-16T21:42:24.377" LastActivityDate="2015-09-16T21:42:24.377" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="12015" PostTypeId="2" ParentId="12011" CreationDate="2015-09-16T13:53:23.493" Score="-3" Body="&lt;p&gt;What you are describing is the deorbit burn. The short answer is that change in velocity allows the Shuttle to slow enough to act as a glider (way oversimplified). During descent the Shuttle brakes by adjusting its angle to further decelerate. The Shuttle also employed a chute. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/launch/landing101.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/mission_pages/shuttle/launch/landing101.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has plenty of information on the subject. Feel free to Google it.&lt;/p&gt;&#xA;" OwnerUserId="11630" LastActivityDate="2015-09-16T13:53:23.493" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12016" PostTypeId="2" ParentId="12011" CreationDate="2015-09-16T14:35:51.317" Score="32" Body="&lt;p&gt;If you're just looking for an intuitive handle on it, try this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In circular LEO, your orbital period is about 90 minutes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you apply a velocity change of 90 m/s, then wait half an orbit -- 45 minutes -- you should expect to be out of position by 90 m/s * 45 min * 60 s/min = 243,000m, or 243km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The distorting effect of Earth's gravity means that the positional offset isn't in the direction you'd expect, of course, but it does explain the magnitude.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-09-16T21:04:26.383" LastActivityDate="2015-09-16T21:04:26.383" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12017" PostTypeId="2" ParentId="12011" CreationDate="2015-09-16T16:08:50.983" Score="16" Body="&lt;p&gt;I think that something visual may be of help&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Auesm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Auesm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a bit more to scale than most peoples pictures, but the shuttle only orbits at 200 miles, while the earth itself is almost 8000 miles across, so the orbit its more like a thick skin on an orange.. we fly very close to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the picture the red dot is the ship, the thick line is earth, thin lines show the expansion of the orbital trajectory, and the arrow is the direction of its burn, if it were not to continue to burn it would be in a suborbital trajectory and hit the ground, in fact even if it made trajectory very large (going across 7,500 of the earths miles, it would still hit the ground on the other side of the earth), its only those final 200 miles that actually will bring it above the earths surface on the other side. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So once it is in orbit, all it has to do is decrease its orbit circle until its just far enough into the atmosphere on the other side to be able to land (as the atmosphere will slow it down further). In the deorbit burn, it burns the opposite direction, which lowers its orbit by just enough to hit the atmosphere at the right angle (the red orbit in the second picture), this would never be more than the height of the orbit (200 miles in this case) which is much less than the 8000 miles of earth it had to lift itself over first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure there is a lot of math to explain this, but i think the practical answer is simply one of being able to conceive the scale.&lt;/p&gt;&#xA;" OwnerUserId="11633" LastEditorUserId="11633" LastEditDate="2015-09-17T20:36:46.120" LastActivityDate="2015-09-17T20:36:46.120" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12018" PostTypeId="2" ParentId="10685" CreationDate="2015-09-16T16:44:16.223" Score="7" Body="&lt;p&gt;In a recent question about &lt;a href=&quot;https://space.stackexchange.com/questions/10087/did-the-pluto-flyby-give-new-horizons-any-significant-gravitational-boost&quot;&gt;New Horizons&lt;/a&gt;, it was shown that the planet Pluto gave NH a gravity assist worth about 21 km/h. Now, Pluto weighs $10^{22}$ kg. If your space junk weighs 10 tons ($10^4$ kg) (just about the largest piece of junk out there today), its gravitational influence would be 22-4 = $10^{-18}$ times the size of Pluto's, i.e. minute.&lt;br&gt;&#xA;$10^{-18}$ km/h is a femtometer per hour, or 1/10000 times the diameter of an atom. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-09-16T16:44:16.223" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12019" PostTypeId="5" CreationDate="2015-09-16T18:00:02.253" Score="0" Body="" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-09-16T18:00:02.253" LastActivityDate="2015-09-16T18:00:02.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12020" PostTypeId="4" CreationDate="2015-09-16T18:00:02.253" Score="0" Body="Questions about the National Space Society, a non-profit organization for space advocacy" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-09-16T18:00:02.253" LastActivityDate="2015-09-16T18:00:02.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12021" PostTypeId="2" ParentId="12013" CreationDate="2015-09-16T19:27:54.690" Score="1" Body="&lt;p&gt;There is &lt;a href=&quot;https://www.youtube.com/user/NationalSpaceSociety/playlists?sort=dd&amp;amp;view=50&amp;amp;shelf_id=4&quot; rel=&quot;nofollow&quot;&gt;this collection of videos&lt;/a&gt; on the NSS's YouTube channel, taken by attendees. They search for such uploaded videos and make them more accessible this way. Right now there are about 150 such videos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However even that is only a smattering of the talks that occurred. ISDCs are huge events, I'd estimate the 25 videos of the most recent one represents maybe 15% of the talks that occurred. Most of those talks were not filmed by the NSS. There were usually 5 or 6 talks happening at any one time, and there weren't enough volunteers to film properly. Proper cameras and often lights were set up in cases where filming was done, and then carted off to other talks. There is no official record of a lot of great talks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The archivist has the complete records of many past conferences, which used to be officially recorded and later sold on DVDs. Apparently there are legal issues with uploading these to a public venue. If you are interested in getting particular talks, try contacting him through their main email: nsshq@nss.org - and put it attn: Dale Amon.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2015-09-16T19:27:54.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12022" PostTypeId="1" CreationDate="2015-09-16T19:55:03.580" Score="5" ViewCount="475" Body="&lt;p&gt;Right now, the cost of a Delta or Atlas is significantly higher than anything else on the market. I could see them being higher than the Soyuz or Long March, as they are made in countries with cheap labor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the Ariane is made using the same first world wages. How could it be &lt;em&gt;so&lt;/em&gt; much cheaper? &lt;/p&gt;&#xA;" OwnerUserId="11640" LastEditorUserId="25" LastEditDate="2015-09-16T22:13:19.370" LastActivityDate="2015-09-16T22:13:19.370" Title="Why do ULA vehicles cost so much more than European?" Tags="|launch|rockets|cost|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12023" PostTypeId="2" ParentId="12022" CreationDate="2015-09-16T20:31:49.070" Score="12" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/United_Launch_Alliance&quot;&gt;ULA had a captive market&lt;/a&gt;: they were the only supplier for US Dept of Defence launches. There was basically no pressure on them to lower prices, and the DOD market was large enough that they never bothered going after the commercial launch market.&lt;br&gt;&#xA;The recent entry into the launcher market by SpaceX is shaking up this situation: ULA is no longer the sole supplier, and they're restructuring their business in an effort to lower cost to a point where they can compete.&lt;br&gt;&#xA;Arianespace is subsidised by ESA to ensure independent access to space for Europe. In 2011, Arianespace had about \$1B in revenue, including \$150M in subsidies. They had 5 Ariane 5 launches in that year, 4 commercial and 1 for ESA. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-16T20:31:49.070" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12025" PostTypeId="2" ParentId="3738" CreationDate="2015-09-17T12:37:05.757" Score="9" Body="&lt;p&gt;The decision to have 4 firing rooms was made in the early 1960s, before the Apollo missions (e.g. the Lunar Orbit Rendezvous plan) were worked out. At the time, &lt;a href=&quot;https://en.wiki2.org/wiki/FLORIDA_Airspace_monitoring_and_management_system&quot; rel=&quot;noreferrer&quot;&gt;some of the options included multiple launches&lt;/a&gt; to assemble the lunar expedition in space. So they built a complex that &lt;a href=&quot;http://apollolaunchcontrol.com/Apollo_Launch_Control/The_LCC.html&quot; rel=&quot;noreferrer&quot;&gt;could accommodate any conceivable mission structure&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Launch Control Center for Launch Complex 39 is attached to the Vehicle Assembly Building (VAB). It originally was designed with four Firing Rooms, one for each High Bay in the VAB.  Up to four Saturn rockets could be stacked in the VAB at the same time, and each would have a dedicated Firing Room for test, checkout, and launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So a firing room would be in use from stacking to launch of a mission. This makes it likely that more rooms were used simultaneously.&lt;/p&gt;&#xA;&lt;p&gt;I seem to remember an aerial photo of Cape Canaveral with two Saturns on two launch pads. Can't find it now, but that would have been an occasion when two rooms were in use at once.&lt;/p&gt;&#xA;&lt;p&gt;In the end, &lt;a href=&quot;http://apollolaunchcontrol.com/Apollo_Launch_Control/The_LCC.html&quot; rel=&quot;noreferrer&quot;&gt;3 rooms were finished as control rooms&lt;/a&gt; during the Apollo program. Firing Room 4 was used as the project management room for construction of LC-39 and the Apollo systems integration.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://books.google.nl/books?id=jlwICgAAQBAJ&amp;amp;pg=PA131&amp;amp;lpg=PA131&amp;amp;dq=LCC%20%22Firing%20Rooms%22%20ASTP&amp;amp;source=bl&amp;amp;ots=RLHjh5uH2U&amp;amp;sig=dz48Olpc1cEDZMHa6EjOSjlm9Y0&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CCsQ6AEwAmoVChMIl4ido4P-xwIVAQgsCh2QMA1u#v=onepage&amp;amp;q=LCC%20%22Firing%20Rooms%22%20ASTP&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Rocket Ranch: The Nuts and Bolts of the Apollo Moon Program at Kennedy Space Center&lt;/a&gt; has a diagram that which room was used for which mission.&lt;/p&gt;&#xA;&lt;p&gt;During the Shuttle era, &lt;a href=&quot;http://www.cnet.com/news/behind-the-scenes-at-kennedy-space-center/&quot; rel=&quot;noreferrer&quot;&gt;2 firing rooms&lt;/a&gt; were used simultaneously:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;... firing room 2, which is still operational as a space shuttle support facility. During a shuttle launch and a mission, NASA will staff the room with several managers and chief engineers, but will not place any command and control people there. Essentially, it seemed like the place for managers to observe the mission while the folks in Houston run it.&lt;/p&gt;&#xA;&lt;p&gt;However, there are two other firing rooms, Nos. 3 and 4, where the prime shuttle operations are run out of KSC.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;These days, &lt;a href=&quot;http://issuu.com/spaceportmagazine/docs/spaceport_magazine_____july_2015_&quot; rel=&quot;noreferrer&quot;&gt;Firing room 4 has been divided into 4 smaller rooms&lt;/a&gt; (page 25).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-09-17T12:37:05.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12026" PostTypeId="2" ParentId="12011" CreationDate="2015-09-17T14:51:41.520" Score="4" Body="&lt;p&gt;The formula relating speed to orbital radius is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;v_c = \sqrt{ \frac{G M}{r} }&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, rearranging this, we get:&#xA;$$&#xA;{v_c}^2r = C&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for some constant C. If your shuttle's speed decreases by 90/7000 m/s = 1.3% approx, the required orbital radius would have to &lt;em&gt;increase&lt;/em&gt; by approx 2.6% ( $= 1.013^2 - 1$ ). If the current orbital radius is 4000 miles, this means the shuttle is now 2.6% x 4000 = roughly 100 miles &lt;em&gt;lower&lt;/em&gt; than where it needs to be to sustain circular orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I realise I've not accurately explained what will happen to the shuttle next, but you can see that's roughly the right order of magnitude speed change to bring it down into the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="11647" LastActivityDate="2015-09-17T14:51:41.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12027" PostTypeId="2" ParentId="6031" CreationDate="2015-09-17T15:41:17.580" Score="12" Body="&lt;p&gt;If the optimal speed is &lt;a href=&quot;https://en.wikipedia.org/wiki/Terminal_velocity&quot; rel=&quot;noreferrer&quot;&gt;terminal velocity&lt;/a&gt;, this is the formula you need: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_t= \sqrt{\frac{2mg}{\rho A C_d }}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where&lt;br&gt;&#xA; - $V_t$ is terminal velocity,&lt;br&gt;&#xA; - $m$ is the mass of the falling object,&lt;br&gt;&#xA; - $g$ is the Earth's gravity|acceleration due to gravity,&lt;br&gt;&#xA; - $C_d$ is the drag coefficient,&lt;br&gt;&#xA; - $\rho$ is the density of the fluid through which the object is falling, and&lt;br&gt;&#xA; - $A$ is the projected area of the object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air density $\rho$ depends on altitude, so you need a table like the &lt;a href=&quot;http://home.anadolu.edu.tr/~mcavcar/common/ISAweb.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Standard Atmosphere&lt;/a&gt; to calculate Vt versus altitude.&lt;br&gt;&#xA;m, Cd and A depend on your rocket.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a sample graph, based on a rocket with 3.66 m diameter, 100 ton weight and a constant Cd of 0.75: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y05Ai.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y05Ai.png&quot; alt=&quot;Terminal velocity vs altitude&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Altitude in m on the X axis, terminal velocity in m/s on the Y axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Russell Borogove pointed out, Cd also changes depending on your speed. Here's a graph that shows how it changes:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/R68QS.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R68QS.gif&quot; alt=&quot;Cd graph&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately this means we have a feedback loop: if you want to travel at Vt, Vt depends on Cd which depends on Vt. So instead of a simple formula, you have to do iterative calculations for each altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But&lt;/strong&gt;, the solution is even more complicated than this. So far, we've ignored gravity losses and looked only at drag. &#xA;&quot;Goddard's Problem&quot; asks basically the same question: which strategy to use to get the maximum altitude out of a fixed amount of rocket fuel. I've found &lt;a href=&quot;http://cas.ensmp.fr/~petit/papers/ifac2008/0637.pdf&quot; rel=&quot;noreferrer&quot;&gt;several papers&lt;/a&gt; that &lt;a href=&quot;http://epubs.siam.org/doi/abs/10.1137/0301020&quot; rel=&quot;noreferrer&quot;&gt;tackle&lt;/a&gt; this problem, all contain solutions far too complicated for the scope of this answer (and for me to understand without spending days on it).&lt;br&gt;&#xA;Many of the solutions for Goddard's problem contain simplifications as well (e.g. the assumption you can do instantaneous thrust, i.e. dump X amount of mass all at once, instead of having to run the engines for Y minutes).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as a final observation, most rockets don't launch in an optimal-velocity profile: every launch I've seen reaches Mach 2 far below 10 km.&lt;br&gt;&#xA;&lt;a href=&quot;http://web.archive.org/web/20131008024739/http://www.princeton.edu/~stengel/MAE342Lecture3.pdf&quot; rel=&quot;noreferrer&quot;&gt;This lecture&lt;/a&gt; contains a decent overview of a practical approach to launch profiles. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-02T16:16:42.637" LastActivityDate="2018-01-02T16:16:42.637" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12028" PostTypeId="1" AcceptedAnswerId="12029" CreationDate="2015-09-17T19:12:33.023" Score="16" ViewCount="75081" Body="&lt;p&gt;Imagine an ICBM with a nuclear warhead is launched from the US to reach Moscow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How long does it take to reach its target? I've read numbers of about 30 minutes (if the missile is launched from America) or even 15 minutes (if the missile is launched from Europe). Is this realistic?&lt;/p&gt;&#xA;" OwnerDisplayName="user3049" LastActivityDate="2019-05-27T11:44:16.563" Title="Is it correct that it takes approx. 30 minutes for an ICBM to reach Russia?" Tags="|icbm|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12029" PostTypeId="2" ParentId="12028" CreationDate="2015-09-17T19:30:40.413" Score="23" Body="&lt;p&gt;Those numbers are realistic, yes. Peak speed for an ICBM is in the ballpark of 6-7km/s (any faster and the payload would go orbital), and it takes about 10 minutes to accelerate to that speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New York to Moscow is 7500km, at 6.5km/s is ~20 minutes. Add in the acceleration time and you're looking at about 30 minutes total. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;London to Moscow is 2500km; most of the flight time there would be accelerating on ascent and decelerating in re-entry rather than coasting; as TildalWave indicates, 15 minutes is about right for that.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-09-17T19:30:40.413" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12030" PostTypeId="2" ParentId="11967" CreationDate="2015-09-17T19:49:43.777" Score="6" Body="&lt;p&gt;You need to make 550 liters of oxygen in 8 hours.  On average in the paper provided they made about 400 milliliters of oxygen per hour (throwing out the high and low outliers).  That's 3.2 liters per 8 hours. At that rate you'd need 171 times more equipment than they had for this test to make this work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They also burned through their electrodes quite often, so you'd need a large supply of these or a way to re-coat them locally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd estimate the equipment weighs between 10 and 25 kg. You've got two crucibles as well as heating elements and insulation. You'd get some economy of scale so you'd need between 855kg and 2,000kg per astronaut, assuming a 100% increase in efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could also look at CO2 photodissociation.  There's been recent research in using ultraviolet light to fully strip CO2 to C and O2.  It could be more efficient and simpler; however it is recycling vs adding new oxygen to the environment so that may not fit your criteria. &lt;/p&gt;&#xA;" OwnerUserId="11654" LastEditorUserId="11654" LastEditDate="2015-09-18T14:37:08.767" LastActivityDate="2015-09-18T14:37:08.767" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12032" PostTypeId="2" ParentId="2907" CreationDate="2015-09-17T23:36:06.393" Score="6" Body="&lt;p&gt;An addendum to the other answers: the flame trench under the Saturn V / Shuttle launch pads was not solely made of concrete, but was lined with refractory brick.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/B5wfm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/B5wfm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This became an issue when &lt;a href=&quot;http://www.nasaspaceflight.com/2008/07/investigation-confirms-vertical-debris-events-during-sts-124-launch/&quot; rel=&quot;noreferrer&quot;&gt;the STS-124 launch blew a lot of the brick out&lt;/a&gt;!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this was in the post-STS-107 era, any kind of debris that might damage the Orbiter tiles got a lot of attention; this was one that hadn't been considered! So it got analysed to death after that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflightnow.com/shuttle/sts124/080612pad/&quot; rel=&quot;noreferrer&quot;&gt;Forensic analysis of the pad&lt;/a&gt; showed that... &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the metal clips that help hold the bricks to the walls of the flame trench were severely rusted and corroded, possibly due to decades of exposure to the acidic byproducts of booster exhaust. Inspectors also found areas where the absence of trowel marks indicates the epoxy intended to help hold the bricks to the underlying concrete wall was not uniformly applied. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;@TidalWave provided the above link and some others; here's another picture of the damage&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/m7xBU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/m7xBU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reading this also reminded me that a special refractory concrete was used with the amusing brand name &lt;a href=&quot;https://en.wikipedia.org/wiki/Fondu_fyre&quot; rel=&quot;noreferrer&quot;&gt;Fondu Fyre&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-17T23:58:47.393" LastActivityDate="2015-09-17T23:58:47.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12033" PostTypeId="2" ParentId="9292" CreationDate="2015-09-18T09:10:15.330" Score="3" Body="&lt;p&gt;It depends on where and when the probe is captured and read. &#xA;If it's within a few thousand years and within our own galaxy, photos as suggested by BAR might be enough. Redshift would also work. NASA put a map on the Voyager spacecraft using the position of &lt;a href=&quot;http://www.johnstonsarchive.net/astro/pulsarmap.html&quot; rel=&quot;nofollow&quot;&gt;pulsars&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the timescale is much longer (millions of years), using a photo of the night sky becomes problematic. The relative position of stars in our galaxy will change a lot. This increases the amount of calculation you'll have to do: basically you'll have to check the view from each star in the galaxy ($10^{11}$) at 1000-year increments. If you have historical records going back that far, good. If not, the calculated positions will become increasingly inaccurate the further back you go.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On these time scales, using redshift from galaxies would be more stable than using individual stars, I suspect.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-18T09:10:15.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12034" PostTypeId="2" ParentId="8792" CreationDate="2015-09-18T10:00:46.980" Score="5" Body="&lt;p&gt;If we can ignore the 'reusable' in the question: commercial launch providers publish a &lt;a href=&quot;http://www.arianespace.com/launch-services-ariane5/Ariane-5-User%27s-Manual.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;user guide&lt;/a&gt; that specifies everything you need to know to make the payload compatible with the launcher.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compatibility consists of several areas, for example: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the spacecraft has to physically fit inside the payload fairing and has to fit on a payload adapter.  &lt;/li&gt;&#xA;&lt;li&gt;if the spacecraft needs communications, electric power etc. it has to fit the provisions made by the launch provider.  &lt;/li&gt;&#xA;&lt;li&gt;the spacecraft has to withstand the launch environment (vibration and noise levels generated by the launcher).  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In some cases, the launch provider can make changes to accommodate your spacecraft, e.g. a new payload fairing (larger or with specific connectors built in) or payload adapter. Changing the launch environment is much harder.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch profile changes depend on the final orbit you need, so every launch has a unique profile based on final destination and weight of the spacecraft.&lt;br&gt;&#xA;The user guide indicates that the amount of propellant can be changed to suit the mission. I suspect this is only done for the liquid stages; solid stages will get a standard amount of propellant and burn time (if not, they'd have to do custom castings for each launch, which would be expensive).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2019-02-16T09:31:28.377" LastActivityDate="2019-02-16T09:31:28.377" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12035" PostTypeId="2" ParentId="11922" CreationDate="2015-09-18T12:22:25.823" Score="4" Body="&lt;p&gt;XKCD has an interesting &lt;a href=&quot;https://what-if.xkcd.com/13/&quot; rel=&quot;nofollow&quot;&gt;discussion of a similar question&lt;/a&gt;, aiming lasers at the Moon instead of Mars. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it takes 5 PW (double of Earth's current electric power production) to light up the moon with the same brightness as the full moon. &lt;/li&gt;&#xA;&lt;li&gt;it takes 50 EW to heat up the surface noticeably. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Mars is bigger than the Moon, but we are within a few orders of magnitude of what'd be required. So we'd need 10,000 times the total power output of all electric power stations on Earth to heat up Mars. &#xA;The laser itself would also have to be gigantic, you'd end up building a planet-sized ship to house the laser (yep, a Death Star). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as PearsonArtPhoto said, you can't heat the core without heating the surface. As David Hammen explains, that's not inconvenient, you want to melt part of the surface anyway to create plate tectonics.&lt;br&gt;&#xA;When you're done, it'd take thousands of years for the surface to cool enough to be livable, so this would be a really expensive and inefficient method of geoengineering. You'd be better off just building the Death Star without the laser and living on that. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-18T12:22:25.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12036" PostTypeId="2" ParentId="12011" CreationDate="2015-09-18T13:31:17.300" Score="13" Body="&lt;p&gt;After the burn, the orbiter goes into an elliptical orbit. To do the math for such an orbit, we can use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot;&gt;&lt;em&gt;vis viva&lt;/em&gt; equation&lt;/a&gt; which relates the semimajor axis to the velocity of the orbiter:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v^2 = GM(\frac{2}{r}-\frac{1}{a})$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where G = gravitational constant, $M$ = mass of earth, $r$ is the instantaneous distance and $a$ is the semi-major axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can use this to compute the change in $a$ when we change the velocity: since $r$ is essentially constant during an instantaneous burn, and $a=r$ at the moment the burn is done, so $v^2=\frac{GM}{r}$, we get&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$2v\;dv = \frac{GM}{a^2}da\\&#xA;\Delta a=\frac{2v a^2}{GM}\Delta v = \frac{2vr}{v^2}\Delta v = \frac{2r\Delta v}{v}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, for every % change in velocity, you get a 2% change in the semi-major axis. And since your apogee is unchanged, that change must be entirely applied to the perigee. This in turn means that &lt;strong&gt;the distance &lt;em&gt;to the center of the earth&lt;/em&gt; will change by 4% for every 1% change in velocity&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in numbers that you used in your question (7 km/s for orbit, 90 m/s deceleration, 7000 km semimajor axis) we obtain a change in height&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta h = 4 \frac{\Delta v}{v} r = \frac{4\cdot 90 \cdot 7000}{7000} \rm{km} = 360\;\rm{km}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the shuttle orbit varies from 300 to 500 km depending on the mission, that is indeed a good fraction of the height. According to &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/basics/landing.html&quot;&gt;this NASA link&lt;/a&gt; the shuttle experiences the force of atmospheric drag at an altitude of about 129 km (80 miles) - so for most of the range of orbits, dropping 360 km is indeed sufficient.&lt;/p&gt;&#xA;" OwnerUserId="10738" LastEditorUserId="10738" LastEditDate="2015-09-18T15:29:36.557" LastActivityDate="2015-09-18T15:29:36.557" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12037" PostTypeId="2" ParentId="12011" CreationDate="2015-09-18T14:47:16.247" Score="6" Body="&lt;p&gt;The retrograde burn removes energy from the orbit. The energy of orbits remain constant when not burning and can be described by:&#xA;$$\frac{E}{m}=\frac12{V^2}-\frac{GM}{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angular momentum also remains constant:&#xA;$$\frac{L}{m}\approx rV$$&#xA;(This is only approximate because it ignores velocity towards or away from the focus of the orbit, but at the aphelion and perihelion it is exact and those are the only places we're going to use it.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starting from a circular radius $R_0$ we have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R_0=\frac{GM}{{V_0}^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{E_0}{m}=\frac12{V_0^2}-V_0^2=-\frac12{V_0^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now let's apply a $90 / 7\,000= 1.3\% =\epsilon$ reduction in velocity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_1=(1-\epsilon) V_0$$&#xA;$$\frac{L_1}{m}=R_0 V_1=(1-\epsilon)R_0 V_0$$&#xA;$$\frac{E_1}{m}=\frac12 V_1^2-\frac{GM}{R_0}=\frac12 V_1^2-V_0^2=\left(\frac{(1-\epsilon)^2}2-1 \right)V_0^2$$&#xA;Now the angular momentum and energy will be constant through from the aphelion to perihelion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$r\,V=(1-\epsilon)R_0 V_0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac12{V^2}-\frac{GM}{r}=\left(\frac{(1-\epsilon)^2}2-1 \right)V_0^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac12{V^2}-V\frac{V_0}{(1-\epsilon)}=\left(\frac{(1-\epsilon)^2}2-1 \right)V_0^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now this is a quadratic equation  with two solutions for velocity. These correspond to the aphelion and perihelion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V=\begin{matrix} V_1 \\ \left(\frac{2}{(1-\epsilon)^2}-1\right) V_1 \end{matrix}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which means the radius at the perihelion would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R=\left(\frac{2}{2\epsilon-\epsilon^2+1}-1\right) R_0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doing a Taylor expansion on $\epsilon=0$ yields:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R=(1-4\epsilon+ ...) R_0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is equivalent to Flouris's Answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks to TildalWave for doing the research for this last section:&#xA;For $\epsilon=1.3\%$ this corresponds to a $5\%$ reduction in orbital radius. So for an initial orbit altitude of $400\ km$ this corresponds to an orbital radius of $6\,760\ km$ which corresponds to a drop of $338\ km$. This will put the perihelion at $62\ km$ which is well below where atmospheric drag will deorbit anything.&lt;/p&gt;&#xA;" OwnerUserId="11659" LastEditorUserId="286" LastEditDate="2015-09-19T16:23:37.383" LastActivityDate="2015-09-19T16:23:37.383" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12038" PostTypeId="2" ParentId="9126" CreationDate="2015-09-18T16:07:45.463" Score="4" Body="&lt;p&gt;Care packages may be sent up once or twice during a six-month period, and they are usually assembled from items sent from the astronaut's home or from the support staff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nztaW.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nztaW.gif&quot; width=&quot;300&quot; height=&quot;244&quot; alt=&quot;ISS crew care package&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The package is sent in a half-size cargo transfer bag (CTB) made of white Nomex fabric and 24.8 cm x 42.5 cm x 23.5 cm. The maximum load is 5 kilograms per crew member.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A NASA article goes into further detail on the content restrictions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The bag is also restricted by content. Food is allowed. But, anything with significant crumbs -- such as potato chips and peanuts, depending on the packaging -- that can stray into the air, causing problems with computers and the ventilation, can't go. Homemade treats like cookies -- besides the crumb factor -- can't go, because they're perishable and their quality can't be monitored. Products containing alcohol -- not just alcohol to drink, but alcohol in perfume, aftershave, and mouthwash -- can't go into space, and neither can cans under pressure -- like shaving cream. Clothing can go, as long as it's 100 percent cotton. Certain electronics, like handheld video games, aren't allowed either, due to concerns about batteries (they have sparking potential) and off-gassing problems. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All objects have to be tested by engineers for off-gassing before being sent up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see in the picture, each item is sealed in a flight-certified plastic zipper bag before it is placed in the CTB.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It isn't specifically addressed, but I would wager that the friends and family of the crew are allowed to submit enough to fill up the package--and that the support staff adds where that is lacking. Though the article does mention that there are certain occasions (like holidays) where psychological support scientists fill the bags with, for example, decorations for Christmas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/audience/foreducators/k-4/features/F_Just_Like_Home_prt.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Just Like Home - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spacecraft.ssl.umd.edu/academics/697S13/697S13L10.habitability2x.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacecraft Habitability - David L. Akin, University of Maryland&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-09-18T16:07:45.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12041" PostTypeId="2" ParentId="5018" CreationDate="2015-09-18T22:00:31.687" Score="6" Body="&lt;p&gt;There are a lot of angles to this question, which is probably why it has taken so long for this post to receive an answer. I'm going to try to break this down into a few different points:&lt;/p&gt;&#xA;&lt;h1&gt;1. The Title Question&lt;/h1&gt;&#xA;&lt;p&gt;There is no single forum for coordinating activities across all space agencies internationally, unless you count the &lt;a href=&quot;https://en.wikipedia.org/wiki/United_Nations_Office_for_Outer_Space_Affairs&quot; rel=&quot;nofollow noreferrer&quot;&gt;United Nations Office for Outer Space Affairs&lt;/a&gt;. Its purpose is to facilitate intergovernmental discussions and the use of space technology by developing countries, so it doesn't really get into the realm of organizing an international space exploration strategy (until September 2016, see &lt;a href=&quot;https://space.stackexchange.com/a/17806/58&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;h1&gt;2. Global Exploration Strategy&lt;/h1&gt;&#xA;&lt;p&gt;There does, however, exist a Global Exploration Strategy which is advanced by the 14 member agencies of the &lt;a href=&quot;http://www.globalspaceexploration.org/wordpress/?page_id=50&quot; rel=&quot;nofollow noreferrer&quot;&gt;International Space Exploration Coordination Group (ISECG)&lt;/a&gt;: ASI (the Italian space agency), CNES (the French space agency), CNSA (China National Space Administration), CSA (Canadian Space Agency), CSIRO (the Australian space agency), DLR (the German space agency), ESA, ISRO (Indian Space Research Organization), JAXA (Japan Aerospace Exploration Agency), KARI (Korea Aerospace Research Institute), NASA, NKAU (the Ukrainian space agency), Roscosmos, and the UK Space Agency. The Global Exploration Strategy was released in May 2007 and it identified the same need you are pointing to here, an international coordination mechanism. The ISECG was instituted to be that coordination mechanism.&lt;/p&gt;&#xA;&lt;p&gt;The ISECG holds regular meetings to establish non-binding products and form working groups for collaboration.&lt;/p&gt;&#xA;&lt;h1&gt;3. Duplication Abounds&lt;/h1&gt;&#xA;&lt;p&gt;Since ISECG products are non-binding, each agency is still free to accomplish its own agenda. There is a strong drive in most states to develop science and technology internally to maintain a competitive advantage against other states. As such, you will see things like ISRO's &lt;em&gt;MOM&lt;/em&gt; and China's &lt;em&gt;Yutu&lt;/em&gt;, which though they have independent purposes, also serve as demonstrations of the national level of development.&lt;/p&gt;&#xA;&lt;h1&gt;4. Is the Duplication Worthless?&lt;/h1&gt;&#xA;&lt;p&gt;Really, though, there is so much data to be gathered and so much development to be done to exploration technology, that it would be hard to assess whether the duplication is really suboptimal. Also, &lt;em&gt;space exploration is risky business&lt;/em&gt;. A little redundancy ensures that progress continues to be made.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-13T19:28:42.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12042" PostTypeId="1" CreationDate="2015-09-18T23:31:37.800" Score="0" ViewCount="182" Body="&lt;p&gt;I would like to know if all spacecraft that have visited other planets first  orbited our own planet and then performed an orbital transfer, or if there have been some that just went directly to the other body. Also, I know that in theory, it's not required to enter orbit first. What are the pros/cons of doing that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you in advance.&lt;/p&gt;&#xA;" OwnerUserId="11665" LastEditorUserId="8693" LastEditDate="2016-03-17T22:43:12.333" LastActivityDate="2016-03-17T22:43:12.333" Title="Do all spacecrafts first enter orbit and then transfer to another body?" Tags="|orbital-mechanics|orbital-maneuver|interstellar-travel|hohmann-transfer|low-energy-transfer|" AnswerCount="0" CommentCount="4" ClosedDate="2015-09-19T00:37:12.990" ContentLicense="CC BY-SA 3.0" />
  <row Id="12044" PostTypeId="1" CreationDate="2015-09-19T20:58:12.013" Score="4" ViewCount="767" Body="&lt;p&gt;I would like to write a script that can calculate the distance between two asteroids at some moment in time, given some object in the &lt;a href=&quot;http://ssd.jpl.nasa.gov/sbdb_query.cgi#x&quot; rel=&quot;nofollow&quot;&gt;JPL database of asteroids and comets&lt;/a&gt;, so available parameters are restricted to the menu they provide there. Just getting consistent spatial coordinates for objects (in any coordinate system, I don't care about the definition of the system) at a time value would be sufficient to do this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trivial conversions are not going to bother me like going between spherical and Cartesian coordinates, or converting date/month/year to floating point. I have more trouble making sense of what those parameters are, and what to do with them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of degrees of freedom, I ultimately expect 3 functions that take one input parameter (time), and give a spatial coordinate (x,y,z). Given this problem specification, I think need 5 free parameters. I say this in order to cover the extent along each coordinate, the time offset for that period, and the angular orientation of the orbital ellipse. Orbital period is redundant with the spatial data since I know GM of the sun, so I don't need another one for that. The JPL database has a great number of parameters with little obvious meaning. My question is about usage of these. Which ones do I need, and how should I use them?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some of the most promising ones, I will need to sort them into categories. Now, I understand that JED is probably a time unit (not hard). I understand that epoch is probably a reference time and might be needed but won't count toward my 5 expected free variables.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;time related&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;tp, time of perihelion passage, JED&lt;/li&gt;&#xA;&lt;li&gt;epoch, epoch of osculation, JED (not counting)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;traditional orbital parameters&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a, semi-major axis, AU&lt;/li&gt;&#xA;&lt;li&gt;e, eccentricity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;spatial offset concepts&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;i, inclination, deg&lt;/li&gt;&#xA;&lt;li&gt;peri, argument of perihelion, deg&lt;/li&gt;&#xA;&lt;li&gt;node, longitude of the ascending node, deg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I know that this question sounds like Wikipedia/textbook kind of stuff that I could easily look up. So sure, let's go to &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_orbit#Keplerian_elements&quot; rel=&quot;nofollow&quot;&gt;Wikipedia for their method of calculating coordinates&lt;/a&gt; of an orbiting body. They rely on true anomaly (or mean anomaly) for setting the temporal start point. JPL does seem to have mean anomaly, but I must be missing something for that method. That parameter deals with the location of the object &lt;em&gt;at some point in time&lt;/em&gt;, and the parameter (variable M) doesn't give any kind of reference point to use. In fact, my idea that epoch is a reference time for other parameters is entirely speculation on my part. That parameter (and only that parameter) is the same for all objects in the database, so I thought that interpretation made sense, but I could be wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that some parameters are redundant, and that either the time of perihelion passage or the Mean anomaly could be used in its own unique calculation sequence. I don't care which approach I use, I just need one that works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the easiest way for me to go about getting a cookbook to start writing this script to get coordinates of objects in the database?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks in advance for entertaining such a detail-oriented question.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Let me take a little more space (in an already crowded question) to respond to a comment.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;you are asking us to write a chapter of a book. That's a bit much for a stackexchange Q&amp;amp;A type of question.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That textbook would have tremendously short chapters. But I recognize the hazard of this question looking like &quot;explain orbital mechanics to me!&quot; because that's not what I have in mind. So instead of trying to say this in words, let me just say it in equations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$  R = a \frac{ 1 - e ^ 2 }{ 1 + e \cos{ TA } } \\&#xA;X = R \left( \cos{ N}  \cos{ (TA + w ) } - \sin{N} \sin{ ( TA+w) } \right) \cos{ i} \\&#xA;Y = R \left( \sin{ N } \cos{ (TA+w) } + \cos{N} \sin{ (TA+w) } \right) \cos{i} \\&#xA;Z = R \sin{ (TA+w) } \sin{i}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just found this from &lt;a href=&quot;https://www.physicsforums.com/threads/calculating-elliptic-orbits-in-cartesian-coordinates.712979/&quot; rel=&quot;nofollow&quot;&gt;physics forms&lt;/a&gt;, and it almost answers my question, aside from a few processing issues... if I interpreted it correctly. The source didn't have consistent parenthesis, but the above form fits my mathematical picture of the problem. Getting TA shouldn't be hard either, considering that all I need is a &lt;em&gt;scriptable&lt;/em&gt; solution. But I still haven't seen a form for TA that could obviously be obtained with the current time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That should be missing just one additional expression or a few other similar details. I wish I could just get some code from the SPICE toolkit that does the same thing, but its source is probably 10,000s of lines of code, doing all kinds of irrelevant things. If the above equations are right, this should really only require just 1 or 2 more small, concise, things.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2015-09-20T03:23:58.347" LastActivityDate="2017-04-24T00:42:59.503" Title="Calculation of coordinates from JPL Small-body database parameters?" Tags="|asteroid|scientific-data|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12045" PostTypeId="2" ParentId="12044" CreationDate="2015-09-20T01:08:28.240" Score="6" Body="&lt;p&gt;Use &lt;a href=&quot;http://ssd.jpl.nasa.gov/x/spk.html&quot;&gt;this page&lt;/a&gt; to generate SPICE kernels for the bodies of interest, and then use &lt;a href=&quot;http://naif.jpl.nasa.gov/naif/toolkit.html&quot;&gt;SPICE routines&lt;/a&gt; to calculate whatever you like to your heart's content.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Aarrr.  I don't be knowin why ye be reinventing th' wheel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alrighty ya scurvy bilge rat, here it be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$x=a\left(\cos\tau-e\right)$$&#xA;$$y=a\sqrt{1-e^2}\sin\tau$$&#xA;$$z=0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That be givin it t' ye in th' plane.  Then ya be rotatin th' plane with that land lubber Euler's transform in $\Omega$, $i$, $\omega$ (longitude of ascending node, inclination, and argument of periapsis), by multiplyin his matrix by yer vector up there:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\left(&#xA;\begin{array}{ccc}&#xA; \cos\omega \cos\Omega-\cos i \sin\omega \sin\Omega &amp;amp; -\cos\Omega \sin\omega&#xA;  -\cos i \cos\omega \sin\Omega &amp;amp; \sin i \sin\Omega \\&#xA; \cos i \cos\Omega \sin\omega+\cos\omega\sin\Omega &amp;amp; \cos i \cos\omega\cos&#xA;  \Omega-\sin\omega\sin\Omega &amp;amp; -\cos\Omega \sin i \\&#xA; \sin i \sin\omega &amp;amp; \cos\omega \sin i &amp;amp; \cos i \\&#xA;\end{array}&#xA;\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ye also be needin th' time it be ($\mu$ being th' $GM$ of the Sun):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$t=\sqrt{a^3\over\mu}\left(\tau-e\sin\tau\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yer $\tau=0$ and $t=0$ being yer time a periapsis.  Ye be seein that ev'ry $2\pi$ in $\tau$, it be one a yer spins about th' Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If yer scurvy rock be gettin close to a planet, then ye be wastin yer time, as th' orbit be changin on ye.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-09-20T15:10:59.363" LastActivityDate="2015-09-20T15:10:59.363" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12046" PostTypeId="1" AcceptedAnswerId="12048" CreationDate="2015-09-20T01:13:27.107" Score="1" ViewCount="1219" Body="&lt;p&gt;I recently saw a documentary about Project Orion - nuclear pulse engine - that would reach a staggering 3-5% of the speed of light. How long would a ship take to get to Mars, Jupiter, and Saturn at that speed, assuming the spacecraft starts its journey form LEO.&lt;/p&gt;&#xA;" OwnerUserId="11671" LastEditorUserId="1142" LastEditDate="2015-09-21T23:28:03.263" LastActivityDate="2015-09-21T23:28:03.263" Title="3-5% lightspeed?" Tags="|mars|propulsion|jupiter|saturn|nuclear-pulse-propulsion|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12048" PostTypeId="2" ParentId="12046" CreationDate="2015-09-20T11:11:19.223" Score="8" Body="&lt;p&gt;3-5% of the speed of light would only be achievable for a really long duration mission. It just isn't efficient enough to do that kind of acceleration, and it takes a really long time to achieve. Okay, so how long would it take? The math is pretty easy, it's just speed/ acceleration. c=300,000,000 m/s, g= 9.8 m/s^2, so just find the speed, divide by 9.8, and you have the time it would take to accelerate that fast (Ignoring relativity, which doesn't really start to apply until 10% of the speed of light)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;3%- 917,000 seconds.&lt;/li&gt;&#xA;&lt;li&gt;5%- 1,529,000 seconds.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, so both are a large time, measured in the large numbers of days. How long would it take to get to the various places? Assuming 3% c, and no acceleration time, here's what it would take:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars- 25,000 seconds&lt;/li&gt;&#xA;&lt;li&gt;Jupiter- 88,000 seconds&lt;/li&gt;&#xA;&lt;li&gt;Saturn- 135,000 seconds&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;See that the acceleration time is much higher than the time it would take to get there. Bottom line is, a constant 1 g accelerate, and then change to 1 g decelerate would be optimal. See &lt;a href=&quot;https://space.stackexchange.com/q/840/25&quot;&gt;this question&lt;/a&gt; for answers to that specific problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also worth mentioning the deceleration problem. You can only go that fast one way, you can't slow down. There are tricks to slowing down using things like the magnetic field, solar sails, etc, which effectively make this possible. It's just worth mentioning to keep in mind that you eventually have to slow down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 3-5% isn't actually Project Orion, BTW, but a derivative called &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Daedalus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Daedalus&lt;/a&gt;. In fact, Daedalus could achieve even higher speeds. Daedalus is a bigger ship, using thermonuclear devices to achieve higher ISP, and really only works for going interstellar.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-09-21T23:24:56.300" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12050" PostTypeId="1" CreationDate="2015-09-20T16:55:34.017" Score="8" ViewCount="376" Body="&lt;p&gt;I was just wondering what variables are involved in a dead satellite deorbiting and fully burning up in the atmosphere. How much does mass/density/entry angle/velocity matter in the situation? If you were purposefully deorbiting something for destructive reentry, what would the ideal conditions be?&lt;/p&gt;&#xA;" OwnerUserId="11677" LastActivityDate="2015-09-20T21:37:29.397" Title="Requirements for full destructive reentry?" Tags="|reentry|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12051" PostTypeId="1" AcceptedAnswerId="12052" CreationDate="2015-09-20T18:34:07.900" Score="3" ViewCount="1175" Body="&lt;p&gt;On the ISS, the breathable air has to include oxygen, but it can't be pure oxygen for safety reasons. So what is the composition of breathable air on the station?&lt;/p&gt;&#xA;" OwnerUserId="11463" LastEditorUserId="11463" LastEditDate="2023-05-30T17:53:41.877" LastActivityDate="2023-05-30T17:53:41.877" Title="What is the composition of breathable air on the International Space Station?" Tags="|iss|crewed-spaceflight|space-station|life-support|eclss|" AnswerCount="1" CommentCount="1" ClosedDate="2015-09-20T22:03:05.003" ContentLicense="CC BY-SA 4.0" />
  <row Id="12052" PostTypeId="2" ParentId="12051" CreationDate="2015-09-20T19:49:07.360" Score="2" Body="&lt;p&gt;You can monitor atmospheric composition, pressure, temperature, and so on of the several station's modules live via &lt;a href=&quot;http://isslive.com/displays/ethosDisplay1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ETHOS (Environmental and Thermal Operating Systems) Life Support System Display&lt;/a&gt;, part of the &lt;a href=&quot;http://isslive.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Live! pages&lt;/a&gt;. It won't always connect, so I'm including a relevant print screen for current data here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://isslive.com/displays/ethosDisplay1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2N7cb.png&quot; alt=&quot;ISS Live! ETHOS Life Support System Display&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;ISS Live! Environmental and Thermal Operating Systems Life Support System Display. Click on image for live data.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, atmospheric composition is mean sea level &lt;em&gt;Earth-like&lt;/em&gt;, meaning it is maintained at roughly 78% nitrogen, 21% oxygen, carbon dioxide produced by astronauts and live animal and plant biologic experiments is kept at small trace values (maximum of 6 mmHg) and scrubbed by the Vozdukh system in Russian Zvezda module, pressure is currently 749.56 mmHg (&lt;a href=&quot;https://encrypted.google.com/#q=749.56+mmHg+in+psi&quot; rel=&quot;nofollow noreferrer&quot;&gt;14.5 psi&lt;/a&gt;) with acceptable range between 14.2 to 14.9 psi, and temperature is, well, &lt;em&gt;user friendly&lt;/em&gt; 65 to 80˚F (18.3 to 26.7°C).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other gases present in trace amounts in Earth's atmosphere are not provided by the system, but are not excluded due to possible outgassing of equipment and experiment materials (where their composition and amount are considered safe for flight to the station during pressurized cargo selection and packaging), or as metabolic byproducts (ammonia, methane,...), and humidity is kept low at 10 ± 5 mmHg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more info on the Temperature and Humidity Control (THC) system, and some additional sources of information, see my answer to &lt;a href=&quot;https://space.stackexchange.com/a/2540/49&quot;&gt;Air temperature and humidity inside the ISS&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-09-20T21:22:16.657" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12053" PostTypeId="2" ParentId="12050" CreationDate="2015-09-20T21:37:29.397" Score="8" Body="&lt;p&gt;Re-entry and orbital debris analysis is complex. Here is the &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/mitigate/technicalstandard.html&quot;&gt;NASA standard for Orbital Debris Mitigation&lt;/a&gt;. For US launches, satellite operators are required to submit a document known as an Orbital Debris Assessment Report (ODAR), an analysis that shows a spacecraft will enter autonomously before 25 years on orbit (for LEO spacecraft), and that there is a small probability of re-entry hazard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This analysis is performed, in part, by using a NASA provided tool known as &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/mitigate/das.html&quot;&gt;Debris Assessment Software&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an example of an ODAR for the &lt;a href=&quot;http://www.southernstars.com/skycube/files/SkyCube%20ODAR.pdf&quot;&gt;Skycube 1U cubesat&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2015-09-20T21:37:29.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12054" PostTypeId="1" AcceptedAnswerId="12067" CreationDate="2015-09-21T13:38:13.620" Score="11" ViewCount="1792" Body="&lt;p&gt;Watching the recent landing video of TMA-16M, I noticed a flashing light on the underside of the soyuz (&lt;a href=&quot;https://youtu.be/-xNAE3qjAMg?t=420&quot; rel=&quot;noreferrer&quot;&gt;video here&lt;/a&gt;). There appears to be an off-centre, bright flash approximately every 10 seconds. There also appears to be another, less intense flash between the brighter flashes, but it is difficult to tell if it is simply on the far side of the capsule instead of actually being dimmer. The light is visible throughout the video linked above but is easiest to see in the last moments leading up to landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BMPTR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BMPTR.jpg&quot; alt=&quot;flash on the underside of the soyuz&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A bit of digging uncovers a &lt;a href=&quot;http://www.spaceref.com/iss/soyuz/SCLSaB.edit.2.pdf&quot; rel=&quot;noreferrer&quot;&gt;document detailing the underside of the soyuz&lt;/a&gt;, listing soft-landing engines, antennae and a caesium altimeter. None of these seem obvious sources of the light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sjx5e.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sjx5e.png&quot; alt=&quot;soyuz underside diagram&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at my own image of Soyuz TM-7 from the Memorial Museum of Cosmonautics shows that space is at a premium under the soyuz heatshield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RIzjC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RIzjC.jpg&quot; alt=&quot;exposed underside of the soyuz&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My best guess would be that the light is integrated into the antennae or altimeter, possibly intended to be a warning/locator light. Does anyone have a better explanation?&lt;/p&gt;&#xA;" OwnerUserId="8323" LastActivityDate="2015-09-22T13:29:38.830" Title="Flashing lights on soyuz underside?" Tags="|landing|soyuz-spacecraft|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12055" PostTypeId="1" AcceptedAnswerId="12056" CreationDate="2015-09-21T14:42:24.710" Score="9" ViewCount="3467" Body="&lt;p&gt;Several space probes have used gravitational slingshots around Earth as part of their mission plan to get to other places in our solar system. Some examples I could find quickly are &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist#Galileo_.E2.80.93_a_change_of_plan&quot;&gt;Galileo&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist#MESSENGER&quot;&gt;Messenger&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist#The_Cassini_probe_.E2.80.93_multiple_gravity_assists&quot;&gt;Cassini&lt;/a&gt;. Many more probes have, to the same end, used gravitational slingshot maneuvers around other planets and moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Borrowing from &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist#Explanation&quot;&gt;Wikipedia's explanation of gravity assists&lt;/a&gt;, my boldface:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To increase speed, the spacecraft flies with the movement of the planet (taking a small amount of the planet's orbital energy); to decrease speed, the spacecraft flies against the movement of the planet. &lt;strong&gt;The sum of the kinetic energies of both bodies remains constant&lt;/strong&gt; (see elastic collision).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Since gravitational slingshots, when used to increase the velocity of a spacecraft, due to conservation of momentum &lt;em&gt;transfer&lt;/em&gt; energy from the astronomical object (the planet or moon) to the spacecraft, this leads to a tiny decrease in the rotational rate (or a tiny increase in the rotational period length) of the astronomical object, and/or correspondingly for the orbital period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that particularly on Earth we have systems that rely on highly accurate timekeeping, but that this effect is almost certainly tiny to begin with:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;What is the extent of this effect?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;To what extent is this effect worth considering in mission planning?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="415" LastEditorUserId="25" LastEditDate="2015-09-22T11:46:48.843" LastActivityDate="2016-09-29T07:31:11.113" Title="What is the effect of gravity slingshots around Earth on Earth's rotation and orbit time, and is this effect worth considering?" Tags="|orbital-mechanics|mission-design|gravity-assist|earth|planning|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12056" PostTypeId="2" ParentId="12055" CreationDate="2015-09-21T14:58:35.023" Score="11" Body="&lt;p&gt;It is completely negligible. Mass of spacecraft: 1000 kg (Thereabouts). Mass of Earth: $5.9e24$. The difference is 21 orders of magnitude. The variation of the rotation of the Earth from day to day is orders of magnitude larger than the difference caused by a gravitational flyby. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, it's the angular momentum of the orbit, not the rotation, which is affected. The momentum of any planet is huge, as momentum is m*v, and the velocity is quite high. You can safely ignore the gravitational affect of a flyby on the host planet/moon completely, unless you are talking about a spaceship the weight of a large asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How big of an asteroid? Here I have to make a few assumptions, which are that the asteroid is spherical, 1/3rd of a second/ year would be problematic, and that the amount varied is proportional to the weight difference. Also assuming 5 g/cm^3 density. Given all of that, an asteroid with a diameter of 130 km would be a candidate for causing a small difference to the year of the Earth. The most problematic assumption is the amount varied is proportional to the weight difference, which I'm sure isn't exactly correct, but is a reasonable assumption.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-21T15:59:03.003" LastActivityDate="2015-09-21T15:59:03.003" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12057" PostTypeId="1" CreationDate="2015-09-21T17:57:54.967" Score="5" ViewCount="56" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5018/how-are-space-agencies-coordinating-their-science-missions-internationally&quot;&gt;Slightly related&lt;/a&gt;.  I am not looking for general coordination, simply on dates and times of non-related launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;HR&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With up to almost daily launches into space, is there (A) any coordination between space-faring countries regarding schedule, and (B) any need for coordination on schedules (e.g. - they may simply have no impact on each other)?&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-09-21T17:57:54.967" Title="Are launch schedules coordinated internationally?" Tags="|launch|" AnswerCount="0" CommentCount="1" ClosedDate="2015-09-21T22:19:15.243" ContentLicense="CC BY-SA 3.0" />
  <row Id="12058" PostTypeId="2" ParentId="10622" CreationDate="2015-09-21T18:15:22.787" Score="24" Body="&lt;h1&gt;Background and Physics&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Note that there are actually two different but related types of actuators that use conservation of angular momentum&lt;sup&gt;1&lt;/sup&gt; to control a spacecraft's attitude (both of which may be lumped into &quot;reaction wheel&quot; by KSP):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_wheel&quot; rel=&quot;noreferrer&quot;&gt;Reaction wheels&lt;/a&gt;&lt;/strong&gt; (RWs, a.k.a &lt;em&gt;momentum wheels&lt;/em&gt;) spin along a fixed axis at a variable speed.  They change angular momentum by speeding up or slowing down.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Control_moment_gyroscope&quot; rel=&quot;noreferrer&quot;&gt;Control moment gyroscopes&lt;/a&gt;&lt;/strong&gt; (CMGs) spin along a rotating axis at a constant speed.  They change angular momentum by rotating their spin axis.&lt;sup&gt;2&lt;/sup&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://web.archive.org/web/20141219040723/http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1080&amp;amp;context=smallsat&quot; rel=&quot;noreferrer&quot;&gt;This paper (pdf)&lt;/a&gt;&lt;sup&gt;3&lt;/sup&gt; talks about the two different technologies, and in particular how they &quot;scale.&quot;  Assuming that two satellites have the same average density, the torque needed required for a given motion scales like $\tau\propto r^5$.  Applying this torque to an object rotating with speed $\omega$ requires a power $P=\tau\omega$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a reaction wheel applies a torque, it does so by changing the speed of the quickly-spinning wheel (typical speeds are thousands of RPM).  However, a CMG rotates the axis of the wheel, so the torque is applied perpendicular to the rotation of the spinning wheel (which moves at a constant speed), so the amount of power used is very small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus from a power consumption standpoint, CMGs win over reaction wheels when very high torques are required.  However, even for relatively large satellites reaction wheels are used for their simplicity (which means low size, low mass, and better reliability).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Reaction Wheels in KSP&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Let's consider the &lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Mk1_Command_Pod&quot; rel=&quot;noreferrer&quot;&gt;Mk1 Command Pod&lt;/a&gt; from KSP.  Using the values from the wiki, we can estimate the moment of inertia as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;I \approx \frac{mr^2}{10} \approx 130~\text{kg}\cdot\text{m}^2&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the same source, the &quot;reaction wheel&quot; torque is $5~\text{kN}\cdot\text{m}$, enough to apply an angular acceleration on the order of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\alpha = \frac\tau I \approx 2000^\circ\cdot\text{s}^{-2} \approx 360~\text{RPM}\cdot\text{s}^{-1}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;reaction wheels&quot; seem to be hugely over-spec'd (this is probably because they are typically used to control the attitude of an entire rocket instead of engine gimbaling).  Indeed, the torque is far beyond the limits of any CMG flying today: for comparison, the &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100021932.pdf&quot; rel=&quot;noreferrer&quot;&gt;CMGs on the ISS (pdf)&lt;/a&gt; have a maximum output torque of just $258~\text{N}\cdot\text{m}$ each.  Thus you are correct that &lt;strong&gt;reaction wheels in KSP have greatly exaggerated performance.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the question remains: are they &lt;em&gt;useless&lt;/em&gt; in real life?&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Attitude Controller Selection&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Although &lt;a href=&quot;https://space.stackexchange.com/questions/4091/what-factors-determine-whether-a-spacecraft-probe-satellite-uses-gyroscopes-or-p/4130#4130&quot;&gt;this answer&lt;/a&gt; covers the topic quite well, I think it's appropriate to have a discussion here with more of an eye towards reaction wheels and CMGs vs. other solutions.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;RW/CMG: Pros&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Doesn't use propellant.&lt;/strong&gt;  Spacecraft that are designed to operate for decades, such as communications and weather satellites, are usually limited by the amount of consumables that they carry on board, particularly fuel.  (This is especially the case for geostationary satellites, where adding mass is very expensive.)  Reaction wheels only use electricity, and solar power can be used indefinitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A secondary reason that this quality is nice is that there are no propellant residues to accumulate on and degrade the spacecraft's optics (if it has any).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;High accuracy.&lt;/strong&gt;  Reaction wheels are controlled by electric motors, and we can make tiny adjustments to their speed with high accuracy.  This enables space telescopes to point within fractions of an arcsecond of their targets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, it is very difficult to scale down thrusters past a certain size.  Similarly, thruster burns cannot be arbitrarily short due to mechanical limitations.  Therefore there is a minimum correction that can be applied by thrusters, and drift below this level cannot be corrected.  Therefore the tighter the pointing requirement, the more frequently thruster firings would be required to keep the spacecraft within the allowable range of error.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;RW/CMG: Cons&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Mechanically limited lifetime.&lt;/strong&gt;  Typically on a spacecraft anything with a moving part is the first to fail.  Reaction wheels in particular, due to their high rotation speed, fail relatively often &lt;a href=&quot;https://space.stackexchange.com/questions/1314/why-do-reaction-wheels-fail-so-frequently&quot;&gt;as this answer covers&lt;/a&gt;.  For example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Kepler&lt;/em&gt;'s&lt;/a&gt; primary mission was ended when two out of four of its reaction wheels failed, although it continues in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_(spacecraft)#Second_Light_.28K2.29&quot; rel=&quot;noreferrer&quot;&gt;new mission mode&lt;/a&gt;.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Hubble&lt;/em&gt;&lt;/a&gt; has also suffered two reaction wheel failures, but the failed units were replaced during Servicing Missions &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-82&quot; rel=&quot;noreferrer&quot;&gt;2&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-109&quot; rel=&quot;noreferrer&quot;&gt;3B&lt;/a&gt; (however, the failures you hear about in the news are actually the &lt;a href=&quot;http://www.stsci.edu/itt/review/2gyro_handbook/c03_gyroscopes2.html&quot; rel=&quot;noreferrer&quot;&gt;rate-sensing gyros&lt;/a&gt;, another mechanical device.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Added complexity.&lt;/strong&gt;  Unfortunately reaction wheels typically cannot take the place of thrusters.  They are still needed to perform maneuvers, particularly orbital corrections or station-keeping over the lifetime of the spacecraft.  In addition, reaction wheels cannot counter a net external torque: the reaction wheels will end up slowly gaining angular momentum until they approach their maximum speed.  At that point a thruster firing will be required to unload, or &quot;dump&quot; the angular momentum overboard (there do exist other methods to manage excess angular momentum, which I'll talk about later, but all add additional systems and complexity to the spacecraft design and operations).  The process of countering the slow buildup of angular momentum from disturbance torques is called &lt;em&gt;momentum management.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that this is in contrast to how reaction wheels work in KSP!  In the game, reaction wheels don't saturate; they can apply torque indefinitely.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Low speed.&lt;/strong&gt;  On large spacecraft, thrusters can be placed far from the axis of rotation to apply larger torques, but reaction wheels apply fixed torque regardless of location.  This means that large spacecraft controlled by reaction wheels will turn slowly.  For example, &lt;em&gt;Hubble&lt;/em&gt; is limited to a slew rate of &lt;a href=&quot;http://www.stsci.edu/hst/proposing/documents/primer/primer.pdf&quot; rel=&quot;noreferrer&quot;&gt;six angular degrees per minute of time (pdf, p. 18)&lt;/a&gt;, due to the fact that its reaction wheels have a maximum torque of &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080023343.pdf&quot; rel=&quot;noreferrer&quot;&gt;$0.82~\text{N}\cdot\text{M}$ each (pdf)&lt;/a&gt;, four orders of magnitude less than the reaction wheels in KSP!&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Examples of Momentum Management Strategies&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;With the above in mind, let's go over a few cases and why reaction wheels are or are not used, in conjunction with other methods:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Manned spacecraft:&lt;/strong&gt; e.g. the Shuttle or &lt;em&gt;Soyuz&lt;/em&gt;, or &lt;em&gt;Apollo&lt;/em&gt;.  These spacecraft perform lots of maneuvers on-orbit, so they already have a large amount of fuel and powerful thrusters.  They also have short mission duration (measured in days; not counting the time they spend docked and inactive) so they don't need a lot of fuel for attitude control.  They also don't have high pointing accuracy requirements since they don't have body-mounted science instruments.  These factors mean that reaction wheels don't provide any mass savings, and are &lt;strong&gt;not used&lt;/strong&gt; on manned missions.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Space stations:&lt;/strong&gt; e.g. the ISS or Mir (but I'll mostly be talking about ISS here).  The amount of fuel required to stabilize such a massive structure through thrusters alone would be tremendous, so the ISS uses four huge CMGs for stabilization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XkGUw.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XkGUw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Canadian mission specialist Dave Williams replacing CMG-3 during &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-118&quot; rel=&quot;noreferrer&quot;&gt;STS-118&lt;/a&gt;; image from &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts118/multimedia/fd6/index_prt.htm&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt; via &lt;a href=&quot;https://physics.stackexchange.com/a/73004/70557&quot;&gt;this answer on physics.stackexchange&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The station does have attitude thrusters to perform momentum dumps and large maneuvers, but it &lt;a href=&quot;http://arc.aiaa.org/doi/pdf/10.2514/6.2007-6734&quot; rel=&quot;noreferrer&quot;&gt;can avoid using them (pdf)&lt;/a&gt; by balancing the gravity-gradient torque and torque from atmospheric drag against any disturbance torques.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Space telescopes:&lt;/strong&gt; e.g. &lt;em&gt;Kepler&lt;/em&gt; and &lt;em&gt;Hubble&lt;/em&gt;.  Due to the long mission duration and (usually) no possibility of refueling, in combination with high pointing accuracy requirements, these spacecraft all use reaction wheels.  Note that spacecraft in low Earth orbit can use gravity-gradient torque and drag torque (like the ISS), but can also torque against the Earth's magnetic field with &lt;a href=&quot;https://space.stackexchange.com/q/2239/&quot;&gt;magnetorquers&lt;/a&gt; to avoid using propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Kepler&lt;/em&gt; (in a heliocentric orbit) and JWST (at L2) are too far from the Earth to use these strategies.  Both &lt;a href=&quot;http://www.stsci.edu/jwst/doc-archive/technical-reports/JWST-STScI-000755.pdf&quot; rel=&quot;noreferrer&quot;&gt;JWST (pdf)&lt;/a&gt; and &lt;em&gt;Kepler&lt;/em&gt; accumulate angular momentum from photon pressure from the Sun, but in its &lt;a href=&quot;https://www.nasa.gov/kepler/keplers-second-light-how-k2-will-work&quot; rel=&quot;noreferrer&quot;&gt;K2 mission mode&lt;/a&gt; (with only two reaction wheels), &lt;em&gt;Kepler&lt;/em&gt; carefully balances this photon pressure to zero the disturbance torque and stabilize itself in its third axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that attitude control using magnetorquers or photon pressure is especially useful in spacecraft like &lt;a href=&quot;https://en.wikipedia.org/wiki/CubeSat&quot; rel=&quot;noreferrer&quot;&gt;cubesats&lt;/a&gt; or solar sails that may be too small or light for thrusters and or even reaction wheels.  )&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Deep-space probes:&lt;/strong&gt; e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;New Horizons&lt;/em&gt;&lt;/a&gt;.  &lt;a href=&quot;https://space.stackexchange.com/a/10108/8996&quot;&gt;This answer&lt;/a&gt; covers in some detail the attitude control system on &lt;em&gt;New Horizons&lt;/em&gt;, but the important takeaway is that it does &lt;em&gt;not&lt;/em&gt; use reaction wheels!  This is because although the mission duration is very long (9 years to Pluto), most of that time is spent in a spin-stabilized state where no active control is required.  Additionally, the very short encounter with the Pluto system means that the spacecraft must be able to make many fast manuvers in a short period.  A reaction wheel system capable of this would be far too heavy for &lt;em&gt;New Horizons&lt;/em&gt;.  Finally, there are reliability issues over such a long mission, and it would be expensive (both in design and testing) to ensure that the reaction wheels would be functional at Pluto, whereas the reliability of the thrusters is required anyway for trajectory correction maneuvers.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So overall, although reaction wheels are not used like they are in KSP, &lt;strong&gt;they do have real-world applications.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; There are two equivalent ways to think about it.  One is that reaction wheels and CMGs apply torque to the spacecraft when they change speed or rotate, respectively, and that torque causes the spacecraft to turn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other way is that the total &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_momentum#Vector_-_angular_momentum_in_three_dimensions&quot; rel=&quot;noreferrer&quot;&gt;angular momentum vector&lt;/a&gt; of the spacecraft &lt;em&gt;including&lt;/em&gt; the actuators must remain the same (both in direction and magnitude).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;When a reaction wheel changes speed, it changes the &lt;em&gt;length&lt;/em&gt; of its angular momentum vector, so the angular momentum of the spacecraft (&lt;em&gt;excluding&lt;/em&gt; the RW) must change along that axis by an opposite amount, causing the spacecraft to turn.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;When a CMG turns its rotation axis, it changes the &lt;em&gt;direction&lt;/em&gt; of its angular momentum vector, so the angular momentum of the spacecraft (&lt;em&gt;excluding&lt;/em&gt; the CMG) must cancel the change in momentum, causing the spacecraft to turn.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt; I realize that the wording here is a bit confusing, because there are two rotations involved.  One is the rotation of a disk spinning about an axle, the other is the rotation of the axle relative to the spacecraft.  &lt;a href=&quot;https://www.youtube.com/watch?v=5cRb0xvPJ2M&quot; rel=&quot;noreferrer&quot;&gt;This demo of precession (youtube)&lt;/a&gt; shows visually the two rotations involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, note that the result of the demo would be different in space, since the chair applies torques to the demonstrator to keep him upright: if he were floating in space, he would rotate about the horizontal axes as well as the vertical axis, while the orientation of the wheel would remain more or less unchanged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;3&lt;/sup&gt; I copied this source from BowlOfRed's answer.&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-09-23T04:06:58.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12059" PostTypeId="1" CreationDate="2015-09-21T23:13:48.847" Score="2" ViewCount="510" Body="&lt;p&gt;I figured out that ten SuperDraco engines have about the same thrust as a Merlin 1D. So if we replace Falcon 9's second stage with a Dragon V2 with 2 additional SuperDracos ( and replace some seats with fuel tanks,) can it plus a Falcon first stage propel itself into orbit?(or, at least, sub-orbit.) &#xA;Plus, Dragon V2 is reusable, while a Falcon second stage is not. So I think replace a Falcon 9 second stage with a Dragon is pretty cost-saving, huh?&lt;/p&gt;&#xA;" OwnerUserId="11622" LastActivityDate="2016-07-06T14:43:56.573" Title="SuperDraco Dragon V2 vs Merlin 1D Falcon" Tags="|spacex|merlin-1d|super-draco|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12060" PostTypeId="2" ParentId="12059" CreationDate="2015-09-21T23:32:49.227" Score="8" Body="&lt;p&gt;The thrust of the engines (for any launcher or spacecraft, really) is nowhere near as important as the amount of fuel feeding them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes about 9400-10000m/s of &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot;&gt;∆v&lt;/a&gt;, depending on trajectory, to reach stable orbit from Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second stage of a Falcon 9 1.1, depending on payload, is responsible for the majority of that speed: 6200-6700 m/s. That gets 10-13 tons into LEO. It achieves this by carrying &lt;a href=&quot;http://www.spaceflight101.com/falcon-9-v11.html&quot;&gt;92 tons&lt;/a&gt; of propellants. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the numbers filed with the EPA, &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/33f75a/just_how_much_deltav_will_the_dragon_2_capsule/cqkecfj&quot;&gt;Dragon V2 carries 450 gallons of hydrazine and NTO propellants&lt;/a&gt;, roughly &lt;em&gt;2&lt;/em&gt; tons. With the dry mass of 4.2 tons, that's about 900 m/s delta-V assuming zero payload. With 3 tons of payload you've got about 565 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the up side, losing the second stage means the first stage is carrying much less weight, so it gets you going a lot faster, yielding an extra 3200m/s of ∆v, but that leaves you 2000 m/s short of orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Replacing the Dragon payload with fuel tankage could get the total system ∆v to 8800m/s, still 1000m/s short of orbit with no useful payload. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-09-21T23:45:19.273" LastActivityDate="2015-09-21T23:45:19.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12061" PostTypeId="2" ParentId="12055" CreationDate="2015-09-21T23:46:40.963" Score="4" Body="&lt;p&gt;This can be worked out through conservation of energy: if a spaceship gains kinetic energy from a flyby, the planet must have lost the same amount of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist#The_Cassini_probe_.E2.80.93_multiple_gravity_assists&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article on gravity assists&lt;/a&gt; shows Cassini as gaining 4000 m/s from the Earth flyby; assuming a mass at flyby of around 4500 kg, this means it gained about 72 GJ of energy.  Earth has a mass of $5.9*10^{24} kg$, so assuming all the energy came from the Earth's orbital velocity rather than from its rotational velocity, the flyby caused the Earth to slow down by 0.00000011 m/s, or about one part in 280,000,000,000.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2015-09-21T23:46:40.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12062" PostTypeId="1" CreationDate="2015-09-22T00:00:25.350" Score="1" ViewCount="300" Body="&lt;p&gt;If a large parachute were attached to the ISS, how long would it last? I know that there is very little atmosphere where the ISS orbits, but also that the atmosphere is thick enough that it needs a few boosts every now and then. Would the ISS survive a few orbits with a parachute? How often would it need to perform reboosts to remain stable? Would the parachutes even expand in the thin atmosphere and would they significantly increase drag? If not, how low would the ISS have to be for the parachutes to have a noticeable effect? If the parachutes would be noticeable at the current altitude, how high would it have to be for the parachutes to go largely unnoticed? In general, are drag characteristics considered at all when looking at low earth orbit vehicles?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The size of the parachute should be big enough that if the ISS were to descend into the thicker parts of the atmosphere (ground level), its terminal velocity should be around 20 m/s (pretty fast, but not terribly fast). The ISS masses approximately 370,000 kg right now. Plugging this in to the drag equation, (and using 1.5 as the drag coefficient for the parachute, I made that number up, I don't know if that is the actual drag coefficients for parachutes), we get that the diameter of the parachutes about 112 meters (which is pretty big so I might have done the math wrong).&lt;/p&gt;&#xA;" OwnerUserId="11681" LastEditorUserId="11681" LastEditDate="2015-09-22T23:41:59.160" LastActivityDate="2015-09-22T23:41:59.160" Title="Parachute on the ISS" Tags="|iss|atmospheric-drag|parachute|" AnswerCount="1" CommentCount="3" ClosedDate="2015-09-22T07:10:26.450" ContentLicense="CC BY-SA 3.0" />
  <row Id="12063" PostTypeId="2" ParentId="12062" CreationDate="2015-09-22T00:17:27.653" Score="3" Body="&lt;p&gt;I'm only going to tackle your final question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In general, are drag characteristics considered at all when looking at low earth orbit vehicles?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, drag characteristics are considered for LEO vehicles that need to stay up for a long time (like the ISS) or are flying particularly low, like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Field_and_Steady-State_Ocean_Circulation_Explorer&quot; rel=&quot;nofollow&quot;&gt;Gravity Field and Steady-State Ocean Circulation Explorer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS rotates its solar panels into a minimum drag position while passing on the night side of Earth.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="415" LastEditDate="2015-09-22T14:38:15.877" LastActivityDate="2015-09-22T14:38:15.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12066" PostTypeId="2" ParentId="12059" CreationDate="2015-09-22T09:59:24.320" Score="5" Body="&lt;p&gt;The critical number for rocket engines is their specific impulse (Isp, a measure of how efficient the engine is).  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Merlin has an Isp of &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29&quot;&gt;311 s&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The SuperDraco Isp is &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/26o6du/spacex_superdraco_thruster_firing/&quot;&gt;estimated&lt;/a&gt; to be around 225 s (which is in line with other &lt;a href=&quot;http://www.astronautix.com/props/hydazine.htm&quot;&gt;hydrazine engines&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This means an upper stage that uses SuperDracos would need to be much heavier than a Merlin-powered upper stage, because it'd need more fuel to achieve the same $\Delta V$. So you'd end up replacing the second stage with a Dragon that's larger and heavier than the second stage (and as a consequence, will be harder to land intact). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-22T09:59:24.320" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12067" PostTypeId="2" ParentId="12054" CreationDate="2015-09-22T11:26:20.023" Score="8" Body="&lt;p&gt;I've been digging based on some of the answers/comments here and here's what I've come up with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/12054/flashing-lights-on-soyuz-underside#comment30964_12054&quot;&gt;Andy's suggestion&lt;/a&gt; that the light was lowered after re-entry, possibly from the parachute compartment sounded pretty good. &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/4/4b/Russian_support_personnel_with_the_Soyuz_TMA-22_capsule_shortly_after_landing.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;This picture of a landed capsule&lt;/a&gt; had me suspecting the white object might be some sort of beacon, but &lt;a href=&quot;http://cryptome.org/info/soyuz-tma19/pict17.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;other pictures&lt;/a&gt; showed it to be part of the parachute. Thinking a little more, it seems unlikely that anything except parachute would be put in the parachute compartment, given that a tangled cable could have life threatening implications for the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.americaspace.com/wp-content/uploads/2013/10/soyuz-23-rescue.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;This artist's impression&lt;/a&gt; from &lt;a href=&quot;http://www.americaspace.com/?p=44182&quot; rel=&quot;nofollow noreferrer&quot;&gt;an article&lt;/a&gt; about Soyuz 23's landing seemed to support the idea of a beacon hanging from above. &lt;a href=&quot;http://www.svengrahn.pp.se/radioind/mirradio/RVdoorX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;A close up of the compartment in question&lt;/a&gt; shows a strobe light and a whip antenna, but the light doesn't appear to be extendible, making me think that the artist misunderstood the difference between the extendible antenna and the static light. &lt;a href=&quot;http://photo.sf.co.ua/g/150/32.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Other pictures of landed capsules&lt;/a&gt; show the light still in place after landing. Furthermore, I couldn't see any trailing lines down the side of the capsule in this &lt;a href=&quot;http://photo.sf.co.ua/g/254/39.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;image which shows the beacon&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://books.google.co.uk/books?id=dbGchpi1HP8C&amp;amp;lpg=PP1&amp;amp;dq=Soyuz%20a%20universal%20spacecraft&amp;amp;pg=PA43#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz: A Universal Spacecraft&lt;/a&gt; lists a 'beacon' on the underside of the capsule, but doesn't go into any more detail (I don't have a full copy of the book).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BUYME.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BUYME.png&quot; alt=&quot;soyuz descent module diagram&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, I came across this image that seemed to settle it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ix8Rb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ix8Rb.jpg&quot; alt=&quot;A landed soyuz capsule&quot;&gt;&lt;/a&gt;&#xA;(&lt;a href=&quot;http://www.seattleastronomy.com/blog1/2011/12/seattle-museum-of-flight-gets-soyuz-capsule-from-simonyi/&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Click to zoom and you can see a beacon/bulb still attached to the underside of the descent module. Perhaps it didn't release properly this time? The length of the cable seems to support the idea that it is stowed under the heatshield. It still isn't entirely clear where it plugs in to the capsule or how it is stowed (I'd hazard a guess at tucked into one of the soft-landing engine wells or as shown in the image), but I'm happy with this as an an answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I was previously working on the assumption that there might be 2 beacons (one brighter, one dimmer), but on closer examination of the video, the flashes do seem to be emanating from the same spot, implying a single beacon.)&lt;/p&gt;&#xA;" OwnerUserId="8323" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-09-22T13:29:38.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12068" PostTypeId="2" ParentId="7877" CreationDate="2015-09-22T12:41:41.097" Score="3" Body="&lt;ul&gt;&#xA;&lt;li&gt;Mobile uplink/downlink are channels between the satellite and 'end users': mobile satphones, satcom terminals on ships, aircraft etc. &lt;/li&gt;&#xA;&lt;li&gt;Feeder uplink/downlink are channels between the satellite and a ground station. The ground station is connected to terrestrial phone/data networks.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;When a satphone user calls someone who uses a 'normal' landline, the call is routed from the satphone to the satellite (through the mobile up/downlink), then from the satellite to a ground station (through the feeder up/downlink), and the ground station connects the call to the landline phone network. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually, a satellite has many mobile links, and one feeder link. The satellite bundles the data from all mobile links into the one feeder link. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-22T12:41:41.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12069" PostTypeId="2" ParentId="6587" CreationDate="2015-09-22T14:11:55.507" Score="14" Body="&lt;p&gt;Keep in mind that a rocket engine is simply a controlled explosion. The explosion is directed at the ground, so most of the heat/flames/exhaust won't reach you unless you were very close. It would be simple to just stand on the side opposite to where it was being directed on launch.&lt;/p&gt;&#xA;&lt;p&gt;However, other than a suppression system consisting of trenches and/or water, nothing will protect you from the &lt;strong&gt;sound&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Sounds are just pressure waves traveling through the air. The sound of the shuttle launching was 215 dB. Saturn V was 220 dB, and was capable of melting concrete through sound alone. For reference the nuclear bombs they dropped on Japan were 248 dB. Studies show that a sound of 210 dB will cause damage to internal organs, probably resulting in death from internal bleeding. How close would you have to be to the Saturn V to get hit with 210 db? Probably a few hundred feet, which would put you beyond most of the smoke and heat, especially if you stood at the right spot.&lt;/p&gt;&#xA;&lt;p&gt;So I'm voting that the sound kills you first.&lt;/p&gt;&#xA;&lt;p&gt;Ref:&lt;a href=&quot;http://www.makeitlouder.com/Decibel%20Level%20Chart.txt&quot; rel=&quot;noreferrer&quot;&gt;http://www.makeitlouder.com/Decibel%20Level%20Chart.txt&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11618" LastEditorUserId="7062" LastEditDate="2022-08-22T06:35:48.410" LastActivityDate="2022-08-22T06:35:48.410" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="12070" PostTypeId="2" ParentId="9217" CreationDate="2015-09-22T15:44:30.973" Score="2" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.raumfahrer.net/forum/smf/index.php?action=media;sa=media;in=2955;dl&quot; rel=&quot;nofollow noreferrer&quot;&gt;the H-IIA user manual&lt;/a&gt; (PDF page 219, figure 5.7.12), in a normal launch, propellant loading stops at 3 hours before launch.&lt;br&gt;&#xA;But there is an option (&quot;98% additional loading&quot;) to continue propellant loading until a few minutes before liftoff. So Geoff's answer was correct. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0cW3c.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0cW3c.png&quot; alt=&quot;H-IIA launch sequence&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-22T15:44:30.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12072" PostTypeId="2" ParentId="11919" CreationDate="2015-09-23T12:09:42.077" Score="11" Body="&lt;p&gt;Thanks to this &lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/press/DA_Schlottke.pdf&quot;&gt;student's Soyuz simulator&lt;/a&gt;:&lt;br&gt;&#xA;Top left:&lt;br&gt;&#xA; - $\Phi$44 : flight mode (44 is final approach)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom left:&lt;br&gt;&#xA; - $\rho$ : range to target&lt;br&gt;&#xA; - $\dot\rho$ : range rate (closing speed)&lt;br&gt;&#xA;These are calculated parameters&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Top right:&lt;br&gt;&#xA; - T: time&lt;br&gt;&#xA; - $\omega$X, Y, Z: angular rates (roll speed on X, Y, Z axes) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Middle right:&lt;br&gt;&#xA; - $\gamma, \psi, \theta$: roll, yaw, pitch attitude&lt;br&gt;&#xA; - $\psi_{\pi}, \theta_{\pi}$: heading, pitch bearing angles (Soyuz position relative to ISS)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom right:&lt;br&gt;&#xA; - $\rho, \dot\rho$ : range and range rate (closing speed)&lt;br&gt;&#xA; - $\Omega Y, \Omega Z$: line of sight angular rates (roll speeds relative to target)&lt;br&gt;&#xA;These are parameters measured by Kurs. So they could compare range and range rate between the calculated course and the actual course. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-09-23T20:03:24.443" LastActivityDate="2015-09-23T20:03:24.443" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12073" PostTypeId="2" ParentId="8380" CreationDate="2015-09-23T13:36:02.290" Score="3" Body="&lt;p&gt;The &lt;a href=&quot;http://spaceflownartifacts.com/flown_ppks.html&quot; rel=&quot;nofollow&quot;&gt;link given by Anthony X&lt;/a&gt; lists the weight of personal items carried on each mission. Their contents were &lt;a href=&quot;http://www.collectspace.com/ubb/Forum14/HTML/001286.html&quot; rel=&quot;nofollow&quot;&gt;considered private so their manifests haven't been published&lt;/a&gt;. So this is the best information we have available. Some astronauts have auctioned some or all of the contents of their PPKs, good luck tracking that information down.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apollo 11: 5 kits, 2.5 lbs.  &lt;/li&gt;&#xA;&lt;li&gt;Apollo 12: 9 kits, 4.5 lbs.  &lt;/li&gt;&#xA;&lt;li&gt;Apollo 13: 15 kits, 7.5 lbs.  &lt;/li&gt;&#xA;&lt;li&gt;Apollo 14: 18 kits, 9 lbs.  &lt;/li&gt;&#xA;&lt;li&gt;Apollo 15: 6 kits, 0.2 lbs? (information on weight was unclear)  &lt;/li&gt;&#xA;&lt;li&gt;Apollo 16: 13 kits, 1.3 lbs? (information on weight was unclear)  &lt;/li&gt;&#xA;&lt;li&gt;Apollo 17: 3 kits, 1.5 lbs.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;On &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15_postage_stamp_incident&quot; rel=&quot;nofollow&quot;&gt;at least one occasion&lt;/a&gt;, astronauts smuggled items on board.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, each mission also carried a number of &lt;a href=&quot;http://www.collectspace.com/ubb/Forum14/HTML/000685.html&quot; rel=&quot;nofollow&quot;&gt;flags&lt;/a&gt;. On Apollo 17, this was manifested as the &lt;a href=&quot;http://www.edgeofdarkness.com/apollo_ppks/ofk.html&quot; rel=&quot;nofollow&quot;&gt;Official Flight Kit&lt;/a&gt;, weight 13.5 lbs. On earlier missions, these are not manifested.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-09-23T15:03:16.320" LastActivityDate="2015-09-23T15:03:16.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12074" PostTypeId="1" CreationDate="2015-09-23T14:36:18.710" Score="5" ViewCount="475" Body="&lt;p&gt;What is the total change in the Earth's Angular Velocity over the past 100 years?  I have read other posts that discuss how much mass has been lifted out of Earth's Atmosphere.  I'm trying to get an idea of the effect of all of our launches on the angular momentum of the Earth.  It would depend not only on the mass of all launches but the weighted-radial separation from the Earth, of all launches, that will ultimately determine the net change in angular velocity.  This should be something that can be looked up as it should have been accounted for early in our space exploration history.&lt;/p&gt;&#xA;" OwnerUserId="11690" LastEditorUserId="4660" LastEditDate="2015-09-23T14:39:53.820" LastActivityDate="2020-02-05T13:53:18.637" Title="How has the angular velocity of Earth changed over the past 100 years?" Tags="|launch|artificial-satellite|earth|rotation|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12075" PostTypeId="2" ParentId="10842" CreationDate="2015-09-23T14:39:11.103" Score="9" Body="&lt;p&gt;This is by no means a definitive answer, as it appears that an exact &quot;first&quot; is difficult to find.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The earliest reference to a large rocket using ammonium perchlorate that I can find is the &lt;a href=&quot;https://en.wikipedia.org/wiki/MIM-14_Nike_Hercules&quot; rel=&quot;noreferrer&quot;&gt;Nike Hercules&lt;/a&gt; which started using an ammonium perchlorate composite propellant in 1958, according to &lt;a href=&quot;http://www.itrcweb.org/Guidance/GetDocument?documentID=60&quot; rel=&quot;noreferrer&quot;&gt;Perchlorate: Overview of Issues, &#xA;Status, and Remedial Options&lt;/a&gt; prepared by The Interstate Technology &amp;amp; Regulatory Council Perchlorate Team.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3YBha.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3YBha.jpg&quot; alt=&quot;Nike Hercules&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears that John Parsons of Aerojet may have been the first person to start considering the use of ammonium perchlorate in solid rocket fuel around 1944, based on &lt;a href=&quot;https://www.google.com/patents/US2783138?dq=solid+fuel+rocket+%22ammonium+perchlorate%22&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CCMQ6AEwAWoVChMIhMLjzbGNyAIVAjk-Ch2wYA0c&quot; rel=&quot;noreferrer&quot;&gt;this patent&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-09-23T14:39:11.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12076" PostTypeId="2" ParentId="10265" CreationDate="2015-09-23T22:45:39.207" Score="6" Body="&lt;p&gt;The main difference between the two is that halo orbits tend to be much larger and are constricted to one plane.  There's a NASA &lt;a href=&quot;https://archive.org/stream/nasa_techdoc_19930015532/19930015532#page/n13/mode/2up&quot; rel=&quot;noreferrer&quot;&gt;tech doc&lt;/a&gt; from 1993 comparing the station keeping costs between the two and finding none.  The authors were quite clear that they did not know if this applied to all station keeping algorithms, but it was clear in the one they tested for both types of orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the JWST, a halo orbit confines the telescope to motion in one plane.  This should give it better ability to compensate for the motion and get higher resolution images.  If it really makes a difference at all.&lt;/p&gt;&#xA;" OwnerUserId="11654" LastActivityDate="2015-09-23T22:45:39.207" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12079" PostTypeId="1" CreationDate="2015-09-24T07:23:28.520" Score="-1" ViewCount="463" Body="&lt;p&gt;We get told &lt;em&gt;&quot;there is nothing in space except planets etc..&quot;&lt;/em&gt; (saying there is no air etc) but is there &quot;nothing&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there is it must be a strong vacuum and how strong will it be? could we use it for more efficient travel? A vacuum pulls, so where is it &quot;pulling&quot; to, it doesn't seem to &quot;go&quot; anywhere? &lt;/p&gt;&#xA;" OwnerUserId="11692" LastEditorUserId="238" LastEditDate="2015-09-24T09:47:03.850" LastActivityDate="2015-09-24T13:05:25.407" Title="What is space made of, and what is nothing?" Tags="|rockets|vacuum|" AnswerCount="3" CommentCount="5" ClosedDate="2015-09-24T14:09:26.463" ContentLicense="CC BY-SA 3.0" />
  <row Id="12080" PostTypeId="2" ParentId="12079" CreationDate="2015-09-24T08:37:09.960" Score="3" Body="&lt;p&gt;I'll give a simple answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An ideal vacuum is simply nothing. It does not pull, it exerts no force on objects. Within a vacuum, far from a gravitational field, objects in motion simply travel in a straight line, the vacuum exerts no force, no resistance, upon the object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm speculating that the idea of &quot;pulling&quot; comes from &quot;explosive decompression&quot; seen in movies, what happens is that atmosphere within a spaceship is pressurized, it is pressurized by the walls of the spaceship which are strong enough to hold the air pressure. Pressure is related to the velocity of molecules within the gas, in air at room temperature and standard atmospheric pressure, the molecules are travelling very fast, about 500m/s, this is 1800km/h or 1100mph (google &quot;speed of air molecules&quot;). All these molecules are zipping around at tremendous velocities and bouncing off each other and solid objects, because the particles are so tiny and there are so many of them and they are bouncing off us from all sides the force evens out, and we just know it as &quot;pressure&quot;, which normally we don't even notice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in the spaceship, there are all these molecules zipping around at 1800km/h, but being contained by the walls. Then something makes a hole in the wall, suddenly the particles are free to shoot out the hole at 1800km/h. And that is exactly what happens, all the particles which &lt;em&gt;would&lt;/em&gt; have bounced off the wall, instead shoot out the hole and travel off into space at 1800km/h.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that &quot;explosive decompression&quot; as seen in movies doesn't happen in reality. You can't get sucked out of a hole. In fact, you could plug a hole with your finger. Rather it's just like I described it above, the air molecules which would have been contained by the wall, instead shoot out into the vacuum, so the air gradually escapes, as air escapes it will create a low pressure zone near the hole, which then creates a wind towards the hole. But it would not be explosively violent.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2015-09-24T13:05:25.407" LastActivityDate="2015-09-24T13:05:25.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12081" PostTypeId="2" ParentId="12079" CreationDate="2015-09-24T10:07:01.060" Score="1" Body="&lt;p&gt;Just to answer your last point. A vacuum on Earth can have an effect, but only because of the pushing of the Atmosphere back to fill that hole. It's the difference in pressure that makes the difference, not anything else. You would have the same effect increasing the outside pressure as decreasing the inside pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as Explosive Decompression, it can be mildly damaging. See &lt;a href=&quot;https://youtu.be/4yG2h1aDB6k&quot; rel=&quot;nofollow&quot;&gt;this Mythbuster's&lt;/a&gt; clip.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-24T10:07:01.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12082" PostTypeId="1" AcceptedAnswerId="12083" CreationDate="2015-09-24T10:33:14.737" Score="22" ViewCount="5371" Body="&lt;p&gt;I recently heard that space suit gloves can cause Astronauts to lose their finger nails.  We have an &lt;a href=&quot;https://space.stackexchange.com/questions/5812/are-we-capable-engineering-wise-of-building-a-spacecraft-in-space/6393#6393&quot;&gt;answer&lt;/a&gt; that includes a link to a &lt;a href=&quot;http://www.space.com/9217-spacesuit-gloves-astronauts-fingernails-fall.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;2010 article&lt;/a&gt; that says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;in extreme cases, detach from their nailbeds or fall off entirely&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The article is on a site with a lot of advertising and it does not explicitly state or reference an example of an astronaut who actually lost a finger nail due to any cause, so not sure how accurate any of it is.  The articles title is &lt;strong&gt;Spacesuit Gloves Can Make Astronauts' Fingernails Fall Off&lt;/strong&gt; with &lt;strong&gt;&lt;em&gt;Can&lt;/em&gt;&lt;/strong&gt; being an operative word suggesting it could happen, but not that it ever has. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do astronauts lose finger nails in space?  Are there any verified occurrences? If so is it still an issue?  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-11-05T20:38:26.187" Title="Do Astronauts lose finger nails in space?" Tags="|health|spacesuits|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12083" PostTypeId="2" ParentId="12082" CreationDate="2015-09-24T10:55:34.823" Score="27" Body="&lt;p&gt;They tended to suffer painful fingernails in the Apollo days, in the &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a17/a17.eva3prep.html&quot;&gt;Lunar Surface Journal&lt;/a&gt; Schmitt and Cernan gave a good description of this on several pages.  There were also references made in the various technical debrief documents.  The problem tended to arise from the fact that the gloves are pressurised, therefore stiff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The journal also links to a photograph which happens to show Schmitt's blackened fngernails after landing:&#xA;&lt;a href=&quot;http://www.hq.nasa.gov/alsj/a17/ap17-S72-56081.jpg&quot;&gt;Picture of landing festivities&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I gather the suits and gloves are being continually improved for this sort of thing. For modern ISS crews they seem to have more nicely designed tools available, so they probably suffer less but it's still a problem as your links note.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional note: A study was done in 2012-14 based on incidents between 1981-2010. &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150003049.pdf&quot;&gt;&quot;Spacesuit Glove Induced Hand Trauma and Analysis of &#xA;Potentially Related Risk Variables &quot;&lt;/a&gt;. This is the most recent one I've been able to find. It doesn't explicitly mention total loss of nails but does describe a wide variety of complaints.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2015-09-24T12:36:54.893" LastActivityDate="2015-09-24T12:36:54.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12084" PostTypeId="2" ParentId="12079" CreationDate="2015-09-24T11:35:29.380" Score="0" Body="&lt;p&gt;Space is not a perfect vacuum. Within the solar system, you can expect about 100 particles/cm$^3$. This ranges from individual atoms to very small dust particles. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-24T11:35:29.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12085" PostTypeId="1" AcceptedAnswerId="12091" CreationDate="2015-09-24T13:06:25.480" Score="57" ViewCount="12425" Body="&lt;p&gt;I can think of a lot of good reasons for both arguments, and the Soviets made the opposite call for their lander, but what I'm interested in here is this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Given the mass penalty of another man (body weight, supplies, oxygen, propellant) and given the LM's extreme optimization for mass, why did NASA decide that two astronauts needed to land on the moon instead of one?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously this decision was made very early on in the LM's development.  NASA must have known what a weight penalty this would impose, and what a crunch this would put on the rest of the vehicle.  Was there just not enough confidence with the guidance systems at that time?  Was there a strong sense of &quot;you're going to need a buddy system for this to be safe&quot;?  Of course, it ended up being the right decision, as Buzz Aldrin came through during the LM's descent for Apollo 11. &lt;/p&gt;&#xA;" OwnerUserId="11694" LastEditorUserId="25" LastEditDate="2015-10-16T13:49:05.320" LastActivityDate="2015-10-16T13:49:05.320" Title="Why did NASA send two astronauts to the moon instead of one?" Tags="|the-moon|crewed-spaceflight|apollo-program|astronauts|lunar-landing|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12086" PostTypeId="2" ParentId="12085" CreationDate="2015-09-24T14:07:12.210" Score="46" Body="&lt;p&gt;EVA suits are very difficult for a single person to put on by themselves, so another person was required to help the other astronaut put on their EVA suit. At the very least, it is much easier to do it with help. A single person would have a very difficult time doing this. On the ground a team of people is usually employed to have this happen. Note there are things on the back of the suit that require connections, zippers, etc. Some of these are in very difficult locations to reach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, this is one of the major assumptions that Andy Weir made in The Martian, that a method of putting on an EVA suit by one's self has been found by the time we try to go to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some references to the difficulty in putting on an EVA suit:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.tested.com/science/space/456645-tested-nasa-how-astronauts-put-space-suits/&quot;&gt;Tested at NASA: How Astronauts Put on Space Suits&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/behindscenes/suit_yourself_prt.htm&quot;&gt;NASA: 'Suit Yourself' is Easier Said Than Done&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://youtu.be/lmUCsoK5x2M&quot;&gt;YouTube: Donning the Shuttle EMU space suit&lt;/a&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One might ask, why not wear the suit the entire time? Well, imagine being trapped inside a suit for over 24 hours (&lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_11i_Timeline.htm&quot;&gt;28 for Apollo 11&lt;/a&gt;, no doubt longer for others), and not being able to take it off at all, except for the helmet. It would get uncomfortable, influence bodily functions, etc. All in all, that is just too long to remain in a space suit if at all possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, there are other advantages to having two people on the surface, for redundancy and to allow more to be done. The systems were only semi-automatic, which means another person was another set of eyes to make things easier. Overall, it just made sense to have two people.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-25T11:38:00.927" LastActivityDate="2015-09-25T11:38:00.927" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12087" PostTypeId="1" AcceptedAnswerId="12088" CreationDate="2015-09-24T17:53:16.463" Score="4" ViewCount="198" Body="&lt;p&gt;I am wondering how much do we lose when launching farther from Earth's equator in the context of reaching Mars? Strict metric might be latitude vs weight on mars transfer orbit for some generic rocket (like Falcon 9 or Soyuz-2 e.t.c) or something like that. I would appreciate any links to works on this topic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am curious about this because not many spaceports are really close to equator, and Russian ones are especially far from it... &lt;/p&gt;&#xA;" OwnerUserId="11701" LastEditorUserId="7062" LastEditDate="2022-06-09T01:49:13.773" LastActivityDate="2022-06-09T01:49:13.773" Title="Graph of spaceport latitude vs Mars transfer orbit weight?" Tags="|launch|mars|launch-site|spaceport|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="12088" PostTypeId="2" ParentId="12087" CreationDate="2015-09-24T21:33:06.943" Score="5" Body="&lt;p&gt;For reference, the same rockets, but different launch sites, gives the following, at Earth escape velocity&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://web.archive.org/web/20130620195416/http://www.starsem.com/services/images/soyuz_users_manual_190401.pdf&quot; rel=&quot;nofollow&quot;&gt;Baikonur&lt;/a&gt;- 1600 kg&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.arianespace.com/launch-services-soyuz/Soyuz-Users-Manual-March-2012.pdf&quot; rel=&quot;nofollow&quot;&gt;French Guiana&lt;/a&gt;- 2200 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You asked about Mars specifically, so let's look at a &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;nofollow&quot;&gt;delta v table&lt;/a&gt;. That requires 600 m/s beyond escape velocity. According to the table, that's a small difference for either location. Thus, you can see that launching from French Guiana has a huge advantage vs Baikonur, lower latitudes give much more payload capacity, taking advantage of Earth's rotation.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="265" LastEditDate="2015-09-24T22:41:50.780" LastActivityDate="2015-09-24T22:41:50.780" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12089" PostTypeId="1" AcceptedAnswerId="12090" CreationDate="2015-09-25T05:57:17.297" Score="0" ViewCount="402" Body="&lt;p&gt;What would be the travel time from Earth orbit to Mars and Mars to Jupiter while accelerating at 3g and 5g (to a top speed of 1% light speed), braking halfway between each transit point?&lt;/p&gt;&#xA;" OwnerUserId="11671" LastEditorUserId="6816" LastEditDate="2015-12-04T19:44:16.110" LastActivityDate="2015-12-04T19:44:16.110" Title="Transit times to Mars and Jupiter" Tags="|mars|jupiter|nuclear-pulse-propulsion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12090" PostTypeId="2" ParentId="12089" CreationDate="2015-09-25T06:33:24.933" Score="8" Body="&lt;p&gt;3 g's is about 30 meters/sec^2 or about .03/km/sec^2. Light speed is about 300,000 km/s. .01 c is 3000 km/s. So it would take about 100,000 seconds to reach .01 c. That's about 28 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With constant acceleration along the same direction, distance traveled is .5at^2. this works out to about 150,000,000 km or about 1 A.U.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Distance between earth and Jupiter can range from 4.2 to 6.2 A.U. I'll go with 4.2 A.U. So if you cut off your rocket engines after accelerating for 1 A.U., you get to coast for 2.2 A.U. At .01 c, it takes 30 hours to travel this distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then the final leg you decelerate for 28 hours over which time you move 1 A.U.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time to accelerate is 28 hours, coasting time is 30 hours, deceleration time is 28 hours. A total of 86 hours, a little less than 4 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Minimum distance between Earth and Mars is about .5 A.U. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth to Mars would be about 28 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If my arithmetic's right.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-09-25T06:42:28.923" LastActivityDate="2015-09-25T06:42:28.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12091" PostTypeId="2" ParentId="12085" CreationDate="2015-09-25T08:11:04.987" Score="34" Body="&lt;p&gt;Even before Kennedy's &quot;we choose to go to the Moon&quot; speech, &lt;a href=&quot;http://www.jsaxon.org/space/book/Apollo.htm&quot; rel=&quot;noreferrer&quot;&gt;NASA was working on concepts&lt;/a&gt; for Moon missions. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Dr. Robert Gilruth, Director of the Manned Spacecraft Center: &quot;Even before the President's decision to land on the moon, we had been working on designs and guidelines for a manned circumlunar mission. This was done in a series of bull sessions on how we would design the spaceship for this purpose if the opportunity occurred. Our key people would get together evenings, weekends, or whenever we could to discuss such questions as crew size and other fundamental design factors. We believed that we would need three men on the trip to do all the work required, even before the complexity of the landing was added.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://books.google.nl/books?id=OGEhBQAAQBAJ&amp;amp;pg=PA9&amp;amp;lpg=PA9&amp;amp;dq=apollo%20Lunar%20module%20%22crew%20size%22&amp;amp;source=bl&amp;amp;ots=bRbh73WSp3&amp;amp;sig=M-6--Yia6UEZ3Wh8wrvj_y0kf2o&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CKkBEOgBMBpqFQoTCNre-rLdkcgCFUi-FAodXngD6g#v=onepage&amp;amp;q=apollo%20Lunar%20module%20%22crew%20size%22&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;One of the reasons a crew of 3 was chosen&lt;/a&gt;: with 3 men, you can have a duty roster of 4 hours on, 8 hours off and have one person on duty at all times. This was seen as a requirement for longer missions. In practice, the crew preferred to synchronize their schedules, leaving the spacecraft to be monitored by ground control at times when the crew was asleep. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With 3 crew members available, it made sense to get &lt;a href=&quot;https://space.stackexchange.com/questions/1302/why-did-apollo-11-need-someone-to-stay-behind-in-lunar-orbit&quot;&gt;as many of them on the surface of the Moon&lt;/a&gt; as possible. More boots on the ground meant more science could be done. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://books.google.nl/books?id=OGEhBQAAQBAJ&amp;amp;pg=PA9&amp;amp;lpg=PA9&amp;amp;dq=apollo%20Lunar%20module%20%22crew%20size%22&amp;amp;source=bl&amp;amp;ots=bRbh73WSp3&amp;amp;sig=M-6--Yia6UEZ3Wh8wrvj_y0kf2o&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CKkBEOgBMBpqFQoTCNre-rLdkcgCFUi-FAodXngD6g#v=onepage&amp;amp;q=apollo%20Lunar%20module%20%22crew%20size%22&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;This book&lt;/a&gt; also confirms the safety aspect of having two crewmembers on the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA had a &lt;a href=&quot;https://books.google.nl/books?id=qE0rAAAAYAAJ&amp;amp;pg=PA220&amp;amp;lpg=PA220&amp;amp;dq=apollo%20Lunar%20module%20%22crew%20size%22&amp;amp;source=bl&amp;amp;ots=e1jo_KcC_W&amp;amp;sig=Nlrc8vQ8QX2EsWEUawTHyxDy1Ms&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CI8BEOgBMBVqFQoTCNre-rLdkcgCFUi-FAodXngD6g#v=onepage&amp;amp;q=apollo%20Lunar%20module%20%22crew%20size%22&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;culture of relying heavily on crewmembers&lt;/a&gt;. In contrast, the Soviets tried to automate their spaceships to a greater extent (and were able to design for a smaller crew as a result).&lt;br&gt;&#xA;This culture came about when the earliest group of astronauts was chosen from the test pilot community; their preference for a hands-on approach influenced the entire space program. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-09-25T08:30:55.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12093" PostTypeId="1" AcceptedAnswerId="12095" CreationDate="2015-09-25T20:14:40.890" Score="47" ViewCount="16458" Body="&lt;p&gt;Let's say we're at T+0, just as the rocket starts to move, what keeps it upright as it clears the tower and gains speed?&lt;/p&gt;&#xA;" OwnerUserId="11720" LastActivityDate="2021-06-30T23:31:59.913" Title="How is a rocket stabilized during the initial, slow speed, portion of launch?" Tags="|launch|attitude|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12094" PostTypeId="2" ParentId="12093" CreationDate="2015-09-25T20:21:55.323" Score="13" Body="&lt;p&gt;Most rockets &lt;a href=&quot;http://exploration.grc.nasa.gov/education/rocket/gimbaled.html&quot;&gt;gimbal their engines&lt;/a&gt; actively to maintain stability. Shifting the axis of thrust slightly works just fine to keep it upright.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2015-09-25T20:21:55.323" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12095" PostTypeId="2" ParentId="12093" CreationDate="2015-09-25T21:30:33.057" Score="39" Body="&lt;p&gt;You can recreate the problem by placing a pencil point-first on your finger. Try to keep the pencil/rocket upright by moving your hand back and forth. If you managed it for more than a few seconds, congratulations! You're doing better than &lt;a href=&quot;https://www.youtube.com/watch?v=vqW0LEcTAYg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Proton 535-43&lt;/a&gt; did.&lt;/p&gt;&#xA;&lt;p&gt;In the very early stages of flight (before aerodynamics has any major effect)  the rocket can be described as an &lt;a href=&quot;https://en.wikipedia.org/wiki/Inverted_pendulum&quot; rel=&quot;nofollow noreferrer&quot;&gt;inverted pendulum&lt;/a&gt;, just like the pencil. In order to keep itself stable the rocket must ensure that the thrust vector from its engines passes directly through its center of gravity.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bHPi3.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bHPi3.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;From &lt;a href=&quot;http://exploration.grc.nasa.gov/education/rocket/gimbaled.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://exploration.grc.nasa.gov/education/rocket/gimbaled.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Most modern rockets gimbal their engines to direct the thrust, but it's not the only way to achieve &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust_vectoring&quot; rel=&quot;nofollow noreferrer&quot;&gt;thrust vectoring&lt;/a&gt;. Here are a few more:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/VYlTH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VYlTH.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/9kcrC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9kcrC.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;See &lt;a href=&quot;https://pritamashutosh.wordpress.com/2014/03/04/staging-and-control-of-rockets-and-missiles/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt; for more info. (Note: Parts of the site seem to be identical to &lt;a href=&quot;http://www.ewp.rpi.edu/hartford/%7Eernesto/S2013/EP/MaterialsforStudents/Lee/Sutton-Biblarz-Rocket_Propulsion_Elements.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Propulsion Elements, George P. Sutton, Oscar Biblarz&lt;/a&gt;.)&lt;/p&gt;&#xA;&lt;p&gt;One of the simplest ways of solving the inverted pendulum problem is with a proportional-integral-derivative controller (PID controller). I'll let &lt;a href=&quot;https://en.wikipedia.org/wiki/PID_controller&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia explain&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A proportional-integral-derivative controller (PID controller) is a&#xA;control loop feedback mechanism (controller) commonly used in&#xA;industrial control systems. A PID controller continuously calculates&#xA;an &amp;quot;error value&amp;quot; as the difference between a measured process variable&#xA;and a desired setpoint. The controller attempts to minimize the error&#xA;over time by adjustment of a control variable, such as the position of&#xA;a control valve, a damper, or the power supplied to a heating element,&#xA;to a new value determined by a weighted sum:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$u(t) = K_pe(t) + K_i\int^t_0e(\tau)d\tau+K_d\frac{de}{dt}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where &lt;span class=&quot;math-container&quot;&gt;$K_p$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$K_i$&lt;/span&gt;, and &lt;span class=&quot;math-container&quot;&gt;$K_d$&lt;/span&gt;, all non-negative, denote the coefficients for the proportional,&#xA;integral, and derivative terms, respectively (sometimes denoted &lt;span class=&quot;math-container&quot;&gt;$P$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$I$&lt;/span&gt;,&#xA;and &lt;span class=&quot;math-container&quot;&gt;$D$&lt;/span&gt;).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Control Solutions, Inc. has a very good (and quite easy to follow) &lt;a href=&quot;http://www.csimn.com/CSI_pages/PIDforDummies.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;explanation on their website&lt;/a&gt; of the bare basics of a PID controller.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2021-06-30T20:04:47.733" LastActivityDate="2021-06-30T20:04:47.733" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="12096" PostTypeId="2" ParentId="12093" CreationDate="2015-09-25T21:35:39.653" Score="17" Body="&lt;p&gt;The other answers here are correct: gimbaling or other active correction measures are used.&lt;/p&gt;&#xA;&lt;p&gt;While most launchers do try and maintain a vertical flight off the pad, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Antares_%28rocket%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Antares&lt;/a&gt; rocket is known for the intentional &amp;quot;Baumgartner Maneuver&amp;quot; it does on takeoff, deliberately gimbaling the engine to maneuver away from the tower in the first seconds of flight, &lt;a href=&quot;https://www.youtube.com/watch?v=K82dosr0L7s&quot; rel=&quot;nofollow noreferrer&quot;&gt;as you can see here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-06-30T23:31:59.913" LastActivityDate="2021-06-30T23:31:59.913" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="12098" PostTypeId="1" CreationDate="2015-09-26T12:35:07.670" Score="12" ViewCount="587" Body="&lt;p&gt;The Mars 2020 rover will drill up real cores decimeter or so deep, in order for them to be returned to Earth by a follow up mission. I hear NASA people now seriously working on having that rover drop off those cores here and there along its route between the sites it will investigate. Whatever could the advantages be of doing it that way?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it be simpler and safer, for both Mars 2020 and for the sample return mission, to store all the samples in a single container which a small fast collecting rover dispatched from a later landed ascent vehicle could go and get, even if the Mars 2020 rover it hangs from is dead by then? Spreading out the samples along an 11 km or more long route, which is what Curiosity has been traveling since three years (I suppose the follow up mission would arrive two conjunctions later). And wouldn't it require a container for each drill core, instead of one for all?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-09-27T12:08:26.870" Title="Why should the Mars 2020 rover drop drilled cores along its route to be picked up later?" Tags="|mars|rovers|sample-return|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12099" PostTypeId="2" ParentId="12098" CreationDate="2015-09-26T14:14:27.017" Score="8" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: on second thought, my initial answer wasn't very good. Here's another go with &lt;a href=&quot;http://www.planetary.org/blogs/casey-dreier/2015/0723-a-new-way-to-prepare-samples-of-mars.html&quot; rel=&quot;noreferrer&quot;&gt;better sources&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;For the Mars 2020 rover, one of the science goals is to collect 20 samples. The rover has room to collect 31 samples.&lt;/p&gt;&#xA;&lt;p&gt;In the initial design, the rover was to carry all its samples on board throughout the mission. But then once the rover has collected 20 samples, the question becomes, what to do with them?&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If you keep going to collect more samples, there's a risk the rover will break down somewhere inaccessible, and you won't be able to retrieve the 20 samples.&lt;/li&gt;&#xA;&lt;li&gt;If you end the mission at 20 samples, you might miss out on valuable information.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;So JPL has come up with a plan where the first 20 samples will be dropped off somewhere accessible, leaving the rover free to go on an extended mission without endangering the 20 samples.&lt;/p&gt;&#xA;&lt;p&gt;The sample return mission is not being designed in detail yet. For now, the number of samples to be returned is set at 31. But it's possible for the rover to carry more than 31 sample tubes, giving the scientists more choice: if sample 32 turns out to be really interesting, they can ditch one of the older samples. Current thinking is that the rover will carry &lt;a href=&quot;http://marsnext.jpl.nasa.gov/documents/Mars_2020_Caching-Strategy_V4.pdf&quot; rel=&quot;noreferrer&quot;&gt;40-50 sample tubes&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;With the old-style cache bucket, you hit an interesting problem once you reach your minimum success goal of 20 samples. By definition: the mission is now a “success.” You have a treasure trove of samples that you could set on the ground for future collection, but you could also fill it with 11 more samples of indeterminate importance. You don’t know what or how important these samples could be, since you’d have to continue looking for interesting rocks to drill. But every meter you rove carries with it risk: risk of breaking down, risk of getting stuck in sand, risk of a computer malfunction, you name it. Despite JPL making it look easy to operate a rover on Mars, it’s not.&lt;/p&gt;&#xA;&lt;p&gt;So there would be an enormous amount of pressure to drop the cache container once minimum success is reached. Do you cash out your winnings or let it ride? Even if you kept going, the levels of risk that the team would accept for roving to interesting places would be very low. This risk-aversion would increase with every sample collected beyond the minimum success point.&lt;/p&gt;&#xA;&lt;p&gt;With the adaptive cache (or cache depot) strategy, you don’t have the same problem. You just tell the rover to drop the 20 samples on the surface, effectively creating a cache depot at that spot on Mars (the rover can carry a certain number of sample tubes with it while it drives). The rover then continues on to other interesting places where it can drill additional samples, safe in the knowledge that it has already deposited a successful mission’s worth for future retrieval.&lt;/p&gt;&#xA;&lt;p&gt;There are additional practical benefits for the cache depot strategy. Since Mars 2020 would no longer be limited by the size of a cache bucket designed to return every sample to Earth, the rover could bring along more sample tubes to use throughout the mission. And since the samples are deposited frequently, which frees the rover to go on in search of more science, the team would have more time in which to debate which samples to return. Without this, the team would have to decide far more quickly about where to drill with less contextual information about the overall geology of the landing site.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This design isn't final yet, JPL are still working on the design concept. But &lt;a href=&quot;http://www.hou.usra.edu/meetings/lpsc2015/pdf/1672.pdf&quot; rel=&quot;noreferrer&quot;&gt;several cache strategies&lt;/a&gt; have been studied, with adaptive caching turning out to be the &lt;a href=&quot;http://marsnext.jpl.nasa.gov/documents/Mars_2020_Caching-Strategy_V4.pdf&quot; rel=&quot;noreferrer&quot;&gt;preferable strategy&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-09-27T12:08:26.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12101" PostTypeId="2" ParentId="6089" CreationDate="2015-09-26T19:32:57.260" Score="3" Body="&lt;p&gt;Yes, SPICE is your best bet.  The learning curve is steep but worth it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can start at the &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/index.html&quot; rel=&quot;nofollow&quot;&gt;SPICE Required Reading&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and look at the PCK, FK and SPK required reading files.  That will lead you to the other required reading files.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You also will need this &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/ROSETTA/kernels/fk/ROS_CGS_AUX_V01.TF&quot; rel=&quot;nofollow&quot;&gt;Frames Kernel (FK)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Caveats:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is not much guidance, and assumes you are a self-starter, but this is not the place for a tutorial, which will only lead you back to the NAIF/SPICE web site anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is the C version of that toolkit; versions also exist for FORTRAN, IDL, Matlab (and Octave), Java and Python.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm busy but I could gin up a quick example for you if you describe your environment in more detail (what does the display look like and/or emulate; what does the camera look like, where are you getting your shape model from).&lt;/p&gt;&#xA;" OwnerUserId="11619" LastActivityDate="2015-09-26T19:32:57.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12104" PostTypeId="1" CreationDate="2015-09-26T19:49:01.637" Score="6" ViewCount="220" Body="&lt;p&gt;On the ISS, &lt;a href=&quot;https://space.stackexchange.com/questions/312/what-time-zone-is-used-on-iss-to-define-a-day&quot;&gt;the timezone used is UTC&lt;/a&gt;. Thus it is reasonnable to assume that all nations implied in the ISS program use UTC for all their space programs to ease ground crew's work while they may work on several space programs. But what about the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_government_space_agencies&quot; rel=&quot;nofollow noreferrer&quot;&gt;other nations with space program&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;: When asking for other nation with space program, I was expecting answer including nation from the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_government_space_agencies&quot; rel=&quot;nofollow noreferrer&quot;&gt;linked wikipedia list&lt;/a&gt; &lt;strong&gt;not&lt;/strong&gt; involved in the ISS space program (e.g. India, North Corea, ...)&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-10-28T08:37:23.797" Title="Is UTC used by all nations for their space programs?" Tags="|space-program|international-politics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12105" PostTypeId="2" ParentId="12104" CreationDate="2015-09-27T05:46:46.247" Score="4" Body="&lt;p&gt;There are two different issues here- what time zone do the people in space use and what time zone the mission control center uses. The crew uses the &lt;em&gt;elapsed time&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote &lt;a href=&quot;https://en.wikipedia.org/wiki/Paul_Lockhart&quot; rel=&quot;nofollow&quot;&gt;Paul Lockhart&lt;/a&gt;'s answer to a &lt;a href=&quot;http://www.spaceflight.nasa.gov/feedback/expert/answer/crew/sts-111/index.html&quot; rel=&quot;nofollow&quot;&gt;similar question&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;....On the shuttle we use mission elapsed time. This is kept by the computers as soon as launch is initiated. And once we are on orbit, we have our own watches that we then set to the computer’s time so that we each are keeping track of our duties according to the mission elapsed time. When we docked to the station, they used Greenwich Mean Time, GMT,  .... on the shuttle we use mission elapsed time, and on the station, they use Greenwich Mean Time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Apparently, almost all of the space industry uses UTC. As ESA explains in a &lt;a href=&quot;http://blogs.esa.int/mex/2012/08/05/what-time-is-it/&quot; rel=&quot;nofollow&quot;&gt;blog&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;....the space industry (and many other organisations facing similar issues) use a standard time zone called UTC – Coordinated Universal Time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Mir used Moscow Standard Time though.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-09-27T05:46:46.247" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12107" PostTypeId="1" CreationDate="2015-09-27T16:44:51.377" Score="5" ViewCount="2317" Body="&lt;p&gt;I'm trying to write code to go back and forth between equatorial KOE (Keplerian Orbital Elements) and ECEI (Cartesian).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ECEI&amp;rarr;KOE I found here: &lt;a href=&quot;https://space.stackexchange.com/questions/1904/how-to-programmatically-calculate-orbital-elements-using-position-velocity-vecto&quot;&gt;How to programmatically calculate orbital elements using position/velocity vectors?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;KOE&amp;rarr;ECEI I found on the Wikipedia page for the Orbital Elements which has the equations for x,y of the ellipse which can then be passed through a DCM transform from the orbital plane to the ECEI system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this is all well-and-good for a spherical earth. However, the purpose of this is to replace my RK4 integration which takes WGS84 into account with KOE parameters for performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd greatly appreciate any pointers leading to a method to account for a non-spherical earth.&lt;/p&gt;&#xA;" OwnerUserId="11740" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2020-12-16T14:07:10.723" Title="Keplerian Orbital Elements ↔ Cartesian ECEI with WGS84 Corrections" Tags="|orbital-elements|mathematics|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12110" PostTypeId="2" ParentId="6316" CreationDate="2015-09-27T22:24:54.577" Score="2" Body="&lt;p&gt;The ideal Lunar Space Elevator and the ideal Earth Space Elevator actually have the ends of their elevators amazingly close, near-Earth astronomically speaking. The centrifugal force at the end of an Earth Elevator and the gravitational force at the end of a Lunar elevator actually makes the transition between the two points have an effective near-zero energy cost. So although we &lt;em&gt;might&lt;/em&gt; be able to put the Lunar Elevator under the additional strain necessary to get it to LEO, it makes more sense to just, after there's a Lunar elevator, to take advantage of pico-gravity station at L1 and precision-create nanotubes for the Earth elevator.&lt;/p&gt;&#xA;" OwnerUserId="11747" LastActivityDate="2015-09-27T22:24:54.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12112" PostTypeId="2" ParentId="2699" CreationDate="2015-09-27T23:12:31.143" Score="0" Body="&lt;p&gt;A space elevator is an tether with climber that connects a planet or other astronomical body (such as the moon) past a balance point (such as a Lagrange point or geosynchronous orbit) to allow for &quot;landing&quot; and &quot;takeoff&quot; of payloads. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although an Earth elevator is largely hypothetical at this point (being dependent on carbon nanotubes, boron nanotubes, or graphene being able to be manufactured to appropriate lengths), a lunar elevator doesn't have as extreme of a material requirement, and vectran string is strong enough to meet its requirements, and so a lunar elevator's issues aren't hypothetical problems so much as practical problems (where to get the funding to build it, specifically.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Lunar elevator has the distinct advantage, also, that it may turn the Earth Elevator from a hypothetical problem into a practical problem as well. The reason being is that a waystation located at the L1 Lagrange point of the Lunar elevator would be able to sustain a pico-gravity facility, which would make precision creation of CNTs much more feasible, allowing creation of the necessary parts of for an Earth Space Elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And space elevators are actually planned. Japan is planning an Earth elevator by 2050, and LiftPort in the U.S. is aiming for a Lunar one by 2020 (although funding problems may prevent the latter.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have also been hints that Russia, EU, and China may pursuing space elevator concepts (the EU poured a lot of money into graphene research referencing both computing and aerospace tech) and Russia has been investing a lot into space development recently, supposedly looking at colonization, and this started shortly after papers on the feasibility of a Lunar Elevator were publicized. China has also declared an interest in setting foot on the Moon, and due to the high expense of shooting up enough fuel for retrograde burn and return, a Lunar elevator would be the cheaper route to pursue that goal. Although it has yet to be formally announced in the case of all three, speculation is that they will turn to a lunar elevator concept due to its feasibility and lower cost and benefit to their other goals.&lt;/p&gt;&#xA;" OwnerUserId="11747" LastActivityDate="2015-09-27T23:12:31.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12113" PostTypeId="2" ParentId="5193" CreationDate="2015-09-27T23:18:49.387" Score="1" Body="&lt;p&gt;It should also be pointed out that L1 Lagrange points are exceedingly useful for a variety of testing and manufacturing purposes due to pico-gravity. Any elevator constructed will likely have a station at this location anyway, with two ribbons tethered to the location instead of just one passing through. And the ribbons don't need to be simply tied, that can easily be on a spool that can shorten and length the ribbon as needed with the tidally locked bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, even with shortening and lengthening distances, its' very feasible, however it won't be a static structure, but an active one.&lt;/p&gt;&#xA;" OwnerUserId="11747" LastActivityDate="2015-09-27T23:18:49.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12114" PostTypeId="1" CreationDate="2015-09-28T01:00:58.863" Score="19" ViewCount="2567" Body="&lt;p&gt;In the video &lt;a href=&quot;https://vimeo.com/55073825&quot; rel=&quot;noreferrer&quot;&gt;Overview&lt;/a&gt; 5 astronauts talk about having powerful spiritual experiences in space, and their descriptions match &lt;a href=&quot;https://en.wikipedia.org/wiki/Enlightenment_(spiritual)&quot; rel=&quot;noreferrer&quot;&gt;enlightenment&lt;/a&gt; experiences. This strikes me as rather more important than you'd think based on how often it is discussed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gene Cernan speaking in the documentary In the Shadow of the Moon:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I felt that the world was just too beautiful to have happened by&#xA;  accident. There has to be something bigger than you and bigger than&#xA;  me... And I mean this in a spiritual sense, not a religious sense...&#xA;  There has to be a creator of the universe who stands above the&#xA;  religions that we ourselves create to govern our lives.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Anousheh Ansari&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The actual experience exceeds all expectations and is something that's&#xA;  hard to put to words… It sort of reduces things to a size that you&#xA;  think everything is manageable.... All these things that may seem big&#xA;  and impossible ... We can do this. Peace on Earth – No problem. It&#xA;  gives people that type of energy ... that type of power, and I have&#xA;  experienced that.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The above quote is one of 7 such accounts listed on &lt;a href=&quot;http://www.starportcafe.com/overview-effect/astronaut-accounts&quot; rel=&quot;noreferrer&quot;&gt;this page&lt;/a&gt; of the Starport Cafe. One of them is by Edgar Mitchell, who described the experience in some detail in Overview:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/CHMIfOecrlo?start=270&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/CHMIfOecrlo?start=646&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other 4 astronauts in Overview were Ron Garan, Nicole Stott, Jeff Hoffman, and Shane Kimbrough. Nicole Stott goes so far near the end of the film as to say everybody who goes to space is changed by it, and puts in terms that, again, to a Buddhist, Taoist, or Hindu would constitute some form of enlightenment experience - an experience of being overwhelmed with awe, of feeling blessed, and connected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quote from David Beaver in the video:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As you go into your mind in a contemplative way, the sense of the&#xA;  living reality of the planet becomes obvious. You become more in tune&#xA;  with the natural world. This is very akin to the direct perception&#xA;  that the astronauts have, so it is no wonder that many people have&#xA;  likened the overview effect to a spiritual or meditative experience&#xA;  although it's not exactly that, it's a cognitive shift that very often&#xA;  can produce a kind of meditative experience.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So far the count is 12 astronauts. Or 9 astronauts, one cosmonaut, and two space tourists, if you prefer. Anyone who has been in space counts here. That is already a substantial percentage of all people who have been there so far. Can more of them be added to this tally? &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-09-29T16:40:33.320" LastActivityDate="2021-08-19T05:04:15.837" Title="How many astronauts have had spiritual experiences while in space?" Tags="|astronauts|health|psychology|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12115" PostTypeId="1" AcceptedAnswerId="12120" CreationDate="2015-09-28T03:05:24.393" Score="8" ViewCount="1663" Body="&lt;p&gt;I read a question on Quora that seemed too simple to not find an answer through Google. Alas, it seems RP-1 is rare enough that there isn't more information than &quot;vendors that sell to NASA&quot; for who supplies RP-1 for rocket launches. I would imagine its Shell or Exxon, but it doesn't say. Since its a hydrocarbon, I would imagine its a private company, and that its not refined on-site at the spaceport, but I may be wrong. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So which company sells RP-1 to companies that launch rockets?&lt;/p&gt;&#xA;" OwnerUserId="11377" LastActivityDate="2015-09-28T19:54:21.347" Title="Who sells RP-1 to SpaceX and other commercial launch companies?" Tags="|launch|rockets|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12116" PostTypeId="1" CreationDate="2015-09-28T03:18:33.577" Score="13" ViewCount="1456" Body="&lt;p&gt;There's a supermoon lunar eclipse ongoing right now (Sept. 27/28, 2015), and it got me thinking that a great way to show to people why the Moon is reddish -- when eclipsed by the Earth -- would be if we have a color photograph taken from a spacecraft in the general vicinity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such a photo could be taken from the Moon while the Moon is being eclipsed by the Earth (which just takes some careful or lucky timing), or from any point in space opposite the Earth (between the Earth and the Earth-Sun L2 point) where the umbra of the Earth still exists. I do know that at the L2 point the Earth is &lt;a href=&quot;https://space.stackexchange.com/a/10360&quot;&gt;not large enough&lt;/a&gt; to cover the Sun, so L2 is too far out, and we also need to get far enough from the Earth to be close to the distance of the Moon's orbit in order to capture the atmospheric ring of dim refracted red light. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think if we got lucky with the timing of any of the manned lunar missions, somebody would have thought to photograph a lunar eclipse event, but it feels like if this happened, I would already have seen the photograph by now. Since it seems like we have no Apollo pictures, all I can hope for are pictures from spacecraft flybys. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, this would be a fantastic thing to work into the planning for the next manned lunar missions, I'm looking at you NASA. :)&lt;/p&gt;&#xA;" OwnerUserId="11750" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2020-11-29T03:42:32.783" Title="Have there been any photos taken of a total Earth-Sun eclipse from the Moon, or its vicinity?" Tags="|the-moon|lunar-landing|photography|celestial-mechanics|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12117" PostTypeId="2" ParentId="12116" CreationDate="2015-09-28T03:52:29.950" Score="16" Body="&lt;p&gt;Yes, well, kind of; JAXA's Kaguya (&lt;a href=&quot;https://en.wikipedia.org/wiki/SELENE&quot; rel=&quot;noreferrer&quot;&gt;SELENE&lt;/a&gt;) took images of the Earth during the February 10, 2009 penumbral lunar eclipse from lunar orbit of roughly 50 km altitude, using its HDTV camera:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ht5NF.jpg&quot; alt=&quot;Japan&amp;#39;s Kaguya probe caught this sight of Earth&amp;#39;s diamond ring as the planet passed in front of the sun as seen from the lunar orbit.&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Japan's Kaguya probe caught this sight of Earth's diamond ring as the planet passed in front of the sun as seen from the lunar orbit. The ring appears incomplete at the bottom, where the night-darkened limb of the moon obscures its view. Credit: JAXA/NHK. Source: &lt;a href=&quot;http://www.space.com/3747-eclipse-moon-probe-earth-blocks-sun.html&quot; rel=&quot;noreferrer&quot;&gt;Space.com&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read a bit more in this Space.com article titled &lt;a href=&quot;http://www.space.com/3747-eclipse-moon-probe-earth-blocks-sun.html&quot; rel=&quot;noreferrer&quot;&gt;Eclipse Seen by Moon Probe as Earth Blocks Sun&lt;/a&gt;. And here's JAXA's video of the event as seen by the Kaguya probe:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ButJ9hqdMq4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm afraid that since this was a penumbral eclipse, and the Moon wasn't in Earth's umbra but in it's penumbral cone surrounding it, it doesn't help your cause of finding images of it showing &lt;a href=&quot;https://en.wikipedia.org/wiki/Rayleigh_scattering&quot; rel=&quot;noreferrer&quot;&gt;Rayleigh scattering&lt;/a&gt; effect of the Earth's atmosphere and why the Moon appears red during umbral lunar eclipses. But, luckily, NASA Goddard prepared for us some animations explaining this effect in more detail:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/lNi5UFpales?start=72&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read a bit more on this topic in &lt;a href=&quot;https://space.stackexchange.com/q/5561/49&quot;&gt;What would a lunar eclipse look like from the surface of the moon?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-09-28T04:21:58.810" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12118" PostTypeId="2" ParentId="182" CreationDate="2015-09-28T04:34:24.277" Score="3" Body="&lt;p&gt;&quot;Made In Space&quot; is the company that built the 3D printer currently on board the ISS. I recently heard a talk from their co-founder at a local commercial spaceflight conference, and he mentioned that they were researching and testing how to do exactly that (though still in the ground test phase). Here's a short video that confirms this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/lp9uIsxmT6k?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6306" LastActivityDate="2015-09-28T04:34:24.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12120" PostTypeId="2" ParentId="12115" CreationDate="2015-09-28T08:50:40.657" Score="6" Body="&lt;p&gt;Part of the problem is that RP-1 is a very generic abbreviation. When you add the actual standard that RP-1 must comply with (&lt;a href=&quot;http://propellants.ksc.nasa.gov/commodities/specs/MIL_DTL_25576D.pdf&quot; rel=&quot;nofollow&quot;&gt;MIL_DTL_25576&lt;/a&gt;), the search results get more specific.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are the first two companies I found that sell RP-1:   &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.shell.com/content/dam/shell/static/aviation/downloads/AeroShell-Book/aeroshell-book-10busspecs.pdf&quot; rel=&quot;nofollow&quot;&gt;Shell&lt;/a&gt;  &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.haltermannsolutions.com/fueltypes/aerospace&quot; rel=&quot;nofollow&quot;&gt;Haltermann solutions&lt;/a&gt;  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(there are probably more, but you get the general idea now) &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-09-28T19:54:21.347" LastActivityDate="2015-09-28T19:54:21.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12121" PostTypeId="2" ParentId="12085" CreationDate="2015-09-28T10:03:30.230" Score="6" Body="&lt;p&gt;I cannot comment, but I just wanted to share some information (as I just wrote a paper on Project Apollo). First, there is an excellent resource available from NASA called &lt;em&gt;&lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4205/cover.html&quot;&gt;Chariots for Apollo&lt;/a&gt;&lt;/em&gt;. The chapter relevant to the question is Chapter 1- &lt;em&gt;Concept to Challenge&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hobbes' quote is correct, historically, that studies had been undertaken by NACA since the mid 1950s. Pearson is also partially correct that NASA wanted the workload split (for redundancy and other reasons). Project Mercury and Gemini were both adapted to the needs of Apollo. However, another great resource, though not official as per the page, is &lt;em&gt;&lt;a href=&quot;http://history.nasa.gov/ap11-35ann/AMSVPguidelines/contents.htm&quot;&gt;Guidelines for Advanced Manned Space Vehicle Program&lt;/a&gt;&lt;/em&gt;. You will want to pay special attention paper written by Stanley C. White &lt;em&gt;Human-Factors Considerations&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA decided on two astronauts for many reasons, with only some given by Hobbes and Pearson. Hopefully, the provided resources will help explain some other reasons aside from the work and abilities of the astronauts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Personally, and without justification, I feel NASA chose multiple astronauts because they did not understand how extended and stressful missions in space might affect a human. I believe this is somewhat shown by the following example. NASA quarantined all Apollo astronauts for 21 days, up until they realized there was no chance for them to bring back anything dangerous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also feel their answers ignore the political ramifications behind Apollo (NASA was pressured to meet the decade goal), but that is a topic for another day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In response to the mass penalty, from the provided resources you can see that NACA and subsequently NASA originally prepared for a mission with far more available resources. Mass in the preparatory stages was not considered to be an issue. The Goett Committee fully expected a two week long mission (against the 8 day Apollo 11 mission). &lt;/p&gt;&#xA;" OwnerUserId="11641" LastActivityDate="2015-09-28T10:03:30.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12122" PostTypeId="2" ParentId="182" CreationDate="2015-09-28T10:23:25.687" Score="2" Body="&lt;p&gt;I cannot respond to other responses, otherwise I would.... I just want to add to the previous posts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are actually quite a few papers written on the subject.  ASCE hosts an annual Earth and Space conference. The paper I am referring to is from 2012 (although I can't find it at the moment). 3d printing is indeed considered a viable suggestion for manufacturing in space, however, it is not considered viable for settlement construction.  The main reason for this is radiation. Unfortunately, there are currently only a few ways to avoid the extreme radiation of a settlement on other celestial bodies. Polametric shielding is one answer, while another is the submersion (or burying) of the colony. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The machines themselves would only have to worry about power and the solar wind (the charged particles cause problems in the systems). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply having a settlement on the Moon or Mars, on the surface and for permanent or extended residence, would subject the inhabitants to extraordinary doses of radiation. The best solution so far is to dig. The technology behind 3d manufacturing can definitely build settlements, but they still need to overcome radiation issues for the inhabitants. &lt;/p&gt;&#xA;" OwnerUserId="11641" LastActivityDate="2015-09-28T10:23:25.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12123" PostTypeId="2" ParentId="10574" CreationDate="2015-09-28T11:02:22.553" Score="1" Body="&lt;p&gt;Arianespace offers secondary payload space on both Ariane 5 and Vega. Their earliest 'microsatellite' launch was using the ASAP structure on an Ariane 40 on January 22, 1990. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Ariane 5 can launch 2 large satellites using the SYLDA (SYstème de Lancement Double Ariane) fairing. &lt;/li&gt;&#xA;&lt;li&gt;Also for the Ariane 5, the &lt;a href=&quot;http://www.brown.edu/Departments/Engineering/Courses/en176/2003%20Lectures/Meeting%203/ASAP5-manual.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane Structure for Auxiliary Payloads&lt;/a&gt; (ASAP) is a ring at the base of the payload stack which contains 4 or 8 positions for small satellites. These can be up to 600 mm x 600 mm x 710 mm and a weight of up to 120 kg each. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is an ASAP with components of the &lt;a href=&quot;http://www.arianespace.com/news-mission-update/2006/333.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;LDREX-2 experiment&lt;/a&gt;. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/2SiHR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2SiHR.jpg&quot; alt=&quot;ASAP&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;For VEGA, the VEga Secondary Payload Adapter (&lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Deploying_multiple_satellites_with_Sylda_and_Vespa/%28print%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;VESPA&lt;/a&gt;) can hold &lt;a href=&quot;http://www.arianespace.com/launch-services-vega/Vega-Users-Manual_Issue-04_April-2014.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;2 payloads of up to 400 kg each/600 kg total&lt;/a&gt; underneath the main payload. &lt;/li&gt;&#xA;&lt;li&gt;for VEGA, development of an &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Deploying_multiple_satellites_with_Sylda_and_Vespa/%28print%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;adapter for smaller payloads&lt;/a&gt; was underway in 2013.&lt;/li&gt;&#xA;&lt;li&gt;VEGA flight VV01 &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2947&amp;amp;context=smallsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;carried a &quot;plate dispenser&quot; for 7 cubesats and a microsatellite&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As for cost, a launch on an Ariane 4 using ASAP is &lt;a href=&quot;http://www.capcomespace.net/dossiers/espace_europeen/ariane/ariane4/caracteristiques.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;reported to cost around \$1.2 M&lt;/a&gt; (1990-2000 timeframe).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, comparing with ULA:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Arianespace's 1990 first launch predates the ULA effort (and ULA itself) significantly.    &lt;/li&gt;&#xA;&lt;li&gt;the range of payloads is similar.  &lt;/li&gt;&#xA;&lt;li&gt;I haven't found a cost for ULA secondary payloads.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-09-28T16:36:16.787" LastActivityDate="2015-09-28T16:36:16.787" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12125" PostTypeId="1" AcceptedAnswerId="12126" CreationDate="2015-09-28T02:07:48.580" Score="12" ViewCount="4220" Body="&lt;p&gt;It seems to me that weight and size would be important in choosing spacefarers for many reasons; I came across this &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronaut_training&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; where the Chinese astronauts (taikonauts) have to be between 50 and 70 kg, which makes sense to me, but other countries don't seem to stipulate a size requirement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would think that it would be easier and cheaper to get the person into space if they weigh less, and could survive on less food for a given time frame so you also save on the provisions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cramped conditions would be less of a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All else being equal, is there any reason why sending bigger people into space would be an advantage?&lt;/p&gt;&#xA;" OwnerUserId="11757" OwnerDisplayName="Kilisi" LastEditorUserId="49" LastEditDate="2015-09-28T18:39:22.563" LastActivityDate="2015-09-28T22:11:25.250" Title="How important is the size of an astronaut?" Tags="|astronauts|medical|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12126" PostTypeId="2" ParentId="12125" CreationDate="2015-09-28T18:49:53.427" Score="11" Body="&lt;p&gt;In the early days of crewed space exploration, size and weight of crew was an issue. The original batch of Soviet cosmonauts selected were restricted to 1.75m height and 72kg mass, for example, and US astronauts to 1.80m and 82kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As launchers got bigger and spacecraft got more complex (and roomier), these restrictions became less important because the crew represented a progressively smaller share of the mass budget. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, in the Mercury program, the astronaut's mass was ~5.8% of the spacecraft mass; in Gemini, the crew was ~4.3%; in Apollo earth-orbit configurations, ~1.2%. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some point in that progression, it makes sense to start relaxing the astronaut height/mass restrictions. China's space program simply hasn't advanced far enough for them to do so yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for inherent advantages of a larger crew member, all other things being equal, larger people in good health are generally stronger, which can be important during certain EVA operations. In most other criteria a smaller crew member would probably be very slightly preferable primarily due to resource consumption rates. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-09-28T22:11:25.250" LastActivityDate="2015-09-28T22:11:25.250" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12127" PostTypeId="2" ParentId="7714" CreationDate="2015-09-28T20:50:13.547" Score="0" Body="&lt;p&gt;Things lacking for Mars to grow plants:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water.  Think that's well covered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atmosphere.  Even if it had a similar composition to earth's, it's simply too thin.  Lack/Loss of magnetic field led to Mars's atmosphere being mostly stripped.  If you were to give it a full 1 atm pressure today, this too would disappear.  So if you plopped an earth plant on Mars, it would be voided of most of it's gases and die.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Loss of Magnetic Field.  Except for small pockets, Mars's magnetic field is too small to protect the surface.  This means far more radiation gets to the surface.  If the above didn't kill plant life, the radiation level would.   The plants cancer would get cancer.&lt;/p&gt;&#xA;" OwnerUserId="11654" LastActivityDate="2015-09-28T20:50:13.547" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12128" PostTypeId="2" ParentId="12125" CreationDate="2015-09-28T21:16:51.230" Score="2" Body="&lt;p&gt;Not a complete answer, but Shuttle height restrictions reflected the initial requirements of not restricting most of the US population and were &lt;a href=&quot;http://www.nytimes.com/1994/07/12/science/astronaut-grows-too-tall.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;generous&lt;/a&gt;: minimum of 4 ft 10 1/2 inches for mission specialists, 5 ft 4 inches for pilots and commanders, maximum of 6 ft 4 inches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is quite a wide range, as this picture from STS-86 indicates.  Russian requirements were not so generous and this fact caused issues for this crew (it was a mission to Mir).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/F3H1W.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/F3H1W.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-09-28T21:16:51.230" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12130" PostTypeId="1" AcceptedAnswerId="12132" CreationDate="2015-09-29T03:37:53.327" Score="7" ViewCount="546" Body="&lt;p&gt;The recent &lt;a href=&quot;https://www.youtube.com/watch?v=1_FXVjf46T8&quot;&gt;SpaceX Pad Abort Test&lt;/a&gt; shows a parachute landing but I'm assuming a pad abort will also utilise the vertical landing capability for regular Dragon vertical landings right?&lt;/p&gt;&#xA;" OwnerUserId="11761" LastActivityDate="2015-09-29T04:37:45.473" Title="Will a Dragon pad abort eventually land vertically or will it always be a parachute landing?" Tags="|spacex|landing|dragon-v2|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12131" PostTypeId="2" ParentId="12130" CreationDate="2015-09-29T03:43:32.063" Score="4" Body="&lt;p&gt;No: Launch abort and propulsive landing both use the eight SuperDraco engines for which there is limited fuel.  The beauty of the design is that the abort system isn't wasted mass when an abort isn't needed because it can be used for precision landing, but reusing the system means that when an abort is necessary there isn't enough fuel to perform the propulsive landing and it reverts back to a parachute landing in water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isn't as explicit a quote as I was looking for, but (from &lt;a href=&quot;http://www.space.com/29329-spacex-tests-dragon-launch-abort-system.html&quot; rel=&quot;nofollow&quot;&gt;space.com&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And SpaceX engineers have designed the SuperDracos to serve as both launch abort engines, as well as descent engines. If a Dragon launched smoothly, the SuperDracos could then be used to make a land-based landing for eventual reuse.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2015-09-29T03:52:16.740" LastActivityDate="2015-09-29T03:52:16.740" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12132" PostTypeId="2" ParentId="12130" CreationDate="2015-09-29T04:37:45.473" Score="7" Body="&lt;p&gt;No, Crew Dragon will splash down in the ocean after an abort sequence. Crew Dragon only has &lt;a href=&quot;http://www.faa.gov/about/office_org/headquarters_offices/ast/media/DragonFly_Final_EA_sm.pdf&quot;&gt;400 gallons of fuel for its SuperDraco thruster system.&lt;/a&gt; All of it is used to accelerate the capsule upwards and away from the Falcon 9 during an abort.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The abort burn was terminated once all propellant was consumed, and Dragon coasted to its highest point. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacex.com/news/2015/05/06/crew-dragon-completes-pad-abort-test&quot;&gt;-SpaceX Pad Abort Test&lt;/a&gt;&lt;br&gt;&#xA;Since Dragon cannot discard the motors after launch like a puller system, powered landings are a residual benefit. Crew Dragon has three landing modes: Water Splashdown, Parachute Assisted Landing, and fully Propulsive Landing. For the first NASA test flights and first crewed missions, Dragon will land with its parachutes in the ocean. Once the &lt;a href=&quot;https://en.wikipedia.org/wiki/DragonFly_(rocket)&quot;&gt;DragonFly test program&lt;/a&gt; progresses, SpaceX will attempt SuperDraco-assisted landings on solid ground. Once DragonFly has conclusively proven the success of Propulsive landings, actual crewed missions will be attempted with them.&lt;/p&gt;&#xA;" OwnerUserId="10398" LastActivityDate="2015-09-29T04:37:45.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12133" PostTypeId="1" CreationDate="2015-09-29T06:08:03.100" Score="8" ViewCount="1623" Body="&lt;p&gt;How does the efficiency of the rocket engine using liquid propellants depend on the throttling setting? (Is it more or less efficient while working on eg.: 85% of its maximum thrust?). The rocket engine is in vacuum.&lt;/p&gt;&#xA;" OwnerUserId="11762" LastEditorUserId="168" LastEditDate="2015-09-30T08:32:07.173" LastActivityDate="2018-06-25T15:03:43.020" Title="Is liquid rocket engine more or less efficient when throttled?" Tags="|rockets|engines|specific-impulse|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12134" PostTypeId="2" ParentId="10829" CreationDate="2015-09-29T10:54:10.967" Score="20" Body="&lt;p&gt;The SRBs are &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html&quot; rel=&quot;noreferrer&quot;&gt;pretty close to burnout when they separate&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SRB separation is initiated when the three solid rocket motor chamber pressure transducers are processed in the redundancy management middle value select and the head- end chamber pressure of both SRBs is less than or equal to 50 psi. A backup cue is the time elapsed from booster ignition.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The separation sequence is initiated, commanding the thrust vector control actuators to the null position and putting the main propulsion system into a second-stage configuration (0.8 second from sequence initialization), which ensures the thrust of each SRB is less than 100,000 pounds. Orbiter yaw attitude is held for four seconds, and SRB thrust drops to less than 60,000 pounds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So at separation, thrust is down to 1.8% of the initial value. Here's a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;noreferrer&quot;&gt;graph that shows the thrust profile&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5B5IN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5B5IN.png&quot; alt=&quot;thust profile on SRB, from Wikipedia&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At liftoff, the entire propellant surface burns and provides thrust. The propellant is cast (mostly *) with a cone-shaped hole down the middle. At the point where the cone is widest, the layer of propellant is thinnest, and at this location the burn time is the shortest. So the SRBs will &lt;a href=&quot;http://www.collectspace.com/ubb/Forum30/HTML/000330.html&quot; rel=&quot;noreferrer&quot;&gt;gradually reduce thrust&lt;/a&gt; for a while before separation.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get a rough approximation of the amount of fuel remaining: &#xA;Thrust in an SRB is related to the propellant area that's burning.&lt;br&gt;&#xA;If you were to coat the inside of an empty SRB casing with a thin layer of propellant, you'd get something close to takeoff performance (3.3 million lbs thrust).&lt;br&gt;&#xA;If you're down to 1.8% of nominal thrust, that means most of the casing is now bare, and about 1.8% is still covered by propellant.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm going to make the following assumptions:&lt;br&gt;&#xA; - length of the propellant stick is 40 m&lt;br&gt;&#xA; - diameter is 3.7 m&lt;br&gt;&#xA; - the SRB is filled with propellant in a simple negative cone shape  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angle of the cone will then be 2.6°, and the length from top to bottom along the inside of the cone will be 40.04 m.&lt;br&gt;&#xA;Now we'll remove propellant until 1.8% of the propellant area is left; the length is then 40.04 * .018 is 72 cm. Thickness at the top is 3.3 cm.  This is equal to a hollowed-out cylinder with a height of 36 cm and a thickness of 1.65 cm.&lt;br&gt;&#xA;The volume of that is: 0.056 $m^3$, or (density is 2 kg/dm$^3$) ca. &lt;strong&gt;100 kg&lt;/strong&gt;. That's down from an initial propellant mass of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;noreferrer&quot;&gt;500 tons&lt;/a&gt;. According to the graph, the SRBs separate 2-3 seconds before they burn out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These won't be very accurate figures. I've taken a number of shortcuts. I've used the length of the cone as a proxy for its internal area. I've ignored the exact shape of the propellant and just assumed a regular cone. So I might be several hundred kg off.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Approaching the problem from another angle: the SRBs burn 4.16 tons of propellant per second at full blast, so about 1/20 that, or 200 kg/s at separation, decreasing to 0 in 3 seconds, or 100 kg/s average for a total of 300 kg.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the amount of propellant left at separation is 0.02% or 0.06% of what the SRB started with. Or something in between, probably. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Russell notes in his comment, you could use more complex methods to get a more accurate answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note&lt;/strong&gt;: I was unable to find a source that listed the propellant mass at separation. The above is an attempt to get to the answer from first principles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*: yes, I know one section of the SRB is cast with a more complex pattern, but that pattern is gone a while before burnout so for this question I'll ignore it. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Edit:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=2aCOyOvOw5c&quot; rel=&quot;noreferrer&quot;&gt;this video taken by cameras on board the boosters&lt;/a&gt;, you can see flames coming out of the nozzles until at least 16 seconds after separation. The first 6 seconds, the flame is pretty solid but reducing rapidly, after that the flame becomes uneven. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-01-17T13:10:11.320" LastActivityDate="2016-01-17T13:10:11.320" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12135" PostTypeId="1" AcceptedAnswerId="12136" CreationDate="2015-09-29T11:17:17.723" Score="21" ViewCount="5802" Body="&lt;p&gt;I would like to know how similar Mars water is to Earth water. &#xA;Can we survive on Mars with Martian water without any major health issues etc?&lt;/p&gt;&#xA;" OwnerUserId="11765" LastEditorUserId="25" LastEditDate="2015-09-29T14:55:19.247" LastActivityDate="2019-01-27T20:30:50.113" Title="Is water on Mars the same as Earth water?" Tags="|mars|planetary-science|astronauts|water|environment|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12136" PostTypeId="2" ParentId="12135" CreationDate="2015-09-29T11:22:54.203" Score="27" Body="&lt;p&gt;Fundamentally, water is water. In its purest form, it is the same anywhere, except perhaps for the isotopes. However, one of the wonderful things about water is the fact that it's a good solvent, and in fact has many things in it that aren't water. For instance, one could not survive off of ocean water: we humans require fresh water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one potential difference is in the isotope of the water. This is a rather complex concept, basically some &lt;a href=&quot;https://en.wikipedia.org/wiki/Heavy_water&quot;&gt;water is heavier&lt;/a&gt; than other water, despite having the same chemistry. It turns out that Mars has a &lt;a href=&quot;https://www.sciencemag.org/content/348/6231/218.abstract&quot;&gt;higher concentration&lt;/a&gt; of heavy water than Earth does. However, it is still within reason to us humans to use. It would make fusion easier, but that's another matter entirely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The water that was discovered on Mars recently is unique. It is believed to be a very salty water; in its natural form humans could not drink it. However, it can go through a desalinization process, where the salt is removed, and in theory it should be drinkable. Basically, the water needs to evaporate, then re-cool. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As was said in the press conference, the best source of water for astronauts would be in the ice caps, which are similar to Earth's ice caps: frozen water on the surface, which would simply require melting to use, along with a small purification process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, there is water on Mars, and through some purification process, it can be made for human consumption.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-09-29T17:12:42.150" LastActivityDate="2015-09-29T17:12:42.150" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="12138" PostTypeId="2" ParentId="5845" CreationDate="2015-09-29T14:35:42.673" Score="11" Body="&lt;p&gt;Interplanetary probes generally don't have enough equipment on board to calculate their own position. So the location of a probe is measured by ground control, using data from the probe.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Distance and speed along the Earth-spacecraft axis is done by sending signals to the probe, having the probe transmit them back immediately and measuring the time elapsed plus Doppler shift. See &lt;a href=&quot;https://space.stackexchange.com/a/2362/265&quot;&gt;Mark Adler's answer&lt;/a&gt; to a related question.  &lt;/li&gt;&#xA;&lt;li&gt;The remaining parameters are derived from the above with some calculation, plus data from the spacecraft. Navigation images can be used, for example: these show the stars and planets from the perspective of the spacecraft, this can be compared to the view from Earth. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For New Horizons' Pluto flyby, this process was complicated by the fact that Pluto's orbit wasn't precisely known. So the NH team used &lt;a href=&quot;http://www.sciencedaily.com/releases/2014/08/140805131708.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;several&lt;/a&gt; methods (*) to measure Pluto's position accurately so they could refine NH's trajectory in turn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;*: the NH team went through &lt;a href=&quot;https://www.washingtonpost.com/national/health-science/new-horizons-is-just-hours-away-from-pluto-after-95-year-journey/2015/07/13/ce4ab61a-294c-11e5-bd33-395c05608059_story.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tombaugh's 1930s photographic plates&lt;/a&gt; and digitized them to get data from as far back as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are &lt;a href=&quot;https://space.stackexchange.com/questions/942/how-can-spacecraft-navigate-without-contact-from-earth?rq=1&quot;&gt;projects to design a method&lt;/a&gt; for space probes to measure their position independently, but none of those are anywhere near 'ready to use'. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-09-30T07:50:09.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12139" PostTypeId="1" CreationDate="2015-09-29T14:51:19.410" Score="5" ViewCount="1790" Body="&lt;p&gt;As title says. We find water on a planet, and we assume that it's more likely life is there. How/why is this the case? Isn't it possible some life could survive without water? Even if the answer is 'no', the fact there's water doesn't mean life could've 'started' does it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm obviously speaking with regard to the latest Mars findings, but the question is bigger than that.&lt;/p&gt;&#xA;" OwnerUserId="8706" LastEditorUserId="25" LastEditDate="2015-09-29T17:18:53.230" LastActivityDate="2015-09-29T21:13:42.950" Title="How does the presence of water on a planet indicate a likelihood of life?" Tags="|mars|life|water|astrobiology|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12140" PostTypeId="1" AcceptedAnswerId="12141" CreationDate="2015-09-29T15:22:41.480" Score="4" ViewCount="216" Body="&lt;p&gt;&lt;a href=&quot;http://www.snopes.com/glurge/communion.asp&quot; rel=&quot;nofollow&quot;&gt;This&lt;/a&gt; article (&lt;a href=&quot;https://en.wikipedia.org/wiki/Buzz_Aldrin#NASA_career&quot; rel=&quot;nofollow&quot;&gt;and&lt;/a&gt; some &lt;a href=&quot;http://www.hoax-slayer.com/communion-on-the-moon.shtml&quot; rel=&quot;nofollow&quot;&gt;others&lt;/a&gt;) confirms &quot;that Buzz Aldrin took communion on the surface of the moon&quot;. How is it possible that he ate and drank on the surface of the moon while wearing a space suit?&lt;/p&gt;&#xA;" OwnerUserId="11768" LastActivityDate="2015-09-30T13:38:37.043" Title="Communion on the Moon" Tags="|the-moon|spacesuits|food|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12141" PostTypeId="2" ParentId="12140" CreationDate="2015-09-29T15:28:39.003" Score="7" Body="&lt;p&gt;He was in the Lunar Excursion Module (LEM) at the time! :)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="33" LastEditDate="2015-09-30T13:38:37.043" LastActivityDate="2015-09-30T13:38:37.043" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12145" PostTypeId="2" ParentId="12139" CreationDate="2015-09-29T17:20:17.950" Score="4" Body="&lt;p&gt;Everywhere on Earth that we find water, we find life. From deep in the crust, to under the Antarctic ice sheets, to the most salty locations on the planet, life seems to follow water. The thought is, if it applies to Earth, it might apply to other planets. Of course, no one has observed life on another planet, so there is not knowing for certain if indeed water= life outside of Earth.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-09-29T17:20:17.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12146" PostTypeId="2" ParentId="10829" CreationDate="2015-09-29T19:21:52.827" Score="11" Body="&lt;p&gt;This is the tail end of the thrust graph given in Hobbes' answer: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/julhg.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/julhg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've added guidelines showing the point where thrust falls below the 60,000 lbf point, about 2.7 seconds before it hits zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The area under the curve there has units of lbf × seconds, which is &lt;a href=&quot;https://en.wikipedia.org/wiki/Impulse_(physics)&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;impulse&lt;/em&gt;&lt;/a&gt;. The pic is a little fuzzy, so it's hard to tell if the arc continues smoothly all the way to zero, or if it falls off sharply at the very end. I made two different estimates, 79500 and 98050 lbf-s, one by modeling the area as a right triangle, one by modeling it as a trapezoid (base at about 14,000 lbf). (It might well be a little less, because of the concavity of the curve.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The specific impulse of the SRBs is known - 242 seconds at sea level. Specific impulse given in seconds is actually &quot;impulse per unit weight&quot;, so all we have to do is divide, and we get 328 to 405 lbs (149-184kg), which is very much in line with Hobbes' estimates. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-09-30T16:47:47.897" LastActivityDate="2015-09-30T16:47:47.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12147" PostTypeId="1" AcceptedAnswerId="12148" CreationDate="2015-09-29T20:18:11.943" Score="17" ViewCount="4115" Body="&lt;p&gt;I think I lack a fundamental knowledge of physics to answer this myself. In many sci-fi stories, there is a rotating spaceship that gives the feeling of being pulled &quot;downwards&quot; against the sides of a cylinder. What I can't seem to grasp is how that works in zero gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is keeping you from floating freely while the cylinder rotates around you? Are there any experiments that can explain this?&lt;/p&gt;&#xA;" OwnerUserId="7027" LastActivityDate="2015-09-30T11:19:26.100" Title="How does centripetal force produce artificial gravity" Tags="|physics|artificial-gravity|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12148" PostTypeId="2" ParentId="12147" CreationDate="2015-09-29T20:53:49.463" Score="19" Body="&lt;p&gt;That is an excellent thought experiment to consider for a spinning vehicle.  You are correct that if you simply enter the open space inside the rotating cylinder, somehow not following the rotation yourself, you will not experience a force to pull you to the outside.  You could just hang there and watch the &quot;floor&quot; rotate below you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However you probably won't be able to just hang there, assuming that there is air in the space.  If you're alive and breathing, then the air will also be moving by you (the boundary effect with the floor and walls will be moving the air around with them), which will start you moving in the same direction as the floor.  As it imparts a force in that direction, you will start moving on a trajectory perpendicular to the radius.  You will then be on a trajectory to intersect the floor, which is curving up in front of you.  As you get closer, the air will be moving faster, further accelerating you to the floor in front of you.  It will seem like falling towards, as well as catching up with, the floor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In what is normally seen in depictions of such a vehicle, you would not float into the space and wait to be directed to the floor by the moving air, but rather you would control your descent on a ladder.  (Seems sensible, as opposed to an uncontrolled fall, however slow.)  Then you are rotating with the floor, with your angular velocity increasing as you descend, increasing the the apparent weight on your feet on the ladder as you descend.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-09-29T22:30:09.110" LastActivityDate="2015-09-29T22:30:09.110" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12149" PostTypeId="2" ParentId="12139" CreationDate="2015-09-29T21:13:42.950" Score="5" Body="&lt;p&gt;From a biochemical standpoint, we don't know of any life that doesn't require water.  Everything on earth requires water as the solvent to a majority of the chemical reactions occurring within it.  Without water those reactions don't happen and the life form dies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, that certainly doesn't make life based on other solvents impossible.  There is always interesting discussion about silicon based life, but the seeming reason we are carbon based is due to the stability of carbon compounds vs silicon compounds in water.  There could very well be another solvent in which silicon makes more stable bonds and then perhaps life would be based around it.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from that, another common potential solvent in our solar system is methane, which is liquid on Titan.  The problem is that for methane to be liquid, it has to be so cold that it would take a very long time for life to evolve (if it could).  So we look to the water oceans of Europa more than the methane oceans of Titan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary we look for water because water based life is the only we know.  But we cannot rule out the possibility of other forms of life. &lt;/p&gt;&#xA;" OwnerUserId="11654" LastActivityDate="2015-09-29T21:13:42.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12151" PostTypeId="2" ParentId="12147" CreationDate="2015-09-29T23:57:50.657" Score="3" Body="&lt;p&gt;To extend Mark Adler's thought experiment a bit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were in contact with the rotating floor and started walking or running in the direction of rotation, you would feel your weight increase. How much depends on the tangential velocity of the habitat floor and its radius. If the velocity is low enough, the added velocity from your own relative motion could noticeably increase your apparent weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely, if you were to walk or run in the direction opposite to that of the floor, you would cancel out some or perhaps all of the centripetal acceleration. You might even find yourself back in a weightless condition.  At least until you drift into something that is moving with the floor, or the air takes effect as in Mark's answer.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2015-09-29T23:57:50.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12152" PostTypeId="2" ParentId="12147" CreationDate="2015-09-30T10:32:42.040" Score="2" Body="&lt;p&gt;If you adopt the rotating reference frame of the spaceship then there are two fictional forces that you will see. One is centrifugal force - this is your artificial gravity. The other force is the coriolis force. In the scenario you describe the coriolis force balances out the centrifugal force and allows you to float above the floor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But its unusual for these two force to exactly balance. Within this reference frame you only have the coriolis force holding you up because you are flying past the floor at high speed.&lt;/p&gt;&#xA;" OwnerUserId="11295" LastActivityDate="2015-09-30T10:32:42.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12153" PostTypeId="2" ParentId="12147" CreationDate="2015-09-30T11:19:26.100" Score="3" Body="&lt;p&gt;Although Mark Adler's thought experiment is nice, I think a better and straighter answer is this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you just happen to appear there near the cylinder, you would simply watch it spin around you while you are standing still in open space. Nothing happens. In void (or something close to it), since there's no air friction (as in Mark Adler's experiment), you would be able to stand there forever (ignore for a moment the gravitation forces around you).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would only experience the artificial gravity if you started rotating together with the cylinder. To do that, you would need to grab onto the rotating &quot;floor&quot;, you would experience some bumps and accelerations, and in the end you would be &quot;one with the floor&quot;. That means that on the tangent direction you would be moving uniformly (equivalent to not moving), while you would only experience the centrifugal force towards outside. It would be perfectly equivalent to standing still on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To complete the answer, the centripetal force produces gravity because it &quot;pushes&quot; you towards the center of the cylinder just like Earth pushes you towards the sky (otherwise you fall down towards the center of the Earth / towards the outside of the cylinder). But to obtain that centripetal force you must be in contact with the rotating floor, and standing still relative to it.&lt;/p&gt;&#xA;" OwnerUserId="11785" LastActivityDate="2015-09-30T11:19:26.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12154" PostTypeId="2" ParentId="11924" CreationDate="2015-09-30T11:53:24.237" Score="4" Body="&lt;p&gt;ThePlanMan was on the right track. The 'On-Orbit Assembly, Modeling, and Mass Properties Data Book' contains inertia tensors for each assembly step of the ISS.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://athena.ecs.csus.edu/~grandajj/ME296M/space.pdf&quot; rel=&quot;nofollow&quot;&gt;On-Orbit Assembly, Modeling, and Mass Properties Data Book volume 1&lt;/a&gt;, 2002  &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://athena.ecs.csus.edu/~grandajj/ME296M/RevAB_Volume%20I%20Signed_updated.pdf&quot; rel=&quot;nofollow&quot;&gt;On-Orbit Assembly, Modeling, and Mass Properties Data Book volume 1&lt;/a&gt;, 2008 &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://athena.ecs.csus.edu/~grandajj/ME296M/RevAB_Volume%20II%20Signed_updated.pdf&quot; rel=&quot;nofollow&quot;&gt;On-Orbit Assembly, Modeling, and Mass Properties Data Book volume 2&lt;/a&gt;, 2008  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I haven't found a more recent edition than 2008. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These don't mention error margins. Since mass and dimensions are listed to 1 mm and 1 kg (and not rounded to larger values) I'd expect the error margins to be in the region of 1 kg*mm$^2$.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-09-30T11:53:24.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12155" PostTypeId="1" AcceptedAnswerId="12169" CreationDate="2015-09-30T13:00:36.260" Score="13" ViewCount="486" Body="&lt;p&gt;Elon Musk talks a lot about Mars. Once a Mars colony is established, we should be able to make a trip to/from Mars every two years when the planets align. However, Mars' gravity is 38% that of the Earth. Won't the deterioration in muscle and bone mass, as a result of residing on Mars for two years, be a problem when returning to Earth gravity? If so, what are the possible solutions?&lt;/p&gt;&#xA;" OwnerUserId="11786" LastEditorUserId="7062" LastEditDate="2015-09-30T15:02:33.067" LastActivityDate="2015-10-01T10:35:42.283" Title="What are the physiological changes that would result from living in Mars gravity for years together?" Tags="|mars|spacex|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12156" PostTypeId="1" AcceptedAnswerId="12157" CreationDate="2015-09-30T13:35:11.127" Score="26" ViewCount="2949" Body="&lt;p&gt;As the title states, if a space shuttle 2.0 were to be built with current technology, in which ways or areas might it be superior to the original space shuttle? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which components are most ripe for improvement using current technology and how big would be the gained advantage by this improvement? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basics requirements: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;vertical launch, horizontal landing orbiter&lt;/li&gt;&#xA;&lt;li&gt;ability to lift at least 25 tons to LEO&lt;/li&gt;&#xA;&lt;li&gt;supports a maximum of 7 crew members for at least 14 days&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Goal of the question: Exercise to illustrate advancements in spaceflight technologies in the last several decades.&lt;/p&gt;&#xA;" OwnerUserId="11787" LastEditorUserId="12102" LastEditDate="2019-12-10T23:59:34.563" LastActivityDate="2021-05-19T07:53:25.833" Title="Technology: How would a state-of-the-art space shuttle be built?" Tags="|space-shuttle|technology|" AnswerCount="5" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="12157" PostTypeId="2" ParentId="12156" CreationDate="2015-09-30T14:00:41.843" Score="22" Body="&lt;p&gt;This could go many ways, depending on whether you mean a clean sheet design or incremental updates to STS.  I'll assume the latter, in a parallel universe where the STS-107 accident never happened and the program continued on and actually did upgrades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can look at &lt;a href=&quot;http://spaceflight.nasa.gov/spacenews/factsheets/pdfs/21stCenturyShuttle.pdf&quot; rel=&quot;noreferrer&quot;&gt;what was planned&lt;/a&gt; for STS:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Improved engines: health monitoring, new controller, new nozzle and combustion chamber design&lt;/li&gt;&#xA;&lt;li&gt;Electric power for the hydraulic pumps instead of toxic and dangerous hydrazine APUs&lt;/li&gt;&#xA;&lt;li&gt;New avionics and cockpit controls &amp;amp; displays&lt;/li&gt;&#xA;&lt;li&gt;Blowdown-type thrust vector control system for the SRBs instead of toxic and dangerous hydrazine APUs.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Other, more expensive and/or speculative updates&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Liquid fueled &lt;a href=&quot;http://www.nap.edu/read/6384/chapter/6#52&quot; rel=&quot;noreferrer&quot;&gt;fully reusable flyback boosters&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/fsSoW.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fsSoW.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Metallic or otherwise hardened Thermal Protection System&lt;/li&gt;&#xA;&lt;li&gt;Operational improvements to reduce cost - reflying crews as a unit to reduce training, for example&lt;/li&gt;&#xA;&lt;li&gt;Forward-to-aft Reaction Control System interconnect&lt;/li&gt;&#xA;&lt;li&gt;Revisit and fully implement the Extended Duration Orbiter project (cryo pallets, Orbital Maneuvering System kits, regenerative CO2 removal, new toilet)&lt;/li&gt;&#xA;&lt;li&gt;Add Shuttle-C-like vehicle as an option for heavy lift missions where crew is not required&lt;/li&gt;&#xA;&lt;li&gt;Crew escape system - via separable crew module?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The second link leads to an entire book titled &lt;strong&gt;Upgrading the Space Shuttle&lt;/strong&gt; which may be of interest.  I have only pulled a few items from it.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-09-30T15:37:25.163" LastActivityDate="2015-09-30T15:37:25.163" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12158" PostTypeId="1" AcceptedAnswerId="12162" CreationDate="2015-09-30T15:24:29.503" Score="6" ViewCount="4544" Body="&lt;p&gt;While with this weeks discovery, I am sure everyone is waiting for the inevitable trip to Palantir Crater, I noticed that one Mars' largest crater, Hellax Planitia has never been visited by a lander or rover. I am curious is as to why this is the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the crater too rough to land on or otherwise too dangerous for a probe? Is there just nothing of interest in that crater? I mean it is also likely that water covered this point on Mars as well, but most of the probes seem centered on the Borealis basin or its edges. It's one of the deepest points on the Martian surface and possibly the largest crater. I am just curious why scientists are not more interested in this area of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A map of where landers have landed on Mars and the proposed InSight landing site:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.space.com/images/i/000/032/395/i02/insight-mars-landing-sites.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://www.space.com/images/i/000/032/395/i02/insight-mars-landing-sites.jpg&quot; alt=&quot;Mars landing sites&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://www.space.com/images/i/000/032/395/i02/insight-mars-landing-sites.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;space.com&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="3344" LastEditorUserId="10273" LastEditDate="2019-07-16T07:57:46.653" LastActivityDate="2019-07-16T07:57:46.653" Title="Why has no lander or rover visited Hellas Planitia on Mars?" Tags="|mars|probe|planetary-science|mission-design|lander|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="12159" PostTypeId="2" ParentId="12133" CreationDate="2015-09-30T15:44:56.597" Score="8" Body="&lt;p&gt;It's surprisingly difficult to find a good answer to this question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, the rated full power level is where the engine is going to be most efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Sutton's &quot;Rocket Propulsion Elements&quot;, typical deep-throttling engines suffer between 1.5% and 9% reduction in specific impulse (fuel efficiency) at low power levels. It mentions an outlier, the engine in the Lance missile, which has an extraordinary 357:1 throttle range with 15% loss at the low end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a poorly labeled and confusing graph that suggests that the &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10#Common_Extensible_Cryogenic_Engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;CECE&lt;/a&gt; upper stage engine suffers about 5% when throttled far down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730023018.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Module Descent Engine is upwards of 97% efficient at 30% throttle&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you cared to, you could design an &quot;afterburning&quot; rocket engine, dumping extra fuel or oxidizer (or any other working fluid, for that matter) into the nozzle; this would give you a big thrust boost, cooler (and possibly dirtier) exhaust, and a large loss in efficiency. Thus power could be increased beyond the point of peak efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Apparently AJR has patented &lt;a href=&quot;http://contest.techbriefs.com/2013/entries/aerospace-and-defense/3364&quot; rel=&quot;nofollow noreferrer&quot;&gt;a variation&lt;/a&gt; on this, injecting both fuel and oxidizer into the nozzle -- effectively using the upper part of the nozzle as combustion chamber, for a more appropriate expansion ratio at sea level, apparently?)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-06-25T15:03:43.020" LastActivityDate="2018-06-25T15:03:43.020" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="12160" PostTypeId="1" AcceptedAnswerId="12163" CreationDate="2015-09-30T15:45:18.927" Score="34" ViewCount="2003" Body="&lt;p&gt;I've heard a rumor that Curiosity rover sent a recording about Phobos covering the Sun on the Martian sky. Did it happen? If yes, how frequently could such things be recorded?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="25" LastEditDate="2015-09-30T18:16:34.303" LastActivityDate="2015-10-01T15:11:57.103" Title="Have any of the rovers ever recorded a solar eclipse on Mars?" Tags="|mars|rovers|the-sun|moons|photography|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12161" PostTypeId="2" ParentId="12133" CreationDate="2015-09-30T15:50:02.113" Score="8" Body="&lt;p&gt;Corroborating Russell Borogove's answer, some Stennis test data I have from 1987 on three different SSMEs shows a small drop in Isp with power level. From 109% to 100% the Isp dropped about 0.08%.  I can't find data at lower power levels but my recollection is that the trend continued, with a small degradation in Isp as you throttled down.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the original SSME could throttle from 109% to 65% of Reference Power Level.  A &quot;bi-stable turbopump anomaly&quot; limited the lower end of the throttle range to 67% in the latter part of the program.  The throttle range actually experienced in flight was from 65% to 104.5%. &lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-09-30T15:50:02.113" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12162" PostTypeId="2" ParentId="12158" CreationDate="2015-09-30T16:36:53.103" Score="13" Body="&lt;p&gt;Hellas basin is certainly an interesting region, and several landing site selection processes evaluated candidate sites in it, mostly in the Northernmost region that's above the 30° South circle of latitude:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/UuU3w.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UuU3w.png&quot; alt=&quot;All Mars Candidate Landing Sites&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;All Mars candidate landing sites that were considered for all NASA missions up to Mars 2020 Rover. Source: &lt;a href=&quot;http://mepag.nasa.gov/meeting/2014-05/18_2020%20First%20Workshop%20Introduction%205-13-14.pdf&quot; rel=&quot;noreferrer&quot;&gt;1st Mars&lt;br /&gt;&amp;nbsp;&amp;nbsp;2020 Landing Site Workshop - Introduction&lt;/a&gt;, John Grant and Matt Golombek, PDF&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Out of some most interesting aspects of it are estimates of large water ice deposits beneath its relatively thick layer of surface regolith in the East, higher surface atmospheric pressure simply because of being so deep and in some Western spots over 8 km below Mars standard datum, large, delta-like alluvial fans spreading into the North of the basin, has generally fairly diverse surface features and is of course large, covering near-equatorial to near-polar latitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of these are also its drawbacks; Sites of areologic interest are far apart, its seasonally induced micro-climate conditions are rough and pronounced due to somewhat larger surface atmospheric pressure, winds are stronger, dust storms longer lasting, it experiences higher seasonal and diurnal surface temperature variations, shorter days and exposure to sunlight during Southern hemisphere winters, and as a deep impact basin, terrain is fairly rugged, inclined, and often covered with deep pockets of sand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in short, any candidate landing sites in the Hellas basin have so far been voted out by better suitability of other - with exceptions being Phoenix Lander and Mars Polar Lander that &lt;em&gt;went rogue&lt;/em&gt; - mostly more temperate latitude landing sites that offer better thermals and solar power year long. Hellas doesn't lack scientific interest, but it comes somewhat short on safety and mission success estimates.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For the Mars 2020 Rover Mission, the two Hellas candidate sites, &lt;em&gt;Hadriacus Palus&lt;/em&gt; (blue dot #101) and &lt;em&gt;Circum-Hellas Hydrothemal&lt;/em&gt; (blue dot, unlabeled) were &lt;a href=&quot;http://mepag.nasa.gov/meeting/2015-02/16_Grant%20MEPAG_2-25-15.pdf&quot; rel=&quot;noreferrer&quot;&gt;ranked #16 and #19 respectively out of 27 sites evaluated&lt;/a&gt; after the first landing site workshop, failing to qualify among top 10 sites evaluated during the &lt;a href=&quot;http://marsnext.jpl.nasa.gov/workshops/wkshp_2015_08.cfm&quot; rel=&quot;noreferrer&quot;&gt;2nd Landing Site Workshop for the 2020 Mars Rover mission&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also NASA's &lt;a href=&quot;http://www.jpl.nasa.gov/missions/insight/&quot; rel=&quot;noreferrer&quot;&gt;InSight&lt;/a&gt; lander &lt;a href=&quot;https://space.stackexchange.com/q/10760/49&quot;&gt;will target a site at Elysium Planitia&lt;/a&gt; even if they fallback to any of the other three landing sites after final evaluation, ESA's &lt;a href=&quot;http://exploration.esa.int/mars/47852-entry-descent-and-landing-demonstrator-module/&quot; rel=&quot;noreferrer&quot;&gt;Schiaparelli EDL demonstrator module&lt;/a&gt; will target a site at &lt;a href=&quot;https://en.wikipedia.org/wiki/Meridiani_Planum&quot; rel=&quot;noreferrer&quot;&gt;Meridiani Planum&lt;/a&gt; (like MER-B Opportunity) at 2016, and also ESA's &lt;a href=&quot;http://exploration.esa.int/mars/48088-mission-overview/&quot; rel=&quot;noreferrer&quot;&gt;ExoMars 2018 Rover&lt;/a&gt; will target either Mawrth Vallis, Oxia Planum, Aram Dorsum, or Hypanis Vallis. Which exactly will be the two final sites (main and backup) will be decided during the &lt;a href=&quot;http://exploration.esa.int/mars/55991-third-exomars-2018-landing-site-selection-workshop/&quot; rel=&quot;noreferrer&quot;&gt;3rd ExoMars 2018 Landing Site Selection Workshop&lt;/a&gt; on 20th to 21st October, 2015:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/3kiAV.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3kiAV.jpg&quot; alt=&quot;ExoMars 2018 mission landing sites&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Four possible landing sites that are being considered for the ExoMars 2018 mission. Source and more info: &lt;a href=&quot;http://exploration.esa.int/mars/54717-rover-landing-site-candidates/&quot; rel=&quot;noreferrer&quot;&gt;ESA&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update (Oct. 24, 2015)&lt;/strong&gt;: &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Landing_site_recommended_for_ExoMars_2018&quot; rel=&quot;noreferrer&quot;&gt;ESA has selected Oxia Planum as the primary candidate site for ExoMars Rover&lt;/a&gt; that is scheduled to launch in May 2018 and land on Mars in January 2019. &lt;a href=&quot;https://en.wikipedia.org/wiki/Oxia_Planum&quot; rel=&quot;noreferrer&quot;&gt;Oxia Planum&lt;/a&gt; has presumably iron-magnesium rich clays, which are a strong evidence of past water activity and may have helped preserve biosignatures of past life.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I want to add a bit of commentary at this point, realizing that my answer probably doesn't seem too inspiring; Most of these landing site constraints make Hellas, so far at least, a weak spot for largely autonomous robotic missions. It doesn't really disqualify it for future advanced robotic and human exploration missions though, albeit lack of more intimate knowledge of the region might prove rather challenging at first. I expect the interest for it to gradually increase as the time for first human missions to the Martian surface approaches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Higher surface atmospheric pressure, according to rather promising preliminary RAD experiment measurement results on MSL, shows potential for further reduced exposure to radiation, very likely large and relatively easily accessible water ice deposits could be utilized to produce potable water, breathable air, propellants for return journey,... sediments under some of the alluvial fans and gullies show potential for clay deposits which, if true, would certainly be a good target to look for signs of past and present life, and having the landing site deep below the standard datum with more of and slightly denser atmosphere means that you get to land a bit heavier payloads at the same cost to the mission, too. And if the subsequent launch site isn't too far South, you don't lose too much of the delta-v boost by the planet's rotation either. So there is certainly the potential for even more interest in Hellas in the, oh, not-too-distant-future (fingers crossed).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-10-24T21:21:10.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12163" PostTypeId="2" ParentId="12160" CreationDate="2015-09-30T18:12:24.697" Score="56" Body="&lt;p&gt;No, because Mars can't have eclipses. Strictly speaking, Mars has only transits. The difference is that Mars's moons are smaller than the Sun as viewed from Mars, thus they don't block out the entire sun. Eclipses are defined as only occurring if the entire sun is blocked, or at least the vast majority. Phobos blocks out only about 60% of the sun at most. This could arguably be called a partial solar eclipse (But definitions are fluid, so...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yes, there have been several transits viewed by the rovers Curiosity and Opportunity and photographed. Here's pictures of some of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2013-263&quot; rel=&quot;noreferrer&quot;&gt;Curiosity&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/QmYnq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QmYnq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Transit_of_Phobos_from_Mars#/media/File:Phobos_Mar_10_2004_from_Opportunity_2.jpg&quot; rel=&quot;noreferrer&quot;&gt;Opportunity&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rJsuB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rJsuB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-01T15:11:57.103" LastActivityDate="2015-10-01T15:11:57.103" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12164" PostTypeId="1" CreationDate="2015-10-01T04:43:42.600" Score="0" ViewCount="182" Body="&lt;p&gt;Is it possible to use magnets to launch small light weight satellites into a orbit around the earth to provide a net like coverage around the earth? &#xA;Is it practical and will it be useful?&lt;/p&gt;&#xA;" OwnerDisplayName="user11792" LastActivityDate="2015-10-04T11:45:16.753" Title="Launching multiple small satellites into orbit using magnets; is it possible?" Tags="|launch|artificial-satellite|" AnswerCount="1" CommentCount="2" ClosedDate="2015-10-01T20:18:01.470" ContentLicense="CC BY-SA 3.0" />
  <row Id="12166" PostTypeId="1" AcceptedAnswerId="12182" CreationDate="2015-10-01T05:02:25.013" Score="17" ViewCount="1011" Body="&lt;p&gt;I see SpaceX making a lot of technological advancements that will eventually lead to Mars settlement, such as development of affordable and reusable rockets. They're actually getting a lot of things &lt;em&gt;done&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Mars One doesn't seem to have done anything more than sign up people who want to go to Mars. SpaceX and Mars One are often compared in the same vein, but Mars One seems to have accomplished very little, if anything at all, for the mission of colonizing Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All that said, what has Mars One actually &lt;em&gt;done&lt;/em&gt; for Mars settlement?&lt;/p&gt;&#xA;" OwnerUserId="11786" LastEditorUserId="1457" LastEditDate="2015-10-01T05:25:26.633" LastActivityDate="2021-05-03T19:53:39.450" Title="Does Mars One actually do anything?" Tags="|mars|spacex|mars-one|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12167" PostTypeId="2" ParentId="12164" CreationDate="2015-10-01T05:56:40.940" Score="4" Body="&lt;p&gt;Theoreticaly? Yes. There are papers about doing this either with a Coilgun or a train-like launch system (the (magnetic floating) &quot;train&quot; would accelerate along a miles-long track which at the end bends upwards).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are three very difficult problems to solve which make it rather unpractical:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Speed vs. drag: To reach a stable orbit you need a start-velocity of Orbitspeed (~8000 m/s) + Loss to gravity drag + loss to &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_%28physics%29&quot; rel=&quot;nofollow&quot;&gt;drag&lt;/a&gt;. The last one is the game-breaker: at ~15 km/s the air at the first few kilometres of atmosphere will be thick like a wall and will:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;a) kill most of your velocity really fast and&lt;br&gt;&#xA;b) create a lot of heat due to shock-heating.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Magnetic shielding: Your payload will be exposed to very strong magnetic fields while in your acceleration system. You either need a very robust satelite (like a simple brick of steel) or need to shield it somehow.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Accelaration: Since you need a huge starting-velocity, you need either really long tracks/barrels (like 100 miles long) or you need to accelerate at several 100s of Gs … Not a problem for a brick of steel, but your satellite will most likely be rather fragile.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="11787" LastEditorUserId="6214" LastEditDate="2015-10-04T11:45:16.753" LastActivityDate="2015-10-04T11:45:16.753" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12168" PostTypeId="2" ParentId="3161" CreationDate="2015-10-01T08:38:31.427" Score="7" Body="&lt;p&gt;SpaceX is not shooting at the moon, SpaceX is shooting at Mars. Logistically, I’m not sure there are viable choices besides methane / LOX and Hydrazine / Tetroxide. The return shot requires fuel stored for an unknown time, which means the default conditions are chilly on Mars. RP-1 is a rock hard solid requiring complex heating to liquify it and LH2 is high pressure H2 requiring complex cooling to liquify it. Most fuels are rock hard solids. Performance is important but secondary. Hydrazine and Nitrogen Tetroxide are what I had expected, with perfect handling and storage properties. Methane and LOX, however, are both materiels with an abundant supply of people experienced in handling them, so they can be handled and stored, it’s just harder than hydrazine and nitrogen tetroxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not LH2 is obvious, the question is why not hydrazine and nitrogen tetroxide. If you were an astronaut, what fuel would you trust to have waiting for you on arrival without leaks so you can come home? To hell with the trip there, get me back and I’ll know we’re ready to go.&lt;/p&gt;&#xA;" OwnerUserId="11796" LastEditorUserId="11796" LastEditDate="2015-10-02T07:15:14.647" LastActivityDate="2015-10-02T07:15:14.647" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12169" PostTypeId="2" ParentId="12155" CreationDate="2015-10-01T10:35:42.283" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Colonization_of_Mars#Differences_from_Earth&quot;&gt;We don't know for sure&lt;/a&gt;. We can make some guesses based on our experience with weightlessness.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;in 1/3 G, muscles and bone will weaken over time, although not as rapidly as they would in weightlessness. This can be mitigated with lots of exercise: on the ISS, a 6 month stay will result in a ~8% loss of bone mass when the astronaut exercises 2.5 hours a day, versus ~20% when he doesn't exercise.  &lt;/li&gt;&#xA;&lt;li&gt;Other effects we see on the ISS (e.g. redistribution and subsequent loss of fluids, changes in the shape of the eyeball) will be less of an issue.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;We haven't done any long-term experiments where humans (or any living creature) was exposed to less than 1 G. There have been proposals, e.g. the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Gravity_Biosatellite&quot;&gt;Mars Gravity Biosatellite&lt;/a&gt; and a &lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module&quot;&gt;centrifuge module for the ISS&lt;/a&gt;, but those have been canceled. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-01T10:35:42.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12170" PostTypeId="1" AcceptedAnswerId="12175" CreationDate="2015-10-01T11:59:59.680" Score="38" ViewCount="9214" Body="&lt;p&gt;SpaceX have put methane on the map as a rocket fuel, but they weren't the first to consider its use. The first experiments in building a rocket engine that uses &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2007/04may_methaneblast/&quot;&gt;methane date back to 2007&lt;/a&gt;.&lt;br&gt;&#xA;Now methane isn't exactly an exotic fuel. It's widely in use in other applications and its combustion process is well understood.&lt;br&gt;&#xA;So why did it take until 2007 for methane to be seriously considered as a rocket fuel?&lt;br&gt;&#xA;Or am I wrong and have there been attempts to build a methane rocket in the past?&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="25" LastEditDate="2015-10-01T17:56:35.007" LastActivityDate="2018-01-14T01:50:51.537" Title="Why did it take so long for methane to be used as a rocket propellant?" Tags="|rockets|spacex|engine-design|fuel|raptor|" AnswerCount="6" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12171" PostTypeId="2" ParentId="12170" CreationDate="2015-10-01T13:25:37.503" Score="38" Body="&lt;p&gt;The three main competitors for liquid fuel choices to date have been:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Hypergolics - easiest to get started with&lt;/li&gt;&#xA;&lt;li&gt;Kerosene/LOX - Good thrust, low performance, but dense&lt;/li&gt;&#xA;&lt;li&gt;LH/LOX - Best performance, hardest to do&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So if you were starting a new space program with a clean sheet design, LOX/LH is out of the question, too hard.  Hypergolics are well understood, storable, great for missiles, horribly corrosive and dangerous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Missiles, converted to launchers were often hypergolics or solids. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kerosene/LOX is a good choice to start with, if hypergolics are not in the running.  LOX is manageable, Kerosene is very easy to manage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gold standard of performance is LH/LOX, especially for upper stages where it matters the most. But liquid hydrogen is very fluffy, needing huge tanks, embrittles metals, leaks very easily, and is just plain hard to work with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most space programs seem to want to jump to LH/LOX when possible, since they are often optimized for performance, not cost.  (India's GSLV, Japan's H-II, Space Shuttle).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has entered with a different goal in mind, cost. I was going to say SpaceX entered this market, but in reality, it was not really a market until SpaceX entered it.  It was more a series of national government prestige projects. This has changed the trade study results. Sure, LH/LOX would have better performance, but if that is not your only goal, then suddenly Methane starts to look really good. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX's primary goal of Mars, where Methane is potentially available for refueling (with appropriate infrastructure to convert CO2 and water to methane) makes it doubly attractive to them.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2015-10-03T18:34:53.700" LastActivityDate="2015-10-03T18:34:53.700" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12172" PostTypeId="2" ParentId="12170" CreationDate="2015-10-01T14:51:47.273" Score="10" Body="&lt;p&gt;Methane, while a high ISP fuel that is relatively easy to work with, does have it's share of difficulties. The hardest thing about working with methane is it requires an ignition source, as methane and oxygen don't burn spontaneously. In fact, there is &lt;a href=&quot;http://www.la.dlr.de/ra/sart/publications/pdf/0095-0212prop.pdf&quot;&gt;a paper&lt;/a&gt; that directly compares Methane and RP-1 as rocket fuel. The major findings include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The ISP of Methane is about 10s higher than RP-1.&lt;/li&gt;&#xA;&lt;li&gt;Motor mass is increased for Methane.&lt;/li&gt;&#xA;&lt;li&gt;Overall, the payload for similar rockets is almost identical.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, while Methane has some advantages, it would require R&amp;amp;D, and end up being more expensive than RP-1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, additional research will help the case for Methane, and more importantly, Methane can be made or is easily available in many places in the Solar System, making it attractive for In-Situ Resource Utilization (ISRU). Aside from that, there isn't a huge advantage today of using Methane as a rocket fuel.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-01T14:57:07.430" LastActivityDate="2015-10-01T14:57:07.430" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12173" PostTypeId="1" AcceptedAnswerId="12174" CreationDate="2015-10-01T15:21:08.647" Score="8" ViewCount="1575" Body="&lt;p&gt;Today (October 1, 2015) the diagnosis codes known as &lt;a href=&quot;https://en.wikipedia.org/wiki/ICD-10#United_States&quot; rel=&quot;nofollow&quot;&gt;ICD-10 went live in the US&lt;/a&gt;. I was wondering if there are any that are specific to spacecraft, space stations, or hypo- or hyper-gravity?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="49" LastEditDate="2015-10-01T18:56:58.943" LastActivityDate="2015-11-11T23:47:56.667" Title="Are there ICD-10 codes for space?" Tags="|crewed-spaceflight|microgravity|medical|hypergravity|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12174" PostTypeId="2" ParentId="12173" CreationDate="2015-10-01T15:31:14.893" Score="11" Body="&lt;p&gt;Yes, &lt;a href=&quot;http://chat.stackexchange.com/transcript/message/24411155#24411155&quot;&gt;shared by MichaelT in The Pod Bay&lt;/a&gt; just recently: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.icd10data.com/ICD10CM/Codes/V00-Y99/V95-V97/V95-/V95.43&quot;&gt;2015/16 ICD-10-CM Diagnosis Code V95.43: Spacecraft collision injuring occupant&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are others that you can search for by keywords on &lt;a href=&quot;http://www.icd10data.com/&quot;&gt;ICD10Data.com&lt;/a&gt;, for example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.icd10data.com/ICD10CM/Codes/V00-Y99/X52-X58/X52-/X52&quot;&gt;2015/16 ICD-10-CM Diagnosis Code X52: Prolonged stay in weightless environment&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.icd10data.com/ICD10CM/Codes/S00-T88/T66-T78/T75-/T75.81XA&quot;&gt;2015/16 ICD-10-CM Diagnosis Code T75.81XA: Effects of abnormal gravitation [G] forces, initial encounter&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some of them are billable items, like &lt;a href=&quot;http://www.icd10data.com/ICD10CM/Codes/S00-T88/T66-T78/T75-/T75.82XA&quot;&gt;2015/16 ICD-10-CM Diagnosis Code T75.82XA: Effects of weightlessness, initial encounter&lt;/a&gt; that &lt;em&gt;can be used to indicate a diagnosis for reimbursement purposes&lt;/em&gt;. There go them &lt;a href=&quot;https://en.wikipedia.org/wiki/Reduced_gravity_aircraft&quot;&gt;vomit comet&lt;/a&gt; profits :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-01T15:48:16.670" LastActivityDate="2015-10-01T15:48:16.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12175" PostTypeId="2" ParentId="12170" CreationDate="2015-10-01T17:19:37.977" Score="43" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Or am I wrong and have there been attempts to build a methane rocket in the past?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Well, if there were, I figured that John D. Clark's famous book &lt;a href=&quot;http://www.goodreads.com/book/show/677285.Ignition_&quot;&gt;&lt;em&gt;Ignition!&lt;/em&gt;&lt;/a&gt; (1972, &lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot;&gt;free online copy&lt;/a&gt;) would be the place to find it.&#xA;And indeed, the index at the end of the book has a convenient entry for &quot;methane, usefulness of&quot; pointing to pages 8 and 191.&#xA;On page 8, Clark describes an early test of a LOX-methane rocket engine in Germany in 1930(!) thus:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Oberth had originally wanted to use methane as fuel, but as it&#xA;  was hard to come by in Berlin, their first work was with gasoline and oxygen. Johannes Winkler, however, picked up the idea, and working independently of the VfR, was able to fire a liquid oxygen-liquid&#xA;  methane motor before the end of 1930. This work led nowhere in&#xA;  particular, since, as methane has a performance only slightly superior&#xA;  to that of gasoline, and is much harder to handle, nobody could see&#xA;  any point to following it up.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, if Clark is to be believed (and I personally have no reason to think otherwise), the main reason why methane has been mostly neglected as a rocket fuel after these early experiments is simply that, with its low boiling point, it's harder to store and handle than more traditional kerosene-based fuels.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;FWIW, the reference to page 191 is much less useful, as that page belongs to the (by now somewhat dated) chapter 13, &quot;What Happens Next&quot;, and merely mentions methane as a possible low-temperature bipropellant fuel for deep space probes.  The use of methane and oxygen difluoride as space-storable bipropellants is also discussed in chapter 6, &quot;Halogens and Politics and Deep Space&quot;, on pages 83 and 86, where Clark notes, summarizing a bunch of NASA studies from the 1960s:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;All the hydrocarbons were good fuels, but methane was in a class by itself as a coolant, transpiration or regenerative, besides having the best performance. The OF&lt;sub&gt;2&lt;/sub&gt;-methane combination is an extremely promising one. (It took a long time for Winkler's fuel of 1930 to come into its own!)&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it seems that methane &lt;em&gt;has&lt;/em&gt; been considered as a rocket fuel long before the 21st century, but mainly for deep-space applications where its high volatility is not such as major issue, and may even be considered a feature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, for completeness, page 146 of Clark's book contains a rather disturbing mention of LOX + liquid methane as a proposed &lt;em&gt;monopropellant&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;If Tannenbaum's mixtures were bad, that proposed at a monopropellant conference in October 1957 by an optimist from Air&#xA;  Products, Inc., was enough to raise the hair on the head of anybody in&#xA;  the propellant business. He suggested that a mixture of liquid oxygen&#xA;  and liquid methane would be an extra high-energy monopropellant,&#xA;  and had even worked out the phase diagrams of the system.* How&#xA;  he avoided suicide (the first rule in handling liquid oxygen is that you&#xA;  never, never let it come in contact with a potential fuel) is an interesting question, particularly as JPL later demonstrated that you could make the mixture detonate merely by shining a bright light on it. Nevertheless, ten years later I read an article seriously proposing an oxygen-methane monopropellant! Apparently junior engineers are&#xA;  allergic to the history of their own business.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I think we can see why &lt;em&gt;this&lt;/em&gt; one never took off.&lt;/p&gt;&#xA;" OwnerUserId="189" LastEditorUserId="49" LastEditDate="2015-10-01T19:00:43.050" LastActivityDate="2015-10-01T19:00:43.050" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12177" PostTypeId="1" CreationDate="2015-10-01T21:19:34.987" Score="2" ViewCount="578" Body="&lt;p&gt;How thin could at atmosphere be before it could not be flown in using a modern plane?&#xA;&lt;br&gt;&#xA;Does speed affect this to a reasonable degree?&lt;/p&gt;&#xA;" OwnerUserId="11809" LastActivityDate="2015-10-02T05:32:40.217" Title="Flying in Thin Atmosphere" Tags="|atmosphere|" AnswerCount="2" CommentCount="3" ClosedDate="2015-10-02T21:53:39.980" ContentLicense="CC BY-SA 3.0" />
  <row Id="12178" PostTypeId="2" ParentId="12177" CreationDate="2015-10-01T21:33:47.173" Score="3" Body="&lt;p&gt;Depends on the plane. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Airliners generally cruise at 10km altitude, where &lt;a href=&quot;http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html&quot; rel=&quot;nofollow&quot;&gt;atmospheric pressure&lt;/a&gt; is about 1/4 of sea level. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speed does indeed matter; lift is proportional to the square of airspeed, and higher-performance planes like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird&quot; rel=&quot;nofollow&quot;&gt;SR-71&lt;/a&gt; can maintain some lift up to 25km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As mentioned by TildalWave, the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow&quot;&gt;Kármán Line&lt;/a&gt; at 100km is the point at which you need to go at orbital speeds to develop any aerodynamic lift. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-10-01T21:33:47.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12179" PostTypeId="2" ParentId="7994" CreationDate="2015-10-02T00:26:55.040" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www.brandeis.edu/graybiel/publications/docs/186_bal%20in%20rotating_ebr.pdf&quot; rel=&quot;noreferrer&quot;&gt;This study&lt;/a&gt;, by Soeda, DiZio &amp;amp;  Lackner (2002), found that - without including the visual effects of gazing out a spacecraft's window - Coriolis forces rising from rotation caused subjects to soon lose some control while doing certain tasks, including just standing and moving about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The results of one of their tests found that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The experimenter counted the number of times the subject had to use the railings in order to avoid losing balance. Each contact involved a momentary push against the rail with the back of the hand. Subjects had to use the rails&#xA;  more frequently during rotation than pre- or post-rotation, and more frequently when their eyes were closed than open. Pre-rotation there were 0.25 touches per trial in the dark and none with vision; per-rotation there were 5.04 touches per trial in the dark and 1.16 when the subject’s eyes were open; post-rotation there were 0.125 touches per trial in both visual conditions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In all the tests, closing eyes did not help subjects retain control; in fact, it lessened their ability to control themselves. However, the latter two of the authors had found (in previous experiments) that adaptation was possible:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, when permitted to make repeated movements, subjects quickly become more accurate and are able to perform near normally within 10–15 reaches&#xA;  (Lackner and DiZio 1994). Such adaptation means that the subjects’ nervous systems have planned anticipatory muscle innervations which cancel the consequences of the Coriolis forces. Aftereffects are present post-rotation with&#xA;  movement paths being mirror image to the initial ones during rotation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/vision/space/livinginspace/23jul_spin.html&quot; rel=&quot;noreferrer&quot;&gt;A NASA page on the researchers' other work&lt;/a&gt; noted that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;DiZio and Lackner have found that people can adapt to rotational speeds as fast as a carnival-ride-like 25 rpm. That's about as fast as people turn their bodies during day-to-day life.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This would seem to be a decent estimate of an upper limit if only Coriolis forces are taken into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this study (and related ones), it seems like simply the rotation - not the visual perception - is the cause of problems. That said, microgravity environments on their own can cause problems (see &lt;a href=&quot;http://www.dsls.usra.edu/meetings/bio2001/pdf/186.pdf&quot; rel=&quot;noreferrer&quot;&gt;Oman et al.&lt;/a&gt;). &lt;a href=&quot;http://www.spacefuture.com/archive/the_architecture_of_artificial_gravity_theory_form_and_function_in_the_high_frontier.shtml&quot; rel=&quot;noreferrer&quot;&gt;This&lt;/a&gt; also states&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Windows continue to be a matter of disagreement. In addition to concerns about structural integrity, environmental control (including radiation shielding), and cost, rotation introduces the issue of dizziness. Payne [20] and others have suggested that, &quot;to avoid disorientation,&quot; windows should not be provided in rotating environments. But depriving the inhabitants of an outside view would do nothing to alleviate the vestibular effects of rotation. On the contrary, it may promote the mismatch between visual and vestibular perception that leads to motion sickness [15]. Windows might provide an obvious, natural aid to orientation, in addition to the abstract, formalist cues discussed previously.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="5963" LastActivityDate="2015-10-02T00:26:55.040" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12180" PostTypeId="2" ParentId="12170" CreationDate="2015-10-02T02:50:01.980" Score="10" Body="&lt;p&gt;In the past, the options were the optimal high tech option (Hydrogen) or the commodity option (RP1, basically kerosene.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methane is certainly much easier to handle than hydrogen, with a boiling point slightly higher than oxygen, nowhere near the -259C (only 14K above absolute zero) of hydrogen. Its energy per unit liquid volume is better than hydrogen too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One factor is that liquid methane is no longer an exotic fuel. Global trade in liquefied natural gas has approximately doubled every decade for the last few decades, as can be seen at &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquefied_natural_gas&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Liquefied_natural_gas&lt;/a&gt;&#xA;(50Mtpa in 1990, 130Mtpa in 2002, 246Mtpa in 2014.) As a result the skills in handling LNG have gone from being specialist to becoming commonplace, which will reduce development costs. Perhaps more importantly, liquid methane/LNG is no longer a specialist material: you can now order it as a regular fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_rocket_propellant&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Liquid_rocket_propellant&lt;/a&gt; shows that CH4/oxygen has only about 3% better specific impulse than RP-1/oxygen, whereas there is a big jump from CH4/oxygen to H2/oxygen. (Both ethylene, and suprisingly, hydrazine, are slightly better than methane.) Therefore I think the other answers are correct in that the main reason for using CH4 is that it is reportedly available elsewhere in the solar system, and Musk would like to gain experience with it, in order to be able to refuel on other worlds. &lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="6117" LastEditDate="2015-10-02T11:13:35.060" LastActivityDate="2015-10-02T11:13:35.060" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12181" PostTypeId="2" ParentId="12177" CreationDate="2015-10-02T05:32:40.217" Score="5" Body="&lt;p&gt;There are two major constraints on aircraft operating in thin atmospheres:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lift produced by aircraft (wing)&lt;/li&gt;&#xA;&lt;li&gt;Thrust produced by the engine&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The lift produced is directly proportional to the density; as altitude increases, density and hence lift reduces. This can be compensated to a certain extent by increasing the speed (or by increasing the lift coefficient). However, as the speed increases, the drag also increases and the required thrust goes up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However,the engines need air to generate thrust (it doesn't matter if they are piston + propeller or gas turbine) and as the density falls, so does the thrust. As a result, the aircraft won't be able to generate enough speed to produce the required thrust to fly in a thin atmosphere. Usually this limitation is reached before the wings stop producing lift and the airplane would never reach theoretical considerations like the Kármán Line.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-10-02T05:32:40.217" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12182" PostTypeId="2" ParentId="12166" CreationDate="2015-10-02T11:25:20.320" Score="17" Body="&lt;p&gt;The biggest thing that Mars One has accomplished is getting people to think about going to Mars. That's actually a difficult process, and so far their efforts have done much towards changing the perspective of people in thinking that way. People talk about Mars One in a way that going to Mars hasn't been talked about much. SpaceX, while it isn't a secret at all that they want to go to Mars, hasn't really hit the public perception like Mars One has. In a way, movies like The Martian owe their existence, in some part, to Mars One, for sparking the public interest in traveling to Mars by humanizing it, by picking astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars One plan, so far as I can tell, is to raise money and purchase the equipment, not to really build things themselves. They have done some whitepaper development, and funded some research. I can't tell the exact relationship, but I believe &lt;a href=&quot;https://www.wageningenur.nl/en/newsarticle/Vegetables-on-Mars-within-ten-years.htm&quot; rel=&quot;noreferrer&quot;&gt;Mars One is funding research&lt;/a&gt; to determining what kinds of crops can grow on Mars. The below text is the evidence that I have for such. It seems at least one of the advisers on Mars One is researching this.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Professor Leo Marcelis of Wageningen University is advisor in the ‘Mars One’ project and one of Wieger Wamelink’s colleagues. He is looking into cultivation systems that should make growing vegetables on Mars possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line, they did a great job at PR for a Mars trip, raising some money, writing some whitepapers, and possibly looking into growing crops on Mars, but not much else. They've pretty much faded away since then as well.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-04-24T09:49:33.807" LastActivityDate="2018-04-24T09:49:33.807" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12183" PostTypeId="1" AcceptedAnswerId="12186" CreationDate="2015-10-02T12:05:10.960" Score="9" ViewCount="865" Body="&lt;p&gt;&lt;strong&gt;Is the source of so much Methane on Titan being overlooked?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tons of very intelligent and knowledgeable people already looked at it, so the answer in undoubtedly &quot;no&quot;, so I guess my question is more along the line: &quot;why so? I don't understand why there's so much confidence that the source of Methane on Titan is not biological.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why am I actually asking this?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember a huge excitement about Methane having been found on Mars' atmosphere before the excitement waned because the amounts were too low and it didn't appear to repeat (ex: with season). Methane is, I learnt, a pretty volatile molecule that will degrade quickly and, thus, needs to be replenished continuously - any methane found needs to have been produced recently (in geological terms). Additionally, it is a known biological &quot;waste&quot; product of biological processes on earth, hence the initial excitement about the discovery on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In early periods of life on Earth (Archaean Eon?) the Methane producing &quot;bacteria&quot; (the Archaea methanogens) possibly ruled the Earth, probably producing lots of Methane that would be released into the atmosphere (akin to Titan's atmosphere of nowadays, I would guess). Oxygen-producing life dramatically changed the atmosphere compositions, making it toxic to the methanogens which became relegated to swamps, underground, guts of animals etc (wherever it's oxygen-free) and made all methane react with the oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doesn't Titan's atmosphere give strong reasons to believe the hypotheses that there's methane-producing life pumping out all that methane? In a similar way to what might have been Earth's past before the great oxygenation event, allowing life millennia of methane production to end up with a thick atmosphere, and similar to how Earth accrued a strong envelope of Oxygen after millennia of oxygen production?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the hypotheses for so much Methane is some geochemical process but .. there's so many bodies in the solar system, from warm to cold, from tiny to large, and yet isn't Titan the only one with (remotely) so much Methane?&lt;/p&gt;&#xA;" OwnerUserId="11812" LastEditorUserId="19335" LastEditDate="2018-04-11T08:49:12.160" LastActivityDate="2018-04-11T08:49:12.160" Title="Titan - Is the source of so much Methane being overlooked?" Tags="|planetary-science|atmosphere|moons|saturn|titan|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12184" PostTypeId="1" AcceptedAnswerId="12185" CreationDate="2015-10-02T12:53:53.740" Score="10" ViewCount="1798" Body="&lt;p&gt;A recent &lt;a href=&quot;http://www.bbc.com/news/science-environment-34412468&quot; rel=&quot;noreferrer&quot;&gt;BBC news article&lt;/a&gt; give a short history on the canals of Mars&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In the late 19th Century, the American astronomer Percival Lowell thought he saw canals on the surface of Mars.&lt;/p&gt;&#xA;&lt;p&gt;He suggested that this was evidence that our near planetary neighbour might not only harbour life, but could possibly be home to an advanced civilisation.&lt;/p&gt;&#xA;&lt;p&gt;This fired the public's imagination - prompting HG Wells's novel, The War of the Worlds, several comics and numerous Hollywood science fiction films depicting Martian invasions.&lt;/p&gt;&#xA;&lt;p&gt;There was a mixture of disappointment and some relief when the existence of canals was finally disproved by orbiting spacecraft sent to the Red Planet in the 1960s and 70s.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;Note: There are a number of other significant series, including &lt;a href=&quot;https://en.wikipedia.org/wiki/John_Carter_of_Mars&quot; rel=&quot;noreferrer&quot;&gt;John Carter of Mars&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Barsoom&quot; rel=&quot;noreferrer&quot;&gt;Barsoom Series&lt;/a&gt; that began in 1912 by Edgar Rice Burroughs in which the canals played a major part.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;If there are no canals on Mars now, what was it that everyone was seeing that created and continued the belief in the Martian Canals?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-02T20:21:42.610" Title="What is the science behind the canals of Mars?" Tags="|mars|planetary-science|history|water|myth-busting|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12185" PostTypeId="2" ParentId="12184" CreationDate="2015-10-02T13:10:08.490" Score="16" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_canal&quot; rel=&quot;nofollow noreferrer&quot;&gt;Canals on Mars&lt;/a&gt; has quite an interesting history, starting with Giovanni Schiaparell. He produced this map of Mars in 1877&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cmwjX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cmwjX.png&quot; alt=&quot;![enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is interesting to compare this map to a &lt;a href=&quot;http://www.msss.com/msss_images/latest_weather.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;more modern map&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7dHiI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7dHiI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the same general features are labeled, however, the lines that run between them don't show up at the modern image. Giovanni was Italian, and called the lines on these chart &quot;canali&quot;, which means Channels. It was erroneously translated as Canals, a small, but significant different, canal implying produced by something. This false believe became quite popular, and fed upon itself, becoming something more than it really was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current theory as to why these channels were seen is that they were optical illusions, followed by the expectation that there should be canals, and nothing more. Keep in mind that the images used to produce such maps looked something like &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_canal#/media/File:Marssep5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DtM2D.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DtM2D.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's amazing that he was as accurate as he was! The channels as seen were simply too small to be accurate, however, and thus were simply optical illusions.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-02T13:10:08.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12186" PostTypeId="2" ParentId="12183" CreationDate="2015-10-02T17:11:23.933" Score="12" Body="&lt;p&gt;It is not being overlooked. There are studies which are being carried out about the feasibility of life on Titan based on methane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is true that Titan has all the necessary conditions for life:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It is not in thermodynamic equilibrium&lt;/li&gt;&#xA;&lt;li&gt;It has more than enough of carbon based molecules&lt;/li&gt;&#xA;&lt;li&gt;It has a fluid environment (ethane/methane) for chemical reactions to take place&lt;/li&gt;&#xA;&lt;li&gt;Its temperature is low enough to permit bonding of molecules (though not like on the Earth)&lt;/li&gt;&#xA;&lt;li&gt;It has other requirements for life, such as metals, a changing geology with (hydrocarbon) oceans and rocky surfaces etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, having &lt;em&gt;necessary&lt;/em&gt; conditions does not mean that life has arisen or it is there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main problem for explaining methanogenic life on Titan is that there is no mechanism that explains its production by process that life as we know is capable of. The most common method of methane production by methonogenic bacteria is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\require{mhchem}\ce{CO2 + 4H2 -&amp;gt; CH4 + 2H2O}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there is little or no $\ce{CO2}$ in the atmosphere of Titan. There is a well understood methane cycle on Titan; however, the origin and replenishment are still mysteries. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a way, there is too much methane on Titan compared to other bodies in solar system. The problem with life in Titan is not how it &lt;em&gt;produces&lt;/em&gt; methane, rather, how it &lt;em&gt;uses&lt;/em&gt; methane. Now we have to develop a way by which lifeforms use methane much like we use water. Though this may be possible, there are some issues with this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Titan is &lt;em&gt;very&lt;/em&gt; cold. It's surface temperature is around -180°C. The main problem is that we know of no catalyst at this temperature, without which life would not be feasible.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Water is a very good solvent, helping in transporting substances in a cell. How methane would behave in such circumstances is not very well understood, though &lt;a href=&quot;http://phys.org/news/2015-02-life-saturn-moon-titan.html&quot;&gt;a model of nitrogen based cell membrane that could exist in methane&lt;/a&gt; has been modeled.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Though methane producing organisms were once dominant on Earth when it had atmosphere similar to Titan, there is one difference &amp;ndash; there is no water vapor on Titan.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Water has one crucial property &amp;ndash; It is denser at ~ 4°C than at its freezing point. This means that while the surface of water may be frozen, the lower part of a liquid body will still be able to support life. Methane has no such property however, and at the low temperatures (on Titan), freezing of whole hydrocarbon lakes can occur, which is not conductive to life. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;At present, geochemical processes offer a better explanation compared to biological processes, and as such is accepted by the scientific community. As a matter of principle, scientific community is conservative in accepting such things (having burnt their fingers more than once). Extraordinary claims require extraordinary proofs. In case of extraterrestrial hydrocarbon based life, no such proof has been offered. However, the hunt for life on Titan goes on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a proposed &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_Saturn_System_Mission&quot;&gt;Titan Saturn System Mission&lt;/a&gt;; however budget constraints have postponed this mission. In a way, the mission selections are biased towards finding water based habitable planets/moons (which would be directly useful for us) rather than find exotic life.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastEditorUserId="49" LastEditDate="2015-10-02T17:32:11.170" LastActivityDate="2015-10-02T17:32:11.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12187" PostTypeId="2" ParentId="12184" CreationDate="2015-10-02T18:57:14.497" Score="7" Body="&lt;p&gt;This &lt;a href=&quot;http://scienceblogs.com/universe/2012/09/28/the-canals-of-mars/&quot; rel=&quot;nofollow&quot;&gt;article about canals on Mars&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://www.skyandtelescope.com/astronomy-news/venus-spokes-an-explanation-at-last/1/?c=y&quot; rel=&quot;nofollow&quot;&gt;More recent&#xA;  scholarship&lt;/a&gt;&#xA;  suggests that what &lt;a href=&quot;https://en.wikipedia.org/wiki/Percival_Lowell&quot; rel=&quot;nofollow&quot;&gt;Percival&#xA;  Lowell&lt;/a&gt; observed as&#xA;  &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_canal&quot; rel=&quot;nofollow&quot;&gt;Martian canals&lt;/a&gt; was merely observing projections of the vein structure of his own&#xA;  eyeball, a &lt;strong&gt;known nuisance among planetary observers using very high&#xA;  magnification.&lt;/strong&gt;&quot; &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(links added and text bolded by me)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IOW, it was artifact of poor instrumentation they used at that time.&lt;/p&gt;&#xA;" OwnerUserId="6305" LastEditorUserId="58" LastEditDate="2015-10-02T20:21:42.610" LastActivityDate="2015-10-02T20:21:42.610" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12188" PostTypeId="1" AcceptedAnswerId="12192" CreationDate="2015-10-02T19:10:03.900" Score="9" ViewCount="706" Body="&lt;p&gt;As of yesterday 01 October 2015, a team is hoping to raise £600k (~$911k) to fund a &lt;a href=&quot;https://www.kickstarter.com/projects/moonspike/moonspike-the-worlds-first-crowdfunded-moon-rocket?ref=space.se&quot; rel=&quot;noreferrer&quot;&gt;Moon impactor via Kickstarter&lt;/a&gt;, which should cover the costs of building and testing a two-stage rocket, a spacecraft with what seems like a custom propulsion subsystem, and a small titanium dart which should impact the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their &lt;a href=&quot;http://moonspike.com/wp-content/uploads/2015/09/MS_feasibilityStudy_Oct01_2015.pdf&quot; rel=&quot;noreferrer&quot;&gt;feasibility study&lt;/a&gt; seems quite imprecise and amateurish to me. Maybe are aerospace engineers not the targeted audience despite the purpose of the document being to &quot;outline the technical requirements for reaching the mission goal and present a starting point for ongoing research and development.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How complete and technically feasible is the information presented in the feasibility study?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below are some extracts which seem imprecise or even incorrect to me. I have added a short comment, which may be very wrong, so please correct me if needed.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Section 6.1&lt;/h1&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The trajectory from LEO towards the Moon is known as a Hohmann Maneuver, by which the&#xA;  spacecraft transitions from LEO to an elliptical orbit, intersecting the trajectory of the Moon. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;An Hohmann transfer is one of the many different trajectories to go from one orbit to another. In actual operations, these transfers do not use the exact theoretical definition of these orbits (due to orbit corrections after adequately ranging the spacecraft while in transit).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Section 6.3&lt;/h1&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;LEO altitude must be high enough to limit drag-induced altitude loss, e.g. 200 km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To start with, what is the inclination and eccentricity of this parking orbit, then we can talk about the three other orbital parameters which are independent from time (i.e. all but the true anomaly)?&#xA;If the vehicle is in such a low orbit for only a very limited amount of time before heading on towards the Moon, drag is not much of a problem in itself.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Section 7&lt;/h1&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Spacecraft shall utilize non-cryogenic propellants, as the expected nominal mission time will otherwise result in excessive propellant boil-off.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Depending on the materials used to insulate the fuel tank, boil-off can be completely negligible for most months-long missions, or am I completely mistaken?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't read the full feasibility study, so I'm sure there are many passages I would question in the rest as well.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="25" LastEditDate="2015-10-02T23:29:02.087" LastActivityDate="2015-10-03T08:15:44.680" Title="How feasible is the Moonspike mission?" Tags="|launch|rockets|the-moon|spacecraft|mission-design|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12189" PostTypeId="1" AcceptedAnswerId="12190" CreationDate="2015-10-02T19:13:19.513" Score="16" ViewCount="2825" Body="&lt;p&gt;Could there be a stable orbit shared by two planets with the same mass and density? Regardless of whether the formation of such system is possible or not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if Earth and Venus could have the same orbit around the sun, but each planet in the opposite side of the orbit, would that orbit be stable?  &lt;/p&gt;&#xA;" OwnerUserId="962" LastEditorUserId="238" LastEditDate="2015-10-02T19:34:05.000" LastActivityDate="2015-10-03T02:28:29.343" Title="Could twin planets exist and share the same orbit?" Tags="|orbit|planet|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12190" PostTypeId="2" ParentId="12189" CreationDate="2015-10-02T19:44:35.313" Score="16" Body="&lt;p&gt;The term for bodies sharing an orbit in this manner is &lt;a href=&quot;https://en.wikipedia.org/wiki/Co-orbital_configuration&quot;&gt;co-orbital configuration&lt;/a&gt;. It is technically possible for two objects of planetary size to share an orbit like this, and we may have even found &lt;a href=&quot;https://www.newscientist.com/article/dn20160-two-planets-found-sharing-one-orbit/&quot;&gt;an example&lt;/a&gt; in an extrasolar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is speculated that early in Earth's history that it shared an orbit with a Mars-sized planet named &lt;a href=&quot;https://en.wikipedia.org/wiki/Theia_%28planet%29&quot;&gt;Theia&lt;/a&gt; which later collided with Earth to produce the Moon.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-10-02T19:44:35.313" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12191" PostTypeId="2" ParentId="12189" CreationDate="2015-10-02T22:13:20.847" Score="23" Body="&lt;p&gt;Saturn has co-orbital moons, &lt;a href=&quot;https://en.wikipedia.org/wiki/Epimetheus_%28moon%29&quot;&gt;Epimetheus&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Janus_%28moon%29&quot;&gt;Janus&lt;/a&gt;. They orbit in the same direction and swap orbits approximately every four years as the inner body catches up with the outer body.&lt;/p&gt;&#xA;" OwnerUserId="11821" LastActivityDate="2015-10-02T22:13:20.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12192" PostTypeId="2" ParentId="12188" CreationDate="2015-10-02T23:12:16.953" Score="6" Body="&lt;p&gt;First of all, in direct response to your questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A Hohmann Maneuver is a perfectly acceptable orbit transfer to get to, say, another planet. From Earth, it doesn't make as much sense to use. I wouldn't expect them to state the exact orbit, but they should have some margin for corrections.&lt;/li&gt;&#xA;&lt;li&gt;200 km is the lowest orbit to stay at for any length of time. I think the orbital lifetime there is measured in days, at most. Personally, I would aim for higher, say, 250 km at a minimum, to give some margin. In fact, they have a chart that shows the minimum length in orbit is about 50 hours, which is indeed what I expected. &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/2848/25&quot;&gt;Cryo fuels&lt;/a&gt; haven't been used for longer than a day. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It seems a bit optimistic, but not unreasonable, as a whole. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Skimming the proposal, I don't see any serious flaws. Of course, a detailed study would be required to really identify if it is capable of doing what they say they can do. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some issues to worry about, however.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What I'd be most worried about is the low margin in the power system. A 10% margin is not very much, considering they have no method of recharging the batteries. They do correctly indicate there is a 10% margin on the worst case time to achieve the mission. &lt;/li&gt;&#xA;&lt;li&gt;The thrust levels are very nominal, I personally haven't seen such even levels. It might be possible (I'm more familiar with Solid rockets than liquid), but... &lt;/li&gt;&#xA;&lt;li&gt;They also don't cover anything about thermal issues, which for a potentially 5 day mission could be problematic.&lt;/li&gt;&#xA;&lt;li&gt;I don't see any requirements on attitude control, other than it is required. This could cause a number of problems, from transmission back to Earth, to not getting the correct attitude for firing the thruster, and likely others.&lt;/li&gt;&#xA;&lt;li&gt;The navigation system could work, but I'm not quite convinced. They have the Earth, Sun, Moon, and IMU. They will be able to correctly identify the attitude if they can get the Earth, Moon, and Sun all at once, then use the IMU if one of the 3 is missing. They definitely need a breakdown on the accuracy of the sensors. The Moon might be difficult to build a sensor to detect, because it has radically different lighting and temperature values. In particular, I'm concerned that they don't seem to be using the sensors until after they do their TLI burn. &lt;/li&gt;&#xA;&lt;li&gt;A COTS GPS doesn't work above a certain altitude/speed, due to anti-SCUD laws. Amateurs have previously build such a device that was capable of receiving GPS from orbit, with special permission. Still, some kind of specialized GPS system is required (And GPS probably isn't needed that much anyways).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It might also be worth comparing this to the Fox-1 project, an &lt;a href=&quot;http://www.amsat.org/amsat-new/symposium/2012/2012_Symposium_Fox_Overview.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Amateur produced satellite&lt;/a&gt;, built by volunteers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I think they could almost get close, but I can't see this mission happening in a short order of time. For their budget, they probably won't get the testing required to make it really sound. In particular, the thermal testing would probably cost a significant part of the entire budget, and with such a variety of thermal situations to manage, I don't think this would happen.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-10-03T08:15:44.680" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12193" PostTypeId="2" ParentId="12189" CreationDate="2015-10-03T02:28:29.343" Score="8" Body="&lt;p&gt;I believe the two bodies could be orbiting a central bodies at the stable &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Lagrange points&lt;/a&gt;. Planet A would be at Planet B's L4 Lagrange point, and Planet B would be at Planet A's L5 point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WvreQ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WvreQ.png&quot; alt=&quot;Lagrange Points&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;from Wikipedia&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2015-10-03T02:28:29.343" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12195" PostTypeId="1" CreationDate="2015-10-03T05:06:11.197" Score="10" ViewCount="1736" Body="&lt;p&gt;Elon Musk has many times announced plans to go (manned) to Mars and it seems strange that investors and stakeholders would be interested in spending money on this just because Elon is interested. There is a possibility of charging scientific institutions to perform research or even take people but this still does not seem viable as a business. Is research the only potential source of income for a commercial manned mission to Mars or are there other business benefits that I've missed?&lt;/p&gt;&#xA;" OwnerUserId="11825" LastEditorUserId="25" LastEditDate="2015-10-03T18:23:17.953" LastActivityDate="2018-05-18T11:27:30.120" Title="Are there serious business benefits to going to Mars?" Tags="|mars|spacex|commercial|crewed-spaceflight|" AnswerCount="5" CommentCount="1" ClosedDate="2018-05-19T08:04:03.610" ContentLicense="CC BY-SA 3.0" />
  <row Id="12196" PostTypeId="2" ParentId="12195" CreationDate="2015-10-03T06:25:45.313" Score="1" Body="&lt;p&gt;Business is what humans want to do. If they want to go to Mars, then it is good business to do so. Why? I don't know, it is a great philosophical mystery why humans want to do anything. Ask people to find out. They do what they want with the profits from what they have created. I don't recommend it, but you might want to search for sexually related stuff online to find out about stranger stuff humans want to do. Going to Mars might become big business just like getting a piercing or baking a cake did. What do you mean &quot;benefits&quot;?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-10-03T06:25:45.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12197" PostTypeId="2" ParentId="5525" CreationDate="2015-10-03T10:38:37.603" Score="8" Body="&lt;p&gt;In addition to Geoff's answer: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was no requirement for the CRS spacecraft to provide ISS reboost. The two reboost methods available now (Progress, and the ISS' own engines on &lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_%28ISS_module%29&quot; rel=&quot;noreferrer&quot;&gt;Zvezda&lt;/a&gt;) were seen as sufficient. NASA was working on a third method: they wanted to install a VASIMR ion engine to test it for reboost purposes. This &lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket#Potential_testing_on_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;plan was canceled in 2015&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SuperDraco is &lt;a href=&quot;https://www.reddit.com/r/spacex/wiki/faq/dragon&quot; rel=&quot;noreferrer&quot;&gt;too powerful&lt;/a&gt;. Reboost is done at lower thrust levels to limit the forces acting on the station.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Progress is connected to the station's guidance system during the reboost. Dragon &lt;a href=&quot;https://www.reddit.com/r/spacex/wiki/faq/dragon&quot; rel=&quot;noreferrer&quot;&gt;doesn't have this capability&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Dragon reboost from the front end of Harmony would require that the station be turned through 180º (twice), &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=26587.0&quot; rel=&quot;noreferrer&quot;&gt;which is doable but increases the station's exposure to micrometeoroids&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A reboost from Cargo Dragon's current berth position would place bending forces on the docking port. This might be an issue, but something similar happened when the Shuttle was used for reboost.    &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-03T10:38:37.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12199" PostTypeId="2" ParentId="12195" CreationDate="2015-10-03T14:38:41.227" Score="3" Body="&lt;p&gt;Well, I'm not sure that I can answer the question of research benefits, although there are major scientific and manufacturing possibilities in low or so-called zero gravity environments. Probably the simplest example for free-fall production is a shot tower for making musket balls. Anyway, there are three very general space ideals (and of course, don't limit yourself by these). You might consider them reasons or rationale for pursuing one type of spaceflight.&lt;/p&gt;&#xA;&lt;p&gt;The first being human spaceflight, with its Utopian thought. Human activities in space, from a public perspective, are inseparable from spaceflight. Going back into the past many popular authors suggested that spaceflight needed to be human oriented. They believe/d that it was destiny. Mankind would first send out simple robots, then launch themselves into the universe and colonize. This would allow mankind to find peace and create many Utopian societies. Some call this the Von Braun Paradigm, but that's another topic. Many groups still ultimately believe that space exploration should ultimately lead to human migration.&lt;/p&gt;&#xA;&lt;p&gt;The second would be that of purely scientific and robot exploration. Those who support space exploration from this perspective tend to point out how expensive and draining a human oriented spaceflight program can be on society. There is some mixing of a pure robotic program and a human program. Those who don't choose one or the other will tend to use one to support the other. For example, some might believe it is too early for human spaceflight, but acknowledge that robotic exploration can help to eventually put humanity in a better position (for migration that is). &lt;a href=&quot;http://www.americanscientist.org/bookshelf/pub/neil-degrasse-tyson&quot; rel=&quot;nofollow noreferrer&quot;&gt;Neil deGrasse Tyson is a perfect example&lt;/a&gt; of the split between human and robotic spaceflight:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;In your autobiography, The Sky Is Not the Limit, you noted the success of robotic space exploration and wrote that &amp;quot;We should not measure our spacefaring era by where footprints have been laid.&amp;quot; Given the high cost and risks, why do you support a manned mission to Mars?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;When I wear my scientist's hat, I do not support a manned mission to Mars. The cost versus the return on such a mission is embarrassingly low. But when I wear my public educator hat, I see and experience the public's vicarious thrills of watching their own species go into orbit and beyond. Astronauts are the only kind of celebrity I know who can have a line of people waiting for their autograph, even if the line of people does not know in advance the astronaut's name.&lt;/p&gt;&#xA;&lt;p&gt;This level of interest runs deep and filters through Congress and on to funding streams. That's why the science programs of NASA have never been more than one-third of the agency's budget. So the social and political reality differs from how the scientific community would rather see it.&lt;/p&gt;&#xA;&lt;p&gt;And since I spend large parts of my time at that intersection, I fully understand that urge to explore with humans and will not try to fight this basic human urge.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The third is that of transhumanism. This one focuses on human modification and adaptation to overcome the rigors of spaceflight. You can stretch the idea of transhumanism by going back to the movie &lt;em&gt;2001: A Space Odyssey&lt;/em&gt; by Kubrick and Clarke. Transhumanism can include the idea of evolution over millions, or billions, of years. In other words, if we are going to eventually evolve, why not do it now?&lt;/p&gt;&#xA;&lt;p&gt;Space exploration isn't always about the money. I imagine this is a &lt;em&gt;small&lt;/em&gt; reason why people will invest in what might be called outlandish schemes. There is something romantic about being a pioneer and being on the frontier. People see space as salvation. It is the only way to save the human race. So, why do people want to go to Mars? For me, I'd go to Mars because I've always been taught to idolize astronauts. Who doesn't want to be a hero? It's a dream. Now imagine having heroes working for your business.&lt;/p&gt;&#xA;&lt;p&gt;A good resource on this topic would be Launius and McCurdy's &lt;em&gt;Robots in Space: Technology, Evolution, and Interplanetary Travel&lt;/em&gt;. It's not the best resource, but it might give some deeper insight into the motivations. If I remember correctly, its been years, the book is split into sections similar to what I've mentioned.&lt;/p&gt;&#xA;&lt;p&gt;Another interesting video, from Werner Von Braun, is the Disney movie &lt;em&gt;Man in Space&lt;/em&gt;. There is a series of videos promoting his early vision.&lt;/p&gt;&#xA;" OwnerUserId="11641" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-03T14:38:41.227" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12200" PostTypeId="1" CreationDate="2015-10-03T16:25:52.390" Score="4" ViewCount="525" Body="&lt;p&gt;Let's say, hypothetically, that I'm thinking about launching a high altitude balloon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to contribute to the bank of mankinds' knowledge, not just take pretty pictures. My (notional) budget is around £300 ($450) for instrument payload, and my maximum payload weight is 2kg. I'm aiming for an altitude of at least 30km, and launching from the south of the UK.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What measuments can I perform that would be genuinely useful to the scientific community?&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="168" LastEditDate="2015-10-03T19:41:10.853" LastActivityDate="2015-10-18T13:09:11.827" Title="Scientifically useful measurements from low-budget high-altitude balloon flights" Tags="|experiment|balloons|science|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12201" PostTypeId="2" ParentId="12195" CreationDate="2015-10-03T18:35:31.230" Score="6" Body="&lt;p&gt;There are a few very interesting benefits, although most of them won't take place for some time. I'll break down benefits into short and long term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Better understanding Earth, sometimes it helps to understand things from a different perspective&lt;/li&gt;&#xA;&lt;li&gt;Inspiration- Many people are inspired by such advances in technology. The Apollo project produced similar benefits, inspiring many people to become Engineers, Scientists, and other technical professions.&lt;/li&gt;&#xA;&lt;li&gt;Setting up a foundation for things to come.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Long term&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars is the best place to farm and get food to outer space. Mars likely will be a major re-supply base for Asteroid mining missions&lt;/li&gt;&#xA;&lt;li&gt;Humanity needs to escape it's cradle (Earth) if it is going to continue to grow.&lt;/li&gt;&#xA;&lt;li&gt;Backup for humanity in case of a global disaster (Asteroid impact, nuclear war, etc)&lt;/li&gt;&#xA;&lt;li&gt;Engineering advances brought by low gravity&lt;/li&gt;&#xA;&lt;li&gt;Engineering advantages by having fewer humans (United States had many advances because there wasn't enough people to do things, for instance)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-03T18:35:31.230" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12203" PostTypeId="2" ParentId="12195" CreationDate="2015-10-03T18:51:08.977" Score="7" Body="&lt;p&gt;It probably isn't good business along the lines of conventional return on investment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However &lt;strong&gt;necessity is the mother of invention&lt;/strong&gt;, building colonies on Mars will inevitably accelerate technological development, not only generally in terms of spaceflight and life support, but also in areas of automation, replication, biosphere management - perhaps in the longer run terraforming.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This kind of technological advance will unquestionably be profitable for humankind, it is difficult to say how much of such advances will become profit for the company responsible for putting humans on Mars, but one might expect that since they are the ones developing most of the technology, they will also have the patents and such and be in the best position to develop and market the technology to an Earth market.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now it is true that in principle any technological advance which could be made by putting humans on Mars, could be made without putting humans on Mars. But humans are lazy and usually need a spur or a goal for development to happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can at least imagine a scenario where some new technology gets developed like self-replicating machinery which makes the owner of said technology fabulously wealthy. Would it happen? Impossible to say. Technological advance is like that, it can't be predicted. But I certainly think it is possible.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-10-03T18:51:08.977" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12205" PostTypeId="2" ParentId="12200" CreationDate="2015-10-03T19:27:48.080" Score="4" Body="&lt;p&gt;Advancement in science comes from&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;repeated (and repeatable)&lt;/strong&gt; observations that are&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;systematically processed&lt;/strong&gt;,&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;hypotheses&lt;/strong&gt; that get tested on experimental data,&lt;/li&gt;&#xA;&lt;li&gt;and  conclusions that get &lt;strong&gt;peer review&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One high-altitude balloon with low-cost off-the-shelf sensor payload will add nothing to existing space/earth sciences if you do not do the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;read principal textbooks on the discipline you want to advance;&lt;/li&gt;&#xA;&lt;li&gt;read scientific journals / arXiv papers going back at least 10 years;&lt;/li&gt;&#xA;&lt;li&gt;formulate a list of unsolved problems in space/earth sciences;&lt;/li&gt;&#xA;&lt;li&gt;choose a few of the problems that can be re-worded into testable hypotheses that need some vertical profile data/high atmospheric measurements;&lt;/li&gt;&#xA;&lt;li&gt;verify that your budget allows you to design a data collection campaign with instrument accuracy and number of samples necessary to test the hypotheses;&lt;/li&gt;&#xA;&lt;li&gt;document all the decisions you make;&lt;/li&gt;&#xA;&lt;li&gt;do Monte Carlo hypothesis testing on mock data;&lt;/li&gt;&#xA;&lt;li&gt;calibrate your instrument package against standard sources/instruments (e.g. radiation sources if you are launching a Geiger counter) at the full range of environmental conditions that your balloon will encounter;&lt;/li&gt;&#xA;&lt;li&gt;publish the results (arXiv is the best bet here, you'd be well advised to team up with one or two subject matter experts - you can get more info by asking at &lt;a href=&quot;https://academia.stackexchange.com&quot;&gt;Academia SE&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As a necessary reminder, you &lt;a href=&quot;https://aviation.stackexchange.com/questions/9931/how-does-one-launch-unmanned-high-altitude-balloons-without-posing-a-safety-risk&quot;&gt;&lt;strong&gt;HAVE TO COORDINATE LAUNCH WITH LOCAL AIR CONTROL AUTHORITY&lt;/strong&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can make the balloon reusable (that is, you can afford going to another country to pick it up or offer an award to any who find it), you stand a much higher chance of getting some stuff done. One vertical profile is unlikely to be useful, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the first couple of papers you can apply for a grant to do more and better stuff without ruining your credit history (unless you are &lt;a href=&quot;https://en.wikipedia.org/wiki/Alfred_Lee_Loomis&quot; rel=&quot;nofollow&quot;&gt;Alfred Lee Loomis&lt;/a&gt; or Howard Hughes-rich). No guarantees, but science enjoys economies of scale nowadays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: for an insight into cutting-edge real-life high-altitude experiment design, please have a look at &lt;a href=&quot;https://aviation.stackexchange.com/a/22102&quot;&gt;this answer&lt;/a&gt; at Aviation SE, describing astronomical observations from a Concorde.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-10-18T13:09:11.827" LastActivityDate="2015-10-18T13:09:11.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12207" PostTypeId="1" CreationDate="2015-10-04T12:49:54.890" Score="9" ViewCount="1380" Body="&lt;h2&gt;Background&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The rovers on Mars are slow and weak. If I am to believe what I read, MSL Curiosity only produces 1/6th of a horsepower electric effect, 125 watt. In order to perform tasks like deep drilling, excavation or fast roving, &lt;em&gt;three orders of magnitude&lt;/em&gt; more power would be nice. It has been proposed to have astronauts orbit Mars and teleoperate a rover in real time, but that is hardly useful with a sleepwalker like MSL with a top speed of 1½ meter per minute and a 2 inch drill.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Questions&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;What power sources will future capable rovers and construction equipment on Mars have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RTG's seem prohibitively expensive to scale up or multiply. Is other nuclear power feasible for caterpillar sized equipment on Mars, as such used for submarines and icebreakers? Could a rover have a tall mast with huge solar panels in the low gravity and weak winds of Mars? Or could chemical fuel, like methane and oxygen from the atmosphere, produced with solar power in advance and stored on Mars be a solution? Or huge stationary solar panels with cables or microwave beams to supply mobile equipment?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-10-04T12:55:55.137" LastActivityDate="2021-03-30T23:46:05.073" Title="How could a fast powerful rover on Mars be powered electrically?" Tags="|mars|rovers|power|solar-power|nuclear|" AnswerCount="9" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12209" PostTypeId="2" ParentId="12207" CreationDate="2015-10-04T13:35:29.477" Score="2" Body="&lt;p&gt;Mars Direct, by the Mars Society/Robert Zubrin have proposed using ISRU (In Situ Resource Utilization) to produce methane and oxygen to power a large faster internal combustion manned rover. They also proposed landing modules within the powered distance of the rover, to allow backup possibilities.  I.e. If there is a second earth return vehicle for the next mission, within the range of the rover away, there is a backup return vehicle for the current crew.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A base station with solar power to run the methane/oxygen generation process, acting as a gas station, could be one way to get to higher power rovers on Mars. Eventually scaling issues would come into play as the rover would be limited to a radius that is half the range of the rover.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-10-26T12:46:42.620" LastActivityDate="2016-10-26T12:46:42.620" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12210" PostTypeId="2" ParentId="12207" CreationDate="2015-10-04T15:20:21.203" Score="4" Body="&lt;p&gt;Nuclear power is not feasible for vehicles. There have been some efforts in the past, but nothing remotely usable.&lt;br&gt;&#xA;The smallest nuclear reactor I'm aware of is the &lt;a href=&quot;http://www.gnnallc.com/pdfs/NPP%2016%20Autonomous%20Power%20System%20for%20Remote%20Locations.pdf&quot; rel=&quot;nofollow&quot;&gt;ELENA&lt;/a&gt; design (still a concept), which produces 100 kWe and weighs 235 tons. A submarine power plant weighs in the region of 1000 tons and produces in the region of 50 MWe.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I don't think power is the main obstacle in increasing a rover's top speed. Autonomous navigation is a difficult problem, and driving at a low speed reduces the risk of a crash. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-04T15:20:21.203" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12211" PostTypeId="2" ParentId="12207" CreationDate="2015-10-04T15:44:05.333" Score="6" Body="&lt;p&gt;The limiting factor on rovers now is the power generation while moving. Incidentally, this same problem exists in Kerbal Space Program, and was covered by Andy Weir in &quot;The Martian&quot;. The problem is, you can either generate power, or move, but it's difficult to do both at the same time. There are a few ways of overcoming this problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best way is to have large amounts of power available while resting (Deploy solar panels, for instance), and take periodic breaks. This is unfeasible for current Mars rovers, because the deploying/ retracting of solar panels over time would prove to be difficult. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second way is to contain the power generation to the vehicle while moving. There are 3 ways of doing this, namely fixed panel solar arrays, like Opportunity, which produce little power, but doesn't involve deploying anything. Secondly, you can have an RTG system (Based off of Radioactive Decay), like Curiosity. They aren't very popular to launch (As it contains radioactive material), have difficult fuel to produce, and produce a fixed amount of power with time (Somewhat fluctuating based on the background temperature). You can never get high power with one of these without high weight. Next you could have a nuclear power plant. These are very fickle, I can't imagine operating one remotely, and they are quite large. This might work for a very large rover (Think the size of, say, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Crawler-transporter&quot; rel=&quot;nofollow&quot;&gt;Crawler&lt;/a&gt;), but is unfeasible for short term. Also, this would probably work better with some form of deployment, to reduce the heat. Lastly, one could do a fuel based system, capturing the exhaust. Unless you can bring enough fuel to be useful, this is a difficult process. Essentially this will work like an efficient battery. Note the exhaust could be captured to re-form fuel. Robert Zubrin in The Case for Mars proposes using Methane/ Oxygen, and capturing the exhaust water for future re-processing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, without more power, you can't move very fast. And power is a difficult thing to do, especially with unmanned vehicles. I believe a manned vehicle, with a deployable solar cell system (As used in The Martian, for instance), would be the best way to drive fast. I believe Robert Zubrin's methane powered combustion engine would be the most effective way to store the power from the solar cells. With that kind of a system, you could drive at a reasonable speed, say 5-10 miles per hour, which is probably best for such an uncharted location. I imagine one could go faster too, if one wanted, it's all in trading off safety and efficiency for speed.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-04T20:59:55.677" LastActivityDate="2015-10-04T20:59:55.677" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12212" PostTypeId="2" ParentId="12207" CreationDate="2015-10-04T18:12:54.807" Score="3" Body="&lt;p&gt;How could we increase the efficiency of vehicle-sized solar cells? One approach might be an orbital concentrating mirror. These have occasionally been proposed on earth, but on earth it's much cheaper just to build a large solar farm. For a Mars mission, it's actually easier to put things in orbit than it is to put them on the ground. Pointing it at a moving vehicle is difficult but not impossible if you're moving smoothly in a straight line. It would certainly be useful for stationary drilling operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Various organisations keep promising compact Thorium reactors, and Lockheed Martin even claim to be working on fusion. If any of these make it into commercial production, they might be good options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it's worth considering how much speed is safe. Curiosity has been gradually accumulating wheel damage. The rocks are sharp as there is no real weather to smooth them. The faster you go, the greater the chances of a crippling accident.&lt;/p&gt;&#xA;" OwnerUserId="1521" LastActivityDate="2015-10-04T18:12:54.807" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12213" PostTypeId="1" CreationDate="2015-10-04T18:48:34.033" Score="1" ViewCount="41" Body="&lt;p&gt;Are there plans on what an orbiter to Pluto might look like? Specifically, in the type of orbit one would do at the system. With the center of mass of Pluto/Charon being outside of either object, it seems to me it would be somewhat difficult to design an orbiter, even if we had the technology to build one today (I would argue we don't). Specifically, here's some things I believe should be considered:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Orbital lifetime- The orbit should be stable over a long period of time.&lt;/li&gt;&#xA;&lt;li&gt;Close- It seems like the ability to be close to both Pluto and Charon would be required. I'm not sure of the orbital dynamics involved, but it seems like it would be desired.&lt;/li&gt;&#xA;&lt;li&gt;Both faces- The orbit should be such that both faces of Pluto and Charon are visible in the light. As Pluto and Charon are tidally locked to each other, this seems difficult.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I could imagine that an ion drive would be involved in such a mission, with some delta v left over to transfer between the orbits. It would also be worthy of figuring out how much delta v would be required to make the orbit of Pluto happen in, say, 10 years time (Similar path to New Horizons, with a Jupiter flyby. For simplicity, you can assume exactly identical until after the Jupiter encounter)&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-04T18:48:34.033" Title="Pluto orbiter mission design" Tags="|orbital-mechanics|spacecraft|mission-design|pluto|moons|" AnswerCount="0" CommentCount="1" ClosedDate="2015-10-04T19:44:36.097" ContentLicense="CC BY-SA 3.0" />
  <row Id="12214" PostTypeId="2" ParentId="12207" CreationDate="2015-10-04T19:09:18.130" Score="10" Body="&lt;p&gt;An option not mentioned so far are solar power satellites.  A large antenna in orbit could focus a tight, high-power microwave beam on the rover.  From a low Mars orbit (300 km), a 1 km antenna could focus 1 cm wavelength microwaves on a 3 m target.  They should be electronically steered to allow fast and accurate slewing.  You would like to have several of these orbiters, with about 20 to 30 minutes of coverage from each orbiter per day.  You could trade higher orbits with more coverage for longer time per orbit.  However the antenna size gets ridiculous (or the wasted power becomes excessive) at areosynchronous orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Make sure that the wire mesh antenna covers the entirety of the rover, so that the beam doesn't cook the rover or its occupants.  And make sure that the beam isn't pointed at you when you're walking around outside the rover.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-10-04T19:09:18.130" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12216" PostTypeId="1" AcceptedAnswerId="12218" CreationDate="2015-10-05T03:01:17.963" Score="10" ViewCount="1448" Body="&lt;p&gt;Space based solar power systems that beam power back to terrestrial receivers have been discussed for decades. However, I've always had a nagging concern that such a system, once built, could be quickly repurposed as a weapon, and so no country would permit another country to launch one.&lt;/p&gt;&#xA;&lt;p&gt;I know the power density on the ground wouldn't be high enough for a dramatic, &lt;a href=&quot;https://www.youtube.com/watch?v=jOKf5r_JMAo&quot; rel=&quot;nofollow noreferrer&quot;&gt;fry-a-city-in-ten-seconds&lt;/a&gt; kind of weapon. In fact, &lt;a href=&quot;https://en.wikipedia.org/wiki/Space-based_solar_power&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; cites expected levels of &lt;span class=&quot;math-container&quot;&gt;$23mW/cm^2$&lt;/span&gt;, which is only about twice the &lt;a href=&quot;https://en.wikipedia.org/wiki/Occupational_Safety_and_Health_Act&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSHA&lt;/a&gt; workplace limits. However, if you follow Wikipedia's reference &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800022396.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;to its source&lt;/a&gt;, on page 14 you'll find that this limit is designed to &amp;quot;avoid potential microwave interference with the D and F layers of the atmosphere.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;For those who don't care about the ionosphere (which probably includes those hoping to create a weapon) it would likely be easy to design a transmission antenna that would usually limit itself to this power density, but could easily be focused to increase it by perhaps an order of magnitude. &lt;span class=&quot;math-container&quot;&gt;$250mW/cm^2$&lt;/span&gt; would be about twice the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar constant&lt;/a&gt;, and would impart significant amounts of heat to anyone in the beam. Given that the beam could cover a large city, and add in infrastructure damage, and that's a pretty significant attack.&lt;/p&gt;&#xA;&lt;p&gt;Is this a legitimate concern? Are designers of potential space solar power systems coming up with ways to eliminate the ability to repurpose them into weapons?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Most proposals put the transmitting satellite in geosynchronous orbit, so that it will always be above its dedicated receiving station. This puts the satellite out of the range of current anti-satellite weapons, and could even make it difficult to destroy in the future; it would be so large, and could be built from redundant sections, so that punching holes through it wouldn't significantly harm it.&lt;/p&gt;&#xA;&lt;p&gt;I also understand the comparison to nuclear weapons. However, the use of nuclear weapons is such a bright line that the attacker would likely receive worldwide condemnation. Nor can you use a &amp;quot;little bit&amp;quot; of nuclear weapons, which also keeps that line bright. However, a divertable microwave beam might be easier to use briefly, and even deniably (&amp;quot;Ooops!&amp;quot;). Imagine a country briefly toasting one of its surrounding countries' capitals, and then immediately apologizing. The effect could be drastic, and could seriously destabilize the attacked country.&lt;/p&gt;&#xA;" OwnerUserId="11823" LastEditorUserId="38" LastEditDate="2023-03-25T13:25:33.090" LastActivityDate="2023-03-25T13:25:33.090" Title="Could a space based solar power system be used as a weapon?" Tags="|artificial-satellite|solar-power|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="12217" PostTypeId="1" CreationDate="2015-10-05T03:44:23.700" Score="3" ViewCount="263" Body="&lt;p&gt;What are the causes of the damaged SARJ on the ISS? This occurred back in 2007, and endangers the lives of astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, what are the solutions to this problem. Does anyone know what types of motors the Solar Panel Arrays use to rotate for optimal solar energy absorption?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a link that might help: &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station_maintenance&quot; rel=&quot;nofollow&quot;&gt;ISS Maintenance Problems&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11850" LastActivityDate="2017-09-30T19:58:27.920" Title="Solutions to Ongoing Problems on the Solar Panel rotary joints" Tags="|iss|solar-power|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12218" PostTypeId="2" ParentId="12216" CreationDate="2015-10-05T11:19:04.053" Score="7" Body="&lt;p&gt;There would be very little you could do to reconfigure a space based power bird to make it more lethal unless you had designed it as a weapon in the first place, and it's unlikely anyone would go down that route as there's much better technology to invest in if you want to destroy things. The best you could do would be to point the power output at a target.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible that space based power systems might be used as disruptive weapons against civilian targets by a desperate nation. Space based power systems require ground based receivers which would be easy targets for conventional weapons, providing the enemy could reach them with their weapons. If the receivers were damaged or destroyed the space power satellites would have nowhere to send the power, and then some big brain might decide if they aren't supplying power to the war economy they could be used to fry a few enemy cities. The power beams could be re-positioned to point at soft enemy targets, providing that they are within reach of the satellite. This would not do that much damage, but it could be disruptive to electrical systems and impact the ability of an enemy to wage war. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd have to be in pretty dire straits to try this though as it would not be very effective and as soon as they are used this way the satellites would be huge, expensive targets. Once a nation uses space-based weapons all their other space infrastructure would likely be fair game as well. It would be very risky and foolish, but as history teaches the human race has an almost limitless capacity for destructive decisions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words this is pretty unlikely and best saved for Thunderbirds Are Go! episodes.  &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2015-10-05T12:13:51.037" LastActivityDate="2015-10-05T12:13:51.037" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12219" PostTypeId="2" ParentId="12217" CreationDate="2015-10-05T11:36:29.833" Score="6" Body="&lt;p&gt;In the article &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015384.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Solar Array Alpha Rotary Joint (SARJ)&#xA;Bearing Failure and Recovery: Technical and&#xA;Project Management Lessons Learned&lt;/a&gt; you can find your answer. Pretty much the problems were in underestimating the amount of use, which caused use of improper lubricant and a few other things. You can see in the paper they believe unclear requirements/specifications for the SARJ were given to everyone.&lt;/p&gt;&#xA;&lt;p&gt;The following are a few brief quotes from the Lessons Learned section of the article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For complex systems it is especially important that everyone have a clear, correct, and basic understanding of the entire system so they can see how their own detailed knowledge fits into the whole. For an engineered system like the SARJ, under development at that time, a frequently updated project description document would have been appropriate.&lt;/p&gt;&#xA;&lt;p&gt;Such a document, however, does not seem to have existed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The first problem was then the lack of proper mission documentation and management. This led to differences in the actual design of the SARJ. The actual design varied from the conceptual design, switching to the use of a gear.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This subtle difference prevents the rollers from self-aligning like a follower-castor wheel and can lead to roller tipping and race-ring pinching (Ref. 5). The conceptual design also took a different approach to the use of “lubricants” for the rollers. In the original design concept, the rollers were&#xA;intended to drive the ring, and sufficiently high sliding friction was thought to be necessary to prevent roller-race slippage (Ref. 3). Thus, surface traction materials were suggested that could ensure high friction and enduring smooth surfaces as opposed to the low-friction surfaces actually required by the&#xA;SARJ.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The article further discusses a misuse or miscommunication of gold foil as a solid lubricant.&lt;/p&gt;&#xA;" OwnerUserId="11641" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-09-30T19:58:27.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12223" PostTypeId="1" CreationDate="2015-10-05T13:07:50.297" Score="2" ViewCount="99" Body="&lt;p&gt;Could two identical or slightly different planets (or planet-like objects if you want to be pedantic) form a binary pair that then orbit some other larger gravitic object (like a star)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was inspired by &lt;a href=&quot;https://space.stackexchange.com/questions/12189/could-twin-planets-exist-and-share-the-same-orbit&quot;&gt;Could twin planets exist and share the same orbit?&lt;/a&gt; which I thought initially was asking the question I'm asking, but wasn't.&lt;/p&gt;&#xA;" OwnerUserId="11853" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-10-05T22:13:40.570" Title="Could two planet-like objects be created-as or form a binary pair that orbits something else?" Tags="|orbit|planet|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12224" PostTypeId="1" AcceptedAnswerId="12235" CreationDate="2015-10-05T15:24:22.507" Score="6" ViewCount="600" Body="&lt;p&gt;Sorry if this is a little broad and hypothetical or too legal.&lt;/p&gt;&#xA;&lt;p&gt;Say I (as a company) (hypothetically) was to build a space gun, reasonably similar to the &lt;a href=&quot;https://web.archive.org/web/20160304071426/http://www.astronautix.com/lvs/7inrpgun.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;7-incher&lt;/a&gt; here in Canada, but with a second stage (hybrid acrylic/Lox) and a bit more complex, and able to reach a good 400km orbit.&lt;/p&gt;&#xA;&lt;p&gt;Besides not shooting it at a Cessna or landing the projectile in an apartment building or something, are the legal hurdles manageable for a very small company with little lobbying power? Would I have to get a temporary no-fly exclusion zone, for instance, and have a range officer with an &amp;quot;explode&amp;quot; button, etc? Essentially, is this something that someone who isn't Elon Musk or Gerard Bull could legally accomplish?&lt;/p&gt;&#xA;" OwnerUserId="11855" LastEditorUserId="11855" LastEditDate="2022-05-10T04:53:30.900" LastActivityDate="2022-05-10T04:53:30.900" Title="If I were to make an orbit-capable space gun, what legal issues could I expect to encounter?" Tags="|launch|law|space-gun|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="12225" PostTypeId="1" AcceptedAnswerId="12226" CreationDate="2015-10-05T17:00:13.223" Score="46" ViewCount="8104" Body="&lt;p&gt;Part of Apollo astronaut training was learning to be a &lt;a href=&quot;http://sterileeye.com/2009/07/23/the-apollo-11-hasselblad-cameras/&quot;&gt;pretty good photographer&lt;/a&gt; without a viewfinder. The Hasselblad 500EL Data Camera used magazines with &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11-hass.html&quot;&gt;160 color/200 B&amp;amp;W&lt;/a&gt; exposures on 70mm film.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the magazine was expended, were the astronauts able to change it while on EVA, or did they have to wait until they were back in the LM?&lt;/p&gt;&#xA;" OwnerUserId="759" LastEditorUserId="25" LastEditDate="2015-10-05T17:26:08.057" LastActivityDate="2018-07-12T11:38:06.530" Title="How did the Apollo astronauts change film in their cameras?" Tags="|the-moon|apollo-program|eva|photography|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12226" PostTypeId="2" ParentId="12225" CreationDate="2015-10-05T17:26:35.427" Score="42" Body="&lt;p&gt;These cameras had magazines that could be exchanged in the middle of a roll (and that was &lt;a href=&quot;http://www.wired.com/2013/07/apollo-hasselblad/&quot; rel=&quot;noreferrer&quot;&gt;one reason NASA chose them&lt;/a&gt;).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a photo of John Young exchanging a magazine during an EVA on Apollo 16:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/TRSgN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TRSgN.jpg&quot; alt=&quot;EVA 2&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, no need to wait until they were back in the LEM.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.attivissimo.net/luna/mhd/mhd-v1.0.1light.pdf&quot; rel=&quot;noreferrer&quot;&gt;long PDF&lt;/a&gt; has more detail on the film change process. A standard Hasselblad magazine has a darkslide: a metal sheet that forms a light-tight seal over the magazine. This means a photographer can swap magazines in the middle of a roll without exposing any film to light. Insert the magazine, then pull out the darkslide and you're ready to go.&lt;br&gt;&#xA;The cameras NASA used were modified: they included a reseau plate. This was a glass plate with cross markings to help gauge distances and sizes from a photograph. The presence of this plate meant that the astronaut had to remove the darkslide before attaching the magazine to the camera. So some film was exposed during a camera swap, and astronauts routinely took some blank shots to get any exposed film out of the way (and test the film transport). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-06T19:37:37.800" LastActivityDate="2015-10-06T19:37:37.800" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12227" PostTypeId="2" ParentId="12225" CreationDate="2015-10-05T17:32:17.527" Score="11" Body="&lt;p&gt;The site you provided states that there were 9 magazines of film used on the Moon for Apollo 11. The astronauts did not return to the LEM until the EVA was complete. Thus, they must have changed the film periodically during the EVA.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-05T17:32:17.527" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12228" PostTypeId="1" CreationDate="2015-10-05T18:43:43.467" Score="7" ViewCount="330" Body="&lt;p&gt;My father told me of a story that while he was young working at an electronics company, they all signed a disc that was shrunk down and mounted to one of the lunar rovers. Does anyone know anything about this?&lt;/p&gt;&#xA;" OwnerUserId="11858" LastEditorUserId="6944" LastEditDate="2020-10-09T00:45:36.347" LastActivityDate="2020-10-09T00:45:36.347" Title="Disc mounted on lunar rover?" Tags="|the-moon|apollo-program|history|lunar-rover|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12229" PostTypeId="2" ParentId="8566" CreationDate="2015-10-05T20:14:07.280" Score="17" Body="&lt;p&gt;To further add to the other answers, I found &lt;a href=&quot;http://www.spacelaunchreport.com/logyear.html&quot; rel=&quot;noreferrer&quot;&gt;some raw data about this&lt;/a&gt;. Here's a visualization of it, broken down by successes/failures and manned/unmanned.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/a27P0.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/a27P0.png&quot; alt=&quot;Orbital Launches Per Year Graph&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h3&gt;Some descriptive statistics:&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;% Manned Failures in Manned, Entire Data Set = &lt;strong&gt;1.64%&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;% Manned Failures in Manned, Last 20 Years = &lt;strong&gt;0.79%&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;% Unmanned Failures in Unmanned, Entire Data Set = &lt;strong&gt;8.08%&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;% Unmanned Failures in Unmanned, Last 20 Years = &lt;strong&gt;6.68%&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;I'd go.&lt;/p&gt;&#xA;&lt;p&gt;Edit: Updated the chart colors to better reflect unmanned failed launches.&lt;/p&gt;&#xA;" OwnerUserId="11859" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-05T20:36:56.073" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12230" PostTypeId="2" ParentId="12228" CreationDate="2015-10-05T21:02:37.847" Score="5" Body="&lt;p&gt;This actually is a very common practice among space hardware. Evidence is seen today in things like &lt;a href=&quot;http://mars.nasa.gov/msl/participate/sendyourname/&quot; rel=&quot;noreferrer&quot;&gt;&quot;Send your name to Mars&quot;&lt;/a&gt; for the general public. Most space missions I'm aware of try to do something like this, put the signatures of the people who made it with the object on some obscure part, a disc usually. These private discs are rarely spoken of in public, and would thus make discovery of them rather difficult. You'd have to find someone who personally worked with the rover and ask them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I would believe your father, but you're unlikely to find any proof.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-05T21:02:37.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12231" PostTypeId="2" ParentId="2304" CreationDate="2015-10-05T21:35:45.533" Score="5" Body="&lt;p&gt;I think its hard to be specific about causes of failure without sight of the formal manufacturer's review. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That pedantically obvious bit aside &lt;em&gt;plausible causes&lt;/em&gt; of failure for a Briz upper stage are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;penetration of any pressurised volume by high velocity debris such that it ruptures,&lt;/li&gt;&#xA;&lt;li&gt;leak between the fuel and oxidiser parts of the system which, being hypergolic for the Briz M (and not cryogenic in this case), will ignite on contact,&lt;/li&gt;&#xA;&lt;li&gt;leaking failure of a regulator from a high pressure gas tank so that the down stream low pressure propellant tank ruptures. This would only apply to the extent that the Briz M incorporates high pressure gas, for example to pressurise the attitude control thrusters. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I hope that tidies things up a little.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Over-pressure of a propellant tank if high temperature from sunlight is experienced after the mission phase. Physically, as the Briz is not a cryogenic stage, it would appear less of a risk, especially as the propellant tank empties. It is plausible therefore that a stage that has already suffered a failure, and thus is not empty, is at higher risk. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I actually have a suspicion that the Briz upper stages that have fragmented could all be the Phase 1 variants which, if I recall correctly, did not have so much in the way of end of mission passivation features. Either of the third or fourth explanations could be promoted if the Phase 1 feature is significant.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-04-18T22:08:57.910" LastActivityDate="2016-04-18T22:08:57.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12232" PostTypeId="1" AcceptedAnswerId="12233" CreationDate="2015-10-05T22:03:41.630" Score="29" ViewCount="3400" Body="&lt;p&gt;I went to &lt;a href=&quot;http://speyer.technik-museum.de/en/&quot;&gt;Technik Museum Speyer&lt;/a&gt;'s space exhibition, which presented an excellent overview of the Apollo missions in a series of posters. The poster describing the Apollo 7 mission (an Earth orbit test flight) contains the following interesting detail:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Only the crew was suffering some tension because [Walter] Schirra developed a cold at an early stage and passed it on to both his crewmates. In the weightlessnes of space, a cold has unpleasant side effects.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No further explanation is given, but I'm curious: Why is having a cold in weightlessness especially unpleasant?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For reference and completeness, here is the relevant section of the poster. I digitally highlighted the above quote in yellow. The German text on the left says the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IkK1b.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IkK1b.jpg&quot; alt=&quot;photograph of the poster with the quote above highlighted in yellow&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11861" LastEditorUserId="25" LastEditDate="2015-10-06T01:45:12.720" LastActivityDate="2015-10-06T19:57:24.123" Title="What are the &quot;unpleasant effects&quot; of having a cold in microgravity?" Tags="|crewed-spaceflight|apollo-program|microgravity|astronauts|health|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12233" PostTypeId="2" ParentId="12232" CreationDate="2015-10-05T22:03:41.630" Score="36" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/apollo/missions/apollo7.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;This description of the Apollo 7 mission from NASA&lt;/a&gt; contains a more detailed description:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mucus accumulates, fills the nasal passages and does not drain from the head. The only relief is to blow hard, which is painful to the ear drums. So the crew of Apollo 7 whirled through space suffering from stopped-up ears and noses. They took aspirin and decongestant tablets, and discussed their symptoms with doctors.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In contrast, gravity on Earth drains nasal mucus out through the nose (where it can be wiped off) or into the lungs (from where it can be coughed up, then swallowed). These processes do not put pressure on the ears.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The piece even suggests that blockage of the ears could cause dangerous levels of pressure during re-entry:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Several days before the mission ended, they began to worry about wearing their suit helmets during re-entry, which would prevent them from blowing their noses. The buildup of pressure might burst their eardrums. Deke Slayton in mission control tried to persuade them to wear the helmets anyway, but Schirra was adamant. They each took a decongestant pill about an hour before re-entry and made it through the acceleration zone without any problems with their ears.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Related: Astronauts in microgravity often experience &lt;a href=&quot;https://space.stackexchange.com/questions/1189/do-the-majority-of-astronauts-experience-space-sickness-while-adapting-to-micro?rq=1&quot;&gt;space sickness&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/2819/how-often-do-astronauts-suffer-headaches-migraines-etc-in-microgravity?rq=1&quot;&gt;other related conditions&lt;/a&gt; already, and the fluid not draining from one's head &lt;a href=&quot;http://www.nasa-usa.de/centers/johnson/pdf/167746main_FS_LivingandWorkinginSpace508c.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;has been described as feeling like &quot;a constant head cold&quot;&lt;/a&gt;. I'd speculate that the discomfort from an additional &lt;em&gt;actual&lt;/em&gt; cold could cumulate with that in a non-linear way.&lt;/p&gt;&#xA;" OwnerUserId="11861" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-10-05T22:03:41.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12234" PostTypeId="2" ParentId="12223" CreationDate="2015-10-05T22:13:40.570" Score="1" Body="&lt;p&gt;It appears what you are describing is a double planet.  There are no examples that we know of where the two planets are practically identical, but I do not believe there is anything prohibiting such an occurrence.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Double_planet&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Double_planet&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11454" LastActivityDate="2015-10-05T22:13:40.570" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12235" PostTypeId="2" ParentId="12224" CreationDate="2015-10-06T01:43:21.257" Score="5" Body="&lt;p&gt;Laws are only as good as the regulators come up with at the time. I doubt there is specific laws governing this, but there are a few analogs which can be used.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Amateur rockets- If you are going to fly a spacecraft really high, you have to get a permit. This will involve a no fly exclusion zone, as anything that flies really high. This will probably involve a larger zone than is typical, owing to the large horizon that would be required (The projectile would remain closer to the Earth for further horizontally than a traditional rocket)&lt;/li&gt;&#xA;&lt;li&gt;Space Ship One- Received first license for suborbital rocket flights to be issued by the US Office of Commercial Space Transportation.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Some issues of particular note: If your spacecraft can survive going 6 km/s in the atmosphere, then it would survive re-entry, without slowing down significantly. That makes it very dangerous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would probably need all of the same setup for a small orbital rocket company. Take a look at SpaceX around 2007, to see the types of things they had to do. They ended up launching from islands in the Pacific because that's where they were allowed to launch from.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-06T01:43:21.257" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12237" PostTypeId="1" CreationDate="2015-10-06T06:29:37.213" Score="9" ViewCount="2338" Body="&lt;p&gt;I read Andy Weir's &quot;The Martian&quot; and saw the movie, which is supposed to be realistic science fiction. And I immediately saw a couple of huge flaws, one of which I'd like to tackle with this question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Warning: Spoilers ahead&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote class=&quot;spoiler&quot;&gt;&#xA;  &lt;p&gt; In one scene a storm occurs on the surface of Mars. The winds are strong enough to cause humans to struggle to stand and to tear a satellite dish from its anchor. However, the atmosphere of Mars is much less dense than Earth's and I don't think even the winds of the strongest of Martian storms could have such an effect. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Could the winds of a Martian storm cause the average human to stumble or fall?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or lift and throw a satellite dish?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="25" LastEditDate="2015-10-06T13:39:49.737" LastActivityDate="2022-10-20T12:13:35.990" Title="Are Martian winds as strong as portrayed in &quot;The Martian&quot;?" Tags="|mars|communication|atmosphere|dust|movement|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12238" PostTypeId="2" ParentId="12237" CreationDate="2015-10-06T09:23:47.233" Score="3" Body="&lt;p&gt;The martian winds are not as strong as portrayed in the Martian. The air pressure on Mars is 1% of Earth sea level, and 200kph, while enough to notice, would not be enough to blow you over. Calculating the earth equivalent speed using the formulas in the related question posted by @1337joe 200kph on mars is 25kph on the earth, or 15mph. That's a breeze, not a gale strong enough to make you stumble even in lower gravity, and I can't imagine it ripping a satellite dish off its mountings. Even if it did come off the dish would be going only 25kph, that's more of a tumbleweed than a projectile. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-10-06T09:23:47.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12239" PostTypeId="2" ParentId="4859" CreationDate="2015-10-06T11:14:50.950" Score="2" Body="&lt;p&gt;There are no laws, but there are &lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/Space_Debris/Mitigating_space_debris_generation&quot; rel=&quot;nofollow noreferrer&quot;&gt;guidelines&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In 2002, the Inter-Agency Debris Coordination Committee published the &amp;quot;IADC Space Debris Mitigation Guidelines,&amp;quot; and presented these to the UNCOPUOS Scientific &amp;amp; Technical Subcommittee (STSC), where they served as a baseline for the &amp;quot;UN Space Debris Mitigation Guidelines.&amp;quot; In 2007 these guidelines were approved by the 63 STSC member nations as voluntary high-level mitigation measures. Since the mid-1990s, space agencies in Europe have developed more technically oriented guidelines as a &amp;quot;European Code of Conduct,&amp;quot; which was signed by ASI, UKSA, CNES, DLR and ESA in 2006.&lt;/p&gt;&#xA;&lt;p&gt;The core elements of this Code of Conduct are in line with the IADC and UN guidelines. In order to tailor the Code of Conduct to the needs of ESA projects, ESA has developed its own &lt;a href=&quot;http://emits.sso.esa.int/emits-doc/ESTEC/AD4RequirementsSpaceDebrisMitigationESA_Projects.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&amp;quot;Requirements on Space Debris Mitigation for Agency Projects&lt;/a&gt;&amp;quot;. These instructions came into force on 1 April 2008. They are applicable to all future procurements of space systems (launchers, satellites and inhabited objects).&lt;/p&gt;&#xA;&lt;p&gt;Space debris mitigation guidelines provide a framework for 'what' needs to be done. The way 'how' mitigation measures must be implemented is specified in a more formal manner, via international standards - or via binding national requirements for the design and operation of space systems.&lt;/p&gt;&#xA;&lt;p&gt;Such common standards guarantee a level field for industrial competition and for safe access to space into the future. International debris mitigation standards have been developed at ISO, such as in ISO-24113. The European Cooperation on Space Standardization (ECSS) adopted ISO-24113 in the space sustainability branch.&lt;/p&gt;&#xA;&lt;p&gt;Experts from ESA regularly support these developments and their harmonisation with existing guidelines and requirements, such as in the ECSS. The ultimate ISO standards on space debris mitigation, however, will remain non-binding (as is true for any ISO standard).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-06T11:14:50.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12240" PostTypeId="1" CreationDate="2015-10-06T12:17:56.543" Score="6" ViewCount="302" Body="&lt;p&gt;How many years can a liquid fuel rocket be safely stored (unfueled) between manufacturing and launch? Are they sensitive to slow processes like corrosion, material degradation from dehydration, other chemical processes, seasonal temperature variation, fatigue of load carrying structures, gathering of dust that can clog or cause a fire, or biological activity by microbes, insects, rodents, or a combination of all above? Would it take much to store a launcher safely? On the other hand, they are robust machines built with high quality materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, an ambitious human mission to Mars would require several launches, preferably within a brief period. Some expect, maybe pessimistically, that only one SLS will be manufactured per year. Could they be saved up for a decade to then be launched as frequently as the space port capacity allows?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-10-06T12:53:50.427" LastActivityDate="2015-10-06T19:35:39.827" Title="Do rockets have a &quot;best before date&quot; after assembly?" Tags="|launch|rockets|materials|sls|storage|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12241" PostTypeId="2" ParentId="12240" CreationDate="2015-10-06T12:47:21.663" Score="1" Body="&lt;p&gt;First of all, I believe the limiting factor on SLS isn't so much the time it takes to build a rocket, but the cost per rocket. As more rockets get launched, the cost will go down per launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the shelf life of a rocket, there are a couple of key things. The main cost is storage, in that the rocket has to be placed in a controlled environment to protect it for the duration. As time passes, the batteries might need to be changed, some basic maintenance is required, etc. But they can store for some time. I don't think you'd have the decade long storage process, but up to months is done frequently. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the most complex parts of a rocket, the engine, has shown to some degree a long lifetime, although it also shows the perils of using such an engine. Antares used the &lt;a href=&quot;https://en.wikipedia.org/wiki/NK-33&quot; rel=&quot;nofollow&quot;&gt;NK-33&lt;/a&gt; engine produced by Russia, which were manufactured at latest the 1990's. They were used 20 years after manufactured. Of course, there were failures, which resulted in them no longer being used.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, if you are willing to store them carefully, and check certain key components before launch, you can probably get away with simply stockpiling rockets. But I wouldn't want to store them for 10 years for a human rated mission!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-06T19:35:39.827" LastActivityDate="2015-10-06T19:35:39.827" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12242" PostTypeId="2" ParentId="12237" CreationDate="2015-10-06T13:21:23.667" Score="13" Body="&lt;p&gt;The winds on Mars are much faster than typically on Earth, however, the atmosphere is much thinner. This can cause some unusual effects, somewhat analogous to high speed/ low torque vs low speed/ high torque. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume the satellite dish had a surface area of $1 m^2$, a reasonable assumption. Also, let's use the math from &lt;a href=&quot;https://space.stackexchange.com/a/9315/25&quot;&gt;this question&lt;/a&gt;, and let's use the Martian listed wind, of 175 kph. That's 48.61 m/s, which is a high wind speed. That creates a pressure of&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${q_M}$ ${=}$ ${0.5(0.02)(48.61)^2}$ ${=}$ ${23.62}$ ${Pa}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For our $1 m^2$ antenna, that exerts a force of 23.6N, or about the force of gravity with a 2.5 kg weight on it. That would probably be enough to bend the dish, if build on Earth. However, there are other things to keep in mind.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;This dish would be built to survive on Mars, and thus would likely be lighter than an analogous Earth antenna. This would make it break more easily.&lt;/li&gt;&#xA;&lt;li&gt;Anything on Mars will have to survive thermal cycles, making it more brittle. &lt;/li&gt;&#xA;&lt;li&gt;Once something is torn, it will tend to move up to the actual wind speed. This could cause damage for anything that actually gets loose.&lt;/li&gt;&#xA;&lt;li&gt;The pressure on Mars effectively goes up in a dust storm, as there is more mass in the air, likely causing the pressure to be higher.&lt;/li&gt;&#xA;&lt;li&gt;The abrasive nature of dust likely reduced the strength of the antenna over time, in particular during the very storm mentioned in the book/movie.&lt;/li&gt;&#xA;&lt;li&gt;The Hermes 3 landing site is one of the lower portions of Mars. In fact, most of the landing sites for landers are in the lower regions (Parachutes work better there). Thus, there is likely higher atmospheric pressure than there would be at other locations, increasing the effect of the wind even more. According to Wikipedia, the highest &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Mars&quot; rel=&quot;noreferrer&quot;&gt;pressure on Mars&lt;/a&gt; is about twice that of the mean.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Taking all of the above into account, it could be possible to have at least a 4 times higher effective pressure, which makes the story a bit more believable. Still, I think this is one of the less scientifically accurate parts of the book, but it's not completely impossible. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-10-06T13:29:38.547" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12244" PostTypeId="1" AcceptedAnswerId="12252" CreationDate="2015-10-06T13:46:41.223" Score="9" ViewCount="1216" Body="&lt;p&gt;This question was inspired by the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/2304/what-exactly-causes-stranded-upper-stages-to-explode&quot;&gt;this question&lt;/a&gt;.  The answer, in part, states that overpressurization of propellant tanks is a cause of upper stage explosions, leading to orbital debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only propellant tanks I am familiar with, those in the STS External Tank, had relief valves that would pop open if the the tank pressure reached a certain value.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After reading about the suspected cause of the latest Falcon 9 anomaly, I assumed that the second stage relief valve was undersized for the rupture of a helium sphere - a design error.  But perhaps there was no relief valve at all?&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2019-02-25T04:01:55.570" Title="Do the propellant tanks of actual (not planned) expendable vehicles' upper stages have relief valves?" Tags="|launch|rockets|design|upper-stage|booster|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12246" PostTypeId="2" ParentId="12228" CreationDate="2015-10-06T17:16:55.263" Score="0" Body="&lt;p&gt;&lt;a href=&quot;http://www.pbs.org/opb/historydetectives/investigation/moon-museum/&quot; rel=&quot;nofollow&quot;&gt;History Detectives on PBS did a show about this&lt;/a&gt;. You will see it there if you look on video.pbs.org.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were looking for rumors of Andy Warhol art going to the moon. I believe this is what your dad was referring to.&lt;/p&gt;&#xA;" OwnerUserId="11870" LastEditorUserId="6816" LastEditDate="2015-10-06T18:36:20.170" LastActivityDate="2015-10-06T18:36:20.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12247" PostTypeId="2" ParentId="12228" CreationDate="2015-10-06T17:34:00.837" Score="3" Body="&lt;p&gt;I was suspicious at the &quot;shrunk down&quot; part of the story, but then I came across &lt;a href=&quot;https://disqus.com/home/discussion/histdet-diy/moon_museum_update_history_detectives_pbs/best/&quot; rel=&quot;nofollow&quot;&gt;this comment&lt;/a&gt; which explains a similar occasion:   &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;My wife's father, Herb Gulbrandsen, (deceased) worked on the LEM.  Along with what we make out to be about 200 other people, he signed a picture of the craft.  In the right hand corner of this pic is a cloverleaf logo with Grumman/NASA/Lunar Module respectively placed on each &quot;petal&quot; of the leaf. At the bottom right is a description of the craft and when it was shipped - October 10/15 1968. A copy of this was put on a piece of microfilm (Mrs. Gulbrandsen's description) and placed on the Apollo 11 lander.  Many of the signatures are fairly legible, including Herb Gulbrandsen's off to the left of the right hand leg pad. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Which sounds plausible enough, a microfiche would be easy to stash somewhere on the LEM (officially or not). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-06T17:34:00.837" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12248" PostTypeId="2" ParentId="12244" CreationDate="2015-10-06T19:34:41.080" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www.astromart.com/auctions/details.asp?auction_id=4885&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Saturn I, IB and Saturn V rockets&lt;/a&gt; had a relief valve system for the S-IVB upper stage. I could not find any documentation of a relief valve system for the Falcon 9 upper stage.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="195" LastEditDate="2019-02-25T04:01:55.570" LastActivityDate="2019-02-25T04:01:55.570" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12250" PostTypeId="2" ParentId="12244" CreationDate="2015-10-06T20:46:20.540" Score="5" Body="&lt;p&gt;There are 2 different systems that are both called relief valves:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;One is pressure-actuated (this is also used on e.g. steam boilers) and makes sure the pressure doesn't rise above a preset value.  &lt;/li&gt;&#xA;&lt;li&gt;The other seems to be more common in rocket stages. While the stage is in use, this valve is inert. After the the stage is discarded, this valve opens to dump any remaining propellant. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The upper stage of the &lt;a href=&quot;http://adsabs.harvard.edu/full/1997ESASP.393..681B&quot; rel=&quot;nofollow&quot;&gt;Ariane 4&lt;/a&gt; had pressure relief valves. These were added after an explosion on an early Ariane 4 upper stage. These valves are used to dump all remaining fuel overboard after the stage has been expended.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ESC-A &lt;a href=&quot;http://eucass2011.conferencecenter.ru/cs/upload/gF76bMq/papers/papers/1194-1879-1-RV.pdf&quot; rel=&quot;nofollow&quot;&gt;upper stage of the Ariane 5&lt;/a&gt; also has valves that are used to dump all remaining fuel overboard after the stage has been expended.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-07T10:23:32.570" LastActivityDate="2015-10-07T10:23:32.570" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12251" PostTypeId="2" ParentId="12244" CreationDate="2015-10-06T21:16:13.213" Score="5" Body="&lt;p&gt;Possibly not what you're looking for, but the Apollo LM's fuel tanks used helium pressurization, and the helium tanks had burst disks (or diaphragms). When pressure in the helium tanks got too high, several days into the mission, they'd pop and vent to space. This means no more pressure in the fuel tank, but this would normally happen well after the lunar landing, after the LM was abandoned. On Apollo 13, they didn't land, and kept the LM on the trip back, so in both the transcript and the film, there's some discussion of when they could expect the &quot;bang&quot; of the burst disk. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-10-06T21:16:13.213" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12252" PostTypeId="2" ParentId="12244" CreationDate="2015-10-06T23:18:36.737" Score="9" Body="&lt;p&gt;Since &lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_%28rocket_stage%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centaur Upper Stage&lt;/a&gt; has a long flight history*, and it did experience hydrogen-venting problems in its early versions resulting in several modifications to its pressurization system, I was fairly certain that it ought to have several &lt;em&gt;relief valves&lt;/em&gt; in place;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730023886.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;CENTAUR D-1T PROPULSION AND PROPELLANT SYSTEMS&lt;/a&gt;, William E, Goette, Lewis Research Center, 1973 AIAA Propulsion Joint Specialist Conference proceedings (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Propellant Tank Venting and Pressurization&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Centaur liquid hydrogen and liquid oxygen propellants are&#xA;  contained in thin-wall, pressure-stabilized tanks. Pressures in each&#xA;  tank are maintained by propellant boiloff. Vent valves are used to&#xA;  control pressure levels during tanking and flight. All of the vent&#xA;  valves are of the same design, differing only in pressure setting. A&#xA;  solenoid in each valve may be energized to place the valve into a&#xA;  shutoff mode to preclude venting.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The liquid oxygen vent system is mounted on the aft bulkhead and&#xA;  consists of one vent valve, insulated ducting, and a vent disconnect.&#xA;  This system is shown in Figure 7. Venting on the launch pad occurs&#xA;  through the disconnect and a duct which penetrates the interstage&#xA;  adapter. After separation from the Titan booster, venting will occur&#xA;  overboard through the ducting. The ducting is aft canted, and&#xA;  adjustable (prior to launch) such that the thrust from the vented gas&#xA;  is directed through the vehicle center-of-gravity. This orientation&#xA;  minimizes disturbing torques while venting.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The liquid hydrogen vent system is located on the forward bulkhead as&#xA;  shown in Figure 8. It consists of two vent valves, a plenum, ducting,&#xA;  two aft canted vent nozzles, and two inflight vent disconnects.&#xA;  Venting on the launch pad and during ascent prior to Centaur Standard&#xA;  Shroud (CSS) separation occurs through one leg of the vent system and&#xA;  overboard through a shroud-mounted vent fin. After CSS jettison&#xA;  venting occurs through both legs of the system.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This system is symmetrical, which provides equal thrust forces from&#xA;  the two vent nozzles. The nozzles are canted aft 30°, so that a&#xA;  positive forward force is produced on the vehicle. In the event the&#xA;  propellants are not completely settled at the time of venting, or if&#xA;  there is some liquid entrained in the vent gas, the additional thrust&#xA;  will help settle the propellants and reduce any problem.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Two vent valves are used to control hydrogen tank venting. The primary&#xA;  vent valve controls the tank pressure during most of the prelaunch and&#xA;  flight operations. However, a requirement exists during boost flight&#xA;  for a higher tank pressure to react flight loads. The secondary valve&#xA;  is set to operate at a higher pressure level than the primary valve,&#xA;  and serves as a relief or safety valve during this period of flight. A&#xA;  solenoid has been added to the secondary vent valve in order to&#xA;  disable the valve during the long zero-gravity coast periods. This&#xA;  permits higher tank pressures to be attained, and reduces the number&#xA;  of tank vent sequences. Except during the boost phase of flight, the&#xA;  vent valves are controlled by a system known as the&#xA;  Computer-Controlled Vent and Pressurization System (CCVAPS). This&#xA;  system will be discussed in detail in the section termed Propellant&#xA;  Management.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HjXHu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HjXHu.png&quot; alt=&quot;Centaur Upper Stage Propellant Vent Valves&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And so on and the same source also describes the use of &lt;a href=&quot;https://en.wikipedia.org/wiki/Solenoid_valve&quot; rel=&quot;nofollow noreferrer&quot;&gt;solenoid valves&lt;/a&gt; on the RCS and Hydrogen Peroxide systems, and explains a bit more the &lt;a href=&quot;https://space.stackexchange.com/a/9196/49&quot;&gt;interstage purging system&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know, this qualifies as vintage, but even today's Centaur screams &lt;em&gt;legacy&lt;/em&gt; throughout, so I wouldn't be surprised if none of this changed substantially at all since they resolved problems with hydrogen-venting system's fire hazard that destroyed the first Atlas-Centaur (F-1) mid-flight. But I'll keep my eyes peeled for anything newer. Also see &lt;a href=&quot;http://history.nasa.gov/SP-4230.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Taming Liquid Hydrogen: The Centaur Upper Stage Rocket 1958-2002&lt;/a&gt;, Virginia P. Dawson and Mark D. Bowles, 2004, The NASA History Series (PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Centaur was &lt;a href=&quot;http://space.skyrocket.de/doc_stage/centaur.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;launched 228 times, as of October 2, 2015&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-10-06T23:31:33.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12254" PostTypeId="1" AcceptedAnswerId="12255" CreationDate="2015-10-07T15:10:19.767" Score="9" ViewCount="1002" Body="&lt;p&gt;Rockets require real-time computers, in that there are operational deadlines between events and system response. How quickly does the rocket's controller need to respond to sensors and events, without fail? What kind of mechanisms are in place, or what kind of practices are avoided, to make sure this happens?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What was the required &lt;a href=&quot;https://en.wikipedia.org/wiki/Guidance%2C_navigation_and_control&quot; rel=&quot;noreferrer&quot;&gt;GNC&lt;/a&gt; reaction time during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Roll_program&quot; rel=&quot;noreferrer&quot;&gt;STS roll maneuver&lt;/a&gt;, and what &lt;a href=&quot;https://en.wikipedia.org/wiki/Attitude_control&quot; rel=&quot;noreferrer&quot;&gt;ADC&lt;/a&gt; parameters were read and polled upon at the fastest rate, that such roll program depended on?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11879" LastEditorUserId="11879" LastEditDate="2015-10-07T20:31:25.827" LastActivityDate="2015-10-07T20:31:25.827" Title="What is the maximum acceptable latency in rocket control computers?" Tags="|attitude|navigation|flight-computer|guidance|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12255" PostTypeId="2" ParentId="12254" CreationDate="2015-10-07T17:24:28.113" Score="10" Body="&lt;p&gt;The STS roll control program controlled attitude in response to sensed velocity, rather than attempting to control the trajectory directly. This portion of the flight wouldn't be particularly latency-sensitive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://archive.org/stream/nasa_techdoc_20060019188/20060019188_djvu.txt&quot;&gt;Spacecraft Guidance, Navigation, and Control Requirements for an Intelligent &#xA;Plug-n-Play Avionics (PAPA) Architecture&lt;/a&gt; :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the first stage after lift-off, the objective of the guidance algorithm is &#xA;  to take the vehicle through the atmosphere while minimizing the fuel spent and keeping the structural loads in their &#xA;  design range. To achieve that, the algorithm provides attitude commands as a function of the vehicle velocity. These &#xA;  values are pre-computed and stored in the computer memory. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;After the roll program completes, the shuttle does go into closed-loop control to &lt;a href=&quot;https://books.google.com/books?id=shajBQAAQBAJ&amp;amp;lpg=PA124&amp;amp;ots=HhCTNavcLO&amp;amp;dq=space%20shuttle%20structural%20load%20relief%20ascent&amp;amp;pg=PA124#v=onepage&amp;amp;q=space%20shuttle%20structural%20load%20relief%20ascent&amp;amp;f=false&quot;&gt;minimize aerodynamic loads&lt;/a&gt;, but the overall trajectory guidance remains open-loop until relatively late in the ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://archive.org/stream/nasa_techdoc_19760024058/19760024058_djvu.txt&quot;&gt;Space Shuttle Digital Flight Control System&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Five seconds after lift-off, commands &#xA;  are issued to accomplish the pitch-over and roll-to-flight-azimuth maneuvers. &#xA;  During regions of high dynamic pressure, a load relief system in both pitch &#xA;  and yaw minimizes air loads on the vehicle... The load relief function is accomplished &#xA;  by lateral and normal accelerometer feedbacks blended into the attitude command system starting at 25 seconds into the flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Regarding latency:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During all flight phases, the fundamental flight control computational &#xA;  cycle is 40 milliseconds. A number of flight control computations take &#xA;  place only every 80 milliseconds, and some take place approximately once per &#xA;  second. In addition to the basic 40-millisecond minor cycle requirement, &#xA;  the total delay between sampling of the flight control sensors and transmittal of the resulting command to the effectors is constrained to be no &#xA;  greater than 20 milliseconds. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If I'm parsing this correctly, it means that every 40ms, there's a sensor-computer-effector cycle that takes no more than 20ms. The shuttle was notoriously aerodynamically unstable, far more so than most launchers, so I imagine that this sort of cycle time would suffice for almost all.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-10-07T20:14:45.337" LastActivityDate="2015-10-07T20:14:45.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12256" PostTypeId="1" AcceptedAnswerId="12257" CreationDate="2015-10-07T17:52:13.027" Score="29" ViewCount="23028" Body="&lt;p&gt;All the photographs I've seen from the Apollo missions show lunar features in good detail and exposure. This of course means that the shutter is not open long enough for light from stars to develop on the film. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence every photograph has a characteristically inky and plain black backdrop. An example is this image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D2cBw.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D2cBw.jpg&quot; alt=&quot;Earth from Moon&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, for the astronauts actually on the Moon, all the stars and galaxies should have been perfectly visible. In fact, they should have been the most clearly visible stars seen by human eyes, since they were further out of Earth's atmosphere than ever before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What did the sky look like from the Moon? Are there any images (real or digitally-altered) that can show us what it looked like?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2020-09-20T01:13:25.047" Title="What did the sky actually look like from the Moon?" Tags="|the-moon|photography|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12257" PostTypeId="2" ParentId="12256" CreationDate="2015-10-07T18:14:29.347" Score="47" Body="&lt;p&gt;The astronauts on the moon had the same problem as the photos - their eyes were adapted to the light levels reflected from the moon's surface, so it was almost as hard to see the stars as it is to see them from Earth during the day.&lt;/p&gt;&#xA;&lt;p&gt;Neil Armstrong said, in &lt;a href=&quot;https://history.nasa.gov/ap11ann/FirstLunarLanding/ch-7.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;a post-Apollo 11 press conference&lt;/a&gt;: &amp;quot;we were never able to see stars from the lunar surface or on the daylight side of the Moon by eye without looking through the optics.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Gene Cernan said that, while standing in the shadow of the Apollo 17 LM, he could see some stars while he was outside.&lt;/p&gt;&#xA;&lt;p&gt;All the landings were done on the daylight side of the moon.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2020-09-20T01:13:25.047" LastActivityDate="2020-09-20T01:13:25.047" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="12258" PostTypeId="1" AcceptedAnswerId="12288" CreationDate="2015-10-07T20:14:22.393" Score="3" ViewCount="508" Body="&lt;p&gt;As a simple project to introduce myself to orbital mechanics I'm attempting to calculate impact times of an object close to Earth. Right now I am using only Keplerian mechanics (no air resistance or other perturbing forces). From &lt;a href=&quot;http://www.amazon.ca/Fundamentals-Astrodynamics-Roger-R-Bate/dp/0486600610&quot; rel=&quot;nofollow&quot;&gt;Fundamentals of Astrodynamics&lt;/a&gt;, given an object with eccentric anomaly $E_0$ at $t_0$ and $E_1$ at $t_1$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$t_1 = t_0 + \sqrt{\frac{a^3}{\mu}}\left( E_1 - e\sin E_1 - (E_0 - e\sin E_0)\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $a$ is the semi-major axis, $e$ is the eccentricity, and $\mu$ is the gravitational parameter. We can then calculate the radius of the periapsis. Assuming a spherical Earth, if this is less than the radius of the Earth the object will impact. If this is the case, letting $r_1 = R = $ Earth radius, we can calculate the eccentric anomalies at $t_0$ and $t_1$ using:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \cos\nu_i =\frac{a(1 - e^2) - r_i}{e r_i}$$ &#xA;$$ \cos E_i = \frac{e + cos\nu_i}{1 + e\cos\nu_i}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\nu_i$ is the true anomaly, and $r_i$ is the distance from the centre of the Earth at $t_i$. This method works well for non-parabolic trajectories (I'm open to modifications for parabolic trajectories). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next step is to loosen the restriction of a spherical Earth and allow Earth to be an ellipsoid, in other words let $R = R(z)$ (symmetric around rotational axis). Specifically I'd like to allow for a bulge at the equator by prescribing an equatorial radius, a polar radius and an eccentricity. Is it possible to solve this problem analytically or does it have to be done numerically?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is my first post on this stack exchange; I felt it was better suited here than scicomp for example. If that's not the case please let me know.&lt;/p&gt;&#xA;" OwnerUserId="11885" LastEditorUserId="11885" LastEditDate="2015-10-09T07:42:35.133" LastActivityDate="2018-02-09T19:21:12.310" Title="Calculate time to impact on elipisoid Earth" Tags="|orbital-mechanics|mathematics|astrodynamics|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="12261" PostTypeId="1" AcceptedAnswerId="12262" CreationDate="2015-10-08T06:27:52.967" Score="7" ViewCount="1007" Body="&lt;p&gt;Suppose you have a hypothetical spacecraft launched from Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it escapes from Earth's gravity it will still be orbiting the sun at roughly the same velocity as the earth, approximately 30km/s relative to the center of the sun.  Say the spacecraft then thrusts in the opposite direction, until its velocity relative to the center of the sun is reduced to 0 while keeping its other positional attributes as close to unchanged as possible.  Now the hypothetical spacecraft is (for some brief instant in time) essentially stationary relative to the sun.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, what happens next?  I'd expect that the spacecraft would immediately begin to fall directly into the sun.  Is that the case, and if so, how long would it take to get there and what would the acceleration profile be like?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if one were to deliberately send a probe into the sun, is there any practical merit to doing it that way as opposed to simply thrusting directly towards the sun while retaining the lateral velocity imparted by the earth?&lt;/p&gt;&#xA;" OwnerUserId="8367" LastEditorUserId="25" LastEditDate="2015-10-08T14:07:35.773" LastActivityDate="2015-10-09T13:08:59.500" Title="Can you reach the Sun by only thrusting horizontally?" Tags="|trajectory|the-sun|thrust|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12262" PostTypeId="2" ParentId="12261" CreationDate="2015-10-08T07:37:05.450" Score="10" Body="&lt;p&gt;You can; burning retrograde with respect to the sun lowers your perihelion.  If you do it long enough your orbit will intersect the sun.  If you ignore gravity assists this is the cheapest way of impacting the sun.  Because the sun is not a point mass you don't even need to reduce your velocity to 0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thrusting directly towards the sun would eventually work, but would be extreme expensive.  (With infinite thrust this approach would be roughly $\sqrt 2$ times as expensive as burning retrograde; you are changing your velocity vector 90 degrees rather than eliminating your velocity.  With less thrust I think it would be even less effective).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more effective way is to make the retrograde(with respect to the sun) burn while still in the Earths sphere of influence.  This in order to utilize the Oberth effect.&lt;/p&gt;&#xA;" OwnerUserId="5934" LastEditorUserId="238" LastEditDate="2015-10-08T14:02:34.593" LastActivityDate="2015-10-08T14:02:34.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12264" PostTypeId="1" CreationDate="2015-10-08T13:33:03.300" Score="19" ViewCount="22702" Body="&lt;p&gt;How was it possible for astronauts to send back tv/radio signals to earth from the moon?  I have just been teaching my son about waves behavior and understand that radio waves are reflected back to earth from the ionosphere. As far as I know microwaves were not used for communication purposes back then. How did the Apollo team then manage to send TV/ Radio signals back from space?&lt;/p&gt;&#xA;" OwnerUserId="11896" LastEditorUserId="26446" LastEditDate="2021-04-06T05:35:41.227" LastActivityDate="2023-03-28T19:34:43.633" Title="Apollo communication between the Earth and Moon" Tags="|the-moon|nasa|apollo-program|communication|radio-communication|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12266" PostTypeId="2" ParentId="12264" CreationDate="2015-10-08T13:58:00.770" Score="32" Body="&lt;p&gt;It can be seen through &lt;a href=&quot;https://en.wikipedia.org/wiki/Unified_S-band&quot;&gt;this Wikipedia&lt;/a&gt; article that all communication with the Apollo missions was done at about 2.2 GHz, which is well above the frequency that reflects on the Ionosphere (No higher than about 30 MHz). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;FYI, the &lt;a href=&quot;http://www.svengrahn.pp.se/radioind/GeminiRadios/GeminiRadio.html&quot;&gt;Gemini radio system&lt;/a&gt; also used frequencies in the UHF range, although it included some VHF, and even some HF. The HF/ VHF signals were either used for voice communication, or nearby operations (Recovery, acquisition). HF was more used then than it is today, and allowed for somewhat of an over the horizon effect. Depending on propagation, it can be difficult, but the 15 MHz used has minimal interference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Microwaves are considered those with a higher frequency than 300 MHz, so technically, they did use microwaves for the Apollo program. These signals work great to penetrate the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-08T15:35:24.363" LastActivityDate="2015-10-08T15:35:24.363" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12267" PostTypeId="2" ParentId="12261" CreationDate="2015-10-08T15:24:09.487" Score="4" Body="&lt;p&gt;Putting on the brakes vs making a left turn? Putting on the brakes is cheaper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would not have to kill all the 30 km/s. Getting your perihelion below the sun's surface takes about 27 km/s. As Taemyr notes, doing the burn deep in earth's gravity well confers an Oberth benefit. From low earth orbit, it'd take about 21.5 km/s for injection to a heliocenric orbit with a sun grazing perihelion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the way I'd do it is send the probe towards Venus. From low earth orbit a trans venus injection takes about 3.5 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus arrival Vinf would be about 2.7 km/s. If the hyperbola's periapsis is at 300 km altitude, turning angle would be 122º. 2*2.7*sin(122º/2)=4.7. So a Venus Hohmann swing by can provide 4.7 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, the 10.5 km/s Venus fly by can provide a nice Oberth benefit. A small burn at the Venus hyperbola's periapsis can get a lot of bang for the buck. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't have time to do the numbers at the moment. Will try to return to this later.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-10-08T16:11:54.067" LastActivityDate="2015-10-08T16:11:54.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12269" PostTypeId="2" ParentId="12261" CreationDate="2015-10-09T04:12:00.800" Score="2" Body="&lt;p&gt;Assuming you can afford to wait quite a while there's a surprising, cheaper option:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Burn for the stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're looking at only about 1/3 of the delta-v as would be needed to kill your orbital velocity.  Now, you go flying way out there, the farther the better (but the slower.)  Lets just figure we start out with a minimum-energy burn for Pluto.  When we get there in a century we burn the rest of our fuel to completely stop.  Even with this we still used only half the delta-v--we can use a rocket of less than 1/10th the size.  Living to see our probe hit the sun is another matter, though...&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-10-09T04:12:00.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12270" PostTypeId="2" ParentId="12261" CreationDate="2015-10-09T06:02:53.673" Score="6" Body="&lt;p&gt;Bottom line, from low-Earth orbit (these are all impulsive burns), you need 21.3 km/s to dive directly into the Sun with a single burn.  (Your &quot;horizontal&quot; thrusting.)  Seems a bit excessive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can do much better than that if you instead raise your aphelion, and once you get to aphelion, lower your perihelion.  Depending on how far out you're willing to go, and how long you're willing to wait to get to the Sun, you can reduce the total of the two burns to as low as 8.75 km/s (the amount need to escape from the Sun directly from low-Earth orbit).  For example, if you're willing to go out to the distance of Uranus, you can get it down to 10 km/s total.  This is about the total $\Delta V$ of the launch vehicle that got you to low-Earth orbit.  Still too high.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can do way better if you use a Jupiter gravity assist.  One of those along with a modest maneuver at Jupiter closest approach will send you right to the surface of the Sun.  (I haven't done the arithmetic, but I'm guessing just a few hundred m/s burn at Jupiter.)  You can go directly to Jupiter from low-Earth orbit for 6.3 km/s.  Now at least we're getting into the realm of something feasible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, you don't need to go directly to Jupiter.  You can use a Venus-Earth-Earth gravity assist to get to Jupiter (like Galileo did).  You just have to live with an extra three years of flight time.  Now you only need 3.5 km/s from low-Earth orbit to get to Venus.  Plus that burn at Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So instead of the 21.3 km/s required for your direct approach of slowing your orbit around the Sun, you can dive into the Sun for less than 4 km/s by stealing tiny bits of energy from Venus, Earth, and Jupiter.  That, and some careful navigation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could also do just Venus flybys, but many of them.  I'm not sure how many, but quite a few.  Solar Probe Plus will take seven Venus flybys and six and a half years to get to 8.5 solar radii.  You want to get to zero solar radii.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-10-09T06:32:06.763" LastActivityDate="2015-10-09T06:32:06.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12271" PostTypeId="1" AcceptedAnswerId="12272" CreationDate="2015-10-09T06:19:40.837" Score="5" ViewCount="514" Body="&lt;p&gt;Moon orbits around Earth and space station orbit around the Earth, can a space station have some objects orbit around it?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="168" LastEditDate="2015-10-09T07:54:57.797" LastActivityDate="2015-10-09T09:30:12.493" Title="Can a space station have its own satellite?" Tags="|artificial-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12272" PostTypeId="2" ParentId="12271" CreationDate="2015-10-09T09:30:12.493" Score="12" Body="&lt;p&gt;No, low Earth orbit satellites can't have their own satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Wikipedia: &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere#Further_examples&quot;&gt;https://en.wikipedia.org/wiki/Hill_sphere#Further_examples&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An astronaut could not orbit the Space Shuttle (with mass of 104&#xA;  tonnes), where the orbit is 300 km above the Earth, since the Hill&#xA;  sphere of the shuttle is only 120 cm in radius, much smaller than the&#xA;  shuttle itself. A sphere of this size and mass would be denser than&#xA;  lead. &lt;strong&gt;In fact, in any low Earth orbit, a spherical body must be more&#xA;  dense than lead in order to fit inside its own Hill sphere, or else it&#xA;  will be incapable of supporting an orbit.&lt;/strong&gt; A spherical geostationary&#xA;  satellite however would only need to be more than 6% of the density of&#xA;  water to support satellites of its own.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6522" LastActivityDate="2015-10-09T09:30:12.493" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12273" PostTypeId="2" ParentId="12261" CreationDate="2015-10-09T13:08:59.500" Score="1" Body="&lt;p&gt;The original question has two parts,&#xA;A) &quot;what happens next?&quot;&#xA;B) &quot;is there any practical merit in doing it that way?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears to me that all the comments and answers have addressed another question, something like &quot;what's the best way to do it?&quot;. I wouldn't knock them beyond that, some of them appear really interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My humble answer to &quot;what happens next?&quot;:&#xA;First, in idealised circumstances of there being just the probe and the Sun then the two objects will move under the acceleration resulting from their gravitational attraction, as always. In this case it will look, to the external observer, as if the probe is simply falling towards the sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the first equation in &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Gravity&lt;/a&gt; for guidance in working out the acceleration profile and eventually the travel time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the real circumstances of the solar system as we know it, I have little idea and would be interested to know, to what extent other solar system bodies might interfere, and this is going to depend a lot on the initial conditions as to what bodies might have some influence.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-10-09T13:08:59.500" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12274" PostTypeId="1" CreationDate="2015-10-09T14:16:29.343" Score="12" ViewCount="1041" Body="&lt;p&gt;In fiction cats are often chosen as the optimal zero/micro gravity pet.  Clearly there is some rational for this, all existing answers at &lt;a href=&quot;https://space.stackexchange.com/questions/2954/how-do-astronauts-turn-in-space&quot;&gt;How do astronauts turn in space?&lt;/a&gt; include cats in them.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While most litter boxes on Earth use sand or some pseudo dirt substance, it seems unlikely this would be a good solution without significant gravity.  It is possible to &lt;a href=&quot;https://pets.stackexchange.com/questions/971/how-can-i-train-my-cat-to-use-the-toilet&quot;&gt;teach a cat to use toilet&lt;/a&gt; so a space cat using a toilet is an option. Though it looks like current &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_toilet&quot; rel=&quot;nofollow noreferrer&quot;&gt;space toilets&lt;/a&gt; use different facilities for liquids and solids so not the best option for a cat. How would a zero gravity cat toilet/litter box work? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While there have been a lot of animals in space they have all been confined, this question assumes a cat with free range of the environment like it's human companions.  This question is about the mechanical/physical solution not the training issues which would be best addressed at &lt;a href=&quot;https://pets.stackexchange.com/questions/10498/overcoming-challenges-in-teaching-a-cat-to-use-a-zero-gravity-toilet-litter-box&quot;&gt;Pets.SE&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:15.717" LastActivityDate="2015-10-13T14:38:55.363" Title="How would a zero gravity cat litter box work?" Tags="|colonization|microgravity|animals|" AnswerCount="3" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="12275" PostTypeId="2" ParentId="8420" CreationDate="2015-10-09T16:47:30.583" Score="5" Body="&lt;p&gt;Just to add to your original question &quot;is this correct?&quot; - yes but only for spirals between two circular orbits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Responding to your request for &quot;guidelines for more general scenarios&quot;: If you wanted to find the ∆V to transfer from an elliptical orbit such as GTO and spiral to GEO then you could follow a method such as in Mark Adler's second paragraph above - with a significant adjustment. For such a transfer its best to stick to a short arc around the apogee and accept that the part of the arc away from the exact apogee will suffer a loss. Evaluating this loss needs numerical integration though if you really want just a ballpark figure you could assume a cosine dependency, i.e. efficiency 1 at the apogee, 0 at the ends of the semi-minor axis (1/4 of an orbit away). &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-10-09T16:47:30.583" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12277" PostTypeId="2" ParentId="12274" CreationDate="2015-10-10T02:32:51.650" Score="7" Body="&lt;p&gt;How about a non-zero-gravity litter box?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While cats have a natural inclination towards the litterbox anything unconventional requires a certain amount of training and kittens require a certain amount of training anyway--you start out with the cat in a fairly small environment that includes the litter box, once it has a good understanding that that's where to go you expand it's environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means you need a station large enough you can confine the cat in the area of it's litter box for a bit--that's a pretty substantial station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus lets give the cat a litter box on a squirrel cage spun to perhaps .05g--we are simply after enough to keep things from floating away.  The &quot;litter&quot; is ferrous material encased in something that will keep it from rusting--magnets are used to keep the cat from tossing it out of the box with it's pawing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The litter box actually comprises the whole ring although the cat only sees a small part of that, the rest of it is behind the scenes.  The cleaning mechanism normally rotates very slowly relative to the litter bed, it stops if it senses a cat in the box and then resumes when the cat has been gone for a minute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such a system would not work for small kittens and might be unsuitable for sufficiently elderly cats--it requires a cat capable of jumping into/out of the rotating ring.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-10-10T02:32:51.650" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12278" PostTypeId="2" ParentId="12274" CreationDate="2015-10-10T03:09:22.827" Score="6" Body="&lt;p&gt;It's not an optimal solution, but fitted cat-diapers might suffice, at least for short periods.&lt;/p&gt;&#xA;" OwnerUserId="8641" LastActivityDate="2015-10-10T03:09:22.827" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12279" PostTypeId="2" ParentId="12256" CreationDate="2015-10-10T03:14:18.677" Score="16" Body="&lt;p&gt;Astronaut William Anders said in an interview that on the way to the moon (the first time men ever made this trip), they could see very few stars, until the Apollo 8 spacecraft passed into the shadow of the moon. With all direct sunlight now blocked, they could see every imaginable star, to the point where constellations became difficult to make out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; the link below is broken, here is a possible replacement: &lt;a href=&quot;https://www.youtube.com/watch?v=PtdcdxvNI1o&amp;amp;app=desktop&quot; rel=&quot;noreferrer&quot;&gt;Neil Armstrong interview, BBC 1970.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Start at 10:20 here:&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/dEkVBmkIyeg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="11905" LastEditorUserId="12102" LastEditDate="2019-03-18T01:10:40.763" LastActivityDate="2019-03-18T01:10:40.763" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="12280" PostTypeId="1" CreationDate="2015-10-10T04:14:09.787" Score="3" ViewCount="601" Body="&lt;p&gt;To what velocity would a spacecraft have to accelerate to launch from the surface of Enceladus into orbit around it? And from the orbital distance of Enceladus, what would be the escape velocity from Saturn, for a return journey to Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding is that a two-man craft the size and power of the Apollo LM ascent stage would have no problem leaving Enceladus or Ceres or most of the moons of Jupiter and Saturn (other than Titan with its thick atmosphere). I'm not so clear on whether a craft with the size and power of the Apollo CSM would be able to escape from the influence of Saturn, with or without a great deal more fuel than that used to escape our Moon during Apollo.&lt;/p&gt;&#xA;" OwnerUserId="11905" LastActivityDate="2015-10-10T05:24:47.720" Title="What is the escape velocity of Saturn at Enceladus distance?" Tags="|interplanetary|crewed-spaceflight|escape-velocity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12281" PostTypeId="2" ParentId="12280" CreationDate="2015-10-10T05:24:47.720" Score="13" Body="&lt;p&gt;No, not even close.  First, you wouldn't choose to escape directly from Enceladus distance.  You would instead lower your perikrone to as close to Saturn as you can get (make sure you miss the rings on the way down!), and then do your escape burn there.  You could use Titan to lower your perikrone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just escaping Saturn is not enough, since you would then be left stranded in a solar orbit much like Saturn's.  You would have to also reduce your perihelion at least as far down as Jupiter, and then use a Jupiter gravity assist to get back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CSM had a &lt;em&gt;total&lt;/em&gt; $\Delta V$ of about 2.8 km/s.  (Much of which was used to get &lt;em&gt;into&lt;/em&gt; lunar orbit.)  Even if I ignore how in the heck you got there in the first place, and you have all that $\Delta V$ available in Enceladus orbit, you're still not getting back.  You'd need 5.3 km/s to escape directly (your original question), 4.5 km/s to lower your perikrone to near Saturn directly, or 3.8 km/s to get up to Titan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if by some swingby hijinks you find a way to get close to Saturn for an escape burn, at that point you will need 10 km/s to escape Saturn on a trajectory that will get you to Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, if you had found a magic elixir on Enceladus that multiplies the Isp of your engine by a factor of four, you are now on a ten-year journey home with a week or two of oxygen, water, food, and CO2 scrubbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sorry, but you and your crew are screwed.  You are living in your coffin.  I don't know how you got there, but just make the best of your remaining time.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-10-10T05:24:47.720" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12282" PostTypeId="1" CreationDate="2015-10-10T06:20:30.860" Score="2" ViewCount="66" Body="&lt;p&gt;I always wanted being an astronaut. I am now going to high school and want to major astronomy in university.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is I live in South Korea, not US.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can you guys tell me how a foreigner can be an astronaut at NASA and if astronomy is not that helpful, which is better?&lt;/p&gt;&#xA;" OwnerUserId="11906" LastEditorUserId="7017" LastEditDate="2015-10-10T07:28:34.593" LastActivityDate="2015-10-10T07:28:34.593" Title="How to be an astronaut?" Tags="|nasa|astronauts|" AnswerCount="0" CommentCount="1" ClosedDate="2015-10-11T10:51:17.077" ContentLicense="CC BY-SA 3.0" />
  <row Id="12283" PostTypeId="1" AcceptedAnswerId="12285" CreationDate="2015-10-10T21:17:51.633" Score="2" ViewCount="4487" Body="&lt;p&gt;Both electric propulsion systems and chemical engines have been used in the history of space travel. What are the specific areas where one would have an advantage over the other?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Basic Principles of Each Type of Engine:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Chemical engines:&lt;/em&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Combustion engines have been used more widely throughout the history of space travel.&lt;/li&gt;&#xA;&lt;li&gt;Chemical engines have a much higher thrust-to-weight  ratio than electric propulsion systems.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;em&gt;Electric Propulsion engines:&lt;/em&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Electric thrusters typically use much less propellant than chemical rockets.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Electric propulsion provide little thrust, but over a long time.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;An electrically powered spacecraft propulsion system uses electrical energy to change accelerate propellant to high speeds.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;This research shows that electric propulsion would be ideal for long duration space flight, but be almost completely ineffective for reaching escape velocity of a planetary body. And on the flip side, that chemical engines would be suitable for reaching escape velocity, but inefficient for long duration space travel due to the amount of fuel consumed by the thrusters. Am I correct in my assumptions?&lt;/p&gt;&#xA;&lt;p&gt;Are their any notable missions that used one distinct kind or the other?&lt;/p&gt;&#xA;" OwnerUserId="11893" LastEditorUserId="8693" LastEditDate="2021-10-17T10:47:47.950" LastActivityDate="2021-10-17T10:47:47.950" Title="Usages of electric propulsion versus chemical engines" Tags="|propulsion|spacecraft-development|electric-propulsion|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12284" PostTypeId="1" CreationDate="2015-10-10T21:27:40.853" Score="7" ViewCount="594" Body="&lt;p&gt;I watched the movie The Martian, and there is a brief scene where there is lightning on Mars. Something about this seems unlikely to me, but I can't remember if it's actually possible or not. Is there actually lightning on Mars?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-10T22:45:38.770" Title="Lightning on Mars?" Tags="|mars|planetary-science|weather|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12285" PostTypeId="2" ParentId="12283" CreationDate="2015-10-10T21:51:56.810" Score="5" Body="&lt;p&gt;Your question really contains most of the answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you note, electrical propulsion is unusable for liftoff from large bodies; the engines have far less than 1:1 thrust-to-weight ratios (in Earth gravity) by themselves, let alone with the added weight of propellant, payload, and power systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you've gotten into orbit on chemical rockets, though, the thrust becomes somewhat less important. Time becomes the next major constraint. Radiation exposure is a significant constraint on any crewed mission beyond low Earth orbit; so are life support consumables. Therefore, chemical rockets will remain the appropriate choice for those missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flyby missions such as New Horizons don't need to carry significant propulsion of their own -- just small thrusters for minor course corrections -- so the mass of a power system for an electric thruster isn't justified. If you need to enter orbit around a distant planet, though, electric propulsion is very attractive because of its specific impulse advantage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding notable missions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a bunch of satellites using ion engines or Hall thrusters for orbit correction. Relatively few interplanetary missions so far have used them: Deep Space 1, Dawn, and Hayabusa 2 are I think the most significant. Some information on those thrusters is &lt;a href=&quot;https://space.stackexchange.com/questions/8105/what-is-the-performance-of-ion-thrusters-in-actual-deployed-spacecraft&quot;&gt;collected here&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-10-10T22:27:45.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12286" PostTypeId="2" ParentId="12284" CreationDate="2015-10-10T22:45:38.770" Score="11" Body="&lt;p&gt;Lightning has been observed in Mars. However, this is &lt;a href=&quot;https://en.wikipedia.org/wiki/Dry_lightning&quot;&gt;'dry' lightning&lt;/a&gt;, unrelated to precipitation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was described in the paper &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1029/2009GL038715/full&quot;&gt;&lt;em&gt;Emission of non-thermal microwave radiation by a Martian dust storm&lt;/em&gt;&lt;/a&gt; by Christopher Ruf and others.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-10-10T22:45:38.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12287" PostTypeId="2" ParentId="12283" CreationDate="2015-10-11T01:47:34.943" Score="4" Body="&lt;p&gt;Once you're in space (let's say at least as far as low-Earth orbit), if you need something like 2 to 3 km/s mission $\Delta V$, then chemical propulsion is probably the cheapest approach.  If you need a fair bit more than that, and you can expend it in the inner solar system with ample sunlight, then solar electric propulsion will be the cheapest.  (This may even be true for missions with time constraints, e.g. with humans -- you may just need to have very high power, i.e. lots of solar array area, to support sufficient thrust.)  If you need lots of $\Delta V$ and you need it in the outer solar system, then you will likely need nuclear electric propulsion, but we have no space nuclear power sources big enough for that at this time.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-10-11T01:47:34.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12288" PostTypeId="2" ParentId="12258" CreationDate="2015-10-11T13:28:26.293" Score="6" Body="&lt;p&gt;In the general case you will need to solve numerically for the radii at which the orbit intersects the ellipsoid.  Then you can solve analytically for the times at which the orbit is at those radii.  That is, if you ignore $J_2$.  Since you made it an ellipsoid, you have introduced a $J_2$, so you will no longer be in a Keplerian orbit.  To take $J_2$ into account, the times would then also need to be solved for numerically.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-10-11T13:28:26.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12289" PostTypeId="1" CreationDate="2015-10-11T13:50:39.743" Score="1" ViewCount="135" Body="&lt;p&gt;I am experimenting with &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_perturbations_models&quot; rel=&quot;nofollow&quot;&gt;SGP4&lt;/a&gt; propagator and &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow&quot;&gt;TLEs&lt;/a&gt; of different satellites looking mainly at long term semi-major axis evolution. I have encountered an issue with with the some of the satellites, e.g. &lt;a href=&quot;http://www.n2yo.com/satellite/?s=40010&quot; rel=&quot;nofollow&quot;&gt;40010&lt;/a&gt;. The problem is that the Semi major axis (SMA) seems to fluctuate long term by +/-2 km around a normal negative trend (for satellites without orbit maintenance). So the the TLE derived SMA is sometimes increasing over several months without any apparent reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this normal for the sort of accuracy the TLE/SGP4 can provide?&lt;/p&gt;&#xA;" OwnerUserId="11907" LastEditorUserId="7017" LastEditDate="2015-10-11T14:43:00.840" LastActivityDate="2015-10-13T13:30:45.937" Title="Issue with SMA evolution derived from TLEs" Tags="|orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12290" PostTypeId="1" CreationDate="2015-10-11T18:08:55.487" Score="7" ViewCount="992" Body="&lt;p&gt;This is part of a &lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/27254/with-what-precision-timing-can-we-send-a-missile-to-strike-the-moon&quot;&gt;short story posted on worldbuilding.se&lt;/a&gt;, but setting my story aside, it was recommended that the part about precision be asked here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;With what precision can an object be put on the moon, with today's technology?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is specifically related to the accuracy of the de-orbit, and the assumption is that there is a will to impact the moon with an object with precision (I'm trying not to stray into worldbuilding territory right now).&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:23.630" LastActivityDate="2016-02-22T21:38:59.383" Title="With what precision can we land an object on the moon, today?" Tags="|the-moon|probe|landing|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12291" PostTypeId="2" ParentId="12290" CreationDate="2015-10-11T18:55:49.627" Score="12" Body="&lt;p&gt;Really precise. &lt;a href=&quot;http://www.lpi.usra.edu/lunar/missions/apollo/apollo_12/landing_site/&quot; rel=&quot;nofollow&quot;&gt;Apollo 12&lt;/a&gt; landed only 600 feet off of target. That's the only landing with precision that I can find, but presumably we could do it at least as well today. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The moon is easier to land precisely on than, say, Mars or Earth, because of the lack of atmosphere. The fact that it is so close to Earth also allows for a spacecraft to be precisely on the correct trajectory, with Mars there is always a small amount of uncertainty, which can really add up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for how accurate with a hard landing, the best analog was &lt;a href=&quot;http://arxiv.org/abs/1103.1687&quot; rel=&quot;nofollow&quot;&gt;LCROSS&lt;/a&gt;, which intentionally targeted a point on the Moon. The miss distance was 146 m for the primary stage, 766m for the less precise part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine if you wanted to, you could probably improve that by an order of magnitude, although I have nothing with which to base that claim. This could be accomplished by using GPS, visual maps, and precise knowledge of it's gravitational field, among others.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-02-22T21:38:59.383" LastActivityDate="2016-02-22T21:38:59.383" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12293" PostTypeId="1" CreationDate="2015-10-12T04:33:26.343" Score="2" ViewCount="443" Body="&lt;p&gt;Is it possible to construct a probe that would float in the Sun's surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The temperature of Sun is 5,800 K and I heared there are alloys that can remain solid at 4500 K. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder if it is possible to develop a better material or use an internal cooling system to keep the shell below the melting point. And have the internal volume filled with light material or vacuum so that it would float. &lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="812" LastEditDate="2015-10-12T04:44:36.327" LastActivityDate="2019-07-20T06:00:05.420" Title="A probe floating in Sun's surface" Tags="|probe|the-sun|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12294" PostTypeId="2" ParentId="12293" CreationDate="2015-10-12T08:10:03.647" Score="13" Body="&lt;p&gt;Thermodynamics says there's no such thing as a completely internal cooling system.  The best you can do is pump heat to a cooler location.  That's very difficult to come by in the photosphere.  Besides trying to keep cool enough for some alloy to remain solid, keeping electronics or a power source cool enough to function would be significantly more difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the photosphere is quite rarefied.  It has a density similar to the earth's atmosphere at an altitude of about 60km.  That's above the level where even high-altitude balloons travel.  So even if the cooling problems weren't there, &quot;floating&quot; at that level with equipment isn't possible.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2015-10-12T08:10:03.647" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12295" PostTypeId="1" AcceptedAnswerId="12299" CreationDate="2015-10-12T13:49:01.773" Score="17" ViewCount="5291" Body="&lt;p&gt;Space is already cold, so why do some space telescopes require cooling? &#xA;Ex: infra-red telescope &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of cryocoolers are used, and why?&lt;/p&gt;&#xA;" OwnerUserId="11586" LastEditorUserId="11586" LastEditDate="2015-10-14T05:23:26.703" LastActivityDate="2015-10-14T05:23:26.703" Title="Why do some space telescopes require cooling (sometimes down to 3K)?" Tags="|telescope|temperature|space-telescope|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12296" PostTypeId="1" CreationDate="2015-10-12T13:58:14.090" Score="12" ViewCount="4783" Body="&lt;p&gt;The ISS has a camera mounted streaming live video from space (&lt;a href=&quot;http://www.ustream.tv/channel/live-iss-stream&quot;&gt;HDEV&lt;/a&gt;). From time to time, there is a blue screen visible, which is noted to be during times of &quot;loss of signal&quot;. What exactly defines a &quot;loss of signal&quot;? Does the ISS have to be in line-of-sight contact with a ground station (which does not seem to the correct answer) or a different satellite? How is the ground communication done for this live video feed? &lt;/p&gt;&#xA;" OwnerUserId="11919" LastActivityDate="2017-10-03T13:49:05.697" Title="When does the ISS have a loss-of-signal?" Tags="|iss|communication|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12297" PostTypeId="2" ParentId="12295" CreationDate="2015-10-12T14:10:53.270" Score="6" Body="&lt;p&gt;In space, the notion of temperature is not the same as on Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Temperature can only apply to matter which vacuum has not.&#xA;Space is near vacuum, so its temperature is irrelevant to large and dense objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites (including telescopes) are made out of matter and therefor can heat or cool.&#xA;They are full of electronics that produce heat, and receive sunlight when not on night side of earth.&#xA;Because space is made of vacuum, heat can not be exchanged by contact with colder matter (convection) , and therefor they must be cooled by radiation (emitting infrared).&#xA;Because this is much less efficient than traditional cooling, a complex cooling system must be added.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TL;dNR: Space is not &lt;code&gt;cold&lt;/code&gt;, and fans don't work well in space since there is no air.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6642" LastEditDate="2015-10-13T13:11:55.797" LastActivityDate="2015-10-13T13:11:55.797" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12299" PostTypeId="2" ParentId="12295" CreationDate="2015-10-12T14:29:30.923" Score="31" Body="&lt;p&gt;Space telescopes like Spitzer, Herschel, Planck, WISE, and in few years Webb, need to observe in the mid and far infrared wavelengths.  The infrared radiation of normal spacecraft temperatures, even if kept cold by our standards, would look like a bright light at those wavelengths.  (Google black-body radiation to learn more.)  The infrared detectors need to be kept at single-digit Kelvin temperatures in order to not radiate into themselves, especially to have the necessary signal-to-noise ratio to be able to detect faint infrared sources in the universe.  Hence the need for cryo-coolers and/or liquid Helium on those telescopes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planck needed to be so cold that it used Helium-3 instead of Helium-4, since the boiling point of He-3 is about one Kelvin lower than He-4.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can look at &lt;a href=&quot;http://webbtelescope.org/webb_telescope/technology_at_the_extremes/keep_it_cold.php&quot;&gt;this page&lt;/a&gt; for more information on the thermal control of Webb.  The Webb cryocooler for the MIRI instrument has been one of the most difficult developments.  Cryocoolers work on the same principles as your refrigerator, using compressors and heat exchangers.  However they use Helium as the exchange fluid, and have multiple stages to get the temperature to single-digit Kelvins.  Here is an &lt;a href=&quot;https://smartech.gatech.edu/bitstream/handle/1853/38740/002.pdf&quot;&gt;informative paper on the MIRI cryocoolers&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-10-12T14:50:46.710" LastActivityDate="2015-10-12T14:50:46.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12301" PostTypeId="2" ParentId="12296" CreationDate="2015-10-12T15:13:47.173" Score="17" Body="&lt;p&gt;The ISS communicates with ground assets through a robust network including ground stations and geostationary satellites (&lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite_System&quot; rel=&quot;noreferrer&quot;&gt;TDRSS&lt;/a&gt;). However, due to line of sight constraints there are brief gaps in coverage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edit - expansion)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These gaps can be seen (at least during the Shuttle era) by looking at the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/438131main_FLT_PLN_131_F_1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Flight Plan&lt;/a&gt;. I've cut-n-pasted the relevant parts of one page from the linked STS-131 Flight Plan to give an idea of how often comm outages occur.  Take a look at the link to see what a full page of the Plan looks like.  Basically I've cut out all the activity information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8uMud.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8uMud.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These timeline sections of the Plan show the GMT and Mission Elapsed time at the top, the Day/Night cycles, the orbit count, etc. (The top values are for the shuttle, the bottom ones are for the ISS). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lines labeled TDRS W (west), E (east), and Z (Zone of Exclusion) show horizontal black bars when the associated Tracking and Data Relay Satellite (TDRS) is in line-of-sight with the shuttle's comm system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lines labeled TDRS AVAIL show when any of the satellites are available to the ISS comm system.  The gold bar is Ku-band (video) availability, the green bar is S-band (voice and data) availability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This graph shows the brief periods during which there is loss of voice or video comm due to line-of-sight constraints with the TDRS system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more info on the ISS comm system see my answer to this question: &lt;a href=&quot;https://space.stackexchange.com/questions/16569/do-antennae-on-the-iss-have-to-constantly-move-to-maintain-data-links/16570#16570&quot;&gt;Do antennae on the ISS have to constantly move to maintain data links?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-10-03T13:49:05.697" LastActivityDate="2017-10-03T13:49:05.697" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12302" PostTypeId="2" ParentId="6043" CreationDate="2015-10-12T16:51:28.803" Score="3" Body="&lt;p&gt;Actually Mazura is wrong, Callisto the moon of Jupiter has a mean temperature of 134 Kelvin, Jupiter has a cloudtop temperature of 165 Kelvin. So it is warmer, but just by 30 degrees. The Jupiter's internal heat really shows below the top clouds, with the core being 36000 Kelvin hot. The freezing point of hydrogen is 13.99 Kelvin. Any &quot;gut feeling&quot; is wrong in astrophysics, the Sun is not luminous enough to melt Mercury, but even Pluto is way too warm to freeze hydrogen at around 40 Kelvin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for this question, I do not really know, but I think there are no gas giants old enough for them to cool to this state in the universe yet. It should be said helium cannot be solid without high pressure, so such frozen gas giants in the far future, billions or even trillions of years in the future would consist of a big core of solid hydrogen surrounded by an envelope of liquid/superfluid and gaseous helium.&lt;/p&gt;&#xA;" OwnerUserId="11921" LastActivityDate="2015-10-12T16:51:28.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12303" PostTypeId="2" ParentId="12170" CreationDate="2015-10-12T17:14:22.150" Score="7" Body="&lt;p&gt;Aside from being, essentially LNG, Methane burns Cleanly. Kerosene produces coking and polymerization within the engine. This does not matter if you use the engine only a few times. If you want to reuse it 40-50 times or more (like Spacex), you need to clean a Kerosene engine every few uses and this gets expensive if you have to dismantle it. Methane is far superior from a &quot;reusability&quot; perspective. LNG is also cheap these days, so RP-1s advantage there is pretty much gone.&lt;/p&gt;&#xA;" OwnerUserId="11922" LastActivityDate="2015-10-12T17:14:22.150" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12304" PostTypeId="1" AcceptedAnswerId="12305" CreationDate="2015-10-12T17:49:54.373" Score="8" ViewCount="689" Body="&lt;p&gt;Are there big differences in the amount of radiation if you would put the crew compartment (on the journey to Mars) inside the LOX/LH2 fuel tank? I know that there are similarities with other question, but this is only meant with its own rocket fuel.&lt;/p&gt;&#xA;" OwnerUserId="7035" LastEditorUserId="238" LastEditDate="2015-10-13T09:21:01.380" LastActivityDate="2015-10-13T09:21:01.380" Title="Could you protect a crew from space radiation with fuel?" Tags="|mars|rockets|fuel|radiation|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12305" PostTypeId="2" ParentId="12304" CreationDate="2015-10-12T18:57:36.927" Score="10" Body="&lt;p&gt;Yes, low-Z shield would work (generally, water or polyethylene). However, LH2 is famously prone to seeping through seemingly solid walls, and making habitable space fit inside the fuel tank would put you at a disadvantage:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Crew habitat has to be at room temperature (293K).&lt;/li&gt;&#xA;&lt;li&gt;LH2 has to be kept at the temperature of 20K.&lt;/li&gt;&#xA;&lt;li&gt;You would have to increase insulation between the habitat and the tank to reduce thermal losses (and correspondingly, hydrogen boil-off which is mass wasted, pure and simple). Instead of a cylinder with hemispherical ends, you'd have to produce a less efficient and more expensive vessel.&lt;/li&gt;&#xA;&lt;li&gt;Even then, the crew would be freezing cold, and thermal design would be unnecessarily bulky and risky.&lt;/li&gt;&#xA;&lt;li&gt;If you spend the last drops of fuel on Trans-Earth injection you'll have no protection from cosmic rays during the return voyage.&lt;/li&gt;&#xA;&lt;li&gt;Another important part of that disadvantage is LH2's low density: 70.85 $kg/m^3$ instead of water's 1000 or ice's 917.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-10-12T19:20:21.023" LastActivityDate="2015-10-12T19:20:21.023" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12307" PostTypeId="2" ParentId="6043" CreationDate="2015-10-13T03:16:17.440" Score="1" Body="&lt;p&gt;No.  Helium won't freeze solid even at the temperature of interstellar space.  Thus even if there's no source of heat you'll still have liquid helium.  Note that hydrogen ice will float on this, the planet could &lt;strong&gt;appear&lt;/strong&gt; solid.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-10-13T03:16:17.440" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12308" PostTypeId="2" ParentId="12293" CreationDate="2015-10-13T03:19:43.903" Score="3" Body="&lt;p&gt;There's an alloy that will just barely be solid even at the temperature at the sun's surface.  If you could make a sufficiently low density foam out of it you might be able to construct something that would float.  Given how close to it's melting point it would be I would be very surprised if it would remain a foam, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you have only one element to work with it would be a pretty useless probe, though.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2015-10-13T03:19:43.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12309" PostTypeId="2" ParentId="12289" CreationDate="2015-10-13T13:30:45.937" Score="2" Body="&lt;p&gt;Seeing a negative trend in your SMA is not unexpected. Consider the specific mechanical energy, &lt;span class=&quot;math-container&quot;&gt;$\epsilon$&lt;/span&gt;, of an orbit, given by&#xA;&lt;span class=&quot;math-container&quot;&gt;\begin{equation}&#xA;\epsilon = -\frac{\mu}{2a}&#xA;\end{equation}&lt;/span&gt;&#xA;where &lt;span class=&quot;math-container&quot;&gt;$\mu$&lt;/span&gt; is the gravitational parameter and &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; is your SMA. Considering the case of a bird in low Earth orbit (LEO) where you would expect air drag and other perturbations to decrease your orbital energy, we can look at the sign of &lt;span class=&quot;math-container&quot;&gt;$d\epsilon/dt$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;\begin{equation}&#xA;\frac{d\epsilon}{dt} = \frac{\mu}{2a^2}\frac{da}{dt}&#xA;\end{equation}&lt;/span&gt;&#xA;Since &lt;span class=&quot;math-container&quot;&gt;$\mu &amp;gt; 0$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$a &amp;gt; 0$&lt;/span&gt;, this relation tells us that a decreasing &lt;span class=&quot;math-container&quot;&gt;$da/dt$&lt;/span&gt; indicates a decrease in orbital energy.&lt;/p&gt;&#xA;&lt;p&gt;Kelso provides an example on &lt;a href=&quot;https://celestrak.org/columns/v01n05/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt; of the Mir's TLE propagation using SGP4 where the SMA is slowly decreasing over time. Figure 1 on that same page also provides an example of SMA variations that are on the same order of magnitude as your example. Bottom line - once you relax the two-body assumptions, you'll start to see short-period and long-period variations in the elements, which is totally and completely expected.&lt;/p&gt;&#xA;" OwnerUserId="1478" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2015-10-13T13:30:45.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12311" PostTypeId="2" ParentId="12274" CreationDate="2015-10-13T14:38:55.363" Score="5" Body="&lt;p&gt;A solution by &lt;a href=&quot;https://en.wikipedia.org/wiki/Jack_McDevitt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jack McDevitt&lt;/a&gt; in &quot;The Cat's Pajamas&quot; (2012) &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;used magnetic gravel and gentle suction to overcome the problems of a zero-gee environment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This was offered as &lt;a href=&quot;https://scifi.stackexchange.com/questions/104754/do-heinleins-novels-mention-a-plausible-zero-gravity-cat-litter-box/104927#104927&quot;&gt;a solution to a related question&lt;/a&gt; by Null On SciFi.SE&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:43:38.653" LastActivityDate="2015-10-13T14:38:55.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12312" PostTypeId="2" ParentId="12295" CreationDate="2015-10-13T17:35:19.053" Score="0" Body="&lt;p&gt;Kanaga, something to consider:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;The Earth is sitting in space!&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth gets hot (because of the Sun); other things sitting in space (the Moon, Mars, satellites, and so on) also get hot because of the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's that simple, really.&lt;/p&gt;&#xA;" OwnerUserId="3289" LastActivityDate="2015-10-13T17:35:19.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12313" PostTypeId="2" ParentId="12295" CreationDate="2015-10-13T18:19:59.697" Score="3" Body="&lt;p&gt;The density of space is really really thin. In the same way that water conducts heat much better than air, so air conducts it much better than space. In fact, even though the temperature might technically be very cold, in fact it doesn't matter, because that heat isn't transferred. Most of the heat that objects feel depends on their surface area, power usage, and area incident to the Sun. An object is really cold if it isn't in the Sunlight in space. Otherwise, it is warmer, typically at similar temperatures to Earth. Basically, the object reaches a temperature known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermodynamic_equilibrium&quot; rel=&quot;nofollow&quot;&gt;Thermal Equilibrium&lt;/a&gt;. Near Earth, that is typically around room temperature (20 degrees C)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft that need to be really cold essentially measure temperature throughout the cosmos. Every object produces a small amount of light, although it might not be visible. You can feel a fire if you are close, even if the wind was blowing the heat away from you. This feeling of heat is infrared, which is what these cold spacecraft measure. If the temperature is too high, then the signal is confused, much in the same way that it is difficult to see out of a dirty windshield if driving towards the sun. Something can be seen, but it is much more difficult to see.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-13T18:19:59.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12314" PostTypeId="2" ParentId="12283" CreationDate="2015-10-13T21:06:18.920" Score="2" Body="&lt;p&gt;Further to the previous answers, it is helpful in early system studies to look at the mass trade-off for various chemical and electric systems. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The hardware mass for an electric system grows with specific impulse because the power source scales with the propulsive beam power. &lt;/li&gt;&#xA;&lt;li&gt;This is offset more readily by propellant savings for missions needing a large delta V&lt;/li&gt;&#xA;&lt;li&gt;This means that the most appropriate choice (in terms of minimum total system mass) of electric specific impulse grows with delta V &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;e.g. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ion thrusters are a better match for large delta V, &lt;/li&gt;&#xA;&lt;li&gt;hall effect thrusters and arc jets match lower delta V missions and&#xA;fit in between ion thrusters and chemical thrusters.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The practical choice is often swayed by other system considerations such as the thrust, total impulse, propulsion needs for other functions such as attitude control, storage volume, presence of solar arrays for other purposes&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2019-03-22T18:47:05.377" LastActivityDate="2019-03-22T18:47:05.377" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="12315" PostTypeId="2" ParentId="2520" CreationDate="2015-10-13T21:45:14.393" Score="0" Body="&lt;p&gt;Try this link for a few small thruster types. Its out of date, the names of manufacturers have changed since this list but the thruster names are probably still in use. &#xA;&lt;a href=&quot;https://engineering.purdue.edu/~propulsi/propulsion/rockets/satellites.html&quot; rel=&quot;nofollow&quot;&gt;https://engineering.purdue.edu/~propulsi/propulsion/rockets/satellites.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-10-13T21:45:14.393" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12316" PostTypeId="2" ParentId="2520" CreationDate="2015-10-14T10:14:34.237" Score="1" Body="&lt;p&gt;Some examples of small engines:&lt;br&gt;&#xA;&lt;a href=&quot;http://www.northropgrumman.com/Capabilities/PropulsionProductsandServices/Pages/default.aspx&quot; rel=&quot;nofollow&quot;&gt;Northrop Grumman&lt;/a&gt; &lt;a href=&quot;http://www.northropgrumman.com/Capabilities/PropulsionProductsandServices/Pages/BipropellantEnginesAndThrusters.aspx&quot; rel=&quot;nofollow&quot;&gt;Bipropellant thrusters&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;&lt;pre&gt;&lt;code&gt;-TR-308 Liquid Apogee Engine, which placed the NASA Chandra X-ray Observatory spacecraft into its final orbit. The Liquid Apogee Engine (LAE) is the world's highest performing apogee engine.&#xA;-TR-312 Liquid Apogee Engines (TR-312-100MN and TR-312-100YN) which have completed DVT testing for commerical geostationary comsat applications.&#xA;-TR-500 Secondary Combustion Augmented Thruster (SCAT) Bi-Modal Thruster, flight-proven on the National Reconnaissance Office's Geosynchronous Lightweight Technology Experiment (GeoLITE) program.&#xA;-TR-711 Gel Boost Engine, which powered the world's first flight of a tactical missile utilizing gelled propellants and achieved a 700:1 Thurst-to-Weight Ratio.&#xA;-LOX/LH2 and LOX/Ethanol RCS engines successfully test fired in 1000 lbf and 870 lbf configurations, respectively.&#xA;-NASA RCE contract for a 100 lbf class LOX/Methane. &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.northropgrumman.com/Capabilities/PropulsionProductsandServices/Pages/MonopropellantThrusters.aspx&quot; rel=&quot;nofollow&quot;&gt;Monopropellant thrusters&lt;/a&gt; - several models between 1 and 86 N, using hydrazine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerojet Rocketdyne:&lt;br&gt;&#xA;&lt;a href=&quot;https://www.rocket.com/propulsion-systems/monopropellant-rockets&quot; rel=&quot;nofollow&quot;&gt;Monopropellant thrusters&lt;/a&gt;, hydrazine, 1-3100 N.&lt;br&gt;&#xA;&lt;a href=&quot;https://www.rocket.com/propulsion-systems/bipropellant-rockets&quot; rel=&quot;nofollow&quot;&gt;Biprop thrusters&lt;/a&gt;, MMH/NTO or Hydrazine (N$_2$H$_4$)/NTO, 22 N-4 kN.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/propulsion-systems/index.html&quot; rel=&quot;nofollow&quot;&gt;EADS Astrium&lt;/a&gt;:&lt;br&gt;&#xA;&lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/bipropellant-thrusters/index.html&quot; rel=&quot;nofollow&quot;&gt;bipropellant thrusters&lt;/a&gt; 4-200 N, MMH and N$_2$O$_4$ or mixed oxides of nitrogen (MON)&lt;br&gt;&#xA;&lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/hydrazine-thrusters/index.html&quot; rel=&quot;nofollow&quot;&gt;hydrazine thrusters&lt;/a&gt; 1-400 N&lt;br&gt;&#xA;&lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/apogee-motors/index.html&quot; rel=&quot;nofollow&quot;&gt;Apogee engines&lt;/a&gt;, MMH and N$_2$O$_4$ or mixed oxides of nitrogen (MON)&lt;br&gt;&#xA;&lt;a href=&quot;http://cs.astrium.eads.net/sp/spacecraft-propulsion/ion-propulsion/index.html&quot; rel=&quot;nofollow&quot;&gt;ion engines&lt;/a&gt;, thrust 50 $\mu$N to 200 mN  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-14T10:35:48.690" LastActivityDate="2015-10-14T10:35:48.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12319" PostTypeId="2" ParentId="1659" CreationDate="2015-10-14T14:51:10.887" Score="7" Body="&lt;p&gt;I'm having a hard time just sorting out NASA, but here's the statistics for every launch, using the source information found &lt;a href=&quot;http://planet4589.org/space/log/launchlog.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. This gives the following breakdown, by decade&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;1950's 43.6%&lt;/li&gt;&#xA;&lt;li&gt;1960's 85.1%&lt;/li&gt;&#xA;&lt;li&gt;1970's 93.5%&lt;/li&gt;&#xA;&lt;li&gt;1980's 96.0%&lt;/li&gt;&#xA;&lt;li&gt;1990's 93.9%&lt;/li&gt;&#xA;&lt;li&gt;2000's 94.7%&lt;/li&gt;&#xA;&lt;li&gt;2010's 94.7%&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note the list was last updated Oct 8, 2015.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-02-13T13:41:59.800" LastActivityDate="2017-02-13T13:41:59.800" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12320" PostTypeId="2" ParentId="10896" CreationDate="2015-10-14T15:45:24.440" Score="7" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gerhard_Thiele&quot;&gt;Gerhard Thiele&lt;/a&gt; confirmed in an old &lt;a href=&quot;http://web.archive.org/web/20110825114726/http://quiz.esa.int/page.php?id=11276&quot;&gt;ESA blog&lt;/a&gt; that the Orlan suit had enough room to lean to one side and pull your arm out of the glove and sleeve to scratch an itch. I couldn't find any further information on this though.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-10-14T15:45:24.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12321" PostTypeId="1" AcceptedAnswerId="12323" CreationDate="2015-10-14T19:51:32.860" Score="46" ViewCount="12011" Body="&lt;p&gt;After reading &lt;a href=&quot;https://aviation.stackexchange.com/questions/21981/how-does-the-space-shuttle-slow-down-on-the-re-entry&quot;&gt;this&lt;/a&gt; answer I found out that the space shuttle normally spent its time upside-down. Why is that? I'd guess it has to do with shielding from the sun, but I can't back that up with anything.&lt;/p&gt;&#xA;" OwnerUserId="3141" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:54.153" LastActivityDate="2018-10-12T11:19:37.463" Title="Why did the Space Shuttle fly upside-down normally" Tags="|nasa|crewed-spaceflight|space-shuttle|attitude|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12322" PostTypeId="2" ParentId="12321" CreationDate="2015-10-14T20:24:14.160" Score="18" Body="&lt;p&gt;Yes, one reason is that it is flown upside down (with the top facing towards Earth) to use the heat shield tiles to shield the astronauts from the sun, but I think a bigger reason is to shield from space debris.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One reason is to protect against space debris. Here on Earth, most of the space junk (small rocks and ice, etc.) that falls towards the planet burns up in the atmosphere before it can get to us. The space shuttle is outside the atmosphere, though, so it is much more exposed to debris (which may be traveling at very high speeds). The space shuttle's belly is designed to take intense heat and pressure so that it doesn't fall apart when it re-enters the atmosphere, and is therefore much better suited for taking hits from flying space junk. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.physlink.com/Education/AskExperts/ae524.cfm&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="690" LastActivityDate="2015-10-14T20:24:14.160" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12323" PostTypeId="2" ParentId="12321" CreationDate="2015-10-15T00:10:03.587" Score="70" Body="&lt;p&gt;The main reason is heat rejection. &lt;a href=&quot;http://www.nasa.gov/missions/highlights/webcasts/shuttle/sts110/shuttle_qa.html&quot;&gt;NASA&lt;/a&gt; was asked this very question, and the answer was identified. Basically, the waste heat from the shuttle is expelled via the cargo bay doors. You don't want to ever point a radiator at the Sun, so the easiest thing is to point it at the Earth. Sometimes, if the heat was too high, they would actually point the shuttle away from the Sun in eclipse, but this was rare.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If space debris was the main reason, then the shuttle would actually fly with the belly towards the direction of motion. Most impacts of space debris will be head on. The shuttle did fly backward, with the tail in the direction of orbital motion. This protected it against debris, as the tail was the less critical portion of the spacecraft for small holes that Debris would cause. See &lt;a href=&quot;http://www.physlink.com/Education/AskExperts/ae524.cfm&quot;&gt;this link&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-15T00:17:30.833" LastActivityDate="2015-10-15T00:17:30.833" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12324" PostTypeId="1" CreationDate="2015-10-15T05:12:56.487" Score="0" ViewCount="453" Body="&lt;p&gt;A lot has been said about human settlements on Mars. However, are there any other options? I'm wondering what other planets/moons/asteroids would be good for settling — where else could humans set up settlements after Mars?&lt;/p&gt;&#xA;" OwnerUserId="11950" LastEditorUserId="1457" LastEditDate="2015-10-15T09:19:59.680" LastActivityDate="2015-10-15T09:19:59.680" Title="Are there any other options for human settlements in the solar system other than Mars?" Tags="|solar-system|" AnswerCount="2" CommentCount="8" ClosedDate="2015-10-15T11:55:01.653" ContentLicense="CC BY-SA 3.0" />
  <row Id="12325" PostTypeId="2" ParentId="12324" CreationDate="2015-10-15T07:36:35.293" Score="2" Body="&lt;p&gt;I think the moon is a prime candidate. It has many advantages to Mars. The single most positive factor is it's closer so support and communication would be less problematic in the critical first years.&lt;/p&gt;&#xA;" OwnerUserId="11757" LastActivityDate="2015-10-15T07:36:35.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12327" PostTypeId="1" CreationDate="2015-10-15T09:05:55.800" Score="3" ViewCount="123" Body="&lt;p&gt;What was the typical&lt;sup&gt;1&lt;/sup&gt; (theoretical) perigee after a shuttle de-orbit burn?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I say 'theoretical' given that presumably the orbiter would already have undergone further deceleration before it reaches the perigee, but I'm just interested in the point they aimed for.  Logic would suggest the perigee did not need to be at ground level, just far enough into the atmosphere to create enough drag to ensure re-entry.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Did the target perigee change per mission or orbit?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="238" LastActivityDate="2015-10-15T09:05:55.800" Title="What was the typical perigee after a shuttle de-orbit burn?" Tags="|orbital-mechanics|low-earth-orbit|reentry|space-shuttle|orbital-maneuver|" AnswerCount="0" CommentCount="1" ClosedDate="2015-10-15T17:24:01.940" ContentLicense="CC BY-SA 3.0" />
  <row Id="12328" PostTypeId="2" ParentId="12324" CreationDate="2015-10-15T09:15:35.227" Score="6" Body="&lt;p&gt;If you define colonization as the capability of forming a self-sustaining society with its own technological base then there are no likely candidates with our current level of technology, even Mars. It's very hard to land on Mars because the atmosphere is thick enough to burn you up but not thick enough to slow you down, provide helpful lift, protect the planet from meteor impacts, or allow movement outside without protective suits. It doesn't have a magnetic field to protect life from harsh radiation, and doesn't have much in the way of easily accessed raw materials. The reason you'd want to put boots on Mars is science - humans would be able to answer a lot of questions that probes cannot. Eventually, with significant technological development, the raw materials Mars provides could be used to create the necessities of life, for now just about everything would need to be sent from earth. That's not a colony, but an outpost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you define colonization as humans living somewhere then just about any body in the solar system is a candidate, or no body at all. If the earth has to supply everything then a space station makes more sense than a planetary body as you don't have to mess with a gravity well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Really once we solve the problems that will allow a self-sustaining colony on mars we would be most of the way to colonies wherever we want provided we can deal with the gravity. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-10-15T09:15:35.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12329" PostTypeId="1" CreationDate="2015-10-15T12:50:24.577" Score="1" ViewCount="1987" Body="&lt;p&gt;What is different about the orbits of Pluto and the comets around the Sun?&lt;/p&gt;&#xA;" OwnerUserId="11956" LastEditorUserId="49" LastEditDate="2015-10-15T13:23:24.053" LastActivityDate="2015-10-15T16:52:01.513" Title="What is different about the orbits of Pluto and the comets around the Sun?" Tags="|orbit|pluto|comet|kuiper-belt|" AnswerCount="2" CommentCount="1" ClosedDate="2015-10-15T23:36:47.893" ContentLicense="CC BY-SA 3.0" />
  <row Id="12331" PostTypeId="2" ParentId="12329" CreationDate="2015-10-15T13:16:49.650" Score="4" Body="&lt;p&gt;A &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet&quot; rel=&quot;nofollow&quot;&gt;comet&lt;/a&gt; has to approach the Sun to within its &lt;a href=&quot;https://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29&quot; rel=&quot;nofollow&quot;&gt;snow line&lt;/a&gt; to start sublimating volatiles and form coma and the tails. For our Sun, its snow (or frost) line is at roughly 5 AU (&lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_unit&quot; rel=&quot;nofollow&quot;&gt;Astronomical Units&lt;/a&gt;), so at about the closest approach of Jupiter to the Sun in its orbit (its perihelion).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto&quot; rel=&quot;nofollow&quot;&gt;Pluto&lt;/a&gt; orbits way past the Sun's snow line in too circular orbit to approach the Sun at such proximity at any point in its orbit (perihelion at roughly 30 AU). For Pluto to ever become &lt;em&gt;a comet&lt;/em&gt;, its orbit would have to be substantially perturbed by other strong gravitational attractors in the system, but since it is in &lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto#Relationship_with_Neptune&quot; rel=&quot;nofollow&quot;&gt;2:3 mean-motion resonance with Neptune&lt;/a&gt;, its closest such neighbor that's massive enough to cause Pluto's orbit to greatly increase its orbital eccentricity and lower perihelion, that won't happen. It actually comes closer to Uranus than Neptune, but still not close enough to cause major destabilization of its orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we have another big difference - Pluto's orbit is long-term stable, while most comets actually evolved much farther from the Sun as icy bodies and were perturbed from their orbit by some other massive object that came close enough to set them on a less permanent (and possibly additionally perturbed, often by Jupiter) and more eccentric orbit that crosses the Solar system's snow line. Some comets are periodic and do that more or less regularly, some are in more chaotic orbits and can be temporarily comets and then move out of snow line completely (e.g. some &lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_%28minor_planet%29&quot; rel=&quot;nofollow&quot;&gt;Centaurs&lt;/a&gt;), and some are &lt;a href=&quot;https://en.wikipedia.org/wiki/Sungrazing_comet&quot; rel=&quot;nofollow&quot;&gt;sungrazers&lt;/a&gt; and one-time comets that are either ejected out of the Solar system entirely on a hyperbolic trajectory, and/or come too close to the Sun, hit it or evaporate and/or are torn apart by tidal forces completely before they get a chance to return for the &lt;em&gt;warmer climate spectacle&lt;/em&gt; once more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are many more &lt;em&gt;unusual comets&lt;/em&gt; that are in so chaotic orbits that they defy single, non-ambiguous classification. That's not the case with Pluto. Well, at least its orbit.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-15T16:52:01.513" LastActivityDate="2015-10-15T16:52:01.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12332" PostTypeId="2" ParentId="12329" CreationDate="2015-10-15T13:34:50.877" Score="0" Body="&lt;p&gt;The major planets all orbit in nearly circular orbits, and near the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic&quot; rel=&quot;nofollow noreferrer&quot;&gt;ecliptic&lt;/a&gt; plane. &#xA;Pluto's orbit is more elliptical (oval), and it makes a larger angle to the ecliptic plane.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/IavIQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IavIQ.jpg&quot; alt=&quot;Planet orbits&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comets also often come in at a large angle to the ecliptic plane, and many comets have an elliptical orbit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-15T13:34:50.877" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12335" PostTypeId="1" CreationDate="2015-10-15T18:18:28.270" Score="36" ViewCount="4907" Body="&lt;p&gt;Looking at the &lt;a href=&quot;http://www.ustream.tv/channel/iss-hdev-payload&quot;&gt;HDEV experiment&lt;/a&gt;, one can see that the orientation of the ISS is always the same with respect to the Earth. This means that the ISS must rotate exactly once per orbit (like the Moon tidally locked in orbit around the Earth); otherwise one would not see always the 'same' surface area of the Earth filling the video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this the case? Because of communication purposes? And how is this locked rotation being achieved?&lt;/p&gt;&#xA;" OwnerUserId="11919" LastEditorUserId="49" LastEditDate="2015-10-15T19:14:36.470" LastActivityDate="2015-10-16T12:21:50.027" Title="Why does the ISS rotate exactly once per orbit?" Tags="|iss|low-earth-orbit|attitude|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12336" PostTypeId="1" AcceptedAnswerId="12340" CreationDate="2015-10-15T18:40:21.223" Score="12" ViewCount="1557" Body="&lt;p&gt;The flight control software for the Space Shuttle was &lt;a href=&quot;https://space.stackexchange.com/questions/9260/how-often-if-ever-was-software-updated-in-the-shuttle-orbiter&quot;&gt;developed to the most rigorous standards&lt;/a&gt; that exist today. They went to enormous lengths to prevent bugs from ever threatening the mission and/or the crew. The towering cost of the software makes it seem unlikely to me that this process is used routinely for scientific space missions.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For scientific satellites and deep space probes, you have a similar problem (although without the lives-at-stake element): you have one chance at making your observations (e.g. planet flyby missions), there are billions of dollars in hardware at stake (flagship missions) and no chance to repair or recover.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet the impression I get is that scientific spacecraft are developed mostly by the science team, i.e. scientists, not e.g. specialists in the development of high-availability software.&lt;br&gt;&#xA;Is that impression correct?&lt;br&gt;&#xA;And what methods are used to reduce the risk of software failure on science missions? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I'm asking specifically about scientific space missions: commercial missions often take advantage of series production so the cost can be spread, and the amount of new functionality on the average commercial mission is small. These missions can also be insured. Scientific space missions tend to be uninsured one-offs. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2020-08-19T13:19:25.717" Title="How is the software for scientific space missions developed?" Tags="|artificial-satellite|probe|software|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12338" PostTypeId="2" ParentId="12335" CreationDate="2015-10-15T18:58:59.683" Score="35" Body="&lt;p&gt;ISS orbits most of its time in what is called a Torque Equilibrium Attitude (TEA). Since gravitational acceleration varies depending on distance from the Earth, non-symmetric objects of any appreciable size have a net torque acting on them that is attitude-dependent, relative to the local vertical/local horizontal. For the most part, the ISS's attitude is controlled through the use of Control-Moment Gyroscopes (CMG), with chemical thrusters used as backup when attitude demands exceed CMG authority or to desaturate CMGs. Flying in TEA minimizes the loads on the CMGs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maintaining a constant attitude relative to Local Vertical Local Horizontal (LVLH) also has the benefit of making thermal and debris constraints more easily met as well, as these environments are relatively constant with respect to the LVLH frame.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="11986" LastEditDate="2015-10-16T12:21:50.027" LastActivityDate="2015-10-16T12:21:50.027" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12340" PostTypeId="2" ParentId="12336" CreationDate="2015-10-15T21:54:09.877" Score="21" Body="&lt;p&gt;No, scientific spacecraft are not developed by scientists on the science team.  They may develop their instruments, but not the spacecraft.  The spacecraft are developed by professional spacecraft engineers in the government or industry.&lt;/p&gt;&#xA;&lt;p&gt;The software engineers are trained in software engineering, and in particular in high-reliability, real-time software.  There are a set of rules and verification approaches used to develop high-reliability software, and there is extensive software testing both in software testbeds and on the spacecraft hardware.&lt;/p&gt;&#xA;&lt;p&gt;You can read the &lt;a href=&quot;http://madler.net/madler/JPL_Coding_Standard_C.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL C coding standard&lt;/a&gt; for an example of such a set of rules.  These rules were produced by JPL's Laboratory for Reliable Software, established specifically to improve the reliability of the critical software on one-off scientific missions.  It was run by &lt;a href=&quot;https://en.wikipedia.org/wiki/Gerard_J._Holzmann&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gerard Holzmann&lt;/a&gt;, an expert on the automatic verification of software correctness.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2020-08-09T01:16:25.717" LastActivityDate="2020-08-09T01:16:25.717" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="12342" PostTypeId="2" ParentId="12336" CreationDate="2015-10-16T01:10:07.453" Score="5" Body="&lt;p&gt;I do want to add that while the spacecraft software is not developed by scientists, it's not uncommon for the scientists to develop (At least in part) post-processing software for the spacecraft that runs on the ground. This software, while important, doesn't require the same level of coding standards as ground software. Still, software engineers will write the more difficult parts of the code, essentially leaving scientists to piece together existing tools to post process the images exactly as is desired.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-16T01:10:07.453" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12344" PostTypeId="1" AcceptedAnswerId="12345" CreationDate="2015-10-16T04:11:16.470" Score="4" ViewCount="444" Body="&lt;p&gt;If there is a stone-size moonlet orbiting around Mars, can we detect it?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="49" LastEditDate="2015-10-16T16:07:48.173" LastActivityDate="2015-10-16T16:07:48.173" Title="Can we detect if Mars has small, stone-size moonlets?" Tags="|mars|orbit|moons|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12345" PostTypeId="2" ParentId="12344" CreationDate="2015-10-16T04:57:18.903" Score="11" Body="&lt;p&gt;Depends on what you mean by a small stone and if it has to be in a long-term stable orbit around Mars. According to &lt;a href=&quot;https://space.stackexchange.com/users/265/mark-adler&quot;&gt;Mark Adler&lt;/a&gt; et al. in the &lt;a href=&quot;http://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4337.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Use Of MRO Optical Navigation Camera To Prepare For Mars Sample Return&lt;/a&gt;, JPL 2012 (PDF), the Mars Reconnaissance Orbiter's Optical Navigation Camera (ONC) could be used to detect small Mars' satellites:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Unknown small satellites may exist in the inner region of the Martian &#xA;  system. The current-best searches (Viking, Hubble) have claimed limits&#xA;  of roughly 50 to 100 meters with no detections. In the region out to&#xA;  Phobos' orbit, the ONC could detect objects as small as one to a few&#xA;  meters diameter during normal spacecraft nadir-pointed operations&#xA;  (that is, with no special pointing or spacecraft attitude&#xA;  maneuvering), and to roughly tens of meters out to Deimos.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Possible use of ONC to detect postulated dust rings through &lt;a href=&quot;https://en.wikipedia.org/wiki/Forward_scatter&quot; rel=&quot;nofollow noreferrer&quot;&gt;forward-scattered light&lt;/a&gt; is also mentioned. So it depends on their size, orbital altitude and albedo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is however no good reason to believe that really tiny, pebble or a small stone-sized moonlets would be long-term stable in orbit around Mars. Mars is still relatively close to the Sun with its semi-major axis at about 1.52 AU (Earth's is 1 AU), unlike the Earth has only weak remnant crustal magnetism, and small orbiting objects would carry small momentum, so they could be relatively easily perturbed by various mechanisms of the Sun's activity (direct interaction with coronal mass ejections, radiation pressure in combination with &lt;a href=&quot;https://en.wikipedia.org/wiki/Yarkovsky_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yarkovsky effect&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Yarkovsky%E2%80%93O&amp;#39;Keefe%E2%80%93Radzievskii%E2%80%93Paddack_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;YORP effect&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Kozai_mechanism&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lidov–Kozai mechanism&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Poynting–Robertson effect&lt;/a&gt;), lose momentum or exchange inclination for eccentricity, and eventually precess out of orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible that shorter-term Mars moonlets exist, or are in stable resonance with Phobos and Deimos, but that's, in my opinion, a long shot. So, in short, we could detect moonlets in the size/mass range that would carry enough momentum to be in long-term stable orbit around Mars, if we systematically searched for them. And smaller than perhaps boulder-sized moonlets most likely don't exist. Last event that had the potential for capture of such small moonlets was &lt;a href=&quot;https://en.wikipedia.org/wiki/C/2013_A1&quot; rel=&quot;nofollow noreferrer&quot;&gt;C/2013 A1 Siding Spring&lt;/a&gt; Mars flyby on 19 October 2014. Its own trajectory and speed would be prohibiting for any gravitational capture of small debris into orbit around Mars, but secondary debris, such as products of collisions with Phobos or Deimos aren't excluded for certain.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-10-16T04:57:18.903" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12347" PostTypeId="1" CreationDate="2015-10-16T09:25:58.363" Score="12" ViewCount="2259" Body="&lt;p&gt;When people visited the Moon, they left sizeable items behind. I know that taking picture of this from Earth is incredibly hard, but: Did someone try? And what pictures do we have?&lt;/p&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="716" LastEditDate="2015-10-16T14:53:16.443" LastActivityDate="2019-09-03T11:27:30.130" Title="Picture of equipment left on the Moon?" Tags="|the-moon|apollo-program|telescope|photography|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12348" PostTypeId="2" ParentId="12347" CreationDate="2015-10-16T11:41:21.013" Score="15" Body="&lt;p&gt;From Earth, you can't. This is actually quite a common question, and was answered by &lt;a href=&quot;http://www.telescopes.com/blogs/helpful-information/18965572-can-you-see-objects-left-behind-on-the-moon&quot;&gt;Telescopes.com&lt;/a&gt;. To see the Lunar Rover, for instance, would require a 75m telescope, far larger than anything ever built! The shadow of the Lunar Decent Stage is a bit larger, but even that is quite difficult to achieve, requiring the perfect timing, although even then, it would barely be detectable, and not as definitive proof.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two additional things that do have some proof of the Lunar Landings having happened. The first is that the &lt;a href=&quot;http://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/apollosites.html&quot;&gt;Lunar Reconnaissance Orbiter&lt;/a&gt; took pictures of the lunar lander sites from orbit, which you can see at the above link. The second is that 3 teams left a retro-reflector, known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment&quot;&gt;Lunar Laser Ranging Experiment&lt;/a&gt; on the surface of the Moon. These can be detected by special laser/ telescope pairs, and are quite distinct from areas without the retro-reflectors on the Moon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="415" LastEditDate="2015-10-16T12:04:36.080" LastActivityDate="2015-10-16T12:04:36.080" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12350" PostTypeId="1" AcceptedAnswerId="12352" CreationDate="2015-10-16T16:21:18.680" Score="6" ViewCount="1600" Body="&lt;p&gt;In examining the upcoming schedule for viewing the ISS, why does it seem like the AM/PM is repetitive. It will appear in the AM several days in a row, then it will switch to the PM several days in a row. &lt;strong&gt;What is it about the ISS orbit that causes AM/PM repetition?&lt;/strong&gt;  Why wouldn't the schedule alternate more often between AM and PM?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some current examples. Notice the repetition of PM or AM based on the four locations.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/37oca.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/37oca.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11991" LastEditorUserId="11991" LastEditDate="2015-10-16T19:46:22.533" LastActivityDate="2016-09-03T10:39:53.513" Title="Why does the ISS viewing times go through repetitive patterns of being always morning or always evening?" Tags="|iss|artificial-satellite|observation|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12351" PostTypeId="2" ParentId="12350" CreationDate="2015-10-16T16:32:30.487" Score="2" Body="&lt;p&gt;Satellites are easiest to see if the satellite is in light, and the sky is dark. This happens most often in evening/morning. In the middle of the night, the satellite is in darkness, and is difficult to see. In the day, the sky is too bright, and again, you can't see it. The satellite, being overhead, has a shorter time without seeing the sun than those of us on the Earth do, thus, it is the best time of day to see a satellite.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-16T16:32:30.487" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12352" PostTypeId="2" ParentId="12350" CreationDate="2015-10-16T16:37:36.937" Score="19" Body="&lt;p&gt;To be visible from the ground, a satellite (including the ISS) must satisfy several conditions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It must be above the horizon&lt;/li&gt;&#xA;&lt;li&gt;It must be illuminated by the sun&lt;/li&gt;&#xA;&lt;li&gt;The observer's sky must be sufficiently dark to not overwhelm the reflected light from the satellite.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Those three conditions basically mean that with the exception of a few extremely bright Iridium flares, all visual satellite observations must take place in fairly narrow windows before sunrise or after sunset (depending on the satellite's reflectiveness) such that the sun is visible to the satellite but not the observer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason successive passes are clustered as all-morning or all-evening has to do with the inclination of the orbit, its orbital period, and the rate of precession of the orbital plane (which also depends on the inclination).  The ISS orbits very nearly divides a 24-hour period, so the timing of overhead passes does not change very much from day to day.  The orientation of its orbit does rotate (completing a full rotation over a span of about two months or so), however, so what you end up with is several days of morning passes (with occasional interruptions), followed by several days of no visible passes, followed by several days of evening passes, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is further complicated by the changing seasons altering how the terminator between illumination and shadow is laid out across the planet.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2015-10-16T16:37:36.937" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12353" PostTypeId="1" CreationDate="2015-10-16T16:42:35.357" Score="23" ViewCount="2150" Body="&lt;p&gt;The James Webb Space Telescope will deploy (unfold mechanically) while on the way to L2. Couldn't it do so in LEO, where it is potentially serviceable? Starliner CST-100 and Dragon are planned to soon allow affordable crewed missions to LEO, and Soyuz could maybe already do it. At least Orion is designed to allow for an EVA, and it is a $bn 8.8 telescope. Even if JWST is not designed to be upgraded, a moving part gotten stuck might be moved manually during an EVA. The upper stage would then bring it to L2. I do think some other spacecrafts have orbited Earth before having set off to their interplanetary destinations. Why won't JWST deploy in LEO where it is potentially serviceable? (like Viking?).&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2015-10-16T18:23:29.633" LastActivityDate="2018-09-23T06:16:21.630" Title="Why won't JWST deploy in LEO where it is potentially serviceable?" Tags="|low-earth-orbit|lagrangian-points|eva|james-webb-telescope|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12354" PostTypeId="2" ParentId="12353" CreationDate="2015-10-16T16:49:14.723" Score="6" Body="&lt;p&gt;Location, location, location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth is too hot for a telescope that needs such low temperatures to operate.  They are building a 5 layered sunshield to protect the JWST from the heat of the Sun. By 'hovering' in Sun-Earth L2, a million miles from Earth, they can avoid essentially all of the radiated heat from Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal is to keep it as cold as possible passively to minimize the active cooling needed, to allow it to work longer. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="265" LastEditDate="2015-10-16T16:57:59.880" LastActivityDate="2015-10-16T16:57:59.880" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12358" PostTypeId="2" ParentId="12353" CreationDate="2015-10-16T18:47:18.440" Score="18" Body="&lt;p&gt;The James Webb Space Telescope will not be deployed in Low Earth Orbit because there is too great a risk of the optics being damaged by debris.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[T]he environment around the ISS is not suitable for the exposed optics that JWST has and would have had the possibility to damage or contaminate the optics. The deployment of JWST happens far above Low Earth Orbit and the debris that resides there.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://jwst.nasa.gov/faqLite.html&quot;&gt;The James Webb Space Telescope Frequently Asked Questions (FAQ) LITE&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-10-16T18:47:18.440" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12359" PostTypeId="1" AcceptedAnswerId="12363" CreationDate="2015-10-16T19:18:55.163" Score="5" ViewCount="757" Body="&lt;p&gt;It is relatively expensive to get things from the Earth to the Moon.  We know there are three Lunar Roving Vehicles (LRV) on the Moon and that there is &lt;a href=&quot;https://space.stackexchange.com/questions/2095/can-i-borrow-a-lunar-rover&quot;&gt;some chance they could be made operational&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there significant amounts of food, fuel, water or other resources that make considering things other than the LRV's an object that might be a viable/valuable resource?   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of all the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_artificial_objects_on_the_Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;man made objects on the Moon&lt;/a&gt; what would be most likely to add value to a modern manned Lunar mission? &amp;nbsp;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-10-17T05:52:35.583" Title="What is the most valuable (potentially useful) man made object on the Moon?" Tags="|the-moon|crewed-spaceflight|apollo-program|colonization|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12360" PostTypeId="2" ParentId="12353" CreationDate="2015-10-16T19:28:13.257" Score="28" Body="&lt;p&gt;JWST is being launched on an Ariane V with a cryogenic upper stage.  That upper stage has to be used immediately to launch it on a trajectory to the Sun-Earth L2.  The stage operates on batteries, and the cryogenic fuel is boiling off.  So there would be no time to do anything even if you deployed the telescope before departure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, the deployed mirror and sunshade would have to be designed to take the loads from the thrust of the upper stage, which would increase their mass and complexity.  Pretty much needlessly increasing the mass for the entire lifetime of the telescope for a 20-minute event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only way to do this would be to have an electric propulsion upper stage.  Then you could have it in a low-Earth orbit for a short time, short enough to mitigate the debris, and still accessible to Orion or Dragon.  You could use the electric propulsion to raise it above most of the debris while waiting for servicing (more than 2000 km), and lower it for the servicing mission.  A few dings in the mirrors is better than no telescope at all.  However you'll have the van Allen belts to consider in the waiting orbit.  The electric propulsion would be very low thrust, so the deployed telescope would hardly notice, and it could wait as long as desired before departing.  The telescope would also have to be designed to be serviceable, which it currently isn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be really nice is if it had an electric propulsion system with enough propellant to take to L2, &lt;em&gt;and&lt;/em&gt; bring it back to LEO.  Then you could service it, refill the Xenon tank, and send it back off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, all of that would just increase the cost of an already rather costly telescope, in order to mitigate a risk that can and has been mitigated through design and test.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-10-16T21:55:00.993" LastActivityDate="2015-10-16T21:55:00.993" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="12363" PostTypeId="2" ParentId="12359" CreationDate="2015-10-17T03:23:27.830" Score="7" Body="&lt;p&gt;Well, there is no food, water or fuel left on the Moon. That leaves purely the equipment. There are a few candidates for interesting objects, namely:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Power sources. Of those available, the best would probably be the RTGs, both for a source of heat and of power. These are likely intact, due to their internal heating, they probably don't change that much in temperature from night to day. They come with dangers, however, of leaking radiation. There was one each for the later 5 Apollo missions. There aren't any large solar panels.&lt;/li&gt;&#xA;&lt;li&gt;Fuel tanks- The best of these would be the Apollo missions, they left the descent stage, which should be pretty cool.&lt;/li&gt;&#xA;&lt;li&gt;Lunar Rovers: As has been mentioned, these were in poor shape, but they are still something of interest.&lt;/li&gt;&#xA;&lt;li&gt;Communication devices- Nothing on the Moon can talk to Earth now, I wouldn't count on anything being particularly useful.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Thus, purely on the basis of what's already there, either Apollo 12 with its Surveyor/ RTG/ Descent stage, or perhaps one of the 3 later Apollo missions, with the addition of the Rovers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should add that none of these compare to liquid water in terms of its usefulness, nor with the solar power potential near the South Pole. So Shackleton Crater remains the best place to land on the Moon for its resources, from Earth or otherwise. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="265" LastEditDate="2015-10-17T05:52:35.583" LastActivityDate="2015-10-17T05:52:35.583" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12364" PostTypeId="2" ParentId="12353" CreationDate="2015-10-17T04:51:09.360" Score="3" Body="&lt;p&gt;My guess is that timing played a role. JWST was planned long before Dragon and CST-100 were. At the time of its planning the shuttle was still in service (I think) but people knew the end of the shuttle was coming.&lt;/p&gt;&#xA;" OwnerUserId="12004" LastEditorUserId="6816" LastEditDate="2015-10-17T09:42:54.753" LastActivityDate="2015-10-17T09:42:54.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12365" PostTypeId="1" AcceptedAnswerId="12367" CreationDate="2015-10-17T06:40:17.833" Score="9" ViewCount="1016" Body="&lt;p&gt;I've read an &lt;a href=&quot;http://sputniknews.com/politics/20151009/1028274915/eu-partially-lifts-anti-russian-sanctions.html&quot;&gt;article&lt;/a&gt; about EU lifting sanctions against Russia related to rocket fuel components. As far as I understand (correct me, if I'm wrong), Russia can now continue to export rocket fuel components to the EU.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another &lt;a href=&quot;http://ria.ru/economy/20151009/1299421965.html&quot;&gt;article&lt;/a&gt; in Russian says that one of the affected components is &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrazine&quot;&gt;hydrazine&lt;/a&gt; (and its derivatives), used in the Russian Proton and French Ariane rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since that fuel is imported from Russia, I conclude that its production is non-trivial (i. e. the EU won't or can't produce it herself). Why? What's so hard about creating hydrazine?&lt;/p&gt;&#xA;" OwnerDisplayName="user3049" LastActivityDate="2016-05-21T10:09:20.523" Title="What's so hard about creating hydrazine?" Tags="|fuel|liquid-fuel|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12367" PostTypeId="2" ParentId="12365" CreationDate="2015-10-17T09:30:18.310" Score="11" Body="&lt;p&gt;@LocalFluff's comment that production scale may have something to do with it has merit. Hydrazine propellant use in Europe is around 20 tonnes/year at the moment, which may be too little to justify investment in production facilities. But good luck finding direct evidence of that. So I looked for other factors that might play a role. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;World production of hydrazine is around 120,000 tonnes per year. A large fraction of that is produced in &lt;a href=&quot;http://www.transparencymarketresearch.com/hydrazine-market.html&quot;&gt;Europe&lt;/a&gt;. Now most of this is hydrazine hydrate, converting that to the anhydrous hydrazine you need for propellant is reasonably complex.&lt;br&gt;&#xA;I found one European supplier of rocket-grade hydrazines, &lt;a href=&quot;http://www.hydrazine.com/&quot;&gt;Arch Lonza&lt;/a&gt; in Switzerland. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EU Regulation (EC) No 1907/2006 - REACH (which regulates health and safety for chemical processes) &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Engineering_Technology/Clean_Space/Space_missions_taking_account_of_fast-changing_chemical_world/%28print%29&quot;&gt;threatens to make working with hydrazine illegal in the EU&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2012, the &lt;a href=&quot;http://eurospace.org/Data/Sites/1/pdf/positionpapers/hydrazinereachpositionpaper_final_14june2012.pdf&quot;&gt;European space industry produced a position paper&lt;/a&gt; arguing for an exemption to REACH for hydrazine used as a propellant. &#xA;The paper also states that annual use of hydrazine propellants amounts to less than 20 tonnes/year. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this &lt;a href=&quot;http://www.reachlaw.fi/images/uploads/files/REACHLaw_Eurospace_Lionnet_29_September_2014.pdf&quot;&gt;presentation&lt;/a&gt;:   &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Space companies use chemicals in small to minute volumes  &lt;/li&gt;&#xA;&lt;li&gt;REACH authorisation process applies regardless of volume  &lt;/li&gt;&#xA;&lt;li&gt;Space industry is a minor market for many suppliers  &lt;/li&gt;&#xA;&lt;li&gt;Limited control of supply chain  &lt;/li&gt;&#xA;&lt;li&gt;Have to rely on registration by upstream manufacturer/importer and authorisation for upstream uses (e.g. formulation)&lt;/li&gt;&#xA;&lt;li&gt;Risk of sudden obsolescence of Space-qualified materials and processes due to REACH-induced alterations of materials and processes upstream  &lt;/li&gt;&#xA;&lt;li&gt;Re-qualification and industrialisation of alternative materials and processes is a long- term and cost intensive undertaking – need to decide as early as possible on the items to include in planning&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-17T11:02:05.623" LastActivityDate="2015-10-17T11:02:05.623" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12368" PostTypeId="2" ParentId="11975" CreationDate="2015-10-17T14:48:35.053" Score="7" Body="&lt;p&gt;Hicks' &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a505342.pdf&quot; rel=&quot;noreferrer&quot;&gt;text&lt;/a&gt; on reentry provides a really good look at the derivations of the reentry problem, and touches on the impacts of top-level design choices. For example, although dependent on the actual entry profile, total heat transfer often has the following form:&#xA;\begin{equation}&#xA;\bar{Q}_{f} \approx \left(\frac{c_{f}A}{C_{D}S}\right)f(V,\vec{p})&#xA;\end{equation}&#xA;where $c_{f}$ is the surface/skin friction, $A$ is the total surface area of the vehicle, $C_{D}$ is the drag coefficient, $S$ is the wetted area, and $f(V,\vec{p})$ indicates a nonlinear function of velocity and other vehicle and environmental parameters $\vec{p}$. If examining heat flux in a vacuum, one would see that heat transfer is minimized for smaller values of surface friction and wetted area, or increase the drag quantity $C_{D}S$. This is the equivalent of saying a blunt vehicle minimizes the total heat absorbed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there's a catch. To demonstrate, consider tne form of the maximum deceleration in terms of kinetic energy $T$ for a shallow gliding entry profile:&#xA;\begin{equation}&#xA;\frac{a_{decel}}{g_{0}} = \frac{1-2T}{\frac{C_{L}}{C_{D}}}&#xA;\end{equation}&#xA;where $a_{decel}$ is the deceleration, $g_{0}$ is a reference gravitational value used to normalize, and $C_{L}$ is a lift coefficient. We saw above that a higher drag quantity minimizes heat flux, but now we see that higher drag coefficients increase the deceleration, which increases the loading on the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cone angle and the bluntness / sharpness of the nose are all design choices that attempt to optimize some design space for a particular mission profile while also meeting a given cost, schedule, and performance. And based on these two simple relations, we already see that it's a very complicated process. Hicks' text reference above is a great introduction to the subject, and most intro to orbital dynamics texts will give the topic some attention as well. &lt;/p&gt;&#xA;" OwnerUserId="1478" LastActivityDate="2015-10-17T14:48:35.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12371" PostTypeId="1" AcceptedAnswerId="12390" CreationDate="2015-10-18T18:45:59.103" Score="2" ViewCount="1220" Body="&lt;p&gt;I &lt;em&gt;believe&lt;/em&gt; Dragon 2 has the same number of &lt;a href=&quot;https://en.wikipedia.org/wiki/Draco_%28rocket_engine_family%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;400N Draco thrusters&lt;/a&gt; that Dragon 1 has, &lt;a href=&quot;http://www.spacex.com/news/2013/04/04/draco-thrusters&quot; rel=&quot;nofollow noreferrer&quot;&gt;which is 18&lt;/a&gt;, but I have not seen any confirmation of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon 1 appears to have them arranged in 2 sets of 4, and 2 sets of 5, as seen in this photo:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Bjejr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Bjejr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet Dragon 2's layout is a bit different, if everything is symmetrical, it appears to have 4 sets of 3, and the 4 lone thrusters on the inside of the SuperDraco mold lines:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XCilm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XCilm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bfqse.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bfqse.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This produces a sum total of &lt;strong&gt;16&lt;/strong&gt; Dracos. &lt;strong&gt;So, what's right? Additionally, if there is a difference, why?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2015-11-20T10:55:47.570" Title="How many Draco engines does Dragon 2/Crew Dragon possess?" Tags="|spacex|dragon|draco|engines|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12372" PostTypeId="1" AcceptedAnswerId="12373" CreationDate="2015-10-18T19:42:37.290" Score="6" ViewCount="3438" Body="&lt;p&gt;I was just looking at a small but impressive &lt;a href=&quot;https://www.youtube.com/watch?v=4r9gmLfpFTg&quot; rel=&quot;noreferrer&quot;&gt;compressed air/water rocket&lt;/a&gt; that launches off of a metal rod that runs through the nozzle and spans 3/4 of the length of the rocket. As the team increased pressure for subsequent launches, I wondered about a great or terrible idea to really make something like it move.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose the rod was an anode, and the water tank was a cathode. Could an extremely high voltage be discharged through the water to quickly split it into hydrogen and oxygen, which would then ignite from compressive heating? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would image that the simplest design, a full tank of water with a large durable anode rod would simply explode, but there are some things we could do to increase the odds of survival for the rocket:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Use a long, narrow body so that there is lots of surface area on the electrodes, compared to the volume of water&lt;/li&gt;&#xA;&lt;li&gt;Ensure that the anode immediately flashes to a plasma, so that it does not obstruct the escaping fuel. For a very short time period, that column of plasma might still function as a conductive anode as the last of the water splits&lt;/li&gt;&#xA;&lt;li&gt;Wrap the tank in the highest ultimate tensile strength &lt;a href=&quot;https://en.wikipedia.org/wiki/Ultimate_tensile_strength&quot; rel=&quot;noreferrer&quot;&gt;material&lt;/a&gt; known to Wikipedia, monolayer graphene (130 GPa!), and expect it to deform during launch&lt;/li&gt;&#xA;&lt;li&gt;Don't fill the tank completely full of water, include a compressible gas like helium to dampen the forces from explosion to a sort-of controlled burn&lt;/li&gt;&#xA;&lt;li&gt;Try to prevent the oxygen and hydrogen from mixing too quickly, so that it burns instead of detonating&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This all hinges on the ability to electrolyse many litres of water in milliseconds. Has this been investigated? Is there any fundamental reason why it couldn't work?&lt;/p&gt;&#xA;" OwnerUserId="10367" LastEditorUserId="49" LastEditDate="2015-10-18T20:42:08.670" LastActivityDate="2022-07-27T13:29:52.457" Title="Splitting water quickly to turn a water rocket into a H/O rocket at launch: could it work?" Tags="|launch|rockets|propulsion|fuel|energy|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12373" PostTypeId="2" ParentId="12372" CreationDate="2015-10-18T20:01:47.380" Score="19" Body="&lt;p&gt;You'll need a nuclear reactor for that because the process of &lt;a href=&quot;https://en.wikipedia.org/wiki/Dissociation_%28chemistry%29&quot; rel=&quot;noreferrer&quot;&gt;dissociation&lt;/a&gt; of water into constituent hydrogen and oxygen consumes vast amounts of energy and for an ascent stage you'll also need a very high energy density to provide sufficient thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Direct anode/cathode system like you describe (i.e. &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrolysis&quot; rel=&quot;noreferrer&quot;&gt;electrolysis&lt;/a&gt;) is wasteful when you don't require separation of produced hydrogen and oxygen for later storage, and all you want is great expansion that steam provides, so it's much simpler to just superheat reaction mass to its dissociation temperature and then expel that at great pressure. Because electrolysis of water is so energy consumptive, most industrial use hydrogen is actually produced through &lt;a href=&quot;http://energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming&quot; rel=&quot;noreferrer&quot;&gt;natural gas reformation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, basically, a better solution to what you're asking about is called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;noreferrer&quot;&gt;Nuclear Thermal Rocket&lt;/a&gt;, and while working directly with hydrogen is preferable (simply because of $E_\text{k} =\tfrac{1}{2} mv^2 $ &lt;a href=&quot;https://space.stackexchange.com/q/5071/49&quot;&gt;small molar mass exhaust products are preferred&lt;/a&gt; where there's linear relationship between their mass and their stored or absorbed energy density that can be later converted into kinetic energy of exhaust products), huge exhaust temperature that it produces often calls for mixing of reaction mass with water to reduce it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat dissociated NTR exhaust products also provide roughly twice the &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;noreferrer&quot;&gt;specific impulse&lt;/a&gt; ($I_\text{sp} \approx 800-900 \text{ s}$, depending on exhaust temperature but it &lt;a href=&quot;http://dash.harvard.edu/handle/1/9569212&quot; rel=&quot;noreferrer&quot;&gt;could go much higher&lt;/a&gt;, say, with &lt;a href=&quot;https://en.wikipedia.org/wiki/Metallic_hydrogen&quot; rel=&quot;noreferrer&quot;&gt;metallic hydrogen&lt;/a&gt;, if you can somehow produce it) of recombining oxygen and hydrogen into water steam in LOX/LH2 cryogenic chemical rockets ($I_\text{sp} \approx 450 \text{ s}$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In orbit, it would be possible to do away with a nuclear reactor and use &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_thermal_rocket&quot; rel=&quot;noreferrer&quot;&gt;solar thermal propulsion&lt;/a&gt;, but you'd probably want to make that a &lt;a href=&quot;https://en.wikipedia.org/wiki/Beam-powered_propulsion&quot; rel=&quot;noreferrer&quot;&gt;beam-powered propulsion&lt;/a&gt; system and separate your source of energy from your rocket to reduce its mass.&amp;nbsp;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-10-19T12:09:51.153" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12374" PostTypeId="2" ParentId="12372" CreationDate="2015-10-19T06:46:32.090" Score="6" Body="&lt;p&gt;A key physics limit is the conservation of energy.  If you split water into hydrogen and oxygen, then let them &quot;burn&quot; to combine back into water, you cannot possibly gain energy from the process.  The only energy you add to the system is that which actually turns into waste heat.  Accordingly, there is nothing you could do with electrolysis here that you could not do with a simple heating element to boil the water nearby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, if you had a free multi-megawatt electrical power supply on board that cost you no weight and all of its fuel is weight-free also, you might be able to get quite a lot of power by high pressure steam with a giant heating element.&lt;/p&gt;&#xA;" OwnerUserId="3532" LastActivityDate="2015-10-19T06:46:32.090" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12375" PostTypeId="1" AcceptedAnswerId="12377" CreationDate="2015-10-19T08:56:42.020" Score="3" ViewCount="312" Body="&lt;p&gt;Recently the Kepler Mission detected a anomaly in a star system (&lt;a href=&quot;https://en.wikipedia.org/wiki/KIC_8462852&quot; rel=&quot;nofollow&quot;&gt;KIC 8462852&lt;/a&gt;) that people are speculating could be a sign of intelligent life (aliens) being cause for anomaly. What was exactly going on there?&lt;/p&gt;&#xA;" OwnerUserId="12019" LastEditorUserId="195" LastEditDate="2015-10-19T20:20:49.153" LastActivityDate="2015-10-20T13:03:37.870" Title="Recent anomaly in star system detected by Kepler mission" Tags="|astronomy|kepler-telescope|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12376" PostTypeId="2" ParentId="12375" CreationDate="2015-10-19T09:17:11.443" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://arxiv.org/abs/1510.04606&quot; rel=&quot;nofollow noreferrer&quot;&gt;This table&lt;/a&gt; gives an overview of possible explanations of the different aspects of anomalies that an artificial superstructure might cause:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pF15s.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pF15s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-10-19T09:17:11.443" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12377" PostTypeId="2" ParentId="12375" CreationDate="2015-10-19T12:29:38.733" Score="4" Body="&lt;p&gt;Kepler detected a light signature on the star (KIC 8462852) that was very unusual. The light from the star appears to be blocked by something, which the amount varies, by up to 20% of the light being blocked. The frequency of the blockage is irregular. This means that there must be a very large amount of small objects inside of that system. There are basically 3 ideas that so far check out:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The debris could be a result of two large planets colliding with each other recently (think the event that created the moon). The main problem with this is the event would have had to happen within a very small window of opportunity.&lt;/li&gt;&#xA;&lt;li&gt;The debris could be a large number of comets, coming from a close encounter of a star recently. In fact, there is currently a close star, which may or may not be a binary. Either case, this could well explain the effect that has been observed. This is the most likely cause, due to the low infrared signature of whatever is causing this.&lt;/li&gt;&#xA;&lt;li&gt;There could be an artificially made cloud of materials there, perhaps a &lt;a href=&quot;https://en.wikipedia.org/wiki/Dyson_sphere#Dyson_swarm&quot; rel=&quot;nofollow&quot;&gt;Dyson Swarm&lt;/a&gt;, bubble, or similar item. This is actually unlikely, as the main reason this star is suspect is a lack of infrared, meaning that whatever is obscuring the star must be cold. In fact, that is the most difficult aspect to explain.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The first two point to something incredible rare happening recently, at a time where we could see it. It's still a possibility, and in fact the most likely item, but the third possibility has captured the public's imagination. It's likely enough that they are going to do a SETI search on the star, but that doesn't mean anything, yet.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="1142" LastEditDate="2015-10-20T13:03:37.870" LastActivityDate="2015-10-20T13:03:37.870" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12378" PostTypeId="1" CreationDate="2015-10-19T12:50:54.653" Score="12" ViewCount="618" Body="&lt;p&gt;I took the &lt;a href=&quot;http://esamultimedia.esa.int/multimedia/virtual-tour-iss/&quot; rel=&quot;noreferrer&quot;&gt;virtual tour of the ISS&lt;/a&gt; - and my first impression was &quot;What a mess!&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/kWBRJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, I understand any elements should be easy to move; any &quot;decorative&quot; elements like locker doors add unnecessary weight to the orbit, without gravity there's no need to attach anything in a really sturdy way...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but as one moves through the station, without means to perform a rapid, unplanned stop, it seems to me it's all too easy to get tangled on all the hundreds of loose wires forming a thick layer of canopy over the walls, and rip something out of its socket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a particular reason the wires aren't trimmed to needed distance, bundled snugly along the walls, or drawn through dedicated ducts, but instead are left coiling all over the place, hard to troubleshoot, easy to damage, and creating visual chaos that is hardly conductive to productive work?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="758" LastEditDate="2015-10-21T21:04:13.333" LastActivityDate="2015-10-21T21:04:13.333" Title="Why all the loose wires in the ISS?" Tags="|iss|" AnswerCount="0" CommentCount="7" ClosedDate="2015-10-20T17:03:38.000" ContentLicense="CC BY-SA 3.0" />
  <row Id="12379" PostTypeId="2" ParentId="12372" CreationDate="2015-10-19T15:00:24.850" Score="7" Body="&lt;p&gt;First we'll see how much energy you would need for the electrolysis:&lt;br&gt;&#xA;Water has a molecular weight of 18.01528 g/mol, so 1 kg of water is 55.55 mol. You need 286 kJ/mol of energy to split water. 55x286=15.8 MJ for 1 kg of water.&lt;br&gt;&#xA;15.8 MJ = 15.8 MW.s = 4.4 kWh of energy. If you wanted to split the water in 1 second, you need 15.8 MW of electrical power.&lt;br&gt;&#xA;Assuming a 200 $\Omega$ resistance between cathode and anode, that translates to 56 kV at 280 A. Normal electrolysis cells operate at a little over 1.5 V. &#xA;At those voltages, you are likely to get arcing between the anode and cathode.&lt;br&gt;&#xA;Because you haven't separated the oxygen and hydrogen, any gas bubbles in contact with the arc will explode.&lt;br&gt;&#xA;Also, the anode and cathode will heat up (in fact, you'd need big, heavy electrodes to keep them from evaporating at this much current), and part of your water will evaporate. This inhibits the electrolysis: water has to be in contact with the electrodes for electrolysis to happen.&lt;br&gt;&#xA;So it's a race between the electrolysis process on one hand and the explosions plus steam formation on the other.&lt;br&gt;&#xA;Finally, &lt;a href=&quot;http://www.powerlabs.org/waterarc.htm&quot; rel=&quot;noreferrer&quot;&gt;something strange happens&lt;/a&gt; when you put a large current through water:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, once the maximum voltage standoff for water is exceeded and the dielectric effect breaks down, something strange happens: The discharge stops being electrolytic (as molecules can no longer move to the electrodes at the rate dictated by the current) and the resistance plummets all at once, allowing massive currents to pass through it. When that occurs a bright flash of light is observed and some of the water in the sample is atomized (atomization here is used to describe a change from the liquid to the gaseous state that does not involve heating, such as in ultrasonic water atomisers) following a very loud report and a powerful shock wave traveling through the liquid.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-20T11:34:30.510" LastActivityDate="2015-10-20T11:34:30.510" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12381" PostTypeId="1" AcceptedAnswerId="12383" CreationDate="2015-10-19T17:27:21.570" Score="14" ViewCount="5395" Body="&lt;p&gt;We all know that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot;&gt;Space Shuttle Solid Rocket Boosters&lt;/a&gt; (SRB) were the pair of large solid rockets used by the United States' NASA Space Shuttle during the first two minutes of its powered flight. Together, they provided about 83% of liftoff thrust for the Space Shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When they are jettisoned, it seems they are still giving off some thrust. In the first seconds after this, would it have been possible for these to turn into the direction of the Space Shuttle and collide with it?&lt;/p&gt;&#xA;" OwnerUserId="10560" LastEditorUserId="49" LastEditDate="2015-10-19T19:09:28.857" LastActivityDate="2015-10-21T05:41:18.913" Title="Is it possible for the Space Shuttle Solid Rocket Boosters (SRB) to hit the Space Shuttle after jettison?" Tags="|rockets|space-shuttle|stages|srb|separation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12382" PostTypeId="2" ParentId="12381" CreationDate="2015-10-19T17:34:07.697" Score="19" Body="&lt;p&gt;The SRBs have smaller solid &lt;a href=&quot;https://en.wikipedia.org/wiki/Booster_separation_motor&quot; rel=&quot;noreferrer&quot;&gt;separation motors&lt;/a&gt;, 8 on each, which simultaneously fire to push the boosters safely away from the orbiter/ET. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's &lt;a href=&quot;https://www.youtube.com/watch?v=AyBfnzX3Q6g&quot; rel=&quot;noreferrer&quot;&gt;a video showing them in action&lt;/a&gt; -- they're powerful enough to give the external tank a good scorching. You can see the separation is designed to turn the boosters slightly outward so any remaining SRB thrust takes them further away from the stack, and what little air there is at 45km altitude also helps to drive them aside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9QKn5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9QKn5.jpg&quot; alt=&quot;Diagram of SRB separation path&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conceivably, if the forward separation motors failed while the aft ones fired, the booster could be driven inward instead of outward. This is a very unlikely scenario.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-10-21T05:41:18.913" LastActivityDate="2015-10-21T05:41:18.913" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12383" PostTypeId="2" ParentId="12381" CreationDate="2015-10-19T17:53:35.580" Score="17" Body="&lt;p&gt;No, &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#Separation&quot;&gt;they don't have sufficient thrust when they're jettisoned&lt;/a&gt; to catch up with Space Shuttle accelerating away:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The SRBs are jettisoned from the space shuttle at high altitude, about&#xA;  146,000 ft (45 km). SRB separation is initiated when the three solid&#xA;  rocket motor chamber pressure transducers are processed in the&#xA;  redundancy management middle value select and the head-end chamber&#xA;  pressure of both SRBs is less than or equal to 50 psi (340 kPa). A&#xA;  backup cue is the time elapsed from booster ignition.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The separation sequence is initiated, commanding the thrust vector&#xA;  control actuators to the null position and putting the main propulsion&#xA;  system into a second-stage configuration (0.8 second from sequence&#xA;  initialization), which ensures the thrust of each SRB is less than&#xA;  100,000 lbf (440 kN). Orbiter yaw attitude is held for four seconds,&#xA;  and SRB thrust drops to less than 60,000 lbf (270 kN).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At the same time, Space Shuttle continues thrusting with its three SSME, each at roughly 418,000 lbf (1,860 kN) for 5,250 kN (1,180,000 lbf) total thrust to the remaining mass of the launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if we assume worst case, SRB dry mass is 82,879 kg, and at maximum separation thrust can achieve acceleration of &lt;a href=&quot;https://encrypted.google.com/#q=270+kN+%2F+82%2C879+kg&quot;&gt;3.26 m/s²&lt;/a&gt; which will rapidly decrease as residual solid propellants burn to exhaustion, while the remaining Space Shuttle Orbiter + External Tank configuration weighs approximately 874,290 kg when full (it wouldn't be even close to that on SRB separation) for a minimum acceleration of &lt;a href=&quot;https://encrypted.google.com/#q=5%2C250+kN+%2F+874%2C290+kg&quot;&gt;6 m/s²&lt;/a&gt;. So even assuming absolute worst case, thrust of Space Shuttle SRBs on jettison isn't enough for them to catch up with the Space Shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used numbers from &lt;a href=&quot;http://www.braeunig.us/space/specs/shuttle.htm&quot;&gt;Braeunig&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-19T18:32:35.040" LastActivityDate="2015-10-19T18:32:35.040" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12384" PostTypeId="1" CreationDate="2015-10-19T18:32:50.437" Score="4" ViewCount="139" Body="&lt;p&gt;I was listening to a Science Friday podcast in which the Martian movie was being discussed. The science contributors stated the radiation problems are the biggest issue for Mars exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reading &lt;a href=&quot;http://physicsworld.com/cws/article/news/2008/nov/06/magnetic-shield-could-protect-spacecraft&quot; rel=&quot;nofollow&quot;&gt;Magnetic shield could protect spacecraft&lt;/a&gt; states that a 1 Tesla magnet would work:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Bamford and colleagues say these results show that a spacecraft could in fact be protected using a bubble just some 100–200 m across. This, they say, would correspond to a magnet of about 1 Tesla, which would be light enough to be transported into space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How much does a 1 Tesla neodymium magnet weigh on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could permanent magnets be installed in the Mars habitat and in the seat of the rover?&lt;/p&gt;&#xA;" OwnerUserId="12022" LastEditorUserId="25" LastEditDate="2015-10-20T14:57:57.007" LastActivityDate="2015-10-20T14:57:57.007" Title="Mars and the radiation problems" Tags="|mars|crewed-spaceflight|radiation|cosmic-radiation|" AnswerCount="0" CommentCount="9" ClosedDate="2015-10-20T17:03:18.177" ContentLicense="CC BY-SA 3.0" />
  <row Id="12387" PostTypeId="2" ParentId="8301" CreationDate="2015-10-19T22:06:29.743" Score="4" Body="&lt;p&gt;It's a rather old question, but let me clarify a bit:&#xA;Rikki-Tikki-Tavi got confused by logic gates versus logic cells - A typical logic cell of a typical FPGA is much more complex than a single logic gate. Some rather rough conversion factor is about 100. Hence, these FPGA are much much more powerfull than needed for this rather purpose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A FPGA that most likely could do the job (despite the space rating and temperature range) can today be bought for few (single digit) Dollars measuring 4x4 mm^2 - and that's just limited by the amount of inputs and outputs and not by the size of the silicon itself.&#xA;Sure, radiation hardness, temperature rating and the ten year old hardware used on the rover pays its toll with respect to size (not mentioning the price rising by many orders of magnitude), but you'll get the point.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2015-10-19T22:06:29.743" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12388" PostTypeId="1" CreationDate="2015-10-20T06:48:12.167" Score="10" ViewCount="362" Body="&lt;p&gt;As part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Aurora_programme#Timeline&quot;&gt;Aurora Exploration Programme&lt;/a&gt;, there is a proposed human mission to the Moon in 2024. The astronaut capsule would be launched atop an Ariane 5 rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the status of this mission? Is it going to take place? Are they on time? What components have been completed so far?&lt;/p&gt;&#xA;" OwnerUserId="761" LastEditorUserId="49" LastEditDate="2015-10-20T14:53:02.783" LastActivityDate="2015-11-06T14:04:06.653" Title="What is the status of ESA's proposed human mission to the Moon?" Tags="|the-moon|reference-request|crewed-spaceflight|esa|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12389" PostTypeId="2" ParentId="8800" CreationDate="2015-10-20T14:27:20.003" Score="2" Body="&lt;p&gt;In addition to the above answers, the oxidizer for the rocket is fed by a pump. This pump is driven by an &lt;a href=&quot;http://www.bloodhoundssc.com/project/car/engines/auxiliary-power-unit&quot; rel=&quot;nofollow&quot;&gt;internal combustion engine&lt;/a&gt;, which needs air to function, so it, like the jet engine, will run out of power at high altitude.&lt;br&gt;&#xA;And the car is designed for a range of 18 km, about half of which is powered (the rest is braking). It will be refueled after every run.&lt;br&gt;&#xA;So you have a vehicle capable of 450 m/s (*) with a powered range of 9 km, while to be in orbit you need 100 km and 7 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*: 450 m/s at ground level, it'd be faster in a vacuum. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-20T14:27:20.003" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12390" PostTypeId="2" ParentId="12371" CreationDate="2015-10-21T10:32:38.723" Score="3" Body="&lt;p&gt;Going through my photos of the Dragon 2s we've seen so far (initial presentations - I've got photos from all angles of that one, and the pad abort test) you're right: the Dragon 2 has 16 Dracos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to a &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/2vvgkz/detailed_images_underneath_cots_2_dragon/?&quot; rel=&quot;nofollow&quot;&gt;thread on Reddit&lt;/a&gt;, Dragon 1 had two extra thrusters pointing 'up' because the 'up' thrusters are used for the reentry burn and SpaceX wanted more redundancy for those.&lt;br&gt;&#xA;Presumably on Dragon 2, with 8 SuperDracos, redundancy for the reentry burn is taken care of.&lt;br&gt;&#xA;All engines in one thruster group &lt;a href=&quot;http://innerspace.net/spacex/failure-is-still-not-an-option-spacex-restores-dragon-systems/&quot; rel=&quot;nofollow&quot;&gt;are fed from one propellant tank&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-21T10:32:38.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12391" PostTypeId="1" AcceptedAnswerId="12394" CreationDate="2015-10-21T10:56:11.287" Score="27" ViewCount="4920" Body="&lt;p&gt;The International Space Station (ISS) is a joint construction of many countries around the world.  Most of whom use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Metric_system&quot;&gt;metric system&lt;/a&gt; for measuring including sizing of bolts, nuts and fasteners.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US seems to be stuck on imperial units, while the rest of the world has moved to Metric and anyone who has worked on both an American automobile and one from just about any other place in the world, has found out the nuts and bolts are different.  In fact you need to have &lt;a href=&quot;http://www.engineeringtoolbox.com/wrenches-inches-metric-us-conversion-comparison-d_1607.html&quot;&gt;two sets of tools,&lt;/a&gt; one SAE (American) tool set and one Metric tool set.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are nuts and bolts on the ISS, metric, SAE, or both?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="26446" LastEditDate="2020-05-22T21:32:38.557" LastActivityDate="2020-05-22T21:32:38.557" Title="ISS nuts and bolts; Metric, SAE or both?" Tags="|iss|unit-systems|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12392" PostTypeId="1" CreationDate="2015-10-21T11:04:01.570" Score="10" ViewCount="439" Body="&lt;p&gt;The Kepler space telescope, during its first mission, rolled over four times per year in order to keep its heat shield facing the Sun. At what dates (in that BJD time format) was this done during the 4+ year duration of the primary mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I note that the three main periods of anomalies in the light curve of KIC 8462852 occurred with exactly two years intervals. About day 40, day 800 and day 1500+. This leads me to believe that the data is the result of some kind of instrument failure, maybe a degrading CCD element, a bad pixel.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Did the three main anomalies occur while the telescope was oriented&#xA;the same way?&lt;/li&gt;&#xA;&lt;li&gt;Obviously, the telescope had the Sun in the same direction during all&#xA;of the three recorded anomalies. But was there some difference in&#xA;orientation in the years in between every other year?&lt;/li&gt;&#xA;&lt;li&gt;While the telescope was held extraordinarily still, could the&#xA;remaining tiny wobbling be enough to make a dead pixel go in and out&#xA;of the light from a star, in order to cause the dips in the light&#xA;curve of KIC 8462852? Maybe more so in the end as the second reaction wheel was failing, explaining the greater anomaly after day 1500+.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Please note that Kepler's orbital period is slightly shorter than an Earth year. It is of course Kepler's orbital period which is of interest here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WhPlQ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WhPlQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source of illustration:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://arxiv.org/pdf/1509.03622v1.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://arxiv.org/pdf/1509.03622v1.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-10-22T19:55:51.063" LastActivityDate="2015-11-04T17:26:07.913" Title="When did Kepler roll over and how does it match the anomalies of KIC 8462852?" Tags="|planetary-science|attitude|failure|scientific-data|kepler-telescope|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12393" PostTypeId="2" ParentId="3663" CreationDate="2015-10-21T11:16:13.147" Score="1" Body="&lt;p&gt;a.i. solutions (www.ai-solutions.com) has multiple tools that handle space mission design. Their high fidelity FreeFlyer is directly comparable to STK/ODTK in every way and is used for many NASA and USAF missions.&#xA;For Deep Space trajectory design like described above they also have a complementary visual tool called the Deep Space Trajectory Explorer which is being built to be very simple to use. It handles Lagrangian Halo, P2 centered Ballistic and Asteroid centered trajectories.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quasi-periodic trajectory search around Europa&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/UfxgCNVTCYY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&#xA;Vail 2015 AAS presentation&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/BE1Urj9uavk?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&#xA;Mixing Earth-Moon L1 Halo and P2 centered Ballistics&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/MotQ1PC1xT8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="11888" LastActivityDate="2015-10-21T11:16:13.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12394" PostTypeId="2" ParentId="12391" CreationDate="2015-10-21T12:06:00.363" Score="27" Body="&lt;p&gt;Both, apparently, at least the ones the crew can access. Their toolbox features both &lt;a href=&quot;http://toolguyd.com/iss-toolbox-tools/&quot; rel=&quot;noreferrer&quot;&gt;systems&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eaXkZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eaXkZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-05-09T23:16:18.893" LastActivityDate="2018-05-09T23:16:18.893" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="12395" PostTypeId="2" ParentId="12392" CreationDate="2015-10-21T12:22:55.517" Score="7" Body="&lt;p&gt;Kepler rolled between the quarter dates found at &lt;a href=&quot;http://keplerscience.arc.nasa.gov/ArchiveSchedule.shtml&quot;&gt;this site.&lt;/a&gt; At first glance, they don't seem to correlate, although I need to do more work to actually line it up. As Kepler rolls the data between data releases, it's possible to look at &lt;a href=&quot;https://archive.stsci.edu/kepler/data_search/search.php&quot;&gt;whole datasets&lt;/a&gt;, and see if they occur at a common point in the cycle. I can tell you the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A dip occurred at the start of KPLR008462852-2009259160929, ending at    KPLR008462852-2009350155506&lt;/li&gt;&#xA;&lt;li&gt;No other dips correspond to transitions in quarters, except possibly one at the very last quarter (KPLR008462852-2013131215648)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-21T13:18:29.170" LastActivityDate="2015-10-21T13:18:29.170" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12396" PostTypeId="1" AcceptedAnswerId="12397" CreationDate="2015-10-21T19:43:13.543" Score="5" ViewCount="507" Body="&lt;p&gt;NASASpaceFlight.com reports that the &lt;a href=&quot;http://www.nasaspaceflight.com/2015/10/spacex-dragonfly-arrives-mcgregor-testing/&quot; rel=&quot;nofollow&quot;&gt;DragonFly test vehicle has arrived at McGregor, TX for testing&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They discuss that the FAA is limiting them to 80 feet above ground for initial testing. That they will hang it off a large crane and test the SuperDracos while hanging off a crane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is, the 8 SuperDracos have 16 Klbf thrust (vacuum) EACH!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Dragon V2 vehicle is around 14,000 lbs mass which the article says includes steel legs attached for testing. I am not sure how low you can throttle a SuperDraco, but even with one SuperDraco firing, that thing is going to be going like a bat out of hell. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SuperDracos are embedded in the side and do not thrust directly down, so firing just one is probably not an option if you do not wish to destroy the test vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How are they going to run this test in a way that makes sense? (Since they clearly have a plan...)  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-10-22T11:24:18.700" LastActivityDate="2016-01-23T02:38:58.473" Title="How can SpaceX test SuperDracos whilst hanging off a crane?" Tags="|spacex|testing|dragon-v2|super-draco|mcgregor-tx|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12397" PostTypeId="2" ParentId="12396" CreationDate="2015-10-21T19:53:41.833" Score="5" Body="&lt;p&gt;Like any good descent engine, SuperDraco throttles deeply; &lt;a href=&quot;https://en.wikipedia.org/wiki/SuperDraco#Design&quot; rel=&quot;nofollow&quot;&gt;Wikipedia says down to 20%&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Various speculative sources put the thrust angle anywhere between 25º and 35º, which determines cosine losses. The sidewall angle is 15º, and the thrust angle is greater than that. Split the difference and assume it's 30º for 0.87 cosine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find a solid reference for the difference between sea level and vacuum thrust; Wikipedia says 240s ISP at SL, and &lt;a href=&quot;http://www.murc.ws/showthread.php?72548-SpaceX-SuperDraco-abort-landing-thrusters&quot; rel=&quot;nofollow&quot;&gt;some unsourced reference from 2013&lt;/a&gt; suggested 300s vacuum, which seems plausible (if a bit low), so let's assume sea level thrust is 80% of vac. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;73kN x 8 x 0.2 x 0.87 x 0.8 = 81kN total thrust, which is in the ballpark of 18,000 lbf. If throttling down that far isn't sufficient, they might have to shut down 4 or 6 of the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With only 2 engines running, they'd lose one axis of differential-thrust control authority, although the Draco RCS thrusters can probably take care of that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has the dry mass of Dragon V2 at 9300 lbs, and your linked article at 14000. Propellant adds another 4000 lbs or so, for a total of something like 18000 lbs at the start of the test. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In January of 2016, SpaceX released a &lt;a href=&quot;https://www.youtube.com/watch?v=07Pm8ZY0XJI&quot; rel=&quot;nofollow&quot;&gt;video of a hover on a crane&lt;/a&gt; from November 2015 showing a successful hover. Information released by SpaceX about that test &lt;a href=&quot;http://spaceflightnow.com/2016/01/21/dragon-crew-capsules-propulsive-landing-system-tested/&quot; rel=&quot;nofollow&quot;&gt;claims 147kN total thrust&lt;/a&gt;, implying about 36% throttle and a heavily ballasted capsule.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-01-23T02:38:58.473" LastActivityDate="2016-01-23T02:38:58.473" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12398" PostTypeId="2" ParentId="1212" CreationDate="2015-10-21T21:01:47.937" Score="6" Body="&lt;p&gt;I note that the question asks about lifespan, not limited to orbital lifetime. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ironically, luni-solar orbital perturbations at GEO have till now had a very strong life limiting effect on GEO satellite lifetimes through the typical mission requirement to control the evolution of orbital inclination, though advances in propulsion technology are easing the limitations of propellant lifetimes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, despite this, the basic premise of the question still holds, design lifetimes of commercial geostationary satellites have crept up from 7 to 15 years since the 1970s whilst those of, for example LEO remote sensing missions have evolved from 3 to 10, though I confess to being a little more shaky on the latter evolution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the reasons relate to the high level emergent behaviour of the customers when faced with their own economic cycle of which I can only suggest some starting ideas:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it costs more to get to GEO so the businesses of these satellites could be more sensitive to economies&lt;/li&gt;&#xA;&lt;li&gt;the functions of most GEO satellites for communications is electrical power intensive, thus mass and launch cost enhancing, thus reinforcing the benefit of longer life to maximise the return on capital &lt;/li&gt;&#xA;&lt;li&gt;some features of LEO, such as ~15 eclipses per day, may have been technically demanding in terms of electrical and thermal stress.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Any other suggestions?&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2015-10-23T12:57:29.860" LastActivityDate="2015-10-23T12:57:29.860" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12399" PostTypeId="2" ParentId="5943" CreationDate="2015-10-21T21:21:05.157" Score="5" Body="&lt;p&gt;To add to Deer Hunter's response:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ITU manages the radio spectrum that satellites use, not their physical location. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many examples of satellites being owned by different organisations whose ITU permitted orbital slots are exactly co-incident but for use with different frequency bands. It is then up to the owners of the satellites to avoid physical encounters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a further detail which is that a satellite does not actually need ITU permission to use certain spectrum in a given orbital slot, and this is recognised by the ITU, if it does so on a &quot;non-interference basis&quot;, i.e. taking their own care not to cause radio friequency interference to neighbours who might already have registered frequency rights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has been an interesting related case in the news very recently, see &lt;a href=&quot;http://spacenews.com/russian-satellite-maneuvers-silence-worry-intelsat/&quot; rel=&quot;noreferrer&quot;&gt;http://spacenews.com/russian-satellite-maneuvers-silence-worry-intelsat/&lt;/a&gt; , in this case Intelsat had become concerned about the presence of a Russian satellite physically close to their own satellites. The article describes their concerns about the difficulties in establishing contact with the Russian satellite control authority but nowhere is there any mention of ITU permissions and, I suspect, this is because there is not necessarily any breaking of those particular conventions.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-10-21T21:21:05.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12400" PostTypeId="1" CreationDate="2015-10-21T21:28:32.677" Score="5" ViewCount="856" Body="&lt;p&gt;I'm writing a novel set 100+ years in the future. I'm looking into future materials from which spacecraft may be built. I read an &lt;a href=&quot;http://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars&quot; rel=&quot;noreferrer&quot;&gt;article about Hydrogenated Boron Nitride Nanotubes&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in knowing if this makes sense. I'm wondering if HBNNTs are suitable for structural elements, for the skin or hull of the spacecraft as radiation shielding, or its ability to withstand high temperatures as surface alloys of reentry bodies.&lt;/p&gt;&#xA;" OwnerUserId="9046" LastEditorUserId="49" LastEditDate="2015-10-22T05:48:27.173" LastActivityDate="2017-06-03T18:18:29.860" Title="Suitability of Hydrogenated Boron Nitride Nanotubes for structural and shielding elements of spacecraft" Tags="|spacecraft|materials|construction|feasibility|shielding|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12401" PostTypeId="2" ParentId="12396" CreationDate="2015-10-21T23:45:26.100" Score="1" Body="&lt;p&gt;Think of it like this: how would you test the vehicle without a crane? They are testing out the powered landing capabilities so the idea is to work with slow, smooth trajectories that may be mostly vertical but will obviously incorporate some lateral movement too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anything were to go wrong the vehicle could diverge wildly off-course and probably necessitate self-destruction. Instead, the crane allows you to hold onto the vehicle with (ideally) minimal reaction between the cable and the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course without any thrust the cable provides a reaction if the vehicle is hanging, but once the test is being run they will likely be adjusting the tension to be minimal (while being careful not to let it go completely slack).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The overall idea is like putting a leash on your dog so it doesn't run away, but still giving the dog slack to walk around comfortably (at least if your dog doesn't try pulling you around like mine!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the overall thrust and throttling, this system is being designed to operate with thrust that reduces to less than 1 g acceleration (otherwise landing would be too dangerous since you would depend on shorter high thrust burns while still descending). That article you link to actually states the dry weight is 14000 lbs, so considering 4000 lbs of propellant you end up with a mass of about 8100 kg so an acceleration of about 1 g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.faa.gov/about/office_org/headquarters_offices/ast/media/dragonfly_final_ea_sm.pdf&quot; rel=&quot;nofollow&quot;&gt;This document&lt;/a&gt; gives some more details about the DragonFly vehicle although it is primarily concerned with the subsequent 4 test phases after using the crane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this document we can pull out these numbers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dry mass = 6364 kg&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Propellant mass = 1756 kg (assuming full 400 gal tank with 50/50 NTO and MMH which have SGs of 1.44 and 0.88 respectively, although it could be anywhere from 1332-2180 kg)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maximum thrust = 546.7 kN&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On page 97 they state &quot;The four engines are modeled as a...&quot;, which could be just some confusion about the fact that the engines are configured in pairs, but might also suggest they are only firing four of the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the masses I've quoted, you would need a thrust of about 79.7 kN to hover, which corresponds to a throttle of 14.6% for all 8 engines or 29.1% for 4 engines.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-10-22T01:43:49.277" LastActivityDate="2015-10-22T01:43:49.277" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12402" PostTypeId="1" AcceptedAnswerId="12404" CreationDate="2015-10-22T01:46:35.973" Score="32" ViewCount="10392" Body="&lt;p&gt;I'm a big fan of &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_%28rover%29&quot; rel=&quot;noreferrer&quot;&gt;Curiosity&lt;/a&gt;&lt;/em&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_(rover)&quot; rel=&quot;noreferrer&quot;&gt;Opportunity&lt;/a&gt;, the Martian rovers. The science is over my head, but the images they've captured are mesmerizing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems unusual to me that we have two active Martian rovers and none on the Moon. And we've send a number of rovers to Mars in the last couple of decades, but we've only sent one rover to the Moon since &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;noreferrer&quot;&gt;Apollo&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DhZau.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DhZau.jpg&quot; alt=&quot;The last Lunar rover - Yutu&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last active rover on the Moon was the Chinese &lt;a href=&quot;https://en.wikipedia.org/wiki/Yutu_%28rover%29&quot; rel=&quot;noreferrer&quot;&gt;Yutu&lt;/a&gt;, in 2014. It died after just one year! Compared to &lt;em&gt;Opportunity&lt;/em&gt; and &lt;em&gt;Curiosity&lt;/em&gt;, with 11 and three years respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why don't we have more rovers on the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, why don't Lunar rovers last as long as Martian rovers?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="9092" LastEditDate="2021-03-15T10:06:15.993" LastActivityDate="2021-03-15T10:06:15.993" Title="Why are there so few lunar rovers?" Tags="|mars|the-moon|rovers|yutu|lunokhod|" AnswerCount="6" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12403" PostTypeId="2" ParentId="12402" CreationDate="2015-10-22T02:00:59.200" Score="7" Body="&lt;p&gt;That is a good question, and this might be better off as a comment but I will answer anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are obviously many reasons why space agencies choose one mission over another. Missions need to fall in line with the overall objectives of the agency, support science that the overall community is investigating, and get the highest value out of every dollar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is a planet, while the Moon is not, so we can't learn the same things from both of them. Mars is also full of surprises while the Moon is perhaps a bit more homogeneous and predictable when it comes to looking at different regions. That is not to say that the Moon is boring or that we already know enough about it, just that there are many more pressing questions that can only be answered by a rover/lander on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that rovers are only one type of spacecraft. Orbiters are much easier to employ and if you can get away with remote sensing instead of in-situ activities then you probably will do so. A lot of what we want to know about the Moon can be learned from orbit while putting off ground measurements for later missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hopefully the growing trends in private space exploration (i.e., the Google Lunar X-Prize) as well as agencies' interest in in-situ resource utilization will result in a surge of new lunar rovers and landers.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-10-22T02:00:59.200" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12404" PostTypeId="2" ParentId="12402" CreationDate="2015-10-22T03:09:01.687" Score="33" Body="&lt;p&gt;That's many questions. &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunokhod_2&quot; rel=&quot;noreferrer&quot;&gt;Lunokhod 2&lt;/a&gt; lasted for nearly 4 months and held the record for longest distance traversed by an off-Earth robot for over 41 years, until MER-B (&lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_%28rover%29&quot; rel=&quot;noreferrer&quot;&gt;Opportunity&lt;/a&gt;) beat that in mid 2014, so I wouldn't really say that lunar rovers don't last. It eventually scooped up some regolith dust with its open lid that was also its solar panel, and as it closed it to prepare for lunar night dumped dust over its radiators and overheated. But it completed its mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Otherwise, we didn't send more rovers to the Moon because it was for a long time after Apollo program considered &lt;em&gt;veni, vidi, vici&lt;/em&gt;, and only became more interesting again after NASA's Mini-SAR on &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandrayaan-1&quot; rel=&quot;noreferrer&quot;&gt;Chandrayaan-1&lt;/a&gt; found evidence of South polar water ice. Sure, you know the phrase, &lt;em&gt;The Moon Is a Harsh Mistress&lt;/em&gt;, and there are certain challenges with robotic exploration of it (talcum powder fine dust and its dielectric, photoelectric and triboelectric properties, long nights, high diurnal surface temperature swings and difficulties with cooling during days and heating during nights, harsh and often highly inclined terrain covered with impact craters and debris, half of it not in direct line of sight with the Earth, difficult lighting conditions with backscatter at high noon, other times long and incredibly contrasting shadows, radiation,...), but other destinations have their own sets of challenges, too. That's not the reason you can't observe recently taken snaps off the lunar surface (tho you can those &lt;a href=&quot;http://wms.lroc.asu.edu/lroc/&quot; rel=&quot;noreferrer&quot;&gt;taken from its orbit&lt;/a&gt;). Simple truth is that since we established that the Moon is a barren rock with no immediate utility to our exploration of the Solar system, our attention focused on other celestial bodies and the search for conditions for life, and soon more direct search for life where we think conditions for it might exists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we'll go back. Actually, any self-respecting space agency has now plans to send rovers on the Moon, with its South polar regions being of most interest, of course;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;JAXA plans &lt;a href=&quot;https://en.wikipedia.org/wiki/SELENE-2&quot; rel=&quot;noreferrer&quot;&gt;SELENE-2&lt;/a&gt;, ISRO &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandrayaan-2&quot; rel=&quot;noreferrer&quot;&gt;Chandrayaan-2&lt;/a&gt;, China &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang&amp;#39;e_4&quot; rel=&quot;noreferrer&quot;&gt;Chang'e 4&lt;/a&gt;, NASA &lt;a href=&quot;https://www.nasa.gov/resource-prospector&quot; rel=&quot;noreferrer&quot;&gt;Resource Prospector&lt;/a&gt; and running a couple of other programs at various levels of maturity, ESA is working on comprehensive lunar exploration plan under the umbrella of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Aurora_programme&quot; rel=&quot;noreferrer&quot;&gt;Aurora programme&lt;/a&gt; and currently cooperating with Roscosmos on &lt;a href=&quot;http://www.hou.usra.edu/meetings/leag2014/presentations/carpenter.pdf&quot; rel=&quot;noreferrer&quot;&gt;PILOT, SPECTRUM, PROSPECT&lt;/a&gt; programs that go from precision landing platforms to sample return missions, and so on. And there is a large number of private ventures with interests in mining the Moon for its resources, and/or to use it as the staging ground for more distant destinations or to service near-Earth / cisluar operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get a bit better perspective of upcoming missions, here is the current (prepared for the currently ongoing &lt;a href=&quot;http://www.hou.usra.edu/meetings/leag2015/&quot; rel=&quot;noreferrer&quot;&gt;LEAG Fall 2015 meeting&lt;/a&gt;) &lt;em&gt;International Cooperation and the Global Exploration Roadmap&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/ZDg14.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZDg14.png&quot; alt=&quot;International Cooperation and the Global Exploration Roadmap&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;strong&gt;International Cooperation and the Global Exploration Roadmap&lt;/strong&gt;, by Jim Carpenter on behalf of the ESA Lunar&lt;br /&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;Exploration Team Directorate of Human Spaceflight and Operations. Source: &lt;a href=&quot;http://www.hou.usra.edu/meetings/leag2014/presentations/carpenter.pdf&quot; rel=&quot;noreferrer&quot;&gt;ESA's plans for Lunar Exploration&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many of these missions include lunar landers and rovers. In fact, there's more lunar surface missions on this roadmap than, say, Mars surface ones. So your question might be, hopefully, short-lived and in a couple of years time, someone might even ask why is there such an apparent overpopulation of robots at and around the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shackleton_%28crater%29&quot; rel=&quot;noreferrer&quot;&gt;Shackleton crater&lt;/a&gt;. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-22T19:31:20.203" LastActivityDate="2015-10-22T19:31:20.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12405" PostTypeId="2" ParentId="12402" CreationDate="2015-10-22T08:59:44.573" Score="1" Body="&lt;p&gt;Mars has enough of an atmosphere to 'weather' its surface. The moon doesn't. This means that the individual grains of moon dust are much sharper and more abrasive. The lack of an atmosphere also makes the day/night temperature swings more extreme. Both of these contribute to wear and failure of rovers.&lt;/p&gt;&#xA;" OwnerUserId="1521" LastActivityDate="2015-10-22T08:59:44.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12406" PostTypeId="1" AcceptedAnswerId="12407" CreationDate="2015-10-22T10:47:36.800" Score="0" ViewCount="2181" Body="&lt;p&gt;Potato chips are a favorite snack food Earth side.  It seems that currently they are not allowed in the ISS &lt;a href=&quot;https://space.stackexchange.com/questions/9126/what-is-in-the-care-packages-sent-to-crew-members-of-the-international-space-sta/12038#12038&quot;&gt;(&lt;em&gt;anything with significant crumbs -- such as potato chips ... can't go.&lt;/em&gt;)&lt;/a&gt; and that chips bags will expand as the surrounding pressure decreases &lt;a href=&quot;https://space.stackexchange.com/questions/6080/how-is-unpressurized-cargo-useful-at-the-iss/6083#6083&quot;&gt;(&lt;em&gt;you notice that your bag of potato chips seems to be about to explode&lt;/em&gt;)&lt;/a&gt;.  We also know that if a package is designed to hold moderate pressure it will not explode for the same reason the &lt;a href=&quot;https://space.stackexchange.com/questions/8269&quot;&gt;Space Shuttle tires don't explode&lt;/a&gt;. At the same time a bag of potato chips needs to be sealed so it can be opened by hand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some point there is going to be more than one place in space, and there will need to be shuttles moving all those chips around &lt;a href=&quot;https://www.google.com/search?q=potato+chip+delivery+truck&amp;amp;espv=2&amp;amp;biw=1680&amp;amp;bih=925&amp;amp;source=lnms&amp;amp;tbm=isch&amp;amp;sa=X&amp;amp;ved=0CAYQ_AUoAWoVChMIr_rOqO3VyAIVghoeCh3-NAsv&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google images 'potato chip delivery truck'&lt;/a&gt; Most of us are familiar with the trucks delivering chips, essentially they are big boxes that keep the chips contained, the driver stops at a store grabs some chips and takes them in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I am really wondering is if the potato chip delivery space truck will be pressurized. Does the driver need to dock at a truck sized air lock, or is he wearing a space suit and takes the chips in through a man sized air lock? He might be making deliveries on the Moon or micro gravity, but in any case where there is enough traffic he is probably going to be making deliveries like his Earth bound counter part, the goods go in the front door of the building. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; In this question, I am assuming multiple populated habitats without interconnecting pressure areas.  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-10-22T14:35:27.940" Title="Will a bag of potato chips explode in space?" Tags="|crewed-spaceflight|colonization|commercial|food|" AnswerCount="1" CommentCount="4" ClosedDate="2015-10-22T17:48:29.273" ContentLicense="CC BY-SA 3.0" />
  <row Id="12407" PostTypeId="2" ParentId="12406" CreationDate="2015-10-22T14:17:37.143" Score="5" Body="&lt;p&gt;The real problem with potato chips isn't the packaging so much as it is the crumbs. Crumbs in zero gravity are a very serious issue, and would get everywhere, the food that they take into space needs to not have many crumbs, thus food like chips is a very bad idea in zero gravity. I can't see having them in space, unless some kind of artificial gravity is created, or at a location where this is natural gravity, such as the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to if the bag would explode, well, a standard pressurized bag would actually not explode, but would pop. See &lt;a href=&quot;https://www.youtube.com/watch?v=JhAxYhyWtiY&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt;. If they thought it a problem due to lower air pressure, they would simply make specially packaged space potato chips, that are packaged in such a way that it would be safe for space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would forsee that all deliveries would be pressurized, because it is much easier to unload them as such.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-22T14:35:27.940" LastActivityDate="2015-10-22T14:35:27.940" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12408" PostTypeId="2" ParentId="12402" CreationDate="2015-10-22T14:58:35.590" Score="18" Body="&lt;p&gt;The Moon has some very serious challenges for rovers. The most serious of these is the fact that the Moon has a 29-day rotation. That means that any spacecraft that will last longer than 14 days must survive a 14-day long night! The temperature swings are incredible in that dark period, from 123 degrees Celsius to -233 degrees Celsius, compared to Mars, which has a -73 to 20 degrees Celsius range over a year, near the equator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This causes a lot of real stress on the spacecraft. This is a serious challenge for those operating the spacecraft, who have to go into a special night mode to survive. Some spacecraft survive without extreme measures, but there hasn't been many that survive long. &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_17&quot; rel=&quot;nofollow&quot;&gt;Luna 17&lt;/a&gt; survived almost a year, but that is rare. Still, there is potential to build a spacecraft that will survive on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, however, that the Moon is by far the most explored part of the Solar System (excluding Earth). No landers have made it to the far side yet, which would be an interesting target, however, it is quite difficult, as one cannot directly reach Earth. However, there have been six manned missions, four unmanned rovers, and 13 other successful landers (If I've done my counting correct), making it a heavily explored body, far more by its surface area (especially considering only the half visible from Earth) than any other body in the Solar System except Earth. Of the &lt;a href=&quot;http://www.space.com/79-distances-driven-on-other-worlds.html&quot; rel=&quot;nofollow&quot;&gt;most traveled&lt;/a&gt; five rovers, four of them went to the Moon (including manned rovers).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is a more interesting subject as well, as it might have life. That has caused much interest in the planet, especially its follow-the-water campaign. Landers can last for longer, if near the equator, due to the more gentile temperature swings. Thus, rovers on Mars make a lot of sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, I don't doubt there will someday be more Lunar rovers. I've even heard of a proposal to launch &lt;a href=&quot;http://www.space.com/23946-golden-spike-private-moon-rover-designer.html&quot; rel=&quot;nofollow&quot;&gt;commercial rovers&lt;/a&gt;. Still, there just isn't enough interest right now in the Moon, at least in the United States, to warrant such attention, at least until we have the capability to land on the Moon again for manned missions.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="286" LastEditDate="2016-08-28T13:51:38.280" LastActivityDate="2016-08-28T13:51:38.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12409" PostTypeId="2" ParentId="3227" CreationDate="2015-10-22T16:27:17.447" Score="8" Body="&lt;p&gt;No, but they are easily seen with a small telescope on a sturdy mount.  March and September are the best times.  Use an app to help you.  My favorite way is to keep M11, the Wild Duck Cluster, in view with a medium power eyepiece.  Every few minutes, a &quot;star&quot; will slowly track through the southern edge! &lt;/p&gt;&#xA;" OwnerUserId="12047" LastEditorUserId="12102" LastEditDate="2016-08-24T05:55:51.183" LastActivityDate="2016-08-24T05:55:51.183" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12410" PostTypeId="2" ParentId="12402" CreationDate="2015-10-22T20:04:55.507" Score="12" Body="&lt;p&gt;Each of the Apollo missions delivered two semi-autonomous, crewed, human-powered roving vehicles with sample-return capability to the moon -- astronauts in suits. The later Apollos also added a wheeled &quot;bus stage&quot; to dramatically extend the range of those rovers. This is only half tongue-in-cheek: the function of the expeditions was broadly similar, and Apollo astronauts have done about twice the mileage as all the Mars rovers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After our sample-return visits to the moon, it was clear that it was not likely to harbor life. On Mars the question is still open, and the Martian landscape seems to be more varied than the moon's, so we generally learn more from putting rovers there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that we have yet to do a sample-return mission to Mars -- Mars' atmosphere, gravity, and distance conspire to make that a much harder prospect than a lunar sample return.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-10-22T20:04:55.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12411" PostTypeId="1" AcceptedAnswerId="12413" CreationDate="2015-10-23T04:55:20.990" Score="0" ViewCount="783" Body="&lt;p&gt;Could you have a large winch in space with a cable hanging down, say, 60 kilometers, connected to a floating station floated by helium? The payload could be floated up by balloons, which would then dock with the floating station. The payload would then be attached to the space-winch, which would bring it up to space. Would that be possible, or is there a gaping hole in my logic?&lt;/p&gt;&#xA;" OwnerUserId="12051" LastEditorUserId="3063" LastEditDate="2015-10-23T07:46:36.620" LastActivityDate="2015-10-23T07:46:36.620" Title="Could a space winch work?" Tags="|launch|space-elevator|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12413" PostTypeId="2" ParentId="12411" CreationDate="2015-10-23T07:33:38.257" Score="11" Body="&lt;p&gt;Is your objective to &quot;get to space&quot; or to &quot;get to orbit&quot;? The two are fairly different - just &quot;getting to space&quot; is not that hard (space starts at about 100 km up), but in order to &lt;em&gt;stay there&lt;/em&gt; (ie be in orbit around the Earth) you need to be going &lt;em&gt;sideways&lt;/em&gt; very very fast (about 8 km/s) - &lt;a href=&quot;https://what-if.xkcd.com/58/&quot;&gt;What If&lt;/a&gt; has a nice visual explanation of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So assuming you want to get to orbit using your method. You therefore need a a satellite in low Earth orbit (say, at an altitude of 200 km - if you're much lower &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot;&gt;than about 160 km&lt;/a&gt;, the drag from the upper atmosphere will slow your satellite down too much and it will re-enter the atmosphere), and you have a 200 km cable hanging down from the satellite. At an altitude of 200 km, the satellite will orbit the Earth about once every 90 minutes (that's how fast it has to go to stay in orbit). Assuming an equatorial orbit, the bottom end of the cable would be dragged through the atmosphere much much faster than the Earth is spinning (the Earth spins once around its axis every 24 hours); since it has to go around the Earth once every 90 minutes, it will be going at about 26,000 km/h (7.4 km/s). Needless to say, not only would the cable burn up almost instantly at that speed, but you would have great difficultly in &quot;catching up to it&quot; in order to attach your payload at the bottom end! And even if you could manage those two, the drag from the cable being pulled through the atmosphere would slow the satellite down so much that it would re-enter the atmosphere in short order.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you move your satellite into a higher orbit, the speed of the bottom end of the cable would decrease (as satellites in higher orbits take longer to go around the Earth). Eventually, once your satellite is at an altitude of about 35,800 km, it will be going around the Earth once every 24 hours (this is called a &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot;&gt;geosynchronous orbit&lt;/a&gt;&quot;). However, by that point your &quot;winch&quot; cable will be very long indeed, and the forces on it will be enormous. So enormous, in fact, that no currently known materials can withstand them, although there are several experimental materials (such as carbon nanotubes) which could, in theory, be used. This concept is called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_elevator&quot;&gt;space elevator&lt;/a&gt;, and while the principle is &lt;a href=&quot;/questions/tagged/space-elevator&quot;&gt;moderately well understood&lt;/a&gt;, it's beyond our current technology to build one on Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that I've oversimplified greatly - a real space elevator would need to extend well beyond geosynchronous orbit to remain stable (as its mass is now non-trivial, and the material needs to stay in tension), so the length of your &quot;winch&quot; would be even longer.)&lt;/p&gt;&#xA;" OwnerUserId="3063" LastEditorUserId="3063" LastEditDate="2015-10-23T07:44:55.723" LastActivityDate="2015-10-23T07:44:55.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12414" PostTypeId="1" CreationDate="2015-10-23T10:16:08.027" Score="4" ViewCount="53" Body="&lt;p&gt;The &lt;a href=&quot;http://spacecollege.org/isee3/we-are-now-in-command-of-the-isee-3-spacecraft.html&quot; rel=&quot;nofollow&quot;&gt;crowdsourced effort&lt;/a&gt; to re-establish communication with ISEE-3 was a spectacular event enabled, amongst all, by serendipitous trajectory, that brought the probe near Earth after 35 years travel, and still mostly working.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are trajectories of any other ongoing or finished missions expected to provide similar opportunities in the future?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2015-10-23T12:48:31.683" LastActivityDate="2015-10-23T12:48:31.683" Title="Are any ISEE-3 like returns expected in the foreseeable future?" Tags="|probe|flyby|cost|public-involvement|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12416" PostTypeId="1" AcceptedAnswerId="12422" CreationDate="2015-10-23T13:13:56.047" Score="13" ViewCount="432" Body="&lt;p&gt;New Horizons is en-route to &lt;a href=&quot;https://en.wikipedia.org/wiki/2014_MU69&quot; rel=&quot;nofollow noreferrer&quot;&gt;2014 MU₆₉&lt;/a&gt;, where it is planned to fly by 1 January 2019.&lt;/p&gt;&#xA;&lt;p&gt;Considering fuel and other considerations, how close to 2014 MU₆₉ can it get?&lt;/p&gt;&#xA;&lt;p&gt;(At the time of writing, October 2015, the IAU has not yet assigned a name to 2014 MU₆₉)&lt;/p&gt;&#xA;&lt;p&gt;For the record: The object has become named as Arrokoth. It had, in fact been nicknamed earler as Ultima Thule, reflecting a Norse conception of the farthest reaches of the world. But that name has become cluttered with racist connotations, and scientists eventually chose to use an Algonqiun word for &amp;quot;sky&amp;quot;.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="28871" LastEditDate="2023-11-13T01:05:04.030" LastActivityDate="2023-11-13T01:05:04.030" Title="How close to 2014 MU₆₉ can New Horizons get?" Tags="|new-horizons|flyby|2014-mu69|kuiper-belt|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="12417" PostTypeId="2" ParentId="12416" CreationDate="2015-10-23T14:11:26.363" Score="5" Body="&lt;p&gt;As close as they think can safely be achieved. I expect that will be in the range of hundreds of km, but that's purely speculation. It should be a fair bit closer than Pluto, however, because there they do not have to worry about flying too close to debris orbiting 2014-MU69. It's Hill Sphere is only about 400,000 km, and it seems likely that there wouldn't be any object of significance much closer. It seems the team hopes to have it's closest approach be less than 1000 km, from social media posts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have plenty of fuel to exact target it (within the uncertainty of it's position and New Horizons), and in fact, with the surplus of fuel they have, I expect it will basically target it spot on. Incidentally, the first burns have already started to aim New Horizons at 2014-MU69, as described in &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20151023&quot; rel=&quot;nofollow&quot;&gt;today's New Horizons mission news article&lt;/a&gt;. The biggest difficulty, at least initially, is likely targeting it precisely, as there is significant uncertainty in it's orbit. No doubt further studies, including from New Horizons, will help to refine its location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT It seems like the planned distance to target is about 1900 miles, per &lt;a href=&quot;http://www.americaspace.com/?p=92994&quot; rel=&quot;nofollow&quot;&gt;American Space&lt;/a&gt;. That is 4 times closer than Pluto's flyby.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-19T13:03:33.503" LastActivityDate="2016-04-19T13:03:33.503" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12418" PostTypeId="1" CreationDate="2015-10-23T14:39:23.383" Score="3" ViewCount="347" Body="&lt;p&gt;What are the fundamental drawbacks to using water ice as an external shell for manned spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="12059" LastEditorUserId="12059" LastEditDate="2015-10-23T15:24:19.933" LastActivityDate="2015-10-23T15:24:19.933" Title="Water ice for spacecraft's external shell material" Tags="|spacecraft|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12420" PostTypeId="2" ParentId="12418" CreationDate="2015-10-23T15:08:44.970" Score="4" Body="&lt;p&gt;Quite a few.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Loss, foremost - water sublimating into space. Sunlight would be turning the craft into a comet anywhere inside Mars orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not heat tolerant - Ejecting heat produced by the systems is another problem. How not to eject it into the ice shell? Performing a burn with the engines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass - getting all that water into orbit. It's heavy and not a nice structural material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Brittleness - if you use weak ion engines, that's okay. If you use rocket thrusters, oops. Also, would it need to hold that 1 bar pressure difference or would it need an extra shell?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reactivity, condensation - it would enable rust in environments with oxygen. You'd have headaches of fighting moisture within the ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many upsides too (you've mentioned them in the comment) but exposing the water to open space creates far too many headaches and it makes a really lousy structural material. A balloon-like shell filled with water is an idea worth considering. A spaceship made of ice - well, maybe in more distant sci-fi universe.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-10-23T15:08:44.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12421" PostTypeId="1" AcceptedAnswerId="12424" CreationDate="2015-10-23T15:48:25.633" Score="2" ViewCount="1538" Body="&lt;p&gt;As a little project in a course of spacecraft design, I have to plan a mission to Saturn's moon Titan.&#xA;The recommended trajectory is Hohmann transfer since it's easy to calculate the delta-V and launch windows. My problem is that after a fast calculation, my delta-v is more than 20 km/s just to get a transfer orbit around Saturn.&#xA;Is it possible to assume that the delta V of departure is entirely given by the launcher wich will reduce it by about 10 km/s?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I inspired myself of documentation of the Cassini mission, and they choose not to use a Hohmann transfer. This decision was motivated by the fact that the most powerfull launcher at that time could not give enough delta V. It was missing about 2 km/s.&#xA;Is it realist to plan this mission with Hohmann transfer considering newer launcher and lighter spacecraft (Cassini weighted 2,523kg) or is it necessary to use gravity assists?&lt;/p&gt;&#xA;" OwnerUserId="12060" LastEditorUserId="25" LastEditDate="2015-10-23T21:05:41.877" LastActivityDate="2015-10-23T21:58:46.130" Title="Titan trajectories options" Tags="|orbital-mechanics|trajectory|saturn|hohmann-transfer|titan|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12422" PostTypeId="2" ParentId="12416" CreationDate="2015-10-23T19:32:46.597" Score="14" Body="&lt;p&gt;The flyby distance will &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/09011608-new-horizons-extended-mission-pt1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;depend on how accurately they can measure&lt;/a&gt; 2014 MU69's orbit. This KBO was discovered only 3 years ago, and because it's so faint we only have a few observations to go on.&lt;br&gt;&#xA;For the Pluto encounter, the NH team went back to the Lowell observatory archives to measure the &lt;a href=&quot;https://www.washingtonpost.com/national/health-science/new-horizons-is-just-hours-away-from-pluto-after-95-year-journey/2015/07/13/ce4ab61a-294c-11e5-bd33-395c05608059_story.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;original plates made by Tombaugh&lt;/a&gt; when he discovered Pluto, in an effort to improve the accuracy of Pluto's position data. With far less data to go on, 2014 MU69's orbit will have some uncertainty.&lt;br&gt;&#xA;This uncertainty will be mainly in the direction along NH's trajectory, so it's the timing of the flyby that will be difficult. As a result, there'll be an uncertainty in flyby distance as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of January 2016, the science team is &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2016/01290930-whats-up-february-2016.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;targeting a flyby distance of 2000-3000 km&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: In &lt;a href=&quot;http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20171209&quot; rel=&quot;nofollow noreferrer&quot;&gt;December 2017&lt;/a&gt;, another course correction was made and the flyby distance is planned to be 3500 km. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The maneuver both refined the course toward and optimized the flyby arrival time at MU69, by setting closest approach to 12:33 a.m. EST (5:33 UTC) on Jan. 1, 2019. The prime flyby distance is set at 2,175 miles (3,500 kilometers); &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's a contingency plan to increase the flyby distance to 10,000 km if obstacles (moons, rings, etc.) are detected. The &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2018/0124-new-horizons-prepares-for-2014mu69.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;final decision will be made in December 2018&lt;/a&gt;, based on observations in the months before. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0llsK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0llsK.jpg&quot; alt=&quot;New Horizons timeline&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-28T10:18:50.497" LastActivityDate="2018-01-28T10:18:50.497" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12423" PostTypeId="2" ParentId="9240" CreationDate="2015-10-23T20:52:41.340" Score="1" Body="&lt;p&gt;So far as I can tell, there is no special procedure to clear any potential toxins from the white room, or anything at the launch pad, after a launch. I can't find any mention of cleaning up the site around a rocket launch after it's launch due to potential toxins, only stuff after failed launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I think the White Room's default environmental protections is sufficient, and no special cleaning of the White Room was required between launches to clean it from any toxic materials.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-23T20:52:41.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12424" PostTypeId="2" ParentId="12421" CreationDate="2015-10-23T21:18:39.877" Score="2" Body="&lt;p&gt;I'm quite confident that for the forseeable future, any mission to Saturn, or for that matter anywhere beyond Jupiter, will involve a flyby of Jupiter, because it gives such a tremendous gravity assist. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best way to figure out a good gravity assist is playing with &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;NASA's trajectory browser&lt;/a&gt;. It unfortunately doesn't allow for finding optimal solutions to Moons, but you can find one to Saturn. From there, you have to add the required delta v to do an orbiting mission, given that you are at C3.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-23T21:18:39.877" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12425" PostTypeId="1" CreationDate="2015-10-23T21:32:22.930" Score="6" ViewCount="1197" Body="&lt;p&gt;What does a new spacesuit and breathing apparatus smell like when you first put it on?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The natural progression of the question is does the scent wear off and does it get to smelling stagnant and nasty in there after a while.&lt;/p&gt;&#xA;" OwnerUserId="14259" OwnerDisplayName="whitepawn" LastEditorUserId="415" LastEditDate="2015-10-23T21:44:40.063" LastActivityDate="2015-10-25T21:27:31.263" Title="What does the interior of a spacesuit smell like?" Tags="|spacesuits|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12426" PostTypeId="2" ParentId="12421" CreationDate="2015-10-23T21:58:46.130" Score="1" Body="&lt;p&gt;It doesn't really make sense to say that &quot;the delta V of departure is entirely given by the launcher which will reduce it by about 10 km/s&quot;. Just because a different stage is being used to accomplish the manoeuvre doesn't mean it reduces the delta-V. You will most likely launch to a parking orbit in LEO before inserting to a trans-Saturn orbit. Once you are in that parking orbit it makes no difference to consider the delta-V coming from an upper stage of the launch vehicle or coming from the spacecraft itself -- you are just distributing mass differently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course overall you &lt;em&gt;would&lt;/em&gt; want to use the launch vehicle to accomplish as much of the manoeuvre as possible so that you can ditch the empty mass to make future manoeuvres more efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question of how realistic a Hohmann transfer is for this scenario is really up to you. As mentioned in the comments, any mission to the outer planets is going to require gravity assists based on the current capabilities of human spacecraft. New improvements in rocket motors and propellants may make it possible to use a simpler approach in the future, but the more significant alternative is the use of ion thrusters (or perhaps solar sails) to achieve interplanetary transfer with a low, continuous thrust. Although using that approach completely changes how you look at the trajectory since we can't assume instantaneous velocity changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this is for a course project, I would suggest this strategy: use a Hohmann transfer anyway but acknowledge the limitations on delta-V and maybe even show sensitivity studies on how the maximum spacecraft mass changes based on different launchers (or maybe just vary the Isp for your propellant), as well as how the required delta-V (and/or Isp) changes based on a given spacecraft mass. Otherwise planning a completely realistic mission to Titan is impossible without gravity assists or a significantly smaller spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-10-23T21:58:46.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12427" PostTypeId="2" ParentId="12425" CreationDate="2015-10-24T03:19:18.203" Score="6" Body="&lt;p&gt;&lt;p&gt;Don't you just love that new spacesuit smell?&lt;/p&gt;&lt;p&gt;Outgassing of volatiles from new materials has been a recognized phenomenon for a long time. All of the materials used in a space suit are selected to be able to withstand vacuum without changing composition or emitting volatiles that might be present when they are new. Astronauts wear special undergarments in a suit. Next to the skin they wear long johns made of a spandex material that wicks up perspiration and ports it into a water reclamation systems. It sounds a bit like a &lt;i&gt;stilsuit&lt;/i&gt;, a la the book &quot;Dune&quot; by Frank Herbert.&lt;/p&gt;&lt;p&gt;The undergarment also has a network of tubes that circulate water to keep the astronaut cooled.&lt;/p&gt;&lt;p&gt;The astronaut wears a &quot;maximum absorption garment&quot; under the undergarment. It's basically an adult diaper. Even in space, if you gotta go, you gotta go.&lt;/p&gt;&lt;p&gt;This means that the smelliest part of the space suit comes out at the end of the EVA. The undergarment is cleaned before it's reused, the, ahem, &quot;maximum absorption garment&quot; goes into the trash, and the suit itself gets appropriate maintenance. This means that by next usage, the suit ensemble is pretty odor neutral.&lt;/p&gt;&lt;p&gt;In Robert Heinlein's book &quot;Have Spacesuit - Will Travel&quot;, a teenager wins a used spacesuit in a contest. When he receives it, it reeks of body odor. This was one of Heinlein's novels for a juvenile audience, but his description of the spacesuit is worth reading.&lt;/p&gt;&lt;/p&gt;&#xA;" OwnerUserId="12071" LastActivityDate="2015-10-24T03:19:18.203" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12428" PostTypeId="1" CreationDate="2015-10-24T03:28:03.663" Score="4" ViewCount="288" Body="&lt;p&gt;It's going to be another year before all of the data that New Horizons gathered is finally transmitted to Earth, but based on current science return, does what's been received so far warrant an advanced orbiter with multiple flybys mission that would navigate the Pluto-Charon system for an extended period of time, the way Cassini is navigating at Saturn?&lt;/p&gt;&#xA;" OwnerUserId="12071" LastEditorUserId="49" LastEditDate="2015-10-24T03:51:51.757" LastActivityDate="2015-10-25T01:53:51.323" Title="Do New Horizons Pluto flyby results justify an advanced orbiter mission to the Pluto-Charon system?" Tags="|orbit|probe|pluto|flyby|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12429" PostTypeId="2" ParentId="12428" CreationDate="2015-10-24T07:31:28.327" Score="7" Body="&lt;p&gt;We simply don't have the launcher system to accomplish &lt;em&gt;that&lt;/em&gt; (see below).&lt;/p&gt;&#xA;&lt;p&gt;EDIT (hat tip to &lt;a href=&quot;https://space.stackexchange.com/users/265/mark-adler&quot;&gt;Mark Adler&lt;/a&gt; for an &lt;a href=&quot;https://space.stackexchange.com/a/9854&quot;&gt;answer&lt;/a&gt; to a related question - although please have a look at his &lt;a href=&quot;https://space.stackexchange.com/questions/12428/do-new-horizons-pluto-flyby-results-justify-an-advanced-orbiter-mission-to-the-p#comment32006_12429&quot;&gt;criticism of my answer here&lt;/a&gt; before judging for yourself and voting):&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;That&lt;/em&gt; means a mission that can achieve Pluto orbit insertion (POI) after being launched from the Earth on a single rocket... and take less than 10 years or thereabouts from launch to first scientific results. ESA's &lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Pluto.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;mission study&lt;/a&gt; from Mark Adler's answer had transfer lasting 18 years. In 18 years Principal Investigators will most likely go senile, retire, or die, and their replacement will lack critical knowledge to troubleshoot payload instruments. (See also &lt;a href=&quot;https://space.stackexchange.com/q/1151&quot;&gt;this question&lt;/a&gt; for more information on this very problem)&lt;/p&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://lesjohnsonauthor.com/yahoo_site_admin/assets/docs/Performance_Requirements_for_a_Nuclear_Electric_Propulsion_System.327200045.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;PERFORMANCE REQUIREMENTS FOR A NUCLEAR ELECTRIC PROPULSION SYSTEM - 2002&lt;/a&gt;&lt;sup&gt;*&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;An orbiter mission is only practical with high-performance propulsion at Pluto. For reasonable trip times, as described below, the encounter velocity at Pluto is about 12 km/s. Pluto does not have enough atmosphere for aerocapture, and its gravity well is of low potential. Therefore, &lt;strong&gt;the entire encounter velocity must be removed propulsively&lt;/strong&gt;. SEP does not work so far from the Sun, thus unless nuclear electric propulsion is available, one is left with chemical propulsion (jet velocity about 4 km/s, optimistically).&lt;/p&gt;&#xA;&lt;p&gt;The practical mass ratio (total mass/payload mass) is about 40 to 50. A reasonable mission spacecraft mass is about 500 kg, &lt;strong&gt;leading to a trans-Pluto mass requirement about 20,000 kg&lt;/strong&gt;. Not since the Saturn V has the U.S. possessed such a launch capability. Nor does any other nation presently possess such large launch capability. NEP is the only propulsion option that can reasonably perform a Pluto orbiter mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;NEP - Nuclear Electric Propulsion.&lt;/p&gt;&#xA;&lt;p&gt;SEP - Solar Electric Propulsion.&lt;/p&gt;&#xA;&lt;p&gt;In addition, you have to choose among scientific missions to maximize their marginal scientific results. Titan, Enceladus, and Europa all look much more promising &lt;sup&gt;#&lt;/sup&gt;, and are much easier to get at with reasonable payload masses.&lt;/p&gt;&#xA;&lt;hr/&gt;&#xA;&lt;p&gt;&lt;sup&gt;*&lt;/sup&gt; Despite the source being 13 years old, nothing has really changed as far as heavy launchers are concerned. Sigh. We don't have nuclear propulsion, either.&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;#&lt;/sup&gt; Liquid something yadda yadda wanna see clash of the titans and the snakes at enceladus &lt;strong&gt;ALL THESE WORLDS ARE YOURS—EXCEPT EUROPA ATTEMPT NO LANDING THERE&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-25T01:53:51.323" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12431" PostTypeId="2" ParentId="12428" CreationDate="2015-10-24T15:22:48.310" Score="6" Body="&lt;p&gt;There are definitely some interesting questions resulting from the Pluto flyby, that I'm sure scientists would love to study. However, there are a number of objects that are higher in the priority to study in the outer solar system. Here is what I think the expected priorities would be, for an orbiter class mission:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Europa, with other observation of Jupiter's moons if possible&lt;/li&gt;&#xA;&lt;li&gt;Enceladus&lt;/li&gt;&#xA;&lt;li&gt;Titan (Might require a dedicated mission just to study this moon)&lt;/li&gt;&#xA;&lt;li&gt;Neptune (Triton in particular is of interest)&lt;/li&gt;&#xA;&lt;li&gt;Uranus&lt;/li&gt;&#xA;&lt;li&gt;Flybys of other major KBOs&lt;/li&gt;&#xA;&lt;li&gt;Pluto&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, while Pluto is a very interesting body, it is only a small body, and not of particular interest such that it would justify an orbiter, particularly in terms of how difficult it is to get there. It is with little doubt the most fascinating KBO, but I just can't see an orbiter for some long period of time, owing to the difficulty in achieving orbit, as well as the fact that there are fewer bodies to study than the moons of any of the giant planets. Mars is more interesting, and closer. Pluto is probably the most interesting rocky object in the outer solar system after Titan, Europa, and Enceladus, but it still doesn't quite beat those 3 objects.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-24T15:22:48.310" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12432" PostTypeId="1" CreationDate="2015-10-24T21:04:03.953" Score="11" ViewCount="1297" Body="&lt;p&gt;In June 2015, SpaceX's mission CRS-7 on Falcon 9 was lost when the rocket exploded on takeoff. SpaceX investigation concluded that the problem was &lt;a href=&quot;http://www.spacex.com/news/2015/07/20/crs-7-investigation-update&quot;&gt;a failure of a single defective strut&lt;/a&gt;. The strut's design load was 2,000 lb and it was rated for 10,000 lb, 5 times higher. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It sounds like a good news for SpaceX — just improve quality control to catch bad parts and you're done. But is it so easy? I wonder if there is a fundamental design flaw there. Why did a failure of a single strut bring down the whole system? Is every strut in Falcon a Single Point of Failure (SPOF) then? Or even just some of them? That would still be a dangerous design. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For space missions, redundancy is expected and engineers take precautions against failing parts, elements and subsystems. Every SPOF is an invitation of trouble. So assuming what SpaceX says is true, is it a sign of bad high-level structural design or what?&lt;/p&gt;&#xA;" OwnerUserId="12080" LastEditorUserId="6816" LastEditDate="2015-10-24T21:25:28.113" LastActivityDate="2018-03-15T01:50:07.027" Title="SpaceX CRS-7 failure because of a bad strut — is it a sign of bad structural design?" Tags="|spacex|falcon-9|design|construction|safety|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12433" PostTypeId="2" ParentId="12432" CreationDate="2015-10-24T21:35:44.887" Score="10" Body="&lt;p&gt;No, it's indicative of insufficient &lt;a href=&quot;https://en.wikipedia.org/wiki/Quality_assurance&quot;&gt;Quality Assurance&lt;/a&gt; (QA), which is supposed to prevent defects, not of bad design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what Factor of Safety (FoS) is used for Falcon 9 struts in question, but judging by numbers given and failure at 1/5th the design limit, it seems to be close to 2.5, assuming maximum load factor of 6 g (page 33 of &lt;a href=&quot;https://www.spaceflightnow.com/falcon9/001/f9guide.pdf&quot;&gt;Falcon 9 Launch Vehicle Payload User's Guide&lt;/a&gt; (PDF), kudos to Brian Lynch in the comments) and load of 3.2 g at the point of failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even at FoS of 2.0, that's well within the industry norms, which, according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Factor_of_safety#Choosing_design_factors&quot;&gt;Wikipedia&lt;/a&gt;, are 1.2 to 3.0 for aircraft and spacecraft, depending on the application and materials. And Elon Musk has been quoted before that SpaceX works with industry leading safety margins, so it's not bad design.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-24T22:47:16.643" LastActivityDate="2015-10-24T22:47:16.643" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12434" PostTypeId="1" AcceptedAnswerId="12919" CreationDate="2015-10-24T23:04:30.130" Score="7" ViewCount="1908" Body="&lt;p&gt;I was reading &lt;a href=&quot;https://en.wikipedia.org/wiki/Heliosphere&quot;&gt;this Wikipedia article on Heliosphere&lt;/a&gt; and was confused a bit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;this supersonic wind must slow down to meet the gases in the interstellar medium&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How can &lt;em&gt;sonic speeds&lt;/em&gt; exist in space? The space is almost vacuum and sound cannot exist there. Also, isn't the heliosphere about electromagnetic wind, not atomic wind?&lt;/p&gt;&#xA;" OwnerUserId="10660" LastEditorUserId="49" LastEditDate="2015-10-24T23:28:50.203" LastActivityDate="2015-12-11T00:18:49.893" Title="How can solar wind be supersonic?" Tags="|physics|space-weather|solar-wind|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12436" PostTypeId="2" ParentId="3435" CreationDate="2015-10-25T02:59:02.690" Score="1" Body="&lt;p&gt;As a matter of fact, people who will land on Mars are not necessarily destined to death, as they will regularly be restocked with supplies from Earth (refer to mars one article in wikipedia). The only element that would not occur would be their return to Earth, because of the lack of its feasibility. Then, morally, the qualified Mars astronauts are not embarking on the mission intending to &quot;commit suicide&quot; on the red planet. Instead, they are willing to explore another probable life sustaining environment in our solar system and, if possible, they will try to create a livable habitat on the second most beloved planet after Earth. Hence, there is no such component of &quot;suicide&quot; in Mars one mission.&lt;/p&gt;&#xA;" OwnerUserId="12083" LastActivityDate="2015-10-25T02:59:02.690" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12437" PostTypeId="1" CreationDate="2015-10-25T02:59:27.783" Score="6" ViewCount="214" Body="&lt;p&gt;This question is motivated by an argument that arose in relation to &lt;a href=&quot;https://space.stackexchange.com/questions/12428&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know there are &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_Science_Decadal_Survey&quot; rel=&quot;nofollow&quot;&gt;decadal surveys&lt;/a&gt; recommending the priorities in planetary exploration. There is also a neat problem summarized by Andrew Kennedy (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Wait_Calculation&quot; rel=&quot;nofollow&quot;&gt;wait calculation&lt;/a&gt;) for interstellar exploration probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each mission study has the following hard-wired quantitative parameters:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;time to develop, test and launch the spacecraft (given by the launch window);&lt;/li&gt;&#xA;&lt;li&gt;maximum payload mass, power, and datarate that limit how much data can be collected and how fast the results can be transferred (given by the chosen trajectory and available launchers);&lt;/li&gt;&#xA;&lt;li&gt;time in transit from launch to destination (or destinations) when the scientific payoff is minimal;&lt;/li&gt;&#xA;&lt;li&gt;cost of launch campaign;&lt;/li&gt;&#xA;&lt;li&gt;cost of mission control over various mission phases;&lt;/li&gt;&#xA;&lt;li&gt;use of otherwise limited resources (like plutonium-238 for RTGs).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are also &quot;soft&quot;, less quantifiable parameters:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;expected value of scientific results;&lt;/li&gt;&#xA;&lt;li&gt;probability of missing the launch window due to the need for redesign;&lt;/li&gt;&#xA;&lt;li&gt;expected development costs.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A naive straightforward approach would be to calculate discounted net present value (NPV) of all the outlays and scientific results (somehow converted into money) and use it or the implicit rate of return to compare missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is twofold:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is there a &lt;strong&gt;widely-accepted handbook&lt;/strong&gt; outlining theory to be used in top-level decision making while choosing the mission portfolio?&lt;/li&gt;&#xA;&lt;li&gt;Are there any studies that analyze &lt;strong&gt;how decisions are really made&lt;/strong&gt; (by NASA or other agencies)? What discounting rates are implicitly used by decision-makers?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="168" LastEditorUserId="25" LastEditDate="2015-10-25T18:36:30.380" LastActivityDate="2015-10-25T18:36:30.380" Title="What is the accepted methodology for prioritizing scientific exploration missions?" Tags="|nasa|planetary-science|mission-design|future-missions|reference-request|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12438" PostTypeId="1" AcceptedAnswerId="12441" CreationDate="2015-10-25T07:39:48.597" Score="5" ViewCount="217" Body="&lt;p&gt;This questions is purely about government agencies, not private companies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typically the massive reams of data that must be produced to design and qualify a satellite or rocket aren't openly shared with the public. Aside from the 'we pay for it, we should get access to the data' I see a few solid reasons why this sort of work should be openly shared;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Promote growth in the sector.&lt;/li&gt;&#xA;&lt;li&gt;Promote the agencies competencies.&lt;/li&gt;&#xA;&lt;li&gt;Show good engineering practice.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I'm sure there are more reasons, these are just the 3 off the top of my head (I'm clearly wearing a weird hat).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I can tell there's only a few reasons against sharing this type of data;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Loss of profitability.&lt;/li&gt;&#xA;&lt;li&gt;Competition.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The second reason is almost a joke, there's so much cross agency cooperation these days that we can barely tighten a bolt without getting the wrench from another country. This leads me to think that profitability is the main reason... is it?&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="25" LastEditDate="2015-10-25T18:11:20.623" LastActivityDate="2015-10-25T18:11:20.623" Title="Do government space agencies capitalise on selling intellectual property/patents etc" Tags="|nasa|technology|law|cost|government|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12440" PostTypeId="2" ParentId="2958" CreationDate="2015-10-25T10:03:41.870" Score="10" Body="&lt;p&gt;The fiberglass (FRP) tubes on the top of the launch platforms are a part of a lightning protection system and are insulators with cables that form a cone of protection for the launch tower. They were designed by my former company CH2M HILL, (Gainesville,FL)an engineering firm, and were manufactured by Starline  Fabricators, Astatula, FL. I brought this design project in for the company from a contact I made with the fabricator during a business development trip to Lake County in the late 70s.&lt;/p&gt;&#xA;" OwnerUserId="12085" LastEditorUserId="12085" LastEditDate="2015-10-25T10:09:34.880" LastActivityDate="2015-10-25T10:09:34.880" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12441" PostTypeId="2" ParentId="12438" CreationDate="2015-10-25T12:37:42.990" Score="2" Body="&lt;p&gt;NASA's &lt;a href=&quot;http://www.nasa.gov/offices/ogc/ip/1210.html&quot; rel=&quot;nofollow&quot;&gt;Intellectual Property policy&lt;/a&gt; contains a lot of information. If I'm reading the legalese correctly, it states that NASA can file for patents, and will license those technologies if appropriate. See &lt;a href=&quot;http://technology.nasa.gov/hot100/&quot; rel=&quot;nofollow&quot;&gt;this site&lt;/a&gt; for a list of patents that NASA is willing to license, for instance. &lt;a href=&quot;http://www.forbes.com/sites/alanboyle/2015/10/24/free-ideas-these-nasa-patents-are-so-crazy-they-just-might-work-for-startups/&quot; rel=&quot;nofollow&quot;&gt;This site&lt;/a&gt; also lists examples of some patents that NASA will give for free, then request royalties after a period of time. It has stated that it will not sue for patent infringement, but will prevent such items from being developed without appropriate licensing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to why they can, it can actually encourage growth. NASA's patents tend to be partially formed. It takes a lot of funding to bring them to fruition. If a company is paying for exclusive use of a patent, then they will be motivated to finish the technology (By having paid for it, and having exclusive rights to said technology). They also use it as a motivator for the individual who created said patent, who earns a significant portion of the money from the patent (25%, according to my research)&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-25T16:34:39.050" LastActivityDate="2015-10-25T16:34:39.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12442" PostTypeId="2" ParentId="9394" CreationDate="2015-10-25T18:21:51.750" Score="5" Body="&lt;p&gt;The paint, as can be seen from &lt;a href=&quot;https://ui.adsabs.harvard.edu/abs/2003DPS....35.3602J/abstract&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, contains a titanium-oxide semiconductor. The spectral measurements of the paint are similar for J002E3 as they were from measurements taken shortly after launch, showing that the paint still performs well even after over 30 years in orbit, for the Apollo 12 mission. The paint simply serves as a thermal control mechanism. From &lt;a href=&quot;https://web.archive.org/web/20170310022023/http://archives.chicagotribune.com/1969/07/05/page/9/article/order-a-new-paint-job-on-apollo-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this news article&lt;/a&gt;, we can see that a primer was used (And apparently, the wrong primer was used for at least one rocket)&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-06-27T17:32:02.593" LastActivityDate="2019-06-27T17:32:02.593" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12443" PostTypeId="2" ParentId="12437" CreationDate="2015-10-25T18:36:06.567" Score="3" Body="&lt;p&gt;This is actually a really good question, but quite a bit more involved. How missions in general are prioritized is as follows (For &lt;a href=&quot;http://discovery.nasa.gov/p_mission.cfml&quot; rel=&quot;nofollow&quot;&gt;discovery class missions&lt;/a&gt;, or the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Scout_Program&quot; rel=&quot;nofollow&quot;&gt;Martian Scout Program&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A scientist (Or group of scientists) decides they want to see a certain kind of mission.&lt;/li&gt;&#xA;&lt;li&gt;Periodically, NASA sends out solicitations for proposals. Typically any mission is allowed, so long as the mission can be done for a particular cost.&lt;/li&gt;&#xA;&lt;li&gt;NASA looks at all of the proposals, along with a team of reviewers from other scientists. They essentially weight each proposal, on the science achieved and the engineering of the project, etc, to see if the mission can succeed.&lt;/li&gt;&#xA;&lt;li&gt;NASA gives a select few teams more money to develop the missions, to ultimately pick the one with the best science/ engineering combination.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The short answer, the more scientists are interested in something, the more highly weighted it will be. This really applies for every class of NASA mission to some extent. #4 is the real key.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-25T18:36:06.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12444" PostTypeId="1" AcceptedAnswerId="12449" CreationDate="2015-10-25T21:27:16.383" Score="3" ViewCount="517" Body="&lt;p&gt;I have to design a space mission to Saturn as part of a course on spacecraft and system engineering. I found a suitable trajectory but, as the Δv is quite high, I would like to adapt it to Ion propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the proper assumptions I have to do to make it &quot;realistic&quot; for low thrust propulsion? How can impulse thrust trajectory be adapted to a low thrust system?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simplest would probably be to find a trajectory with multiple gravity assists, like Cassini did (EVVEJS), but since this is just an assignment project, I haven't the tools nor the time to compute such a trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php?maxMag=25&amp;amp;maxOCC=4&amp;amp;chk_target_list=on&amp;amp;target_list=Saturn&amp;amp;mission_class=oneway&amp;amp;mission_type=rendezvous&amp;amp;LD1=2023&amp;amp;LD2=2024&amp;amp;maxDT=8.0&amp;amp;DTunit=yrs&amp;amp;maxDV=7&amp;amp;min=DV&amp;amp;wdw_width=365&amp;amp;submit=Search#a_load_results&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tM8R0.png&quot; alt=&quot;Example trajectory to Saturn employing DSM and Earth flyby&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&lt;sup&gt;&lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php?maxMag=25&amp;amp;maxOCC=4&amp;amp;chk_target_list=on&amp;amp;target_list=Saturn&amp;amp;mission_class=oneway&amp;amp;mission_type=rendezvous&amp;amp;LD1=2023&amp;amp;LD2=2024&amp;amp;maxDT=8.0&amp;amp;DTunit=yrs&amp;amp;maxDV=7&amp;amp;min=DV&amp;amp;wdw_width=365&amp;amp;submit=Search#a_load_results&quot; rel=&quot;nofollow noreferrer&quot;&gt;Example trajectory to Saturn&lt;/a&gt; employing Deep Space Maneuver and Earth flyby. Made using &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Trajectory Browser&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12060" LastEditorUserId="12060" LastEditDate="2015-10-25T22:09:55.430" LastActivityDate="2015-10-26T13:30:06.600" Title="Modifications to impulse burn trajectory for low thrust propulsion" Tags="|orbital-mechanics|propulsion|trajectory|saturn|ion-thruster|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12445" PostTypeId="2" ParentId="12425" CreationDate="2015-10-25T21:27:31.263" Score="0" Body="&lt;p&gt;Obviously an astronaut would be the best person to answer.  Not being an astronaut and assuming you are referring to conventional space suits, not some futuristic sci-fi suit, I will take my best guess.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you go to a bowling alley and received the shoes they rent out, they spray them down quite often with a cleaning/disinfecting solution, but they still have a smell.  I clean my motorcycle helmet every now and then, but it definitely takes on a smell.  NASA space suits are designed for the person wearing it, so I believe that after time, the plastics and such would mingle with the personal smell of the occupant to create a definite smell.  I also, think the strength of that smell would increase with use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are writing a story and want to include a smell, who's going to argue your wrong?  Your knowledgeable pool of critics is pretty small.  Good luck.&lt;/p&gt;&#xA;" OwnerUserId="12093" LastActivityDate="2015-10-25T21:27:31.263" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12447" PostTypeId="1" CreationDate="2015-10-26T04:05:53.637" Score="14" ViewCount="798" Body="&lt;p&gt;I was watching a documentary recently on the development of space suits, both of IVA and EVA. The program made it clear that the most difficult thing was the development of the gloves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sbZZm.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sbZZm.jpg&quot; alt=&quot;EVA Gloves&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The complexities involve the need to be airtight but still be able to feel tactile sensation, strong enough to withstand the rigors of, not only space, but manual labor while still being maneuverable to perform delicate technical operations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My first thought was to seal the hands in a tube and control &quot;robot hands&quot; via a controller. That way the strength and dexterity can be increased.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like robotic hands are a thing&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4UENp.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4UENp.jpg&quot; alt=&quot;robot hands&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would also protect the gloves from possible punctures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this a viable idea? Has it been considered before?&lt;/p&gt;&#xA;" OwnerUserId="723" LastActivityDate="2017-01-20T01:49:09.947" Title="Why use fingers in EVA suits?" Tags="|spacesuits|robotics|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12448" PostTypeId="2" ParentId="12447" CreationDate="2015-10-26T08:26:30.807" Score="5" Body="&lt;p&gt;The Apollo program involved pretty heavy investigations into spacesuit designs since walking around on the Moon was obviously a lot more demanding than sitting in a capsule or even EVAs. Check out &lt;a href=&quot;https://www.youtube.com/watch?v=Ce84hkvN7KA&quot;&gt;this Moon Machines episode&lt;/a&gt; if you are interested. Some of the concepts that were considered included hard-shell suits like those used for deep ocean dives, and those typically have the type of hands you describe -- although they are more like a simple claw.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vuyys.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vuyys.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-10-26T08:26:30.807" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12449" PostTypeId="2" ParentId="12444" CreationDate="2015-10-26T13:30:06.600" Score="2" Body="&lt;p&gt;To make this happen, you have to write a simulation. There are a couple of ways you could approximate this, but to give the best example, I would have 3 gravity point sources, one for each Saturn, Earth, and the Sun. You could add objects if desired for higher accuracy, or remove them if you get to be a sufficiently far distance from them. Then calculate at some appropriate interval (Smaller for near Earth/ Saturn, larger if further away from them would work), increasing the delta v in that direction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suggest you first find the path that will take you to Saturn's orbit, then find the Earth/ Saturn angle that will get you there. Then find a point in the orbit where the Earth/ Saturn angle is that angle, and you should be fine.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-26T13:30:06.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12452" PostTypeId="1" CreationDate="2015-10-26T18:33:04.980" Score="6" ViewCount="198" Body="&lt;p&gt;This Wednesday, October 28 2015 at 11:22 a.m. EDT (15:22 UTC), &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-teleconference-to-preview-historic-flyby-of-icy-saturn-moon&quot;&gt;Cassini spacecraft will flyby through the Enceladus plume&lt;/a&gt; at only 30 miles (49 km) altitude to analyze plume chemistry with its ion and neutral mass spectrometer (INMS). While the plume particle size is estimated at only 30 microns, and it is fairly tenuous, relative velocity on &lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/flybys/enceladus20151028/&quot;&gt;flyby E-21&lt;/a&gt; will be 19,014 mph (8.5 km/sec)!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/fezGaCnWOWM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Animation of Cassini's Oct. 28, 2015 flyby of Enceladus, during which the spacecraft will make its deepest dive through the moon's active plume of icy material.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question is, how risky has this been assessed to be to the spacecraft and its instruments?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that even if the momentum of the plume particulate during the flyby, due to their low mass and density, isn't sufficient to cause, say, sizable dents and tearing of the spacecraft's multi-layer insulation, since Cassini will also attempt to take close proximity images of Enceladus' South polar region where these vents are forming the plume, this could cause darkening of optics to occur through contact with ionized molecular hydrogen and other trace gasses in the plume. Especially since Enceladus will only be illuminated by Saturn's shine, which should require longer exposure times. And perhaps it could cause other problems that I haven't envisioned here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure JPL did risk assessment, so I'm mostly interested in any published papers discussing this flyby in greater detail from perspective of risk management.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-10-28T15:24:56.657" Title="Cassini flyby through Enceladus plume - How risky is that for the spacecraft and its instruments?" Tags="|reference-request|flyby|risk-management|cassini|enceladus|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12453" PostTypeId="1" AcceptedAnswerId="21857" CreationDate="2015-10-26T18:44:43.067" Score="7" ViewCount="6195" Body="&lt;p&gt;It seems that first rabbit was launched into space around 1959. There are differences in sources &lt;a href=&quot;https://en.wikipedia.org/wiki/Korabl-Sputnik_2&quot;&gt;(&lt;em&gt;Wikipedia, Korabl-Sputnik 2, does not mention rabbits&lt;/em&gt;)&lt;/a&gt; but it appears several rabbits have been launched into space.  As prey animals rabbits are not built as tough as dogs and cats, they don't take physical abuse well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have rabbits been launched into space without notable injury? If so how were they secured for the trip? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: To qualify as &quot;without notable injury&quot; the live rabbit would need to be examined, in space or on the ground.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;References &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Animals_in_space&quot;&gt;Animals in space (Wikipedia)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://history.nasa.gov/animals.html&quot;&gt;Animals in space (NASA)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="58" LastEditDate="2017-06-07T13:48:10.480" LastActivityDate="2022-05-29T15:59:04.793" Title="Rabbits in Space" Tags="|launch|research|medical|animals|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12454" PostTypeId="1" AcceptedAnswerId="12457" CreationDate="2015-10-27T04:01:40.740" Score="7" ViewCount="256" Body="&lt;p&gt;Why was 2015 TB145 detected just weeks before its closest approach, while so many other asteroids (which will make closer approach or farther approach) are detected years before? What made this asteroid difficult to detect?&lt;/p&gt;&#xA;" OwnerUserId="12019" LastEditorUserId="897" LastEditDate="2015-10-27T15:02:33.963" LastActivityDate="2015-10-27T15:02:33.963" Title="Detection of 2015 TB145 (Halloween asteroid)" Tags="|asteroid|astronomy|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12456" PostTypeId="2" ParentId="12454" CreationDate="2015-10-27T08:21:38.173" Score="4" Body="&lt;p&gt;I bet that the high inclination, 40 degrees, is the explanation. &lt;a href=&quot;http://jtg.sjrdesign.net/advanced/asteroid.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Very few asteroids&lt;/a&gt; are, or have been found, at such high inclination. But I don't specifically know how asteroid searches work with inclinations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Auzbl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Auzbl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-10-27T08:21:38.173" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12457" PostTypeId="2" ParentId="12454" CreationDate="2015-10-27T12:06:58.770" Score="8" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/2015_TB145&quot;&gt;2015 TB145&lt;/a&gt; had an apparent magnitude of 20 when it was discovered. This is pretty faint and may account for it not being found earlier.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most asteroids have a small inclination (angle relative to the ecliptic plane). Some of the early asteroid surveys didn't look at high inclinations, because it can take a long time to do a survey of the entire sky. These programs need to look at the same area of sky several times per year to detect which objects have moved, so they made a decision to limit the area they looked at. With increasing computing power and better CCDs it became possible to do full-sky surveys. These detection programs tend to get hardware upgrades every few years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pan-STARRS&quot;&gt;Pan-STARRS&lt;/a&gt; (which discovered 2015 TB145) does full-sky surveys and has been operational since 2010. So even with this inclination, Pan-STARRS has looked at the region of sky where 2015 TB145 was several times already without detecting it.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-27T14:43:26.313" LastActivityDate="2015-10-27T14:43:26.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12458" PostTypeId="2" ParentId="12452" CreationDate="2015-10-27T17:37:43.260" Score="2" Body="&lt;p&gt;I don't have the source for this latest flyby, but I do have one for the &lt;a href=&quot;http://saturn.jpl.nasa.gov/faq/FAQMission/#q1&quot; rel=&quot;nofollow&quot;&gt;50 km&lt;/a&gt; pass that was previously executed in 2008, only slightly further than the 49 km pass this go-around. The two stated dangers were &quot;The two threats to the spacecraft were identified to be an inadvertent impact with Enceladus, and damage resulting from the environment within the plume.&quot; The impact with Enceladus was deemed to be very unlikely. As for the particles, they used data to determine there wasn't any particles of sufficient size in the plume to damage the spacecraft. Optics were not included in the publicly released danger considerations. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-27T17:37:43.260" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12459" PostTypeId="1" CreationDate="2015-10-27T23:28:41.037" Score="3" ViewCount="183" Body="&lt;p&gt;One of the missions NASA is planning is to redirect a small asteroid to lunar orbit and land astronauts on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it can redirect asteroids, why not just send it to LEO and send a Dragon to it?&lt;/p&gt;&#xA;" OwnerUserId="12112" LastEditorUserId="6816" LastEditDate="2015-10-27T23:44:37.320" LastActivityDate="2016-01-05T03:22:33.947" Title="Why is NASA redirecting an asteroid to Lunar Orbit instead of LEO?" Tags="|asteroid|planning|orion-spacecraft|" AnswerCount="1" CommentCount="1" ClosedDate="2015-10-27T23:49:35.500" ContentLicense="CC BY-SA 3.0" />
  <row Id="12460" PostTypeId="2" ParentId="12459" CreationDate="2015-10-27T23:43:31.593" Score="3" Body="&lt;p&gt;The reason is that a lunar orbit (specifically, a &lt;em&gt;distant retrograde orbit&lt;/em&gt; or DRO) is much more stable compared to a low Earth orbit so you can leave it there for a long time and study it in the future. You also don't have to worry about it decaying and colliding with the Earth -- although the size of the target asteroid (actually only a piece of an asteroid) &lt;em&gt;probably&lt;/em&gt; makes it nothing to worry about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2013/Johnson_Kirstyn/finalorbit.html&quot; rel=&quot;nofollow&quot;&gt;this project&lt;/a&gt; for a decent description of DRO's and how they relate to NASA's asteroid redirect mission.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2016-01-05T03:22:33.947" LastActivityDate="2016-01-05T03:22:33.947" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12461" PostTypeId="2" ParentId="12388" CreationDate="2015-10-28T00:37:25.090" Score="6" Body="&lt;p&gt;I asked Jim Carpenter, lead scientist for ESA on the &lt;a href=&quot;http://www.bbc.com/news/science-environment-34504067&quot; rel=&quot;nofollow noreferrer&quot;&gt;Luna 27 lander&lt;/a&gt; project, about this. He said that although the plans and protocols for Aurora are still in place, it is currently on hold, outside of the &lt;a href=&quot;https://en.wikipedia.org/wiki/ExoMars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Exomars&lt;/a&gt; program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ESA announced the &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/ESA_Moon_challenge&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heracles&lt;/a&gt; program a few weeks ago. The winners of that competition will present their plans at the &lt;a href=&quot;http://spaceflight.esa.int/humanrobotics/about/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moon 2020-2030&lt;/a&gt; symposium in December.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ypXIc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ypXIc.jpg&quot; alt=&quot;infrastructure system for lunar exploration&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The subtitle of the symposium is 'A New Era of Human and Robotic Exploration'. &#xA;ESA's position on human exploration of the Moon is summed up in one of the slides from &lt;a href=&quot;http://www.hou.usra.edu/meetings/leag2014/presentations/carpenter.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;last year's LEAG meeting&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5dp3S.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5dp3S.png&quot; alt=&quot;ESA position on human exploration of the Moon&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-11-06T14:04:06.653" LastActivityDate="2015-11-06T14:04:06.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12463" PostTypeId="1" AcceptedAnswerId="12465" CreationDate="2015-10-28T09:22:52.937" Score="13" ViewCount="2002" Body="&lt;p&gt;Cassini is to fly &lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-34593944&quot;&gt;within 50 km&lt;/a&gt; of Enceladus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the closest flyby a spacecraft has ever made? What object was targeted and when was it? Is 50 km a new record?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(For the purposes of this question, an ultra-close orbit is also included, but lithobraking, landing, or any procedure that ultimately resulted in contact or was designed to, is not)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="49" LastEditDate="2015-10-28T10:43:31.463" LastActivityDate="2015-10-28T20:04:29.047" Title="What is the closest flyby to date?" Tags="|probe|spacecraft|flyby|record|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12464" PostTypeId="2" ParentId="12463" CreationDate="2015-10-28T09:38:00.033" Score="5" Body="&lt;p&gt;The closest I know of is &lt;a href=&quot;http://www.universetoday.com/118777/rosetta-to-snuggle-up-to-comet-67p-for-closest-encounter-yet/&quot;&gt;Rosetta&lt;/a&gt;, which orbited 67P at altitudes down to 6 km. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-28T09:38:00.033" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12465" PostTypeId="2" ParentId="12463" CreationDate="2015-10-28T09:59:10.017" Score="15" Body="&lt;p&gt;JAXA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa&lt;/a&gt; came to within &lt;strong&gt;44 meters&lt;/strong&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/25143_Itokawa&quot; rel=&quot;nofollow noreferrer&quot;&gt;25143 Itokawa&lt;/a&gt;, which, in an unfortunate sequence of events, lead to its lander &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa#MINERVA_mini-lander&quot; rel=&quot;nofollow noreferrer&quot;&gt;MINERVA&lt;/a&gt; to miss the target:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;MINERVA was deployed on 12 November 2005. The lander release command&#xA;  was sent from Earth, but before the command could arrive, Hayabusa's&#xA;  altimeter measured its distance from Itokawa to be 44 m (144 ft) and&#xA;  thus started an automatic altitude keeping sequence. As a result, when&#xA;  the MINERVA release command arrived, MINERVA was released while the&#xA;  probe was ascending and at a higher altitude than intended, so that it&#xA;  escaped Itokawa's gravitational pull and tumbled into space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is possible that MINERVA itself came closer to Itokawa than 44 meters, but it was an unpowered mini lander, so that probably doesn't count:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/qjQ2k.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qjQ2k.jpg&quot; alt=&quot;Hayabusa&amp;#39;s MINERVA lander missing the asteroid 25143 Itokawa&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;MINERVA lander (in yellow circle) missing the asteroid 25143 Itokawa as seen by Hayabusa probe. Image: &lt;a href=&quot;https://translate.google.com/translate?sl=ja&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=en&amp;amp;ie=UTF-8&amp;amp;u=http%3A%2F%2Fwww.isas.jaxa.jp%2Fj%2Fsnews%2F2005%2F1113.shtml&amp;amp;edit-text=&amp;amp;act=url&quot; rel=&quot;nofollow noreferrer&quot;&gt;JAXA&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Perhaps also interesting (tho it doesn't fit requirements of your question), a week later and as a result of a separate set of approach maneuvers, Hayabusa also &lt;a href=&quot;http://www.isas.jaxa.jp/e/snews/2005/1124_hayabusa.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;landed on the asteroid, collected samples, and then took off again&lt;/a&gt;. Next probe to perform a similar feat of briefly touching an asteroid to collect samples will be NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx#Sample_acquisition&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-REx&lt;/a&gt; at &lt;a href=&quot;https://en.wikipedia.org/wiki/101955_Bennu&quot; rel=&quot;nofollow noreferrer&quot;&gt;101955 Bennu&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-28T20:04:29.047" LastActivityDate="2015-10-28T20:04:29.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12466" PostTypeId="2" ParentId="12463" CreationDate="2015-10-28T10:37:09.900" Score="10" Body="&lt;p&gt;Chinese &lt;a href=&quot;https://en.wikipedia.org/wiki/4179_Toutatis&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chang'e 2&lt;/a&gt; flew 3.2 km from the asteroid 4179 Toutatis (size: 4.7×2.4×1.9 km) in 2012 after having ended its primary Lunar mission. Maybe a record for intentional flyby's?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GCnWo.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GCnWo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jBSWm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jBSWm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_minor_planets_and_comets_visited_by_spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here's a list&lt;/a&gt; of the 21 asteroids and comets and minor planets visited by spacecrafts, with closest approach altitude. One way to define it is in number of object radii, and then Dawn wins at Ceres according to that table (but many Earth, Moon, Mercury, Venus and Mars orbiters beat that).&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-10-28T11:10:42.330" LastActivityDate="2015-10-28T11:10:42.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12467" PostTypeId="1" AcceptedAnswerId="12469" CreationDate="2015-10-28T12:41:54.990" Score="18" ViewCount="10341" Body="&lt;p&gt;According to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton&amp;#39;s_law_of_universal_gravitation&quot; rel=&quot;nofollow&quot;&gt;Newton's law of universal gravitation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In view of this, astronauts in a space station should be attracted by nearby objects. So they must be attracted towards heavy objects like machines etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How are they floating without the attraction of gravitational force of nearby objects?&lt;/p&gt;&#xA;" OwnerUserId="12113" LastEditorUserId="1477" LastEditDate="2016-02-23T16:20:10.147" LastActivityDate="2017-05-29T17:09:08.293" Title="How can astronauts float in space without being affected by the gravitational force of nearby objects?" Tags="|gravity|microgravity|astronauts|" AnswerCount="6" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12468" PostTypeId="2" ParentId="12467" CreationDate="2015-10-28T12:47:49.267" Score="5" Body="&lt;p&gt;Douglas Adams once said:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Gravity works well with very massy things. With low mass things it has very very minor effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A US aircraft carrier, floating next to a battleship, experiences a pull of gravity between the two masses but even then it is so low as to be immaterial. (About the biggest mobile structures on earth by mass, maybe an oil tanker masses more). Scale that down to a person in a fairly low mass ISS and you will see that yes, they do experience a pull, but it is incredibly low. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-10-28T15:15:48.887" LastActivityDate="2015-10-28T15:15:48.887" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12469" PostTypeId="2" ParentId="12467" CreationDate="2015-10-28T13:15:30.847" Score="78" Body="&lt;p&gt;Objects in orbit &lt;strong&gt;are&lt;/strong&gt; attracted to each other, it's just their mass is small enough that the force of gravity between them is infinitesimal. Gravitational acceleration is dependent on mass and distance. In a scenario where a 150 kg astronaut is 10 m from a 80,000 kg Space Shuttle, the astronaut would be pulled toward the Shuttle at 5.336e-8 m/second squared. That's 0.00000005336 as opposed to the Earth's surface gravity of 9.8 m/second squared, and ~ 8.7 m/second squared at orbital altitude of 375 km. In other words, the Earth's force on the astronaut would be over 160 million times stronger, if it wasn't balanced by the exactly opposing centrifugal force due to their orbital speed of 7,682.5 m/s at that same orbital altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter's influence on the astronaut (and the Shuttle and the Earth) when the Earth and Jupiter are close together is 3.2e-7 m/second squared, six times more than gravitational attraction of the Shuttle on the astronaut, but still very little.&lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="49" LastEditDate="2015-10-30T14:57:24.397" LastActivityDate="2015-10-30T14:57:24.397" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12470" PostTypeId="1" AcceptedAnswerId="12471" CreationDate="2015-10-28T13:16:13.730" Score="9" ViewCount="918" Body="&lt;p&gt;I've been having a hard time understanding what flying through Enceladus's plume accomplishes. Cassini is an orbiter. It seems odd that it would have instruments that could even analyze the plume at all. I know dust counters are pretty common for similar missions, but it seems like Cassini would avoid flying through anything that would have enough material to sample. What can Cassini accomplish by flying through the Plume, and why was the capability designed in the first place?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-03-04T13:02:56.640" Title="What does flying in Enceladus's plume accomplish?" Tags="|water|saturn|instrument|cassini|enceladus|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12471" PostTypeId="2" ParentId="12470" CreationDate="2015-10-28T13:44:51.603" Score="16" Body="&lt;p&gt;Cassini's INMS, the Ion Neutral Mass Spectrometer, is an in situ instrument that measures the neutral and plasma gas composition of what it ingests.  It was intended for the measurement of Titan's atmosphere, Saturn's magnetosphere plasma, ring composition, and in fact the composition of icy satellite effluents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;http://lasp.colorado.edu/~horanyi/graduate_seminar/VortragINMS.pdf&quot;&gt;good presentation on the basics of the INMS instrument&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yes, the Cosmic Dust Analyzer, CDA, also is an in situ instrument that detects ice particles as it flies through the plume.  The CDA was intended mainly to measure ring particles.  The CDA is sensitive enough to detect particle fluxes that would not harm the spacecraft.  Otherwise there would be little sense in having such an instrument.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-10-28T13:44:51.603" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12472" PostTypeId="2" ParentId="12467" CreationDate="2015-10-28T13:48:01.657" Score="18" Body="&lt;p&gt;Consider &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton&amp;#39;s_law_of_universal_gravitation#Modern_form&quot; rel=&quot;nofollow noreferrer&quot;&gt;this equation&lt;/a&gt; for gravitational attraction between two bodies:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$$F = G \frac{m_1 m_2}{r^2}$$&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;where:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;$F$ is the force between the masses;&lt;/li&gt;&#xA;  &lt;li&gt;$G$ is the gravitational constant (6.674×10&lt;sup&gt;−11&lt;/sup&gt; N · (m/kg)&lt;sup&gt;2&lt;/sup&gt;);&lt;/li&gt;&#xA;  &lt;li&gt;$m1$ is the first mass;&lt;/li&gt;&#xA;  &lt;li&gt;$m2$ is the second mass;&lt;/li&gt;&#xA;  &lt;li&gt;$r$ is the distance between the centers of the masses.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So if we say that an astronaut in an EVA suit has a mass of 150 kg, the International Space Station (ISS) itself has a mass of &lt;a href=&quot;http://isslive.com/displays/adcoDisplay1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;390,000 kg&lt;/a&gt;, and the distance between their centers of mass is 5 meters, then their gravitational attraction between them is &lt;a href=&quot;https://encrypted.google.com/#q=%28%28%286.674%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+%28390000+kg+%2B+150+kg%29%29%2F%28%285+m%29%5E2%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;$1.042 × 10^{-6} \text{ m/s}^2$&lt;/a&gt;, or roughly one ten millionth (1/10,000,000) that of &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravity&quot; rel=&quot;nofollow noreferrer&quot;&gt;standard gravity&lt;/a&gt; at the surface of the Earth ($9.80665 \text{ m/s}^2$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, neither the astronaut nor the station are point masses, so at such close proximity to each other, their mass distribution will play a major role and we have to account for it. Problem is, we don't really have exact mass distribution of the station, even if we neglected astronauts own non-uniformity as largely irrelevant due to small relative mass to the station. But, since we also don't have astronaut's angle to it, I'll just assume uniform mass for the 100 m long and 5 m in girth (r=2.5 m) station, and astronaut's position tangential to the station and orthogonal to its center of mass. I.e. the astronaut will be outside the station, somewhere near &lt;a href=&quot;https://en.wikipedia.org/wiki/Unity_%28ISS_module%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Node 1&lt;/a&gt;'s outer truss;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In our case (with a few assumptions making this easier to calculate), combined gravity vector amplitude will change with cosine to the mean angle to the station's mass. That is, angle to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Centroid&quot; rel=&quot;nofollow noreferrer&quot;&gt;centroid&lt;/a&gt; of each 50 m sides. In our case, using the &lt;a href=&quot;http://www.mathwords.com/s/sohcahtoa.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHCAHTOA mnemonic&lt;/a&gt;, that would be, 78.69°. So our acceleration to our uniform mass center of given dimensions and distribution would be $\text{cos}(78.69°) \cdot 1.042 × 10^{-6} \text{ m/s}^2$ or &lt;a href=&quot;https://encrypted.google.com/#q=cos%2878.69+deg%29+*+%28%28%286.674%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+%28390000+kg+%2B+150+kg%29%29%2F%28%285+m%29%5E2%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;$2.04264874 × 10^{-7} \text{ m/s}^2$&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is only &lt;a href=&quot;https://encrypted.google.com/#q=%28cos%2878.69+deg%29+*+%28%28%286.674%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+%28390000+kg+%2B+150+kg%29%29%2F%28%285+m%29%5E2%29%29%2F%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%28%286378.1+km+%2B+412.5+km%29%5E2%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;$2.36349805 × 10^{-8}$&lt;/a&gt; times (24 one billionths) the Earth's gravity at ISS mean orbital altitude (semi-major axis) of &lt;a href=&quot;https://encrypted.google.com/#q=%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%28%286378.1+km+%2B+412.5+km%29%5E2&quot; rel=&quot;nofollow noreferrer&quot;&gt;$\approx 8.64 \text{ m/s}^2$&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So not exactly nothing, but you'll grow a really long beard waiting for any noticeable change in your position relative to the station to happen due to your mutual gravitational attraction alone. Slight difference in your orbit relative to the station, combined with gravity gradient with respect to the station's center of mass will move you relative to it much faster than that, and we still call it a &lt;a href=&quot;https://space.stackexchange.com/a/3906/49&quot;&gt;microgravity environment&lt;/a&gt; (i.e. difference measurable on a micro-g scale for its entire volume).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, for what is worth, distant gravitational perturbers like other celestial bodies will act on the station in exactly the same way as on our EVA astronaut, so their position relative to each other doesn't change because of that. There might be other, non-gravitational perturbing effects like solar wind, radiation pressure and exospheric drag, affecting station's movement slightly differently than our astronaut's, but that's not what the question asks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravity&lt;/a&gt; is essentially a really weak force and it takes a whole lot of mass for its effects to be appreciable without doing extremely precise measurements (and growing a long beard). If you don't believe me, consider this: You can pick up relatively heavy objects off the Earth's surface while the whole planet is gravitationally pulling onto them. Conversely, take two small rare earth magnets, join them together by opposite magnetic poles, and you'll have really big problems separating them again.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-10-30T15:06:27.247" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12473" PostTypeId="2" ParentId="12470" CreationDate="2015-10-28T15:21:26.813" Score="5" Body="&lt;p&gt;Concise version from the &lt;a href=&quot;http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20151022/&quot; rel=&quot;nofollow noreferrer&quot;&gt;pre-flyby media teleconference announcement&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Cassini scientists are hopeful the flyby will provide insights into&#xA;  how much hydrothermal activity is occurring within Enceladus, and how&#xA;  this hot-water chemistry might impact the ocean’s potential&#xA;  habitability for simple forms of life. If the spacecraft’s ion and&#xA;  neutral mass spectrometer instrument (INMS) detects molecular hydrogen&#xA;  as it travels through the plume, scientists may get the measurements&#xA;  they need to answers these questions.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;Confirmation of molecular hydrogen in the plume would be an&#xA;  independent line of evidence that hydrothermal activity is taking&#xA;  place in the Enceladus ocean, on the seafloor,&quot; said Hunter Waite,&#xA;  INMS team lead at Southwest Research Institute in San Antonio. &quot;The&#xA;  amount of hydrogen would reveal how much hydrothermal activity is&#xA;  going on.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Using Cassini's cosmic dust analyzer (CDA) instrument, scientists&#xA;  expect the flyby will lead to a better understanding of the chemistry&#xA;  of the plume. The low altitude of the encounter is, in part, intended&#xA;  to increase the spacecraft’s access to heavier, more massive molecules&#xA;  -- including organics -- than the spacecraft has observed during previous, higher altitude passes through the plume. The CDA&#xA;  instrument, which is capable of detecting up to 10,000 particles per&#xA;  second from the plume, also is expected to reveal how much material&#xA;  the plume is spraying from the moon's ocean into the space around&#xA;  Saturn.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And more detailed explanation of scientific expectations can be found in this &lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/flybys/enceladus20151028/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Enceladus Flyby 'E-21' page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Key scientific expectations for this flyby&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Scientists are looking forward to several important scientific results from the Oct. 28 flyby. These results will not be available immediately -- they will take several months of careful analysis, and would be published in a peer-reviewed journal.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;1. Confirm presence of molecular hydrogen (H2)&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;This measurement will be accomplished using Cassini's sensor that sniffs the gases in the plume (&lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instrumentscassiniinms/&quot; rel=&quot;nofollow noreferrer&quot;&gt;called INMS&lt;/a&gt;)&lt;/li&gt;&#xA;  &lt;li&gt;Confirmation of H2 would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean, on the seafloor&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Amount of H2 Cassini measures would reveal how much hydrothermal activity is going on in the ocean.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;• This has implications for the amount of energy available for creating a habitable environment in the ocean&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;2. Better understand the chemistry of material in the plume&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;Cassini's dust detector (&lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instrumentscassinicda/&quot; rel=&quot;nofollow noreferrer&quot;&gt;called CDA&lt;/a&gt;) will obtain spectra of the heavier particles only found at low altitudes nearer to the plume's source&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;•  Among these heavier particles, Cassini may detect new, more complex organic molecules (albeit with not enough resolution to confirm if they are biological in nature)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;•  Scientists think these heavier particles carry material from the sub-surface ocean&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;•  Scientists are doing laboratory experiments to create a catalog they can refer to of chemical fingerprints (or spectra) for fragments of complex organic molecules Cassini might detect &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;3. Determine the nature of the plume sources&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Is the plume made up of tight, column-like jets or curtain-like eruptions that run along the length of the tiger stripe fractures (or both)?&lt;/li&gt;&#xA;  &lt;li&gt;How much icy material are the plumes actually spraying out? Scientists are still not sure, and the amount has major implications for how long the moon might have been active.&lt;/li&gt;&#xA;  &lt;li&gt;This measurement will be accomplished by part of Cassini's CDA instrument called the high-rate detector, which can count the impacting ice particles from the plume (over 10,000 per second) in real-time.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I know, pretty dry reporting* from me this time, but the rest is already in Mark's answer. I just thought to share this since I watched &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-teleconference-to-preview-historic-flyby-of-icy-saturn-moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;October 26th flyby teleconference&lt;/a&gt;. Recording of it is &lt;a href=&quot;http://www.ustream.tv/recorded/76308203&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, accompanying visuals &lt;a href=&quot;https://www.nasa.gov/cassini/telecon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;*OK, OK, here's a contrast-enhanced crop of a nice image of Enceladus' fairly recent South polar plume activity, taken by Cassini on October 14, 2015 (&lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=342476&quot; rel=&quot;nofollow noreferrer&quot;&gt;N00249604&lt;/a&gt;), two weeks before its flyby &lt;em&gt;in the thick of it&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4WaTk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4WaTk.jpg&quot; alt=&quot;How&amp;#39;s that for a footer, huh? :P&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cassini Solstice Mission scientists might have their own expectations, but I'll be also hoping for some awesome closeup images. :) It has been mentioned during the teleconference that they might suffer from substantial motion smear due to such high relative velocity (8.5 km/s) and bad lighting conditions since the Enceladus' South polar region will only be illuminated by Saturn shine during the flyby, but that it &lt;em&gt;should&lt;/em&gt; be possible to digitally enhance them in post-processing and get some nice vistas of its surface anyway.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-10-28T16:03:26.263" LastActivityDate="2015-10-28T16:03:26.263" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12474" PostTypeId="2" ParentId="12452" CreationDate="2015-10-28T15:24:56.657" Score="0" Body="&lt;p&gt;I wish NASA would make transcripts of their audio broadcasts, because this question was raised by someone in the media during the question &amp;amp; answer part of last Monday's teleconference. (I searched, couldn't find any transcripts -- can someone do better?) In short, and working from memory, there's apparently no risk at all to the spacecraft or its instruments. I remember the flight director saying something along the lines of, &quot;we know where Enceladus is and where Cassini is&quot; and the risk to the spacecraft is virtually nil.&lt;/p&gt;&#xA;" OwnerUserId="9041" LastActivityDate="2015-10-28T15:24:56.657" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12475" PostTypeId="2" ParentId="1336" CreationDate="2015-10-28T15:44:34.437" Score="4" Body="&lt;p&gt;A 1 km cube with room for 1,000,000 people each with  16 rooms of 20 Square meters, would require 18 tons per person for a 3 meter shield, which would reduce radiation levels to less than the .000001 R/Hr of the Earths surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Allocating each person 1000 cubic meters. If a floor height is 3 meters the 1000 cubic meters provides 333 square meters of floor space. This space can be divided  16 rooms of 20 square meters (200 square feet). The assumption is that people will share part of that space for common areas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 1 km cube has 6 sides each 1000 x 1000 meters. The total surface area is thus 6 * 1000 * 1000 or 6 million square meters . Divided among the 1 million occupants means 6 meters of shielding each. Three meters of shielding is required for each square meter of surface. This is a Stack of 3, 1 meter cubes. A cubic meter of water weighs/has a mass of 1 metric tonne (very close to 1 ton). The stack of 3 * surface area of 6 totals 18 tons.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a 100 meter cube (1,000 people) the figure is 180 tons per person.&#xA;The surface area is 6 * 100 * 100 = 60,000 square meters , which is 60  square meters for each of the 1000 people. Thus 3 * 60 or 180 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 3 meter shield size is derived from the information in keithS's post for the same question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is required to reduce 100,000 R/hr to .000001 (1 millionth ) R/hr which is a reduction of 100 thousand million times - which is under $2^{38}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each 7cm (&lt;a href=&quot;http://www.nist.gov/pml/data/xraycoef/index.cfm&quot; rel=&quot;nofollow&quot;&gt;http://www.nist.gov/pml/data/xraycoef/index.cfm&lt;/a&gt;) of  water divides the radiation by a factor of 2. Thus the total required is 38 * 7cm or 266 cm.&#xA;Use  300 cm / 3 meters to make calculation easier and it provides an extra safety margin which makes the calculation easier.&lt;/p&gt;&#xA;" OwnerUserId="12131" LastEditorUserId="12131" LastEditDate="2015-11-01T10:55:21.177" LastActivityDate="2015-11-01T10:55:21.177" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12476" PostTypeId="1" CreationDate="2015-10-28T18:49:40.920" Score="1" ViewCount="510" Body="&lt;p&gt;What happens to the old satellites and other debris that is left in space? Why is it even acceptable to leave our trash / garbage out there?&lt;/p&gt;&#xA;" OwnerUserId="12127" LastEditorUserId="415" LastEditDate="2015-10-28T20:45:42.340" LastActivityDate="2018-04-20T05:51:08.363" Title="What happens to man-made objects in space after their service life ends?" Tags="|orbit|debris|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12477" PostTypeId="1" AcceptedAnswerId="12480" CreationDate="2015-10-28T18:53:53.123" Score="7" ViewCount="5928" Body="&lt;p&gt;I have read somewhere that to-be astronauts are trained on Earth to be deft enough when they are in space which has zero gravity. How do we create a zero gravity situation on Earth? On Earth, even if you are in a vacuum, you still experience the force of gravity, so exactly how do we engineer such a situation which actually defies gravity on Earth itself?&lt;/p&gt;&#xA;" OwnerUserId="12128" LastEditorUserId="47149" LastEditDate="2022-06-05T05:14:11.490" LastActivityDate="2022-06-05T05:14:11.490" Title="How do we create a zero gravity environment on Earth?" Tags="|gravity|astronauts|microgravity|technology|training|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="12478" PostTypeId="1" AcceptedAnswerId="12481" CreationDate="2015-10-28T19:06:36.620" Score="17" ViewCount="3485" Body="&lt;p&gt;Early in &lt;a href=&quot;http://www.imdb.com/title/tt0112384/&quot;&gt;the &lt;em&gt;Apollo 13&lt;/em&gt; movie&lt;/a&gt;, Lovell (Tom Hanks) is showing off the Saturn V and states that a part of what has made the moon landings possible is a belief that anything is possible, &quot;things like a computer that can fit into a single room, and hold millions of pieces of information&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia's article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Core_rope_memory&quot;&gt;core rope memory&lt;/a&gt; and the article on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Guidance_Computer&quot;&gt;Apollo Guidance Computer&lt;/a&gt;, the Block II AGC had a total storage (ROM plus RAM) of 40,960 16-bit words, or in modern terms 81,920 bytes. (We sent people to the moon with less computer code than is needed on my system to &lt;em&gt;list files in a directory!&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any merit to the statement that the spacecraft computer could hold &quot;millions of pieces of information&quot;, or is it simply an attempt at setting the date for when the events took place?&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2015-10-29T19:38:30.880" Title="Is there any merit to the Apollo 13 (movie) statement &quot;a computer that can hold millions of pieces of information&quot;?" Tags="|apollo-program|myth-busting|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12479" PostTypeId="2" ParentId="12476" CreationDate="2015-10-28T19:06:57.017" Score="6" Body="&lt;p&gt;Once something is in space, there's only a few ways it'll ever come back down.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It rockets down to re-enter Earth&lt;/li&gt;&#xA;&lt;li&gt;It is in the upper part of the atmosphere, which slowly will bring it down.&lt;/li&gt;&#xA;&lt;li&gt;Sunlight pushes it into an orbit where it intersects the atmosphere, or even leaves Earth orbit.&lt;/li&gt;&#xA;&lt;li&gt;It hits another object, changing it's velocity.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Once debris is up there, it's hard to get down. Spacecraft are required to put themselves into either a graveyard orbit, or an orbit where the spacecraft will naturally return to Earth shortly thereafter. However, some spacecraft fail, others were built before this was a known problem, and overall, it's expensive to ensure that it happens.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-10-28T19:06:57.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12480" PostTypeId="2" ParentId="12477" CreationDate="2015-10-28T19:10:23.770" Score="15" Body="&lt;p&gt;There are two methods which are used:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Aircraft can fly on a trajectory that will simulate zero gravity for a few seconds, usually around 30. This is used for most short term zero gravity tests. It is commonly known as the &quot;vomit comet&quot;, especially the earlier versions.&lt;/li&gt;&#xA;&lt;li&gt;Underwater testing can be done, in a neutral buoyancy lab. Basically, you make it so you weigh as much as the water you displace does, making it so you &quot;float&quot; in zero gravity. If you pushed away from something, you would continue to move (Although being in water, you have drag and swimming that could move you as well)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Both are approximations, but they do work well enough to do something at least.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-10-29T16:14:53.250" LastActivityDate="2015-10-29T16:14:53.250" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12481" PostTypeId="2" ParentId="12478" CreationDate="2015-10-28T19:25:41.840" Score="35" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;things like a computer that can fit into a single room, and hold&#xA;  millions of pieces of information&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;was not about the Apollo computer, but about the general-purpose computers that were available around the start of the Apollo program. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1960, the &lt;a href=&quot;https://en.wikipedia.org/wiki/IBM_7090&quot;&gt;IBM 7090&lt;/a&gt; could store 32 kB. In 1964, &lt;a href=&quot;http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_PR360.html&quot;&gt;IBM introduced the System/360&lt;/a&gt;, which could store up to 8 MB of data. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-28T19:58:19.537" LastActivityDate="2015-10-28T19:58:19.537" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="12482" PostTypeId="2" ParentId="4416" CreationDate="2015-10-28T19:26:39.730" Score="3" Body="&lt;p&gt;According to a yahoo group conversation, they can measure effects of solar radiation pressure on the object's orbit.  The force would be roughly proportional to the area, and the resulting acceleration is inversely related to the mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See: &lt;a href=&quot;https://groups.yahoo.com/neo/groups/mpml/conversations/topics/31398&quot; rel=&quot;nofollow&quot;&gt;https://groups.yahoo.com/neo/groups/mpml/conversations/topics/31398&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I presume this is how they estimated the density of WT1190F.&lt;/p&gt;&#xA;" OwnerUserId="12129" LastEditorUserId="12129" LastEditDate="2015-10-29T17:36:53.273" LastActivityDate="2015-10-29T17:36:53.273" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12483" PostTypeId="1" CreationDate="2015-10-28T22:20:16.970" Score="2" ViewCount="185" Body="&lt;p&gt;I have been trying to obtain the pitch-yaw-roll errors induced by known errors in the in-track, cross-track and radial coordinates or LVLH errors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is the relation of these error sets derived?&lt;/p&gt;&#xA;" OwnerUserId="12133" LastEditorUserId="6816" LastEditDate="2016-05-29T20:48:18.810" LastActivityDate="2016-06-28T20:56:54.017" Title="Orbit-induced attitude errors" Tags="|orbit|attitude|" AnswerCount="0" CommentCount="6" ClosedDate="2016-06-29T08:47:06.187" ContentLicense="CC BY-SA 3.0" />
  <row Id="12484" PostTypeId="1" CreationDate="2015-10-29T04:24:54.220" Score="0" ViewCount="50" Body="&lt;p&gt;In a future manned Mars mission, how bad a sand storm going to affect a mission? What are the measures taken to prevent damage from these sand storms?&lt;/p&gt;&#xA;" OwnerUserId="12019" LastEditorUserId="238" LastEditDate="2015-10-29T04:41:17.773" LastActivityDate="2015-10-29T04:41:17.773" Title="How violent are sand storms on Mars?" Tags="|mars|" AnswerCount="0" CommentCount="4" ClosedDate="2015-10-29T04:39:26.510" ContentLicense="CC BY-SA 3.0" />
  <row Id="12485" PostTypeId="2" ParentId="12467" CreationDate="2015-10-29T11:20:08.153" Score="6" Body="&lt;p&gt;Don't worry, they &lt;em&gt;are&lt;/em&gt; attracted by other objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only problem is, that attraction is ridiculously weak, so you won't notice it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most high school students, when they learn about the four forces, are a bit surprised that &lt;em&gt;gravity&lt;/em&gt; is the weakest one – it seems so strong?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, consider this: it takes &lt;strong&gt;a whole planet's&lt;/strong&gt; gravity for you to even notice it. A planet is a very big and heavy thing...not so much a space station or a piece of machinery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, as someone mentioned, the magnetism of a tiny magnet can easily overcome the gravitational pull of the whole planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, gravity is there, and it does exist even between small objects. And it does affect us in interesting if unexpected ways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take pool, for example. If you try to do a trick shot, gravity is there to mess it up for you. Well, you wouldn't notice normally, but with 4-5 or more collisions, you are hosed – there is no way for you to predict where a ball will go after that. Not even a pro can do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason is that &lt;em&gt;gravity&lt;/em&gt; affects the ball's trajectory. Yes, even the tiny gravity of the people around the table is big enough to make it impossible to know where things will end up after more than five or so bounces :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;https://michaelberryphysics.files.wordpress.com/2013/07/berry174.pdf&quot;&gt;Berry, M V, 1988, ‘The electron at the end of the universe’, p. 44&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, don't think gravity has no effect between small objects!&lt;/p&gt;&#xA;" OwnerUserId="12141" LastActivityDate="2015-10-29T11:20:08.153" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12489" PostTypeId="1" AcceptedAnswerId="12491" CreationDate="2015-10-29T18:49:52.207" Score="13" ViewCount="8040" Body="&lt;p&gt;As far as i understand we are going on a route shown in the picture.&#xA;When we can actually let Mars do the travelling for us and just take the short route and wait for Mars to arrive... Won't that be faster?&#xA;I have added a picture with some arrows to show what i mean. &lt;a href=&quot;https://i.stack.imgur.com/2nHIn.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2nHIn.jpg&quot; alt=&quot;alternative route to Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original picture can be found here -&gt; &lt;a href=&quot;http://www.extremetech.com/wp-content/uploads/2013/03/Inspiration_Mars_trajectory.svg_.png&quot; rel=&quot;noreferrer&quot;&gt;http://www.extremetech.com/wp-content/uploads/2013/03/Inspiration_Mars_trajectory.svg_.png&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12147" LastActivityDate="2017-04-03T20:51:41.813" Title="Why don't we take the shortest route to Mars?" Tags="|mars|trajectory|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12490" PostTypeId="2" ParentId="12489" CreationDate="2015-10-29T18:55:35.760" Score="6" Body="&lt;p&gt;Fuel mass and time of flight are the two main variables to be traded off for deep-space spacecraft. In this case, they decided to design their spacecraft and orbit around a minimum-fuel operation (and also a unique opportunity to do a human flyby of Venus, &lt;a href=&quot;https://en.wikipedia.org/wiki/Inspiration_Mars_Foundation#Mission&quot;&gt;&quot;Plan B&quot;&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you had a lot (A LOT) of fuel mass to spare, you could use thrusters during the entire flight (accelerating for halfway, decelerating for the rest), you could drastically decrease the time of flight, but the amount of usable mass you'd have at the end of the flight would be negligible (or the takeoff mass would be insane)&lt;/p&gt;&#xA;" OwnerUserId="11653" LastActivityDate="2015-10-29T18:55:35.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12491" PostTypeId="2" ParentId="12489" CreationDate="2015-10-29T19:18:19.690" Score="21" Body="&lt;p&gt;When a spacecraft is launched from Earth, it's moving counterclockwise across your diagram at a speed of 30 km/s (the same speed as Earth itself), plus a speed of 10-15 km/s in the direction of the launch.&lt;br&gt;&#xA;The trajectory you propose is only possible if you reduce the spacecraft's counterclockwise speed to zero. The fastest launch ever (New Horizons) reached about 16 km/s, so at the moment we can't fly the trajectory you've drawn.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-10-29T19:18:19.690" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12494" PostTypeId="2" ParentId="12467" CreationDate="2015-10-30T00:58:21.867" Score="0" Body="&lt;p&gt;Gravity has a tendency to sneak up on you. &#xA;Consider the 150 Kg EVA Suited astronaut hanging about the 390 ton space station about 50 meters from the station and lulled by the beautiful starscape falls asleep. &#xA;So what happens ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Relevant Equations . &#xA;&lt;br&gt;&lt;b&gt; Acceleration due to Gravity &lt;/b&gt; &lt;br&gt;&#xA;    ISS mass (350,000) * Gravitation constant (6.674 ) * 1/(100,000,000,000) / (distance * distance ) &#xA;&lt;br&gt;&lt;b&gt;Distance Moved at Constant Acceleration&lt;/b&gt;&lt;br&gt;&#xA;Acceleration due to Gravity * (time * time)/2 + starting velocity * time &#xA;&lt;br&gt; &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;  &#xA;  slot     time   distance  acceleration velocity &#xA;    1       0      50       .000 000 0093    0&#xA;    2     12 hours 42       .000 000 014    0.0004&#xA;    3     18 hours 29       .000 000 027    0.0007&#xA;    4     24 hours 10       .000 000 29     0.0013&#xA;&#xA;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So in less than 24 hours, the sleeper is dragged to within 10 meters of station and drifting into the station at  1.3 mm per second. Just by &quot;weak&quot; gravity.&#xA;Less than another 1.5 hours to reach the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I apologize, it is late here, and I only calculated the acceleration at each slot, instead of continuous change. The actual results would significantly reduce the time to cross the distance by at least 25%. &lt;/p&gt;&#xA;" OwnerUserId="12131" LastActivityDate="2015-10-30T00:58:21.867" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12495" PostTypeId="1" AcceptedAnswerId="12496" CreationDate="2015-10-30T03:55:04.430" Score="9" ViewCount="663" Body="&lt;p&gt;I was reading about thorium reactors &lt;a href=&quot;http://www.world-nuclear.org/info/current-and-future-generation/thorium/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. It says 'Th-232 is fissionable with fast neutrons of over 1 MeV energy'. Most cosmic rays have &lt;a href=&quot;http://www.srl.caltech.edu/personnel/dick/cos_encyc.html&quot; rel=&quot;noreferrer&quot;&gt;lots more energy&lt;/a&gt; than that. I haven't found a reference for uranium but I suppose it must be similar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I was reading about current NTR designs &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140013259.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The NTR uses a compact fission reactor core containing 93 percent&#xA;  “enriched” Uranium (U)-235 fuel to generate 100’s of megawatts of&#xA;  thermal power (MWt) required to heat the LH2 propellant to high&#xA;  exhaust temperatures for rocket thrust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It mentions elements that sound like maybe they exist to address this issue, but they don't make sense to me:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Multiple control drums, located in the reflector region surrounding&#xA;  the reactor core, regulate the neutron population and reactor power&#xA;  level over the NTR’s operational lifetime. The internal neutron and&#xA;  gamma radiation shield, located within the engine’s pressure vessel,&#xA;  contains its own interior coolant channels. It is placed between the&#xA;  reactor core and key engine components to prevent excessive radiation&#xA;  heating and material damage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GEJkI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GEJkI.png&quot; alt=&quot;nuclear thermal reactor engine cutaway schematic&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The parts marked shields really don't seem adequate to deal with cosmic rays. So are GCRs not enough to matter? Or maybe they are of little enough significance that minor shielding is all that is needed? Are GCRs considered in such designs?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2017-02-24T10:10:31.667" Title="How are nuclear thermal reactors designed for cosmic rays in space?" Tags="|engine-design|safety|cosmic-radiation|ntr|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12496" PostTypeId="2" ParentId="12495" CreationDate="2015-10-30T04:52:20.103" Score="9" Body="&lt;p&gt;Reactors are, in general, tuned so that the proportion of delayed neutrons (on the order of seconds to minutes after each fissioning) out of all neutrons is enough to make the reactor more or less dynamically stable. (Specifically, the prompt neutrons released within nanoseconds, which are upwards of 95% of all neutrons, aren't enough to match the neutrons &lt;em&gt;consumed&lt;/em&gt;, so the delayed neutrons are essential to sustain the reaction for any length of time.) It takes some time for the delayed-critical chain reaction to speed up or slow down, enough time for the control elements to react automatically. So if there's a larger flux of cosmic rays that knock off more neutrons, nothing special happens: the control drums shift position as normal to compensate and the reaction continues at the same pace. Given the very large flux of neutrons in the core normally, it would take an enormous surge of cosmic rays to cause enough of an increase that the reactor became prompt-critical, i.e. explosive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's more, one of the defining features of most cosmic ray particles is that they are individually very high-energy. That makes them quite dangerous to living creatures, but because the total energy is spread across fewer particles, less dangerous to reactors. A given GCR can trigger a cascade of multiple particles, spreading their energy between them, but the probability of doing so in a way that creates the maximum number of neutrons with enough energy to trigger fissioning is low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://three.jsc.nasa.gov/articles/Heilbronn_Neutron_Supplement.pdf&quot;&gt;There are no neutron GCRs; free neutrons have a half-life of about 15 minutes&lt;/a&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2015-10-30T05:03:36.713" LastActivityDate="2015-10-30T05:03:36.713" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12497" PostTypeId="2" ParentId="12476" CreationDate="2015-10-30T06:32:47.250" Score="-2" Body="&lt;p&gt;Currently there are thousands to millions of objects orbiting around earth's surface. These items are just as &quot;acceptable&quot; as Saturn's rings. Whether everything will stay there or come down to earth depends on the object.&lt;/p&gt;&#xA;" OwnerUserId="12153" LastActivityDate="2015-10-30T06:32:47.250" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12498" PostTypeId="1" AcceptedAnswerId="12499" CreationDate="2015-10-30T07:13:07.337" Score="15" ViewCount="2479" Body="&lt;p&gt;Recently I learned that the Soyuz crew ship can take up to three days to reach ISS. I know that most of the time the travel takes only about 6 hours, but still:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case something happens, you have to spend about 3 days in relatively small ship. It looks to me really small to do anything else than sit in your chair.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if you have to choose 3 day travel to ISS, how and where would you sleep?&lt;/p&gt;&#xA;" OwnerUserId="5754" LastEditorUserId="512" LastEditDate="2015-10-30T16:01:40.513" LastActivityDate="2015-10-30T16:01:40.513" Title="How do astronauts and cosmonauts sleep on a Soyuz?" Tags="|soyuz-spacecraft|sleep|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12499" PostTypeId="2" ParentId="12498" CreationDate="2015-10-30T11:19:37.163" Score="21" Body="&lt;p&gt;There are two modules in the Soyuz that are habitable.  The Soyuz stack is three modules in total. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DDKoa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DDKoa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;The crew launches in the middle module, which is also where they land and has the heat shield on its bottom.  The bottom module is the propulsion modules. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The top module is the Utility module and has the washroom facilities and the airlock. They sleep in the Utility module, while the non sleepers stay in the descent module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, the 6 hour ISS rendezvous is very new and only been used in the last few years, the normal transit time is 3 days. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-10-30T11:19:37.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12503" PostTypeId="1" AcceptedAnswerId="12505" CreationDate="2015-10-30T13:25:11.187" Score="8" ViewCount="776" Body="&lt;p&gt;I would like to know the performance for escape mission of the Ariane 5 launcher. More precisely, I would like &quot;LV Performance vs. C 3&quot; curve.&#xA;My problem is, I can find this info for a lot of launchers but not Ariane. Does anybody have such plot or, at least the value for low C3 (around 0-4)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2fSLp.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2fSLp.png&quot; alt=&quot;Soyuz curve&quot;&gt;&lt;/a&gt;&#xA;Here the kind of plot I would like. This one is for Soyuz and I found it in the &lt;a href=&quot;http://www.arianespace.com/launch-services-soyuz/Soyuz_Users_Manual_CSG_June06.pdf&quot; rel=&quot;noreferrer&quot;&gt;Soyuz user's manual&lt;/a&gt; but this is not in the &lt;a href=&quot;http://www.arianespace.com/launch-services-ariane5/Ariane5_users_manual_Issue5_July2011.pdf&quot; rel=&quot;noreferrer&quot;&gt;Ariane one's&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12060" LastEditorUserId="49" LastEditDate="2015-10-30T15:37:57.490" LastActivityDate="2015-10-30T20:09:22.043" Title="Ariane 5 performance data for escape missions" Tags="|interplanetary|reference-request|ariane|performance|specifications|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12505" PostTypeId="2" ParentId="12503" CreationDate="2015-10-30T14:09:45.503" Score="6" Body="&lt;p&gt;The Ariane 5 user manual has the following data: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Using a storable propellant upper stage, through a delayed ignition of this upper stage, Ariane 5, in the A5G version, has demonstrated its ability to carry a satellite weighing 3065 kg, leading to a total required performance of 3190 kg, towards the following earth escape orbit:&lt;br&gt;&#xA;   - infinite velocity V∞ = 3545 m/s&lt;br&gt;&#xA;   - declination δ = - 2°&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The typical Ariane 5ECA performance on a similar orbit is 4.3 t.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No graph though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;C$_3$ = (V∞)$^2$. And V∞ is related to payload mass via the rocket equation, so you should be able to draw a plot using this one data point. I've no time to do that today though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-10-30T20:09:22.043" LastActivityDate="2015-10-30T20:09:22.043" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12506" PostTypeId="1" AcceptedAnswerId="12509" CreationDate="2015-10-30T14:20:16.750" Score="12" ViewCount="4341" Body="&lt;p&gt;I'm attempting to do some basic planet position calculations using the JPL ephemeris data. The first step is to properly parse the files. Here's my understanding so far:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The header file gives a matrix, for example:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; 3   171   231   309   342   366   387   405   423   441   753   819   899&#xA;14    10    13    11     8     7     6     6     6    13    11    10    10&#xA; 4     2     2     1     1     1     1     1     1     8     2     4     4&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The documentation says that this can be interpreted as follows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Word (1,i) is the starting location in each data record of the chebychev &#xA;  coefficients belonging to the ith item.  Word (2,i) is the number of chebychev &#xA;  coefficients per component of the ith item, and Word (3,i) is the number of &#xA;  complete sets of coefficients in each data record for the ith item.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And later on:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first two double precision words in each data record contain the &#xA;  Julian date of earliest data in record and the Julian date of latest data in record.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The remaining data are chebychev position coefficients for each component of &#xA;  each body on the tape.  The chebychev coefficients for the planets represent &#xA;  the solar system barycentric positions of the centers of the planetary systems.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;There are three Cartesian components (x, y, z), for each of the items #1-11&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So each planet has $N$ coefficients in each of $n$ subintervals for each of the 3 coordinates ($N = 14$ and $n=4$ for the first planet in the data above). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question now is, what is the order of the coefficients in each subinterval? For example if $X_i^k, Y_i^k$ and $Z_i^k$ are $k$th coefficient for $(x,y,z)$ in the $i$th subinterval, the coefficients could be logically ordered in any of the following ways (and probably others):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$X_1^1, ..., X_1^N, Y_1^1, Y_1^2, ..., Y_1^N, Z_1^1, ..., Z_1^N, X_2^1, ...$&lt;/li&gt;&#xA;&lt;li&gt;$X_1^1, ..., X_1^N, X_2^1, ... X_n^N, Y^1_1, ..., Y_n^N, Z^1_1, ..., Z_n^N$&lt;/li&gt;&#xA;&lt;li&gt;$X_1^1, Y_1^1, Z_1^1, X_1^2, Y_1^2, Z_1^2, ...$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Is there any documentation that goes over this order?&lt;/p&gt;&#xA;" OwnerUserId="11885" LastActivityDate="2020-07-15T15:04:23.653" Title="What is the exact format of the JPL ephemeris files?" Tags="|orbital-mechanics|planet|ephemeris|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12507" PostTypeId="2" ParentId="12506" CreationDate="2015-10-30T15:49:49.853" Score="13" Body="&lt;p&gt;No, the first step is to &lt;a href=&quot;http://naif.jpl.nasa.gov/naif/toolkit.html&quot; rel=&quot;noreferrer&quot;&gt;download the SPICE Toolkit&lt;/a&gt;.  It will read the ephemeris and generate the data you need for you, as well as do &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/&quot; rel=&quot;noreferrer&quot;&gt;much, much more&lt;/a&gt;.  There are many formats used in SPICE kernel files, not just the particular one you're looking at, which is probably a Chebyshev position-only array (type 2).  There are 16 types.  The values can be stored little or big endian.  There can be large comment sections in the file.  There are several bodies in each file.  The bodies will have different degrees of resolution in time.  It will take some work for you to decode the thing yourself, which is just wasted time since the toolkit will do all that for you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Download the toolkit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer to your question, if you have successfully found a single type 2 Chebyshev position-only array record and have arrived at the polynomial coefficients (having read MID and RADIUS for that record), is that: the sequence of doubles starts with the constant term of the polynomial for $x$, and continues with the linear coefficient of $t$, until that polynomial is complete, followed by the same thing for $y$ and $z$.  The doubles are IEEE 64-bit floating point numbers in little or big-endian order depending on what it says in the header.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$t$ is ephemeris time in seconds, and the results are in km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The format is documented &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/req/spk.html#Type%202:%20Chebyshev%20(position%20only)&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However you should download the toolkit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-10-30T20:35:00.903" LastActivityDate="2015-10-30T20:35:00.903" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12508" PostTypeId="1" CreationDate="2015-10-30T18:29:20.773" Score="13" ViewCount="390" Body="&lt;p&gt;Whilst checking the latest images of the &lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/flybys/enceladus20151028/&quot; rel=&quot;noreferrer&quot;&gt;E-21 flyby&lt;/a&gt; returned by the Cassini probe, I've noticed that some of them show interesting optical phenomena that I'm not sure I can explain;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, on &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=342976&quot; rel=&quot;noreferrer&quot;&gt;this one&lt;/a&gt;, there are two spotlight-like beams of light at the top part of the image, crossing each other just above the Enceladus' plume at the image center:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/h5gQX.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h5gQX.jpg&quot; alt=&quot;Cassini Solstice Mission image of Enceladus plume&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Cassini Solstice Mission image &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=342976&quot; rel=&quot;noreferrer&quot;&gt;N00250060&lt;/a&gt;, taken on October 28, 2015 and pointing towards Enceladus (non-filtered)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My thinking is that the beam going from the top left towards image center is &lt;a href=&quot;https://en.wikipedia.org/wiki/Forward_scatter&quot; rel=&quot;noreferrer&quot;&gt;forward scattered light&lt;/a&gt; of Saturn shine through the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Saturn#E_Ring&quot; rel=&quot;noreferrer&quot;&gt;E Ring&lt;/a&gt;, and the beam going from top right towards image center could be &lt;a href=&quot;https://en.wikipedia.org/wiki/Coherent_backscattering&quot; rel=&quot;noreferrer&quot;&gt;coherent backscatter&lt;/a&gt; amplified reflection of the Sun by Saturn's main rings, but the latter makes less sense to me as coherent backscatter should be visible when the spacecraft is directly between the Sun and the surface reflecting its light, so maybe there's some other type of light amplification going on there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, since I fail to explain it better and I doubt these beams are actual features of the E Ring as they don't appear in all subsequent images (e.g. they're &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=343002&quot; rel=&quot;noreferrer&quot;&gt;not visible here&lt;/a&gt;), neither do I think these are lens flares or other image artifacts as they still appear on several other images (e.g. &lt;a href=&quot;http://saturn.jpl.nasa.gov/photos/raw/rawimagedetails/index.cfm?imageID=342979&quot; rel=&quot;noreferrer&quot;&gt;this one&lt;/a&gt; only shows one of them, but not the other), here's my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Could these optical effects be explained with the help of &lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/presentposition/&quot; rel=&quot;noreferrer&quot;&gt;Cassini's position relative to Saturn and the Sun&lt;/a&gt; when images they appear on were taken? Or are they some other optical phenomena? Or are they actual tangible features, perhaps the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Saturn#/media/File:PIA17191-SaturnMoon-Enceladus-TendrilSims-20150414.jpg&quot; rel=&quot;noreferrer&quot;&gt;E Ring tendrils&lt;/a&gt;?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="897" LastEditDate="2015-10-30T18:49:54.810" LastActivityDate="2015-11-17T15:37:36.323" Title="What are the &quot;crossing beams of light&quot; in the Cassini images taken during Enceladus plume flyby?" Tags="|flyby|imaging|optical|cassini|enceladus|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12509" PostTypeId="2" ParentId="12506" CreationDate="2015-10-30T18:56:06.630" Score="14" Body="&lt;p&gt;You can parse and use those ascii JPL ephemeris files.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Warning&lt;/strong&gt;: This is an exercise for people who like to torture themselves. If you aren't into self-flagellation, you should do what Mark Adler wrote and use the SPICE toolkit. I went through this self-flagellation over a decade ago, when SPICE was closed and wasn't what it is now. Now people are thinking of killing a thousand lines of code or so by replacing my work with SPICE. And that is exactly what they should do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Consider the file &lt;a href=&quot;ftp://ssd.jpl.nasa.gov/pub/eph/planets/ascii/de405/ascp2200.405&quot;&gt;ascp2200.405&lt;/a&gt;. (I chose that file because it's relatively small.) The first line is&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;     1  1018&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The &quot;1&quot; denotes the block number within the file. The &quot;1018&quot; denotes the number of numbers in the block. The next 340 lines contain those 1018 numbers (plus two more for padding). The first two numbers represent the start and end Julian dates of the block, in JPL's own ephemeris time. If you don't care about millisecond-ish errors, you can use TAI in lieu of JPL's $T_{\text{eph}}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first 168 numbers in that block pertain to Mercury, the next 60 to Venus, the next 78 to the Earth-Moon barycenter, and so on. This is specified in the &lt;code&gt;GROUP 1050&lt;/code&gt; section of the header file. You can deduce the dimensionality of each item from that section. Alternatively, you should know that position is three-dimensional, while librations are two dimensional.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 168 numbers that describe the position of Mercury are organized in four sub-blocks, each comprising 3*14=42 numbers. (Regarding those magic numbers: 14 is from the GROUP 1050 data for Mercury, and 3 is the dimensionality of the position data.) Of those 42 numbers, the first 14 are the coefficients for the 13&lt;sup&gt;th&lt;/sup&gt; order Chebyshev polynomial coefficients that describes the x position, the next 14 describe the y position, and the last 14 describe the z position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those 14 numbers are Chebyshev polynomial coefficients with time (properly scaled) as the independent variable. Chebyshev polynomials are functions limited to the domain [-1,1]. To find the position of Mercury at some time $t$, you'll first need to find the block that pertains to that time. Then you'll need to find the sub-block (Mercury has four sets of coefficients) that pertains to that time. Next you'll have to offset the time from the middle of that sub-block and scale the result so that -1 represents the start of the sub-block and 1 represents the end. Finally, you'll need to compute $\sum_{k=0}^{13} C_n T_n(t)$ for each of the three elements of the position vector, where $C_n$ are the Chebyshev polynomial coefficients and the $T_n(t)$ are the Chebyshev polynomials: $T_0(t)=1$, $T_1(t)=t$, and $T_{n+1}(t) = 2tT_n(t)-T_{n-1}(t)$. The results are in kilometers, except for items pertaining to rotation. This may or may not be what you want.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make matters even more fun (self-flagellation fun, that is), what if you want to know the position of Mars relative to the Earth? The JPL ephemeris file doesn't have coefficients for the Earth. It has coefficients for the Earth-Moon barycenter (relative to the solar system barycenter) and for the Moon (relative to the center of the Earth). You'll need to calculate the position of the Earth by calculating the position of Mars, the position of the Earth-Moon barycenter, and the position of the Moon relative to the Earth. Next you'll need to calculate the position of the Earth (relative to the solar system barycenter) using the Earth-Moon mass ratio (a magic number that's in the header file). With this, you can finally compute the position of Mars relative to the Earth. That's the instantaneous relative position. If you care about where you would see Mars in the sky, you'll need to worry about where Mars was at the time the light reaches you on the Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Alternatively, you could follow Mark Adler's advice: Use the SPICE toolkit.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2015-10-30T19:07:11.793" LastActivityDate="2015-10-30T19:07:11.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12510" PostTypeId="1" AcceptedAnswerId="12511" CreationDate="2015-10-30T21:33:29.537" Score="4" ViewCount="1506" Body="&lt;p&gt;In &lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/the-pod-bay&quot;&gt;The Pod Bay&lt;/a&gt;, we were talking about some pictures that Cassini took of Enceladus recently. We were discussing the &lt;a href=&quot;http://saturn.jpl.nasa.gov/multimedia/images/raw/casJPGFullS91/W00094898.jpg&quot; rel=&quot;nofollow&quot;&gt;calibration pictures&lt;/a&gt; and the filters that the spacecraft uses. A &lt;a href=&quot;http://saturn.jpl.nasa.gov/faq/FAQRawImages/#q15&quot; rel=&quot;nofollow&quot;&gt;NASA FAQ page&lt;/a&gt; for Cassini shows a table of the filters it uses, and I've been trying to understand some of them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right now I'm trying to understand the Continuum Band filters. There is a sentence on the FAQ page that says &quot;For the Continuum Bands, Band 1 and 2 are in the red part of the spectrum while Band 3 is in the infrared.&quot; What is the difference between these filters and the other filters that are just labeled for &quot;methane&quot;, &quot;green&quot;, &quot;infrared&quot;, and so on? For instance, what is the difference between Continuum Band 3 (Infrared) and one of the filters simply labeled &quot;Infrared&quot;?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2015-10-31T04:44:37.927" Title="Cassini's camera continuum band filters" Tags="|photography|camera|cassini|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12511" PostTypeId="2" ParentId="12510" CreationDate="2015-10-30T21:42:27.147" Score="4" Body="&lt;p&gt;They are very narrow &lt;a href=&quot;https://en.wikipedia.org/wiki/Passband&quot; rel=&quot;nofollow noreferrer&quot;&gt;passband&lt;/a&gt; filters with &lt;a href=&quot;https://en.wikipedia.org/wiki/Full_width_at_half_maximum&quot; rel=&quot;nofollow noreferrer&quot;&gt;FWHM&lt;/a&gt; pass-through of usually only a few nanometers and a sharp &lt;a href=&quot;https://en.wikipedia.org/wiki/Cutoff_frequency&quot; rel=&quot;nofollow noreferrer&quot;&gt;cutoff&lt;/a&gt;. Say, a red continuum filter might be centered on ~ 620–740 nm with a FWHM of, say, 5–20 nm. They can be used standalone to only pass through emissivity in that narrow spectral band, or to use produced images as a mask for other exposures and then subtract the continuum band from it, for example to remove background stars from the final image.&lt;/p&gt;&#xA;&lt;p&gt;Other Cassini filters are either wide band, clear (no filtering), polarizing filters, or a &lt;a href=&quot;https://en.wikipedia.org/wiki/H-alpha&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hydrogen Alpha&lt;/a&gt; which is also a single-band filter passing through a narrow band centered at 656.28 nm frequency range, and a Methane Band filter, which is likely a multi-band filter with visible spectrum band at 880-900 nm and a near-IR band centered at ~ 1.3 μm.&lt;/p&gt;&#xA;&lt;p&gt;Cassini uses two filter wheels per camera (narrow angle camera has 12 filters, wide angle camera has 9 filters per wheel) and can combine up to two such filters for each single frame.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;OK, I found a Cassini Imaging Science Subsystem (ISS) reference so we can see how well I did above with my guesstimates. Fairly good I think. Most of it is spot on, some fair (CB continuum band filters aren't really used standalone), and the things that are off are:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;There's a two-lobed narrow-band filter, the CB1. CB1-CB3 serve as continuum filters to MT methane filters from 619 nm (weak) to 898 nm (strong) methane absorption bands.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Methane filters don't go up to 1.3 μm into the near-IR frequency range, but up to 1 μm due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Quantum_efficiency&quot; rel=&quot;nofollow noreferrer&quot;&gt;quantum efficiency&lt;/a&gt; for the ISS of &amp;gt; 1% limiting it to that EM frequency floor. But Cassini also has &lt;a href=&quot;http://saturn.jpl.nasa.gov/spacecraft/cassiniorbiterinstruments/instrumentscassinivims/&quot; rel=&quot;nofollow noreferrer&quot;&gt;VIMS&lt;/a&gt;, Visual and Infrared Mapping Spectrometer, that, while it has smaller spatial resolution than ISS, has better spectral coverage, so there's that.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;From words to figures, the following is all from &lt;a href=&quot;http://ciclops.org/sci/docs/CassiniImagingScience.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini Imaging Science: Instrument Characteristics And Anticipated Scientific Investigations At Saturn&lt;/a&gt;, Carlyn C. Porco et al., Space Science Reviews 115:363–497, 2004 (PDF), please refer to the document for more detailed information:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h3&gt;Cassini ISS filter characteristics&lt;/h3&gt;&#xA;&lt;pre&gt;&lt;code&gt;Filter  λcen,NAC  λeff,NAC  λcen,WAC  λeff,WAC  Science justification&#xA;------------------------------------------------------------------------------------------&#xA;UV1     258W      264       –         –         Aerosols&#xA;UV2     298W      306       –         –         Aerosols, broad-band color&#xA;UV3     338W      343       –         –         Aerosols, broad-band color, polarization&#xA;VIO     –         –         420SP     420       Broad-band color&#xA;BL2     440M      441       –         –         Medium-band color, polarization&#xA;BL1     451W      455       460W      463       Broad-band color&#xA;GRN     568W      569       567W      568       Broad-band color&#xA;MT1     619N      619       –         –         Methane band, vertical sounding&#xA;CB1     619N      619       –         –         Two-lobed continuum for MT1&#xA;CB1a    635       635       –         –&#xA;CB1b    603       603       –         –&#xA;RED     650W      649       648W      647       Broad-band color&#xA;HAL     656N      656       656N      656       H-alpha/lightning&#xA;MT2     727N      727       728N      728       Methane band, vertical sounding&#xA;CB2     750N      750       752N      752       Continuum for MT2&#xA;IR1     752W      750       742W      740       Broad-band color&#xA;IR2     862W      861       853W      852       Broad-band color; ring absorption band&#xA;MT3     889N      889       890N      890       Methane band, vertical sounding&#xA;CB3     938N      938       939N      939       Continuum for MT3; see thru Titan haze&#xA;IR3     930W      928       918W      917       Broad-band color&#xA;IR4     1002LP    1001      1001LP    1000      Broad-band color&#xA;IR5      –        –         1028LP    1027      Broad-band color&#xA;CL1     611       651       635       634       Wide open, combine with wheel 2 filters&#xA;CL2     611       651       635       634       Wide open, combine with wheel 1 filters&#xA;P0      617       633       –         –         Visible polarization, 0°&#xA;P60     617       633       –         –         Visible polarization, 60°&#xA;P120    617       633       –         –         Visible polarization, 120°&#xA;IRP0    746       738       705       705       IR polarization; see through Titan haze&#xA;IRP90   –         –         705       705       IR polarization; see through Titan haze&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;strong&gt;TABLE  VIII&lt;/strong&gt;: All wavelengths in nm. Central wavelengths (‘cen’) are computed&#xA;using the full system transmission function. These numbers are taken&#xA;to be the numerical name assigned to the filter. Effective wavelengths&#xA;(‘eff’) are computed using the full system transmission function&#xA;convolved with a solar spectrum. Bandpass types: SP: short wavelength&#xA;cutoff; W: wide; N: narrow; LP: long wavelength cutoff.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Cassini ISS WAC &amp;amp; NAC Broad-band and Medium-band Filters&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EoaIV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EoaIV.png&quot; alt=&quot;Cassini ISS WAC &amp;amp; NAC Broad-band and Medium-band Filters&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Figure 20&lt;/strong&gt;: System transmission functions for NAC and WAC broad- and&#xA;medium-band filters. Filters for the NAC are as follows: UV1, UV2, UV3&#xA;(purple: solid, dotted, and dashed); BL1, BL2 (blue: solid and&#xA;dashed); GRN (green); RED, (red); IR1, IR2, IR3, and IR4 (orange:&#xA;solid, dot-dash, dashed, and dotted). For the WAC: VIO (purple), BL1&#xA;(blue), GRN (green), RED (red), IR1, IR2, IR3, IR4 and IR5 (orange:&#xA;solid, dashed, dotted, and the latter two not shown). The CL1/CL2&#xA;filter combination is given by the solid black line in both plots.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Cassini ISS WAC &amp;amp; NAC Narrow-band Filters&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iDnve.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iDnve.png&quot; alt=&quot;Cassini ISS WAC &amp;amp; NAC Narrow-band Filters&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Figure 21&lt;/strong&gt;: System transmission functions for the NAC and WAC narrow-band filters. NAC filters are as follows: MT1, MT2, MT3&#xA;(purple: solid, dotted, and dashed); CB1, CB2, CB3 (green: solid,&#xA;dotted, and dashed); and HAL (red). WAC filters are HAL (red), MT2,&#xA;MT3 (purple: solid and dashed); CB2 and CB3 (green: solid and dashed).&#xA;The geometric albedo of Titan is also given (solid black) to&#xA;illustrate the placement of the methane band and continuum filters&#xA;relative to the methane features in the spectrum of Titan.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h3&gt;Cassini ISS WAC &amp;amp; NAC Polarizing Filters&lt;/h3&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zUHGB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zUHGB.png&quot; alt=&quot;Cassini ISS WAC &amp;amp; NAC Polarizing Filters&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Figure 22&lt;/strong&gt;: System transmission functions for the NAC visible and infrared, and the WAC infrared, polarizers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Some additional explanations can also be found in &lt;a href=&quot;http://www.sciencemag.org/content/suppl/2003/03/06/299.5612.1541.DC1/Porco.SOM.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini Imaging of Jupiter's Atmosphere, Satellites and Rings - Supplemental Online Material&lt;/a&gt;, C. Porco et al. (PDF).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-10-31T04:44:37.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12512" PostTypeId="1" AcceptedAnswerId="12515" CreationDate="2015-10-31T13:56:42.663" Score="5" ViewCount="794" Body="&lt;p&gt;Suppose we detect an object on an orbit that will pass close to Earth, entering our SOI, but escaping after just a few days.  If we wanted to mount a mission to intercept that object, how would we go about determining the most delta-v efficient trajectory to accomplish it?  Full disclosure, this is a question inspired by a &lt;a href=&quot;https://gaming.stackexchange.com/questions/241535/mounting-a-time-limited-rescue-mission-for-kerbal-inbound-to-kerbin&quot;&gt;situation in a game&lt;/a&gt;, but it seemed like it might be an interesting situation in real life as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, have we ever actually done a real mission like this?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;My current thought is that, intuitively, you'd want to time it so that your interceptor's apoapsis intersects the periapsis of the inbound object, then match velocity at closest approach, but I have no idea if this is actually the most efficient way to do it.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Per request for more information (estimating how the game's situation would map to more real world units):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Periapsis of inbound object to Earth is about 0.17 lunar distances.&lt;/li&gt;&#xA;&lt;li&gt;Inbound orbital inclination (relative to sun) is 0.168 degrees.&lt;/li&gt;&#xA;&lt;li&gt;Eccentricity of orbit is 0.388.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I'm willing to do some math to figure things out, but I'm not sure where to get started.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Per request, here are images from the game situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar orbit of the inbound object (teal arc is inbound, violet is the estimated post-encounter orbit):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zfcOM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zfcOM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbit near Kerbin:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QR9gn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QR9gn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12168" LastEditorUserId="-1" LastEditDate="2017-04-13T12:10:06.060" LastActivityDate="2015-10-31T19:00:40.033" Title="Intercepting an object on flyby past Earth?" Tags="|orbital-mechanics|orbital-maneuver|kerbal-space-program|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12513" PostTypeId="1" CreationDate="2015-10-31T15:09:53.307" Score="19" ViewCount="4766" Body="&lt;p&gt;The ignite command for the Solid Rocket Boosters (SRB) would not be issued unless &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/srb/ignition.html&quot;&gt;all three Space Shuttle Main Engines (SSME) were at least 90% of thrust&lt;/a&gt;. The launch would have been aborted if one or more of the SSMEs failed to reach required performance before T-minus zero. Abort was possible because the SSMEs could be shutdown after starting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRBs, of course, couldn't be shutdown after ignition. Once the SRB ignition command was issued, abort was impossible (unless maybe both SRBs failed to ignite?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If only one ignited, what would have happened?&lt;/p&gt;&#xA;" OwnerUserId="12169" LastEditorUserId="49" LastEditDate="2015-10-31T15:58:11.990" LastActivityDate="2021-01-14T03:09:50.330" Title="What would have happened if only one Space Shuttle SRB ignited?" Tags="|launch|crewed-spaceflight|space-shuttle|failure|launch-sequence|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12514" PostTypeId="2" ParentId="12513" CreationDate="2015-10-31T15:52:29.597" Score="8" Body="&lt;p&gt;Everything I can find says the crew would have died. Depending on which one lit, the space shuttle would have either ended up in the ocean, or smacking the Launch Control Center. Of course, before either of those events would happen, the Flight Termination System would have been activated, which would have caused all 3 of the rocket stages to explode. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if somehow the SRBs could be ejected when thrusting, the orbiter didn't have enough thrust to lift itself up without SRBs at the launch pad. As a result, the orbiter would have crashed in to the ground, which while it might have been at least slightly more likely to survive than if the FTS had been activated, it still would have almost certainly resulted in a very bad day.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-11-03T11:33:21.033" LastActivityDate="2016-11-03T11:33:21.033" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="12515" PostTypeId="2" ParentId="12512" CreationDate="2015-10-31T18:34:36.740" Score="3" Body="&lt;p&gt;The flyby would probably not be the best time to intercept (Unless manned). Usually the point where the object is furthest from the Sun is the key point to intercept. Specifically, a Hohmann transfer orbit. Of course, if the object has a particularly high inclination, it might help to intercept close to Earth. Of course, for these, a fly-by of Earth actually works better to adjust the inclination to a suitable approach vector for the asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a manned mission, you would do something like the KSP path. The optimal point would probably be the point of closest approach. You would set up an orbit such that you were moving in the same direction (but not velocity) as the asteroid, and then accelerate near the point of closest approach to intercept the asteroid. This is exactly the optimal KSP mission, BTW.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Kerbal Space program, it's much easier to intercept close because planning a maneuver for that lengthy of a time in advance is rather challenging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for real life, the closest I'm aware of is the &lt;a href=&quot;http://www.asteroidmission.org/&quot; rel=&quot;nofollow&quot;&gt;Osiris-Rex&lt;/a&gt; mission, which will put a transmitter on an asteroid destined to pass close to Earth in about 200 years.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2015-10-31T19:00:40.033" LastActivityDate="2015-10-31T19:00:40.033" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12519" PostTypeId="2" ParentId="1144" CreationDate="2015-11-01T04:15:04.763" Score="7" Body="&lt;p&gt;It was DoD, not NASA, but there was a post-NERVA nuclear rocket research program named &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Timberwind&quot; rel=&quot;noreferrer&quot;&gt;Project TIMBERWIND&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The summary of the project at its conclusion can be gleaned from this &lt;a href=&quot;https://web.archive.org/web/20130408131611/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA305996&quot; rel=&quot;noreferrer&quot;&gt;report&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12174" LastEditorUserId="168" LastEditDate="2015-11-02T07:10:06.537" LastActivityDate="2015-11-02T07:10:06.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12520" PostTypeId="1" CreationDate="2015-11-01T07:11:18.827" Score="5" ViewCount="2775" Body="&lt;p&gt;If an asteroid collided with the Moon, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_A119&quot; rel=&quot;nofollow noreferrer&quot;&gt;an atomic bomb was blasted on it&lt;/a&gt;, would it affect the orbit of the Moon? The Moon's gravity plays a major role in ocean tides. If the Moon's orbit changed, what kind of consequences would the Earth experience? Would the Moon remain in an altered orbit, or would it precess and hit the Earth, or even recess out of Earth's orbit?&lt;/p&gt;&#xA;" OwnerUserId="12113" LastEditorUserId="7062" LastEditDate="2022-10-15T17:31:57.657" LastActivityDate="2022-12-22T06:09:10.410" Title="Would an asteroid collision affect the Moon's orbit, and what consequence would that have for Earth?" Tags="|the-moon|asteroid|earth|celestial-mechanics|tides|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="12521" PostTypeId="2" ParentId="12520" CreationDate="2015-11-01T10:57:43.840" Score="12" Body="&lt;p&gt;The Moon's mass is 7.34 × 10&lt;span class=&quot;math-container&quot;&gt;$^{22}$&lt;/span&gt; kg. For an asteroid to have a noticeable effect, its mass must be in that ballpark.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the asteroid mass needed also depends on its speed (kinetic energy).&lt;/li&gt;&#xA;&lt;li&gt;what time frame do you want to consider? On a timescale of a billion years, a tiny asteroid will have measurable effects: the change in orbit will be tiny, but over millions of orbits that change will add up.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;A very big asteroid would be needed to produce effects that would be noticeable in the short term. An asteroid big enough to affect the Moon's orbit would break off large chunks of Moon, which would result in a meteor bombardment on Earth (see Neal Stephenson's novel Seveneves for an example of that), so we'd have more immediate concerns than its influence on tides.&lt;/p&gt;&#xA;&lt;p&gt;A bomb can displace &lt;a href=&quot;https://en.wikipedia.org/wiki/Operation_Plowshare&quot; rel=&quot;nofollow noreferrer&quot;&gt;~100 tons of soil per ton of TNT&lt;/a&gt;. So detonating a 1 MT nuclear bomb would displace (optimistically, rock will be harder to move than soil) 100 MT or 10&lt;span class=&quot;math-container&quot;&gt;$^{11}$&lt;/span&gt; kg of rock, or a fraction of 10&lt;span class=&quot;math-container&quot;&gt;$^{-11}$&lt;/span&gt; of the moon's mass. Also, most of that mass would stay on the Moon, so the total gravitational pull of the Moon would remain the same. The blast would exert some thrust on the Moon, so its orbit could be altered. But again that effect would be tiny, just going by the orders of magnitude we're considering here.&lt;/p&gt;&#xA;&lt;p&gt;What would happen to the Moon really depends on the details: a retrograde impact slows the moon down and widens its orbit, a prograde impact speeds it up and tightens its orbit. To get the Moon to collide with Earth, I'd expect an asteroid the size of the Moon would be needed, or lots of patience.&lt;/p&gt;&#xA;&lt;p&gt;As it is, the Moon recedes slowly: the tides rob orbital energy from the Moon. Small effects like detonating a bomb would be swamped by this effect.&lt;/p&gt;&#xA;&lt;p&gt;A wider orbit means smaller tides.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="7062" LastEditDate="2022-10-15T17:41:15.993" LastActivityDate="2022-10-15T17:41:15.993" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="12522" PostTypeId="1" AcceptedAnswerId="12527" CreationDate="2015-11-01T20:41:33.100" Score="12" ViewCount="1165" Body="&lt;p&gt;I am working on the on-board computer for a CubeSat. Our computer will be vulnerable to radiation, hence single event upsets, e.g. bit flips are likely to occur. Would a lighter, smaller OS like FreeRTOS bring more stability and a lower probability of failure over a full-blown Linux operating system? &lt;/p&gt;&#xA;" OwnerUserId="8692" LastActivityDate="2016-01-08T20:50:39.700" Title="RTOS vs Linux against single event upsets" Tags="|radiation|cubesat|flight-computer|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12523" PostTypeId="1" CreationDate="2015-11-01T22:33:30.043" Score="5" ViewCount="2745" Body="&lt;p&gt;I just read an &lt;a href=&quot;http://space.about.com/od/Space-and-Astronomy-Star-Trek/a/Matter-Antimatter-Power.htm&quot; rel=&quot;nofollow&quot;&gt;article&lt;/a&gt; by John P. Millis, Ph.D which says that antimatter is created in the Van Allen radiation belts. I am writing a novel where a future starship with FTL capability, powered by a matter/antimatter power plant, needs to harvest antimatter from the radiation belts around a distant Earth-like planet in another planetary system. How might that happen?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think I'll create a large gas giant similar but larger than Saturn in my fictional universe. I should be able to collect more anti-protons in a much stronger radiation belt. I found a great article on anti-matter as it pertains to propulsion &lt;a href=&quot;http://www.centauri-dreams.org/?p=1569&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="9046" LastEditorUserId="9046" LastEditDate="2015-11-04T21:55:40.243" LastActivityDate="2023-03-28T19:33:19.383" Title="Collecting antimatter from Van Allen radiation belts" Tags="|interstellar-travel|advanced-propulsion|isru|antimatter|ftl|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12524" PostTypeId="2" ParentId="12522" CreationDate="2015-11-01T22:49:13.837" Score="7" Body="&lt;p&gt;I don't know about the probability, but the RTOS will reboot a lot faster. When you run into a problem, and you will run into a problem, you reboot, and you want that to be fast.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-11-02T03:45:11.523" LastActivityDate="2015-11-02T03:45:11.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12525" PostTypeId="2" ParentId="12523" CreationDate="2015-11-01T22:56:35.800" Score="24" Body="&lt;p&gt;The &lt;em&gt;total&lt;/em&gt; antimatter in the van Allen belts is estimated to be &lt;a href=&quot;https://astronomynow.com/news/n1108/19antimatter/&quot; rel=&quot;nofollow noreferrer&quot;&gt;160 nanograms&lt;/a&gt;. Annihilating that with matter would produce a whopping 8 kW-hr of energy. A quarter of a gallon of gasoline has that much energy. The star ship would be better off getting a quick spurt from a gas station pump before heading out.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2023-03-28T19:33:19.383" LastActivityDate="2023-03-28T19:33:19.383" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="12526" PostTypeId="2" ParentId="12392" CreationDate="2015-11-02T00:46:15.220" Score="7" Body="&lt;p&gt;Section 4.1 on page 8 of the &lt;a href=&quot;http://arxiv.org/pdf/1509.03622v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;original report&lt;/a&gt; on the phenomenon discards this possibility, albeit indirectly:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Kepler light curve for KIC 8462852 is unique, and we have&#xA;thoroughly explored the raw data for defects/instrumental effects,&#xA;which could cause the observed variations in KIC 8462852’s flux. We&#xA;use the P Y K E software tools for Kepler data analysis to check the&#xA;data for instrumental effects. We check the following possibilities:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;We checked that the same flux variations, i.e., the ‘dips’, are&#xA;present in the SAP FLUX data set&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;We verified that data gaps and&#xA;cosmic rays events do not co- incide with the dipping events, as&#xA;they are prone to produce glitches in the corrected fluxes.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;We&#xA;verified at the pixel-level that there are no signs of peculiar&#xA;photometric masks used in making the light curves.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;We verified at&#xA;the pixel level that the image light centroid does not shift during&#xA;the ‘dipping’ events&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;We inspected light curves of neighboring&#xA;sources and find that they do not show similar variability patterns in&#xA;their light curves.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;We determined that CCD cross talk and reflection&#xA;cannot be the cause (Coughlin et al. 2014).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;We verified with the&#xA;Kepler team mission scientists that the data were of good quality.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;This analysis concludes that instrumental effects or artifacts in the&#xA;data reduction are not the cause of the observed dipping events, and&#xA;thus the nature of KIC 8462852’s light curve is astrophysical in&#xA;origin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The light curves obtained by Kepler were consistent with data from the Nordic Optical Telescope (though just for the star, not for the times when there are dips in its light) - section 2.2, page 4 of original report:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;we used the co-added FIES spectrum to determine the stellar effective&#xA;temperature &lt;em&gt;T&lt;sub&gt;eff&lt;/sub&gt;&lt;/em&gt; , surface gravity log &lt;em&gt;g&lt;/em&gt;, projected&#xA;rotational velocity &lt;em&gt;v&lt;/em&gt; sin &lt;em&gt;i&lt;/em&gt;, metal abundance [M/H], and spectral&#xA;type of KIC 8462852... The temperature we derive (&lt;em&gt;T&lt;sub&gt;eff&lt;/sub&gt;&lt;/em&gt; =&#xA;6750 ± 140 K) is consistent with the photometric estimate of&#xA;&lt;em&gt;T&lt;sub&gt;eff&lt;/sub&gt;&lt;/em&gt; = 6584+178 −279 K from the revised Kepler Input Catalog properties (Huber et al. 2014), as well as with&#xA;&lt;em&gt;T&lt;sub&gt;eff&lt;/sub&gt;&lt;/em&gt; = 6780 K derived from the empirical (V − K) color-temperature relation from Boyajian et al. (2013). The projected&#xA;rotational velocity we measure&#xA;&lt;em&gt;v&lt;/em&gt; sin &lt;em&gt;i&lt;/em&gt; = 84 ± 4 km s&lt;sup&gt;−1&lt;/sup&gt; is also well in line with the one predicted from rotation in Section 2.1, if the 0.88 d signal is in&#xA;fact the rotation period&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Several of the verification techniques would have revealed whether the dips were due to Kepler's positioning - the masks would have been out of alignment, other stars would have shown the same phenomenon. There is no known mechanism by which these readings could have been caused by Kepler itself.&lt;/p&gt;&#xA;&lt;p&gt;The pixels of Kepler's CCD are calibrated according to procedures described in &lt;a href=&quot;http://kepler.nasa.gov/science/ForScientists/papersAndDocumentation/SOCpapers/CAL-SPIE_Aug19_copyright.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pixel-Level Calibration in the Kepler Science&#xA;Operations Center Pipeline&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;the flight data is subject to numerous data processing steps that are&#xA;performed in a complex automated pipeline... [it] is composed of a&#xA;number of modules that operate sequentially (Figure 1)... Additional&#xA;modules operate in parallel to monitor the instrument performance and&#xA;provide target management... The raw data include photometric (target&#xA;and background) pixels, along with a subset of the CCD termed&#xA;“collateral data” which includes masked and virtual (over-clocked)&#xA;rows and columns that are used primarily for calibration... Many of&#xA;the primary corrections use external models of each CCD that were&#xA;developed from pre-flight hardware tests and FFI data taken during&#xA;commissioning...  We discuss how these models are applied within CAL&#xA;to correct for 2D bias structure, gain and nonlinearity of the&#xA;conversion from analog-to-digital units (ADU) to photoelectrons, local&#xA;detector electronics effects (undershoot and overshoot), and flat&#xA;field (variations in pixel sensitivity). Other signals that are&#xA;corrected include excess charge from saturated stars that leak into&#xA;the masked and virtual regions, cosmic ray events, dark current and&#xA;smear.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZYmkk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZYmkk.png&quot; alt=&quot;flowchart of automated procedures to calibrate Kepler's CCD&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-11-04T17:26:07.913" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12527" PostTypeId="2" ParentId="12522" CreationDate="2015-11-02T02:54:32.310" Score="9" Body="&lt;p&gt;In general, you want as simple of an OS for any embedded systems application. You want as few features as you can get away with. You are less likely to run into problems that way, don't really need all of the features, and it'll reboot quicker, and use less memory. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two things you want for sure. You want a realtime operating system, you probably want threads and similar protections, but you probably don't want device drivers, for instance, especially for GUIs. &lt;a href=&quot;https://en.wikipedia.org/wiki/Rodos_%28operating_system%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rodos&lt;/a&gt; is one OS I found in my search that meets the criteria, I'm sure there are others. RTOS is a reasonable choice as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BTW, as a bonus tip, there are several architectures used to keep satellites safe in case of an emergency. See &lt;a href=&quot;https://space.stackexchange.com/q/3637/25&quot;&gt;How does one build software for a Satellite such that a new build doesn&amp;#39;t break it?&lt;/a&gt; , for instance.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-11-02T02:54:32.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12528" PostTypeId="1" AcceptedAnswerId="12532" CreationDate="2015-11-02T10:21:42.977" Score="28" ViewCount="3668" Body="&lt;p&gt;I was looking at &lt;a href=&quot;http://www.alexras.info/code/orbital_objects/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this 3D map of satellites orbiting earth&lt;/a&gt; and noticed something interesting. Around the green ring of active geosynchronous satellites, there is a ribbon of what I assume are obsolete geosynchronous satellites. The strange thing is that this ring is skewed - above the Atlantic Ocean, most inactive satellites are positioned north of the active ones, while they are positioned to the south above the Pacific Ocean. Why is that? I'd expect the satellites to &quot;average out&quot; and form a band north and south of the equator regardless of longitude.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Not very clear when viewed at reduced magnification, open in new window so the pixel-sized spacecraft are clearer... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fLYDb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fLYDb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or check &lt;a href=&quot;http://apps.agi.com/SatelliteViewer/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site's representation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sUPpb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sUPpb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12186" LastEditorUserId="12102" LastEditDate="2019-07-19T23:24:58.120" LastActivityDate="2020-02-08T01:44:11.910" Title="Why is the ribbon of decommissioned geosynchronous satellites skewed?" Tags="|orbital-mechanics|artificial-satellite|debris|geosynchronous|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="12529" PostTypeId="2" ParentId="12528" CreationDate="2015-11-02T11:58:15.940" Score="8" Body="&lt;p&gt;You'll see those satellites drift over the equator every 12 hours. They are almost geosynchronous instead of geostationary, i.e. they are in an orbit that's 24 hours and a few minutes long, and inclined instead of right on the equator. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those decommissioned satellites are in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Graveyard_orbit&quot; rel=&quot;noreferrer&quot;&gt;graveyard orbit&lt;/a&gt;, 235+ km above the geostationary orbit. After decommissioning, they can no longer maneuver to stay in their orbit so they'll slowly drift away from the equator and to the ecliptic plane under the influence of the Sun's and Moon's gravity.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-11-03T07:31:59.677" LastActivityDate="2015-11-03T07:31:59.677" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12530" PostTypeId="1" AcceptedAnswerId="12537" CreationDate="2015-11-02T14:33:39.847" Score="4" ViewCount="143" Body="&lt;p&gt;&lt;a href=&quot;http://arxiv.org/abs/1510.07704&quot; rel=&quot;nofollow noreferrer&quot;&gt;The image of Hydra&lt;/a&gt; recently released shows a sharply edged black half oval cutting deep into its bottom as in the image below. Is it dark sky behind an open gap we see, or just unreflective matter on that surface patch? If the latter, could it be similar with the upper right black part of it too, that gives it its apparent shape of an axes head? I've not read or heard any comments about this yet. Is it likely to be figured out from New Horizon data?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dYhVr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dYhVr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(B) LORRI panchromatic (350 to 850 nm) image of Hydra at 1.13&#xA;  km/pixel, solar phase angle 34°&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;roughly 43 km × 33 km&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The feature then I suppose is about 6-9 km wide and high, as seen. But with pixels at 1 km its regularity can maybe not be trusted. Hydra is about 10 times as large as comet 69/P, and looks almost more incredibly irregularly shaped.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2015-11-02T16:00:18.463" LastActivityDate="2015-11-03T07:45:50.727" Title="Is Hydra of Pluto cut unusually, or just darkened on its surface?" Tags="|planetary-science|new-horizons|pluto|moons|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12531" PostTypeId="2" ParentId="12530" CreationDate="2015-11-02T15:58:26.177" Score="2" Body="&lt;p&gt;It seems like it is dark, probably due to a hole (But not all the way through). I imagine I see a dark spot, but not quite the same levels as the black of space. So it's either a dark spot, or a non-penetrating hole, which I suspect the later to be the truth.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-11-02T15:58:26.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12532" PostTypeId="2" ParentId="12528" CreationDate="2015-11-02T19:57:27.497" Score="31" Body="&lt;p&gt;The apparent source of the data (as it is linked in the source linked in the description of the author of this map) is &lt;a href=&quot;http://apps.agi.com/SatelliteViewer/&quot;&gt;http://apps.agi.com/SatelliteViewer/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this animated view you can see that these are the real orbits: Satellites that currently are in the northern hemisphere show up in the southern hemisphere half a day later. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As active station keeping is not possible for decommissioned satellites, their orbit will change more with every year they stay in orbit. &#xA;One of the main influences is the sun: The GEO, aligned with the equator of Earth, is inclined by 23° with respect to the sun. Due to the gravitational pull of the sun, the inclination increases slowly until the orbit of the satellite is in the same plane as Earths' orbit around the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_station-keeping#Station-keeping_in_geostationary_orbit&quot;&gt;Wikipedia says:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the effect of the lunar/solar gravitation [...] perturbs the orbit&#xA;  pole with typically 0.85 degrees per year&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2015-11-02T21:26:54.820" LastActivityDate="2015-11-02T21:26:54.820" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12533" PostTypeId="2" ParentId="12528" CreationDate="2015-11-03T00:10:04.437" Score="15" Body="&lt;p&gt;Here is a stab at explaining the &quot;average out&quot; part of the question, hopefully tying together some of the points brought up in the other answers and comments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites that are not undergoing control of their orbital inclination will gradually adopt both an inclination that increases up to ~15 degrees (and after a long time, back down to 0 deg) and also a progressive change of Right Ascension of the Ascending Node (RAAN). This is driven by a combination of lunar and solar perturbations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't matter whether they are at geosynchronous radius or in the normal graveyard, they are all affected. Operational satellites have to expend a lot of propellant to remain geostationary. As an aside, whilst re-orbiting (graveyarding) practices have improved over the years there are hundreds of abandoned satellites in the operational zone. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is easier to visualise the combined behaviour of inclination and RAAN by imagining that the satellite's orbit is a spinning top with an axis at the middle which we'll call the orbit pole. In a controlled geo&lt;strong&gt;stationary&lt;/strong&gt; orbit the orbit pole is closely aligned with the Earth's spin axis.  In a drifting orbit this pole traces out a circle whose own centre is ~7.5 degrees from the polar axis over a period of ~56 years. The inclination and RAAN can be thought of as the magnitude and direction of the orbit pole (OK, but for an inconvenient 90 degrees which I will gloss over, it's just a metaphor). Thus the maximum change in inclination is from 0 to about 15 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key point of the question was, as I understand it, &quot;why are all these satellites 'in phase'&quot;?  It is because they all adopt the same spinning top motion, i.e. all the orbit poles are in the process of tracing out the same big circle offset from the Earth's pole in the same direction in inertial space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that at any instant all those satellites at similar longitudes will be at the same point in their approx daily cycle of latitude, north or south of the equator.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For all those that, at some time of day, are at their maximum latitude:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;90 degrees of longitude around the equator there are others at 0 degrees latitude &lt;/li&gt;&#xA;&lt;li&gt;and another 90 degrees further away there are more that are at there greatest negative latitude&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The idea of it being related to the Atlantic and Pacific however is just a co-incidence of the snapshot. The orbit pole motion is relative to inertial space under which the Earth turns regardless.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="6863" LastEditDate="2020-02-08T01:44:11.910" LastActivityDate="2020-02-08T01:44:11.910" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="12534" PostTypeId="1" CreationDate="2015-11-03T00:44:26.003" Score="3" ViewCount="609" Body="&lt;p&gt;So I understand what was said &lt;a href=&quot;https://space.stackexchange.com/questions/2993/how-did-nasa-achieve-their-live-tv-broadcast-in-1969&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/9168/why-wasnt-there-a-delay-when-neil-landed-on-the-moon/9170#9170&quot;&gt;here&lt;/a&gt;, but not everything is clear to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they were able to transmit from the Moon, then why in 1985 were we not able to have &lt;a href=&quot;https://en.wikipedia.org/wiki/Dancing_in_the_Street#David_Bowie_and_Mick_Jagger_version&quot; rel=&quot;nofollow noreferrer&quot;&gt;an audio/video song&lt;/a&gt; performed by Mick Jagger in Philadelphia and David Bowie in London.&lt;/p&gt;&#xA;" OwnerUserId="12197" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-11-03T15:43:51.727" Title="Why were we able to have a more or less real-time conversation with the Moon?" Tags="|the-moon|communication|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12535" PostTypeId="2" ParentId="12520" CreationDate="2015-11-03T02:13:36.620" Score="5" Body="&lt;p&gt;Strangely enough, an asteroid missing the moon by a small distance might actually deflect the moon by more than a collision!&lt;/p&gt;&#xA;&lt;p&gt;When a small body moves past a larger body, it's direction is changed by 100%*, meaning that its momentum has been changed by almost double the original falling momentum. This momentum must be conserved, so the moon must gain twice the momentum of the falling asteroid.&lt;/p&gt;&#xA;&lt;p&gt;You can think of it as the same effect as if the asteroid was made of rubber and bounced off the moon, as compared to being made of putty (or rock) and sticking to it.  A putty (or rock) asteroid collision transfers its momentum, but loses kinetic energy in the form of deformation, heat, shock. An asteroid that bounces off without expending energy in the collision contributes more energy to the moon's motion.&lt;/p&gt;&#xA;&lt;p&gt;*&lt;em&gt;I should say &amp;quot;up to 100%&amp;quot;, because 100% is only if the asteroid starts the interaction with negligible relative speed. I'm talking about the optimal case, here.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12198" LastEditorUserId="12198" LastEditDate="2022-12-22T06:09:10.410" LastActivityDate="2022-12-22T06:09:10.410" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="12537" PostTypeId="2" ParentId="12530" CreationDate="2015-11-03T07:40:08.840" Score="3" Body="&lt;p&gt;&lt;a href=&quot;https://blogs.nasa.gov/pluto/2015/10/05/plutos-small-moons-nix-and-hydra/&quot; rel=&quot;nofollow noreferrer&quot;&gt;According to New Horizons team&lt;/a&gt;, it is most likely a bilobed body produced by a slow collision of two older moons, and is somewhat similar in shape to &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow noreferrer&quot;&gt;comet 67P/Churyumov-Gerasimenko&lt;/a&gt;. Here's an enhanced version of same image you attach to your question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/Wokq8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wokq8.png&quot; alt=&quot;Pluto&amp;#39;s moon Hydra as seen by New Horizons&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Pluto's moon Hydra as seen from NASA's New Horizons spacecraft, July 14, 2015. Credit: NASA/JHUAPL/SwRI&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This processing reveals a couple of subtle differences to the raw image. Mainly, with respect to your question, the lower dark blotch appears less sharp (despite the image being sharpened, likely &lt;a href=&quot;https://en.wikipedia.org/wiki/Deconvolution&quot; rel=&quot;nofollow noreferrer&quot;&gt;deconvoluted&lt;/a&gt;) and the front and back lobe edges don't align any more, suggesting it is a saddle-shaped col more than a cut-through impact site or a darkened, lower albedo surface feature. A sharp (sharper but somewhat shorter after processing) graben-like antipodal depression and no global feature seamlessly transitioning between both lobes further suggest that Hydra might indeed be a result of a low speed collision of two smaller bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Either that, or it's a rather big figurine of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Chow_Chow&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chow Chow&lt;/a&gt;. :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-11-03T07:45:50.727" LastActivityDate="2015-11-03T07:45:50.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12538" PostTypeId="2" ParentId="12523" CreationDate="2015-11-03T10:01:42.100" Score="5" Body="&lt;p&gt;The Enterprise uses Bussard-Collectors to harvest hydrogen from interstellar space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the Enterprise is science fiction, the Bussard-Collector - or more the Bussard ramjet&lt;sup&gt;1&lt;/sup&gt; - is a theoretical concept from a physicist named Bussard from 1960. In this concept, the spacecraft uses very strong EM fields to deflect and focus the hydrogen it's flying through into a (nuclear) reactor. Inside this reactor, the hydrogen is heated (accelerated) and jetted out backwards, which results in forward thrust.&lt;br&gt;&#xA;However, Bussard's concept is more about how much hydrogen must be collected, how much it must be heated, and what initial speed is needed. It does not say anything about the field geometry, its strength or how to generate it.&lt;br&gt;&#xA;It has to be noted that the fields must be incredible strong, because a not charged hydrogen molecule would interact with this fields only due to its (comparable weak) dipole momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About anti-matter: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Matter consists of elementary particles, and for each of this particle types, an anti-type exists.&lt;br&gt;&#xA;Positrons as anti-particle of the electrons are the most prominent ones and are for example released during a nuclear decay mechanism called inverse beta decay. Sodium-22 is an isotope which decays this way and is used PET, a medical diagnostics method. (Yes, anti-matter in your body!)&lt;br&gt;&#xA;Neutrons and protons are not elementary, but consist of quarks. If you manage to put together three quarks, each being the counter-part of a quark in a neutron, you would get an anti-neutron. But this is really hard to achieve, due to physical constraints.&lt;br&gt;&#xA;And once you have an anti-proton, it's also hard to combine it with a positron to an anti-hydrogen atom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://arxiv.org/PS_cache/arxiv/pdf/1107/1107.4882v1.pdf&quot; rel=&quot;nofollow&quot;&gt;PAMELA experiment&lt;/a&gt; states that the ratio of anti-protons to protons is 1:10000 in the van Allen belt. As to my knowledge, there are about 11g of protons in the belt, there should be 1.1mg of anti-protons. If you collect all of them, you get the energy a 1GW power plant delivers in three minutes - yet not so much. &#xA;Other sources state 10µg of anti-protons, and the answer from @Mark Adler states even less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However... if your anti-matter actually consists of anti-protons or positrons (i.e. charged particles), it's (a little) easier to collect them. The fields must not be that strong, and it's easy to separate protons, anti-protons, electrons and positrons by their mass and charge. May be, you read about mass spectrometers to get an idea on how this might work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Storing antimatter is another issue. If it's charged, you can use magnetic mirrors - in fact, the van Allen belt &lt;em&gt;is&lt;/em&gt; a giant magnetic mirror.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As said, the amount of antimatter in the van Allen belt of the earth is quite small, you may have more luck with an other planet around an other star. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stars also don't contain much anti-matter, but (as Mark Adler wrote), it may be possible (and is subject to current research), that there are larger amounts of pure anti-matter somewhere out there.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;) Today, a ramjet is a special type of aircraft engine without any rotating parts. Air is compressed by ram pressure at the inlet only, not by a fan. Fuel is added and burned, and the hot gas leaves through the exhaust nozzle, providing thrust. But the aircraft must already be moving, before this engine can be started. When it is, it allows extreme high speeds. &lt;/p&gt;&#xA;" OwnerUserId="11749" LastEditorUserId="11749" LastEditDate="2015-11-03T21:22:53.600" LastActivityDate="2015-11-03T21:22:53.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12539" PostTypeId="1" AcceptedAnswerId="26074" CreationDate="2015-11-03T12:27:03.597" Score="7" ViewCount="1085" Body="&lt;p&gt;Following my previous question about &lt;a href=&quot;https://space.stackexchange.com/questions/12503/ariane-5-performance-data-for-escape-missions&quot;&gt;Ariane 5 performance data for escape missions&lt;/a&gt;, I would like to know how the plot of launch vehicle capabilities like the next one are obtained:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oEOnf.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oEOnf.png&quot; alt=&quot;Capabilities of various Delta IV launchers&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More specifically, I would like to know how I can generate an approximation of these curves by using discrete data like the payload mass for a given C3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My first idea was to use the rocket equation, but there is a lot of unknowns and the assumption that there are no external forces is clearly not respected for low altitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions come down to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What model can I use to generate such a curve?&lt;/li&gt;&#xA;&lt;li&gt;What data are needed?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12060" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2018-03-17T14:40:50.187" Title="Generating plot of LV's performance as a function of the C3 parameter" Tags="|launch|interplanetary|mathematics|performance|statistics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12540" PostTypeId="2" ParentId="12534" CreationDate="2015-11-03T15:41:21.463" Score="13" Body="&lt;p&gt;For two people to sing together, they have to be synchronized. This is difficult when sending a signal from Philadelphia to London takes at least 0.2 seconds one way (*).&lt;br&gt;&#xA;If you have access to an audio delay effect you can try this for yourself.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the live broadcast from the Moon, the delay was even longer, but the people on the Moon weren't trying to sing together with people in Mission Control.&lt;br&gt;&#xA;When you listen to the unedited recordings from Moon broadcasts, you'll notice long delays when the astronauts have to respond to a question from mission control. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;*: it's 0.2 seconds when the signal is sent via a geostationary satellite. If you used a cable link the delay would be shorter, but broadcast-quality cable links were in short supply in 1985. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-11-03T15:43:51.727" LastActivityDate="2015-11-03T15:43:51.727" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12541" PostTypeId="1" AcceptedAnswerId="12544" CreationDate="2015-11-03T18:07:47.540" Score="40" ViewCount="12689" Body="&lt;p&gt;I would like to find out why there is a straight hole down the middle in all solid rocket engine motors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6mxdV.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6mxdV.gif&quot; alt=&quot;The hole is shown here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought it only makes sense in hybrid engines where pure oxygen needs to be blown down the tube.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And why isn't there a hole down the middle in model rocket engines?&lt;/p&gt;&#xA;" OwnerUserId="12205" LastEditorUserId="49" LastEditDate="2015-11-03T23:34:04.443" LastActivityDate="2016-02-17T21:19:21.277" Title="Why is there a hole in solid rocket engines?" Tags="|rockets|fuel|solid-fuel|thrust|flight-profile|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12542" PostTypeId="2" ParentId="12541" CreationDate="2015-11-03T18:12:16.970" Score="37" Body="&lt;p&gt;It depends on the particular engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thrust from a solid rocket is approximately proportional to the burning surface area of the fuel (also called the &lt;em&gt;grain&lt;/em&gt;). A long solid rocket motor with a channel along its length is burning more surface area than an &quot;end-burning&quot; motor, so produces more thrust. Typically solid rocket boosters are used to provide very high levels of thrust at liftoff, so the long channel in the grain is needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the channel is a simple circular bore, the area increases as fuel burns away, making thrust increase over time. This generally isn't what you want, because mass is decreasing as the fuel is spent, so the rocket accelerates too fast. For this reason, other grain profiles are used as described in TildalWave's answer to control the thrust-versus-time curve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Model rockets usually fly with a much higher thrust-to-weight ratio than full-sized orbital rockets - higher than 5:1 TWRs are common for model rockets, while large rockets are often below 1.5:1 at ignition. End-burning grain, therefore, is more than sufficient for them.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-02-17T21:19:21.277" LastActivityDate="2016-02-17T21:19:21.277" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12544" PostTypeId="2" ParentId="12541" CreationDate="2015-11-03T18:32:32.720" Score="78" Body="&lt;p&gt;A bore in the solid propellant grain increases exposed surface area and allows for a higher burn rate to increase thrust. There might be several grain geometries used, to meet launch vehicle's ascent profile needs through grain regression and with it control flow rate as the solid propellant core burns. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid-fuel_rocket#Grain_geometry&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Solid-fuel rocket - Grain geometry&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Solid rocket fuel &lt;a href=&quot;https://en.wikipedia.org/wiki/Deflagrates&quot; rel=&quot;noreferrer&quot;&gt;deflagrates&lt;/a&gt; from the surface of exposed&#xA;  propellant in the combustion chamber. In this fashion, the geometry of&#xA;  the propellant inside the rocket motor plays an important role in the&#xA;  overall motor performance. As the surface of the propellant burns, the&#xA;  shape evolves (a subject of study in internal ballistics), most often&#xA;  changing the propellant surface area exposed to the combustion gases.&#xA;  The &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_flow_rate&quot; rel=&quot;noreferrer&quot;&gt;mass flow rate&lt;/a&gt; (kg/s) [and, therefore, pressure] of&#xA;  combustion gases generated is a function of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Instantaneous&quot; rel=&quot;noreferrer&quot;&gt;instantaneous&lt;/a&gt;&#xA;  &lt;a href=&quot;https://en.wikipedia.org/wiki/Surface_area&quot; rel=&quot;noreferrer&quot;&gt;surface area&lt;/a&gt; $A_s$, (m2), and linear burn rate $b_r$ (m/s):&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$$\dot{m} = \rho \cdot A_s \cdot b_r$$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\dot{m}$ is here a &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_flow_rate&quot; rel=&quot;noreferrer&quot;&gt;mass flow rate&lt;/a&gt; in kg/s,&lt;/li&gt;&#xA;&lt;li&gt;$\rho$  is mass density of the fluid,&lt;/li&gt;&#xA;&lt;li&gt;$A_s$ instantaneous surface area and&lt;/li&gt;&#xA;&lt;li&gt;$b_r$ linear &lt;a href=&quot;https://en.wikipedia.org/wiki/Burn_rate_%28chemistry%29&quot; rel=&quot;noreferrer&quot;&gt;burn rate&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some of the grain geometries and their corresponding &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust_curve&quot; rel=&quot;noreferrer&quot;&gt;thrust curves&lt;/a&gt; might look like below examples:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/BdPrt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BdPrt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Common solid propellant core cross-sections of grain geometries, including circular, finocyl and non-circular bores.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other profiles in use are a C-slot (wedge cut out at the side of the core), Moon burner (off-center circular bore), and so on. Grain geometry can also use three-dimensional cross-sections, usually achieved by stacking two-dimensional cross-section bored core segments one atop the other. E.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5&quot; rel=&quot;noreferrer&quot;&gt;Ariane 5&lt;/a&gt; SRBs (EAP P238 and P241) use such stacked segments to achieve a three-dimensional grain profile, but there are other launch vehicles that use that, too.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2015-11-07T01:12:06.800" LastActivityDate="2015-11-07T01:12:06.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12545" PostTypeId="1" AcceptedAnswerId="12546" CreationDate="2015-11-03T20:49:25.700" Score="24" ViewCount="6960" Body="&lt;p&gt;Why is the Reaction Engines Limited &lt;a href=&quot;https://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29&quot; rel=&quot;noreferrer&quot;&gt;SABRE&lt;/a&gt; (Synergistic Air-Breathing Rocket Engine) curved along it's length?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qwLiD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qwLiD.jpg&quot; alt=&quot;SABRE Engine Diagram&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Illustration of the SABRE engine with the pre-cooler heat exchanger highlighted in blue. Credit: REL/Adrian Mann&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="10890" LastEditorUserId="12102" LastEditDate="2021-01-01T10:52:06.247" LastActivityDate="2021-01-01T10:52:06.247" Title="Why is the SABRE engine curved?" Tags="|engines|engine-design|ssto|ramjet|sabre-engine|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12546" PostTypeId="2" ParentId="12545" CreationDate="2015-11-03T21:10:11.433" Score="25" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mark_Hempsell&quot;&gt;Mark Hempsell&lt;/a&gt;, formerly Future Programmes Director at Reaction Engines Ltd., now CEO of Hempsell Astronautics Ltd., explaining the reason for SABRE's curved nacelle over at &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=22434.80&quot;&gt;NasaSpaceFlight.com forum&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why a Curved nacelle? – the most frequently asked technical question.&#xA;  The answer is: the air intake on the front of the nacelle needs to&#xA;  point directly into the incoming airflow whereas SKYLON’s wings and&#xA;  body need to fly with an angle of incidence to create lift, so the&#xA;  intake points down by 7 degrees to account for this. The rocket thrust&#xA;  chambers in the back of nacelle need to point through the centre of&#xA;  mass of the vehicle so are angled down; again by 7 degrees but it is a&#xA;  coincidence the angle is the same.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-11-03T21:15:22.303" LastActivityDate="2015-11-03T21:15:22.303" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12547" PostTypeId="1" CreationDate="2015-11-04T00:59:43.510" Score="2" ViewCount="732" Body="&lt;h2&gt;Background&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Tsiolkovski's rocket equation says that change of speed is proportional to the exhaust velocity. Chemical propulsion seems to have a strictly limited and since long achieved maximum exhaust velocity in its combustion products. But applied ion propulsion today already has tens of times higher exhaust velocity and, I think, theoretically has no distinct upper limit. The problem with ion propulsion is that it is electric, and solar and nuclear power plants do not produce power at all at such high effect as chemical combustion/explosion does. (Such is my uneducated understanding.)&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Question&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Could chemical combustion be used to power an ion engine to shorten travel times? That instead of gradually capturing the rays of the Sun or the decay of plutonium, an interplanetary spacecraft could accelerate to high speed in minutes instead of during months, by suddenly combusting hydrogen and oxygen, in some kind of thermal chemical power plant on the spacecraft, into electric power for ion acceleration? Joining the suddenness of chemics with the efficiency of ions.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2022-10-22T15:11:49.563" Title="Could chemical energy quicken ion propulsion?" Tags="|propulsion|power|combustion|ion-thruster|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12548" PostTypeId="2" ParentId="12547" CreationDate="2015-11-04T01:46:03.670" Score="4" Body="&lt;p&gt;The idea of using a high-power source of electricity to increase the thrust of an ion thruster is definitely sound, but inefficiency in transforming chemical energy to electrical energy is really the limiting factor. Add to that the complexity of the propulsion system, which translates into increased mass and hence lower overall Delta-V for the same amount of propellant, and you diminish the benefits of using ion propulsion in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the moment, ion thrusters depend on solar or nuclear power supplies for electricity, and those sources have limits on the amount of power available -- therefore limiting your thrust. Larger power supplies could be used to provide higher thrust, so if your hydrogen fuel cell (or other chemical energy converter) can provide higher power at a manageable mass then it would be feasible. However, carrying the chemical propellants to generate that electricity is a major problem -- so maybe your idea would be more suitable for futuristic interstellar spacecraft that could harvest hydrogen and oxygen along the way.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-11-04T01:46:03.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12549" PostTypeId="2" ParentId="12547" CreationDate="2015-11-04T03:08:36.890" Score="7" Body="&lt;p&gt;Hydrogen-oxygen &lt;a href=&quot;https://en.wikipedia.org/wiki/Fuel_cell&quot; rel=&quot;nofollow noreferrer&quot;&gt;fuel cells&lt;/a&gt;, such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid_oxide_fuel_cell&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOFC&lt;/a&gt;, have maximum specific &lt;a href=&quot;https://en.wikipedia.org/wiki/Energy_density&quot; rel=&quot;nofollow noreferrer&quot;&gt;energy density&lt;/a&gt; of ~ 17.9 MJ/kg (using 142 MJ/kg for compressed hydrogen and including oxygen at 1:7.93 2H-O &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_fraction_%28chemistry%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;mass fraction&lt;/a&gt; yielding water at a &lt;a href=&quot;https://en.wikipedia.org/wiki/Stoichiometry&quot; rel=&quot;nofollow noreferrer&quot;&gt;stoichiometrically&lt;/a&gt; perfect &lt;a href=&quot;https://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;air to fuel ratio&lt;/a&gt;). Other chemical components yield lower specific energy density since they're using &lt;a href=&quot;https://space.stackexchange.com/a/5084/49&quot;&gt;higher molar mass&lt;/a&gt; fuels. Incidentally, this is also the reason why LOX/LH2 achieve highest &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific impulse&lt;/a&gt; among chemical rockets. Now, theoretically highest fuel cell efficiency, assuming heat recapture, is at 85–90%. So we're down to maximum theoretical specific energy density of ~ 16 MJ/kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely, plutonium (&lt;a href=&quot;https://en.wikipedia.org/wiki/Plutonium-238&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pu-238&lt;/a&gt;) powered &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;RTGs&lt;/a&gt; have specific energy density of 2,239,000 MJ/kg. If we assume they generate sufficient power for one quarter its half-life of 87.7 years, let's round it up to 20 years, we've made use of 14.6% of its total power density, and efficiency of 7% (pretty good for a RTG, most of its energy ends up as waste heat), we end up with our system power density of 22,916 MJ/kg. That's over 1,400 times more than our best chemical fuels when it comes to power density (which is what matters when it comes to moving mass around).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And it's not that RTGs are that much more efficient, they're not, it's just that chemical energy doesn't come in very mass efficient form. Fuel cells will have some advantages over RTGs, such as being able to dump no longer needed reaction products overboard or even find its use elsewhere (coolant, potable water, biological shielding,...), not having to shield against own fuel's radioactive decay, and so on, but it will also come with disadvantages, such as still requiring thick-walled or heavily heat shielded pressure vessels not to lose too much of it to space due to fuel boil-off, energy efficient fuel cells will still generate a lot of excess heat that needs to be efficiently radiated away, etc. Not that RTGs don't need that, but fuel cells won't have so many advantages over them to offset those 1,400 times the difference in power density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And since we're already discussing nuclear, RTGs are completely overshadowed by nuclear fission reactors with specific energy density of 79,420,000 (thorium) to 80,620,000 (uranium). And there are &lt;a href=&quot;http://www.eoearth.org/view/article/154970/&quot; rel=&quot;nofollow noreferrer&quot;&gt;compact, space application ready nuclear fission reactor designs&lt;/a&gt;, like &lt;a href=&quot;https://en.wikipedia.org/wiki/Safe_Affordable_Fission_Engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;SAFE&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/SP-100&quot; rel=&quot;nofollow noreferrer&quot;&gt;SP-100&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/SNAP-10A&quot; rel=&quot;nofollow noreferrer&quot;&gt;SNAP-10A&lt;/a&gt;, &lt;a href=&quot;http://www.researchgate.net/publication/236531658_Nuclear_Design_of_the_HOMER-15_Mars_Surface_Fission_Reactor&quot; rel=&quot;nofollow noreferrer&quot;&gt;HOMER-15&lt;/a&gt; to name a few, mostly using Heatpipe Power Systems (HPS) design. We just have to overcome &lt;a href=&quot;https://space.stackexchange.com/a/2365/49&quot;&gt;our aversion to anything nuclear&lt;/a&gt; and start regulating their development, testing and use so they're as much of an option as they're safe to use.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-11-04T03:08:36.890" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12550" PostTypeId="1" CreationDate="2015-11-04T03:49:18.373" Score="7" ViewCount="778" Body="&lt;p&gt;It seems that all nuclear propulsion concepts extract fission energy as heat, and somehow convert a portion of that heat to energy in the desired form, such as electrical (e.g. to power an ion thruster). It always amounts to creating heat in one part of the system and dumping waste heat somewhere else. For example: using a working fluid that is heated, perhaps discharged through a turbine, cooled, and sent back to the heat source. As the turbine (for example) extracts mechanical energy from the working fluid, the equivalent heat energy is removed. Nevertheless, there is still a lot of waste heat to be dissipated somewhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: in the higher power concepts, how is all that waste heat removed from the spacecraft? In the vacuum of space, it can only leave by radiation. So wouldn't that require a very large array of radiators? Wouldn't that add a significant amount of mass to the spacecraft, affecting the overall mass efficiency?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a secondary question: would the radiation pressure from &quot;waste&quot; heat for a nuclear powered electric engine be enough to make a worthwhile contribution to spacecraft thrust?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2019-02-13T06:51:21.103" Title="For a hypothetical high-power nuclear spacecraft, where does the waste heat go?" Tags="|nuclear|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12551" PostTypeId="2" ParentId="12550" CreationDate="2015-11-04T05:31:00.873" Score="8" Body="&lt;p&gt;As you stated, radiation is the only method of removing excess heat from a conventional spacecraft in vacuum. Designing a spacecraft to radiate heat appropriately is important and yes, it does add mass to the overall thermal control system. The best way to reduce that mass is to try and ensure the spacecraft doesn't &lt;em&gt;need&lt;/em&gt; to radiate as much heat. For example, heat is desirable for maintaining electronics, propellant, or other components within required temperature ranges, so taking advantage of a nuclear source to provide that heat makes it an asset instead of waste (or at least some of it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, if you are talking about a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot;&gt;nuclear thermal rocket engine&lt;/a&gt; then most of the heat generated by the nuclear source is ejected along with the exhaust. (See Deer Hunter's comments below.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For your secondary question: using heat radiation pressure for thrust has definitely been considered (check out &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_photonic_rocket&quot;&gt;nuclear photonic propulsion&lt;/a&gt;). Also, that kind of effect has been observed as a perturbation on spacecraft already (see the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_anomaly&quot;&gt;Pioneer effect&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-11-04T11:31:38.803" LastActivityDate="2015-11-04T11:31:38.803" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12552" PostTypeId="1" AcceptedAnswerId="12553" CreationDate="2015-11-04T06:00:16.450" Score="39" ViewCount="14439" Body="&lt;p&gt;If you've been reading the news lately, you've probably been reading about &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylon_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Skylon&lt;/a&gt; and it being &lt;strong&gt;Single Stage To Orbit&lt;/strong&gt; (SSTO).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've had a think back and to the best of my knowledge humanity has always used multi-stage rockets. Am I wrong? Has humanity ever launched to Earth orbit carrying the initial stage?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what the technical definition of a &quot;stage&quot; is but I take it to mean jettisoning anything other than propellant - so engines, tanks, etc.&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="198" LastEditDate="2015-11-11T10:47:59.600" LastActivityDate="2015-11-11T10:47:59.600" Title="Has humanity launched a SSTO ... ever?" Tags="|record|ssto|" AnswerCount="7" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12553" PostTypeId="2" ParentId="12552" CreationDate="2015-11-04T07:30:38.500" Score="72" Body="&lt;p&gt;Yes, technically humans have launched to orbit with a single stage -- the Apollo lunar excursion modules launched from the Moon's surface to lunar orbit using a single stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for Earth orbit, there have not been any true single-stage-to-orbit launches since the vehicle should not jettison any components to be considered true SSTO. However, both the Atlas-B and Space Shuttle did carry the first stage to orbit in some sense (sometimes referred to as &quot;stage-and-a-half&quot; configuration). The Atlas-B is definitely the closest to SSTO since it only jettisoned booster engines (not entire stages or strap-on boosters). The Space Shuttle Orbiter contributes its main engines to the entire flight so you could definitely say it is part of the first stage and of course makes it to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: the Atlas-B was pointed out by 2012rcampion and Mark Adler, the Space Shuttle was pointed out by Organic Marble.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-11-05T02:02:24.360" LastActivityDate="2015-11-05T02:02:24.360" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12554" PostTypeId="1" AcceptedAnswerId="12561" CreationDate="2015-11-04T10:19:35.117" Score="11" ViewCount="2891" Body="&lt;p&gt;I am interested in what the surface of Venus looks like in true color, but have discovered that true color images of Venusian surface are rare, and am hoping that a lander mission is planned to take surface photos of Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have found that, up to now, most missions to Venus were flybys, orbiters, and some of these included atmospheric probes. But, are there currently any planned lander or rover missions to Venus?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="49" LastEditDate="2015-11-04T12:48:27.160" LastActivityDate="2020-12-28T19:09:03.563" Title="Are there currently any plans for a Venus lander?" Tags="|rovers|venus|future-missions|lander|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12555" PostTypeId="2" ParentId="9412" CreationDate="2015-11-04T10:24:56.350" Score="2" Body="&lt;h2&gt;Interplanetary launch windows&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The efficiency of interplanetary trips - the fuel and time required - varies significantly in time due to orbital mechanics. This means that if we assume some far future with significant traffic between, say, Earth and Mars, then we would expect to see most of ships in one direction being launched in the same few days where the trip is efficient, and then a pause for many months with trips made only if absolutely neccessary, as they require far more fuel and thus a different ship.&lt;/p&gt;&#xA;" OwnerUserId="2667" LastActivityDate="2015-11-04T10:24:56.350" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12556" PostTypeId="1" AcceptedAnswerId="12564" CreationDate="2015-11-04T11:45:23.693" Score="13" ViewCount="16450" Body="&lt;p&gt;Looking at Venus, Earth and Mars, the closer you are to the Sun the warmer you are.  At the same time the warmer planets also have thicker atmospheres.&lt;/p&gt;&#xA;&lt;div class=&quot;s-table-container&quot;&gt;&#xA;&lt;table class=&quot;s-table&quot;&gt;&#xA;&lt;thead&gt;&#xA;&lt;tr&gt;&#xA;&lt;th&gt;Planet&lt;/th&gt;&#xA;&lt;th&gt;Minimum&lt;/th&gt;&#xA;&lt;th&gt;Maximum&lt;/th&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/thead&gt;&#xA;&lt;tbody&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Venus&lt;/td&gt;&#xA;&lt;td&gt;870 °F  (465 °C)&lt;/td&gt;&#xA;&lt;td&gt;870 °F  (465 °C)&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Earth&lt;/td&gt;&#xA;&lt;td&gt;-129 °F  (- 89 °C)&lt;/td&gt;&#xA;&lt;td&gt;136 °F  (58 °C)&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;Mars&lt;/td&gt;&#xA;&lt;td&gt;-195 °F  (- 125 °C)&lt;/td&gt;&#xA;&lt;td&gt;70 °F  (20 °C)&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/tbody&gt;&#xA;&lt;/table&gt;&#xA;&lt;/div&gt;&#xA;&lt;p&gt;Source for temperatures in the table: &lt;a href=&quot;http://earthguide.ucsd.edu/eoc/special_topics/teach/sp_climate_change/p_planet_temp.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;earthguide.ucsd.edu&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Mars does not have an significant &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth%27s_magnetic_field&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetic field&lt;/a&gt; which is problematic on several levels for terraforming. There are several &lt;a href=&quot;https://space.stackexchange.com/questions/634&quot;&gt;issue with keeping an atmosphere on Mars&lt;/a&gt; and some &lt;a href=&quot;https://space.stackexchange.com/questions/2423/&quot;&gt;potential solutions.&lt;/a&gt; For this question please assume the issues with maintaining an atmosphere are addressed.&lt;/p&gt;&#xA;&lt;p&gt;Given sufficient atmosphere, would Mars have temperature ranges similar to Earth? If so, would the atmosphere required to keep it warm, be problematic for any human to breathe?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="7062" LastEditDate="2022-06-16T11:47:06.920" LastActivityDate="2023-05-12T04:28:08.420" Title="If Mars had a denser atmosphere, how warm would it be?" Tags="|mars|planetary-science|atmosphere|temperature|terraforming|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="12557" PostTypeId="2" ParentId="12552" CreationDate="2015-11-04T12:06:11.850" Score="9" Body="&lt;p&gt;You could say that Shuttle was a single stage to orbit...albeit with very large strap-on boosters! The concept we knew as Shuttle was originally called TAOS for Thrust Augmented Orbital Shuttle.  This was genned up after the original fully reusable concepts turned out to be unaffordable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it does meet your criterion of &quot;launching to orbit carrying the inital stage&quot;.  Just not the strap-ons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But...a craft that launches from Earth, and arrives in orbit without shedding any structure, only consuming propellant....no. That has never been done.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-11-04T12:11:39.047" LastActivityDate="2015-11-04T12:11:39.047" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12558" PostTypeId="1" AcceptedAnswerId="12562" CreationDate="2015-11-04T12:31:11.190" Score="12" ViewCount="859" Body="&lt;p&gt;I read up articles on both types - EVA suits meant for spacewalks, and Escape Suits used in launch and reentry - but other than bright orange color for ACES making it easy to find and the fact that Sokol doesn't have life-support backpacks (only due to room limitations though, not as a desirable design choice) I couldn't really pinpoint which differences between EVA and Escape suits are just arbitrary engineering choices and which ones are deliberate distinction due to their intended purpose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - what design considerations and engineering details set an escape suit apart from an EVA suit?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2015-11-04T14:42:11.017" LastActivityDate="2015-11-20T17:09:24.707" Title="What are the requirements differences between Escape Suit and EVA Suit?" Tags="|crewed-spaceflight|reentry|spacesuits|eva|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12559" PostTypeId="2" ParentId="12552" CreationDate="2015-11-04T12:54:27.770" Score="11" Body="&lt;p&gt;No, humanity has never before launched to Earth orbit using single stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's practically impossible to reach the dV required if you carry all the oxygen with you. Skylon is hydrogen-oxygen propelled. In this propellant system the oxygen weighs 8 times as much as the hydrogen (with a chemically balanced mix), if you carry it all with you. (Rocket engines tend to run a bit fuel rich and oxygen poor but anyway, that's a big difference.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By breathing air for the first part of the ascent, one can gain significant velocity and height before one switches to oxygen from the tank. This is how Skylon makes the single stage to orbit concept theoretically possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the engine concept is critical to the success of the vehicle and there are very few organizations in the world with experience with such engines.&lt;/p&gt;&#xA;" OwnerUserId="6117" LastActivityDate="2015-11-04T12:54:27.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12560" PostTypeId="2" ParentId="12552" CreationDate="2015-11-04T13:28:21.950" Score="26" Body="&lt;p&gt;The closest from Earth was the Atlas B, which I would call a 1.25 stage-to-orbit.  It got to orbit with a single set of extremely light propellant tanks.  It launched with three engines fed by those tanks, and dropped two of the engines on the way up.  Several Mercury missions were on the Atlas (Atlas D).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would call the Space Shuttle a 1.5 stage to orbit, where it went all the way to orbit on the same main propulsion system and tanks, but dropped two entire booster propulsion systems on the way up, which were a significant fraction of the mass at launch.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-11-04T20:06:44.190" LastActivityDate="2015-11-04T20:06:44.190" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="12561" PostTypeId="2" ParentId="12554" CreationDate="2015-11-04T13:42:40.930" Score="12" Body="&lt;p&gt;NASA's New Frontiers program has been calling for a Venus lander (&lt;a href=&quot;http://www.lpi.usra.edu/vexag/VISE/&quot;&gt;Venus In Situ Explorer&lt;/a&gt;) on every solicitation.  Venus landers are proposed every time, sometimes making it to Step 2 in the competition (e.g. &lt;a href=&quot;https://solarsystem.nasa.gov/docs/pr450.pdf&quot;&gt;SAGE&lt;/a&gt;), but a Venus lander has not been selected yet.  Maybe next time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/v13_vg261_262.html&quot;&gt;color images of the surface of Venus from Venera&lt;/a&gt;.  I'm not sure what &quot;true color&quot; would be at the surface of Venus, since there's so little light it is essentially equivalent to early night on Earth.  Our color vision is diminished in low light, so a human on the surface of Venus (soon to be dead) would not see a lot of color without a bright flashlight.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-11-04T13:42:40.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12562" PostTypeId="2" ParentId="12558" CreationDate="2015-11-04T14:41:52.947" Score="9" Body="&lt;p&gt;The main consideration is the lifetime. An escape suit only needs to function for maybe 10 minutes. While they do need some life support provided through launch, it can easily be disconnected if required. For only 10 minutes, you only need a small supply of oxygen, a thinner suit will do, that is more mobile, etc. Also, the reentry suits are designed to be small, they have to fit in the usually small reentry space, while larger suits are okay for EVAs. Heat probably isn't a major concern, one can survive okay for 10 minutes if required, although they do still use a similar system to EVA suits for cooling. The suit also needs to be able to be put on quickly. They are designed for more shock absorption than an EVA suit (See &lt;a href=&quot;https://en.wikipedia.org/wiki/Advanced_Crew_Escape_Suit#Design&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An EVA suit has to last for a longer time, usually around 8 hours. There has to be some kind of water/nourishment is such a process. Harder work is also required, along with higher levels of redundancy, to sustain such a lengthy excursion. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-11-04T14:41:52.947" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12564" PostTypeId="2" ParentId="12556" CreationDate="2015-11-04T14:54:03.200" Score="16" Body="&lt;p&gt;Other factors being equal, planetary temperature (measured on an absolute scale e.g. degrees Kelvin) falls with the square root of distance from the sun, so a merely Earth-like atmosphere isn't enough to warm Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;http://www.astro.indiana.edu/gsimonel/temperature1.html&quot;&gt;planetary temperature calculator&lt;/a&gt;, an Earth-like planet (similar albedo and greenhouse-effect atmosphere) at Mars's orbital distance would have an average surface temperature of -40ºC as compared to Earth's 15ºC. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can check this; Mars' semi-major axis is 1.52 AU, Earth's average temperature is about 288K (15ºC). So we expect:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{288K}{\sqrt{1.52}} = 233K = -40ºC$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmosphere would moderate the extreme high and low temperatures on Mars substantially, though. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars's albedo (reflectivity) is slightly lower than Earth, which would increase temperatures a few degrees above that, but not enough to make it a comfortable place. If you increased the greenhouse gases in the atmosphere substantially (~5 times the greenhouse effect seen on Earth) the temperature would be similar to Earth's. There's probably a mixture of gases which would provide that effect while remaining human-breathable, but I couldn't tell you what it was. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Producing and maintaining that atmosphere, of course, is far beyond our capabilities today.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-11-04T15:12:27.513" LastActivityDate="2015-11-04T15:12:27.513" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12565" PostTypeId="2" ParentId="12552" CreationDate="2015-11-04T16:09:34.287" Score="2" Body="&lt;p&gt;No, but the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_Martin_X-33&quot; rel=&quot;nofollow noreferrer&quot;&gt;X-33&lt;/a&gt; almost did it. Its 120 tons of starting weight contained around 95 tons of fuel...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kIrcVm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kIrcVm.jpg&quot; alt=&quot;Lockheed X-33&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1054" LastActivityDate="2015-11-04T16:09:34.287" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="12566" PostTypeId="2" ParentId="12554" CreationDate="2015-11-04T17:06:46.200" Score="11" Body="&lt;p&gt;The Russian &lt;a href=&quot;https://en.wikipedia.org/wiki/Venera&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera missions&lt;/a&gt; 9 (1975), 10 (1975), 13 (1982), and 14 (1982) all took images of the Venusian surface. Venera 13 and 14 took color images.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LnoLy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LnoLy.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Z5oJf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z5oJf.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Russia is also planning another Venera mission called &lt;a href=&quot;https://en.wikipedia.org/wiki/Venera-D&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera-D&lt;/a&gt;. It was initially proposed in 2003, but the initial proposal has gone through several changes. The plan is currently much smaller than it was before. From Wikipedia:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In its original conception, it had a large orbiter, sub-satellite, two balloons, two small landers, and a large, long-lived lander.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Now the plan is to send only an orbiter with a subsatellite orbiter, and the lander. Initially the lander was considered long-lasting because it was supposed to survive for 30 days. The current plan is that the lander should survive about 3 hours. Right now it's planned to be launched in 2024.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="38577" LastEditDate="2020-12-28T19:09:03.563" LastActivityDate="2020-12-28T19:09:03.563" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12567" PostTypeId="1" CreationDate="2015-11-04T17:39:32.943" Score="4" ViewCount="960" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FNyaF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FNyaF.jpg&quot; alt=&quot;Stanford Torus&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Artist's conception of a Stanford Torus. [&lt;a href=&quot;http://new3.fjcdn.com/comments/Reminded+me+of+this+wallpaper+_aa5fd8ac5435376a76469a8fe81758a6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;]&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a Stanford Torus of radius $ 4km $, with the inner radius of the torus ring being $1km$. Disregard structural stability issues. To simulate $1g$ of centripetal acceleration, it would rotate at around $0.5 rpm$ generating a rotational speed of ~$200m/s$ at the edge of the torus. This structure is then filled with air to maintain $1 atm$ of pressure, with a mixture of gases matching Earth's own atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What would happen if the hull is punctured? How rapidly would air escape from the structure? How would this variable change depending on the size and location of the puncture? How long would it take for the entire structure to fully decompress?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-12-05T23:35:30.903" Title="How rapid would air loss be in the event that a supermassive torus is punctured?" Tags="|space-station|artificial-gravity|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12568" PostTypeId="1" AcceptedAnswerId="12581" CreationDate="2015-11-04T18:21:20.943" Score="4" ViewCount="200" Body="&lt;p&gt;Imagine a self-contained city in space with a usable surface area of $50 sq. km.$ (we're talking about the distant future, of course). Over 20,000 residents, industries like agriculture and fisheries, and reasonably humid air to maintain comfort. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We're now faced with the task of plumbing this behemoth. My rough calculations estimate that around 50 million litres of water must be &lt;em&gt;stored&lt;/em&gt; at all times to sustain the station for at least a month in the event of a failure of some sort. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During normal operation, the water would be constantly replenished, using techniques similar to those on the ISS. Moisture will be extracted from the air, waste, and other by-products, and be filtered and added to system again. Some water will also be manufactured on the station itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How should the plumbing be managed on a station like this? Should there be constant replenishment of the water supply through the means discussed above? Are my estimates for water consumption correct? How should drinking water be managed?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1028" LastActivityDate="2015-11-05T14:51:03.053" Title="How would one go about plumbing in a large space outpost?" Tags="|space-station|water|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12569" PostTypeId="2" ParentId="12541" CreationDate="2015-11-04T18:44:31.377" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;And why isn't there a hole down the middle in model rocket engines?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Model rocket engines are available in many &quot;sizes&quot;--varying thrust and duration.  One configuration has the hole down the middle.  They do make a lot of thrust for a short time. These were designed to be used for booster stages.&lt;/p&gt;&#xA;" OwnerUserId="12220" LastActivityDate="2015-11-04T18:44:31.377" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12570" PostTypeId="1" CreationDate="2015-11-04T19:34:35.263" Score="2" ViewCount="92" Body="&lt;p&gt;I've started looking into the use of compressible foams used in space (specially in approximately 500 km SS orbit). I've tried to find some form of heritage or proof of concept missions that have used low density compressible foam, but I struggle to find anything of merit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any search term that relates to this topic that's specific to space that I should be searching for? I don't usually find it this hard t find things that do exist, but I'd be surprised if this didn't exist.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-11-04T19:34:35.263" Title="Compressible foam in space" Tags="|mission-design|payload|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12571" PostTypeId="2" ParentId="12541" CreationDate="2015-11-04T21:41:10.700" Score="0" Body="&lt;p&gt;Fwoosh instead of boom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Others noted that the model engines may or may not have a bore, and that bores may or may not be round. To that I'll point out that some of the bores aren't actually cylindrical, either. Various tapers are used, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's to maximize thrust, certainly, but that's not &lt;em&gt;just&lt;/em&gt; burn rate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have to toss mass away from (as in separate and remove, creating a reactive force) or smack mass into the mass you want to move. Gases wandering about don't help, excepting as they increase the efficiency of the parts that &lt;em&gt;are&lt;/em&gt;. (or happen to hit the mass appropriately) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, the other biggie is shaping the flume. With a naked burning cylinder end, you do get thrust, but the lateral expansion is almost utterly wasted. With burning from the center out (or plugged with &lt;em&gt;appropriate&lt;/em&gt; nozzle) that lateral expansion increases the pressure of the main body of the flume and, thus, the thrust. (Any nozzle used needs to be fitted to this purpose correctly or it will get popped right off.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check out &quot;shaped charge&quot; for further edification. See also why guns have a chamber. These are all linked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Ed.: My friend pointed out that insulation against the burn is quite important, as well, to prevent the destruction of the shaft and also reduce the benefit of the nozzle.)&lt;/p&gt;&#xA;" OwnerUserId="12224" LastEditorUserId="12224" LastEditDate="2015-11-06T05:07:01.413" LastActivityDate="2015-11-06T05:07:01.413" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12572" PostTypeId="2" ParentId="8674" CreationDate="2015-11-04T22:21:42.227" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/pdf/148072main_DART_mishap_overview.pdf&quot; rel=&quot;nofollow&quot;&gt;DART&lt;/a&gt;, or Demonstration for Autonomous Rendezvous Technology, had a &lt;strong&gt;low speed&lt;/strong&gt; &lt;a href=&quot;https://www.nasa.gov/mission_pages/dart/main/&quot; rel=&quot;nofollow&quot;&gt;collision&lt;/a&gt; with MUBLCOM arising from a problematic autonomous rendezvous manoeuvre. &#xA;The following details are only from &lt;a href=&quot;https://en.wikipedia.org/wiki/DART_%28satellite%29&quot; rel=&quot;nofollow&quot;&gt;Wiki&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two satellites were launched separately. Both were US owned satellites, MUBLCOM being owned by DARPA, DART by NASA. The collision apparently occurred at about 1.5 m/s ( ~ 3 miles per hour), DART had a mass of 360kg and MUBLCOM about 50kg and the satellites both apparebtly survived the event without damage though other factors lead to the rendezvous manoeuvres being abandoned.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2015-11-05T23:23:28.107" LastActivityDate="2015-11-05T23:23:28.107" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12573" PostTypeId="2" ParentId="12568" CreationDate="2015-11-04T23:38:55.273" Score="2" Body="&lt;p&gt;It probably wouldn't be all that different from how things are on Earth. Any water storage would probably happen in some kind of lakes. For a space city of that size, I would assume that one would have sufficient land for some lakes, which would bring food, regulate the atmosphere, and otherwise help the station to thrive. As you mentioned, it would be captured from all sources, etc. Such a station will have multiple redundant facilities, but at least to some extent things will happen as on Earth. With artificial gravity, water will flow somewhat similarly as on Earth, but will have some slight difficulties due to the spinning of the station. Still, this will result in some slight correction depending on which way the water flows. The treatment will probably be similar to what is done on Earth, but to a higher extent.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-11-04T23:38:55.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12574" PostTypeId="2" ParentId="7929" CreationDate="2015-11-05T03:06:01.173" Score="8" Body="&lt;p&gt;One of the key control algorithms involved is called G-FOLD, or Guidance for Fuel-Optimal Large Diverts. This is something that JPL has been working on for Mars landings, and some people involved have actually went to work for SpaceX a few years ago.&#xA;Here is a high level description:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4193.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4193.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And you can find more here :&#xA;&lt;a href=&quot;https://scholar.google.com/citations?hl=en&amp;amp;user=xZvK9z0AAAAJ&amp;amp;view_op=list_works&amp;amp;sortby=pubdate&quot; rel=&quot;noreferrer&quot;&gt;https://scholar.google.com/citations?hl=en&amp;amp;user=xZvK9z0AAAAJ&amp;amp;view_op=list_works&amp;amp;sortby=pubdate&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2015-11-05T03:06:01.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12576" PostTypeId="1" AcceptedAnswerId="12577" CreationDate="2015-11-05T11:57:09.567" Score="35" ViewCount="14275" Body="&lt;p&gt;When we think of terraforming a body in our galaxy &lt;a href=&quot;https://space.stackexchange.com/questions/tagged/mars+terraforming&quot;&gt;Mars gets lots of consideration&lt;/a&gt; as the closest potential body.  But what about the Moon?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is currently beyond our ability to add an atmosphere to a planet or a moon, but it is not impossible, and will likely be done in the future. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we add an atmosphere to Earth's Moon, how stable would it be? Would the Earth's Gravity steal it? Would the &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth%27s_magnetic_field&quot; rel=&quot;noreferrer&quot;&gt;Earth's magnetic field&lt;/a&gt; protect it? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2023-05-24T16:17:05.857" Title="Could the Moon keep an atmosphere?" Tags="|the-moon|planetary-science|atmosphere|terraforming|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12577" PostTypeId="2" ParentId="12576" CreationDate="2015-11-05T13:55:25.943" Score="41" Body="&lt;p&gt;It can keep an atmosphere, and &lt;a href=&quot;https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphere.html#.VjtdQ0ZlxNU&quot; rel=&quot;nofollow noreferrer&quot;&gt;in fact does&lt;/a&gt;. The atmosphere is something akin to a high grade Earth-based vacuum. But that's probably not what you are looking for.&lt;/p&gt;&#xA;&lt;p&gt;What would happen with an Earth sized atmosphere on the Moon? A lot of really interesting things would happen actually. First of all, in longest days of the night, the atmosphere might freeze. It would also have a high wind volume, at the day/night terminator. It would lose a lot of its atmosphere (See &lt;a href=&quot;https://physics.stackexchange.com/questions/31909/why-is-titan-able-to-maintain-an-atmosphere-and-not-luna&quot;&gt;this Physics Question&lt;/a&gt;) due to Solar Wind and simple lose by having too much energy.&lt;/p&gt;&#xA;&lt;p&gt;Bottom line - the atmosphere would stay - but only for maybe 1000 years (See &lt;a href=&quot;http://www.universetoday.com/121140/could-we-terraform-the-moon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;). Domes are probably the best way for the foreseeable future on the Moon.&lt;/p&gt;&#xA;&lt;p&gt;To illustrate this, Wikipedia provides a &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_escape&quot; rel=&quot;nofollow noreferrer&quot;&gt;really neat chart&lt;/a&gt; showing atmospheric particles that would stick vs temperature and size.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kcZuz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kcZuz.png&quot; alt=&quot;Chart illustrating what molecules will stay vs temperature and size&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="51366" LastEditDate="2023-05-24T16:17:05.857" LastActivityDate="2023-05-24T16:17:05.857" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="12578" PostTypeId="1" AcceptedAnswerId="12579" CreationDate="2015-11-05T13:57:52.613" Score="1" ViewCount="152" Body="&lt;p&gt;What would be the best way to move the earth if possible by human intervention via only using photons from the sun as a means of momentum force to move earth.Even a mile would be considered success so let's start small.This movement would increase its semi-major axis.The sun moves earth 15 cm away each year without our intervention.How could we increase the distance. &#xA; &lt;a href=&quot;http://ib-physics-ii-6b-e.aspen.high.schoolfusion.us/modules/locker/files/get_group_file.phtml?gid=4397333&amp;amp;fid=19334358&quot; rel=&quot;nofollow&quot;&gt;http://ib-physics-ii-6b-e.aspen.high.schoolfusion.us/modules/locker/files/get_group_file.phtml?gid=4397333&amp;amp;fid=19334358&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12094" LastEditorUserId="12094" LastEditDate="2015-11-05T17:29:50.877" LastActivityDate="2015-11-05T18:26:32.607" Title="Increasing the earth's distance from the sun with only the sun's photons as a force" Tags="|travel|" AnswerCount="2" CommentCount="0" ClosedDate="2015-11-05T15:08:15.603" ContentLicense="CC BY-SA 3.0" />
  <row Id="12579" PostTypeId="2" ParentId="12578" CreationDate="2015-11-05T14:36:02.987" Score="1" Body="&lt;p&gt;&lt;a href=&quot;http://qntm.org/moving&quot; rel=&quot;nofollow&quot;&gt;Earthmoving&lt;/a&gt; is impossible by all practical means, because it takes too much energy.  &lt;a href=&quot;http://qntm.org/moving&quot; rel=&quot;nofollow&quot;&gt;This fully unscientific but amusing page&lt;/a&gt; lists a number of theoretical methods, none of which would work.  The one that is least ridiculous is the method through gravitational fly-bys:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow&quot;&gt;Gravity assistance&lt;/a&gt;. This is a method originally proposed as a means of moving Earth to a higher orbit around the Sun in order to save it from the Sun's inevitable Red Giant expansion. It involves asteroids, like the above method, only instead of direct impacts, this time we just steer them past the Earth, allowing rock and planet to exchange a little momentum, with the result of an Earth moving on a slightly different track and an asteroid moving on a significantly different one. You could reuse the same asteroid again and again, looping it around a few gas giants and back to gain lots more kinetic energy from those gas giants in the same way that Earth just gained velocity from the rock. You could repeat this thousands of times over the course of millions of years. Better, you could use many, many asteroids one after the other in a steady stream, and cut down the total time significantly. You could of course use this method to steer the Earth in any direction you wanted, not just away from the Sun... heh heh heh...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, no.  It's not plausible.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2015-11-05T14:36:02.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12581" PostTypeId="2" ParentId="12568" CreationDate="2015-11-05T14:51:03.053" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;My rough calculations estimate that around 50 million litres of water must be stored at all times to sustain the station for at least a month in the event of a failure of some sort.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I believe that the &quot;one month of storage&quot; logic is not correct. A space city will be utterly reliant on its technology working properly. There are so many things which would require technology, water, atmosphere, day/night cycle, thermal management.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here on Earth, if the machinery keeping our cities alive broke down people would be dying in droves. Water, electricity, gas, petrol, telecommunications, we are super reliant on them working smoothly and having excellent uptime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I wouldn't dispute the usefulness of a buffer, and 50 million litres does seem reasonable or a bit on the generous side.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Redundancy and decentralization&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;On a space colony, the best way to manage the processing of atmosphere and water would be to do it with hundreds of parallel units all of which are capable of working independently, that way if a unit or three fail there is no danger to the colony.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Decentralization could be taken to the extreme by having each dwelling have its own independent water supply and recycling. This is not nearly as extreme as it may seem at first glance. These days Air Conditioning window units are a common sight on the sides of buildings. One function they can perform is dehumidification, which with the right engineering can produce drinkable water. An appropriately designed unit can easily produce 15 gallons (60L) of water per day, this is rather more than the minimum amount of water a person needs per day (according to UN standards) and is a reasonable daily usage for someone on a rainwater (rather than townwater) scheme and so practicing frugality in water user.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of using dehumidifiers or other condensing methods to draw water from the atmosphere introduces the intriguing concept that a space colony might not even need plumbing for water, or another way of looking at it is can use the atmosphere for plumbing. It sure eliminates a whole lot of pipes and pumps and if any single unit breaks down that unit is very easily repaired or replaced. There is a reason the small window units are popular these days and it has a lot to do with it being the most economical approach - it's cheaper and easier to mass produce 20,000 small identical units than a single big custom built unit and it is by far the more robust solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is using the atmosphere for water buffering and storage crazy? Not at all! The atmosphere of the Earth contains &lt;a href=&quot;https://en.wikipedia.org/wiki/Air_well_(condenser)#background&quot; rel=&quot;nofollow&quot;&gt;a lot of fresh water&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The atmosphere contains 12,900 km3 (3,000 cubic miles) of fresh water, composed of 98 percent water vapour and 2 percent condensed water (clouds): a figure comparable to the renewable liquid water resources of inhabited lands (12,500 km3).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The water cycle is vital to the functioning of Earth's biosphere, so the idea of using a mechanically-augmented water cycle rather than having pipes running everywhere is certainly not complete madness. In this case, the primary mechanical augmentation is using dehumidifiers in place of rainfall. To a certain extent humidity is self-balancing, evaporation is faster when humidity is lower, so a few lakes might help with passively maintaining balance, but it would be wise to invest in active central climate control which is responsible for either adding or removing water from the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much water could the atmosphere hold? That depends how big it is of course. The area is stated at $50 sq. km.$ and a cubic km of air at 20C and 50% Humidity contains 10 million L of water, so if the height of the atmosphere was 200m ($10 cub. km.$ of air) the atmosphere would hold 100million L of water which would seem to be a pretty solid buffer.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Waste Recycling&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Of all challenges, I think that waste recycling is one of the greatest.&#xA;If we are going with an augmented water cycle approach which minimizes the need for plumbing, the obvious solution is to use evaporation extensively and return water to the atmosphere as quickly as possible, a common technique to reduce the volume of waste is to just evaporate off the water using heat and fans. This is quite a popular design for toilets in remote locations, the urine and excrement simply falls into a pit, where heat (passive solar and/or heat from decomposition) and a fan (solar powered) evaporate off the water, and the solids decompose. I'm not saying that exact design would be good, just that it scales well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Removing the water isn't too hard, but the solid component of human waste is more problematic, the low tech solution is &lt;em&gt;essentially&lt;/em&gt; a wetland, let microbes and plants recycle the waste back into water, CO2 and N2. Plants are great at pumping water into the atmosphere so this can double as an evaporator. The main problem with an approach like this is that it requires considerable space and plants are terribly inefficient at utilizing light, but you may want a wetland as part of the city for aesthetic reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A related way to close the water cycle is to have waste water from residences go not to a wetland but instead to agriculture where it nourishes useful crops, and the plants return the water to the atmosphere. This isn't quite as easy as it sounds because it is also desirable to grow crop plants under tightly controlled conditions, but it would definitely be a feasible solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The high tech solution to waste disposal is to use an electric plasma arc to directly dissociate the molecules (plasma pyrolysis) then release the resulting gases to the atmosphere. Human waste is moderately energy rich so this process can generate rather than consume energy and might actually be the less energy intensive solution to disposal of solid waste. It's a sterile solution which would go well with intensive forms of agriculture which aren't compatible with organic farming styles.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Water Use Reduction&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The most water-efficient way to grow plants is aeroponics (growing plants with their roots dangling in a mist of water and nutrients), any kind of plant can be grown using aeroponics and it works in microgravity too. Compared with conventional agriculture water use is dramatically reduced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For animal protein, smaller animals are more efficient than large ones, at the extreme many insects are capable of growing using humidity in the air alone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For personal hygiene, I've seen some interesting ideas in science fiction, including using ultrasound to clean skin by knocking off particles of dirt and dead skin. Ultrasound is already used in beauty care so it is a real thing. I suspect many people will prefer water even if only as a form of recreation/comfort, but as noted earlier the water usage for someone on a rainwater scheme is very reasonable and wouldn't be difficult to provide. There are toilets which don't rely on water, but a simple way to economize on water is to re-use shower water for toilet flushing.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2015-11-05T14:51:03.053" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12582" PostTypeId="2" ParentId="12578" CreationDate="2015-11-05T15:22:32.480" Score="2" Body="&lt;p&gt;You want to move Earth by about 0.5 AU. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the easy thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to move it in prograde direction, wait a month. If you want to move it in retrograde direction, wait 11 months. Done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now radial/antiradial option, somewhat harder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's orbital velocity is 30km/s. Mass of Earth is 6*10^24kg. From Kinetic Energy equation, that's 2.7e33 Joules. Potential Energy is -5.3e33 Joules (with 0 in infinity) for a total of -2.6e+33. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After moving it to 1.5 AU away, potential energy will be -3.53e33 Joules and kinetic - 1.77e33 J for a total of -1.76e+33&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's 8.4e32 Joules of energy difference in the easier of directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.2e23 Joules is the combined energy of all Uranium, so we still need about four billion times more. Luckily that's not all we have - we're on our way to develop controllable nuclear fusion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 Deuterium atom an 1 Tritium atom creates  3.5 MeV (Helium)  + 14.1 MeV (Neutron).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since operating on atomic energies is cumbersome, let's transform that to moles. 1 mole of Deuterium (2 grams) + 1 mole of Tritium (3 grams) binding would produce 1.7e12 Joules. That's promising. We need 5e20 moles of both to produce the energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1.5e18 kg of tritium and 1e18 kg of deuterium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have 1.35e21 kg of water in all our oceans. 0.0312% deuterium by content is 4.2e17 kg of deuterium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ouch, bummer. We have only half the needed amount of deuterium and we still don't even have the tritium which would need to be produced. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's even before we begin to consider just how to efficiently utilize the energy for propulsion, with quite certain losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shortly put, we don't have enough resources on Earth for that. Gerrit suggests gravity assists, but the timing would go rather into centuries, locating, encountering and redirecting them by an enormous fleet of extremely powerful starships. And the combined mass of the ones that would be able to be redirected into useful trajectories isn't all that big either...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, no. It's not plausible. :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Using sunlight...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we took all the Sun energy that hits Earth and converted it to acceleration (somehow?) it would take 150 mln years to grow the orbit by 0.5AU.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Replacing the magical &quot;somehow&quot; with actual light pressure of 9.08 micronewtons/m^2 of solar sail thrust. will give us 0.27 Watt/m^2 at Earth's original orbital speed (and will grow worse with distance=sunlight and orbital speed dropping). (versus 1361 W/m^2 used in the above...) - but let's not limit ourselves to the size of Earth :) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll simplify the calculations by assuming constant speed and sunlight - it's a bad cheat but the interals I'd need to use otherwise give me creeps. But I'll happily multiply the requirement by square root of 2 to account for loss due to &lt;a href=&quot;https://space.stackexchange.com/a/379/103&quot;&gt;tacking against the solar wind&lt;/a&gt; by turning the sail 45 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over 40 years, a square meter of sail perpendicular to the Sun will produce 34e7 Joules of energy, be it kinetic or potential by changing your orbital altitude. So, to reach our 8.4e32J needed for the 1.5AU orbit over that time we need 2.46e24 m^2 of area perpendicular to the Sun. Tilted 45 degrees - 3.48e24 m^2. A square sheet of 1,860,000,000km side. Or just over a billion by billion miles square :) Or 12x12AU which... um, seriously invalidates the &quot;constant sun exposure&quot; assumption. Well, let's just say solar sail is not a good planet propulsion. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-11-05T18:26:32.607" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12584" PostTypeId="2" ParentId="7813" CreationDate="2015-11-05T17:41:22.290" Score="12" Body="&lt;p&gt;There are multiple possible improvements for moving the coolant, in pumping systems. There are piezoelectric pump and electrohydrodynamic pumping systems being worked on, with main advantage being increased reliability. One other candidate would be thermoelectric aka Peltier effect pumps. Similar tech would have ample applications in terrestrial cooling solutions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find state of the art and proposed future solutions discussion in NASA technology roadmaps, for this it is the &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/2015_nasa_technology_roadmaps_ta_14_thermal_management_final.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;technology area 14, thermal management systems&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been multiple different issues with the external ammonia cooling loops, from what I understand. Two that I know of was a pump failure, that had several contributing causes. Pump was returned to earth and taken apart. Basically operating conditions pushed several components (bearings, seals) of the mechanical pump outside of the design margins, resulting in wear. There are also issues with valve failures inside PVTCS, which I don't think has been fully root-caused. In each instance, contributing causes have been mechanical complex systems that are quite hard to get right on orbit, even though electronics failures have been suspected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At a very high level, the possible mitigations are iterating and improving the mechanical subsystems, by understanding the operating conditions better one can design better margins, use better materials and subassembly designs etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other possible mitigation is to try and minimize moving parts as much as possible and use solid state systems where possible. That's where the future currently TRL 3-4 level technologies come in, mentioned above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been a plethora of operational lessons as well of course. Ability to return failed subsystems to earth for quick analysis and design improvements is a key one, as the best way to improve technology is still to operate it and learn from the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some references:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.iapmonline.org/Documents/archive/20140106_International_Space_Station_Cooling.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.iapmonline.org/Documents/archive/20140106_International_Space_Station_Cooling.aspx&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20150004079&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ntrs.nasa.gov/search.jsp?R=20150004079&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140000513&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ntrs.nasa.gov/search.jsp?R=20140000513&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140005425&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ntrs.nasa.gov/search.jsp?R=20140005425&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20150018325&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ntrs.nasa.gov/search.jsp?R=20150018325&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3140" LastEditorUserId="6816" LastEditDate="2017-08-26T15:11:33.283" LastActivityDate="2017-08-26T15:11:33.283" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12586" PostTypeId="2" ParentId="12567" CreationDate="2015-11-05T19:34:39.073" Score="1" Body="&lt;p&gt;Plugging my values into the Wolfram Alpha &lt;a href=&quot;https://www.wolframalpha.com/input/?i=Venturi%20flowmeter&amp;amp;lk=3&quot; rel=&quot;nofollow&quot;&gt;Venturi flow equation calculator&lt;/a&gt;, I got my result:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;320,000 litres per second&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Formula:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Q = (\pi (D_1^2 \sqrt{\frac{2 (P_1-P_2)}{\rho}}\over(4 \sqrt{\frac{D_1^4}{D_2^4}-1))}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Q | flow rate&lt;br&gt; &#xA;rho | fluid density&lt;br&gt; P_1 | upstream pressure&lt;br&gt; P_2 | downstream pressure &lt;br&gt;D_1 | upstream pipe diameter &lt;br&gt;D_2 | downstream pipe diameter &lt;br&gt;(Q is the flow rate as measured by a Venturi flow meter)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot verify this answer as correct.&lt;/p&gt;&#xA;" OwnerUserId="1028" LastEditorUserId="4660" LastEditDate="2015-11-05T23:15:07.677" LastActivityDate="2015-11-05T23:15:07.677" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12587" PostTypeId="1" CreationDate="2015-11-05T21:00:39.807" Score="10" ViewCount="343" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#Itinerary&quot;&gt;Cassini spacecraft&lt;/a&gt; has been exploring Saturn's system since 2004 and is scheduled to be sent into Saturn's atmosphere at the end of its mission following a final encounter with Titan in 2017.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the craft makes its journey into Saturn, for how long will it be able to gather and report data back to Earth? Which instruments will still be operational and what kinds of data will they be able to collect?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it will not be able to report any data during its entry, for how long will the craft itself survive until it is destroyed?&lt;/p&gt;&#xA;" OwnerUserId="6" LastActivityDate="2015-11-05T23:39:22.047" Title="How long will Cassini survive its dive into Saturn's atmosphere?" Tags="|probe|saturn|cassini|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12588" PostTypeId="2" ParentId="12576" CreationDate="2015-11-05T21:22:17.733" Score="15" Body="&lt;p&gt;PearsonArtPhoto covers the basic issue, which is that light molecules move so fast that they reach lunar escape velocity.&lt;/p&gt;&#xA;&lt;p&gt;The average speed of the molecules of a particular gas &lt;a href=&quot;https://en.wikipedia.org/wiki/Root-mean-square_speed&quot; rel=&quot;nofollow noreferrer&quot;&gt;is proportional to the square root of (temperature in Kelvin divided by molecular mass)&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;I propose a couple of gases with a molecular weight higher than xenon (mw 54)&lt;/strong&gt; that might be able to sustain an atmosphere on the Moon, given that there is very little xenon in the Solar System.&lt;/p&gt;&#xA;&lt;p&gt;I think the chances of terraforming the Moon with an atmosphere are very slim, but it's an interesting thought experiment.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Butane (mw 58.)&lt;/strong&gt; Made of abundant elements. May decompose to lighter hydrocarbons over time, especially if interacting with hydrogen from the solar wind. Butenes or heavier, more unsaturated hydrocarbons (such as benzene) might last longer.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Chlorine Cl&lt;sub&gt;2&lt;/sub&gt; (mw 71.)&lt;/strong&gt; Not one of the top ten most common elements in the Solar System, so would need to be specifically mined, or imported (Earth seawater is about 2% chlorine by mass.) Would be stable on its own, but would oxidise rocks and thus leave the atmosphere if the rocks were oxygen deficient. Forms a number of gaseous oxides, but these are unstable with respect to decomposition to chlorine and oxygen, so would not accumulate. Would react with hydrogen from the solar wind to form hydrogen chloride, which has a lower molecular mass.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;EDIT 1: according to Wikipedia, the solar wind is a mass flow of about &lt;span class=&quot;math-container&quot;&gt;$10^{9} \frac{kg}{s}$&lt;/span&gt;, most of which is hydrogen. At Earth's orbital radius of &lt;span class=&quot;math-container&quot;&gt;$150 \cdot 10^{6} km$&lt;/span&gt;, this is just:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{10^{9}}{4\cdot\pi\cdot(150 \cdot 10^{6})^2} = 3.54\cdot 10^{-9} \frac{kg}{km^2 \cdot s}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;or &lt;span class=&quot;math-container&quot;&gt;$0.11 \frac{kg}{km^2 \cdot year}$&lt;/span&gt;. The Moon's radius is 1740 km, so it presents a &lt;span class=&quot;math-container&quot;&gt;$9.511 \cdot 10^6 km^2$&lt;/span&gt; surface to the solar wind. The total hydrogen received by the moon from the solar wind is therefore about &lt;span class=&quot;math-container&quot;&gt;$3.367 \cdot 10^{-2} \frac {kg}{s}$&lt;/span&gt; so the replenishment rate of chlorine Cl&lt;sub&gt;2&lt;/sub&gt; that has been converted to HCl would be quite trivial, in comparison to the unimaginably large task of putting a chlorine atmosphere on the Moon in the first place. I'm surprised this number is as small as it is.&lt;/p&gt;&#xA;&lt;p&gt;Photodissociation of Cl&lt;sub&gt;2&lt;/sub&gt; into two Cl· radicals (which each have a lower mass than CO&lt;sub&gt;2&lt;/sub&gt;) may be a far more significant mechanism of loss of chlorine, but I don't know how to quantify it.&lt;/p&gt;&#xA;&lt;p&gt;EDIT 2: another possibility is &lt;strong&gt;sulphur dioxide SO&lt;sub&gt;2&lt;/sub&gt; (mw 64.)&lt;/strong&gt; This is probably chemically stabler than either of my previous proposals. There's also a possible non-intentional source for it: If massive building programs on the Moon were undertaken using &lt;a href=&quot;https://en.wikipedia.org/wiki/Sulfur_concrete&quot; rel=&quot;nofollow noreferrer&quot;&gt;sulphur concrete&lt;/a&gt;, it might degrade in the presence of oxygen to SO&lt;sub&gt;2&lt;/sub&gt;. Sulphur concrete is similar to regular concrete in that it contains aggregate, but molten sulphur is used as a binder instead of traditional cement.&lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="7062" LastEditDate="2022-05-24T03:54:49.733" LastActivityDate="2022-05-24T03:54:49.733" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="12589" PostTypeId="2" ParentId="12587" CreationDate="2015-11-05T23:39:22.047" Score="11" Body="&lt;p&gt;Define &quot;destroyed&quot;. It will no longer be able to maintain attitude control due to torque from the atmosphere at a radius of about 61,700 km. From there we will get no more data from the spacecraft since it can't point the antenna. From our point of view, at that moment, Cassini is buh bye.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-11-05T23:39:22.047" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12592" PostTypeId="2" ParentId="12552" CreationDate="2015-11-06T04:06:26.093" Score="-3" Body="&lt;p&gt;What about Orbital Sciences Pegasus rocket? It was a single stage that took a payload (admittedly, small) to orbit. However, it was dropped from a B-52 at around 40,000 ft, not launched from the ground.&lt;/p&gt;&#xA;" OwnerUserId="12252" LastEditorUserId="6816" LastEditDate="2015-11-06T07:55:12.623" LastActivityDate="2015-11-06T07:55:12.623" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12593" PostTypeId="1" AcceptedAnswerId="12596" CreationDate="2015-11-06T06:09:07.333" Score="5" ViewCount="139" Body="&lt;p&gt;Recently a &lt;a href=&quot;http://news.discovery.com/space/acid-fog-likely-dissolved-rocks-on-mars-151102.htm&quot; rel=&quot;noreferrer&quot;&gt;study&lt;/a&gt; on the data from the &lt;strong&gt;Spirit&lt;/strong&gt; rover in Gusev crater suggested that there may be acid fog changing the rocks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What exactly is this acid fog?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is it formed on Mars and where?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it a potential hazard for astronauts and Mars missions?&lt;/p&gt;&#xA;" OwnerUserId="12019" LastEditorUserId="168" LastEditDate="2015-11-06T09:46:29.210" LastActivityDate="2015-11-06T09:46:29.210" Title="Acid fog on Mars?" Tags="|mars|weather|spirit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12594" PostTypeId="1" AcceptedAnswerId="12595" CreationDate="2015-11-06T07:02:10.160" Score="2" ViewCount="612" Body="&lt;p&gt;Is it possible to design a rocket, such that if anything goes wrong, the crew and people on the ground will always be in no harm, or are there certain limitations that we will never be able to overcome?&lt;/p&gt;&#xA;" OwnerUserId="12254" LastEditorUserId="25" LastEditDate="2015-11-06T15:30:05.303" LastActivityDate="2015-11-08T13:45:46.743" Title="Is it possible to design a completely fail-safe rocket?" Tags="|launch|rockets|crewed-spaceflight|safety|" AnswerCount="2" CommentCount="5" ClosedDate="2015-11-06T20:04:44.640" ContentLicense="CC BY-SA 3.0" />
  <row Id="12595" PostTypeId="2" ParentId="12594" CreationDate="2015-11-06T09:17:40.377" Score="4" Body="&lt;p&gt;No, there will always be failure modes that may lead to injury or death with any design (see Nathan Tuggy's comment!). Launch and space travel involve conditions that you have to actively control to avoid catastrophe. For example, once the rocket has launched it is also now a wayward missile if guidance and control systems were to fail -- hence why there are explosives installed to self-destruct, as well as why launches occur over the ocean or other uninhabited areas (mostly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most important thing that your question alludes to is the idea that failures are bound to happen so the system should fail gracefully. Unfortunately, we learn the most about how to best handle failures by studying catastrophes that have happened. A tragic example is the loss of the Apollo 1 crew, where an in-ward opening hatch was used because it was feared it might accidentally open in the vacuum of space but in the end made it too difficult to escape during an emergency (of course this was by no means the cause or the sole contributing factor).&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-11-06T09:17:40.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12596" PostTypeId="2" ParentId="12593" CreationDate="2015-11-06T09:25:20.403" Score="4" Body="&lt;p&gt;It's anybody's guess until we get a chemical analysis - but we can make some informed guesses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One obvious solution is $\mathrm{CO_2 + H_2O \rightarrow H_2CO_3} $ which is a very weak acid, but an acid nevertheless, though I doubt it could contribute much to erosion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The detected perchlorates in soil though are a different matter. There's a range of reactions of $\mathrm{KClO_4}$ and $\mathrm{NaClO_4}$ with water and input of electrons (from cosmic radiation) that result in a range of chlorine-based acids:  $\mathrm{HCl}$, $\mathrm{HClO}$, $\mathrm{HClO_2}$, $\mathrm{HClO_3}$ - and these are acids that can cause serious damage.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="168" LastEditDate="2015-11-06T09:43:04.050" LastActivityDate="2015-11-06T09:43:04.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12597" PostTypeId="1" AcceptedAnswerId="12598" CreationDate="2015-11-06T12:15:54.113" Score="6" ViewCount="1844" Body="&lt;p&gt;The Moon is not capable of &lt;a href=&quot;https://space.stackexchange.com/questions/12576/could-the-moon-keep-an-atmosphere&quot;&gt;keeping an Earth like atmosphere&lt;/a&gt;, nor is it practical to &lt;a href=&quot;https://space.stackexchange.com/questions/221&quot;&gt;keep satellites in lunar orbit&lt;/a&gt;.  Which means all transportation and communication would need to be ground based. No airplanes, no satellites, no GPS, decreased radio communication, etc.&lt;/p&gt;&#xA;&lt;p&gt;Xenon is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Xenon&quot; rel=&quot;nofollow noreferrer&quot;&gt;heavy, colorless, dense, odorless noble gas.&lt;/a&gt; You would not be able to breath it, but if you inhaled some it would not kill you.  It is heavy enough to make a &lt;a href=&quot;https://space.stackexchange.com/a/12577/25&quot;&gt;fairly stable atmosphere&lt;/a&gt; on the Moon. If you had enough of it you could fly a kite, fly a plane, or &lt;a href=&quot;http://www.google.com/loon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;float a balloon.&lt;/a&gt; It is rare on Earth, but assuming you found an asteroid or other source of significant amounts of xenon, how much would you need to make atmospheric plane and/or balloon operations possible?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; The boiling point of xenon is 165 K and the asteroid belt seems to have a temperature of around 200 K, so finding a frozen ball of xenon seems unlike, but if we did, how much would we need?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="47149" LastEditDate="2022-05-15T15:20:32.510" LastActivityDate="2022-05-15T15:20:32.510" Title="Could we achieve aerodynamic and aerostatic lift on the Moon if it had an atmosphere of xenon?" Tags="|the-moon|atmosphere|aerodynamics|terraforming|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="12598" PostTypeId="2" ParentId="12597" CreationDate="2015-11-06T13:10:34.020" Score="8" Body="&lt;p&gt;First of all, while xenon would be the most stable due to its molecular weight being the largest of any elemental non-radioactive gas, there are other gases that would work as well. Sulfur hexafluoride would be a particularly interesting choice, and that should be easier than xenon to find.&lt;/p&gt;&#xA;&lt;p&gt;Okay, that notwithstanding, how well would it work to fly in an atmosphere of xenon or similarly heavy gas? As a source, I am going to use &lt;a href=&quot;https://what-if.xkcd.com/30/&quot; rel=&quot;nofollow noreferrer&quot;&gt;XKCD's interplanetary Cessna flying chart&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Bernoulli%27s_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bernoulli's principle from Wikipedia&lt;/a&gt;. Of particular note are a few things. One is that the ability to fly largely depends on the pressure, not on the weight of the gas. So you would need a whole lot of gas, making the atmosphere quite heavy (&lt;a href=&quot;https://en.wikipedia.org/wiki/Ideal_gas_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;ideal gas law&lt;/a&gt;). You would then be able to fly, but only rocket powered items, which would probably be easier to do without the atmosphere anyways, although precision landing would be easier.&lt;/p&gt;&#xA;&lt;p&gt;The other advantage of an atmosphere would be the protection against small micro-meteors, which is one of the major obstacles of lunar colonies. The temperature would also be somewhat regulated, etc. However, I still wouldn't recommend this.&lt;/p&gt;&#xA;&lt;p&gt;Bottom line, I'd invest in some kind of high speed train, which should be much cheaper and easier to manage than filling the atmosphere with xenon.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mtoAF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mtoAF.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="47149" LastEditDate="2022-05-15T15:20:00.363" LastActivityDate="2022-05-15T15:20:00.363" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="12599" PostTypeId="2" ParentId="12594" CreationDate="2015-11-06T15:38:36.677" Score="2" Body="&lt;p&gt;At most, I could see rocket travel being something akin to commercial airline travel is today. Commercial airline travel, while safe, is hindered by poor pilot judgement, mechanical defects, etc. And don't even start about the terrorist potential! Still, for the most part, it is quite safe, and in fact, I personally don't think anything of the safety factor of flying.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today, it isn't possible to build rockets as reliable as commercial airliners. Why? First of all, the technology is still in it's infancy. There have only been around 5,000 orbital launches in all history (See Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Timeline_of_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;count of rocket launches&lt;/a&gt;). Compare that to the 30,000 flights in the United States every day! The reliability has to be up to allow for such a convenience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/stmbO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/stmbO.jpg&quot; alt=&quot;launches per year&quot;&gt;&lt;/a&gt;&#xA;Furthermore, it should be noted that rockets are controlled explosions. Yes, in theory you can say that of a jet or car engine, but they are on a much smaller scale (And don't contain oxidizer). With the oxidizer and fuel, there is much more potential for catastrophic damage. Furthermore, there are huge heat loads makes things even more difficult. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, space travel can no doubt be made more reliable, but achieving a fail safe rocket isn't likely to be done anytime soon. One could hope for the reliability of commercial aircraft, which hopefully will happen someday soon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="1142" LastEditDate="2015-11-08T13:45:46.743" LastActivityDate="2015-11-08T13:45:46.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12601" PostTypeId="2" ParentId="12576" CreationDate="2015-11-06T16:22:48.717" Score="3" Body="&lt;p&gt;If I remember correctly, Stephen Baxter's novel Moonseed (1998) explores this question and answers yes, it could maintain an atmosphere sufficient to support human life on the surface for a couple of hundred years before it attrited away. Hardly definitive, but he's no slouch when it comes to research.&lt;/p&gt;&#xA;" OwnerUserId="12258" LastActivityDate="2015-11-06T16:22:48.717" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12602" PostTypeId="2" ParentId="9412" CreationDate="2015-11-06T18:56:10.133" Score="6" Body="&lt;p&gt;Space &quot;convoys&quot; will, in my fondest dreams, one day be extraordinarily common.  The reason has very little to do with the traditional advantages of traveling together, i.e. Companionship, redundancy, etc.  Docking spacecraft is hard enough to make that impractical.  Instead you will see the convoys as a natural consequence of orbital mechanics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get from Earth to Mars or any other planet, you will probably use a &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Hohmann Transfer orbit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are highly elliptical orbits that stretch way out so that your aphelion reaches the target planet's orbit.  Missing something as large as a planet is very embarrassing, so you plan a Hohmann transfer so that the planet arrives near your aphelion at the same time as the spacecraft.  The lowest delta-V/thrust/fuel Hohmann transfers occur on a repeating &quot;transfer window&quot; cycle based on the orbits of the source and destination planets.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Mars, the low delta-v transfer window occurs every two years.  For Venus, it's every 15 months*.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://alexmoon.github.io/ksp/&quot; rel=&quot;nofollow&quot;&gt;Alex Moon's Launch Window Planner&lt;/a&gt; is a launch window calculator for Kerbal Space Program that illustrates this nicely.  Getting from Kerbin to Duna can take as little as 1.7 km/s of delta-v or as more than 10 times that for the same travel time, all depending on when you do your trans-Duna-injection burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we become a truly space faring civilization I expect you will see many craft that launch to LEO in the weeks before a colony transfer window.  They'll shake down each craft, testing all the knobs and widgets before their long voyage.  (Hopefully they'll fuel up from an orbiting depot too.)  Finally, as the magic time draws close, you'll see them each light off one-by-one and slowly disappear from your window/viewscreen/telescope.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Check the value for Venus before swearing by it.&lt;/li&gt;&#xA;&lt;li&gt;All of this is moot if EmDrive technology can scale up by a few orders of magnitude.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12262" LastEditorUserId="1303" LastEditDate="2016-09-30T15:57:35.610" LastActivityDate="2016-09-30T15:57:35.610" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12604" PostTypeId="1" AcceptedAnswerId="12605" CreationDate="2015-11-06T23:37:18.937" Score="3" ViewCount="328" Body="&lt;p&gt;Is it possible to move yourself &lt;strong&gt;at a practical, helpful speed&lt;/strong&gt; (leaving that definition up to mathematician's discretion) using a can of air on the ISS? Let's say the astronaut is in a large room, stuck in the middle. It will take quite a while to reach the side of the room by other means; is canned air a good way to move around?&lt;/p&gt;&#xA;" OwnerUserId="12266" LastEditorUserId="6816" LastEditDate="2015-11-07T00:33:16.820" LastActivityDate="2015-11-08T10:43:50.740" Title="Is it practical to propel oneself in space with canned air?" Tags="|iss|crewed-spaceflight|microgravity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12605" PostTypeId="2" ParentId="12604" CreationDate="2015-11-07T00:30:06.953" Score="9" Body="&lt;p&gt;Let's assume you are using &lt;a href=&quot;https://www.tedpella.com/cleaning_html/dusters.htm&quot; rel=&quot;nofollow&quot;&gt;this can&lt;/a&gt; that produces bursts up to 70 psi (482.6 kPa) -- and let's assume that is gauge pressure. We could be conservative and say 300 kPa for a consistent long-duration burst. Let's also assume that the can's nozzle has a diameter of 3 mm. With those ballpark numbers, you would have a thrust force of about 2.12 N.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's assume our astronaut has a &lt;a href=&quot;https://www.google.ca/search?q=iss+kibo+internal+dimensions&amp;amp;ie=utf-8&amp;amp;oe=utf-8&amp;amp;gws_rd=cr&amp;amp;ei=nj49Vv_kMMPue_uYjvgO#safe=off&amp;amp;q=average+body+mass+of+a+human&quot; rel=&quot;nofollow&quot;&gt;body mass of 75 kg&lt;/a&gt;. That would mean an acceleration of 0.028 m/s2 (0.00288 g).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then let's assume we're talking about being stuck in the centre of the Kibo module on ISS (the largest open space). It has dimensions of about &lt;a href=&quot;https://en.wikipedia.org/wiki/Kibo_%28ISS_module%29&quot; rel=&quot;nofollow&quot;&gt;11.2 m length and 4.4 m width&lt;/a&gt;. I believe that is external so let's reduce those dimensions to estimate the internal size -- say 10 m length and 3.5 m width. If you just care about getting from the centre of the room to any reachable surface then we can ignore the length and focus on the width -- which gives you a required traverse distance of 1.75 m.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even though we only have a small constant acceleration of 0.028 m/s2, it would take only about 11 seconds to travel the necessary 1.75 m -- at the end of which you would be moving at 0.32 m/s. That is a very slow walk but not insignificant and certainly seems like enough to get you from being stuck in the centre to the nearest wall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, let's say you want to conserve your pressurized air and instead rely on a 1 second burst to speed up slightly and then coast. That would propel you to 0.028 m/s, which would then cover the 1.75 m distance in 63 seconds -- so just over 1 minute. Again, this doesn't seem unrealistic and suggests using this method to move around might work pretty well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that this has all been calculated using a very simplified model. Also, there are numerous reasons you wouldn't want to do this at all, let alone allow astronauts to make it a habit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another option would be to take off your socks and throw them in the opposite direction, propelling you towards the wall to conserve momentum. If you assume &lt;a href=&quot;http://www.cockeyed.com/science/weight/socks-mens-dress.html&quot; rel=&quot;nofollow&quot;&gt;a pair of socks has a mass of 50 g&lt;/a&gt;, and &lt;a href=&quot;https://ca.answers.yahoo.com/question/index?qid=20070422143111AAgfJlS&quot; rel=&quot;nofollow&quot;&gt;you can easily throw them at 10 m/s&lt;/a&gt;, then you would end up moving at a speed of about 0.0067 m/s and reach the wall in about 260 seconds -- just over 4 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe the easiest thing to do is just carry one of those mini personal fans around -- although that would cause you to start spinning slightly when you used it!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another interesting method pointed out by DJohnM is to take advantage of the gravity gradient, which will cause a relative acceleration with respect to the ISS centre of mass. Judging from &lt;a href=&quot;http://iss.jaxa.jp/iss/kibo/develop_status/pict/038/meit3_20030917_pic02_e.jpg&quot; rel=&quot;nofollow&quot;&gt;available images&lt;/a&gt;, Kibo appears to be something like 15 m away from the centre of mass. So if we assume that distance is aligned along the radial axis, it would actually take only about 69 seconds to drift 1.75 m.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-11-08T10:43:50.740" LastActivityDate="2015-11-08T10:43:50.740" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12606" PostTypeId="1" CreationDate="2015-11-07T00:44:59.547" Score="4" ViewCount="511" Body="&lt;p&gt;I have been working on &lt;a href=&quot;http://www.moonwards.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;a project&lt;/a&gt; to build a realistic digital model of a Moon base. I'd assumed that during the initial phase of the base, the power source would be photovoltaic panels. But now that I think about it, could ones available today make it through a week or so of -170&lt;sup&gt;o&lt;/sup&gt;C temperatures at night? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something about the size of the ISS solar array is probably what is called for, but that array only spends something like 40 minutes at a time in the cold of orbital darkness, and I guess the Earth is always throwing some warmth their way. The solar panels on Opportunity have lasted 10 years on Mars, but they only deal with temperature swings of around 100 K, and it only gets down to about -100&lt;sup&gt;o&lt;/sup&gt;C, and only for 12 hours or so. No rover or instrument on the Moon has ever made it through the night - nobody has even tried yet. (&lt;em&gt;Edit&lt;/em&gt; - Chang'e 3 has lasted for almost 2 years, and Surveyor and Lunokhod missions lasted through lunar nights, all these on solar power. Apollo seismometers lasted for years using RTGs. See comments and answers.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has it been worked out what would happen to the kinds of solar panels used in space today, if they had to work on the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a great picture of the ISS panels, just for the mood...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0XEiS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0XEiS.jpg&quot; alt=&quot;ISS solar panel array close-up&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-11-07T16:21:13.143" LastActivityDate="2015-11-07T16:21:13.143" Title="Are there solar panels available today that would last through various lunar days?" Tags="|the-moon|mission-design|solar-power|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12607" PostTypeId="1" AcceptedAnswerId="12680" CreationDate="2015-11-07T03:53:01.440" Score="8" ViewCount="3287" Body="&lt;p&gt;The ISS and currently used spacecraft have toxic stuff (ammonia, propellants) that makes donning gas masks a life-saver in emergency. Beards may interfere with the seal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kLniZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kLniZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photo: Wikimedia Commons, NASA (Public Domain).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in exact pointers to standing NASA/ESA/Roskosmos (and perhaps other spacefaring nations) rules that limit lower facial hair growth and perhaps stipulate which types of beards can and cannot be worn in space (and probably limit the length of mustache).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QMi5x.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QMi5x.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.shavingstuff.com/archives/2006_12.php&quot; rel=&quot;noreferrer&quot;&gt;http://www.shavingstuff.com/archives/2006_12.php&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="58" LastEditDate="2018-04-13T19:14:23.927" LastActivityDate="2018-04-13T19:25:07.927" Title="What are the regulations governing (against) beards in space?" Tags="|iss|reference-request|astronaut-lifestyle|medical|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12609" PostTypeId="2" ParentId="12606" CreationDate="2015-11-07T14:06:11.873" Score="5" Body="&lt;p&gt;Well, first a correction regarding no (solar powered) rover or instrument on the Moon ever making it through the night and nobody even trying. There has been several instruments and two rovers that made it through at least one, if not several lunar nights:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_1&quot; rel=&quot;noreferrer&quot;&gt;Surveyor 1&lt;/a&gt; was solar powered, landed on the Moon on June 2, 1966 and continued returning data through January 7, 1967.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_3&quot; rel=&quot;noreferrer&quot;&gt;Surveyor 3&lt;/a&gt; landed on April 20, 1967 and was active until May 3, 1967.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_5&quot; rel=&quot;noreferrer&quot;&gt;Surveyor 5&lt;/a&gt; from September 11, 1967 to December 17, 1967&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_6&quot; rel=&quot;noreferrer&quot;&gt;Surveyor 6&lt;/a&gt; that landed on November 10, 1967 barely made it through a single lunar night and didn't return any useful data after it, but it managed &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_7&quot; rel=&quot;noreferrer&quot;&gt;Surveyor 7&lt;/a&gt; that landed on January 09, 1968 also suffered battery damage during its first lunar night, but it managed to return some useful data and not just tease that it's still alive then die like its predecessor&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lunokhod_1&quot; rel=&quot;noreferrer&quot;&gt;Lunokhod 1&lt;/a&gt; landed on November 17, 1970 and was last contacted on September 14, 1971&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lunokhod_2&quot; rel=&quot;noreferrer&quot;&gt;Lunokhod 2&lt;/a&gt; landed on 15 January 1973, achieved record for off-Earth roving distance only beaten by Opportunity rover on Mars 41 years later, and it is estimated to have died due to overheating on June 3, 1973.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This doesn't tell us much about durability of today's solar panels on the surface of the Moon, or effects of thermal cycling though, since all of these achievements are probably older than your average readership here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Luckily, NASA comes to the rescue in &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090015377.pdf&quot; rel=&quot;noreferrer&quot;&gt;Analysis of Stationary, Photovoltaic-Based Surface Power System Designs at the Lunar South Pole&lt;/a&gt;, Joshua E. Freeh, Glenn Research Center, 2009 (PDF). Environment selected for for the study is arguably even harsher, though thermal cycling periods average out to about once per year instead of once per month closer to lunar equator. Still, it gives good overview of challenges and assumed lifetime of photovoltaic-based regenerative (using hydrogen fuel cells) power systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I'm reading the paper correctly, total regenerative power system's durability is estimated to 10 years, with lowest hydrogen tank's state of charge at 10% for a 9.9 kW RFC (Regenerative Fuel Cell) &lt;em&gt;option 0&lt;/em&gt; with total power system mass of 2,860 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing worth noting here is that fuel cells generate substantial amount of heat energy (more efficient hydrogen fuel cells operate at roughly 1,000 K), so thermal management could also be designed to be self-sufficient while fuel cells last. Additional problems affecting lifetime of solar array cells are also dust, radiation, and micrometeorites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dust problem is explained in more detail in &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140002505.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Kicking Up Some Dust&quot;: An Experimental Investigation Relating Lunar Dust Erosive Wear to Solar Power Loss&lt;/a&gt;, Jeremiah N. Mpagazehe et al., 2013 (PDF) but could be mitigated with, say, &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130012895.pdf&quot; rel=&quot;noreferrer&quot;&gt;Optimization of Photovoltaic Performance through the Integration of Electrodynamic Dust Shield Layers&lt;/a&gt;, S. Nason et al., 2014 (PDF) and strategic placement of arrays where they wouldn't be a subject to bombardment of kicked up dust. Radiation environment is somewhat harsher than for solar arrays in GEO due to secondary reflected off the surface radiation, so PV durability would likely be closer to those of GPS satellites orbiting within the Van Allen radiation belts and I'd assume ~ 2% degradation per year instead of more average 1%, tho that's my own assumptions, and micrometeorites are something you'll have to design to live with. Certain PV designs (like, say, &lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-junction_solar_cell&quot; rel=&quot;noreferrer&quot;&gt;multi-junction cells&lt;/a&gt; that NASA loves to use for its probes and orbiters despite larger mass than, say, &lt;a href=&quot;https://en.wikipedia.org/wiki/Thin-film_solar_cell&quot; rel=&quot;noreferrer&quot;&gt;thin film&lt;/a&gt; PVs) will tolerate single cell defects better than others, but you'll want to design with some reasonable margin and redundancy in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I had to design what you're doing, I'd probably go with &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576506001044&quot; rel=&quot;noreferrer&quot;&gt;Stretched Lens Array Squarerigger&lt;/a&gt; (SLASR) and state-of-the-art multi-junction cell design. For example, one proposed Europa Orbiter design described in &lt;a href=&quot;http://www.lpi.usra.edu/opag/nov_2007_meeting/presentations/solar_power.pdf&quot; rel=&quot;noreferrer&quot;&gt;Solar Power for Outer Planets Study&lt;/a&gt; (PDF) uses &lt;em&gt;Advanced SLASR Arrays SOA MJ Cell&lt;/em&gt; at 45 kW BOL at 1 AU, using four 7.0 m diameter Ultraflex arrays at 513 kg total mass. And that was in 2007, you can assume that state-of-the-art somewhat improved by now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some other documents worth considering:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940006939.pdf&quot; rel=&quot;noreferrer&quot;&gt;Computation of photovoltaic parameters under lunar temperature variation&lt;/a&gt;, Neelkanth G. Dhere &amp;amp; James V. Santiago, Florida Solar Energy Center, 1993 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19910016733&quot; rel=&quot;noreferrer&quot;&gt;Solar cells for lunar applications by vacuum evaporation of lunar regolith materials&lt;/a&gt;, Alex Ignatiev, Houston Univ., TX, 1991&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And of course &lt;a href=&quot;http://ntrs.nasa.gov&quot; rel=&quot;noreferrer&quot;&gt;NASA Technical Reports Server&lt;/a&gt; (NTRS) is your friend whenever you're looking for information on something that might have been considered before.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-11-07T14:20:25.173" LastActivityDate="2015-11-07T14:20:25.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12610" PostTypeId="1" CreationDate="2015-11-07T14:26:18.913" Score="0" ViewCount="447" Body="&lt;p&gt;I remembered that I once read an article about the &lt;strong&gt;design of the controller&lt;/strong&gt;, but now I can't google it out. In short, it talked about the control panel of one Moon landing mission was full of technical number, and not a good design for human. When landing on the surface on the Moon, those numbers went wildly too much that they could not rely on them. In the end, they had to use their own senses to land. In an interview, they said that this would almost kill them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this ever happen? &lt;/p&gt;&#xA;" OwnerUserId="12270" LastEditorUserId="12270" LastEditDate="2017-10-28T15:06:26.293" LastActivityDate="2017-10-28T15:06:26.293" Title="Was there a Moon landing mission that the astronauts had to land by hand?" Tags="|the-moon|apollo-program|landing|design|death|" AnswerCount="2" CommentCount="8" ClosedDate="2015-11-08T00:25:28.377" ContentLicense="CC BY-SA 3.0" />
  <row Id="12611" PostTypeId="2" ParentId="12610" CreationDate="2015-11-07T15:55:24.513" Score="2" Body="&lt;p&gt;It seems pretty clear from this photo that everything was visible. There were definitely some difficult aspects of the journey. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ndhwm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ndhwm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The truth is, the LEM was largely computer controlled. Humans helped in the landing spot selection, and could adjust the aimpoint, but that was really all they were doing. There were some adjustments made by virtue of some written numbers that might have been illegible in flight, but they could get used to the numbers in training and adjust, even if they couldn't read the number. Essentially the windows had numbers in them to program into the computer to adjust the landing site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might read &lt;a href=&quot;http://amzn.to/1kBiq8y&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Digital Apollo: Human and Machine in Spaceflight&quot;&lt;/a&gt;, which is where I learned how this works.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-11-07T17:01:00.697" LastActivityDate="2015-11-07T17:01:00.697" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12612" PostTypeId="2" ParentId="12610" CreationDate="2015-11-07T18:07:37.113" Score="8" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Guidance_Computer&quot; rel=&quot;noreferrer&quot;&gt;digital computer&lt;/a&gt; on the Apollo spacecraft (virtually identical units in the CM and the LM) used a keypad input and numeric display output which was not very intuitive to use, although the intensive training the crews underwent probably helped it feel relatively natural. In theory, the crew could designate a landing point, and the computer could use the ship's radar to watch the surface and land in a completely automatic mode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the LM also had joystick controls and windows, and in practice, for each of the Apollo landings, at around 500 feet altitude the commander switched the computer into a semi-automatic mode called P66 attitude hold. In this mode the commander could adjust the rate of descent by 1 foot-per-second at a time, and adjust the attitude (orientation) of the lander via joystick -- turning it slightly toward any direction would cause it to accelerate in that direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LM didn't have a good view directly downward, however, so P66 required coordination between the commander (looking out the window and flying the ship) and the LM pilot (looking at the computer displays). If you read the transcripts of &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11transcript_tec.html&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11&lt;/a&gt; around the 04 06 43 11 timestamp, you'll see the LMP (Aldrin) calling out altitude and speed figures, while Armstrong is steering around, looking for a good flat spot to put down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was absolutely a challenging and scary process, as evidenced by Armstrong's heart rate at the time, but the crews had practiced it, and managed 6 out of 6 safe landings over the course of the Apollo program.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-11-07T18:16:54.367" LastActivityDate="2015-11-07T18:16:54.367" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12613" PostTypeId="1" CreationDate="2015-11-07T18:26:03.060" Score="5" ViewCount="159" Body="&lt;p&gt;China's new mission &lt;strong&gt;&lt;a href=&quot;http://www.spaceflightinsider.com/missions/space-observatories/china-to-launch-its-first-dark-matter-probe-by-the-end-of-2015/&quot; rel=&quot;nofollow&quot;&gt;DAMPE&lt;/a&gt;&lt;/strong&gt; will launch a dark matter particle telescope to detect and study the evidence of dark matter. How does this telescope work?&#xA;What is expected from the mission? &lt;/p&gt;&#xA;" OwnerUserId="12019" LastEditorUserId="4660" LastEditDate="2015-11-07T19:03:24.113" LastActivityDate="2019-09-25T07:01:25.190" Title="What is the Dark Matter Particle Explorer telescope?" Tags="|mission-design|telescope|astronomy|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12614" PostTypeId="2" ParentId="8440" CreationDate="2015-11-07T22:44:04.363" Score="12" Body="&lt;p&gt;The Alcubierre bubble could get to Alpha Centauri in less than 4 years according to outside observers, not just observers riding inside the bubble--the exact time depends on the exact 'shape' of the warped spacetime in the bubble, but the theory allows you to fine-tune the bubble to give as short a time as desired. The fact that observers outside the bubble also see it travel faster than light is stated on p. 117 of the book &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/022604548X&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Time Travel and Warp Drives&lt;/em&gt;&lt;/a&gt; by physicists Allen Everett and Thomas Roman:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The bubble and its contents could travel through spacetime at a speed faster than light, as seen by observers outside the bubble.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And pages 118-119 elaborate:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;So essentially what we've done here is &quot;speed up&quot; light inside the bubble relative to observers outside the bubble. Observers inside the bubble can thus travel at faster than light speeds relative to observers outside, but still slower than the local light speed inside the bubble.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, there is a complication--the person inside the bubble is essentially riding a sort of &quot;track&quot; of distorted spacetime from Earth to Alpha Centauri, and although once the track is &quot;laid down&quot; the person can then travel from Earth to Alpha Centauri as fast as desired, work on laying down this track must begin more than 4 years before the date they want to arrive. This is explained on p. 121 of the same book:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Y]ou can't create and control warp bubbles on demand, they have to be prepared in advance before you can use them, as also pointed out by Krasnikov. Suppose you want to create a warp bubble that will get you from Earth to Alpha Centauri 4 light-years away (it's actually about 4.2, but let's use 4 to make the arithmetic simpler) in one day, say, on January 1, 2200. You can't do this by starting on December 31, 2199. By then the spacetime point on Alpha Centauri on January 1, 2200 is far outside your future &lt;a href=&quot;http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/spacetime/#Light&quot; rel=&quot;noreferrer&quot;&gt;light cone&lt;/a&gt;, and the earliest time at which you can affect anything happening on the star is December 31, 2203. Remember from chapter 4, you can't do anything to affect what happens outside your future light cone. If you want a warp bubble to arrive at the star on January 1, 2200, the latest date at which you can arrange for this is Jan. 1, 2196. Starting then, you can in principle arrange for a warp bubble to leave your location, on December 31, 2199, and arrive at the star on January 1, 2200. If you wanted to, you could then arrange for a daily warp bubble service to Alpha Centauri arriving every day &lt;em&gt;after&lt;/em&gt; January 1, 2200.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12273" LastEditorUserId="12273" LastEditDate="2015-11-07T23:06:39.620" LastActivityDate="2015-11-07T23:06:39.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12615" PostTypeId="1" CreationDate="2015-11-07T23:04:17.437" Score="3" ViewCount="338" Body="&lt;p&gt;Related to &lt;a href=&quot;https://space.stackexchange.com/questions/7813/are-there-any-promising-next-generation-space-station-cooling-technologies&quot;&gt;Are there any promising next-generation space station cooling technologies?&lt;/a&gt;, I'm wondering specifically about a technology that hasn't been mentioned there: Zeeman slowers. Since these are a) used to cool atom beams from at least 290 K to 5-10 K (if not lower) and b) operate without convection or conduction by triggering increased thermal radiation from cooled atoms, it seems that they would be not only suitable for general-purpose cooling in space, but if anything, a good deal more efficient there compared to expansion-compression cooling systems than on Earth, since they skip the whole &quot;pumping heat to enormous radiators&quot; step. And they've been in use for something like three decades in lab setups. What am I missing?&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-04-01T10:23:33.693" Title="How close are Zeeman slowers to being useful for general cooling in space?" Tags="|thermal|thermal-control|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12616" PostTypeId="2" ParentId="12552" CreationDate="2015-11-08T05:36:15.630" Score="-1" Body="&lt;p&gt;We haven't launched into LEO with an SSTO because of the rocket equation- basically, for every ounce of fuel, we have to have fuel to lift that fuel plus the entire rocket- I'm not good enough to calculate the max mass of a planet for an SSTO, but I'm sure someone whose good with physics and rocket science can work it out &lt;/p&gt;&#xA;" OwnerUserId="12276" LastActivityDate="2015-11-08T05:36:15.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12617" PostTypeId="1" CreationDate="2015-11-08T09:16:38.753" Score="6" ViewCount="1709" Body="&lt;p&gt;For my own interest I'm trying to implement the EGM96 gravitational model. I found the coefficients &lt;a href=&quot;http://cddis.nasa.gov/926/egm96/getit.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. The readme file says that they are &quot;fully normalized&quot;, but I'm not 100% sure what this means. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance &lt;a href=&quot;http://fr.mathworks.com/help/matlab/ref/legendre.html?refresh=true&quot; rel=&quot;noreferrer&quot;&gt;MATLAB&lt;/a&gt; defines fully normalized associated Legendre polynomials as:&#xA;$$ N_\ell^m = (-1)^m\sqrt{\frac{(\ell+0.5)(\ell-m)!}{(\ell+m)!}}P_\ell^m$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;whereas the &lt;a href=&quot;https://en.wikipedia.org/wiki/Spherical_harmonics#Conventions&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; on spherical harmonics says that geodesy often uses the normalization:&#xA;$$ Y_\ell^m = \sqrt{\frac{(2\ell + 1)(\ell-m)}{(\ell +m)!}}P_\ell^m $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The readme file uses the notation Ynm, which leads me to believe I should be using the second normalization. Is that correct?&lt;/p&gt;&#xA;" OwnerUserId="11885" LastActivityDate="2019-05-29T19:44:03.700" Title="How are the coefficients in the EGM96 model normalized?" Tags="|gravity|mathematics|geophysics|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12618" PostTypeId="1" CreationDate="2015-11-08T14:20:19.123" Score="4" ViewCount="1021" Body="&lt;p&gt;Vandenberg and Plesetsk and Satish Dhawan (India) have obviously been used for polar orbits. Palmachim (Israel) launches westward due to political and safety reasons. But what are the constraints on what orbit which spaceport can launch to? Has or could it be done to launch to polar orbit from Cape Canaveral, Baikonur and Palmachim? This considering current safety policies and regulations.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2015-11-08T14:55:49.507" Title="How are different spaceports restrained in terms of orbits reachable?" Tags="|launch-site|spaceport|polar-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12619" PostTypeId="2" ParentId="12618" CreationDate="2015-11-08T14:55:49.507" Score="7" Body="&lt;p&gt;Any time a craft launches from a launch site at any azimuth other than due east there is a performance penalty.  But the constraints you mention are largely due to safety or political constraints.  Here are the limits for the major continental US launch sites.  (These writeups from &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/sts/launch.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; are shuttle-centric but the information is still largely relevant)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Kennedy Space Center launches have an allowable path no less than 35&#xA;  degrees northeast and no greater than 120 degrees southeast. These are&#xA;  azimuth degree readings based on due east from KSC as 90 degrees.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A 35-degree azimuth launch places the spacecraft in an orbital&#xA;  inclination of 57 degrees. This means the spacecraft in its orbital&#xA;  trajectories around the Earth will never exceed an Earth latitude&#xA;  higher or lower than 57 degrees north or south of the equator.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A launch path from KSC at an azimuth of 120 degrees will place the&#xA;  spacecraft in an orbital inclination of 39 degrees (it will be above&#xA;  or below 39 degrees north or south of the equator).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;These two azimuths - 35 and 120 degrees - represent the launch limits&#xA;  from the KSC. Any azimuth angles further north or south would launch a&#xA;  spacecraft over a habitable land mass, adversely affect safety&#xA;  provisions for abort or vehicle separation conditions, or present the&#xA;  undesirable possibility that the SRB or external tank could land on&#xA;  foreign land or sea space.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Launches from Vandenberg have an allowable launch path suitable for&#xA;  polar insertions south, southwest and southeast. The launch limits at&#xA;  Vandenberg are 201 and 158 degrees. At a 201-degree launch azimuth,&#xA;  the spacecraft would be orbiting at a 104-degree inclination. Zero&#xA;  degrees would be due north of the launch site, and the orbital&#xA;  trajectory would be within 14 degrees east or west of the north-south&#xA;  pole meridian. At a launch azimuth of 158 degrees, the spacecraft&#xA;  would be orbiting at a 70-degree inclination, and the trajectory would&#xA;  be within 20 degrees east or west of the polar meridian. Like KSC,&#xA;  Vandenberg has allowable launch azimuths that do not pass over&#xA;  habitable areas or involve safety, abort, separation and political&#xA;  considerations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/centers/wallops/about/launchrange.html&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; are the Wallops limits&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Wallops offers a wide array of launch vehicle trajectory options. The&#xA;  coastline of Wallops Island is oriented such that a launch azimuth of&#xA;  135 degrees is perpendicular to the shoreline. In general, launch&#xA;  azimuths between 90 and 160 degrees can be accommodated depending on&#xA;  impact ranges. For most orbital vehicles, this translates into orbital&#xA;  inclinations between 38 degrees and approximately 60 degrees.&#xA;  Trajectory options outside of these launch azimuths, including polar&#xA;  and sun-synchronous orbits, can be achieved by inflight-azimuth&#xA;  maneuvers. For example, wider northerly options are possible by&#xA;  maneuver around Assateague Island after passing 5 nm downrange. The&#xA;  North Carolina Outer Banks are generally the restricting landmass for&#xA;  southern launch azimuths. Specific trajectory options and the&#xA;  operational impact area are determined in consultation with our Flight&#xA;  Safety Group.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-11-08T14:55:49.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12621" PostTypeId="1" AcceptedAnswerId="12622" CreationDate="2015-11-08T19:03:10.560" Score="23" ViewCount="6491" Body="&lt;p&gt;During the Apollo 11 landing, a misconfiguration caused the guidance computer to activate the 1201 and 1202 program alarms, signifying that the computer was overloaded and dropping low-priority tasks. This was connected to the master alarm system to get the crew's attention; the alarms distracted the crew and may have contributed to the long hover time and marginal fuel state on that landing. In actuality the 1201 and 1202 errors were not extremely serious, though the uncertainty of their cause at the time was a big concern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In November 1968, NASA engineer Bill Tindall wrote &lt;a href=&quot;http://www.lettersofnote.com/2011/06/gee-whiz-that-master-alarm-certainly.html&quot;&gt;a delightful memo&lt;/a&gt; recommending against tying a low fuel warning to the master alarm precisely because it was an expected occurrence: &quot;just at the most critical time in the most critical operation of a perfectly nominal lunar landing mission, the master alarm with all its lights, bells, and whistles will go off.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The film Apollo 13 &lt;a href=&quot;https://youtu.be/kAmsi05P9Uw?t=60&quot;&gt;portrays the master alarm&lt;/a&gt; as a red light with a loud repeating buzzer. Is that reasonably accurate? Would that buzzer have gone off during the 1201 and 1202 alarms during the Apollo 11 landing?&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2022-06-08T02:03:11.550" Title="How intrusive was the Apollo Master Alarm system?" Tags="|apollo-program|lunar-landing|safety|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12622" PostTypeId="2" ParentId="12621" CreationDate="2015-11-08T20:11:18.503" Score="23" Body="&lt;p&gt;The Apollo Operations Handbook, &lt;a href=&quot;http://history.nasa.gov/afj/aoh/aoh-v1-2-10-c+w.pdf&quot; rel=&quot;noreferrer&quot;&gt;Caution and Warning section&lt;/a&gt;, identifies the alarm tone as &quot;a square wave that is alternately 750 cps and 2000 cps, changing at a rate of 2.5 times a second&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps someone can gen that up?  It appears to me to be more like an alternating type sound rather than a buzzer.  If the Shuttle copied this sound for its master alarm (likely), I've heard that one a bunch.  It is indeed kind of a warbling sound that to me sounds like a French police car (at least as they are portrayed in American movies).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: just found the Shuttle spec, and it's 400 Hz for .4 seconds, then 1024 Hz for 0.4 seconds.  So roughly similar (alternating) but not the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit2: Russell Borogove synthesized 5 seconds of each alarm at &lt;a href=&quot;https://soundcloud.com/borogove-41558917/sets/apollo-sts-master-alarms-updated&quot; rel=&quot;noreferrer&quot;&gt;this link&lt;/a&gt;.  Very neat!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit3: The above information related to the Command Module C&amp;amp;W system, my error, since the question is really about the &lt;em&gt;Lunar Module&lt;/em&gt; C&amp;amp;W system.  That, in fact, seems to have been a buzzer, although not through a loudspeaker:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A  malfunction  also  activates  a  3-kc  signal  that  provides&#xA;  a  tone  in  the  astronaut  headsets and  supplies  the  PCMTEA  with  a  telemetry  signal .&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I got this from an incredible resource I just found: the &lt;a href=&quot;http://www.hq.nasa.gov/alsj/LM10HandbookVol1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Lunar Module Operations Handbook&lt;/a&gt;, 800 pages of Apollo-y goodness.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-11-11T16:09:46.323" LastActivityDate="2015-11-11T16:09:46.323" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="12626" PostTypeId="1" CreationDate="2015-11-09T05:46:31.080" Score="16" ViewCount="5420" Body="&lt;p&gt;Once again the impossible drive is in the forefront of public news.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But thus far, I am still unimpressed. NASA seems to have thrown a lot of resources, to test the viability of the EmDrive. But I am not sure we are any closer to knowing if it &quot;works&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why don't we build a cube sat, launch it into orbit and try and push it out to Pluto?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seems like we would get much more useful data much quicker.&lt;/p&gt;&#xA;" OwnerUserId="6157" LastActivityDate="2018-05-22T12:55:46.287" Title="Why don't we put an EmDrive in space?" Tags="|nasa|testing|emdrive|" AnswerCount="7" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12627" PostTypeId="2" ParentId="12626" CreationDate="2015-11-09T09:41:42.883" Score="3" Body="&lt;p&gt;NASA doesn't work like that. It might be quicker, but it's also a lot riskier, and it has very little value. A Cubesat can't carry much instrumentation, so after flying it we'd be no closer to understanding how and why the EM drive works, which is the more important question at the moment.  &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Edit&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;And in 2018 we found out that &lt;a href=&quot;https://space.stackexchange.com/questions/24509/what-is-the-current-status-of-emdrive-engine&quot;&gt;the EmDrive doesn't work&lt;/a&gt;, without ever having to take it off Earth.&lt;br&gt;&#xA;Tests on Earth have some big advantages over tests in space. On Earth, we can easily examine and modify the test system, to progressively rule out errors. We can progressively add more test equipment and e.g. shielding to refine the test.&lt;br&gt;&#xA;If you launch an EmDrive on a small satellite, you get only one chance to build a test system. Any follow-ups with a changed test system would be very expensive as they would require another launch. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-05-22T12:55:46.287" LastActivityDate="2018-05-22T12:55:46.287" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="12628" PostTypeId="2" ParentId="12626" CreationDate="2015-11-09T09:44:32.833" Score="40" Body="&lt;p&gt;EmDrive takes ~300W. You won't get it from a cubesat. You need over a meter of solar panels, or good 56kg of RTG battery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's been tested on Earth, made with materials and electronics meant to work in Earth ambient conditions: temperature, pressure, radiation. Putting things in space is not as simple as loading them onto a rocket. If they are to work, they need &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a complex thermal management system (dissipating 300W using only radiators will again require a good few m^2 of radiator area), &lt;/li&gt;&#xA;&lt;li&gt;power source and management (no nice 230V out of wall socket) and a thermal management system for &lt;em&gt;that&lt;/em&gt; (frozen batteries don't work)&lt;/li&gt;&#xA;&lt;li&gt;all the electronics must be toughened against space radiation (meaning replaced with special chips made in a technology of thick traces, high currents, and a lot of redundancy, so that errant particles don't change the bits). These are complex, expensive and require special expertise, way different than common electronics. &lt;/li&gt;&#xA;&lt;li&gt;There's the matter of pressure - create a neat component safely embedded in epoxy, get a bubble of air trapped, your neat component explodes in orbit.&lt;/li&gt;&#xA;&lt;li&gt;There's the whole telemetry and radio matter - so you sent your neat drive away and nobody knows what happened to it. Space is big.&lt;/li&gt;&#xA;&lt;li&gt;Attitude control and assuring that the drive's thrust is in line with the center of mass. Otherwise instead of flying to Pluto you'll have the fastest-spinning satellite in the world, spewing pieces around as it breaks apart.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;...and funds. How many thousands dollars per kilogram? And this thing won't be a cubesat. It will be something of order of tons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And for what? If it doesn't work, we won't know if it doesn't due to some failure or because it's a faulty concept. Let's first make sure we have something we can send into space.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-11-09T09:44:32.833" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="12629" PostTypeId="2" ParentId="12626" CreationDate="2015-11-09T10:41:21.643" Score="1" Body="&lt;p&gt;Paul March (if I remember well) explained that just to have Nasa GRC to accept working on EmDrive test, they needed to have reach a minimum thrust (50 or 100µN as I remember).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting straight to space with an untested device may lead to a fatal false negative.&#xA;It happened that way with Cold Fusion, with two influential labs who failed , for reason that were unknown at that time, and because they made bad assumptions and refused to ask to experts (Nasa EW did similar error relative to Shawyer).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For me that false negative risk is a much more serious risk, thank the delay and the budget. If nasa can accept failure without dumping the idea , it is OK, but my perception is that it does not work that way.&lt;/p&gt;&#xA;" OwnerUserId="5888" LastActivityDate="2015-11-09T10:41:21.643" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12630" PostTypeId="2" ParentId="12626" CreationDate="2015-11-09T12:06:53.443" Score="12" Body="&lt;p&gt;We might, it depends on the scale of the project, and if someone proposed a proof of concept mission and is ready to finance it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a small scale &lt;a href=&quot;https://www.nasa.gov/mission_pages/tdm/main/index.html&quot;&gt;technology demonstration mission&lt;/a&gt; such as the ones regularly performed aboard the ISS, it could be, for example, proposed through &lt;a href=&quot;http://www.iss-casis.org/&quot;&gt;CASIS&lt;/a&gt; as a &lt;a href=&quot;http://www.iss-casis.org/CASISBasics/ForBusinesses/PhysicalSciences.aspx&quot;&gt;Physical and Materials Science R&amp;amp;D project&lt;/a&gt;, but it likely won't win any government sponsored grants (competition is fierce).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for NASA, it depends to what &lt;a href=&quot;http://www.esto.nasa.gov/files/trl_definitions.pdf&quot;&gt;TRL&lt;/a&gt; (Technology Readiness Level) can you qualify it. For a full-scale system component, it is currently only at TRL2 and lacks breadboard tests for independent verification and validation (IV&amp;amp;V) of the technology (TRL3), full-scale experiments (TRL4), and validation in representative environment (TRL5), before it can move to prototype demonstration in relevant environment (TRL6) and beyond.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA Eagleworks, in the &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140009930&quot;&gt;most recent related paper that NTRS will return&lt;/a&gt;, in &lt;em&gt;section V. Application of Technology to Space Exploration Missions&lt;/em&gt;, interestingly avoids description of small-scale technology demonstration missions and jumps right to potential full-scale application of the technology with a couple of interplanetary mission examples (to Mars and Titan/Enceladus at Saturn). Those serve as demonstration that what currently stands on paper (and has yet to be independently verified) has potential real-world applications, but that's it. Paper's summary clearly states that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The near term objective is to complete a Q-thruster breadboard test&#xA;  article that is capable of being shipped to other locations which&#xA;  possess the ability to measure low thrust for independent verification&#xA;  and validation  (IV&amp;amp;V) of the technology. The current plan is to&#xA;  support an IV&amp;amp;V test campaign at the Glenn Research Center (GRC) using&#xA;  their low thrust torsion  pendulum followed by a repeat campaign at&#xA;  the Jet Propulsion Laboratory  (JPL) using their low thrust torsion&#xA;  pendulum. The Johns Hopkins University  Applied Physics Laboratory has&#xA;  also expressed an interest in performing a  Cavendish Balance style&#xA;  test with the IV&amp;amp;V shipset.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="2910" LastEditDate="2015-11-09T15:41:11.167" LastActivityDate="2015-11-09T15:41:11.167" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12631" PostTypeId="1" AcceptedAnswerId="12632" CreationDate="2015-11-09T13:17:28.720" Score="15" ViewCount="7359" Body="&lt;p&gt;ISS constantly loses altitude to air drag and other forces (tidal, electromagnetic). While finding that rate in the sources isn't that hard, with orbital mechanics of altitude loss actually increasing the linear speed despite growing drag, finding the decelerating force isn't nearly as easy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say we can develop a thruster of very high ISp that would keep the ISS at constant altitude without need for re-boosts from the delivery vehicles - what thrust would it need to have to let it maintain altitude?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2015-11-09T14:31:38.467" LastActivityDate="2021-03-15T12:09:23.840" Title="What is the ISS drag?" Tags="|iss|orbital-mechanics|atmospheric-drag|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12632" PostTypeId="2" ParentId="12631" CreationDate="2015-11-09T15:05:15.900" Score="21" Body="&lt;p&gt;First of all, let's figure out what the drag actually is. For that, &lt;a href=&quot;http://www.heavens-above.com/IssHeight.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heavens-above&lt;/a&gt; has a nice chart.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/eLQOS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eLQOS.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Of some note is the fact that the &lt;a href=&quot;http://www.esa.int/esapub/sp/sp1201/a3.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;atmospheric drag rate&lt;/a&gt; changes over time, most notably with the solar cycle, but it can change for a variety of reasons, especially for a body as dynamic as the ISS. With the current altitude ranging around 400. The time that it took to go from 414 to 406 km was about 2.5 months, or, say, 75 days. That means that the drag on the spacecraft is about 106 m/day. Orbital energy can be calculated by &lt;span class=&quot;math-container&quot;&gt;$e_k=m \cdot v^2/2$&lt;/span&gt;. The energy at 406km is 29400301 J/kg, and at 406.1 is 29399868 J/kg. Thus, 433 J/kg of energy is lost per day. The force is applied over all that time period to make that energy lost. &lt;span class=&quot;math-container&quot;&gt;$F=m\cdot A$&lt;/span&gt; provides us that 433 J/kg / (Distance traveled in a day) = m * a. Thus, the constant acceleration that would keep it at the same orbit is about &lt;span class=&quot;math-container&quot;&gt;$6.56\cdot 10^{-7}~\rm m/s^2$&lt;/span&gt;, or &lt;span class=&quot;math-container&quot;&gt;$0.656 ~\rm \mu m/s^2$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Given the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/onthestation/facts_and_figures.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;station mass&lt;/a&gt; of 419455kg, the decelerating force would be 0.275 newtons.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="12024" LastEditDate="2021-03-15T10:10:51.777" LastActivityDate="2021-03-15T10:10:51.777" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="12633" PostTypeId="2" ParentId="12626" CreationDate="2015-11-09T17:14:05.550" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why don't we build a cube sat, launch it into orbit and try and push it out to Pluto? Seems like we would get much more useful data much quicker&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Well, ignoring the details of low earth orbit, lack of space for power and problems of monitoring a small dark object over vast distance ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To traverse 7.5 billion kilometers starting at 0 m/s and using an accelleration of 50 micronewtons on a 1Kg cubesat means you would get your data in about 38 years. That doesn't seem very fast.&lt;/p&gt;&#xA;" OwnerUserId="3101" LastActivityDate="2015-11-09T17:14:05.550" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12634" PostTypeId="1" CreationDate="2015-11-09T18:13:53.337" Score="26" ViewCount="1956" Body="&lt;p&gt;The recent question about &lt;a href=&quot;https://space.stackexchange.com/questions/1479/could-you-take-a-cessna-from-the-iss-to-earth&quot;&gt;Cessna reentering from ISS&lt;/a&gt; got the answers that all imply a rapid drop.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But from what I know, air drag is proportional:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;to square of airspeed&lt;/li&gt;&#xA;&lt;li&gt;to air density&lt;/li&gt;&#xA;&lt;li&gt;to attack surface (with all the fancy caveats of aerodynamics).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, lift of a plane is proportional&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;to square of airspeed&lt;/li&gt;&#xA;&lt;li&gt;to air density&lt;/li&gt;&#xA;&lt;li&gt;to wing area (with angle of attack and all the fancy caveats of aerodynamics).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That means if an airplane in air experiences X newtons of air drag, its wings are capable of producing Y newtons of lift, with Y being usually considerably higher than X, and mostly proportional. The actual ceiling limit comes from the engine performance which has not enough air to push against.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there, then, a specific reason why a plane couldn't enter from the sufficiently low, circular orbit, without need for heatshields? The moment it starts experiencing significant drag - that means considerable energy is being dissipated - wouldn't the same speed and air density that create that drag, contribute equally to its lift, allowing it to maintain altitude where the power dissipation would be kept in check? In other words, instead of dropping steeply into thick atmosphere like most reentry vehicles do, couldn't it just glide down, losing altitude very slowly, to keep both drag and lift to survivable/usable values? Or am I missing something here?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2021-02-11T06:25:53.110" Title="Gliding into the atmosphere" Tags="|spacecraft|reentry|aerobraking|spaceplane|aerodynamics|" AnswerCount="5" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="12635" PostTypeId="2" ParentId="12634" CreationDate="2015-11-09T19:15:19.837" Score="13" Body="&lt;p&gt;It most certainly won't hurt anything. From &lt;a href=&quot;https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/cami/library/online_libraries/aerospace_medicine/tutorial/media/iii.4.1.7_returning_from_space.pdf&quot; rel=&quot;noreferrer&quot;&gt;FAA's document on returning from space&lt;/a&gt;, there is a very interesting chart, which I've included below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/R7EE1.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R7EE1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bFFhK.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bFFhK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the maximum g load is almost always at around 4500 m, for this particular flight trajectory. From other charts, we find that a lower angle will spread out the re-entry forces, and specifically make the max deceleration happen at a higher altitude. Bottom line is, for our Cessna to have a higher change of landing, it would have to be capable of flight at high velocity around 10,000 m. The max flight ceiling is 13500 feet, roughly 4000 m, however, it should be capable of some lift at that high speeds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think you would run into a problem, however. The aircraft would effectively be going supersonic, and wouldn't have much steering capability. I don't believe the design of a Cessna would be capable of maintaining stability at such a high altitude and speed. A space plane would have it's wings further back, like the Space Shuttle, to allow it to have more stability. And if the stability can't be maintained, the lift can't be maintained.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, this could assist, but I still think the Cessna wouldn't be around for long.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="4660" LastEditDate="2017-11-01T22:05:58.570" LastActivityDate="2017-11-01T22:05:58.570" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12636" PostTypeId="1" AcceptedAnswerId="12637" CreationDate="2015-11-09T21:34:10.810" Score="17" ViewCount="1072" Body="&lt;p&gt;This image from &lt;a href=&quot;http://www.heavens-above.com/IssHeight.aspx&quot; rel=&quot;noreferrer&quot;&gt;Heavens Above&lt;/a&gt; that @PearsonArtPhoto links to in &lt;a href=&quot;https://space.stackexchange.com/a/12632/758&quot;&gt;this answer&lt;/a&gt; shows the orbital altitude of the International Space Station:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/KdSqk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KdSqk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, the ISS altitude decreases gradually with time due to drag, and increases almost as a step function (in the time-scale of this graph) as the ISS is boosted back up to keep it within normal parameters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However on or around January 27th 2015, there is a sudden loss of altitude of about 1.5km.  Given the shape of the rest of this graph, I assume that this was not due to dynamic atmospheric conditions, but more likely a planned maneuver.  If it was a planned maneuver, it appears to be in the wrong direction - it looks like the use of (precious) fuel to achieve a result that could have been achieved just by waiting 10 days or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was this a planned maneuver or was it due to natural atmospheric conditions?  If it was planned, what was its reason?&lt;/p&gt;&#xA;" OwnerUserId="758" LastEditorUserId="758" LastEditDate="2017-05-12T20:51:41.203" LastActivityDate="2019-08-08T00:40:17.383" Title="What caused the ISS's sudden loss of altitude in January 2015?" Tags="|iss|orbital-mechanics|orbit|low-earth-orbit|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12637" PostTypeId="2" ParentId="12636" CreationDate="2015-11-09T21:39:31.417" Score="15" Body="&lt;p&gt;It was a decision made to &lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2931725/ISS-moves-closer-home-Space-station-lowers-orbit-make-commute-Earth-space-shorter.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;lower the orbit of the ISS&lt;/a&gt; of the &lt;a href=&quot;https://blogs.nasa.gov/stationreport/2015/01/28/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS on Jan 28, 2015&lt;/a&gt;. The goal was to make it easier for spacecraft to make it to the ISS. Rumor is this had something to do with the loss of Orbital Science's &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cygnus spacecraft&lt;/a&gt; on 28 October 2014, which resulted in the cargo resupply missions needing to carry more supplies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of note is that this was performed by the ATV, about 2 weeks before the last one ended it's mission. Likely they had surplus fuel that they wanted to use, and decided to use it to lower the orbit of the ISS.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user10373" LastEditDate="2019-08-08T00:40:17.383" LastActivityDate="2019-08-08T00:40:17.383" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="12638" PostTypeId="1" CreationDate="2015-11-09T22:15:52.147" Score="27" ViewCount="8425" Body="&lt;p&gt;I am curious what the most accurate, versus most inaccurate things that were depicted in the space movie, &quot;The Martian&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought the part where he punctures his suit and jets over to the rescue vehicle seemed implausible. Unless you had the hole directly jetting gas directly in line with your center of gravity, wouldn't you just spin around?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the way the homemade &quot;explosive&quot; blasted a hole in the door seemed unbelievable to me, but I guess that would depend how tough the door is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I don't see how having a small hole in the habs tent would cause it to suddenly and violently blast apart the way it did.&lt;/p&gt;&#xA;" OwnerUserId="6871" LastActivityDate="2015-12-26T22:00:15.507" Title="What were the most inaccurate or accurate things in &quot;The Martian&quot;?" Tags="|mars|science|" AnswerCount="10" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12639" PostTypeId="2" ParentId="12638" CreationDate="2015-11-09T22:24:13.433" Score="28" Body="&lt;p&gt;Watney &lt;em&gt;did&lt;/em&gt; have trouble controlling himself with the &quot;iron man&quot; suit puncture trick; it's plausible that he could quickly get the hang of positioning his hand to thrust through his center of mass (though that all happened fast enough that I didn't see if it was so depicted). Mastering that technique in the few seconds he had available was pretty unlikely, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The least plausible thing about the story was the force of Martian winds -- at less than 1% of Earth atmospheric pressure, even a big storm on Mars wouldn't lift anything other than the finest grains of dust. Both the initial accident and the flapping of the makeshift hab repair job make no sense given that fact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next least plausible thing IMO is the decision by the Hermes crew to return to Mars to rescue Watney. Given that there was already a workable plan to keep him alive and get him back, it was criminally insane to divert a multi-billion dollar mission and risk five more lives in the process. If I remember correctly, the book devotes some attention to mechanical breakdowns aboard the Hermes during its extended mission; while it was designed to be reused as the interplanetary carrier for subsequent Ares missions, it was supposed to get some maintenance while parked in Earth orbit between trips. If Hermes had fallen apart en route, that would have been the end not only of the crew but the entire Ares program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The convenient ability to electrically interface the hab solar panels to both Pathfinder's and the rover's electrical systems seems a bit unlikely, but not physically implausible per se.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrazine and its decomposition products are pretty toxic, so his water source would have killed him.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The film doesn't address the problem of radiation exposure - with little atmosphere and magnetosphere, radiation shielding for human explorers is a necessity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pathfinder detached from its parachutes while still a ways up (it landed via airbag), but in real Martian weather I suppose it wouldn't have wandered too far, and Watney might have had access to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Pathfinder#/media/File:MPT_Hardware_on_the_Surface_MRO_picture.jpg&quot;&gt;this map&lt;/a&gt; to lead him from the chute to the lander. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a bunch of stuff that is barely physically plausible, but almost certainly wouldn't work out in practice, like improvising an orbital rendezvous using vented cabin air for thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All that said, it's a fun movie, and the grossly plausible aspects of the science are interesting to see. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-11-09T22:47:31.960" LastActivityDate="2015-11-09T22:47:31.960" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12640" PostTypeId="2" ParentId="12638" CreationDate="2015-11-09T22:39:01.450" Score="25" Body="&lt;p&gt;The most inaccurate &quot;artistic license&quot; thing was the Martian storm - Andy Weir admitted that he misrepresented it for dramatic effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most blatant error though was in growing the potatoes. In particular, the matter of lighting. For the depicted indoor growth, Mark would need 60 kilowatt worth of LED growth lights, and a proportionally sized solar farm. Potatoes are light-hungry. The sunlight levels on Mars are insufficient for direct sunlight growth, and for artificial light you'd need about 4m^2 of solar panels per m^2 of farmland.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-11-09T22:39:01.450" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12642" PostTypeId="2" ParentId="12638" CreationDate="2015-11-09T22:57:49.160" Score="5" Body="&lt;p&gt;With 2 mm/s^2 acceleration, Hermes' 124 day trip from Low Earth Orbit to Low Mars Orbit is impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Low earth orbit?&quot; a Weir defender might object, &quot;It's all hyperbolic fly bys.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is wrong, of course. The hyperbolic rendezvous were extraordinary maneuvers made under unusual circumstances (Watney needing rescue). The 124 day earth to Mars trip preceded the Sol 6 windstorm that stranded Watney. There aren't hyperbolic fly bys either at the beginning or end of this leg of the journey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;The 124 day trip wasn't depicted in the movie&quot;. It was described in Weir's book as well as the &lt;a href=&quot;http://s3.foxfilm.com/foxmovies/dev/films/108/assets/ares-mission-guide.pdf.zip-1442438953.zip&quot; rel=&quot;nofollow noreferrer&quot;&gt;movie's back story provided by Fox&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also Neil DeGrasse Tyson's trailer for the movie. &lt;a href=&quot;http://www.youtube.com/watch?v=-fdKyszL1Zo&amp;amp;t=1m15s&quot; rel=&quot;nofollow noreferrer&quot;&gt;1:15 of Neil Degrasse Tyson's trailer&lt;/a&gt; has Hermes departing from low earth orbit. 2:20 gives the trip at 124 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition here's a graphic from &lt;a href=&quot;https://www.insidescience.org/content/inside-spaceflight-martian/3251&quot; rel=&quot;nofollow noreferrer&quot;&gt;Inside Science&lt;/a&gt; (thank you Pearson Art Photo). I underlined the relevant phrase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/l69dr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/l69dr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Weir defender writes &quot;The 124 day mission seems to not include LEO, but otherwise is fine.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is like saying the 5 hour drive from Spokane Washington to Great Falls Montana seems to not include the Rocky Mountains but otherwise is fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cities are 300 miles apart so a 5 hour drive seems plausible until you take into account that they're separated by the Rocky Mountains.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And so it is with the Hermes 124 day trip. When you're only accelerating 2 mm/s^2, getting out of LEO is huge. Here is an illustration from Mark Adler's answer to the Stack Exchange question &lt;a href=&quot;https://space.stackexchange.com/questions/8420/general-guidelines-for-modeling-a-low-thrust-ion-spiral&quot;&gt;General guidelines for modeling a low thrust ion spiral?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WFOkk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WFOkk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mark Adler's description of above spiral:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Here is an example of a spiral from a circular orbit to escape (C3=0):&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This is normalized to the starting circular orbit, where the distances&#xA;  are in units of the initial orbit radius, and the acceleration is&#xA;  constant at 10−3 of the gravitational acceleration of the body at the&#xA;  initial orbit radius. The total ΔV to escape is 0.856 of the initial&#xA;  orbit velocity, as compared to 1.0 for the rule of thumb. The total&#xA;  time to escape is 136 initial orbit periods. It goes around the body&#xA;  about 40 times before escaping.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In Hermes' case the 2 mm/s^2 constant acceleration would be about 2*10^-4 of the gravitational acceleration at the initial body radius, so the delta V would be &lt;em&gt;more&lt;/em&gt; than .856 * 7.73 km/s. But I'll be kind and go with this under estimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;.856 * 7.73 km/s is about 6.6 km/s. At 2 mm/s^2, it would take Hermes 3.3 million seconds to achieve this delta V. 3.3 million seconds is ~38 days. That leaves about 90 days to go from a 1 A.U. to a 1.52 heliocentric orbit. Which isn't doable at 2 mm/s^2 acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of that 38 days would be a slow spiral through the Van Allen Belts. Not only is the 124 day trip from LEO to LMO impossible, but it would also cook Watney and friends.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-12-26T22:00:15.507" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12643" PostTypeId="2" ParentId="12638" CreationDate="2015-11-09T23:39:04.683" Score="17" Body="&lt;p&gt;The most obvious thing in my opinion was the orientation of Hermes in the scene right before they receive the hidden transmission with instructions for the Rich Purnell manoeuvre. They had just discussed the fact that Hermes is currently slowing down in its approach to Earth, which would mean the vehicle is pointing retrograde. However, when it cuts to Hermes you can clearly see it traveling towards Earth bow first with the engines brightly lit firing in the prograde direction. Either they intended that shot to be used after the manoeuvre was discussed and agreed upon by the crew but then edited it later, or the usual Hollywood lack of understanding is to always show a spacecraft pointing in the direction it is moving.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-11-09T23:39:04.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12644" PostTypeId="1" CreationDate="2015-11-10T01:10:42.457" Score="4" ViewCount="197" Body="&lt;p&gt;Infrared telescopes need to be very cold (often below 10 K) to be effective. So do a few other miscellaneous things, including superconductors, if we ever use them in space for e.g. mass drivers or radiation shielding. When it's launched, JWST will have &lt;a href=&quot;https://smartech.gatech.edu/bitstream/handle/1853/38740/002.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;a 500W-550W four-stage cooling system&lt;/a&gt; that manages to reject a measly 76mW at 6.2 K &amp;mdash; roughly a 7000 to 1 input power to useful cooling load ratio. However, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Zeeman_slower&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zeeman slower&lt;/a&gt; seems tailor-made for this, since it cools only with radiated heat, can cool down to the required temperature range, and is already sometimes used instead of that same basic refrigeration design (Joule-Thomson etc) in Earth-bound labs, where there's convection and conduction to dump the extra heat more efficiently than via radiation alone. Given this, are there any obstacles to Zeeman slowers being seriously considered for cold-instrument mission design in the near future?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(This is a more focused version of a similar question I recently asked, about &lt;a href=&quot;https://space.stackexchange.com/questions/12615/how-close-are-zeeman-slowers-to-being-useful-for-general-cooling-in-space&quot;&gt;general systems cooling&lt;/a&gt;.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-08-26T14:34:45.303" Title="Could IR space telescopes and other very cold devices use Zeeman slowers effectively in the near term?" Tags="|mission-design|cryogenics|thermal|space-telescope|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12645" PostTypeId="1" CreationDate="2015-11-10T05:05:57.023" Score="6" ViewCount="190" Body="&lt;p&gt;After looking at the &lt;a href=&quot;https://space.stackexchange.com/questions/10749/could-liquid-airlocks-work&quot;&gt;previous question on liquid airlocks&lt;/a&gt;, I noted that the main problem with technology is that the force of gravity is far too weak on any body that an airlock would be a useful piece of equipment for the liquid airlock to work effectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, lets replace gravity with the magnetic force, and the liquid with a ferroliquid, and see what happens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious advantages of such an airlock would be that it could be used in a continuous manner, perhaps for mining operations (for moving large machinery and for moving a conveyor belt of minerals).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would this work in a vacuum?&lt;/p&gt;&#xA;" OwnerUserId="6157" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-11-10T05:05:57.023" Title="Could a Ferroliquid airlock work?" Tags="|habitat|airlock|" AnswerCount="0" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="12646" PostTypeId="2" ParentId="11969" CreationDate="2015-11-10T09:24:25.867" Score="9" Body="&lt;p&gt;Roman aqueduct engineers used a &lt;a href=&quot;https://en.wikipedia.org/wiki/Roman_aqueduct&quot;&gt;typical gradient&lt;/a&gt; of 1:4800, which is about 20cm per km. The equivalent acceleration is about $2 mm/s^2$. So not very much is required to keep the water flowing.&lt;/p&gt;&#xA;" OwnerUserId="10417" LastActivityDate="2015-11-10T09:24:25.867" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12647" PostTypeId="2" ParentId="12626" CreationDate="2015-11-10T12:59:25.937" Score="6" Body="&lt;p&gt;The real information, not the &quot;public news&quot;, is what NASA uses.  Spinning a mundane negative result into hype, or describing the taking of measurements near the noise level of the instruments and then claiming it's a profound and confusing result, does not fool or impress the real engineers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does not work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no reason to suppose anything fishy is going on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://io9.com/no-german-scientists-have-not-confirmed-the-impossibl-1720573809&quot; rel=&quot;nofollow noreferrer&quot;&gt;Popular-press articles that claim otherwise&lt;/a&gt;, and articles written by the cranks themselves, &lt;a href=&quot;https://plus.google.com/117663015413546257905/posts/WfFtJ8bYVya&quot; rel=&quot;nofollow noreferrer&quot;&gt;don't change the actual facts&lt;/a&gt; even if they create a public mythology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now would a similar measurement in space &lt;em&gt;help&lt;/em&gt;?  A characteristic article I recall described small force measurements seemingly at random including those in the wrong direction or when the machine was off.  Tiny effects in the environment and noise in the instruments swamp any real reading.  If you tried it in a free orbiting platform, you would &lt;em&gt;also&lt;/em&gt; get random changes due to variable atmospheric drag, solar particles, magnetic fields, outgassing of parts, differential cooling, and light pressure, not to mention peturbations from other satellites and bodies in the solar system and irregularities in the Earth (for a good description read up on &lt;a href=&quot;https://einstein.stanford.edu/TECH/technology1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravity Probe B and the drag-free orbit effect&lt;/a&gt;).  Taking data, whether by careful positioning information or on-board accelerometers, you would have random changes that cannot be controlled for, exactly like the previous bench tests.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And you will still have &lt;a href=&quot;http://arstechnica.com/science/2015/08/heres-why-scientists-havent-invented-an-impossible-space-engine/&quot; rel=&quot;nofollow noreferrer&quot;&gt;reports that acceleration during the &quot;control&quot; phase (turned off)&lt;/a&gt; or in the wrong direction is somehow mysterious and suggestive, when it actually means that the trial run cannot be distinguished from the control and supports the &quot;does nothing&quot; hypothesis to enough sigmas to rule out any interesting effects.&lt;/p&gt;&#xA;" OwnerUserId="6471" LastEditorUserId="6471" LastEditDate="2017-04-26T21:07:41.077" LastActivityDate="2017-04-26T21:07:41.077" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12648" PostTypeId="2" ParentId="10877" CreationDate="2015-11-10T13:45:55.343" Score="4" Body="&lt;p&gt;There is a related question, one &lt;a href=&quot;https://space.stackexchange.com/questions/6167/what-is-the-accuracy-uncertainty-of-two-line-elements-tles?lq=1&quot;&gt;answer&lt;/a&gt; gives a rule of thumb. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This, present, answer addresses the first part of the question &quot;&lt;em&gt;how far into the future can I make accurate predictions for that satellites location using that TLE&lt;/em&gt;&quot; specifically the satellite orbit parameters rather than the consequences for the ground track line of sight etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a crude assessment, think &quot;system engineer without the budget to pay for an orbit analyst&quot;, that attempts to generalise to any orbit of interest.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Identify a non manoeuvering object with similar characteristics to the satellite of interest and obtain a short TLE history, or pick a stretch of time between known manoeuvres.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;From the first TLE, propagate forwards and compare with each of the later TLEs. The difference at each comparison is a proxy for the progressively worsening uncertainty due to propagation. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Repeat for several different start points and different objects to obtain an average.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Note that this doesn't account for the inaccuracy of the first TLE, only the degree of worsening through propagation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that the options to &quot;&lt;em&gt;to use a more accurate propagator&lt;/em&gt;&quot; are limited because a TLE gives you &quot;mean&quot; elements whereas other propagators may expect to be fed &quot;osculating&quot; elements. I don't know what would result from trying such a mix though I suspect it may be no improvement or worse than TLE/SGP4 which at least have the advantage of being designed together.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-11-10T13:45:55.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12649" PostTypeId="1" CreationDate="2015-11-10T13:55:09.917" Score="3" ViewCount="4977" Body="&lt;p&gt;I'm trying to get some information on this and its proving difficult as the information is usually acquired experimentally from what I've found. A single number at a specific altitude and speed would be more than enough.&lt;/p&gt;&#xA;" OwnerUserId="12307" LastActivityDate="2018-09-16T02:54:50.570" Title="How can I estimate the Coefficient of Drag on a Saturn V rocket, a simulator or some data would be pretty awesome" Tags="|rockets|atmospheric-drag|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12650" PostTypeId="2" ParentId="12649" CreationDate="2015-11-10T14:12:12.037" Score="7" Body="&lt;p&gt;Bob Braeunig's &lt;a href=&quot;http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V simulation page&lt;/a&gt; (gone missing; &lt;a href=&quot;https://web.archive.org/web/20170313142729/http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;archive.org link&lt;/a&gt;) has good information on how he estimated the drag coefficient, and a convenient C&lt;sub&gt;D&lt;/sub&gt; vs Mach number plot. The key point:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;No information has been found regarding the drag coefficient of the Saturn V, however at least one data point can be calculated – that at the time of maximum dynamic pressure. The Saturn V News Reference, August 1967, states &quot;At approximately 69 seconds into the flight the vehicle experiences a condition of maximum dynamic pressure. At this time, the restraining drag force is approximately equal to 460,000 pounds.&quot; From Ascent Data we see that, for Apollo 11, maximum dynamic pressure (also called maximum q) occured at a time of 83.0 s, an altitude of 44,512 feet, and had a value of 735.17 lb/ft&lt;sup&gt;2&lt;/sup&gt;. There are two methods we can use to estimate the drag coefficient at maximum q.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;First, given that q = rv&lt;sup&gt;2&lt;/sup&gt;/2, we substitue q for rv&lt;sup&gt;2&lt;/sup&gt;/2 in the drag force equation, obtaining F&lt;sub&gt;D&lt;/sub&gt; = q C&lt;sub&gt;D&lt;/sub&gt; A. We now rearrange the equation, plug in our known values (assuming 460,000 lbf is applicable to Apollo 11), and solve for C&lt;sub&gt;D&lt;/sub&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;C&lt;sub&gt;D&lt;/sub&gt; = 460,000 / (735.17 × 1,216) = 0.515&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The C&lt;sub&gt;D&lt;/sub&gt; versus mach number plot is based off data from the Atlas rocket, so its applicability to Saturn V is questionable, but here it is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qXnqu.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qXnqu.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-09-16T02:54:50.570" LastActivityDate="2018-09-16T02:54:50.570" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="12651" PostTypeId="1" AcceptedAnswerId="12654" CreationDate="2015-11-10T19:57:41.207" Score="4" ViewCount="573" Body="&lt;p&gt;A &lt;a href=&quot;https://physics.stackexchange.com/questions/217545/does-turning-a-spoon-in-water-raise-the-temperature#comment468096_217545&quot;&gt;comment on recent popular question&lt;/a&gt; brought me to &lt;a href=&quot;https://what-if.xkcd.com/71/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stirring Tea&lt;/a&gt; and this entry&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To boil a cup of water, you'd have to drop it from higher than the top of the atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If that is true, in theory a nice coating of ice could make a viable heat shield. The biggest problem that comes to mind is that water ice is relatively fragile.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming I could keep the ice from breaking away on re-entry could I use it for a heat shield on Earth re-entry?  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-11-10T21:02:37.067" Title="Can I use an ice cube as a re-entry heat shield?" Tags="|reentry|physics|atmospheric-drag|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12653" PostTypeId="2" ParentId="12638" CreationDate="2015-11-10T20:23:20.230" Score="2" Body="&lt;p&gt;I find it improbable that earth would be making the call to abort the mission when it takes 12 minutes x 2 for them to get the Mars weather report and to relay their decision. Whether in the scenario portrayed in the movie or any potential scenario, the astronauts would need to be empowered to make their own decisions.&#xA;This is relevant for the book and the movie.&lt;/p&gt;&#xA;" OwnerUserId="12313" LastActivityDate="2015-11-10T20:23:20.230" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12654" PostTypeId="2" ParentId="12651" CreationDate="2015-11-10T20:49:09.740" Score="7" Body="&lt;p&gt;Nice idea, but no, it doesn't work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's true that a cube of ice in 100km height has a potential energy of just about a third of what is needed to melt and boil it completely. First point, as you already mentioned: How to keep the liquid water in place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, talking about reentry usually applies coming from an orbit and hence having a speed of at least 7.8 km/s. This kinetic energy is about 20 times higher than the potential energy in 100km height. Hence, you need to get rid of about 6 times the energy the ice needs to boil.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, even worse: So far we just talked about the ice itself. A heat shield is put in place to protect something - and we need to get rid of both potential and kinetic energy of this something as well. If we assume a payload the same size as your block of ice - and that's still no good ratio at all - then we have to spend about 13 times more energy than the ice can take while evaporating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, when bringing some material back from an Asteroid it might be a viable option to bury it deep inside a huge ball of ice - but you better don't hit the wrong spot on earth as something similar already messed up our beloved dinosaurs life some time ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When talking about heat shielding, one also has to keep in mind that its purpose is not to absorb all the kinetic energy at reentry. It merely prevents that heat enters the payload taking advantage of its low heat conductivity (a feature water does not have) and it optimizes heat transfer back to the air rushing by. The ablative features of most materials are mostly helping to keep the material and its properties intact, though getting thinner with time.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2015-11-10T20:55:19.207" LastActivityDate="2015-11-10T20:55:19.207" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12656" PostTypeId="2" ParentId="12638" CreationDate="2015-11-10T21:45:26.070" Score="1" Body="&lt;p&gt;In the book (original epub version, anyway) the chemistry of making water had serious flaws.  The narrative assumes that all materials have the same number of molecules per unit volume.  I looked up density and molecular formulas in Wikipedia while I was reading, and found that the moles for the fuel was off by a factor of 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also found that the name of the Chinese rocket translates as “The Titanic”, which I thought was inauspicious and would be remarked upon in the story.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the Big One:  the air is so thin that even a hurricane air speed would not knock over the rocket or push the people around like that.  It seems unlikely that the rockets stay there for years, for multiple missions in staging, and can't handle the weather that occurs just as the people show up!  The worse wind to be expected would be a known factor.  Now they could have made this more plausible by coming up with an unexpected factor, like the wind-bourne dust is giving an impact force amplified compared to the wind itself.&lt;/p&gt;&#xA;" OwnerUserId="6471" LastActivityDate="2015-11-10T21:45:26.070" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12657" PostTypeId="1" AcceptedAnswerId="12659" CreationDate="2015-11-11T01:56:49.807" Score="1" ViewCount="303" Body="&lt;p&gt;While sending a space probe to orbit around a gas giant is often heard of, are there plans to send a space telescope to orbit around gas giants, so that it can operate far away from the sun, or even see the outer solar system clearer?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="6816" LastEditDate="2015-11-13T19:36:09.620" LastActivityDate="2015-11-13T19:36:09.620" Title="Is there any plan to send a space telescope to orbit around gas giants?" Tags="|mission-design|future-missions|jupiter|astronomy|space-telescope|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12658" PostTypeId="2" ParentId="12638" CreationDate="2015-11-11T02:56:35.613" Score="0" Body="&lt;p&gt;The hacking away at the rocket bothered me a lot. Thin atmosphere aside, I just can't stomach that you could hack that much off a a rocket and still have it work aerodynamically.&lt;/p&gt;&#xA;" OwnerUserId="12320" LastActivityDate="2015-11-11T02:56:35.613" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12659" PostTypeId="2" ParentId="12657" CreationDate="2015-11-11T03:31:21.240" Score="6" Body="&lt;p&gt;No, Jupiter is simply too far away (Distance = less data transmitted), it's a hard environment (Lots of radiation), and relatively little is accomplished by sending it out so far. But I do know of at least one serious proposal to send a telescope to Mars. A bare bones spy satellite (Telescope only really) was donated to NASA, and a proposal called &lt;a href=&quot;http://www.space.com/21064-nasa-donated-spy-telescope-mars.html&quot;&gt;Mars Orbiting Space Telescope&lt;/a&gt; (MOST). The idea is to send the telescope to Mars, where it could take photos of Mars, and take photos of deep space. It was tossed around almost 3 years ago, no idea what the current status is of it, but it seems likely it isn't going anywhere. A complete study of what might be done can be found at the &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/van-kane/20130220-brother-can-you-spare-1b.html&quot;&gt;Planetary Society&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two advantages of a telescope at Mars. The first is that it could get high resolution pictures of Mars. The second is that it would give a wider baseline, allowing us to determine where an object is with much higher precision than we can today, both inside and outside of the Solar System. I would imagine that we could extend the &lt;a href=&quot;http://lcogt.net/spacebook/parallax-and-distance-measurement/&quot;&gt;parallax distance&lt;/a&gt; from 1000 parsecs to perhaps 2000 or more. It would not be particularly useful for looking at dim objects, to keep the costs down, but would be great for looking at the gas giants, for instance.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-11-11T03:36:29.797" LastActivityDate="2015-11-11T03:36:29.797" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12660" PostTypeId="1" CreationDate="2015-11-11T04:00:47.530" Score="2" ViewCount="11489" Body="&lt;p&gt;I'm making a Mars probe in &lt;a href=&quot;http://forum.kerbalspaceprogram.com/threads/55145-1-0-4-Real-Solar-System-v10-3-1-Nov-3&quot; rel=&quot;nofollow&quot;&gt;Kerbal Space Program Real Solar System&lt;/a&gt;, and I have 13 km/s of Delta-V in the lifter itself. I have a cryogenic second stage with a KVD-1 engine, it has 7.36 km/s of Delta-V, and about 4 km/s is used for the orbital injection. Does my lifter have enough Delta-V? I can upload a picture with the delta-v overall, if that helps.&lt;/p&gt;&#xA;" OwnerUserId="12321" LastEditorUserId="12321" LastEditDate="2015-11-11T04:10:43.017" LastActivityDate="2015-11-12T22:31:32.697" Title="Delta-V to Low Mars Orbit" Tags="|mars|low-earth-orbit|delta-v|hohmann-transfer|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12661" PostTypeId="5" CreationDate="2015-11-11T04:34:02.223" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-11-11T04:34:02.223" LastActivityDate="2015-11-11T04:34:02.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12662" PostTypeId="4" CreationDate="2015-11-11T04:34:02.223" Score="0" Body="Use this tag for questions relating to missions that are manned by a crew." OwnerUserId="6" LastEditorUserId="6" LastEditDate="2015-11-11T10:48:43.200" LastActivityDate="2015-11-11T10:48:43.200" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12663" PostTypeId="2" ParentId="12660" CreationDate="2015-11-11T07:00:28.793" Score="8" Body="&lt;p&gt;Let's break down your problem into a very simplified launch phase from the ground to low Earth orbit at 250 km altitude. Then we'll use patch conics to get an estimate of the $\Delta v$ necessary to reach a low Mars orbit of 80 km altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One method that I like to use to get ball-park estimates of required launch $\Delta v$ is to first consider an instantaneous impulse on the ground that will give you enough velocity to reach the target orbit altitude (like throwing up a ball with enough speed that it just reaches the orbit altitude at it's peak), then consider another instantaneous impulse at that point that gives you enough velocity to reach orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Potential energy on the ground and in orbit (where $\mu_{Earth}$ is the gravitational parameter for Earth -- $\mu_{Earth} = 398601.2$ $\frac{\text{km}^3}{s^2}$):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{ground} = -\frac{\mu_{Earth}}{r_{ground}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{orbit} = -\frac{\mu_{Earth}}{r_{orbit}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We'll assume we start at the mean Earth radius of 6378 km (the actual radius at the Kennedy Space Centre could be substituted but probably won't make a significant difference considering our crude analysis). That gives us a difference in potential energy of 2.357 kJ/kg, which translates to a required initial speed of 2.171 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once we reach our peak height of 250 km, we'll be at zero velocity and need to accelerate to an orbit velocity of 7.755 km/s based on the following equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{orbit} = \sqrt{\frac{\mu_{Earth}}{r_{orbit}}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that gives us a total launch $\Delta v$ of about 9.926 km/s. If you launch from the equator to the East then you will have an initial speed of 0.465 km/s from the Earth's rotation, so that could reduce your launch $\Delta v$ to 9.461 km/s. This is probably about 5-10% higher than the true value, but a good conservative approximation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next we use patch conics to analyze the interplanetary transfer. The figure below shows the interplanetary trajectory using a Hohmann transfer from Earth to Mars in the centre, where we can just assume our heliocentric radii are those of the planets. The spheres of influence for Earth and Mars are expanded on the left and right, respectively, to show the hyperbolic trajectories in each planet's reference frame as well as the planetary orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OcnTI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OcnTI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First we find the semi-major axis of the Mars transfer orbit or MTO, $a_{MTO}$, based on the radii of the orbits of Earth and Mars (which we assume are circular for this analysis).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$a_{MTO} = \frac{1}{2} \left( r_{Earth} + r_{Mars} \right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we can find the velocities of the spacecraft as it departs Earth and when it reaches Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{MTO,E} = \sqrt{\mu_{Sun} \left( \frac{2}{r_{Earth}} - \frac{1}{a_{MTO}}\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{MTO,M} = \sqrt{\mu_{Sun} \left( \frac{2}{r_{Mars}} - \frac{1}{a_{MTO}}\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Earth reference frame, we consider the spacecraft to &quot;depart&quot; when it leaves the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sphere_of_influence_%28astrodynamics%29&quot; rel=&quot;noreferrer&quot;&gt;sphere of influence&lt;/a&gt; or SOI (at a radius of about 924000 km). Assume we can set up our hyperbolic escape trajectory so that our velocity in the heliocentric frame is parallel to Earth's. So that means that in the Earth frame, our velocity at the edge of the SOI will be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{SOI,E} = v_{MTO,E} - v_{Earth}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $v_{Earth} = \sqrt{\frac{\mu_{Sun}}{r_{Earth}}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the radius and velocity at the SOI we can find the semi-major axis of the escape trajectory as well as the required MTO insertion velocity to leave low Earth orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$a_{MTO,E} = \left( \frac{2}{r_{SOI,E}} - \frac{v^2_{SOI,E}}{\mu_{Earth}}\right)^{-1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{Insertion} = \sqrt{\mu_{Earth} \left( \frac{2}{r_{LEO}} - \frac{1}{a_{MTO,E}}\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that our velocity in LEO is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{LEO} = \sqrt{\frac{\mu_{Earth}}{r_{LEO}}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in our numbers, we get $v_{Insertion} = 11.318$ km/s and $v_{LEO} = 7.755$ km/s, resulting in a required $\Delta v$ of 3.563 km/s to leave LEO and enter MTO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next we use the same analysis at Mars, where the right-hand side of the above figure shows the spacecraft hyperbolic rendezvous trajectory as well as the target low Mars orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First we determine the relative velocity that the spacecraft will have at the edge of the sphere of influence (again assuming that the velocity is initially parallel to Mars'):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{SOI,M} = v_{Mars} - v_{MTO,M}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next we determine the hyperbolic orbit semi-major axis and rendezvous velocity at the periapsis (which will be at low Mars orbit radius):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$a_{MTO,M} = \left( \frac{2}{r_{SOI,M}} - \frac{v^2_{SOI,M}}{\mu_{Mars}}\right)^{-1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{Rendezvous} = \sqrt{\mu_{Mars} \left( \frac{2}{r_{LMO}} - \frac{1}{a_{MTO,M}}\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that our velocity in LMO is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{LMO} = \sqrt{\frac{\mu_{Mars}}{r_{LMO}}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in our numbers, we get $v_{Rendezvous} = 5.618$ km/s and $v_{LMO} = 3.514$ km/s, resulting in a required $\Delta v$ of 2.105 km/s to enter LMO from the Mars transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With those two manoeuvres, the total $\Delta v$ for interplanetary transfer is 5.668 km/s. Adding the approximated launch $\Delta v$ results in a grand total of 15.129 km/s, which should be within reach of your design.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-11-12T22:31:32.697" LastActivityDate="2015-11-12T22:31:32.697" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12664" PostTypeId="2" ParentId="12657" CreationDate="2015-11-11T08:56:25.257" Score="2" Body="&lt;p&gt;If you're going to send a telescope far away, you shouldn't put it in orbit around a planet.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a telescope orbits a planet, the planet blocks part of the sky, limiting your observations. It also reflects light, limiting your observations some more. In most orbits, the telescope travels through the planet's shadow cone, which leads to temperature swings.&lt;br&gt;&#xA;So these days, space telescopes are sent to Lagrange points instead (the James Webb telescope will be parked at the Earth-Sun L2 point, for example).   &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-11-11T08:56:25.257" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12665" PostTypeId="2" ParentId="1943" CreationDate="2015-11-11T10:07:48.030" Score="2" Body="&lt;p&gt;In the eighties there was this &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Longshot&quot; rel=&quot;nofollow&quot;&gt;project 'Longshot' to go to Alpha Centauri B&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Project Longshot was a conceptual design for an interstellar&#xA;  spacecraft, an unmanned probe, intended to fly to and enter orbit&#xA;  around Alpha Centauri B powered by nuclear pulse propulsion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Longshot#Mission_profile&quot; rel=&quot;nofollow&quot;&gt;It was expected to reach and start orbiting Alpha Centauri B&lt;/a&gt; in ~100 years.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The journey to Alpha Centauri B orbit would take about 100 years, at&#xA;  an average velocity of approximately 13411 km/s, about 4.5% the speed&#xA;  of light, and another 4.39 years would be necessary for the data to&#xA;  reach Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12304" LastActivityDate="2015-11-11T10:07:48.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12666" PostTypeId="1" AcceptedAnswerId="12667" CreationDate="2015-11-11T12:18:57.787" Score="10" ViewCount="2440" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/12347/picture-of-equipment-left-on-the-moon&quot;&gt;After reading this question:&lt;/a&gt; I got an itch to to test out the mirrors myself (and maybe record whole process to irritate conspiracy nuts).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What equipment would I need?&lt;/strong&gt; I assume a high powered laser that would go through Earths atmosphere on clear night, some sort of mechanism that would be able to aim it very precisely (is there something that is commercially available), and a detector that would read the laser bounced back. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How would I have to set it up?&lt;/strong&gt; I would need to go out of the city get on some rocky place so mechanism wouldn't 'wobble' to much then point laser to the moon (not sure how would I know where to point) then place detector on the ground at short distance from laser (again not sure how I would know where to position it) and &lt;a href=&quot;http://www.bbc.co.uk/education/clips/zhxmhyc&quot; rel=&quot;nofollow noreferrer&quot;&gt;then if after ~2.6s later detector lights up&lt;/a&gt; - It Works. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How would I know that reflection I got is from mirrors and not some random 90 degree angled shiny Moon rock?&lt;/strong&gt; I would assume for this I would need to calculate how much dimmer signal has to come back (not sure how to do the calculation)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Updated by request in comment:&#xA;&lt;strong&gt;Can I just do this or will this get me into trouble with aviation authorities?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is such an experiment possible (for an individual), would I be able to get my hands on equipment and data necessary, or I am missing something?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="12304" LastEditorUserId="7062" LastEditDate="2021-09-13T16:03:57.857" LastActivityDate="2021-09-15T12:39:00.260" Title="Doing an experiment to point a laser to the moon and back" Tags="|the-moon|apollo-program|myth-busting|laser|calculation|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="12667" PostTypeId="2" ParentId="12666" CreationDate="2015-11-11T13:40:05.503" Score="12" Body="&lt;p&gt;This question was asked on Physics.SE with the question &lt;a href=&quot;https://physics.stackexchange.com/q/82276/871&quot;&gt;Amateur moon laser ranging&lt;/a&gt;. The answer is that in order to do so, you'd have to have a really precise laser beam (1 mRad divergence), along with a very accurate time measurement system (Nanosecond scale), and need a reasonably large telescope (On the order of a meter or two seems to be large enough). It's really just not practical to do, unless you are willing to drop a whole lot of change on the project (I'm guessing \$100,000 to \$1,000,000 range, roughly)&lt;/p&gt;&#xA;&lt;p&gt;As for the false alarm, well, the best trick is to simply repeat the measurement a day later, while pointing the laser at the same spot. The moon moves enough that if a rock directly faced you one day, it wouldn't the next day.&lt;/p&gt;&#xA;&lt;p&gt;For an individual, I'd say just settle for this video. &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/dsRsap2_RAc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2021-09-14T14:52:05.240" LastActivityDate="2021-09-14T14:52:05.240" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="12668" PostTypeId="2" ParentId="12660" CreationDate="2015-11-11T15:22:01.600" Score="3" Body="&lt;p&gt;I've made a &lt;a href=&quot;http://www.clowder.net/hop/railroad/Hohmann.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;spreadsheet&lt;/a&gt; for stuff like this. It assumes circular, coplanar orbits so the numbers are ball park.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a screen capture:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ckLxX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ckLxX.png&quot; alt=&quot;Screen Capture Spreadsheet&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've set ellipse's apoaerion altitude at the Sphere of Influence. This is a capture orbit, the sun's influence won't tear it away any time soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've set ellipse's periaerion altitude at 100 km. The ship passes through Mars upper atmosphere each periaerion. Atmospheric friction will shed velocity each pass serving to lower the apoaerion. When the apoaerion reaches 400 km or so, a tiny burn will serve to raise the periapsis above the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So at Mars arrival it will take a .7 km/s periaerion burn to achieve capture and eventually a low circular orbit.  &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2015-11-11T15:22:01.600" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12669" PostTypeId="2" ParentId="12638" CreationDate="2015-11-11T16:16:55.993" Score="3" Body="&lt;p&gt;I liked the film and found it quite realistic overall, but what I found very unrealistic was his repair of the hab using plastic foil and duct tape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has already been mentioned that the martian atmosphere is very thin. Inside the hab the air is breathable, so it's pressure must be substantially higher than that of the martian atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the pressure inside the hab was about the same as on earth, the force acting on the plastic foil would be very large:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;F = p*A&#xA;F force in Newton [N]&#xA;p Pressure difference in Pascal [N/m^2]&#xA;A Area in square meters [m^2]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;With p = 100'000 N/m^2, A = 4 m^2, the force would be&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;F = 400'000 N&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;about equivalent to the pull of &lt;strong&gt;40'000 kg&lt;/strong&gt; on earth!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if the pressure was lower and the area smaller, it's completely impossible that this repair would have worked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another unrealistic thing that was not mentioned yet was the &quot;oxygenator&quot;, it was never explained how it was supposed to works.&lt;/p&gt;&#xA;" OwnerUserId="12328" LastActivityDate="2015-11-11T16:16:55.993" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12670" PostTypeId="2" ParentId="12173" CreationDate="2015-11-11T20:34:04.257" Score="0" Body="&lt;p&gt;Spacecraft collision injuring occupant is actually listed as V95.42XD. In ICD-9 the code is E845.0  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find the info here: &lt;a href=&quot;http://icd10.nuemd.com/icd-10/codes/V95.42XD.&quot; rel=&quot;nofollow&quot;&gt;icd10.nuemd.com/icd-10/codes/V95.42XD.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12330" LastEditorUserId="109" LastEditDate="2015-11-11T23:47:56.667" LastActivityDate="2015-11-11T23:47:56.667" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12671" PostTypeId="1" AcceptedAnswerId="12674" CreationDate="2015-11-11T22:52:39.583" Score="12" ViewCount="1857" Body="&lt;p&gt;What exactly is that orange/rust coloured insulation foam on the Space Shuttle external tank, the H-IIA/B, Delta-IV, and so many other launch vehicles? I remember finding the name of it in a NASA document somewhere but it has since escaped my memory (it was a combination of two different compounds if I remember correctly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that I am asking what type of material this is, I understand very well the purpose of thermal insulation foam.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="6944" LastEditDate="2015-11-11T23:47:18.473" LastActivityDate="2015-11-11T23:47:18.473" Title="What exactly is the orange insulation foam used on many different launch vehicle stages?" Tags="|rockets|space-shuttle|materials|insulation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12672" PostTypeId="1" CreationDate="2015-11-11T23:13:02.553" Score="3" ViewCount="68" Body="&lt;p&gt;Although this seems to apply to older launch vehicles, many of them have checkerboard patterns on the stages and/or interstages. Is it cosmetic or functional? If functional, then what is the purpose? Fiduciary marking for camera tracking? Thermal control? Some kind of colour scheme for construction and assembly purposes?&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-11-11T23:13:02.553" Title="Why do some launch vehicle stages have checkerboard patterns?" Tags="|rockets|" AnswerCount="0" CommentCount="3" ClosedDate="2015-11-11T23:18:17.473" ContentLicense="CC BY-SA 3.0" />
  <row Id="12674" PostTypeId="2" ParentId="12671" CreationDate="2015-11-11T23:42:55.367" Score="14" Body="&lt;p&gt;At least for the Shuttle ET:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The tank’s foam is a polyurethane-type foam composed of five  primary&#xA;  ingredients: polymeric isocyanate, a flame retardant, a  surfactant, a&#xA;  blowing agent, and a catalyst. A surfactant controls  the surface&#xA;  tension of a liquid and thus cell formation. The blowing agent, HCFC&#xA;  141b, creates the foam’s cellular structure by  making millions of&#xA;  tiny bubbles or foam cells.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are four specially engineered closed-cell foams used on  the&#xA;  tank. The larger sections of the tank are covered in NCFI  24-124,&#xA;  which accounts for 77 per cent of the total foam used on  the tank.&#xA;  NCFI 24-57, which has a slightly different formulation than  NCFI&#xA;  24-124, is used on the aft dome, or bottom, of the liquid  hydrogen&#xA;  tank. PDL 1034, hand-poured foam used for filling  odd-shaped&#xA;  cavities, and BX 250/265 foam is used on the tank’s  “closeout”&#xA;  areas. During the early days of the External Tank’s  development,&#xA;  PDL was an acronym for Product Development  Laboratory, the first&#xA;  supplier of that foam.  NCFI 24-124 and NCFI 24-57 are mechanically&#xA;  sprayed foams;  BX 250/265 are applied mechanically on the feedlines&#xA;  and domes  inside of the intertank and manually-applied, or&#xA;  hand-sprayed in  the closeout areas.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note: NCFI stands for &lt;a href=&quot;http://www.ncfi.com/&quot;&gt;North Carolina Foam Industries&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.nasa.gov/sites/default/files/114022main_TPS_FS.pdf&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-11-11T23:45:23.160" LastActivityDate="2015-11-11T23:45:23.160" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12675" PostTypeId="2" ParentId="9421" CreationDate="2015-11-12T02:09:43.353" Score="3" Body="&lt;p&gt;In answer to &quot;what's taking so long&quot;, LMP-103S has actually made steady progress since that time, but the space industry is very conservative, so change takes time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One issue a friend of mine in the field mentioned that may potentially be slowing things down is that LMP-103S apparently currently costs about $5000/kg (about 15x the cost of hydrazine).&lt;/p&gt;&#xA;" OwnerUserId="12332" LastActivityDate="2015-11-12T02:09:43.353" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12676" PostTypeId="2" ParentId="12638" CreationDate="2015-11-12T11:06:16.660" Score="1" Body="&lt;p&gt;Aside from the enormous pressure that would have torn the makeshift repair on the air lock apart, I think the hydrazine burning would have poisoned Watney. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reaction happens in a more or less open setup, and hydrazine is toxic. Some would have evaporated or carried up as tiny droplets by the hydrogen and nitrogen, and escaped into the hab atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="12336" LastActivityDate="2015-11-12T11:06:16.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12678" PostTypeId="1" CreationDate="2015-11-11T12:20:40.707" Score="2" ViewCount="344" Body="&lt;p&gt;Imagine rovers collecting samples on Mars and launching them to dock with a Mars-Earth transfer spacecraft. This vehicle would be in a Mars cycler orbit which passes near Mars and Earth every 7 years, for example. The sample in a cubesat sized probe and put in Earth orbit, possibly landing on Earth. It could also carry terabytes of data in solid memory chips brought together with the rock samples from instruments on the surface of Mars. All being low weight and cheap to accelerate, using the fuel stored in the Cycler.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point would be to lower mass and complexity of the Mars ascent vehicle, so that for each return to Earth only one Atlas V class launch, or so, would be required to land it on Mars. Plus the one time larger launch of the reusable cycler. The cycler could provide fuel for Earth orbit insertion of the sample, and during the transfer also solar power, shielding, Earth communication, keeping the samples at Mars temperature if desirable. And maybe a capsule for direct Earth landing (storing one for each return mission foreseen). A sample return mission once every third conjunction (about 7 years) doesn't seem overly ambitious.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could sample ascent, Earth landing and cycler transfer vehicles be made light enough to make this feasible today? Or would they need too much overhead equipment? Would the high velocities required for rendezvous with a cycler orbit eat up the benefits?&lt;/p&gt;&#xA;" OwnerUserId="1457" OwnerDisplayName="LocalFluff" LastEditorUserId="1457" LastEditDate="2015-11-12T15:29:10.050" LastActivityDate="2020-12-28T17:27:05.270" Title="Could a small Mars Cycler be used for repeated sample and big data transfers?" Tags="|mars|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12679" PostTypeId="2" ParentId="12678" CreationDate="2015-11-12T16:16:53.373" Score="4" Body="&lt;p&gt;I think a more efficient method of data transfer would be relay from Mars orbit to Earth orbit. Of course this would involve beefing up both Mars and Earth satellites (most substantially, Mars). Of course, if you're talking about 100 TB of data, then we won't have any quick and efficient means of delivering that for a while.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as sample return, I can't see the need for sample return being that high anywhere in the near future. It is a lot more likely that we'll do more &lt;em&gt;in situ&lt;/em&gt; lab work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, &lt;em&gt;could you?&lt;/em&gt; Sure, I don't see any reason why you couldn't have cargo instead of crew. I just doubt a cycler will be set up specifically for data and sample purposes, due to the cost of setup. If you already had a cycler system set up, it might be used for that as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;More information on Mars data rates:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mars.nasa.gov/msl/mission/communicationwithearth/data/&quot; rel=&quot;nofollow&quot;&gt;Data Rates/Returns - Mars Science Laboratory&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astrosurf.com/luxorion/qsl-mars-communication3.htm&quot; rel=&quot;nofollow&quot;&gt;Space Communications with Mars - Luxorion&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2015-11-12T17:14:24.437" LastActivityDate="2015-11-12T17:14:24.437" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12680" PostTypeId="2" ParentId="12607" CreationDate="2015-11-12T16:48:24.447" Score="5" Body="&lt;p&gt;To some extent the protective equipment is designed around facial hair:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Additionally, the crew could not guarantee &lt;strong&gt;facial hair&lt;/strong&gt; and eyeglasses would not interfere with the integrity of the face seal of the mask. A second design of mask was reviewed to eliminate those issues.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;emphasis added, from &lt;a href=&quot;http://research.jsc.nasa.gov/BiennialResearchReport/PDF/SliSci-1.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Space Station Ammonia Respirator Kit - NASA&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does not seem that there is much regulation around facial hair. Astronauts are allowed to maintain their own style within reason, and because of the astronaut culture (with many coming from the military) most are clean shaven or have a well trimmed mustache at most. I'm sure that practically there is a length limit, but I doubt there is regulation yet since it probably has not been an issue thus far.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2015-11-12T16:48:24.447" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12681" PostTypeId="1" AcceptedAnswerId="12683" CreationDate="2015-11-12T18:25:46.293" Score="-2" ViewCount="385" Body="&lt;p&gt;Disregarding the distance from Earth, would Mars be more habitable than the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or better put, disregarding the distance from Earth, which of Mars or the Moon would be easier (in cost and/or effort) to make temporarily habitable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the Martian (from the book/ Movie) have fared better on the Moon?&lt;/p&gt;&#xA;" OwnerUserId="10160" LastEditorUserId="25" LastEditDate="2015-11-12T22:28:41.330" LastActivityDate="2015-11-12T22:28:41.330" Title="Disregarding the distance from Earth, would Mars be more habitable than the Moon?" Tags="|mars|the-moon|" AnswerCount="1" CommentCount="3" ClosedDate="2015-11-13T08:37:01.690" ContentLicense="CC BY-SA 3.0" />
  <row Id="12682" PostTypeId="2" ParentId="4330" CreationDate="2015-11-12T19:11:24.283" Score="2" Body="&lt;p&gt;This seems a complicated way of doing it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;theoretical&lt;/em&gt; minimum is surely the kinetic energy of a kg travelling at about 7.8km/sec, the speed required for a minimal LEO. This is given by 1/2.m.v^2 and comes out at 30 MegaJoules. Rather less than the thermal energy in a kg of gasoline (about 45MJ) If you want to go a bit higher and travel at 10,000km/sec it comes out at 50MJ.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to escape from the earth completely you'll have to move at 40.27 km/sec and the theoretical minimum energy to get to that speed is 811MJ.&lt;/p&gt;&#xA;" OwnerUserId="12340" LastActivityDate="2015-11-12T19:11:24.283" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12683" PostTypeId="2" ParentId="12681" CreationDate="2015-11-12T20:33:21.227" Score="3" Body="&lt;p&gt;I think the simplest answer is this: the Moon is a much harsher environment compared to Mars and has fewer usable resources. The only thing that makes the Moon more attractive for colonization/exploration (apart from being so much closer), is its smaller gravity well. That makes it much easier to descend to the surface as well as ascend to orbit. Without an atmosphere, you cannot take advantage of aerobraking or parachute assisted descent -- but I still believe the Moon's lower gravity still makes it easier for descent and ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would argue that Mark Watney would not have fared nearly as well on the Moon! He depended on Mars' atmosphere for survival and thermal problems nearly killed him (those conditions are even worse on the Moon). Not sure if lunar regolith could be used as soil for his potatoes, but maybe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it would be a very different story since a rescue mission would be much easier and the tools and supplies he would have available would be completely different. Also, this is all assuming you are taking the Ares III mission and supplanting it on the Moon, which is like asking how an Amazon exploration party would fare if suddenly moved to the Sahara. See kim holder's comment above -- it just isn't comparing apples to apples... not even apples to oranges... more like apples to dishwashers.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-11-12T21:45:46.923" LastActivityDate="2015-11-12T21:45:46.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12684" PostTypeId="2" ParentId="12678" CreationDate="2015-11-12T21:55:36.723" Score="7" Body="&lt;p&gt;I'm not sure data return in a physical format is worthwhile. Samples are another question, of course.&lt;/p&gt;&#xA;&lt;p&gt;If we're already willing to wait seven years for it all to come in, why not transmit data slowly throughout that period via a relay satellite? This means you can start interpreting it early (and help inform the ongoing sample collection work); you avoid the &amp;quot;all your eggs in one basket&amp;quot; problem; you escape the risk of physical data corruption; and, best of all, you get flexibility. You can relay information from other orbiters, from other (non-sample-return) vehicles, etc.&lt;/p&gt;&#xA;&lt;p&gt;But surely a relay satellite is too slow? Well, not really. Conveniently, we have a figure for the amount of data returned by the Mars Reconnaissance Orbiter over its first seven and a bit years; &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2013-324&quot; rel=&quot;nofollow noreferrer&quot;&gt;over 200 terabits&lt;/a&gt;, or 25 terabytes (TB). It's reasonable to assume that a dedicated relay satellite would have the ability to exceed this, and if we can transfer dozens of TB every seven years, we're seriously competing with the physical storage approach.&lt;/p&gt;&#xA;&lt;p&gt;Only a small fraction of the data sent through MRO is relayed from the surface, but if we knew there would be a dedicated and high-capacity relay satellite, rovers and landers would no doubt be built to rely on that infrastructure, and high-bandwidth relay uplinks on them could be prioritised.&lt;/p&gt;&#xA;&lt;p&gt;Yes, we'd still have to keep the data relay satellite working. But we're doing pretty well with Mars orbiters - every orbiter that's made it to Mars in the past fifteen years is still going - and if we built a cycler, we'd have to worry about keeping &lt;em&gt;that&lt;/em&gt; going.&lt;/p&gt;&#xA;&lt;p&gt;All told, from the data transfer perspective, it's probably unnecessary.&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="1235" LastEditDate="2020-12-28T17:27:05.270" LastActivityDate="2020-12-28T17:27:05.270" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="12688" PostTypeId="2" ParentId="10904" CreationDate="2015-11-13T09:18:03.927" Score="2" Body="&lt;p&gt;An aspect not yet mentioned, is that I don't think solar panels work too close to the Sun. &lt;a href=&quot;http://www.techbriefs.com/component/content/article/ntb/tech-briefs/physical-sciences/18899&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Probe Plus&lt;/a&gt;, at 8.5 solar radii or 6 million km at the closest, will actually fold back its solar panels into the shadow of the spacecraft while near the Sun. Also, optically concentrating photovoltaic cells are among the most efficient today, and I think that overheating is a limit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eIXJw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eIXJw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-11-13T09:32:39.143" LastActivityDate="2015-11-13T09:32:39.143" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12689" PostTypeId="1" AcceptedAnswerId="12691" CreationDate="2015-11-13T09:23:45.910" Score="7" ViewCount="1870" Body="&lt;p&gt;Why aren't more parts of spacecraft commonly reusable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I'm looking at the Soyuz plans, the three parts don't seem all that different in terms of survivalibility. The orbital module resembles Voskhod which was meant for reentry. The Service module seems quite similar in shape to the Descent module. I understand the fragile solar panels wouldn't withstand the reentry, nor would most of external parts, but wouldn't a coating of ablator and a parachute allow to recover most of the systems for refurbishing, even with the structural elements bent out of shape and burned?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the hardest part of making a module (partially) reusable? What's so expensive that letting it burn in the atmosphere is cheaper?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="2981" LastEditDate="2015-11-13T12:46:58.900" LastActivityDate="2016-12-30T23:33:21.593" Title="What is the hardest part of making spacecraft (mostly) reusable?" Tags="|spacecraft|reentry|reuse|" AnswerCount="4" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12690" PostTypeId="2" ParentId="12689" CreationDate="2015-11-13T13:45:10.247" Score="2" Body="&lt;p&gt;I suspect it's mostly a problem of weight vs. drag. If you wanted to land all 3 modules of a Soyuz, you'd have 3 times the weight of the descent module, but the heat shield area would remain the same. So the ballistic coefficient would drop and your speed during reentry would be higher. This means you need a heavier heat shield.&lt;br&gt;&#xA;You also need larger parachutes to land the increased mass. The extra mass of parachutes and heat shield has to be launched etc. and you have a weight spiral on your hands. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-11-13T13:45:10.247" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12691" PostTypeId="2" ParentId="12689" CreationDate="2015-11-13T14:06:16.930" Score="14" Body="&lt;p&gt;The hardest part of making a spacecraft reusable is making it cheaper to re-use than to let it burn up. Every gram you add in weight makes launch more expensive, and a spacecraft that can be re-used is going to be considerably heavier than a disposable one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to launch costs you have to factor in the costs of refurbishing and re-certifying a spacecraft after its been through the rigors of launch, space travel, re-entry and recovery. You can't just take the spacecraft, brush it off, re-fill it with hydrazine and stick it back up on top of a rocket, you have to test and repair all the systems on board. The shuttle program showed that these costs can be higher than outright replacement of the spacecraft every time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the newer spacecraft designs are for re-usable spacecraft, using lessons learned from the STS program and new technologies and materials to make it costs effective. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2015-11-13T14:06:16.930" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12693" PostTypeId="1" AcceptedAnswerId="12695" CreationDate="2015-11-13T15:53:46.683" Score="2" ViewCount="348" Body="&lt;p&gt;Obviously in the simplest variant it's not complex at all. I've seen amateur rocket engine nozzles shaped through applying a nail to packed clay layer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How complex is the science behind the shape of fixed nozzles used professionally - in space-faring rockets, in military missiles, solid rocket boosters etc? How much work - comparing to difficulty of other factors - is spent on designing the nozzle for a rocket?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2015-11-13T16:52:48.457" LastActivityDate="2015-11-14T02:11:57.990" Title="How complex is the science of a fixed nozzle shape?" Tags="|nozzle|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12695" PostTypeId="2" ParentId="12693" CreationDate="2015-11-13T17:09:54.160" Score="7" Body="&lt;p&gt;The general &lt;em&gt;shape&lt;/em&gt; of fixed nozzles is pretty close to a solved problem. Per &lt;a href=&quot;https://books.google.com/books?id=TKdIbLX51NQC&amp;amp;pg=PA76&amp;amp;lpg=PA76&amp;amp;dq=conical%20nozzle%20efficiency&amp;amp;source=bl&amp;amp;ots=sl829Gdrs2&amp;amp;sig=O9m9GODbQIIQHQtgHJiSpE94qjc&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CGAQ6AEwCWoVChMI0_mc-vCNyQIVxyqICh2fswXp#v=onepage&amp;amp;q=conical%20nozzle%20efficiency&amp;amp;f=false&quot; rel=&quot;nofollow&quot;&gt;Huzel&lt;/a&gt;, even a conical nozzle of sufficient length and 15º half-angle gives better than 95% efficiency. Bell nozzles can exceed 99% efficiency in much shorter form. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual engineering design of a nozzle for a large liquid-fueled rocket is more complex; cooling the nozzle becomes a large challenge, and in &lt;a href=&quot;https://en.wikipedia.org/wiki/Regenerative_cooling_%28rocket%29&quot; rel=&quot;nofollow&quot;&gt;regeneratively cooled&lt;/a&gt; engines, it interacts with the propellant pump system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In solid rockets, regenerative cooling isn't an option, so I believe ablative cooling is more common, helped by the fact that burn times are often shorter -- typically around 2 minutes instead of 4-6 minutes for liquid first stages. Substantial engineering issues involved in interfacing the casing to vectored nozzles, but again, shape-wise, it's generally a near-optimal bell nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Either the Huzel or Sutton book will take you through the next couple of levels of complexity on the topic. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-11-14T02:11:57.990" LastActivityDate="2015-11-14T02:11:57.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12696" PostTypeId="1" CreationDate="2015-11-13T18:18:24.873" Score="7" ViewCount="568" Body="&lt;p&gt;I've been looking everywhere for the detailed drivers for why it costs so much to launch cargo of any kind into LEO. From all the people I've talked with and the research through google, labor for getting the rocket ready to launch is the cost driver, since people cost a lot to pay per hour, and a lot of them are needed to build and inspect the rocket before launch. However, it never gives a breakdown as to where most of the man-hours go. I suspect it goes into building and inspecting the engines, since a lot of the assembly and qualification testing is not automated, but I haven't been able to find a good answer so I don't know. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the time breakdown for getting a rocket like the Falcon 9 ready for launch? Is most of the time spent in building the rocket, in assembling components for the engines? Or is it mostly spent inspecting and qualification testing those components/assemblies for flight?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I know there are significant differences between the Falcon 9 and Atlas 5, but since data from SpaceX and ULA is hard to get, I figured I'd not pick only one rocket, since there might be next to no useful information out there. &lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="11377" LastEditDate="2015-11-13T18:24:37.487" LastActivityDate="2015-11-13T22:22:42.767" Title="What is the time breakdown for getting a rocket like the Falcon 9 or Atlas 5 ready for launch?" Tags="|launch|cost-to-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12699" PostTypeId="2" ParentId="12693" CreationDate="2015-11-13T20:28:54.657" Score="3" Body="&lt;p&gt;&quot;How complex&quot; sounds like a judgement call, but if you wish to make up your own mind, take a look at Chapter 3 &quot;Nozzle Theory and Thermodynamic Relations&quot; in &lt;a href=&quot;https://web.archive.org/web/20140722111108/http://web.mit.edu/e_peters/Public/Rockets/Rocket_Propulsion_Elements.pdf&quot; rel=&quot;nofollow&quot;&gt;Sutton&lt;/a&gt;.  Although this edition of the book is somewhat infamously &quot;watered down&quot;, this particular chapter still seems good, and should give a handle on the basic theory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that you asked about the &quot;science&quot;, which is covered in the work I cite.  If you want to talk about the &quot;engineering&quot; of rocket nozzles, which involves turning this theory into a workable device, that's another question entirely.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-11-13T20:28:54.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12700" PostTypeId="2" ParentId="12696" CreationDate="2015-11-13T21:34:39.637" Score="3" Body="&lt;p&gt;This is very broad and fairly complex question. There is a good &lt;a href=&quot;http://www.airspacemag.com/space/is-spacex-changing-the-rocket-equation-132285884/?all&quot; rel=&quot;nofollow&quot;&gt;article here&lt;/a&gt; that goes in some details, but basically boosters (which are the most expensive part of the rocket) have traditionally been very complex, high performance, single use items.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there is not much incentive to lower the cost and risk aversion is a big thing in space systems, it's much easier to stick with costly systems that are flight proven, than develop something new that's cheaper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you add in the overhead of the huge bureaucracies of NASA and the contractors involved the costs balloon to what they are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to answer your question - the reason for the high cost is the high complexity of the engines, the lack of reusability and the high overhead, not necessarily the labor of the engineers (even though that increases proportionally to the bureaucracy involved). &lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2015-11-13T22:22:42.767" LastActivityDate="2015-11-13T22:22:42.767" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12701" PostTypeId="2" ParentId="12689" CreationDate="2015-11-13T22:23:25.397" Score="9" Body="&lt;p&gt;Recovering the physical material is easy enough, but making it fly again is harder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The price of a space craft failing is spectacular.  This is doubly true for manned missions.  The required reliability of parts scales in kind.  Its much easier to develop highly reliable parts to work once than it is to make parts that achieve the same reliability time after time after time.  There's a lot of inelastic deformations and irreversible changes which occur in hardware in flight, especially when you play with ideas like a parachute assisted landing!  Building to make these elastic and reversable increases weight and increases cost.  You also have to build them to be re-inspectable, which is not always easy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, challenging weight or shape requirements create highly customized solutions.  A closer to home example is the cellphone market.  Batteries used to be replaceable, so when one got used up (from too many launches.. I mean phone calls), you could swap it out.  Now days, as phones have gotten thinner and battery demands have gotten higher, you see batteries glued into the case.  This is abysmal for the replacement battery business, but it shaves off a fraction of a millimeter from the phone dimensions!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now a cell phone is a commodity device.  Expand that effect correspondingly for a vehicle that has to hurtle into space, with a small market.&lt;/p&gt;&#xA;" OwnerUserId="3532" LastActivityDate="2015-11-13T22:23:25.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12702" PostTypeId="2" ParentId="12689" CreationDate="2015-11-14T05:27:54.830" Score="1" Body="&lt;p&gt;There was a &lt;a href=&quot;https://space.stackexchange.com/questions/12696/what-is-the-time-breakdown-for-getting-a-rocket-like-the-falcon-9-or-atlas-5-rea&quot;&gt;similar question&lt;/a&gt; earlier today regarding rockets.&#xA;Basically in space systems risk aversion is a very big thing, so there is a tendency to reuse systems that have been proven to work, so there is reusability in that sense. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real reason though is that there is no incentive for companies to design such systems. Those are government funded contracts, they tend to be cost plus (cost plus guaranteed profit), so you end up with custom built one off vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space shuttle was reusable but only after extensive rebuild after each flight.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-11-14T05:27:54.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12703" PostTypeId="1" AcceptedAnswerId="12704" CreationDate="2015-11-14T22:13:53.767" Score="2" ViewCount="494" Body="&lt;p&gt;I have a situation where I have proceduraly generated planets and moons in a solar system.  These bodies have varying mass and radius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now what I want to do is be able to define a &lt;strong&gt;circular&lt;/strong&gt; orbital path around each (Planet and Moons) that would be conceptually equivalent to Low Earth Orbit, High Earth Orbit, and Geosynchronous Earth Orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't want to get into high math calculating things exactly, and like I stated previously these orbits are circular not elliptical anyway.  What I do need is really just a way to fake it so it seems somewhat logical and semi-believable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone have any idea on how I might best go about this?  I would guess that the main, or maybe only necessary, variable in the equation would be the mass of the planet or moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also as a side but semi-related question would there be something similar I should take into account when placing moons around a planet to keep it semi logical?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for any help!&lt;/p&gt;&#xA;" OwnerUserId="11474" LastActivityDate="2015-11-14T22:49:43.227" Title="Calculate Low/High/Geo orbits for other planets? - math lite - gamedev" Tags="|orbital-mechanics|orbit|low-earth-orbit|geosynchronous|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12704" PostTypeId="2" ParentId="12703" CreationDate="2015-11-14T22:41:54.003" Score="5" Body="&lt;p&gt;For low/high orbit, you could just make them proportional to the diameter of the planet -- low orbit 250km for Earth, 125km for one half Earth's diameter, and so on -- the lower bound is dependent on atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For synchronous orbit, you can use the equations derived &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit#Derivation_of_geostationary_altitude&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; or &lt;a href=&quot;https://physics.stackexchange.com/questions/48698/what-is-geostationary-orbit-radius&quot;&gt;here&lt;/a&gt;; keep in mind the difference between altitude (above the surface of the body) and radius (from the center of the body). It's dependent on the rotational speed of the body, though, which is widely variable. Planets close to the star might be tidally locked or in an stellar-orbital resonance (like Mercury) in which case their synchronous orbit altitude would be outside their sphere of influence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For placement of moons, a lower bound would be the &lt;a href=&quot;https://en.wikipedia.org/wiki/Roche_limit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Roche limit&lt;/a&gt;; an upper bound would be the planet's &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hill sphere&lt;/a&gt;. Moon-to-moon interaction would be dependent on the moons' Hill spheres. In reality the moons would have complicated resonant interactions like those of Jupiter's, but those are more complicated than I'm willing to deal with in my space game. ;)  &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-11-14T22:49:43.227" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12706" PostTypeId="1" AcceptedAnswerId="17338" CreationDate="2015-11-15T12:50:06.923" Score="8" ViewCount="2429" Body="&lt;p&gt;There are websites that present home projects where people have successfully received weather satellite imagery from NOAA satellites (such as &lt;a href=&quot;http://www.asciimation.co.nz/bb/2014/03/19/using-rtl-sdr-to-automatically-receive-weather-satellite-images&quot;&gt;this one&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to know if there are other satellites in orbit, which transmit data in a form that can be (with proper setup of course) received by enthusiasts? It doesn't need to be imagery only, it can be any kind of data that is generated by the craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a plus, a particularly nice thing to have would be a website with a (updated) list of such satellites in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; &lt;br&gt;&#xA;My question can be considered a generalisation of an &lt;a href=&quot;https://space.stackexchange.com/questions/4485/which-high-resolution-earth-facing-satellites-transmit-data-to-the-public-in-rea&quot;&gt;existing one&lt;/a&gt;. I am asking for any data, not only imagery. And it doesn't need to be &quot;simply&quot; decodable by means of general consumer products.&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="6750" LastEditDate="2015-11-15T12:56:32.853" LastActivityDate="2016-07-21T00:06:17.993" Title="Satellites in orbit which transmit data available to public?" Tags="|artificial-satellite|data-transmission|noaa|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12708" PostTypeId="1" AcceptedAnswerId="14844" CreationDate="2015-11-15T20:13:45.980" Score="12" ViewCount="1805" Body="&lt;p&gt;This is a photo of the soyuz capsule posted by the asker of a previous question. One of the labels reads &quot;Not to stand near this side&quot;. What is the reason for this? Is it because of active ionizing or RF radiation sources on that side? Or a caution against the possibility of yet not fired retro rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RIzjC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RIzjC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12356" LastActivityDate="2016-04-15T08:18:33.917" Title="Stand clear sign on Soyuz capsule underside" Tags="|soyuz-spacecraft|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12709" PostTypeId="1" CreationDate="2015-11-16T01:15:52.087" Score="6" ViewCount="1264" Body="&lt;p&gt;Almost all leaves are a pretty similar shade of green, and from the altitude of a satellite a single pixel must be bigger than a whole bush. Yet satellite imagery has become &lt;a href=&quot;http://www.nytimes.com/1999/03/28/world/un-to-create-own-satellite-program-to-find-illegal-drug-crops.html&quot;&gt;central to detecting&lt;/a&gt; illegal drug crops - even though farmers take extensive steps to hide the crops by interspersing the plants among legal crops and planting small, irregular fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://books.google.com.mx/books?id=HB9PuEhHahQC&amp;amp;pg=PA250&amp;amp;lpg=PA250&amp;amp;dq=satellite%20coca&amp;amp;source=bl&amp;amp;ots=tlGrLcxPqI&amp;amp;sig=HXcfb80H_WWQ7gQK43IioJJtwHU&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0CCwQ6AEwA2oVChMIroyzjM2TyQIVwzEmCh1rswxZ#v=onepage&amp;amp;q=satellite%20coca&amp;amp;f=false&quot;&gt;World Drug Report 2010&lt;/a&gt; explains a little, but also makes it sound extremely hard:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Satellite images contain more information than a normal photo, since&#xA;  the satellites have extra sensors to register infrared colours.&#xA;  Precisely these infrared colours help to distinguish vegetation types.&#xA;  [From footnote:] Coca is a bush and the leaves can be harvested&#xA;  throughout the year. Therefore, the crop stage can vary by field but&#xA;  also within a field. At the same time a satellite image is taken, a&#xA;  field can contain different crops in different growing stages, which&#xA;  give a different appearance in the satellite image.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it has something to do with infrared information, but how does that give such specific information that an individual kind of plant can be distinguished with any confidence at all, even despite efforts to hide it, and confounding factors such as weather, slopes, and a wide range of mixed developmental stages? &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2015-11-16T04:25:10.677" LastActivityDate="2015-11-16T04:25:10.677" Title="How is satellite data used to find drug crops?" Tags="|artificial-satellite|imaging|camera|earth-observation|remote-sensing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12710" PostTypeId="1" AcceptedAnswerId="12711" CreationDate="2015-11-16T01:28:30.907" Score="15" ViewCount="1622" Body="&lt;p&gt;I'm building an electrolysis cell myself, and got to wondering. On Earth, there is a significant force pushing bubbles of hydrogen and oxygen upward, so your electrodes still contact mainly water. However, in space, I imagine the bubbles would either diffuse around the electrode tank, or cluster around the electrode itself, and thus the speed of reaction would be significantly lowered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, I imagine that it would be hard to conduct electrolysis in zero gravity because you lose essentially all control of where the output gas goes. How do they solve this?&lt;/p&gt;&#xA;" OwnerUserId="11967" LastEditorUserId="49" LastEditDate="2015-11-16T03:27:35.177" LastActivityDate="2015-11-16T03:27:35.177" Title="How do they conduct electrolysis in zero gravity?" Tags="|microgravity|technology|water|oxygen|hydrogen|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12711" PostTypeId="2" ParentId="12710" CreationDate="2015-11-16T03:10:49.380" Score="11" Body="&lt;p&gt;I guess the generic answer for any mass separation system would be that you spin it, but mass separation isn't really effective for electrolysis since that requires larger separation between anode and cathode which leads to protonic ohmic losses, i.e. increased internal resistance. So a more optimal species separation is done not by their mass but by their size and charge, usually through a &lt;a href=&quot;https://en.wikipedia.org/wiki/Proton_exchange_membrane&quot; rel=&quot;noreferrer&quot;&gt;Proton Exchange Membrane&lt;/a&gt; (PEM).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Polymer_electrolyte_membrane_electrolysis&quot; rel=&quot;noreferrer&quot;&gt;PEM electrolysis&lt;/a&gt; allows for tighter electrode placement to reduce &lt;a href=&quot;https://en.wikipedia.org/wiki/Polymer_electrolyte_membrane_electrolysis#Ohmic_Losses&quot; rel=&quot;noreferrer&quot;&gt;ohmic losses&lt;/a&gt;, allows for higher current and is tolerant to relatively high pressures. Water if fed on the anode end, and as current flows from cathode towards the anode, water is separated into hydrogen and oxygen, positively charged hydrogen is attracted through PEM towards the negatively charged cathode, and the negatively charged oxygen is repelled away from cathode towards the positively charged anode:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/XYm9a.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Diagram of PEM electrolyzer cell and the basic principles of operation. Animation source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Polymer_electrolyte_membrane_electrolysis&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on PEM electrolysis&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PEM electrolysis is &lt;a href=&quot;https://www.asme.org/engineering-topics/articles/aerospace-defense/making-space-safer-with-electrolysis&quot; rel=&quot;noreferrer&quot;&gt;also main source of oxygen on the International Space Station&lt;/a&gt;, and they use &lt;a href=&quot;https://en.wikipedia.org/wiki/Chemical_oxygen_generator&quot; rel=&quot;noreferrer&quot;&gt;perchlorate candles&lt;/a&gt; as backups (for emergencies or when the electrolysis machine is not operational):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The generator operates on a simple principle — electrolysis, the&#xA;  dissociation of oxygen and hydrogen from water through the application&#xA;  of electricity. What makes the generator safe? The device uses a&#xA;  proton-exchange membrane (PEM) to separate the generated hydrogen and&#xA;  oxygen.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In a PEM electrolyzer, water is oxidized at the oxygen electrode, or&#xA;  cell anode, to produce oxygen gas, releasing hydrogen ions (protons)&#xA;  and electrons. The hydrogen ions migrate from the cell anode to the&#xA;  cell cathode, or hydrogen electrode, under the effect of the electric&#xA;  field imposed across the cell, while the electrons are transferred by&#xA;  a dc power source. The protons and electrons recombine at the cell&#xA;  cathode to produce hydrogen. Liquid water is also released at the&#xA;  cathode due to a process called electro-osmotic drag. Oxygen and&#xA;  hydrogen are generated in a stoichiometric ratio—two volume units of&#xA;  hydrogen for every one of oxygen—at a rate proportional to the applied&#xA;  cell current.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-11-16T03:10:49.380" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12712" PostTypeId="2" ParentId="12709" CreationDate="2015-11-16T04:01:49.240" Score="7" Body="&lt;p&gt;If you're not asking specifically about the near IR range photogrammetry but more in a general sense, then one obvious method of detection of covert drug plantations is simply human activity around them and lack of any public records about activities in the area. So even satellite imagery in the visible spectrum could reveal access roads, air strips, piers and traffic in the middle of nowhere. Imagery in the IR range could then help detect such activities also during nights, reveal hot spots where electricity or open fire are used extensively, even that vehicles at rest have recently been on the move. A coffee or peanuts plantation simply won't have same patterns of activity than a coca farm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In near IR spectrum though, it's possible to distinguish between plant species in a similar way than we can all see the difference in foliage colors between certain tree species at certain seasons of the year, the difference only becomes much more apparent in near IR range because that spectrum extends to where plant species differentiate more than &lt;em&gt;it's all green to me&lt;/em&gt; in the visible spectrum. On average, foliage photosynthesizes most intensively in the yellow to orange spectrum, i.e. that's the light frequency they absorb, while they reflect the rest and that's why they appear to us (we see reflected light but not absorbed one) as the exactly contrasting dark green.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But plants at different stages of maturity also shift this absorbed spectrum. Most obvious is the shift to lower frequency (more red and into IR range) absorption during flowering. So if you know what season it is somewhere, and when cultures flower in that season, you can then extrapolate what crops you might be looking at. Couple this with growth rate (by measuring thermal inertia of the area you can estimate vegetation density), and you're one step closer to identifying one plantation's purpose. In other words, plant species' biological markers are more distinct if you can also include observations in the IR / near IR range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Repeat observations will then give you more clues, up to the point when you decide that you need boots on the ground.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-11-16T04:07:12.597" LastActivityDate="2015-11-16T04:07:12.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12714" PostTypeId="1" AcceptedAnswerId="12715" CreationDate="2015-11-16T06:50:27.073" Score="6" ViewCount="479" Body="&lt;p&gt;So a question came up on our photography forum on facebook... Will Nikon's VR work in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're not aware Nikon and most brands have an image stabilizer lens (generally at the rear of the lens barrel) that attempts to compensate for hand shake.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've got no idea exactly what sensor they use and the most expedient search reveals this article on nikon's site.  It mentions &quot;Angular Velocity&quot; sensors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nikonusa.com/en/learn-and-explore/article/ftlzi4s6/vibration-reduction.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nikonusa.com/en/learn-and-explore/article/ftlzi4s6/vibration-reduction.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TWCuM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TWCuM.jpg&quot; alt=&quot;Oleg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12377" LastActivityDate="2015-11-16T16:21:36.850" Title="Would Nikon VR work in Space?" Tags="|iss|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12715" PostTypeId="2" ParentId="12714" CreationDate="2015-11-16T16:21:36.850" Score="10" Body="&lt;p&gt;Nikon uses angular velocity sensors (rate gyros*) for measuring the lens shake and determine the compensation to be applied (from the link you provided). &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nikon VR lenses use two angular velocity sensors, one that detects vertical movement (pitch), the other, horizontal movement (yaw), with diagonal motion handled by both sensors working together.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Alj49.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Alj49.jpg&quot; alt=&quot;Nikon VR&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: nikon.com&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As angular velocity is same in earth or space for same motions, there should not be a problem in operating VR in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nikon's VR lenses &lt;a href=&quot;http://www.nikon.com/news/2010/0614_energia_01.htm&quot; rel=&quot;noreferrer&quot;&gt;are in use in space&lt;/a&gt; (ISS) without any modifications. According to Nikon,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nikon Corporation is pleased to announce that orders for one Nikon D3S and two Nikon D3X digital-SLR cameras, four interchangeable lenses including AF-S NIKKOR 400mm f/2.8G ED VR ... This equipment will be transported to the ISS via the Soyuz TMA-19 spacecraft, to be launched from the Baikonur Cosmodrome on June 16, 2010. ... No special improvements or modifications will be made to these products. They will be the same products available to consumers, ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;from &lt;em&gt;An Introduction to Feedback Control for Optical Systems&lt;/em&gt; by Keith B. Powell&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-11-16T16:21:36.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12716" PostTypeId="1" CreationDate="2015-11-16T16:52:45.877" Score="4" ViewCount="124" Body="&lt;p&gt;I've been trying to find figures for the total ionising dose and potential for single event upsets for spacecraft operations but I've ran into a bit of a brick wall. It seems like there is ample information regarding &lt;a href=&quot;https://www.nasa.gov/pdf/284277main_Radiation_MS.pdf&quot; rel=&quot;nofollow&quot;&gt;manned missions&lt;/a&gt; but very little 'raw' information (the manned mission information details the total dose received by the astronaut who will inevitably be protected by the spacecraft to some degree. Am I missing some source of data on this topic? Can anyone suggest a place to look or a paper to gain access to?&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2015-11-17T15:26:18.950" Title="Total ionising dose and single event upsets in orbit" Tags="|radiation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12717" PostTypeId="1" CreationDate="2015-11-16T20:29:36.100" Score="-2" ViewCount="642" Body="&lt;p&gt;Was reading &lt;a href=&quot;https://space.stackexchange.com/questions/176/which-planet-or-moon-would-be-most-feasible-to-terraform&quot;&gt;Which planet or moon would be most feasible to terraform?&lt;/a&gt; and specially &lt;a href=&quot;https://space.stackexchange.com/a/206/12379&quot;&gt;https://space.stackexchange.com/a/206/12379&lt;/a&gt; . I am not really good at science or maths so please excuse below and look at the figures and ideas shared as a simple person's analogy to what he thinks. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I don't know if we have technology to add mass to a planet. Let's say we decide to go for it so to have somewhat like Earth's gravity (from 1/5th say it becomes 1/3rd or better due to doubling size of Mars as well as having more real-estate for Mars colonists) while also presuming or assuming that such technology is not expensive to sustain (which probably is/would be wrong) wouldn't it change the rotational planet of Mars itself vis-a-vis in relation to other planets due to its attraction and repulsion by it, thereby also changing rotations of other planets, which will have unknown ripple effects on the solar system as a whole (at least the nine planets that are there ?), maybe also changing/risking Earth's atmosphere in the process for the worse? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a what-if fantasy idea probably might have tried in some sci-fi B-Movie. &lt;/p&gt;&#xA;" OwnerUserId="12379" LastActivityDate="2015-11-17T02:04:53.640" Title="wouldn't adding mass to Mars be problematic for its rotation as well as other planets?" Tags="|mars|terraforming|rotation|mass|" AnswerCount="1" CommentCount="3" ClosedDate="2015-11-17T04:11:33.000" ContentLicense="CC BY-SA 3.0" />
  <row Id="12718" PostTypeId="2" ParentId="3448" CreationDate="2015-11-16T22:14:26.073" Score="10" Body="&lt;p&gt;This answer addresses the headline question, rather than the detailed content that suggests a connection between HST and a KH9. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be a connection between the Hubble and a &lt;strong&gt;KH11&lt;/strong&gt;. Refer &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; and  &lt;a href=&quot;https://en.wikipedia.org/wiki/KH-11_Kennan&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. These links identify the prime contractor as Lockheed in both cases and also suggest a rough gemoetric similarity based on an anecdote about the shipping container having been used, with some difference such as the mirror size. This &lt;a href=&quot;http://www.spacesafetymagazine.com/space-debris/astrophotography/view-keyhole-satellite/&quot; rel=&quot;noreferrer&quot;&gt;link&lt;/a&gt; shows a ground photo though little can be discerned from it, even the scale, other than the outline geometrical shape. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;HST is an electro-optical system, where as the KH9 used photographic film returned in a series of &lt;a href=&quot;https://en.wikipedia.org/wiki/KH-9_Hexagon&quot; rel=&quot;noreferrer&quot;&gt;re-entry&lt;/a&gt; vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also this &lt;a href=&quot;http://www.americaspace.com/?p=20825&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; suggests a few differences rather than similarities and asserts that two KH11 based imagers were offered to NASA by the NRO - though this point seems to be directly refuted in the Wiki KH11 article, which states that the hardware was related to a different NRO program and even goes a little way to describing the optical path. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its interesting, certainly, though I would be rather circumspect about all of this information unless you are prepared to spend a while auditing each paper/web trail and, being realistic, it wouldn't surprise me if you find nothing you can be sure of. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, apologies for being vague but I think that &lt;em&gt;comes with the territory&lt;/em&gt;. I don't expect you are going to get much in the way of hard facts until sometime in the distant future when and if some declassification occurs. &lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2015-11-17T10:56:19.817" LastActivityDate="2015-11-17T10:56:19.817" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12719" PostTypeId="2" ParentId="12717" CreationDate="2015-11-17T02:04:53.640" Score="1" Body="&lt;p&gt;There are issues with trying to added mass to Mars, or any other celestial object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sourcing the material to add to Mars is the main problem. Getting the material to Mars will be the next problem and financing the operation will be another major problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring those problems, material would have to be added uniformly to the surface. A high concentration of very dense material in one location may introduce a very slight wobble in the spin of Mars, not its orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the time frames you are assuming, any material added to it the would most likely resemble unconsolidated sand or rubble on the surface of Mars and would not necessarily increase the density of the planet due to the density of the material used and the void spaces between the particles of sand/rubble.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-11-17T02:04:53.640" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12723" PostTypeId="2" ParentId="12716" CreationDate="2015-11-17T15:26:18.950" Score="4" Body="&lt;p&gt;You can run your own &lt;a href=&quot;https://creme.isde.vanderbilt.edu/&quot; rel=&quot;nofollow&quot;&gt;CREME96 models&lt;/a&gt;.  As for information, there is a ton of it at that site, or just search on CREME96.&lt;/p&gt;&#xA;" OwnerUserId="12389" LastActivityDate="2015-11-17T15:26:18.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12724" PostTypeId="2" ParentId="12508" CreationDate="2015-11-17T15:37:36.323" Score="3" Body="&lt;p&gt;The very structured nature of the features strongly suggests stray light.  In fact, &lt;a href=&quot;http://www.ciclops.org/media/sp/2011/6788_16338_0.pdf&quot; rel=&quot;nofollow&quot;&gt;in-flight calibration&lt;/a&gt; has an example of this (ref. Figure 18).  From the text:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Also, when a bright object is&#xA;  located near to the corners of the NAC frame, a bright streak can&#xA;  be seen to extend diagonally across the field of view&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12389" LastActivityDate="2015-11-17T15:37:36.323" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12725" PostTypeId="1" AcceptedAnswerId="17250" CreationDate="2015-11-17T16:39:10.840" Score="9" ViewCount="9842" Body="&lt;p&gt;I can't count how many science fictions stories I have read where someone is strapping on magnetic boots to walk in or on a space ship without gravity. How realistic of a solution is this? Just for reasons of mass I assume pretty much everything currently in space is &lt;a href=&quot;https://en.wikipedia.org/wiki/Non-ferrous_metal&quot;&gt;Non-ferrous&lt;/a&gt; which also means non-magnetic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming for some reason (not sure why) you actually wanted to walk in/on a space ship/station without gravity would magnetic boots or something else be the best way to go? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2023-03-22T07:08:13.240" Title="Are magnetic boots in space practical?" Tags="|microgravity|magnetic-field|feasibility|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12726" PostTypeId="1" CreationDate="2015-11-17T17:13:13.013" Score="2" ViewCount="1014" Body="&lt;p&gt;I am trying to calculate the orbit parameters for a cubesat (Duration, eclipse time and sun time) can someone help please &lt;/p&gt;&#xA;" OwnerUserId="12390" LastActivityDate="2015-11-17T19:08:08.207" Title="Cubesat Orbit calculation" Tags="|low-earth-orbit|cubesat|calculation|orbital-lifetime|" AnswerCount="1" CommentCount="2" ClosedDate="2015-11-18T08:03:20.657" ContentLicense="CC BY-SA 3.0" />
  <row Id="12727" PostTypeId="2" ParentId="12726" CreationDate="2015-11-17T19:08:08.207" Score="3" Body="&lt;p&gt;The easiest way to get this information is to use a tool such as the Satellite Tool Kit (STK). Essentially, this information varies over time, especially for high inclination orbits. A good orbital simulator will allow you to enter mean altitude, inclination, and eccentricity (Or alternatively apogee/ perigee/inclination)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you already have the satellite in orbit, then you can import the satellite into STK, by looking up the satellite from their orbit. The eclipse times are an available report.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-11-17T19:08:08.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12728" PostTypeId="1" CreationDate="2015-11-16T20:44:05.557" Score="1" ViewCount="94" Body="&lt;p&gt;What are the prerequisites to utilize the data from 'astrosat' or 'mangalyaan'. My aim is to check how useful the data generated by an Indian scopes are? I could not make any sense in the catalogs. &lt;/p&gt;&#xA;" OwnerDisplayName="quarkonium" LastEditorUserId="4660" LastEditDate="2015-11-17T21:04:49.313" LastActivityDate="2015-11-17T21:04:49.313" Title="Utilization of data from satellites mantained by India" Tags="|isro|mars-orbiter-mission|raw-data|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12729" PostTypeId="2" ParentId="12728" CreationDate="2015-11-17T01:00:59.433" Score="1" Body="&lt;p&gt;Right now, the data from ASTROSAT is not available as the &lt;a href=&quot;http://astron-soc.in/outreach/2015/09/astrosat-faq/&quot; rel=&quot;nofollow&quot;&gt;instruments are still being checked&lt;/a&gt;. According to Astronomical Society of India,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;ASTROSAT is a public observatory and is therefore available for any potential researcher in India and abroad. The first 6 months after launch will be devoted to extensive tests of all the systems. The next 6 months will be for observations of the sky by the teams that built the instruments. However, a year after the launch, a certain fraction of ASTROSAT time will be available for any scientist in India who proposes an observation that passes review. Two years after the launch, international scientists can apply as well. In addition, once an observation is done by ASTROSAT and some time has passed, the entire data will be made public to anyone who is interested !&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For using data from Mangalyaan, &lt;a href=&quot;http://www.isro.gov.in/pslv-c25-mars-orbiter-mission/announcement-of-opportunity-utilising-mars-orbiter-mission-data-mcc&quot; rel=&quot;nofollow&quot;&gt;a proposal has to be submitted to ISRO&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Proposals could be submitted by individuals or a group of scientists, academicians belonging to recognized institutions, universities and government organizations of India. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The present deadline for submission of proposals has already passed (Oct 10). I think the rules for using data from Mars Orbiter Mission (MOM) is similar to ASTROSAT. First, the data will be available to Indian and partner institutions and then for others. In general, the data will be made available to the public after some time, usually after the results have been published.&lt;/p&gt;&#xA;" OwnerUserId="11361" OwnerDisplayName="aeroalias" LastActivityDate="2015-11-17T01:00:59.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12730" PostTypeId="2" ParentId="10699" CreationDate="2015-11-17T21:29:25.780" Score="7" Body="&lt;p&gt;This wiki article on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;noreferrer&quot;&gt;molniya orbit&lt;/a&gt;  provides this helpful starting explanation from which one can begin to tie together both previous answers by Hobbes and Mike: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Much of the area of the former Soviet Union, and Russia in particular,&#xA;  is located at high latitudes. To broadcast to these latitudes from a&#xA;  geostationary orbit (above the Earth's equator) would require&#xA;  considerable power due to the low elevation angles. A satellite in a&#xA;  Molniya orbit is better suited to communications in these regions&#xA;  because it looks directly down on them. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An additional advantage is that considerably less launch energy is&#xA;  needed to place a spacecraft into a Molniya orbit than into a&#xA;  geostationary orbit. Disadvantages are that, as opposed to a&#xA;  spacecraft in a geostationary orbit, the ground station needs a&#xA;  steerable antenna to track the spacecraft and that the spacecraft will&#xA;  pass the Van Allen radiation belt four times per day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So there are two immediate advantageous features that suited the former Soviet Union, namely improved elevation angles for communications and the launch energy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To explain the latter a little more: the propellant mass required to achieve a given orbit depends in part on the difference in degrees between the latitude of the launch site and the inclination of the target orbit. For example, the lowest latitude launch site used by the former Sovient Union was at Baikonur with a latitude of 46 degrees, hence it would take least energy to go into a 46 degree orbit, more to get to 63.4 degrees and much more to get to GEO at 0 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next point is &quot;why choose 63.4 degrees specifically?&quot;. The same wiki article goes on to say: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In general, the oblateness of the Earth perturbs the argument of&#xA;  perigee, so that even if the apogee started near the north pole, it&#xA;  would gradually move unless constantly corrected with station-keeping&#xA;  thruster burns. To avoid this expenditure of fuel, the Molniya orbit&#xA;  uses an inclination of 63.4°, for which these perturbations are zero.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Put simply, the orbit is most stable there. To determine which feature amongst those mentioned was the driving feature when the former Soviet Union planned the Molniya communications satellite programme would be a big historical pursuit in its own right though it appears that dealing with the perturbation through the satellite life was a significant challenge as they chose 63.4 degrees. I haven't done the sums to compare the launch mass under the three scenarios for propellant and transmitter power by the way. &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-11-17T21:29:25.780" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12731" PostTypeId="1" AcceptedAnswerId="12732" CreationDate="2015-11-18T00:33:41.997" Score="4" ViewCount="6676" Body="&lt;p&gt;The ISS is about 330-410 km above earth surface and within the thermosphere layer, in which according to Wikipedia the temperature can reach 1500 °C. However the gas inside this layer is highly diluted so it feels like 0 °C, which I find very confusing (what happens to the sun light that hits the object in that layer?!). So I'm wondering how much is the average temperature on the surface of ISS.&lt;/p&gt;&#xA;" OwnerUserId="12391" LastEditorUserId="6816" LastEditDate="2015-11-18T01:10:28.667" LastActivityDate="2015-11-18T11:11:22.230" Title="What's the average temperature on the surface of ISS (International Space Station)" Tags="|iss|temperature|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12732" PostTypeId="2" ParentId="12731" CreationDate="2015-11-18T11:11:22.230" Score="6" Body="&lt;p&gt;At the altitude of ISS, the temperature depends on whether it is measured in sunlight or otherwise. In sunlight, the temperature can reach 121$^{\circ}$C, while in darkness, it can plunge to -156$^{\circ}$C. &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2001/ast21mar_1/&quot;&gt;According to NASA&lt;/a&gt;, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Without thermal controls, the temperature of the orbiting Space Station's Sun-facing side would soar to 250 degrees F (121 C), while thermometers on the dark side would plunge to minus 250 degrees F (-157 C). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-11-18T11:11:22.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12733" PostTypeId="1" CreationDate="2015-11-18T11:19:45.187" Score="8" ViewCount="2182" Body="&lt;p&gt;Do sun-synchronous orbit pass &lt;strong&gt;any&lt;/strong&gt; spot around Earth at a specified local time?&#xA;If yes or no, what is the reason?&#xA;(I know that this type of orbit passes everywhere at the equator at the same local time.)&#xA;thank you so much.&lt;/p&gt;&#xA;" OwnerUserId="12396" LastEditorUserId="1457" LastEditDate="2015-11-18T11:51:18.253" LastActivityDate="2018-03-28T08:16:07.190" Title="Are sun synchronous orbits possible for any place on Earth?" Tags="|orbit|sun-synchronous|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12734" PostTypeId="1" AcceptedAnswerId="12735" CreationDate="2015-11-18T12:11:38.587" Score="25" ViewCount="6408" Body="&lt;p&gt;I just saw a show that featured an incident in which an astronaut on a spacewalk to install the Canada 2 arm on the ISS was temporarily blinded by contaminated water inside his helmet.  His colleague said something like &quot;I was getting ready to help him get inside and out of his suit in a &lt;em&gt;relative hurry&lt;/em&gt; so he could get help&quot;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would imagine that it takes at least 20 minutes to get into the airlock and close the door, and about as long to get out of the suit.  Am I in the right ballpark?  How long would it take in an emergency?&lt;/p&gt;&#xA;" OwnerUserId="11184" LastActivityDate="2016-02-27T11:52:22.737" Title="How long does it take to get from a spacewalk to being inside the ISS and out of your suit in an emergency?" Tags="|iss|astronauts|eva|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12735" PostTypeId="2" ParentId="12734" CreationDate="2015-11-18T13:25:27.323" Score="31" Body="&lt;p&gt;Although not the incident you are referring to, an incident outside the ISS in 2013 during EVA 23 is a good reference point for your question -- where Italian astronaut Luca Parmitano had water leaking into his helmet. My answer is based primarily on information in &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/Suit_Water_Intrusion_Mishap_Investigation_Report.pdf&quot;&gt;the official NASA report regarding the incident&lt;/a&gt;. There is also a good summary on &lt;a href=&quot;http://weebau.com/iss/exp_36.htm&quot;&gt;this site&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During this emergency, it seems there was some debate about whether or not to repressurize the airlock faster but in the end it was decided to repressurize at a normal rate -- with the two options for ending an EVA prematurely being &quot;terminate&quot; and &quot;abort&quot;, they chose to &quot;terminate&quot;, making sure to tie up any loose ends before returning. It took 31 minutes from the time they decided to return until the time they opened up the inner hatch. Parmitano took 6 minutes to get inside the outer hatch, his EVA partner Chris Cassidy took another 9 minutes, and then it took another 5 minutes until they closed the outer hatch. Repressurization began 3 minutes later and lasted 8 minutes. After that, they performed an &quot;expedited suit doffing&quot; (faster-than-normal removal of a spacesuit), but I cannot track down how long that is supposed to take.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It took quite a while because nobody was really sure exactly how bad the situation was becoming. Let's say they chose to abort instead of terminate, in which case I imagine both astronauts would have returned immediately to the airlock (that gets rid of 9 minutes). The 8 minutes between the time they were both in the airlock and when repressurization began was most likely spent going through checklists of protocols -- let's say we can cut that down to 1 minute in an emergency. Repressurization took 8 minutes at the nominal rate, but according to recommendations in the linked report, this could be achieved in just over 1 minute (in fact, the system is capable of doing it even faster but it would likely cause adverse health effects on the astronauts). So that reduces the 31 minutes to only 8 or 9 minutes, followed by the unknown time to doff the suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose in certain situations just getting inside the pressurized station is good enough, in others you might only need to get the helmet off (as in the case of EVA 23), then in more dire situations you'd need to remove the entire suit or at least expose the chest.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-11-18T15:04:50.570" LastActivityDate="2015-11-18T15:04:50.570" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12736" PostTypeId="2" ParentId="12733" CreationDate="2015-11-18T14:42:48.140" Score="12" Body="&lt;p&gt;Sun-synchronous orbits take advantage of the &lt;em&gt;precession&lt;/em&gt; of the ascending node due to orbit perturbations (primarily the $J_2$ spherical harmonic due to the slight bulge in Earth's shape at the equator). By choosing the right combination of orbit altitude and inclination, that precession can be set to take 1 year so that the local time remains the same at each location &lt;em&gt;in the orbit&lt;/em&gt;. The required inclination is typically quite high, resulting in a polar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That orbit is not synchronized with the Earth's &lt;em&gt;rotation&lt;/em&gt;, so the satellite does not pass over the same longitude each time it crosses the equator. See the last figure on &lt;a href=&quot;http://earthobservatory.nasa.gov/Features/OrbitsCatalog/page2.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/l4qHg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/l4qHg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember that &lt;em&gt;local time&lt;/em&gt; is... well... local! There is continuously a spot (an entire arc, in fact) sweeping across the Earth where it is noon -- the best time to take photographs with the Sun at your back, which is usually why you use a sun-synchronous orbit in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, are sun-synchronous orbits possible for any place on Earth? Well, there is nothing really special about sun-synchronous orbits when it comes to coverage in terms of location (just the timing). For an orbit to pass over a particular location on Earth, you just need to have the right orbit inclination for that location's latitude (Deer Hunter hinted at this). How often you pass over the location depends on the orbit period, and it &lt;em&gt;is&lt;/em&gt; possible to have orbits that are in some resonance with Earth's rotation (hence they only pass over particular regions repeatedly... like geo-synchronous orbits) -- but let's ignore that since most orbits will eventually cover all longitudes. Sun-synchronous orbits are high-inclination, and the tilt of the Earth's rotation axis means that the orbit will eventually cover all latitudes as well. So sun-synchronous orbits can cover &lt;em&gt;any&lt;/em&gt; point on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, &lt;strong&gt;yes, sun-synchronous orbits are possible for any location on Earth&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12102" LastEditDate="2018-03-28T07:24:22.207" LastActivityDate="2018-03-28T07:24:22.207" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12737" PostTypeId="1" AcceptedAnswerId="23318" CreationDate="2015-11-18T20:42:51.397" Score="3" ViewCount="331" Body="&lt;p&gt;The Ku-band system consisted of a directional antenna mounted in the orbiter payload bay and provided high BW communication with a ground station. This page: &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-ovcomm.html#sts-kuband&quot; rel=&quot;nofollow&quot;&gt;KU-BAND SYSTEM&lt;/a&gt; describes the system as having uplink at 13.755 GHz and downlink at 15.003 GHz. The various modes and data rates are described as well, but there is no mention of the modulation / coding that was used. There seem to be a few IEEE articles that may explain the system in detail, however they require purchase. My guess is it should have been PM, but if someone could confirm it would be great.&lt;/p&gt;&#xA;" OwnerUserId="12356" LastActivityDate="2017-10-08T23:42:49.733" Title="What was the modulation used in the Space Shuttle Ku-band comm system?" Tags="|communication|space-shuttle|radio-communication|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12738" PostTypeId="2" ParentId="8736" CreationDate="2015-11-18T22:45:20.107" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=36507.0&quot; rel=&quot;nofollow&quot;&gt;There's one for SpaceX here&lt;/a&gt;. SpaceX has been under a great deal more scrutiny than other launch providers; most of the time people simply do not care enough about launch scrubs to put them in a list, since what's more important is a successful launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that to compile a similar list would involve combing through decades' worth of individual reports on launches, and even then uncertainty about whether or not reports simply didn't bother to mention any launch holds or scrubs would be very high. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The link to the PDF in the previous comment of the Shuttle missions summary does include the number of scrubs per mission, though the document is 300 pages long and it would take some time to go through that.&lt;/p&gt;&#xA;" OwnerUserId="12404" LastEditorUserId="6816" LastEditDate="2015-11-18T23:11:33.317" LastActivityDate="2015-11-18T23:11:33.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12741" PostTypeId="1" CreationDate="2015-11-19T12:45:20.470" Score="8" ViewCount="2297" Body="&lt;p&gt;I imagine that engineers have to decide where an inertial measurement unit (or just an accelerometer, gyro, etc.) is placed in a rocket or spacecraft. I'd like to know how are these positions decided? Do they put them in zones of least vibrations for the smoothest signal, etc.? Particularly for sensor fusion, how would one decide on sensor location for the best attitude estimation of a rocket? Thanks for your input!&lt;/p&gt;&#xA;" OwnerUserId="11551" LastEditorUserId="1142" LastEditDate="2015-11-23T07:14:44.697" LastActivityDate="2021-10-15T18:36:46.650" Title="How are IMU positions decided for rockets and spacecraft?" Tags="|rockets|attitude|spacecraft-development|measurement|guidance|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12742" PostTypeId="1" CreationDate="2015-11-19T14:30:26.183" Score="0" ViewCount="50" Body="&lt;p&gt;Assuming we just spotted an asteroid, of a similar size to the one that killed the dinosaurs, that is approaching earth and will hit it in an equally fatal way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presuming we detected this asteroid as early as possible: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would Humanity be able to ensure the prevention of this event within the given time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if so what would be reasonable steps to prevent this?&lt;/p&gt;&#xA;" OwnerUserId="11099" LastEditorUserId="195" LastEditDate="2015-11-19T16:10:51.113" LastActivityDate="2015-11-19T16:10:51.113" Title="If an extinction capable asteroid was approaching Earth, would we be capable of preventing the collision?" Tags="|asteroid|asteroid-redirect-mission|" AnswerCount="0" CommentCount="4" ClosedDate="2015-11-19T14:33:23.013" ContentLicense="CC BY-SA 3.0" />
  <row Id="12743" PostTypeId="2" ParentId="12741" CreationDate="2015-11-19T16:24:25.583" Score="6" Body="&lt;p&gt;For spacecraft it's &quot;where they fit and can be oriented properly&quot;. Harnessing constraints are somewhat considered (how do you get wires to it) and thermal constraints might be pretty important (ir telescopes need the detector to be very very cold so anything that produces heat is as far away as possible), but in general for satellites all motion is pretty slow so vibration isn't much of an issue. I can't talk about rocket design.  &lt;/p&gt;&#xA;" OwnerUserId="12004" LastActivityDate="2015-11-19T16:24:25.583" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12744" PostTypeId="2" ParentId="12741" CreationDate="2015-11-19T17:05:13.530" Score="5" Body="&lt;p&gt;It depends upon the mission.  Ideally you try to put them as close to the center of mass as you can so that you don't have that offset between the IMU and the center of mass to worry about, but these days with everything modeled up to the millimeter in CAD, even that isn't that important any more.  In practice, unless there is a mission driver, you put it where it makes sense for practical reasons, like to balance the mass distribution.  &lt;/p&gt;&#xA;" OwnerUserId="12389" LastActivityDate="2015-11-19T17:05:13.530" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12745" PostTypeId="1" AcceptedAnswerId="12746" CreationDate="2015-11-19T17:56:06.853" Score="7" ViewCount="2116" Body="&lt;p&gt;I've been reading around a lot and I haven't been able to find any clear explanations. Maybe I'm a bit confused or maybe I am just looking in the wrong places. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, what are some basic principles or guidelines to re-entry conditions on a spacecraft? Let's say I have a vehicle in a circular orbit at X km and I perform a retro burn to put my perigee at the tip of Earths atmosphere. Using a blunt-faced capsule design, what are some basic assumptions or relationships I can use to estimate initial velocity, entry angle (corridor bounds), max deceleration, heat flux, etc.? I have found several resources online, however I keep getting wrong numbers from my calculations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the very least, could somebody point me in the right direction? Or even explain some concepts I might be lacking? I've dealt with orbital mechanics in the past but this seems like a whole new ball game.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point I just need a few numbers to work with (max g load, heat flux and max temp) so that I can use those parameters for material and structural design. Plotting flight profiles will also need to be done however is not a top priority right now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a worked example for finding max deceleration. According to &lt;a href=&quot;https://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section%20III.4.1.7%20Returning%20from%20Space.pdf&quot; rel=&quot;noreferrer&quot;&gt;this FAA PDF&lt;/a&gt;, max deceleration can be found using $a_\max = (V^2)*(H*sin(\gamma)) / (2*e)$ where V is initial velocity, H is Earth scale height and $\gamma$ is flight path angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's say I'm entering Earths atmosphere at 10 km/s and I want a max g load of 8 g's, or 78.48 m/s&lt;sup&gt;2&lt;/sup&gt;. Using this equation, I will need a flight path angle of 1.75 degrees which seems very low to me. The Apollo missions had similar velocities and yet their entry corridor sat at around 7 degrees.&lt;/p&gt;&#xA;" OwnerUserId="12412" LastEditorUserId="6816" LastEditDate="2015-11-19T18:26:35.363" LastActivityDate="2023-02-07T04:35:52.827" Title="Re-entry Dynamics" Tags="|orbital-mechanics|orbit|spacecraft|reentry|atmosphere|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12746" PostTypeId="2" ParentId="12745" CreationDate="2015-11-20T00:40:41.967" Score="8" Body="&lt;p&gt;There aren't good back-of-the-envelope relations to get you what you want. However all you need is a simple numerical integrator, an atmosphere model with dispersions, and a vehicle model with mass, size, and some aerodynamic coefficients.  You can then play with the entry conditions (speed and flight-path angle) to see the trajectories. Ballistic entry is a good place to start, and once you get that working, you can add a small lift-to-drag ratio for control. The dispersions are important, as well as uncertainty in the entry flight-path angle, to assess whether you are too shallow to reliably avoid unintentional skip-out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google &quot;Sutton-Graves&quot; for simple formulae to compute the stagnation point convective heating, which gives you an order of magnitude on the peak heating and integrated heat load. Much harder to calculate is the radiative heating, which can be as much as the convective heating for an Earth entry. The codes to get a real prediction of the heating are quite complicated.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-11-20T03:25:06.547" LastActivityDate="2015-11-20T03:25:06.547" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12747" PostTypeId="1" CreationDate="2015-11-20T01:03:09.770" Score="5" ViewCount="427" Body="&lt;p&gt;During reentry, it's routine for the ionized atmosphere surrounding the spacecraft to cut off radio communications for a certain period. As far as I know, this can't be avoided, except perhaps by using aerodynamic lift to avoid sudden reentry deceleration and therefore heating. But that's a &lt;em&gt;radio&lt;/em&gt; blackout. What about using higher wavelength EM communications &amp;mdash; visible light, UV, or the like? Would those also be blocked by the reentry sheath typical of e.g. Apollo, Shuttle, or similar &quot;flying brick&quot; craft?&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="58" LastEditDate="2020-02-26T14:03:54.680" LastActivityDate="2020-02-26T14:03:54.680" Title="Would plasma sheathes from reentry heating block optical/UV communications as well as radio?" Tags="|reentry|communication|optical-communication|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12752" PostTypeId="1" CreationDate="2015-11-20T11:33:04.063" Score="2" ViewCount="182" Body="&lt;p&gt;There have been quite a few missions to libration points. As far as the dynamical possibilities are concerned, libration point vicinity offers a spectrum of orbit profiles.&#xA;What science data is not available and is relevantly needed by the space science community, that can be fulfilled by a spacecraft placed in an orbit around libration points (Both Earth-Moon and Sun-Earth L-points)?&#xA;In other words what all relevant science missions can be executed at libration points ?&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="25" LastEditDate="2015-11-20T16:57:55.173" LastActivityDate="2015-11-20T16:57:55.173" Title="Libration points - Science prospects" Tags="|orbit|spacecraft|mission-design|lagrangian-points|astrodynamics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12753" PostTypeId="1" CreationDate="2015-11-20T11:51:59.607" Score="-2" ViewCount="956" Body="&lt;p&gt;I have knowledge about basic control systems, mathematical modelling and deriving state space and transfer functions. I'm a part of a team designing a small satellite. Control will be achieved using RCS thrusters and reaction wheels. How do I design an attitude control system for such a satellite? References to lectures and/or books would be helpful. &#xA;To be more specific, I'd like to learn more about the mathematical and analytical part, where the controller algorithm is designed, analysed and tested.&lt;/p&gt;&#xA;" OwnerUserId="12386" LastActivityDate="2015-11-20T12:50:42.867" Title="How to design a spacecraft attitude control system?" Tags="|design|attitude|" AnswerCount="1" CommentCount="3" ClosedDate="2015-11-20T22:54:20.467" ContentLicense="CC BY-SA 3.0" />
  <row Id="12754" PostTypeId="2" ParentId="12747" CreationDate="2015-11-20T11:59:23.773" Score="7" Body="&lt;p&gt;The problem is the plasma is VERY bright.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find all the numbers easily but let's find a ballpark figure cobbling some available data together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A reentry capsule of $m=3$ tons (like Soyuz). Blackout of $t=4$ minutes (like Gemini 2). Initial blackout velocity $v_0 = 7$ km/s, final $v_1 = 4$ km/s (like the shuttle)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The produced plasma is created using kinetic energy of the craft being dissipated during the blackout. How fast is the energy is being dissipated on the average?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;{ {1 \over 2} m {v_0}^2 - {1 \over 2} m {v_1}^2 } \over t&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After plugging the numbers we have &lt;a href=&quot;http://www.wolframalpha.com/input/?i=%28%28+%28+0.5*%287km%2Fs%29+%5E2+*+3+metric+tons+%29+-+%28+0.5*%284km%2Fs%29%5E2+*+3+metric+tons+%29%29+%29+%2F+4+minutes&quot;&gt;0.2 gigawatt&lt;/a&gt; of energy output on the average. This is the kind of interference we are obtaining, spread over multiple spectra including electromagnetic, visual, UV and sound.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any broadcast would need to overcome this level of noise - or broadcast on bandwidth way, way low to be discernable from that noise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, there would be some way to win: instead of overcoming the noise, harness it. Imagine some kind of spoilers/fins coated in ablator, that can be extended/turned in modulated way, burning away at a modulated rate. The trail of plasma itself would become the message. But do we really need it? Is there anything the ground control could actually do - help somehow during the reentry if something goes wrong? Would it help with anything other than anxiety?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-11-20T11:59:23.773" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12755" PostTypeId="2" ParentId="12753" CreationDate="2015-11-20T12:50:42.867" Score="2" Body="&lt;p&gt;Take a look at &lt;em&gt;Space Mission Analysis and Design&lt;/em&gt; by Larson and Wertz, it's standard literature on systems engineering. Additionally &lt;em&gt;&lt;a href=&quot;https://books.google.co.uk/books?id=GtzzpUN8VEoC&amp;amp;printsec=frontcover&amp;amp;dq=inauthor:%22Computer%20Sciences%20Corporation.%20Attitude%20Systems%20Operation%22&amp;amp;hl=de&amp;amp;sa=X&amp;amp;ved=0ahUKEwjqpP3lgZ_JAhUGmg4KHVLqAzAQ6AEIJjAA#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow&quot;&gt;Spacecraft Attitude Determination and Control&lt;/a&gt;&lt;/em&gt; by Wertz will give you more information on the design of an ACS. &lt;/p&gt;&#xA;" OwnerUserId="11391" LastActivityDate="2015-11-20T12:50:42.867" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12756" PostTypeId="2" ParentId="12752" CreationDate="2015-11-20T14:38:41.147" Score="4" Body="&lt;p&gt;Two possibilities:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;An SEL2 infrared telescope&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the point of view of a space craft at Sun Earth L2, the sun, earth and moon all dwell in the same region of the sky. A small shade could shield heat from all three sources. This leaves almost a full 4 pi steradians of 4 K sky an infra red scope can radiate heat into.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;An EML2 asteroid mining test bed&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many near earth asteroids that can be reached with just a small amount of delta V. Unfortunately launch windows to these accessible asteroids are rare. Windows can be years or even decades apart. Seeing that we have zero experience in asteroid mining, a trial and error process would require multiple missions to a single destination. Rarity of launch windows could stretch this learning period to decades or even the better part of a century. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also telerobotics is more difficult for rocks in heliocentric orbit. Signal strength falls with inverse square of distance. Therefore good bandwidth is hard when the rock is one or two astronomical units from earth. Light lag latency for a rock on the other side of the sun can be 30 to 40 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast, the lunar neighborhood enjoys frequent launch windows. Every two weeks from a given low earth orbit. Trip time is less than a week. Light lag latency is about 3 seconds. Signal is hundreds of thousands times stronger than a rock 1 or 2 A.U. distant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rock in high lunar orbit makes a much better learning test bed than an asteroid in heliocentric orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If proximity to the earth moon neighborhood makes for a better test bed, why not park the rock in low earth orbit? Here's a screen capture from my &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;blog post on EML2&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0myw6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0myw6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Energetically speaking you can see EML2 is not that far from asteroid 2008 HU4. It is plausible to park a rock at EML2. Parking a rock in low earth orbit, not so much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides EML2, distant retrograde lunar orbits have also been proposed for parking asteroids.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-11-20T15:07:40.677" LastActivityDate="2015-11-20T15:07:40.677" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12757" PostTypeId="2" ParentId="12752" CreationDate="2015-11-20T15:56:44.260" Score="2" Body="&lt;p&gt;One additional mission that is best performed at a Lagrange point is a solar observing mission at SEL1. This will have good focus on the Sun while remaining near Earth. It also shows the daylight portion of Earth at all times. See also &lt;a href=&quot;http://www.space.com/30302-lagrange-points.html&quot; rel=&quot;nofollow&quot;&gt;this article on Lagrange Points.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-11-20T15:56:44.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12758" PostTypeId="2" ParentId="12558" CreationDate="2015-11-20T17:09:24.707" Score="7" Body="&lt;p&gt;PearsonArtPhoto's answer is correct, as far as it goes.  I'd like to add two more major differences between the two types of suits in question (FYI, I'm basing my answer here on the Launch and Entry Suits (LES) and the Extravehicular Maneuvering Units (EMU) used during the Space Shuttle era).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mobility:&lt;/strong&gt; The EMU's were designed in such a fashion so as to allow the crewmember inside to perform rather intricate tasks during long spacewalks.  The EMU gloves were custom fitted for to each astronaut, while the LES gloves were generic.  Also, the EMU's were designed to allow &lt;em&gt;relatively&lt;/em&gt; unhindered arm and leg movement when compared to the LES.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Fit:&lt;/strong&gt; In general, the EMU's, being modular in construction, allowed for a better fit.  One could mix and match torso, leg, arm, boot sections.  The LES's, on the other hand were mostly of unit construction.  Sometimes, this led to compromises being made.  For example, a particular crewmember might have considered arm length the primary factor in choosing a size, even though said suit may have been too short in the shoulder to crotch dimension (ouch!).  It was generally accepted that long EVA's would be quite difficult if one were wearing a standard LES.&lt;/p&gt;&#xA;" OwnerUserId="12422" LastActivityDate="2015-11-20T17:09:24.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12759" PostTypeId="2" ParentId="12747" CreationDate="2015-11-20T18:36:26.140" Score="6" Body="&lt;p&gt;As it happens, reentry temperatures peak at approximately the same temperature in kelvins as reentry speed in m/s, so assuming roughly 8 km/s, 8000 K is the figure to work from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronomy.ohio-state.edu/~pogge/Ast161/Unit4/spectra.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;This diagram from Ohio State&lt;/a&gt; shows that roughly half the emitted black-body radiation at 8000 K is in the UV spectrum:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/3wM0P.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3wM0P.jpg&quot; alt=&quot;Black-body spectrum diagram&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;Nearly all of that is in the near and middle UV range (200-400nm); excimer lasers are easily capable of transmitting far UV, so that's not a problem. (Even at 10000 K, the amount emitted at 126 nm for $Ar_2$ is fairly low.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the atmosphere absorbs middle and far UV quite efficiently, which would be a problem. The absorption mostly takes place in the ozone layer, at around 100000 feet and below. But we can see that Apollo, at least, exited its blackout at nearly twice that altitude, which means that there's no real absorption until quite a ways below the spacecraft at that point:&lt;br&gt;&#xA;&lt;a href=&quot;http://www.kenrockwell.com/Images/nasa/apollo-11/24-july/reentry-curve.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ox6rR.gif&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it would seem there's no inherent reason UVC (~120 nm) communications would not be possible using satellite or high-flying balloon relays. This could be more generally useful than the Shuttle technique of shaping a hole in the sheath to beam relayed radio transmissions through, since it doesn't rely on specific body shapes.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2015-11-24T03:13:21.820" LastActivityDate="2015-11-24T03:13:21.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12760" PostTypeId="1" CreationDate="2015-11-20T21:07:00.037" Score="4" ViewCount="332" Body="&lt;p&gt;Suppose mare regolith is heated to 2400 K, then left to cool slowly in a tank. Samples of powder regolith from the mare are about half glasses and half crystals of composition as shown below. Would some pure minerals separate into their own layer? Might one hope to find layers of pure metal oxides before the mix started to crystallize at 1700 K according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Bowen&amp;#39;s_reaction_series&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bowen's reaction series&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZwP9j.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZwP9j.jpg&quot; alt=&quot;mineral oxide composition of lunar soils&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they wouldn't separate as they cooled, what would they do? Is the low gravity an issue?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been working on very rough ideas for solar furnaces on the Moon, to model a &lt;a href=&quot;http://moonwards.com/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;virtual colony&lt;/a&gt;, and I'm trying to work out its general format and applications. Suggestions for reading at an introductory level are also welcome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2400 K is above the melting point of all the constituent minerals, and all the metal oxides except MgO and CaO, while being below the temperature at which some of the oxygen would separate (at about 2700 K). It's my best guess at the appropriate temperature to try this. Bowen's reaction series  begins at about 1700 K, so until the mix cools to that temperature, it shouldn't crystallize (other than tiny crystals of MgO and CaO floating around). &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-12-20T22:27:28.337" LastActivityDate="2015-12-20T22:27:28.337" Title="If a high-temperature melt of lunar soil cooled slowly, what layers might appear?" Tags="|the-moon|planetary-science|mining|geology|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12762" PostTypeId="2" ParentId="12760" CreationDate="2015-11-21T02:11:13.167" Score="3" Body="&lt;p&gt;The concentration of metals with the regolith will be critical a factor to the amount of effort, energy required and regolith required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other important factors will be what other metals will be in the regolith and how this may influence the formation and degree of alloyed metals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also what metal do you want to concentrate by melting regolith?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting the metal you want from its particular layer with a melt will be a problem. One of the problems will be the thickness, or thinness of the layer, and its proximity to layers of other metals or alloys and the need to get the metals from layers without contamination.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2015-11-21T02:11:13.167" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12763" PostTypeId="1" AcceptedAnswerId="12767" CreationDate="2015-11-21T20:39:49.880" Score="1" ViewCount="76" Body="&lt;p&gt;I'm reading an &lt;a href=&quot;http://phys.org/news/2015-10-saturn-moon-titan.html&quot; rel=&quot;nofollow&quot;&gt;article&lt;/a&gt; and it says &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It's [sic] internal makeup is likely differentiated into several layers, with a 3,400-kilometre (2,100 mi) rocky center surrounded by layers composed of different forms of crystalized [sic] ice.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Does this mean the diameter of the &quot;center&quot; is 3,400 km? It's a bit ambiguous where this &quot;center&quot; ends. Anywhere I can find more details on Titan's interior?&lt;/p&gt;&#xA;" OwnerUserId="12432" LastEditorUserId="6816" LastEditDate="2015-11-21T20:49:08.240" LastActivityDate="2015-11-22T13:49:04.380" Title="Titan's interior" Tags="|interplanetary|cassini|titan|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12764" PostTypeId="2" ParentId="7773" CreationDate="2015-11-22T00:36:49.137" Score="2" Body="&lt;p&gt;A practical way to deflect large asteroids could be kinetic impact with a smaller asteroid. That smaller asteroid would in turn be deflected to collide with the larger asteroid using either a spacecraft that landed on it and used e.g. ion thrusters, or it could be done via a kinetic impact using a yet smaller asteroid.  In the latter case, the accuracy of the deflection may be a problem, but it's fun to speculate about the possibility of having to deflect only a very small asteroid to make it hit a larger one which in turn will then hit an even larger one etc. etc...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose that the Earth is going to be hit in $\tau = 2$ years time by an asteroid of size $D_a$, and that on average impacts by asteroids of size larger than $D_a$ happen once every $T = $ million years.  To make the asteroid miss the Earth requires a change in its velocity of the order of $\frac{R}{\tau}$ where $R$ is the Earth's radius. Then from conservation of momentum, you can derive that diameter of the smaller asteroid $D_s$ that needs to collide with the asteroid we want to deflect is given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$D_s = \left(\frac{R}{\tau v_s}\frac{\rho_a}{\rho_s}\right)^{\frac{1}{3}}D_a$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $v_s$ is the relative velocity of the small asteroid w.r.t. to the large one and $\rho_s$ and $\rho_s$ are the densities of the large and small asteroids, respectively. If we take $v_s = 30$ km/s, then for the assumed figures this works out as approximately $D_s = 0.015 D_a$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is then what the typical closest approach of an asteroid with diameter $D_s$ to the large one is within a time frame of the order of $\tau$, as this determines by how much we need to deflect a smaller asteroid to hit the large one. The cumulative size distribution of asteroids $N(D)$ decays approximately as a power law:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$N(D)\sim D^{-1.32}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The number of asteroids with diameters larger than $D_s$ is thus larger than the number of asteroids with diameters larger than $D_a$ by a factor of 256. If the Earth will on average collide with an asteroid of size $D_a$ or larger once every $T = $ million years, then a closest approach of an asteroid of diameter larger than $D_s$ within Earth's radius of the large asteroid will happen once every $\frac{T}{256}\approx 4000$ years. The typical closest approach within a two year period will thus be of the order of $r$ such that $\left(\frac{r}{R}\right)^2 = 2000$, so the small asteroid will typically have to be deflected by a distance of the order of 45 earth radii to make it hit the large one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the deflection needed is 45 times larger, the mass of the smaller asteroid is much smaller, about 300,000 times smaller, so it's much more feasible to achieve than deflecting the large one directly.&lt;/p&gt;&#xA;" OwnerUserId="11234" LastActivityDate="2015-11-22T00:36:49.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12765" PostTypeId="2" ParentId="12638" CreationDate="2015-11-22T00:48:36.513" Score="4" Body="&lt;p&gt;One addition that I struggle with was the rapid way he was able to use poop and Martian regolith to create soil to grow the potatoes.  In the book at least (didn't see the movie).  I can't &lt;em&gt;anywhere&lt;/em&gt; find data on how long it takes to make organic soil for plants; sure hydroponics are great, but that's not what Mr. Whatney did.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soil takes a long time to create and it most certainly takes a long time if all you have is poop, bacteria, and the fine dust of Mars.  Composting with worms can make some pretty good nutrition for plants, but that is a lot more ingredients than poop and sand.  Once you've spent that time, you must then wait out the time to grow the potatoes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, I thoroughly enjoyed the quick read, and I hope to see the movie soon.  Moreso, I hope it inspires people to believe more in the possibility of going to Mars sooner rather than later.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2015-11-22T00:48:36.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12766" PostTypeId="1" AcceptedAnswerId="12773" CreationDate="2015-11-22T12:37:17.267" Score="2" ViewCount="2756" Body="&lt;p&gt;After research, I knew that a certain TLE data can be transformed into another format (I thought about ECI) so geodetic conversion can be applied (to fetch coordinates which can be plotted) .&lt;/p&gt;&#xA;&lt;p&gt;I wanted to build a software that ground tracks the ISS (I saw many online; so I thought about developing my own).&lt;/p&gt;&#xA;&lt;p&gt;Though I read here that this conversion should not be applied (because it produces errors) :&#xA;&lt;a href=&quot;https://celestrak.org/columns/v04n05/index.asp#FAQ05&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://celestrak.org/columns/v04n05/index.asp#FAQ05&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I want to ask about the amount of the error the author was meaning.&lt;/p&gt;&#xA;" OwnerUserId="10359" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2015-11-23T19:38:31.960" Title="Ground tracking : converting TLE to another format" Tags="|artificial-satellite|ground-track|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12767" PostTypeId="2" ParentId="12763" CreationDate="2015-11-22T13:49:04.380" Score="3" Body="&lt;p&gt;There are two different models for the internal structure of Titan, the wholly undifferentiated model, where the ice and rock are mixed together uniformly from surface to core and wholly differentiated model, where there is a separate rocky core surrounded by an ice shell. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0032063311001401&quot; rel=&quot;nofollow noreferrer&quot;&gt;Titan’s internal structure and the evolutionary consequences&lt;/a&gt; by A.D. Fortes gives a detailed overview of interior of Titan. From the paper:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sPXgb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sPXgb.png&quot; alt=&quot;Titan interior&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interior looks something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4YrRK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4YrRK.jpg&quot; alt=&quot;Titan interior&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: nasa.gov&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-11-22T13:49:04.380" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12768" PostTypeId="2" ParentId="12747" CreationDate="2015-11-22T14:46:24.277" Score="2" Body="&lt;p&gt;The re-entry blackout problem was resolved for the Space Shuttle with the implementation of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite_System&quot; rel=&quot;nofollow&quot;&gt;TDRSS&lt;/a&gt; system. It consists of (currently) 7 satellites in geostationary orbit. During re-entry, the shuttle was able to communicate with a satellite &quot;through a &quot;hole&quot; in the ionized air envelope at the tail end of the craft, created by the Shuttle's shape&quot;.&lt;/p&gt;&#xA;" OwnerUserId="12356" LastActivityDate="2015-11-22T14:46:24.277" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12769" PostTypeId="1" AcceptedAnswerId="12770" CreationDate="2015-11-22T15:34:52.213" Score="7" ViewCount="621" Body="&lt;p&gt;The first stage of the Saturn V, for example, burned about 26 tons of material in total per second (&lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1#Specifications&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Rocketdyne_F-1#Specifications&lt;/a&gt;). Therefore the rocket dynamics are changing... &lt;em&gt;very quickly&lt;/em&gt;. How does the control/guidance system deal with this? Do they design 100 controllers that they gain-schedule? Thanks for your input!&lt;/p&gt;&#xA;" OwnerUserId="11551" LastActivityDate="2015-12-06T08:42:53.777" Title="How does rocket guidance deal with changing mass?" Tags="|rockets|guidance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12770" PostTypeId="2" ParentId="12769" CreationDate="2015-11-22T16:41:03.723" Score="6" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.ibiblio.org/apollo/Documents/tn_d-5869_1970023342.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Description and Performance of the Saturn Launch Vehicle's Navigation, Guidance, and Control System&quot;&lt;/a&gt;, the control gain values make scheduled changes twice in each of the first two stages of ascent, and one more during the TLI burn, presumably reducing gain to avoid oversteering the lightened vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k11sm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k11sm.png&quot; alt=&quot;Saturn V Scheduled Control Gains Table&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that Saturn V first-stage guidance was a pitch-vs-time program, concerned more with staying in a safe aerodynamic regime than with the exact trajectory. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know how this compares with more modern systems; presumably the gain could be made a simple linear or polynomial function of time (since mass versus time should be fairly predictable). &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-11-22T16:41:03.723" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12771" PostTypeId="2" ParentId="3197" CreationDate="2015-11-22T17:33:34.767" Score="1" Body="&lt;p&gt;It depends on a few choices made by the designer of the spacecraft. If it follows a lifting skipping reentry trajectory, the spacecraft can potentially get away without a heat shield (since the airframe serves as the heat sink) and only have a small amount of propellant to deorbit. (around 10% of the spacecraft mass is the reentry propellant in this case)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a ballistic reentry trajectory is used, then the heat fluxes rise enormously because all of the spacecraft kinetic energy is being dumped into the air via shock wave heating. Because of this, with existing materials, ablative heat shields are the only designs that work for this reentry trajectory, meaning the heat shield gets eaten away by reentry. (here it could be almost 25% of the spacecraft mass could be the heat shield/deorbit propellant. It depends on the specific heat and heat of vaporization of the heat shield material. Currently the state of the art here is the PICA-X material SpaceX uses on its Dragon capsules)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a powered reentry trajectory is used, where the spacecraft uses reverse thrust to slow down throughout the whole trajectory, the heat loads experienced during reentry can be greatly reduced since if done right, most of the spacecraft kinetic energy can be dumped at altitudes above 200,000 ft, turning a hypersonic reentry into a slightly hypersonic reentry. However, the propellant cost of such a trajectory is high. Current rocket engines, with a maximum Isp of 465, would drive the spacecraft to be nearly 90% propellant if this reentry mode is picked.   &lt;/p&gt;&#xA;" OwnerUserId="11377" LastActivityDate="2015-11-22T17:33:34.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12773" PostTypeId="2" ParentId="12766" CreationDate="2015-11-22T20:51:36.953" Score="4" Body="&lt;p&gt;The &lt;strong&gt;circumspect answer&lt;/strong&gt; is that I think you can only find this by specific evaluation for the type of object and orbit that you have in mind. Apologies, as I enthusiastically wrote most of what follows before remembering that you said that you want to track the ISS position. The &lt;strong&gt;short answer&lt;/strong&gt; is, give it a whirl, it may well be just fine! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest of this is just if you'd like to take more of an interest in it.  I don't know where you have got with your own understanding but, as a start, here a couple of points from the reference you cited that are worth looking at more closely:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, TLEs are already in an ECI frame (see further down the page on the reference):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;To be precise, the reference frame of the Earth-centered inertial&#xA;  (ECI) coordinates produced by the SGP4/SDP4 orbital model is true&#xA;  equator, mean equinox (TEME) of epoch.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;With that you may be wondering why they need to be converted at all, this leads to the second point:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The elements in the two-line element sets are mean elements&#xA;  calculated to fit a set of observations using a specific model...&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Try this little thought experiment to help with the concept of &quot;mean elements&quot; which leads to a very rough way to evaluate the error:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Imagine an ideal keplerian orbit defined by the six orbital elements and time. You can convert this into cartesian co-ordinates and you will see that the orbit is elliptical, by definition.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If you instead look at the orbit of a real object and took observations to plot its course in cartesian co-ordinates then you would see that the &quot;ellipse&quot; was just an approximation and that the real satellite path has lumps in it. It isn't possible to represent this orbit as an ellipse because it isn't one. Typically orbits are either specified as a position/velocity vector for that instant in time, or are converted back to keplerian-looking elements on the express understanding that such ephemeris, the eccentricity, semi major axis, angles etc only apply at that specific instant in time. Strictly this ephemeris would be described as &quot;&lt;strong&gt;osculating&lt;/strong&gt;&quot; elements rather than &quot;&lt;strong&gt;keplerian&lt;/strong&gt;&quot; ones. If you had a propagator to see what the orbit would look like a few minutes later, the ephemeris would have changed.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Imagine now conjuring up some simplfied view of the real satellite path that can still be defined by the 6 keplerian orbit elements, time and a couple of other details. A TLE describes an orbit for which some, but not all, of the bumps have been smoothed out. These are &quot;&lt;strong&gt;mean&lt;/strong&gt;&quot; elements.  This simplification will will only be valid if the reader has access to the same underlying set of assumptions that were used to generate it, namely the SGP4 model.  If you produce a different tpe of mean elements then, metaphorically, you will get a different mean. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If you try to use elements from one mean algorithm in software for another then you will have errors. If you go a step further and blindly use mean elements, say TLEs, in a propagator meant for osculating elements then the errors will be still larger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you try converting each of the above three orbits into cartesian coordinates you should see that 1. is an elipse, 2 is bumpy and 3 is somewhere between the others. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That will give you some idea of the errors that are mentioned. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to get more into this there is another Q&amp;amp;A on this stack exchange &lt;a href=&quot;https://space.stackexchange.com/questions/4211/calculate-satellite-coordinates-from-tle-data?rq=1&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2015-11-23T19:38:31.960" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12774" PostTypeId="1" AcceptedAnswerId="12776" CreationDate="2015-11-23T07:40:04.573" Score="8" ViewCount="13119" Body="&lt;p&gt;If a meteorite the size of a pea landed on the road in front of you, what sort of impact would it have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How big would it have been when it entered the atmosphere and would it have been visible during the day? &lt;/p&gt;&#xA;" OwnerUserId="12434" LastEditorUserId="4660" LastEditDate="2015-11-24T00:00:59.760" LastActivityDate="2017-11-01T11:40:08.150" Title="What impact would a pea size meteor have when it hits the ground?" Tags="|impact|meteorites|reentry|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12776" PostTypeId="2" ParentId="12774" CreationDate="2015-11-24T05:31:50.903" Score="11" Body="&lt;p&gt;&lt;a href=&quot;https://books.google.com.mx/books?id=K-0xBsMsursC&amp;amp;pg=PA50&amp;amp;lpg=PA50&amp;amp;dq=meteorite%20terminal%20velocity&amp;amp;source=bl&amp;amp;ots=uAOfWJxr_-&amp;amp;sig=Ufz4ivKvCVtA9u6G3s-mXCJsgeY&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwirxeHGh6jJAhWJ8CYKHVUxBGA4ChDoAQgaMAA#v=onepage&amp;amp;q=meteorite%20terminal%20velocity&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Asteroids, Meteorites, and Comets&lt;/a&gt; has this to say:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Meteorites between the size of a grain of sand and about four inches&#xA;  (10 cm) in diameter create a bright light trail and usually burn up in&#xA;  the atmosphere. Meteorites between about four inches (10 cm) and 3&#xA;  feet (1 m) may survive the burning of entry to land on the Earth's&#xA;  surface. Meteorites larger than a meter or two usually detonate in the&#xA;  upper atmosphere, releasing energy equivalent to several thousand tons&#xA;  of TNT, though they may make it to the surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It goes on to say meteorites of this size lose about 30 to 60 percent of their mass as they fall, so a pea-size one wouldn't have been much bigger when it started - something like an iron marble, which would be likely to survive entry. Unless what landed was only a fragment of something bigger, if that was the case it could have come from something huge. An example &lt;a href=&quot;https://www.psi.edu/epo/faq/meteor.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; (section 9) cites &lt;a href=&quot;https://en.wikipedia.org/wiki/2008_TC3&quot; rel=&quot;noreferrer&quot;&gt;2008 TC&lt;sub&gt;3&lt;/sub&gt;&lt;/a&gt;, of 10&lt;sup&gt;4&lt;/sup&gt; to 10&lt;sup&gt;5&lt;/sup&gt; kg, of which only 280 pieces totalling 4 kg were recovered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As they pass through the air, meteoroids lose almost all their speed (unless they are really big). Most weigh &lt;a href=&quot;http://www.amsmeteors.org/meteor-showers/meteor-faq/#3&quot; rel=&quot;noreferrer&quot;&gt;less than 2 grams&lt;/a&gt; when they hit the atmosphere, and yet they create a bright streak of light kilometers long, visible to us on the ground dozens of kilometers away. That is the energy involved, but it is all spent high in the atmosphere, heating it and the air it passes through glowing hot. That friction shortly slows the object to its terminal velocity. A meteorite starts at &#xA;&lt;a href=&quot;http://www.armaghplanet.com/html/meteorites.html&quot; rel=&quot;noreferrer&quot;&gt;11 to 70 km/s&lt;/a&gt; (25 000 to 160 000 mph), slows to &lt;a href=&quot;http://www.amsmeteors.org/fireballs/faqf/#8&quot; rel=&quot;noreferrer&quot;&gt;2 to 4 km/s&lt;/a&gt; (4500 to 9000 mph) by the time it stops glowing, and slows further to 0.1 to 0.2 km/s (200 to 400 mph) before atmospheric drag balances the pull of gravity, and then it stays at that speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, an average bullet is travelling about twice as fast when it leaves a gun. Kinetic energy rises with the square of velocity, so the pea meteorite would have about a quarter of its energy, if it was iron, meaning it would be about the same size and density. If it hit a road, it would put a small pit in the surface of it maybe, barely noticeable. A stone meteorite that size could have maybe a third of the energy of an iron pea one. You wouldn't want to be hit by it, but it probably wouldn't kill you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.huffingtonpost.com/2013/11/28/wayne-lippard-son-hit-by-meteorite_n_4356246.html&quot; rel=&quot;noreferrer&quot;&gt;This&lt;/a&gt; was a credible report of a small boy being hit possibly by a meteorite fragment, as other fragments were found around. It only gave him a gash that needed 3 stitches, so probably not quite pea size. &lt;a href=&quot;http://news.nationalgeographic.com/news/2003/05/0513_030513_tvmeteorites.html&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; is an article on a bunch of fragments falling in a Chicago suburb. Some of them were large enough to do significant damage.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-11-24T15:52:12.920" LastActivityDate="2015-11-24T15:52:12.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12777" PostTypeId="1" AcceptedAnswerId="12786" CreationDate="2015-11-24T06:32:55.193" Score="15" ViewCount="386" Body="&lt;p&gt;Last weekend I attended a prelection at Polish Aviation Museum, on modern trends in aviation. The lecturer talked quite a bit of history too, and Soviet achievements were in as much focus as the western ones. Among all, he talked about missile guidance systems, including infrared homing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He mentioned that materials for the superior (for its time) homing systems of air-to-air missiles were produced in a small furnace on board of Salyut, and later MIR space stations, as the manufacturing would be impossible in standard gravity; about 0.5kg of the material manufactured over a month of operation of the furnace would allow manufacturing of about 500 missiles with the superior guidance system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could someone provide more information - what substance exactly was being manufactured, and what about the process was so subtle that it required freefall conditions?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="12102" LastEditDate="2019-07-18T07:48:26.070" LastActivityDate="2019-07-18T07:48:26.070" Title="What was being manufactured on Salyut and Mir?" Tags="|russia|mir|reduced-gravity-manufacturing|salyut|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12778" PostTypeId="1" CreationDate="2015-11-24T08:13:47.160" Score="10" ViewCount="2024" Body="&lt;p&gt;For example: Japanese H-IIA rocket only jettisons its boosters 10 seconds after burnout, which I image would waste fuel as the spent boosters are like &quot;dead-weight&quot; dragging the rocket down. So why don't they jettison the boosters right after they burn out like many other rockets do? Thanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight101.com/spacerockets/h-iia-202/&quot;&gt;http://spaceflight101.com/spacerockets/h-iia-202/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Weighing 77,000 Kilograms, each SRB-A is 2.5 Meters in Diameter and 15.1 Meters long. The Boosters burn for the first 100 seconds of the flight and are jettisoned 10 seconds after burnout.&quot;&lt;/p&gt;&#xA;" OwnerUserId="1262" LastEditorUserId="4660" LastEditDate="2015-11-24T23:30:02.743" LastActivityDate="2016-05-11T07:11:44.657" Title="Why do some rockets jettison their boosters much after they are burned out?" Tags="|launch|rockets|booster|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12779" PostTypeId="2" ParentId="12778" CreationDate="2015-11-24T14:05:04.553" Score="13" Body="&lt;p&gt;The jettison of spent boosters depends on a number of factors like the design, safety etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Japanese H-IIA has a rather unique design, where the thrust developed by the solid fuel rocket motor (SRB-A) passes through the thrust struts and is&#xA;received by the lug which forms an integral part of the cross beam fixed on the LE-7A engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fXSvU.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fXSvU.png&quot; alt=&quot;Thrust strut&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image from &lt;em&gt;Successful Return to Flight of the H-IIA Launch Vehicle&lt;/em&gt; by Takashi Maemura et al.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://global.jaxa.jp/countdown/f18/overview/h2a_e.html&quot; rel=&quot;noreferrer&quot;&gt;jettison procedure&lt;/a&gt; is initiated when the combustion pressure reaches 2%. The forward and aft yaw braces (which transfer the radial component of thrust) are cut off by the ordnance system, and the booster assembly (that is held by thrust struts) departs from the launch vehicle, and is completely separated after about two seconds at the point where the thrust struts are cut off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S0n3l.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S0n3l.png&quot; alt=&quot;H IIA seperation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image from &lt;em&gt;New H-IIA Launch Vehicle Technology and Results of Maiden Flight&lt;/em&gt; by Takashi Maemura et al.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main concern during jettison is safety, so that (either of) the boosters don't have any residual thrust while the jettison process is under way. otherwise, the transfer of thrust through the thrust strut will make the vehicle unstable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another end of delayed separation is the Delta II rockets, which carry their spent casings till they reached their preset drop zones (well after burnout) &lt;a href=&quot;http://spaceflightnow.com/delta/d357/ascent.html&quot; rel=&quot;noreferrer&quot;&gt;to clear offshore oil platforms&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-11-24T14:05:04.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12780" PostTypeId="2" ParentId="9124" CreationDate="2015-11-24T18:43:36.183" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/IAmA/comments/14schr/i_am_astronaut_chris_hadfield_commander_of/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chris Hadfield&lt;/a&gt; responded to this question this way:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Soyuz is very small and the weight balance affects how it flies,&#xA;  so we are very restricted in what we can bring. I thus chose small&#xA;  items for my family and close friends: a new wedding ring for my wife,&#xA;  commemorative jewellery, a watch for my daughter (I flew a watch each&#xA;  for my sons on previous flights), a full family photo for my Mom and&#xA;  Dad, and some mission emblem guitar picks.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The video below mentions some of the items two astronauts listed - Scott Kelly mentioned 'a little tool pouch... certain kinds of sweat shirts and sweat pants... certain kinds of shoes for the weight-lifting device&quot; Reid Wiseman said &quot;I'd make sure I had some local baseball, local football shirts&quot;, he showed a toy giraffe and said &quot;I got two kids, and so this one actually flew with me in space, this is giraffity&quot;, also &quot;here is my yo-yo, that I had up there&quot;. He showed some of his kids' drawings that he had. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/JfiTfsT_FSE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The food he showed likely didn't count as personal luggage, but as '&lt;a href=&quot;https://space.stackexchange.com/q/9126/4660&quot;&gt;bonus foods&lt;/a&gt;'. Each astronaut has a supply provided, as a personal comfort, which is helpful for morale. They come up with the care packages shown later in the video. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sunita Williams had these items&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Indian American is busy packing up her personal things which she&#xA;  took up with her, including a yo yo, a crew notebook with pictures,&#xA;  speciality t-shirts and a family photo album.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cosstores.com/gb/Magazine/AW15/Anousheh-Ansari&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anousheh Ansari&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Yes, each person is allowed five kilograms of personal items,&#xA;  including clothes. I took a few personal items from people close to me&#xA;  so I could return those to them from space: wedding rings, little&#xA;  pieces of jewellery.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-11-24T18:43:36.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12781" PostTypeId="2" ParentId="10546" CreationDate="2015-11-24T19:27:59.203" Score="20" Body="&lt;p&gt;In answer to one of the OP's questions (&lt;strong&gt;&lt;em&gt;&quot;If they had known (or even suspected) the damage to Columbia would have resulted in catastrophe upon re-entry, what process would have taken place?&quot;&lt;/em&gt;&lt;/strong&gt;), if NASA had had insight into the extensive damage Columbia had been dealt during ascent, they would have certainly considered a different reentry plan - most logically to &lt;a href=&quot;https://en.wikipedia.org/wiki/Edwards_Air_Force_Base#Space_Shuttle_support&quot; rel=&quot;noreferrer&quot;&gt;Edwards AFB&lt;/a&gt;.  The most vulnerable section of the flight path, in terms of producing debris, would then have been at least a few hundred miles west of the California coast, over the Pacific Ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If nothing else, it would have been viewed as highly irresponsible to route the reentry over populated areas, as it ended up being...and a one-orbit wave-off (which was discussed, due to weather concerns at the Cape - I was there) would have put the reentry trajectory over the heavily populated Houston metro area.&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="12422" LastEditDate="2020-04-06T18:16:36.173" LastActivityDate="2020-04-06T18:16:36.173" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="12782" PostTypeId="2" ParentId="2890" CreationDate="2015-11-24T19:44:25.717" Score="25" Body="&lt;p&gt;The Space Shuttle Orbiter had a flight mode whereby, at or below ~50,000 feet of altitude during the reentry (or, in the event of an ascent abort, the gliding, unpowered phase of flight), the commander could command an autopiloted, wings level glide at about 190 knots equivalent airspeed (KEAS).&lt;/p&gt;&#xA;&lt;p&gt;The plan was then, while the Orbiter was gliding, that the crew could execute a manual bailout procedure via the side hatch, utilizing the egress pole system.  Said bailout could be initiated at or below ~30,000 feet of altitude during the glide.&lt;/p&gt;&#xA;&lt;p&gt;Said bailout procedures were initiated after the Challenger mishap.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;To reference some of the bailout procedures (what we called a &amp;quot;Mode 8 egress&amp;quot;), please see the &lt;a href=&quot;https://archive.org/details/main-shuttle-crew-operations-manual/page/n431/mode/2up&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt; (page 2.10-13 for information about the Egress Pole System and starting on page 2.10-18 for information about the crew bailout procedures).&lt;/p&gt;&#xA;&lt;p&gt;FYI, the 190 KEAS figure I stated above is from a crew training workbook that I still have on hand - specifically, there is a depiction therein of the &amp;quot;&lt;strong&gt;BAILOUT MODE 8&lt;/strong&gt;&amp;quot; crew cue card that states (among other things) that the commander is to take manual control of the Orbiter, establish level flight, then, as the airspeed bleeds down to 185-195 KEAS, is to engage the autopilot.  Said autopilot would then establish a glide at about those airspeeds.&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="-1" LastEditDate="2024-03-17T16:42:55.890" LastActivityDate="2024-03-17T16:42:55.890" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="12783" PostTypeId="1" CreationDate="2015-11-24T21:03:53.040" Score="5" ViewCount="113" Body="&lt;p&gt;Are there advantages to observing the universe from interstellar space? What kind of scientific experiments would be useful for a spacecraft outside of the Solar System's Oort cloud?&lt;/p&gt;&#xA;" OwnerUserId="12432" LastEditorUserId="12038" LastEditDate="2015-11-25T16:01:59.473" LastActivityDate="2015-11-25T16:01:59.473" Title="What to study in interstellar space?" Tags="|spacecraft|interstellar-travel|" AnswerCount="0" CommentCount="3" ClosedDate="2015-11-25T19:53:03.480" ContentLicense="CC BY-SA 3.0" />
  <row Id="12784" PostTypeId="2" ParentId="10890" CreationDate="2015-11-24T23:02:56.250" Score="44" Body="&lt;p&gt;The answer to the question, &lt;strong&gt;&lt;em&gt;&quot;Do the astronauts feel the station moving?&quot;&lt;/em&gt;&lt;/strong&gt; is yes, definitely, but sometimes in an &quot;indirect&quot; fashion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During Space Shuttle mission &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-109&quot; rel=&quot;noreferrer&quot;&gt;STS-109&lt;/a&gt;, when floating in my sleeping bag and waiting for slumber to come, I would notice that occasionally my body would softly brush up against one side or the other of said sleeping bag.  A quick glance at a laptop computer nearby, with, among other things, thruster status on its screen, would validate that a vernier jet (a small, 24 lb thrust attitude control jet) had just fired, in response to the autopilot, which was keeping the Orbiter in its previously-commanded attitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, really, it was not a case of my body brushing up against the side of my sleeping bag, but a case of said sleeping bag (which was attached to the Orbiter) brushing up against my stationary body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A measly little 24 lb thruster pushing around a 220,000 lb. spaceship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cool stuff!&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="12422" LastEditDate="2019-11-02T15:56:11.653" LastActivityDate="2019-11-02T15:56:11.653" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12785" PostTypeId="2" ParentId="8451" CreationDate="2015-11-24T23:25:58.440" Score="3" Body="&lt;p&gt;To add a few names to the above excellent answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jsc.nasa.gov/Bios/htmlbios/caldeiro.html&quot; rel=&quot;nofollow&quot;&gt;Frank Caldeiro&lt;/a&gt; and &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/htmlbios/loria.html&quot; rel=&quot;nofollow&quot;&gt;Gus Loria&lt;/a&gt;, both from NASA's astronaut class of 1996, never flew in space, due to health reasons.&lt;/p&gt;&#xA;" OwnerUserId="12422" LastActivityDate="2015-11-24T23:25:58.440" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12786" PostTypeId="2" ParentId="12777" CreationDate="2015-11-25T04:44:29.700" Score="8" Body="&lt;p&gt;The &lt;a href=&quot;http://www.astronautix.com/craft/kristall.htm&quot;&gt;Kristall (crystal) module&lt;/a&gt; of Mir carried a number of payloads for carrying out metallurgy and crystal growth experiments and was dubbed a factory in Soviet Press, as it was supposed to manufacture semiconductor samples for Soviet industries. The instruments abroad Kristall included,&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Krater-V electrical furnace- for producing perfect gallium arsenide and zinc oxide crystals of under microgravity conditions (~ $10^{-3}$ - $10^{-5}$ g).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Optizon furnace- for semi-industrial production of perfect cremnium monocrystals.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Zona-02 and Zona-03 furnaces- for semiconductor production experiments. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are no reports of the actual production figures or even if anything commercial (beyond experiments) was produced.&lt;/p&gt;&#xA;" OwnerUserId="11361" LastActivityDate="2015-11-25T04:44:29.700" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12787" PostTypeId="1" AcceptedAnswerId="32447" CreationDate="2015-11-25T05:28:03.090" Score="10" ViewCount="6496" Body="&lt;p&gt;I have been researching the genesis of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Clohessy-Wiltshire_equations&quot; rel=&quot;nofollow noreferrer&quot;&gt;Clohessy-Wiltshire&lt;/a&gt; (C-W, or Hill's) equations. These equations are used to describe relative motions between the &amp;quot;chaser&amp;quot; and &amp;quot;target&amp;quot; spacecraft in a space rendezvous situation. A fairly decent derivation (I presume, as it is over my head - read on) is given in &lt;a href=&quot;http://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwi8jKDV5arJAhVSLIgKHaVKDigQFggdMAA&amp;amp;url=http%3A%2F%2Fwww.dept.aoe.vt.edu%2F%257Ecliff%2Faoe4134%2Fc-w.pdf&amp;amp;usg=AFQjCNHPqm6Wc1GYHIrSs3de6bIhlQlxmA&amp;amp;bvm=bv.108194040,d.cGU&amp;amp;cad=rja&quot; rel=&quot;nofollow noreferrer&quot;&gt;Clohessy - Wiltshire Analysis&lt;/a&gt;, Eugene M. Cliff, October 23, 1998&lt;/p&gt;&#xA;&lt;p&gt;For reference, here are the rectangular coordinates NASA uses for space rendezvous considerations:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wAkLy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wAkLy.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The origin of the depicted coordinate system is located at the &amp;quot;target&amp;quot; vehicle (depicted in red). The &amp;quot;chaser&amp;quot; vehicle is not depicted here.&lt;/p&gt;&#xA;&lt;p&gt;The version of the C-W equations that is consistent with these coordinate axes is:&lt;/p&gt;&#xA;&lt;h2&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \ddot{x}-2\omega\dot{z}=F_x $$&lt;/span&gt;&lt;/h2&gt;&#xA;&lt;h2&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \ddot{y}+\omega^2y=F_y $$&lt;/span&gt;&lt;/h2&gt;&#xA;&lt;h2&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \ddot{z}+2\omega\dot{x}-3\omega^2z=F_z$$&lt;/span&gt;&lt;/h2&gt;&#xA;&lt;p&gt;Note that omega (&lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt;) is, of course, angular velocity. Also, the &lt;span class=&quot;math-container&quot;&gt;$F$&lt;/span&gt; terms represent component forces due to &amp;quot;chaser&amp;quot; spacecraft thruster firings.&lt;/p&gt;&#xA;&lt;p&gt;When rearranged into the familiar &lt;span class=&quot;math-container&quot;&gt;$F=ma$&lt;/span&gt; format, the equations become:&lt;/p&gt;&#xA;&lt;h2&gt;&lt;span class=&quot;math-container&quot;&gt;$$F_x=m(\ddot{x}-2\omega\dot{z}) $$&lt;/span&gt;&lt;/h2&gt;&#xA;&lt;h2&gt;&lt;span class=&quot;math-container&quot;&gt;$$ F_y=m(\ddot{y}+\omega^2y)   $$&lt;/span&gt;&lt;/h2&gt;&#xA;&lt;h2&gt;&lt;span class=&quot;math-container&quot;&gt;$$  F_z=m(\ddot{z}+2\omega\dot{x}-3\omega^2z)$$&lt;/span&gt;&lt;/h2&gt;&#xA;&lt;p&gt;Note that I've &amp;quot;added&amp;quot; the mass term (&lt;span class=&quot;math-container&quot;&gt;$m$&lt;/span&gt;) back into the equations for clarification (I can do this with impunity, since these equations are only of interest to me when the thrust force components are all zero, implying a &amp;quot;free drift&amp;quot; situation for the &amp;quot;chaser&amp;quot; spacecraft - note that the &amp;quot;target&amp;quot; spacecraft is assumed to be always in free drift here).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Now my question:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Do the &amp;quot;extra&amp;quot; terms (the ones involving angular velocity) come about because you are dealing with objects that are moving in orbits?  The forces referenced all act along orthogonal axes, and each of the acceleration terms include a linear acceleration along one of said axes.&lt;/p&gt;&#xA;&lt;p&gt;But, we still have those &amp;quot;extra&amp;quot; acceleration terms to deal with.&lt;/p&gt;&#xA;&lt;p&gt;My supposition is that the &amp;quot;extra&amp;quot; acceleration terms arise because there is orbital motion that is driven by a centripetal force (gravity).&lt;/p&gt;&#xA;&lt;p&gt;Put into simple dirt farmer terms, am I too far off base here?&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="18948" LastEditDate="2020-11-18T16:27:13.763" LastActivityDate="2020-11-18T16:27:13.763" Title="Clohessy-Wiltshire/Hill's Equations - F=ma?" Tags="|orbital-mechanics|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="12788" PostTypeId="1" CreationDate="2015-11-25T08:27:32.213" Score="0" ViewCount="834" Body="&lt;p&gt;In theory, the orbit around a single mass always has the same amount of total energy (Kinetic + Potential). Therefore, if a satellite was launched from earth, it should always require the same amount of energy (and therefore fuel) to attain orbit, no matter at which altitude this orbit is. Of course this assumes all other things (efficiency, total energy at launch, mass of rocket, etc.) are equal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus my question: are there significant differences between the amount of fuel needed to reach different types of earth orbits? And why is this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would imagine that the majority of the fuel burned by the rockets is outside of any significant atmosphere and therefore the efficiency is relatively constant.&lt;/p&gt;&#xA;" OwnerUserId="12445" LastActivityDate="2016-02-23T11:26:38.620" Title="How are fuel requirements different for different types of orbits?" Tags="|orbit|gravity|fuel|escape-velocity|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12789" PostTypeId="1" CreationDate="2015-11-25T08:33:55.333" Score="0" ViewCount="202" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kShil.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kShil.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Spaceship with a concave bottom is kept on top of a hole which contains water. A nuclear blast then converts the water to steam which gives the thrust to the spaceship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is that is this method feasible, if not then why? &lt;/p&gt;&#xA;" OwnerUserId="6753" LastEditorUserId="6753" LastEditDate="2015-11-26T05:56:50.310" LastActivityDate="2015-11-26T05:56:50.310" Title="Is it feasible to launch spacecraft/payload by water steam generated by a nuclear blast?" Tags="|launch|rockets|advanced-propulsion|" AnswerCount="1" CommentCount="7" ClosedDate="2015-11-25T20:38:09.827" ContentLicense="CC BY-SA 3.0" />
  <row Id="12791" PostTypeId="2" ParentId="12789" CreationDate="2015-11-25T09:04:14.053" Score="5" Body="&lt;p&gt;First, heavy water and its steam would rather dampen the blast than amplify it. It's not how thermonuclear charges work. But that's a technicality...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to deliver raw materials into space, this is semi-viable (incurring a significant loss during travel through the atmosphere as the outer surface will melt into plasma). But with proper ablators, heat shielding etc, it would be &lt;em&gt;somewhat&lt;/em&gt; doable, though not easy. And there's risk of significant irradiation of the cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to deliver anything slightly fragile (say, more fragile than a solid block of of steel) it will shatter due to impact of the blast, the acceleration exerting force of a concentrated thermonuclear blast. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;No humans, no electronics, no rocket engines, no systems or structures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And due to lack of these it won't be able to circularize the orbit once above the atmosphere, so unless you launch it at a speed that would allow escape from Earth's gravity, it will just fall back to Earth, even if it was to cross Moon orbit on the way. Simply, any &quot;orbit&quot; achieved by a single, point acceleration passes through that point (approaching from the opposite side) - and since that point is on Earth surface, it means the payload will hit Earth somewhere &quot;on the other side&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although, as I wrote - this is semi-viable. At higher orbits the energy to change orbit (circularize) is relatively low, so if the payload was encountered by an orbital &quot;tug&quot; that could give it an acceleration (or some kind of propulsion was developed that would survive the nuclear blast) - yes,the payload could be brought to orbit.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2015-11-25T11:20:59.083" LastActivityDate="2015-11-25T11:20:59.083" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12792" PostTypeId="2" ParentId="12788" CreationDate="2015-11-25T09:19:30.103" Score="1" Body="&lt;p&gt;First, your assumption is wrong for the simple reason we need nearly 8km/s to bring any payload - and all the fuel to accelerate it any further - to LEO. And that means burning enormous amounts of fuel burned during the launch phase; for every kilogram of fuel in orbit you need several kilograms of fuel burned in the atmosphere. This is known as the Tyranny of Rocket Equation, and this is an unavoidable, fixed initial cost to get anything into space - and bring it up there fast, because on top of all the acceleration you give your payload to bring it to orbital speed, every second you're losing 9.8m/s of that speed to &quot;gravitational drag&quot;, fighting Earth's pull to keep from falling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But once in orbit, all the fun begins with orbital mechanics. And there's a lot of savings and caveats there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your primary saving is the Oberth Effect. There were many questions on this site about understanding it, and I encourage you to look them up. The essence is that if your fuel has a high kinetic energy (that is - it's moving fast; the ship is flying fast) - then by burning it, you harness this energy into more acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital mechanics works in such a way, that a body in elliptical orbit moves faster when it's close to the body it's orbiting - so there's more to be gained by burning in LEO than any farther.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTOH direction corrections are easier at low speeds - near apoapsis of an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then there are gravitational slingshots; using other bodies (planets, moons) to steal a part of their kinetic energy, entirely for free.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can encourage you to play Kerbal Space Program if you want to explore all these intricacies in an intuitive way, not through dry equations but more like a gravitational theme park :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to give a &quot;tl;dr&quot; answer: &lt;em&gt;the orbit around a single mass always has the same amount of total energy (Kinetic + Potential).&lt;/em&gt; This is true, but only the Kinetic energy can be usefully transformed into speed by spacecraft engines. Therefore burns performed where the Kinetic component is highest are most efficient.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-11-25T09:19:30.103" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12793" PostTypeId="1" AcceptedAnswerId="12794" CreationDate="2015-11-25T11:14:44.393" Score="23" ViewCount="5226" Body="&lt;p&gt;If one equipped the SRB like the ones used with the Shuttles with minimal attitude control systems and launched it as a standalone rocket with no payload - what speed would it reach? Would it be capable of entering the Low Earth Orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, what is the viability of (staged or not) SRB-only launch vehicles?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-11-27T11:29:03.357" Title="Could the shuttle's SRB alone reach orbit?" Tags="|low-earth-orbit|srb|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12794" PostTypeId="2" ParentId="12793" CreationDate="2015-11-25T12:25:56.400" Score="31" Body="&lt;p&gt;No, a quick calculation yields a $\Delta v$ of about 4.6 km/s and you need about 9 km/s to get to low-Earth orbit. You'll lose a lot of that velocity to aerodynamic drag as well as the vertical portion of the flight, so a rough estimate of the final speed at burnout would be somewhere around 3 km/s -- or about 10,000 km/h.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the specifications from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;noreferrer&quot;&gt;SRB Wikipedia article&lt;/a&gt; (which admittedly have some conflicts):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_{full} = 590000 kg$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_{empty} = 91000 kg$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$I_{SP} = 250 s$ (242 at sea-level, 268 in a vacuum, most of the flight is in the atmosphere so we'll round up to 250)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v = g_0 I_{SP}\ln \frac{m_{full}}{m_{empty}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a simulation of a launch composed of vertical flight until reaching 100 m altitude, then pitching over to a constant pitch of 85 deg until reaching 1000 m altitude, then reverting to a gravity turn at 0 deg angle-of-attack. This sends the SRB to an altitude over 500 km, with a peak speed around 3.7 km/s at burnout.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vEQu0.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vEQu0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wK6D7.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wK6D7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a look at the pitch, flight-path angle (direction of motion), and angle-of-attack (difference between pitch and flight-path angle) as a function of altitude. Although it might seem strange that the rocket starts pitching over after only 100 m of flight, this is necessary to start gaining enough velocity to reach orbit -- the rocket is still climbing throughout most of the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QEfCX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QEfCX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that I'm using a simple atmospheric model for the air density:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\rho = 1.225 e ^\frac{h}{8000}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And a constant drag coefficient of $C_D = 0.8$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$D = \frac{1}{2} \rho v^2 C_D A$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass flow rate is assumed to be constant, computed from the stated sea-level thrust (12 MN) and specific impulse (242 s), whereas specific impulse is computed based on a simple mixture rule:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$I_{SP} = I_{SP,sea} \frac{\rho}{\rho_{sea}} + I_{SP,vac}\left( 1 - \frac{\rho}{\rho_{sea}}\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then thrust is computed from the specific impulse and mass-flow:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$T = \dot{m} g_0 I_{SP}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the equations of motion in 2D are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\ddot{r} = -\frac{\mu}{r^2} + r \dot{\theta}^2 + \frac{T}{m} \sin \phi - \frac{D}{m} \sin \gamma$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\ddot{\theta} = -\frac{2 \dot{r} \dot{\theta}}{r} + \frac{T}{m r} \cos \phi - \frac{D}{m r} \cos \gamma$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $r$ and $\theta$ are the polar coordinates about the Earth's centre, $m$ is the vehicle mass, $\phi$ is the vehicle pitch and $\gamma$ is the flight-path angle (velocity direction) -- both of those angles are with respect to the local horizon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for all-solid fuel launch vehicles, there are a number of them that have been and are still in use (such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_%28rocket_family%29&quot; rel=&quot;noreferrer&quot;&gt;Minotaur family&lt;/a&gt;). But, as LocalFluff pointed out, they are usually only sensible for smaller payloads. &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Prospector&quot; rel=&quot;noreferrer&quot;&gt;Lunar Prospector&lt;/a&gt; was launched to the moon on an all-solid &lt;a href=&quot;https://en.wikipedia.org/wiki/Athena_II&quot; rel=&quot;noreferrer&quot;&gt;Athena II&lt;/a&gt; although it looks like the upper stage used hydrazine (not solid, but not a complex liquid motor either).&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-11-26T12:55:10.350" LastActivityDate="2015-11-26T12:55:10.350" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="12795" PostTypeId="1" AcceptedAnswerId="12796" CreationDate="2015-11-25T15:20:12.967" Score="4" ViewCount="558" Body="&lt;p&gt;Related to &lt;a href=&quot;https://space.stackexchange.com/questions/12552/has-humanity-launched-a-ssto-ever/12553#12553&quot;&gt;this question&lt;/a&gt; about single-stage-to-orbit vehicles, the Atlas-B launch vehicle seems to have been the closest to a true SSTO solution. Although it did jettison booster engines, the vehicle was basically a single stage -- often referred to as &quot;stage-and-a-half&quot; configuration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the Atlas-B have made it to a relatively stable orbit without jettisoning those engines? Even if it carried no payload? Note that &lt;a href=&quot;http://www.astronautix.com/lvs/atlasb.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Astronautix page&lt;/a&gt; provides some numbers we can use for analysis and simulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I welcome any modifications to this question in regards to what defines a &quot;relatively stable orbit&quot;, but let's say we just want to reach an orbit that won't decay for at least 24 hours.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-11-25T15:55:09.950" Title="Could the Atlas-B (SM-65B) have made it to orbit without jettisoning the first-stage engines?" Tags="|rockets|stages|ssto|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12796" PostTypeId="2" ParentId="12795" CreationDate="2015-11-25T15:55:09.950" Score="5" Body="&lt;p&gt;Given the values on the &lt;a href=&quot;http://www.astronautix.com/lvs/atlasb.htm&quot; rel=&quot;noreferrer&quot;&gt;linked Astronautix page&lt;/a&gt;, it looks like jettisoning those booster engines &lt;em&gt;is&lt;/em&gt; necessary to get into orbit -- even without any payload. Splitting the flight into two $\Delta v$ calculations, we get this value for the &quot;first stage&quot; (i.e., the two booster engines and main engine firing in tandem, all pulling fuel out of the main tank):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v_A = 4753 \frac{m}{s}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we jettison the booster engines, we lose 3050 kg and end up with a $\Delta v$ for the &quot;second stage&quot; of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v_B = 4469 \frac{m}{s}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if we retain that 3050 kg as empty mass for the second stage flight, we get a new value of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v_B = 3235 \frac{m}{s}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for the Atlas-B with no payload (which is only 70 kg nominal anyway), we have a total $\Delta v$ of about 9.2 km/s if the booster engines are jettisoned, and 8.0 km/s if they are not. Getting to low-Earth orbit usually requires at least around 8.7 km/s, so it would seem that jettisoning those engines was critical for a successful flight.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-11-25T15:55:09.950" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="12797" PostTypeId="1" CreationDate="2015-11-25T16:45:38.477" Score="1" ViewCount="40" Body="&lt;p&gt;I understand that the asteroid mining proposals are looking to mine platinum, paladium, and other rare-earth metals that have a high economic value. What are the proposals for getting the mined material back to earth in an economic manner?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me it would be cheap if they could some 'drop' the material back, but of course, they need to avoid disasters and make sure its retrievable. Controlled returning, such as on a spacecraft, would add to the cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there compromise options, such as manufacturing the material into a 'glider' that can go to safely to the Earth in a controlled, inexpensive manner?&lt;/p&gt;&#xA;" OwnerUserId="10846" LastActivityDate="2015-11-25T16:45:38.477" Title="How will asteroid mining projects get the heavy material back to Earth?" Tags="|mining|" AnswerCount="0" CommentCount="0" ClosedDate="2015-11-25T17:49:46.907" ContentLicense="CC BY-SA 3.0" />
  <row Id="12798" PostTypeId="2" ParentId="12793" CreationDate="2015-11-25T20:34:11.147" Score="16" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I-X&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ares I-X&lt;/a&gt; was a &lt;strong&gt;modified STS SRB&lt;/strong&gt; with with an upper-stage simulator. So far as I know, this was the only standalone launch of SRB hardware. Even the original SRBs &lt;a href=&quot;https://space.stackexchange.com/questions/4146/have-srbs-or-ssmes-ever-launched-without-a-space-shuttle&quot;&gt;were not flight tested&lt;/a&gt; before manned launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Ares I-X flight had an &lt;strong&gt;apogee of 46 kilometers&lt;/strong&gt;, less than half the 100 kilometer altitude of the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow noreferrer&quot;&gt;Karman line&lt;/a&gt;, which is considered the edge of space and below which orbital trajectories are impossible due to air resistance. Due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;the rocket equation&lt;/a&gt; we can clearly see that no amount of stacked SRB components would so much as double the altitude, therefore we can conclude that &lt;strong&gt;a single SRB could not fly alone to orbit&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;User &lt;a href=&quot;https://space.stackexchange.com/users/103/sf&quot;&gt;SF.&lt;/a&gt; &lt;a href=&quot;http://pastebin.com/a4XZnyLx&quot; rel=&quot;nofollow noreferrer&quot;&gt;calculates&lt;/a&gt; that &lt;strong&gt;a staged SRB&lt;/strong&gt; may be able to launch considerable payload to orbit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Alone - okay. Staged - quite likely doable. From peak speed of 1.7km/s, and mass of the 3 stages vs 2 stages (from here ) I can find the ISp. Replacing second stage with another such thruster, I'm getting 1.8km/s from stage 1, another 4.9 from stage 2, for a total of 6.7km/s for 172 tons of stage 3. Now if we leave 50 ton of dry mass, and sacrifice 122 tons to SRB fuel (same ISp still!) we're getting another 3.9km/s for a total of 10.6km/s. There, 50 tons to orbit on 3 SRBs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;https://www.youtube.com/watch?v=H0ZHzAvFuYc&quot; rel=&quot;nofollow noreferrer&quot;&gt;video of the launch&lt;/a&gt;, complete with on-board camera views:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/H0ZHzAvFuYc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-11-26T14:26:37.067" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="12799" PostTypeId="1" AcceptedAnswerId="12801" CreationDate="2015-11-25T22:48:04.620" Score="7" ViewCount="210" Body="&lt;p&gt;Is there a site with the tracking information for the old space station Mir? Or some place with the data location tracking information still available? Maybe even a site with orbital mechanics backwards in time?&lt;/p&gt;&#xA;" OwnerUserId="12450" LastEditorUserId="49" LastEditDate="2015-11-25T23:37:46.953" LastActivityDate="2015-11-26T03:17:06.167" Title="Tracking of Mir space station backwards in time?" Tags="|space-station|tracking|orbital-elements|mir|ephemeris|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12801" PostTypeId="2" ParentId="12799" CreationDate="2015-11-25T23:27:24.347" Score="8" Body="&lt;p&gt;&lt;a href=&quot;https://celestrak.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;CelesTrack&lt;/a&gt; has Mir ephemerides (as &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE&lt;/a&gt;, two-line element sets) in its &lt;a href=&quot;https://celestrak.org/NORAD/archives/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NORAD Two-Line Element Sets Historical Archives&lt;/a&gt;. It's a 755 KB in size ZIP archive (&lt;a href=&quot;https://celestrak.org/NORAD/archives/mir.zip&quot; rel=&quot;nofollow noreferrer&quot;&gt;direct link&lt;/a&gt;) packing a text file with 22,333 TLE spanning time period from February 19, 1986 to March 23, 2001 when it was &lt;a href=&quot;https://en.wikipedia.org/wiki/Mir#Final_days_and_deorbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;deorbited&lt;/a&gt;. If these don't go far enough back in time for your needs, you can request even earlier TLE by filling and submitting &lt;a href=&quot;https://celestrak.org/NORAD/archives/request.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;this form&lt;/a&gt; (use &lt;em&gt;16609&lt;/em&gt; as Mir's 5-digit NORAD catalog number).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2015-11-26T03:17:06.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12802" PostTypeId="2" ParentId="12787" CreationDate="2015-11-26T05:07:56.220" Score="9" Body="&lt;p&gt;OK, I'll take a stab at this and try not to get too deeply buried in the math...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks to the above comments by members &lt;a href=&quot;https://space.stackexchange.com/users/12038/brian-lynch&quot;&gt;Brian Lynch&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/users/12389/dave&quot;&gt;Dave&lt;/a&gt;, I think I can convey a pretty good intuitive feel for what is going on.  Really, the credit for this answer goes to them (however, if the answer is deemed lame, then I'll take the &quot;credit&quot;)!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-07-dynamics-fall-2009/lecture-notes/MIT16_07F09_Lec18.pdf&quot; rel=&quot;noreferrer&quot;&gt;This link&lt;/a&gt;, provided by Dave, examines what is referred to therein as the &quot;Restricted Three-Body Problem&quot;.  While it is acknowledged that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Three-body_problem&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;general&lt;/em&gt; three-body problem&lt;/a&gt; has no analytical solution, the case considered in said reference is one wherein one of the masses is so small as to not have any significant gravitational effect on the other two masses, while the other two masses are large and gravitationally significant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, in order to more easily understand and analyze said restricted three-body system, a three-dimensional rectangular coordinate system is proposed such that the origin is located at the center of mass of the two largest masses and whose axes are stationary with respect to said two largest masses, even though said two masses are orbiting each other in inertial space.  So, said adopted coordinate system is rotating in inertial space with a certain, constant angular velocity &lt;code&gt;Ω&lt;/code&gt; (note that the equations I posted in the OP refer to the angular velocity as &quot;&lt;code&gt;ω&lt;/code&gt;&quot;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/y6on6.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y6on6.png&quot; alt=&quot;Coordinate System&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note in the above diagram that the smallest mass (mentioned above) is designated as &lt;a href=&quot;https://i.stack.imgur.com/YYwZ5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YYwZ5.png&quot; alt=&quot;Msub3&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much math is then furiously invoked, and some cool equations are arrived at, but the telling phrase in the whole treatment is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vF7Xr.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vF7Xr.png&quot; alt=&quot;Phrase&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(Sorry for the small font - &lt;a href=&quot;https://meta.stackexchange.com/questions/90054/image-preview-or-expand-option-after-it-is-posted&quot;&gt;I don't know how to enlarge this image&lt;/a&gt;.)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For those reading along, said phrase occurs just above Equation (8) in said reference.  It says essentially what Brian Lynch was trying to tell me in his above comments to my OP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, &lt;a href=&quot;http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-346-astrodynamics-fall-2008/lecture-notes/lec_26.pdf&quot; rel=&quot;noreferrer&quot;&gt;this link&lt;/a&gt; (again, provided by Dave), goes on to show how the C-W equations can be derived from the results of the above treatment (essentially addressing a special case of said &quot;Restricted Three-Body Problem&quot; wherein one of the two larger masses becomes small enough to be gravitationally insignificant - thus qualifying as the case of a &quot;chaser&quot; spacecraft attempting a space rendezvous with a &quot;target&quot; spacecraft whilst both said spacecraft are orbiting the Earth - the very case I was interested in in my OP).  Again, much math is invoked and, voila, the C-W equations pop out (for the case of no thrusting taking place).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bingo!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to put it in a nutshell, the &quot;extra&quot; acceleration terms I was curious about in my OP arise because my coordinate system is rotating in inertial space.&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-11-26T12:14:14.200" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12803" PostTypeId="1" AcceptedAnswerId="12804" CreationDate="2015-11-26T11:17:57.833" Score="2" ViewCount="1986" Body="&lt;p&gt;Having a TLE data, how do I update it with -let's say- today's date ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have the TLE of the ISS, I concluded that in order to have the location of the satellite at the moment, columns 21 to 32 should be altered with the date at the moment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no problem with columns 21,22,23, but the rest are the problem (up to 32).&#xA;I tried to convert to epoch time, but it's not giving me &quot;8 slots&quot;; rather 10. &#xA;I couldn't understand what is meant by &quot;    Epoch (day of the year and fractional portion of the day&quot;. I tried to convert the time to many forms, but it either yields &quot;6 slots&quot; or &quot;10 slots&quot;.&lt;/p&gt;&#xA;" OwnerUserId="10359" LastEditorUserId="10359" LastEditDate="2015-11-26T11:24:43.927" LastActivityDate="2017-10-26T12:01:00.710" Title="TLE data: how to update the date?" Tags="|artificial-satellite|ground-track|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12804" PostTypeId="2" ParentId="12803" CreationDate="2015-11-26T12:43:29.103" Score="7" Body="&lt;p&gt;TLEs are a product of fitting observations using the SGP4 propagator. So you can't really change them around the way it seems you're trying to. The TLE catalog is updated regularly, so if you want updated information for the ISS for instance, you can grab catalog data from &lt;a href=&quot;https://www.space-track.org&quot; rel=&quot;noreferrer&quot;&gt;https://www.space-track.org&lt;/a&gt;. There's also an API there so you can fetch the data programmatically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, once you have the TLE data for a given epoch, you can use the SGP4 propagate to propagate to a given epoch. This then gives you the equivalent Cartesian state at the requested epoch. There are a number of implementations of the SGP4 propagator, e.g., for Python: &lt;a href=&quot;https://pypi.python.org/pypi/sgp4/&quot; rel=&quot;noreferrer&quot;&gt;https://pypi.python.org/pypi/sgp4/&lt;/a&gt;. Bare in mind that the accuracy of the TLEs is limited, so if you propagate over long time-scales, the results are unlikely to be meaningful.&lt;/p&gt;&#xA;" OwnerUserId="720" LastActivityDate="2015-11-26T12:43:29.103" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12805" PostTypeId="1" CreationDate="2015-11-26T16:16:44.610" Score="3" ViewCount="643" Body="&lt;p&gt;How far could a heavily modified SpaceX Dragon capsule fly just by using its SuperDraco engines?&#xA;By heavily modified i mean unmanned, unpressurized and full of mono propellant.&lt;/p&gt;&#xA;" OwnerUserId="8925" LastEditorUserId="51" LastEditDate="2015-11-26T17:41:23.660" LastActivityDate="2015-11-26T18:31:01.837" Title="Could a modified Dragon reach Low Earth Orbit just using SuperDraco engines?" Tags="|launch|rockets|spacex|dragon-v2|super-draco|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12806" PostTypeId="2" ParentId="12805" CreationDate="2015-11-26T16:42:21.900" Score="1" Body="&lt;p&gt;We've seen how high a Dragon can reach with its superdraco engines on its pad abort test, and it was about 5000 feet. If you filled it with propellant instead of cargo and passengers it might get a bit higher, but not that much. If you hooked up a big fuel tank to it the engines would not be powerful enough to lift it. It's not going to get anywhere near space on its own. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2015-11-26T17:10:49.883" LastActivityDate="2015-11-26T17:10:49.883" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12807" PostTypeId="2" ParentId="12805" CreationDate="2015-11-26T17:23:32.523" Score="12" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_V2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dragon V2&lt;/a&gt; (Crew Dragon) uses 8 &lt;a href=&quot;https://en.wikipedia.org/wiki/SuperDraco&quot; rel=&quot;nofollow noreferrer&quot;&gt;SuperDraco&lt;/a&gt; engines with maximum of 73,000 N thrust each. So, in theory, it could produce enough thrust to lift about 50,000 kg off the pad at &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust-to-weight_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;thrust-to-weight ratio&lt;/a&gt; (T/W) of ~ &lt;a href=&quot;https://encrypted.google.com/#q=%2873%2C000+N+*+8%29+%2F+%2850%2C000+kg+*+9.807+m%2Fs%5E2%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;1.19&lt;/a&gt;. You need T/W larger than 1 to lift off the ground, and some more. T/W of ~ 1.2 is low for a launch vehicle and is usually closer to ~ 1.5, but let's run with this number and see where we get if we stretch Crew Dragon and its 8 SuperDraco engines to near senseless theoretical limits;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we have vehicle's maximum wet mass of 50,000 kg. Its dry mass is, according to linked Wikipedia article, around 4,200 kg. Assuming no modifications that will add dry mass to the vehicle are needed to accommodate that much of &lt;a href=&quot;https://en.wikipedia.org/wiki/Dinitrogen_tetroxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;NTO&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/Monomethylhydrazine&quot; rel=&quot;nofollow noreferrer&quot;&gt;MMH&lt;/a&gt; propellants at &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific impulse&lt;/a&gt; of 240 seconds (yes, a ludicrous notion, but we'll let this one slide too and let's assume that we dropped solar panels, heat shield, nearly everything inside and used saved mass to extend its storage to accommodate that much more fuel - i.e. about three times its nominal volume), we now have its maximum propellant mass it could possibly lift off the ground of 45,800 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We now have everything we need to plug our numbers in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt;, which is good enough as it is for a single stage vehicle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://encrypted.google.com/#q=%28240+s+*9.807+m%2Fs%5E2%29ln%2850%2C000%2F4%2C200%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;$$\Delta v = v_\text{e} \ln \frac {m_0} {m_1} = (240\text{ s} \cdot 9.807\text{ m/s}^2)\ln\frac {50,000\text{ kg}} {4,200\text{ kg}}\approx5,830 \text{ m/s}$$&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So our generously (ridiculously?) light and highly modified Crew Dragon filled with 45,800 kg of NTO/MMH reaches a maximum delta-v of about half the change in velocity needed to reach orbit from the ground, and about &lt;a href=&quot;https://encrypted.google.com/#q=%28240+s+*9.807+m%2Fs%5E2%29ln%2850%2C000%2F4%2C200%29%2F%28sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%286378.1+km+%2B+412.5+km%29%29%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;76%&lt;/a&gt; of the orbital velocity of the ISS if it was deployed in vacuum and didn't lose any of that thrust to atmospheric drag. Note that this also assumes no thrust loss to multiple engine combustion instability and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming all that, it probably could fly close to the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kármán line&lt;/a&gt; on a suborbital flight (it would have enough fuel for about 103 seconds of powered flight at full thrust), and reach altitude &lt;a href=&quot;https://space.stackexchange.com/q/2076/49&quot;&gt;officially declared as the boundary of outer space&lt;/a&gt;, but then again, this is a generous back of an envelope notion and not really achievable in real world. Such paper thin propellant tanks simply wouldn't withstand dynamic pressure and ascent heating during launch, likely not even the acoustic shock during engine ignition.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-11-26T18:31:01.837" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12808" PostTypeId="1" AcceptedAnswerId="12809" CreationDate="2015-11-26T18:52:53.080" Score="4" ViewCount="2387" Body="&lt;p&gt;How does a spacecraft know its orientation while in orbit? For example, turning from prograde to retrograde when starting a descent? I am wondering if there is any way to sense this without using an external reference, like recording two known points on earth some time apart and then using this to calculate the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related:&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/3631/what-are-these-orientations-called-in-orbit&quot;&gt;What are these orientations called in orbit?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U5Eoq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U5Eoq.png&quot; alt=&quot;Kerbel Space Program: Orientation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12461" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-11-27T09:46:39.777" Title="How does a spacecraft know its orientation in orbit?" Tags="|orbit|spacecraft|attitude|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12809" PostTypeId="2" ParentId="12808" CreationDate="2015-11-26T19:23:25.260" Score="5" Body="&lt;p&gt;In LEO, nowadays? Easy, GPS and compass (actually, a multi-axis magnetometer or a gyrocompass).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if it's in orbit of other bodies, or in interplanetary space... or was a craft before GPS... that gets more interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me paste a rather long but quite comprehensive summary of the devices used, from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Attitude_control#Absolute_attitude_sensors&quot;&gt;Attitude Control&lt;/a&gt; article on Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Horizon sensor&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A horizon sensor is an optical instrument that detects light from the 'limb' of Earth's atmosphere, i.e., at the horizon. Thermal infrared sensing is often used, which senses the comparative warmth of the atmosphere, compared to the much colder cosmic background. This sensor provides orientation with respect to Earth about two orthogonal axes. It tends to be less precise than sensors based on stellar observation. Sometimes referred to as an Earth sensor.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Orbital gyrocompass&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Similar to the way that a terrestrial gyrocompass uses a pendulum to sense local gravity and force its gyro into alignment with Earth's spin vector, and therefore point north, an orbital gyrocompass uses a horizon sensor to sense the direction to Earth's center, and a gyro to sense rotation about an axis normal to the orbit plane. Thus, the horizon sensor provides pitch and roll measurements, and the gyro provides yaw.[citation needed] See Tait-Bryan angles.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Sun sensor&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A sun sensor is a device that senses the direction to the Sun. This can be as simple as some solar cells and shades, or as complex as a steerable telescope, depending on mission requirements.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Earth sensor&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;An Earth sensor is a device that senses the direction to Earth. It is usually an infrared camera; nowadays the main method to detect attitude is the star tracker, but Earth sensors are still integrated in satellites for their low cost and reliability.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Star tracker&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A star tracker is an optical device that measures the position(s) of star(s) using photocell(s) or a camera.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Magnetometer&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A magnetometer is a device that senses magnetic field strength and, when used in a three-axis triad, magnetic field direction. As a spacecraft navigational aid, sensed field strength and direction is compared to a map of Earth's magnetic field stored in the memory of an on-board or ground-based guidance computer. If spacecraft position is known then attitude can be inferred.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In case of probes in interplanetary space these are insufficient. In this case the probe can use a star tracker to determine its own attitude, but the position is determined on Earth - radio signals from the probe are triangulated, precise distance (to within meters or less!) can be determined using phase shift, and then the probe receives its calculated position back from Earth.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-11-26T19:23:25.260" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12811" PostTypeId="1" CreationDate="2015-11-28T07:11:22.043" Score="2" ViewCount="352" Body="&lt;p&gt;There is abundant $\require{mhchem}\ce{CO2}$ in martian atmosphere, so we could use that in a modified gas turbine. Can this, combined with PV cells (a hybrid power source), be an efficient energy source? It also turns out that supercritical $\ce{CO2}$ gas turbines are &lt;a href=&quot;http://www.scientificamerican.com/article/can-carbon-dioxide-replace-steam-to-generate-power/&quot; rel=&quot;nofollow&quot;&gt;smaller&lt;/a&gt; than water steam turbines.&lt;/p&gt;&#xA;" OwnerUserId="12467" LastEditorUserId="49" LastEditDate="2015-11-28T08:29:08.200" LastActivityDate="2015-11-28T09:04:24.677" Title="What are the challenges in building a supercritical CO2 gas turbine power plant for a Mars habitat of 6 astronauts?" Tags="|mars|colonization|power|solar-power|habitat|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12812" PostTypeId="1" CreationDate="2015-11-28T07:54:57.613" Score="4" ViewCount="3906" Body="&lt;p&gt;Does anyone know where I can find more detailed dimensions of the Mars Curiosity rover?  For example, what are various dimensions of the boogie rocker system?  Or the dimensions of the main body itself, or the dimensions of the arm?&lt;/p&gt;&#xA;" OwnerUserId="12468" LastActivityDate="2019-03-08T13:22:17.567" Title="Dimensions of the Curiosity Rover?" Tags="|mars|curiosity|dimensions|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12813" PostTypeId="2" ParentId="12811" CreationDate="2015-11-28T07:55:50.203" Score="5" Body="&lt;p&gt;Any type of steam cycle of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Stirling_engine&quot; rel=&quot;nofollow&quot;&gt;Stirling engine&lt;/a&gt; or a &lt;a href=&quot;https://en.wikipedia.org/wiki/Closed-cycle_gas_turbine&quot; rel=&quot;nofollow&quot;&gt;closed-cycle gas turbine&lt;/a&gt; would face significant heat exchange challenges on Mars, so what little you saved on size and mass in improved heat transport efficiency for using a higher &lt;a href=&quot;https://en.wikipedia.org/wiki/Heat_capacity&quot; rel=&quot;nofollow&quot;&gt;heat capacity&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Expansion_ratio&quot; rel=&quot;nofollow&quot;&gt;expansion ratio&lt;/a&gt; working fluid (sCO2 thermal efficiency is close to 45%), would be lost to cooling due to operation in extremely tenuous atmosphere and requiring massive heat pipes and large radiators, also adding significant &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_lag&quot; rel=&quot;nofollow&quot;&gt;thermal lag&lt;/a&gt; to the steam cycle and thus decreasing its capacity and efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So short of building such regenerative power systems in situ, and for a small settlement of few people, it would be too heavy to transport to the surface of Mars and there's a large number of other regenerative power systems available and in development that are more mass efficient (from heat pipe nuclear reactors coupled with some energy storage to electrolysis of the atmosphere systems).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One other comment tho, that the heating part of such a system that you inquire about would be better off with &lt;a href=&quot;https://en.wikipedia.org/wiki/Concentrated_solar_power&quot; rel=&quot;nofollow&quot;&gt;solar concentrators&lt;/a&gt; than photovoltaics, since there's no &lt;a href=&quot;https://en.wikipedia.org/wiki/Energy_conversion_efficiency&quot; rel=&quot;nofollow&quot;&gt;energy conversion inefficiencies&lt;/a&gt; (tho there are still losses, of course).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-11-28T09:04:24.677" LastActivityDate="2015-11-28T09:04:24.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12814" PostTypeId="2" ParentId="12812" CreationDate="2015-11-28T10:15:43.100" Score="11" Body="&lt;p&gt;This PDF has &lt;a href=&quot;http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/41421/1/08-0699.pdf&quot; rel=&quot;noreferrer&quot;&gt;a side view with some dimensions&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lePR8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lePR8.png&quot; alt=&quot;MSL side view&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.jpl.nasa.gov/news/press_kits/MSLLanding.pdf&quot; rel=&quot;noreferrer&quot;&gt;Press kit&lt;/a&gt; has lots of information. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And NASA has made a &lt;a href=&quot;http://nasa3d.arc.nasa.gov/detail/mars-rover-curiosity&quot; rel=&quot;noreferrer&quot;&gt;3D model&lt;/a&gt; which you can open and measure. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-11-28T10:15:43.100" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12815" PostTypeId="1" CreationDate="2015-11-28T19:58:13.847" Score="3" ViewCount="189" Body="&lt;p&gt;We know that the Spaceship One and Spaceship Two designs from Virgin Galactic/The Spaceship Company/Scaled Composites employ &lt;a href=&quot;https://space.stackexchange.com/a/1855/10856&quot;&gt;feathering&lt;/a&gt; for aerobraking and maneuverability. I can't seem to track down any information on when or exactly how (beyond 'hydraulics') the transitions occur in the flight profile. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;At what velocity or altitude&lt;/em&gt; does the ship transfer from the 'capsule' configuration to the 'plane' configuration and also what is the orientation of the plane with respect to the nose and atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it reach the plane configuration in a dive which it must then pull out of or does it transfer to a plane in belly-down freefall where the hydraulic feathering must push against substantial drag?&lt;/p&gt;&#xA;" OwnerUserId="10856" LastEditorUserId="12102" LastEditDate="2018-04-06T05:15:14.320" LastActivityDate="2018-04-06T14:35:15.763" Title="Feathering Profile Change for Virgin/TSC/Spaceship Two" Tags="|reentry|aerobraking|aerodynamics|virgin-galactic|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12816" PostTypeId="1" AcceptedAnswerId="12821" CreationDate="2015-11-28T23:42:40.487" Score="9" ViewCount="1646" Body="&lt;p&gt;Consider using solar electric propulsion for, e.g. Earth-Mars trip, or for asteroid retrieval mission. Is there a benefit to first entering an eccentric orbit so that part of the time is spent relatively close to the Sun, to maximize the available solar power &lt;em&gt;and&lt;/em&gt; the effect of thrust, similar to the Oberth effect?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does circular -&gt; eccentric -&gt; circular have potential benefits compared to simply spiraling from one circular orbit to another?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, is there any software that can calculate optimal solar electric trajectories and taking effects like this into account?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume the solar cells can withstand being closer to the Sun, and operate at the same % efficiency at any light level; also assume the engine can effectively use all available power.&lt;/p&gt;&#xA;" OwnerUserId="8315" LastEditorUserId="49" LastEditDate="2015-11-29T09:52:23.807" LastActivityDate="2015-11-30T19:24:40.763" Title="Optimal trajectory for solar electric propulsion?" Tags="|orbital-mechanics|propulsion|trajectory|solar-power|ion-thruster|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12817" PostTypeId="1" CreationDate="2015-11-29T00:18:20.203" Score="2" ViewCount="590" Body="&lt;p&gt;SpaceX and Blue Origin have demonstrated the feasibility of modern reusable rockets. Elon Musk claims that a reusable rocket is 100 times cheaper than a conventional one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The average success rate of a rocket launch is around 94%. &#xA;What will be the average loss in this reliability/success rate after each launch/reuse?&#xA;How many times can they reuse a rocket(say falcon 9)?&#xA;If SpaceX was to provide rocket launch service at a 1% of the actual overall cost(lets assume), will they make profits? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lets assume NASA wants to launch a satellite to Earths low orbit. Will they trust a used rocket?&lt;/p&gt;&#xA;" OwnerUserId="12471" LastActivityDate="2015-12-22T04:32:01.467" Title="Can we trust reusable rockets?" Tags="|rockets|spacex|nasa|falcon-9|blue-origin|" AnswerCount="1" CommentCount="7" ClosedDate="2015-11-29T03:28:18.453" ContentLicense="CC BY-SA 3.0" />
  <row Id="12818" PostTypeId="2" ParentId="12817" CreationDate="2015-11-29T00:31:51.720" Score="5" Body="&lt;p&gt;Quoting an average success rate for rocket launches as a whole is not very useful. Atlas V has a near-perfect record, for instance, while Zenit and Proton have about 88% rates. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presently, Falcon 9's partial success rate is 95% and its full success rate is 88% as an expendable, but it's still quite a young launcher. We won't know how it performs as a reusable until there's much more of  a track record. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has trusted not only satellites but humans to used rockets, many, many times. The space shuttles were used an average of 27 times each, and the two failures were not related to the reuse per se (although design decisions made in the name of reusability were major contributing factors). So it wouldn't be surprising for Falcon 9 first stages to manage 10 to 100 reuses, with some degree of engine overhaul or replacement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you mean provide rocket launch service at 1% of the per rocket lifetime costs, almost certainly not; that would be gambling on more than 100 successful reuses, a success rate which almost no expendable launch systems manage. It's much more likely that SpaceX will charge a large fraction of the price they would for an expendable rocket. Even charging half the current cost of a Falcon launch would be a win for the customers who currently pay full price and a win for SpaceX if they can use the first stage three or four times successfully. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that Falcon 9 throws away its second stage, which is apparently &lt;a href=&quot;http://shitelonsays.com/transcript/spacex-press-conference-september-29-2013-2013-09-29&quot; rel=&quot;nofollow&quot;&gt;around 25% of the total cost of the rocket&lt;/a&gt; (it accounts for half the guidance systems, half the tank assemblies, 1/7 of the dry mass, 1/10 of the engines, etc.) -- on that basis even a sixfold cost reduction would be a big challenge. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-22T04:32:01.467" LastActivityDate="2015-12-22T04:32:01.467" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12821" PostTypeId="2" ParentId="12816" CreationDate="2015-11-29T04:38:58.640" Score="10" Body="&lt;p&gt;Optimal low-thrust trajectories for interplanetary missions are complicated to calculate. It's a little too broad of a topic for a Q&amp;amp;A. In fact, I spent an entire internship at NASA Johnson Space Center working on this problem (building of the work of many others years before and hence), and wrote my first (non-conference) academic paper on exactly this topic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://veridical.net/raptor.html&quot; rel=&quot;nofollow&quot;&gt;A Genetic Algorithm and Calculus of Variations-Based Trajectory Optimization Technique&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A &lt;a href=&quot;https://en.wikipedia.org/wiki/Genetic_algorithm#Methodology&quot; rel=&quot;nofollow&quot;&gt;genetic algorithm&lt;/a&gt; is used cooperatively with the Davidon–Fletcher–Powell penalty function method and the calculus of variations to optimize low-thrust, Mars-to-Earth trajectories for the Mars Sample Return Mission. The return trajectory is chosen thrust-coast-thrust a priori, has a fixed time of flight, and is subject to initial and final position and velocity equality constraints. The global search properties of the genetic algorithm combine with the local search capabilities of the calculus of variations to produce solutions that are superior to those generated with the calculus of variations alone, and these solutions are obtained more quickly and require less user interaction than previously possible. The genetic algorithm is not hampered by ill-behaved gradients and is relatively insensitive to problems with a small radius of convergence, allowing it to optimize trajectories for which solutions had not yet been obtained. The use of the calculus of variations within the genetic algorithm optimization routine increased the precision of the final solutions to levels uncommon for a genetic algorithm.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;TLDR: As first-year orbital mechanics would suggest, it was marginally more efficient to perform the plane change during the Mars escape. The general solution was using electrical propulsion to spiral to escape velocity, coast to Earth, and spiral back in.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="127" LastEditDate="2015-11-30T19:24:40.763" LastActivityDate="2015-11-30T19:24:40.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12822" PostTypeId="2" ParentId="12816" CreationDate="2015-11-29T04:47:26.253" Score="8" Body="&lt;p&gt;Solar Electric Propulsion (SEP) is unlikely to benefit on many missions from periapsis burns since, albeit it can achieve high &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;noreferrer&quot;&gt;specific impulse&lt;/a&gt; (high exhaust exit velocity), is a relatively small thrust (small exhaust mass flow rate) propulsion method. We have yet to see actual SEP implementations for human missions, and development of those is aiming for much higher thrust than what we're seeing currently, but it will still be considered low thrust and constant acceleration systems compared to impulse burn chemical rockets. E.g. NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/NEXT_%28ion_thruster%29&quot; rel=&quot;noreferrer&quot;&gt;NEXT&lt;/a&gt; (Evolutionary Xenon Thruster) is currently at about one quarter Newton thrust. So SEP will operate pretty much all the of the time, meaning there's no net gain due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;noreferrer&quot;&gt;Oberth effect&lt;/a&gt; with highly eccentric orbits during periapsis burn, since it would only linger longer at apoapsis then (where no net gain of periapsis burns at constant thrust is a direct consequence of &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;noreferrer&quot;&gt;orbital-energy-invariance law&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that sense, from thrust efficiency perspective, there is therefore no incentive to start &lt;em&gt;the climb&lt;/em&gt; from a highly eccentric orbit. From solar power's perspective, any eccentricity will result in more or less same insolation since it will still be close to the primary with orbit's semi-major axis within the primary's Hill sphere, otherwise it's already on its way somewhere else anyway. It will be more important to pay attention to other orbital elements such as inclination and node and make sure that the spacecraft stays for as short period as possible in the primary's shadow (both umbra and penumbra), or that it avoids other potentially damaging regions (such as, e.g., Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;noreferrer&quot;&gt;Van Allen radiation belts&lt;/a&gt;). Equally, it's important to realize that constant thrust will naturally reduce initial eccentricity anyway for the same reason I already mention in the first paragraph (efficiency of shorter periapsis burn will be offset by inefficiency of longer apoapsis burn - actually, come to think of it, this can be visualized pretty well with &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler&amp;#39;s_laws_of_planetary_motion#Second_law&quot; rel=&quot;noreferrer&quot;&gt;Kepler's second law of planetary motion&lt;/a&gt;, and why $\Delta A/\Delta t$ remains constant - but i.e. difference between apoapsis and periapsis will remain more or less the same while semi-major axis constantly increases, thus decrease in eccentricity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So using SEP in deep gravity wells will be more akin to slowly &lt;em&gt;climbing out of orbit&lt;/em&gt; in many spiraling out orbits up to reaching &lt;a href=&quot;https://en.wikipedia.org/wiki/Characteristic_energy&quot; rel=&quot;noreferrer&quot;&gt;characteristic energy&lt;/a&gt; needed to escape the gravity well ($C_3$) and be on its way to some interplanetary destination. At that point any additional trust translates to &lt;a href=&quot;https://en.wikipedia.org/wiki/Hyperbolic_trajectory#Hyperbolic_excess_velocity&quot; rel=&quot;noreferrer&quot;&gt;hyperbolic excess velocity&lt;/a&gt; ($v_\infty$) with respect to the primary body and shortens transfer time, but it doesn't orbit it any more. I.e. it's the point where the spiraling geometry of climbing out of the gravity well changes to either a heliocentric &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;noreferrer&quot;&gt;Hohmann transfer orbit&lt;/a&gt;'s hyperbole without any additional thrust for the interplanetary transfer leg, or the first accelerating leg of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Brachistochrone_curve&quot; rel=&quot;noreferrer&quot;&gt;brachistochrone curve&lt;/a&gt; with continuous propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for if there are any tools that can calculate optimal solar electric trajectories, yes, some previous versions of NASA's &lt;a href=&quot;http://gmatcentral.org&quot; rel=&quot;noreferrer&quot;&gt;General Mission Analysis Tool (GMAT)&lt;/a&gt; already had third party Electric Propulsion plugins available, but that's now built into the latter releases, starting with R2015a:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;GMAT now supports modelling of electric propulsion systems. Below is&#xA;  an example showing GMAT modelling a cube-sat with electric propulsion&#xA;  in a lunar weak-stability orbit. You can model electric tanks,&#xA;  thrusters, and power systems (both Solar and nuclear).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xZZv1.png&quot; alt=&quot;GMAT Electric Propulsion&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;See the &lt;a href=&quot;http://gmat.sourceforge.net/docs/R2015a/html/Tut_ElectricPropulsion.html&quot; rel=&quot;noreferrer&quot;&gt;Electric Propulsion&lt;/a&gt; tutorial for more information.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-11-29T09:47:56.790" LastActivityDate="2015-11-29T09:47:56.790" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12823" PostTypeId="1" AcceptedAnswerId="13230" CreationDate="2015-11-29T18:38:11.423" Score="25" ViewCount="5241" Body="&lt;p&gt;According to the &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot;&gt;October 2015 revision of the Falcon 9 User's Guide&lt;/a&gt;, the Falcon 9's first stage Merlin 1D engines have a 70%-100% throttle range, while the second stage Merlin 1D Vac engine has a 38.5%-100% throttle range. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two engines are very similar; the major difference is that the second stage engine uses a large radiatively cooled nozzle extension which produces substantially higher thrust in vacuum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 100% throttle rating for the engines (934kN for the vacuum engine in vacuum, 756kN for the first stage engine at sea level) represents the same fuel flow rate when adjusted for specific impulse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question: Why is the upper stage engine so much more deeply throttleable than the first stage engine?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the common problems with deep throttling engines include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Combustion stability&lt;/li&gt;&#xA;&lt;li&gt;Pump flow stability&lt;/li&gt;&#xA;&lt;li&gt;Exhaust flow adhering to one side of the nozzle, causing uneven nozzle erosion and off-center thrust&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Given the common design of the pumps and combustion chamber between the engines, the first two seem unlikely. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is a nozzle operating in vacuum less susceptible to the flow adherence problem?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the first-stage engine perhaps require more fuel flow through the regeneratively cooled nozzle to maintain cooling?&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-11-30T01:09:34.337" LastActivityDate="2015-12-31T06:02:07.010" Title="What makes the Merlin vacuum engine so deeply throttleable?" Tags="|spacex|falcon-9|engines|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12824" PostTypeId="1" AcceptedAnswerId="19499" CreationDate="2015-11-29T22:02:03.947" Score="12" ViewCount="3842" Body="&lt;p&gt;I'd like to know what is the lightest possible solar array per unit power with current or near future technology.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The arrays I have been able to find:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Vanguard Space &lt;a href=&quot;http://techport.nasa.gov/file/15225&quot; rel=&quot;noreferrer&quot;&gt;THINS&lt;/a&gt; 400-500 W/kg (2014; thin film)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.2067&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;noreferrer&quot;&gt;SLASR&lt;/a&gt; 362 W/kg (2006; concentrator with GaAs cells)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sbn.pds.nasa.gov/holdings/ds1-c-micas-3-rdr-visccd-borrelly-v1.0/document/doc_Apr04/int_reports/Scarlet_Integrated_Report.pdf&quot; rel=&quot;noreferrer&quot;&gt;SCARLETT&lt;/a&gt; 45 W/kg (1998; concentrator with GaAs cells; flown on DS-1)&lt;/li&gt;&#xA;&lt;li&gt;Orbital/ATK &lt;a href=&quot;https://www.orbitalatk.com/space-systems/space-components/solar-arrays/docs/FS007_15_OA_3862%20UltraFlex.pdf&quot; rel=&quot;noreferrer&quot;&gt;UltraFlex&lt;/a&gt; 150 W/kg (since 1995-98; triple junction GaAs; flown on Mars Phoenix and Mars Insight)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Thin film solar cells (for example CIGS) can have efficiencies around 10% with a thickness of 1-3 microns (excluding substrate).  A 10 micron laminate with a thin film solar cell would weigh 30-50 g/m^2 depending on composition; this would put the output at 2600-4300 W/kg, which might be a reasonable ballpark near-term goal without major technology advances.  With a bit of a stretch, a 3-5 micron laminate with a 1-2 micron cell could be over 10 kW/kg.  From that perspective, the numbers for existing arrays are a bit low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the state of the art and most promising research for ultra-lightweight solar arrays?  What are the major technical challenges to higher power/mass ratios?&lt;/p&gt;&#xA;" OwnerUserId="8315" LastEditorUserId="8315" LastEditDate="2015-11-30T19:15:53.310" LastActivityDate="2019-08-19T11:29:03.070" Title="Lightest possible solar array?" Tags="|solar-power|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12825" PostTypeId="2" ParentId="12816" CreationDate="2015-11-29T22:42:32.483" Score="4" Body="&lt;p&gt;You do not mention what acceleration your SEP is capable of. Presently power sources have to be rather massive. When, for example, your thrust is 1 newton but spacecraft mass is 10 tonnes, acceleration is .0001 meters/sec^2. See &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/the-need-for-better-alpha.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Need For a Better Alpha&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If acceleration from the ion engines is a tiny fraction of the acceleration the sun exerts, the trajectory is a gradual spiral. The delta V from one circular orbit to another is well approximated by |speed of departure orbit - speed of destination orbit|. See the Stack Exchange question &lt;a href=&quot;https://space.stackexchange.com/questions/8420/general-guidelines-for-modeling-a-low-thrust-ion-spiral&quot;&gt;General guidelines for modeling a low thrust ion spiral?&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example earth's average speed is about 29.79 km/s. Mars average speed is about 24.13 km/s. 29.79 - 24.13 = 5.66. A slow ion spiral from a 1 A.U. circular orbit to a 1.52 circular orbit takes about 5.66 km/s. (Here I'm assuming circular coplanar orbits and disregarding the planetary gravity wells).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's look at a Hohmann orbit for the same scenario, a Hohmann from a 1 A.U. circular orbit to a coplanar circular 1.52 A.U. circular orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Departure &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vinf&lt;/a&gt; is 2.94 km/s. Arrival Vinf is 2.65 km. These total 5.59 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;5.59 km/s vs 5.66 km/s? That's a difference of only .07 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a graphic comparing sum of Hohmann Vinfs to the delta V needed for a gradual ion spiral:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8DRFV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8DRFV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The blue and red portion is like a sand chart. The red part laid on top of the blue gives the sum of the two Vinfs.&#xA;The grey part in the back is |destination orbit speed minus earth orbit speed|.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see for neighboring planets like Venus and Mars, they're pretty close. But a Hohmann to Jupiter beats slow ion spirals by a good margin. A Hohmann to Uranus wins by a larger margin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as you move from the sun, burns can be much more leisurely. Something in low earth orbit is moving about 4 degrees per minute. If you wanted your burn within 8 degrees of perigee, you'd have less than two minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast earth moves around the sun at about a degree per day. At 1 A.U., a burn within 8 degrees of perihelion would be about 8 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter moves around the sun at about a degree every 12 days. So if perihelion is 5.2 A.U. from the sun, you'd have more than three months to execute a burn within 8 degrees of perihelion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as you move outward from the sun, even ion burns start looking like impulsive burns. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-11-29T22:53:34.033" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12826" PostTypeId="1" CreationDate="2015-11-29T22:48:39.297" Score="1" ViewCount="204" Body="&lt;p&gt;I once read an article about proposed project to move a NEO (near earth object) using a small satellite that would land on its surface and drill out small pieces of the object, slinging it out into space. This force would &quot;eventually&quot; alter its orbit (trajectory). Is there any possibility that this could happen to Earth, as we continue to launch rockets out into space?&lt;/p&gt;&#xA;" OwnerUserId="12475" LastEditorUserId="6816" LastEditDate="2015-11-29T22:55:55.143" LastActivityDate="2015-11-29T23:15:14.627" Title="Possibility of moving Earth's orbit from continual rocket ejections" Tags="|launch|rockets|orbit|earth|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12827" PostTypeId="2" ParentId="12826" CreationDate="2015-11-29T23:15:14.627" Score="3" Body="&lt;p&gt;In principle, yes, but practically, no. For one thing, a NEO is much smaller than Earth, so the amount of effort required to slingshot the necessary mass is within reason (whereas the relative amount of mass of rockets being launched from Earth is infinitesimal).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is also assuming that rockets are &quot;slung&quot; into space, but they aren't. The majority of a rocket's thrust is due to the exchange of momentum between the rocket and the exhaust -- not reactions between the rocket and the Earth (although you could say there is some due to atmospheric effects).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, even if you were slinging mass from the Earth (or if the reaction forces from conventional rockets were significant enough), you would have to make sure each launch is timed correctly so that momentum is exchanged in the same direction -- otherwise the launches will generally cancel out.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-11-29T23:15:14.627" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12828" PostTypeId="1" CreationDate="2015-11-30T03:38:10.473" Score="4" ViewCount="416" Body="&lt;p&gt;Chang'e 3 now on the Lunar surface has imaged the Earth. Is it possible for that same camera to take a time exposure and show us the stars?&lt;/p&gt;&#xA;" OwnerUserId="12406" LastEditorUserId="103" LastEditDate="2015-11-30T06:12:48.573" LastActivityDate="2016-08-22T11:23:30.250" Title="Can Chang'e 3 take photos of the stars from the moon" Tags="|the-moon|astronomy|chang-e|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12829" PostTypeId="1" AcceptedAnswerId="12832" CreationDate="2015-11-30T09:37:51.153" Score="7" ViewCount="708" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/10230/what-is-so-hard-about-making-a-throttleable-large-rocket-engine&quot;&gt;Some&lt;/a&gt; &lt;a href=&quot;https://space.stackexchange.com/questions/12823/what-makes-the-merlin-vacuum-engine-so-deeply-throttleable&quot;&gt;questions&lt;/a&gt; popped up about difficulties of creating throttleable liquid fuel engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder why the obvious alternative isn't more common: multiple engines, from which only some will be lit; possibly ones of varied thrust so that you can &lt;em&gt;fine-tune your thrust&lt;/em&gt; by selecting which engines are active simultaneously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several Soviet launchers used four engines. The Shuttles used three engines. Falcon Heavy uses 27 Merlin engines. Soyuz uses 20 combustion chambers across five engines - so the multi-engine solution is not unheard of - but not quite as common as single-engine ones. What factors decide upon its relatively low popularity?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2018-03-21T21:47:39.733" Title="What challenges are there for more, smaller liquid fuel engines instead of one large?" Tags="|propulsion|engine-design|liquid-fuel|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12830" PostTypeId="1" CreationDate="2015-11-30T09:51:50.117" Score="10" ViewCount="2210" Body="&lt;p&gt;What is the fundamental difference between a construction of &lt;em&gt;n&lt;/em&gt; engines vs one engine with &lt;em&gt;n&lt;/em&gt; combustion chambers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, Energia rocket used four boosters, each with a 4-chamber &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-170&quot;&gt;RD-170&lt;/a&gt; engine, and four separate single-chamber &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-0120_(rocket_engine)&quot;&gt;RD-0120&lt;/a&gt; engines for its core stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess one difference may be a pump - for designs that utilize one, instead of using compressed neutral gas for displacing the fuel, but in designs that don't use turbopumps I fail to see any difference... &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2023-07-30T21:16:22.290" Title="Multiple engines vs one engine with multiple combustion chambers" Tags="|engine-design|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12831" PostTypeId="2" ParentId="12828" CreationDate="2015-11-30T11:09:59.687" Score="3" Body="&lt;p&gt;Looking through &lt;a href=&quot;http://moon.bao.ac.cn/multimedia/img2dce3.jsp&quot; rel=&quot;nofollow noreferrer&quot;&gt;this image archive&lt;/a&gt;, they haven't taken photos where stars are the main subject. All photos in that archive were done in daylight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vQ6my.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vQ6my.jpg&quot; alt=&quot;Chang&amp;#39;e 3 photo of Earth&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've removed the processed photo - I found 2 bright pixels in the shadowed region of the Earth, so those can't possibly be stars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chang'e 3 &lt;a href=&quot;http://spaceflight101.com/change/change-3/&quot; rel=&quot;nofollow noreferrer&quot;&gt;hibernates&lt;/a&gt; during the lunar night. It's &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/c-missions/chang-e-3&quot; rel=&quot;nofollow noreferrer&quot;&gt;not clear&lt;/a&gt; if the probe contains an RTG that can provide electrical power, or if it has radionucleide heaters only.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The majority of power provided by the RTG is used for the heaters and a small fraction is used to sustain the lander's Lunar Night Sleep Mode in which all systems except for crucial control and housekeeping systems are shut down.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I tried to find out a bit more about the &lt;a href=&quot;http://spaceflight101.com/change/change-3/&quot; rel=&quot;nofollow noreferrer&quot;&gt;camera&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Three panoramic cameras are installed on the lander, facing different&#xA;  directions to allow the lander to acquire images of the lunar terrain&#xA;  surrounding the landing site and take photos of the rover as it&#xA;  departs the landing site. The cameras can take still images as well as&#xA;  video. &lt;em&gt;Exact technical details for the cameras were not released&lt;/em&gt;. (emphasis mine)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Camera technology tested on previous missions that is employed on&#xA;  Chang’e 3 include auto-exposure, focus adjustments, high-speed&#xA;  compression of color imagery and static gray image, and sub-sampling&#xA;  methods.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I also found a paper (&lt;a href=&quot;https://www.raa-journal.org/docs/papers_accepted/ms1939.pdf.gz&quot; rel=&quot;nofollow noreferrer&quot;&gt;PDF&lt;/a&gt;) that analyzes the performance of the cameras on Yutu in more detail. It doesn't mention exposure times specifically. It does say the camera has both manual and automatic exposure modes. The PDF contains a few graphs that show camera performance, these have an X-axis marked 0-1200 with a label of &quot;0.5 ms&quot;. I think that means the X-axis shows an exposure time of 0-0.6 seconds, but I'm not entirely sure. If this is true, the camera has been designed to use exposures of up to 0.6 s, which sounds long enough to see a few stars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b2g6N.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b2g6N.png&quot; alt=&quot;Yutu camera performance&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information on the Chang'e 3 image archive can be found in these blog posts:&lt;br&gt;&#xA;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2016/01281656-fun-with-a-new-data-set-change.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;one&lt;/a&gt;&lt;br&gt;&#xA;&lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2016/01221450-china-invites-public-on-board.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;two&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The archive itself is &lt;a href=&quot;http://moon.bao.ac.cn/ceweb/datasrv/datadesclevel.jsp&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; but is available in Chinese only. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-08-22T11:23:30.250" LastActivityDate="2016-08-22T11:23:30.250" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12832" PostTypeId="2" ParentId="12829" CreationDate="2015-11-30T12:39:57.407" Score="7" Body="&lt;p&gt;Having a large range of thrust settings is important for landings, not so much for launches. During a launch, you want some throttlability to reduce loads around max-Q, but otherwise a launch is most efficient when it's done as quickly as possible. So in non-reusable launchers, there's no point in being able to shut down some of your engines.&lt;br&gt;&#xA;When you use more engines, your parts count, cost and weight goes up and your reliability goes down.&lt;br&gt;&#xA;If you have enough engines (about 5 engines seems to be enough), you gain the option to continue the mission if one engine fails, but for most designs that decision hasn't been made (more launchers with a small number of engines than with engine-out capability). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-11-30T12:39:57.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12834" PostTypeId="1" CreationDate="2015-11-30T18:51:29.207" Score="10" ViewCount="419" Body="&lt;p&gt;I'm trying to estimate the depth of the soil around this 300 m crater about 150 m from the lip of Lalande crate on the Moon's equator. In the image below &lt;a href=&quot;http://wms.lroc.asu.edu/lroc/view_lroc/LRO-L-LROC-2-EDR-V1.0/M1169579935RE&quot; rel=&quot;nofollow noreferrer&quot;&gt;from the LRO's camera&lt;/a&gt; the sun's incidence angle is 65&lt;sup&gt;o&lt;/sup&gt; (it is 25&lt;sup&gt;o&lt;/sup&gt; above the horizon). This image shows the crater doesn't have a floor, the reglith &#xA;was loose enough and deep enough to slump down and fill it in after the impact. At this shallow sun angle, the sun also reaches all of it. Based on that I guesstimate that on the darker side the crater has an incline of no more than maybe 15&lt;sup&gt;o&lt;/sup&gt;. So the crater is about 35 to 40 m deep, and there is no sign it penetrates a layer of coarser or denser regolith.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yCewP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yCewP.jpg&quot; alt=&quot;small crater beside lalande crater on moon&amp;#39;s equator taken by Lunar Reconnaissance Orbiter&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here it is in context of Lalande and with the sun a bit higher, in the area where it is shown. The image is a screenshot of the &lt;a href=&quot;http://target.lroc.asu.edu/q3/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Quickmap&lt;/a&gt; service, you can go directly to the spot on the map with &lt;a href=&quot;http://bit.ly/1OA7zpK&quot; rel=&quot;nofollow noreferrer&quot;&gt;this link&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OWNYZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OWNYZ.jpg&quot; alt=&quot;lip of lalande crater taken by Lunar Reconnaissance Orbiter&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that one I traced a ridge a couple hundred meters away from the lip which seems to indicate where denser or more consolidated material begins. I couldn't find the LROC image file for the right side where the small crater is, so I don't know the sun-incidence angle, but it looks like the sun was a little higher. The wall of Lalande is in deep shadow at that sun angle, so the slope there is at least that steep - something over about 30&lt;sup&gt;o&lt;/sup&gt;, between the lip and that first ridge. So, the ridge is over 100 m below the crater lip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can it be concluded from this that the fine-grain, upper layer of regolith in this area near the lip of Lalande is over 100 m deep? &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-11-30T19:00:34.580" LastActivityDate="2016-05-05T23:50:35.323" Title="How deep is the regolith around this crater?" Tags="|the-moon|planetary-science|regolith|crater|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12835" PostTypeId="1" CreationDate="2015-11-30T23:26:04.483" Score="7" ViewCount="3090" Body="&lt;p&gt;I understand ISS is using gyroscopes (CMG) to maintain its attitude. At the same time, I understand that ISS maintains constant attitude relative to Earth, i.e. its Z axis always points toward the center of the planet. Given the station's speed, it means ISS has to continuously change its attitude with speed of 4 degrees per minute (see &lt;a href=&quot;https://www.quora.com/Does-the-ISS-always-face-the-Earth&quot;&gt;1&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is where I am missing something. Gyroscopes, by definition, maintain certain attitude. Being stabilized by gyroscopes, shouldn't the station always be in the same &quot;absolute&quot; position, which would mean it has to be spinning relative to Earth?&lt;/p&gt;&#xA;" OwnerUserId="10643" LastEditorUserId="25" LastEditDate="2015-12-01T12:34:23.250" LastActivityDate="2016-05-16T17:47:58.023" Title="Why is ISS spinning even though stabilized by gyroscopes?" Tags="|iss|attitude|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12836" PostTypeId="2" ParentId="12835" CreationDate="2015-11-30T23:38:31.333" Score="10" Body="&lt;p&gt;The term &quot;gyroscope&quot; is a bit general and applies to two devices in the context of spacecraft orientation: a &lt;em&gt;gyroscope sensor&lt;/em&gt; and a &lt;em&gt;control moment gyroscope&lt;/em&gt;. A gyroscope sensor is &lt;em&gt;measuring&lt;/em&gt; the rotation rate of the spacecraft meanwhile a control moment gyroscope is &lt;em&gt;controlling&lt;/em&gt; the rotation rate (by generating torque).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft like the ISS require a complete attitude control system that uses sensor measurements to estimate the true orientation, compare that to the desired orientation, and then apply the required torque. The answer to your question really lies in how you define that &lt;em&gt;desired orientation&lt;/em&gt;. Yes, the ISS is rotating to be aligned with the Earth, but it is also being controlled to maintain that desired orientation. It is no problem that the desired orientation is changing, you don't &lt;em&gt;need&lt;/em&gt; to be stabilized to a &quot;certain attitude&quot; when using gyroscopes. You can just as easily track a spinning desired attitude using gyroscopes, and you can also track a moving target with changing spin as well.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-11-30T23:38:31.333" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12837" PostTypeId="2" ParentId="12835" CreationDate="2015-12-01T03:36:40.097" Score="5" Body="&lt;p&gt;ISS uses a torque equilibrium attitude which by definition requires the minimum input from the control moment gyros. &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100024204_2010020499.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100024204_2010020499.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-12-01T03:36:40.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12839" PostTypeId="2" ParentId="12835" CreationDate="2015-12-01T09:23:42.957" Score="5" Body="&lt;p&gt;Let me add:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS without gyroscopes would follow Newton's 1st law of rotary motion - maintain its spin; if the spin speed is 1 revolution per orbital period, it will face Earth with the same side at all times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CMGs with axis that are perpendicular to the rotation plane would not be affected, maintaining their speed; their torque doesn't act in that plane if their speed is unchanged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CMGs with axis in the rotation plane though would exert powerful torque if they are only spinned up - flipping a spinning gyroscope upside down in a straight arc requires quite a bit of force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But they don't need to be flipped. The CMGs are gimballed. So while the station spins (just following its own spin inertia), the CMGs that would exert torque, remain in a fixed orientation, slipping in their gimbals relative to the station. Only when the station's spin needs to be changed, the motors of the CMGs and the motors of their gimbals are engaged to provide the desired torque - outside of that, they only account for friction/losses.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-12-01T09:23:42.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12840" PostTypeId="1" AcceptedAnswerId="20591" CreationDate="2015-12-01T18:06:37.380" Score="13" ViewCount="3257" Body="&lt;p&gt;I would like informations on the flight controls of the Soyuz spacecraft.&#xA;Especially the two control sticks, as shown in this picture (of a Soyuz simulator).&#xA;&lt;a href=&quot;https://i.stack.imgur.com/3VZtz.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3VZtz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm particularly interested by the user interactions.&#xA;To my understanding the left one is used to control the attitude of the spacecraft: yaw, pitch, roll (the latter by rotating the stick).&#xA;The right stick is used for translation control (forward/backward up/down right/left)&#xA;The third axis is controlled by pulling/pushing the control.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Did I understood well how the sticks function?&lt;/li&gt;&#xA;&lt;li&gt;Does the sticks allow for multi-directional inputs?&lt;/li&gt;&#xA;&lt;li&gt;Does the sticks have a potentiometre (are the thrusters throttlable)?&lt;/li&gt;&#xA;&lt;li&gt;What are the green and brown plots on the left stick?&lt;/li&gt;&#xA;&lt;li&gt;Which axis on the right controller controls which direction?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Image credit: &lt;a href=&quot;https://www.flickr.com/photos/cmichel67/14305335555&quot; rel=&quot;noreferrer&quot;&gt;https://www.flickr.com/photos/cmichel67/14305335555&lt;/a&gt; licensed under CC.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="415" LastEditDate="2015-12-02T09:10:35.313" LastActivityDate="2017-03-13T20:22:44.967" Title="How do the Soyuz flight control sticks operate?" Tags="|soyuz-spacecraft|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12841" PostTypeId="1" CreationDate="2015-12-01T21:12:34.803" Score="11" ViewCount="2157" Body="&lt;p&gt;I was curious while designing a spacecraft for doing a flyby of the solar gravitational focus. I know that power is currently the limiting factor for ion drives (since solar power and radioisotope batteries are still the predominant energy sources for them, and both require considerable area or mass to reach power levels above 1 kW). But still despite that, this is not a problem for ground testing ion thrusters, and 1 MW power levels could be attained during ground testing. While most people assume higher power can be easily handled, I know from personal experience that at those high power levels, effects like erosion, wall impacts, and cooling will become big problems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From my research on Google, it seems the highest power ion thruster ever built was built by the folks at Ad Astra Rocket with their VASIMR coming to 200 kW. Is this the record for the highest power output achieved with an ion thruster? Or is there something higher?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also related, what is the highest Isp achieved by an ion thruster? I know for a fixed power, higher Isp means lower thrust, and that is ultimately limited by acceleration voltage, but still I am curious. &lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="12102" LastEditDate="2021-06-14T04:29:33.277" LastActivityDate="2021-06-14T04:29:33.277" Title="What is the highest power output ever achieved with an ion thruster?" Tags="|propulsion|ion-thruster|record|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12842" PostTypeId="1" CreationDate="2015-12-01T21:53:44.367" Score="5" ViewCount="1019" Body="&lt;p&gt;If I have a booster capable of reaching LEO, and want to launch it from a sailing platform in the middle of the South Atlantic (on the equator) do I have to notify some agency or state beforehand? Can it be mistaken for SLBM launch?&lt;/p&gt;&#xA;" OwnerUserId="12356" LastEditorUserId="6944" LastEditDate="2020-09-17T00:22:45.107" LastActivityDate="2020-09-17T00:22:45.107" Title="If you launch a booster from international waters, who do you have to notify beforehand?" Tags="|launch|low-earth-orbit|law|regulatory|sea-launch|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12843" PostTypeId="2" ParentId="12842" CreationDate="2015-12-01T22:02:00.060" Score="6" Body="&lt;p&gt;It seems the host country of the company requires a permit. This has been done by only one company that I'm aware of, Sea Launch. Sea Launch was &lt;a href=&quot;http://www.faa.gov/about/office_org/headquarters_offices/ast/faq/#s2&quot; rel=&quot;nofollow&quot;&gt;licensed by the FCC&lt;/a&gt;, as it was managed by Boeing. If it's not a company, then it would be the host country. And quite frankly, if a person don't have citizenship of any country, then they seem unlikely to be in such a position. In any such case, the UN should be notified, along with any country over which a piece of the rocket might drop a stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could fairly easily be differentiated from a ballistic missile, long before it would get to a point of any real danger. Most rockets burn out after around 10 minutes, while it would take a full 40 minutes or so to get across the world. Once the rocket is burned out, it's very easy to tell, and I suspect the profile is radically different even for a rocket not yet burnt out. You should also notify whatever entity manages the air traffic in the area. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-12-01T22:32:49.687" LastActivityDate="2015-12-01T22:32:49.687" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12844" PostTypeId="1" AcceptedAnswerId="12845" CreationDate="2015-12-02T00:41:43.167" Score="11" ViewCount="4032" Body="&lt;p&gt;Let's consider a hypothetical situation where a communications satellite is launched into orbit. However, no launch is perfect so after burnout the satellite has picked up some roll. In order to correct this roll, the reaction wheels that the satellite is equipped with are used to apply the appropriate torque necessary to negate the rotation of the spacecraft. This is done by rotating the reaction wheels in the opposite direction to the rotation of the spacecraft, until enough angular momentum is applied that the spacecraft's rotation is nullified.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in this situation the reaction wheel would still be spinning even when the angular velocity of the craft is corrected. There are a couple problems here...&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;If we assume friction is negligible...&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;If we assume friction is negligible or non-existent, then the reaction wheel which is now spinning will continue to spin until the reaction wheel is powered up again (as per Newton's first law of motion). Now lets say that due to external factors the spacecraft picks up more rotation in the same direction as before, and the reaction wheel must be sped up further in order to counter this. Eventually, this cumulative application of angular force to the reaction wheel will speed up the reaction wheel so fast that it goes beyond its limits of structural integrity - which is bad.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;If we consider friction...&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;If we consider friction to be a factor in the aforementioned hypothetical situation, then the reaction wheel will begin to slow down as the force of friction is applied on it. However, this force of friction would not only be applied to the wheel, it's equal and opposite reaction would also apply a rotation to the spacecraft and destabilize it, making it necessary to use the reaction wheels even more...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it true that reaction wheels are not, on their own, capable of stabilizing a spacecraft? If not, why not?&lt;/p&gt;&#xA;" OwnerUserId="12484" LastEditorUserId="12484" LastEditDate="2015-12-02T01:50:35.213" LastActivityDate="2015-12-02T20:16:55.067" Title="Why don't reaction wheels destabilize spacecraft over time?" Tags="|reaction-wheel|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12845" PostTypeId="2" ParentId="12844" CreationDate="2015-12-02T01:04:07.493" Score="14" Body="&lt;p&gt;To answer your question &quot;Is it true that...&quot; then it is best to understand the context. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reaction wheel will be in a loop with a sensor that detects one or more dynamic properties of the satellite, such as an angle through a Earth or star sensor or a rate from a gyroscope. There are also likely to be some &quot;external&quot; actuators in a related control loop, such as thrusters or magnetorquers.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you pointed out if a disturbance torque is continually applied then the loop will command the reaction wheel to run faster until its speed saturates. There is a choice, long before this point is reached, to prevent saturation by off-loading the momentum stored in the wheels via one of the external actuators.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of an Earth pointing mission some external torques are periodic over the satellite orbit, some not. Depending upon the application, the wheels and control-loop may be sized so as to take up and give back the periodic disturbance in time with its periodicity (i.e. net speed change over a cycle = 0) and to only go to the bother of off-loading with a thruster where the buildup is continual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To address your point about friction, there is some and it occurs between the satellite structure and the wheel. Putting a little energy into keeping the wheel speed constant has just that affect, by definition using &quot;the reaction wheels even more...&quot; just keeps them at their commanded speed.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-12-02T01:04:07.493" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12846" PostTypeId="2" ParentId="12844" CreationDate="2015-12-02T01:59:55.190" Score="10" Body="&lt;p&gt;I am just an enthusiast but I did work for a time as an intern at an aerospace company and had some exposure to satellite designs, mostly from the attitude control system programming viewpoint.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That particular satellite had reaction wheels, &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetorquer&quot; rel=&quot;nofollow&quot;&gt;magnetic bars&lt;/a&gt; and thrusters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnets were very useful to press against the Earth's magnetic field and allow the reaction wheels to slow down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thrusters were used for extreme situations when the rotation rate is very high. That could be a bad separation from the bus or a micrometeor/debris strike or even a bearing freeze on a wheel. This was aerospace engineering: &lt;em&gt;every possible&lt;/em&gt; failure was on a checklist. At any rate, thrusters were the last possible choice since every use was a big hit to mission time. There's nowhere to get more fuel out there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the spacecraft and its orientation requirements, reaction wheels can be used together to slow down and spread the rotation rate of the wheels. The spacecraft can spin around so that the wheel can begin spinning the other direction. Also, wheels have gyroscopic effects which have to be managed, and these can also be used to counteract spin of another wheel.&lt;/p&gt;&#xA;" OwnerUserId="12485" LastEditorUserId="10625" LastEditDate="2015-12-02T20:16:55.067" LastActivityDate="2015-12-02T20:16:55.067" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12847" PostTypeId="2" ParentId="12844" CreationDate="2015-12-02T05:57:55.490" Score="5" Body="&lt;p&gt;Reaction wheels can store angular momentum up to a certain limit. If you Fourier analyze the torques on the satellite, some are constant and some are sinusoidal.  The reaction wheels can store the sinusoidal part and deliver it back when the torque is the other direction.  They don't help (in the long run) for the constant torques.  For that, you need either thrusters (which use fuel) or magnetic torquers (which react against the earth's field).  Both of these can deliver torque to the satellite that does not need to be stored.  You can generally assume the friction on the wheels is negligible-you overcome that with a small amount of electrical power.  Generally you size the reaction wheels to cover the worst case sinusoidal torque, then deal with the secular torque some other way. If you use thrusters, this is one entry in the propellant budget, which can be the limit to the lifetime of the satellite.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2015-12-02T05:57:55.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12848" PostTypeId="1" CreationDate="2015-12-02T07:32:41.290" Score="6" ViewCount="1537" Body="&lt;p&gt;It would seem to me that a &lt;a href=&quot;https://en.wikipedia.org/wiki/Phased_array&quot;&gt;phased array antenna&lt;/a&gt; would be great for spacecrafts since it removes the need to turn the entire spacecraft around in order to point the radio dish towards Earth. Many spacecrafts already have to turn to point their instruments, solar panels and radiation shield in the right direction, so getting rid of antenna pointing should be helpful for both spacecraft and operations design. An interplanetary probe like New Horizons could've been able to send data while making observations and more importantly always been able to receive a last minute command. Station keeping costs could be lowered for orbiting satellites, including the ISS. So why do spacecrafts still carry a fixed dish instead of a phased array?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I note that the 285 kg and 90 Watt &lt;a href=&quot;https://en.wikipedia.org/wiki/BepiColombo#Mercury_Magnetospheric_Orbiter_.28MMO.29&quot;&gt;Mercury Magnetospheric Orbiter&lt;/a&gt;, a planned JAXA Mercury orbiter, will have a phased array for communicating with Earth at 160 Gbit/year, and Mercury is about 90 to 210 million km from Earth. So cost, mass, size, power requirements, data rate and distances don't seem to be obstacles for phased arrays in space. So what is the problem with them? Or are they maybe already taking over?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-12-02T07:38:20.097" LastActivityDate="2015-12-02T22:41:00.893" Title="How are phased array antennas used in spaceflight? Benefits and limitations" Tags="|spacecraft|radio-communication|antenna|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12849" PostTypeId="1" AcceptedAnswerId="12850" CreationDate="2015-12-02T16:47:27.393" Score="16" ViewCount="4867" Body="&lt;p&gt;In this &lt;a href=&quot;https://youtu.be/WotVw5FDVHY&quot;&gt;SpaceX promotional video&lt;/a&gt;, they indicate their plans to land a crewed capsule without a parachute, using landing thrusters. I know that Soyuz does this too, though in combination with a landing chute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why, in either case, but especially the no-chute-at-all configuration, is it desirable to replace a (apparently) hyper-reliable and inexpensive chute system with an (apparently) expensive and complex landing thruster system? What's the advantage?&lt;/p&gt;&#xA;" OwnerUserId="5965" LastEditorUserId="12038" LastEditDate="2015-12-02T21:12:02.983" LastActivityDate="2015-12-04T06:41:48.243" Title="Why is it even desirable for a spacecraft to land without a parachute?" Tags="|spacex|dragon-v2|parachute|landing-gear|" AnswerCount="5" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12850" PostTypeId="2" ParentId="12849" CreationDate="2015-12-02T17:21:25.047" Score="16" Body="&lt;p&gt;Note that this answer refers to the final flight version of the Dragon V2. The first powered landings will be using parachutes before the system is validated for powered-only landing (see &lt;a href=&quot;http://www.nasaspaceflight.com/2014/08/dragon-v2-rely-parachutes-landing/&quot;&gt;this article&lt;/a&gt;, thanks to Mark Adler for pointing this out).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parachutes are &lt;em&gt;passive&lt;/em&gt; devices that slow the vehicle but do not steer it (some parachutes can steer a bit, others more, but not the kind used for spacecraft entry, descent, and landing). So if you want to precisely land the vehicle at a designated target then you need an active system like what you are describing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could be possible to add parachutes to assist with the descent but the added mass and complexity is likely not worth it since the vehicle itself is already outfitted with a propulsion system that can be leveraged for descent and landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also keep in mind that a parachute landing is not the softest, whereas a powered landing allows you to make a much cleaner touchdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would seem that it was a design decision to build the system so that it reliably used a powered landing alone, that way the same vehicle could possibly be used for other missions (such as to Mars or the Moon).&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-12-03T14:35:48.223" LastActivityDate="2015-12-03T14:35:48.223" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12851" PostTypeId="2" ParentId="12844" CreationDate="2015-12-02T18:14:05.177" Score="1" Body="&lt;p&gt;Reaction wheels are fully capable of stabilizing the spacecraft from small perturbations such as gravity gradient torque, atmospheric drag and solar radiation pressure, however coming to your initial scenario after the orbital insertion, it is quite difficult to use reaction wheels to stabilize. For that we use passive control such as yo-yo despinning or libration damper. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reaction wheels having a zero initial angular momentum will take relatively more time to destabilize that, thus have a less preference. An alternate to them would be to use Inertia wheels that have some initial angular momentum which can be pre-adjusted before launch to have maximum effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Let's talk about the case of no-friction&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You had a small disturbance which activated the reaction wheels, once the satellite is stabilized and the wheels are turned off they will keep rotating infinitely, but there is a limit to the maximum angular momentum that can be reached after which they are no longer effective thus not giving any output torque and this would require some momentum dumping mechanism to operate. This is where thrusts come into picture. Once they are fired in the opposite direction, the wheels gets desaturated and are now back in action, but since they are still rotating in the same direction, this will cause the satellite to rotate along with it, which to the system is fed as a perturbation in the opposite direction as before thus sending the signal to the controller to rotate the wheels in the opposite direction. Thus a satellite will never be at zero position but will move about an equilibrium point with a prescribed amplitude and frequency that is predetermined considering the case you mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In your scenario after the momentum dumping if there is another perturbation in the same direction as before that would activate the wheels again but the system is aware of its current angular velocity and the velocity needed to counter that perturbation so it will not necessarily accelerate the wheels, rather it might slow them down instead, because there is also a limit to the maximum ang. vel. that can be achieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an alternate approach to this which involves configuring the wheels not along the axes but like a pyramid with 4RWs such that each wheel corresponds to some contribution in control, thus further reducing the harmonic motion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a control engineer it's your job to minimize this harmonic motion. And thrusters are the worst ones requiring a robust non-linear controller. &lt;/p&gt;&#xA;" OwnerUserId="10558" LastActivityDate="2015-12-02T18:14:05.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12852" PostTypeId="1" AcceptedAnswerId="12855" CreationDate="2015-12-02T19:32:53.850" Score="29" ViewCount="4228" Body="&lt;p&gt;When/where did the cosmonauts fight wolves? or was it bears? With a gun? Could someone provide a more complete story?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of comments on the &lt;a href=&quot;http://jalopnik.com/5883458/how-to-fly-a-soyuz-space-capsule&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; TidalWave linked in the &lt;a href=&quot;https://space.stackexchange.com/questions/12840/how-do-the-soyuz-flight-control-sticks-operate&quot;&gt;Soyuz joysticks&lt;/a&gt; question had links to a pair of articles &lt;sup&gt;&lt;a href=&quot;http://www.jamesoberg.com/russiangun_tec.html&quot; rel=&quot;noreferrer&quot;&gt;1&lt;/a&gt; &lt;a href=&quot;http://gunsite.narod.ru/tp82.htm&quot; rel=&quot;noreferrer&quot;&gt;2&lt;/a&gt;&lt;/sup&gt; on a gun that is a standard issue in the survival pack in all Soyuz capsules. The second article mentions the only non-training instance when the gun was used:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Every Soyuz spacecraft carries such a gun, although none of these guns have ever been unpacked in flight. And they have never been needed, with the exception of an incident in 1965, when bears (or wolves—the story varies) chased two far-off-course cosmonauts. The guns are often presented to crew members as postflight souvenirs. Although several survival kit bags have shown up at space auctions, I’ve never seen any of the guns for sale. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hK6Hr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hK6Hr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I tried to find more on that topic, but...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first mission with a crew, Soyuz 1, launched on 23 April 1967 &lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So either it was not Soyuz or the date is off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could someone provide the full story?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-09-18T06:58:37.150" Title="When/where did the cosmonauts fight wolves?" Tags="|history|astronauts|russia|recovery|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12854" PostTypeId="2" ParentId="12849" CreationDate="2015-12-02T20:08:06.450" Score="23" Body="&lt;p&gt;If you're just looking at it as a mass trade, then yes, you will definitely get a mass savings by replacing a portion of your propellant with a bag full of nylon. Depending on your speed, the nylon can have a much higher &quot;Isp&quot; than the rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However at the system level, there are other things to consider besides mass. First, parachutes are not &quot;hyper-reliable&quot;. You need to consider the consequences of parachute failure, which is why you usually see three of them in a cluster on crewed vehicles. (Soyuz is an exception with one large parachute, though there is a smaller backup parachute you don't see. The single Soyuz 1 cosmonaut died on impact due to both parachutes failing.) Second, if you're doing a propulsive landing for the last bit, then you may need to get rid of the parachute and also make sure that it doesn't then interfere with the landing. Third, you wouldn't reuse these kinds of parachutes, so while they are relatively inexpensive for space systems, it does cost more to replace the parachutes than to refill the tanks, and they take a fair amount of integration time due to the care needed in making sure that the rigging will deploy as intended. Filling tanks, even with hypergols, can go pretty quick. As noted in another answer, you are more at the mercy of the winds while on a parachute, so if you are targeting a very small landing location (e.g. a pad), then you will have more correction to do once you go propulsive, and will need more altitude in which to do that. Or you could have a steerable parachute, which has its own efficiency and reliability issues, and you would still be correcting at the end with the rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even with all that, if you really need the down-mass capability, you may find yourself working to solve all those problems in order to incorporate a parachute system and reduce mass. But if you're in a happy place for mass, then you would want to avoid the complexity, cost, and reliability issues of parachutes.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-12-02T20:08:06.450" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12855" PostTypeId="2" ParentId="12852" CreationDate="2015-12-02T20:15:05.993" Score="37" Body="&lt;p&gt;The first manned Soviet flight was Vostok 1 in 1961, and the first Soviet flight with a multiperson crew was Voskhod 1 in 1964. The Wikipedia article that you linked intends to say that Soyuz 1 was the first &lt;em&gt;Soyuz&lt;/em&gt; mission with a crew.&lt;/p&gt;&#xA;&lt;p&gt;The anecdote about the wolves and bears comes from Voskhod 2 on March 19, 1965:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For the Soviet crew of Voshod 2 &lt;em&gt;[sic]&lt;/em&gt;, their landing back on Earth in an isolated, very snowy forest marked a harrowing start to a new mission: survival. Getting through the ordeal would end up requiring a gun to ward off wild bears, some tricks to staying warm in below zero temperatures and cross country skiing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Voskhod 2 landed off target in the Siberian forest, and according to the account it was in the middle of mating season for the bears and wolves which lived there. Moscow had no idea where they landed for some time, so the two cosmonauts had to survive in the forest until they were retrieved by a rescue team.&lt;/p&gt;&#xA;&lt;p&gt;You can read the full story in the &lt;a href=&quot;http://news.discovery.com/space/history-of-space/voskhod-2s-perilous-landing-130321.htm&quot; rel=&quot;noreferrer&quot;&gt;Discovery article&lt;/a&gt; I snagged this from. Wikipedia says the landing site was 59°34′N 55°28′E.&lt;/p&gt;&#xA;&lt;p&gt;The first-hand account captured in &lt;a href=&quot;http://www.airspacemag.com/space/the-nightmare-of-voskhod-2-8655378/?no-ist=&amp;amp;c=y%3Fno-ist&amp;amp;page=3&quot; rel=&quot;noreferrer&quot;&gt;Air and Space&lt;/a&gt; neglects to mention any use of the gun:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;We were only too aware that the taiga where we had landed was the habitat of bears and wolves. It was spring, the mating season, when both animals are at their most aggressive. We had only one pistol aboard our spacecraft, but we had plenty of ammunition. As the sky darkened, the trees started cracking with the drop in temperature—a sound I was so familiar with from my childhood—and the wind began to howl.&lt;/p&gt;&#xA;&lt;p&gt;Even though mission control had no idea where we were or whether we had survived, our families were informed that we had landed safely and were resting in a secluded dacha before returning to Moscow.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That seems like an extraordinary event to leave out (and if the gun had been fired, this would have been in a report), so it is most likely they just took comfort in the fact that they had a gun.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;In response to some questions about radio -- the craft was equipped with a radio beacon, and though Moscow did not receive the signal others did:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;We had no idea if our rescue signal had been received. It turned out later that Moscow had not received it, but it had been picked up by listening posts as far away as Bonn, Germany. More importantly, a cargo plane flying close to our landing site had also picked it up. A search party had been dispatched, and late in the afternoon, we picked up the sound of a helicopter approaching.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-18T06:58:37.150" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12857" PostTypeId="2" ParentId="12848" CreationDate="2015-12-02T21:25:53.577" Score="7" Body="&lt;p&gt;To address the last part of your question &quot;.. or are they already taking over&quot; phased arrays have been used in commercial communications and remote sensing satellites for a while already though I don't think they are &quot;taking over&quot; as such. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These examples aren't the interplanetary use you have suggested but they do serve to demonstrate that the technology is well established for satellite use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inmarsat 4 uses a phase array for its main &lt;a href=&quot;http://www.microwavejournal.com/legacy_assets/FigureImg/9053_f13.gif&quot;&gt;L band receive and transmit&lt;/a&gt; antenna: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/KhlFG.gif&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KhlFG.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Synthetic Aperture Radar is closely related to a phased array. The Sentinel 1 satellite uses a phased array as the basis for its &lt;a href=&quot;https://sentinel.esa.int/c/wiki/get_page_attachment?p_l_id=250898&amp;amp;nodeId=250994&amp;amp;title=Instrument&amp;amp;fileName=Instrument%2FSAR_instrument.png&quot;&gt;C-band SAR&lt;/a&gt; payload:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Xq1j9.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xq1j9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the costs of the solution have been higher in the past, perhaps improving now with wider use and that this has probably driven the trade off for science missions to date.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-12-02T21:25:53.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12858" PostTypeId="2" ParentId="12848" CreationDate="2015-12-02T22:41:00.893" Score="7" Body="&lt;p&gt;Phased arrays have mathematical limits, so you may still need to have gimbals or orient the array.  In particular they suffer from decreased gain off-boresight.  The more elements to the antenna, the tighter the beam, but the worse the off-boresight issues.  NASA has &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100020864.pdf&quot;&gt;studied&lt;/a&gt; these issues, and there are solutions (such as having multiple arrays).  In the example from that paper, they needed 8 separate antennae, each a phased array.  That's a lot of mass and power, though it certainly has its advantages!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real power of phased arrays is their ability to be steered electronically.  The best use of this is in situations where one needs to scan an area quickly, but those situations rarely arise in spacecraft communications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several satellites which communicate with a phased array, but from what I have been able to tell, they're usually either there because they needed the array for another purpose (such as SAR imaging), or because they needed to rapidly switch between multiple communication channels with different craft without involving a physical gimbal movement.&lt;/p&gt;&#xA;" OwnerUserId="3532" LastActivityDate="2015-12-02T22:41:00.893" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12859" PostTypeId="1" AcceptedAnswerId="12861" CreationDate="2015-12-02T23:18:54.453" Score="14" ViewCount="2984" Body="&lt;p&gt;What dry mass could have soft-landed on the Moon if a Saturn V had been reconfigured to launch a robotic mission going entirely to the surface? Obviously, that wasn't possible at the time, but for a return to the Moon it might make a lot of sense. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though there are no more Saturn Vs, if SpaceX produces their Mars Colonial Transporter design, &lt;a href=&quot;http://www.universetoday.com/108966/mars-space-colony-rockets-could-be-ready-in-10-years-spacex-ceo/&quot;&gt;Elon says&lt;/a&gt; it'll &lt;em&gt;'make the Apollo moon rocket look small'&lt;/em&gt;. So to mock-up &lt;a href=&quot;http://moonwards.com/index.html&quot;&gt;large-scale missions&lt;/a&gt; to the Moon based on robotics, I'd like to base the payload on what a Saturn V could have done if it had been reconfigured with that in mind. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's what there is to work with:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1969-059C&quot;&gt;Lunar Modules&lt;/a&gt; - Ascent - 4800 kg (can add all that, nothing is ascending)&lt;/li&gt;&#xA;&lt;li&gt;Descent Module - 2000 kg dry mass, 8212 kg propellant,&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Command_Module_.28CM.29&quot;&gt;Command and Service Module&lt;/a&gt; - 11,900 kg dry mass, 16,900 kg fuel&#xA;(these masses need to be combined to calculate what mass could be landed if there was only one engine system)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.apollosaturn.com/asnr/escape.htm&quot;&gt;Launch Escape System&lt;/a&gt; - 3600 kg (discarded soon after launch, so some portion of that can be added to payload to the Moon's surface)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So there are a few wrinkles here. What kind of mass savings could be gotten by combining the engines of the LEM and CSM and resizing for something that would land the resultant configuration on the surface? How much of the launch escape system can be regarded as extra payload mass to the Moon? I haven't found a figure for its mass, but would it be reasonable to tack on the mass of the &lt;a href=&quot;http://airandspace.si.edu/explore-and-learn/multimedia/detail.cfm?id=5205&quot;&gt;Lunar Module Adapter&lt;/a&gt;?  &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;A note - as this is about construction, I'm not really making a distinction between the spacecraft themselves and their payloads. In the context of ongoing missions directed towards settlement, spacecraft are useful materials that can be reused - even leftover fuel. So if it lands, it counts.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="49" LastEditDate="2015-12-03T01:30:08.467" LastActivityDate="2018-09-23T10:12:43.700" Title="How much mass could the Saturn V rockets have landed on the Moon if nothing was coming back?" Tags="|the-moon|lunar-landing|payload|saturn-v|mass-fraction|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12860" PostTypeId="2" ParentId="12849" CreationDate="2015-12-02T23:22:02.927" Score="15" Body="&lt;p&gt;E. Musk has answered this multiple times in interviews. This comes down to the economics of it. They are putting this entire system together to haul cargo, and the reusable portion is there to increase profits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are putting in place a reusable restartable engine in order to have the maximum duplication of function for the components onboard. Previously you had one system, the engine, responsible for going up, and another system, parachutes, responsible for coming down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parachutes are not capable of doing job #1. Rocket engines are capable of doing both job #1 and #2. You've just reduced the mass, the failure points, and complexity of your vehicle with a minimum of redesign.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parachutes only slow the vehicle to a certain speed. That speed isn't slow enough to land a ten-story hollow metal tube with breakable components inside on land. And landing in water adds new complications for reuse of the motors.&lt;/p&gt;&#xA;" OwnerUserId="12498" LastEditorUserId="6816" LastEditDate="2015-12-02T23:30:56.240" LastActivityDate="2015-12-02T23:30:56.240" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12861" PostTypeId="2" ParentId="12859" CreationDate="2015-12-02T23:51:14.087" Score="23" Body="&lt;p&gt;According to Wikipedia, &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;noreferrer&quot;&gt;Saturn V could launch 48600 kg&lt;/a&gt; to translunar injection. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From there, you need about &lt;a href=&quot;https://i.stack.imgur.com/piNqI.png&quot; rel=&quot;noreferrer&quot;&gt;2410m/s of ∆v to soft-land on the moon&lt;/a&gt;. Let's take a little additional fuel for safety margin and call it 2700m/s.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;noreferrer&quot;&gt;rocket equation&lt;/a&gt;, assuming you're using a rocket that uses storable hypergolic propellants with a specific impulse of 312s (like the &lt;a href=&quot;http://www.astronautix.com/engines/aj10137.htm&quot; rel=&quot;noreferrer&quot;&gt;AJ-10&lt;/a&gt; used on the Apollo Service Module), you need a mass ratio of 2.42 to provide that amount of ∆v. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus you can soft-land a total of 48600/2.42 = ~20100kg dry mass of ship.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has considered the &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10#Common_Extensible_Cryogenic_Engine&quot; rel=&quot;noreferrer&quot;&gt;CECE&lt;/a&gt;, a deep-throttling evolution of the RL-10 hydrogen-fueled engine, for future lunar landing missions. This would provide much better specific impulse, but the associated fuel tankage would be much larger in volume and boil-off of cryogenic propellants would be a big concern. With a specific impulse of 455s, the landed dry mass would be about 26200kg. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-03T04:14:45.537" LastActivityDate="2015-12-03T04:14:45.537" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12862" PostTypeId="1" CreationDate="2015-12-03T05:27:11.063" Score="9" ViewCount="1201" Body="&lt;p&gt;Where can I find the actual radiation pattern and signal strength of transmitting antennas used by GNSS navigation satellites/constellations (GPS, GLONASS, IRNSS, etc)?&lt;/p&gt;&#xA;" OwnerUserId="8501" LastEditorUserId="49" LastEditDate="2015-12-03T05:56:54.297" LastActivityDate="2015-12-03T06:22:05.537" Title="Radiation pattern and signal strength of navigation satellite antennas" Tags="|navigation|reference-request|communication-satellite|gnss|antenna|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12863" PostTypeId="2" ParentId="12862" CreationDate="2015-12-03T06:22:05.537" Score="9" Body="&lt;p&gt;Exact &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pattern&quot; rel=&quot;noreferrer&quot;&gt;radiation patterns&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Antenna_gain&quot; rel=&quot;noreferrer&quot;&gt;gain&lt;/a&gt; (you're probably not asking about &lt;a href=&quot;https://en.wikipedia.org/wiki/Signal_strength&quot; rel=&quot;noreferrer&quot;&gt;signal strength&lt;/a&gt; since that depends on distance which isn't constant) will vary across all the different &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_navigation&quot; rel=&quot;noreferrer&quot;&gt;GNSS&lt;/a&gt; contellations, even individual generations / blocks of same systems, but for a fairly detailed analysis of GPS blocks, you can refer to e.g. &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20150011650&quot; rel=&quot;noreferrer&quot;&gt;GPS Space Service Volume: Ensuring Consistent Utility Across GPS Design Builds for Space Users&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, side lobe gain patterns could look like this (for GPS Block IIF and Block IIR-M):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/7cqsB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7cqsB.jpg&quot; alt=&quot;GPS Block IIF and Block IIR-M side lobe gain patterns&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And hemispherical gain patterns could look like this (For GPS Block IIR):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/EAqbX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EAqbX.png&quot; alt=&quot;GPS constellation hemispherical gain patterns&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find more relevant information e.g. by searching the &lt;a href=&quot;http://ntrs.nasa.gov&quot; rel=&quot;noreferrer&quot;&gt;NASA Technical Report Server&lt;/a&gt; (NTRS).&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-12-03T06:22:05.537" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12864" PostTypeId="2" ParentId="12849" CreationDate="2015-12-03T17:44:34.647" Score="6" Body="&lt;p&gt;One thing that hasn't been mentioned thus far - the powered landing engines also serve as the launch abort system. Without them, they'd need something like &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_(spacecraft)#Launch_Escape_System_.28LES.29&quot;&gt;the Apollo Launch Escape System&lt;/a&gt; to carry the crewed spacecraft away from an exploding launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck landing a Dragon with parachutes on the Moon, as well. :-)&lt;/p&gt;&#xA;" OwnerUserId="5793" LastActivityDate="2015-12-03T17:44:34.647" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12865" PostTypeId="2" ParentId="10798" CreationDate="2015-12-03T20:44:21.660" Score="6" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;Ultimately, I'm trying to figure out if NASA's decrease in funds is responsible for fewer missions...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The decrease in launches and/or planned missions is due to budget cuts.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;...what the funding required to keep those missions alive ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The operational cost of maintaining contact with and retrieving data from any given spacecraft depends upon the age of the spacecraft and distance of the spacecraft from Earth.  The former cost difference results from differences in designs/software/specs/etc. than the newer spacecraft (things change with time), which requires different personnel.  Sometimes this can decrease the cost if things are simple and well understood by a younger, less senior engineer.  Other times it can increase the cost if the only people that still understand such things happen to be very senior level engineers.  It really depends upon the team and the mission.&lt;/p&gt;&#xA;&lt;p&gt;The latter arises from the costs associated with maintaining and using either the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; or the &lt;a href=&quot;https://en.wikipedia.org/wiki/Near_Earth_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Near Earth Network&lt;/a&gt;.  Using the &lt;a href=&quot;http://deepspace.jpl.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSN&lt;/a&gt; is expensive for several reasons that involve the use and maintaince of large area &lt;a href=&quot;https://en.wikipedia.org/wiki/Beam_waveguide_antenna&quot; rel=&quot;nofollow noreferrer&quot;&gt;antennas&lt;/a&gt;, power, security, etc.&lt;/p&gt;&#xA;&lt;p&gt;In general, operations costs are typically more expensive for spacecraft located farther from Earth than nearer (not always true, but a decent hand-wavy guideline).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;...or if it's that the current missions cost more...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Yes, newer missions can cost more than older missions, but this is not always true either.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Changes in Requirements&lt;/strong&gt;&lt;br /&gt;&#xA;Much of the reason for the increased cost is due to the increase risk reduction requirements.  Meaning, newer missions are often held to a very high standard where failure is effectively not an option.  The result is an increase in redundancy, testing, and quality of parts.  Another significant fraction of the increased cost came from the increased price of launch vehicles bought from the &lt;a href=&quot;https://en.wikipedia.org/wiki/United_Launch_Alliance&quot; rel=&quot;nofollow noreferrer&quot;&gt;United Launch Alliance (ULA)&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Early Space Age Advantages&lt;/strong&gt;&lt;br /&gt;&#xA;In the early days of the space age, missions were often approved based upon ~3-5 page proposals and they were designed, tested, and constructed by small teams (often a dozen or less university faculty and researchers).  The missions had a lot of risk (i.e., little-to-no redundancies and/or the instruments being flown had no previous flight experience in space), were typically smaller thus requiring a smaller (i.e., cheaper) launch vehicle, and many missions flew fewer instruments.  Most of the earlier missions were just much more simple in design and scope.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Changes in Opportunities&lt;/strong&gt;&lt;br /&gt;&#xA;There has also been a significant decrease in the number of &lt;a href=&quot;https://en.wikipedia.org/wiki/Small_Explorer_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;small explorer program (SMEX)&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Explorers_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;explorer program&lt;/a&gt; mission opportunities.  Many of the recent (i.e., last ~10 years) announcements have been for the much larger and more complicated &lt;a href=&quot;https://en.wikipedia.org/wiki/Flagship_Program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flagship Program&lt;/a&gt; missions.  These are more expensive for multiple reasons and the larger price results in fewer SMEX opportunities because there just isn't enough funding.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Summary&lt;/strong&gt;&lt;br /&gt;&#xA;There are a semi-infinite number of reasons for why newer missions can be more expensive, but budget cuts often affect different parts of NASA differently.  Sometimes budget cuts are specifically directed at one program (e.g., when congress cut funding for the manned spaceflight programs) while other times it is an across the board cut.&lt;/p&gt;&#xA;&lt;p&gt;Regardless, you can see for yourself that &lt;a href=&quot;https://en.wikipedia.org/wiki/Budget_of_NASA&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's budget&lt;/a&gt; peaked in 1966 and has been steadily declining since the mid 1990's until the most recent fiscal year, where the budget slightly increased.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastEditorUserId="12508" LastEditDate="2021-09-02T14:29:38.663" LastActivityDate="2021-09-02T14:29:38.663" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12866" PostTypeId="1" CreationDate="2015-12-03T21:32:45.683" Score="0" ViewCount="472" Body="&lt;p&gt;There are some resource questions posed by the book and film &lt;em&gt;The Martian&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How much oxygen can you take with you? &lt;/li&gt;&#xA;&lt;li&gt;How much $CO_2$ can be recycled back into oxygen? &lt;/li&gt;&#xA;&lt;li&gt;How much energy would you need for that? &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In the movie, they show some high-tech buildings on Mars &amp;hellip; the lead character spends more than a year bouncing around those, yet his oxygen supply never seems to be a problem. Is this a realistic view of $O_2$ management? Is there a real-life system that could be operated like this?&lt;/p&gt;&#xA;" OwnerUserId="12267" LastEditorUserId="6816" LastEditDate="2015-12-04T18:49:02.287" LastActivityDate="2015-12-04T23:05:21.633" Title="Does an Oxygen regeneration system as depicted in the Martian exist in real-life?" Tags="|mars|oxygen|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12867" PostTypeId="1" CreationDate="2015-12-04T06:01:34.640" Score="2" ViewCount="98" Body="&lt;p&gt;With regards to Project Orion, nuclear pulse engine, could the 0.015 kiloton up to 0.35 kiloton Deuterium/Lithium bombs proposed by Freeman Dyson be constructed? Could they be built to leave a small to insignificant amount of radioactive fallout in space?&lt;/p&gt;&#xA;" OwnerUserId="11671" LastEditorUserId="1142" LastEditDate="2015-12-14T15:54:39.313" LastActivityDate="2015-12-14T15:54:39.313" Title="Small Deuterium bombs" Tags="|nuclear-pulse-propulsion|" AnswerCount="0" CommentCount="4" ClosedDate="2015-12-04T16:43:05.177" ContentLicense="CC BY-SA 3.0" />
  <row Id="12868" PostTypeId="2" ParentId="12849" CreationDate="2015-12-04T06:41:48.243" Score="3" Body="&lt;p&gt;Atmosphere. Parachutes don't work well with thin atmospheres, like Mars', and they don't work at all with no atmosphere like the Moon. The goal is to land on other bodies in our solar system - not just Earth - so we need precision and technology that works regardless of atmosphere so rockets are much more preferential than parachutes for this reason: universal landing technology.&lt;/p&gt;&#xA;" OwnerUserId="1087" LastActivityDate="2015-12-04T06:41:48.243" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12869" PostTypeId="1" CreationDate="2015-12-04T07:34:16.893" Score="13" ViewCount="15535" Body="&lt;p&gt;As I understand it, liquid-fueled rockets typically burn some amount of their fuel in a separate low-pressure combustion chamber, and then use the combustion products produced to drive the turbopump. High-efficiency engines then pump this exhaust into the rocket's combustion chamber to ensure complete combustion and get more efficiency from the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be feasible to instead use some of the high-pressure gas from the rockets main combustion chamber to drive the turbopump, having passed it through a nozzle to reduce the pressure?&lt;/p&gt;&#xA;" OwnerUserId="12512" LastEditorUserId="6816" LastEditDate="2015-12-04T07:41:54.113" LastActivityDate="2016-04-13T23:00:58.240" Title="Why don't rockets drive the turbopump from the combustion chamber?" Tags="|rockets|engine-design|engines|liquid-fuel|combustion|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12870" PostTypeId="2" ParentId="12869" CreationDate="2015-12-04T07:59:22.033" Score="15" Body="&lt;p&gt;The main reason for this is the temperature. Depending on the actual fuel the temperature in the main combustion chamber can be above 3000K. However, in the gas generator the temperature is kept below 1400-1600K. These lower gas temperatures allow uncooled chamber construction and prevent melting or limit&#xA;the erosion of turbine blades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: Sutton - Rocket Propulsion Elements&lt;/p&gt;&#xA;" OwnerUserId="12514" LastEditorUserId="6816" LastEditDate="2015-12-04T08:12:22.697" LastActivityDate="2015-12-04T08:12:22.697" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12871" PostTypeId="2" ParentId="12869" CreationDate="2015-12-04T12:34:00.500" Score="19" Body="&lt;p&gt;You've got it slightly incorrect. Staged combustion engines pre-burn the propellants at a higher, not lower pressure than the main chamber. The exhaust from the preburner isn't pumped into the main chamber but flows through the turbine, dropping in pressure there and in the ducting before it enters the main chamber. The preburners generally run at a lower mixture ratio than the main chamber to keep things cooler for the turbine blades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: I extracted this schematic with example pressure and temperature numbers from &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;noreferrer&quot;&gt;this link&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4ZDiY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4ZDiY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-12-04T15:33:47.713" LastActivityDate="2015-12-04T15:33:47.713" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12872" PostTypeId="1" CreationDate="2015-12-04T13:17:00.150" Score="2" ViewCount="456" Body="&lt;p&gt;When working on a project, it is often useful to predict what will happen in the future, 5 or 10 years from now. Past data is needed in order to extrapolate them and make accurate predictions. How can I manage to get the area density of spatial solar panels from year 1970? This is used to calculate how much it costs to bring a square meter to orbit.&lt;/p&gt;&#xA;" OwnerUserId="10836" LastActivityDate="2015-12-04T14:29:10.553" Title="Spatial solar panels area density through history" Tags="|history|solar-power|cost-to-orbit|" AnswerCount="1" CommentCount="8" ClosedDate="2015-12-11T00:13:29.910" ContentLicense="CC BY-SA 3.0" />
  <row Id="12873" PostTypeId="2" ParentId="12872" CreationDate="2015-12-04T14:21:50.807" Score="4" Body="&lt;p&gt;I did a search using these keywords: solar panel spacecraft &quot;specific power&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://solarsystem.nasa.gov/docs/D-24454%20(2)%20-%20Solar%20Cell%20and%20Array%20Technology%20%2020022.pdf&quot; rel=&quot;nofollow&quot;&gt;a 2002 JPL report on solar array technology&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At the start of the space age, solar arrays produced about 15W/kg and the cells were about 10% efficient.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-12-04T14:29:10.553" LastActivityDate="2015-12-04T14:29:10.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12874" PostTypeId="1" AcceptedAnswerId="13664" CreationDate="2015-12-04T19:46:54.803" Score="9" ViewCount="1437" Body="&lt;p&gt;This question is inspired by an answer to &lt;a href=&quot;https://space.stackexchange.com/questions/12848/how-are-phased-array-antennas-used-in-spaceflight-benefits-and-limitations&quot;&gt;this&lt;/a&gt; question, where it is stated that: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The real power of phased arrays is their ability to be steered&#xA;  electronically.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This set me wondering about the antenna on &lt;a href=&quot;http://www.space-airbusds.com/media/image/inmarsat-2-f1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; satellite, Inmarsat 2 F1&#xA;&lt;a href=&quot;https://i.stack.imgur.com/oh2i1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oh2i1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have been looking online for any description that recognises the antenna as a phased array without luck. However, just looking at the antenna it seems if there is no controlled phase relationship between the horns then at L-band their beams will be so large that most of the power from the antenna will miss the Earth. It appears to me that the horns must all be phase linked so that the effective antenna aperture is the size of the whole satellte Earth face and thus the beam will be much narrower. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Does this count as a phased array or must the beam be electrically steerable once in orbit as well? Was it considered to be a phased array when it was launched back in 1990?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm aware some people may think this question is off topic, though I thought I'd start here as I'd be more likely to find someone with specific knowledge of the Inmarsat 2 satellites here.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-01-24T03:40:12.893" Title="When is a phased array antenna not a phased array?" Tags="|spacecraft|radio-communication|antenna|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12875" PostTypeId="2" ParentId="12866" CreationDate="2015-12-04T21:04:57.890" Score="8" Body="&lt;p&gt;Okay, first things first: You can take as much oxygen as you're willing to pay for. One liter of LO2 at 1.41kg converts to 810 liters of oxygen at 1 bar, so space isn't that much of a problem; the weight is - but you only need it for the trip, because you can manufacture as much as you desire on Mars from the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that means question 2 only matters during the trip, and currently the numbers for that aren't too good: about 30% if you have the CO2 separated from air already. But again, that's for the trip. On Mars we have all the CO2 we'd ever wish for, so we can be as wasteful as we wish, providing we keep our energy demands in check.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now for question 3, where things are getting interesting.&#xA;The &lt;a href=&quot;http://adsabs.harvard.edu/full/1995LPI....26...49A&quot; rel=&quot;noreferrer&quot;&gt;paper&lt;/a&gt; on electrolysis of CO2 gives us some numbers to work with:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With 2V of power supply, drawing 0.3A of current, processing 10cm^3 of CO2 at atmospheric pressure, heated to 950C, per minute, they were able to obtain about 1.5cm^3 of pure oxygen per minute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0.3A * 2V is 0.6 Watt. That's energy demand for 1.5cc/min of O2 from 10cc of CO2 at 1 bar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Human needs &lt;a href=&quot;http://health.howstuffworks.com/human-body/systems/respiratory/question98.htm&quot; rel=&quot;noreferrer&quot;&gt;550 liters&lt;/a&gt; of pure oxygen per 24h. That's 380cc/minute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To obtain 380cc of oxygen per minute we must scale up both power consumption and CO2 intake. 152 Watt, and 2533cc of CO2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;152W looks very promising. The electrolysis really doesn't cost us all that much. There are two problems left though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the CO2 on Mars is not at 1 bar, but at 0.006 bar. And then, its temperature rarely rises above 0C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we're scaling our intake up again. 2533cc/0.006 is 0.42 m^3. That volume of martian atmosphere must be processed each minute - but that's still a pretty reasonable intake. But we need to pressurize it to 1 bar, not only to improve electrolysis efficiency but primarily because we must pump that oxygen into the habitat!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using some figures from &lt;a href=&quot;https://en.wikipedia.org/wiki/Compressed_air_energy_storage&quot; rel=&quot;noreferrer&quot;&gt;Compressed air energy storage&lt;/a&gt; Wikipedia page, I found compressing 1m^3 of gas per minute, by 1 bar (from ~0, more precisely 0.006, to 1 bar) takes about 7.16KW. If we match it with our required 0.43m^3, we're obtaining almost precisely 3KW. Add another 1KW of losses on pumping efficiency (1/3 is a reasonable estimate), for 4KW and here's our primary energy cost, totally dwarfing the 152W of electrolysis. The good news though is that most of that energy will appear as heat in the newly compressed CO2. I don't know how much you will heat 2.5 liter of CO2 at atmospheric pressure if you apply 4KW for a minute to it, but with lousy heat capacity of gasses, I can only assume that's a lot. Somebody correct me if I'm wrong here, but our 950C temperature problem seems to be solved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That way, in a little over 4 kilowatt, we have oxygen for one human indefinitely. It's not cheap but it's definitely doable with solar panels and we're getting a lot of free heat for habitat heating, as the discarded heated CO2+CO mix can be used to heat up the hab before being depressurized and released back into the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible we could do much better by working at 0.006bar with the electrolysis and pressurizing only the produced O2, but I didn't find any studies of efficiency of that process in such low pressure, so what I presented is what I know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, for the last question:  Is this a realistic view of O2 management? Yes, it is. The produced oxygen can be liquified for storage, transport and use on the return trip or as oxidizer for launch. It's just a matter of enough energy input - the device in the article worked for 253 hours and didn't show any signs of wear - it seems that the pump might be more vulnerable than the electrolytic separator, but compressing gas by 1 bar, at less than 1m^3 per minute is not really such a heavy task for a compressor; building one that can last years is an engineering challenge, but definitely not an impossible one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a real-life system that could be operated like this? - yes, with 1.5cc/minute oxygen output. Definitely not sufficient for Mars survival, but definitely possible for upscaling. Since &lt;em&gt;The Martian&lt;/em&gt; takes place in 2035, we can safely assume reliable, efficient systems of required throughput will be made by then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: When I said a lot, I still underestimated the actual heat. The machinery will definitely require some very serious cooling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our CO2 will get very hot; its specific heat changes with temperature but at some 7000K it's around 1.5 kJ/kg K. At 1.98 kg/m^3 @ 1 bar, 2.5l of CO2 is about 5 grams. 7.5J/K for our minute dose. 4KW for 1 minute is 240 kJ. 240,000/7.5... 32,000 Kelvins. So, we're about 31,000 Kelvins ahead of the target. An extra cooling system is a must.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2015-12-04T23:05:21.633" LastActivityDate="2015-12-04T23:05:21.633" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="12876" PostTypeId="1" CreationDate="2015-12-04T21:38:56.577" Score="1" ViewCount="136" Body="&lt;p&gt;I was wondering if it's possible, when we have a satellite in L1, or going to the L1 direction from the earth, to use a gravity assistance with the moon in order to go in L2.&#xA;I read that it's possible for a satellite going from the earth to the L1 point to be redirected to the L2 point with a cost of more or less 750m/s. If the gravity assist using the moon is feasible, do you think this cost is anyway the speed to add with thruster?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for your answer, and sorry if this question has already be asked, I read a lot of topic but I can't find the answer. The L1 and L2 point are the point of the earth-sun problem.&lt;/p&gt;&#xA;" OwnerUserId="12518" LastEditorUserId="12518" LastEditDate="2015-12-05T00:30:13.840" LastActivityDate="2015-12-05T00:30:13.840" Title="from L1 to L2 (earth-sun system) using gravity assistance with the moon" Tags="|orbit|lagrangian-points|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12877" PostTypeId="2" ParentId="12876" CreationDate="2015-12-04T23:05:07.820" Score="1" Body="&lt;p&gt;The &lt;a href=&quot;http://wind.nasa.gov&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;Wind&lt;/em&gt;&lt;/a&gt; spacecraft did just this back in 2002-2004 over a series of several orbits.  It used the moon to change its angular momentum, but it also used its &lt;a href=&quot;https://en.wikipedia.org/wiki/Vernier_thruster&quot; rel=&quot;nofollow&quot;&gt;thrusters&lt;/a&gt; during the maneuver.  So technically, it was not entirely a moon-assisted launch into the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#L2&quot; rel=&quot;nofollow&quot;&gt;$L_{2}$&lt;/a&gt; point.  Several spacecraft, including &lt;em&gt;Wind&lt;/em&gt;, have used the moon to help them get to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#L1&quot; rel=&quot;nofollow&quot;&gt;$L_{1}$&lt;/a&gt; point as well.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2015-12-04T23:05:07.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12878" PostTypeId="1" AcceptedAnswerId="12884" CreationDate="2015-12-04T23:31:45.887" Score="7" ViewCount="603" Body="&lt;p&gt;The Lagrangian points are points of unstable balance (at least gravitationally; L4 and L5 are stable thanks to Coriolis force.), and that means an object not stabilized actively will fall out of them and start accelerating down the steepest gradient of whichever gravity well it toppled into.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But with the gravity wells being as irregular as they are...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J8hJs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J8hJs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the object gaining momentum, never mind neither of the two bodies remaining at rest, the trajectory becomes less intuitive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, assuming a minimal nudge, just a smallest loss of stability, where would the object end up starting at different Lagrangian points? Which nudges out of L-points make the object crash into Earth or the Moon, which will let it escape the 2-body system, and will any set it on a mostly cyclic orbit? (possibly unstable due to tidal forces, but still one that will take some time to decay.)&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-12-05T21:21:52.123" Title="Where will objects end up, after losing stability at Lagrangian points?" Tags="|orbital-mechanics|lagrangian-points|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12879" PostTypeId="1" CreationDate="2015-12-05T07:33:57.910" Score="7" ViewCount="1007" Body="&lt;p&gt;With the current infrastructure available to SETI, how distant would it be feasible to pick up routine radio communications and noise from a planet emitting radio with identical power and signal characteristics to that of present-day Earth?&lt;/p&gt;&#xA;" OwnerUserId="11095" LastEditorUserId="1142" LastEditDate="2016-08-10T23:42:47.463" LastActivityDate="2016-08-10T23:42:47.463" Title="How far can SETI's ear listen?" Tags="|data-transmission|seti|radio-telescope|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12880" PostTypeId="1" AcceptedAnswerId="12881" CreationDate="2015-12-05T07:37:53.363" Score="4" ViewCount="227" Body="&lt;p&gt;A lot of material about extraterrestrial life seems to suggest that life essentially cannot survive in environments with more radiation beyond what's on Earth. For example, when red dwarfs planets are discussed, the fact they seem to have more flares and radioactive output and the planets are closer, therefore it's assumed by some life is unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen this as the case too with life coming near the surface of Europa, and of course being on the surface of Mars (assume any was actually there).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that a few things are often overlooked or ignored:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Maybe the super-earths around these red dwarfs (or any star really) could have a larger atmosphere with more substances like ozone, and more powerful magnet fields?&lt;/li&gt;&#xA;&lt;li&gt;Even on Earth we have bacteria which can survive extreme radioactivity, and some that even thrive on it, couldn't something like this exist elsewhere?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Are these things considered when thinking of the potential in the same way it often seems to me that (up until recently) atmospheric thickness and planet size wasn't even considered to be a factor in placement in the goldilocks zone? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a known absolute maximum level of UV or other radioactivity that all Earth-DNA can stand, and are these logical to assumed the same elsewhere?&lt;/p&gt;&#xA;" OwnerUserId="3443" LastEditorUserId="4660" LastEditDate="2015-12-05T15:42:47.963" LastActivityDate="2015-12-05T15:42:47.963" Title="What is the potential limit extraterrestrial life can survive and under what factors?" Tags="|astrobiology|exoplanet|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12881" PostTypeId="2" ParentId="12880" CreationDate="2015-12-05T11:06:44.383" Score="4" Body="&lt;p&gt;The Goldilocks Zone is indeed a rough approximation of the region in which a planet may be habitable. It essentially evaluates the stellar energy output in the context of a planet like Earth -- how close or far could you move the Earth within that system in order to still maintain habitable temperatures. The Planetary Habitability Laboratory at the University of Puerto Rico (at Arecibo) has a great graphic to put some visuals (I know this is tangential to your question, I'm getting there...):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UeNYd.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UeNYd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you go to &lt;a href=&quot;http://phl.upr.edu/projects/habitable-exoplanets-catalog&quot; rel=&quot;nofollow noreferrer&quot;&gt;the webpage from where I sourced this image&lt;/a&gt; you will see a chart listing various habitable planet candidates. It is listed according to the &quot;Earth Similarity Index&quot;, which accounts for stellar flux as well as planet mass and radius. The reason it is listed in that order is directly linked to your question: the Goldilocks Zone is really looking at where an &lt;em&gt;Earth-like&lt;/em&gt; planet could be habitable. So they &lt;em&gt;are&lt;/em&gt; accounting for planet size, and you can also see in the definition of the equilibrium temperature that they admit that exoplanet atmospheres are unknown and thus an Earth-like atmosphere is assumed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://rationalwiki.org/wiki/Astrobiology#Habitable_zone&quot; rel=&quot;nofollow noreferrer&quot;&gt;RationalWiki article on astrobiology&lt;/a&gt; points out another important factor: icy moons like Enceladus and Europa that are heated by tidal friction are also considered to be in the Goldilocks Zone. Therefore, the Goldilocks Zone could be considered to have extra &quot;pockets&quot; around places such as those moons. Arguably, ozone layers and magnetospheres &lt;em&gt;are&lt;/em&gt; also considered when evaluating habitability -- as more information about the planet is gathered and processed, every new clue is added to the analysis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also possible that planets inside the Goldilocks Zone may be &lt;em&gt;inhabitable&lt;/em&gt;. For example, &lt;a href=&quot;http://www.space.com/13950-habitable-alien-planets-tidal-lock-life.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;tidal locking may cause wild climate effects&lt;/a&gt;... although &lt;a href=&quot;http://www.gizmag.com/tidally-locked-exoplanets-potentially-habitable/39407/&quot; rel=&quot;nofollow noreferrer&quot;&gt;recent work&lt;/a&gt; suggests that may be less of a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Onto your next point about radiation resistance. Welcome to the field of astrobiology! Studying life that can thrive in extreme conditions is essentially all that an astrobiologist does. You mentioned &lt;a href=&quot;https://en.wikipedia.org/wiki/Deinococcus_radiodurans&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiation resistant bacteria&lt;/a&gt;, there is also &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiotrophic_fungus&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiation resistance fungus&lt;/a&gt; which had been discovered thriving around the Chernobyl disaster site, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tardigrade&quot; rel=&quot;nofollow noreferrer&quot;&gt;tardigrade&lt;/a&gt; is an example of an actual animal that can withstand around 1000 times the amount of radiation as other animals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately I cannot quote a number for a maximum UV or radiation dose that DNA in general can withstand. However, different organisms have varying degrees of resistance to UV based on their respective evolutionary history, so I expect it would be inappropriate to suggest that the maximum dose that DNA alone can withstand corresponds to the maximum dose that the organism can withstand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my opinion (caution, I'm not an astrobiologist by training), the rate at which new discoveries are being made regarding extreme survivability of life on Earth suggests that something as simple as a minimum/maximum distance from a particular star is insufficient. So I think your question itself is part of the answer, it is an ongoing debate.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-12-05T11:06:44.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12882" PostTypeId="2" ParentId="4003" CreationDate="2015-12-05T12:56:48.193" Score="19" Body="&lt;p&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1964-083D&quot;&gt;Transit 5B-5&lt;/a&gt; a.k.a. Oscar 2 (not to confuse with OSCAR ham radio satellites), part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transit_(satellite)&quot;&gt;Transit / Navsat&lt;/a&gt; navigational satellite program was launched 51 years ago on Dec. 13th 1964 and its telemetry transmitter is still transmitting on 136.650 MHz when it's passing through sunlight.  It's not operational anymore (in fact it quit its job 19 days after launch) but it's most likely the oldest piece of space scrap that's still kind of &quot;active&quot;.&lt;/p&gt;&#xA;" OwnerUserId="12523" LastEditorUserId="4660" LastEditDate="2015-12-05T16:07:23.237" LastActivityDate="2015-12-05T16:07:23.237" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12883" PostTypeId="2" ParentId="3759" CreationDate="2015-12-05T15:07:05.293" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;...when a spacecraft stationed there diverges toward either side of the saddle, and station-keeping is no longer maintained, where will it go?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Earth-Sun &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#L1&quot; rel=&quot;nofollow&quot;&gt;$L_{1}$&lt;/a&gt; point is unstable, just like the rest of the Lagrange points.  If a spacecraft degrades from a quasi-stable orbit at $L_{1}$, it will most likely just orbit the sun in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Heliocentric_orbit&quot; rel=&quot;nofollow&quot;&gt;heliocentric orbit&lt;/a&gt;.  This happened with the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Cometary_Explorer&quot; rel=&quot;nofollow&quot;&gt;ISEE 3&lt;/a&gt; (or ICE) spacecraft which was sent after &lt;a href=&quot;https://en.wikipedia.org/wiki/21P/Giacobini%E2%80%93Zinner&quot; rel=&quot;nofollow&quot;&gt;Comet Giacobini-Zinner&lt;/a&gt; in 1985.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISEE 3 orbited the sun for ~30 years and eventually came back around to Earth in 2014.  Remember, the Earth-Sun $L_{1}$ is centered at something like ~230 $R_{E}$ away from Earth.  If a spacecraft &quot;falls out&quot; of orbit there, it will just keep moving along in its normal tangential motion around the sun.  Note that the ISEE 3 spacecraft was sent into an &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#L2&quot; rel=&quot;nofollow&quot;&gt;$L_{2}$&lt;/a&gt; transfer orbit prior to leaving Earth for Comet GZ.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The JWST won't be at the Sun-Earth L2 point &lt;em&gt;forever&lt;/em&gt;...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, it will not.  I do not recall if there are plans to degrade the orbit and crash the spacecraf to avoid debris build-up.  However, if we assume JWST were to lose communication with the ground and go into safe-mode, its orbit would degrade.  It, like ISEE 3, would most likely enter into a heliocentric orbit as well, but it would lag the Earth, instead of lead it like ISEE 3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that &lt;a href=&quot;https://en.wikipedia.org/wiki/Wilkinson_Microwave_Anisotropy_Probe&quot; rel=&quot;nofollow&quot;&gt;WMAP&lt;/a&gt; stayed at the $L_{2}$ point from 2001-2010.  Then the &lt;a href=&quot;https://en.wikipedia.org/wiki/Planck_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Planck spacecraft&lt;/a&gt; was launched and sent to the $L_{2}$ point in 2010.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I'm most interested in the Earth-Moon system...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I seem to recall that there are spacecraft debris that are being tracked that are in some of the Earth-Moon &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow&quot;&gt;Lagrangian points&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a consensus that there are small asteroids and dust clouds in the Earth-Sun &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#L4&quot; rel=&quot;nofollow&quot;&gt;$L_{4}$&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#L5&quot; rel=&quot;nofollow&quot;&gt;$L_{5}$&lt;/a&gt; points.  These are called &lt;a href=&quot;https://en.wikipedia.org/wiki/Trojan_(astronomy)&quot; rel=&quot;nofollow&quot;&gt;trojans&lt;/a&gt;.  The Earth has a relatively large trojan that was found in &lt;a href=&quot;https://en.wikipedia.org/wiki/2010_TK7&quot; rel=&quot;nofollow&quot;&gt;2010&lt;/a&gt; by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer&quot; rel=&quot;nofollow&quot;&gt;WISE&lt;/a&gt; spacecraft.  We have known about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter_trojan&quot; rel=&quot;nofollow&quot;&gt;Jovian trojans&lt;/a&gt; since the early 1900's.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Which of these would crash into the moon and the Earth?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This actually depends upon the phase and speed of the orbiting object when its orbit finally degrades.  I am not so sure that one can definitively show, for instance, that the Earth-Moon $L_{1}$ orbit will always result in the object crashing into the Earth or the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, a spacecraft inserted into a Lagrangian point orbits in an elliptical pattern in one plane (i.e., the plane of the two bodies creating the Lagrange points), and the third component of the orbit is decoupled, called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;nofollow&quot;&gt;Lissajous orbit&lt;/a&gt;.  The amplitude of the out-of-plane component matters here as well, because if too large the orbit will degrade in a completely different manner than if the in-plane amplitudes become too large.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, the system is very complicated.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2015-12-05T15:07:05.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12884" PostTypeId="2" ParentId="12878" CreationDate="2015-12-05T20:58:35.487" Score="10" Body="&lt;p&gt;&lt;strong&gt;EML1 and EML2&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From EML2 and EML1 it is possible to collide with earth or the moon. It is also possible to sail out of earth's Hill Sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are a range of pellets from &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;noreferrer&quot;&gt;EML2&lt;/a&gt; nudged away from the earth and moon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bUxzr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bUxzr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some sail out of earth's Hill Sphere. Note the orange pellet makes a close approach to earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some trajectories from EML1:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0KFUN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0KFUN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nudging towards the earth results in an approximately 300,000 x 100,000 km earth orbit. Third or fourth apogee the moon pulls the pellets backward lowering perigee. Over time it is possible for pellets to collide with earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are pellets from EML1 nudged towards the moon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/btNgX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/btNgX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice at 3rd apolune they sail through EML2 out towards the outer reaches of earth's Hill Sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When nudged moonward they fall into an about 60,000 km x 5,000 km lunar elliptical orbit. While this ellipse's line of apsides remains constant, EML1 and EML2 rotate. At the third apolune the pellets find themselves in the neighborhood of EML2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A small braking burn at that 3rd apolune could park at EML2. Thus it is possible to go from EML1 to EML2 with little delta V. And vice versa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EML4, EML5, EML3&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe something nudged away from a Trojan point will form a tadpole orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ce8G2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ce8G2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bigger the nudge, the bigger the tadpole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the tadpole's tail extends past EML3, you get a horse shoe:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kxhji.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kxhji.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the earth, moon and pellet were an ideal 3 body system, these tadpole and horseshoe orbits could be long lived.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the sun is a major influence. The sun's influence can wreck what would otherwise be fairly stable orbits.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-12-05T21:21:52.123" LastActivityDate="2015-12-05T21:21:52.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12888" PostTypeId="2" ParentId="12879" CreationDate="2015-12-06T19:35:29.420" Score="4" Body="&lt;p&gt;A brief search led me to the conference proceedings for the &quot;Life in the Universe&quot; conference held at Ames Research Centre in 1979 (yes, this is old, but serves as a good baseline from which to begin). The paper &quot;Eavesdropping Mode and Radio Leakage from Earth&quot; notes that early warning systems for ballistic missiles beam intense radio signals that could be detected up to 15 light years away. It is also mentioned that the proposed radio array for &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Cyclops&quot; rel=&quot;nofollow&quot;&gt;Project Cyclops&lt;/a&gt; would have been detectable up to 250 light years away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course this does not answer your question regarding &quot;present day Earth&quot;. &lt;a href=&quot;http://www.straightdope.com/columns/read/3058/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth&quot; rel=&quot;nofollow&quot;&gt;This 2012 article&lt;/a&gt; from The Straight Dope is certainly rather opinionated but the author throws out an estimate of 30-50 light years. They also mention that radio astronomy signals being used to survey the sky might be detectable up to 1000 light years away (citing Seth Shostak of SETI).&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-12-06T19:35:29.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12889" PostTypeId="1" CreationDate="2015-12-06T19:59:41.633" Score="11" ViewCount="3714" Body="&lt;p&gt;I &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Roll_program&quot; rel=&quot;noreferrer&quot;&gt;read recently&lt;/a&gt; that the roll maneuver that rockets perform shortly after clearing the tower to become tilted serves a few purposes (Shuttle and Apollo Saturn V alike). I understand most of these reasons like generating lateral velocity, getting a better view, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing mentioned was that it allowed the vehicle to lift more mass. All things being equal, I fail to see how tilt can increase the ability to increase the potential energy of the payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone explain how tilt of a rocket allows it to lift more mass into orbit?&lt;/p&gt;&#xA;" OwnerUserId="12538" LastEditorUserId="6816" LastEditDate="2015-12-07T07:51:07.390" LastActivityDate="2020-01-21T17:25:39.497" Title="How does the roll maneuver allow more mass to be lifted into orbit?" Tags="|launch|orbit|mass|performance|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12890" PostTypeId="1" AcceptedAnswerId="12891" CreationDate="2015-12-06T22:00:30.907" Score="3" ViewCount="344" Body="&lt;p&gt;On Earth, at some times and places, we can see many stars in the sky. When I watch rocket launch videos, or look at NASA imagery from the Apollo missions, I rarely see any stars. Why is this? The stars are obviously there, why can't they be seen in these circumstances?&lt;/p&gt;&#xA;" OwnerUserId="10712" LastEditorUserId="4660" LastEditDate="2015-12-06T22:41:14.177" LastActivityDate="2015-12-07T10:40:51.290" Title="Seeing stars in space" Tags="|imaging|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12891" PostTypeId="2" ParentId="12890" CreationDate="2015-12-06T22:14:51.947" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Aperture&quot; rel=&quot;noreferrer&quot;&gt;Aperture&lt;/a&gt;. The aperture diameter is too small and frame rate too great to capture dim distant sources of light. It's really that simple. Consider how the rocket's engine, its ejecta and the illuminated Earth in the background would look if the camera's aperture was set to detect background stars. Any foreground lit object would cause frame overexposure and everything would appear white, possibly even cause permanent sensor damage if it caught a ray of light from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can try this at home with any point and shoot camera. Point it towards a full moon (or close to being full) during night, set it on auto-exposure, and then see if it also caught any stars appearing around the Moon. It won't. Even the Moon at night is too bright and capturing stars requires larger aperture, longer exposure and/or greater sensor sensitivity. You might succeed in capturing Venus (&lt;a href=&quot;https://en.wikipedia.org/wiki/Apparent_magnitude&quot; rel=&quot;noreferrer&quot;&gt;apparent magnitude&lt;/a&gt; of up to −4.89) when it's particularly bright next to a half or less lit Moon, but that's not a star. &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_brightest_stars&quot; rel=&quot;noreferrer&quot;&gt;Brightest star&lt;/a&gt; on the night sky, as seen from Earth or indeed anywhere near our Solar system, if we exclude the Sun itself, is Sirius at mag. −1.46 (&lt;a href=&quot;https://encrypted.google.com/#q=2.512%5E%28-1.46+-+-4.89%29&quot; rel=&quot;noreferrer&quot;&gt;23.55 times dimmer&lt;/a&gt; than Venus at its brightest, and &lt;a href=&quot;https://encrypted.google.com/#q=2.512%5E%28-1.46+-+-12.92%29&quot; rel=&quot;noreferrer&quot;&gt;over 38,000 times dimmer&lt;/a&gt; than full moon at apparent magnitude of −12.92).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="33" LastEditDate="2015-12-07T10:40:51.290" LastActivityDate="2015-12-07T10:40:51.290" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12892" PostTypeId="1" CreationDate="2015-12-06T22:58:53.280" Score="2" ViewCount="125" Body="&lt;p&gt;After learning what apogee and perigee are, I'm curious to know if/when these stages align with particular phases of the Moon? I'm obviously assuming that the orbit has a repeating pattern, but maybe I'm wrong?&lt;/p&gt;&#xA;" OwnerUserId="10712" LastEditorUserId="49" LastEditDate="2015-12-06T23:27:00.743" LastActivityDate="2015-12-06T23:27:00.743" Title="Moon's orbit and its phases" Tags="|the-moon|orbit|observation|astronomy|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12893" PostTypeId="2" ParentId="12892" CreationDate="2015-12-06T23:25:12.450" Score="1" Body="&lt;p&gt;In general, no. Most lunar cycles won't align with its orbital distance to Earth (perigee at its closest or apogee at its farthest). There is however an &lt;em&gt;orbital pattern&lt;/em&gt;, or a resonance if you will, that combines both periodically and it's called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Full_moon_cycle&quot; rel=&quot;nofollow&quot;&gt;full moon cycle&lt;/a&gt; which is roughly 14 lunations long period when the Moon will be full at roughly its perigee. When the Moon is both the closest to Earth (perigee) and also in &lt;a href=&quot;https://en.wikipedia.org/wiki/Opposition_%28planets%29&quot; rel=&quot;nofollow&quot;&gt;opposition&lt;/a&gt; (on the other side of the Earth than the Sun) at exactly the same time, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Supermoon&quot; rel=&quot;nofollow&quot;&gt;supermoon&lt;/a&gt; eclipse happens. This is another such cycle and it is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Saros_%28astronomy%29&quot; rel=&quot;nofollow&quot;&gt;saros&lt;/a&gt;. It occurs every 18 years and a bit, or 223 &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_month#Synodic_month&quot; rel=&quot;nofollow&quot;&gt;synodic months&lt;/a&gt; (time between one full moon and another).&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-12-06T23:25:12.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12894" PostTypeId="2" ParentId="12889" CreationDate="2015-12-06T23:55:58.687" Score="15" Body="&lt;p&gt;First, let's get terminology straight: &quot;Tilt maneuver&quot;, or &quot;Gravity turn&quot;, sometimes also called &quot;Pitch maneuver&quot;. It was called &quot;Roll Program&quot; in case of Space Shuttles, because it was connected with a roll, necessary for technical reasons but not contributing to flight efficiency directly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All rockets (and all flying bodies on Earth for that matter) are subject to gravity drag. That's a jargon name for pull of gravitational force that makes everything fall to Earth if not suspended somehow. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, a rocket just to hover above launchpad, without rising, each second must burn enough fuel to grant it 9.8m/s^2 acceleration - just to break even with Earth acceleration. That's all a wasted fuel that doesn't contribute to accelerating the rocket or lifting it, it just prevents it from falling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Understandably, we want to minimize this waste - and the only way to do this is to minimize the time between launch and the moment we reach orbital speed, at which gravitational drag is reduced to zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch trajectory is a balance between horizontal acceleration which serves exactly that, and vertical, which is to reduce atmospheric drag by lifting the rocket into increasingly thinner atmosphere and eventually out of it entirely. So the thrust is divided between the horizontal factor (reducing gravitational drag) and vertical (reducing atmospheric drag). This is why the rocket is tilted, to start gaining horizontal speed immediately, and this tilt increases as atmospheric drag becomes weaker - the higher the horizontal speed, the lower the gravitational drag as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Optimizing this means we can reach orbit using less fuel than we'd use if we took an unoptimal trajectory (e.g. launch vertically up, and only later start accelerating horizontally, while gravitational drag costs us a fortune in fuel), and of course - instead of actually &lt;em&gt;burning less fuel&lt;/em&gt; we just &lt;em&gt;increase the payload&lt;/em&gt; - a rocket flying on optimal trajectory can take a larger payload than one with the same amount of fuel not flying optimally.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In case of Space Shuttles, the Tilt Maneuver was called &quot;Roll Program&quot;. That is because the maneuver of pitching the shuttle to the right inclination was connected with it rolling to position &quot;shuttle below, tank and boosters above&quot;. That roll was necessitated by structural and other technical requirements - it was a quirk of the space shuttle construction, the irregular shape, utilizing its wings etc. It's not needed in great most launch vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even in the shuttle it wasn't exactly necessary - it was dictated by the launchpad and crawlerway location, as rotating the shuttle on the ground, to launch in proper roll orientation would be too cumbersome - so it was done only after launch. But despite the name, the &quot;roll&quot; part of the &quot;roll program&quot; was merely a technical quirk of this particular construction. Tilt into proper inclination was the general &quot;rocket science&quot; part though, contributing to craft efficiency.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2015-12-07T09:35:06.790" LastActivityDate="2015-12-07T09:35:06.790" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12895" PostTypeId="1" AcceptedAnswerId="12897" CreationDate="2015-12-07T00:56:50.840" Score="2" ViewCount="919" Body="&lt;p&gt;It seemed to me that today's (Dec. 6, 2015) launch of ULA's Atlas V carrying Orbital ATK's Cygnus spacecraft in orbit and towards the International Space Station on the &lt;a href=&quot;http://www.ulalaunch.com/ula-successfully-launches-oa4-cygnus.aspx?title=United%20Launch%20Alliance%20Successfully%20Launches%20OA-4%20Cygnus%20to%20International%20Space%20Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;OA-4 mission&lt;/a&gt; had a particularly slow liftoff and the &lt;a href=&quot;http://www.ulalaunch.com/ula-successfully-launches-gps-iif11.aspx?title=United%20Launch%20Alliance%20Successfully%20Launches%20GPS%20IIF-11%20Satellite%20for%20U.S.%20Air%20Force&amp;amp;archived=True&amp;amp;Category=all&amp;amp;Page=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;previous Atlas V launch of GPS IIF-11&lt;/a&gt; on Oct. 31, 2015 was faster off the pad. I thought at first that it might have been some frame lag of the live stream during webcast, but the launch vehicle appeared equally slow off the pad in later repeats of the liftoff from various angles;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I checked with ULA's mission brochures (for &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Mission_Booklets/AV/av_oa4_mob.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;OA-4 mission&lt;/a&gt; and for &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Mission_Booklets/AV/av_gpsiif11_mob.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;GPS IIF-11 mission&lt;/a&gt;, both PDF), and event timeline shows that today's launch was slower to accelerate to Mach 1 and Maximum Dynamic Pressure (Max Q), but that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Common_Core_Booster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Common Core Booster&lt;/a&gt; of today's OA-4 launch also cutoff nearly 12 seconds later than during the GPS IIF-11 launch, despite having same mission time ignition and liftoff:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/6xyBj.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it's not simple maths of heavier payload on otherwise equivalent flight profile and same launch vehicle (this is as expected really), and the OA-4's Atlas V did indeed launch with a smaller thrust-to-weight ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is simply, what this T/W was? Additional kudos for pointing me to an online resource where such information can be found for all ULA launches, so I can refer to it in the future.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If it helps visualize what I'm asking about, here are the videos of the two launches:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Uh5YnXyjT5c?start=30&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/NPcRziWDigQ?start=82&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-07T01:06:38.693" LastActivityDate="2015-12-07T05:13:50.183" Title="What was the thrust-to-weight ratio during liftoff of the Atlas V OA-4 launch?" Tags="|launch|reference-request|ula|performance|atlas-v|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12896" PostTypeId="2" ParentId="12889" CreationDate="2015-12-07T02:06:35.287" Score="8" Body="&lt;p&gt;Almost all the launch vehicles lift off vertically and are designed to reach orbital speed, altitude and orientation as the upper stage completes its injection burn.&lt;/p&gt;&#xA;&lt;p&gt;Consider a launch vehicle lifting off vertically- The vehicle accelerates to overcome two forces- earth's gravity and the atmospheric drag.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xEKBA.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xEKBA.gif&quot; alt=&quot;Forces on rocket&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image from rocketmime.com&lt;/p&gt;&#xA;&lt;p&gt;If the launch vehicle goes vertically up the whole way, it will reach the needed altitude, but will fall down as it won't enter into an orbit around the earth. However, if the vehicle is launched horizontally, the time spent in the atmosphere will make the fuel requirement prohibitive.&lt;/p&gt;&#xA;&lt;p&gt;An ideal trajectory will reach the required position with minimum expenditure of fuel and minimum load on the rocket structure (which saves weight). This is usually achieved through the use of 'gravity turn' or zero-lift turn', which uses the gravity (which is perpendicular to the launch vehicle's longitudinal axis initially) to turn the velocity vector as it ascents toward orbit, instead of using the vehicles on-board propellant.&lt;/p&gt;&#xA;&lt;p&gt;Basically, as soon as the vehicle clears the launch platform, a pitchover maneuver is executed (usually by gimbaling the rocket engines), directing some of the thrust to one side and creating a net torque on the vehicle.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KI93r.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KI93r.gif&quot; alt=&quot;Rocket forces&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image from spaceflightsystems.grc.nasa.gov&lt;/p&gt;&#xA;&lt;p&gt;Now, a small part of the gravitational force is directed perpendicular to the longitudinal axis. This is the beginning of the gravity turn. After the pitchover is complete, the engines are reset to point straight down the axis of the rocket again. From this point until orbit injection, the transverse gravity component continues to grow (as gravity is basically turning the rocket about its lateral axis) and causes the vehicle's velocity vector to rotate toward the horizon as it ascends (by turning the rocket nose towards ground).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BmssL.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BmssL.png&quot; alt=&quot;Gravity Turn&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Gravity_turn_-_phase_2.svg#/media/File:Gravity_turn_-_phase_2.svg&quot; rel=&quot;noreferrer&quot;&gt;Gravity turn - phase 2&lt;/a&gt;&amp;quot; by &lt;a href=&quot;//commons.wikimedia.org/wiki/User:AndrewBuck&quot; rel=&quot;noreferrer&quot; title=&quot;User:AndrewBuck&quot;&gt;AndrewBuck&lt;/a&gt; - Own work. Licensed under CC BY-SA 3.0 via &lt;a href=&quot;https://commons.wikimedia.org/wiki/&quot; rel=&quot;noreferrer&quot;&gt;Commons&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;For a properly executed gravity turn, this gravity is the only force acting on the rocket that acts to turn the rocket, thus saving fuel (there may be some corrections due to wind, however, these are negligible). Also, the angle of attack is reduced to (near) zero, thus reducing transverse aerodynamic forces and enabling a lighter vehicle.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The exact initial pitchover angle depends on the specific launch vehicle, time of launch, orbital destination etc.&lt;/p&gt;&#xA;&lt;p&gt;For &lt;a href=&quot;http://www.apollosaturn.com/s5flight/sec2.htm&quot; rel=&quot;noreferrer&quot;&gt;Saturn V&lt;/a&gt;, this maneuver is initiated  000:00:13 seconds after launch, after the rocket has lifted off some 450 ft,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;After clearing the tower, a tilt and roll maneuver is initiated to achieve the flight attitude and proper orientation for the selected flight azimuth. Launch azimuth is 90 degrees; flight azimuth may vary between 72 and 108 degrees, depending upon time and date of launch. From the end of the tilt maneuver to tilt-arrest, the vehicle flies a pitch program (biased for winds of the launch month) to provide a near zero-lift (gravity-tum) trajectory.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;http://www.aerospaceweb.org/question/spacecraft/q0127a.shtml&quot; rel=&quot;noreferrer&quot;&gt;the case of the space shuttle&lt;/a&gt;, the maneuver is initiated before the spacecraft experiences the maximum dynamic pressure (termed &amp;quot;max q&amp;quot;).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The roll maneuver is performed shortly before max q is reached because this &amp;quot;heads-down&amp;quot; orientation helps alleviate the stresses that the dynamic pressure loads cause on the vehicle's structure.&lt;/p&gt;&#xA;&lt;p&gt;The second factor we need to consider is that for each mission, the Shuttle must launch at a certain azimuth angle in order to be inserted into the correct orbital plane. Since the launch pad (and therefore the Shuttle) sits in a fixed position, the Shuttle must perform a roll maneuver during ascent in order to orient itself to achieve the desired launch azimuth angle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="11361" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-12-07T10:36:53.913" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12897" PostTypeId="2" ParentId="12895" CreationDate="2015-12-07T05:07:51.433" Score="2" Body="&lt;p&gt;Per &lt;a href=&quot;http://spaceflight101.com/spacerockets/atlas-v-401/&quot; rel=&quot;nofollow&quot;&gt;Spaceflight101&lt;/a&gt;, the Atlas 401 has a mass at launch of 334.5 tons (this likely varies with fuel load) and thrust is 3827kN. Cygnus is &quot;just over 8 tons&quot;; GPS IIF is 1.63 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for the GPS IIF launch, we have $\frac{3827}{334.5+1.63} = 11.38 m/s^2$ acceleration, less Earth's gravity of $9.81 m/s^2$,  gives acceleration off the pad of $1.57 m/s^2$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Cygnus, $\frac{3827}{334.5+8.1} = 11.17 m/s^2$, less g, for $1.36 m/s^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The length of the Atlas itself is 55m. Time to travel under constant acceleration is $\sqrt{ \frac{2 d}{a} }$ gives us 9 seconds for the Cygnus launch to travel one rocket-length; 8.4 seconds for the GPS launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in the videos you link, it looks to me more like 6 and 5 seconds to clear the tower, which is almost the entire length of the rocket, so something's amiss here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some rocket engines run above 100% of their so-called rated thrust for portions of their ascent, but I don't know if the Atlas's RD-180 is one of them. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-07T05:13:50.183" LastActivityDate="2015-12-07T05:13:50.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12898" PostTypeId="1" CreationDate="2015-12-07T05:21:56.067" Score="1" ViewCount="1013" Body="&lt;p&gt;How feasible is it to use a cyclotron in this manner for space propulsion?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inject small amounts (aliquots) of ionized matter into cyclotron at some high integer multiple n of the cyclotron frequency $\omega=qB/m$ so that there are n aliquots at each radius in the cyclotron. Since the time it takes to complete one revolution is the same for all of these aliquots, one could use the frequency $2n\omega$ of the alternating electric field to accelerate all these aliquots in a synchronous manner. &#xA;This would give essentially a continuous stream of very high speed plasma or material to accelerate spacecraft.&#xA;Of course, the aliquots could move at relativistic speeds which would affect the cyclotron frequency by $\gamma$. But one could design a magnetic field that gets stronger with increasing radius especially toward outer portions where relativistic effects would be more prominent.&#xA;There would also be electrostatic repulsions between aliquots that would need to be taken into account.&#xA;The spacecraft would likely need nuclear power source.&#xA;Is this a feasible idea?&#xA;Has this been thought of already?&lt;/p&gt;&#xA;" OwnerUserId="11095" LastActivityDate="2022-07-26T09:00:19.587" Title="Cyclotron engines as a means of thrust for interplanetary travel?" Tags="|interplanetary|interstellar-travel|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12899" PostTypeId="1" AcceptedAnswerId="13012" CreationDate="2015-12-07T07:37:17.650" Score="1" ViewCount="164" Body="&lt;p&gt;I was reading about a rover concept ' Hedgehog ' which operates by spinning flywheels very fast and then braking quickly which transfers the momentum thereby causing the rover to move by tumbling.&#xA;My question is so how can this method be effective( what am I missing? ) as by transferring momentum  the rover would just fly off( as the gravitational force exerted by the asteroid on the rover is very small).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=FPyVGRwegvU&quot; rel=&quot;nofollow&quot;&gt;YouTube video on the working of the rover  ( 2 min 11 sec) &lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=4712&quot; rel=&quot;nofollow&quot;&gt;An article by JPL NASA on the hedgehog rover&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6753" LastActivityDate="2015-12-15T23:42:14.360" Title="How can a hedgehog rover be used to explore an asteroid, wouldnt it just fly away?" Tags="|asteroid|rovers|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12901" PostTypeId="2" ParentId="12898" CreationDate="2015-12-07T10:05:26.903" Score="13" Body="&lt;p&gt;A cyclotron is a type of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot;&gt;Ion engine&lt;/a&gt;. Cyclotrons (in the classical sense of the word) haven't been used because they have a very low power-to-weight ratio (tons of magnet to accelerate tiny amounts of matter).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are ion engines that use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Electron_cyclotron_resonance&quot;&gt;Electron cyclotron Resonance&lt;/a&gt; principle; these use the cyclotron princple at a tiny scale to create ions that are then accelerated in a linear accelerator. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-07T10:05:26.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12902" PostTypeId="2" ParentId="1147" CreationDate="2015-12-07T14:36:27.667" Score="3" Body="&lt;p&gt;Land, no... Study, yes!  There is a new mission called &lt;a href=&quot;http://sci.esa.int/juice/50068-science-objectives/&quot; rel=&quot;nofollow&quot;&gt;JUICE&lt;/a&gt; lead by &lt;a href=&quot;http://www.esa.int/ESA&quot; rel=&quot;nofollow&quot;&gt;ESA&lt;/a&gt; and in partnership with NASA to study the &lt;a href=&quot;https://en.wikipedia.org/wiki/Galilean_moons&quot; rel=&quot;nofollow&quot;&gt;Galilean moons&lt;/a&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter&quot; rel=&quot;nofollow&quot;&gt;Jupiter&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The entire purpose of the JUICE mission is to:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;to characterise the conditions that may have led to the emergence of habitable environments among the Jovian icy satellites, with special emphasis on the three ocean-bearing worlds, Ganymede, Europa, and Callisto...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2015-12-07T14:36:27.667" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12903" PostTypeId="2" ParentId="12889" CreationDate="2015-12-07T14:44:22.443" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;One thing mentioned was that it allowed the vehicle to lift more mass. All things being equal, I fail to see how tilt can increase the ability to increase the potential energy of the payload.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All things weren't equal. The Shuttle was not an axially symmetric vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle roll program was performed starting about ten seconds after launch and lasted for only eight seconds or so. It was called the roll program because the primary effect was to roll the vehicle by about 180 degrees. Without this maneuver, the layout of the launch pad would have dictated a heads-up orientation of the Orbiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle stack comprised a number of parts, including the Orbiter itself, the external tank, and the solid rocket boosters. Flying heads sideways would not have made any sense at all. The stresses on the connectors between those components would have torn the vehicle apart. Flying in a heads-up configuration also would not have made much sense. This would have resulted in tensile stresses between the Orbiter and the external tank. The heads-down configuration that was flow resulted in compressive stresses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While flying heads-up might have been controllable, that the forces would have been tensile rather than compressive would have mandated a significant reduction in payload mass lest the vehicle tear itself apart at &lt;a href=&quot;https://en.wikipedia.org/wiki/Max_Q&quot; rel=&quot;noreferrer&quot;&gt;maximum dynamic pressure (max Q)&lt;/a&gt;. This was part of what the cited wikipedia article meant by &quot;Increasing the mass that can be carried into orbit.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another aspect of the roll program was to orient the vehicle so that it was on the correct heading. A third aspect was to start the vehicle pitching downward. To attain orbit, a vehicle has to gain altitude and a huge amount of horizontal velocity. A vehicle that is launched vertically from the ground has to pitch downward throughout the course of the launch to eventually achieve this mostly horizontal course eight to ten minutes after liftoff. Ideally, this is done so as to constantly keep the vehicle on a zero angle of attack throughout the launch. Doing so is called a gravity turn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No real spacecraft performs a true gravity turn. The vehicle has to make it through the thickest part of the atmosphere first. Minimizing stresses at maximum dynamic pressure plays a key role here. The short Shuttle roll program set the stage for when max Q occurred. In addition to the stresses on the connectors and structure, it was very important to minimize stress on the Orbiter's wings, particularly at max Q. The wings were more of a hindrance than an aid during launch. The Shuttle flew at a negative angle of attack to keep the stresses on the wings down to a minimum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle roll program, short as it was, set the stage for the rest of the flight. Getting that just right did indeed affect how much mass could be carried into orbit.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-12-07T14:44:22.443" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12904" PostTypeId="2" ParentId="1147" CreationDate="2015-12-07T17:50:24.627" Score="3" Body="&lt;p&gt;In addition to NASA's plans, a key figure pushing for Europa missions is Congressman &lt;a href=&quot;http://blog.chron.com/sciguy/2014/03/heres-why-the-europa-mission-is-real-and-could-very-well-happen/&quot; rel=&quot;nofollow&quot;&gt;Culberson&lt;/a&gt;, who is in line to become the next chairman of the House Appropriations subcommittee responsible for NASA’s budget. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interesting part of Europa (its ocean) is buried under 30 km of ice. Getting to that ocean is a challenge we haven't solved on Earth, let alone halfway across the Solar system. So this is what's being worked on at the moment. Europa Clipper's results will help solve the challenge, it is hoped (it will look for vents where liquid water comes to the surface, for example). &#xA;Once we have a feasible solution, it's time to start planning a lander mission. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-07T17:50:24.627" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12905" PostTypeId="1" CreationDate="2015-12-07T18:54:11.247" Score="4" ViewCount="456" Body="&lt;p&gt;I am hoping someone can point me to images of Pluto and Charon with latitude and longitude lines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know it's the convention to designate the near point of a tidelocked moon as 0°, 0°. Is the same true of a planet tidelocked with his moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am doing some drawings of the Pluto Charon neighborhood. For example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VZWAu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VZWAu.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drawings would be more interesting and informative if the Pluto and Charon circles included some features. I am hoping a lobe of Pluto's heart is close to the near point.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-12-08T00:46:52.747" LastActivityDate="2015-12-13T08:30:51.907" Title="Latitude and longitude lines for Pluto and Charon" Tags="|pluto|mapping|charon|coordinates|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12906" PostTypeId="1" AcceptedAnswerId="12907" CreationDate="2015-12-08T13:14:14.780" Score="6" ViewCount="1270" Body="&lt;p&gt;I asked this in other boards but no one could give me answers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, can someone tell me:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Why can New Horizons (which is older than Dawn) take photos in a&#xA;way higher resolution than Dawn can (NH: 70m/pixel at an altitude of&#xA;17.000km; Dawn: 140m/pixel at an altitude of 1400km)?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Why does a probe which should observe objects optical not take&#xA;the latest camera technique along?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Why isn't there a minimum standard equipment on each probe that&#xA;makes results and images of missions comparable to each other?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="12553" LastEditorDisplayName="user10509" LastEditDate="2015-12-08T14:34:35.920" LastActivityDate="2016-10-20T21:28:31.633" Title="Why do New Horizons and Dawn have such different imagers/cameras aboard?" Tags="|new-horizons|camera|dawn-mission|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12907" PostTypeId="2" ParentId="12906" CreationDate="2015-12-08T14:13:36.870" Score="20" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;New Horizons was designed to do a fly-by of Pluto in a relatively large distance. It was only in range of Pluto for a couple of hours. Dawn is designed to orbit Ceres at much closer range. To accomplish its mission, New Horizons had to have such a high resolution camera to get the desired results.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Instruments are designed specifically for a certain mission. Imaging alone is only one of several mission objectives that is accomplished by a space probe. A probe is severely limited by the amount of weight and available space it has for all its instruments. So sometimes to make room for other experiments, some things have to be replaced, made smaller or dropped altogether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A camera like with New Horizons wasn't needed for Dawn because Dawn can get much much closer to Ceres than New Horizons ever got to Pluto.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And last but not least, the project budget must be met. A higher resolution camera that fits the probe costs a lot more than a low resolution camera.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Because there is a lot of planning time involved for each mission and each mission is designed specifically for the technology safely available and affordable at that time.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Don't forget that the costs for New Horizons were already much higher than for Dawn. The project cost for New Horizons was about double that of Dawn, so it's easier to squeeze in another million when the project budget is already rather large and you only get one shot at this in 20 years than on a small budget that could possibly be repeated in half that time. ;)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="12555" LastEditorUserId="6816" LastEditDate="2015-12-09T08:38:21.650" LastActivityDate="2015-12-09T08:38:21.650" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12908" PostTypeId="2" ParentId="12725" CreationDate="2015-12-08T14:54:49.263" Score="1" Body="&lt;p&gt;As long as we use ultra-light materials in space, it isn't realistic. But even if we assume the space ship to be built of iron or steel:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd want the boots to simulate earth gravity towards your spacecraft while you are on an EVA to facilitate movement etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, if you'd want that, you probably would line the whole suit with magnetic fibres so that the entire suit is being pulled towards the spaceship to simulate gravity on your whole body. &#xA;The boots alone would just mean that your feet get pulled down while your body won't. I doubt that this would really help with working in a 0G envionment when just your feet get pulled down. Would feel a lot like in a water tank with weights around your ankles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, to be viable this suit would not disengage and reengage like a magnetic lock that is visible in several movies everytime you try to lift your foot. &#xA;It would simply be an always on like earth gravity - which you might have guessed by now - is also always on and would cause your suit to accelerate with 9.81 m/s² towards the spaceship. &#xA;On the plus side, this would almost negate the requirement of training every day on a spaceship to combat muscle atrophy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;INSIDE a station, the easiest way to simulate gravity would be to rotate it. No need for anything magnetic around you to give you &quot;gravity&quot;. Also inside you'd have the problem that you'd need one plane to be your &quot;anchor&quot;. Otherwise you'd lift a foot and it would suddenly get pulled to the wall because that happens to be made of steel just like the &quot;floor&quot;.&lt;/p&gt;&#xA;" OwnerUserId="12555" LastEditorUserId="12555" LastEditDate="2015-12-08T15:20:56.207" LastActivityDate="2015-12-08T15:20:56.207" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12909" PostTypeId="2" ParentId="12906" CreationDate="2015-12-08T19:04:27.303" Score="7" Body="&lt;p&gt;SWaP and SWaP-C - Size, Weight, and Power (and Cost) can almost always explain decisions beyond the mission objectives.  Reducing any one of these for one instrument means they are available resources for another instrument, either an existing one, or an additional instrument. Once you achieve the minimum specifications for each mission objective, you balance the rest according to the desired outcome.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some missions do prioritize visual imagers, but the hard reality is that they provide limited scientific value.  The vast majority of money that goes into visual imagers is usually justified by PR needed to keep space activities funded.  A lot of science is more readily accomplished by instruments that aren't visual in nature, and giving them more space, power, weight, or funds may exponentially increase the scientific value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you get to a certain point in visual imaging, increasing beyond it significantly doesn't significantly increase the science gained.&lt;/p&gt;&#xA;" OwnerUserId="248" LastActivityDate="2015-12-08T19:04:27.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12910" PostTypeId="2" ParentId="12741" CreationDate="2015-12-09T01:55:54.380" Score="6" Body="&lt;p&gt;See &lt;a href=&quot;http://www.ibiblio.org/apollo/Documents/tn_d-5869_1970023342.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA Technical Note D-5869&lt;/a&gt;.&#xA;This document includes a discussion about choosing the location of the navigation instruments of the Saturn V rocket. See the figures on pages 11 and 13 and nearby text (page 12).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several considerations were mentioned, including the effects of bending modes, and other environmental factors of possible locations, length of cabling needed, and so on. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bending mode shapes show the magnitude of the amplification of lateral motion. You would prefer not to place motion sensors at a position that bends (a lot) at a similar frequency to control motions of the rocket, as the bending could feed back into the control loop and lead to instability.&lt;/p&gt;&#xA;" OwnerUserId="12562" LastEditorUserId="6816" LastEditDate="2015-12-09T02:25:38.807" LastActivityDate="2015-12-09T02:25:38.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12913" PostTypeId="2" ParentId="7824" CreationDate="2015-12-09T13:06:53.827" Score="2" Body="&lt;p&gt;Usually, the rocket trajectory would be pre-planned on the ground to effectively point into the (expected) wind somewhat to reduce the angle-of-attack caused by the winds. Some rockets adjust their planned trajectory the day of launch to take into account the expected winds over a shorter duration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the trajectory was shaped for no wind, and day-of-launch you have the jet stream running overhead instead, I recall the difference might be up to 75 m/s of wind. Some rockets were designed with adaptive systems to adjust the pre-launch designed attitude profile into the wind based on in flight estimates of angle of attack, to reduce the angle attack effect of both the winds and sudden gusts on top of that, and perhaps even the effect of having lost one engine. But, having done so is a severe change to the trajectory for the rest of the flight after that so you have to have a fairly strong rocket (to survive the gusts) and extra propellant margin available to use to correct the trajectory after you fly through the maximum dynamic pressure area where the angle of attack limit is severe. All this is expensive to the payload capability of the rocket.&lt;/p&gt;&#xA;" OwnerUserId="12562" LastActivityDate="2015-12-09T13:06:53.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12918" PostTypeId="2" ParentId="12869" CreationDate="2015-12-09T20:33:33.013" Score="8" Body="&lt;p&gt;There is one flight-tested engine that uses the design you're suggesting: the &lt;a href=&quot;https://en.wikipedia.org/wiki/BE-3&quot;&gt;Blue Origin BE-3&lt;/a&gt;.  It is a liguid hydrogen/liguid oxygen fueled engine that produces 110,000 lbf of thrust (throttleable to 20,000 lbf).  It powers the New Shepard suborbital launch vehicle, which was first flown on April 29, 2015, and then contributed in the first ever soft vertical landing of a rocket returning from space on November 23, 2015.  It is also being considered for use on the ACES upper stage for ULA's Vulcan rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.blueorigin.com/technology#engine_stories_1&quot;&gt;Blue Origin's website&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The BE-3 is the first tapoff engine to fly. We’ve designed a simple rocket engine, where hot gasses from combustion are tapped from the main combustion chambers and fed back to spin the turbopumps in flight. Having only one combustion chamber with a single ignition event enhances reliability.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have no information on how they achieved this, as the high temperatures of the gasses in a combustion chamber (6,000+ºF) make it a challenge, like other people have mentioned here.  Blue Origin is a notoriously secretive company and hardly releases any information about their operations, let alone information about the design of their engine.&lt;/p&gt;&#xA;" OwnerUserId="11160" LastActivityDate="2015-12-09T20:33:33.013" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12919" PostTypeId="2" ParentId="12434" CreationDate="2015-12-10T01:50:33.380" Score="5" Body="&lt;p&gt;The &lt;em&gt;speed of sound&lt;/em&gt; in space has multiple meanings because space is not a vacuum (though the number density of Earth's magnetosphere can be ~6-12 orders of magnitude more tenuous than the best vacuums produced in labs), it is full of ionized particles, neutral and charged &lt;a href=&quot;https://physics.stackexchange.com/a/160627/59023&quot;&gt;dust&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_medium&quot; rel=&quot;nofollow noreferrer&quot;&gt;interplanetary medium&lt;/a&gt; or IPM, there are five relevant speeds that can all be considered a type of &lt;em&gt;sound&lt;/em&gt; in a way, because each are related to the speed of information transfer in the medium.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;The &quot;speeds of sound&quot;&lt;/h1&gt;&#xA;&#xA;&lt;h2&gt;Sound Speed&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Since a plasma can act collectively like a fluid, it can have a &lt;a href=&quot;http://en.wikipedia.org/wiki/Speed_of_sound&quot; rel=&quot;nofollow noreferrer&quot;&gt;sound speed&lt;/a&gt; in the classic form of $C_{s}^{2} = \partial P/\partial \rho$, where $P$ is the thermal pressure and $\rho$ is the mass density.  In a plasma, this takes the slightly altered form of:&#xA;$$&#xA;C_{s}^{2} = \frac{ k_{B} \left( Z_{i} \ \gamma_{e} \ T_{e} + \gamma_{i} \ T_{i} \right) }{ m_{i} + m_{e} }&#xA;$$&#xA;where $k_{B}$ is &lt;a href=&quot;http://en.wikipedia.org/wiki/Boltzmann_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Boltzmann's constant&lt;/a&gt;, $Z_{s}$ is the charge state of species $s$, $\gamma_{s}$ is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Heat_capacity_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;adiabatic or polytrope index&lt;/a&gt; of species $s$, $m_{s}$ is the mass of species $s$, and $T_{s}$ is the average &lt;a href=&quot;http://en.wikipedia.org/wiki/Temperature&quot; rel=&quot;nofollow noreferrer&quot;&gt;temperature&lt;/a&gt; of species $s$.  In a tenuous plasma, like that found in the IPM, it is often assumed that $\gamma_{e}$ = 1 (i.e., isothermal) and $\gamma_{i}$ = 2 or 3, or that $\gamma_{e}$ = 1 and $T_{e} \gg T_{i}$.  The above form of the sound speed is known as the ion-acoustic sound speed because it is the phase speed at which linear &lt;a href=&quot;http://en.wikipedia.org/wiki/Ion_acoustic_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;ion-acoustic waves&lt;/a&gt; propagate.  Thus, $C_{s}$ is a legitimate type of sound speed in space.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Alfvén Speed&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The Alfvén speed is defined as:&#xA;$$&#xA;V_{A} = \frac{ B_{o} }{ \sqrt{ \mu_{o} \ \rho } }&#xA;$$&#xA;where $B_{o}$ is the magnitude of quasi-static, ambient magnetic field, $\mu_{o}$ is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Permeability_(electromagnetism)&quot; rel=&quot;nofollow noreferrer&quot;&gt;permeability of free space&lt;/a&gt;, and $\rho$ is the plasma mass density (which is roughly equivalent to the ion mass density unless it's a pair plasma).  This speed is typically associated with transverse &lt;a href=&quot;http://en.wikipedia.org/wiki/Alfv%C3%A9n_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alfvén waves&lt;/a&gt;, but the speed is relevant to information transfer in plasmas.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Magnetized Sound Waves&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;In a magnetized fluid like a plasma, there are fluctuations that are compressive whereby they compress the magnetic field in phase with the density, called &lt;a href=&quot;http://en.wikipedia.org/wiki/Magnetosonic_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetosonic&lt;/a&gt; or fast mode waves.  The phase speed for a fast mode wave is given by:&#xA;$$&#xA;2 \ V_{f}^{2} = \left( C_{s}^{2} + V_{A}^{2} \right) + \sqrt{ \left( C_{s}^{2} + V_{A}^{2} \right)^{2} + 4 \ C_{s}^{2} \ V_{A}^{2} \ \sin^{2}{\theta} }&#xA;$$&#xA;where $\theta$ is the angle of propagation with respect to $\mathbf{B}_{o}$.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Thermal Speeds&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;There are also the &lt;a href=&quot;http://en.wikipedia.org/wiki/Thermal_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermal speeds&lt;/a&gt; one often thinks about in regards to gases.  In a plasma, there is a thermal speed for each particle species, e.g., electrons and ions.  The one-dimensional &lt;a href=&quot;http://en.wikipedia.org/wiki/Root_mean_square&quot; rel=&quot;nofollow noreferrer&quot;&gt;rms&lt;/a&gt; speed is given by:&#xA;$$&#xA;V_{Ts}^{rms} = \sqrt{\frac{ k_{B} \ T_{s} }{ m_{s} }}&#xA;$$&#xA;where $s$ can be $e$(electrons) or $i$(ions).  The three dimensional &lt;em&gt;most probable speed&lt;/em&gt;, which is given by:&#xA;$$&#xA;V_{Ts}^{mps} = \sqrt{\frac{ 2 \ k_{B} \ T_{s} }{ m_{s} }}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Answer&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The reason the solar wind becomes supersonic is actually a bit of a mystery and a large part of the motivation for the &lt;a href=&quot;http://solarprobe.gsfc.nasa.gov/spp_science.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Probe Plus&lt;/a&gt; mission.  One of the original theories was that the change in the ratio of the $B_{o}/n_{o}$ with increasing distance from the solar surface caused an effect similar to that of a &lt;a href=&quot;https://en.wikipedia.org/wiki/De_Laval_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;de Laval nozzle&lt;/a&gt;.  There are several other aspects that complicate things (i.e., multiple non-Maxwellian particle distributions), but more recently, &lt;a href=&quot;https://en.wikipedia.org/wiki/Alfv%C3%A9n_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alfvén waves&lt;/a&gt; have been proposed as the possible acceleration mechanism.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why subpopulations of particles in a plasma can become supersonic is less of a mystery.  There are several mechanisms from wave-particle interactions, &lt;a href=&quot;https://en.wikipedia.org/wiki/Fermi_acceleration&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fermi acceleration&lt;/a&gt;, different types of shock acceleration, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once supersonic, a gas does not necessarily have any reason to slow down unless it encounters an obstacle, whether a solid body (e.g., asteroid) or a slower flowing fluid.  Without some impeding force/resistance, there is no reason that a gas cannot maintain a supersonic speed.  Shock waves in a collisional fluid, like Earth's atmosphere, do not last long because they &quot;run into&quot; slower moving fluid, which impedes its propagation.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2015-12-11T00:18:49.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12920" PostTypeId="1" AcceptedAnswerId="12921" CreationDate="2015-12-10T04:07:39.683" Score="19" ViewCount="3101" Body="&lt;p&gt;In the TV show Last Man On Earth we follow a small group of immune survivors after an apocalyptic plague has essentially wiped out humanity. It is revealed that a single astronaut on the ISS has also survived albeit he is not very hopeful about his future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably there is nobody left at NASA, ESA, Roscosmos, or any other agency that would be monitoring and managing space activities. So that leads to my question: what options exist for this lone astronaut? Would it be possible to return to Earth? Does the Soyuz capsule need ground support? Can a single astronaut even leave the ISS in Soyuz by themselves? Obviously astronauts need physical therapy and support after returning from flight so that is also an element to be included in your answer.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="49" LastEditDate="2015-12-10T11:12:46.387" LastActivityDate="2015-12-10T11:12:46.387" Title="Would it be possible to get back to Earth from the ISS without any ground support?" Tags="|iss|crewed-spaceflight|reentry|astronauts|ground-station|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12921" PostTypeId="2" ParentId="12920" CreationDate="2015-12-10T06:55:04.423" Score="22" Body="&lt;p&gt;Yes, Soyuz spacecraft can serve as a lifeboat and return a single crew to Earth. From Anatoly Zak's &lt;a href=&quot;http://www.russianspaceweb.com/soyuz_flight.html&quot;&gt;Russian Space Web page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In case of emergency on the station, the Soyuz can be sent up unmanned&#xA;  or piloted by a single cosmonaut to serve as a lifeboat; or be used as&#xA;  an unmanned cargo ship to return 250 kilograms from orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If for some reason contact with ground control is lost, vehicle can't use ground-based telemetry for guidance, or its autonomous flight mode failed, it will default to a high g ballistic entry. This means it will likely miss its landing site by quite some margin, but it has happened on several occasions before. Soyuz TMA is quite a robust crew return vehicle. Refer e.g. to James Oberg's &lt;a href=&quot;http://www.jamesoberg.com/soyuz.html&quot;&gt;Soyuz Landing Historical Reliability Study&lt;/a&gt; for more details, it sounds nearly impossible in what all circumstances Soyuz managed to return crew back home alive, though deadly accidents during deorbit burn and landing did, sadly, also happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for post landing support, well, if there isn't anyone left to await the returning single person crew and thus no recovery forces to meet him or her at the launch site, that kind of forfeits any physical therapy and post-landing support, too. That person is forced to manage on their own.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a bit of luck however, depending on when the emergency happened, that person wouldn't have been on orbit and in microgravity conditions for a very long time. Deorbit burn will have to happen in up to 200 days after Soyuz TMA was launched into orbit, since that's it's maximum on-orbit standby time. A single crew without all the Mission Control Center's flight controllers keeping an eye on all the sensor readings is also unlikely to maintain the station safe for a very long time, so it would have to be evacuated pretty much the same day as the emergency scenario happened. It also depends on each individual how &lt;a href=&quot;http://www.scientificamerican.com/article/how-does-spending-prolong/&quot;&gt;adverse effects of a prolonged stay in a microgravity environment&lt;/a&gt; will be, and how fast they acclimate back to Earth gravity conditions.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-12-10T06:55:04.423" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12922" PostTypeId="1" AcceptedAnswerId="12923" CreationDate="2015-12-10T09:19:01.107" Score="78" ViewCount="12835" Body="&lt;p&gt;It must be absolutely torturous if your nose starts itching and you can't scratch it for several hours, when you're on EVA. How do astronauts deal with it? Are they undergoing a special training, or take some kind of desensitizing drugs? Or apply some special anti-itching cosmetics?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="4660" LastEditDate="2015-12-12T22:19:00.250" LastActivityDate="2016-07-24T11:29:46.630" Title="How do astronauts on EVA deal with nose itching?" Tags="|eva|spacesuits|astronaut-lifestyle|psychology|" AnswerCount="4" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="12923" PostTypeId="2" ParentId="12922" CreationDate="2015-12-10T09:33:39.700" Score="75" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.reddit.com/r/IAmA/comments/14schr/i_am_astronaut_chris_hadfield_commander_of/c7g0btu&quot;&gt;Chris Hadfield's answer&lt;/a&gt; during &lt;a href=&quot;https://www.reddit.com/r/IAmA/comments/14schr/i_am_astronaut_chris_hadfield_commander_of/&quot;&gt;his Reddit AMA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;We have a squishy thing inside we jam our nose into while we clear our ears — we scratch our nose on that.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;squishy thing&quot; is formally known as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Valsalva_device&quot;&gt;Valsalva device&lt;/a&gt; (used by both astronauts and divers for equalizing pressure in their sinuses. (Thanks to Organic Marble's comment).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, astronauts wearing &lt;a href=&quot;https://en.wikipedia.org/wiki/Orlan_space_suit&quot;&gt;Orlan spacesuits&lt;/a&gt; can actually pull their arm inside the suit to freely scratch an itch. See called2voyage's answer to &lt;a href=&quot;https://space.stackexchange.com/questions/10896/orlan-suit-inside-arm-hand-access&quot;&gt;this question&lt;/a&gt; as well as &lt;a href=&quot;https://web.archive.org/web/20110825114726/http://quiz.esa.int/page.php?id=11276&quot;&gt;this entry&lt;/a&gt; on Thomas Reiter's EVA blog.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="5946" LastEditDate="2015-12-12T20:22:34.827" LastActivityDate="2015-12-12T20:22:34.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12925" PostTypeId="2" ParentId="12922" CreationDate="2015-12-10T10:40:56.377" Score="18" Body="&lt;p&gt;How about Willpower? Every Soldier learns to stand still at a parade or when they got to pledge loyalty. When I had my  inauguration at the Austrian Military forces, we all had to stand still for quite some time (2-3 hours no nose or butt scratching) and it wasn't really a problem. &#xA;Also, if you're concentrated on something serious (like doing work in a completely hostile environment) it is very likely that this distracts you from any upcoming itching.&lt;/p&gt;&#xA;" OwnerUserId="12156" LastActivityDate="2015-12-10T10:40:56.377" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12926" PostTypeId="1" AcceptedAnswerId="12928" CreationDate="2015-12-10T13:23:52.667" Score="23" ViewCount="2640" Body="&lt;p&gt;Let's say that the ISS has to be immediately evacuated and that one of the two Soyuz is broken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the crew quickly remove &quot;unnecessary weight&quot; (seats maybe?) and fit six person to return back to Earth?&lt;/p&gt;&#xA;" OwnerUserId="8925" LastEditorUserId="49" LastEditDate="2015-12-10T15:14:09.230" LastActivityDate="2015-12-30T15:08:48.593" Title="Can a single Soyuz return a crew of six back to Earth?" Tags="|crewed-spaceflight|reentry|landing|soyuz-spacecraft|feasibility|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12927" PostTypeId="1" CreationDate="2015-12-10T13:37:06.870" Score="9" ViewCount="357" Body="&lt;p&gt;Imagine a spacecraft which orbits or follows an object like Dawn, Rosetta and Cassini do today, not being decommissioned and deliberately crashed, but instead put in hibernation. With enough fuel and other consumables left in store for this purpose: To be reactivated for a while whenever useful. For example as the seasons vary or when the next spacecraft approaches, or as has happened on Jupiter and the Sun when a comet strikes. Imagine reactivating the Galileo orbiter to have a look at those European plumes observed from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Are very long term intermittently activated space probes feasible?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meaning that they could work on and off during many decades. JPL already designs some die hard missions. But it seems almost unintentional, with 3 years or so planned lifetimes, but still working a decade and more later. Why not plan for multi-decadal probes, to have every orbiter and lander of object &quot;X&quot; pile up with time, each with its specialized instruments activated briefly as needed. Especially for minisats with limited resources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the problems with this kind of concept? Long term cosmic radiation exposure? Storage of the consumables? Cost of Earth management? Instruments becoming outdated? Could the current mode of &quot;&lt;em&gt;throw-away&lt;/em&gt;&quot; space missions be replaced with &quot;&lt;em&gt;sustainable&lt;/em&gt;&quot; dito?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I completely stole and vandalised this question from &lt;a href=&quot;http://www.science20.com/robertinventor&quot; rel=&quot;noreferrer&quot;&gt;Robert Inventor&lt;/a&gt; asking it &lt;a href=&quot;https://thespaceshow.wordpress.com/2015/12/08/dr-marc-rayman-monday-12-7-15/#comment-53019&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; on the Space Show blog, to give credit to where it belongs.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-12-10T14:21:33.993" LastActivityDate="2017-05-26T15:06:30.550" Title="Why not leave interplanetary probes on standby after primary mission, activating them at will decades later?" Tags="|probe|interplanetary|long-duration|robotic-missions|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12928" PostTypeId="2" ParentId="12926" CreationDate="2015-12-10T13:37:49.030" Score="28" Body="&lt;p&gt;No. And it doesn't even matter that there's no space for a crew of six no matter how small they might be or if they threw away some other pieces and try to make do. The life support of the descent module of the Soyuz TMA can only support up to three people. Since descent time is more or less constant regardless of the descent mode, and there's no way to descent faster after module separation without killing the crew in the process, you could only transport additional crew of three if they don't depend on module's available life support's provisions. I.e. if they're already dead and essentially cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They wouldn't fit anyway, &lt;a href=&quot;http://wsn.spaceflight.esa.int/docs/Factsheets/35%20Soyuz%20LR.pdf&quot; rel=&quot;noreferrer&quot;&gt;Soyuz TMA descent module&lt;/a&gt; can fit 50 kg of cargo for a crew of three or 150 kg for a crew of two, with total capacity of 305 kg. And with habitable volume of only 3.5 cubic meters, it gets pretty tight in there with all the essential equipment, spare chute and astronauts / cosmonauts in their &lt;a href=&quot;https://en.wikipedia.org/wiki/Sokol_space_suit&quot; rel=&quot;noreferrer&quot;&gt;Sokol IVA suits&lt;/a&gt; as it is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/QNxyV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;ISS crew members, U.S. astronaut Daniel Burbank and Russian cosmonauts Anton Shkaplerov and Anatoly Ivanishin,&lt;br/&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;crowded inside their Soyuz capsule shortly after landing in Kazakhstan, on April 27, 2012. (Reuters/Sergei Remezov)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-10T13:55:14.167" LastActivityDate="2015-12-10T13:55:14.167" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="12929" PostTypeId="2" ParentId="12927" CreationDate="2015-12-10T13:46:56.117" Score="5" Body="&lt;p&gt;It has been attempted, but is very difficult. The &lt;a href=&quot;http://www.collectspace.com/news/news-052214a-isee3-reboot-project.html&quot; rel=&quot;noreferrer&quot;&gt;ISEE-3 mission&lt;/a&gt; was put into a hibernation mode where it made a close Earth approach last year. When that close approach happened, it was determined that the spacecraft's thruster did not work, which resulted in a failure to anticipate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More practically, such missions are not usually attempted because hibernation is tough. Getting the location of the spacecraft is difficult, as they tend to drift with time. The electronics degrade, as does the propulsion. Things get more efficient in operations with a continuing operation, as does getting science out of the mission. It's more practical to get as much science out of the mission as can be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, putting a satellite in hibernate that is at a potentially interesting science objective isn't very useful. Hibernate is only done between interesting targets. A mission is only destroyed at the end of the life if it doesn't have enough fuel to continue a useful mission beyond the useful lifetime of the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a more practical sense, how many people know how to use a slide rule today? Or paper tape, or punch cards? We don't know what technology will be like in 20 years, using that technology could become quite tricky. Better technology comes out all the time as well, making it more useful to send updated missions in 20 years if required.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-12-10T13:46:56.117" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12930" PostTypeId="1" CreationDate="2015-12-10T14:10:23.767" Score="14" ViewCount="830" Body="&lt;p&gt;Inspired by &lt;a href=&quot;https://space.stackexchange.com/questions/3010/what-is-the-farthest-a-spacecraft-has-traveled-away-from-earth&quot;&gt;this question&lt;/a&gt;, I've been wondering how far a probe has been from Earth, conducted its mission, then subsequently returned to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm aware of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stardust&lt;/a&gt; missions, but I've not found any information on how far away they travelled, either at maximum distance or during the encounters with Itokawa or the various asteroids and comets of the Stardust mission.  Which of these went further away from Earth, and was either of them the furthest ever, or is there another mission I've not found?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm mainly interested in physical separation from Earth, though distance measured by differences in orbital distance from the Sun would also be interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &quot;returned to Earth&quot; I mean re-entering the atmosphere, either purposefully or accidentally / fatally, not for the many missions that have used Earth for a gravity assist.&lt;/p&gt;&#xA;" OwnerUserId="10686" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2023-09-27T15:12:40.887" Title="What's the furthest a probe has travelled from Earth before returning?" Tags="|probe|record|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12931" PostTypeId="2" ParentId="12927" CreationDate="2015-12-10T14:40:16.870" Score="3" Body="&lt;p&gt;Spacecraft are designed for a mission: &quot;We want to measure X, Y and Z&quot;. The spacecraft is designed to fulfill this objective. For many missions, once you've completed that, the mission is over and there's little point in doing it again. E.g. if the objective is to provide a surface map, photographing the entire surface once is enough, doing it again won't yield new information in many cases (unless the planet has weather or active geology). &#xA;The exceptions are e.g. the Voyagers, which continue to discover new territory. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many spacecraft are in unstable orbits. Missions like Cassini and Rosetta require the spacecraft to correct its course at regular intervals, or it's going to drift away (Rosetta) or crash somewhere (Cassini). So a decades-long mission requires decades worth of fuel and constant monitoring. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's the logistics and funding. If you hibernate a spacecraft for a decade, you need to find something else to do for the science team for that decade, and then be sure the team has something to come back to. I.e. you need continued funding to keep the operations center intact, to keep receivers around for the next time you activate the spacecraft. You need a core team to continue convincing your political masters that funcing this mission remains a good idea, and that is hard when there have been no new results for years because the spacecraft is in hibernation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft becomes more expensive (an RTG that can power all the instruments for 30 years is much larger than one that only needs to work for 5 years, for example.)&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2015-12-10T19:17:40.123" LastActivityDate="2015-12-10T19:17:40.123" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12932" PostTypeId="2" ParentId="12922" CreationDate="2015-12-10T14:56:45.033" Score="17" Body="&lt;p&gt;According to an &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a17/a17.trvsta6.html#1633918&quot;&gt;audio transcript&lt;/a&gt; of the Apollo 17 lunar mission, some astronauts had a Velcro patch somewhere on the inside of the helmet to scratch their nose on.&lt;/p&gt;&#xA;" OwnerUserId="12579" LastActivityDate="2015-12-10T14:56:45.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12933" PostTypeId="1" AcceptedAnswerId="13035" CreationDate="2015-12-10T15:18:03.493" Score="4" ViewCount="248" Body="&lt;p&gt;I've been researching what drives the cost of spacecraft, to see why many spacecraft still use very antiquated electronics and power systems developed in the 1980's instead of up to date electronics and power systems. It seems the cost (I.E. man-hours) needed to qualify the new components outweighs the cost savings from buying off the shelf. I researched more deeply into why qualification testing is expensive, and it seems the main reason for high costs is the parts are put through a variety of what-if scenarios on Earth through simulated environments instead of just putting the part into the actual environment they'll be operating in. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That got me to thinking if cubesats could be launched with up-to-date parts to qualify them for use on future spacecraft by having them launch through the high vibration environment of a rocket, bake and cool in orbit, and deal with hard vacuum and radiation, taking advantage of being there instead of meticulously simulating it on the ground. The parts would be launched into orbit without the extensive ground qualification testing, instead doing a simple check to see if they work or not in orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I understand I may be overestimating the cost of qualification testing, if this is, please correct this misunderstanding. But if it is very expensive, have cubesats been used to build up a list of new electronics, power systems, valves, etc. that have been proven to work in space through flights? Or would this be more expensive than testing them on the ground?&lt;/p&gt;&#xA;" OwnerUserId="11377" LastActivityDate="2015-12-16T23:38:45.270" Title="Can cubesats be used to qualify parts for spaceflight more cheaply than ground testing?" Tags="|spacecraft-development|cubesat|qualification-testing|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12934" PostTypeId="2" ParentId="12933" CreationDate="2015-12-10T20:38:40.963" Score="5" Body="&lt;p&gt;In qualification testing, you want to be able to examine the parts after the test. If you can learn why a part failed, you can improve the design.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also becomes harder to identify the cause of failure since you're doing all your tests at once. Was it a supply voltage spike, temperature changes or a cosmic ray that made the part fail? Again, not being able to examine the part afterward makes this harder to determine.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't control all the environmental factors. A debris strike can disable the part you're testing and all you know is 'it stopped working'. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-10T20:38:40.963" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12935" PostTypeId="1" AcceptedAnswerId="12958" CreationDate="2015-12-11T00:37:20.717" Score="13" ViewCount="1832" Body="&lt;p&gt;SpaceX has &lt;a href=&quot;http://spaceflightnow.com/2015/09/25/first-static-fire-completed-on-upgraded-falcon-9/&quot; rel=&quot;nofollow noreferrer&quot;&gt;announced&lt;/a&gt; an upgrade to their Falcon 9 rocket, achieving greater thrusts with, as far as I can tell, the same rocket motors (at least, they apparently still call the motors &quot;Merlin 1D&quot;). See &lt;a href=&quot;https://space.stackexchange.com/questions/8009/what-is-changing-between-the-merlin-1d-engine-and-the-uprated-merlin-1d-engine&quot;&gt;this question&lt;/a&gt;, which had no conclusive answer on engine changes. The change takes the Merlin 1D from 147K lb thrust at sea level to 170K lb, an increase of 15.6%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The increased thrust is apparently due to switching to using extremely chilled fuel (RP-1 kerosene) and oxygen. Not just liquid, but barely above the temperature at which it would freeze solid. This increases the density of the propellants, presumably allowing more mass of fuel to be contained in the tanks and increasing the flow rate (mass per unit time) through the turbopumps and rocket nozzles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, why are they only doing this now? 15% more thrust from the same engines, and increased fuel capacity (even though it also increases launch weight), seem like highly desirable changes. What are the risks of this approach that led to it not being used previously?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few guesses:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is the change in propellant temperature sufficient to significantly increase the risk of failures in components that must handle cryogenic fluids?&lt;/li&gt;&#xA;&lt;li&gt;Is it significantly harder to build spacecraft-usable tanks that can hold near-frozen O2 (and RP-1), instead of &quot;merely&quot; liquid O2?&lt;/li&gt;&#xA;&lt;li&gt;Does increasing the mass/time flow rate through the motors (assuming that's actually what's happening) pose a greater risk to the motors? If so, does the fact that the motors can take it imply they were overbuilt for the old propellant density? (Merlin 1D already had the highest T/WR of any production liquid-fueled rocket motor.)&lt;/li&gt;&#xA;&lt;li&gt;Are there other changes that are required to support this upgrade, and was engineering those changes difficult or very costly?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;While I'm obviously hoping to get as full a picture as possible, feel free to post partial answers (i.e. one thing you know of that poses a significant challenge) as answers rather than comments.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="11532" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-12-21T12:31:53.713" Title="What are the challenges in Falcon 9 &quot;full thrust&quot; (v1.2) with densified propellant?" Tags="|spacex|propulsion|falcon-9|fuel|merlin-1d|" AnswerCount="5" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12936" PostTypeId="1" AcceptedAnswerId="13037" CreationDate="2015-12-11T02:25:20.630" Score="4" ViewCount="363" Body="&lt;p&gt;From what I have read and from what I have heard from people who have taken a spacecraft through from design to flight, qualification testing is very expensive and time consuming, and cite this as the reason parts and systems don't evolve quickly in the space industry. However, I've never gotten a clear detailed answer as to why qualification tests are so expensive (and time consuming). Is it because of the need to operate at high vacuum, and those vacuum chambers/vacuum pumps require a lot of maintenance to keep working properly? Or is it because a person needs to be involved to place sensors in the proper locations and interpret the data coming from the sensor?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is the case, why can't qualification testing be automated?&lt;/p&gt;&#xA;" OwnerUserId="11377" LastActivityDate="2015-12-17T00:13:25.330" Title="Why are spacecraft qualification tests expensive and time consuming?" Tags="|spacecraft-development|qualification-testing|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12937" PostTypeId="2" ParentId="7967" CreationDate="2015-12-11T04:23:48.510" Score="2" Body="&lt;p&gt;In one particular case, astronauts staged a strike when they felt they were being overworked with a schedule filled with tedious tasks without break.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During its 84 day mission in 1973-1974, the crew of Skylab 4, consisting of &#xA;Gerald P. Carr, Edward G. Gibson, and William R. Pogue complained of exhaustion from the endless tedium of chores assigned by ground controllers. Six weeks into the mission things came to a head and the crew decided to take an unscheduled vacation day. They turned off the communications radio with Mission Control during this time and spent the day relaxing and taking in the view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Following this &quot;strike&quot; NASA agreed to decrease their workload and not disturb the crew during meals and rest periods. The astronaut's productivity subsequently improved and the remainder of the mission passed uneventfully. But apparently NASA was quite upset about the sit-down strike, and (petulantly in my opinion) made sure Pogue, Carr, and Gibson would never fly in space again. &lt;/p&gt;&#xA;" OwnerUserId="12583" LastEditorUserId="12583" LastEditDate="2015-12-11T04:45:44.983" LastActivityDate="2015-12-11T04:45:44.983" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12939" PostTypeId="1" CreationDate="2015-12-11T09:33:05.373" Score="3" ViewCount="1242" Body="&lt;p&gt;My basic understanding of what achieves orbit is: &#xA;The gravitational pull of the larger object (Earth) + the natural force of the mass object moving forward (The Moon) = orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this being true, how come there are different levels of orbit, like the Low Earth Orbit which I keep reading about? Doesn't the mass of the satellite dictate the height/location of the orbit?&lt;/p&gt;&#xA;" OwnerUserId="10712" LastActivityDate="2015-12-12T12:07:43.457" Title="How can there be different heights of orbit?" Tags="|orbit|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12941" PostTypeId="2" ParentId="12939" CreationDate="2015-12-11T11:19:39.860" Score="0" Body="&lt;p&gt;Orbits are possible due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifugal_force&quot; rel=&quot;nofollow&quot;&gt;centrifugal force&lt;/a&gt;. The faster something is going (in &lt;em&gt;angular velocity&lt;/em&gt;, which you can think of as &quot;the speed at which it goes around something&quot; or &quot;the times it goes around a thing in a minute/day/year/whatever&quot;), the stronger the centrifugal force and the closer to earth you can orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass doesn't matter, or rather it cancels itself out. There's a mass term in the formula for centrifugal force: &lt;strong&gt;F = m * [d^2 &lt;em&gt;r&lt;/em&gt;]/[d t^2]&lt;/strong&gt; which means &lt;strong&gt;F&lt;/strong&gt;orce = &lt;strong&gt;m&lt;/strong&gt;ass times the &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_velocity&quot; rel=&quot;nofollow&quot;&gt;angular velocity&lt;/a&gt; (the change in direction around something divided by time, just like normal velocity is the change of position divided by time), squared (meaning multiplied by itself). Angular velocity is often written as a lower-case Greek letter omega: &lt;strong&gt;ω&lt;/strong&gt;. Thus, we can re-write the formula for centrifugal force as &lt;strong&gt;F = m * ω * ω&lt;/strong&gt;. There's also a mass term (actually, two of them, one for each object, but for this purpose we treat the earth as fixed and are only concerned about the satellite's mass) in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow&quot;&gt;formula for gravity&lt;/a&gt;: &lt;strong&gt;F = G * m1 * m2 / r^2&lt;/strong&gt; which means that the &lt;strong&gt;F&lt;/strong&gt;orce of gravity is equal to the &lt;strong&gt;G&lt;/strong&gt;ravitational constant (you need to know this to determine the actual force as a number, but for now, just ignore it) times the &lt;strong&gt;m&lt;/strong&gt;ass of the &lt;em&gt;first&lt;/em&gt; object (earth) times the &lt;strong&gt;m&lt;/strong&gt;ass of the &lt;em&gt;second&lt;/em&gt; object (the satellite), all divided by the &lt;strong&gt;r&lt;/strong&gt;adius (the distance between the objects' centers) squared (multiplied by itself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to orbit at a specific height (commonly called an &quot;altitude&quot;), your outward (centrifugal) force must equal your inward (gravitational) force. Thus, to orbit at height &lt;strong&gt;r&lt;/strong&gt; (from the center of the earth) you solve this formula: &lt;strong&gt;m * ω^2 = G * m1 * m2 / r^2&lt;/strong&gt;. Since the &lt;strong&gt;m&lt;/strong&gt;ass of the satellite is on both sides of the equation, you can simply divide both sides of the equation by that mass - it doesn't matter what it is! - and you have a simpler equation that gives the same result: &lt;strong&gt;ω^2 = G * m1 / r^2&lt;/strong&gt;. If you plug in all the numbers into that, it will tell you how fast &lt;em&gt;anything&lt;/em&gt; needs to be going around the earth in order to orbit the Earth at a given height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since gravity falls off rapidly as you go further out - something twice as far as you from the center of the earth experiences only one fourth as much gravity - stuff that is relatively close to earth (things in low earth orbit, such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow&quot;&gt;International Space Station&lt;/a&gt;) have to go around the earth really fast; the ISS (and everything else at the same height) does it in about 90 minutes. That's only a few hundred miles (or a few hundred kilometers) above the earth's surface. Many times that far away - about 26.2 thousand miles, or 46.2 thousand kilometers - stuff &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;nofollow&quot;&gt;orbits in exactly one day&lt;/a&gt;, which means they always pass over the same spot on the equator every day. If they orbit over the equator directly, they don't appear (from those of us on earth) to move at all. This is useful for communication and TV satellites; you can point a fixed satellite dish at them. Much further out still, at 385 thousand kilometers, you have the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_of_the_Moon&quot; rel=&quot;nofollow&quot;&gt;orbit of the moon&lt;/a&gt;, which takes about 27.3 days (just shy of four weeks). These are just three examples of different orbital heights around earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can demonstrate this easily yourself if you have a yo-yo or similar (really, any string with a weight on the end). Start with a long section of string, at least two feet long (60 cm) and spin it around your hand &lt;em&gt;just&lt;/em&gt; fast enough to keep the string tight and have the weight make a full circle. Now shorten the string a bunch, to maybe eight inches (20 cm) and again, spin it fast enough to make a full circle. You should notice that, with the shorter string, you have to spin the object a &lt;em&gt;lot&lt;/em&gt; faster to keep the string tight, but you &lt;em&gt;can&lt;/em&gt; keep it tight at the different distances. The tension in the string stands in for gravity, here; if the string is loose, gravity would overcome your centrifugal force and crash your satellite. If you spin (orbit) so fast that the string is under too much tension and breaks, that's like going so fast you leave earth's orbit (though unlike a string, gravity never &quot;breaks&quot;; it's more like it would just stretch out, letting the satellite fly away and exerting a weaker and weaker pull to try and bring it back).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope that helps!&lt;/p&gt;&#xA;" OwnerUserId="11532" LastActivityDate="2015-12-11T11:19:39.860" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12942" PostTypeId="1" AcceptedAnswerId="12943" CreationDate="2015-12-11T14:59:31.490" Score="5" ViewCount="868" Body="&lt;p&gt;Why does ISS have &lt;a href=&quot;https://space.stackexchange.com/questions/1527/are-there-any-humanoid-robots-on-board-the-iss&quot;&gt;two humanoid robots&lt;/a&gt; (rather impractical for microgravity) instead of something not human shaped?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A robot with &quot;RCS&quot; composed of a set of propellers could efficiently navigate the inside of ISS, aiding with &quot;fetch&quot; tasks, inspections and experiments (which could be performed remotely from Earth through the robot instead of involving the overworked crew) and essentially be of broad use as a mobile unit, unlike the robots on the station which are working in fixed locations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A small remote-controlled drone with some simple rocket engines (hydrazine, ion, even compressed gas), reaction wheels, camera, and a set of tools/manipulators could be stationed permanently on the outside of ISS and save weeks of cumbersome EVA time by performing inspections, simple repairs, assisting astronauts during EVA, or even flying a couple kilometers away for photoshoot opportunities, examining (or capture) of objects like the loose &lt;a href=&quot;http://www.spacesafetymagazine.com/news/cover-breaks-iss-floats/attachment/iss-debris-antenna-cover/&quot; rel=&quot;nofollow noreferrer&quot;&gt;antenna cover&lt;/a&gt; - or even serve as a makeshift &lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;EMU&lt;/a&gt; unit to pull astronauts around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dedicated manipulators with tools would be more useful than fake arms, never mind the completely useless legs. When I read Chris Hadfield's AMA, I couldn't help an impression he was &lt;a href=&quot;https://www.reddit.com/r/IAmA/comments/14schr/i_am_astronaut_chris_hadfield_commander_of/c7fzkwh&quot; rel=&quot;nofollow noreferrer&quot;&gt;underwhelmed&lt;/a&gt; by KIROBO and the Robonaut. OTOH, he was quite enthusiastic about Canadarm 2, which is a genuine help for the astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why the decision to go with totally impractical &quot;humanoid&quot; option?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-12-12T06:45:30.140" Title="Why a humanoid robot and not a dedicated drone style one?" Tags="|iss|robotics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12943" PostTypeId="2" ParentId="12942" CreationDate="2015-12-11T15:30:55.037" Score="10" Body="&lt;p&gt;There are other robotic tools similar to what you describe being developed for assisting astronauts such as the &lt;a href=&quot;http://ssl.mit.edu/spheres/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SPHERES&lt;/a&gt; project at MIT.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary reason &lt;a href=&quot;http://robonaut.jsc.nasa.gov/default.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robonaut&lt;/a&gt; is humanoid is that it can essentially use tools and grab onto components exactly how an astronaut would -- meaning no special considerations are required when designing components for servicing (if it can be serviced by an astronaut then it can be serviced by Robonaut). To quote NASA more elegantly:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The value of a humanoid over other designs is the ability to use the same workspace and tools - not only does this improve efficiency in the types of tools, but also removes the need for specialized robotic connectors.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Another reason is out of some concern that astronauts need a robot that is less intimidating and easier to work alongside. Quoting &lt;a href=&quot;http://link.springer.com/article/10.1023%2FA%3A1022231703061&quot; rel=&quot;nofollow noreferrer&quot;&gt;this research paper&lt;/a&gt; by the creators:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Having a head on Robonaut gives human co-workers a feel that Robonaut is more human-like, allowing it to more readily become one of the team.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In my opinion this is not the way to go, but NASA insists that there is a major psychological barrier when it comes to astronauts trusting and working with robots. As far as I have seen, Chris Hadfield's similar reaction is common among the astronaut corps. Canadarm, Canadarm 2, and especially &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/iss/dextre/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dextre&lt;/a&gt; are entirely invaluable for operations on the ISS and supporting EVA servicing -- the astronauts work with them fine and they aren't humanoid in the least bit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who would you trust?...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yStsE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yStsE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MoviF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MoviF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tKVOy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tKVOy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="238" LastEditDate="2015-12-12T06:45:30.140" LastActivityDate="2015-12-12T06:45:30.140" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="12944" PostTypeId="2" ParentId="3523" CreationDate="2015-12-11T22:11:05.177" Score="7" Body="&lt;p&gt;When the sun goes down on the Moon, Earth is waxing and at half full as seen from its surface on the near side. At lunar dawn Earth is a waning 'half-Earth', if you will. This is noticeable to us in the way you can see the full disc of a young waxing crescent Moon in the early evening, because Earth is lighting it up. This is known as earthshine.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://webserver.dmt.upm.es/%7Eisidoro/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Isidoro Martinez&lt;/a&gt; wrote an overview of the &lt;a href=&quot;http://webserver.dmt.upm.es/%7Eisidoro/tc3/Space%20environment.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;space environment&lt;/a&gt; that has this to say on page 26:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Earth has a larger albedo than the Moon &lt;span class=&quot;math-container&quot;&gt;$(\rho_E/\rho_M=0.30:0.12=2.5)$&lt;/span&gt;, and larger size &lt;span class=&quot;math-container&quot;&gt;$((R_E/R_M)^2=(6370/1740)^2=13.4)$&lt;/span&gt;. An observer on the Moon will see ‘full Earth’ (at new moon) 4 times wider&#xA;than the Sun disc, and 100 times brighter than we see full moon,&#xA;corresponding to an apparent magnitude (if the observer reference&#xA;point is exchanged in the definition of apparent magnitude) of &lt;span class=&quot;math-container&quot;&gt;$m= -17.7$&lt;/span&gt;&#xA;(as can be checked from the full moon magnitude, &lt;span class=&quot;math-container&quot;&gt;$m= -12.7$&lt;/span&gt;, and the&#xA;‘brightness’ ratio, &lt;span class=&quot;math-container&quot;&gt;$I_\text{E}/I_\text{M}=2.512^{-(mE-mM)}=2.512^5=100)$&lt;/span&gt;; notice that the&#xA;illuminance it creates on the lunar surface is around &lt;span class=&quot;math-container&quot;&gt;$20\ \text{lx}$&lt;/span&gt;, a good&#xA;value for ‘ambient light’ in a living room, and for outdoor night&#xA;lighting (Apollo 8 astronauts described relief features flying over&#xA;the dark side of the Moon); full moon shine yields around &lt;span class=&quot;math-container&quot;&gt;$0.25\ \text{lx}$&lt;/span&gt; on&#xA;the Earth surface (although it may reach &lt;span class=&quot;math-container&quot;&gt;$1\ \text{lx}$&lt;/span&gt; at great altitudes near&#xA;the equator); good reading light is about &lt;span class=&quot;math-container&quot;&gt;$200\ \text{lx}$&lt;/span&gt; (up to &lt;span class=&quot;math-container&quot;&gt;$2000\ \text{lx}$&lt;/span&gt; for&#xA;precision work, matching natural diffuse light; maximum Sunlit yields&#xA;nearly &lt;span class=&quot;math-container&quot;&gt;$100\ 000\ \text{lx}$&lt;/span&gt;). Notice also that the brightness ratio &lt;span class=&quot;math-container&quot;&gt;$(100:1)$&lt;/span&gt; does&#xA;not coincide with the product of Bond albedo ratio &lt;span class=&quot;math-container&quot;&gt;$(0.30:0.12=2.5)$&lt;/span&gt;&#xA;times area ratio &lt;span class=&quot;math-container&quot;&gt;$(13.4)$&lt;/span&gt;, which is &lt;span class=&quot;math-container&quot;&gt;$2.5·13.4 =34$&lt;/span&gt;, because of atmospheric&#xA;absorption and directional effects.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Monitoring of earthshine from here on Earth has been used to monitor changes in Earth's albedo. Such study reveals that its albedo is rather variable depending on the part of the planet that is visible. The &lt;a href=&quot;http://astrogeo.oxfordjournals.org/content/49/3/3.15/F6.expansion.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;graph&lt;/a&gt; is from &lt;a href=&quot;http://astrogeo.oxfordjournals.org/content/49/3/3.15.full&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earthshine: not just for romantics&lt;/a&gt;. It is based on a mathematical model which takes into account the difference in albedo of land versus ocean, but ignores clouds because modeling for them is much more complex. Ocean is much darker than most land (except where the sun is glinting off it towards the observer). Modeled like this, when the Pacific is facing the Moon the planet reflects half as much light as it does over Eurasia and Africa. Differences are especially big when the Moon is crossing its ascending or descending node, aligned with Earth's equator so the polar ice caps aren't very visible. The clouds distributed over the Earth dampen this variation a lot though. At any rate, the graph shows that the range of results over a month is from near 0 to 0.06 W/m&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Iwrdk.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Iwrdk.gif&quot; alt=&quot;earthshine intensity over a month&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Peter Thejll, Chris Flynn, Hans Gleisner and Andrew Mattingly&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;The image below shows the albedo of all of Earth's land masses, with data taken by the &lt;a href=&quot;http://visibleearth.nasa.gov/view.php?id=60636&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moderate Resolution Imaging Spectroradiometer.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TXtrl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TXtrl.jpg&quot; alt=&quot;albedo of earth land masses&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Credit: Image courtesy Crystal Schaaf, Boston University, based upon data processed by the MODIS Land Science Team&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CWaXG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CWaXG.jpg&quot; alt=&quot;albedo of different substances&quot; /&gt;&lt;/a&gt;&#xA;&lt;sup&gt;From &lt;a href=&quot;http://www.eoearth.org/view/article/149954/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Encyclopedia of the Earth: Albedo&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;Unlike the values in lux mentioned in the Martinez overview, these earthshine studies measure the energy of the light reaching the Moon. The studies seek to understand how climate and albedo interact, and so need to see all the energy in earthshine. The &lt;a href=&quot;http://archive.sbig.com/pdffiles/STL_catalog2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;camera used&lt;/a&gt; in the study quoted above extends into the near infra-red range, and the energy from that invisible range would accounts for about a third of the values it obtains. The peak result in that graph is closely matched by simply multiplying Earth's average albedo, its projected area, and the energy in sunlight at this distance from the Sun, and dividing that by the area of a hemisphere at the average distance of the Moon's orbit. That takes the energy reflected off the sunlit half of the planet and spreads it out to what it would be at some spot at the distance of the Moon.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac {(0.3) \pi (6.37 * 10^6)^2\,m^2)(1366\, W/m^2)}{\pi (3.85 * 10^8)^2\,m^2} = 0.112\;W/m^2$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;" OwnerUserId="4660" LastEditorUserId="58" LastEditDate="2022-06-08T16:18:07.407" LastActivityDate="2022-06-08T16:18:07.407" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12946" PostTypeId="1" AcceptedAnswerId="12947" CreationDate="2015-12-12T06:37:13.447" Score="4" ViewCount="141" Body="&lt;p&gt;With regards to project Orion the nuclear pulse engine,&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Would the fallout from the nukes detonated in space remain in that corridor that the orion spacecraft passed?&lt;/li&gt;&#xA;&lt;li&gt;Would it be pushed away by the solar wind?&lt;/li&gt;&#xA;&lt;li&gt;Would it fall behind the movement of our solar system around the center of Milky Way?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="11671" LastActivityDate="2015-12-12T07:07:02.953" Title="Fallout in space" Tags="|radiation|nuclear-pulse-propulsion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12947" PostTypeId="2" ParentId="12946" CreationDate="2015-12-12T07:07:02.953" Score="7" Body="&lt;p&gt;First consider what fallout actually &lt;em&gt;is&lt;/em&gt;: basically any material blown into the upper atmosphere during a nuclear blast -- which eventually &lt;em&gt;falls out&lt;/em&gt; of the sky. Essentially fallout is a bunch of ejecta flying outwards from the epicentre of the explosion. (In fact, fallout in space is kind of a contradiction, but let's ignore that!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what happens to a bunch of ejecta flying outwards from an explosion in space? The answer to that becomes obvious if you simply consider each particle separately and dust off your old celestial mechanics textbook. Each particle will have its own orbital trajectory, with most of them flying backwards in a lower orbit than the spacecraft (this is how all rocket motors generate thrust, through momentum transfer). Some will also be tossed outwards laterally into more eccentric or inclined orbits, but it will not simply remain suspended in the vehicle's wake.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, solar wind &lt;em&gt;will&lt;/em&gt; most certainly have an effect. However, it is difficult to say whether or not that effect is significant enough that you could easily discern it from the effects of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, it depends on how close you are to the nearest celestial bodies, but the fallout will generally enter into orbits around whatever central body the vehicle is traveling around. So if you were traveling in interplanetary space the fallout would orbit the Sun, whereas if you were traveling in interstellar space then the fallout would orbit the Milky Way. Some may fall back to the solar system (hey, maybe fallout in space isn't quite contradictory!) but yes, it would also fall behind the solar system.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-12-12T07:07:02.953" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12949" PostTypeId="2" ParentId="12939" CreationDate="2015-12-12T12:07:43.457" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Doesn't the mass of the satellite dictate the height/location of the orbit?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, at least not to first order. There is some effect in the case of objects whose mass is close to that of the object it is orbiting, such as the Moon orbiting the Earth. I'll deal with that later. I'll focus at first on objects that are many orders of magnitude smaller than the object they are orbiting. For example, large as it is, the mass of the International Space Station is less than 10&lt;sup&gt;-19&lt;/sup&gt; Earth masses. The mass of the Space Station plays no role in determining it's orbit. The key to determining the orbit of some satellite about a primary (e.g., the Space Station about the Earth, or the Earth about the Sun) is the satellite's velocity with respect to the primary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no need to invoke fictitious forces such as centrifugal force to explain why things orbit. The first thing to do to understand orbits is to look to Newton's first law of motion. A satellite's orbit is a curved path (i.e., not a straight line) about the primary. Since the path is curved rather than a straight line, there must necessarily be some net force acting on the satellite to make it follow that path. That force is of course gravitation. No other forces are needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine an airless, non-rotating planet with a cannon that shoots cannonballs horizontally mounted atop the planet's tallest mountain. (This is Newton's cannon.) If you use but a small amount of gunpowder to fire a cannonball, the cannonball will appear to follow a parabolic arc, just as you learned as a kid. If you use more gunpowder you'll find that the path is no longer parabolic. It is instead a segment of an ellipse. If you use even more gunpowder, the ellipse will no longer intersect the planet's surface. The cannonball is in orbit. If you use just the right amount of gunpowder, the cannonball will orbit at a constant height above the planet. This is a circular orbit. (But note that a circle is just a special case of an ellipse.) With even more gunpowder, the orbit is once again elliptical, but now the top of the mountain is the closest point on the cannonball's trajectory about the planet. Finally, if you use more gunpowder yet, the cannonball will have enough energy to escape the gravitational clutches of the planet. In this case, the path is a parabola or hyperbola rather than an ellipse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose you can raise the cannon hundreds or even thousands of kilometers above the top of the mountain. As you do so, you'll find that the amount of gunpowder needed to have a cannonball orbiting circularly about the planet decreases. The circular orbit velocity is solely a function of the planet's mass $M$ and the radial distance $r$ from the center of the planet, $v_\text{circ} = \sqrt{\frac{GM} r}$, where $G$ is Newton's universal gravitational constant. You'll also find that the escape velocity similarly decreases with radial distance, $v_\text{escape} = \sqrt{2 \frac{GM} r}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, what about the Moon? Unlike the Space Station, the Moon's mass compared to that of the Earth is not negligible. It is more than 1/100 of the Earth's mass. (To be more precise, it's about 0.0123 Earth masses, a conveniently easy number to remember.) The above expression for the circular orbital velocity is not quite correct. In this case, it's better to use $v_\text{circ} = \sqrt{\frac{G(m_1+m_2)} r}$, where $m_1$ and $m_2$ are the masses of the primary and the satellite.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-12-12T12:07:43.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12950" PostTypeId="2" ParentId="12930" CreationDate="2015-12-12T16:13:10.153" Score="7" Body="&lt;p&gt;The best I could find was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_(spacecraft)&quot;&gt;Stardust mission&lt;/a&gt;, which went a little over ~&lt;a href=&quot;http://stardust.jpl.nasa.gov/mission/scnow.html&quot;&gt;2.5-3.0&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_unit&quot;&gt;AU&lt;/a&gt; away from Earth before returning (it landed so dust from comet &lt;a href=&quot;https://en.wikipedia.org/wiki/81P/Wild&quot;&gt;Wild 2&lt;/a&gt; could be analyzed).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are more examples of previous and potential future missions on &lt;a href=&quot;https://en.wikipedia.org/wiki/Sample_return_mission&quot;&gt;this&lt;/a&gt; Wikipedia page as well.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2015-12-12T16:13:10.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12951" PostTypeId="2" ParentId="10875" CreationDate="2015-12-12T17:01:15.150" Score="7" Body="&lt;p&gt;&lt;a href=&quot;http://www.nakedeyeplanets.com/mars-seasons.htm#duststorms&quot; rel=&quot;noreferrer&quot;&gt;This reference&lt;/a&gt; provides this handy table:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Tc0oG.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Tc0oG.gif&quot; alt=&quot;dust storm table&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There hasn't been a global dust storm since 2007.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www-mars.lmd.jussieu.fr/oxford2014/abstracts/kass_dust_oxford2014.pdf&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; classifies regional dust storms in Martian years without a global dust storm into three categories, with starting L&lt;sub&gt;S&lt;/sub&gt;'s of 210° to 240°, 245° to 260°, and 305° to 320°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that we have been watching dust storms on Mars for a limited time, so the data is for a relatively small number of events. We can draw conclusions from what we have seen, but any one dust storm is a weather event and so can easily fall outside of the bounds of what we have seen before.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-12-12T17:07:40.113" LastActivityDate="2015-12-12T17:07:40.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12953" PostTypeId="1" AcceptedAnswerId="12954" CreationDate="2015-12-12T22:32:19.097" Score="14" ViewCount="1746" Body="&lt;p&gt;How big would a nuclear explosion in open space around 1 AU from Proxima Centauri have to be for it to be detectable with current technology? Assume best-case scenarios, such as we're specifically looking for it, it's at the closest approach and best angle, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to clarify, I'm more interested in an answer based on existing technology.&lt;/p&gt;&#xA;" OwnerUserId="8641" LastEditorUserId="6" LastEditDate="2015-12-13T17:16:43.200" LastActivityDate="2017-10-31T20:50:08.133" Title="Could we detect a nuclear explosion near Proxima Centauri?" Tags="|astronomy|nuclear|seti|technomarkers|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12954" PostTypeId="2" ParentId="12953" CreationDate="2015-12-12T22:49:41.230" Score="16" Body="&lt;p&gt;&lt;strong&gt;Assuming best case?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://nuclearweaponarchive.org/Nwfaq/Nfaq5.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; page, about half the energy from a normal nuclear explosion is radiation. That means we can simplify it to all energy radiated for an order of magnitude estimate.&#xA;For reference I will use the Tzar bomba, the largest nuclear weapon ever tested. It was about 50 megatons worth of TNT. According to &lt;a href=&quot;http://www.unitconversion.org/energy/joules-to-megatons-conversion.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;unit conversion&lt;/a&gt;&#xA;, a megaton of TNT is about $4.18 \cdot 10^{15} J$.&#xA;50 megatons is then around 200 petajoules.&#xA;Alpha Centauri is about $3.9 \cdot 10^{16}$ meters away, so by dividing all that energy on a shell with such a radius, 0.0014 Joules would hit the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this too small to detect?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming an average of $10^{-19}$ Joules per photon, we get about 100 photons per square meter.&#xA;According to &lt;a href=&quot;https://physics.stackexchange.com/questions/73687/how-many-watts-of-light-do-instruments-in-modern-telescopes-need&quot;&gt;this answer to a related question&lt;/a&gt;, modern sensors can detect single photons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if we were looking for it, we could maybe detect it.&#xA;However, detecting single photons in the flash light from Alpha Centaur, is a real best case scenario.&#xA;A star emits photons over the whole spectrum, so we can not sort out which of them came from the nuclear explosion and which from the star.&#xA;So the real challenge is moving the explosion away from the star, not making it bright enough.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2016-01-31T22:31:23.300" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12955" PostTypeId="2" ParentId="12953" CreationDate="2015-12-12T23:26:30.210" Score="17" Body="&lt;p&gt;If you can detect unusually large quantities (superabundance) of Xe-129 or higher &lt;a href=&quot;https://en.wikipedia.org/wiki/Isotopes_of_xenon&quot;&gt;isotopes of Xenon&lt;/a&gt; that would naturally only be present in trace amounts without explosions of thermonuclear weapons, then yes. Potentially even millions of years after the event (Xe-129 has a half-life of 16 million years). It is hard to estimate what weapon yield would be detectable at what distance, since that also depends on sensitivity of equipment used, but high sensitivity &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_spectroscopy&quot;&gt;astronomical spectroscopy&lt;/a&gt; is something within our reach and to some extent already used to such precision to detect chemical composition of exoplanetary atmospheres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Emissions of visible light, heat, and in radio frequencies by such weapons would likely wash out in the glare of the star and remain undetectable compared to star's own emission &lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_star&quot;&gt;variability&lt;/a&gt; in both intensity and spectral emission lines or simply producing large enough flares though, unless you happen to have a large orbital &lt;a href=&quot;https://en.wikipedia.org/wiki/Occulting_disk&quot;&gt;occulting disk / coronograph&lt;/a&gt; deployed with telescopes pointing directly at the event, or we're talking of weapon yield in the order of being capable of nuking a whole planetary system with a single blow. Let's call it &lt;em&gt;nova class&lt;/em&gt; for the fun of it, since it would produce a debris cloud not unlike one produced by a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nova&quot;&gt;nova&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's an, erm, &lt;em&gt;interesting&lt;/em&gt; related document you could read that discusses detection of thermonuclear explosions after the fact, written by John E. Brandenburg and titled &lt;a href=&quot;http://www.scribd.com/doc/248404603/Evidence-of-a-Massive-Thermonuclear-Explosion-on-Mars-in-the-Past-The-Cydonian-Hypothesis-and-Fermi-s-Paradox&quot;&gt;Evidence of a Massive Thermonuclear Explosion on Mars in the Past, The Cydonian Hypothesis, and Fermi’s Paradox&lt;/a&gt;. It's a tad eccentric, but nonetheless an interesting read. And the science of detecting such weapons is sound, even if it might be better to take conclusions drawn with ample reservation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another possibility is looking for flashes of gamma rays produced by nuclear weapon detonations where we know such emissions shouldn't exist naturally (say, no background &lt;a href=&quot;https://en.wikipedia.org/wiki/Gamma-ray_burst&quot;&gt;gamma-ray burst&lt;/a&gt; emitters). And such explosions, if they occurred on planetary surfaces, would result in other changes to their atmospheres, such as change in their opacity due to onset of nuclear winter, and similar &lt;a href=&quot;https://en.wikipedia.org/wiki/Technosignature&quot;&gt;technosignatures&lt;/a&gt; we might be able to detect given long enough observations. See e.g. &lt;a href=&quot;http://arxiv.org/ftp/arxiv/papers/1507/1507.08530.pdf&quot;&gt;Observational Signatures of Self-Destructive Civilisations&lt;/a&gt;, A. Stevens et al. (PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this paper, rephrasing, we're currently capable or have good prospects of soon becoming capable of (with launch and deployment of &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope&quot;&gt;JWST&lt;/a&gt;) detecting signatures of active or dead civilizations that are or were using nuclear weapons within 5 years between destructive event and observations, and for planetary scale nuclear annihilation around nearest stars, by using gamma ray detection and transit spectroscopy techniques.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-13T00:32:51.790" LastActivityDate="2015-12-13T00:32:51.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12956" PostTypeId="1" CreationDate="2015-12-12T23:31:06.247" Score="1" ViewCount="250" Body="&lt;p&gt;Calculating the regular synodic period of two orbiting bodies is pretty straight forward, but what if I wanted to know when they would meet in a specific position? For example if they share a periapse. (By &quot;same position&quot; I mean that the two orbiting objects and the central body are on the same line)&lt;br&gt;&#xA;I know this only applies if you can express one of the bodies period as a fraction of the other, so a 1 and a sqrt(2) orbit would never sync.&lt;br&gt;&#xA;But what if I decided an acceptable margin? (like 10 degrees)&lt;br&gt;&lt;br&gt;&#xA;Currently I am brute forcing this.&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;My question is:&lt;/strong&gt; Is it a way to calculate the next time two bodies would meet each other at the same location, given their orbital period and an acceptable error?&lt;br&gt;&lt;br&gt;&#xA;The objects does not have a noticeable effect on each other, nor on the central body. Restrict to circular or co-planar if necessary.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-13T06:53:07.123" LastActivityDate="2020-10-18T04:02:15.223" Title="Calculating synodic periods at a given point in the orbit" Tags="|orbital-mechanics|orbit|calculation|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12957" PostTypeId="2" ParentId="7769" CreationDate="2015-12-13T00:27:04.633" Score="6" Body="&lt;p&gt;'What could go wrong' questions are hard to answer, but I would like to attempt it.&lt;br&gt;&#xA;The reason the depressuarization happened is important. If it was planned, they would likely focus on pressurizing it quietly, minimizing risk.&lt;br&gt;&#xA;If it was an accident, the worst case scenario is evacuation. Then it would take months getting things back to normal if the problem can not be fixed remotely, and repairing must wait for a new crew of cosmonauts&lt;br&gt;&lt;br&gt;&#xA;Machines do not need air, but they need cooling. For instance the &lt;a href=&quot;https://en.wikipedia.org/wiki/ISS_ECLSS#Elektron&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elektron&lt;/a&gt; units of the Russian segment, a major component of the oxygen regenerating system, are known to be prone to overheating. A lot of systems must be shut down to avoid that. But electronics may also suffer due to too low temperatures, and must if not kept at room temperature by the air be kept warm with heaters. (The Chinese lunar rover finally failed due to the extremely cold temperatures during the lunar night.)&lt;br&gt;&#xA;To protect everything from overheating or freezing is thus a careful balance of shutting down, cooling and warming.&lt;br&gt;The major components of the ISS are not air cooled. instead, they are cooled by pipes containing coolant. A depressuarization may not affect those pipelines, but again, that really depends on what happened to the station. Leaks draining the pipes will render them useless.&lt;br&gt;Vacuum is also a good insulator, thus heat is likely to travel through the walls instead of radiating. That may in turn cause unpredictable cold and hot spots. See for instance &lt;a href=&quot;http://seradata.com/SSI/2014/08/soyuz-fregat-launch-failure-dooms-two-galileo-satellites-to-useless-orbit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this launch failure due to unexpecteed freezing of propellant&lt;/a&gt;.&lt;br&gt;A lot of smaller electric and electronic equipment is entirely dependent on air cooling (laptops, portable monitors). The best way of protecting them is simply turning them off. Even if not, like during an emergency, they are lightweight and easy to replace on the next resupply mission.&lt;br&gt;&lt;br&gt;&#xA;The next big issue is air. According to &lt;a href=&quot;http://www.nasa.gov/sites/default/files/104840main_eclss.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;, the air system is capable of replacing the air. But because of the risk of leaks, the station would likely be slowly re-pressurized to detect them. Because even small leaks are a huge problem, they would need to monitor the station for quite a long time at a low pressure. So a re-pressurization would likely take a couple of days. If the station happens to be without air for more than a brief period of time, on-board experiments, especially living, might suffer. If the accident in addition to puncture the station also involves a shock or explosion, experiments not relying on air but otherwise sensitive, like &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/230.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;crystal growth&lt;/a&gt;, are vulnerable too.&lt;br&gt;&lt;a href=&quot;https://i.stack.imgur.com/AsTSz.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AsTSz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&lt;br&gt;&#xA;There might be a lot of parts not vacuum compatible, but it is difficult to find such data for all the systems. I assume equipment are placed in pressurized compartments because of that, otherwise, these areas would be used for habitation instead.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-14T19:05:08.753" LastActivityDate="2015-12-14T19:05:08.753" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12958" PostTypeId="2" ParentId="12935" CreationDate="2015-12-13T01:08:46.533" Score="8" Body="&lt;p&gt;The Merlin 1D's are claimed to have been able to run at the Falcon 9 1.1 Full Thurst level all along. However to get them into initial use on the Falcon 9 1.1 earlier, they were initially certified for only 85% of the design maximum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus this evolution is just to run them at full settings. If I understand it correctly, the densified propellant and oxidizer is not to achieve the extra thrust but rather to take advantage of it, with minimal tank stretches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current first stage is reported to be at the very limits of simple trucking on the US Interstate system. They need to be able to legally and safely drive it from Hawthorne, CA to McGregor, TX, and from either to Cape Caneveral, FL. Based on those roads, they are loathe to stretch the first stage much further to avoid hitting those limits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you increase the thrust, you increase the flow through the engine, which means you use the fuel/oxidizer up faster, and run through the tank quicker, which means you cannot thrust as long. Densifying helps but packing more into the same sized place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are probably many issues with cooling the propellant, but at least one is that you have less time on the pad from fueling to launch before the fuel and oxidizer warm up too much to be able to get the needed performance out of the vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is significantly expensive hardware needed, pretty near the launch site (Thus needing to be hardened to survive launches) to chill the fuel and oxidizer. (Running insulated pipes is hard, but doable, as NASA discovered with the LH2 lines for Saturn V).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-12-13T01:08:46.533" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12959" PostTypeId="1" AcceptedAnswerId="12960" CreationDate="2015-12-13T01:48:59.037" Score="7" ViewCount="718" Body="&lt;p&gt;Assuming a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion&quot;&gt;nuclear pulse propulsion&lt;/a&gt; spaceship would be launched to space with conventional chemical rockets and on escape trajectory from Earth, and it would only then start the nuclear pulse engine,&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could an individual or a company in such conditions restart &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29&quot;&gt;Project Orion&lt;/a&gt;, the nuclear pulse engine?&lt;/li&gt;&#xA;&lt;li&gt;Would relocating to one of the countries that didn't sign the &lt;a href=&quot;https://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty&quot;&gt;Partial Nuclear Test Ban Treaty&lt;/a&gt; help?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11671" LastEditorUserId="49" LastEditDate="2015-12-13T02:15:17.273" LastActivityDate="2015-12-23T02:34:33.457" Title="Does the Partial Nuclear Test Ban Treaty apply to nuclear pulse propulsion and its use in outer space?" Tags="|law|nuclear-pulse-propulsion|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12960" PostTypeId="2" ParentId="12959" CreationDate="2015-12-13T02:08:42.583" Score="11" Body="&lt;p&gt;Using nuclear power, for any reason whatsoever, is looked upon with great suspicion by the existing nations with nuclear capabilities. There are a lot of countries, including the US, that are skeptical of Iran's nuclear program, that they are currently just building reactors, not bombs.&lt;br&gt;&lt;br&gt;&#xA;Project Orion included nuclear pulse units that are in practice small nuclear warheads. Also, this combined with orbital capability, would mean a potential for building ICBMs with nuclear warheads.&lt;br&gt;&lt;br&gt;&#xA;Even if you circumnavigate the test ban treaty, the international agreements on not spreading nuclear warheads will apply.&lt;br&gt;&lt;br&gt;&#xA;However, several countries have not signed the &lt;a href=&quot;http://en.wikipedia.org/wiki/Comprehensive_Nuclear-Test-Ban_Treaty&quot; rel=&quot;nofollow&quot;&gt;Comprehensive Test Ban Treaty&lt;/a&gt;, of them the nuclear powers Pakistan, India, North Korea and Israel. They have instead signed the &lt;a href=&quot;http://en.wikipedia.org/wiki/Partial_Nuclear_Test_Ban_Treaty&quot; rel=&quot;nofollow&quot;&gt;Limited Test Ban Treaty&lt;/a&gt;. On the other hand countries like France and China have signed the Comprehensive Test Ban Treaty, but not the Limited version.&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An example of a country not signing any of them is Saudi Arabia.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="1142" LastEditDate="2015-12-23T02:34:33.457" LastActivityDate="2015-12-23T02:34:33.457" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12961" PostTypeId="2" ParentId="12959" CreationDate="2015-12-13T04:51:13.280" Score="7" Body="&lt;p&gt;The Outer Space Treaty (&lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot;&gt;https://en.wikipedia.org/wiki/Outer_Space_Treaty&lt;/a&gt;) bars &quot;weapons of mass destruction&quot; from being placed or used in space.  As the propulsion capsules of a nuclear-pulse spaceship are basically atomic bombs, I suspect the OST would apply.&lt;/p&gt;&#xA;" OwnerUserId="12174" LastActivityDate="2015-12-13T04:51:13.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12962" PostTypeId="2" ParentId="12926" CreationDate="2015-12-13T08:04:31.823" Score="9" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/structure/elements/soyuz/landing.html&quot; rel=&quot;noreferrer&quot;&gt;the NASA fact sheet on Soyuz landing&lt;/a&gt;, it takes about 3.5 hours from undocking to a landing on Earth. But in the three first hours they have the otbital module, and you can likely then cram up to six people into Soyuz.&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;Mass limit&lt;/strong&gt;&lt;br&gt;&#xA;The Soyuz has a cargo capacity of 50 kg in addition to three crew members. If you dropped the Sokol suits, which are only a backup system, you can fit in one more without going over limit. But how the mass is distributed is also important, as the capsule has to be balanced&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;Volume&lt;/strong&gt;&lt;br&gt;&#xA;As for volume, it is pretty tight though. Is it enough space to fit one more person in the descent module? Finding pictures which clearly shows the spare room is difficult, so I decided to do some original research. As it turns out, crowding a 3.5 cube meter room with four people is not all that difficult. The spare room of the capsule is also pretty much concentrated between the commander and the hatch.&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;Life support&lt;/strong&gt;&lt;br&gt;&#xA;Surviving in a closed space is always a question of CO2 removal. Assuming worst case, that the system can only support three people, carbon dioxide from the forth crewman will build up in the atmosphere of the capsule. At normal human carbon dioxide emission, the concentration after half an hour would be between 0.5 and 1 %. That is still within acceptable limits. Using the equation found &lt;a href=&quot;http://kimberlymoynahan.com/2012/04/friday-fiction-facts-trapped-in-an-airtight-room/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, I get a comparable ressult.&#xA;&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;Acceleration&lt;/strong&gt;&lt;br&gt;&#xA;The Soyuz has custom fit seats for the Cosmonaut, and that indicates high acceleration is a big deal. But like @SF said, they are likely to get away with it with only broken bones or minor damages.&lt;br&gt;&lt;br&gt;&#xA;In conclusion, it is not possible to land the Soyuz safely with a six person crew, but it may be possible to fit in one more.&lt;br&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;This is not to say that the Soyuz can actually carry four people, but it may be possible in theory. Real world safety procedures will never allow this as it has a too high risk.&lt;/strong&gt;&lt;br&gt;&#xA;Even &lt;a href=&quot;http://news.softpedia.com/news/ISS-Crew-Takes-Refuge-in-Soyuz-039-Lifeboat-039-106707.shtml&quot; rel=&quot;noreferrer&quot;&gt;emergency events&lt;/a&gt; have never been enough for trying to stretch the capacity of the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-30T15:08:48.593" LastActivityDate="2015-12-30T15:08:48.593" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12963" PostTypeId="2" ParentId="12905" CreationDate="2015-12-13T08:30:51.907" Score="2" Body="&lt;p&gt;I am not aware of any particular standard for the primary body, however for the Pluto system, in this image the 0,0 point is on the opposite side of the hearth:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/IYhAp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IYhAp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;And in this image, the hearth is shown to also be at the opposite side from Charon.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/b41le.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b41le.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;br&gt;&#xA;So I assume you are right.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2015-12-13T08:30:51.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12964" PostTypeId="2" ParentId="12922" CreationDate="2015-12-13T08:52:02.877" Score="23" Body="&lt;p&gt;From &lt;a href=&quot;http://www.retrospaceimages.com&quot; rel=&quot;noreferrer&quot;&gt;Retro Space Images&lt;/a&gt;'s &lt;a href=&quot;https://www.facebook.com/132774600078701/photos/a.134464163243078.16363.132774600078701/1002581646431321/?type=3&amp;amp;theater&quot; rel=&quot;noreferrer&quot;&gt;FaceBook post&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Gene Cernan scratches the itch during an Apollo 17 training session at KSC.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XEx6c.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XEx6c.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4558" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-24T11:29:46.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12965" PostTypeId="2" ParentId="10879" CreationDate="2015-12-13T09:34:19.777" Score="13" Body="&lt;p&gt;Getting hard numbers about how accurate measures we can get from current systems, adapted to the Sun instead of far away stars is difficult, bordering to impossible. But we can get data about the relative difficulty of the solar system planets.&lt;br&gt;&lt;br&gt;&#xA;First off, we can do some cheating for Mercury and Venus, as they occasionally go in front of the Sun. Given your 20 year observation span, you have observed multiple Mercury transits. &lt;a href=&quot;https://en.wikipedia.org/wiki/Transit_of_Venus&quot; rel=&quot;noreferrer&quot;&gt;Venus transits&lt;/a&gt; on the other hand happen about twice a century, and thus you have had a less than 50% chance of observing one, although you have had two of them in the last two decades.&lt;br&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Doppler_spectroscopy&quot; rel=&quot;noreferrer&quot;&gt;Doppler&lt;/a&gt; detection relies on the planet making their star moving slightly back and forth, causing the light from it to vary in wavelength.&lt;br&gt;&lt;br&gt;&#xA;Therefore, for a planet to be easy to detect, we want the difference of the Sun moving towards us and away from us to be as large as possible. The larger objects distance to the barycentre is determined by the smaller objects share of the mass. (&lt;a href=&quot;http://hopsblog-hop.blogspot.no/2015/06/mass-parameter-and-itn.html&quot; rel=&quot;noreferrer&quot;&gt;the µ&lt;/a&gt;)&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/a9cm6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/a9cm6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;br&gt;&#xA;The objects in the solar system has a negligible mass compared to the Sun, so the radius of the Sun's orbit around the barycentre is proportional to the planet's mass and the distance from the Sun.&lt;br&gt;&lt;br&gt;&#xA;In a two body system, the two bodies and the barycentre are always on a straight line, so the actual velocity is also proportionally to that of the planet.&lt;br&gt;&lt;br&gt;&#xA;Combining this parameters for all solar system planet gives the following list of relative Doppler shifts:&lt;br&gt;&lt;br&gt;&#xA;Jupiter: 1.00&lt;br&gt;&#xA;Saturn:  0.41&lt;br&gt;&#xA;Neptune: 0.13&lt;br&gt;&#xA;Uranus:  0.088&lt;br&gt;&#xA;Earth:   0.0014&lt;br&gt;&#xA;Venus:   0.00095&lt;br&gt;&#xA;Mars:    0.00018&lt;br&gt;&#xA;Mercury: 0.000047&lt;br&gt;&lt;br&gt;&#xA;For comparison, the values for Pluto and Ceres are 0.000019 and 0.00000034 respectively.&lt;br&gt;&lt;br&gt;The parameters for the relatively small &lt;a href=&quot;http://exoplanets.org/detail/GJ_581_c&quot; rel=&quot;noreferrer&quot;&gt;Gliese 581c&lt;/a&gt; gives about 0.25 on this scale, indicating that extraterrestrials with the capabilities of current Earth technology could detect Jupiter and Saturn in our solar system.&lt;br&gt;&lt;br&gt;&#xA;An important note is that from Saturn and out, you are not able to observe the whole Doppler shift cycle in 20 years due to their long orbital periods. Also you are able to get data from multiple revolutions from the inner planets, making it easier to confirm the observations.&lt;br&gt;&lt;br&gt;&#xA;Splitting the combined signal of all the solar system planets from each other is a task requiring a &lt;a href=&quot;http://mathworld.wolfram.com/FourierTransform.html&quot; rel=&quot;noreferrer&quot;&gt;Fourier Transform.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="1142" LastEditDate="2016-01-14T12:34:13.557" LastActivityDate="2016-01-14T12:34:13.557" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12966" PostTypeId="1" AcceptedAnswerId="12967" CreationDate="2015-12-13T12:11:02.390" Score="3" ViewCount="368" Body="&lt;p&gt;How would a spaceship traveling through interplanetary space, let's say to Mars, avoid all the space debris or there isn't any interplanetary space debris at all, is it only orbital debris around the earth?&lt;/p&gt;&#xA;" OwnerUserId="12608" LastEditorUserId="25" LastEditDate="2015-12-14T03:11:28.920" LastActivityDate="2015-12-15T00:26:14.133" Title="Interplanetary space debris" Tags="|crewed-spaceflight|spacecraft|debris|interplanetary|interstellar-travel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12967" PostTypeId="2" ParentId="12966" CreationDate="2015-12-13T14:38:06.783" Score="12" Body="&lt;p&gt;Space is in general an empty place, and debris is only a problem in Earth orbit. Even there, the amount of debris is only considerable in popular orbits like &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;GEO&lt;/a&gt;. The reason is in part that there have only been a two digit number of probes sent away from Earth's influence, compared to the thousands of satellites currently orbiting Earth.&lt;br&gt;&lt;br&gt;&#xA;The other part of the reason is the vast size of interplanetary space. Near Earth space is measured in thousands of kilometres, interplanetary space in  millions. The volume of a cube with sides a thousand times longer is a billion times as big. The debris is therefore so  scattered that we do not actually have to pay attention to it.&lt;br&gt;&lt;br&gt;&#xA;However, not all small objects out there are man-made, a lot of them are natural rocks and gravel. They are too small to observe from Earth.&lt;br&gt;&lt;br&gt;&#xA;To answer your question about how to avoid them, you really can't. We have no way of detecting them, and spacecraft in interplanetary space are mostly on a predetermined course just coasting without changing their orbit.&lt;br&gt;&#xA;Shielding is an option though, mostly protecting from very small particles. To shield from larger ones, you would need a heavier shield, eating into your spacecraft mass budget. A bit more about micro meteorite shielding can be found &lt;a href=&quot;http://www.smithsonianmag.com/smart-news/how-do-you-shield-astronauts-and-satellites-from-deadly-micrometeorites-3911799/?no-ist&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;br&gt;&#xA;&lt;a href=&quot;http://www.universetoday.com/13587/space-debris-illustrated-the-problem-in-pictures/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Universe today&lt;/a&gt; has some excellent visualisations of the orbital debris problem.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/QwSeUm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QwSeUm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;Damage on shuttle after micrometeorite impact&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-15T00:26:14.133" LastActivityDate="2015-12-15T00:26:14.133" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="12968" PostTypeId="2" ParentId="8152" CreationDate="2015-12-13T15:25:28.833" Score="4" Body="&lt;p&gt;50 years old abort routines are hard to come by, but the &lt;a href=&quot;http://fas.org/nuke/space/nerva-spec.pdf&quot; rel=&quot;nofollow&quot;&gt;NERVA-specs&lt;/a&gt; (unclassified 9/8/1970) clearly specify a malfunction mode in which the engine should be able to provide a minimum of thrust and impulse. The values stated (thrust of 30000 pounds, impulse of 10⁸ lb-seconds) are enough to direct the upper stage away from densely populated areas, towards a landing in the ocean. The engine should also be capable of operating with just one turbo pump&lt;br&gt;Normal procedure for saving the crew during a launch failure is separating the capsule from from the rocket, and fire a solid rocket escape tower to get away, followed by a normal descent with parachutes. The Mercury capsules, pre-dating the NERVA program, was equipped with escape towers.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-13T17:17:37.757" LastActivityDate="2015-12-13T17:17:37.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12969" PostTypeId="2" ParentId="10399" CreationDate="2015-12-13T16:11:33.877" Score="3" Body="&lt;p&gt;In a system with libration points, none of the bodies move relative to the point. Therefore, the Earth/Moon points are not valid 4-body solutions, because they move relative to the Sun. For true 4-body or higher n-body systems that are not simply coupled 3-body systems, we have to look at &lt;a href=&quot;https://en.wikipedia.org/wiki/Klemperer_rosette&quot; rel=&quot;nofollow noreferrer&quot;&gt;Klemperer rosettes&lt;/a&gt; and similar arrangements. If the barycentre in the middle is not occupied, this would be a libration point. If we use a rotating frame of reference, we can model inertia as a fictitious force acting away from the middle. If an object occupies the barycentre, an equivalent of the L1 point will exist between the centre and one of the objects in the orbiting polygon. That is not a stable point, and the many other possible points are not stable either. As the inertial force of an object stationary in a rotating frame of reference grows linear with distance, and the gravitational influence from the object with the inverse square law, a equilibrium where this forces cancel out exist on every axis of symmetry of the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0gS0qm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0gS0qm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-16T05:36:11.347" LastActivityDate="2015-12-16T05:36:11.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12971" PostTypeId="2" ParentId="8599" CreationDate="2015-12-13T19:30:17.430" Score="1" Body="&lt;p&gt;Ignoring the energy question that has been covered very nicely already...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ion engines are &quot;low thrust&quot; engines, which means their thrust-weight ratio is much lower than 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, ion engines are scalable! You can make them any size, but you can't really increase the thrust-to-surface area ratio. So if you want to double the thrust you have to make the engine twice as wide, or use two engine right next to each other. Practically this means that we can 1) scale up an ion-engine spacecraft and 2) add even more engines to get a better thrust-weight ratio but it's still going to be a fundamentally &quot;low thrust&quot; spacecraft with those graceful spiral trajectories. That doesn't mean they aren't useful, it just means whoever operates them needs patience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Volume increases with a cube of size, and area with the square of size. So if you make a spaceship 10 times longer than Dawn, you'd end up with something like 1000 times the weight. To get the same thrust-to-weight ratio, you'd need engines with 1000 times the area, or sqrt(1000)=32 times larger. Also, the solar panels would need to be 32 times as long for the same shape. There's no limit but eventually your ship is all solar panels and engines!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now this is pure speculation... an ion engine is a particle accelerator, and electrostatic accelerators are a &lt;em&gt;very&lt;/em&gt; basic way to do the job. A small cyclotron might give you a better thrust-weight ratio, but it would still be &quot;low thrust&quot;!&lt;/p&gt;&#xA;" OwnerUserId="12611" LastEditorUserId="12611" LastEditDate="2015-12-13T19:39:39.877" LastActivityDate="2015-12-13T19:39:39.877" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12972" PostTypeId="2" ParentId="12959" CreationDate="2015-12-14T00:16:56.030" Score="6" Body="&lt;p&gt;One of the countries that hasn't signed the treaty would make things quite difficult. So far as I can tell, it seems likely that a launch of Orion would be acceptable with the Treaty, but any testing relating to the treaty would be forbidden. It's hard to do without testing, so it seems unlikely to be of much help. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29&quot;&gt;Wikipedia&lt;/a&gt; states that the test ban treaty is generally accepted to be the end of Project Orion.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Partial Test Ban Treaty of 1963 is generally acknowledged to have ended the project.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-12-14T00:16:56.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12973" PostTypeId="1" CreationDate="2015-12-14T17:37:56.583" Score="3" ViewCount="196" Body="&lt;p&gt;The &lt;a href=&quot;http://www.russianspaceweb.com/acts.html&quot; rel=&quot;nofollow&quot;&gt;PTK NP&lt;/a&gt; spacecraft seems to be the future successor of the Soyuz spacecraft. Does anyone know when the first launch will likely be? The estimates I have seen vary a lot, so what will be realistic?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-14T18:08:55.710" LastActivityDate="2019-09-28T11:03:46.980" Title="When is the PTK NP spacecraft likely to be launched?" Tags="|launch|crewed-spaceflight|spacecraft|russia|human-rating|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12974" PostTypeId="1" AcceptedAnswerId="12975" CreationDate="2015-12-14T17:45:58.853" Score="2" ViewCount="736" Body="&lt;p&gt;I'm watching the British movie &lt;a href=&quot;http://www.imdb.com/title/tt1535107&quot; rel=&quot;nofollow&quot;&gt;The First Men in the Moon&lt;/a&gt; and it got me wondering if it was scientifically possible to have sent humans to land and explore the moon (or even just orbit) before the NASA Apollo missions of the late 1960s. Was it?&lt;/p&gt;&#xA;" OwnerUserId="12620" LastActivityDate="2019-06-11T20:59:32.697" Title="Moon Exploration Before the Apollo Missions?" Tags="|the-moon|nasa|apollo-program|" AnswerCount="1" CommentCount="2" ClosedDate="2015-12-14T21:05:31.680" ContentLicense="CC BY-SA 3.0" />
  <row Id="12975" PostTypeId="2" ParentId="12974" CreationDate="2015-12-14T18:07:37.147" Score="9" Body="&lt;p&gt;The science behind getting to the Moon is a rather complex question. Most of the challenges were in the engineering. Some of the key science points include:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Orbital mechanics- Around since &lt;a href=&quot;https://en.wikipedia.org/wiki/Johannes_Kepler&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's laws of motion&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket equation&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;All of those have been around since the 1800's or longer. There is also a number of science topics related directly to space (Albeit ones that could have been worked around given enough resources) that include the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Surface of the Moon- Could it support weight of a lander? Was it too rocky?&lt;/li&gt;&#xA;&lt;li&gt;What was the radiation environment like between the Earth and the Moon.&lt;/li&gt;&#xA;&lt;li&gt;Could humans survive in zero gravity?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are a number of engineering challenges that were required, including:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Optimal propellant- (&lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;RP-1&lt;/a&gt; first stage, LH2/LO2 for others) (1950's)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid-propellant_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;Liquid rocket engine&lt;/a&gt;- 1926.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry#History&quot; rel=&quot;nofollow noreferrer&quot;&gt;Re-entry design&lt;/a&gt;- 1920 for concept, 1950's for practice.&lt;/li&gt;&#xA;&lt;li&gt;Computers, while strictly not required, make many of the things much easier. For the size they were required, 1950's to 1960's.&lt;/li&gt;&#xA;&lt;li&gt;Light materials, including ceramics, composites.&lt;/li&gt;&#xA;&lt;li&gt;Thermal control, including mylar.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There were a number of operational items as well that were required. These were developed at the beginning of the space program by both the USSR and USA, which include:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Orbital Rendezvous&lt;/li&gt;&#xA;&lt;li&gt;EVA&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the basic science has been around for hundreds of years. The specific science to aid in engineering, along with the engineering to do so was developed largely as a result of the space program, and the cold war ICBM development. A few years earlier might have been possible if the effort was expended, but not many.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-06-11T20:59:32.697" LastActivityDate="2019-06-11T20:59:32.697" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="12979" PostTypeId="1" CreationDate="2015-12-14T21:46:46.533" Score="7" ViewCount="932" Body="&lt;p&gt;Which planets or large moons of our solar system are currently known to have the raw materials needed for us to produce rocket fuel?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars, for example, has plentiful CO₂, and &lt;a href=&quot;http://www.geoffreylandis.com/propellant.html&quot; rel=&quot;noreferrer&quot;&gt;we know how to produce&lt;/a&gt; methane and oxygen from it.&lt;/li&gt;&#xA;&lt;li&gt;Titan has &lt;a href=&quot;https://en.wikipedia.org/wiki/Lakes_of_Titan&quot; rel=&quot;noreferrer&quot;&gt;lakes of liquid hydrocarbons&lt;/a&gt; (including methane) on its surface.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Where else is already known to have what we need, readily available?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &quot;readily available&quot;, I mean that the material is lying out freely on large areas of the surface (like the lakes on Titan) or is available throughout the atmosphere at the surface (like on Mars) so that no searching or mining is needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &quot;rocket fuel&quot;, I mean the material used to launch a liquid-fueled rocket from the surface of that planet/moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &quot;planets or large moons&quot;, I mean roughly those bodies that are in hydrostatic equilibrium.  (Since there are only 40 or so in the solar system, this can't be a very long list!)&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="26446" LastEditDate="2019-05-17T22:26:21.847" LastActivityDate="2019-08-07T19:58:33.303" Title="Which heavenly bodies have stuff to make rocket fuel?" Tags="|fuel|isru|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="12980" PostTypeId="2" ParentId="8153" CreationDate="2015-12-14T22:00:20.510" Score="1" Body="&lt;p&gt;Some others not previously mentioned:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.nap.edu/search/?term=space&quot; rel=&quot;nofollow&quot;&gt;National Academies Press&lt;/a&gt; has a fairly large collection of free ebooks on space topics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA's websites are very poorly coordinated and badly maintained, but there is good material there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.jsc.nasa.gov/history/&quot; rel=&quot;nofollow&quot;&gt;NASA History Portal&lt;/a&gt; although clumsy to navigate and locate items in, trees out to a vast collection of documentation on US manned spaceflight.  In looking at this, I just noticed that the &lt;a href=&quot;http://www.spaceflight.nasa.gov/shuttle/reference/green/&quot; rel=&quot;nofollow&quot;&gt;Shuttle Operational Data Book&lt;/a&gt; (in part) is there, which I wasn't aware of.  There's some junk linked under this portal but a lot of gold.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/connect/ebooks/index.html&quot; rel=&quot;nofollow&quot;&gt;NASA ebooks&lt;/a&gt;, what it says.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/series95.html&quot; rel=&quot;nofollow&quot;&gt;NASA history ebooks&lt;/a&gt;, also what it says.  Lots of good stuff here including &lt;em&gt;Rockets and People&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://images.jsc.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;JSC Digital Image Collection&lt;/a&gt;  Searchable image index.  For some reason only goes up to STS-79.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-12-14T22:00:20.510" CommentCount="0" CommunityOwnedDate="2015-12-14T22:00:20.510" ContentLicense="CC BY-SA 3.0" />
  <row Id="12981" PostTypeId="2" ParentId="12979" CreationDate="2015-12-14T22:04:59.147" Score="11" Body="&lt;p&gt;To start off, literally everything can be used as rocket propellant. For a more practical approach though, water is a good resource for making hydrogen and oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Inner solar system we have water on Mars, bound as ice on the poles, and buried &lt;a href=&quot;http://www.astrobio.net/topic/solar-system/mars/buried-ice-on-mars/&quot; rel=&quot;noreferrer&quot;&gt;just under the surface&lt;/a&gt; in some places.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dAOuft.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dAOuft.jpg&quot; alt=&quot;ice on Mars&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;Ice on Mars&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://http:/http://nssdc.gsfc.nasa.gov/planetary/ice/ice_moon.html&quot; rel=&quot;noreferrer&quot;&gt;Recent findings&lt;/a&gt; by the Clementine Lunar Orbiter strongly suggest water in cold traps in the polar regions of the Moon. Ideas about mining the water of the cold traps is often combined with using the &lt;a href=&quot;http://www.astrobio.net/news-exclusive/peaks-of-eternal-light/&quot; rel=&quot;noreferrer&quot;&gt;peaks of eternal light&lt;/a&gt; as the required energy source to crack the water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the outer Solar system, most small objects partially consist of ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You also mention producing methane and oxygen from carbon dioxide, using imported hydrogen. This is good for planets with a carbon dioxide atmosphere, like Mars and &lt;a href=&quot;http://www.space.com/18527-venus-atmosphere.html&quot; rel=&quot;noreferrer&quot;&gt;Venus&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even for bare rock bodies without an atmosphere, there are still options, like producing solid rocket fuel from aluminium. &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0306261915011071&quot; rel=&quot;noreferrer&quot;&gt;Other&lt;/a&gt; commonly occurring metals like iron or calcium may also be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are not so strict about &quot;fuel&quot;, just &lt;a href=&quot;http://the-difference-between.com/propellant/fuel&quot; rel=&quot;noreferrer&quot;&gt;propellant&lt;/a&gt;, there have been proposals for &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_mining&quot; rel=&quot;noreferrer&quot;&gt;scooping the atmospheres of the gas giants&lt;/a&gt;. This may not be feasible due to the high delta-v required to climb out of their gravity wells. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrogen, and maybe more important, water may be useful for a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;noreferrer&quot;&gt;NTR&lt;/a&gt;, especially since you do not have to electrolyse the water before use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket fuels does in essence just require a chemical process producing a significant amount of energy and gasses, and NTR propellants are just anything liquid you can pump through your engine. (John Clarke even mentions mercury in his book &lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;noreferrer&quot;&gt;Ignition&lt;/a&gt; due to its high density impulse).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good site for learning about rocket propellants and ISRU is Winchell Chung's &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/engines.php&quot; rel=&quot;noreferrer&quot;&gt;Atomic Rockets&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an informal introduction to rocket fuel I recommend this video by Scott Manley:&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/jI8TuufCp0M?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&#xA;&lt;br&gt;&#xA;When mass is hard to come by, like in empty space, other methods are used, for example &lt;a href=&quot;http://sail.planetary.org/&quot; rel=&quot;noreferrer&quot;&gt;using a sail to let the Sun push you&lt;/a&gt;. In interstellar space without any strong light source, methods for &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;noreferrer&quot;&gt;collecting&lt;/a&gt; the extremely low density interstellar clouds of hydrogen have been proposed.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-15T16:36:26.743" LastActivityDate="2015-12-15T16:36:26.743" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12982" PostTypeId="1" AcceptedAnswerId="12983" CreationDate="2015-12-15T01:19:16.173" Score="3" ViewCount="332" Body="&lt;p&gt;The ideal rocket sled launch is building a track that accelerates a rocket (via linear motors or something like that) up the side of a tall mountain. Not only could this save huge amounts of fuel, but also could remove the need for the components needed to store and burn that fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what problems still need to be solved for this idea to become a reality?&lt;/p&gt;&#xA;" OwnerUserId="12624" LastEditorUserId="49" LastEditDate="2015-12-15T02:58:05.397" LastActivityDate="2015-12-15T02:58:05.397" Title="What barriers are there to creating a reusable rocket sled lanchpad?" Tags="|launch|technology|feasibility|launch-assist|rocket-sled|" AnswerCount="1" CommentCount="1" ClosedDate="2015-12-15T03:24:55.703" ContentLicense="CC BY-SA 3.0" />
  <row Id="12983" PostTypeId="2" ParentId="12982" CreationDate="2015-12-15T02:33:04.800" Score="3" Body="&lt;p&gt;In a nutshell, &lt;a href=&quot;https://en.wikipedia.org/wiki/Dynamic_pressure&quot; rel=&quot;nofollow noreferrer&quot;&gt;dynamic pressure&lt;/a&gt;. There are also other serious limitations, but this one is the hardest to overcome. Say, at 10 km altitude, atmospheric pressure is still a bit over one quarter that of atmospheric pressure at mean sea-level. A conventional launch vehicle will at that altitude &lt;em&gt;only&lt;/em&gt; have reached velocity of approximately 850 km/h and stay subsonic. As it gradually increases speed and &lt;em&gt;climbs&lt;/em&gt; out of the denser atmosphere, it will reach Max-Q (maximum dynamic pressure) at an altitude of about 40 kilometers at velocity of 900 m/s (3,240 km/h). Here's a Douglas T. Jackson's example for a Space Shuttle launch:&lt;/p&gt;&#xA;&lt;p&gt;                  &lt;img src=&quot;https://i.stack.imgur.com/KoFmw.jpg&quot; alt=&quot;Space Shuttle Max-Q&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;   &lt;sup&gt;Variation in air density (ρ), velocity (V), altitude (h), and dynamic pressure (q) during a Space Shuttle launch. &lt;a href=&quot;http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;Now, let's be perfectly clear here, that other launch vehicles will have different ascent profiles, but all would have reached only a small fraction of orbital speed at an altitude of 10 km, which is probably quite a bit higher than you could bargain for as a release altitude of a rocket sled launch assist system. Orbital speed is roughly 7.6 km/s (that's per second!), and our Space Shuttle has reached at 10 km altitude only roughly 3% of that, but is already experiencing approx. 70% of its flight profile's maximum dynamic pressure.&lt;/p&gt;&#xA;&lt;p&gt;So, if we want to release our launch vehicle from the ramp at much greater speed, we have a bit of problem here, and it goes like this:&lt;/p&gt;&#xA;&lt;h3&gt;&lt;span class=&quot;math-container&quot;&gt;$$q = \tfrac12\, \rho\, v^{2}$$&lt;/span&gt;&lt;/h3&gt;&#xA;&lt;p&gt;Where &lt;span class=&quot;math-container&quot;&gt;$q$&lt;/span&gt; is dynamic pressure in pascals, &lt;span class=&quot;math-container&quot;&gt;$\rho$&lt;/span&gt; is fluid density in kg/m&lt;sup&gt;3&lt;/sup&gt;, and &lt;span class=&quot;math-container&quot;&gt;$v$&lt;/span&gt; is fluid velocity in m/s.&lt;/p&gt;&#xA;&lt;p&gt;You can see that it's quite similar to the formula for kinetic energy and just like it, changes exponentially with velocity &lt;span class=&quot;math-container&quot;&gt;$v$&lt;/span&gt;. So if we assume same &lt;span class=&quot;math-container&quot;&gt;$\rho$&lt;/span&gt;, and we want to only double our vehicles speed at release altitude, we have doubled release point's dynamic pressure it has to withstand. Or increased it 4.5 times if we want to triple release speed. Or made it 8 time worse by quadrupling release speed, and so on. Basically, for each &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; increase in release speed, we have &lt;span class=&quot;math-container&quot;&gt;$n^2/2$&lt;/span&gt; as much of pressure to withstand.&lt;/p&gt;&#xA;&lt;p&gt;And with 5 times the speed at 10 km altitude and 12.5 times the dynamic pressure (nearly 9 times the Max-Q of Space Shuttle), we've barely reached a good one third of the orbital speed, still have quite some atmospheric and gravity drag to overcome, for which our released vehicle will require own propulsion. It will also experience more friction heating, acoustic vibration from ground effect and as it reaches Mach 1 if launched from a non-evacuated tube, or a strong barrier shock if launched from evacuated tube once it exits it.&lt;/p&gt;&#xA;&lt;p&gt;It will also have to tolerate much stronger lateral forces than a vertically launched vehicle, so its construction will have to be much, much stronger. All of that will eventually make it heavier and have a much lower wet-to-dry &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;mass ratio&lt;/a&gt;, and have greater difficulties reaching orbital speed from release point, whereon it is still a subject to the tyranny of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket equation&lt;/a&gt;. Conventional, vertically launched rockets are really so elegant when it comes to managing extreme power at small increments of stress on the vehicle and where it can tolerate them best. You forfeit any of that elegance once you &lt;em&gt;shoot out as a bullet&lt;/em&gt;. It will go bang, and quite likely, your vehicle will go with it.&lt;/p&gt;&#xA;&lt;p&gt;There are however few of these problems that remain if you constructed such &lt;em&gt;launch ramp&lt;/em&gt; on bodies with no atmosphere, like for example on the Moon.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-12-15T02:38:33.993" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12984" PostTypeId="1" AcceptedAnswerId="12985" CreationDate="2015-12-15T02:41:42.900" Score="9" ViewCount="7152" Body="&lt;p&gt;I have a question regarding the types of motors that were used on the Martian rovers, specifically the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Exploration_Rover&quot; rel=&quot;noreferrer&quot;&gt;Mars Exploration Rovers&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_%28rover%29&quot; rel=&quot;noreferrer&quot;&gt;Curiosity&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am wondering about the main 6 motors that turn the wheels, so the rover can move forward or backwards, and the 4 motors that turn the outer wheels around their axes, so the rover is maneuverable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am pretty sure they used DC motors for both types of motors, but did they use brushed or brushless motors, and what specifications did those motors have? How did they make the motors have such a low RPM, and can they control the speed and motion of every motor's torque, voltage, current, etc.?&lt;/p&gt;&#xA;" OwnerUserId="12625" LastEditorUserId="49" LastEditDate="2015-12-15T03:16:53.987" LastActivityDate="2015-12-15T04:26:53.960" Title="The types of motors used on the Martian rovers?" Tags="|mars|rovers|design|robotic-missions|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12985" PostTypeId="2" ParentId="12984" CreationDate="2015-12-15T04:26:53.960" Score="17" Body="&lt;p&gt;MER uses brushed motors, Curiosity brushless. They both have a gear box on each motor with a fixed, very high gear ratio (a few thousand) for low speed and very high torque. The current to each motor is independently controlled and monitored.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-12-15T04:26:53.960" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="12987" PostTypeId="1" CreationDate="2015-12-15T05:58:14.950" Score="3" ViewCount="1840" Body="&lt;p&gt;I'm trying to figure out how to calculate Coordinated Mars Time (MTC). Can it be calculated directly from Earths current time/date, or calculated from the current Mars Sol Date (MSD)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Timekeeping_on_Mars&quot; rel=&quot;nofollow&quot;&gt;this wiki page&lt;/a&gt; I can calculate the current Mars Sol Date using:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;MSD = (Julian Date using International Atomic Time - 2451549.5 + k)/1.02749125 + 44796.0, where k is a small correction of approximately 0.00014 d (or 12 s)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;but how would one calculate Coordinated Mars Time?&lt;/p&gt;&#xA;" OwnerUserId="12627" LastActivityDate="2016-02-13T18:30:34.120" Title="How to calculate Coordinated Mars Time (MTC)?" Tags="|mars|curiosity|time|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12989" PostTypeId="1" CreationDate="2015-12-15T08:07:12.547" Score="12" ViewCount="1050" Body="&lt;p&gt;How easy is it to disconnect and remove the helmet of a space suit? More specifically...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How long does it take for one to remove his own helmet?&lt;/li&gt;&#xA;&lt;li&gt;Would it be feasible for somebody to remove another's helmet in a relatively quick manner?&lt;/li&gt;&#xA;&lt;li&gt;Would it be possible for somebody in vacuum to reattach his own helmet if it were removed?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I am aware that there are a variety of space suits, and I am under the impression that some EVA suits are more of a hard shell without a separately removable helmet. On that point, I welcome answers relating to any and every variety of suit you are familiar with.&lt;/p&gt;&#xA;" OwnerUserId="12628" LastEditorUserId="6816" LastEditDate="2015-12-15T08:18:06.280" LastActivityDate="2015-12-15T20:11:30.070" Title="Ease of Detachment of a Space Helmet" Tags="|spacesuits|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12990" PostTypeId="2" ParentId="12987" CreationDate="2015-12-15T14:26:03.073" Score="1" Body="&lt;p&gt;On the page you refer:&amp;nbsp;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Timekeeping_on_Mars#Formulas_to_compute_MSD_and_MTC&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Timekeeping_on_Mars#Formulas_to_compute_MSD_and_MTC&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MTC is simply the fractional part of MSD, in hours, minutes and seconds:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;MTC = (MSD mod 1) × 24 h &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12619" LastEditorUserId="8162" LastEditDate="2015-12-15T18:03:54.733" LastActivityDate="2015-12-15T18:03:54.733" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="12992" PostTypeId="1" CreationDate="2015-12-15T15:11:44.630" Score="3" ViewCount="3647" Body="&lt;p&gt;I think the answer to this question probably lies more in biology than astrophysics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were in space with an oxygen mask on (ignoring the fact that you will get quite cold for the moment) you wouldn't be able to hear sound transmitted through the air (or lack of it). But from the experience of standing in an anechoic chamber, when outside noise is removed you notice more the internal sounds of your body. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So would you be able to hear your blood pumping, lungs, etc whilst floating in the vacuum of space?&lt;/p&gt;&#xA;" OwnerUserId="2849" LastActivityDate="2022-12-05T03:15:47.420" Title="Could you hear the sound of your own body in the vacuum of space?" Tags="|vacuum|life|sound|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="12993" PostTypeId="2" ParentId="12987" CreationDate="2015-12-15T15:25:58.567" Score="1" Body="&lt;p&gt;The fractional part of MTC corresponds to a period of 24 Martian hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;${HH \over 24} + {MM \over 60 \cdot 24} +  {SS \over 60 \cdot 60 \cdot 24} = frac(MSD)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversion in the opposite direction is a little more complicated: If $frac$ is the fractional part of a number, and $floor$ is the integer part (rounding by truncation), then,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA; F_h = frac(MSD); \\&#xA;HH = floor(24 \cdot F_h) \\&#xA; F_m = frac(24 \cdot F_h) \\&#xA;MM = floor(60 \cdot F_m) \\&#xA; SS = 60 \cdot frac(60 \cdot F_m) \\&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;This is according to the convention o &lt;em&gt;using a 24-hour &quot;Mars clock&quot; on which the hours, minutes and seconds are 2.7% longer than their standard (Earth) durations.&lt;/em&gt; That means these aren't SI system seconds - these are arbitrary periods of time corresponding to 1/(24*60*60)th of Sol.&lt;/li&gt;&#xA;&lt;li&gt;HH and MM are integers. SS are given as a Real, including the fractional part of the &quot;second&quot;. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt; we should really find some better names for these time units&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-12-15T15:25:58.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12995" PostTypeId="2" ParentId="12992" CreationDate="2015-12-15T15:43:05.470" Score="4" Body="&lt;p&gt;Your ear drums are connected to your ear canal, and vibrations are also passed to it by that means. So they would perceive your heart thumping in a terrified panic, and your lungs trying to haul in some air from the oxygen tank, though the seal on the mask better be good or most of it will escape into the vacuum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But your eardrums won't last long. They will rupture under a pressure difference of more than &lt;a href=&quot;http://hypertextbook.com/facts/2007/ShemikaWhite.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;5 to 10 psi&lt;/a&gt;. Atmospheric pressure is 14.7 psi, so if you hold your breath, or hold the mask on tight and get all its air, they will explode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9UlkY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9UlkY.png&quot; alt=&quot;anatomy of the ear&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;CC 2.5 Chittka L, Brockmann, from Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Ear&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ear article&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will still hear a little, as it is the inner hear which actually does the hearing, and it will still receive vibrations from surrounding bones. (That's the purple part on the diagram.)  Until the &lt;a href=&quot;https://en.wikipedia.org/wiki/Round_window&quot; rel=&quot;nofollow noreferrer&quot;&gt;round window&lt;/a&gt;, which is a membrane on the cochlea, also bursts. It might last longer, and after it bursts it might take a few seconds for the fluid inside it to boil away. The cochlea needs that fluid to pass vibrations to tiny hairs that then trigger nerve endings, which is what your brain interprets as sound. During that time you might not hear your heart pounding any more, but you could probably hear yourself scream, for instance, in a dim, distorted way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you had planned for this though, and used an oxygen system that was only 4.4 psi of pure oxygen, that would be enough for your body's needs and wouldn't explode your eardrums. Then if you spent adequate time in a specially designed airlock, that also held pure oxygen at that pressure,  your body would expel the nitrogen from its bloodstream so it &lt;a href=&quot;https://en.wikipedia.org/wiki/Decompression_sickness&quot; rel=&quot;nofollow noreferrer&quot;&gt;doesn't bubble&lt;/a&gt; as soon as you are in vacuum and kill you. Then you could perform this experiment and survive, if it lasted no more than a minute or so before you retreated back into the airlock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also, that you wouldn't necessarily get cold. If you were near Earth and in sunlight, your sunlit side would get very hot. But that wouldn't be much of an issue in the time span considered.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2015-12-15T16:41:35.187" LastActivityDate="2015-12-15T16:41:35.187" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="12996" PostTypeId="2" ParentId="12989" CreationDate="2015-12-15T16:08:21.893" Score="3" Body="&lt;p&gt;For the Shuttle / ISS Extravehicular Mobility Unit, it is not too difficult:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The connecting ring of the helmet is similar to the rings used for the&#xA;  body-seal closure. Mobility is not needed in this ring because the&#xA;  inside of the helmet is large enough  for  the  crew  member’s   head &#xA;  to  move around. To open or lock any of the connecting rings, one or&#xA;  two sliding , rectangular-shaped knobs are moved to the right or the&#xA;  left. When opened, the two halves of the connecting rings come apart&#xA;  easily. To close and lock, one of the rings slides part way into the&#xA;  other against an O-ring seal.The knob is moved to the right, and small&#xA;  pins inside the outer ring protrude into a groove around the inside&#xA;  ring, thereby holding the two together . All suit openings have&#xA;  locking provisions that require a minimum of three independent motions&#xA;  to open.This feature prevents any accidental opening of suit&#xA;  connections.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.nasa.gov/pdf/188963main_Extravehicular_Mobility_Unit.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-12-15T16:08:21.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="12997" PostTypeId="1" AcceptedAnswerId="13026" CreationDate="2015-12-15T16:18:50.440" Score="4" ViewCount="1311" Body="&lt;p&gt;When I was calculating what delta-v was required to transfer between two orbits of radius 1 and 2, with a relative inclination of 20 degrees, I found, as expected, that it was best to combine the plane change with arriving at the outer orbit. (0.411756 compared to 0.533744 doing it at the inner one). However, doing a little of the plane change at the first burn got me a slightly better result. (0.391922 for a 4.42 degree angle). What is the reason for this? And how do I calculate the optimum split?&#xA;&lt;a href=&quot;https://i.stack.imgur.com/KtJ6J.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KtJ6J.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-15T16:42:11.723" LastActivityDate="2015-12-16T22:19:54.877" Title="Saving delta-v by splitting plane changing manoeuvre?" Tags="|orbital-mechanics|orbital-maneuver|trajectory|delta-v|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="12998" PostTypeId="1" AcceptedAnswerId="13034" CreationDate="2015-12-15T16:40:50.713" Score="11" ViewCount="41606" Body="&lt;p&gt;Which engineering branch will be most suitable for becoming an astronaut, and what other options do I have if I want to become an astronaut with an engineering background?&lt;/p&gt;&#xA;" OwnerUserId="12517" LastEditorUserId="10234" LastEditDate="2015-12-16T17:02:27.053" LastActivityDate="2020-09-20T09:42:08.873" Title="Becoming an astronaut with engineering background" Tags="|astronauts|engineering|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="12999" PostTypeId="1" AcceptedAnswerId="13011" CreationDate="2015-12-15T16:41:27.963" Score="4" ViewCount="205" Body="&lt;p&gt;Imagine having a tank of compressed air to feed an organ. Would the pipes still vibrate when played? If so, would the frequency be different? And how many decibels quieter would it sound if you put your ear/helmet directly in the path of the exiting air?&lt;/p&gt;&#xA;" OwnerUserId="12745" OwnerDisplayName="Matt Thomas" LastEditorUserId="8162" LastEditDate="2016-01-02T19:59:07.330" LastActivityDate="2016-02-10T04:31:53.347" Title="Would a woodwind instrument still play in outer space?" Tags="|sound|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13000" PostTypeId="2" ParentId="12997" CreationDate="2015-12-15T17:02:21.270" Score="3" Body="&lt;p&gt;I think I see what is going on. Doing a plane change is in general better to do farther away from the planet, where the velocity is lower.&lt;br&gt;&#xA;However, combining parts of the inclination change with the initial burn to enter the transfer orbit saves you delta-v because adding a small thrust vector normal to the main burn is for a large ratio between them virtually free. To illustrate this, the highlighted part of the hypotenuse is enough for adding the thrust vector of the smaller side.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/eSfAU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eSfAU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;This advantage become smaller if we add more side way thrust, eventually reaching an equilibrium.&lt;br&gt;&lt;br&gt;&#xA;Finding this equilibrium is harder. As a point of departure, the delta-v of a Hohmann transfer with inclination change is.&lt;br&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{\sqrt{ \left(\operatorname{sin} \left( x \right) \; \sqrt{\frac{2}{r1} - \frac{2}{r1 + r2}} \right)^{2} +  \left(\operatorname{cos} \left( x \right) \; \sqrt{\frac{2}{r1} - \frac{2}{r1 + r2}} - \frac{1}{\sqrt{r1}} \right)^{2}} + \sqrt{ \left(\frac{\operatorname{sin} \left( inc - x \right)}{\sqrt{r2}} \right)^{2} +  \left(\frac{\operatorname{cos} \left( inc - x \right)}{\sqrt{r2}} - \sqrt{\frac{2}{r2} - \frac{2}{r1 + r2}} \right)^{2}}}$$&#xA;&lt;br&gt;&lt;br&gt;&#xA;Where r1 and r2 are the radii of the departure and target orbit respectively, inc the total angle of the plane change, and x is the plane change done on departure. The issue is then solving for x when the derivative equals zero. That is the nasty equation:&lt;br&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{\frac{\left(-\sqrt{2} \; \sqrt{\frac{1}{r2} - \frac{1}{r1 + r2}} + \frac{\operatorname{cos} \left( inc - x \right)}{\sqrt{r2}} \right) \; \frac{\operatorname{sin} \left( inc - x \right)}{\sqrt{r2}} - 2 \; \operatorname{sin} \left( inc - x \right) \; \frac{\operatorname{cos} \left( inc - x \right)}{r2}}{\sqrt{ \left(\frac{\operatorname{sin} \left( inc - x \right)}{\sqrt{r2}} \right)^{2} +  \left(-\sqrt{2} \; \sqrt{\frac{1}{r2} - \frac{1}{r1 + r2}} + \frac{\operatorname{cos} \left( inc - x \right)}{\sqrt{r2}} \right)^{2}}} + \frac{2 \;  \left(\frac{1}{r1} - \frac{1}{r1 + r2} \right) \; \operatorname{sin} \left( x \right) \; \operatorname{cos} \left( x \right) - 2 \; \sqrt{2} \;  \left(\sqrt{2} \; \sqrt{\frac{1}{r1} - \frac{1}{r1 + r2}} \; \operatorname{cos} \left( x \right) - \frac{1}{\sqrt{r1}} \right) \; \sqrt{\frac{1}{r1} - \frac{1}{r1 + r2}} \; \operatorname{sin} \left( x \right)}{\sqrt{ \left(\sqrt{2} \; \sqrt{\frac{1}{r1} - \frac{1}{r1 + r2}} \; \operatorname{sin} \left( x \right) \right)^{2} +  \left(\sqrt{2} \; \sqrt{\frac{1}{r1} - \frac{1}{r1 + r2}} \; \operatorname{cos} \left( x \right) - \frac{1}{\sqrt{r1}} \right)^{2}}}} = 0$$&#xA;&lt;br&gt;&lt;br&gt;&#xA;@jah138 posted an answer on how to go further with this.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-16T22:19:54.877" LastActivityDate="2015-12-16T22:19:54.877" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13001" PostTypeId="1" AcceptedAnswerId="13003" CreationDate="2015-12-15T18:09:50.093" Score="2" ViewCount="870" Body="&lt;p&gt;When we look at the Moon in the night sky, why do we always see the same side every night?&lt;/p&gt;&#xA;" OwnerUserId="12626" LastEditorUserId="49" LastEditDate="2015-12-15T23:32:44.490" LastActivityDate="2015-12-16T05:50:06.987" Title="Why does the Moon only show its near side while orbiting the Earth?" Tags="|the-moon|orbit|observation|astronomy|tidal-locking|" AnswerCount="2" CommentCount="1" ClosedDate="2015-12-22T15:01:50.550" ContentLicense="CC BY-SA 3.0" />
  <row Id="13002" PostTypeId="1" AcceptedAnswerId="13006" CreationDate="2015-12-15T18:16:12.617" Score="0" ViewCount="126" Body="&lt;p&gt;If the orbit of the Moon was elliptical or in someway different from what it is currently, would that change affect the climate of the Earth?&lt;/p&gt;&#xA;" OwnerUserId="12626" LastActivityDate="2015-12-15T19:08:00.577" Title="Would the orbit of the Moon affect Earth's climate?" Tags="|the-moon|orbit|climate|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13003" PostTypeId="2" ParentId="13001" CreationDate="2015-12-15T18:17:17.250" Score="5" Body="&lt;p&gt;The Moon is &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_locking&quot; rel=&quot;nofollow&quot;&gt;Tidally locked&lt;/a&gt; to the Earth.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This effect is known as synchronous rotation. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We have a few questions centering around the concept of &lt;a href=&quot;/questions/tagged/tidal-locking&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;tidal-locking&amp;#39;&quot; rel=&quot;tag&quot;&gt;tidal-locking&lt;/a&gt; you may want to take a look at them.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="6302" LastEditDate="2015-12-15T19:53:14.417" LastActivityDate="2015-12-15T19:53:14.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13004" PostTypeId="2" ParentId="13001" CreationDate="2015-12-15T18:17:56.463" Score="5" Body="&lt;p&gt;Because of &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_locking&quot;&gt;tidal locking&lt;/a&gt;. The gravitational forces between two massive bodies like the Earth and the moon will cause their rotations to slow down over long periods of time, eventually stopping them relative to one another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The smaller body will lock first because the larger body's gravitational effect is stronger, but at some point in the distant future, Earth's rotation will slow to once a month as well, showing the same face to the moon at all times.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-15T18:19:23.640" LastActivityDate="2015-12-15T18:19:23.640" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13005" PostTypeId="1" AcceptedAnswerId="13007" CreationDate="2015-12-15T18:49:12.733" Score="1" ViewCount="2183" Body="&lt;p&gt;We know that according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_fusion&quot; rel=&quot;nofollow&quot;&gt;Wikipedia on &lt;strong&gt;Nuclear fusion&lt;/strong&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Sun is a &lt;strong&gt;main-sequence&lt;/strong&gt; star, and thus generates its &lt;strong&gt;energy&lt;/strong&gt; by nuclear fusion of &lt;strong&gt;hydrogen&lt;/strong&gt; nuclei into &lt;strong&gt;helium&lt;/strong&gt;. In its core, the Sun fuses 620 million &lt;strong&gt;metric tons&lt;/strong&gt; of hydrogen each second.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What would happen if we sent a nuke into the Sun on doomsday?&lt;/p&gt;&#xA;" OwnerUserId="12626" LastActivityDate="2019-11-01T13:18:54.713" Title="Nuking the Sun?" Tags="|the-sun|nuclear|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13006" PostTypeId="2" ParentId="13002" CreationDate="2015-12-15T18:56:51.213" Score="2" Body="&lt;p&gt;The most important effect of the Moon on the Earth is tides. The act of transporting huge amounts of water around is important for circulation and exchange of matter, and is also making the shore line a wide and ecological interesting area. If the Moon had another orbit, the tides would have another strength or frequency.&lt;br&gt;&#xA;Microbiological life in the top layer of the ocean is responsible for a lot of the &lt;a href=&quot;http://science.opposingviews.com/photosynthesis-place-ocean-5104.html&quot; rel=&quot;nofollow&quot;&gt;photosynthesis&lt;/a&gt;, important for the composition of the atmosphere. They are dependant on the transport of nutrients between coastal and open ocean zones.&lt;br&gt;&#xA;The Moon may also &lt;a href=&quot;http://www.intelligentdesigntheory.info/moon_comet_meteor_shower_asteroid_planet_earth.htm&quot; rel=&quot;nofollow&quot;&gt;protect the Earth from meteorites&lt;/a&gt;, event though that is &lt;a href=&quot;http://www.answers.com/Q/Does_the_moon_protect_the_earth_from_asteroids&quot; rel=&quot;nofollow&quot;&gt;disputed.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-15T19:08:00.577" LastActivityDate="2015-12-15T19:08:00.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13007" PostTypeId="2" ParentId="13005" CreationDate="2015-12-15T18:58:42.287" Score="12" Body="&lt;p&gt;What would happen? Not much. The Sun is mindbogglingly vast. Even our biggest nuclear bombs don't fuse more than 1 ton of hydrogen. Compare that with the Sun's 620 million tons burned per second: the nuke would add $10^{-9}$ to the Sun's output. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="3065" LastEditDate="2015-12-15T19:19:19.350" LastActivityDate="2015-12-15T19:19:19.350" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13008" PostTypeId="1" AcceptedAnswerId="13009" CreationDate="2015-12-15T19:20:53.857" Score="3" ViewCount="108" Body="&lt;p&gt;I got lost down the wiki hole and ended up reading a page on &lt;a href=&quot;https://en.wikipedia.org/wiki/Sergei_Avdeyev&quot; rel=&quot;nofollow&quot;&gt;Sergei Avdeyev&lt;/a&gt; when I noticed that he has 2 very short spacewalks listed:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol start=&quot;6&quot;&gt;&#xA;  &lt;li&gt;MIR EO-20 – 8 December 1995 – 0 hours, 37 minutes&lt;/li&gt;&#xA;  &lt;li&gt;MIR EO-26 – 15 September 1998 – 0 hours, 30 minutes&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As far as I understand preparing from and then cleaning up after a spacewalk takes a lot of time and effort, so having such short spacewalks seems like a bit of a waste. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why where these spacewalks so short? Did something happen during the walks or where they designed to be this short?&lt;/p&gt;&#xA;" OwnerUserId="3141" LastActivityDate="2015-12-15T19:33:23.990" Title="Why were these spacewalks so short?" Tags="|eva|russia|mir|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13009" PostTypeId="2" ParentId="13008" CreationDate="2015-12-15T19:27:28.703" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Mir_spacewalks&quot; rel=&quot;nofollow&quot;&gt;wiki page on Mir space walks&lt;/a&gt; has a note for each of these spacewalks. Nothing went wrong, but the reason for them were small routine tasks not taking much time. He just returned after the tasks were done.&lt;br&gt;&#xA;A little more detail on the EO-20 spacewalk can be found on page 42 of &lt;a href=&quot;http://ston.jsc.nasa.gov/collections/TRS/_techrep/TP-1998-208920.pdf&quot; rel=&quot;nofollow&quot;&gt;this pdf.&lt;/a&gt;&lt;br&gt;&#xA;More about the EO-26 spacewalk is &lt;a href=&quot;http://www.spacefacts.de/mir/english/mir-26.htm&quot; rel=&quot;nofollow&quot;&gt;here.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-15T19:33:23.990" LastActivityDate="2015-12-15T19:33:23.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13010" PostTypeId="2" ParentId="12989" CreationDate="2015-12-15T19:48:39.533" Score="4" Body="&lt;p&gt;For the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orlan_space_suit&quot; rel=&quot;nofollow&quot;&gt;Orlan&lt;/a&gt; spacesuits this is not possible. From &lt;a href=&quot;http://www.astronautix.com/craft/orlan.htm&quot; rel=&quot;nofollow&quot;&gt;Encyclopedia Astronautica&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It consisted of flexible limbs attached to a one-piece rigid body / helmet unit.&#xA;  The suit was entered through a hatch in the rear of the torso.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The helmets of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sokol_space_suit&quot; rel=&quot;nofollow&quot;&gt;Sokol&lt;/a&gt; suits are easy to open, like shown in this video: (3:15)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/R2MJD60he8g?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Strizh_%28space_suit%29&quot; rel=&quot;nofollow&quot;&gt;Strizh&lt;/a&gt; suit seems to have the &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/b/b7/Strizh_spacesuit_4148047368_c19cec3782_o.jpg&quot; rel=&quot;nofollow&quot;&gt;same mechanism.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-15T20:11:30.070" LastActivityDate="2015-12-15T20:11:30.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13011" PostTypeId="2" ParentId="12999" CreationDate="2015-12-15T21:09:06.963" Score="9" Body="&lt;p&gt;They wouldn't work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a pipe, wind blowing over the fipple, or past a reed causes vortices which give lots of different frequencies. Then at the open end of the pipe (or open finger holes) the change to a fixed pressure causes most of the vibration to be reflected back down the pipe, setting up standing waves at various harmonics, which produce the tone. With no atmosphere in space, the air would rush explosively down the pipe, there would be no reflection, no establishment of standing waves, hence no tone formed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you put a microphone on the body of the instrument you would probably pick up a lot of noise, of rushing air. You would not get a tone. The air down the pipe would be moving &lt;a href=&quot;http://www.geoffreylandis.com/higgins.html&quot; rel=&quot;nofollow&quot;&gt;quite fast&lt;/a&gt;. Don't put your head in the way!&lt;/p&gt;&#xA;" OwnerUserId="11966" LastEditorUserId="49" LastEditDate="2016-02-10T04:31:53.347" LastActivityDate="2016-02-10T04:31:53.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13012" PostTypeId="2" ParentId="12899" CreationDate="2015-12-15T21:50:07.043" Score="2" Body="&lt;p&gt;The problem with a wheeled rover on an asteroid is the difficulty of making the wheels obtain a proper grip. A good grip is on Earth obtained by the vehicle experiencing a down-force towards the ground, provided by the gravity. When high acceleration is needed, for example in race-cars, additional down-force is provided by a spoiler.&lt;br&gt;&#xA;When the wheels pushes of from the ground, &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;a similar force&lt;/a&gt; is experienced by the vehicle. This is causing it to turn. With almost no gravity to hold it back, it will continue to turn, eventually falling over. That requires the rover to have wheels on all sides, complicating the design.&lt;/p&gt;&#xA;&lt;p&gt;The hedgehog design is designed to tumble, and the lack of wheels make it possible to have vulnerable moving parts within a protective hull. The hedgehog is also capable of stopping quickly, simply cancelling out the tumbling with the flywheels. Braking with wheels is again depending on traction with the ground.&lt;/p&gt;&#xA;&lt;p&gt;However, the tiny gravity of these small objects are limiting the rover's maximum velocity, regardless of design. For example, the velocity of an orbit around the smaller of Mars' moons, Deimos is less than 4 m/s. If the rover is travelling over this velocity, it will be orbiting and refuse to fall back to the ground.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-12-15T21:57:09.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13013" PostTypeId="2" ParentId="12879" CreationDate="2015-12-15T22:32:40.723" Score="9" Body="&lt;p&gt;Using &lt;em&gt;current technology&lt;/em&gt; (and by that I mean experiments and telescopes that are available now) we would probably be unable to detect life on Earth even if observed from a distance of a few light years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &quot;blind&quot; search could look for radio signatures and of course this is what SETI has been doing. If we are talking about detecting &quot;Earth-like&quot; signals, then we must assume that we are not talking about deliberate beamed attempts at communication, and so must rely on detecting random radio &quot;chatter&quot; and accidental signals generated by our civilisation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SETI Phoenix project was the most advanced search for radio signals from other intelligent life. Quoting from &lt;a href=&quot;http://adsabs.harvard.edu/full/2000ASPC..213..451C&quot;&gt;Cullers et al. (2000)&lt;/a&gt;: &quot;&lt;em&gt;Typical signals, as opposed to our strongest signals fall below the detection threshold of most surveys, even if the signal were to originate from the nearest star&lt;/em&gt;&quot;. Quoting from &lt;a href=&quot;http://adsabs.harvard.edu/abs/2001ARA%26A..39..511T&quot;&gt;Tarter (2001)&lt;/a&gt;: &quot;&lt;em&gt;At current levels of sensitivity, targeted microwave searches could detect the equivalent power of strong TV transmitters at a distance of 1 light year (within which there are no other stars)...&lt;/em&gt;&quot;. The equivocation in these statements is due to the fact that we &lt;em&gt;do&lt;/em&gt; emit stronger beamed signals in certain well-defined directions, for example to conduct metrology in the solar system using radar. Such signals have been calculated to be observable over a thousand light years or more. But these signals are brief, beamed into an extremely narrow angle and unlikely to be repeated. You would have to be very lucky to be observing in the right direction at the right time if you were performing targeted searches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been suggested that new radio telescope projects and technology like the Square Kilometre Array may be capable of serendipitously detecting radio &quot;chatter&quot; out to distances of 50 pc ($\sim 150$ light years) - see &lt;a href=&quot;http://adsabs.harvard.edu/abs/2007JCAP...01..020L&quot;&gt;Loeb &amp;amp; Zaldarriaga (2007)&lt;/a&gt;. This array, due to begin full operation some time after 2025 could also monitor a multitude of directions at once for beamed signals. A good overview of what might be possible in the near future is given by &lt;a href=&quot;http://adsabs.harvard.edu/abs/2009astro2010S.294T&quot;&gt;Tarter et al. (2009)&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8792" LastActivityDate="2015-12-15T22:32:40.723" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13015" PostTypeId="2" ParentId="12899" CreationDate="2015-12-15T23:30:02.597" Score="-1" Body="&lt;p&gt;The 'Hedgehog' robot is designed to explore an asteroid by the use of flywheels. The concept behind the use of the flywheel is to use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kinetic_energy&quot; rel=&quot;nofollow&quot;&gt;kinetic energy&lt;/a&gt; it produces and repurpose it for the task of its unique mode of transport. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the robot to return to the comet, we must refer to &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton.27s_first_law&quot; rel=&quot;nofollow&quot;&gt;Newton's First Law&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first law states that if the net force (the vector sum of all forces acting on an object) is zero, then the velocity of the object is constant. Velocity is a vector quantity which expresses both the object's speed and the direction of its motion; therefore, the statement that the object's velocity is constant is a statement that both its speed and the direction of its motion are constant.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Newton's First Law also applies to space. We take into example of how rocket's thrusters guide it to a specified target. Therefore, we can assume that the flywheels are designed to move the robot to its destination by upsetting its balance of equilibrium in the middle of its brief flight in space.  &lt;/p&gt;&#xA;" OwnerUserId="12626" LastEditorUserId="12626" LastEditDate="2015-12-15T23:42:14.360" LastActivityDate="2015-12-15T23:42:14.360" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13016" PostTypeId="2" ParentId="12998" CreationDate="2015-12-15T23:54:51.987" Score="4" Body="&lt;p&gt;The obvious engineering discipline would be aerospace engineering. Though mechanical engineering and some of the applied sciences would also be good options to consider. It would probably also be a good idea to obtain an advanced degree in your field. Many astronauts have held PhD's.&#xA;I say aerospace because it will give you a solid foundation in many of the topics astronauts must know - orbital mechanics, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you don't want to go down the military flight path, it wouldn't hurt to get some civilian flying experience to help your application. &#xA;You should also consider becoming involved in activities such as mountaineering or SCUBA diving to show you can work in a hostile environment.&#xA;Since current and future space missions are likely to be multinational, you should also learn a foreign language such as Russian. You should also aim to be very physically fit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, that was a very long list. Becoming an astronaut is no easy task and very few who set out to get there actually make it. Best of luck to you.&lt;/p&gt;&#xA;" OwnerUserId="6254" LastActivityDate="2015-12-15T23:54:51.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13017" PostTypeId="1" CreationDate="2015-12-16T01:06:35.323" Score="2" ViewCount="467" Body="&lt;p&gt;Is there readily available video of Space Shuttle missions showing the crew:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;During the entire time period from ignition to shortly after the shuttle has passed through planet Earth's atmosphere?&lt;/li&gt;&#xA;&lt;li&gt;During the entire time period just before reentry to just after landing?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I have interviewed multiple astronauts from Space Shuttle missions, and they thought such footage should be available on NASA's website, but I have yet to find it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know if such footage exists, and where it is located? If not, does anyone know why NASA does not make it available?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(Note that I am also interested in similar footage from all other spacecraft starting from the first human spaceflight (Vostok 1) until present.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12180" LastActivityDate="2015-12-16T01:40:55.903" Title="Footage of Space Shuttle astronauts during launch and reentry" Tags="|crewed-spaceflight|space-shuttle|humans|physical-activity|crew-module|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13018" PostTypeId="2" ParentId="13017" CreationDate="2015-12-16T01:40:55.903" Score="4" Body="&lt;p&gt;There's loads of them, use e.g. &lt;a href=&quot;https://www.google.com/videohp&quot; rel=&quot;noreferrer&quot;&gt;Google's video search&lt;/a&gt; with some related search terms like &lt;em&gt;&lt;a href=&quot;https://encrypted.google.com/#q=space+shuttle+inside+cabin+launch+landing&amp;amp;tbm=vid&quot; rel=&quot;noreferrer&quot;&gt;space shuttle inside cabin launch landing&lt;/a&gt;&lt;/em&gt;. This returned for example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/NsmW_y04z_Y?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/98SbI5Wv1Ck?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;... and so on. Replace search term to find videos from inside the crew cabin of other launch vehicles. Happy viewing! :)&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-12-16T01:40:55.903" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13019" PostTypeId="2" ParentId="9156" CreationDate="2015-12-16T02:51:07.563" Score="7" Body="&lt;p&gt;The main barrier to making a cost effective rocket sled launch is the atmosphere. It's a huge barrier to making a cost effective rocket sled launch for a few reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Atmospheric drag at terrestrial altitudes reduce the effectiveness of building up speed on a terrestrial track.&lt;/li&gt;&#xA;&lt;li&gt;Stresses of moving a rocket through the atmosphere at higher speeds requires stronger rocket structures that are necessarily heavier.&lt;/li&gt;&#xA;&lt;li&gt;Reaching altitudes above mountain tops is a huge engineering and materials science challenge&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;An example is if you built a track up Mt Denali, you could easily make it up to 700 m/s at just 3 gees if you started horizontally before ascending from the base of the mountain. The problem is that that mountain is only at an altitude of 6.2km (20,300ft). Even if you got to 700 m/s at the top of mt Everest, you'd only be at 8.84km (29,000ft). To compare to a shuttle launch, at 8.84km the shuttle is moving at about 230 m/s (~750 ft/s), so 700 m/s would be about 3 times as fast and have air drag on it of 4.5 times as much (for why see this answer for a very helpful graph showing various atmospheric variables in a space shuttle launch: &lt;a href=&quot;https://space.stackexchange.com/questions/12982/what-barriers-are-there-to-creating-a-reusable-rocket-sled-lanchpad/12983?noredirect=1#comment33956_12983&quot;&gt;What barriers are there to creating a reusable rocket sled lanchpad?&lt;/a&gt; ). That extra drag would suck away a lot of the velocity gained via the sled, and would require a heavier rocket structured to withstand that speed at that altitude. I don't know how to estimate the extra fuel cost and rocket construction cost of a heavier rocket, so I'll have to end that thought there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly for reaching higher altitudes where the atmosphere would pose less of an issue, I don't know how to estimate the feasibility of building the structure it would take to get much higher than earth's mountains. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But lets say you didn't want a stronger, heavier rocket, and just wanted to get up to the speed the shuttle usually got to at mountain-altitude. I've been told the amount of fuel it takes to get up to orbital height is only a small fraction of the amount of fuel it takes to get up to orbital speed (8 km/s), which would mean you can't save much fuel from a mountain-altitude rocket sled. It would certainly save some, but possibly not much. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there might be easier avenues at the moment for cost savings. Designing fully reusable vehicles might be able to save more money in launches than the minimal fuel and structure reductions a rocket sled launch could make. Also, a working scramjet engine would be able to use aircraft to get rockets up beyond the atmosphere &lt;em&gt;and&lt;/em&gt; to orbital speed, which would replace almost all the rocket fuel needed to get to LEO with much cheaper aircraft fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in summary, the barriers to economically feasible rocket sleds are problems that have yet to be overcome in the design of rocket sled launches and more promising alternatives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the above said, since the atmosphere is the main barrier, on bodies like the moon with no atmosphere, a rocket sled is a much more attractive option. &lt;/p&gt;&#xA;" OwnerUserId="12624" LastEditorUserId="12624" LastEditDate="2017-08-31T07:39:24.350" LastActivityDate="2017-08-31T07:39:24.350" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13022" PostTypeId="1" AcceptedAnswerId="13842" CreationDate="2015-12-16T10:37:15.390" Score="11" ViewCount="1360" Body="&lt;p&gt;In the long run Mars needs a magnetic field to keep a thick atmosphere and for radiation shielding, if humanity were to terraform it. Starting a dynamo in Mars' core seems forever impossible, but a magnetic field could instead be induced by the Solar wind. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus has &lt;a href=&quot;http://www.astrobio.net/topic/solar-system/venus/a-magnetic-surprise-from-venus/&quot; rel=&quot;noreferrer&quot;&gt;a naturally induced&lt;/a&gt; magnetic field. Could an ionosphere be densified to create a much stronger magnetic field? For example by bringing material from Phobos and ionising it above Mars?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="15685" LastEditDate="2021-03-31T03:22:57.317" LastActivityDate="2021-03-31T03:22:57.317" Title="How could an induced magnetosphere be created at Mars for terraforming?" Tags="|mars|radiation|terraforming|magnetic-field|magnetosphere|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13024" PostTypeId="2" ParentId="3490" CreationDate="2015-12-16T10:52:58.513" Score="1" Body="&lt;p&gt;An explanation to the equatorial ridge of Iapetus must also explain the similar, yet even more striking feature of two other Saturn moons, &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_%28moon%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Pan_%28moon%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pan&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bRmpi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bRmpi.jpg&quot; alt=&quot;Atlas&quot;&gt;&lt;/a&gt;&lt;a href=&quot;https://i.stack.imgur.com/i2aLMs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i2aLMs.jpg&quot; alt=&quot;Pan&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This may rule out the explanations involving geological activity, because Atlas and Pan are too small to have any.&#xA;This leaves the explanation that the ridges are material from the rings of Saturn &lt;a href=&quot;http://www.space.com/4711-flying-saucers-saturn-explained.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;swept up by the moons&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To address the second part of your question, regarding cold-erosion, this is due to glaciers. Glaciers consist of ice, like Iapetus, but this ice is not moving like on Earth, thereby causing no erosion due to friction.&#xA;Water in the outer solar system is best viewed as just another type of rock. Like most objects in space, the mountain range of Iapetus has not received any notable erosion.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2015-12-16T10:52:58.513" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13026" PostTypeId="2" ParentId="12997" CreationDate="2015-12-16T14:38:00.293" Score="4" Body="&lt;p&gt;Using @Hohmanfan's expression, the total $\Delta V$ for the Hohmann transfer with inclination change $\Delta i$ is assumed as&#xA;\begin{equation}&#xA;\Delta V = \sqrt{\left(\zeta_{1}s_{x}\right)^2 + \left(\zeta_{1}c_{x}-\lambda_{1}\right)^2} + \sqrt{\left(\lambda_{2}s_{ix}\right)^2 + \left(\lambda_{2}c_{ix}-\zeta_{2}\right)^2}&#xA;\end{equation} &#xA;where the coefficients are used for analytical simplicity and are given as&#xA;\begin{equation}&#xA;\begin{split}&#xA; \zeta_1 &amp;amp;= \sqrt{\frac{2}{r_1}-\frac{2}{r_1+r_2}}\\&#xA; \zeta_2 &amp;amp;= \sqrt{\frac{2}{r_2}-\frac{2}{r_1+r_2}}\\&#xA; c_{i} &amp;amp;= \cos{(i)}\\&#xA; s_{i} &amp;amp;= \sin{(i)}\\&#xA;\end{split}&#xA;\quad\quad&#xA;\begin{split}&#xA; \lambda_1 &amp;amp;= \frac{1}{\sqrt{r_1}}\\&#xA; \lambda_2 &amp;amp;= \frac{1}{\sqrt{r_2}}\\&#xA; c_{ix} &amp;amp;= \cos{(i-x)}\\&#xA; s_{ix} &amp;amp;= \sin{(i-x)}&#xA;\end{split}&#xA;\end{equation}&#xA;Using Matlab, we can take the first and second partials of $\Delta V$ with respect to $x$, the initial inclination change to find $\frac{\partial\Delta V}{\partial x}$ and $\frac{\partial^2\Delta V}{\partial x^2}$, which are not presented here. @Hohmanfan's expression for the first partial gives us a nonlinear transcendental function that likely does not have an analytical solution in general. However, some simple root finding techniques (Newton-Raphson, etc.) are quite adequate here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the given $r_1 = 1, r_2 = 2$ scenario, I found the minimizing $x$ for a range of total inclination changes $i$. This is accomplished by iterating through the total inclination change vector, finding the min $\Delta V$ and the corresponding minimizing $x$, and then verifying local optimality by checking the value of the first derivative is zero, and the sign on the second derivative is positive. Figure 1 shows the minimizing $x$ and minimum $\Delta V$ for a given total inclination change $\Delta i$. Matching the OP's intuition, it appears that the largest portion of the plane change is saved at the end of the transfer, while there is some benefit to performing an initial inclination change with the first burn. The graph also makes intuitive sense when looking at the coplaner case hen $\Delta i = 0$, which is the case of a coplaner Hohmann transfer.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ZcR4A.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZcR4A.jpg&quot; alt=&quot;Figure 1: Minimizing inclination change and minimum burn value&quot;&gt;&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, Fig. 2 displays the minimum $\Delta V$ as a function of the minimizing inclination angle change $x$. We can see there is some benefit to the initial plane change, but for very large inclination changes, the burn is incredibly expensive and the effect of the initial change is much less significant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RxQU8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RxQU8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1478" LastActivityDate="2015-12-16T14:38:00.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13027" PostTypeId="2" ParentId="12998" CreationDate="2015-12-16T16:52:38.183" Score="6" Body="&lt;p&gt;I will be applying for the recently opened Astronaut Candidate position, and I have a Ph.D. in optical engineering -- everything from cameras, detectors, optical systems, radiometry.  However, and more importantly, I have had the pleasure of speaking 1-on-1 with 2 astronauts who both alluded the following: While NASA &lt;em&gt;does&lt;/em&gt; look favorably on an advanced degree, your initial field-specific education (like mine, in optics) had 0% influence on what they wound up doing during the mission. NASA, I think, trains you in what they think you should know. If you haven't learned it by then, they will teach you (granted your application gets accepted). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, a Ph.D. in an engineering field can take the place of years of flight experience that you would have built by going the route of a military pilot. I do agree w/ @Daniel Cleaver 's answer.&lt;/p&gt;&#xA;" OwnerUserId="12646" LastActivityDate="2015-12-16T16:52:38.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13028" PostTypeId="1" AcceptedAnswerId="13030" CreationDate="2015-12-16T20:13:05.090" Score="0" ViewCount="460" Body="&lt;p&gt;There have been &lt;a href=&quot;http://io9.gizmodo.com/5923828/dont-worry-people--nasa-has-a-plan-for-moving-the-earth&quot; rel=&quot;nofollow&quot;&gt;proposals to change planetary orbits using gravity assists&lt;/a&gt;. Could this be used to bring Venus close to the sun, burn off its toxic atmosphere, then move it into the habitable zone for terraforming?&lt;/p&gt;&#xA;" OwnerUserId="12649" LastEditorUserId="195" LastEditDate="2015-12-16T21:07:39.260" LastActivityDate="2019-02-11T01:05:38.710" Title="Would it be possible to burn off Venus' atmosphere using the sun?" Tags="|terraforming|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13029" PostTypeId="2" ParentId="10239" CreationDate="2015-12-16T20:31:33.117" Score="2" Body="&lt;p&gt;How quickly human adapts to other kinds of force fields is hard to answer, because no one has ever lived in one. For weightlessness however, we have a lot of data showing that humans quickly learn how to move around and predicting movement of objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let us consider some of the interesting effects in a rotating cylinder. For a hab with a radius of 10 meters and simulated Earth gravity, walking against the direction of rotation at 1.5 m/s will make you feel about 28% lighter. In the opposite direction you will feel 32% heavier.&#xA;Throwing a ball towards the direction of rotation will cause it to fall down as expected, but the harder you throw it, the faster it falls. In the other direction we have a lot of confusing behaviour. If you throw the ball gently, it would just arc and fall like normal. Throwing it harder causes the ball to fall slower, up to a point where it is not falling at all, orbiting inside the hab. After six and a half second the ball returns to you, making it possible to catch your own throw. An orbiting ball will be slowed by the air eventually falling to the floor.&lt;br&gt;&#xA;Throwing the ball even harder would cause it to at first fall, then falling slower when air has slowed it down a bit, after that it would start to orbit, and finally it will fall and hit the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Throwing the ball straight up would make it rise at first, then its trajectory will be bended to the side and fall down. No matter how hard you throw it, it would always miss the centre. If you instead throw it at an angle sidewards it follows the type of trajectory showed in your diagram. This makes it possible for the ball to pass the middle using a loop, or even a spiral path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This effects are adding a great deal of unpredictability to a ball game, especially if the ball ends up slowly floating in the middle, and then returns after an unpredictable time depending on how close it was to the middle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A somewhat related non-uniform force field environment is a rotating parabola constructed on Earth. Here it will always feel like you are on a flat floor too, with similar weird effects like your weight increasing towards the edge, and ball trajectories apparently bending to the side.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2015-12-16T20:31:33.117" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13030" PostTypeId="2" ParentId="13028" CreationDate="2015-12-16T20:46:31.483" Score="4" Body="&lt;p&gt;If you somehow have the capability to move masses of planetary size, then moving only the atmosphere would be easier than moving the planet itself, being thousands of times less massive. Exactly how this can be accomplished is a concept belonging so far into the future that it is in essence impossible to say anything meaningful about it. Our current capability of sending equipment to Venus is only several tons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the sun to heat the gasses of the atmosphere can be accomplished by large mirrors in space. If the gasses have a high enough velocity, they can &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_escape&quot; rel=&quot;nofollow noreferrer&quot;&gt;escape&lt;/a&gt; the gravity of Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mZMF5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mZMF5.png&quot; alt=&quot;graph of escape velocity vs temperature and gas&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This chart shows how heating the atmosphere can make gasses escape, but the energy requirements are enormous. In order to be able to heat the exosphere, you must focus the mirrors on a tiny part of it, because most of the radiation will just pass through the low density gas. Removing the atmosphere in this way is possible in theory, but how you should be able to do it in practice I have not the foggiest idea of due to the enormous scale of the operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Making Venus more habitable is possible with other means than relocating it; the lead melting temperatures can for instance be reduced by a large &lt;a href=&quot;http://www.orionsarm.com/fm_store/TerraformingVenusQuickly.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;sunshade&lt;/a&gt; blocking some of the light from the Sun. That does not get rid of the high pressure at the surface though, but there is &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_sea_communities&quot; rel=&quot;nofollow noreferrer&quot;&gt;life&lt;/a&gt; capable of tolerating a pressure of 92 atmospheres, corresponding to a depth of &lt;a href=&quot;http://oceanservice.noaa.gov/facts/pressure.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;910 meters&lt;/a&gt; in the ocean.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="6816" LastEditDate="2019-02-11T01:05:38.710" LastActivityDate="2019-02-11T01:05:38.710" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="13034" PostTypeId="2" ParentId="12998" CreationDate="2015-12-16T22:49:15.657" Score="22" Body="&lt;p&gt;I thought it might be interesting (and hopefully helpful) to take a look at the education that astronauts actually have. For that, I decided to look at the list of &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/astrobio.html&quot;&gt;current NASA astronauts&lt;/a&gt; (and &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/astrobio_mgmt.html&quot;&gt;current but not flight-eligible&lt;/a&gt;), and &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/astrobio_former.html&quot;&gt;former NASA astronauts&lt;/a&gt; (there are some extremely interesting people in there, by the way).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sake of this answer, I'm going to simplify a few things and lump similar degrees together. In some of these instances degrees have similar, but different, names. They seem to be similar enough for the purpose of this answer, and it'll make this easier to write and read if I list them together. Because this question is specifically asking about engineering I'm also going to leave out anything that isn't related to engineering, unless it came up a lot and I think it's interesting. I'm also excluding any honorary degrees, and I tried to include only degrees earned before the astronaut's career began because anything following that seemed less relevant to becoming an astronaut (not guaranteeing perfection here, though). Only the highest degrees earned by each astronaut made it into my list.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Current astronauts&lt;/strong&gt; (including those ineligible for flight):&#xA;Of the current astronauts, five of them have earned Bachelor degrees, 43 have earned Master degrees, and 31 have earned a Doctorate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of the Bachelor degrees, one is a Civil Engineering degree, and two are Aerospace Engineering degrees. The only other two are in Mathematics. It seems that, for the most part at least, astronauts with only a Bachelor degree also had successful military careers. Most astronauts have earned at least a Master degree.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most common Master degrees seem to be in Aerospace Engineering (including Aeronautics and/or Astronautics). Here is a list:&lt;br /&gt;&#xA;Aerospace Engineering: 16&lt;br /&gt;&#xA;Electrical Engineering: 4&lt;br /&gt;&#xA;Mechanical Engineering: 3&lt;br /&gt;&#xA;Ocean Engineering: 2&lt;br /&gt;&#xA;Systems Engineering: 2&lt;br /&gt;&#xA;Flight Test Engineering: 1&lt;br /&gt;&#xA;Industrial Engineering: 1&lt;br /&gt;&#xA;Engineering Management: 1&lt;br /&gt;&#xA;The rest of the Master degrees were in other science, physics, and technology fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several astronauts with doctorate degrees have them in medicine. Currently there are eight astronauts with medical degrees (and one veterinarian). There are four with physics degrees. For engineering:&lt;br /&gt;&#xA;Aerospace Engineering: 3&lt;br /&gt;&#xA;Mechanical Engineering: 2&lt;br /&gt;&#xA;Chemical Engineering: 1&lt;br /&gt;&#xA;Polymer Science and Engineering: 1&lt;br /&gt;&#xA;Electrical Engineering: 1&lt;br /&gt;&#xA;Bioengineering: 1&lt;br /&gt;&#xA;The rest are in different biology fields, chemistry, and geology.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Past Astronauts&lt;/strong&gt;: There were 49 past astronauts who earned a Bachelor degree, 146 with Master degrees, and 102 with Doctorates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Bachelor degrees, Aerospace Engineering degrees dominated this category as well.&lt;br /&gt;&#xA;Aerospace Engineering: 23&lt;br /&gt;&#xA;Mechanical Engineering: 5&lt;br /&gt;&#xA;degrees listed only as &quot;Engineering&quot;: 3&lt;br /&gt;&#xA;Electrical Engineering: 2&lt;br /&gt;&#xA;Engineering Physics: 1&lt;br /&gt;&#xA;Engineering Science: 1&lt;br /&gt;&#xA;Systems Engineering: 1&lt;br /&gt;&#xA;Engineering Management: 1&lt;br /&gt;&#xA;The rest of the degrees here were in mathematics, physics, sciences, aviation, and business.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of the 146 Master degrees, a very high number of them were in engineering, and nearly half of them were in Aerospace Engineering (including Aeronautical Engineering, Astronautical Engineering, etc).&lt;br /&gt;&#xA;Aerospace Engineering: 69&lt;br /&gt;&#xA;Mechanical Engineering: 13&lt;br /&gt;&#xA;Engineering Management: 6&lt;br /&gt;&#xA;Electrical Engineering: 5&lt;br /&gt;&#xA;Nuclear Engineering: 3&lt;br /&gt;&#xA;Engineering Science: 3&lt;br /&gt;&#xA;Listed only as &quot;Engineering&quot;: 2&lt;br /&gt;&#xA;Flight Structures Engineering: 1&lt;br /&gt;&#xA;Ocean Engineering: 1&lt;br /&gt;&#xA;Metallurgical Engineering: 1&lt;br /&gt;&#xA;Materials Science Engineering: 1&lt;br /&gt;&#xA;Electronics Engineering: 1&lt;br /&gt;&#xA;Engineering Mechanics: 1&lt;br /&gt;&#xA;Ceramic Engineering: 1&lt;br /&gt;&#xA;There are also several degrees in this category in Management/Administration (15), Computer Science, Physics, Aviation, Mathematics, Education, and other sciences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Past astronauts with Doctorates are varied, but the highest number of the 102 are in Medicine (23). As for engineers:&lt;br /&gt;&#xA;Aerospace Engineering: 10&lt;br /&gt;&#xA;Mechanical Engineering: 7&lt;br /&gt;&#xA;Electrical Engineering: 5&lt;br /&gt;&#xA;Chemical Engineering: 3&lt;br /&gt;&#xA;Material Science and Engineering: 2&lt;br /&gt;&#xA;Civil and Environmental Engineering: 1&lt;br /&gt;&#xA;Listed only as &quot;Engineering&quot;: 1&lt;br /&gt;&#xA;Biomedical Engineering: 1&lt;br /&gt;&#xA;Hydraulic Engineering: 1&lt;br /&gt;&#xA;Other high numbers in this category include: Physics (14) and Astronomy/Astrophysics (10). The rest include Meteorology, Geology, Law, Computer Science, and other fields in physics and science.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;From what I can see, they seem to favor aerospace engineering, but that shouldn't really come as too much of a surprise. What did surprise me is that about a quarter of all astronauts who earned a doctorate degree earned it in medicine. The number of degrees at that level in medicine is about equal to the degrees in all the disciplines of engineering combined. It looks like most of the doctorate degrees are in physics, mathematics, and sciences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For engineers, it looks like most of them earn Master degrees. Nearly three quarters of all astronauts with Master degrees have them in Engineering. Aerospace, Mechanical, and Electrical Engineers have the highest numbers by far, especially Aerospace. It looks like if you want to be an astronaut, having a Master degree in one of those disciplines is most valuable.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2015-12-16T22:49:15.657" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13035" PostTypeId="2" ParentId="12933" CreationDate="2015-12-16T23:38:45.270" Score="3" Body="&lt;p&gt;You have clearly already come up against the dilemma of &quot;its expensive because its expensive&quot;. Your question is reasonable, notwithstanding the first answer about learning from hands-on post-test inspection and measurement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Don't lose hope, but here are some other points that confound things:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is often a strong emphasis on qualification testing being &quot;&lt;strong&gt;representative&lt;/strong&gt;&quot;. Where this applies it means that equipment that has prior flight experience is of little value unless that flight experience is directly representative.  e.g. take a solar cell and wire it into a different circuit on a different substrate in a different orbit and little relevance remains. Even the vacuum around it can change if adhesives and grouting are different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that if a planned mission implies an environmental temperature range of, say, -10 degC to + 40 degC then a candidate item of equipment must be taken through its paces, usually having to demonstrate its entire range of functions, at both temperature extremes. Equipment subject to life testing for whatever reason, be it radiation dose or mechanical movement, would thus be expected to still show that it can meet its specification (or some agreed, degraded but still useful state) at the end of the mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the cost multipliers is the need to show that the design is understood and that there are &lt;strong&gt;no critical uncontrolled parameters&lt;/strong&gt;. This means a lot of time is put into planning tests, measuring all the right parameters before and after an environmental exposure (temperature, vacuum, vibration, radiation) and documenting it all so that the customer is finally happy. Think then of the many levels of customer and supplier relationships and its a wonder that the industry works &quot;so well&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going back to the benefits of a flight test, you can see that it could be hard to commercially demonstrate that a cubesat flight will have measured everything (just what was the temperature of and electron fluence onto that wirebundle in eclipse?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That all said, there may well be some good cubesat demonstration mission ideas that still tick all these boxes. Even if not, microsatellite missions have been used for decades to gain reputation as much as for technical qualification. We're all human and at all levels in the business the &quot;proof of the pudding&quot; concept holds a strong grip on people's imaginations from satellite operator's buying a complete satellite to several levels of equipment integration before that.  &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-12-16T23:38:45.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13036" PostTypeId="1" AcceptedAnswerId="13038" CreationDate="2015-12-17T00:06:07.547" Score="4" ViewCount="862" Body="&lt;p&gt;Is there some combination of a plane that flies high enough, with a railgun small enough to fit into the fuselage, and a satellite small and durable enough, to make this feasible?&lt;/p&gt;&#xA;" OwnerUserId="12606" LastEditorUserId="6816" LastEditDate="2015-12-17T00:19:57.217" LastActivityDate="2017-12-17T22:43:03.227" Title="Is it possible to launch picosatellite into orbit from railgun on high-altitude plane?" Tags="|launch|artificial-satellite|escape-velocity|rail-gun|air-launch|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13037" PostTypeId="2" ParentId="12936" CreationDate="2015-12-17T00:13:25.330" Score="3" Body="&lt;p&gt;This answer addresses the last part of the question: &quot;why can't qualification testing be automated&quot;.  You have a good point, but we need to clarify some terminology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, the earlier parts of the question have some inevitable overlap with this question &lt;a href=&quot;https://space.stackexchange.com/questions/12933/can-cubesats-be-used-to-qualify-parts-for-spaceflight-more-cheaply-than-ground-t/13035#13035&quot;&gt;here&lt;/a&gt; by the same OP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back to automation.  There are two distinct concepts in testing for satellites. There is &lt;strong&gt;Qualification&lt;/strong&gt; testing and &lt;strong&gt;Acceptance&lt;/strong&gt; testing. Where a piece of equipment will have a &quot;large&quot; production run, i.e. hundreds (thrusters, amplifiers) or thousands (solar cells) then &lt;em&gt;acceptance testing is a much more intuitive candidate for automation than qualification&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are probably others who could make this distinction more formally though this is it in human terms: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The aim of qualification is to demonstrate that the design works and has sufficient margin to be tolerant of allowable build variations. It is usually done on just one or a small number of flight-standard production items.&lt;/li&gt;&#xA;&lt;li&gt;The aim of acceptance is to demonstrate that there are no workmanship errors in each item in the production run.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Loosely, qualification testing is more severe and intrusive than acceptance testing and the tested item would usually be considered to be degraded by the qualification experience, e.g. already having suffered a lifetime of use, and would not subsequently be used for flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a relatively commonly understood range of names given to different levels in the integration process: piece part, tray, sub-assembly, equipment, subsystem, spacecraft and the concepts of qualification and acceptance can be found at every level. Whole spacecraft are often (not exclusively) subject to a middle ground in test-severity terms, called proto-flight testing, which reflects the customer's risk-tolerance and agreement that it would cost too much to build an entire satellite, test it and then never use it.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-12-17T00:13:25.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13038" PostTypeId="2" ParentId="13036" CreationDate="2015-12-17T00:55:19.980" Score="11" Body="&lt;p&gt;That depends upon what would satisfy your needs as an &quot;orbit&quot;. I think its unlikely to do what you are hoping. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good way of thinking about the problem to make it more intuitive is to say that there is a rule that applies to all satellite orbits that, aside from  the effects of the atmosphere, from the point that a manoeuvre ends the satellite will conduct a revolution in &quot;some ellipse&quot; and must pass through the same point in space on the second orbit, no matter how high it goes in the mean time. Atmosphere and other perturbations aside it will continue to do that indefinitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even without an atmosphere this idealisation isn't right because the Earth isn't a point mass, but it will do for the thought experiment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In our case the end of the manoeuvre is the point that the satellite leaves the railgun. This means that however high the aircraft and however big the railgun the satellite will return to the same altitude after one orbit. If we include the atmosphere, as its necessary for the aircraft, then we have to accept that the first orbit will probably be the last for the satellite due to atmospheric drag. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two ways of getting past this constraint.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The railgun gives the satellite such an almighty kick that it takes up a parabolic trajectory at escape velocity and will never be seen again. &lt;/li&gt;&#xA;&lt;li&gt;The satellite has some means of performing a second big manoeuvre, when it has gained a lot more altitude.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In case 2 this has the effect of &quot;raising the perigee&quot;, causing the satellite not to dip back into the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going back to our thought experiment rule about orbits now causes the satellite to return to the engine cut-off point of the second manoeuvre and it is &lt;em&gt;that&lt;/em&gt; point that now defines where the satellite must now return on subsequent orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, an orbital delivery system has to impart velocity increments at at least two points in the trajectory to go from rest at the Earth's surface to a stable orbit. Just in case you are wondering, I've described an idealised impulsive system, if you had a very long thrust duration, with an arc extending most of an orbit, then you'd find a way to do it also. &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2015-12-17T00:55:19.980" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13039" PostTypeId="1" AcceptedAnswerId="13040" CreationDate="2015-12-17T05:55:04.993" Score="4" ViewCount="673" Body="&lt;p&gt;In a two body system, there are a lot of orbital parameters not changing over time, like eccentricity or the orbital plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A constant parameter can be a combination of two or more, like even though kinetic and potential energy is always changing in an elliptical orbit, they always sum up to a constant energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any equivalent parameter that does not change for the smaller satellite in a three body system? Can that parameter be used to confirm if you are observing the same asteroid after a fly-by of, let's say Jupiter?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-17T16:21:24.357" LastActivityDate="2015-12-17T19:29:13.653" Title="A constant parameter in a three body system?" Tags="|orbital-mechanics|flyby|calculation|three-body|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13040" PostTypeId="2" ParentId="13039" CreationDate="2015-12-17T07:21:13.407" Score="4" Body="&lt;p&gt;That would be the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jacobi_integral&quot; rel=&quot;nofollow&quot;&gt;Jacobi integral&lt;/a&gt; ($C_\text{J}$, or $C_\text{H}$ in &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;nofollow&quot;&gt;Hill's problem&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In &lt;a href=&quot;https://en.wikipedia.org/wiki/Celestial_mechanics&quot; rel=&quot;nofollow&quot;&gt;celestial mechanics&lt;/a&gt;, Jacobi's integral (also The Jacobi Integral or&#xA;  The Jacobi Constant; named after &lt;a href=&quot;https://en.wikipedia.org/wiki/Carl_Gustav_Jacob_Jacobi&quot; rel=&quot;nofollow&quot;&gt;Carl Gustav Jacob Jacobi&lt;/a&gt;) is the only&#xA;  known conserved quantity for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Euler&amp;#39;s_three-body_problem&quot; rel=&quot;nofollow&quot;&gt;circular restricted three-body&#xA;  problem&lt;/a&gt;. Unlike in the two-body problem, the energy and momentum of&#xA;  the system are not conserved separately and a general analytical&#xA;  solution is not possible. The integral has been used to derive&#xA;  numerous solutions in special cases.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For a worked example, as requested specifically about application of CRTBP conserved quantity for a Jupiter flyby (in this case a time-frequency analysis of a Sun-Jupiter-comet system), see e.g. &lt;a href=&quot;http://www.cds.caltech.edu/~marsden/bib/2004/01-VeMa2004/VeMa2004.pdf&quot; rel=&quot;nofollow&quot;&gt;Time-frequency analysis of the restricted three body problem: Transport and resonance transitions&lt;/a&gt;, Luz V. Vela-Arevalo and Jerrold E. Marsden, Georgia Institute of Technology (PDF).&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-17T07:33:56.223" LastActivityDate="2015-12-17T07:33:56.223" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13041" PostTypeId="1" CreationDate="2015-12-17T08:12:57.873" Score="1" ViewCount="302" Body="&lt;p&gt;Is the optimization process for fuel used, response time, thruster activity different? How is the trade-off between these done? Also, is there a better controller for on-off RCS thruster system?&lt;/p&gt;&#xA;" OwnerUserId="12386" LastEditorUserId="49" LastEditDate="2015-12-17T09:13:53.197" LastActivityDate="2015-12-17T09:13:53.197" Title="How to optimize a Pulse-Width Pulse-Frequency (PWPF) modulated attitude control system?" Tags="|attitude|design|engineering|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13044" PostTypeId="1" CreationDate="2015-12-17T14:21:00.667" Score="7" ViewCount="1181" Body="&lt;p&gt;A random question. At the very first try we succeeded. Though it might not be strange, What was the probability of surviving so long with so many external variables and constants.?&lt;/p&gt;&#xA;" OwnerUserId="12654" OwnerDisplayName="Bandeep Rabha" LastActivityDate="2017-05-30T16:41:42.970" Title="Is it pure luck that the voyager 1 survived to travel beyond our solar system in interstellar space?" Tags="|solar-system|interstellar-travel|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13045" PostTypeId="1" CreationDate="2015-12-17T17:16:06.937" Score="6" ViewCount="626" Body="&lt;p&gt;I've been studying the resonant return and &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_keyhole&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational keyhole analysis&lt;/a&gt; for &lt;a href=&quot;https://en.wikipedia.org/wiki/Near-Earth_object&quot; rel=&quot;nofollow noreferrer&quot;&gt;NEOs&lt;/a&gt; and using the extension of Öpik's theory of close encounters (aka Öpik-Valsecchi theory, Valsecchi et al., Astron Astrophys 408:1179–1196, 2003), developed to identify the keyholes on the encounter b-plane for an asteroid. Understanding the analytical part is relatively easy. However it's numerical implementation got me confused;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once I have the locus of Earth for next encounter corresponding to a particular semi-major axis, using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Minimum_orbit_intersection_distance&quot; rel=&quot;nofollow noreferrer&quot;&gt;Minimum orbit intersection distance (MOID)&lt;/a&gt;, I can identify if the impact or close encounter is possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Analytical theory suggests that my keyholes for next encounter should lie near the intersection of the circle and MOID line represented on b-plane (target plane). They have shown numerical results corresponding to this but did not mention how the transformations were obtained. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I tried taking the coordinates of the intersection for various resonances and propagate them under both restricted (n+1) body motion and Keplerian model, but have not been able to obtain similar results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How can this particular part can be verified?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is to identify if the points that lie on my first encounter b-plane near the MOID line will impact on next encounter. To do that, those points have to be numerically propagated from current encounter to next. That requires heliocentric state vector of those points &lt;span class=&quot;math-container&quot;&gt;$(\vec{r}, \vec{v})$&lt;/span&gt;. And I do not have/know the transformations that can convert my b-plane coordinates &lt;span class=&quot;math-container&quot;&gt;$(\xi, \eta, \zeta)$&lt;/span&gt; that lie near the MOID to &lt;span class=&quot;math-container&quot;&gt;$(\vec{r}, \vec{v})$&lt;/span&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The case here is when my original set of VAs do not touch the resonance circle but lie close to it, but my MOID intersect those circles, unlike the case where I can use rotation matrix to obtain reverse transformations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YZzFh.png&quot; alt=&quot;b-plane for Apophis 2029 encounter&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see in the image, for &lt;a href=&quot;https://en.wikipedia.org/wiki/99942_Apophis&quot; rel=&quot;nofollow noreferrer&quot;&gt;99942 Apophis&lt;/a&gt; (2004MN4), I do not have any impact with the Earth (red circle) in 2029. But My MOID (black line) intersects with 6:7 resonance circle indicating the existence of keyholes. I am stuck at obtaining the heliocentric coordinates of the point lying near the intersection of black line and blue circle.&lt;/p&gt;&#xA;" OwnerUserId="10558" LastEditorUserId="5775" LastEditDate="2019-05-02T02:06:04.520" LastActivityDate="2019-05-02T02:06:04.520" Title="Understanding gravitational keyhole analysis for Near-Earth Objects" Tags="|trajectory|asteroid|impact|celestial-mechanics|near-earth-object|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13046" PostTypeId="2" ParentId="13044" CreationDate="2015-12-17T17:55:39.273" Score="5" Body="&lt;p&gt;A thing works fine until something goes wrong.&lt;br&gt;&#xA;As far as I can see, the main external risks of the voyage were the radiation belts of Jupiter and Saturn, but those were already survived by the &lt;a href=&quot;http://lasp.colorado.edu/education/outerplanets/missions_pioneers.php&quot; rel=&quot;nofollow&quot;&gt;Pioneer 10 and 11&lt;/a&gt; missions. After the planetary fly-bys, there are close to nothing external affecting the spacecraft.&#xA;The important risks therefore lies in the potential failure of of one of the spacecraft’s subsystems:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Power&lt;/strong&gt;&lt;br&gt;&#xA;The power source of the Voyager missions was an &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow&quot;&gt;RTG&lt;/a&gt;. This requires no moving parts, and the decay of radioactive isotopes is not dependent on any external factors. This will probably work just fine until the power level has decreased enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Computer hardware&lt;/strong&gt;&lt;br&gt;&#xA;A computer has multiple potential weaknesses, the most common ones being overheating and memory malfunction, like what happened on the &lt;a href=&quot;http://www2.jpl.nasa.gov/sl9/gll16.html&quot; rel=&quot;nofollow&quot;&gt;Galileo mission.&#xA;&lt;/a&gt; Radiation in outer space will over time slowly degrade electronic components. Magnetic memory will over time &lt;a href=&quot;http://www.extremetech.com/computing/170748-how-long-do-hard-drives-actually-live-for&quot; rel=&quot;nofollow&quot;&gt;degenerate anyway.&lt;/a&gt;&lt;br&gt;&#xA;It seems like what eventually will likely  cause the Voyagers to fail is their nearly four decade old electronics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Manoeuvring&lt;/strong&gt;&lt;br&gt;&#xA;The probes needed propellant for changing attitude, as well as for trajectory corrections. Corrosive propellant staying in the tanks for years are a major cause of spacecraft failures. See for instance the &lt;a href=&quot;http://www.nbcnews.com/id/40595752/ns/technology_and_science-space/t/japanese-investigate-venus-probe-failure/#.VnL1pV6Le1E&quot; rel=&quot;nofollow&quot;&gt;Akatsuki probe&lt;/a&gt;.&amp;nbsp;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-17T22:20:42.077" LastActivityDate="2015-12-17T22:20:42.077" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13047" PostTypeId="2" ParentId="13045" CreationDate="2015-12-17T18:22:57.053" Score="5" Body="&lt;p&gt;Your encounter must by definition occur somewhere around the MOID line, if you do not consider to significantly change the trajectory. A solution can be obtained using Keplerian analysis, but that is only an approximation, due to the hard-to-restrict three body nature of the problem. If I understand your question correctly, you want a higher degree of accuracy.&lt;br&gt;&#xA;In this particular case, you are not left with many other possibilities than to brute force it with numerical integration, because of the general lack of analytical solutions to n-body problems. (except for certain exceptions)&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Response to the edit:&lt;/strong&gt;&lt;br&gt;&#xA;Until I can find something better, it seems to me that you can use the Laplace parameters to calculate a position (Either in a co-rotating reference frame or other). This can easily by transformed to a position state vector $(\vec{r})$. Using that again, you can obtain the velocity state vector $(\vec{v})$from the resulting change in the Hill potential from before the encounter. This is however a rather annoyingly complicated way of doing it. I am still looking for a more intuitive solution.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-17T20:00:50.443" LastActivityDate="2015-12-17T20:00:50.443" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13048" PostTypeId="2" ParentId="13044" CreationDate="2015-12-17T19:11:49.150" Score="14" Body="&lt;p&gt;It can't be pure luck, seeing as how &lt;em&gt;both&lt;/em&gt; Voyager spacecraft are still operating today. If it were just one, you might chalk it up to luck. Both still working means they must have been built really well. Which they were.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is essential of course that they are nuclear powered, and also that we have really big antennas on Earth with which to talk to them. And that there was attitude control fuel leftover after their planetary flybys. But none of those are luck.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things have failed on them, mostly moving parts and some capacitors, but we have found ways to work around all of the problems. Again, not luck.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-12-17T19:11:49.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13049" PostTypeId="2" ParentId="13039" CreationDate="2015-12-17T19:29:13.653" Score="3" Body="&lt;p&gt;There is another parameter known as Tisserand's parameter which stays constant for a given body in 3 body probelm. &#xA;This is more useful in identifying any given asteroid or comet. &#xA;Given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$T = \dfrac{a_e}{a}+2\sqrt{\dfrac{a}{a_e}(1-e^2)}cos(i)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where e subscript represents parameters of perturbing body and no subscript for small body. &#xA;It stays constant for a body even after the close encounter and is very useful in identifying if a newly discovered asteroid is an old body that underwent close encounter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://farside.ph.utexas.edu/teaching/336k/Newtonhtml/node122.html&quot; rel=&quot;nofollow&quot;&gt;http://farside.ph.utexas.edu/teaching/336k/Newtonhtml/node122.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10558" LastActivityDate="2015-12-17T19:29:13.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13050" PostTypeId="2" ParentId="12998" CreationDate="2015-12-17T19:56:42.767" Score="13" Body="&lt;p&gt;This will be a subjective answer to a subjective question (I was not going to answer at all, but @called2voyage's link ([Good Subjective/Bad Subjective])&lt;a href=&quot;https://blog.stackoverflow.com/2010/09/good-subjective-bad-subjective/&quot; title=&quot;Good Subjective/Bad Subjective&quot;&gt;1&lt;/a&gt; (see @called2voyage's comment to the OP, above) emboldened me...).  Please keep in mind that the experiences shared below were attained during a period of time lasting from the mid 90's to the early 00's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on my personal experience in NASA's Astronaut Office, I'd say the best &lt;em&gt;educational&lt;/em&gt; course to follow, if one wants to make oneself most eligible for selection to said Astronaut Corps, is to first bone up on NASA's &lt;a href=&quot;http://astronauts.nasa.gov/content/broch00.htm&quot; rel=&quot;noreferrer&quot;&gt;current education requirements&lt;/a&gt;.  Gotta meet &lt;em&gt;those&lt;/em&gt; requirements!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, said requirements encompass a vast array of educational degrees.  How to choose?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's what I've seen with my own eyes and still believe to be true:  &lt;strong&gt;Within the above boundaries, the choices are mostly irrelevant.&lt;/strong&gt;  What matters most is that one excel at one's chosen field.  So, AFAIC, the choice becomes simple - choose a field that you are both passionate about and very good at.  A superior candidate with a Master's degree in Engineering Mechanics (for example) will be chosen over a mediocre candidate with a Master's degree in Aerospace Engineering (for example).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Choosing a field you are both passionate and very good in will maximize your changes of excelling in said field, which &lt;em&gt;then&lt;/em&gt; maximizes your chances of being selected to join NASA's Astronaut Corps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck!&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="12422" LastEditDate="2019-06-03T17:43:11.633" LastActivityDate="2019-06-03T17:43:11.633" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13051" PostTypeId="1" AcceptedAnswerId="13052" CreationDate="2015-12-17T20:16:30.813" Score="19" ViewCount="15296" Body="&lt;p&gt;On Apollo 8, following the first trans-Earth injection, when the crew came back into contact with Houston, Lovell said: &quot;Please be informed there is a Santa Claus.&quot; (&lt;a href=&quot;http://history.nasa.gov/ap08fj/16day4_final_orbit_tei.htm&quot;&gt;Transcript&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that Apollo 8 had already been in contact with ground through Christmas Eve, what prompted Lovell to make this statement at the time that he did?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Note: Since motivations cannot be known without the account of the individual, I will take common sense answers speculating on likely reasons that include a statement to the effect that we cannot truly know why he did it. That's assuming we don't have an account from Lovell, of course.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="17839" LastEditDate="2017-12-13T00:02:53.917" LastActivityDate="2019-07-07T02:54:22.713" Title="What prompted Lovell on Apollo 8 to announce &quot;there is a Santa Claus&quot;?" Tags="|the-moon|apollo-program|orbital-maneuver|astronauts|santa-claus|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13052" PostTypeId="2" ParentId="13051" CreationDate="2015-12-17T20:35:26.827" Score="21" Body="&lt;p&gt;The first trans-Earth injection from the moon was a high-stakes maneuver; if the Apollo's SPS didn't fire, the crew would have been stuck very far from home. I always took &quot;please be informed there is a Santa Claus&quot; to mean something like &quot;we got what we wanted for Christmas&quot; -- that is, a good burn and a good chance to get home. Basically, a seasonally appropriate way to say &quot;we have good news.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You mention the &lt;a href=&quot;https://en.wikipedia.org/wiki/NORAD_Tracks_Santa&quot;&gt;NORAD Santa tradition&lt;/a&gt;, though, and that predates Apollo by a good ways (dating to 1955!), so it might also just be an obscure joke to the effect that Santa prepares for his Christmas Eve run behind the moon where we can't see him. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lovell himself confirms that it's an allusion to the famous &lt;a href=&quot;https://en.wikipedia.org/wiki/Yes,_Virginia,_there_is_a_Santa_Claus&quot;&gt;&quot;Yes Virginia&quot;&lt;/a&gt; letter &lt;a href=&quot;https://www.youtube.com/watch?v=9XRrznkoWZA&quot;&gt;in this interview&lt;/a&gt;, though he isn't explicit about what he meant when he said it. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-17T20:47:13.693" LastActivityDate="2015-12-17T20:47:13.693" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13053" PostTypeId="2" ParentId="13051" CreationDate="2015-12-17T20:37:02.463" Score="3" Body="&lt;p&gt;From reading the transcript context it seems like Lovell is somewhat jokingly reporting from what they actually saw on their expedition, being the first humans in the vicinity of the Moon. Because they were the first, no one could actually debunk anything, including a claim of finding Santa on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another humorous comment can be found in the &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/apollo12.htm&quot; rel=&quot;nofollow&quot;&gt;Apollo 12 transcript&lt;/a&gt;, where ground control order the astronauts to look for the Chinese Moon goddess &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang&amp;#39;e&quot; rel=&quot;nofollow&quot;&gt;Chang'e&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-17T21:31:40.267" LastActivityDate="2015-12-17T21:31:40.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13054" PostTypeId="2" ParentId="13051" CreationDate="2015-12-17T21:39:51.840" Score="2" Body="&lt;p&gt;Since you comment that &lt;em&gt;this is a semi-serious question intended to allow for some lightheartedness&lt;/em&gt;, here goes;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By December 21, 1968 when Apollo 8 was launched and nearly 4 days later as the events described in your questions took place, we only had one and over three years old account of encounter of Santa Claus from outer space. Gemini 6 astronauts Wally Schirra and Thomas Stafford, while orbiting the Earth, saw &lt;em&gt;an object that looked like a satellite going from North to South probably in a polar orbit, on a very low trajectory...&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/HqfIEQKnkJU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this was but one encounter and you know science, it demands repeatability. With the second observation by Lovell good three years later, we finally got a confirmation that Santa Clause and his reindeer exist and that they're on a once per year periodic trajectory with closest approach to Earth just before Christmas each year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What prompted Lovell on Apollo 8 to announce “there is a Santa Claus”?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Love for the truth!&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-17T21:53:03.890" LastActivityDate="2015-12-17T21:53:03.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13055" PostTypeId="2" ParentId="13044" CreationDate="2015-12-17T21:49:08.290" Score="0" Body="&lt;p&gt;You give a good group of engineers enough time, money and resources to build a decent piece of equipment, and its going to last for a while.&#xA;Proper practice is to overdesign for known stresses by a factor of three or more. That gives you leeway with unexpected stresses to.&#xA;No proper engineer wants to see his creation fail because he bought the 29 cent capacitor instead of the $1.69 one.&lt;/p&gt;&#xA;" OwnerUserId="12655" LastActivityDate="2015-12-17T21:49:08.290" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13056" PostTypeId="1" AcceptedAnswerId="13058" CreationDate="2015-12-17T22:25:58.553" Score="0" ViewCount="223" Body="&lt;p&gt;Would it be possible to lay a fiber optic cable over long distances and use it to transfer information- for example, as a sort of telescope?&lt;/p&gt;&#xA;" OwnerUserId="12649" LastEditorUserId="25" LastEditDate="2015-12-17T22:36:13.603" LastActivityDate="2015-12-17T22:43:10.613" Title="Would it be practical to lay a fiber optic cable to Alpha Centauri?" Tags="|communication|interstellar-travel|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13057" PostTypeId="2" ParentId="13056" CreationDate="2015-12-17T22:35:32.820" Score="4" Body="&lt;p&gt;That is absurd just by the required mass, a fibre cable can be made 1000 microns thin, and that is a total volume of 600000 cube kilometres for the whole length.&#xA;A fibre cable works by sending information as light, and that can be done just as well through empty space.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2015-12-17T22:35:32.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13058" PostTypeId="2" ParentId="13056" CreationDate="2015-12-17T22:35:34.603" Score="4" Body="&lt;p&gt;No, for a number of reasons.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The distance varies somewhat, which would make keeping it straight difficult&lt;/li&gt;&#xA;&lt;li&gt;There is a small loss in light with fiber optics, making it somewhat redundant. If you put repeaters in place (Which is done on Earth, and would really be required), they would need to be powered. Solar power isn't really an option so far out there.&lt;/li&gt;&#xA;&lt;li&gt;Orbital parameters would make it difficult.&lt;/li&gt;&#xA;&lt;li&gt;The amount of high quality fiber required would be enormous.&lt;/li&gt;&#xA;&lt;li&gt;I suspect the temperature in the void of space would cause undesirable effects on the cable.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Far better would be to simply use an open aired system, as there would be no loss. Fiber optics is really only helpful with cable that can bend, for such a straight distance, a large telescope on each end would work better, and far cheaper.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-12-17T22:43:10.613" LastActivityDate="2015-12-17T22:43:10.613" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13059" PostTypeId="2" ParentId="13056" CreationDate="2015-12-17T22:38:00.450" Score="2" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many reasons.  But here is a fun one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Volume of a cylinder is V=pi r^2 h. So Pi times the radius squared, times the height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So lets say 10 mm optical fibre that is 4 light years high.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I get Pi * 0.01m^2 * 3.784e+16=11.9 trillion CUBIC meters of optical fibre.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let us just say, that is an impractical amount of matter.  Volume of the Earth is: 1,097,509,500,000,000,000,000  or about 1.1 septillion cubic meters?  Ok, so that is like 10 to the negative 8 percent of the planets volume. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot think of any large impractical volumes to compare that too.  But almost 12 trillion cubic meters is a LOT of material.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-12-17T22:38:00.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13060" PostTypeId="2" ParentId="2971" CreationDate="2015-12-17T23:07:09.803" Score="0" Body="&lt;p&gt;Using data from Wikipedia rocket launch's, the highest ISP for a first stage engine that has publicly available data is the Vulcain engine, used by Ariane 5. The value would be somewhere between 318 to 446, depending on exactly how you count it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The highest stage is GSLV Mk I or the Delta III/IV upper stage, which both achieve 462 ISP.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-12-17T23:07:09.803" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="13061" PostTypeId="1" CreationDate="2015-12-18T00:15:41.607" Score="3" ViewCount="557" Body="&lt;p&gt;I was thinking about the answers and comments I got to this question (&lt;a href=&quot;https://space.stackexchange.com/questions/12933/can-cubesats-be-used-to-qualify-parts-for-spaceflight-more-cheaply-than-ground-t&quot;&gt;Can a cubesat be used to qualify parts in space?&lt;/a&gt;) and came across one that said satellites can't be fixed once launched into orbit (except for a few exceptions like the Hubble Space Telescope and International Space Station) because of the extreme cost of launching astronauts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I got to thinking of the advances in robotics over the past ten years, and how first person view robots have become cheap enough for kids. I know the bandwidth or some regulations may prevent this, and that this wouldn't fix every problem, but could a cubesat with robotic arms be controlled in real-time from the ground via laptop/internet connection?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems many of the problems that occur in orbit come down to the satellite having something jammed, clogged, or shorted, and having a satellite with arms would help a lot to fix these problems for lower costs than an astronaut. And since these satellites are near Earth, the time lag issue shouldn't be a problem, so real time control could be done.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;*I am not worrying about the delta v issues this satellite would have, that is another whole huge problem for how the cubesat would rendezvous with many crippled satellites to fix them, I am just looking at seeing if the infrastructure exists to allow real time control of satellites by average people with internet connections or satellite phones.  &lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-02-17T06:57:12.343" Title="Can a cubesat in orbit be operated like a R/C airplane in real time?" Tags="|cubesat|in-space-repairs|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13062" PostTypeId="2" ParentId="4042" CreationDate="2015-12-18T01:09:09.583" Score="1" Body="&lt;p&gt;There exists a &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon_Treaty&quot; rel=&quot;nofollow&quot;&gt;Moon Treaty&lt;/a&gt;, but  this is only signed and ratified by 16 countries, none of them capable of space travel. India is a signatory, and now has its own space program, but has not ratified it. &#xA;Even though treaties exist, the UN has no overruling power over independent nations, thus making the actual relevance of the treaty disputable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon Treaty does not strictly ban commercial use of Lunar resources, but it is insufficient to actually provide a good framework if the issue turns up. When Lunar resource harvesting is becoming a reality, it must be replaced with a more solid international agreement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Together with many other fields of space law, there are a lot of ambiguity and untested cases related to the Moon treaty. It may not be possible to determine if the treaty would hold in the case of a dispute, being hugely dependent on international politics.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-18T23:17:42.233" LastActivityDate="2015-12-18T23:17:42.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13064" PostTypeId="2" ParentId="8297" CreationDate="2015-12-18T06:36:18.980" Score="3" Body="&lt;p&gt;I can't give you a quantitative answer to your question but I'll try to give you a feel for how CFD has affected the creation of engines.  I'm going to start by discussing air breathing engines as a proxy for rocket engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm an aero/astro engineer and so was my father (who worked on several air breathing engine programs).  In my father's era (J79 -&gt; F110 time frame), engineers would &quot;design&quot; an engine, put it together, and then start a test program to determine its performance.  Often the engines didn't work all that well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether the engine worked well or not was an art on top of the science used to put it together.  Once an engine was found to perform well, the engineers more or less kept reusing the same engine core but used different fans &amp;amp; AB sections with it to create engines for other uses (e.g. J79).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I started working in the late 80s / early 90s there was still some of that going on (F101 -&gt; F110 by reusing the core but changing the fans).  However, big engine companies became less reliant upon reusing old cores for new engine families.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Don't get me wrong.  Engine companies &lt;strong&gt;DO&lt;/strong&gt; still make new engines out of proven cores but the new engines they design work far better on average than the ones that they developed in earlier decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether CFD actually reduces testing, I can't say.  But it does permit engine companies to have a much greater chance of developing high performance engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And to bring it full circle, rocket engine reuse was common in earlier decades.  The &lt;A HREF=&quot;https://en.wikipedia.org/wiki/RD-107&quot; rel=&quot;nofollow&quot;&gt;Russian's Soyuz RD-107 rocket engines&lt;/a&gt; are direct descendants of V2 rocket engines.  Even the US still uses the descendants of the now &lt;A href=&quot;https://en.wikipedia.org/wiki/RL10&quot; rel=&quot;nofollow&quot;&gt;55+ year old RL10 engine&lt;/a&gt; for the same reason the engine companies reused air breathing engine designs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But with CFD, companies like SpaceX are now able to design new engines from scratch.  These have performed well so far.&lt;/p&gt;&#xA;" OwnerUserId="8630" LastActivityDate="2015-12-18T06:36:18.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13065" PostTypeId="1" AcceptedAnswerId="13068" CreationDate="2015-12-18T07:06:36.070" Score="7" ViewCount="389" Body="&lt;p&gt;American, Japanese and ESA space travellers are astronauts, Russian or Soviet space travellers are cosmonauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But are there exceptions from this? Americans visiting the ISS the last years have used the Soyuz, can they be referred to as &quot;American cosmonauts&quot;? Similarly, can the Russian citizens returning from Mir in the shuttle also be &lt;a href=&quot;http://history.nasa.gov/SP-4225/sts60/sts-60.htm&quot;&gt;&quot;Russian astronauts&quot;&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, is it allowed to use the term cosmonaut for a space traveller not from a specific country?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2015-12-18T12:08:57.363" Title="Cosmonauts and Astronauts" Tags="|crewed-spaceflight|terminology|astronauts|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13066" PostTypeId="2" ParentId="13061" CreationDate="2015-12-18T07:24:07.563" Score="1" Body="&lt;p&gt;Fitting all that the equipment you describe into a 1.33 kg spacecraft, like robotic arms and a propulsion system is not possible, nor are cubesats &lt;a href=&quot;http://browncubesat.org/wp-content/uploads/2013/01/Cubesat-Reqs.pdf&quot; rel=&quot;nofollow&quot;&gt;allowed to have propulsion.&lt;/a&gt; But your question is not about that, it is about the internet connection. It is nothing wrong about the current systems used for communication with satellites receiving their information via the use of the internet. This is however not likely due to security concerns. The distance should not cause any significant delay, but bandwidth limitation and the redirecting of the commands may give a lag in the order of tens of seconds or minutes.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2015-12-18T07:24:07.563" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13067" PostTypeId="2" ParentId="13065" CreationDate="2015-12-18T08:52:35.237" Score="0" Body="&lt;p&gt;When you speak English, use the term English astronaut. Using the Russian word 'cosmonaut' as an honorific was a nice idea when there were only 2 nations with manned space programs, but is becoming untenable. Your own examples make this clear, and it'll only get worse as more nations start manned space programs.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-18T08:52:35.237" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13068" PostTypeId="2" ParentId="13065" CreationDate="2015-12-18T10:57:01.573" Score="7" Body="&lt;p&gt;Today, we speak of astronauts, cosmonauts, angkasawan, vyomanauts, and so on as terms adopted by specific countries to have official meanings. I.e. they're titles. So if you're addressing someone by their official title, a single specific term should be used and these terms aren't interchangeable. They denote a job position and whoever manages that job also decides on terminology to be adopted for the purpose of an official title. Otherwise, most terms are roughly equivalent in meaning:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Astronaut: from Greek &lt;em&gt;astron&lt;/em&gt; or Latin &lt;em&gt;aster&lt;/em&gt; for a star, and Greek &lt;em&gt;nautilos&lt;/em&gt; for a sailor, navigator - a traveler to stars, or a star traveler&lt;/li&gt;&#xA;&lt;li&gt;Cosmonaut: from Greek &lt;em&gt;kosmos&lt;/em&gt; for world or universe, and Greek &lt;em&gt;nautilos&lt;/em&gt; - a traveler of worlds, a traveler in the universe&lt;/li&gt;&#xA;&lt;li&gt;Taikonaut: from Chinese &lt;em&gt;Wài tàikōng (外太空)&lt;/em&gt; for outer space, and Greek &lt;em&gt;nautilos&lt;/em&gt; - a traveler to outer space. This is not a Chinese word!&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If you called an American astronaut a cosmonaut, and you weren't addressing their official position, you wouldn't be offending anyone. Both terms are ancient Greek in origins and have equivalent meaning today. It certainly doesn't translate to &lt;em&gt;an astronaut specifically of some other country&lt;/em&gt;. Neither does &lt;em&gt;cosmonaut&lt;/em&gt;, and both of them are used as titles by many different countries. The only one of the three that has a root in a language of one specific country is taikonaut, and even that term is Western replacement for Chinese &lt;em&gt;Yǔháng yuán (宇航員)&lt;/em&gt; (interchangeably translated as both astronaut or cosmonaut) and officially, Chinese astronauts are called &lt;em&gt;háng tiān yuán&lt;/em&gt; (航天员).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, TL;DR, if you're addressing a person of some specific nationality and position by their title, then only one of these terms would be correct. Otherwise, it wouldn't be &lt;em&gt;inappropriate&lt;/em&gt; to use a different term to describe what they do or where they've been to / came from. &lt;em&gt;American cosmonaut&lt;/em&gt; sounds odd in most circumstances because it's not a generally adopted term in English speaking parts of the world, or indeed most of the countries of what used to be the Western bloc. But if you said an equivalent Американский космонавт (amerikanskiy kosmonavt) in Russian and in Russia, or similar in what used to be predominately Eastern bloc countries (e.g. Romania uses &lt;em&gt;cosmonaut&lt;/em&gt; even though the Romanian language is predominately Neo-Latin), that would be just fine.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Do note though that specific cultures might have adopted their own naming conventions for this. Here, for example, we've decided on it by community vote in &lt;a href=&quot;https://space.meta.stackexchange.com/q/174/49&quot;&gt;What term should be used for *naut?&lt;/a&gt; Also see &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronaut#Terminology&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Astronaut / Cosmonaut - Terminology&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:04.403" LastActivityDate="2015-12-18T12:08:57.363" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13069" PostTypeId="1" AcceptedAnswerId="13077" CreationDate="2015-12-18T11:11:09.163" Score="5" ViewCount="343" Body="&lt;p&gt;In the olden days most of space-faring electronics would be embedded in resin. Hey, that's where the term &quot;embedded software&quot; comes from! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That came with a number of advantages like pressure-proofing the system, making it way more durable against accelerations and allowing for more balanced temperature distribution. It also came with a slew of new problems, making the boards unserviceable, heat removal harder at times, and the whole thing much heavier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does the situation look like nowadays? How often are electronics meant for space embedded in resin?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2015-12-19T04:48:05.577" LastActivityDate="2022-08-09T09:29:05.280" Title="How common is resin/epoxy sealing of space-faring equipment nowadays?" Tags="|artificial-satellite|spacecraft|materials|electronics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13072" PostTypeId="2" ParentId="7749" CreationDate="2015-12-18T15:50:49.057" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;To my knowledge, there isn't anything for the gas to push against.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That is not how a reaction engine - a family of which rockets and thrusters are a part - works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short: you got it backwards. The gas that is expelled is not pushing against a medium (such as the atmosphere)... the gas is pushing against the &lt;strong&gt;thruster&lt;/strong&gt;. Or - to be accurate - they are pushing at each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The long explanation, in layperson terms: when the thruster &quot;throws out&quot; the gas, the work is already done. Anything that happens &lt;strong&gt;after&lt;/strong&gt; the gas molecules leave the nozzle of the thruster is irrelevant. It is the process of getting the gas molecules up to speed that causes thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And why is that? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because in order to get the gas molecules up to speed, the thruster must push at them. And when that happens, the gas molecules push back at the thruster. They are both pushing the other away from them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The long explanation, in slightly more scientific terms: when inside the fuel tank, the gas molecules are in constant motion... they are bouncing hard against the walls of the gas tank, creating pressure as they recoil back into the tank every time they impact the walls. But since the tank is closed, the net forces on the tank balance each other out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens when you use the thruster is essentially that you open a hole in the side of the tank. Now there is no wall there for the gas molecules to bounce off of; instead they just escape. That means that on the &lt;strong&gt;other&lt;/strong&gt; side of the tank, where gas molecules are still bouncing against the wall, there is a force that doesn't have a balancing counterpart. That means that suddenly you have a net force on the tank, and so it starts accelerating.&lt;/p&gt;&#xA;" OwnerUserId="10493" LastActivityDate="2015-12-18T15:50:49.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13073" PostTypeId="2" ParentId="8451" CreationDate="2015-12-18T18:48:04.033" Score="17" Body="&lt;p&gt;As of 2013, 29 of the 330 (8.8%) NASA astronauts have had 0 flights.  Removing the 2009 rookies, 20 of 321 (6.2%) have had 0 flights.  Incidentally, none of these 20 are still astronauts.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;11 of 312 (3.5%) that survived long enough to have a chance never flew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/pdf/740566main_current.pdf&quot;&gt;http://www.nasa.gov/pdf/740566main_current.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;See, Elliot M., Jr.;  Rookie year: 1962; DIED: February 28, 1966 in crash of T-38 jet &lt;br&gt;&#xA;Bassett, Charles A., II;  Rookie year: 1963; DIED: February 28, 1966 in crash of T-38 jet &lt;br&gt;&#xA;Chaffee, Roger B.;  Rookie year: 1963; DIED: January 27, 1967 in the Apollo 1 spacecraft fire&lt;br&gt;&#xA;Freeman, Theodore C.;  Rookie year: 1963; DIED: October 31, 1964 in a T-38 jet crash&lt;br&gt;&#xA;Williams, Clifton C., Jr.;  Rookie year: 1963; DIED: October 5, 1967 in T-38 jet crash&lt;br&gt;&#xA;Graveline, Duane E.;  Rookie year: 1965&lt;br&gt;&#xA;Michel, F. Curtis;  Rookie year: 1965&lt;br&gt;&#xA;Bull, John S.;  Rookie year: 1966&lt;br&gt;&#xA;Givens, Edward G., Jr.;  Rookie year: 1966; DIED: June 6, 1967 in an automobile accident&lt;br&gt;&#xA;Chapman, Philip K.;  Rookie year: 1967&lt;br&gt;&#xA;Holmquest, Donald L.;  Rookie year: 1967&lt;br&gt;&#xA;Llewellyn, John A.;  Rookie year: 1967&lt;br&gt;&#xA;O'Leary, Brian T.;  Rookie year: 1967&lt;br&gt;&#xA;Smith, Michael J.;  Rookie year: 1980; DIED: January 28, 1986 in the STS-51-L accident &lt;br&gt;&#xA;Thorne, Stephen D.;  Rookie year: 1985; DIED: May 24, 1986 in airplane crash&lt;br&gt;&#xA;Cagle, Yvonne D.;  Rookie year: 1996&lt;br&gt;&#xA;Caldeiro, Fernando;  Rookie year: 1996&lt;br&gt;&#xA;Loria, Christopher J.;  Rookie year: 1996&lt;br&gt;&#xA;Robertson, Patricia Hilliard;  Rookie year: 1998; DIED: May 24, 2001 of injuries from the crash of a private plane&lt;br&gt;&#xA;Woodward, Neil W., III;  Rookie year: 1998&lt;br&gt;&lt;/p&gt;&#xA;" OwnerUserId="12668" LastEditorUserId="12668" LastEditDate="2015-12-18T19:06:13.393" LastActivityDate="2015-12-18T19:06:13.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13076" PostTypeId="2" ParentId="10050" CreationDate="2015-12-18T23:03:34.840" Score="3" Body="&lt;p&gt;Your major concern is going to be how easy it is to ignite the fuel, because the fire, ideally, must be contained by the rocket to produce any useful thrust. If the fuel is just flowing out of the bottle and then ignites at a distance, the only thrust you are going to get is a slight push. Easy to ignite pretty much always mean more dangerous to handle, so a good propellant is mostly dependent on what risks you are willing to take, or what you are actually allowed to acquire in your area. That said, I can recommend gasoline, maybe with additives for it to burn more easily, as a powerful fuel. That is surprisingly close to the RP-1 used in real rockets. Ethanol is another example of good commonly available fuel. Using other oxidisers than air is making your design at least an order of magnitude more complex, and falls outside the scope of a woosh rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, the shape of the nozzle is important, an ideal shape to aim for is a &lt;a href=&quot;https://en.wikipedia.org/wiki/De_Laval_nozzle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lavall nozzle&lt;/a&gt;. This turns the random motion of the gas molecules into a relatively straight stream. As you correctly point out, a smaller stream is indeed giving you a more efficient use of the pressure, but this advantage is at some point less important than the reduced thrust. Merely just hoovering is not getting it anywhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To maximize your altitude per unit &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;nofollow noreferrer&quot;&gt;delta-v&lt;/a&gt;, the optimal speed is &lt;a href=&quot;http://www.wisegeek.org/what-is-terminal-velocity.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;terminal velocity&lt;/a&gt;. Therefore, you want your rocket to accelerate quickly at first, in order to reach it and not loosing a lot of delta-v to gravity drag, but then slow down the acceleration to not exceed it and loose delta-v to air drag. In a model rocket, where you can not adjust the thrust much in flight, this is always a trade off. Giving the rocket a more aerodynamic shape rise the terminal velocity. Then the optimal thrust is going to be higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Chemical rockets are dangerous!&lt;/strong&gt; I hope you know what you are actually doing.&lt;br&gt;&#xA;If you want a more gentle start to hobby rocketry, &lt;a href=&quot;http://aircommandrockets.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;advanced water rockets&lt;/a&gt; can provide a lot of fun challenges, without the risk of you loosing an eye.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2019-08-15T22:37:50.527" LastActivityDate="2019-08-15T22:37:50.527" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13077" PostTypeId="2" ParentId="13069" CreationDate="2015-12-19T01:01:45.473" Score="4" Body="&lt;p&gt;I know for certain epoxy is still used today, perhaps not as commonly as in days past, but still are used. They are mostly used for delicate parts, to keep them from falling off, or otherwise hold them in place. I'm sure they aren't quite as heavily used as in previous days, but I imagine that they are used in almost every spacecraft (Every one that I'm aware of, at least)&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-12-19T01:01:45.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13078" PostTypeId="2" ParentId="844" CreationDate="2015-12-19T04:10:09.610" Score="3" Body="&lt;p&gt;Many launches have a designated launch time window necessitating a narrow period in which the launch must be made. The launch might fall only during the night or day period depending on these constraints. A limited launch window is a result of one or more mission objectives occurring later in orbit that can be met by the vehicle only being at a specific location in orbit at a specific time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Examples might include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the need for the launch vehicle to make a planned rendezvous with an existing orbiting object.&lt;/li&gt;&#xA;&lt;li&gt;a requirement to be at a specific time and location later in orbit to deploy a mission payload.&lt;/li&gt;&#xA;&lt;li&gt;a requirement to later be positioned over a specific earth location at a specific time to fulfill a mission objective such as surveillance or provide communications or provide some other service.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12674" LastEditorUserId="6816" LastEditDate="2015-12-19T04:21:14.190" LastActivityDate="2015-12-19T04:21:14.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13079" PostTypeId="2" ParentId="5409" CreationDate="2015-12-19T05:15:29.783" Score="1" Body="&lt;p&gt;The early orbiter design for Columbia included emergency ejection seats useful for a late aborted orbiter landing for two cockpit located astronauts.  Early Shuttle Astronauts trained in test seats and the flight cabin mockups at JSC/Houston fully included the related seat hardware and means of activation.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The propellants though at Kennedy Launch facility were never loaded and the seats were never available for arming in any landing emergency.  This decision was reported to be a direct request of STS-1 astronaut John Young as he had early concerns for the seats operational safety as well as a possible accidental activation during their times of non-use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle crews for the following flights also abandoned the emergency egress seat design altogether citing that the seats only afforded a flight crew of two emergency egress, though crews were exceeding that number.  On &#xA;Columbia's later makeover, the emergency seat design was fully removed. &lt;/p&gt;&#xA;" OwnerUserId="12674" LastEditorUserId="12674" LastEditDate="2015-12-19T05:20:32.620" LastActivityDate="2015-12-19T05:20:32.620" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13080" PostTypeId="2" ParentId="4561" CreationDate="2015-12-19T05:42:57.940" Score="4" Body="&lt;p&gt;Because of the needed orbital inclinations, the prime TAL site frequently was a remote site in North Africa.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interesting footnote is that the landing strip could support an emergency Shuttle Orbiter landing, but it could at the time not support the orbiter's later removal on the back of 747.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it had ever been used, it was understood in doing so only the crew and payload could be saved and recovered while the orbiter would have been potentially lost. &lt;/p&gt;&#xA;" OwnerUserId="12674" LastEditorUserId="12674" LastEditDate="2015-12-20T18:38:10.503" LastActivityDate="2015-12-20T18:38:10.503" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13081" PostTypeId="1" CreationDate="2015-12-19T06:51:17.387" Score="2" ViewCount="245" Body="&lt;p&gt;A couple of years ago I heard Neil Armstrong interviewed by Terry Gross on NPR's Fresh Air.  He was talking about manually piloting the Apollo 11 lunar module in extremely stressful conditions.  Terry asked him if he was nervous.  He said no, because he was able to rely on his training.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, my spouse tells me he read somewhere that Armstrong's heart rate and breathing were monitored, and they really didn't go up during the hair-raising manual navigation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have googled the death out of this topic, and I can't for the life of me find this interview.  I have saved hundreds of Fresh Air podcasts on my hard drive, and if I knew what date to look for, I would probably find this interview in my archive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please don't tell me I imagined the whole interview.  It had a big personal influence on my life.&lt;/p&gt;&#xA;" OwnerUserId="12675" LastActivityDate="2015-12-21T10:05:29.750" Title="Where is the Neil Armstrong interview I heard on Fresh Air with Terry Gross?" Tags="|apollo-11|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13082" PostTypeId="1" AcceptedAnswerId="13083" CreationDate="2015-12-19T07:41:45.650" Score="4" ViewCount="8965" Body="&lt;p&gt;Falcon 9 uses Merlin 1 Kerosene based engines. Are there 4 Merlin 1 engines? This seems simple to calculate:&#xA;Number of Merlin Engines x Merlin 1 fuel capacity(gallons) x Kerosene to CO2 Production.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Falcon_9&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;eia.gov says:&#xA;19.64 pounds of carbon dioxide (CO2) are produced from burning a gallon of gasoline that does not contain ethanol. About 22.38 pounds of CO2 are produced by burning a gallon of diesel fuel.&lt;/p&gt;&#xA;" OwnerUserId="12677" LastEditorUserId="51" LastEditDate="2015-12-22T19:53:16.200" LastActivityDate="2022-08-04T14:53:56.810" Title="Calculate Falcon 9 CO2 emissions" Tags="|rockets|spacex|falcon-9|fuel|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13083" PostTypeId="2" ParentId="13082" CreationDate="2015-12-19T08:08:11.200" Score="8" Body="&lt;p&gt;The mass of the RP-1 fuel of the first stage of Falcon 9 is &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/&quot; rel=&quot;noreferrer&quot;&gt;119,100 kg&lt;/a&gt;. That is around 100,000 kg of carbon, corresponding to 360,000 kg of carbon dioxide. However, according to&#xA;&lt;a href=&quot;http://web.ornl.gov/info/reports/1991/3445603520893.pdf&quot; rel=&quot;noreferrer&quot;&gt;This pdf&lt;/a&gt;, only a little less than half the carbon is completely burnt in hydrocarbon based rocket propellants, the rest becoming carbon monoxide or mono atomic carbon. The figure is then closer to 170,000 kg. The RP-1 fuel of the second stage is 27,850 kg, adding another 40,000 kg of carbon dioxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total is then somewhere around 210,000 kg of carbon dioxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CO will eventually oxidise further to carbon dioxide, so the final amount of CO&lt;sub&gt;2&lt;/sub&gt; added to the atmosphere is roughly 440,000 kg.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-20T11:58:24.543" LastActivityDate="2015-12-20T11:58:24.543" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13084" PostTypeId="2" ParentId="13082" CreationDate="2015-12-19T08:34:48.167" Score="0" Body="&lt;p&gt;This is what I came up with:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stage 1:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;9 Merlin1D Engines 540gps x 180 secs = 97200 gallons of LOX/RP-1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stage 2:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 Merlin1D Engine burns 60 gps&#xA;60gps x 375secs = 22500 gallons of LOX/RP-1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;119,700 gallons of LOX / RP-1 (Kerosene)&#xA;119,700gals x 22.38lbs/gallon = 2,678,900lbs&lt;/p&gt;&#xA;" OwnerUserId="12677" LastEditorUserId="8693" LastEditDate="2015-12-19T09:06:05.357" LastActivityDate="2015-12-19T09:06:05.357" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13085" PostTypeId="2" ParentId="10659" CreationDate="2015-12-19T11:30:22.847" Score="4" Body="&lt;p&gt;Sorry, but this comment is rather long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding the Saturn V LEO payload questions...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check out this source:&#xA;&lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=12519.20&quot; rel=&quot;nofollow&quot;&gt;http://forum.nasaspaceflight.com/index.php?topic=12519.20&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V launch vehicle was upgraded throughout its lifetime. Its engines were uprated and numerous measured were taken to reduce the weight of the rocket. Thus, by the time Apollo 17 came around, the Saturn was quite an improved machine. The link above details its LEO throw weight as having been 140,893kg. Some sources indicate lower or higher than this value, but this one was at hand when I wrote this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, the Saturn V was capable of delivering over 140t to LEO, but we must remember that this was after a bit of improvement. The final Saturn Vs were pumping out very close to 7.9 million pounds of thrust, which is also a lot more than is usually quoted (7.5 million). I believe the initial lunar landing missions involved LEO masses of somewhere around 133t, with less powerful Saturns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, we must outline what makes up the LEO payload. The 140,893kg figure includes the partially-fuelled S-IVB third stage of the Saturn V. However, upper stages are usually excluded from earth orbital mission payload masses.  So why is it included in the LEO payload… &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because Apollo 17’s mission was to go to the moon!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is where the Saturn V’s quoted LEO payload definition differs from other launch vehicles. In the case of lunar landing missions, LEO is not the end of the line; something is needed to push the Apollo hardware to TLI. This is the S-IVB’s purpose. Upon reaching its low-earth parking orbit, the third stage is still fuelled and is not just inert mass. It still constitutes usable payload, as it is used to go to the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another important matter to consider is the height and inclination of the earth parking orbit. Early Apollo lunar missions entered LEO at around 185km at around 28.5 degrees. But on later ‘Apollo J’ missions, higher masses were delivered to LEO partly because they orbited at a lower altitude. Apollo 15, 16 and 17 parked at around 167km with similar inclination, meaning there would be more fuel left in the S-IVB upper stage for the TLI burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for the other values (127t and 118t respectively)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;127t is the LEO payload mass given for the Saturn C-5… Take note!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn C-5 is not the Saturn V!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn C-5 was Von Braun’s initial design, developed while Apollo mission planners were still deciding on a basic method with which to reach the moon. When ‘Lunar Orbit Rendezvous’ was selected as the way to go, the Apollo CSM and LM combo was developed in greater detail. The payload weights grew, however, and eventually the original Saturn C-5 was found to be too weak to deliver them to TLI. Thus, the C-5 was improved and became the Saturn V. The first stage was kept very nearly identical, but the second stage was lengthened by around 3.5m for increased propellant. Also, the third stage was increased in diameter, again to hold more fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn C-5 was able to deliver up to 127t (including the third stage) to a 185km LEO, and around 41t to TLI. The later Saturn Vs were able to deliver over 140t to a 167km LEO, and almost 50t to TLI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;118t (and 120t) is up for debate, as far as I know. Some say that it is the theoretical usable LEO payload of the Saturn V three-stage vehicle if the third stage is not included and is depleted upon reaching earth orbit (Apollo weighed up to 48.6t in total, leaving fuel left over for TLI, whereas 118t of payload would not). This is explored in the link at the top of this comment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I have been led to wonder if this could be the maximum 185km LEO payload of a Saturn V two-stage vehicle to LEO, similar to the approach that would have occurred with non-TLI Saturn MLV-V-1 and Saturn MLV-V-3 launches (look these variants up, they’re brilliant). Such a rocket was designated the Saturn INT-21 and was never launched, although the Skylab Saturn V was essentially just that. Yes, the Skylab launcher didn’t deliver that much usable payload to its orbit, but it launched to 434km and over 50 degrees inclination, which requires more performance than 185km at 28.5 degrees. For this orbit, the INT-21 was quoted for 255,000lbs, or 115.7t… Pretty close, and that was studied before even the early Saturn Vs and as such did not include late model uprated engines and weight savings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, things to remember:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Think about what the mission requires. Missions going beyond LEO can still include the mass of the upper stage and its fuel for their quoted LEO masses. Missions only going to LEO cannot if the upper stage is depleted, because there is no more use for the empty fuel tank and engines.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Consider the altitude and inclination of the payload’s orbit. Higher orbits require more energy to achieve, and as such less mass will be delivered for a given rocket. Also, higher inclinations take more energy for a given altitude.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Think about developments and improvements made to the model of rocket. A rocket that has been in service for a long time will generally be more capable later in its operating life than its initial variants.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For error margins, I can’t say a whole lot on that subject. However, I would believe that it is recommended not to load a rocket to its designated limit. Safety margins are included, and as such a rocket should launch with a little bit more fuel than is absolutely needed, just in case something doesn’t turn out as planned. This would allow the vehicle to reach orbit even in the case of, let’s say, a single engine failure on a multi-engine upper stage (maybe, kind of like Apollo 6).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For historians, I would say that average payload for a launch vehicle variant over its lifetime would be useful knowledge. However, I would say that whatever the current LEO payload rating is for a rocket is the more important value. Assuming the vehicle has been improved over its lifetime (likely), it will generally be more capable than its earlier iterations. But that’s just my opinion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have read all of this, thank you for taking the time to do so. Any feedback is welcome. If you notice anything incorrect about the information listed, please let me know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sorry for the ‘essay,’&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alastair. &lt;/p&gt;&#xA;" OwnerUserId="12678" LastEditorUserId="12678" LastEditDate="2015-12-24T02:48:58.423" LastActivityDate="2015-12-24T02:48:58.423" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13086" PostTypeId="1" CreationDate="2015-12-19T11:41:36.370" Score="13" ViewCount="8789" Body="&lt;p&gt;What determines if a launch has to be done instantaneously or has a launch window of minutes or hours? To what parts is it about celestial and ground issues?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just &lt;a href=&quot;http://www.nasaspaceflight.com/2015/12/spacex-static-fire-falcon-9-og-2-rtf/&quot;&gt;one example&lt;/a&gt; of alleged switching between a window and instantaneously, Falcon 9 planned launch of multiple communication satellites to LEO in 2015-12-19:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;That window was expected to be 20:02-23:02 Eastern. However, that has&#xA;  since been updated to an instantaneous window of 20:20 Eastern.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="49" LastEditDate="2015-12-19T17:55:19.527" LastActivityDate="2015-12-20T03:10:30.267" Title="What decides if a launch has to be done instantaneously or during a window?" Tags="|launch|launch-site|law|time|procedure|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13087" PostTypeId="2" ParentId="13086" CreationDate="2015-12-19T12:08:08.453" Score="4" Body="&lt;p&gt;The target is usually the key, although the range might have a say as well. For instance, an ISS mission must be lined up to get to the ISS, requiring a small window. For the purpose of LEO communication satellites, the key is to get to an orbit with a time of day that is particularly useful. For most of the Falcon 9 launches, this has been the case. There have been some very long windows, for instance. It could be restricted on either the rocket side (Very unlikely), launch site support, or blocking Air Traffic in the area, all of which might require a smaller window.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that in this case, it's probably not the customer. One might recall that &lt;a href=&quot;http://spacecoastdaily.com/2014/07/spacex-orbcomm-launch-set-monday-morning/&quot; rel=&quot;nofollow&quot;&gt;ORBCOMM previously had a 2 hour 33 minute launch window&lt;/a&gt;. As the goal seems to be &lt;a href=&quot;http://www.decodesystems.com/orbcomm.html&quot; rel=&quot;nofollow&quot;&gt;4 planes with the two launches&lt;/a&gt;, most of the satellites are going to have to do some traveling to achieve the goal. I suspect it has more to do with SpaceX, or more likely, FAA regulating the time of their launch during a fairly busy travel season. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-12-19T12:13:26.093" LastActivityDate="2015-12-19T12:13:26.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13088" PostTypeId="2" ParentId="13086" CreationDate="2015-12-19T12:25:33.030" Score="16" Body="&lt;p&gt;In the case of Falcon 9 / Orbcomm OG2 launch from SLC-40, Cape Canaveral Air Force Station, FL that's now scheduled for Dec. 20th at 8:29 p.m. EST (01:29 a.m. UTC), according to &lt;a href=&quot;http://spaceflightnow.com/2015/12/19/spacex-plans-to-return-falcon-9-rocket-to-flight-sunday/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sources said only an instantaneous launch opportunity is available&#xA;  Sunday due to restrictions imposed by the Federal Aviation&#xA;  Administration, which may be hesitant to re-route holiday week air&#xA;  traffic around the Cape Canaveral launch base for several hours in&#xA;  case of launch delays.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So in this case it's range imposed restrictions. On top of other answers in this thread, here are some others that are related:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/6191/short-eft-1-missions-launch-window&quot;&gt;Short EFT-1 missions launch window&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/8081/why-would-a-mission-to-sun-earth-l1-have-an-instantaneous-launch-window&quot;&gt;Why would a mission to Sun-Earth L1 have an instantaneous launch window?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/844/day-versus-night-launches&quot;&gt;Day versus night launches&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So to recap, and adding a few points to what has already been previously discussed, possible restrictions could be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Range&lt;/strong&gt;: Launch times have to be coordinated with other local and sometimes international air and maritime authorities to redirect traffic along the route that a launch vehicle would take, and safety exclusion zone has to be cordoned off for debris and returning stages along its ground track. This requires a lot of coordination and scrambling of hardware that has to happen prior to launch and for the duration of its launch window and then some until the range is cleared after the last launch opportunity.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Tracking&lt;/strong&gt;: Time of onsite and downrange tracking facilities needs to be allocated, and temporary permissions to use frequencies used for telemetry and other broadcasts need to be acquired. Since these are shared resources, operators and authorities might only allocate short time slots to support the needs of your operation. This might also depend on availability and accessibility of orbital facilities, like &lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_data_relay_satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;TDRS&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Target&lt;/strong&gt;: Oftentimes a launched spacecraft will either chase another vehicle in orbit for rendezvous and both vehicles' orbital phases have to align over the launch site, or the vehicle will target a specific local time-dependent orbit like &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;SSO&lt;/a&gt; or specific orbital plane on interplanetary trajectory as the Earth rotates on its axis. Suborbital launches might also target specific phenomena like auroras as they occur.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Weather&lt;/strong&gt;: Some launch sites (and especially Florida) have predictable weather cycles and specific times of the day or night could be targeted for launch to maximize the chance that weather cooperates and meets launch criteria. Depending on the time of the year, there might also be only a limited period of time when environmental constraints (like temperature, humidity) stay within safety threshold.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Payload&lt;/strong&gt;: Some payloads (e.g. small live cargo) can only be supported on the pad for limited periods of time, especially those that in any way rely on limited life support provisions or internal power and for whichever reason can't be connected with umbilicals externally for power and supplies, or the fairing sufficiently ventilated to prevent hazard of bled off propellants, and similar.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Launch hardware&lt;/strong&gt;: Some facilities and the launch vehicle itself might only be capable of continuous support on the pad and once fueled for a very limited period of time, like e.g. high pressure pumps, cryogenic storage, ability to purge bled off propellants, ice condensing on cryogenic surfaces, and so on.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Collision avoidance&lt;/strong&gt;: Launched object will intersect air and orbital space of other aircraft, satellites and orbital debris, so conjunction assessment (CA), collision avoidance (COLA), and likely launch collision avoidance (LCOLA) procedures will have to be done to avoid that. Since that requires awareness of flight paths, activities and orbits of classified objects, it is done (in US) on request through &lt;a href=&quot;https://en.wikipedia.org/wiki/Joint_Space_Operations_Center&quot; rel=&quot;nofollow noreferrer&quot;&gt;Joint Space Operations Center&lt;/a&gt;, part of &lt;a href=&quot;https://en.wikipedia.org/wiki/Joint_Functional_Component_Command_for_Space&quot; rel=&quot;nofollow noreferrer&quot;&gt;Joint Functional Component Command for Space&lt;/a&gt; of US Strategic Command. See &lt;a href=&quot;http://static.e-publishing.af.mil/production/1/af_se/publication/afi91-217/afi91-217.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;AFI91-217&lt;/a&gt; (PDF) for more info. These analyses may give recommendations and limits with regard to launch window and assumed trajectory, but they won't specify reasons for them, so you might not know why your launch window might be &lt;em&gt;instantaneous&lt;/em&gt; even if you'd otherwise be ready to launch during a longer launch window.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Other&lt;/strong&gt;: Eventually, people will want to go home and return to their families, and sitting in constant readiness for hours on end isn't easy. It might be more prudent to split launch opportunities into smaller chunks of up to some number of hours per day, than to increase risk of human factor affecting safety. Theirs and the launch vehicle's.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Additionally, in our specific case and if I'm allowed a bit of conjecture, SpaceX is with Orbcomm OG2 launch both returning to flight after a RUD (rapid unscheduled disassembly) event during the CRS-7 launch, and they're doing this with an upgraded launch vehicle (Falcon 9 v1.2) that uses higher propellant density and deep cryogenic liquid oxygen (which, &lt;a href=&quot;https://twitter.com/elonmusk/status/677660460230041600&quot; rel=&quot;nofollow noreferrer&quot;&gt;according to Elon Musk himself&lt;/a&gt;, is a bit of a challenge). They might have estimated that if they'll have to hold the launch for whichever reason, there's a good chance they would have to scrub it for the day and have only requested a very short opportunity to launch. If they're working with some quota (by any authority or service, from FAA, FCC, USCG, MARAD, AST, JSpOC, Space wing, TDRS, Bermuda downrange tracking station,...), and their customer isn't pushing for launch as soon as possible, it makes sense not to &lt;em&gt;upset the deities&lt;/em&gt; with unreasonable demands that might not be met, or indeed needed.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Just for reference, here are the safety zones (&lt;a href=&quot;http://www.patrick.af.mil/shared/media/document/AFD-151219-003.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Launch Hazard Area&lt;/a&gt; and &lt;a href=&quot;http://www.patrick.af.mil/shared/media/document/AFD-151219-004.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Airspace Closure Area&lt;/a&gt;, respectively) and their schedule for Dec. 20th launch attempt, announced by &lt;a href=&quot;http://www.patrick.af.mil/&quot; rel=&quot;nofollow noreferrer&quot;&gt;45th Space Wing, Patrick Air Force Base&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.patrick.af.mil/shared/media/document/AFD-151219-003.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/G7Jham.png&quot; alt=&quot;Launch Hazard Area&quot;&gt;&lt;/a&gt; &lt;a href=&quot;http://www.patrick.af.mil/shared/media/document/AFD-151219-004.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jOvgQm.png&quot; alt=&quot;Airspace Closure Area&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, issued documents list launch window of 26 minutes for 20th:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;20 Dec 2015 8:29pm - 8:55pm&lt;/li&gt;&#xA;  &lt;li&gt;21 Dec 2015 0129Z – 0155Z&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's the same launch window, just in local EST (Eastern Standard Time) time zone followed by &quot;&lt;a href=&quot;http://www.navy.mil/navydata/questions/zulutime.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zulu&lt;/a&gt;&quot; time (today, just a military way of saying &lt;a href=&quot;https://en.wikipedia.org/wiki/Coordinated_Universal_Time&quot; rel=&quot;nofollow noreferrer&quot;&gt;UTC&lt;/a&gt; really). Just announced &lt;a href=&quot;http://livestream.com/spacex/events/4600161&quot; rel=&quot;nofollow noreferrer&quot;&gt;launch webcast&lt;/a&gt; is also an hour long, ending 31 minutes after the &lt;em&gt;instantaneous&lt;/em&gt; launch on 8:29 p.m. EST (1:29 UTC), which would be unusually long for &lt;a href=&quot;http://www.spacex.com/webcast/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX hosted webcasts&lt;/a&gt; if they really stayed on-air for that long after liftoff. Unless it was a CRS launch to ISS for NASA, they would usually wrap up minutes after second stage separation. I'm not exactly sure what to make of this, but all that talk of &lt;em&gt;instantaneous&lt;/em&gt; launch might have been for naught. I couldn't find any official word on it, not from &lt;a href=&quot;http://www.spacex.com/news&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt; and neither from &lt;a href=&quot;https://twitter.com/elonmusk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elon Musk&lt;/a&gt; that tweeted all the updates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;26 minutes long launch window would permit for up to two countdown clock recycles of 13 minutes that were usual during previous SpaceX Falcon 9 launches with vehicle's terminal countdown of 10 minutes, so possibly up to three attempts to launch on that same day, assuming no issues are worked on that extend the hold (like issues with the vehicle or a boat breaching the safety exclusion zone which did happen before and even resulted in a &lt;a href=&quot;https://twitter.com/WaywardBoat&quot; rel=&quot;nofollow noreferrer&quot;&gt;parody Twitter account&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Just another quick update that an &lt;a href=&quot;http://www.spacex.com/news/2015/12/11/orbcomm-2-mission-update&quot; rel=&quot;nofollow noreferrer&quot;&gt;announcement of tomorrow's launch&lt;/a&gt; now appeared also on SpaceX pages and it reads (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX is currently aiming for a December 20th launch of the Falcon 9&#xA;  rocket, carrying 11 satellites for &lt;a href=&quot;http://www.orbcomm.com/en/company-investors/news/2015/orbcomm-announces-target-launch-schedule-for-second-og2-mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;ORBCOMM&lt;/a&gt;. The launch is part of&#xA;  ORBCOMM's second and final OG2 Mission and will lift off from SpaceX's&#xA;  launch pad at Cape Canaveral Air Force Station in Florida.  &lt;strong&gt;This&#xA;  mission also marks the first time SpaceX will attempt to land the&#xA;  first stage of the Falcon 9 rocket on land&lt;/strong&gt;. The landing of the first&#xA;  stage is a secondary test objective.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The launch webcast is targeted to begin at 5:05pm PT with liftoff at&#xA;  5:29pm PT.  The launch webcast can be viewed at &lt;a href=&quot;http://www.spacex.com/webcast&quot; rel=&quot;nofollow noreferrer&quot;&gt;spacex.com/webcast&lt;/a&gt;. &#xA;  For updates, visit &lt;a href=&quot;http://www.spacex.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;www.spacex.com&lt;/a&gt; and &lt;a href=&quot;http://www.orbcomm.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;www.orbcomm.com&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So here's another explanation, that the launch might still be &lt;em&gt;instantaneous&lt;/em&gt; and those 26 minutes are now allocated for both launch of Falcon 9R from &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Space_Launch_Complex_40&quot; rel=&quot;nofollow noreferrer&quot;&gt;SLC-40&lt;/a&gt; and landing of its first stage at &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Launch_Complex_13&quot; rel=&quot;nofollow noreferrer&quot;&gt;Landing Complex 1&lt;/a&gt;. How cool is that?&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-12-20T03:10:30.267" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13089" PostTypeId="1" CreationDate="2015-12-19T12:37:30.330" Score="2" ViewCount="79" Body="&lt;p&gt;I'm trying to roughly estimate dimensions and masses of propellant and pressurant ducts for conventional upper- &amp;amp; transfer stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My basic approach follows Barlow's formula for cylindrical vessels (hoopstress), but that results in wall thicknesses that seem very small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example: A stainless steel pipe with a diameter of 30mm and a pressure of 12bar(~175psi) would only have a 0.1mm wall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My usual sources for the preliminary design of propulsion systems (Huzel/Huang, Sutton/Biblarz, Humble/Henry/Larson) unfortunately don't cover that topic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore I'm asking if you could recommend some links, papers or books with reliable, &quot;rocket-related&quot; information or best practices for dimensioning pressurant and propellant ducts.&lt;/p&gt;&#xA;" OwnerUserId="8724" LastEditorUserId="8724" LastEditDate="2015-12-19T20:17:26.997" LastActivityDate="2015-12-19T20:17:26.997" Title="Preliminary design of Propellant ducts" Tags="|rockets|propulsion|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13090" PostTypeId="2" ParentId="13086" CreationDate="2015-12-19T12:42:26.447" Score="3" Body="&lt;p&gt;Length of launch windows is determined by many things but one that hasn't been mentioned yet is performance margin of the booster. As the Earth rotates away from the desired orbital plane, the booster has to provide extra performance to change the plane. This effect was often a launch window length constraint for Shuttle rendezvous missions.  If the vehicle has little performance margin an instantaneous window is likely.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-12-19T12:42:26.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13091" PostTypeId="2" ParentId="4561" CreationDate="2015-12-19T12:50:50.390" Score="8" Body="&lt;p&gt;Shuttles &lt;a href=&quot;https://en.m.wikipedia.org/wiki/List_of_space_shuttle_landing_sites#Edwards_Air_Force_Base&quot; rel=&quot;noreferrer&quot;&gt;landed on the lakebed runways&lt;/a&gt; at Edwards many times, which had no concrete at all, just markings painted on the desert floor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SsSyd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SsSyd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These runways are separate from the concrete and asphalt runways also located at Edwards (also used by the Shuttles). The dry lakebed is located at the upper left of the 2nd picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7aDwk.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7aDwk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-04-05T22:36:35.223" LastActivityDate="2017-04-05T22:36:35.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13092" PostTypeId="2" ParentId="13086" CreationDate="2015-12-19T15:06:37.207" Score="8" Body="&lt;p&gt;A constraint not mentioned is the launch vehicle capability to target a variable azimuth. The Delta II launch vehicle did not have a variable launch azimuth capability. As a result, Mars missions on the Delta II (there were several) would generally have two instantaneous launch windows each day at two azimuths. Two were considered sufficient to address possible range issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It took half an hour to reload and verify the parameters on the Delta II, so the distance between the two had to be at least that. The launch vehicle could only wait for 83 minutes before the risk of freezing the feed system, so the distance had to be less than that. The windows for MER ended up about 37 to 44 minutes apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The windows were not technically instantaneous, but actually one second long, so the guy on the button didn't have to be perfect. (Yes, there was a guy with a button.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-12-19T15:06:37.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13094" PostTypeId="2" ParentId="13081" CreationDate="2015-12-19T22:41:07.617" Score="2" Body="&lt;p&gt;Here is a &lt;a href=&quot;http://www.npr.org/player/v2/mediaPlayer.html?action=1&amp;amp;t=1&amp;amp;islist=false&amp;amp;id=241830872&amp;amp;m=241857425&quot; rel=&quot;nofollow&quot;&gt;Terry Gross interview with astronaut Chris Hadfield &lt;/a&gt; about remaining calm in stressful situations. Maybe you are looking for this one?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only time Armstrong has been on Fresh Air is &lt;a href=&quot;http://www.npr.org/player/v2/mediaPlayer.html?action=1&amp;amp;t=1&amp;amp;islist=false&amp;amp;id=332760088&amp;amp;m=332760089&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt;, but that is not it. Scott Simon did the interview, not Terry Gross.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-20T11:38:43.420" LastActivityDate="2015-12-20T11:38:43.420" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13095" PostTypeId="1" AcceptedAnswerId="15303" CreationDate="2015-12-19T23:05:52.057" Score="11" ViewCount="2941" Body="&lt;p&gt;As I observed ignition of several rocket engines (&lt;a href=&quot;https://youtu.be/ShRa2RG2KDI?t=9m52s&quot; rel=&quot;noreferrer&quot;&gt;Shuttle&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/watch?v=gSkEu9_Wg7Y&quot; rel=&quot;noreferrer&quot;&gt;AMDE&lt;/a&gt;, &lt;a href=&quot;https://youtu.be/vec1z8dJ5as?t=45s&quot; rel=&quot;noreferrer&quot;&gt;Lynx Suborbital&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/watch?v=DKtVpvzUF1Y&quot; rel=&quot;noreferrer&quot;&gt;Saturn V&lt;/a&gt;) they all come through a couple phases - emitting various vapors, unfocused flame, etc, before the proper &quot;full thrust&quot; flame appears - sometimes they come slower, sometimes faster (in case of &lt;a href=&quot;https://www.youtube.com/watch?v=XaVC3r8yw6g&quot; rel=&quot;noreferrer&quot;&gt;Bantam&lt;/a&gt; that's nearly instantaneous).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are these phases? What am I observing - could someone describe step by step, what are the phases between the countdown &quot;zero&quot; and the engine firing at full thrust? And conversely (like in case of AMDE), what are the phases of engine shutdown?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="49" LastEditDate="2015-12-19T23:42:12.580" LastActivityDate="2018-06-05T19:22:44.153" Title="What are the phases of ignition (and flameout) of a liquid fuel engine?" Tags="|propulsion|fuel|engine-design|engines|liquid-fuel|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13096" PostTypeId="2" ParentId="13095" CreationDate="2015-12-19T23:53:51.150" Score="8" Body="&lt;p&gt;My &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;noreferrer&quot;&gt;go-to open source for SSME&lt;/a&gt; information has nice graphs for startup and shutdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a bit more going on than is shown in this diagram such as the spark ignitors coming on and off, and the main engine controller stepping through its start sequence, verifying parameters to ensure that start is progressing normally, but these pictures give a very good overall look at start and shutdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some words of explanation (refer to the link for details)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;CCV is the chamber coolant valve, routes hydrogen through the nozzle&#xA;cooling circuits.&lt;/li&gt;&#xA;&lt;li&gt;MOV is Main Oxidizer Valve, basically just a shut-off.&lt;/li&gt;&#xA;&lt;li&gt;MFV is Main Fuel Valve, ditto.&lt;/li&gt;&#xA;&lt;li&gt;OPOV is the Oxidizer Preburner Oxidizer Valve, this valve is&#xA;controlled by the chamber pressure (aka thrust) control loop.  It&#xA;controls the amount of oxygen entering the Oxidizer Preburner and&#xA;thereby the amount of O2 pumped by the high pressure ox turbopump.&lt;/li&gt;&#xA;&lt;li&gt;FPOV is the Fuel Preburner Oxidizer Valve, this valve is controlled&#xA;by the mixture ratio control loop.  It controls the amount of oxygen&#xA;entering the Fuel Preburner and thereby the amount of H2 pumped by&#xA;the high pressure fuel turbopump.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MhCQP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MhCQP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/iutts.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iutts.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2015-12-19T23:59:19.057" LastActivityDate="2015-12-19T23:59:19.057" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13097" PostTypeId="1" AcceptedAnswerId="13796" CreationDate="2015-12-20T00:19:12.870" Score="11" ViewCount="542" Body="&lt;p&gt;I know some of the components at SpaceX are manufactured using metal additive manufacturing (3D printing), but how much of the entire Falcon 9 launch vehicle or Dragon capsule is made up of printed components? Obviously there are more sensible techniques for manufacturing parts like thin-walled panels, but how much of the vehicle could potentially be 3D printed? How about an entire engine?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note this question is regarding all variants of the Falcon 9 and Dragon capsule, and I also welcome any relevant information about test hardware like the Grasshopper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the SuperDraco engine block shown below was manufactured with 3D printing (see &lt;a href=&quot;http://www.designnews.com/author.asp?doc_id=274432&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; for some more information).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ttYIJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ttYIJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorDisplayName="user20636" LastEditDate="2018-05-13T08:24:46.683" LastActivityDate="2018-05-13T08:24:46.683" Title="How much of SpaceX's Falcon 9 launch vehicle and Dragon capsule are 3D printed?" Tags="|spacex|falcon-9|dragon|manufacturing|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="13098" PostTypeId="2" ParentId="13095" CreationDate="2015-12-20T00:34:42.220" Score="5" Body="&lt;p&gt;Considering the Space Shuttle main engines as an example and building on Organic Marble's answer, the start-up phases amount to some transient effects while mixing the fuel and oxidizer while building up thrust and developing a steady-state flow out of the nozzle. You'll often see un-burnt hydrogen and clouds of oxygen as the mixing occurs and the initial flame builds up. Although it happens very quickly, it still takes some time to develop the flow to steady-state, so you get a bunch of different phenomena happening like shock cones and plasma.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/W2VygftZSCs?t=2m40s&quot; rel=&quot;noreferrer&quot;&gt;This video&lt;/a&gt; commemorating the Space Shuttle from an engineering perspective has probably the best available footage of the SSME's starting up. The commentary does not go into detail regarding the various phases but they provide some information about what you're seeing.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-12-20T00:34:42.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13099" PostTypeId="1" AcceptedAnswerId="13102" CreationDate="2015-12-20T14:44:35.797" Score="4" ViewCount="1585" Body="&lt;p&gt;If skipping off the atmosphere during re-entry runs a real risk of of the spacecraft &quot;escaping&quot; from orbit, why don't deep-space missions use this method instead of using power to reach escape velocity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(I am not a physicist and my knowledge in this field is limited so I apologize if the question was asked wrong or is based on a false premise)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12683" LastEditorUserId="49" LastEditDate="2015-12-20T16:09:35.597" LastActivityDate="2015-12-20T17:37:46.740" Title="Why can't deep-space missions escape orbit by skipping off the atmosphere?" Tags="|orbit|reentry|trajectory|escape-velocity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13100" PostTypeId="2" ParentId="13099" CreationDate="2015-12-20T15:06:05.367" Score="6" Body="&lt;p&gt;During reentry, spacecraft like Apollo run a risk of skipping due to an excess of lift. (The blunt body shape doesn't generate a lot of lift, but it does generate some at hypersonic velocities.) But lift generates drag: you can't get lift for free, you have to have extra speed to create it. Apollo in particular reentered with a lot of extra velocity, since it was generally returning from a translunar orbit, which turns all the potential energy from a high altitude into velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, you can use lift to get mostly out of the atmosphere. It's not a free lunch, though, and generally the mass of making wings that will give enough lift without falling apart from drag stresses is a serious problem for spacecraft, so there are at present no orbital spaceplanes. Ultimately, too, that's only &lt;em&gt;mostly&lt;/em&gt;. A stable orbit has to be quite some distance above any concentration of air thick enough to provide measurable lift, or else it will decay from drag and reenter early.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2015-12-20T15:06:05.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13101" PostTypeId="1" CreationDate="2015-12-20T16:33:13.607" Score="3" ViewCount="320" Body="&lt;p&gt;How much helium and nitrogen are used on SpaceX's Falcon 9?&lt;/p&gt;&#xA;" OwnerUserId="12684" LastEditorUserId="6944" LastEditDate="2021-03-28T18:17:22.173" LastActivityDate="2021-03-28T18:17:22.173" Title="How much helium and nitrogen are used in SpaceX's Falcon 9?" Tags="|spacex|falcon-9|helium|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13102" PostTypeId="2" ParentId="13099" CreationDate="2015-12-20T17:37:46.740" Score="14" Body="&lt;p&gt;Skipping off the atmosphere can only lead to an escape if your speed going in was above the escape speed. The skip changes your direction, and it reduces your speed a bit. So if you enter a skip below escape speed, skipping will send you into an elliptical orbit.&lt;br&gt;&#xA;So for a deep-space mission there's no advantage to doing a skip. You need to build up velocity some other way. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-20T17:37:46.740" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13103" PostTypeId="1" AcceptedAnswerId="13105" CreationDate="2015-12-20T17:54:31.450" Score="3" ViewCount="423" Body="&lt;p&gt;What was the plan if the flight controller received a no-go prior to Apollo 8's traverse of the far side of the Moon? How would they have returned to Earth without the gravity assist from the Moon?&lt;/p&gt;&#xA;" OwnerUserId="12685" LastEditorUserId="49" LastEditDate="2015-12-20T18:13:53.133" LastActivityDate="2015-12-21T02:33:35.583" Title="Abort plan for Apollo 8" Tags="|orbital-mechanics|the-moon|apollo-program|abort|free-return|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13105" PostTypeId="2" ParentId="13103" CreationDate="2015-12-20T18:12:29.550" Score="8" Body="&lt;p&gt;Apollo 8 was on a free-return trajectory when approaching the moon. If there was a no-go for the lunar orbit insertion, or if the engine failed to fire, they would have returned to Earth needing only minor course correction (doable with the RCS thrusters, which had a terrific amount of redundancy) to get home.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Via &lt;a href=&quot;http://www.americaspace.com/?p=28278&quot; rel=&quot;nofollow&quot;&gt;Americaspace.com&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During the translunar coast, the large Service Propulsion System (SPS) engine in the service module would be test-fired for a few seconds. If it refused to work, the astronauts could still be brought home safely, thanks to a safety feature built into Apollo 8’s trajectory design. Known as the “free return,” it would allow the crew to essentially loop around the Moon and use its gravitational influence to slingshot them back to Earth without using the SPS. In fact, if Borman, Lovell, and Anders did find themselves with a useless engine, they would only need to perform a couple of mid-course correction burns, using the service module’s thruster quads, to keep them on track for home.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Your question is a bit confused, though: the free return trajectory &lt;em&gt;is&lt;/em&gt; a gravity assist trajectory, using the moon's gravity to sling the ship back towards Earth. It's simply set up long before the ship reaches the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.braeunig.us/apollo/free-return.htm&quot; rel=&quot;nofollow&quot;&gt;This page has a nice animation&lt;/a&gt; of the trajectory - after the ship leaves Earth, gravity is doing all the work. The Apollo missions after 11 switched to &lt;a href=&quot;http://www.braeunig.us/apollo/hybrid-profile.htm&quot; rel=&quot;nofollow&quot;&gt;a different profile&lt;/a&gt;, starting with a free return trajectory falling well short of the moon, then switching to a non-free-return with a midcourse correction. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-21T02:33:35.583" LastActivityDate="2015-12-21T02:33:35.583" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13109" PostTypeId="1" CreationDate="2015-12-21T05:02:54.953" Score="4" ViewCount="104" Body="&lt;p&gt;High Intensity Interval Training (HIIT) has had a lot of attention over the past few years as an exercise method because studies show better improvement in various fitness parameters than traditional, steady state training (though I am still skeptical as to whether this benefit is retained long-term).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of these benefits is that there is an improvement in fitness without a loss in muscle mass, whereas traditional endurance training slowly causes a loss of muscle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would seem like a huge benefit on the ISS or any other long-term space mission as it would cut back on workout times and frequency while yielding better results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am wondering if this method of exercise has ever been trialed in space and if so what were the results?&lt;/p&gt;&#xA;" OwnerUserId="6254" LastActivityDate="2015-12-21T18:11:12.543" Title="Has high intensity training been tested in space?" Tags="|iss|research|astronaut-lifestyle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13110" PostTypeId="1" CreationDate="2015-12-21T06:08:17.863" Score="8" ViewCount="1420" Body="&lt;p&gt;In the television show &quot;The Expanse&quot;, we see the Solar System having been partially colonized and developed during the 23rd century. The spacecraft in the show appear to depend on conventional chemical rocket engines (or possibly a much more efficient high-thrust ion drive). Ceres has been colonized as a primary station within the asteroid belt and various spacecraft are depicted as hauling ice from other parts of the Solar System. In particular, we see an &quot;ice hauler&quot; spacecraft in orbit around Saturn which then departs to return to Ceres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although there has not been any explicit timeline shown, it is essentially implied that this type of mission is commonplace and simply a way of life for many of the colonists (known as &quot;belters&quot;). However, considering the expected trajectories for transferring between Ceres and places such as Saturn, I would have thought that it would be unrealistic to expect a crew to commit to this kind of lifestyle -- most importantly the time it takes to go back and forth between those two locations (or others).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Hohmann transfer from Ceres to Saturn would take about 7.64 years (one-way) at a minimum Delta-V of about 7.57 km/s, but you can reduce that duration immensely at the cost of more fuel by transferring with higher-energy trajectories. Here is a plot of the one-way transfer time as a function of Delta-V:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that a Delta-V above 19 km/s corresponds to an escape trajectory from the Solar System. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/flfw3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/flfw3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here is a look at the family of trajectories -- each rendezvous with Saturn occurs at the same anomaly location, whereas the departure anomaly from Ceres varies. I'm assuming that Saturn and Ceres are in circular orbits (they have eccentricities of about 0.055 and 0.075, respectively), and that they are coplanar (Ceres is at about 7.5 degree inclination compared to Saturn, so that is not insignificant but neglected anyway).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7bQsJ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7bQsJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what we see here is that a mission from Ceres to Saturn and back would take somewhere between about at least 4 years and at most about 15 years. Clearly there are careers on Earth that take people away from their homes and families for a year or two at a time, but the travel time for these missions does not seem like something that would be undertaken as a career choice. Does this suggest that the kind of rough-and-tumble crewed space mining concepts are unrealistic?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question is rather broad, so I'll say that answers should really focus on (a) the elements of propulsion and orbital mechanics that define how long these transfers would take, and (b) what kind of outlook we might expect for manned spaceflight as a career (probably most comparable to tours in the military or jobs at sea).&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2015-12-21T18:47:49.333" LastActivityDate="2017-04-20T10:41:28.257" Title="Is space mining and development as shown in the television show &quot;The Expanse&quot; realistic?" Tags="|orbital-mechanics|future-missions|mining|time|" AnswerCount="3" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="13111" PostTypeId="2" ParentId="13081" CreationDate="2015-12-21T10:05:29.750" Score="2" Body="&lt;p&gt;There is information about heart rates for most of the Apollo landings (I think this data was recorded for five of the six landings, though I don't have a reference right now for which one wasn't recorded.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Armstrong's heart rate was elevated for that first landing, up to 150 bpm - see the &lt;a href=&quot;http://history.nasa.gov/alsj/a11/a11.landing.html&quot; rel=&quot;nofollow&quot;&gt;Apollo Lunar Surface Journal&lt;/a&gt; notes at around 102:46:23, including an interesting comment from Aldrin about how some people &quot;jazz up&quot; the tension.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On that first landing lots of things didn't go to plan - computer error reports, a (misleading) low fuel reading, and a surprisingly rocky landing site among them. All of the astronauts were accustomed to dealing with the extra excitement though...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also there's a graph and other data in an excerpt from the &lt;a href=&quot;http://history.nasa.gov/alsj/a11/a11MIssionReport_1971015566_Sec12BiomedEvaluation.pdf&quot; rel=&quot;nofollow&quot;&gt;Mission Report&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally commanders on the later flights experienced much lower heart rates, they had the benefit of encouragement from the previous successful landings.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2015-12-21T10:05:29.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13112" PostTypeId="1" AcceptedAnswerId="13797" CreationDate="2015-12-21T10:14:57.410" Score="8" ViewCount="1234" Body="&lt;p&gt;I am researching for a student nano-cubesat program, and came across the use of flywheels for attitude control. I looked up some successful projects and found that one university had used 4 reaction wheels in place of 3. Why would that be? Wouldn't using 3 be simpler? Is it just because of the redundancy? If so, isn't this a heavy price to pay in terms of operating complexity for the safety gained? &lt;/p&gt;&#xA;" OwnerUserId="12692" LastActivityDate="2016-02-04T00:13:55.717" Title="Use of 4 reaction wheels in place of 3" Tags="|artificial-satellite|attitude|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13113" PostTypeId="1" CreationDate="2015-12-21T10:20:36.617" Score="3" ViewCount="643" Body="&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/questions/7771/why-does-the-falcon-9-consume-hydraulic-fluid&quot;&gt;discussion over Falcon 9 failure and its hydraulic system&lt;/a&gt; involved many points over difficulties of pumping the liquid, reuse of the fluid as fuel, nomenclature etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I missed was one crucial point: What's the benefit of using gas-pressurized hydraulic fluid instead of making the system pneumatic in the first place? It seems using gas pressurization beats all benefits of hydraulic over pneumatic - the incompressible fluid vs compressible gas - when the fluid can still back up and compress the pressurization gas.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-12-22T21:35:52.853" Title="Why does Falcon 9 use open-system gas-powered hydraulic system instead of pneumatic?" Tags="|spacex|falcon-9|avionics|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13114" PostTypeId="2" ParentId="4764" CreationDate="2015-12-21T10:52:50.243" Score="1" Body="&lt;p&gt;First, it is nothing special with the geostationary orbit regarding gravity, this is just the location where an object seems to hoover in the same spot in the sky. That is dependent on the rotation of the Earth, and has thus no forces attracting it. A normal station keeping budget for a satellite in GEO is typically 50 m/s of delta-v a year. If I interpret your question as how much delta-v it is going to cost us to still hover, even though we are a little bit off, the delta-v cost is 0.5 m/s per year per meter. This is not exactly scalable, but accurate enough within a few hundred kilometres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for L-points, they come in two types. Stable and unstable. In the stable points, like L4 and L5, an object that is a little bit off is still hold in place and therefore &quot;orbiting&quot;. In the unstable points, an error is growing until the object escapes. Imagine this as the difference of balancing marbles on a sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V291cm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V291cm.png&quot; alt=&quot;ball on top and inside a sphere&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because any error grows over time, the unstable points do not have a size, they are just points. But the delta-v cost does not have to be high, and satellites can indeed orbit them for some time, like the &lt;a href=&quot;http://genesismission.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Genesis mission&lt;/a&gt; did. This is necessary if you want to accommodate multiple satellites in the same L-point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the size of the L4 and L5 points, this question has been asked before, see &lt;a href=&quot;https://astronomy.stackexchange.com/questions/2230/maximum-amplitude-of-a-lissajous-orbiting-object-in-a-l4-or-l5-position&quot;&gt;Maximum Amplitude of a Lissajous Orbiting Object in a L4 or L5 Position&lt;/a&gt; at astronomy. A particular amplitude is not stated though, but from the common illustration of L-points, it seems like an object can go almost back to the L3 point in an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pZj5Um.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pZj5Um.png&quot; alt=&quot;l-points&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that for example Venus is at sometimes closer to the SEL4 and SEL5 points than the Earth, thereby causing significant perturbations, necessary to compensate.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2015-12-21T11:08:21.717" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13116" PostTypeId="2" ParentId="13109" CreationDate="2015-12-21T13:22:33.527" Score="2" Body="&lt;p&gt;A major focus of the long stay Mir missions was to monitor the long term effects of weightlessness on the human body. As a part of that, cosmonaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Valeri_Polyakov&quot; rel=&quot;nofollow&quot;&gt;Valeri Polyakov&lt;/a&gt; stayed 437 days in space, following an intensive training program much like the HIIT. He was able to counteract the loss of muscle mass typically experienced on long space flights, but it did not solve the problem of &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2001/ast01oct_1/&quot; rel=&quot;nofollow&quot;&gt;weakening of the skeleton&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the workout time required did still consume a considerable time of the daily routine, even though it was of the high intensity type. The Mir missions showed that it is possible to keep a good health in space, but at the cost of having to follow the training program of a top athlete just to stay fit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A currently ongoing project, the &lt;a href=&quot;https://en.wikipedia.org/wiki/ISS_year_long_mission&quot; rel=&quot;nofollow&quot;&gt;year long mission&lt;/a&gt; by Scott Kelly and Mikhail Korniyenko, will provide more data on the effects of long stays in space. They are however following a slightly less intensive workout program than the Mir missions.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-21T18:11:12.543" LastActivityDate="2015-12-21T18:11:12.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13117" PostTypeId="1" CreationDate="2015-12-21T13:51:38.567" Score="11" ViewCount="870" Body="&lt;p&gt;A possible way of heating up Mars is to use mirrors to direct more sunlight towards the planet. I am not asking about the feasibility or cost of such a gigantic project, but I have a question regarding the placement of the mirror in order to make it stay in place. If the mirror is placed trailing behind Mars, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pressure&quot; rel=&quot;noreferrer&quot;&gt;light pressure&lt;/a&gt; required to compensate for the gravity of Mars is also adding a component force vector accelerating the mirror outward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Oy2eK.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Oy2eK.png&quot; alt=&quot;mirror trailing Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two different locations were this is not a problem, between Mars and the Sun, and directly behind Mars, both relatively useless for directing more light towards Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder, can the mirror orbit in circles outside the shadow cone, thereby adding the missing compensating force by inertia?&#xA;&lt;a href=&quot;https://i.stack.imgur.com/UXeDw.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UXeDw.png&quot; alt=&quot;mirror orbiting shadow cone&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this possible? And if not, are there other ways of placing it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I am not looking for a solution were the mirrors orbit the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To clarify, I want a location similar to L-points, but that also consider radiation pressure as well as inertia, gravity of Mars and gravity of the Sun, while still pointing the reflected light beam at Mars. In the terminology of Forward, this is a special kind of near Mars &lt;a href=&quot;https://en.wikipedia.org/wiki/Statite&quot; rel=&quot;noreferrer&quot;&gt;statite&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2020-01-18T11:11:02.587" LastActivityDate="2020-02-28T23:29:14.933" Title="Balancing a mirror" Tags="|mars|orbital-mechanics|terraforming|three-body|statite|" AnswerCount="4" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="13118" PostTypeId="2" ParentId="13117" CreationDate="2015-12-21T15:16:06.237" Score="5" Body="&lt;p&gt;You have missed another option for mirror placement, orbit them around the Mars.  In much the same way as &lt;a href=&quot;https://en.wikipedia.org/wiki/Ringworld&quot; rel=&quot;noreferrer&quot;&gt;Larry Niven's, Ringworld&lt;/a&gt; is built.  Your circle of mirrors are in a polar orbit of Mars. The mirrors always face the sun and direct the light to the surface of Mars. You might need to do something with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Moons_of_Mars&quot; rel=&quot;noreferrer&quot;&gt;moons of Mars&lt;/a&gt; to prevent issues between the moons and your mirrors, but the upside is that they might be good material sources.  &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2015-12-21T15:16:06.237" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13120" PostTypeId="2" ParentId="10820" CreationDate="2015-12-21T17:14:21.603" Score="2" Body="&lt;p&gt;It's a tug of war between three accelerations: Central body gravity, orbiting body gravity, and centrifugal acceleration (inertia in a rotating frame).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Centrifugal acceleration is $\omega^2*r$ where $\omega$ is how fast the system's spinning and r is distance from barycenter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ci8hi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ci8hi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of L1, centrifugal force and orbiting body gravity are on the same team. These two exactly cancel central body gravity. In this case all accelerations are parallel or anti-parallel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At L4 and L5 none of these forces are parallel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wtm4d.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wtm4d.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Centrifugal pushes away from barycenter. Central body's gravity pulls towards center of central body. Orbiting body's gravity pulls towards center of orbiting body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way to add vector forces is to put the foot of one vector on the head of the other vector. The sum is a vector from from the first vector's foot to the 2nd vector's head.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nPWL5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nPWL5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding the two gravities we get a vector that exactly cancels centrifugal acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we shrink the orbiting body's mass, barycenter moves closer to center of larger body. Central body's gravity becomes more dominant and the orbiting body gravity vector gets smaller. But the two gravities still sum to cancel the $\omega^2r$ which points from barycenter to L4.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2015-12-21T17:21:47.280" LastActivityDate="2015-12-21T17:21:47.280" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13121" PostTypeId="5" CreationDate="2015-12-21T18:47:42.260" Score="0" Body="&lt;p&gt;Originally invented by Robert Forward, a statite is now a part of aero-space terminology. A typical usage of a statite is providing communication for the poles of a planet.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-21T20:42:15.370" LastActivityDate="2015-12-21T20:42:15.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13122" PostTypeId="4" CreationDate="2015-12-21T18:47:42.260" Score="0" Body="Questions about artificial objects hovering by the pressure of light." OwnerUserId="8693" LastEditorUserId="58" LastEditDate="2019-04-23T16:09:01.063" LastActivityDate="2019-04-23T16:09:01.063" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13123" PostTypeId="1" AcceptedAnswerId="13124" CreationDate="2015-12-21T19:22:42.337" Score="1" ViewCount="683" Body="&lt;p&gt;Computing a delta-V budget from one heliocentric orbit to another can be done as a Hohmann transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does one calculate delta-V for transferring from a heliocentric orbit to be captured by another planet?  Is it as simple as computing the Hohmann delta-V and subtracting the velocity of the orbit around the destination planet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question is inspired by comments in &lt;a href=&quot;https://space.stackexchange.com/questions/13110/is-space-mining-and-development-as-shown-in-the-television-show-the-expanse-re&quot;&gt;Is space mining and development as shown in the television show “The Expanse” realistic?&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus: Is the reverse process symmetrical? (I.e. same delta-V)&lt;/p&gt;&#xA;" OwnerUserId="11479" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-12-22T08:44:53.357" Title="Transfer to Capture Orbit Delta-V" Tags="|orbital-mechanics|hohmann-transfer|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13124" PostTypeId="2" ParentId="13123" CreationDate="2015-12-21T19:37:32.460" Score="2" Body="&lt;p&gt;Your capture delta-v is calculated by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\sqrt{v_e^2+v_{inf}^2}-v$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $v_e$ is the escape velocity at your target periapsis, and $v_{inf}$ is the &lt;a href=&quot;http://hopsblog-hop.blogspot.no/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow&quot;&gt;velocity at infinity.&lt;/a&gt;&#xA;For any given target orbit, replace the $-v$ after the square root with the velocity of your orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A minimal capture is when your periapsis is as low as possible, and the apoapsis just within the sphere of influence. If you do not know the SOI, using $v_e$ for $v$ gives a good aproximation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a symmetrical process. You can also be captured by passing through the atmosphere of the planet, if it exist. In that case, it is not a reversible process.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-22T08:44:53.357" LastActivityDate="2015-12-22T08:44:53.357" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13125" PostTypeId="1" CreationDate="2015-12-21T20:59:39.297" Score="2" ViewCount="430" Body="&lt;p&gt;The LGM-30 Minuteman is the only land based ICBM in the US strategic arsenal. Gradually these missiles will have to be retired. My question is, can they be used to launch commercial satellites into LEO? What is the highest orbit they can achieve? &lt;/p&gt;&#xA;" OwnerUserId="12356" LastActivityDate="2015-12-21T21:11:26.047" Title="Can Minuteman LGM-30 booster be used to launch commercial satellites?" Tags="|low-earth-orbit|icbm|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13126" PostTypeId="2" ParentId="13125" CreationDate="2015-12-21T21:11:26.047" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;http://gutenberg.us/articles/lgm-30_minuteman#Satellite_launching_role&quot; rel=&quot;nofollow&quot;&gt;this page&lt;/a&gt; at the Gutenberg Self-Publishing Press, the LGM-30 Minuteman III has been considered for launching satellites. It also states that Orbital Sciences currently already modifies the Minuteman series &#xA;for use in the Minotaur launch vehicle (although it seems this is the Minuteman II).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like most of the necessary specifications are provided &lt;a href=&quot;http://www.astronautix.com/lvs/mineman3.htm&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, so I will try and simulate a launch to get an idea of what kind of payloads could be lifted to LEO.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-12-21T21:11:26.047" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13127" PostTypeId="1" CreationDate="2015-12-21T21:47:32.487" Score="15" ViewCount="6331" Body="&lt;p&gt;One of the changes made to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9_v1.1#Falcon_9_v1.1_Full_Thrust&quot;&gt;Falcon 9 v1.1 Full Thrust&lt;/a&gt; was using supercooled liquid oxygen at 66 K (-340 F) as well as chilled RP-1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen various claims that this is a first in the industry. Clearly liquid oxygen has been chilled this low in a lab before, but is using this level of supercooled LOX a first for a real launch carrying a payload?&lt;/p&gt;&#xA;" OwnerUserId="6772" LastEditorUserId="12102" LastEditDate="2021-09-13T00:21:04.327" LastActivityDate="2021-09-13T00:21:04.327" Title="Is the supercooled LOX used in Falcon 9 Full Thrust a first?" Tags="|spacex|falcon-9|fuel|spaceflight-firsts|sub-cooled|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13128" PostTypeId="1" AcceptedAnswerId="13129" CreationDate="2015-12-21T22:42:25.767" Score="3" ViewCount="973" Body="&lt;p&gt;For a one-way robotic or cargo mission, with current technology, how much of a lander that touches down on the Moon wouldn't be propellant or rocket parts? Modern engines like the &lt;a href=&quot;http://www.rocket.com/common-extensible-cryogenic-engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;CECE&lt;/a&gt; by Aerojet Rocketdyne could be used, or any other that could realistically be ready for a mission in the next few years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After asking &lt;a href=&quot;https://space.stackexchange.com/q/12859/4660&quot;&gt;how much a Saturn V today could soft-land&lt;/a&gt; for such a mission, eventually I realized I'm still unclear how much of that is rocket. &lt;a href=&quot;http://www.spudislunarresources.com/Bibliography/p/102.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This paper&lt;/a&gt; by Spudis and Lavoie says this about their lunar cargo lander parameters: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The lander has a dry mass of 8300 kg, a propellant mass of 22000 kg&#xA;  and a payload capacity of 12000 kg.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I had penciled in a much higher payload fraction than that. One CECE would be enough to land the mass a Saturn V equivalent could launch, and masses only 250 kg. I looked up that stages for chemical rockets are roughly 8% inert mass and went with a 4 ton lander with 21 tons of payload for that Saturn V lander. Am I way off?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-12-22T20:10:33.147" Title="What proportion of a space craft soft-landing on the Moon would be payload?" Tags="|rockets|the-moon|lander|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13129" PostTypeId="2" ParentId="13128" CreationDate="2015-12-21T23:29:16.267" Score="2" Body="&lt;p&gt;The Apollo LM fueled ascent stage, which could be considered payload, was about 30% of the total fueled mass of the LM. However it departed from lunar orbit, so you'd first have to insert into orbit, which takes another stage. That's about 900 m/s. For storable propellants you can assume an Isp of 325 s. Let's say you can make a 15% dry mass stage. (The Apollo service module was 25%, but it did a lot of other things.) Then overall you would get 21% of your TLI mass to the surface as payload. 25% would be an optimistic number, perhaps getting rid of some things on the descent stage. However you need landing gear and a platform to support the payload, or a skycrane with a cable and gear on the payload. Whatever it is, you need more mass than just a stage to land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you could keep cryogens from boiling off for long enough, you could do better on the Isp.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your numbers are about 28%. In the same ballpark.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://www.hq.nasa.gov/alsj/LM04_Lunar_Module_ppLV1-17.pdf&quot; rel=&quot;nofollow&quot;&gt;Lunar Module Quick Reference Data&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-12-22T20:10:33.147" LastActivityDate="2015-12-22T20:10:33.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13130" PostTypeId="2" ParentId="13128" CreationDate="2015-12-21T23:31:43.747" Score="2" Body="&lt;p&gt;Let's consider Chang-e 3 as an example, where we'll assume that the lander is our payload and our spacecraft is designed to start in lunar orbit (at 100 km altitude) with enough fuel to make the descent. According to &lt;a href=&quot;http://spaceflight101.com/change/change-3/&quot; rel=&quot;nofollow&quot;&gt;this Spaceflight 101 reference&lt;/a&gt;, the specifications are as follows:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dry mass of lander: 1200 kg&lt;/li&gt;&#xA;&lt;li&gt;Rover: 120 kg&lt;/li&gt;&#xA;&lt;li&gt;Wet mass of entire probe: 3780 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That wet mass includes the lander, rover, and all the fuel required to transfer from the final launch vehicle trajectory, insert into lunar orbit, and perform the landing. If we consider that to be our spacecraft design, the payload fraction is obviously 3.2%. However, let's do a bit of math and assume that another stage has done the work of inserting us into a lunar orbit -- so how much fuel do we need for descent and landing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To do this, we'll simplify the problem as being composed of a burn to de-orbit and descend (with apolune at the initial altitude and perilune at the surface), followed by a burn to bring the speed down to null.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In orbit at 100 km (at a radius of 1837.4 km), the speed of the spacecraft is 1.63 km/s from (where $\mu_{Moon} = 4904.9 \frac{km^3}{s^2}$):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{orbit} = \sqrt{\frac{\mu_{Moon}}{r_{Orbit}}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our descent trajectory is an elliptical orbit with semi-major axis $a = 1787.4$ km from:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$a = \frac{1}{2}\left( r_{Orbit} + R_{Moon} \right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From that we can compute our speed at the start and end of descent, which gives us $v_{start} = 1.61$ km/s and $v_{end} = 1.70$ km/s:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{start} = \sqrt{\mu_{Moon} \left(\frac{2}{r_{Orbit}} - \frac{1}{a}\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{end} = \sqrt{\mu_{Moon} \left(\frac{2}{R_{Moon}} - \frac{1}{a}\right)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we'll need an initial $\Delta v$ of about 0.02 km/s to insert into our descent and then a landing $\Delta v$ of about 1.70 km/s to stop the vehicle. That gives a total of 1.72 km/s, and then assuming a specific impulse of $I_{SP} = 300$ s, we can compute our mass fraction. Note we'll assume the final mass is the lander (1200 kg), the rover (120 kg), and some fuel left over that totals 1400 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\frac{m_{initial}}{m_{final}} = e^{\frac{\Delta v}{g_0 I_{SP}}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting those values, we get a mass fraction of about 1.8, and that yields an initial mass of 2520 kg. So for that spacecraft you would have a payload fraction of about 4.8%.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-12-21T23:31:43.747" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13132" PostTypeId="1" AcceptedAnswerId="13137" CreationDate="2015-12-22T01:52:12.770" Score="18" ViewCount="1356" Body="&lt;p&gt;As I was watching the SpaceX launch today on YouTube, I have noticed a bright distant light as the satellites were being deployed. What was that light?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DNLFg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DNLFg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, after each deployment there was something white and small flying away from the main body. What was that?&lt;/p&gt;&#xA;" OwnerUserId="5968" LastEditorUserId="12102" LastEditDate="2021-08-07T00:12:31.260" LastActivityDate="2021-08-07T00:12:31.260" Title="What was the distant bright light in the SpaceX webcast of Orbcomm OG2 deployment?" Tags="|spacex|low-earth-orbit|camera|payload-deployment|orbcomm-2|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13133" PostTypeId="1" CreationDate="2015-12-22T01:58:54.410" Score="10" ViewCount="1770" Body="&lt;p&gt;As of a short time ago, SpaceX made history as the first entity *ever* to successfully re-land the initial stage of an orbital rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Up until this point, the dominating factor in the cost of Rocket Launches has been that you have to build a new Rocket for every launch (about \$50 Million for a Falcon 9 launch, I've seen estimates of \$10 - \$20 Million purely to manufacture the rocket), whereas a typical launch burns just $200,000 of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, in theory, Falcon 9 can continually re-use its initial rocket stage without having to build a new one each time, how much might this reduce the cost of a Rocket Launch by?&lt;/p&gt;&#xA;" OwnerUserId="12698" LastEditorUserId="12698" LastEditDate="2015-12-22T05:49:03.657" LastActivityDate="2015-12-22T14:56:33.957" Title="Roughly what percentage (by cost) of a Rocket Launch does stage 1 of the Falcon 9 Rocket represent?" Tags="|rockets|spacex|landing|cost|cost-to-orbit|" AnswerCount="1" CommentCount="3" ClosedDate="2015-12-23T06:42:12.907" ContentLicense="CC BY-SA 3.0" />
  <row Id="13134" PostTypeId="1" AcceptedAnswerId="13140" CreationDate="2015-12-22T02:10:57.320" Score="74" ViewCount="11895" Body="&lt;p&gt;So, SpaceX has finally landed their booster back to the land. &#xA;BlueOrigin has achieved the same thing in a recent past, but I have read so many people commenting and criticizing the comparison of BlueOrigin and SpaceX landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, Jeff Bezos has just published the following &lt;a href=&quot;https://twitter.com/JeffBezos/status/679116636310360067&quot;&gt;tweet&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Congrats @SpaceX on landing Falcon's suborbital booster stage. Welcome&#xA;  to the club!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;but people are commenting on his tweet that your landing was nothing as compared to SpaceX's landing and it was easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone please explain the difference between the two landings in layman's term ?&lt;/p&gt;&#xA;" OwnerUserId="12699" LastEditorUserId="25" LastEditDate="2015-12-22T15:58:32.940" LastActivityDate="2020-12-22T03:18:03.757" Title="Difference between BlueOrigin and SpaceX rocket landings?" Tags="|rockets|spacex|landing|falcon-9|blue-origin|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13136" PostTypeId="2" ParentId="13134" CreationDate="2015-12-22T02:27:38.027" Score="33" Body="&lt;p&gt;Blue Origin's flight was straight-up, straight-down, with a fairly small rocket that can't carry much useful payload. It's a great demonstration of technology, but it's only practical for space tourism.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today's Falcon flight was a paid orbital payload mission. In order to get the payload to orbit, the first stage has to not only bring the second stage to high altitude, but start it going horizontally at fairly high speed; for the return-to-launch-site landing, it then has to cancel all that speed and make its way back, and all this with a much bigger rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX did low altitude VTVL flights starting as far back as 2012; they could have matched what Blue Origin did within the last year or so if that had been the goal, but their preferred strategy is to take their big technological steps as part of paid payload missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of this is intended to belittle Blue Origin's accomplishment. The two companies just have different goals, and very different ways of approaching them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Also, 1982 called to tell Blue Origin and SpaceX they both need to quit fronting.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-23T15:25:16.853" LastActivityDate="2015-12-23T15:25:16.853" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13137" PostTypeId="2" ParentId="13132" CreationDate="2015-12-22T02:31:04.183" Score="18" Body="&lt;p&gt;It was the Moon. It was illuminated at roughly 85-86% as seen from low Earth's orbit at the time of deployment, so pretty bright, and roughly directly overhead of the Americas. Deployment camera was pointing more or less backwards with respect to upper stage's velocity vector and the upper stage was tracking roughly over the Atlantic ocean West of Europe 17 minutes into the launch. Earth's, Moon's and Sun's geometry, made with &lt;a href=&quot;http://www.solarsystemscope.com/&quot; rel=&quot;noreferrer&quot;&gt;Solar System Scope&lt;/a&gt; orrery for exact time and in realistic size mode centered on Earth, looked like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/kDB3u.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that how the Moon appears to track relative to the camera's frame also depends on upper stage's own attitude during deployment, not just orbit's geometry. Its orientation relative to the Earth would slightly change in between each deployment stage. To appreciate this easier, here's the video at roughly the time that the image in your question displays:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/O5bTbVbe4e4?start=2270&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short of having an officially made ground track so we can see precisely over which area Orbcomm OG2 satellites were deployed, here's a simulated ground track, made with &lt;a href=&quot;http://www.flightclub.io/&quot; rel=&quot;noreferrer&quot;&gt;Flight Club simulator&lt;/a&gt; that should be fairly close to the real deal:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/kdZCq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Small things flying away from the main body are mostly transient debris from the upper stage and payloads themselves, things like droplets of hydraulic fluid or actuators pressurization gas (depending on design) for the stage engine's gimbals, lubrication oils of the deployment system to prevent cold welding (stiction) of similar metallic surfaces or friction of the spring release mechanism, residual fuel from reaction control system thrusters firing up, ice condensed on cryogenic surfaces now exposed to vacuum chipping off as its surface starts outgassing, and similar volatile fluids that would &quot;bake-off&quot; and disperse in the heat of sunlight as soon as their orbit enters daylight (at that altitude, one orbit is approximately &lt;a href=&quot;https://goo.gl/CFmDB9&quot; rel=&quot;noreferrer&quot;&gt;97 minutes&lt;/a&gt; long). And some of it would be camera noise and you're seeing ionizing radiation striking its sensor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But short of an unexpected event, there wouldn't be many, if any at all long-lasting orbital debris released by the deployment mechanism or the upper stage itself once they've all reached orbital speed, since a lot of effort is made to reduce the number and long lasting impact of orbital debris. But if there will be any, we'll see soon enough either via &lt;a href=&quot;http://satellitedebris.net/Database/index.php&quot; rel=&quot;noreferrer&quot;&gt;NORAD Orbital Debris Catalog&lt;/a&gt; and/or &lt;a href=&quot;http://www.unoosa.org/oosa/osoindex/search-ng.jspx&quot; rel=&quot;noreferrer&quot;&gt;UNOOSA Index of Objects Launched into Outer Space&lt;/a&gt;, which is a registrar that all member state parties to the Outer Space Treaty report to.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-22T14:23:31.620" LastActivityDate="2015-12-22T14:23:31.620" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13139" PostTypeId="1" AcceptedAnswerId="13159" CreationDate="2015-12-22T03:36:26.147" Score="21" ViewCount="9888" Body="&lt;p&gt;SpaceX's original plan was landing the 1st stage to a landing ship, that waits far away from the launch site, and it was like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/nrPia.jpg&quot; alt=&quot;initial plan&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But tonight, the rocket's first stage landed at Cape Canaveral. Looking at this original plan slowing down and coming back to launch site seems not possible. So how did it made all the way back? Was it a different maneuver than landing on a ship.&lt;/p&gt;&#xA;" OwnerUserId="688" LastEditorUserId="688" LastEditDate="2018-01-12T22:12:26.630" LastActivityDate="2018-01-12T22:12:26.630" Title="How did the Falcon 9's 1st stage return back to Cape Canaveral?" Tags="|spacex|falcon-9|reuse|first-stage|booster-flyback|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13140" PostTypeId="2" ParentId="13134" CreationDate="2015-12-22T03:58:44.563" Score="89" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8SGVV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8SGVV.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;You see that tiny thing on the far left?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;That's the Falcon 1. It's a comparable size to Blue Origin's New Shepard spacecraft and SpaceX's Grasshopper (which accomplished a similar feat 6 times, around 3 years ago, but didn't *technically* enter space).&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;You see those 3 in the middle?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;That's what SpaceX landed today. Grasshopper / Blue Origin were comparatively short, slow, experimental proof-of-concept flights. The Falcon 9 launch was a fully-fledged orbital rocket, delivering a real payload on an actual mission, operating at a level of difficulty an order of magnitude greater than the previous achievements of either company.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;That's the difference&lt;/strong&gt;. *Technically*, both landings count as &amp;quot;Launched a rocket into space and successfully re-landed&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;But the first two were, essentially, &amp;quot;Fly straight up and come straight back down at sub-orbital speeds&amp;quot;. Basically a scaled-up water-rocket. Specifically, New Shepard accelerates straight up to 40km, at 100,000 pounds of thrust, then coasts to 100.5km and comes straight down again.&lt;/p&gt;&#xA;&lt;p&gt;The Falcon 9 has to handle &lt;em&gt;1.5 million&lt;/em&gt; pounds of thrust, reaches a maximum altitude of &lt;em&gt;300 km&lt;/em&gt;, &lt;em&gt;takes a payload into space&lt;/em&gt;, at the time of detachment is travelling mostly &lt;em&gt;horizontal&lt;/em&gt; to the earth's surface, at &lt;em&gt;twice&lt;/em&gt; the velocity (and 10x the mass so &lt;strong&gt;40x&lt;/strong&gt; &lt;em&gt;the kinetic energy&lt;/em&gt;), and has to then &lt;strong&gt;turn around!&lt;/strong&gt;, &lt;strong&gt;re-enter the atmosphere&lt;/strong&gt;, and land back at the launch site.&lt;/p&gt;&#xA;&lt;p&gt;In terms of the difficulty involved and the commensurate level of achievement, today's landing is in a whole different league to anything that's come before.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;In really simplified terms:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Grasshopper / New Shepard were &amp;quot;throwing a &lt;em&gt;tennis&lt;/em&gt; ball high in the air and catching it&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;Falcon 9 was &amp;quot;Shoot a &lt;em&gt;cannonball&lt;/em&gt; at supersonic speed, sideways, with topspin, so that it curves through the air, bounces off that building over in the distance, comes back at you at 3x the speed and still lands exactly where you threw it from.&amp;quot;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Addendum:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;It should also be noted that New Shepard is small/nimble enough that it can throttle down its engine and hover above a landing site, allowing for a controlled landing and/or aborting a landing attempt and re-trying if it's not right. It can control its descent velocity at will, coming down slowly and gently to the point where you could &lt;em&gt;almost&lt;/em&gt; do it by hand-held remote.&lt;/p&gt;&#xA;&lt;p&gt;Because the falcon 9 needs much larger engines (because it's &lt;em&gt;far&lt;/em&gt; heavier at launch), and has lost almost all its fuel mass by this point, it cannot hover. This means it has to land by a method called a &lt;strong&gt;suicide burn&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;A suicide burn means that it fires its engines at a &lt;strong&gt;very&lt;/strong&gt; precise altitude, such that it decelerates and slows to a stop at the &lt;strong&gt;exact&lt;/strong&gt; point where it touches down on the landing pad.&lt;/p&gt;&#xA;&lt;p&gt;If the burn starts a tenth of a second too late, the rocket will hit the launch pad at several hundred m/s and explode. A tenth of a second too early, and it will level out a few hundred meters above the launch pad, run out of fuel and fall out of the sky.&lt;/p&gt;&#xA;&lt;p&gt;It is &lt;strong&gt;easily&lt;/strong&gt; an order of magnitude harder to do, and you don't get any re-takes.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;In short, though still a historic achievement in its' own right, landing New Shepard is &lt;strong&gt;nothing&lt;/strong&gt; compared to landing Falcon 9.&lt;/p&gt;&#xA;" OwnerUserId="12698" LastEditorUserId="12698" LastEditDate="2020-12-22T03:18:03.757" LastActivityDate="2020-12-22T03:18:03.757" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="13141" PostTypeId="2" ParentId="13139" CreationDate="2015-12-22T04:11:13.383" Score="16" Body="&lt;p&gt;I think this is pretty self-explanatory:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9C4JJ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9C4JJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Thanks to @TildalWave for finding an updated graphic)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's important to note that the landing pad was a different site to the launch pad (6 miles away), so it didn't return *exactly* to its' original point of take-off, as can be seen here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D3qfh.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D3qfh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12698" LastEditorUserId="12698" LastEditDate="2015-12-22T15:10:28.700" LastActivityDate="2015-12-22T15:10:28.700" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13142" PostTypeId="2" ParentId="13133" CreationDate="2015-12-22T04:17:30.250" Score="3" Body="&lt;p&gt;It depends really, on a number of factors that we simply don't know right now. From &lt;a href=&quot;https://space.stackexchange.com/a/8461/25&quot;&gt;this answer&lt;/a&gt;, we know that the propellant is only about 0.3% of the cost of the launch. Furthermore, the first stage is about 3/4th of the cost of the rocket. So, what will the cost be of a re-used rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, first of all, the lifetime is currently estimated at around 10 launches per rocket. The cost of the booster is around 75% of the cost. Fuel is negligible. A certain amount of cost is fixed in the mission planning aspects, which I suspect costs around $1 million (Quite possibly less, but let's just go with that). All in all, I imagine the cost of a re-used rocket would be about 35% of a non-reused rocket, give or take. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bigger than the propellant cost, especially for the first attempts, will be the re-certification cost. A significant part of any space mission is this aspect. I would give this a 15% overall mission cost for the first few attempts, and less of a cost for all future missions.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2015-12-22T14:56:33.957" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13144" PostTypeId="2" ParentId="13127" CreationDate="2015-12-22T09:31:10.990" Score="5" Body="&lt;p&gt;I have not been able to find a reliable source for this, but according to &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/000539.html&quot;&gt;this&lt;/a&gt; forum post, the Atlas ICBM used lox at 78 K in the first part of the launch, referred  to as &quot;supercooled&quot;. For comparison, the &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/launch/LOX-LH2-storage.html&quot;&gt;shuttle&lt;/a&gt; used lox at 90 K.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-22T09:39:19.757" LastActivityDate="2015-12-22T09:39:19.757" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13145" PostTypeId="1" CreationDate="2015-12-22T09:51:15.827" Score="3" ViewCount="189" Body="&lt;p&gt;Hydrogen, for rocket fuel, tends to leak even through metal containers. Launchers carry hydrogen cooled to liquid in order to save storage space for aerodynamical reasons, but that is unnecessary in free space. Hydrogen is the most efficient rocket fuel and needs to be stored in space during long travel times until for example a landing burn, or in the future maybe after having been extracted from airless celestial bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would hydrogen gas in inflated huge balloons leak less than liquified hydrogen in pressure tanks?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2022-10-25T18:28:00.423" Title="Would a low pressure container lower hydrogen leakage in space?" Tags="|fuel|hydrogen|fuel-depots|expandable|storage|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13146" PostTypeId="1" CreationDate="2015-12-22T10:00:58.970" Score="1" ViewCount="114" Body="&lt;p&gt;It is a (maybe marginal) problem for long duration space flights that hydrogen fuel leaks through its containing tank. I suppose it is because a hydrogen atom some of the time loses its electron and a free proton is so small that it is hard to stop with a physical barrier. But &lt;em&gt;if the container had a positive electric charge&lt;/em&gt;, it should repel protons. One might lose electrons and get ionized hydrogen, but electrons have almost no mass and it should not be too difficult to recombine ions to atoms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how does this work, I have no clue, as you who do obviously realize by reading this ;)&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-12-22T13:36:37.047" LastActivityDate="2015-12-22T13:38:21.973" Title="Would electrically charged hydrogen tanks lower leakage?" Tags="|hydrogen|storage|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13147" PostTypeId="1" CreationDate="2015-12-22T10:49:53.123" Score="2" ViewCount="111" Body="&lt;p&gt;In a tweet &lt;a href=&quot;https://twitter.com/elonmusk/status/678679083782377472&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://twitter.com/elonmusk/status/678679083782377472&lt;/a&gt; Mr Musk states that they are using Monte Carlo analysis to determine mission success.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My Question(s):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;how does that work exactly?&lt;/li&gt;&#xA;&lt;li&gt;what parameters flow into this simulation?&lt;/li&gt;&#xA;&lt;li&gt;is this standard in the space industry?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And now the really important part:&#xA;I'm in software engineering and I'd like to know if and how this is applicable to software engineering (project success, failure rates, etc) too? Where do i start if I would like to do this?&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="25" LastEditDate="2017-12-19T13:14:58.960" LastActivityDate="2017-12-19T13:14:58.960" Title="How does SpaceX determine mission success" Tags="|mission-design|failure|" AnswerCount="0" CommentCount="5" ClosedDate="2015-12-22T15:01:19.143" ContentLicense="CC BY-SA 3.0" />
  <row Id="13149" PostTypeId="2" ParentId="8060" CreationDate="2015-12-22T12:15:50.380" Score="21" Body="&lt;p&gt;The top-level answer, as others said, is &quot;because it was decided so&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But following that, we have actual design decisions that simply make SLS completely impractical to recover. It focuses on getting the big payload to a high orbit, and through ignoring the necessity for reusability, it follows design principles that make recovery completely impractical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRBs of SLS could be reused, but there's little benefit in that - they are cheap, simple devices and once the fuel has burned there's really little left to recover - a big, hard shell and some avionics. It never saved much money in case of the Shuttle, and the recovery and refurbishing process was costly and complex enough that it was deemed it's not worth the headaches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's the core stage. A huge, orange tank of cryofuels and four advanced engines. It would be worth to recover these engines, but it's not possible. The tank is actually quite soft, mostly made of foam with a thin aluminum lining. It carries a lot of fuel, meaning a lot of delta-V. That means it reaches a very high speed and very high altitude. And then it needs to lose that speed and altitude if it's to be recovered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is quite opposite to Falcon 9's recoverable first stage, which barely reaches barely 1.5km/s (out of needed 8) and only shortly dips above the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 can brake and descend at reasonable, survivable speed, without burning up and without spending excessive amounts of fuel on braking, not to burn up. The second, non-recoverable stage carries most of the responsibility for reaching the orbital speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTOH, SLS core stage pretty much reaches the orbit. It would take enormous amount of energy to slow it down - or some extremely complex (and probably very heavy) technical solutions to make it survive the descent and use air to brake. It's not worth the trade-off. It would either need to be even bigger, or it would need to sacrifice a lot of payload weight to survive reentry from that speed. F9's first stage doesn't speed up a lot, so it can be slowed down quite easily. SLS gets so fast, that slowing it down in a controlled way is simply too much effort and cost to bother.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-05-18T02:13:44.663" LastActivityDate="2017-05-18T02:13:44.663" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13150" PostTypeId="2" ParentId="13146" CreationDate="2015-12-22T12:32:37.293" Score="3" Body="&lt;p&gt;Giving the tank a positive charge is in the most extreme case to partially ionize it, but if it is possible to do it in another way, it is going to take a lot of energy anyway. Even with only a little energy usage, a bigger problem is to avoid heating of the hydrogen, and the resulting boil-off. I can not think of a way to charge either the tank nor the hydrogen without producing a significant amount of waste heat.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-22T13:38:21.973" LastActivityDate="2015-12-22T13:38:21.973" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13151" PostTypeId="2" ParentId="13128" CreationDate="2015-12-22T13:08:07.610" Score="1" Body="&lt;p&gt;In addition to the above answers presenting realistic examples, there is one more interesting consideration. Would it be beneficial to re-use the cargo lander? In other words, when is the mass of the propellant needed to return the lander to orbit, and the propellant needed to bring that propellant to the surface of the Moon, smaller than the dry mass of the lander?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We must also consider the ratio of total mass to dry mass (excluding payload), because a lander only capable of landing on the Moon and return is of no use. Most of modern upper stages has a mass ratio between 10 and 15, and because a lander is a bit more complex than an upper stage, I choose a ratio of 10.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plotting this, we see that re-using the lander is better than single use when the ISP is larger than 680s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QgMTY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QgMTY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;X-axis is ISP of lander, y-axis is proportion of mass in lunar orbit delivered to lunar surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Re-using the lander is not economical with chemical rockets, but it gives a slight advantage for a hydrogen NTR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we must also consider the cost of propellant relative to building an entire new lander. For that matter, the number of possible reuses is also important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of cargo in lunar orbit delivered to the surface is for a single use lander&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{\frac{\frac{r}{ℯ^{\frac{\Delta v}{I_{sp}g}}} - 1}{r}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $r$ is the landers ratio of dry mass to total mass, $I_{sp}$ is the specific impulse of the lander and $\Delta v$ is one way delta-v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a re-usable lander, the formula is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{\frac{r- \left(ℯ^{\frac{\Delta v}{I_{sp} g}} \right)^{3}}{r \; ℯ^{\frac{\Delta v}{I_{sp} g}} - ℯ^{\frac{\Delta v}{I_{sp} g}}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting $ℯ^{\frac{\Delta v}{I_{sp} g}}$ with $\rho$, we get&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{\frac{r- \rho^{3}}{\rho \left(r-1 \right)}}$$&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-22T18:56:41.413" LastActivityDate="2015-12-22T18:56:41.413" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13152" PostTypeId="1" CreationDate="2015-12-22T13:18:54.200" Score="26" ViewCount="2775" Body="&lt;p&gt;As most of you, I am super stoked by the amazing success of SpaceX with their Falcon 9 first stage landing back on land!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also know that they have a huge backlog (over 50 flights) and are building an F9 rocket apparently every three weeks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know what are the next milestones regarding reusability of the F9 stages recovered? Will they actually test one and have it fly again? Will they reuse the nine engines (worth millions)? Will they fly Jeff Bezos for free to space just charging him the cost of fuel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's next on the F9R side?&lt;/p&gt;&#xA;" OwnerUserId="8925" LastEditorUserId="51" LastEditDate="2015-12-22T19:53:00.590" LastActivityDate="2016-02-10T20:54:03.920" Title="Now that they can land a Falcon 9, what will they do with it?" Tags="|spacex|landing|reuse|falcon-9|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13153" PostTypeId="1" AcceptedAnswerId="13158" CreationDate="2015-12-22T13:46:05.453" Score="3" ViewCount="278" Body="&lt;p&gt;What kind of software quality assurance techniques are employed in the development of space systems? How are they different from other techniques in aviation, nuclear or other high stake systems?&lt;/p&gt;&#xA;" OwnerUserId="12712" LastActivityDate="2015-12-22T15:18:08.927" Title="software quality in space systems" Tags="|qualification-testing|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13155" PostTypeId="2" ParentId="13152" CreationDate="2015-12-22T13:49:44.007" Score="30" Body="&lt;p&gt;Nobody knows, but it seems &lt;strong&gt;pretty&lt;/strong&gt; safe to assume, for a while, things will continue as normal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX will continue launching rockets and attempting to land them. They will take the one they have (and any more they acquire) and analyse them extensively to determine if they are, in fact, flightworthy again and what effects being used has had on their integrity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, at some point in the future, they will announce that they're ready to test-fly with a re-used Rocket. At which point things get &lt;strong&gt;really&lt;/strong&gt; interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacenews.com/spacex-early-adopter-ses-ready-to-reuse-falcon-9-%C2%AD-for-the-right-price/&quot;&gt;At least 1 company&lt;/a&gt; has already expressed a willingness to fly on a re-used rocket, for the right discount.&lt;/p&gt;&#xA;" OwnerUserId="12698" LastActivityDate="2015-12-22T13:49:44.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13156" PostTypeId="2" ParentId="13152" CreationDate="2015-12-22T14:12:36.073" Score="19" Body="&lt;p&gt;An earlier plan, years ago, when they thought they would have landed much sooner was to take the used first stage to SpacePort America in the Mojave Desert, after inspection, and refly it there several times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But current plans seem to be focused on expediency and they will use it for fit checks and ground support equipment tests at the LC-39A pad they are building. They may even use it for a static fire test at LC-39A as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once there they might refly it first at Spaceport America. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A recent tweet by Musk suggests they might not refly it considering its historic nature. But plans are in flux. Apparently it was mentioned in the post flight conference call as well:&#xA;&lt;a href=&quot;http://www.theverge.com/2015/12/21/10642028/spacex-falcon-9-landing-elon-musk-wont-fly&quot; rel=&quot;nofollow&quot;&gt;http://www.theverge.com/2015/12/21/10642028/spacex-falcon-9-landing-elon-musk-wont-fly&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally they did a static fire test at LC-40 and returned the core to LC-39A for continued fit testing. They claim to have discovered an issue upon refire (which is a historic first, refiring a used orbital booster) that they then delayed SES-9 and CRS-8 to resolve. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-02-10T20:54:03.920" LastActivityDate="2016-02-10T20:54:03.920" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13157" PostTypeId="2" ParentId="13152" CreationDate="2015-12-22T14:42:58.767" Score="6" Body="&lt;p&gt;Well, SpaceX's main goal is to reuse stages in order to lower costs. Now that they've successfully landed the first stage, they'll examine it, refurbish whatever need refurbishing and fly it again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have 7 launches scheduled in the first quarter of 2016 but I don't know how long it will take them to examine the stage, test it and make sure it's OK to fly again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SES has expressed willingness to fly on &quot;recycled&quot; rocket, but their launch is scheduled for January 2016 and that might not be enough time to have the stage ready.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then again, Musk has a bit of a reputation of a slave driver and launches are often delayed, so I won't be surprised if they have the stage ready for it.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2015-12-22T14:50:04.230" LastActivityDate="2015-12-22T14:50:04.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13158" PostTypeId="2" ParentId="13153" CreationDate="2015-12-22T15:08:17.657" Score="4" Body="&lt;p&gt;The normal stuff - code reviews, unit tests, black box tests, load tests etc, etc. The biggest difference is the vigour. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have to go through detailed design review, a code review, there is often a requirement that certain percentage (often 80% or 90%) of your code be covered by automated tests, there are usually performance requirements you have to meet, you have to run static and dynamic code analysis the results of which are also peer reviewed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of that is the factory testing, once the software is delivered, you go through a round of system level tests which are sometimes designed by a third party with third party data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My experience is all in ground software, for flight software, they have an additional step where they load the software to the actual flight hardware, hook it up to hardware simulators and test it.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2015-12-22T15:18:08.927" LastActivityDate="2015-12-22T15:18:08.927" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13159" PostTypeId="2" ParentId="13139" CreationDate="2015-12-22T15:28:11.607" Score="13" Body="&lt;p&gt;The &quot;return to launch site&quot; flight profile flown on December 21, 2015, requires that the first stage re-light three of its engines after separation, turn around, cancel out all its speed, and accelerate back towards the launch site. To do this, it has to keep a lot of fuel in reserve after the separation -- less than you might think, though, because the first stage is quite light once the ascent fuel, second stage, and payload are gone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this launch, there were three significant factors that allowed them to do this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;They &quot;supercooled&quot; the fuel and oxidizer to much lower temperatures than before, which makes them denser, allowing them to carry more fuel in the same volume of tankage on the first stage; &lt;/li&gt;&#xA;&lt;li&gt;They lengthened the second stage, allowing more tankage there, so the second stage could provide more of the necessary speed to get the payload to orbit (meaning the stage separation could occur at a shorter distance from the launch site and at lower speed);&lt;/li&gt;&#xA;&lt;li&gt;The payload was relatively small, consisting of 11 functional and 1 dummy satellite of about 175kg each -- with mounting and ejection hardware and so on, probably less than 4 tons of payload, compared to the 13 ton payload Falcon 9 is normally capable of launching to LEO.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For full-payload missions, the first stage will have to do more work to push towards orbit, so return-to-launch-site may not be possible, so I believe those missions will use the Florida-to-Atlantic-barge or Texas-to-Florida flight profiles, which require much less fuel in reserve after separation.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-22T17:06:07.300" LastActivityDate="2015-12-22T17:06:07.300" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13160" PostTypeId="1" CreationDate="2015-12-22T15:46:43.583" Score="10" ViewCount="725" Body="&lt;p&gt;The attempted landings on the drone ship all failed, while the landing on land was successful. As far as I'm aware, all major changes to the rocket were made to allow it to land on land, so I assume the two rockets are very similar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any significant differences between landing on the drone ship vs landing on land? Is landing on the drone ship somehow harder?&#xA;Would the successful landing on land indicate increased chance of landing successfully on the drone ship?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it know if SpaceX will attempt another drone ship landing?&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="51" LastEditDate="2015-12-22T16:06:23.637" LastActivityDate="2015-12-23T14:14:40.113" Title="SpaceX landing on land vs landing on drone ship" Tags="|rockets|spacex|falcon-9|landing|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13161" PostTypeId="2" ParentId="13134" CreationDate="2015-12-22T15:47:52.320" Score="27" Body="&lt;h1&gt;Blue Origins&lt;/h1&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Flew to just over 100 km (100.5), just enough to say it went to space.&lt;/li&gt;&#xA;&lt;li&gt;Landed at the same site, presumably went straight up and down.&lt;/li&gt;&#xA;&lt;li&gt;Carried a suborbital payload.&lt;/li&gt;&#xA;&lt;li&gt;Announced only after the fact.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h1&gt;SpaceX&lt;/h1&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://youtu.be/O5bTbVbe4e4&quot; rel=&quot;noreferrer&quot;&gt;Separation from 1st stage&lt;/a&gt; at 79.3 km, 5929 km/hr (From webcast video)&lt;/li&gt;&#xA;&lt;li&gt;At that point, the upward speed was about (86.6-75.3)/7 or 1.6 km/ second, as measured from the video. That being very rough, but still under the speed (5800 km/hr), indicates to me that likely most of the speed was in the upward direction, but there was a cross range speed of non-negligible difference.&lt;/li&gt;&#xA;&lt;li&gt;Landing occurred at 9:42. Using Earth's surface gravity, and assuming no acceleration (Both bad assumptions), that means the booster likely topped 400 km, much higher than Blue Origins test.&lt;/li&gt;&#xA;&lt;li&gt;SpaceX broadcast it to the world live&lt;/li&gt;&#xA;&lt;li&gt;The Falcon 9 is much taller than Blue Origins.&lt;/li&gt;&#xA;&lt;li&gt;SpaceX was performing a commercial mission while this was happening.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4QCUa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4QCUa.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Bottom line, Blue Origin was a technology demonstration mission, which while quite cool, and worthy of some attention, SpaceX's is a more difficult, more useful demonstration of the technology. Both companies deserve merits for accomplishing this difficult task!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2015-12-22T18:55:53.413" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13162" PostTypeId="1" AcceptedAnswerId="13168" CreationDate="2015-12-22T15:52:33.833" Score="20" ViewCount="7217" Body="&lt;p&gt;The Lunar Roving Vechicle (LRV), also known as Moon Buggy, was used as a form of transportation on the surface of the Moon during the Apollo program. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tHihs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tHihs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What system did the astronauts use for navigation purposes whilst in the LRV?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume the astronauts lost sight of the Lunar Module (LM) and they had maps with them and could have used an inertial navigation system to produce selenographic coordinates telling them where they are on the surface of the Moon.&lt;/p&gt;&#xA;" OwnerUserId="12710" LastEditorUserId="6944" LastEditDate="2020-10-09T00:12:10.063" LastActivityDate="2020-10-09T00:12:10.063" Title="How did astronauts navigate the Lunar Roving Vehicle (LRV) over the surface of the Moon?" Tags="|the-moon|apollo-program|rovers|navigation|lunar-rover|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13163" PostTypeId="2" ParentId="13160" CreationDate="2015-12-22T16:05:34.790" Score="6" Body="&lt;p&gt;Generally the information available indicates that there is no difference between the landing on a barge or on land in terms of the rocket itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 1.1 Full Thrust (SpaceX continues to deny this is a 1.2 version, the theory behind that is that a change of 1.1 to 1.2 might imply a need to recertify a new version. It seems to assume people are stupid, which may be a correct assumption on this subject.) has all the modifications for landing period. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As they attempted the Grasshopper/F9DevR1 programs, they added and modified hardware to get better and better at landing. Then they tested it on a several flights, where the ASDS was not there, due to weather or other reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They attempted two landings, and ran into two different hardware issues (Lack of hydraulic fluid for the grid fins first attempt, slow reacting valve second attempt) that were resolved in later builds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was just the next test in a long line of tests, and it succeeded.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing on the ASDS has complexities beyond that of LZ-1. The ASDS is a bit smaller, but if you look at the targeting in the second ASDS attempt, they got to the barge no problem. If you look at the video of the OG2 landing attempt, they hit the X marks the spot pretty much spot on.  (I really want to know HOW close, and divide that by 160K to get a percentage of accuracy!) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a concern with the ASDS moving while a landing is under attempt. It has been noted that the ASDS is big enough, that a wave would have to be very wide in order to significantly make the ASDS tilt. Generally the arm chair critics do not consider this aspect to be a real issue. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-12-22T16:05:34.790" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13164" PostTypeId="2" ParentId="8628" CreationDate="2015-12-22T16:07:23.867" Score="5" Body="&lt;p&gt;Nothing is wrong with,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\frac{m_0}{m_1}=e^{\frac{\sqrt{\frac{\mu_S}{r_1}\left(\sqrt{\frac{2r_2}{r_1+r_2}}-1\right)^{2}+\frac{2\mu_1}{a_1}}-\sqrt{\frac{\mu_1}{a_1}}+\sqrt{\frac{\mu_S}{r_2}\left(\sqrt{\frac{2r_1}{r_1+r_2}}-1\right)^{2}+\frac{2\mu_2}{a_2}}-\sqrt{\frac{\mu_2}{a_2}}}{v_e}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;assuming that the orbits of the planets are circular and coplanar, and your parking orbits too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is basically combining the equation for the $\Delta v$ required for the Hohmann transfer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v =\sqrt{\frac{\mu_S}{r_1}\left(\sqrt{\frac{2r_2}{r_1+r_2}}-1\right)^{2}+\frac{2\mu_1}{a_1}}-\sqrt{\frac{\mu_1}{a_1}}+\sqrt{\frac{\mu_S}{r_2}\left(\sqrt{\frac{2r_1}{r_1+r_2}}-1\right)^{2}+\frac{2\mu_2}{a_2}}-\sqrt{\frac{\mu_2}{a_2}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the inverse form of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;rocket equation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{m_0}{m_1}=e^{\frac{\Delta v}{v_e}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some cases, a bi-elliptic transfer might give a better result. For a bi-elliptical transfer of the same type, use:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v=\sqrt{\left(3-2\sqrt{2}\right)\frac{\mu_S}{r_1}+\frac{2\mu_1}{a_1}}-\sqrt{\frac{\mu_1}{a_1}}+\sqrt{\left(3-2\sqrt{2}\right)\frac{\mu_S}{r_2}+\frac{2\mu_2}{a_2}}-\sqrt{\frac{\mu_2}{a_2}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That can be combined with the rocket equation in the same way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\frac{m_0}{m_1}=e^{\frac{\sqrt{\left(3-2\sqrt{2}\right)\frac{\mu_S}{r_1}+\frac{2\mu_1}{a_1}}-\sqrt{\frac{\mu_1}{a_1}}+\sqrt{\left(3-2\sqrt{2}\right)\frac{\mu_S}{r_2}+\frac{2\mu_2}{a_2}}-\sqrt{\frac{\mu_2}{a_2}}}{v_e}}$&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-31T22:29:30.517" LastActivityDate="2016-03-31T22:29:30.517" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13165" PostTypeId="5" CreationDate="2015-12-22T16:39:45.907" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2015-12-22T16:39:45.907" LastActivityDate="2015-12-22T16:39:45.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13166" PostTypeId="4" CreationDate="2015-12-22T16:39:45.907" Score="0" Body="Questions regarding cost and funding. Please note that this questions may be hard to answer, or opinion based." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-22T17:20:05.637" LastActivityDate="2015-12-22T17:20:05.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13168" PostTypeId="2" ParentId="13162" CreationDate="2015-12-22T17:02:41.647" Score="30" Body="&lt;p&gt;The navigation system used a combination of magnetic pickups in the wheels (to count how many turns each one has taken) and a directional gyro. This allowed both turn angles and distances travelled to be calculated on board.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See this section of the &lt;a href=&quot;http://history.nasa.gov/alsj/LRV_OpsNAS8-25145Pt2.pdf&quot;&gt;manual&lt;/a&gt;, section 1.3.7 &quot;Drive Control Electronics&quot; (page 5 onwards.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also page 18 of the above link shows the instrument panel, which included a heading indicator and distance back to the LM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More sections of the manual can be downloaded from the &lt;a href=&quot;http://history.nasa.gov/alsj/lrvhand.html&quot;&gt;Apollo Lunar Surface Journal&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.ninfinger.org/karld/LRV%20Documents/19740003321_1974003321.pdf&quot;&gt;Performance Review&lt;/a&gt; was written, which includes more technical information and traverse plots for all trips made.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2015-12-22T17:15:26.030" LastActivityDate="2015-12-22T17:15:26.030" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13169" PostTypeId="1" CreationDate="2015-12-22T17:25:52.080" Score="5" ViewCount="931" Body="&lt;p&gt;On the space shuttle, engine ignition was delayed for the main engines. Not all of them were started at once, presumably to avoid vibrations. How does that work for the Falcon 9? I imagine that the rather symmetrical layout could cause issues.&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="51" LastEditDate="2015-12-22T17:29:23.483" LastActivityDate="2016-05-30T13:46:45.067" Title="Falcon 9 engine ignition sequence" Tags="|spacex|falcon-9|engine-design|merlin-1d|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13170" PostTypeId="2" ParentId="8330" CreationDate="2015-12-22T17:26:28.603" Score="2" Body="&lt;p&gt;Something I just read on space.com:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The Falcon 9 rocket costs about \$16 million to build ... but the&#xA;  cost of the propellant, which is mostly oxygen and a gas, is only&#xA;  about $200,000,&quot; Musk said. &quot;So that means that the potential cost&#xA;  reduction in the long term is probably in excess of a factor of a&#xA;  hundred.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/31444-spacex-falcon-rocket-landing-epic-photos.html&quot; rel=&quot;nofollow&quot;&gt;http://www.space.com/31444-spacex-falcon-rocket-landing-epic-photos.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The quote is attributed to Elon Musk, but there must be a mistake, I find it highly doubtful it only costs $16 million...&lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2015-12-22T17:26:28.603" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13171" PostTypeId="2" ParentId="13169" CreationDate="2015-12-22T17:31:16.087" Score="4" Body="&lt;p&gt;Some launchers stagger engine starts for a variety of reasons. As you noted, the Shuttle started the SSMEs one at a time, then the SRBs together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Delta 4 Heavy starts one side booster then the middle the the last one, in order to burn off some of the extra hydrogen from chill down. The extra hydrogen has been setting the insulation on fire, leading to awesome shots of the booster on fire as it soars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX does hold-down starts, so they run the engines up for 1-2 seconds before releasing, and they seem to start the engines all pretty close to each other; what with 9 starting, any particular sequence would be tricky to get right every time.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="1419" LastEditDate="2016-05-30T13:46:45.067" LastActivityDate="2016-05-30T13:46:45.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13172" PostTypeId="1" AcceptedAnswerId="13174" CreationDate="2015-12-22T17:36:53.150" Score="18" ViewCount="12663" Body="&lt;p&gt;The big news yesterday and today is that SpaceX had a successful launch, payload delivery, and landing of the Falcon 9 stage 1 rocket. My question is, where is the second stage of Falcon 9 now, and where will it ultimately end up? I read on Wikipedia that SpaceX would like to be able to land all stages of the spacecraft, but I'm having a hard time discovering what happened (or what will happen) to stage two of Falcon 9's flight 20? Is it supposed to land vertically as well, or parachute into the ocean at some point (or did it already)? Is it a big deal either way what happens to stage two after successful payload delivery?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note to flaggers, this is not a dupe of &lt;a href=&quot;https://space.stackexchange.com/questions/7814/what-happens-to-the-falcon-9-second-stage-after-payload-separation&quot;&gt;this question,&lt;/a&gt; as I am asking what actually happened, not for all the different possibilities. &lt;/p&gt;&#xA;" OwnerUserId="4645" LastEditorUserId="25" LastEditDate="2017-12-19T13:27:18.853" LastActivityDate="2017-12-19T13:27:18.853" Title="Where is the other half (stage two) of the SpaceX Falcon 9 rocket?" Tags="|spacex|falcon-9|upper-stage|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13173" PostTypeId="2" ParentId="13172" CreationDate="2015-12-22T17:39:34.360" Score="15" Body="&lt;p&gt;The payload penalty to recover the second stage turned out to be more than they initially expected, and as such, SpaceX has given up on second stage recovery for the Falcon 9 booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is, the first stage is only 160K up, Mach 5 or so which is very fast and high, but the second stage is basically orbital, which is much higher (maybe) and faster. Which means it needs to return all the way from orbit, which is much trickier.  Need a heat shield, and more fuel/oxidizer to get back to the launch site (or really anywhere under control). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has hinted that their next major booster (BFR/MCT not Falcon Heavy) will have second stage recovery, since that is meant to be their Mars booster, and they intend to recover the entire vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a really fun discussion of the math of second stage recovery on &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3xzdyy/2nd_stage_reusability_math/&quot; rel=&quot;nofollow&quot;&gt;Reddit&lt;/a&gt; that is worth reading. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-08-04T13:34:12.053" LastActivityDate="2016-08-04T13:34:12.053" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13174" PostTypeId="2" ParentId="13172" CreationDate="2015-12-22T17:44:54.783" Score="25" Body="&lt;p&gt;The second stage performed an engine relight test and re-entered over the Indian Ocean about an hour after launch. Below is the reentry region defined by &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3xojmp/i_received_this_notice_today_at_work/&quot; rel=&quot;noreferrer&quot;&gt;a NOTAM (Notice to Airmen)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Blnu5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Blnu5.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacenews.com/falcon-9s-second-stage-restart-was-just-as-important-as-sticking-the-landing/&quot; rel=&quot;noreferrer&quot;&gt;Relighting the second stage is an important step&lt;/a&gt; in launching satellites to geostationary orbit, so their successful test is promising.&lt;/p&gt;&#xA;" OwnerUserId="881" LastActivityDate="2015-12-22T17:44:54.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13175" PostTypeId="1" CreationDate="2015-12-22T18:08:54.883" Score="10" ViewCount="1745" Body="&lt;p&gt;If you look at the daylight photos of the landed first stage of the Orbcomm G2 Falcon 9 mission, there is all sorts of soot covering stuff in interesting patterns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xlOOW.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xlOOW.jpg&quot; alt=&quot;Night Time image&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ggz8K.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ggz8K.jpg&quot; alt=&quot;Day time photo&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What elements of the soot pattern can you explain?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-01T02:42:31.807" Title="Why the strange bands of soot of the landed F9 first stage?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13176" PostTypeId="2" ParentId="13175" CreationDate="2015-12-22T18:23:24.020" Score="9" Body="&lt;p&gt;At the bottom you can see the outline of the legs, which protected the white paint since they extend only in the last 10 seconds of flight or so.  (Oddly late, considering how we thought they would have an aero role in landing)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LOX tank was cold being Liquid Oxygen, but then even colder, so ice likely formed on the white areas, and the soot that was picked up on landing (Flying into your own plume will do that to ya) was picked up on the ice, which fell off or melted protecting the paint job.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tom Mueller, main designer of the Merlin engines tweeted that with Methane, the soot won't stick to either tank, due to the cold.&#xA;&lt;a href=&quot;https://twitter.com/lrocket/status/681556076731351040&quot; rel=&quot;noreferrer&quot;&gt;Tweet by Mueller&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lines near the top of the band are said to be the oxygen vent areas which would of course have been either cold, and left ice behind or else blew off the soot.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-01-01T02:42:31.807" LastActivityDate="2016-01-01T02:42:31.807" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13178" PostTypeId="1" AcceptedAnswerId="13183" CreationDate="2015-12-22T19:47:23.680" Score="8" ViewCount="1169" Body="&lt;p&gt;On December 21st, 2015 on the Orbcomm OG2 mission, Falcon 9's first stage landed 6 miles from its launchpad. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was the highest altitude reached by the first stage, and how far did it go from the launchpad?&lt;/p&gt;&#xA;" OwnerUserId="12736" LastEditorUserId="12102" LastEditDate="2021-08-07T00:31:16.277" LastActivityDate="2021-08-07T00:31:16.277" Title="How far from launchpad did Falcon 9's first stage go?" Tags="|spacex|falcon-9|landing|reuse|orbcomm-2|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13179" PostTypeId="1" AcceptedAnswerId="13180" CreationDate="2015-12-22T20:00:02.757" Score="10" ViewCount="1431" Body="&lt;p&gt;How big was the acceleration during the different phases after the stage separation? Would it be safe to land a human being that way? (at least taking only the g-forces into account, as I suppose there might be too much heat anyway by design).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can think of 4 different phases:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;boostback burn&lt;/li&gt;&#xA;&lt;li&gt;re-entry aerobraking (maybe this one is not significant enough on its own if it &quot;happens&quot; at the same time as the burn)&lt;/li&gt;&#xA;&lt;li&gt;re-entry burn&lt;/li&gt;&#xA;&lt;li&gt;hover-slam&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="49" LastEditDate="2015-12-22T20:15:27.503" LastActivityDate="2015-12-23T15:21:42.367" Title="How rough was the redirecting, re-entry and landing of the first stage of the Falcon 9?" Tags="|spacex|falcon-9|landing|first-stage|booster-flyback|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13180" PostTypeId="2" ParentId="13179" CreationDate="2015-12-22T20:18:45.413" Score="9" Body="&lt;p&gt;The whole process is probably very human-survivable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The very end of the hover-slam is about 2g acceleration (dry mass of stage ~23 tons, single Merlin 1D rated at 723kN, throttled down to 70%, so 723 x 0.7 / 23 = 22m/s&lt;sup&gt;2&lt;/sup&gt; assuming it lands bone-dry). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The boost-back is done on three engines but with some fuel remaining in the tanks. Our user Hobbes computes that given the speed at separation and the duration of the boostback burn that it exceeds 4.6g acceleration; it's potentially as much as 8g. That would make it the roughest part, but comparable to the ascent forces of Mercury or Gemini manned flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerodynamic braking force will be much less than it would for an orbital reentry (because of the lower speed); at a guess it would contribute less than 1g of force to the experience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would also guess that the jolt at touchdown is substantially less than that of an Apollo-style space capsule splashdown, which could be pretty rough, particularly if the capsule hits a rising wave. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-23T15:21:42.367" LastActivityDate="2015-12-23T15:21:42.367" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13181" PostTypeId="2" ParentId="12256" CreationDate="2015-12-22T21:12:09.223" Score="0" Body="&lt;p&gt;This doesn't cover all cases, but you can load a planetarium app like Stellarium, go do a date/time with a full moon at the right position in the sky, and what the sky looks like from the Earth would be the starfield as seen from the dark side of the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, going to the opposite side of the Earth at that same time will show you what the celestial sphere looks like from the light side, minus the Earth in the sky. &lt;/p&gt;&#xA;" OwnerUserId="10846" LastActivityDate="2015-12-22T21:12:09.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13182" PostTypeId="2" ParentId="13113" CreationDate="2015-12-22T21:35:52.853" Score="5" Body="&lt;p&gt;The hydraulic fluid used to steer the fins is actually rocket fuel. After performing the function of steering the fins, the fuel drains into the main tanks to be burned by the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This setup saves the weight of a hydraulic pump and dedicated fluid. Since the fins are useless at low airspeeds, i.e. at final landing, nitrogen (cold gas) thrusters are used instead at that stage. At this point, all the remaining hydraulic fluid can be dumped into the fuel tank, so the fuel budget is in fact constant no matter how much fin control was needed and no additional weight for a dedicated hydraulic fluid was needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Arguably, the fins could have been electrically controlled. However, that would require batteries and electric motors, which are more finicky than hydraulic motors.&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2015-12-22T21:35:52.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13183" PostTypeId="2" ParentId="13178" CreationDate="2015-12-22T22:22:03.810" Score="9" Body="&lt;p&gt;The first part of this answer is data provided by a reasonably high fidelity simulation of the launch. While not perfect, it is the best data that anyone outside of SpaceX has at this point. &lt;a href=&quot;http://www.flightclub.io/results.php?id=45ed6809-0d1c-4ff4-891d-7d405203a691&amp;amp;code=OG22&quot;&gt;More information can be found here.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At its highest point, stage one was just above 180 km. At its farthest point, it was ~95 km downrange. Again, this data isn't perfect, but we can hopefully expect SpaceX to release more information in the coming days and weeks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as velocity goes, at MECO, the vehicle is traveling at around 6000 km/h, or mach 4.87. This data comes from direct vehicle telemetry that was &lt;a href=&quot;https://youtu.be/O5bTbVbe4e4?t=1525&quot;&gt;displayed on the webcast&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12746" LastActivityDate="2015-12-22T22:22:03.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13184" PostTypeId="2" ParentId="13162" CreationDate="2015-12-22T23:53:56.687" Score="15" Body="&lt;p&gt;Well if they did slip their navigation they can follow their tracks back to the LM. They planned for this in case of total breakdown, how much more for nav error.&lt;/p&gt;&#xA;" OwnerUserId="10442" LastActivityDate="2015-12-22T23:53:56.687" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13185" PostTypeId="1" AcceptedAnswerId="13186" CreationDate="2015-12-23T00:26:39.173" Score="9" ViewCount="1498" Body="&lt;p&gt;Thanks to their now recently official reusability capability, and after looking at the SpaceX flight manifest that showcases at least five new v1.1 FT before the Demo, it made me ponder:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the time of the planned Falcon Heavy Demo launch (May 2016), could SpaceX have enough 1st stages to possibly make a refurbished Falcon Heavy (ie: the hardware aspect would be covered)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is possible, it would make sense for SpaceX would go down that road — that launch is paid for from SpaceX's own pocket (explaining why Falcon Heavy was pushed back when in 2014 they saw 1st stage recover was doable on paper).  Also it could solve their client’s initial worries about the quality of their refurbished rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here it could put SpaceX in a position of strength saying &quot;No rebates on refurbishments, we go by cost of launch only like we’ve always done. Expect the price to drop in the future though&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I’m asking to know if the added risk is worth it.&#xA;Also to know if there are any science/practical reasons why it wouldn’t be doable. Examples that comes to mind are: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Refurbishment time frame, knowing SpaceX's track record, is not realistic.&lt;/li&gt;&#xA;&lt;li&gt;the v1.1 FT would require excessive rebuild to accommodate 'Heavy' stuff (ex: cross-feed capability).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6852" LastEditorUserId="6537" LastEditDate="2017-12-19T09:11:37.417" LastActivityDate="2018-02-08T06:07:38.093" Title="Is it possible the Falcon Heavy Demo flight could be done with three previously flown boosters?" Tags="|spacex|reuse|falcon-9|falcon-heavy|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13186" PostTypeId="2" ParentId="13185" CreationDate="2015-12-23T02:19:05.987" Score="7" Body="&lt;p&gt;Let's take a look at the &lt;a href=&quot;http://spaceflightnow.com/launch-schedule/&quot; rel=&quot;noreferrer&quot;&gt;upcoming flight manifest&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;January- SES- GTO- Will attempt landing on drone ship&lt;/li&gt;&#xA;&lt;li&gt;January- Jason 3- LEO, likely will attempt landing on barge&lt;/li&gt;&#xA;&lt;li&gt;February- CRS 8- Likely attempt landing on land&lt;/li&gt;&#xA;&lt;li&gt;Early 2016- Eutelsat 117- GTO, Will likely attempt landing on drone ship&lt;/li&gt;&#xA;&lt;li&gt;Early 2016- JCSAT14- GTO, Will likely attempt landing on drone ship&lt;/li&gt;&#xA;&lt;li&gt;1st quarter 2016- Amos 6- GTO, Will likely attempt landing on drone ship&lt;/li&gt;&#xA;&lt;li&gt;March 21- CRS 9, likely landing attempt on land&lt;/li&gt;&#xA;&lt;li&gt;April- Falcon 9 heavy attempt.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;First of all, this is a very optimistic schedule. The fastest SpaceX has recycled a launch was about 3 weeks, if they can keep that pace up, then the 8 listed missions would be complete in 24 weeks, or about 6 months. Okay, but how about with a more realistic schedule? Most of the landing attempts will likely be on the barge, for those missions requiring the higher performance. There will likely be one more land attempt of the early missions, for the CRS mission. Otherwise, barges seems to be where the landing attempts will occur, due to the more stressing mission attempts. Hopefully we'll get a barge landing soon, but so far that hasn't been demonstrated successfully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next step is to see how different a F9 heavy core is from the lower stage of a Falcon 9. Let's take a look at &lt;a href=&quot;http://www.spacex.com/falcon-heavy&quot; rel=&quot;noreferrer&quot;&gt;SpaceX's image of the Falcon 9 Heavy&lt;/a&gt;. The biggest difference that I notice is that there is a framework on the top part of the booster. Also, there is a nosecone that is unique to the heavy configuration. This appears to be bolted together. It could be retrofitted in the factory, but it wouldn't be using the exact same booster, just 90% the same. Looking at the image, it also appears there is several connecting points along the structure, all of which likely need to be purpose built.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0cre0.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0cre0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it could potentially be done, but I'm not confident that will be the approach taken. More likely the F9 and F9 heavy cores will be different, although they are almost identical. It would be cool, but I don't think that will be the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's see if these modifications appear to be present with &lt;a href=&quot;http://www.orbcomm.com/en/networks/satellite/orbcomm-og2&quot; rel=&quot;noreferrer&quot;&gt;the recent launch&lt;/a&gt;. I don't see any obvious modifications that would support a Falcon 9 core. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/62uub.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/62uub.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-12-24T01:50:46.517" LastActivityDate="2015-12-24T01:50:46.517" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13187" PostTypeId="2" ParentId="13152" CreationDate="2015-12-23T03:06:07.123" Score="1" Body="&lt;p&gt;Well, they can launch 11 satellites at a time while maintaining and reusing resources of old rockets first stages. Determining how to make a manned returning rocket of the same style seems like a logical next step, but the time frames on that are not known to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are on the cutting edge of space research,  and have contracts from NASA to fulfill. The things to come will be exciting and unprecedented,  just like last night's launch. The old engines become available for both analysis for wear and tear, as well as becoming part mines, since interchangeable parts is likely somewhere high on the goal list of SpaceX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once they do achieve a successful relaunch the cycle of analysis, part replacement, and determining standards for safe relaunching criteria will begin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no doubt a stage 1 will be reused before end of 2016.&lt;/p&gt;&#xA;" OwnerUserId="12757" LastEditorUserId="1142" LastEditDate="2015-12-23T15:15:41.777" LastActivityDate="2015-12-23T15:15:41.777" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13188" PostTypeId="2" ParentId="13134" CreationDate="2015-12-23T04:14:00.410" Score="49" Body="&lt;p&gt;Here is an image of the two trajectories. (From &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3xzn7s/my_previous_spacex_falcon_9_vs_blue_origin_new/&quot;&gt;Reddit&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vLZQG.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vLZQG.png&quot; alt=&quot;Blue New Sheppard vs Falcon 9&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a nice infographic explaining the differences between the two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oM4uT.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oM4uT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kudos for both images above to &lt;a href=&quot;https://twitter.com/zlsadesign&quot;&gt;Jon Ross&lt;/a&gt; of &lt;a href=&quot;https://zlsa.github.io/&quot;&gt;ZLSA Design&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here is another fun size comparison (source unknown):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Vq22Z.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vq22Z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://youtu.be/2YldQM1Wk_s?t=32s&quot;&gt;video&lt;/a&gt; especially where I cued that link, should also show an aspect of the discussion.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2015-12-26T07:56:05.137" LastActivityDate="2015-12-26T07:56:05.137" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13189" PostTypeId="2" ParentId="13185" CreationDate="2015-12-23T04:17:21.640" Score="12" Body="&lt;p&gt;The side boosters and the center core will be different. Gwynne Shotwell discussed this in an interview, which you can read a report of in this &lt;a href=&quot;http://spacenews.com/spacex-aims-to-debut-new-version-of-falcon-9-this-summer/&quot; rel=&quot;noreferrer&quot;&gt;SpaceNews article&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“Falcon Heavy is two different cores — the inner core and the two side sticks,” Shotwell said. “The new Falcon 9 will basically be a Falcon Heavy side booster. So we’re building [only two different] cores to make sure we don’t have a bunch of configurations around the factory so we can streamline operations and hit a launch cadence of one or two a month from every launch site we have.” - See more at: &lt;a href=&quot;http://spacenews.com/spacex-aims-to-debut-new-version-of-falcon-9-this-summer/#sthash.mhopYFhr.dpuf&quot; rel=&quot;noreferrer&quot;&gt;http://spacenews.com/spacex-aims-to-debut-new-version-of-falcon-9-this-summer/#sthash.mhopYFhr.dpuf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;SpaceX will be building two core types, the center core, and side boosters. The side boosters will be based on the Falcon 9 single stick and the core will be a separate design, but they will make only two core types to keep production simplified.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, revisiting this years later, the two side boosters for the Falcon Heavy demo flight are previously flown boosters, modified to be side boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an image of a side booster wrapped (The cone shaped top is the giveaway). &lt;a href=&quot;https://i.stack.imgur.com/OiPMM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OiPMM.jpg&quot; alt=&quot;Wrapped Side Core in transit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Core B1023, previously flown on the Thaicomm 8 GTO mission. (Thus&#xA;very hard landing on OCISLY) &lt;/li&gt;&#xA;&lt;li&gt;Core B1025, previously flown on CRS-9,&#xA;which landed at LZ-1, a very easy landing. &lt;/li&gt;&#xA;&lt;li&gt;Core B1033 a new build, will be the center core of the mission.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.reddit.com/r/spacex&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Reddit page&lt;/a&gt;, has a couple of &lt;a href=&quot;https://www.reddit.com/r/spacex/wiki/cores&quot; rel=&quot;noreferrer&quot;&gt;good Wiki's&lt;/a&gt; they are maintaining with all this information.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-11-06T19:39:49.420" LastActivityDate="2017-11-06T19:39:49.420" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13191" PostTypeId="1" CreationDate="2015-12-23T10:03:12.197" Score="0" ViewCount="6380" Body="&lt;p&gt;Consider an object placed in space, where there is no gravity. What is the amount of force needed to move the object by one unit length?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does that relate to the mass of the object and the time spent?&lt;/p&gt;&#xA;" OwnerUserId="12113" LastEditorUserId="8693" LastEditDate="2015-12-23T14:10:20.830" LastActivityDate="2015-12-23T14:10:20.830" Title="What is the amount of force needed to move an object in space?" Tags="|microgravity|travel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13192" PostTypeId="2" ParentId="13191" CreationDate="2015-12-23T10:16:06.020" Score="9" Body="&lt;p&gt;That depends on how much time you want to spend. Space is frictionless, you could move a planet by applying a force of 1 N. It would just take a very long time for the planet to move 1 m in response to that force.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because space is frictionless, you can use &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot;&gt;basic physics&lt;/a&gt; to calculate the information you need. $F=ma$, and from $a$ you can derive the object's speed and position over time. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-23T10:16:06.020" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13193" PostTypeId="1" AcceptedAnswerId="13194" CreationDate="2015-12-23T12:17:56.970" Score="17" ViewCount="2843" Body="&lt;p&gt;Falcon 9 Flight 20 has successfully deployed 11 ORBCOMM satellites into orbit. I can easily imagine a rocket deploying 1 or 2 satellites. But &lt;strong&gt;how did Falcon deploy 11 satellites&lt;/strong&gt; in a single flight? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if they orbit at the same altitude I expect they have to be properly spaced between one another. Is the answer specific to Falcon 9, or even to this particular launch? &lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="12102" LastEditDate="2021-08-07T00:16:06.120" LastActivityDate="2021-08-07T00:16:06.120" Title="How did Falcon 9 deploy 11 satellites?" Tags="|spacex|artificial-satellite|falcon-9|payload-deployment|orbcomm-2|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13194" PostTypeId="2" ParentId="13193" CreationDate="2015-12-23T13:22:25.953" Score="19" Body="&lt;p&gt;In the &lt;a href=&quot;http://www.spacex.com/webcast/&quot; rel=&quot;noreferrer&quot;&gt;webcast&lt;/a&gt;, you can see the mechanism that deploys the satellites: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IaBjg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IaBjg.jpg&quot; alt=&quot;Deploying the Orbcomm constellation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellites are attached to the sides of a tower structure. They are probably held in place by clamps. When the clamps release, a spring pushes the satellite away from the rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an earlier launch, a similar tower looked like this:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/QnvEG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QnvEG.jpg&quot; alt=&quot;Orbcomm 2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellites all &lt;a href=&quot;http://personal.ee.surrey.ac.uk/Personal/L.Wood/constellations/orbcomm.html&quot; rel=&quot;noreferrer&quot;&gt;share the same orbit&lt;/a&gt;. After launch, the satellites will use their own engines to achieve the proper spacing in this orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/luNB3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/luNB3.png&quot; alt=&quot;orbits&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-23T13:22:25.953" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13195" PostTypeId="2" ParentId="13160" CreationDate="2015-12-23T14:14:40.113" Score="6" Body="&lt;p&gt;Landing on a barge is more difficult for 2 reasons: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;the size of the target is much smaller.&lt;/li&gt;&#xA;&lt;li&gt;the barge moves in 3 dimensions. Especially the pitching motion (up/down) &lt;a href=&quot;http://science.slashdot.org/comments.pl?sid=8513185&amp;amp;cid=51163659&quot;&gt;gave them trouble&lt;/a&gt;: &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The magic to a smooth landing is to make it so that you reach EXACTLY zero altitude at precisely the same moment when your forward motion puts you at the beginning of the runway, at the same instant that your lateral adjustment, with wind, puts you in the middle of the runway, while at the same instant you have ceased lateral motion against the wind and brought the yaw exactly parallel to the runway, at the same time roll goes to zero, while maintaining proper flare (pitch). In other words, the craft is moving in six dimensions* and you try to hit just the right mark in all six dimensions at precisely the same time. It's awfully tough to hit zero AGL at exactly the right time when the ground is moving up towards you, then down away from you. Too difficult for me to try in real life. SpaceX has had much trouble with this. They had the rocket perfectly vertical, and they were able to reach 0 AGL, but they couldn't do both at the same time - touch down while the vehicle was vertical. It's much easier to do that if zero AGL remains constant, rather than having the ocean move the barge up and down.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Having been on board a 30,000 ton ferry crossing the English Channel: wave motion is definitely a concern even in those ships which weigh approx. 15 times as much as the ASDS barge. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-23T14:14:40.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13196" PostTypeId="2" ParentId="13193" CreationDate="2015-12-23T16:29:16.533" Score="20" Body="&lt;p&gt;A few additional points.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The satellites were deployed 2 at a time, to keep the rocket from tipping while deploying.&lt;/li&gt;&#xA;&lt;li&gt;The satellites were on a spring system, as can be seen. They were held seem to be held on by a few bolts, as can be seen in the below image, of what looks like a vibration test.&lt;/li&gt;&#xA;&lt;li&gt;I suspect they were deployed at various angles to keep them apart. The &lt;a href=&quot;https://youtu.be/O5bTbVbe4e4?t=38m32s&quot; rel=&quot;noreferrer&quot;&gt;video shows fairly clearly (Focus on the moon)&lt;/a&gt; that the rocket rotated after each deployment, likely to send them at different angles&lt;/li&gt;&#xA;&lt;li&gt;There exist different strengths of springs. &lt;/li&gt;&#xA;&lt;li&gt;Each pair of spacecraft goes in opposite directions.&lt;/li&gt;&#xA;&lt;li&gt;After deployment, the spacecraft will use their thrusters to properly space. I suspect this will be done similar to how &lt;a href=&quot;http://www.satnews.com/story.php?number=461344891&quot; rel=&quot;noreferrer&quot;&gt;the previous satellites&lt;/a&gt; were done.&lt;/li&gt;&#xA;&lt;li&gt;It seems the tower is known as a &lt;a href=&quot;http://www.moog.com/news/operating-group-news/2015/moog-has-shipped-the-satellite-dispenser-to-cape-canaveral-air-force-station-to-support-orbcomm-generation-2-satellites/&quot; rel=&quot;noreferrer&quot;&gt;Satellite Dispenser&lt;/a&gt;, and is provided by Moog. Specifically, the tower is build from a number of ESPA rings (Secondary Payload Adapter), and they provided the harness and other wiring as well. &lt;/li&gt;&#xA;&lt;li&gt;Of some note are the size of these satellite. These satellites have &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbcomm_%28satellite%29#Orbcomm-OG2&quot; rel=&quot;noreferrer&quot;&gt;172 kg mass&lt;/a&gt;. Time 12 (For the mass simulator), that's 2064 kg. The mass to LEO for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9#Falcon_9_v1.1&quot; rel=&quot;noreferrer&quot;&gt;Falcon 9 is about 13500 kg&lt;/a&gt;. Even with the support structure, that's 10,000 kg surplus in capacity. Compare the size of the structure on the bottom of the SES launch (Pictured below) to that of the ESPA ring. Note the diameter of the rings is the same.  SES easily fit inside the fairing, and I suspect even with 3 stacks high, the OG2 satellites only are about 1/5th the height of SES inside the fairing.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It's pretty standard fair that the rocket company does analysis to make sure the satellites won't recontact each other for a specified time post-launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sbFCm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sbFCm.jpg&quot; alt=&quot;OG2 during testing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4rs38.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4rs38.jpg&quot; alt=&quot;SES inside fairing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2015-12-24T01:43:33.430" LastActivityDate="2015-12-24T01:43:33.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13197" PostTypeId="2" ParentId="10820" CreationDate="2015-12-23T17:42:30.213" Score="1" Body="&lt;p&gt;Let's start by looking at the equations of motion for R3BP.&lt;br&gt;&#xA;$\ddot x -2\Omega\dot y-\Omega^2x = -\dfrac{\mu_1}{r_1^3}(x+\pi_2r_{12})-\dfrac{-\mu_2}{r_2^3}(x-\pi_1r_{12})$&lt;br&gt;&#xA;$\ddot y+2\Omega\dot x-\Omega^2y = -\dfrac{\mu_1}{r_1^3}y-\dfrac{\mu_2}{r_2^3}y$&lt;br&gt;&#xA;$\ddot z = -\dfrac{\mu_1}{r_1^3}z-\dfrac{\mu_2}{r_2^3}z$&lt;br&gt;&#xA;Where $\pi_1 = \dfrac{m_1}{m_1+m_2}$ and $\pi_2 = 1-\pi_1$ are the mass ratio, $\Omega$ is the angular velocity of reference frame wtih respect to inertial frame; $r_1$ and $r_2$ are distances of third body located at $(x,y,z)$ from $m_1$ and $m_2$ respectively. And the center of coordinate axes lies at the barycenter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your solution for $L_4$ and $L_5$ (trojan points) comes from equilibrium condition thus $\dot x= \dot y = \dot z = 0$ and $\ddot x = \ddot y= \ddot z=0$. Which on solving gives&lt;br&gt;&#xA;$x = \dfrac{r_{12}}{2}-\pi_2 r_{12}; \hspace{10pt} y = \pm\dfrac{\sqrt{3}}{2}r_{12}; \hspace{10pt} z = 0$&lt;br&gt;&#xA;The remaining 3 points solution requires moving one step further by imposing the constrains $y=0$ and $z=0$ and solving the following polynomial for given mas ratio to obtain $\gamma$&lt;br&gt;&#xA;$\dfrac{1-\pi_2}{|\gamma+\pi_2|^3}(\gamma+\pi_2)+\dfrac{\pi_2}{|\gamma+\pi_2-1|^3}(\gamma+\pi_2-1)-\gamma = 0$&lt;br&gt;&#xA;giving 3 unique solutions; where $\gamma = \dfrac{x}{r_{12}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at $L_4$ and $L_5$ the $y$ coordinate the general solution tells you that they lie along the vertex of an equilateral triangle with other two vertices lying at two masses. This is because the function $\pi_2$ is generally $&amp;lt;&amp;lt;1$ so $x$ becomes essentially $r_{12}/2$. Thus they are function of majorly $r_{12}$.&lt;br&gt;&#xA;This unique geometric position enables the force acting on $3^{rd}$ body to pass through the barycenter, which essentially makes it motionless wrt to one of the bodies as seen from other. For better understanding you consider two bodies of comparable masses, then your barycenter position $\rightarrow r_{12}/2$ and your mass ratio $\pi_2 \rightarrow 1/2$, so your $x$ becomes zero. Based on vector diagram it is easier to see that net force will then pass through the barycenter in such case.&#xA;So all points are equilibrium points and none of them have something special attached to them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On effect of Earth gravitation at $L_4$ and $L_5$, using Newton's Law  we can show that on a unit mass at those points  $\dfrac{F_e}{F_s} = 0.000003003 $. This small yet significant force arising due to Earth changes the application of resultant force on unit mass to act on barycenter instead of pointing directly at the focus (the Sun center).&lt;/p&gt;&#xA;" OwnerUserId="10558" LastEditorUserId="10558" LastEditDate="2015-12-23T17:56:57.630" LastActivityDate="2015-12-23T17:56:57.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13198" PostTypeId="1" CreationDate="2015-12-23T19:05:20.433" Score="2" ViewCount="379" Body="&lt;p&gt;Europa has 13½% of Earth's surface gravity and no atmosphere, so an optimal landing on Europa might be similar to a classic propulsive Lunar landing. But with up to an hour communication time lag, very different surface materials and topology, radiation and gravitational environment, insolation, temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the main similarities and differences of a Europa lander compared to Surveyor, Viking, Philae, Huygens and other landers? How would it be designed? Congressman Culberson who has promoted a NASA Europa lander says 17 minutes into &lt;a href=&quot;http://www.planetary.org/multimedia/planetary-radio/show/2015/1221-culberson_nasa_budget.html&quot; rel=&quot;nofollow&quot;&gt;this interview&lt;/a&gt; that it could use both the skycrane of MSL and the &quot;airbags&quot; of the MERs. I'm not sure that would be suitable for an airless world, and especially not in combination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a mothership Jupiter orbiter have to reconnaitre Europa's surface during flybys before releasing its lander, or did Galileo map good landings sites? Does the terrain on Europa allow for a rover?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-12-23T19:50:19.503" LastActivityDate="2015-12-23T20:11:56.337" Title="How would a robotic landing on an icy moon like Europa differ from landings on the Moon, Mars, Venus, Jupiter, Titan, asteroids, comets?" Tags="|lander|jupiter|europa|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13200" PostTypeId="2" ParentId="13198" CreationDate="2015-12-23T19:34:48.507" Score="2" Body="&lt;p&gt;Galileo appears to have done &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2015/0218-mapping-europa.html&quot; rel=&quot;nofollow&quot;&gt;optical mapping mostly at 200 meter resolution&lt;/a&gt;, with some areas covered at lower resolutions and some as fine as 10-20 meter resolution, which doesn't seem ideal for anything beyond choosing a general area to land in. I don't believe there's been any radar/lidar altimeter mapping done, so (as PearsonArtPhoto notes) a combined orbiter/lander mission would be sensible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The surface of Europa appears to have some clay-like minerals on it in addition to ice; tectonic activity causes the moon to produce water vapor geysers, which may yield some snowfall, so the terrain is going to be varied and quite different from either the moon or Mars, so I would guess design of a rover would have to be a little different -- spikes on the wheels? &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2015-12-23T19:34:48.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13201" PostTypeId="2" ParentId="13198" CreationDate="2015-12-23T20:11:56.337" Score="5" Body="&lt;p&gt;Yes, you would need to get higher resolution data than we have in order to pick a landing site, even if only for science reasons. You want to get to the &quot;good stuff&quot;, which would be exposure of material somewhat recently brought up from the ocean. You would also need to pick a site that meets the requirements of the lander, which would be designed to be as forgiving as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A skycrane is a very good way to get a very low landing velocity, which is step one in a successful landing. Once you have a skycrane, airbags are not necessary since they solve the problem of higher landing velocities. However buried in the airbags was another system that might be useful, which is the ability to right the vehicle from any orientation. Step two is deploying whatever scientific investigation is needed on a hostile surface never before seen, which will require a very robust system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current plan is to not have a Europa orbiter, but rather a Jupiter orbiter that does many flybys. This maximizes the science return in the face of the short lifetime of the electronics in the high-radiation environment near Europa. So if there are data relays from the lander to the Jupiter orbiter, they might only be a few passes, and they will need to get a lot of data transferred on each short pass. I suspect that the lander would have both relay comm and a direct-to-Earth link, as our current rovers do on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing and communicating are only part of what needs to be dealt with. The science instruments, what they do, what support they need, what they need to access, how long they need to operate, how they will survive the radiation for that long, how the system will be thermally maintained without vaporizing the terrain around you, all of that will drive the design of the Europa lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We will soon begin to understand Clarke's wisdom ...&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2015-12-23T20:11:56.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13202" PostTypeId="1" CreationDate="2015-12-23T20:32:04.193" Score="10" ViewCount="1365" Body="&lt;p&gt;If you watch the landing footage of the Orbcomm OG2 mission, where the first stage of the F9 1.1 Full Thrust lands, just after it touches down, shuts down the engine, a triple boom is heard.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most videos of people watching it, seem to suggest they thought it exploded, but with no fire/explosion visible they get over that fear quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even Elon Musk &lt;a href=&quot;https://youtu.be/LgNekVW0pps?t=4m45s&quot; rel=&quot;noreferrer&quot;&gt;initially thought the booms were an explosion&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at the various vudeos, it really looks that way, due to distance and speed of sound hitting the timing just right. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this &lt;a href=&quot;https://youtu.be/FdYGu9bTLhQ&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt; the boom arrives before the landing, so still up high, less to possibly reflect off. &#xA;But why a triple boom?  Shuttle had a double boom, one from the leading edge of the wings, the second from the top of the tail.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-12-24T12:39:44.937" LastActivityDate="2015-12-24T15:51:40.393" Title="Why did the returning F9 first stage generate a triple sonic boom?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13203" PostTypeId="1" AcceptedAnswerId="13204" CreationDate="2015-12-23T20:48:26.570" Score="4" ViewCount="605" Body="&lt;p&gt;My Googling skills may be bad, but I have found no map or reference for the Spacex landing pad location. Probably somewhere near Cape Canaveral, but where?&lt;/p&gt;&#xA;" OwnerUserId="8278" LastEditorUserId="51" LastEditDate="2015-12-23T20:59:24.557" LastActivityDate="2015-12-23T21:00:53.737" Title="Where is the landing pad SpaceX used to land a first stage on Dec 21, 2015?" Tags="|spacex|falcon-9|landing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13204" PostTypeId="2" ParentId="13203" CreationDate="2015-12-23T20:52:40.557" Score="4" Body="&lt;p&gt;The new &lt;a href=&quot;https://en.wikipedia.org/wiki/Landing_Zone_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Landing Zone 1&lt;/a&gt;, is the old LC-13, at the south end of Missile Row. Interestingly the initial SpaceX name was Landing Complex 1 (LC-1) which was confusing and just before this landing the sign was renamed to Landing Zone 1, LZ-1, less confusing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has been updated, so that Launch Complex 13 now redirects to Landing Zone 1, which is kind of fun!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;https://www.google.ca/maps/place/Launch+Complex+13/@28.4883142,-80.5624057,7306m/data=!3m1!1e3!4m2!3m1!1s0x88e0a4802dc1919f:0x98f792466f190cfd!6m1!1e1?hl=en&quot; rel=&quot;nofollow noreferrer&quot;&gt;link on Google Maps&lt;/a&gt;, which is not yet updated with imagery showing the LZ-1 configuration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an image of the various launch complexes at Cape Canaveral. You can see down at the bottom end that LC-13 is shown as Inactive in 2003, which is true. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eWNdy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eWNdy.jpg&quot; alt=&quot;Map of Cape Caneveral Launch Complexes&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch was from LC-40, further north, and SpaceX has a lease on LC-39A, for manned launches and Falcon Heavy launches.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2015-12-23T21:00:53.737" LastActivityDate="2015-12-23T21:00:53.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13206" PostTypeId="2" ParentId="13202" CreationDate="2015-12-23T21:07:55.860" Score="5" Body="&lt;p&gt;Current speculation seems to be one boom for the leading edge, one for the hyper-sonic grid fins, and the third for the trailing edge. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-12-23T21:07:55.860" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13207" PostTypeId="2" ParentId="2496" CreationDate="2015-12-23T21:38:19.647" Score="5" Body="&lt;p&gt;Apollo 10 LM ascent module is indeed in solar orbit, most likely initially in a shorter orbital period then the earth's 365 days.Every 15 years or so a close earth approach, this however presents a possibility of changing Snoopy's solar orbit as described by &quot;Astrogator Mike&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://astrogatorsguild.com/?p=240&quot; rel=&quot;nofollow&quot;&gt;Astrogators guild;Snoopy's future orbits&lt;/a&gt;&lt;br&gt;&#xA;A NASA history document shows air to ground between transcript of Apollo 10 crew during the mission&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/ap10fj/as10-day5-pt23.htm&quot; rel=&quot;nofollow&quot;&gt;Snoopy airlock accidently blows during undocking&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Snoopy then fires its engine into solar orbit 23 May 1969.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/ap10fj/video/a10-lm-jettison.mov&quot; rel=&quot;nofollow&quot;&gt;Archived Apollo 10 LM ascent stage jettison&lt;/a&gt;   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Listen in to air to ground as crew prepares to jettison Snoopy into solar orbit&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/ap10fj/audio/a10a-10816XX.mp3&quot; rel=&quot;nofollow&quot;&gt;Apollo 10 LM ascent stage jettisoned into solar orbit&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have found another great resource of possible ephemerides of Apollo 10 LM ascent stage&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://talk.asteroidzoo.org/#/boards/BAZ0000002/discussions/DAZ0000889&quot; rel=&quot;nofollow&quot;&gt;Asteroid Zoo &lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Data sheet on Landed LM&#xA;&lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_18-27_LM_Lunar_Landing.htm&quot; rel=&quot;nofollow&quot;&gt;Landed LM data sheet&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lunar Module impact from orbit data sheet&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_18-29_LM_Lunar_Impact.htm&quot; rel=&quot;nofollow&quot;&gt;LM impacter data sheet&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3297" LastEditorUserId="3297" LastEditDate="2015-12-24T19:06:05.283" LastActivityDate="2015-12-24T19:06:05.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13208" PostTypeId="1" CreationDate="2015-12-24T04:28:19.147" Score="8" ViewCount="799" Body="&lt;p&gt;The videos of the test and live landings appear to show smoke and a few times flames coming from the landing legs. Will this be engineered out in later development or is it something else? Would it become an issue if a long term hover was required, e.g. on an unexpected landing diversion?&lt;/p&gt;&#xA;" OwnerUserId="12779" LastEditorUserId="49" LastEditDate="2016-03-10T20:19:16.420" LastActivityDate="2016-03-10T20:19:16.420" Title="Is it a problem that Falcon 9R landing legs are getting sizzling hot and catching flames during booster landing?" Tags="|spacex|landing|reuse|falcon-9|booster-flyback|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13209" PostTypeId="2" ParentId="13208" CreationDate="2015-12-24T10:14:38.930" Score="13" Body="&lt;p&gt;Landing legs &lt;em&gt;catching fire&lt;/em&gt; as they extend in the final second prior to landing isn't really unexpected, so they would be designed to withstand that. And this being potentially a problem unless &lt;em&gt;engineered around it&lt;/em&gt; has already been made more than apparent during &lt;a href=&quot;https://en.wikipedia.org/wiki/Grasshopper_(rocket)&quot; rel=&quot;noreferrer&quot;&gt;Grasshopper and F9R Dev&lt;/a&gt; test flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adiabatic flame temperature of RP-1 (rocket grade kerosene) and LOX (liquid cryogenic oxygen) that Falcon 9 boosters use can reach anywhere between 3,300 to 3,700 K, depending on chamber pressure and air-to-fuel mixture ratio, but tends to go lower with smaller pressure and likely (unconfirmed, judging by flame color and residual burn) somewhat more fuel rich mixture at deep throttle during F9R booster landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/yjCiY.gif&quot; alt=&quot;Adiabatic flame temperature of LOX/Kerosene at different chamber pressures and mixture ratios&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Adiabatic flame temperature of LOX/Kerosene at different chamber pressures and mixture ratios. Source: &lt;a href=&quot;http://www.braeunig.us/space/comb.htm&quot; rel=&quot;noreferrer&quot;&gt;Braeunig&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the booster &lt;em&gt;flies&lt;/em&gt; (well, it's called a &lt;a href=&quot;https://space.stackexchange.com/questions/10307/what-is-a-suicide-burn&quot;&gt;suicide burn&lt;/a&gt;, there's not much &lt;em&gt;flying&lt;/em&gt; involved since the transition to subsonic speeds and as the booster reaches its terminal velocity) into its own exhaust as it gradually reduces vertical speed to near 0 approaching touchdown, all this heat of the exhaust flame will reflect from the flame's boundary layer upwards and around the supersonic flame by the surrounding airflow, and also reflect off the ground on final approach. So engineers knew exactly what they're dealing with. It can be anticipated and simulated to good precision even before any metal is bent for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So design was made such that landing legs tolerate expansion from temperature difference that they would be exposed to (from freezing temperatures of higher altitude atmosphere and touching the booster body with deep chilled propellants in it, to up to ~ 3,500 K on final approach and touchdown), mostly by moving all their actuators and hinges upwards and away from the heat of the thrusters along the length of the booster, and landing legs are extended only about 10 seconds prior to landing to further reduce such thermal effects. They also need to tolerate such stress only once before they're inspected and serviced again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, like mentioned in the comments, F9R boosters can't hover. Even a single &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29&quot; rel=&quot;noreferrer&quot;&gt;Merlin engine&lt;/a&gt; at deep throttle down is too powerful and produces more thrust than needed to support the weight of a returning booster that is by then nearly empty of propellants. Or in field parlance, its &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust-to-weight_ratio&quot; rel=&quot;noreferrer&quot;&gt;thrust-to-weight ratio&lt;/a&gt; is larger than 1. It wouldn't have enough of propellants left for such maneuvers anyway, to maintain altitude while &lt;em&gt;fighting&lt;/em&gt; against the gravity drag. If a &lt;em&gt;diversion&lt;/em&gt; will be required and its largely ballistic trajectory is off the mark, it will land on any of the four contingency landing pads around the main one, or the booster will be destroyed either by ditching it into the ocean or by activating its &lt;a href=&quot;https://en.wikipedia.org/wiki/Range_safety&quot; rel=&quot;noreferrer&quot;&gt;flight termination&lt;/a&gt; system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/ziidg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ziidg.jpg&quot; alt=&quot;Landing Zone 1, former Launch Complex 13 at Cape Canaveral Air Force Station, Florida&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Landing_Zone_1&quot; rel=&quot;noreferrer&quot;&gt;Landing Zone 1&lt;/a&gt;, former Launch Complex 13 at Cape Canaveral Air Force Station, Florida. Source: &lt;a href=&quot;http://www.patrick.af.mil/shared/media/document/AFD-141107-004.pdf&quot; rel=&quot;noreferrer&quot;&gt;Patrick AFB&lt;/a&gt; (PDF)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what you saw during the historic first successful landing of an orbital launch vehicle's booster after it has boosted upper stage and payload towards orbit is pretty much the most thermal stress that landing legs of this specific design will ever be subjected to. Small improvements aren't excluded, and SpaceX has already redesigned them a bit at least once (on flown Falcon 9Rs, more often since Grasshopper), but we were &lt;a href=&quot;https://chat.stackexchange.com/transcript/message/19401777#19401777&quot;&gt;hard-pressed in our chat to find any differences&lt;/a&gt;. &amp;nbsp;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="8162" LastEditDate="2015-12-24T19:25:55.987" LastActivityDate="2015-12-24T19:25:55.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13210" PostTypeId="1" CreationDate="2015-12-24T13:49:56.673" Score="9" ViewCount="381" Body="&lt;p&gt;I was wondering if anyone may have useful links to the mathematics used by the F9R that allowed it to fly back to LZ-1 and land successfully. I'm hoping to find some papers that will give the broadest (i.e. easiest) mathematical approximation, where the F9R is treated as a point-mass (i.e. no need to model the rigid-body dynamics, cold-gas thrusters, grid-fin aerodynamics or nozzle thrust vectoring).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to be able to build a 3D Earth model and use some differential equations to model the F9R's motion, but would also like to be able to model a simplified version of its flyback and landing maneuver to be incorporated into the DEs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any help would be greatly appreciated, thanks!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Having found a webpage listing Lars Blackmore's (SpaceX flyback engineer) publications (&lt;a href=&quot;http://web.mit.edu/larsb/www/&quot; rel=&quot;nofollow&quot;&gt;http://web.mit.edu/larsb/www/&lt;/a&gt;), it seems that he did a lot of work on powered descent guidance for Mars landings. I'm assuming this has been generalized for use in the Falcon 9 flyback maneuver. Unfortunately optimization isn't one of my fortes.&lt;/p&gt;&#xA;" OwnerUserId="10130" LastEditorUserId="1142" LastEditDate="2016-08-09T23:49:25.223" LastActivityDate="2016-08-09T23:49:25.223" Title="Mathematics used for F9R flyback and landing" Tags="|spacex|reuse|falcon-9|mathematics|booster-flyback|" AnswerCount="0" CommentCount="3" ClosedDate="2016-08-10T02:01:55.573" ContentLicense="CC BY-SA 3.0" />
  <row Id="13211" PostTypeId="1" CreationDate="2015-12-24T14:57:41.617" Score="7" ViewCount="702" Body="&lt;p&gt;How long is the first stage of a landing Falcon 9 remain hypersonic, and under control of its grid fins? Specifically the hypersonic component is of interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Future second question will be, based on 6 attempts so far with grid fins, has SpaceX exceeded the record for controlled hypersonic flight yet?)&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-12-18T19:56:49.887" Title="How long does the F9 first stage remain hypersonic under control of its grid fins?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="2" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13212" PostTypeId="1" CreationDate="2015-12-24T15:30:57.217" Score="9" ViewCount="992" Body="&lt;p&gt;The pad at Landing Zone 1 (Former LC-13 at Cape Canaveral) has a big giant SpaceX logo consisting of a stylized X. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceNews reposted this picture of a guy on the X marks the spot, as distributed by SpaceX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rToeC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rToeC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How close did the landed first stage, on Dec 21, 2015 (Orbcomm OG2 mission) come to the X?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is relevant, since the X on the ASDS barges is a smaller space so accuracy in hitting the target matters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Udu0q.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Udu0q.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the NasaSpaceflight.com forums, there is a running Barge Landing Bingo thread, where the have divided the surface of the ASDS into a grid, and allowed people to pick grid coordinates to predict where the first landed stage will touch down. It looks like the LZ-1 landing was sufficiently unexpected that the LZ-1 Landing Bingo did not happen.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-08-28T09:06:22.853" Title="How close did the landed first stage of the Falcon 9 get to the X on the landing pad?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13213" PostTypeId="2" ParentId="13212" CreationDate="2015-12-24T15:45:04.673" Score="14" Body="&lt;p&gt;I don't know how precise an answer you're looking for, but looking at this frame from the &lt;a href=&quot;https://www.youtube.com/watch?v=ZCBE8ocOkAQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;helicopter video&lt;/a&gt;, it looks to me like it came down within one stage diameter of the center of the X:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TWYs2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TWYs2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combined with the accuracy of the two failed barge landings, I assume this is not a fluke, and the stage can land this accurately on a regular basis.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;As far as I know, the Jason-3 mission barge landing is the first time SpaceX has announced their landing accuracy, in this case within an impressive 1.3 meters (~4 feet) of dead center of the target. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here's a picture looking down the first stage of the Thaicom-8 mission after landing. If the landing leg span is 18 meters, the rocket looks to be around 3 meters off center. This was a &quot;hot and fast&quot; landing from a geosynchronous launch mission.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jGQn8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jGQn8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Musk claimed an &lt;a href=&quot;https://twitter.com/elonmusk/status/900951975671373824&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.7m off-target distance&lt;/a&gt; for the FORMOSAT-5 flight.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-25T16:33:52.937" LastActivityDate="2017-08-25T16:33:52.937" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13214" PostTypeId="2" ParentId="13202" CreationDate="2015-12-24T15:46:05.703" Score="0" Body="&lt;p&gt;If I had to guess, the first boom is the from the tip of the flame from the rocket engine, the second boom is the bottom of the rocket and the third is the wake shockwave. Also, I would like to add, this Shockwave is probably unique, in that is a normal shockwave generated at Mach 1. Normally, we don't hear normal shockwaves because they are not aimed at the ground, but in the direction the aircraft is moving.&lt;/p&gt;&#xA;" OwnerUserId="12784" LastEditorUserId="12784" LastEditDate="2015-12-24T15:51:40.393" LastActivityDate="2015-12-24T15:51:40.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13217" PostTypeId="1" AcceptedAnswerId="13218" CreationDate="2015-12-24T16:30:29.017" Score="5" ViewCount="578" Body="&lt;p&gt;I'm trying to watch the Orbcomm-2 satellites evolve into a constellation. They were recently placed in orbit by SpaceX (their video &lt;a href=&quot;https://www.youtube.com/watch?v=O5bTbVbe4e4&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;) and the details are discussed in this excellent  &lt;a href=&quot;https://space.stackexchange.com/q/13132/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/13137/12102&quot;&gt;answer&lt;/a&gt; pair. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a way to get latitude, longitude, and altitude by clicking on each satellite listed &lt;a href=&quot;http://www.satview.org/lista_sat.php?cat=tle-new&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (Norad 41179U through 41189U). The process was awkward - I made a series of screen shots (twenty two of them - argh!) like this, two for each satellite, and interpolated to a single point in time (14:00:00 UTC 24-DEC-2015)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iHpTy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iHpTy.png&quot; alt=&quot;screen shot of satview.org&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Python I converted to X, Y, Z in space, &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;for sat in sats:&#xA;&#xA;    r = r_earth + sat.alt&#xA;&#xA;    x = r * np.cos(rads*sat.lat) * np.cos(rads*sat.lon)&#xA;    y = r * np.cos(rads*sat.lat) * np.sin(rads*sat.lon)&#xA;    z = r * np.sin(rads*sat.lat)&#xA;&#xA;    sat.r, sat.x, sat.y, sat.z = r, x, y, z&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;and got this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ui4zF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ui4zF.png&quot; alt=&quot;X, Y, Z of constellation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those are kilometers, relative to the average of the group - just to get a  rough idea what is happening. The color lines are motion over 5 seconds of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question:&lt;/strong&gt; Is there an easier way to download latitude, longitude, and altitude for all of the satellites in the constellation, possibly as a text file at a single point in time? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: There are many nice programs and packages to plot astronomical and orbital data, but this question is about getting the data now and plotting it myself.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-07T00:13:56.873" LastActivityDate="2021-08-07T00:13:56.873" Title="A nicer way to download the positions of the Orbcomm-2 satellites" Tags="|spacex|launch|low-earth-orbit|tracking|orbcomm-2|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13218" PostTypeId="2" ParentId="13217" CreationDate="2015-12-24T17:15:48.140" Score="5" Body="&lt;p&gt;There are a few ways&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Use a tool like &lt;a href=&quot;http://www.agi.com/products/stk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satellite Tool Kit&lt;/a&gt;, which will allow you to plot the spots with time.&lt;/li&gt;&#xA;&lt;li&gt;Get the &lt;a href=&quot;https://celestrak.org/columns/v04n03/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Two Line Elements (TLEs)&lt;/a&gt;, and use them to find the positions directly. I believe &lt;a href=&quot;https://celestrak.org/NORAD/elements/orbcomm.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt; will have them sometime soon. I think they are updated daily. There's a nice packaged called &lt;a href=&quot;https://programmingforresearch.wordpress.com/2012/10/30/using-pyephem-to-get-the-ground-coordinates-of-a-satellite/&quot; rel=&quot;nofollow noreferrer&quot;&gt;PyEphem&lt;/a&gt; that helps with such things. &lt;a href=&quot;https://www.space-track.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Track&lt;/a&gt; also has the data, with a free account, although that account has to be verified. It's run by the US military, I don't know what their criteria is for accounts, but it's the best data source if you can get it.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2015-12-25T01:38:59.607" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13221" PostTypeId="2" ParentId="8107" CreationDate="2015-12-24T23:53:04.117" Score="4" Body="&lt;p&gt;It's a precarious balance between heating and heat dissipation - it seems that heat dissipation - heat loss - is significantly lower in thin atmosphere, than in much denser atmosphere. Meanwhile, heating from air resistance grows much sooner.&lt;/p&gt;&#xA;&lt;p&gt;If you &amp;quot;plunge&amp;quot; into deep atmosphere, it can remove produced heat very fast. How fast? Well, the ballpark figure is &lt;span class=&quot;math-container&quot;&gt;$0.2$&lt;/span&gt; gigawatt during the communicational blackout. This is the energy that is being dissipated as the glowing plasma surrounding the plunging capsule.&lt;/p&gt;&#xA;&lt;p&gt;Now, if you want to descend slowly, you'll need to mostly depend on radiative cooling. And that takes time. How much time?&lt;/p&gt;&#xA;&lt;p&gt;Let's be generous, and let our craft dissipate 20 kilowatt of energy during the descent. And let's make it pretty light, a mere 5 tons.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$E_k = \frac{1}{2} \cdot 5000\ \text{kg} \cdot (7800 \frac{\text{m}}{\text{s}})^2 = 152\ \text{GJ} = 1.52\cdot10^{11}\ \text{J}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;This is what we have to dissipate at &lt;span class=&quot;math-container&quot;&gt;$20\ \text{kW}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\frac{1.5\cdot 10^{11}\ \text{J}}{t} = 20000\ \text{W}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$t= \frac{1.5 \cdot 10^{11}}{20000\phantom{0}}\ [\text{s}]$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$t = 7.5 \cdot 10^6\ \text{s}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;That comes up as almost &lt;strong&gt;quarter of a year&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Of course over time convective cooling will increase so the heat dissipation will grow and rate of descent could be increased, so the figure wouldn't be &lt;em&gt;that&lt;/em&gt; bad, but we're still looking at &lt;strong&gt;weeks&lt;/strong&gt; of descent time if we don't &amp;quot;dunk&amp;quot; the craft into atmosphere dense enough to remove the heat as fast as it's created.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="15906" LastEditDate="2022-06-07T11:10:33.377" LastActivityDate="2022-06-07T11:10:33.377" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="13222" PostTypeId="1" AcceptedAnswerId="13227" CreationDate="2015-12-25T01:54:59.520" Score="8" ViewCount="341" Body="&lt;p&gt;As I was thinking about the process of stage separation, I realized the empty stage dropped, often many tons, is usually separated quite gently, moving at sluggish couple of m/s or less away from the upper stage. The immediate thought was &quot;What a waste! Even ejected at moderate speed, so much mass could provide a lot of delta-V!&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then I began thinking on how this could be done from engineering point of view and besides some totally impractical ideas, came up blank. But c'mon, there are people smarter than me and surely I'm not the first who thought of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - were there any projects (realized or not) to gain extra boost from the process of separation - by ejecting the dropped stage as a reaction mass, with a considerable force?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-06-30T00:25:08.553" Title="Using lower stage as reaction mass" Tags="|propulsion|stages|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13225" PostTypeId="1" AcceptedAnswerId="13236" CreationDate="2015-12-25T15:21:00.817" Score="12" ViewCount="792" Body="&lt;p&gt;Is there a difference between the effect of different ionizing wavelengths on electronics? Can a &lt;a href=&quot;https://en.wikipedia.org/wiki/Linear_particle_accelerator#Medical_linacs&quot;&gt;medical accelerator&lt;/a&gt; be used for simulation of conditions a cubesat will be exposed to? If not what would be an acceptable solution?&lt;/p&gt;&#xA;" OwnerUserId="12797" LastEditorUserId="4660" LastEditDate="2015-12-25T19:48:12.430" LastActivityDate="2015-12-26T15:24:37.813" Title="How to test the impact of radiation on cubesat's electronics?" Tags="|radiation|cubesat|electronics|cosmic-radiation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13226" PostTypeId="1" AcceptedAnswerId="13239" CreationDate="2015-12-25T15:50:50.120" Score="0" ViewCount="486" Body="&lt;p&gt;I need to determine the torque that should be provided by the RCS thrusters for a slew maneuver of a 3axis-stabilized spacecraft (say 50x50x60cm and 50kg). Too much torque can cause unwanted structural vibrations and too little results in a very high thruster activity. Is there a method or systematic/analytical approach to determine the torque output required by the Reaction Control Subsystem?&lt;/p&gt;&#xA;" OwnerUserId="12386" LastEditorUserId="12386" LastEditDate="2015-12-28T10:19:36.613" LastActivityDate="2015-12-28T10:19:36.613" Title="How does one determine the optimal torque output for a satellite attitude control system?" Tags="|attitude|design|reaction-wheel|engines|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ClosedDate="2015-12-27T10:10:00.417" ContentLicense="CC BY-SA 3.0" />
  <row Id="13227" PostTypeId="2" ParentId="13222" CreationDate="2015-12-25T21:56:58.377" Score="14" Body="&lt;p&gt;The additional speed you can gain from this is too small to be of any use. Here is why:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, the stage and payload is not designed to withstand acceleration larger than few g, e.g. 5g. That means, ejection of the first stage must take place over an extended amount of time. Let's assume a time of 1s - at 5 g this gives an additional 50 m/s to the second stage - not too much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, let's see if at least this is feasible: The empty Falcon9 first stage seems to have a mass of about 30 tons, while the full second stage including payload amounts to about 120 tons, four times more. That is, if we want to accelerate our second stage with 5 g, we need to accelerate the first stage at 20 g - much more than it is built to withstand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, during our second of acceleration the distance between both stages increases to 125 meters - and we need to keep a mechanical connection between both, e.g. with a spring that is able to withstand the immense force needed for acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the same time, all masses involved in this need to weigh less than one ton - that's about the amount of fuel we can save. The conclusion can only be that it is rather impossible to make use of the first stage or any other large part of the vehicle as reaction mass. Ejecting burned fuel on the other hand works very well as we can use comparably tiny amounts at a time and accelerate it to high speed.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2015-12-25T21:56:58.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13228" PostTypeId="2" ParentId="6089" CreationDate="2015-12-25T22:47:12.310" Score="3" Body="&lt;p&gt;As Brian Carcich noted, you will need to learn and use SPICE. There is a bit of a learning curve, but it is well worth it. You can find the latest kernel &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/ROSETTA/kernels/ck/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. The latest &lt;code&gt;CATT_*.BC&lt;/code&gt; file in that directory has the detailed orientation and rotation information for the comet. It looks like a new one is uploaded every three or four days. Look at the &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/ROSETTA/kernels/ck/aareadme.txt&quot; rel=&quot;nofollow&quot;&gt;readme&lt;/a&gt; there for some information on how to use it.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2015-12-26T00:02:56.723" LastActivityDate="2015-12-26T00:02:56.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13229" PostTypeId="1" CreationDate="2015-12-26T03:59:32.983" Score="2" ViewCount="305" Body="&lt;p&gt;I am curious to know how you calculate a slingshot direction change &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In both angle you are able to change per encounter (degrees) &amp;amp; the velocity ($m/s$) When $V_{inf}$ is higher than the planets escape velocity by a significant amount. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the first encounter, the craft is climbing up to the planets orbit from a perfect transfer orbit from a planet closer to the sun (pure retrograde).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The end goal is to be in a pure prograde trajectory after the last encounter with the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to know, if the planet escape velocity is say only half of the Infinite encounter Velocity  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_{inf}= 10 km/s$$&#xA;$$$V_{esc}= 5 km/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Prograde initial angle = $180 \deg$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Prograde final angle = $0 \deg$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many $m/s$ are we allowed to change at a time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many encounters with the same planet would be nesscary to completely change the direction of the Encounter velocity from retrograde to prograde assuming we change direction from Retrograde through Radial out to Prograde?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the last encounter have to be done farther away with an even lower escape velocity at closest approach to avoid going into the radial in direction? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so; the escape velocity on the final encounter would be close to how many $m/s$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initial velocities;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_{Pro}= -10km/s$$&#xA;$$V_{Rad}= 0km/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$RadAngle= 180 degrees$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;velocity after 1st encounter with planet&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$V_{1Pro}= ?$$&#xA;$$V_{1Rad}= ?$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$RadAngle= ?$$&lt;/p&gt;&#xA;" OwnerUserId="12803" LastEditorUserId="8162" LastEditDate="2016-05-02T19:38:09.473" LastActivityDate="2016-05-02T19:38:09.473" Title="Repeated slingshot encounters with the same planet" Tags="|orbital-mechanics|gravity-assist|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13230" PostTypeId="2" ParentId="12823" CreationDate="2015-12-26T04:04:52.683" Score="17" Body="&lt;p&gt;You are correct, it is the atmospheric pressure at sea level that limits the minimum power setting on the ground started Merlin engines. At atmospheric pressure, the exhaust can only be over expanded up to a point before the engine begins producing negative thrust or very strong oscillating thrust loads (from the exhaust separation from the nozzle walls) which can load the engine past the fatigue life of the nozzle wall and thrust frames.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To help you understand where the cross over point is, here is some math for understanding the gas dynamics inside the nozzle. Essentially, the nozzle is an accelerator, turning omnidirectional static pressure into directional velocity. In vacuum, there is nothing pushing against the exhaust, so this expansion can theoretically be taken to a maximum where the static pressure of the exhaust is zero. However, inside the atmosphere, the outside air pushes against the exhaust flow. As long as the static pressure of the exhaust is higher than atmospheric, the flow accelerates, however, once the flow is expanded past atmospheric, the atmospheric pressure begins to slow down the exhaust. This wouldn't matter so much if the flow in the exhaust were uniformly high. However, this doesn't happen because of something called a boundary layer. A boundary layer is a thin layer of fluid along a wall that brings the flow from full exhaust velocity down to zero at the wall. It is here where the problems of exhaust wall separation start, because this flow begins slowing down then reversing after the nozzle flow static pressure drops below atmospheric. This flow reversal then starts lifting the gas flow off of the wall. Now, due to viscosity, this gas flow will feel shear forces, which spin up eddies in the gas layer. These eddies act like scavenging pumps, helping the gas flow claw its way back to the wall. However, because the gas has mass, this clawing will go too far, alternating between hitting the wall and lifting far off the wall each time, causing an oscillation to start. Because the flow velocity to flow viscosity ratio is really high (this is called the Reynolds Number), this oscillation will grow not die out with time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A designer has two choices here to ensure this oscillation stays manageable:&#xA;1) Minimize the nozzle length so that the oscillation doesn't have a chance to grow too big and generate huge side loads.&#xA;2) Shape the nozzle properly to lift the gas flow slightly off the wall to accelerate it by reducing the friction experienced by the flow. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Option 1 is doable up to a point, because the flow will still have to be turned, and that has to be done somewhat gradually to avoid shock wave formation inside the nozzle. The bell nozzle developed by Rocketdyne in the 60's is still the best design for this, but some aerospike nozzle designs have promise to reduce the length further. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Option 2 is where a lot of advancements have been made using unsteady gas dynamics. A class of shapes have been discovered that structure the boundary layer in such a way as to form a series of fluid rollers to behind backward facing steps that lift the flow off the wall and allow it to accelerate, helping delay the point where the flow reversal starts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if you want to avoid the damaged engines you'll have to go through during testing, it's best to avoid this problem altogether. Hence, it's best to operate the engines at or close to full power (I.E. full chamber pressure) than throttle them deeply within the atmosphere. This is probably why the Merlin engine has a smaller power range in atmosphere than out because SpaceX didn't want to spend the time and money on working out the nozzle geometry through static engines tests.    &lt;/p&gt;&#xA;" OwnerUserId="11377" LastEditorUserId="11377" LastEditDate="2015-12-31T06:02:07.010" LastActivityDate="2015-12-31T06:02:07.010" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13232" PostTypeId="2" ParentId="7997" CreationDate="2015-12-26T05:53:07.747" Score="5" Body="&lt;p&gt;The wheels were designed with minimum weight to reduce stress on landing gear deployment motors and on total payload by 10-20 kilos; they are 20&quot; in diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wheel material was a bad choice: aluminium has a fast fatigue fracture potential compared to other metals. It doesn't elastically rebound into shape, but instead it memorizes stress and microscopic fractures at stress points accumulate rapidly over time. The lifetime of a good steel frame bicycle is 10 times longer than that of an aluminium one. So aluminium is not a good choice for wheels. Spirit and Opportunity had 10&quot; aluminium wheels which were obviously more rigid and less burdened.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The aluminium between the ridges is 0.75&amp;nbsp;mm; the contractors who made them must have known the wheels were not suitable for long distance rocky terrain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The small stones on Mars are called &quot;ventifacts&quot; and are sharp like walking on smashed roof tiles. The design team had not expected to have such sharp ground and the rover was designed entirely for sand and pavings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here on earth no one would ever use 0.75&amp;nbsp;mm aluminium for a volcano/desert rover to do 50&amp;nbsp;km (2.25&amp;nbsp;mm equivalent for earth gravity). We use cable reinforced rubber or &lt;a href=&quot;https://en.wikipedia.org/wiki/Kevlar&quot; rel=&quot;noreferrer&quot;&gt;Kevlar&lt;/a&gt; foam, and aluminium is just about &lt;strong&gt;the least suitable&lt;/strong&gt; wheel material for a cross country rover, because it memorizes stress and its lack of springiness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tests have proven total failure at 8&amp;nbsp;km of all rock terrain, and limitless lifetime on sand.&#xA;The &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_(rover)&quot; rel=&quot;noreferrer&quot;&gt;Opportunity&lt;/a&gt;&lt;/em&gt; rover covered 42&amp;nbsp;km in 11 years, so they will still be able to run Curiosity for many years with limited terrain choice, and it should still be a very successful mission with a very long lifespan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The design criteria of the wheels at the time of launch were publicised to be its good traction on sand and the distance measurements using the grooves, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory&quot; rel=&quot;noreferrer&quot;&gt;JPL&lt;/a&gt; written in Morse code in the grooves. The design mindset seems to have misjudged the surface of Mars to be strangely softer and less earth-like than it actually is, and to have not done distance testing on rocky surfaces. That's born out by the fact that the wheels fail after only 8&amp;nbsp;km of rocky terrain. That's how long they take to be split in two all the way around.&lt;/p&gt;&#xA;" OwnerUserId="12806" LastEditorUserId="286" LastEditDate="2016-08-27T21:32:54.450" LastActivityDate="2016-08-27T21:32:54.450" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13234" PostTypeId="1" CreationDate="2015-12-26T09:32:31.133" Score="2" ViewCount="316" Body="&lt;p&gt;&lt;a href=&quot;http://www.dlr.de/pf/Portaldata/6/Resources/lcpm/abstracts/Abstract_Rivkin_A.pdf&quot; rel=&quot;nofollow&quot;&gt;MANTIS&lt;/a&gt; was in 2015 a &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/van-kane/20150727-proposals-to-explore-the-solar-systems-smallest-worlds.html?referrer=https://www.google.se/&quot; rel=&quot;nofollow&quot;&gt;proposed Discovery class&lt;/a&gt; mission which were to fly by seven different asteroids including at least one NEA. I think the spacecraft which holds the record in number of flyby's is Voyager 2 which flew by four planets. Also New Horizons flew by an asteroid, Jupiter and Pluto and is heading towards a KBO. There have been spacecrafts which have done many flyby's of Earth and Venus for gravity assists, but asteroids are in different orbits and do not offer gravity assist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of trajectory or tour would allow for as many as 7 flyby's? Is it easy to pick and choose asteroids of interesting types and families, or would only a few alignments be available each year?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2015-12-26T09:39:17.773" LastActivityDate="2019-09-07T16:17:33.580" Title="How can one spacecraft (MANTIS) fly by multiple (7) asteroids?" Tags="|orbital-mechanics|asteroid|mission-design|flyby|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13235" PostTypeId="2" ParentId="13234" CreationDate="2015-12-26T10:04:33.543" Score="2" Body="&lt;p&gt;By early 2014, we had discovered &lt;a href=&quot;https://www.youtube.com/watch?v=2k2vkLEE4ko&quot; rel=&quot;nofollow noreferrer&quot;&gt;630,000 asteroids&lt;/a&gt; (Youtube video visualizing asteroid discoveries) in the main asteroid belt. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kjpmq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kjpmq.png&quot; alt=&quot;Asteroid belt&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MANTIS team &lt;a href=&quot;http://nesf2015.arc.nasa.gov/sites/default/files/downloads/pdf/61.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;has found&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Many trajectories exist that provide required diversity&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;but I haven't found more information on trajectories (or much of anything beyond the two PDFs linked by Localfluff and me).  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-26T10:04:33.543" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13236" PostTypeId="2" ParentId="13225" CreationDate="2015-12-26T10:42:24.130" Score="9" Body="&lt;p&gt;&lt;strong&gt;TL;DR:&lt;/strong&gt; The medical accelerators are not suitable.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;There are basically two effects of radiation on electronics:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Single event upsets (SEUs)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;RAM Memory cells are usually small capacitors (caps) which are charged or not to represent a &lt;code&gt;1&lt;/code&gt; or &lt;code&gt;0&lt;/code&gt;. If an ionizing particle crosses the dielectric between the cap plates, it forms a channel of ionized, and so conducting, material. A charged cap may discharge through this, and a &lt;code&gt;1&lt;/code&gt; becomes a &lt;code&gt;0&lt;/code&gt;. Due to recombination, the channel becomes an insulator again within a short period. So, this effect is reversible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is counter-intuitive, but making the cap larger can help: The cap holds a larger charge, which will not fully discharge until the channel &quot;closes&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(By the way, this  also applies for field effect transistors, but let's stick with memory)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more effective way is to use more than one cell to store a single bit. For example, you use three cells and compare their contents and let the  majority win. This is way, a single discharged cap doesn't matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, at some level of radiation flux you still get SEUs. Multiple particles hitting the chip at the same time may discharge also a larger cap, or two of the three caps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To test if a piece of electronics is immune to SEUs in space, it is radiated by the maximum flux occurring in space, plus some margin.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Material degeneration&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;While SEUs don't have a permanent influence on electronics, radiation also has a degenerating or aging effect on semiconductors. Semiconductors are made e.g. of pure silicon, doped with traces of other elements to give them unique properties. If radiation smashes silicon nuclei, it creates other elements which behaves differently. For example, sensor elements constructed as diodes may invert their doping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Counter-measures include choice of special material combinations and special doping. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this is a cumulative long-term effect, electronics have to be exposed to a total dose equivalent to the dose expected to be collected in space over their entire life time. As satellites can stay in space for several years but you don't want to run radiation tests that long, you need a high intensity radiation source which will apply the expected dose within days.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Type of radiation&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As already mentioned in the comment, you have to distinguish between EM (X-Ray, Gamma-Ray) and particle radiation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;People think gamma radiation is the most dangerous one because you need really thick shielding and one man now gets green when upset. But that's not the case. Gamma rays don't react that much with matter they fly through, so you need lots of matter to let most of it react with it. So, the effect of gamma rays on electronics is quite small, and since gamma rays can't destroy nuclei, it will only cause SEUs from time to time, but no degradation. X-Rays reaction with matter is more likely and so causes more SEUs, but also doesn't degenerate matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electrons deposit their energy by ionizing atoms of the matter they are traveling through. But they also don't destroy nuclei, and the density of ionized atoms left along their track is rather low. So, again, no long term effect, just SEUs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Protons, alpha particles (helium nuclei) and nuclei of heavier elements have a much severe effect on matter, as they deposit large amounts of energy due to ionization in short distances, and they can destroy nuclei inside the electronics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, radiation in space consists of about 87% protons, 12% alpha particles and 1% heavier ions and electrons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A large amount of this radiation is in an energy range similar to that of radioactive elements, which is usually easily shielded by the hull of the satellite. But there's lots of high energy radiation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OQBLo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OQBLo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;em&gt;By Sven Lafebre (own work, after Swordy.) GFDL, CC-BY-SA-3.0 or CC BY-SA 2.5-2.0-1.0, via &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Cosmic_ray_flux_versus_particle_energy.svg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The unit of the y-axis is a bit hard to understand, but the labels give a better idea: The diagram starts with about 1000 particles per square meter per second at $10^9$eV (1000MeV), while the highest energy from radioactive decay is about 10MeV.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Test devices / facilities&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Coming back to your medical accelerators, these are mainly X-Ray sources. As said, this may cause some SEUs, but no aging. You need a particle source! &#xA;Even if you disassemble a medical accelerator and use the electron accelerator inside, you still get just electrons of some 10MeV.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New types of cancer therapy use accelerators for &lt;a href=&quot;https://en.wikipedia.org/wiki/Proton_therapy&quot; rel=&quot;noreferrer&quot;&gt;protons&lt;/a&gt;, alpha particles and heavier nuclei. The link states protons up to 250MeV, which isn't so much compared to the spectrum in space. And although I can't find data about the intensity, it's most likely far to low. Human tissue is far more sensitive to radiation than silicon, so these facilities will have a  far lower intensity than you need, especially to study degrading effects. &#xA;However, these facilities are quite new and very large compared to your accelerators, which makes them very expensive and anything but common.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you really want to test your electronics, go to one of the larger scientific accelerator facilities like CERN,  the Paul-Scherrer-Institute (PSI) or DESY in Europe or Fermilab etc. in the US. They provide several kinds of radiation facilites with all kinds of particle sources over wide energy and intensity ranges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, here is a &lt;a href=&quot;http://pif.web.psi.ch/&quot; rel=&quot;noreferrer&quot;&gt;website&lt;/a&gt; from PSI where they offer a proton irradiation facility. This is a project from PSI and ESA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(OK, this got a little elaborate... )&lt;/p&gt;&#xA;" OwnerUserId="11749" LastEditorUserId="4660" LastEditDate="2015-12-26T15:24:37.813" LastActivityDate="2015-12-26T15:24:37.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13237" PostTypeId="1" CreationDate="2015-12-26T17:41:17.057" Score="0" ViewCount="1100" Body="&lt;p&gt;I'm working on a simulation with 2 bodies - one a star and the other an orbiting planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I calculated the orbital energy from  the first part of ${v^{2} \over {2}}-{\mu  \over {r}}=-{\frac {\mu }{2a}}=\epsilon&amp;lt;0$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the simulation I have omitted $G$ from the $\mu$, so the numbers are nicer to deal with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I calculate the orbital energy at different points around the orbit, the value is different. Using the second part of the equation I can calculate the semi-major axis, however this will vary as the orbital energy changes...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't understand why, could somebody please explain?&lt;/p&gt;&#xA;" OwnerUserId="10688" LastEditorUserId="49" LastEditDate="2015-12-26T20:14:41.990" LastActivityDate="2015-12-26T20:46:31.443" Title="Is specific orbital energy a constant? How does this affect the semi-major axis?" Tags="|orbital-mechanics|simulation|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="13238" PostTypeId="2" ParentId="13237" CreationDate="2015-12-26T20:46:31.443" Score="2" Body="&lt;p&gt;Conservation of energy is universal across all mechanics, including orbital mechanics. The orbital energy will be constant as long as there are no non-conservative external forces acting on the body (gravity is a conservative force). Semi-major axis is directly related to orbital energy as you have shown, therefore it will also remain constant. The semi-major axis defines the size of the orbit and therefore it also makes sense that the orbit doesn't change without any external forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are only working on a two-body problem, then you can simply &lt;em&gt;choose&lt;/em&gt; an orbit based on the set of orbital elements (semi-major axis, eccentricity, and argument of perigee, as well as inclination and right ascension of the ascending node in full 3D cases). With those elements you don't have to simulate the orbit with gravity forces and equations of motion, you can just simply map out the position of the body at any time you wish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, let's assume you still want to simulate gravity for the two-body case since it can be useful and is easily extended to more than two bodies. Looking at your code, the biggest thing that stands out to me is: why are you dividing the distance by 3000? Remove that scaling from your function and only apply it for plotting.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-12-26T20:46:31.443" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13239" PostTypeId="2" ParentId="13226" CreationDate="2015-12-26T21:04:49.733" Score="2" Body="&lt;p&gt;If you are asking about how to determine the torque output from a given system with reaction wheel and/or thrusters, then you need to start with the specifications of those devices. Reactions wheels will generate torque only along their specific axis of rotation, otherwise you can simply look up what the rated and maximum torques are for that wheel. Thrusters generate torque based on their offset distance away from the vehicle centre of mass. To compute the torque you need to look up the rated thrust for that thruster then use the cross-product of the thrust &lt;em&gt;vector&lt;/em&gt; and the position vector from the centre of mass to the thruster position. You can then combine all of the torques from each device to determine the next torque in all three directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, if you are asking about how to determine &lt;em&gt;how much torque you need&lt;/em&gt;, then you will have to follow Deer Hunter's comment and break down your problem some more so that you can determine what &lt;em&gt;motions&lt;/em&gt; are needed and then convert that to torque requirements based on angular acceleration. For example, you could assume you'll be slewing from one orientation to another using a bang-bang maneouvre (apply some constant torque to start moving, then do nothing while moving, then apply some constant torque to stop moving). Then you can define how quickly you want to accelerate/decelerate and determine how much torque is required.&lt;/p&gt;&#xA;" OwnerUserId="12038" LastActivityDate="2015-12-26T21:04:49.733" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13241" PostTypeId="1" CreationDate="2015-12-26T22:20:18.100" Score="4" ViewCount="968" Body="&lt;p&gt;I generally understand the concept of the LDZ, but would like to look at actual examples for past Falcon and Antares orbital launches. How are private boaters and the general public warned about KSC/Canaveral launches and the corresponding downrange exclusion zones?  Is there a link to public notices and Notice to Mariners/Airmen from past launches at KSC and Wallops?&lt;/p&gt;&#xA;" OwnerUserId="12814" LastActivityDate="2023-12-08T23:13:09.450" Title="Where can I find examples of down-range launch exclusion zones for Falcon-9 and Antares orbital launches?" Tags="|falcon-9|antares|range-safety|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13242" PostTypeId="1" CreationDate="2015-12-27T00:11:35.687" Score="1" ViewCount="122" Body="&lt;p&gt;Is there footage of the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_2&quot; rel=&quot;nofollow&quot;&gt;NASA Astronaut Group 2&lt;/a&gt; press conference online? I can watch the press conference for the Mercury astronauts &lt;a href=&quot;https://www.youtube.com/watch?v=FXj5lc_QUOM&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, but can't find &lt;a href=&quot;http://www.spacefacts.de/groups/photo2/nasa-2.jpg&quot; rel=&quot;nofollow&quot;&gt;Group 2&lt;/a&gt; footage. I've seen snippets of the 1962 Group 2 press conference in Nasa documentaries, so I know it exists somewhere.&lt;/p&gt;&#xA;" OwnerUserId="12620" LastEditorUserId="4660" LastEditDate="2015-12-27T00:59:50.707" LastActivityDate="2015-12-27T00:59:50.707" Title="NASA Astronaut Group 2 Press Conference" Tags="|nasa|astronauts|reference-request|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13243" PostTypeId="1" AcceptedAnswerId="13244" CreationDate="2015-12-27T00:53:10.317" Score="9" ViewCount="1227" Body="&lt;p&gt;As a follow-up question to this &lt;a href=&quot;https://space.stackexchange.com/a/13137/12102&quot;&gt;answer&lt;/a&gt; by @TildalWave, &lt;strong&gt;exactly what frame is used&lt;/strong&gt; to calculate the &quot;speed&quot; in the inset display of speed and altitude in the SpaceX broadcast of the Falcon 9 launch and Orbcomm-2 deployment. Since it starts at zero before launch, is it the ground speed of the orbit projected on the Earth's surface, or something else? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm trying a simple physics exercise to convert speed to velocity under some assumptions (fixed inclination, radial velocity from $\Delta altitude/\Delta t$) and then integrate it to try to reconstruct the orbit for fun. I know there are other ways, but this is what I want to try. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, as a rough test of the numbers (in Python):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import math&#xA;&#xA;pi     = math.pi&#xA;two_pi = 2. * pi&#xA;&#xA;r_earth  = 6371. # km from google&#xA;altitude = 629.  # km from screen shot&#xA;r_orbit  = r_earth + altitude&#xA;&#xA;speed_screen = 25947.  # km/hr from screen shot&#xA;&#xA;print 60. * two_pi * r_orbit / speed_screen, &quot;minutes&quot; &#xA;&#xA;r_florida = r_earth * math.cos((pi/180.)*23.4)&#xA;speed_florida = two_pi * r_florida / 24.&#xA;&#xA;print speed_florida, &quot;kph&quot;&#xA;&#xA;print 60. * two_pi * r_orbit / (speed_screen + speed_florida), &quot;minutes&quot;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;gives:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;101.704930397 minutes&#xA;1530.74487421 kph&#xA;96.0390978625 minutes&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Adding the &quot;speed&quot; from the screen shot to the &quot;speed&quot; of Florida (to get a speed closer to something in an inertial frame) gives 96 minutes - close to the 97.4 minute period of the Orbcomm-2 satellites currently. You can use &lt;a href=&quot;http://www.satview.org/lista_sat.php?cat=tle-new&quot; rel=&quot;nofollow noreferrer&quot;&gt;this link&lt;/a&gt; and click on any of the Norad IDs between 41179U through 41189U. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is just very rough back of the envelope (for example, inclination is 47 degrees), but it suggests that the speed is in the rotating frame of the Earth. Imagine a giant x, y, z axes draw from the Earth and rotating with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this actually what it means?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XsW5u.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XsW5u.png&quot; alt=&quot;SpaceX Broadcast Inset&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S6MoW.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S6MoW.jpg&quot; alt=&quot;SpaceX Orbcomm 2 Launch and Deploy Screen Capture&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-07T00:14:45.173" LastActivityDate="2021-08-07T00:14:45.173" Title="2nd stage speed - with respect to what? (SpaceX webcast of Orbcomm OG2 deployment)" Tags="|orbital-mechanics|launch|falcon-9|low-earth-orbit|orbcomm-2|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13244" PostTypeId="2" ParentId="13243" CreationDate="2015-12-27T11:31:28.897" Score="5" Body="&lt;p&gt;On-screen telemetry readings caused &lt;a href=&quot;https://chat.stackexchange.com/transcript/message/26355512#26355512&quot;&gt;a bit of confusion in our chat&lt;/a&gt; as we were observing the launch, too:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;PearsonArtPhoto:&#xA;  &lt;em&gt;Stage 2 seems low...&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So we started calculating what the speeds for target semi-major axis of 625 km (639 x 611 km / 2) and then current altitude of 622 km for circular orbits should be. Using $v_o \approx \sqrt{\mu/a}$, that came out at &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%286378.14+km+%2B+625+km%29%29+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;27,157&lt;/a&gt; and &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%286378.14+km+%2B+622+km%29%29+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;27,163&lt;/a&gt; km/h, respectively. And for the 629 km altitude that your screen is showing, that comes out at &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%286378.14+km+%2B+629+km%29%29+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;27,149.35&lt;/a&gt; km/h, but telemetry shows 25,947 km/h and slowly dropping, while the altitude was slowly climbing. Something isn't quite right, and this can't have been readings for inertial frame of reference. If the upper stage was 1,202.35 km/h or 4.4% short of its orbital speed, it would start reducing its altitude and increasing speed as it falls in a wide arc towards the Earth. Yet we saw the opposite;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I started plugging some more values into the formula, suspecting the values given were for rotating frame of reference (i.e. downrange speed), which would make sense since the launch vehicle started at 0 speed and not the Earth's eastward rotation at &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Space_Launch_Complex_40&quot; rel=&quot;nofollow noreferrer&quot;&gt;28.562106° N for SLC-40&lt;/a&gt; latitude and accounting for Earth's flattening:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_\text{LS} = 2 \cdot \pi \cdot 6378.14 \text{ km } \cdot cos(28.562106°) \cdot (1 - 0.0033528)\ /\ 0.99726968 \text{ day}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which comes out at &lt;a href=&quot;https://encrypted.google.com/#q=%28%282+*+pi+*+6378.1+km+%2F+0.99726968+day%29+*+cos%2828.5240583+deg%29+*+%281+-+0.0033528%29%29+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;1,465.65 km/h&lt;/a&gt;. Subtracting this value from previous results, as the launch site is rotating towards the upper stage, I get &lt;a href=&quot;https://encrypted.google.com/#q=sqrt%28%28%286.673%C3%9710%5E%E2%88%9211+N*%28m%2Fkg%29%5E2%29+*+5.97219%C3%9710%5E24+kg%29%2F%286378.1+km+%2B+629+km%29%29+-+%28%282+*+pi+*+6378.1+km+%2F+0.99726968+day%29+*+cos%2828.5240583+deg%29+*+%281+-+0.0033528%29%29+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;25,683 km/h&lt;/a&gt; for a circular orbit at 629 km altitude. That's better! Now my calculations are off only by 264 km/h, which can be attributed to small difference in stage's azimuth with respect to the launch site during deployment, and explains why the upper stage is still slowly climbing without propulsion while telemetry readings show its speed to be slowly dropping. I.e. that is still has some excess orbital energy from the impulse burns and it will take a bit of time for it to &lt;em&gt;climb the orbit&lt;/em&gt;, while the Orbcomm OG-2 satellites will have to remove that much speed to later circularize their final orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, while I don't have any official sources for this, it does appear that the telemetry readings were downrange speed, given in a rotating frame of reference. And the next best thing to official sources of information, here's the simulated upper stage graphs of the SpaceX Orbcomm OG-2 launch profile, made by &lt;a href=&quot;http://www.flightclub.io&quot; rel=&quot;nofollow noreferrer&quot;&gt;FlightClub.io&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aSmnp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aSmnpm.png&quot; alt=&quot;Upper Stage Altitude&quot;&gt;&lt;/a&gt;&lt;a href=&quot;https://i.stack.imgur.com/D0QhG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D0QhGm.png&quot; alt=&quot;Upper Stage Acceleration&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Target orbit there is described as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;apoapsis: 639 km&lt;br /&gt;&#xA;  periapsis: 611 km&lt;br /&gt;&#xA;  inclination: 47.6°&lt;br /&gt;&#xA;  eccentricity: 0.0020&lt;br /&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;with upper stage engine cut-off at T+2004 seconds.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-27T13:02:07.507" LastActivityDate="2015-12-27T13:02:07.507" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13245" PostTypeId="2" ParentId="13229" CreationDate="2015-12-27T16:01:18.230" Score="2" Body="&lt;p&gt;If I understand your question right, you want to calculate how many fly-bys of the planet you must perform to turn your velocity  vector 180 degrees around. If the spacecraft does not interact with any other object between the fly-bys, the $v_{inf}$ (encounter velocity) stays constant. The minimum number of encounters is therefore calculated by dividing 180 degrees on the maximum turn angle. (Ceil the resulting number)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Turn angle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Angle =\mathbf{2 \; \operatorname{sin}^{-1} \left( \frac{GM}{P \; v_{inf}^{2} + GM} \right)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $P$ is the distance of periapsis, often limited by the radius of the planet or its atmosphere, $GM$ is the product of the planets mass and the gravitational constant, and $v_{inf}$ is the &lt;a href=&quot;http://hopsblog-hop.blogspot.no/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow&quot;&gt;velocity at infinity&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-02T19:03:09.627" LastActivityDate="2016-05-02T19:03:09.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13247" PostTypeId="1" AcceptedAnswerId="13249" CreationDate="2015-12-27T18:55:07.723" Score="3" ViewCount="537" Body="&lt;p&gt;Considering launches from the two major cosmodroms are launched into inclinations quite far from equatorial, and that inclination change in LEO is awfully expensive in terms of delta-v, what trajectory do rockets take to deploy satellites into a low orbit above Earth's equator?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-12-28T10:52:31.313" Title="How are satellites deployed into equatorial LEO?" Tags="|artificial-satellite|low-earth-orbit|orbital-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13248" PostTypeId="1" AcceptedAnswerId="13251" CreationDate="2015-12-27T20:21:44.163" Score="13" ViewCount="1279" Body="&lt;p&gt;So I was curious about why the Atlas V rocket has so many versions, with three-digit designators like &quot;401&quot; and &quot;551&quot;. The obvious differentiator is the number of solid rocket boosters you can add to the core rocket. Also, there's an option for extra engine for the second stage (though that hasn't flown yet AFAICT).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing I don't get though is how many different payload fairing options they offer. They have a 4-meter and a 5.4m fairing types, with very different designs, but both have 3 different length versions. And while, say, 5-m Long is &lt;em&gt;a lot&lt;/em&gt; longer than 5-m Short (by almost 5 meters), the three different versions of the 4-meter fairing have almost negligible differences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is why. Wouldn't it be better to just have maybe two versions, one standard and one extra long? Perhaps this is a stupid, opinion-based question. But is there an obviously good reason for this? Do the 4m and 5m fairing types feature a different design because of some important trade-off? Is the weight difference between the length options significant enough to warrant making them?&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="29286" LastEditDate="2020-08-05T09:20:48.120" LastActivityDate="2020-08-05T09:20:48.120" Title="Why does Atlas V have so many different types of payload fairings?" Tags="|atlas-v|ruag-space|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13249" PostTypeId="2" ParentId="13247" CreationDate="2015-12-27T20:44:27.880" Score="4" Body="&lt;p&gt;The answer is that they do not, the delta-v cost is as you state high. Plesetsk is 63 degrees north, and Baikonur is 43 degrees. Plesetsk is actually so inclined that it costs less to do a bi-elliptic manoeuvre, meaning that you accelerate almost up to escape velocity, do the inclination change at a great distance, and then fall back and brake into low Earth orbit.&#xA;To have a satellite in low equatorial orbit, you have to launch from the equator or close to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For that matter, almost no satellites are launched to equatorial LEO from Cape Canaveral either.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-28T10:52:31.313" LastActivityDate="2015-12-28T10:52:31.313" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13250" PostTypeId="2" ParentId="13248" CreationDate="2015-12-27T21:06:17.127" Score="8" Body="&lt;p&gt;You're forgetting that on top of their mass being different, larger ø also means larger cross sectional area, and with it increase in aerodynamic &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_%28physics%29&quot; rel=&quot;noreferrer&quot;&gt;drag&lt;/a&gt;. So larger ø ones are heavier, subject to stronger atmospheric resistance, have larger contact area subjected to ascent heating, and with a more contoured launch vehicle's body deviate from &lt;a href=&quot;https://en.wikipedia.org/wiki/Sears%E2%80%93Haack_body&quot; rel=&quot;noreferrer&quot;&gt;ideal aerodynamic shape&lt;/a&gt; and are subject to more vibration due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Bernoulli&amp;#39;s_principle#Bernoulli_equations&quot; rel=&quot;noreferrer&quot;&gt;pressure differential&lt;/a&gt; and transonic &lt;a href=&quot;https://en.wikipedia.org/wiki/Wave_drag&quot; rel=&quot;noreferrer&quot;&gt;wave drag&lt;/a&gt;. It makes sense to have many different ø and length payload fairings to choose from so they fit requirements of different payloads and can achieve best performance and mass to orbit.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2015-12-27T21:06:17.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13251" PostTypeId="2" ParentId="13248" CreationDate="2015-12-27T21:08:53.933" Score="9" Body="&lt;p&gt;You can find lots of information in the &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/AtlasVUsersGuide2010.pdf&quot;&gt;User Guide&lt;/a&gt; (420-page PDF). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Page 57 shows the performance effects:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Atlas V 400 series performance is based on the use of the 4-m EPF. GTO performance with the LPF is approximately 35 kg (77.2 lb) more than the EPF with the 401 configuration and 44 kg (97 lb) more with the 431 configuration. GTO performance with the XEPF is approximately 35 kg (77.2 lb) less than the EPF with the 401 configuration and 44 kg (97 lb) less with the 431 configuration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;abbreviations are: Large Payload Fairing (LPF), Extended Payload Fairing (EPF), and Extra Extended Payload Fairing (XEPF). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They've also included the reason for the 3 fairing sizes (same PDF, page 267): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Atlas V developed the EPF to support launches of larger-volume spacecraft by adding a 0.9-m (36-in.) high cylindrical plug atop the cylindrical section of the LPF. The XEPF is a modified version of the EPF that incorporates an additional 0.9-m (36-in.) high cylindrical plug to further increase the available payload volume.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So they started with the shortest fairing, then developed larger versions as the need arose. I suspect they saw no sense in throwing away a proven design, so all 3 were kept around. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-27T21:08:53.933" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13252" PostTypeId="1" AcceptedAnswerId="13256" CreationDate="2015-12-28T03:19:12.263" Score="1" ViewCount="1174" Body="&lt;p&gt;For an unmanned probe which can theoretically withstand 100G acceleration, from Lunar orbit, in regards to &lt;a href=&quot;http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)&quot; rel=&quot;nofollow&quot;&gt;Project Orion&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="11671" LastEditorUserId="1142" LastEditDate="2016-01-31T15:25:47.770" LastActivityDate="2024-03-26T09:56:33.960" Title="How many pulse units would be required to reach 1% and 10% the speed of light?" Tags="|interstellar-travel|nuclear-pulse-propulsion|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13253" PostTypeId="1" AcceptedAnswerId="13265" CreationDate="2015-12-28T09:32:06.170" Score="1" ViewCount="664" Body="&lt;p&gt;This (amateur) question has always been on my mind.&#xA;We send probes to leave the solar system, but when we reach there, their battery is almost empty, so when finally after ~30 years we reach there, we can hardly &quot;see&quot; anything.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What do we need? Higher speed. If we could reach where Voyager is right now in 5 years, we could collect much better and more useful information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What can we do today with the &lt;strong&gt;current technology&lt;/strong&gt;? &#xA;Almost all rocket fuel is spent to escape the Earth's gravity, so the probe is not really accelerated in space after that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why isn't rocket/fuel sent to space first, instead of the probe (I know transport capacity is limited, so we need to send fuel in chunks), and when desirable amount of fuel is there (don't know, 10-20 launches?) we connect them, attach the probe, and ignite in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Couldn't we reach much much higher speed there?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why isn't this method used? I know it's expensive, but that shouldn't be the blocking issue, should it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;--Edit--&lt;/strong&gt;&#xA;I found this: &lt;a href=&quot;https://space.stackexchange.com/questions/9211/why-cant-we-launch-from-space&quot;&gt;Why can&amp;#39;t we launch from space?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12828" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-12-29T03:32:49.683" Title="Why aren't deep-space probes launched from space?" Tags="|probe|fuel|voyager|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13254" PostTypeId="2" ParentId="13253" CreationDate="2015-12-28T09:48:42.033" Score="2" Body="&lt;p&gt;The main reason this isn't done, is cost. A single rocket launch costs $100-300 million, so it's very expensive to create a faster deep-space mission. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-28T09:48:42.033" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13255" PostTypeId="2" ParentId="13248" CreationDate="2015-12-28T10:38:10.587" Score="6" Body="&lt;p&gt;While other answers are perfect from an engineering point of view, there is another aspect. The 4m and 5m fairings have different designs and are produced at different sites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5m fairings have aluminum honeycomb core and graphite epoxy face panels and are made in Emmen, Switzerland (&lt;a href=&quot;https://www.ruag.com/space/ruag-space-switzerland/emmen-site/&quot; rel=&quot;noreferrer&quot;&gt;RUAG&lt;/a&gt;), the same manufacturer that makes Ariane 5 PLFs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, this is a business decision to outsource production to the producer which has the experience, technology, and frequent enough orders to justify keeping the line open. Mind you, it is only 23 employees, and mostly skilled manual labor (not a Chinese consumer plastics sweatshop, and not a high-volume auto assembly line either).&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-12-28T16:37:52.390" LastActivityDate="2015-12-28T16:37:52.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13256" PostTypeId="2" ParentId="13252" CreationDate="2015-12-28T11:18:55.140" Score="5" Body="&lt;p&gt;Is that possible at all? Given the relatively low accuracy of all estimates, 1% or 10% of the speed of light would experience only minor relativistic effects. I will therefore treat them as 3.000.000 m/s and 30.000.000 m/srespectively.&#xA;Given those extreme velocities, the fact that you start in lunar orbit is not important. Also, the exact number of pulse units is highly dependent on the payload mass and the implementation of the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000096503.pdf&quot; rel=&quot;noreferrer&quot;&gt;The paper on project Orion&lt;/a&gt; state a maximum ISP of 1.3 * 10⁶ s. Plugging this, and the target velocity into the inverse rocket equation, gives a mass ratio of 10 for the 1% of speed of light spacecraft, and aprox. 10 billions for the 10% spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, the paper also gives ISP values several orders of magnitudes lower for any actual implementation of a nuclear pulse rocket. So fractions of the speed of light are even inaccessible for the Orion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course this is hard to estimate accurately, but a few percent of the speed of light is achievable. The actual number of pulse units is more difficult to answer, as their decided size is a bit arbitrary. As bigger nuclear charges are more effective, I choose pulse units giving your stated 100g of acceleration. Then the number is around 10000 for 1% of the speed of light.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-29T19:03:13.927" LastActivityDate="2015-12-29T19:03:13.927" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13257" PostTypeId="1" AcceptedAnswerId="13260" CreationDate="2015-12-28T11:35:11.290" Score="21" ViewCount="14575" Body="&lt;p&gt;Considering how important mass ratio is for the rocket equation, why are rockets built still out of aluminium, and not some lighter material, perhaps carbon/glass fiber composites?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm aware composites are used here and there for some parts of rockets, but the heaviest part — main structures and tanks all use aluminium as far as I can see. (Also, the payload fairings seem like a good candidate for composites, and I know Atlas V 500 series use a fairing that's composite on the outside, but the main structure is still Al)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it be useful to switch to composites? Are there big downsides that make it a no-no, or is it just that it's hard to develop this technology and metal is simpler?&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="58" LastEditDate="2016-07-26T15:36:52.067" LastActivityDate="2022-08-29T13:56:45.527" Title="Why not build rockets out of composite materials?" Tags="|rockets|materials|composite-materials|" AnswerCount="6" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13258" PostTypeId="1" CreationDate="2015-12-28T11:41:40.643" Score="5" ViewCount="526" Body="&lt;p&gt;So I remember reading that there's a 15% performance penalty to landing Falcon 9's first stage on the barge, and 30% penalty for RTLS (Return to Launch Site).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But of course it would be very preferable to land on land. More room for error, no need to worry about weather on the ocean. Simpler.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So… if SpaceX is building their own launch site at the very south-east tip of Texas, could they just do the land-landing someplace else than the launch site? The CC Landing Zone 1, perhaps, and if that's too far away, then maybe a bit north, like Louisiana.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does that make any sense at all?&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="28587" LastEditDate="2019-05-27T22:23:41.750" LastActivityDate="2019-05-27T22:23:41.750" Title="Would it make sense for SpaceX to launch from Boca Chica and land in Florida (or somewhere else)?" Tags="|spacex|falcon-9|landing|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="13259" PostTypeId="2" ParentId="13258" CreationDate="2015-12-28T11:53:47.987" Score="6" Body="&lt;p&gt;The Boca Chica launch site has a very narrow flight path it will be allowed, that avoids land overflight.  The LZ-1 pad would require flying over the width of the state to get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A downrange landing site is ideal, thus the barge ideas.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2015-12-28T11:53:47.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13260" PostTypeId="2" ParentId="13257" CreationDate="2015-12-28T12:55:50.137" Score="27" Body="&lt;p&gt;Composite materials work well in temperatures commonly encountered in all weathers on Earth, but rocket elements swing between temperatures of liquid hydrogen (where polymer components become very brittle) and thousands of degrees of air resistance and combustion radiation (where composites simply burn), and brutal temperature shifts once in space. Never mind limited durability against UV and space radiation, often worse absolute durability values (better &quot;per gram&quot; but worse &quot;per cc&quot;, meaning a bigger rocket with more air resistance), worse chemical resistance (hydrazine, SRB fuels), oxidation in high concentrations of oxygen (that evaporates from low pressure cryofuel tanks), and worse long-term durability (this is not as important actually).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They certainly have their place - eg. in interior of manned spacecraft, infrastructure surrounding the launch, prototyping - but currently their characteristics in environments that the craft must withstand during launch and in orbit aren't quite satisfactory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes though, their shortcomings are simply &lt;em&gt;accepted&lt;/em&gt;, e.g. the Shuttle liquid fuel tank, built from a composite of quite thin aluminiu and two kinds of foams, would arrive in space badly scorched in several places, and definitely in a worse condition than before the launch - but still good enough to keep the remainder of the fuel until it's depleted.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2015-12-28T12:55:50.137" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="13261" PostTypeId="2" ParentId="13257" CreationDate="2015-12-28T13:06:50.993" Score="19" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: added more information on why composites aren't common yet.&lt;/p&gt;&#xA;&lt;p&gt;Most of a rocket's structure consists of LOX and fuel tanks. Historically, carbon composites were viewed as too flammable to be used safely for tanks. Carbon composites failed standard tests used by e.g. NASA to determine flammability. In 2001, &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20010020209.pdf&quot; rel=&quot;noreferrer&quot;&gt;a study was done&lt;/a&gt; to re-examine this decision.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To be considered LO&lt;sub&gt;2&lt;/sub&gt; compatible, materials must be selected that will resist any type of detrimental, combustible reaction when exposed to usage environments. This is traditionally evaluated using a standard set of tests. However, materials that do not pass the standard tests can be shown to be safe for a particular application. This paper documents the approach and results of a joint NASA/Lockheed Martin program to select and verify LO&lt;sub&gt;2&lt;/sub&gt; compatible composite materials for liquid oxygen fuel tanks.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(the program was VentureStar, by the way)&lt;/p&gt;&#xA;&lt;p&gt;And there are other issues: at LOX temperatures, carbon composites can do &lt;a href=&quot;http://www.compositesworld.com/articles/an-update-on-composite-tanks-for-cryogens&quot; rel=&quot;noreferrer&quot;&gt;strange things&lt;/a&gt;, including igniting spontaneously when something impacts the tank. Preventing that was a matter of finding the right materials:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Tom DeLay, a researcher in the area of nonmetallic materials and processes at NASA's Marshall Space Center (Huntsville, Ala.), notes that the many composite cryogenic tank development programs initiated over the years have certainly resulted in technology improvements, yet most had budgetary and schedule constraints that did not permit researchers to identify or qualify the optimum materials for cryogenic environments.&lt;/p&gt;&#xA;&lt;p&gt;Lockheed Martin Space Systems - Michoud Operations (New Orleans, La.) has worked for more than 20 years on various NASA programs to adapt composites for cryogenic applications.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Another &lt;a href=&quot;http://aviationweek.com/space/advances-lightweight-composite-tanks-launchers&quot; rel=&quot;noreferrer&quot;&gt;roadblock&lt;/a&gt; in the use of composites is the need for giant autoclaves if you want to build large structures out of a composite (e.g. the SLS tanks).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Manufacturing, especially for composites, is limited by available facility size, and the more complicated the design, the greater the cost and difficulty of manufacturing,” states the new NASA document, which was released May 11. “So concepts that are enabled by non-autoclave processing of composites and with integrated or low-cost tooling are of great importance.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Developing a new rocket is a long and expensive process. The lack of price pressure meant companies could afford to keep using existing designs instead of having to develop new, cheaper rockets. The rocket market is far less cutthroat than the aviation market, and far smaller (so your expensive development has to be paid for by fewer sales).&lt;/p&gt;&#xA;&lt;p&gt;With the arrival of several commercial ventures, that is changing. These days, composites are being considered by &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20120009355&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;, &lt;a href=&quot;http://www.esa.int/TEC/Structures/SEMNQG91M9H_0.html&quot; rel=&quot;noreferrer&quot;&gt;ESA&lt;/a&gt; and &lt;a href=&quot;http://www.tandfonline.com/doi/abs/10.1163/156855101753396726&quot; rel=&quot;noreferrer&quot;&gt;JAXA&lt;/a&gt; for interstage structures, &lt;a href=&quot;http://aviationweek.com/space/advances-lightweight-composite-tanks-launchers&quot; rel=&quot;noreferrer&quot;&gt;cryogenic fuel tanks&lt;/a&gt; and other rocket parts.&lt;/p&gt;&#xA;&lt;p&gt;SpaceX already uses carbon fiber composites for the &lt;a href=&quot;http://www.spacex.com/falcon9&quot; rel=&quot;noreferrer&quot;&gt;payload fairing and interstage&lt;/a&gt; on the Falcon 9. Here's the interstage:&lt;br /&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/VH7b1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VH7b1.jpg&quot; alt=&quot;Falcon 9 interstage&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;noreferrer&quot;&gt;Falcon 9 user guide:&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Falcon vehicles’ interstage, which connects the first and second stages, is a composite structure consisting of an aluminum honeycomb core surrounded by carbon fiber face sheet plies. The interstage is fixed to the forward end of the first-stage tank.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;ULA uses the same combination for their Atlas V fairing. The Atlas SRBs use a &lt;a href=&quot;http://www.ulalaunch.com/products_atlasv.aspx&quot; rel=&quot;noreferrer&quot;&gt;carbon-fiber casing&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;This is a Falcon 9 fairing after landing:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/u08BZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/u08BZ.jpg&quot; alt=&quot;F9 fairing&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-19T16:42:59.193" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13262" PostTypeId="2" ParentId="9062" CreationDate="2015-12-28T15:15:37.757" Score="1" Body="&lt;p&gt;I would also think that in a failure where the rocket either explodes or must be detonated, having the trunk attached might act as shielding from penetrating debris. There would only be a short window where that might be needed, and it would seem to coincide with the duration that the trunk remains attached.&lt;/p&gt;&#xA;" OwnerUserId="10641" LastActivityDate="2015-12-28T15:15:37.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13263" PostTypeId="2" ParentId="4411" CreationDate="2015-12-28T15:36:47.967" Score="3" Body="&lt;p&gt;Usable Mercury cyclers exist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the large inclination of Mercury, we want the encounter to happen along the line of apsides, thereby requiring a quasi-periodic stationary cycler. The simplest type of cycler in this family is a high periapse fly by ellipse, ideally close to a Hohmann transfer orbit. This requires that the planets' synodic period is a simple fraction of the orbital period of either of the planets. Furthermore, the orbital period of the cycler must be close to the previously stated quasi-period, divided by an integer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interesting quasi-periods include:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3, -17.36375&lt;br&gt;&#xA;19,  10.02959&lt;br&gt;&#xA;22,  -7.33416&lt;br&gt;&#xA;41,   2.69542  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Synodic periods, inaccuracy in degrees)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for actual appearance of the planetary alignments, a usable launch window would in worst case appear within a decade in the Mercury-Earth system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 22 and the 41 synodic period cycler are the two most interesting. The 22 (7 year) orbit offers a trajectory suitable for reuse 4-5 times, at a relatively low delta-v cost. This is because the orbital period of 0.5385 years, compared to the 0.5327 years for a perfect Hohmann transfer to the ascending node, has a close to minimal vinf.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is also the case for the even better 41 (13 year) orbit, where the same comparison is 0.61905 years to 0.61853 years for a descending node transfer. (The deviation from a Hohmann transfer is negligible.) A slight boost to a 0.63636 empty 7 year cycle every 46 years reduces the angular drift to 1.635 degrees per century, allowing stability for maybe as long as a millennium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This type of cycler does not offer two-way transportation, so two cyclers are necessary. (One inbound, one outbound.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="6816" LastEditDate="2019-03-13T09:31:12.263" LastActivityDate="2019-03-13T09:31:12.263" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13265" PostTypeId="2" ParentId="13253" CreationDate="2015-12-28T17:33:46.740" Score="4" Body="&lt;p&gt;This is the basis for several concepts of interstellar probes. The most awesome is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;nofollow&quot;&gt;Bussard ramjet&lt;/a&gt;, aimed to exploit the 'fuel' already present in the interstellar space - protons (hydrogen ions) (or other ions, in some variants) for magnetically compressed fusion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than that, gathering reaction mass (e.g. water, methane, or carbon dioxide for processing) &lt;a href=&quot;https://en.wikipedia.org/wiki/ISRU&quot; rel=&quot;nofollow&quot;&gt;on distant worlds&lt;/a&gt; is one of the most important shortcuts in return missions to avoid launching &lt;em&gt;megarocket trains&lt;/em&gt; at exorbitant (no pun intended) cost. The complexity of such collection and processing is still one step beyond current technology (Bussard ramjets are at least seven+ steps beyond).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fueling on the way to the destination is not done due to absence of maintained &lt;a href=&quot;https://en.wikipedia.org/wiki/Propellant_depot&quot; rel=&quot;nofollow&quot;&gt;orbital fuel depots&lt;/a&gt; and lower mission success probability as soon as one provides for orbital rendezvous and refueling. This may become an option once these technologies are perfected, for low-thrust ion engine etc. exploration probes.&lt;/p&gt;&#xA;" OwnerUserId="168" LastEditorUserId="168" LastEditDate="2015-12-29T03:32:49.683" LastActivityDate="2015-12-29T03:32:49.683" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13266" PostTypeId="2" ParentId="6204" CreationDate="2015-12-28T18:14:15.240" Score="1" Body="&lt;p&gt;When working with the &lt;a href=&quot;http://www.dept.aoe.vt.edu/~lutze/AOE4134/patchedconiceqs.pdf&quot; rel=&quot;nofollow&quot;&gt;patched conics&lt;/a&gt; approximation, it is not possible to brake into orbit using gravity alone. The rule used is that &lt;a href=&quot;http://hopsblog-hop.blogspot.co.uk/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow&quot;&gt;vinf&lt;/a&gt; in equals vinf out. But this is only an approximation used to divide a n-body problem into a more easily solved two-body problem. It assumes a distance, a sphere of influence, where an object stops orbiting one body (for example the Sun), and starts to orbit another body (for example the Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality, this transition is more smooth, giving a spacecraft some wiggling room when it orbits very close to edge of the planets influence. Ballistic capture is thus not an exception from the usual rules, it is just a phenomenon that the traditional approximations fails to handle properly. They are limited though, saving only a little bit of delta-v, and have been &lt;a href=&quot;http://hopsblog-hop.blogspot.co.uk/2015/04/potholes-on-interplanetary-superhighway.html&quot; rel=&quot;nofollow&quot;&gt;criticised&lt;/a&gt; for giving people an impression of a new revolutionary concept in space travel.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2015-12-28T18:14:15.240" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13267" PostTypeId="2" ParentId="13253" CreationDate="2015-12-28T18:39:20.810" Score="3" Body="&lt;p&gt;The needed fuel has to be launched into space either way, so a refueling option would add complexity without reducing the total launch mass requirement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get to where the Voyagers are today in only 5 years would require them to leave Earth going more than 115km/s. Because of the nonlinearity in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;rocket equation&lt;/a&gt;, reaching that speed with chemical rockets would require about one &lt;em&gt;trillion&lt;/em&gt; tons of fuel, which would take about 7 billion Saturn V launches to bring into Earth orbit (or 100 billion Falcon 9 reusables). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using more fuel-efficient ion thrusters, the fuel requirement to reach 115km/s would be greatly lessened, but acceleration would be too low to reach that speed in less than decades. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, the Voyager spacecraft aren't that severely compromised by the decay of their &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow&quot;&gt;RTG&lt;/a&gt; power sources. They may not be able to use all their instruments at once, but they don't really need to. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-28T19:26:43.263" LastActivityDate="2015-12-28T19:26:43.263" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13268" PostTypeId="2" ParentId="13257" CreationDate="2015-12-28T18:44:03.020" Score="6" Body="&lt;p&gt;Parts of newer launchers have been made out of composites. For example the &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Vega_main_engine_test_in_Kourou&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vega first stage&lt;/a&gt; (P80) uses composites for the casing and some nozzle parts:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, instead of the steel outer structure used for the Ariane boosters, the P80 has a lightweight, filament-wound composite casing. It also incorporates a new, simplified design of igniter with a carbon fibre structure.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A new, steerable nozzle fabricated from composite material has been developed, with a simplified architecture made up of fewer elements, to reduce production costs. It also includes complex-formed cast metal parts and a new thermal insulation material.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;ELV are aiming to develop a larger first stage, the P120, to upgrade Vega. These are also expected to be used as the Ariane 6 boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to why these materials aren't used more already, remember that the technology is relatively new (at least for use in such demanding applications as rocketry, as described in &lt;a href=&quot;https://space.stackexchange.com/a/13260/12831&quot;&gt;SF's answer&lt;/a&gt;) and the launcher industry is fairly conservative and evolves slowly.&lt;/p&gt;&#xA;" OwnerUserId="12831" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2015-12-28T19:43:57.683" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13269" PostTypeId="1" CreationDate="2015-12-28T19:38:10.000" Score="3" ViewCount="690" Body="&lt;p&gt;The frequently used reasoning for why the re-entry should always be performed at neck-breaking speeds is that it would take almost as much fuel to slow the craft down as it takes to launch it into orbit, and that's a whole lot of extra weight to carry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now correct me if I'm wrong, but the Apollo crafts were, in fact, extremely fast; they reached the Moon and inserted themselves into orbit in mere days; after which the lunar module would detach itself from the mothership and begin a &lt;em&gt;hovering&lt;/em&gt; descent to the surface (in the final stage, the descent was vertical). They slowed down sufficiently to make their landings smooth (i.e. from orbital speed to near-zero) &lt;em&gt;and&lt;/em&gt; had enough fuel left to get back into orbit afterwards. And this was nearly half-a-century ago. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yet each time someone returns from orbit these days, they run the risk of getting bounced back into space and getting lost there forever, or burning up before they reach the surface, due to their great speed. And then they use parachutes to perform an uncontrolled splashdown or touchdown wherever, and have to be &quot;rescued&quot; every time. Why?&lt;/p&gt;&#xA;" OwnerUserId="12267" LastActivityDate="2015-12-28T20:44:45.687" Title="Ways to mitigate the forever violent re-entry? (Moon landings vs Earth landings)" Tags="|landing|reentry|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13270" PostTypeId="2" ParentId="13269" CreationDate="2015-12-28T20:12:37.047" Score="9" Body="&lt;p&gt;In the lunar landing case, there's simply no choice; there's no atmosphere available to decelerate the landing craft, so powered landing is the only option. Fortunately, with 1/6 of Earth's gravity, and starting from about 1/5 the speed of Earth orbit, decelerating, hovering, and landing on the moon is much cheaper in fuel -- but we still had to use the largest successful rocket launcher ever built, the Saturn V, to send that fuel mass to the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Earth return case, the deceleration is achieved almost for free using atmospheric drag; the mass of parachutes is tiny compared to the mass of fuel that would be required, and even tinier compared with the launch fuel that would be needed to boost the landing fuel into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 54 years of manned space flight, no ship has ever gotten &quot;bounced back into space and lost there forever&quot;; only one has &quot;burned up&quot; (Columbia), though other failures during reentry killed Soyuz crews in the early days (1967 and 1971). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the last 44 years, blunt-capsule reentry and parachute landing has been 100% safe. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Between 1981 and 2011, 99.25% of manned spaceplane reentries from orbit were successful. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unlikely that powered rocket landings would prove to be any safer.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-28T20:39:49.680" LastActivityDate="2015-12-28T20:39:49.680" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13271" PostTypeId="2" ParentId="13269" CreationDate="2015-12-28T20:20:34.707" Score="7" Body="&lt;p&gt;The Apollo craft started its trip to the Moon at a high speed. But as it got further away from Earth (climbing out of Earth's gravity well), its speed dropped steadily, and it orbited the Moon at a speed of about 1.5 km/s. That's much lower than Earth orbit (around 8 km/s), which makes a propulsive landing much easier. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2015-12-28T20:20:34.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13272" PostTypeId="2" ParentId="13269" CreationDate="2015-12-28T20:44:45.687" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Now correct me if I'm wrong, but the Apollo crafts were, in fact, extremely fast; they reached the Moon and inserted themselves into orbit in mere days; after which the lunar module would detach itself from the mothership and begin a hovering descent to the surface (in the final stage, the descent was vertical).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The flight from lunar orbit to the surface was mostly ballistic. The Apollo lander made a small burn (33 meters/second) to depart from that lunar orbit, placing the vehicle on an elliptical orbit whose perilune was slightly inside the Moon. Then it simply fell for about an hour. It was only in the final twelve minutes of descent where the lander expelled a sizable amount of fuel to cancel that orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even then, the total delta V from lunar orbit to landing was only 1750 meters/second. That's a fraction of the delta V that would be needed to have a vehicle enter the Earth's atmosphere at sub-orbital speeds -- and then the vehicle would still have about 100 km to fall before landing on the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only technology currently available to dump the 6750 meters/second (or more) of orbital velocity is to use the Earth's atmosphere to perform most of the braking.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2015-12-28T20:44:45.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13273" PostTypeId="1" CreationDate="2015-12-28T22:09:37.927" Score="4" ViewCount="77" Body="&lt;p&gt;I am writing a paper about a possible mission to Saturn but I am a business and Spanish student and am quite ignorant when it comes to the engineering and science behind probes and their instruments; I have had a very hard time researching this topic myself. So I have two questions, first, Why would it be important to the scientific community to send a probe to Saturn in order to figure out how exactly the planet generates its internal heat?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, What kind of instruments would the probe need to have in order to make these observations?&lt;/p&gt;&#xA;" OwnerUserId="12835" LastEditorUserId="6816" LastEditDate="2015-12-28T22:48:51.130" LastActivityDate="2015-12-28T22:48:51.130" Title="What instruments are necessary to measure/determine the process that generates Saturn's internal heat?" Tags="|probe|saturn|instrument|measurement|heat|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13278" PostTypeId="2" ParentId="4551" CreationDate="2015-12-29T01:07:59.360" Score="4" Body="&lt;p&gt;I like a location on the rim of Shackleton crater near a so-called Peak of Eternal Light (PEL) which has eight small craters (1-3 km diameter) very much nearby.  A power cable from a peak to the near side of a crater floor looks to be about 1-2 km distant.  That area is also about equidistant between two PELs which if connected would provide continuous solar power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the picture, I am referring to the craters located about at 8:30:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/fD5qx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="12846" LastEditorUserId="49" LastEditDate="2015-12-29T01:31:34.797" LastActivityDate="2015-12-29T01:31:34.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13280" PostTypeId="2" ParentId="10466" CreationDate="2015-12-29T01:23:58.733" Score="0" Body="&lt;p&gt;From a delta-v perspective, everything from LLO to LEO is closer to the lunar surface than to the surface of the Earth. So one could argue that sending propellant made on the lunar surface should cost less than launching it from Earth.  But the downside of launching from Earth can be partially overcome by staging and using partially or wholly reusable launchers.  Then from LEO on out, it is very hard to beat ion propulsion.  Compare that with the cost to set up and maintain ice harvesting propellant production on the Moon and one sees that the case for lunar-derived propellant is more difficult.  However, lunar-derived propellant for accessing lunar surface is still a sure bet.  And since lunar resources contain the elements needed for settlement, I think that the case remains favorable for lunar ice harvesting.&lt;/p&gt;&#xA;" OwnerUserId="12846" LastEditorUserId="22203" LastEditDate="2023-12-10T12:35:12.357" LastActivityDate="2023-12-10T12:35:12.357" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="13283" PostTypeId="1" AcceptedAnswerId="13285" CreationDate="2015-12-29T05:19:57.717" Score="0" ViewCount="714" Body="&lt;p&gt;I'm working on a project and I was wondering if there is a comprehensive archive or a database available out there of all of the spacecrafts and human made probes that were ever launched into space (specially on missions to other celestial bodies than Earth, such as the sun, the inner and outer planets, planet's moons and asteroids) including USSR, ESA, JAXA etc. missions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about future missions?&lt;/p&gt;&#xA;" OwnerUserId="3456" LastEditorUserId="49" LastEditDate="2015-12-29T13:42:02.723" LastActivityDate="2015-12-29T13:42:02.723" Title="Is there a comprehensive list of all space probes ever launched?" Tags="|probe|spacecraft|history|future-missions|reference-request|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13284" PostTypeId="2" ParentId="5967" CreationDate="2015-12-29T10:59:27.547" Score="5" Body="&lt;p&gt;Due to the discussion around &lt;a href=&quot;https://space.stackexchange.com/questions/13257/why-not-build-rockets-out-of-composite-materials&quot;&gt;this question&lt;/a&gt;, I've done some more digging to find out whether or not the epoxy layer shown in pericynthion's answer can be considered a structural part of the tank. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In many NASA documents, the &lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/584728main_Wings-ch4b-pgs182-199.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;epoxy layer isn't mentioned&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA applied two basic types of Thermal Protection System materials to the ET. One type was a low-density, rigid, closed-cell foam. This foam was sprayed on the majority of the tank’s “acreage”—larger areas such as the liquid hydrogen and liquid oxygen tanks as well as the intertank—also referred to as the tank “sidewalls.”&lt;br&gt;&#xA;  The other major component was a composite ablator material (a heat shield material designed to burn away) made of silicone resins and cork.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/sites/default/files/114022main_TPS_FS.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Facts - External Tank Thermal Protection System&lt;/a&gt; doesn't mention it either: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are two basic Thermal Protection Systems on the External Tank: One is low-density, closed-cell foam; the other is a denser composite material called ablator, made of silicone resins and cork. An ablator is a material that dissipates heat by eroding.&lt;br&gt;&#xA;  The closed-cell foam used on the tank acreage is a Spray-On- Foam-Insulation often referred to by its acronym as SOFI (pro- nounced sō-FEE). The composite material is Super Lightweight Ablator, known as SLA (pronounced slaw).&lt;br&gt;&#xA;  The External Tank uses ablators on areas that are subjected to extreme heat, such as the aft dome near the engine exhaust and on protuberances that are exposed to aerodynamic heating, such as the cable trays.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have found &lt;a href=&quot;https://news.google.com/newspapers?nid=1310&amp;amp;dat=20030304&amp;amp;id=Q1RWAAAAIBAJ&amp;amp;sjid=_-sDAAAAIBAJ&amp;amp;pg=6916,803842&quot; rel=&quot;nofollow noreferrer&quot;&gt;references that mention an epoxy primer&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The application of the (epoxy) primer, used largely as an adhesive for foam insulation,...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It seems unlikely to me that you'd need a layer of primer that's 12 mm thick, but I'm no expert in this area. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/584728main_Wings-ch4b-pgs182-199.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thermal Protection Systems&lt;/a&gt; document contains this graphic: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/x4egM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/x4egM.png&quot; alt=&quot;Foam application&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which lists a resin/cork combination as the ablator, used in high heating areas. From the graphic this ablator appears to be used instead of SOFI foam, not underneath it.&lt;br&gt;&#xA;The resin/cork material was applied in sheets. This material caused some &lt;a href=&quot;http://www.nytimes.com/1981/03/08/us/crucial-repairs-set-for-space-shuttle.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;problems before the first launch&lt;/a&gt;, with panels coming off the tank as it contracted when it was filled with cryogenics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Provisionally, I'd have to conclude that the epoxy layer is not a structural part of the tank. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2015-12-29T10:59:27.547" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13285" PostTypeId="2" ParentId="13283" CreationDate="2015-12-29T12:13:52.140" Score="3" Body="&lt;p&gt;Depends on what you mean with &lt;em&gt;comprehensive&lt;/em&gt;, but &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Solar_System_probes&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia's List of Solar System probes&lt;/a&gt; seems fairly up to date (date for &lt;a href=&quot;https://en.wikipedia.org/wiki/InSight&quot; rel=&quot;nofollow noreferrer&quot;&gt;InSight&lt;/a&gt; is still 2016 and it has &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=4805&quot; rel=&quot;nofollow noreferrer&quot;&gt;recently been announced&lt;/a&gt; that the launch date slipped to no earlier than May 2018) and also includes &lt;em&gt;confirmed future probes&lt;/em&gt; (subject to interpretation) and cancelled missions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;You should be able to also find infographics showing our Solar system exploration endeavors, but while visually appealing, unless they present past missions only and you pay attention to when they were created, those obviously won't age so gracefully as some community edited list. You could find some at &lt;a href=&quot;http://www.jpl.nasa.gov/infographics/index.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Infographics&lt;/a&gt;, &lt;a href=&quot;http://www.space.com/infographics/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space.com Infographics&lt;/a&gt;, or in our &lt;a href=&quot;https://space.meta.stackexchange.com/questions/472/collection-of-space-exploration-related-infographics&quot;&gt;Collection of space exploration related infographics&lt;/a&gt; on Space Exploration Meta (contributions welcome). For example, here's &lt;a href=&quot;http://www.jpl.nasa.gov/infographics/infographic.view.php?id=11049&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Mission Paths&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;http://www.jpl.nasa.gov/infographics/infographic.view.php?id=11049&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lQ2aR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize that infographics aren't really what you asked for, but since you mention that you're working on a project, perhaps some such visualizations wouldn't go amiss towards building awareness on the topic. Maybe for some poster session. ;)&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:04.403" LastActivityDate="2015-12-29T12:34:15.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13286" PostTypeId="1" AcceptedAnswerId="13289" CreationDate="2015-12-29T12:28:57.283" Score="7" ViewCount="9845" Body="&lt;p&gt;I found it quite fascinating that the Shuttle External Tank was &lt;a href=&quot;https://space.stackexchange.com/questions/5967/structural-composition-of-shuttle-liquid-fuel-tank-wall&quot;&gt;as thin as 2.5mm&lt;/a&gt;. That seems very thin for a structure over 40 meters tall and weighing as much as 760 000kg fueled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm curious how this compares to other rockets. I know original Atlas had very thin &quot;balloon&quot; tanks — but how thin was it actually, and how much did it weigh? And what about other rockets in use, say Falcon 9 or Atlas V? Or something really big like Saturn V? How much thickness and weight you need in the tank to support the whole rocket?&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-05-23T14:43:55.007" Title="What's the thickness and weight of rocket tanks?" Tags="|rockets|falcon-9|space-shuttle|atlas-v|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13287" PostTypeId="1" AcceptedAnswerId="13288" CreationDate="2015-12-29T14:09:08.987" Score="7" ViewCount="338" Body="&lt;p&gt;Among other reasons, the high pressure of Venus and the gas giants atmospheres are often stated as one of the major problems for an unmanned lander. They say the pressure will &quot;crush&quot; the probe. Why is high pressure a problem? Of course you can not have a confined space of gas or vacuum , or foam containing small bubbles of air on-board, but that should be easy to avoid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Just to clarify, I am totally aware of the terrible environment of Venus, I just wonder about why high pressure in general is a problem.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-29T19:31:25.813" LastActivityDate="2015-12-29T22:15:47.383" Title="What is the problem with pressure?" Tags="|atmosphere|venus|uncrewed-spaceflight|high-pressure-environment|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13288" PostTypeId="2" ParentId="13287" CreationDate="2015-12-29T17:58:53.710" Score="7" Body="&lt;p&gt;The pressure at the surface of Venus is about 92 atmospheres -- 1400 lbs per square inch, almost 100kg per square centimeter (9.2 MPa). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many solid, homogenous materials can withstand that, but consider something like an integrated circuit chip, where different metals are layered within a plastic or ceramic carrier. If the different material layers flex by slightly different amounts under the pressure, something is going to crack. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More of a problem than the pressure itself, though, is heat. The surface temperature on Venus is about 460ºC. A lander won't be able to keep its instruments and other electronics cool enough to function for very long. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="4577" LastEditDate="2015-12-29T22:15:47.383" LastActivityDate="2015-12-29T22:15:47.383" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13289" PostTypeId="2" ParentId="13286" CreationDate="2015-12-29T18:19:27.077" Score="9" Body="&lt;p&gt;Old school Atlas -- &lt;a href=&quot;http://www.warrenmuseum.com/missiles/atlas-sm-65/&quot;&gt;0.1 to 0.4 inches&lt;/a&gt; (2.5 to 10 mm) thick -- so, actually, similar to the metal portion of the shuttle ET. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Centaur upper stage tanks -- &lt;a href=&quot;http://www.alternatewars.com/BBOW/Boosters/Centaur/Centaur.htm&quot;&gt;0.014&quot;--0.016&quot;&lt;/a&gt; (0.36 to 0.41 mm) thick (!). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saturn V's 1st stage tanks varied in thickness from &lt;a href=&quot;http://www.apollosaturn.com/ascom/s5flight/sec4.htm&quot;&gt;0.170&quot;-0.254&quot;&lt;/a&gt; (4.32 to 6.45 mm).&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2015-12-30T20:36:17.653" LastActivityDate="2015-12-30T20:36:17.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13291" PostTypeId="1" CreationDate="2015-12-29T21:40:39.877" Score="5" ViewCount="204" Body="&lt;p&gt;In the 1960s, James Van Allen's &lt;em&gt;Outer Planets Panel&lt;/em&gt; made a proposal that NASA begin planning missions to the outer planets, that eventually led to Pioneer 10 and 11.  I've found many sources that mention it briefly.  &lt;a href=&quot;https://books.google.com/books?id=9WfMExUYtvIC&amp;amp;pg=PA16&amp;amp;lpg=PA16&quot; rel=&quot;nofollow&quot;&gt;This one&lt;/a&gt; is characteristic:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During this same period an Outer Planets Panel associated with NASA's Lunar and Planetary Missions Board (also chaired by Van Allen) recommended in 1962 that planning should start for low-cost exploratory missions to the outer planets.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Does the text of this recommendation survive?  What exactly did they say?&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="12102" LastEditDate="2021-08-03T23:47:55.330" LastActivityDate="2021-08-03T23:47:55.330" Title="What exactly did the Outer Planets Panel say in their recommendation for outer planets missions?" Tags="|nasa|history|deep-space|outer-planets|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13294" PostTypeId="2" ParentId="12926" CreationDate="2015-12-30T12:05:16.000" Score="19" Body="&lt;p&gt;I found an actual reference for this, although it's in a memoir, not a technical book.  Jerry Linenger's book &lt;a href=&quot;https://books.google.com/books/about/Off_the_Planet.html?id=o37S0BIftyQC&quot;&gt;Off The Planet&lt;/a&gt; states&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Unfortunately there is no way to squeeze more than three people into a&#xA;  Soyuz capsule.  There are only three seats in the spacecraft.  Even if&#xA;  one other person, without a seat, sprawled across the sitting crew,&#xA;  the presence of that fourth person would shift the center of gravity&#xA;  of the vehicle, most likely causing the capsule to tumble during&#xA;  reentry.  A tumbling vehicle means that the capsule's heat shield&#xA;  would not be pointed in the proper direction.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Page 112 of the McGraw-Hill 2000 printing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was in reference to the fire emergency on Mir during the Mir 22-23 crew handover period.  There were two Soyuzes docked and six people on board.  The fire blocked access to one of the Soyuzes.  Linenger concludes that it would have been impossible for all six to evacuate in the accessible Soyuz.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2015-12-30T12:05:16.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13295" PostTypeId="2" ParentId="8976" CreationDate="2015-12-30T15:02:50.540" Score="2" Body="&lt;p&gt;A starting point is the smallest sample return probe ever built, the Luna 24.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jDFQTt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jDFQTt.jpg&quot; alt=&quot;Luna 24 return stage accent&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Massing &lt;a href=&quot;http://russianspaceweb.com/luna24.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;5300 kg&lt;/a&gt; in lunar orbit it was pretty much bare bones. To see where we can improve this, we must first split the mission it its separate parts. This follows the standard procedure of doing the mission backwards. First, I would not choose ISS as the target, as braking into orbit and then rendezvous and dock with it is a complicated task. To simply hit the Earth is much easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of the 514kg for the return stage of the Luna 24, around 300kg was propellant. Perhaps a slightly more efficient engine exist today, but the technology used now for landing and ascent would still be hypergolic propellants. That is the most important limit for scaling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The re-entry capsule was only 34kg, but you might still be able to shave off a few kilograms. The main savings are in the remaining 180kg though, including electrical systems, control systems, the engine and the propellant tanks. With the miniaturization of electronic equipment since the seventies, and some new lighter materials, you may be able to squeeze everything required into a dry mass of 100kg. That is about 220kg at the lunar surface, roughly halved. A similar miniaturization of the descent stage and drilling equipment yields a spacecraft of about 2 metric tonnes in Lunar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To transport the spacecraft to the Moon, the minimal solution is a spiralling ion-craft. That is within current technology, take for instance the engine powering Dawn. At the cost of a long transfer time, the total mass in Earth orbit is only going to be around 3 metric tonnes minimum. However, considering the relatively high drag at the altitude of the ISS, the craft must start from a higher orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, it is more effective to combine multiple related goals, like a lunar rover, and on-site experiments, than to launch multiple minimal missions with high risk of failure.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2015-12-30T15:02:50.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13296" PostTypeId="1" AcceptedAnswerId="13297" CreationDate="2015-12-30T16:06:57.970" Score="17" ViewCount="2653" Body="&lt;p&gt;Whilst watching &lt;a href=&quot;https://www.nasa.gov/multimedia/nasatv/&quot; rel=&quot;noreferrer&quot;&gt;NASA TV&lt;/a&gt; over the years, I've often seen the map on the wall of Mission Control:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/LScVT.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LScVT.jpg&quot; alt=&quot;Main Screen in the ISS Mission Control Center&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&quot;Main Screen in the ISS Mission Control Center&quot;, by &lt;a href=&quot;https://www.flickr.com/photos/wildtexas/5170431156&quot; rel=&quot;noreferrer&quot;&gt;AGeekMom&lt;/a&gt;, CC BY 2.0&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this image, as well as the orbital track of the ISS, we see a number of other regions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A light blue line enclosing North and South America, and much of the Pacific and Atlantic Oceans&lt;/li&gt;&#xA;&lt;li&gt;A green line enclosing Europe, Africa, parts of Asia, parts of South America, the Atlantic Ocean and possibly the South Pole&lt;/li&gt;&#xA;&lt;li&gt;A yellow line enclosing Asia, Australia and much of the Pacific&lt;/li&gt;&#xA;&lt;li&gt;Various smaller yellow lines enclosing labelled areas, mostly over Russia&lt;/li&gt;&#xA;&lt;li&gt;A smaller area in the southern Atlantic labelled SAA&lt;/li&gt;&#xA;&lt;li&gt;A circle around the ISS itself.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What exactly do these represent?&lt;/p&gt;&#xA;" OwnerUserId="10686" LastEditorUserId="10686" LastEditDate="2015-12-30T22:05:52.593" LastActivityDate="2015-12-31T09:41:09.103" Title="What are these enclosed regions on the Mission Control Center map?" Tags="|iss|nasa|tracking|mission-control|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13297" PostTypeId="2" ParentId="13296" CreationDate="2015-12-30T16:30:48.150" Score="26" Body="&lt;ul&gt;&#xA;&lt;li&gt;The three big areas (light blue, green, yellow) outside of what looks like the enclosed area on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Equirectangular_projection&quot; rel=&quot;noreferrer&quot;&gt;equirectangular projection&lt;/a&gt; are geosynchronous satellite &lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite_System&quot; rel=&quot;noreferrer&quot;&gt;TDRSS&lt;/a&gt; (Tracking and Data Relay Satellite System) coverage areas for &lt;a href=&quot;https://en.wikipedia.org/wiki/S_band&quot; rel=&quot;noreferrer&quot;&gt;S Band&lt;/a&gt; communications *,&lt;/li&gt;&#xA;&lt;li&gt;Smaller yellow areas over Russia are ground-based tracking and data relay stations,&lt;/li&gt;&#xA;&lt;li&gt;The white area in South Atlantic is the &lt;a href=&quot;https://en.wikipedia.org/wiki/South_Atlantic_Anomaly&quot; rel=&quot;noreferrer&quot;&gt;South Atlantic Anomaly&lt;/a&gt; (SAA)&lt;/li&gt;&#xA;&lt;li&gt;The white circle around the ISS is area of direct line of sight to the station from the ground,&lt;/li&gt;&#xA;&lt;li&gt;Thin line between a dimmed and illuminated region of the Earth is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Terminator_%28solar%29&quot; rel=&quot;noreferrer&quot;&gt;solar terminator&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;White square brackets over ISS ground track are indications of orbital sunset and sunrise (thanks &lt;a href=&quot;https://twitter.com/spaceguy87/status/682249072393109504&quot; rel=&quot;noreferrer&quot;&gt;Ben Honey&lt;/a&gt;!)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;*&amp;nbsp;Coverage areas of each of the three TDRSS satellites (also shown color-coded on the map) are bigger than half the Earth's surface on the map because of adjustment to ISS orbital altitude. So the &lt;em&gt;light blue line enclosing&lt;/em&gt; is the no cover zone that's antipodal to T275W that is shown just South of Sri Lanka and below the ISS on the image. I realize that this might look odd on a equirectangular projection map, but in a sense both sides of these lines are circles on a globe of the Earth. The bigger area side is inclusion area where communications are possible with that satellite before the station sets behind the Earth from satellites vantage point, the smaller one (I guess it could be deemed the &lt;em&gt;enclosed&lt;/em&gt; one on the projection) is the exclusion zone where coverage has to be provided by the other two TDRSS satellites spread roughly 120° around the globe in a small inclination &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;geosynchronous orbit&lt;/a&gt;, or ground based stations where available and in direct line of sight with the station.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2015-12-31T09:41:09.103" LastActivityDate="2015-12-31T09:41:09.103" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13299" PostTypeId="1" AcceptedAnswerId="13300" CreationDate="2015-12-30T21:22:55.620" Score="2" ViewCount="881" Body="&lt;p&gt;Jupiter's outer, equatorial atmosphere rotates at 12,517.12 meters/second, once every 9.5 hours, at a radius of 71,492 km. Geostationary orbital speeds for earth are around 3,000 meters/second, at a radius of 42,164 km. From what I understand, the gravitational acceleration in the upper atmosphere of Jupiter is around 2.4g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a stationary probe (relative to the atmosphere) that maintained its altitude with something like a hot hydrogen air balloon or vacuum-filled sphere experience noticeably less gravity than 2.4g from the extremely fast rotation of Jupiter at the equator? Or to ask in another way, is there noticeably less gravity at the equator than the poles, due to the high rotation rate of the gas giant?&lt;/p&gt;&#xA;" OwnerUserId="12859" LastEditorUserId="6816" LastEditDate="2015-12-30T21:30:53.747" LastActivityDate="2015-12-31T02:47:38.187" Title="Does Jupiter's high rotation speed affect its gravitational strength at the surface?" Tags="|gravity|jupiter|rotation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13300" PostTypeId="2" ParentId="13299" CreationDate="2015-12-30T21:31:30.653" Score="4" Body="&lt;p&gt;The felt centripetal acceleration is about 0.22gs, quite high compared to the gravity of the planet, 2.53gs at the equator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Net acceleration is calculated by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{m}{r^2}-w^2r$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or, using velocity instead of angular velocity,&#xA;$$\frac{m}{r^2}-\frac{v^2}{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing lighter than hydrogen is hot hydrogen (except for vacuum, which is difficult to confine with a lightweight structure), so it is the only possibility to make an airship. For the buoyancy, that is just an issue about sinking low enough in the atmosphere to get a good density gradient between the hot hydrogen and the surrounding gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A possible durable heat source may be an &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_heater_unit&quot; rel=&quot;nofollow&quot;&gt;RHU&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="6816" LastEditDate="2015-12-31T02:47:38.187" LastActivityDate="2015-12-31T02:47:38.187" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13302" PostTypeId="1" AcceptedAnswerId="13476" CreationDate="2015-12-31T09:14:25.300" Score="2" ViewCount="468" Body="&lt;p&gt;From the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; on solar sails:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The force on a sail and the actual acceleration of the craft vary by the inverse square of distance from the Sun (unless extremely close to the Sun)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is when you are accelerating straight away from the Sun, and &quot;proportional to inverse square of distance&quot; is $\frac{1}{r^2}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that this is only valid when you are far from the Sun, and can model it as a point source of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course the Sun is not a point, and its radius is important when close to it. After a little thinking, I came up with this modification to the proportionality law:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{2}{3}\left(1-\cos^3\left(\sin^{-1}\left(\frac{r_S}{r}\right)\right)\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, if you prefer it non-trig:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{2}{3}\left(1-\sqrt{\left(1-\left(\frac{r_S}{r}\right)^2\right)^3}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $r$ is the distance from the centre of the Sun and $r_S$ is the radius of the Sun. This follows the $\frac{1}{r^2}$ law at a distance, but accounts for the fact that some of the light hits the sail at an angle when it is close to the Sun, following the squared cosine rule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have not been able to find a modified relation anywhere, but is this correct? Also, is there a way to include the diameter of the Sun in a formula taking the orientation of the sail into account? That seems difficult, as light is hitting both sides of the sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a visualisation of the difference, this plots the simple proportionality in red, and the modified in grey. Distance from the middle of the Sun along the x-axis, and proportionality along the y-axis.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ZSg7t.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZSg7t.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-31T11:58:42.950" LastActivityDate="2016-01-31T15:39:06.943" Title="A more accurate way of solar sail calculation?" Tags="|solar-sail|calculation|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13303" PostTypeId="1" AcceptedAnswerId="13310" CreationDate="2015-12-31T12:37:03.887" Score="21" ViewCount="2213" Body="&lt;p&gt;Re-reading Linenger's &lt;a href=&quot;https://books.google.com/books/about/Off_the_Planet.html?id=o37S0BIftyQC&quot;&gt;Off the Planet&lt;/a&gt; and encountered a question from long ago that I never had found an answer to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During Mir's troubled old age, the attitude control computer would fail frequently.  The station then immediately started rotating and the solar panels stopped tracking the sun. This resulted in a long, multi-orbit process of slowly recharging the batteries and essentially rebooting the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But...why did Mir start to rotate immediately when the computer failed?  The lack of control in and of itself wouldn't cause the station to rotate.  There must have been some external torque that the system was constantly compensating for, but what?&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="25" LastEditDate="2015-12-31T17:49:21.863" LastActivityDate="2015-12-31T19:26:38.870" Title="When Mir's attitude control computer failed, why did the station immediately start rotating?" Tags="|crewed-spaceflight|attitude|space-station|mir|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13304" PostTypeId="2" ParentId="13303" CreationDate="2015-12-31T12:59:18.003" Score="11" Body="&lt;p&gt;It doesn't have to be an external torque, consider for example the crew changing position inside the space station, although the maximal torque provided by moving the entire crew to the Kvant-1 is only going to change the rotation rate by 0.15%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; This is not violating the conservation of momentum, as they are simply changing the centre of mass in an already rotating station. (A space station does usually have one side always facing in the orbital direction.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2015-12-31T19:26:38.870" LastActivityDate="2015-12-31T19:26:38.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13306" PostTypeId="1" AcceptedAnswerId="13308" CreationDate="2015-12-31T15:17:06.053" Score="3" ViewCount="1260" Body="&lt;p&gt;I know that near the equator where the inclination is 0° it is easier to put payloads to low earth orbit because of the advantage of rotation speed of earth which saves more fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saving more fuel means more fuel for the payload, so you can burn longer and put a heavier payload in low earth orbit compared to a launch in a higher place where inclination is for example 56° or 82° and you can't save as much fuel as inclinations near 0° or for example 28.5° (Cape Canaveral US).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some web sites I've read that a heavier payload is attained at a higher inclination: Soyuz 2.1v, &lt;a href=&quot;http://spaceflight101.com/spacerockets/soyuz-2-1v/&quot; rel=&quot;nofollow&quot;&gt;2800kg in 62.8° and 3000kg in 82.4°&lt;/a&gt; or &lt;a href=&quot;http://www.russianspaceweb.com/soyuz1_lv_development.html&quot; rel=&quot;nofollow&quot;&gt;3000kg in 62.8°, 3100kg in 67.1°&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess in fact it should be the opposite: heavier payload in lower inclination. Is that due to rocket ability or trajectory and why?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="6816" LastEditDate="2015-12-31T19:03:10.837" LastActivityDate="2015-12-31T19:03:10.837" Title="Space rocket payload by inclination" Tags="|orbital-mechanics|low-earth-orbit|payload|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13308" PostTypeId="2" ParentId="13306" CreationDate="2015-12-31T15:46:24.913" Score="5" Body="&lt;p&gt;Most optimal is launching into same target orbital inclination as the site's latitude, so Earth's rotation on its axis can be used to maximum advantage (&lt;em&gt;free&lt;/em&gt; delta-v is roughly equatorial speed of Earth's rotation times cosine of the latitude, and small adjustment for Earth's flattening which has minimal effect) and no &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination_change&quot; rel=&quot;nofollow noreferrer&quot;&gt;plane change&lt;/a&gt; needs to be done once in orbit. Those links you provide include payload mass for launches from &lt;a href=&quot;https://en.wikipedia.org/wiki/Baikonur_Cosmodrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Baikonur&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Plesetsk_Cosmodrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Plesetsk&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plesetsk is at roughly 63° North latitude (&lt;em&gt;free&lt;/em&gt; delta-v of ~ &lt;a href=&quot;https://encrypted.google.com/#q=%28%282+*+pi+*+6378.14+km+%2F+0.99726968+day%29+*+cos%2862.8+deg%29+*+%281+-+0.0033528%29%29+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;763 km/h&lt;/a&gt; if launched with 0° azimuth), so it's indeed surprising that the highest mass to orbit is given by &lt;a href=&quot;http://spaceflight101.com/spacerockets/soyuz-2-1v/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight101&lt;/a&gt; with 3,000 kg for inclination of 82.4° and 2,800 kg for 62.8°, and without explanation and cited source for those figures, I would assume that it's a typo, as &lt;a href=&quot;http://www.russianspaceweb.com/soyuz1_lv_development.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anatoly Zak's page&lt;/a&gt; suggests (3,000 kg to 62.8°, quoting &lt;em&gt;Soyouz-2, de la phase 1V (in French), Samara Space Centre, 2010&lt;/em&gt; as his source). The difference of 100 kg more to 67.1° orbit is minimal and could be attributed to launch vehicle optimizations as it evolved. Other payload masses to different inclinations on record match such explanation, but the payload mass goes on average lower with difference between launch site latitude and target orbital inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Baikonur is however a bit of an exception with optimal target inclination of 56.8° despite the site being at roughly 46° (for which &lt;em&gt;free&lt;/em&gt; delta-v would be ~ &lt;a href=&quot;https://encrypted.google.com/#q=%28%282+*+pi+*+6378.14+km+%2F+0.99726968+day%29+*+cos%2846+deg%29+*+%281+-+0.0033528%29%29+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;1,159 km/h&lt;/a&gt;), because there's China's territory directly due East. So the launch azimuth is adjusted a bit more North to avoid overflying Chinese airspace (minimum adjustment of 5.65° would mean &lt;em&gt;free&lt;/em&gt; delta-v of ~ &lt;a href=&quot;https://encrypted.google.com/#q=%28%282+*+pi+*+6378.14+km+%2F+0.99726968+day%29+*+cos%2846+deg%29+*+sin%285.65+deg%29+*+%281+-+0.0033528%29%29+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;114 km/h&lt;/a&gt; less, so roughly 1,045 km/h). That's also one of the &lt;a href=&quot;https://space.stackexchange.com/a/5298/49&quot;&gt;reasons why the ISS orbit is at such high inclination&lt;/a&gt; (tho there are other advantages to it, too):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/eEWBl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Rockets coming from the Baikonur Cosmodrome, Kazakhstan and launching towards the International Space Station are sent into orbits inclined 51.65 degrees to avoid overflying the territory of China.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some other launch sites also always launch at an azimuth, for example the   Palmachim Airbase launching &lt;a href=&quot;https://en.wikipedia.org/wiki/Shavit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shavit rockets&lt;/a&gt; always into polar inclination to, well, avoid inadvertently starting war with its neighbors. And some other launch sites might require slight adjustment to launch azimuth or employ the so-called dogleg maneuvers for similar reasons.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2015-12-31T16:25:21.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13310" PostTypeId="2" ParentId="13303" CreationDate="2015-12-31T16:32:38.540" Score="26" Body="&lt;p&gt;Most such spacecraft, including Skylab and the ISS, have their attitude maintained by reaction wheels. These wheel essentially convert the rotational energy of the entire spacecraft into a smaller reaction wheel. If the attitude failed while these wheels were spun up, then they would eventually slow down (In a timeframe of minutes). As the wheels slowed down, the station would begin rotating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MIR did have reaction wheels, as can be seen from &lt;a href=&quot;http://www.russianspaceweb.com/mir_2001.html&quot;&gt;this site&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is very common for these wheels to be spun up, as the attitude of the station must be properly maintained for thermal and power reasons, and varies depending on where in the orbit it is found.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2015-12-31T16:32:38.540" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13312" PostTypeId="1" CreationDate="2015-12-31T19:04:58.743" Score="5" ViewCount="203" Body="&lt;p&gt;After reading the answers from &lt;a href=&quot;https://space.stackexchange.com/q/13303&quot;&gt;this&lt;/a&gt; question and reading the excellent article &lt;a href=&quot;http://www.russianspaceweb.com/mir_2001.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; that goes through the de-orbiting in detail, I wondered if any debris had been found after it plunged into the ocean?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was skeptical when I read &lt;a href=&quot;http://boston.cbslocal.com/2013/06/14/rock-found-in-amesbury-backyard-came-from-space-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; CBS News report of a person in Massachusetts, USA finding a piece of Mir. (How did it get there?) Most of the other Google results are about debris in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the ocean is probably pretty deep where Mir came down and it is ~1,100 miles from the coast of New Zealand, so perhaps it is not very likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has any other debris been found?&lt;/p&gt;&#xA;" OwnerUserId="7041" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2015-12-31T19:04:58.743" Title="Has any (additional) debris from Mir de-orbiting been found?" Tags="|reentry|debris|mir|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13313" PostTypeId="1" CreationDate="2015-12-31T21:36:53.240" Score="9" ViewCount="455" Body="&lt;p&gt;On the floors of young craters of the Moon, smoothly rounded domes are visible in some spots. &lt;a href=&quot;http://bit.ly/1mmfRZJ&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lalande crater&lt;/a&gt; has 4 of them, each around 200 m across.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LO3Z8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LO3Z8.jpg&quot; alt=&quot;domes on floor of Lalande crater&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nuGPH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nuGPH.jpg&quot; alt=&quot;Lalande crater floor in the evening&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://bit.ly/1kwXYFq&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here are&lt;/a&gt; less regular ones on the floor of Copernicus crater. The most pronounced one, in the middle, is 500 m across.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qC9Fl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qC9Fl.png&quot; alt=&quot;domes on floor of Copernicus crater&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ones in Lalande in particular are quite round and regular, and make me think of bubbles on the surface of a thick liquid. I have looked for material on such domes without luck. I asked &lt;a href=&quot;http://www.spudislunarresources.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Paul Spudis&lt;/a&gt; if they could be hollow when he appeared on a recent &lt;a href=&quot;http://www.thespaceshow.com/show/11-dec-2015/broadcast-2602&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Show episode&lt;/a&gt; , and he said yes. (Relevant portion starts at 36:45, with a discussion of impact melts, and then gets into the domes.) He indicated we can't know but it is possible. I wish I'd followed up with more questions, but I thought I'd be taking too much time away from other people with questions, and left it. I won't make that mistake again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anything more specific be said about such domes, for instance based on similar dome morphology on the Earth or other bodies? If they are hollow, would it be the case of a hollow disk at the top of them, or a whole spherical bubble deep inside? If they are solid, then what would be the nature of their development? &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2018-07-19T20:20:41.897" LastActivityDate="2018-07-19T20:20:41.897" Title="How do domes in lunar impact melts develop, and might they be hollow?" Tags="|the-moon|planetary-science|" AnswerCount="0" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="13314" PostTypeId="1" CreationDate="2015-12-31T23:03:53.720" Score="8" ViewCount="555" Body="&lt;p&gt;SpaceX got access to LC-13, a former Atlas launch pad, to convert to LZ-1 (Landing Zone 1) for landing Falcon 9 first stages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What considerations went into the decision on using LC-13? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly the Cape Canaveral folk were involved in deciding which pad was safest. SpaceX obviously had requirements in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I initially thought it would be the furthest south, and thus furthest away from LC-39A/B (Old Shuttle pads, now SpaceX LC-39A and 39B multipurpose/SLS), the VAB, LC-41 (Atlas 5 pad), LC-40 (SpaceX pad), and all the active infrastructure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But looking at this map, it is clear that there are many more further south options in Missile Row.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eWNdy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eWNdy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-01T05:43:27.667" Title="Why did SpaceX use LC-13 as LZ-1?" Tags="|spacex|landing|reuse|falcon-9|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13315" PostTypeId="2" ParentId="13314" CreationDate="2016-01-01T01:07:57.213" Score="4" Body="&lt;p&gt;It's nowhere near a complete answer, but according to the &lt;a href=&quot;http://www.patrick.af.mil/shared/media/document/AFD-141107-004.pdf&quot; rel=&quot;nofollow&quot;&gt;environmental assessment&lt;/a&gt;, it looks like they just chose one far from other active sites:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Complexes at KSC were considered; however, no sites were readily available or within reasonable distances from the launch and stage refurbishment location. Therefore, there were no reasonable landing sites available at KSC. Several complexes at CCAFS were also considered, many of which have been inactive for numerous years. Specific facilities considered but removed from further analysis include LC-1, LC-36A and B, LC-46 (both of which are currently used by Space Florida), LC-2, LC-12, and LC 47. These facilities are located along the coast and away from inhabited areas and structures; however they are all not available due to existing operational requirements and incompatibility with previously planned actions. The USAF would have to end operations or have them curtailed. Based on current USAF operational needs, and the potential for significant additional demolition and re-construction at some facilities, therefore these locations were not considered reasonable alternatives.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-01-01T01:07:57.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13317" PostTypeId="2" ParentId="13314" CreationDate="2016-01-01T05:43:27.667" Score="0" Body="&lt;p&gt;The furthest to the South is also the closest to Cape Canaveral, making it less likely to be the chosen location. LC13 or 14 are about the same distance away from active launch pads, and thus the best choice among the listed locations.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-01T05:43:27.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13318" PostTypeId="1" CreationDate="2016-01-01T11:56:30.673" Score="4" ViewCount="88" Body="&lt;p&gt;Decompression sickness is a significant risk to both current and future astronauts and currently significant time is lost in preparation for extra-vehicular activity on the ISS [including airlock utilisation for nitrogen-washout and pure oxygen cycle ergometer pre-breathe]. In the event of sudden decompression [such as micro meteor strike] astronauts in the ISS could seal off a section of the ISS or retire to the Soyuez. Theoretically, if this was not an option [or not a possibility such as aboard a smaller space-craft on an ECM] astronauts would be at significant risk of decompression sickness and subsequent morbidity/mortality if they just donned their EVA suits. [Suits at 100% O2 and 4.3psi].&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have been reading the information on both the Dragon and Orion module. The Dragon states an 'internal pressurised atmosphere' and the Orion module states 'pressurised' and then clearly shows Nitrogen storage which could only be for atmospherics. In my opinion this suggests both are using an Earth-equivalent atmosphere [21% O2, 79% N2, at 14.7psi or roughly sea-level pressure]. This is the same as the ISS and reduces risk of fire [such as the 100% oxygen atmosphere of Apollo 1].&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is as follows; Given the time lost in EVA / future surface activity preparation with the current atmospherics, would a change in internal atmosphere not significantly reduce this 'lost time'. If so would it not also be of great benefit in reducing risk during sudden decompression [in the event of emergency egress/EVA suit use] or even in an extra-vehicular emergency requiring immediate, unplanned EVA [such as an injured astronaut on EVA]. I have found one example of work using an atmosphere tri-mix [Helium] to reduce this risk, but little other work. Does anyone else have any knowledge on this subject?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note I know that there are some 'zero-prebreathe' suits in development such as the Mark III and Z-series. I am more interested in the possible changes/research in atmospherics or environmentals, or the prevention of decompression sickness for current spacecraft, if any is ongoing at all now that these suits are in development.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TLDR: Is anyone researching or developing new ways to prevent decompression sickness for astronauts other than the new 'zero-prebreathe' suits currently in development?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many thanks for any responses and I hope this makes sense!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Links below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Helium Tri-mix discussion: &lt;a href=&quot;https://astronautics.usc.edu/assets/003/70954.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://astronautics.usc.edu/assets/003/70954.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion Capsule Datasheet:&#xA;&lt;a href=&quot;https://www.nasa.gov/pdf/306407main_orion_crew%20_expl_vehicle.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://www.nasa.gov/pdf/306407main_orion_crew%20_expl_vehicle.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12871" LastActivityDate="2016-01-01T11:56:30.673" Title="Atmospherics for 'next generation' manned space flight / Exploratory Class Missions" Tags="|spacex|crewed-spaceflight|research|orion-spacecraft|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13319" PostTypeId="1" AcceptedAnswerId="13320" CreationDate="2016-01-01T14:07:51.740" Score="9" ViewCount="264" Body="&lt;p&gt;I've always been a fan of satellites like &lt;a href=&quot;http://www.csr.utexas.edu/grace/&quot; rel=&quot;noreferrer&quot;&gt;GRACE&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Recovery_and_Interior_Laboratory&quot; rel=&quot;noreferrer&quot;&gt;GRAIL&lt;/a&gt;, where two satellites orbit a body, precisely measuring the changing distance between them in order to sense gravitational anomalies caused by near-surface density changes. I also understand how you can do the same with a single satellite such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Dawn&lt;/a&gt;, by doing sensitive &lt;a href=&quot;https://en.wikipedia.org/wiki/Doppler_effect&quot; rel=&quot;noreferrer&quot;&gt;Doppler&lt;/a&gt; measurements on the radio telemetry link to precisely measure its movements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Dawn is reported to have used this technique to determine that &lt;a href=&quot;https://en.wikipedia.org/wiki/4_Vesta&quot; rel=&quot;noreferrer&quot;&gt;Vesta&lt;/a&gt; has a &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/marc-rayman/20130131-vesta-mountains-gorges-craters.html&quot; rel=&quot;noreferrer&quot;&gt;dense iron core&lt;/a&gt;. This surprises me, because by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shell_theorem&quot; rel=&quot;noreferrer&quot;&gt;shell theorem&lt;/a&gt;, a spherically symmetric body's gravity should be identical to that of a different body whose mass is concentrated at its center. As long as the mass is spherically symmetric, you shouldn't be able to determine its distribution, because spherically symmetric changes in the mass distribution shouldn't change the resulting gravitational field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Somehow, my understanding is wrong. How can the motions of an orbiting spacecraft be used to measure the deep structure of mass in the orbited body?&lt;/p&gt;&#xA;" OwnerUserId="11823" LastEditorUserId="49" LastEditDate="2016-01-01T19:46:41.137" LastActivityDate="2016-01-04T05:02:57.370" Title="How can an orbiting spacecraft's motions yield the orbitee's deep structure?" Tags="|artificial-satellite|planetary-science|gravity|measurement|gravity-fields|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13320" PostTypeId="2" ParentId="13319" CreationDate="2016-01-01T16:40:02.293" Score="8" Body="&lt;p&gt;Gravimetric Doppler measurements can actually confirm that Vesta has an iron core, as it eliminates the alternatives. This works in the following way:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The mass of Vesta is first measured accurately, for example using the orbital period of a satellite, for instance the Dawn spacecraft.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Then, based on observed geological features, its average density, the oblateness of Vesta compared to its rotation rate, the location of the poles, and various assumptions about its internal structure, based on other objects in hydrostatic equilibrium, you can give a good estimate on its &lt;a href=&quot;https://en.wikipedia.org/wiki/Moment_of_inertia_factor&quot; rel=&quot;nofollow&quot;&gt;moment of inertia&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;From that, you can conclude that the inner structure of Vesta must contain one or more regions with higher density, and get a relatively good impression on how they are located relative to each other.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If this more dense matter is arranged in some unsymmetrical way (e.g. not spherical), Doppler observations will then detect the non-uniformity of Vesta’s gravitational field, based on small changes in Dawn's orbit, as the orbit then does not follow that of a spherical body. The observations by Dawn indicated that the dense matter must be assembled in a spherical core in the middle.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;That confirms the existence of a core. But as you state, Newtons shell theorem makes it difficult to measure accurately the size of the core, or if it is layered in regions with different densities. To achieve more data on Vesta's internal structure, you have to land probes on the surface and measure how the shock waves from an earthquake propagate through the planetoid. Exact gravimetry has its limitations, and good seismic data is definitely going to improve the geological assumptions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some useful references:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www-geodyn.mit.edu/konopliv_dawngravity_ssr11.pdf&quot; rel=&quot;nofollow&quot;&gt;The Dawn Gravity Investigation at Vesta and Ceres&lt;/a&gt;: a detailed paper on the gravimetric techniques used by the Dawn science team&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.astronomynotes.com/solarsys/s8.htm&quot; rel=&quot;nofollow&quot;&gt;Planetary Interiors&lt;/a&gt;: a chapter from the Astronomy Notes website that talks about the various techniques for investigating the interiors of other planets&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="11823" LastEditDate="2016-01-03T14:27:27.123" LastActivityDate="2016-01-03T14:27:27.123" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13321" PostTypeId="2" ParentId="13286" CreationDate="2016-01-01T19:58:37.723" Score="6" Body="&lt;p&gt;Adding to Russell's answer, Falcon 9 apparently has &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3yyx13/elon_on_twitter_falcon_9_back_in_the_hangar_at/cyhw6xn&quot; rel=&quot;nofollow&quot;&gt;3/16''-thick walls&lt;/a&gt; (4.7mm). This is interesting, because it's more than Shuttle's ET and about as much as parts of S-IC, despite being a smaller rocket. Presumably this is for extra structural margins to make the stage survive multiple reuse.&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="10274" LastEditDate="2016-05-23T06:51:08.590" LastActivityDate="2016-05-23T06:51:08.590" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13322" PostTypeId="2" ParentId="13302" CreationDate="2016-01-02T14:47:13.917" Score="2" Body="&lt;p&gt;The usual proportionality law for solar sails is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\cos^2\left(\alpha\right)}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\alpha$ is the sails angle from zenith, and $r$ is the distance from the Sun. This assumes that the Sun a point source of light, and that the size of the sail is negligible compared to the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can simply be decomposed into&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\cos^3\left(\alpha\right)}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for the vertical component, and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\cos^2\left(\alpha\right)\sin\left(\alpha\right)}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for the horizontal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A naive solution for this is simply combining the $\cos^2\left(\alpha\right)$,$\cos^3\left(\alpha\right)$ and $\cos^2\left(\alpha\right)\sin\left(\alpha\right)$ with the modified law, however this is not completely accurate, but it is a little bit better than the point source assumption.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a per unit of angle solution, we must make a decision about if we assume that the backside of the sail is reflective or not. Why this is important can be illustrated in the following way:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GeAwUm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GeAwUm.png&quot; alt=&quot;light hitting both sides of the sail&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can be decomposed multiplying with either $\cos\left(\alpha\right)$ or $\sin\left(\alpha\right)$ as usual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Important&lt;/strong&gt; this is a formula per unit of angle, so to get a law similar to $\frac{\cos^3\left(\alpha\right)}{r^2}$, you must integrate your decided function per degree angle for the whole range. This is possible, as the function obviously has an enclosed area. Total acceleration in green, vertical in blue and horizontal in orange. Y-axis is proportionality, X-axis is the different angles of illumination. In this case, the angle of the sail is 20 degrees, and the distance from the centre of the Sun is 2 solar radii. Note the part where the horizontal acceleration is negative, due to light hitting the backside of the sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IkH1b.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IkH1b.png&quot; alt=&quot;graph showing decomposed acceleration&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I gave up calculating the integral after about an hour, as did my CAS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For any practical calculation, the usual solar sail equations are accurate enough, as you have to be extremely close to the Sun for any error by the point light source to be detectable.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="13262" LastEditDate="2016-01-31T15:39:06.943" LastActivityDate="2016-01-31T15:39:06.943" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13323" PostTypeId="1" CreationDate="2016-01-02T17:12:21.020" Score="9" ViewCount="265" Body="&lt;p&gt;I was talking to someone who anticipated digging for water ice and other volatiles would be easy in these areas because Pete Schultz of the &lt;a href=&quot;http://lcross.arc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;LCROSS&lt;/a&gt; science team described them as 'fluffy'.&lt;/p&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;http://www.universetoday.com/76365/understanding-the-unusual-lcross-ejecta-plume/&quot; rel=&quot;noreferrer&quot;&gt;this Universe Today article&lt;/a&gt; he explains that he concludes that based on how the surface reacted when the empty Centaur stage hit it to create the debris plume the LCROSS instruments measured. There was a delay before the flash of that impact in the infra-red range was detected:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;We knew when it was going to hit the surface – we know how fast how we&#xA;were going and where we were above the surface — and it turned out&#xA;there was a delay before we saw the flash and that was really a&#xA;surprise... It was about a half second delay and then it&#xA;took about a third of second delay before it began to rise and get&#xA;brighter. The whole thing took seven-tenths of a second before it&#xA;began to get bright. That is hallmark of a fluffy surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Having heard a few talks by experts on polar volatile deposits, I had the impression the nature of the deposits is still unknown, and had thought it likely that digging for them would be more like digging into frozen tundra than into sort of a dirty snow. Is that fluffy surface layer a generally accepted conclusion, or are there different viewpoints?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-10-09T08:21:53.533" Title="Are the surfaces in permanent shadow in Moon craters 'fluffy'?" Tags="|the-moon|water|lcross|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13324" PostTypeId="5" CreationDate="2016-01-02T17:31:44.023" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-01-02T17:31:44.023" LastActivityDate="2016-01-02T17:31:44.023" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13325" PostTypeId="4" CreationDate="2016-01-02T17:31:44.023" Score="0" Body="The Lunar CRater Observation and Sensing Satellite mission, which measured the ejecta plume of a planned impact in a polar crater of the Moon that is in permanent shadow.The plume contained some water, and compounds containing nitrogen, hydrogen, and carbon." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-01-02T17:31:44.023" LastActivityDate="2016-01-02T17:31:44.023" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13326" PostTypeId="1" AcceptedAnswerId="13327" CreationDate="2016-01-02T13:28:43.810" Score="1" ViewCount="2535" Body="&lt;p&gt;Why do the satellites never run out of power do they have batteries that are charged by sunlight everyday? Also they transmit so much information across the globe.&lt;/p&gt;&#xA;" OwnerUserId="12770" OwnerDisplayName="sonu kumar" LastEditorUserId="8162" LastEditDate="2016-01-02T19:59:02.807" LastActivityDate="2016-03-16T03:04:10.220" Title="Why do satellites never run out of power?" Tags="|artificial-satellite|power|solar-power|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13327" PostTypeId="2" ParentId="13326" CreationDate="2016-01-02T16:48:44.533" Score="5" Body="&lt;p&gt;Actually, satellites may kind of run out of power when their solar panels stop working properly because they have degraded with age. Then we lose connection with them and they become just trash in orbit which is &lt;a href=&quot;http://www.bu.edu/synapse/2012/10/26/space-debris-a-universal-problem/&quot; rel=&quot;nofollow&quot;&gt;endangering&lt;/a&gt; other satellites and most importantly the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, satellites use solar energy and it is enough, since those in space are much more efficient. At 500 kilometers a satellite is not exposed to &lt;strong&gt;direct&lt;/strong&gt; sunshine for more than 38% of the time. Also on Earth you encounter with cloudy weather, storms and so on.&lt;/p&gt;&#xA;" OwnerUserId="12888" OwnerDisplayName="Vojta Klimes" LastEditorUserId="1142" LastEditDate="2016-03-16T03:04:10.220" LastActivityDate="2016-03-16T03:04:10.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13328" PostTypeId="1" CreationDate="2016-01-02T20:11:07.220" Score="0" ViewCount="77" Body="&lt;p&gt;I recently watched, The Martian and was wondering about some of the manoeuvres and propulsion in space.&lt;/p&gt;&#xA;&#xA;&lt;blockquote class=&quot;spoiler&quot;&gt;&#xA;  &lt;p&gt; Specifically  the manoeuvre the Ares 3 crew use in the scene where they pick up Mark Watney after he leaves Mars' atmosphere. The crew blow an airlock with an IED in order to slow their relative speed with the MAV and in the same vein, Watney then uses the same principle when he does his 'Iron Man' stunt, using the escaping air to propel himself towards the Ares 3.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;My question is, how is this possible? How can escaping an escaping gas (from a pressurised container, a spacecraft or a suit), venting into a vacuum, produce thrust with no particles to act against?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen similar depictions of this in, for example, Battlestar Galactica. The Colonial Vipers vent some kind of gas to aid manoeuvrability. &lt;/p&gt;&#xA;" OwnerUserId="12879" LastEditorUserId="12879" LastEditDate="2016-01-02T20:58:49.233" LastActivityDate="2016-01-02T21:33:49.250" Title="Does releasing a gas propellant in a vacuum produce thrust? [Martian Spoilers]" Tags="|propulsion|spacecraft|thrust|" AnswerCount="0" CommentCount="9" ClosedDate="2016-01-02T23:09:44.407" ContentLicense="CC BY-SA 3.0" />
  <row Id="13329" PostTypeId="1" AcceptedAnswerId="13344" CreationDate="2016-01-02T20:22:07.447" Score="2" ViewCount="214" Body="&lt;p&gt;If you are orbiting below the surface of a planet, and ignore drag from rocks (or orbit in a vacuum tunnel), what is the shape of the orbit? Usually, the proportionality law for gravity is $\frac{1}{r^2}$, but in this case, it scales with the radius $r$, increasing in strength when you move away from the centre. The potential energy is simple to calculate, as is the angular momentum, but the overall shape is not completely obvious to me. From simulating this using time increments, it seem to me that the shape is an ellipse, with the centre of the planet as the centre (not focus). However, that is just speculation on my part as such a method does not give any reliable results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Simplifications made:&lt;/strong&gt; Uniform density, planet not rotating, orbiting body's mass negligible to main, and orbiting object not influenced by other forces like drag. Of course I also appreciate answers covering those factors too, and also how an object orbiting both inside and outside the planet behaves. But that is just a bonus&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example of an orbit with gravity proportional to $r$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WMUFMm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WMUFMm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-03T17:10:10.563" LastActivityDate="2016-01-03T17:10:10.563" Title="Shape of an orbit inside a planet" Tags="|orbital-mechanics|sub-orbital|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13331" PostTypeId="1" AcceptedAnswerId="13375" CreationDate="2016-01-02T22:37:37.227" Score="14" ViewCount="4216" Body="&lt;p&gt;Wikipedia claims that space suits that use pure oxygen are pressurized to 4.7 psi, instead of 3.0 psi, to account for carbon dioxide and water vapor pressure as per the &lt;a href=&quot;https://en.wikipedia.org/wiki/Alveolar_gas_equation&quot;&gt;alveolar gas equation&lt;/a&gt; (although noting that the calculation has slight overcorrection).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA EMU is pressurized to 4.3 psi according to many sources. The rationale given for this is the same: to give the astronaut the same amount of oxygen as for normal air per the alveolar gas equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for others, A7L Skylab space suits were pressurized to 3.7 psi. Russian Orlan space suits are pressurized to 5.8 psi.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, none of these values seem to fit when actually viewing the alveolar gas equation. If breathing pure oxygen, the alveolar gas equation becomes simply:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\require{mhchem}p_A\ce{O2} = P_{atm} - p\ce{H2O} - p_a\ce{CO2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $p\ce{H2O}$ is 6.28 kPa at 37 °C and $p_a\ce{CO2}$ is normally 40 mmHg. Given that 104 mmHg is considered normal for $p_A\ce{O2}$, this is achieved with a pressure of 3.7 psi for pure oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why 4.3 psi specifically? Are there benefits to having a slightly hyperoxic environment? Is this to allow for slow leaks or partial depressurization without the astronaut becoming hypoxic?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will obviously be very happy to receive corrections if I have made a mistake in my calculations or if there's some physiological phenomena here that I don't understand. I would love to get a proper answer from someone who actually &lt;em&gt;knows&lt;/em&gt; the real reason, but forgoing that I would be thankful for any well-founded arguments as to why this might be so.&lt;/p&gt;&#xA;" OwnerUserId="2631" LastEditorUserId="49" LastEditDate="2016-01-02T23:17:59.057" LastActivityDate="2019-02-02T23:30:37.930" Title="Why is the EMU space suit pressurized to 4.3 psi specifically?" Tags="|spacesuits|oxygen|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13333" PostTypeId="1" AcceptedAnswerId="13336" CreationDate="2016-01-03T01:44:39.427" Score="1" ViewCount="735" Body="&lt;p&gt;My main question is whether GPS receivers use trilateration or the distances between satellites. On GPS.gov and their videos, it says you receivers use trilateration. But, I have seen people say that they use the distances between satellites. I would like to know which is correct, I have a question if GPS receivers use trilateration. How do GPS receivers get time synchronization if they need to use how long it takes for a signal to make it to them to calculate their location (I know it has been explained in other threads (&lt;a href=&quot;https://space.stackexchange.com/questions/5423/how-does-gps-receiver-synchronize-time-with-gps-satellites&quot;&gt;Here&lt;/a&gt;) but none have been real clear to me how it does it (Explain it if you can please.))? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is my other question if GPS receivers use the distance between satellites, how does it do it? A diagram would be useful if anyone has it. Also, why would GPS.gov say that receivers use trilateration if they actually don't?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Again, I know this is a 'duplicate thread' but no other threads have explained to me in a way that I can understand it.&lt;/strong&gt; &lt;/p&gt;&#xA;" OwnerUserId="12885" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-06-25T17:31:47.610" Title="Do GPS receivers use trilateration or distances between satellites?" Tags="|navigation|gps|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13334" PostTypeId="2" ParentId="13333" CreationDate="2016-01-03T01:58:34.130" Score="0" Body="&lt;p&gt;GPS receives use the distance between the user and the satellite, not the distance between satellites. In fact, the name for this is &lt;a href=&quot;https://en.wikipedia.org/wiki/Trilateration&quot; rel=&quot;nofollow&quot;&gt;trilateration&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If time isn't well known, then one can use an additional satellite to figure out the time, much in the same way that 3 satellites gives the position in 3 axis, the 4th allows for figuring out the time, the 4th dimension.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-01-03T02:16:05.607" LastActivityDate="2016-01-03T02:16:05.607" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13335" PostTypeId="2" ParentId="13333" CreationDate="2016-01-03T02:45:47.427" Score="2" Body="&lt;p&gt;Each satellite provides the receiver with a time value $t_s$ and its (the satellites) position, $x_s, y_s$ and $z_s$. Consider that you are receiving data from $4$ such satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time value will be larger the further away the satellite is. Using the time values, you can find the difference in delays between the satellites. If you know the difference in delays between the satellites and know the position of each satellite, you can accurately calculate your own position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This process does not require being perfectly synced to the timing of a satellite, since it's the time differences that are the key.&lt;/p&gt;&#xA;" OwnerUserId="12877" LastActivityDate="2016-01-03T02:45:47.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13336" PostTypeId="2" ParentId="13333" CreationDate="2016-01-03T03:57:30.173" Score="6" Body="&lt;p&gt;While PearsonArtPhoto answered this correctly the discussion on his answer shows real question seems to be &lt;strong&gt;how&lt;/strong&gt; it works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lets start with a simple case here on Earth.  You have a bell in the town square that rings at noon.  You hear the bell, look at your watch and it says 12:00:02.  (I'm assuming perfect timepieces and zero reaction time here.)  What can you conclude from that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A reasonable value for the speed of sounds is 1125 ft/sec.  You heard it 2 seconds after it rang, you can conclude that you are somewhere on the circle 2250 feet from the bell--not very useful unless you want to know how long it will take to walk to the town square.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now lets add another bell to the picture.  This bell is placed 2000' from the original bell and rings at the same time but on a different frequency so you can tell them apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bells ring.  You hear the first bell at 12:00:02 and the second at 12:00:03.  Now we have something useful.  We know we are 2250 from the first bell and 3375 from the second.  Take a map, draw both circles on it.  You are standing where they intersect.  Two circles inherently intersect at exactly two points (assuming they intersect at all, something we know will happen if everybody's timepiece is accurate.)  You are on one of those intersections, although you can't tell which without additional data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, lets make it more complex--you're a bird instead of a person.  The bells ring, you get the same measurements.  Now, however, instead of drawing circles around each bell you have to draw spheres.  The intersection of two spheres is a circle--we are back to the same problem we had on the ground with one bell.  Once again, we fix it the same way--add another bell.  Now we are looking at the intersection of that circle with the third sphere--once again, two points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note, however, that this requires everyone to have an accurate timepiece.  Since the GPS system requires time accuracy on the order of a few nanoseconds for normal civilian accuracy that's a tall order.  You don't put such timepieces in your pocket!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do we fix this?  The same way we keep fixing it as we add dimensions--add another bell.  We don't know how big to draw the spheres anymore but we still do know the differences between their sizes.  We start out assuming the sound we heard first was right there and try to solve the problem--of course we can't.  Keep increasing the size until we do find a solution.  We will end up with four spheres that intersect in exactly two points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the GPS system uses a radio broadcast instead of a bell but other than the vastly higher transmission speed the math is exactly the same.  Yes, the GPS system produces two possible answers for your location but we do have one more piece of data available that resolves it--your receiver assumes you are on Earth.  The satellites are 12,550 miles up--which puts the second solution roughly 25,000 miles up.  The receiver discards this obviously bogus answer and gives you the one that's somewhere near the Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the real system is more complex than this because lightspeed is only constant in a vacuum.  Thus the satellites also transmit an atmospheric weather report (not sun/wind/rain, but how much slowing is going to happen) and your receiver listens to these reports and figures out how to correct for this slowing.  (These weather reports are on a 30 second cycle which imposes a 30 second minimum time for your GPS to acquire after it's turned on.  In practice the acquisition time is longer because of the games they play to allow the system to work at space-transmitter power levels while using simple omnidirectional antennas on the receivers.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mil-spec GPS systems use a different approach.  There are actually two signals being broadcast by every GPS satellite, your civilian unit ignores the second.  A military unit has the crypto keys to read that second signal.  This is on a substantially different frequency than the primary signal and thus is affected differently by the atmosphere.  The mil-spec receiver can use the difference between the two signals to figure out it's own atmospheric corrections without waiting for the weather report.  This also allows the government to deliberately introduce error into the civilian signals while not messing up the military signals--useful in wartime if you want to deny the enemy accurate navigation.  This capability has never been used, though--too many soldiers had their own civilian GPS units in Desert Storm.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-01-03T03:57:30.173" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13337" PostTypeId="1" AcceptedAnswerId="13338" CreationDate="2016-01-03T06:30:22.793" Score="8" ViewCount="1814" Body="&lt;p&gt;In Kerbal Space Program, most liquid fueled engines include a generator which provides electricity to systems on the rocket, keeping the batteries topped up while on ascent. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do any real rockets (contemporary or historic) use such a system? I would imagine it's easier to just size the batteries or fuel cells appropriately for the duration of the launch and avoid the complexity of putting a generator on the turbopumps. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-01-10T13:55:45.347" Title="Alternator/generator on launcher engines?" Tags="|rockets|power|liquid-fuel|kerbal-space-program|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13338" PostTypeId="2" ParentId="13337" CreationDate="2016-01-03T06:36:20.273" Score="4" Body="&lt;p&gt;Apparently &lt;a href=&quot;https://en.wikipedia.org/wiki/Flux_switching_alternator&quot; rel=&quot;nofollow&quot;&gt;flux switching alternators&lt;/a&gt; are used on some guided missiles (e.g. AIM-7 Sparrow), which may have to push a lot of power through a radar antenna during their short flight. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-01-03T06:36:20.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13339" PostTypeId="1" CreationDate="2016-01-03T09:20:56.430" Score="1" ViewCount="155" Body="&lt;p&gt;Is the speed of gravity instant? By the speed of gravity, I'm not talking about acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I mean is something like this:&#xA;If I take a vacuum, for example, and put in two objects of the same size, shape, and mass, exactly 1 light year apart. In this vacum, no other objects exist anywhere. Therefore, these two objects will have gravity towards each other. How long will it take before the force of gravity to &lt;em&gt;start&lt;/em&gt; acting? One year, (as it's the speed of light) or instantly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under both conditions, I have question expanding on them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is instant, then could we theoretically have a machine sensitive enough to pick up gravity of objects outside of our observable universe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's not, then what would happen if an object with high enough gravity (Like super-super-super massive black hole) were to suddenly enter our observable universe. Then would we be hugely affected? What would happen?&lt;/p&gt;&#xA;" OwnerUserId="12887" LastActivityDate="2016-01-03T09:20:56.430" Title="Speed of Gravity" Tags="|gravity|" AnswerCount="0" CommentCount="9" FavoriteCount="0" ClosedDate="2016-01-03T13:52:39.753" ContentLicense="CC BY-SA 3.0" />
  <row Id="13340" PostTypeId="1" AcceptedAnswerId="14936" CreationDate="2016-01-03T09:59:47.113" Score="8" ViewCount="730" Body="&lt;p&gt;If I loosely define &quot;constellation&quot; here as a group of earth satellites where different ones are visible at different times, and you need all or most of them to get round-the-clock coverage, some examples might be: &lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&quot;&gt;Iridium&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Planet_Labs&quot;&gt;Doves&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)&quot;&gt;Galileo&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/GLONASS&quot;&gt;Glonass&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbcomm_(satellite)#Orbcomm-OG2&quot;&gt;OG2&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Quasi-Zenith_Satellite_System&quot;&gt;QZSS&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_GPS_satellites&quot;&gt;GPS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering, were the &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_(satellite)&quot;&gt;Molniya's&lt;/a&gt; the first practical use of the constellation concept?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; for this question geosynchronous is fine, but not geostationary.  Millitary, commercial, and scientific constellations are all fine.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-18T01:49:31.513" LastActivityDate="2021-08-18T01:49:31.513" Title="What was the first practical use of a satellite constellation?" Tags="|orbital-mechanics|artificial-satellite|low-earth-orbit|spaceflight-firsts|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13341" PostTypeId="1" AcceptedAnswerId="13343" CreationDate="2016-01-03T10:28:02.037" Score="0" ViewCount="777" Body="&lt;p&gt;The question is self explanatory. Which of the two should be considered when selecting a reaction wheel(which does not function as a momentum wheel)?&lt;/p&gt;&#xA;" OwnerUserId="12386" LastActivityDate="2016-01-03T10:47:16.577" Title="What is the difference in Speed Control Mode and Torque Demand mode of a Reaction Wheel?" Tags="|design|reaction-wheel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13342" PostTypeId="1" CreationDate="2016-01-03T10:34:43.010" Score="12" ViewCount="550" Body="&lt;p&gt;When combustion takes place in a chemical rocket (LOX/LH2), do the gases ionize due to high temperatures? If they do, how do they get around the problem?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ionization is a waste of energy I believe, which could be used to increase thrust.&lt;/p&gt;&#xA;" OwnerDisplayName="user12889" LastEditorUserId="49" LastEditDate="2016-01-03T13:32:17.640" LastActivityDate="2016-01-04T13:29:43.393" Title="Do the propellants ionize in chemical rockets?" Tags="|rockets|propulsion|fuel|thrust|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13343" PostTypeId="2" ParentId="13341" CreationDate="2016-01-03T10:47:16.577" Score="2" Body="&lt;p&gt;Both function as reaction wheel because mechanically it's still the same device. Since what you're looking for in a reaction wheel is torque, the torque control mode offloads the need of torque control from your system onto the reaction wheel controller, meaning less work - but also less control over the device. In speed control mode you must derive the necessary speed changes from torque demands - more work (calculations) for your attitude control system, but then you can account for factors manufacturers of the controller didn't and apply in their algorithms that e.g. take precession into account or anticipate disturbances from other reaction wheels even before the sensors pick them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shortly, &quot;torque control&quot; is &quot;easy mode&quot;, where you forfeit some of the control flexibility in favor of ease of use. &quot;Speed control&quot; will necessitate somewhat more complex algorithms, but allow for far more advanced algorithms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(technically you &lt;em&gt;could&lt;/em&gt; use these more advanced algorithms with torque control mode but that would add yet another layer of complexity, in revealing the variables that are hidden by this method of control; completely unnecessary).&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-01-03T10:47:16.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13344" PostTypeId="2" ParentId="13329" CreationDate="2016-01-03T11:40:23.890" Score="4" Body="&lt;p&gt;You are right, it's a perfect ellipse with the center of the planet in the geometrical center of the ellipse.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;A little physics:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The force on a body with mass $m$ in the gravitational field of a planet with radius $R$ and gravity $g$ at its surface is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{F}(\vec{r})=\begin{cases}-m\cdot g\cdot \frac{\vec{r}}{R} &amp;amp;\text{for} &amp;amp;|\vec{r}|\le R\\ -m\cdot g\cdot R^2\cdot\frac{\vec{r}}{|\vec{r}|^3} &amp;amp;\text{else}\end{cases}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside of the planet, you have to solve the differential equation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$m\ddot{\vec{r}}=-m\cdot g\cdot R^2\cdot\frac{\vec{r}}{|\vec{r}|^3}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which is a bit difficult due to the $\frac{1}{|\vec{r}|^3}$. (In fact, one uses other approaches)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, inside the planet, it's simply&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$m\ddot{\vec{r}}=-m\cdot g\cdot \frac{\vec{r}}{R}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or with $\vec{r}=\begin{pmatrix}x\\y\end{pmatrix}$ in the plane of the moving body:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}\ddot{x}(t)&amp;amp;=-\frac{g}{R}x(t) \\ \ddot{y}(t)&amp;amp;=-\frac{g}{R}y(t)\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is are two independent linear differential equations, the solution is just&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{r}(t)=\begin{pmatrix}x\\y\end{pmatrix}=\begin{pmatrix}A\sin(\omega t+\varphi)\\B\sin(\omega t+\psi)\end{pmatrix}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an ellipse with semi-axes A and B. $\omega=\sqrt{g/R}$ defines the period &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T=\frac{2\pi}{\omega}=2\pi\sqrt{\frac{ R}{g}}$$&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Notes:&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Of course, this assumes the body is always inside the planet, i.e. $A,B\le R$. It would be interesting to see trajectories where the body leaves the planet.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;It's remarkable that the period is constant for a given planet and does not depend on the orbit parameters.&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;If you set e.g. $A=0$ and $B=R$, you get the special case @hopDavid mentioned in his comment&lt;/li&gt;&#xA;&lt;li&gt;If you have trouble to understand / imagine this: The equations for a pendulum are identical (with $\omega=\sqrt{g/l}$), so a pendulum and the body behave the same way.&lt;/li&gt;&#xA;&lt;li&gt;Like for normal orbits, body and planet will both have an elliptical trajectory with the geometrical centers in the barycenter.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11749" LastActivityDate="2016-01-03T11:40:23.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13345" PostTypeId="2" ParentId="13342" CreationDate="2016-01-03T13:48:45.363" Score="8" Body="&lt;p&gt;Yes, a proportion of the propellant products attain excited molecular states, ionize or dissociate and the fraction that doesn't recombine before leaving the nozzle, termed &quot;frozen flow&quot;, results in a performance loss. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's as far as I'll go as the topic of how to manage the problem is out of my field. You could try &lt;a href=&quot;http://www.ewp.rpi.edu/hartford/~ernesto/S2013/EP/MaterialsforStudents/Lee/Sutton-Biblarz-Rocket_Propulsion_Elements.pdf&quot; rel=&quot;nofollow&quot;&gt;Rocket Propulsion Elements by Sutton&lt;/a&gt; for background and perhaps something like &lt;a href=&quot;http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-512-rocket-propulsion-fall-2005/lecture-notes/lecture_14.pdf&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; if you want to get into it further.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-01-04T13:29:43.393" LastActivityDate="2016-01-04T13:29:43.393" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13346" PostTypeId="1" AcceptedAnswerId="13348" CreationDate="2016-01-03T13:59:12.147" Score="11" ViewCount="1070" Body="&lt;p&gt;For example, why was the 19th Shuttle mission named STS-51F instead of something more obvious like STS-19?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side note: One reason I'm asking is because in the DVD commentary of &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13_(film)&quot; rel=&quot;noreferrer&quot;&gt;Apollo 13&lt;/a&gt;, Tom Hanks or someone like that mentioned that after that mission, &lt;em&gt;&quot;you won't find the number 13 anywhere in official NASA titles or launch date/times&quot;&lt;/em&gt; (my rough paraphrasing). Not quite sure if this is true, although it seems to be for Space Shuttle missions at least.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="49" LastEditDate="2016-01-03T14:26:41.620" LastActivityDate="2016-01-03T18:06:06.527" Title="How were the Space Shuttle missions named?" Tags="|nasa|space-shuttle|terminology|crewed-spaceflight|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13347" PostTypeId="2" ParentId="13337" CreationDate="2016-01-03T14:02:54.933" Score="5" Body="&lt;p&gt;A quick search didn't turn up any alternators on rocket engines.&lt;br&gt;&#xA;I did find a list that showed the &lt;a href=&quot;http://www.lr.tudelft.nl/en/organisation/departments/space-engineering/space-systems-engineering/expertise-areas/spacecraft-engineering/design-and-analysis/subsystems/electric-power/power-table/&quot; rel=&quot;noreferrer&quot;&gt;power source on a few rockets&lt;/a&gt;. &#xA;Nothing complete, but it shows that e.g. the Ariane 5 has one power source (batteries) in the VEB, which is attached to the second stage. The entire rocket is controlled and powered from here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This brings us to two problems an alternator would have:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;the rocket needs electrical power for its entire mission (could be several hours for a GTO or multiple-satellite mission), but its engines will only run part of the time. This means you need batteries anyway. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;a rocket generally has multiple stages. If you want alternators to provide the majority of the rocket's electrical needs, you need an alternator on every stage. You'd be switching between power sources multiple times. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It should be possible to run an alternator off the turbopump. On aircraft, the auxiliary power unit is usually a small turbine with an alternator. But from the above you can see you'd be adding a lot of complexity for little weight gain.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-03T14:02:54.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13348" PostTypeId="2" ParentId="13346" CreationDate="2016-01-03T14:31:57.447" Score="12" Body="&lt;p&gt;Wikipedia has &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Space_Shuttle_missions&quot; rel=&quot;noreferrer&quot;&gt;a good summary&lt;/a&gt; of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STS-1 to STS-9 were numbered sequentially by order of launch, then it switched to a number based on fiscal year (5 for FY 1985), launch site code (1 for KSC, 2 for Vandenberg), and letter. The letters were assigned sequentially at the time of planning, but schedule slips etc meant that they were not launched in this order.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After &lt;em&gt;Challenger&lt;/em&gt; they dropped this system, and returned to a numeric STS-xx name. As with the letter codes, these were assigned sequentially but got out of sequence as time went on and schedules were moved around - the first flights were 26, 27, 29, 30, 28...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The STS-xx numeric codes were still used internally during the &quot;letter sequence&quot;, referring to the specific payload/mission rather than the launch, but these wouldn't usually be public. This meant that there were some duplicates on internal paperwork, eg two STS-26s, and so the new ones were sometimes listed as STS-26R.)&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="6944" LastEditDate="2016-01-03T18:06:06.527" LastActivityDate="2016-01-03T18:06:06.527" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13349" PostTypeId="2" ParentId="1359" CreationDate="2016-01-03T21:11:30.807" Score="6" Body="&lt;p&gt;As of Early 2015, the maximum payload to GEO is 6750 KG using &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV_Heavy&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Delta IV Heavy&quot;&gt;Delta IV Heavy&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="10273" LastEditDate="2019-05-21T14:44:54.397" LastActivityDate="2019-05-21T14:44:54.397" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13350" PostTypeId="2" ParentId="13257" CreationDate="2016-01-04T01:55:36.240" Score="4" Body="&lt;p&gt;Composites work fine in space, and for cryogenic fuels. There was some trouble with the X-33's liquid hydrogen tanks which is why people say they &quot;don't work at cold temperatures&quot; but those problems have long been resolved. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Making them radiation resistant is a matter of selecting the right resin, and resins that hold up in space are available. Of course, rockets only operate in space for a few days at most, so this wouldn't be a factor in any case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea that composites are less corrosion resistant than aluminum is absurd, because almost anything is more corrosion resistant than aluminum. Corrosion resistance is actually one of the main selling points of composites. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Composites are used extensively in modern satellites to store propellents, among other things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason they aren't used in rockets is simple: even the newest rockets (like the Falcon 9) were designed before composites were proven to be able to hold cryogenic propellents. Switching to a new material for the fuel tanks of the rocket would require them to re-qualify the rocket, which takes years and is very expensive. You can expect rockets to begin using these materials for fuel tanks as new rockets are developed in the coming years and decades.&lt;/p&gt;&#xA;" OwnerUserId="12893" LastEditorUserId="6816" LastEditDate="2016-01-04T02:16:45.563" LastActivityDate="2016-01-04T02:16:45.563" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13351" PostTypeId="2" ParentId="13331" CreationDate="2016-01-04T01:57:58.727" Score="9" Body="&lt;p&gt;Apparently, it's because putting a person in the suit prevents the atmosphere from being pure O&lt;sub&gt;2&lt;/sub&gt;. The &lt;a href=&quot;https://archive.org/details/flight_rules_generic&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Flight Rules&lt;/a&gt; (Rule A13-53) state:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;ONE-GAS ENVIRONMENT (EMU) - IF THE PRESSURE IN THE SUIT FALLS BELOW&#xA;3.15 (3.3) PSI, THE EVA CREWMEMBER WILL ABORT THE EVA. ®[050400-7197A]&lt;/p&gt;&#xA;&lt;p&gt;At 92 percent O&lt;sub&gt;2&lt;/sub&gt;, 3.15 (3.3) psia corresponds to a pressure altitude&#xA;of 8000 feet and the rationale of paragraph A.1 applies.  Exhaled&#xA;nitrogen from the crewmember dilutes the oxygen atmosphere in the suit&#xA;to 92 percent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Breaking the code here, it says that the real limit is 3.15, with instrumentation error it's 3.3, and the canned crewmember exhales enough N&lt;sub&gt;2&lt;/sub&gt; to dilute the suit atmosphere down by 8%.&lt;/p&gt;&#xA;&lt;p&gt;It doesn't say specifically, but if 3.3 is the working limit, 4.3 sounds like a nice 1 psi margin over that.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-02-02T23:30:37.930" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="13353" PostTypeId="2" ParentId="13319" CreationDate="2016-01-04T05:02:57.370" Score="2" Body="&lt;p&gt;I'm going to give an opposing answer.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As long as the mass is spherically symmetric, you shouldn't be able to determine its distribution, because spherically symmetric changes in the mass distribution shouldn't change the resulting gravitational field.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If the Sun, the planets, and the other bodies in the solar system truly were spherically symmetric, and that could be determined about those objects by observing how objects orbit another would be their masses. (More precisely, what would be determined are the standard gravitational parameters $\mu=GM$, the product of the universal gravitational constant and  mass. The large uncertainty in $G$ means that mass can only be determined to four decimal places of accuracy.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Masses in the solar system aren't spherically symmetric. The Sun is the closest thanks to its large size and its slow rotation rate. The giant planets have a marked spherical bulge, but also apparently have some gravity anomalies due to uneven mass distribution. The terrestrial planets  are even less spherical than are the giant planets, the asteroids, even less so. It's best to describe asteroids and moons with a radius smaller than 200 to 300 kilometers as lumpy potatoes rather than spherical cows. Vesta is right in the middle of that range. While not near as lumpy as 25143 Itokawa (visited by JAXA's Hayabusa a decade ago), Vesta is still far from spherical. This gives a handle for measuring characteristics within Vesta, just by observing how Dawn orbits Vesta. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Numerical orbit determination and propagation techniques account for the non-spherical nature of solar system bodies and for perturbing effects from other bodies. The Earth-based tracking stations used to communicate with Dawn provided measurements that give clues regarding how Dawn was orbiting Vesta, which in turn gives clues to Vesta's gravitational field. The most precise measurement is the Doppler shift in signals sent from Earth to Dawn which Dawn simply relays back to Earth. (Every deep space probe launched by the US and Europe is outfitted with a device for just that purpose.) Enough of these observations, spread out over time, coupled with models of the solar system, models of how gravity works, plus lots of computing horsepower, gives a picture of the interior of a solar system body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to model the gravitational field of not-quite spherical objects is with spherical harmonics (or sometimes elliptical harmonics). The mathematics is very well established. This approach works nicely for planets, somewhat nicely for Moon-sized object (but the Moon has five large mascons (mass concentrations) on the near side and a two kilometer offset between its center of mass and geometrical center). They might work even for a Vesta-sized object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another approach is to use the body's geometrical shape to model the gravitational field. Dawn took lots of pictures of Vesta. These pictures collectively yield a 3D shape model of Vesta. A commonly used approach in computer graphics is to represent the surface of some object via polygons (typically triangles). These shape models can be used as a gravitational model by projected those polygons to some well-chosen center to create polyhedra. The gravitational field for a constant density polyhedron is easy to calculate (with a computer, that is), and from that, you have a polyhedral gravity model of the object in question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbits from the harmonics-based and polyhedra-based gravity models inevitably won't agree with one another, or with the observations. The biggest problem is almost certain to be the assumption of a constant density in the polyhedral gravity model. A Vesta-sized body should have some amount of differentiation. Polyhedral gravity models can be augmented with a density profile. Modifying that model so it is more consistent results with the observations gives a clue as to what's inside Vesta.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;References:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www-geodyn.mit.edu/konopliv_dawngravity_ssr11.pdf&quot; rel=&quot;nofollow&quot;&gt;Alex S. Konopliv, et al., &quot;The Dawn gravity investigation at Vesta and Ceres,&quot; &lt;em&gt;The Dawn Mission to Minor Planets 4 Vesta and 1 Ceres&lt;/em&gt;, Springer New York, 2012. 461-486.&lt;/a&gt; (Same reference used by Hohmannfan)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://articles.adsabs.harvard.edu/full/1997cemda..65..313w/0000313.000.html&quot; rel=&quot;nofollow&quot;&gt;Robert A. Werner and Daniel J. Scheeres, &quot;Exterior gravitation of a polyhedron derived and compared with harmonic and mascon gravitation representations of asteroid 4769 Castalia,&quot; &lt;em&gt;Celestial Mechanics and Dynamical Astronomy&lt;/em&gt; 65.3 (1996): 313-344.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-01-04T05:02:57.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13354" PostTypeId="1" AcceptedAnswerId="13357" CreationDate="2016-01-04T13:12:28.973" Score="1" ViewCount="230" Body="&lt;p&gt;Five years ago, a teacher said there's sound in space, called &quot;the music of the space.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this true or false?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think it's true because there's nothing in the void.&lt;/p&gt;&#xA;" OwnerUserId="12899" LastEditorUserId="1142" LastEditDate="2016-01-04T17:01:23.817" LastActivityDate="2016-01-04T17:01:23.817" Title="There's sound in space?" Tags="|sound|" AnswerCount="1" CommentCount="3" ClosedDate="2016-01-04T22:40:43.757" ContentLicense="CC BY-SA 3.0" />
  <row Id="13355" PostTypeId="1" AcceptedAnswerId="13358" CreationDate="2016-01-04T14:53:00.253" Score="5" ViewCount="1804" Body="&lt;p&gt;Between this two LH2/LOX rocket engines the SSME(RS-25 different) of the Space Shuttle rocket and RD-0120 of the Energia rocket, which one has higher performance and better technology to launch space rockets in low earth orbit?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-07-19T17:54:49.370" LastActivityDate="2021-08-22T01:23:51.563" Title="Space rocket engines performance and technology: SSME vs. RD-0120" Tags="|engines|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13356" PostTypeId="2" ParentId="13134" CreationDate="2016-01-04T15:18:23.370" Score="7" Body="&lt;p&gt;Several of the answer have stressed the difference in size of the vehicles, and that the trajectories are very different. While those are all good points, I think the fundamental thing which makes SpaceX's feat a good deal more impressive though is that &lt;strong&gt;it was done with a lower stage which was being used as part of a regular mission, and not by a vehicle specifically designed for suborbital VTOL-flights&lt;/strong&gt;. That makes a world of difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a launcher, roughly every 10kg of extra weight in the lower stage (landing gear, propellant, grid fins, everything) make 1kg less payload to orbit. It would be very easy to end up with a system which is so heavy that you can't carry your satellites to orbit anymore. A rocket designed specifically for suborbital VTOL hops faces similar control issues, and must for instance also have engines capable of deep throttling (not easy). It can afford to invest far more weight into those systems however, for all the required gear and the propellant needed to get it back to the ground safely, than a launcher can. It also doesn't have to supply all that extra delta-V for the horizontal velocity. On a launcher, weight is everything, up to their first actual attempt to land on the barge, many space experts would have said that what SpaceX was trying to do was close to impossible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;VTOL under rocket power with a specifically designed vehicle on the other hand, had been done a couple of times before. Examples include the low-gravity Apollo lunar landings, but also vehicles on earth such as the Grumman Lunar Lander XChallenge landers (though the distances and velocities here are much smaller), and perhaps most prominently the &lt;strong&gt;DC-X prototype&lt;/strong&gt; in the 90s. There's a great video of the DC-X online: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/JzXcTFfV3Ls?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What SpaceX did with the 1st stage of their launcher is so difficult from a technical standpoint that it will probably be a long time, perhaps a decade or more, until anyone else manages to pull that off.&lt;/p&gt;&#xA;" OwnerUserId="12900" LastActivityDate="2016-01-04T15:18:23.370" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13357" PostTypeId="2" ParentId="13354" CreationDate="2016-01-04T16:25:58.457" Score="6" Body="&lt;p&gt;Of course there is no sound in the vacuum of space, as there isn't a sufficient medium to propagate sound waves. Your teacher was referring to a medieval concept known as the &lt;a href=&quot;http://en.wikipedia.org/wiki/Musica_universalis&quot;&gt;music of the spheres&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Musica universalis (lit. universal music, or music of the spheres) or Harmony of the Spheres is an ancient philosophical concept that regards proportions in the movements of celestial bodies—the Sun, Moon, and planets—as a form of musica (the Medieval Latin term for music). This &quot;music&quot; is not usually thought to be literally audible, but a harmonic, mathematical or religious concept. The idea continued to appeal to thinkers about music until the end of the Renaissance, influencing scholars of many kinds, including humanists.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is philosophy, not physics.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2016-01-04T16:25:58.457" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13358" PostTypeId="2" ParentId="13355" CreationDate="2016-01-04T17:33:00.343" Score="22" Body="&lt;p&gt;The two big performance considerations for a liquid fueled engine are thrust and &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot;&gt;specific impulse&lt;/a&gt;. On these criteria, the two engines are very comparable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot;&gt;SSME&lt;/a&gt; has a bit more thrust: 1860kN at sea level, increasing to 2279kN in vacuum, versus &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-0120&quot;&gt;RD-0120&lt;/a&gt;'s 1526kN to 1961kN -- a 22% difference at sea level and 16% in vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other major factor is specific impulse, which is a measure of fuel efficiency. The SSME's specific impulse is 366 seconds at sea level, increasing to 452 seconds in vacuum; the RD-0120's goes from 353 seconds to 455 seconds. RD-0120 is spending 3% more fuel per unit of thrust during takeoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, once out of atmosphere, the Russian engine is almost 1% more efficient; in all other cases, the SSME outperforms it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you swapped the engines between &lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot;&gt;Energia&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot;&gt;shuttle&lt;/a&gt; -- keeping the number of engines the same, so 3 RD-0120 on the shuttle and 4 SSME on the Energia -- the re-engined Energia would be able to lift a little more payload, and the shuttle could only make it to orbit with a reduced payload -- about 15 tons instead of 25. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle would still be able to take off despite the reduction in thrust, because at liftoff the solid rocket boosters are doing most of the work. It would ascend a little more slowly at the start, which would cost a little more fuel due to gravity losses, but it turns out that the shuttle throttles its engines down substantially over the course of the ascent to reduce aerodynamic loads and peak g-forces, so the RD-0120s would have more than enough power during much of the ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its slight edge in performance doesn't necessarily make the SSME &quot;better technology&quot;. It's one of the most complicated and expensive rocket engines ever flown. RD-0120 gives you more bang for the buck (or maybe &quot;more rumble for the ruble&quot;).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here's a method to roughly estimate the payload hit. Ignore the SRBs for the moment and consider a 68 ton orbiter, 25 ton payload, 30 ton external tank, and 730 tons of propellant: 853 tons total, 123 tons with propellant expended. Apply the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot;&gt;rocket equation&lt;/a&gt; with the 366 second sea level SSME figure:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$∆v = 366s \cdot 9.81m/s^2 \cdot \ln \frac{853}{123} = 6953 m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now determine the change in payload mass (∆m) needed to get the same result from the RD-0120's specific impulse:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$6953 m/s = 353s \cdot 9.81m/s^2 \cdot \ln \frac{853+∆m}{123+∆m}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ 7.447 = \frac{853+∆m}{123+∆m} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rearranging and solving, we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ ∆m = -9.77 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if the sea level specific impulse held throughout the flight, and the SRBs weren't involved, shaving 9.77 tons off the payload would produce the same ∆v. This is a worst case because we expect the specific impulse of the RD-0120 to &quot;catch up&quot; with and even exceed that of the SSME over the course of the flight, so the actual payload hit should be smaller -- but we'll also lose some ∆v to gravity during the slightly slower ascent, so 10 tons is a reasonable estimate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can run similar calculations on the Energia side to work out the additional payload mass it could lift if equipped with SSMEs, but I can't find clear numbers for its dry, wet, and payload masses; probably it's a fairly similar delta.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-01-04T22:48:16.660" LastActivityDate="2016-01-04T22:48:16.660" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13359" PostTypeId="1" CreationDate="2016-01-04T19:56:25.397" Score="12" ViewCount="577" Body="&lt;p&gt;SpaceX has several launch sites (and landing sites!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Florida they started at LC-40, and are working to activate LC-39A for launches, and LC-13 has been re-labelled LZ-1 for landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In California, they have SLC-4E for launch and SLC-4W for landings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boca Chica, Texas has seen little direct work yet, with the report being they will work on it post-LC-39A activation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will ignore Omelek (Falcon-1 launch site in the Kwajellin Atoll) since it is no longer in use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the cost drivers on the space shuttle program was a standing army of staff, paid, regardless of flight rate that had to be maintained. SLS is likely replicating the jobs program aspects of this, for one launch every 3-4 years. SpaceX being dedicated to lower costs is clearly looking to minimize the standing army problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How has SpaceX structured their launch site teams?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LC-40 is clearly where most of the staff has been, since there has only been a single Vandenberg flight (Cassioppe) as I write this. However SLC-4 is due to get busier as there are a number of flights on the 2016-2017 manifest set. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When they launch from SLC-4 do they reuse the team from LC-40? Is it a separate team? What is the plan for LC-39A? Share the LC-40 team or have its own dedicated team? The structure of the launch team seems like it would be an interesting thing to understand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer might include it, but a possible secondary question might be, how is their mission control team structured?  One team to rule them all?  Red team/Blue team?  Or a problem to be solved when they have enough missions that they need more staff?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-01-04T20:54:28.273" LastActivityDate="2016-08-11T00:09:57.653" Title="With SpaceX running multiple launch sites, how are their launch teams structured?" Tags="|launch|spacex|falcon-9|launch-site|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13360" PostTypeId="1" AcceptedAnswerId="13362" CreationDate="2016-01-04T21:00:36.083" Score="13" ViewCount="1948" Body="&lt;p&gt;All the orbital launch-sites I know of are to some degree also military bases, like Cape Canaveral and Plesetsk, and they have in any case restricted access. However, which of them can I visit, without breaking any law? I am most interested in a manned launch, if that helps to narrow down the question. (although I am then left with only Russia and China.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-01-05T16:26:29.877" Title="What orbital launch-site can I lawfully visit?" Tags="|launch-site|law|baikonur-cosmodrome|cape-canaveral|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13361" PostTypeId="2" ParentId="13360" CreationDate="2016-01-04T21:50:43.657" Score="8" Body="&lt;p&gt;It is certainly possible to visit Kennedy Space Center and see a launch.  This &lt;a href=&quot;https://www.nasa.gov/centers/kennedy/launchingrockets/viewing.html&quot;&gt;link&lt;/a&gt; gives viewing sites for KSC, Vandenberg, and Wallops as well as a link to information on purchasing tickets for on-site viewing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, since you wish to see a human spaceflight launch, KSC is not going to fit the bill for quite a while.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-01-04T21:50:43.657" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13362" PostTypeId="2" ParentId="13360" CreationDate="2016-01-04T23:28:49.383" Score="15" Body="&lt;p&gt;The United States launch sites are the most accessible, both in terms of location (near population centers and accessible) and political environment. Kennedy Space Center in particular is welcoming of visitors, and to a lesser extent Wallops, and you can get into &lt;a href=&quot;http://newpreview.afnews.af.mil/vandenberg/publictours.asp&quot;&gt;Vandenburg occasionally if you ask nicely&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as other countries, the major centers are &lt;a href=&quot;http://www.theguardian.com/world/2014/oct/21/-sp-kazakhstan-space-launch-tourism-baikonur-cosmodrome&quot;&gt;Baikonour&lt;/a&gt;, which doesn't allow tourists (Yet), &lt;a href=&quot;http://www.arianespace.com/spaceport-facility/practical-informations/&quot;&gt;Guiana Space Center,&lt;/a&gt;  which does have some launch site visitation (But is in a fairly remote part of the world anyways), and the &lt;a href=&quot;http://www.china.org.cn/english/travel/226048.htm&quot;&gt;various Chinese sites&lt;/a&gt;, all of which seem to allow some form of public visitation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, the US and China seem to be the most open about visiting their launch sites.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-01-04T23:36:38.497" LastActivityDate="2016-01-04T23:36:38.497" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13363" PostTypeId="1" AcceptedAnswerId="13889" CreationDate="2016-01-04T23:43:43.423" Score="7" ViewCount="6479" Body="&lt;p&gt;For a rocket (and its stages, if it's a multistage rocket) to successfully deploy its payload into the desired orbit, it has to follow a specific trajectory with a specific velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know rockets alter/maintain their course with &lt;a href=&quot;https://space.stackexchange.com/questions/2830/how-does-a-launch-vehicle-control-its-trajectory-during-the-first-stage&quot;&gt;gimbaled engines&lt;/a&gt;, which can be considered the actuator in a control loop. &lt;strong&gt;But what is rocket's reference in the control system loop? From where do gimbals get the information on how they should position themselves?&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the desired trajectory transmitted to the vehicle during flight? Does the vehicle itself have the trajectory uploaded/computed on board? &#xA;(this is the crucial piece I'm waiting answer for, some references would be nice)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/questions/4968&quot;&gt;possible duplicate question&lt;/a&gt; is in many ways similar to mine, but it is more general. Answers there do not address the specifics of feeding desired trajectory (i.e. the reference) in launcher's control loop. That said, answers there are still very informative and the OP apparently got what he needed. Answers (or comments) in other questions do touch the subject, but again not the specifics I would like to know. See discussion in comments with Brian Lynch, it may help clarify. &lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="6750" LastEditDate="2016-01-07T13:20:50.540" LastActivityDate="2016-02-04T12:08:42.880" Title="How are rockets guided to follow specific trajectory?" Tags="|rockets|trajectory|" AnswerCount="4" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="13364" PostTypeId="1" CreationDate="2016-01-05T00:33:37.627" Score="2" ViewCount="312" Body="&lt;p&gt;As more material weight leads to higher stress for a space elevator I have been asking myself if a kind of tower thats based on magnetic repulsion would not be an alternative.&#xA;While I know that magnetic levitation can be hard to stabilize there are 2 scenarios I have in mind and which I am curious to what prevents their execution.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;A series of disks is put on top of each other changing magnetic fields are induced in these disks(maybe the first disks induces a phase shifted version of its field in the disk above, or each disk generates the field independent of each other, or all disks above the first are permanently magnetic and rotate due to the magnetic field created by the base) the disk repel each other, the tower extends.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In a coil/spool an alternating current is induced such that one curl of the coil is pi in terms of frequency, such curls should repel each other(if I not misconstrued something)and as the spool stretches frequency is adjusted.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Even if such a tower might not be stabilized indefinitely, this still could be used to lift an object a certain distance and accelerate it at the same time(when placed on top of the tower during the &amp;quot;powering up&amp;quot; process)&lt;/p&gt;&#xA;&lt;p&gt;So where did I go awry? Where are the problems an expert would point out?&lt;/p&gt;&#xA;" OwnerUserId="12901" LastEditorUserId="6944" LastEditDate="2021-10-28T14:27:06.680" LastActivityDate="2021-10-29T12:32:20.567" Title="Space elevator/tower through magnetic repulsion? What are the technical problems?" Tags="|design-alternative|launchpad|space-elevator|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="13365" PostTypeId="1" AcceptedAnswerId="13374" CreationDate="2016-01-05T00:35:47.267" Score="5" ViewCount="365" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.spacepolicyonline.com/news/nasa-gets-big-boost-in-final-fy2016-appropriations-bill&quot; rel=&quot;nofollow&quot;&gt;Marcia S. Smith of SpacePolicyOnline.com in &quot;NASA Gets Big Boost in Final FY2016 Appropriations Bill&quot; news article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Planetary Science&lt;/strong&gt;. Funding for planetary missions is \$1.631 billion, an increase of \$270 million above the request and \$193&#xA;  million more than FY2015. It includes \$175 million for a mission to&#xA;  Jupiter's moon Europa, a priority of House Appropriations CJS&#xA;  subcommittee chairman Rep. John Culberson (R-TX).  He added&#xA;  substantial funding for the mission in FY2013 (\$75 million) and&#xA;  FY2014 (\$80 million) although NASA did not request any. For FY2015,&#xA;  NASA requested \$15 million and Congress provided \$100 million. For&#xA;  this year, NASA requested \$30 million. Not only did Congress add&#xA;  \$145 million to that figure, but it directs NASA to build a lander as&#xA;  well as an orbiter. It reiterates that it wants the mission launched&#xA;  in 2022. NASA has been envisioning a later launch date to match the&#xA;  projected funding levels in the President's budget request.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, don't get me wrong, I'm all for an advanced Europa mission, but deciding so unilaterally, by a single congressman that happens to chair the &lt;a href=&quot;http://appropriations.house.gov/subcommittees/subcommittee/?IssueID=34794&quot; rel=&quot;nofollow&quot;&gt;House Committee on Appropriations Subcommittee on Commerce, Justice, Science, and Related Agencies&lt;/a&gt;, seems wrong to me. Countless advanced missions are proposed each year and compete for attention of the statesmen and budgetary support, and which ones succeed and which ones don't shouldn't be decided on by lone policymaker's wits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might backfire, and having watched the two day &lt;a href=&quot;https://exep.jpl.nasa.gov/exopag/exopag13/&quot; rel=&quot;nofollow&quot;&gt;Exoplanets Program Analysis Group (ExoPAG) meeting&lt;/a&gt; that just ended today, it appears it already has. This unilateral budget increase for a single specific mission has been a subject of some ridicule, and Culberson's &quot;vision&quot; of focusing on Europa as one single world with highest likelihood of harboring life was corrected with the dioptric power of exoplanetology snark - that surely Earth-analog exoplanets have the highest likelihood of harboring life besides the Earth itself. I'm not going to comment if I agree or not (Enceladus! oops), that's not the point of my question, which is simply:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is &lt;a href=&quot;https://en.wikipedia.org/wiki/John_Culberson&quot; rel=&quot;nofollow&quot;&gt;Culberson&lt;/a&gt; within his rights to unilaterally bypass all the mission prioritization and selection processes set in place so the scientific community and the society at large decide which ones to support, to what extent and when based on merit, feasibility, benefit, cost and other criteria that might apply? Can a single well-seated policymaker come up with and self-approve his own policy?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that I'm not seeking an open discussion. I expect answers to be substantiated with verifiable sources of information, perhaps there's a precedent that I'm not aware of, but I also welcome some conjecture if you want to comment on this from your own perspective, as long as you first answer the question.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="265" LastEditDate="2016-01-05T16:10:43.683" LastActivityDate="2016-01-05T16:10:43.683" Title="&quot;Culberson's&quot; Europa mission: How can mission proritization be done so unilaterally?" Tags="|nasa|cost|policy|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13366" PostTypeId="1" CreationDate="2016-01-05T01:48:39.893" Score="34" ViewCount="10231" Body="&lt;p&gt;Let's say you have a spaceship on its way to Mars; halfway there you decide you need to come back to Earth instead. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any reason a spacecraft would be unable to do this? Let's assume it began in Earth orbit with enough fuel/engines to leave orbit and head for Mars, and that it was already planning to come back after orbiting Mars.&lt;/p&gt;&#xA;" OwnerUserId="12902" LastEditorUserId="699" LastEditDate="2016-01-05T19:03:31.453" LastActivityDate="2016-01-08T18:45:39.290" Title="Is there any reason why a spacecraft would not be able to turn around without a gravity assist?" Tags="|orbital-mechanics|abort|propulsive-deceleration|" AnswerCount="7" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13367" PostTypeId="2" ParentId="13366" CreationDate="2016-01-05T02:33:15.823" Score="53" Body="&lt;p&gt;Let's say you start rolling down from the top of a half-pipe skateboard ramp, and you plan to get back to your starting point. If you stop in the middle of the pipe, it is much harder to climb back up to your starting point then if you ride up the other side of the ramp and let gravity accelerate you back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, even if you planned to get into an orbit in Mars, most orbits are chosen so you only expend a small amount of energy decelerating to get into orbit and only expend a small amount of energy to leave the orbit and slingshot back. Compare this to stopping your spacecraft mid-space and accelerating back. After all, you conserve the momentum your spaceship has while orbiting around the planet.&lt;/p&gt;&#xA;" OwnerUserId="12901" LastEditorUserId="49" LastEditDate="2016-01-06T13:40:54.640" LastActivityDate="2016-01-06T13:40:54.640" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="13368" PostTypeId="2" ParentId="13366" CreationDate="2016-01-05T03:06:20.810" Score="25" Body="&lt;p&gt;I assume that &quot;halfway there&quot; refers to being halfway along the transfer orbit from Earth to Mars, and that by &quot;turn around&quot; you mean reverse direction immediately. There is no reason a spacecraft &quot;cannot&quot; do this, but the reason it is entirely impractical is because of the propellant required to change the spacecraft trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aborting an Earth-to-Mars transfer to return to Earth requires the spacecraft continue along its current interplanetary transfer orbit until it swings back to Earth -- but most importantly it requires that the transfer orbit be selected with a period so that the spacecraft encounters Earth. Without the appropriate timing, the spacecraft will return to where Earth &lt;em&gt;was&lt;/em&gt; at launch, not where Earth &lt;em&gt;will be&lt;/em&gt; when it reaches the same point. This type of abort scenario uses what is called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Free_return_trajectory&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;free-return trajectory&lt;/em&gt;&lt;/a&gt;, which was used during Apollo 13 (along with a subsequent propulsive manoeuvre to hasten the return and shift the splashdown location).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find a discussion of Earth-to-Mars abort scenarios in &lt;a href=&quot;http://www.marsjournal.org/contents/2007/0002/files/wooster_mars_2007_0002.pdf&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt; -- see page 20. Both a free-return trajectory as well as a propulsive or gravity-assist return trajectory are discussed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a comparison of a 2-year free-return trajectory on the left and a propulsive abort trajectory on the right (from the above linked paper). Note the dotted green line on the right indicates the trajectory without any propulsive manoeuvre (which would not reach Earth at the correct timing).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i0wDj.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i0wDj.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2016-01-05T07:40:24.440" LastActivityDate="2016-01-05T07:40:24.440" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13369" PostTypeId="2" ParentId="13360" CreationDate="2016-01-05T06:26:49.680" Score="11" Body="&lt;p&gt;I'm not sure if it'd be exciting enough for you, but I visited &lt;a href=&quot;https://en.wikipedia.org/wiki/Tanegashima_Space_Center&quot;&gt;Tanegashima&lt;/a&gt; with a friend in January 2013. It's a bit of a way off the west end of Japan, but not quite as far as Okinawa. They mostly do satellite launches and tracking. (this qualifies it as an orbital launch site for you, yes?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you've made the ferry, hydrofoil or plane trip to get there from Kagoshima, it's easy to get around. Tanegashima Space Centre is super welcoming and they run guided tours of the facilities as well as leaving you free to walk around the sizeable public areas. TSC is a beautiful place, and they're justifiably proud of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://global.jaxa.jp/about/centers/tnsc/&quot;&gt;JAXA's website&lt;/a&gt; has info in English on visiting and booking tours. Wikitravel has good details on &lt;a href=&quot;http://wikitravel.org/en/Tanegashima&quot;&gt;travelling there&lt;/a&gt;. Note that the guided tour around the facilities is all in Japanese, but don't let that stop you. For what it's worth, you'll have the opportunity to get eyes on the launchpad from about... 1km away? Maybe a bit further; I don't recall the bus route so I can't confirm that on google maps.&lt;/p&gt;&#xA;" OwnerUserId="12906" LastActivityDate="2016-01-05T06:26:49.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13370" PostTypeId="2" ParentId="13364" CreationDate="2016-01-05T08:28:58.487" Score="5" Body="&lt;p&gt;In theory, there is nothing wrong with a space elevator resting on the ground, instead of hanging from a counterweight. However, magnetic repulsion is relatively much weaker than the force holding materials together. The simple version first: grab two magnets, and try to push them together. That is relatively easy. After that, take a steel rod of comparable size, and try to stretch it. Not quite as easy.&lt;/p&gt;&#xA;&lt;p&gt;The forces present in a space-elevator are enormous, but not the whole way. After all, the first meter does only have to take the weight of the payload, and can therefore be relatively thin. The last meter must take the weight of the payload, plus the entire elevator, and must therefore be a lot thicker. How large the increase in cross-section area is, is known as the taper ratio. If we accept that the size of magnetic discs increases over 3000 times over the whole elevator, a one kilogram magnet must still exert a force of 6 million Newtons at a one meter distance. (A Saturn-V hovering over a three kilogram magnet). We can have the discs closer, and that can reduce the requirements a bit.&lt;/p&gt;&#xA;&lt;p&gt;If we follow this strategy, we can take advantage of another fact:&#xA;It is actually repulsion between the electrons of your feet and the ground that keeps you from falling towards the centre of the Earth. That is, when the distance between the discs is sufficiently small, it is actually just as good to stack bricks on top of each other. Materials can tolerate a stronger tension than compression, so therefore the design of choice for a space-elevator is now a long tether hanging from geostationary orbit.&lt;/p&gt;&#xA;&lt;p&gt;In a sense, space-elevators are already based on electromagnetic forces.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="38535" LastEditDate="2021-10-29T12:32:20.567" LastActivityDate="2021-10-29T12:32:20.567" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="13371" PostTypeId="2" ParentId="13366" CreationDate="2016-01-05T09:10:00.910" Score="5" Body="&lt;p&gt;There's no singular reason why a spacecraft would be &lt;em&gt;unable&lt;/em&gt; to change its trajectory to return to Earth, it just requires the appropriate delta-v budget and time.&lt;/p&gt;&#xA;&lt;p&gt;Depending on what the craft was initially designed for, it may not have the delta-v to execute such a course change.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;For example, the delta-v remaining on a one-way Mars ship, even one that used a rocket-powered soft landing on Mars, would certainly not have the delta-v to return to Earth much sooner, if at all, compared to a slingshot.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;By contrast, the delta-v remaining on a two-way Mars ship without a separate lander (from Earth, go to Mars, land, get back to orbit, return to Earth) could likely pull of the maneuver, though not necessarily in the most expedient way (dipping in closer to the sun than Earth's orbit to 'catch up' to it).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;As a balancing point, a ship that was designed to go to Mars, stay in orbit while a lander dropped to the surface and returned, then take the next convenient transfer back to Earth is a toss up on whether or not it has the delta-v required to 'turn back'.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;With all this in mind, the &lt;em&gt;most time expedient&lt;/em&gt; return would be very expensive, delta-v wise, and the crew or supplies may run out of time waiting for a more fuel efficient transfer; especially if any refueling or resupplying was to be done at Mars.&lt;/p&gt;&#xA;&lt;p&gt;This would be quite easy to try out yourself, just download the &lt;a href=&quot;https://kerbalspaceprogram.com/en/&quot; rel=&quot;nofollow noreferrer&quot;&gt;KSP trial&lt;/a&gt; and use a pre-built ship. The solar system isn't entirely the same, but you can see the relative merits of each maneuver with Duna, or download a mod to do it in a decent approximation of ours.&lt;/p&gt;&#xA;&lt;p&gt;If you ask nicely enough in the forums, someone might record the answer for you in video form.&lt;/p&gt;&#xA;&lt;h3&gt;EDIT&lt;/h3&gt;&#xA;&lt;p&gt;A short note on what delta-v is, in case you are not familiar: delta-v is a measure of the amount of change in velocity your craft can perform. If you are riding a bicycle, and going 4 miles per hour, and want to go 10 MPH, you need 6 MPH delta-v. You can provide this to your 'craft' on Earth's surface from energy expended against the friction of the surface due to gravity.&lt;/p&gt;&#xA;&lt;p&gt;Because there is so little matter in the near-vacuum of space, there is hardly any friction with which to push off. The only (current) method of changing your velocity in space is by &lt;a href=&quot;https://what-if.xkcd.com/85/&quot; rel=&quot;nofollow noreferrer&quot;&gt;tossing mass in the opposite direction&lt;/a&gt;. It helps if you 'toss' the mass fast, which is why we use rocket fuel which burns and gets subsequently 'tossed' rather fast.&lt;/p&gt;&#xA;&lt;p&gt;The other thing to note is that trajectories between objects in space are typically some variant of transfer orbits, not simply 'heading toward' something. Once an orbit is established, a maneuver is plotted that changes that orbit into one that will intercept another orbit at the time very close to when the target object will be there. &amp;quot;Turning around&amp;quot; then becomes not simply reversing course or 'pulling a 180', but again altering the orbit for a return transfer orbit that intersects the original body.&lt;/p&gt;&#xA;&lt;p&gt;A very common one is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann Transfer Orbit&lt;/a&gt;, and the wiki article is a good place to start.&lt;/p&gt;&#xA;" OwnerUserId="6821" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-01-05T19:29:46.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13372" PostTypeId="2" ParentId="10777" CreationDate="2016-01-05T09:16:52.170" Score="5" Body="&lt;p&gt;The anomaly may not be that the number of missions is declining, but that it's been so high in the past few years.&lt;br&gt;&#xA;Here are some &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/12311322-planetary-exploration-timelines.html&quot; rel=&quot;noreferrer&quot;&gt;new graphs from Emily Lakdawalla at the Planetary Society&lt;/a&gt;. First the missions for the 2003-2018 timeframe. Dark blue = NASA, I haven't looked at cooperative missions.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s1LTg.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s1LTg.png&quot; alt=&quot;Missions by agency, 2003-2018&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://planetary.s3.amazonaws.com/assets/images/charts-diagrams/2015/20151231_planetary-exploration-timeline-version-8-by-agency_Artboard-1.png&quot; rel=&quot;noreferrer&quot;&gt;large version&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the missions for the 15 years before that:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Z5Aau.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z5Aau.png&quot; alt=&quot;Missions 1987-2002&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://planetary.s3.amazonaws.com/assets/images/charts-diagrams/2015/20151231_planetary-exploration-timeline-version-8-by-agency-02.png&quot; rel=&quot;noreferrer&quot;&gt;large version&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2015, I count 11 active NASA missions, in 2018 that declines to 7. That includes Opportunity, which was unexpectedly long-lived. &#xA;But in 2005, there were 8 active missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can also see the highest number of missions at any point in the 1987-2002 timeframe was 8 (in 1999), with most years at 6 missions or less. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-01-05T09:22:12.350" LastActivityDate="2016-01-05T09:22:12.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13373" PostTypeId="2" ParentId="13366" CreationDate="2016-01-05T13:52:42.780" Score="49" Body="&lt;p&gt;All the other answers are great, but I think one explanation is still missing: how an interplanetary orbital transfer actually works in practice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thing is, space is rather big, and things keep moving. At the same time, you're being tugged on constantly by all the other bodies in a planetary system (we can ignore other stars for interplanetary transfers). So travelling between planets is not as easy as pointing your ship in the direction of planet B and pressing the speed pedal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, things keep moving. The historic Mars missions have taken anywhere from about 100 days to more than 300 days - the latter actually meaning that by the time you arrive, Mars is &lt;em&gt;on the opposite side of the Solar system&lt;/em&gt; compared to when you started. You need to aim for where Mars &lt;em&gt;will&lt;/em&gt; be, rather then where it is. Now, this isn't too big of a problem for the trip itself, but think about what happens when you abort at some point during the trip - the Earth has moved as well. It's not as easy as pointing your spaceship in the opposite direction and accelerating again - the Earth is no longer where it was when you started.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does it take so long to get to Mars? Well, space is big. Those probes were usually travelling at around 20 000 km/h - quite fast in terrestrial terms. But the closest distance between Earth and Mars is still over fifty &lt;em&gt;million&lt;/em&gt; kilometers. The only way to get there faster is to get your speed up. Here lies the main problem - our spaceship engines are actually extremely inefficient. Jet engines are much less efficient than car engines, but they're still peanuts to rocket engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's two main reasons - &lt;em&gt;propellant&lt;/em&gt; and &lt;em&gt;fuel&lt;/em&gt;. When you drive your car, the wheels are pushing of from the Earth - basically, you're either &quot;stealing&quot; or &quot;giving&quot; momentum to Earth, which means your car only needs to produce enough energy to supply your kinetic energy, give or take. With 100% efficiency in a vacuum, a 1 ton car would only need about 300 kJ of energy to accelerate to 100 km/h - that's about 10 grams of gasoline worth of energy. The first annoying thing is that kinetic energy grows quadratically with respect to speed - so accelerating to 200 km/h takes around 1400 kJ of energy, rather than 600 kJ. So getting even to that 20 000 km/h would take around 1400 MJ of energy (1.4E10 J) - the equivalent of about 280 kg of gasoline, which is more than a quarter of the mass of the car itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quarter of the mass is quite a lot - a spaceship can't just stop every few kilometers to refill its fuel tank. It needs to carry all of its fuel from the very beginning - so our sample car wouldn't actually be 1 ton, it would be 1 ton + 280 kg of gasoline. And those 280 kg of gasoline would need even more gasoline to accelerate &lt;em&gt;itself&lt;/em&gt; - this is known as the tyranny of the rocket equation - the more fuel you bring, the more fuel you need to bring the fuel, etc. Now, if we actually had a rocket that would be able to accelerate 1 ton to 20 000 km/h with just 280 kg of fuel, we'd be peachy. But we don't have anything close to that. Why? The two reasons mentioned above - fuel and propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be saying, wait, we already accounted for fuel - that's the 280 kg of gasoline (plus the gasoline for the gasoline, ...), right? Nope. Gasoline doesn't just spontaneously break down, releasing energy - it's being burned, which is just a simple way of saying it chemically interacts with oxygen. Your car can easily use the ambient oxygen, getting its mass for free, but rockets tend to travel in a vacuum - they need to bring their oxygen along for the ride as well. That's extra mass, of course. How much mass? Well, using a very simplistic calculation (considering pure octane burning perfectly), you need about 3.5kg of oxygen to burn 1kg of gasoline. I think you can see the problem now - while we can barely ignore the 280kg compared to our 1 ton car, the problem becomes glaringly obvious when you actually need more fuel than your vehicle weighs (280kg of gasoline + 980kg of oxygen)!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But wait, this isn't over yet. This explains the fuel part of the issue, but there's still the propellant. Cars don't need propellant, since they're pushing off the ground. Even jets don't need to &lt;em&gt;carry&lt;/em&gt; their own propellant, since they're again using ambient air as both fuel and propellant. But again, our rocket engine doesn't have the same luxury. It needs to carry its propellant along the trip. This is the place where rocket science gets really complicated (and we didn't even &lt;em&gt;glance&lt;/em&gt; on orbits or interplanetary transit yet!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conservation of momentum basically says that to accelerate in one direction, you need to accelerate something else with the same mass in the opposite direction with the same acceleration. You can also trade mass for velocity - the faster you throw away your propellant, the less mass does it need to have to give you the same acceleration. This is the &lt;em&gt;exhaust velocity&lt;/em&gt; of a rocket - the speed of the propellant relative to the rocket. To give you a bit of a scale, a typical liquid oxygen+liquid hydrogen rocket has an exhaust velocity around 18 000 km/h. This basically means that to accelerate a 1 ton spaceship to 18 000 km/h, you need 1 tons of propellant (ignoring the mass of the fuel and propellant itself and the mass of the engine). If we stop ignoring the mass of the fuel and propellant, we need to turn to the rocket equation - again, you need more fuel to accelerate the fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket equation has the following unknowns:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The amount of velocity change required (known as &lt;em&gt;delta-V&lt;/em&gt;). In our case, that's 20 000 km/h - or around 5 500 m/s.&lt;/li&gt;&#xA;&lt;li&gt;The exhaust velocity of our rocket. In our case, 18 000 km/h, or about 4 900 m/s.&lt;/li&gt;&#xA;&lt;li&gt;The &quot;dry&quot; mass of the spaceship - that is, excluding the fuel and propellant. In our case, we'll stick with the 1 ton value.&lt;/li&gt;&#xA;&lt;li&gt;The &quot;initial&quot; mass of the spaceship, including fuel and propellant. This is what we want to determine.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Putting the known values in our equation gives us an initial mass of about 3.2 tons - so you need about 2.2 tons of fuel to get your 1 ton spaceship to 20 000 km/h. That's already pretty bad - don't forget that we also need to get the spaceship into orbit in the first place, where the payload/fuel ratio is even worse. But wait, it gets even worse. To stop the ship, you &lt;em&gt;again&lt;/em&gt; need to accelerate the same amount. Another 5 500 m/s brings us to a fuel mass of 9.3 tons - and that's just stopping at the target. To enable a mid-transit return trip, you need 5 500 m/s for the initial acceleration, 5 500 m/s to stop, 5 500 m/s to start back again and 5 500 m/s to stop at the end. This gives you a monstrous 22 000 m/s delta-V requirement, requiring 106 tons of fuel for your tiny 1 ton spaceship. And that ignores the mass required to actually &lt;em&gt;store&lt;/em&gt; all that fuel, as well as all the fuel lines etc. needed to get it to the engine!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be obvious that with our current rocket engine technology, what you're proposing is pretty much outright impossible. There's two ways around this - one is staging (throwing away &quot;spent&quot; parts of your spaceship), one is keeping your delta-V requirements as low as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Staging allows you to build rockets with higher fuel/ship mass ratios. For a single stage rocket, a ratio of about 15 is already pretty difficult, and anything above 20 is very unrealistic given our technology. This is mostly due to the fact that you need all kinds of supports etc. to make sure your rocket doesn't crumble (or explode - the common fuel mix is liquid hydrogen and liquid oxygen). The cool thing about staging is that you basically multiply the mass ratios of the different stages - so to get at a 100+ mass ratio, you could use three stages, each with a mass ratio of 5, for example. The drawback is that you end up littering your spent stages all over the place (their price is pretty much thrown away, and spaceship stages aren't exactly cheap), and you still need to use huge amounts of fuel to deliver a bit of payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is where the delta-V tricks come in. One great trick is using the target's atmosphere for the final braking - this is used all the time by craft returning to Earth, for example. Instead of having to nullify your whole orbital speed, you only need to shave off about 100 m/s or so to get you low enough into the atmosphere to get enough drag to &quot;steal&quot; the remaining orbital velocity. Doing the same with interplanetary velocities is a fair bit trickier, though - you only have a limited time to get rid of enough velocity to get you in a reasonable orbit. And the faster you need to brake, the stronger your spaceship structure must be - and stronger structure means more mass. Yet another reason to keep the ship/probe as tiny as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another great trick is using the gravity of other planets to modify your velocity - this needs a lot of planning, but allows you to both speed up and slow down without expending any fuel (other than the bit required for path corrections). This was used to great effect with the Voyager missions of the 70s - Voyager 1 has achieved a velocity of over 17 km/s thanks to its numerous gravity assists. Even then, this is only about half of Earth's orbital velocity - so you really want to make sure to steal as much velocity from Earth as possible on launch. The tricky part about this? For one, it's kind of slow. The path you take is likely going to be much longer than a direct path, especially if you need multiple gravity assists from multiple planets. The Voyager missions were possible mainly because in the 70s, the outer planets were aligned in a way that allowed the Voyagers to use Jupiter and Saturn (+ Uranus for Voyager 2) in a sequence of assists that increased their velocity &quot;for free&quot;. The trick there? Those alignments don't happen all that often. Another alignment that would allow the same kind of mission happens about once in 170 &lt;em&gt;years&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the planet position issue isn't that much of a problem when travelling from Earth to Mars - for one, you don't need nearly as much delta-V as the Voyagers, and second, both Earth and Mars have much shorter orbits. But still, if we keep to modern-style rockets (as opposed to the so-called &quot;Torchships&quot;, which would be capable of accelerating the whole trip, avoiding a lot of the issues mentioned here thanks to their massive exhaust velocities), position matters a lot. This gives rise to the familiar concept of &lt;em&gt;launch windows&lt;/em&gt; (which are also used to great effect in drama - &quot;if we don't go now, our next option is in 3 years, by which time we'd starve&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to get somewhere on the lowest delta-V budget possible, Hohmann transfer orbits are your friend. However, they are also the slowest, and the most dependent on the alignment of the planets. Launch windows are quite periodic - again, the Voyager missions have launch windows every 175 years or so, and Mars missions around 780 days. The more delta-V you're willing to spend, the wider your launch window and the shorter your trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example, getting from Earth to Mars and back requires a minimum of about 30 km/s of delta-V. For comparison, the massive Saturn V rocket is only capable of about 18 km/s. It shouldn't be very surprising that we make the interplanetary probes as light as possible, and we don't bring them back again :) This trip would take about 17 months. That's quite long - it doesn't matter much for an unmanned probe, but any human crew would need to bring a lot of supplies, be pretty good at recycling and not get mad. Adding more delta-V to the budget allows us to trim this considerably - 53 km/s will allow you to do the trip in just two months. Of course, getting a delta-V of 53 km/s is well beyond our current capabilities. Ignoring the mass of the rather massive fuel tanks, you'd need a mass ratio of about 80 000. Yikes. Interestingly, increasing the budget to 94 km/s will only save you about half a month - adding more delta-V gives diminishing returns, since you're still relying heavily on using tricks - the more delta-V, the less effective these tricks are in comparison.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what about absolutely &lt;em&gt;absurd&lt;/em&gt;, outrageous delta-Vs? The next obvious step is something that can accelerate the whole trip. We're already experimenting with ion drives for exactly this, but with the kind of acceleration we're getting, it's not really useful for humans. But let's imagine we have some fusion torch drive that gives us enough reachable delta-V. Pushing the delta-V to 370 km/s (remember how I said &lt;em&gt;absurd&lt;/em&gt; delta-V?) makes the roundtrip about one month long. This corresponds to a constant acceleration of about 0.01 g. By this point, orbital mechanics are no longer much of a limitation, so adding more delta-V tends to give you close to a linear speedup - 0.1g acceleration requires about 1 100 km/s for a 12 day roundtrip, and 1g requires &quot;only&quot; 3 500 km/s for a 4 day roundtrip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, once you get to a torchship, your original problem disappeared. Even for the 0.01g constant acceleration scenario, your velocity is huge compared to orbital velocities, so the cost of turning your ship around is relatively tiny (compared to going the whole way and back again), and the impact of all those manenuvering tricks are quite tiny as well (so you can shave off 1 km/s of my delta-V budget? Big deal...).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as long as you're flying in a modern rocket, that takes 9 months to get to Mars... once you're on the interplanetary trajectory, there's no going back. Even if you expend all your remaining fuel, you're not even going to stop your ship, much less turn it around. You'll just have to sit out the rest of the trip :) &lt;/p&gt;&#xA;" OwnerUserId="2861" LastEditorUserId="2861" LastEditDate="2016-01-06T09:59:36.973" LastActivityDate="2016-01-06T09:59:36.973" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13374" PostTypeId="2" ParentId="13365" CreationDate="2016-01-05T15:27:07.113" Score="10" Body="&lt;p&gt;Any lawmaker involved in writing a bill is entitled to change the bill in any way he or she sees fit. That change alone cannot be enacted by that single representative, but must be voted on by the entire Congress. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, the process of inserting one's own pet project into such a bill is known as an &lt;a href=&quot;https://en.wikipedia.org/wiki/Earmark_%28politics%29&quot;&gt;&quot;Earmark&quot;&lt;/a&gt;. Usually this is done to benefit one's home location, but can be used to support any project that the congressmen wishes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, NASA Earmarks have a long history. For instance, the &lt;a href=&quot;https://oig.nasa.gov/audits/reports/FY07/IG-07-028.pdf&quot;&gt;Office of Inspector General&lt;/a&gt; did a report on NASA's earmarks in 2006. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also worth noting that the President can specifically request NASA to earmark money for some project. The two most notable examples are Kennedy's 1960 request to get a man to the Moon, and less famously President Bush's &lt;a href=&quot;https://www.nasa.gov/pdf/55522main_FY05_Budget_Briefing020304.pdf&quot;&gt;2004 exploration program&lt;/a&gt;. These are usually done in concert with NASA missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, the &lt;a href=&quot;http://news.discovery.com/space/private-spaceflight/europa-mission-wins-big-in-new-nasa-budget-141226.htm&quot;&gt;NASA Europa Exploration&lt;/a&gt; mission as a part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_Science_Decadal_Survey&quot;&gt;Planetary Sciences Decadal Survey&lt;/a&gt;, published in 2011. Initially it was allocated \$75 million, presumably over a period of time, \$15 million being a part of the first year. It has since grown every time it has passed through Congress. And in fact, the &lt;a href=&quot;http://www.space.com/29713-europa-mission-approved-for-development.html&quot;&gt;mission has been approved by NASA&lt;/a&gt; for continuing studies, in a mission concept phase. This mission is a Jupiter orbiter concept, which makes frequent close approaches to Europa, an anticipated 45 times in the primary mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the lander bit specifically, it is within Congress's rights to do so, and part of the appropriation they have for the mission has been devoted to studying a lander. There is evidence that NASA has been interested, such as &lt;a href=&quot;http://spaceflightnow.com/2015/04/10/nasa-invites-esa-to-build-europa-piggyback-probe/&quot;&gt;NASA inviting ESA&lt;/a&gt; to send a lander with the Europa mission. In my personal opinion, making such a change to the mission is not wise, but not completely unreasonable either. Europa has received considerable attention in the past, far more than Titan had when the Huygen's probe landed on it. At a later date, if it becomes clear that NASA cannot include the lander, then they can return to Congress to explain why, nothing is set in stone yet. It is still worthwhile to explore such concepts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, NASA would prefer to pick their own missions, at least according to this &lt;a href=&quot;http://www.washingtonpost.com/wp-dyn/content/article/2006/04/25/AR2006042501702.html&quot;&gt;Washington Post article in 2006&lt;/a&gt;. They prefer to have Congress set the priorities, and perhaps a very general budget, and let NASA figure out the specifics themselves. In this instance, it seems that NASA laid out the missions that it would like to do, and a particular Congressman took it into his own hands to prioritize one of them. Granted the Europa mission is one of the higher priority missions on the list, and as it was extra money allocated to NASA, and not taken from another pile of money allocated to them, I'm sure they aren't complaining about it too much.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-01-05T16:02:59.160" LastActivityDate="2016-01-05T16:02:59.160" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13375" PostTypeId="2" ParentId="13331" CreationDate="2016-01-05T16:06:17.520" Score="14" Body="&lt;p&gt;This is what Kenneth S. Thomas and Harold J.McMann have to say about it in U.S. Spacesuits:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Operating pressures:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Shuttle extravehicular mobility unit (EMU) has an operating&#xA;  pressure of 4.3 psi (30 kPa) and the Shuttle crew escape/launch/entry&#xA;  suits operates at a maximum of 3.5 psi (24 kPA). All Russian&#xA;  spacesuites, in comparison, operate at 5.8 psi (40kPa) to minimize or&#xA;  avoid decompression sickness or other risks.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Decompression:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;U.S. Studies have indicated that people could rapidly decompress from&#xA;  14.7 psi (1 atm) to 8pis (55 kPa) with minimal risk. Similar Russian studies viewed decompression sickness risk slightly differently. From&#xA;  14.7 psi, a russian suit pressure of 5.8 psi (40 kPa) is judged to be sufficient to avoid decompression sickness after half and hour of&#xA;  breathing pure oxygen, which is approximately how long it takes to&#xA;  perform a suit checkout before going out to do an extravehicular&#xA;  activity. As a result, all Russian spacesuits feature a 5.8 psi&#xA;  operating pressure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So as well as the specific job that the suit is designed for, it seems that studies about decompression have largely guided these operating pressures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the other variables will be down to maneuverability, comfort, fatigue and cost. As long as the variables are within safe limits, then there is room to move with other factors in mind. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earlier spacesuits had lower pressures as described below:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Humans require environmental parameters to be within prescribed limits&#xA;  for comfort and to effectively perform work. One significant parameter&#xA;  is oxygen concentration. In a sea level atmosphere of 14.7 psia, the&#xA;  oxygen partial pressure is 3.08 psia. This results in an oxygen&#xA;  partial pressure with the alveoli of the lungs of 2.0 psia. NASA&#xA;  selected this as the lower limit of alveolar pressure for nominal&#xA;  human space operations. To maximize spacesuit joint mobility and to&#xA;  minimize leakage and loads of the pressure suit, spacesuits are&#xA;  designed to operate at the lowest pressure consistent with other&#xA;  requirements. Hence all spacesuits systems provide a breathing&#xA;  atmosphere of 100% oxygen (discounting small amounts of carbon dioxide&#xA;  and water vapor). However, because breathing efficiency decreases as&#xA;  pressure decreases, the normal operating pressure for U.S. spacesuits&#xA;  in the 1960s and early 1970s was established at 3.7psi.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There was a competitive review between systems and in February 1990 the decision was made to use 4.3 psi. The main reasons were to be able to support both exrtavehicular activity (EVA) and provide more effective crew escape and survival in intravehicular emergencies.&lt;/p&gt;&#xA;" OwnerUserId="3433" LastActivityDate="2016-01-05T16:06:17.520" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13377" PostTypeId="2" ParentId="13363" CreationDate="2016-01-05T16:18:55.123" Score="4" Body="&lt;p&gt;The desired trajectory is computed on the ground before launch. The rocket determines how well it is following the planned path, and may make some adjustments if required (Most notably the Falcon 9). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two reference points of interest. The first is how position is modeled, the second is how the attitude is modeled. Position references are usually done in a coordinate system called Earth Centered Inertial. This basically calls the center of the Earth 0,0,0 with axis being the poles, 0 and 90 degree longitude equator points. The I comes into effect because while the ECI coordinates are set at a point in time, they adjust based on the rotation of the Earth. So an object on the ground not moving will be moving with the rotational speed of the Earth. &#xA;This is found either by a high precision accelerometer (Part of a device called an &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_measurement_unit&quot; rel=&quot;nofollow&quot;&gt;IMU&lt;/a&gt;) or GPS. Usually the IMU is used for primary navigation, supported by GPS as a backup in case of serious problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the orientation, one can use a number of systems. For rockets, I believe the most common is Quaternions with a reference point of the position straight down. This can be measured by a number of methods, usually an IMU is used, which is a device that basically figures the difference in pointing from the start point.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="415" LastEditDate="2016-02-04T12:08:42.880" LastActivityDate="2016-02-04T12:08:42.880" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13379" PostTypeId="5" CreationDate="2016-01-05T17:32:14.870" Score="0" Body="&lt;p&gt;The Salyut stations were a series of manned Soviet space stations, the first space stations after the American station Skylab. After Salyut 7, they were succeeded by the Mir station.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-05T17:34:03.097" LastActivityDate="2016-01-05T17:34:03.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13380" PostTypeId="4" CreationDate="2016-01-05T17:32:14.870" Score="0" Body="Questions regarding the Soviet Salyut space stations" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-05T17:34:10.023" LastActivityDate="2016-01-05T17:34:10.023" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13381" PostTypeId="5" CreationDate="2016-01-05T17:50:57.900" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-01-05T17:50:57.900" LastActivityDate="2016-01-05T17:50:57.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13382" PostTypeId="4" CreationDate="2016-01-05T17:50:57.900" Score="0" Body="Questions regarding torque generation and momentum storage by the means of rotating wheels" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-05T19:03:26.397" LastActivityDate="2016-01-05T19:03:26.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13383" PostTypeId="5" CreationDate="2016-01-05T18:02:24.870" Score="0" Body="&lt;p&gt;The bottom section of a rocket with multiple sections, that lifts the mass of the entire loaded rocket off the ground and accelerates it through the lower atmosphere. Once its fuel is spent, it separates from the rocket and falls, and the next section - the second or upper stage - ignites to complete the journey to orbit. &lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="4660" LastEditDate="2016-01-05T19:29:14.353" LastActivityDate="2016-01-05T19:29:14.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13384" PostTypeId="4" CreationDate="2016-01-05T18:02:24.870" Score="0" Body="The bottom section of a rocket with multiple sections, that lifts the mass of the entire loaded rocket off the ground and accelerates it through the lower atmosphere. Once its fuel is spent, it separates from the rocket and falls, and the next section - the second or upper stage - ignites to complete the journey to orbit." OwnerUserId="8693" LastEditorUserId="4660" LastEditDate="2016-01-05T19:30:44.110" LastActivityDate="2016-01-05T19:30:44.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13385" PostTypeId="1" CreationDate="2016-01-05T21:06:32.350" Score="1" ViewCount="310" Body="&lt;p&gt;Today I learned about &lt;a href=&quot;https://en.wikipedia.org/wiki/Sea_Launch&quot; rel=&quot;nofollow&quot;&gt;Sea Launch&lt;/a&gt;, essentially a launch pad floating on the sea. (Pretty crazy, right?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's cool about that is that you can launch from the equator without the tricky part about your country being on the equator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, say, hypothetically, SpaceX* made their own floating launch pad on the equator. Obviously it would be costly to make, costly to transport the rocket there, and there's possible pain in the neck with ITAR requirements, etc…&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what's the upside? How much extra performance could you squeeze this way vs launching from Cape Canaveral (or even more southern Boca Chica)? Say you want to launch to a geostationary orbit, how much more payload could you carry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;* - Or anyone else for that matter&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="51" LastEditDate="2016-01-05T21:53:39.277" LastActivityDate="2016-01-05T21:53:39.277" Title="How much extra performance could Falcon 9 have if launched from the equator?" Tags="|launch|falcon-9|geostationary|performance|" AnswerCount="0" CommentCount="3" ClosedDate="2016-01-07T19:56:28.367" ContentLicense="CC BY-SA 3.0" />
  <row Id="13386" PostTypeId="1" AcceptedAnswerId="13387" CreationDate="2016-01-05T22:45:46.740" Score="7" ViewCount="497" Body="&lt;p&gt;How are SpaceX supposed to return the rocket's first stage to the launch pad/hangar for re-fitting after it has landed on a landing zone on land, or on a barge?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since they aren't landing on a launch pad, they must get it back to a launchpad for the next launch? Or do they really need to do that? There seem to be a lot of equipment involved at the launchpads to keep the cooled down fuel, oxidizers and whatnot, that you don't have at landingzones or barges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will they fill the tanks just a little so the first stage can hover by itself, or lay it down horizontally and move it by trucks and cranes or even some &lt;a href=&quot;https://en.wikipedia.org/wiki/Mil_Mi-26&quot;&gt;russian helicopter&lt;/a&gt;? ;)&lt;/p&gt;&#xA;" OwnerUserId="12923" LastActivityDate="2016-01-07T14:21:25.707" Title="Move landed rocket stage back to launch pad" Tags="|rockets|spacex|reuse|first-stage|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13387" PostTypeId="2" ParentId="13386" CreationDate="2016-01-05T22:52:41.960" Score="10" Body="&lt;p&gt;SpaceX's &lt;a href=&quot;https://en.wikipedia.org/wiki/Landing_Zone_1&quot; rel=&quot;noreferrer&quot;&gt;Landing Zone 1&lt;/a&gt; is just a few miles down the road from their launch sites at &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station&quot; rel=&quot;noreferrer&quot;&gt;Cape Canaveral&lt;/a&gt;, so they use a crane to put it on a truck horizontally and drive it back to a hangar near the launch site (at &lt;a href=&quot;https://en.wikipedia.org/wiki/Kennedy_Space_Center_Launch_Complex_39&quot; rel=&quot;noreferrer&quot;&gt;LC-39A&lt;/a&gt;) for cleanup, inspection, and refurbishment. Eventually, presumably, they'll be able to prep a landed stage for reflight there and mate it with a new upper stage and payload; right now they launch out of &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Space_Launch_Complex_40&quot; rel=&quot;noreferrer&quot;&gt;LC-40&lt;/a&gt; next door.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For barge landings, similarly, they'll bring it back to land, then drive it to the refurbishment or relaunch site by truck. (This is how they bring their rockets to the launch site to begin with.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For safety reasons, the landing site consists of just large bare pads of concrete -- nothing to crash into. While SpaceX hit within a couple of meters of bullseye on their first LZ1 landing, that's no guarantee they can repeat it consistently. The launch sites have quite a bit of infrastructure installed, including lightning-rod towers surrounding the launch pad, so they aren't appropriate as landing sites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We're a long way away from being able to simply land, refuel, add a new payload, and take off from the same location as we do with commercial aircraft; I don't think there's any expectation that Falcon rockets will be able to do this. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-01-06T03:24:10.893" LastActivityDate="2016-01-06T03:24:10.893" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13388" PostTypeId="1" CreationDate="2016-01-06T00:44:52.717" Score="6" ViewCount="740" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_II#cite_note-16&quot;&gt;Delta II&lt;/a&gt; is a rocket capable of up to 6 metric tons to LEO, and has the best safety record of any launcher in history. ULA still has the &lt;a href=&quot;http://www.ulalaunch.com/products_deltaii.aspx&quot;&gt;launcher listed&lt;/a&gt; as one of its products because it has one unsold rocket, but it has stopped making them, despite the rocket's great success. From &lt;a href=&quot;http://spaceflightnow.com/2015/04/08/what-to-do-with-the-final-delta-2-rocket/&quot;&gt;SpaceFlightNow&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The United Launch Alliance vehicle has flown 153 times since 1989,&#xA;  accumulating 151 successes and currently rides a string of 98&#xA;  consecutive flawless missions. Only three vehicles remain in&#xA;  inventory, with NASA the customer for two of them to launch the JPSS 1&#xA;  weather satellite in 2016 and the ICESat 2 environmental spacecraft in&#xA;  2017, both from Vandenberg Air Force Base in California. That leaves&#xA;  one rocket up for grabs, but no one appears to have a payload in the&#xA;  class range of the Delta 2 from the sole remaining launch site to&#xA;  polar orbit interested in purchasing the ride. “I’m afraid, no, I&#xA;  don’t have a sale for it,” said ULA CEO and President Tory Bruno. “We&#xA;  are keeping it in the factory for now in case somebody appears or we&#xA;  need a spare part for the last two birds that are sold.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;SpaceFlightNow also has &lt;a href=&quot;http://www.spaceflightnow.com/news/n0908/29mediumlift/&quot;&gt;an article&lt;/a&gt; explaining that the Taurus 2 and Falcon 9 have out-competed the Delta II on price:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Both rockets are designed to be less expensive than current launchers,&#xA;  an important factor for science missions in a tight budget&#xA;  environment. Lower launch costs would allow more missions to be&#xA;  started for less money.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Alright, but is ULA really not going to put up a fight somehow? Is there really no other market for them for the world's most reliable rocket? Is there some strategic reason why they aren't interested in cutting the price for the sake of maintaining a market share?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2016-04-15T08:55:25.783" Title="Why are there no more Delta II rockets?" Tags="|rockets|commercial|ula|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13389" PostTypeId="2" ParentId="8412" CreationDate="2016-01-06T02:24:47.487" Score="2" Body="&lt;p&gt;It isn't entirely clear to me whether you are doing a series of impulsive (zero time duration) maneuvers with coasts between, or an actual &quot;finite burn&quot;, a more realistic maneuver that takes some non-zero time to accomplish. In either case, however, you might be carrying some of the propellant to a higher potential energy before burning it, relative to an instantaneous burn. This &quot;gravity loss&quot; is noticeable. For an extreme example, this effect was used by the onboard &quot;Gamma Guidance&quot; algorithm to adjust the timing of the third of three solid rocket motor burns by the IUS/PAM-S upper stage to counter dispersions in the first two burns to place the ESA spacecraft Ulysses more accurately onto its initial hyperbolic trajectory away from Earth in 1990. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Ph.D. dissertation of Greg A. Dukeman, Georgia Institute of Technology, 2005, &quot;Closed-Loop Nominal and Abort Atmospheric Ascent Guidance for Rocket-Powered Launch Vehicles&quot; in part (Chapter 3) discusses finite burns giving equations for &quot;Thrust Integrals&quot; (analytic solutions for the change in radius and velocity vectors over the course of a finite burn), for constant thrust or constant thrust acceleration for simplified equations of motion (burn direction changes linearly with time, and gravity is taken as constant). See the thesis for why this is useful even with these simplifications. (Beware the typo in equation 3-3. I believe f11*T*A should instead be f11*A in the first term for delta R sub T.) Unless your maneuvers are very long (and perhaps even then) this technique could be used to target your burns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is much more of interest in this very fine dissertation, available online. &lt;a href=&quot;http://hdl.handle.net/1853/6820&quot; rel=&quot;nofollow&quot;&gt;http://hdl.handle.net/1853/6820&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12562" LastActivityDate="2016-01-06T02:24:47.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13390" PostTypeId="2" ParentId="10169" CreationDate="2016-01-06T02:37:48.033" Score="2" Body="&lt;p&gt;I always thought the term was defined by the relative influence of the 2 major forces perturbing a satellite's orbit.  Namely:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The sun.  The sun's gravity causes the plane of the orbit to rotate slightly with a period roughly equal to the 1.5 times the planet's year squared divided by the satellite's orbital period.  For our moon this is 18.6 years.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The oblateness of the planet, which tends to push satellites into equatorial orbits.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Now, for our moon #1 is much bigger than #2, but for most our our artificial satellites, #2 is bigger than #1 if they are in approximately equitorial orbits.  So their orbits do not rotate.  If this were not true, then geo-synchronous satellites would be impossible, as the sun would rotate the orbit about the ecliptic, and the satellite would not be stationary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if the satellite is sufficiently close and is sufficiently equitorial, then its orbit does not rotate and it is a &quot;near equitorial&quot;; otherwise its orbit does rotate and it is not.  My understanding is that the majority (but definitely not all) the moons in the solar system are near equitorials; the fact that the outer 4 planets are gas giants with huge oblateness relative to the earth is undoubtedly responsible for this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I probably don't have all this exactly correct; perhaps someone can clean up this rough definition.  But I think this is the general idea.&lt;/p&gt;&#xA;" OwnerUserId="12927" LastActivityDate="2016-01-06T02:37:48.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13391" PostTypeId="2" ParentId="13366" CreationDate="2016-01-06T10:32:51.480" Score="0" Body="&lt;h2&gt;Spacecraft cannot take a less efficient path than originally intended&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As a rule, spacecraft will have enough fuel to accomplish it's intended mission path and some safety margin, but not more than that. Furthermore, since fuel is so huge part of the total weight, you will optimize the mission to take the most efficient suitable path, often relying various factors like choosing transit moment to fit a particular position of planets, using gravity assist and the like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, due to the way orbital mechanics work, if you're en route to Mars then the cheapest way (fuel-wise, not time-wise) to turn around is to go to Mars orbit and burn your engines there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even simply &quot;stopping&quot;(e.g. relative to sun) on the spot in middle of nowhere already requires more fuel than it took you to get up to speed, since the mission definitely was planned to use earth orbital speed and earth rotation speed to the advantage of the initial mission plan; and reversing also takes more fuel than expected since the earth is moving away from you in its orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that if you have enough fuel for &lt;em&gt;a particular&lt;/em&gt; trip to Mars and back, after you burn nearly half of it to start the initial trip, any significant changes will require more fuel - which you simply won't have.&lt;/p&gt;&#xA;" OwnerUserId="2667" LastActivityDate="2016-01-06T10:32:51.480" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13392" PostTypeId="2" ParentId="13366" CreationDate="2016-01-06T10:44:23.967" Score="3" Body="&lt;p&gt;Orbital mechanics is weird. When on Earth, turning around and heading home is usually a good solution when something goes wrong. This isn't the case in space because of some strange quirks.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Stopping (in space) is REALLY REALLY hard.&lt;/li&gt;&#xA;&lt;li&gt;The shortest distance is CURVED.&lt;/li&gt;&#xA;&lt;li&gt;Home is constantly moving (really really fast)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Stopping is hard&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So with the first item, stopping is hard. This is because you can't build brakes in space. Which means to stop, you need a rocket just as big as the one that you used to get you going in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you need to send a really big rocket, then you need an even bigger rocket to get that going in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We call this problem the &quot;tyranny of the rocket equation&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is why we almost always stop on planets where we can use brakes (or parachutes as you call them).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The shortest distance is curved&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, earth is pretty flat. But if need to walk from a-b and there is a hill in the way, you would generally walk around the hill.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space is the same. With orbital mechanics, there is a MASSIVE hill in the middle we call the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Home is constantly moving&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the time scale of a trip to Mars, Earth will move. If you turned around and headed back again, it won't be there. It is possible that it is now &quot;on the other side&quot; of Mars, depending on the alignment of the stars.&lt;/p&gt;&#xA;" OwnerUserId="6157" LastActivityDate="2016-01-06T10:44:23.967" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13393" PostTypeId="1" CreationDate="2016-01-06T11:42:17.227" Score="10" ViewCount="1866" Body="&lt;p&gt;Turning $\require{mhchem}\ce{CO2}$ and some hydrogen into $\ce{O2}$ and $\ce{CH4}$ seems pretty straightforward and has been known since early chemistry in the 19th century. But what about the quality, the purity of the fuel produced? Kerosene rockets use highly refined kerosene, would they still work with low grade kerosene? How sensitive is a methane rocket engine (like the Raptor under development), and especially reusable engines, to impurities in $\ce{CH4}$ and $\ce{O2}$? Is it challenging to produce pure enough rocket fuel and oxidizer on Mars, compared to just producing some of it? And doesn't it require quite a gear to cool and pressurize them to liquids in fuel tanks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The illustration below, of the oxygen extracting MOXIE payload planned for the Mars 2020 rover, kind of hints that it isn't as trivial as the chemical formula suggests. Will MOXIE produce (not liquefied I suppose) oxygen pure enough for use in a rocket engine?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DUiBX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DUiBX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2016-01-26T14:58:26.143" LastActivityDate="2016-01-26T14:58:26.143" Title="What is required to produce rocket quality methane fuel and oxygen from Mars' atmosphere?" Tags="|mars|rockets|fuel|oxygen|chemistry|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13395" PostTypeId="2" ParentId="13363" CreationDate="2016-01-06T13:34:35.927" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;But what is rocket's reference in the control system loop? From where do gimbals get the information on how they should position themselves?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is a collaborative effort split amongst the design of the flight software, pre-launch flight planning, commands from the ground, and the operation of the flight software. The key components of the flight software involved in this process are moding, guidance, navigation, and control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moding (which goes by various names) determines overall spacecraft operations. Even ignoring the myriad failure, recovery, and abort modes, launch vehicles switch operating modes a number of times during the course of a launch. The mode dictates which algorithms and magic numbers (e.g., control gains) the guidance, navigation, and control systems use to take the spacecraft to the desired orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The navigation software uses a variety of sensors to keep track of the vehicle's state. This state includes position and velocity, attitude and attitude rate, plus other parameters such as angle of attack and sideslip. The vehicle's inertial measurement unit, which senses acceleration and angular velocity, is one of the key inputs to the navigation system. The Saturn V had a gimbaled IMU, so it reported acceleration with respect to some inertial frame. This was very expensive and prone to error. Modern accelerometers are fixed with respect to the vehicle and this report sensed acceleration in a frame fixed with respect to the vehicle. This sensed acceleration needs to be transformed to some inertial frame to be of use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The accelerometers sense acceleration to thrust and drag, but not gravity. (Accelerometers cannot sense gravity.) The navigation system needs to augment those sensed accelerations with a model of the Earth's gravitational field. Integrating the computed acceleration to yield velocity, and then integrating that to yield position is called dead reckoning. Without correction, the estimated state would drift from the true state. Modern navigation systems use GPS to provide an alternative estimate of position. Reconciling the conflict between these disparate measurements is the job of the navigation system's Kalman filter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The navigation system feeds the estimated state to the guidance system. The guidance system uses the flight plan (calculated on the ground, prior to launch) to determine the error between the planned and navigated state. This error might be due to thrusters not behaving quite as planned, a change in the wind, a specifically-planned maneuver such as the roll program initiated shortly after launch, or errors in the navigated state. Whatever the cause, the vehicle's planned and navigated state don't agree with one another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The guidance system feeds this state error to the control system. The control system uses the state error as a hint to issue commands to various actuators. The error has to be used as just a hint; small errors are best left uncorrected, large errors can't be corrected instantaneously, and some errors just aren't corrected at all. In the case of vehicles with throttleable engines, changing the thrust level can help reduce the errors in velocity and position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Correcting errors in attitude and attitude rate is the job of the attitude controller. A number of different approaches have been and continue to be used for this. One widely-used approach is a phase plane controller. I'll use roll as an example. Suppose the roll error is negative and the roll rate error is positive. The best thing to do might be well be to do nothing. The positive rate error will eventually result in the vehicle having the correct roll angle. A phase plane controller has dead bands where nothing is done. Outside of these dead bands, the phase plane control indicates that something does need to be done. The controller's gain settings translates this something into commands to the gimbals, if the rocket has gimbaled engines. Some rockets don't have gimbaled engines; they instead use vernier jets or thrust vectoring. Whatever the case, errors outside of the dead band result in actuator commands that move the spacecraft toward the desired attitude / attitude rate.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-01-06T13:34:35.927" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13396" PostTypeId="1" CreationDate="2016-01-06T13:58:49.513" Score="6" ViewCount="7226" Body="&lt;p&gt;Based on an XKCD comic, &lt;a href=&quot;http://xkcd.com/1626/&quot; rel=&quot;noreferrer&quot;&gt;XKCD 1626&lt;/a&gt; which has the image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ud4gW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ud4gW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do any of the active duty ICBM's have the ability to actually target the Sun?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that the R-7 booster that is the basis used by Soyuz/Progress was an ICBM, as were the original Atlas and Titan programs boosters, perhaps orbital. But Earth escape, and then towards the Sun?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-08T12:11:19.193" Title="Do any current ICBM's have the delta-V to target the sun?" Tags="|launch|delta-v|performance|icbm|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13397" PostTypeId="2" ParentId="13396" CreationDate="2016-01-06T14:04:18.707" Score="3" Body="&lt;p&gt;No, nowhere near. Hohmannfan's answer describes various time-consuming and complex routes to the sun which take over 3200m/s ∆v beyond that needed to get to LEO, and ICBMs can't generally get to LEO. They are built to go from point A to point B on Earth's in about half an hour, and don't have substantial performance margin beyond that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/questions/3612/calculating-solar-system-escape-and-and-sun-dive-delta-v-from-lower-earth-orbit&quot;&gt;Q&amp;amp;A touches on the topic&lt;/a&gt; a little bit. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-01-06T22:01:58.490" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13398" PostTypeId="2" ParentId="13366" CreationDate="2016-01-06T16:13:52.620" Score="4" Body="&lt;p&gt;I'll try a less technical answer, because most people are unfamiliar with the orders of magnitudes involved in space travel, and sci-fi action movies taught us that flying in space is just like a ship cruising on the sea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To escape the gravity of Earth, you need a lot of speed. You will need over 40000 km/h, this is more than 40 times faster than a jet airliner, and more than 10 times faster than the fastest fighter jet has ever flown. To reach this speed, you need a huge rocket engine and an insane amount of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, if you don't use gravity assist but want to rely on your own engines to turn back, you have to expend (almost) the same amount of energy to stop what you used to speed up at the beginning, and then spend that energy again to achieve the same speed in the other direction. So why not just carry three times as much fuel? Because unlike a car where you can just put a small barrel of extra fuel in your trunk, most of the weight of a spacecraft is made up by the fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you look at a picture of a spacecraft before launch, you'll see that most of it is fuel:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mu5Dg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mu5Dg.jpg&quot; alt=&quot;Saturn V&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See that tiny little trapezoid near the top? That is the actual spaceship, all the rest is mostly fuel. So we'll just build our rocket 3 times larger, for the eventuality we want to turn back, right? Wrong! The humongous rocket on the picture was used to speed up that tiny little spacecraft at the top of it! To lift up all the extra fuel (which is heavier than the original entire rocket), you will need an even bigger rocket. And to lift off that even bigger rocket, you will need a much bigger rocket, and so on ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an &lt;a href=&quot;https://what-if.xkcd.com/38/&quot; rel=&quot;nofollow noreferrer&quot;&gt;interesting article on xkcd&lt;/a&gt;, describing what you will need to return one of our small probes which is on the way to leave the solar system: it would require a rocket at least 60 times bigger than the one we used to send it up!&lt;/p&gt;&#xA;" OwnerUserId="273" LastEditorUserId="273" LastEditDate="2016-01-08T18:45:39.290" LastActivityDate="2016-01-08T18:45:39.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13399" PostTypeId="1" AcceptedAnswerId="13402" CreationDate="2016-01-06T16:52:52.203" Score="13" ViewCount="3786" Body="&lt;p&gt;So in the light of some technical failures last year, I recently had a discussion about engineering endeavours throughout human history. An interesting recent adventure is the revival of rocketry. Naturally the question 'what are we still doing wrong after 60 yrs of experience with this technology?' arises.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Machines become ever more complex and thus more complicated to handle and control. I however discard the 'argument' that we will at some point hit rock bottom because we're not able to grasp the ever-increasing complexity of technology. Humans are able to handle much, much more, at least that's what I believe in my naivety.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is:&#xA;&lt;strong&gt;What are the major (engineering) difficulties that prevent us from building airplane-safe space-rocketry of accident rates of $\sim 2-4 \cdot 10^{-6}$ (&lt;a href=&quot;https://en.wikipedia.org/wiki/Aviation_safety#Accidents_and_incidents&quot; rel=&quot;nofollow noreferrer&quot;&gt;see here&lt;/a&gt;) per  launch?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. Yes, I have researched other similar questions like &lt;a href=&quot;https://space.stackexchange.com/questions/12594/is-it-possible-to-design-a-completely-fail-safe-rocket&quot;&gt;this one&lt;/a&gt;, but the simple reason &quot;the technology is still in it's infancy&quot; or #RocketsAreTricky is not enough for me. I'm able to understand the hard, nerdy reasons, so give me those please if you have them.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-10-28T17:00:57.390" Title="Engineering difficulties towards near-perfect rockets?" Tags="|rockets|safety|" AnswerCount="6" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13400" PostTypeId="2" ParentId="13399" CreationDate="2016-01-06T17:43:57.153" Score="15" Body="&lt;p&gt;The main problem is that in order to have enough thrust to get a rocket out of the atmosphere and up to orbital velocity, 85% to 90% of the rocket has to be fuel. So you have a controlled explosion, essentially, in which thousands of pounds of fuel is combusting each second and the force of that is being directed out of nozzles, pushing a rocket upwards with forces of several gravities, through supersonic winds, and you have to make the structure and engines that support all those forces as light as possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a good overview on the topic by astronaut Don Pettit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/uWjdnvYok4I?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even very minor failures can cause payload to fail to reach orbit or the loss of the vehicle, because there is so little margin for error. A part slightly out of place in a supersonic wind will make the whole structure unstable, which quickly makes things much worse until the vehicle tumbles. Fuel that burns with even slight unevenness will cause vibrations that cause other things to fail, which then cause more things to fail and soon something explodes. An engine that takes even a few seconds too long to light means the vehicle doesn't attain the speed to reach orbit. The slightest flaw in structures that have to withstand extreme pressures, extreme temperatures, severe vibrations and multiple gravities can become worse very quickly again leading to explosions.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-01-06T18:22:41.027" LastActivityDate="2016-01-06T18:22:41.027" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13401" PostTypeId="2" ParentId="13399" CreationDate="2016-01-06T18:12:00.063" Score="9" Body="&lt;p&gt;The biggest thing keeping us from having airplane-level accident rates is the lack of airplane-level flight rate.  Not recovering/reusing flown rockets doesn't help either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To date SpaceX has launched 25 rockets in 9 years (according to &lt;a href=&quot;https://spacexstats.com/#LaunchCount&quot;&gt;spacexstats&lt;/a&gt;) to orbit, with the most per year being 7 in 2015.  Every flight has been a completely new vehicle, and there have been incremental upgrades to the design over time so it's not like they've even got 25 launches of flight data on the same hardware.  Also, until the last flight they didn't get the flight hardware back so they couldn't analyze the flown rockets for potential issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, Boeing has delivered over 350 787-series airplanes in 4 years (&lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_787_Dreamliner#Orders_and_deliveries&quot;&gt;wikipedia&lt;/a&gt;) and has hundreds of flights per day, all of which are recovered and serviced so wear and stress are very well understood.  Beyond that, they had over 8 months of full-up flight testing before the first delivery, which isn't a luxury available to a rocket company that's not able to recover its rockets.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2016-01-06T18:12:00.063" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13402" PostTypeId="2" ParentId="13399" CreationDate="2016-01-06T18:23:56.127" Score="26" Body="&lt;p&gt;Practice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unusual for the same fault to cause more than one rocket launch to fail. Flaws are found and corrected, and launchers get more reliable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In all our history, there have been something like 5000 orbital launches. Each one of those was an opportunity for things to go wrong, for a flaw to be exposed and eventually corrected. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today, airlines fly 90,000 flights per &lt;em&gt;day&lt;/em&gt;. Over the last 50 years, we've had more plane &lt;em&gt;crashes&lt;/em&gt; than orbital launches. The industries of aircraft development and operation have simply had far more chances to learn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket builders and operators are learning plenty fast, though. The Atlas series has had 120 launches with only one partial failure since 1991, for instance -- just about as close to a perfect record as that small a sample size can get. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(In other words, &quot;the technology is still in its infancy&quot; -- the same answer you saw and didn't like on a previous similar question.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-01-06T18:23:56.127" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13403" PostTypeId="1" AcceptedAnswerId="13404" CreationDate="2016-01-06T19:03:13.723" Score="13" ViewCount="1292" Body="&lt;p&gt;Given that SpaceX's Dragon has been designed to be reusable (&quot;with minimal rework and fueling&quot;), why hasn't it been, in fact, reused?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've had 6 CRS missions bringing Dragon to ISS and back home (and a few Dragon test missions before that), but as far as I know (can't find source now), only CRS-11 will fly on a reused Dragon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This might be speculative, but is there a good reason not to? Is there a NASA decision that prevents them from doing so? Or is it just risk management? (Or, heavens forbid, are recovered Dragons in a really bad condition?)&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorDisplayName="user20636" LastEditDate="2018-06-15T06:16:02.057" LastActivityDate="2018-06-15T06:16:02.057" Title="Why hasn't Dragon been reused before CRS-11?" Tags="|spacex|reuse|dragon|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13404" PostTypeId="2" ParentId="13403" CreationDate="2016-01-06T19:10:12.977" Score="15" Body="&lt;p&gt;The NASA contract for the CRS missions specified a new Dragon for each flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That would explain why no Dragon has been reused on an ISS logistics mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that does not answer why SpaceX has not reflown any of the others, which they retain ownership of, post flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has proposed a commercial free flyer, where a Dragon would be launched with experiments and called DragonLab, reside in orbit some period of time, then return.  The current launch manifest for SpaceX shows two such missions on the list, but without dates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I heard &lt;a href=&quot;http://www.thespaceshow.com/show/04-mar-2011/broadcast-1522-special-edition&quot; rel=&quot;nofollow noreferrer&quot;&gt;Max Vozhoff&lt;/a&gt; on the &lt;a href=&quot;http://thespaceshow.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceShow&lt;/a&gt; talk about plans for DragonLab.  They had suggested offering the Department of Ed, an option to buy an entire Dragon flight, and offer a competition for students from all states to supply experiments.  But that never seems to have materialized. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been reported that components of Dragon's have been reflown, but not specifics of which components. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CRS-11 missions in 2017 and pretty much all future Dragon Cargo are expected to be the first to refly a Dragon to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally the first few Dragon attempts were learning experiences, and changes were made based on lessons learned. There was a leaking problem with water after landing in one of the Dragons, and changes to the power/refrigeration systems in later models. It may be they need to stabilize on a design as mainly static before reusing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon V2 will probably be a different story, being planned for reuse from the start, based on many previous Dragon cargo flights and years of testing and upgrading.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of Jun 2018, 3 Dragons have been reflown with number 4 due at the end of the month. CRS-11 and CRS-13 used the same booster (Core B1035).  CRS-12 and CRS-14 used the same booster (Core B1039). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;CRS-15 is reusing the booster (Core B1045, last Block 4) from the TESS mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reused capsules on reused boosters oh my!&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-06-14T17:38:16.200" LastActivityDate="2018-06-14T17:38:16.200" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="13406" PostTypeId="2" ParentId="13396" CreationDate="2016-01-06T20:06:38.463" Score="13" Body="&lt;p&gt;The delta-v needed from low Earth orbit to a Hohmann transfer orbit with a periapse inside the Sun is actually &quot;just&quot; 21,300 m/s. But there is a better option. A bi-elliptic transfer to just hit a central body is better when the ratio between the orbital radius and the radius of the central body is larger than 4.82. The orbital radius of the Earth  divided by the radius of the Sun is larger than 200. Therefore, accelerating up to solar system escape velocity, and then do a minuscule manoeuvre at a great distance is only going to cost 8750 m/s of delta-v from LEO. Even that is impossible for an ICBM. A fly by of Jupiter is even better, and combined with a multi fly hit-the-sun trajectory, it is only going to cost around 6300 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The theoretically lowest is however not going to cost more than a transfer orbit towards the Moon (3120 m/s). With three Moon flybys, you can escape the Earth-Moon system with a little $v_\text{inf}$. After a long time waiting for the then long synodic period to sync up, you can do a flyby of the Earth-Moon system to increase the $v_\text{inf}$. (You can not increase the $v_\text{inf}$ just with a flyby of a planet, however, you can if it is a two body system. That is the same effect as a binary star flyby, only much weaker.) Repeat this over again several times for the next century, and the warhead would hit the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3120 m/s is maybe achievable if you replace parts of the warhead with an upper stage, as a regular ICBM has trouble even going into orbit. Even then, major upgrades are required. But at least, it is within the same magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, why are you going to nuke the Sun? I assume you have a good reason for that. (Kind of not assuming that actually)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="415" LastEditDate="2016-01-08T08:48:21.753" LastActivityDate="2016-01-08T08:48:21.753" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13407" PostTypeId="2" ParentId="6448" CreationDate="2016-01-06T20:38:21.940" Score="4" Body="&lt;p&gt;The winners of the Commercial Crew were announced after this question was asked. At that point, both winners, Boeing's CST-100 and SpaceX's Dragon V2 will use the PMA based docking ports.  (There will be in 'insert', the IDA, International Docking Adapter between the vehicle and the older PMA's.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cargo resuppliers, Orbital Science's Cygnus and SpaceX Dragon use the CBM docking ports. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the Visiting Vehicle schedule for 2016 looking very busy, it looks like there will be several times when both providers will have vehicles at the station for cargo and using both CBM ports. With the initial SpaceX return to flight schedule, NASA was showing video of the Cygnus and Dragon docked to the station at the same time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That still leaves outstanding the issue of policy for the PMA's, when dealing with manned flights.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-06T20:38:21.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13408" PostTypeId="2" ParentId="13399" CreationDate="2016-01-07T01:01:16.910" Score="20" Body="&lt;p&gt;Just another analogy to maybe help explain why #EverythingIsTrickyNotJustRockets (in addition to the great answers already here).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take Formula 1 racing. It's a sport with some extremely complex machines, built to the very edge of engineering and manufacturing capabilities to give as much performance as possible. Just like rockets. Others have mentioned airplanes, but those are generally built to be safe and comfortable, too. F1 cars just have the imperatives of &quot;win race&quot; and &quot;try to not kill driver&quot;. Again, much like rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's plenty of experience with race cars. F1 cars as we see them today (rear-mounted engine, monocoque chassis) have a lineage back to the 1960s, and cars in general have of course been around much longer. And there's a lot more F1 racing than rocket launches, so there's more practice as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even so, take a look at any race from &lt;a href=&quot;https://en.wikipedia.org/wiki/2015_Formula_One_season&quot; rel=&quot;nofollow&quot;&gt;last year's season&lt;/a&gt;. For instance, &lt;a href=&quot;https://en.wikipedia.org/wiki/2015_Malaysian_Grand_Prix&quot; rel=&quot;nofollow&quot;&gt;the Malaysian Grand Prix&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of 20 cars, 5 failed to complete the race. And none of this was due to plain ol' collisions. One car spun off the track, so you could call that driver error, rather than #RacecarsAreTricky, but the remaining 4 were all due to technical issues. Specifically: Brake failure, turbo failure, power unit failure, and fuel system failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, &lt;em&gt;at best&lt;/em&gt;, that's still a failure rate of &lt;em&gt;20%&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's a pretty common failure rate in a Formula 1 race. There're &lt;em&gt;always&lt;/em&gt; cars that break down during races. And they're &lt;em&gt;just cars!&lt;/em&gt; Yes, they're advanced and insanely complex cars, but things like brakes, fuel systems, turbos - those are all things that regular, everyday cars have too. We know these systems well; the F1 versions are just the very best that money can buy. And yet they fail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare that to rockets, which have nothing in common with everyday vehicles, and where there's just nowhere near the same amount of practice to begin with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if we have a hard time making &lt;em&gt;a car&lt;/em&gt; go reliably around a squiggly circle for a few hours, it's amazing that we're able to make rockets at all.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To respond to your earlier comment:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I was under the impression, that the most fragile parts in any construction should be known at the time of designing it, thus the question of project success would boil down to material and engineering quality. Is that impression totally wrong?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Alright, let's say you build a liquid fuel rocket. You know exactly how much thrust it'll produce, and you make the structure strong enough to withstand more than that. Good safety margin. You use the best materials and all that. It's all completely by the book.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you ignite it. Inertia keeps the rocket on the ground at first, with the engines pushing hard against their support struts, so the struts flex a little. They have to: They can't be infinitely strong. But you've accounted for that, so it's fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fuel combustion is almost perfect. Almost. You simply cannot guarantee that it's 100% perfect, because it's a chaotic, violent chemical reaction. However, it's well within tolerance, and the rocket takes off beautifully. Just as planned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that tiny bit of combustion instability causes some vibration. The vibration makes the engine supports flex a tiny bit more. Nothing to be alarmed about, they're built for it. If they weren't, they'd snap instead of flex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But when they flex, the fuel line to the engine also gets compressed and stretched too. This changes the flow rate of the fuel, which changes engine pressure, and, in turn, thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now, your engine's initial combustion instability has has caused &lt;em&gt;more&lt;/em&gt; instability, and &lt;em&gt;more&lt;/em&gt; vibration. The engine starts to vibrate more. It doesn't (yet) flame out or explode or anything, but it's not producing a stable amount of thrust. Engine supports and fuel lines flex more and more, making the problem worse and worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sooner or later the struts or fuel lines flex too much, and, in technical terms, rocket goes boom. Or maybe you've accounted for &lt;em&gt;a lot&lt;/em&gt; of vibration in the struts, but the entire body of the rocket starts shaking and bending like a pool noodle from all this vibration, so it snaps in two, and... rocket goes boom. Or a million other things could break as the vibrations grow and grow. Maybe the engines just experience fuel starvation, flame out, and the whole thing falls to the ground (and goes boom).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This type of situation is called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Pogo_oscillation&quot; rel=&quot;nofollow&quot;&gt;pogo oscillation&lt;/a&gt;, and it famously plagued the Saturn V rocket for quite some time. It's something that any large rocket has to deal with. And ways have indeed been found to deal with it. But before it was first experienced, who would have thought of it? You'd pretty much have to experience a failure first. And &lt;em&gt;then&lt;/em&gt; you can fix it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, your design &lt;em&gt;will&lt;/em&gt; contain assumptions. Assumption like &quot;thrust is X&quot;. True - until the vibrations started. After that, you might as well assume that &quot;thrust is infinite&quot;, but then you'd have to make the struts infinitely strong (good luck with that). Or maybe you have an assumption like &quot;liquid fuel settles at the bottom of the tank&quot;. That's true if there's gravity (and the rocket is upright). It's true if there no gravity, but the rocket is accelerating. But it's not true in microgravity with no acceleration. There, the fuel sloshes around, which can &lt;a href=&quot;https://en.wikipedia.org/wiki/Slosh_dynamics&quot; rel=&quot;nofollow&quot;&gt;causes problems all on its own&lt;/a&gt;, besides the fuel not being near the pump intake. Of course, you've already thought of this, so you add &lt;a href=&quot;https://en.wikipedia.org/wiki/Ullage_motor&quot; rel=&quot;nofollow&quot;&gt;ullage motors&lt;/a&gt; to add a bit of acceleration, and make the fuel settle again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Except now, of course, you've actually just strapped another set of rockets to your rocket, and you have new problems and assumptions to deal with, because #RocketsAreTricky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your comment was itself based on an assumption: That it's possible to know everything.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; As Michael Kjörling adds in the comments, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13#Mission_highlights&quot; rel=&quot;nofollow&quot;&gt;Apollo 13 experienced pogo oscillations on launch&lt;/a&gt;. Even though the engineers knew of the phenomenon at this point, they were &lt;em&gt;still&lt;/em&gt; surprised by the oscillations, because &quot;they were amplified by an unexpected interaction with turbopump cavitation.&quot; That's the sort of complexity you have to figure out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That article is worth a read, but its references are even more interesting. It cites an aerospace industry publication, that just happens to contain the aptly named article: &lt;strong&gt;&lt;a href=&quot;http://aerospace.wpengine.netdna-cdn.com/wp-content/uploads/crosslink/V5N1.pdf&quot; rel=&quot;nofollow&quot;&gt;That's why they call it rocket science&lt;/a&gt;&lt;/strong&gt;, which also attempts to answer the question: &quot;Why is it so hard to launch a rocket into space with absolute assurance of success?&quot;. Pretty much exactly the thing you're looking for! Among other things, it offers this quote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Launch system designers base their designs on the best data available—but sometimes, the best data are just good estimates. True measurements of the launch environment can only be obtained in flight, and that’s hardly an option at the design stage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="11257" LastEditorUserId="11257" LastEditDate="2016-01-09T18:25:59.623" LastActivityDate="2016-01-09T18:25:59.623" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13409" PostTypeId="1" CreationDate="2016-01-07T01:12:21.203" Score="4" ViewCount="243" Body="&lt;p&gt;Rocket Lab announced the &lt;a href=&quot;http://www.stuff.co.nz/business/industries/75361113/rocket-lab-begins-work-on-electron-rocket-launch-site-in-mahia&quot; rel=&quot;nofollow&quot;&gt;new launch site&lt;/a&gt; at Mahia last year. Which is due for completion soon. According to their &lt;a href=&quot;http://book.rocketlabusa.com/&quot; rel=&quot;nofollow&quot;&gt;booking system&lt;/a&gt; commercial launches will happen in quarter 4 of 2016. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we know the exact date of the first commercial launch? I.e. excluding any test launches.&lt;/p&gt;&#xA;" OwnerUserId="12947" LastEditorUserId="12102" LastEditDate="2021-09-13T00:01:31.410" LastActivityDate="2021-09-13T00:01:31.410" Title="When is the first commercial Rocket Lab launch occurring out of Mahia Peninsula, New Zealand?" Tags="|launch-site|spaceflight-firsts|rocketlab|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13415" PostTypeId="2" ParentId="13386" CreationDate="2016-01-07T14:21:25.707" Score="4" Body="&lt;p&gt;Russell Borogove's answer is excellent, so I just want to address a small part of your question that isn't answered there:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Will they fill the tanks just a little so the first stage can hover by itself&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It can't hover. Even with 8 engines shut down and the 9th throttled back to minimum, the thrust is much bigger than the weight of the (mostly) empty first stage without the second stage and payload. That's why they have to do that insane-looking &quot;hover-slam&quot; landing, where the rocket basically free-falls to the landing pad and then at the very last second the engine fires so that the rocket will reach a velocity of zero at exactly the same time it reaches an altitude of zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What they &lt;em&gt;could&lt;/em&gt; do is &quot;hop&quot;: fire the engine so the rocket takes off on a ballistic trajectory towards the launch pad, then do a hover-slam there. (Or do multiple &quot;sideways hops&quot;, re-starting the engine multiple times.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know if it would make sense to put enough ballast (i.e. propellant/oxidizer) into the rocket in order to bring the thrust-to-weight-ration below 1 to make it hover-capable.&lt;/p&gt;&#xA;" OwnerUserId="6156" LastActivityDate="2016-01-07T14:21:25.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13416" PostTypeId="2" ParentId="5951" CreationDate="2016-01-07T16:33:51.797" Score="1" Body="&lt;p&gt;Doing this efficiently requires knowing exactly where the lander is. A laser will generate a tiny beam over the distances we're talking about, so you have to aim the laser accurately.&lt;br&gt;&#xA;Rosetta doesn't have enough equipment on board to find where Philae is. You could use a radio direction finding system, for example.&lt;br&gt;&#xA;An additional problem: the orbiter will be in sight of the lander for only a fraction of its orbit, so you need to transfer enough power for an entire orbit in that small window. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-07T16:33:51.797" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13417" PostTypeId="1" AcceptedAnswerId="13420" CreationDate="2016-01-07T16:34:14.113" Score="12" ViewCount="4499" Body="&lt;p&gt;Several times when I followed a space launch being broadcasted in the internet, they also included live video from the launch vehicle itself. Such as this &lt;a href=&quot;https://www.youtube.com/watch?v=3PA-yjYJ-pg&quot;&gt;ESA ATV launch&lt;/a&gt;. Now I don't know if this exact video was streamed online during launch, but usually it does look much like that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;High altitude balloon experiments sometimes attempt video (or rather image) live streaming during balloon flight. But the frame rates are very low. Here is &lt;a href=&quot;http://www.timzaman.com/live-space-webcam/&quot;&gt;an example&lt;/a&gt; of such balloon flight experiment. They say it took between 10 and 60 seconds to send a single image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The huge difference in budget and systems complexity between a space rocket and such balloon is obvious. I would guess it is a high frequency transmitter that makes live video streaming during rocket launches possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But what exactly is it aboard a rocket, that allows live video streaming? And since its power requirements are probably higher, how is it powered?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6750" LastActivityDate="2016-01-07T22:41:53.927" Title="How do rockets stream live video during launches?" Tags="|launch|rockets|data-transmission|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13418" PostTypeId="2" ParentId="13417" CreationDate="2016-01-07T19:35:12.257" Score="8" Body="&lt;p&gt;Transmitting live video is done just like any other wireless data transmission, except that the video encoding must be fault tolerant. The frequency used is chosen so that &lt;a href=&quot;http://www.rfcafe.com/references/electrical/atm-absorption.htm&quot;&gt;attenuation through the atmosphere&lt;/a&gt; is tolerable, and will not interfere with other communications systems (as decided by the FCC and other regulating bodies). There are various video encoding protocols that are currently used to stream information, both live and on-demand. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Amateur balloon launches certainly could stream live but here are some complications. The general public is restricted in &lt;a href=&quot;https://en.wikipedia.org/wiki/Amateur_radio_frequency_allocations&quot;&gt;which frequencies can be used&lt;/a&gt; and in transmitter power (1500 Watts maximum in most cases). Rocket launches are typically given authority to use frequency bands controlled by the Government. Because the general public is not allowed to use these frequencies there is little interference to be concerned with. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The communication system would be powered like any other, using on board batteries.&lt;/p&gt;&#xA;" OwnerUserId="12966" LastActivityDate="2016-01-07T19:35:12.257" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13419" PostTypeId="2" ParentId="13417" CreationDate="2016-01-07T22:23:16.827" Score="3" Body="&lt;p&gt;The transmitter of the balloon you mentioned is very low speed, caused by several points: &#xA;The transmitter uses only 10mW because these modules are cheap and can be bought everywhere. They can be operated from a small battery for hours. The 434MHz band it uses is used in many applications and hence has a lot of interference and noise. It used an omnidirectional antenna because using a directional antenna would mean to include mechanics to move the antenna in the right position.&#xA;Last but not least the whole assembly has to be very lightweight, few 100g usually, not allowing for larger batteries or antennas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The transmitter used in rockets (no matter how exactly it works or if it transmits directly to earth or via a relay satellite) can be much larger and use a bit more power. But this power requirement is not as high as you might think: Television satellites located more than 36000km away from your dish have transmitters of about 100W only. Powering such a transmitter from a battery for few hours is no big challenge.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-01-07T22:23:16.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13420" PostTypeId="2" ParentId="13417" CreationDate="2016-01-07T22:41:53.927" Score="4" Body="&lt;p&gt;These are custom commercial video systems developed by aerospace companies that specialize in telemetry.  For instance, the linked video includes the credit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The video system was developed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Kayser-Threde&quot; rel=&quot;nofollow&quot;&gt;Kayser-Threde GmbH&lt;/a&gt; for ESA and DLR and integrated on Ariane by &lt;a href=&quot;https://en.wikipedia.org/wiki/Astrium&quot; rel=&quot;nofollow&quot;&gt;Astrium GmbH&lt;/a&gt;. Usage for ATV-4 was financed by DLR and ESA and supported by Arianespace and CNES.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;These are two aerospace companies that provide a wide range of telemetry, data capture, video, and other solutions for space flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cameras such as these have been included recently for a variety of reasons.  On manned spaceflights, they need to look for safety events, such as falling ice, to know if they need to inspect the spacecraft before return, or if an abort is required. On commercial flights these are included to understand failures better.  They are also useful for marketing and publicity, but the bandwidth and cost (not just financial, but space, weight, and power on the craft) have been too high for regular inclusion.  Now they are smaller, cheaper, lighter, and require less power to transmit an HD image in real time, so the added data gained from them makes them a worthwhile investment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally speaking, these videos aren't much different than radio video signals used on the earth, with the exception that launches always have continuous antenna dishes pointed at them, even as they rotate around the earth.  Creating the video, encoding the data, and sending it via a chosen RF frequency isn't hard, but by the time the launch &quot;ends&quot; the vehicle may be halfway around the world, and well out of sight of the original launch site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore all data communications are done by first coordinating a set of frequencies that will need to be received on earth, and sent from earth.  Then NASA works with other space agencies with receiving and transmission arrays arranged around the earth along the launch path to get time on their arrays.  As the space ship moves away from the launch site, they coordinate switchover so it appears seamless to the launch control center, but in reality may have switched several times from one site to another as the spacecraft accelerates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The live video feed from the moon was done similarly, except the moon travels very slowly, so they didn't have to switch from site to site during the transmission (if I recall, it was all received in Australia and then re-transmitted around the world using other communications channels).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With large receiving dishes, and very powerful radio transmitters on earth (because space, weight, and power are cheap on the ground) you don't need much space, weight, or power for the receivers and transmitters on the rocket.  They are powered like any other radio system on the rocket, and probably transmit at a relatively low power mode.&lt;/p&gt;&#xA;" OwnerUserId="248" LastActivityDate="2016-01-07T22:41:53.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13421" PostTypeId="1" AcceptedAnswerId="13422" CreationDate="2016-01-08T01:35:03.757" Score="-2" ViewCount="139" Body="&lt;p&gt;So just had an idea that could work but would require billions to build:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretically we could build a giant highway for satellites to orbit on but as they are orbiting they generate energy as they are on a treadmill which generates energy from the satellites movement. Would this work if it were built if so doesn't this beat the law of conservation of energy.&lt;/p&gt;&#xA;" OwnerUserId="12970" LastActivityDate="2016-01-08T09:36:28.020" Title="Would it be possible to generate electricty in space with a satellite in orbit" Tags="|energy|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13422" PostTypeId="2" ParentId="13421" CreationDate="2016-01-08T01:45:06.923" Score="15" Body="&lt;p&gt;No, it wouldn't work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to drive the treadmill, momentum would have to be transferred from the satellites to the treadmill. The sats would slow down and eventually stop.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;highway&quot; itself wouldn't be stable, either. Since it's moving at a different speed from the satellites, it cannot be moving at circular orbital speed for its altitude by definition; any tiny nudge on it will cause it to drift into the atmosphere and break apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a general rule, if you think you have a scheme for free energy, assume the answer is &quot;no&quot; and then figure out why it's &quot;no&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-01-08T01:45:06.923" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13423" PostTypeId="2" ParentId="13399" CreationDate="2016-01-08T04:41:34.623" Score="3" Body="&lt;p&gt;Safety margins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rocket built with the sort of safety margins that are normal practice in aircraft would be too heavy to reach orbit.  Not only do you build your aircraft reliable but you build it so that when something goes wrong you still get your airplane back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are barreling down the runway and just pulled the stick back when one of your engines goes south.  What do you do?  If the runway is long, land and stop.  If it's short, firewall the rest and promptly get your plane back on the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have just cleared the launch gantry and one of your engines goes south.  What do you do?  Fly the rocket over empty space, separate the payload section if it can be recovered and push the big red button.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of comparing lost aircraft to lost rockets you need to compare a wide variety of minor aircraft failings to lost rockets.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-01-08T04:41:34.623" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13424" PostTypeId="2" ParentId="13421" CreationDate="2016-01-08T09:36:28.020" Score="2" Body="&lt;p&gt;Apart from the impossibility of harvesting energy, here's another way to sanity-check your grand designs.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A highway all the way around Earth would be a structure at least 40,000 km long. Let's optimistically say you can build such a highway by launching it in sections 100 m long each. That means 400,000 launches to create the highway.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have only launched 6,600 satellites to date, and they're in all kinds of different orbits. The most densely-populated orbit is the geostationary orbit, with about 300 satellites.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of energy put into 1 satellite is exactly 1 rocket launch. You can never recover more energy than what was put in. &#xA;So in geostationary orbit, you can harvest at most 300 launches worth of energy. For an energy cost of 400,000 launches. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-08T09:36:28.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13425" PostTypeId="1" AcceptedAnswerId="13438" CreationDate="2016-01-08T10:51:59.303" Score="13" ViewCount="1984" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/questions/2470/iss-attitude-during-docking&quot;&gt;question from 3 years ago&lt;/a&gt; got an answer that would be a great answer to a different question (and concluded with a comment that more would be posted later.) Let me re-ask, making the question clearer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally, ISS spins at 1 revolution per orbit, effectively one side always facing the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With two bodies in &lt;em&gt;almost&lt;/em&gt; the same orbit, they move in an elliptical trajectory respective to each other (for example, the &lt;a href=&quot;http://www.china.org.cn/world/2013-08/22/content_29794731.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;loose antenna cover&lt;/a&gt; seems to orbit ISS, even if it's not gravitationally bound to it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The docking maneuver takes good several minutes, which comprise a good part of an orbit. If I brought Soyuz to a dead stop relative to the station 200m below the bottom port while ISS is facing Earth with it, the orbital mechanics would move me to 200m above the station within 45 minutes (half the orbital period) - or move me 45 degrees off course within some 11 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is this dealt with? Is ISS rotation relative to Earth stopped (ISS attitude becoming constant relative to distant stars, changing relative to Earth) or does the docking ship deal with it using RCS to account for the drift? Or is this even different or I made some wrong assumptions?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-01-08T22:15:15.067" Title="Does ISS stop rotation for docking?" Tags="|iss|attitude|docking|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13426" PostTypeId="1" AcceptedAnswerId="13433" CreationDate="2016-01-08T11:07:49.850" Score="3" ViewCount="908" Body="&lt;p&gt;...or &quot;How precisely does ISS face Earth with one side?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know &quot;ISS always faces Earth with one side&quot; - &quot;spins at 1 revolution per orbit&quot; which accounts for staying quite precisely parallel to Earth in the east-west axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But since the orbit is quite steeply inclined (51.6 degrees), if it's parallel to Earth in the north-south axis while passing over the equator, at northernmost point of its orbit the cupola would be facing 51.6 degrees north from the local &quot;directly down&quot;, and respective 51.6 degrees south in the southernmost point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does ISS compensate for that with its CMGs or does it &quot;look at Earth sideways&quot; when away from the equator?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="10132" LastEditDate="2016-01-08T18:11:11.233" LastActivityDate="2019-06-27T09:20:41.077" Title="Does ISS wobble north/south?" Tags="|iss|orbital-mechanics|attitude|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13427" PostTypeId="2" ParentId="13396" CreationDate="2016-01-08T12:11:19.193" Score="5" Body="&lt;p&gt;Borrowing from Wikipedia's article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Intercontinental_ballistic_missile#Modern_ICBMs&quot; rel=&quot;noreferrer&quot;&gt;modern ICBMs&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One particular weapon developed by the Soviet Union (&lt;a href=&quot;https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment_System&quot; rel=&quot;noreferrer&quot;&gt;Fractional Orbital Bombardment System&lt;/a&gt;) had a partial orbital trajectory, and unlike most ICBMs its target could not be deduced from its orbital flight path. It was decommissioned in compliance with arms control agreements, which address the maximum range of ICBMs and prohibit orbital or fractional-orbital weapons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Orbital and fractional-orbital weapons were prohibited by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Strategic_Arms_Limitation_Talks#SALT_II&quot; rel=&quot;noreferrer&quot;&gt;SALT II&lt;/a&gt; agreement in 1979, and the FOBS missile was phased out in early 1983.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember that the &quot;B&quot; in ICBM stands for &lt;em&gt;ballistic&lt;/em&gt;; other than the boost phase, &lt;a href=&quot;https://en.wikipedia.org/wiki/Intercontinental_ballistic_missile#Flight_phases&quot; rel=&quot;noreferrer&quot;&gt;ICBM flight is largely unpowered&lt;/a&gt; and suborbital.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that breaking free of Earth's gravity, which would be necessary in order to reach the Sun, requires a higher velocity than attaining Earth orbit, and by implication of the SALT II agreement no current ICBMs are able to attain even Earth orbit, we can conclude that no matter the transfer orbit trickery involved, &lt;strong&gt;no current ICBMs have the capability to target the Sun.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2016-01-08T12:11:19.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13428" PostTypeId="2" ParentId="4765" CreationDate="2016-01-08T13:52:43.643" Score="7" Body="&lt;p&gt;Specifically for the oxidizer/fuel combinations you specified, this appears to be unlikely for the simple reason of reaction kinetics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have&lt;/p&gt;&#xA;&#xA;&lt;p&gt;O&lt;sub&gt;2&lt;/sub&gt; + 2 H&lt;sub&gt;2&lt;/sub&gt; &amp;rarr; 2 H&lt;sub&gt;2&lt;/sub&gt;O&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2 O&lt;sub&gt;2&lt;/sub&gt; + CH&lt;sub&gt;4&lt;/sub&gt; &amp;rarr; 2 H&lt;sub&gt;2&lt;/sub&gt;O + CO&lt;sub&gt;2&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note how in the first reaction we burn 1 part oxidizer with 2 parts fuel; in the second it 2 parts oxidizer with 1 part fuel (i.e. &lt;em&gt;four&lt;/em&gt; times the oxidizer in relation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even taking different densities into account, this would require the turbo pumps to deal with such different mixture rates. I'm not a rocket scientist but this appears to be an engineering challenge. Consider that often the pumps for both fuel and oxidizer are driven by a single shaft, i.e. you can't simply open a valve to get some more of that oxidizer in, and less of that fuel.&lt;/p&gt;&#xA;" OwnerUserId="716" LastActivityDate="2016-01-08T13:52:43.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13429" PostTypeId="1" AcceptedAnswerId="13431" CreationDate="2016-01-08T15:47:32.980" Score="6" ViewCount="1277" Body="&lt;p&gt;The answer by &lt;em&gt;PearsonArtPhoto&lt;/em&gt; in &lt;a href=&quot;https://space.stackexchange.com/questions/633/what-resources-could-be-gained-from-asteroid-mining-that-would-be-worth-the-effo&quot;&gt;this question&lt;/a&gt; got me thinking, particularly this line:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Asteroids would be cheaper to go to than the Moon&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I suppose the subject here are asteroids that pass in the zone between Earth and Moon's orbit around Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that such asteroids pass by Earth at high velocity and in a relatively short time. Would a near-Earth asteroid pass give enough time to complete a mining operation and generate profit?  &lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="8693" LastEditDate="2016-01-11T14:01:08.123" LastActivityDate="2016-01-11T14:01:08.123" Title="Mining a near-Earth passing asteroid with profit?" Tags="|the-moon|asteroid|mining|feasibility|near-earth-object|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13430" PostTypeId="2" ParentId="13429" CreationDate="2016-01-08T16:12:03.833" Score="3" Body="&lt;p&gt;The trick isn't to mine them when they are close, necessarily. Asteroids are cheaper to get to than the Moon because the Moon has quite a bit of gravity. If you aim correctly, you can avoid all of the loss of rocket fuel associated with gravity. Where it is doesn't matter that much (So long as it is in a nearby orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, an Asteroid mining mission will have a lot of time to mine, not just the few hours of closest approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, asteroids make frequency close encounters. Of particular note is &lt;a href=&quot;https://en.wikipedia.org/wiki/99942_Apophis&quot; rel=&quot;nofollow&quot;&gt;Apophis&lt;/a&gt;, which will have a close encounter in 2029, and another one in 2035. One could sent the equipment during the initial encounter, mine the asteroid, tweak the orbit slightly to return to Earth thereafter. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-08T16:12:03.833" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13431" PostTypeId="2" ParentId="13429" CreationDate="2016-01-08T16:13:02.903" Score="5" Body="&lt;p&gt;The real issue here is how much rocket propellant you have to spend to get to the location in question. In one way, asteroids are easier to get to due to the fact that landing on the Moon takes a lot of velocity change. On the other hand, an asteroid flying through the Earth-Moon system is going really fast, so catching up with it is going to take a lot of propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One problem with mining NEOs is that the closer the orbit is to Earth, and therefore the velocity change needed, the longer is the &lt;a href=&quot;http://www.physics.unlv.edu/~jeffery/astro/glossary/synodic.html&quot; rel=&quot;nofollow&quot;&gt;synodic period&lt;/a&gt;. That is, you have to wait a long time before you can return. (If you do not want to waste a lot of propellant, effectively cancelling the advantage.) However, asteroids gives you the ability to choose among them, so you can target &lt;a href=&quot;https://en.wikipedia.org/wiki/M-type_asteroid&quot; rel=&quot;nofollow&quot;&gt;those high in ore content&lt;/a&gt;. Ores on the Moon are generally more equally distributed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A better option may be to capture an asteriod in orbit, so you have more time to mine it. If mining in space is going to be profitable is difficult to answer though.&#xA;This may not actually be all that difficult, see for instance: &lt;a href=&quot;http://hopsblog-hop.blogspot.no/2013/04/catching-asteroid.html&quot; rel=&quot;nofollow&quot;&gt;Catching an asteroid&lt;/a&gt;. That proposes a way to catch it with only 250m/s of delta-v.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-08T16:33:12.100" LastActivityDate="2016-01-08T16:33:12.100" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13432" PostTypeId="2" ParentId="13429" CreationDate="2016-01-08T16:21:39.880" Score="7" Body="&lt;p&gt;That's a one trillion dollars question! Proximity of near-Earth flybys of asteroids is largely irrelevant when it comes to feasibility of matching their orbit and rendezvousing with them, what is more important is their hyperbolic excess velocity with respect to Earth, and how much delta-v is needed to do that. There are many &lt;a href=&quot;https://en.wikipedia.org/wiki/Aten_asteroid&quot;&gt;Athen&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Apollo_asteroids&quot;&gt;Apollo&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Amor_asteroid&quot;&gt;Amor&lt;/a&gt; groups of asteroids that have similar orbit to Earth's in that respect, for example, here's a nice chart of JPL's &lt;a href=&quot;http://echo.jpl.nasa.gov/~lance/delta_v/delta_v.rendezvous.html&quot;&gt;Near-Earth Asteroid Delta-V for Spacecraft Rendezvous&lt;/a&gt;, with many NEA listed with smaller delta-v requirement to rendezvous with them than for a lunar surface, so Pearson's answer is correct. And even prettier than a long text file, here's &lt;a href=&quot;http://www.asterank.com/&quot;&gt;Asterank&lt;/a&gt; (Asteroid Database and Mining Rankings) that should give you a fairer impression of what it would take to rendezvous with them and their estimated worth. And you can plug names of many of these into NASA's &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php&quot;&gt;Trajectory Search&lt;/a&gt; tool to also see requirements for round-trip rendezvous.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2016-01-08T16:21:39.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13433" PostTypeId="2" ParentId="13426" CreationDate="2016-01-08T16:59:46.610" Score="7" Body="&lt;p&gt;The rotation matches the orbit inclination so cupola down is always towards the earth.  You can watch the live stream from &lt;a href=&quot;http://www.ustream.tv/channel/live-iss-stream&quot; rel=&quot;nofollow noreferrer&quot;&gt;ustream&lt;/a&gt; to watch this (or at least you can when it's showing the exterior view), or there are any number of videos on youtube that show down being stable.  Here's one collection of timelapse videos:&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/nod7rUzIX8w?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbits are ellipses, regardless of inclination.  Rotating once per orbit around the same axis as the orbit can maintain down, regardless of what axis that is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For help explaining this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Consider the case of an equatorial orbit: the axis normal to the orbit is the north-south line through the earth, so rotating around that axis will match the orbit and down remains pointing directly at the earth.&lt;/li&gt;&#xA;&lt;li&gt;Now consider the case of a 90 degree polar orbit: the earth rotates so we need an external reference frame.  For simplicity let the axis normal to the orbit match the line from the earth to the sun (which itself changes as the earth orbits the sun, but only at a rate of about one degree per day so it's reasonably fixed short-term).  If the station also rotates around the axis defined by the line from the sun to the earth then down will always point directly at the earth.&lt;/li&gt;&#xA;&lt;li&gt;Now wait three months and due to the axial tilt of the earth relative to its orbit an orbit around the earth that's normal to the line from the earth to the sun will have shifted by about 23 degrees.  The same rotation around the line between the earth and the sun will still maintain down as pointing at the earth.&lt;/li&gt;&#xA;&lt;li&gt;Regardless of inclination the orbit there is a single axis that is normal to the orbit (defined in inertial space, which is fixed in space and doesn't rotate with the earth).  Because of that, there is always a simple rotation axis that can be used to maintain down pointing at the center of the orbit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2019-06-27T09:20:41.077" LastActivityDate="2019-06-27T09:20:41.077" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="13435" PostTypeId="2" ParentId="12522" CreationDate="2016-01-08T20:30:52.300" Score="2" Body="&lt;p&gt;I do know that FreeRTOS, on an Pumpkin MSP430-F216 microprocessor platform, has been working successfully on the NASA Firefly Cubesat project that I wrote the flight and ground software for, and has been communicating for over 2 years.&lt;/p&gt;&#xA;" OwnerUserId="12983" LastEditorUserId="6816" LastEditDate="2016-01-08T20:50:39.700" LastActivityDate="2016-01-08T20:50:39.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13436" PostTypeId="1" CreationDate="2016-01-08T20:46:09.720" Score="12" ViewCount="2656" Body="&lt;p&gt;Has anyone had any experience with successful Linux-based flight computer on Cubesat missions?&#xA;What Linux hardware was used and what problems did you encounter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am contracted to write flight/ground software for another NASA Cubesat mission, and am investigating the options for using a Linux-capable 32 bit processor.  The flight computer hardware selection is one of my responsibilities.&lt;/p&gt;&#xA;" OwnerUserId="12983" LastEditorUserId="4660" LastEditDate="2016-01-10T02:15:26.473" LastActivityDate="2018-03-13T21:01:28.903" Title="Successful Linux-based Cubesat missions?" Tags="|software|cubesat|flight-computer|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13438" PostTypeId="2" ParentId="13425" CreationDate="2016-01-08T22:15:15.067" Score="8" Body="&lt;p&gt;ISS rotation is not changed for docking. The orbital drift is only a problem if you're not actively maneuvering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the approach consists of long periods of drift, with specific points where burns are planned.  Orbital mechanics are well planned out so that the burns and resulting drift combine to deliver the visiting craft to the approach point at the right time.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But for final approach, you can almost ignore the orbital issues and thrust to keep the craft in the correct spot in the trajectory relative to the station.  The phase is short enough, and the craft is close enough, that this won't result in a lot of fuel waste.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another answer points to this nasa document with details about &lt;a href=&quot;http://www.nasa.gov/centers/johnson/pdf/567076main_RNDZ_135_F.pdf&quot;&gt;STS-135 Rendezvous&lt;/a&gt;.  Several charts in chapter 1 show the planned approach up to MC4.  After that point, the shuttle was flown manually by the pilot.  The chart shows ISS in the LVLH frame, which means it keeps the Z-axis pointed at the earth (and is therefore still rotating).&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2016-01-08T22:15:15.067" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13440" PostTypeId="1" AcceptedAnswerId="13441" CreationDate="2016-01-09T00:52:51.730" Score="9" ViewCount="1265" Body="&lt;p&gt;In his Edge.org Annual Question response, &lt;a href=&quot;http://edge.org/response-detail/26688&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Progress in Rocketry&lt;/em&gt;&lt;/a&gt;, George Dyson exults in the success of Blue Origin and SpaceX in landing their rockets after pushing them past the Kármán line, and then goes on to predict that this will help pave the way to rockets with separate substances as propellant and as fuel:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is no reason the source of reaction mass (propellant) has to be the same as the source of energy (fuel). Burning a near-explosive mix of chemicals makes the process inherently dangerous and places a hard limit on specific impulse (ISP), a measure of how much acceleration can be derived from a given amount of propellant/fuel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;All the advances in autonomous control, combustion engineering, and computational fluid dynamics that allowed these two rockets to make a controlled descent, after only a handful of attempts, are exactly what will be needed to develop a new generation of launch vehicles that leave chemical combustion behind to ascend on a pulsed energy beam. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Leaving aside whether, and to what extent, the Blue Origin and SpaceX results get us in that direction, are rockets like that even contemplated as a realistic possibility in the accessible future? &lt;a href=&quot;https://www.youtube.com/watch?v=aSNDQHnjc5s#t=8m02s&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kerbal beamed power&lt;/a&gt; aside, is there any way for a rocket to climb from the surface to Earth orbit without physically carrying the source of energy that will get it there? Or what is Dyson talking about?&lt;/p&gt;&#xA;" OwnerUserId="653" LastEditorUserId="25" LastEditDate="2017-12-19T13:29:16.497" LastActivityDate="2017-12-19T23:04:37.723" Title="Do any realistic rocket systems use different substances for propellant and fuel?" Tags="|rockets|propulsion|fuel|energy|beamed-propulsion|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13441" PostTypeId="2" ParentId="13440" CreationDate="2016-01-09T01:03:21.700" Score="16" Body="&lt;p&gt;To beam the power to the rocket, especially when needing high acceleration, is not what I will expect to show up in the near future. However, separating the energy source from the propellant is the idea behind the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;nuclear thermal rocket&lt;/a&gt;. Here, energy comes from a reactor, and you can choose more freely in higher performing propellants. Because a NTR is limited by the temperature at which the reactor starts to melt, you want an exhaust-gas with low molecular mass, most commonly hydrogen. &lt;a href=&quot;http://www.astronautix.com/engines/rd0410.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Working prototypes&lt;/a&gt; are built. A separate energy source is also the idea behind an &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;ion thruster&lt;/a&gt;, where high efficiency is achieved by accelerating the propellant (often xenon) using electrical energy. That is the technology used by the ongoing &lt;a href=&quot;http://dawn.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dawn mission.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SYVe7m.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SYVe7m.jpg&quot; alt=&quot;ion thruster&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this is also actually a possibility with chemical rockets. In his book &lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;!Ignition&lt;/a&gt;, John D. Clarke tells the story about how mercury was seriously considered as a part of the fuel in order increase the density impulse of a missile, trading ISP for delta-v. That is achieved because you can then fit more propellant into the same tanks. Delta-v is then calculated by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v=ln\left(\frac{m_0+V*\rho}{m_0}\right)v_e$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $m_0$ is the dry mass of the rocket, $V$ is tank volume, $\rho$ is propellant density and $v_e$ is exhaust velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for a &lt;a href=&quot;http://sail.planetary.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar sail&lt;/a&gt;, it carries neither the energy source nor any propellant. Note that you can also point a laser on the sail to increase the thrust.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.islandone.org/LEOBiblio/SPBI115.HTM&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heating propellant&lt;/a&gt; with an external laser may also be a feasible concept, because you can then have the same efficiency as a NTR, but not having to carry a heavy reactor around. Several concept for this can be found &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/enginelist.php#laserthermal&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. The temperature of the exhaust gas is also then no longer limited the melting point of the fuel elements in a nuclear reactor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A chemical rocket is actually just a special case, where the fuel and the propellant happens to be the the same thing. In a jet engine, for example, the fuel brought along is half the energy source, and the oxidizer is collected from the air. The air passing through the engine is also the propellant used.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="1128" LastEditDate="2016-12-19T02:48:39.650" LastActivityDate="2016-12-19T02:48:39.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13443" PostTypeId="2" ParentId="13440" CreationDate="2016-01-09T01:48:33.240" Score="8" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Kevin_L.G._Parkin&quot; rel=&quot;noreferrer&quot;&gt;Kevin Parkin&lt;/a&gt; has been the lead researcher into &lt;a href=&quot;http://thesis.library.caltech.edu/2405/&quot; rel=&quot;noreferrer&quot;&gt;Microwave Thermal Thrusters&lt;/a&gt;, which was his PhD thesis at CalTech. He continues to work on the technology. From an interview with &lt;a href=&quot;http://nextbigfuture.com/2011/02/nasa-researcher-kevin-parkin-discusses.html&quot; rel=&quot;noreferrer&quot;&gt;NextBigFuture&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We are scheduled to take delivery of the gyrotron in April 2012. We&#xA;  hope to have a subscale demonstration by 2018, and to prove suborbital&#xA;  capability within a decade. Within two decades, we should have true&#xA;  orbital, single-stage-to-orbit capacity. Of course, if we had higher&#xA;  funding, we could reduce those timeframes substantially.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The gyrotrons produce the microwaves that hit a heat exchanger on the rocket, which heats liquid hydrogen, which propels the rocket. It's been a while since I looked at his paper, but if I recall correctly, the heat exchanger is the biggest problem. It has to withstand very high temperatures and efficiently pass heat to the propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5bkur.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5bkur.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parkin was for a time Deputy Director of Ames Research Center, so he is highly regarded in the field. In 2012, the &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20130000009&quot; rel=&quot;noreferrer&quot;&gt;Millimeter-Wave Thermal Launch System&lt;/a&gt; program at Ames was created jointly by NASA and DARPA, based on Parkin's work.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2016-01-09T01:48:33.240" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13444" PostTypeId="1" AcceptedAnswerId="13445" CreationDate="2016-01-09T13:17:38.213" Score="5" ViewCount="2384" Body="&lt;p&gt;In terms of engineering documentation for space-qualified electronic components, what is an &quot;SCD&quot;? See, for example, page 6 of this &lt;a href=&quot;http://www.vectron.com/products/brochures/Vectron_Space_Brochure.pdf&quot;&gt;document&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is an SCD a common document and does it apply to other (non-electronic) components?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at this particular vendor's &lt;a href=&quot;http://www.vectron.com/products/space/space.htm&quot;&gt;corresponding&lt;/a&gt; web page, there is a link to their company's generic &lt;a href=&quot;http://www.vectron.com/products/military/xo/os68338.pdf&quot;&gt;SCD&lt;/a&gt; for the particular component class in question, but it sheds no light on the meaning of the &quot;SCD&quot; abbreviation.&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="19946" LastEditDate="2019-11-22T12:06:03.393" LastActivityDate="2019-11-22T12:06:03.393" Title="For space-qualified electronic components, what is an &quot;SCD&quot;?" Tags="|engineering|electronics|specifications|compliance|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13445" PostTypeId="2" ParentId="13444" CreationDate="2016-01-09T15:02:39.307" Score="10" Body="&lt;p&gt;&lt;a href=&quot;http://nepp.nasa.gov/docuploads/FFB52B88-36AE-4378-A05B2C084B5EE2CC/EEE-INST-002_add1.pdf&quot;&gt;Specification Control Drawing/Document or Source Control Drawing&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;page 9:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Source Control Drawing (SCD) - Provides an engineering description (including configuration, part number, marking, reliability, environmental, functional / performance characteristics), qualification requirements and acceptance criteria for commercial items or vendor developed items procurable from a specialized segment of industry that provides for application critical or unique characteristics.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;page 12:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When parts cannot be procured to Military Specifications, a drawing (such as a Source Control Drawing) should be prepared by the developer to control procurement requirements. The drawing shall include the screening and qualification requirements specified in Tables 2 and 3 of this document for the applicable part type. The drawing shall also include performance parameters, absolute maximum ratings, dimensions, terminal descriptions, materials, and other unique requirements. Existing developer SCDs proposed for use shall be evaluated for compliance to Table 2 and 3 requirements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-09T15:02:39.307" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13446" PostTypeId="1" CreationDate="2016-01-09T16:11:47.690" Score="4" ViewCount="1241" Body="&lt;p&gt;What I am looking for is an example where real persons in the real world &lt;strong&gt;really need&lt;/strong&gt; to mathematically model a situation before they &lt;strong&gt;need&lt;/strong&gt; to calculate the average and instantaneous rate of change. I want to to use this example as an entry point to calculus for students. It's supposed to be a big question to pursue/problem to solve while learning about those three aspects. Every time we move to the next aspect we learn something new to solve this big problem.&lt;/p&gt;&#xA;&lt;p&gt;I already asked this questions over at matheducators.stackexchange and received an &lt;a href=&quot;https://matheducators.stackexchange.com/a/10361/6215&quot;&gt;answer&lt;/a&gt; I thought was worth investigating:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;What about something like a rocket taking off into outer space?&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The process needs to be modelled up front because of the cost and&#xA;risk to life involved in a failed mission.&lt;/li&gt;&#xA;&lt;li&gt;The average rate of change needs to be calculated in order to ensure that the rocket gains enough speed to reach escape velocity, otherwise the mission will fail.&lt;/li&gt;&#xA;&lt;li&gt;The instantaneous rate(s) of change need to be calculated in order to ensure that the rocket materials and crew can cope with the stress of acceleration.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This answer seems somewhat convincing to me, although I am fully aware that I would need to use a simplified version of this example. Simplifying this example while keeping it plausible however raises some question:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;How&lt;/em&gt; is the process of launching a rocket modeled up front (in a simplified sense)?&lt;/li&gt;&#xA;&lt;li&gt;I know there have been &lt;a href=&quot;https://youtu.be/liO3blZWv8w&quot; rel=&quot;nofollow noreferrer&quot;&gt;engine tests for the Saturn V rocket&lt;/a&gt;. Did NASA – in simple words – use the data from these tests to project the velocity the rocket would reach in order to calculate the average and instantaneous rate of change?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h2&gt;Rough design-sketch of my explanation for the students so far&lt;/h2&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;When a rocket is launched into space it has to be ensured, that its acceleration for any given moment doesn't drop under a certain value, since that would mean that the rocket's velocity gets to low for escaping the earth's gravitational pull. On the other hand the acceleration can't be to high, since the resulting g-forces could damage the rocket itself or injure the crew.&lt;/li&gt;&#xA;&lt;li&gt;This can't be solved in a try-and-error attempt, because of the cost and&#xA;risk to life involved in such an attempt&lt;/li&gt;&#xA;&lt;li&gt;So what do NASA-Engineers do?&lt;/li&gt;&#xA;&lt;li&gt;By testing the rocket engine, the velocity of the rocket for any given moment of its voyage into space can be projected.&lt;/li&gt;&#xA;&lt;li&gt;[insert time-velocity data].&lt;/li&gt;&#xA;&lt;li&gt;NASA-Engineers take these data and insert them into a coordinate system. If we compare the time-velocity-pairs we'll notice that the velocity-values can always be described by inserting the time value into a specific formula&lt;/li&gt;&#xA;&lt;li&gt;[insert plausible function and add some explanation what a function and a graph is]&lt;/li&gt;&#xA;&lt;li&gt;[I would then go on an explain how to find the average rate of change (aka average acceleration) and how to find the steepest point of that curve (aka highest acceleration)]&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h2&gt;So the big question is:&lt;/h2&gt;&#xA;&lt;p&gt;Is this introduction &lt;em&gt;anywhere&lt;/em&gt; near &amp;quot;the real thing&amp;quot;?&lt;/p&gt;&#xA;" OwnerUserId="12988" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-01-09T16:21:46.903" Title="How is the process of launching a rocket mathematically modeled up front?" Tags="|launch|rockets|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13447" PostTypeId="1" AcceptedAnswerId="13449" CreationDate="2016-01-09T18:19:03.247" Score="9" ViewCount="357" Body="&lt;p&gt;In a short &lt;a href=&quot;https://www.youtube.com/watch?v=oljpxs7YhCM&quot; rel=&quot;nofollow&quot;&gt;video&lt;/a&gt; I just watched, it's stated that the SNC Dream Chaser can remain docked with the ISS for &quot;up to 210 days&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What determines this limit?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2016-01-11T04:45:02.203" LastActivityDate="2016-01-11T04:45:02.203" Title="ISS dock duration limit for SNC Dream Chaser" Tags="|iss|crewed-spaceflight|docking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13449" PostTypeId="2" ParentId="13447" CreationDate="2016-01-09T19:41:23.467" Score="11" Body="&lt;p&gt;210 days is a NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Commercial_Crew_Development&quot;&gt;CCDev&lt;/a&gt; (Commercial Crew Development) requirement:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The key high-level requirements for the Commercial Crew vehicles&#xA;  include:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Deliver and return four crew members and their equipment to International Space Station (ISS).&lt;/li&gt;&#xA;  &lt;li&gt;Provide assured crew return in the event of an emergency.&lt;/li&gt;&#xA;  &lt;li&gt;Serve as a 24-hour safe haven in the event of an emergency.&lt;/li&gt;&#xA;  &lt;li&gt;Capable of remaining docked for 210 days. The Space Shuttle could only remain docked for a maximum of 12 days.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The 210 day is also know as the &lt;em&gt;quiescent docking mode&lt;/em&gt;, during which the visiting spacecraft is not consuming its own life support provisions, but is made available to the astronauts as a &lt;em&gt;functional volume&lt;/em&gt; and has to thus provide air circulation, light, and so on. To enable this, it requires ability to draw station's power through the docking adapter, and conserve its own autonomous power and provisions (24 hour &lt;em&gt;safe haven&lt;/em&gt; plus time to deorbit and land). I.e. if the craft will use batteries, those have to keep sufficient charge even after 210 days on-orbit once they're disconnected from station's power, so they can power the craft for the required duration of the autonomous mode. Life support provisions, propellant tanks, and other volatiles have to maintain minimum required pressure, craft's Environmental Control and Life Support Systems (ECLSS) has to handle mission length &lt;em&gt;metabolic rates&lt;/em&gt;, external truss will have to tolerate within design limits 210 days of on-orbit thermal cycling, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Refer to &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/560761main_CHSIP_Baseline_JSC%2065995_FINAL_05182011.pdf&quot;&gt;Commercial Human Systems Integration Processes (CHSIP)&lt;/a&gt; for more detailed information. Also of use is &lt;a href=&quot;http://ston.jsc.nasa.gov/collections/TRS/_techrep/SP-2010-3407.pdf&quot;&gt;Human Integration Design Handbook (HIDH)&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-01-09T19:54:23.757" LastActivityDate="2016-01-09T19:54:23.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13450" PostTypeId="1" CreationDate="2016-01-10T07:41:48.770" Score="2" ViewCount="202" Body="&lt;p&gt;I am researching for a student developed nanosat mission, and would like to know which companies to buy from. The sensors are sun sensors and magnetometers. The actuators are magnetorquers. Can anyone please point me to a site that does a comparative study of all commercially available components and helps me choose? &lt;/p&gt;&#xA;" OwnerUserId="12692" LastActivityDate="2016-01-10T10:04:16.680" Title="Comparison of commercially available components" Tags="|artificial-satellite|commercial|" AnswerCount="2" CommentCount="3" ClosedDate="2016-01-11T00:59:26.293" ContentLicense="CC BY-SA 3.0" />
  <row Id="13451" PostTypeId="2" ParentId="13436" CreationDate="2016-01-10T08:57:52.103" Score="5" Body="&lt;p&gt;Planet Labs has flown more than 100 cubesats using Intel Atom computer-on-modules running Linux (a stock server distribution).&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2016-01-10T08:57:52.103" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13452" PostTypeId="2" ParentId="13450" CreationDate="2016-01-10T09:40:41.110" Score="3" Body="&lt;p&gt;I take you mean space-qualified parts? So far as I know, there isn't one specific site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the mission duration and profile?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Try the product, supplier and company searches &lt;a href=&quot;http://www.epiqspace.com/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;http://www.edn.com/search?keyword=sun%20sensors&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;http://www.edn.com/search?keyword=magnetometer&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. Also try &lt;a href=&quot;http://mwrf.com/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://sourceesb.com/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can try also the industry one-stop shops like this &lt;a href=&quot;http://mouser.com&quot; rel=&quot;nofollow&quot;&gt;one&lt;/a&gt; - search several countries, this &lt;a href=&quot;http://element14.com&quot; rel=&quot;nofollow&quot;&gt;one&lt;/a&gt;, or this &lt;a href=&quot;http://rscomponents.com&quot; rel=&quot;nofollow&quot;&gt;one&lt;/a&gt;. There's lot's more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an national space agency's perspectives, try &lt;a href=&quot;http://nepp.nasa.gov&quot; rel=&quot;nofollow&quot;&gt;http://nepp.nasa.gov&lt;/a&gt;, and the NASA Parts Selection List &lt;a href=&quot;https://nepp.nasa.gov/npsl/&quot; rel=&quot;nofollow&quot;&gt;https://nepp.nasa.gov/npsl/&lt;/a&gt;. I don't know what the equivalents are for ESA, CNES, or DLR, or the Italian national space agency. If you find out, please let me know in a comment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A reference of interest to you might be this &lt;a href=&quot;https://nepp.nasa.gov/DocUploads/2ED1FADF-B443-4038-8292DC44387FC3B3/commspce.pdf&quot; rel=&quot;nofollow&quot;&gt;paper&lt;/a&gt;, for starters.&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2016-01-10T09:52:20.250" LastActivityDate="2016-01-10T09:52:20.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13453" PostTypeId="2" ParentId="13450" CreationDate="2016-01-10T10:04:16.680" Score="3" Body="&lt;p&gt;I do not know of any all-inclusive review of the available COTS hardware for small satellites. NASA does, however, have a nice paper detailing/discussing current technology for small spacecraft, which I would recommend checking out -&lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/Small_Spacecraft_Technology_State_of_the_Art_2014.pdf&quot; rel=&quot;nofollow&quot;&gt;https://www.nasa.gov/sites/default/files/files/Small_Spacecraft_Technology_State_of_the_Art_2014.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Small Spacecraft&quot; in the paper are defined to be &amp;lt;180kg, however it has an emphasis on CubeSat-related technology. It is not exclusively focused on COTS hardware, but even if you do not find a specific sensor or magnetorquer you want to use, I think it is worth it to read through the sections on the subsystem(s) you are in charge of. I had a similar project to you and found their discussion on the subsystem I was charge of to be quite helpful. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do not find the specific hardware you are looking for, I would recommend you check out Clyde Space (&lt;a href=&quot;http://www.clyde-space.com/&quot; rel=&quot;nofollow&quot;&gt;http://www.clyde-space.com/&lt;/a&gt;) and Innovative Solutions In Space (&lt;a href=&quot;http://www.isispace.nl/cms/&quot; rel=&quot;nofollow&quot;&gt;http://www.isispace.nl/cms/&lt;/a&gt;), which both have a strong selection of CubeSat/nanosat components to choose from.&lt;/p&gt;&#xA;" OwnerUserId="12877" LastActivityDate="2016-01-10T10:04:16.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13454" PostTypeId="1" AcceptedAnswerId="14598" CreationDate="2016-01-10T13:22:11.090" Score="13" ViewCount="967" Body="&lt;p&gt;The ISP of a propellant is important when mass is the limiting factor; however, that is not exactly the case when launching a rocket. For the first part of the flight, the density of the propellant is also important.&lt;/p&gt;&#xA;&lt;p&gt;Higher density → more propellant in the same tank → higher mass ratio → more &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;A better metric for propellant performance for the first km/s is therefore &lt;a href=&quot;http://encyclopedia2.thefreedictionary.com/density+specific+impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;density impulse&lt;/a&gt;.&#xA;It is easy to notice that the extremely low density of liquid hydrogen (70.8 kg/m³) makes methane, ammonia and water outperform it as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;NTR&lt;/a&gt; propellant in the early stages of the flight.&lt;/p&gt;&#xA;&lt;p&gt;In the search for an even better density impulse, I tried to find high density metals with a low enough boiling point. Mercury and zinc turned out as possible candidates. For some reason, I am a bit afraid of glowing-hot, vaporized, radioactive mercury rain, so I choose instant galvanization instead. Zinc has an excellent density impulse. At an estimated 130s of ISP, the 7140kg/m³ density implies a density impulse 15 times better than hydrogen, and over twice that of water. Furthermore, as thrust is inversely proportional to exhaust velocity at the same reactor power, this is also giving the rocket a better thrust to weight ratio.&lt;/p&gt;&#xA;&lt;p&gt;But, zinc is not a fluid, so I wonder, &lt;strong&gt;is it possible to have a mix of zinc powder and water or ammonia as a propellant?&lt;/strong&gt; Is it possible to pump such a mix through the engine and prevent the reactor from clogging? Do you know of any research related to this?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Worked example of density impulse:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Sample rocket:&lt;/p&gt;&#xA;&lt;p&gt;Dry mass: 10,000kg&lt;br /&gt;&#xA;Tank volume: 100m³&lt;/p&gt;&#xA;&lt;p&gt;Sample propellants:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Hydrogen&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Exhaust velocity: 9000 m/s&#xA;Density: 70.8 kg/m³&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\Delta v=9000m/s \cdot ln\left(\frac{10,000kg + 100m^3 \cdot 70.8 kg/m^3}{10,000kg}\right)$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$=4818 m/s$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Water&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Exhaust velocity: 4000 m/s&#xA;Density: 1000 kg/m³&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\Delta v=4000m/s \cdot ln\left(\frac{10,000kg + 100m^3 \cdot 1000 kg/m^3}{10,000kg}\right)$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$=9592 m/s$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Feel free to leave out any discussion on the feasibility of using a NTRs, or potential benefits from a high density impulse from your answer.&lt;/p&gt;&#xA;&lt;p&gt;What I really want to know is if it is possible to pump metal powder through an engine when mixed with water, ammonia or methane. If it is possible, how high concentrations of metal powder I can have, and still be able to pump it?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2023-01-31T22:35:03.777" LastActivityDate="2023-01-31T22:35:03.777" Title="Is it possible to use zinc powder as a NTR propellant?" Tags="|launch|propulsion|fuel|nuclear|ntr|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="13455" PostTypeId="2" ParentId="13337" CreationDate="2016-01-10T13:55:45.347" Score="1" Body="&lt;p&gt;Another concept for a rocket engine alternator is the &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Solid_Core--Bi-Modal_NTR&quot; rel=&quot;nofollow noreferrer&quot;&gt;bi-modal NTR&lt;/a&gt;. It is a conventional NTR used for the high thrust manoeuvres, but when coasting the reactor is used for electricity production. The electricity produced can either be used for on-board systems, cryogenic refrigeration, or it can be used to power an ion engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qwbUGm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qwbUGm.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-01-10T13:55:45.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13456" PostTypeId="5" CreationDate="2016-01-10T18:07:01.597" Score="0" Body="&lt;p&gt;A breathing mix containing oxygen is necessary for humans, and all manned space-travel must therefore deal with that. On a space-flight, oxygen is a consumable, unless you find a way of recycling it or produce more of it. A common method is to electrolyse water to liberate the bounded oxygen. Producing oxygen by the means of photosynthesis has also been considered, but have not yet been tested further than closed ecosystems experiments on Earth (Bios 1,2 and 3, Biosphere 2).&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-10T18:52:47.703" LastActivityDate="2016-01-10T18:52:47.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13457" PostTypeId="4" CreationDate="2016-01-10T18:07:01.597" Score="0" Body="Questions regarding consumption and production of oxygen" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-10T18:52:38.697" LastActivityDate="2016-01-10T18:52:38.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13458" PostTypeId="2" ParentId="10527" CreationDate="2016-01-10T18:37:30.470" Score="7" Body="&lt;p&gt;There has been a starting point for international law in place since the 1967 Outer Space Treaty and specifically the detail given in the 1972 &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Liability_Convention&quot;&gt;Convention on International Liability for Damage Caused by Space Objects&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an lay-persons and engineering risk perspective and I am not licensed to give legal advice! To this extent the convention asserts that the &quot;launching state&quot; is responsible for liabilities, assessed on a fault basis. Also, its present interpretation is that state signatories are liable for the actions of their citizens and corporations and are expected to regulate accordingly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note in particular that whilst &lt;em&gt;&quot;Until recently space travel has been something governmental entities take on.&quot;&lt;/em&gt; may have been intended to refer to crewed flight it is not correct in the liability sense. Commercial space operations and launches have existed since the 1960's and '70s respectively and the largest commercial satellite operators control many tens of satellites each. As context to the commercial interest that exists the privately sponsored &lt;a href=&quot;http://www.space-data.org/sda/&quot;&gt;Space Data Association&lt;/a&gt;  was created in 2009 by Inmarsat, Intelsat and SES to share data to assist in collision avoidance and radio frequency problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The liability convention still gives rise to imponderables about which party bears liability. In the case of a satellite operator from state A, procuring a satellite from a manufacturer in state B which is then placed on a launch vehicle built in state C but launched from state D then each of those countries can be liable for third party losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for national regulation, many countries have had something in place for decades. I'm less familiar with the US practice though I understand, rather than the EPA, that three bodies, the FAA, NOAA and the FCC participate in the regulatory activity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;@jvriesem is right that asymmetries have arisen between countries in terms of how they implement their own regulations. If you keep your eyes open you may see corporations lobbying their own governments on the basis that they believe their local laws are stricter than elsewhere and there have been unilateral moves to harmonise matters. Furthermore, I suspect that &quot;flag of convenience&quot; filing of satellite assets under shell companies set up in diverse regimes may well already occur though it might be hard to trace as to whether it was motivated by lighter regulations.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-01-10T18:37:30.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13459" PostTypeId="1" AcceptedAnswerId="13462" CreationDate="2016-01-10T19:37:43.817" Score="5" ViewCount="518" Body="&lt;p&gt;Picture a system with three bodies: a central star and two planets in orbits around the central star. The planets' orbits are perfectly circular (&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_eccentricity&quot;&gt;eccentricity&lt;/a&gt; 0) and their orbital radii are 1.0 units and 1.5 units, respectively. The two planets have identical mass, and the mass of the planets is much smaller than that of the central star. (We could make the system more complex, but I am pretty sure that this is good enough for now. If you want a more real-life example, take the Sun, Earth and Mars, but make sure to state that you're using that in your answer.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, a spacecraft is launched onto a transfer orbit from one planet to the other. The instant the spacecraft attains escape velocity from the origin planet, its engines are shut down. (The velocity along the transfer orbit is equal to the origin planet's escape velocity.) We can simplify by handwaving: there is no venting or anything else like that which would impact the transfer orbit after the point of engine shutdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this scenario, at the instant when it enters the destination planet's sphere of influence, will a spacecraft launched from the inner planet toward the outer planet have a &lt;strong&gt;velocity appreciably different&lt;/strong&gt; from that which would be the case if the spacecraft was launched from the outer planet toward the inner planet?&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2016-01-11T19:14:38.207" Title="Do transfer orbits toward the central star necessarily result in a higher velocity on arrival due to the star's gravity?" Tags="|orbital-mechanics|hohmann-transfer|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13460" PostTypeId="2" ParentId="13459" CreationDate="2016-01-10T19:46:30.220" Score="8" Body="&lt;p&gt;When you've reached escape velocity relative to the planet, the star becomes the dominant gravitational influence: you're in orbit around the star.&lt;br&gt;&#xA;And as always when you're in orbit, your orbit's radius and speed are interchangable. So when you travel outward from the star, your speed will decrease, and when you move closer to the star, your speed will increase. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-10T19:46:30.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13461" PostTypeId="2" ParentId="13459" CreationDate="2016-01-10T21:01:47.580" Score="1" Body="&lt;p&gt;Perhaps this animation will make it more clear, but basically, the craft that leaves a lower/inner orbit planet to arrive at one further fro a star (without further propulsive effort) will arrive there at a slower velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6I3T5.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6I3T5.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shows the orbits of Mercury through Jupiter, with the 'mid point' of the asteroid belt thrown in for good measure, together with the elliptical orbits that pass between Earth and the other planets.  Note how much faster the dots are travelling at the low point of the elliptical orbits, compared to the high point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;N.B.  That image is intended to be shown on a dark background.  The orbit of Earth is shown in white.&lt;/p&gt;&#xA;" OwnerUserId="238" LastEditorUserId="238" LastEditDate="2016-01-10T21:11:37.887" LastActivityDate="2016-01-10T21:11:37.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13462" PostTypeId="2" ParentId="13459" CreationDate="2016-01-10T22:12:13.550" Score="7" Body="&lt;p&gt;You didn't say anything about how you chose the transfer orbit, nor did you specify velocity relative to what. And of course, &quot;appreciably&quot; is not defined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we will have to make some assumptions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, I will assume that you chose a minimum energy transfer orbit between the two bodies. For bodies with this small of a ratio of radii, that minimum transfer energy is a Hohmann transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second there are two choices for what the velocity is relative to, one more meaningful than the other.  The first choice is relative to the central star. The Hohmann transfer orbit to go from the inner body to the outer body is identical to the Hohmann transfer orbit the other way. Except for the direction, of course (which half of the orbit you are using). By their nature, the orbit will be faster closer to the star than farther.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much faster?  That orbit has an apoapsis just touching the orbit of the outer planet, and a periapsis just touching the orbit of the inner planet. So we have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$-{\mu\over a_i+a_o}={v^2\over 2}-{\mu\over r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $a_i$ is the radius of the inner planet orbit, the perapsis, and $a_o$ is the radius of the outer planet orbit, the apoapsis. The velocity $v$ is determined by the current radius from the central star, $r$.  So we have for the velocity of the vehicle at arrival at the two ends of the transfer orbit are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_o=\sqrt{2\mu{a_i\over a_o\left(a_i+a_o\right)}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_i=\sqrt{2\mu{a_o\over a_i\left(a_i+a_o\right)}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in your example, assuming that we have picked dimensions for which the radius of the inner planet is one and the $\mu=GM$ of the central star is $30$ (chosen just to make the results a little simpler), we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_o=4$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_i=6$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ratio of the velocities is $3\over 2$, equal to one over the ratio of radii. Is that appreciable? I'd say yes. I'll call anything more than about 10% appreciable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second choice for what the velocity is relative to has more import. That is the velocity relative to the arrival body. That tells you something about how much you'll have to slow down to stop or how hard you'll hit the atmosphere (though for the full answer you'll also need the mass and size of the planet).  The arrival orbit will be faster than the inner planet and slower than the outer planet, so at arrival you will be catching up with the inner planet, or the outer planet will be catching up with you from behind. So we will look at the magnitude of those two velocities.  They are (where we will denote the magnitude of the relative velocity as $v_\infty$):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{\infty o}=\sqrt{\mu\over a_o}-\sqrt{2\mu{a_i\over a_o\left(a_i+a_o\right)}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{\infty i}=\sqrt{2\mu{a_o\over a_i\left(a_i+a_o\right)}}-\sqrt{\mu\over a_i}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In our handy units, they are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{\infty o}=\sqrt{20}-4\approx 0.472$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{\infty i}=6-\sqrt{30}\approx 0.523$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The arrival velocity at the inner planet is higher, and the ratio is 1.11, so just a smidge over my definition of 10%. So, yes, it's appreciable, but just barely so. Since the arrival energy ratio is the square of the velocity ratio, and really it's the energy that matters, then it's definitely appreciable at over 20%.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-01-10T22:40:51.997" LastActivityDate="2016-01-10T22:40:51.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13463" PostTypeId="2" ParentId="3238" CreationDate="2016-01-11T00:18:38.613" Score="2" Body="&lt;p&gt;I could not find any diagram, but here is one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s384Q.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s384Q.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_rocket_engines&quot; rel=&quot;nofollow noreferrer&quot;&gt;list of orbital rocket engines&lt;/a&gt; is a good place to start, just click the link to their main article. Most of them have their diameter in their fact box:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ocdx3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ocdx3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Be aware that determining the optimal nozzle diameter is complicated, especially for the first stage, due to atmospheric backpressure&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="4660" LastEditDate="2016-01-13T16:08:20.153" LastActivityDate="2016-01-13T16:08:20.153" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13464" PostTypeId="2" ParentId="10807" CreationDate="2016-01-11T00:49:54.597" Score="3" Body="&lt;p&gt;The density of the atmosphere at the surface of Venus is about 65kg/m³, much higher than on Earth (1.2 kg/m³). That gives airships a lot more lifting-force. Furthermore, the Venus atmosphere is mostly carbon dioxide, which has a high molar mass. That allows use of other gasses for an airship, for example nitrogen or oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An important question is of course if any work at all can be done on the surface, due to the extremely high temperature and pressure. Higher up in the atmosphere, the conditions are &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030022668.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;a little more&lt;/a&gt; friendly, and once leaving the ground, flying vehicles become a necessity. Given the fact that lighter than air technology has always been close to the surface, but never quite made it, here on Earth, gives airships a good chance in the more dense atmosphere of Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/t1WCM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t1WCM.png&quot; alt=&quot;density of atmospheres&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Density of the Earth's and Venus' atmospheres in kg/m³&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-11T04:52:58.020" LastActivityDate="2016-01-11T04:52:58.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13465" PostTypeId="1" AcceptedAnswerId="13466" CreationDate="2016-01-11T04:40:14.537" Score="13" ViewCount="668" Body="&lt;p&gt;What are the reasons for deciding to use a 100% $ O_2 $ cabin atmosphere in the early US space programme?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was danger of nitrogen narcossis a factor?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did all missions prior to Apollo 1 use a 100% $ O_2 $ atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="8693" LastEditDate="2016-01-14T08:18:42.650" LastActivityDate="2016-01-14T08:18:42.650" Title="Decision Factors for using 100% $ O_2 $ cabin atmosphere in early US space program" Tags="|crewed-spaceflight|humans|environment|oxygen|capsule|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13466" PostTypeId="2" ParentId="13465" CreationDate="2016-01-11T05:16:53.110" Score="11" Body="&lt;p&gt;Despite a higher risk of a fire, pure oxygen also has some advantages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the internal pressure of the vessel is only a fifth of a normal breathing mix, allowing less structural load on the hull of the spacecraft. The resupply system is also simplified, because a system including nitrogen must have an extra tank for the nitrogen. (If you had them mixed, you end up with a higher and higher nitrogen pressure over time). A small mass saving is therefore achieved. For a minimal spacecraft where you simply open the hatch and vent the cabin air when performing an EVA, pure oxygen simply means less air wasted. Nitrogen narcosis seems to not be an issue, as I find it difficult to imagine an accident of &lt;em&gt;increase&lt;/em&gt; in the pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the decision making process in the early US space programme, the slightly higher complexity of a nitrogen system must have felt a little redundant. The early US Mercury and Gemini also used pure oxygen, but the early Soviet spacecraft, like Vostok, used a normal atmospheric breathing mix. Note that modern EVA suits do still use pure oxygen.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-11T13:54:36.967" LastActivityDate="2016-01-11T13:54:36.967" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13467" PostTypeId="1" AcceptedAnswerId="13529" CreationDate="2016-01-11T15:09:05.123" Score="7" ViewCount="104" Body="&lt;p&gt;Besides monoatomic oxygen, what other monoatomic or chemically reactive atmospheric or environmental constituents, if any, are present at orbital altitudes that aren't present at lower altitudes, and how do they affect spacecraft materials, instruments and equipment?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2016-01-11T15:33:40.537" LastActivityDate="2016-01-14T19:36:44.143" Title="Monoatomic species at orbital altitudes" Tags="|low-earth-orbit|atmosphere|materials|engineering|chemistry|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13469" PostTypeId="1" AcceptedAnswerId="13481" CreationDate="2016-01-11T15:39:13.897" Score="8" ViewCount="2596" Body="&lt;p&gt;In the article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_1#Command_Module_redesign&quot; rel=&quot;noreferrer&quot;&gt;Apollo command module design&lt;/a&gt;, I read:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The cabin atmosphere at launch was changed to 60% oxygen and 40% nitrogen at sea-level pressure: 14.7 psi (10.1 N/cm2). During ascent the cabin rapidly vented down to 5 psi (3.4 N/cm2), releasing approximately 2/3 of the gas originally present at launch. The vent then closed and the environmental control system maintained a nominal cabin pressure of 5 psi (3.4 N/cm2) as the spacecraft continued into vacuum. The cabin was then very slowly purged (vented to space and simultaneously replaced with 100% oxygen), so the nitrogen concentration fell asymptotically to zero over the next day. Although the new cabin launch atmosphere was significantly safer than 100% oxygen, it still contained almost three times the amount of oxygen present in ordinary sea level air (20.9% oxygen). This was necessary to ensure a sufficient partial pressure of oxygen when the astronauts removed their helmets after reaching orbit. (60% of 5 psi is 3 psi, compared to 20.9% of 14.7 psi (10.1 N/cm2), or 3.07 psi (2.12 N/cm2) in sea-level air.)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The environment within the astronauts' pressure suits was not changed. Because of the rapid drop in cabin (and suit) pressures during ascent, decompression sickness was likely unless the nitrogen had been purged from the astronauts' tissues prior to launch. &lt;strong&gt;They would still breathe pure oxygen, starting several hours before launch, until they removed their helmets on orbit.&lt;/strong&gt; Avoiding the &amp;quot;bends&amp;quot; was considered worth the residual risk of an oxygen-accelerated fire within a suit.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(emphasis mine)&lt;/p&gt;&#xA;&lt;p&gt;The article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Oxygen_toxicity#Signs_and_symptoms&quot; rel=&quot;noreferrer&quot;&gt;Oxygen Toxicity&lt;/a&gt; has a following table:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/V4Wai.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V4Wai.jpg&quot; alt=&quot;Oxygen poisoning at 90 feet (27 m) in the dry in 36 subjects in order of performance&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Several hours&amp;quot; seem like much longer than the 96 minutes the most persistent test subject withstood, of exposure to pure oxygen at atmospheric pressure, and risk of oxygen-accelerated fire within a suit seems like the least of worries the astronauts would have.&lt;/p&gt;&#xA;&lt;p&gt;How were they dealing with oxygen toxicity before and during the launch, before the pressure was vented to 5 psi?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-12-21T04:06:31.543" Title="Oxygen toxicity vs Apollo mission preparation" Tags="|apollo-program|astronauts|health|life-support|oxygen|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13470" PostTypeId="2" ParentId="13465" CreationDate="2016-01-11T15:53:30.387" Score="3" Body="&lt;p&gt;It is very difficult to make a soft or hybrid space suit manouverable with higher pressure (over 5 or 6 psi for example), because they are effectively just big balloons which just get stiffer and stiffer with more pressure. With such a low pressures, using pure oxygen is mandatory to get a human breathable partial pressure for oxygen. For this reason, all western space suits use pure oxygen at lower pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using higher pressures for cabin atmospheres requires adding nitrogen in to the mixture to prevent oxygen toxicity from higher oxygen partial pressure. This means that decompression sickness (the bends), which results from nitrogen inside the bloodstream being released as bubbles inside the body when moving to a lower pressure, is a real problem and requires hours of preparation to avoid it for an EVA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using pure oxygen also for the cabin atmosphere is an optimal choice with almost no downsides - except for the fire hazard. Fires burn hotter based on the proportion of gas that is oxygen, not based on partial pressure, so 100% oxygen atmosphere is very flammable. Unfortunately the fire hazard is so great that it is the limiting factor why 100% oxygen atmospheres are no longer used in cabins.&lt;/p&gt;&#xA;" OwnerUserId="2631" LastActivityDate="2016-01-11T15:53:30.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13473" PostTypeId="1" AcceptedAnswerId="20752" CreationDate="2016-01-11T16:54:10.047" Score="12" ViewCount="419" Body="&lt;p&gt;It has been found (from missions like the &lt;a href=&quot;http://setas-www.larc.nasa.gov/LDEF/&quot; rel=&quot;noreferrer&quot;&gt;Long Duration Exposure Facility&lt;/a&gt;, LDEF) that some metals, especially tin and cadmium in solders, used in space-electronics assemblies, form crystal &lt;a href=&quot;https://en.wikipedia.org/wiki/Whisker_%28metallurgy%29&quot; rel=&quot;noreferrer&quot;&gt;whiskers&lt;/a&gt; over time, which lead to short circuits in the circuitry when the whiskers reach other components or tracks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What mitigation procedures are in place to prevent whisker growth?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2016-02-07T08:31:59.283" LastActivityDate="2018-08-14T18:11:02.097" Title="What mitigation procedures are in place in space systems to prevent whisker growth?" Tags="|materials|engineering|electronics|chemistry|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13474" PostTypeId="2" ParentId="13436" CreationDate="2016-01-11T18:23:57.270" Score="6" Body="&lt;p&gt;I worked directly on several university cubesat missions that ran linux in a semi-custom main flight computer, as well as more traditional embedded software on small processors such as the MSP-430. I don't want to say which university I supported because I no longer work there and I don't know how open they are currently about their system design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Through interacting with other universities and small businesses I know that there are several fully COTS solutions available that are capable of running linux. &lt;a href=&quot;http://www.cubesat.org/index.php/collaborate/suppliers&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; and &lt;a href=&quot;http://www.cubesatshop.com/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; are the two most comprehensive lists I could find of suppliers marketing towards micro/small sat missions. There is also &lt;a href=&quot;http://www.digikey.com/product-search/en/embedded-computers/single-board-computers-sbcs/5047549&quot; rel=&quot;noreferrer&quot;&gt;a wide selection of products&lt;/a&gt; that not satellite oriented that are equally suitable for running linux and commanding a satellite (such as the Intel Atom, Raspberry Pi, and other computers designed for used in more typical industrial applications). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't really address the specifics of hardware selection as it is so tightly coupled with other missions design concerns. What I can say is that running linux has the potential to simplify the software design (increased clock speed, cashe, memory, higher level interfaces like USB, etc.) and shift complexity into interfacing with the hardware and designing for reliability (timing accuracy, linux complexities, multiple threads, etc.). Look for a system that is physically designed to be fault tolerant, or where it can be added on in another way (internal &amp;amp; external watchdog timers).&lt;/p&gt;&#xA;" OwnerUserId="12966" LastActivityDate="2016-01-11T18:23:57.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13475" PostTypeId="2" ParentId="13459" CreationDate="2016-01-11T19:14:38.207" Score="1" Body="&lt;p&gt;I will use the scenario modeled in my &lt;a href=&quot;http://clowder.net/hop/railroad/Hohmann.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann Spreadsheet&lt;/a&gt;. The spreadsheet calls Mars' orbit 1.52 A.U. and earth's orbit 1 A.U. The sheet assumes circular, coplanar orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth to Mars:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Departure Vinf 2.94 km/s.&lt;br&gt;&#xA;Arrival Vinf 2.65 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screen capture from my spreadsheet:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4sRRl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4sRRl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mars to Earth:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Departure Vinf 2.65 km/s.&lt;br&gt;&#xA;Arrival Vinf 2.94 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quantities are the same but switched. The total Vinfinity is the same regardless if the Hohmann is going out or in. The Vinf at the outer planet is smaller. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bigger the distance between planets, the more pronounced the difference. For example,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mercury to Neptune:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Departure Vinf 19.4 km/s.&lt;br&gt;&#xA;Arrival Vinf 4.57 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vinf&lt;/a&gt; is a hyperbolic orbit's speed at infinity. For practical purposes a Hohmann transfer becomes hyperbolic when entering a planet's sphere of influence and Vinf refers to the difference in speed between heliocentric orbits.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-01-11T19:14:38.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13476" PostTypeId="2" ParentId="13302" CreationDate="2016-01-11T20:19:37.110" Score="5" Body="&lt;p&gt;In his text &lt;em&gt;&lt;a href=&quot;https://www.springer.com/us/book/9783540210627&quot; rel=&quot;nofollow&quot;&gt;Solar sailing: Technology, Dynamics and Mission Applications&lt;/a&gt;&lt;/em&gt; (see &lt;a href=&quot;https://www.researchgate.net/profile/Colin_Mcinnes3/publications&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; for some of his open access titles), Colin R. McInnes derives the pressure on a perfect sail face on to Sol as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P(r) = (L/(3 \pi c R^2)) * (1 -[1 - (R/r)^2]^{1.5})$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$P$ is photon pressure&lt;br&gt;&#xA;$c$ is speed of light&lt;br&gt;&#xA;$r$ is distance from center of Sol&lt;br&gt;&#xA;$L$ is luminosity of Sol&lt;br&gt;&#xA;$R$ is radius of Sol  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In another form, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F(r) = (2/3) * (r/R)^2 * (1 -[1 - (R/r)^2]^{1.5})$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;meaning at $R$ the force experienced is 2/3 that expected from the inverse square law.  After 10 $R$, $F(r)$ becomes very close to one.  McInnes also covers limb darkening. This is a good text, and I hope an updated version will be out soon. If using the original version, watch out for printer's errors:  missing parentheses, incorrect or misplaced exponents, dropped parameters, and so on.&lt;/p&gt;&#xA;" OwnerUserId="12999" LastEditorUserId="49" LastEditDate="2016-01-11T20:55:13.593" LastActivityDate="2016-01-11T20:55:13.593" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13477" PostTypeId="1" AcceptedAnswerId="13485" CreationDate="2016-01-12T03:32:57.787" Score="20" ViewCount="1448" Body="&lt;p&gt;Before the loss of Columbia during the STS-107 mission on February 1 2003, the International Space Station was scheduled to reach &quot;Core Complete&quot; status in February 2004. (Although this was increasingly unlikely at the time, and the Columbia Accident Investigation Board &lt;a href=&quot;http://s3.amazonaws.com/akamai.netstorage/anon.nasa-global/CAIB/CAIB_lowres_chapter6.pdf&quot;&gt;cited this schedule pressure&lt;/a&gt; as a contributing factor to the accident). The ISS Assembly Complete planned date at that time was likely a few years further on, &lt;a href=&quot;http://www.jsc.nasa.gov/roundup/online/2002/0102_p6_8.pdf&quot;&gt;perhaps 2005&lt;/a&gt; (also increasingly unlikely).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, were any shuttle manifests or plans ever published showing proposed utilization of the fleet &lt;em&gt;after&lt;/em&gt; ISS Assembly Complete?  Would the schedule have been reduced to two or three launches a year to rotate ISS crewmembers and refresh supplies? Or would the Orbiters have, in addition to ISS support, returned to launching other payloads?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't ever remember seeing this addressed, perhaps because before STS-107 the focus was solely on Core and Assembly Complete, and after STS-107 everything was thrown into Return to Flight and then Assembly complete.  But surely NASA did some long range planning? &lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-08-29T23:46:18.530" LastActivityDate="2020-08-29T23:46:18.530" Title="Before the Columbia accident, were there shuttle manifests/plans for after ISS assembly complete?" Tags="|space-shuttle|history|planning|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13478" PostTypeId="1" CreationDate="2016-01-12T04:50:17.183" Score="-1" ViewCount="390" Body="&lt;p&gt;Trying to work out a rough time estimate for an ASAT rocket to reach GEO height?&lt;/p&gt;&#xA;" OwnerUserId="13004" LastActivityDate="2016-01-12T08:20:04.107" Title="min time a direct ascent ASAT rocket would take to reach GEO" Tags="|orbital-mechanics|geostationary|geosynchronous|" AnswerCount="1" CommentCount="22" ClosedDate="2016-01-12T08:20:22.633" ContentLicense="CC BY-SA 3.0" />
  <row Id="13479" PostTypeId="2" ParentId="13478" CreationDate="2016-01-12T08:00:39.523" Score="3" Body="&lt;p&gt;To just intercept the orbit, but not remain there, you can travel at any speed above that required for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Hohmann transfer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words your travel time can be reduced to as short as you wish, provided you have the fuel and acceleration available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can't handle the maths, you could try some trial-and-error simulations with &lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/&quot; rel=&quot;nofollow&quot;&gt;Orbiter&lt;/a&gt; for example (just look at the time needed to rise to the altitude you want).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Out of interest (not really an answer), there is a &lt;a href=&quot;http://www.satsig.net/orbit-research/delta-v-geo-injection-calculator.htm&quot; rel=&quot;nofollow&quot;&gt;calculator&lt;/a&gt; though it only gives the fuel and delta-vee requirements for the Hohmann transfer orbit, not the transfer time.  Also it does not do any calculations for the higher energy case you require.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-01-12T08:20:04.107" LastActivityDate="2016-01-12T08:20:04.107" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13480" PostTypeId="1" AcceptedAnswerId="13486" CreationDate="2016-01-12T12:54:11.440" Score="5" ViewCount="314" Body="&lt;p&gt;As I understand, Solar Sail runs on dual energy: light pressure and solar wind. While the former is quite deterministic and rigidly following calculations, solar flares create huge bursts of solar wind - at unpredictable points in time, of unpredictable intensity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would they affect solar sail propulsion?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;solar sail needs to be huge and light - that means quite fragile. Would it be possible they could damage (tear) it?&lt;/li&gt;&#xA;&lt;li&gt;even if it withstands them physically, wouldn't they throw the craft off painstakingly calculated trajectory?&lt;/li&gt;&#xA;&lt;li&gt;anything else I missed?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(and generally, what is the proportion between &quot;propulsive power&quot; between the two accelerating factors of solar sail?)&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-01-13T00:29:49.667" Title="How would Solar Flares affect Solar Sail navigation?" Tags="|solar-sail|advanced-propulsion|cosmic-radiation|solar-flares|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13481" PostTypeId="2" ParentId="13469" CreationDate="2016-01-12T13:27:48.553" Score="8" Body="&lt;p&gt;Reading the listed Wikipedia article, it is evident that these experiments were done at pressure, the same pressure that would be experienced at 90 feet of water. I've been trying to find a better source, and I looked at the &lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2053251/?page=1&quot; rel=&quot;noreferrer&quot;&gt;source article for that chart&lt;/a&gt;. It contains the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... were safe breathing oxygen ... for at least three hours at 3 atmospheres&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It seems the toxicity only really happens once one gets above 3 atmospheres, the safe exposure for 4 atm is around 30 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it isn't recommended for everyone, there seems to be no adverse effects to breathing pure oxygen at atmospheric pressures for several hours, as was done for the Apollo program.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-12T13:27:48.553" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13482" PostTypeId="1" CreationDate="2016-01-12T15:22:30.007" Score="18" ViewCount="2457" Body="&lt;p&gt;DARPA has proposed orbital maintenance and salvage robots. I had gathered a list of links that was lost when DARPA rearranged their website. But I was able to find this page:&#xA;&lt;a href=&quot;http://www.darpa.mil/news-events/2014-04-02&quot;&gt;Phoenix Makes Strides in Orbital Robotics and Satellite Architecture Research&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose someone successfully designs an orbital maintenance and salvage robot that can rendezvous with many satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What parts of a dead satellite can be salvaged?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="12788" LastEditDate="2016-01-12T19:31:56.280" LastActivityDate="2016-01-13T20:52:41.363" Title="What's salvageable from a dead satellite?" Tags="|artificial-satellite|low-earth-orbit|geostationary|robotic-missions|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13483" PostTypeId="2" ParentId="13482" CreationDate="2016-01-12T16:45:13.460" Score="10" Body="&lt;p&gt;It really depends on what stage the satellite is in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the satellite still functions at a reduced capacity, then likely everything except the batteries, and fuel, is salvageable. Some of the transmitters, instruments, etc could also work, although it really depends on why the satellite failed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the satellite isn't functioning, then some components may be working. These components would need to be tested. The heater may not be functioning, which might make the components less likely to work after being salvaged. In such a case, the fuel, solar panels, housing, and possibly others might be salvageable, but it would be more difficult. More likely everything except the listed items would best be used as scrap material, and without manufacturing on orbit would be difficult. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar panels degrade at a rate of about 2-3% per year. After 20 years, they will be about half of what they were when made. There might be some use in collecting them, but overall they will have issues due to radiation, thermal, and debris degradation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, the best thing is to repair working satellites, as opposed to putting two or more together. You might get some commonality for satellites of the same design, but even then, it might be difficult.  I'd say the best bet is components of satellites of the same design, fuel, power generation, possibly batteries, and scrap metal is all that would really be reusable in a salvage situation for now.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-01-12T16:56:06.207" LastActivityDate="2016-01-12T16:56:06.207" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13484" PostTypeId="2" ParentId="13482" CreationDate="2016-01-12T17:47:45.060" Score="7" Body="&lt;p&gt;A few things I come to think of:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Heat radiators for cooling a power source.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Physical structure needed in-space but not during launch. Maybe for example such needed to spin a spacecraft for simulated gravity. (Once that kind of stuff has been standardized)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A memory chip with petabytes of science and engineering data stored on it, to be collected and soft landed on Earth. I think this is very doable, data has low mass and travels lightly through space. Even in &quot;cartridges&quot;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Equipment, such as a large radio parabola and sensitive instruments. Like taking the optical mirror off an old space telescope and putting it on a new Kind of a Hubble servicing mission++.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Espionage! Since it is about DARPA, I suspect that they want to bring parts of enemy spy satellites and such back to their laboratories on Earth. To assess and reverse engineer whatever the foe has done.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Finally, Apollo 12 brought back parts of a previous robotic Surveyor lander to Earth. Just to examine how it survived up there.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Versions of that above could maybe be useful to do in-space instead of being brought back to Earth as has yet been the prefered way to do it AFAIK.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2016-01-12T20:07:12.830" LastActivityDate="2016-01-12T20:07:12.830" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13485" PostTypeId="2" ParentId="13477" CreationDate="2016-01-12T19:29:12.013" Score="8" Body="&lt;p&gt;Not that I recall ever seeing!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spider.seds.org/shuttle/sts_all.txt&quot;&gt;This&lt;/a&gt; is a copy of the 2001 Consolidated Launch Manifest, which only goes out to 2006; it has two non-ISS flights, STS-107 and the fourth Hubble flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spider.seds.org/shuttle/shu-f_030202.html&quot;&gt;This list&lt;/a&gt; isn't an official document, but apparently compiled from various NASA sources. It was not updated after Columbia, so is probably a good snapshot of contemporary plans as at the start of 2003. Again, only the fourth Hubble flight other than ISS construction/utilization flights, but projected dates go out to 2009.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither seem to go beyond the end of construction, though...&lt;/p&gt;&#xA;" OwnerUserId="11259" LastActivityDate="2016-01-12T19:29:12.013" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13486" PostTypeId="2" ParentId="13480" CreationDate="2016-01-12T20:40:16.483" Score="5" Body="&lt;p&gt;This response addresses the last part of the question: &quot;what is the proportion between &quot;propulsive power&quot; between the two accelerating factors of solar sail?&quot;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind#Solar_wind_pressure&quot;&gt;Solar Wind&lt;/a&gt; is between 1-6x10&lt;sup&gt;-9&lt;/sup&gt; N/m&lt;sup&gt;2&lt;/sup&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The wind exerts a pressure at 1 AU typically in the range of 1–6 nPa (1–6×10&lt;sup&gt;−9&lt;/sup&gt; N/m&lt;sup&gt;2&lt;/sup&gt;), although it can readily vary outside that range.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pressure#Solar_radiation_pressure&quot;&gt;Solar Radiation Pressure&lt;/a&gt; is about 9x10&lt;sup&gt;-6&lt;/sup&gt; N/m&lt;sup&gt;2&lt;/sup&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;9.08 µPa (µN/m&lt;sup&gt;2&lt;/sup&gt;) at 1AU.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Thus the latter is &quot;typically&quot; a thousand times larger. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This at least seems consistent with operational experience on conventional geostationary satellites. In this scenario solar radiation pressure is an orbit disturbance. Solar wind on the other hand, or rather the flares and CMEs that modulate it, is the bringer of ESD, SEUs and long anomaly management meetings.  I'm unable to offer guidance as to the magnitude of the solar wind under flare/CME conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its not beyond belief that there could by a dynamic effect from the solar wind on a very large area to mass object, particularly if its approach vector to the object isn't radial from the Sun (don't count on CMEs being radially directed). However this is speculation placed here as a straw-man for someone else to pick up!&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorDisplayName="user5892" LastEditDate="2016-01-13T00:29:49.667" LastActivityDate="2016-01-13T00:29:49.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13487" PostTypeId="2" ParentId="13482" CreationDate="2016-01-12T21:18:31.473" Score="2" Body="&lt;p&gt;As the other question responses have hinted, it depends upon what you mean by &quot;&lt;em&gt;can&lt;/em&gt; be salvaged&quot;. This field, robotic intervention, is a mixture of  bright ideas, immature technology and poor but growing cost effectiveness.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The technically &quot;easiest&quot; salvage and most readily exploitable commercially would be the whole satellite. This idea has been around for a decade or more for the limited situation of a satellite that has residual function in terms of power, command access etc but which has, or will soon, run out of propellant.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The first step in salvage is rendezvous and docking. If the salvager has attitude control authority over the target then it can point it towards the Earth or move it to another location to give it a new lease of life. This has already been considered to be the end goal in a number of studies.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The more adventurous steps of salvage, which has included repair, refueling, or now in the DARPA Phoenix project, dismantling for re-use elsewhere will all need some additional robotic capability over and above the initial rendezous and docking. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Thus all the ideas suggested in other responses and comments could be workable, just depending on the level of robotic sophistication and price, both unknown at the moment. I've heard the antenna removal option specifically in conjunction with the DARPA programme recently and suspect it relates to reuse of large mesh antennas though I haven't yet found any references to flesh it out.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-01-12T21:18:31.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13488" PostTypeId="1" CreationDate="2016-01-12T23:26:13.127" Score="3" ViewCount="333" Body="&lt;p&gt;Inspired by this question, &lt;a href=&quot;https://space.stackexchange.com/questions/88/total-mass-sent-into-orbit-over-all-history&quot;&gt;total mass ever launched to orbit&lt;/a&gt; (this is a different question, as many of the things launched have decayed by now.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much of it is still up there? Out of that again, how much is still operational?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Rules&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spent upper stages are valid and count as launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Still there&quot; does not include spacecraft landed or crashed on the Moon, Mars or other bodies. They may however be included in a separate number. Spacecraft &quot;landed on&quot; (docked to) another artificial object, like the ISS, does not count as landed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Operational&quot; means two-way communication with Earth, propulsion capability is not necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An asteroid must be larger than the spacecraft for it to count as &quot;landed&quot;, meaning a spacecraft struck by interplanetary dust or pebbles is not &quot;landed&quot;. &lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-01-13T20:46:46.340" Title="What is the total mass of everything launched that are still in space?" Tags="|launch|spacecraft|history|mass|orbital|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13489" PostTypeId="1" CreationDate="2016-01-12T23:36:35.143" Score="1" ViewCount="2295" Body="&lt;p&gt;I'm wondering if it'd be possible to tape a bottle rocket to one of these &lt;a href=&quot;https://www.kickstarter.com/projects/zacinaction/kicksat-your-personal-spacecraft-in-space/posts/1247352&quot; rel=&quot;nofollow&quot;&gt;chipsat things&lt;/a&gt; and carry it to space with a balloon. Once the balloon hits max altitude, the small rocket is ignited and takes the chipsat into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Feasible? Stupid? Are there much easier/better ways to get a chipsat into space? Are there laws against doing such a thing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt; After doing further research on it I've discovered this is one of the stupidest questions I've ever asked.&lt;/p&gt;&#xA;" OwnerUserId="13016" LastEditorUserId="25" LastEditDate="2016-01-13T03:18:24.690" LastActivityDate="2016-01-13T03:18:24.690" Title="Is it possible to put something small in orbit with a balloon?" Tags="|launch|rockets|orbit|balloons|" AnswerCount="1" CommentCount="1" ClosedDate="2016-01-13T03:21:26.033" ContentLicense="CC BY-SA 3.0" />
  <row Id="13490" PostTypeId="2" ParentId="13489" CreationDate="2016-01-13T00:14:46.090" Score="7" Body="&lt;p&gt;You can get things to the edge of space with a balloon fairly easily (in fact quite a few people have done this as amateur science projects)  but getting into a stable orbit is an entirely different matter as a low earth orbit requires a relative velocity of something like 7km/s. In fact for a conventional rocket the energy to achieve the altitude to get into space is a fairly small fraction of that required to accelerate to the orbital velocity needed to stay there. &lt;/p&gt;&#xA;" OwnerUserId="13018" LastActivityDate="2016-01-13T00:14:46.090" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13491" PostTypeId="2" ParentId="13482" CreationDate="2016-01-13T01:50:53.243" Score="4" Body="&lt;p&gt;Even if everything is dead and cannot be repaired or salvaged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A momentum exchange tether could be used to throw it into the atmosphere and applying momentum to something useful. &lt;/p&gt;&#xA;" OwnerUserId="3277" LastActivityDate="2016-01-13T01:50:53.243" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13492" PostTypeId="1" AcceptedAnswerId="21402" CreationDate="2016-01-13T04:20:33.250" Score="7" ViewCount="1491" Body="&lt;p&gt;The Shuttle main engines used LH2 and LOX, and used sparklers to burn off the hydrogen that would be exhausted, unburned, during engine startup. (They pre-chill the engine components so that they are not shocked by the cold LH2.  Avoiding shrinkage is important, even if you are an SSME not a guy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I asked about the Delta 4 Heavy, why they could not do the same in this question.&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/3241/why-not-use-sparklers-on-delta-4-heavy-launches-to-burn-off-the-hydrogen&quot;&gt;Why not use Sparklers on Delta-4 Heavy launches to burn off the hydrogen?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some discussion centered around not invented here syndrome, which raised a question.  Who provided the sparklers for the Space Shuttle program? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockwell (now Boeing) as part of the Orbiter?  Aerojet as part of the SSME? Or KSC as part of the launch pad? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2020-04-24T18:47:58.117" Title="Who built the sparklers used by the Shuttle program during launch?" Tags="|space-shuttle|hydrogen|delta-iv-heavy|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13493" PostTypeId="1" CreationDate="2016-01-13T09:28:08.717" Score="4" ViewCount="233" Body="&lt;p&gt;What are my options (ie purchasable hardware) for a COTS 32-bit Cubesat motherboard, that has successfully flown in LEO, running Linux? Do not suggest any hardware that has never flown in space before.&lt;/p&gt;&#xA;" OwnerUserId="12983" LastEditorUserId="12102" LastEditDate="2021-03-25T23:14:20.293" LastActivityDate="2021-03-25T23:14:20.293" Title="COTS 32-bit Cubesat flight mother-board options" Tags="|cubesat|cots|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13494" PostTypeId="1" CreationDate="2016-01-13T09:56:42.327" Score="2" ViewCount="441" Body="&lt;p&gt;It looks to me like the camera used for the deployment part of the Webcast shown on YouTube is located near the &quot;bottom&quot; of the payload area  (closer to the engine &amp;amp; tanks)  - in which case, did the spacecraft rotate so that it was pointing roughly &quot;backwards&quot; by the time deployment started? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graphics and explanations in this &lt;a href=&quot;https://space.stackexchange.com/q/13132/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/13137/12102&quot;&gt;answer&lt;/a&gt; pair are excellent, so I'll just link to them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; It maybe be helpful to look at this &lt;a href=&quot;http://gph.is/1TWo7dm&quot; rel=&quot;nofollow noreferrer&quot;&gt;GIF here&lt;/a&gt; which is too large to upload to SE, and I'm adding another captured image below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is something sticking up at either side of the bottom of the image - possibly a boom for the camera and floodlights? It sticking out farther from the axis than the satellites. It seems it would be easier if these were deployed from the bottom of the payload area, not the top (but anything is possible). Also looking at the top of the image there is no hint of the engine or spacecraft structure. It &quot;looks like&quot; the top of the structure which holds the satellites. (moon still there at the top).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX second stage after all Orbcomm-2 satellites deployed&#xA;Location ~ roughly Europe, where moon is setting in the West. Direction of motion is (roughly speaking) East.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Click to expand:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UlzhU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UlzhU.jpg&quot; alt=&quot;SpaceX second stage after all Orbcomm-2 satellites deployed&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the nominal sizes of the Orbcomm-2 satellites (roughly 1 x 1.3 x 0.5 meters) and the nominal size of a human, this can't be much more than 3 meters wide, and if there is still some external tube around the 2nd stage fuel tanks that should be about 3.6 meters in diameter, and clearly visible if it were at the far end in the deployment video. The image below is linked to &lt;a href=&quot;http://www.spaceflight101.net/orbcomm-g2-launch-1---falcon-9-launch-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflight101&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT 2:&lt;/strong&gt; I found this sentence in the &lt;a href=&quot;http://www.spaceflight101.net/orbcomm-g2-launch-1---falcon-9-launch-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight101 article&lt;/a&gt; (my bold) in reference to the previous SpaceX launch of 6 Orbcomm satellites:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;Second stage shutdown came just after passing the T+9-minute 30-second mark to allow the stage to begin a short coast that included the &lt;strong&gt;re-orientation to the proper attitude for the spacecraft separation sequence&lt;/strong&gt; that kicked off at T+14 minutes and 35 seconds.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7NPhS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7NPhS.jpg&quot; alt=&quot;SpaceX second stage after all Orbcomm-2 satellites deployed&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; that image (above) is actually Orbcomm-1, a previous mission. Here is an image of the Orbcomm-2 configuration prior to launch, from &lt;a href=&quot;http://www.orbcomm.com/en/networks/satellite/orbcomm-og2&quot; rel=&quot;nofollow noreferrer&quot;&gt;ORBCOMM&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xKqTz.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xKqTz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-07T00:10:54.397" LastActivityDate="2021-08-07T00:10:54.397" Title="DId the Falcon 2nd stage turn around [UPDATED] and point &quot;backwards&quot; before the Orbcomm-2 deployement started?" Tags="|spacex|falcon-9|low-earth-orbit|payload-deployment|orbcomm-2|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="13495" PostTypeId="1" AcceptedAnswerId="13499" CreationDate="2016-01-13T10:43:22.930" Score="9" ViewCount="16379" Body="&lt;p&gt;I understand that these days a spacecraft design is very much centred on what can be lifted into space by current rockets, hence the modular, cylindrical shape of the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/main/index.html&quot; rel=&quot;noreferrer&quot;&gt;ISS&lt;/a&gt;. So, with current manufacturing and design technologies and under the huge (unrealistic) assumption that there were no restrictions on lifting materials/components into space, what would be the most optimal shape for a spacecraft in terms of some of the following example factors (others are welcome)?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Efficiency&lt;/li&gt;&#xA;&lt;li&gt;Construction practicality&lt;/li&gt;&#xA;&lt;li&gt;Internal navigation by crew&lt;/li&gt;&#xA;&lt;li&gt;Etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Would it be a sphere? cube? (I can smell the Borg...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should make clear that I'm not interested in a spacecraft that has to go near any atmosphere so there doesn't need to be any consideration for aerodynamics.&lt;/p&gt;&#xA;" OwnerUserId="3384" LastEditorUserId="8693" LastEditDate="2016-01-13T21:24:07.117" LastActivityDate="2021-08-19T09:23:38.600" Title="What is the optimum shape for a spacecraft?" Tags="|spacecraft|design|spacecraft-development|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13496" PostTypeId="2" ParentId="13495" CreationDate="2016-01-13T11:46:58.577" Score="2" Body="&lt;p&gt;A large-enough donut or cylinder would allow for artificial gravity through rotation. Diameter would have to be several hundred m at least. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-13T11:46:58.577" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13497" PostTypeId="1" CreationDate="2016-01-13T12:07:54.677" Score="5" ViewCount="1063" Body="&lt;p&gt;I hadn't noticed it before, but after seeing &lt;a href=&quot;https://www.youtube.com/watch?v=ANv5UfZsvZQ&quot; rel=&quot;noreferrer&quot;&gt;yesterday's recap video&lt;/a&gt; of the recent Launch and Landing of the Falcon 9, I'm wondering what the plume visible during ascent is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see it clearly immediately after launch, when the video changes camera angles at around 1:04, but I've highlighted it below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/vEaQn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume it's the fuel gases continuing to be vented, and the plume appears to extend/originate from up the stack.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, is the lower, circled part, just being illuminated by the light from the engines, or is it actually being ignited? Does the plume have any measurable affect on the attitude that the rocket has to correct for, either through the venting or ignition (if indeed it is alight)?&lt;/p&gt;&#xA;" OwnerUserId="10686" LastEditorUserId="10686" LastEditDate="2016-01-13T12:36:23.680" LastActivityDate="2016-03-23T13:05:31.397" Title="What is this additional plume on the Falcon 9, and is it being ignited/corrected for?" Tags="|spacex|falcon-9|plume|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13498" PostTypeId="2" ParentId="13495" CreationDate="2016-01-13T12:15:23.353" Score="0" Body="&lt;p&gt;I think that spaceships in the far future will be analogous to biology. They will constantly reassemble and reorganize themselves to repair damages from radiation and decay and to adapt to changing environments or desires. Or simply because one structure obtained reveals that another structure is more attractive to attain in an evolutionary fashion of discovery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Life on &quot;spacecraft Earth&quot; has done this spontaneously. Fundamental logics of physics, geometry and economics set some constraints, but lifeforms still vary from tiny archean bacteria to huge reptiles and mammals to distributed ant and bee colonies as genetic unit. The wisdom of this billion year long evolution seems hard to beat. Lack of gravity and lack of interaction with atmosphere/hydrosphere removes important constraints to shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tethers can be very useful. Microbiology seems to be full of microscopic &quot;hairs&quot; which guide transports and communication. In microgravity tethers are useful to simulate gravity by rotation, and maybe for launching or capturing smaller spacecrafts.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-01-13T12:15:23.353" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13499" PostTypeId="2" ParentId="13495" CreationDate="2016-01-13T12:19:15.777" Score="11" Body="&lt;p&gt;&lt;strong&gt;Spheres are best, but cylinders are easiest to manufacture&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the most internal volume compared to the surface area, and hence the smallest possible mass, a sphere is the ideal shape for a spacecraft. The orbital module of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz&lt;/a&gt; spacecraft is approximately a sphere:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BWQbwt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BWQbwt.jpg&quot; alt=&quot;soyuz&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most construction techniques in use are based on &lt;a href=&quot;https://en.wikipedia.org/wiki/Sheet_metal&quot; rel=&quot;nofollow noreferrer&quot;&gt;metal plates&lt;/a&gt;. Therefore, spacecraft shaped like cubes or cylinders (a bent plate) are easy to manufacture. The sharp edges are however less optimal than the sphere to withstand internal pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For long stays in space, it is beneficial to have some sort of artificial gravity induced by rotation. Cylinders and toruses are then good options. They are also rather easy to inflate. (that is a launch constraint, but an important one)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/t7zJzm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t7zJzm.jpg&quot; alt=&quot;http://www.darkgovernment.com/news/wp-content/uploads/2013/01/Space_station.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A spacecraft with artificial gravity must not necessary have to be of a curved shape, two spacecraft, or one spacecraft and a counterweight with a tether in between can basically do the same thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For some types of propulsion, most importantly nuclear-electric, solar sails, or solar-electric, huge areas of radiators, sails, or solar cells are necessary. That pretty much determines their shape.&#xA;(huge but flat, with a long truss-segment in the middle) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/u2qHyt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/u2qHyt.jpg&quot; alt=&quot;electric spacecraft&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Different parts of a spacecraft has different requirements, hence, you may use different shapes for them instead of an all-in-one trade off. Again, the Soyuz is an excellent example of geometry, using a sphere for the orbital module, an aerodynamically blunt body shape for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry&quot; rel=&quot;nofollow noreferrer&quot;&gt;return capsule&lt;/a&gt;, and a cylinder for the equipment module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Landers&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the top priorities for a lander is to have a large base to prevent it from tipping when landing on the surface. Despite initial surveys from unmanned probes in orbit of for example the Moon, you can never be totally sure about the usefulness of a particular landing site. The Apollo 11 did for example change the target landing site in flight. Therefore, landers are  typically very wide, so they do not fall over. The descent stage is often-shaped like a polygonal prism, with a symmetry depending on the number of landing legs. (three for a minimum configuration, four for a little more stability , or five or more for redundancy and extra stability).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo lunar module descent stage had an octagonal shape, corresponding to its four-fold symmetry:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fggw5m.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fggw5m.png&quot; alt=&quot;Apollo descent stage&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-31T22:14:26.113" LastActivityDate="2016-01-31T22:14:26.113" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13500" PostTypeId="2" ParentId="13494" CreationDate="2016-01-13T13:28:50.780" Score="3" Body="&lt;p&gt;It is very likely there are two separate cameras. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have consistently had a camera focused on the bell of the second stage, that appears also able to do IR imagery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would not be mounted in the airflow, since we see images of the second stage engine before MECO and with the interstage still attached.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus it would not likely be able to have a view forward of the payload, which is at the other end of the second stage (now longer due to the new Falcon design stretch).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-01-13T15:28:15.830" LastActivityDate="2016-01-13T15:28:15.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13501" PostTypeId="1" AcceptedAnswerId="13503" CreationDate="2016-01-13T14:34:37.523" Score="23" ViewCount="9086" Body="&lt;p&gt;Why did the Apollo Lunar Module have four landing legs? Is not just three legs enough? Also, three legs guaranties they always form a plane, four does not, as the fourth leg may dangle. To have the centre of mass over the landing leg polygon, even if one of the legs broke, you would need at least five. So why four?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-13T14:46:23.667" LastActivityDate="2022-02-20T06:16:29.957" Title="Why did the Apollo Lunar Module have four landing legs?" Tags="|apollo-program|landing|lunar-landing|landing-gear|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13502" PostTypeId="2" ParentId="13482" CreationDate="2016-01-13T15:06:54.210" Score="3" Body="&lt;h2&gt;Mass&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Mass is the costly thing to accelerate to orbital velocity, so it could easily be cheaper to put up a droid and some new parts to inhabit the existing shell. This is pretty much the hermit crab approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider the size of historic satellites; an iridium satellite is 689 kg of stuff - that is a lot of mass. A small satellite could make use of the basic shell of a lump of matter like that for protection. Historically, small satellites have been cheap to launch because they can be loaded into spare capacity on existing launches, whereas something like an iridium satellite would bear the main cost of a launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be likely that a lot of subsystems would still work; there might be some propulsion still available, gyros and so on which are vital for certain types of mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A particularly ruthless scavenger could simply cut up the existing satellite body and use it as a reaction mass; some solar panels, capacitors and some sputtering could provide a usable railgun and by alternating shots toward Earth and away it could propel itself without adding to the debris cloud.&lt;/p&gt;&#xA;" OwnerUserId="519" LastActivityDate="2016-01-13T15:06:54.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13503" PostTypeId="2" ParentId="13501" CreationDate="2016-01-13T15:28:27.727" Score="27" Body="&lt;p&gt;There is some interesting information &lt;a href=&quot;http://www.astronautix.com/craft/lmlggear.htm&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three legs were probably not robust enough, they do not appear to have been considered seriously for any length of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Five &lt;strong&gt;fixed&lt;/strong&gt; legs was the main option considered at first, however this wasn't thought to have a wide enough footprint for reliable landings. Eventually they selected the final design of four widely-splayed legs, which were stored folded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though the four leg design does not seem suited to an uneven surface (not all in the same plane), part of each leg was collapsible to absorb shock.  In fact Apollo 15 did land at a moderate angle:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The vehicle at-rest attitude, as determined from the gimbal angles, was 6.9 degrees pitch up and 8.6 degrees roll to the left, resulting in a vehicle tilt angle on the lunar surface of approximately 11 degrees from the horizontal&#xA;  (&lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/a15mr-7.htm&quot;&gt;Source&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-01-13T15:28:27.727" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13505" PostTypeId="2" ParentId="8509" CreationDate="2016-01-13T15:43:57.853" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://youtu.be/2qsIRnaCXy8?t=39&quot; rel=&quot;nofollow noreferrer&quot;&gt;HERE&lt;/a&gt; is an instructive video of the procedure if a fire breaks out on &lt;strong&gt;Soyuz&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The astronauts stay cool, at least in their minds, and use their sticks to push the right buttons. That's the plan. Reminds me of the time I was a conscript. But I think that the Russian have a tradition of being able to remotely control most of what happens on their spacecrafts from the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, first, after having closed their visors, they arrest the ventilation to stop feeding the fire with oxygen. Then they shut down the electrical supplies(!). Finally, if necessary, they decompress the spacecraft and then have to land within two hours which is how long their space suits can supply them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QiZ0Y.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QiZ0Y.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-01-13T15:43:57.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13506" PostTypeId="1" AcceptedAnswerId="13507" CreationDate="2016-01-13T16:39:40.660" Score="11" ViewCount="3243" Body="&lt;p&gt;Some launchers, like Falcon 9 recently as in a video clip linked to below, and I think some other launchers since long ago in the space race history, test their engines before launch by firing them while remaining strapped on to the launch pad. I mean firing the actual vehicle days or hours before a real launch, not any horizontal engine test under development. F9 weighs about 500 tons on the launch pad and the engines firing are powerful enough to go to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is it possible to hold something like that anchored to the ground without breaking it? Are such launchers especially sturdy and heavy, or is it maybe not as hard as it looks because the forces are similar to those a launcher encounters during a real launch anyway?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the main trade offs when deciding if a launcher should be able to do such static engine firing before each launch? How large is the penalty to the payload it can deliver to orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/CmnkZ_FSYts?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="8693" LastEditDate="2016-01-13T21:29:04.700" LastActivityDate="2016-01-13T21:29:04.700" Title="How does static test firing before launch affect design of rocket mass and structure?" Tags="|launch|spacex|testing|launchpad|first-stage|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13507" PostTypeId="2" ParentId="13506" CreationDate="2016-01-13T17:07:11.250" Score="9" Body="&lt;p&gt;Thrust to weight ration of boosters at takeoff needs to be greater than 1, else they do not go up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it is usually not greatly higher than 1.  Usually 1.2-1.8 perhaps. This is obvious in boosters that seem slow or fast off the pad. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the boost is gravity plus a bit but not that much. So the stage is able to handle gravity just to stand up. This is really not that much more.  It also needs to be able to handle the load on the way up as well.  (I.e. The failure in Falcon 9 CRS-7 mission was a strut that could not handle the load). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus it comes down to, where on the booster can you hold it, without damage to withstand the thrust. In some ways, gravity and some of the boost cancel out. It is only the difference that is the issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fully fueled, pressurized, the stages are pretty strong. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider as well you need a pad strong enough to not be ripped out of the ground by all that thrust either!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does need to be designed in from the get go, not the sort of thing you add after the booster is complete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME's on the Shuttle would actually ignite and reach full power while held down, before the SRB's fired. The famous 'twang' of the Shuttle was the SSME's hitting full power, and moving the entire stack before leaving the pad. (Once the SRB's fire, you go, or you die. There is no try).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-13T17:07:11.250" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13508" PostTypeId="2" ParentId="13506" CreationDate="2016-01-13T17:48:11.150" Score="5" Body="&lt;p&gt;STS did Flight Readiness Firings in the early days where the SSMEs would light up for a few seconds.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/QZ9iDqCni3s?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The stack was bolted to the Mobile Launcher by eight large bolts at the base of the SRBs.  Here's a picture of the nuts both before and after they were blown apart by pyros. (&lt;a href=&quot;http://www.rocketryforum.com/archive/index.php/t-57082.html&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember discussion about what would happen if the SRBs fired and the nuts didn't split.  There were 2 independent pyro systems to blow the nuts apart.  On a flight late in the program one of the systems failed to fire which led to a lot of analysis and Powerpoint.  I believe, but cannot find a reference, that the consensus was that the vehicle would still lift off but performance would be degraded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hEBaI.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hEBaI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-01-13T18:00:47.703" LastActivityDate="2016-01-13T18:00:47.703" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13509" PostTypeId="2" ParentId="13497" CreationDate="2016-01-13T19:46:33.200" Score="3" Body="&lt;p&gt;I checked the &lt;a href=&quot;https://www.youtube.com/watch?v=csVpa25iqH0&quot; rel=&quot;nofollow&quot;&gt;CRS-6 launch&lt;/a&gt; (which launched at daytime, so you can see more on the video).    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Around T+1m, the camera pans up and you can see vapor coming out of at least two ports at the top of the first stage. These ports are 90 degrees apart, so they aren't both umbilical connections. In the Jason-3 launch, you can see these plumes before liftoff (&lt;a href=&quot;https://www.youtube.com/watch?v=vkz_lclGXNg&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt; from 15:10 to 20:00). It looks like 2 plumes from the top of the second stage, and 2 from the top of the first stage. There is at least one &lt;a href=&quot;https://www.quora.com/Why-is-there-steam-coming-off-a-rocket-before-it-launches&quot; rel=&quot;nofollow&quot;&gt;LOX vent&lt;/a&gt; at the top of the first stage. Some redundancy would be logical, so 2 LOX vents is a plausible explanation as the source of the 2 plumes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vapor occurs during the entire sequence from T+0 to T+1m, I'd say that rules out ascent heating as the main driver for the vapor. It surprises me that LOX boiloff is rapid enough to require venting even while the tank is being drained at a gigantic rate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also some ice coming off the LOX tanks, you can see chunks coming off from launch to T+1 minute. But that doesn't seem to be the source of the vapor. &#xA;The gas generator exhausts are right next to the engine bells, so that's definitely not it. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-03-23T13:05:31.397" LastActivityDate="2016-03-23T13:05:31.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13510" PostTypeId="2" ParentId="13492" CreationDate="2016-01-13T20:11:45.293" Score="3" Body="&lt;p&gt;According to descriptions in the &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/stsover-prep.html&quot; rel=&quot;nofollow&quot;&gt;KSC's Mission Preparation and Prelaunch Operations&lt;/a&gt; and the &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/launch/launch-pad-mlp.html&quot; rel=&quot;nofollow&quot;&gt;Launch Pad/Mobile Launcher Platform Interfaces&lt;/a&gt; pages, the hydrogen burn-off system for the Shuttle was located on the tail service masts on the launch pad, suggesting that the system was built by the Kennedy Space Center. While the site doesn't go into further detail about who specifically designed the system, it was probably done by NASA themselves.&lt;/p&gt;&#xA;" OwnerUserId="13033" LastEditorUserId="6816" LastEditDate="2016-01-13T20:29:52.437" LastActivityDate="2016-01-13T20:29:52.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13511" PostTypeId="2" ParentId="13467" CreationDate="2016-01-13T20:31:47.243" Score="1" Body="&lt;p&gt;Above 100km altitude, there are few enough atmospheric particles that they can largely travel without collision. Because of this, chemical reactions mostly cease and become mostly irrelevant; without particle collisions, no reactions take place. In this region, called the Heterosphere, different species are spread into layers based on their atomic mass. &lt;a href=&quot;http://history.nasa.gov/NP-119/p71.htm&quot; rel=&quot;nofollow&quot;&gt;This chart&lt;/a&gt; shows the general layers in which each species of particle inhabits. To answer the second part of the question, the particles in this region are so far apart that they likely have very little impact on spacecraft passing through them, as there simply aren't enough particles to have a noticeable effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/NP-119/ch6.htm#n71&quot; rel=&quot;nofollow&quot;&gt;http://history.nasa.gov/NP-119/ch6.htm#n71&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.worldlibrary.org/articles/heterosphere&quot; rel=&quot;nofollow&quot;&gt;http://www.worldlibrary.org/articles/heterosphere&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13033" LastActivityDate="2016-01-13T20:31:47.243" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13513" PostTypeId="5" CreationDate="2016-01-13T21:19:51.803" Score="0" Body="&lt;p&gt;Usually, the trajectory of a spacecraft is modelled as if it is orbiting just one body at a time, using the patched-conics approximation based upon the well known two-body equations. This is however not strictly true, and the errors might be noticeable at the edge of a planets sphere of influence, or during a fly by manoeuvre. Furthermore, the concept of Lagrangian points does not exist in the two-body model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simplifications often made in the three-body include:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass of third body is negligible&lt;br&gt;&#xA;The two primaries orbit in a circular barycentric orbit&lt;br&gt;&#xA;Orbits are co-planar&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is known as the circularly restricted three body problem (CRTBP or CR3BP)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-13T21:23:57.393" LastActivityDate="2016-01-13T21:23:57.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13514" PostTypeId="4" CreationDate="2016-01-13T21:19:51.803" Score="0" Body="Questions regarding the relative motion of three masses." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-13T21:23:00.090" LastActivityDate="2016-01-13T21:23:00.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13515" PostTypeId="5" CreationDate="2016-01-13T22:23:32.167" Score="0" Body="&lt;p&gt;Mass is the unit for an objects resistance to force (inertia) and gravitational influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space exploration, mass is often the limiting factor of a spacecraft, due to the high cost of launching payloads to orbit.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-13T22:25:58.480" LastActivityDate="2016-01-13T22:25:58.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13516" PostTypeId="4" CreationDate="2016-01-13T22:23:32.167" Score="0" Body="Questions regarding the mass of spacecraft or other objects" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-13T22:25:50.340" LastActivityDate="2016-01-13T22:25:50.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13518" PostTypeId="1" AcceptedAnswerId="13522" CreationDate="2016-01-13T22:51:55.903" Score="2" ViewCount="3129" Body="&lt;p&gt;Alright, I know that &quot;Sol&quot; is used on Mars to count days. But is the sun called: &quot;Sol&quot;? And can you use that term on Earth as well?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for the question is that the term &quot;sol&quot; is used in two ways, but never clarified. Because it also means the sun, and the sun is what time (day/night) is based on on Earth as well as Mars. So why is Sol only used to count time on mars while it is also what time is based on on Earth? And can you use Sol to count time on Earth? &lt;/p&gt;&#xA;" OwnerUserId="13036" LastEditorUserId="13036" LastEditDate="2016-01-27T03:49:07.020" LastActivityDate="2016-01-27T09:10:09.230" Title="Where can the term &quot;Sol&quot; used?" Tags="|mars|terminology|earth|time|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13519" PostTypeId="2" ParentId="7718" CreationDate="2016-01-14T03:04:25.033" Score="16" Body="&lt;p&gt;I thought I had heard Musk say that a rocket landing was for another very good reason, over and above those already listed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Practice/development for landing large mass on the moon, Mars, asteroids, etc.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No atmosphere (or little) means parachutes are simply not an option, especially as the mass being delivered goes up. And the mass will go up a lot with anything past the current slew of 'sensor drones'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I have not heard it, I still reckon it is probably true. A vendor with a proven rocket-landing solution is in the box seat for the next generation of space cargo. As if Musk doesn't know that...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Said another way, this is a case of 'design re-usability'. Get this design working well on Earth, and it can be taken to other off-Earth destinations with  relatively minor adjustments (Mostly scale). - Minor compared with totally changing the landing paradigm.&lt;/p&gt;&#xA;" OwnerUserId="13037" LastEditorUserId="13037" LastEditDate="2016-01-14T04:09:43.190" LastActivityDate="2016-01-14T04:09:43.190" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13520" PostTypeId="1" CreationDate="2016-01-14T03:23:53.607" Score="3" ViewCount="727" Body="&lt;p&gt;I've looked for better explanation, and all I can get is:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Stage separation occurred as planned, but because residual fuel in the new Merlin 1C engine evaporated and provided transient thrust, the first stage recontacted the second stage, preventing successful completion of the mission&lt;/p&gt;&#xA;&lt;p&gt;- &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Falcon_1_launches#Third_flight&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/List_of_Falcon_1_launches#Third_flight&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;However, why couldn't the second stage still get to orbit?&lt;/p&gt;&#xA;&lt;p&gt;Did the 'recontact':&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Damage the second stage so much that it failed to light or was unsafe to light?&lt;/li&gt;&#xA;&lt;li&gt;Knock its attitude so far that it couldn't make corrections? What would prevent the second stage reorient itself?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6821" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-08T15:33:12.083" Title="Why couldn't the Falcon 1 flight 3 launch make it to orbit?" Tags="|launch|spacex|failure|falcon-1|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13521" PostTypeId="2" ParentId="13520" CreationDate="2016-01-14T04:00:53.573" Score="7" Body="&lt;p&gt;According to Elon Musk, the problem was in a &lt;em&gt;recontact between the stages&lt;/em&gt;. From a SpaceRef's &lt;em&gt;&lt;a href=&quot;http://www.spaceref.com/news/viewnews.html?id=1302&quot;&gt;SpaceX Determines Cause of Falcon 1 Launch Failure&lt;/a&gt;&lt;/em&gt; article after the media teleconference, Elon Musk was quoted:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;We have a definitive understanding of what went wrong on Flight 3. The problem was due to a design error not a production or quality&#xA;  assurance issue. The thrust transient was longer than it was for the&#xA;  prior flight. The previous flight had an ablatively cooled engine.&#xA;  Flight 3 had a regeneratively cooled engine. The gap between engine&#xA;  cut off and staging was 1.5 seconds - which was fine for the&#xA;  ablatively cooled engine on Flight 2. But on Flight 3, with the&#xA;  regeneratively cooled engine, there was some residual thrust after&#xA;  engine shut down and this caused the first stage to be pushed back&#xA;  toward the second stage after separation and there was a recontact&#xA;  between the stages.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note that Musk talks about a recontact, not a bump. I.e. the separation failed with the inverse of it, a recontact of the two stages. Here's a video of the relevant part of the SpaceX Falcon 1 launch 3 from the Omelek Island, the Kwajalein Atoll:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Qz0yJ8N3cA0?start=168&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last couple of frames just before the video jumps to fairing separation show upper stage engine ignition while the first stage was again in contact with it, catching up with up till that point inertial upper stage due to the mentioned residual burn. Since the upper stage was not designed for such engine ignition (some launch vehicles do exactly that and while lower stage still burns so the upper stage doesn't require ullage motors, but no US ones as far as I'm aware, and those that are use an open mesh interstage &lt;em&gt;skirt&lt;/em&gt; at sufficient separation from the lower stage to allow exhaust gas expansion), this could have only resulted in two scenarios: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Recontact wasn't symmetric and exhaust of the upper stage engine ignition with lateral thrust vector forced the upper stage into a violent spin, or&lt;/li&gt;&#xA;&lt;li&gt;upper stage exhaust reflecting upwards into itself and creating a pressure spike, resulting in a &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hard_start&quot;&gt;hard start&lt;/a&gt;&lt;/em&gt;, followed by &lt;em&gt;rapid unscheduled disassembly&lt;/em&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Final few seconds of the video during fairing separation suggest the first scenario was the final demise of the upper stage, from which the stage apparently couldn't recover and likely eventually disintegrated as it reentered denser lower altitude atmosphere. There's only that much of translational control that gimballed main engine and RCS thrusters can provide to the stage, which is at that point far below orbital speed, and soon starts falling back towards the surface.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-01-14T04:42:42.893" LastActivityDate="2016-01-14T04:42:42.893" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13522" PostTypeId="2" ParentId="13518" CreationDate="2016-01-14T07:33:41.127" Score="12" Body="&lt;p&gt;A &quot;sol&quot; is only commonly used for a Martian day, and it seems pretty redundant to use this term for Earth, as it is already covered by the concept 'day'. Also, it may induce ambiguity, as people think you are referring to a Martian day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the other planets in the inner solar system, Mercury and Venus, they have a so long day that their orbital period may be a more useful measure of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.thefreedictionary.com/sol&quot; rel=&quot;nofollow&quot;&gt;Sol&lt;/a&gt; is simply another English word for 'the Sun', and it is also the main word for it in several other languages, including my own (Norwegian).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some languages use the same word for 'sun' and 'day', including &lt;a href=&quot;https://en.wikipedia.org/wiki/M%C4%81ori_language&quot; rel=&quot;nofollow&quot;&gt;Maori&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Hungarian_language&quot; rel=&quot;nofollow&quot;&gt;Hungarian&lt;/a&gt; and my second language, &lt;a href=&quot;https://en.wikipedia.org/wiki/Southern_Sami_language&quot; rel=&quot;nofollow&quot;&gt;Southern Sami&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-27T08:22:15.560" LastActivityDate="2016-01-27T08:22:15.560" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13523" PostTypeId="1" AcceptedAnswerId="13671" CreationDate="2016-01-14T09:05:12.820" Score="10" ViewCount="4651" Body="&lt;p&gt;I recently started using &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt; (python) to find (x, y, z) coordinates of things in space. Now I'd like to transform between Earth-Centered, Earth-Fixed (rotating) and Earth-Centered Inertial (non-rotating) coordinates, since &quot;rocket speeds&quot; may sometimes be webcasted in earth-fixed (rotating) frames . See &lt;a href=&quot;https://stackoverflow.com/a/34532049&quot;&gt;this answer&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/13244&quot;&gt;this answer&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what I have so far. The barycentric positions are straight from Skyfield, but I'm just subtracting the earth position from the moon's position to get earth-centered inertial position - I think this is OK for non-precision results, I don't know if it is really that easy or if there is something subtle I'm missing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I'm wondering if there is an &quot;official&quot; Earth-fixed (rotating) frame, and if Skyfield has a method to convert to it, or to any rotating frame in general?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Python:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from skyfield.api import load, JulianDate&#xA;&#xA;de421 = load('de421.bsp')&#xA;&#xA;earth = de421['earth']&#xA;moon  = de421['moon']&#xA;slc40 = earth.topos('28.562 N', '80.577 W')&#xA;&#xA;jd = JulianDate(utc=(2015,12, 22, 1, 48, 0))&#xA;&#xA;epos     = earth.at(jd).position.km # barycentric (ICRS) position of earth&#xA;mpos     = moon.at(jd).position.km  # barycentric (ICRS) position of moon&#xA;slc40pos = slc40.at(jd).position.km # barycentric (ICRS) position of SLC-40&#xA;&#xA;mpos_ec     = mpos     - epos # earth centered (inertial) position of moon&#xA;slc40pos_ec = slc40pos - epos # earth centered (inertial) position of SLC-40&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-05-23T12:37:21.217" LastActivityDate="2022-01-18T02:21:56.227" Title="How to get earth-centered, earth-fixed coordinates from Skyfield?" Tags="|orbital-mechanics|artificial-satellite|low-earth-orbit|python|skyfield|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13528" PostTypeId="1" CreationDate="2016-01-14T19:18:32.537" Score="5" ViewCount="423" Body="&lt;p&gt;After the successful ORBCOMM OG2 mission on a Falcon 9 v1.1 Full Thrust (aka 1.2, aka 'just' Falcon 9) and the recovery of its first stage at LZ-1, SpaceX indicated it survived well, and trucked it to the HIF (Horizontal Integration Facility) at LC-40. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX initially said they intended to hot fire the stage to demonstrate the engines were still good, even after a full flight including RTLS, at LC-39A. This had the added benefit of not affecting flight operations at LC-40, and would allow some dry runs, with no real time constraints for LC-39A.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But then, suddenly, images of the stage on a truck, driving to LC-40 arrived (Imagine the F-9 stage 1 as Slim Pickens and the truck as the nuclear bomb) on the web. Then we found out they were going to hot fire it at LC-40, and sneak it in before the SES-9 launch end of January.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the move, we started to hear that the SES-9 flight was delayed for undisclosed reasons. But regardless, why the switch from LC-39A to LC-40 for the hot fire?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Possible theories abound:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;LC-39A ground support equipment (GSE) not ready&lt;/li&gt;&#xA;&lt;li&gt;LC-39A supply of LOX/RP1 not ready, or not on hand in time.&lt;/li&gt;&#xA;&lt;li&gt;Other LC-39A issues.&lt;/li&gt;&#xA;&lt;li&gt;With delay of SES-9 known earlier to SpaceX, LC-40 was free and would provide another dry run of the GSE for sub-cooled LOX and RP1 adding to the confidence of an on-time launch for SES-9&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2016-01-14T19:42:21.883" LastActivityDate="2016-01-14T21:26:22.370" Title="When SpaceX hot fired the previously used ORBCOMM OG2 first stage, why did they suddenly switch from LC-39A to LC-40?" Tags="|spacex|falcon-9|reuse|testing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13529" PostTypeId="2" ParentId="13467" CreationDate="2016-01-14T19:36:44.143" Score="3" Body="&lt;p&gt;Here I found a document that details the composition of the upper atmosphere:&#xA;&lt;a href=&quot;http://www.springer.com/cda/content/document/cda_downloaddocument/9783642251283-c2.pdf?SGWID=0-0-45-1269313-p174259871&quot; rel=&quot;nofollow&quot;&gt;Empirical Modelling of the Thermosphere&lt;/a&gt;.&#xA;In 300 km height, the main constituent of air is atomic oxygen at a density of about 10^14 atoms per cubic meter. There is some percentage of nitrogen, but all other elements seem to be negligible.&#xA;There might be some more aggressive gases being released from the materials of the satellite itself - which is one of the reasons every component needs to be tested before it is allowed to be used in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To elaborate a bit on the influence of oxygen:&#xA;As a satellite travels at about 7800 m/s, each square meter of its surface cuts through 7800 m^3 of air each second. This amounts 25 µg/s or 2 g/day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't give details on the influence on the material of the satellite though. But &lt;a href=&quot;http://www.nasa.gov/topics/technology/features/atomic_oxygen.html&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt; reports on the effect on the shuttle surfaces that &quot;looked frosty because they were actually being eroded and textured&quot;.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-01-14T19:36:44.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13530" PostTypeId="2" ParentId="13528" CreationDate="2016-01-14T21:26:22.370" Score="6" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/spacex/2016/01/13/spacex-poised-fire-landed-falcon-rockets-engines/78731184/&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;, LC-39a isn't ready for testing at this time. SpaceX wanted to also do an integration test at that site using the new rocket, but were unable to do so, and they really want to see it fire sooner than later. With the delay of SES-9, there is a sufficient time window to test at LC-40, and it seems they wanted to take advantage of that time window.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-14T21:26:22.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13531" PostTypeId="5" CreationDate="2016-01-14T21:30:42.000" Score="0" Body="&lt;p&gt;A fly-back booster returns to the launch site after separation from the rocket, allowing recovery and possible reuse. A fly-back booster must include a propulsion system to reverse its trajectory, and for a powered landing. A fly-back first stage is an important part of the spaceX concept for commercial launch.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-14T21:55:53.437" LastActivityDate="2016-01-14T21:55:53.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13532" PostTypeId="4" CreationDate="2016-01-14T21:30:42.000" Score="0" Body="Questions regarding boosters returning to the launch site after separation" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-14T21:55:23.483" LastActivityDate="2016-01-14T21:55:23.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13533" PostTypeId="2" ParentId="3260" CreationDate="2016-01-14T22:24:49.473" Score="6" Body="&lt;p&gt;The first part of the ascent profile is nearly vertical so as to get out of the thick atmosphere as soon as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When accelerating vertically, the ship suffers gravity loss:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UNnuZ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UNnuZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every 102 seconds spent in vertical ascent gives you a gravity loss hit of 1 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's desirable to reduce time of vertical ascent so as to minimize gravity loss. How fast a ship gets above the atmosphere is related to the thrust to weight ratio, a.k.a. T/W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the rocket's thrust exactly equals gravity, 9.8 meters/sec^2, T/W=1 and the rocket just hovers. A big T/W helps the ship ascend faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, better ISP means less thrust. Ion engines have fantastic ISP but miniscule thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it's not really a question of the upper stage needing more ISP. We want good ISP for all the stages. But the booster stage needs more thrust.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-01-15T04:22:01.447" LastActivityDate="2016-01-15T04:22:01.447" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13534" PostTypeId="2" ParentId="13501" CreationDate="2016-01-15T00:51:48.233" Score="32" Body="&lt;p&gt;Three legs offers less safety margin for steep slopes and sideways landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a three-legged lander touches down on a steep slope such that the center of gravity of the ship is outside the triangle formed by the contact points, it will tip over. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The four-legged lander has a substantially larger area inside the quadrangle formed by the contact points; as long as the CG stays above that area, the ship is stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Projected to the ground, the shortest horizontal distance the CG could tip and become unsupported:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;for the three legged lander is half the landing gear radius; &lt;/li&gt;&#xA;&lt;li&gt;for the four-legged, it's $ \frac { \sqrt 2 }{ 2 } = 0.7071 $ of the radius. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hEbMk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hEbMk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the time of landing, with the tanks in the descent stage empty, and the ascent stage full, the LM's CG is fairly high, and the &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=32246.20&quot; rel=&quot;noreferrer&quot;&gt;absolute stability limit is about 43º&lt;/a&gt;. A three-legged lander with the same gear-radius-to-CG-altitude ratio could tip over at 29º. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously the LM was never intended to land on that steep a slope (I think 15º was the design limit, which A15 got close to). The larger concern was stability in cases where the lander touched down with a substantial horizontal velocity component. In such cases, the fourth leg buys a lot of stability margin.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-08-18T03:37:56.550" LastActivityDate="2016-08-18T03:37:56.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13535" PostTypeId="1" CreationDate="2016-01-15T01:19:08.520" Score="4" ViewCount="440" Body="&lt;p&gt;In some ways, this is a question in general about docking port compatability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians have two types of ports, and the Americans are standardizing on the NDS (NASA Docking System, sometimes called LIDS - Low Impact Docking System) via the IDA (International Docking Adapter) that is being attached to the end of the PMA (Pressurized Mating Adapter).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are three PMA's on the ISS.  PMA-2 and PMA-3 will get the IDA for Dragon V2 and CST-100 dockings (The first IDA was lost in the SpaceX CRS-7 failure, IDA2 is launching on SpaceX CRS-9 mission). But PMA-1 is the interesting one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A PMA is designed to reduce the diameter of the CBM (Common Berthing Method) large diameter port for attaching modules to the US segment to a smaller diameter docking port that the Shuttle could use. But PMA-1 actually docks one end to the CBM on Node-1 (Unity) and to the Russian module Zarya.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That implies that the Russian docking system can dock with the end of a PMA. The IDA is meant to stay mostly compatible with the PMA style connector, just allow for softer dockings and some other upgrades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the question, can a CST-100 or Dragon V2 dock to a Russian docking port? Active or passive. Secondarily, the port on a Soyuz, are they still androgynous, so could two Soyuz'es dock nose to nose?  If so, could the US crew vehicles dock to a Soyuz?  Or each other?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-15T02:34:37.757" Title="Can the US Manned vehicles (CST-100, Dragon V2) dock to a Russian vehicle?" Tags="|soyuz-spacecraft|docking|dragon-v2|cst-100|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13536" PostTypeId="2" ParentId="13535" CreationDate="2016-01-15T01:35:34.090" Score="5" Body="&lt;p&gt;No, two incompatible systems are used on the ISS. (For docking...many other incompatible systems are used as well). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The FGB could dock to the PMA because the FGB has an APAS (Androgynous Peripheral Attachment System)** on one end of it.  The other Russian docking ports don't have this system, they use a probe and drogue mechanism. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;** The shuttles used an APAS as well.  Ironically, it was a Russian system.  So, while you are correct that &quot;the Russian docking system can dock with the end of a PMA&quot; it isn't the same Russian docking system that is used on the Russian side of the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Androgynous_Peripheral_Attach_System#/media/File:STS-76_docking_with_MIR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;APAS system&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UFaUV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UFaUV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://thumbs.media.smithsonianmag.com//filer/FM-2011-how-things-work-2-FLASH.jpg__800x600_q85_crop.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Probe and drogue system&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/byJNy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/byJNy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a nice picture of the European ATV (which docked to the Russian segment of the ISS) showing the docking probe extended.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7NBTC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7NBTC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, here's a picture from STS-88 showing the FGB just before attachment to the PMA.  You can see that it has the APAS style mechanism on the near end.  &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/d/d7/S99_03776.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;  If you view the image full size you can clearly see the APAS petals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iIajC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iIajC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-01-15T02:34:37.757" LastActivityDate="2016-01-15T02:34:37.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13537" PostTypeId="1" CreationDate="2016-01-15T02:44:46.680" Score="31" ViewCount="5493" Body="&lt;p&gt;Each of the lunar Apollo missions (Apollo 8-17) entered Earth orbit immediately after launch. Each mission then left Earth after a few orbits by burning the S-IVB engine, and headed off to the Moon. (&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program#Lunar_mission_profile&quot;&gt;https://en.wikipedia.org/wiki/Apollo_program#Lunar_mission_profile&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wikpedia page says orbiting Earth was necessary &quot;to verify readiness of spacecraft systems&quot;, but I'm curious how this affected fuel requirements and other aspects of navigation. Would heading toward the Moon directly from the launch pad have required more or less fuel? If orbiting earth first is less efficient, how much extra fuel does it require over leaving directly?&lt;/p&gt;&#xA;" OwnerUserId="13043" LastEditorUserId="2752" LastEditDate="2016-01-15T13:49:35.877" LastActivityDate="2016-01-16T14:31:40.137" Title="Orbiting Earth before heading to Moon" Tags="|orbital-mechanics|apollo-program|astrodynamics|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13538" PostTypeId="2" ParentId="13537" CreationDate="2016-01-15T03:32:43.247" Score="24" Body="&lt;p&gt;Going from Earth's surface direct to Moon orbit without stopping in LEO would offer negligible savings -- depending on your assumptions, maybe 20 m/s worth of ∆v (and 2-3 hours worth of consumables).&lt;sup&gt;1&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://georgetyson.com/files/apollostatistics.pdf&quot; rel=&quot;noreferrer&quot;&gt;Apollo By The Numbers&lt;/a&gt; and &lt;a href=&quot;http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;noreferrer&quot;&gt;Bob Braeunig's simulations&lt;/a&gt;, the ∆v budget for launch to orbit plus trans-lunar injection for the Apollo missions totalled about 12250 m/s, so any difference would be far less than 1% of the fuel budget. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Assuming ascent to circular orbit at the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;noreferrer&quot;&gt;Kármán line&lt;/a&gt; in either case, then in the first case doing two &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Calculation&quot; rel=&quot;noreferrer&quot;&gt;Hohmann transfers&lt;/a&gt;, from Kármán line to 160km parking orbit then from parking orbit to lunar altitude; and in the second case a single Hohmann transfer from Kármán line to lunar altitude. The Apollo missions did a faster trajectory than Hohmann (saving 2 days travel time each way) but the difference between the parking orbit and non-stop versions is probably very similar. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-01-15T19:06:56.943" LastActivityDate="2016-01-15T19:06:56.943" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="13539" PostTypeId="1" AcceptedAnswerId="13569" CreationDate="2016-01-15T10:27:34.143" Score="20" ViewCount="19399" Body="&lt;p&gt;I've seen some of the articles mention that ISS switched from Windows to Linux. But that's just considering the laptops. What OS are the other computers present on ISS running? I would think that e.g. life support isn't running on a laptop and isn't using an off-the-shelf operating system.&lt;/p&gt;&#xA;" OwnerUserId="13045" LastActivityDate="2016-02-04T20:03:47.477" Title="Which operating systems is the International Space Station running?" Tags="|iss|flight-computer|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13541" PostTypeId="2" ParentId="13537" CreationDate="2016-01-15T13:44:23.013" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;The wikpedia page says orbiting Earth was necessary &quot;to verify readiness of spacecraft systems&quot;, but I'm curious how this affected fuel requirements and other aspects of navigation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The use of a parking orbit most likely &lt;strong&gt;saved&lt;/strong&gt; fuel compared to a direct translunar insertion. A direct insertion into a translunar trajectory would have saved a tiny amount of fuel compared to that need by adding a parking orbit if everything went perfectly. However, nothing ever works perfectly. Rocket thrust varies, and navigation sensors are imperfect. Launch in the Apollo era was largely a &lt;a href=&quot;https://en.wikipedia.org/wiki/Dead_reckoning&quot;&gt;dead reckoning&lt;/a&gt; process; there was no such thing as GPS in the Apollo era.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This meant that navigation errors built up during launch. A launch directly into a translunar trajectory would have meant correcting for those errors and the injection error after the launch+injection. This would have more than offset the tiny extra cost of placing the vehicle into a parking orbit before undergoing the translunar injection. With a parking orbit, most of the launch errors were corrected for by the translunar injection burn. Correction burns were still needed, but these were much smaller than those that would have been needed for a direct launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Much more importantly, the use of a parking orbit made the missions possible. A direct translunar insertion launch would have required an instantaneous launch window. The launch would have had to have been delayed by a day (or perhaps by several months) if something went wrong during the countdown in the case of a direct insertion into a translunar trajectory. Mission planners deemed that a 2.5 hour launch window was the minimum needed to have a reasonable chance of success. This alone ruled out the possibility of a launch directly into a translunar trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something did indeed go wrong during the countdown in two of the Apollo missions. Apollo 14 launched 40 minutes late due to weather problems, and Apollo 17 launched 2 hours and 40 minutes late due to an automatic cutoff at the T-30 second mark.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;That NASA used the parking orbit as a means for verifying the readiness of spacecraft systems for the continuation of the mission was an added benefit of using a parking orbit. This was not the primary driver. The primary driver was that the use of a parking orbit made the missions feasible.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-01-15T13:44:23.013" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13542" PostTypeId="2" ParentId="13539" CreationDate="2016-01-15T16:04:29.463" Score="14" Body="&lt;p&gt;In short, I am pretty sure that no Windows systems are being used anymore. &#xA;All/most laptops used by the astronauts have a Linux based operating systems (Debian). The Command and Data Handling (CDH) system, controls all major functions of the US segment, which contains  Multiplexer/DeMultiplexers that mainly routes the data to and from other systems in the ISS. It is however not clear to me if these systems do have an operating system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&lt;sub&gt;20 Jan 2016: Changed based on further research, and David Hammen's comments.&lt;/sub&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In both &lt;a href=&quot;http://amedleyofpotpourri.blogspot.com.es/2015/02/international-space-station.html&quot; rel=&quot;noreferrer&quot;&gt;this blog article&lt;/a&gt; and this &lt;a href=&quot;https://www.quora.com/Does-ISS-use-Linux-or-Microsoft-Windows-for-its-computers&quot; rel=&quot;noreferrer&quot;&gt;quora question&lt;/a&gt; they mention that the telemetry, commanding, and key station functions are controlled by systems with the Linux operating system, but no clear reference is given. &lt;a href=&quot;http://www.telegraph.co.uk/technology/news/10049444/International-Space-Station-to-boldly-go-with-Linux-over-Windows.html&quot; rel=&quot;noreferrer&quot;&gt;Telegraph&lt;/a&gt; mentions that: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Dozens of laptops on the ISS's 'opsLAN' network - which provides the&#xA;   ship's crew with vital capabilities for day-to-day operations, from&#xA;   telling the astronauts where they are to interfacing with onboard&#xA;   cameras - will be switched, removing Windows entirely from the ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But Linux was already present on most systems:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Linux is already used to run various systems aboard the ISS, including the world's first 'Robonaut', sent to the Space Station in 2011. 'R2' can be manipulated by astronauts as well as ground controllers and is designed to carry out tasks &quot;too dangerous or mundane&quot; for astronauts in microgravity, according to the Linux Foundation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Furthermore &lt;a href=&quot;https://www.quora.com/Does-ISS-use-Linux-or-Microsoft-Windows-for-its-computers&quot; rel=&quot;noreferrer&quot;&gt;one quora answer&lt;/a&gt; refers to &lt;a href=&quot;http://www.militaryaerospace.com/articles/print/volume-16/issue-9/product-applications/software/space-shuttle-uses-qnx-software-for-camera.html&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; which mentions that &lt;a href=&quot;http://www.qnx.com/products/neutrino-rtos/neutrino-rtos.html&quot; rel=&quot;noreferrer&quot;&gt;QNX RTOS&lt;/a&gt;, a micro-kernel real-time OS, was used on Space Shuttle missions to guide the &lt;a href=&quot;https://en.wikipedia.org/wiki/Canadarm&quot; rel=&quot;noreferrer&quot;&gt;Canadarm&lt;/a&gt; and on the International Space Station. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/International_Space_Station/DMS-R_ESA_s_Data_Management_System&quot; rel=&quot;noreferrer&quot;&gt;ESA's Data Management System: DMS-R&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;is a set of computers, that provides for the overall control, mission and failure management of the entire Russian segment of the International Space Station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However it is not clear to me which Operating System they are using.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An &lt;a href=&quot;http://er.jsc.nasa.gov/seh/td9702.pdf&quot; rel=&quot;noreferrer&quot;&gt;outdated NASA document of 1998&lt;/a&gt; shows in Table 2-2 (p.40) some used computer systems as &lt;em&gt;Crew Interface&lt;/em&gt;, but only two are listed with operating system:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Portable Computer System (PCS):&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Purpose:&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Execute Station mode changes&lt;/li&gt;&#xA;  &lt;li&gt;Manage Station C&amp;amp;W&lt;/li&gt;&#xA;  &lt;li&gt;Command and Control (C&amp;amp;C) U.S. systems&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;li&gt;Hardware: IBM Thinkpad 760XD laptop at 8A&lt;/li&gt;&#xA;  &lt;li&gt;OS: Solaris UNIX operating system&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;li&gt;Station Support Computer (SSC)&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Purpose:&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;View U.S. and multisegment electronic procedures&lt;/li&gt;&#xA;  &lt;li&gt;Use inventory management system&lt;/li&gt;&#xA;  &lt;li&gt;View and edit onboard short term plan&lt;/li&gt;&#xA;  &lt;li&gt;Provide standard office automation tools and other crew support software&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;li&gt;Hardware: IBM Thinkpad 760XD laptop at 8A&lt;/li&gt;&#xA;  &lt;li&gt;OS: Windows 95 operating system&lt;/li&gt;&#xA;  &lt;li&gt;Additional Thinkpad will serve as file server for RF Local Area Network that allows SSCs to communicate to server&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note:&lt;/em&gt; PCS and SSC are both laptops, not the flight computers as commented by Organic Marble.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; As commented by David Hammen, the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;International Space Station&lt;/a&gt; was designed mid to end 1990s. &lt;a href=&quot;https://en.wikipedia.org/wiki/History_of_Linux&quot; rel=&quot;noreferrer&quot;&gt;Linux&lt;/a&gt; was also in development these days, starting with Linus' announcement in 1991, and Linux version 1.0 was launched in 1994. &lt;a href=&quot;https://en.wikipedia.org/wiki/RTLinux&quot; rel=&quot;noreferrer&quot;&gt;Real Time Linux&lt;/a&gt; was presented in 1997. This makes it improbable that the systems run, or at least ran, Linux or Real-Time Linux.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/sites/default/files/atoms/files/np-2015-05-022-jsc-iss-guide-2015-update-111015-508c.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA's ISS guide update 2015&lt;/a&gt; mentions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The system for storing and transferring information essential to operating the ISS&#xA;  has been functioning at all stages of assembly and provides control from various&#xA;  segments of the ISS. The Enhanced Processor and Integrated Communications&#xA;  upgrade in some of the Multiplexer/Demultiplexers (MDMs) has vastly improved&#xA;  the processing and memory margins; in addition to adding a new Ethernet&#xA;  interface. The Portable Computer System laptops provide the crew interface for&#xA;  commanding and monitoring the ISS Core Systems hardware and associated&#xA;  software.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Quoting part of &lt;a href=&quot;https://www.quora.com/What-are-computers-used-for-on-the-ISS&quot; rel=&quot;noreferrer&quot;&gt;Robert Frost's answer on Quora&lt;/a&gt; about the ISS' computers:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are two, or six, main computers, depending on how you want to think about it.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The US segment has a computer called the C&amp;amp;C MDM (Command and Control Multiplexer DeMultiplexer).  Well, it's actually three nearly identical computers.  They all run at the same time, and at any one time, one of them will be considered the primary, another the backup, and the third a standby.  There are an additional forty or so lower level MDMs responsible for systems or hardware.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Russian segment has a computer called the ЦВМ (Центральная вычислительная машина) (central computing machine).  Well, it's actually three nearly identical computers, too.  They operate a little differently, though.  They operate simultaneously and tasks may be assigned to the different lanes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.nasaspaceflight.com/2014/04/imminent-spacewalks-external-mdm-failure-iss/&quot; rel=&quot;noreferrer&quot;&gt;this nasaspaceflight.com article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A Multiplexer/Demultiplexer (MDM) is essentially an electronics unit that sends and receives multiple streams of data, and thus all incoming and outgoing data to and from the ISS is routed to its correct destination via an MDM.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The ISS MDMs are part of the Command and Data Handling (CDH) system, which controls all major functions of the US segment of the ISS, including power generation and distribution, attitude control, environmental control, communications systems, and monitoring of scientific payloads. &lt;strong&gt;The MDMs contain all the software needed to control these systems&lt;/strong&gt;, hence why they can essentially be thought of as computers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit 2:&lt;/strong&gt; &#xA;Two interesting videos:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=sx6k0sLjjPE&quot; rel=&quot;noreferrer&quot;&gt;ISS Update: Computer Upgrade on Station&lt;/a&gt; about the computer upgrade of the computer systems in May 2013. It was a circuit board upgrade to improve processing performance (memory, CPUs, etc.).&#xA;They upgraded 7 of the 47 MDMs which control the space station.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=Xlck7GsHNf8&quot; rel=&quot;noreferrer&quot;&gt;NASA ISS Stream Computer Reboot&lt;/a&gt;: this is the computer that streams the cameras, and seems to be running &lt;a href=&quot;https://en.wikipedia.org/wiki/VxWorks&quot; rel=&quot;noreferrer&quot;&gt;VxWorks&lt;/a&gt; from &lt;a href=&quot;http://www.windriver.com/products/vxworks/&quot; rel=&quot;noreferrer&quot;&gt;Wind River Systems&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11962" LastEditorUserId="11962" LastEditDate="2016-02-04T20:03:47.477" LastActivityDate="2016-02-04T20:03:47.477" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13544" PostTypeId="1" AcceptedAnswerId="13549" CreationDate="2016-01-15T18:32:52.660" Score="5" ViewCount="243" Body="&lt;p&gt;Browsing &lt;a href=&quot;http://www.asterank.com/&quot;&gt;Asterank&lt;/a&gt;, it seems that people believe many asteroids have large amounts of iron or magnesium silicate.  These compounds may come in a hydrated form which contains water, but it is chemically fixed into the solids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the composition of the NEA named Anteros is listed as magnesium silicate, aluminum, iron silicate. Magnesium Silicate is $Mg_3Si_4O_{10}(OH)_2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it feasible to extract the hydrogen and oxygen for a return trip?  Assume sufficient, reliable, automation is in place to perform this complicated task.  With energy provided by solar panels, and enough time, could fuel be created?  How much time and energy would be required to produce a unit (e.g. kg) of fuel?&lt;/p&gt;&#xA;" OwnerUserId="11479" LastEditorUserId="11479" LastEditDate="2016-01-15T21:02:47.480" LastActivityDate="2016-01-16T05:59:38.463" Title="How much energy does it take to extract water from silicate?" Tags="|fuel|asteroid|chemistry|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13546" PostTypeId="2" ParentId="13537" CreationDate="2016-01-15T23:56:34.167" Score="6" Body="&lt;p&gt;Achieving orbital velocity on earth's surface is not practical due to earth's atmosphere. First a ship must get above the atmosphere and then achieve orbital velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once altitude is gained, the most efficient way to achieve orbital velocity is by a horizontal burn. You could do the major burn along a non zero flight path angle but then the vertical component of the thrust suffers gravity loss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UNnuZ.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UNnuZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typically, the first part of a space ship's trajectory is nearly vertical but then leans to the east to give the thrust vector a bigger horizontal component as the atmosphere gets thinner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the ship's above the atmosphere it does the major burn and is traveling at a near zero flight path angle (In other words, horizontally). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a direct insertion to lunar orbit would be doing burn to achieve 10.9 km/s horizontal velocity when above the atmosphere. But at sometime during this burn, the ship will be traveling at a horizontal velocity of 7.8 km/s. At this point I would say the ship is in orbit. After achieving orbital velocity the ship could keep on firing to achieve another 3.1 km/s for Trans Lunar Insertion (TLI).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or it could cut off the engines after achieving orbital velocity and the do the remaining 3.1 km/s TLI burn later. What is the difference is delta V? Zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From John Schilling's &lt;a href=&quot;http://www.silverbirdastronautics.com/LaunchMethodology.pdf&quot;&gt;launch simulator methodology pdf&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Townsend technique begins by assuming that all space launches&#xA;  consist of a direct ascent to a low circular parking orbit, followed&#xA;  by a series of on-orbit maneuvers to the final destination orbit. In&#xA;  fact, many launch vehicles fly only a direct-ascent trajectory, even&#xA;  to a high or non- circular orbit. &lt;strong&gt;However, an observation of these&#xA;  trajectories almost invariably finds the launch vehicle, at an&#xA;  altitude of a few hundred kilometers, accelerating almost horizontally&#xA;  through the local circular orbit velocity.&lt;/strong&gt; One may simplify the&#xA;  problem by treating this as an instantaneous &quot;parking orbit&quot;, reached&#xA;  by direct ascent, and with all subsequent powered flight treated as an&#xA;  &quot;on-orbit maneuver&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Emphasis added mine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, virtually all trajectories are in low earth orbit for a time. Sometimes very briefly, some times in an extended parking orbit. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-01-16T00:05:35.807" LastActivityDate="2016-01-16T00:05:35.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13547" PostTypeId="1" CreationDate="2016-01-16T00:36:11.853" Score="5" ViewCount="2409" Body="&lt;p&gt;I know terraforming Mars has been a topic of discussion for a long time. One of the larger obstacles is the planet's weak magnetic field, which if I understand correctly is integral to containing an atmosphere and deflecting harmful solar radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could this problem be solved by deploying a network of satellites that contain large superconducting electromagnets that would work together to create the needed electromagnetic field so that the planet could be terraformed?&lt;/p&gt;&#xA;" OwnerUserId="13051" LastEditorUserId="6816" LastEditDate="2016-01-16T00:43:10.703" LastActivityDate="2016-01-17T09:02:30.773" Title="Would it be possible to deploy a man-made magnetic field in Mars orbit for terraforming?" Tags="|mars|magnetic-field|terraforming|" AnswerCount="5" CommentCount="3" ClosedDate="2016-01-17T17:53:11.870" ContentLicense="CC BY-SA 3.0" />
  <row Id="13548" PostTypeId="2" ParentId="13547" CreationDate="2016-01-16T05:00:33.117" Score="9" Body="&lt;p&gt;If you are able to terraform Mars in some reasonable amount of time, let's say in 100 years, then you don't need a magnetic field. Just do whatever you did to terraform the planet, but at &lt;em&gt;one billionth&lt;/em&gt; of that rate, in order to counter the loss of atmosphere to the solar wind. Or don't even bother at all, leaving the problem to your great&lt;sup&gt;1000000&lt;/sup&gt;-grandchildren to deal with. It's a very slow process, taking tens of millions of years to even have a noticeable loss.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-01-16T05:13:05.543" LastActivityDate="2016-01-16T05:13:05.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13549" PostTypeId="2" ParentId="13544" CreationDate="2016-01-16T05:59:38.463" Score="2" Body="&lt;p&gt;Hydrated silicates will give up the trapped water by simply &lt;a href=&quot;http://www.nature.com/articles/srep06003&quot; rel=&quot;nofollow&quot;&gt;heating to 600C&lt;/a&gt; (873K).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A black body at 1 AU would have a temperature of 279K by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law&quot; rel=&quot;nofollow&quot;&gt;Stefan-Boltzmann law&lt;/a&gt;.  So, heat sufficient to increase temperature by 594 degrees is required. Specific heat of magnesium silicate is &lt;a href=&quot;http://ottp.fme.vutbr.cz/vyuka/thermomechanics/therm_prop_non-met.pdf&quot; rel=&quot;nofollow&quot;&gt;885 J/C/kg&lt;/a&gt;, so it takes 525,690 J of energy to bake the water out of 1kg of hydrated magnesium silicate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Magnesium silicate is $XMgO:YSiO_2$ where X:Y is a molar ratio.  Hydrated, it becomes $XMgO:YSiO_2 * H_2O$.  Talc, a terrestrial mineral hydrated magnesium silicate is $Mg_3Si_4O_{10}(OH)_2$, or written as a synthetic silicate, $3MgO:4SiO_2*H_2O$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ratio of the atomic weight of water to hydrated magnesium silicate is 4.75%, so 1 kg of hydrated magnesium silicate (aka talc) would yield 47.5 grams of water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.lpi.usra.edu/books/AsteroidsIII/pdf/3031.pdf&quot; rel=&quot;nofollow&quot;&gt;paper by A.S. Kivkin&lt;/a&gt; of Massachusetts Institute of Technology suggests that hydrated minerals are present in the mid-asteroid belt.  In particular CI, CM, and CR types, but also in M and E-class asteroids.&lt;/p&gt;&#xA;" OwnerUserId="11479" LastActivityDate="2016-01-16T05:59:38.463" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13550" PostTypeId="2" ParentId="13547" CreationDate="2016-01-16T11:31:25.167" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;One of the larger obstacles is the planet's weak magnetic field, which if I understand correctly is integral to containing an atmosphere and deflecting harmful solar radiation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That is far from one of the larger obstacles. That is, per Mark Adler's answer, a problem for our children's children's ... children to solve. The larger obstacles are&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How do we terraform Mars?&lt;br&gt;&#xA;There are lots of science fiction stories about terraforming Mars. For now, the concept of terraforming Mars is just that, science fiction. Sans fiction, we don't know how to do that.&lt;/li&gt;&#xA;&lt;li&gt;Can we afford to terraform Mars?&lt;br&gt;&#xA;Terraforming Mars will cost lots and lots and lots of money. There are perhaps better ways to spend all of the money than spending it on making Mars habitable.&lt;/li&gt;&#xA;&lt;li&gt;Do we want to terraform Mars?&lt;br&gt;&#xA;Sending a small number of people to Mars would fall under the category of &quot;we do this not because it is easy, but because it is hard.&quot; Terraforming Mars would fall under the category of the much larger question, &quot;is this profitable?&quot;&lt;/li&gt;&#xA;&lt;li&gt;What if Mars still harbors life?&lt;br&gt;&#xA;If this is the case, I'd argue that terraforming Mars would make each of us stuck on the Earth many times worse than the very worst despots of all of the previous millennia.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-01-16T14:07:31.747" LastActivityDate="2016-01-16T14:07:31.747" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13551" PostTypeId="2" ParentId="13547" CreationDate="2016-01-16T12:25:29.350" Score="1" Body="&lt;p&gt;To address your actual question, it might be extremely hard to do that. Individual satellites would make individual dipole fields. Even if you deploy a million satellites in low orbit, thats still &gt;40 square kilometers for each satellite to try to affect. It would be mostly holes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe something upstream at Lagrange L1 has slightly better numbers? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;...but the other answers are the best answers - it won't matter for a zillion years and there are other more pressing questions about terraforming Mars in the first place.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-01-16T12:25:29.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13554" PostTypeId="2" ParentId="13547" CreationDate="2016-01-16T14:36:46.680" Score="3" Body="&lt;p&gt;Let's use Earth's magnetic field as a starting point. If we wanted to replicate that using magnetic iron, we'd need $10^{20}$ kg of iron, give or take a few orders of magnitude.&lt;br&gt;&#xA;Supercondicting magnets can be &lt;a href=&quot;https://en.wikipedia.org/wiki/Orders_of_magnitude_%28magnetic_field%29&quot; rel=&quot;nofollow&quot;&gt;$10^5$ times stronger&lt;/a&gt; than magnetic iron.&lt;br&gt;&#xA;So we'd need to install $10^{15}$ kg of magnets on Mars, plus their power sources. Again, give or take a few orders of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far we've launched something like $10^6$ kg into orbit. The biggest construction projects in Earth weigh $10^9$ kg. You're proposing a project a million times larger. You can see how far from possible this is. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-01-17T09:02:30.773" LastActivityDate="2016-01-17T09:02:30.773" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13555" PostTypeId="1" AcceptedAnswerId="13556" CreationDate="2016-01-16T14:56:20.277" Score="9" ViewCount="1349" Body="&lt;p&gt;There were multiple mentions of the future ideal first stage ASDS landings - if the mission won't have enough deltaV to do a RTLS, then just land on the barge, refuel and &lt;em&gt;&quot;hop&quot;&lt;/em&gt; back to the launch site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much fuel is needed for such a &lt;strong&gt;hop&lt;/strong&gt; and how can it be done? Would it be most effective to go &quot;back&quot; to space and endure a &quot;full&quot; reentry and landing again? Or might it be possible to return through atmospheric-only flight (I am thinking about multiple engine ignitions maybe)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it make any sense to fill the first stage full of fuel to work as ballast and allow hover or would it burn most of it anyway to get to the destination (rocket equation is such a tyrant)? &lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2016-01-16T18:49:46.477" LastActivityDate="2016-02-03T22:01:46.590" Title="Falcon 9 - fuel needed to hop back from a barge" Tags="|spacex|falcon-9|first-stage|asds|recovery|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13556" PostTypeId="2" ParentId="13555" CreationDate="2016-01-16T18:00:16.930" Score="3" Body="&lt;p&gt;It's likely that the most efficient flight path is the mirror of the one that got the 1st stage to the barge in the first place - a powered ascent phase to high altitude (a little ways out of the atmosphere) followed by a ballistic coast phase, hypersonic reentry, and powered descent and landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did some number crunching, making a &lt;em&gt;lot&lt;/em&gt; of unwarranted assumptions, and came up with a figure of about 200 tons of fuel for the return flight -- half the tankage of the 1st stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The return trajectory might be shaped somewhat differently from the ascent, and drag effects would be very different (even if SpaceX puts a small conical fairing on the stage for the return flight), so I could be off by a lot, but I think it has to be something like half the tank.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-01-16T18:00:16.930" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13557" PostTypeId="2" ParentId="13555" CreationDate="2016-01-16T18:10:06.093" Score="16" Body="&lt;p&gt;It seems to me very unlikely that this would actually occur. One would have to get the fuel out to the barge, fill the rocket, monitor the mission to return, and run the risk of losing the rocket, etc. The cost to have the barge return is minimal in comparison, and in fact, the cost is probably about the same regardless of if there is a booster on top or not.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-16T18:10:06.093" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13559" PostTypeId="2" ParentId="13547" CreationDate="2016-01-16T19:20:16.640" Score="-1" Body="&lt;p&gt;The fairly obvious answer is not to use individual satellites. Instead, create an orbital ring. It would be about 20.000 kilometer long. This isn't an &lt;a href=&quot;https://en.wikipedia.org/wiki/Areostationary_orbit&quot; rel=&quot;nofollow&quot;&gt;areostationary orbit&lt;/a&gt; precisely because the ring is intended to rotate. Otherwise, the sunny side would overheat. It needs to rotate to balance the temperature.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2016-01-16T19:20:16.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13561" PostTypeId="2" ParentId="13061" CreationDate="2016-01-17T10:03:39.760" Score="2" Body="&lt;p&gt;For most of its orbit, a direct radio connection is not possible because you don't have a line of sight to the satellite. So you'd have to set up radio relays on the ground or via other satellites (NASA's TDRSS, for example).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using a satellite phone is actually not a bad idea. A sat phone would fill most of the space in a 3U cubesat, though. And I don't know how well a sat phone works when it's traveling at orbital speeds. You may have to synchronize the cubesat's orbit with that of the communications satellite. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-17T10:03:39.760" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13568" PostTypeId="1" AcceptedAnswerId="13570" CreationDate="2016-01-17T17:17:01.160" Score="6" ViewCount="2501" Body="&lt;p&gt;So far, we have gotten rovers on Mars to take samples and analyze them and send data back to Earth. However, why haven't we gotten a rocket to land on Mars, collect a sample, and blast it off back to Earth? Surely we can learn much more using our better equipment on Earth and we don't have to worry about what equipment we need to pack on the rover because of weight.&lt;/p&gt;&#xA;" OwnerUserId="12689" LastEditorUserId="49" LastEditDate="2016-01-17T17:24:45.070" LastActivityDate="2016-01-18T16:20:33.463" Title="Why haven't we returned samples from Mars?" Tags="|mars|feasibility|sample-return|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13569" PostTypeId="2" ParentId="13539" CreationDate="2016-01-17T17:22:42.937" Score="21" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I would think that e.g. life support isn't running on a laptop and isn't using an off-the-shelf operating system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That is correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heart and soul of the computational system in the Russian segment of the International Space Station is the Data Management System - Russia (DMS-R), which comprises a pair of fault tolerant computers (each of which has three independent CPUs that operate by voting) plus another pair of control post computers. These computers collectively operate life support, electrical power, communications, and guidance, navigation and control for the Russian segment. The DMS-R computers use VxWorks 5.3, an old (vintage late 1990s) version of the VxWorks operating system, as the OS. VxWorks is a proprietary realtime operating system that mostly looks like Unix.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heart and soul of the computational system for the US segment is a set of 50+ computers that go under (IMHO) the terrible name of &quot;Multiplexer / Demultiplexer&quot;. The USOS multiplexer/demultiplexers are programmed in Ada and use a bare Ada runtime environment. Depending on perspective, one can look at the MDMs as not having an operating system (which is quite common for embedded systems) or that the Ada RTE is the operating system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side note: A multiplexer/demultiplexer (MDM) is typically a rather simple and very low-level communications device that combines multiple signals into one (a multiplexer) and splits a single signal into multiple signals (a demultiplexer). The ISS MDMs are anything but simple and low-level. They collectively run over two million lines of Ada code.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;What about all those laptops on the station? Originally, the space station was not supposed to have any commercial laptops. It was instead supposed to have &quot;multipurpose application consoles&quot; which were to be programmed in Ada by the same group that developed the USOS MDM software. That didn't work out for a number of reasons. Those laptops are used to do many different things, but they do not operate the life support system, or any of the other safety critical systems on the Station.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-01-17T17:22:42.937" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13570" PostTypeId="2" ParentId="13568" CreationDate="2016-01-17T17:35:45.093" Score="11" Body="&lt;p&gt;You said it yourself when you mentioned weight. So far, we managed to soft land roughly two metric tons on Mars in one go, if we count both the Curiosity rover and its &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Science_Laboratory#Sky_crane&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sky Crane&lt;/a&gt; landing system (Curiosity itself is about one metric ton in mass, but both the rover and the Sky Crane actually achieved close to zero vertical velocity with respect to the martian surface before the rover was released). Now consider how much mass it would take to also launch something back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Mars surface, &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;nofollow noreferrer&quot;&gt;required delta-v&lt;/a&gt; to launch something into orbit is roughly half that required to launch something from Earth to low Earth orbit, so ~ 6 km/s (your mileage might vary, depending on launch site's altitude, latitude,...). Once in Mars orbit, you require additional ~ 3 km/s to launch something into most energy efficient &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann transfer orbit&lt;/a&gt; towards the Earth. So we'd need a launch system capable of achieving roughly 9 km/s in likely two or more stages. With just the one stage needed on the surface, that's still quite a rocket even if the payload is fairly small. Depending on its performance (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific impulse&lt;/a&gt; of propellants used), its mass would be over 100 times what we so far managed to land on Mars in one go, if it later docks with an orbital stage to return the capsule with samples back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mind that even the upcoming &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020_%28rover%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars 2020 Rover&lt;/a&gt; will only stash samples (&lt;a href=&quot;http://mepag.nasa.gov/meetings.cfm?expand=m30&quot; rel=&quot;nofollow noreferrer&quot;&gt;that's the plan so far&lt;/a&gt;), and they might, or might not ever be recovered for launch to Earth. I'm afraid it's simply too difficult to bring Mars samples to Earth, and we don't really have the necessary technology at this point in time. When it comes to samples from Mars, for the moment, we're stuck with either &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_meteorite&quot; rel=&quot;nofollow noreferrer&quot;&gt;meteorites that have been established to have come from Mars&lt;/a&gt;, or analyzing them there, &lt;em&gt;in-situ&lt;/em&gt;, with probes, &lt;a href=&quot;https://space.stackexchange.com/a/4338/49&quot;&gt;landers&lt;/a&gt; and rovers, and remote observations with orbiters and flyby missions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some additional comments:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of the mass that needs to be landed on Mars could be reduced with &lt;em&gt;in-situ&lt;/em&gt; production of propellants (what's commonly referred to as &lt;a href=&quot;https://en.wikipedia.org/wiki/In_situ_resource_utilization&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISRU&lt;/a&gt; - In-Situ Resource Utilization), perhaps just the oxidizer by electrolysis of the atmosphere to extract oxygen like the &lt;a href=&quot;https://space.stackexchange.com/questions/5094/how-do-you-make-oxygen-on-mars/5096#5096&quot;&gt;MOXIE&lt;/a&gt; (Mars Oxygen ISRU Experiment) on the Mars 2020 Rover will attempt to demonstrate on a tiny scale, which would constitute over two thirds the mass of a liquid hydrogen and oxygen stage, and seems a lot simpler to do than finding and purifying sources of hydrogen for rocket fuels with no infrastructure there to do that. But even that still means landing a lot more mass than we currently know how to, and holding onto hydrogen as fuel or a fuel component, which, because it's in tiny molecules and doesn't particularly appreciate thermal cycling, really likes to bleed-off through light and thin walls of rocket propellant tanks. Perhaps using imported hydrogen to produce hydrocarbon fuels with the carbon from the mostly carbon dioxide atmosphere, such as methane, would improve &lt;em&gt;shelf life&lt;/em&gt; and improve performance of such rocket stages despite lower specific impulse better than shielding cryogenic liquid hydrogen against thermal radiation, and at the same time reduce landed mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA/JPL (and Mark Adler that provided you an answer here as its project manager) are currently developing &lt;a href=&quot;https://en.wikipedia.org/wiki/Low-Density_Supersonic_Decelerator&quot; rel=&quot;nofollow noreferrer&quot;&gt;LDSD&lt;/a&gt; (Low-Density Supersonic Decelerator) that could decelerate more mass down to transonic speeds with the supersonic parachutes and the SIAD (Supersonic Inflatable Aerodynamic Decelerator) parts without spending any additional propellants for that part of Entry, Descent and Landing (EDL), but that isn't yet at sufficient &lt;a href=&quot;https://en.wikipedia.org/wiki/Technology_readiness_level#NASA_definitions&quot; rel=&quot;nofollow noreferrer&quot;&gt;technology readiness level&lt;/a&gt; and the supersonic parachute development has hit a snag now two times during testing and has yet to demonstrate a successful deployment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Retropulsive landing techniques are also being developed to some extent; NASA's budget is tiny for that at this moment, so it's mostly student work on the dirt cheap &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Morpheus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Morpheus&lt;/a&gt; that is pushing the &lt;a href=&quot;https://en.wikipedia.org/wiki/VTVL&quot; rel=&quot;nofollow noreferrer&quot;&gt;VTVL&lt;/a&gt; envelope with technologies such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_Landing_Hazard_Avoidance_Technology&quot; rel=&quot;nofollow noreferrer&quot;&gt;Autonomous Landing Hazard Avoidance Technology&lt;/a&gt; (ALHAT), and of course SpaceX is making big strides towards landing whole rocket stages propulsively, and a big part of what they're now doing on Earth is exactly what's needed to land a rocket stage on Mars, too (and NASA is extremely interested in not just because they're one of their launch providers).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, we'll also need either bigger rockets, or many of the ones we have now and practice once again orbital assembly, to even send that much mass towards Mars first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to wrap things up, there's a lot of technology that still needs to be developed and brought to required technology readiness level, so we're even confident that this is feasible from a technical standpoint. And that doesn't even address political and financial aspects of it (see Mark Adler's answer). But it would most certainly be a welcome learning step before we send sample bringing astronauts to Mars and back, if we could put much of the technology needed to do that first to the test on a robotic mission. Though, maybe we'll opt for a different path, and establish presence on the two satellites of Mars, Phobos and Deimos, use them as a staging ground and learn to produce propellants there. Who knows? We have a long way to go.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-01-17T23:55:33.123" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13571" PostTypeId="1" AcceptedAnswerId="13583" CreationDate="2016-01-17T18:07:29.053" Score="8" ViewCount="2336" Body="&lt;p&gt;You would think that with the quest for aerodynamic efficiency in current spacecraft that the nose-cones at the pointy end of the launch-vehicle would have a sharp taper, more so for craft that aren't going to re-enter the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However this doesn't appear to be the case. Why?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2016-01-17T18:13:58.107" LastActivityDate="2018-12-10T20:06:52.140" Title="Why are the nose cones of current spacecraft less pointy?" Tags="|spacecraft|engineering|aerodynamics|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13572" PostTypeId="1" CreationDate="2016-01-17T18:39:48.690" Score="11" ViewCount="884" Body="&lt;p&gt;On the Jason-3 mission, several times the spokeskids said that this is the &#xA;last time we will hear the T-13 readiness check on a SpaceX webcast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, &lt;a href=&quot;https://www.youtube.com/watch?v=ivdKRJzl6y0&amp;amp;start=1643&quot;&gt;at 27:23&lt;/a&gt; they say:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;...this is going to be the first and last time you are going to hear a readiness poll come on the air for a SpaceX webcast&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They did not say why. Why will they no longer allow us to hear the poll?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="49" LastEditDate="2016-01-17T21:35:10.600" LastActivityDate="2016-01-17T21:35:10.600" Title="Why was Jason-3 the last launch we heard the T-13 readiness poll during a SpaceX Webcast?" Tags="|launch|spacex|falcon-9|launch-sequence|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13573" PostTypeId="2" ParentId="13572" CreationDate="2016-01-17T18:55:34.037" Score="13" Body="&lt;p&gt;The go/no go poll is the last major hurdle before the launch. If one of the polled stations says 'no go', the launch will be delayed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the rocket is full of subcooled propellant, that delay means the propellant will warm up and start to expand. They want to avoid that, so they decided to move propellant loading to after the go/no go poll, which means the poll ends up at T -30 min, before the webcast begins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current version (Falcon 9 v1.2?) uses subcooled propellants.&#xA;Jason-3 used the last F9 v1.1 rocket, which didn't use subcooled propellants. This meant they could start loading earlier, and have the poll later: having a delay after loading the propellant isn't as critical in this configuration. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-01-17T19:28:32.400" LastActivityDate="2016-01-17T19:28:32.400" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13574" PostTypeId="1" AcceptedAnswerId="14666" CreationDate="2016-01-17T19:54:30.780" Score="9" ViewCount="2126" Body="&lt;p&gt;SpaceX's attempts to land involve using the first stage engines to slow the booster down, and use grid fins to perform trajectory adjustments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parachutes generally aren't rigid and can throw the rocket off course when landing on something as small as a barge, but is it an option to use airbrakes - possibly resembling the grid fins, but with a solid surface - to slow the rocket down during descent, saving fuel, and helping prevent hard landings?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, what are the drawbacks preventing SpaceX from using rigid airbrakes?&lt;/p&gt;&#xA;" OwnerUserId="12046" LastEditorUserId="51" LastEditDate="2016-01-17T20:30:03.200" LastActivityDate="2016-04-07T13:35:11.007" Title="Why doesn't the Falcon 9 booster use rigid air brakes during landing attempts?" Tags="|spacex|falcon-9|landing|aerodynamics|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13575" PostTypeId="2" ParentId="13574" CreationDate="2016-01-17T20:29:42.873" Score="7" Body="&lt;p&gt;SpaceX needs a solution that can work at hypersonic speeds, and then down as slow as possible. They have enough fuel allocated to use the engine to kill velocity, and they need steering fins anyway.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus grid fins do the trick for steering across the performance domains with the engine providing sufficient deceleration as needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air brakes would likely only work at the very end of the flight, where the engine is mostly controlling the speed of descent.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-17T20:29:42.873" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13576" PostTypeId="1" CreationDate="2016-01-17T20:32:09.227" Score="2" ViewCount="500" Body="&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=ivdKRJzl6y0&amp;amp;start=2485&quot; rel=&quot;nofollow&quot;&gt;today's SpaceX launch&lt;/a&gt;, at &lt;a href=&quot;https://www.youtube.com/watch?v=ivdKRJzl6y0&amp;amp;start=2485&quot; rel=&quot;nofollow&quot;&gt;41:25&lt;/a&gt;-41:28 there are a lot of dark objects which appear to be flying debris, almost like they might be parts of the light poles etc. which are visible on/near the launch pad before liftoff:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ivdKRJzl6y0?start=2485&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are those flying pieces, and how much of a launch pad site typically has to be reconstructed after a launch because of damage from the launch? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much of the immediate &quot;post-launch [pad] operations&quot; they are talking about in that clip is about collecting the debris and/or repairing that damage? &lt;/p&gt;&#xA;" OwnerUserId="11181" LastEditorUserId="49" LastEditDate="2016-01-17T21:25:25.373" LastActivityDate="2016-01-17T22:26:08.520" Title="What is all the stuff that goes flying during a launch?" Tags="|launch|rockets|falcon-9|launch-site|launchpad|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13577" PostTypeId="2" ParentId="13576" CreationDate="2016-01-17T21:00:04.553" Score="4" Body="&lt;p&gt;Most likely it is light debris, stuff like leaves, dirt, anything that literally is not bolted hard to the ground. They clear the pad of obvious loose stuff, almost like on an aircraft carrier where they sweep for FOD (Foreign object debris) but there is a lot of ground around a launch pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, recall the engines of a rocket, unlike a plane on a carrier or even a normal runway, do not inject air rather it exhausts huge amounts, so almost anything that does blow, will blow away (far away) instead of being sucked into an engine. (Eagles may soar, but weasles don't get sucked into jet engines). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look a bit earlier you will see a splash of dark stuff come up the left side of the rockets as well.  That is water from the Niagara deluge system, which is designed to protect the pad from acoustic damage. The system dumps amazingly large amounts of water, which the heat boils, instead of the concrete, and the water absorbs the sound (also causing it to heat up) instead of vibrating the concrete. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other launches have had the dirty looking water splash pretty high up and get the pretty booster dirty.  But as we saw in Orbcomm OG2 launch and landing, coming back, through your own RP1/LOX exhaust is pretty dirty work from all the soot on the first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the water minimizes the physical damage to a lot of the pad surface. However, a 1.3 million lb thrust rocket just lit on top of the pad and flew away, stuff is going to be damaged.  The trick is to make the expensive stuff resilient and the stuff that is going to get destroyed anyway, cheap and easily replaceable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is clearly far along in the learning phase as they just completed their 21st launch campaign, and they seem to be getting better at resetting for the next launch.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-01-17T21:57:58.043" LastActivityDate="2016-01-17T21:57:58.043" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13578" PostTypeId="1" CreationDate="2016-01-17T21:53:32.730" Score="7" ViewCount="1383" Body="&lt;p&gt;This question is moderately inspired by &lt;a href=&quot;https://space.stackexchange.com/questions/13576/what-is-all-the-stuff-that-goes-flying-during-a-launch&quot;&gt;this one&lt;/a&gt;, or at least the speculative answer &amp;quot;birds&amp;quot; that is ruled out on a second review of the video.&lt;/p&gt;&#xA;&lt;p&gt;Various animals have learned signals of severe immediate danger that causes them to flee certain hazards.  However, the hazard of &amp;quot;rocket launch&amp;quot; is sufficiently rare and modern that most wildlife have probably not learned how to get away from a rocket that is about to launch, or even be able to tell that a rocket is about to launch.&lt;/p&gt;&#xA;&lt;p&gt;Prior to launch, the range is cleared of humans and certain human vehicles (drones, etc.) and that's part of the check sequence.  Also, a certain number of insects etc. are just expected to die in the firing of rocket engines that are boosting payloads to orbit.&lt;/p&gt;&#xA;&lt;p&gt;Prior to certain other events (e.g. demolitions by implosion), there are warning signals sounded so that any creature close enough to feel alarmed will run away.&lt;/p&gt;&#xA;&lt;p&gt;The question is, is there any siren or warning signal sounded before rocket launches, to give wildlife a heads up to get out of there?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;em&gt;Related:&lt;br /&gt;&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/6248/are-there-any-indicative-statistics-about-animal-loss-during-launches&quot;&gt;Are there any indicative statistics about animal loss during launches?&lt;/a&gt;&lt;br /&gt;&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/6587/how-close-can-i-get-to-a-rocket-launch-and-not-get-killed&quot;&gt;How close can I get to a rocket launch and not get killed?&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="11181" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-01-18T05:04:36.983" Title="Are sirens (etc.) sounded before launches to warn wildlife?" Tags="|launch|launch-site|launchpad|launch-sequence|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13579" PostTypeId="2" ParentId="13568" CreationDate="2016-01-17T22:09:55.250" Score="5" Body="&lt;p&gt;Money. Commitment. Confidence. Insufficient amounts of those three are why we haven't yet returned samples from Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is certainly motivation. The last planetary science decadal survey, &lt;a href=&quot;http://solarsystem.nasa.gov/docs/Vision_and_Voyages-FINAL.pdf&quot; rel=&quot;nofollow&quot;&gt;Visions and Voyages&lt;/a&gt;, where the science community gets together and decides on priorities, rated as its &lt;em&gt;number one&lt;/em&gt; flagship mission priority the first of three missions to accomplish Mars Sample Return, where that mission is now referred to as Mars 2020. It has the job of carefully selecting and collecting rock cores and other samples of the Martian surface over about three years, for eventual return to Earth later in the decade. (&quot;Return&quot; is probably the wrong word here — unless we picked the wrong samples, they certainly didn't come from Earth — but anyway &quot;return&quot; is the word everyone uses after &quot;Mars sample&quot;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if NASA commits to the Mars 2020 mission (which could be in the near future), NASA will not be committing at that time to the two subsequent missions that would be needed to transport the collected samples to Earth. The Mars 2020 mission was &lt;a href=&quot;http://mars.nasa.gov/mars2020/files/mars2020/SDT-Report%20Finalv6.pdf&quot; rel=&quot;nofollow&quot;&gt;conceived&lt;/a&gt; to have a very robust and novel &lt;a href=&quot;https://www.nasa.gov/press/2014/july/nasa-announces-mars-2020-rover-payload-to-explore-the-red-planet-as-never-before&quot; rel=&quot;nofollow&quot;&gt;in situ investigation capability&lt;/a&gt; that would justify the mission even if the samples are never picked up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the subsequent missions go forward, one of them would be a lander with a fetch rover to go get the retrieved samples and a Mars ascent vehicle (a rocket) to launch the fetched samples into a low Mars orbit. The other mission would be a Mars orbiter that rendezvous with and captures the orbiting sample container and then departs Mars orbit, returns to Earth (there &quot;return&quot; works), and as currently conceived send the samples down to the surface of the Earth in a small entry vehicle. Each of those missions are of the flagship class, which would dominate the planetary science budget for a decade or more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost in the context of NASA's planetary budget is daunting. Furthermore the ability to estimate the cost of such an endeavor in the face of all of the significant technical challenges is limited. There is justified concern over the magnitude of the cost &lt;em&gt;risk&lt;/em&gt;, even if initially the cost is considered to be palatable. That is where the deficit in confidence comes in. Not so much about a lack of confidence in it working — we always have to accept some risk there, and we know how to mitigate it — but rather a lack of confidence in our ability to limit budget and schedule overruns to a reasonable level for such an unprecedented technical undertaking. (See the James Webb Space Telescope.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;International partnerships could mitigate the cost, but two attempts at that around the late 1990's and late 2000's both were eventually abandoned. Maybe we're due to try to kick that football again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost and the cost risk, plus the very long time scale of investments over more than a decade with limited payoff unless and until it is successful at the very end of the campaign (longer than any administration), has led to a lack of commitment to Mars Sample Return in the leadership of the US government, including the White House and the Office of Management and Budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be the dominant fraction of the total planetary budget over more than a decade, where even that fraction might not be enough, so it is difficult for a government that works on year-by-year funding uncertainty to commit to something like that where the payoff doesn't come until the end. Still there is hope that this might get pulled off someday, one mission at a time. Once we have samples on the surface ready to be picked up, we have more motivation ahead of us and more cost behind us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the technology, there is time to develop what's needed even if there were a commitment today to develop and launch all three missions in a serial fashion, picking up on the next mission as the funding wedge on the previous one starts to drop. Nothing about the mission is insurmountable. The greatest challenges are in the area of planetary protection, which impact every phase of the mission from sample collection to Earth entry. Some of those planetary protection challenges are being tackled by the Mars 2020 mission already, as is the significant challenge of collecting intact rock cores. The orbital rendezvous and capture and the Mars ascent vehicle are engineering challenges, not new technology. Overall, there is a &lt;em&gt;ton&lt;/em&gt; of work to do, but there are no apparent showstoppers so long as there is flexibility in requirements. This can happen, but only if there is a serious commitment and money to go along with it.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-01-18T16:20:33.463" LastActivityDate="2016-01-18T16:20:33.463" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13580" PostTypeId="2" ParentId="13576" CreationDate="2016-01-17T22:26:08.520" Score="4" Body="&lt;p&gt;All kinds of crazy stuff blew off the shuttle pads: handrails, lights, signs, rust, ice, etc.  Even flying bricks!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ErPNK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ErPNK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-01-17T22:26:08.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13581" PostTypeId="2" ParentId="13578" CreationDate="2016-01-18T00:40:15.463" Score="5" Body="&lt;p&gt;This is actually a non-trivial problem, one that &lt;a href=&quot;http://www.spacedaily.com/reports/NASA_Seeking_Ways_To_Keep_Wildlife_Away_From_Shuttle_Launches_999.html&quot; rel=&quot;noreferrer&quot;&gt;NASA was seeking help for in 2006&lt;/a&gt;. It seems the plan mostly involves firing a number of &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/behindscenes/clearbirds.html&quot; rel=&quot;noreferrer&quot;&gt;propane cannons in the area&lt;/a&gt; to scare away birds (Which are the animal of most concern, as they could damage the vehicles) They have even &lt;a href=&quot;http://environmental.ksc.nasa.gov/projects/documents/KSCBirdAbateEfforts.pdf&quot; rel=&quot;noreferrer&quot;&gt;released a complete paper on the subject&lt;/a&gt;, which also mentions &quot;Sound Deterrent Systems&quot; installed in 2005. The bird of most concern is Vultures, and they key to reducing their population in the area seems to be reducing roadkill. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-18T00:40:15.463" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13583" PostTypeId="2" ParentId="13571" CreationDate="2016-01-18T00:58:57.190" Score="3" Body="&lt;p&gt;The blunt nose produces a &lt;a href=&quot;https://books.google.nl/books?id=Zjv0CgAAQBAJ&amp;amp;pg=PT100&amp;amp;lpg=PT100&amp;amp;dq=nose+cone+shockwave&amp;amp;source=bl&amp;amp;ots=oWyf7oOa89&amp;amp;sig=BNL0ydE8UxQTp4rg4YQ5DzgVin0&amp;amp;hl=nl&amp;amp;sa=X&amp;amp;ved=0ahUKEwi9g4bGk7LKAhVH1hQKHeeACYkQ6AEIPTAD#v=onepage&amp;amp;q=nose%20cone%20shockwave&amp;amp;f=false&quot; rel=&quot;nofollow&quot;&gt;shock wave&lt;/a&gt; which diverts heat away. The sharp tip you envisage would stick out in the hypersonic air stream and melt right off.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2016-01-18T00:58:57.190" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13584" PostTypeId="1" CreationDate="2016-01-18T03:35:50.297" Score="1" ViewCount="91" Body="&lt;p&gt;Spacex has already landed its falcon rocket on land. Why exactly are they coming for a barge landing. What are its advantages as compared to landing on land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/688842435955261440&quot; rel=&quot;nofollow&quot;&gt;https://twitter.com/elonmusk/status/688842435955261440&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6753" LastEditorUserId="6753" LastEditDate="2016-01-18T03:48:35.880" LastActivityDate="2016-01-18T03:48:35.880" Title="Why is spacex trying to land on a barge?" Tags="|spacex|falcon-9|landing|" AnswerCount="0" CommentCount="3" ClosedDate="2016-01-18T04:38:15.663" ContentLicense="CC BY-SA 3.0" />
  <row Id="13585" PostTypeId="2" ParentId="13578" CreationDate="2016-01-18T05:04:36.983" Score="4" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts119/launchbat.html&quot; rel=&quot;nofollow&quot;&gt;this NASA article&lt;/a&gt;, &quot;Because the Merritt Island National Wildlife Refuge coexists inside Kennedy Space Center, the launch pads have a number of measures available, including warning sirens, to deter birds and other creatures from getting too close. The launch team also uses radar to watch for birds before a shuttle liftoff.&quot;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems to be specific to KSC.&lt;/p&gt;&#xA;" OwnerUserId="11181" LastActivityDate="2016-01-18T05:04:36.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13586" PostTypeId="1" AcceptedAnswerId="13588" CreationDate="2016-01-18T06:33:54.710" Score="17" ViewCount="12763" Body="&lt;p&gt;In many launch sequences I see the phrase 'engine' chilldown' coming up. What is this and why is it done? &lt;/p&gt;&#xA;" OwnerUserId="3372" LastActivityDate="2018-02-06T14:15:59.883" Title="What is the rocket engine chilldown?" Tags="|engines|launch-sequence|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13588" PostTypeId="2" ParentId="13586" CreationDate="2016-01-18T07:35:34.927" Score="28" Body="&lt;p&gt;Chilldown is needed for engines using cryogenic propellants -- liquid oxygen as the oxidizer most commonly, sometimes in conjunction with liquid hydrogen as the fuel. Pumping large amounts of very cold fluids into an engine resting at ambient temperature causes a variety of problems, so the plumbing and pumps need to be brought down to a working temperature before the engine starts. Chilldown is performed by allowing relatively small amounts of the cryogenic propellants through the system until it reaches a safely low temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://books.google.com/books?id=_wpes7aZ1REC&amp;amp;pg=PA27&amp;amp;lpg=PA27&amp;amp;dq=engine%20chilldown&amp;amp;source=bl&amp;amp;ots=ZlnqdD38Fs&amp;amp;sig=mhiyVafcaLr9KvqG9O4iHHeCByE&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjAs8io6LLKAhVG0WMKHXqKDsQQ6AEISjAJ#v=onepage&amp;amp;q=engine%20chilldown&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Operation of a Cryogenic Rocket Engine&lt;/a&gt; by Wolfgang Kitsche:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A propulsion system running on cryogenic fluids has to pass a chill down phase before ignition. This chill down is indispensable, particularly if the propulsion system has turbo pumps to deliver the extremely cold fluids. The feed lines, turbo pumps, valves and ducts of the engine have to be brought down to the temperature of the cryogenic fluid. During the hot run the turbo pump delivers the fluid at high rotational speed. Starting the pump without prior chill down would cause cavitation [vapor bubbles produced in the stream], the bearings would not have sufficient cooling, and the fuel or oxidiser would be pumped into the chamber with an unacceptably high fraction of bubbles and gas. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastEditorUserId="6944" LastEditDate="2016-01-19T04:10:55.447" LastActivityDate="2016-01-19T04:10:55.447" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13591" PostTypeId="1" CreationDate="2016-01-18T10:12:42.540" Score="8" ViewCount="4596" Body="&lt;p&gt;What is the mechanism and what medium is used in the actuators to open the legs of the Falcon 9 First Stage on landing?&lt;/p&gt;&#xA;" OwnerUserId="13078" LastEditorUserId="6473" LastEditDate="2016-01-19T20:12:08.813" LastActivityDate="2016-01-19T20:12:08.813" Title="How are the Spacex Falcon 9 landing legs actuated?" Tags="|spacex|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13592" PostTypeId="2" ParentId="13591" CreationDate="2016-01-18T10:21:07.063" Score="7" Body="&lt;p&gt;The mechanism is a pneumatic cylinder. The cylinder is &lt;a href=&quot;https://twitter.com/elonmusk/status/330053450261987328/photo/1&quot; rel=&quot;noreferrer&quot;&gt;actuated&lt;/a&gt; by a bottle of &lt;a href=&quot;http://www.spaceflightinsider.com/space-flight-news/spacexs-falcon-9-to-sport-landing-legs-for-nasa-crs-3-mission-to-iss/&quot; rel=&quot;noreferrer&quot;&gt;high-pressure helium&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YXCrb.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YXCrb.jpg&quot; alt=&quot;F9 Landing leg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-01-18T18:21:24.113" LastActivityDate="2016-01-18T18:21:24.113" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13593" PostTypeId="1" CreationDate="2016-01-18T10:58:15.533" Score="4" ViewCount="216" Body="&lt;p&gt;I realize there is a whole lot of stuff in low earth orbit, but didn't imagine that it would be so visible. Here are the two brighter objects that passed by during the deployment of Jason-3 by SpaceX. I've nicknamed them &quot;Elvis&quot; and &quot;David Bowie&quot;, but are they satellites - or just nearby debris associated with the spacecraft? In the original SpaceX &lt;a href=&quot;https://www.youtube.com/watch?v=vkz_lclGXNg&quot; rel=&quot;nofollow noreferrer&quot;&gt;webcast&lt;/a&gt; you can see that Elvis seems to be rotating at about 1.8 revs per second, which might help in the identification.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I've removed the phrase &quot;educated guess&quot; from the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note the mission time in the upper right corner to you can refer back to the video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&quot;Elvis:&quot;&lt;/strong&gt;  the faint spot from left to right below Jason-3 (click to enlarge)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/eUdyu.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eUdyu.gif&quot; alt=&quot;SpaceX_Jason-3_deploy_GIF A&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&quot;David Bowie:&quot;&lt;/strong&gt; (click to enlarge)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/y9s7P.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y9s7P.gif&quot; alt=&quot;SpaceX_Jason-3_deploy_GIF B&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-01-18T15:01:28.783" LastActivityDate="2016-01-18T16:25:23.123" Title="Are these objects other satellites in the SpaceX webcast of Jason-3 deployment?" Tags="|spacex|low-earth-orbit|upper-stage|payload-deployment|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13594" PostTypeId="2" ParentId="13501" CreationDate="2016-01-18T11:11:10.950" Score="13" Body="&lt;p&gt;There's some interesting discussion of the Lunar Module design in chapter 6 of &lt;a href=&quot;http://www.hq.nasa.gov/pao/History/SP-4205/contents.html&quot;&gt;&lt;em&gt;Chariots For Apollo&lt;/em&gt;&lt;/a&gt; (Brooks, Grimwood &amp;amp; Swenson 1979, NASA SP-4205).  In particular, &lt;a href=&quot;http://www.hq.nasa.gov/pao/History/SP-4205/ch6-4.html&quot;&gt;chapter 6.4&lt;/a&gt; notes that a five-legged design was initially considered, but was changed to a four-legged one both to save weight and to simplify the design, which was already based on a four-way symmetric frame:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Grumman had first considered five legs but, during 1963, decided on four. The change was dictated by the weight-versus-strength tradeoff that had produced the cruciform descent stage, with its four obvious attachment points. The revised gear pattern also greatly simplified the structural mounting of the vehicle within the adapter. Four legs set on the orthogonal axes of the lander (forward, aft, left, and right) mated ideally with the pattern of four reaction control &quot;quads&quot; (the basic four-engine package). The quads were rotated 45 degrees so the downward-thrusting attitude control engine fired between the two nearest gear legs, overcoming a severe thermal problem of the five-leg arrangement.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and also:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A retractable gear therefore replaced the simpler fixed-leg gear. Retractability also figured in the shift from five to four legs - the fewer to fold, the better.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Obviously, three legs would've been even lighter and easier to fold than four, but would've had the same issues with mounting and thruster quad placement as five legs: the LM design was based on a cross-shaped frame, with four simple and sturdy attachment points, and it had four symmetrically placed packs of attitude control thrusters whose exhaust had to be directed away from the legs.  Also, a three-legged design would've had a significantly smaller stability margin when landing on a slope, and/or if any residual horizontal velocity remained while landing.&lt;/p&gt;&#xA;" OwnerUserId="189" LastActivityDate="2016-01-18T11:11:10.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13595" PostTypeId="1" CreationDate="2016-01-18T11:11:57.690" Score="7" ViewCount="376" Body="&lt;p&gt;While the image appears bright, the light is just reflected sunlight (from the earth, per the webcast). The circularization burn doesn't start until about T+ 00:55:16 per the SpaceX &lt;a href=&quot;https://www.youtube.com/watch?v=vkz_lclGXNg&quot; rel=&quot;nofollow noreferrer&quot;&gt;webcast&lt;/a&gt;. But at about T+ 00:52:58 it looks like an object produces a moving shadow that passes from left to right across the spacecraft. If so, what was it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the shadow is caused by blocking the light from the earth, then this puts constraints on its size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QZFvO.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QZFvO.gif&quot; alt=&quot;SpaceX_Jason-3_shadow_GIF cropped quarter speed&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-06-03T05:52:14.460" LastActivityDate="2016-06-03T05:52:14.460" Title="What caused this shadow on Falcon 9 stage 2 w/Jason-3 before circularization burn?" Tags="|spacex|low-earth-orbit|upper-stage|payload-deployment|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13596" PostTypeId="2" ParentId="13593" CreationDate="2016-01-18T14:13:14.923" Score="4" Body="&lt;p&gt;I could not find any reference to a secondary payload on that Falcon.&#xA;&lt;a href=&quot;http://www.n2yo.com/satellites/?c=latest-launches&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.n2yo.com/satellites/?c=latest-launches&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I therefore assume that because of its low relative speed, Elvis is a piece of the satellite. For example, there might be equipment on the sattelite that is sensitive to the rocket exhaust, so it is covered while the engines are still firing. Once the engines shut down, they jettison the cover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since David Bowie seems to be moving relative to the apparent background (although this is not 100% clear from the video) I would assume that it is a passing object. From the fact that it takes a while to pass the spacecraft, I would guess that it is a larger satellite passing in considerable distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see in the screenshot below, the sky south east of Madagascar was quite crowded at that time. My money would be on Iridium 11 being David Bowie, because it has a large radio dish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jason 3 is not pictured, but the position on the map would be almost precisely where Saudisat (1C) is pictured. However, Saudisat passed far underneath Jason, so if it was in the frame, the earth would also be in the frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking through the instruments of Jason-3, Elvis may be a cover for the Carmen-3 experiment. It fits the general shape, and since it is a sensitive electromagnetic device, it may need protection while crossing the inner Van-Allen-Belt. &lt;a href=&quot;https://jason-3.cnes.fr/en/JASON3/carmen3.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://jason-3.cnes.fr/en/JASON3/carmen3.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YP1ak.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YP1ak.jpg&quot; alt=&quot;Approximate Location of David Bowie&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-01-18T16:25:23.123" LastActivityDate="2016-01-18T16:25:23.123" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13597" PostTypeId="1" AcceptedAnswerId="13598" CreationDate="2016-01-18T15:30:48.853" Score="29" ViewCount="2279" Body="&lt;p&gt;If you watch the truly &lt;a href=&quot;https://www.instagram.com/p/BAqirNbwEc0/&quot; rel=&quot;noreferrer&quot;&gt;awesome video of the Jason-3 first stage landing&lt;/a&gt; on the ASDS Barge Just Read The Instructions, it is pretty clear that for one thing, they landed it.  Then  the leg gave out, gosh darn it!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However watch when it explodes, the side of the booster has just barely begun to touch the deck and it blows. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What part of the vehicle causes that explosion? The LOX tank rupturing and spilling LOX probably would not explode that fast, would it?  Is it an engine getting crumpled and the TEA/TEB spilling out along with leaking LOX?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It happens like in a movie, which is never how things really happy, so what is the cause of the explosion?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-12-27T23:04:51.847" LastActivityDate="2017-12-28T21:28:45.517" Title="What started the explosion on the crashed Jason-3 stage landing on the barge?" Tags="|spacex|falcon-9|landing|asds|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13598" PostTypeId="2" ParentId="13597" CreationDate="2016-01-18T15:42:38.697" Score="26" Body="&lt;p&gt;I reviewed the video frame-by-frame. The stage falls on the collapsed leg (which was hot enough to be on fire during the landing). The leg penetrated the RP-1 tank. The tank is under pressure, so it ruptures violently and takes the oxygen tank with it. You have both gushing out under pressure (i.e. at high speed) over a hot deck and towards the even hotter engines. &#xA;5 frames later (0.2 seconds), a small fire erupts near the top of the collapsed landing leg. Then it's another 10 frames (0.4 s) before the big fireball.&lt;br&gt;&#xA;So the explosion predates the flames and is purely due to the pressure in the tanks.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the effect LOX has on a fire, see this '&lt;a href=&quot;https://www.youtube.com/watch?v=sab2Ltm1WcM&quot;&gt;don't try this at home&lt;/a&gt;' video.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-01-18T18:55:08.960" LastActivityDate="2016-01-18T18:55:08.960" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13599" PostTypeId="1" CreationDate="2016-01-18T16:44:48.267" Score="2" ViewCount="683" Body="&lt;p&gt;when rockets are put in orbit, could they stay in orbit forever? if there was no one to man it, and it was not programmed to come back down, what would happen? would it stay up there, or would earth's gravity pull it back down? &lt;/p&gt;&#xA;" OwnerUserId="12917" LastActivityDate="2016-01-18T21:52:11.720" Title="could rockets stay in orbit forever?" Tags="|rockets|orbit|spacecraft|" AnswerCount="1" CommentCount="6" ClosedDate="2016-01-18T22:46:57.313" ContentLicense="CC BY-SA 3.0" />
  <row Id="13600" PostTypeId="2" ParentId="13599" CreationDate="2016-01-18T16:54:00.183" Score="10" Body="&lt;p&gt;There's the simplistic theory, and the more complicated practice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In theory, yes, an object put in orbit would stay there forever. Earth's gravity pull keeps it in orbit, spinning it around the planet instead of flying off. It doesn't pull it back down, because the forces are already balanced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in practice, there's other forces in play. Most importantly, on low Earth orbits, there's just a tiny bit of atmosphere leftovers slowing down the object. Depending on the altitude, it might take months, years, or decades, but eventually it will slow down enough to reenter. Also, there are minute forces from solar radiation and gravitational fields of other objects that might slow down the object (though that might take an awfully long time).&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="10274" LastEditDate="2016-01-18T21:52:11.720" LastActivityDate="2016-01-18T21:52:11.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13601" PostTypeId="2" ParentId="13597" CreationDate="2016-01-18T16:59:56.253" Score="12" Body="&lt;p&gt;If you look and listen closely to the video, you'll hear two bangs, not one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the LOX tank at the top of the rocket ruptures. Since it's pressurized, it doesn't just spill out, but quite loudly shoots liquid oxygen in all directions. But no explosion, only a small fire, likely combusting residue kerosene on the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A moment later, the kerosene tank at the bottom ruptures. &lt;em&gt;Now&lt;/em&gt; we see a fireball!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there was plenty of liquid oxygen around, sprayed into the air half a second before, and already a small fire to initiate the reaction, this allowed the RP-1 to explore quite rapidly.&lt;/p&gt;&#xA;" OwnerUserId="10274" LastEditorUserId="10274" LastEditDate="2016-01-18T17:05:38.380" LastActivityDate="2016-01-18T17:05:38.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13602" PostTypeId="2" ParentId="13518" CreationDate="2016-01-18T17:48:56.077" Score="5" Body="&lt;p&gt;The term &quot;sol&quot; is short for the word &quot;solar day&quot;, which is the amount of time it takes for mars to revolve once around its own axis. A &lt;a href=&quot;http://www.universetoday.com/14717/how-long-is-a-day-on-mars/&quot; rel=&quot;nofollow&quot;&gt;martian sol&lt;/a&gt; is approximately &lt;a href=&quot;http://www.giss.nasa.gov/tools/mars24/help/notes.html&quot; rel=&quot;nofollow&quot;&gt;24 hours, 39 minutes, and 35.244 seconds&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The English word is just sun, the Spanish word for sun is sol. the french word is soleil. It would seem sort of redundant to use this term for earth, as it is already used by the concept day.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, Earth's days are shorter: they are approximately 23 hours, 56 minutes, and 4 seconds, which is roughly 40 minutes shorter than a day on mars. If you want more information, check out the links. I thought they were helpful :)&lt;/p&gt;&#xA;" OwnerUserId="12917" LastEditorUserId="897" LastEditDate="2016-01-26T09:08:53.237" LastActivityDate="2016-01-26T09:08:53.237" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13603" PostTypeId="1" CreationDate="2016-01-18T20:27:58.993" Score="5" ViewCount="232" Body="&lt;p&gt;Are there plans to keep building the ISS? In particular adding more modules and generally expanding it.&#xA;If not, are there some fundamental limitations on its expansion? Like power, cooling, or something else.&lt;/p&gt;&#xA;" OwnerUserId="8903" LastEditorUserId="51" LastEditDate="2016-01-19T03:25:40.827" LastActivityDate="2016-01-19T03:25:40.827" Title="Will ISS grow further" Tags="|iss|space-station|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13604" PostTypeId="2" ParentId="13603" CreationDate="2016-01-18T20:53:01.063" Score="5" Body="&lt;p&gt;The USOS (US Operating Segment) is basically complete with some minor exceptions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX CRS-8 mission (Due Feb 2016) is scheduled to bring a BEAM (Bigelow Expandable Module) to the station for a test of inflatables at the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX CRS-7 was carrying the IDA-1 (International Docking Adapter) and was lost.  CRS-9 will carry IDA-2. They will build IDA-3 out of spare for launch on CRS-14 I think.  These are minor plugins for the PMA (Pressurized Mating Adapters) so that the commercial crew vehicles can dock using the NDS/LIDS docking systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ROS (Russian Operating Segment) is still waiting to replace the Pirs docking module with a new module known as Nauka, that will be earth facing. This module will then have a docking port at the bottom and a new airlock. The OM module (stands for something in Russian I cannot type) that will be a ball module with 6 docking ports.  Then at least two more modules.  Good examples and images in this answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2245/will-the-iss-need-more-docking-ports&quot;&gt;Will the ISS need more docking ports?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-01-19T03:19:29.763" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13605" PostTypeId="1" CreationDate="2016-01-18T21:06:12.143" Score="2" ViewCount="372" Body="&lt;p&gt;Building off of &lt;a href=&quot;https://space.stackexchange.com/questions/13597/what-started-the-explosion-on-the-crashed-jason-3-stage-on-the-barge&quot;&gt;this question&lt;/a&gt;, I am curious as to &lt;strong&gt;how much (just a simple magnitude of order) propellant remains inside a booster to cause such an explosion&lt;/strong&gt;?  Considering cost, I would expect very little would have remained after main engine cutoff on the barge, preventing such a hollywood-style explosion.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-01-19T03:24:55.900" Title="How much fuel remains in a booster such as the one used for Jason-3" Tags="|spacex|landing|fuel|reuse|booster-flyback|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13607" PostTypeId="1" AcceptedAnswerId="13610" CreationDate="2016-01-19T03:22:13.120" Score="8" ViewCount="1306" Body="&lt;p&gt;As we know, gas giants do not have a solid surface, but on Earth we can float a balloon in air. Are there any plans to send balloon-like probes to gas giants so that they can float in its atmosphere and explore the gas giant longer?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="1142" LastEditDate="2016-01-20T06:25:52.470" LastActivityDate="2016-01-20T19:43:32.800" Title="Is there any plan to send balloon like probes to gas giants to explore interior of gas giants?" Tags="|probe|gas-giant|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13608" PostTypeId="2" ParentId="13605" CreationDate="2016-01-19T03:24:55.900" Score="1" Body="&lt;p&gt;I do not have a quote to source it, but recall Elon Musk saying it was on the order of a 100 lbs of fuel and oxidizer left. Almost dry, but clearly enough to burn.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-19T03:24:55.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13609" PostTypeId="2" ParentId="13597" CreationDate="2016-01-19T05:39:05.263" Score="5" Body="&lt;p&gt;Liquid Oxygen has an expansion ratio of 1:861 (This means that 1 liter of liquid oxygen is equivalent to 861 liters of gaseous oxygen). Having worked with it on military aircraft, and witnessed a VERY SMALL amount being introduced to a flame source, I can tell you it very much could have just been the LOX going up. (I won't speculate if that's what it actually was)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At atmospheric pressure, LOX under pressure can convert to gaseous very quickly. If it's not under pressure, it converts much slower, but still creates a very dense fog in a matter of minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat it up from a fire, and things get significantly more violent, in an incredibly small amount of time.&lt;/p&gt;&#xA;" OwnerUserId="13093" LastActivityDate="2016-01-19T05:39:05.263" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13610" PostTypeId="2" ParentId="13607" CreationDate="2016-01-19T07:01:52.577" Score="4" Body="&lt;p&gt;I've seen it in &lt;a href=&quot;http://www2.jpl.nasa.gov/adv_tech/balloons/outer.htm&quot; rel=&quot;nofollow&quot;&gt;viewgraphs&lt;/a&gt;, but not in a serious proposal. I suspect that there will be more traditional probes (entry vehicles on parachutes) to Saturn and one of Neptune or Uranus, since we haven't done those yet, before a balloon would be attempted.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-01-19T15:48:18.537" LastActivityDate="2016-01-19T15:48:18.537" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13611" PostTypeId="2" ParentId="7753" CreationDate="2016-01-19T08:07:42.737" Score="7" Body="&lt;p&gt;Now that we've seen 4 landing attempts, it's time to revisit this question.  We've seen both options (landing on a barge and on land) being used, albeit not yet successfully for the barge landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX wants both options.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;They prefer to land the stage on land, if that's possible. This can only happen if there is sufficient margin to account for the extra fuel needed by the boostback burn. The Falcon 9 has some flexibility: the first stage cutoff point can be at speeds between 6000-9000 km/h (1670-2500 m/s). The speed chosen depends on the mission: missions to a high-energy orbit (GEO, heavy payloads) need a higher staging speed. &lt;/li&gt;&#xA;&lt;li&gt;At the high end of that speed range, there's not enough fuel left for a return to the launch site, so the second option is to land the stage on the barge.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Quoting &lt;a href=&quot;https://twitter.com/elonmusk/status/688842435955261440&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ship landings are needed for high velocity missions.&lt;br&gt;&#xA;  &lt;a href=&quot;https://twitter.com/elonmusk/status/688844190826942465&quot; rel=&quot;noreferrer&quot;&gt;Ship landings&lt;/a&gt; are not needed for flexibility or to save fuel costs.&lt;br&gt;&#xA;  Just not physically possible to return to launch site&lt;br&gt;&#xA;  &lt;a href=&quot;https://twitter.com/elonmusk/status/688845689023901696&quot; rel=&quot;noreferrer&quot;&gt;If speed&lt;/a&gt; at stage separation (higher than) ~6000 km/hr. With a ship, no need to zero out lateral velocity, so can stage at up to ~9000 km/h.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They also used the first two barge landings to test the landing process and build confidence they can land the stage accurately. This may have been a requirement imposed by Cape Canaveral, but I haven't seen evidence either way.&lt;br&gt;&#xA;The first landing on land (Falcon 9 Flight 20, Orbcomm-2) showed SpaceX can provide a safety margin for landings on land: after the reentry burn, the stage is on a trajectory towards a point just off the coast. If the landing burn fails (engine doesn't start), the stage will splash down harmlessly. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-01-19T12:33:41.457" LastActivityDate="2016-01-19T12:33:41.457" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13612" PostTypeId="2" ParentId="7753" CreationDate="2016-01-19T11:57:18.650" Score="6" Body="&lt;p&gt;As this question has resurfaced following the Jan-2016 launch of Jason-3 it is worth flagging &lt;a href=&quot;http://spacenews.com/spacex-tests-recovered-falcon-9-stage-and-prepares-for-next-launch/&quot;&gt;this&lt;/a&gt; news article that says, precisely, why a barge landing was selected &lt;em&gt;in this case&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It states &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Koenigsmann said SpaceX is doing the ship landing on this mission because it was not able to secure environmental permissions in time to permit a landing back at Vandenberg. “We could land back on land” were it not for the paperwork issue, he said. “We have enough energy on this mission to come back to land. It’s not anything technical.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The source of the quote, Hans Koenigsmann, is vice president of mission assurance at SpaceX.  &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-01-19T11:57:18.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13613" PostTypeId="1" CreationDate="2016-01-19T13:36:38.883" Score="4" ViewCount="1062" Body="&lt;p&gt;I'm trying to make a little application to draw ground station coverage circles (before anyone suggests using other software, I enjoy making these kinds of things myself!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I already have code that converts the lat and long coordinates of a point on the surface to points on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercator_projection&quot; rel=&quot;nofollow&quot;&gt;Mercator projection map&lt;/a&gt; so now I'm drawing the 'circles' around these.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also have the equation to calculate the slant range (maximum distance between the satellite and the ground station at minimum elevation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's what I do currently:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Convert lat, long and Re of ground stations (spherical coords) to x1, y1, z1 (Cartesian coords)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Convert elevation, azimuth and slant range (spherical coords) to x2, y2, z2 (Cartesian coords) in ground station centred reference frame&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Translate x2,y2,z2 from part 2 to Earth centred (x2+x1, y2+y1, z2+z1) lets call these x3, y3, z3&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Rotate x3, y3, z3 to align with earth centred reference frame lets say x4, y4, z4&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Convert x4, y4, z4 to spherical coordinates and then plot the lat and long that I've found.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;To my eye the above should give me the correct points to plot however there is an error somewhere. I know I step 1 is correct (the magnitude of the vector is Re). I know step 2 is correct since the magnitude of the vector is the slant range. But here's where I hit a problem.... No idea where the problem is though.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastEditorUserId="415" LastEditDate="2016-01-20T10:40:07.597" LastActivityDate="2016-01-20T10:40:07.597" Title="Drawing ground station coverage circles" Tags="|ground-station|ground-track|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13614" PostTypeId="1" AcceptedAnswerId="29974" CreationDate="2016-01-19T15:43:29.537" Score="3" ViewCount="292" Body="&lt;p&gt;NASA and other space exploration programs are funded by respective Governments as we know. Are funds also raised by publishing books on space exploration and related subjects?&lt;/p&gt;&#xA;" OwnerUserId="13094" LastEditorDisplayName="user20636" LastEditDate="2018-08-07T20:47:16.327" LastActivityDate="2018-08-07T20:47:16.327" Title="Are NASA Space Programs Government funded only or at times financed by publishing books as well?" Tags="|nasa|cost|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13615" PostTypeId="2" ParentId="13613" CreationDate="2016-01-19T17:05:07.343" Score="1" Body="&lt;p&gt;Here's how I would solve it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the math is easier if proper spherical coordinates $(r, \theta, \phi)$ are used instead of longitude and latitude are used. The difference is latitude is the angle from the equator, whereas $\phi$ is the angle from the north pole. To find $\theta$ and $\phi$ from the longitude and latitude,&#xA;$$\theta = \text{Long}$$&#xA;$$\phi = 90 - \text{Lat}$$&#xA;All angles are in degrees.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Convert the elevation and azimuth of the satellite into absolute angles $\theta$ and $\phi$. This will depend on both the position of the satellite in the sky, and the location of ground station.&#xA;$$\phi_\text{satellite} = \phi_\text{station} - (90-\text{Elev}) \cos(\frac{2 \pi}{360} \text{Azim})$$&#xA;$$\theta_\text{satellite} = \theta_\text{station} + (90-\text{Elev}) \sin(\frac{2 \pi}{360} \text{Azim})$$&lt;/li&gt;&#xA;&lt;li&gt;Convert the vector between the center of the earth and the ground station, and the vector between the station and the satellite to Cartesian coordinates&#xA;$$x = \text{Range} \cos(\frac{2 \pi}{360}\theta) \sin(\frac{2 \pi}{360}\phi)$$&#xA;$$y = \text{Range} \sin(\frac{2 \pi}{360}\theta) \sin(\frac{2 \pi}{360}\phi)$$&#xA;$$z = \text{Range} \cos(\frac{2 \pi}{360}\phi)$$&#xA;For vector from the center of the earth to the ground station, $\text{Range}$ is the radius of the earth.&lt;/li&gt;&#xA;&lt;li&gt;Add the two vectors together, convert back to spherical coordinates, and plot them.&#xA;$$r = \sqrt{x^2 + y^2 + z^2}$$&#xA;$$\theta = \frac{360}{2 \pi}\tan^{-1}(y/x)$$&#xA;$$\phi = \frac{360}{2 \pi}\cos^{-1}(z/r)$$&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Enjoy!&lt;/p&gt;&#xA;" OwnerUserId="13043" LastEditorUserId="13043" LastEditDate="2016-01-19T20:29:23.823" LastActivityDate="2016-01-19T20:29:23.823" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13616" PostTypeId="1" CreationDate="2016-01-19T18:51:16.070" Score="1" ViewCount="1188" Body="&lt;p&gt;Is it technically possible to build a ship that could keep a constant 1G acceleration / deceleration on a flight from Earth to Mars? What could provide energy? What could provide accelerant? (I am not asking if it would be affordable, though a guess at the cost would be fun to see )&lt;/p&gt;&#xA;" OwnerUserId="13106" LastActivityDate="2016-01-20T12:08:06.573" Title="What would be required to fly four astronauts at constant 1G to Mars?" Tags="|mars|crewed-spaceflight|advanced-propulsion|" AnswerCount="1" CommentCount="4" ClosedDate="2016-01-20T21:42:28.670" ContentLicense="CC BY-SA 3.0" />
  <row Id="13617" PostTypeId="2" ParentId="13616" CreationDate="2016-01-19T19:01:01.010" Score="1" Body="&lt;p&gt;There are simply two options: either you provide 1 g of acceleration on the spacecraft the entire flight (from thrust) or you use a rotating habitat to provide centrifugal acceleration. Applying 1 g of acceleration is probably not possible given the fact your trajectory will have to be planned to accommodate that (i.e., you will be expending a ridiculous amount of propellant to achieve this), so a rotating habitat would be the most feasible option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case, the energy required is minimal -- either just enough to overcome friction between the rotating and non-rotating components of the spacecraft, or essentially zero if the entire spacecraft itself rotates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But... Let's consider some ballpark estimates if you still insist on trying to achieve a 1 g continuous acceleration from thrust...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to do this with chemical rocket motors, you'll be looking at a manoeuvre that needs so much fuel my calculator simply says &quot;error&quot; (basically you'll need a $\Delta v$ of over 2500 km/s, assuming a 3 day flight).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to use ion thrusters then you'll still need to achieve the same $\Delta v$ but your specific impulse will be much higher (say 6,000 s instead of 350 s). This leads to a mass ratio of about 6E18, or rather 6 billion billion (so you will need over 6,000,000,000,000,000,000 kg of propellant for every 1 kg of empty mass). Not to mention the assumptions made here in terms of how you will power this and provide the thrust on that huge mass to get 1 g of acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that I'm approaching this question in terms of accessible technology. You could look into what kind of specific thrust would make this achievable for a more acceptable mass ratio, or some kind of external source of propulsion, but at that point you are making up propulsion technology that does not exist!&lt;/p&gt;&#xA;" OwnerUserId="12038" LastEditorUserId="12038" LastEditDate="2016-01-20T12:08:06.573" LastActivityDate="2016-01-20T12:08:06.573" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13618" PostTypeId="2" ParentId="13607" CreationDate="2016-01-19T21:50:13.940" Score="1" Body="&lt;p&gt;While not a bad idea, there are several problems with implementing this in reality. For starters, scale: balloons don't tend to move very fast, and Jupiter is &lt;em&gt;huge.&lt;/em&gt; Just from the top of the atmosphere to the first layer of clouds is nearly 1000 kilometers (10x Earth's atmospheric depth). As the balloon would dive deeper, other factors become a problem. Although Jupiter's atmosphere is not dense, there is a lot of it, meaning that temperature and pressure would quickly become a major concern for the balloon in question. The balloon might be able to take readings from the upper atmosphere, but would likely have significant trouble going deeper than the &lt;em&gt;Galileo&lt;/em&gt; probe already did (about 1000km). Providing the balloon &lt;em&gt;somehow&lt;/em&gt; survived this, it would eventually reach an environment that a balloon is completely unsuited for: liquid. The concept in the question that there is no solid land in a gas giant is not necessarily true. Deep within the planet's atmosphere, pressure builds and builds until the surrounding gas became liquid. There is no surface, but the pressure increases until it is defined as a liquid and not a gas. Theoretically, at lower depths, the liquid would eventually be under so much pressure as to become a solid, again without a surface. So while a balloon might not be an awful idea for exploring the &lt;em&gt;very&lt;/em&gt; top of a gas giant's atmosphere, a more traditional, heat and pressure shielded probe (such as the submarine in Ben Bova's &lt;em&gt;Jupiter&lt;/em&gt; and &lt;em&gt;Leviathans of Jupiter&lt;/em&gt;) would likely be more effective at exploring anything below the top thousand kilometers of the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="13033" LastActivityDate="2016-01-19T21:50:13.940" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13620" PostTypeId="1" CreationDate="2016-01-20T17:00:14.350" Score="8" ViewCount="1549" Body="&lt;p&gt;I am familiar with all different orbits but I struggle with the new launch injection method in GTO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the differences between sub-synchronous and super-synchronous transfer orbits compared to a traditional GTO?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have read that SpaceX and Proton are offering such tailored services; what are the benefits for the payload?&lt;/p&gt;&#xA;" OwnerUserId="13115" LastEditorUserId="6816" LastEditDate="2016-01-20T17:13:13.103" LastActivityDate="2016-01-20T20:51:25.673" Title="Differences between launch injection and transfer orbits for geostationary satellites" Tags="|launch|orbital-mechanics|orbital-maneuver|geostationary|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13621" PostTypeId="2" ParentId="13620" CreationDate="2016-01-20T17:39:28.007" Score="8" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Supersynchronous_orbit&quot;&gt;Supersynchronous orbits&lt;/a&gt; are sometimes used because of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination_change#Efficiency&quot;&gt;more efficient inclination change at high apogee&lt;/a&gt; compared to a typical Hohmann transfer orbit to GEO (aka &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_transfer_orbit&quot;&gt;GTO&lt;/a&gt;) with apogee at GEO altitude, before a GEO satellite is placed into its allocated orbital slot. This would be useful for launches from high latitude launch sites. Technically, the GEO disposal or &lt;a href=&quot;https://en.wikipedia.org/wiki/Graveyard_orbit&quot;&gt;graveyard orbit&lt;/a&gt; is also a supersynchronous orbit, since it has a longer orbital period than a &lt;a href=&quot;https://en.wikipedia.org/wiki/Synchronous_orbit&quot;&gt;synchronous orbit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Subsynchronous_orbit&quot;&gt;Subsynchronous orbits&lt;/a&gt; can be any transfer orbits to GEO that don't achieve apogee with a single &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot;&gt;Hohmann transfer&lt;/a&gt; impulse to GTO to later circularize the orbit with another, such as transfer orbits that slowly increase apogee with multiple perigee burns, using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot;&gt;Oberth effect&lt;/a&gt; to their advantage. This might be especially useful to &lt;em&gt;all electric&lt;/em&gt; geostationary satellites that use highly mass efficient but low thrust propulsion. They can also be used as intermediate orbits for deactivated but not decommissioned GEO satellites, or during change of allocated orbital slot with a so-called &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_phasing&quot;&gt;orbit phasing&lt;/a&gt; maneuver.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2016-01-20T17:39:28.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13622" PostTypeId="2" ParentId="13607" CreationDate="2016-01-20T19:43:32.800" Score="0" Body="&lt;p&gt;There is VICE (Venus In Situ Explorer) which is proposed mission to be launched around 2022 (if it gets approved of course). I think the two major approaches here are to use a balloon or airplane. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I seem to remember reading something about a similar mission being discussed to Saturn or Titan, but I can't seem to find any references.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2016-01-20T19:43:32.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13623" PostTypeId="2" ParentId="13620" CreationDate="2016-01-20T20:51:25.673" Score="4" Body="&lt;p&gt;I think Tildalwave's answer captures the essence. Here are some peripheral points specifically on the super-synchronous part: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Its worth clarifying that such an injection requires a minimum of two manoeuvres to get to GEO, the first to raise the perigee/remove the inclination and the second to drop the apogee. The idea is that the extra cost of the apogee reduction is more than offset by the saving on the inclination change. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;As a historical point, its not such a new method. My first memory of it was in connection with the Orion 1 / Telstar 11 launch in 1994, though it could easily have been applied earlier. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The common applied terminology &quot;super-synchronous&quot; in this context may seem confusing. By injection into an orbit with an apogee higher than GEO this new transfer orbit is called a &quot;super-synchronous transfer orbit&quot;. However it does not itself have to have an orbital period greater than 24 hours, it earns the &quot;super-synchronous&quot; label simply from having an apogee radius greater than GEO.  The term is slightly inconsistent with other orbit names because neither the transfer orbit itself nor the final destination are super-synchronous, just the intermediate point along the way. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-01-20T20:51:25.673" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13624" PostTypeId="1" AcceptedAnswerId="13628" CreationDate="2016-01-21T04:23:54.833" Score="4" ViewCount="268" Body="&lt;p&gt;When searching for signs of extra-terrestrial life, the key primary challenge as of now seems to be searching for signs of water on other planets. Presumably, this is because on Earth, water is necessary for all life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, what if we discovered organisms on another planet which exhibited respiration, reproduction, or even social interaction, in a similar way life on Earth does, but which has evolved without the need for water? We would still consider this a form of &quot;life&quot;, and certainly it would be a worthy find.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems perfectly possible that other substances besides water could come together to create something that we would perceive as &quot;life&quot;. This could either be through other substances which contain Hydrogen and Oxyen, or because there are other elements (including elements not yet discovered) which could combine in a way that appears to create &quot;life&quot;, just from a different direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Life&quot;, in a broad sense -- and certainly in the sense that extra-terrestrial scientists would be interested in -- is just a series of chemical reactions which happen to have allowed for reproduction / respiration etc.; but this has no relation to the existence of water. In fact, given the vastness of the universe, the vastness of the number of potential new chemicals and new chemical reactions out there, and the relative insignificance of Earth and its beloved water amongst all of this, it seems unlikely that if there is extra-terrestrial life, it would be water-based.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why, then, do we focus so much of our efforts on searching for extra-terrestrial water? Is it because it's a good starting point and we don't know what else to look for? Or is it because I am wrong in thinking that other forms of &quot;life&quot; could have evolved without water?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(This question relates only to the search for extra-terrestrial life, not for other uses of extra-terrestrial water, such as colonisation.)&lt;/p&gt;&#xA;" OwnerUserId="13118" LastEditorUserId="13118" LastEditDate="2016-01-21T04:29:12.520" LastActivityDate="2016-01-21T18:50:15.760" Title="Why do we search for extra-terrestrial water?" Tags="|astrobiology|astronomy|chemistry|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13625" PostTypeId="1" AcceptedAnswerId="13626" CreationDate="2016-01-21T04:36:58.813" Score="32" ViewCount="3443" Body="&lt;p&gt;A sounding rocket is a rocket that ascends into the high atmosphere in order to take all sorts of measurements so we can learn more about the upper atmosphere. They don't go into orbit, so they can be smaller than what we normally think of space rockets. However, they can still go above 100km fairly easily, and they still go very fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I want to know is, how can it accurately measure the ambient temperature at whatever altitude it's at? The rocket will be flying very fast, incurring a lot of heat from air friction.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2016-01-21T17:41:19.993" Title="How can a sounding rocket accurately take the temperature of the atmosphere while flying through it at high speed?" Tags="|temperature|measurement|science|sounding-rocket|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13626" PostTypeId="2" ParentId="13625" CreationDate="2016-01-21T06:11:26.177" Score="41" Body="&lt;p&gt;According to the &lt;a href=&quot;https://www.google.co.in/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=2&amp;amp;ved=0ahUKEwiGxKOanrrKAhWBKCYKHSkUCvoQFggpMAE&amp;amp;url=https%3A%2F%2Fwww.wmo.int%2Fpages%2Fprog%2Fwww%2FIMOP%2Fpublications%2FCIMO-Guide%2FEd2008Up2010%2FPart-II%2FWMO8_Ed2008_PartII_Ch6_Up2010_en.pdf&amp;amp;usg=AFQjCNHorkyEtJNGISsQtdKHTuL9Z93nGA&amp;amp;sig2=E7r6-LQc9o-IOXuNmsD0Xg&quot;&gt;link&lt;/a&gt; here,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In meteorological rockets, the temperature sensor is not measured as the rocket ascends but a payload containing the sensor is ejected from a high altitude and as it parachutes down measurements are beamed back.&lt;/p&gt;&#xA;" OwnerUserId="12467" LastActivityDate="2016-01-21T06:11:26.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13627" PostTypeId="2" ParentId="13624" CreationDate="2016-01-21T08:03:37.540" Score="5" Body="&lt;p&gt;Life depended on water is what we know. Until we find life not dependent on water we don't know what to look for. We have to act within our experience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cosmic exploration is expensive and the risks are high. To minimize the chances of failure in looking for life it's less risky to look for &quot;life as we know it&quot;.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2016-01-21T08:03:37.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13628" PostTypeId="2" ParentId="13624" CreationDate="2016-01-21T08:52:05.990" Score="5" Body="&lt;p&gt;Virtually all chemical reactions take place in a liquid or gaseous phase.  Uncontained gasses aren't a likely candidate for life, so we're looking for solvents for life-related reactions to take place in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good solvent is (a) common, (b) dissolves a wide variety of substances, and (c) is liquid at a wide range of temperatures.  Very few liquids meet even two of these criteria, much less all three.  The likely solvents for life are water and ammonia, and maybe methane.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2016-01-21T08:52:05.990" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13630" PostTypeId="1" AcceptedAnswerId="13634" CreationDate="2016-01-21T11:54:39.900" Score="19" ViewCount="1229" Body="&lt;p&gt;Since Pluto and Charon have distinct orbits around their barycenter (located approximately 960 km above Pluto's surface), the centripedal acceleration must be different on the outer hemisphere of Pluto (in relation to the barycenter).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does that mean I would weigh less standing on the top of the outer hemisphere than I would standing on top of the inner hemisphere (facing the barycenter)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is that cancelled out by the pull of Charon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wCIHl.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wCIHl.gif&quot; alt=&quot;Pluto-Charon System&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12215" LastEditorUserId="12215" LastEditDate="2016-01-22T07:01:21.347" LastActivityDate="2016-01-22T07:01:21.347" Title="Would my weight be constant on Pluto?" Tags="|gravity|pluto|charon|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13632" PostTypeId="1" CreationDate="2016-01-21T12:58:19.523" Score="12" ViewCount="9471" Body="&lt;p&gt;For circular orbits the most efficient transfer is achieved with a hohmann transfer or a bi-elliptic transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what is the most efficient way to transfer between elliptical orbits?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume in the general case this isn't easily solvable (correct me if I'm wrong) but for the reduced case where only one orbit is elliptical:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PE4WH.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PE4WH.png&quot; alt=&quot;Two orbits around a central mass and two possible transfer orbits between them&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would (intuitively) assume that the optimal transfer is either the red or the blue one but I don't know how to calculate this myself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Googling my question title I found &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/3.20909&quot;&gt;these&lt;/a&gt; &lt;a href=&quot;http://link.springer.com/chapter/10.1007%2F978-1-4615-2425-0_12&quot;&gt;two&lt;/a&gt; links which probably answer my question but aren't publicly accessible and &lt;a href=&quot;http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-07-dynamics-fall-2009/lecture-notes/MIT16_07F09_Lec17.pdf&quot;&gt;this paper&lt;/a&gt; though this only seems to talk about circular orbits.&lt;/p&gt;&#xA;" OwnerUserId="10488" LastActivityDate="2016-11-15T11:40:36.463" Title="Transfer between elliptical orbits" Tags="|orbital-mechanics|orbital-maneuver|delta-v|low-energy-transfer|" AnswerCount="5" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13633" PostTypeId="2" ParentId="13574" CreationDate="2016-01-21T13:47:45.637" Score="3" Body="&lt;p&gt;Air brakes would have to withstand very high temperatures, since they basically convert the kinetic energy (speed) into friction (heat).  So they would need to be big and expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket already has a side which is very heat-resistant (the engine side, with the exhaust nozzles).  So they can use that as a kind of very small air brake.  The grid fins are tiny air brakes as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since these don't provide enough braking power, they simply light the engines one more time to cancel out the biggest part of the kinetic energy.&lt;/p&gt;&#xA;" OwnerUserId="6033" LastActivityDate="2016-01-21T13:47:45.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13634" PostTypeId="2" ParentId="13630" CreationDate="2016-01-21T14:44:32.787" Score="20" Body="&lt;p&gt;Let's try and do some math with this. Will use &lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto&quot;&gt;Wikipedia&lt;/a&gt; for numbers.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Gravity at Pluto's surface: 0.61711215789 $m/s^2$&lt;/li&gt;&#xA;&lt;li&gt;Gravity from Charon- Pluto's near side- 0.0002724276 $m/s^2$&lt;/li&gt;&#xA;&lt;li&gt;Gravity from Charon- Pluto's far side- 0.00027242753 $m/s^2$&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://theory.uwinnipeg.ca/physics/circ/node6.html&quot;&gt;Centripetal Acceleration&lt;/a&gt;- Near side- 0.00001749536&#xA;$m/s^2$&lt;/li&gt;&#xA;&lt;li&gt;Centripetal Acceleration- Far side- 0.00006647414 $m/s^2$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For reference, the formulas to calculate these are as follows:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Gravity- $a_g=G* m/d^2$ where $G=6.673×10^{-11} N m^2 kg^{-2}$. As $F=m*a$, this is basically the same as the gravitational equation, assuming a much smaller body than the primary mass.&lt;/li&gt;&#xA;&lt;li&gt;Centripetal- $a_c=v_t^2/r$. $v_t=2*\pi*r_{orbit}/orbital Period$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line, Charon's gravity has more of an effect than the Centripetal acceleration, but both are at least 3 orders of magnitude smaller than the base gravity. This is less than the &lt;a href=&quot;http://curious.astro.cornell.edu/about-us/42-our-solar-system/the-earth/gravity/94-does-your-weight-change-between-the-poles-and-the-equator-intermediate&quot;&gt;difference of weight in a person&lt;/a&gt; from the equator to the poles.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-01-21T23:19:02.577" LastActivityDate="2016-01-21T23:19:02.577" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="13635" PostTypeId="2" ParentId="13632" CreationDate="2016-01-21T14:51:47.510" Score="2" Body="&lt;p&gt;I don't know the answer for certain, but am willing to hazard a guess that the most efficient way of transferring between a circular orbit and an elliptical orbit, assuming for a moment that they are coplanar, &lt;strong&gt;remains a single Hohmann transfer.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's why: Nominally, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Hohmann transfer&lt;/a&gt; takes you from one circular orbit to another. For every point on an elliptical orbit, there exists a circular orbit which intersects that point with the body moving in the same direction (though not at the same velocity). A Hohmann transfer works by first turning your orbit from circular to elliptical, &lt;em&gt;then back into a circular orbit.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Changing the eccentricity of an orbit requires a burn delivering the proper velocity change (including the burn attitude) at the appropriate point in the orbit. This is done at the end of a traditional Hohmann transfer orbit to re-circularize the orbit at the desired distance from the barycenter, which becomes our new orbital radius. Once our orbit is circular, we could, in principle, apply another burn to change to a different elliptical orbit with the semi-major axis being the circular orbit's radius. By allowing the time between the burns to go to 0 and recalculating the second de-circularization burn based on the new point in the orbit at which it happens, we can turn these two burns into a single burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, by tuning the Hohmann re-circularization impulse, it should be possible to transform your Hohmann transfer orbit directly into an elliptical orbit of the desired shape, while maintaining the Hohmann transfer's (delta-v) efficiency in getting between the two corresponding circular orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It follows from the same line of reasoning that this should work equally well whether none, either or both of the endpoint orbits involved are circular or elliptic. (If the origin orbit is elliptical, you simply, at some point during the first burn, are instantaneously in a circular orbit.)&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2016-01-21T14:51:47.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13636" PostTypeId="2" ParentId="2294" CreationDate="2016-01-21T15:05:47.100" Score="2" Body="&lt;p&gt;Lift = Cl*0.5*(density)*(velocity)^2*Area. There are three variables that can be optimized to achieve the proper lift to remain in flight in Jupiter; density, velocity, and wing area. Increasing wing area adds weight and makes the vehicle more difficult to launch from earth. Maintaining a high velocity will require higher thrust and would expend more oxidizer on board, and add more drag. Finally flying in a lower altitude with higher density of Hydrogen will add more drag on the vehicle but will save on power requirements for compression before combustion. There is no true answer except for the optimization of all parameters. There's also design constraints of nozzle area ratios and operating pressures. As you increase the back pressure on the nozzle (at lower altitudes), it's more difficult to achieve fully expanded flow, since shocks may develop at the nozzle exit (assuming C-D nozzle)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The game changer for a mission like this would be to fly in a specific cloud type with a naturally high composition of oxydizer and engineer a way to extract it on board. Or, find a different power source all together (solar, thermal, radiation).&lt;/p&gt;&#xA;" OwnerUserId="13124" LastActivityDate="2016-01-21T15:05:47.100" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13637" PostTypeId="1" AcceptedAnswerId="13646" CreationDate="2016-01-21T16:01:24.337" Score="10" ViewCount="818" Body="&lt;p&gt;I heard the other day that to take cargo from Earth to a Lagrange point or the Moon using solar electric propulsion would be a problem because the Van Allen radiation belts would degrade the solar panels excessively. How long would the ship be in the radiation belts, how quickly would this occur and what specifically would be the kind of damage done?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Geoff Landis mentioned this during his &lt;a href=&quot;http://www.thespaceshow.com/show/18-jan-2016/broadcast-2626&quot;&gt;interview Monday&lt;/a&gt; on The Space Show.)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2022-04-24T12:49:14.997" Title="How quickly do the Van Allen radiation belts degrade solar panels?" Tags="|the-moon|radiation|solar-power|ion-thruster|van-allen-belt|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13639" PostTypeId="2" ParentId="13632" CreationDate="2016-01-21T16:51:08.473" Score="5" Body="&lt;p&gt;I can give you an upper bound for the necessary $\Delta v$, between all elliptical orbits, regardless of inclination. You can always do a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot; rel=&quot;noreferrer&quot;&gt;bi-elliptical transfer&lt;/a&gt;, performed by almost reaching escape velocity, then do a close to 0 $\Delta v$ manoeuvre at infinity, and then fall back to insert into the target orbit. Both the large burns are done at periapse. You can then always do the transfer in:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\sqrt{\frac{2\mu}{r_{p1}}}-\sqrt{\mu\left(\frac{2}{r_{p1}}-\frac{2}{r_{a1}+r_{p1}}\right)}+\sqrt{\frac{2\mu}{r_{p2}}}-\sqrt{\mu\left(\frac{2}{r_{p2}}-\frac{2}{r_{a2}+r_{p2}}\right)}$$&#xA;or better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $r_a$ and $r_p$ are radius of apoapse and periapse for the two orbits, and $\mu$ is the gravitational parameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, the red one is the most efficient one on your illustration. If the orbits are coplanar and one is circular, your manoeuvre can be performed in:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\left|\sqrt{\mu\left(\frac{2}{r_{a2}}-\frac{2}{r_{a2}+r_{p2}}\right)}-\sqrt{\mu\left(\frac{2}{r_{a2}}-\frac{2}{r_{a2}+r_{1}}\right)}\right|+\left|\sqrt{\mu\left(\frac{2}{r_{1}}-\frac{2}{r_{a2}+r_{1}}\right)}-\sqrt{\frac{\mu}{r_{1}}}\right|$$&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-22T07:34:23.337" LastActivityDate="2016-01-22T07:34:23.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13640" PostTypeId="2" ParentId="13625" CreationDate="2016-01-21T17:41:19.993" Score="12" Body="&lt;p&gt;&lt;a href=&quot;https://aviation.stackexchange.com/a/1642/11953&quot;&gt;Here is an answer from over on aviation.stackexchange.com&lt;/a&gt; which addresses this issue. I've quoted a portion of it below, but see that link for more info.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As you measure temperature moving at high velocities, your outside&#xA;  thermometer will measure a higher temp than what is actually outside&#xA;  (what a non-moving thermometer would get). That's because as the air&#xA;  rams into your thermometer it gets a little bit compressed, and that&#xA;  makes it heat up a little bit. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Amazingly, some smart people have even calculated how much that &quot;ram&#xA;  rise&quot; is, and you can actually compensate (well, in theory) from the&#xA;  indicated temperature to calculate what the actual outside air temp&#xA;  is: &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$$Ram~Rise=SAT\times0.2\times{M}^2$$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to Mathav Raj's answer, temperature isn't measure during ascent, but this explains how it could be done.&lt;/p&gt;&#xA;" OwnerUserId="12742" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:54.153" LastActivityDate="2016-01-21T17:41:19.993" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13641" PostTypeId="2" ParentId="13637" CreationDate="2016-01-21T17:57:15.993" Score="4" Body="&lt;p&gt;It turns out that &lt;a href=&quot;https://ieeexplore.ieee.org/document/4323248&quot; rel=&quot;nofollow noreferrer&quot;&gt;IEEE Xplore Nuclear Science&lt;/a&gt; published a paper on the subject of solar cell degradation in the Van Allen belts. While I can't read the paper, it mentions that N-P solar cells (The most common) are what is susceptible to any damage by being in this region in the abstract. This paper was published in 1963. In any case, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt#Inner_belt&quot; rel=&quot;nofollow noreferrer&quot;&gt;most damaging part of the belt&lt;/a&gt; is the inner belt, which ranges from 600 to 3700 miles, which, correctly, a satellite going from LEO to Lunar orbit would spend significant time in.&lt;/p&gt;&#xA;&lt;p&gt;In fact, there are spacecraft that fly in this region. The most high profile is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_Probes#Spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;Van Allen Probes&lt;/a&gt;, which were intended to study the region. I can't actually find anything about the solar panels being a concern.&lt;/p&gt;&#xA;&lt;p&gt;My thoughts is that this would degrade the overall lifetime of the panels, but wouldn't significantly affect the capacity. At least 2 years of operating in the worst part of the Van Allen belts is possible, and I believe this would work for some period of time, as only a relatively small portion of the mission would be in the belts. A single cargo mission? Well, there's been plenty of missions that have gone through the belts using ion drives, I don't think that's even a major concern.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="10273" LastEditDate="2022-04-24T12:49:14.997" LastActivityDate="2022-04-24T12:49:14.997" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="13642" PostTypeId="2" ParentId="5225" CreationDate="2016-01-21T18:01:41.810" Score="1" Body="&lt;p&gt;We should also consider two things:&#xA;First, the computer on the Voyager probes is an ancient, CPU-less system with a bunch of kilobytes of core memory (actually sheet memory, a type of core memory) and the processing speed of a 8-bit home computer. It would be unable to react autonomously. Second, the distance to earth is a bunch of light-hours, meaning that even if the cameras were powered, we would see any object hours after the probe saw it. A command to avoid the object would then take the same amount of hours to reach the probe. Furthermore, as the baud rate at these distances is very low, the probe can deliver images with a framerate of maybe an image per hour or less. So any object colliding with it would have to be big enough to be seen at least some hours before or the probe wouldn't have time to transfer the image. Won't happen though, there's almost nothing out there.&lt;/p&gt;&#xA;" OwnerUserId="13128" LastEditorUserId="6816" LastEditDate="2016-01-21T18:14:05.710" LastActivityDate="2016-01-21T18:14:05.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13643" PostTypeId="2" ParentId="13632" CreationDate="2016-01-21T18:23:41.117" Score="8" Body="&lt;p&gt;It is my belief that a bitangential transfer between two coplanar orbits is best. For such a transfer, no direction change is needed at departure or arrival since velocity vectors are parallel at these two points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k4gGL.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k4gGL.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The red orbit pictured above is tangent to the circular departure orbit as well as the elliptical destination orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a picture of many possible bitangential transfer orbits:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pHwJ7.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pHwJ7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As with the earlier illustration, possible transfer orbits are colored, departure and destination orbits black.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For all these, the departure and arrival &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot;&gt;Vinfinity&lt;/a&gt; sum to very close to the same quantity (if my arithmetic's correct).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if the circular departure orbit is earth's and the elliptical destination orbit is an asteroid's, the longest transfer is the best. This makes rendezvous at the asteroid's aphelion. At this point the departure Vinf is at a maximum and arrival Vinf is at a minimum. But the big departure Vinf is largely mitigated by the &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/10/what-about-mr-oberth.html&quot;&gt;Oberth benefit&lt;/a&gt; since earth has a deep gravity well. So of the two orbits you illustrate, the red one is less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I write about these sorts of transfers at &lt;a href=&quot;http://clowder.net/hop/railroad/TangentEllipses.pdf&quot;&gt;Tangent Ellipses&lt;/a&gt;. I made a spreadsheet looking at these orbits. A caveat: The spreadsheet is very complicated and no one has reviewed it. I am not confident it's error free. With that disclaimer out of the way, &lt;a href=&quot;http://clowder.net/hop/railroad/MarsTanEll.xls&quot;&gt;here's&lt;/a&gt; a link to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may be interested in this often cited paper by Shoemaker and Helin: &lt;a href=&quot;http://clowder.net/hop/railroad/Shoemaker_Helin_1978.pdf&quot;&gt;Earth-Approaching Asteroids as Targets for Exploration&lt;/a&gt;. The delta Vs they list assume a bitangential transfer with rendezvous at the asteroid's aphelion.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-01-21T18:23:41.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13644" PostTypeId="2" ParentId="13624" CreationDate="2016-01-21T18:50:15.760" Score="4" Body="&lt;p&gt;The other two answers give one basic, primary reason: If we're looking for signs of life, water is assumed to be an essential building block and a good hint the place has any chance to host life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there are two secondary concerns:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;if we ever plan manned exploration, water will be one of the scarce, expensive resources needed to sustain human life. Lifting it off Earth is very costly. Any bases/stations/etc would be much easier to sustain with local water source.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;water is hydrogen and oxygen, and that means rocket fuel. In more distant perspective, water-rich destinations are possible locations where the craft could refuel. We're still pretty far from that point, but having a good list of the places already won't hurt.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-01-21T18:50:15.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13645" PostTypeId="1" AcceptedAnswerId="13647" CreationDate="2016-01-21T21:43:30.310" Score="2" ViewCount="535" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist#Explanation&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article on Gravity assist&lt;/a&gt; has this terrestrial analogy:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Imagine standing on a train platform, and throwing a tennis ball at 30&#xA;  km/h toward a train approaching at 50 km/h. The driver of the train&#xA;  sees the ball approaching at 80 km/h and then departing at 80 km/h&#xA;  after the ball bounces elastically off the front of the train. Because&#xA;  of the train's motion, however, that departure is at 130 km/h relative&#xA;  to the train platform; the ball has added twice the train's velocity&#xA;  to its own.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have two problems with this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The train's speed relative to the platform is 50 km/h. The ball, traveling in the opposite direction relative to the same platform, is going at 30 km/h. Thus, the train operator will indeed see the ball at&lt;/p&gt;&#xA;&#xA;&lt;p&gt;50 + 30 = 80 km/h.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far, so good. However, once the ball has hit the front of the train (let's assume that no speed at all is lost in the process), it'll have the train's velocity (50 km/h) added to its own (30 km/h) relative to the platform:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;50 + 30 = 80 km/h.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the train's velocity relative to the platform is 50 km/h, and the ball's now 80 km/h, the operator will see it departing from him at&lt;/p&gt;&#xA;&#xA;&lt;p&gt;80 - 50 = 30 km/h.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As already mentioned above, the ball's velocity relative to the platform after the bounce will be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The train's velocity (50 km/h) + the ball's initial velocity (30 km/h) = 80 km/h, and &lt;strong&gt;&lt;em&gt;not&lt;/em&gt;&lt;/strong&gt; twice the train's velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What am I missing?&lt;/p&gt;&#xA;" OwnerUserId="12267" LastEditorUserId="49" LastEditDate="2016-01-22T00:21:24.700" LastActivityDate="2016-01-22T00:21:24.700" Title="Gravity assist and the terrestrial analogy: why does the velocity double?" Tags="|gravity-assist|physics|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13646" PostTypeId="2" ParentId="13637" CreationDate="2016-01-21T22:13:29.963" Score="8" Body="&lt;p&gt;This response addresses the generic degradation of solar arrays passing through Van Allen belts, rather than those specifically bound for the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its a topic that has stirred up a lot of interest in the last few years as Boeing, a manufacturer of commercial communications satellites, announced a launch of two satellites designed to be deployed in GTO and transferred to GEO with electric propulsion. They (Eutelsat 115 West B and ABS 3A) were launched in 2015 and are now on station. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Prior to this I am only aware of two other satellites having made such a transfer with thrusters that were not intended for that part of the mission. One was Artemis and the other was AEHF-1, and this was in both cases because of earlier failures that would otherwise have prevented deployment in GEO. I believe that all other satellites to date employing ion thrusters or hall effect thrusters have done so with their primary use being for station-keeping once delivered to GEO and they have not spent a long time in transit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this recent paper is relevant: &lt;a href=&quot;http://nora.nerc.ac.uk/510795/1/Horne_et_al-2015-Space_Weather.pdf&quot; rel=&quot;noreferrer&quot;&gt;Space Weather Concerns for All-Electric&#xA;Propulsion Satellites&lt;/a&gt; from Ref 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have extracted the following figure from it:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/5SDIE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5SDIE.png&quot; alt=&quot;Figure 1 from Ref 1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[NB &quot;EOR&quot; refers to &quot;Electric Orbit Raising&quot;]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites are designed with shielding against specifications which in turn depend upon a good picture of the environment. Solar arrays can, to degree, be protected by thicker cover-glasses, hence the 6.7 year part of the caption &lt;strong&gt;does not apply to solar arrays&lt;/strong&gt; other than to show the concept. I believe the Van Allen probes, mentioned in PearsonArtPhoto's response will have featured allowances for uncertainties in the very phenomenon they are there to measure.  In the case of geostationary communications satellites which are usually relatively power-hungry compared to science satellites, the effect of mis-judging the uncertainties would be felt later in the mission as the available power dropped. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, referring to a part of the &lt;a href=&quot;http://www.thespaceshow.com/show/18-jan-2016/broadcast-2626&quot; rel=&quot;noreferrer&quot;&gt;Dr. Geoffrey Landis article&lt;/a&gt; referred to in the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;He noted the slow speed used by this form of propulsion to transit the Van Allen radiation belts saying that for HSF, one must go through the belts much faster.  He said nuclear propulsion was needed, mentioned the fear factor that several have but that we should develop and implement it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;His solution may be consistent with some lunar mission scenarios though there isn't enough context given to conclude that it is &quot;needed&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ref 1. Horne, R. B., and D. Pitchford (2015),&#xA;Space Weather Concerns for All-Electric&#xA;Propulsion Satellites,&#xA;Space Weather&#xA;,&#xA;13&#xA;,&#xA;430&#xA;–&#xA;433, doi:10.1002/2015SW001198&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-01-21T22:13:29.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13647" PostTypeId="2" ParentId="13645" CreationDate="2016-01-21T22:25:14.647" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;However, once the ball has hit the front of the train (let's assume&#xA;  that no speed at all is lost in the process), it'll have the train's&#xA;  velocity (50 km/h) added to its own (30 km/h) relative of the platform&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, the platform is not participating in this collision.  You could have just as well asked what the speeds were relative to a faster train on another track and gotten yet another answer.  That frame of reference would be just as relevant as the frame of reference of the platform, which is not relevant at all, and the resulting answer would be equally wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only things participating are the front of the train and the ball.  So all that matters is the speed of the front of the train relative to the ball, or the speed of the ball relative to the front of the train.  That's 80 km/hr.  Since the collision has been stated to be elastic (which we will take to be 100% elastic), and since the train weighs much, much more than the ball, then the train won't change speed at all (ok, it will change by the ratio of the mass of the ball to the mass of the train, out in the ninth decimal place), whereas the ball will depart the front of the train at 80 km/hr in the opposite direction, relative to the front of the train.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest follows.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-01-21T22:25:14.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13648" PostTypeId="2" ParentId="13645" CreationDate="2016-01-21T22:26:16.700" Score="4" Body="&lt;p&gt;In your step two, you're considering the speed relative to the station for an interaction that doesn't involve the station any more. The ball travels towards the train at 80 km/h, and it will bounce off it at 80 km/h with respect to the train just like a ball that drops towards the floor at 80 km/h rebounds at 80 km/h (in a fully &lt;a href=&quot;https://en.wikipedia.org/wiki/Elastic_collision&quot; rel=&quot;nofollow noreferrer&quot;&gt;elastic collision&lt;/a&gt; at least, but that's the assumption of the analogy in your question). That is then 130 km/h with respect to the station, assuming a head-on collision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://solarsystem.nasa.gov/basics/grav/primer.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Gravity Assist Primer&lt;/a&gt; for additional explanation and where that analogy likely originates from (just in mph instead of km/h):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/lrXL9.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lrXL9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image credit: Charles Kohlhase (Concept) &amp;amp; Gary Hovland (Artwork). Source: &lt;a href=&quot;http://solarsystem.nasa.gov/basics/grav/primer.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA/JPL&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TL;DR is that in the reference frame of the gravity assisting body, the assisted body neither gains nor loses any speed. That is the same for the ball with respect to the train in your analogy.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-01-21T22:31:24.297" LastActivityDate="2016-01-21T22:31:24.297" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13650" PostTypeId="1" CreationDate="2016-01-22T01:37:31.360" Score="4" ViewCount="604" Body="&lt;p&gt;I am looking for a mission profile including the delta-v for burns for an interplanetary mission to Mars. I have been unable to find any free return trajectories between 2018 and 2024, are there any?&lt;/p&gt;&#xA;" OwnerUserId="13132" LastEditorUserId="49" LastEditDate="2016-01-22T02:08:14.613" LastActivityDate="2016-01-22T02:08:14.613" Title="Are there any free return trajectories for a manned Mars flyby mission between 2018 and 2024?" Tags="|mars|trajectory|interplanetary|flyby|free-return|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13653" PostTypeId="1" AcceptedAnswerId="13654" CreationDate="2016-01-22T06:10:43.410" Score="8" ViewCount="384" Body="&lt;p&gt;Is it possible to have a stable orbit comprised of many moons(&gt;2) that keep themselves roughly equal distance apart?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sort of like Epimetheus and Janus around Saturn but comprised of many moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea being that an image sort of like this would be able to be seen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FVmbu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FVmbu.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3277" LastEditorUserId="238" LastEditDate="2016-01-23T12:04:09.233" LastActivityDate="2019-05-12T22:48:40.800" Title="Many moons in same orbit" Tags="|orbital-mechanics|moons|n-body-problem|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13654" PostTypeId="2" ParentId="13653" CreationDate="2016-01-22T07:24:39.383" Score="17" Body="&lt;p&gt;You can have small moons in the two stable (L4 and L5) &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lagrangian points&lt;/a&gt; of a main moon, like the Saturn moon &lt;a href=&quot;https://en.wikipedia.org/wiki/Dione_%28moon%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dione's&lt;/a&gt; companions Helene and Polydeuces, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Tethys_%28moon%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tethys'&lt;/a&gt; Telesto and Calypso.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want more than three moons, you can organize them in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Klemperer_rosette&quot; rel=&quot;nofollow noreferrer&quot;&gt;Klemperer rosette&lt;/a&gt;, although this type of configuration is not stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another option is to have a huge number of small fragments orbiting inside the&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Roche_limit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Roche limit&lt;/a&gt;, preventing them from forming a larger object. In a way, the rings of Saturn can be thought of like many small moons orbiting in the same orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p0ab3t.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p0ab3t.jpg&quot; alt=&quot;rings of Saturn&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="28871" LastEditDate="2019-05-12T22:48:40.800" LastActivityDate="2019-05-12T22:48:40.800" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="13657" PostTypeId="1" AcceptedAnswerId="13658" CreationDate="2016-01-22T21:22:54.137" Score="27" ViewCount="16536" Body="&lt;p&gt;I am aware that the Saturn V is the largest rocket ever built and was capable of lifting more weight into space than anything that exists today. Modern rockets often carry several satellites up at once that together make up its full payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I am wondering is: &lt;strong&gt;what is the largest single object ever lifted into space and what is its weight&lt;/strong&gt;. Let's assume for this question that the LEM/CM or similar combinations are single multistage vehicles. Is the LEM/CM the largest yet, or is there something else?&lt;/p&gt;&#xA;" OwnerUserId="6392" LastActivityDate="2019-12-14T11:12:35.247" Title="What's the largest single object payload ever lifted into space?" Tags="|rockets|payload|record|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13658" PostTypeId="2" ParentId="13657" CreationDate="2016-01-22T21:26:10.673" Score="36" Body="&lt;p&gt;Depending on your definitions, the contenders seem to be the US Space Shuttle, Buran, Apollo 17, or Skylab.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Apollo 17 + S-IVB translunar     143 t?&#xA;STS, maximum payload             115 t&#xA;Discovery STS-82                 106 t&#xA;STS, no payload                   90 t&#xA;Buran + payload                   87 t?&#xA;Polyus                            80 t?&#xA;Skylab                            77 t&#xA;Buran, no payload                 75 t? &#xA;Apollo 17 CSM/LM                  47 t&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot; rel=&quot;noreferrer&quot;&gt;Skylab&lt;/a&gt; was 77 tons, launched as a single payload on a two-stage version of a Saturn V (in fact, the station was converted from a Saturn V third stage). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot; rel=&quot;noreferrer&quot;&gt;Apollo CM/LM&lt;/a&gt; stack was under 45 tons, but for the lunar missions, in low Earth orbit it remained attached to the Saturn's S-IVB third stage, which provided the fuel and engine for the translunar injection burn. The S-IVB burned a small amount of its fuel to get into LEO, but that left the combined translunar spacecraft massing over 115 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollos grew slightly in weight over the course of the program, as the launcher engines were uprated, confidence in safety margins improved, and more equipment was brought along. &lt;a href=&quot;http://georgetyson.com/files/apollostatistics.pdf&quot; rel=&quot;noreferrer&quot;&gt;Apollo 16 and 17 were the heaviest, with 16 weighing a little more at launch, and 17 a little more in LEO (having used a bit less fuel getting there).&lt;/a&gt; Adding up the launch weights of components listed in &lt;em&gt;Apollo By The Numbers&lt;/em&gt;, then subtracting the LES and the fuel used in the initial third-stage burn suggests an orbital mass over 140 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_orbiter#Shuttle_Orbiter_Specifications_.28OV-105.29&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle orbiter&lt;/a&gt; masses over 68 tons dry, and carries a payload of up to 25 tons; its maximum takeoff weight ranged as high as 115 tons. I found a source giving &lt;a href=&quot;https://books.google.com/books?id=6aO9aGNBAgIC&amp;amp;pg=PA52&amp;amp;lpg=PA52&amp;amp;dq=space%20shuttle%20orbiter%20mass%20LEO&amp;amp;source=bl&amp;amp;ots=1EUAALnVAM&amp;amp;sig=ukASXC86LR5d5qD2cjap0GvuPPY&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjtyNf1xr7KAhVBF2MKHXScApYQ6AEIWTAN#v=onepage&amp;amp;q=space%20shuttle%20orbiter%20mass%20LEO&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Discovery's orbital mass as 106 tons&lt;/a&gt; for the STS-82 mission. As with the S-IVB, the orbiter is ambiguously part launcher and part payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soviet &lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot; rel=&quot;noreferrer&quot;&gt;Energia&lt;/a&gt; booster was second to the Saturn V for payload-to-LEO, with approximately 100-ton capability. It flew twice: once with the Polyus payload described in Puffin's answer, once with the Buran spaceplane. Astronautix manages to give two very different weight figures for &lt;a href=&quot;http://www.astronautix.com/craft/buran.htm&quot; rel=&quot;noreferrer&quot;&gt;Buran&lt;/a&gt; on a single page. It's either a little lighter or a little heavier than STS. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-11-25T19:02:12.907" LastActivityDate="2019-11-25T19:02:12.907" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="13660" PostTypeId="2" ParentId="13657" CreationDate="2016-01-22T22:27:03.483" Score="14" Body="&lt;p&gt;Following the theme of &quot;depending on your definitions&quot; one could also consider the &lt;a href=&quot;https://en.wikipedia.org/wiki/Polyus_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Polyus&lt;/a&gt; spacecraft launched in 1987. The mass of the spacecraft was 80 tonnes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thoughts about definitions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;slightly heavier then Skylab, less than the SIVB/LM/CSM stack&lt;/li&gt;&#xA;&lt;li&gt;it didn't get to a stable orbit, perhaps to &lt;a href=&quot;http://www.buran-energia.com/polious/polious-desc.php&quot; rel=&quot;noreferrer&quot;&gt;155km&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Energia vehicle functioned correctly and the Polyus separated correctly. However the mission plan called for the Polyus to complete the final orbit insertion itself, which didn't happen due to a fault in Polyus itself and it re-entered into the South Pacific.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FMaf3.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FMaf3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image is from &lt;a href=&quot;http://www.buran-energia.com/polious/polious-photos.php?file_dbt_=1&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-01-22T22:27:03.483" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13661" PostTypeId="1" CreationDate="2016-01-23T11:34:41.987" Score="6" ViewCount="308" Body="&lt;p&gt;What is the working concept for dispersal of a constellation of cubesats launched together? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that for the Flock satellites to be useful in providing frequent Earth observation revisit opportunities that they should all be in different orbits, whether spaced out within the same plane or in different planes altogether. EDIT: This spacing appears to me to be crucial to their business objectives, especially given the short orbital lifetimes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two working examples I'm aware of are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Flock 1C deployment of 11 satellites from a Dnepr in June 2014&lt;/li&gt;&#xA;&lt;li&gt;all other Flock deployments which I understand have been from the ISS&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As there is no propulsion I can't see why all the released satellites won't simply accompany the dispenser (i.e. Dnepr upper stage or ISS) around in very slowly expanding cloud. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Is a substantial delta-V imparted by the dispenser release? (EDIT: 2m/s, thanks to the answer from PearsonArtPhoto)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) How is deployment managed (within one launch batch) in terms of release timing so as to achieve satellite spacing?  EDIT: What outcome in terms of orbital spacing was actually targeted and achieved, and by what means - is the dispenser's angle adjusted on the ISS or is the main control achieved by release at different times? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Background: I've been looking at Flock TLEs. There has been some dispersion though they are still largely grouped according to their delivery to the ISS. Having an idea of the deployment strategy as per this question is simply a piece of the puzzle.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-01-23T12:15:42.650" LastActivityDate="2016-01-23T12:15:42.650" Title="Cubesat dispenser operation for fleet deployment" Tags="|launch|orbital-mechanics|cubesat|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13662" PostTypeId="2" ParentId="13661" CreationDate="2016-01-23T11:49:00.147" Score="4" Body="&lt;p&gt;The most common Cubesat deployment mechanism is called a &lt;a href=&quot;http://www.cubesat.org/index.php/documents/launch-providers&quot; rel=&quot;nofollow noreferrer&quot;&gt;P-POD&lt;/a&gt;. The release velocity is up to 2 m/s, which is actually pretty substantial. The main goal is the launch them fast enough where re-contact with the primary launcher is unlikely. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to how they are deployed, it is the &lt;a href=&quot;https://space.stackexchange.com/a/13196/25&quot;&gt;same as for any satellite&lt;/a&gt;. Change the angle, the exit velocity, and you'll get different deployments, sufficient to protect against re-contact with either the launcher or the other satellites. The analysis is done on the ground to prevent that. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-01-23T11:49:00.147" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13663" PostTypeId="1" CreationDate="2016-01-23T15:34:50.580" Score="12" ViewCount="833" Body="&lt;p&gt;I assume that if everyone left the ISS, there is a method that can be used to put people back on it.   For example to open the “door” from the outside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However how long could the ISS remain functioning without anyone on it?&lt;/p&gt;&#xA;" OwnerUserId="12645" LastActivityDate="2016-07-19T11:57:32.277" Title="If all people come home from the ISS for how long would it remain usable?" Tags="|iss|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13664" PostTypeId="2" ParentId="12874" CreationDate="2016-01-23T18:25:27.667" Score="6" Body="&lt;p&gt;I think your question boils down to weather or not people associated with the design, fabrication or operation of Inmarsat 2F1 refer to the L-band transmit antenna specifically as a &amp;quot;phased array&amp;quot;. I think we can all agree that the phases are specified and managed. Further, based on the article below (and others), I think they are designed to be fixed once the satellite is deployed. But I think the question is asking &amp;quot;&lt;em&gt;Is this &lt;strong&gt;array antenna&lt;/strong&gt; called a &lt;strong&gt;phased array antenna&lt;/strong&gt; by those associated with the satellite?&lt;/em&gt;&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Since you are interested in a timely answer, I'm going to do a partial post right now. The amplitude sketch in the article's Figure 7 is actually not 100% correct. I think it's just an example. You'd use something like that (an Airy pattern) at the focal point of a reflector if you wanted to produce a flat beam. I'll see if I can reverse engineer a better hypothetical phase profile.&lt;/p&gt;&#xA;&lt;p&gt;I see terms like &amp;quot;&lt;em&gt;Beam Forming Network&lt;/em&gt;&amp;quot; and &amp;quot;&lt;em&gt;Shaped Coverage Earth Beam&lt;/em&gt;&amp;quot;, but not &lt;em&gt;&amp;quot;phased array&amp;quot;&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Boeing seems to be knee-deep in satellite shaped beam patents, as are other companies, but trying to go through all of that is a serious time sink.&lt;/p&gt;&#xA;&lt;p&gt;Anyway, I found &lt;a href=&quot;http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&amp;amp;arnumber=82422&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;, and one more below for specifics:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Gambaruto, E.; Banks, D.K.; Krinsky, B., &amp;quot;&lt;em&gt;INMARSAT second generation satellites for mobile communications&lt;/em&gt;&amp;quot; in Aerospace Applications Conference, 1989. Digest., 1989 IEEE , vol., no., pp.13 pp.-, 12-17 Feb. 1989&#xA;doi: 10.1109/AERO.1989.82422&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I'll include snippets here. Most university libraries and many public libraries can obtain full IEEE journal articles for you.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/svzRE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/svzRE.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V79uL.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V79uL.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/87Rdc.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/87Rdc.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yK6fs.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yK6fs.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RyLrz.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RyLrz.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/thxwz.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/thxwz.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8WUOv.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8WUOv.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XrI1m.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XrI1m.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;What is a Turnstile Cup Dipole?&lt;/strong&gt; I'm glad you asked! The crossed dipole resembles a turnstile, and they are driven 90° out of phase to obtain circular polarization. There is a nice description of their advantages. Along with two figues and a paragraph, I've included the front pages from this &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/769907.pdf&quot; rel=&quot;noreferrer&quot;&gt;1973 MIT report&lt;/a&gt; at dtic.mil so you can see it says: &amp;quot;&lt;em&gt;Approved for public release. Distribution unlimited.&lt;/em&gt;&amp;quot;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZjXt4.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZjXt4.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mX2Su.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mX2Su.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZkIUk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZkIUk.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JIh90.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JIh90.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wFUBR.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wFUBR.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-01-24T03:40:12.893" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13665" PostTypeId="1" AcceptedAnswerId="13672" CreationDate="2016-01-23T18:34:24.783" Score="24" ViewCount="10032" Body="&lt;p&gt;People often talk about docking and berthing. They both seem to be connecting a spacecraft to the ISS, yet seem to be very different. Why?&lt;/p&gt;&#xA;" OwnerUserId="12645" LastEditorUserId="6816" LastEditDate="2016-01-24T02:44:46.690" LastActivityDate="2016-01-24T02:44:46.690" Title="What is the difference between docking and berthing on the ISS?" Tags="|iss|terminology|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13666" PostTypeId="2" ParentId="13665" CreationDate="2016-01-23T18:37:29.643" Score="12" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacecraft&quot;&gt;Berthing is when the vehicle is grappled by the robot arm and moved to a port.  Docking is when it flies in on its own.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-01-23T18:37:29.643" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13667" PostTypeId="2" ParentId="9329" CreationDate="2016-01-23T18:47:04.870" Score="1" Body="&lt;p&gt;The difference is that under the &quot;old&quot; set up, NASA paid for a company to build rocket to NASA’s design and NASA owned the design. Under the new system, NASA pays for the use of the rocket and the design is owned by the company.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;This does not look like much of a difference until&amp;hellip;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You release that the companies can sell the usage of the rocket to other companies and nations to launch satellites. As the cost of building satellites has come down, if the launch costs also come down, the demand increases greatly&amp;hellip; This then creates the beginning of a market.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the market is created, the companies will start to create new rockets without needing funding from NASA &amp;mdash; or at least that is what NASA hopes.&lt;/p&gt;&#xA;" OwnerUserId="12645" LastEditorUserId="6816" LastEditDate="2016-01-24T02:44:18.490" LastActivityDate="2016-01-24T02:44:18.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13668" PostTypeId="2" ParentId="12734" CreationDate="2016-01-23T20:01:07.963" Score="7" Body="&lt;p&gt;If there is a true emergency, a spacewalk will be &quot;aborted&quot;. An &lt;a href=&quot;http://www.spaceref.com/iss/ops/7A.EVA.checklist.pdf&quot; rel=&quot;noreferrer&quot;&gt;EVA Abort procedure&lt;/a&gt; seems to be pretty simple:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Both astronauts go in crew lock&lt;/strong&gt; (yes, only one can go if desired)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Close and lock outer hatch&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Turn inner hatch pressure equalization valve to emergency setting&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Open manual pressure equalization valve from inside the station&lt;/li&gt;&#xA;&lt;li&gt;Ensure manual pressure equalization valve on outer hatch is closed&lt;/li&gt;&#xA;&lt;li&gt;Turn water off in the suit control system&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Open inner door&lt;/strong&gt; (require maximum 0.5 psi pressure differential)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Open space suit purge valve&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Turn off suit oxygen&lt;/li&gt;&#xA;&lt;li&gt;Turn off suit fan&lt;/li&gt;&#xA;&lt;li&gt;Check that relative pressure in suit is less than 0.4 (psi I assume)&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Remove gloves&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Remove helmet&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are a couple of checklist items, but those are placed so that they can be done while waiting for something else to complete. The things that require waiting are &lt;strong&gt;airlock pressurization&lt;/strong&gt; (at emergency speed) and &lt;strong&gt;space suit pressure equalization&lt;/strong&gt; (purge valve open).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The emergency speed airlock pressurization to a normal atmosphere is said (in the report) is said to be +0.34 psi/sec, which makes it last around 45 seconds. The high flow rate setting on the suit oxygen purge valve is rated as 7.75 lb/hr (of pure oxygen vented to space from 4.3 psia), which through numerous quick approximations would mean that it would take around a minute to get from 4.3 psia to 14.7 psia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to summarize: there are two waiting periods of around one minute, that cannot be sped up, before you can remove the helmet, but otherwise it is pretty much limited to as fast as you can move.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Also of note is that &lt;a href=&quot;http://blogs.esa.int/luca-parmitano/2013/08/20/eva-23-exploring-the-frontier/&quot; rel=&quot;noreferrer&quot;&gt;Parmitano mentions&lt;/a&gt; that during airlock repressurization he is able to open the helmet if necessary. If this is true, then in an extreme emergency I would guess that an astronaut can open the helmet immediately after the outer hatch on the airlock is closed, probably losing consciousness but surviving.&lt;/p&gt;&#xA;" OwnerUserId="2631" LastEditorUserId="2631" LastEditDate="2016-01-23T20:18:12.763" LastActivityDate="2016-01-23T20:18:12.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13669" PostTypeId="2" ParentId="13665" CreationDate="2016-01-23T21:17:04.893" Score="29" Body="&lt;p&gt;The basic difference between docking and berthing methods is as described by @OrganicMarble and the linked &lt;a href=&quot;https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacecraft&quot;&gt;article&lt;/a&gt;, that berthing involves the robot arm and docking does not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reasons why one is used rather than the other was discussed by the NASA PRO in a recent webcast. Obviously, if the vehicle is not capable of docking it must be berthed; however it must be equipped with a berthing connector and a robot arm connector. These things must then be designed and built-in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the reasons why one is selected in the design rather than the other is cargo size. The berthing and docking ports on the ISS are of different shapes and dimension and can thus be used to transfer different volumes of cargo. In particular the &lt;a href=&quot;https://en.wikipedia.org/wiki/Common_Berthing_Mechanism&quot;&gt;Common Berthing Mechanism (CBM)&lt;/a&gt; used by the &lt;a href=&quot;https://en.wikipedia.org/wiki/H-II_Transfer_Vehicle&quot;&gt;H-II Transfer Vehicle (HTV)&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_(spacecraft)&quot;&gt;Dragon spacecraft&lt;/a&gt; have a square opening of 1.3m (50&quot;) which can transfer whole equipment racks. By contrast &lt;a href=&quot;https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacecraft#List_of_mechanisms.2Fsystems&quot;&gt;the Russian and new US docking port&lt;/a&gt; is a circular port of 0.8m (31&quot;), which is too small for racks.&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="5946" LastEditDate="2016-01-23T23:43:47.653" LastActivityDate="2016-01-23T23:43:47.653" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13670" PostTypeId="1" AcceptedAnswerId="13697" CreationDate="2016-01-23T23:18:03.547" Score="11" ViewCount="422" Body="&lt;p&gt;Planet 9, the &lt;a href=&quot;http://www.findplanetnine.com/p/blog-page.html&quot; rel=&quot;noreferrer&quot;&gt;hypothetical planet predicted Batygin and Brown&lt;/a&gt;, is expected to have an apoasis near the galactic center. Is there a spacecraft that is heading in that direction? Specifically, they state:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;perihelion position is straight overhead in late May. ... aphelion (the furthest point from the sun) at about 4 hours, or straight overhead in late November&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Note that I realize that we don't know where this object is, and thus would have great difficulty pinpointing this exactly, but are any of the known objects leaving the solar system heading in that direction?&lt;/p&gt;&#xA;&lt;p&gt;Also, of some note is that the inclination is expected to be 30 degrees, plus or minus 20 degrees.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-01-25T17:31:39.213" Title="Are any spacecraft heading remotely where Planet 9 is expected to be?" Tags="|probe|trajectory|planet|interstellar-travel|coordinates|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13671" PostTypeId="2" ParentId="13523" CreationDate="2016-01-23T23:38:18.450" Score="10" Body="&lt;p&gt;&lt;strong&gt;Skyfield ≥1.34&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The most recent versions of Skyfield introduce an explicit &lt;code&gt;itrs&lt;/code&gt; reference frame that rotates with the Earth. You can learn about it starting here:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://rhodesmill.org/skyfield/coordinates.html#eci-versus-ecef-coordinates&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://rhodesmill.org/skyfield/coordinates.html#eci-versus-ecef-coordinates&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://rhodesmill.org/skyfield/coordinates.html#geographic-itrs-latitude-and-longitude&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://rhodesmill.org/skyfield/coordinates.html#geographic-itrs-latitude-and-longitude&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;strong&gt;Skyfield &amp;lt;1.34&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;First: the main problem with your technique of subtracting the two positions is that it does not account for light-time travel. In the case of the Moon observation that you set up in your code, the error is only about 38 km, which might be well within your tolerances. The way to ask for a relative position that is properly light-time backdated is through the &lt;code&gt;observe()&lt;/code&gt; method:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;mpos_ec = earth.at(jd).observe(moon).position.km&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Second: You are right that Skyfield currently has no built-in first-class support for a reference frame that rotates with the Earth. I should probably add one. A workaround that should get you up and running immediately, though, is to tell Skyfield that you want to observe:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;From an Earth location, so the Skyfield does the rotation calculation that you want.&lt;/li&gt;&#xA;&lt;li&gt;But from an Earth location whose latitude and longitude are zero, so that Skyfield applies no further rotations beyond moving into the Earth's rotating reference frame.&lt;/li&gt;&#xA;&lt;li&gt;And from a location whose elevation is negative enough to place it exactly at the center of the Earth, so that you are not applying an offset to the position of the Moon or whatever.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The one trick is that we have to do the rotation ourselves because, as you note, the way Skyfield is currently written it's only willing to apply the rotation if it's about to reduce the coordinates to polar alt/az for you. So try out the following:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;# Create an observer at the center of the Earth,&#xA;# rotated zero degrees from the Earth's reference frame:&#xA;&#xA;gcrs = earth.topos(&#xA;    latitude_degrees=0,&#xA;    longitude_degrees=0,&#xA;    elevation_m=-6378136.6,&#xA;    )&#xA;&#xA;# Ask where the observer was at a given time:&#xA;&#xA;g = gcrs.at(jd)&#xA;&#xA;# Then ask for a relative position as usual:&#xA;&#xA;p = g.observe(moon).position.km&#xA;print p&#xA;&#xA;# Now the tricky part: each observer.at() has&#xA;# an attribute `altaz_rotation` which is the rotation&#xA;# matrix you want to apply:&#xA;&#xA;print einsum('ij...,j...-&amp;gt;i...', g.altaz_rotation, p)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Let me know if the numbers you get back seem to work! And once I have the time-system rewrite finished up and can release Skyfield 1.0, I'll turn my attention to make this computation a bit more convenient.&lt;/p&gt;&#xA;" OwnerUserId="8590" LastEditorUserId="8590" LastEditDate="2022-01-18T02:21:56.227" LastActivityDate="2022-01-18T02:21:56.227" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="13672" PostTypeId="2" ParentId="13665" CreationDate="2016-01-24T01:13:28.770" Score="20" Body="&lt;p&gt;The terms docking and berthing have a nautical origin. Smaller ships come into port under their own authority and dock. Large ships instead are berthed. They come to a stop outside of the port, relinquish control to the port authority, and are towed into port by tug boats.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Docking with the International Space Station is essentially a controlled collision with the station. This controlled collision has to be extremely precise in order to be nondestructive. Because the space station is an extremely expensive resource, approving a vehicle that plans to dock with the station is something done with extreme care (and extreme expense).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Berthing opened up the door to automated operations by the Japanese HTV, SpaceX's Dragon, and Orbital's Cygnus. With berthing, the designers of a vehicle need to prove to NASA that their vehicle can safely approach the station, that it can hold position and attitude in well-designed berthing box, that it recognizes and responds to problems along the way, and that it relinquishes control to the ISS upon command. That the ports used for berthing are larger than the ports used for docking is a side benefit.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-01-24T01:13:28.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13673" PostTypeId="1" AcceptedAnswerId="13674" CreationDate="2016-01-24T01:41:10.943" Score="3" ViewCount="448" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yWPAh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yWPAh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/YYaVM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YYaVM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In each footage from ISS I see this thick tube going through the hatches into Soyuz spacecraft, ending in the descent module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is it for?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-01-24T01:47:56.797" Title="What's the thick tube going into Soyuz?" Tags="|iss|soyuz-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13674" PostTypeId="2" ParentId="13673" CreationDate="2016-01-24T01:47:56.797" Score="7" Body="&lt;p&gt;Ventilation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In microgravity, hot/cold air does not move the same way, so you do not get natural mixing or movement of air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sleeping astronauts get a air stream pointed at them so that they do not get enveloped in a cloud of carbon dioxide and die.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When docked at the ISS the Soyuz is mostly powered down, or runs of the ISS power and life support systems. After all they are designed to stay docked for 200= days. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mir was full of these things between the modules.  The ISS design was a bit better and seems to require fewer.  But the Soyuz vehicles still need the ventilation.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-24T01:47:56.797" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13676" PostTypeId="1" AcceptedAnswerId="13678" CreationDate="2016-01-24T05:44:15.933" Score="12" ViewCount="2637" Body="&lt;p&gt;In an answer to a similar &lt;a href=&quot;https://space.stackexchange.com/questions/2102/karman-line-analog-on-other-celestial-bodies&quot;&gt;question&lt;/a&gt;, the Karman Line for Mars is stated as being at 125km compared to 122km for Earth. However, Mars has an atmospheric pressure about 1/10 that of Earth, its differences in mass, rotational period and gravitational acceleration not withstanding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is it higher than for Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: D'Ohhh!!! Just reread the original prior question again, as per @Nakedible. But I still can't see the justification for the 125km figure.&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-01-24T10:02:44.213" Title="Why is the Karman line for Mars defined higher than for Earth?" Tags="|mars|reentry|atmosphere|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13677" PostTypeId="1" AcceptedAnswerId="13681" CreationDate="2016-01-24T07:40:14.463" Score="13" ViewCount="1174" Body="&lt;p&gt;Videos of rocket launches often show exhaust flames engulfing at least the upper parts of a launch tower or strongback.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do these towers need to be reconstructed or at least repainted after a launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the umbilical cords and fuel hoses seen retracting during a launch survive the launch or do they need to be replaced? What brands are available?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2016-01-26T06:42:38.927" LastActivityDate="2016-01-26T06:42:38.927" Title="Do strongbacks/launch towers have to be repainted or refurbished after a launch?" Tags="|launch|launch-site|launchpad|exhaust|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13678" PostTypeId="2" ParentId="13676" CreationDate="2016-01-24T07:46:19.063" Score="14" Body="&lt;p&gt;The Kármán line in Venus is around 250 km high, and in Mars about&#xA;80 km (&lt;a href=&quot;http://webserver.dmt.upm.es/~isidoro/tc3/Space%20environment.pdf&quot; rel=&quot;noreferrer&quot;&gt;reference&lt;/a&gt;). An answer on the question you have linked places it at 88 km on Mars. So I believe you have just misread the answers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, what is discussed is that &lt;em&gt;entry interface altitude&lt;/em&gt; for Mars starts at 125 km, where as for Apollo program it was 122 km (400,000 ft). This is because even though the Kármán line is much lower on Mars, the lower gravity on Mars makes the &lt;em&gt;scale height&lt;/em&gt; for Mars atmosphere much higher. This means that atmospheric drag starts to have an effect on spacecraft at around the same height as it does for earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know if there is any specific rationale for those exact values (125 km vs 400,000 ft) of entry interface altitude though - I think they are just nice round numbers that are sufficiently above where the actual effects start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rough Mars atmosphere model can be seen &lt;a href=&quot;http://exrocketman.blogspot.fi/2012/06/atmosphere-models-for-earth-mars-and.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GCsx4.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GCsx4.jpg&quot; alt=&quot;Mars Atmosphere Model&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2631" LastEditorUserId="2631" LastEditDate="2016-01-24T07:59:52.210" LastActivityDate="2016-01-24T07:59:52.210" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13679" PostTypeId="1" AcceptedAnswerId="13680" CreationDate="2016-01-24T07:48:26.130" Score="5" ViewCount="1682" Body="&lt;p&gt;What's the difference between a launch tower and a strongback?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="49" LastEditDate="2016-01-24T10:14:46.213" LastActivityDate="2016-01-24T10:14:46.213" Title="Strongbacks vs Launch towers: Difference and definition?" Tags="|launch|terminology|launchpad|exhaust|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13680" PostTypeId="2" ParentId="13679" CreationDate="2016-01-24T09:36:28.387" Score="7" Body="&lt;p&gt;Strongback is a part of &lt;a href=&quot;https://en.wikipedia.org/wiki/Transporter_erector_launcher&quot;&gt;TEL&lt;/a&gt; (Transporter Erector Launcher) that facilitates horizontal transport from horizontal Vehicle Assembly Building to the launch site, erecting the launch vehicle vertically, and holding umbilical connectors for launch vehicle fueling, power and purging gas. On its own, it doesn't enable crew or service personnel access to the launch vehicle's payload once it's erect, albeit it might be used together with a &lt;a href=&quot;https://en.wikipedia.org/wiki/Service_structure&quot;&gt;launch tower&lt;/a&gt; (like e.g. Soyuz-TMA do). Launch tower on the other hand never supports the launch vehicle horizontally. It is either transported to it with a TEL, or the whole launch platform is mobile, fixed to the launch pad, and the vehicle is assembled in a vertical &lt;a href=&quot;https://en.wikipedia.org/wiki/Vehicle_Assembly_Building&quot;&gt;Vehicle Assembly Building&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2016-01-24T09:36:28.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13681" PostTypeId="2" ParentId="13677" CreationDate="2016-01-24T12:22:00.047" Score="15" Body="&lt;p&gt;On e.g. the Saturn and Shuttle launches, vulnerable items like umbilicals are retracted into closed spaces, with a door closing over them in time to protect them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://www.youtube.com/watch?v=vFwqZ4qAUkE&quot;&gt;very detailed video&lt;/a&gt; of a Shuttle launch shows some of those (at 9:40, for example). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The audio commentary mentions (around 9:00) that the cameras on the platform and tower are inside explosion-proof boxes, in a nitrogen atmosphere. The lenses are protected by quartz glass covers. These get damaged sometimes and have to be reground and polished. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/shuttle/behindscenes/postlaunchinspectteam.html&quot;&gt;On almost every launch&lt;/a&gt;, items like tools were left behind and got blasted away, melted or otherwise damaged by the rocket exhaust. Even parts attached to the tower structure sometimes get blown off. NASA does a post-launch inspection after every launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.nasa.gov/mission_pages/shuttle/flyout/launchpadflyout.html&quot;&gt;entire tower is designed to explosion-proof&lt;/a&gt; standards. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;They typically spend a few weeks making repairs, re-painting rails and other sections that have been stripped of their coatings by the launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Everything has to be painted to prevent corrosion in the salty sea air at Canaveral. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concrete of the flame trench has been known to need occasional repairs too. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-01-24T12:56:56.157" LastActivityDate="2016-01-24T12:56:56.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13682" PostTypeId="1" AcceptedAnswerId="13683" CreationDate="2016-01-24T13:39:45.987" Score="3" ViewCount="278" Body="&lt;p&gt;Are electronic telemetry and control links (of TT&amp;amp;C - telemetry, tracking and control) to the launch vehicle incorporated into the launch pad umbilical lines and handed off at launch, or is it all done through radio links exclusively?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What sort of control and telemetry data transmission rates are typical for a launch vehicle prior to and at launch?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastActivityDate="2016-01-24T21:03:27.957" Title="LV Telemetry links on the launch pad" Tags="|launch|launchpad|launch-sequence|electronics|data-transmission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13683" PostTypeId="2" ParentId="13682" CreationDate="2016-01-24T16:38:56.377" Score="4" Body="&lt;p&gt;Yes, control and telemetry links are incorporated into the umbilicals. &lt;a href=&quot;https://www.youtube.com/watch?v=vFwqZ4qAUkE&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle launch video, at 9:00&lt;/a&gt;, and &lt;a href=&quot;http://www.nasaspaceflight.com/2012/11/nasa-sls-mobile-launcher-umbilical-plans/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here are plans for the umbilicals for SLS&lt;/a&gt;.&lt;br /&gt;&#xA;During the launch sequence there's a point where control of the rocket is handed over &lt;a href=&quot;http://www.americaspace.com/?p=32859&quot; rel=&quot;nofollow noreferrer&quot;&gt;from the launch control center to the internal systems&lt;/a&gt;. For the &lt;a href=&quot;http://everything2.com/title/Space+Shuttle+Terminal+Count&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle, this starts at T -20 minutes&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Up to this point, control over the countdown and all launch systems resides with a set of computers on the ground, known as the Launch Processing System (LPS). At T-20:00, the Orbiter general purpose computers (GPCs) are loaded with their first set of operations software (OPS1) and formed into a cluster of four GPCs with a fifth online as backup and 'tiebreaker'. From this point on, launch commands pertaining to the vehicle are triggered by the LPS issuing commands to the onboard GPCs and awaiting their report. All backup flight systems are commanded to 'flight' status. Fuel cell thermal conditioning - essentially warmup - begins, and all orbiter cabin vent valves are closed.&lt;/p&gt;&#xA;&lt;p&gt;The navigation systems come further online; the orbiter's Inertial Measurement Units (IMUs) are initialized. An external bias to compensate for the Earth's rotation is fed into them until just before launch as the onboard IMUs won't show any of the Earth's rotation, due to gravity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;At T -31 seconds, the onboard computers assume full control:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Sequencing and control of the launch process is handed off from the LPS to the GPCs aboard the Orbiter. The GPCs assume control of all launch activity; however, they will still respond to 'hold', 'resume count' and 'recycle' (re-do part of the countdown) commands from the LPS. Note that at and after this point, any 'hold' or anomalous events will cause the countdown to recycle to the T minus 20 minute mark. After this, only one ground command is required to launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It makes sense to switch over to radio links after that, but before the launch proper, so you can test the links before you commit to the launch. But I &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/comm/&quot; rel=&quot;nofollow noreferrer&quot;&gt;haven't found data on this&lt;/a&gt; for the Shuttle.&lt;/p&gt;&#xA;&lt;p&gt;As for &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-ovcomm.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;data rates&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;During ascent, space shuttle main engine interface unit data are sent to the S-band FM system to be transmitted in real time to the ground. These data also are routed to the operations recorders for non-real-time transmission. On orbit, one of the other six services may be selected and routed through the S-band FM signal processor.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The FM signal processor is commanded to select one of these sources at a time for output to the S-band FM transmitter, which transmits it to the S-band FM return link through the STDN ground station used for launch, lift-off, ascent or landing or the DOD AFSCF ground station. The S-band FM return link transfers real-time SSME data from the engine interface units during launch at 60 kbps each or real-time video or operations recorder dumps of high- or low-data-rate telemetry at 1,024 kbps or one-track dumps of 60-kbps SSME data at 1,024 kbps or payload recorder at 25.5 kbps or 1,024 kbps or payload analog at 300 hertz or 4 MHz or payload digital data at 200 bps or 5 mbps or DOD data at 16 kbps or 256 kbps in real time or 128 kbps or 1,024 kbps of playback.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The S-band PM forward link transfers a high data rate of 72 kilobits per second, consisting of two air-to-ground voice channels at 32 kbps each and one command channel at 8 kbps, two-way Doppler and two-way tone ranging.&lt;/p&gt;&#xA;&lt;p&gt;The S-band PM forward link transfers a low data rate of 32 kbps, consisting of one air-to-ground voice channel at 24 kbps and one command channel of 8 kbps, two-way Doppler and two-way ranging. The two-way ranging does not operate through the TDRS.&lt;/p&gt;&#xA;&lt;p&gt;The S-band return link high data rate of 192 kbps consists of two air-to-ground voice channels at 32 kbps each and one telemetry link of 128 kbps, two-way Doppler and two-way ranging. In the high-data-rate mode, incoming signals are directed to a preamplifier before they reach the transponder, and outgoing signals go to the preamplifier after leaving the transponder. The two-way Doppler and two-way ranging are operative only when in view of the NASA STDN ground stations at launch, lift-off, ascent or landing or in view of the Department of Defense AFSCF ground stations. The two-way ranging does not operate with the TDRSS.&lt;/p&gt;&#xA;&lt;p&gt;The Ku-band system return link consists of channel 1, modes 1 and 2, plus one channel 2, modes 1 and 2, and one channel 3. Channel 1, modes 1 and 2, consists of 192 kbps of operational data (128 kbps of operational data telemetry and payload interleaver plus two air-to-ground voice links at 32 kbps each) plus one of the following selections from channel 2, modes 1 and 2: (1) payload digital data from 16 kbps to 2 Mbps, (2) payload digital data from 16 kbps to 2 Mbps, (3) operations recorder playback from 60 kbps to 1,024 kbps, or (4) payload recorder playback from 25.5 kbps to 1,024 kbps. It also includes one of the following from channel 3: mode 1 attached payload digital data (real-time or playback) from 2 Mbps to 50 Mbps, mode 2 television (color or black and white) composite video, or mode 2 real-time attached payload digital data or payload analog data.&lt;/p&gt;&#xA;&lt;p&gt;The Ku-band system forward link consists of a mode 1 and 2 through the TDRS in view. Mode 1 consists of 72-kbps data (two air-to-ground voice streams at 32 kbps each and 8 kbps of command), 128-kbps TAGS (used in place of the teleprinter) and 16-kbps synchronization. Mode 2 consists of 72-kbps operational data (two air-to-ground voice streams at 32 kbps each and 8 kbps of command).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-01-24T21:03:27.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13684" PostTypeId="1" AcceptedAnswerId="13694" CreationDate="2016-01-24T19:11:03.977" Score="7" ViewCount="1572" Body="&lt;p&gt;Over 45 years ago, we know that USA/Russia were able to send satellites/humans and return them to the Earth. &#xA;Isn't that supposed to be much more difficult to do than what SpaceX is doing now by returning (landing) the rockets/boosters on the Earth (not the Moon)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is more difficult to land on the Moon or the Earth? I would say that it is easier to land stuff on the Earth because it is closer to us, it's easier to exchange signals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is it the mass of the lander/rockets we are landing that makes things harder? The rocket such as Falcon 9 is much heavier than the rovers/landers that were landed on the moon and then returned back to the earth.&lt;/p&gt;&#xA;" OwnerUserId="13166" LastEditorUserId="6816" LastEditDate="2016-01-24T19:52:32.107" LastActivityDate="2018-10-21T04:13:29.767" Title="Wasn't the moon landing + reentry much harder to do than SpaceX's reusable rockets/boosters?" Tags="|the-moon|landing|earth|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13685" PostTypeId="2" ParentId="12028" CreationDate="2016-01-24T19:15:36.347" Score="10" Body="&lt;p&gt;This response addresses the practical warning time for ordinary citizens. It isn't quite the scenario asked in the original question and is a response to the various comments that clarify that citizen perspective is of interest and also that the flight path Russia -&gt; Western Europe is of interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the 70's and 80's in the UK there was an awareness of the &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Four-minute_warning&quot; rel=&quot;noreferrer&quot;&gt;four minute warning&lt;/a&gt;&quot;. The wiki article is quite chilling. Some salient points: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;the four-minute warning was a public alert system conceived by the British Government during the Cold War and operated between 1953 and 1992&quot;&lt;/li&gt;&#xA;&lt;li&gt;&quot;From the early 1960s, initial detection of attack would be provided primarily by the RAF BMEWS station at Fylingdales in North Yorkshire.&quot;&lt;/li&gt;&#xA;&lt;li&gt;The British government was not the main beneficiary of BMEWS, given that it would only receive ...   ... &quot;no more than 5 minutes warning time&quot; of an attack. The United States ... ..., however, and its Strategic Air Command would have thirty minutes warning from the Fylingdales station.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As a peripheral point there may also be some inspiration for the novel in the book and film &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/On_the_Beach_%28novel%29&quot; rel=&quot;noreferrer&quot;&gt;On the beach&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-01-24T19:15:36.347" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13686" PostTypeId="2" ParentId="13684" CreationDate="2016-01-24T19:19:07.370" Score="4" Body="&lt;p&gt;Landing on the Moon is simultaneously easier and more difficult than SpaceX's landing of a first stage: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Easier: there's no atmosphere, so you don't have to worry about aerodynamic heating and pressure.  &lt;/li&gt;&#xA;&lt;li&gt;Easier: because the Moon has a much lower gravity, you need far less energy to ascend from the Moon to an orbit around the Moon than you need for a launch from Earth. This also makes the landing on the Moon easier. &lt;/li&gt;&#xA;&lt;li&gt;More difficult: The hard part about the Moon landing is getting off the surface of the Earth and into Moon orbit. We needed a gigantic 2000-ton Saturn V to get that far, but we could land and ascend again in a 10-ton LM. &lt;/li&gt;&#xA;&lt;li&gt;More difficult: In the 1960, computers were barely up to the task of helping a pilot land safely. These days, we have enough computer power that we can allow a rocket to land autonomously without human intervention. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-24T19:19:07.370" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13687" PostTypeId="1" CreationDate="2016-01-24T19:25:39.567" Score="1" ViewCount="1221" Body="&lt;p&gt;Most of what I read says there is not any phosphorus on Venus, which I take to mean none in the atmosphere or none detected. I mean, it gets hit by iron-nickel meteorites and probably has some apatite in the crust.&lt;/p&gt;&#xA;&lt;p&gt;There are &lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt; soluble phosphorus compounds, like &lt;span class=&quot;math-container&quot;&gt;$PH_3$&lt;/span&gt;, which could be leached out of rocks by the near supercritical lower atmosphere moved around and redeposited.&lt;/p&gt;&#xA;&lt;p&gt;I am just throwing some thinking at it; what do you, people in the know, know?&lt;/p&gt;&#xA;" OwnerUserId="13167" LastEditorUserId="47149" LastEditDate="2022-05-22T17:46:43.697" LastActivityDate="2022-05-22T17:46:43.697" Title="Could there be a phosphorus cycle on Venus?" Tags="|venus|chemistry|geology|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13688" PostTypeId="2" ParentId="13684" CreationDate="2016-01-24T19:51:25.523" Score="2" Body="&lt;p&gt;It depends on how you want to define &quot;difficulty&quot;. Using cost as a proxy for difficulty, the Apollo program cost more than the equivalent of US\$100 billion. SpaceX has probably spent under ten billion to date, maybe closer to 5.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-01-25T04:23:46.173" LastActivityDate="2016-01-25T04:23:46.173" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13689" PostTypeId="2" ParentId="13687" CreationDate="2016-01-24T21:39:20.367" Score="5" Body="&lt;p&gt;Yes, there is phosphorus on Venus, just like on Earth. The elemental composition is not that different. However, you can not find many phosphorus compounds in the atmosphere, due to few of them being gasses even at Venus high temperatures. The one you use as an example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Phosphine&quot;&gt;Phosphine&lt;/a&gt;, suffers from the fact that Venus almost completely lacks hydrogen. Phosphine does also have a positive enthalpy of formation, making it even less likely to occur naturally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, phosphorus is a lot more concentrated in a phosphorus cycle, due to the strange phenomenon &lt;strong&gt;life&lt;/strong&gt;. The ratio between the concentration of phosphorus in living organisms and in the crust is one of the highest for any element (kind of making it a bottleneck resource). As far as we know, there is no life on Venus, so no such concentration has taken place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Phosphorus exists on Venus, but it is a lot more scattered.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-01-24T21:39:20.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13690" PostTypeId="2" ParentId="13684" CreationDate="2016-01-25T06:35:41.850" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Over 45 years ago, we know that USA/Russia were able to send satellites/humans and return them to the Earth. Isn't that supposed to be much more difficult to do than what SpaceX is doing now by returning (landing) the rockets/boosters on the Earth (not the Moon)?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Landing a rocket containing people on the Moon in the 1960s was orders of magnitude harder than what SpaceX is trying to do now. The Apollo program would have been shut down shortly after Apollo 13 had that mission been preceded by Apollo 11 and 12 punching the barge. The Moon landings had to work, the first time, and every time. Failure was not an option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were people on those rockets, and national prestige was at stake. None of the Apollo missions punched the barge. That failure is an option for SpaceX (until they get it right), that people's lives aren't at stake, and that corporate success is at stake compared to national prestige makes SpaceX's task much, much easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over two percent of the entire United States federal budget went to NASA from 1963 to 1969 (peaking at over 4% in 1965 and 1966), and over half of NASA's budget from 1964 to 1970 went to the Apollo program. A sizable fraction of those monies went to the problem of landing a rocket on the Moon, first time and every time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In comparison, NASA currently receives less than half a percent of the federal budget, and it only sends a tiny fraction of that tiny fraction to SpaceX. SpaceX in turn only dedicates a small fraction of its incomes to the problem of landing a rocket on a barge.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-01-25T06:35:41.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13691" PostTypeId="1" AcceptedAnswerId="13696" CreationDate="2016-01-25T07:09:05.207" Score="19" ViewCount="3897" Body="&lt;p&gt;The space shuttle used three SSME, organized in a triangle with two next to each other closest to the external tank, and one on top of those. I am pretty sure the first two engines were close to identical, in order to maintain symmetry. But the third engine did not have to be, as it was offset in the unsymmetrical direction of the orbiter. Because it was a little bit more distant from the centre of mass than the two others, i would expect it to have more of a control function, having a larger gimbal range and throttability. Was that the case? And were there other differences, like in the propellant feed system or in nozzle shape?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2018-11-04T19:52:09.820" Title="Was the third shuttle engine any different from the others?" Tags="|launch|space-shuttle|engine-design|ssme|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13692" PostTypeId="5" CreationDate="2016-01-25T08:32:05.637" Score="0" Body="&lt;p&gt;At some altitudes, the Earth's magnetic field traps radiation, causing regions with high levels of radiation. They are particularly important for manned space flight, but may degrade electronics as well. Discovered by the first American artificial satellite, Explorer 1.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-25T10:15:23.420" LastActivityDate="2016-01-25T10:15:23.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13693" PostTypeId="4" CreationDate="2016-01-25T08:32:05.637" Score="0" Body="Questions regarding the van Allen radiation belts around the Earth." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-25T10:15:27.293" LastActivityDate="2016-01-25T10:15:27.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13694" PostTypeId="2" ParentId="13684" CreationDate="2016-01-25T09:33:15.570" Score="5" Body="&lt;p&gt;Comparing the whole Saturn V mission vs an orbital delivery is totally apples to oranges. There's no doubt that developing Saturn V was vastly harder but they are so different that the comparison is pointless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one point where comparing them makes sense: Vertical landing of a rocket-powered vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's still an unfair comparison for one fundamental reason: one was man-controlled, while the other was automatic. There's a human at the heart of the lunar lander controls, where Falcon 9 uses a computer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In one hand, the tasks before the computer are harder. There's a precise point of landing vs any random semi-flat piece of Lunar surface. There's a vehicle with a much higher point of mass and poorer landing legs configuration. There's a much higher gravity to brake against. In case of barge landing the landing point moves. A computer to land a lunar lander would be much, much easier to build.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But on the other hand you have a human in a terribly unwieldy spacesuit, with only rudimentary instrumentation (heavily depending on observing shadows on the ground!), subjected to all kinds of accelerations, doing something he had never done it outside of simulation, under immense amount of stress, and having only one go at it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while &lt;em&gt;theoretically&lt;/em&gt; the computer's task is harder than the human's, the number of handicaps the human suffered while performing the landing well balances it out.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-01-25T09:33:15.570" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13695" PostTypeId="2" ParentId="13691" CreationDate="2016-01-25T10:23:01.623" Score="12" Body="&lt;p&gt;The 3 engines were identical. Of the 46 SSME engines produced, at &lt;a href=&quot;http://www.collectspace.com/ubb/Forum30/HTML/000409.html&quot; rel=&quot;noreferrer&quot;&gt;least 3 were installed in 3 different positions on various flights&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;engine 2012 on OV-103 Discovery&lt;/li&gt;&#xA;&lt;li&gt;engine 2019 on OV-104 Atlantis&lt;/li&gt;&#xA;&lt;li&gt;engine 2054 on OV-105 Endeavour&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If you really want to know which engine flew where, &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#/media/File:SSME_Flight_History.png&quot; rel=&quot;noreferrer&quot;&gt;there's a chart&lt;/a&gt; that tracks the flight history of each engine. Engine manufacturer Pratt &amp;amp; Whitney made press kits for each flight that detailed which engine was installed in which position.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A better list than these two is included in the &lt;a href=&quot;http://www.spaceshuttlealmanac.com/&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Almanac&lt;/a&gt;, page 1031-1034 (not available for free though). This lists the position of each engine per flight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fun fact: As the SSME was developed during the Shuttle program, there were various flights with 2 &quot;production&quot; engines and one upgraded engine.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This photo of an SSME being installed shows what's part of the SSME (i.e. which is identical on all 3 engines). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZE9JE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZE9JE.jpg&quot; alt=&quot;SSME installation. Engine bell, turbopump and lots of plumbing are installed as a unit &quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-07T07:48:42.817" LastActivityDate="2016-06-07T07:48:42.817" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13696" PostTypeId="2" ParentId="13691" CreationDate="2016-01-25T11:08:59.903" Score="32" Body="&lt;p&gt;The engines themselves were identical within manufacturing tolerances, but there were some installation differences, mostly due to &quot;packaging&quot; constraints in the crowded aft compartment of the shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The propellant feedlines were not identical in shape and this resulted in some minor performance differences.  For example, the left SSME LH2 line had a slightly larger pressure drop and thus the left SSME would be the first to fail in a low Net Positive Suction Pressure (NPSP) malfunction case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ku2TR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ku2TR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;(picture from Rocketdyne &quot;&lt;a href=&quot;https://archive.org/download/PocketDataRIRD87142SpaceShuttleMainEngine/Pocket_Data_RI-RD87-142_Space_Shuttle_Main_Engine.pdf&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Main Engine Pocket Data RI/RD87-142&lt;/a&gt;&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another difference on the left engine is that its thrust vector control actuators were mounted in a different orientation from that of the other two, which drove some software changes, but was operationally transparent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZgVzd.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZgVzd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjIx9Db6cTKAhUjlYMKHWzTDhYQFggdMAA&amp;amp;url=https%3A%2F%2Fwww.nasa.gov%2Fcenters%2Fjohnson%2Fpdf%2F390651main_shuttle_crew_operations_manual.pdf&amp;amp;usg=AFQjCNENi1Wfkn12N9uV7BJ3FfCXYBLgag&amp;amp;sig2=NORmqVOw-tZ9hUoKt8T4lQ&quot; rel=&quot;noreferrer&quot;&gt;TVC source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the TVC attach points on the engines themselves were identical, this different arrangement of the actuators for the left engine meant that it was actually installed in a different rotational attitude from the other two.  In this personal photo of the Main Propulsion Test Article (a functionally identical shuttle aft compartment used for testing) taken at Marshall Space Flight Center one can see how the mounting interfaces are rotated 90 degrees for the left engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UFEry.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UFEry.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And a schematic (this is, maybe confusingly, looking &lt;strong&gt;aft&lt;/strong&gt;, so the left engine is on the right of this picture).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dhBAj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dhBAj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(picture from Rocketdyne &quot;Space Shuttle Main Engine Pocket Data RI/RD87-142&quot;)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-11-04T19:52:09.820" LastActivityDate="2018-11-04T19:52:09.820" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13697" PostTypeId="2" ParentId="13670" CreationDate="2016-01-25T14:33:08.273" Score="5" Body="&lt;p&gt;Galactic centre:&lt;br&gt;&#xA;&#xA;-29.01° declination and 17.76 hours right ascension,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 1&lt;br&gt;&#xA;12.44° declination and 17.163 hours right ascension, missing by ~40°&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 2&lt;br&gt;&#xA;−55.29° declination and 19.888 hours right ascension, missing by ~50°&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pioneer 11&lt;br&gt;&#xA;&#xA;-8.80° declination and 18.83 hours right ascension, missing by ~30°&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pioneer 10&lt;br&gt;&#xA;&#xA;25.99° declination and 5.2 hours right ascension, missing by nearly ~180°&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons:&lt;br&gt;&#xA;&#xA;-20.73° declination and 19.07 hours right ascension, missing by ~20°&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we do not know where in its orbit the potential ninth planet is, so we may hit it by pure luck ;)&#xA;However, even an error of even one degree is going to be a miss of over 2 AU. (and the Pioneer probes are not operational).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Loctp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Loctp.png&quot; alt=&quot;stuff escaping&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-25T15:47:08.570" LastActivityDate="2016-01-25T15:47:08.570" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13698" PostTypeId="1" AcceptedAnswerId="13699" CreationDate="2016-01-25T14:39:25.597" Score="6" ViewCount="290" Body="&lt;p&gt;4 of the 5 spacecraft listed in the &lt;a href=&quot;https://space.stackexchange.com/a/13697/25&quot;&gt;answer to this question&lt;/a&gt; have a Right Ascension of 18.5 hours, plus or minus 1.4 hours. That seems a bit of a coincidence. Is there a reason why spacecraft leave the Solar System with such a Right Ascension?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-06-07T13:09:06.040" Title="Why are the Right Ascension of spacecraft leaving the Solar System so similar?" Tags="|trajectory|interstellar-travel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13699" PostTypeId="2" ParentId="13698" CreationDate="2016-01-25T15:05:13.987" Score="6" Body="&lt;p&gt;It's a coincidence caused by the position of the planets they visited and the moment they visited them. &#xA;Pioneer 10 only visited Jupiter so it could be launched any time they wanted. The other 3 had to have a reasonably close conjunction of at least Jupiter and Saturn.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Voyager timeframe, there was a rare 5-planet lineup of Jupiter, Saturn, Uranus, Neptune and Pluto. Everything launched to take advantage of that, had to go in the same general direction.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Voyager 1 passed Saturn on November 12, 1980  &lt;/li&gt;&#xA;&lt;li&gt;Pioneer 11 passed Saturn on September 1, 1979  &lt;/li&gt;&#xA;&lt;li&gt;Voyager 2 passed Saturn on August 25, 1981&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That's less than 2 years, or 1/15 of Saturn's orbital period. So they were still quite close at that point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto, being the slowest of them all, is still in the same general area 30 years later. Jupiter, being the fastest of the group, was on the other side of the solar system just a few years earlier. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This graph shows their &lt;a href=&quot;http://www.nasa.gov/centers/ames/missions/archive/pioneer.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;trajectories&lt;/a&gt;:   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZMtqD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZMtqD.jpg&quot; alt=&quot;Voyager and Pioneer trajectories, viewed from above. Pioneer 10 visited Jupiter and went off to the left (9 o&amp;#39;clock). Voyager 1 is at 4 o&amp;#39;clock, Pioneer 11 at half past 2 and Voyager 2 at 1 o&amp;#39;clock, all fairly close to each other&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image doesn't tell the whole story: what you don't see is that all 4 probes have different inclinations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U07OE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U07OE.jpg&quot; alt=&quot;3D map of Pioneer and Voyager trajectories&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pioneer 10 is on the ecliptic plane  &lt;/li&gt;&#xA;&lt;li&gt;Pioneer 11 is slightly above it  &lt;/li&gt;&#xA;&lt;li&gt;Voyager 1 is far above  &lt;/li&gt;&#xA;&lt;li&gt;Voyager 2 is far below, due to its Triton flyby at Neptune  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-07T13:09:06.040" LastActivityDate="2016-06-07T13:09:06.040" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13700" PostTypeId="1" AcceptedAnswerId="13708" CreationDate="2016-01-25T17:21:49.887" Score="6" ViewCount="3330" Body="&lt;p&gt;I'm trying to figure out the timing of the 3 recovery burns and the total fuel reserve needed to perform them.  [Answers Inlined]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boostback: [30] seconds of 3-engines at full-thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reentry: [20] seconds of 3-engines at full-thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing: [32] seconds of 1-engine throttled between 70-100% (Merlin 1-D throttle range).  They probably target somewhere between the midpoint (max margins) and higher throttle (better efficiency), so maybe 90% on average?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Merlin 1-D+ (F9FT) fuel flow rate is 270.7 kg/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;S1: 270.7*3*30 = 24,363kg (wow)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;S2: 270.7*3*20 = 16,242kg&lt;/p&gt;&#xA;&#xA;&lt;p&gt;S3: 270.7*0.90*32 = 7,796kg&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Total: 48,400kg of fuel reserve (wow!).&lt;/p&gt;&#xA;" OwnerUserId="13177" LastEditorUserId="12102" LastEditDate="2021-08-07T00:23:07.897" LastActivityDate="2021-08-07T00:23:07.897" Title="What is the burn time for the F9 boostback / reentry / landing burns?" Tags="|spacex|falcon-9|recovery|orbcomm-2|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13701" PostTypeId="2" ParentId="13393" CreationDate="2016-01-25T18:18:43.570" Score="7" Body="&lt;p&gt;That is the beauty of rocket engines, you can put almost whatever you want into them, and they still kinda work. (That said, &lt;em&gt;I&lt;/em&gt; see that as beauty, engineers just scream in terror.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Joking aside, there is actually a little bit of truth in that, rocket propellant specifications are not that strict. For example, the original specifications for RP-1 just limited the amount of sulphur and components inducing polymerization. Pretty much everything could pass as rocket fuel. (&lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot;&gt;Ignition&lt;/a&gt;, by John D. Clarke has some fascinating information on that.) For methane fuel, contamination would likely be larger hydrocarbons, falling inside the range of kerosene, thereby providing no harm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Oxygen_ISRU_Experiment&quot;&gt;MOXIE&lt;/a&gt; is an oxygenator, splitting $CO_2$ into $CO$ and $O_2$. The diagram has something labelled &quot;cryocooler&quot;, so I presume it does actually produce liquid oxygen. Here is the thing: Even regular rockets does not burn their fuel completely! It actually helps performance. Most extreme is liquid hydrogen burning rockets, only using about half the fuel. So in a hydrocarbon propellant combination, you already have to deal with half-burned fuel, with species like $CO$ present. That means that really pure oxygen is not that important, and limited purity ISRU propellants are fine, within reasonable limits. Note that even if a little contamination is OK, it must be a homogeneous mix in your tanks, otherwise you have to deal with random combustion instability. The MOXIE $O_2$ is good enough for rockets, the real issue is producing enough of it. MOXIE is just a small scale test, and even a full scale system is going to require months if not years to produce enough fuel to ascend back into orbit.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-01-25T18:18:43.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13703" PostTypeId="1" CreationDate="2016-01-25T20:02:02.160" Score="11" ViewCount="198" Body="&lt;p&gt;Videos of people on the ISS make it clear how little they need to bend or stretch, and how their joints stay in their neutral position most of the time as they float around - elbows, knees and hips slightly flexed, shoulders lifted towards their heads. It seems to me that if they didn't specifically stretch to ensure that their joints don't tighten up, by the time they got back to Earth they would have lost a lot of flexibility. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quick search doesn't show anything in particular in the ISS exercise routines regarding stretching. Is it something the crew just does on their own now and then, as the urge arises? Has loss of flexibility ever been noted?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2021-12-15T05:59:46.697" Title="Is loss of flexibility a concern with crew on the ISS?" Tags="|health|astronaut-lifestyle|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13705" PostTypeId="1" CreationDate="2016-01-25T21:30:09.933" Score="5" ViewCount="627" Body="&lt;p&gt;If a spacecraft is entering the Martian atmosphere (say Curiosity rover in a Sky Crane capsule), is the wind strong enough to actually produce forces on the spacecraft? (like you would see during an Earth re-entry?) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I’ve read that some Martian wind storms can get up to 60 miles/hr and create dust storms. But I also know that the Martian atmosphere is not really dense. So how can we have such strong winds when the atmosphere is not very dense?&lt;/p&gt;&#xA;" OwnerUserId="13183" LastEditorUserId="4660" LastEditDate="2016-07-31T00:33:44.457" LastActivityDate="2016-07-31T15:57:14.920" Title="Is Martian wind strong enough to produce aerodynamic forces on a spacecraft?" Tags="|mars|reentry|aerodynamics|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13706" PostTypeId="2" ParentId="2470" CreationDate="2016-01-25T23:48:24.173" Score="2" Body="&lt;p&gt;There are multiple things going on here. First, the ISS does maintain a constant attitude in a rotating LVLH (Local-Vertical-Local-Horizontal) frame (cupola pointing towards earth), but it uses a combination of CMGs and the jets on the Russian segment for attitude control. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The attitude of the ISS does change during docking, but that attitude is still constant in a rotating LVLH frame. It is changed to an attitude that has minimal torque from drag so that the CMGs can hold the attitude for as long as possible (minimum 180 seconds). Usually what happens is the CMGs are controlling attitude, but due to non-secular torques, the CMGs have to be desaturated by the jets on the Russian segment every so often. This causes non-desired effects during berthing and docking, and so the jets are not used during capture (hence the 180 second rule of only using the CMGs)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To hold a constant attitude in a rotating LVLH frame, the vehicle will rotate once per orbit, so the attitude rate is nearly constant. This is not difficult to maintain for a vehicle as long as there are no external torques. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The docking port is in the orbital trajectory of vehicles that dock from the rear (Progress, ATV, etc), but in a curvilinear LVLH frame, not a rectilinear LVLH frame. Once the vehicle is close (around 500m or a kilometer), the differences between the two reference frames are negligible. Farther than that, the docking port can't be &quot;seen&quot; anyways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But neither the station nor the approaching vehicle maintain a constant attitude in an inertial frame at any time during docking (or berthing).&lt;/p&gt;&#xA;" OwnerUserId="12979" LastActivityDate="2016-01-25T23:48:24.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13707" PostTypeId="1" AcceptedAnswerId="13709" CreationDate="2016-01-26T12:13:17.570" Score="21" ViewCount="8990" Body="&lt;p&gt;In the &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Martian_(film)&quot;&gt;Martian movie&lt;/a&gt; Watney patched a broken door to the martian surface with plastic tarp and duct tape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pressure on Mars is 0.5 kPa, which is 200 times smaller, that on Earth (100 kPa), i.e. it is approximately vacuum outside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pressure inside is from 0.5 to 1 of atmospheres (depending on it's composition which is not clear from the movie).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, we have 0.5 atmosphere pressing onto tarp.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 atmosphere is 10 metric tons per square meter. The surface of the opening was more than 6 square meters. I don't remember visually the radius of the opening, but probably it was enough to human height passing, i.e. 2 * 1 meters. Pi * 1^2  ~ 3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, we have 30 tons pressing onto the door.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although, I can agree that plain new plastic can handle such a pressure, I can't believe it is possible to tie edges so that this huge tension would distribute equally. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is: was there some examples of closing such big openings with plastic in reality?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or what specialists are saying?&lt;/p&gt;&#xA;" OwnerUserId="5734" LastEditorUserId="25" LastEditDate="2016-01-26T13:31:18.877" LastActivityDate="2020-01-26T09:23:10.917" Title="Is it real to patch an opening to the vacuum with plastic tarp and duct tape as in Martian?" Tags="|mars|habitat|myth-busting|vacuum|" AnswerCount="2" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="13708" PostTypeId="2" ParentId="13700" CreationDate="2016-01-26T12:43:40.830" Score="6" Body="&lt;p&gt;These are the data for the Orbcomm-2 mission where &lt;a href=&quot;http://www.spacelaunchreport.com/falcon9ft.html&quot; rel=&quot;noreferrer&quot;&gt;the stage landed back on Cape Canaveral&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Meanwhile, the first stage performed a roughly 30 second boostback burn beginning about 3 minutes 50 seconds after launch and a 20 second reentry burn about 8 minutes after launch, both using three engines. A final, roughly 32 second single-engine landing burn using only the center engine took place just before the landing, about 10 minutes after liftoff.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2nR5G.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2nR5G.png&quot; alt=&quot;Falcon 9 first stage trajectory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-26T12:43:40.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13709" PostTypeId="2" ParentId="13707" CreationDate="2016-01-26T13:27:42.987" Score="21" Body="&lt;p&gt;The fix as show in the movie wouldn't work. To seal off the atmosphere in a more permanent manner, one would need to have something much stronger than Duct tape, or any tape. That wouldn't hold a seal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The book mentions a resin that they had to patch the hab. I suspect the tape was shown in the movie because it looked more jury-rigged, but in reality, they would have a solution, some sort of a resin, that would seal firmly that would be available for such an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One could make the assumption that the resin was applied off camera, and the tape was merely added to provide some additional structural support, which it indeed might do. I'll go with this option as it sounds the best.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-26T13:27:42.987" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="13710" PostTypeId="2" ParentId="13393" CreationDate="2016-01-26T13:56:46.987" Score="5" Body="&lt;p&gt;What is in Mars atmosphere? About 95.3 % CO2, 2.7 % nitrogen, 1.6 % argon and some water vapour and a lot of dust. The produced liquid oxygen should not contain water ice or dust, but a little nitrogen or argon would not harm, they would be liquid too. Liquid CH4 should not contain water ice or dust, but a little nitrogen or argon would be gaseous at the temperature of liquid CH4. CO2 is solid at the temperatures of LOX and LCH4. Therefore all remaining solids like dust, water ice and frozen CO2 should be removed.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-01-26T13:56:46.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13711" PostTypeId="1" CreationDate="2016-01-26T14:52:26.807" Score="2" ViewCount="1814" Body="&lt;p&gt;I was wondering if anyone knew a rough, ball park estimate of the cost per pound to send something to Mars. I know it can vary depending on mission, but I'm having trouble on finding an estimate. Thanks, &lt;/p&gt;&#xA;" OwnerUserId="13183" LastActivityDate="2016-01-26T15:53:02.580" Title="Cost per pound to Mars?" Tags="|mars|" AnswerCount="0" CommentCount="6" FavoriteCount="0" ClosedDate="2016-01-26T18:09:58.427" ContentLicense="CC BY-SA 3.0" />
  <row Id="13713" PostTypeId="2" ParentId="10687" CreationDate="2016-01-26T15:08:31.623" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Have there been any measurements on neutron radiation performed by the Surveyor (or later) missions?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://lunar.gsfc.nasa.gov/science.html&quot;&gt;Lunar Reconnaissance Orbiter (LRO)&lt;/a&gt; has several instruments designed to determine the radiation environment of the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Cosmic Ray Telescope for the Effects of Radiation (&lt;a href=&quot;http://crater.sr.unh.edu/lro_payload.shtml&quot;&gt;CRaTER&lt;/a&gt;) was specifically designed to &quot;...characterize the lunar radiation environment and its biological impacts...&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Lunar Exploration Neutron Detector (&lt;a href=&quot;http://l503.iki.rssi.ru/LEND-en.html&quot;&gt;LEND&lt;/a&gt;) instrument is a dedicated neutron detector and has the capability to measure up to ~15 MeV neutrons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer is yes.&lt;br&gt;&#xA;[I am not at work (still snowed-in after Janus) so I do not have access to most journal publications.  Thus, I am unable to elaborate further on the results/conclusions of the neutron radiation surveys.]&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is it possible to derive an upper bound on neutron radiation energies and flux from models of interaction of nuclei with cosmic radiation?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (&lt;a href=&quot;http://pamela.roma2.infn.it/index.php?option=com_mjfrontpage&amp;amp;Itemid=159&quot;&gt;PAMELA&lt;/a&gt;) spacecraft has a dedicated &lt;a href=&quot;http://pamela.roma2.infn.it/index.php?option=com_content&amp;amp;task=view&amp;amp;id=32&amp;amp;Itemid=133&quot;&gt;neutron detector&lt;/a&gt; to measure energies above &lt;a href=&quot;http://adsabs.harvard.edu/abs/2008ICRC....2..325S&quot;&gt;20 MeV&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far as I can tell, the detectors can go up to $&amp;gt; 10^{3}$ GeV/nucleon, but I am not sure if they are pointed at the moon.  I do know it is one of the few space born cosmic ray detectors (most cosmic ray detectors require a great deal of mass, which places significant limits on space-based missions).  They have characterized the absolute flux of protons and helium up to ~1.2 TeV, electrons up to ~600 GeV, anti-proton energy spectrum up to ~200 GeV.  There are several detailed observations in the arXiv eprint &lt;a href=&quot;http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:0904.4692&quot;&gt;0904.4692&lt;/a&gt; showing the observed cosmic ray spectrum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, I do not know the answer to your second question, but my best guess as a good start would be to look into results from PAMELA.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2016-01-26T15:08:31.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13715" PostTypeId="2" ParentId="13518" CreationDate="2016-01-26T18:36:48.500" Score="1" Body="&lt;p&gt;Some people haven't actually mentioned that sol is not the name for the sun in English. It is the name for the sun in Latin and you won't hear astronomers say &quot;Oh, look! It's the sol!&quot;. However, sol is the name of a Martian day in English, so you will hear astronomers say &quot;The astronauts will be there for approximately 53 sols!&quot;.&lt;/p&gt;&#xA;" OwnerUserId="12689" LastActivityDate="2016-01-26T18:36:48.500" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13717" PostTypeId="1" AcceptedAnswerId="13719" CreationDate="2016-01-26T19:45:04.490" Score="1" ViewCount="121" Body="&lt;p&gt;I know there are some satellites in space that monitor specific things in order for us to be able to forecast the weather.&#xA;Next to this I would like to know what other elements (mostly related to nature) do satellites monitor on earth? Deforestation?&lt;/p&gt;&#xA;" OwnerUserId="13091" LastActivityDate="2020-01-24T12:19:17.147" Title="what exactly from our ecological system is monitored by satellites?" Tags="|artificial-satellite|spacecraft|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13719" PostTypeId="2" ParentId="13717" CreationDate="2016-01-26T20:23:20.020" Score="2" Body="&lt;p&gt;Deforestation, sea levels (Jason), snow/ice cover(JPSS), amount of water vapor in the air (JPSS, GOES), lightning (GOES), cloud cover (JPSS, GOES), forest fires (JPSS, GOES), geomagnetic storms (DSCOVR), ozone levels (Jason)...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure there is a bunch of other stuff that I'm forgetting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are mostly NOAA operated satellites (though I think they have some NASA instruments on them). They are mostly weather / atmospheric science related.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some link you might find interesting:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://eospso.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;http://eospso.nasa.gov/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nesdis.noaa.gov/about_satellites.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nesdis.noaa.gov/about_satellites.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-01-26T20:36:01.730" LastActivityDate="2016-01-26T20:36:01.730" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13720" PostTypeId="1" CreationDate="2016-01-26T20:42:07.367" Score="3" ViewCount="302" Body="&lt;p&gt;Renewable energy, such as wind, solar, and geothermal, can sometimes be difficult to find optimal locations for. How are, or can, satellites be used to assist in finding the optimal locations for renewable energy generation?&lt;/p&gt;&#xA;" OwnerUserId="13091" LastEditorUserId="25" LastEditDate="2016-01-26T21:21:03.060" LastActivityDate="2016-01-27T13:52:14.383" Title="Do (Or can) satellites help renewable energy companies?" Tags="|artificial-satellite|energy|natural-resources|" AnswerCount="3" CommentCount="0" ClosedDate="2016-01-27T15:23:41.623" ContentLicense="CC BY-SA 3.0" />
  <row Id="13721" PostTypeId="2" ParentId="13720" CreationDate="2016-01-26T21:25:38.237" Score="1" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Solar- Satellites commonly measure the appearance radiation of the Earth as reflected from the Sun. They also measure cloud cover. This can assist in finding areas with few clouds and bright sunlight. Furthermore, they can help find areas where there will be minimal impact&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Wind- &lt;a href=&quot;http://science.nasa.gov/earth-science/oceanography/physical-ocean/winds/&quot; rel=&quot;nofollow&quot;&gt;Wind speeds can be detected&lt;/a&gt; using a scatterometer. The speeds are over large areas, and are primarily used to detect storms, but could be used to assist in finding optimal areas for wind farms. The winds detected tend to be higher than wind farms typically can measure.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Geothermal- &lt;a href=&quot;http://phys.org/news/2015-04-goce-gravity-satellite-geothermal-energy.html&quot; rel=&quot;nofollow&quot;&gt;GOCE&lt;/a&gt; is a satellite that detects unusually warm spots in the crust, which allow for easy geothermal energy production. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-01-26T22:24:06.003" LastActivityDate="2016-01-26T22:24:06.003" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13722" PostTypeId="1" CreationDate="2016-01-26T23:07:41.023" Score="6" ViewCount="1023" Body="&lt;p&gt;Current space suits carry oxygen bottles for oxygen and some sort of non-regenerative carbon dioxide scrubbers for removing carbon dioxide. This obviously means that a space suit is only able to provide life support for a very limited time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Around 30 liters of algae and water is able to provide enough oxygen for a human to breathe and remove the equivalent amount of carbon dioxide. This has been experimentally verified first by Shepelev and then many others. However, in some experiments this algae was spread over 8 m&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, my question is - would it (theoretically) be possible to build a suit that would use algae as regenerative life support?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is it possible to carry the algae in a wide enough spread layout to rely on natural sunlight, for example, on Mars?&lt;/li&gt;&#xA;&lt;li&gt;Is it possible to carry a power source capable of producing enough artificial light for the algae for extended periods of time?&lt;/li&gt;&#xA;&lt;li&gt;Atmospheric leakage would have to be replenished somehow, but perhaps this could be a smaller oxygen bottle lasting for a very long time?&lt;/li&gt;&#xA;&lt;li&gt;Water vapor would have to be condensed but that could work like it does on current suits?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2631" LastActivityDate="2016-01-27T02:37:51.283" Title="Is space suit using algae for oxygen production possible?" Tags="|spacesuits|life-support|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13723" PostTypeId="1" CreationDate="2016-01-26T23:54:40.257" Score="2" ViewCount="182" Body="&lt;p&gt;There are standard launchers and standard upper stages used for many different missions within cis-lunar space. Deep space missions are so rare maybe it makes no sense to try and come up with something like that for them. Then again, maybe if one existed there would be more deep space missions. Look at how many people have started using Cubesats.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a modular platform be created that could adapt to the all the kinds of payloads that exist, maybe by using small engines and solar cell units so sizing is very flexible? Would it help to save development and fabrications costs over many missions if there was a standard platform that could adequately shield instruments and integrate heaters and transmitters and such?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="25" LastEditDate="2016-01-27T01:58:17.803" LastActivityDate="2016-01-27T03:11:09.410" Title="Would it make sense to design a standard stage for deep space missions?" Tags="|rockets|probe|deep-space|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13724" PostTypeId="2" ParentId="13722" CreationDate="2016-01-27T00:07:09.487" Score="3" Body="&lt;p&gt;Spacesuits use rebreather technology. Exhaled air is passed through CO2 scrubbers, then measured for oxygen content. Oxygen from a 100% oxygen source is added to the air stream to bring it back up to normal.&lt;/p&gt;&lt;p&gt;Exhaled air still has oxygen in it, so the air stream only needs to be 'topped up'.&lt;/p&gt;&lt;p&gt;There's actually more to it - the air handling system also removes excess humidity, odors, and other possible contaminants. Water condensed from the humidity goes into a storage pocket that the astronaut can draw from. There's a secondary O2 system in case the main life support system fails. It's good for at least 30 minutes, depending on the design. That's barely enough time to get someone back into the ISS atmosphere.&#xA;While using an algae system separately to generate the oxygen used in the space suits might be feasible, at the very least it would highly increase the complexity of the air handling system in a space suit, making it much more prone to disastrous failure.&lt;/p&gt;&#xA;" OwnerUserId="12071" LastEditorUserId="12071" LastEditDate="2016-01-27T00:14:35.233" LastActivityDate="2016-01-27T00:14:35.233" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13725" PostTypeId="2" ParentId="13723" CreationDate="2016-01-27T01:58:04.867" Score="2" Body="&lt;p&gt;The problem is, all deep space missions have their own missions, which have different requirements. The masses change significantly, depending on the payload. Does it need to have fast acceleration, or slow? How much delta v? How long does the fuel need to be stored? All deep space missions have various amounts for all of these, making it difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some modularity, however. There are common thrusters, tanks, lines, etc, all of which are relatively easy to get a hold of. These same parts can be used for satellites as well, reducing their cost. Overall, this is probably a better approach, to have standardized parts that can be fitted together, until we start sending a large number of nearly identical ships.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-27T01:58:04.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13726" PostTypeId="2" ParentId="13722" CreationDate="2016-01-27T02:37:51.283" Score="6" Body="&lt;p&gt;The one critical problem is heat dissipation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine components of such a space suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;8m^2 utilizing sunlight is far too unwieldy to carry around. This needs to be far more compact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;30l is bad, but not very bad. It can be done in a way that would be useful in microgravity. Think multiple thin modules composed of moisturized algae caked between layers of thin LED growth lights. An active pumping system (sorry, no gravitational bubbling separation in microgravity) with water reclamation/reintroduction, a good RTG battery to keep it powered, extra filters for moisture and other contaminations. Everything works... except for the heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar irradiation on Earth in a moderate climate in reasonable though not excellent weather would be of order of 200-300W per m^2. So, 1000-1500W. Efficiency - let's extremely generously assume 10% (normally 3-7%). So the RTG alone creates at least 9kW of heat. Even if we use different power sources we still have 1-1.5kW for growth lights, and that's still not including the pumping, any active thermoregulation systems, mixing etc. The spacesuit is an oven that would require simply huge radiators - something far too unwieldy to use as a portable unit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could see a system like that used even with very small spacecraft. I could picture it used on a space station with non-autonomous space suits tethered with air hoses. But the heat dissipation needed just won'tfit in the volume of any practical spacesuit.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-01-27T02:37:51.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13727" PostTypeId="2" ParentId="13723" CreationDate="2016-01-27T03:11:09.410" Score="4" Body="&lt;p&gt;For deep space missions there's rarely any &lt;em&gt;stage&lt;/em&gt; at all. It's usually a probe launched into cislunar space using standard cislunar stages, and then the probe uses its own propulsion system to reach its destination in deep space, no separate stage for that. But then, there &lt;em&gt;could&lt;/em&gt; be a standarized system/platform for deep space propulsion, which isn't a stage, just a standarized probe module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While we got around to get a pretty consistent set of launchers, the development work for deep space propulsion is constantly ongoing. There are dozens of various ion engine designs, more or less standarized RCS propulsion systems, alternative propulsions like solar sail, and so on. It's all very much a work in progress - practically each new mission on top of the new mission goals is testing a new propulsion system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while creating a common, standard deep space propulsion system would come with some benefits, it would come with one critical disadvantage: depriving us of test platforms for new deep space propulsion systems. Any probe flying on the new standarized platform would be NOT testing a new, newly developed propulsion system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe once the development reaches some stalemate/cooldown situation where not much new can be designed and new designs will cease to sprout more frequently than new deep space missions occur that will make sense. If, similarly to current chemical engines, we reach a situation where there's quite hard to invent anything significantly better than existing designs, where developing a new platform around the new engine is less beneficial than going with &quot;tried and true&quot; (because the incremental advantages of the new engine aren't a significant improvement over the old), such platform - a standarized propulsion system for deep space - will become considerably advantageous. Currently there are simply too many good, new, untested systems and systems even better (and considerably so!) still in development to justify freezing the progress through adopting one standarized platform.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-01-27T03:11:09.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13728" PostTypeId="2" ParentId="13720" CreationDate="2016-01-27T03:30:37.387" Score="1" Body="&lt;p&gt;Let me add there are satellites that monitor the oceans, in particular waves, tides and currents. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are all sources of renewable energy, either used already or with systems for harvesting them being developed, and the satellite data can definitely help with placing these.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-01-27T03:30:37.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13729" PostTypeId="1" AcceptedAnswerId="13731" CreationDate="2016-01-27T05:30:48.320" Score="56" ViewCount="9127" Body="&lt;p&gt;If the Saturn V rocket along with its Apollo spacecraft was miniaturized, for example to 1/72 scale so it was five feet tall, could it still perform a moon landing like the Apollo missions and get back to Earth? The rocket equation only involves the percent mass of the rocket as propellant for a certain delta-v, so it seems that a miniature Saturn V would have the same total delta-v as the real thing.&lt;/p&gt;&#xA;" OwnerUserId="11820" LastEditorUserId="195" LastEditDate="2016-01-29T03:57:56.210" LastActivityDate="2016-01-30T19:26:24.930" Title="Can a miniature Saturn V get to the moon and back?" Tags="|launch|rockets|delta-v|" AnswerCount="6" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13730" PostTypeId="1" AcceptedAnswerId="13733" CreationDate="2016-01-27T06:24:05.247" Score="5" ViewCount="349" Body="&lt;p&gt;My interest was piqued by this &lt;a href=&quot;https://space.stackexchange.com/q/13518/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/13522/12102&quot;&gt;answer&lt;/a&gt;. For several reasons NASA needed to refer frequently and extensively to a &lt;em&gt;Martian solar day&lt;/em&gt;, and  &quot;sol&quot; became the &lt;em&gt;nom du jour.&lt;/em&gt; There have been remotely controlled rovers on the Moon as well, but I don't know in the Russian or Chinese if the corresponding word for &lt;em&gt;month&lt;/em&gt; was used for convenience, or if a new term for &lt;em&gt;Lunar day&lt;/em&gt; was coined for the occasion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar days are important for exploring bodies. Oblique lighting (coming from a direction other than the direction of the camera) is extremely important for 3D reconstruction of the landscape for navigation. Surface conditions (liquids, slushes, venting, boiling) may be very important for a rovers mission. Except for water's freakishly high melting and boiling points (273K, 373K) the materials on the surfaces of many solar system bodies can often have phase changes at the much lower temperatures the sun produces far beyond earth's orbit. Potential ionospheric phenomena may also be of interest, and of course on some bodies it can get prohibitively &lt;em&gt;cold&lt;/em&gt; at night.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While some landers have used and will use solar power, this is usually used to charge batteries which could easily deliver power at night. Of course some landers will cary other sources of power, including radioisotope based thermo-electric.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is, besides &lt;code&gt;body_name_as_adjective + &quot;day&quot;&lt;/code&gt; (e.g. Lunar day, Martian day, etc.) has any other term been used in space exploration to refer to a solar day on a solar system body besides &lt;em&gt;sol&lt;/em&gt; on Mars?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-01-27T16:47:21.967" Title="Has any other term been used for a solar day on a solar system body besides &quot;sol&quot; on Mars?" Tags="|terminology|planet|moons|time|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13731" PostTypeId="2" ParentId="13729" CreationDate="2016-01-27T07:26:54.320" Score="64" Body="&lt;p&gt;If you could miniaturize each component uniformly, you're correct that the rocket equation terms would all balance out and the rocket would be capable of the same delta-v performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the impact of atmospheric drag would be much worse; drag force is proportional to cross sectional area, not to volume. A Saturn V loses less than 1% of its delta v potential to drag; a five foot version would probably lose so much to drag that it couldn't reach orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine there would be other square-versus-cube-scaling problems involved in ridiculously tiny fuel pumps, injectors, and combustion chambers as well, probably making the engines vastly less efficient or preventing them from working altogether, but since I don't know much about those issues we will assume some sort of elfin magic is taking care of that end of things.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-01-27T07:26:54.320" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13732" PostTypeId="2" ParentId="13518" CreationDate="2016-01-27T09:10:09.230" Score="1" Body="&lt;p&gt;First some definitions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sol is short for Solar day, that is, the time it takes to get from a point on day one to the same point on day two, based on the Earth's motion relative to the sun (i.e., from the point where the sun is directly overhead one day to the sun in the same position the next day). What is usually used is the average over time, as the length of a solar day can vary (and does, on Earth and Mars, due to axial tilt and elliptical orbits). One mean solar day is 24 hours on Eath, and 24 hours, 39 minutes, 35.2 seconds on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Sidereal day, on the other hand, is the time it takes to get from a point on day one to the same point on day two, based on the Earth's motion relative to the stars. It is the time it takes a planet to rotate completely around its axis. It likewise varies. One mean sidereal day on Earth is 23 hours, 56 minutes, 4.1 seconds; on Mars is 24 hours, 37 minutes, 22 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sol is the Latin name for the sun (and some other languages derived from Latin), but context should quickly distinguish the two meanings; I can think of no examples (that are not needlessly convoluted) where the two could be mixed up in usage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shorter 'Sol' could also be used for the solar days of other planets rather than the full 'Solar day', but is not likely to be, just to avoid confusion. (Mars got dibs on that one!) It might be used with the name of the planet or moon to sufficiently distinguish it. So far the issue hasn't come up, as there is rarely need to reference the length of days on other planets. It only arose with Mars because of the rovers and probes that have been sent there that operate on solar power, making it more convenient for the people who worked with them to operate on the same solar schedule that Mars does. Humans being humans, &quot;Solar day&quot; got shortened to &quot;Sol&quot;, and after enough use that became the recognized word for a solar day on Mars (and 'yestersol' as the solar day prior to the current Sol).&lt;/p&gt;&#xA;" OwnerUserId="13067" LastActivityDate="2016-01-27T09:10:09.230" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13733" PostTypeId="2" ParentId="13730" CreationDate="2016-01-27T11:31:51.547" Score="4" Body="&lt;p&gt;&quot;Lunation&quot; is informally used as a &quot;calendar&quot; for lunar days. However I'm not sure it can be said to be an equivalent of &quot;sol&quot; because it's more properly known as the lunation &lt;em&gt;number&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So probably the martian &quot;sol&quot; is the only commonly used example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, I believe people working on the current probes started to use &quot;sol&quot; because the Martian day is so close to the Earth day; there is always a good chance there will be confusion over exactly which day type is being referred to...&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-01-27T11:31:51.547" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13734" PostTypeId="2" ParentId="13720" CreationDate="2016-01-27T12:00:04.403" Score="3" Body="&lt;p&gt;Earth observation from space is &lt;strong&gt;crucial&lt;/strong&gt; in many ways for renewable energy companies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth observation is crucial for &lt;strong&gt;weather forecasts&lt;/strong&gt;.  Most forms of renewable energy are weather-dependent:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Solar energy performs best when the sun is shining.&lt;/li&gt;&#xA;&lt;li&gt;Wind energy performs best when it's windy (within limits).&lt;/li&gt;&#xA;&lt;li&gt;Hydro energy requires well-filled reservoirs, which requires rainfall and snowfall.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For short-term planning of electricity, it is crucial to know the short-term weather forecast.  In fact, if you're better at forecasting the weather than the market, you can get seriously rich by trading in wind and solar energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For long-term planning of locations for any of those, Earth observation from space is equally crucial, as we need to know the &lt;em&gt;climate&lt;/em&gt;.  Ground-based observations are only limited to specific spots, and reanalysis data relies heavily on satellite measurements.  Reanalysis data in turn informs decision-takers of where is a suitable spot for solar, wind, or hydro plants.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2016-01-27T13:52:14.383" LastActivityDate="2016-01-27T13:52:14.383" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13736" PostTypeId="2" ParentId="13729" CreationDate="2016-01-27T13:37:19.277" Score="41" Body="&lt;p&gt;You've got a few problems:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;When a model is scaled to 1:2 it's size, its area drops by 1/4 but its volume by 1/8. This is known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Square-cube_law&quot; rel=&quot;noreferrer&quot;&gt;square-cube scaling problem&lt;/a&gt;. So you will have one quarter the drag but one eighth the impulse. &lt;a href=&quot;https://space.stackexchange.com/a/13731/472&quot;&gt;Russell expands on this&lt;/a&gt;, so I'll address the other issues.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Reynolds_number&quot; rel=&quot;noreferrer&quot;&gt;Reynolds number&lt;/a&gt; of the air does not scale with the model. So you not only have the square-vs-cube scaling problem (which could be mitigated overcome by making the model a bit longer), you've got probably an order of magnitude more drag per unit of area. You can think of this as the air being thicker from the craft's point of view. At extreme (nano) scaling the air molecules can even act as discrete particles colliding with the model!&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If acceleration is scaled then the model will spend much more time in the atmosphere. Each second not in orbit will use up to 9.81 m/s of delta-v on &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_drag&quot; rel=&quot;noreferrer&quot;&gt;gravity drag&lt;/a&gt;, no matter how much your craft weighs or how much propellant you brought with you.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="472" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-01-27T19:19:41.103" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13737" PostTypeId="2" ParentId="13729" CreationDate="2016-01-27T15:05:06.000" Score="25" Body="&lt;p&gt;In addition to the other answers, you'd also have to take temperature into the equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your cryogenics tanks have much thinner thermal insulation, primarily LH2 (LOX was &quot;insulated&quot; by the tank hull + &lt;a href=&quot;http://history.nasa.gov/afj/s-ii/s-ii-insulation.html&quot;&gt;ice, yes, really&lt;/a&gt;) but the cryogenic is still at the same low temperature. Much more ice build-up on the outside in relation to the Saturn's total weight. Much more pressure build up in the tanks due to increased boiling. Instead of a smooth lift-off I'd expect some crackling, rupturing and hydrogen/oxygen rivers on the pad...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you'd definitely need not worry about &lt;a href=&quot;https://en.wikipedia.org/wiki/Pogo_oscillation&quot;&gt;pogo&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_engine#Combustion_instabilities&quot;&gt;combustion instability&lt;/a&gt; in the F-1 engines. :-)&lt;/p&gt;&#xA;" OwnerUserId="716" LastEditorUserId="716" LastEditDate="2016-01-29T12:53:29.717" LastActivityDate="2016-01-29T12:53:29.717" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13738" PostTypeId="2" ParentId="13729" CreationDate="2016-01-27T15:30:32.593" Score="7" Body="&lt;p&gt;Also don't forget that the tanks would be thinner in general, making it much more likely to break through the tank, and suffer problems. The tanks need to be able to withstand the pressure and the weight of the payload and rocket. Thin walls would probably still hold the weight, but they probably wouldn't hold the pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall, it'd be nice if this would work, but there are a lot of problems in terms of miniaturization, and all orbital rockets are big for a reason.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-01-27T17:02:44.250" LastActivityDate="2016-01-27T17:02:44.250" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13739" PostTypeId="2" ParentId="13730" CreationDate="2016-01-27T15:53:49.727" Score="3" Body="&lt;p&gt;There are only 2 &quot;regular&quot; bodies that have had long term landers visit them, Mars and the Moon. Small bodies, asteroids and comets, have unpredictable light cycles. Titan and Venus, which have both had landers, had very short term landers. Bottom line is, if this was going to apply, it would only apply to either Mars or the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon is somewhat difficult, because few spacecraft survive the lunar night. Also, it is so long that it doesn't mean as much. As Andy mentioned in his answer, it is called Lunation, the day/month of the Moon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-27T15:53:49.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13740" PostTypeId="2" ParentId="13730" CreationDate="2016-01-27T16:47:21.967" Score="0" Body="&lt;p&gt;&quot;Sol&quot; is the word for &quot;Sun&quot; in several languages. Since ancient times, humans measured the time by counting how many times the Sun rised and setted in the sky, which is what is known as day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other planets, measuring time using Earth-days would be complicated due to not being in sync with the planet's rotation, so a simple solution is to count how many times the Sun rised and setted.&lt;/p&gt;&#xA;" OwnerUserId="8140" LastActivityDate="2016-01-27T16:47:21.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13741" PostTypeId="1" AcceptedAnswerId="13760" CreationDate="2016-01-27T18:57:55.453" Score="9" ViewCount="1754" Body="&lt;p&gt;Air-augmented rockets have not historically been very successful. In most cases, adding a shroud around the outside of an existing rocket was a large weight cost in exchange for only a modest increase in thrust specific fuel consumption. Because they weren't optimized for using the air as reaction mass, most of the added thrust was the result of secondary combustion between the fuel-rich rocket exhaust and the atmospheric air, making them essentially very inefficient ramjets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, however, an air-augmented rocket engine were designed in an inside-out configuration with a central bypass rather than an external bypass, you'd could end up with a simpler, lower-drag design which could be scaled up and would likely be pretty high in efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OZgWE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OZgWE.png&quot; alt=&quot;central bypass air-augmented rocket&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such a design could allow a really, really high bypass ratio, causing thrust specific fuel consumption to drop ridiculously low. The combination of really high thrust and really high specific impulse is pretty nice. How high of a bypass ratio would be possible before diminishing returns made the added drag too high?&lt;/p&gt;&#xA;" OwnerUserId="13211" LastActivityDate="2016-01-28T22:16:34.213" Title="Limits of bypass ratio in air-augmented (ejector-jet/ducted) rocket" Tags="|rockets|atmosphere|design|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13742" PostTypeId="1" AcceptedAnswerId="13743" CreationDate="2016-01-27T19:04:46.153" Score="8" ViewCount="1331" Body="&lt;p&gt;In the landing and landing attempts I've seen from the Falcon 9 first stage boosters, the landing legs are always deployed just a split second before landing, right when they are about to touch down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any idea why this is? It would seem that deploying them earlier would provide a greater drag surface, reducing the fuel needed to slow down. If they can hold the whole rocket up, I'd imagine they should be able to deal with the stresses of opening in flight.&lt;/p&gt;&#xA;" OwnerUserId="13211" LastEditorUserId="25" LastEditDate="2016-01-27T19:08:46.657" LastActivityDate="2016-01-27T20:40:29.977" Title="SpaceX Falcon 9 landing leg deployment time" Tags="|rockets|spacex|falcon-9|landing|booster|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13743" PostTypeId="2" ParentId="13742" CreationDate="2016-01-27T19:08:31.493" Score="8" Body="&lt;p&gt;Stability is one big reason. Fins are typically placed near the bottom of rockets, which means they are in the opposite direction of movement. This is because that position tends to make the rocket more stable. If the legs were deployed, it would tend to make the rocket flip around, as the leg &quot;fins&quot; would tend to want to be furthest away from the direction of motion, or upside-down. The rocket is poorly balanced as it is, so adding such an instability would make things even more difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, note that if the legs deployed earlier, they would be subject to more stress, which might cause them to fail.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-01-27T19:31:37.220" LastActivityDate="2016-01-27T19:31:37.220" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13744" PostTypeId="1" CreationDate="2016-01-27T19:19:33.680" Score="1" ViewCount="342" Body="&lt;p&gt;What's the absolute minimum time in which spacecraft docking can be achieved? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question isn't so much about the time to match orbits as it is about the time it takes to maneuver into the proper orientation and execute the individual docking steps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a better example than my original one -- suppose you have a Dragon V2 approaching the ISS when they suddenly realize that their CO&lt;sub&gt;2&lt;/sub&gt; scrubbers have failed. They need to dock as rapidly as possible; every second counts. The capsule is already on a course which will take it right up into alignment for docking, but it will still have to be oriented, mated, and so forth. If they act as rapidly as they possibly can, how long will it take before they can breathe fresh air again (ignoring the remaining time it takes to close the distance with the ISS)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously there would be redundancies in place for the failure of the CO&lt;sub&gt;2&lt;/sub&gt; scrubbers (if the Dragon V2 even has those). This scenario is just for the sake of discussion.&lt;/p&gt;&#xA;" OwnerUserId="13211" LastEditorUserId="13211" LastEditDate="2016-02-02T16:44:45.323" LastActivityDate="2016-03-03T19:36:18.433" Title="Minimum spacecraft docking time" Tags="|spacex|reentry|dragon|docking|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13745" PostTypeId="2" ParentId="13742" CreationDate="2016-01-27T20:40:29.977" Score="7" Body="&lt;p&gt;At the start of the landing burn, the stage is still supersonic. If they deploy the legs earlier, they'd get lots of aerodynamic heating. Now the legs can withstand a fair amount of heat: the Orbcomm-2 landing video showed the legs were on fire just before the landing. But that's just a few seconds. A long heat soak may induce structural failure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, SpaceX does plan to refine the design so they can open the legs &lt;a href=&quot;https://twitter.com/elonmusk/status/536268250620125185&quot;&gt;earlier&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Using legs as air brakes to drop terminal velocity in half requires slight redesign &amp;amp; more data. Maybe flight 21.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-01-27T20:40:29.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13746" PostTypeId="1" CreationDate="2016-01-27T21:52:27.397" Score="3" ViewCount="364" Body="&lt;p&gt;I know I might get an answer like &quot;the sky is not the limit&quot;...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But considering the actual state of the art of modern satellites &lt;a href=&quot;http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-851-satellite-engineering-fall-2003/lecture-notes/l9_acs.pdf&quot; rel=&quot;nofollow&quot;&gt;ADC systems&lt;/a&gt;. Haven't we reached a &quot;maximum&quot; yet? Or are there still a lot of improvements which have to be made? What is being worked on for the moment? What are some major difficulties engineers developing those systems are facing nowadays?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are some mission failures caused by the ADCS (Attitude Determination and Control Systems)?&lt;/p&gt;&#xA;" OwnerUserId="13091" LastEditorUserId="8693" LastEditDate="2016-01-27T22:46:41.770" LastActivityDate="2019-03-15T15:55:53.940" Title="State of the art of satellite Attitude Determination and Control Systems" Tags="|attitude|navigation|guidance|mission-control|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13747" PostTypeId="2" ParentId="13729" CreationDate="2016-01-27T22:01:24.993" Score="13" Body="&lt;p&gt;Even setting aside the other problems, rocket engines themselves do not scale indefinitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You decrease the scale of a rocket motor, you decrease the thickness of the rocket chamber walls, which in turn decreases the bursting pressure of said rocket chamber. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you scale the rocket down enough, this will become a limiting factor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At that point you will have to drop the pressure of the rocket chamber to avoid bursting it, which, since the maximum specific impulse achievable by a chemical rocket is limited by (among other things) chamber pressure, means that there will come a point where your specific impulse has to drop as you decrease the scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And sooner or later (read: sooner), it will reach a point where the specific impulse is low enough that the effective delta-v is too low to reach orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have analogous problems with thrust (pressure limited, again), temperature inside the rocket chamber (surface area to volume ratio increases as the scale decreases, causing the percentage heat loss to rise), and tankage (pressure limited, yet again).&lt;/p&gt;&#xA;" OwnerUserId="12210" LastActivityDate="2016-01-27T22:01:24.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13749" PostTypeId="1" AcceptedAnswerId="13750" CreationDate="2016-01-27T23:35:25.610" Score="15" ViewCount="2409" Body="&lt;p&gt;After reaching a peak altitude of 222 km the Ariane 5 launcher with Intelsat 29e starts to lose altitude. What is the reason for this trajectory?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=cJBzLGljmg8&quot; rel=&quot;noreferrer&quot;&gt;Youtube video&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/l2jdj.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/l2jdj.png&quot; alt=&quot;altitude&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.arianespace.com/wp-content/uploads/2016/01/Dossier-de-presse-VA-228-final-GB.pdf&quot; rel=&quot;noreferrer&quot;&gt;Paper on Ariane 5&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-28T05:09:32.873" LastActivityDate="2016-01-28T05:09:32.873" Title="What is the reason for the Ariane 5 launcher with Intelsat 29e losing altitude?" Tags="|launch|trajectory|geostationary|altitude|ariane|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13750" PostTypeId="2" ParentId="13749" CreationDate="2016-01-27T23:59:42.043" Score="24" Body="&lt;p&gt;Altitude drops like that are common when the orbital stage has a high-efficiency, low-thrust engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes a few minutes for the upper stage to bring the craft up to orbital speed. During that period, the craft is indeed starting to fall back towards Earth. The rocket's travel over the curvature of the Earth contributes an effective altitude gain that increases as the rocket's speed increases, slowing the fall. At circular orbit speed, the fall and the curvature cancel out exactly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lower the acceleration of the stage that does the orbital insertion, the more dramatic the fall is. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, the HM-7B engine with a thrust of 67kN is pushing the 19 ton ESC-A second stage -- mostly fuel -- plus more than 7 tons of payload. This yields roughly 0.25 g of acceleration at the start of the burn, increasing to something like 0.65 g at the end.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is in contrast to something like Falcon 9, where the second stage&#xA;has an initial acceleration around 0.75g, increasing to 3g or so as fuel runs out. That would produce a much shallower dip, if any. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-01-28T00:19:37.000" LastActivityDate="2016-01-28T00:19:37.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13751" PostTypeId="2" ParentId="13749" CreationDate="2016-01-28T00:11:48.213" Score="10" Body="&lt;p&gt;We know that the tough part of getting to space isn't getting there, it's &lt;a href=&quot;https://what-if.xkcd.com/58/&quot; rel=&quot;noreferrer&quot;&gt;going fast enough to stay there&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3o1tI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3o1tI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the key reasons is that the rocket is concentrating more on gaining horizontal speed than vertical height. So the rocket will eventually start being pulled down vertically. The atmosphere is such at that altitude that the rocket won't be affected by it much, so it continues to function the same. Eventually, when orbital speeds are approached, the small vertical motion will keep the spacecraft accelerating upward, in addition to having the nearly orbital horizontal speed. And then orbit is achieved, with ever increasing vertical altitude.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-28T00:11:48.213" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13752" PostTypeId="2" ParentId="13746" CreationDate="2016-01-28T00:20:45.897" Score="1" Body="&lt;p&gt;The major improvements in ADCS are caused when there are failed components, and one has to work around these limitations. For instance, Kepler has only 2 functioning reaction wheels, when it originally required 3. Advances in ADCS have allowed it to retain some stability utilizing the 2, using the Sun as a third force. Hubble used similar technology as well, for broken reaction wheels, until they were replaced in a Shuttle servicing mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basics are already known, as you stated, and in fact, have been known for some time. There are refinements to further improve the systems, but the algorithms are known.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-28T00:20:45.897" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13753" PostTypeId="1" AcceptedAnswerId="13754" CreationDate="2016-01-28T01:38:15.573" Score="5" ViewCount="498" Body="&lt;p&gt;Let's say I won the jackpot on the lottery and want to finally launch the satellite I've been building in my garage for past few years and just finished yesterday.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How long would it take between me contacting the space agency with the shortest queue, and the satellite reaching LEO? Assuming I pay the regular fare, no skipping the line or premium express services, but my sat is top-notch, attach it to the fairing and it's good to go, if any inspections are required, the sat would pass them with flying colors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(more seriously, just how long is the usual backlog of commercial satellites awaiting launch, currently - how far into the future do payload manifests reach?)&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2016-01-28T12:39:45.810" LastActivityDate="2016-01-30T08:25:54.107" Title="How long are the wait queues for commercial payloads?" Tags="|launch|artificial-satellite|economics|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13754" PostTypeId="2" ParentId="13753" CreationDate="2016-01-28T08:31:26.703" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;http://www.arianespace.com/wp-content/uploads/2015/09/Ariane5_users_manual_Issue5_July2011.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5 user manual&lt;/a&gt; contains a mission integration schedule that's 24 months long (page 140). This includes a pile of analysis and tests to ensure spacecraft compatibility with the launcher.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9 user manual&lt;/a&gt; has a schedule that's 22 months long. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More user manuals &lt;a href=&quot;https://space.meta.stackexchange.com/questions/249/resources-and-references-considered-as-literary-canons-on-the-topic-of-interpl/319#319&quot;&gt;can be found here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: fixed the links.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:04.403" LastActivityDate="2016-01-28T13:09:46.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13755" PostTypeId="1" CreationDate="2016-01-28T12:31:58.200" Score="1" ViewCount="571" Body="&lt;p&gt;What are the mechanisms of &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_wheel#Failures_and_replacement&quot; rel=&quot;nofollow&quot;&gt;reaction wheel anomalies&lt;/a&gt;? Are there any &lt;a href=&quot;https://en.wikipedia.org/wiki/Ishikawa_diagram&quot; rel=&quot;nofollow&quot;&gt;fish bone&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Fault_tree_analysis&quot; rel=&quot;nofollow&quot;&gt;fault tree analyses&lt;/a&gt; on these issues?&lt;/p&gt;&#xA;" OwnerUserId="13224" LastEditorUserId="49" LastEditDate="2016-01-28T14:14:18.423" LastActivityDate="2022-04-27T11:43:19.723" Title="What are the causes of Reaction Wheel Failures/Anomalies?" Tags="|attitude|reference-request|engineering|failure|reaction-wheel|" AnswerCount="1" CommentCount="2" ClosedDate="2016-01-30T09:36:10.933" ContentLicense="CC BY-SA 3.0" />
  <row Id="13756" PostTypeId="2" ParentId="13753" CreationDate="2016-01-28T12:38:08.370" Score="3" Body="&lt;p&gt;It really depends on how much cash you are willing to put up, and how much testing you've already done. There are a series of tests that must be done prior to a rocket launch to ensure compatibility. These include fit checks, flight path analysis, vibration analysis, and others. If you were to be the primary load on a spacecraft, then you'd have to make sure all of these were done. If you paid extra cash to ensure that you were given priority, and bumped other payloads, then you might be able to accomplish the task in 6 months, especially if you were willing to be flown on an experimental flight, such as the Falcon 9, that happened to be aligned with your time table.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most parallel example in recent years that I've seen to your situation was the launch of the Cygnus capsule on an Atlas 5. The failure of &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;Cygnus&lt;/a&gt; was at the end of Oct 2014, the launch of Atlas 5 was Dec 2015. I suspect just figuring out which provider would launch required a month or two, making that time to launch at about a year. I doubt that they paid a premium to do this launch, although I would argue that any ULA launch comes at a premium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, note that with SpaceX in particular, the manifest has been in play for a long time. Several of the payloads launched had been waiting on the ground for extended periods of time, ready to launch. Some satellites also aren't completely ready. You might be able to slip in the cracks and have a launch sooner, if you are completely ready to go, or you might get bumped for some time. There is a lot of randomness to rocket launches still to this day.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-28T12:38:08.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13757" PostTypeId="2" ParentId="13755" CreationDate="2016-01-28T14:11:45.747" Score="2" Body="&lt;p&gt;I'm going to presume that a &lt;a href=&quot;https://space.stackexchange.com/questions/1314/why-do-reaction-wheels-fail-so-frequently&quot;&gt;related question&lt;/a&gt; linked in the comments answers the first part of the question and will focus on the latter part of it as a reference request follow-up to the linked one. Since no specific mission's methodology was inquired about, here are some arbitrarily selected publications on the topic of fault detection and management:&lt;/p&gt;&#xA;&lt;p&gt;Example:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.trylam.com/files/2001IEEE_A_Systematic_Risk_Management_Approach_Employed_on_the_CloudSat_Project.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Systematic Risk Management Approach Employed on the CloudSat Project&lt;/a&gt;, R. R. Basilio et al., Jet Propulsion Laboratory, California Institute of Technology, California, 2000 (PDF)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;General references:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://spectrum.library.concordia.ca/id/eprint/8103/1/MQ94691.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Fault-Tree Approach for Identifying Causes of Actuator Failure in Attitude Control Subsystems of Space Vehicles&lt;/a&gt;, A. Barua, Department of Electrical and Computer Engineering, Concordia University Montreal, Canada, 2004 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://ieeexplore.ieee.org/document/1507339&quot; rel=&quot;nofollow noreferrer&quot;&gt;A novel fault-tree approach for identifying potential causes of satellite reaction wheel failure&lt;/a&gt;, A. Barua et al., Department of Electrical and Computer Engineering, Concordia University Montreal, Canada, 2005&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140002994.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Goal-Function Tree Modeling for Systems Engineering and Fault Management&lt;/a&gt;, S. B. Johnson et al., Jacobs ESSSA Group, American Institute of Aeronautics and Astronautics, 2014 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.dglr.de/publikationen/2013/281268.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Innovative Fault Detection, Isolation and Recovery Strategies On-Board Spacecraft: State of the Art and Research Challenges&lt;/a&gt;, A. Wander &amp;amp; R. Förstner, Bundeswehr University Munich, Institute of Space Technology and Space Applications, Neubiberg, Germany, 2012 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://web.archive.org/web/20180417054203/http://www.aerospace.org:80/wp-content/uploads/2015/04/TOR-2014-02202-Root-Cause-Investigation-Best-Practices-Guide.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Root Cause Investigation Best Practices Guide&lt;/a&gt;, R. J. Duphily, Acquisition Risk and Reliability Engineering Department, National Reconnaissance Office, 2014 (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://web.archive.org/web/20150708181400/http://www.aerospace.org/wp-content/uploads/2015/04/TOR-20098591-14-Effective-Fault-Management-Guidelines.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Effective Fault Management Guidelines&lt;/a&gt;, S. L. Hogan, Digital and Integrated Circuit Electronics Department, Air Force Space Command, El Segundo, California, 2009 (PDF)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="10273" LastEditDate="2022-04-27T11:43:19.723" LastActivityDate="2022-04-27T11:43:19.723" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="13758" PostTypeId="1" CreationDate="2016-01-28T14:51:49.110" Score="6" ViewCount="237" Body="&lt;p&gt;The CRS (Cargo Resupply Services) 2 contract was announced with three winners, SpaceX for Dragon, Orbital for Cygnus, and SNC for Dream Chaser. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How will the cargo version of Dream Chaser differ from proposed manned version?  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-27T18:47:06.617" Title="How does the Dream Chaser manned differ from Dream Chaser cargo?" Tags="|uncrewed-spaceflight|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13760" PostTypeId="2" ParentId="13741" CreationDate="2016-01-28T20:04:50.630" Score="7" Body="&lt;p&gt;For an upper bound, the exhaust velocity of of the rocket sets an absolute limit. If you are flying faster than that, the intake air is actually slowed down when added to the mix, because &lt;strong&gt;the rocket exhaust is moving so slowly&lt;/strong&gt;. For the most efficient propellant combination in use, LOX/LH2, the exhaust velocity is about &lt;span class=&quot;math-container&quot;&gt;$4,500 m/s$&lt;/span&gt;, and for hydrocarbons, about &lt;span class=&quot;math-container&quot;&gt;$3,500 m/s$&lt;/span&gt;. That places the limit approximately in the same range as scramjets.&lt;/p&gt;&#xA;&lt;p&gt;However, even if the extra air is increasing the ISP, it steals energy from the propellant, reducing the exhaust velocity. (when using air-breathing engines, ISP and exhaust velocity is not equivalent)&#xA;The limit is then also dependent on the ratio of rocket propellant and intake air flow.&lt;/p&gt;&#xA;&lt;p&gt;Furthermore, for the extra efficiency to matter, the throughput of intake air has to stay high for quite some time, necessary meaning that the rocket has to fly for a long time experiencing high drag.&lt;/p&gt;&#xA;&lt;p&gt;At high velocities, minimizing the drag of the intakes is important, calling for a straight-through design. At that point you can ask yourself the qustion: What is preventing the rocket exhaust from escaping in that direction as well? If the rockets are firing in the wrong direction, you are having a bad problem, and &lt;a href=&quot;https://xkcd.com/1133/&quot; rel=&quot;noreferrer&quot;&gt;you will not go to space today.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In short, the limit is at most &lt;span class=&quot;math-container&quot;&gt;$4,500 m/s$&lt;/span&gt;, and certainly quite a bit less.&lt;/p&gt;&#xA;&lt;p&gt;As a rule of thumb for an acceptable bypass ratio, you can assume that the exhaust velocity is divided by the square root of the airflow to total flow ratio.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-01-28T22:16:34.213" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13761" PostTypeId="1" CreationDate="2016-01-28T23:47:04.173" Score="6" ViewCount="400" Body="&lt;p&gt;Here is some renditions of &lt;a href=&quot;https://en.wikipedia.org/wiki/216_Kleopatra&quot; rel=&quot;nofollow noreferrer&quot;&gt;216 Kleopatra&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Txit4.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Txit4.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Kleopatra is a relatively large asteroid, measuring 217 × 94 × 81&#xA;  km.Calculations from its radar albedo and the orbits of its &lt;strong&gt;moons&lt;/strong&gt; show&#xA;  it to be a rubble pile, a loose amalgam of metal, rock, and 30–50%&#xA;  empty space by volume, likely due to a disruptive impact prior to the&#xA;  impact that created its moons.&#xA;  [from wikipedia]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is a fairly large asteroid with moons. Could a person or vehicle with treads circumnavigate it the long way round? If traveling along the narrow bit which way would be down?&lt;/p&gt;&#xA;" OwnerUserId="13167" LastEditorUserId="8693" LastEditDate="2016-01-29T14:05:19.147" LastActivityDate="2016-01-29T14:05:19.147" Title="What would it be like to walk on the Asteroid 216 Kleopatra?" Tags="|gravity|asteroid|eva|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13762" PostTypeId="2" ParentId="13761" CreationDate="2016-01-29T00:43:49.683" Score="10" Body="&lt;p&gt;&lt;strong&gt;Really weird&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the neck region between the lobes, the gravity is almost cancelling out. On the other parts of the asteroid, the surface gravity is going to be about $30 \space mm/s^2$. But 216 Kleopatra is spinning really fast, providing up to $20\space mm/s^2$ of radial acceleration due to inertia. $10 \space mm/s^2$ is barely enough to walk. If you are not careful with your steps, you are going to fly for a while before returning to the ground. (Seven minutes airtime for a normal Earth step). You are however not able to throw rocks into orbit, like on Phobos or Deimos, except around the neck where an unstable orbit is just requiring $14 \space m/s$. Just manageable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for circumnavigating the asteroid, it is simply going to take too much time. Being patient with the low gravity, moving at say $0.1 \space m/s$, the whole $500km$ trip is going to take about 60 days. Orbiting it is much simpler, only taking a few hours and a minuscule amount of $\Delta v$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Down&quot; is going to shift a great deal depending on your location, but your acceleration vector is never going over the local horizon. Otherwise pieces of this rubble pile would have flown into space.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-29T01:01:18.807" LastActivityDate="2016-01-29T01:01:18.807" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13763" PostTypeId="1" CreationDate="2016-01-29T01:13:22.927" Score="15" ViewCount="4931" Body="&lt;p&gt;When I graph the rocket equation, substituting arbitrary values for v(exhaust) and m1, so m0 because m1 - m0, the graph implies that increasing propellant mass past a certain point does not increase the delta V by very much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, if I plot the inverse of the function, the graph implies that for increasing delta V values, extremely high propellant masses are required, and there is a vertical asymptote. Does this not imply that a rocket has a maximum delta V?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, what is the significance of the graph of the derivative? of the integral?&lt;/p&gt;&#xA;" OwnerUserId="8825" LastEditorUserId="49" LastEditDate="2016-01-29T10:38:47.540" LastActivityDate="2016-12-13T05:38:35.843" Title="Is this a correct understanding of Tsiolkovsky's rocket equation?" Tags="|rockets|delta-v|specific-impulse|mass-fraction|rocket-equation|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13764" PostTypeId="2" ParentId="13729" CreationDate="2016-01-29T01:45:05.710" Score="2" Body="&lt;p&gt;While you can miniaturize most of the brute-force parts (insulation, no.  Engine walls???) that's not all that's in the rocket.  There's a bunch of electronics on board and almost all of it is as small as they could build already, no miniaturization possible.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-01-29T01:45:05.710" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13765" PostTypeId="2" ParentId="13763" CreationDate="2016-01-29T02:22:50.323" Score="15" Body="&lt;p&gt;The implication of the rocket equation is that &lt;em&gt;linear&lt;/em&gt; increases in ∆v require &lt;em&gt;exponential&lt;/em&gt; increases in mass ratio for a single stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's not strictly a maximum delta-v -- if you redo your plot on a log scale, you'll see that it doesn't go vertical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting very high mass ratios (much above 10:1) is difficult to do on a &lt;em&gt;single&lt;/em&gt; stage, so there is a practical ∆v limit there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Multiple stage rockets can reach arbitrarily high total mass ratios (if you have the budget), so linear increases in ∆v simply require exponential increases in overall mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The derivative of that plot is a rate of change of mass with respect to rate of change in velocity. For a rocket at a particular point in flight, it tells how much propellant mass needs to be expended in order to increase its speed by a given amount; for a rocket on the drawing board, it tells how much more propellant needs to be carried in order to increase its capability as defined by ∆v. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know if there are any useful interpretations of the integral.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-01-29T03:07:34.140" LastActivityDate="2016-01-29T03:07:34.140" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13766" PostTypeId="2" ParentId="13763" CreationDate="2016-01-29T05:21:39.920" Score="6" Body="&lt;p&gt;No, that is not quite right. Let's first state and describe the Tsiolkovsky Rocket Equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\displaystyle \Delta v = V_e \times \ln(\frac{m_i}{m_f})$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v$ is delta v, the change in velocity in km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_e$ is the effective exhaust velocity in km/s (it's another way of measuring specific impulse)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\ln()$ is just &lt;strong&gt;the natural logarithm&lt;/strong&gt;, or log base e (e is an interesting mathematical constant approximately equal to 2.7). Almost all scientific and graphing calcs has an ln() function.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_i$ and $m_f$ are initial and final masses in kg&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ratio of masses is just that, &lt;strong&gt;a ratio.&lt;/strong&gt; So if we start with 100 kg and end with 10, or start with 1,000 kg and end with 100, the change in velocity will stay the same. This of course assumed that Ve didn't change either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So the equation is basically the natural log of a ratio, r, then multiplied by a constant.&lt;/strong&gt; To analyze any asymptote behavior all you really need to understand is the behavior of the natural log function.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Go to &lt;a href=&quot;http://www.fooplot.com&quot;&gt;fooplot.com&lt;/a&gt; and type in &quot;ln(x)&quot; without quotes. You will see a black curve. In this graph, our ratio &lt;strong&gt;r&lt;/strong&gt; is now called &lt;strong&gt;x&lt;/strong&gt;, and y represents the delta v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes there is a vertical asymptote &lt;strong&gt;on the left&lt;/strong&gt;, coinciding with the y-axis itself. This would imply some really weird stuff about an unbounded negative limit of delta v (so you could go infinitely fast with a negative speed!)...&lt;strong&gt;except that our ratio cannot be less than 1&lt;/strong&gt;...because initial mass is always greater than final mass. That's just how chemical rockets work. They burn mass and thus lose mass. You always start with more fuel than you end with. In mathematical terms, this means that the valid domain starts at x &gt; 1. x &amp;lt;= 1 is just not physically possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you should notice that the curve gets shallower as it goes to the right. You may be wondering if the curve has &lt;strong&gt;a horizontal asymptote?&lt;/strong&gt; If so, this would indeed set a max on delta v as you suspect. But the answer is, &lt;strong&gt;no, there's no horizontal asymptote&lt;/strong&gt; in the graph. The curve always gets higher and higher as it goes to the right...just at shallower and shallower angles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way of saying that, is that logarithms behave with exponential decay. &lt;strong&gt;However&lt;/strong&gt;, &quot;exponential&quot; is used very loosely these days. &lt;strong&gt;I don't recommend that terminology!&lt;/strong&gt; Just to confuse you more, a horizontal asymptote is &lt;strong&gt;also&lt;/strong&gt; called exponential decay! (of another kind).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BTW you can multiply &quot;ln(x)&quot; by whatever Ve you want and it will not change the graph's behavior. Fair warning though, if you type in a typical number that real-world rockets use, such as 3,000 (m/s), you will have to zoom way, way out in order to see the curve. That's why I like working in km/s so you only multiply by 3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bottom line about the equation:&lt;/strong&gt; there is no max delta v. If you're willing to exponentially increase the mass ratio, you can linearly increase the delta v to any high value you want. This does not mean, however, that it's physically possible. There are of course technological limits on how much thrust, heat, and other stuff we can controllably achieve in our rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You asked about the derivative and integral too. You also mentioned the inverse of the Tsiolkovsky Rocket Equation in your second paragraph. I will try to explain what those mean in a future edit.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2016-01-29T05:28:52.273" LastActivityDate="2016-01-29T05:28:52.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13767" PostTypeId="2" ParentId="13763" CreationDate="2016-01-29T05:26:19.447" Score="15" Body="&lt;p&gt;Your question is about the behavior of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt; itself, in the limit of very small final mass (&lt;em&gt;dry mass&lt;/em&gt;). Roughly: &quot;&lt;em&gt;is there any limit to delta-v in theory?&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using &lt;a href=&quot;http://meta.math.stackexchange.com/questions/5020/mathjax-basic-tutorial-and-quick-reference&quot;&gt;MathJax&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta v=v_e \ln\frac{m_0}{m_f}.&#xA;$$&#xA;If you just look at the velocity ratio and the mass ratio:&#xA;$$&#xA;\frac{\Delta v}{v_e}=\ln\frac{m_0}{m_f}=-\ln\frac{m_f}{m_0},&#xA;$$&#xA;you can see that one is just the log of the other. Log keeps increasing forever, but more and more slowly. It never reaches an asymptote. It shoots up fast on the linear plot, but if you change the x-axis from linear to log, the velocity ratio just continues to increase. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;click to enlarge:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/1c071.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1c071.png&quot; alt=&quot;Tsiolkovsky rocket equation lin log&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the final mass is &lt;em&gt;one billionth&lt;/em&gt; of the start mass, you can get a delta-v that is over 20 times the exhaust velocity. Theoretically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I think that the answer to your question, just from the mathematical point of view, based on the rocket equation is no. There's no limit here. If your rocket is made of pure fuel, with a magic massless  nozzle, it can go very very fast!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But for sure, as the other answers suggest, a physical rocket we can build and launch does have a very real (and so far very expensive) physical limit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is some Python code for the plot:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;mf_over_m0 = np.logspace(-0.5, -10, 20)&#xA;&#xA;delta_v_over_ve = -np.log(mf_over_m0)&#xA;&#xA;# that's it - the rest is just plotting @@!&#xA;&#xA;fig = plt.figure(figsize=[16, 8])&#xA;&#xA;xlab,   ylab   = &quot;mf / m0&quot;,             &quot;delta v / v exhaust&quot;&#xA;title1, title2 = &quot;x-axis linear scale&quot;, &quot;x-axis log scale&quot;&#xA;&#xA;ax1 = plt.subplot(121, xlabel=xlab, ylabel=ylab, title=title1)&#xA;ax2 = plt.subplot(122, xlabel=xlab, ylabel=ylab, title=title2)&#xA;&#xA;# from here: http://stackoverflow.com/a/14971193/3904031&#xA;for ax in [ax1, ax2]:&#xA;    for item in ([ax.title, ax.xaxis.label, ax.yaxis.label] +&#xA;                 ax.get_xticklabels() + ax.get_yticklabels()):&#xA;        item.set_fontsize(20)&#xA;    ax.invert_xaxis()&#xA;&#xA;for ax in [ax1, ax2]:&#xA;    ax.plot(mf_over_m0, delta_v_over_ve, 'ok')&#xA;    ax.plot(mf_over_m0, delta_v_over_ve, '-b')&#xA;ax2.set_xscale('log')&#xA;&#xA;plt.savefig(&quot;rocket equation question&quot;)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="11649" LastEditDate="2016-12-13T05:38:35.843" LastActivityDate="2016-12-13T05:38:35.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13768" PostTypeId="2" ParentId="1314" CreationDate="2016-01-29T11:07:05.683" Score="5" Body="&lt;p&gt;A bit more detail on the causes of mechanical failures:&#xA;&lt;a href=&quot;http://www.grc.nasa.gov/WWW/spacemech/vol1&quot; rel=&quot;nofollow&quot;&gt;Bearing cage instability&lt;/a&gt; (scroll to the sections on Momentum and reaction wheels) is a common phenomenon. It states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Bearing lubricant depletion between the ball race retainer causes cage instability and subsequent pointing errors, increased bearing torque, and wheel vibration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If this isn't immediately obvious,try &lt;a href=&quot;https://en.wikipedia.org/wiki/Ball_bearing&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; as a primer. The &quot;ball race&quot; is the circular track of a ball bearing that contains the balls. The retainer or cage is a non-metallic, usually phenolic, that sits in the ball race and keeps the balls spaced apart. The balls therefore bear against both the race, which takes the main load of the mechanism, and the cage, which takes no load. Bearing cage instability refers to unintended vibrational modes of the cage caused by a lack of lubricant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has affected at least the &lt;a href=&quot;http://llis.nasa.gov/lesson/1598&quot; rel=&quot;nofollow&quot;&gt;Cassini&lt;/a&gt;, also &lt;a href=&quot;http://arc.aiaa.org/doi/10.2514/1.A33047&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;http://arc.aiaa.org/doi/pdfplus/10.2514/6.2014-1875&quot; rel=&quot;nofollow&quot;&gt;XMM-Newton&lt;/a&gt; missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typical solutions mentioned in the latter papers, learned on a trial and error basis, include stopping the wheel for a while to allow the lubricant to redistribute, and also avoiding certain operational speeds, either very high or near zero.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-04-19T22:50:42.940" LastActivityDate="2016-04-19T22:50:42.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13769" PostTypeId="1" AcceptedAnswerId="13770" CreationDate="2016-01-29T14:40:20.117" Score="8" ViewCount="3605" Body="&lt;p&gt;I am wondering if there is a telescope in or outside the ISS that has been used for astronomical imaging? I have seen &lt;a href=&quot;http://www.celestron.com/about-us/public-relations/press-releases/celestron-telescope-onboard-international-space-station&quot;&gt;here&lt;/a&gt; that there is already a Celestron telescope on board but it is only used for Earth observation.&lt;/p&gt;&#xA;" OwnerUserId="13176" LastActivityDate="2016-01-29T21:37:14.977" Title="Is there a telescope on board the ISS?" Tags="|iss|astronomy|telescope|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13770" PostTypeId="2" ParentId="13769" CreationDate="2016-01-29T15:25:15.673" Score="7" Body="&lt;p&gt;The Celestron telescope is not stationary. It can be moved around inside the space station to look through any window.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is however just used to observe the Earth, and the reason for that is the orbital velocity of the ISS. Moving at almost $8km/s$, the view change with an angular velocity of 3.88 arc-degrees per second because the station rotates to keep the same side facing the Earth. If the ISS had instead stayed fixed relative to the stars, like the Hubble telescope, you get the same problem with Earth observations. You can not simply do both.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is what makes it hard to focus on one part of the sky. Given the limited view from one of the windows, you can only observe a region of the sky for about 10 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That does not mean that The ISS does not have equipment to observe the sky, it does for instance have the &lt;a href=&quot;https://en.wikipedia.org/wiki/Alpha_Magnetic_Spectrometer&quot; rel=&quot;nofollow&quot;&gt;Alpha Magnetic Spectrometer&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-29T21:37:14.977" LastActivityDate="2016-01-29T21:37:14.977" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13771" PostTypeId="2" ParentId="13758" CreationDate="2016-01-29T15:53:12.250" Score="1" Body="&lt;p&gt;The information I have been able to find so far is below:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Folding wings, so it can launch inside a fairing.&lt;/li&gt;&#xA;&lt;li&gt;External cargo unit at the rear&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Please edit in more info if you find it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: added image from &lt;a href=&quot;http://www.sncspace.com/Mediakit/Images&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; showing external cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jAvWS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jAvWS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6944" LastEditDate="2016-07-27T18:47:06.617" LastActivityDate="2016-07-27T18:47:06.617" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13772" PostTypeId="1" CreationDate="2016-01-29T19:49:40.060" Score="4" ViewCount="618" Body="&lt;p&gt;Does anyone know what the dry mass and mass ratio of the Falcon 9v1.1 Full Thrust Stage 1 is? The best info I've been able to find was by taking the specified burn time and multiplying it by the mass flow (which in turn was obtained by combining thrust specific fuel consumption and known thrust), but since I'm assuming there is first stage downthrottling at some point before MECO, that's not a sure thing.&lt;/p&gt;&#xA;" OwnerUserId="13211" LastEditorUserId="51" LastEditDate="2016-01-29T20:18:30.453" LastActivityDate="2016-01-29T20:18:30.453" Title="Falcon 9 v1.1 Full Thrust Specifications" Tags="|spacex|falcon-9|merlin-1d|mass-fraction|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13773" PostTypeId="2" ParentId="8759" CreationDate="2016-01-29T19:56:10.603" Score="1" Body="&lt;p&gt;As we saw in the last few launches, cold gas thrusters (the F9 first stage uses nitrogen for recovery maneuvers) seem to be sufficient to maneuver the stage before fins engage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But yes, Superdracos could be used to help land the first stage but are not needed since the center engine can throttle down far enough to allow landing (and that's already with a much lowered thrust being a single engine versus nine). The second stage, however, has a single engine already, so would need to throttle down nearly an order magnitude more than the first stage center engine. In that case, Superdracos would make sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, auxillary landing engines (perhaps SuperDracos) for the second stage are shown in this 2011 video from SpaceX: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/sSF81yjVbJE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk mentioned they decided to hold off on second stage reuse for the time since a kerosene/oxygen vehicle doesn't quite have the performance margin to make it worthwhile for high-energy payloads to geosynchronous transfer orbit, but SpaceX recently won a contract from the Air Force to help develop their new high-performance methane/oxygen Raptor engine (eventually for their &quot;BFR&quot;) for a Falcon 9 or Falcon Heavy upper stage. Maybe that's just the Air Force talking, but it could mean that they intend a higher performing upper stage, and it would be consistent with previous SpaceX statements if this stage were reusable like shown in the above video, and probably would also require auxiliary landing engines to have low enough thrust to allow reliable landing.&lt;/p&gt;&#xA;" OwnerUserId="13240" LastEditorUserId="13240" LastEditDate="2016-01-29T20:02:04.637" LastActivityDate="2016-01-29T20:02:04.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13774" PostTypeId="1" CreationDate="2016-01-29T20:30:33.973" Score="6" ViewCount="1588" Body="&lt;p&gt;Please edit, if 'engines' is not the correct term (layman here).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I understand it, you have your SRB's, then they detached, and the External Tank supplied the liquid oxygen and hydrogen fuel to the main engines of the Orbiter.  Then, after main engine cutoff, it is detached.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why didn't the External Tank just have its own engines/boosters?&lt;/strong&gt;  The Orbiter maintained a bit of fuel for orbital maneuvering, but otherwise didn't need to be responsible for the thrust provided by the ET.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="49" LastEditDate="2016-01-29T20:54:19.273" LastActivityDate="2016-01-30T02:28:40.603" Title="Why did the External Tank not have its own engines?" Tags="|space-shuttle|design|engines|launch-vehicle|external-tank|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13775" PostTypeId="1" AcceptedAnswerId="13777" CreationDate="2016-01-29T20:44:10.720" Score="12" ViewCount="590" Body="&lt;p&gt;Because of all the heat fueling convection cells, Venus has some brisk winds especially in the upper atmosphere (up to 370 km/h).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize the winds are quite slow at the surface, but are there any latitudes with more moderate winds at the 50 km level?   &lt;/p&gt;&#xA;" OwnerUserId="13167" LastEditorUserId="13167" LastEditDate="2016-01-30T16:09:54.193" LastActivityDate="2016-01-30T16:09:54.193" Title="Does Venus have doldrums or horse latitudes (latitudes with lower winds)?" Tags="|planetary-science|atmosphere|venus|weather|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13776" PostTypeId="2" ParentId="13774" CreationDate="2016-01-29T20:50:45.413" Score="9" Body="&lt;p&gt;As a design decision, if you throw away the External Tank and it has engines attached, you are throwing away the engines.  Since the Orbiter was returning for sure anyway, the decision was to leave the engines on the orbiter. Once the ET is done its job, the engines are not needed to make orbit.  The OMS pods provide enough punch for the needed orbital manuevers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians on the other hand, with Energia, which was the booster for the Buran space shuttle, had liquid fueled side boosters, and the engines were on the main body of the vehicle. (No longer a tank, it is now the core booster). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were ok with throwing away all the engines on each flight, contemplating wings on the side booster to try and recover them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME's were however so expensive that throwing them away seemed to defeat the point of a reusable space ship.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-29T20:50:45.413" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13777" PostTypeId="2" ParentId="13775" CreationDate="2016-01-29T21:55:08.173" Score="11" Body="&lt;p&gt;There are no &lt;a href=&quot;https://en.wikipedia.org/wiki/Horse_latitudes&quot; rel=&quot;nofollow noreferrer&quot;&gt;horse latitudes&lt;/a&gt; on Venus, only equatorial &lt;a href=&quot;https://en.wikipedia.org/wiki/Doldrums&quot; rel=&quot;nofollow noreferrer&quot;&gt;doldrums&lt;/a&gt; and polar fronts / collars. Convection driven &lt;a href=&quot;https://en.wikipedia.org/wiki/Hadley_cell&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hadley cells&lt;/a&gt; on Venus stretch to &amp;pm;60° in latitude from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Intertropical_Convergence_Zone&quot; rel=&quot;nofollow noreferrer&quot;&gt;intertropical convergence zone&lt;/a&gt; to North and South polar collars, so no subtropical latitudes high pressure areas. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Circulation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Atmosphere of Venus - Circulation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0hHCG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is quite a bit different to formation of subtropical anticyclone zones between trade winds and westerlies on Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/8WVEl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And average wind speed at 50 km altitude on Venus is between 55 and 70 m/s (200 to 250 km/h). From &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090001338.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Geoffrey A. Landis' Exploring Venus&lt;/a&gt; (PDF):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/ZqWwq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Comparison of winds measured by radio tracking of probes below 60 km&#xA;  with winds estimated from the measured pressure by means of the&#xA;  assumption of cyclostrophic balance. Above 60 km, the winds are&#xA;  inferred entirely from measured pressures.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you're curious about this for the purpose of &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobot&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerobot&lt;/a&gt; exploration or &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerostat&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerostat&lt;/a&gt; colonization, you would be advised to increase your altitude above the sulfuric acid clouds reaching to about 60 km altitude, but properly shielded craft (e.g. high density &lt;a href=&quot;https://en.wikipedia.org/wiki/Polyethylene&quot; rel=&quot;nofollow noreferrer&quot;&gt;PE plastics&lt;/a&gt; are resistant to it) shouldn't have these problems also lower, unless they're solar powered, at which point you'll again have to aim higher. Refer to &lt;a href=&quot;http://ntrs.nasa.gov/?N=0&amp;amp;Ntk=All&amp;amp;Ntt=landis%20geoffrey%20colonization%20venus&amp;amp;Ntx=mode%20matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;Geoffrey A. Landis' work on Venus aerostat colonies&lt;/a&gt; and the latter part of my answer to &lt;a href=&quot;https://space.stackexchange.com/a/9159/49&quot;&gt;What useful materials can be extracted from Venusian atmosphere?&lt;/a&gt; for more info.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Equatorial doldrums, even if they're fast, should be fairly stable though, again something I already discuss in my answer to &lt;a href=&quot;https://space.stackexchange.com/a/9162/49&quot;&gt;A cloud-top colony on Venus, will it drift to the poles?&lt;/a&gt;, in particular in the part about the observed path taken by the two &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_program#Balloon&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vega program balloons&lt;/a&gt;, both released at near-equatorial latitudes. Balloons traveled at an average altitude of 54 km for roughly 46 hours and navigated about 1/3 the Venusian circumference. That comes out at average speed of &lt;a href=&quot;https://encrypted.google.com/#q=%286%2C051.8+km+%2B+54+km%29+*+2+*+pi+%2F+46+hours+%2F+3+in+km%2Fh&quot; rel=&quot;nofollow noreferrer&quot;&gt;~ 278 km/h&lt;/a&gt;. Not 370 km/h, but not exactly a mild breeze either.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-01-29T22:14:23.607" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13778" PostTypeId="1" CreationDate="2016-01-29T22:41:38.797" Score="15" ViewCount="2188" Body="&lt;p&gt;Why does NASA use the position title &quot;Pilot&quot;' for crew members who are not in charge of piloting the spacecraft? For example famously Buzz Aldrin's position was Lunar Module Pilot but his spacecraft was piloted by Neil Armstrong as the Commander of the Lunar Module. This terminology frequently causes confusion in the public since people commonly and (somewhat understandably) assume the dictionary definition of a pilot as &quot;a person who operates the flying controls of an aircraft&quot;.&lt;/p&gt;&#xA;" OwnerUserId="11397" LastEditorUserId="19946" LastEditDate="2019-11-22T12:02:48.393" LastActivityDate="2019-11-22T12:02:48.393" Title="Why does NASA use the position title &quot;Pilot&quot;' for crew members who are not in charge of piloting the spacecraft?" Tags="|nasa|history|terminology|compliance|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13779" PostTypeId="1" AcceptedAnswerId="13780" CreationDate="2016-01-29T22:58:21.727" Score="6" ViewCount="1822" Body="&lt;p&gt;The worst obstacle and the primary roadblock to using superconductors on Earth is the ambient temperature which necessitates a coolant like LN2 or even liquid Helium (and a lot of energy for creating these).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Achieving these temperatures in space is dirt cheap, a simple sun shield keeping the superconductor in shadow is all that's needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I read the &lt;a href=&quot;https://space.stackexchange.com/questions/811/what-contributes-the-majority-of-the-mass-in-an-ion-drive&quot;&gt;mass breakdown&lt;/a&gt; of some ion engines, the cabling contributed quite a bit to the weight. There's a lot of other applications where using superconductors would be superior. Maybe we could get ultra-low-power superconducting electronics. What about superconductor-based bearings for all the inertia wheel needs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why aren't they used more commonly? What are some other obstacles?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2020-07-22T23:12:58.113" Title="Why aren't superconductors used more in space?" Tags="|development|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13780" PostTypeId="2" ParentId="13779" CreationDate="2016-01-29T23:26:57.390" Score="12" Body="&lt;p&gt;&lt;strong&gt;Although space is cold, vacuum is a pretty good insulator.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;That means it is difficult to get rid of waste heat. Almost any action your spacecraft performs does produces heat that must be radiated away. Radiators are very sensitive to temperatures, being victims of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tyranny of the Stefan-Boltzmann Equation&lt;/a&gt;. Their efficiency depends on the fourth power of their absolute temperature, so a spacecraft operating at 140K (The current temperature limit for &lt;a href=&quot;https://en.wikipedia.org/wiki/High-temperature_superconductivity&quot; rel=&quot;nofollow noreferrer&quot;&gt;high-temperature superconductors&lt;/a&gt;) has 16 times less efficient radiators. If you must have sixteen times the area of radiators to provide the same cooling, any mass savings by using the superconductors are efficiently cancelled out.&lt;/p&gt;&#xA;&lt;p&gt;Another concern is reliability. If the thermal system fails, your spacecraft is dead.&lt;/p&gt;&#xA;&lt;p&gt;Using superconductors will of course reduce the heat load a bit, but you still have waste heat production from on-board electronics, actuators and reaction wheels, and of course the Sun.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="29286" LastEditDate="2020-07-22T23:12:58.113" LastActivityDate="2020-07-22T23:12:58.113" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="13781" PostTypeId="2" ParentId="13774" CreationDate="2016-01-29T23:28:30.843" Score="7" Body="&lt;p&gt;Reusability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole idea of the shuttle was to discard all the parts that are simple, cheap and easy to replace and recover everything expensive, complex and hard to replace.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course the reality, involving meddling by parties other than NASA, never mind failures in the process the shuttle was designed (not so much the design itself as the process of designing it; as detailed in the &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/missions/51-l/docs/rogers-commission/Appendix-F.txt&quot; rel=&quot;noreferrer&quot;&gt;Feymann Report&lt;/a&gt;) made the effect a mockery of the intention, with costs of refurbishing, replacing and repairing returned reusable parts overshadow what building single-use ones would be, but the initial concept was sound: throw away the simple tank, keep the complex, expensive engine with the orbiter. SSME was (despite some construction/safety flaws) one of the most efficient engines in history of rocketry, but that was at expense of complexity - it was a true miracle of technology. It was designed to be reusable and far too expensive to be disposable. And the Shuttles were the first big step towards reusability.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-01-30T02:28:40.603" LastActivityDate="2016-01-30T02:28:40.603" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13782" PostTypeId="2" ParentId="13753" CreationDate="2016-01-30T08:25:54.107" Score="3" Body="&lt;p&gt;Trailblazer was awarded and got to the launch pad in 4 months, it was an 80 kg microsatellite and the launch attempt (unsuccessful) was on a Falcon 1 in 2008, but it was flying as a backup for another satellite that had been scheduled and couldn't make it.  Some Russian launch vehicles (Dnepr, Soyuz) get last minute openings from customers who have a delay and can't make the launch and it is not uncommon to find openings for CubeSats or microsatellites within 4-6 months of the launch date.  More typically, the Russian vehicles prefer to have a full manifest at least 12 months prior to launch.  Most other large launch vehicles set their manifests nearly 24 months prior to launch, with the exception of secondary payload slots for (mostly) CubeSats of various sizes.  These slots often don't firm up until about 9-12 months prior to liftoff.  &lt;/p&gt;&#xA;" OwnerUserId="13247" LastActivityDate="2016-01-30T08:25:54.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13783" PostTypeId="2" ParentId="3405" CreationDate="2016-01-30T08:53:36.943" Score="2" Body="&lt;p&gt;Solar sails are generally envisioned from very lightweight reflective materials like aluminized mylar and this is what the Planetary Society vehicle used.  This material is regularly used in multi-layer insulation (MLI) blankets which are on the outside of a great number of spacecraft.  Structural degradation of these materials is slow but steady, even in relatively high radiation environments. Optical degradation occur quicker and can be quite rapid if you have contamination from the rest of your spacecraft.  Because outgassing from the spacecraft gets less as time goes on, most of the optical degradation from contamination occurs in the first few weeks.  Radiation darkening is steady, of course.  To put time scales on it, it would likely take many years, maybe a decade, to see any significant perforation of the sail that would show up in its performance, say 25%.  Some loss of reflectivity and thus performance will occur due to contamination, but it will take a very large flux of protons to affect the aluminum coating so the performance hit is likely to only be around 10% after a decade or two.  Some interesting data on MLI performance can be found looking at material collected from Hubble servicing missions &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015287.pdf&quot; rel=&quot;nofollow&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015287.pdf&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="13247" LastActivityDate="2016-01-30T08:53:36.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13784" PostTypeId="2" ParentId="8225" CreationDate="2016-01-30T08:59:05.753" Score="8" Body="&lt;p&gt;The dragon capsule has limited painting on it because it has to stay in space for long periods of time and the addition of logos would affect the thermal performance of the exterior at that spot, possibly leading to a hot spot on that location.  The sides of the launch vehicle however, don't get exposed long enough during the time they are active to really care as much about the solar heating difference, so big logos can easily be added to these stages.&lt;/p&gt;&#xA;" OwnerUserId="13247" LastActivityDate="2016-01-30T08:59:05.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13785" PostTypeId="1" CreationDate="2016-01-30T10:23:42.007" Score="4" ViewCount="387" Body="&lt;p&gt;A number of companies are working on suborbital flight with Blue Origin, Virgin Galactic and XCOR being among the most recognized today. I don't ask here about what actual plans different companies have, but at first look a plausible strategy could be to start out with a suborbital launch system with the aim of developing an orbital launch system out of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is suborbital flights a natural stepping stone to orbit? (In our time, the early space race is a bit OT). Can the same engines, whole stages, crew capsules be used both in and sub orbit? Or are there difficult compromises which risk leading to both the suborbital and the orbital systems becoming suboptimal? For Virgin Galactic and Virgin Airline (suborbital and airlines) the synergies are not obvious to my mind, maybe the step to orbit is as big.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sub-orbital systems which are up and coming, if they become successful, are mostly crew oriented (while sounding rockets with science payloads and ICBM's are well established and less commercial). So a strategy to begin with sub-orbital flights seems to me to require crewed flights, if the sub-orbital business leg is to generate a profit of its own. If that is correct then I suppose my question is whether it is basically rational to approach crewed orbital launches from crewed sub-orbital launches, and maybe even building on airline experiences. Will sub-orbitals get important advantages over those who go for orbit directly, or are they suboptimizing by trying to do two different things?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Airborne launches (like VG is planning), VTOL and winged spacecrafts and parachutes into the ocean and whatnot are all plausible technologies. I don't want to directly address them per se in this question about the synergies and differences in going from (or via) sub-orbital to orbital. Could for example the same launch pad and ground operations be used?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-01-30T12:52:14.410" LastActivityDate="2016-05-29T19:40:31.487" Title="What are the main synergies between sub-orbital and orbital space launch?" Tags="|commercial|sub-orbital|launch-vehicle|orbital|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13786" PostTypeId="1" CreationDate="2016-01-30T11:44:47.637" Score="4" ViewCount="79" Body="&lt;p&gt;Imagine that an asteroid is detected on an incoming trajectory, or some scientifically very interesting phenomenon occurs somewhere in the Solar System, prompting us to &lt;strong&gt;quickly&lt;/strong&gt; send something there to learn more about it. Something off the shelf, a science probe which already awaits launch for some other intention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How soon could a planned launcher with a prepared payload be rescheduled to launch on a completely different trajectory than initially intended? I suppose the military can do this within days or hours for their suborbital ICBM's, but I wonder how this could be done with today's most frequent orbital launchers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, how soon can the payload of a prepared launch be changed? Could for example the Osiris-Rex probe (to be launched in about 8 months from when this is written) be put on the next scheduled Delta IV Heavy launcher and sent to a threatening asteroid? Or is everything just too mingled together and optimized for such dramatic changes to be feasible? Such things seem to have been simpler 50+ years ago. If so, maybe it was because of the world war sense of urgency then, and the increasing complexity now.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-01-30T12:51:11.917" Title="How fast can a launcher be retargeted?" Tags="|launch|planning|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13787" PostTypeId="2" ParentId="13786" CreationDate="2016-01-30T12:51:11.917" Score="5" Body="&lt;p&gt;It depends, on how much testing you want to do, etc. Each launch has a specified vibration testing that is done to ensure that there won't be any problems. Swapping out a payload, depending on the type, can actually be fairly easy with many of today's spacecraft, that have a modular design. It depends somewhat on the type of payload, mind you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the launch itself, it could be done within a matter of a few days. The most difficult thing about changing the launch is to ensure that the &quot;&lt;a href=&quot;https://www.faa.gov/about/office_org/headquarters_offices/ast/licenses_permits/media/Ac4311fn.pdf&quot;&gt;Expected Casualty&lt;/a&gt;&quot; figures are still met by the flight path, and if there was something truly important, that might even be bypassed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The military is rumored to have the capability to launch spy satellites in a very short period of time, through a contract with ULA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an interplanetary object, there are a lot more concerns. Launch windows, figuring out the trajectory, etc. These would likely take priority.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the book &quot;The Martian&quot; does a reasonably good example of what it would take to launch a payload really quickly, and that likely involved items already in place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, this kind of re-direction could probably be done, but it might be difficult. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-01-30T12:51:11.917" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13788" PostTypeId="2" ParentId="5221" CreationDate="2016-01-30T12:52:08.637" Score="0" Body="&lt;p&gt;There seem to be a number of directions to take with this question. This response tries to address the final part of the question &quot;Can a launch-provider determine..&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two approaches that will occur in parallel during a launch:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;position and velocity measurement, by what ever method&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;telemetry regarding the physical health and operation of the vehicle&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Position and velocity.&lt;/strong&gt; You can get a very clear idea of the difficulties of ranging by considering that the orbital period of Galileo is 14 hours. I don't know what the transfer orbit would be but I suspect a substantial fraction. This means that at 30 minutes, just to use the example in Ross Millikan's answer, the satellite would only have done a small fraction of an orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ask a friend to start drawing a circle and see if you can guess &quot;exactly&quot; how big it will be after they have only drawn ten percent of the way around. You might get it roughly right, but the difference between the first orbit estimate and one done later, after perhaps a half orbit, clearly in this case has been significant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Telemetry.&lt;/strong&gt; In principal this should provide a good idea of whether the mission is broadly going to plan. The apparent outcome is that the real failure, as described by David Hammen's answer was either not spotted (telemetry not observing that area) or that the information wasn't passed on to be made public until much later. &lt;em&gt;I've not seen any discussion of the latter though its plausible given human nature.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Points for the future:&lt;/strong&gt; Take a glance through these press releases from Arianespace:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.arianespace.com/press-release/first-two-operational-satellites-in-the-galileo-constellation-galileo-foc-m1-sat-5-6-the-european-unions-flagship-program-successfully-launched-by-arianespace-vs-09-soyuz/&quot; rel=&quot;nofollow&quot;&gt;success&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.arianespace.com/press-release/vs09-soyuz-launch-galileo-satellites-orbital-injection-anomaly/&quot; rel=&quot;nofollow&quot;&gt;anomaly&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.arianespace.com/press-release/galileo-satellites-experience-orbital-injection-anomaly-on-soyuz-launch-initial-report/&quot; rel=&quot;nofollow&quot;&gt;the next day&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this is simply the normal human-organisational difficulty. The &quot;Success&quot; is released prematurely in this case, but perhaps this was done on the basis of very early ranging. The latter two show progressive awareness developing and in this case, measurement by the customer, ESA, and CNES rather than the launch service provider, Arianespace.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-02-07T00:27:10.947" LastActivityDate="2016-02-07T00:27:10.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13789" PostTypeId="2" ParentId="9329" CreationDate="2016-01-30T13:41:04.587" Score="0" Body="&lt;p&gt;Some companies are paid for their work on a pay as you go basis. For every 100 dollars of work, they might get 15 dollars for profit. If it takes longer or problems are found, or if the customer decides it wants something different and is willing to pay for the plans to change... they get even more money. While seemingly ludicrous in the business 'real world' outside large government contracting, this can actually be fair for the companies involved - especially if the customer is uncertain about what it wants or how long it should take, is prone to changing it's mind, and to do something (charitably) totally new. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine if my wife wanted to hire a contractor to build something she couldn't describe in the back yard and wanted to pay the contractor a day at a time for an uncertain amount of work that she could halt at any time. What kind of terms would be negotiated? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, how long it takes for this system, once entrenched, to resemble a Dilbert cartoon... I leave to the reader to judge.&lt;/p&gt;&#xA;" OwnerUserId="12562" LastActivityDate="2016-01-30T13:41:04.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13790" PostTypeId="2" ParentId="13785" CreationDate="2016-01-30T15:34:08.030" Score="2" Body="&lt;p&gt;Sub-orbital space flight as a step on the road makes sense, at least for a commercial space program. You can then scale up your business along with your development process. The most notable disadvantage is that you will not be able to launch satellites, forcing the company to focus more on manned space flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast, the main focus of the early space-programs was to develop an ICBM as soon as possible (In the business world, nuking competing companies are frowned upon). That means that resources are channelled into completing an orbital launcher. When the Soviet union had an orbital launcher, they decided to just skip a manned sub-orbital program. As a consequence, the first human in space, Yuri Gagarin, went on an orbital mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the manned spacecraft developed for sub orbital flights are not necessarily fit for orbital missions. They tend to be more like aircraft and less like minimum mass capsules, and in any case, a lot of new requirements must be met, like heat shields. Winged orbiters are not necessarily a good idea, as they tend to mass more and have a more complicated design. The space shuttle for instance was never capable of providing a reasonable launch cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, a space program is not just the spacecraft on top, almost everything else, like launch pads, rocket engines and mission control can be transferred directly into an orbital space program. Sub orbital flight is definitely not a dead end.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="10791" LastEditDate="2016-01-30T18:01:09.530" LastActivityDate="2016-01-30T18:01:09.530" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13792" PostTypeId="2" ParentId="8114" CreationDate="2016-01-30T18:53:06.680" Score="2" Body="&lt;p&gt;At even lower air density the drag induced would be proportionally less.  Any speed you do reduce though would reduce the peak heating dramatically because heat rate is proportional to the square of the speed at any time.  The net result is likely that you shift the peak heating point to earlier when you're going faster but still less heating than before because you're at the max heating point when the air is thinner.  So, the answer is to feather as soon as possible in your profile if your objective is to have the maximum impact on peak heating rate.  Whether this improvement in peak heating is sufficient to make this technique feasible in other applications is a trade between the weight and complexity (risk) of the feathering system and what it saves you in weight of insulation and risk of thermal issues for that particular application's flight profile.  For reentry from orbit, it seems unlikely the excess mass of the feathering system would be worth it since you have so much velocity to shed that feathering will only take a little bit out so you still have to have a very robust thermal protection system.&lt;/p&gt;&#xA;" OwnerUserId="13247" LastActivityDate="2016-01-30T18:53:06.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13793" PostTypeId="1" CreationDate="2016-01-30T19:06:46.997" Score="11" ViewCount="1310" Body="&lt;p&gt;I've been looking at the ISS's &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_International_Space_Station_expeditions&quot;&gt;List of International Space Station expeditions&lt;/a&gt;, and I'm curious about how the commanders are chosen. I can't really find anything that stands out. Sometimes the commander is military (or former military) and sometimes they're civilian, so military rank doesn't seem to be what decides it. It also doesn't even seem to be that the most experienced astronaut/cosmonaut becomes commander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, for &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_40&quot;&gt;Expedition 40&lt;/a&gt; Steven R. Swanson was the commander, but Aleksandr Skvortsov went up with him for Expedition 39. Skvortsov had military experience as a leader, he logged more time in space than Swanson, he was a cosmonaut before Swanson became an astronaut, and Skvortsov even had experience as a commander on the ISS during Expedition 24, while Expedition 40 was Swanson's first time aboard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't really find anything that says why a particular person is chosen to be the commander of an Expedition. How are they chosen, and is there any difficulty among countries/agencies when deciding who will be commander?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2016-02-08T17:17:46.447" Title="How is ISS commander chosen?" Tags="|iss|astronauts|mission-design|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13794" PostTypeId="2" ParentId="8262" CreationDate="2016-01-30T19:12:16.020" Score="3" Body="&lt;p&gt;Rickhg12hs has the correct conclusion.  Flares are produced when large, flat reflective surfaces are sunlit so there's enough light directed back at an observer to be visible.  The curved and mesh of the antenna scatters sunlight in many directions so the intensity at the observer's location is never sufficient for visibility.  The solar arrays are single sided, so only the sunward facing side of the array will have cells on it and the array is fixed in position on the satellite (not gimballed) (&lt;a href=&quot;http://spaceflight101.com/smap/&quot; rel=&quot;nofollow&quot;&gt;http://spaceflight101.com/smap/&lt;/a&gt;).  The flare will only be visible to obeservers on sunward side of the ground track which means at dawn, look to the west for it (away from the rising sun) and at dusk look to the east for it (away from the setting sun).  This geometry is typical of most satellite flares, but can vary based on the particular spacecraft and where they have big reflective surfaces placed.&lt;/p&gt;&#xA;" OwnerUserId="13247" LastActivityDate="2016-01-30T19:12:16.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13796" PostTypeId="2" ParentId="13097" CreationDate="2016-01-30T19:27:50.157" Score="7" Body="&lt;p&gt;Additive manufacturing has a high payoff for complex parts that are hard or impossible to machine using traditional methods, however, the process often requires significant trial and error to get correct and getting consistent material properties on the finished part and precision are difficult.  For these reasons it's generally only advantageous to do with complex parts that can tolerate the precision and material property variations and that typically for launch vehicles means limiting it to the engines.  Structural pieces have strength requirements that don't tolerate material voids well.  Mechanisms typically need both consistent strength and precision in critical locations.  Engine components that don't move, or at least that don't need super tight precision, can all be printed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The company I work for, Parabilis, just tested an entire engine (RCS thruster) that was printed as a single unit &lt;a href=&quot;http://sbir.gsfc.nasa.gov/SBIR/abstracts/15/sbir/phase1/SBIR-15-1-H2.01-9296.html&quot;&gt;http://sbir.gsfc.nasa.gov/SBIR/abstracts/15/sbir/phase1/SBIR-15-1-H2.01-9296.html&lt;/a&gt;&lt;br&gt;&#xA;The folks at Rocket Labs USA are similarly 3D printing pretty much the entire main engine of their vehicle with the exception of things like pneumatics and valves due to the precision sealing needed there.  &lt;a href=&quot;https://www.rocketlabusa.com/about-us/propulsion/&quot;&gt;https://www.rocketlabusa.com/about-us/propulsion/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13247" LastActivityDate="2016-01-30T19:27:50.157" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13797" PostTypeId="2" ParentId="13112" CreationDate="2016-01-30T19:33:41.183" Score="5" Body="&lt;p&gt;Three is indeed simpler. However, reaction wheels, being moving parts, are more prone to failure than purely electrical systems, especially for missions lasting many years where bearings may eventually wear out, especially if lubrication retention is not adequately addressed.  Adding a 4th wheel makes some of the math more complex for the ADCS software, but it's typically a trivial issue compared to the rest of the problem and it's a nice way to add just one part that backs up three potential failure points, so it can be a good trade for missions needing a little more reliability.  For most cubesats, the short lifetimes and limited volume mean that 3 wheels is the best choice, especially if the wheels come from a commercial provider who has a good heritage of success on orbit.  If the wheels are student built or experimental, you might still want a 4th just in case, even for a cubesat.&lt;/p&gt;&#xA;" OwnerUserId="13247" LastEditorUserId="1142" LastEditDate="2016-02-04T00:13:55.717" LastActivityDate="2016-02-04T00:13:55.717" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13798" PostTypeId="2" ParentId="1364" CreationDate="2016-01-30T20:01:35.207" Score="13" Body="&lt;p&gt;The key is that you don't need to make a satellite impossible to see (e.g. cooling it down to 20 Kelvin); You just need to make it harder to see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Misty&quot; is supposedly the codename of a stealth satellite project run by the National Reconnaissance Office (NRO). There are at least two suspected Misty satellites in orbit (See the &lt;a href=&quot;https://en.wikipedia.org/wiki/Misty_(satellite)&quot; rel=&quot;noreferrer&quot;&gt;&quot;Misty&quot; article on Wikipedia&lt;/a&gt; for more).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Misty design supposedly uses a conical reflective balloon, patented as a &quot;satellite signature suppression shield&quot; by Teledyne, who did the design for the NRO, in &lt;a href=&quot;http://www.google.com/patents/US5345238&quot; rel=&quot;noreferrer&quot;&gt;patent # US5345238 A&lt;/a&gt;.  The balloon is designed to &quot;suppress laser, radar, visible and infrared signatures of satellites to make it difficult or impossible for hostile enemy forces to damage or destroy satellites in orbit.&quot;  Basically, when the cone is pointed in the general direction of a laser sensor or radar, the laser beams and radar waves are reflected away from the Earth.  Optical tracking is suppressed because (according to the patent) most visible light reflecting off a satellite comes from the Earth, and that light is reflected by the cone into space instead of back toward the Earth.  The balloon reflects most energy from the sun, so its infrared signature is lowered by two orders of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hwlr7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hwlr7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cone must be pointed in the general direction of the sensor you want to hide the satellite from in order to be effective, and the following charts show how the degree of stealth changes as the cone's aspect angle changes.  Fig. 3B from the patent shows how the radar cross section (RCS) is about 15-30 dB smaller than that of a typical satellite at an aspect angle of 0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1vpq5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1vpq5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that it's important to aim the cone at the sensor you want to hide from.  Some amateur observers &lt;a href=&quot;http://www.wired.com/2006/02/spy-3/&quot; rel=&quot;noreferrer&quot;&gt;even think that their locations are programmed into Misty satellites as &quot;hostile&quot;&lt;/a&gt;, so that the satellite will do its best to hide itself from them when it's overhead.&lt;/p&gt;&#xA;" OwnerUserId="6569" LastActivityDate="2016-01-30T20:01:35.207" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13799" PostTypeId="1" AcceptedAnswerId="14919" CreationDate="2016-01-30T20:14:42.667" Score="12" ViewCount="3515" Body="&lt;p&gt;I have read about the common berthing mechanism on ISS and how the attachment process works and how the pressure seal is established, and I think I have that part figured out. There is a passive CBM and an active CBM which mate together and provide a pressure tight seal against space around them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/578Ni.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/578Ni.jpg&quot; alt=&quot;Passive CBM&quot;&gt;&lt;/a&gt;&lt;a href=&quot;https://i.stack.imgur.com/YBdvS.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YBdvS.jpg&quot; alt=&quot;Active CBM&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have also seen plenty of diagrams of the hatches, so I know how they look and what parts they have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LbSMs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LbSMs.jpg&quot; alt=&quot;CBM Hatch&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, what I haven't been able to figoure out from the documents is exactly how the hatches work with regards to modules in general. I have a lot of questions that I'm just going to list here - but I think a better answer might be to just explain in very concrete terms how the CBMs work.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is there a hatch on both the passive and the active CBM? If so, does that mean there are two hatches to go through when going from one module to another?&lt;/li&gt;&#xA;&lt;li&gt;Or are the hatches somehow just halves of a single hatch that is somehow combined during the docking?&lt;/li&gt;&#xA;&lt;li&gt;Can (both of?) the hatches be opened from both sides?&lt;/li&gt;&#xA;&lt;li&gt;Can the manual pressure equalization valve (MPEV) be operated from both sides? Are there two valves (one per hatch) or only one?&lt;/li&gt;&#xA;&lt;li&gt;If there are two hatches, do both of them open towards the inside of the module they are in?&lt;/li&gt;&#xA;&lt;li&gt;Can a module be moved from one place to another on the ISS while it is pressurized?&lt;/li&gt;&#xA;&lt;li&gt;What happens to the air that is in between the passive and active CBM when they are detached? Is there a way to use a depress pump on it or is it simply lost to space? Is the air vented to space somehow before the detaching operation or does it just whoosh out when the seal is opened?&lt;/li&gt;&#xA;&lt;li&gt;Do all the modules using CBM have exactly the same kind of hatch? Does the cupola have a hatch at all? Is the pressurized mating adapter also using the same hatch on the CBM side?&lt;/li&gt;&#xA;&lt;li&gt;Harmony module is said to have six CBMs, with only the one connecting to Destiny being passive and the rest active. Yet in &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/d/d7/Harmony_detail.jpg&quot; rel=&quot;noreferrer&quot;&gt;schematics&lt;/a&gt; and &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/8/8a/Node_2_-_STS-134.jpg&quot; rel=&quot;noreferrer&quot;&gt;pictures&lt;/a&gt;, the CBM towards PMA-2 (forward) looks different (no covers). Is there any difference there, or is it a 100% standard CBM with active side on the Harmony side and passive on the PMA-2?&lt;/li&gt;&#xA;&lt;li&gt;Oxygen, power, water, etc. are routed through connectors in the vestibule, but how does it concretely work? Do the routings require that the hatches stay open? What is needed to route them? Is it just a wire/pipe between the passive and active CBMs, or is there something to connect on the module side of the hatch as well? Are the connectors automatically sealed if there is nothing connecting to them (for example, in oxygen lines)?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2631" LastActivityDate="2016-04-21T22:36:28.897" Title="How are hatches set up in the common berthing mechanism (CBM)?" Tags="|iss|cbm|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13800" PostTypeId="1" AcceptedAnswerId="13801" CreationDate="2016-01-30T21:39:24.930" Score="14" ViewCount="3294" Body="&lt;p&gt;Mercury's day is only a little bit shorter than its orbital period, the relationship is a 3:2 resonance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I noticed that the planet's angular velocity at perihelion, 0.265 arc-degrees per hour, is slightly larger than the angular velocity of its rotation, 0.256 arc-degrees per hour. Does that mean that the Sun briefly goes in the opposite direction across the sky when Mercury is at perihelion? If so, are strange phenomena like a double sunset or double sunrise possible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angular velocity of the orbit is larger for 25.5 degrees before and after perihelion, making the total retrograde motion 1.2 degrees.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="49" LastEditDate="2016-01-31T14:56:18.750" LastActivityDate="2016-02-01T22:01:16.150" Title="Can the Sun set twice during the same day on Mercury?" Tags="|planetary-science|observation|mercury|sunlight|diurnal-cycle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13801" PostTypeId="2" ParentId="13800" CreationDate="2016-01-30T22:12:13.633" Score="15" Body="&lt;p&gt;If memory serves correctly*, then the angular diameter of the Sun as it appears on the &lt;em&gt;skies&lt;/em&gt; of Mercury from its surface is larger than its total angular displacement during its apparent retrograde motion at Mercury's perihelion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So assuming that's correct and my memory didn't suffer too much in the 80's, the disco ball, erm, the Sun won't set anywhere on Mercury's surface completely assuming it also rose first completely during the same early part of a day on Mercury. But it is possible to be at a specific point on Mercury's surface where a complete sunset can follow a partial sunrise or vice versa. Otherwise yes, your calculations are correct. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Apparent_retrograde_motion#From_Mercury&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Apparent retrograde motion - From Mercury&lt;/a&gt; (citing Strom, Robert G.; Sprague, Ann L. (2003). Exploring Mercury: the iron planet. Springer. ISBN 1-85233-731-1):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At specific points on Mercury's surface, an observer would be able to&#xA;  see the Sun rise part way, then reverse and set before rising again,&#xA;  all within the same Mercurian day. This apparent retrograde motion of&#xA;  the Sun occurs because, from approximately four Earth days before&#xA;  perihelion until approximately four Earth days after it, Mercury's&#xA;  angular orbital speed exceeds its angular rotational velocity.&#xA;  Mercury's elliptical orbit is farther from circular than that of any&#xA;  other planet in the Solar System, resulting in a substantially higher&#xA;  orbital speed near perihelion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If this is difficult to imagine, I've prepared a small animation of one Mercury's orbit around the Sun, starting at aphelion (farthest distance from the Sun) where animation restarts:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/MKGvY.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note the apparent motion of Mercury's surface with respect to the Sun's center immediately after perihelion (closest distance to the Sun). Sizes of the Sun and Mercury are of course highly exaggerated and not to scale, but the orbit's geometry and Mercury's rotation rate are correct. Animation made with realistic orbit model setting in &lt;a href=&quot;http://www.solarsystemscope.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar System Scope&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're having problems seeing the effect of the apparent retrograde motion in the animation I prepared, then this &lt;a href=&quot;http://www.skymarvels.com/infopages/vids/Mercury%20Spin-Orbit%20Resonance.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;animation of Mercury's spin-orbit resonance on Sky Marvels&lt;/a&gt; should demonstrate it better. Sadly, it's copyrighted so I can't reproduce it here. Similar animation could be made with &lt;a href=&quot;http://www.shatters.net/celestia/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestia&lt;/a&gt; and a &lt;a href=&quot;http://www.celestiamotherlode.net/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestia Motherlode&lt;/a&gt; surface map of Mercury, if someone wants to create and publish it under our license.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;*&amp;nbsp;Memory served, and after a bit of insisting that it does &lt;a href=&quot;https://chat.stackexchange.com/rooms/9682/conversation/suns-angular-displacement-at-apparent-retrograde-motion-on-mercury-during-perihe&quot;&gt;in our chat&lt;/a&gt; and many recalculations after by both OP and I, we finally arrived to matching numbers;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Total Sun's angular displacement during apparent retrograde motion as seen from the surface of Mercury is ~ 1.2° (I got 1.2345° or 1° 14' 4.2&quot;, if a more precise number is needed to check against), and at Mercury's 46,778,000 km distance to the Sun when apparent retrograde motion begins, Sun's angular diameter is &lt;a href=&quot;https://encrypted.google.com/#q=2*arctan%28696%2C342%2F46%2C778%2C000%29+in+deg&quot; rel=&quot;nofollow noreferrer&quot;&gt;~ 1.706°&lt;/a&gt; and increasing to &lt;a href=&quot;https://encrypted.google.com/#q=2*arctan%28696%2C342%2F46%2C001%2C200%29+in+deg&quot; rel=&quot;nofollow noreferrer&quot;&gt;~ 1.734°&lt;/a&gt; at perihelion. So apparent retrograde motion is roughly two thirds of Sun's angular diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This goes slightly against Wikipedia's mention of &lt;em&gt;about half&lt;/em&gt; the Sun's angular diameter in &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury_%28planet%29#Orbit.2C_rotation.2C_and_longitude&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mercury - Orbit, rotation, and longitude&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At certain points on Mercury's surface, an observer would be able to&#xA;  see the Sun rise &lt;strong&gt;about halfway&lt;/strong&gt;, then reverse and set before rising&#xA;  again, all within the same Mercurian day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But let's settle for &lt;em&gt;about halfway&lt;/em&gt; being close enough to &lt;em&gt;about two thirds&lt;/em&gt;, and the initial point about the Sun's apparent motion stands.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Paying it forward a bit, here's another quote of the following paragraph from the same Wikipedia page that I thought is interesting enough to reproduce here:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the same reason, there are two points on Mercury's equator, 180&#xA;  degrees apart in longitude, at either of which, around perihelion in&#xA;  alternate Mercurian years (once a Mercurian day), the Sun passes&#xA;  overhead, then reverses its apparent motion and passes overhead again,&#xA;  then reverses a second time and passes overhead a third time, taking a&#xA;  total of about 16 Earth-days for this entire process. In the other&#xA;  alternate Mercurian years, the same thing happens at the other of&#xA;  these two points. The amplitude of the retrograde motion is small, so&#xA;  the overall effect is that, for two or three weeks, the Sun is almost&#xA;  stationary overhead, and is at its most brilliant because Mercury is&#xA;  at perihelion, its closest to the Sun. This prolonged exposure to the&#xA;  Sun at its brightest makes these two points the hottest places on&#xA;  Mercury. Conversely, there are two other points on the equator, 90&#xA;  degrees of longitude apart from the first ones, where the Sun passes&#xA;  overhead only when the planet is at aphelion in alternate years, when&#xA;  the apparent motion of the Sun in Mercury's sky is relatively rapid.&#xA;  These points, which are the ones on the equator where the apparent&#xA;  retrograde motion of the Sun happens when it is crossing the horizon&#xA;  as described in the preceding paragraph, receive much less solar heat&#xA;  than the first ones described above.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-01T22:01:16.150" LastActivityDate="2016-02-01T22:01:16.150" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13802" PostTypeId="2" ParentId="5062" CreationDate="2016-01-30T23:46:42.763" Score="2" Body="&lt;p&gt;The precautions you take will vary according to the application/orbit etc. It would be prudent to spend some time looking into this; its a big topic and a cause of plenty of modern day failures. Don't rely on advice from this forum! That disclaimer made, here's some ideas to get you started.&lt;/p&gt;&#xA;&lt;p&gt;Read up about the environment and agency standards for mitigation. Really, you, or someone in your organisation, should be on top of this, its worth asking around. Try this for a list of &lt;a href=&quot;http://descanso.jpl.nasa.gov/SciTechBook/series3/17JKeyDocs.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;documents&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;As a very rough guide the body of a satellite is usually considered to provide some shielding. Thus it takes high energy electrons to get inside - termed &amp;quot;internal charging&amp;quot; whilst low energy electrons still cause surface charging.&lt;/p&gt;&#xA;&lt;p&gt;Three very broad failure modes come to mind:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Charging of dielectric material&lt;/strong&gt; This can occur from surface charging or internal charging. This is hard to do in practice, its everywhere in the harness, though having a slightly leaky insulator can help.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Charging of floating metal&lt;/strong&gt;  Effects can be larger and more damaging discharges than from a build-up in dielectric though again it can be from surface or internal charging. Real world examples of design features that present a sufficient risk of ESD trigger (in GEO) that they would each warrant a grounding connection:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;each layer of aluminised kapton in a blanket&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;metallic badge on an IC package (or removal of the badge)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;unused connector pins&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;strong&gt;Three phase&lt;/strong&gt; In locations where there is a high voltage between two conductors a small discharge from a nearby dielectric can produce a brief puff of plasma and thus trigger a sustained discharge between the power rails. Known to cause plenty of damage. I &lt;em&gt;believe&lt;/em&gt; its more often associated with surface charging.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2020-08-11T20:54:32.777" LastActivityDate="2020-08-11T20:54:32.777" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13803" PostTypeId="1" AcceptedAnswerId="13831" CreationDate="2016-01-30T23:54:23.560" Score="-2" ViewCount="228" Body="&lt;p&gt;For a small Orion nuclear pulse probe that is accelerating to 10% light speed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the upper G force acceleration limit for electronics, a small thorium based nuclear reactor, pulse units and for the structural integrity to survive and function properly? &lt;/p&gt;&#xA;" OwnerUserId="11671" LastActivityDate="2019-12-12T09:37:29.023" Title="Upper G acceleration limit for a small thorium nuclear reactor and electronics" Tags="|nuclear|nuclear-pulse-propulsion|g-force|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13804" PostTypeId="2" ParentId="13799" CreationDate="2016-01-31T01:12:35.367" Score="2" Body="&lt;p&gt;I can comment that there are two hatches. Each side has its own and they are independently opened.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-01-31T01:12:35.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13805" PostTypeId="5" CreationDate="2016-01-31T01:28:14.403" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-01-31T01:28:14.403" LastActivityDate="2016-01-31T01:28:14.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13806" PostTypeId="4" CreationDate="2016-01-31T01:28:14.403" Score="0" Body="Questions regarding patterns repeating once per rotation relative to the Sun" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-01-31T01:35:52.803" LastActivityDate="2016-01-31T01:35:52.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13807" PostTypeId="2" ParentId="12956" CreationDate="2016-01-31T08:17:36.667" Score="1" Body="&lt;p&gt;This is what eventually solved my problem. This JavaScript function takes the parameters: &lt;code&gt;radiusRatio&lt;/code&gt;, the outermost objects radius divided by the innermosts. &lt;code&gt;innerAnomaly&lt;/code&gt;, the innermost objects true anomaly from the reference direction, same for &lt;code&gt;outerAnomaly&lt;/code&gt;, &lt;code&gt;errorMargin&lt;/code&gt; is the maximal angle between either of the two radius vectors or the reference direction, and &lt;code&gt;limit&lt;/code&gt; is how many of the innermost objects orbits to simulate for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the angle measure used for &lt;code&gt;innerAnomaly&lt;/code&gt;, &lt;code&gt;outerAnomaly&lt;/code&gt; and &lt;code&gt;errorMargin&lt;/code&gt; is in fractions of an orbit, not degrees nor radians.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;sameLine = function (radiusRatio,innerAnomaly,outerAnomaly,errorMargin,limit){&#xA;    results = [];&#xA;    newMargin = errorMargin;&#xA;    periodRatio = Math.pow(radiusRatio,3/2);&#xA;    for (i = 1; i &amp;lt; limit; i++){&#xA;        anomaly = (outerAnomaly + (i - innerAnomaly)/periodRatio) % 1;&#xA;        if (anomaly &amp;gt; 1 - anomaly){&#xA;            anomaly = 1 - anomaly;&#xA;        };&#xA;        if (anomaly &amp;lt;= newMargin){&#xA;            results.push([i - innerAnomaly,anomaly]);&#xA;            newMargin = anomaly;&#xA;        };&#xA;    };&#xA;    return results;&#xA;};&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;It outputs the an array containing sub arrays with the encounter data in the format of 1 numbers of orbits the innermost object has performed, and 2 what the error margin was. The next entry is the next time the error is smaller than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is of course restricted to coplanar, circular orbits, and the mass of the two orbiting objects is negligible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a more detailed explanation for a related problem at &lt;a href=&quot;https://physics.stackexchange.com/a/232918/102747&quot;&gt;https://physics.stackexchange.com/a/232918/102747&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2016-01-31T17:45:00.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13809" PostTypeId="1" AcceptedAnswerId="13810" CreationDate="2016-01-31T12:13:33.333" Score="6" ViewCount="5202" Body="&lt;p&gt;I am trying to understand the relationship (quantitatively) between JulianDate, UTC, and the epoch of Two Line Elements. It's a deep subject, especially if you include relativity, and a long subject - there is a lot of history. But here what I need is just a bare-bones understanding of how these three things are related quantitatively, on earth, plus/minus a few years from now, to milli-second accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I roughly understand that UTC increase by 1 second every &lt;a href=&quot;http://www.bipm.org/en/publications/si-brochure/second.html&quot; rel=&quot;noreferrer&quot;&gt;second&lt;/a&gt;, with the usual rules for hours, minutes, days, years, &lt;a href=&quot;https://en.wikipedia.org/wiki/Leap_year&quot; rel=&quot;noreferrer&quot;&gt;leap years&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Leap_second&quot; rel=&quot;noreferrer&quot;&gt;leap seconds&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't understand &lt;a href=&quot;https://en.wikipedia.org/wiki/Julian_day&quot; rel=&quot;noreferrer&quot;&gt;JulianDate&lt;/a&gt;. That links to a long article which states many facts, but after reading, I still don't understand what it is quantitatively. If I line up UTC agains Julian Day are they parallel (except for UTC's leap years/seconds)? Is there a mapping that I can understand or do I have to use a computer program or web site or some other black box to convert between the two?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are two examples of situations where I feel lost. I print additional information to make sure I understand some things - the questions are in &lt;strong&gt;bold&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Example 1&lt;/strong&gt; - time on my computer and in &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;noreferrer&quot;&gt;Skyfield&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from skyfield.api import now, JulianDate, load&#xA;import time&#xA;def test():&#xA;    a, b =  now(), time.gmtime()   # check the times&#xA;    print a.tt_tuple()&#xA;    print time.asctime(b)&#xA;test()&#xA;&#xA;(2016.0, 1.0, 31.0, 11, 45, 28.927114605903625)&#xA;Sun Jan 31 11:44:20 2016&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Both of those are derived from my computer's system time (I think!) which is up-to-date. &lt;strong&gt;There is a difference of about 69 seconds. What is that?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Example 2&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;noreferrer&quot;&gt;TLE&lt;/a&gt; epoch:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from skyfield.api import now, JulianDate, load&#xA;import math&#xA;&#xA;data = load('de421.bsp')&#xA;earth = data['earth']&#xA;&#xA;TLE = &quot;&quot;&quot;&#xA;1 25544U 98067A   16031.25992506  .00006019  00000-0  97324-4 0  9994&#xA;2 25544  51.6430  25.8646 0006733  62.5910  66.7566 15.54344726983528&#xA;&quot;&quot;&quot;&#xA;&#xA;ISS = earth.satellite(TLE)&#xA;&#xA;print ISS._sgp4_satellite.satnum, &quot;  NORAD satellite number of ISS in the TLE&quot;&#xA;print ISS._sgp4_satellite.epoch,  &quot;  The epoch of the TLE&quot;&#xA;&#xA;frac = 0.25992506                 #  fraction of day in the epoch of the TLE&quot;&#xA;&#xA;print round((((frac*24.-6.)*60.-14.)*60.-17.)*1E+06), &quot;  the microseconds of the epoch&quot;&#xA;&#xA;print ISS._sgp4_satellite.mo*(180./math.pi),    &quot;  TLE Mean Anomaly - of ISS *at* epoch?&quot;&#xA;&#xA;print JulianDate(utc=(2016, 1, 31, 6, 14, 17.525184)).tt,  &quot;  What is this number??&quot;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;output:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;25544   NORAD satellite number of ISS in the TLE&#xA;2016-01-31 06:14:17.525184   The epoch of the TLE&#xA;525184.0   the microseconds of the epoch&#xA;66.7566   TLE Mean Anomaly - of ISS *at* epoch?&#xA;2457418.76071   What is this number??&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is the exact meaning of the last number, and how can I convert the value of this epoch to UTC (if I wanted to)?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-02-01T06:19:03.507" Title="Time, UTC, Julian Date, TLE epoch - how are they related quantitatively?" Tags="|artificial-satellite|time|skyfield|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13810" PostTypeId="2" ParentId="13809" CreationDate="2016-01-31T13:32:01.140" Score="4" Body="&lt;p&gt;JulianDate basically counts the number of days since Jan 1, 4713 BC 12:00 UTC in the Julian calendar. However, that is a bit hard to understand since it is so long ago, uses a different calendar, and you must take the transition form BC to AD into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we use Reduced JulianDate, which is another official standard, related by $\text{Reduced JD} = \text{JD} - 2400000$. This counts the number of days since Nov 16, 1858 12:00 TT (Terrestrial Time) in the (usual) Gregorian calendar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we may add 45.5 days to reach Jan 1, 1859 00:00 UTC. We can add 51499 more days to reach Jan 1, 2000 00:00 (Terrestrial Time), the beginning of a new 400-years cycle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We may now compute:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$$\text{JD} = 2451544.5 + 365Y + \lfloor 0.25Y \rfloor - \lfloor 0.01Y \rfloor + \lfloor 0.0025Y \rfloor +A \\ +D + \tfrac1{24}H+ \tfrac1{1440}M + \tfrac1{86400}S$$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$Y$ is the current Gregorian year minus 2000&lt;/li&gt;&#xA;&lt;li&gt;$\lfloor x \rfloor$ is the floor function, the largest integer smaller than $x$.&lt;/li&gt;&#xA;&lt;li&gt;$D$ gives the current day number. This is 1 on Jan 1, 32 on Feb 1, etc.&lt;/li&gt;&#xA;&lt;li&gt;$H$ is the current hour in Terrestrial Time.&lt;/li&gt;&#xA;&lt;li&gt;$M$ is the current minute in Terrestrial Time.&lt;/li&gt;&#xA;&lt;li&gt;$S$ is the current second in Terrestrial Time, including milliseconds, etc.&lt;/li&gt;&#xA;&lt;li&gt;$A$ is a correction factor. If the year is a leap year, it is -1, otherwise it is zero.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is just a formula that works. There are many other formulas that would work, but I picked this one because it was the easiest to explain. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference of 69 seconds is caused by the difference between Terrestrial Time and UTC, currently 68.184 seconds. This is because a diffence of 36 seconds in leap seconds and a historical offset of 32.184 seconds. &lt;/p&gt;&#xA;" OwnerUserId="13262" LastEditorUserId="13262" LastEditDate="2016-02-01T06:19:03.507" LastActivityDate="2016-02-01T06:19:03.507" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="13811" PostTypeId="1" AcceptedAnswerId="13814" CreationDate="2016-01-31T14:21:51.403" Score="4" ViewCount="296" Body="&lt;p&gt;This &lt;a href=&quot;http://spaceflightnow.com/news/n1403/05spacexula/#.Vq4RNXr7t0x&quot; rel=&quot;nofollow&quot;&gt;news item&lt;/a&gt; quotes Elon Musk as saying &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;it would make sense for the government to rely on ULA's Delta 4 and the SpaceX Falcon 9, phasing out the venerable Atlas 5.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;You don't need both of those rocket families,&quot; he said. &quot;I think it&#xA;  would make sense for the long-term security interests of the country&#xA;  to probably phase out the Atlas 5, which depends on the Russian&#xA;  engine, and have ULA operate the Delta family,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I understand that since then ULA have followed quite a different &lt;a href=&quot;http://spacenews.com/lawmakers-seek-pentagon-review-of-ula-decisions-on-delta-4-aerojet-rocketdyne/&quot; rel=&quot;nofollow&quot;&gt;approach&lt;/a&gt;, that of keeping the Atlas 5 and the Delta 4 Heavy and just retiring the Delta 4 medium, which happens to use solid boosters not used on the heavy version. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its very easy to get bogged down in corporate mud-slinging regarding the strategies of both ULA and SpaceX to engage with government clients. That said, it does seem sensible from an assurance point of view to keep the Delta 4 medium instead of the Atlas 5, given the latter's dependence on the RD-180 engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know of any technical reasons, or technical and manufacturing/economic drivers that explain ULA's approach. E.g. the solid booster topic may be relevant? Please try to keep it to rationale that is &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;referenced&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;avoid dipping into speculation about their business strategy, tax payer value for money etc &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="51" LastEditDate="2016-01-31T18:58:56.783" LastActivityDate="2016-01-31T19:57:00.103" Title="Technical reasons for Delta 4 medium phase out" Tags="|launch|falcon-9|atlas-v|manufacturing|delta-iv|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13812" PostTypeId="1" AcceptedAnswerId="13813" CreationDate="2016-01-31T16:47:54.233" Score="4" ViewCount="899" Body="&lt;p&gt;Recently Tim Peake and Tim Kopra carried out an EVA to replace a failed Sequential Shunt Unit, some form of electrical regulator for the solar panels, on the outside of the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why aren't these electrical boxes on the inside of the ISS, which would make replacing them much easier?&lt;/p&gt;&#xA;" OwnerUserId="13263" LastEditorUserId="1142" LastEditDate="2016-01-31T17:12:13.577" LastActivityDate="2016-01-31T17:15:44.917" Title="Why are the ISS' Sequential Shunt Units not on the inside?" Tags="|iss|low-earth-orbit|design|eva|solar-power|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13813" PostTypeId="2" ParentId="13812" CreationDate="2016-01-31T17:15:44.917" Score="5" Body="&lt;p&gt;In &lt;a href=&quot;https://en.wikipedia.org/wiki/Short_circuit&quot; rel=&quot;nofollow noreferrer&quot;&gt;short&lt;/a&gt;: electrical and fire hazard. From &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050215641.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Options Studied for Managing Space Station Solar Array Electrical Hazards for Sequential Shunt Unit Replacement&lt;/a&gt; (PDF), explaining a bit what SSU are:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The U.S. solar array strings on the International Space Station are&#xA;  connected to a sequential shunt unit (SSU). The job of the SSU is to&#xA;  shunt, or short, the excess current from the solar array, such that&#xA;  just enough current is provided downstream to maintain the 160-V bus&#xA;  voltage while meeting the power load demand and recharging the&#xA;  batteries.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If SSU units malfunction, as the sunlight hits the station's solar panels, it could present substantial risk to anyone or anything near them:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The open-circuit voltage of cold solar-array strings can exceed 320 &#xA;  V, and warm solar-array strings could feed a short circuit with a&#xA;  total current level exceeding 240 A.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's the reason why Tim Peake and Tim Kopra had to carry out replacement of a faulty SSU when the station was in Earth's shadow and there was no current going through the units, and this is why they had to work fast:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/fV28s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Normalized short-circuit current, $I_{sc}$, versus orbit solar $\beta$&#xA;  angle (angle between the orbit plane and the Earth-Sun line). Velocity&#xA;  vector for the International Space Station (ISS) is aligned with the&#xA;  ISS +x-axis in this figure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Should such units be installed on the inside of the station, with oxygen in the breathable atmosphere, on top of being an electrical hazard due to high current and voltage, it could also present fire hazard if any of them shorted. And as you can imagine, pumping up to 76,800 W through them, they would also get pretty hot, so there is also a good reason to move them far along the station's main truss and away from the habitable and experiment area, and that's why Tim Peake and Tim Kopra took so much time to translate during their EVA to get to the unit.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2016-01-31T17:15:44.917" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13814" PostTypeId="2" ParentId="13811" CreationDate="2016-01-31T18:58:37.757" Score="3" Body="&lt;p&gt;The design of the Delta 4 is such that each CCM (Common Core Module) has fittings to support thrust through the strap on mount points. It has to support 2 other CCMs attached for the Delta Heavy model, and then lower mount points for the various combinations of SRBs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For some reason I do not fully understand the Delta 4 pays a heavier price in terms of both manufacture (more mount points required) and payload penalty for these mount points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Atlas 5 of course has mount points for up to 5 SRBs and each core has support for 0-5 without the same penalty that the Delta 4 seems to pay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring even that, with Delta 4 being a LOH/LOX vehicle, it is more expensive to build than a Ker/Lox  vehicle.  Hydrogen is much colder than LOX, causes metal to become brittle, and it leaks much more (being a significantly smaller molecule) and whatnot.  These are all surmountable problems (witness the fact the D-4/D-4H work, the Shuttle worked, all other LH vehicles in history have worked) but they add cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the Delta 4 from a pure cost perspective is more expensive than the Atlas 5. (Now I get into the speculation you wished to avoid) It seems like ULA actually made a decision based on cost for once, in the EELV program. As unlikely as that seems based on past history. They could not dump the Delta 4 Heavy since some occasional NRO payloads require that size, so we end up with this odd decision.  Cancel the D4 Medium but keep all the costs associated with making cores for it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-01-31T19:57:00.103" LastActivityDate="2016-01-31T19:57:00.103" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13815" PostTypeId="1" AcceptedAnswerId="13816" CreationDate="2016-01-31T19:52:24.597" Score="1" ViewCount="1132" Body="&lt;p&gt;Say, Barry, can you abuse this site's &quot;answer your own question&quot;&#xA;feature to provide insight into this oft-asked question (in various&#xA;different forms):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You launch a projectile upwards at velocity v0 on a airless spherical&#xA;planet with a radius of r, a rotation period of p, and a surface&#xA;acceleration of g.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Set up differential equations to find how far from the launch point&#xA;the projectile will land? You may ignore the planet's revolution&#xA;around its primary (if any), in your calculations.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Numerically solve these differential equations in some interesting cases.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Ways to make this problem harder:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Allow for directional launches, not just straight up&#xA;launches. This is probably the easiest and most obvious improvement&#xA;to the question.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Allow for ellipsoid planets, where the radius varies with&#xA;latitude, and the surface normal (&quot;up&quot; direction) is not exactly&#xA;opposite to the pull of gravity (&quot;down&quot; direction)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user7073" LastActivityDate="2016-01-31T19:52:24.597" Title="Trajectory of projectile launched from planet's surface" Tags="|orbit|trajectory|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13816" PostTypeId="2" ParentId="13815" CreationDate="2016-01-31T19:52:24.597" Score="1" Body="&lt;p&gt;Without loss of generality, we can draw our axes so the projectile's&#xA;initial position is in the xz plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Allowing &lt;code&gt;phi&lt;/code&gt; to be the launch site's latitude, the projectile's&#xA;initial position is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\{r \cos (\phi ),0,r \sin (\phi )\}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch site completes a rotation in time p, so its position&#xA;at time t is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$&#xA;   \left\{r \cos (\phi ) \cos \left(\frac{2 \pi  t}{p}\right),r \cos (\phi )&#xA;    \sin \left(\frac{2 \pi  t}{p}\right),r \sin (\phi )\right\}&#xA;$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Differentiating, we have the site's velocity as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$   &#xA;   \left\{-\frac{2 \pi  r \cos (\phi ) \sin \left(\frac{2 \pi &#xA;    t}{p}\right)}{p},\frac{2 \pi  r \cos (\phi ) \cos \left(\frac{2 \pi &#xA;    t}{p}\right)}{p},0\right\}&#xA;$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, the velocity at time 0 is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\left\{0,\frac{2 \pi  r \cos (\phi )}{p},0\right\}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, the projectile's starting velocity will be &lt;code&gt;v0&lt;/code&gt; plus the&#xA;velocity imparted from the launch site, as above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Excluding the site velocity for a moment, the launch will be away from&#xA;the center of the planet, and thus in the direction:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\{\cos (\phi ),0,\sin (\phi )\}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this is a unit vector, the launch velocity (excluding the launch site's velocity for the moment) is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\{\text{v0} \cos (\phi ),0,\text{v0} \sin (\phi )\}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding the launch site's initial velocity back in, we have the&#xA;projectile's initial velocity as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$&#xA;   \left\{\text{v0} \cos (\phi ),\frac{2 \pi  r \cos (\phi )}{p},\text{v0} \sin&#xA;    (\phi )\right\}&#xA;$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To simplify, we write separate equations for the x, y, and z&#xA;components of the projectile. From the above, we have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$x(0)=r \cos (\phi )$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$y(0)=0$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$z(0)=r \sin (\phi )$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$x'(0)=\text{v0} \cos (\phi )$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$y'(0)=\cos (\phi )$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$z'(0)=\text{v0} \sin (\phi )$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration acts towards the center of the planet, so the direction&#xA;of acceleration at time t is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\{-x(t),-y(t),-z(t)\}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Converting this to a unit vector:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$&#xA;\left\{-\frac{x(t)}{\sqrt{x(t)^2+y(t)^2+z(t)^2}},-\frac{y(t)}{\sqrt{x(t)^2+y(&#xA;t)^2+z(t)^2}},-\frac{z(t)}{\sqrt{x(t)^2+y(t)^2+z(t)^2}}\right\}&#xA;$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The acceleration due to gravity is g at the surface, which is distance&#xA;r from the planet's center, and is thus g*(r/d)^2 at distance d from the&#xA;planet (provided that d &gt; r, which it is in our problem). Using the&#xA;distance formula, the magnitude of gravity at time t is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\frac{g r^2}{x^2+y^2+z^2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, the vector of acceleration due to gravity is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$&#xA;   \left\{-\frac{g r^2 x(t)}{\left(x(t)^2+y(t)^2+z(t)^2\right)^{3/2}},-\frac{g&#xA;    r^2 y(t)}{\left(x(t)^2+y(t)^2+z(t)^2\right)^{3/2}},-\frac{g r^2&#xA;    z(t)}{\left(x(t)^2+y(t)^2+z(t)^2\right)^{3/2}}\right\}&#xA;$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we'll be solving these equations numerically, we need to limit&#xA;the values of t. We obviously want to start at t=0, but where to end? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roughly speaking, if an object has initial velocity v0 and accelerates&#xA;at g (in the negative direction), it will reach 0 velocity at time&#xA;v0/g, and will return to its initial position at time&#xA;2*v0/g. Therefore, computing until t=4*v0/g should be sufficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(of course, this entire problem is about refining the &quot;roughly&#xA;speaking&quot; paragraph above)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that simply finding the approximate function for the projectile's&#xA;position is only step 1. We then need to find when the projectile&#xA;lands back on the planet: in other words, when it's distance from the&#xA;center is once again r.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mathematica code follows.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;(* TODO: I am not returning functions properly here *)&#xA;&#xA;(* &#xA;&#xA;This module returns (in this order):&#xA;&#xA;s[t]: the xyz position of the projectile at time t, 0 &amp;lt; t &amp;lt; root&#xA;&#xA;site[t]: the position of the launch site at time t, just as a reminder&#xA;that the planet is rotating.&#xA;&#xA;root: The time at which the projectile lands&#xA;&#xA;dlat: The projectile's change in latitude (radians)&#xA;&#xA;dlon: The projectile's change in longitude (radians)&#xA;&#xA;nsdist: north/south distance traveled by projectile&#xA;&#xA;ewdist: east/west distance traveled by projectile&#xA;&#xA;tdist: total distance traveled by projectile (on surface of sphere)&#xA;&#xA;Some helper functions used below are found in:&#xA;&#xA;https://github.com/barrycarter/bcapps/blob/master/bclib.m&#xA;&#xA;*)&#xA;&#xA;launch[r_, p_, g_, phi_, v0_] := launch[r,p,g,phi,v0] = Module[&#xA; {s, site, root, dlat, dlon, garb, nsdist, ewdist, tdist, maxtime, maxheight},&#xA;&#xA;s[t_] = {x[t], y[t], z[t]} /. NDSolve[{&#xA; x[0] == r*Cos[phi], y[0] == 0, z[0] == r*Sin[phi],&#xA; x'[0] == v0*Cos[phi], y'[0] == 2*Pi*r*Cos[phi]/p, z'[0] == v0*Sin[phi],&#xA; x''[t] == -g*((r^2*x[t])/(x[t]^2 + y[t]^2 + z[t]^2)^(3/2)),&#xA; y''[t] == -g*((r^2*y[t])/(x[t]^2 + y[t]^2 + z[t]^2)^(3/2)),&#xA; z''[t] == -g*((r^2*z[t])/(x[t]^2 + y[t]^2 + z[t]^2)^(3/2))&#xA;}, {x[t],y[t],z[t]}, {t,0,4*v0/g}][[1]];&#xA;&#xA;maxtime = FindMaximum[Norm[s[t]]-r,{t,v0/g}];&#xA;maxheight = maxtime[[2,1,2]];&#xA;maxtime = maxtime[[1]];&#xA;&#xA;site[t_] = r*{Cos[2*Pi*t/p]*Cos[phi], Sin[2*Pi*t/p]*Cos[phi], Sin[phi]};&#xA;root = t /. FindRoot[Norm[s[t]]-Norm[s[0]], {t, 2*v0/g}];&#xA;{dlon,dlat,garb} = xyz2sph[s[root]]-xyz2sph[site[root]];&#xA;{nsdist, ewdist} = {dlat*r, Cos[phi+dlat]*dlon*r};&#xA;tdist = 2*r*ArcSin[Norm[s[root]-site[root]]/2/r];&#xA;Return[{s[t], site[t], root, dlat, dlon, nsdist, ewdist, tdist, maxtime,&#xA; maxheight}];&#xA;];&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Now, let's use this code to answer some existing questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/48287/&quot;&gt;https://physics.stackexchange.com/questions/48287/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To jump for 1 second, I need an initial velocity of 4.9m/s, so we compute&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;launch[6371000, 86400, 9.8, 0*Degree, 4.9]&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The jump takes 1.003 seconds (3 milliseconds longer than expected),&#xA;and I land 116 micrometers west of my original position (no change in&#xA;my north/south position, since I started at the equator).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To check @NowIGetToLearnWhatAHe's answer: jumping 1m on Earth requires&#xA;an initial velocity of 4.43 m/s, so we run&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;launch[6371000, 86400, 9.8, 45*Degree, 4.43]&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The jump lasts 0.9053 seconds, and I land 6.9mm south and 62.5&#xA;micrometers west of where I started, verifying&#xA;@NowIGetToLearnWhatAHe's answers up to roundoff errors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This effectively also answers&#xA;&lt;a href=&quot;https://physics.stackexchange.com/questions/80090/&quot;&gt;https://physics.stackexchange.com/questions/80090/&lt;/a&gt; if we assume jumping&#xA;1m up and hovering 1 second is close enough to the cases above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It unfortunately doesn't answer&#xA;&lt;a href=&quot;https://physics.stackexchange.com/questions/89276&quot;&gt;https://physics.stackexchange.com/questions/89276&lt;/a&gt; Even though the&#xA;train's ceiling is probably no more than 1m higher than the thrower's&#xA;hand, the 600mph velocity of the train might be sufficient to affect&#xA;the equation.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/214008&quot;&gt;https://physics.stackexchange.com/questions/214008&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Let's solve the problem at 45 degrees north latitude, where the&#xA;bullet's motion is greatest:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;launch[6371000, 86400, 9.8, 45*Degree, 1200]&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bullet will land 249.148 seconds later, 507m west and 1260m south&#xA;of its original position, reaching a maximum height of 74455m at the&#xA;halfway point in its journey (124.574s)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/226882&quot;&gt;https://physics.stackexchange.com/questions/226882&lt;/a&gt; actually asks&#xA;about air resistance, so my answer explicitly doesn't apply, but&#xA;let's run the numbers anyway. I did&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;launch[3389500, 88643, 3.711, 0*Degree, v0]&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;with various v0's to see how fast the coin must be flipped to miss the&#xA;flipper's hand (assuming hand radius of 12cm).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the Martian equator, you would have to flip a coin at 25.9m/s for&#xA;it to miss your hand on the way down. This is equivalent to flipping a&#xA;coin at 9.8m/s (about 22 miles per hour) on Earth, so, yes, you could&#xA;concievably flip a coin fast enough that it would miss your hand on&#xA;the way down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 45 degrees latitude on Mars, we have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;launch[3389500, 88643, 3.711, 45*Degree, v0]&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and it turns out an initial velocity of 10m/s (equivalent to 3.76m/s&#xA;or 8.5 mph on Earth) would suffice to have the coin miss your hand.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Other questions this helps answers (which don't have numerical&#xA;quantities in the question):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/166853&quot;&gt;https://physics.stackexchange.com/questions/166853&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/174385&quot;&gt;https://physics.stackexchange.com/questions/174385&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/137191&quot;&gt;https://physics.stackexchange.com/questions/137191&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/14993&quot;&gt;https://physics.stackexchange.com/questions/14993&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/126469&quot;&gt;https://physics.stackexchange.com/questions/126469&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/148008&quot;&gt;https://physics.stackexchange.com/questions/148008&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/7479&quot;&gt;https://physics.stackexchange.com/questions/7479&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/20424&quot;&gt;https://physics.stackexchange.com/questions/20424&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/136351&quot;&gt;https://physics.stackexchange.com/questions/136351&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/12766&quot;&gt;https://physics.stackexchange.com/questions/12766&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;It doesn't answer the following questions, but may have enough&#xA;relevance that they're worth linking to this answer:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/16390&quot;&gt;https://physics.stackexchange.com/questions/16390&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/133801&quot;&gt;https://physics.stackexchange.com/questions/133801&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/214614&quot;&gt;https://physics.stackexchange.com/questions/214614&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;We can also use this code to get an idea of how much accounting for&#xA;gravity and rotation affects trajectory at different initial&#xA;velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the below, I use:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;launch[6371000, 86400, 10, 45*Degree, v0]&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;estimating Earth's surface gravity at 10m/s^2 for convenience.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Landing time (in seconds):&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/12vU8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/12vU8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The red line represents t/5, the landing time we'd expect if the Earth&#xA;didn't rotate and gravity didn't decrease with distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The blue line represents actual landing time.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Landing spot how far west of launch site (in meters):&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lOy8O.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lOy8O.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Landing spot how far south of launch site (in meters):&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MtiHY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MtiHY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The total distance between landing spot and launch site mirrors&#xA;the graph above, since most of the motion is north-south.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The apparent surface velocity of the projectile: ie, the distance&#xA;traveled on the surface divided by the landing time (in meters/second):&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/B28LY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/B28LY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Mathematica can't solve the differential equation when the initial&#xA;velocity exceeds 7000m/s, possibly since that's fairly close to the&#xA;velocity required to achieve Earth orbit (though not enough to&#xA;escape the Earth's gravitational pull entirely, which would require&#xA;a velocity of 11186 m/s)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Longer-term, I'd like to work out first-order approximations to&#xA;the quantities above.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user7073" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2016-01-31T19:52:24.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13817" PostTypeId="1" AcceptedAnswerId="17449" CreationDate="2016-01-31T20:01:14.420" Score="14" ViewCount="1805" Body="&lt;p&gt;The Delta 4 vehicle is a common booster core (CBC) design, where a single booster design can be used to launch in single stick mode, or in triple stick (Heavy) mode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The single stick model also has mount points for strap on solid rocket boosters of varying numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the Delta 4 Heavy has the highest payload to LEO (Falcon Heavy is not active as of time of writing) of all active US boosters, what if they needed a heavier version?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the side cores of a Delta 4 Heavy launch with strap on SRBs?  Could additional strap on SRB's be mounted on the center core? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously flame trench issues would ensue and require modifications, but since my understanding is that each CBC has all the mount points for both strap ons and side CBC cores, could you use both at once? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, how much of a payload boost would that add?  (The obvious correlary is how much closer could it come to a Falcon Heavy?)&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-24T18:15:01.817" Title="Could a Delta 4 Heavier be built?" Tags="|launch|falcon-heavy|delta-iv-heavy|delta-iv|cbc|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13820" PostTypeId="2" ParentId="13817" CreationDate="2016-01-31T22:09:46.717" Score="9" Body="&lt;p&gt;There are a number of differences between the two systems. One of them is the cross-feed system of the Falcon Heavy. The cross-feed is only expected to be used on the Falcon Heavy with the heaviest loads, &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-heavy/&quot; rel=&quot;noreferrer&quot;&gt;over 45 tons&lt;/a&gt;. The Delta IV has a LEO capability of about 28 tons. Falcon Heavy's max capacity is 54 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV#Future_variants&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; states that a Delta Heavy with cross-feed, extra cores, and solid rocket boosters could lift 100 tons to LEO. There is also &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Evolution/DeltaIVLaunchVehicle_GrowthOptionstoSupportNASAsSpaceExplorationVision.pdf&quot; rel=&quot;noreferrer&quot;&gt;a paper&lt;/a&gt; about a number of proposed Delta IV Heavy configurations, from which the following comes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7aAF5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7aAF5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The general thought is to fist increase the number of cores, then attach some solid boosters. The solid boosters don't seem to add much performance wise. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="49" LastEditDate="2016-02-01T06:37:52.617" LastActivityDate="2016-02-01T06:37:52.617" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13821" PostTypeId="2" ParentId="13809" CreationDate="2016-01-31T22:34:32.390" Score="10" Body="&lt;p&gt;I often have to go read &lt;a href=&quot;http://rhodesmill.org/skyfield/time.html&quot; rel=&quot;noreferrer&quot;&gt;the Skyfield guide to dates and times&lt;/a&gt; to keep everything straight, but, briefly:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;A Julian Date is simply a rival way to name moments, that is simpler than our customary date-plus-time system. Normally, to specify a date and time requires six different numbers — year, month, day, hour, minute, and seconds — and comparing two dates takes a terrible amount of math, where you find yourself borrowing 31 days from some months but 30 from others, and having to carry the leap day in your head if you want the math to work out. Not so with Julian Dates: they simply assign a simple floating-point number to each date-plus-time on our calendar, and are much easier to do math with.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The question of whether you like date-and-time names for moments, or instead love Julian Date numbers for moments, is entirely separate from the question of &lt;em&gt;time scales&lt;/em&gt; — the question, given a given moment, of “what time is it”? Whether you use the name “2457418.5” or the name “2016 January 31 00:00:00.0” (the two are exactly equivalent) for a given moment, that still leaves the question “but &lt;em&gt;how&lt;/em&gt; do we agree on when that moment was?”&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A &lt;em&gt;time scale&lt;/em&gt; is a system for assigning names to moments in time. UT1, UTC, TAI, TT, and TDB all have a different answer for, “What is the best name for RIGHT NOW?” Whether you express TT as a Julian Date because you like big numbers that make math easy, or as a date-plus-time because the big numbers don't mean much to you, you are simply expressing in two different ways the answer to the question “What does TT call this particular moment?”&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You might wonder why we would have so many different names (because there are actually several more beyond the basic five I named above) for the exactly same moment. Briefly:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UT1 — because we wonder where the Sun and stars are above our heads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UTC — because we want our beside clocks to kind-of be synchronized with sunrise and sunset, but it's hard to explain to everyone's watches that seconds need to speed up and slow down to track the sun, so instead let's keep seconds a constant length and add a leap second when we start to fall behind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TAI — just forget about the sun. Atomic clocks need seconds that march straight ahead with no leap calculations ever.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TT — great idea, TAI people! Only, astronomers already had that idea, years ago, and had already started doing calculations against an imaginary clock that didn't slow up with the Earth's slowing rotation, so it's best that they maintain a timescale parallel with TAI but that stays 30+ seconds different so they don't have to rewrite all their math and tables.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TDB — but if you are interested in the celestial clockwork of the Solar System, all those other clocks are useless! Because any Earth based clock will speed up and slow down as the Earth accelerates and decelerates each year, because: relativity. If you are going to study planet motions, you have to use an imaginary clock that is stationary at the middle of the Solar System, not the crazy Mickey Mouse clocks we carry on Earth that accelerate BOTH with our orbital revolution AND with our daily rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, refer to the Skyfield documentation linked above, and to the &lt;a href=&quot;http://aa.usno.navy.mil/publications/docs/Circular_179.php&quot; rel=&quot;noreferrer&quot;&gt;USNO circular 179&lt;/a&gt; for a more technical write-up of these concepts.&lt;/p&gt;&#xA;" OwnerUserId="8590" LastActivityDate="2016-01-31T22:34:32.390" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13822" PostTypeId="1" AcceptedAnswerId="13824" CreationDate="2016-01-31T22:49:58.703" Score="39" ViewCount="7646" Body="&lt;p&gt;During a recent NASA broadcast, I noticed something in the background:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6t2GZ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6t2GZ.png&quot; alt=&quot;eye chart?&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like a &lt;a href=&quot;https://en.wikipedia.org/wiki/Snellen_chart&quot; rel=&quot;noreferrer&quot;&gt;Snellen chart&lt;/a&gt; to me, but why would they have that on board? If that's not it, what is it then?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="49" LastEditDate="2016-01-31T23:14:33.303" LastActivityDate="2016-01-31T23:14:33.303" Title="Why do they have a Snellen eye chart on the ISS?" Tags="|iss|crewed-spaceflight|health|medical|ocular|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13823" PostTypeId="2" ParentId="13822" CreationDate="2016-01-31T22:57:35.110" Score="37" Body="&lt;p&gt;NASA has been studying the effects of microgravity on astronauts' eyes for at least a few years. This article from &lt;a href=&quot;http://www.space.com/14876-astronaut-spaceflight-vision-problems.html&quot;&gt;Space.com&lt;/a&gt; from 2012 talks about some of the findings from that time. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a new study, researchers used magnetic resonance imaging (MRI) to study the eyes and brains of 27 astronauts who spent an average of 108 days in space aboard NASA's space shuttle and/or the International Space Station. They found optical abnormalities similar to those that can occur in patients with intracranial hypertension, a potentially serious condition in which pressure builds up inside the skull. - See more at: &lt;a href=&quot;http://www.space.com/14876-astronaut-spaceflight-vision-problems.html#sthash.h3YPDnGZ.dpuf&quot;&gt;http://www.space.com/14876-astronaut-spaceflight-vision-problems.html#sthash.h3YPDnGZ.dpuf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For example, nine of the 27 astronauts (33 percent) exhibited expansion of the cerebrospinal fluid space surrounding the optic nerve, and six (22 percent) showed flattening of the back of the eyeball - See more at: &lt;a href=&quot;http://www.space.com/14876-astronaut-spaceflight-vision-problems.html#sthash.h3YPDnGZ.dpuf&quot;&gt;http://www.space.com/14876-astronaut-spaceflight-vision-problems.html#sthash.h3YPDnGZ.dpuf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They say it's a high priority, so having the ability to perform tests aboard the ISS makes sense, especially for the astronauts who are up there for a year.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2016-01-31T22:57:35.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13824" PostTypeId="2" ParentId="13822" CreationDate="2016-01-31T22:57:59.710" Score="55" Body="&lt;p&gt;Eyes do strange things in microgravity (when you consider they're deformable bags of fluid, this isn't too surprising). &lt;a href=&quot;https://geiselmed.dartmouth.edu/news/2015/an-eye-on-microgravity/&quot;&gt;This report&lt;/a&gt; outlines the changes that can be identified after just a short parabolic flight. Eye test charts provide a way to investigate this without requiring heavy equipment or specialists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/204.html&quot;&gt;This study&lt;/a&gt; appears to be an on-going project to study this on the ISS (Mar 13 - Sep 16), and is probably the reason that chart's there...&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="11259" LastEditDate="2016-01-31T23:05:39.977" LastActivityDate="2016-01-31T23:05:39.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13825" PostTypeId="1" AcceptedAnswerId="13826" CreationDate="2016-02-01T01:18:58.000" Score="14" ViewCount="20332" Body="&lt;p&gt;I want to convert the epoch time of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE&lt;/a&gt; to UTC. At this moment I am less interested in explanations and history - I just want to know quantitatively, with certainty, how to convert.&lt;/p&gt;&#xA;&lt;p&gt;I'm confused because I see references to both UT and JD in NASA and Celestrak sites as show below. Since Julian Date currently leads UT by approximately &lt;a href=&quot;https://space.stackexchange.com/a/13810/12102&quot;&gt;68.184 seconds&lt;/a&gt; (as well as a 0.5D offset), I am stuck.&lt;/p&gt;&#xA;&lt;p&gt;The epoch time in the TLE is used in at least two ways. First - it's the time when the prediction from the TLE is most accurate. Second - (I think) it is the moment in time corresponding to the mean anomaly value in line 2 of the TLE, which is 66.7566 degrees in this example:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;TLE = &amp;quot;&amp;quot;&amp;quot;&#xA;1 25544U 98067A   16031.25992506  .00006019  00000-0  97324-4 0  9994&#xA;2 25544  51.6430  25.8646 0006733  62.5910  66.7566 15.54344726983528&#xA;&amp;quot;&amp;quot;&amp;quot;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;strong&gt;my question:&lt;/strong&gt; what is the exact relationship between the epoch time in a TLE and the corresponding time in UTC?&lt;/p&gt;&#xA;&lt;p&gt;The current difference can also be shown with &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;from skyfield.api import JulianDate&#xA;&#xA;jd = JulianDate(utc=(2016,1,1))&#xA;&#xA;print jd.tt, &amp;quot;days&amp;quot;&#xA;print jd.tt % 1, &amp;quot;day fraction&amp;quot;&#xA;print ((jd.tt % 1) - 0.5) * 24. * 3600, &amp;quot;seconds&amp;quot;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;gives&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;2457388.50079 days&#xA;0.500789166894 day fraction&#xA;68.1840196252 seconds&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;this &lt;a href=&quot;http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/tle_def.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; page calls the fraction &amp;quot;Julian Day Fraction&amp;quot;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/13u3a.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/13u3a.gif&quot; alt=&quot;NASA TLE 2line.gif&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dYlms.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dYlms.png&quot; alt=&quot;NASA TLE screenshot&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;but this &lt;a href=&quot;https://celestrak.org/columns/v04n03/#FAQ03&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt; page states that&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Obez8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Obez8.png&quot; alt=&quot;Celestrak TLE screenshot&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;&amp;quot;Note that the epoch day starts at UT midnight (not noon) and that all times are measured mean solar rather than sidereal time units (the answer to our third question).&amp;quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2023-10-03T18:00:35.830" Title="How to obtain UTC of the epoch time in a satellite TLE (two line element)?" Tags="|artificial-satellite|time|orbital-elements|skyfield|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13826" PostTypeId="2" ParentId="13825" CreationDate="2016-02-01T01:42:22.077" Score="10" Body="&lt;p&gt;Given: 16031.25992506&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 16 corresponds to 2016. As 1957 was the first year with satellites launched, 57 would be 1957, and in 2057 this might change, as there will be an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 31 means the 31st day of the year (January 31)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The .25992506 is the fractional day from midnight. This means 6.2382 hours, 14.292 minutes, 17.52 seconds, basically accomplished by multiplying by 24 first, then removing the hours, then 60, then removing the minutes, then 60 again to get the seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for leap seconds, it isn't actually defined. &lt;a href=&quot;http://cdf.gsfc.nasa.gov/html/leapseconds_requirements.html&quot; rel=&quot;noreferrer&quot;&gt;NASA has stated the practiced standard is&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In practice, it generally holds UTC times, but leap seconds are overloaded onto the first second of the next day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the bottom line is, that conversion should give you the time in UTC. Leap seconds are annoying, but the time should still be UTC.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-01T01:42:22.077" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13828" PostTypeId="2" ParentId="13825" CreationDate="2016-02-01T03:24:36.133" Score="14" Body="&lt;p&gt;The best guess available about the time scale used in TLE files is from the epic &lt;em&gt;Revisiting Spacetrack Report #3&lt;/em&gt; report from Celestrak, which they have put online here:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://celestrak.org/publications/AIAA/2006-6753/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://celestrak.org/publications/AIAA/2006-6753/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;They took the old SGP4 software, collected every single patch and improvement they could find in the dozens of versions online and being passed around various laboratories, and issued a new cleaned up version of the algorithm (the one used in Skyfield, in fact).&lt;/p&gt;&#xA;&lt;p&gt;In the report's “Section E” they guess that the time is either in UTC or UT1, and conclude that the difference does not really matter because SGP4 just isn't accurate enough in predicting satellite positions for &amp;lt;1 second to matter. (The difference between UTC and UT1 is always less than a second.) They say:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Time accounting within SGP4 is referenced to the epoch of the TLE data. … The time system is assumed here to be UTC, but no formal documentation exists and UTC, as currently defined, was only introduced in 1972. UT1 is needed to calculate GMST for the coordinate transformations discussed in the appendix, but it is unknown whether UT1 or UTC is what is required by the software, although we assume UT1 for this paper. The error associated with approximating UT1 with UTC is within the theoretical uncertainty of the SGP4 theory itself. Except for the GMST calculation, this paper and code assumes time to be realized as UTC.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So you can ignore TAI, TT, and TDB when dealing with satellites. All of the times you deal with will be probably be UTC.&lt;/p&gt;&#xA;" OwnerUserId="8590" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2016-02-01T03:24:36.133" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13829" PostTypeId="2" ParentId="13825" CreationDate="2016-02-01T04:14:21.577" Score="9" Body="&lt;p&gt;The term &quot;Julian day&quot; (or date) has three very distinct meanings. To historians, it typically means a date that is expressed in the Julian calendar, as opposed to the Gregorian calendar. For example, the Julian date 3 September 1752 is the same day as the Gregorian date 14 September 1752. To astronomers, it means the number of days since noon (in some time scale, typically TT or UT) on January 1, 4713 BC (Julian). For example, JD 2357569 is refers to noon UT on 14 September 1752.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third meaning is the day number of the year, with 1 meaning January 1. This third meaning is the meaning intended at the cited NASA page.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-02-01T04:14:21.577" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13831" PostTypeId="2" ParentId="13803" CreationDate="2016-02-01T07:12:36.073" Score="2" Body="&lt;p&gt;For a nuclear pulse rocket, the peak acceleration is due to the shock of the push provided by each pulse unit. I struggled a bit trying to find a reference for the G-limit of what shocks electronic equipment can tolerate, but finally found it &lt;a href=&quot;http://edge.rit.edu/content/P10662/public/old/Mechanical/DesigningElectronics.doc&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. The paper mentions a spaceborne computer system that must tolerate shock levels of up to 450 G's&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-01T07:12:36.073" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13832" PostTypeId="2" ParentId="10025" CreationDate="2016-02-01T08:51:28.997" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Operating costs must be taken into account, from production to transportation to loading and launching the rocket.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...and since the question is about application in the &lt;strong&gt;final&lt;/strong&gt; rocket stage, that &lt;strong&gt;must&lt;/strong&gt; include cost of getting the final stage to altitude and speed where that stage is ignited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here the comparison to cryofuels crashes and burns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UDMH/$N_2O_4$ has a poor density impulse, 316-kg s/l. LOX/LH2 - 124 kg-s/l.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means you need to load up much more of it onto the final stage, than say, LOX/LH2 to achieve the same delta-V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that means, that regardless of costs of production, transportation, loading and fueling up the final stage, the cost of about the most expensive part of the rocket - the launch stage - goes up by strides. Any benefits to reduction of cost or complexity of the final stage will be completely overshadowed by the increased cost of the initial stages that need to handle the increased payload.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-02-01T09:02:32.630" LastActivityDate="2016-02-01T09:02:32.630" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13833" PostTypeId="1" CreationDate="2016-02-01T10:29:08.893" Score="24" ViewCount="3208" Body="&lt;p&gt;The traditional explanation for rocky inner planets and large gaseous outer planets is that the outer planets are farther from the Sun, and therefore can hold onto large atmospheres that don't get eroded by the solar wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay so let's look at Venus. 30% closer to the Sun and has a much weaker magnetic field than Earth. So we would expect much less of an atmosphere, if any at all. But Venus' atmosphere is &lt;strong&gt;92 times thicker&lt;/strong&gt; than ours. How?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I know the Venusian atmosphere is 96% carbon dioxide, which is heavier than O2 or N2, so it's a little more &quot;stalwart&quot; against the solar wind. But honestly I don't see how that's enough. CO2 molecular weight compared to O2 is 44/32 = 1.375 times heavier. But again, Venus is 30% closer to the Sun and has an extremely weak magnetic field. Let's also not forget &lt;strong&gt;how darn hot&lt;/strong&gt; Venus is, which means its air molecules are bouncing around much harder, making it easier for the solar wind particles to come along and knock out that CO2 into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's also not forget Mars, which also has an atmosphere 96% CO2 (very interesting why they are both 96%). But Mars' atmosphere is extremely thin, just a few hundred pascals. If the solar wind can do that all the way out to Mars, which is 2.1 times farther than Venus...then what is Venus doing with all that air?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Some things I tried to think of&lt;/strong&gt; that make Venus different from Earth or Mars: Venus rotates extremely slowly. Venus rotates in the opposite direction. Venus has no moon(s). Venus has no water, no tectonic or volcanic activity, and no people. &lt;strong&gt;I don't see how&lt;/strong&gt; any of this would help hold its atmosphere against the solar wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Any joke comments on an extinct venusian people making foolish use of carbon fuels, hence the runaway greenhouse effect, 96% CO2, and cooked planet, will be met with skepticism.)&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2017-04-24T10:57:04.753" Title="How does Venus' thick atmosphere survive against the solar wind?" Tags="|atmosphere|venus|solar-wind|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13835" PostTypeId="2" ParentId="13833" CreationDate="2016-02-01T11:28:57.513" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www-ssc.igpp.ucla.edu/personnel/russell/papers/interact_solwind/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This&lt;/a&gt; looks like a good read if your really interested in the subject but the simple answer appears to be that the ionosphere of Venus is strong enough to sufficiently deflect the solar wind, without the need of a strong magnetic field. Although this doesn't completely protect the planet, Venus still looses some of it's atmosphere into space.&lt;/p&gt;&#xA;" OwnerUserId="13176" LastEditorUserId="13176" LastEditDate="2017-04-24T10:57:04.753" LastActivityDate="2017-04-24T10:57:04.753" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13836" PostTypeId="2" ParentId="13833" CreationDate="2016-02-01T11:28:58.107" Score="18" Body="&lt;p&gt;The mass of Mars is so small, and its magnetic field so weak that it cannot hold onto carbon for a long period of time, making &lt;a href=&quot;http://science.sciencemag.org/content/350/6261/643.full&quot; rel=&quot;noreferrer&quot;&gt;almost all of it to escape.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/K2cs6t.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/K2cs6t.jpg&quot; alt=&quot;Venus Earth and Mars&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;(Venus and Earth are close to identical in size, Mars is much smaller)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus does not really leak that much of its atmosphere, the only noticeable exception is that all hydrogen compounds are virtually absent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is the Venus atmosphere so much denser, compared to Earth? Surely we can resist the solar wind even better than Venus can. Both Venus and Earth are massive enough, and has both has significant magnetic fields, to resist the solar wind, and hold a significant atmosphere (Mars can not, it is just too small, and the magnetic field is too weak). So why the difference?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass of the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html&quot; rel=&quot;noreferrer&quot;&gt;Venus atmosphere&lt;/a&gt; is $4.8 \cdot 10^{20}kg$, or about $1.2 \cdot 10^{20}kg$ of carbon, as it is mainly carbon dioxide. Where is all that carbon on Earth? Our atmosphere does only contain $8.1 \cdot 10^{14}kg$. A lot of carbon is also dissolved in the oceans, approximately $3.6 \cdot 10^{16}kg$, but that is still over three orders of magnitude less. Adding all coal, oil, etc. we are still below $5 \cdot 10^{16}kg$, so a Venus civilization burning all their fossil fuels can not be the explanation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where is that carbon on Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of that missing carbon on Earth is in limestone. The total amount of carbon in limestone in the crust is about $4 \cdot 10^{19}kg$. Now we are talking! Limestone is just 10% of the sedimentary rock, so there are probably a lot more carbon in the crust. Such carbon containing compounds releases carbon at high temperatures, thus they are already decomposed on Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth has a thick atmosphere too, just like Venus. But it is currently bound to the crust&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As strange as it may sound, the higher temperature of Venus is the cause of the dense atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8162" LastEditDate="2017-04-18T11:47:19.783" LastActivityDate="2017-04-18T11:47:19.783" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13837" PostTypeId="1" AcceptedAnswerId="13840" CreationDate="2016-01-31T10:04:49.397" Score="3" ViewCount="794" Body="&lt;p&gt;I have read (&lt;a href=&quot;https://astronomy.stackexchange.com/a/11489&quot;&gt;here&lt;/a&gt;) and heard (on astronomy podcast) that radio waves decay or are indistinguishable the more distance they travel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do any of man made satellites communicate with Earth solely by laser?&lt;/p&gt;&#xA;" OwnerUserId="13273" OwnerDisplayName="Kyslik" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2016-02-01T15:06:09.350" Title="Does any satellite communicate only through laser?" Tags="|artificial-satellite|communication|laser|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13838" PostTypeId="2" ParentId="13837" CreationDate="2016-01-31T12:23:57.343" Score="2" Body="&lt;p&gt;It's not the case that radio waves decay. Radio waves are a very good choice for most long distance communication, owing to their penetration of the atmosphere and the relative ease with which even distant, low-power sources can be received. In these ways, radio waves are superior to visible laser light. Visible light is blocked by clouds, and low intensity sources are swamped by the brightness of reflected sunlight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However for high-bandwidth communication lasers do have a place. No spacecraft has been entirely dependent on lasers for communication though.&lt;/p&gt;&#xA;" OwnerUserId="11966" OwnerDisplayName="James Kilfiger" LastActivityDate="2016-01-31T12:23:57.343" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13839" PostTypeId="2" ParentId="13837" CreationDate="2016-01-31T14:38:49.990" Score="4" Body="&lt;p&gt;The main reason for reduced received signal with radio communication in space is the spreading loss, that is intensity declines with the square of the distance from source to receiver. This is true even with beamed radio signals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Laser communications beams suffer from exactly the same loss once the cross-sectional area of the beam exceeds size of the receiving sensor. Which for practical purposes will always be the case for space communications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The comms. system you choose is more related to bandwidth, absorption losses, immunity to interference, the practicality of the technology, ... but not spreading losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For SETI laser communications signals will be more difficult to detect unless they are deliberated attempts to contact others by an alien civilisation own SETI program, as lasers usually have much narrower beam widths than radio transmitters.&lt;/p&gt;&#xA;" OwnerUserId="10998" OwnerDisplayName="Conrad Turner" LastActivityDate="2016-01-31T14:44:24.367" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13840" PostTypeId="2" ParentId="13837" CreationDate="2016-02-01T14:53:33.657" Score="3" Body="&lt;p&gt;Lasers have two advantages over radio waves for communication&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;They operate at a higher frequency, allowing for higher data rate transfer.&lt;/li&gt;&#xA;&lt;li&gt;They are more focused, not dispersing the energy as much as radio waves, making their effective distance higher.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are also a few disadvantages:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The beam width is so narrow that exact pointing is required, both for the satellite and the receiver. It is said that from Mars, you not only have to point at the Earth, but point at an area a few US states wide to maintain contact.&lt;/li&gt;&#xA;&lt;li&gt;Clouds will obscure the laser communication, while they don't affect radio waves the same.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;No spacecraft has used exclusively Laser Communications, but there have &lt;a href=&quot;https://en.wikipedia.org/wiki/Laser_communication_in_space&quot; rel=&quot;nofollow&quot;&gt;been a number of tests&lt;/a&gt;. There is at least one satellite that uses lasers for a relay communication, using lasers on a LEO satellite to communicate with a satellite in GEO orbit, although the communication with the ground still occurs over RF. See &lt;a href=&quot;http://spaceflight101.com/european-laser-communications-terminal-set-for-liftoff-atop-proton-rocket/&quot; rel=&quot;nofollow&quot;&gt;European Laser Communication Satellite&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-02-01T15:06:09.350" LastActivityDate="2016-02-01T15:06:09.350" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13841" PostTypeId="1" CreationDate="2016-02-01T16:42:03.200" Score="5" ViewCount="118" Body="&lt;p&gt;ULA gets a so called subsidy (currently purported to be $800 million a year, since it is being considered to be cancelled) for Assured Access to space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their lack of a bid on the GPS3 launch contract has bothered congress critters, who say, if we pay for this assured access, and you do not bid, what are we getting out of the money?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the question: What do we get out of the funding?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-02-02T08:11:51.177" Title="What does the US Govt get out of its Assured Access funding of ULA?" Tags="|ula|delta-iv-heavy|atlas-v|delta-iv|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13842" PostTypeId="2" ParentId="13022" CreationDate="2016-02-01T17:35:21.223" Score="6" Body="&lt;p&gt;The premise of the question is incorrect. If you were able to terraform Mars on a human civilization time scale in the first place, then you can replenish the atmosphere, if you like, at a rate several orders of magnitude faster than the rate of loss to the solar wind. Or you can simply ignore that loss, since it won't be noticeable for many millions of years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The terraformed atmosphere, assuming Earth sea-level density or even mile-high density, would provide ample radiation protection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You don't need a magnetosphere to terraform Mars.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-02-01T17:35:21.223" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13844" PostTypeId="1" AcceptedAnswerId="13845" CreationDate="2016-02-01T20:43:35.470" Score="0" ViewCount="99" Body="&lt;p&gt;Is there any planet outside solar system that has almost same characters that of earth? If not found till now,will it be possible ever to shift the whole human race to a new planet,because as Stephen Hawkins stated,in the years to come the world will be unfit for living beings,and a need for a new planet to reside will come.So if it is so, is there any idea of how to shift humans to a new planet or is there no such ways at all?&lt;/p&gt;&#xA;" OwnerUserId="13277" LastActivityDate="2016-02-01T21:22:23.883" Title="Similar planet to earth" Tags="|future-missions|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13845" PostTypeId="2" ParentId="13844" CreationDate="2016-02-01T20:57:16.687" Score="2" Body="&lt;p&gt;The Earth similarity index (ESI) is a way to measure how similar other planets are to Earth. Mars has similarity index or 0.7 (1 being Earth) and Kepler-438b 0.88.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ESI depends on planet's density, radius, surface temperature and escape velocity.&#xA;A planet with high ESI value is not necessarily habitable (and there a there is some uncertainty of it's accuracy because some of the values going in might not be very accurate) but it stands to reason that if they are in the habitable zone of their star, there is a chance they are habitable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, those planets are light years away. Flying there with current technology will take thousands of years. To make this trip reasonable you have to reach a significant fraction of the speed of light (10% - 20% maybe) which would require a (very large) leap in propulsion technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gliese 667 Cc is probably the best candidate at the moment similarity wise that's relatively close by - &quot;mere&quot; 24 light years away.&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gliese_667_Cc&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Gliese_667_Cc&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-02-01T21:22:23.883" LastActivityDate="2016-02-01T21:22:23.883" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13846" PostTypeId="2" ParentId="12834" CreationDate="2016-02-02T08:01:33.620" Score="1" Body="&lt;p&gt;Using @TildalWave's suggested method of estimating the depth of the regolithe, I looked for boulders penetrating the surface layer. There are a couple of them:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kra3U.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kra3U.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/oP5m3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oP5m3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/1TKfs.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1TKfs.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most notable however are the large (~10m) boulders towards the north rim of the crater. They seem to be located in a pretty shallow place, so I would say that the depth of the soil is no more than a few meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VAkqg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VAkqg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So close to the Lalande crater, the crater must have been created afterwards, making a &quot;dusty&quot; impact in the depositions from the much larger event that created Lalande.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;I use the assumption that boulders must lay on solid rock. That may be horribily wrong.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-05T22:22:50.200" LastActivityDate="2016-02-05T22:22:50.200" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13847" PostTypeId="2" ParentId="13841" CreationDate="2016-02-02T08:11:51.177" Score="5" Body="&lt;p&gt;The &lt;a href=&quot;https://www.hsdl.org/?view&amp;amp;did=764787&quot; rel=&quot;noreferrer&quot;&gt;goals of Assured Access to Space are&lt;/a&gt; (PDF of a hearing by the House Committee on Armed Services, page 14): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The existing policy, codified in federal law, requires that assured&#xA;  access policy and spending, at a minimum, achieve the following two objectives:  &lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;the availability of at least two space launch vehicles (or families of space launch vehicles) capable of delivering into space any payload designated by the Secretary of Defense or the Director of National Intelligence as a national security payload; and,&lt;/li&gt;&#xA;  &lt;li&gt;a robust space launch infrastructure and industrial base.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In other words: if necessary, the government pays to keep the launch provider from going bankrupt. The benefit to the Government is the knowledge they'll be able to launch their satellites when they need to, rather than having to wait in line with commercial customers, or worse, not be able to launch at all because a link in the supply chain controlled by foreign governments is blocked from delivering critical components. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The program failed in one respect: it continued to rely on critical components from foreign companies (i.e. the Russian RD-180) via waivers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assured Access is a common construct in the defence world. Companies that are strategically important are kept afloat instead of being left to fend for themselves on the open market. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-02T08:11:51.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13848" PostTypeId="2" ParentId="7927" CreationDate="2016-02-02T08:31:23.083" Score="2" Body="&lt;p&gt;Wikipedia states \$500 million per launch, with &lt;a href=&quot;http://www.nbcnews.com/id/49019843/ns/technology_and_science-space/#.VrBlqSaLe1F&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; citation. The development budget is &lt;a href=&quot;http://www.spacepolicyonline.com/news/new-nasa-crew-transportation-system-to-cost-18-billion-through-2017&quot; rel=&quot;nofollow&quot;&gt;$18 billion&lt;/a&gt; (\$10 billion for the actual SLS rocket), and that is a more reliable number as it is a budget. However, a development budget is always a lower limit. &lt;a href=&quot;http://www.nbcnews.com/id/44517102/ns/technology_and_science-space/t/nasa-unveils-giant-rocket-design-future-odysseys/#.VrBowCaLe1E&quot; rel=&quot;nofollow&quot;&gt;This site&lt;/a&gt; for instance, provides an total development cost estimate of \$35 billion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.thespacereview.com/article/2330/1&quot; rel=&quot;nofollow&quot;&gt;The space review&lt;/a&gt; says \$1 billion per launch, \$1.5 billion if development costs are included. Their citation is &lt;a href=&quot;http://www.nature.com/nature/journal/v472/n7341/full/472038d.html&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; study&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-02T08:31:23.083" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13849" PostTypeId="1" AcceptedAnswerId="13850" CreationDate="2016-02-02T13:12:00.043" Score="3" ViewCount="1067" Body="&lt;p&gt;Helium-3 is a substance that is very useful for nuclear fusion, which can generate a lot of power. Unfortunately we don't have a lot of it on Earth, but on the Moon it can be processed from basic moon rocks. Say that we figured out a way how to mine large amounts of the stuff in relative safety, but we still need to process the rock into Helium-3 Where would be the best place to do that, on the Moon itself or on Earth? The ratio of moon rocks to Helium-3 is not that spectacular (&lt;a href=&quot;http://www.popularmechanics.com/space/moon-mars/a235/1283056/&quot; rel=&quot;nofollow&quot;&gt;according to Popular Mechanics a patch of three quarters of a square mile to a depth of about 9 feet nets about 220 pounds of Helium-3&lt;/a&gt;), and moving enough moon rocks to Earth for processing is rather costly. Moving the processed Helium-3 back to Earth would require a lot less moving things around in space ships. However, it would require the facilities and knowledge to process this to be present on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So which is the more viable option, to move all the moon rocks back to Earth and process them here, or have the processing done on the Moon and move the Helium-3 back here?&lt;/p&gt;&#xA;" OwnerUserId="11575" LastEditorUserId="11575" LastEditDate="2016-02-02T23:45:17.507" LastActivityDate="2021-12-17T12:19:53.000" Title="Would it be more viable to process moon rocks into Helium-3 on the Moon than doing so on Earth?" Tags="|the-moon|mining|chemistry|helium-3|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13850" PostTypeId="2" ParentId="13849" CreationDate="2016-02-02T13:20:50.140" Score="9" Body="&lt;p&gt;The Moon would be a much better place most likely. As you said, 220 pounds of Helium-3 in a mass of many many tons of rock, makes it so that even a few tons of equipment to be dropped on the Moon would vastly reduce the price to return it home. Lifting 220 pounds from the Moon to return to Earth is relatively easy, all of the Apollo missions did it, and then some. How much mass is in that 3/4ths of a square mile, 9 feet deep? Well, the &lt;a href=&quot;http://curator.jsc.nasa.gov/lunar/letss/regolith.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;density of lunar regolith&lt;/a&gt; is about &lt;span class=&quot;math-container&quot;&gt;$1.5 g/cm^3$&lt;/span&gt; The volume of 3/4 square miles 9 feet deep is &lt;span class=&quot;math-container&quot;&gt;$4.370625*10^{12} cm^2$&lt;/span&gt;, or about 6,500,000 metric tons! I can't even imagine how much rocket power would be required to lift that much!&lt;/p&gt;&#xA;&lt;p&gt;I suspect the machinery to process the rock, and the power, would amount to maybe 100 tons, if not a bit more. Even if I'm off by a factor of 100, it is still &lt;strong&gt;vastly&lt;/strong&gt; cheaper to process the Helium-3 on the moon rather than return the rocks to Earth. According to &lt;a href=&quot;http://io9.gizmodo.com/5908499/could-helium-3-really-solve-earths-energy-problems&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, the main thing involved in the separation is heating the rock to 600 °C, which wouldn't require much in the way of expensive equipment.&lt;/p&gt;&#xA;&lt;p&gt;Next, is there something &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_soil&quot; rel=&quot;nofollow noreferrer&quot;&gt;inherit in the other Moon rocks&lt;/a&gt; that would be valuable? Not really. Most of it is similar to composition on Earth, including somewhat more aluminum and volcanic rocks. Also, the Moon is very dry.&lt;/p&gt;&#xA;&lt;p&gt;The other argument is that the filtered Moon rock could then be used to make structures on the Moon, which could likely make things even easier to continue the processing in the future.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="7062" LastEditDate="2021-12-17T12:18:21.620" LastActivityDate="2021-12-17T12:18:21.620" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="13851" PostTypeId="1" AcceptedAnswerId="13856" CreationDate="2016-02-02T14:28:09.970" Score="4" ViewCount="4652" Body="&lt;p&gt;Basically if it becomes a rock, how long until it reenters and burns up?&lt;/p&gt;&#xA;" OwnerUserId="13285" LastEditorUserId="25" LastEditDate="2016-07-19T17:26:52.250" LastActivityDate="2016-07-19T17:26:52.250" Title="How long can a LEO Satellite maintain its orbit if it loses all power?" Tags="|artificial-satellite|low-earth-orbit|reentry|orbital-decay|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13852" PostTypeId="2" ParentId="13851" CreationDate="2016-02-02T14:49:44.180" Score="5" Body="&lt;p&gt;It depends entirely on the orbit and the &quot;aerodynamic&quot; properties of the satellite.  For example, the ISS is often quoted as descending between 70 to 100 metres &lt;strong&gt;per day&lt;/strong&gt; and needs frequent boosts.  (As geoffc has pointed out, it is an exceptional case due to the large area it covers.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another interesting case was &lt;a href=&quot;http://m.esa.int/Our_Activities/Observing_the_Earth/GOCE/Satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;GOCE&lt;/a&gt;. This earth observation satellite was designed to operate in low orbit, as low as 229km. According to &lt;a href=&quot;http://m.esa.int/Our_Activities/Observing_the_Earth/GOCE/Satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;, its engine ran out of fuel on 21 October 2013, and it re-entered on 11 November 2013. (Between those two, a 155 km perigee was reported around 9 November though the exact timing of this is not necessarily precise.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, there are lots of old things still in orbit - some even operating, see &lt;a href=&quot;https://space.stackexchange.com/questions/1564/what-is-the-oldest-artificial-satellite-still-in-use&quot;&gt;this question&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-02-02T15:16:04.140" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13854" PostTypeId="2" ParentId="10625" CreationDate="2016-02-02T15:46:04.703" Score="1" Body="&lt;p&gt;&lt;em&gt;The Case for Mars&lt;/em&gt; by Robert Zubrin suggests making clay bricks on Mars. Most of the surface of Mars contains materials that would be excellent to produce clay, with such fine grained dust everywhere on the surface. This would be a lot easier than messing with stones and getting them into the right shape. The potential for these bricks is quite high, and they could be used to build even pressurized structures.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-02T15:46:04.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13856" PostTypeId="2" ParentId="13851" CreationDate="2016-02-02T18:40:58.847" Score="8" Body="&lt;p&gt;This really depends on your altitude first, and second your aerodynamic properties, the point in the solar cycle, and the mass of the object. The peak of a solar cycle increases drag on satellites as the upper atmosphere grows during that period of time. LEO varies dramatically, the altitude of the ISS is only stable for at most maybe a year without any kind of boost, while the first US satellite, launched into LEO, is still there after nearly 70 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary determination of lifetime is the altitude of the periapsis, although the apoapsis has an affect as well.  While there are a number of variables, I found &lt;a href=&quot;http://aiaa.kavi.com/apps/group_public/download.php/3172/ISO_27852_%28E%294.pdf%27&quot; rel=&quot;noreferrer&quot;&gt;a paper&lt;/a&gt; discussing in great detail all of this, and it also has the following chart that gives a range of orbital lifetimes based on the orbital altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uPpT0.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uPpT0.png&quot; alt=&quot;Orbital lifetime&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-02-04T13:44:44.693" LastActivityDate="2016-02-04T13:44:44.693" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13857" PostTypeId="2" ParentId="13744" CreationDate="2016-02-02T18:44:16.487" Score="1" Body="&lt;p&gt;This primarily depends on starting distance and relative velocity, but nevertheless, it's so dangerous as to point of total impracticality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Total delta-V of Dragon V2 is hard to find, but &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/33f75a/just_how_much_deltav_will_the_dragon_2_capsule/&quot; rel=&quot;nofollow&quot;&gt;some estimates&lt;/a&gt; put it at about 390m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the capsule accelerates up to half of its delta-V, so to speed of 195m/s. We might try to get the time taken from thrust and mass of the capsule, but &lt;a href=&quot;https://en.wikipedia.org/wiki/SuperDraco&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; conveniently provides us with &quot;Burn time - 25 seconds&quot; for the engines, let's run with it - so acceleration takes 12.5s. At 15.6m/s^2 we've covered 1218m out of initial distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's inert flight at 195m/s until we're 1218m away from the station. The time in seconds is the distance in meters divided by 195&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we brake for 12.5s and just gently bump right into the docking port as the engines go off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course if we made any slightest mistake along the way, we either crash into ISS or float inertly in space with no propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, for a distance $d$ greater than 2436m the time will be $t = 25+(d-2436)/195 [s]$. For a lower distance it will be correspondingly less than 25s, you can find it yourself for 15.6m/s^2 acceleration. And of course any minimal mistake will end with a disaster.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-02T18:44:16.487" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13858" PostTypeId="2" ParentId="13849" CreationDate="2016-02-02T18:51:11.467" Score="11" Body="&lt;p&gt;Two big ifs here. IF we achieved viable commercial fusion power (other than the sun) and IF Helium-3 was an indispensable part of this process. But for the sake of argument, let's say Helium-3 is the fusion fuel of the future.&lt;/p&gt;&#xA;&lt;p&gt;I'll quote &lt;a href=&quot;http://www.transterrestrial.com/?p=41338&quot; rel=&quot;nofollow noreferrer&quot;&gt;John Schilling's comment&lt;/a&gt; from Rand Simberg's Transterrestrial Musings blog.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Helium-3 mining on the moon simply does not pass the arithmetic test.&#xA;The highest 3He concentration ever recorded in lunar regolith is&#xA;fifteen parts per billion, and the process by which it is deposited is&#xA;inherently resistant to geologic concentration. Assuming someone&#xA;manages to invent a 3He fusion reactor that operates at 50% efficiency&#xA;(giggle), that translates to net energy output of 4.5E6 joules per&#xA;kilogram of high-grade regolith. The energy output of a kilogram of&#xA;the lowest grade of coal burned in a good 19th-century reciprocating&#xA;steam engine, is about 4.5E6 joules per kilogram. And that doesn’t&#xA;change if you substitute dried peat for the coal. So, the proposal is&#xA;to set up an enormous mining infrastructure on the Moon, and invent a&#xA;fundamentally new kind of engine backed by fifty years of failed&#xA;promises, for the sake of an energy source roughly as good as burning&#xA;high-grade dirt in a type of engine obsolete for over a century. And&#xA;no, that analysis doesn’t change significantly if we include&#xA;accessible reserves or environmental impact. I understand that you&#xA;want desperately to believe that there are immense riches to be had in&#xA;space, as soon as the suits see the light and come up with the money.&#xA;The good news is, this is probably true. But the list of great riches&#xA;to be had in space, does not include lunar helium-3 (or helium-4, for&#xA;that matter). The numbers do not add up, no matter what the glossy&#xA;magazine articles say, and math trumps faith.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So a huge volume and mass of regolith that contains only minute amounts of Helium-3.&lt;/p&gt;&#xA;&lt;p&gt;To answer your question, It'd be less expensive to extract the Helium-3 on the moon. That would save the transportation expense of delivering mountains of mass to earth's surface.&lt;/p&gt;&#xA;&lt;p&gt;But I don't think Helium-3 will ever be an economic driver for lunar development.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="7062" LastEditDate="2021-12-17T12:19:53.000" LastActivityDate="2021-12-17T12:19:53.000" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13859" PostTypeId="2" ParentId="11932" CreationDate="2016-02-02T19:59:54.553" Score="2" Body="&lt;p&gt;Asking an expert at my work, the answer is that it's not a strong risk for hybrids.  The reasoning is that pogo is typically due to an interaction between a pump and a long vertical run of tubing on a rocket (like where a propellant line goes from the top tank around or through the bottom tank to get to the engine).  Since hybrids only have a single liquid tank there typically are not long vertical runs of piping and most hybrids don't use pumps.&lt;/p&gt;&#xA;" OwnerUserId="13247" LastActivityDate="2016-02-02T19:59:54.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13860" PostTypeId="2" ParentId="2716" CreationDate="2016-02-02T21:30:42.407" Score="3" Body="&lt;p&gt;GOCE entered the atmosphere on November 11, 2013. Its reentry took place over the South Atlantic, someone on the Falklands shot this photo of the reentry:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SyT0D.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SyT0D.jpg&quot; alt=&quot;GOCE as a bright trail across the sky&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;JSOC released this map of the reentry path and the debris field. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7W5D0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7W5D0.jpg&quot; alt=&quot;reentry path starts over the Falklands and continues North, the path ends before it makes landfall in South America. The entire reentry corridor is over the ocean.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as expected, the satellite broke up and any debris that made it all the way down ended up harmlessly splashing into the ocean. &#xA;The &lt;a href=&quot;http://www.spaceflight101.net/goce-re-entry.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101 post-reentry report&lt;/a&gt; lists the decay location as &quot;approximate&quot;, I suspect because there's no space tracking installations in that area, so no direct imaging apart from the flares photographed by amateurs. That also explains why I haven't found more about the amount of debris that actually made it to sea level (rather than the estimates made beforehand). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;GOCE's outer shell was made up of thin panels that would rapidly heat up as GOCE entered the atmosphere. Its aerodynamic shape may have delayed the inevitable a bit (1), but it was going to heat up to structural failure in short order anyway, making its aerodynamic qualities irrelevant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1: remember the Shuttle? Despite rather more refined aerodynamics and active control during reentry, it needed an exquisite heat shield to keep from being fried. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://puma.isti.cnr.it/rmydownload.php?filename=cnr.isti/cnr.isti/2014-A3-010/2014-A3-010.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;pre-launch reentry analysis&lt;/a&gt; (PDF also contains a lot of data taken in the days before reentry):  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A pre-launch destructive analysis was carried out for ESA by Hyperschall Technologie Göttingen (HTG) using the Space-Craft Atmospheric Re-entry and Aerothermal Break-up (SCARAB) software tool.   &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The beginning of the satellite fragmentation was predicted at an altitude of ∼ 95 km and the end at ∼ 35 km, but most of the debris generation was expected between 80 and 45 km.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The most massive fragment would have had a mass just below 95 kg&#xA;  There were no hazardous materials on-board and, according to ESA, the components suspected to survive reentry were a tank and the magnetotorquers. The rest of the falling debris would have been just irregular fragments.   &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Even if not zero, the individual risk for any inhabitant of the Earth was very small: 65,000 times lower than being hit by lighting, or 1.5 million times lower than being killed in a home accident.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Overall, 43 macroscopic fragments, totaling approximately 270 kg, were expected to survive reentry, hitting the ground along a 900 km footprint during a 17 minutes time interval.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A minimum casualty expectancy (Ec ) of ∼ 0.0002 was estimated at ISTI/CNR implying a probability of ∼ 1/5000 of causing 1 casualty over the whole world&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-07T07:28:53.337" LastActivityDate="2016-06-07T07:28:53.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13861" PostTypeId="1" AcceptedAnswerId="13863" CreationDate="2016-02-02T23:51:12.930" Score="3" ViewCount="186" Body="&lt;p&gt;Given recent advances in both engineering achievements - reusable rockets technology, launch frequency, fabrication/3D printing; and the growth in the general public awareness of space exploration due to ISS, Rosetta, SpaceX etc., what are the possibilities of manned missions, and/or ambitions for establishing semi-habitable/permanent stations on the Moon in the next two decades?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will we have to wait for another &quot;Space Race&quot; in order to become multi-planetary? Most effort in this regard is being motivated by highly ambitious private entrepreneurs and businessmen not national space agencies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is lack of involvement from established players here due to a lack of scientific value or fundamental budgetary constraints?&lt;/p&gt;&#xA;" OwnerUserId="13272" LastEditorUserId="8693" LastEditDate="2016-02-03T00:13:43.840" LastActivityDate="2016-02-03T00:54:15.617" Title="What factors hinder establishing a moon-base mission? Enterprise or scientific value?" Tags="|mars|the-moon|cost|multi-launch|" AnswerCount="2" CommentCount="3" ClosedDate="2016-02-03T04:07:22.333" ContentLicense="CC BY-SA 3.0" />
  <row Id="13862" PostTypeId="2" ParentId="13861" CreationDate="2016-02-03T00:38:24.847" Score="1" Body="&lt;p&gt;One limiting factor is the lack of a heavy launcher capable of sending a mission to the Moon (50 tons+). The historical vehicles with such capabilities are not many: &lt;strong&gt;Saturn V&lt;/strong&gt;, &lt;strong&gt;N1&lt;/strong&gt;, and &lt;strong&gt;Energia&lt;/strong&gt;, none of which are still operating. Development of a new huge launcher is going to use a lot of a space agency's budget, without giving them a lot of new capabilities. A heavy launcher is almost single purpose to send humans towards the Moon or Mars, or in the case of Skylab, launching a space station in one piece.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The major factor seems to be a budgetary one, if a single space station in low Earth orbit takes the main part of the budget for not one, but many of the worlds space agencies, how are they going to have enough money for one on the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for scientific value, what are they going to accomplish with a manned moon base? Almost any measurements and surface sampling can be performed by unmanned probes at a cost more than an order of magnitude less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What seems like the main driving force behind the conquering of space is national pride. The USA and Russia have already showed their capabilities, but China has yet to do anything spectacular. Watch out for them in the next ten to fifteen years.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-03T00:38:24.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13863" PostTypeId="2" ParentId="13861" CreationDate="2016-02-03T00:48:07.030" Score="3" Body="&lt;p&gt;(Started writing this before your edits — decided to keep going since it still answers the questions)&lt;/p&gt;&#xA;&lt;p&gt;Here are the major problems as I see them.&lt;/p&gt;&#xA;&lt;h3&gt;Technical&lt;/h3&gt;&#xA;&lt;p&gt;A lunar colony would need a lot of help from Earth before it could be self-sufficient. The bare necessities are a habitat, oxygen, food, and water. It would take a long time to get just the habitat up there — look how long it's been since we started making the ISS, and getting things to Low Earth Orbit is much easier than landing them on the moon. Similarly, we'd need ways to get oxygen, food, and water up there until the colony can become self-sufficient — if it ever gets that far. We're currently working on growing food in low-gravity environments, so that could become sustainable. There is a lot of water ice at the lunar poles, but that would require more help from Earth to get a mining operation going.&lt;/p&gt;&#xA;&lt;p&gt;Long story short, technically it would require advancements in &amp;quot;space survival&amp;quot;, meaning generation/recapturing of food, water, and oxygen in environments not naturally conducive to survival, as well as (probably) advancements in rocketry that would make it cheaper to launch very large machinery and parts to the moon. Which we're working on, but we aren't quite there yet.&lt;/p&gt;&#xA;&lt;h3&gt;Scientific&lt;/h3&gt;&#xA;&lt;p&gt;We've been to the moon. We have rocks from the moon. We've been doing science directly on rocks from the moon for almost fifty years. You know what we haven't been doing science on directly? Rocks from Mars. Rocks from asteroids. Comets. The best we have is remote-control robots on 2/3 of those. From a scientific standpoint, there is more to learn from going to Mars than the moon.&lt;/p&gt;&#xA;&lt;h3&gt;Social&lt;/h3&gt;&#xA;&lt;p&gt;As I said above, we've been to the moon. It's not new anymore. It's not exciting. Andy Weir summed it up pretty well in The Martian (paraphrased because I don't have the book in front of me):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;When the crew of the Ares I returned to Earth, they were greeted with parades all around the planet. When the crew of Ares II did the same thing four years later, they were greeted   with a firm handshake and a hot cup of coffee.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;We like new things more than we like things that have already been done. So, frankly, people aren't as likely to support another moon mission as they are to support a Mars mission. Mars is new. Mars is cool. The moon is sooo 1969. The benefits to society would be there for both, obviously, but it's easier to sell something new and exciting.&lt;/p&gt;&#xA;&lt;h3&gt;So How Do We Deal With It?&lt;/h3&gt;&#xA;&lt;p&gt;Frankly, we already are. The simple fact that SpaceX and such private space companies exist is good news for interplanetary space travel, and the way you worded it in your question makes me think that you might have a major misunderstanding of what's going on. SpaceX is currently doing Low Earth Orbit launches; they're currently able to run resupply missions to the ISS. They've been in the news a lot more than NASA, that's very true. But SpaceX is currently stuck in Low Earth Orbit — if LEO is like running a 5K, getting to the moon is running a marathon, and getting to Mars is an ultra-marathon. SpaceX will not get us to Mars.&lt;/p&gt;&#xA;&lt;p&gt;But the fact that they exist, that they can do resupply missions, means that NASA doesn't have to worry about resupply missions and can instead focus on longer-term plans.&lt;/p&gt;&#xA;&lt;p&gt;NASA doesn't have to focus on resupply missions every few months, so they can devote more time, money, etc to development of new programs that can actually &lt;strong&gt;get&lt;/strong&gt; to Mars or beyond, like Orion. It's just that the general public doesn't really care about the many, many phases of testing that a spacecraft has to go through before it can be declared ready for launch, so they don't get as much coverage.&lt;/p&gt;&#xA;&lt;p&gt;Add in the fact that that lack of coverage tends to lead to a cycle of budget cuts (&amp;quot;What have you done to deserve all this money? Here's a 3% cut!&amp;quot; &amp;quot;Okay, so now we have less money to work with… We're going to have to push testing back to spread over more time to deal with that.&amp;quot; &amp;quot;Well you keep pushing back the delivery date, so this part of the budget looks like a nice place to cut back a bit more….&amp;quot;) and you end up with programs that end up taking a long time to come to fruition. Budgetary constraints are huge for an organization like NASA, but infinite money won't solve problems instantaneously.&lt;/p&gt;&#xA;&lt;p&gt;TLDR, it's a combination of everything that leads to a lunar colony being unlikely any time soon. But we have our eyes on the rest of space, and we are actively working on getting there.&lt;/p&gt;&#xA;" OwnerUserId="12312" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-02-03T00:54:15.617" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13865" PostTypeId="1" AcceptedAnswerId="13869" CreationDate="2016-02-03T02:42:33.707" Score="4" ViewCount="1317" Body="&lt;p&gt;What are the possible methods of in-situ extraction of oxygen from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus&quot; rel=&quot;noreferrer&quot;&gt;atmosphere of Venus&lt;/a&gt;, which is mostly carbon dioxide?&lt;/p&gt;&#xA;" OwnerUserId="13292" LastEditorUserId="49" LastEditDate="2016-02-03T05:59:44.167" LastActivityDate="2016-02-05T15:22:02.697" Title="How to produce oxygen out of Venusian atmosphere's carbon dioxide?" Tags="|planetary-science|atmosphere|venus|isru|oxygen|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13866" PostTypeId="1" CreationDate="2016-02-03T03:14:52.273" Score="-1" ViewCount="207" Body="&lt;p&gt;Suppose one needs to travel to Somalia from Indonesia(which are approximately at a distance of $4000$ miles from each other on the line of equator). To accomplish this task, I suggest the following method:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Design an airplane that can surpass the earth's atmosphere radially(~ $300$ miles thick) and stand still in the air till the landing point of Somalia is just about to arrive radially beneath the plane's stationary point(earth rotates from west to east) at which time instant, the landing process is initiated. In this way, the total time taken would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{equation}&#xA;T=\frac{4000(mi)}{1000(mi/h)}  + \epsilon&#xA;\end{equation}&#xA;Here, $1000(mi/h)$ is speed of any point on earth's equator and $\epsilon$ is the small time the aircraft took to travel to and fro through the earth's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total approximated journey time with this method would not exceed $6(hr)$ which is considerably smaller than the actual time taken of about $12(hr)$. My apprehension is why this technique is not being used to facilitate travelling between two different points when the absolute speed of the aircraft(w.r.t space) can be reduced to zero by surpassing the earth's atmosphere?  &lt;/p&gt;&#xA;" OwnerUserId="13293" LastActivityDate="2016-02-03T03:50:42.037" Title="Faster alternative to travel to any location in the west" Tags="|atmosphere|earth|rotation|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13867" PostTypeId="2" ParentId="13866" CreationDate="2016-02-03T03:49:14.030" Score="5" Body="&lt;p&gt;Summary: It's not a great idea. It will take you over 8 hours, and&#xA;you'll end up 5400+ miles above your destination. Details follow&#xA;image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/A98iL.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/A98iL.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;Since you don't specify the speed of the plane, let's assume for the&#xA;moment that you can move 300 miles up from your current location&#xA;instantaneously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To avoid the air/wind problem, let's also assume we are doing this on&#xA;an airless planet that is otherwise similar to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, since you can &quot;stand still in the air&quot;, we'll ignore the&#xA;effect of gravity as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since your starting point is rotating, its (x,y) position in the diagram&#xA;above is modeled as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$&#xA;  \left\{\text{eer} \sin \left(\frac{2 \pi  t}{\text{sidday}}\right),\text{eer}&#xA;    \cos \left(\frac{2 \pi  t}{\text{sidday}}\right)\right\}&#xA;$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where &lt;code&gt;t&lt;/code&gt; is the time in seconds since the &quot;launch&quot;, &lt;code&gt;eer = 6378.137&lt;/code&gt;&#xA;is the Earth's equitorial radius in kilometers and &lt;code&gt;sidday = 86164.1&lt;/code&gt;&#xA;is the length of the sidereal day in seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And your destination is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$&#xA;   \left\{-\text{eer} \sin \left(\frac{\text{dist}}{\text{eer}}-\frac{2 \pi &#xA;    t}{\text{sidday}}\right),\text{eer} \cos&#xA;    \left(\frac{\text{dist}}{\text{eer}}-\frac{2 \pi &#xA;    t}{\text{sidday}}\right)\right\}&#xA;$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where &lt;code&gt;dist = 4000*1.609344&lt;/code&gt; is the distance to your destination in kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since your launchpoint has an initial velocity of about 1000mph, so do&#xA;you. This means your position at time t is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\left\{\frac{2 \pi  \text{eer} t}{\text{sidday}},\text{eer}+\text{hi}\right\}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where &lt;code&gt;hi= 300*1.609344&lt;/code&gt; is your initial height in kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With these conditions, you will be over your destination about 28911&#xA;seconds after you start (8 hours, 1 minute and 51 seconds), at a&#xA;height of about 8718 kilometers (about 5417 miles).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your average surface velocity would be right around 500 miles per&#xA;hour, slower than a supersonic airplane, and even slower if you&#xA;include the time to ascend 300 miles at the start and descend 5417&#xA;miles at the end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, if your magic plane can move 300 miles in time $\epsilon$,&#xA;you could just as easily apply a westward thrust of ~1000mph to cancel&#xA;our your initial eastward velocity, in which case your calculations&#xA;would be correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A plane as powerful as this, however, could probably travel anywhere&#xA;in the world rapidly, without help from the Earth's rotation, so you'd&#xA;be best off aiming it west, just as you would a normal airplane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I toyed with the idea that you could launch at a high velocity and&#xA;land back on Earth at your destination purely due to gravity (albeit&#xA;at a very high speed) as per:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/13815&quot;&gt;Trajectory of projectile launched from planet&amp;#39;s surface&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but haven't been able to get the numbers to work. My work on the&#xA;high-velocity launch idea is at:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://github.com/barrycarter/bcapps/blob/master/MATHEMATICA/bc-solve-physics-232844.m&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://github.com/barrycarter/bcapps/blob/master/MATHEMATICA/bc-solve-physics-232844.m&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user7073" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-02-03T03:49:14.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13868" PostTypeId="2" ParentId="13866" CreationDate="2016-02-03T03:50:42.037" Score="3" Body="&lt;p&gt;The first problem is just that &lt;strong&gt;there is no free lunch&lt;/strong&gt; in decelerating/accelerating; in particular, there's nothing in space to slow you down, nothing that is &quot;unmoving&quot; in any real sense to push against, so if you go straight up, you're still moving along with the earth's rotation at the same speed. (Not the same angular velocity, but the same mph.) So just going up and then coming down doesn't buy you all that much: you cannot &quot;reduce absolute speed to zero&quot; that way without &lt;em&gt;actually&lt;/em&gt; reducing speed in exactly the same way you normally do (by burning fuel).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By going outward, you can reduce your angular velocity somewhat, meaning that you'd no longer quite keep up with the number of (micro)radians per second that the earth is traversing. Unfortunately, the earth's radius is a good few thousand miles, and going a measly few hundred out will do very little that way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second problem is the challenge of hovering for that long; you're not going nearly fast enough to use orbital mechanics to fall around the earth continuously for free, so instead you have to burn fuel, and a lot of it, to keep up a continuous 1g vertical acceleration just to keep from falling. You can save a bit by simply accelerating in a higher arc and then letting yourself drop down. That way, gravity is reduced a little at the top. But it takes quite a distance before that saves you much, and if you go too far, you'll take too long to fall back down and you'll land in the wrong place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third problem, although far smaller, is that to the comparatively simple challenge of keeping airliners pressurized you have now added the thornier problem of maintaining atmosphere in space for hours. Worse, you also have to manage temperature. That's harder than it sounds in space, where there's glaring sunlight and no air to carry away heat. (With hundreds of warm bodies in a small enclosure, freezing would not be a problem.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, you have to use rockets for a lot of this. Rockets are less efficient than jet engines by a large margin, because they have to carry their air with them, and that requires extra fuel and extra oxygen, which requires &lt;em&gt;more&lt;/em&gt; fuel and oxygen, and so forth. When you're also doing far more acceleration than a normal airliner, the result is burning many times as much fuel, which also requires a much larger (and more expensive) vehicle to carry all that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oh yeah, and you're probably going to be breaking the speed of sound at various stages during your flight. This is already a practical problem for airliners; Concorde was retired largely because of problems getting clearances where people wouldn't be bothered too much by sonic booms, and the fuel flow at supersonic speeds is ... you guessed it, much higher than subsonic, where airliners normally are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In sum, fuel costs are going to be perhaps hundreds of times as much, the vehicle will cost far more to design and build, you'll have to train pilots in new and exciting ways to handle problems... and you won't actually save all that much time, if only because you'll have to fly complicated paths around anywhere that's populated.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2016-02-03T03:50:42.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13869" PostTypeId="2" ParentId="13865" CreationDate="2016-02-03T03:57:55.777" Score="5" Body="&lt;p&gt;Some of your options are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Electrolysis of the atmosphere (see e.g. &lt;a href=&quot;https://space.stackexchange.com/a/5096/49&quot;&gt;how MOXIE will do it on Mars&lt;/a&gt;), for which you will need a source of electricity, a catalyst (e.g. zirconia), and which produces carbon monoxide and oxygen:&#xA;$$\require{mhchem}\ce{2 CO2} + \text{energy} → \ce{2 CO + O2}$$&#xA;Carbon monoxide can be further reduced into elemental carbon and oxygen by the same process, but that requires more input energy to break the &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbon%E2%80%93oxygen_bond&quot; rel=&quot;nofollow noreferrer&quot;&gt;carbon-oxygen trivalent bond&lt;/a&gt;:&#xA;$$\ce{2 CO + energy → 2C + O2}$$&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Photosynthesis&quot; rel=&quot;nofollow noreferrer&quot;&gt;Photosynthesis&lt;/a&gt;, for which you will need water, light and photosynthesizing plant life which might require additional nutrients, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Artificial_photosynthesis&quot; rel=&quot;nofollow noreferrer&quot;&gt;artificial photosynthesis&lt;/a&gt; technology:&#xA;$$\ce{CO2 + 2 H2O + photons → CH2O + O2}$$&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Bosch_reaction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bosch reaction&lt;/a&gt; followed by &lt;a href=&quot;https://space.stackexchange.com/a/12711/49&quot;&gt;electrolysis of water&lt;/a&gt;, which requires input hydrogen, likely some catalyst (e.g. iron), and high reaction temperature (450 to 600 °C), producing molecular oxygen, elemental carbon, and molecular hydrogen that you can recycle back into your oxygen production:&#xA;$$\ce{CO2 + 2 H2 → C + 2 H2O}$$&#xA;$$\ce{2 H2O + energy → O2 + 2 H2}$$&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If you really have to do this on Venus, be prepared to deal with sulfuric acid ($\ce{H2SO4}$) and hydrogen sulfide ($\ce{H2S}$), because they're pretty much the only viable sources of hydrogen. You will likely need also other materials to sustain your endeavors on Venus, so see some suggestions which ones to look for and where e.g. &lt;a href=&quot;https://space.stackexchange.com/a/9159/49&quot;&gt;in this answer&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-02-03T04:53:16.207" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13872" PostTypeId="1" AcceptedAnswerId="13873" CreationDate="2016-02-03T04:59:00.910" Score="6" ViewCount="2310" Body="&lt;p&gt;From what I understand, an object has to keep moving in an orbit to not come crashing down to Earth. (The ISS is the first thing I think of for this.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How then, do satellites which are purpose built for a particular region (satellite internet for example) stay in a fixed place?&lt;/p&gt;&#xA;" OwnerUserId="10712" LastEditorUserId="238" LastEditDate="2016-02-03T05:14:03.013" LastActivityDate="2016-02-03T05:35:05.543" Title="How do communication satellites remain positioned above a particular region?" Tags="|orbit|geostationary|communication-satellite|geosynchronous|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13873" PostTypeId="2" ParentId="13872" CreationDate="2016-02-03T05:16:31.770" Score="17" Body="&lt;p&gt;Such satellites are in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;geosynchronous orbit&lt;/a&gt; (GSO), orbiting at an orbital altitude where &lt;a href=&quot;https://www.desmos.com/calculator/owynnyfb1s&quot; rel=&quot;noreferrer&quot;&gt;orbital period matches Earth's rotation&lt;/a&gt; on its axis. Their orbital speed is roughly 3 km/s at mean orbital altitude of 35,786 km above the Earth's surface:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/M7d1A.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_speed&quot; rel=&quot;noreferrer&quot;&gt;Orbital speeds&lt;/a&gt; ($v_o \approx \sqrt{\mu/r}$) at mean altitudes above the Earth's surface (blue) and required speed to keep up with Earth's rotation on its axis, aka the geosynchronous speed (green). Both speeds match at ~ 3,074 m/s at an altitude of ~ 35,783 km above the Earth's surface. Graph is first order approximation, real values are ever so slightly different.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most commonly, &lt;a href=&quot;https://en.wikipedia.org/wiki/Communications_satellite&quot; rel=&quot;noreferrer&quot;&gt;communications satellites&lt;/a&gt; are placed in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;noreferrer&quot;&gt;geostationary orbit&lt;/a&gt; (GEO), which is a circular equatorial GSO, so the satellites stay roughly above the same longitude over the Earth's equator, while inclined circular GSO orbits produce a &lt;em&gt;figure eight&lt;/em&gt; ground track, and highly eccentric inclined GSO like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Tundra_orbit&quot; rel=&quot;noreferrer&quot;&gt;Tundra orbits&lt;/a&gt; produce an elongated &lt;em&gt;figure eight&lt;/em&gt;. Note that some communications satellites might also operate in lower, non-geosynchronous orbits and depend on multiple satellites in &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_constellation&quot; rel=&quot;noreferrer&quot;&gt;constellations&lt;/a&gt; to assure constant coverage over specific regions on the Earth's surface, or globally.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-03T05:35:05.543" LastActivityDate="2016-02-03T05:35:05.543" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13874" PostTypeId="2" ParentId="13872" CreationDate="2016-02-03T05:21:11.250" Score="5" Body="&lt;p&gt;It's important to pay close attention to your language when talking about matters involving reference frames. Communications satellites do not remain in a &quot;fixed place&quot; in the absolute sense, but they do &quot;remain positioned above a particular region.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the Earth is rotating, if a satellite is in an orbit whose period is equal to one sidereal day, its movement will match the rotation of the Earth and it will remain positioned over the same location day in and day out. The orbit where this equivalency occurs is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot;&gt;Geostationary orbit&lt;/a&gt; (GEO) and is frequently the destination of communications and weather satellites.&lt;/p&gt;&#xA;" OwnerUserId="12877" LastEditorUserId="12877" LastEditDate="2016-02-03T05:29:52.907" LastActivityDate="2016-02-03T05:29:52.907" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13875" PostTypeId="1" CreationDate="2016-02-03T09:24:34.917" Score="2" ViewCount="2166" Body="&lt;p&gt;A satellite is in orbit at an altitude of 62.5km, with a speed along the earth-relative position vector of 3.98km/s, and a speed perpendicular to the earth-relative position vector of 7.763km/s. &#xA;What is the eccentricity of its orbit?&lt;/p&gt;&#xA;" OwnerUserId="13296" LastActivityDate="2016-09-19T15:56:35.923" Title="Determine the eccentricity of the orbit" Tags="|orbital-mechanics|orbit|trajectory|astronomy|astrodynamics|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13876" PostTypeId="1" AcceptedAnswerId="13980" CreationDate="2016-02-03T14:46:52.957" Score="8" ViewCount="17014" Body="&lt;p&gt;I have to size the electric power system for a satellite on a circular, sun-synchronous orbit at 800 km altitude. In the problem I have been given the LTAN (Local Time of the Ascending Node) also, which is 11:00 am.&#xA;How do I use the LTAN information? I think I should use it to compute the relative position between satellite and sun in order to check for eclipses during the orbit (which in turn will allow me to size the batteries), but I do not know how to use it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I forgot to say that I have to perform all the calculations by hand&lt;/p&gt;&#xA;" OwnerUserId="6608" LastEditorUserId="6608" LastEditDate="2016-02-08T12:55:23.643" LastActivityDate="2019-09-24T21:09:52.283" Title="Sun synchronous orbit and LTAN" Tags="|orbital-mechanics|sun-synchronous|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13877" PostTypeId="2" ParentId="13875" CreationDate="2016-02-03T16:22:31.713" Score="5" Body="&lt;p&gt;You should write everything in vector form and then it is just a matter of applying a couple of formulas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Use the given position and velocity values to write the position and velocity vectors, $\vec{r}$ and $\vec{v}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Compute $\vec{h} = \vec{r} \times \vec{v}$ (where $\times$ is the cross product)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Compute the eccentricity $\vec{e} = \dfrac{1}{\mu}(\vec{v} \times \vec{h})-\dfrac{\vec{r}}{|\vec{r}|}$&lt;/p&gt;&#xA;" OwnerUserId="6608" LastActivityDate="2016-02-03T16:22:31.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13878" PostTypeId="1" AcceptedAnswerId="13892" CreationDate="2016-02-03T18:36:35.953" Score="10" ViewCount="1340" Body="&lt;p&gt;Counting down the seconds leading up to a rocket launch is iconic - certainly it seems that every child knows that after &quot;5, 4, 3, 2, 1&quot; comes &quot;Blast off!&quot; However, the countdown can't always have been associated with rocket launches. My impression is that rockets originated as weapons, and I have a hard time imaging the countdown in the midst of a heavy battle. Therefore, when did rocket launches first start using a countdown, and why did it become so well-known?&lt;/p&gt;&#xA;" OwnerUserId="13179" LastEditorUserId="12102" LastEditDate="2021-09-13T00:00:27.627" LastActivityDate="2021-09-13T00:00:27.627" Title="What's the first recorded use of a countdown associated with a rocket launch?" Tags="|rockets|history|launch-sequence|spaceflight-firsts|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13879" PostTypeId="2" ParentId="13878" CreationDate="2016-02-03T19:24:18.793" Score="3" Body="&lt;p&gt;NASA has always had someone announce milestones in the countdown, here's &lt;a href=&quot;https://www.youtube.com/watch?v=Vm9ez-6Njgo&quot; rel=&quot;nofollow&quot;&gt;Mercury 3&lt;/a&gt; (first manned flight with Alan Shepard, around 0:30) for example.&#xA;And &lt;a href=&quot;https://www.youtube.com/watch?v=NfMIrKkzRx8&quot; rel=&quot;nofollow&quot;&gt;Explorer 1&lt;/a&gt; (around 2:26), the first US satellite.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure about the Russian space program, but the Vostok 1 launch doesn't seem to have an audible 10-0 countdown.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before that, most of the V-2 launch footage I've seen was silent or overdubbed with later commentary, so no idea if the Germans used a countdown. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-03T19:24:18.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13880" PostTypeId="1" AcceptedAnswerId="13886" CreationDate="2016-02-03T19:26:45.100" Score="13" ViewCount="2490" Body="&lt;p&gt;One of more successful tactics in Kerbal Space Program in order to get big missions with a lot of delta-V going is to launch the big interplanetary craft (with landers, rovers, service modules and everything needed on the mission) on top of a relatively weak disposable first stage(s). The craft reaches the orbit almost completely depleting its fuel supply, and then consecutive launches deliver fuel, refueling the craft in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course real life is not KSP, but on top of prohibitive cost of such a mission in general (even if launched &quot;traditionally&quot; it would be vastly more expensive) what are the factors that would make this approach impractical?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(which I assume it is, since smarter minds than us decided not to go with it.)&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="12102" LastEditDate="2017-08-15T16:22:57.520" LastActivityDate="2017-08-15T16:22:57.520" Title="Viability of orbital refueling" Tags="|launch|interplanetary|refueling|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13881" PostTypeId="2" ParentId="13880" CreationDate="2016-02-03T19:55:57.357" Score="19" Body="&lt;p&gt;This is basically the approach being considered for getting all of the hardware to Mars for a crewed mission.  However instead of refueling an upper stage that was used to get you to orbit, it is considered much easier with almost the same benefit to launch an entire fueled stage with engines instead of just the tanks with propellant. The only penalty is that you have the mass and cost of some engines attached to the bottom of the tanks that you were going to transfer propellant from, which is a relatively small fraction of the mass. The benefit is that a) you don't have to transfer propellant between tanks -- you just dock with the stage, and b) you can now design the stage for its interplanetary mission instead of having to modify an upper stage for two different missions (that and getting to orbit).  For example you may want to keep cryogens from boiling off in a long cruise, which is never a concern for a launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, there is research in both preventing cryogen boil off and in the transfer of propellants in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The International Space Station is regularly refueled in orbit.  Though it's not so that it can depart Earth orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-02-04T00:04:03.387" LastActivityDate="2016-02-04T00:04:03.387" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13883" PostTypeId="1" CreationDate="2016-02-03T20:40:26.900" Score="4" ViewCount="564" Body="&lt;p&gt;ISS is actionally falling towards the earth and therefor they don't feel gravity. But ISS is falling in a direction towards the earth. So the astronauts living in ISS probably live in ISS in a manner which would be directed how ISS is organised. All the instruments are probably easier to work with when you are up (at the 'ceiling') or down. Perhaps also the beds have a preferred site, but I don't know for sure. So is there is preferrenced site or does is it really not  mater?&lt;/p&gt;&#xA;" OwnerUserId="13303" OwnerDisplayName="Marijn " LastActivityDate="2016-02-04T05:36:44.943" Title="Is there really no difference between up and down in ISS?" Tags="|gravity|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13884" PostTypeId="2" ParentId="13883" CreationDate="2016-02-03T21:42:58.273" Score="3" Body="&lt;p&gt;I can think of one place where you would prefer to have your head in a specific direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nadir (Earth facing, or &quot;down&quot;) &lt;a href=&quot;https://en.wikipedia.org/wiki/Cupola_%28ISS_module%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cupola module&lt;/a&gt; is a popular place among the cosmonauts, where you can look down at the Earth. Here, you want to stand in such a way that your head is able to watch out of the windows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jZ5AO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jZ5AO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beds do not have a preferred direction, in fact, the sleeping compartments are stacked in a square around the node-2 module in the American segment.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-03T21:42:58.273" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13885" PostTypeId="2" ParentId="13555" CreationDate="2016-02-03T22:01:46.590" Score="1" Body="&lt;p&gt;Hop back to the launch site? What????&#xA;They either land it on the barge or at the launch site.&#xA;If they land it on the barge they just sail the barge back!!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing is hard enough without them having to do it TWICE per flight. The barge is of course movable so flying the rocket back would be ridiculous!&lt;/p&gt;&#xA;" OwnerUserId="13305" LastActivityDate="2016-02-03T22:01:46.590" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13886" PostTypeId="2" ParentId="13880" CreationDate="2016-02-04T01:11:58.783" Score="7" Body="&lt;p&gt;This is a short answer but in KSP pumping fuel is totally costless, whereas in reality pumping fuel between stages is difficult even in the case where the stages are mated together on the ground. This is especially true when the propellants are cryogenic and therefore more akin to a thick slush than easily-pumped liquid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, I have found a NASA study which seems to validate the possibility of (some degree of) orbital re-fueling:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120009208.pdf&quot;&gt;&quot;Cryogenic Propellant Storage and Transfer Technology Demonstration for Long Duration In-Space Missions&quot;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13306" LastActivityDate="2016-02-04T01:11:58.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13887" PostTypeId="2" ParentId="13883" CreationDate="2016-02-04T01:57:31.467" Score="4" Body="&lt;p&gt;ISS does have a sort of 'up and down', deliberately created as part of the design. All the modules apparently have a consistent 'up' orientation in one direction. The majority of lights are in the 'ceiling' and are set up to shine 'downward'. Furthermore, all the writing on the walls is pointed in the same direction, so that it is 'right side up' in relation to the 'floor' and 'ceiling'.&lt;/p&gt;&#xA;" OwnerUserId="13067" LastEditorUserId="8996" LastEditDate="2016-02-04T05:36:44.943" LastActivityDate="2016-02-04T05:36:44.943" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13888" PostTypeId="2" ParentId="4926" CreationDate="2016-02-04T02:09:39.837" Score="2" Body="&lt;p&gt;So you've done all the work to pick a trajectory type, picked the optimum point to hit on the arrival, decided on a 60 day launch window,... but then you still have to tell your launch vehicle or upper stage where you prefer to be delivered at any point in that entire period. More solutions needed! Almost 30 years ago the process for that step for use with the Inertial Upper Stage (IUS) for Magellan, Galileo and Ulysses missions involved JPL creating dozens of time limited overlapping sets of desired energy and outgoing asymptote specifications in polynomial form that were loaded as data into the IUS guidance system as targets. At this point you still can't go on vacation. JPL also constructed figure-of-merit matrices that were useful to estimate the cost to the spacecraft from injection errors (basically estimating the delta-V of a maneuver 10 days after launch). These were used to make decisions in real time during the upper stage mission. It probably took them a couple of months just to do all this mission design. (Actually, I assume they did it twice. Once for development use and once as close to launch as possible... finishing at something like L-6 months.) &lt;/p&gt;&#xA;" OwnerUserId="12562" LastActivityDate="2016-02-04T02:09:39.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13889" PostTypeId="2" ParentId="13363" CreationDate="2016-02-04T02:27:47.087" Score="2" Body="&lt;p&gt;If a controls engineer was designing a rocket guidance algorithm, he would load a desired trajectory into the algorithm and try to follow it. :) There is a place for that type of algorithm, but ascent rockets don't typically face enough complicated optimization or &quot;path constraints&quot; to need that. They can be both less and more adaptive to events during the mission. Early on while in the atmosphere, they can (e.g.: Saturn V, STS, any number of other systems I'm not familiar with) use fixed tables of attitude vs time, altitude, or velocity. These are pre-calculated on the ground, sometimes using wind estimates only hours old, to fly the rocket through the maximum dynamic pressure zone at very low angles of attack. Once the atmosphere is mostly gone (usually around the time of first stage separation), they can use a closed loop scheme that simulates the rest of the flight, sees where it is expected to end up, and try to make &lt;em&gt;that&lt;/em&gt; state match the desired end state. There is a subtle difference here: they aren't trying to follow a particular reference trajectory, but rather find the best trajectory (or at least a feasible one) to get to where they are going at the end. This is like the difference between driving on the shoulder when the main line of the freeway is bogged down and choosing an alternate route entirely if that is better. &lt;/p&gt;&#xA;" OwnerUserId="12562" LastActivityDate="2016-02-04T02:27:47.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13890" PostTypeId="1" AcceptedAnswerId="13895" CreationDate="2016-02-04T04:33:27.887" Score="0" ViewCount="214" Body="&lt;p&gt;If one wants to move an object from orbit around one planet in the solar system and have it collide with another, what information is needed, and how would one go about calculating such a thing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, lets say:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;I can build stuff around Jupiter, and money and logistics are no&#xA;object. Whatever piece of physically possible technology I want, I&#xA;can have there. &lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;For experimental purposes, I want to crash Hydrogen filled bubbles of iron at Venus, so that they fall through the atmosphere as&#xA;meteors.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;My first thought is to use a huge mass driver to fire the bubbles through space so that most of them crash into Venus,like a giant&#xA;electromagnetic shotgun firing across the solar system.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Even though I have all these resources, I somehow need the help of StackExchange to learn what exactly I will need to accomplish my goal. So what do I need to do to aim and fire the projectiles?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How do I calculate a trajectory from one planet to another, taking into account motion of the planets etc? Is there an app for that?&lt;/li&gt;&#xA;&lt;li&gt;What is the relationship between precision of aim, mass and speed of the projectile, and vulnerability to deflection by solar wind or gravitational pull of passing objects?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In other words, how do I figure out how fast I need to shoot my projectiles, and what do I need to know/do to aim them so that I have a reasonable chance of hitting my target at that distance, assuming no further adjustments to the trajectory once fired?&lt;/p&gt;&#xA;" OwnerUserId="10979" LastActivityDate="2016-02-04T12:35:21.600" Title="Help me build a space shotgun so I can shoot Venus from Jupiter" Tags="|trajectory|physics|uncrewed-spaceflight|ballistics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13891" PostTypeId="2" ParentId="13880" CreationDate="2016-02-04T05:13:06.293" Score="-2" Body="&lt;p&gt;If you use a VASIMR you wouldn't have to refuel because the amount you could possibly carry(weight to thrust) is much much much greater than the amount you'd  burn on the launch and trip.  Using one you could make the trip to Mars in 39 Days instead of six months or more with a conventional rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometime this year NASA will install a VASIMR on the ISS for testing.&#xA;The ISS boosts are currently provided by spacecraft with conventional thrusters, which consume about 7.5 tonnes of propellant per year. A VASIMR would cut this amount down to 0.3 tonnes.&lt;/p&gt;&#xA;" OwnerUserId="13308" LastActivityDate="2016-02-04T05:13:06.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13892" PostTypeId="2" ParentId="13878" CreationDate="2016-02-04T06:31:07.110" Score="7" Body="&lt;p&gt;According to &lt;em&gt;Rockets, Missiles, and Space Travel&lt;/em&gt; by Willy Ley, the Fritz Lang movie &lt;em&gt;Girl in the Moon&lt;/em&gt; (1926) featured a fictional rocket launch countdown for dramatic purposes and it was adopted by the experimenters in the German Rocket society for thier launches.  Werner Von Braun and others became engineers in the German military rocket program and then in the US military and civilian rocket programs and kept on using countdowns.&lt;/p&gt;&#xA;" OwnerUserId="10618" LastActivityDate="2016-02-04T06:31:07.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13893" PostTypeId="1" AcceptedAnswerId="13897" CreationDate="2016-02-04T08:15:36.600" Score="34" ViewCount="6112" Body="&lt;p&gt;How is the matter of fresh clothes managed on ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a local washing machine or are fresh clothes delivered and dirty ones burned with trash on delivery craft reentry? How long do astronauts wear one change of clothes?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-09-24T18:34:18.900" Title="Laundry on the ISS" Tags="|iss|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13894" PostTypeId="2" ParentId="13878" CreationDate="2016-02-04T08:41:44.943" Score="1" Body="&lt;p&gt;This V2 launch footage has audio and is not overdubbed, there is no audible countdown. The sound of birds chirping adds a surreal touch.&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/94T8VxOOvdI?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="10979" LastActivityDate="2016-02-04T08:41:44.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13895" PostTypeId="2" ParentId="13890" CreationDate="2016-02-04T08:45:55.817" Score="3" Body="&lt;p&gt;You'd better install some &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_control_system&quot; rel=&quot;nofollow&quot;&gt;RCS&lt;/a&gt; on your projectiles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is you need quite a bit of energy, accelerating your projectiles to something of order of 20-50km/s depending on alignment. And if your calculation program tells you you need to launch at 25746.32m/s it better not be 25746.28m/s or you'll miss. And keep the angle accurate to about a second (1/3600th of a degree) to keep the direction in check.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planets are really tiny compared to the vastness of space. Just look at Venus or Mars on the night sky and imagine shooting that with a gun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless you can correct in flight. Very weak engines can go a long way correcting all inaccuracies of the launch system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lambert's problem is good for two-body solutions. For case where you need to pass Mars, Earth, Jupiter's moons, and other bodies, you need a numerical solution. An example of a program to provide it is &lt;a href=&quot;http://gmatcentral.org/display/GW/GMAT+Wiki+Home&quot; rel=&quot;nofollow&quot;&gt;GMAT&lt;/a&gt;. You will also need &lt;a href=&quot;http://www.mathworks.com/products/matlab/?requestedDomain=www.mathworks.com&quot; rel=&quot;nofollow&quot;&gt;Matlab&lt;/a&gt; to perform the actual optimization on top of mission planning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravitational influences of so many moving bodies along the way are too complex to just obtain a neat equation for the trajectory, so instead it's done by simulating flight of the spacecraft and adjusting parameters by a small amount for desired result. (and that of course requires quite a bit of computational power).&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-02-04T12:35:21.600" LastActivityDate="2016-02-04T12:35:21.600" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13896" PostTypeId="2" ParentId="13663" CreationDate="2016-02-04T09:18:33.213" Score="5" Body="&lt;p&gt;The main factor that would limited unmanned lifetime of the ISS is a  breakdown of a critical system. The thermal control system comes to mind: without cooling, you'll get a much wider range of temperatures which can lead to other failures. This is one of the more problematic systems on board as well, requiring regular repairs.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-04T09:18:33.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13897" PostTypeId="2" ParentId="13893" CreationDate="2016-02-04T11:34:06.427" Score="39" Body="&lt;p&gt;&lt;em&gt;Summary:&lt;/em&gt; &lt;strong&gt;They do not wash their clothes&lt;/strong&gt; since there is not enough water to do that. They use the same clothes with which they came to the ISS. They get their new clothes with the resupply vessels, and their old clothes are burned.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/vision/space/livinginspace/Astronaut_Laundry.html&quot;&gt;This article explains&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... clothes don't get dirty as quickly on the Space Station as they do on Earth. Astronauts on the Station are living in a controlled environment, so the temperature stays at a constant, comfortable level. And when everything around you is virtually weightless, you don't have to exert yourself physically the same way you do in the gravity on Earth's surface. However, astronauts do have to spend a substantial amount of time each day exercising so that their bodies don't atrophy in microgravity, so they do still get a workout. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;ISS Expedition Six Science Officer Don Pettit wrote that he changes his underwear once every 3 or 4 days.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In an interview in February, Pettit said that he was still wearing the same pair of shorts he had been wearing since he first arrived on the Station - in November! Even though they have more shorts to change into. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They get their clothes with the resupply vessel, and the dirty clothes are burned together with the garbage:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Russian Space Agency’s Progress resupply vessels are non-reusable, meaning they only conduct one-way trips to the ISS and then burn up upon reentry into Earth’s atmosphere. Before they depart the station, they're loaded with trash—including astronauts' clothes. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To clean or treat the clothes, several options have been thought of:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Cleaning with a plastic bag:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Expedition Six Commander Ken Bowersox also has a favorite pair he chooses to wear frequently. Even though there's no laundry facility on the Station, Bowersox even figured out a way to wash his shorts using a plastic bag.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;He washes the clothes in a bag with a bit of water and soap. See &lt;a href=&quot;http://spaceflight.nasa.gov/gallery/video/station/expedition6/qtlow/bowersoxshorts.mov&quot;&gt;this video&lt;/a&gt; (last video &lt;a href=&quot;http://spaceflight.nasa.gov/gallery/video/station/expedition6/ndxpage4.html&quot;&gt;of this page&lt;/a&gt;).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;Grow some tomato and basil plants, Don Pettit once folded an old pair of underwear into a sphere to use as a planter. The idea was remarkably resourceful, considering the lack of soil aboard the ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;blockquote&gt;&#xA;  &lt;p&gt;Russian scientists have also tried using special kinds of bacteria to literally digest cotton and paper underwear. Theoretically, the methane produced by the related biochemical reactions could also be harnessed to power spacecraft. But the offbeat idea was never actually put into use, since it could take up to a decade to determine the right combination of bacteria for the job.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1084.html&quot;&gt;NASA's Intravehicular Activity Clothing Study&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Intravehicular Activity Clothing Study (IVA Clothing Study) dresses crew members in commercially available lightweight clothes that have been designed to resist odors.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/vision/space/livinginspace/Astronaut_Laundry.html&quot;&gt;Astronauts' Dirty Laundry&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://laundry.reviewed.com/features/how-do-astronauts-clean-their-dirty-clothes&quot;&gt;How Do Astronauts Clean Their Dirty Clothes?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.space.com/22337-space-station-dirty-laundry-is-it-washed-video.html&quot;&gt;Space Station Dirty Laundry - Is It Washed? (video)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=1JfqdBJDNZc&quot;&gt;How do you wash your clothes in space? (video)&lt;/a&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11962" LastEditorUserId="11962" LastEditDate="2016-02-04T17:11:26.167" LastActivityDate="2016-02-04T17:11:26.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13898" PostTypeId="2" ParentId="13363" CreationDate="2016-02-04T11:54:31.080" Score="1" Body="&lt;p&gt;Folowing the sequence of comments to date, I think the universe of possible answers comes down to just two:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Dead-reckoning&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Measurement&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And that's it. I don't think I need to exand on the first, and you can make it as complicated as you like. The latter can be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Measurement from the vehicle&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;inertial (accelerometer or gyroscope, these days called IMU)&lt;/li&gt;&#xA;&lt;li&gt;external (star-tracker, ground based radio navigation, GPS)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;(For completeness) Measurement from the ground and telecommand&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;optical, radar, radio tracking&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Hopefully this might provoke some of the other knowledgeable folks here to provide some specific examples of different approaches for &quot;rockets&quot; which can include missiles and satellite launch vehicles. Some admittedly old-world examples of measurement include &lt;a href=&quot;http://fas.org/nuke/guide/uk/slbm/chevaline.html&quot; rel=&quot;nofollow&quot;&gt;Polaris&lt;/a&gt; (inertial) and &lt;a href=&quot;https://en.wikipedia.org/wiki/Trident_%28missile%29&quot; rel=&quot;nofollow&quot;&gt;Trident&lt;/a&gt; (inertial and star-tracker). There is an interesting look at &lt;a href=&quot;http://scpnt.stanford.edu/pnt/PNT08/Presentations/2_Czopek_PNT_2008.pdf&quot; rel=&quot;nofollow&quot;&gt;Pre GPS History of Satellite Navigation&lt;/a&gt; which touches upon missile launch (radio time difference of arrival, slide 9) as it goes along.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-02-04T12:08:35.633" LastActivityDate="2016-02-04T12:08:35.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13899" PostTypeId="1" AcceptedAnswerId="13903" CreationDate="2016-02-04T13:13:26.970" Score="8" ViewCount="1032" Body="&lt;p&gt;I saw a video today of a flash drive being thrown into space from the ISS (&lt;a href=&quot;http://www.bbc.co.uk/news/science-environment-35487441&quot;&gt;http://www.bbc.co.uk/news/science-environment-35487441&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What will happen to this flash drive? Will it burn up in the atmosphere, drift off into deep space or land back on earth?&lt;/p&gt;&#xA;" OwnerUserId="13314" LastEditorUserId="49" LastEditDate="2016-02-04T15:27:28.827" LastActivityDate="2016-02-04T15:35:53.503" Title="What will happen to the Russian messages flash drive?" Tags="|iss|orbit|low-earth-orbit|debris|orbital-decay|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13900" PostTypeId="1" AcceptedAnswerId="13910" CreationDate="2016-02-04T13:13:49.277" Score="3" ViewCount="457" Body="&lt;p&gt;Sounding rockets or suborbital launches basically go straight up and then fall down without entering orbit. That requires much less fuel mass than to achieve 8,000 m/s or so orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;For how long could such a launch put a payload in freefall&#xA;microgravity (which could be of interest to science, manufacturing,&#xA;tourism)? I suppose that there won't be much microgravity once&#xA;atmospheric braking sets in below 100 km altitude or so.&lt;/li&gt;&#xA;&lt;li&gt;How would today's most powerful launcher, Delta IV Heavy, perform as&#xA;a sounding rocket in terms of altitude and payload time in freefall? Or a Saturn V? The EFT-1 with Orion on a DIVH lasted for over 4 hours, but was actually designed to accelerate the payload down through Earth's atmosphere.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-02-04T13:18:57.380" LastActivityDate="2016-02-04T19:35:42.790" Title="How high can a sounding rocket launch and for how long time can it put a payload in freefall microgravity?" Tags="|microgravity|sub-orbital|sounding-rocket|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13901" PostTypeId="2" ParentId="2894" CreationDate="2016-02-04T13:36:09.703" Score="2" Body="&lt;p&gt;Perhaps. &#xA;There seems to be persistent interest, see &lt;a href=&quot;http://www.ceoi.ac.uk/index.php?option=com_content&amp;amp;view=article&amp;amp;id=149:smalllauncherandsub-orbital-call&amp;amp;catid=8:latest&quot; rel=&quot;nofollow&quot;&gt;NSTP2&lt;/a&gt;, which is an invitation from the UK Space Agency for &quot;proposals for industrial research projects that will contribute to the introduction of sub-orbital flight and satellite launch operations in the UK&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Proposals have to address:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;determine how sub-orbital or small satellite launch vehicles primarily designed for operation outside of the UK can be adapted to operate on a commercial basis in the UK, and/or;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;develop capabilities in the UK that will increase the UK’s ability to operate sub-orbital or small satellite launch services, and/or;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;identify the technology developments that will be needed and how UK companies can play a significant role in the supply chain for future sub-orbital and small satellite launch systems&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So who knows where it will lead, one can't predict the future.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-02-04T13:36:09.703" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13902" PostTypeId="2" ParentId="13899" CreationDate="2016-02-04T13:40:19.190" Score="8" Body="&lt;p&gt;It will burn up in the atmosphere. Without and source of internal thrust it will decay into a lower orbit due to micro-drag forces, and it is too small to survive re-entry. There is no way it could land back on earth without a form of heat shield at the very least. Also from the video it appears that the Cosmonaut threw it against the direction of travel which would cause it to fall back to earth slightly quicker than if he had thrown it &quot;in-front&quot; of the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a reference you can see what happened to &lt;a href=&quot;http://www.space.com/7088-tool-bag-lost-space-meets-fiery.html&quot;&gt;this tool-bag&lt;/a&gt; in 2009 when a NASA astronaut accidentally let go of it while repairing one of the solar panels. It took about 9 months for the bag to de-orbit and burn up.&lt;/p&gt;&#xA;" OwnerUserId="13176" LastActivityDate="2016-02-04T13:40:19.190" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13903" PostTypeId="2" ParentId="13899" CreationDate="2016-02-04T13:45:43.847" Score="8" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.spaceflightinsider.com/missions/iss/second-spacewalk-2016-conducted-cosmonaut-duo/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Insider&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... After that, Volkov was the first to leave the module. He had with&#xA;  him a flash drive containing messages from last year’s 70th&#xA;  anniversary of Russia’s Victory Day. &lt;strong&gt;It was thrown retrograde from the&#xA;  space station so as to ensure that it will not collide with the&#xA;  station in future orbits before its orbit eventually degrades into the&#xA;  Earth’s atmosphere.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight101.com/russian-eva-42-success/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt; adds that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[the flash drive will] &lt;strong&gt;re-enter in a period of only a few months&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And includes this image of the release:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/F2KIv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/F2KIv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Symbolic jettisoning of the video capsule containing messages from last year’s Victory Day celebration. Photo: NASA TV&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, the flash drive was put into a sealed reflective bag similar to a candy bar wrapper. This will increase its surface area while not greatly increase its mass, making sure it loses its orbital speed and decays faster because of the nature of the drag equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a_D = - {1\over2}({C_D{A_v(t)\over{m_s}}})\ {pv_r}^2e_v$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $a_D$ is total acceleration due to atmospheric drag (mind the sign, it is a deceleration), $p$ is atmospheric density (at ISS orbital altitude its density is around $10^{-8}\text{ Pa}$ or $1.45 \times 10^{-12}\text{ psi}$), $C_D$ is &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_coefficient&quot; rel=&quot;noreferrer&quot;&gt;drag coefficient&lt;/a&gt;, $A_v(t)$ is the cross-sectional area of the object in the direction of travel, $m_s$ is the total object's mass, $v_r$ is the velocity  magnitude relative to the ambient atmosphere, and $e_v$ is a unit vector in the  relative velocity direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the flash drive loses its orbital speed and altitude, it will burn up in the upper Earth's atmosphere as it reenters it at at roughly 6.5 km/s, producing &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_ionization&quot; rel=&quot;noreferrer&quot;&gt;contact ionization&lt;/a&gt; heat due to friction with the atmosphere of over 6,000 °C (rule-of-thumb for peak shock layer temperature is as many Kelvins as speed of reentry in meters per second). And it takes a whole lot less of heat to completely melt and evaporate a typical flash drive reentering the atmosphere. Flash drive will start melting on its surface, fragment by ablation and outgas until it's completely gone. It might produce a small &lt;em&gt;shooting star&lt;/em&gt; visible from the ground, if someone is lucky enough to see it, but that's about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reflective bag (likely a metallized &lt;a href=&quot;https://en.wikipedia.org/wiki/BoPET&quot; rel=&quot;noreferrer&quot;&gt;boPET&lt;/a&gt; film, better known as Mylar) that the flash drive was put in also increases its radar signature, so the object can be tracked by ground facilities like e.g. &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/measure/radar.html&quot; rel=&quot;noreferrer&quot;&gt;NASA's Haystack&lt;/a&gt;, and we can expect that its predicted reentry location will be announced beforehand. As technically a space launch, this object also, eventually, has to be registered with the &lt;a href=&quot;http://www.unoosa.org/oosa/index.html&quot; rel=&quot;noreferrer&quot;&gt;UNOOSA&lt;/a&gt; (United Nations Office for Outer Space Affairs), as per the &lt;a href=&quot;https://en.wikipedia.org/wiki/Registration_Convention&quot; rel=&quot;noreferrer&quot;&gt;Launch Registration Convention&lt;/a&gt; which was signed and ratified by the launching nation (in our case Russia), and published in the &lt;a href=&quot;http://www.unoosa.org/oosa/osoindex/search-ng.jspx&quot; rel=&quot;noreferrer&quot;&gt;OSOidx&lt;/a&gt; (Online Index of Objects Launched into Outer Space). And since it's not a classified object, it should eventually also find its way into the NORAD (North American Aerospace Defense Command) catalog and have published &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;noreferrer&quot;&gt;TLE&lt;/a&gt;, so the community at large can track it and predict its reentry.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-04T15:35:53.503" LastActivityDate="2016-02-04T15:35:53.503" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13904" PostTypeId="1" CreationDate="2016-02-04T13:45:50.797" Score="1" ViewCount="240" Body="&lt;p&gt;Aerobraking has always been thought of as a way to economically decelerate a spacecraft before landing or orbit insertion. I'm asking here if it could instead be used to accelerate a spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Passing close by a massive planet, like Jupiter, a spacecraft seeking maximum speed as it heads outwards enjoys two benefits and one problem. The benefits are the gravitational assist and the Oberth effect of firing its engines at periapsis. The drawback is the resistance from the atmosphere if one gets too close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the friction heat from aerobraking in an atmosphere be used to ignite a propulsion system, of any kind which transforms external heat to a fast ejection of mass? Rocket engines use oxidizers to chemically create heat that ejects hydrogen at high velocity. Could the oxygen, which represents the large majority of fuel mass, be replaced by the friction heat from passing through a planetary atmosphere? Somehow transferring the heat from the shield to some kind of hydrogen acceleration mechanism. It would come with the bonus of happening when the Oberth effect is the greatest. And would it be useful for igniting some kind of fission thermal short term booster?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Chelyabinsk meteor produced an explosion of about 500 kilotons of TNT when it entered Earth's atmosphere. Could that kind of effect be harnessed and used to substantially accelerate a spacecraft through an atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-02-04T14:13:05.137" LastActivityDate="2016-02-05T10:47:29.763" Title="Could friction heat from aerobraking be used to power propulsion?" Tags="|propulsion|gravity-assist|aerobraking|oberth-maneuver|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13905" PostTypeId="2" ParentId="13904" CreationDate="2016-02-04T14:52:30.560" Score="4" Body="&lt;p&gt;Can it be done - I think yes. Does it make any sense - I think no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concept of a non-chemical thermal rocket is not new. Other types of such rockets would be a nuclear thermal rocket or a resistojet. Both work in principle by heating a fluid and then expelling it, just like the concept you suggest. The fluid can be anything, but hydrogen is best in terms of ISP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how would a use case of your rocket look like? Obviously, you would not try to use it to escape earth orbit, because the energy dissipating at re-entry is just the motion energy you put in your spacecraft in the first place. Furthermore, most of the energy is not transferred to the spacecraft but stays in the atmosphere (if I remember correctly, it was about 0.2% going into the spacecraft).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, as you were saying, maybe your want to use this energy to fly a maneuver in the Jovian system. When you enter the system, you have escape velocity, so you want to shed some of that energy. In this case by aerobrakeing in Jupiter (may not be smart for other reasons), yes, you may encounter some heat, and that heat, in theory, can be used as energy for later course changes. You could, for example pressurize a lot of hydrogen, then use it to cool a heat shield, and then release the hydrogen for thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would be a very bad design. You would have to store a large amount of pressurized hydrogen somewhere, because you don't want to fire your thruster right away, while still in Jupiter. You want to have propulsion hours, or days, or years later. This makes your tanks massively heavy. And that is just the least of your problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to get any sort of decent ISP, you would need to heat your hydrogen to thousands of kelvin. Therefore, it would not be an effective coolant. Actually, you would somehow have to actively cool your storage tank, because there is no material in the world that could resist the temperatures needed for a gas to be an effective rocket propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And even that ignores the largest flaw in this concept, that energy isn't even the largest concern in spacecraft propulsion, it's momentum. There are many sources of energy available, even in outer space. There are radioisotope thermoelectric generators, solar panels, chemical reactions, even nuclear reactors. The tricky part is to use this energy to expel a fluid backward at the greatest speed (read: ISP) possible, so according to the momentum balance, your space ship gets the most forward momentum. In this regard, there is nothing better than ion thrusters, and your concept would do worse than the simplest thruster, the resistojet. This is a thruster that does nothing but heat something (anything really - hydrogen, water, poop, you name it) electrically to expel it through a nozzle. So just like your idea, the resistojet also also heats up the propellant, but it doesn't have to store it and it is not dependent on a specific part of the mission in order to function.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, even if it was possible to recover some energy from aerobrakeing, I don't think that it can be a viable part of any propulsion system.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-02-05T10:47:29.763" LastActivityDate="2016-02-05T10:47:29.763" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13906" PostTypeId="1" AcceptedAnswerId="13907" CreationDate="2016-02-04T15:21:38.080" Score="0" ViewCount="443" Body="&lt;p&gt;How does SpaceX get their rockets to stay vertical when hovering without tips over and eventually crashing into the ground? I don't mean the initial launch. I mean when its coming back down I feel like the gravity would take advantage of one tiny wobble and send it to the ground because the rocket is top heavy. &lt;/p&gt;&#xA;" OwnerUserId="13317" LastEditorUserId="13317" LastEditDate="2016-02-04T15:54:09.607" LastActivityDate="2016-02-04T15:54:09.607" Title="SpaceX hovering rocket?" Tags="|rockets|spacex|propulsion|falcon-9|aerodynamics|" AnswerCount="1" CommentCount="2" ClosedDate="2016-02-05T00:56:41.093" ContentLicense="CC BY-SA 3.0" />
  <row Id="13907" PostTypeId="2" ParentId="13906" CreationDate="2016-02-04T15:36:43.147" Score="4" Body="&lt;p&gt;If you mean the Falcon 9 then that cannot hover. It has TWR &gt;1 during the landing even when using just 1 of its 9 engines. But yes it can stay vertical during the descent by using thrust gimbal. The Grasshopper test rocket is able to hover that way. The nozzle of the engine can point a bit to the side to balance the long rocket - similarly as you can balance a stick on your finger.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-02-04T15:36:43.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13908" PostTypeId="2" ParentId="13904" CreationDate="2016-02-04T16:17:33.987" Score="2" Body="&lt;p&gt;The fundamental problem is in law of conservation of energy. All the energy you might expend on propulsion obtained that way comes from one singular source: kinetic energy of your ship, braking against the atmosphere. You won't extract more energy than you're losing, since you can't collect it at any reasonable efficiency as it's generated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might help yourself with an oxidizer, or save up a certain percent of the descent energy for consecutive ascent if you want to enter Jupiter orbit and later depart it, but in the current form you've presented it, it's an &lt;strong&gt;aerodynamic perpetuum mobile&lt;/strong&gt;. (just imagine instead of propelling yourself out of the atmosphere you enter an orbit through the atmosphere and somehow create all the heat without ever slowing down, just maintaining speed).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There &lt;em&gt;might&lt;/em&gt; be a way though. Not now and not soon, possibly a couple centuries away. You can extract more energy from pure hydrogen than you spend, but you must find spend &lt;em&gt;a lot&lt;/em&gt;, and there are no solids that can survive this order of temperatures involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Create a magnetic &quot;air intake&quot; - out of powerful magnetic field. Compress the incoming hydrogen into superheated plasma. Make it so hot that it undergoes nuclear fusion, turning into helium and expending a lot of energy in the process. In short, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;nofollow&quot;&gt;Bussard Ramjet&lt;/a&gt; that is intended for gas giants instead of interplanetary medium. This would work, but the engineering task behind shaping such a magnetic field makes my head spin.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-04T16:17:33.987" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="13909" PostTypeId="1" CreationDate="2016-02-04T16:38:32.793" Score="2" ViewCount="41" Body="&lt;p&gt;Does the magnitude and character of an airbursting meteor depend substantially on whether it hits an atmosphere with much O2, N2 or CO2? Does the oxygen in Earth's atmosphere affect meteors?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume that density and everything's the same. Does the chemistry of the atmosphere matter much for how the impactor behaves on entry? Are for example meteors different at Venus and Mars than on Earth?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-02-04T16:38:32.793" Title="Does the airburst effect of a meteor depend on the chemical composition of the atmosphere?" Tags="|reentry|meteors|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13910" PostTypeId="2" ParentId="13900" CreationDate="2016-02-04T19:35:42.790" Score="5" Body="&lt;p&gt;A good start is to find the upper limit, so I am going to first oversimplify it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I launch a rocket straight upwards with the same $\Delta v$ as required for a circular orbit, my free-fall time is $\frac{\pi +2}{2\pi}$ times the orbital period of the circular orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means that the maximum amount of time I can spend in free-fall is about 80 minutes, before it makes more sense to launch into orbit instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the atmosphere actually matter? At $8 km/s$ the last 100 km is only a loss of thirteen seconds, negligible compared to the total flight time. Of course the $\Delta v$ lost at ascent is significant, but a rocket launched to orbit also experiences that drag. Thus, the longest meaningful free-fall time a suborbital launch can give you is a little more than an hour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for what payloads modern rockets can set into this trajectory, the $\Delta v$ required for the longest meaningful free-fall time is the same as for a launch into orbit. Simply use their payload capacity into LEO. As for a minimal sub-orbital launch, about $1.4 km/s$ of $\Delta v$ is required. For such low requirements, the launcher's thrust to weight ratio is the limit.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-04T19:35:42.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13911" PostTypeId="2" ParentId="2894" CreationDate="2016-02-04T23:19:06.433" Score="2" Body="&lt;p&gt;Although not orbital, which was the explicit question asked, a rocket has been launched from the UK to beyond the atmosphere in October 2015.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is &lt;a href=&quot;http://www.bbc.co.uk/news/uk-scotland-highlands-islands-35482244&quot; rel=&quot;nofollow noreferrer&quot;&gt;reported by the BBC today&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://ichef-1.bbci.co.uk/news/624/cpsprodpb/AEC1/production/_88073744_missilethree.jpg&quot; alt=&quot;BBC news image from Qinetic&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The BBC article also writes about the sites for planned spaceports, including an &lt;a href=&quot;http://www.bbc.co.uk/news/uk-scotland-highlands-islands-35438350&quot; rel=&quot;nofollow noreferrer&quot;&gt;article about a proposed vertical rocket launch site in Sutherland&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of these proposals are still vapourware.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I presume this flurry of BBC articles result from journalists attending the &lt;a href=&quot;http://www.aerosociety.com/Events/Event-List/2148/Spaceport-UK&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceport UK conference&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.747" LastActivityDate="2016-02-04T23:19:06.433" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13913" PostTypeId="1" AcceptedAnswerId="13914" CreationDate="2016-02-05T02:02:55.163" Score="19" ViewCount="6140" Body="&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=Nw_PyrsGHuY&amp;amp;t=5m5s&quot;&gt;Armageddon&lt;/a&gt;, two space shuttles launch from adjacent pads mere seconds after one another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems highly implausible for several reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Pressure, heat, vibration and debris from the first launch might damage the second shuttle's launch mechanisms&lt;/li&gt;&#xA;&lt;li&gt;Exhaust of the first shuttle would make navigational calculations extremely difficult once the second shuttle is airborne&lt;/li&gt;&#xA;&lt;li&gt;Command has limited staff and a clean handoff between two teams would be unlikely given the time constraints&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Are simultaneous, close-proximity dual shuttle launches really feasible, and do they have historical precedent?&lt;/p&gt;&#xA;" OwnerUserId="13325" LastEditorUserId="6944" LastEditDate="2016-06-07T19:44:03.763" LastActivityDate="2016-06-07T19:44:03.763" Title="Are Armageddon's simultaneous shuttle launches feasible?" Tags="|launch|space-shuttle|launch-site|feasibility|multi-launch|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13914" PostTypeId="2" ParentId="13913" CreationDate="2016-02-05T02:21:43.463" Score="16" Body="&lt;p&gt;There's no historical precedent for simultaneous launch, but some early space program rendezvous experiments had flights launched somewhat close together.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Gemini&quot; rel=&quot;noreferrer&quot;&gt;Gemini&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_8&quot; rel=&quot;noreferrer&quot;&gt;8&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_10&quot; rel=&quot;noreferrer&quot;&gt;10&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_11&quot; rel=&quot;noreferrer&quot;&gt;11&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_12&quot; rel=&quot;noreferrer&quot;&gt;12&lt;/a&gt; each launched within a couple of hours of an unmanned Agena spacecraft for a rendezvous exercise. They would launch from separate pads at Cape Canaveral, &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Launch_Complex_14&quot; rel=&quot;noreferrer&quot;&gt;LC-14&lt;/a&gt; for the Agenas and &lt;a href=&quot;https://en.wikipedia.org/wiki/Cape_Canaveral_Air_Force_Station_Launch_Complex_19&quot; rel=&quot;noreferrer&quot;&gt;LC-19&lt;/a&gt; for the Geminis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo and Soyuz craft used in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot; rel=&quot;noreferrer&quot;&gt;ASTP&lt;/a&gt; rendezvous mission launched about 8 hours apart, but obviously from very different launch sites. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the other rendezvous missions I'm aware of launched days apart. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, I don't think it's &lt;em&gt;completely&lt;/em&gt; infeasible if there were a compelling reason to do it (but there isn't; safely rendezvousing in space is going to take hours whether the ships are launched simultaneously or not). Of your concerns:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Direct interference between the launches: probably not a problem; the big launchpads at Canaveral are something like a mile apart, and the pressure, heat, and vibration from rocket A are way, way, way more troublesome for rocket A than for rocket B. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Exhaust disturbance in the air: probably not a problem for the same reason; if the craft were close enough to one another for exhaust plume impingement to be a concern, the chance of collision would be a far more significant concern. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Command/coordination issues: probably one of the big reasons it's never been done. A lot of activity has to be coordinated for a rocket launch, and doubling that workload without a damned good reason is pointless and hazardous. According to &lt;a href=&quot;https://space.stackexchange.com/questions/3738/nasa-launch-control-was-all-of-its-capacity-ever-used&quot;&gt;this Q&amp;amp;A&lt;/a&gt; though, the infrastructure to support simultaneous launch exists; the redundancy is mainly there for testing, training, rehearsal and maintenance reasons.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If NASA was determined to do a simultaneous launch of two shuttle-sized rockets it could probably replace a couple of the smaller, idle launch pad complexes at Canaveral with one big one, in order to put more distance between the launch pads. That could get maybe 8 miles of separation between the pads, but I don't think NASA would ever be comfortable with the collision risk implied there.   &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-02-05T03:06:00.770" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13915" PostTypeId="2" ParentId="13913" CreationDate="2016-02-05T02:35:14.333" Score="27" Body="&lt;p&gt;Most of your concerns can be put to rest by this image:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dDsKD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dDsKD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Image credit: NASA, Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Kennedy_Space_Center_Launch_Complex_39#/media/File:Space_shuttles_Atlantis_%28STS-125%29_and_Endeavour_%28STS-400%29_on_launch_pads_again.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;For the last time in the Shuttle program, Space Shuttles Atlantis and&#xA;  Endeavour are placed at LC39A and LC39B in preparation for the &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-125&quot; rel=&quot;noreferrer&quot;&gt;STS-125&lt;/a&gt;&#xA;  mission to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt;.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;Space shuttle Atlantis on Launch Pad 39A (left) is accompanied by&#xA;  space shuttle Endeavour after Endeavour's rollout to Launch Pad 39B.&#xA;  This was the last time two shuttles were on launch pads&#xA;  simultaneously. Atlantis mission STS-125 was the final servicing&#xA;  mission to upgrade NASA's Hubble Space Telescope. Endeavour was&#xA;  prepped for contingency support (Launch On Need) standing by at pad 39B&#xA;  in the event Atlantis was damaged during flight and unable to safely&#xA;  return to earth, necessitating an emergency rescue mission (STS-400.)&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Endeavour&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Endeavour&lt;/a&gt; was on its Launch Pad 39B when the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Atlantis&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Atlantis&lt;/a&gt; launched on its STS-125 HST servicing mission. That said, while the supporting Space Shuttle Endeavour never launched on that mission, it was in readiness to do so should the need arise. That means also ground support for both launch vehicles. They wouldn't launch in tandem like I think was portrayed in the movie, if I remember correctly, but a back-to-back launch was a contingency (&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-400&quot; rel=&quot;noreferrer&quot;&gt;STS-400 contingency support mission&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while there isn't any exact historical precedent, there is about, oh, shall we say 90% of it as it pertains to portrayal in that movie that you mention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For easier appreciation of how far from each other the two launch sites are, here's an aerial view:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/abJ7U.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/abJ7U.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Photo credit: Cory Huston, Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/File:LC39A_and_LC39B.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This aerial view looking north shows space shuttle Complex 39 Launch&#xA;  Pads A (foreground) and B at NASA's Kennedy Space Center. To the right&#xA;  is the Atlantic Ocean.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-05T02:46:04.513" LastActivityDate="2016-02-05T02:46:04.513" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13917" PostTypeId="2" ParentId="4465" CreationDate="2016-02-05T07:14:22.147" Score="0" Body="&lt;p&gt;Theoretically, a satellite could be in the correct position but travelling in the opposite direction (or at some other angle). In practice though, that's not how satellite launches work. Satellites are launched on rockets that fly into a particular orbit. Orbits are characterised by various parameters but the most important are the apogee (the highest point in the orbit above Earth), perigee (the lowest point) and inclination (the angle of the orbital path, with respect to the equator). The inclination is determined by the direction the rocket flies so this is already well monitored during the flight. By the time the payload satellite is released from the rocket, the main concern for the ground controllers is to check that the satellite reaches the right apogee and perigee, which is determined by the duration and timing of the rocket motor burns. The direction is no longer in doubt. Mission controllers informally refer to the satllite having reached the correct 'position' but what they really mean is that the entire orbital track has the right shape.&lt;/p&gt;&#xA;" OwnerUserId="13327" LastActivityDate="2016-02-05T07:14:22.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13918" PostTypeId="1" AcceptedAnswerId="13920" CreationDate="2016-02-05T07:27:07.273" Score="23" ViewCount="9071" Body="&lt;p&gt;A group of Russian geeks &lt;a href=&quot;https://boomstarter.ru/projects/zelenyikot/proekt_sputnika_dlya_fotosemki_luny&quot;&gt;intends&lt;/a&gt; to shoot a small vehicle to the moon, which is supposed to photograph the places of the Apollo landings. This will be the ultimate proof that people actually visited the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In one of the articles about the topic I've read that you could use a low-cost onboard computer, even an Arduino, provided that it is protected from radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the purpose of the spacecraft is to&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;separate itself from a group of satellites (several small vehicles will be launched with one rocket),&lt;/li&gt;&#xA;&lt;li&gt;navigate to and fly over a particular place,&lt;/li&gt;&#xA;&lt;li&gt;shoot a couple of photos and&lt;/li&gt;&#xA;&lt;li&gt;transmit them to Earth,&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;can you really use such a low-end device as Arduino as the onboard computer? If not, why not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update 1 (06.02.2016 12:34 MSK):&lt;/strong&gt; I asked the project lead on how they intend to prove that the images are real. You can find his answer &lt;a href=&quot;http://altruix.cc/sources/how-the-authenticity-of-the-photos-sent-by-moon-photographing-vehicle-will-be-ensured/&quot;&gt;here&lt;/a&gt;. Short version:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The images will have a much higher quality than those generated by previous analogs. This means that the new vehicle will &quot;see&quot; things (such as details of the Moon surface), which previous didn't. Therefore, the images will contain additional information, which is not known yet (like a small crater, which was unrecognizable on earlier photographs).&lt;/li&gt;&#xA;&lt;li&gt;Radio amateurs will be able to detect (but not to decode) the signal from the vehicle.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerDisplayName="user3049" LastEditorDisplayName="user3049" LastEditDate="2016-02-06T09:41:58.567" LastActivityDate="2022-05-14T13:20:43.840" Title="Can you really use Arduino for a small spacecraft?" Tags="|spacecraft|design|flight-computer|electronics|" AnswerCount="10" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="13919" PostTypeId="1" CreationDate="2016-02-05T07:31:03.473" Score="2" ViewCount="400" Body="&lt;p&gt;I'm trying to understand if it is only fuel efficiency that makes us use gravity assist, or we don't have enough rocket fuel in an average spacecraft to reach farther ends of solar system. Also, is there some other reason why gravity assists are used?&lt;/p&gt;&#xA;" OwnerUserId="13329" LastEditorUserId="25" LastEditDate="2016-02-05T13:28:40.307" LastActivityDate="2022-10-07T00:51:01.863" Title="For what reasons are gravity assists used? What objects can be visited without such measures?" Tags="|launch|rockets|orbital-mechanics|gravity-assist|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13920" PostTypeId="2" ParentId="13918" CreationDate="2016-02-05T07:56:46.567" Score="35" Body="&lt;p&gt;Can you have a spacecraft based on an Arduino? Sure you can! &lt;a href=&quot;https://www.kickstarter.com/projects/575960623/ardusat-your-arduino-experiment-in-space/posts/669200&quot;&gt;ArduSat&lt;/a&gt; was two kickstarter funded cubesats that were eventually launched from the International Space Station in November 2013. When you think about it, an Arduino easily outperforms for instance the over forty year old &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Guidance_Computer&quot;&gt;Apollo Guidance Computer&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of your requirements should be doable, if it is simply a flyby mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, images of the Apollo landing sites are already taken, for instance by the &lt;a href=&quot;http://solarviews.com/cap/moon/apollo16.htm&quot;&gt;Clementine probe&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-05T08:03:03.950" LastActivityDate="2016-02-05T08:03:03.950" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13921" PostTypeId="2" ParentId="13918" CreationDate="2016-02-05T11:59:28.487" Score="6" Body="&lt;p&gt;From a computational point of view, it is feasible to pack all the algorithms in there.  However, there are issues with radiation shielding as you point, but integrated circuits are also affected by temperature, so proper thermal insulation must be considered too, as these are not definitely graded to operate in &quot;harsh&quot; environments.&lt;/p&gt;&#xA;" OwnerUserId="13333" LastActivityDate="2016-02-05T11:59:28.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13922" PostTypeId="2" ParentId="13918" CreationDate="2016-02-05T12:00:22.100" Score="16" Body="&lt;p&gt;I concur with Hohmannfan's response. This answer addresses the wider issue of computers in satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Who needs a computer?&lt;/strong&gt; &#xA;I don't think there is anything about the mission that you have described in the question that actually requires any &quot;digital computer&quot; at all. It might seem as if image handling and navigation are highly demanding in computing terms, but that is largely because we are accustomed to the idea of a world enabled by high-level software. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think it is a good starting point in terms of systems engineering education to actually step through the processes involved and ask oneself &quot;&lt;em&gt;what is the most basic implementation possible?&lt;/em&gt;&quot;, particularly in terms of &quot;&lt;em&gt;what decisions absolutely must be taken on board, rather than by ground command?&lt;/em&gt;&quot;. Designers of missions in the 60's through 80's often came down on the side of &quot;no computer needed&quot;. Its only since the weight, performance and cost of such things has come down that we take it for granted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Digital&lt;/strong&gt;&#xA;All of the logical decisions that need to be made on a satellite could in principal be made by discrete logic gates. Its an engineering judgement as to when the &quot;digital finite state machine&quot; so created has become so complicated that it would be better replaced by a CPU/address bus/data bus architecture. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, its not obvious that there is any clear dividing line between discrete electronics and a &quot;computer&quot; in the modern sense. This article regarding &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_10&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pioneer 10&lt;/a&gt; hints at the in-between possibilities.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Much of the computation for the mission was performed on Earth and transmitted to the probe, where it was able to retain in memory up to five commands of the 222 possible entries by ground controllers. The spacecraft included two command decoders and a command distribution unit, a very limited form of processor, to direct operations on the spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Analogue&lt;/strong&gt;&#xA;Furthermore, decisions relating to progressive quantities - sensor outputs and control loops are in the first instance analysed in control engineering terms. How they are implemented is again a design choice and the old world was full of analogue computer elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Environmental compatibility&lt;/strong&gt;&#xA;There is the launch environment and then natural radiation once in orbit. The latter includes ESD damage (see &lt;a href=&quot;https://space.stackexchange.com/questions/5062/strategies-for-combating-esd-and-ground-plane-potential-shifts-on-spacecraft-cha&quot;&gt;here&lt;/a&gt;) as well as radiation dose and displacement damage. One interesting anecdote is that the progressively higher performance computers using physically smaller gates and switching times are more vulnerable to these effects than their older cousins. When it comes to ESD, shielding doesn't help when there are peripheral bits of the circuit on the spacecraft skin. The designers need to take it carefully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, thank you @uhoh for the &lt;a href=&quot;https://space.stackexchange.com/questions/4922/space-computing-general-question/4929#4929&quot;&gt;link&lt;/a&gt;, that was interesting.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-02-05T12:52:08.327" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="13923" PostTypeId="2" ParentId="13918" CreationDate="2016-02-05T13:11:45.667" Score="8" Body="&lt;p&gt;You don't need that much processing power or RAM to explore space. The Arduino is far more powerful than the processors which have been used on spacecraft historically. The basic Arduino is 16&amp;nbsp;MHz and 256&amp;nbsp;KB of flash memory. You can add RAM or flash in significant amounts. The computers used in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;nofollow&quot;&gt;Apollo space program&lt;/a&gt; come nowhere near this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;Galileo&lt;/em&gt;'s&lt;/a&gt; processors are 8&amp;nbsp;MHz, while &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Spirit_%28rover%29&quot; rel=&quot;nofollow&quot;&gt;Spirit&lt;/a&gt;&lt;/em&gt; and &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_%28rover%29&quot; rel=&quot;nofollow&quot;&gt;Opportunity&lt;/a&gt;&lt;/em&gt; are 20&amp;nbsp;MHz processors. The code on these probes was highly optimized and specially developed for the purpose at hand, making very efficient use of the computing resources available. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's entirely do-able.&lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="286" LastEditDate="2016-02-07T14:52:35.873" LastActivityDate="2016-02-07T14:52:35.873" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13924" PostTypeId="2" ParentId="13919" CreationDate="2016-02-05T13:20:24.697" Score="7" Body="&lt;p&gt;New Horizons was launched on a direct escape trajectory, so in theory we can go anywhere in the Solar System, and beyond, without a gravity assist, except for very close to the Sun, which we do not have the fuel to do without such measures (At least, not in a reasonable amount of time.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity assists just make the process cheaper, and allow for heavier payloads. New Horizons was able to do so because it was very small, to send a larger payload would be difficult. It did a flyby of Jupiter, not because it was required, but because it allowed the spacecraft to get there a bit faster. And quite frankly, there's almost no reason to go to the Outer solar system without doing a flyby of Jupiter first, as the speed improvement is quite large. The main exception would be that it limits when you can launch the mission, but that isn't a huge factor typically.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-02-05T13:30:37.337" LastActivityDate="2016-02-05T13:30:37.337" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13925" PostTypeId="2" ParentId="13918" CreationDate="2016-02-05T13:55:55.690" Score="26" Body="&lt;p&gt;You can, but it will suffer from a number of problems. These problems can probably be overcome with a short term mission. Problems include:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Radiation- Degrading the long term effect of the electronics.&lt;/li&gt;&#xA;&lt;li&gt;Single event upsets- This is probably the biggest danger, a high energy cosmic ray strike could cause a bit flip, potentially changing the code running in a mission critical way.&lt;/li&gt;&#xA;&lt;li&gt;Temperature- If the temperature isn't carefully managed, could see some significant damage.&lt;/li&gt;&#xA;&lt;li&gt;Vacuum- Probably not much of an issue, but could cause out-gassing which might have a long term effect.&lt;/li&gt;&#xA;&lt;li&gt;Vibration- Parts could be vibrated off during the launch.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;These probably can mostly be overcome by adding in extra protection to the spacecraft. But there isn't anything in particular that would prevent an Arduino from being used to control a spacecraft, especially a very short term mission. It has been demonstrated in LEO, but the radiation effects will be more severe for a mission to the Moon. Bottom line, it could be done, but I wouldn't recommend it.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2022-05-14T13:20:43.840" LastActivityDate="2022-05-14T13:20:43.840" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="13926" PostTypeId="2" ParentId="13919" CreationDate="2016-02-05T13:57:03.500" Score="4" Body="&lt;p&gt;You need to understand that &quot;enough fuel&quot; is an incredibly crucial matter in space travel. Spacecraft are subject to &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow&quot;&gt;Tyranny of Rocket Equation&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, if you want to bring your spacecraft and use fuel, say, somewhere around Mars, you need to bring it there, using more fuel to accelerate it out of Earth's orbit. To have that fuel, in order, you need more fuel in the second stage. And to accelerate the extra fuel in the second stage (plus the fuel in Earth orbit, and fuel around Mars), you need a lot more fuel in the first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much? On launch, the fuel and launch stages mass to actual scientific payload for distant solar system missions oscillates around 98-99%. Extra kilogram of payload around Pluto can be several tons of fuel on the launchpad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that all converts to rocket size - and enormous costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, any techniques that can save mass later in the mission - especially fuel mass - are absolutely essential to the budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity assists mean free acceleration. You can save many tons of fuel on launchpad, and just use a rocket that is smaller and cheaper by a couple hundred million dollars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any mass can be accelerated, and the higher mass of the body you're making a fly-by against, the more acceleration can be achieved. Keyword: fly-by. You can't just launch from Earth and do a gravity assist while in LEO. But you can depart into &quot;deep space&quot; on an elliptic orbit (using engines and precious fuel) and encounter Earth later, performing a gravity assist. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;About anything in Solar System, except for the Moon can be reached with benefit from gravity assists - although to reach nearest planets (Mars, Venus) they are often skipped, because the only thing we can really beneficially perform a gravity assist against, while going there, is the Moon, and it's too light to make any significant difference - launching at the perfect time to take least fuel on a direct transfer will cost less than waiting for the Moon and the target planet to align, then using the gravity assist. Of course, if the Moon happens to be in the right alignment at the right time, the orbital dynamics people happily tackle the opportunity and perform the assist anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thing is, planets move on their own schedule, and a trajectory that encounters one without spending a lot of fuel takes time. Therefore flights that use gravity assists usually take longer than direct encounters, like Hohmann Transfer. But they are significantly less expensive, and really, the savings go into many, many millions dollars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have a look at the &lt;a href=&quot;http://www.esa.int/esatv/Videos/2013/12/Rosetta_s_Journey_B-Roll/Solar_system_animation_showing_Rosetta_trajectory&quot; rel=&quot;nofollow&quot;&gt;trajectory of Rosetta&lt;/a&gt;. It performed 4 gravity assists and took 12 years to reach its destination. But it did reach the destination and sent good science. Probably if it just flew there directly, without all these assists, it would have taken some 4 years. But it couldn't, because that would about double the mission costs, and such money just isn't there.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-02-05T14:03:05.613" LastActivityDate="2016-02-05T14:03:05.613" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13930" PostTypeId="1" CreationDate="2016-02-05T15:54:28.620" Score="1" ViewCount="402" Body="&lt;p&gt;I am trying to fully understand the TRIAD algorithm for attitude determination of a satellite. &#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Triad_method&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Triad_method&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So so far I know that the vectors $\vec{r}_1$ and $\vec{r}_2$ can be constructed using measurements from different sensors. For example: if you have a sunsensor on each side of a satellite (in the x- ,y- and z-direction) and only the x-side is facing the sun you could eventually have a vector as follows $\vec{r}_1 = [100,0,0,]^T$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about the vectors which are not fixed to the body of the satellite itself? How do you construct vectors for such a reference frame (orbital reference frame)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/smgIu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/smgIu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any alternatives are welcome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final goal is to get the rotation matrix A.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$A = [\vec{R}_1;\vec{R}_2; (\vec{R}_1 \times \vec{R}_2)] [\vec{r}_1; \vec{r}_2;(\vec{r}_1 \times \vec{r}_2)]^T$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I know it has something to do with spherical harmonics&lt;/p&gt;&#xA;" OwnerUserId="13091" LastEditorUserId="13091" LastEditDate="2016-02-07T05:11:38.973" LastActivityDate="2016-02-07T05:11:38.973" Title="attitude determination TRIAD - how are orbital reference frame vectors constructed?" Tags="|artificial-satellite|attitude|calculation|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13932" PostTypeId="1" CreationDate="2016-02-05T16:55:30.843" Score="3" ViewCount="537" Body="&lt;p&gt;The American space shuttle program was, from what I understand, a radically different approach to space exploration at the time of its inception. With the enormous changes in technology and knowledge accrued in the 30+ years since then, I want to know: &lt;strong&gt;is the space shuttle program considered a financial success compared to other programs?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be more explicit: looking back, would it have been more cost-effective to use other technologies and approaches to explore space rather than the 'reusable' Space shuttles? Did they enable us to do anything we could not do with &quot;conventional&quot; technologies? Were conventional tactics any cheaper to manufacture/launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To cite an example (&lt;a href=&quot;https://space.stackexchange.com/questions/10350/why-didnt-nasa-use-the-shuttle-to-make-a-profit?rq=1&quot;&gt;Why didn&amp;#39;t NASA use the shuttle to make a profit?&lt;/a&gt;) - the shuttle's primary advantage was &quot;cost efficiency&quot;, but apparently this was proven to be a fiction after only one launch. If the greatest advantage of a program disappeared after just one launch, one would imagine the program would become worthless, yet it continued for thirty years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related:&#xA;&lt;br&gt; &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/1173/what-made-nasa-shut-down-the-shuttle-program&quot;&gt;What made NASA shut down the Shuttle program?&lt;/a&gt;&#xA;&lt;br&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Criticism_of_the_Space_Shuttle_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia's article on space shuttle criticisms&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13339" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-02-06T17:33:48.880" Title="Was the shuttle program a success?" Tags="|spacex|nasa|space-shuttle|space-program|" AnswerCount="1" CommentCount="6" ClosedDate="2016-02-07T06:13:39.040" ContentLicense="CC BY-SA 3.0" />
  <row Id="13934" PostTypeId="2" ParentId="13918" CreationDate="2016-02-05T17:04:23.400" Score="8" Body="&lt;p&gt;Others have covered the hardware difficulties, but I would like to mention the software difficulties. Getting enough margin (CPU and memory) is difficult on arduino class processors. Spacecraft I have worked have required anything between 50% and 90% &lt;em&gt;margin&lt;/em&gt; that means that you only get to use between 50% and 10% of the processor. The margin is for things like schedulability and memory scrubbing. The other factor is it is just harder (and therefore more expensive) to write reliable software with less resources. There is a fine line between too simple (can't do all the checking you really should do) and too complex (impossible to test) and the arduino probably favours more of the former. &lt;/p&gt;&#xA;" OwnerUserId="12004" LastEditorUserId="12004" LastEditDate="2016-02-06T14:53:05.983" LastActivityDate="2016-02-06T14:53:05.983" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13935" PostTypeId="1" AcceptedAnswerId="13936" CreationDate="2016-02-05T18:22:20.643" Score="57" ViewCount="27725" Body="&lt;p&gt;Now that a Falcon 9 first stage has successfully landed after a launch mission, I want to know how the first stage can avoid burning up when coming back down to earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There doesn't appear to be any heat shield on the bottom of the rocket, and I assume the first stage would be well above the atmosphere, meaning that it would have to re-enter.&lt;/p&gt;&#xA;" OwnerUserId="13032" LastEditorUserId="51" LastEditDate="2016-02-05T19:37:45.547" LastActivityDate="2018-05-13T18:33:12.540" Title="How does the Falcon 9 first stage avoid burning up on re-entry?" Tags="|spacex|landing|reentry|falcon-9|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13936" PostTypeId="2" ParentId="13935" CreationDate="2016-02-05T18:33:09.400" Score="59" Body="&lt;p&gt;The heat of re-entry is highly dependent on speed. The second stage of the rocket is responsible for providing most of the speed needed for orbit, after the first stage lifts it out of dense atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 separates its first and second stages at relatively low speed, so its reentry starts off drastically slower than a reentry from orbit -- about 1650 m/s for the return-to-launch-site flight in December 2015, compared to orbital speed of 7700 m/s. Stage separation can be quite a bit faster in their downrange, barge landing flights, but the first stage is still moving much slower than orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's still up around Mach 5 or 6, though, which produces a lot of heat. So the rocket fires three of its engines to slow down further before entering the thicker part of the atmosphere. The exhaust plume from that burn, as well, forces the atmospheric compression that creates reentry heat to occur well away from the rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The end result is that the heat load is light enough that the body of the rocket can survive it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a protective shell on the underside of the rocket, rather than a heavyweight ablative heat shield. The engine bells themselves are bearing the brunt of what reentry heat there is, and they are obviously able to cope with very high temperatures. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-02-15T01:26:07.193" LastActivityDate="2018-02-15T01:26:07.193" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13937" PostTypeId="2" ParentId="13913" CreationDate="2016-02-05T18:51:26.930" Score="-1" Body="&lt;p&gt;Launching two rockets would be insane. A rocket is about the size of a commercial airliner. When a commercial airliner uses the same runway, they wait a period of a few minutes between launch, for safety purposes (And to let the turbulence to settle down). The closest this has been done in real life was the Gemini/Agena launches happened. What they did was to launch one, then wait an orbit, then launch the second (See the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_8#Mission_parameters&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is really the only thing that makes sense. Dual launches is difficult for a multitude of reasons, which includes range safety, the boosters being too close to each other, debris fallout, collision, and quite a few other things. As in the movie it was ported to be done due to redundancy, waiting an orbit for the second to launch would have made much more sense. Or even better would be to simply launch each one when it was ready. One of the two would inevitably be ready a bit quicker, so concentrating on launching one, then concentrating on the other, would give the best chance of success. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-02-05T20:06:10.283" LastActivityDate="2016-02-05T20:06:10.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13938" PostTypeId="2" ParentId="13935" CreationDate="2016-02-05T18:58:15.090" Score="35" Body="&lt;p&gt;Here's an image of the bottom of the stage before launch.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/e5IjC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/e5IjC.jpg&quot; alt=&quot;F9 first stage&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, the entire bottom is covered in white panels. I suspect those panels are a heat shield.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.spacex.com/news/2013/10/14/upgraded-falcon-9-mission-overview&quot; rel=&quot;noreferrer&quot;&gt;SpaceX press release&lt;/a&gt; on the introduction of the Falcon 9 v1.1 refers to a heat shield. The reference is a bit oblique, but I think this refers to the first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the same area after a successful landing: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GZUts.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GZUts.jpg&quot; alt=&quot;after landing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The panels are now charred black, but are still intact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an &lt;a href=&quot;https://www.youtube.com/watch?v=riU3DZmU-jE&quot; rel=&quot;noreferrer&quot;&gt;infrared video&lt;/a&gt; that shows the rocket gets pretty hot - at one point it glows red-hot, and that's &lt;em&gt;before&lt;/em&gt; the engines are started for the reentry burn. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-02-05T20:36:03.647" LastActivityDate="2016-02-05T20:36:03.647" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13939" PostTypeId="1" AcceptedAnswerId="13959" CreationDate="2016-02-05T19:52:55.473" Score="5" ViewCount="2107" Body="&lt;p&gt;How would a constellation of GPS satellites differ for Mars? For Earth the system is supposed to work with at least 24 satellites orbiting at ~20.000 km. Would Martian equivalent need less of them? At what orbit height?&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-02-07T18:38:20.100" Title="GPS constellation for Mars" Tags="|mars|gps|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13940" PostTypeId="1" CreationDate="2016-02-05T20:08:49.973" Score="0" ViewCount="68" Body="&lt;p&gt;My question is similar to one concerning 3d printing for enhnaced performance with hybrid rockets, but concerns using it to improve performance of solid chemical rockets by 3d Printing the solid rocket fuel &#xA;into more efficient geometry,to maximize burn surface and thus allow for more precise and greater thrust control. I am wondering if any material on the subject exist and maybe were to find it?&lt;/p&gt;&#xA;" OwnerUserId="13342" LastEditorUserId="13342" LastEditDate="2016-02-06T14:45:50.197" LastActivityDate="2016-02-06T14:45:50.197" Title="Solid rocket boosters and 3d printing" Tags="|rockets|booster|" AnswerCount="0" CommentCount="3" ClosedDate="2016-02-06T15:20:10.570" ContentLicense="CC BY-SA 3.0" />
  <row Id="13941" PostTypeId="1" AcceptedAnswerId="13942" CreationDate="2016-02-05T20:10:37.130" Score="7" ViewCount="379" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_8#Mission_parameters&quot; rel=&quot;nofollow&quot;&gt;this Wikipedia article&lt;/a&gt;, the time to launch between the two missions was 1 hour 41 minutes (15:00 and 16:41 UTC, respectively), which is pretty much 1 orbit in LEO. That would give a nodal separation of about 15 degrees. How did the two missions manage to rendezvous? I suspect the answer lays that they didn't exactly copy each other's launch trajectory, but I'm curious what the thought process was to overcome this problem.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="23891" LastEditDate="2019-12-31T00:49:54.610" LastActivityDate="2019-12-31T00:49:54.610" Title="How did Gemini/Agena rendezvous?" Tags="|orbital-mechanics|launch|rendezvous|project-gemini|agena|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="13942" PostTypeId="2" ParentId="13941" CreationDate="2016-02-05T20:29:48.940" Score="8" Body="&lt;p&gt;A very detailed timeline is &lt;a href=&quot;http://history.nasa.gov/SP-4203/ch13-6.htm&quot;&gt;provided by NASA&lt;/a&gt;. Some of the key elements:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Agena was in a 299 km circular orbit.&lt;/li&gt;&#xA;&lt;li&gt;Gemini launched 1 orbit later.&lt;/li&gt;&#xA;&lt;li&gt;Gemini's trajectory was such that it was in a 160x272 km orbit. This was well below Agena, and was thus moving in nodal point, although the effect was small.&lt;/li&gt;&#xA;&lt;li&gt;Several maneuvers were done on orbit. The first was to more circularlize the orbit. The second was to adjust the nodal plane to match, which required an 8.5 m/s burn at precisely the right point in the orbit. This was done 90 degrees to the flight path, which would have corrected the discrepancy.&lt;/li&gt;&#xA;&lt;li&gt;The inclination was low to begin with, the nominal out of the Cape, which allowed for the low correction to plane shift.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Once this was accomplished, they were in the same plane, and the rest is straightforward rendezvous.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-05T20:29:48.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13943" PostTypeId="2" ParentId="13918" CreationDate="2016-02-05T20:57:11.263" Score="9" Body="&lt;p&gt;It should be possible. ESA has tested the rad-hardness of some cousin processors to that used in the Arduino and they turned out fairly well, at least for a relatively short mission. Some current stuff is actually using ancient 8051-architecture chips. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be enough processing power to do navigation, maybe even enough to stream out recorded fake pictures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: A powerpoint summarizing the ESA findings can be found &lt;a href=&quot;http://microelectronics.esa.int/conferences/mesa2010/06_S2_1120_ASTRIUM_GmbH_Andreas_Schuettauf.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Heavy Ion result for ATMega128&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The ATMega128-AU16 has an acceptable behavior for a ISS LEO environment.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;SEL once in 481 years&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;SEU once in 690 years&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The AT90CAN128-AU16 is not acceptable for space application.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The chip used in many Arduinos is the ATMega328, which is a cousin of the ATMega128. &lt;/p&gt;&#xA;" OwnerUserId="10976" LastEditorUserId="10976" LastEditDate="2016-02-06T18:18:07.180" LastActivityDate="2016-02-06T18:18:07.180" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13944" PostTypeId="1" AcceptedAnswerId="14081" CreationDate="2016-02-05T22:49:00.610" Score="6" ViewCount="1035" Body="&lt;p&gt;I know that we already have satellites in position at our Lagrange points, but what if we want to use them to park spacecraft sections for assembly reasons, or possibly even a meteorite for mining.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are the 10 points big/stable enough to have multiple objects stationed there without causing collisions?  If not for all 10, which ones?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we already have points that have more than one objects?&lt;/p&gt;&#xA;" OwnerUserId="13032" LastEditorUserId="13032" LastEditDate="2016-02-08T15:19:48.967" LastActivityDate="2016-02-12T21:17:14.040" Title="Are the Earth's 10 Lagrange points stable and large enough to park multiple satelites/space vessels" Tags="|artificial-satellite|spacecraft|lagrangian-points|" AnswerCount="2" CommentCount="22" ContentLicense="CC BY-SA 3.0" />
  <row Id="13946" PostTypeId="1" AcceptedAnswerId="13950" CreationDate="2016-02-06T11:39:54.680" Score="4" ViewCount="529" Body="&lt;p&gt;This &lt;a href=&quot;http://www.nasa.gov/feature/jpl/small-asteroid-to-pass-close-to-earth-march-5&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA webpage&lt;/a&gt; describes the close approach of asteroid 2013 TX68 on March 5, 2016 and shows a probability distribution of the point of closest approach based on the only data available at this moment - three or four days of measurements in October of 2013. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screenshot (Feb 5, 2016) from [&lt;a href=&quot;http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2013%20TX68;orb=1]&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2013%20TX68;orb=1]&lt;/a&gt; which I &lt;a href=&quot;http://neo.jpl.nasa.gov/ca/&quot; rel=&quot;nofollow noreferrer&quot;&gt;found here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UbuS0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UbuS0.png&quot; alt=&quot;screenshot of NASA webpage about 2013 TX68&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I tried to load DE431 into Skyfield out of curiosity to see if I can learn how to use these kinds of solutions (which can include substantial uncertainty sometimes) but I was stopped immediately by an error shown below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question: How to handle this error? And, more broadly, how to start using orbital solutions of asteroids in Skyfield?&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;&amp;gt;&amp;gt;&amp;gt; data421 = load('de421.bsp')  # this worked&#xA;&amp;gt;&amp;gt;&amp;gt; data431 = load('de431.bsp')  # few seconds delay, and then:&#xA;Traceback (most recent call last):&#xA;  File &quot;&amp;lt;stdin&amp;gt;&quot;, line 1, in &amp;lt;module&amp;gt;&#xA;  File &quot;/Users/yournamehere/anaconda2/anaconda/envs/myenv/lib/python2.7/site-packages/skyfield/iokit.py&quot;, line 43, in load&#xA;    download(url, path, verbose=verbose)&#xA;  File &quot;/Users/yournamehere/anaconda2/anaconda/envs/myenv/lib/python2.7/site-packages/skyfield/iokit.py&quot;, line 74, in download&#xA;    raise IOError('cannot get {1} because {2}'.format(url, e))&#xA;IndexError: tuple index out of range&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-02-08T14:03:22.897" LastActivityDate="2016-02-08T14:03:22.897" Title="Asteroid 2013 TX68 March 5, 2016 close approach, and calculating with Skyfield" Tags="|orbital-mechanics|asteroid|near-earth-object|skyfield|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="13947" PostTypeId="1" CreationDate="2016-02-06T16:12:24.063" Score="7" ViewCount="1714" Body="&lt;p&gt;This &lt;a href=&quot;http://map.gsfc.nasa.gov/mission/observatory_l2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; website states &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;NASA is unlikely to find any use for the L3 point since it remains hidden behind the Sun at all times. The idea of a hidden &quot;Planet-X&quot; at the L3 point has been a popular topic in science fiction writing. The instability of Planet X's orbit (on a time scale of 150 years) didn't stop Hollywood from turning out classics like The Man from Planet X.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;where &quot;The Man from Planet X&quot; links to &lt;a href=&quot;http://www.imdb.com/title/tt0043778/&quot; rel=&quot;nofollow noreferrer&quot;&gt;IMDB: The Man from Planet X (1951)&lt;/a&gt;. Since Planet X supposedly passes by the earth &lt;a href=&quot;https://scifi.stackexchange.com/questions/117611/in-the-man-from-planet-x-was-planet-x-ever-directly-behind-the-sun-w-same-orb/117617#117617&quot;&gt;after leaving it's own sun&lt;/a&gt;, this should probably point to &lt;a href=&quot;http://www.imdb.com/title/tt0064519/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Journey to the Far Side of the Sun (1969)&lt;/a&gt; instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sake of discussion of the question of stability: if there &lt;em&gt;hypothetically&lt;/em&gt; were a planet, with say a mass roughly equivalent to earth, then would it really be unstable in the same way that a low-mass object would be unstable at Earth's L3? And wouldn't that mean that both it and earth would be similarly unstable? Or does this only refer to the idea that it &lt;em&gt;could't remain hidden behind the sun&lt;/em&gt; for hundreds of years, as a very old calculation summarized on &lt;a href=&quot;http://files.ncas.org/condon/text/appndx-e.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this webpage&lt;/a&gt; which I found &lt;a href=&quot;https://en.wikipedia.org/wiki/Counter-Earth#Scientific_analysis&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; seems to suggest. Has this calculation been repeated and reported more recently?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; this all stared from a conversation below &lt;a href=&quot;https://space.stackexchange.com/q/13944/12102&quot;&gt;this question&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note 2:&lt;/strong&gt; while this question is about &lt;em&gt;the statement about stability of a planet opposite earth&lt;/em&gt;, from &lt;a href=&quot;https://scifi.stackexchange.com/a/117617/51174&quot;&gt;this answer&lt;/a&gt; it seems that the quote above should point to &lt;a href=&quot;http://www.imdb.com/title/tt0064519/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Journey to the Far Side of the Sun (1969)&lt;/a&gt; and &lt;strong&gt;not&lt;/strong&gt; point to &lt;a href=&quot;http://www.imdb.com/title/tt0043778/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Man from Planet X (1951)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-06-14T20:55:59.170" Title="Why would Hollywood's Planet X (at earth's L3) be unstable?" Tags="|orbital-mechanics|lagrangian-points|planet|myth-busting|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13948" PostTypeId="2" ParentId="13932" CreationDate="2016-02-06T17:33:48.880" Score="5" Body="&lt;p&gt;The Shuttle had some unique capabilities that would have been difficult or expensive to replicate using expendable rockets. The combination of accommodation for 8 people plus the ability to carry ~20 tons of cargo enabled missions like Spacelab and the Hubble service missions. The Shuttle also served as a construction base for the ISS.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other missions (satellite launches) would have been cheaper to do on an expendable rocket. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-06T17:33:48.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13950" PostTypeId="2" ParentId="13946" CreationDate="2016-02-06T20:57:36.187" Score="1" Body="&lt;p&gt;Although the universe does revolve around me, I'm assuming you meant the barycentric celestial reference system :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HORIZONS will give you these elements if you use these settings:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZksyE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZksyE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user7073" LastActivityDate="2016-02-06T20:57:36.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13951" PostTypeId="1" AcceptedAnswerId="13952" CreationDate="2016-02-06T23:43:57.627" Score="8" ViewCount="3396" Body="&lt;p&gt;Is it possible for ordinary table salt (sodium chloride) to form on a lifeless planet / moon / comet / other body? Is liquid water required? Would it be common or nearly-impossibly rare? Part of &lt;a href=&quot;http://www.goodreads.com/review/show/26826589&quot; rel=&quot;nofollow&quot;&gt;a novel I'm reading&lt;/a&gt; takes place in a salt mine on a moon with liquid water (similar to current theories about Europa.)&lt;/p&gt;&#xA;" OwnerUserId="8641" LastEditorUserId="1142" LastEditDate="2016-02-07T14:56:16.707" LastActivityDate="2016-02-07T14:56:16.707" Title="Is there salt in space?" Tags="|planetary-science|chemistry|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13952" PostTypeId="2" ParentId="13951" CreationDate="2016-02-07T01:02:51.587" Score="10" Body="&lt;p&gt;Chlorine is a relatively rare element, so the concentrations of it is not going to be high anywhere. It is also very reactive, so all of it is likely to only occur as a part of different chemical compounds. Chlorine is especially likely to react with sodium as it one of the most common elements, and also has a very low electronegativity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That makes sodium chloride a very &lt;a href=&quot;https://en.wikipedia.org/wiki/Chemical_polarity&quot;&gt;polar&lt;/a&gt; compound, and since water is also a polar molecule, salt dissolves easily in water. Water flowing over the crust of a planet for thousands of years slowly extract the salt from the ground, and can leave it as concentrated deposits when it evaporates. Thus liquid water is required for any significant concentrations of salt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This creates a border between the worlds that have liquid water (or had, &lt;a href=&quot;http://www.americaspace.com/?p=33382&quot;&gt;Mars has salt deposits&lt;/a&gt;), and those that do not. One notable exception is the &lt;a href=&quot;http://www.colorado.edu/news/releases/1999/06/02/chlorine-discovery-near-jupiter-moon-hints-salt-presence-surface&quot;&gt;chlorine detected in the erupted material from Io&lt;/a&gt;, making table salt at its surface a possibility.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-07T01:02:51.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13953" PostTypeId="1" CreationDate="2016-02-07T09:48:01.647" Score="12" ViewCount="7954" Body="&lt;p&gt;Do they use a tungsten wires or something else? &lt;/p&gt;&#xA;" OwnerUserId="13366" LastEditorUserId="189" LastEditDate="2016-02-07T14:52:45.010" LastActivityDate="2018-02-13T17:46:53.977" Title="How do the Falcon 9 engines re-ignite?" Tags="|spacex|falcon-9|engines|merlin-1d|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13954" PostTypeId="2" ParentId="13953" CreationDate="2016-02-07T10:22:27.320" Score="19" Body="&lt;p&gt;Falcon 9 uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Triethylaluminium&quot; rel=&quot;noreferrer&quot;&gt;TEA&lt;/a&gt;-&lt;a href=&quot;https://en.wikipedia.org/wiki/Triethylborane&quot; rel=&quot;noreferrer&quot;&gt;TEB&lt;/a&gt; to ignite its engines both on launch and during all the landing burns. The mixture is &lt;a href=&quot;https://en.wikipedia.org/wiki/Pyrophoricity&quot; rel=&quot;noreferrer&quot;&gt;pyrophoric&lt;/a&gt; - it ignites spontaneously when it comes to contact with oxygen (both in air or the liquid supercooled).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket carries &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3xieex/falcon_9_launch_and_landing_infographic/cy4xxdw&quot; rel=&quot;noreferrer&quot;&gt;limited&lt;/a&gt; amount of the stuff so the number of possible re-ignitions is limited too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently it needs to re-ignite 3 engines for the boostback burn (when doing RTLS or limited downrange landing - usually for LEO missions, GTO ones do not execute this burn), 3 again for the re-entry burn and 1 or 3 for the landing. That means it should have enough for 7-9 uses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second stage may need to reignite its engines too, depending on the mission so it needs to carry some amount of the igniter fluids too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the first launch of Falcon Heavy there was some problem on the center core which caused &lt;a href=&quot;https://twitter.com/elonmusk/status/963107229523038211&quot; rel=&quot;noreferrer&quot;&gt;the outer two engines to run out of igniter fluid&lt;/a&gt; and not start for the landing burn, meaning the core was not able to slow down enough and hit the water at about 500 km/h (~300 mph). The cause is currently unknown or at least unpublished - possibilities including bad valve, human error or some FH specifics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I gathered reading various comments about that incident, it seems that there is no specific &lt;em&gt;&quot;dosage&quot;&lt;/em&gt; of  the TEA-TEB mixture per engine startup, instead the mixture is allowed to flow to the engine until proper startup is detected. But I do not know about any official source for this and it may be just wrong understanding on my side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another speculation on my part: because SpaceX uses 1-3 or 1-3-1 scheme for multi-engine landing burns and according to currently public information &lt;em&gt;both&lt;/em&gt; outer engines failed to ignite, it seems reasonable that there is just one common reservoir of the TEA-TEB mixture per stage, supplying all 3 restartable engines, instead of a reservoir per engine. That way the center engine would start fine because it was first to get started during the burn, but some problem (possibly greater airflow because of heavier and thus faster center core?) made it use more fluids for the last ignition, leaving just scraps for the outer engines.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2018-02-13T17:46:53.977" LastActivityDate="2018-02-13T17:46:53.977" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13955" PostTypeId="2" ParentId="13946" CreationDate="2016-02-07T11:30:28.563" Score="1" Body="&lt;p&gt;Asteroid TX 68 2013 will pass near the earth on March 5 , 2016&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/qoqTPJc3qbs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="13367" LastActivityDate="2016-02-07T11:30:28.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13956" PostTypeId="1" AcceptedAnswerId="13958" CreationDate="2016-02-07T14:32:11.077" Score="8" ViewCount="1994" Body="&lt;p&gt;I'm a software developer and I'm trying to move towards astrophysics. I wanted to create an accurate solar system model using real-time accurate data. I've found the NASA data site, however I can only find data for small bodies, nothing for the planets and their moons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anyone come across such a data set before? &lt;/p&gt;&#xA;" OwnerUserId="13369" LastEditorUserId="8693" LastEditDate="2016-02-07T14:50:27.750" LastActivityDate="2016-02-08T02:19:29.743" Title="Is there free, accurate and updated data on planetary body positions in the solar system?" Tags="|orbital-mechanics|reference-request|solar-system|scientific-data|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13957" PostTypeId="2" ParentId="13939" CreationDate="2016-02-07T15:14:22.987" Score="5" Body="&lt;p&gt;From the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler&amp;#39;s_laws_of_planetary_motion&quot; rel=&quot;noreferrer&quot;&gt;3rd Kepler's law&lt;/a&gt; you can find that the orbit radius for the satellite in a circle orbit can be found as &#xA;$$&#xA;R = \frac{G^{1/3}}{(2\pi)^{2/3}} T^{2/3} M_{planet}^{1/3}&#xA;$$&#xA;So if you take $G=6.67 \times 10^{-11}$ SI units, $M = 6.4171\times 10^{23}$ kg, and, like GPS satellites, $T$ is equal to half of the Martian sidereal day (which is 24h 37m 22s or 86644 seconds), you finally get&#xA;$$ &#xA;R = 12672~\text{km}&#xA;$$&#xA;from the center of the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also means that the orbit should be about $9282~\text{km}$ above the &lt;em&gt;average&lt;/em&gt; surface of Mars. In case of Earth, such an orbit is more than twice as far out since Earth is more massive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The number of satellites may depend on actual configuration and requirements for the minimum number of satellites visible from a certain point of surface (if you need 24/7 performance), but it can't be much less than the same in case of Earth. I believe something about 18 (6 in three orbital planes) is a necessary minimum.&lt;/p&gt;&#xA;" OwnerUserId="13370" LastEditorUserId="6816" LastEditDate="2016-02-07T15:24:10.133" LastActivityDate="2016-02-07T15:24:10.133" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13958" PostTypeId="2" ParentId="13956" CreationDate="2016-02-07T15:59:56.797" Score="11" Body="&lt;p&gt;For integration into software, I would recommend the &lt;a href=&quot;https://naif.jpl.nasa.gov/naif/toolkit.html&quot;&gt;SPICE toolkit&lt;/a&gt;, available with interfaces for C, Fortran, IDL, and MATLAB, and the &lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/&quot;&gt;many SPK kernels that can be loaded into SPICE&lt;/a&gt; containing the most accurate ephemerides available for the planets and their satellites. For specific small bodies you can use the &lt;a href=&quot;http://ssd.jpl.nasa.gov/?horizons&quot;&gt;HORIZONS system&lt;/a&gt; to generate SPK kernels, or you can use a generic SPK from the previous link with 300 small bodies at lower accuracy.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-02-07T19:11:06.167" LastActivityDate="2016-02-07T19:11:06.167" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13959" PostTypeId="2" ParentId="13939" CreationDate="2016-02-07T18:38:20.100" Score="6" Body="&lt;p&gt;GPS requires four satellites being visible from any point on the surface at any time (one less if the receivers carry a high-precision atomic clock). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a perfect spherical world this can (theoretically) be provided by eight satellites in a very high orbit - from far away every observer can see exactly half of the planet. This constellation would use four different orbits with two satellites positioned opposite to each other in each orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;High orbits might not be favorable due to the large distance to the receivers and accordingly high transmitting power needed and also larger measurement errors. Additionally, satellites that are barely visible above the horizon can be obstructed by mountains (do exist on Mars) or high buildings (don't exist on Mars yet). So, one might want to place 3 satellites in each orbit and use more than four different orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A GPS that is available at any time at any point of the surface and that guarantees a minimum elevation (10 degrees according to my very rough estimate) above the horizon can likely be built with about 15 satellites (e.g. 5 orbits with 3 satellites each). &#xA;The 24 satellites used for the Earth-GPS are due to lower orbits, high minimum elevation (atmosphere is thick at low elevation angles causing large errors) and redundancy. Note that four satellites visible is the absolute minimum to do any position estimation. A fifth one is already needed to make any estimation on precision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Mars I'd assume one is fine with a lower precision than on Earth and one does not want to spend too many satellites and hence goes with a lower count of satellites in higher orbits. 15 seem likely to me. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that in no calculation the size of the planet plays a role - it's only the ration between planetary radius and orbital radius that determines the area a satellite can be seen from.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-02-07T18:38:20.100" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13961" PostTypeId="2" ParentId="8124" CreationDate="2016-02-07T19:00:32.107" Score="3" Body="&lt;p&gt;&lt;strong&gt;Solar arrays&lt;/strong&gt; ... as the other answers mentioned, its not worth improving over a few degrees. There is an exception of course, for particularly high power satellites (20kW) it might be worthwhile.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Antennas&lt;/strong&gt; Its worth adding to the other answers that the idea of &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2006-5304&quot; rel=&quot;nofollow noreferrer&quot;&gt;&amp;quot;RF autotrack&lt;/a&gt;&amp;quot; exists. This basically means the needs to be a feedback loop that adjusts the beam pointing according to how well centred the beam is. In principle there are lots of ways of creating this. Geostationary communications satellites with narrow spot beams have used an on board system, for example as mentionend in the link, that monitors a ground based beacon sited within the antenna pattern.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2022-03-16T21:05:33.217" LastActivityDate="2022-03-16T21:05:33.217" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13962" PostTypeId="1" AcceptedAnswerId="13965" CreationDate="2016-02-07T19:21:33.680" Score="10" ViewCount="443" Body="&lt;p&gt;I can not find any reference for this, but it seems to me that &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot;&gt;Skylab&lt;/a&gt;, unlike the International Space Station, had a fixed attitude instead of rotating to have the same side always facing the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reasons I suspect that was the case:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;On all the photographs I can find of the station, the unscrambled solar panel of the station and the panels of the Apollo service module are pointing in the same fixed direction.&lt;/li&gt;&#xA;&lt;li&gt;The provisional sunshade was installed at the same side as the solar panels was pointing.&lt;/li&gt;&#xA;&lt;li&gt;It had a telescope mount for observation of the Sun and the sky, something the ISS does not (the ISS only does Earth observations). That would not have been possible if the station was rotating relative to the sky.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Did Skylab actually have a fixed orientation relative to the Sun? I can not verify if that is true or not.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-07T19:59:15.007" LastActivityDate="2016-02-07T21:09:34.950" Title="Did Skylab have a fixed orientation?" Tags="|history|attitude|space-station|skylab|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13963" PostTypeId="1" CreationDate="2016-02-07T19:55:50.667" Score="1" ViewCount="248" Body="&lt;p&gt;For about 15 years the astronauts are doing scientific research in the ISS for future space population and traveling.The wide variety of research fields include astrobiology, astronomy, human research including space medicine and life sciences, physical sciences, materials science, space weather, and weather on Earth. But what till now are the most remarkable results of this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps one important example of each field can be mentioned?&lt;/p&gt;&#xA;" OwnerUserId="13303" LastEditorUserId="13303" LastEditDate="2016-02-07T20:24:07.560" LastActivityDate="2016-02-07T23:32:09.213" Title="What are the major results of ISS experiments?" Tags="|iss|" AnswerCount="2" CommentCount="1" ClosedDate="2016-02-08T00:09:51.813" ContentLicense="CC BY-SA 3.0" />
  <row Id="13964" PostTypeId="2" ParentId="13963" CreationDate="2016-02-07T20:16:07.230" Score="3" Body="&lt;p&gt;Since you ask a broad question, you get a broad answer. All the publications and results of experiments run on the ISS to date are available &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/Publications.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE: A shorter list of highlights and Utilisation Reports up to August 2014 are &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/research_library_rev1_no_proposals2014(7).pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; from this &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/overview.html&quot; rel=&quot;nofollow&quot;&gt;site&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Happy Hunting!&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2016-02-07T21:24:17.227" LastActivityDate="2016-02-07T21:24:17.227" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13965" PostTypeId="2" ParentId="13962" CreationDate="2016-02-07T21:09:34.950" Score="9" Body="&lt;p&gt;Yes, &lt;a href=&quot;http://history.nasa.gov/SP-400/ch2.htm&quot;&gt;here&lt;/a&gt; it is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Two precise control systems permitted the astronauts to orient the workshop to collect experimental data and to position Skylab so that its solar arrays faced the Sun.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That means that the solar panels were fixed, and that the whole station was oriented so they faced the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This kind of rotation is &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19730032401&quot;&gt;quasi-inertial&lt;/a&gt;, as fixed relative to the Sun is not the same as fixed relative to the stars. The difference is however just one rotation a year, meaning that it is easy to switch between the two different modes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that the Apollo Telescope Mount could &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950003609.pdf&quot;&gt;rotate independently&lt;/a&gt; to  track an object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More on the Skylab space station can be found &lt;a href=&quot;http://viking.coe.uh.edu/~gkitmacher/_content/spacecraft/Space%20Stations%20folder/skylab.pdf&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-07T21:09:34.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13966" PostTypeId="1" AcceptedAnswerId="13971" CreationDate="2016-02-07T21:47:37.820" Score="9" ViewCount="432" Body="&lt;p&gt;If you compare the pressurized volume of all the space stations launched to their mass, their volume-efficiency (in this context volume divided by mass) seem to decrease over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In chronological order:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Salyut_1&quot;&gt;Salyut:&lt;/a&gt;&lt;/strong&gt; (actually seven stations, but practically identical. Salyut 1 numbers are used)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pressurized volume: 99 m³&lt;/li&gt;&#xA;&lt;li&gt;Mass     18,425 kg  &lt;/li&gt;&#xA;&lt;li&gt;Cubic meters per metric ton: &lt;strong&gt;5.37&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot;&gt;Skylab:&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pressurized volume: 319.8 m³&lt;/li&gt;&#xA;&lt;li&gt;Mass     77,088 kg&lt;/li&gt;&#xA;&lt;li&gt;Cubic meters per metric ton: &lt;strong&gt;4.15&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mir&quot;&gt;Mir:&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pressurized volume: 350 m³&lt;/li&gt;&#xA;&lt;li&gt;Mass     129,700 kg&lt;/li&gt;&#xA;&lt;li&gt;Cubic meters per metric ton: &lt;strong&gt;2.70&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot;&gt;International Space Station:&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pressurized volume: 916 m³&lt;/li&gt;&#xA;&lt;li&gt;Mass     390,378 kg&lt;/li&gt;&#xA;&lt;li&gt;Cubic meters per metric ton: &lt;strong&gt;2.35&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tiangong-1&quot;&gt;Tiangong:&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Pressurized volume: 15 m³&lt;/li&gt;&#xA;&lt;li&gt;Mass     8,506 kg&lt;/li&gt;&#xA;&lt;li&gt;Cubic meters per metric ton: &lt;strong&gt;1.76&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What is going on? This is quite the opposite of what one would expect from technological improvements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the reason for the volume-efficiency of space stations dropping when the overall technological progression improves?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-07T22:16:36.687" LastActivityDate="2016-02-08T23:21:27.867" Title="Why have space stations become less volume-efficient despite technological progression?" Tags="|crewed-spaceflight|design|technology|space-station|mass|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="13967" PostTypeId="1" CreationDate="2016-02-07T22:12:44.993" Score="4" ViewCount="1006" Body="&lt;p&gt;I've had an idea floating in my head recently and would love if someone more knowledgeable with physics or engineering could review it/debunk the idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been many ideas related to using balloons as launch platforms, but what about a small 'rocket' that utilizes the gas from a hydrogen balloon as a fuel to reach orbital velocity, once it's out of the dense atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea, if at all viable, would only make sense for tiny payloads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main challenges in this idea seem to be:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Is there enough energy in the hydrogen gas to begin with?  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Can you convert the gas into a liquid state quickly, in flight?  What kind of mechanism could be used and what would be the minimum mass of it?  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Would you need to carry up liquid oxidizer?  I'm assuming a liquid-fuel rocket, but perhaps there are better options?  (Something utilizing hydrogen, or maybe helium).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is there a current technology that can use the hydrogen in it's gaseous form, combined with captured atmospheric oxygen, as a propulsion mechanism?  (Avoiding the need for conversion all together).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So to those of you who understand the physics and engineering, is this possible -- or practical?  Thanks!&lt;/p&gt;&#xA;" OwnerUserId="13376" LastEditorUserId="13376" LastEditDate="2016-02-07T22:22:50.510" LastActivityDate="2016-02-08T00:05:20.780" Title="Hydrogen balloon 'rocket' that uses the hydrogen as fuel?" Tags="|rockets|balloons|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13968" PostTypeId="1" AcceptedAnswerId="13973" CreationDate="2016-02-07T23:28:00.077" Score="5" ViewCount="2701" Body="&lt;p&gt;I have a GPS module which outputs NMEA messages at 1Hz. In the data sheet it is mentioned that at cold start, it will get a fix in 29s. I run it cold start and indeed it gets the fix in 29th second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However in around 12 seconds, I start getting the time in the GPRMC and GPGGA and GPGLL. Also after a few more seconds I get the exact date in GPRMC.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is it possible that without even a fix,(which is supposed to happen after complex calculations and we get time and lat/long value in 29s) I am getting this time and date?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does everything is not calculated simultaneously?&lt;/p&gt;&#xA;" OwnerUserId="13377" LastActivityDate="2016-02-08T00:21:20.593" Title="How does GPS module gets time even before a fix?" Tags="|gps|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="13969" PostTypeId="2" ParentId="13963" CreationDate="2016-02-07T23:32:09.213" Score="3" Body="&lt;p&gt;In 2013 ISS chief scientist Julie Robinson listed the Top 10 ISS Research Results.  So in the project's own assessment, these were the most important as of 2013. They are linked &lt;a href=&quot;http://spaceksc.blogspot.com/2013/11/the-iss-top-ten.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; so I won't copy the links but here are the subjects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I find this list kind of disappointing (especially # 4 which isn't a research result at all) but perhaps great progress has been made since 2013!&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;New Targeted Method of Chemotherapy Drug Delivery; Clinical Breast Cancer Trials Now in Development&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Robotic Assist for Brain Surgery&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Dark Matter is Still Out There&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;43 Million Students and Counting&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Pathway for Bacterial Pathogens to Become Virulent&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;New Process of “Cool Flame” Combustion&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Colloid Self Assembly Using Electrical Fields for Nanomaterials&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Hyperspectral Imaging for Water Quality in Coastal Bays&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Understanding Mechanisms of Osteoporosis and New Drug Treatments&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Preventing Loss of Bone Mass in Space Through Diet and Exercise &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-02-07T23:32:09.213" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13970" PostTypeId="2" ParentId="13967" CreationDate="2016-02-07T23:32:20.567" Score="2" Body="&lt;p&gt;The primary problem is the amount of hydrogen we're talking about. LH2 density is 70.8 kg/m³. Gaseous hydrogen at sea level is 0.09 kg/m³. Take a good high-altitude balloon like &lt;a href=&quot;https://en.wikipedia.org/wiki/Explorer_II&quot; rel=&quot;nofollow&quot;&gt;Explorer II&lt;/a&gt;, it has 100,000 m³ of volume, you're loading up only 9 tons of hydrogen - at sea level! And if you want it to remain buoyant (and not to burst) you must vent hydrogen as you go, so that the density inside the balloon remains lower than density of the surrounding air! (and the pressure not much higher).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's for vehicle (payload+oxidizer+balloon+etc.) of 6,800 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you need means to vent that hydrogen from the balloon to the engine. You still need oxidizer, optimally in mass ratio of about 6:1 relative to hydrogen. You need some means of driving the hydrogen out of the balloon fast, or it becomes a half-filled rag dragging after your rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...and a whole slew of other headaches. I'm not going to try to dig any deeper - what you have already should be enough to show how impractical it is.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-07T23:32:20.567" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13971" PostTypeId="2" ParentId="13966" CreationDate="2016-02-08T00:01:54.820" Score="8" Body="&lt;p&gt;The International Space Station is a far more capable vehicle than any of its predecessors. This comes at a cost, and that cost is mass. There's a problem here: Doubling the electrical power more than doubles the mass needed to produce that power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things don't scale linearly. You can see this in the animal kingdom. Given equal sized images of the skeletons of an elephant and a mouse, you can immediately tell which is which. Scaling a mouse to the size of an elephant would not work. An elephant-sized mouse would fail under its own weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the ISS does not need to sustain its own weight, it does need to withstand things such as thermal stresses. A flimsy tin can cannot scale up to the size of the ISS, without structural changes. Another name for this is the cube-square law.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="8693" LastEditDate="2016-02-08T23:21:27.867" LastActivityDate="2016-02-08T23:21:27.867" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13972" PostTypeId="2" ParentId="13967" CreationDate="2016-02-08T00:05:20.780" Score="2" Body="&lt;p&gt;For a better use of that hydrogen, you can use a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;nofollow&quot;&gt;Nuclear thermal rocket&lt;/a&gt;. They can use both hydrogen and helium, without the use of any oxidizer. They even gives you a better efficiency than a chemical engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that escaping the atmosphere is only a tiny fraction of the way to an orbit, so the rocket has to be substantial even with your proposed scheme. Over all, you only get a small gain from a far more complicated design.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-08T00:05:20.780" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13973" PostTypeId="2" ParentId="13968" CreationDate="2016-02-08T00:11:19.323" Score="8" Body="&lt;p&gt;Each satellite broadcasts time. Only one is needed to determine the time with accuracy to distance to the satellite in light seconds - under 0.088s of error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you need at least three, preferably more satellites to get the location fix and the high-precision time (adjusted for distance from the satellite = time it takes for the signal to reach you.) As your GPS obtains more signals, it calculates its position (fix) and adjusts the clock, but the initial first broadcast is enough to establish timestamps on messages.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-02-08T00:21:20.593" LastActivityDate="2016-02-08T00:21:20.593" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="13974" PostTypeId="2" ParentId="13956" CreationDate="2016-02-08T02:04:46.927" Score="2" Body="&lt;p&gt;The quickest way to get your feet wet is to use the python package &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt; which can load and read ephemerides for you and give you both (x, y, z) positions in space and relative apparent positions (altitude, azimuth) observed from a point on earth, corrected for the finite speed of light. But if you want to get in deeper, the SPICE toolkit mentioned in the other answers may be more what you want. Just for example, the answers to &lt;a href=&quot;https://space.stackexchange.com/q/10490/12102&quot;&gt;this question&lt;/a&gt; may be worth reading.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual ephemerides can be found &lt;a href=&quot;ftp://ssd.jpl.nasa.gov/pub//eph/planets/ascii/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. These are produced by fitting to a large body of observational data - mostly optical, radar, and telemetry from exploration satellites where available, and then integrating equations of motion with high precision and attention to detail, forwards and backwards in time (future and past). They contain positions at fixed time intervals, and coefficients so you can interpolate positions to any point in time between to the same level of accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FREE: Yes! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ACCURATE: In a word, Yes, but it really depends what you mean by &quot;accurate&quot;, and which body in which ephemeride. For practical purposes - &quot;&lt;em&gt;where are the planets right now, and where were they and where will they be?&lt;/em&gt;&quot; they are all good, and you can choose one that is not too large and covers the time span and list of bodies you need. If you are navigating a satellite close fly-by of a planet or moon, or predicting a planetary occultation of a star seen from a point on earth, then you should look deeper into the various accuracies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATED: These are not updated, they are fixed, published tables, that apply to any time within their range. If something happens in the solar system - for example a previously undiscovered comet visits and hits a small moon of a planet and changes its orbit, the existing ephemerides will not be updated to reflect that. Of course a revision may be published as a new ephemeride at some point - or some method or &quot;patch&quot; to handle those moons may be issued - that's another question altogether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For artificial satellites, it's a whole different story. They are frequently observed and their orbits are best described using the most recently published set of two-line elements, which is accurate for a short period of time. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-02-08T02:19:29.743" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13975" PostTypeId="1" AcceptedAnswerId="16757" CreationDate="2016-02-08T02:34:05.270" Score="9" ViewCount="749" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/q/13939/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/13959/12102&quot;&gt;answer&lt;/a&gt; has got me thinking. Would one or more ground stations be necessary (or even just very beneficial) for a GPS-like system to operate around Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmospheric issues are much smaller, but is a planet-fixed set of clocks still required or at least of strong benefit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: I'm thinking that &lt;em&gt;if a ground station is necessary computationally&lt;/em&gt;, then as long as the satellites could communicate amongst themselves in some way, they could construct a mathematical (virtual) ground station and solve for its location without it actually existing. Wouldn't the solution be unique? If not, what am I missing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But &lt;em&gt;if it's necessary observationally&lt;/em&gt; - I mean - if it's a fixed radar station measuring the position of the satellites, I still don't understand why they can't simply measure each other. It still seems like it would be sufficiently constrained to have a unique solution for all of their orbital elements. What is the unknown that a ground station is needed to find?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2020-08-08T10:20:15.297" Title="Would a GPS type system on Mars really need a ground station?" Tags="|mars|gps|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="13976" PostTypeId="1" CreationDate="2016-02-08T04:13:11.757" Score="1" ViewCount="93" Body="&lt;p&gt;New Horizons has sent back some amazing, unexpected photos of Pluto.  I'm sure there are people who would love to build a larger version of &lt;a href=&quot;https://space.stackexchange.com/a/183/898&quot;&gt;Hubble&lt;/a&gt; that could continue imaging Pluto at a similar resolution to what &lt;a href=&quot;https://space.stackexchange.com/a/9935/898&quot;&gt;New Horizons did with LORRI&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring any impracticalities of actually funding and launching such a telescope, how big would it need to be?&lt;/p&gt;&#xA;" OwnerUserId="898" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-02-08T04:18:47.483" Title="How powerful a telescope would be needed to equal New Horizons'?" Tags="|new-horizons|pluto|telescope|" AnswerCount="0" CommentCount="3" ClosedDate="2016-02-08T17:04:17.210" ContentLicense="CC BY-SA 3.0" />
  <row Id="13977" PostTypeId="2" ParentId="13946" CreationDate="2016-02-08T06:04:51.547" Score="1" Body="&lt;p&gt;The error &lt;code&gt;IndexError: tuple index out of range&lt;/code&gt; is simply because I mistyped the two indexes in the error message in that source file. Python is zero-indexes so I ought to have typed:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;IOError('cannot get {0} because {1}'.format(url, e))&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Instead, as you can see, I seem to have produced the utterly wrong:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;IOError('cannot get {1} because {2}'.format(url, e))&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;This fix will be in the next version of Skyfield (and thanks for pointing out the problem you ran into!). You can edit the source file by hand for now if you want to see the error message which your code is trying to print, which is:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;OSError: cannot get ftp://ssd.jpl.nasa.gov/pub/eph/planets/bsp/de431.bsp ...&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The problem is that no &lt;code&gt;de431.bsp&lt;/code&gt; exists in that directory:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;ftp://ssd.jpl.nasa.gov/pub/eph/planets/bsp/&quot; rel=&quot;nofollow&quot;&gt;ftp://ssd.jpl.nasa.gov/pub/eph/planets/bsp/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe you intended to name the &lt;code&gt;de431t.bsp&lt;/code&gt; file? (In which case you must have more disk space free than I do — it’s 3.4 GB in size!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that the asteroid approach is in 2016, you can use the much smaller DE430 ephemeris. And unless you are trying to predict its location to within, say, 1 meter, you should be able to use the much smaller ephemerides DE421 or DE405 without any problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do you have a source for an ephemeris for the asteroid? Or will you be wanting to produce one from orbital elements?&lt;/p&gt;&#xA;" OwnerUserId="8590" LastActivityDate="2016-02-08T06:04:51.547" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13979" PostTypeId="2" ParentId="4295" CreationDate="2016-02-08T10:26:13.677" Score="6" Body="&lt;p&gt;Using LEGO is also an option:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://lego.wikia.com/wiki/7467_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;7467-1: International Space Station&lt;/a&gt;: a &lt;em&gt;Discovery&lt;/em&gt; set of 162  pieces, released in 2003.&lt;/li&gt;&#xA;&lt;li&gt;A &lt;a href=&quot;https://ideas.lego.com/projects/121473&quot; rel=&quot;noreferrer&quot;&gt;small ISS&lt;/a&gt; proposed on &lt;a href=&quot;https://ideas.lego.com&quot; rel=&quot;noreferrer&quot;&gt;ideas.lego.com&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;A &lt;a href=&quot;https://ideas.lego.com/projects/61057&quot; rel=&quot;noreferrer&quot;&gt;bigger ISS version&lt;/a&gt; also proposed on &lt;a href=&quot;https://ideas.lego.com&quot; rel=&quot;noreferrer&quot;&gt;ideas.lego.com&lt;/a&gt;, and it got the required 10,000 supporters, but it was rejected by the reviewers.&lt;/li&gt;&#xA;&lt;li&gt;There are also &lt;a href=&quot;http://www.collectspace.com/news/news-010815a-lego-minifigures-space-station.html&quot; rel=&quot;noreferrer&quot;&gt;LEGO minifigures&lt;/a&gt; of astronauts on the space station.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The official models of the ISS for NASA were created by &#xA;&lt;a href=&quot;http://www.lifeinscale.net/ISS_model_story.asp&quot; rel=&quot;noreferrer&quot;&gt;Gamla Model Makers&lt;/a&gt;. They provided several 1:100 scale models of the International Space Station for their Headquarter, and Space Centers. They, however, took more than a year to create the model, and they are &lt;a href=&quot;http://www.lifeinscale.net/mm_terms.asp&quot; rel=&quot;noreferrer&quot;&gt;not cheap&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="11962" LastActivityDate="2016-02-08T10:26:13.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13980" PostTypeId="2" ParentId="13876" CreationDate="2016-02-08T12:41:34.990" Score="5" Body="&lt;p&gt;Let me try. I'm not sure if I'm right so take everything with a grain of salt, but that's how I'd approach it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of trying to find the precise eclipses that will occur (based on LTAN) and trying to adapt to their duration, I'd choose a worst scenario eclipse: the satellite moving in the same direction as the Moon, as result staying in the shadow the longest, and also, with Moon at the perigee, resulting in largest umbra diameter and largest Moon linear velocity (again prolonging the eclipse).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ErqF8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ErqF8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's some simple trigonometry to obtain the diameter of umbra and penumbra at Earth's terminator line distance, with Moon at perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest of the calculations happen in the orbital plane of the satellite, or the plane of terminator, or plane normal to the Sun-Moon line, at Earth's distance - which one is pretty insignificant as they are all very close to each other and differing by cosine of angle between them -&gt; minimal error. Let's assume they are all the same plane so that you don't need to e.g. angle the umbra cross-section.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculate the curve of the trajectory of the satellite (vs time) in the reference frame of the (moving) center of umbra in that plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you have your optimization task:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xUitw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xUitw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculate the density of penumbra, say, in range [0,1] 1=full sun, 0=full shadow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will be awfully nonlinear - decreasing from outer edge to umbra edge with surface of circle partially obscured by another circle (relative displacement of the circles proportional to distance through penumbra, diameters of the circles proportional to angular size of the Sun and the (Super-)Moon as seen from Earth respectively). And a flat zero through Umbra. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Multiply by solar irradiation constant. That way you obtain the irradiation density field. Again, multiply by panel efficiency and you have your power input density field. Subtract your power output (constant) from the whole field function, then discard the positive part (more input than output while the batteries are full, power wasted). You have a field of &quot;power debt&quot;; how much power your satellite draws at each point of eclipse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then you apply your trajectory (f(t)-&gt;x,y) , offset (translated) by parameters &lt;span class=&quot;math-container&quot;&gt;$x_0,y_0$&lt;/span&gt; relative to the field center. Apply integral (dt) along the curve over the scalar field, and optimize the resulting function for such &lt;span class=&quot;math-container&quot;&gt;$x_0,y_0$&lt;/span&gt; that the integral value is lowest (most negative). It's important to integrate over time, not just location as your velocity isn't constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The value of the integral is your energy output during eclipse - the value of energy that must be kept in batteries to &quot;survive&quot; between break even points between input from the solar panels and power requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As per your edit: the integral over such a nonlinear field would be a total overkill for calculation by hand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, assume:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;curvature of the trajectory across penumbra = straight line. The diameter in shadow relative to the diameter of the orbit is small. This reduces the whole optimization task to crossing the diameter of the shadow.&lt;/li&gt;&#xA;&lt;li&gt;ballpark diameter of &quot;debt circle&quot; within penumbra as ratio of power input to power requirement. Say, if your solar batteries produce 20% more power than necessary, reduce penumbra diameter by 20%. That way you drop the awful task of calculating &quot;surface of circle partially obscured by another circle&quot;.&lt;/li&gt;&#xA;&lt;li&gt;Assume all, umbra and penumbra within the &quot;debt circle&quot; is flat zero - solar batteries don't contribute anything.&lt;/li&gt;&#xA;&lt;li&gt;assume constant velocity. That way your energy requirement will be just &quot;time spent in 'debt circle'&quot; times power requirements of the satellite.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Don't skip:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;penumbra and umbra size calculation&lt;/li&gt;&#xA;&lt;li&gt;satellite velocity in the umbra frame of reference if the Moon and the satellite move in the same direction.&lt;/li&gt;&#xA;&lt;li&gt;irradiation, panel efficiency, reducing size of penumbra to where panels don't suffice (although with much simplified function).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Explain what simplifications you've made. And also, that you'd want a bigger battery anyway, as a safety margin. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="11653" LastEditDate="2019-05-28T21:09:17.967" LastActivityDate="2019-05-28T21:09:17.967" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="13981" PostTypeId="1" AcceptedAnswerId="13983" CreationDate="2016-02-08T14:52:04.270" Score="13" ViewCount="2047" Body="&lt;p&gt;The idea of surface rendezvous for supply missions to a Mars expedition (either before or after arrival of a manned expedition) seems to come up every now and then. This would allow a manned spacecraft to either land near a supply craft, or would allow a supply craft to land near a manned base.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, that obviously requires close to pinpoint accuracy in landing position. Anything worse than that, and the two spacecraft might be too far away from each other for any supplies to be of significant use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For our Mars missions of recent past, &lt;strong&gt;what has been the landing position error compared to the landing position intended during the final mission planning stage?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2021-05-07T14:04:48.260" Title="How precise are our Mars landings?" Tags="|mars|landing|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13982" PostTypeId="2" ParentId="13981" CreationDate="2016-02-08T15:11:08.537" Score="4" Body="&lt;p&gt;For &lt;a href=&quot;http://mars.nasa.gov/msl/mission/timeline/edl/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity, the target area&lt;/a&gt; was a 20-km ellipse: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Where the Mars Exploration Rovers could have landed anywhere within their respective 150 by 20 kilometers (about 93 miles by 12 miles) landing ellipses, Mars Science Laboratory landed within a 20-kilometer (12-mile) ellipse.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hqtkx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hqtkx.jpg&quot; alt=&quot;Curiosity landing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-08T15:11:08.537" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13983" PostTypeId="2" ParentId="13981" CreationDate="2016-02-08T15:14:42.030" Score="13" Body="&lt;p&gt;Curiosity was targeted at a small area later named &lt;a href=&quot;https://en.wikipedia.org/wiki/Bradbury_Landing&quot; rel=&quot;noreferrer&quot;&gt;Bradbury Landing&lt;/a&gt; and came down 2.4 km from the centre.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also: &lt;a href=&quot;http://www.nasa.gov/mission_pages/msl/multimedia/pia16039.html&quot; rel=&quot;noreferrer&quot;&gt;landing ellipse comparison&lt;/a&gt; (earlier probes were of much lower accuracy.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, for projected future landings, carrying a better fuel reserve will allow much better landings.  (Edit: I've removed my suggestion that landing from a parking orbit would improve accuracy; Mark Adler has convinced me in comments that this doesn't make any difference, aero manoeuvres during entry can accommodate reasonable errors in the entry window.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I'd add that Apollo 12 landed within walking distance of an old Surveyor probe - obviously that was the Moon, not Mars, but take that as an indication that pinpoint landings are possible if really needed..)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-02-09T08:58:51.213" LastActivityDate="2016-02-09T08:58:51.213" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13984" PostTypeId="2" ParentId="13793" CreationDate="2016-02-08T17:17:46.447" Score="4" Body="&lt;p&gt;&lt;strong&gt;Partial answer:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I can not say anything on the individual qualifications for the cosmonauts and astronauts that became commanders, I do have a clue about the nationality pattern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It starts off in a pretty simple pattern, with alternating Russian and American commanders for the first 19 expeditions. (With a few exceptions, the schedule was a bit messy in the early construction phase)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, on expedition 20 in 2009, a Russian commander followed the previous Russian commander in order to compensate for the ESA commander, Frank De Winne. An American commander followed for compensation. This was the start of the next pattern, with commanders alternating in pairs (Russian, Russian, American, American).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That continued on to expedition 35, when the Canadian cosmonaut Chris Hadfield became commander. Also, the JAXA cosmonaut Koichi Wakata became the commander on expedition 39, and this two events must be compensated. That was done by having three Russian commanders in a row on 36, 37 and 38, and the pattern is now back to alternating American and Russian commanders. The American compensation is currently ongoing, with astronaut Scott Kelly's double reign as commander. (Also connected with the &lt;a href=&quot;http://www.nasa.gov/content/a-year-in-space&quot; rel=&quot;nofollow&quot;&gt;year in space experiment&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last compensating event scheduled is an American commander on both expedition 50 and 51.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-08T17:17:46.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13985" PostTypeId="1" CreationDate="2016-02-08T17:46:20.803" Score="3" ViewCount="635" Body="&lt;p&gt;I was reading a &lt;a href=&quot;http://www.nasa.gov/offices/marsplanning/faqs/&quot; rel=&quot;nofollow&quot;&gt;FAQ on Mars missions&lt;/a&gt; when I came across the following snippet:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA does have the rocket technology to get humans to Mars. However, current technology is not advanced enough to get us there efficiently. We are continuing to develop technologies to improve that efficiency. Another challenge we are actively working to overcome is the heat shield that will be needed to protect the returning capsule as it passes through Earth's atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The last sentence is what gets me the most. I know the atmosphere is the most effective way we have to shed excess velocity, and if something made it back to Earth from Mars it would have to be going pretty darned fast. But are there other alternatives? Could a Mars mission instead shed its velocity with an anti-gravity-assist on the Moon or some other means and proceed to dock up with the ISS? Or is that simply too much excess velocity to shed given mission constraints...&lt;/p&gt;&#xA;" OwnerUserId="1537" LastActivityDate="2021-04-12T21:23:56.683" Title="Could a Mars return mission go directly back to the ISS?" Tags="|iss|mars|delta-v|docking|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13986" PostTypeId="1" AcceptedAnswerId="13987" CreationDate="2016-02-08T17:57:45.417" Score="9" ViewCount="5333" Body="&lt;p&gt;The SLS and the STS seem to be the only rockets utilizing SRBs...  Why did they choose to go with them over LH/LOX or Kerosene/LOX?  Wouldn't it also be easier to upgrade those rockets in the future?&lt;/p&gt;&#xA;" OwnerUserId="13387" LastEditorUserId="51" LastEditDate="2016-02-08T18:07:20.860" LastActivityDate="2016-12-29T16:08:51.787" Title="Why were Solid Rockets chosen for the SLS" Tags="|launch|rockets|nasa|sls|srb|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13987" PostTypeId="2" ParentId="13986" CreationDate="2016-02-08T18:04:59.620" Score="16" Body="&lt;p&gt;SRBs have a very high thrust to weight ratio, even if they have an only modest ISP. That makes it complementary with LH2/LOX systems that have a relatively low thrust. Early on acceleration is almost as important for over all efficiency as exhaust velocity. For the first few hundred m/s of $\Delta v$, a combined metric of ISP and propellant density, called density impulse, is a good way to rate performance. Considering that, SRBs have an excellent performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason to continue with them for the SLS is to use off-the-shelf technology already developed for the space shuttle in order to save money. However, they are going to be upgraded from four to five booster segments for a larger total impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The space shuttle SRBs were the rocket engines with the highest thrust ever made, and that was achieved without particularly cutting-edge technology. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The STS and the SLS are not the only launchers utilizing them, the European &lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5&quot;&gt;Ariane 5&lt;/a&gt; uses solid rocket boosters too. Most ICBMs are also currently based on solid propellants for storage reasons.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-08T20:49:36.037" LastActivityDate="2016-02-08T20:49:36.037" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13988" PostTypeId="2" ParentId="13986" CreationDate="2016-02-08T18:07:13.080" Score="5" Body="&lt;p&gt;Ariane 5 uses similarly large SRBs. Atlas V and Delta 4 have strap on, smaller SRB's to help boost larger payloads than a bare core can provide. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heck Vega is entirely made out of solids. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is, you need to get off the pad. What do you do, when your initial stack, as needed to hit your payload/orbital targets, is too heavy to get off the ground. You add on a strap on first stage, usually SRB's because they are simple, can be built large to provide copious thrust early on, then get thrown away as they are not very efficient in terms of ISP. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-02-08T18:07:13.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13989" PostTypeId="2" ParentId="13985" CreationDate="2016-02-08T18:20:00.273" Score="5" Body="&lt;p&gt;No. Not with a lunar gravity assist for LEO at least. Moon's average orbital speed is about 1 km/s. Mars return for a rendezvous mission to Mars has an Earth reentry speed of about 11.4 km/s and more, depending on exact chosen trajectory. Since the Moon could never offer large declination angle for a retrograde gravity assist at such large inbound hyperbolic excess velocity (it's surface escape velocity is only ~ 2.4 km/s, decreasing in inverse square relationship to its radius as you select a higher periselene for your gravity assist flyby), it could only offer a reduction of a tiny fraction of its orbital speed for Earth reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's atmosphere could be used for aerocapture into elliptical orbit and later propulsive circularization at apogee, but this hasn't been done yet at such high entry speeds and at Earth. See &lt;a href=&quot;https://space.stackexchange.com/q/4668/49&quot;&gt;Could lunar mining be used to service Low Earth Orbit?&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/11941/49&quot;&gt;How can aerobraking be used to enter high orbit without landing?&lt;/a&gt; where this is discussed a bit more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also been some work done on Mars sample return capture trajectory for later retrieval from lunar trailing orbit, following Earth propulsive flyby and lunar gravity assist (see &lt;a href=&quot;https://solarsystem.nasa.gov/docs/7B.6%20Faber%20Mars%20Sample%20Return%20Using%20Commercial%20Capabilities%20ERV%20Trajectory%20and%20Capture%20Requirements.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Sample Return Using Commercial Capabilities: Concept for the Earth Return Vehicle Trajectory and Related Capture Requirements&lt;/a&gt;, N. Faber et al., NASA Ames, PDF), but that's not LEO either.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-02-08T19:05:27.717" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="13990" PostTypeId="1" AcceptedAnswerId="14022" CreationDate="2016-02-08T21:52:21.107" Score="6" ViewCount="608" Body="&lt;h3&gt;Problem&lt;/h3&gt;&#xA;&lt;p&gt;One of the biggest problems of a space elevator concept is that the elevator would have to be centered on the equator, and its tethers would pass through the Van Allen belts that contain super charged particles that are caught in eddies in Earth's magnetic field.  A space elevator traveling up and down the tether would have to pass directly through the wide belts at a relatively slow rate in comparison to a spaceship, meaning that any passengers on the elevator would have to pass through the belts of highly charged particles for a dangerously long amount of time.&lt;/p&gt;&#xA;&lt;h3&gt;Question&lt;/h3&gt;&#xA;&lt;p&gt;&lt;strong&gt;The belts are constantly changing with a mix of super-charged particles.&lt;/strong&gt;  Could the belts discharge each other if we connect them with a conductive conduit?  If so, then would it be possible to make parts of the space elevator/tether conductive so the two belts would dissipate each other?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;What if there is only one large belt at a particular time?  Would we need to stop the elevator until the belt splits into two?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="13032" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-02-23T14:19:55.040" Title="Space Elevator Hurdle: Can you ground out the Van Allen belts?" Tags="|space-elevator|van-allen-belt|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="13991" PostTypeId="1" AcceptedAnswerId="13992" CreationDate="2016-02-08T22:00:54.233" Score="3" ViewCount="637" Body="&lt;p&gt;My city is a bit to the north of ISS orbit's northmost point (inclination is 51°, my city is N55°). I observed ISS around 23:xx in summer at its heighest point over horizon, 45°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YHmji.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YHmji.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I supposed that the orbit northern point (lets call it ONP) is stable and with season it changes it's side: it may be on the lit side, half year later it's in shadow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An observer like me sees ONP pass every day (Earth rotates &quot;inside&quot; the orbit), and with season it should be later every day. Apparently, in my winter it passes at noontime, I don't see it in the bright sky. In summer it passes around midnight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least this observation concords with my theory: it's a bit more than 1/4 of year till summer solstice, and it's visible 1/4 of day till midnight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I right with the conclusion? Is ISS orbital plane stable as I describe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image shows the opposite picture. ONP is on the far side of this image, and with seasons Sun should move in counter-clockwise manner (in geocentric system). So, with season, the time I pass near this point should move to earlier, to daytime rather than to midnight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3yoNT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3yoNT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia illustration: same side (for example, a distant star) in inertial reference system should be seen earlier and earlier with seasons changing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TXKSj.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TXKSj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="612" LastEditorUserId="612" LastEditDate="2016-02-08T22:16:46.173" LastActivityDate="2016-02-09T00:03:11.463" Title="ISS observation and orbit plane" Tags="|iss|orbital-mechanics|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="13992" PostTypeId="2" ParentId="13991" CreationDate="2016-02-09T00:03:11.463" Score="5" Body="&lt;p&gt;No. The ISS has a cycle of when it rises and sets, that is about 2 months. The following plot shows an example of the beta cycle. The movement is the precession around the Earth. Note that the cycle time is about 2 months. That means that about every 2 months, the satellite will be overhead at the same time. Note that there is some variation with the day, but the overall trend is such that it repeats every 2 months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5V8TM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5V8TM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://heavens-above.com/PassSummary.aspx?satid=25544&amp;amp;lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&quot; rel=&quot;noreferrer&quot;&gt;Heavens Above&lt;/a&gt;, I have found the 10 day orbital predictions for the ISS at 55N 0E, UTC. There are many passes per day, but the best prediction comes from the first pass in a cycle. The first is the day in February 2016. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;9- 16:45&lt;/li&gt;&#xA;&lt;li&gt;10- 15:53&lt;/li&gt;&#xA;&lt;li&gt;11- 16:35&lt;/li&gt;&#xA;&lt;li&gt;12- 15:43&lt;/li&gt;&#xA;&lt;li&gt;13- 14:51&lt;/li&gt;&#xA;&lt;li&gt;14- 15:34&lt;/li&gt;&#xA;&lt;li&gt;15- 14:41&lt;/li&gt;&#xA;&lt;li&gt;16- 13:49&lt;/li&gt;&#xA;&lt;li&gt;17- 12:58&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note that in 10 days, the satellite went from its first appearance at about 1700 to about 1300. That's about 1/6th of the day, which 10 days is about 1/6th of 2 months.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-09T00:03:11.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13993" PostTypeId="1" AcceptedAnswerId="13995" CreationDate="2016-02-09T01:10:32.573" Score="3" ViewCount="195" Body="&lt;p&gt;I understand that the Soyuz 2.1v launcher will switch from using the NK33 engine to the new RD-193 when stocks of the former are finished. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know how many launches, beyond the two so far, are planned using the NK33 type?&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-02-09T02:30:07.063" Title="For how long will the Soyuz 2.1v launcher use the old NK33 engines?" Tags="|engines|launch-vehicle|nk-33|rd-19x|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13994" PostTypeId="1" CreationDate="2016-02-09T01:32:29.393" Score="2" ViewCount="655" Body="&lt;p&gt;I've been scouring the internet to try to find information on this. Unfortunately most of what I've found has indicated that there are currently no plans for another international alliance project in space. I was wondering if anyone has managed to find information about future orbital manned space stations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the complexity of these projects, I understand that it would take a long time to design and construct the components for a new station, which really saddens me as it implies that its allowed the continual inhabitation of space by man for over 15 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I feel that it really is a great symbol of union on Earth, and a healthy reminder of what we can do when we work together as one species. It'd be a huge shame to lose such an asset, with no vision for further discoveries/experiments of this unique type. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess maybe the general public has lost interest in space since the moon landings. I do find it incredibly frustrating that America decides to spend more money on &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_countries_by_military_expenditures&quot; rel=&quot;nofollow&quot;&gt;its military every year&lt;/a&gt; than has been spent throughout &lt;a href=&quot;https://en.wikipedia.org/wiki/Budget_of_NASA&quot; rel=&quot;nofollow&quot;&gt;NASA's total history&lt;/a&gt;. Obviously, some of the military research does benefit humanities knowledge and understanding (e.g. NIF), but I cant help but imagine what the world would be like if the leaders of nations focused their efforts on advancing together instead of developing ever more destructive weapons and focusing on killing each other. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd love to hear the thoughts of others on how to get people to wake up and realise people across the world are not all that different. I'm adamant that international projects like the ISS (besides providing vast improvements to our technologies and basic understanding of science) are an incredible way to help people see how insignificant their squabbles are in the infinity of the cosmos.&lt;/p&gt;&#xA;" OwnerUserId="13391" LastEditorUserId="13391" LastEditDate="2016-02-10T12:55:57.450" LastActivityDate="2016-02-10T12:55:57.450" Title="What plans exist for another future international space station after the ISS is retired, and what stage of development are the plans at?" Tags="|iss|crewed-spaceflight|space-station|research|" AnswerCount="1" CommentCount="2" ClosedDate="2016-02-09T15:43:36.317" ContentLicense="CC BY-SA 3.0" />
  <row Id="13995" PostTypeId="2" ParentId="13993" CreationDate="2016-02-09T02:19:26.617" Score="3" Body="&lt;p&gt;According to now almost three years old news article (&lt;a href=&quot;http://novosti-kosmonavtiki.ru/news/7229/&quot; rel=&quot;nofollow&quot;&gt;in Russian&lt;/a&gt;, &lt;a href=&quot;https://translate.google.com/translate?hl=en&amp;amp;sl=ru&amp;amp;tl=en&amp;amp;u=http%3A%2F%2Fnovosti-kosmonavtiki.ru%2Fnews%2F7229%2F&quot; rel=&quot;nofollow&quot;&gt;Google Translate version&lt;/a&gt;) from April 2013 that quotes Energomash CEO Vladimir L. Solntsev, there were at the time in total of &lt;em&gt;about 20&lt;/em&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/NK-33&quot; rel=&quot;nofollow&quot;&gt;NK-33&lt;/a&gt; engines left in the warehouse, which would be enough for &lt;em&gt;about 10 launches&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is a bit difficult to predict how long the NK-33 supplies will last. Launch vehicle is now awaiting return to flight after the partial failure with Kosmos 2511 (Kanopus-ST) &lt;a href=&quot;http://www.russianspaceweb.com/kanopus-st.html&quot; rel=&quot;nofollow&quot;&gt;failing to separate from the upper stage&lt;/a&gt; in December 2015, and it is also unclear when we'll see the first &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz-2-1v&quot; rel=&quot;nofollow&quot;&gt;Soyuz-2-1v&lt;/a&gt; launch from &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostochny_Cosmodrome&quot; rel=&quot;nofollow&quot;&gt;Vostochny&lt;/a&gt;. First Soyuz-2.1a is &lt;a href=&quot;http://spaceflightnow.com/launch-schedule/&quot; rel=&quot;nofollow&quot;&gt;currently scheduled&lt;/a&gt; to launch from there on April 25, but I haven't seen any announcement of Soyuz-2-1v launch from Vostochny yet. So far, only three Soyuz-2-1v launches from &lt;a href=&quot;https://en.wikipedia.org/wiki/Plesetsk_Cosmodrome&quot; rel=&quot;nofollow&quot;&gt;Plesetsk&lt;/a&gt; are expected till 2018 according to &lt;a href=&quot;http://spaceflight101.com/calendar/&quot; rel=&quot;nofollow&quot;&gt;Spaceflight 101&lt;/a&gt;, and usually well informed Gunter Dirk Krebs currently only lists &lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/soyuz-2-1v_volga.htm&quot; rel=&quot;nofollow&quot;&gt;one planned launch&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.russianspaceweb.com/vostochny_soyuz.html&quot; rel=&quot;nofollow&quot;&gt;According to Anatoly Zak&lt;/a&gt;, the new Soyuz-2-1v (previously referred to as Soyuz-1) configuration with RD-193 engines will be called Soyuz-2-1d, but searching the web by that moniker didn't yield any more information, I'm afraid.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If I had to guess, then my bet is that Baikonur will never see a Soyuz-2-1v launch at all (because of the nature of its typical payloads and target orbital inclinations, and because Baikonur isn't in Russia), and we'll see inaugural RD-193 on Soyuz-2-1v (Soyuz-2-1d?) launch from Vostochny, likely before Plesetsk runs out of NK-33 engines, somewhere around 2020. Sooner, if they decide they'll only supply Vostochny with RD-193's and keep all NK-33's at Plesetsk.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-09T02:30:07.063" LastActivityDate="2016-02-09T02:30:07.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13996" PostTypeId="1" CreationDate="2016-02-09T08:00:07.500" Score="4" ViewCount="520" Body="&lt;p&gt;Suppose I start on Earth and make a Hohmann transfer to Neptune.  After traveling along my initial trajectory, I arrive at the Neptunian system.  However, the objects in the Neptune system are going faster than I am (they approach from behind me).  So if I want to fall into orbit around Neptune itself, I need to do another burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I want to orbit one of the &lt;em&gt;moons&lt;/em&gt; of Neptune, not Neptune itself, directly.  I don't care which direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say I pick &lt;a href=&quot;https://en.wikipedia.org/wiki/Triton_%28moon%29&quot; rel=&quot;nofollow&quot;&gt;Triton&lt;/a&gt;, which happens to be the largest retrograde moon in the solar system.  My question: &lt;strong&gt;would Triton's orbiting retrograde made it easier (less delta-V) to capture into an orbit around it?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="2707" LastActivityDate="2016-02-09T22:01:53.007" Title="Is It Easier To Hohmann Transfer to a Retrograde Satellite or to a Prograde Satellite?" Tags="|moons|hohmann-transfer|retrograde-orbit|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="13997" PostTypeId="2" ParentId="13994" CreationDate="2016-02-09T15:15:42.607" Score="4" Body="&lt;p&gt;The Russians have a plan (that is slowly disappearing as the oil crunch kills their budgets) to detach the future OM and MLM modules (Nauka) and all that go with it as a standalone station once the ISS is retired (aka Splashed into the Pacific). Who knows if that will happen. The Russians are not in great shape financially, but they could succeed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bigelow has serious plans for a commercial research station made out of inflatables. They have mostly been waiting for commercial crew to happen. They have launched two subscale modules (Genesis I and II). They have BEAM launching on CRS-8 in the Dragon's unpressurized trunk to be berthed to Node 3 and stay on the station for 2 years. This will prove the technology to NASA even further, and get Bigelow serious experience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They need a heavy lifter to launch their main/big module (BA-330), which is Falcon Heavy and is always just a few months away from first launch. They need a crew vehicle other than Soyuz to get there, and have contracts with Boeing (CST-100/Starliner) and SpaceX (Dragon V2/Crew). They just need those other vendors to deliver. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than that, the US has no serious plans that are funded and look real right now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;China is planning on Tiangong, in a model similar to Mir in the near future.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-02-09T17:46:31.290" LastActivityDate="2016-02-09T17:46:31.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="13999" PostTypeId="2" ParentId="13996" CreationDate="2016-02-09T16:39:55.007" Score="3" Body="&lt;p&gt;There are two ways of entering the system, depending on what side of the planet you fly by. The two are entirely equivalent, so you can chose what is going to be &quot;prograde&quot;, and what is going to be &quot;retrograde&quot;. Nothing special about the Triton case then. That is NOT to say that what direction you chose to enter is not important, it just mean that the general motion of the moon system is not important. You can just enter through the mirrored fly-by trajectory to inverse the rotation. The same rules as for regular approaches to gas giant moons apply then: Do not encounter the moon head on, but instead go parallel with it in the same rotational direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zQccgm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zQccgm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-09T22:01:53.007" LastActivityDate="2016-02-09T22:01:53.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14001" PostTypeId="1" AcceptedAnswerId="14005" CreationDate="2016-02-09T18:15:06.497" Score="9" ViewCount="1864" Body="&lt;p&gt;We know that the SLS is going to use solid rocket boosters that are decedents of the boosters used on the Space Shuttle.  The new version is supposed to be bigger and better, but will NASA be able to recover them to cut down on cost?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does NASA have any plans to adopt the active recovery systems that SpaceX are working on?&lt;/p&gt;&#xA;" OwnerUserId="13032" LastEditorUserId="13032" LastEditDate="2016-02-09T18:35:07.693" LastActivityDate="2018-05-23T15:08:52.493" Title="Can the SLS reuse its solid rocket boosters?" Tags="|sls|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14002" PostTypeId="1" AcceptedAnswerId="14007" CreationDate="2016-02-09T18:29:00.330" Score="8" ViewCount="2692" Body="&lt;p&gt;Is it possible for one satellite to image another at high resolution?   Would this information be useful from an intelligence perspective?&lt;/p&gt;&#xA;" OwnerUserId="13397" LastActivityDate="2023-04-28T02:40:50.543" Title="Could a spy satellite image another satellite?" Tags="|artificial-satellite|imaging|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14004" PostTypeId="2" ParentId="13996" CreationDate="2016-02-09T18:41:15.157" Score="2" Body="&lt;p&gt;It certainly makes a difference - entering on the prograde direction relative to the moon's orbit is much more beneficial in outward Hohmann transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper image shows the needed trajectory in the planet's frame of reference. The bottom one is from the Sun's reference frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XMNE9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XMNE9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Relative to the Sun, given moon moves in an &lt;a href=&quot;https://en.wikipedia.org/wiki/Deferent_and_epicycle&quot; rel=&quot;nofollow noreferrer&quot;&gt;epicycle&lt;/a&gt; trajectory. Epicycles have a part that is much faster than &quot;standard&quot; orbital speed around the Sun, when the moon and the planet move in accord, and drops of speed - going into negatives sometimes - when the planet and the Moon go in opposite directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since your spacecraft is at apogeum of its elliptical orbit, its speed is much slower than that of bodies in circular orbit at the same altitude. That means matching speed with a body that moves below speed of circular orbit is easier than with one that is even faster than the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole situation, obviously, is reversed if you arrive from outer planets towards inner ones. That is, you still want to meet the moon in its prograde orbit direction, but you meet it on the side where it moves fast relative to the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, there's still the matter of whether to enter prograde relative to the moon's spin. This really doesn't matter unless you plan to land (or deploy a lander) in which case prograde is beneficial, meaning the moon's rotary speed may be substracted from delta-V needed to land, instead of added.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-02-09T19:14:28.603" LastActivityDate="2016-02-09T19:14:28.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14005" PostTypeId="2" ParentId="14001" CreationDate="2016-02-09T18:56:11.607" Score="11" Body="&lt;p&gt;It can but it won't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was decided that the costs of recovery, inspection and refurbishing the boosters plus increased risk of disaster due to unnoticed wear is just not worth it - the boosters will not be recovered for economical, not technical reasons.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-09T18:56:11.607" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14006" PostTypeId="2" ParentId="14002" CreationDate="2016-02-09T19:46:42.633" Score="5" Body="&lt;p&gt;It is quite possible. HiRISE has attempted to image other satellites in orbit around Mars (Specifically MGS, although the attempt was not successful). &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Global_Surveyor&quot; rel=&quot;noreferrer&quot;&gt;Mars Global Surveyor&lt;/a&gt; could, and in fact was successful at taking a few pictures, for instance, Mars Express seen below. MGO wasn't really intended to take such photographs, but the photo is still quite high resolution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GGytR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GGytR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the high resolution, it is theoretically possible, but one is probably better off using ground based assets, as you can get a larger mirror.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, you could probably do it, it would take some skill, perfect timing, a bit of luck, etc. There must be some valuable information about satellites gathered from such, such as their optics size, antenna size, design, etc. I have almost no doubt that there is information being gathered about orbiting satellites, some small part may be gathered by orbiting satellites, and that the practice will continue for some time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-09T19:46:42.633" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14007" PostTypeId="2" ParentId="14002" CreationDate="2016-02-09T20:27:30.583" Score="13" Body="&lt;p&gt;You don't have to expect of the intelligence community to divulge classified information to see if this is possible, it has been done before by civilian satellites:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/z6Wdz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;SPOT 5 by Pléiades, an unusual picture provided by CNES, Source: &lt;a href=&quot;http://www.space-airbusds.com/en/2011/spot-5-celebrates-10-years-in-orbit.html&quot;&gt;Airbus Defence &amp;amp; Space&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking images of satellites in different orbits isn't easy though, and depending on wavelength, it might not be of much use to infer information that otherwise isn't yet known and is considered valuable. So it somewhat depends on what you mean with &quot;imaging&quot;;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, observations in visual range aren't useful beyond providing another reference point where and when the observed object was and updating your knowledge of its orbit with it. It might be useful if the satellite is fragmented, or otherwise in a bad shape (e.g. you could infer that it failed to deploy one or more solar panels, that its multi-layer insulation might be torn, or that it's in unrecoverable tumble), but you'll also be dealing with a lot of glare and other optical aberrations when the satellite is illuminated either by the Sun or the Moon, and insufficient illumination when it's in both these objects' shadows or in transit across them and all you're able to get is the object's contours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In infrared wavelengths, things get a bit more useful and you can potentially even infer if the satellite is even still functional, and even get an estimate of how much of propellants it has left based on its thermal signature (by measuring its thermal inertia you can estimate how much mass is subject to thermal cycling, assuming of course good enough imaging resolution and several observations of the same object).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With active radar imaging, it should be possible to get a lot more detailed geometry of the object that isn't affected by light and thermal cycling, assuming it has good radar signature and isn't deliberately made stealth and less visible to radars. You can probably remember some of the recent animations of active radar imaging of certain Near Earth Asteroids (like the Halloween / Pumpkin / Skull &lt;a href=&quot;https://en.wikipedia.org/wiki/2015_TB145#Radar_imagery&quot;&gt;asteroid 2015 TB145&lt;/a&gt; or the &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=4459&quot;&gt;asteroid 2004 BL86&lt;/a&gt; that was imaged with the DSN radar and was discovered to have a small moonlet).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But all this imaging means that you'll have to first get a good sense of its orbit, so you can conduct imaging at various wavelengths for sufficiently long exposure times and maintain fixed attitude relative to it for the duration of imaging, to infer whatever information you need about it that you didn't yet obtain by other means. And those &lt;em&gt;other means&lt;/em&gt; are often much easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, an active satellite would likely produce compromising emanations that you could detect by simply listening to it in radio wavelengths. Some old Soviet satellites tried very hard to avoid that and were taking reconnaissance photographs on film and then releasing those films in tiny capsules so they fell back on Earth over the Soviet territory. But that takes a lot of time to get information you need, and since data is only valuable information while it's still relevant, such obscure methods of concealing the object's purpose should be a thing of the past by now. And &lt;em&gt;in your face&lt;/em&gt; reconnaissance comes with an added value of your adversary having to resort to more expensive and technologically advanced means of concealment, anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in short, direct imaging of satellites doesn't have too much value to the intelligence community, or the information it could provide could be inferred with easier and cheaper means than large orbital facilities. And since you either have to match observed object's orbit (something that's rather prohibitive in terms of required reaction mass), or it will still rise and set from your vantage point just like it would to ground observatories, it makes little sense to make such investments. It is unclear what exact NRO projects were cancelled that &lt;a href=&quot;https://en.wikipedia.org/wiki/2012_National_Reconnaissance_Office_space_telescope_donation_to_NASA&quot;&gt;resulted in NASA getting a couple of Hubble-size telescopes for free&lt;/a&gt;, but it's entirely possible that, among other possible tasks, they were also meant to image orbital assets of other nations. If that bears some truth, then they must have come to the same conclusion than I did at the start of this paragraph.&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2016-02-09T20:27:30.583" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14008" PostTypeId="1" AcceptedAnswerId="14011" CreationDate="2016-02-09T21:36:55.807" Score="13" ViewCount="2263" Body="&lt;p&gt;How big a thing was able to land or splash down? Was there ever anything bigger or heavier than the Space Shuttle? If no then whats next on the list? And secondary question: what is the maximal payload which can be returned from orbit (LEO if it matters).&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2023-02-12T21:30:57.383" Title="What were the biggest and heaviest spacecrafts to safely return from orbit?" Tags="|landing|record|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14009" PostTypeId="2" ParentId="14008" CreationDate="2016-02-09T21:49:18.453" Score="5" Body="&lt;p&gt;The US space shuttle would be the biggest; the Soviet Buran spaceplane would be the runner-up. I think the shuttle could land just about as much payload as it could lift, about 25 tons. (maximum landing weight is 5 tons less than max takeoff weight, but that includes maneuvering fuel). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We don't have that capability any more, of course; I think the only thing we're routinely reentering these days is Soyuz, with three astronauts and a miniscule amount of cargo. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-02-09T21:49:18.453" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14010" PostTypeId="1" CreationDate="2016-02-09T22:34:13.197" Score="4" ViewCount="740" Body="&lt;p&gt;Analyzing data coming from outside Earth (such as the temperature around Curiosity on Mars, or the radiation level on geosynchronous orbit around Earth, etc.) might be an interesting task and/or hobby for those not working in the space industry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do NASA, ESA or any other organizations that owns devices outside Earth (= in space or on the surface of other bodies) provide public access to any real-time data stream to analyze anc such?&lt;/p&gt;&#xA;" OwnerUserId="306" LastActivityDate="2016-02-09T22:34:13.197" Title="Are there any publicly available real-time data stream containing measurements outside Earth?" Tags="|data-transmission|scientific-data|public-involvement|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14011" PostTypeId="2" ParentId="14008" CreationDate="2016-02-09T23:21:01.327" Score="17" Body="&lt;p&gt;The heaviest space shuttle mission at landing (and an STS Orbiter is by far the heaviest craft to return from space intact) was &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-83&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-83&lt;/a&gt; at 235,421 lbs (106,785 kg).  This was a Spacelab mission which ended early due to fuel cell problems so it still had a lot of consumables onboard as well as the Spacelab.&lt;/p&gt;&#xA;&lt;p&gt;Data from Appendix A of &lt;a href=&quot;https://ntrs.nasa.gov/api/citations/20110001406/downloads/20110001406.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;the ever-useful Space Shuttle Missions Summary&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2023-02-12T21:30:57.383" LastActivityDate="2023-02-12T21:30:57.383" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14012" PostTypeId="1" AcceptedAnswerId="14013" CreationDate="2016-02-10T02:40:03.197" Score="-1" ViewCount="390" Body="&lt;p&gt;I was just googling on how does radio waves travel in space and stumbled upon this reddit post &lt;a href=&quot;https://www.reddit.com/r/askscience/comments/1r15bm/if_sound_cant_travel_through_space_how_can_radio/&quot; rel=&quot;nofollow&quot;&gt;If sound can't travel through space, how can radio waves?&lt;/a&gt; and the idea that sound can't travel in space caught my attention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although it makes absolute sense that sound needs medium to travel from one end to another, has this actually been tested in space somehow? if &quot;theoratically&quot; I was able to shout in a void space would I look like this dog?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/gNMHF9hdVyQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I still have my doubts.&lt;/p&gt;&#xA;" OwnerUserId="12391" LastEditorUserId="12391" LastEditDate="2016-02-10T02:52:07.390" LastActivityDate="2016-02-10T03:21:33.377" Title="So if I could shout in space, no one can ever hear me including myself?" Tags="|sound|" AnswerCount="1" CommentCount="6" ClosedDate="2016-02-10T10:19:02.973" ContentLicense="CC BY-SA 3.0" />
  <row Id="14013" PostTypeId="2" ParentId="14012" CreationDate="2016-02-10T03:21:33.377" Score="2" Body="&lt;p&gt;You couldn't shout without air to expel from your lungs, so presumably you're in a space suit with air in it. Therefore you can hear yourself shout just fine. (Even if you didn't have a helmet's worth of air around your head, you'd hear the shout via vibration through your own body.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vibrations in the air, that make up the sound of your shout, would vibrate your helmet as well. If your helmet was touching another astronaut's helmet, that would conduct sound reasonably well, and that astronaut could hear you. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another astronaut floating in space next to you wouldn't hear a thing, because there's no medium to conduct vibrations between you and them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know if astronauts have done this experiment; I'm sure the equivalent has been done in vacuum chambers on Earth. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-02-10T03:21:33.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14014" PostTypeId="1" AcceptedAnswerId="14015" CreationDate="2016-02-10T04:34:21.793" Score="14" ViewCount="2476" Body="&lt;p&gt;I've just read &lt;a href=&quot;http://arstechnica.com/information-technology/2016/02/north-koreas-successful-satellite-in-orbit-but-tumbling-and-useless/&quot;&gt;North Korea’s “successful” satellite in orbit, but tumbling and useless&lt;/a&gt;. How can we tell a satellite is tumbling or not? It's so far away!&lt;/p&gt;&#xA;" OwnerUserId="13400" LastEditorUserId="6816" LastEditDate="2016-02-10T04:45:08.267" LastActivityDate="2016-02-10T06:21:31.917" Title="How to tell a satellite is tumbling and out of control?" Tags="|orbital-mechanics|artificial-satellite|attitude|observation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14015" PostTypeId="2" ParentId="14014" CreationDate="2016-02-10T05:40:13.433" Score="18" Body="&lt;p&gt;First, let's clear the part about how far away Kwangmyongsong 4 (KMS-4, NORAD ID 41332) is. It was inserted into a 472.6 km × 508.5 km near-polar &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot;&gt;sun-synchronous orbit&lt;/a&gt; with inclination of 97.5°. It is not geosynchronous, so its orbit takes it over all the longitudes of the globe, and latitudes of 80.5° North to 80.5° South. That means that roughly once per day, it will be nearly directly overhead over any spot of the Earth's surface, at an altitude of 472.6 - 508.5 km above mean sea-level. You can see its orbit for yourself with tools such as &lt;a href=&quot;http://www.satflare.com/track.asp?q=41332#TOP&quot;&gt;Satflare&lt;/a&gt;, &lt;a href=&quot;http://www.n2yo.com/satellite/?s=41332&quot;&gt;N2YO&lt;/a&gt;, &lt;a href=&quot;http://www.heavens-above.com/orbit.aspx?satid=39026&quot;&gt;Heavens Above&lt;/a&gt;, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's orbital period is 94.3 minutes, and its &lt;a href=&quot;https://en.wikipedia.org/wiki/Nodal_precession&quot;&gt;nodal precession&lt;/a&gt; rate is approximately 1° (360/365.25) per day. That means that maximum equatorial separation between its ground tracks is 23.5° or 2,616 km. Half of that is our worst case observational angle to it relative to zenith from any point on the Earth's surface at least once per day, as long as it's within the &amp;pm;80.5° latitude. That means that we can observe its passes with fixed ground facilities every day at a distance of between 472.6 and 1,405 km and at higher latitudes, this ground track separation decreases further still.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's not too bad, and we can track so distant objects down to about a couple of centimeters in size even with civilian ground based radars, such as e.g. &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/measure/radar.html&quot;&gt;NASA's Haystack Auxiliary Radar&lt;/a&gt;, and who knows what capability military has, but I wouldn't put it past them that they use active radar and are &lt;em&gt;illuminating&lt;/em&gt; the object with phased array microwave systems to get even better resolution and increase the object's radar signature. In fact, NASA's Deep Space Network is capable of achieving a similar feat on their own, and that's not a classified system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, so how do we know it's tumbling out of control? We can simply infer that via periodically changing radar signature. A tumbling object that isn't spherical and reflecting in observational wavelengths uniformly in all directions, will produce sinusoidal dips and increases in brightness of the reflected photons (in whichever wavelength). If that spin rate is too fast to be deliberate, you can reasonably conclude that it is spinning out of control and it isn't simply a case of a curious attitude control. The object is roughly 200 kg in mass, and there is a limit to its ability to neutralize its angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, if the intelligence community &lt;em&gt;leaks&lt;/em&gt; such information despite the fact that it would suit them better that the object remains a threat to someone's national security and they get some nice new toys, erm, assets and/or permission to scratch an itch, pardon, to mitigate the threat, I would tend to believe them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if you prefer to believe the civilian society, there's even no need to believe anyone focused on their own national interests. Remember &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_M-27M&quot;&gt;Progress M-27M&lt;/a&gt;? While we knew it was tumbling out of control because we saw the start of that in a live webcast from its own camera, and there was no reason to doubt official press releases, &lt;a href=&quot;http://www.astrobin.com/177632/&quot;&gt;even amateurs&lt;/a&gt; and &lt;a href=&quot;http://www.news.com.au/technology/science/space/look-up-theres-an-outofcontrol-spacecraft-tumbling-overhead/news-story/2fab71a87b44490b910e65bafb292440&quot;&gt;all over the world&lt;/a&gt; were able to confirm its spin rate. While it would take a bit better equipment to do the same for around 40 times less massive object at twice the orbital altitude, I wouldn't put it past the more persistent enthusiasts to confirm this for KMS-4 as well. Stay tuned.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-10T06:21:31.917" LastActivityDate="2016-02-10T06:21:31.917" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14017" PostTypeId="1" AcceptedAnswerId="14018" CreationDate="2016-02-10T08:40:33.697" Score="6" ViewCount="1351" Body="&lt;p&gt;The lower section of Dragon, holding the solar panels, are ditched during descent.&lt;/p&gt;&#xA;&lt;p&gt;So, is Dragon carrying cargo in that section during ascent?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DcVim.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DcVim.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Source of the image: &lt;a href=&quot;https://vimeo.com/41871982&quot; rel=&quot;nofollow noreferrer&quot;&gt;Launch &amp;amp; Spacecraft Visit to Space Station - SpaceX&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6415" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-02-10T23:16:29.430" Title="Is that Dragon's cargo exposed to space environment in this picture?" Tags="|spacex|dragon|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14018" PostTypeId="2" ParentId="14017" CreationDate="2016-02-10T09:02:10.497" Score="11" Body="&lt;p&gt;Yes, that section is usually called &lt;em&gt;trunk&lt;/em&gt; and it may contain &lt;em&gt;unpressurized cargo&lt;/em&gt; - up to 14 m&lt;sup&gt;3&lt;/sup&gt; (compared to 10 m&lt;sup&gt;3&lt;/sup&gt; of pressurized payload) (&lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see it being loaded with &lt;a href=&quot;https://en.wikipedia.org/wiki/Bigelow_Expandable_Activity_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;BEAM&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/umoha.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/umoha.jpg&quot; alt=&quot;BEAM being loaded to Dragon trunk for CRS-8&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;https://twitter.com/BigelowSpace/status/694714898123661312&quot; rel=&quot;nofollow noreferrer&quot;&gt;Twitter post&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here is what seems like a real photo of the open section from the ISS &lt;a href=&quot;https://i.stack.imgur.com/XsJhw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XsJhw.jpg&quot; alt=&quot;Inspection of Dragon aft by remote cameras&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://www.nasaspaceflight.com/2013/03/ssrms-removes-payload-dragon-trunk-new-milestone/&quot; rel=&quot;nofollow noreferrer&quot;&gt;nasaspaceflight.com article&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-02-10T09:02:10.497" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14020" PostTypeId="1" AcceptedAnswerId="14021" CreationDate="2016-02-10T14:57:07.097" Score="-1" ViewCount="733" Body="&lt;p&gt;I have a doubt regarding rocket engines. Now which is the most responsible factor for propulsion using a rocket engine? Thrust / Specific Impulse? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I need a comparison b/w three engines:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;MPDT1 [Magneto Plasma Dynamic Thruster]&lt;/li&gt;&#xA;&lt;li&gt;MPDT2&lt;/li&gt;&#xA;&lt;li&gt;VASIMR [Variable Specific Impulse Magnetoplasma Rocket]&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Body Weight [On Earth] = &lt;strong&gt;&lt;em&gt;50k Ton&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launched from Earth&lt;/strong&gt;, I want this body to reach space and &lt;strong&gt;escape the&lt;/strong&gt; &lt;strong&gt;Earth's Gravitational Field&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;MPDT1 Specifications:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Propellant&lt;/em&gt;: Hydrogen&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Thrust&lt;/em&gt; = 88,500 mN&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Specific Impulse&lt;/em&gt; = 3,500 s&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Power Required&lt;/em&gt; = 3,750 kW&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;MPDT2 Specifications:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Propellant&lt;/em&gt;: Hydrogen&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Thrust&lt;/em&gt; = 26,300 mN&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Specific Impulse&lt;/em&gt; = 4,900 s&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Power Required&lt;/em&gt; = 1,500 kW&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;VASIMR Specifications:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;Propellant&lt;/em&gt;: Argon&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Thrust&lt;/em&gt; = 5,000 mN&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Specific Impulse&lt;/em&gt; = 3,000-12,000 s&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Power Required&lt;/em&gt; = 200 kW&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Source for all this data: &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;nofollow&quot;&gt;Wikipedia- Ion Thruster&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11904" LastActivityDate="2016-02-11T00:44:31.930" Title="Thrust, Specific Impulse and Power in Ionic Thrusters" Tags="|rockets|artificial-satellite|propulsion|ion-thruster|" AnswerCount="1" CommentCount="1" ClosedDate="2016-02-11T20:54:56.397" ContentLicense="CC BY-SA 3.0" />
  <row Id="14021" PostTypeId="2" ParentId="14020" CreationDate="2016-02-10T15:03:24.657" Score="5" Body="&lt;p&gt;For the initial ascent from Earth into space, thrust is most important. Once in orbit and free of the atmosphere, specific impulse becomes more important, though thrust will still be a factor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electric propulsion engines of the types you're interested in, however, make an extreme trade-off of minimal thrust in favor of very high specific impulse. They are very fuel efficient, but they don't have anywhere near enough thrust to lift their own weight against Earth's gravity, let alone a useful payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You simply can't get off the ground with any of those engines.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-02-10T15:03:24.657" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14022" PostTypeId="2" ParentId="13990" CreationDate="2016-02-10T15:42:18.160" Score="1" Body="&lt;p&gt;My gut feeling says no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem you face is that the total electric charge in the belts is immense. Your cable is a few meters in diameter tops, while the bands themselves are thousands of kilometers long. Outside of the direct area of the cable, you're not seriously affecting any electron or ion. Sure, you may bend its path, but you don't remove it from orbit. The electron has to hit the cable for any grounding to occur. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In numbers: the van Allen belts have an volume in the order of 10^23 m3. The area directly surrounding your cable is about 10^5 m3. This sweeps much larger volume, of course: a disk about 10^10 m3. Still you're looking at 13 orders of magnitude difference.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2016-02-10T15:42:18.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14023" PostTypeId="1" AcceptedAnswerId="14026" CreationDate="2016-02-10T16:16:45.450" Score="26" ViewCount="5932" Body="&lt;p&gt;From what I understand, the engine is usually the most expensive part of each rocket, excluding payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm struggling to understand how SpaceX can have one of the cheapest launchers on the market when it flies on a 9 engine configuration, while many other launchers have fewer engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't the cost of producing nine separate Merlin engines &quot;theoretically&quot; nine times more expensive than making one big engine?&lt;/p&gt;&#xA;" OwnerUserId="8925" LastEditorUserId="1142" LastEditDate="2016-02-11T02:02:06.710" LastActivityDate="2016-02-25T05:16:31.850" Title="How can a 9-engine Falcon rocket be cheaper than single engine rockets?" Tags="|spacex|falcon-9|merlin-1d|" AnswerCount="7" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14024" PostTypeId="2" ParentId="14023" CreationDate="2016-02-10T16:34:11.740" Score="10" Body="&lt;p&gt;Traditionally, engines are expensive because they require lots of manual labor to build. Because they are built in small numbers, there's no point in investing in an automated factory that can build them cheaper. They are also optimized for maximum performance, requiring complex constructions and expensive materials.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX wants to change that equation. By building 10 engines per rocket, it becomes cost-effective to set up a production line. They've also designed the engines to be cheap to produce, even if that reduces performance a bit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-10T16:34:11.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14025" PostTypeId="2" ParentId="14023" CreationDate="2016-02-10T16:34:21.347" Score="8" Body="&lt;p&gt;It depends on so many things. SpaceX chose many instances of a smaller engine, which is generally simpler than a single larger one to build and complexity is related to cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They chose a fuel and fuel cycle that is simpler than most. They chose an injector that is reliable and simple. Liquid Hydrogen is very hard to work with and very expensive. So an RS-68/68A in comparison would be much more expensive. An SSME is in a class of its own for costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the real comparison would be something like a pair of RD-193's or a single RD-180 whose aggregate thrust is on the order of 800,000lbs of thrust. Each Merlin is approaching 180,000lbs of thrust, having started at 75,000lbs of thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, SpaceX has said that they brought in process engineers from Tesla to focus on issues related to building large numbers of engines, taking advice from the auto industry on reducing costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you make a grand total of 10 engines a year, then all the staff costs for the facility is spread as the cost of those 10 engines. If you make 400 (SpaceX's stated goal) it is spread over more. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-02-10T16:34:21.347" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14026" PostTypeId="2" ParentId="14023" CreationDate="2016-02-10T16:41:33.707" Score="35" Body="&lt;p&gt;No, on the contrary. The larger the engines get, the more expensive they get. Just look at the massive amount of full-scale tests they needed to get the F-1 engines of the Saturn V main stage running smoothly. Now imagine, you spend all this money to design a huge engine, and you want to manufacture it. You will need a top-notch machine park to do that. Making such an engine includes a lot of steps, like getting a huge chunk of space alloy, and milling almost all of it away, closing the cooling channel using a slow galvanic process, maybe some electron ray welding here and there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply put, to make a huge engine, you need huge amounts of raw materials, huge machines and a huge permanent staff to operate all of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, on the other hand, if you want to make several small engines, the design becomes less risky and you don't need such huge machines. What's more, since you now need many more engines, you have something resembling a production line, and all machines are operating at the same time, rather than each engine going through a process like in a manufactory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At almost every step of the way you save money, but several smaller engines are generally heavier. An even greater downside is that, if you need every engine to work for your rocket to reach orbit, you end up with a larger chance of failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has some redundancy planned in, but that comes at a further cost in terms of payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end it comes down to cost per kilogram to a target orbit to the customer, and the amount of factors that have an influence here is way too large for an SE answer. SpaceX bet on clustering, Europe will continue to go with large engines. Only time will tell who made the better call.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-02-16T14:43:40.340" LastActivityDate="2016-02-16T14:43:40.340" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14027" PostTypeId="1" AcceptedAnswerId="14028" CreationDate="2016-02-10T18:42:09.997" Score="-1" ViewCount="579" Body="&lt;p&gt;So I was building a rocket. I tried to find a more efficient way of attaching the fins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So then I realized how easy it would be if I just attached them like this:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/EzUCF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EzUCF.png&quot; alt=&quot;Possible fin configuration.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So instead of  having the fins perpendicular to the body, what if we had fins parallel to the body?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why would this be a bad idea?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about 4 fins?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NmWpu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NmWpu.png&quot; alt=&quot;Possible fin configuration.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I could not resist but ask if this will actually affect anything:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b2A5g.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b2A5g.png&quot; alt=&quot;Possible fin configuration&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12205" LastActivityDate="2016-02-10T19:43:36.333" Title="Why won't this configuration of fins work?" Tags="|construction|model-rocketry|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14028" PostTypeId="2" ParentId="14027" CreationDate="2016-02-10T19:43:26.650" Score="9" Body="&lt;p&gt;The purpose of the fins is to keep the rocket on course. When your rocket goes off course, the rocket body is off-axis relative to the rocket's velocity vector. This means the side of two fins is exposed to the wind streaming down the rocket body. This creates a force that pushes the rocket back on course. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In your configuration, when there's a force on the fins, the fins exert a torque on the rocket, making it spin. Sometimes you want that (spin adds stability), but generally you want constant spin, not a spin rate that depends on how much your rocket goes off-axis.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, the fins have to be much larger to have the same effective area: some of the fin area is in front of the rocket body, meaning any force acting on this area would have acted on the body anyway. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-10T19:43:26.650" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14029" PostTypeId="2" ParentId="14027" CreationDate="2016-02-10T19:43:36.333" Score="4" Body="&lt;p&gt;We don't know if it might work or not, it entirely depends on what effect are you after. But such a fin configuration will cause spin along the long axis (might be desirable for all we know, smaller launch vehicles are often intentionally spun for flight stability), and pressure on the fins from both sides and lateral to them won't be equal in magnitude, which might cause vibration, destabilize the vehicle and snap the fins right off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both effects are due to pressure of compressed air as the vehicle gains speed and ascends through the denser part of the atmosphere not being equal at both sides of each fin:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/avrE5.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/avrE5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastActivityDate="2016-02-10T19:43:36.333" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14030" PostTypeId="1" AcceptedAnswerId="27387" CreationDate="2016-02-10T19:48:47.087" Score="12" ViewCount="1484" Body="&lt;p&gt;It’s common knowledge that SpaceX goal is to create a fleet of spacecraft shuttling from Earth to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One milestone they intend to achieve beforehand is having a fleet of 1st stage boosters shuttling from Earth to LEO/GTO. They'll use their two designs as they reach maturity, the Falcon 9 (also used as the heavy boosters) and Falcon Heavy core (with partial cross-feed). This will allow SpaceX to have firsthand experience on how to manage a fleet of 1st stage boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Together with this information:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Client flight manifests are piling up, 25 launches for 2016 that are bound to spill over into 2017. &lt;/li&gt;&#xA;&lt;li&gt;In partnership with Google (and its $1B influx), doubling the Internet capacity with 4000 new satellites over 15 years.&lt;/li&gt;&#xA;&lt;li&gt;Current 3 weeks average launch pad and new build turnaround.&lt;/li&gt;&#xA;&lt;li&gt;Ideally need 12 flights yearly to keep the &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3f4idq/elons_email_to_spacex_employees_regarding_taking/&quot; rel=&quot;noreferrer&quot;&gt;company financially afloat&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Reusability in its infancy, with Elon currently saying they've passed the 70% recovery success rate mark and ~90% in 2017 (was at 40% just 7 months ago). It’s not a factual metric, just &lt;a href=&quot;https://twitter.com/elonmusk/status/689299216607232000&quot; rel=&quot;noreferrer&quot;&gt;Elon’s educated guess&lt;/a&gt;. With the goal of being well at ~100% before 2024, obviously.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Their yearly production capacity is of 40 1st stage reusable -9 boosters (and 20 2nd stage but with no intention of reusability), bringing it to 10 new builds for Falcon 9 and Heavy a year.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;The Merlin rocket has a &lt;a href=&quot;http://aviationweek.com/blog/nasa-cnes-warn-spacex-challenges-flying-reusable-falcon-9-rocket&quot; rel=&quot;noreferrer&quot;&gt;limited number of re-ignitions&lt;/a&gt; in them (engine cycle). Making the boosters reusable up to 10 times.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If the mature aircraft industry can be of an analogy here, &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/EasyJet&quot; rel=&quot;noreferrer&quot;&gt;fleet size to destination ratio&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/United_Airlines&quot; rel=&quot;noreferrer&quot;&gt;is slightly below 2:1&lt;/a&gt;&lt;/em&gt; &#xA;(Although a commercial aircraft is reusable 40 000 times ‘metal fatigue gets it after 30 years’ and has a turnaround of well under 1 day.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Lower bound answer:&lt;/strong&gt; That would mean that their 1st stage booster fleet could be as low as 2 Falcon 9s and 2 Falcon Heavies, provided the turnaround is at least as low as the demand of the fleet to be reflowed.  Bearing in mind that those 8 boosters will need to be replaced at a yearly rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Upper bound answer:&lt;/strong&gt; The other extreme is that they will keep producing to their max capacity, ever expanding their fleet. But I find that unsustainable as economically they will be producing higher than the demand (only two destinations). Also they would need at some point to shift some of this manpower into building the BFR instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which brings me to ask, how big of a booster fleet will SpaceX be settling for? They’re anticipating an even demand for Falcon 9 and Heavy. Did SpaceX provide any info on the subject?&lt;/p&gt;&#xA;" OwnerUserId="6852" LastEditorUserId="16312" LastEditDate="2017-12-13T01:09:01.347" LastActivityDate="2018-05-22T11:26:16.170" Title="How many 1st stage boosters will SpaceX settle for, for its LEO/GTO fleet?" Tags="|spacex|falcon-9|reuse|falcon-heavy|booster|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14031" PostTypeId="1" CreationDate="2016-02-10T20:16:45.050" Score="3" ViewCount="289" Body="&lt;p&gt;When I look up in the sky, I can obviously see the day side of the moon clear as, well, day, cloud cover permitting, but a thought occurred to me:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I &lt;em&gt;believe&lt;/em&gt; I can (barely) see the night side of the Moon as well, I can't tell if it's my eyes playing tricks with me and my mind &quot;filling in&quot; the space to make the moon round, or if enough light is &lt;em&gt;actually&lt;/em&gt; falling on the Moon to illuminate it to such a degree that I can see it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a specific situation is required, say a crescent moon in December/January in Northern Europe at six in the evening, watching with the naked eye and no particular low-light impairment.&lt;/p&gt;&#xA;" OwnerUserId="5898" LastEditorUserId="1142" LastEditDate="2016-02-11T01:51:59.860" LastActivityDate="2016-02-11T01:51:59.860" Title="Can I see the night side of the Moon?" Tags="|the-moon|observation|" AnswerCount="2" CommentCount="2" ClosedDate="2016-02-11T04:15:23.173" ContentLicense="CC BY-SA 3.0" />
  <row Id="14032" PostTypeId="1" AcceptedAnswerId="14036" CreationDate="2016-02-10T20:21:45.450" Score="6" ViewCount="476" Body="&lt;p&gt;The image I have is that if we turn off heating in a space vehicle, the cold will be immense. However, I started figuring in terms of energy levels of particles (which is the temperature) and realized that a cooling process is basically fast moving matter particles bouncing themselves slow by collisions with slower particles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in space, if the heat (the quickies) can't bounce of the cold (the slowies) because there's vacuum outside of the space ship, so there's no matter at all, then the energy should be conserved withing the material domain of the vessel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I wrong about it getting cold or am I misleading myself in the energy preservation reasoning? &lt;/p&gt;&#xA;" OwnerUserId="13409" LastActivityDate="2019-07-10T13:13:58.453" Title="Does it get cold in a container in space?" Tags="|life-support|heat|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14033" PostTypeId="2" ParentId="14031" CreationDate="2016-02-10T20:29:54.757" Score="4" Body="&lt;p&gt;Yes, you can often see the non-directly lit part of the moon; it's mostly lit by light reflected from Earth. A google image search for &lt;a href=&quot;https://www.google.com/search?q=earthshine%20moon&amp;amp;safe=off&amp;amp;biw=1245&amp;amp;bih=832&amp;amp;source=lnms&amp;amp;tbm=isch&amp;amp;sa=X&amp;amp;ved=0ahUKEwjNnY3IhO7KAhUN4WMKHX4ZCokQ_AUIBigB&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;earthshine moon&lt;/em&gt;&lt;/a&gt; shows many photos of the phenomenon (most of which are &quot;real&quot; pictures). &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-02-10T20:29:54.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14034" PostTypeId="2" ParentId="14031" CreationDate="2016-02-10T20:30:53.990" Score="4" Body="&lt;p&gt;Yes, if you are reasonably close to either sunset or sunrise. The 'dark' part of the moon can be seen due to earthshine, the light of the sun reflected from the earth to the moon. It makes the part of the moon that is in lunar night faintly visible if you are in an area without heavy light pollution. (Earthshine also hits the part of the moon that is in sunlight, you just can't see it.)&lt;/p&gt;&#xA;" OwnerUserId="13067" LastActivityDate="2016-02-10T20:30:53.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14035" PostTypeId="2" ParentId="14023" CreationDate="2016-02-10T21:56:48.540" Score="10" Body="&lt;p&gt;In addition to the other answers, I think its also worth noting a few extra points:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Landing&lt;/strong&gt;.  It takes just one of those engines, throttled all the way down to its minimum thrust to land the stage for re-use (a key design feature).  Any more than this amount of thrust would make the landing even more difficult.  Thus if one large engine were used instead of the 9 merlins, then it would have the additional design requirement of being able to throttle down all to an equivalent level of a merlin at minimum thrust, which would presumably add to the cost.  Or have a dedicated small engine for landing, which would add weight and cost.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Stage 2&lt;/strong&gt;. The 2nd stage of this launch vehicle &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9#Comparison&quot;&gt;uses one merlin&lt;/a&gt;.  If one large engine was used for stage 1, then there would be 2 engine designs instead of one.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Redundancy&lt;/strong&gt;. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9#Engine-out_capability&quot;&gt;wikipedia&lt;/a&gt;:&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Like the Saturn series from the Apollo program, the presence of multiple first-stage engines can allow for mission completion even if one of the first-stage engines fails mid-flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A single large engine could not satisfy this safety/reliability design requirement by means of redundancy.  So it would have to have that extra required reliability as an inherent part of its design, which would also increase cost.&lt;/p&gt;&#xA;" OwnerUserId="758" LastActivityDate="2016-02-10T21:56:48.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14036" PostTypeId="2" ParentId="14032" CreationDate="2016-02-10T22:42:08.623" Score="7" Body="&lt;p&gt;All objects in a vacuum will have a decreasing temperature, unless new heat is supplied. The reason for that is energy lost to &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermal_radiation&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermal radiation&lt;/a&gt;, which is electromagnetic radiation carrying energy away. In fact, this is the same thing that allows the Sun to emit light even though surrounded by vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnitude of the radiated energy is proportional to the forth power of the absolute surface temperature. (The &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stefan-Boltzmann law&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An object in space would eventually stabilize at a temperature where the energy radiated away is equal to the radiation absorbed, for example from sunlight. This temperature may as well be above liveable temperatures. For that reason, it is common to add extra surface area for radiation to a spacecraft, in order to keep the temperature within acceptable limits.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="10273" LastEditDate="2019-07-10T13:13:58.453" LastActivityDate="2019-07-10T13:13:58.453" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14037" PostTypeId="2" ParentId="14023" CreationDate="2016-02-11T00:29:06.170" Score="-1" Body="&lt;p&gt;SpaceX's cost, in addition to all of the other items noted, is cheaper because they build everything in house, and have made a deliberate effort over the years to reduce their cost. Most of the other players aren't working much to reduce their costs, and not nearly as hard as SpaceX. SpaceX uses many non-space rated components which have been found to work through experimentation. All of this reduces the costs&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-11T00:29:06.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14038" PostTypeId="2" ParentId="14023" CreationDate="2016-02-11T00:42:19.213" Score="-3" Body="&lt;p&gt;There is the fact that they already designed the rocket engine for single engine launches. Strapping 9 together is the kind of agile work done by a company that disrupts slumbering Giants. &lt;/p&gt;&#xA;" OwnerUserId="13413" LastActivityDate="2016-02-11T00:42:19.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14041" PostTypeId="1" AcceptedAnswerId="14043" CreationDate="2016-02-11T10:50:57.050" Score="15" ViewCount="2298" Body="&lt;p&gt;I have spent a lot of hours with Kerbal Space Program recently and I am curious about one thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I got into orbit around the Moon and then I was able to get back to an orbit of &quot;Kerbin&quot; (the home planet in this game, similar to Earth) and land successfully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did not have much fuel left, therefore I wanted to save as much as possible for anything that might happen during landing. When I was at my Apoapsis (which was as far away as the Moon) from Kerbin I used fuel to get my periapsis to only 50km, while Apoapsis remained the same. The trajectory was very &quot;ellipse-like&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happened? When I was aerobraking at 50km with more than 3000m/s speed, the apoapsis was decreasing, while periapsis remained almost the same. Then I was catapulted &quot;back into space&quot;, but with a shorter trajectory around Kerbin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did this multiple times and after some time, my speed at periapsis decreased to 2400m/s and after that, the apoapsis got as low that I stayed in the atmosphere and landed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point is - I did not have to use as much fuel to get low orbit, I slowed down a lot by repeating &quot;slow a little with aerobreaking and then go back to space&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am curious - why this is not used in reality? At least I did not hear about it. I am thinking that probably real materials do not take lightly to &quot;burning and freezing&quot; multiple times...?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS : This is a copy of an &lt;a href=&quot;https://astronomy.stackexchange.com/questions/13669/multiple-aerobraking&quot;&gt;astronomy question&lt;/a&gt; This site is more relevant.&lt;/p&gt;&#xA;" OwnerUserId="13421" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2016-02-12T00:33:07.880" Title="Multiple aerobraking" Tags="|orbital-mechanics|spacecraft|landing|aerobraking|kerbal-space-program|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14042" PostTypeId="2" ParentId="14041" CreationDate="2016-02-11T11:09:13.560" Score="16" Body="&lt;p&gt;This works well in reality too, and helps to reduce heat loads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason this is not used is the fact that you must then pass through the &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;nofollow&quot;&gt;Van Allen radiation belts&lt;/a&gt; multiple times, where you ideally want the crew to spend as little time as possible due to the high levels of radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason is redundancy. If the heat shield is not able to the whole deceleration in one go, how can you be sure it is safe enough? The final part of the aero-braking from orbit that you must do in one take is close to 8000 m/s, and that is not so much smaller than the 11000 m/s you have when hitting the atmosphere when returning from the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A less extreme variant of multiple re-entries is &lt;a href=&quot;https://en.wikipedia.org/wiki/Skip_reentry&quot; rel=&quot;nofollow&quot;&gt;skip re-entry&lt;/a&gt;, which uses aero-dynamical lift to pass through the atmosphere multiple times to bleed of speed, but also radiate heat in-between.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-12T00:33:07.880" LastActivityDate="2016-02-12T00:33:07.880" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14043" PostTypeId="2" ParentId="14041" CreationDate="2016-02-11T12:47:28.543" Score="15" Body="&lt;p&gt;This is used in reality, although it is not as easy to do in real life as it is in Kerbal Space Program. Such passes dramatically increase the heat that the capsule is forced to deal with, and as a result, it might simply fail. The only time it is done is when orbiting bodies with an atmosphere, specifically Mars and Venus. What is done is that the spacecraft enters a highly elliptical orbit around the body. Repeated passes through the very upper part of the atmosphere will slow down the satellite, dragging the apoapsis down. This is then continued until a short burn will bring it above the atmosphere, circularizing the orbit, and saving a massive amount of fuel. To do this safely takes a long time: the &lt;a href=&quot;http://mars.nasa.gov/mro/mission/timeline/mtaerobraking/&quot; rel=&quot;noreferrer&quot;&gt;Mars Reconnaissance Orbiter&lt;/a&gt; took almost 6 months to complete the aerobraking process. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To a lesser extend this was used in Apollo's re-entry. The re-entry was aimed at such a point where after the lowest part of the atmosphere was reached, it continued to rise, although it slowed down before a complete orbit was accomplished. This allowed for extending the period of time which the speed was dropped, improving the re-entry process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The limiting factor, which does not apply to KSP, is that the higher atmosphere of bodies isn't well known, and in fact is quite variable over time. If you push aerobraking too far, you've lost the spacecraft, so one has to be cautious. But it can, and it has, been used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted that in reality the first spacecraft to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobraking&quot; rel=&quot;noreferrer&quot;&gt;aerobraking&lt;/a&gt; for a real mission was Magellan, in 1993. 4 spacecraft have since used it around Mars, and there was a demonstration mission around Earth in 1991. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2016-02-11T19:17:40.750" LastActivityDate="2016-02-11T19:17:40.750" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14044" PostTypeId="1" CreationDate="2016-02-11T13:37:40.827" Score="7" ViewCount="438" Body="&lt;p&gt;A flyby probe at Jupiter gets its inclination, relative to the ecliptic, changed if it passes above or below Jupiter's equator. How sensitive is a trajectory to this effect?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that there could exist a function which approximately describes the change in a flyby probe's trajectory given the perijove latitude, the altitude and whether on the leading or trailing side of Jupiter's orbit around the Sun. &lt;em&gt;(I don't request exact calculations, just some hand which waves a ball in the park, or however the sayings go).&lt;/em&gt; Would a polar passage at Jupiter throw a probe into a trajectory perpendicular to the ecliptic?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-02-11T13:47:23.783" LastActivityDate="2016-12-30T13:53:27.883" Title="What are the gravitational assist effects on the inclination of a probe's trajectory, from passing by Jupiter at different latitudes?" Tags="|gravity-assist|flyby|jupiter|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14045" PostTypeId="2" ParentId="14044" CreationDate="2016-02-11T15:39:18.373" Score="12" Body="&lt;p&gt;Yes, Jupiter can make a trajectory perpendicular to the ecliptic, which is exactly what it did for &lt;a href=&quot;http://solarsystem.nasa.gov/missions/ulysses&quot; rel=&quot;noreferrer&quot;&gt;Ulysses&lt;/a&gt;, which observed the poles of the Sun:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zKQky.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zKQky.jpg&quot; alt=&quot;Ulysses trajectory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The function is just vector addition. It depends on the $V_\infty$ with respect to Jupiter and the flyby distance, from which you get a change in direction in the plane of the trajectory relative to Jupiter, which is a $\Delta V$ vector. You change from the Jupiter-centered plane to to the Sun-centered frame to get the $\Delta V$ vector in that frame and add that to the velocity vector of the spacecraft in that frame before the swing-by. Then the new plane formed by that vector and the Sun is the plane of the new orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2012/reid/&quot; rel=&quot;noreferrer&quot;&gt;this reference&lt;/a&gt; for the calculations.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-02-11T15:45:35.323" LastActivityDate="2016-02-11T15:45:35.323" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14046" PostTypeId="1" CreationDate="2016-02-11T16:01:53.750" Score="1" ViewCount="509" Body="&lt;p&gt;Is there any leaked information about the real purpose of the X-37B. It is said that it could be used as a multipurpose space-plane such as a spy satellite or demonstrating technologies for re-usability, reliability, test of new  propulsion technology for longevity, testing avionics, flight systems, guidance and navigation, thermal protection, or insulation  and re-entry systems. In previous space flights information was tended to be secret and not many declarations are done by Boeing, Air Force and NASA for the purpose of each launch or program at all. Most of information remain just as possible but not confirmed. So i was searching if are some proofs,kind of proofs such as confirmed declarations published by wikileaks for example or leaked by inside people of the program, if X-37B is used for spying other countries for example, intercepting or capturing confidential secret information of other countries, if X-37B is completely a spy satellite itself, if it testing new propulsion system then what kind of it is testing and keeping so secret to other agencies,or doing other experimental technologies what exactly new technology is operating.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-02-11T20:09:48.033" LastActivityDate="2016-02-11T20:09:48.033" Title="Is there any leaked information about the real purpose of the X-37B spaceplane" Tags="|nasa|spacecraft-development|spaceplane|x-37|millitary|" AnswerCount="1" CommentCount="1" ClosedDate="2016-02-11T20:54:38.777" ContentLicense="CC BY-SA 3.0" />
  <row Id="14047" PostTypeId="2" ParentId="14046" CreationDate="2016-02-11T17:49:36.147" Score="1" Body="&lt;p&gt;The official name of the vehicle is the &quot;Orbital Test Vehicle&quot;, and there is an information sheet produced by the &lt;a href=&quot;http://www.af.mil/AboutUs/FactSheets/Display/tabid/224/Article/104539/x-37b-orbital-test-vehicle.aspx&quot; rel=&quot;nofollow&quot;&gt;US Airforce&lt;/a&gt;. The two stated purposes are:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_X-37#cite_note-USAF_factsh-5&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; has a pretty detailed list of some of the items that have been tested on the various flights. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most likely candidate, as stated by &lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-3106293/Is-X-37B-secret-spy-mission-Air-Force-Amateur-astronomers-spot-mystery-space-plane-unusual-orbit.html&quot; rel=&quot;nofollow&quot;&gt;Daily Mail&lt;/a&gt;, is that the space plane is being used to test technologies that will improve spy satellites. Such things could include camera testing, thrusters, surviving at low altitude, among others. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-11T17:49:36.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14048" PostTypeId="2" ParentId="14041" CreationDate="2016-02-11T18:19:19.327" Score="5" Body="&lt;p&gt;This works just fine. This is not normally used for manned returns to Earth because of the amount of life support equipment required to be in the return module, making it much heavier (and slower return -- as you probably discovered the first elliptical orbit may well have a period of &gt; 1 day). Also, this makes recovery annoying due to the low precision involved.&lt;/p&gt;&#xA;" OwnerUserId="10442" LastActivityDate="2016-02-11T18:19:19.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14049" PostTypeId="1" CreationDate="2016-02-11T18:28:52.977" Score="8" ViewCount="127" Body="&lt;p&gt;Being announced as the discovery of the decade (sucks to work in science for the next 4 years I guess) the recent announcement of the discovery of gravitational waves from the collision of two black holes seems to usurp the eLISA mission entirely. Is the hope that the eLISA mission can do more accurate or finer measurements? May the mission be scrapped?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I considered putting this on Physics stack exchange but it seems part of the answer will be about the satellite funding etc.&lt;/p&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2016-02-11T21:41:36.920" Title="What do the gravitational wave findings mean for Evolved Laser Interferometer Space Antenna satellite" Tags="|future-missions|cost|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14050" PostTypeId="1" CreationDate="2016-02-11T19:04:00.657" Score="2" ViewCount="1173" Body="&lt;p&gt;If I had the time and the financial means to do so (and legal rights), could I design, build, launch, and deliver a small (very small)&#xA;payload to low Earth Orbit?&#xA;I think it would be awesome to put a small camera up in LEO and have it radio back a picture or two of the Earth before it de-orbits and burns up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the biggest hurdle to overcome?  My gut feeling tells me that the sheer amount of fuel I would need would be the biggest prohibittor (all legal issues aside :) ).&lt;/p&gt;&#xA;" OwnerUserId="13032" LastEditorUserId="13032" LastEditDate="2016-02-11T19:47:46.757" LastActivityDate="2016-02-11T19:47:46.757" Title="Can I launch a homemade rocket from the ground that can reach LEO?" Tags="|low-earth-orbit|amateur-rocketry|" AnswerCount="1" CommentCount="4" ClosedDate="2016-02-11T19:19:15.680" ContentLicense="CC BY-SA 3.0" />
  <row Id="14051" PostTypeId="2" ParentId="14050" CreationDate="2016-02-11T19:15:14.807" Score="3" Body="&lt;p&gt;Of course you &lt;em&gt;could&lt;/em&gt; design, build, launch and deliver a payload, given all your needs are met. I believe Elon Musk said that fuel only costs about .3% of the total cost. Granted, he's paying employees to do the work for him, but it should be relatively low cost compared to the rest of the components&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The largest hurdle you will have to overcome is actually being able to launch it legally. See &lt;a href=&quot;https://space.stackexchange.com/questions/6071/am-i-allowed-to-send-a-rocket-to-leo?lq=1&quot;&gt;Am I allowed to send a rocket to LEO?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11380" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-02-11T19:15:14.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14052" PostTypeId="1" AcceptedAnswerId="14055" CreationDate="2016-02-11T19:35:07.880" Score="1" ViewCount="225" Body="&lt;p&gt;I was thrilled watching 'Interstellar'. It was only much later that I came to realize certain anomalies in the movie (like the scene in which McConaughey approaches the Gorgantua &amp;mdash; his body theoretically should have been ripped apart &amp;mdash; much, much before he could reach the singularity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ever since, I have been wondering &amp;mdash; should there really be a time dilation between Earth and a planet similar in size and mass to Earth &amp;mdash; revolving around a supermassive blackhole (like the Miller's Planet described in Interstellar). If an object experiences a similar weight on that planet, what factors cause the dilations shown in the movie?&lt;/p&gt;&#xA;" OwnerUserId="13365" LastEditorUserId="6816" LastEditDate="2016-02-11T19:40:30.703" LastActivityDate="2016-02-11T21:20:22.357" Title="Time dilation between Earth and a similar planet revolving around a supermassive Black hole" Tags="|gravity|time|black-hole|spacetime|" AnswerCount="1" CommentCount="4" ClosedDate="2016-02-17T08:27:17.263" ContentLicense="CC BY-SA 3.0" />
  <row Id="14054" PostTypeId="2" ParentId="13986" CreationDate="2016-02-11T20:34:24.940" Score="-2" Body="&lt;p&gt;Because congress wants pork to go to Utah and congress wants to keep engineers who know how to make large solid rocket ICBMs in jobs in case we ever need to build a lot more of them in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid rockets are a terrible fit for SLS, not only do they have form for having killed seven astronauts but they are no cheaper than a liquid fueled booster without any hope of having an economically viable way of reusing them (sts booster reuse saved nothing money wise).&#xA;Block 1b SLS is a heavier rocket with greater thrust than Saturn V yet still manages to deliver less cargo to tli or gto than a 50 year old design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid boosters work great to tailor performance on expendable rockets like atlas and they brilliant for ICBMs. They are pretty naff IMO for high performance heavy lift - they are too dangerous, uncontrollable, expensive, poorly performing ( mainly with regard to ISP).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not a fan.&lt;/p&gt;&#xA;" OwnerUserId="13426" LastActivityDate="2016-02-11T20:34:24.940" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14055" PostTypeId="2" ParentId="14052" CreationDate="2016-02-11T21:20:22.357" Score="1" Body="&lt;p&gt;You are a lot further down a gravity well when you are in the vicinity of a black hole. That alone can explain time dilation factors without even  considering the extreme velocity that object must have in order to remain in orbit. Both factors help 'slow' the time.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-11T21:20:22.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14056" PostTypeId="2" ParentId="13986" CreationDate="2016-02-11T21:28:55.470" Score="3" Body="&lt;p&gt;Cost is the primary advantage of SRBs. Mass specific impulse and density specific impulse are largely unimportant to a liftoff stage (and low mass specific impulse actually provides an advantage in acceleration and thus reduced gravity losses on ascent). The &quot;cost specific impulse&quot; of solid rockets is excellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle SRBs produced 11-13MN of thrust for a cost of \$23 million; the hydrogen-fueled SSME (an exceptionally complex and expensive engine, to be sure) cost \$50 million and produced a sixth as much thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SLS boosters are longer, 5 segments instead of 4, but any recovery-and-reuse features can be omitted, so it should cost less than 125% of the shuttle SRB; I'd guess something like \$28M per shot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most feasible replacement for solids for the SLS booster would be the &quot;Pyrios&quot; booster proposed by Dynetics and Rocketdyne; each booster would be a kerosene/LOX tank with a pair of cost-reduced F-1B engines. It's unclear what Pyrios boosters would cost, but it's hard to imagine it would be much less than $20 million per &lt;em&gt;engine&lt;/em&gt;, plus the structure.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-12-29T16:08:51.787" LastActivityDate="2016-12-29T16:08:51.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14057" PostTypeId="2" ParentId="14049" CreationDate="2016-02-11T21:41:36.920" Score="5" Body="&lt;p&gt;The scientific method require experimental results to be confirmed by other researchers. Even though &lt;a href=&quot;https://en.wikipedia.org/wiki/LIGO&quot; rel=&quot;noreferrer&quot;&gt;LIGO&lt;/a&gt; has detected gravitational waves, that is still a weak theory until others can reproduce the experiment. Ideally, you also want another detection method to be substantially different from previous experiments, in order to confirm the hypothesis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that the &lt;a href=&quot;https://www.elisascience.org/articles/lisa-pathfinder/lpf-mission/lpf-mission-journal-approaching-launch&quot; rel=&quot;noreferrer&quot;&gt;LPF&lt;/a&gt; is already launched, I doubt that it will be cancelled. I expect quite the contrary, eLISA is a perfect opportunity to do the confirmation of the results found by the LIGO team, and will gain more public interest from the recent news.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-11T21:41:36.920" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14059" PostTypeId="1" AcceptedAnswerId="14060" CreationDate="2016-02-11T23:52:48.503" Score="23" ViewCount="4739" Body="&lt;p&gt;On the Sci-Fi Stack Exchange there was a &lt;a href=&quot;//scifi.stackexchange.com/questions/118181/if-the-ships-self-destruct-is-such-a-great-idea-why-dont-real-navies-do-this&quot;&gt;question&lt;/a&gt; about implementing self-destruct on spaceships and comparing that to scuttling on naval vessels. One person in a comment said, &quot;They do implement a self-destruct on real spacecraft. If a launch is off of intended trajectory and is a potential danger they will order the rocket to self destruct.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first I was suspicious of this statement because it seems to me that causing parts of a rocket to go careening off in random directions is no way at all going to increase the safety of the situation. Then I found &lt;a href=&quot;//space.stackexchange.com/questions/923/do-all-launches-include-self-destruct-mechanisms&quot;&gt;this question here&lt;/a&gt; on this Stack Exchange, where the one and only answer verifies that most nations do implement self-destruct mechanisms and includes a recap on the situation with the Challenger disaster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But now I am still wondering: why is it safer to have many small parts of the rocket come down separately than a fewer number of very large pieces? It's still the same mass overall and might just as well do &lt;em&gt;more&lt;/em&gt; damage since it will surely fall over a larger area. This same reasoning has been used to disqualify the idea of blowing up an incoming asteroid as stated in &lt;a href=&quot;http://www.universetoday.com/16066/bad-idea-blowing-up-asteroids-with-nuclear-missiles/&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; which quotes Apollo astronaut Rusty Schweickart:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Another problem I can see is blowing up a large piece of rock only to&#xA;  create many smaller (but just as deadly) pieces of rock, doesn’t&#xA;  really extinguish the destructive power of an asteroid on collision&#xA;  course, in fact, it might increase it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So what makes a rocket different? Are there any examples where the self-destruction of a vehicle was undoubtedly safer than the alternative?&lt;/p&gt;&#xA;" OwnerUserId="6392" LastEditorUserId="6816" LastEditDate="2016-02-13T00:13:23.423" LastActivityDate="2023-11-26T19:11:59.793" Title="How can a rocket's self-destruct make for a safer situation than being out-of-control?" Tags="|rockets|range-safety|flight-termination-system|" AnswerCount="6" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14060" PostTypeId="2" ParentId="14059" CreationDate="2016-02-12T00:13:47.553" Score="31" Body="&lt;p&gt;Self destructs do not result in smaller deadly pieces hitting people!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What the self destruct is designed to do is prevent &lt;strong&gt;powered&lt;/strong&gt; chunks of rocket or payload getting to somewhere dangerous. If you look at any destruct videos, you will see that the explosion takes out any form of propulsion. Imagine leaving the craft to travel unguided until propellant ran out - you could hit a major city, or a nuclear reactor, or something equally as critical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, there may still be big or smaller pieces of the craft (generally much smaller, but not always) but they will come down within the safe exclusion zone near the platform or downrange.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you end up with burning propellant and hot shards of metal over the deserted area around the launch platform, it's not a fatal problem (if we exclude manned launches...) as all spectators, civilians etc are well away from the area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oh, and it's entirely different to the blowing an asteroid into small chunks of asteroid concept. There we have a potential target of a hemisphere of the Earth.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2016-02-12T00:13:47.553" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14061" PostTypeId="2" ParentId="14059" CreationDate="2016-02-12T00:20:20.720" Score="6" Body="&lt;p&gt;In most scenarios in which a rocket flight needs to be deliberately terminated, the rocket is nearly full of fuel and oxidizer. The range-safety self-destruct system opens the propellant tanks rapidly, allowing the propellant to mix and burn while the rocket is still up in the air.&lt;/p&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=ycRVAcZC5R4&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video of a Proton rocket guidance failure&lt;/a&gt;, it's clear by 0:15 seconds into the flight, we are not going to space today. The flight should have been terminated at that point,but apparently Proton doesn't have a self-destruct. The rocket starts to come apart, presumably due to aerodynamic stresses, around 0:25, and the body of the rocket ruptures and burns. Even so, there's a huge fireball when it hits the ground. If it could have been terminated earlier, the ground fireball would have been smaller, and closer to the launch pad (i.e. closer to the center of the area cleared of people for safety); if it hadn't exploded in the air, it would have been a bigger fireball, possibly travelling along the ground, and likely much further from the pad.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2023-11-26T19:11:59.793" LastActivityDate="2023-11-26T19:11:59.793" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="14062" PostTypeId="2" ParentId="14059" CreationDate="2016-02-12T01:24:32.433" Score="11" Body="&lt;p&gt;Self destruct is used in order to burn all the fuel in the air, preventing it from landing on the ground. The debris is not so dangerous in comparison. Most of a rocket is fuel, like most of the weight of a soda tin can is soda. Also to make sure that the event takes place over an evacuated area in a prepared launch range (although the Chinese don't seem to give that part of it high priority).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That spectacular 2011 Proton launch failure was caused by all the gyroscopes having been installed upside down, so the test of them before launch showed all correct because they were all equally wrong in agreement. Self destruct in that case would've been counter productive since the altitude was too low. The fuel would just have been spread out even more than by a crashing rocket. They shut down one of the six engines in the first stage in order to make it steer away from the launch pad and crash somewhere safe. Proton uses very toxic hypergolic fuel which adds to the problem of not self destructing. The landing ground had to be decontaminated.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-02-12T01:24:32.433" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14063" PostTypeId="2" ParentId="14059" CreationDate="2016-02-12T01:28:26.990" Score="20" Body="&lt;p&gt;As for whether it's implemented on real spacecraft:  This &lt;a href=&quot;https://www.youtube.com/watch?v=PDVN78hdqTk&quot;&gt;amateur video of the Challenger accident&lt;/a&gt; captures the destruction of the SRBs (about 1:45 in).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three factors as  to why:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you push the button you convert one big rocket into a bunch of little pieces.  Little pieces fall much more slowly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you blow the rocket you know where the debris is going to fall--and that's almost certainly an area that's already been cleared for the launch.  Rather than on inhabited area like the &lt;a href=&quot;https://www.youtube.com/watch?v=8_EnrVf9u8s]&quot;&gt;Chinese Long March 3 catastrophe&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you blow the rocket you burn up the fuel.  That's a whole bunch of mass that doesn't come back down at all and the energy in that fuel is up there with small atomic weapons.  That's an awful lot of destruction you prevented.  (Admittedly, the fuel isn't as damaging as a nuke of the same power but it can still be devastating.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="6944" LastEditDate="2016-02-12T03:50:01.460" LastActivityDate="2016-02-12T03:50:01.460" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="14064" PostTypeId="2" ParentId="14059" CreationDate="2016-02-12T07:43:54.487" Score="10" Body="&lt;p&gt;If you check for example the &lt;em&gt;Jason 3&lt;/em&gt; satellite launch &lt;a href=&quot;http://www.spacex.com/webcast&quot; rel=&quot;noreferrer&quot;&gt;webcast&lt;/a&gt; starting around 23 minutes, you will see this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KbWRe.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KbWRe.png&quot; alt=&quot;Jason 3 launch&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://www.spacex.com/webcast&quot; rel=&quot;noreferrer&quot;&gt;SpaceX webcast&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;white&lt;/em&gt; path shows the past trajectory of the rocket and the &lt;em&gt;blue&lt;/em&gt; is a projection of ballistic path in case of total engine cutoff. The projection is always updated as can be seen in the video. For each launch there is a trajectory which the rocket should follow and then a safe flight launch corridor (not showed in there) where ships and airplanes etc. are forbidden during the launch. If there is any problem, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Range_safety&quot; rel=&quot;noreferrer&quot;&gt;Range Safety Officer&lt;/a&gt; monitors the projection and in case it would get outside of the specified zone, they terminate the flight (autodestruct or just thrust termination) so the rocket or its remains have no chance of getting outside the safe zone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That way the destruction of the rocket ensures that it does not just fly randomly still being powered by the powerful engines but it instead goes down where it is expected to not do any harm.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-02-12T07:43:54.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14065" PostTypeId="1" AcceptedAnswerId="14067" CreationDate="2016-02-12T07:52:03.360" Score="0" ViewCount="165" Body="&lt;p&gt;I'm guessing as if its an optical reflecting telescope. How in general does NASA and other research centres detect far away bodies? What telescopes do they use, and in general, wouldn't the event be over before we'd discovered it?&lt;/p&gt;&#xA;" OwnerUserId="13431" LastActivityDate="2016-02-12T08:29:53.593" Title="If a telescope detects death of a star which is 1 million light years away today, doesn't that mean it's dead 1 million years ago?" Tags="|nasa|space-telescope|stars|" AnswerCount="1" CommentCount="5" ClosedDate="2016-02-12T09:48:47.567" ContentLicense="CC BY-SA 3.0" />
  <row Id="14066" PostTypeId="1" CreationDate="2016-02-12T08:18:30.047" Score="5" ViewCount="426" Body="&lt;p&gt;I'm looking for the record shortest and longest times a real rocket took to get its payload into LEO, such that it didn't need any additional delta-v to stay in orbit, starting from launch at the Earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that shorter times mean less gravity drag, but heavier engines. I want to compare the launch mass and payload mass for the rockets with the shortest and longest times to LEO to see which method has the lowest launch mass to payload mass ratio.&lt;/p&gt;&#xA;" OwnerUserId="11820" LastEditorUserId="11820" LastEditDate="2016-02-12T16:16:47.470" LastActivityDate="2016-02-12T17:00:18.727" Title="Shortest and longest times a real rocket got into LEO from liftoff?" Tags="|launch|orbit|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14067" PostTypeId="2" ParentId="14065" CreationDate="2016-02-12T08:29:53.593" Score="2" Body="&lt;p&gt;Yes, that event was &lt;em&gt;roughly&lt;/em&gt; 1mln years ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At scales of &quot;merely&quot; a million light years, the inaccuracy would be of order of 10 days. On distances of billions of light years the inaccuracy can climb considerably due to space expansion - an event we have observed today, that ocurred a couple billion years ago, occurred a considerably shorter time ago than its distance in light years would indicate. After all, the radius of the observable universe is 46.5 billion light years but the age of the universe is 13.82 billion years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Various astronomical institutes worldwide mostly use radiotelescopes and telescopes of infrared and higher spectra. Due to the Red Shift phenomenon (caused by space expansion) they are more useful at picking radiation of distant objects than visible spectrum ones. And yes, at these distances they observe events that are long long past.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-12T08:29:53.593" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14068" PostTypeId="1" AcceptedAnswerId="14076" CreationDate="2016-02-12T08:51:19.640" Score="7" ViewCount="436" Body="&lt;p&gt;I enjoy spotting and occasionally attempting to photograph Iridium Flares:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YBdsa.jpg&quot; alt=&quot;Iridium flare&quot;&gt;&#xA;(Iridium flare with airplane trail)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They're caused by sunlight reflecting off the antennaes of the satellites that make up the Iridium communication satellite constellation, which presumably has a limited lifetime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next generation of Iridium satellites, Iridium NEXT, I believe are due to begin being launched this year.  Will these continue to cause flares in the same predictable manner, and about as regularly?  If not, how much longer can we expect to see flares from the existing constellation before they're either unpredictable because the satellites are no longer stable, or because the satellites are brought down when no longer needed.&lt;/p&gt;&#xA;" OwnerUserId="10686" LastEditorUserId="10686" LastEditDate="2016-02-12T09:03:34.740" LastActivityDate="2016-03-04T12:33:52.170" Title="What is the expected lifetime of observable Iridium flares?" Tags="|communication-satellite|" AnswerCount="1" CommentCount="2" ClosedDate="2017-04-26T13:36:41.587" ContentLicense="CC BY-SA 3.0" />
  <row Id="14069" PostTypeId="1" AcceptedAnswerId="14071" CreationDate="2016-02-12T09:06:05.497" Score="0" ViewCount="89" Body="&lt;p&gt;I was thinking about asteroids which travel through solar system and I have a question. &#xA;Is it possible to inject small object into an orbit around one of them and use momentum of this asteroid to increase range of our object?&lt;/p&gt;&#xA;" OwnerUserId="10151" LastActivityDate="2016-02-12T10:09:57.633" Title="How feasible will be to inject an object into the asteroid orbit" Tags="|artificial-satellite|asteroid|" AnswerCount="2" CommentCount="9" ClosedDate="2016-02-12T13:22:35.157" ContentLicense="CC BY-SA 3.0" />
  <row Id="14070" PostTypeId="2" ParentId="14069" CreationDate="2016-02-12T09:48:48.277" Score="3" Body="&lt;p&gt;This is possible, but entirely pointless. Spacecraft are not limited by range, only by how much they can change their velocity, usually called &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;nofollow&quot;&gt;delta-v&lt;/a&gt;. If you can change your velocity enough to catch up with the asteroid, you are already in the same orbit as it. That means, &lt;strong&gt;You can do that even if the asteroid was not there.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are perhaps also thinking about a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow&quot;&gt;gravity assist&lt;/a&gt;, by which a spacecraft gain momentum from a fly by of a planet. But that only works with massive objects, like planets, and the gain from an asteroid fly-by is incredibly small. Also, if this manoeuvre is performed by first braking into orbit, as you describe, more delta-v is already wasted than you can possibly gain from the manoeuvre.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-12T09:48:48.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14071" PostTypeId="2" ParentId="14069" CreationDate="2016-02-12T09:49:49.970" Score="2" Body="&lt;p&gt;Orbiting does not work that way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting into orbit around an object means to get close to it and then change your speed and direction to be almost the same as the object. That means you can't &quot;hitchhike&quot; with a fast asteroid, because the energy you would need to rendezvous with it would be the same energy you need to get onto the same trajectory it already has.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What might give you a speed boost is going onto a collision course with the asteroid and having it ram you (like a billard ball). But building a spacecraft which can survive such a collision would be quite challenging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What might give you a much safer speed boost is doing a fly-by with the asteroid to give you a gravity assist. But the speed-gain by a gravity assist depends on the mass of the object, so you won't get much from an asteroid. A gravity assist is usually only worth the trouble when it's with a planet or at least a large moon.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2016-02-12T10:09:57.633" LastActivityDate="2016-02-12T10:09:57.633" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14075" PostTypeId="1" AcceptedAnswerId="14758" CreationDate="2016-02-12T17:39:20.483" Score="14" ViewCount="1825" Body="&lt;p&gt;When I saw &quot;50 seconds of weightlessness&quot; (see quote below) I made a &lt;em&gt;very rough&lt;/em&gt; estimate based on 45 degrees nose-up to 45 degrees nose-down &lt;a href=&quot;https://en.wikipedia.org/wiki/Reduced_gravity_aircraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reduced Gravity Flight&lt;/a&gt; centered at about 35,000 feet where the speed of sound is about 295 m/s, and an airspeed of mach 0.85 at that altitude:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;t = 2 * 0.707 * 0.85 * 295 (m/s) / 9.8 (m/s^2) &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;which gives about 36 seconds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After watching &lt;a href=&quot;http://www.npr.org/event/music/466443587/ok-gos-new-video-is-as-incredible-as-youd-expect&quot; rel=&quot;nofollow noreferrer&quot;&gt;this incredible video&lt;/a&gt; which is also &lt;a href=&quot;https://www.youtube.com/watch?v=LWGJA9i18Co&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; a sufficient number of times, I read &lt;a href=&quot;http://www.redbull.com/us/en/music/stories/1331776046202/ok-go-how-they-made-their-zero-gravity-video&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; which was used as a citation &lt;a href=&quot;https://en.wikipedia.org/wiki/Upside_Down_%26_Inside_Out&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; for technical discussion of the video timing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article left me somewhat upside-down and confused, asking myself &quot;&lt;em&gt;How many seconds of near-zero gravity are practical on a Reduced Gravity flight?&lt;/em&gt;&quot; as well as &quot;&lt;em&gt;Do they do things like this on the ISS when the cameras are off?&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Kulash: We also came up with a system for doing a single take over eight parabolas. In each flight you have 15 parabolas and &lt;strong&gt;in each parabola you have 20 seconds of double gravity, then 50 seconds of weightlessness&lt;/strong&gt; and few minutes of setting it all up again. So to make it one take, we took eight of these in a row over 40-45 minutes.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Sie: We also we &lt;strong&gt;slowed our playback&lt;/strong&gt; of the song down a bit (28.5 percent, to be exact) and performed each portion of the dance a little slower. This way, the &lt;strong&gt;21 seconds of song fit neatly into the 27 seconds of weightlessness&lt;/strong&gt;. The pilots — there are 10 OF THEM flying the plane at the same time, by the way — pull out of the parabola when the plane has enough downward speed and momentum in order to &quot;scoop&quot; itself up out of the downward acceleration.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;strong&gt;bold highlight&lt;/strong&gt; is mine. In the first quote - perhaps the numbers are reversed and it's 50 seconds of &gt;1G and 20 seconds of weightlessness, which would then be more consistent with &lt;a href=&quot;http://www.ew.com/article/2016/02/11/ok-go-zero-gravity-music-video&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, which also supports the 27 seconds in the second quote.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But maybe I am wrong and somehow the Russian company &lt;a href=&quot;https://en.wikipedia.org/wiki/S7_Airlines#Current_fleet&quot; rel=&quot;nofollow noreferrer&quot;&gt;S7 Airlines&lt;/a&gt; (mentioned &lt;a href=&quot;https://en.wikipedia.org/wiki/Upside_Down_%26_Inside_Out&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; ) found a way to go mach 1.2 to get 50 seconds?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So &lt;strong&gt;my question:&lt;/strong&gt; &quot;How many seconds of near-zero gravity are practical on a Reduced Gravity flight?&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I count seven &quot;parabolas&quot; instead of eight, but between 02:00 and 02:31 it looks like partial gravity - not a typical parablola.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;video time - they are clearly modulating frame rate to reduce the high G periods&#xA;begins  ends&#xA;00:26   01:04&#xA;01:06   01:24 (normal walking)&#xA;01:26   01:42 (normal walking)&#xA;01:48   02:05 &#xA;02:07   02:12 weak gravity?  02:27 stronger gravity?&#xA;02:31   02:47 &#xA;02:51   03:08&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-19T09:37:46.007" LastActivityDate="2020-06-25T13:23:58.533" Title="How many seconds of near-zero gravity are practical with a Reduced Gravity Aircraft?" Tags="|microgravity|astronauts|training|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14076" PostTypeId="2" ParentId="14068" CreationDate="2016-02-12T17:42:06.080" Score="5" Body="&lt;p&gt;Responding to &quot;&lt;em&gt;Will these continue to cause flares in the same predictable manner, and about as regularly?&lt;/em&gt;&quot; (SEE EDIT at foot of answer) the original Iridium satellites and the Iridium Next generation are of different physical design, see the following photos from these sources:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;nofollow noreferrer&quot;&gt;Original&lt;/a&gt;  (Earth is to the right as this photo is oriented)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.iridium360.com/2013/04/19/iridium-next-a-global-effort-to-launch-the-future-of-global-communications/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NEXT&lt;/a&gt;   (despite the artists image of the planet, this satellite points to Earth in the lower left foreground)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current Iridium satellites aren't unique in producing strong flares though I think it would be a coincidence, give the huge variety of satellite designs, if the replacement constellation was similar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Four the part of your question &quot;&lt;em&gt;If not, how much longer can we expect to see flares from the existing constellation&lt;/em&gt; &quot; I think this will be driven a) from the deployment schedule of the new generation adhering to plans and b) the remaining propellant of the older satellites and how they are managed after retirement from the operating fleet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H3uMc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H3uMc.jpg&quot; alt=&quot;Original Iridium based on LM700 satellite platform&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MRNP2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MRNP2.jpg&quot; alt=&quot;Iridium-NEXT based on Thales Alenia Elite bus&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: This &lt;a href=&quot;http://www.iridium360.com/2011/08/17/video-roundup-iridium-flares/&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt; is a bit more direct:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Iridium flares will be gone in a few years when we launch our next generation of satellites, Iridium NEXT. Due to design improvements, our new satellites unfortunately are not expected to create flares, so enjoy Iridium flares while you can. The Iridium NEXT satellites will launch in 2015-2017 and will have only one antenna instead of three. The new antennas offer increased data speed and capacity but are angled differently than the current design, and are therefore not likely to reflect much sunlight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-03-04T12:33:52.170" LastActivityDate="2016-03-04T12:33:52.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14077" PostTypeId="1" CreationDate="2016-02-12T17:42:35.050" Score="7" ViewCount="408" Body="&lt;p&gt;North Korea recently launched &lt;a href=&quot;https://en.wikipedia.org/wiki/Kwangmy%C5%8Fngs%C5%8Fng-4&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kwangmyongsong-4&lt;/a&gt; but it wasn't entirely successful and is &lt;a href=&quot;https://space.stackexchange.com/q/14014/10234&quot;&gt;tumbling out of control&lt;/a&gt;. I read on a &lt;a href=&quot;http://www.wired.com/2016/02/tracking-north-korea-satellite/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wired&lt;/a&gt; article that &lt;em&gt;“This thing is probably a failed attempt to get higher up, and is going to re-enter soon.”&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3Q3zp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3Q3zp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How soon is soon? Does anyone know when this satellite is likely to re-enter?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2020-01-11T17:15:05.070" Title="When (roughly) will North Korea's Kwangmyongsong-4 satellite re-enter?" Tags="|orbital-mechanics|artificial-satellite|reentry|north-korea|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14078" PostTypeId="1" AcceptedAnswerId="14103" CreationDate="2016-02-12T18:53:04.910" Score="8" ViewCount="507" Body="&lt;p&gt;Let's imagine we have finally developed buckytube rope. A couple hundred kilometers of rope, able to tug a 100-ton craft at 6g acceleration, in a package packable on said craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A near-earth asteroid is passing by. A craft is launched on a nearby flyby trajectory. When the flyby point approaches, the craft launches a &quot;missile&quot; - either an impactor one with a harpoon style hook, or some kind of net, or one that could create a loop... with the rope trailing. It catches the asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The craft unrolls the rope from a spool, while strongly braking the spool spin, so that the acceleration of the craft pulled by the rope is survivable for the crew/payload and doesn't break anything. This is until the spool stops or the whole rope unrolls (in which case it's allowed to fly loose with the asteroid, disengaging from the craft).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the spool came to a standstill, the craft is anchored to the asteroid, having gained good several km/s essentially free.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What (beyond inventing a buckytube rope) obstacles could stand behind such a method of propulsion? Would heat dissipation of the brake be manageable? (say, some kind of ablator/sublimator, that's a one-off affair). Would that kind of propulsion make sense?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2019-07-16T15:45:55.513" Title="Harpoon propulsion - what would be the problems?" Tags="|propulsion|advanced-propulsion|" AnswerCount="3" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14079" PostTypeId="2" ParentId="14077" CreationDate="2016-02-12T19:01:59.427" Score="4" Body="&lt;p&gt;No one can say for sure, but the appropriate ISO standard can be used to make an estimate:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://aiaa.kavi.com/apps/group_public/download.php/3172/ISO_27852_%28E%294.pdf&quot; rel=&quot;nofollow&quot;&gt;Space systems — Estimation of orbit lifetime (ISO 27852)&lt;/a&gt; (see page 20)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Figure B5 says that an object in polar orbit with about 500 km perigee (taken from your picture) will reenter in about 25 years. Depending on how much drag it produces, it could be longer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to put in some effort to get a more precise answer, try &lt;a href=&quot;http://repository.tudelft.nl/assets/uuid:41892dd4-427e-4afe-b6ef-aa1b258341b0/043-2.pdf&quot; rel=&quot;nofollow&quot;&gt;UTIAS Review 43&lt;/a&gt;. It tells you that at 500 km altitude, the free path length of a molecule is 77km, so it's not so much a gas, but rather some few stray molecules. From this alone, we can see that it won't reenter &quot;soon&quot;. To make a calculation, you would need to estimate the area of the surfaces exposed to the orbital direction and the mass. The review specifically talks about spinning satellites. If you can't make a good estimate, you could try to get the orbital parameters as precisely as possible now, and then again in a year and modify the drag coefficient to fit the change. With this you can make a better guess at the orbital lifetime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do this, please tell us what you found! If you post it as a comment, I will include it in the answer.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-02-15T12:54:08.423" LastActivityDate="2016-02-15T12:54:08.423" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14080" PostTypeId="2" ParentId="13944" CreationDate="2016-02-12T19:13:52.267" Score="2" Body="&lt;p&gt;I presume you are referring to the five Lagrange points for the earth and the sun (SE) and the five points for the earth and the moon (EM), to get to your tally of ten points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Broadly speaking for systems like SE and EM, L1-3 lie in a straight line between the bodies and are unstable. L4 and L5 are pretty stable when the larger body is ~&gt;25 times the size of the smaller one. This is just about true for EM and is obviously true for the earth and sun.&#xA;Practically speaking l4 and l5 are generally not as useful as the other Lagrange points in terms of sun earth observation, getting shielding from the sun or as a staging point for BEO exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may have heard of the three body problem, and this is the reason why there is no definitive long term stable orbit at L1-3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some relatively predictable associated orbits called Lissajous orbits which don't solve the three body problem but can let you park a spacecraft in an orbit around any of the L1-3 points with some very minimal station keeping. These are the orbits that have been used to date for satellites, but are not very useful for storing a large asteroid unless we also add a large enough propulsion system to keep it in the Lissajous orbit.&lt;/p&gt;&#xA;" OwnerUserId="13426" LastActivityDate="2016-02-12T19:13:52.267" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14081" PostTypeId="2" ParentId="13944" CreationDate="2016-02-12T19:23:08.353" Score="4" Body="&lt;p&gt;When the media talks about spacecraft at the Lagrange point, what they really are saying is that the spacecraft will be somewhere in the general area of the Lagrange point, not at the point itself which is infinitely small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravitational pull of one of the bodies will be a bit higher, but in practical term it does not make any difference, so yes, &lt;strong&gt;the Lagrange &quot;points&quot; are big enough&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are they stable? Again, for practical purposes they are i.e. with some modest station keeping you can keep a spacecraft there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More details can be found here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronomy.com/magazine/ask-astro/2013/10/lagrangian-points&quot; rel=&quot;nofollow&quot;&gt;Lagrangian points article [astronomy.com]&lt;/a&gt; &lt;br&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#Stability&quot; rel=&quot;nofollow&quot;&gt;Lagrangian points - stability [wikipedia]&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="6750" LastEditDate="2016-02-12T21:17:14.040" LastActivityDate="2016-02-12T21:17:14.040" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14082" PostTypeId="1" AcceptedAnswerId="14083" CreationDate="2016-02-12T19:54:06.917" Score="2" ViewCount="137" Body="&lt;p&gt;I understand that flying in either direction on a planet that has an atmosphere doesn't effect flight times, but on a body such as the moon, would it improve flight times?&lt;/p&gt;&#xA;" OwnerUserId="13438" LastActivityDate="2016-02-12T20:12:43.247" Title="Would flying westwards on the Moon be more efficient than east?" Tags="|rockets|the-moon|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14083" PostTypeId="2" ParentId="14082" CreationDate="2016-02-12T19:55:53.883" Score="4" Body="&lt;p&gt;Yes, your suborbital hop stays fixed in an inertial frame of reference, and the Moon rotate slowly underneath you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is not that important though, as the rotation rate of the surface of the Moon is only $4.6m/s$, relatively small compared to the orbital velocity of $1720m/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, a similar but even weaker effect exists on Earth too, only eastwards instead. With the same velocity relative to the air, you are then moving at a higher fraction of orbital velocity, and it is then easier to counteract gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most efficient way of performing a flight on the Moon is a suborbital ellipse, with foci in the centre of the Moon, and directly between the two endpoints:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/z0FtGt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/z0FtGt.jpg&quot; alt=&quot;suborbital hop&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-12T20:12:43.247" LastActivityDate="2016-02-12T20:12:43.247" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14084" PostTypeId="1" CreationDate="2016-02-12T20:41:41.600" Score="1" ViewCount="315" Body="&lt;p&gt;The planets orbiting the sun are always depicted as ovals or circles going around the sun in the center, and it throws me off at how two-dimensional it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this really how the planets go, or do some planets, say, orbit 'vertically' compared to us?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it really does look that way, why? If not, then why do we always use the '2-D' imagery to show it?&lt;/p&gt;&#xA;" OwnerUserId="13441" LastEditorUserId="12781" LastEditDate="2016-02-13T00:45:43.590" LastActivityDate="2016-02-13T18:13:18.643" Title="Are the orbits of the planets really all on the same (or a similar) plane? If so, why? If not, why do we depict it this way?" Tags="|orbit|" AnswerCount="1" CommentCount="2" ClosedDate="2016-02-12T22:40:28.293" ContentLicense="CC BY-SA 3.0" />
  <row Id="14085" PostTypeId="2" ParentId="14084" CreationDate="2016-02-12T20:51:42.000" Score="7" Body="&lt;p&gt;It's pretty close to flat.  Close enough to draw it that way, anyhow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way of measuring the &quot;flatness&quot; of the Solar System is to compare the inclination of the planets' orbits to a measure known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Invariable_plane&quot; rel=&quot;nofollow noreferrer&quot;&gt;invariable plane&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the planets' orbits are inclined by less than 2°.  Venus is inclined at 2.19°.  Mercury has the strongest inclination, at 6.34°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oFhsz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oFhsz.png&quot; alt=&quot;diagram of orbital inclinations&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(image from &lt;a href=&quot;http://clearscience.tumblr.com/post/1132218393/so-since-the-planets-all-came-from-our-flattened&quot; rel=&quot;nofollow noreferrer&quot;&gt;clearscience.tumblr.com&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="265" LastEditDate="2016-02-13T18:13:18.643" LastActivityDate="2016-02-13T18:13:18.643" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14086" PostTypeId="1" AcceptedAnswerId="14088" CreationDate="2016-02-13T15:57:57.587" Score="3" ViewCount="322" Body="&lt;p&gt;I was wondering, what if one of the satellites that were previously serviced by the space shuttle, like the Hubble Space Telescope, needs repairs now that the shuttle program is off?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there other ways to fix them, or if not, what is then going to happen with them? &lt;/p&gt;&#xA;" OwnerUserId="13445" LastEditorUserId="8693" LastEditDate="2016-02-13T17:22:51.337" LastActivityDate="2016-02-13T19:12:31.033" Title="Space maintenance after Space Shuttle Program Shutdown" Tags="|artificial-satellite|space-shuttle|maintenance|in-space-repairs|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14087" PostTypeId="1" CreationDate="2016-02-13T16:06:16.760" Score="8" ViewCount="1240" Body="&lt;p&gt;With the overwhelming majority of heat shields being ablative and expendable in nature, it is fascinating to read about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system&quot; rel=&quot;noreferrer&quot;&gt;Shuttle's TPS&lt;/a&gt;, its design and some of the problems that it faced. I believe that at some point in the (likely distant) future, developing reusable heat shielding systems will be of interest. Assuming for a moment that my speculation is accurate (please comment if you think that it is not), what would be the primary design challenges facing such a development?&lt;/p&gt;&#xA;" OwnerUserId="12877" LastEditorUserId="12877" LastEditDate="2016-02-13T16:56:08.883" LastActivityDate="2020-02-19T10:08:23.020" Title="What are the major challenges faced in designing a reusable heat shield?" Tags="|reentry|reuse|design|heat-shield|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14088" PostTypeId="2" ParentId="14086" CreationDate="2016-02-13T19:12:31.033" Score="3" Body="&lt;p&gt;There have been proposals for automated servicing systems, and NASA is doing exercises with teleoperated repair systems on the ISS. But as of today, if the Hubble shut down due to a power supply failure, there is nothing that could fix it.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-02-13T19:12:31.033" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14089" PostTypeId="1" AcceptedAnswerId="14094" CreationDate="2016-02-13T19:42:05.013" Score="8" ViewCount="706" Body="&lt;p&gt;In &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Martian_%28Weir_novel%29&quot;&gt;&lt;em&gt;The Martian&lt;/em&gt;&lt;/a&gt; by Andy Weir, the main character Mark Watney takes a laptop out of the habitation module to take notes. But when he comes out, he discovers that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid-crystal_display&quot;&gt;LCD&lt;/a&gt; monitor is broken, because the liquid crystals have froze.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Small monitors are going to be handy in many cases, for example as a control panel on an unpressurised rover (Apollo style), or for various indicators on the outside of a habitation module or a lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But can we make computer screens work at such low temperatures? Which technologies can be used, and which can definitely be ruled out?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Factors that make it tricky for monitors to work includes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Very low temperatures, often lower than -60°C.&lt;/li&gt;&#xA;&lt;li&gt;An extremely thin atmosphere preventing proper heat flow.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-14T10:07:48.833" Title="Which computer monitors will work on Mars?" Tags="|mars|crewed-spaceflight|technology|environment|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14091" PostTypeId="1" AcceptedAnswerId="14093" CreationDate="2016-02-13T23:05:30.630" Score="6" ViewCount="1044" Body="&lt;p&gt;It would be the same blimp on Earth, except with some modifications like anti-corrosion coating. The blimp will use helium as the gas to keep it aloft. Would it last long enough to be useful?&lt;/p&gt;&#xA;" OwnerUserId="12420" LastEditorUserId="49" LastEditDate="2016-02-14T08:47:21.667" LastActivityDate="2016-02-14T11:54:26.760" Title="How long would a blimp last in Venusian atmosphere at an altitude of 65 km?" Tags="|atmosphere|venus|balloons|aerobot|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14093" PostTypeId="2" ParentId="14091" CreationDate="2016-02-14T08:45:41.193" Score="9" Body="&lt;p&gt;So far, we have only two actual examples of balloons deployed into Venusian atmosphere, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_program#Balloon&quot; rel=&quot;noreferrer&quot;&gt;two Vega program balloons&lt;/a&gt; from June 1985. They were identical in design, 3.4 m diameter helium filled balloons made of teflon cloth matrix coated with teflon film, each with a gondola suspended on a 13 m long tether. You can read more about them on Goddard Space Flight Center pages: &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1984-125F&quot; rel=&quot;noreferrer&quot;&gt;Vega 1 Balloon&lt;/a&gt; and &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1984-128F&quot; rel=&quot;noreferrer&quot;&gt;Vega 2 Balloon&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both released to near-equatorial latitudes into night side of Venus, Vega 1 Balloon was active for 46 hours and 32 minutes and traversed distance of 11,600 km, of which 3,100 km in Venusian day side. Vega 2 Balloon, released 4 Earth days later, was active for roughly 60 hours (GSFC page quotes same final transmission time as for Vega 1 Balloon, but that seems to be an error), and traveled in total of 11,100 km, 3,700 km of that in the day side. Both of their westward paths were tracked by 20 ground radars using &lt;a href=&quot;https://en.wikipedia.org/wiki/Very-long-baseline_interferometry&quot; rel=&quot;noreferrer&quot;&gt;Very Long Baseline Interferometry&lt;/a&gt; (VLBI) technique:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/6F0Yd.png&quot; alt=&quot;Diagram of the paths of the two Vega balloons&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Diagram of the paths the two Vega balloons took in Venusian atmosphere. Source: &lt;a href=&quot;http://ipnpr.jpl.nasa.gov/progress_report/42-85/85Q.PDF&quot; rel=&quot;noreferrer&quot;&gt;The Venus Balloon Project&lt;/a&gt; (PDF)&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is unclear for how long balloons themselves lasted, since VLBI requires a radio source which were housed in the balloons' gondolas, but we know that both maintained average flight altitude of 53 to 54 km with variation of ~ 3 km (likely due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Lee_wave&quot; rel=&quot;noreferrer&quot;&gt;lee waves&lt;/a&gt;) and stable latitude, mean environmental pressure of 535 mbar (0.53 atm) and temperature range of 300 to 316 Kelvin (~ 27 to 43 °C).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we know that relatively stable flight of &lt;a href=&quot;https://en.wikipedia.org/wiki/Airship&quot; rel=&quot;noreferrer&quot;&gt;aerostat dirigibles&lt;/a&gt; is possible on Venus at altitudes within the layers of sulfuric acid clouds and above most of them, and this flight can last for days with over 30 years old technology and design based on just as old knowledge about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus&quot; rel=&quot;noreferrer&quot;&gt;atmosphere of Venus&lt;/a&gt;. We could probably, we should, do a lot better nowadays;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One proposed design is the use of &lt;a href=&quot;http://www2.jpl.nasa.gov/adv_tech/balloons/venus.htm&quot; rel=&quot;noreferrer&quot;&gt;Phase Change Balloons for Venus&lt;/a&gt; which could navigate at altitudes between 38 and 64 km, if filled with water/ammonia and helium mixture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/b0xRU.gif&quot; alt=&quot;Phase Change Balloons for Venus&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Phase Change Balloons for Venus. Image source and credit: &lt;a href=&quot;http://www2.jpl.nasa.gov/adv_tech/balloons/venus.htm&quot; rel=&quot;noreferrer&quot;&gt;Dr. Ronald Ross&lt;/a&gt;, JPL&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such design could be &lt;em&gt;stabilized&lt;/em&gt; (in liberal interpretation of the word, extreme altitudes would be quite stable, but it would still oscillate up and down in altitudes between gas to liquid and vice versa phase change temperatures) to higher average flight altitudes by using dark, heat absorbing layer &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_balloon&quot; rel=&quot;noreferrer&quot;&gt;solar balloons&lt;/a&gt; beneath the transparent teflon-coated and sulfuric acid resistant surface, and then depending on the sun to heat up the phase-transitioning water/ammonia mixture faster to gaseous phase at higher altitudes, where there's higher solar flux on Venusian day side. It would still depend on heat from the atmosphere alone on the dark side of Venus, but one good thing about such design is that water and ammonia are much larger molecules than helium and will escape the balloon of same basic materials much slower, so technically the balloon part of the setup could last longer. Lowest altitude could be increased by increasing ammonia to water ratio of the mixture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another design is simply improving on the Vega balloons, like e.g. &lt;a href=&quot;https://www.nasa.gov/vision/universe/solarsystem/venus-20070827.html&quot; rel=&quot;noreferrer&quot;&gt;ILC Dover and NASA Wallops were doing&lt;/a&gt; back in 2007:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/6k4py.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6k4py.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an 18 ft (5.5 m) diameter balloon that is also teflon covered and helium filled, but includes a more reflective aluminum layer, a layer of &lt;a href=&quot;https://en.wikipedia.org/wiki/BoPET&quot; rel=&quot;noreferrer&quot;&gt;mylar&lt;/a&gt; to prevent helium from escaping it, and a &lt;a href=&quot;https://en.wikipedia.org/wiki/Vectran&quot; rel=&quot;noreferrer&quot;&gt;Vectran&lt;/a&gt; inner layer with &lt;a href=&quot;https://en.wikipedia.org/wiki/Polyurethane&quot; rel=&quot;noreferrer&quot;&gt;polyurethane&lt;/a&gt; coating for structural strength and adhesion. They were designed to coast at about 56 km altitude, stable to under &amp;pm;1 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are a couple of other balloon designs on the table for exploration of Venus and at various &lt;a href=&quot;https://en.wikipedia.org/wiki/Technology_readiness_level#NASA_definitions&quot; rel=&quot;noreferrer&quot;&gt;Technology Readiness Levels&lt;/a&gt;, like e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus_In_Situ_Explorer&quot; rel=&quot;noreferrer&quot;&gt;Venus In Situ Explorer&lt;/a&gt; that is in fierce competition for selection under the &lt;a href=&quot;https://discoverynewfrontiers.nasa.gov/program/index.cfml&quot; rel=&quot;noreferrer&quot;&gt;New Frontiers Program&lt;/a&gt; (see more in e.g. &lt;a href=&quot;http://www.nap.edu/read/12175/chapter/1&quot; rel=&quot;noreferrer&quot;&gt;2008 Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity&lt;/a&gt; or &lt;a href=&quot;http://www.lpi.usra.edu/vexag/reports/&quot; rel=&quot;noreferrer&quot;&gt;VEXAG documentation&lt;/a&gt;) like &lt;a href=&quot;http://www.lpi.usra.edu/vexag/VISE/&quot; rel=&quot;noreferrer&quot;&gt;this one&lt;/a&gt;, but those are low altitude balloons and don't directly apply to your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in conclusion, we can say with some confidence that days long flights at altitudes above most sulfuric acid clouds with small and in atmosphere deployable balloons is possible, and with smart design learning from past experience, they could last a whole lot longer. The NASA Wallops design above was meant to last for several circumnavigations of Venus, each taking approximately four days to complete. It then depends on the choice of your mission objectives and their prioritization how successful you would consider such a mission and what you can do with its science return. But there are designs, some materialized and being tested, others still having to achieve that, that would enable all kinds of flight profiles, from selection of injection latitudes to vertical and zonal ranges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For some more information on the environment you'll be dealing with and references linked therein, also see:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/9160/a-cloud-top-colony-on-venus-will-it-drift-to-the-poles&quot;&gt;A cloud-top colony on Venus, will it drift to the poles?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/13775/does-venus-have-doldrums-or-horse-latitudes-latitudes-with-lower-winds&quot;&gt;Does Venus have doldrums or horse latitudes (latitudes with lower winds)?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/9158/what-useful-materials-can-be-extracted-from-venusian-atmosphere&quot;&gt;What useful materials can be extracted from Venusian atmosphere?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And other questions tagged as &lt;a href=&quot;/questions/tagged/venus&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;venus&amp;#39;&quot; rel=&quot;tag&quot;&gt;venus&lt;/a&gt; and &lt;a href=&quot;/questions/tagged/atmosphere&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;atmosphere&amp;#39;&quot; rel=&quot;tag&quot;&gt;atmosphere&lt;/a&gt; (&lt;a href=&quot;https://space.stackexchange.com/questions/tagged/venus+atmosphere&quot;&gt;link to questions including both tags&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other references:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://media.cleveland.com/science_impact/other/Landis%20Venus%20atmospheric%20flight%20study%202002.pdf&quot; rel=&quot;noreferrer&quot;&gt;Atmospheric Flight on Venus&lt;/a&gt;, Geoffrey A. Landis (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050192264.pdf&quot; rel=&quot;noreferrer&quot;&gt;Solar Airplane Concept Developed for Venus Exploration&lt;/a&gt;, Geoffrey A. Landis (PDF)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.geoffreylandis.com/papers.html&quot; rel=&quot;noreferrer&quot;&gt;Geoffrey A. Landis: Scientific Papers Available on the Web - Venus Related Papers&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Observations_and_explorations_of_Venus&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Observations and explorations of Venus&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/pioneer_venus.html&quot; rel=&quot;noreferrer&quot;&gt;Pioneer Venus Project Information&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-14T11:54:26.760" LastActivityDate="2016-02-14T11:54:26.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14094" PostTypeId="2" ParentId="14089" CreationDate="2016-02-14T10:07:48.833" Score="4" Body="&lt;p&gt;I think LCDs are going to be just fine on Mars. We have LCD displays meant to withstand temperatures as low as -50°C here on earth. Usually the displays do not work at such temperatures, though, so a small heating element would have to be used to bring the necessary parts to a more manageable -20°C when the displays need to be active.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is taking equipment designed for room temperatures to such extremes - none of the components used are rated for it, so the LCD screen is likely not the only component that is going to break.&lt;/p&gt;&#xA;" OwnerUserId="2631" LastActivityDate="2016-02-14T10:07:48.833" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14095" PostTypeId="1" AcceptedAnswerId="14110" CreationDate="2016-02-14T11:45:48.740" Score="8" ViewCount="1856" Body="&lt;p&gt;I came across the following algorithm to calculate the cartezian vectors from the orbital elements:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First calculate some coefficients that will be used in determining the position:&#xA;$$&#xA;r_x' = a (\cos E - \varepsilon)\\&#xA;r_y' = b \sin E\\&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then calculate coefficients for the velocity:&#xA;$$&#xA;\dot{\varepsilon} = \sqrt{\frac{\mu}{ar}}\\&#xA;v_x' = -a\dot{\varepsilon}\sin E\\&#xA;v_y' = b\dot{\varepsilon}\cos E&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, what appears to be some common coordinates for position and velocity:&#xA;$$&#xA;p_x = \cos\omega \cos\Omega - \sin\Omega \sin\omega \cos i\\&#xA;p_y = \cos\omega \sin\Omega + \cos\Omega \sin\omega \cos i\\&#xA;p_z = \sin\omega \sin i\\&#xA;q_x = -\sin\omega \cos\Omega - \sin\Omega \cos\omega \cos i\\&#xA;q_y = -\sin\omega \sin\Omega + \cos\Omega \cos\omega \cos i\\&#xA;q_z = \cos\omega \sin i&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, position and velocity are calculated:&#xA;$$&#xA;r_x = r_x'p_x + r_y'q_x\\&#xA;r_y = r_x'p_y + r_y'q_y\\&#xA;r_z = r_x'p_z + r_y'q_z\\&#xA;v_x = v_x'p_x + v_y'q_x\\&#xA;v_y = v_x'p_y + v_y'q_y\\&#xA;v_z = v_x'p_z + v_y'q_z&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$a$ and $b$ are the semi-major and semi-minor axes, &lt;/li&gt;&#xA;&lt;li&gt;$E$ is the eccentric anomaly, &lt;/li&gt;&#xA;&lt;li&gt;$\varepsilon$ is the eccentricity, &lt;/li&gt;&#xA;&lt;li&gt;$\mu$ is the gravitational parameter, &lt;/li&gt;&#xA;&lt;li&gt;$i$ is the inclination,&lt;/li&gt;&#xA;&lt;li&gt;$\omega$ is the argument of periapsis,&lt;/li&gt;&#xA;&lt;li&gt;$\Omega$ is the longitude of the ascending node, and&lt;/li&gt;&#xA;&lt;li&gt;$r$ is the distance from the barycenter to the orbiter.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Does anybody recognize this algorithm? I'd like to understand what $\dot{\varepsilon}$ represents, and how those coefficients are combined to produce position and velocity.&lt;/p&gt;&#xA;" OwnerUserId="13454" LastEditorUserId="13454" LastEditDate="2016-02-14T12:07:47.497" LastActivityDate="2016-02-15T15:00:30.123" Title="What is this algorithm to calculate orbital state vectors?" Tags="|orbital-mechanics|orbit|orbital-elements|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14096" PostTypeId="2" ParentId="14087" CreationDate="2016-02-14T14:00:46.723" Score="8" Body="&lt;p&gt;&lt;strong&gt;Making it cost effective&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the large expenses in the shuttle program was to inspect and overhaul the heat shield after each flight. A major condition for re-use to be feasible is that the equipment is expensive, and still useful after flight. Even if they are specialized and single-purpose, thermal tiles are not exactly expensive to manufacture, and may benefit more from a higher production rate than re-use.&#xA;(A small market for &lt;a href=&quot;http://www.ebay.com/sch/items/?_nkw=space%20shuttle%20heat%20tile&amp;amp;_sacat=&amp;amp;_ex_kw=&amp;amp;_mPrRngCbx=1&amp;amp;_udlo=&amp;amp;_udhi=&amp;amp;_sop=12&amp;amp;_fpos=&amp;amp;_fspt=1&amp;amp;_sadis=&amp;amp;LH_CAds=&amp;amp;rmvSB=true&quot; rel=&quot;nofollow noreferrer&quot;&gt;used shuttle tiles&lt;/a&gt; seems to exist, but it is not that important for the overall economy.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another issue is reliability, you have to be tediously careful to find that one invisible crack or weakening that may end in a catastrophe. A heat shield defect is not something to be careless about, and to guaranty quality, an enormous amount of manual labour is required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We remember &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Columbia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2020-02-19T10:08:23.020" LastActivityDate="2020-02-19T10:08:23.020" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14097" PostTypeId="2" ParentId="14059" CreationDate="2016-02-14T18:19:23.553" Score="7" Body="&lt;p&gt;The purpose of self destruct is not necessarily to make it safer in all scenarios.  It's to make it more predictable.  As Rory mentioned, one of the major purposes of self destruct is to make sure there are no &quot;powered chunks&quot; flying in unpredictable directions.  Predictability is the goal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The self destruct is designed to be a floor on how bad things can get.  We can run simulated scenarios as to how the pieces of a self-destructed vehicle behave in hundreds of scenarios, and make sure they all work out.  Trying to do that with unpredictable hardware that was designed to be capable of reaching space is much harder.  If it is decided that that unpredictability is unacceptable, and the predictable fallout of a self-destruct is acceptable, the button is pressed.&lt;/p&gt;&#xA;" OwnerUserId="3532" LastActivityDate="2016-02-14T18:19:23.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14098" PostTypeId="2" ParentId="14087" CreationDate="2016-02-14T19:36:10.987" Score="3" Body="&lt;p&gt;Hohmannfan detailed the basic problem behind developing a fully reusable heat shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe a more viable alternative developing an inexpensive, simple to apply ablator. Even some kind that an astronaut could spray on, while on a spacewalk or after landing on a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ablator as such is really not a very complex idea chemically, and leaves a lot of room for improvement in other aspects than 'ablative'. Applying it to shuttle tanks was done by a spray-on method. So - make the firm, hard non-ablative part of the heat shield reusable, but reapply the ablator whenever needed - even in field conditions. Just treat it as consumable like fuel or food, not as a part, like an engine - the heat shield &quot;consumes&quot; ablator, the way engine consumes fuel.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-14T19:36:10.987" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14100" PostTypeId="1" CreationDate="2016-02-15T07:52:30.993" Score="6" ViewCount="1734" Body="&lt;p&gt;Currently, we have good methods of sustained, slow dissipation of energy through radiators. They take quite large area (and mass) per watt dissipated though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know sublimators were used at least in the Apollo missions, as exhaustible but long-term means of heat dissipation. I'm not sure how efficient they were though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, I wonder what means we have or have plans for, that could be used for rapid dissipation of huge amounts of power in a very short time - possibly in exhaustible, non reusable way. Ablators come to mind, but all I know about ablators is that they are used in the atmosphere, on reentry, and they strongly depend on convection, most of heat dissipated into the air, not into the ablator. I have no clue how one would work in vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is related to my question on &lt;a href=&quot;https://space.stackexchange.com/questions/14078/harpoon-propulsion-what-would-be-the-problems&quot;&gt;Harpoon Propulsion&lt;/a&gt; - in particular, brakes of the spool. Their energy output would be of orders of a megawatt, over a period of a couple minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are allowed to burn, melt, evaporate or explode afterwards, but they must keep on braking at constant force while they work. It may be a friction brake, or electrodynamic or any other, applied to linear motion (along the ribbon) or rotary (of the axis). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, how can we sink that amount of energy?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2019-07-16T05:49:58.563" Title="Dissipating large amounts of energy fast, in space" Tags="|energy|heat|" AnswerCount="8" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14101" PostTypeId="1" AcceptedAnswerId="14105" CreationDate="2016-02-15T08:43:07.693" Score="11" ViewCount="3032" Body="&lt;p&gt;I just saw the term &lt;a href=&quot;https://space.stackexchange.com/a/4313/12102&quot;&gt;&quot;Double Iridium Flare&quot;&lt;/a&gt; and I'm wondering how close two Iridium satellites might actually be (in distance (km) and angle). I found this video as an example. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/-XE3LuD5htM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Iridium 94 (00:05) and Iridium 23 (00:51)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since they are flying in a spread-out formation for more uniform coverage, why would they become so close?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-20T13:08:12.733" LastActivityDate="2023-10-01T00:10:43.970" Title="What is a &quot;Double Iridium Flare&quot; and how close can the two satellites be?" Tags="|artificial-satellite|low-earth-orbit|observation|iridium|satellite-flare|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14102" PostTypeId="2" ParentId="14100" CreationDate="2016-02-15T09:25:26.613" Score="2" Body="&lt;p&gt;Obviously we cannot use ordinary radiator panels to increase the rate of energy dissipation (other than adding more of them)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is always the option of using this excess energy:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;accelerate reaction wheels&lt;/li&gt;&#xA;&lt;li&gt;power a laser or some other energy transmission device&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;etc&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2016-02-15T09:25:26.613" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14103" PostTypeId="2" ParentId="14078" CreationDate="2016-02-15T09:43:02.077" Score="5" Body="&lt;p&gt;Let's say the comet passes by at 10 km/s and your vehicle speed is 0. Then your propulsion system has to deliver a delta-V of 10 km/s. You can do this either by burning rocket fuel, or by your harpoon/winch/brake system. The amount of kinetic energy you have to generate/dissipate is the same in either case.  &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;First step: guesstimate&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;A 100-ton craft requires something like 2000 tons of rocket fuel to accelerate to 10 km/s (roughly the performance of a Saturn V).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we limit ourselves to the temperatures found in chemical rockets, I suspect your winch brake will need to dissipate enough energy to evaporate 2000 tons of water.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can use higher temperatures, the system becomes more mass-efficient, but you'd have to add an intermediary step: convert the brake energy to electricity, and use that to drive an ion engine or thermal rocket.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Second step: calculate&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The kinetic energy of 100 tons traveling at 10 km/s is $1/2 *m * v^2$ is  $5 * 10^{12}$ J is $1.38 * 10^9$ Wh. That's the amount of energy you have to put in to accelerate to 10 km/s, so that's the amount of energy your brake will have to dissipate.&lt;br&gt;&#xA;At 6 G, that $1.38 * 10^9$ Wh is generated in 166 (10,000 / 60) seconds, for an average of 30 GW. On Earth, that kind of power dissipation requires a river and dozens of giant cooling towers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water requires 2.2 MJ/kg to evaporate, so my guesstimate of 2000 tons was accurate within 10%. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-02-16T10:02:06.673" LastActivityDate="2016-02-16T10:02:06.673" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14104" PostTypeId="2" ParentId="14100" CreationDate="2016-02-15T09:50:12.450" Score="3" Body="&lt;p&gt;Water has one of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Heat_capacity#Table_of_specific_heat_capacities&quot; rel=&quot;nofollow&quot;&gt;highest heat capacities&lt;/a&gt; available, so this would be the most efficient material to use as dissipation mass (the least mass necessary per MW of heat dissipated).&lt;br&gt;&#xA;You could use the steam in various ways: build a steam engine to drive a generator, heat the spacecraft and/or or use the steam exhaust as propulsion. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-15T09:50:12.450" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14105" PostTypeId="2" ParentId="14101" CreationDate="2016-02-15T10:25:47.203" Score="12" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&quot; rel=&quot;noreferrer&quot;&gt;Iridium satellite constellation&lt;/a&gt; consists of 72 satellites, 6 of those on-orbit spares, and since May 1997 to June 2002, in total of 95 Iridium satellites were launched to orbit. Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation#Defunct_satellites&quot; rel=&quot;noreferrer&quot;&gt;currently lists&lt;/a&gt; 17 defunct Iridium satellites. Two of them decayed, &lt;a href=&quot;https://en.wikipedia.org/wiki/2009_satellite_collision&quot; rel=&quot;noreferrer&quot;&gt;Iridium 33 collided with Kosmos 2251&lt;/a&gt;, 6 in uncontrolled orbit, one presumed partially operational, and additional 6 in their nominal orbit. That list seems a bit outdated though, and searching through NORAD catalog, at the time of writing this answer, I managed to find only 70 Iridium satellites that aren't tracked as debris and haven't yet decayed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All that means that not all of their orbits will have identical orbital period, i.e. some will be in slightly higher orbits than others. Operator might have also moved some of the on-orbit spares to closely matching (trailing or leading) orbits to some of by now defunct satellites to ensure global coverage. So the Iridium satellites, defunct on-orbit ones and active ones, can appear to converge over a small area of the sky as observed from a location on Earth, and would produce a flare at a specific angle to the observer at nearly the same time, due to same or similar orbital geometry, small angular separation as observed from the ground, and reflecting light from the same distant source.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How close can they get? Well, theoretically, angular separation between two Iridium satellites as observed from the ground can be 0. So, &lt;em&gt;really close&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mind that not only Iridium satellites produce &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;noreferrer&quot;&gt;satellite flares&lt;/a&gt;, and there's a lot of junk up there, from tumbling upper stages, to other defunct satellites, many of them in similar orbital inclinations, so chances that two or more of such object produce a flare close to each other increases. For example, just at this exact moment, there's this many object in orbit over the area I live at and directly overhead (there's many more at shallower angle to the horizon):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;img src=&quot;https://i.stack.imgur.com/KkBOO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;sup&gt;&lt;strong&gt;Objects in orbit currently appearing directly overhead to an observer in central Europe. Made with &lt;a href=&quot;http://www.satflare.com/track.asp&quot; rel=&quot;noreferrer&quot;&gt;SatFlare&lt;/a&gt;&lt;/strong&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-15T11:22:23.507" LastActivityDate="2016-02-15T11:22:23.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14107" PostTypeId="1" AcceptedAnswerId="21563" CreationDate="2016-02-15T12:18:14.497" Score="7" ViewCount="1481" Body="&lt;p&gt;Different grain geometries in solid-fuel rockets produce various thrust curves: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt; &#xA;&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/PzDnY.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PzDnY.gif&quot; alt=&quot;thrust curves of various grain geometries&quot;&gt;&lt;/a&gt;&#xA;&lt;br&gt;&#xA;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can imagine it's not too difficult to obtain a thrust curve of a specific grain geometry with a measurement while the engine burns. But are thrust curves also obtained in any way through calculation, without physical testing? And if yes, with what methods or formulae?&lt;/p&gt;&#xA;" OwnerUserId="6750" LastActivityDate="2017-05-18T14:37:30.647" Title="Ways to obtain thrust curves of different grain geometries" Tags="|engines|solid-fuel|thrust|burn|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14108" PostTypeId="2" ParentId="14077" CreationDate="2016-02-15T12:56:35.307" Score="0" Body="&lt;p&gt;The answer Rikki-Tikki-Tavi provided is right, with one small exception. The periapsis, not the apoasis, is the key to using such charts for reentry purposes. The 466 km periapsis should be used. Using the same source, I would estimate the lifetime to be no more than 5 years, 2-3 being more likely. I suspect the satellite was a bit low, it certain was compared to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kwangmy%C5%8Fngs%C5%8Fng-3_Unit_2&quot; rel=&quot;nofollow&quot;&gt;previous satellite North Korea launched&lt;/a&gt;, and would also be consistent with it's tumbling. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-15T12:56:35.307" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14109" PostTypeId="2" ParentId="14078" CreationDate="2016-02-15T14:18:26.090" Score="3" Body="&lt;p&gt;Instead of having only one harpoon with a single wire which has to deal with all of the enormous stress, maybe one could use a whole spiderweb of threads. Each of which snaps in a series of relatively moderate chocks over some time period during the fast flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A very close flyby would lower the required length of wires. A big problem is how to attach the harpoons in the comet's surface. But anchoring points or nets could be pre placed by soft landers which are lighter, slower and launched to meet the comet when it is more accessible than when the spacecraft that later uses this installation for deflection and acceleration passes by on its way to target X.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe a pre placed soft landed asset could melt its surrounding on the comet and blow gasses in a long stream towards the trajectory of the flyby spacecraft, providing aerobraking for it as it approaches.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-02-15T14:18:26.090" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14110" PostTypeId="2" ParentId="14095" CreationDate="2016-02-15T14:52:29.963" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Then calculate coefficients for the velocity:$$\dot{\varepsilon} = \sqrt{\frac{\mu}{ar}}$$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The eccentricity ($\varepsilon$) is constant. That should be $\dot E$ rather than $\dot\varepsilon$, and that expression is incorrect. The correct expression is $$\dot E = \sqrt{\frac{\mu}{ar^2}}$$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I'd like to understand what $\dot{\varepsilon}$ represents, and how those coefficients are combined to produce position and velocity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;First, I'll look at what the eccentric anomaly represents. In terms of the eccentric anomaly, the orbital plane cartesian coordinates of an orbiting body are (as noted in the question) given by $$\begin{aligned}r_x &amp;amp;= a(\cos E - \varepsilon)\\r_y &amp;amp;= a\sqrt{1-e^2}\sin E\end{aligned}$$ The radial distance in terms of the eccentric anomaly is $$r = a(1- \varepsilon\cos E)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time derivative of the eccentric anomaly can be calculated by differentiating Kepler's equation, $M = E - \varepsilon \sin E$, with respect to time: $\dot M = \dot E\,(1 - \varepsilon \cos E)$. Note the common term $1-\varepsilon \cos E$ in the expressions for the radial distance and the time derivative of Kepler's equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time derivative of the mean anomaly is the mean motion: $\dot M = n = \sqrt{\frac \mu {a^3}}$. Thus the time derivative of the eccentric anomaly is $$\dot E = \frac {\dot M}{1-\varepsilon\cos E} = \sqrt{\frac \mu {a^3}} \frac 1 {1-\varepsilon\cos E} = \sqrt{\frac \mu {a^3}} \frac a r = \sqrt{\frac \mu {a r^2}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Differentiating the coordinates $r_x$ and $r_y$ with respect to time yields $$\begin{aligned}\dot r_x = v_x &amp;amp;= \dot E a\sin E\\ \dot r_y = v_y &amp;amp;= \dot E a\sqrt{1-e^2}\cos E\end{aligned}$$&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-02-15T15:00:30.123" LastActivityDate="2016-02-15T15:00:30.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14111" PostTypeId="2" ParentId="14100" CreationDate="2016-02-15T16:51:05.120" Score="6" Body="&lt;p&gt;&lt;strong&gt;In a vacuum, there are only two ways to get rid of the heat, radiators and sublimators.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can not simply get the energy in the heat back to useful forms like electricity, due to the laws of thermodynamics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The efficiency of both is dependent on the temperature they operate at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For radiators, we have the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stefan-Boltzmann law&lt;/a&gt;, that says that the power radiated is proportional to the fourth power of the temperature. That means that if you want to get rid of large amounts of heat, you can let the radiators run hotter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The power level of a sublimator is proportional to the temperature and the &lt;a href=&quot;http://www2.ucdsb.on.ca/tiss/stretton/Database/Specific_Heat_Capacity_Table.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;heat capacity&lt;/a&gt; of the evaporated material. The best heat capacity possible is that of hydrogen at &lt;span class=&quot;math-container&quot;&gt;$14.267 J/gK$&lt;/span&gt;. Water is good too. Generally, go for light molecules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the systems scale differently, sublimators are good for small, short duration missions with a low operational temperature, and radiators for larger, long duration missions with a higher operational temperature. The borders are fuzzy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are planning to use heat to produce useful energy, you can do so if you have a temperature difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The overall efficiency is determined by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Endoreversible_thermodynamics#Novikov_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chambadal-Novikov efficiency&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\eta = 1 - \sqrt{\frac{T_L}{T_H}}$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where &lt;span class=&quot;math-container&quot;&gt;$T_L$&lt;/span&gt; is the lower temperature and &lt;span class=&quot;math-container&quot;&gt;$T_H$&lt;/span&gt; the higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a constant high-end operation temperature, you can combine this efficiency with the Stefan-Boltzmann law to find the ideal radiator temperature for the highest energy production to radiator mass ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It occurs at:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$T_L=\frac{64}{81}T_H$$&lt;/span&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="10273" LastEditDate="2019-04-18T07:15:10.270" LastActivityDate="2019-04-18T07:15:10.270" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="14112" PostTypeId="2" ParentId="14087" CreationDate="2016-02-15T18:29:57.097" Score="1" Body="&lt;p&gt;The OP is missing the point of what an ablative heat shield is actually doing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heat shield's primary function isn't to protect the capsule. An aerodynamic shell would fare far better than a heat shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main function of a heat shield is to slow a capsule down from orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does this by converting kinetic energy into a huge amount of heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, much of the heat generated never touches the heat shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To protect the capsule by pure insulation is rather difficult. As a huge amount of energy needs to be dissipated, or the heat shield would have to insulate the huge temperate differences for the lengthy reentry sequence (over 10 minutes long).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally the skin temperature is at about 1600C, which is above melting point of many materials. Most materials would structurally fail at that temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://h2g2.com/approved_entry/A6381038&quot; rel=&quot;nofollow&quot;&gt;http://h2g2.com/approved_entry/A6381038&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My point is of course is that you are in effect trying to build a reusable meteorite catcher (in terms of energy).&lt;/p&gt;&#xA;" OwnerUserId="6157" LastEditorUserId="6157" LastEditDate="2016-02-15T19:06:23.327" LastActivityDate="2016-02-15T19:06:23.327" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14113" PostTypeId="2" ParentId="9158" CreationDate="2016-02-15T20:13:06.927" Score="8" Body="&lt;p&gt;Old topic, but for those just showing up:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;There are two leading candidates for the &amp;quot;unknown UV absorber&amp;quot; in Venus's atmosphere, and probably the most likely answer is some degree of both.  One of these is elemental sulfur.  The other?  Ferrous chloride.  Indeed, one of the Venera probes detected iron during its descent.  All evidence points to there being a small amount of iron in the air in Venus.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&amp;quot;(e.g. cadmium, lead, some magnesium alloys, mercury, phosphorus, potassium, selenium, ...)&amp;quot;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Just because something can boil off in some form doesn't mean that it actually will in practice (chemical reactions come into play), nor that it could manage to be transported all the way up a whopping 50+ kilometers (on Earth that would be halfway to the Karman line) through an atmosphere which at the base is 90 atm, without precipitating out.  On the other hand, the world isn't made of pure elements, there's all sorts of compounds, and many have vastly lower boiling points than their elemental forms.  It gets complicated.&lt;/p&gt;&#xA;&lt;p&gt;The honest fact is, we don't know what minor constituents make up the Venusian cloud decks.  Back in the 1960s, it was theorized that the clouds were a wide range of mercury compounds.  Now we know that mercury can't be but a trace element in the clouds.  Another theorized one that's been fairly well ruled out is aluminum chloride.   Some antimony compounds, bromine compounds, and iodine compounds are considered possible if not likely, but only in small quantities.&lt;/p&gt;&#xA;&lt;p&gt;There are a number of chemicals that are well known to exist in Venus's atmosphere that are commonly not listed in lists of Venus's atmospheric components.  For example, the lower haze and possibly a lot of the lower cloud deck are rich in to dominated by phosphoric acid.  A wide variety of acidic/corrosive chlorine compounds also exist in Venus's atmosphere (most lists show only HCl), include some that make sulfuric acid seem tame (for example, perchloric acid is believed to make up a small component of the cloud particles).  By the way, note that the cloud particles are a fine mist, it's not like a bath of acid... more like a really bad smog/vog that tends to eat away at things over time. There's some evidence, contrary to the oversimplified picture often presented, that there might even be &amp;quot;surface hazes&amp;quot; - there was some dimming experienced by the landers during their last 1-2km that some interpret as hazes.  These would be some interesting compounds.&lt;/p&gt;&#xA;&lt;p&gt;There's pretty good evidence that Venus's surface has exotic frosts or snows, although the composition is still unknown.  Venus's surface appears to be made of lavas that on their own tend to be rather rich in interesting minerals (for example, there's good evidence of carbonatites - go check out what sort of economic deposits of minerals they're associated with on Earth).  But it's also exposed to some intense chemical weathering processes, which tend to concentrate specific minerals in different environments.  So the surface of Venus is probably a mining treasure trove.  It's easy to overstate the difficulty of accessing the surface; no, it's no walk in the park, but on the other hand, it's not vastly more difficult than a submarine.  Temperature issues only affect the outside; the inside is insulated and relies largely on thermal inertia to maintain itself at a comfortable operating temperature.  The Soviets landed functional and capable probes with 1960s technology.  The Soviets never lifted off the surface and went back up to the cloud deck, but we know how to do that - it's called a phase change balloon.&lt;/p&gt;&#xA;&lt;p&gt;No, you don't need to do electrolysis to extract oxygen from H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; - you just have to heat it.  H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; breaks down at high temperatures to H&lt;sub&gt;2&lt;/sub&gt;O + SO&lt;sub&gt;3&lt;/sub&gt;, and the SO&lt;sub&gt;3&lt;/sub&gt; reduces to SO&lt;sub&gt;2&lt;/sub&gt; + O&lt;sub&gt;2&lt;/sub&gt;. So you get both H&lt;sub&gt;2&lt;/sub&gt;O and O&lt;sub&gt;2&lt;/sub&gt; from a simple process.  Of course, the details make it tricky, because you have the other mixed compounds in there, you have to separate the acid gases (many of which are hygroscopic) from the water stream, etc.  But on the upside, Venus's atmosphere is really giving one the gift of providing all but one of the major industrial acids (all but nitric) in a relatively easy to acquire form, and nitric is relatively easy to produce.  A fun side benefit of Venus is that many major industrial processes (the Haber process, the Sabatier reaction, etc.) require elevated temperatures and pressures for the thermodynamics to work out.  On Venus, all that means is &amp;quot;descend&amp;quot;.  :)&lt;/p&gt;&#xA;&lt;p&gt;Concerning plastics: there's really no question that the perfect skin material is a PTFE copolymer with IR-reflective additives.  PTFE is almost perfectly designed for the Venus environment, in terms of its amazing chemical resistance, heat tolerance, UV stability, relatively simple production process, etc.  It of course must be bonded with a ripstop.  Normally on Earth we'd use nylon, but it's really not ideal for Venus, the production chains are rather long.  UHMWPE (spectra) is probably a much better choice; its monomer is easy to make, it's incredibly high tensile strength (higher than Kevlar), very light (even floats in water), and very abrasion resistant.  Also, both UHMWPE and PTFE, while neither of them like to adhere to anything, are both thermoplastics and can thus be thermally bonded.&lt;/p&gt;&#xA;&lt;p&gt;Note, contrary to what was stated above, HDPE is not long-term stable with concentrated H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; (it's about 80% in the middle cloud deck). Although that's for liquids, not mists; it might do better in mists.  But the key point is, the PTFE layer is what you have exposed to the clouds.&lt;/p&gt;&#xA;&lt;p&gt;There's so vastly much more I could write on the topic, but I'll leave it at this for now.  :)&lt;/p&gt;&#xA;" OwnerUserId="13465" LastEditorUserId="47149" LastEditDate="2022-05-29T01:09:12.580" LastActivityDate="2022-05-29T01:09:12.580" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="14115" PostTypeId="1" AcceptedAnswerId="14131" CreationDate="2016-02-16T12:54:27.157" Score="1" ViewCount="356" Body="&lt;p&gt;I would like to see some comparison between different launchers wrt how quickly their mass is reduced over time (or altitude or velocity). There are differences in overall size class, propellant type, number and size of solid fuel boosters, common cores, burn time of the first stage and certainly more. It would be interesting to learn something about how current and historic launchers differ in getting rid of mass over time.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-02-18T00:11:32.167" Title="How does the mass of launchers change over time between ground and orbit?" Tags="|launch|mass-fraction|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14117" PostTypeId="1" CreationDate="2016-02-16T16:01:51.790" Score="4" ViewCount="135" Body="&lt;p&gt;With Philae dead on the comet, how will Rosetta's mission change? Obviously there isn't anything more that the lander can do, so Rosetta doesn't have to use its time and resources trying to wake it up. Will Rosetta be moved into positions or given tasks that it otherwise wouldn't if Philae was still functional? How is Philae's power failure going to affect the mission as a whole?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2023-12-14T22:14:23.220" Title="How will Philae's power failure affect Rosetta's mission?" Tags="|mission-design|rosetta|failure|philae|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14118" PostTypeId="2" ParentId="14117" CreationDate="2016-02-16T16:22:03.410" Score="4" Body="&lt;p&gt;Philae's &lt;a href=&quot;https://en.wikipedia.org/wiki/Philae_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;primary mission&lt;/a&gt; was planned to last for a week, with possible extensions.&lt;br&gt;&#xA;This primary mission is considered successful:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Prior to falling silent, the lander was able to transmit all science data gathered during the First Science Sequence.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Has Philae landed as planned...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Had Philae landed at the planned site of Agilkia in November 2014, its mission would probably have ended in March 2015 due to the higher temperatures of that location as solar heating increased.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the mission plan already counted on Philae not being available for most of last year. They've made some changes to try and contact Philae at regular intervals, until the comet came too close to perihelion for Rosetta to operate in radio range of Philae.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To summarize, no. Nothing will change. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-16T16:22:03.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14119" PostTypeId="1" CreationDate="2016-02-16T16:32:11.583" Score="10" ViewCount="2023" Body="&lt;p&gt;What does it cost for an astronaut to spend an hour outside the ISS, in terms of&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;astronaut time&lt;/li&gt;&#xA;&lt;li&gt;mission preparation time&lt;/li&gt;&#xA;&lt;li&gt;ground support personnel&lt;/li&gt;&#xA;&lt;li&gt;space suit &quot;rent&quot; (?)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Does this cost vary for a Russian/European/American cosmo/astronaut?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br/&gt; Regarding cost of space suits, according to &lt;a href=&quot;https://www.quora.com/How-do-the-Russian-EVA-space-suits-on-the-ISS-compare-with-the-American-ones&quot;&gt;this comparison of US and Russian spacesuits (EMU and Orlan respectively)&lt;/a&gt;, EMUs are returned to Earth every 25 EVAs for refurbishment, whereas Orlans are used 12 times then destroyed (!). For the price of an EMU I have seen quotes between 2 and 12 million USD. No idea how much the return to Earth and refurbishment cost, nor how much an Orlan costs. &lt;br/&gt; As you said, the astro/cosmonauts' time spent in EVA preparation, donning, and doffing, is all time during which they are not &quot;productive&quot;, and therefore that has to be taken into account when computing the price per &quot;productive&quot; astronaut work. Any idea how much time these would take? &lt;br/&gt; Finally, do you have any idea how many more people sit on console during EVA compared to a normal situation? &lt;br/&gt; Thanks!&lt;/p&gt;&#xA;" OwnerUserId="13473" LastEditorUserId="13473" LastEditDate="2016-02-17T18:40:21.157" LastActivityDate="2016-10-13T23:10:43.650" Title="Cost of an EVA on the ISS?" Tags="|iss|eva|cost|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14120" PostTypeId="1" CreationDate="2016-02-16T17:23:49.427" Score="1" ViewCount="808" Body="&lt;p&gt;Doing some digging online, but I was unable to come up with an answer to this...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are ballpark thrust-to-weight ratios for typical hydrazine monopropellant thrusters? I'm looking for a value that would be comparable to the quoted T/W ratios for conventional bipropellant engines (I'm not sure whether those typically factor in the mass of the turbopump or not). Also, what would the value be for a typical hydrogen peroxide monopropellant thruster?&lt;/p&gt;&#xA;" OwnerUserId="13211" LastActivityDate="2016-02-17T03:40:04.103" Title="Thrust/weight ratio of monopropellant thruster engine" Tags="|engines|monopropellant|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14121" PostTypeId="2" ParentId="14120" CreationDate="2016-02-16T19:22:31.437" Score="6" Body="&lt;p&gt;The thrust-to-weight ratio is going to vary greatly for small thrusters (and hydrazine monoprops are generally quite small), with the very smallest having much lower thrust-to-weight ratios; mass doesn't scale down linearly with thrust at the low end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, Airbus makes &lt;a href=&quot;http://www.space-propulsion.com/spacecraft-propulsion/hydrazine-thrusters/1n-thruster.html&quot; rel=&quot;nofollow&quot;&gt;1N&lt;/a&gt;, &lt;a href=&quot;http://www.space-propulsion.com/spacecraft-propulsion/hydrazine-thrusters/20n-thruster.html&quot; rel=&quot;nofollow&quot;&gt;20N&lt;/a&gt;, and &lt;a href=&quot;http://www.space-propulsion.com/spacecraft-propulsion/hydrazine-thrusters/400n-thruster.html&quot; rel=&quot;nofollow&quot;&gt;400N&lt;/a&gt; hydrazine thrusters, all pressure fed with similar operating characteristics:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Thrust (N)    Mass (g)       TWR&#xA;         1        290       0.35&#xA;        20        650       3.14&#xA;       400       3800      10.73&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Aerojet Rocketdyne also offers a &lt;a href=&quot;https://www.rocket.com/propulsion-systems/monopropellant-rockets&quot; rel=&quot;nofollow&quot;&gt;family of hydrazine monoprops&lt;/a&gt; with generally similar TWR trends.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronautix &lt;a href=&quot;http://www.astronautix.com/props/hydazine.htm&quot; rel=&quot;nofollow&quot;&gt;doesn't list any hydrazine thrusters above 1kN&lt;/a&gt;. The most powerful I could find in my search were the AJR MR-80 used on the Viking Mars lander (3.1kN, 37:1 TWR), and its derivative, the &lt;a href=&quot;http://matthewwturner.com/uah/IPT2008_summer/baselines/LOW%20Files/Payload/Downloads/AIAA-2007-5481-979.pdf&quot; rel=&quot;nofollow&quot;&gt;Mars Lander Engine&lt;/a&gt; used for Curiosity's skycrane, at 3.6kN. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find much info on peroxide monoprops. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quoted masses for pump-driven bipropellant engines should include the pump mass, by the way. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-02-17T03:40:04.103" LastActivityDate="2016-02-17T03:40:04.103" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14123" PostTypeId="1" AcceptedAnswerId="14250" CreationDate="2016-02-16T23:45:04.987" Score="13" ViewCount="1548" Body="&lt;p&gt;We are familiar with the first groups of NASA astronauts, like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury_Seven&quot; rel=&quot;nofollow&quot;&gt;Mercury Seven&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_2&quot; rel=&quot;nofollow&quot;&gt;the New Nine&lt;/a&gt;. Something about that struck me as (literary) odd; All the first twelve astronaut groups had an odd number of astronauts, except for &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_3&quot; rel=&quot;nofollow&quot;&gt;Astronaut Group 3&lt;/a&gt; and 4. The probability for such a coincidence is quite low, so there was perhaps a reason for this policy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why did NASA hire an odd number of astronauts in almost all the first astronaut groups?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;7 ,&#xA;9 , &#xA;14 , &#xA;6 , &#xA;19 , &#xA;11 , &#xA;7 , &#xA;35 , &#xA;19 , &#xA;17 , &#xA;13 , &#xA;15  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, the same pattern does not exist in the Soviet space program. A list of the cosmonaut groups can be found &lt;a href=&quot;http://www.worldspaceflight.com/bios/cosgroups.php&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-10-13T07:49:51.487" LastActivityDate="2021-03-07T02:35:17.590" Title="What was the reason for an odd number of astronauts in almost every group of the early US space program?" Tags="|nasa|crewed-spaceflight|history|astronauts|astronaut-recruitment|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14124" PostTypeId="1" CreationDate="2016-02-17T00:56:49.720" Score="12" ViewCount="3678" Body="&lt;p&gt;One kind of 'popular science' fun facts that is very popular is to show how many of the individuals belonging to a notable group of people that are left handed, often showing a large disproportionally from the rest of the population. (&lt;a href=&quot;http://www.lefthandersday.com/super/super-astronauts#.VsPFoCbiVz0&quot; rel=&quot;noreferrer&quot;&gt;example&lt;/a&gt; with Apollo astronauts, but that is a small sample size) This has been asked about &lt;a href=&quot;https://en.wikipedia.org/wiki/Handedness_of_Presidents_of_the_United_States&quot; rel=&quot;noreferrer&quot;&gt;presidents&lt;/a&gt; and &lt;a href=&quot;http://www.eharmony.com/dating-advice/about-you/15-reasons-to-date-a-lefty/#.VsPCQibiVz0&quot; rel=&quot;noreferrer&quot;&gt;Nobel prize winners&lt;/a&gt; etc., But are there any &lt;a href=&quot;https://en.wikipedia.org/wiki/Handedness&quot; rel=&quot;noreferrer&quot;&gt;handedness&lt;/a&gt; skewing regarding astronauts too? &lt;a href=&quot;http://www.collectspace.com//ubb/Forum38/HTML/000622.html&quot; rel=&quot;noreferrer&quot;&gt;This discussion&lt;/a&gt; suggest such a correlation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I can see, handedness may have a slight impact on the design of control panels and equipment with &lt;a href=&quot;https://en.wikipedia.org/wiki/Chirality_%28chemistry%29&quot; rel=&quot;noreferrer&quot;&gt;chiral&lt;/a&gt; properties, like scissors. (In the later case calling for a more neutral design in order to maintain uniform productivity.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EagHDm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EagHDm.jpg&quot; alt=&quot;Buzz Aldrin in space suit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Buzz Aldrin, left handed&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="40257" LastEditDate="2022-02-03T02:38:49.753" LastActivityDate="2022-08-15T03:10:43.207" Title="How many left handed astronauts have there been?" Tags="|astronauts|life|statistics|astronaut-recruitment|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="14125" PostTypeId="2" ParentId="13061" CreationDate="2016-02-17T06:57:12.343" Score="5" Body="&lt;p&gt;At a previous employer I set up a live video feed from a cubesat's star camera and hooked up a joystick to the attitude control system. I guess that counts. You only get a few minutes during a groundstation pass and it was a lot slower to respond than an R/C plane!&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2016-02-17T06:57:12.343" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14126" PostTypeId="5" CreationDate="2016-02-17T07:42:15.590" Score="0" Body="&lt;p&gt;Astronauts have an unusual job, where they often have to deal with special situations and carefully planed operations. This calls for a special set skills and personality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early era, pilot experience was a mandatory skill, but that was later relaxed to also recruit people with a scientific education, and payload specialists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On long duration missions, social compatibility of the crew is an important consideration.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-17T07:42:15.590" LastActivityDate="2016-02-17T07:42:15.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14127" PostTypeId="4" CreationDate="2016-02-17T07:42:15.590" Score="0" Body="Questions regarding how astronauts are selected, and their required qualifications" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-17T07:42:15.590" LastActivityDate="2016-02-17T07:42:15.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14128" PostTypeId="1" AcceptedAnswerId="14129" CreationDate="2016-02-17T09:29:37.117" Score="4" ViewCount="2117" Body="&lt;p&gt;If like to visualize the launch and landing, maybe tied into the audio. &lt;/p&gt;&#xA;" OwnerUserId="13241" LastEditorUserId="25" LastEditDate="2016-02-17T13:29:35.510" LastActivityDate="2020-06-24T17:41:01.057" Title="Is the Falcon 9 launch/landing telemetry data available for visualisation purposes?" Tags="|spacex|falcon-9|raw-data|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14129" PostTypeId="2" ParentId="14128" CreationDate="2016-02-17T13:34:01.870" Score="4" Body="&lt;p&gt;No, it is not public. This level of detailed information in the United States would fall under ITAR, and as a point of fact, SpaceX is more secretive with this kind of data than other launch providers, you have to pretty much be a SpaceX employee, or contractor, or government agency, to see any of the information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, &lt;a href=&quot;http://fas.org/spp/starwars/offdocs/itar/p121.htm&quot; rel=&quot;nofollow&quot;&gt;ITAR Item 12 Category II includes&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(3) Software which processes post-flight, recorded data, enabling determination of vehicle position throughout its flight path (see § 121.1, Category IV(i)). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-17T13:34:01.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14131" PostTypeId="2" ParentId="14115" CreationDate="2016-02-17T16:55:41.713" Score="10" Body="&lt;p&gt;In almost all cases, the overall mass versus time curve for a multistage launcher will approximate an exponential decay curve; this is the nature of the rocket equation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Within each stage, if all the stage's engines fire at a continuous throttle setting, the mass decline over that portion of the flight will be linear. Many launchers throttle down over the course of a stage's burn in order to control g-loading, which makes the mass decline more closely approach the exponential decay curve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's one simulation result, for the Saturn V with Apollo payload:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CoN63.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CoN63.png&quot; alt=&quot;Saturn Mass versus Time&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each stage is generally linear, though there's a slight kink near the end of the first-stage run when the center engine cuts off to limit G-load, and two kinks near the end of the second stage run for center engine cutoff and propellant mix ratio shift. The vertical jumps in the plot are the stage drop events. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some more modern examples: Falcon 9, Delta IV Medium (4,2), Vega, and Proton M. For the F9 and D4, I've simulated abrupt throttle step-downs, so the stage mass curves are kinked. This is accurate for the Delta, but possibly not for the Falcon; if the engine throttles down gradually the stage mass curve would be smooth, but the overall shape would be essentially the same. Vega's first three stages are solids, with poor specific impulse, so its mass curve is quite a bit deeper than the others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uo9N9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uo9N9.png&quot; alt=&quot;Mass versus Time curves&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Individual launcher design choices appear to account for more variation in the exact shape of the curve than historical trends. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, there is a tendency in modern launchers to use very low-thrust upper stages (Falcon 9 being a bit of an outlier), so their graphs tend a little steeper at the start and shallower at the end. Ariane 5 ES is an exemplar of the trend:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QCcvD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QCcvD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Atlas LV-3B used for the Mercury orbital flights is the opposite of that trend, with high acceleration throughout and a very short time to orbit; its mass curve is somewhat shallower, but not dramatically so:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3VkFJ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3VkFJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-02-18T00:11:32.167" LastActivityDate="2016-02-18T00:11:32.167" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14132" PostTypeId="2" ParentId="1594" CreationDate="2016-02-17T18:42:02.123" Score="4" Body="&lt;p&gt;There is also the issue of eccentricity- accelerating only on one side of a planet would, as far as I know, gradually increase your eccentricity, until it eventually would escape the orbit it was in entirely. However, I'm no expert, so I could easily be wrong.&lt;/p&gt;&#xA;" OwnerUserId="13485" LastActivityDate="2016-02-17T18:42:02.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14133" PostTypeId="1" CreationDate="2016-02-17T23:27:44.600" Score="3" ViewCount="559" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/14115/how-does-the-mass-of-launchers-change-over-time-between-ground-and-orbit#14131&quot;&gt;Most fuel&lt;/a&gt; on board a launcher is used during the first couple of minutes. Most of the propellant is needed to carry other propellant. For example, Saturn V (kerosene) and Delta IV (hydrogen) use about a tenth of their launch mass during the first half of a minute. That's more than their payload delivered to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not hose (I mean pumping through a flexible tube) liquid oxygen or kerosene to the rocket during the first tens of seconds or so, in order to reduce its mass? While it still moves slowly at low altitude. Not having the hose with explosive fuel destroyed by the rocket engine exhaust might be an engineering challenge, but not as crazy as the often advocated sci fi space elevator tether idea. I could invoke the carbon nanotube dream as solution.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2018-04-17T00:46:14.550" Title="Could fuel be &quot;hosed&quot; (pumped) from the ground to a launcher?" Tags="|launch|advanced-propulsion|mass-fraction|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14135" PostTypeId="2" ParentId="14133" CreationDate="2016-02-18T02:14:30.480" Score="7" Body="&lt;p&gt;You'll lose more energy hauling that hose up than you could possibly save by pumping fuel in.  A hose full of fuel is going to be &lt;strong&gt;heavy&lt;/strong&gt;, not to mention you'll have quite a time keeping it out of the rocket exhaust.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-02-18T02:14:30.480" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14136" PostTypeId="1" CreationDate="2016-02-18T03:21:50.423" Score="3" ViewCount="611" Body="&lt;p&gt;Let's say we want to build a rocket that has the maximum amount of delta-v in deep space, but its launch mass has to be less than some number. We have a finite list of rocket components that can be used to build the rocket, such as engines, fuel tanks, stage separators, etc, and know all the details about each component such as mass, fuel capacity, thrust, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a non brute-force algorithm that can tell us exactly what combination of the rocket components will make the rocket have the highest delta-v possible while staying under a certain launch mass?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I talk about a brute-force algorithm, I mean simply trying out every possible combination of rocket components whose total mass and fuel is under the limit, and then seeing which one has the highest delta-v.&lt;/p&gt;&#xA;" OwnerUserId="11820" LastActivityDate="2016-02-18T13:55:22.630" Title="Is there a non brute-force algorithm for finding the combination of rocket components to maximize delta-v for a certain launch mass?" Tags="|rockets|delta-v|mass|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14137" PostTypeId="2" ParentId="14133" CreationDate="2016-02-18T05:01:24.690" Score="5" Body="&lt;p&gt;Additional problems I see are having a pump that can move the required amount of fuel at the rate required to the height required while overcoming shock losses as the hose unrolls and travels upwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, a trailing hose would be problematic with rocket stability and streamlining the rocket as it moves through the air.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2016-02-18T05:01:24.690" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14138" PostTypeId="2" ParentId="14133" CreationDate="2016-02-18T08:35:38.353" Score="6" Body="&lt;p&gt;The simple solution is to launch from much higher up.  Either an air launched rocket or launch from a raised platform.  (Both have been described, and one has been used.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or just begin the launch with a supergun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Really, there are so many engineering problems associated with hoses... just put the fuel in detachable boosters.  (If you think about it, putting the fuel in a strap on booster and making it support its own weight is equivalent to using your hoses.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-02-18T08:43:42.207" LastActivityDate="2016-02-18T08:43:42.207" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14139" PostTypeId="2" ParentId="2591" CreationDate="2016-02-18T09:30:20.830" Score="0" Body="&lt;p&gt;Another major consideration is the tilt in the axis of Mercury. The are some regions on which the surface of the planet never receives direct overhead heating from the sun, much like our polar region. Therefore, the temperature variation would not be as great. Clearly, there are curtain regions where the maintaining of acceptable temperatures would be far less difficult. Since Mercury rotates quite slowly, a landing in one of these temperature transition zones would allow for an extended stay.  &lt;/p&gt;&#xA;" OwnerUserId="13489" LastActivityDate="2016-02-18T09:30:20.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14140" PostTypeId="2" ParentId="14136" CreationDate="2016-02-18T13:14:07.277" Score="2" Body="&lt;p&gt;Outside of games like Kerbal Space Program, application for such an algorithm is limited, because components like tanks and interstages are geneally purpose-designed and built for a particular rocket, rather than being selected off-the-shelf.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, your problem description strongly hints that you're talking about KSP, so I'm going to run with that. I don't believe there's a general magic algorithm for optimal rockets, but there are several heuristic shortcuts that can allow you to reduce the size of the brute force search space. Wikipedia has some relevant &lt;a href=&quot;https://en.wikipedia.org/wiki/Multistage_rocket#Optimal&quot; rel=&quot;nofollow&quot;&gt;hints on optimal staging&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take the initial mass limit divided by the payload mass to get the overall mass ratio you're working with. For an n-stage rocket, the nth root of the overall mass ratio gets you a stage-to-stage mass ratio target; now you can break the problem down into building individual stages that fit approximately those target numbers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if your payload is 10 tons, your launch mass limit is 200 tons, then a 2-stage rocket should have a $\sqrt 20 = 4.472$ stage mass ratio; 10 tons payload, 45 tons 2nd stage+payload; 200 tons 1st+2nd+payload. Thus your first stage would be aiming at 155 tons, and your second 35 tons. You'd probably want to give some wiggle room to each stage search, say +/- 20% of the mass target. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Brute force search for an individual stage is pretty straightforward. Iterate over the engine types available, figure out how many of that engine you need to give the stage-plus-upper-stages better than 1:1 initial thrust-to-weight ratio, allocate the remaining mass after engines and decoupler to tankage, compute the stage ∆v, repeat and sort, discard the low performers. There might be better solutions using heterogenous engines (e.g. 1 of engine X and 2 of engine Y) it's up to you if you want to accept that complexity in the search. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you just take each combination of high-performing stages that remain under the mass limit and find the winner.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-02-18T13:14:07.277" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14142" PostTypeId="2" ParentId="5297" CreationDate="2016-02-18T14:03:12.650" Score="4" Body="&lt;p&gt;The orbit if the ISS stays within the envelope of the lower Van Allen belt.  If the inclination were greater, above 52°, the radiation exposure of the crew would be elevated severely, to the point where the health risk would be unacceptable.  As is, the exposure is four times that permitted of nuclear plant workers.  If the orbit were higher or the inclination greater, the crew would only be able to stay aboard the station for a few days before the radiation exposure would force their return to the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;////&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The greatest exposure of radiation to the crew of the station in the current orbit is from South Atlantic Anomaly and EVA.  The SAA is only a light concentration of protons at the current orbit, a mere grazing of the belt compared to the full density of the protons in the lower belt.  And the station/proton collisions generate a small amount of hull penetrating gamma radiation on three to four orbits a day, with varying intensity.  To venture into the full density of the lower Van Allen belt thirty two times a day (twice an orbit) would bring at least a fifty fold exposure, if not several hundred to thousands of times the exposure, depending on the time spent in the belt each orbit.  It is a very serious constraint on any manned orbit path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Traveling once through the Van Allen to higher orbit or escape velocity is not nearly as hazardous as facing it thirty two times a day.  It is however a concern, and manned craft leaving low earth orbit is something that is no longer done without sufficient reason.  The ISS covers the maximum range of space that is considered an acceptable radiation risk.&lt;/p&gt;&#xA;" OwnerUserId="13491" LastEditorUserId="13491" LastEditDate="2016-02-19T19:59:14.210" LastActivityDate="2016-02-19T19:59:14.210" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14143" PostTypeId="1" AcceptedAnswerId="14144" CreationDate="2016-02-18T19:54:43.623" Score="10" ViewCount="3468" Body="&lt;p&gt;From &lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/14499/what-are-the-effects-of-a-planet-staying-long-term-inside-of-a-nebula&quot;&gt;this&lt;/a&gt; question in World Building, it is speculated that planets in nebulas are not greatly affected by the presence of the nebula. This being the case, how do we know that the earth itself is not in a nebula? Has this been tested?&lt;/p&gt;&#xA;" OwnerUserId="13494" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:23.630" LastActivityDate="2021-08-07T15:39:56.700" Title="Is it possible that the earth is in a nebula?" Tags="|earth|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14144" PostTypeId="2" ParentId="14143" CreationDate="2016-02-18T22:42:11.860" Score="10" Body="&lt;p&gt;This depends a lot on exactly how you define a nebulae, but we are actually in a very dense region of the interstellar medium, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Local_Interstellar_Cloud&quot; rel=&quot;noreferrer&quot;&gt;local interstellar cloud&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9p4r5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9p4r5.png&quot; alt=&quot;local cloud&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Observing it directly from Earth is very difficult, due to sunlight and the solar wind, but its magnetic field has been measured by the &lt;a href=&quot;http://voyager.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Voyager 2&lt;/a&gt; probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming it is mostly hydrogen and helium, there are only about 1-2 ng/km³, so it is not a lot to observe. The actual properties of the cloud are still very uncertain.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-18T22:42:11.860" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14145" PostTypeId="2" ParentId="14133" CreationDate="2016-02-18T23:02:39.927" Score="8" Body="&lt;p&gt;&lt;strong&gt;There exist one way for this to work&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As pointed out by others before, a hose is heavy to carry along. However, if the propellant station had the same altitude and velocity as the rocket, it may be pretty simple engineering. And the obvious way of propelling the tank is by the means of a rocket engine. This is known as &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Modular_rocket#Falcon_Heavy&quot;&gt;propellant cross feed&lt;/a&gt;&quot; or &quot;asparagus staging&quot;. Although not strictly a hose, the concept is substantially different from traditional strap on rocket boosters. The first actual use of this scheme is planned to for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_Heavy&quot;&gt;Falcon Heavy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another concept based on cross-feed is a &lt;a href=&quot;http://www.sei.aero/downloads/SEI_Bimese_15Apr01.pdf&quot;&gt;bimese&lt;/a&gt;, in which two close-to-identical rockets travel together, sharing propellant, until the empty one returns, and the other one continues with full tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do ground support have to be on ground?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-02-18T23:02:39.927" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14146" PostTypeId="1" CreationDate="2016-02-19T12:05:06.960" Score="2" ViewCount="537" Body="&lt;p&gt;I have read a book claiming that the speed of the Voyager probes do not fit neither the Newton's laws of motion nor Einstein's relativity theory. Is it a recognized scientific fact? Where can I find the relevant information?&lt;/p&gt;&#xA;" OwnerUserId="13498" LastEditorUserId="415" LastEditDate="2016-02-19T13:21:34.463" LastActivityDate="2021-04-03T03:39:36.843" Title="Is the speed of the Voyager probes accurately described by Newton's or Einstein's theories?" Tags="|voyager|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14148" PostTypeId="1" CreationDate="2016-02-19T18:25:15.357" Score="6" ViewCount="202" Body="&lt;p&gt;I gather that the Virgin Galactic planes use feathered reentry systems with a maximum velocity of about 1.4 Mach, or about .5 km/s. I understand this won't work from full orbital speed, 7km/s, because the friction is too great for the feathered system to be effective. Does anyone know what is the maximum possible velocity (for Earth's atmosphere) at which such a reentry system could be effective? Wondering, that is, if it would work for higher suborbital flights, or for orbital flights initially slowed down by some other braking system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Love the first comment, btw--I should've expected that!)&lt;/p&gt;&#xA;" OwnerUserId="13502" LastEditorUserId="13502" LastEditDate="2016-02-26T16:09:47.633" LastActivityDate="2016-02-26T16:09:47.633" Title="What is the maximum velocity for feathered reentry systems?" Tags="|spacecraft|reentry|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14149" PostTypeId="1" AcceptedAnswerId="14215" CreationDate="2016-02-19T20:46:31.360" Score="13" ViewCount="3054" Body="&lt;p&gt;Before sending first dog to space, the only animals sent to space were fruit flies. Flies made it back alive, but in case of dog we are talking about a much more complex organism.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GH34K.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GH34K.jpg&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://timedotcom.files.wordpress.com/2014/11/laika.jpeg?quality=75&amp;amp;strip=color&amp;amp;w=1012&quot; rel=&quot;nofollow noreferrer&quot;&gt;wordpress.com&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;Image Source: &lt;a href=&quot;https://time.com/3546215/laika-1957/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Time&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;What were expectations of scientists working on the project of sending a dog to space? Did they expect Laika to survive &lt;strong&gt;until&lt;/strong&gt; reentry? What effects of space flight did they consider most threatening to dog's life in the situation? Did they perform any (not necessarily humane) experiments to determine the chances of success?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;I should probably add extra clarification to my question. I knew Laika had no chance of making it back to Earth and would eventually die, during reentry if not earlier. But since the scientists were willing to risk her for this experiment, I assume they had some doubts whether a dog can survive a spaceflight even &lt;strong&gt;until&lt;/strong&gt; the point of reentry (or they wouldn't do it in the first place, I believe...). So what is it that they expected to threaten Laika's life? Exposure to radiation? Effects of large forces excerted? What did her flight (dis)prove that was in question before? Did they perform any experiments in order to anticipate the outcome before the actual flight? I hope this helps to clear things up a bit and get answers to original question above.&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="10273" LastEditDate="2022-12-30T12:50:46.393" LastActivityDate="2022-12-30T12:50:46.393" Title="What did they expect to happen with Laika?" Tags="|orbit|history|research|animals|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="14150" PostTypeId="1" CreationDate="2016-02-19T21:05:35.847" Score="7" ViewCount="1492" Body="&lt;p&gt;I've seen multiple sources attribute the clear flames to the use of hypergolic fuels for propulsion, but I have been unable to find any explanation for why hypergolic fuels burn clearly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://spaceflightnow.com/titan/g9/images/titan2quikscat.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;&#xA;(Image Source: &lt;a href=&quot;https://spaceflightnow.com/titan/g9/031014titan2.html&quot;&gt;Spaceflight Now&lt;/a&gt;)&#xA;&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="13503" LastEditorUserId="8162" LastEditDate="2016-02-20T14:23:56.380" LastActivityDate="2018-10-16T20:42:57.097" Title="Why were the Titan II's Flames Clear?" Tags="|launch|rockets|fuel|liquid-fuel|exhaust|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14153" PostTypeId="1" CreationDate="2016-02-20T10:36:34.580" Score="1" ViewCount="123" Body="&lt;p&gt;I am interested in the history of space-flight and was wondering if anyone had any good book suggestions. I have no scientific background, but am more interested in the science behind the space missions than the personalities of the astronauts/cosmonauts etc. Thanks&lt;/p&gt;&#xA;" OwnerUserId="13511" LastEditorUserId="415" LastEditDate="2016-02-22T09:17:02.557" LastActivityDate="2016-02-22T09:17:02.557" Title="Book recommendation for history of spaceflight" Tags="|history|reference-request|" AnswerCount="1" CommentCount="2" ClosedDate="2016-02-23T15:52:54.727" ContentLicense="CC BY-SA 3.0" />
  <row Id="14154" PostTypeId="1" AcceptedAnswerId="14156" CreationDate="2016-02-20T10:54:52.350" Score="6" ViewCount="1223" Body="&lt;p&gt;According to the award's requirements, the Congressional Space Medal of Honor may be awarded to &quot;any astronaut who, in the performance of his duties, has distinguished himself by exceptionally meritorious efforts and contributions to the welfare of the Nation and mankind.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any specific known reason why Aldrin, and indeed, even Collins, were not afforded this honor alongside Armstrong? Of course Armstrong was the mission commander, and the first man to set foot on the moon, but it's difficult to not consider all of their efforts &quot;exceptionally meritorious&quot; as major pioneers in mankind's space exploration endeavors.&lt;/p&gt;&#xA;" OwnerUserId="13509" LastActivityDate="2016-02-20T12:54:48.170" Title="Why was Buzz Aldrin not Awarded the Congressional Space Medal of Honor?" Tags="|nasa|apollo-program|astronauts|lunar-landing|apollo-11|" AnswerCount="1" CommentCount="0" ClosedDate="2016-02-21T23:07:46.233" ContentLicense="CC BY-SA 3.0" />
  <row Id="14155" PostTypeId="2" ParentId="14153" CreationDate="2016-02-20T11:54:15.777" Score="1" Body="&lt;p&gt;I believe &lt;a href=&quot;https://space.meta.stackexchange.com/a/250/6750&quot;&gt;this is the resource&lt;/a&gt; Organic Marble mentioned in the comment. Somewhere down the answer there is a section &lt;em&gt;History books and collections&lt;/em&gt;. A couple more things I can think of:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;It's ONLY Rocket Science: An Introduction in Plain English&lt;/em&gt; is a nice read on pretty much everything involved in space missions and&#xA;astronautics. It is not really focused on history. But when&#xA;discussing topics (which it does in a &quot;friendly&quot; way, as title&#xA;suggests) it uses historic examples in many cases.&lt;/li&gt;&#xA;&lt;li&gt;If you are interested in Apollo missions you may find interesting&#xA;topics on technology and design processes in &lt;em&gt;Digital Apollo: Human&#xA;and Machine in Spaceflight&lt;/em&gt;. Though it might not be exactly what you&#xA;are looking for now.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I know these aren't books, but still for future reference I would&#xA;   recommend two Wiki sources: &lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;For a short overwiew I like Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/History_of_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;History of spaceflight&lt;/a&gt;,&#xA;with links pointing you to more specific topics. &lt;br&gt;&lt;/li&gt;&#xA;&lt;li&gt;And if you want a chronological walk through the events Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Timeline_of_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;Timeline of spaceflight&lt;/a&gt; is a good place to check out. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;If you can provide some feedback I may be able to give you more recommendations.&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:04.403" LastActivityDate="2016-02-20T12:24:28.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14156" PostTypeId="2" ParentId="14154" CreationDate="2016-02-20T12:54:48.170" Score="2" Body="&lt;p&gt;The best I can find is from &lt;a href=&quot;https://www.nasa.gov/topics/people/galleries/armstrong_oct1978.html&quot; rel=&quot;nofollow&quot;&gt;this photograph's caption&lt;/a&gt;, which states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Astronaut Neil Armstrong received the first Congressional Space Medal of Honor from President Jimmy Carter, assisted by Captain Robert Peterson. Armstrong, one of six astronauts to be presented the medal during ceremonies held in the Vehicle Assembly Building, was awarded for his performance during the Gemini 8 mission and the Apollo 11 mission when he became the first human to set foot upon the moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It could also be because Apollo 11 had a number of difficulties, and Neil Armstrong was the pilot. It was his quick thinking that allowed the mission to be successful. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-20T12:54:48.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14157" PostTypeId="1" AcceptedAnswerId="14167" CreationDate="2016-02-21T04:57:53.910" Score="11" ViewCount="1792" Body="&lt;p&gt;How would you get precious metals (like platinum, lithium, uranium, etc) from an asteroid safely back to Earth?&lt;/p&gt;&#xA;&lt;p&gt;First, assume we don't need the metals in space for in-space manufacturing.&#xA;Second, assume we want to get the metals back safely, and not pound the Earth like a meteoroid.&#xA;Third, ideally this method could deliver multiple tons of materials safely back to Earth.&lt;/p&gt;&#xA;&lt;h3&gt;Parameters&lt;/h3&gt;&#xA;&lt;p&gt;Using a heat shield and atmospheric braking is okay.&lt;br /&gt;&#xA;Using rocket propellant to slow down is fine too.&lt;br /&gt;&#xA;Semi-hard impacts are okay as long as the materials and containers can handle it.&lt;br /&gt;&#xA;This needs to be guided, and land in a roughly accurate landing zone.&lt;/p&gt;&#xA;&lt;h3&gt;Possible solution&lt;/h3&gt;&#xA;&lt;p&gt;One possible method is to extract hydrogen from water-rich asteroids, and use that for fuel to slow the descent of a capsule, along with parachutes and a heat shield. But I wonder just how much could be delivered at a time with this method.&lt;/p&gt;&#xA;&lt;p&gt;Still, I'd like to know what could be a practical method for delivering precious metals (by the ton) from space to Earth.&lt;/p&gt;&#xA;" OwnerUserId="13514" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-07-18T02:20:47.027" Title="How to get processed asteroid metals back to Earth in a safe manner" Tags="|landing|reentry|asteroid|mining|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14158" PostTypeId="1" CreationDate="2016-02-21T13:57:11.927" Score="4" ViewCount="129" Body="&lt;p&gt;I've been looking into the polyus laser system launched (almost!) by energia at the end of the cold war by the USSR.&#xA;I've read on astronautix and wiki that this massive 80 ton vehicle was a tks derived design. &#xA;My question is, was this a manned laser weapon system or an unmanned system based on a space station module?&#xA;If it was manned, was it supposed to get supplied the way Mir was? &#xA;How was the weapon supposed to work in terms of tracking, guidance (i.e was it meant to hit the booster as it ascends or is it meant to hit the MIRVs as they were coming down) and firing (i.e. details of the laser itself).&#xA;I realise that this was all super top secret thirty years ago but was hoping a bit more information was available.&lt;/p&gt;&#xA;" OwnerUserId="13439" LastActivityDate="2016-02-21T20:42:32.473" Title="Polyus system questions" Tags="|energia|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14159" PostTypeId="2" ParentId="14158" CreationDate="2016-02-21T20:42:32.473" Score="3" Body="&lt;p&gt;There seem to be all kinds of threads as to what it might have been intended for, I don't dispute your view though here is another &lt;a href=&quot;http://k26.com/buran/Info/Polyus/polyus-energia.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;view&lt;/a&gt;, it says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Statements made at that time by Gorbachev confirm the impression left by this article and another by General Designer V.V. Pallo of Design Bureau Salyut that the Polyus was indeed a test bed for the Soviet counter measure to the United States &quot;Stars Wars&quot;  program, an Orbital Weapons Platform. Platforms of this type would have been capable of delivering nuclear warheads from orbit to any point in the US in six minutes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Thus something consistent, the last bit rather less so. The article doesn't give any references though so the trail stops there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, as everyone likes a picture, this is from the same source:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5WMJE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5WMJE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-02-21T20:42:32.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14161" PostTypeId="1" CreationDate="2016-02-21T22:29:49.667" Score="16" ViewCount="3040" Body="&lt;p&gt;In case the Apollo Lunar module had approached the landing site at way too high velocity, could the ascent stage have separated and returned to the command module before a crash? Thus performing a task similar to a launch abort tower. One challenge would be to synch with the orbit of the command module.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6117" LastEditDate="2016-02-22T11:17:37.653" LastActivityDate="2022-09-27T18:35:05.000" Title="Could the Apollo lunar ascent stage have separated from the descent stage before touchdown in an emergency?" Tags="|apollo-program|lunar-landing|abort|launch-escape-system|" AnswerCount="2" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="14162" PostTypeId="2" ParentId="14161" CreationDate="2016-02-21T23:15:27.860" Score="32" Body="&lt;p&gt;Yes.  There is a lot of good information in this &lt;a href=&quot;https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.207.7507&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;presentation&lt;/a&gt; from the June 1966 Apollo Lunar Landing Mission Symposium relative to landing flight design including abort planning.  The crucial figure is this one:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/d0F55.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d0F55.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;which shows the capability of the ascent engine to abort all the way down to landing.  It assumes a 4 second delay to separate the landing stage.&lt;/p&gt;&#xA;&lt;p&gt;As far as returning to the orbit of the command module, &lt;a href=&quot;http://klabs.org/history/apollo_experience_reports/tn-d6847_apollo_abort_planning.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Apollo Experience Report - Abort Planning document&lt;/a&gt; explains on page 24 (33 of the pdf) that the Lunar Module would attempt to attain a safe orbit and the Command Module would rendezvous with it.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2022-09-27T18:35:05.000" LastActivityDate="2022-09-27T18:35:05.000" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="14163" PostTypeId="1" CreationDate="2016-02-22T11:24:01.660" Score="12" ViewCount="756" Body="&lt;p&gt;What is the distance two spacecraft will spark/arc at in SSO? Eg if one has just launched and one has been there a long time (eg a 10-20kV worst case voltage difference). I think Paschen's Law is the right equation, but I'm not sure how to deal with this in the space environment... &lt;/p&gt;&#xA;" OwnerUserId="13524" LastActivityDate="2020-09-18T19:09:08.493" Title="Sparking/arcing distance between two spacecraft?" Tags="|orbit|docking|electronics|rendezvous|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14164" PostTypeId="1" CreationDate="2016-02-22T12:20:07.437" Score="6" ViewCount="191" Body="&lt;p&gt;I've been looking at this &lt;a href=&quot;https://celestrak.org/events/collision/&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt;. This question is about the motion of fragments in the immediate aftermath of the collision. I've drawn the following figures and quotes from the link.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Figure 9 shows the evolution of the debris clouds 180 minutes post-collision, almost two revolutions later. The spread of each debris cloud around its respective orbit is already becoming apparent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Figure 9:&lt;/strong&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/GHzdn.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GHzdn.gif&quot; alt=&quot;Figure 9. View of Iridium 33 and Cosmos 2251 orbits and debris 180 minutes post-collision&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The next extract relates to a snapshot of about 30 seconds after the collision.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Examination of the interactive 3D scenario (provided below Figure 10) shows large out-of-plane relative velocity components, the apparent result of coupling of the two masses, despite the hypervelocity nature of the collision. There are also a large number of pieces of Cosmos 2251 debris with significant radial (downward) relative velocities, although it is not apparent why this situation would be the case. It is hoped that the availability of this data set will help researchers with expertise in hypervelocity impacts develop a more complete description of the collision geometry for this event.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Figure 10:&lt;/strong&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/KKRxk.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KKRxk.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This question is &lt;strong&gt;not&lt;/strong&gt; about the mentioned radial (downward) velocities, though I grant it is curious.&lt;/p&gt;&#xA;&lt;p&gt;The implication of the two screenshots, where figure 9 is three hours after figure 10, is that the spectrum of relative velocities is very small compared to original satellite velocities. The &lt;strong&gt;question&lt;/strong&gt; is, why are there not more fragments immediately generated with a range of high velocity headings between the two original trajectories?&lt;/p&gt;&#xA;&lt;p&gt;If it helps, think of the impact of two billiard-balls, in such a case neither ball retains its original heading and both depart in very different directions, according to the normal expectation of a near elastic collision.&lt;/p&gt;&#xA;&lt;p&gt;I can see a possible explanation, that the collision was a glancing blow, the main bodies continued as they were and the spectrum of ejected fragments is near inelastic, in the sense that much collision energy is lost and so the relative velocities are low.&lt;/p&gt;&#xA;&lt;p&gt;Does anyone have any more insight into this?&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2016-02-24T01:22:28.587" Title="Iridum-Cosmos post-collision vectors" Tags="|debris|impact|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14165" PostTypeId="1" AcceptedAnswerId="14177" CreationDate="2016-02-22T16:02:35.777" Score="6" ViewCount="396" Body="&lt;p&gt;I'm working on a science fiction piece set on Enceladus, and I'd like the characters (human) to do a fair amount of swimming (they've managed to terraform a small bit of ocean for themselves). However, since the phase velocity of the wave would be greatly reduced, as pointed out in the swimming on Mars post, I'm wondering if swimming in Enceladus' .113G would be feasible at all, or if there might be a more desirable means of moving oneself across a body of water in extreme low-G scenarios. &lt;/p&gt;&#xA;" OwnerUserId="2802" LastEditorUserId="12102" LastEditDate="2016-09-28T04:21:17.933" LastActivityDate="2016-09-28T04:21:17.933" Title="With so little gravity, would swimming on Enceladus be feasible for humans?" Tags="|colonization|gravity|physical-activity|enceladus|reduced-gravity-sports|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14166" PostTypeId="1" CreationDate="2016-02-22T16:12:06.777" Score="6" ViewCount="469" Body="&lt;p&gt;In the &lt;a href=&quot;http://www.techinsider.io/nasa-apollo-10-crew-recording-of-music-on-the-dark-side-of-moon-2016-2&quot; rel=&quot;noreferrer&quot;&gt;news&lt;/a&gt; today, are reports of &quot;outer-space-type music&quot; that the Apollo 10 crew heard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the mission &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/AS10_LM.PDF&quot; rel=&quot;noreferrer&quot;&gt;transcript&lt;/a&gt; (page 197 of the PDF):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;CDR Commander Thomas P. Stafford&#xA;CMP Command module p_lot John W. Young&#xA;LMP Lunar module pilot Eugene A. Cernan&#xA;SC Unidentifiable crewmembcr&#xA;CC Capsule communicator (CAP COMM)&#xA;&#xA;04 06 13 02 LMP That music even sounds outer-spaeey, doesn't it?&#xA;You hear that? That whistling sound?&#xA;04 06 13 06 CDR Yes.&#xA;04 06 13 07 _4P Who0ooooo. Say your - -&#xA;04 06 13 12 CMP Did you hear that whistling sound, too?&#xA;04 06 13 14 _P Yes. Sounds like - you know, outer-space-type&#xA;music.&#xA;04 06 13 18 _4P I wonder what it is.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;NASA's explanation of this sound is that the radios on the command module and lunar module were interfering with one another. Cassini also had a similar experience, and that was explained as charged particles moving through the magnetic environment around Saturn creating distortions in ways that cause those odd sounds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have either of these &quot;problems&quot; been corrected with newer space craft or is such interference rare enough that it's not actually a problem?&lt;/p&gt;&#xA;" OwnerUserId="6868" LastEditorUserId="6868" LastEditDate="2016-02-22T16:39:23.850" LastActivityDate="2016-02-22T16:39:23.850" Title="Does the Apollo 10 &quot;music&quot; still occur on modern spacecraft?" Tags="|apollo-program|cassini|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14167" PostTypeId="2" ParentId="14157" CreationDate="2016-02-22T19:30:24.463" Score="6" Body="&lt;p&gt;&lt;strong&gt;Asteroidal water as a prerequisite to precious metals&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something to keep in mind is the effect your delta V budget has on your propellent to dry &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2012/08/mf-is-mofo-tyranny-of-rocket-equation_21.html&quot; rel=&quot;noreferrer&quot;&gt;mass fraction&lt;/a&gt;. Larger Delta V budgets mean less dry mass that can be devoted to structure, thermal protection, parachutes, and/or wings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DdmSP.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DdmSP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I picked 4.4 km/s exhaust velocity because that's near the best you're going to get with chemical propellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes about 9 to 10 km/s to get to Low Earth Orbit (LEO). Remember, some of the dry mass fraction must also be payload. So just for getting to LEO, your dry mass is around 4 or 5%. This puts constraints on how robust you can make the structure or how much thermal protection you can add.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On re-entry, the space shuttle would shed around 8 km/s over an hour's time. This would subject it to high temperatures and large dynamic pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For ascent shuttle's Max Q was around 35 kilo pascals. A strong hurricane has a dynamic pressure of around 3 kilo pascals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These constraints have made it impractical to re-use rockets. The space shuttle did this in limited fashion (although it would throw away dry mass enroute). But maintaining the thermal protection tiles was so labor intensive little if any money was saved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So water is one of the first commodities Planetary Resources hopes to mine. Given an extra-terrestrial propellent source, it would no longer be necessary to climb in and out of earth's steep gravity well to get the propellent needed for a spacecraft's continued use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And economic, reusable spacecraft are a prerequisite for mining platinum, gold or any other precious metal. If a space craft is thrown away each trip, I believe even an asteroid made out of pure crack cocaine would be a money pit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Oxygen is more storable in LEO&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrogen and oxygen propellent depots are a problem in low earth orbit. Not only do you have the sun for a heat source, but the earth is a ~300 Kelvin heat source that takes up nearly half the sky. The Multi Layer Insulation (MLI) heat shades would need to be cones that cover most the depot's sky, leaving only a small area to radiate heat into. Also some aerobraking &lt;a href=&quot;http://hop41.deviantart.com/art/Drag-Pass-Maneuver-Movie-127420694&quot; rel=&quot;noreferrer&quot;&gt;drag passes&lt;/a&gt; would probably be used to park the propellent tanker in LEO. For parking in LEO, drag passes can save up to 3.1 km/s but they are time consuming and heat the rocket, two factors that exacerbate cryogenic boil off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both oxygen and hydrogen are cryogens, but hydrogen needs to be kept much colder and is harder to make an effective seal against. Oxygen typically accounts for the lion's share of propellent mass is a lot easier to store. So if we get water from asteroids, I'd expect the propellent sent to LEO would be mostly oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But ULA's Kutter and Zegler have proposed propellent depots that use hydrogen boil off for station keeping: &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/published_papers/extended_duration/propellantdepots2009.pdf&quot; rel=&quot;noreferrer&quot;&gt;Realistic Near-Term Propellent Depots&lt;/a&gt;. Hydrogen boiling off acts like evaporative cooling. So the inevitable hydrogen boil off not only serves for station keeping -- but also keeps remaining cryogenic oxygen cold.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Getting the platinum to earth's surface&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'm assuming asteroidal propellent in LEO. How much is needed for a soft landing? That is an open question. I'll note that SpaceX looks like they're well on their way to recovering their boosters. But these don't re-enter the atmosphere at 8 km/s as an orbital cargo would.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we can look at energy savings. Kinetic energy goes with square of velocity. Assuming the usual aereo braking re-entry sheds 8 km/s of velocity we can look at energy differences for different scenarios.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shed 2 km/s with propellent leaves 75% percent of the velocity to shed. .75^2 = .56. So 56% of the energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shed 4 km/s with propellent leaves 50% percent of the velocity to shed. .50^2 = .25. So 25% of the energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shed 6 km/s with propellent leaves 25% percent of the velocity to shed. .25^2 = .0625. So 6% of the energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides propellent for reducing energy, it is possible asteroidal stuff could beef up structure and thermal protection thus making the space craft more able to endure re-entry.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-02-22T19:30:24.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14168" PostTypeId="1" CreationDate="2016-02-23T00:04:44.003" Score="3" ViewCount="217" Body="&lt;p&gt;Do gravitational forces affect how sound travels?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example would two different planets with similar atmospheres, but greatly differing in gravity make any differences to how sound travels?&lt;/p&gt;&#xA;" OwnerUserId="13528" LastEditorDisplayName="user10509" LastEditDate="2016-02-23T14:15:49.690" LastActivityDate="2016-02-24T08:53:35.710" Title="Does gravity affect sound on different mass planets?" Tags="|gravity|sound|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14172" PostTypeId="1" AcceptedAnswerId="14176" CreationDate="2016-02-23T09:54:23.220" Score="37" ViewCount="15113" Body="&lt;p&gt;I've always been curious how did the Apollo 11's &lt;em&gt;Eagle&lt;/em&gt; ascent stage of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot;&gt;Apollo Lunar Module&lt;/a&gt; navigate back to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot;&gt;Apollo Command/Service Module&lt;/a&gt; (CSM) that was in lunar orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was it simply a case of being able to see it and thrust in the general direction, or was there something more sophisticated at work? Was the CSM itself in a selenostationary orbit, or was it moving with respect to the lunar surface, thus making it even more difficult?&lt;/p&gt;&#xA;" OwnerUserId="13535" LastEditorUserId="32645" LastEditDate="2019-08-01T18:10:27.980" LastActivityDate="2019-08-01T18:44:39.987" Title="How did the Apollo 11's Eagle ascent stage find the CSM after leaving the Moon?" Tags="|the-moon|apollo-program|navigation|apollo-11|rendezvous|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14173" PostTypeId="2" ParentId="14168" CreationDate="2016-02-23T10:15:28.080" Score="5" Body="&lt;p&gt;Yes and no. It's a yes in that gravity is often responsible for the density of the atmosphere, at least on a planet or gas cloud. Higher air density often means a higher speed of sound for the same composition of atmosphere, although there are other factors involved. It's a no in that gravity does not change the path of sound. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say you had 2 identical habitats pressurized to 1 earth atmosphere at the same temperature and humidity levels. One habitat is on a 1G planet, the other is on a 5G planet. Sound would travel the same speed and reach the same point on the other side of the hab at the same time. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2016-02-24T08:53:35.710" LastActivityDate="2016-02-24T08:53:35.710" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14175" PostTypeId="2" ParentId="14164" CreationDate="2016-02-23T16:17:26.600" Score="0" Body="&lt;p&gt;Satellites are not billiard balls, they are complex structures designed to withstand the expected stresses in their lifetimes.  As an example, imagine the impact was between the bodies of the satellites.  The arms holding the solar arrays are not very strong, so would snap before imparting much velocity change to the arrays.  This would cause the arrays (or maybe pieces of them) to leave the impact with similar velocity to what the satellite had before the impact.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2016-02-23T16:17:26.600" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14176" PostTypeId="2" ParentId="14172" CreationDate="2016-02-23T16:28:04.390" Score="54" Body="&lt;p&gt;The process was a great deal more sophisticated than pointing and thrusting, and the CSM was cooking along at over 1600 m/s, circling the moon every 2 hours. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mission control, however, had fairly precise tracking of the positions of both the CSM and the landing site, and they computed the correct time of launch to begin the rendezvous using powerful Earth-side computers. The CSM was tracked using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Unified_S-band#Ranging&quot; rel=&quot;noreferrer&quot;&gt;Unified S-Band&lt;/a&gt; system. For Apollo 11, the LM's position was determined by &lt;a href=&quot;http://www.hq.nasa.gov/alsj/a11/a11.launch.html&quot; rel=&quot;noreferrer&quot;&gt;using the LM's rendezvous radar to determine the direction and distance to the CSM&lt;/a&gt; (see commentary at 121:00:34) as it passed overhead, but for the later Apollo missions, I believe there was better tracking of the LM down to the surface and landing sites could be pinpointed by comparing observations from the surface against &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Orbiter_program&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbiter&lt;/a&gt; imagery. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LM ascent stage launched initially into a 18km x 87km elliptical orbit somewhat behind the CSM, which was orbiting at 105km x 116km (a higher, slower orbit). At the high point of that elliptical orbit, about an hour after liftoff, the LM circularized its orbit using its RCS thrusters; this is called the &quot;Coelliptic Sequence Initiation&quot; burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe from this point on, Eagle could detect Columbia on rendezvous radar. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two and a half hours later, with the LM about 72km from the CSM and approaching at 34 m/s, they did another RCS burn called &quot;Terminal Phase Initiation&quot; to intercept the CSM. 10 minutes after that, Armstrong was able to see the CSM. Until the ships were very close together, the maneuvering was all done under computer control using radar data; once they were close together and velocity-matched, they manually flew the approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the maneuvering of the approach was done by the LM, but the CSM was at each point prepared to make the reciprocal maneuver, such that if the LM's crew, computers, or RCS failed, the CSM would have taken over as the active participant in the rendezvous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.hq.nasa.gov/alsj/a11/a11.launch.html&quot; rel=&quot;noreferrer&quot;&gt;This transcript and commentary covers the ascent&lt;/a&gt; and &lt;a href=&quot;http://history.nasa.gov/ap11fj/19day6-rendezvs-dock.htm&quot; rel=&quot;noreferrer&quot;&gt;this one covers the rendezvous and docking&lt;/a&gt;. The &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_11i_Timeline.htm&quot; rel=&quot;noreferrer&quot;&gt;mission timeline here&lt;/a&gt; summarizes the LM maneuvers from ascent starting at 124:22.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the rendezvous approach plan was fairly leisurely, and the RCS thrusters capable of quite precise small adjustments, any small deviation in the initial ascent (due to errors in the estimated positions of the craft, the timing of the launch, or variation in the performance of the ascent engine) would be easily taken care of in the following maneuvers. This incremental approach is the key to any orbital rendezvous, whether in Earth orbit or lunar.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-08-01T18:44:39.987" LastActivityDate="2019-08-01T18:44:39.987" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="14177" PostTypeId="2" ParentId="14165" CreationDate="2016-02-23T16:28:12.610" Score="6" Body="&lt;p&gt;Sure. Whatever the pressure-based limit is, they can dive proportionally deeper by the amount that the gravitational acceleration is less. For Enceladus, that would be 87 times deeper. The pressure is simply the weight of the water above you, and the weight is proportional to gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pressure-based limit on Earth depends on what effect you are worrying about. If it's nitrogen narcosis, it's about 30 meters. If it's oxygen toxicity, because you've mitigated nitrogen problem by reducing the nitrogen but increased the oxygen, it's about 56 meters. To go deeper you add helium to the mix to reduce the oxygen partial pressure. Then the limit is around 300 meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So multiply those by 87.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or there is no limit, except that due to materials strength, if you &lt;a href=&quot;http://www.deepseachallenge.com/the-sub/&quot;&gt;build your own pressure chamber&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Make sure you bring a lot of light.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-02-23T16:28:12.610" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14180" PostTypeId="2" ParentId="14168" CreationDate="2016-02-23T21:59:03.677" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;For example would two different planets with similar atmospheres, but greatly differing in gravity make any differences to how sound travels?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That depends on what you mean by &quot;similar atmospheres.&quot; If you mean similar chemical constituents, similar surface temperature, and similar surface pressure, then the answer is no. In an ideal gas, the speed of sound depends only on temperature and chemical constituents of the atmosphere. Pressure does come into play in a non-ideal gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How the speed of sound varies with pressure depends very much on the components of the non-ideal gas. With dry air, the speed of sound increases very slightly as pressure increases (and temperature remains constant). Water vapor makes this slight increase in the speed of sound with increasing pressure change into a decrease, a rather marked decrease with high humidity. That said, if the surface pressure is &quot;similar&quot;, then there is no effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one significant effect in these similar atmospheres, and that's how the speed of sound varies with altitude. The dry adiabatic lapse rate (the rate at which temperature nominally drops with altitude outside of clouds) is proportional to &lt;em&gt;g&lt;/em&gt;. Assuming similar atmospheres, the planet with a higher surface gravity will have a much higher lapse rate than the planet with a lower surface gravity. The speed of sound in air strongly depends on temperature, proportional to the square root of absolute temperate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that the planet with the greater value of &lt;em&gt;g&lt;/em&gt; will see a slower speed of sound at altitude, which in turn means sounds will attenuate faster (at least during daytime) on the planet with the greater value of &lt;em&gt;g&lt;/em&gt;. A decreasing speed of sound with altitude refracts sound up, away from the surface. (Aside: On Earth, a temperature inversion often sets up at night. This has the opposite effect, refracting sound down. This is why you can hear a far-away train whistle at night but not during the day.)&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-02-23T21:59:03.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14181" PostTypeId="1" AcceptedAnswerId="14182" CreationDate="2016-02-23T22:10:18.893" Score="16" ViewCount="3768" Body="&lt;p&gt;Apollo 11 used the Saturn V rocket, which is no longer produced. As far as I can tell, Atlas V rockets are not as powerful and would not be able to deliver the lunar lander to the surface of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we were to launch another Apollo mission to land on the Moon today, do we have a rocket powerful enough to do it?&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="1142" LastEditDate="2016-02-23T23:06:42.617" LastActivityDate="2016-02-25T04:38:59.587" Title="Are any rockets currently in use capable of putting a man on the Moon?" Tags="|rockets|the-moon|crewed-spaceflight|apollo-program|lunar-landing|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="14182" PostTypeId="2" ParentId="14181" CreationDate="2016-02-23T22:39:09.740" Score="17" Body="&lt;p&gt;From the questions wording you mean, could we lift the Apollo CM/LM stack on any existing boosters? In that case no. Largest heavy lift vehicle is probably Delta-4-Heavy.  Falcon Heavy is not yet operational and will exceed it, but has a low Isp upper stage so is not optimal for such missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a different moon mission be launched? Almost certainly, just a matter of launching in smaller pieces and likely refueling in orbit. Would it be worth it? Well clearly no one has thought it worthwhile to date. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would launch something like the Command Module and Service module (In one or two modules), where the Service module would act as an upper stage, and then refuel it in orbit from following launches. Then another launch for a Lunar Module, assemble in orbit and onwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a lot of speculation that the Falcon Heavy demo payload might be an Apollo 8 style mission, where a capsule is sent to go around the moon and come back.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there was a true desire to do so, it is likely possible, but no one has decided it is necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, consider costs. A Saturn V in modern dollars has been estimated in \$2 billion dollar range a launch. A Delta 4 Heavy is around \$400 million a launch.  A Falcon Heavy launch is advertised at \$125 million. (Assuming they ever develop cross feed). Thus even 5 Delta 4 Heavy launches is competitive with a Saturn V, and as many as 16 Falcon Heavy launches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the experience of building the ISS a lot of technology and tools for in space assembly has become much more feasible than in the 1960s on a tight deadline. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We probably no longer need a giant super booster just for the moon, even though SpaceX plans to build one as the BFR/MCT combo for Mars missions.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="897" LastEditDate="2016-02-24T13:51:50.913" LastActivityDate="2016-02-24T13:51:50.913" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14183" PostTypeId="1" AcceptedAnswerId="17107" CreationDate="2016-02-23T22:49:21.827" Score="13" ViewCount="931" Body="&lt;p&gt;According to &lt;a href=&quot;http://georgetyson.com/files/apollostatistics.pdf&quot;&gt;Apollo By The Numbers&lt;/a&gt;, the LM ascent stage of Apollo 11 (&quot;Eagle&quot;, LM-5) had maximum rated thrust of 3218 lbf (14.31 kN), and subsequent missions had very similar figures. The engine itself is usually advertised as producing 3500 lbf (15.56 kN). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo 9's &quot;Spider&quot; (LM-3), on the other hand, is shown as having maximum thrust of 2524 lbf (11.23 kN), and Apollo 10's &quot;Snoopy&quot; (LM-4) as 1650 lbf (7.34 kN). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why were the LM ascent thrust ratings so dramatically different between Apollo 9, Apollo 10, and the subsequent missions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that the original Bell-designed injector for the ascent engine was causing problems, and Rocketdyne stepped in with a new injector very late in the program, but according to &lt;a href=&quot;http://www.friends-partners.org/mwade/craft/lmalsion.htm&quot;&gt;this timeline&lt;/a&gt;, the new injector was flown on all manned LM flights starting with Apollo 9's LM-3, and it seems very unlikely that an injector with stability problems would have been flown on a critical manned test flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can think of a few possible explanations:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Simple errors in the figures from either Apollo press kits or the transcriptions of the press kits to ABTN.&lt;/li&gt;&#xA;&lt;li&gt;Testing of the Rocketdyne injectors at full power had not been completed, so the rated power levels were fixed at artificially low values. This seems unlikely to me, particularly with LM-3 being more powerful than LM-4. &lt;/li&gt;&#xA;&lt;li&gt;The ascent engine power on LM-4 was deliberately restricted so that the acceleration of the ascent stage with its light fuel load would be similar to that of LM-5. However, the math doesn't seem to work: the loaded weight of the LM-4 ascent stage was more than half that of LM-5, so acceleration would be substantially lower across the entire ascent stage burn. &lt;/li&gt;&#xA;&lt;li&gt;The ascent engine power on LM-4, along with the fuel load, was tuned such that the staging and ascent from the LM's 9-mile periapsis would leave the ship in a similar orbit to LM-5's ascent from the lunar surface. This leaves the LM-3 figure mysterious. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-02-23T22:57:07.663" LastActivityDate="2016-07-10T00:17:45.783" Title="What were the differences on the LM ascent stage engines on Apollo 9, 10, and 11?" Tags="|apollo-program|lunar-landing|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14184" PostTypeId="2" ParentId="14181" CreationDate="2016-02-24T00:32:39.290" Score="6" Body="&lt;p&gt;No. But two ideas about how to do it aren't far off, although I doubt it will be done that way.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Falcon Heavy, which would be three Falcon 9 first stages put together&#xA;and which will test fly in September 2016 (if not postponed again and&#xA;again). It would require &lt;a href=&quot;http://shitelonsays.com/transcript/spacex-next-falcon-heavy-press-conference-2011-04-05&quot; rel=&quot;nofollow&quot;&gt;two launches&lt;/a&gt;. One that lands an empty ascent&#xA;and return vehicle on the Moon, and a second which lands an astronaut&#xA;or two next to it.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Russian Angara 5, which would be the heaviest launcher in the world today and has been successfully &lt;a href=&quot;http://www.russianspaceweb.com/angara5_flight1.html&quot; rel=&quot;nofollow&quot;&gt;test launched&lt;/a&gt; in 2014, but in a yet uncomplete configuration, has officially been proposed to send kosmonauts to the Moon with &lt;a href=&quot;http://sputniknews.com/russia/20151029/1029277505/russia-moon-mission-angara.html&quot; rel=&quot;nofollow&quot;&gt;four launches&lt;/a&gt; that assemble the lunar mission in low Earth orbit.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-02-24T20:50:05.327" LastActivityDate="2016-02-24T20:50:05.327" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14185" PostTypeId="2" ParentId="6607" CreationDate="2016-02-24T00:48:23.160" Score="7" Body="&lt;p&gt;There's one consideration probably neglected by most answers: with water viscosity unchanged, it will be much easier to remain on/above the surface through actively &quot;pushing against the water&quot; - unlike on Earth, where it takes a lot of (futile) effort to remain immersed any less than what your buoyancy gives you, a couple strokes will get you sliding along the surface of water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With strokes like front crawl you'll be easily able to bring yourself to the surface, and then only immerse your hands for propulsion, while your body skims the surface, barely creating any wake wave. Of course if you want to remain immersed or dive, it just requires you to angle your hands differently and &quot;pull yourself under&quot;, but if you want to move fast, you can move like a speed boat, barely skimming the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess with gravity low enough and the right technique, even running on water could be possible - not due to buoyancy but because so little force is needed to stay airborne.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-24T00:48:23.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14186" PostTypeId="2" ParentId="14164" CreationDate="2016-02-24T01:22:28.587" Score="2" Body="&lt;p&gt;With the satellites in LEO, relatively few debris will end up with velocity higher than at the impact moment, while moving at roughly similar trajectory as initially. That means most debris scattered in random directions would enter elliptical orbits with velocity roughly similar to initial at the point of impact. And that means apoapsis going significantly up, and periapsis - down. And lowering the periapsis significantly in LEO means one thing: reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About all the debris that were knocked out of circular orbits, by the time of two revolutions later were already burned up, whether going directly down from the impact, or going up towards the new apoapsis, and then heading down almost a revolution later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What remained were debris that didn't participate in the core part of the collision, torn away and shattered by the vibration of the stressed structure, not gaining or losing much of the initial velocity, plus scarce few that achieved enough velocity at just the right angle not to fall - apoapsis rising, periapsis not - certainly not many of them, because hypervelocity collisions tend to be of inflexible nature, metal &lt;em&gt;&lt;a href=&quot;https://i.stack.imgur.com/df9hT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;splashing&lt;/a&gt;&lt;/em&gt; instead of shattering, meaning gaining speed vs losing it is unlikely, - lower periapsis much more likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, putting a satellite in a stable LEO requires quite a bit of mathematics, and if you change its velocity randomly by a large factor, the chance it will remain in orbit is quite small. The wiggle room isn't much. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It isn't that most debris entered these orbits shown by the &quot;clouds&quot;. Simply, most debris that didn't continue along these orbits burned up on reentry.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-24T01:22:28.587" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14187" PostTypeId="1" CreationDate="2016-02-24T04:56:01.883" Score="8" ViewCount="2158" Body="&lt;p&gt;How did the first manned Moon mission spacesuits block the deadly radiation of both Van Allen belts and direct sunlight?&lt;/p&gt;&#xA;" OwnerUserId="13544" LastEditorUserId="12519" LastEditDate="2016-02-25T21:51:42.220" LastActivityDate="2018-10-26T20:59:10.197" Title="How did the first US manned Moon mission suits block radiation?" Tags="|the-moon|nasa|radiation|spacesuits|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14188" PostTypeId="1" AcceptedAnswerId="14192" CreationDate="2016-02-24T06:46:10.870" Score="13" ViewCount="4743" Body="&lt;p&gt;For the Falcon 9R, and the Dragon spacecraft, the use thrusters for attitude adjustments on both the stages returning and to control the rotation of the descending Dragon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/sSF81yjVbJE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the return stage thrusters at 0:40, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering what would make reaction wheels impractical here - for the desired torque, would the weight of the reaction wheel and its associated motor and hardware outweigh the weight of the fuel/gas they are using?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or perhaps the thrusters are more reliable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's logical that at a certain quantity of thrust ability, a reaction wheel could weigh less than the associated fuel and thrusters and piping. Is it just that the quantity of thrust they need is relatively low? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By 'reaction wheel' I do mean any currently available non-fuel using attitude controls - &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_wheel&quot;&gt;reaction wheels&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_wheel&quot;&gt;momentum wheels&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Control_moment_gyroscope&quot;&gt;control moment gyroscopes&lt;/a&gt;, (anything else?).&lt;/p&gt;&#xA;" OwnerUserId="6821" LastEditorUserId="51" LastEditDate="2016-02-24T12:50:29.543" LastActivityDate="2017-09-08T07:13:04.330" Title="Why did SpaceX choose gas thrusters over reaction wheels/CMGs?" Tags="|spacex|falcon-9|reaction-wheel|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14189" PostTypeId="1" CreationDate="2016-02-24T07:28:18.103" Score="1" ViewCount="123" Body="&lt;p&gt;How long can a balloon stay at a certain altitude (~14km) and what are the techniques to control the altitude? Are there low-cost ($300) methods to maintain an altitude of 14km for weeks? The foreseen payload would be around 1kg.&lt;/p&gt;&#xA;" OwnerUserId="13548" LastEditorUserId="415" LastEditDate="2016-02-24T11:58:22.897" LastActivityDate="2016-02-24T11:58:22.897" Title="How practical is a low-cost balloon that stays at 14 km with attitude control, carrying a 1 kg payload?" Tags="|balloons|" AnswerCount="0" CommentCount="11" ClosedDate="2016-02-24T16:59:37.260" ContentLicense="CC BY-SA 3.0" />
  <row Id="14190" PostTypeId="2" ParentId="14187" CreationDate="2016-02-24T07:35:30.017" Score="15" Body="&lt;p&gt;The Van Allen belt radiation was primarily shielded by the hull of the spacecraft, not the space suits. The electrons were mostly absorbed by the aluminium. Of the protons, however, some would penetrate the hull, and the flight path was chosen in such a way as to avoid the most intense radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Moon, there's no Van Allen belt radiation, so only solar and cosmic radiation had to be dealt with, plus a small dose of secondary radiation produced by absorption processes in the regolith. A space suit cannot protect against all of this radiation, but it was good enough to not be of major concern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was however a constant danger of solar &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot;&gt;coronal mass ejections&lt;/a&gt; while a mission was flown, which would create huge amounts of high energy radiation.&#xA;This was a risk that simply was accepted. This is one of the major obstacles for a manned mission to Mars. While the probability of a CME during a one week mission is acceptably low, it's almost a certainty for a one year mission. &lt;/p&gt;&#xA;" OwnerUserId="716" LastActivityDate="2016-02-24T07:35:30.017" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14191" PostTypeId="1" AcceptedAnswerId="14193" CreationDate="2016-02-24T08:21:42.807" Score="8" ViewCount="1084" Body="&lt;p&gt;In the &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/spacex_ses9_press_kit_final.pdf&quot; rel=&quot;nofollow&quot;&gt;SES-9 mission press kit&lt;/a&gt;, SpaceX mentions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Given this mission’s unique GTO profile, a successful landing is not expected.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is considered &quot;unique&quot; about this transfer orbit's profile?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the end of this &lt;a href=&quot;http://spaceflightnow.com/2016/02/23/flight-timeline-for-falcon-9s-launch-of-ses-9/&quot; rel=&quot;nofollow&quot;&gt;spaceflightnow&lt;/a&gt; article, it states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The SES 9 satellite separates from the Falcon 9 rocket in an orbit with a predicted high point of about 39,300 kilometers (24,400 miles), a low point of 290 kilometers (180 miles) and an inclination of 28 degrees.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In that case, the satellite carries enough propulsion to move to a geostationary orbit (zero eccentricity, zero inclination) &lt;s&gt;from there&lt;/s&gt; ? Where (at what point in the orbit) will the satellite separate from the second stage?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-03-17T08:31:25.573" LastActivityDate="2021-03-17T08:31:25.573" Title="What will be &quot;unique&quot; about upcoming SES-9 GTO transfer profile by SpaceX?" Tags="|spacex|falcon-9|orbital-maneuver|communication-satellite|ses|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14192" PostTypeId="2" ParentId="14188" CreationDate="2016-02-24T08:32:53.847" Score="22" Body="&lt;p&gt;Torque and/or mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can go with small reaction wheels which can only barely affect the attitude, or with massive ones, which... still barely affect the attitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are good for satellites that stay in the orbit for years, where you can afford hours per maneuver, but need years of operation. Meanwhile, RCS will run out of fuel quite fast, but it can provide a good torque rapidly. With a tight schedule for reentry at the right trajectory, Falcon 9 needs that quick reaction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any docking vessels require RCS for translation maneuvers for docking - it certainly does have some reaction wheels besides that, but RCS is still a must.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Edit: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's have a look at the &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1080&amp;amp;.context=smallsat&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; linked from the Wikipedia page that talks about &quot;a few hundred watts and 100kg of mass... thousands of newton meters of torque&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's ballpark the F9 1st stage at the moment of separation - before the most torque-intensive maneuver of turning 180 degrees - as &lt;a href=&quot;https://space.stackexchange.com/questions/4466/how-is-spacex-going-to-land-their-falcon-first-stage&quot;&gt;100 ton&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's approximate it as a rod of length &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/&quot; rel=&quot;noreferrer&quot;&gt;41.2 meters&lt;/a&gt;. The &lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/mi2.html&quot; rel=&quot;noreferrer&quot;&gt;moment of inertia&lt;/a&gt; will be around 14,000,000 $kg m^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know how many &lt;em&gt;thousands of newton meters of torque&lt;/em&gt; the sentence in Wikipedia meant, but let's generously assume &lt;em&gt;a hundred&lt;/em&gt; thousands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/n2r.html&quot; rel=&quot;noreferrer&quot;&gt;angular acceleration&lt;/a&gt; will then be 0.007rad/s^2. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ \phi = 0.5 \alpha t^2$. We need to accelerate for 1/4 turn, then decelerate for the remaining 1/4 turn to complete a 180 degree turn. Therefore half of the time taken will occur after $0.5 \pi$ radians of the turn. $t = \sqrt{ 2\phi \over \alpha} = \sqrt{\pi/0.007} s = 21s$. Double that for the braking half of the turn, for 42s for the whole maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;42 seconds is still doable within time budget of F9 launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTOH, the paper shows the CMG with highest available torque commercially available to be of order of 100Nm, not 100,000Nm. A CMG of 1000Nm (ten times stronger than the top one) will then take 420s to turn the stage around. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, on arm of, say, 20m (41.2m/2 minus margin not to place them not on the very tips) 100,000Nm (the same 42s from above) can be achieved with two 2500N rocket engines, which is a very modest value.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-02-24T13:17:31.903" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14193" PostTypeId="2" ParentId="14191" CreationDate="2016-02-24T12:49:29.320" Score="12" Body="&lt;p&gt;SpaceX has stated a goal that every launch going forward will attempt to land. The expectation initially is not 100% success, but an attempt at the very least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Large GTO payloads do not leave enough fuel reserve for a Return to Launch Site (RTLS) at Landing Zone 1. Instead they will use the ASDS (Autonomous Spaceport Drone Ship) as far downrange as needed to land upon. This cuts down on the required fuel used in the first reentry burn since you only have to cancel most of the stages forward motion, not also boost it back towards the launch site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of SES-9, the launch was delayed numerous times and thus entry into revenue service is delayed costing the owner money every day it is not orbited. In order to keep the customer happy SpaceX has agreed to cut the margins they reserve for recovery to the bare minimum and likely risk not recovering the stage. The fuel not reserved for recovery can then be used for additional performance in the first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal is to put the payload into the more energetic orbit, so that transitioning to full GEO is faster and simpler. This means it gets there faster (less distance to go/energy to impart by the built in thrusters) and uses less onboard fuel getting there, which means a longer life span in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time is money, since SpaceX cost them time/money on launch dates, they are trying to give some back with performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual orbit they will be delivering the payload into is basically a standard GTO (Geosync Transfer Orbit) where the payload itself must finish the job of getting to GEO. There are several accepted versions GTO-1500, GTO-1800 where additional m/s of delta V are required to finish the job.  They had contracted for GTO-1800 I think, and SpaceX is offering a better orbit.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-02-24T14:00:40.107" LastActivityDate="2016-02-24T14:00:40.107" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="14194" PostTypeId="2" ParentId="14188" CreationDate="2016-02-24T13:38:34.603" Score="1" Body="&lt;p&gt;&lt;s&gt;Somebody check my math.&lt;/s&gt; &lt;em&gt;I have checked the math, shown edits.&lt;/em&gt; To turn the first stage around in one minute using a 2 meter diameter steel (&lt;em&gt;or lead or tungsten or depleted uranium&lt;/em&gt;) wheel with a mass equal to a typical payload (~5000kg), you'd need to discharge a Tesla &lt;s&gt;model-S&lt;/s&gt; &lt;em&gt;Powerwall&lt;/em&gt; battery (about 5 kWh) in 10 seconds to spin it up to &lt;s&gt;1800&lt;/s&gt; &lt;em&gt;180&lt;/em&gt; RPM, then dump the energy back into the battery again a &lt;em&gt;half&lt;/em&gt;-minute later to stop it. Assume the first stage has 10% propellants remaining.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can take a minute to pause and enjoy that image, but the thrusters have one thing going for them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nice thing about thrusters is that you can put them at the ends, where you can take advantage of the length of the body (r^2). That &quot;little&quot; 5000kg wheel in the middle is at a distinct disadvantage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Structural engineers will shudder at the thought of applying the torque to the middle while most of the mass is at the ends - another advantage of having the thrusters at places where the mass is concentrated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/syWUs.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/syWUs.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-03-04T08:18:04.437" LastActivityDate="2016-03-04T08:18:04.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14197" PostTypeId="2" ParentId="14181" CreationDate="2016-02-24T15:43:33.653" Score="-3" Body="&lt;p&gt;I remember reading in &lt;a href=&quot;http://www.billbryson.co.uk/index.php/a-short-history-of-nearly-everything/&quot; rel=&quot;nofollow&quot;&gt;Bill Bryson's - a short history of nearly everything&lt;/a&gt; that we no longer possess a rocket powerful enough to send a man to the moon! Apparently the historic Apollo 11 plans were destroyed sometime in 90's by NASA during a spring clean.&lt;/p&gt;&#xA;" OwnerUserId="13552" LastActivityDate="2016-02-24T15:43:33.653" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14198" PostTypeId="1" CreationDate="2016-02-24T16:21:37.657" Score="4" ViewCount="1216" Body="&lt;p&gt;The Spaceflight Now article &lt;a href=&quot;http://spaceflightnow.com/2016/02/23/flight-timeline-for-falcon-9s-launch-of-ses-9/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flight timeline for Falcon 9’s launch of SES 9&lt;/a&gt; says near the end that the Falcon 9 2nd stage will leave SES-9 in an elliptical, inclined transfer orbit.&lt;/p&gt;&#xA;&lt;p&gt;And &lt;a href=&quot;https://spaceflightnow.com/2016/02/24/falcon-9-rocket-to-give-ses-9-telecom-satellite-an-extra-boost/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9 rocket to give SES 9 telecom satellite an extra boost&lt;/a&gt; says that this is a super-synchronous orbit, and that the original plan was for a sub-synchronous orbit with apogee of only 26,000km which would have then left SES-9 with a 93 day &amp;quot;climb&amp;quot; to geostationary.&lt;/p&gt;&#xA;&lt;p&gt;I'm a little overwhenlmed by acronyms so I've left them out. but first of all &lt;em&gt;what exactly&lt;/em&gt; do these represent: &amp;quot;GTO&amp;quot;, &amp;quot;GSO&amp;quot;, and &amp;quot;GEO&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;But the main part of my question is how &lt;s&gt;will&lt;/s&gt; *&lt;em&gt;did&lt;/em&gt;  SES-9 reach its target geostationary orbit from the inclined elliptical orbit described in the first article? I found this in &lt;a href=&quot;https://github.com/NathanKell/RealSolarSystem/wiki/Equatorial-Orbits-from-Non-Equatorial-Launch-Sites&quot; rel=&quot;nofollow noreferrer&quot;&gt;GitHub&lt;/a&gt; which points to the image below, but that's clearly for a different initial condition.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LKw0A.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LKw0A.jpg&quot; alt=&quot;Github NathanKell Equatorial-Orbits-from-Non-Equatorial-Launch-Sites&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image linked from &lt;a href=&quot;https://github.com/NathanKell/RealSolarSystem/wiki/Equatorial-Orbits-from-Non-Equatorial-Launch-Sites&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; in reference to a different transfer orbit strategy.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-03-17T08:30:53.097" LastActivityDate="2021-03-17T08:30:53.097" Title="How did SES-9 reach geostationary orbit from inclined transfer orbit (from F9 2nd stage)" Tags="|spacex|falcon-9|orbital-maneuver|communication-satellite|ses|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="14199" PostTypeId="2" ParentId="14198" CreationDate="2016-02-24T16:49:03.890" Score="3" Body="&lt;p&gt;GEO and GSO are synonyms for geostationary (earth) orbit.  They are orbits that are in the equatorial plane at an altitude that has a $24$ hour period so that the satellite appears to stay at one point in the sky.  Many communications satellites are placed in one of these orbits and SES-9 is going to one at $108.2^\circ$ East longitude.  GTO is geostationary transfer orbit.  It is typically an orbit with a low perigee (a few hundred km), an apogee at about 42000 km, close to the altitude of GEO, and some inclination.  The satellite carries a rocket to circularize the orbit.  It fires the rocket when the satellite is at apogee, raising perigee and cancelling the inclination.  If the initial apogee is not at synchronous altitude, that needs to be corrected as well by firings.  Supersynchronous orbits have the apogee higher than 42000 km, so it must be lowered.  If the launch rocket has enough performance to provide an orbit like that, it can reduce the fuel consumption of the satellite by reducing the fuel needed to cancel the inclination.&lt;/p&gt;&#xA;" OwnerUserId="176" LastActivityDate="2016-02-24T16:49:03.890" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14201" PostTypeId="1" CreationDate="2016-02-24T23:00:11.337" Score="8" ViewCount="455" Body="&lt;p&gt;&lt;a href=&quot;http://www.lr.tudelft.nl/?id=26232&quot; rel=&quot;nofollow noreferrer&quot;&gt;Resistojets&lt;/a&gt; heat a compressed gas or vaporize a liquid to expand it through a nozzle for thrust. They improve a bit on the performance of cold gas thrusters. &lt;a href=&quot;http://www.mtc.edu.eg/ASAT13/pdf/PP24.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This experimental water resistojet&lt;/a&gt; managed an &lt;em&gt;I&lt;/em&gt;&lt;sub&gt;sp&lt;/sub&gt; of 140 s.&lt;/p&gt;&#xA;&lt;p&gt;I'm thinking about what sort of simple rocket engines might be feasible in the initial stages of a permanent lunar base, made on-site mostly from local materials. Oxygen is the only volatile that can be extracted in volume anywhere. Obviously the example shows water is a fuel option, but the paper cited above answers most of my questions regarding that, and also I wonder if oxygen might have advantages.&lt;/p&gt;&#xA;&lt;p&gt;What materials that might be available early on for such complex fabrication is unclear, however I'm going to suggest that something can be&#xA;come up with, that can handle storing liquid oxygen and gaseous elemental oxygen at the nozzle up to, say, 500 °C. Let's say iron combined with metal oxides or basalt. The resistance heater and other complex bits come from Earth. The LOX flows to the resistance heater and then out the nozzle.&lt;/p&gt;&#xA;&lt;p&gt;Could such a system work? What sort of &lt;em&gt;I&lt;/em&gt;&lt;sub&gt;sp&lt;/sub&gt; and thrust might it have?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;For what it's worth, the idea is that these engines only need to circularize an orbit around the Moon, and do other orbital maneuvers or a  burn to enter orbit around EML1. Launch is by a &lt;a href=&quot;https://bis-space.com/shop/product/analysis-of-a-lunar-sling-launcher/&quot; rel=&quot;nofollow noreferrer&quot;&gt;sling launcher&lt;/a&gt; using basalt fiber cable, that imparts orbital or escape speed. So low &lt;em&gt;I&lt;/em&gt;&lt;sub&gt;sp&lt;/sub&gt; isn't a problem in itself.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="6944" LastEditDate="2022-05-05T22:36:53.777" LastActivityDate="2022-05-05T22:36:53.777" Title="Could oxygen be used in a resistojet in a vacuum environment?" Tags="|fuel|engine-design|engines|feasibility|performance|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14202" PostTypeId="1" CreationDate="2016-02-25T00:06:07.043" Score="5" ViewCount="895" Body="&lt;p&gt;I think most of the CubeSats have an &lt;a href=&quot;https://en.wikipedia.org/wiki/ARM_Cortex-M&quot; rel=&quot;nofollow&quot;&gt;ARM Cortex-M&lt;/a&gt; microcontroller. &#xA;Like, e.g. &lt;a href=&quot;http://www.cubesatshop.com/index.php?page=shop.product_details&amp;amp;product_id=106&amp;amp;option=com_virtuemart&amp;amp;Itemid=65&quot; rel=&quot;nofollow&quot;&gt;CubeComputer&lt;/a&gt; or &lt;a href=&quot;http://www.cubesatshop.com/index.php?page=shop.product_details&amp;amp;product_id=68&amp;amp;category_id=8&amp;amp;option=com_virtuemart&amp;amp;Itemid=75&quot; rel=&quot;nofollow&quot;&gt;NanoMind&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why are the Cortex-M the most frequently used microcontrollers for On-Board Computers (OBC)? Or are they? I have even seen some OBCs having a combination of an M0 and an M4 in some way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which aspects are the most important when choosing the microcontroller for a CubeSat?&lt;/p&gt;&#xA;" OwnerUserId="8968" LastEditorUserId="8968" LastEditDate="2016-06-30T13:39:17.797" LastActivityDate="2016-06-30T14:21:55.463" Title="Based on which aspects should the on-board computer microcontroller of a CubeSat be chosen?" Tags="|cubesat|electronics|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14203" PostTypeId="2" ParentId="14201" CreationDate="2016-02-25T01:36:30.763" Score="7" Body="&lt;p&gt;This response addresses just the last part of your question &lt;em&gt;&quot;what sort of $I_\text{sp}$ and thrust might it have&quot;&lt;/em&gt;, and does so in the sense of an ideal resistojet. I hope this isn't too basic an approach but its worth stepping through it to develop a more intuitive view of the very high level physics. There are similar equations in the link you gave though, given your question, I'm guessing that the meaning is lost in lower level details. If not, it will be useful to someone out there!:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The power in a rocket exhaust, the &quot;beam power&quot;, is the rate of flow of kinetic energy, i.e. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{1}{2}  \cdot \dot m  \cdot v_e^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\dot m$ is the mass flow rate and $v_e$ the exhaust velocity. Also $v_e = I_{\text{sp}} \cdot g_0$. I wrote it in that form to start with, as kinetic energy is a relatively familiar concept.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;More usefully for your question: $\text{Thrust} = \dot m \cdot v_e$ (Eq 7 in your reference) and so: &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;$$\text{Beam power} = \frac{1}{2} \cdot \dot m \cdot v_e^2  =  \frac{1}{2} \cdot \text{Thrust} \cdot v_e$$&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;A pure resistojet, i.e. without any other energy source such as from chemical reactions, means that we can directly relate the electrical power in the resistance heater to the beam power thus (Eq. 1 in your reference): &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P_e \cdot \eta =  \frac{1}{2} \cdot \dot m \cdot v_e^2$$&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;where $P_e$ is the power dissipated in the heater and $\eta$ is some efficiency factor allowing that some power will be lost, radiating to space and conducting back into the structure, e.g. $50 \text{%}$ (guessed). In the case of $\require{mhchem}\ce{LOX}$ storage, this will also need to allow for the $\ce{LOX-O2}$ phase change, but don't worry about that for now.&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;4&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Putting this all together we can choose our values of Thrust and $I_\text{sp}$ for &lt;strong&gt;any&lt;/strong&gt; non-reacting propellant, and then work out the corresponding $dot m$ and $P_e$. This is the basic concept, it is meant to be as simple as it sounds. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;OK, OK, it's bound to be more complicated, but you have to start somewhere. This will equip you to begin to very roughly understand someone else's thruster. Check out your reference though also try &lt;a href=&quot;http://microsat.sm.bmstu.ru/e-library/SSTL/Subsys_h20rjet_HQ.pdf&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt;, as it just gives the headlines without getting lost in $C^*$ (characteristic exhaust velocity), chamber pressure, etc. It gives $\text{Thrust} = 0.045 \text{ N}$, $I_\text{sp} = 152 \text{ s}$, which I calculate gives a beam power of $33 \text{ W}$ for $100  \text{ W}$ electrical, i.e. $\eta = 33 \text{%}$. In this particular case, they are paying over the odds, because they are supplying heat for the water-steam phase change, too. You can check the published mass flow rate of $0.03 \text{ g/s}$ and it neatly comes back to about $45 \text{ mN}$, too.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A problem: You can't arbitrarily increase the electrical power without finding that you have losses from the gas becoming so energetic that the power goes into exciting rotational and vibrational modes of the gas molecules, rather than keeping them zinging around (translational modes). When this begins to happen for $\ce{O2}$ as a propellant, is something you'd have to look into, but I'd &lt;strong&gt;guess&lt;/strong&gt; not until you were up beyond an $I_\text{sp}$ of $500 \text{ seconds}$.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Back to the idea of $\ce{LOX -&amp;gt; O2}$ as a propellant, there are a couple of other concerns:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\ce{O2}$ specifically might be quite a hard choice for materials compatibility as it's very reactive, more so the hotter it is. As a first pass at temperature, assume the energy per molecule is $\frac{3}{2}  k \cdot T$ where $k$ is Boltzmann's constant, $\ 1.38 \cdot 10^-23$.&lt;/li&gt;&#xA;&lt;li&gt;As mentioned earlier, by storing the propellant as $\ce{LOX}$, you are directly creating more of a challenge as the electrical power input has to boil the $\ce{LOX}$ (a phase change) and then raise its temperature. This is another step you will need to do sums on. This area, and particularly the logistics of $\ce{LOX}$ production and storage aren't my territory at all, so I'll leave it there. &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-02-25T15:48:02.803" LastActivityDate="2016-02-25T15:48:02.803" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14204" PostTypeId="1" CreationDate="2016-02-25T04:19:54.690" Score="3" ViewCount="356" Body="&lt;p&gt;Sulfur concrete is a mix of sulfur and aggregates that make a strong, quick-hardening concrete that is likely to be used for construction on Mars, which has a great deal of sulfur. Most experiments, however, seem to use commercial sulfur, which is relatively pure, while a great deal of sulfur on Mars is in the form of sulfate salts such as Ca-, Mg- and Fe-sulfates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get an initial structure built, are there any places on Mars:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;With sulfur deposits having some pure or purifiable sulfur, or any form of sulfur that can be quickly and easily converted to at least moderately pure sulfur? (Aggregates in the mix is fine, they would just add less later.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;That are relatively easily accessible (no deep digging or apparent risk of life and limb).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Are near the Terra Meridiani or Arabia Terra area, where I'm hoping to place my proto-colony? It is one of the areas of Mars that is high in sulfur, although primarily in sulfate form, as seen in one of the images in this &lt;a href=&quot;http://www.eurekalert.org/multimedia/pub/84638.php&quot; rel=&quot;nofollow&quot;&gt;eurekalert article&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="13067" LastActivityDate="2016-02-25T08:33:09.423" Title="Mars: Readily usable sulfur for sulfur concrete?" Tags="|mars|planetary-science|colonization|construction|chemistry|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14206" PostTypeId="2" ParentId="14023" CreationDate="2016-02-25T05:16:31.850" Score="-2" Body="&lt;ul&gt;&#xA;&lt;li&gt;draw a graph cost per pound thrust vs. engine size an you surely will find a minimum &lt;/li&gt;&#xA;&lt;li&gt;economy of scale - mass production of similar parts &lt;/li&gt;&#xA;&lt;li&gt;redundancy&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10218" LastActivityDate="2016-02-25T05:16:31.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14207" PostTypeId="2" ParentId="14204" CreationDate="2016-02-25T08:33:09.423" Score="3" Body="&lt;p&gt;Don't be too hasty to disregard the sulfur salts as a construction material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gypsum is a calcium sulfate that has been used to create concrete-like paths, artificial pillars underground and for the construction of walls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For paths, confining form work is placed on the ground and filled with gypsum which is then sprayed with water to set off the gypsum so it hardens into a concrete-like pathway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make artificial pillars, the gypsum needs to be conveyed pneumatically to a hose nozzle at which point water is added and the resulting wet gypsum is sprayed on to the base structure upon which the pillar will be formed. Gypsum cannot be conveyed wet as it starts to set immediately upon getting wet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For walls, bags of gypsum are  briefly submerged in water and then place side by side and eventually on top of previous wet bags to form a wall. Steel reinforcing, or similar, can be used to give the wall more strength. To seal the wall and to give it smoother finish, it could even be sprayed with wet gypsum as used in making artificial pillars.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2016-02-25T08:33:09.423" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14208" PostTypeId="2" ParentId="12935" CreationDate="2016-02-25T13:27:59.683" Score="4" Body="&lt;p&gt;It is interesting that, according to &lt;a href=&quot;http://livestream.com/spacex/events/4862005&quot; rel=&quot;nofollow&quot;&gt;SES-9 launch webcast page on Livestream&lt;/a&gt;, they scrubbed yesterday's launch because the propellants were not cold enough:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Launch update&lt;/strong&gt; (25 Feb. 2016):&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Falcon 9 remains healthy in advance of SpaceX and SES’s mission to&#xA;  deliver the SES-9 satellite to Geostationary Transfer Orbit. Out of an&#xA;  abundance of caution, the team opted to hold launch for today to&#xA;  ensure liquid oxygen temperatures are as cold as possible in an effort&#xA;  to maximize performance of the vehicle. SpaceX is now targeting&#xA;  tomorrow, Thursday Feb. 25, at 6:46pm ET for launch of SES-9.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Probably not a serious concern in many cases, but sometimes scrubs can be a big problem for schedule and cost.&lt;/p&gt;&#xA;" OwnerUserId="13564" LastEditorUserId="49" LastEditDate="2016-02-25T14:45:03.953" LastActivityDate="2016-02-25T14:45:03.953" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14209" PostTypeId="2" ParentId="14202" CreationDate="2016-02-25T13:39:36.283" Score="0" Body="&lt;p&gt;I have not specifically worked cubesats, but generally the cheapest, lightest, smallest hardware that will meet requirements is used.  Cheapest would include the entire lifecycle, including programming.  And those requirements include reliability, and the environments are taken into account.  So it depends on the requirements of the mission, including duration and environments, and what functions the controller must perform.  I'm sure many of those requirements change for the different missions of cubesats.&lt;/p&gt;&#xA;" OwnerUserId="13564" LastActivityDate="2016-02-25T13:39:36.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14210" PostTypeId="2" ParentId="14187" CreationDate="2016-02-25T13:45:32.673" Score="5" Body="&lt;p&gt;Yes, the short answer is, there wasn't much effective shielding.  The risk was relatively low due to the short mission duration.  A trip to Mars would significantly increase the risk at the same level of shielding.&lt;/p&gt;&#xA;" OwnerUserId="13564" LastActivityDate="2016-02-25T13:45:32.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14211" PostTypeId="2" ParentId="12935" CreationDate="2016-02-25T16:30:18.600" Score="0" Body="&lt;p&gt;You don't want to carry more fuel fuel than what you need because that's just extra weight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing on land however requires extra fuel (the stage has to fly back) and thus they had to make chill the fuel further so they can fit more of it inside the tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the thrust, I think geoffc is right and they are simply running the engine at higher setting. They probably had to do that to be able to carry the extra fuel weight.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2016-02-25T16:30:18.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14212" PostTypeId="2" ParentId="14149" CreationDate="2016-02-25T16:32:51.547" Score="3" Body="&lt;p&gt;According to the &lt;a href=&quot;http://usatoday30.usatoday.com/news/world/2008-04-11-177105809_x.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; on the unveiling of her statue:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;All dogs used in the Soviet space program were stray mongrel dogs -- doctors believed they were able to adapt quicker to harsh conditions. All were small so they could fit into the tiny capsules.&lt;/p&gt;&#xA;&lt;p&gt;The 2-year-old Laika was chosen for the flight just nine days before the launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JoK34.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JoK34.jpg&quot; alt=&quot;Laika, first space dog in orbit&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Stories about how she was selected varied: Some said Laika was chosen for her good looks -- a Soviet space pioneer had to be photogenic. Others indicated the top choice for the mission was dropped because doctors took pity on her: Since there was no way to design a re-entry vehicle in time for the launch, the flight meant a certain death.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Laika was quiet and charming,&amp;quot; Dr. Vladimir Yazdovsky wrote in his book chronicling the story of Soviet space medicine. He recalled that before heading to the launch pad, he took the dog home to play with his children. &amp;quot;I wanted to do something nice for her: She had so little time left to live,&amp;quot; Yazdovsky said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://en.wikipedia.org/wiki/Laika&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wiki&lt;/a&gt; goes on to say that they intended to euthanize Laika with an injection, but she perished due to the overheating of the capsule hours into the flight.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-02-25T16:32:51.547" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14213" PostTypeId="1" AcceptedAnswerId="14224" CreationDate="2016-02-25T18:09:39.583" Score="5" ViewCount="779" Body="&lt;p&gt;I'm working through Sutton's &quot;Rocket Propulsion Elements,&quot; (8th Ed.) and it is mostly gibberish. However, I understand a little bit. Anyways, the question is: When using RP-1 as a fuel, your oxidizer shipment contains 15% nitrogen by mistake. What effects would this have on performance, combustion gasses, etc? What would the optimum mix ratio be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for performance, I can only assume that specific impulse would lessen due to unburnt nitrogen conveying heat to the supersonic gas flow in the nozzle. Combustion gasses would ultimately have a concentration of nitrogen, monatomic nitrogen, nitrous oxide and nitric acid, and possibly some nitrogen tetroxide, as well as the typical combustion gasses (I'm bad at chemistry...). For ratios, I am guessing that the optimum of 2.3 or 2.5 would need to be adjusted to be more oxidizer rich due to nitrogen being inert.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thoughts? Suggestions? Anything?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: For the mix ratio, I did a simple calculation accounting for the decrease in oxidation efficiency and came up with a new ratio of 2:1 (for frozen equilibrium). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also should have pointed out that these ratios are: 2.3:1 for frozen equilibrium expansion and 2.5:1 for shifting equilibrium expansion with gas expansion to sea-level pressure.&lt;/p&gt;&#xA;" OwnerUserId="13563" LastEditorUserId="13563" LastEditDate="2016-02-26T14:37:50.923" LastActivityDate="2022-12-06T01:22:28.990" Title="What would adding 15% nitrogen do to pure oxygen as an oxidizer?" Tags="|rockets|chemistry|combustion|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14214" PostTypeId="1" CreationDate="2016-02-25T18:14:33.890" Score="6" ViewCount="421" Body="&lt;p&gt;Rovers normally roll off a lander, or are otherwise deployed by one. When I look at the design of &lt;a href=&quot;https://www-robotics.jpl.nasa.gov/systems/system.cfm?System=11&quot; rel=&quot;nofollow noreferrer&quot;&gt;ATHLETE&lt;/a&gt;, it isn't hard to imagine a rocket engine bolted underneath it, and ATHLETE's legs doing the job of absorbing landing impact and steadying itself. Given ATHLETE's robotic abilities, it also isn't hard to imagine it then unbolting the rocket engine and driving away from it. This seems like it would allow a mass savings. Is it that it is too hard to protect against the heat and vapors of the engines? Have designs like this been considered?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sVBc5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sVBc5.jpg&quot; alt=&quot;ATHLETE robotic rover design&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/krAxq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/krAxq.jpg&quot; alt=&quot;mock-up of ATHLETE as lander&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2016-02-25T18:28:12.837" Title="Can a lander on an airless body BE a rover?" Tags="|landing|design|rovers|lander|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14215" PostTypeId="2" ParentId="14149" CreationDate="2016-02-25T18:18:55.867" Score="7" Body="&lt;p&gt;Laika was never expected to make it back alive because the Sputnik 2 capsule it was launched in didn't have any means of deorbiting. From Anatoly Zak's &lt;a href=&quot;http://www.russianspaceweb.com/sputnik2_preflight.html&quot; rel=&quot;noreferrer&quot;&gt;Russian Space Web page on Sputnik-2&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As remembered by Yevgeniy Shabarov, &lt;em&gt;&quot;after placing Laika in the&#xA;  container and before closing the hatch, we kissed her nose and wished&#xA;  her bon voyage, knowing that she would not survive the flight.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Its orbit eventually, in &lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_2&quot; rel=&quot;noreferrer&quot;&gt;a bit under half a year&lt;/a&gt; (November 3, 1957 - April 14, 1958), decayed and the capsule reentered the Earth's atmosphere and most of it if not all would have burnt on reentry. It didn't have a reentry heat shield, no parachutes, and even if it did, the 500 kg something capsule wouldn't be able to keep sufficient life support (breathable air, scrubbing carbon dioxide, food, water, temperature and humidity control) provisions to keep even a small dog alive for such a long time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soviets, motivated to some degree also by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Laika#Controversy&quot; rel=&quot;noreferrer&quot;&gt;public outcry of mostly the Western media&lt;/a&gt; and later &lt;a href=&quot;http://www.bbc.com/news/magazine-24837199&quot; rel=&quot;noreferrer&quot;&gt;JFK's protest to Nikita Khrushchev in Vienna in 1961&lt;/a&gt;, even though there's documents attesting that such decision to sacrifice Laika for science was difficult to make on its own, nobody enjoyed it, some scientists working on the project publicly declared that &lt;a href=&quot;https://en.wikipedia.org/wiki/Laika#Controversy&quot; rel=&quot;noreferrer&quot;&gt;science return did not justify her sacrifice&lt;/a&gt;, and later made every conceivable effort (and it has to be said they were years ahead at the time compared to US efforts) to equip the following capsules with deorbit motors, heat shield and parachutes, something that had yet to be developed, and they succeeded in returning canine visitors to the outer space back home alive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nikita Khrushchev later presented &lt;a href=&quot;https://space.stackexchange.com/questions/5301/why-exactly-did-khrushchev-present-pushinka-to-jfks-daughter-caroline&quot;&gt;a gift to JFK's daughter Caroline&lt;/a&gt;, a puppy named Pushinka (Fluffy) that was one of Stelka's (Arrow) offspring. &lt;a href=&quot;https://en.wikipedia.org/wiki/Soviet_space_dogs#Belka_and_Strelka&quot; rel=&quot;noreferrer&quot;&gt;Strelka and Belka&lt;/a&gt; (Whitey) were in 1960 second and third dogs in Earth's orbit.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To answer your questions more directly:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;What were expectations of scientists working on the project of sending a dog to space?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To learn more about untill then mostly just theoretical limitations of travel to space and conditions in Earth's orbit. That &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_2#Mission_profile&quot; rel=&quot;noreferrer&quot;&gt;Laika survived for only a few hours instead of the planned ten days&lt;/a&gt;&lt;/em&gt; and died due to overheating attests to our lack of knowledge of it at the time. Of course, there was also the prestige of sending first living being into orbit, but that was somewhat spoiled by the fact that Laika didn't survive it.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Did they expect Laika to survive until reentry?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;No. See above. Laika was selected by Vladimir Yazdovsky ten days before flight out of ten available dogs, one of which (but not Laika) had two previous &lt;em&gt;suborbital flight experiences&lt;/em&gt;. Oleg Gazenko, one of project scientists, was later &lt;a href=&quot;https://web.archive.org/web/20060816092218/http://www.moscowanimals.org/laika/laika.html&quot; rel=&quot;noreferrer&quot;&gt;quoted&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Work with animals is a source of suffering to all of us. We treat them&#xA;  like babies who cannot speak. The more time passes, the more I'm sorry&#xA;  about it. We shouldn't have done it ... We did not learn enough from&#xA;  this mission to justify the death of the dog.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Reentry was roughly half a year away from the date of the launch. While the mission also served to measure orbital decay rate, Sputnik-1 as a precursor orbital mission did teach us enough not to expect Sputnik-2 to reenter any time soon enough for Laika's life support provisions to last. As rumor has it (and this isn't confirmed!), her last food ration was poisoned to euthanize her so she doesn't have to suffer any of the number of worse ways to go. It was &lt;a href=&quot;https://en.wikipedia.org/wiki/Laika#Voyage&quot; rel=&quot;noreferrer&quot;&gt;one of the options being considered&lt;/a&gt;, but it is unknown if it has actually been implemented.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;What effects of space flight did they consider most threatening to dog's life in the situation?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nobody really knew, but there were many theories. We didn't really know if microgravity is even survivable on its own for such a long time (Laika was expected to survive for 10 days). All microgravity experience with mammals we had before Sputnik-2 flight was on short, suborbital flight experiments. But most expected that either solar and cosmic rays radiation dose, or her heart giving up will end Laika's life, and there was instrumentation aboard Sputnik 2 to measure these sources of radiation and Laika's heart rate, blood pressure and breathing were measured.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Did they perform any (not necessarily humane) experiments to determine the chances of success?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Small surgical procedure was &lt;a href=&quot;http://www.russianspaceweb.com/sputnik2_preflight.html&quot; rel=&quot;noreferrer&quot;&gt;necessary before flight&lt;/a&gt; &lt;em&gt;to route cables from breathing, pulse and blood-pressure sensors to the transmitters&lt;/em&gt;. As stated before, these procedures wouldn't be &lt;em&gt;inhumane&lt;/em&gt; and they're routinely performed nowadays on dogs before more difficult surgical procedures. I've seen it done many times. It involves tiny punctures through epidermis and shaving off a small spot of the dog's fur. It might be a bit unpleasant for a minute or two, but that's it.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-02-25T19:22:34.740" LastActivityDate="2016-02-25T19:22:34.740" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14216" PostTypeId="2" ParentId="14214" CreationDate="2016-02-25T18:28:12.837" Score="8" Body="&lt;p&gt;Yes, though not necessarily as the landing pads. One of its main imagined applications was to take payloads off of landers, as shown in the cartoon below. A single Athlete could do this for several payloads at the same site so that you wouldn't need another Athlete for every lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6IWRu.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6IWRu.jpg&quot; alt=&quot;athlete&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that Curiosity landed on its wheels, so it is a lander that is a rover. (Once that system got to the terminal descent phase, Mars was essentially airless to that system.) The rocket engines were above, cut away, and flew away so there was no need to unbolt the engines from the rover.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-02-25T18:28:12.837" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14217" PostTypeId="2" ParentId="12935" CreationDate="2016-02-26T01:23:14.667" Score="7" Body="&lt;p&gt;We saw one of the major drawbacks of using the condensed propellant tonight. First, the LOX and RP-1 are not loaded until last minute (-10:00) so that they heat up at a minimum. This led to tonight's scrub, I imagine. (My guess is they were ready to pull the umbilical and the LOX wasn't fully loaded.) Another big drawback we are seeing is that once the LOX/RP-1 are loaded if there is any hold to the launch, they must be unloaded and reloaded before another attempt, which takes too much time for most launch windows. This is why, when there's a halt at -1:40, they scrub the whole launch, which we saw tonight.&lt;/p&gt;&#xA;" OwnerUserId="13563" LastActivityDate="2016-02-26T01:23:14.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14219" PostTypeId="2" ParentId="14213" CreationDate="2016-02-26T14:50:18.933" Score="1" Body="&lt;p&gt;Your performance also decreases. Either you burn 15% less propellant, or if you really want to burn all your propellant you need to put 15% more oxidizer in the tank to have the amount of oxygen you need, so you start out with more mass. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-02-26T17:53:44.437" LastActivityDate="2016-02-26T17:53:44.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14220" PostTypeId="2" ParentId="14213" CreationDate="2016-02-26T14:51:15.780" Score="1" Body="&lt;p&gt;Did you specify LOX for the oxidizer?  I will assume that you are using LOX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I certainly do not know all of those answers, but if you are already fuel rich, then your engine is burning 15% less propellants than expected.  It seems that the performance hit would be proportional to the inert nitrogen concentration, although I do not know if it would be exactly 15%.&lt;/p&gt;&#xA;" OwnerUserId="13564" LastActivityDate="2016-02-26T14:51:15.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14222" PostTypeId="1" AcceptedAnswerId="14223" CreationDate="2016-02-26T15:34:49.033" Score="4" ViewCount="250" Body="&lt;p&gt;Is it a real-time communication rate measure? As in it would take one second to receive 1 kbit from Pluto? &lt;/p&gt;&#xA;" OwnerUserId="13574" LastActivityDate="2016-02-26T15:39:36.663" Title="What is the meaning of: New Horizons' communication rate at 1kbit/s between the probe and Earth?" Tags="|communication|new-horizons|pluto|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14223" PostTypeId="2" ParentId="14222" CreationDate="2016-02-26T15:39:36.663" Score="5" Body="&lt;p&gt;The rate is the maximum data rate from New Horizons to Earth, assuming the 70m dish is being used, and there isn't significant data loss. See &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2015/01300800-talking-to-pluto-is-hard.html&quot;&gt;this article from the Planetary Society&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-26T15:39:36.663" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14224" PostTypeId="2" ParentId="14213" CreationDate="2016-02-26T15:45:24.807" Score="5" Body="&lt;p&gt;I'm guessing, you are familiar with &lt;a href=&quot;http://www.grc.nasa.gov/WWW/CEAWeb/&quot; rel=&quot;nofollow&quot;&gt;CEA&lt;/a&gt;, right? It's a nifty tool that you can use to calculate equilibrium chemistry and its effect on rocket combustion. By fiddling with the inputs, you should be able to answer all your questions.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-02-26T17:12:17.040" LastActivityDate="2016-02-26T17:12:17.040" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14225" PostTypeId="1" CreationDate="2016-02-26T17:16:24.420" Score="7" ViewCount="580" Body="&lt;p&gt;Are there any ion thrusters which could conceivably provide thrust inside the atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not that they'd have enough thrust to take off under their own power, of course; I'm just wondering whether all ion thruster designs require a vacuum to operate, or if any could be theoretically designed to operate at ambient pressure. I know about electrohydrodynamic thrusters; thinking more about something which actually ejects a propellant stream of its own.&lt;/p&gt;&#xA;" OwnerUserId="13211" LastEditorUserId="13211" LastEditDate="2016-02-26T19:51:34.243" LastActivityDate="2016-06-13T12:35:30.337" Title="Non-vacuum ion propulsion" Tags="|propulsion|atmosphere|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14226" PostTypeId="2" ParentId="14225" CreationDate="2016-02-26T19:39:25.853" Score="8" Body="&lt;p&gt;Yes, you can even easily &lt;a href=&quot;https://www.youtube.com/watch?v=PvnF2bMnJtU&quot;&gt;build your own.&lt;/a&gt; And it uses atmospheric air as propellant too!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's nothing inherent about ion thrusters that demands vacuum - of course there are engineering concerns like the size of spark gap, so that you don't have an arc between the electrodes, or pressure of the propellant and giving it a clear path to move, and so on - so ion engines designed for vacuum may, or may not operate in air. But designing one that works in air is definitely possible - if not quite practical, for the reasons you have mentioned.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-26T19:39:25.853" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14227" PostTypeId="1" AcceptedAnswerId="14228" CreationDate="2016-02-27T01:37:47.767" Score="4" ViewCount="402" Body="&lt;p&gt;A proposal for lunar development, &lt;a href=&quot;http://www.cislunarone.com/&quot; rel=&quot;nofollow&quot;&gt;LunarCOTS&lt;/a&gt;, relies on a reusable shuttle running on LH&lt;sub&gt;2&lt;/sub&gt; and LOX to work. It lands, is refueled on the lunar surface, and then takes off again. Presumably it could sit for days between firings, and would fire many times over the course of months or years. I'm working on adapting that mission architecture, and this part makes me more nervous than usual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know the J2s on the Saturn 3rd stages &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_J-2#Engine_restart&quot; rel=&quot;nofollow&quot;&gt;restarted&lt;/a&gt; once up to 6 hours after shutting down during Apollo missions. Have liquid hydrogen engines been restarted more extensively since then? Is this a big technical challenge?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2016-04-27T18:32:03.220" LastActivityDate="2016-04-27T18:32:03.220" Title="How often can liquid hydrogen engines be restarted?" Tags="|engine-design|feasibility|hydrogen|liquid-fuel|restartable-engine|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14228" PostTypeId="2" ParentId="14227" CreationDate="2016-02-27T02:38:49.970" Score="6" Body="&lt;p&gt;My understanding is that designing for multiple restarts isn't particularly difficult; it's just that most engines don't have the need for it, so they don't bother. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RL-10 mentioned in the Lunar COTS paper you linked has been restarted at least 7 times in a single real mission, and depending on the model, is rated for &lt;a href=&quot;http://yarchive.net/space/rocket/rl10.html&quot; rel=&quot;noreferrer&quot;&gt;10 starts and 4000 seconds&lt;/a&gt; of run time without maintenance. The RL10 is a relatively simple design, with modest chamber pressure and thrust-to-weight ratio; getting longer run times out of a more sophisticated and powerful engine is more of a challenge, but I don't think starts per se are difficult to design for. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocketdyne thinks the &lt;a href=&quot;http://www.rocket.com/common-extensible-cryogenic-engine&quot; rel=&quot;noreferrer&quot;&gt;CECE evolution of the RL-10 will be capable of 50 starts&lt;/a&gt;, but I don't know if they've proven that capability. &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100032918.pdf&quot; rel=&quot;noreferrer&quot;&gt;This paper says 27 starts&lt;/a&gt; over the course of multiple test series. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-02-27T02:45:54.707" LastActivityDate="2016-02-27T02:45:54.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14231" PostTypeId="1" CreationDate="2016-02-27T14:13:18.397" Score="6" ViewCount="701" Body="&lt;p&gt;So we all know that the Apollo missions used a water landing, as will Orion, while the Soyuz lands on land but can land on water if needed. What would have happened if an Apollo crew began reentry in at the wrong place due to a navigational error, and the capsule was heading for land? What about Orion, under the same circumstances?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing on water greatly reduces the impact force, so the Soyuz has retrorockets that fire a quick burst just before landing to soften the blow. Would an Apollo or Orion crew be doomed if they missed their ocean landing site, or would he impact be survivable?&lt;/p&gt;&#xA;" OwnerUserId="13579" LastActivityDate="2019-02-20T17:22:25.997" Title="Could the Orion or Apollo command module safely land on land in an emergency due to a navigation error?" Tags="|apollo-program|landing|reentry|soyuz-spacecraft|orion-spacecraft|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14233" PostTypeId="2" ParentId="14231" CreationDate="2016-02-27T17:07:25.563" Score="1" Body="&lt;p&gt;A descent rate of 10 m/s into the ground equals a collision into a solid wall at 36 kilometers per hour / 20 miles per hour. Add to that the reclining, form-fitted (molded even?) seats and it should preset no significant hazard. It would be uncomfortable and decidedly unpleasant, but definitely survivable, even without any major injury unless something pierces the cabin.&lt;/p&gt;&#xA;" OwnerUserId="10493" LastActivityDate="2016-02-27T17:07:25.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14234" PostTypeId="1" AcceptedAnswerId="14241" CreationDate="2016-02-27T19:28:40.677" Score="5" ViewCount="422" Body="&lt;p&gt;Is there any (current, or planned, or hypothesized) practical application for engines of low thrust - like, below 40N - in the atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm asking this in relation to the &lt;a href=&quot;https://space.stackexchange.com/questions/14225/non-vacuum-ion-propulsion&quot;&gt;Non-vacuum ion propulsion&lt;/a&gt; question: it can be used as an educational tool or technology demonstrator, or a hobby model, but I can't imagine a scenario where someone would find a ion engine or similar actually useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I missing something or are they simply a thing destined for space and will never find actual use down on Earth?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-02-28T21:05:46.130" Title="Is there any application for very low thrust engines within atmosphere?" Tags="|propulsion|atmosphere|ion-thruster|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14235" PostTypeId="2" ParentId="14234" CreationDate="2016-02-27T20:11:02.800" Score="8" Body="&lt;p&gt;This is perhaps not exactly what you are asking about, but one possible use is to counteract &lt;a href=&quot;https://space.stackexchange.com/q/12631/8693&quot;&gt;drag on the ISS&lt;/a&gt;. In order to experience aero-dynamical drag, you technically have to be inside an atmosphere, and so an ISS orbital-decay-fighting thruster is not in a complete vacuum, thereby qualifying. This scheme is &lt;a href=&quot;http://www.space.com/23613-advanced-space-propulsion-vasimr-engine.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;proposed&lt;/a&gt; for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;VASIMR&lt;/a&gt; ion thruster. This has a few advantages, firstly a lot smaller use of fuel due to a higher specific impulse than chemical rockets, and secondly it is much gentler to microgravity experiments due to the much lower accelerations. (Actually it cancels out most of the acceleration caused by drag.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For real atmospheric use, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_orbital_rocket_engines&quot; rel=&quot;nofollow noreferrer&quot;&gt;power consumption&lt;/a&gt; is the main reason to not us an ion  thruster. The NSTAR uses almost 24 kW per N, and then you are much better off pushing air with a propeller.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-02-28T21:05:46.130" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14236" PostTypeId="1" AcceptedAnswerId="14238" CreationDate="2016-02-27T20:50:05.020" Score="4" ViewCount="1038" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot; rel=&quot;noreferrer&quot;&gt;Apollo service module&lt;/a&gt; carried a lot of propellant for performing the following tasks on a Moon mission:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mid-course correction manoeuvres to and from the Moon.&lt;/li&gt;&#xA;&lt;li&gt;Braking the entire CSM/LM stack into orbit around the Moon.&lt;/li&gt;&#xA;&lt;li&gt;Escaping low Moon orbit to transfer the crew back to Earth.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Especially the two later are relatively expensive manoeuvres, and the same propellant in a CSM configuration without the LM sums up to multiple $km/s$ of $\Delta v$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was all that propellant used for when the Apollo spacecraft did &lt;strong&gt;&lt;em&gt;not&lt;/em&gt;&lt;/strong&gt; travel to the Moon? Examples are the early hardware tests in LEO before the Moon missions, the three missions to &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot; rel=&quot;noreferrer&quot;&gt;Skylab&lt;/a&gt; and finally the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot; rel=&quot;noreferrer&quot;&gt;Apollo-Soyuz test project&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was the SM close to empty? Was it used as a last stage to reach orbit? Did the Apollo spacecraft perform some expensive plane change manoeuvre? Was a lot of fuel launched just to be ditched at re-entry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of those possibilities makes very much sense.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-05-02T02:31:09.807" Title="What was the Apollo service module propellant used for on the LEO missions?" Tags="|propulsion|apollo-program|low-earth-orbit|mission-design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14237" PostTypeId="1" CreationDate="2016-02-27T20:53:23.760" Score="2" ViewCount="280" Body="&lt;p&gt;Do we know of any astronomical objects--planets, dwarf planets, moons, etc.--that have a higher average temperature at their poles than on their equator?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we haven't observed this, is it still a possible phenomenon?&lt;/p&gt;&#xA;" OwnerUserId="47" LastActivityDate="2016-02-28T21:58:44.587" Title="Objects hotter at their poles than their equators?" Tags="|temperature|pole|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14238" PostTypeId="2" ParentId="14236" CreationDate="2016-02-27T21:01:11.250" Score="11" Body="&lt;p&gt;According to &lt;a href=&quot;http://georgetyson.com/files/apollostatistics.pdf&quot; rel=&quot;nofollow&quot;&gt;Apollo By The Numbers&lt;/a&gt;, the loaded weight of the CSM was substantially lower for the LEO Apollo 7 and Apollo 9 missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo 7 (SM loaded: 19,730 lbs / 8950kg) was a no-LM mission launched on a Saturn 1B; Apollo 9 (SM loaded: 36,159 lbs / 16400kg) was CSM+LM launched on a Saturn V. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The translunar missions by comparison had loaded SM weighing upwards of 51,000 lbs / 23000kg. The full SPS propellant load is 40,590 lb / 18,410 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both 7 and 9 were 10-day missions, so would have had a full load of consumables other than fuel; the mass difference must be almost all SPS propellant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo 9 did &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_9#Summary_of_maneuvers&quot; rel=&quot;nofollow&quot;&gt;7 test burns of the SPS&lt;/a&gt;, some quite lengthy, in addition to the deorbit burn, so that accounts for a great deal of fuel usage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise Apollo 7 did &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_7#On-orbit_operations&quot; rel=&quot;nofollow&quot;&gt;several SPS burns&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in each case they loaded a reasonable amount of fuel for the mission they intended to fly.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn 1B didn't have the capability to lift a fully loaded SM, so the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Modifications_for_Saturn_IB_missions&quot; rel=&quot;nofollow&quot;&gt;Skylab and ASTP missions actually removed some tankage&lt;/a&gt; and other unneeded components to save weight. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-05-02T02:31:09.807" LastActivityDate="2016-05-02T02:31:09.807" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14239" PostTypeId="2" ParentId="14237" CreationDate="2016-02-27T21:04:39.513" Score="3" Body="&lt;p&gt;That is completely possible, consider for example a planet tilted 90°. There, a pole gets maximal sunlight insolation comparable to $\frac{1}{\pi}$ of the time, compared to $\frac{1}{4}$ for the planet overall (the ratio between the area of a circle and a sphere). For bodies without an atmosphere, that therefore conducts heat poorly, this difference should result in a higher average temperature at the poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 1 to $\pi$ ratio can be intuitively understood by the fact that the pole rotates around the planet by one rotation a year relative to the Sun, because it is fixed in a inertial frame of reference. The Sun only fully illuminates $\frac{1}{\pi}$ of this path:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rurUrm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rurUrm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, at the poles of a tilted planet, the Sun can be in zenith in long periods of time, something that is not possible at equator as it rotates away. Note that such a planet have seasons where the equator may be warmer, when the rotational axis is parallel with the orbital motion. Then, the general trend of Earth-like planets holds briefly.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-02-27T21:56:51.177" LastActivityDate="2016-02-27T21:56:51.177" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14240" PostTypeId="2" ParentId="14234" CreationDate="2016-02-28T01:53:01.073" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www.jpaerospace.com/&quot; rel=&quot;noreferrer&quot;&gt;JP Aerospace&lt;/a&gt;, a company which provides airships for advertising and who says they hold the world altitude record for airships, is working on using electric propulsion in the atmosphere as part of their &lt;a href=&quot;http://www.jpaerospace.com/atohandout.pdf&quot; rel=&quot;noreferrer&quot;&gt;Airhip-To-Orbit&lt;/a&gt; project. As high up in the atmosphere as buoyancy can get, a solar powered low thrust rocket engine would slowly accelerate an airship to orbital velocity, they propose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5axDZ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5axDZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-02-28T01:53:01.073" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14241" PostTypeId="2" ParentId="14234" CreationDate="2016-02-28T01:56:44.523" Score="5" Body="&lt;p&gt;Yes. &#xA;They are being considered as a smaller, more quiet, no moving parts alternative to fans for cooling electronic devices. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.patentlyapple.com/patently-apple/2012/01/apple-reinvents-the-ionic-wind-generator-cooling-system.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.patentlyapple.com/patently-apple/2012/01/apple-reinvents-the-ionic-wind-generator-cooling-system.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-02-28T01:56:44.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14242" PostTypeId="1" AcceptedAnswerId="14850" CreationDate="2016-02-28T03:55:42.337" Score="10" ViewCount="1353" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;An additional four 150 feet (46 m) diameter pads are planned to&#xA;  support the simultaneous recovery of additional boosters of the Falcon&#xA;  Heavy. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;on &lt;strong&gt;Cape Canaveral SpaceX landing Complex/Zone 1&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Landing_Zone_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;wiki&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The falcon heavy having 2 booster, one would think having 2 contingency landing pads would be enough. (See the pictures below for illustration). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IFeZT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IFeZT.jpg&quot; alt=&quot;http://www.collectspace.com/ubb/Forum35/HTML/000701.html&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.collectspace.com/ubb/Forum35/HTML/000701.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX demo heavy video, img source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4L2dc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4L2dc.jpg&quot; alt=&quot;http://www.universetoday.com/124020/successful-spacex-launch/&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.universetoday.com/124020/successful-spacex-launch/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google maps, img source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6852" LastEditorUserId="6537" LastEditDate="2017-12-12T23:33:10.123" LastActivityDate="2017-12-12T23:33:10.123" Title="Why does SpaceX's Launch Complex 1 have four contingency landing pads?" Tags="|rockets|spacex|falcon-9|landing|falcon-heavy|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14243" PostTypeId="1" AcceptedAnswerId="14244" CreationDate="2016-02-28T07:13:33.010" Score="8" ViewCount="661" Body="&lt;p&gt;The Vostok spacecraft used by Yuri Gagarin seems to have had an &quot;ejection seat&quot;. The Soyuz launcher has an escape tower, which at least once saved its crew. The space shuttle of course did not have any other option than to land the orbiter intact. Were the 135 space shuttle launches the only times when astronauts did not have a launch escape system to rely on if their spacecraft disintegrated?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="7062" LastEditDate="2016-02-28T08:02:54.247" LastActivityDate="2016-02-28T15:26:44.507" Title="When have astronauts been launched without launch escape system (tower)?" Tags="|crewed-spaceflight|space-shuttle|safety|launch-escape-system|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14244" PostTypeId="2" ParentId="14243" CreationDate="2016-02-28T13:38:06.603" Score="12" Body="&lt;p&gt;US Gemini capsules had ejection seats, not escape towers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dCAov.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dCAov.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle had ejection seats (commander and pilot only) for the first 4 flights (albeit with limited useful envelope: close to launch and landing). And after STS-51L the shuttle had a bailout system (although it was useful in only a minuscule number of possible scenarios.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle bailout testing video:&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/lVWcuoQ1HgY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screen shot of shuttle display with bailout mode engaged&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ljyg7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ljyg7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-02-28T15:26:44.507" LastActivityDate="2016-02-28T15:26:44.507" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14245" PostTypeId="1" AcceptedAnswerId="14246" CreationDate="2016-02-28T14:22:42.407" Score="6" ViewCount="575" Body="&lt;p&gt;This question just occured to me while I was setting up my first manned (err, kerbalized?) Duna mission in KSP: In order to test my basic lander design I did not want to start a whole Duna Mission but thought of using it on Minmus first. So the ascent engines are used for landing/starting from a closer body with lower gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, what would NASA do with a manned Mars mission? Where there ever (or are there) plans to test a Mars lander on the moon first? Would that even work (is landing on and starting from the Moon possible with the delta-v required for landing on and starting from Mars)? How about the atmospheric reentry? Is there a way to test a Mars lander in Earth' atmosphere or would it simply not work here (heat shield, parachute design)? So how would NASA ensure that after a year long mission the lander/ascent stage actually works as expected?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="25" LastEditDate="2016-02-28T14:43:48.857" LastActivityDate="2016-02-28T14:43:48.857" Title="How would a manned Mars Lander be tested" Tags="|mars|crewed-spaceflight|spacecraft|landing|testing|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14246" PostTypeId="2" ParentId="14245" CreationDate="2016-02-28T14:41:48.223" Score="7" Body="&lt;p&gt;No, testing a Mars lander on the Moon wouldn't work for a number of reasons. The main thing is Mars's atmosphere, which is one of the more difficult things in managing a Martian lander. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are parts of a Martian lander which can, and are frequently, tested on Earth. A few items include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/centers/glenn/about/history/marspbag.html&quot;&gt;Airbag tests for Pathfinder&lt;/a&gt;- Done in a vacuum chamber, with Mars air pressure, and dropped at a height to have the same force on impact as on Mars.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/centers/glenn/about/history/marspbag.html&quot;&gt;Mars Supersonic Parachute System&lt;/a&gt; Tested and failed using a rocket sled.&lt;/li&gt;&#xA;&lt;li&gt;Many parachute systems are tested in high atmosphere, where the atmospheric pressure is similar to that on Mars.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As to your original question, well, I think it will be tested with at least an unmanned lander, most likely carrying supplies, prior to the launch. There might be a test of an ascent rocket, perhaps the Mars Sample Return Mission. But testing on the Moon is impracticable, and wouldn't be useful.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-02-28T14:41:48.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14247" PostTypeId="1" AcceptedAnswerId="14385" CreationDate="2016-02-28T17:38:04.000" Score="9" ViewCount="637" Body="&lt;p&gt;The RCS thrusters on the Apollo Service Module were positioned approximately around the center of mass of the ship, such that translational maneuvers wouldn't unnecessarily rotate the ship. While the position of the center of mass would vary as propellant and other consumables were used, a small amount of differential thrust from the RCS thrusters could maintain the ship's attitude against the resulting torque. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those mass shifts were small, however, compared to the movement of the center of mass with the Apollo LM attached. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the Apollo CSM and LM were docked en route to the moon, did the control systems automatically utilize the LM's RCS thrusters in combination with the CSM's to maintain attitude during translation maneuvers, or was the LM entirely dead weight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After LM extraction, translational RCS maneuvers may not have been needed; any small midcourse corrections that were performed on the RCS could be done by rotating the ship into the correct orientation, then using pure forward thrust (i.e. thrusting through the shifted center of mass).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo 13 crew commented on the difficulty of maneuvering the joined ship using only the LM's thrusters; presumably the CSM RCS was entirely shut down at that point, but if the CSM were undamaged and powered-up, could the ship be flown with coordinated RCS using the LM's controls?&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-02-28T17:47:44.333" LastActivityDate="2016-03-11T04:22:45.597" Title="Coordination of RCS on Apollo CSM/LM" Tags="|apollo-program|orbital-maneuver|attitude|apollo-13|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14248" PostTypeId="1" AcceptedAnswerId="14607" CreationDate="2016-02-28T19:55:09.343" Score="10" ViewCount="700" Body="&lt;p&gt;I understand there is a propulsion system on each of Zvezda and Zarya (NTO-UDMH I assume, given their heritage, though set me straight if you know better). Do any of the other permanent ISS space station modules have a propulsion system and what type is it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no need to count the past and present visiting vehicles: Shuttle, Soyuz,  ATV, HTV, Progress, Cygnus and Dragon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, it would be interesting to know which permanent ISS propulsion systems can be resupplied by each visitor. Does the resupply vehicle only resupply the module to which it is attached or is there plumbing between permanent modules?&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-03-31T17:55:42.157" Title="What are the various propulsion systems on the ISS?" Tags="|iss|propulsion|space-station|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14249" PostTypeId="2" ParentId="14237" CreationDate="2016-02-28T21:58:44.587" Score="1" Body="&lt;p&gt;For what duration? Uranus, with its axis tilted 98 degrees, faces the Sun nearly head-on with one of the poles for a considerable part of its orbital period, while the other remains in the darkness - nearly all of the planet is subjected to lengthy polar day/polar night, and the vicinity of the pole facing the sun at given time will be the hottest area.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-02-28T21:58:44.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14250" PostTypeId="2" ParentId="14123" CreationDate="2016-02-28T22:52:42.077" Score="6" Body="&lt;p&gt;Can't answer the overall question, but for &lt;strong&gt;Group 1&lt;/strong&gt; specifically, there's a discussion of the recruitment program &lt;a href=&quot;http://www.nasa.gov/mission_pages/mercury/missions/astronaut.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. The original plan was to recruit twelve, to allow for some dropping out during the program. When it became clear there were unlikely to be many dropouts, they revised the target to six. However, when making the final selection, they got stuck on bringing the shortlist down to 6:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sitting in judgment over 18 finalists, Donlan, White, and North pared down the final pool of selectees, choosing each to complement the rest of the group. The going was so difficult that they could not reach the magic number six, so Gilruth decided to recommend seven. Donlan then telephoned each of the seven individually to ask whether he was still willing to accept a position as a Mercury astronaut. Each one gladly volunteered again. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So that one's as much an accident as anything...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edit:) &lt;strong&gt;Group 4&lt;/strong&gt; was &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_4&quot; rel=&quot;nofollow&quot;&gt;originally six strong&lt;/a&gt;, but one (Graveline) resigned before being assigned to a mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edit:) &lt;strong&gt;Group 7&lt;/strong&gt; was essentially arbitrary - it was &quot;&lt;a href=&quot;http://www.nasa.gov/centers/kennedy/about/history/molsuits.html&quot; rel=&quot;nofollow&quot;&gt;all the Manned Orbiting Laboratory astronauts under 35&lt;/a&gt;&quot;. It so happened there were an odd number of them, but there was no set number initially aimed for.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;After accounting for these, we end up with:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Planned even, actually odd (7)&lt;/li&gt;&#xA;&lt;li&gt;Odd (9)&lt;/li&gt;&#xA;&lt;li&gt;Even (14)&lt;/li&gt;&#xA;&lt;li&gt;Even (6)&lt;/li&gt;&#xA;&lt;li&gt;Odd (19)&lt;/li&gt;&#xA;&lt;li&gt;Odd (11)&lt;/li&gt;&#xA;&lt;li&gt;No plan for numbers, actually odd (7)&lt;/li&gt;&#xA;&lt;li&gt;Odd (35)&lt;/li&gt;&#xA;&lt;li&gt;Odd (19)&lt;/li&gt;&#xA;&lt;li&gt;Odd (17)&lt;/li&gt;&#xA;&lt;li&gt;Odd (13)&lt;/li&gt;&#xA;&lt;li&gt;Odd (15)&lt;/li&gt;&#xA;&lt;li&gt;Odd (23)&lt;/li&gt;&#xA;&lt;li&gt;Even (24)&lt;/li&gt;&#xA;&lt;li&gt;Odd (23)&lt;/li&gt;&#xA;&lt;li&gt;Even (44)&lt;/li&gt;&#xA;&lt;li&gt;Even (32)&lt;/li&gt;&#xA;&lt;li&gt;Odd (17)&lt;/li&gt;&#xA;&lt;li&gt;Odd (11) &lt;/li&gt;&#xA;&lt;li&gt;Even (14)&lt;/li&gt;&#xA;&lt;li&gt;Even (8)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So across all 21 groups, and taking account of the fact we know two were intended to be different sizes, this is eight even, twelve odd, one &quot;whoever happens to qualify&quot; (but happened to be odd). This certainly feels like there's no intentional focus on odd numbers.&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="11259" LastEditDate="2016-10-13T16:09:34.923" LastActivityDate="2016-10-13T16:09:34.923" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14251" PostTypeId="2" ParentId="13291" CreationDate="2016-02-29T01:02:25.873" Score="2" Body="&lt;p&gt;In general, NASA published selected documents as to their exploration policies since inception in Exploring the Unknown Vol. 5: Exploring the Cosmos (copy at &lt;a href=&quot;http://history.nasa.gov/SP-4407/ETUv5.pdf&quot; rel=&quot;nofollow&quot;&gt;http://history.nasa.gov/SP-4407/ETUv5.pdf&lt;/a&gt;). You should be able to find specifics as to your question there.&lt;/p&gt;&#xA;" OwnerUserId="13563" LastActivityDate="2016-02-29T01:02:25.873" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14252" PostTypeId="1" AcceptedAnswerId="14253" CreationDate="2016-02-29T01:07:28.003" Score="11" ViewCount="904" Body="&lt;p&gt;On February 28 2016, Falcon 9 attempted a second launch for its &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9_Flight_22&quot; rel=&quot;nofollow&quot;&gt;22nd flight&lt;/a&gt;. However, it was put on hold for about 20 minutes at T-01:33 and had its countdown reset to T-11:44, giving a total of about 30 minutes between fueling and ignition. The liquid oxygen is &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-ft/&quot; rel=&quot;nofollow&quot;&gt;cooled to -207 °C&lt;/a&gt; before being transferred to the rocket where, I'm guessing, it starts to warm up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How long does it take for LOX to get to a temperature where launch becomes unfeasible?&lt;/p&gt;&#xA;" OwnerUserId="13591" LastEditorUserId="13591" LastEditDate="2016-03-01T21:11:19.490" LastActivityDate="2016-03-02T08:38:26.227" Title="How long can a vehicle stay fueled before the liquid oxygen gets too warm?" Tags="|spacex|falcon-9|fuel|liquid-fuel|oxidizer|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14253" PostTypeId="2" ParentId="14252" CreationDate="2016-02-29T01:24:07.547" Score="5" Body="&lt;p&gt;Normal vehicles that use LOX, use it at the normal temp (-183C) and what they do is let the LOX boil off, and refill it right up to the latest possible moment their hardware allows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 1.1 Full Thrust (or whatever it is finally called) uses the LOX supercooled to -207C which is a different problem. The answer is unclear and is obviously something SpaceX is working on figuring out since it seems based on an Elon Musk &lt;a href=&quot;https://twitter.com/elonmusk/status/704102461766676481&quot;&gt;tweet&lt;/a&gt; that the ignition issue was due to rising LOX temperatures. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-02-29T01:24:07.547" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14254" PostTypeId="1" AcceptedAnswerId="14255" CreationDate="2016-02-29T02:43:01.877" Score="1" ViewCount="297" Body="&lt;p&gt;Isn't it better to colonize on Venus than Mars? Due to similar gravity and the atmosphere blocking a lot of the radiation? &lt;/p&gt;&#xA;" OwnerUserId="12420" LastActivityDate="2016-02-29T03:18:31.583" Title="What are the problems stopping us from making a &quot;cloud-city&quot; on Venus?" Tags="|venus|" AnswerCount="1" CommentCount="5" ClosedDate="2016-02-29T13:25:42.807" ContentLicense="CC BY-SA 3.0" />
  <row Id="14255" PostTypeId="2" ParentId="14254" CreationDate="2016-02-29T03:18:31.583" Score="2" Body="&lt;p&gt;Unfortunately not. There is no water. Venus has lost the vast majority of its hydrogen to space, as evidenced by a very high D/H ratio. Most of the remaining hydrogen is bound in sulfuric acid and hydrogen sulfide.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-02-29T03:18:31.583" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14256" PostTypeId="1" CreationDate="2016-02-29T05:15:47.740" Score="8" ViewCount="4375" Body="&lt;p&gt;I want to know what is the temperature range of solar panel used in space missions. What is the maximum temperature of solar panels reached in space? How does it affect the efficiency of solar panel? Do they use any cooling methods?&lt;/p&gt;&#xA;" OwnerUserId="13594" LastEditorUserId="33864" LastEditDate="2020-09-08T23:28:07.223" LastActivityDate="2020-09-10T09:04:35.280" Title="what is the temperature of solar panels used in space missions such as ISS" Tags="|artificial-satellite|solar-power|heat|cooling|heat-flow|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="14257" PostTypeId="2" ParentId="14256" CreationDate="2016-02-29T11:46:39.467" Score="5" Body="&lt;p&gt;Unfortunately telemetry readings aren't shown on ISSLive.com (Tristan has actually pointed out to me there is no telemetry, which to me is a surprise, but there you go).&lt;/p&gt;&#xA;&lt;p&gt;See this &lt;a href=&quot;https://web.mscsoftware.com/support/library/conf/auc97/p01997.pdf#page=13&amp;amp;zoom=auto,-226,571&quot; rel=&quot;nofollow noreferrer&quot;&gt;analysis for the ISS&lt;/a&gt;. It is an analysis rather than a measurement and was performed in 1997 and so take it with a pinch of salt. Headline is -100 degF (-73°C) in eclipse to +150 degF (70°C) sunlit +- 50degF (28°C) to account for other conditions. Its going to depend to a first order on orientation.&lt;/p&gt;&#xA;&lt;p&gt;Typically, the higher the temperature the poorer the performance. There can be a slightly hazardous moment upon eclipse exit, which needs to be accounted for in design, when the cold arrays produce more power than there normal sun facing maximum.&lt;/p&gt;&#xA;&lt;p&gt;In typical designs with extended panels the back face of the panel should have a high emissivity and, ideally, good thermal conduction from front to back. The back face then radiates to deep space, or the Earth.&lt;/p&gt;&#xA;&lt;p&gt;Note that body mounted solar panels, such as on a cubesat, will not experience such large temperature swings during eclipse to the extent that the spacecraft body effectively increases the thermal mass of the array.&lt;/p&gt;&#xA;&lt;p&gt;I wasn't aware of active cooling design examples until LocalFluff provided the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_Probe_Plus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Probe Plus&lt;/a&gt; link. This case seems special because of the very close approach to the sun: roughly 1/8 of the perihelion of Mercury. The article goes on to say that the solar intensity is 520 times that at Earth's orbit, which will be extremely demanding in design terms.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2020-09-10T09:04:35.280" LastActivityDate="2020-09-10T09:04:35.280" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="14258" PostTypeId="1" AcceptedAnswerId="14260" CreationDate="2016-02-29T12:30:01.133" Score="7" ViewCount="571" Body="&lt;p&gt;There is a class of planets called ice giants. Let's talk about Neptune. The top layer is atmosphere, consisting of hydrogen and helium. The amount of these gases is about 10% but they produce high pressure onto the second layer. The top of second layer consists of water and methane with temperature about 1700K. It's obvious that it is not ice but liquid! So why &lt;strong&gt;ice&lt;/strong&gt; giants? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have also heard that small amounts of ice exist on Jupiter. I would also like to know where and what is the real form of them.&lt;/p&gt;&#xA;" OwnerUserId="10660" LastActivityDate="2019-07-14T01:19:08.700" Title="Where do ice giants contain ice?" Tags="|planetary-science|atmosphere|gas-giant|ice|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14259" PostTypeId="1" AcceptedAnswerId="14261" CreationDate="2016-02-29T12:55:17.383" Score="18" ViewCount="4008" Body="&lt;p&gt;Is SpaceX really the only launch service provider, who can attempt another launch the very next day after a scrub? While other providers may need weeks of preparations before another try.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that is the case, why is it so? Because of SpaceX's better technology, management...?&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="198" LastEditDate="2016-02-29T18:26:27.700" LastActivityDate="2016-03-01T13:55:46.030" Title="Is SpaceX the only launch service provider who can attempt another launch the next day after a scrub?" Tags="|launch|spacex|falcon-9|failure|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14260" PostTypeId="2" ParentId="14258" CreationDate="2016-02-29T13:43:21.193" Score="5" Body="&lt;p&gt;The term &quot;Ice Giant&quot; is given to planets which are thought to have formed from materials in their ice phase, not because they are made of ice. The ices changed to gas and liquid during planet formation. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-02-29T13:43:21.193" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14261" PostTypeId="2" ParentId="14259" CreationDate="2016-02-29T14:56:56.373" Score="30" Body="&lt;p&gt;This mainly depends on the cause of the scrub, it seems. &#xA;I went back through Ariane 5 launches. When a scrub was caused by a technical problem, it'd take several days or weeks to make another attempt. The vehicle has to be drained of fuel, rolled back to the assembly building, failure analysis etc.&lt;br&gt;&#xA;When the cause is external (weather, nimrods driving their boat into the exclusion zone), Arianespace has done launches on the &lt;a href=&quot;http://www.space.com/3555-ariane-5-rocket-launch-scrubbed-due-technical-glitch.html&quot;&gt;next day&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And why the next day? For many orbits, there's only one time of day when that orbit is above the launch site and reachable. So for any launch where the scrub and retry would put you outside of the launch window, you have to wait 24h to try again. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-02-29T14:56:56.373" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14262" PostTypeId="2" ParentId="14123" CreationDate="2016-02-29T15:00:34.303" Score="1" Body="&lt;p&gt;I can't remember who told me this. But I've heard that the reason astronauts go up in odd numbers is so they don't form &quot;teams&quot; against each other. For example, if six astronauts spend a year in space together, three astronauts might disagree with the three other astronauts on some problem. This would create a lot of tension with no easy way to solve the problem. But if seven astronauts were to go up, opinions would never be split evenly. A decision would always have a deciding vote, and everyone could move on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't give a source right now. But that's what I've heard. &lt;/p&gt;&#xA;" OwnerUserId="13596" LastActivityDate="2016-02-29T15:00:34.303" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14263" PostTypeId="2" ParentId="14259" CreationDate="2016-02-29T16:15:50.337" Score="12" Body="&lt;p&gt;SpaceX being one of the few launch providers doing hot fire testing before launches means they have to be able to quickly fuel and defuel their vehicles as needed. If it takes a week to recover from a fuel/defuel cycle before the next launch attempt it is hard to do a hot fire test 2-3 days before hand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So whether this was a design consideration from the outset or one that became clear later, it does seem to be baked into their design. Likely the reuse goal overall influenced it as well. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-02-29T16:15:50.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14264" PostTypeId="1" CreationDate="2016-02-29T16:41:02.090" Score="1" ViewCount="85" Body="&lt;p&gt;I was just reading an article about how the main factor preventing ion propulsion from being more effective is power generated from solar arrays. Solar arrays typically only generate 1-5 kW of power, which severely limits the thrust of ion thrusters. The article also predicted that if solar arrays could generate 100 kW, thrust would be on the order of pounds. Well lets say some magical device could efficiently and compactly generate 1000 kW. And then, 1 GW. Would thrust just keep rising and rising? Would it ever plateau? &lt;/p&gt;&#xA;" OwnerUserId="13596" LastActivityDate="2016-02-29T16:41:02.090" Title="Is there an upper limit for thrust generation by ion propulsion?" Tags="|propulsion|power|solar-power|" AnswerCount="0" CommentCount="1" ClosedDate="2016-02-29T17:24:59.447" ContentLicense="CC BY-SA 3.0" />
  <row Id="14267" PostTypeId="2" ParentId="14259" CreationDate="2016-02-29T20:23:29.523" Score="11" Body="&lt;p&gt;No,&amp;nbsp;it is not. At the very least, the Space Shuttle has had its launch aborted at 20 minutes prior to launch, and attempted to launch the next day. See &lt;a href=&quot;http://www.space.com/6959-thunderstorms-delay-space-shuttle-launch.html&quot; rel=&quot;noreferrer&quot;&gt;this Space.com article&lt;/a&gt;. As mentioned, it really depends on the cause of the scrub. If the scrub is weather, or range, related, it's very common to try the next day. If the scrub is technical, it could be something that is very quick to replace (Which SpaceX is far more likely than most to try), or it could be something that takes longer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, we know that the following have attempted the next day, for weather issues:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;STS&lt;/li&gt;&#xA;&lt;li&gt;Ariane 5&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.usatoday.com/story/tech/nation-now/2015/12/05/windy-weather-plagues-third-atlas-v-launch-attempt/76831914/&quot; rel=&quot;noreferrer&quot;&gt;Atlas 5&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6471" LastEditDate="2016-03-01T13:08:02.900" LastActivityDate="2016-03-01T13:08:02.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14268" PostTypeId="1" CreationDate="2016-02-29T23:27:52.390" Score="13" ViewCount="8469" Body="&lt;p&gt;I'm looking for information on commonly used geocentric artificial satellite ephemeris file formats.  I'm working on extending an analysis tool for earth orbiting satellites to add the ability to read an additional ephemeris format (OEM), and thought I might add other common formats while I have it opened up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To expand a bit, an ephemeris is a representation of the trajectory of an object over a period of time.  It typically consists of a list of times and state vectors spanning the period of interest, and may include various meta-data regarding units, coordinate frames, time systems, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In contrast to element sets (such as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;noreferrer&quot;&gt;TLE&lt;/a&gt;, VCM, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;noreferrer&quot;&gt;classical elements&lt;/a&gt;, etc.), which require the use of a propagator, object positions represented by an ephemeris are typically interpolated.  This can facilitate sharing of data where all users may not have access to the same propagation models, and allows the inclusion of non-Keplerian considerations such as maneuvers, specific decay models, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I have so far:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.agi.com/resources/help/online/stk/10.1/index.html?page=source%2Fstk%2Fimportfiles-02.htm&quot; rel=&quot;noreferrer&quot;&gt;STK&lt;/a&gt; Satellite Tool Kit, Analytical Graphics&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://public.ccsds.org/Pubs/502x0b2c1.pdf&quot; rel=&quot;noreferrer&quot;&gt;OEM (Orbital Ephemeris Message)&lt;/a&gt; CCSDS Recommended Standard&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://file.space-track.org/documents/Launch%20CA%20Trajectory.docx&quot; rel=&quot;noreferrer&quot;&gt;CALIPER&lt;/a&gt; JSPOC launch ephemeris format&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://file.space-track.org/documents/Ephemeris_Format_10Apr14.pdf&quot; rel=&quot;noreferrer&quot;&gt;JSPOC Ephemeris&lt;/a&gt; JSPOC on-orbit format&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://igscb.jpl.nasa.gov/igscb/data/format/sp3_docu.txt&quot; rel=&quot;noreferrer&quot;&gt;SP3&lt;/a&gt; National Geodetic Survey - may be specific to GPS&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html&quot; rel=&quot;noreferrer&quot;&gt;SPK&lt;/a&gt; Ephemeris format associated with &lt;a href=&quot;http://naif.jpl.nasa.gov/naif/&quot; rel=&quot;noreferrer&quot;&gt;SPICE&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ilrs.gsfc.nasa.gov/docs/2006/cpf_1.01.pdf&quot; rel=&quot;noreferrer&quot;&gt;CPF&lt;/a&gt; - Consolidated Prediction format, used by the ILRS&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://gmat.sourceforge.net/doc/R2015a/html/EphemerisFile.html&quot; rel=&quot;noreferrer&quot;&gt;Code 500&lt;/a&gt; - Source document appears to be GSFC Document &quot;Flight Dynamics Division (FDD) Generic Data Products Formats Interface Control Document (document number 553-FDD-91/028&quot; which I haven't been able to locate yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure I'm missing a lot of stuff here, but my google-fu isn't finding it.  Of course, every organization seems to have their own internal formats &amp;mdash; I'm more interested in what folks are using to publish or otherwise exchange data between organizations.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="12483" LastEditDate="2017-09-20T19:50:43.210" LastActivityDate="2017-09-20T19:50:43.210" Title="Satellite Ephemeris File Formats" Tags="|artificial-satellite|ephemeris|raw-data|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14269" PostTypeId="1" AcceptedAnswerId="14271" CreationDate="2016-03-01T04:59:58.107" Score="5" ViewCount="567" Body="&lt;p&gt;I sometime see this quote in forum arguments against &quot;Klingon/Romulan-style&quot; cloaking in video games that tries to be close as possible to real-life. It is quite difficult to ascertain where the actual quote originally came from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It goes something like this &quot;You can detect a nuclear powered submarine on earth from as far as Saturn orbit&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However Saturn's own magnetic/lightning storm and occultation (from ocean water and earth and various moon thereof) make me seriously question if this is even remotely viable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming worst scenario case, the submarine is on Earth, the opposite side from Saturn, and Saturn itself is not occultating Earth but causing interference to some degree. What would it take to detect such a vehicle (assuming it is the only nuclear powered present on Earth)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the detection based on electric activities as opposite to the nuclear power source? I am curious how much the detection is from the nuclear power itself vs EM fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the nuclear powered submarine had the nuclear system taken out and used something else that is based on chemical batteries or some non-radiation-emitting based power source does it make harder to detect?&lt;/p&gt;&#xA;" OwnerUserId="13603" LastEditorUserId="13603" LastEditDate="2016-03-02T21:44:02.563" LastActivityDate="2016-03-02T21:44:02.563" Title="Detecting a Nuclear powered vehicle from outer space (at least 8 AU or further out) feasible?" Tags="|physics|nuclear|stealth|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14271" PostTypeId="2" ParentId="14269" CreationDate="2016-03-01T08:01:24.830" Score="8" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I sometime see this quote in forum arguments against &quot;Klingon/Romulan-style&quot; cloaking in video games that tries to be close as possible to real-life&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I gather the &quot;World Building&quot; Stack exchange might be a place for this, if you want that sort of speculative answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want an engineering answer: first of all nuclear subs are very hard to detect... this is the whole point of them in fact. They would be useless if the enemy could easily track them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, if the Nuclear plant (of any vehicle) put out enough radiation to be easily detectable at distance, the crew would have received a very high dose and be dead. Also not very useful in a combat situation :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want a way to detect &quot;Cloaked&quot; sci-fi vessels, look for their engine emissions. A (real world) rocket engine would put out streams of hot gas or plasma, and even a sci-fi &quot;drive&quot; of some sort would obviously have some sort of measurable effect, otherwise it wouldn't impart any measurable speed on the craft.  &lt;strong&gt;You canna' change the laws of physics!&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-03-01T08:01:24.830" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14272" PostTypeId="1" AcceptedAnswerId="14273" CreationDate="2016-03-01T08:12:22.897" Score="4" ViewCount="559" Body="&lt;p&gt;I am reading this &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/d/dscovr&quot;&gt;article&lt;/a&gt; about DSCOVR which links (ref 16) to this: &lt;a href=&quot;http://spaceref.com/news/viewpr.html?pid=45046&quot;&gt;&lt;em&gt;DSCOVR: Mission Success for Moog Engines Over a Decade Later&lt;/em&gt;&#xA;&lt;/a&gt;, about the recertification by MOOG of the MONARC 5N monopropellant engines and thruster valves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I read these two, I don't understand - are there &lt;em&gt;both&lt;/em&gt; engines &lt;em&gt;and&lt;/em&gt; thrusters? Could someone explain the distinction - and possibly locate a clear diagram with DSCOVRs various propulsion elements used for the mid-course correction (MCC), Lissajous orbit insertion (LOI), and all station-keeping tasks? Here is a sample paragraph from &lt;a href=&quot;http://spaceref.com/news/viewpr.html?pid=45046&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;&quot;The 5N engines will be used for attitude control and minor adjustments to maintain the satellite’s Lagrange-1 (L-1) orbit one million miles from Earth; DSCOVR is the nation’s first operational spacecraft in deep space. DSCOVR has two key burns: Mid-Course Correction (MCC) and Lissajous Orbit Insertion (LOI). The thrusters will be used to perform the MCC for 50 minutes and LOI for 5.3 hours. After reaching L-1, DSCOVR will perform station-keeping and momentum unloading with the thrusters.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-03-01T08:24:45.237" LastActivityDate="2016-03-01T08:53:48.033" Title="What is the Distinction between DSCOVR's &quot;engines&quot; and &quot;thrusters&quot;?" Tags="|artificial-satellite|propulsion|station-keeping|dscovr|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14273" PostTypeId="2" ParentId="14272" CreationDate="2016-03-01T08:53:48.033" Score="5" Body="&lt;p&gt;In this case, the &lt;a href=&quot;http://spaceflight101.com/dscovr/&quot; rel=&quot;noreferrer&quot;&gt;terms are being used interchangeably&lt;/a&gt;. DISCOVR has one type of engine, i.e. the MOOG Monarc 5N thruster. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;DSCOVR is equipped with ten thrusters that can be used for spacecraft attitude control as well as translational maneuvers that are required for mid-course corrections and orbital adjustment maneuvers around L1. Ten thrusters are fed from a pressurized Hydrazine tank, making use of the catalytic decomposition of hydrazine over a metallic catalyst bed to create a hot gas that generates thrust. With a total propellant mass of 145 Kilograms, the mission has a total delta-v budget of 600m/s which is sufficient for the transfer to L1, orbital adjustment maneuvers and regular reaction wheel momentum dumps.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A more common usage is to use the term &quot;engines&quot; for large, main propulsion units, and &quot;thrusters&quot; for small units that do maneuvering and attitude control. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-01T08:53:48.033" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14274" PostTypeId="1" AcceptedAnswerId="14276" CreationDate="2016-03-01T10:00:55.690" Score="4" ViewCount="263" Body="&lt;p&gt;From this &lt;a href=&quot;https://space.stackexchange.com/a/14273/12102&quot;&gt;answer&lt;/a&gt; to my previous DSCOVR question, and &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/d/dscovr&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;http://spaceref.com/news/viewpr.html?pid=45046&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;http://spaceflight101.com/dscovr/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; article, it seems that DSCOVR used one or more of its ten thrusters for the long burns for mid course correction (MCC) and Lissajous orbit insertion (LOI). These were to be 50 minutes and 5.3 hours, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did the spacecraft maintain the proper thrust direction? If it were a single engine located on a central axis, it might have been slightly gimbaled. But for these 5N thrusters, were they distributed around an axis and used in groups of 3 or more? During a long burn, it's important to maintain the thrust vector in the correct direction. Were they pulsed in order to maintain the proper direction, or other thrusters on the sides used, or attitude adjusted with reaction wheels (gimbaling the entire spacecraft)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would also like to see a drawing of some kind indicating the locations of the ten thrusters and the directions they point, so I can visualize this better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;note: &quot;&lt;em&gt;They could have...&lt;/em&gt;&quot; type answers are sometimes helpful, but here I'd like something definitive.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-06-07T07:56:22.453" Title="How did DSCOVR use its ten thrusters for the long burn maneuvers MCC and LOI?" Tags="|artificial-satellite|propulsion|station-keeping|dscovr|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14276" PostTypeId="2" ParentId="14274" CreationDate="2016-03-01T10:50:16.517" Score="6" Body="&lt;p&gt;The report &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150022915.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;'Resurrected DSCOVR Propulsion System – Challenges and&#xA;Lessons Learned'&lt;/a&gt; contains a schematic of the propulsion system:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/K7uuV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/K7uuV.png&quot; alt=&quot;DSCOVR thrusters schematic diagram. Thrusters 1-8 are installed in pairs, 9 and 10 are axial thrusters.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and indicates thruster locations:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/d8DeD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d8DeD.png&quot; alt=&quot;DSCOVR is a cylinder, thruster 1-8 are installed around the sides of the cylinder, 9-10 are on the bottom.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;thruster 9 and 10 are on the bottom: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xF81l.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xF81l.png&quot; alt=&quot;more locations&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrusters were used as follows:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... the DSCOVR propulsion system, which utilizes ten 4.5 N thrusters in blowdown mode to perform Midcourse Correction (MCC) maneuvers, Lissajous Orbit Insertion (LOI) at Lagrangian point L1, momentum unloading maneuvers, and station keeping delta-v maneuvers at L1.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The first planned thruster maneuver was the MCC burn, which was designed to correct any launch dispersion in the spacecraft trajectory. ... The MCC #1 maneuver was actually performed using Thrusters 9 and 10 for +Z delta-V direction and Thrusters 1 through 8 for attitude control.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;On June 7, 2015 (115 days after launch), the Lissajous Orbit Insertion (LOI) burn placed DSCOVR in orbit at the Sun-Earth L1 Lagrangian point. The burn consisted of two segments: one burn approximately 4 hours long (152.4 m/s) and a second burn approximately 35 minutes long (13.9 m/s). The burn was performed in two segments so the Flight Dynamics team could re-calibrate the second segment based on the information from the first segment. The thruster configuration used Thrusters 9 and 10 for +Z delta-V and the Thrusters 1 through 8 as needed for attitude control.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-07T07:56:22.453" LastActivityDate="2016-06-07T07:56:22.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14277" PostTypeId="2" ParentId="14269" CreationDate="2016-03-01T11:01:47.627" Score="6" Body="&lt;p&gt;Romulan warbirds and klingon bird of preys (as well as federation ships) are made of materials absolutely unknown today and we don't know the properties of that materials. Furthermore, we don't know much about their engines and what kind of radiation they would emit. And all we know about their sensor technology is that it's really fancy. So, let's stay in our century. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In principle, a submarine does not emit anything that allow to locate it from a larger distance. The only methods so far are sonar,  and listening to  noise from the submarine, and measuring the distortion of the magnetic field by this large piece of metal in the ocean. The first two options work under water and close proximity only, the third can be used by deep flying aircraft, but still works for low distances only. :-(&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You mentioned radioactive stuff. Most people know α, β⁠ and γ radiation. It is also known that the first two can be shielded easily (and completely) by a thin layer of metal, while γ radiation needs more care. Typically, thick shields of lead or similar heavy materials are used to drop the radiation level down to a value comparable to the level of the environment. (But this is to protect the staff!) Furthermore, water isn't the best shielding material, but the thickness of the water layer above the submarine compensates this. So, no chance to detect this. :-(&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, α, β⁠ and γ radiation aren't the only types, there are more. &#xA;Neutrons play a key role in nuclear reactors, but can easily be shielded - and it turns out water is the best material. :-(&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a β-decay occurrs, not only a β-particle (an electron) escapes from the nucleus, but also an anti-neutrino . The interesting fact is that (anti-)neutrinos can not be shielded: You would need a stack of lead as thick as the diameter of earths orbit around the sun to catch just 50% of the neutrinos...&lt;br&gt;&#xA;A 1GW nuclear power plant releases about $10^{21}$ anti-neutrinos per second. It will be less for a submarine, but this is a number I found, and it can easily be scaled down.&#xA;This number seems to be large, but in a distance of 1km, this leads to a flux of $8\cdot10^9$ anti-neutrinos per cm² and second. For comparison: The sun emits large amounts of neutrinos, the flux here is $6\cdot10^{10}$ per cm² and s on earth. (Keep in mind: sun: neutrinos. Reactor: anti-neutrinos)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Saturn, the flux from our reactor is just about $3\cdot10^{-9}$ per cm² and s (or 1 anti-neutrino per squaremeter and 10 hours), while the flux from the sun is just 1/100 of the value on earth, i.e. $6\cdot10^8$ per cm² and s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An observer on Saturn needs a large detector and a lot of time to get enough neutrinos. AND he somehow hast to distinguish all the different submarines and power plants, as well as the natural radiation of the earth. :-/&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the initial advantage of neutrinos of extreme rare interaction with matter becomes a disadvantage. These interactions are also what a sensor measures. This means you need a really really vast sensor volume and a really really long observation time to observe just a really really tiny fraction of all the anti-neutrinos flowing through this volume.  :-(&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, the path (origin) of low energy anti-neutrinos as emitted by radioactive processes can not be determined, so distinguishing between several submarines is not possible. :-(&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way: There have been ideas to use large supertankers filled with a liquid detector material to discover nuclear power plants. Those would be sent to the coast of a suspicious country to check if they have nuclear power plants. But that's just an idea to my knowledge, and I don't know how they should work. I guess, they would measure the rate only.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Finally, it is not possible to detect a nuclear driven submarine from saturn today. Even if we assume that in the future we will be able to detect EACH anti-neutrino as well as its origin, the flow from earth on saturn is so low that we still need much observation time and vast detectors. At least, it might be possible to detect that there is an anti-neutrino source somewhere in space, which would be enough to detect a cloaked submarine in space... &lt;/p&gt;&#xA;" OwnerUserId="11749" LastActivityDate="2016-03-01T11:01:47.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14278" PostTypeId="1" AcceptedAnswerId="14279" CreationDate="2016-03-01T11:41:18.250" Score="3" ViewCount="88" Body="&lt;p&gt;With the discovery of gravitational waves, I keep hearing that we now have a &quot;totally new way to explore the universe.&quot; But as I understand it, gravitational waves are only detectable when they come from super massive objects (like two black holes colliding). So are gravitational waves just a new way to observe black hole mergers? Could we observe a single black hole? Could stars ever be observed through gravitational waves?&lt;/p&gt;&#xA;" OwnerUserId="13596" LastEditorUserId="415" LastEditDate="2016-03-01T12:26:33.587" LastActivityDate="2016-03-01T13:38:09.263" Title="What kinds of objects could we potentially observe through gravitational waves?" Tags="|gravity|observation|black-hole|gravity-fields|" AnswerCount="1" CommentCount="1" ClosedDate="2016-03-01T14:34:02.037" ContentLicense="CC BY-SA 3.0" />
  <row Id="14279" PostTypeId="2" ParentId="14278" CreationDate="2016-03-01T13:38:09.263" Score="3" Body="&lt;p&gt;There are two search channels in LIGO, the first is for CBCs (Compact Binary Coalescence). These are binary systems of very dense objects, either BH-BH, BH-NS or NS-NS mergers (BH: Black Hole, NS: Neutron Star). They produce well known signals that increase in frequency over time, an example of this is shown here: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/EGiUc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EGiUc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second search channel is looking for &quot;Bursts&quot;, this involves more unpredictable signals such as the one shown below, which are harder to model and know the characteristics about, but such a signal is expected from supernovae explosions and possibly Gamma Ray Bursts.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/wp2fs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wp2fs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravitational waves are produced by all binary systems to a certain extent, but the current sensitivity of LIGO means it is only sensitive to the most extreme events, like a black hole merger. So individual stars themselves cannot produce them, only quadruople sources can produce gravitational waves, however if most stars have planets orbiting them then in theory they are producing very tiny gravitational waves. The same goes for a single black hole, it alone cannot be a source of gravitational waves.&lt;/p&gt;&#xA;" OwnerUserId="13176" LastActivityDate="2016-03-01T13:38:09.263" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14280" PostTypeId="2" ParentId="14259" CreationDate="2016-03-01T13:55:46.030" Score="6" Body="&lt;p&gt;No, there is an(other) example:&#xA;ESA satellite GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) was launched one day after an aborted attempt (see  &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Field_and_Steady-State_Ocean_Circulation_Explorer#Launch&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, revision id=701703277):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first launch attempt on 16 March 2009 was aborted due to a malfunction with the launch tower.&#xA;  Liftoff occurred successfully at 14:21 GMT on 17 March 2009.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line: &lt;strong&gt;&lt;a href=&quot;http://www.eurockot.com&quot; rel=&quot;noreferrer&quot;&gt;Eurockot&lt;/a&gt;&lt;/strong&gt; can do it, too.&lt;/p&gt;&#xA;" OwnerUserId="13609" LastActivityDate="2016-03-01T13:55:46.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14282" PostTypeId="1" CreationDate="2016-03-01T18:18:50.433" Score="9" ViewCount="2677" Body="&lt;p&gt;The SpaceX launch attempt on Sunday 02/28/16 had problems partly because of a &lt;a href=&quot;http://www.space.com/32076-spacex-ses9-satellite-launch-abort.html&quot;&gt;delay due to a boat&lt;/a&gt; in the launch exclusion zone. Elon Musk tweeted &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Launch aborted on low-thrust alarm. Rising oxygen temps due to hold&#xA;  for boat and helium bubble triggered alarm,&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The cost to SpaceX for that is significant, as is the delay for the customer with the payload. Is there a cost to the owner of the boat? Is there a fine or other consequence? Does this vary according to the nation or state where the launch complex is sited?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2016-03-02T00:39:42.970" Title="What are the consequences for a person who violates a launch exclusion zone?" Tags="|launch|range-safety|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14284" PostTypeId="1" AcceptedAnswerId="14286" CreationDate="2016-03-01T19:23:11.527" Score="27" ViewCount="5263" Body="&lt;p&gt;There are answers about Voyager 1 sending messages to Earth, but can Earth still send signals to Voyager 1? I am just wondering if the signal of the Sun will eventually overcome the signal from Earth or if the signal from Earth will become too attenuated for Voyager 1's antenna to pick up.&lt;/p&gt;&#xA;" OwnerUserId="13617" LastEditorUserId="6816" LastEditDate="2016-03-01T19:41:56.720" LastActivityDate="2016-04-30T06:42:41.977" Title="Can Voyager 1 receive signals from Earth?" Tags="|communication|voyager|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14285" PostTypeId="2" ParentId="14284" CreationDate="2016-03-01T19:36:31.717" Score="6" Body="&lt;p&gt;Yes, but it's not sure &lt;a href=&quot;http://www.popularmechanics.com/space/deep-space/a8029/how-much-longer-will-we-talk-to-the-voyagers-11479518/&quot;&gt;how long&lt;/a&gt; that will last. The problem is that Voyager needs to aim its receiver towards earth, and it's losing its capability to do so.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2016-03-01T19:36:31.717" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14286" PostTypeId="2" ParentId="14284" CreationDate="2016-03-01T19:55:28.963" Score="26" Body="&lt;p&gt;Yes, the Earth can send Voyager 1 a message as easily as we can receive a message. There are a few differences between the uplink and the downlink paths. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;We cannot upgrade the radio on Voyager 1 to newer equipment (but an&#xA;upgrade to the Earth station is equally beneficial),&lt;/li&gt;&#xA;&lt;li&gt;The gain of the antennas on the spacecraft and on Earth are constant, and the free space path loss is the same for both uplink and downlink. This means that for a given transmit power (either direction) the received power will be the same (But not the intensity of the wave entering the receive antenna. &lt;a href=&quot;https://en.wikipedia.org/wiki/Friis_transmission_equation&quot;&gt;See here for details&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;li&gt;The performance of the spacecraft's transmitter and receiver may be slightly different from those on earth,&lt;/li&gt;&#xA;&lt;li&gt;The noise seen by Earth and Voyager 1 will be slightly different (Voyager is pointed towards the sun, dishes on Earth are pointed into&#xA;space).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Phew! Now that all of that is stated, YES signals from Earth will become more attenuated as distance increases. BUT, we have the advantage that we can use more and more powerful amplifiers (to a point).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As MSalters pointed out, the limiting factor will likely end up being a lack of electrical power from the RTG to point the spacecraft accurately and/or power the spacecrafts LNA. We can continue improving the ground stations here for quite a while. We even have developed better error correcting codes since the Voyagers were launched (turbo codes vs Reed-Solomon), so any future deep space missions will be able to go even further (all else equal).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Updated to clarify several points I had glossed over initially. Thanks SF.&#xA;EDIT 2: Fixed typo. Thanks JDługosz.&lt;/p&gt;&#xA;" OwnerUserId="12966" LastEditorUserId="12966" LastEditDate="2016-03-02T18:35:01.927" LastActivityDate="2016-03-02T18:35:01.927" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14287" PostTypeId="2" ParentId="14284" CreationDate="2016-03-01T21:13:49.603" Score="18" Body="&lt;p&gt;Yes. The onboard sequence on both Voyagers is updated every month and a half or so. What's more, there is a DSN pass every few days, in which there is ranging requiring the receipt and turn-around of a pseudo-noise signal, and there are simple commands in every pass, like resetting the uplink loss timer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Voyagers' power will run down well before we can no longer communicate with them. Were it not for power, we could communicate with them for another century, or more (even with no Earth antenna upgrades over that time, which could make it go even longer). The money will likely run out before the power to communicate, once we can no longer operate any science instruments due to insufficient power. This is estimated to be around 2025.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-03-02T19:26:41.603" LastActivityDate="2016-03-02T19:26:41.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14288" PostTypeId="2" ParentId="14252" CreationDate="2016-03-01T21:20:46.087" Score="5" Body="&lt;p&gt;They're subcooling the LOX in order to get more oxygen in the tank. The lower temperature translates to something like 3% more oxygen (by weight) in the same volume. When the LOX warms up, it expands so tank pressure will rise rapidly unless you vent the oxygen. So if you use subcooled oxygen, a launch delay will leave you with less oxygen than you need.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Geoff said, once the LOX reaches its boiling point, temperature stabilizes until all the oxygen has boiled off, and you can keep it at that temperature by replenishing. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-03-02T08:38:26.227" LastActivityDate="2016-03-02T08:38:26.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14290" PostTypeId="2" ParentId="14282" CreationDate="2016-03-02T00:39:42.970" Score="7" Body="&lt;p&gt;&lt;a href=&quot;http://www.floridatoday.com/story/news/local/2016/02/29/tug-boat-contributed-spacex-launch-scrub/81102952/&quot; rel=&quot;noreferrer&quot;&gt;This article from Florida Today&lt;/a&gt; has a comprehensive look at the particular violation on Sunday. In this case, the violator was a tugboat hauling a barge. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Coast Guard said the tug boat and barge were about 30 miles form&#xA;  shore and within the safety zone... violators of the safety zone can&#xA;  be fined if they are within 12 miles U.S. territorial waters. The&#xA;  Coast Guard does not have the authority to fine boats outside&#xA;  territorial waters. A boat could, however, still face fines for&#xA;  equipment violations... A commercial vessel such as the tugboat is&#xA;  required to have an automatic identification system and radio. If the&#xA;  vessel strayed into the area because it did not have the required&#xA;  equipment it could be fined $1,000, even if it was outside territorial&#xA;  waters. &quot;A lot of people are asking if they could be fined,&quot; he said.&#xA;  &quot;They could be fined for not using AIS system.&quot; West said the Coast&#xA;  Guard had not yet determined what, if any, action would be taken&#xA;  against the tugboat owners.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nao.usace.army.mil/Media/PublicNotices/tabid/3060/Article/489104/nao-2009-00949-wallops-island-launch-july-2014.aspx&quot; rel=&quot;noreferrer&quot;&gt;A notice regarding a launch&lt;/a&gt; from Wallops Flight Center listed the penalty for violation of the launch exclusion zone:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Every person and every corporation which shall violate such&#xA;  regulations shall be deemed guilty of a misdemeanor and, on conviction&#xA;  thereof in any district court of the United States within whose&#xA;  territorial jurisdiction such offense may have been committed, shall&#xA;  be punished by a fine not exceeding $500, or by imprisonment not&#xA;  exceeding six months, in the discretion of the court.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Because it mentions 'any district court in the United States', this would seem to be a federal regulation in the U.S.. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A case of a leisure boat that caused the scrub of the Antares launch of Cygnus last October (which later exploded at liftoff) was described in &lt;a href=&quot;http://www.airspacemag.com/daily-planet/whats-deal-boat-scuttling-last-nights-antares-rocket-launch-180953176/?no-ist&quot; rel=&quot;noreferrer&quot;&gt;this Air and Space article&lt;/a&gt; this way: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“Boats out that far in the ocean are supposed to be monitoring marine&#xA;  band channel 16,” Beutel says. The effort to warn away at-risk boasts&#xA;  is robust: Nine hours prior to launch time, Wallops begins&#xA;  broadcasting a warning on channel 16. Four hours before, a complement&#xA;  of “seven or eight” boats from the U.S. Coast Guard, Coast Guard&#xA;  Auxiliary, and Virginia Marine Police begins patrolling and clearing&#xA;  the launch area. An hour after that, they’re joined by three&#xA;  aircraft—a radar airplane, a spotter airplane, and a helicopter—all on&#xA;  the lookout for stray boats.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The leisure boat that scuttled last night’s planned launch was spotted&#xA;  early in the launch count, but it did not respond to numerous attempts&#xA;  at radio contact. The spotter plane’s subsequent attempt to signal to&#xA;  the boat by circling it and dipping one wing, as is standard practice&#xA;  when radio contact fails, also went unanswered. The boat was traveling&#xA;  at only about four knots. “There was no way it was going to be able to&#xA;  clear the hazard zone in time for the launch,” Beutel says.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There was no mention of anyone being fined. Elsewhere it was implied that violators are given some leeway and fines may be deferred depending on circumstances. From &lt;a href=&quot;http://spaceflightnow.com/2014/10/27/errant-boat-scrubs-antares-launch-from-virginia/&quot; rel=&quot;noreferrer&quot;&gt;SpaceFlightNow&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Boats violating restricted rocket safety zones periodically cause&#xA;  launch delays, but missions with long launch windows give vessels more&#xA;  time to vacate restricted areas... An Atlas rocket launch from Cape&#xA;  Canaveral — a spaceport run by the U.S. Air Force — was scrubbed in&#xA;  2000 after boats strayed into restricted waters during a fishing&#xA;  tournament scheduled the same day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2016-03-02T00:39:42.970" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14293" PostTypeId="1" CreationDate="2016-03-02T05:01:41.927" Score="0" ViewCount="513" Body="&lt;p&gt;Watching Astronaut Scott Kelly and his crew members emerge from the Soyuz capsule tonight, I'm curious why television experts say their free-fall in orbit is at a whopping 5 miles per second instead of the more surface-appropriate 18,000 miles per hour?&lt;/p&gt;&#xA;" OwnerUserId="13626" LastActivityDate="2016-03-02T22:46:10.640" Title="Miles-per-second around the earth vs. miles-per-hour?" Tags="|orbit|crewed-spaceflight|nasa|" AnswerCount="3" CommentCount="1" ClosedDate="2016-03-02T22:46:22.843" ContentLicense="CC BY-SA 3.0" />
  <row Id="14295" PostTypeId="2" ParentId="14293" CreationDate="2016-03-02T08:04:27.187" Score="3" Body="&lt;p&gt;'five' is easier to say and wastes less space than 'eighteen thousand'. With numbers this far out of normal human experience, I don't think people have an easier time understanding 18,000 mph versus 5 miles/s.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, in the spaceflight community, using miles/s or km/s is common. Again, brevity plays a role. km/s is also easier to use in calculations (saves having to divide by 3600 before you can get anywhere). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In engineering, it's common to use SI prefixes to shorten numbers. Unfortunately 1 Mm (=1000 km) never got any traction. Prefixes for miles even less so (kilomile?). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-03-02T21:41:33.217" LastActivityDate="2016-03-02T21:41:33.217" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14296" PostTypeId="2" ParentId="14293" CreationDate="2016-03-02T12:10:04.233" Score="0" Body="&lt;p&gt;On Earth we use &quot;Miles per hour&quot; because it gives us a good idea of how long it will take to get to our destination as well as we are used to car/train speeds in miles per hour. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We do know pretty much how long a second is (a very short time) and can conceptualise how far 5 miles is (a longish distance) so telling us it moves 5 miles in just one second gives a great idea of the ridiculously high speed of the spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And even though 18,000 is a very large number, 18,000 miles is almost meaninglessly large as very few of us have experienced that distance, so it doesn't really help to visualise the speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, however, one asked how long it took to orbit the earth, then the answerer might start to talk in miles per hour since the orbit is thousands of miles and we want to know how many hours it will take.&lt;/p&gt;&#xA;" OwnerUserId="13629" LastActivityDate="2016-03-02T12:10:04.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14297" PostTypeId="1" AcceptedAnswerId="25736" CreationDate="2016-03-02T12:49:59.617" Score="4" ViewCount="1340" Body="&lt;p&gt;I was watching the return of Scott Kelly to earth from the ISS, I noticed that when the crew entered the Soyuz, they didn't wear spacesuits (https://youtu.be/saE0JGtIEKE?t=7m13s) but when they are back on earth they are wearing spacesuits (https://youtu.be/dWCqzPksYyY?t=9m48s). I noticed a boot in the Soyuz (you can spot it at the hatch of the first video) so I am guessing they suit up in the Soyuz. Since there isn't a lot of space in the Soyuz, my question is: How (and when) do they suit up?&lt;/p&gt;&#xA;" OwnerUserId="13186" LastEditorUserId="3200" LastEditDate="2018-02-28T03:17:40.163" LastActivityDate="2018-02-28T03:17:40.163" Title="How (and when) do astronauts suit up when leaving ISS?" Tags="|iss|soyuz-spacecraft|spacesuits|sokol-spacesuit|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14298" PostTypeId="1" CreationDate="2016-03-02T12:30:39.630" Score="10" ViewCount="518" Body="&lt;p&gt;Does any probe or lander track the temperature on Mars, and is this data published anywhere?  I recall the MGS used to issue a &quot;Martian Weather Report&quot;, but &lt;a href=&quot;http://nova.stanford.edu/projects/mgs/dmwr.html&quot;&gt;it has ceased operating&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13636" OwnerDisplayName="tomh" LastEditorUserId="25" LastEditDate="2016-03-02T15:59:54.677" LastActivityDate="2019-02-20T01:21:43.573" Title="How can I find a daily record of the temperature on Mars?" Tags="|mars|planetary-science|temperature|weather|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14299" PostTypeId="2" ParentId="14268" CreationDate="2016-03-02T15:17:28.897" Score="11" Body="&lt;p&gt;The list in your question is a decent starting list, but there are more to add, and I'd go so far as to say they should be ranked by total usage.  My list is:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://public.ccsds.org/Pubs/502x0b2c1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCSDS OEM&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://igscb.jpl.nasa.gov/igscb/data/format/sp3_docu.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;SP3&lt;/a&gt; - Heavily used for GPS (NGA and IGS)&lt;/li&gt;&#xA;&lt;li&gt;SPK - Often for interplanetary/deep space, but a lot of use&lt;/li&gt;&#xA;&lt;li&gt;STK - Internal format for AGI, but widely used, and read by other tools&lt;/li&gt;&#xA;&lt;li&gt;JSpOC - JSpOC internal on-orbit format used for debris tracking&lt;/li&gt;&#xA;&lt;li&gt;FreeFlyer - Internal format for a.i. solutions, used at NASA JSC/GSFC&lt;/li&gt;&#xA;&lt;li&gt;Code 500 - NASA GFSC Binary format&lt;/li&gt;&#xA;&lt;li&gt;CPF - Consolidated Prediction format, used by the ILRS&lt;/li&gt;&#xA;&lt;li&gt;JSpOC CALIPER - JSpOC Launch format&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are lots of internal or proprietary formats used by various tools and centers (SOGS-MOGS, OASYS, etc), and pretty much every launch provider has their own ephemeris format for launch data.  &lt;/p&gt;&#xA;" OwnerUserId="12483" LastEditorUserId="12483" LastEditDate="2017-09-20T18:41:06.987" LastActivityDate="2017-09-20T18:41:06.987" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14300" PostTypeId="2" ParentId="14298" CreationDate="2016-03-02T16:01:53.730" Score="10" Body="&lt;p&gt;There are a number of ways. Basically, you can either get a global record, or a record from a spacecraft. Curiosity provides us with the clearest cut version, available from &lt;a href=&quot;http://curiosityrover.com/rems/&quot;&gt;its website&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars Climate Sounder, on MRO, shows daily maps of the temperature as recorded from MRO. That data isn't readily available, but can be found from the &lt;a href=&quot;https://pds.nasa.gov/ds-view/pds/viewInstrumentProfile.jsp?INSTRUMENT_ID=MCS&amp;amp;INSTRUMENT_HOST_ID=MRO&quot;&gt;Planetary Data System&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-03-02T16:07:09.410" LastActivityDate="2016-03-02T16:07:09.410" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14301" PostTypeId="1" CreationDate="2016-03-02T17:09:31.957" Score="7" ViewCount="912" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/132/would-wearing-heavy-clothing-be-enough-to-solve-the-health-effects-of-reduced-gr&quot;&gt;Related question&lt;/a&gt;...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why don't astronauts do isometrics or timed static contractions against immovable objects (like fixed bars at different heights) to sustain muscle mass and bone density in zero, micro, or low (to Earth) gravity conditions?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems like a cheep and effective way to mitigate a lot of the problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, does anyone know if the do?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems like a very simple solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JKqRl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JKqRl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; or this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eqXqa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eqXqa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and won't take up much space in space.&lt;/p&gt;&#xA;" OwnerUserId="13637" LastEditorUserId="12102" LastEditDate="2021-08-26T01:31:01.370" LastActivityDate="2021-12-14T14:41:31.070" Title="Isometrics in zero gravity" Tags="|microgravity|health|reduced-gravity-sports|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14302" PostTypeId="1" AcceptedAnswerId="14305" CreationDate="2016-03-02T17:14:18.033" Score="14" ViewCount="1452" Body="&lt;p&gt;Scott Kelly just returned from nearly a year of orbiting the Earth in the space station. How much younger is he than his twin, now that he has spent a significant amount of time, traveling at high speeds?&lt;/p&gt;&#xA;" OwnerUserId="6627" LastEditorUserId="6627" LastEditDate="2016-03-03T12:18:19.457" LastActivityDate="2016-03-03T12:18:19.457" Title="Scott Kelly age vs Twin's" Tags="|orbit|astronauts|space-station|time|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14303" PostTypeId="2" ParentId="14301" CreationDate="2016-03-02T17:22:53.080" Score="-2" Body="&lt;p&gt;We have many muscles, and not all are easy to train (respiratory, heart, and many inner muscles). It take also much place and time to build and operate a gymn, both of which are taken from experiments (so not so cheap), and there is also no &quot;fix&quot;: the actual space training machines are designed so that they don't give regular impulse to the ISS structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But astronauts do a lot of exercises to mitigate the problem.&lt;/p&gt;&#xA;" OwnerUserId="13424" LastActivityDate="2016-03-02T17:22:53.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14305" PostTypeId="2" ParentId="14302" CreationDate="2016-03-02T17:31:49.450" Score="18" Body="&lt;p&gt;Neil DeGrasse Tyson &lt;a href=&quot;https://twitter.com/neiltyson/status/705064381420343296&quot;&gt;says that Scott Kelly is now 1/100 second younger&lt;/a&gt; than he would have been otherwise, which almost certainly isn't enough to alter the birth order of the two; I don't know which of the two was born first. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-03-02T17:31:49.450" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14306" PostTypeId="2" ParentId="14297" CreationDate="2016-03-02T18:15:39.753" Score="2" Body="&lt;p&gt;There's an interesting &lt;a href=&quot;http://blogs.esa.int/iriss/2015/09/11/return-to-earth-the-ride-of-a-lifetime/&quot; rel=&quot;nofollow&quot;&gt;blog entry&lt;/a&gt; describing the return to Earth for an earlier expedition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a quote from the most relevant part:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Gennady, Andreas and Aidyn will put on medical telemetry belts and tight-fitting shorts called the Kentavr to stop blood from the legs flowing to the head during reentry. Afterwards they will put on their Sokol pressure suits in the orbital module.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All of these procedures seem to occur after hatch closing (there isn't an accurate timeline.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some other interesting items described too, such as various cabin leak checks, a suit leak check, breathing exercises for re-entry...&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-03-02T18:15:39.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14307" PostTypeId="5" CreationDate="2016-03-02T20:05:31.410" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-03-02T20:05:31.410" LastActivityDate="2016-03-02T20:05:31.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14308" PostTypeId="4" CreationDate="2016-03-02T20:05:31.410" Score="0" Body="Questions originating from a stated interest in some concept of science fiction and seeking its relation to real world space exploration or technology" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-03-03T01:03:37.977" LastActivityDate="2016-03-03T01:03:37.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14309" PostTypeId="1" CreationDate="2016-03-02T21:58:27.073" Score="6" ViewCount="252" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Mesosphere&quot; rel=&quot;noreferrer&quot;&gt;mesosphere&lt;/a&gt; (the part of the atmosphere above the stratosphere where temperatures increase with altitude) is generally above the altitude reachable by balloon.  It begins somewhere around 50 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;When did a rocket first reach the mesosphere?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="12102" LastEditDate="2021-09-05T23:15:55.787" LastActivityDate="2021-09-05T23:15:55.787" Title="First sounding rocket to reach the mesosphere?" Tags="|history|atmosphere|spaceflight-firsts|sounding-rocket|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14310" PostTypeId="2" ParentId="14293" CreationDate="2016-03-02T22:46:10.640" Score="0" Body="&lt;p&gt;Miles per second, or more commonly, km/second, is used because it makes the math easier. Miles per Hour is used on Earth because it makes the math easier. Common tasks in earth travel are in miles per hour because it takes hours to get from place to place, or at least many minutes. Miles per Hour will allow you to roughly estimate how long it takes to get to places on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for space, it makes sense because the units of force are typically measured in seconds, and many of the tasks involved in spaceflight thrusting. The units of thrust are either pound or newtons, both of which convert thrust using seconds. Acceleration typically m/s^2, and so hours makes the numbers very large, besides, no thrust keeps constant for time period of that long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it makes more sense.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-03-02T22:46:10.640" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14311" PostTypeId="2" ParentId="14309" CreationDate="2016-03-02T22:55:23.843" Score="6" Body="&lt;p&gt;This was almost certainly the German V-2 rocket. According to &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_V-2_test_launches&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, on the 3rd of October 1942, a V-2 reached an altitude of between 85 to 90 kilometers. The second, and the first true &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/Sounding_rocket&quot; rel=&quot;nofollow&quot;&gt;sounding rocket&lt;/a&gt;,&quot; was James Van Allen's &lt;a href=&quot;http://en.wikipedia.org/wiki/Aerobee&quot; rel=&quot;nofollow&quot;&gt;Aerobee&lt;/a&gt;:    &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first instrument-carrying Aerobee was the A-5, launched on March 5, 1948 from White Sands, carrying instruments for cosmic radiation research, reaching an altitude of 117.5 km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2016-03-30T02:09:42.927" LastActivityDate="2016-03-30T02:09:42.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14312" PostTypeId="2" ParentId="14284" CreationDate="2016-03-03T01:38:16.257" Score="0" Body="&lt;p&gt;While we may not have constant signal, we could easily plan when we'd have the best opportunities for a signal and perhaps get better quality during specific times of the year. If deemed valuable enough, we could implement the lifeline method - send another probe as a signal relay, along with the newest version of technology to, and then we can continue such missions, upgrading as we move outward.  It could perhaps follow a slightly different trajectory, and travel in a parallel (yet curved) path, and still stay in contact with the Voyager 1, plus have better reception for the rest of its service days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the mathematical geniuses we have in this forum, it would be conceivable to simply upload a short table of dates/times/angles for best signal reception, refreshing it over time, as dates roll off.&lt;/p&gt;&#xA;" OwnerUserId="13649" LastActivityDate="2016-03-03T01:38:16.257" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14313" PostTypeId="1" AcceptedAnswerId="21454" CreationDate="2016-03-03T05:23:22.847" Score="9" ViewCount="1333" Body="&lt;p&gt;&lt;strong&gt;NOTE:&lt;/strong&gt; I'm asking if this material property is ever considered. I'm &lt;strong&gt;not&lt;/strong&gt; asking &quot;do &lt;em&gt;you&lt;/em&gt; think it should be considered&quot;. I'm pretty sure someone has considered it, and I am hoping to track down some information on that - a report, paper, even an anecdote. As a whole, people who launch rockets are an extremely cautious bunch and rarely if ever think &quot;&lt;em&gt;well that probably won't matter so we better not calculate it.&lt;/em&gt;&quot; One might look at &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110016758.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; before saying there is no effect. Hunting the &quot;free&quot; internet (not behind a paywall) for the susceptibility of LOX, I found a value of Chi=0.0035 &lt;a href=&quot;https://www.pmmh.espci.fr/fr/gouttes/Publications_files/Piroird2013.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; which isn't really &quot;small&quot; at all! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this &lt;a href=&quot;https://physics.stackexchange.com/a/23528/83380&quot;&gt;answer&lt;/a&gt; to this &lt;a href=&quot;https://physics.stackexchange.com/q/23523/83380&quot;&gt;physics question&lt;/a&gt; I found a link to this &lt;a href=&quot;https://www.youtube.com/watch?v=Isd9IEnR4bw&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt; where liquid oxygen is poured between the poles of a laboratory permanent magnet. Liquid oxygen sticks between the poles, holding against gravity and the pressure of the boiling at the pole surfaces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was as surprised as the guy in the video!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Isd9IEnR4bw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since LOX is &quot;magnetic&quot; (colloquial use), are there design considerations in the tank, transport, and engine? Components themselves might have magnetic fields - motors and solenoids for example. In the &quot;weightlessness&quot; of orbit, is there any significant interaction between a &lt;a href=&quot;https://space.stackexchange.com/a/14194/12102&quot;&gt;partial tankful of LOX&lt;/a&gt; and the earth's magnetic field?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another &lt;a href=&quot;https://www.youtube.com/watch?v=Lt4P6ctf06Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; with more &quot;stuck&quot; LOX this time (read &lt;a href=&quot;http://sciencedemonstrations.fas.harvard.edu/presentations/paramagnetism-oxygen&quot; rel=&quot;nofollow noreferrer&quot;&gt;more here&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8qw2h.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8qw2h.gif&quot; alt=&quot;magnetic LOX anyone?&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Lt4P6ctf06Q?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="415" LastEditDate="2017-05-11T13:44:31.870" LastActivityDate="2019-07-28T02:05:10.267" Title="Is the Paramagnetism of Liquid Oxygen Ever Considered in Engine or Tank Design?" Tags="|rockets|low-earth-orbit|fuel|magnetic-field|lox|" AnswerCount="4" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14314" PostTypeId="1" AcceptedAnswerId="14316" CreationDate="2016-03-03T08:51:21.530" Score="11" ViewCount="3642" Body="&lt;p&gt;As a seasoned KSP player, I am aware of how important it is during ascent to have center of thrust and center of mass perfectly aligned. If you don't, your rocket will deviate from its course and finally crash.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle seems like a pretty obvious asymmetric launch stack. I assume that at liftoff balance was perfect, but how was it maintained when the big tank loses more and more mass? How about the boosters? When they were jettisoned, wouldn't that mean a rather dramatic shift in balance?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="6944" LastEditDate="2016-03-03T14:29:13.933" LastActivityDate="2016-03-03T14:29:13.933" Title="How did the Space Shuttle maintain thrust/weight balance" Tags="|space-shuttle|mass|thrust|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14315" PostTypeId="2" ParentId="14313" CreationDate="2016-03-03T08:53:44.160" Score="1" Body="&lt;p&gt;Well, at least I would not be concerned about the Earth's magnetic field. The reason is as follows: There are a lot of small magnets on-board the ISS, used in hundreds of small experiments. Those magnets are transported there with rockets. A typical refrigerator magnet is around 5mT, about a 100 times stronger than the magnetic field of Earth. If those magnets was dangerous to transport to the station, they would not have done it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides, there are lots of other magnetic parts in a rocket, for example those made out of steel. I have never heard about the magnetic field of the Earth causing trouble because of that. Small metal parts are of course not liquid though, and does not represent most of the mass of the rocket, so that is not necessary a valid argument.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Magnets around a pipeline may increase the resistance in it, but as shown in the video, the magnets there barely counteract normal gravity. Pipelines are generally high pressure, making the force pushing the LOX much higher than what the magnets can resist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has it ever been considered? Well, now you have :)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-03T12:07:08.127" LastActivityDate="2016-03-03T12:07:08.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14316" PostTypeId="2" ParentId="14314" CreationDate="2016-03-03T09:03:54.273" Score="15" Body="&lt;p&gt;The variations in the centre of mass was handled by the huge gimbal range of over 20 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GlEGhm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GlEGhm.jpg&quot; alt=&quot;gimbal&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the heaviest part of the propellant, the liquid oxygen, was placed in the upper part of the external tank. That means that the centre of mass was pretty high, placing it far from the engines, and thereby reducing the deviation angle.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-03-03T09:03:54.273" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14317" PostTypeId="1" AcceptedAnswerId="51305" CreationDate="2016-03-03T11:28:23.347" Score="26" ViewCount="3240" Body="&lt;p&gt;This is a follow-up question to &lt;a href=&quot;https://space.stackexchange.com/q/14284/12102&quot;&gt;Can Voyager 1 receive signals from Earth?&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/14286/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/14287/12102&quot;&gt;this&lt;/a&gt; answer.  &lt;strong&gt;UPDATED:&lt;/strong&gt; see additional information and discussion below.&lt;/p&gt;&#xA;&lt;p&gt;As it continues to move farther from the Sun, the angular separation between the Earth and Sun continues to decrease, can its antenna actually resolve the two and limit the noise from the sun? (seems to be about 0.4 degrees at opposition now) For that matter, how strong IS the noise from the sun relative to earth transmissions, considering the passband of Voyager's electronics - is is it a serious issue to begin with? As earth oscillates in its orbit - is there a seasonal effect?&lt;/p&gt;&#xA;&lt;p&gt;Besides the mind-boggling large distances an weak signal, the problem I'm talking about here is that - as seen from Voyager 1 (and 2), the earth is only a small fraction of 1° away from the sun, which is a powerful and noisy radio source.&lt;/p&gt;&#xA;&lt;p&gt;Voyager receives at around 2 GHz&lt;sup&gt;1&lt;/sup&gt;, so its 3.7m diameter dish can not separate the two. Even a quiet sun is almost &lt;span class=&quot;math-container&quot;&gt;$\text{10}^6$&lt;/span&gt; Jy. The electronics is circa 1970, if it has an input bandwidth presented to the front-end of 10MHz, the sun will be a million times stronger than the 20kW &lt;a href=&quot;http://deepspace.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; DSN signal.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fp1gO.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fp1gO.gif&quot; alt=&quot;Voyagers from 1969 until 2018&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; data for the Sun, planets, Pluto, Voyager 1 and Voyager 2, from January 1, 1969 (a good year to start things) until now. Dots are now. Data is from &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA JPL Horizons&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tt3K0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tt3K0.png&quot; alt=&quot;Angular separation of Voyagers from the Sun&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Angular separation between the earth and the sun in degrees, as seen from Voyager 1 (heavy, blue) and Voyager 2 (light, green). The dips to near-zero stop happening once the spacecraft left the plane of the ecliptic.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/565eu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/565eu.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tbm0Q.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tbm0Q.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; example of the type of settings I used to get the data in ecliptic coordinates.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZoINh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZoINh.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Partial screen shots from DSN Now: &lt;a href=&quot;https://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://eyes.nasa.gov/dsn/dsn.html&lt;/a&gt;, just for fun because this is downlink at 8 GHz rather than uplink at 2 GHz.&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; from All About Circuits' &lt;a href=&quot;https://www.allaboutcircuits.com/news/voyager-mission-anniversary-celebration-long-distance-communications/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Communicating Over Billions of Miles: Long Distance Communications in the Voyager Spacecraft&lt;/a&gt; (a fun read!):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The uplink carrier frequency of Voyager 1 is 2114.676697 MHz and 2113.312500 MHz for Voyager 2. The uplink carrier can be modulated with command and/or ranging data. Commands are 16-bps, Manchester-encoded, biphase-modulated onto a 512 HZ square wave subcarrier.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CDDGJ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CDDGJ.png&quot; alt=&quot;Voyager's antennas' radiation patterns&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;from DESCANSO Design and Performance Summary Series &lt;a href=&quot;https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Article 4: Voyager Telecommunications&lt;/a&gt; as discussed in &lt;a href=&quot;https://space.stackexchange.com/a/24343/12102&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2022-09-10T09:51:08.907" LastActivityDate="2023-05-26T18:08:57.610" Title="How well can Voyager 1 separate Earth signals from Solar noise these days?" Tags="|communication|voyager|" AnswerCount="5" CommentCount="15" ContentLicense="CC BY-SA 4.0" />
  <row Id="14318" PostTypeId="2" ParentId="14313" CreationDate="2016-03-03T13:19:23.900" Score="5" Body="&lt;p&gt;LOX is paramagnetic, not fully magnetic. What does that mean? It takes an extremely large magnet to have any effect at all. The only design consideration which would need to occur at all would be to not have a powerful magnet near the thruster valve, or really anywhere near it. There would serve no real useful purpose for this, and thus it isn't required. The &lt;a href=&quot;http://www.school-for-champions.com/science/magnetic_materials.htm#.Vtg6N0afZhM&quot; rel=&quot;noreferrer&quot;&gt;magnetism of such paramagnetic materials&lt;/a&gt; is barely measurable with very sensitive equipment.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-03-03T13:19:23.900" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14323" PostTypeId="1" AcceptedAnswerId="14331" CreationDate="2016-03-03T19:51:29.197" Score="6" ViewCount="530" Body="&lt;p&gt;Many websites tell me effects of microgravity on the human body over a period of months. However nothing about the immediate effects of it. Could it be because there isn't any? I once read that fluid redistribution due to microgravity make you want to relieve yourself within two hours time.&lt;/p&gt;&#xA;" OwnerDisplayName="user12889" LastEditorUserId="4660" LastEditDate="2016-03-03T19:57:15.787" LastActivityDate="2016-08-11T02:48:01.467" Title="Effects of microgravity on human body in a days time" Tags="|microgravity|health|humans|crewed-spaceflight|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14324" PostTypeId="2" ParentId="14323" CreationDate="2016-03-03T21:02:39.843" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/audience/forstudents/5-8/features/F_How_Youve_Grown_5-8.html&quot; rel=&quot;nofollow&quot;&gt;Spinal elongation&lt;/a&gt; happens right away and the onset of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_adaptation_syndrome&quot; rel=&quot;nofollow&quot;&gt;space adaptation syndrome&lt;/a&gt; (for the unlucky 50%) can also be immediate. I think the fluid transfer to the head also starts then but haven't found a source.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-03-03T21:02:39.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14327" PostTypeId="1" CreationDate="2016-03-04T10:58:14.570" Score="5" ViewCount="874" Body="&lt;p&gt;Since go-pros are very popular and you see them everywhere, has anyone sent them to space orbiting? Or any plans?&lt;/p&gt;&#xA;" OwnerUserId="12420" LastEditorUserId="4660" LastEditDate="2016-03-04T15:43:54.073" LastActivityDate="2022-07-21T22:43:56.667" Title="Has there ever been a go-pro satellite?" Tags="|artificial-satellite|camera|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14329" PostTypeId="1" CreationDate="2016-03-04T16:23:32.397" Score="4" ViewCount="118" Body="&lt;p&gt;Looking for online databases or printed compilations of launch vehicle accidents, mistakes done while fabricating rockets and preventive measure taken.&lt;/p&gt;&#xA;" OwnerUserId="13661" LastEditorUserId="4660" LastEditDate="2016-03-04T16:26:22.943" LastActivityDate="2020-12-08T16:06:53.940" Title="Is there a website or database which lists launch vehicle accidents and reasons in detail?" Tags="|reference-request|failure|launch-vehicle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14330" PostTypeId="1" CreationDate="2016-03-04T17:40:08.860" Score="4" ViewCount="289" Body="&lt;p&gt;First there's the annoying part where everyone says it was a year but the actual flight time was less than a year. It is only a year in the sense that they launched and landed in March.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But getting past that, I am having trouble deciphering all the media to understand exactly what they were studying that hasn't already been studied or proven in some way before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, there was the flight by the Russian cosmonaut that actually lasted for more than a year. We already know that it's physically possible. So what were we studying in that respect and why not try to beat his record instead of undercutting it substantially? Just more thorough data collection that we have available with today's technology? What does that get us?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Expanding on that we know that your body degrades from the lack of exercise required in microgravity but lack of exercise also degrades your body no matter how much gravity you are subjected to. And we know and have proven that the cure is to exercise. So was it more about trying to find the best/minimal exercise routine/equipment that works in microgravity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The psychological aspect has been mentioned a lot too yet we have done numerous studies involving isolation in recent years and have already lived through the practicality of it through the use of boats and submarines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there is the effects of radiation and microgravity on DNA replication. We have already seen radiation effects in nuclear disaster zones like Chernobyl and have done studies sending seeds and animals to orbit to expose them to these types of conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then there is whatever else I'm not thinking of at the moment. So in conclusion, what science did we gain from this expedition that helps us get to Mars?&lt;/p&gt;&#xA;" OwnerUserId="1087" LastEditorUserId="6816" LastEditDate="2016-03-07T04:52:20.503" LastActivityDate="2016-03-07T04:57:02.727" Title="What advancements towards Mars were made during Scott Kelly and Mikhail Kornienko's &quot;1 year&quot; journey?" Tags="|iss|mars|crewed-spaceflight|low-earth-orbit|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14331" PostTypeId="2" ParentId="14323" CreationDate="2016-03-04T18:08:01.283" Score="9" Body="&lt;p&gt;Organic Marble, as usual, is correct in his answer, as far as it goes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anecdotally, I can add a few primary effects and one secondary effect to the list:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fluid transfer to the head (a primary effect) is generally noted within minutes after the onset of microgravity conditions.  It feels as if you are hanging upside down in Earth's gravity well and manifests itself as a feeling of fairly severe head stuffiness and sinus congestion.  We referred to the phenomenon informally as &quot;launch face.&quot;  The externally visible effects seemed to remain for at least two weeks, but the uncomfortable feeling was generally mostly gone after the first 48-72 hours in said environment.  I'm not sure if the externally visual effects ever go away - I've noticed that my friends who are serving long duration missions on the ISS seem to never quite get back to looking exactly as they do on Earth as long as they remain in Space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Frequent urination (another primary, but acute effect) is generally noted within two hours or so after the onset of microgravity conditions.  Because of this, some crewmembers will intentionally launch in a dehydrated state.  However, I understood (in 2002) that this practice was discouraged by the NASA flight doctors.  After about two days in Space, the body has achieved a new fluid equilibrium and urination frequencies stabilize to a &quot;new normal.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few words on spinal elongation (a primary effect mentioned by Organic Marble in his answer): For me, some lower back pain became evident after about four days into my mission.  I'd say that over half of all Space Shuttle crewmembers complained of lower back pain during a mission.  I used to sleep with my knees strapped to my chest - this seemed to keep my discomfort at bay.  It was generally accepted, around the 2002 time frame, that said pain was directly related to spinal elongation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few words on space adaptation sickness (SAS) (a primary effect mentioned by Organic Marble in his answer):  Anecdotally, it was acknowledged (2002 time-frame) that most crewmembers largely got past the effects of SAS by about day three or so in space.  I was pretty &quot;green&quot; throughout day two, but felt much better during later days.  However, not everyone was so lucky, notably &lt;a href=&quot;https://en.wikipedia.org/wiki/Jake_Garn#Spaceflight&quot; rel=&quot;noreferrer&quot;&gt;Senator Jake Garn&lt;/a&gt;.  As the referenced link states, the unit of SAS is jokingly referred to as a Garn.  It was also said that the &lt;em&gt;second&lt;/em&gt;-highest level of SAS affliction ever attained in the history of manned spaceflight barely registered as one nano-Garn!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eye irritation (mostly a secondary effect, I believe), is noted by most crewmembers within a few hours after the onset of microgravity conditions.  We believed (as of 2002) that this effect was caused primarily by dust and lint in the air.  Although the Orbiter crew cabins were quite clean before launch, any particulate matter present, as well as lint displaced from crew clothing, remained suspended in the air until removed by air filters.  I found this eye irritation to be moderately uncomfortable.  Some crewmembers used eye drops to mitigate the discomfort.  While this effect never quite went away for me during my &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-109&quot; rel=&quot;noreferrer&quot;&gt;10 day mission&lt;/a&gt;, it didn't bother me too much after day two or so.  However, I do remember that the eye discomfort was cumulative throughout a crew duty day and, by the time bedtime arrived, I was ready for some relief in the form of some &quot;shut eye!&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One curiosity to add:  My initial feeling after the onset of microgravity condition was not a feeling of zero-G, it was a overwhelming sensation of &lt;em&gt;negative&lt;/em&gt;-G (i.e. I felt as if I were being drawn to the ceiling of the flight deck).  Although several other NASA Astronauts I've spoken with had a similar experience, I can't comment as the the overall prevalence of this phenomenon.&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="12422" LastEditDate="2016-08-11T02:48:01.467" LastActivityDate="2016-08-11T02:48:01.467" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14332" PostTypeId="2" ParentId="14330" CreationDate="2016-03-04T21:59:05.237" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Just more thorough data collection that we have available with today's technology? What does that get us?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What we had was a very small sample size (4 people). Adding to that data is valuable in itself. We also had the benefit of 30 years of advancement so we could gather &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/news/top_ten_1YM&quot; rel=&quot;nofollow&quot;&gt;better data than before&lt;/a&gt;. The research done on Mir was a valuable starting point. With the information gathered there, scientists could identify areas that need more study, and that has informed the research plan for this mission.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This is the first time that extensive research using exciting new techniques like genetic studies has been conducted on very long-duration crew members. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Kelly having a twin is a big help: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One of the unique aspects of Kelly’s participation in the one-year mission is that he has an identical twin brother, Mark, who is a former astronaut. The pair have taken part in a suite of studies that use Mark as a human control on the ground during Scott’s year-long stay in space. The Twins Study is comprised of 10 different investigations coordinating together and sharing all data and analysis as one large, integrated research team. The investigations focus on human physiology, behavioral health, microbiology/microbiome and molecular/omics. The Twins Study is multi-faceted national cooperation between investigations at universities, corporations, and government laboratories.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA's research goals for this mission are &lt;a href=&quot;https://www.nasa.gov/1ym/research&quot; rel=&quot;nofollow&quot;&gt;pretty easy to find&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mission actually continues for another year, monitoring Kelly's recovery. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. Radiation effects in space are rather different from conditions found on Earth (higher energies). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6816" LastEditDate="2016-03-07T04:57:02.727" LastActivityDate="2016-03-07T04:57:02.727" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14333" PostTypeId="1" CreationDate="2016-03-05T00:10:21.380" Score="7" ViewCount="448" Body="&lt;p&gt;In the &lt;a href=&quot;https://www.youtube.com/watch?v=sIkPP2LM8DU&amp;amp;t=3083s&quot; rel=&quot;nofollow noreferrer&quot;&gt;SES launch on March 4, 2016&lt;/a&gt;, when the satellite deployed, there was quite a bit of junk that was released when the satellite was released. What was that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3rYqO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3rYqO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-03-05T00:42:36.137" LastActivityDate="2016-06-03T05:39:33.173" Title="What was all of the junk in the SES satellite deployment?" Tags="|launch|spacex|falcon-9|debris|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14334" PostTypeId="2" ParentId="14333" CreationDate="2016-03-05T03:19:31.563" Score="1" Body="&lt;p&gt;Probably ice. As the spacecraft goes through the atmosphere water condenses, then freezes. When the satellite is released, some of that ice breaks off. &lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2016-03-05T03:19:31.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14335" PostTypeId="5" CreationDate="2016-03-05T03:51:21.390" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-03-05T03:51:21.390" LastActivityDate="2016-03-05T03:51:21.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14336" PostTypeId="4" CreationDate="2016-03-05T03:51:21.390" Score="0" Body="information collected during a space mission, in particular from instruments observing a phenomenon, person, or object in space." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-03-05T03:51:21.390" LastActivityDate="2016-03-05T03:51:21.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14337" PostTypeId="1" AcceptedAnswerId="14339" CreationDate="2016-03-05T05:23:10.417" Score="2" ViewCount="457" Body="&lt;p&gt;This is one of my all-time YouTube favorite &lt;a href=&quot;https://www.youtube.com/watch?v=z52WWLE8bBo&quot; rel=&quot;nofollow&quot;&gt;videos&lt;/a&gt;. It's an illustration of the Earth-Moon Lagrange points in orbit around the sun for a year. Put on your headphones, dim the lights, and set YT to HD.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 1:17, two new green lines appear, associated with EML2. Does anyone know what these represent exactly? You can see them  on the left and bottom in the still image below as well. Also - where was this animation produced, and why - what is the background?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/z52WWLE8bBo?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-03-05T05:30:13.850" LastActivityDate="2016-03-05T05:55:52.470" Title="What do the green lines represent in this Lagrange Point animation?" Tags="|orbital-mechanics|lagrangian-points|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14338" PostTypeId="1" CreationDate="2016-03-05T05:27:21.913" Score="21" ViewCount="4175" Body="&lt;p&gt;The rocket would keep same descent system as now (no parachute). But instead of landing on feet, it would have long grid fins on its top that would be caught by an elevated cable net.&lt;/p&gt;&#xA;&lt;p&gt;The previous SpaceX attempts showed that the landing position is very precise, but that it's hard to land slowly and stable enough to avoid falling on its side or break a leg. This solution objective would solve this issue, plus reduce the rocket weight.&lt;/p&gt;&#xA;&lt;p&gt;The global idea is to transfer as much as possible the landing system (and its weight) from the rocket to the ground.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V4KEm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V4KEm.png&quot; alt=&quot;Elevated net for rocket first stage landing&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;No parachutes, keep the current advantages of controlled descend targeting a precise and optimal position in the sea.&lt;/li&gt;&#xA;&lt;li&gt;No more heavy legs.&lt;/li&gt;&#xA;&lt;li&gt;Cannot fall after landing or break a leg anymore.&lt;/li&gt;&#xA;&lt;li&gt;The long top grid fins would help stabilize the descent, and slow down a little the rocket (opposite of the current bottom legs that makes it harder to control).&lt;/li&gt;&#xA;&lt;li&gt;The landing impact force would be absorbed by the elasticity of the fin (and the whole platform on water) on several meters; much longer than the current brutal impact on feet. So the rocket could land with a higher speed, saving more fuel.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;So, surely there must be flaws and issues with this idea... waiting for your opinion and ideas!&lt;/p&gt;&#xA;&lt;p&gt;EDIT: Top-view of the net below. Metal structure would be 50m wide (same as the current landing platform), the net hole 15m in diameter, and top legs 15m length, so it cannot fall through the hole nor be in contact with the solid structure.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BIKy5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BIKy5.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;About some very interesting issues reported in the replies:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Rocket precision to enter the net hole&lt;/strong&gt;: the hole would be 15m in diameter, and the target precision of SpaceX is 10m, so it should be already OK.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Net would burn/melt from the rocket fire&lt;/strong&gt;: as seen in the point above, flames should not touch the cables - and cables should be quite resistant to them anyway.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Top-Legs resistance&lt;/strong&gt; (15m instead of the actual 7m):&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I think a lot of the weight of the currently used legs which comes from the huge hydraulic cylinders required to open the legs against the air friction (not the case for that in top-legs).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Also, they need to be very strong to support the instant landing impact, that would be smoothed by the net elasticity.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;We can also imagine some &amp;quot;legs&amp;quot; a bit different: if the structure is high enough, it could be some hooks that catch the net, not horizontal by at 45° vertically, and so take all the force mostly in tension and not compression, more like cables and so very resistant for a very low weight.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;As a note, at this point I made an error of calling them &amp;quot;grid fins&amp;quot;, they would just be top-legs, automatically doing the same stabilizing job as the grid fin hanks to their length.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Stage structure resistance&lt;/strong&gt;. The stage is not conceived to support top-legs, but the rocket weights 325t at lift-off, so I suppose 20t on a smoother landing should be OK without too much reinforcement&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Movement of the barge&lt;/strong&gt; that would make the top of the net very unstable. Good point. But the barge is very big (50 &lt;span class=&quot;math-container&quot;&gt;$\times$&lt;/span&gt; 90m) so it is quite stable, and we could add very heavy and deep keel (not sure of the word) to stabilize it more, as it doesn't need to move fast at this time (it may already be the case).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;The net would break if rocket would come too fast&lt;/strong&gt;. Absolutely, but the rocket would still slow down to almost 0, like now. It would just have a little more flexibility, could come slightly faster or less straight.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="13665" LastEditorUserId="47149" LastEditDate="2022-06-01T20:19:59.753" LastActivityDate="2022-06-01T20:19:59.753" Title="SpaceX first stage landing: Why not use an elevated net above the landing platform to catch the rocket?" Tags="|spacex|falcon-9|landing|reuse|first-stage|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="14339" PostTypeId="2" ParentId="14337" CreationDate="2016-03-05T05:55:52.470" Score="2" Body="&lt;p&gt;According to one of the comments, the animation was created by the user who posted it (user '3D4U'), &quot;using two commercial applications: Cinema4D and After Effects.﻿&quot; The about page for this user merely states &quot;Channel hosting a collection of CGI.&quot; If you wish to know more about why they made this particular animation, why not just ask them directly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The green lines you ask about (they are vaguely heart-shaped curves encircling earth, one of which makes a sharp corner at L2) are called equipotential surfaces. Think of them like terrain contours. Each curve has an associated energy level, and any item that is placed on one of the curves (and that is co-rotating with the system, I think...) will have the same amount of gravitational and potential energy regardless of where on the curve it is placed. It will also feel a net gravitational (or centripetal) force towards (or away from) the system that is perpendicular to the curve. Where these curves intersect, the forces balance out and create a Lagrange point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a link in the video description box that takes you to a page that provides more information on equipotenital surfaces, and draws many more than just two of them. Looking at the more detailed set of curves here should be much more intuitive: &#xA;&lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/lagpt.html&quot; rel=&quot;nofollow&quot;&gt;http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/lagpt.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6306" LastActivityDate="2016-03-05T05:55:52.470" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14340" PostTypeId="2" ParentId="14338" CreationDate="2016-03-05T06:22:32.850" Score="16" Body="&lt;p&gt;Issue #1: the rocket body is really thin (like a soda can) and the smallest force in the direction the net would apply would probably dent it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Issue #2: there was never a problem with stability which would need solving with some extra contraptions. It was:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;not enough hydraulic fluids for steering&lt;/li&gt;&#xA;&lt;li&gt;stuck valve and algorithms not accounting for the latency it created&lt;/li&gt;&#xA;&lt;li&gt;leg not locked&lt;/li&gt;&#xA;&lt;li&gt;not sure yet (maybe coming too fast because not enough fuel margins?)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The first 3 just needed fixing the specific problem and the 4th attempt was a test of upper limits anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Issue #3: the net and surrounding structures would have to withstand &lt;em&gt;rocket exhausts&lt;/em&gt; of 1 or even 3 (supposedly in the 4th attempt) engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Issue #4: SpaceX is training for Mars and you won't have such &lt;em&gt;benefits&lt;/em&gt; there.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2016-03-05T07:11:07.423" LastActivityDate="2016-03-05T07:11:07.423" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="14341" PostTypeId="1" CreationDate="2016-03-05T10:20:48.713" Score="0" ViewCount="214" Body="&lt;p&gt;I am guessing it's about the same as having a medical X-ray each day, is this about right?&lt;/p&gt;&#xA;" OwnerUserId="13668" LastEditorUserId="38" LastEditDate="2016-03-05T10:50:02.907" LastActivityDate="2016-03-05T12:41:01.390" Title="If I spend one day beyond the Van Allen belt in an unshielded craft/spacesuit how much radiation will I be exposed to?" Tags="|radiation|cosmic-radiation|van-allen-belt|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14342" PostTypeId="2" ParentId="14341" CreationDate="2016-03-05T10:49:14.263" Score="1" Body="&lt;p&gt;From this &lt;a href=&quot;https://travel.stackexchange.com/q/13877/1381&quot;&gt;answer&lt;/a&gt; on Travel.SE&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a flight from New York to Tokyo is probably about 150 μSV. For comparison, natural background radiation is about 2,000-7,000 μSv per year, a chest x-ray is about 50 μSv.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;By contrast, outside the Van Allen belt, say on a trip to Mars, you are looking at around 1.84 mSv per day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's about 37 times as much as a medical X-Ray each day!&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:14.817" LastActivityDate="2016-03-05T10:49:14.263" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14343" PostTypeId="2" ParentId="14338" CreationDate="2016-03-05T12:23:40.270" Score="21" Body="&lt;h2&gt;Edit 2: added a paragraph about what happens after the landing (see at the bottom)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;So I've got a pile of arguments against this scheme, but this is the big one: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You've made the legs twice as long (the real ones are about 7 m), which means they're at least twice as heavy. Probably more, since the load will be applied further out so the legs need to be stronger. So far from saving weight, you've just added several tons to the empty weight of the stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other arguments: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The legs are at the bottom of the stage, conveniently located where the stage already has to be strongest to withstand the loads from the engines. The bottom hinges are attached to the Octaweb engine frame. The top hinge has a compression load, which is easy enough to engineer for (a band of thicker skin is enough). Due to the leg geometry, the load is spread between the bottom and top hinges.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you move the legs to the top, you have to strengthen the stage at the top, where you don't already have a strong structure inside. When the legs are straight out, the top hinges won't take any load so all of it has to be absorbed by the lower hinge. The lower hinge will be pulling outwards with a force of several tons, so you need to strengthen the skin here.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The top of the stage is made to support weight in the vertical direction, not to support large horizontal loads. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So... &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;you don't save any weight, because you still need legs that can support the stage, you've just moved them from the bottom to the top. &lt;/li&gt;&#xA;&lt;li&gt;you need to strengthen the top of the stage, which adds weight. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;You still have the failure mode of 'a leg breaks after landing' or 'a leg fails to lock into position', because you're using a very similar mechanism. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hydraulic cylinders are still there: this time they're not pushing against the wind, they're preventing the wind from snapping the legs back violently. Longer legs = greater aerodynamic forces, so again more weight to prevent stuff from breaking. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A net is inherently less resistant to the rocket's flames than a slab of asphalt on top of a slab of steel. The net also creates point loads where a cable meets the landing leg. As the rocket settles in the net, it will tend to move around. The leg will slide over the cables, which will abrade the legs (steel cable is a decent saw blade).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The barge moves in the water, rocking on the waves. Any movement is amplified by the height of the poles that hold up the net: a few degrees of tilt ends up putting the poles several m out of place. This makes the hole a moving target that's much harder to hit than the barge itself.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hoverslam maneuver aims to get the rocket's vertical speed to 0 at the moment of touchdown. Catching the rocket while it still has some vertical speed increases the loads on the rocket. So you have to make the legs stronger and heavier again.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the stage uses its main engine to maneuver, the engine gimbals, spewing a ten-meter column of flame in a large off-vertical angle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible to build a platform that's not affected by wave motion. You can add a large underwater structure that provides the buoyancy. Offshore drilling rigs are sometimes built like this. This would make the barge far less mobile, and 10x heavier (=10x more expensive) than it is now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get flexibility in the landing, it'd be easier to replace the current leg design with one that doesn't lock, and adds some shock absorption to the hydraulic system. This requires more complex, heavier hydraulics though. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And another thing: what happens after the landing? In the current design, the crew (presumably) comes aboard and welds some clamps over the legs to secure the rocket for the return voyage. Simple.&lt;br&gt;&#xA;In your scenario, the rocket's dangling from a net. It'll have to be secured, and pretty quick, or it'll bang into something and be destroyed. You can't lower it vertically (no legs at the bottom), so you'll have to devise a complex mechanism to set it down horizontally on a cradle. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-03-06T19:24:54.297" LastActivityDate="2016-03-06T19:24:54.297" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14344" PostTypeId="2" ParentId="14341" CreationDate="2016-03-05T12:41:01.390" Score="2" Body="&lt;p&gt;This must relate to real-time solar conditions. e.g. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a quiet day&lt;/li&gt;&#xA;&lt;li&gt;a storm day&lt;/li&gt;&#xA;&lt;li&gt;some mission time weighted average&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Any of these could be misleading without specifying the assumptions. Note that the latter has to account for the fact that large events occur on longer time-scales than a year or two to Mars and back hence it is a probabilistic problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interesting way of looking at this is that near polar aircraft flights are very much more sensitive to radiation than at lower latitudes. &lt;a href=&quot;http://www.nasa.gov/topics/aeronautics/features/airline-radiation.html#.VtrGVHr7vtQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;This link&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Air crew are classed as radiation workers&lt;/li&gt;&#xA;&lt;li&gt;The dose during one particular storm corresponded to 12% of the normal yearly limit (note, limit, not normal dose). &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Anecdotally, I have heard there have been rather worse storms in this respect where the single flight does exceeded the yearly limit, if I find a reference I will post it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence, back to the question, it could be very much worse beyond the protection of the Earth's magnetosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is related to &lt;a href=&quot;https://space.stackexchange.com/questions/9487/could-an-apollo-crew-have-been-killed-by-solar-radiation&quot;&gt;this one&lt;/a&gt;. On of the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/tnD7080RadProtect.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;linked references&lt;/a&gt; gives two interesting figures, which I have reproduced below. As a caution, out of context they are not so meaningful, and even look rather cavalier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J3Zse.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J3Zse.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Gsjv4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Gsjv4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-03-05T12:41:01.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14345" PostTypeId="2" ParentId="14338" CreationDate="2016-03-05T13:20:23.757" Score="3" Body="&lt;p&gt;You're using way too high speeds to make it feasible, similar to a water landing (you could say, why not &quot;dive&quot; into the water). At the speeds a rocket touches down, if not heavily controlled, hitting a net is equal to hitting the ground/water surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the landing legs (which anchor to the net) need to withstand the full force, AND dissipate all kinetic energy, without creating a high momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what makes this &quot;easier&quot; than a ground landing according to you? The net needs to have a very high yielding strength to withstand those forces. But not only that: either the net needs to be really flexible, or the landing anchors simply need to withstand the same forces as a ground landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A high yield strength, highly ductile material (low &lt;a href=&quot;http://en.wikipedia.org/wiki/Young%27s_modulus&quot; rel=&quot;nofollow&quot;&gt;Young's modulus&lt;/a&gt;) that can be created of large enough size simply does not exist yet. Sure, theoretical materials such as graphene might prove useful in the future, but if you allow that, why not make the whole rocket from that material?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now there is also an actual drawback to using a net compared to a landing on land such as the Soyuz do. (This is similar to why water landings are so much harder to both air and spacecraft). Land is well defined structure, you know its shape and it's (probably) nicely flat. A net (and sea) are not that way, they deform (waves) and are nearly never flat. This effect is, in a net, exaggerated by the workings of netted structures: you never land perfectly, so one part of the rocket will touch the net first, this creates a wave which will actually create an upward force at other parts that are landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Try simulating the last part.&lt;/p&gt;&#xA;" OwnerUserId="10127" LastEditorUserId="1142" LastEditDate="2016-03-05T16:55:21.337" LastActivityDate="2016-03-05T16:55:21.337" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14346" PostTypeId="1" AcceptedAnswerId="18727" CreationDate="2016-03-05T15:17:13.267" Score="28" ViewCount="6707" Body="&lt;p&gt;The James Webb Space Telescope has a &lt;a href=&quot;https://www.youtube.com/watch?v=vpVz3UrSsE4&quot;&gt;complicated deployment sequence&lt;/a&gt;.  Beyond data from sensors aboard the spacecraft, does it have any cameras to take pictures of the process?&lt;/p&gt;&#xA;" OwnerUserId="13397" LastEditorUserId="12102" LastEditDate="2017-01-24T05:32:12.923" LastActivityDate="2022-01-06T20:45:16.020" Title="Does the JWST have a camera to monitor its deployment progress?" Tags="|artificial-satellite|camera|james-webb-telescope|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14347" PostTypeId="2" ParentId="14330" CreationDate="2016-03-05T15:25:59.453" Score="3" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;I am having trouble deciphering all the media to understand exactly what they were studying that hasn't already been studied or proven in some way before.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;You don't want to decipher the media. The lay media does a lousy at presenting science and engineering. As Hobbes wrote in his answer, getting past the media is not that hard with just a bit of Google Fu.&lt;/p&gt;&#xA;&lt;p&gt;What this study offers that previous studies did not include&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Use of human guinea pigs rather than real guinea pigs (or other non-human test subjects). Sometimes animal studies translate to humans, but other times they don't.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;An identical control. Scientists have performed controlled studies related to the deleterious effects of space on animals, this has never been done with humans.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Modern techniques. Sequencing the Kelly twins' genomes would have cost about \$200,000,000 dollars 15 years ago. Now its about \$2000. This five order of magnitude drop in price opens all kinds of research avenues that didn't exist in the previous millennium.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Investigators of the NASA Twins Study (and other investigators from Russia, Europe, and Japan performing related research) will collectively have conducted 383 different experiments on the Kelly twins and on Mikhail Kornienko by the time this is all over. The ten principal investigators of the NASA Twins Study their principal areas of studies are&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Susan Bailey (Colorado State University): Differential effects on telomeres and telomerase in twin astronauts associated with spaceflight.&lt;/li&gt;&#xA;&lt;li&gt;Andrew Feinberg (Johns Hopkins University School of Medicine): Comprehensive whole genome analysis of differential epigenetic effects of space travel on monozygotic twins.&lt;/li&gt;&#xA;&lt;li&gt;Christopher Mason (Weill Medical College of Cornell University): The Landscape of DNA and RNA Methylation Before, During, and After Human Space Travel.&lt;/li&gt;&#xA;&lt;li&gt;Scott Smith (NASA Johnson Space Center): Homozygous twin control for a 12 month Space Flight Exposure.&lt;/li&gt;&#xA;&lt;li&gt;Emmanuel Mignot (Stanford University School of Medicine): Immunome Changes in Space.&lt;/li&gt;&#xA;&lt;li&gt;Stuart Lee (Wyle Laboratories): Metabolomic And Genomic Markers Of Atherosclerosis As Related To Oxidative Stress, Inflammation, And Vascular Function In Twin Astronauts.&lt;/li&gt;&#xA;&lt;li&gt;Brinda Rana (University of California)&#xA;Proteomic Assessment of Fluid Shifts and Association with Visual Impairment and Intracranial Pressure in Twin Astronauts.&lt;/li&gt;&#xA;&lt;li&gt;Mathias Basner (University of Pennsylvania School of Medicine): Cognition on Monozygotic Twin on Earth.&lt;/li&gt;&#xA;&lt;li&gt;Fred Turek (Northwestern University): Metagenomic Sequencing of the Bacteriome in GI Tract of Twin Astronauts.&lt;/li&gt;&#xA;&lt;li&gt;Michael Snyder (Stanford University): Longitudinal integrated multi-omics analysis of the biomolecular effects of space travel.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;For details, see &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150008172.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Twins Study: NASA’s First Foray into 21st Century Omics Research&lt;/a&gt; and &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160001730.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;One-Year Mission &amp;amp; Twins Study&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;br&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;So in conclusion, what science did we gain from this expedition that helps us get to Mars?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;You are jumping the gun. The NASA Twins Study is not complete and will not be complete for some time. Post-flight sample collections will occur about three months and six months after landing. After that, it will take researchers time to make sense of all of the collected results, and it will take yet more time for &lt;strong&gt;some&lt;/strong&gt; of that data to be published in scientific journals. (The reason I wrote &amp;quot;some&amp;quot; is because some of the results might never be released. This is a human guinea pig study, and human guinea pigs, unlike real guinea pigs, have medical privacy rights.)&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-05T15:25:59.453" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14348" PostTypeId="2" ParentId="14301" CreationDate="2016-03-05T15:47:41.950" Score="2" Body="&lt;p&gt;It seems like simple equipment to support isometric exercises would indeed be logical. There might be such a thing. But I couldn't find a quick answer for this. I almost expect to see multiple experimental solutions, given that combating muscle atrophy in zero G is a major concern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably the most famous piece of exercise of equipment on the ISS is the Treadmill with Vibration Isolation Stabilization (TVIS). As indicated by the name, the treadmill's chief asset is its ability to be used without propagating virbration to the vast number of micro-gravity experiments on the station.&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Treadmill_with_Vibration_Isolation_Stabilization&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Treadmill_with_Vibration_Isolation_Stabilization&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isolation turns out to be a major design requirement. Equipment put on the station should not induce measurable vibration to the station. In fact, this is a major reason why there is no human-sized centrifuge or rotating section connected to the station, even though this would go far toward understanding any proposed partial gravity prescriptions and probably combat atrophy. (There are vibration problems on the station as is. NASA and its partners probably don't want to compound the problem.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for why the TVIS is so famous... For very strange reasons related to the magical hold of Stephen Colbert on parts of humanity, it is also known as the Combined Operational Load Bearing External Resistance Treadmill (COLBERT). It was a little weird while it was happening, but it's a good story, and not just because of Colbert. &lt;a href=&quot;https://en.wikipedia.org/wiki/Tranquility_(ISS_module)#Naming_contest&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Tranquility_(ISS_module)#Naming_contest&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were it not for the naming controversy, many of us probably wouldn't even be aware of that particular treadmill.&lt;/p&gt;&#xA;" OwnerUserId="13671" LastActivityDate="2016-03-05T15:47:41.950" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14349" PostTypeId="1" AcceptedAnswerId="14350" CreationDate="2016-03-05T17:40:54.233" Score="6" ViewCount="508" Body="&lt;p&gt;I've just watched the SES-9 Launch Webcast and was wondering what is visible at about &lt;a href=&quot;https://m.youtube.com/watch?v=muDPSyO7-A0&amp;amp;t=2235&quot; rel=&quot;nofollow noreferrer&quot;&gt;37:15&lt;/a&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SClle.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SClle.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the Falcon 9 have cameras in the kerosene (or LOX) tank? &lt;/p&gt;&#xA;" OwnerUserId="13674" LastEditorUserId="8162" LastEditDate="2016-03-05T18:04:08.123" LastActivityDate="2016-03-05T18:04:08.123" Title="SpaceX Webcast - What is visible in this clip?" Tags="|spacex|falcon-9|fuel|" AnswerCount="1" CommentCount="0" ClosedDate="2016-03-06T09:18:07.020" ContentLicense="CC BY-SA 3.0" />
  <row Id="14350" PostTypeId="2" ParentId="14349" CreationDate="2016-03-05T17:50:43.997" Score="7" Body="&lt;p&gt;Yes, that's a look inside the kerosene or LOX tank. SpaceX has cameras inside both tanks. &lt;a href=&quot;https://space.stackexchange.com/questions/7737/spacex-crs-5-launch-what-is-the-footage-that-appears-to-contain-bubbles/7738#7738&quot;&gt;This question&lt;/a&gt; has more detail (incl. how you can see which tank you're looking at). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-03-05T18:04:07.843" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14352" PostTypeId="1" CreationDate="2016-03-06T04:14:19.207" Score="18" ViewCount="9700" Body="&lt;p&gt;Instead of &lt;a href=&quot;//space.stackexchange.com/questions/13547/would-it-be-possible-to-deploy-a-man-made-magnetic-field-in-mars-orbit-for-terra&quot;&gt;placing many magnetic satellites close to Mars to artificially generate a magnetic field to protect Martian colonists from harming radiation&lt;/a&gt;, we could place a magnetic satellite at Lagrangian point L1. The magnetic field does not need to be as strong to deflect the solar wind enough, since the deflection takes place over millions of km before reaching the planet. What would this require to be practical?&lt;/p&gt;&#xA;" OwnerUserId="13677" LastEditorUserId="6816" LastEditDate="2016-03-06T04:23:37.663" LastActivityDate="2020-08-11T12:08:10.710" Title="Place a satellite at Sun-Mars L1 to shield Mars from Sun radiation" Tags="|mars|magnetic-field|terraforming|solar-wind|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14353" PostTypeId="1" AcceptedAnswerId="14356" CreationDate="2016-03-06T14:59:43.327" Score="4" ViewCount="1654" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/14352/place-a-satellite-at-sun-mars-l1-to-shield-mars-from-sun-radiation&quot;&gt;This question&lt;/a&gt; proposes building a satellite to deflect solar ionizing radiation from Mars. But there are two sources of radiation: the sun and 'everything else' (cosmic radiation). How much radiation can you expect from these sources in orbit around Mars?   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does a shield that removes solar radiation alone (1), result in a survivable environment, either in orbit or on Mars' surface? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1: not just the satellite from the linked question. You could build a ship that has a thick shield in the sun-facing direction only, for example. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-03-06T18:29:31.153" Title="Sources of radiation on Mars" Tags="|mars|radiation|health|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14354" PostTypeId="2" ParentId="14352" CreationDate="2016-03-06T15:13:03.573" Score="22" Body="&lt;p&gt;This answers the question on how to block ions coming directly from the Sun from hitting Mars by a satellite stationed at L1. It does not cover the fact whether such a shield is effective in reducing the radiation level on Mars surface. See e.g. &lt;a href=&quot;https://space.stackexchange.com/questions/14353/sources-of-radiation-on-mars&quot; title=&quot;here&quot;&gt;here&lt;/a&gt; for more details on radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, let's have a look at the magnetic field needed. Inside a magnetic field (gray area), the particles follow a circular trajectory (green). The radius depends on magnetic field strength and particle momentum while the total deflection angle also depends on the length of the field.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/SBGDr.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SBGDr.png&quot; alt=&quot;Trajectory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using a distance estimate of 1 million kilometer between Mars and L1, and a Mars diameter of 6000 km we see that we need to deviate the solar wind by about 0.3°. Further assuming a thickness of the magnetic field of 10 km we see that the radius of the path of particles in the magnetic field needs to be about 1500 km. Now, we can use the formula for the magnetic rigidity&#xA;$$B= \frac{m v}{q r}$$&#xA;and plug in the known values of mass and charge of alpha particles, we get a magnetic field strength of 5 nT - or about 10000 times weaker than the Earth magnetic field. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sounds simple enough? Let's have a look at pure geometry. In the drawing all values are given in thousands of kilometers - not to scale for obvious reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5k5No.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5k5No.png&quot; alt=&quot;Shield size - geometry&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sun (yellow) diameter is about 1.4 million kilometer and we need to fully cover it.&#xA;A shield (blue) positioned 1 million kilometers from Mars (red) that covers the sun completely has a diameter of:&#xA;$$R = \frac{R_{sun}d_{L1}}{d_{sun}} = 6100~\rm{km} $$&#xA;But, we want to cover the whole surface of Mars (green shield), so we need to add another 3000 km in each direction, yielding a total diameter of 12000 km. That is equivalent to build a ring around the equator of Earth!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately it is not enough to build a single coil with this diameter - the magnetic field needs to be perpendicular to the solar wind and the intended direction of deflection. That is, pointing away from the viewer, into the screen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1rFIA.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1rFIA.png&quot; alt=&quot;Dual coil setup&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A single coil of reasonable size is not able to produce such a field that extends over thousands of kilometers along the axis of the coil.&#xA;A simple way to provide it would be a pair of Helmholtz coils (orange). &#xA;Again assuming a diameter of 12000 km (which is already beyond the validity of simple formulas as the distance between the two coils needs to be smaller than their diameter) and a strand of copper with a cross-section of 1 square meter, the whole setup would need a reasonable 10 MW of power but weigh about a billion tons, or in other words, the world-wide copper production of 50 years launched to space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that it is not strictly necessary to use a cross-section of 1 square meter. We can always trade mass versus power needed due to increased resistance of a thinner copper &quot;wire&quot;: Half the cross-section requires double the power. &lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-03-12T13:49:08.357" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="14355" PostTypeId="2" ParentId="14327" CreationDate="2016-03-06T16:14:06.083" Score="2" Body="&lt;p&gt;If the intent of the question is, has a GoPro camera itself ever been packaged into a nanosatellite (i.e., 10 kg or less), notably CubeSats, I believe the answer is, no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A GoPro camera generates video at a bitrate of 25 Mbits/sec or greater. The data typically goes onto an SD card, which is extracted for later playback. For a GoPro to be useful in space, you’d have to either recover the SD card or transmit the data down. Even one second of video is a lot of data to transmit. Data rates pretty low, and there are only a few minutes of downlink time per orbit. Nanosatellites such as STRaND-1, PhoneSat and other CubeSats are very low power. Their ability to transmit is very weak. From what I can tell, STRaND-1 depended on amateur radio, and did either 9.6 or 19.2 Kbits/sec for downlink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PhoneSat 1.0 was built around a Nexus One smartphone, with virtually no modification to the phone. Subsequent generations have taken the phone apart and repackaged it for flight. (Among other things, the display and outer case are not very useful inside a satellite.)  &lt;a href=&quot;http://www.nasa.gov/phonesat&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/phonesat&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The technology that is the basis of smartphones and arguably GoPro is being tested and flown by Planet Labs. They take images of the Earth, but these are not videos.  &lt;a href=&quot;http://www.planet.com&quot; rel=&quot;nofollow&quot;&gt;http://www.planet.com&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planet Labs has launched on  the order of 100 3U CubeSats into space. These may be launched directly off a rocket; or flown first to the International Space Station, and then launched weeks or months later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I attended a Planet Labs technical presentation a couple of years ago; the engineer was asked what sort of optics were used. They basically used a small, but good quality telescope. However, I could imagine (no pun intended) the imaging element being the same kind of CCD or CMOS camera sensor used in GoPro or smartphones. &lt;/p&gt;&#xA;" OwnerUserId="13671" LastActivityDate="2016-03-06T16:14:06.083" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14356" PostTypeId="2" ParentId="14353" CreationDate="2016-03-06T18:16:32.420" Score="8" Body="&lt;p&gt;Galactic cosmic rays (GCRs) provide a steady background radiation that poses health risks to humans on the year time scale. That is, over a year with typical spacecraft shielding you will get on the order of the total lifetime dose allowed in Earth occupations. Not really a big deal at all, seeing as how you are on a once-in-a-lifetime adventure of epic proportions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar proton events (SPEs) on the other hand are episodic and huge, ranging up to 100,000 times the flux of the GCRs, with each lasting on the order of days. They are directional and not all that common. But with a little bad luck, they can kill you. We were somewhat fortunate that none of the Apollo missions saw any major solar proton events. It helped of course that the missions only lasted a week or two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While flying the Mars Exploration rovers to Mars, we experienced such an event around Halloween of 2003. It resulted from one of the largest solar flares ever observed. The spacecraft survived, since they were designed for that (unlike humans), but the radiation took out our star scanners for about two weeks. The star scanners were seeing thousands of stars that weren't stars, preventing them from recognizing star field patterns. So no turns or maneuvers were possible during that time. Several FPGAs also were taken out of service due to single-event upsets, but we were able to reset those to restore their operation. Here is a plot of the event as seen by the GOES 11 satellite at Earth at the time:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lSXrs.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lSXrs.png&quot; alt=&quot;GOES 11 Oct 30, 2003 solar flare event&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the vertical scale is logarithmic, showing about 10,000 times the background during the event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At your location, the SPE protons are essentially omnidirectional, so you can't just shield on one side. The directionality is on the order of 10%, so no help there.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-03-06T18:29:31.153" LastActivityDate="2016-03-06T18:29:31.153" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14357" PostTypeId="1" CreationDate="2016-03-06T19:29:13.860" Score="8" ViewCount="3369" Body="&lt;p&gt;Given that the barycenter of the Earth-Moon system lies somewhat below the surface of the Earth, but not at its geometric center, why do our man-made satellites still orbit the Earth's geometric center, and not the barycenter of the Earth-Moon system?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shouldn't all bodies in a system orbit around the barycenter of that system?  In other words, the Earth, the Moon, and any satellites are actually all satellites of this common barycenter?&lt;/p&gt;&#xA;" OwnerUserId="13686" LastActivityDate="2016-03-07T10:53:35.617" Title="Earth-Moon system barycenter" Tags="|low-earth-orbit|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14358" PostTypeId="2" ParentId="14357" CreationDate="2016-03-06T19:51:49.063" Score="7" Body="&lt;p&gt;That is not a safe assumption.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine a satellite orbiting close to Earth. If the Earth was the only mass that mattered, the geometric centre would be at the focus of the orbit. If we include the Moon, the distance between it and the satellite is very similar to the distance between it and the Earth. Thus, even if the gravitational influence of the Moon is noticeable, &lt;strong&gt;the Moon then accelerates both the Earth and the satellite at almost exactly the same magnitude and direction&lt;/strong&gt;. This is like how the Moon seems to have a normal orbit around the Earth, even though the gravitational influence of the Sun is strong. An external force does not matter if it affects both the objects in the same way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the distance ratio between the Moon's orbital radius and the satellite's is not infinite, so there would be some minor tidal effects. But they are way smaller than making the satellite orbiting the barycentre.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="4660" LastEditDate="2016-03-06T23:23:09.870" LastActivityDate="2016-03-06T23:23:09.870" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14359" PostTypeId="1" AcceptedAnswerId="14360" CreationDate="2016-03-06T21:27:44.380" Score="8" ViewCount="2443" Body="&lt;p&gt;Whenever a lander enters an atmosphere, it generates a lot of friction and that heats up the heat shield. On Earth we have seen videos of spacecraft with red-hot shields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Saturn's moon Titan, wouldn't the ignition of some of those hydrocarbons kick-start a run-away inferno? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can fast moving meteorite not do the same?&lt;/p&gt;&#xA;" OwnerUserId="3137" LastEditorUserId="3137" LastEditDate="2016-03-07T18:42:26.730" LastActivityDate="2016-03-07T18:42:26.730" Title="How could a hot lander enter Titan's atmosphere without setting its hydrocarbons ablaze?" Tags="|reentry|atmosphere|lander|atmospheric-drag|titan|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14360" PostTypeId="2" ParentId="14359" CreationDate="2016-03-06T21:53:40.047" Score="27" Body="&lt;p&gt;In order for a combustion process to happen, you do not only need fuel, you also need an &lt;a href=&quot;https://en.wikipedia.org/wiki/Oxidizing_agent&quot; rel=&quot;noreferrer&quot;&gt;oxidizer&lt;/a&gt;. On Earth, that is usually the oxygen in the air. In Titan's atmosphere, there is no oxygen. This applies to other atmospheres too, like the hydrogen dominated atmospheres of Jupiter and Saturn. Hydrogen, just like the methane in Titan's atmosphere, is flammable too, but only in the combination with oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basic check, the fire triangle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lyh2gm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lyh2gm.png&quot; alt=&quot;fire triangle&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a meteor impacting Titan:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fuel: YES&lt;/li&gt;&#xA;&lt;li&gt;Heat: YES&lt;/li&gt;&#xA;&lt;li&gt;Oxygen: NO&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-06T22:02:06.750" LastActivityDate="2016-03-06T22:02:06.750" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="14362" PostTypeId="2" ParentId="14247" CreationDate="2016-03-06T22:35:50.637" Score="3" Body="&lt;p&gt;The book &lt;a href=&quot;http://:%20Human%20and%20Machine%20in%20Spaceflight&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;Digital Apollo: Human and Machine in Spaceflight&lt;/em&gt;&lt;/a&gt;, in the context of the Apollo 9 LM flight testing, says that the digital autopilot on the LM computer supported three modes: CSM+LM, complete LM, and LM ascent stage, which suggests that even at that point they were anticipating the option of flying the CSM+LM stack from within the LM, as done during Apollo 13. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not explicit that the CSM RCS thrusters would be driven from the LM computer, but it seems likely that they would be, since that sort of thing had already been done during the Gemini program. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="1142" LastEditDate="2016-03-11T04:22:45.597" LastActivityDate="2016-03-11T04:22:45.597" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14363" PostTypeId="1" AcceptedAnswerId="14365" CreationDate="2016-03-07T05:28:22.510" Score="5" ViewCount="5953" Body="&lt;p&gt;When I was watching The Martian I really couldn't make a clue of this thing.&#xA;And sadly all in the film that was stated about it (at least in the language version I watched) was, because they thought it would be a nice idea to drill a hole in his roof. But that's all and there was nothing else stated after that about it anymore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a picture of the thingy I'm talking about:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YLZd7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YLZd7.jpg&quot; alt=&quot;airbag on roof&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So earlier problems that had not been mentioned how to be solved:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The temperature&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;He had at that time the radioactive module in the rover &quot;spending heat&quot; but I had the impression it could still get dangerously cold at night. So does this thing have any purpose of regulating temperature?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Generating oxygen&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;He has an oxygen generator but earlier it was mentioned that he will have trouble with the length of the journey as the oxygen won't last long enough, what let me assume no way to generate it on the rover. As after it no word was dropped how they solved this problem: Does this thing maybe some how support the regeneration of oxygen?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Or is it just solving a problem I didn't even notice coming up?&#xA;I dare to doubt they didn't do it just because they thought it would be fancy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this isn't somehow on topic here, please be so kind to migrate it to movies SE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I assumed even it is based on a film this is probably better fitting here.&lt;/p&gt;&#xA;" OwnerUserId="11099" LastEditorUserId="897" LastEditDate="2016-03-07T15:51:27.720" LastActivityDate="2016-09-30T21:55:22.887" Title="What is the &quot;airbag&quot; on the roof of the Mars rover in 'The Martian'?" Tags="|mars|air-bag|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14364" PostTypeId="1" AcceptedAnswerId="14367" CreationDate="2016-03-07T06:34:28.473" Score="4" ViewCount="471" Body="&lt;p&gt;We live in the &lt;a href=&quot;https://www.youtube.com/watch?v=rENyyRwxpHo&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laniakea supercluster&lt;/a&gt;.  Galaxies including the Milky Way are being pulled toward the &quot;Great Attractor&quot; in its center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two questions (note that the answer to one can be used to derive the answer to the other):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;How fast are we moving toward the Great Attractor?&lt;/li&gt;&#xA;&lt;li&gt;What is the escape velocity of the Laniakea supercluster?  That is, assume it is the only supercluster in the universe.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;As in &lt;a href=&quot;https://space.stackexchange.com/questions/3948/escape-velocity-for-our-galaxy&quot;&gt;this question&lt;/a&gt; I understand that the answer depends on uncertain data.  A reasonable guess is perfectly fine for me.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="2707" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-03-07T07:17:37.937" Title="Laniakea Delta-V and Escape Velocity" Tags="|escape-velocity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14365" PostTypeId="2" ParentId="14363" CreationDate="2016-03-07T06:52:36.907" Score="10" Body="&lt;p&gt;I don't remember if it was mentioned in the film but the main problem was that all the equipment he needed to bring for life support did not fit into the rover. So he needed more space and had to modify the rover (cut open the roof) to fit all the equipment. But he also needs the rover to be airtight (the equipment is the primary means of scrubbing CO2 and generating oxygen after all) so he had to plug the hole with HAB canvas material.&lt;/p&gt;&#xA;" OwnerUserId="6280" LastActivityDate="2016-03-07T06:52:36.907" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14366" PostTypeId="2" ParentId="14363" CreationDate="2016-03-07T06:53:20.360" Score="4" Body="&lt;p&gt;The book explains the issue. The cab of the rover was too small for the duration of his trip. He wanted a 'spare room' that he could inflate while waiting for charging to complete. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a subplot not handled in the movie where he shorts out the Pathfinder rover by leaning the drill he was using against the cab, with an exposed wire. Then how he recovers from the loss of communication.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-03-07T06:53:20.360" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14367" PostTypeId="2" ParentId="14364" CreationDate="2016-03-07T07:17:37.937" Score="4" Body="&lt;p&gt;Finding the escape velocity seems simpler than to find the &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;, for the reason that our movement relative to the centre of the cluster is in best case close to pure guesswork.&lt;/p&gt;&#xA;&lt;p&gt;A starting point is to use the estimated distance (&lt;span class=&quot;math-container&quot;&gt;$250×10^6$&lt;/span&gt; ly) and the estimated mass (&lt;span class=&quot;math-container&quot;&gt;$10^{17}$&lt;/span&gt; M☉), and plug that into the escape velocity equation:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$v_e=\sqrt{\frac{2\mu}{r}}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;That gives &lt;span class=&quot;math-container&quot;&gt;$3.35×10^6$&lt;/span&gt; m/s.&lt;/p&gt;&#xA;&lt;p&gt;Of course, the shell theorem does not exactly fit, as the shape of the structure is highly irregular, and the fact that we are actually inside the cluster, so you can perhaps assume that the actual escape velocity is a little bit lower.&lt;/p&gt;&#xA;&lt;p&gt;Back to velocity estimation, we can use &lt;a href=&quot;https://en.wikipedia.org/wiki/Laniakea_Supercluster&quot; rel=&quot;nofollow noreferrer&quot;&gt;the stated Hubble constant&lt;/a&gt;.&#xA;At &lt;span class=&quot;math-container&quot;&gt;$h^{−1}_{0.6780 ± 0.077}$&lt;/span&gt; for 159 Mpc, we can calculate a relative velocity of &lt;span class=&quot;math-container&quot;&gt;$1.08×10^5$&lt;/span&gt; m/s, a much smaller number than the escape velocity.&lt;/p&gt;&#xA;&lt;p&gt;In conclusion, the escape velocity and &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; are both around  &lt;span class=&quot;math-container&quot;&gt;$0.01 c$&lt;/span&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-07T07:17:37.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14368" PostTypeId="1" CreationDate="2016-03-07T07:26:39.437" Score="1" ViewCount="378" Body="&lt;p&gt;As probes that we build travel farther, won't more problems occur? For example, as the nuclear energy thing (&lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow&quot;&gt;RTG&lt;/a&gt;) it goes on gets weaker over time. Or, solar panels will receive less light. Or even, the delay between Earth and the satellite communication. Are there any probes that address some of these issues? Are there any planned probes in the future to go interstellar?&lt;/p&gt;&#xA;" OwnerUserId="12420" LastEditorUserId="8693" LastEditDate="2016-03-07T08:38:28.830" LastActivityDate="2020-05-25T08:55:58.363" Title="Interstellar probe travel problems?" Tags="|artificial-satellite|interstellar-travel|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14369" PostTypeId="2" ParentId="14368" CreationDate="2016-03-07T08:08:32.057" Score="3" Body="&lt;p&gt;As satellites travel further from the sun, available solar energy decreases proportional to the square of the distance. This means that satellites we build for much operation past Mars require supplemental energy sources, such as thermonuclear generators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right now, if we were to build something for interstellar travel, we'd either need a fission reactor onboard, a very, very large thermonuclear generator, or very deep sleep cycles. We'd also need a mechanism for generating the required thrust, and a way to shed it upon arrival at a target. The short answer: we're not quite ready.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not aware of any planned or even announced interstellar satellite or probe missions.&lt;/p&gt;&#xA;" OwnerUserId="6821" LastActivityDate="2016-03-07T08:08:32.057" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14370" PostTypeId="2" ParentId="14357" CreationDate="2016-03-07T10:53:35.617" Score="12" Body="&lt;p&gt;First, a simple question which proves your assumption wrong:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;If you could always replace planet and moon by their barycenter and all satellites would orbit this, how could the Apollo command modules (and others) orbit the moon? Further more, how could anything orbit the earth, if the barycenter of earth and sun is deep inside the sun?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point is, you have to look at the distances. If you are really far away from the planet-moon system, you can approximate them as as single mass located at their barycenter.&lt;br&gt;&#xA;But if you are in close proximity to one object, the gravitational force of the other object is more or less neglible.  For example, here is the acceleration by Earth/Moon on an object on Earth/Moon: &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                     | acc. by Earth | acc. by Moon&#xA;---------------------+---------------+-------------&#xA;Object on the Earth  |     9.81      |   0.00003&#xA;Object on the Moon   |     0.003     |   1.62&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;To make the influence of the distance more clear, I made some plots of the gravitational field of Earth and Moon. They just show the direction of the field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the field near the Earth. I have added the geostationary orbit for size comparison and some lines through the center of the earth and the barycenter:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4HueJ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4HueJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All arrows point to the center of the earth, not to the barycenter. It looks as if there is no moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a more extreme example, here is the field near the moon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MhDGt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MhDGt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also here, far far away from the barycenter, there is no evicence of the big, heavy earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, let's zoom out:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8CuDH.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8CuDH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some distance to earth and moon, the field points to the barycenter (though it'shard to distinguish from center of the earth). &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Finally...&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;this shows what I wrote in the beginning: Close to an object, the effect of other is more or less negligible, and the field points to the center of the object. And from far away, the field points to the barycenter, so you can approximate the objects by a single one.&lt;br&gt;&#xA;So, satellites orbit the center of the earth, not the barycenter. But of course, the moon will affect the orbit a little And I agree, this could become interesting when planet and moon are more of the same size, which results in bigger influences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Also note that I neglected the rotation of the Earth-Moon system. This will also affect an orbit, but since the typical orbital period of a satellite is in the order of an hour, and that of Earth-Moon is 28d, the effect should be not that large.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="11749" LastActivityDate="2016-03-07T10:53:35.617" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14371" PostTypeId="1" AcceptedAnswerId="14792" CreationDate="2016-03-07T12:15:09.277" Score="8" ViewCount="568" Body="&lt;p&gt;I once heard an audio tape compiled from data downloads from a high rate accelerometer on the Olympus satellite, from which one could hear thruster firings, valve switching and reaction wheel operation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have astronauts reported hearing audible creaking sounds from any extended or docked structures? Its easy to imagine that the docking adaptors would be solid but that extremities such as antennas and solar arrays might transmit vibrations through the structure as their temperature changed through each orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm less interested in the noises made by cyclic mechanical machinery such as fans.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="13871" LastEditDate="2016-04-10T10:03:07.817" LastActivityDate="2016-04-12T19:39:06.703" Title="Do astronauts hear audible creaking from spacecraft structures?" Tags="|crewed-spaceflight|vibration|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14372" PostTypeId="1" AcceptedAnswerId="14373" CreationDate="2016-03-07T14:13:27.697" Score="16" ViewCount="5371" Body="&lt;p&gt;I have been looking at different threads. It seems like the receiver clock on Earth doesn't have to be synchronized with the satellite clocks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I believe the satellite clocks need to be synchronized with each other in the earth's frame of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is this achieved?&lt;/p&gt;&#xA;" OwnerUserId="13693" LastEditorUserId="6816" LastEditDate="2016-03-07T18:44:22.870" LastActivityDate="2016-03-07T23:01:12.877" Title="How are GPS satellite clocks synchronized with each other in the earth's frame of reference?" Tags="|artificial-satellite|communication|navigation|time|gps|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14373" PostTypeId="2" ParentId="14372" CreationDate="2016-03-07T15:47:23.990" Score="13" Body="&lt;p&gt;The satellites' location is very accurately determined using ranging techniques, where a pulse is sent from a ground station to the satellite and the satellite responds very quickly, with a known time between receive/transmit. This allows one to determine exactly how far away the satellite is. Combining this information with orbital tracks allows one to determine exactly where the satellite is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of time required to do a time update from the time the signal is sent to the time it is received and in use is also well known. What is actually sent to the satellite is the time deltas, to tell the clock to jump forward or backward a certain amount of time. For GPS, this update can occur every 8 hours, but in practice is done every 24 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellites each have onboard atomic clocks, for very accurately determining the time. Relativity is managed in essence by correcting for the clock based on the expected time. They &lt;a href=&quot;http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html&quot; rel=&quot;noreferrer&quot;&gt;slowed the clocks down to correct for this effect&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Signals are sent up to the satellite via the &lt;a href=&quot;http://www.navipedia.net/index.php/GPS_Ground_Segment&quot; rel=&quot;noreferrer&quot;&gt;GPS ground network&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time standard used for GPS clocks is a whole number of seconds from &lt;a href=&quot;https://en.wikipedia.org/wiki/Coordinated_Universal_Time&quot; rel=&quot;noreferrer&quot;&gt;UTC&lt;/a&gt;. GPS time does not account for leap seconds, thus the whole number of seconds time difference. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-03-07T23:01:12.877" LastActivityDate="2016-03-07T23:01:12.877" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="14374" PostTypeId="2" ParentId="14372" CreationDate="2016-03-07T16:14:41.903" Score="8" Body="&lt;p&gt;&lt;strong&gt;Receiver clock synchronisation:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GPS or GNSS applications are only dependend on the travel time of the signal. To put it simply: You only need the travel time of four GNSS satellites to calculate the position. Three satellites for determination of the position (x,y,z) and one for the receiver clock error. This clock error is the synchronisation between the receiver (e.g. car navigation) and the GNSS satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Satellite clock synchronisation:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite clocks are synchronised regularly by an associated ground station of the provider.&lt;br/&gt;&#xA;For example &lt;em&gt;Galileo:&lt;/em&gt; There is a ground station (Galileo Mission system) and there are several Sensor stations worldwide. &lt;br/&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The GMS ( Galileo Mission System ) will use the GSS ( Galileo Sensor&#xA;  Stations ) network in two independent ways. The first is the&#xA;  Orbitography Determination and Time Synchronisation (OD&amp;amp;TS) function,&#xA;  which will provide batch processing every ten minutes of all the&#xA;  observations of all satellites over an extended period and calculates&#xA;  the precise orbit and clock offset of each satellite, including a&#xA;  forecast of predicted variations (SISA - Signal-in-Space Accuracy)&#xA;  valid for the next hours. The results of these computations for each&#xA;  satellite will be up-loaded into that satellite nominally every 100&#xA;  minutes using a scheduled contact via a Mission Up-link Station&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.navipedia.net/index.php/Galileo_Ground_Segment#Galileo_Mission_System_.28GMS.29&quot;&gt;Stolen here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Best regards &#xA;Ben&lt;/p&gt;&#xA;" OwnerUserId="13695" LastActivityDate="2016-03-07T16:14:41.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14375" PostTypeId="1" CreationDate="2016-03-07T22:53:03.380" Score="0" ViewCount="186" Body="&lt;p&gt;Through this link : &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_probe&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, it is possible to get an idea of regions where space probes have been sent since the beginning of space exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But my question is, what are the bare bones essential parts that are compulsorily included in any space probe design, and secondly, how does the construction and operation of a space probe differ based on its intended journey.&lt;/p&gt;&#xA;" OwnerUserId="13699" LastActivityDate="2016-03-08T11:19:52.010" Title="Space probe design based on intended exploration trajectory?" Tags="|probe|design|mission-design|" AnswerCount="3" CommentCount="2" ClosedDate="2016-03-09T15:20:41.167" ContentLicense="CC BY-SA 3.0" />
  <row Id="14376" PostTypeId="2" ParentId="14375" CreationDate="2016-03-07T23:42:06.847" Score="2" Body="&lt;p&gt;Making probes are generally not like mass-production, it is more like crafting. Launch costs are high, and therefore minimizing the mass of the probe is important, even if it takes a lot of work. Therefore, probes are often very individually constructed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are some things probe must have:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A power supply&lt;/li&gt;&#xA;&lt;li&gt;Communication equipment&lt;/li&gt;&#xA;&lt;li&gt;A scientific payload (This one will naturally vary a lot)&lt;/li&gt;&#xA;&lt;li&gt;Propulsion (usually)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But even those can be achieved in different ways, like how solar panels are used in the inner solar system where there are enough sunlight, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow&quot;&gt;RTG&lt;/a&gt;'s where the Sun is too weak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a criterion, a probe has to send back data to Earth, in order to be usable. Thus, one of the few things probes must have in common is antennas.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-03-07T23:42:06.847" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14377" PostTypeId="2" ParentId="14375" CreationDate="2016-03-08T09:54:17.833" Score="0" Body="&lt;p&gt;In short there is nothing that is &lt;strong&gt;compulsory&lt;/strong&gt; beyond actually getting something with mass up there. The equipment is entirely dependant on the mission at hand. This must be made slightly more generic or we will have to start listing all the different types of missions that a space probe can be sent on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of things that are &lt;strong&gt;common&lt;/strong&gt; on a space craft however. @Hoffmanfan have already mentioned a few. I will add to this: the &quot;bus&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_bus&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Satellite_bus&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10493" LastActivityDate="2016-03-08T09:54:17.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14378" PostTypeId="2" ParentId="14375" CreationDate="2016-03-08T11:19:52.010" Score="2" Body="&lt;p&gt;There are a few general criteria that influence probe design:  &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Distance&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;How far from the Sun will the probe operate? This dictates:   &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;power &lt;/li&gt;&#xA;&lt;li&gt;thermal design&lt;/li&gt;&#xA;&lt;li&gt;radio design&lt;/li&gt;&#xA;&lt;li&gt;launcher size &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Anything out to Jupiter can be powered by solar panels, go further out and you'll need an RTG. Depending on this distance you may also need heaters to keep the interior of the spacecraft warm enough to function. Or if you go to the inner solar system, you need to think about removing excess heat. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The further away you go, the better your radio needs to be. This drives e.g. amplifier size, antenna size, receiver quality. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Radiation&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;A probe that orbits Jupiter or Saturn needs more radiation hardening than one that's destined for Mars or Pluto. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Launcher size&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;To get further out, you need a bigger launcher (and even then you may need gravity assists). The size of your launcher determines the maximum weight of your probe, which in turn drives all other aspects of the design. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Instruments&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The instruments are driven by the mission (what science do we want to do?) but are constrained by power, weight etc. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Bare bones&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;So, what do you really need?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Instruments,&lt;/li&gt;&#xA;&lt;li&gt;A radio,&lt;/li&gt;&#xA;&lt;li&gt;Enough power to run 1. and 2., so solar panels or an RTG,&lt;/li&gt;&#xA;&lt;li&gt;(usually) some maneuvering capability, so thrusters and/or reaction wheels,  &lt;/li&gt;&#xA;&lt;li&gt;A chassis that holds all of these.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-08T11:19:52.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14379" PostTypeId="1" CreationDate="2016-03-08T13:01:46.940" Score="16" ViewCount="1917" Body="&lt;p&gt;I have found this interesting report &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/Section403%28b%29CommercialMarketAssessmentReportFinal.pdf&quot;&gt;Commercial Market Assessment for Crew and Cargo Systems Pursuant to Section 403 of the NASA Authorization Act of 2010 (P.L. 111-267)&lt;/a&gt;, published in April 2011. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Appendix B there is this text:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the Falcon 9 analysis, NASA used NAFCOM to predict the development cost for the Falcon 9 launch vehicle using two methodologies:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;1) Cost to develop Falcon 9 using traditional NASA approach, and&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;2) Cost using a more commercial development approach.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Under methodology #1, the cost model predicted that the Falcon 9 would&#xA;  cost \$4 billion based on a traditional approach. Under methodology #2, NAFCOM predicted \$1.7 billion when the inputs were adjusted to a more commercial development approach. Thus, the predicted the cost to develop the Falcon 9 if done by NASA would have been between \$1.7 billion and \$4.0 billion.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;SpaceX has publicly indicated that the development cost for Falcon 9  launch vehicle was approximately \$300 million. Additionally, approximately \$90 million was spent developing the Falcon 1 launch vehicle which did contribute to some extent to the Falcon 9, for a total of \$390 million. NASA has verified these costs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;After this there is the key statement:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is difficult to determine exactly why the actual cost was so&#xA;  dramatically lower than the NAFCOM predictions. It could be any number&#xA;  of factors associated with the non-traditional public-private&#xA;  partnership under which the Falcon 9 was developed (e.g., fewer NASA&#xA;  processes, reduced oversight, and less overhead), or other factors not&#xA;  directly tied to the development approach. NASA is continuing to&#xA;  refine this analysis to better understand the differences.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And to be fair, it finishes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Regardless of the specific factors, this analysis does indicate the&#xA;  potential for reducing space hardware development costs, given the&#xA;  appropriate conditions. It is these conditions that NASA hopes to&#xA;  replicate, to the extent appropriate and feasible, in the development&#xA;  of commercial crew transportation systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The question:&lt;/strong&gt; &#xA;Has NASA published any reports, or updates, recently saying that they (hopefully) understand better the apparent problems of their cost models and how they have adjusted their models for the future?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To be clear&lt;/strong&gt;, I'm not looking for SE member's opinions of why their model over-predicted the Falcon 9 development cost.  Nor am I looking for any entertaining display of wit regarding well-worn NASA jibes, I can go elsewhere to find that easily. What I'm looking for is published output that says, from NASA, what they've learned.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10686" LastEditDate="2016-03-08T15:50:36.030" LastActivityDate="2023-12-14T19:41:51.227" Title="NASA cost estimating and Falcon 9" Tags="|launch|spacex|nasa|falcon-9|cost-to-orbit|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14381" PostTypeId="2" ParentId="14379" CreationDate="2016-03-08T19:17:44.867" Score="17" Body="&lt;p&gt;I did not find any report published by NASA, but I found a quite interesting paper of the &lt;a href=&quot;https://www.spacesymposium.org/sites/default/files/downloads/tt_papers/G.Oleson_31st_Space_Symposium_Tech_Track_paper.pdf&quot;&gt;31st Space Symposium.&lt;/a&gt;&lt;br&gt;&lt;br&gt;&#xA;&lt;strong&gt;Short summary / most important facts if you do not read the paper&lt;/strong&gt;: &lt;br&gt;&#xA;For cost estimation of space systems mainly three modells are used:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The Parametric method (e.g. NAFCOM),&lt;/li&gt;&#xA;&lt;li&gt;the Analogy method&lt;/li&gt;&#xA;&lt;li&gt;and the Engineering (Bottom-Up) method.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;All methods depend on the&#xA;  knowledge obtained through previous program experience. This is not unlike any type of manufacturing industry.&#xA;  The spacecraft industry, however, can experience significant technological advancements that render the&#xA;  dependence on historical programs somewhat useless.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is why SpaceX has not used one of these conventional methods.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Elon Musk and his&#xA;  team at SpaceX apparently rejected both analogy and parametric methods to estimate the cost of the Falcon&#xA;  launch vehicles. They determined that no prior launch vehicles were sufficiently similar for either method to work.&#xA;  Instead the SpaceX team used a bottom-up approach from the beginning, improving the estimates as engineering&#xA;  data accumulated and became more precise.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So what can the goverment agencies (e.g. &lt;strong&gt;NASA&lt;/strong&gt;, ESA) do?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Government agencies are often handicapped in their use of engineering estimates early in the acquisition&#xA;  process both by a frequent intention to develop and use cutting edge technologies and by inadequate access to&#xA;  the needed component-level data. This puts the government in a difficult position as rapid innovation reduces the&#xA;  utility of parametric models. The government needs a third way.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Another possibility is for government agencies to gain access to engineering cost data or estimates based on&#xA;  engineering costs by the simple expedient of paying for them. Vendors could be contracted to produce&#xA;  engineering cost estimates on their own projects in advance of new procurements, while showing their work to&#xA;  the customer. In any case, government agencies will need to be as nimble and innovative as their vendors if they&#xA;  are to keep up with the new pace of development.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Although it is not published by NASA, I hope it helped.&lt;/p&gt;&#xA;" OwnerUserId="13695" LastEditorUserId="13695" LastEditDate="2016-03-08T20:57:31.287" LastActivityDate="2016-03-08T20:57:31.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14382" PostTypeId="2" ParentId="3565" CreationDate="2016-03-08T23:32:27.180" Score="5" Body="&lt;p&gt;Other than Lagrange points (since you asked &quot;such as Lagrange points&quot;), another option would be to enter a retrograde orbit around a moon with the same period as the prograde orbital period round its planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the Earth/Moon system; go &quot;backwards&quot; round the Moon and take 27&amp;#8531; days doing it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can work out the distance your satellite would be from the Moon with this handy equation:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/B15kK.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/B15kK.png&quot; alt=&quot;From https://en.wikipedia.org/wiki/Orbital_period&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;G = 6.67 x 10&lt;sup&gt;-11&lt;/sup&gt; Nm&lt;sup&gt;2&lt;/sup&gt;kg&lt;sup&gt;2&lt;/sup&gt;&lt;/li&gt;&#xA;&lt;li&gt;M&lt;sub&gt;L&lt;/sub&gt; = 7.34 x 10&lt;sup&gt;22&lt;/sup&gt; kg&lt;/li&gt;&#xA;&lt;li&gt;T = 2.36 x 10&lt;sup&gt;6&lt;/sup&gt; s (27&amp;#8531; days)&lt;/li&gt;&#xA;&lt;li&gt;a = distance (m)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Turns out you need to be 88,436 km away. This will be in shade as we know the umbra extends to the Earth (384,400 km) during a total eclipse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure how stable this would be when it passes between the Moon and Earth. It may be stable enough for long enough for the mission and it's less of a trek than Mars L2 ... use the fuel saved to keep it in orbit. The Earth/Moon neutral point (where the gravity of each cancels out) is 70,000 km from the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or course, there may be other systems where the numbers work out better ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example Phobos:&#xA; - G = 6.67 x 10&lt;sup&gt;-11&lt;/sup&gt; Nm&lt;sup&gt;2&lt;/sup&gt;kg&lt;sup&gt;2&lt;/sup&gt;&#xA; - M&lt;sub&gt;Ph&lt;/sub&gt; = 1.066 x 10&lt;sup&gt;16&lt;/sup&gt; kg&#xA; - T = 27,554s s (7 h 39.2 min)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This works out at 23.9 km ... better, but Phobos' gravity is so weak, I'm sure it'd only be stable for &amp;frac34; of a month. For Earth's moon, that's a 3 week total eclipse which some astronomers would get excited about.&lt;/p&gt;&#xA;" OwnerUserId="13712" LastEditorUserId="-1" LastEditDate="2016-03-10T00:10:14.920" LastActivityDate="2016-03-10T00:10:14.920" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14383" PostTypeId="1" AcceptedAnswerId="17576" CreationDate="2016-03-09T07:45:39.930" Score="118" ViewCount="63205" Body="&lt;p&gt;&lt;strong&gt;hint:&lt;/strong&gt; &lt;em&gt;Apparently the Tsiolkovsky rocket equation&lt;/em&gt; does not actually say &lt;em&gt;that you can launch a conventional rocket into orbit around an arbitrarily large and massive body.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for a number based on scaling the earth radius and maintaining the same average density. Must attain LEO, &lt;em&gt;which also gets faster&lt;/em&gt; as the planet grows. Don Pettit's &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;noreferrer&quot;&gt;Tyranny&lt;/a&gt; mentioned in &lt;a href=&quot;https://space.stackexchange.com/a/17426/12102&quot;&gt;this nice answer&lt;/a&gt; is fun, but does not present enough math.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On &lt;em&gt;this&lt;/em&gt; Earth, rockets barely work. Payloads can only be a few percent of the total mass for &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;LEO&lt;/a&gt;, and less than one percent for deep space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we define slightly heavier Earths, say Earth&lt;sub&gt;1.1&lt;/sub&gt;, Earth&lt;sub&gt;1.2&lt;/sub&gt;... where the radii were 1.1, 1.2, etc. times that of Earth and the masses were 1.1&lt;sup&gt;3&lt;/sup&gt;, 1.2&lt;sup&gt;3&lt;/sup&gt;, etc. times the Earth's mass (in other words same average density, same &quot;iron/rock ratio&quot;) what happens? Is there some point where chemical rockets simply will no longer be able to put things in space, or does the payload mass simply become ridiculously tiny? If there is a cut-off, is it different for LEO and deep space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For our purposes, let's not explore alternative or hybrid launch systems or boost systems (such as balloons, planes, laser beams, space elevators etc.). Just stick to chemical propellant rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; here is a guide. So for a scaling factor &lt;span class=&quot;math-container&quot;&gt;$f$&lt;/span&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;r = f r_{earth}&#xA;$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;m = f^3 m_{earth}&#xA;$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;g = G \frac{m}{r^2} = \frac{f^3}{f^2}g_{earth} = f g_{earth}&#xA;$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;H = \frac{kT}{gm_{molecule}} = f^{-1}H_{earth}&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We catch a little break here. Assuming same surface atmosphere composition, temperature and pressure (STP), the &lt;a href=&quot;http://spacemath.gsfc.nasa.gov/astrob/7Page15.pdf&quot; rel=&quot;noreferrer&quot;&gt;scale height&lt;/a&gt; H actually &lt;em&gt;decreases&lt;/em&gt; with increasing &lt;span class=&quot;math-container&quot;&gt;$f$&lt;/span&gt;. (If we were &quot;world builders&quot; we should probably increase pressure to get more oxygen needed for moving in the higher gravity, but that's &lt;a href=&quot;https://worldbuilding.stackexchange.com/&quot;&gt;a different Stack Exchange&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as LEO &lt;strong&gt;altitude&lt;/strong&gt; is concerned (thanks @Lex for &lt;a href=&quot;https://space.stackexchange.com/questions/14383/how-much-bigger-could-earth-be-before-rockets-wouldt-work?noredirect=1#comment73460_14383&quot;&gt;catching&lt;/a&gt; that) one might define it as the same number of scale heights as would be on Earth. That's not really so useful because the density profiles of the bits of the atmosphere responsible for drag (&lt;a href=&quot;https://en.wikipedia.org/wiki/Thermosphere&quot; rel=&quot;noreferrer&quot;&gt;Thermosphere&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Exosphere&quot; rel=&quot;noreferrer&quot;&gt;Exosphere&lt;/a&gt; are affected by many phenomenon, including the solar wind, and don't scale at all like the lower layers. Nonetheless for historical reasons I'll leave the following, as it is not essential to the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;h_{LEO} = h_{LEOearth} \frac{H}{H_{earth}} = f^{-1} h_{LEOearth}&#xA;$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;v_{LEO}=fv_{LEOearth}&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LEO period is independent of the size of a planet, if the average density is fixed. However, the velocity of LEO &lt;em&gt;does&lt;/em&gt; scale with radius!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-05-08T10:10:27.157" LastActivityDate="2021-04-06T08:30:22.110" Title="How much bigger could Earth be, before rockets wouldn't work?" Tags="|launch|rockets|payload|escape-velocity|performance|" AnswerCount="6" CommentCount="31" ContentLicense="CC BY-SA 4.0" />
  <row Id="14384" PostTypeId="2" ParentId="14383" CreationDate="2016-03-09T08:48:55.863" Score="32" Body="&lt;p&gt;First, let us look at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;noreferrer&quot;&gt;rocket equation&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\Delta v=\ln \left(\frac{m_0}{m_f}\right)v_e$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;That tells how much a rocket can change its velocity (the &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;). The requirements for reaching a higher velocity for a minimal orbit would increase on your heavier Earth. (For constant density it is proportional to the radius.)&lt;/p&gt;&#xA;&lt;p&gt;How can we increase the &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; of the rocket to keep up? We can increase the exhaust velocity, &lt;span class=&quot;math-container&quot;&gt;$v_e$&lt;/span&gt;, of the engine, but that cut-off is around 5000 m/s for chemical engines. The other thing we could do is increasing the mass ratio of the rocket &lt;span class=&quot;math-container&quot;&gt;$\left(\frac{m_0}{m_f}\right)$&lt;/span&gt;. That is problematic too, as we can not really make the fuel tanks out of soap bubbles. Staging is the option left, you could place a big rocket under a small rocket to get a little more change in velocity. Then you are getting a linear benefit for an exponential expense.&lt;/p&gt;&#xA;&lt;p&gt;As an example, the Saturn V rocket got into LEO (~9000 m/s), sent a payload towards the Moon (3120 m/s), the service module slowed the stack into LMO (820 m/s), and finally the LM landed and took off again (2*1720 m/s). There are still some unused fuel left in the service module then, so let us just call the total &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; of the Saturn V/Apollo 17 km/s. That is less than the requirements for a 2x radius Earth. The Apollo program was pretty expensive [citation needed], so it may take a while before a nation of a 2x Earth world attempts to go into orbit. The limit is, as you state, the ridiculously low payload ratio.&lt;/p&gt;&#xA;&lt;p&gt;Another consideration is the increased surface gravity. (That scales linearly with diameter at constant density). That requires the rocket to have a higher thrust to weight ratio, and that will increase the dry mass, reducing the possible &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;. (It also increases gravity losses, but that is mostly compensated by the lower scale height of the planet, reducing drag losses).&lt;/p&gt;&#xA;&lt;p&gt;Eventually, the gravity is so high that even the most powerful engine can not lift itself from the ground. That at least is a definitive limit.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;A more theoretical consideration, is &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; requirements actually a finite limit?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Surprisingly, it is not. Remember what I said about staging earlier: &amp;quot;you are getting a linear benefit for an exponential expense&amp;quot;. But there is not limit to what we can expend! Consider the following scenario: We add more and more stages at the bottom of the rocket, each of them has the same mass as all the stages on top of it. Then burning each of them gives the same mass ratio between before and after, therefore each of them are supplying&#xA;the same amount of &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;. To add 10 times that amount, you need 10 stages each doubling the mass. To add 100 times that amount, you need to double a hundred times. The mass grows ridiculously fast, even doubling 10 times are over a thousand times more. But why should we stop :)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;But can we &lt;em&gt;really&lt;/em&gt; continue to add exponentially larger stages for ever?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;After a while, other problems show up. For instance: Rockets are long and thin, to minimize drag. That shape can not be kept for very large rockets. The reason not is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Square-cube_law&quot; rel=&quot;noreferrer&quot;&gt;square cube law&lt;/a&gt;. Conserving the same dimensional proportions, a rocket twice the height has 8 times more mass. But the base area of the rocket has only increased 4 times. That means that each unit of area has to support more mass. Sooner or later, even the strongest materials must give up, and you must give up the traditional rocket shape in favour of a wider base. That adds a lot to the drag! Problems like that are going to continue to show up:&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;More mass means more problems, exponentially more mass means exponentially more problems.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Summarized:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;A modern design, larger rocket than the Saturn V, with modifications to increase the T/W ratio could probably make it to orbit on a 2x radius, 8x mass Earth. That is a feasibility limit, rockets that are ridiculously much larger may have a few km/s extra &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;, but that does not alter the numbers a lot. In theory though, rockets can grow until the drag stops them, or the engines can no longer lift even themselves.&lt;/p&gt;&#xA;&lt;p&gt;Or perhaps you at some point want to use the available resources of the planet to launch a single rocket to orbit.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="40073" LastEditDate="2021-04-06T08:30:22.110" LastActivityDate="2021-04-06T08:30:22.110" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="14385" PostTypeId="2" ParentId="14247" CreationDate="2016-03-09T16:39:59.197" Score="3" Body="&lt;p&gt;The CSM computer could not use the LM RCS and vice versa. The different control modes are for the RCS DAP filter gains, and the Thrust Vector Control. Additionally to the control modes (CSM, CSM/LM, CSM/LM ascent stage or off) the computer had to be given information about the CSM and LM mass. Then, depending on the vehicle configuration, the correct moments of inertia were calculated and used for the RCS or TVC Digital Autopilot. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So on Apollo 13 they indeed had to maneuver around the stack with full SPS propellant tanks, just with the LM RCS engines. If you are interested, here the document describing the three DAPs (CSM RCS, CSM TVC and CM RCS) of the Command Module computer: &lt;a href=&quot;http://www.ibiblio.org/apollo/Documents/HSI-208472.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.ibiblio.org/apollo/Documents/HSI-208472.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13716" LastActivityDate="2016-03-09T16:39:59.197" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14386" PostTypeId="1" AcceptedAnswerId="14389" CreationDate="2016-03-09T19:12:22.107" Score="-1" ViewCount="408" Body="&lt;p&gt;Fed up with clouds ruining clear nights here on earth, I point my car upwards, and head for the moon. My car can provide the same amount of relative thrust it can going 65 mph, simply in whatever direction I choose. I can burn continuously at this rate. This is going to take awhile. Thankfully, I brought plenty of snacks and gas, so fuel is not a concern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hoping to get there as fast as humanly possible, without ignoring gravity, what is going to be my best lunar transfer orbit? Will a Hohmann serve best, or will my low thrust mean another orbit will be better? What is my Odometer going to show upon arrival?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit 1: The car begins in low earth orbit. Very low earth orbit, simply ignoring the atmosphere, so distance doesn't change, but escaping the earth and entering an initial orbit is no longer an issue.&lt;/p&gt;&#xA;" OwnerUserId="13720" LastEditorUserId="8693" LastEditDate="2016-03-10T09:11:38.913" LastActivityDate="2016-03-10T09:11:38.913" Title="I'm planning a roadtrip to the moon. What is going to be my best transfer orbit? How far will it be?" Tags="|orbital-mechanics|crewed-spaceflight|orbital-maneuver|trajectory|hohmann-transfer|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14388" PostTypeId="2" ParentId="5774" CreationDate="2016-03-09T21:15:47.093" Score="5" Body="&lt;p&gt;From what I've read, the main goal is to learn how to operate a long term base that far from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon's main advantage in that is that it's much closer to Earth. You can have real time communications with the Moon base and provide material support fairly quickly if something goes wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That also makes it much more convenient for science. Not having any atmosphere makes the Moon perfect for optical telescopes and the far side being shielded from RF interference from Earth is ideal for radio telescopes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this is not directly related to Mars, that extra value makes the Moon much more attractive destination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, some of the technology will be transferable - habitats, life support systems, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further down the road, the Moon can be mined and fuel produced that can then be deposited in LEO for in orbit refueling thus significantly lowering the cost of spaceflight.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-03-09T21:36:21.777" LastActivityDate="2016-03-09T21:36:21.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14389" PostTypeId="2" ParentId="14386" CreationDate="2016-03-09T21:26:02.690" Score="6" Body="&lt;p&gt;Car accelerations vary a lot, but from &lt;a href=&quot;http://www.zeroto60times.com/vehicle-make/toyota-0-60-mph-times/&quot;&gt;this table&lt;/a&gt;, 3 m/s² seems like a safe bet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hohmann transfer burn towards the Moon from LEO is around 3100 m/s, so the &quot;burn&quot; is only going to take about 17 minutes. That is quick enough for a transfer orbit burn, so a low acceleration spiral is not required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the odometer, that is going to display half the perimeter of the transfer ellipse, that is around 400000 km depending on where in its orbit the Moon is.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-09T22:27:48.470" LastActivityDate="2016-03-09T22:27:48.470" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14390" PostTypeId="1" CreationDate="2016-03-09T22:08:19.473" Score="4" ViewCount="141" Body="&lt;p&gt;In the past I (think) I've read of artificial satellites which are designed to go into orbit quickly to deal with a short term need for observation of some type.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm now struggling to find a reference to this idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure whether what I read was referring to the past or a system which is currently in place ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I seem to remember that the idea was that it would be launched within days from the need occurring and wouldn't be intended to be used for longer than weeks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also have an idea that the orbit might be an unconventional one in order to provide optimal coverage of the area of interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming I haven't imagined this - can anyone point me at examples of such systems ?&lt;/p&gt;&#xA;" OwnerUserId="13722" LastActivityDate="2016-03-10T12:59:54.767" Title="Examples of short satellite durations?" Tags="|artificial-satellite|observation|earth-observation|military|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14391" PostTypeId="1" CreationDate="2016-03-09T23:33:44.720" Score="15" ViewCount="1204" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Barycenter&quot; rel=&quot;noreferrer&quot;&gt;barycenter&lt;/a&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto&quot; rel=&quot;noreferrer&quot;&gt;Pluto&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Charon_(moon)#Orbit&quot; rel=&quot;noreferrer&quot;&gt;Charon&lt;/a&gt; is in space between the two bodies, close to Pluto. It is the center point of the animation below. The image is roughly to scale. Note that the two bodies are mutually tidally locked, with the same sides always facing each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V3CQE.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V3CQE.gif&quot; alt=&quot;animation of orbits of Pluto and Charon&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Charon is 17,536 km from the system barycenter, and 19571 km from the center of Pluto. Its surface gravity is 0.288 m/s&lt;sup&gt;2&lt;/sup&gt;. It has a mean radius of 606 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto has a surface gravity of 0.62 m/s&lt;sup&gt;2&lt;/sup&gt; and a mean radius of 1,187 km. &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Pluto#Pressure&quot; rel=&quot;noreferrer&quot;&gt;The atmosphere of Pluto&lt;/a&gt;, though very thin, is also an issue over time. At the surface it is 1 Pa, 0.001% the pressure of Earth's atmosphere (right now, while it is close to the sun). Above the bottom layer of the atmosphere, its scale height is estimated at 50 to 60 km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be difficult to put an orbiter in a stable orbit around Pluto? Or around Charon? What kind of station-keeping might be involved?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-03-11T02:55:14.333" LastActivityDate="2021-04-23T02:13:02.057" Title="If we could send an orbiter to the Pluto-Charon system, could we put it in a stable orbit?" Tags="|orbital-mechanics|probe|pluto|" AnswerCount="2" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="14392" PostTypeId="1" CreationDate="2016-03-10T11:39:34.123" Score="0" ViewCount="200" Body="&lt;p&gt;I want to write a young adults fiction book about a girl who buys an astronauts helmet from EBay and decides to see if she can then build a rocket to get to the moon. I'm a materials engineer, and with aircraft salvage yards and other resources I would think it is (just) within the realms of possibility if not probability, but obviously there are massive risk/barriers/etc. But, hypothetically, if you were to try it, what would you need. What would be the biggest hurdles, how you be creative to get around them.&lt;/p&gt;&#xA;" OwnerUserId="13724" LastActivityDate="2016-03-10T17:18:32.347" Title="Is it possible to build a rocket from EBay" Tags="|launch|crewed-spaceflight|" AnswerCount="2" CommentCount="6" ClosedDate="2016-03-10T15:18:27.980" ContentLicense="CC BY-SA 3.0" />
  <row Id="14393" PostTypeId="2" ParentId="14392" CreationDate="2016-03-10T11:56:08.760" Score="1" Body="&lt;p&gt;Sadly not, the time is against you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It took the labour of 200,000 people several years to accomplish the Apollo lunar landing. If you were to do that all by yourself, it would take you pretty close to 1 million years. Even if you could live for that long, the part of the rocket you started with would have rusted away long ago when you are finished.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As people have pointed out in the comments, the Apollo program was far from bare bones, and a lot of those person-years was used on research. But even if you could cut down the time 500x, the argument still holds.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-10T17:18:32.347" LastActivityDate="2016-03-10T17:18:32.347" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14394" PostTypeId="2" ParentId="14391" CreationDate="2016-03-10T12:56:33.980" Score="6" Body="&lt;p&gt;Let us try to see what happens if we place the satellite in an orbit with and altitude of about 320 km over Pluto, so we get above most of the atmosphere, and get a nice round number for the orbital radius at 1500 km. Let us also normalize Pluto's gravitational acceleration on the satellite to &quot;1&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, when closest to Charon, the satellite is 18000 km away from it, making its gravitational influence $\frac{1}{1180}$ of Pluto's, dragging in the opposite direction. When farthest from Charon, its gravitational influence is down to $\frac{1}{1610}$, dragging in the same direction as Pluto. We then have a total gravitational influence in the range 0.99915 to 1.00062.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;That does not say a lot about stability, but:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let us compare that to a system we are more familiar with. What orbital radius has a satellite in the Earth-Moon system with a similarly large range?&#xA;That happens to be at around 87000 km. That is about twice as high as GEO. Given the very low impact of the Moon on GEO satellites, I would expect that to be pretty stable. Same so for the low Pluto orbit.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-03-10T12:56:33.980" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14395" PostTypeId="2" ParentId="14390" CreationDate="2016-03-10T12:59:54.767" Score="2" Body="&lt;p&gt;These are almost certainly for military applications, as they are the only ones that require short turn around and short term usage. This has been a dream of military for some time. Most of this research is spearheaded by DARPA. DARPA tests stuff like &lt;a href=&quot;http://www.space.com/28504-darpa-satellite-launching-jet-alasa-xs-1.html&quot; rel=&quot;nofollow&quot;&gt;launching satellites from a fighter jet&lt;/a&gt;, &lt;a href=&quot;http://www.pcworld.com/video/47829/how-darpa-is-planning-fast-cheap-satellite-launches&quot; rel=&quot;nofollow&quot;&gt;$1 million satellites&lt;/a&gt;, and other related technology. Practically speaking, this isn't really being done at this time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-03-10T12:59:54.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14396" PostTypeId="2" ParentId="14392" CreationDate="2016-03-10T14:55:26.433" Score="0" Body="&lt;p&gt;It's not possible for a single person to make a moon mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo program is estimated to have taken somewhere around 15.5 billion man hours, even though some of the problems would be easier to solve now it would still take hundreds of millions of man hours (at least) to do it again. It takes engines providing millions of pounds of thrust, 7 million pounds of fuel and oxidizer, and vast amounts of specialized knowledge in hundreds of fields to make something like that work. None of that's on eBay. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-03-10T14:55:26.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14398" PostTypeId="1" AcceptedAnswerId="14399" CreationDate="2016-03-10T17:56:12.927" Score="6" ViewCount="152" Body="&lt;p&gt;A few days ago, NASA announced that, due to a vacuum leak in one of the instruments, they would have to delay InSight's launch for about two years.  This is longer than it would take to fix the instrument, but they have to wait for Mars's orbit to line up with Earth's again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering if anybody knew if NASA has any plans at this point to tinker with the craft's payload or design, since they now have two more years to work on it.  I have not seen any news articles saying this, but I doubt that they would give up this extra time.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-03-10T22:47:02.187" Title="Since NASA's InSight Lander's launch has been delayed by two years, are there any plans to edit its payload?" Tags="|mars|lander|payload|insight|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14399" PostTypeId="2" ParentId="14398" CreationDate="2016-03-10T20:15:38.970" Score="4" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/press-release/nasa-targets-may-2018-launch-of-mars-insight-mission&quot; rel=&quot;noreferrer&quot;&gt;The NASA press&lt;/a&gt; release contains the details. Specifically, they state:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, will redesign, build and conduct qualifications of the new vacuum enclosure for the Seismic Experiment for Interior Structure (SEIS), the component that failed in December.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Aside from that, no changes are planned, at least, that have publicly been announced. I suspect they will do more testing, fix a few minor issues that they would have flown with, maybe do some software updates, but overall that they won't do any major hardware changes. 2 years just isn't long enough to do that effectively.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-03-10T22:47:02.187" LastActivityDate="2016-03-10T22:47:02.187" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14400" PostTypeId="1" AcceptedAnswerId="14402" CreationDate="2016-03-11T02:54:22.587" Score="6" ViewCount="498" Body="&lt;p&gt;I was recently reading a very good sci-fi story recounting, in loving detail and with a good deal of accuracy, the first space program of an alien race, with strong (intentional) similarities to the American and Soviet endeavors. One thing caught my attention, though: a capsule in distress vented extra $O_2$ in order to spin-stabilize itself in preparation for extremely-manual retro-rocket activation (which was expected to be much less precise than normal, as the retro-rockets had to be activated with a jury-rigged flare gun). But as far as I could tell, the $O_2$ system had not been designed for the purpose, so presumably had only a single vent (rather than two deliberately placed to create spinning, which would be very strange, or two placed to cancel thrust, which would not have been useful). Would a continuous vent over some time actually create a proper spin and create a stable attitude, or would it instead just spiral outward and become uncontrollable?&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2016-03-11T04:36:37.973" LastActivityDate="2020-12-31T23:13:21.270" Title="Does a single off-axis thruster firing continuously eventually stabilize attitude gyroscopically?" Tags="|attitude|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14402" PostTypeId="2" ParentId="14400" CreationDate="2016-03-11T04:05:11.693" Score="6" Body="&lt;p&gt;The short answer is &amp;quot;maybe, if they were spectacularly lucky, but it's a story, so that's OK.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Longer answer:&lt;/p&gt;&#xA;&lt;p&gt;There is quite a lot buried in the question in terms of assumptions, but here's a go.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Just for a vent, if the thrust direction is radially out from the centre of mass then there will be no torque and no spin up. It would need to be off radial.  Details: it would be most efficient if tangential.&lt;/li&gt;&#xA;&lt;li&gt;I presume what you mean by &amp;quot;rather than two deliberately placed…&amp;quot; is that if you have two vents you could arrange them so that you get a &amp;quot;pure&amp;quot; torque with no vehicle translation. If there is just one vent then there will be some translation regardless of whether there is some torque as per the first point. If there is nothing to crash into nearby then it's probably not a problem.&lt;/li&gt;&#xA;&lt;li&gt;The point of spinning for stability during a rocket motor firing is so that the thrust vector is in a known, deliberate direction and doesn't drift because of minor thrust misalignments or an initial attitude drift rate from another cause such as asymmetric drag. The significance being:&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You have to know (i.e. measure) which direction you want to fire the retro-rocket in the first place.&lt;/li&gt;&#xA;&lt;li&gt;You have to be able to point the spin axis along the direction of the intended retro-rocket firing to better than the plausible thrust misalignment. That could mean a reaction wheel (which could give you your stabilisation spin anyway), another thruster, or another vent in the right place and a means of confirming (measuring) the achieved direction of the spin axis.&lt;/li&gt;&#xA;&lt;li&gt;The degree of spin-up has to be meaningful for the size of retro-rocket, potential misalignment and the amount of drift that could otherwise take place, plus drift from initial rates. This could be quite uncomfortable if crewed (think 60rpm).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;Objects don't automatically become stable when they spin. A solid flat spinning-top would, but anything with a different shape (e.g. pencil like) and some sort of energy damping from appendages, liquid slosh etc could actually become less stable, or rather transfer its spin axis away from the desired one.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;I have a dim recollection from college (a long time ago) of being taught that an early US unmanned mission actually suffered from precisely the last problem because it had whip-aerials and it was not foreseen that this would cause the pencil-shaped upper stage to become unstable.&lt;/p&gt;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&lt;p&gt;See SF and Uhoh's answers for one extra constraint I missed, viz. relationship of the created spin to the axis along which the motor is mounted. My points above about measuring angles etc addressed the also vital but distinct concept of the orientation of the spin to the desired thrust vector in inertial space.&lt;/p&gt;&#xA;&lt;p&gt;EDIT 2 (2020):&lt;/p&gt;&#xA;&lt;p&gt;Here is the related answer to a question on spin stabilisation - the one that I refer to above regarding &amp;quot;the unmanned US mission&amp;quot; &lt;a href=&quot;https://space.stackexchange.com/questions/20754/why-does-spinning-help-stabilisation/24105#24105&quot;&gt;Why does spinning help stabilisation - Explorer 1&lt;/a&gt;    I wrote that in 2017 long after I'd forgotten that I wrote this answer (originally in 2016).&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2020-12-31T22:57:35.790" LastActivityDate="2020-12-31T22:57:35.790" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="14403" PostTypeId="2" ParentId="14400" CreationDate="2016-03-11T13:29:33.290" Score="4" Body="&lt;p&gt;Off-axis yes, but in-plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust vector must be in the plane perpendicular to the spin axis and passing through the center of gravity of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And of course oriented opposite to the spin direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this picture, the spin axis goes lengthwise through the rocket, so a thruster located at the height of the center of mass of the rocket will work. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/jV3vv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jV3vv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't need to be tangent to the rocket surface (though it will be most efficient if it is), but if the thrust vector is either pointing off the plane surface, or not attached to the plane surface, the rocket, while losing its original spin, will begin entering a new spin, with its plane of rotation passing through the center of mass and containing the thrust vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it will introduce some translational motion too, but it will be quite minimal comparing to what you normally face with orbital motion - nothing to worry about. That would be an issue when docking, but with one thruster you're definitely not docking on your own anyway.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-03-11T13:29:33.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14404" PostTypeId="1" AcceptedAnswerId="14412" CreationDate="2016-03-11T14:37:31.050" Score="16" ViewCount="2977" Body="&lt;p&gt;In case of a very specific rotation, even &lt;a href=&quot;https://space.stackexchange.com/questions/14400/does-a-single-off-axis-thruster-firing-continuously-eventually-stabilize-attitud&quot;&gt;one&lt;/a&gt; suffices, but it must be located just right for that specific axis of rotation, and if you want to rotate the satellite, it can do so in that specific axis only, and one direction only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, first, assumptions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;RCS is the only method of rotational stabilization. If there are any gyroscopes, magnetorquers etc, we don't take them into account.&lt;/li&gt;&#xA;&lt;li&gt;we don't care about translation. &lt;/li&gt;&#xA;&lt;li&gt;the thrusters are not gimballed, vectorable or anything like that - each provides thrust in one direction only. &lt;/li&gt;&#xA;&lt;li&gt;but the moment and power at which they are fired can be very precisely controlled. No negative thrust though, that requires a second thruster.&lt;/li&gt;&#xA;&lt;li&gt;we also have a good information about the attitude changes from the sensors, but the spin/tumble we have entered is arbitrary (within reason, not destructive).&lt;/li&gt;&#xA;&lt;li&gt;for simplicity let's assume a spherical satellite with all mass concentrated in the center, massless thrusters, they may be installed on (massless) trusses extending from the satellite, if need be. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One solution I came up while thinking about this problem requires 5 thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wCp2N.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wCp2N.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cluster of three thrusters can stop any spin/tumble except one going straight through their axis of symmetry (the blue axis). This can be negated by the two thrusters on the &quot;equator&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a strong suspicion this can be done with four thrusters though - for example, offsetting the three away from the &quot;pole&quot;, that they create a &quot;turbine&quot; pattern, always creating a &quot;polar&quot; spin next to their normal activity, and remove one of the &quot;equatorial&quot; ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder if it can be done with even less, or what other layouts would be beneficial.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2022-12-03T17:18:01.760" Title="What is the minimum number of RCS thrusters capable of stabilizing a satellite against an arbitrary rotation?" Tags="|attitude|rcs|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14406" PostTypeId="1" AcceptedAnswerId="14428" CreationDate="2016-03-11T15:02:32.540" Score="29" ViewCount="5297" Body="&lt;p&gt;Why was radio contact lost with the Pioneer probes much earlier than with the Voyager probes? The Voyagers were launched only 4 years after the Pioneers, yet the Voyagers are projected to last into the 2020s, while the Pioneers stopped transmitting in 1995 and 2003 respectively. Both were powered by RTGs as well.&lt;/p&gt;&#xA;" OwnerUserId="13732" LastActivityDate="2016-03-14T21:15:51.047" Title="Why was radio contact with Pioneer lost earlier than with Voyager?" Tags="|voyager|pioneer|rtg|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14407" PostTypeId="2" ParentId="14406" CreationDate="2016-03-11T15:11:34.420" Score="37" Body="&lt;p&gt;Interplanetary communication is mainly dependent on signal strength (for transmission) and antenna size (for reception). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Pioneers use a 9-foot antenna and an 8-watt transmitter. The Voyagers use a 12-foot antenna and a 20-watt transmitter, allowing a substantially stronger signal to be received on Earth. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-03-11T17:27:14.153" LastActivityDate="2016-03-11T17:27:14.153" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14408" PostTypeId="2" ParentId="14406" CreationDate="2016-03-11T15:23:21.890" Score="28" Body="&lt;p&gt;In addition to a better transmitter, the Voyagers have better power reserves: their RTGs supplied &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1#Power&quot;&gt;470 W&lt;/a&gt; at launch, while the &lt;a href=&quot;https://www.nasa.gov/centers/ames/missions/archive/pioneer.html&quot;&gt;Pioneer RTGs&lt;/a&gt; supplied 160 W at launch. So the Voyager RTGs will take much longer to decay to a point where they can't power the spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA &lt;a href=&quot;http://www.nasa.gov/centers/ames/missions/archive/pioneer.html&quot;&gt;seems to think&lt;/a&gt; RTG decay is the primary reason we can't receive Pioneer 10 any more: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Pioneer 10 made valuable scientific investigations in the outer regions of our solar system until the end of its science mission on 31 March 1997. The Pioneer 10 weak signal continued to be tracked by the DSN as part of an advanced concept study of communication technology in support of NASA's future interstellar probe mission. The power source on Pioneer 10 finally degraded to the point where the signal to Earth dropped below the threshold for detection in 2003. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We're not sure about &lt;a href=&quot;http://www.nasa.gov/centers/ames/missions/archive/pioneer.html&quot;&gt;Pioneer 11&lt;/a&gt;'s fate:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Pioneer 11 Mission ended on 30 September 1995, when the last transmission from the spacecraft was received. There have been no communications with Pioneer 11 since. The Earth's motion has carried it out of the view of the spacecraft antenna. The spacecraft cannot be maneuvered to point back at the Earth. It is not known whether the spacecraft is still transmitting a signal. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-03-11T18:50:46.433" LastActivityDate="2016-03-11T18:50:46.433" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14411" PostTypeId="1" AcceptedAnswerId="14415" CreationDate="2016-03-11T21:54:42.493" Score="9" ViewCount="1273" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.nasa.gov/centers/ames/missions/archive/pioneer.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Pioneer Missions&lt;/a&gt;, my emphasis,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Pioneer 11 Mission ended on 30 September 1995, when the last transmission from the spacecraft was received. There have been no communications with Pioneer 11 since. &lt;strong&gt;The Earth's motion has carried it out of the view of the spacecraft antenna.&lt;/strong&gt; The spacecraft cannot be maneuvered to point back at the Earth. It is not known whether the spacecraft is still transmitting a signal.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If I am interpreting NASA's depiction of the &lt;a href=&quot;https://www.nasa.gov/mission_pages/voyager/multimedia/pia14112.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;relative positions of distant spacecraft&lt;/a&gt; correctly, it appears that Pioneer 11 is roughly in the plane of the solar system ecliptic:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/rHBVx.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/voyager/multimedia/pia14112.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Image source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Given that Pioneer 11 was launched (presumably on a heliocentric orbit) &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_11#Launch_and_trajectory&quot; rel=&quot;nofollow noreferrer&quot;&gt;directly toward Jupiter&lt;/a&gt;, and assuming for a moment that we didn't deliberately tell it to aim its antenna away from Earth, I don't understand why the Earth's motion would have carried it out of the view of the spacecraft's antenna. Even if the Earth's revolution around the Sun would periodically degrade the signal received, possibly to the point of providing insufficient signal-to-noise ratio to decode the signal, it would seem like the signal strength should peak once a year, with an additional lower peak once a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/14284/can-voyager-1-receive-signals-from-earth#comment37432_14285&quot;&gt;Hobbes' claim of a 0.7 AU beamwidth at Earth even for Voyager&lt;/a&gt; (based on -3 dB received power and a &lt;a href=&quot;http://pds-rings.seti.org/voyager/rss/vg1sinst.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.32° beam angle (from center to half power)&lt;/a&gt;), and &lt;a href=&quot;https://space.stackexchange.com/questions/14406/why-was-radio-contact-with-pioneer-lost-earlier-than-with-voyager#comment37787_14408&quot;&gt;wider for Pioneer&lt;/a&gt;, would seem to be plenty wide enough that precise aiming would not be required; being in the general ballpark should be enough to be able to establish communications with the spacecraft at least once a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We do of course &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_11#NASA_ends_operations&quot; rel=&quot;nofollow noreferrer&quot;&gt;have good reason to believe that the transmitter is no longer working&lt;/a&gt; (&lt;a href=&quot;https://space.stackexchange.com/a/3586/415&quot;&gt;also here&lt;/a&gt;), but that is not what NASA claims in the quote at the top of this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is the basis for NASA's statement that the Earth's motion has carried Earth out of the view of Pioneer 11's antenna?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-03-13T09:00:52.450" Title="Why has the Earth's motion carried it out of view of Pioneer 11's antenna?" Tags="|orbital-mechanics|data-transmission|radio-communication|pioneer|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14412" PostTypeId="2" ParentId="14404" CreationDate="2016-03-11T23:24:45.933" Score="6" Body="&lt;p&gt;Your suspicion that it can be done with 4 is correct, and the method you suggest will work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More formally, to prove that a set of thrusters allows arbitrary torque is equivalent to showing that you can produce torque in each direction about the 3 orthogonal axes shown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ones drawn allow +blue and -blue (the two equatorial thrusters), +green (the one at the top pointing opposite to the red direction) and -green (firing the other two at the top equally). We can then get +red or -red by firing one of these two together with enough of the first one to counteract the -green torque that they also deliver. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words the torques from the thrusters are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;+B&lt;/li&gt;&#xA;&lt;li&gt;-B&lt;/li&gt;&#xA;&lt;li&gt;+G&lt;/li&gt;&#xA;&lt;li&gt;-aG + bR&lt;/li&gt;&#xA;&lt;li&gt;-aG - bR&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(If the thrusters are identical, symmetrically arranged and equidistant from the centre of gravity then a and b are 1/2 and $\sqrt3/2$, but the proof works as long as the two thrusters are symmetric, whatever the values of a and b are.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you offset each of the top set by moving each one to the right, each one will also create some +blue torque:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;+G + cB&lt;/li&gt;&#xA;&lt;li&gt;-aG + bR + cB&lt;/li&gt;&#xA;&lt;li&gt;-aG - bR + cB&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Those 3 are still enough to produce any torque you like about the G or R axis (in either direction) but now they will also produce an uncontrolled amount of +blue torque. You can always produce more +blue torque (just increase the thrust of all 3 equally) but never reduce it just by firing these three. However, we now only need one more thruster, -B, to get arbitrary torques in all directions.&lt;/p&gt;&#xA;" OwnerUserId="12831" LastActivityDate="2016-03-11T23:24:45.933" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14414" PostTypeId="2" ParentId="14400" CreationDate="2016-03-12T05:40:22.543" Score="2" Body="&lt;p&gt;Luckily their capsule was not space-station-shaped!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/q5RkC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/q5RkC.png&quot; alt=&quot;venting into space - uhoh!&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;@SF.'s answer points out that the vent needs to be on the plane containing the center of mass to avoid adding spin about a different axis. If the vent is ON the station, it better not be narrow at that point!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-03-12T12:50:35.453" LastActivityDate="2016-03-12T12:50:35.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14415" PostTypeId="2" ParentId="14411" CreationDate="2016-03-12T10:08:47.513" Score="6" Body="&lt;p&gt;Looking at the &lt;a href=&quot;http://codexgalactic.com/2011/03/24/the-pioneer-anomaly/&quot; rel=&quot;nofollow noreferrer&quot;&gt;trajectories of the Pioneers&lt;/a&gt;, the wording &quot;The Earth's motion has carried it out of the view of the spacecraft antenna&quot; seems to be a poor choice of words. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pS5We.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pS5We.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;you can see that in the long term, Pioneer has to rotate to keep Earth in view. Once it's out of fuel, it can't do this maneuver any more. For Pioneer 11's last transmission, it's possible the antenna beam just grazed one segment of Earth's orbit. As Earth continued on its orbit, it'd move out of the antenna's field of view for most of the year. The following year, Pioneer's changed position has moved the antenna's field of view away from Earth's orbit entirely. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Pioneers' high gain antenna has a &lt;a href=&quot;http://relativity.livingreviews.org/Articles/lrr-2010-4/articlesu32.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;beam width of 3°&lt;/a&gt; (gain at the edges is -10 dB relative to the center). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For &lt;a href=&quot;http://pds-rings.seti.org/voyager/rss/vg1sinst.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The transmit boresight gain of the HGA was 36 dB at S-band and 47 dB at X-band. The half-power half-width of the antenna beam was 0.32 degrees at X-band and 1.1 degrees at S-band.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A halving in power translates to a 3 dB loss. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-03-13T09:00:52.450" LastActivityDate="2016-03-13T09:00:52.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14416" PostTypeId="2" ParentId="13127" CreationDate="2016-03-12T10:51:34.330" Score="8" Body="&lt;p&gt;The Soviet N1 rocket used subcooled liquid oxygen. The driving force behind this was to use a common shaft on the turbopump for the fuel and the oxidiser. By subcooling the liquid oxygen such that it's density was equal to the RP1 fuel they were able to simplify the rocket engine design and reduce weight. It's one of the reasons behind the very high thrust to weight ratio of the &lt;a href=&quot;http://www.astronautix.com/engines/nk15.htm&quot; rel=&quot;nofollow&quot;&gt;NK-15&lt;/a&gt; and &lt;a href=&quot;https://en.m.wikipedia.org/wiki/NK-33&quot; rel=&quot;nofollow&quot;&gt;NK-33&lt;/a&gt; engines.&#xA;The subcooled liquid oxygen was also required to lubricate the turbopump.&#xA;It never made it to orbit unfortunately, but it had four real launches with payloads as per the question.&lt;/p&gt;&#xA;" OwnerUserId="13439" LastEditorUserId="16521" LastEditDate="2016-10-26T15:34:48.430" LastActivityDate="2016-10-26T15:34:48.430" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14417" PostTypeId="1" CreationDate="2016-03-12T12:38:06.877" Score="4" ViewCount="1316" Body="&lt;p&gt;After reading into the topic a little, I've come across a number of increases to the Saturn V's LEO and TLI performance over the course of the Apollo program. Early moon missions sent between 133t and 137t to 185km parking orbits, and could then throw about 47t to the moon. Later missions (Apollo 15, 16 and 17) orbited 140t to 141t at 167km, and each sent around 48.6t to the moon. These later Saturns were slightly uprated compared to those of earlier flights. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, had the SA-514 and SA-515 Apollo Saturn Vs been improved a little further for the later planned missions (Which were, of course, cancelled), &lt;strong&gt;how much mass would they have been able to send to 167km LEO and to the moon?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, &lt;strong&gt;could these LEO and TLI masses be increased by lowering the earth parking orbit to 150km?&lt;/strong&gt; (This would depend on whether S-IVB hydrogen venting could overcome atmospheric drag during the time before the TLI burn...)&lt;/p&gt;&#xA;" OwnerUserId="12678" LastActivityDate="2022-02-10T17:21:20.600" Title="Would the Saturn V have been able to send more mass to TLI if it had a lower earth parking orbit?" Tags="|the-moon|apollo-program|low-earth-orbit|payload|saturn-v|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14418" PostTypeId="2" ParentId="14417" CreationDate="2016-03-12T15:52:09.877" Score="5" Body="&lt;p&gt;It's not clear what improvements you're thinking about for late Apollo missions. It's unlikely that significant further changes would have been made for two additional missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The performance increases across the flown missions came mostly from minor modification of the first-stage engine fuel injectors allowing a higher propellant flow rate and hence greater thrust, combined with somewhat greater fuel loads on all three launcher stages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA was able to increase the weight of the Apollo payload stacks simply because they gained confidence over the course of the program that the launcher was reliably meeting and exceeding its performance specifications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A number of future &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19650020081.pdf&quot; rel=&quot;nofollow&quot;&gt;evolutionary developments of the Saturn V&lt;/a&gt; were considered. The first of these involved using uprated F-1A engines, plus stretched first and second stages, and would have increased payload to LEO by some 7 tons, but this would be an entirely new rocket, not a modification to the already built SA-514 or SA-515 vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As described in Mark Adler's answer, a lower parking orbit can give about 1% payload mass advantage on a translunar or Mars mission. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-03-13T16:45:07.013" LastActivityDate="2016-03-13T16:45:07.013" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14421" PostTypeId="1" CreationDate="2016-03-12T21:53:14.580" Score="2" ViewCount="399" Body="&lt;p&gt;In the question &lt;a href=&quot;https://space.stackexchange.com/questions/5335/how-strong-is-the-electric-charge-capacity-of-martian-dust-storms&quot;&gt;How strong is the electric charge capacity of Martian dust storms?&lt;/a&gt; the possibility of electrically-charged dust and its inconvenience or danger to Mars explorers is described. The triboelectric effect (commonly named static electricity) is described in &lt;a href=&quot;https://en.wikipedia.org/wiki/Triboelectric_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus Mars explorers, re-entering their habitat or spacecraft, could find themselves coated in dust which until now has unknown composition. The effects of a Martian dust and its possible risks to health is discussed at length in &lt;a href=&quot;https://space.stackexchange.com/questions/2621/what-would-it-feel-like-to-be-in-a-martian-dust-storm&quot;&gt;What would it feel like to be in a Martian dust storm?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be feasible to design a device to remove the statically-charged dust using an electric field? (Would a handheld device be more effective than a frame that a Marstronaut could walk through?)&lt;/p&gt;&#xA;" OwnerUserId="13543" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2019-08-12T18:46:56.150" Title="Is a system to remove the charged Mars dust from settlers' suits feasible?" Tags="|mars|spacesuits|dust|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14422" PostTypeId="1" CreationDate="2016-03-13T01:57:18.187" Score="8" ViewCount="2350" Body="&lt;p&gt;I have heard presentations on lunar lava tubes that estimated the temperatures of the ground surrounding them as being well below zero. An article in &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2010/12jul_rabbithole/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Science News&lt;/a&gt; says this: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once you get down to 2 meters under the surface of the Moon, the&#xA;  temperature remains fairly constant, probably around -30 to -40&#xA;  degrees C.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But this graph of measurements taken on Apollos 15 and 17 paints a very different picture: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Mv4eN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mv4eN.png&quot; alt=&quot;temperatures below the lunar surface, ranging from 250 to 256 K&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above is from &lt;a href=&quot;http://www.lpi.usra.edu/publications/books/lunar_sourcebook/pdf/Chapter03.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ch. 3&lt;/a&gt; of the &lt;a href=&quot;http://www.lpi.usra.edu/publications/books/lunar_sourcebook/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Sourcebook&lt;/a&gt;, page 12 of 34. All measurements taken showed rising temperatures with depth of about 1 &lt;sup&gt;o&lt;/sup&gt;C per meter. The largest known lava tubes on the Moon have skylights revealing &lt;a href=&quot;http://blogs.esa.int/caves/2015/05/06/caves-the-hidden-side-of-planets/&quot; rel=&quot;nofollow noreferrer&quot;&gt;depths of 100 m&lt;/a&gt;, so if that rate of temperature increase is taken, then the floors of such tubes should be about 80 to 100 &lt;sup&gt;o&lt;/sup&gt;C warmer than the temperature near the surface, which means something like 60 or 70 &lt;sup&gt;o&lt;/sup&gt;C. I've never heard any such figures talked about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, is there a cooling mechanism involved with these tubes? I wouldn't have thought so, being in a vacuum and only open to the surface through gaps that are probably very small relative to their full size. Why this apparent discrepancy?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-04-15T19:38:59.270" LastActivityDate="2016-04-15T19:38:59.270" Title="Does the temperature near the surface of the Moon rise rather quickly with depth?" Tags="|the-moon|planetary-science|temperature|lava-tubes|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14423" PostTypeId="5" CreationDate="2016-03-13T02:01:40.257" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-03-13T02:01:40.257" LastActivityDate="2016-03-13T02:01:40.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14424" PostTypeId="4" CreationDate="2016-03-13T02:01:40.257" Score="0" Body="Tubular voids beneath the ground created when lava flows drain away. The Moon and Mars are theorized to have possibly many and very large ones, suitable for conversion to habitats as they protect from radiation and can be easily sealed." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-03-13T02:01:40.257" LastActivityDate="2016-03-13T02:01:40.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14425" PostTypeId="5" CreationDate="2016-03-13T02:04:17.333" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-03-13T02:04:17.333" LastActivityDate="2016-03-13T02:04:17.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14426" PostTypeId="4" CreationDate="2016-03-13T02:04:17.333" Score="0" Body="Reaction Control Systems, small thrusters to control attitude and occasionally translation of a spacecraft." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-03-13T02:04:17.333" LastActivityDate="2016-03-13T02:04:17.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14427" PostTypeId="1" AcceptedAnswerId="14434" CreationDate="2016-03-13T02:11:32.340" Score="3" ViewCount="367" Body="&lt;p&gt;A twist on the Archimedes law. Any object in Earth's atmosphere should weigh less than in vacuum, by amount of mass of air it displaces. At 1.2 kg/m³ of air at sea level, that is a bit of a difference, especially in case of light materials like insulation foam.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this difference taken into account when weighing payloads, parts etc? Or is it simply sunk into allowable error margins?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-03-13T12:28:03.143" Title="Is buoyancy of rockets a factor taken into account?" Tags="|mass|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14428" PostTypeId="2" ParentId="14406" CreationDate="2016-03-13T02:19:00.120" Score="13" Body="&lt;h1&gt;Pioneers&lt;/h1&gt;&#xA;&lt;p&gt;Several factors lead to the loss of signal.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Radiated power:  Pioneer 10's broadcast power is particularly low. 8 W at ~2.2 GHz ①&lt;/li&gt;&#xA;&lt;li&gt;Very narrow beam; the antenna gain is +65dB, which means 1/(10^6.5) the broadcast surface area... which means a pretty narrow cone. (I don't know the math for the actual beam angle.)&lt;/li&gt;&#xA;&lt;li&gt;Input power decreases - the RTGs still produce plenty of energy, but the corroding of the thermocouples results in progressively less input power to run the transmitters. (Corroded thermocouples are less efficient at converting heat to electricity.)&lt;/li&gt;&#xA;&lt;li&gt;Continued drag on the spacecraft. ② This is relevant because Pioneer cannot continue to reorient indefinitely - fuel is a finite resource.&lt;/li&gt;&#xA;&lt;li&gt;Budget cuts.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The combination of these factors leads to the cumulative failure.&lt;/p&gt;&#xA;&lt;p&gt;Primary is that Pioneer is not producing intelligible data due to weak signal, so it was no longer cost effective to track Pioneer 10. Pioneer 11 shutdown was primarily due to power loss. ③&lt;/p&gt;&#xA;&lt;p&gt;In looking at how the decision works - at the range they are at, the signal is very weak, and antenna accuracy (to keep Earth in the center of the antenna lobe) is a function of rotational control over attitude. Even so, the required dish size to get data to or from Pioneer 10 was a minimum 64m dish to have the required signal strength. As it goes further out, the minimum antennae to receive its signal increases. At present, only a handful of scopes can communicate with Pioneer 10 at all, and the data is so weak that it's unreliable.&lt;/p&gt;&#xA;&lt;p&gt;The Pioneers used gas thrusters as their reaction control ④, which provides a fundamental limit to their pointing accuracy.&lt;/p&gt;&#xA;&lt;p&gt;The drag of the interplanetary medium (IPM) and the solar wind are also issues. ② because the medium is not actually uniform, this results in slight differentials across the spacecraft, which, given the decades, are able to slow the spin and alter the angle. Keeping in mind that the IPM includes chunks of dust and even small pebbles, and the Pioneer velocities make a pebble impact comparable to a small caliber firearm hit, one at the right spot could wobble the spacecraft out of communication.&lt;/p&gt;&#xA;&lt;p&gt;The exact cause of Pioneer 11 failure isn't actually known, but it's presumed to be power failure (as the telemetry showed decreasing power on a decay curve), possibly coupled with an impact event and/or short circuit. If it were knocked off axis, it might still be broadcasting but have earth outside the antenna's broadcast lobe. The knocked off axis hypothesis isn't likely, as the location is still on the predicted course.&lt;/p&gt;&#xA;&lt;p&gt;The expectation of Pioneer 10 to continue to degrade in power consumption contributed in the public explanations of termination of the Pioneer 10 mission, equally as much as the expense of the 65m, 100m, and Arecibo antenna time, and the ability to spend that money on missions returning more data.&lt;/p&gt;&#xA;&lt;h1&gt;Voyagers&lt;/h1&gt;&#xA;&lt;p&gt;Voyager, on the other hand, has better thermocouples, younger thermocouples, more power produced, more power (18 w)⑤,  and while only a 48 dB gain, it's in a better frequency noise-wise (~ 8 GHz), it's still sending useful data from instruments, and had much higher surplus power at launch.&lt;/p&gt;&#xA;&lt;p&gt;Voyager power losses have been a shallower curve than the pioneer program; more of the initial power available is still available. Further, the original power was much higher.&lt;/p&gt;&#xA;&lt;p&gt;The instrumentation for Voyager included several more experiments that are not of use in the extended mission; they can be turned off with no loss of data gain. This allows more of the available power to be used on the ones that are relevant.&lt;/p&gt;&#xA;&lt;p&gt;The Voyager missions as originally conceived of also included a longer operational period - more than 10 years was expected, versus Pioneer's 2 years.&lt;/p&gt;&#xA;&lt;h1&gt;To recap:&lt;/h1&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Voyagers 1 &amp;amp; 2&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Better frequency (less noise).&lt;/li&gt;&#xA;&lt;li&gt;Bigger dish &amp;amp; better aim.&lt;/li&gt;&#xA;&lt;li&gt;More power in ship (both at start and at present)&lt;/li&gt;&#xA;&lt;li&gt;More working instruments&lt;/li&gt;&#xA;&lt;li&gt;working instruments on mission goals for current mission&lt;/li&gt;&#xA;&lt;li&gt;not as far along the loss of power curve ⑥&lt;/li&gt;&#xA;&lt;li&gt;still have significant fuel reserves so can stay pointed.&lt;/li&gt;&#xA;&lt;li&gt;Designed for a longer operational period.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;Pioneer 11&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Power failure monitored resulted in expected failure to be able to compute and communicate about the time signal was lost&lt;/li&gt;&#xA;&lt;li&gt;power failure curve precluded useful instrumentation for extended mission.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;Pioneer 10&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Instrumentation not suited for extended mission.&lt;/li&gt;&#xA;&lt;li&gt;Signal strength and accuracy not conducive to the transmission of data.&lt;/li&gt;&#xA;&lt;li&gt;Fuel reserves nearly depleted&lt;/li&gt;&#xA;&lt;li&gt;probability of failure made continued contact not worth the money.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;References&lt;br&gt;&#xA;① &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1972-012A&quot; rel=&quot;noreferrer&quot;&gt;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1972-012A&lt;/a&gt; &lt;br&gt;&#xA;② &lt;a href=&quot;http://arxiv.org/pdf/1307.0537.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://arxiv.org/pdf/1307.0537.pdf&lt;/a&gt; &lt;br&gt;&#xA;③ &lt;a href=&quot;http://nineplanets.org/spacecraft.html&quot; rel=&quot;noreferrer&quot;&gt;http://nineplanets.org/spacecraft.html&lt;/a&gt; &lt;br&gt;&#xA;④ &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009039.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009039.pdf&lt;/a&gt; &lt;br&gt; ⑤ &lt;a href=&quot;http://www.uhf-satcom.com/misc/datasheet/dh2va.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.uhf-satcom.com/misc/datasheet/dh2va.pdf&lt;/a&gt; &lt;br&gt;&#xA;⑥ &lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html&quot; rel=&quot;noreferrer&quot;&gt;http://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="158" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-14T21:15:51.047" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14429" PostTypeId="1" CreationDate="2016-03-13T02:33:27.147" Score="6" ViewCount="347" Body="&lt;p&gt;It occurred to me that EVAs on the Martian surface would be liable to bring biologicals from Earth out into the Martian environment. A habitat will become contaminated with bacteria from the humans occupying it... Skin and GI flora, skin flakes, and other types of biologicals from experiments going on in the habitat. I was wondering if anyone has given any thought to how we're going to prevent this stuff from going out the airlock every time someone goes EVA.&lt;/p&gt;&#xA;" OwnerUserId="12071" LastEditorUserId="-1" LastEditDate="2016-07-06T14:12:34.977" LastActivityDate="2016-07-06T14:12:34.977" Title="How would a manned Mars mission keep from contaminating Mars?" Tags="|mars|crewed-spaceflight|forward-contamination|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14431" PostTypeId="2" ParentId="14429" CreationDate="2016-03-13T06:47:27.713" Score="5" Body="&lt;p&gt;While measures would be taken to limit the contamination for scientific investigation reasons, it is generally accepted that 100% containment would be impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, when the planetary protection regulations were originally established in the mid 1960's, a time limit of 50 years was set as the time, for example, required that Mars orbiters remain in orbit. Once they de-orbit, their unsterilized innards would potentially contaminate Mars. Why 50 years? Because it was considered obvious at the time that, surely, humans would be on Mars in less than 50 years. If humans are there, then we have clearly given up on preventing biological contamination, so let the orbiters auger in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, it has been 50 years, and there are no humans on Mars. But when there are, they and their habitats will certainly be a source of biological contamination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even so, there are approaches for collecting sub-surface samples in a contaminated environment that are not contaminated, so unsullied scientific investigations could and would be conducted.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-03-13T06:47:27.713" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14432" PostTypeId="2" ParentId="14417" CreationDate="2016-03-13T07:12:13.487" Score="7" Body="&lt;p&gt;Yes, you would get a little more performance from a lower parking orbit. The standard is 100 nmi (185.2 km), but on MER (Mars Exploration Rover) we got the launch vehicle folk to lower the parking orbit to 80 nmi for, as I recall, on the order of a 1% increase in delivered mass to Mars injection. 1% might not sound like a lot, but every kg counts. Later we backed off to 90 nmi when we had better knowledge of the spacecraft mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also got them to reduce the altitude at which the fairing was ejected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We considered, but rejected, making the parking orbit elliptical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also allowed for a slightly lower probability of commanded shutdown, i.e. an increased probability of running out of propellant on the second stage. This could be accommodated probabilistically by spacecraft maneuvers, resulting in a net win.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are all small changes, but when you are trying to eke out the last few kg as we were at one point on MER, you try stuff like this.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-03-13T07:12:13.487" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14433" PostTypeId="2" ParentId="14427" CreationDate="2016-03-13T10:20:00.187" Score="1" Body="&lt;p&gt;I think it is very likely that the buoyancy of the rocket is taken into account. First, let us see what savings it gives in terms of mass:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the density of a rocket? That is actually quite difficult to find, but considering a kerosene/LOX rocket like Soyuz, it is mostly propellant anyway. Kerosene has a density a bit less than 1 kg/L, and LOX a little bit more. As the rest of the rocket is metal, that should average out a little over 1 kg/L. Continuing the rough approximation, that means that the Soyuz rocket displaces around 350 kg of air. Is that much? For a comparable number, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_external_tank&quot; rel=&quot;nofollow&quot;&gt;Space Shuttle External Tank&lt;/a&gt; was painted with reflective paint on the first two flights. Then they decided that it could actually be skipped, &quot;saving approximately 272 kg&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket science is all about small margins, and that means there is a fight for every kilogram. As the effect of buoyancy is within the magnitude of other things considered, it is almost for sure taken into account.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="10791" LastEditDate="2016-03-13T12:05:58.070" LastActivityDate="2016-03-13T12:05:58.070" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14434" PostTypeId="2" ParentId="14427" CreationDate="2016-03-13T12:28:03.143" Score="4" Body="&lt;p&gt;Yes, we calculated the bouyancy for the External Tank in the code for the Shuttle Mission Simulator. I was dubious that it really mattered since you get out of the dense atmosphere so quickly, and made at least one run to see if I could tell. It's been a long time but my memory is that I could not tell any difference at Main Engine Cut-off...but I just may not have spotted it. It's all ancedotal anyway, sadly.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-03-13T12:28:03.143" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14435" PostTypeId="1" CreationDate="2016-03-13T22:10:23.723" Score="6" ViewCount="2171" Body="&lt;p&gt;I'm trying to estimate the delta-v (and, by applying the Rocket Equation, the propellant) required to lift a payload from LEO to a higher orbit. As I understand it, the standard way to do this is with a Hohmann Transfer: first, boost from a circular orbit at &lt;em&gt;r1&lt;/em&gt; (which in this case is LEO) to an elliptical orbit with apogee at &lt;em&gt;r2&lt;/em&gt; (the altitude of my target orbit); then, boost the elliptical orbit into a circular orbit at &lt;em&gt;r2&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the equations described here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Calculation&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Calculation&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;r1&lt;/em&gt; = 6451000m (i.e., an orbital altitude of 80km + the radius of the earth)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;r2&lt;/em&gt; = 95896000m (orbital altitude of 89,500km + earth radius)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I come up with a delta-v of about 4,215 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if I am instead transferring from my 80km LEO altitude to 179,000km, I get a delta-v of about 4,150 m/s. I.e., increasing the altitude of my target orbit by 80,000km &lt;em&gt;decreases&lt;/em&gt; the total delta-v by about 75m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mathematically, I can see why it's happening. The value of &lt;em&gt;Δv2&lt;/em&gt; is decreasing faster than &lt;em&gt;Δv1&lt;/em&gt; is increasing. But it doesn't seem right. Am I misunderstanding the purpose of the delta-v value in the Hohmann Transfer equation? Or is that actually the way it is?&lt;/p&gt;&#xA;" OwnerUserId="13756" LastActivityDate="2016-03-15T12:28:33.253" Title="Question about the Hohmann Transfer: why does delta-v go down when transferring to a higher orbit?" Tags="|orbital-mechanics|physics|mathematics|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14436" PostTypeId="2" ParentId="14435" CreationDate="2016-03-13T22:14:55.137" Score="1" Body="&lt;p&gt;I may have found the answer to my own question. From the &quot;Example&quot; section of the same page I linked to:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is interesting to note that this is greater than the Δv required for an escape orbit: 10.93 − 7.73 = 3.20 km/s. Applying a Δv at the LEO of only 0.78 km/s more (3.20−2.42) would give the rocket the escape speed, which is less than the Δv of 1.46 km/s required to circularize the geosynchronous orbit. This illustrates that at large speeds the same Δv provides more specific orbital energy, and energy increase is maximized if one spends the Δv as quickly as possible, rather than spending some, being decelerated by gravity, and then spending some more to overcome the deceleration (of course, the objective of a Hohmann transfer orbit is different).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, if I'm understanding this correctly, a Hohmann transfer to a higher orbit is more energy-efficient because approximately half of the &quot;lifting&quot; work happens at or near the apogee of the elliptical orbit, where the gravitational force of the earth is a bit weaker. Therefore, if the apogee of the elliptical orbit is higher, less delta-v is required because there is less gravitational force to fight against.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that a reasonably accurate explanation?&lt;/p&gt;&#xA;" OwnerUserId="13756" LastActivityDate="2016-03-13T22:14:55.137" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14439" PostTypeId="2" ParentId="14435" CreationDate="2016-03-14T01:15:33.407" Score="8" Body="&lt;p&gt;While the lower destination orbits don't require a big insertion burn, they do need a big circularization burn at apogee of the transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The higher the apogee, the longer and skinnier the ellipse and the more parabola-like the transfer orbit becomes. So, as the ellipse's apogee goes to infinity, perigee velocity approaches escape velocity. That would be sqrt(2) * circular orbit velocity at departure orbit. So as apogee goes up, insertion burn approaches (sqrt(2) - 1) * circular orbit velocity (assuming you're already in the circular departure orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Circularization burn at apogee approaches zero as apogee goes to infinity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Worst_case.2C_maximum_delta-v&quot; rel=&quot;noreferrer&quot;&gt;Maximum Hohmann delta-v&lt;/a&gt; occurs when the destination orbit radius is 15.58 times the departure orbit radius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the radii of the departure and destination orbits differ by a factor of 11.94 or more, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot; rel=&quot;noreferrer&quot;&gt;bi-elliptic&lt;/a&gt; transfer is more economical (in terms of delta-v, but not in transfer time) than a Hohmann transfer.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="49" LastEditDate="2016-03-14T15:14:25.247" LastActivityDate="2016-03-14T15:14:25.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14440" PostTypeId="1" CreationDate="2016-03-14T02:20:19.823" Score="2" ViewCount="476" Body="&lt;p&gt;I finished reading the novel &amp;quot;Saturn Run&amp;quot; by John Sandford and Ctein.  Putting aside storyline and content of the novel there was a very interesting &lt;a href=&quot;http://epubbookonline.com/b/2317/john-sandford/saturn-run-2015-/71&quot; rel=&quot;nofollow noreferrer&quot;&gt;Author's note&lt;/a&gt; that explains how the science portion was written and that it may be plausible.&lt;/p&gt;&#xA;&lt;p&gt;Let me focus my question.  &lt;em&gt;Is it possible with our current VASIMR spaceship technology that a manned trip to Saturn and back to Earth is plausible?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;The book gives a 6 month timeline from launch to Saturn and about the same time frame to get back to Earth.    In order to avoid too much speculation I will stick with this time.&lt;/p&gt;&#xA;&lt;p&gt;Please note any spoilers with the link I posted.&lt;/p&gt;&#xA;&lt;p&gt;The United States ship is described below.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Americans need something better. Enter the VASIMR engines. VASIMR stands for “Variable Specific Impulse Magnetoplasma Rocket.” “Specific impulse” is how rocket scientists refer to exhaust velocity. We didn’t make the VASIMR up. They’re being tested on Earth, fairly small ones. Ours are a lot bigger, and a little better-performing, but it’s fifty years from now. For a trip time of four to five months, we were able to get the ship down to a mass ratio of 10 with an exhaust velocity that varied from 35 up to 300 km/s. That’s about half the mass ratio we could come up with for a fixed-specific-impulse ship of any remotely plausible design.&lt;/p&gt;&#xA;&lt;p&gt;Exhaust velocity is the magic number. As long as the total delta-vee you want is less than your exhaust velocity, the amount of reaction mass you need isn’t too bad.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="13765" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-16T01:49:02.103" Title="VASIMR technology for a manned trip to Saturn" Tags="|engines|saturn|magnetoplasmadynamic|" AnswerCount="0" CommentCount="16" ClosedDate="2016-03-14T10:19:54.493" ContentLicense="CC BY-SA 3.0" />
  <row Id="14442" PostTypeId="1" AcceptedAnswerId="14446" CreationDate="2016-03-14T10:12:23.813" Score="7" ViewCount="1156" Body="&lt;p&gt;I know that ESA is cooperating with Roscosmos on the ExoMars project. But what were the reasons of selecting a Proton launch vehicle instead of Ariane? Is it because Baikonur is a more favourable launch site (than French Guyana)?&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="6816" LastEditDate="2019-02-08T12:09:08.357" LastActivityDate="2019-02-08T12:09:08.357" Title="Why Proton launcher for ExoMars, and not Ariane?" Tags="|mars|esa|roscosmos|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="14444" PostTypeId="2" ParentId="14442" CreationDate="2016-03-14T13:28:27.693" Score="7" Body="&lt;p&gt;Baikonur is definitely less favorable launch site than Guiana, with highly inclined orbits, but that doesn't play such a big role with interplanetary missions as inclination correction on very high orbits is relatively inexpensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But first, there are simple, direct budget concerns: ExoMars doesn't require the large payload capacity of Arianne (and it's difficult to share payload space with commercial missions on an interplanetary launch), and then Proton-M &lt;a href=&quot;https://space.stackexchange.com/questions/2015/what-is-the-current-cost-per-kg-to-send-something-into-gso-geo&quot;&gt;cost per kg to orbit&lt;/a&gt; is significantly lower - almost half the price of Ariane 5ES.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then - considering that Roscosmos provides the launchers, the only costs of ESA are development of the actual payloads. Surely Roscosmos wouldn't be willing to fund an Arianne launch instead...&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-03-14T13:28:27.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14446" PostTypeId="2" ParentId="14442" CreationDate="2016-03-14T14:29:34.530" Score="8" Body="&lt;p&gt;As @Antzi said in his now-deleted answer, initially ExoMars was to be a collaboration between ESA and NASA, with NASA providing the launches. When NASA left the program, &lt;a href=&quot;http://www.russianspaceweb.com/exomars_2016.html&quot; rel=&quot;noreferrer&quot;&gt;ESA sought a new deal&lt;/a&gt;, this time with Roscosmos.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As of 2009, in April 2018, NASA's Atlas rocket would launch the European Space Agency's long-delayed ExoMars rover and a smaller NASA rover (Mars Astrobiology Explorer-Cacher, MAX-C) to look for signs of life on Mars. The rovers were scheduled to land in the same location on the Red Planet in January 2019. Thanks to a highly intelligent navigation systems, the mobile robots would travel for several miles across the Martian landscape. The European spacecraft would use a ground-penetrating radar to locate subsurface features associated with the past presence of water. It would then drill as deep as two meters below the surface to extract samples for in-situ analysis.&lt;/p&gt;&#xA;&lt;p&gt;By 2011, Russian and European scientists did discuss coordination between ExoMars and Mars-NET project, which at the time was expected to coincide with ExoMars. A joint launch of both spacecraft on a single Russian Proton rocket had been considered, however no agreement was reached. Russian scientists also proposed an infrared spectrometer and particle detectors for installation on the ExoMars rover. Russian-built radioactive heat generators could also be integrated into landers to provide thermal control on the frozen martian surface.&lt;/p&gt;&#xA;&lt;p&gt;However all plans for the ExoMars mission in 2016, along with the follow-on launch in 2018, ran into trouble by 2011. NASA failed to secure funds for the procurement of the rocket to launch the orbiter and the EDM spacecraft and possibly for the already long-delayed ExoMars rover. As a result, in October 2011, European space officials were quoted in the Western media as saying that Russia had been &amp;quot;invited&amp;quot; into the project in exchange for... providing a heavy-lifting Proton rocket to launch the troubled US-European mission in 2016. According to European space officials, Russians would be &amp;quot;compensated&amp;quot; with the &amp;quot;right&amp;quot; to install their experiments onboard the EDM and ExoMars spacecraft and access the scientific return from the missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Despite initial misgivings (Russia had phased out Proton for scientific missions), they agreed after all:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;By the Spring of 2012, Russian space officials agreed to provide a pair of Proton rockets to launch both ExoMars missions -- in 2016 and 2018.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-14T14:29:34.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14447" PostTypeId="1" CreationDate="2016-03-14T14:43:22.627" Score="5" ViewCount="767" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/14442/why-proton-launcher-for-exomars-and-not-ariane&quot;&gt;This question&lt;/a&gt; got me thinking about launch site selection for missions to Mars. I assumed the optimal launch site inclination would be the one where Mars is due East. Depending on where Mars is in its orbit, this would be at 23.4° ± 0-1.85° (Mars's orbit has an angle of 1.85° wrt the ecliptic). Or would you be better off launching from the equator at an angle of 23.4° ± 0-1.85°?   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is the solution different altogether? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-03-15T09:12:41.303" Title="Is there an optimal launch inclination for interplanetary launches?" Tags="|launch|mars|orbital-mechanics|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14448" PostTypeId="2" ParentId="14447" CreationDate="2016-03-14T17:20:49.590" Score="4" Body="&lt;p&gt;I would say, anywhere where the line of the ecliptic is directly overhead. In other words, anywhere between the line of the Tropics (with two launch windows per day), so your suggestion of near the equator is good.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though it has to be said that as Earth's velocity along the ecliptic is already pretty high, launching from outside the optimal tropical area is a relatively small penalty...&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-03-14T17:20:49.590" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14449" PostTypeId="2" ParentId="14421" CreationDate="2016-03-14T19:15:57.207" Score="2" Body="&lt;p&gt;Yes, it's feasible. I'm uncertain that it's important but it could be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the dust is charged, and that is why it clings, it would be trivial to remove it. You'd just need to apply the same charge to the suit to repulse it. (this could be done to prevent it from clinging, as well)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternately, apply the opposite charge to something else and it would pull the stuff over. (within limits for range and intensity, obviously)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More likely, the issue would be simply a built-up static charge, so a discharge station would make sense. (Perhaps the difference could be harvested for a teensy power boost.) Essentially, this would be a post or panel that is touched as part of the re-entry procedure. A ground strap equivalent could be useful for this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It bears noting that blowing or sucking the dust off after partially neutralizing the charge might be (read: probably is) more reasonable than trying to do this through pure EM force.&lt;/p&gt;&#xA;" OwnerUserId="12224" LastEditorUserId="12224" LastEditDate="2016-03-14T19:28:41.490" LastActivityDate="2016-03-14T19:28:41.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14450" PostTypeId="2" ParentId="8103" CreationDate="2016-03-14T19:59:37.360" Score="1" Body="&lt;p&gt;John Mankins is one of the top SSP researchers today. In an interview he said that he could put together an SSP &quot;pilot plant&quot; (in orbit) given ten years and $10 Billion. I'm not sure what the size of the plant would be, but it would be a full system architecture technology demonstrator that could then serve as the blueprint for additional commercial scale facilities. &#xA;A working moon base with resource extraction, refining, and electromagnetic launch to supply materials for space colonies and SSP systems? Likely trillions. You may have read &quot;Colonies in Space&quot; by TA Heppenheimer. It was based on the work of Glaser and a grad student summer study. &#xA;Of course as solar cell technology gets better and launch costs drop, we should eventually reach the price point where SSP makes sense to everyone. I'm surprised Elon Musk has talked so much about Mars and never mentioned (to my knowledge) Space Solar Power, because the first SSP projects may very well launch on recycled Falcon 9s. &lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-03-14T19:59:37.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14451" PostTypeId="2" ParentId="12216" CreationDate="2016-03-14T20:11:57.107" Score="1" Body="&lt;p&gt;John Mankins is one of the top SSP researchers. Some power beaming architectures call for diffuse lasers and color-tuned solar cells, while others stick with microwaves. In an interview he reported some of the findings of a recent (2013 or 2014) European conference on Space Solar Power. One finding was that recent modeling suggests that beaming the power back with a relatively diffuse laser could still cause dangerous levels of heating if it was mis-targeted or intentionally targeted at cities. It seemed to be the death knell of the light-based power beaming architectures. Meanwhile, microwaves are not only diffuse but most designs build the frequency into the hardware. If it isn't built as a weapon then it can't be re-tuned remotely to increase the frequency and weaponize it.&#xA;So in short, if you are using a microwave beamed power system then you probably can't repurpose it as a weapon unless you build that capability in before it launches, quite possibly requiring a lot of extra hardware and weight.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-03-14T20:11:57.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14452" PostTypeId="1" AcceptedAnswerId="14453" CreationDate="2016-03-14T21:06:56.797" Score="2" ViewCount="429" Body="&lt;p&gt;I only found sources (&lt;a href=&quot;http://www.braeunig.us/space/propel.htm&quot; rel=&quot;nofollow&quot;&gt;like this&lt;/a&gt; ) giving impulse density of hydrazine used as a bipropellant in conjunction with various oxidizers, but I was unable to find any that would give me density impulse of monopropellant hydrazine... does anyone know?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-03-14T21:43:17.513" Title="Density impulse of monopropellant hydrazine?" Tags="|fuel|liquid-fuel|monopropellant|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14453" PostTypeId="2" ParentId="14452" CreationDate="2016-03-14T21:43:17.513" Score="2" Body="&lt;p&gt;Well, the density of hydrazine is $1.021 \frac{kg}{L}$, and its $I_{sp}$ is $220 s$ as a mono propellant. (Both values from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrazine&quot; rel=&quot;nofollow&quot;&gt;Wiki page&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is a total of $224.6 \frac{kg \cdot s}{L}$, using the same units as your source.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-03-14T21:43:17.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14454" PostTypeId="1" AcceptedAnswerId="14457" CreationDate="2016-03-15T07:06:41.620" Score="8" ViewCount="964" Body="&lt;p&gt;When planning a new architecture for lunar missions, one always faces the potential of &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot;&gt;nuclear thermal engines&lt;/a&gt;. Having a much higher $I_{sp}$, they offer a much higher propellant efficiency, causing a more elegant design over all. NTRs have their pros and cons, but also a question: &quot;Where do you get rid of the engine after it is used?&quot; It would be very relaxing if you could just let the engine crash into the Moon, but is that a good idea? The Moon is an interesting place, and some people have &lt;a href=&quot;http://moonwards.com/&quot;&gt;great plans for it&lt;/a&gt;. As a rule of thumb, it is not a good idea to cover something that may come in handy with radioactive dust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens when a spacecraft crash into the Moon? In the case of a nuclear reactor, it does really matter if all the debris is limited to a few kilometres, making a &quot;nuclear graveyard&quot; possible, or not. High velocity impacts are hardly intuitive, and the velocities here are high. (2500 m/s in case of a direct impact from a transfer orbit, 1700 m/s from LMO). There should at least be some data available, as a lot of probes have been crashed into the Moon, both intentionally and by accident.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-03-16T18:10:40.593" Title="What happens to a spacecraft crashing into the Moon?" Tags="|the-moon|debris|impact|nuclear|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14455" PostTypeId="2" ParentId="5522" CreationDate="2016-03-15T07:29:40.230" Score="0" Body="&lt;p&gt;A retrograde orbit can help in achieving a sun-synchronous orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;nofollow&quot;&gt;wikipedia page on sun-synchronous orbits&lt;/a&gt; shows that a (small) &lt;a href=&quot;https://en.wikipedia.org/wiki/Nodal_precession&quot; rel=&quot;nofollow&quot;&gt;nodal precession&lt;/a&gt; (movement of the orbit's point of crossing the equatorial plane) ensures that the satellite is overhead the same location at the same local time every day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can use the ellipsoidal nature of the central body to &quot;pull&quot; on the orbit and generate the nodal precession that is required for a sun-synchronous orbit. The rate of nodal precession depends on the inclination of the orbit, and a slightly retrograde orbit yields the desired nodal precession.&lt;/p&gt;&#xA;" OwnerUserId="13771" LastActivityDate="2016-03-15T07:29:40.230" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14456" PostTypeId="1" AcceptedAnswerId="14462" CreationDate="2016-03-15T08:09:44.980" Score="26" ViewCount="4763" Body="&lt;p&gt;It was mentioned in this &lt;a href=&quot;https://space.stackexchange.com/a/14416/12102&quot;&gt;answer&lt;/a&gt; and in several other places on the internet that the kerosene fuel used by NK-33 engines needed to be subcooled enough so that it reached the same density as the LOX, and that this is required so that the turbopumps can run on the same shaft. These statements all trace back to &lt;a href=&quot;http://www.astronautix.com/engines/nk33.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, where I can not actually find any discussion of subcooled kerosene, much less a discussion of the need to match the density of LOX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mentioned this &lt;a href=&quot;https://space.stackexchange.com/questions/13127/is-the-supercooled-lox-used-in-falcon-9-full-thrust-a-first/14416?noredirect=1#comment37815_14416&quot;&gt;here&lt;/a&gt; - it bothers me for two reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;I think it would be extremely hard to obtain liquid kerosene at a density of 1.17 or 1.18 g/cm&lt;sup&gt;3&lt;/sup&gt;, which is the density of LOX at -310 ℉, the temperature that the NK-33 engines require in order to cool the turbopump bearings. The density of RP-1 is about 0.8 g/cm&lt;sup&gt;3&lt;/sup&gt; at 25 ℃, and the largest value I’ve seen &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;noreferrer&quot;&gt;anywhere&lt;/a&gt; is 1.02 g/cm&lt;sup&gt;3&lt;/sup&gt;.&lt;/li&gt;&#xA;&lt;li&gt;I can’t think of any reason why the densities should be equal to run the two turbopumps on the same shaft. The ratio of the mass flows is quite large - perhaps 2.62 LOX/kerosene (from &lt;a href=&quot;http://www.russianspaceweb.com/nk33.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;) so I don’t understand why matching their densities in the two pumps so closely is important.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; I use the generic “kerosene” since the Russian fuel used to develop the engines can’t officially be called “RP-1”.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I’ve tried to collect as much information as I can on-line, and plot it here. Since I've had to deal with four different temperature scales, and people are accustomed to using different ones, I've just brute-force plotted with all four since I can't do the conversions &lt;a href=&quot;https://space.stackexchange.com/questions/13127/is-the-supercooled-lox-used-in-falcon-9-full-thrust-a-first/14416#comment34399_13144&quot;&gt;in my head&lt;/a&gt; either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i2fSM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i2fSM.png&quot; alt=&quot;Cold LOX and kerosene - delicious!&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RP-1 solid blue line from &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/290659.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.dtic.mil/dtic/tr/fulltext/u2/290659.pdf&lt;/a&gt;. Other data points I’ve found seem to match.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RP-1 dashed red line is an extrapolation of the same line below it’s valid range of -45 ℃ to +25 ℃, just to guide the eye and give the brain something to thing about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RP-1 comments on consistency (“gel”, “wax”, etc.) are from &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020018567.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020018567.pdf&lt;/a&gt;. I have included a screenshot of the relevant section below the plots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LOX solid line from &lt;a href=&quot;http://booksite.elsevier.com/9780750683661/Appendix_C.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://booksite.elsevier.com/9780750683661/Appendix_C.pdf&lt;/a&gt;. A screen shot of the entry and equation is given below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LOX data points from &lt;a href=&quot;http://oxygen.atomistry.com/liquid_oxygen.html&quot; rel=&quot;noreferrer&quot;&gt;http://oxygen.atomistry.com/liquid_oxygen.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; additional LOX data points can be found at &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-ft/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt; and &lt;a href=&quot;http://spaceflight101.com/spacerockets/wp-content/uploads/sites/17/2015/11/loxdensity.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, credit: NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question is:&lt;/strong&gt; Does the operation of the NK-33 engine really require equal densities of LOX and kerosene? If so, WHY?? Also if so, what is the actual temperature of the kerosene to acheive this density, and what is its consistency really like, liquid, gel, wax?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;NOTE:&lt;/strong&gt; below is from: &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020018567.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020018567.pdf&lt;/a&gt; &lt;strong&gt;R&lt;/strong&gt; is temperature in Rankine - which could be called &quot;&lt;em&gt;absolute Fahrenheit&lt;/em&gt;&quot; I suppose.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GQJZb.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GQJZb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;  &lt;a href=&quot;https://i.stack.imgur.com/KpU2X.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KpU2X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;NOTE:&lt;/strong&gt; below is from &lt;a href=&quot;http://lpre.de/resources/articles/AIAA-1998-3361.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://lpre.de/resources/articles/AIAA-1998-3361.pdf&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5Gn8s.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5Gn8s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;  &lt;a href=&quot;https://i.stack.imgur.com/Gbhbr.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Gbhbr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;  &lt;a href=&quot;https://i.stack.imgur.com/y2CX3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y2CX3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;  &lt;a href=&quot;https://i.stack.imgur.com/O7PCy.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/O7PCy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;NOTE:&lt;/strong&gt; below is from: &lt;a href=&quot;http://booksite.elsevier.com/9780750683661/Appendix_C.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://booksite.elsevier.com/9780750683661/Appendix_C.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/y8d7z.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y8d7z.png&quot; alt=&quot;LOX liquid density&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2019-11-07T00:20:40.333" Title="Does the NK-33 engine require subcooled kerosene so cold that it turns to wax?" Tags="|fuel|engine-design|cryogenics|nk-33|sub-cooled|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14457" PostTypeId="2" ParentId="14454" CreationDate="2016-03-15T08:25:58.963" Score="4" Body="&lt;p&gt;Smashing into something at those velocities, even metals can behave pretty much like putty or a really wet snowball, &lt;a href=&quot;https://www.youtube.com/watch?v=QfDoQwIAaXg&quot; rel=&quot;nofollow&quot;&gt;as exemplified here&lt;/a&gt;. Judging by the appearances of previous impacts, I would hazard to say that an impact on the Moon is pretty much like that: like a wet snowball smacking into an inch thick layer of wet snow/slush. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also on the Moon you have no atmosphere, meaning that anything that gets tossed up and out go in a nice Newtonian parabola without any air getting into the way. With initial speeds of 1000 m/s or more, some particles may go very far. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Hence: your nuclear engine would be pulverized and throw &quot;hot&quot; dust and chunks wide and far. This is a bad idea.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And since we cannot expect perfectly uniform distribution, but instead that the spent nuclear fuel may come out in sizable chunks (&quot;sizable&quot; in this context meaning a few millimeters), you may end up with some very nasty &quot;hot-spots&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is exacerbated — again — by the fact that the Moon has no atmosphere, meaning that the radiation intensity of these hot-spots will not diminish by the cube of the distance as on Earth but &quot;only&quot; by the square of the distance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Better to just &lt;a href=&quot;https://en.wikipedia.org/wiki/US-A&quot; rel=&quot;nofollow&quot;&gt;put the nuclear engine in a parking/disposal orbit&lt;/a&gt; until you feel like bringing it home in a controlled manner.&lt;/p&gt;&#xA;" OwnerUserId="10493" LastEditorUserId="3065" LastEditDate="2016-03-15T21:52:36.393" LastActivityDate="2016-03-15T21:52:36.393" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14458" PostTypeId="2" ParentId="14456" CreationDate="2016-03-15T08:39:21.607" Score="5" Body="&lt;p&gt;First, the only place I have been able to find the equal density claim is in the NK-33 Wikipedia article. (In fact it originates from the &lt;a href=&quot;https://en.wikipedia.org/w/index.php?title=NK-33&amp;amp;oldid=36281337&quot; rel=&quot;nofollow&quot;&gt;first version&lt;/a&gt;). There have never been provided any source for it. On the other hand, &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/2.5210?journalCode=jpp&quot; rel=&quot;nofollow&quot;&gt;this paper&lt;/a&gt; seems to suggest that that is indeed the cases, although indirectly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Polymerization does not necessarily have to be a problem, as &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;nofollow&quot;&gt;RP-1&lt;/a&gt; is by default low on sulphur, alkenes and aromatics. The Russians may have used a stricter specification as well. (The point of which hydrocarbons polymerize is strongly dependent on composition, and pushing it past the &quot;supercooled&quot; range should not be a problem.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The claim that the NK-33 needed similar densities in order to operate seems odd, as it is not the only closed-cycle engine, just one out of many with a great variety of propellants. The article does however mention a single &lt;em&gt;rotating&lt;/em&gt; shaft, so the issue may be a ressult of centrifugal separation of the liquids.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-15T08:55:54.723" LastActivityDate="2016-03-15T08:55:54.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14459" PostTypeId="2" ParentId="14447" CreationDate="2016-03-15T09:07:40.047" Score="4" Body="&lt;p&gt;In fact there is no penalty for launching an interplanetary vehicle from a higher latitude, apart from the rotation velocity benefit. Even in a polar orbit, it is easy to achieve an escape trajectory parallel to the ecliptic, or if you prefer, at ± 1.85°:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HjvV9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HjvV9.png&quot; alt=&quot;parallel&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the launch window for that only appears two times a day.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-15T09:12:41.303" LastActivityDate="2016-03-15T09:12:41.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14460" PostTypeId="2" ParentId="14456" CreationDate="2016-03-15T11:22:59.277" Score="6" Body="&lt;p&gt;According to Sutton's Rocket Propulsion Elements:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If the two propellants have similar densities (say within 40%) such as NTO &amp;amp; UDMH or LOX and kerosene, and the volume flow of oxidizer and fuel are similar, then the same type of impeller (running at the same speed) can be used on both of them on a single shaft. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It goes on to say that with hydrogen/LOX you definitely want separate shafts. There seems to be a general relationship between impeller speed and propellant density where efficiency is optimal; hydrogen can be pumped on a higher-speed axial impeller while denser propellants want a slower radial or semi-radial impeller. Disclaimer: I am not a hydrodynamicist!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So with a 2.6 LOX:kerosene mass ratio, this seems to say that, within limits, you'd actually want the kerosene &lt;em&gt;less&lt;/em&gt; dense in order to achieve more equal volume flow. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-03-15T11:22:59.277" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14461" PostTypeId="2" ParentId="14435" CreationDate="2016-03-15T12:28:33.253" Score="2" Body="&lt;p&gt;There are already good answers here,  but I'll discuss this without being too mathematical.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's start from a circular orbit.  The velocity in this case is constant, let's call it $v_c$.&lt;br&gt;&#xA;When the necessary delta v is imparted to a body transferring to a higher orbit,  the transfer orbit always have its periapse at the circular orbit and apoapse at new orbit.  From basic orbital mechanics you can tell that the speed of a body is fastest at the periapse.&lt;br&gt;&#xA;Let's say the new periapse speed is given by $v_t$.&lt;br&gt;&#xA;For a body in elliptical orbit that velocity will be given by &#xA;$v_t = \sqrt{(\dfrac{2\mu} {r_p}-\dfrac{\mu} {a})} $&lt;br&gt;&#xA;Which as you can see will increase as the semi major axis of the orbit increases.  Which concludes that the necessary delta v needed at that point will decrease when transferring to a higher orbit. &lt;/p&gt;&#xA;" OwnerUserId="10558" LastActivityDate="2016-03-15T12:28:33.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14462" PostTypeId="2" ParentId="14456" CreationDate="2016-03-15T13:45:13.383" Score="16" Body="&lt;p&gt;Confusion abounds. &lt;a href=&quot;http://spaceflight101.com/spacerockets/soyuz-2-1v/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt; has this to say about the NK-33: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The NK-33 requires sub-cooled Oxygen with a temperature below its boiling point of -183 degrees Celsius to cool the turbopump bearings that would otherwise fail. Also, sub-cooled LOX has a higher density, close to that of Kerosene, reducing required tank volume and overall launch vehicle mass and allowing the engine to use a single rotating shaft for both turbopumps.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So they're flipping the statement, saying the LOX has to be subcooled to get the right density, not the kerosene. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia (and corroborated by uhoh's diagrams), LOX at any temperature has a higher density than kerosene. So subcooling the LOX increases the density difference. But Spaceflight 101 may have a point. The engine has burns 2.8 kg of oxygen with 1 kg of kerosene. When you make the LOX denser, you bring the volumes of the two closer together, which might make pump design on a common shaft easier.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Aerojet experiments&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Let's see if we can get a statement from source: this &lt;a href=&quot;http://lpre.de/resources/articles/AIAA-1998-3361.pdf&quot; rel=&quot;noreferrer&quot;&gt;AIAA paper by Aerojet&lt;/a&gt; details the modifications done by them. It mentions density only once (table 6 on page 16), and that same table lists fuel temperature as -30 °F (-34 °C), so it'd be nowhere near the same density as the LOX. Anyway, this paper confirms the kerosene does not have to be subcooled for the engine to work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They did tests running the engine with ambient-temperature RP1 and boiling-point LOX, as far as I can see they did burns of up to 140 seconds with this combination. They did no tests with RP-1 subcooled below -37 °F (-38 °C). They also mention the Russians normally used kerosene at above -30 °F (-34 °C).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this I conclude subcooling of either LOX or propellant is possible, but not necessary on this engine. So the question &quot;Does the NK-33 engine require subcooled kerosene?&quot; can be answered with 'No'.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Other engines&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Single-shaft turbopumps are common in all sorts of rocket engines. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Kerosene/LOX engines like the &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-0110&quot; rel=&quot;noreferrer&quot;&gt;RD-0110&lt;/a&gt;, which does not use subcooling&lt;/li&gt;&#xA;&lt;li&gt;hydrogen/LOX engines like the &lt;a href=&quot;http://www.rocket-propulsion.info/resources/articles/AIAA-2006-4377.pdf&quot; rel=&quot;noreferrer&quot;&gt;RD-0120&lt;/a&gt;, where the density of LOX and LH are much farther apart than the densities of LOX/RP-1. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6642" LastEditDate="2016-10-05T07:19:41.507" LastActivityDate="2016-10-05T07:19:41.507" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14463" PostTypeId="1" AcceptedAnswerId="14464" CreationDate="2016-03-15T14:36:44.237" Score="2" ViewCount="426" Body="&lt;p&gt;I was reading the &lt;a href=&quot;http://www.nasa.gov/press-release/engine-test-marks-major-milestone-on-nasa-s-journey-to-mars&quot; rel=&quot;nofollow&quot;&gt;press release&lt;/a&gt; for the recent test firing of the RS-25 engine No. 2059 at Stennis Space Center and I came across the quote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The next time rocket engine No. 2059 fires for that length of time, it will be carrying humans on their first deep-space mission in more than 45 years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To me this implies that the next time this engine will be used is in around 8 (maybe more) years time when they use SLS to launch astronauts on the Asteroid Redirect Mission &lt;a href=&quot;http://www.nasa.gov/press/2015/march/nasa-announces-next-steps-on-journey-to-mars-progress-on-asteroid-initiative&quot; rel=&quot;nofollow&quot;&gt;(ARM)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is there such a delay in using this engine (which has flown several Shuttle missions before) again? Or is it an incorrect statement that they have made in this press release? &lt;/p&gt;&#xA;" OwnerUserId="13176" LastActivityDate="2018-09-03T22:02:09.567" Title="Why is there such a long delay in between test firing and launching the SLS engine No. 2059?" Tags="|nasa|engines|sls|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14464" PostTypeId="2" ParentId="14463" CreationDate="2016-03-15T20:20:13.953" Score="1" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: my original answer was incorrect. NASA has already allocated engines to specific launches: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.nasaspaceflight.com/2014/05/four-shuttle-veterans-drive-sls-uphill-maiden-flight/&quot; rel=&quot;nofollow&quot;&gt;May 2014, NASA allocated four engines&lt;/a&gt; for the first launch of SLS: ME-2045, ME-2056, ME-2058, and ME-2060. Around the same time, they were wrapping up preparations on the SSME engine test stand. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In May 2014, NASA also selected 4 contingency engines for SLS EM-1: ME-2047, ME-&lt;strong&gt;2059&lt;/strong&gt;, ME-2062, and ME-2063. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a series of test fires of development engines (to test the new engine controller, for example).&lt;br&gt;&#xA;ME-2059 was the first flight engine to be tested in this new configuration, &lt;a href=&quot;https://www.nasaspaceflight.com/2016/01/sls-engine-tests-ar-focus-rs-25/&quot; rel=&quot;nofollow&quot;&gt;this served as a calibration run&lt;/a&gt; of the new configuration. The other engines will also be tested, then there'll be a test of the entire first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If ME-2059 is not needed in its contingency role, it'll be used for SLS &lt;a href=&quot;https://www.nasaspaceflight.com/2016/01/sls-engine-tests-ar-focus-rs-25/&quot; rel=&quot;nofollow&quot;&gt;EM-2&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.nasaspaceflight.com/2015/01/nasa-ready-power-up-rs-25-engine/&quot; rel=&quot;nofollow&quot;&gt;early 2015&lt;/a&gt;, the plan was to test the &lt;a href=&quot;https://www.nasaspaceflight.com/2015/01/nasa-ready-power-up-rs-25-engine/&quot; rel=&quot;nofollow&quot;&gt;EM-1 engines first&lt;/a&gt;. I don't know what has changed that, maybe something as simple as scheduling of the upgrade work to the various engines. There have also been delays in &lt;a href=&quot;https://www.nasaspaceflight.com/2016/01/sls-engine-tests-ar-focus-rs-25/&quot; rel=&quot;nofollow&quot;&gt;getting the test stand ready&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-05-17T12:14:40.617" LastActivityDate="2016-05-17T12:14:40.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14467" PostTypeId="1" AcceptedAnswerId="14469" CreationDate="2016-03-16T06:55:20.543" Score="4" ViewCount="705" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; See my &lt;a href=&quot;https://space.stackexchange.com/a/14489/12102&quot;&gt;supplementary answer&lt;/a&gt; below for additional information. @@SE - stop firing the good guys nailed it.&lt;/p&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/q/5522/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/14455/12102&quot;&gt;answer&lt;/a&gt; started me wondering - how big is the J2 (oblateness) term of Mars' gravity compared to earth? Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit#Technical_details&quot; rel=&quot;nofollow noreferrer&quot;&gt;sun-synchronous&lt;/a&gt; orbits are usually around 98 degrees inclination (retrograde). That question states MOM is at about 150 degrees, which is &amp;quot;very&amp;quot; retrograde, almost equatorial retrograde. Is this because Mars has a non-zero J2, the same sign but significantly weaker than that of earth?&lt;/p&gt;&#xA;&lt;p&gt;If so, &lt;em&gt;what is the value of J2 for Mars and Earth&lt;/em&gt;? Does MOM's 150 degree inclination make sense quantitatively (need &amp;quot;math&amp;quot; answer, not just &amp;quot;words&amp;quot; answer) This &lt;a href=&quot;https://space.stackexchange.com/a/5314/12102&quot;&gt;answer&lt;/a&gt; has helpful explanations, as does the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit#Technical_details&quot; rel=&quot;nofollow noreferrer&quot;&gt;wiki&lt;/a&gt; page. This ArXiv paper &lt;a href=&quot;http://arxiv.org/abs/1108.4737&quot; rel=&quot;nofollow noreferrer&quot;&gt;Five Special Types of Orbits Around Mars&lt;/a&gt; also compares Earth and Mars orbits.&lt;/p&gt;&#xA;&lt;p&gt;(also it looks like that &lt;a href=&quot;https://space.stackexchange.com/questions/5522/why-is-mom-in-a-very-retrograde-orbit&quot;&gt;question&lt;/a&gt; is still waiting for a more complete answer)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; here is some information about the orbit from &lt;a href=&quot;https://planetary.s3.amazonaws.com/assets/resources/ISRO/Mars-atlas-MOM.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this pdf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Periapsis:&lt;/strong&gt;    350km&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Apo-apsis:&lt;/strong&gt;    71,000km&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Inclination:&lt;/strong&gt;  150 degrees&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Sun Elevation:&lt;/strong&gt;  6.8 degrees  (what is this??)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Period:&lt;/strong&gt;   72 hours&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shot from page 6 of:  &lt;a href=&quot;https://planetary.s3.amazonaws.com/assets/resources/ISRO/Mars-atlas-MOM.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://planetary.s3.amazonaws.com/assets/resources/ISRO/Mars-atlas-MOM.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/61tfQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/61tfQ.png&quot; alt=&quot;MOM Mars Orbiter Mission (orbit around Mars)&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-06-09T02:24:54.503" LastActivityDate="2021-06-09T02:24:54.503" Title="MOM's NOT-so-sun-synchronous orbit about Mars - how large is it's J2?" Tags="|mars|low-earth-orbit|mars-orbiter-mission|sun-synchronous|python|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14468" PostTypeId="1" AcceptedAnswerId="14470" CreationDate="2016-03-16T10:27:52.267" Score="10" ViewCount="2114" Body="&lt;p&gt;Assume you have a sophisticated nuclear (thermal or whatever comes to mind) propelled spaceship for your summer vacation on Mars. Half way there Houston calls and informs you about an inbound solar storm. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, you bought your ship at &quot;Honest John's used space ship and spare parts&quot; and you have no radiation shelter on board. So in a desperate attempt to save you and your crew, you turn the ship so that the nuclear reactor points at the sun in the hope that its radiation shielding saves you from the solar storm.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is this in principle even possible, i.e. is the radiation emanating from a solar storm directed? Maybe when paired with additional magnetical shielding?&lt;/li&gt;&#xA;&lt;li&gt;Is it practically feasible, i.e. can you turn your ship in the time you have after a warning? Could you have your ship always pointing away from the sun?&lt;/li&gt;&#xA;&lt;li&gt;Is it economical, i.e. would there be additional requirements on the shielding of your nuclear reactor and would they outweigh a typical storm shelter? I know this depends on your reactor size, so I am just asking for order of magnitudes here.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="8693" LastEditDate="2016-03-16T19:37:31.910" LastActivityDate="2016-03-16T22:58:18.063" Title="Would a nuclear propelled spaceship still need a storm shelter?" Tags="|radiation|nuclear|advanced-propulsion|shielding|solar-wind|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14469" PostTypeId="2" ParentId="14467" CreationDate="2016-03-16T10:41:31.343" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;https://janus.astro.umd.edu/astro/calculators/pcalcframe.html&quot; rel=&quot;nofollow&quot;&gt;planet calculator&lt;/a&gt; lists a J2 of $0.001960454$ for Mars, higher than the Earth's $0.001082627$. That makes sense, as Mars has a smaller mass, but still approximately the same rotation rate. That means achieving a sun-synchronous around Mars is slightly easier than around Earth. However, MOM is not in such an orbit.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-16T11:05:08.763" LastActivityDate="2016-03-16T11:05:08.763" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14470" PostTypeId="2" ParentId="14468" CreationDate="2016-03-16T10:58:41.927" Score="17" Body="&lt;p&gt;The basic idea here is to turn to have the shield you have towards the Sun. That does actually work, because the radiation from the Sun is directed, with a few exceptions: First, inside a planetary magnetosphere, charged particles are bent, and form radiation belts, for example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Van_Allen_radiation_belt&quot; rel=&quot;noreferrer&quot;&gt;Van Allen belts&lt;/a&gt;. There, shielding is a bit more difficult. Secondly, that is also a problem close to the poles of the Sun, but there &quot;avoiding melting&quot; is a bigger concern. As most spacecraft have attitude control systems, like RCS thrusters, it can be rotated in just a few minutes. That is by far shorter than the warning time for solar flares. (Even in the worst cases several hours.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But!&lt;/strong&gt; Solar flares are not the only radiation concern in space, as the cosmic radiation is large enough to pose a serious threat to the health of the cosmonauts. In order to protect them from that, you are going to need a shelter in either case, making use of just the engine shield insufficient.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="6816" LastEditDate="2016-03-16T22:58:18.063" LastActivityDate="2016-03-16T22:58:18.063" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="14472" PostTypeId="2" ParentId="5522" CreationDate="2016-03-16T13:25:13.257" Score="5" Body="&lt;p&gt;According to the abstract &lt;a href=&quot;http://issfd.org/2015/files/downloads/abstracts/016_B-S.pdf&quot; rel=&quot;noreferrer&quot;&gt;Target Martian Orbit Selection for ISRO Mars Orbiter Mission&lt;/a&gt;, a number of constraints were taken into account:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The other orbit parameters of Inclination, Right Ascension of Ascending Node and Argument of Perigee were selected after a detailed analysis to meet the following Mission constraints:- maximum eclipse period should be less than 100 min; imaging of Mars full disc from apoapsis region should be possible from Day 1 after MOI; orbit periapsis altitude should be stable at least for one year considering all the perturbative forces. The achievable range of Martian orbit inclination values were found to be from 29 deg to 151 deg in Mars IAU frame of reference, since the declination of the V-infinity of the Mars arrival hyperbola was -29 deg. Seven orbits, two direct, one polar and four retrograde orbits were targeted in the maneuver design and studied with respect to the delta-V cost of achieving the orbit, the sun-orbit geometry after MOI, the eclipse characteristics, orbit stability and imaging opportunities. The star sensor mounting configuration requirement and occult management was also studied for each of the orbits. The retrograde orbit of size 500 x 80000 km, with 151 deg inclination, ascending node of 61 deg and argument of perigee of 206 deg, was found to be the most suitable orbit satisfying all the specified constraints as well as from the viewpoint of star sensor mounting and management of star sensor occults. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This abstract is on the &lt;a href=&quot;http://issfd.org&quot; rel=&quot;noreferrer&quot;&gt;ISSFD website&lt;/a&gt; in the 2015 abstracts folder, but I can't find the full paper, nor any evidence on the site that the paper was presented in the 2015 conference. There's another abstract from the ISRO flight dynamics group on &lt;a href=&quot;http://issfd.org/2015/files/downloads/abstracts/018_B-S.pdf&quot; rel=&quot;noreferrer&quot;&gt;the overall mission&lt;/a&gt;, again with no full paper that I can find; it's possible that the ISRO FDG planned to attend and present these and then had to cancel.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-03-16T13:51:40.710" LastActivityDate="2016-03-16T13:51:40.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14474" PostTypeId="2" ParentId="8698" CreationDate="2016-03-16T15:18:59.320" Score="6" Body="&lt;p&gt;Xenon, as a rare gas, has high fluctuations in price over time.   Factors including who sources it, what purity is required, and how much is being purchased at once will all impact the cost.  For something like fueling an ion propulsion engine, my guess is there would be a large bulk purchase with specially negotiated rates directly from the air separation units.  The general population can buy xenon by the liter in gaseous form from most compressed gas companies.  Over the last 15 years that I have worked in the compressed gas and cryogenic liquid field, I have seen 1 liter of xenon sell at US\$5.50/L for industrial grade up to US\$19.50/L for grade 5.5 (99.9995% pure).  I do not know what grade of material is required for these engines.&lt;/p&gt;&#xA;" OwnerUserId="13784" LastEditorUserId="4660" LastEditDate="2016-03-16T15:38:18.133" LastActivityDate="2016-03-16T15:38:18.133" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14475" PostTypeId="1" CreationDate="2016-03-16T15:26:59.127" Score="4" ViewCount="137" Body="&lt;p&gt;What purity of xenon is required for the xenon ion propulsion system?  Does the purity impact the efficiency of the engine? &lt;/p&gt;&#xA;" OwnerUserId="13784" LastActivityDate="2016-03-16T15:48:42.777" Title="XIPS purity requirements?" Tags="|propulsion|fuel|advanced-propulsion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14476" PostTypeId="2" ParentId="14475" CreationDate="2016-03-16T15:48:42.777" Score="3" Body="&lt;p&gt;This depends on &lt;a href=&quot;http://descanso.jpl.nasa.gov/SciTechBook/series1/Goebel__cmprsd_opt.pdf&quot; rel=&quot;nofollow&quot;&gt;details of the engine construction&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;One of the major drawbacks of using BaO dispenser cathodes in electric propulsion applications is the extremely high feed gas purity specified to avoid these poisoning and tungsten-material transport issues, which has resulted in a special “propulsion-grade” xenon with 99.9995% purity to be specified by some users of these cathodes for flight.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Because chemistry is involved in the formation of the low work function&#xA;  surface, dispenser cathodes are subject to poisoning that can significantly&#xA;  increase the work function [17]. Some care must be taken in handling the&#xA;  inserts and in the vacuum conditions used during operation of these  cathodes to avoid poisoning by impurities in the gas that produce unreliable emission and shorten the lifetime. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The most potent poisons for both (dispenser cathodes [17] and LaB&lt;sub&gt;6&lt;/sub&gt; cathodes [55] -ed) cathodes are oxygen and water...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Oxygen partial pressures in the 10&lt;sup&gt;–7&lt;/sup&gt; torr range can completely poison the dispenser cathode at temperatures of 1100˚C...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;For the case of xenon ion thrusters, LaB&lt;sub&gt;6&lt;/sub&gt; cathodes can tolerate the crudest grade of xenon available (~99.99% purity) without affecting the LaB&lt;sub&gt;6&lt;/sub&gt; electron emission or life. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-16T15:48:42.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14477" PostTypeId="2" ParentId="14454" CreationDate="2016-03-16T17:05:27.937" Score="4" Body="&lt;p&gt;Why do you want to get rid of your engine in the first place? A trip to the moon would hardly use all the nuclear fuel you got on board. Of course it depends on the intensity of your burns, but current wet-navy ships (submarines or carriers) reactors are only refueled every year or so. So your engine should last for several trips. After that you can probably afford to park it on some remote orbit.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="11256" LastEditDate="2016-03-16T18:10:40.593" LastActivityDate="2016-03-16T18:10:40.593" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14478" PostTypeId="2" ParentId="14468" CreationDate="2016-03-16T19:21:26.457" Score="3" Body="&lt;p&gt;Yes, such a ship would still need a storm shelter. The reactor shielding is not enough: when you're in a solar storm (i.e. a cloud of particles ejected by the Sun), the particles &lt;a href=&quot;https://space.stackexchange.com/questions/14353/sources-of-radiation-on-mars&quot;&gt;will come at you from all directions&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Warning in time is possible: &lt;a href=&quot;http://www.swpc.noaa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA already does this&lt;/a&gt;. The &lt;a href=&quot;http://soho.nascom.nasa.gov/explore/faq.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHO&lt;/a&gt; satellite observes solar flares and CMEs. Because the flare/CME moves at in the region of &lt;a href=&quot;http://www.swpc.noaa.gov/phenomena/coronal-mass-ejections&quot; rel=&quot;nofollow noreferrer&quot;&gt;300-3000 km/s&lt;/a&gt; (i.e. up to 1% of light speed) we get ~ 12 hours advance warning on Earth, more if you're further out in the solar system.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of using mass for shielding, I suspect a nuclear-powered spaceship would rely on a &lt;a href=&quot;https://space.stackexchange.com/questions/3772/how-much-power-would-a-spacecrafts-magnetic-shield-require/8666#8666&quot;&gt;magnetic shield.&lt;/a&gt; This would require on the order of 500 kW, well within the power generation capability of even a small nuclear reactor. This also avoids nasty &lt;a href=&quot;https://space.stackexchange.com/questions/4706/how-are-astronauts-protected-from-radiation-in-a-spacecraft/4712#4712&quot;&gt;secondary radiation&lt;/a&gt; effects you can get from mass shielding. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-03-16T20:57:54.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14480" PostTypeId="1" AcceptedAnswerId="14481" CreationDate="2016-03-16T22:35:43.733" Score="3" ViewCount="515" Body="&lt;p&gt;I just read a &lt;a href=&quot;https://space.stackexchange.com/questions/14161/could-the-apollo-lunar-ascent-stage-have-separated-from-the-descent-stage-before#comment37105_14161&quot;&gt;comment&lt;/a&gt; by @RussellBorogove about the Apollo 11 abort options that the landing module had about 700 pounds of fuel left on touchdown. I assume that this is the intended behavior for any detached craft (you probably always plan in a little reserve that you do not want to use up), I wonder if it is generally possible to use that leftover fuel in the next stage:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In an Apollo-style mission, can you recover leftover fuel from the descent stage for the ascent stage and from the ascent stage for the command module? I.e. can you pump it from the earlier stages to the current stage? Did the Apollo missions do that or did they leave fuel on the moon (and whereever the ascent module went)?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-03-17T00:43:23.393" Title="Can landing modules use left over fuel from earlier stages?" Tags="|apollo-program|landing|fuel|stages|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14481" PostTypeId="2" ParentId="14480" CreationDate="2016-03-16T23:39:59.723" Score="10" Body="&lt;p&gt;In general, no; rockets aren't equipped to pump propellant between stages. You load each stage with enough fuel to do its job, plus a safety margin, and if you discard leftover fuel you congratulate yourself for having built a system with reliable performance, and on future missions, you might load a little less propellant and/or take a little more payload!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the specific case of the Apollo architecture, after the descent, there was known to be more than enough propellant in the LM ascent stage to do the ascent. If the ascent stage underperformed for some reason and launched into too low an orbit, the command/service module carried enough extra propellant to come and get it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Building additional unused tankage capacity into the ascent stage, anticipating tapping some propellant from the descent stage, would have made the ascent stage bigger and heavier, and keeping the weight of the LM to a minimum was a major engineering challenge in the development of Apollo. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in the end, the Apollo spacecraft either vented or abandoned propellant from the descent stage (on the moon), the ascent stage (after CSM/LM rendezvous), and the service module (before separation from the CM prior to reentry). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the LM ascent stage did have separate tankage for the small RCS thrusters, which used the same propellants as the ascent engine, and it was possible to pump between those tanks! According to &lt;a href=&quot;http://georgetyson.com/files/apollostatistics.pdf&quot;&gt;Apollo By The Numbers&lt;/a&gt;, Apollo 9, 10, 11, and 12 transferred some ascent engine fuel (~20-45kg) to the RCS tanks, and 16 transferred some from the RCS tanks to the ascent tanks. The ascent engine could fire only once, and RCS was used to complete the rendezvous maneuvering with the CSM after ascent, so it makes sense that some unused fuel might be more useful in the RCS tanks at that point, but 130-190kg of propellants remained unused in the ascent tanks on all the landing missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As SF. reminds me, the propellant left in the descent stage could have been used by simply reigniting the descent stage when it was time to leave and beginning the ascent with that, then staging onto the ascent stage when descent fuel ran out. This would be a similar flight plan to the LM's abort-from-descent option. All it would accomplish would be saving more fuel to discard later in the mission. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-03-17T00:42:39.347" LastActivityDate="2016-03-17T00:42:39.347" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14482" PostTypeId="2" ParentId="14480" CreationDate="2016-03-16T23:40:28.563" Score="3" Body="&lt;p&gt;The Apollo Lunar Module had no fuel transfer capabilities, nor did the astronauts have any equipment to transfer it. Therefore, any left-over fuel was vented, in the case of both stages. (venting is mentioned multiple places in the &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/AS11_TEC.PDF&quot; rel=&quot;nofollow&quot;&gt;mission transcripts&lt;/a&gt;, for example page 319 of the linked PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A concept for using fuel from the ascent stage to power a smaller &quot;life boat&quot; was considered during the program, but never implemented. (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Escape_Systems&quot; rel=&quot;nofollow&quot;&gt;Lunar Escape System&lt;/a&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="195" LastEditDate="2016-03-16T23:45:21.677" LastActivityDate="2016-03-16T23:45:21.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14483" PostTypeId="1" CreationDate="2016-03-17T04:14:34.163" Score="0" ViewCount="209" Body="&lt;p&gt;What exoplanet atmospheres might support life beside our nitrogen and oxygen atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related question: Could an exoplanet atmosphere be very similar to our own, but contain some component toxic to human life (free chlorine or hydrogen sulfide, for example)? &lt;/p&gt;&#xA;" OwnerUserId="13794" LastActivityDate="2016-03-17T08:38:41.777" Title="What exoplanet atmospheres might support life beside our nitrogen and oxygen atmosphere?" Tags="|atmosphere|life|astrobiology|exoplanet|" AnswerCount="1" CommentCount="3" ClosedDate="2016-03-17T16:13:23.933" ContentLicense="CC BY-SA 3.0" />
  <row Id="14484" PostTypeId="2" ParentId="14483" CreationDate="2016-03-17T08:38:41.777" Score="2" Body="&lt;p&gt;That depends on your definition of life. If you include any self-replicating automata, you probably do not need an atmosphere at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the question should rather be: Which atmospheres do &lt;strong&gt;not&lt;/strong&gt; support the spontaneous &lt;strong&gt;development&lt;/strong&gt; of life. I would put my bet on anything that is either:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;too dense, i.e. of such a high pressure that no complex molecular structure can surive. In the very extreme, I do not expect life in the &quot;atmosphere&quot; of a neutron star.&lt;/li&gt;&#xA;&lt;li&gt;too empty, i.e. distances between atmons are so large that a merging into more complex molecules is extremely unlikely (think of the moon) &lt;/li&gt;&#xA;&lt;li&gt;too monomorphic, i.e. so pure that there simply are not enough different kind of atoms to form complex molecules. I have no example for that, though.&lt;/li&gt;&#xA;&lt;li&gt;too cold, i.e. there is simply not enough energy to create complex molecules from the existing matter&lt;/li&gt;&#xA;&lt;li&gt;too hot, i.e. there is so much energy that any complex molecule is instantaneously destroyed (e.g. by radiation) again&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All these seem to be rather extreme conditions, but in any other case there is at least the possibility of some form of life.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-03-17T08:38:41.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14485" PostTypeId="1" CreationDate="2016-03-17T08:39:05.027" Score="4" ViewCount="364" Body="&lt;p&gt;Microwave radiometers (MWR) can be used passively, to get regolith's thickness, brightness temperature and dielectric constant. Apart from that, I have little idea of how the instrumentation's parameters affect the measurements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the situation is as such. There are quite a few Earth-based remote sensing MWRs. Can I use them directly for a lunar orbiter? If not, what changes would these have to incorporate in order to be useful, if possible at all? Also, References would be helpful, since I am new to this.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="49" LastEditDate="2016-03-17T11:40:00.710" LastActivityDate="2016-03-17T11:40:00.710" Title="Earth-based microwave radiometer for the Moon" Tags="|the-moon|spacecraft|earth|instrument|remote-sensing|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14486" PostTypeId="1" CreationDate="2016-03-17T10:40:20.177" Score="3" ViewCount="187" Body="&lt;p&gt;Is there a NASA or a Roscosmos safety guidelines or something, stating the minimum crew required to safely operate the TMA-M? I.e. get it into orbit and safely re-enter.&lt;/p&gt;&#xA;" OwnerUserId="13796" LastEditorUserId="6816" LastEditDate="2016-03-18T16:37:12.240" LastActivityDate="2016-03-19T11:31:44.733" Title="Minimum Crew of Soyuz TMA-M" Tags="|soyuz-spacecraft|crew-size|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14487" PostTypeId="2" ParentId="14486" CreationDate="2016-03-17T10:53:07.477" Score="5" Body="&lt;p&gt;In theory 0, as the spacecraft can be &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz-TMA#Soyuz_TMA-M&quot; rel=&quot;noreferrer&quot;&gt;remotely controlled&lt;/a&gt; by the Teleoperated Control Mode present in all Soyuz spacecraft. This has not been used beyond a few rendezvous tests though, so an onboard pilot is still recommended.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-03-17T10:53:07.477" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14488" PostTypeId="2" ParentId="14485" CreationDate="2016-03-17T11:10:38.133" Score="6" Body="&lt;p&gt;China's &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang&amp;#39;e_1&quot;&gt;Chang'e 1&lt;/a&gt; (CE-1) and &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang&amp;#39;e_2&quot;&gt;Chang'e 2&lt;/a&gt; (CE-2) lunar orbiters were both equipped with multi-channel microwave radiometers, measuring thermal emissions from the lunar surface and near-surface down to 30 meters of depth. Similarly, Chinese &lt;a href=&quot;https://en.wikipedia.org/wiki/Yutu_%28rover%29&quot;&gt;Yutu rover&lt;/a&gt; from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang&amp;#39;e_3&quot;&gt;Chang'e 3 mission&lt;/a&gt; is equipped with a &lt;a href=&quot;https://en.wikipedia.org/wiki/Ground-penetrating_radar&quot;&gt;GPR&lt;/a&gt; that also operates in microwave frequencies. NASA, to my knowledge, hasn't yet used &lt;a href=&quot;http://instrumentsystems.jpl.nasa.gov/farir/microrad/index.cfm&quot;&gt;microwave radiometers&lt;/a&gt; in lunar orbit, but they do use them for other than Earth science missions, such as &lt;a href=&quot;http://map.gsfc.nasa.gov/mission/observatory.html&quot;&gt;WMAP&lt;/a&gt; and &lt;a href=&quot;http://juno.wisc.edu/spacecraft_instruments_MWR.html&quot;&gt;Juno&lt;/a&gt;, has flown &lt;a href=&quot;http://www.spudislunarresources.com/Bibliography/p/94.pdf&quot;&gt;MiniSAR&lt;/a&gt; microwave Synthetic Aperture Radar on ISRO's &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandrayaan-1&quot;&gt;Chandrayaan-1&lt;/a&gt; lunar probe, will include a &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0273117715007486&quot;&gt;dual-frequency SAR&lt;/a&gt; on &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandrayaan-2&quot;&gt;Chandrayaan-2&lt;/a&gt;, and MWRs might be included on future lunar polar landers such as &lt;a href=&quot;https://www.nasa.gov/resource-prospector&quot;&gt;Resource Prospector&lt;/a&gt;. Similar design instrument, but of course in a different electromagnetic spectra, is also &lt;a href=&quot;http://diviner.ucla.edu/specifications.shtml&quot;&gt;LRO's Diviner&lt;/a&gt; in IR frequencies from 0.3 to 400 μm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the two Chang'e probes used the closest to if not exactly what you're inquiring about in lunar orbit. There's many papers published and freely accessible online on their mission, science return and to some extent discussing specifics of their design:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;For a broad overview: &lt;a href=&quot;http://sservi.nasa.gov/articles/the-first-microwave-image-of-the-complete-moon/&quot;&gt;SSERVI's article on The First Microwave Image of the Complete Moon&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Poster session presentation: &lt;a href=&quot;http://cahsi.cs.utep.edu/cahsifiles/Files/PostersFinal/LunarSurfaceMicrowaveObservationsBrightness.pdf&quot;&gt;Lunar Surface Microwave Observations of Brightness Temperature Collected with the Chang’E 2&lt;/a&gt;, PDF, Y. M. Rivera-López et al., &lt;/li&gt;&#xA;&lt;li&gt;44th LPSC proceedings paper: &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2013/pdf/1472.pdf&quot;&gt;Analysis of High Frequency Brightness Temperature of Lunar Surface from Chang'e 2 Microwave Radiometer and Investigations of Mean Diurnal Temperature of Regolith Layer&lt;/a&gt;, PDF, W. Fa&#xA;and T. Fang&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also search &lt;a href=&quot;http://t95009.eos-intl.net/T95009/OPAC/Index.aspx&quot;&gt;Lunar and Planetary Institute's library&lt;/a&gt;, &lt;a href=&quot;http://ntrs.nasa.gov/&quot;&gt;NASA Technical Reports Server&lt;/a&gt;, &lt;a href=&quot;https://scholar.google.com/&quot;&gt;Google Scholar&lt;/a&gt; and so on for papers on related topics. Potentially also of interest might be the upcoming &lt;a href=&quot;http://www.hou.usra.edu/meetings/lpsc2016/&quot;&gt;47th Lunar and Planetary Science Conference&lt;/a&gt;, &lt;a href=&quot;http://lunarvolatiles.nasa.gov/&quot;&gt;ISC's Lunar Polar Volatiles portal&lt;/a&gt; with schedule to &lt;a href=&quot;http://lunarvolatiles.nasa.gov/calendar/&quot;&gt;own publicly accessible workshops and other related events of interest&lt;/a&gt;, and so on.&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-03-17T11:34:35.887" LastActivityDate="2016-03-17T11:34:35.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14489" PostTypeId="2" ParentId="14467" CreationDate="2016-03-17T12:32:37.970" Score="2" Body="&lt;p&gt;@SE - stop firing the good guys's &lt;a href=&quot;https://space.stackexchange.com/a/14469/12102&quot;&gt;answer&lt;/a&gt; states MOM is not in a sun-synchronous orbit. I just though I'd add some math to that. Using the equation for &lt;a href=&quot;https://space.stackexchange.com/a/5314/12102&quot;&gt;rotation of ascending node&lt;/a&gt; posted by @MarkAdler, I calculated the &lt;code&gt;delta_OMEGA&lt;/code&gt; - how much the ascending node rotates in one year, for two cases: earth LEO and mars MOM.&lt;/p&gt;&#xA;&lt;p&gt;For earth LEO (400km) I get a little more than 97 degrees, which is about what we expect, suggesting this calculation might be on the right track. Plugging in the numbers for MOM (from information in the question and answer above), it turns out that for MOM's orbit with an inclination of 150 degrees, the rate is only 0.45 of what is needed for sun synchrony.&lt;/p&gt;&#xA;&lt;p&gt;Blue line which has a large amplitude is earth LEO, red line which never exceeds +/- 3.3 radians/year is mars MOM.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zPLLh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zPLLh.png&quot; alt=&quot;MOM is not sun-synchronous&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here is some python I used: (tried to add &lt;a href=&quot;https://meta.stackexchange.com/a/22189/303080&quot;&gt;language-sensitive highlighting&lt;/a&gt; but doesn't seem to work. Suggestions?)&lt;/p&gt;&#xA;    &#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;pi, twopi = np.pi, 2*np.pi&#xA;&#xA;# all distances in km (they cancel, it's OK!)&#xA;&#xA;r_earth = 6371.&#xA;r_mars  = 3393.&#xA;&#xA;# from: https://planetary.s3.amazonaws.com/assets/resources/ISRO/Mars-atlas-MOM.pdf&#xA;alt_peri = 350.&#xA;alt_apo  = 71000.&#xA;r_peri = alt_peri + r_mars&#xA;r_apo  = alt_apo  + r_mars&#xA;&#xA;a_LEO   = r_earth + 400.&#xA;a_MOM   = 0.5*(r_peri + r_apo)&#xA;&#xA;e_LEO = 0.&#xA;e_MOM = (r_apo - r_peri) / (r_apo + r_peri)&#xA;&#xA;# from: https://en.wikipedia.org/wiki/Standard_gravitational_parameter&#xA;GM_earth  = 3.98600442E+14  # m^3/s^2&#xA;GM_earth *= 1E-09           # km^3/s^2&#xA;&#xA;GM_mars   = 4.2828E+13      # m^3/s^2&#xA;GM_mars  *= 1E-09           # km^3/s^2&#xA;&#xA;print &amp;quot;GM_earth, GM_mars: &amp;quot;, GM_earth, GM_mars, &amp;quot;km^3/s^2&amp;quot;&#xA;&#xA;# from: https://space.stackexchange.com/a/14469/12102&#xA;# and: https://janus.astro.umd.edu/astro/calculators/pcalcframe.html&#xA;J2_earth = 0.001082627&#xA;J2_mars  = 0.001960454&#xA;&#xA;inc_deg = np.linspace(0, 180, 181)&#xA;inc     = (pi/180.) * inc_deg&#xA;&#xA;# earth LEO&#xA;term_1 = -1.5 * J2_earth&#xA;term_2 = (r_earth/(a_LEO*(1-e_LEO**2)))**2&#xA;term_3 = np.sqrt(GM_earth/a_LEO**3)&#xA;&#xA;OMEGA_dot_LEO = term_1*term_2*term_3*np.cos(inc)&#xA;&#xA;# mars MOM&#xA;term_1 = -1.5 * J2_mars&#xA;term_2 = (r_mars/(a_MOM*(1-e_MOM**2)))**2&#xA;term_3 = np.sqrt(GM_earth/a_MOM**3)&#xA;&#xA;OMEGA_dot_MOM = term_1*term_2*term_3*np.cos(inc)&#xA;&#xA;year_earth = 365.25 * 24. * 3600.  # sec&#xA;year_mars  = 686.97 * 24. * 3600. &#xA;&#xA;delta_OMEGA_LEO = OMEGA_dot_LEO * year_earth&#xA;delta_OMEGA_MOM = OMEGA_dot_MOM * year_mars&#xA;&#xA;print inc_deg[150], delta_OMEGA_MOM[150], delta_OMEGA_MOM[150]/twopi&#xA;&#xA;thing = {'ha': 'left',  'va': 'bottom', 'bbox': None,&#xA;         'fontsize':16}&#xA;plt.figure(figsize=[8,10])&#xA;ax1 = plt.subplot(2, 1, 1)&#xA;for item in (ax1.get_xticklabels() + ax1.get_yticklabels()):&#xA;    item.set_fontsize(15)&#xA;plt.plot(inc_deg, delta_OMEGA_LEO, '-b', lw=2)&#xA;plt.plot(inc_deg, delta_OMEGA_MOM, '-r', lw=2)&#xA;plt.plot(97, twopi, 'ok')&#xA;plt.plot(inc_deg, np.zeros_like(inc_deg), '-k')&#xA;plt.plot(inc_deg, twopi*np.ones_like(inc_deg), '--k')&#xA;plt.plot([90, 90], [-100, 100], '-k')&#xA;plt.xlim(80, 100)&#xA;plt.ylim(-10, 10)&#xA;plt.text(91, twopi+0.5, &amp;quot;2 pi&amp;quot;, thing)&#xA;ax2 = plt.subplot(2, 1, 2)&#xA;for item in (ax2.get_xticklabels() + ax2.get_yticklabels()):&#xA;    item.set_fontsize(15)&#xA;plt.plot(inc_deg, delta_OMEGA_LEO, '-b', lw=2)&#xA;plt.plot(inc_deg, delta_OMEGA_MOM, '-r', lw=2)&#xA;plt.plot(97, twopi, 'ok')&#xA;plt.plot(inc_deg, np.zeros_like(inc_deg), '-k')&#xA;plt.plot(inc_deg, twopi*np.ones_like(inc_deg), '--k')&#xA;plt.plot([90, 90], [-100, 100], '-k')&#xA;plt.xlim(80, 180)&#xA;plt.ylim(-20, 60)&#xA;plt.text(120, twopi+2.0, &amp;quot;2 pi&amp;quot;, thing)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-06-09T02:23:37.373" LastActivityDate="2021-06-09T02:23:37.373" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14490" PostTypeId="1" CreationDate="2016-03-17T13:45:54.503" Score="3" ViewCount="230" Body="&lt;p&gt;How effective are different types of radar in space over large distances?  Is radar significantly different outside of the atmosphere of Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would you need to generate a more powerful signal to cover multiple AU distances?  Or would you need a more sensitive type of receiver?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would passive radar be less effective?  Do signals of opportunity even bounce off objects in space well enough to register in a passive radar system?  There are a lot of loud objects in space that constantly generate noise, so I assume passive radar might actually be fairly accurate.&lt;/p&gt;&#xA;" OwnerUserId="2709" LastEditorUserId="6816" LastEditDate="2016-07-23T08:18:55.737" LastActivityDate="2019-06-24T03:40:22.460" Title="Is radar &amp; passive radar significantly different in space?" Tags="|instrument|radar|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14491" PostTypeId="1" CreationDate="2016-03-17T15:28:42.083" Score="0" ViewCount="2007" Body="&lt;p&gt;I'm just wondering how long it takes satellites from launch to orbiting in MEO.&lt;/p&gt;&#xA;" OwnerUserId="13799" LastEditorUserId="6816" LastEditDate="2016-03-17T15:49:50.327" LastActivityDate="2016-03-17T15:49:50.327" Title="How long does it take for a satellite to reach MEO?" Tags="|launch|orbit|time|" AnswerCount="1" CommentCount="1" ClosedDate="2016-03-18T02:42:53.563" ContentLicense="CC BY-SA 3.0" />
  <row Id="14492" PostTypeId="2" ParentId="14491" CreationDate="2016-03-17T15:49:47.913" Score="2" Body="&lt;p&gt;It will vary quite a bit with the exact MEO orbit and launcher used, but typically it would be something like:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;10-15 minutes launch from surface to a MEO transfer orbit&lt;/li&gt;&#xA;&lt;li&gt;Coasting to apogee, anywhere from half an hour to several hours&lt;/li&gt;&#xA;&lt;li&gt;Circularization burn taking a few minutes&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That gets the satellite into the desired orbit, but the satellite's mission may require it to be in a particular position in that orbit as well; depending on the satellite's capabilities, the phasing maneuvers could take days or even weeks (for ion thruster-powered sats, e.g.) to carry out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's &lt;a href=&quot;http://spaceflight101.com/atlas-v-gps-iif11/launch-profile/&quot; rel=&quot;nofollow&quot;&gt;a typical example&lt;/a&gt; of a launch to 20,000km circular orbit inclined at 55º: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;~4 minutes ascent on the first stage&lt;/li&gt;&#xA;&lt;li&gt;~13 minutes on the second stage&lt;/li&gt;&#xA;&lt;li&gt;3 hour coast to apogee&lt;/li&gt;&#xA;&lt;li&gt;about 1.5 minute circularization burn &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-03-17T15:49:47.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14493" PostTypeId="1" AcceptedAnswerId="14494" CreationDate="2016-03-17T15:54:15.780" Score="28" ViewCount="5086" Body="&lt;p&gt;I just saw this &lt;a href=&quot;http://cdn.phys.org/newman/gfx/news/hires/2016/1-whattheweath.jpg&quot; rel=&quot;noreferrer&quot;&gt;image&lt;/a&gt; in this &lt;a href=&quot;http://phys.org/news/2016-03-weather-star-life.html&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; in Phys.org, which is generally a great source of physics news. Normally I would expect them to post real images, or if they use artist's conceptions, they would flag them as such, but this is just &quot;blowing me away&quot; so to speak. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The caption, including credit: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;&lt;em&gt;Our sun burps, on August 31, 2012, flinging material into space as part of a coronal mass ejection. Credit: Flickr/NASA Goddard Space Flight Centre, CC BY&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is this a real image? If so, what satellite took the image?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UJu6w.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UJu6w.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-03-17T16:11:05.020" LastActivityDate="2016-03-18T01:53:04.303" Title="Is this really an image of the sun, or an &quot;artist's conception&quot;?" Tags="|observation|imaging|the-sun|solar-dynamic-observatory|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14494" PostTypeId="2" ParentId="14493" CreationDate="2016-03-17T16:04:31.367" Score="28" Body="&lt;p&gt;You can find the image on &lt;a href=&quot;https://www.flickr.com/photos/gsfc/7931831962/in/photolist-d5UHAf-d5UHiu-d5UU7h&quot;&gt;Flickr&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On August 31, 2012 a long filament of solar material that had been hovering in the sun's atmosphere, the corona, erupted out into space at 4:36 p.m. EDT. The coronal mass ejection, or CME, traveled at over 900 miles per second. The CME did not travel directly toward Earth, but did connect with Earth's magnetic environment, or magnetosphere, causing aurora to appear on the night of Monday, September 3.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Pictured here is a lighten blended version of the 304 and 171 angstrom wavelengths. Cropped&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The picture was taken by &lt;a href=&quot;https://www.nasa.gov/mission_pages/sdo/spacecraft/index.html&quot;&gt;the Solar Dynamics Observatory&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, it's a real picture, but taken at UV wavelengths that has been processed to get a visible-light image. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-17T16:04:31.367" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14495" PostTypeId="2" ParentId="14490" CreationDate="2016-03-17T16:42:08.827" Score="8" Body="&lt;p&gt;Many questions!&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How effective are different types of radar in space over large&#xA;  distances. Is radar significantly different outside of the atmosphere&#xA;  of Earth?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The only difference between a radar inside the atmosphere and in space is the lack of air to dim the signal. Instead, a usually small angular size of the target, caused by the large distances.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would you need to generate a more powerful signal to cover multiple AU&#xA;  distances? Or would you need a more sensitive type of receiver?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Radar is, after all, just the same as shining a flash-light at something, just with radio waves instead of light. Both increasing the brightness of the flash-light, and the sensitivity of the receiver would naturally help, however, hitting the target accurately over such large distances is difficult, which brings us to the next question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would passive radar be less effective? Do signals of opportunity even&#xA;  bounce off objects in space well enough to register in a passive radar&#xA;  system?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is in fact a lot of radio noise in space, some of which is bouncing off other objects, or disturbed by for example clouds of gas. Listening to that noise is a field known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Radio_astronomy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radio Astronomy&lt;/a&gt;, one of the major ways of observing far away objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For planetary science, radar has also been used, most notably by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Magellan_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Magellan probe&lt;/a&gt; for penetrating the thick atmosphere of Venus to generate usable images of the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AArET.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AArET.jpg&quot; alt=&quot;Arecibo&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="10273" LastEditDate="2019-06-24T02:56:06.693" LastActivityDate="2019-06-24T02:56:06.693" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14496" PostTypeId="1" CreationDate="2016-03-17T17:48:25.367" Score="2" ViewCount="96" Body="&lt;p&gt;We are designing a small satellite lunar mission. In the preliminary design submitted for approval, most of the focus was on technological demonstration of acquired COTS hardware, flight systems, guidance navigation and control, propulsion and telemetry, tracking and command. The proposal, along with preliminary design report, is to be submitted to &lt;a href=&quot;http://www.isro.gov.in/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Indian Space Research Organisation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some time back, it was suggested, unofficially, by administration responsible for approval, that a scientific objective was necessary. So, we are currently looking at prospects of on-orbit science that can be conducted aboard a small satellite in a lunar orbit. This is again constrained by the fact that payload cannot be developed and must be acquired.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what strategies could be taken up in this problem? How do we go about narrowing down to a viable payload of scientific worthiness?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Lunar Exploration Roadmap and other documents and preliminary idea on mass and power budgets, we did narrow down to surface/sub-surface measurements using Microwave Radiometer (thus, my &lt;a href=&quot;https://space.stackexchange.com/questions/14485/earth-based-microwave-radiometer-for-the-moon&quot;&gt;question&lt;/a&gt;) or Hyper-spectral imaging, specifically over few crucial craters and SPA basin (which is a common attribute for lunar orbiters). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence, any help as to how we should approach this problem of selection of commercially acquirable scientific payload and any other suggestions to form a viable mission?&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-05-24T02:25:47.047" Title="Strategies for scientific objectives aboard small lunar orbiting satellites" Tags="|the-moon|artificial-satellite|planetary-science|design|science|" AnswerCount="0" CommentCount="6" ClosedDate="2016-04-15T18:41:58.187" ContentLicense="CC BY-SA 3.0" />
  <row Id="14497" PostTypeId="1" CreationDate="2016-03-17T18:20:27.397" Score="2" ViewCount="143" Body="&lt;p&gt;Why are the cameras no longer switching views? They have been stuck on down view for several weeks.&lt;/p&gt;&#xA;" OwnerUserId="13803" LastActivityDate="2016-03-17T18:20:27.397" Title="International space station camera views" Tags="|iss|" AnswerCount="0" CommentCount="3" ClosedDate="2016-04-05T19:44:41.290" ContentLicense="CC BY-SA 3.0" />
  <row Id="14498" PostTypeId="2" ParentId="14490" CreationDate="2016-03-17T21:10:56.843" Score="4" Body="&lt;p&gt;The basic problem with using radar in space is range. When you send out a radar pulse, &lt;a href=&quot;https://en.wikipedia.org/wiki/Radar&quot; rel=&quot;nofollow&quot;&gt;the amount of power that returns&lt;/a&gt; to the transmitter is proportional to 1/range&lt;sup&gt;4&lt;/sup&gt;. &#xA;So double the distance means power drops to 1/16.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, that's not a big problem. To detect objects in the atmosphere (e.g. aircraft), you can build a transmitter powerful enough to have full coverage to the edge of space and the horizon: every object that's not obscured by the Earth itself (and not very small or very stealthy, etc.) will be detected.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But space is really, really big. &lt;a href=&quot;https://en.wikipedia.org/wiki/Radar_astronomy&quot; rel=&quot;nofollow&quot;&gt;Radar astronomy&lt;/a&gt; is feasible, but limited to the solar system, and you need a giant antenna like the Arecibo dish (see the photo in Hohmannfan's answer) coupled to a powerful transmitter (the most powerful radars use transmitter in the 1-5 MW range) and a very sensitive receiver to get that far. And you can only detect large objects this way (asteroids, yes. non-giant spaceships, no).&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-17T21:10:56.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14499" PostTypeId="1" AcceptedAnswerId="14501" CreationDate="2016-03-17T22:56:32.000" Score="18" ViewCount="1988" Body="&lt;p&gt;Since only one Space Shuttle orbiter was ever in space at a time, it seems logical that only two would be needed: one for the mission and the other as a backup for an emergency rescue. Perhaps a third could prove useful if one is in for repairs while the other two are mission-ready. Why, then were four orbiters—Columbia, Challenger, Discovery, and Atlantis—initially built?&lt;/p&gt;&#xA;" OwnerUserId="12358" LastActivityDate="2020-10-13T16:34:35.397" Title="Why were four Space Shuttle orbiters initially built?" Tags="|space-shuttle|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14500" PostTypeId="2" ParentId="14499" CreationDate="2016-03-17T23:03:10.633" Score="16" Body="&lt;p&gt;The Space Transportation System was intended to provide the United States with a low-cost, reliable means of transportation into low Earth orbit.  The expectation was that all United States space launches would take place on the STS fleet.  Four Orbiters were required to meet the expected demand and flight rate.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.presidency.ucsb.edu/documents/fact-sheet-outlining-united-states-space-policy&quot; rel=&quot;nofollow noreferrer&quot;&gt;The official US government policy in 1982 was that STS would be the primary launch system for both military and civil space missions.&lt;/a&gt;  It stated that &amp;quot;Expendable launch vehicle operations shall be continued by the United States Government &lt;em&gt;until the capabilities of the STS are sufficient to meet its needs and obligations&lt;/em&gt;.&amp;quot;  (italics mine, showing that expendable launch vehicles were to be phased out)&lt;/p&gt;&#xA;&lt;p&gt;None of this played out as planned.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-10-13T16:34:35.397" LastActivityDate="2020-10-13T16:34:35.397" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14501" PostTypeId="2" ParentId="14499" CreationDate="2016-03-17T23:08:39.750" Score="22" Body="&lt;p&gt;As projected development costs for the shuttle climbed, the only way for it to appear to be more cost-effective than expendable launcher options was if it flew a very high mission tempo, about one launch per week. With such rapid turnaround times, four orbiters probably wouldn't have been enough, and more would have been ordered. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, launch tempo was more like 7 launches a year, which was appropriate for a four-ship fleet. Maintenance of each orbiter between launches was extensive, taking &lt;a href=&quot;https://space.stackexchange.com/questions/2025/how-long-did-it-take-at-a-minimum-to-prep-a-shuttle-for-launch-after-it-landed?rq=1&quot;&gt;at least two to three months&lt;/a&gt;, so it wouldn't have been impossible to need one on-mission, one being stacked for the next mission, one undergoing normal between-flight refurbishment, and one undergoing more extensive repair if anything went wrong.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-01-02T23:18:44.203" LastActivityDate="2018-01-02T23:18:44.203" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14502" PostTypeId="1" AcceptedAnswerId="14504" CreationDate="2016-03-18T00:09:24.190" Score="1" ViewCount="1016" Body="&lt;p&gt;If a spacecraft heading to Alpha Centauri at 10% light speed, given a close approach to the main star, would deploying large solar sails, just before entering the solar system, slow the spacecraft enough to remain in orbit of that star ? &lt;/p&gt;&#xA;" OwnerUserId="11671" LastActivityDate="2016-03-18T01:37:23.277" Title="Can a solar sail reduce the speed from 1-10% light speed to achieve a solar orbit?" Tags="|interstellar-travel|solar-sail|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14503" PostTypeId="1" AcceptedAnswerId="14506" CreationDate="2016-03-18T00:51:04.557" Score="1" ViewCount="125" Body="&lt;p&gt;As far as I've been able to find (please correct me if I am wrong!), there is no current technology that would permit an EVA suit that is being worn to be sterilized without risk of damage to the suit or the astronaut. For example, infrared could be used, but would be at risk of damaging the suit after repeated exposures, and could damage the astronaut if the helmet did not block the IR sufficiently. This would be for situations such as when the astronaut is preparing to go out onto a planet surface, so doing a wipe-down in a clean room is not a practical solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any process that is currently being developed, that is known about but not currently possible, or that is likely to be discovered, which could allow the exterior of an EVA suit to be sterilized immediately prior to an EVA?&lt;/p&gt;&#xA;" OwnerUserId="13067" LastActivityDate="2016-03-18T07:19:33.383" Title="Probable future tech to sterilize an EVA suit as it is being worn" Tags="|design|feasibility|forward-contamination|contamination|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14504" PostTypeId="2" ParentId="14502" CreationDate="2016-03-18T01:03:48.323" Score="4" Body="&lt;p&gt;&lt;strong&gt;No&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1-10% of the speed of light is really, really fast. A solar sail would not receive a noticeable amount of light before it is about 5 AU away from the star (even then much less light than at 1 AU from the Sun), and from there, it is just 6-60 hours before the probe hits the star or flies past it, with your given velocity. That is a too short time, as solar sails use weeks or months to do even much smaller changes in velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, here are a few things you can check:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Determine if the sail can even slow you down:&lt;br&gt;&#xA;Pick a place at any distance from the star, and calculate the force of the gravitational pull. $\frac{G(M_{star}M_{sail})}{r^2}$. Compare that with the force of the light pushing the sail. $\frac{2E}{c}$, where $E$ is the energy hitting the sail per second, and c is the speed of light. If the gravitational pull is stronger, it is so everywhere, as both forces scale the same way. Then it would not be able to slow down at all.  &lt;/li&gt;&#xA;&lt;li&gt;Determine how much it is going to slow down in the case it is strong enough:&lt;br&gt;&#xA;As the force from the sail acts essentially the same as if the star had a lower mass, we can pretend so to keep the calculations simple. Use the previously calculated forces, and give the star a &quot;new&quot; mass: $M_{new}=M_{star} \cdot \frac{F_{gavity}-F_{sail}}{F_{gavity}}$. (Do not panic if you get a negative value, that is perfectly normal).&#xA;Now, you can calculate how much energy the probe can get rid of from infinity, to a target radius $r$: $E = M_{sail} \cdot -\frac{M_{new}}{r}$.&#xA;Subtract that from the initial kinetic energy of the probe, $E_{initial}=M_{sail} \cdot \frac{v^2}{2}$ and you can calculate the new velocity, $v_{final}=\sqrt{2 \cdot E_{final}}$.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-03-18T01:37:23.277" LastActivityDate="2016-03-18T01:37:23.277" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14505" PostTypeId="2" ParentId="14493" CreationDate="2016-03-18T01:47:19.007" Score="13" Body="&lt;p&gt;I'm posting these images as a supplement @Hobbes's accepted answer and @TildalWave's comments (which includes links to these images). I started reading some of those links. The &lt;a href=&quot;http://soho.nascom.nasa.gov/gallery/&quot; rel=&quot;nofollow noreferrer&quot;&gt;gallery&lt;/a&gt; is a good starting place but there are different tabs to check out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The values &lt;strong&gt;171&lt;/strong&gt; and &lt;strong&gt;304&lt;/strong&gt; represent the central wavelengths used, in Angstrom units. Our visible spectrum is roughly 4000 to 7000 Angstroms (400 to 700 nm) so these images are taken quite far into the ultraviolet range. In fact it's called &quot;extreme ultraviolet&quot; and the &lt;a href=&quot;http://umbra.nascom.nasa.gov/eit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;EIT&lt;/a&gt; instrument is called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Extreme_ultraviolet_Imaging_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Extreme Ultraviolet Imaging Telescope&lt;/a&gt;. This &lt;a href=&quot;http://soho.nascom.nasa.gov/explore/faq.html#EIT&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHO link&lt;/a&gt; has more info on the color coding of the images.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the images that @TildalWave linked, which were combined to make the image in the original question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OB6Az.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OB6Az.jpg&quot; alt=&quot;171 Angstroms&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/o1iqA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/o1iqA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-03-18T01:53:04.303" LastActivityDate="2016-03-18T01:53:04.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14506" PostTypeId="2" ParentId="14503" CreationDate="2016-03-18T07:19:33.383" Score="2" Body="&lt;p&gt;Vacuum is a harsh environment, so having the suit exposed to it should get rid of most bacteria. It is not a bulletproof method though, as confirmed by a &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/1998/ast01sep98_1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;famous case&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another good opportunity is to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation&quot; rel=&quot;nofollow noreferrer&quot;&gt;UV sterilization&lt;/a&gt;, which offers to kill at least a good chunk of the bacteria. This can not possibly harm the suit, as they are designed to withstand sunlight, which is not filtered for ultra violet radiation in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space suits are also by design tolerant to huge temperature differences, which can be helpful as a typical bacteria would not be tolerant to both temperature extremes. Combines with other methods, this can provide a little extra safety.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Micro organisms are though, and even clean-room sterilizing of probes is known to fail at complete sterilization. With more primitive methods in the field, there would always be a risk of contamination. A possible solution to that is to never have the external part of the space suit in touch with the habitat environment in the first place, and simply have it connected to an airlock:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ANMUSm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ANMUSm.jpg&quot; alt=&quot;external suit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-03-18T07:19:33.383" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14507" PostTypeId="2" ParentId="4140" CreationDate="2016-03-18T08:21:38.797" Score="0" Body="&lt;p&gt;The basic physics of circular motion require 2 directions of motion, with vectors of 90 deg. to each other. In orbit, the forward vector is provided by the initial propellant burn that got you into orbit. When you are in a circular orbit, this force is no longer providing acceleration, so you have a steady velocity (see Newton's first law). Gravity is the vector at 90 degrees. It is providing constant acceleration in the direction of the center of mass of the planet. Of course, since this is a stationary point from the perspective of the orbiter, the direction of the pull keeps changing, since they are also moving along the other vector. These two directions of motion are balanced such that the resulting path curves into a circle.  Orbits can essentially be considered constantly falling, but moving forward just fast enough that you keep missing the point you are falling towards by a constant amount. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a pro-grade burn, you are adding acceleration to the forward plane of this two vector system, essentially increasing the forward vector in relation to the downward vector. Since the balance of the vectors has changed, the resulting path will move closer to a straight line the more forward velocity you add. While you will initially move closer to your target because you appear to be moving in the same direction, you are now actually accelerating on both vectors, while your target continues to accelerate on only one. Your paths will quickly diverge, as your increased forward motion moves you in a flatter trajectory, while the other object maintains its curved, circular path. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get closer to an object while remaining in the same orbit, you would have to increase the gravity acting on you at the same time you are increasing your forward velocity. This could be simulated by adding a thruster to the top of your orbiting vehicle, and applying a downward force as you accelerated forward. However, the vertical thrust would have to remain constant, even after you cut forward acceleration, to simulate the increase in constant gravitic acceleration you would need to remain in that orbit at the higher forward velocity. Most current orbiter vehicles could not carry the amount of fuel necessary for the type of constant burn this would require. Hence the far more fuel efficient use of altered orbits described in the answers above to catch up to another object.&lt;/p&gt;&#xA;" OwnerUserId="13809" LastEditorUserId="13809" LastEditDate="2016-03-18T09:04:42.890" LastActivityDate="2016-03-18T09:04:42.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14508" PostTypeId="1" AcceptedAnswerId="14509" CreationDate="2016-03-18T13:07:08.987" Score="14" ViewCount="3542" Body="&lt;p&gt;And how the heck would an astronaut have opened it with his gloves on? Is that bag likely to still be there in good condition, or would it have been broken down by radiation by now? Was it normal to just toss stuff that is no longer needed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Upper right corner:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.physics.ucsd.edu/~tmurphy/apollo/a14.jpg&quot; alt=&quot;image of retroreflector array and Ziploc bag on Moon surface&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://www.physics.ucsd.edu/~tmurphy/apollo/lrrr.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.physics.ucsd.edu/~tmurphy/apollo/lrrr.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10367" LastEditorUserId="10273" LastEditDate="2019-01-16T08:06:58.223" LastActivityDate="2019-01-16T08:06:58.223" Title="What was this Ziploc bag used for on an Apollo 14 moonwalk?" Tags="|the-moon|apollo-program|environment|procedure|moonwalking|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14509" PostTypeId="2" ParentId="14508" CreationDate="2016-03-18T14:09:59.213" Score="26" Body="&lt;p&gt;That's not a &lt;a href=&quot;https://en.wikipedia.org/wiki/Ziploc&quot; rel=&quot;noreferrer&quot;&gt;Ziploc&lt;/a&gt; bag, but the retroreflector's dust cover. Here is a larger image of it removed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/YsJJ5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YsJJ5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here's an image with the cover still attached to the retroreflector:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/jhmaW.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jhmaW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The clue is in the use of the red markings along the edges of the part in question, in spaceflight commonly used to indicate parts that need to be removed before use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both images from &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Apollo Lunar Surface Experiments Package&lt;/a&gt;, credit &lt;a href=&quot;https://www.hq.nasa.gov/office/pao/History/alsj/&quot; rel=&quot;noreferrer&quot;&gt;NASA History Office's Apollo Lunar Surface Journal&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I wasn't able to find what the covers were made out of, but they appear to be clear PVC (Polyvinyl chloride). &lt;a href=&quot;https://en.wikipedia.org/wiki/Materials_for_use_in_vacuum#Materials_to_avoid&quot; rel=&quot;noreferrer&quot;&gt;PVC doesn't do terribly well in space&lt;/a&gt; and exposure to UV radiation, thermal cycling and &lt;a href=&quot;https://en.wikipedia.org/wiki/Sputtering&quot; rel=&quot;noreferrer&quot;&gt;sputtering&lt;/a&gt; due to exposure to high energy radiation. Cover's current state is of course unknown, but it's a safe bet that it is by now nothing more than a thin layer of dust covered by more lunar surface dust moved around by photocharging, bombardment by hard and particle radiation,...&lt;/p&gt;&#xA;" OwnerUserId="49" LastEditorUserId="49" LastEditDate="2016-03-18T14:45:42.463" LastActivityDate="2016-03-18T14:45:42.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14510" PostTypeId="2" ParentId="14508" CreationDate="2016-03-18T14:16:35.583" Score="12" Body="&lt;p&gt;To answer the last part of your question: yes, it was normal to throw away stuff that was no longer needed. On Apollo 11 alone, the &lt;a href=&quot;http://www.spiegel.de/international/zeitgeist/saving-moon-trash-urine-containers-space-boots-and-artifacts-aren-t-just-junk-argue-archaeologists-a-684221.html&quot;&gt;astronauts left behind more than 100 items&lt;/a&gt;, including&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;four urine containers, several airsickness bags, a Hasselblad camera, lunar overshoes and a complete moon-landing step.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The astronauts left behind as much as possible, to &lt;a href=&quot;http://gizmodo.com/this-hasselblad-camera-used-on-the-moon-just-sold-nea-1550241803&quot;&gt;save weight&lt;/a&gt; on the return trip. The Apollo spacecraft were on a very tight weight budget, and they had to allow for the ~ 380 kg of Moon samples they'd bring back over 6 mission.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-18T14:16:35.583" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14514" PostTypeId="1" AcceptedAnswerId="14515" CreationDate="2016-03-18T20:49:13.650" Score="3" ViewCount="221" Body="&lt;p&gt;Based on the situation in 2016:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Which vehicles that are in use for ISS resupply missions are human-rated by any space agency? (Bonus points for a list broken down by country of origin and/or the space agency/agencies that consider the vehicles acceptable for human occupants.)&lt;/li&gt;&#xA;&lt;li&gt;How many astronauts can each support on a normal ascent and descent respectively, without any significant retrofitting efforts?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="415" LastEditorUserId="7062" LastEditDate="2016-03-19T01:11:59.227" LastActivityDate="2016-03-19T01:11:59.227" Title="Which current (2016) ISS resupply vehicles are human-rated, and how many astronauts can they each support?" Tags="|iss|crewed-spaceflight|human-rating|crew-size|supply-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14515" PostTypeId="2" ParentId="14514" CreationDate="2016-03-18T21:18:46.957" Score="8" Body="&lt;p&gt;At the moment, the only man-rated vehicle, in the sense you mean, is Soyuz. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, all the visiting vehicles that dock to the station have to be man-rated in a sense, since they become station extensions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of the other currently flying vehicles, since only Dragon (cargo) can reenter without burning up (HTV, ATV, Progress, Cygnus all destruct on reentry) it is the only potentially that could carry humans back. However it would be a very bad day indeed before they would seriously consider doing that. They are not designed to carry people, from a seating perspective, and taking G's while sitting unsecured is bad for the body. But also the life support system is not meant for supporting people, although perhaps that could be upgraded if desired.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The future vehicles are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Boeing CST-100 Starliner which is specced for 7 but will fly with 4 for NASA&lt;/li&gt;&#xA;&lt;li&gt;Dragon v2 (Manned) which is specced for 7 but will fly with 4 for NASA.&lt;/li&gt;&#xA;&lt;li&gt;Dream Chaser (Cargo) unmanned. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="51" LastEditorUserId="415" LastEditDate="2016-03-18T23:55:37.633" LastActivityDate="2016-03-18T23:55:37.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14517" PostTypeId="2" ParentId="13257" CreationDate="2016-03-19T09:55:13.693" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www.rocketlabusa.com/&quot; rel=&quot;noreferrer&quot;&gt;Rocket Lab&lt;/a&gt;'s Electron launch vehicle is (as far as I know) constructed almost completely from carbon composite including the fuel tanks. I hear there have been issues with carbon composites being suitable for holding liquid propellants but if this company is confident enough to produce a vehicle entirely of the stuff then I'm sure they've done their homework. &lt;/p&gt;&#xA;" OwnerUserId="13821" LastEditorUserId="897" LastEditDate="2016-03-20T16:08:16.780" LastActivityDate="2016-03-20T16:08:16.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14519" PostTypeId="1" CreationDate="2016-03-19T15:49:07.837" Score="2" ViewCount="91" Body="&lt;p&gt;An air lock in a space craft is a door used to move between the sealed environment of a space craft to the vacuum of space (or other craft) with minimal impact to the sealed environment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I always think of airlocks on space craft, like cabin doors in the Alaskan bush, never physically locked.  If you have managed to get to the door alive, and you want in, come on in. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are any airlocks actually locked (circa 2016)?&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2016-03-19T15:49:07.837" Title="Are there any locked Airlocks in space?" Tags="|spacecraft|space-station|airlock|" AnswerCount="0" CommentCount="5" ClosedDate="2016-03-19T22:53:45.097" ContentLicense="CC BY-SA 3.0" />
  <row Id="14520" PostTypeId="2" ParentId="13127" CreationDate="2016-03-19T17:49:03.297" Score="15" Body="&lt;p&gt;In websites it is written a lot for the SpaceX and Falcon 9 FT space rocket because this rocket uses the liquid oxygen at -340 Fahrenheit (°F) or 66 Kelvin (K) which is far below compared to other operational rockets used in our days. SpaceX decided to operate different in colder temperatures because it was needed an extra performance for Falcon 9, to have in this way more fuel available for first stage landings. Since Falcon 9 FT is a rocket that could be used to transport astronauts in space and also using the supercooled LOX is not an easy process which could be risky and needs a very secure technology to complete this process, it gives attention and fame to Falcon 9 FT for this part. But the Falcon 9 FT is not the first using supercooled LOX. Other rockets have used it before the SpaceX’s rocket and according the information of some websites even in lower temperatures than Falcon 9 FT rocket uses. So the supercooled is not used only in laboratories for studying it, but also in real rockets putting in to practice and being operational for years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket for which I am referring is the two stage R-9A intercontinental ballistic missile (ICBM) also known as SS-8 Sasin. We know that most of the missiles use very toxic hypergolic propellant or solid propellant. But the R-9A was a launcher that used cryogenic propellants, kerosene and liquid oxygen, for its first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.arms-expo.ru/upload/medialibrary/668/668ae8c341fc19d029b6da20776c2b21.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://www.arms-expo.ru/upload/medialibrary/668/668ae8c341fc19d029b6da20776c2b21.jpg&quot; alt=&quot;R-9A launch pad&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Transportation and installation of the missile R - 9A / 8K75 on the launch pad . The launch complex &quot; Desna -N&quot; at Baikonur Cosmodrome 1961-1963&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The R-9 represented Korolev's last attempt to develop a practical military rocket and to prove the military worth of the non-toxic LOX/kerosene propellant combination.&#xA;  &lt;sup&gt;(&lt;a href=&quot;http://www.astronautix.com/lvs/r9.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It was projected and being operational at the beginning of the space rockets developments when a lot of experiments and test were done for new and unknown technologies at that time. So was even for R-9A rocket in which new technology was applied for the supercooled LOX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Designed in 1959 and first Flight tests of rocket R-9A started on April 9, 1961 and the first successful launch took place on April 21, 1961. It entered in service on July 21, 1965 and remained in service for more than 10 years. A total of only 27 operational launchers and 70 missiles were built. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Previous Soviet designs, fuelled with cryogenic LOX and kerosene, commonly took hours to fuel and launch, around 2 hours. The R-9A, on the other hand, could be launched after 20 minutes from the time a launch order was given. So for R-9A new achievements were the capability of using the supercooled oxygen and also the process of fueling and launching quickly a production missile using cryogenic LOX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to achieve this, new technologies had to be mastered.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A system for overcooling, durable storage, and high-speed filling of the rocket with supercooled LOX has been developed and the problem of durable storage of oxygen without losses has been solved.&#xA;  &lt;sup&gt;(&lt;a href=&quot;http://www.energia.ru/english/energia/launchers/rocket-r9.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;3&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Vasily Mishin developed new materials and machinery to precool the liquid oxygen to around -210 Celsius (°C), then devised vacuum isolation storage systems that reduced boil-off losses during storage, transport, and fueling operations by 500 times. All the processes followed before launching are described &lt;a href=&quot;https://www.youtube.com/watch?v=q7lBg1YWhyo&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (audio is in Russian language). Using the supercooled LOX was very important for this project. Studies have shown that there are additional opportunities to improve the efficiency of the supercooled LOX. Using it gave a lot of advantages to R-9A. First it takes a smaller volume, secondly sharply decrease evaporation losses and thirdly can be implement high-speed refueling in this way the oxygen getting into a warm tank will not boil rapidly, as it usually happens. In this case supercooled LOX at a temperature of  -210 Celsius (°C),  strongly increases its fluidity and allowed them to drastically reduce refueling time which was very important for the missile to be fueled and launched quickly as fast as possible for emergency or tactical launches in case that will be needed. We know how the tension was at its peak in the beginning of 1960s. The R-9A despite that it had nuclear weapon as payload was a launcher, which probably after some modifications done to it and with a 3rd stage mounted to reach orbital speed, could have the capability to launch small payloads in orbit according to its specifications. Information of websites confirms that the LOX used in R-9A was supercooled. But some websites say -186 °C, but I doubt this because at some of them the temperature is shown with (?) mark. According to &lt;a href=&quot;http://tonnel-ufo.ru/eanglish/weapon/intercontinental-ballistic-missile-r-9a-8k75.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;tonnel-ufo.ru&lt;/a&gt; and &lt;a href=&quot;http://www.astronautix.com/lvs/r9.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;astronautix.com&lt;/a&gt;, the temperature of liquid oxygen was -210 Celsius (°C) = 346 Fahrenheit (°F) = 64 Kelvin (K), which means lower than Falcon 9 FT 66 K (-340 F). So by this information it means that the R-9A could be the launcher that uses the LOX in the lowest temperatures ever. Anyway the most important thing that has to do also with your question, is to be sure that the Falcon 9 FT is not the first real launch carrying a payload that uses supercooled LOX, because at least one another launcher has been using it long before than Falcon 9 FT.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="10273" LastEditDate="2019-03-13T00:10:22.030" LastActivityDate="2019-03-13T00:10:22.030" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14521" PostTypeId="1" CreationDate="2016-03-19T22:51:27.967" Score="3" ViewCount="1786" Body="&lt;p&gt;In the movie &quot;Alien&quot; by Ridley Scott, there's a &lt;a href=&quot;http://giphy.com/gifs/alien-cinematics-drinking-bird-OcU63zhX0cEmc&quot; rel=&quot;nofollow&quot;&gt;Drinking Bird&lt;/a&gt; which, presumably, the crew activated when entering stasis, and which is still going after they emerge. However anyone who has had one of these birds will know that they don't last long at all. But it made me curios to know if there are better examples of perpetual motion style machines that could actually last years or maybe even decades in a spacecraft, given vacuum and micro-gravity? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only one I can think of is a spinning sphere or disc suspended in a vacuum. Should last a very long time, but it's not very flashy or interesting. Any ideas?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: To clarify, I'm looking for a device that could fit inside a spacecraft and provide the same symbolism for a real space flight crew as the Drinking Bird does in Alien. Something you can set off, and wake up to still moving years later.&lt;/p&gt;&#xA;" OwnerUserId="13299" LastEditorUserId="13299" LastEditDate="2016-03-20T08:28:16.287" LastActivityDate="2016-12-29T23:02:48.140" Title="What is the closest to a perpetual motion machine that could be made to work in space?" Tags="|crewed-spaceflight|spacecraft|interstellar-travel|technology|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14522" PostTypeId="2" ParentId="14521" CreationDate="2016-03-20T01:26:27.910" Score="4" Body="&lt;p&gt;&quot;Solar fan&quot; - a turbine with one side of the blades reflective (white) and the other absorptive (black). The differential in light pressure would keep its spin accelerating. Sure it would also drift, but there are a plenty of ways to counteract it, or just neglect the drift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For deep space counterpart you could exploit cosmic background radiation by selectively shielding or exposing areas of the propeller with large enough block of absorptive material.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-03-20T01:26:27.910" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14523" PostTypeId="2" ParentId="14521" CreationDate="2016-03-20T02:00:57.617" Score="6" Body="&lt;p&gt;The Earth's been going around the Sun for 4 and a half billion years. It's still not a perpetual motion machine, but it's doing a good imitation.&lt;/p&gt;&#xA;" OwnerUserId="12071" LastActivityDate="2016-03-20T02:00:57.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14524" PostTypeId="1" AcceptedAnswerId="14529" CreationDate="2016-03-20T12:56:37.357" Score="2" ViewCount="1152" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Androgynous_Peripheral_Attach_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;APAS&lt;/a&gt; docking system was first used on the Apollo Soyuz Test Project mission. Both parties (US and USSR) already had developed and used docking mechanisms by then. Is there any truth to the &lt;a href=&quot;https://space.stackexchange.com/questions/14519/are-there-any-locked-airlocks-in-space&quot;&gt;claim&lt;/a&gt; (see the comments in that question) that the US and USSR were unwilling to share details of their existing systems with each other, so they had to develop something new instead? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6944" LastEditDate="2020-09-04T15:48:44.840" LastActivityDate="2020-09-04T15:48:44.840" Title="Why was the APAS docking system developed?" Tags="|docking-port|mechanisms|apollo-soyuz|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14525" PostTypeId="1" CreationDate="2016-03-19T00:34:10.790" Score="2" ViewCount="445" Body="&lt;p&gt;I have the orbital elements of a satellite orbit around earth. It is an inclined elliptical orbit. I have plotted the orbit in 3D and have all the position vectors of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am now trying to plot a ground track of the orbit on MATLAB and I am unsure where to start. I am looking for guidance on the steps required to achieve this&lt;/p&gt;&#xA;" OwnerDisplayName="EddieP" LastActivityDate="2016-03-20T13:31:20.873" Title="Determining orbit ground track" Tags="|orbit|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14526" PostTypeId="1" AcceptedAnswerId="14527" CreationDate="2016-03-19T19:53:31.887" Score="4" ViewCount="772" Body="&lt;p&gt;Exactly as stated in the title. I guess it is not a trivial subject but I believe a rough algorithm or process description may be provided.&lt;/p&gt;&#xA;" OwnerUserId="13832" OwnerDisplayName="freefall" LastEditorUserId="897" LastEditDate="2016-03-23T19:06:51.223" LastActivityDate="2016-12-05T17:36:08.623" Title="When sending a probe to Mars, how is the optimal travel path calculated?" Tags="|orbital-mechanics|probe|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14527" PostTypeId="2" ParentId="14526" CreationDate="2016-03-20T17:26:32.950" Score="14" Body="&lt;p&gt;To define &quot;optimal&quot; you need an objective function that you are maximizing or minimizing. What is your objective function?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For real Mars missions, the objective function can be quite involved, since many factors are considered. Let's assume an impulsive trajectory (i.e. very close to on target to Mars immediately after launch). Then there is the mass that can be delivered to the target, which is a function of the launch energy and departure declination. There is the arrival velocity, which determines the orbit insertion propellent for an orbiter, or the heat shield capability and aspects of the entry trajectory for a lander. There is the approach declination, which determines what orbits you can get into with one burn, or what landing site latitudes you can access. There is the visibility of the insertion burn or entry from Earth for telecommunications during critical events, so that you have data if something goes wrong. There is Mars relay orbiter coverage constraints for the entry and landing event, again to get more data in case something goes wrong. You will need to define a few weeks of available launch days (usually three weeks) to allow for weather, range, launch vehicle, or spacecraft delays. Over that launch period, you will need to satisfy all the other constraints. You may want to have the arrival day be the same for all launch days, in order to simplify planning. You may want to not have that arrival day be on Super Bowl Sunday, so as to get better press coverage and to not annoy the crew. (I seriously took that into account once.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could go on. Does this answer your question?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Update:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To address the comment on optimizing for a specific parameter, e.g., $C_3$, the process for an impulsive trajectory is to make a porkchop plot. Like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Qu3WO.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qu3WO.gif&quot; alt=&quot;porkchop plot&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a given departure date from Earth and arrival date at Mars, there is one short, prograde orbit that connects them (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Lambert%27s_problem&quot; rel=&quot;noreferrer&quot;&gt;Lambert's problem&lt;/a&gt;). You then make a contour plot over a range of departures and arrivals of the parameters of interest. In the plot above, the blue contours are the injection energy. You can see two local minima, both around a $C_3$ of 16, for the 2005 opportunity. Though as noted, the $C_3$ doesn't tell the whole story. The departure declination, if greater than the launch site latitude, will reduce the injected mass at the same $C_3$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-12-05T17:36:08.623" LastActivityDate="2016-12-05T17:36:08.623" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14528" PostTypeId="1" AcceptedAnswerId="14541" CreationDate="2016-03-20T19:28:40.260" Score="2" ViewCount="398" Body="&lt;p&gt;On the concept art pictured in &lt;a href=&quot;http://www.nasa.gov/exploration/home/antimatter_spaceship.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; link, we see a very sleek looking ship that would supposedly be powered by antimatter.&lt;a href=&quot;https://i.stack.imgur.com/S6uXS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S6uXS.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;What is the large disk present on the side of what appears to be the second stage division of the craft?&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="11893" LastActivityDate="2016-05-28T13:12:12.713" Title="What is the disk on NASA's anti-matter concept design?" Tags="|crewed-spaceflight|spacecraft|design|conceptual|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14529" PostTypeId="2" ParentId="14524" CreationDate="2016-03-20T20:46:40.263" Score="6" Body="&lt;p&gt;Prior to ASTP, both Apollo and Soyuz spacecraft used a probe-and-drogue docking system, which requires different hardware at each end of the docking interface. This means that if you want two spacecraft to dock with the same space station, for example, the spacecraft can't dock with each other. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to support ship-to-ship docking, therefore, an all-new &lt;em&gt;androgynous&lt;/em&gt; docking interface was required.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-03-20T20:46:40.263" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14530" PostTypeId="1" AcceptedAnswerId="21542" CreationDate="2016-03-20T22:15:06.293" Score="8" ViewCount="414" Body="&lt;p&gt;Are there any androgynous docking/berthing systems currently in use for &lt;strong&gt;docking&lt;/strong&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've browsed &lt;a href=&quot;https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacecraft#List_of_mechanisms.2Fsystems&quot;&gt;the list&lt;/a&gt; and it seems all the androgynous systems fall into three groups:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;obsolete / used on crafts no longer in use&lt;/li&gt;&#xA;&lt;li&gt;used for berthing only&lt;/li&gt;&#xA;&lt;li&gt;planned / in development, or craft they are to be used on is in development.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Maybe I missed something, but are currently any androgynous systems used for docking (and not just berthing)? Any spacecraft out there that could dock to other spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-17T00:18:54.150" Title="Are any androgynous docking systems currently in use?" Tags="|docking|docking-port|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14532" PostTypeId="1" CreationDate="2016-03-21T12:41:58.503" Score="26" ViewCount="3194" Body="&lt;p&gt;Since most rockets are 'expendable', and perform re-entry over sea, their parts either burn up or end up in the world's oceans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It there any legal basis for the waste (pollution) that ends up in the ocean?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, if I have an empty, useless and worthless rocket on land, and I want to save on disposal costs.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Can I rent a ship, sail out to sea, and dump the rocket - legally - in the ocean?  (I hope not?)&lt;/li&gt;&#xA;&lt;li&gt;What if I refurbish the same rocket, fill it with fuel, and launch it straight into the ocean?&lt;/li&gt;&#xA;&lt;li&gt;What if I launch a payload to orbit with it, and &lt;em&gt;then&lt;/em&gt; let the first stage crash in the ocean?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Do launching countries themselves have regulations for this, or is it covered by international treaties such as exist for &lt;a href=&quot;https://en.wikipedia.org/wiki/Ocean_disposal_of_radioactive_waste&quot;&gt;radioactive waste&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="6033" LastEditorUserId="58" LastEditDate="2018-09-27T14:39:35.317" LastActivityDate="2020-01-20T23:28:34.387" Title="What is the legal status of waste rocket parts ending up in the ocean?" Tags="|stages|regulatory|pollution|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="14533" PostTypeId="1" CreationDate="2016-03-21T13:58:56.767" Score="5" ViewCount="195" Body="&lt;p&gt;There are proposals and maybe serious plans to send astronauts to asteroids or Mars' moons, objects which are a few kilometers or just a few meters in diameter. I call them &quot;milligravity&quot; objects since their gravities are operationally relevant, but maybe medically equivalent to microgravity. Some of those objects should be loose rubble piles, never before encountered by humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it be a valuable preparation to launch 20 tons or so of such a rubble pile, a bunch of sand and rock loosely bound together, to (a short lived) low Earth orbit and have astronauts test out handling technologies and operations on it? The structure of the specimens would be completely controlled by design. It could have sensors planted inside of it. The operations could be performed in relative safety. No time consuming transport of some boulder on some NEA would be needed, as with the ARM mission. A boulder which could be picked up from a NEA won't be an exotic rubble pile anyway, I suppose it would be more like some meter sized meteorites found in museums on Earth's surface.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2022-01-16T13:28:06.300" Title="Could we launch a rubble pile simulant to LEO for astronauts to practice on before visiting natural milligravity objects?" Tags="|low-earth-orbit|asteroid|microgravity|crewed-spaceflight|asteroid-redirect-mission|" AnswerCount="4" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14534" PostTypeId="2" ParentId="14532" CreationDate="2016-03-21T14:41:49.840" Score="9" Body="&lt;p&gt;Are you interested in the liability for damage and contamination or the ownership? Either way there are holes you could drive a bus through. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most importantly in what follows, please bear in mind that I am not a lawyer. However it is edifying to recall that barristers are in aggregate wrong 50% of the time by definition, as it is an adversarial role with a winning and losing party. Hence any of the following could be on the nail, or not!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is the Outer Space treaty (&lt;a href=&quot;https://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967#Article_VIII&quot;&gt;1967&lt;/a&gt;) and the Space liability convention (&lt;a href=&quot;http://www.unoosa.org/pdf/gares/ARES_26_2777E.pdf&quot;&gt;1971&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Liability&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The liability convention is clear on the idea that a space object includes its launch vehicle and that the launching state (notoriously poorly defined): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;shall be absolutely liable to pay compensation for damage caused by&#xA;  its space object on the surface of the Earth or to an aircraft in&#xA;  flight.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A &quot;space object&quot; is defined in the liability convention:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;includes component parts of a space object as well as its launch vehicle and parts thereof.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Hence this appears to include first and second stages of launch vehicles and perhaps carrier aircraft and rail-mounted rocket sledges too, these weren't defined. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It also says nothing about about the damage caused (e.g. leakage of toxic or radioactive material) under the surface, within the oceans. I mention this simply because the oceans are referred to in the question. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ownership&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The liability convention says nothing about ownership, e.g. for salvage. The Outer Space Treaty does touch upon ownership and says: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A State Party to the Treaty on whose registry an object launched into outer space is carried shall retain jurisdiction and control over such object, and over any personnel thereof, while in outer space or on a celestial body. Ownership of objects launched into outer space, including objects landed or constructed on a celestial body, and of their component parts, is not affected by their presence in outer space or on a celestial body or by their return to the Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However the Outer Space Treaty does not define &quot;objects launched into outer space&quot; so well. It appears that it does not obviously include non-orbiting rocket stages to the extent that they are not &quot;launched into outer space&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether these are considered abandoned from an ownership (not liability) perspective would have to be checked against other forms of law, national or international, and whether that even mattered for salvage, according to whether the object landed in national or international waters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is more, even for objects launched briefly into space and then returning it appears that there are two sub groups:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Registered objects, which apparently are permanently subject to jurisdiction and control&lt;/li&gt;&#xA;&lt;li&gt;Other objects are simply &quot;unaffected by their presence in outer space / return to Earth&quot; and thus can apparently can be abandoned in the sense of customary law.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-03-21T14:41:49.840" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14535" PostTypeId="2" ParentId="14533" CreationDate="2016-03-21T16:54:36.520" Score="6" Body="&lt;p&gt;In short: yes, it could be. But it is needlessly expensive, so they will not do it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First you have to get the junk up there. Then you need to get the astronauts up there, and essentially put them in the same situation as they need training for to deal with. Then get them down again. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same could easily be achieved close enough &lt;a href=&quot;https://en.wikipedia.org/wiki/Neutral_Buoyancy_Laboratory&quot; rel=&quot;nofollow noreferrer&quot;&gt;in a pool&lt;/a&gt; or in a &lt;a href=&quot;https://www.youtube.com/watch?v=cJCsomGwdk0&quot; rel=&quot;nofollow noreferrer&quot;&gt;cable-robot&lt;/a&gt; for a tiny fraction of the cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also: &lt;a href=&quot;https://space.stackexchange.com/questions/12467/how-can-astronauts-float-in-space-without-being-affected-by-the-gravitational-fo?rq=1&quot;&gt;this question&lt;/a&gt;, and its answer, are relevant. In short: at 80 tons and 10 meters out, the Space Shuttle exerted almost 10 times less gravitational pull on the astronauts than did &lt;strong&gt;Jupiter&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you are going to need to put a &lt;strong&gt;LOT&lt;/strong&gt; of junk up there before you have anything even remotely useful for training in milligravity. &lt;/p&gt;&#xA;" OwnerUserId="10493" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-03-21T16:54:36.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14536" PostTypeId="2" ParentId="14533" CreationDate="2016-03-21T17:41:59.900" Score="4" Body="&lt;p&gt;There is already too much junk in orbit which could collide with satellites and spacecraft, adding 20 tons of loose garbage would be extremely risky - if it got loose it could be a serious hazard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get the benefits of any training you would need to build a very, very big pile of rubble. 20 tons would not be enough, you would need more on the order of 2000 tons. Launching that much mass would be incredibly expensive, far more than any mission to go to a real space rock in the first place. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-03-21T17:41:59.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14537" PostTypeId="1" CreationDate="2016-03-22T03:02:38.320" Score="5" ViewCount="169" Body="&lt;p&gt;If a suborbital flight from NY to Beijing will cover about 7k miles in an hour, what will that trajectory be and at what speed (including the Earth's rotation)? Will most of the flight be above the Kármán line?&lt;/p&gt;&#xA;" OwnerUserId="13846" LastEditorUserId="6816" LastEditDate="2016-03-22T03:09:21.180" LastActivityDate="2016-03-22T03:09:21.180" Title="How will a suborbital flight country to country work?" Tags="|sub-orbital|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14538" PostTypeId="1" AcceptedAnswerId="14544" CreationDate="2016-03-22T09:47:14.283" Score="8" ViewCount="2668" Body="&lt;p&gt;The telescope itself will operate in wavelengths from 0.6 µm (orange) to 28.5 µm (mid-infrared), so it reaches a bit into the visible spectrum, but it's still going to be aimed away from Earth, and photograph distant galaxies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I'm interested in is, will the satellite contain any cameras that can take common visible light photos of Earth? Just like &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory&quot; rel=&quot;noreferrer&quot;&gt;DSCOVR&lt;/a&gt; can take &lt;a href=&quot;http://apod.nasa.gov/apod/ap150807.html&quot; rel=&quot;noreferrer&quot;&gt;impressive images&lt;/a&gt; of Earth and the Moon from $L_1$, will we have similar images from James Webb at $L_2$, of the permanent corona eclipse of the Sun by Earth, or the always-crescent Moon? &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="415" LastEditDate="2016-03-22T10:19:17.107" LastActivityDate="2016-10-20T20:55:49.027" Title="Will the James Webb telescope have any visible spectrum cameras?" Tags="|camera|instrument|james-webb-telescope|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14540" PostTypeId="2" ParentId="14538" CreationDate="2016-03-22T13:30:11.753" Score="2" Body="&lt;p&gt;As far as I've been able to find, &lt;a href=&quot;http://www.jwst.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;JWST&lt;/a&gt; will not contain any cameras other than its main instruments. I won't rule out some small cameras to monitor the unfolding of the sunshield and primary mirror, but haven't found anything definitive on that. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-22T13:30:11.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14541" PostTypeId="2" ParentId="14528" CreationDate="2016-03-22T14:05:42.673" Score="8" Body="&lt;p&gt;That looks like a simplified drawing of a standard parabolic radio antenna.  Presumably pointed 'down' because the ship will be flying away from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&lt;a href=&quot;https://i.stack.imgur.com/EgTfY.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EgTfY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&lt;sup&gt;Image produced for NASA's Exploration Office, Johnson Space Center (JSC) by John Frassanito and Associates&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="49" LastEditDate="2016-03-22T15:51:36.220" LastActivityDate="2016-03-22T15:51:36.220" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14542" PostTypeId="1" CreationDate="2016-03-22T16:50:11.537" Score="7" ViewCount="2652" Body="&lt;p&gt;If enviromental concern was not taken into account, could you use a nuclear thermal rocket such as NERVA or Pewee to take off from Earth's surface into low Earth orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is a launch failure the only enviromental concern?&lt;/p&gt;&#xA;" OwnerUserId="13854" LastActivityDate="2023-09-28T16:32:25.710" Title="Could a spacecraft take off from Earth using nuclear thermal rockets?" Tags="|launch|propulsion|nuclear|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14543" PostTypeId="2" ParentId="14542" CreationDate="2016-03-22T17:05:12.407" Score="4" Body="&lt;p&gt;I would say it's a bad idea even without taking environmental impact into consideration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For vertical ascent from earth's surface, a good Thrust to Weight ratio (T/W) is desirable. Else gravity loss can inflict a big delta V penalty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Kirk Sorensen NTR's T/W isn't that great. See &lt;a href=&quot;http://selenianboondocks.com/2010/06/ssto-ntr-bad/&quot; rel=&quot;nofollow&quot;&gt;SSTO is a bad idea, but NTR SSTO is worse&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-03-22T17:05:12.407" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14544" PostTypeId="2" ParentId="14538" CreationDate="2016-03-22T17:42:33.453" Score="8" Body="&lt;p&gt;Two things here:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The main physical processes JWST is concerned about take place in the Infrared, and&lt;/li&gt;&#xA;&lt;li&gt;JWST is not a Earth observatory&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It will have the NIRCam that can reach slightly into the visible spectrum, but this will definitely not be used to make photos of Earth. All of its science instruments will absolutely have to point away from Earth (also Sun and Moon) in order to be cooled down sufficiently to 50K operational temperatures.&lt;br&gt;&#xA;The part of JWST's servicing module that penetrates through the sun-shield contains mainly reaction wheels and Earth communication antennae, but no other optical camera.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastEditorUserId="6816" LastEditDate="2016-10-20T20:55:49.027" LastActivityDate="2016-10-20T20:55:49.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14545" PostTypeId="2" ParentId="14542" CreationDate="2016-03-22T17:50:05.353" Score="6" Body="&lt;p&gt;You'd need something with a better thrust-to-weight ratio than NERVA for that. But that might be within the realm of possibility. The NERVA engine mass was about 20,000 lbm, and the thrust about 75,000 lbf, which doesn't leave near enough for the rest of the vehicle and propellant. It was really designed for an in-space stage, where the benefit would be substantial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for environmental, use of NERVA in Earth's atmosphere does pose a problem even if there is no accident. In normal operation, bits of the reactor fuel come out in the heated hydrogen thrust, leaving a radioactive trail in the atmosphere. You would need to do this very far from populated areas (perhaps a sea launch), and do analyses of the distribution and eventual fate of the radioactive material to show, hopefully, that it would not significantly increase health risks to humans.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-03-22T17:50:05.353" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14547" PostTypeId="1" AcceptedAnswerId="14548" CreationDate="2016-03-23T04:22:58.953" Score="11" ViewCount="3309" Body="&lt;p&gt;As I understand it, the Space Shuttle used cryogenic fuel in its main engines and it kept a supply onboard for the duration of its mission for a deorbit burn. Since Space Shuttle missions often lasted upwards of a week, how was this kept cold?&lt;/p&gt;&#xA;" OwnerUserId="12358" LastActivityDate="2016-03-25T18:38:24.457" Title="How did the Space Shuttle keep its cryogenic fuel cold?" Tags="|fuel|space-shuttle|cryogenics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14548" PostTypeId="2" ParentId="14547" CreationDate="2016-03-23T04:38:17.813" Score="24" Body="&lt;p&gt;The fuel used in the shuttle's &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Orbital_Maneuvering_System&quot;&gt;Orbital Maneuvering System&lt;/a&gt; engines and used for the deorbit burn was not cryogenic; it was storable hypergolic fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cryogenic hydrogen and oxygen fuel burned by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot;&gt;main engines&lt;/a&gt; was used only during the ascent, which took only about 10 minutes.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-03-25T18:38:24.457" LastActivityDate="2016-03-25T18:38:24.457" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14549" PostTypeId="1" AcceptedAnswerId="14881" CreationDate="2016-03-23T07:15:33.300" Score="8" ViewCount="780" Body="&lt;p&gt;If you haven't seen it, you can monitor the status of the Deep Space Network's radio dishes (which communicate with pretty much all of our space probes and satellites)  from &lt;a href=&quot;https://eyes.nasa.gov/dsn/dsn.html&quot;&gt;this website&lt;/a&gt;.  At times, there will be dishes which have a three or four letter code that doesn't correspond to any space mission, and the description is merely &quot;Testing.&quot;  For example, I have seen 'TEST', 'MCOB', and 'SGP' all used as Testing codes.  Does anyone know what these different codes may mean?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did send an email to the DSN Now contact address a few weeks ago, but have yet to receive a response.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="9060" LastEditDate="2016-03-23T17:54:19.533" LastActivityDate="2016-04-18T14:23:10.710" Title="What do the different &quot;Testing&quot; codes mean on NASA's Deep Space Network website?" Tags="|nasa|communication|deep-space-network|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14550" PostTypeId="2" ParentId="2423" CreationDate="2016-03-23T15:37:32.777" Score="6" Body="&lt;p&gt;Mars's has an exceedingly low flux magnetic field generated by its core because there is very little convection of its conductive core material. This is likely because the natural nuclear fission process in the core has stalled. When planets form they become molten, which causes the constituent materials to differentiate. Lighter materials e.g. silicates gravitate to the surface, heavy materials like iron sink to the core. However, the ultra dense materials, like uranium, end up at the absolute centre of the planet where they form a georeactor, which is a naturally moderated breeder fission reactor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nuclearplanet.com/Herndon&quot; rel=&quot;noreferrer&quot;&gt;http://nuclearplanet.com/Herndon&lt;/a&gt;'s%20Nuclear%20Georeactor.html&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heat from this georeactor drives the convection which creates the magnetosphere. However, on Mars the georeactor seems to have stalled, presumable because it has run out of nuclear fuel and suitable fertile breeder materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to answer you question, in order to kick start Mars Magnetic field you would need to inject a relatively large quantity of fissionable and fertile materials into the core. Convection is restarted along with volcanism and plate tectonic motion. Fortunately the pyroclastic gases produce an atmosphere and, when they condense, an ocean. I'd say shake and bake terraforming except I believe someone already has that trademarked...  &lt;/p&gt;&#xA;" OwnerUserId="5761" LastActivityDate="2016-03-23T15:37:32.777" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14551" PostTypeId="1" AcceptedAnswerId="14562" CreationDate="2016-03-23T20:31:14.100" Score="11" ViewCount="3144" Body="&lt;p&gt;According to the &lt;a href=&quot;http://jwst.nasa.gov/faq.html#pointing&quot; rel=&quot;noreferrer&quot;&gt;JWST FAQ&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To turn and point at different objects in space, Webb uses six reaction wheels to rotate the observatory. The reaction wheels are basically flywheels, which store angular momentum. The effect of angular momentum is familiar in bicycle riding. It is much easier to stay up on the bike when it is moving than when it is standing still, and the bicycle will tend to go straight in 'no hands' mode thanks to the angular momentum of the spinning wheels. Slowing down or speeding up one or more of the Webb's reaction wheels alters the total angular momentum of the whole observatory and consequently the observatory turns to conserve angular momentum. Hubble uses reaction wheels also to turn to point at different objects.&lt;/p&gt;&#xA;&lt;p&gt;The reaction wheels work in combination with three star trackers and six gyroscopes that provide feedback on where the observatory is pointing and how fast it is turning. This enables coarse pointing sufficient to keep the solar array pointed at the Sun and the high-gain antenna pointed at the Earth. To take images and spectra of astronomical targets (i.e., galaxy, star, planet, etc.) finer pointing is needed. Additional information for finer pointing from the Fine Guidance Sensor in Webb's integrated science instrument module (ISIM) is used to move the telescope's fine steering mirror (FSM) to steady the beam of light coming from the telescope and going into the science instruments. Webb's reaction wheels, star trackers, gyroscopes, Fine Guidance Sensor, and fine steering mirror work together in the observatory's attitude control system (ACS) to precisely point and stare at targets so that the science instruments can see them and see them clearly. The system works much the same way your body uses multiple methods of differing precision (&amp;quot;your inner ears and eyes and nervous system and muscles&amp;quot;) to catch a baseball in the outfield.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;To me, this implies the JWST's mirrors are rigidly attached to the base of the spacecraft, so they're in a fixed orientation. The telescope always looks in a plane roughly parallel to the sunshield.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eoH62.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eoH62.jpg&quot; alt=&quot;JWST&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Because the sunshield has to be normal to the Sun (more or less), that limits the field of view available at any one time to a circular strip of the sky. If you wanted to look at a particular star, you'd have to wait for up to 6 months for it to become available. That seems too long for many interesting transient phenomena.&lt;/p&gt;&#xA;&lt;p&gt;The key here is the 'more or less' part: how far out of the normal can you tilt the JWST without losing the sunshield's effectiveness? How much of the sky can the JWST observe from any given position in its orbit?&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-07-29T01:46:19.683" Title="How much of the sky can the JWST see?" Tags="|james-webb-telescope|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14553" PostTypeId="1" AcceptedAnswerId="14554" CreationDate="2016-03-24T02:50:35.500" Score="17" ViewCount="2113" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_(rocket)&quot; rel=&quot;noreferrer&quot;&gt;Vanguard rocket&lt;/a&gt; had three stages. The first two were liquid-fueled and the third was solid-fueled.&#xA;That struck me as odd, as solid fuel engines are almost exclusively used in atmosphere, and you'd probably want the finest control for your last stage, when placing a satellite into orbit (especially one with no propulsion of its own).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would have driven that design decision?&lt;/p&gt;&#xA;" OwnerUserId="13871" LastEditorUserId="13871" LastEditDate="2016-03-25T17:14:25.353" LastActivityDate="2022-08-04T16:11:21.463" Title="Why did the Vanguard rocket use a solid engine for its third stage?" Tags="|rockets|history|solid-fuel|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14554" PostTypeId="2" ParentId="14553" CreationDate="2016-03-24T03:25:46.617" Score="17" Body="&lt;p&gt;Solid fuel motors aren't unknown for upper stage use (e.g &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_Upper_Stage&quot;&gt;IUS&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Star_48&quot;&gt;Star-48&lt;/a&gt;, among others). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While solids generally have the drawbacks of poor Isp and controllability, their simplicity keeps cost and stage dry mass down on small upper stages. Note that in early launchers such as Vanguard, the specific impulse of the solid stage wasn't much less than that of the liquid stages (248, 271, and 230 seconds respectively for the three stages); there likely weren't any very small, efficient liquid engines available for the third stage role.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Control of orbital insertion, also, wasn't that critical for the Vanguard series; the satellites were launched into fairly high, elliptical orbits, and it was challenging enough to get them up at all without worrying about their exact trajectories.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-03-24T16:42:21.637" LastActivityDate="2016-03-24T16:42:21.637" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14555" PostTypeId="2" ParentId="14553" CreationDate="2016-03-24T03:30:53.853" Score="20" Body="&lt;p&gt;A solid stage can have a very low dry mass fraction, and can be spin stabilized avoiding the need for heavy guidance systems in the final stage. Both add mass directly to what is available for the payload. The Isp of solids is competitive with liquids other than LOX/LH2. A little bit of inaccuracy due to uncertainty in the total impulse of the motor can be made up by the spacecraft with a small amount of propellant. Overall a win.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The three-stage Delta II rockets used for many interplanetary and other missions in the 1990's and 2000's did the same thing, where the third stage was a spin-stabilized Star-48 motor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons used a Star-48 motor as the final stage on the Atlas V-551 to get its high-speed boost to Jupiter and on to Pluto.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-03-24T03:30:53.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14556" PostTypeId="1" AcceptedAnswerId="14565" CreationDate="2016-03-24T04:27:12.210" Score="2" ViewCount="249" Body="&lt;p&gt;Many people say that a Mars colony is economically unsustainable as goods from Earth will be considerably cheaper for the foreseeable future and Mars won't have anything to trade in return. This got me thinking, that due to Mars' lower gravity and lower density atmosphere launching satellites into Earth orbit might be cheaper to do from Mars rather then from Earth, despite the need for a much longer journey. Is this correct from the point of view of orbital mechanics?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If feasible this would also require the satellites to be manufactured on Mars. What sort of level of development (colony size, equipment) would be needed on the surface of Mars before the Martian satellite industry can be cost competitive with the Terran one?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any other examples of industries where this could be the case?&lt;/p&gt;&#xA;" OwnerUserId="13872" LastEditorUserId="6816" LastEditDate="2016-03-24T04:37:29.733" LastActivityDate="2017-12-19T13:30:57.333" Title="Manufacturing Earth satellites on Mars" Tags="|orbital-mechanics|artificial-satellite|colonization|economics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14558" PostTypeId="1" AcceptedAnswerId="14559" CreationDate="2016-03-24T06:54:41.033" Score="-1" ViewCount="114" Body="&lt;p&gt;In regards to project Orion, using conventional explosives for a radiation free launch and in the range of 0.03 up to 0.35 kilotons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the smallest amount of fissionable material needed to start nuclear fusion of Deuterium + Helium3?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not able to initiate fusion with conventional explosives i am looking to find out the minimum amount of fissionable material to initiate fusion for a nuclear pulse pellet with the least radioactive fallout possible. &lt;/p&gt;&#xA;" OwnerUserId="11671" LastEditorUserId="11671" LastEditDate="2016-03-25T07:09:23.890" LastActivityDate="2016-03-25T07:09:23.890" Title="Could fusion of Deuterium + Helium3 be started using conventional explosives?" Tags="|nuclear-pulse-propulsion|" AnswerCount="1" CommentCount="2" ClosedDate="2016-03-24T10:41:40.947" ContentLicense="CC BY-SA 3.0" />
  <row Id="14559" PostTypeId="2" ParentId="14558" CreationDate="2016-03-24T07:53:07.173" Score="2" Body="&lt;p&gt;The amount of fusionable material isn't a problem: you can initiate fusion inside a tiny pellet containing &lt;a href=&quot;https://en.wikipedia.org/wiki/National_Ignition_Facility#ICF_basics&quot; rel=&quot;nofollow&quot;&gt;milligrams of fusionable material&lt;/a&gt; if you compress it enough. But you need a facility the size of the &lt;a href=&quot;https://en.wikipedia.org/wiki/National_Ignition_Facility&quot; rel=&quot;nofollow&quot;&gt;National Ignition Facility&lt;/a&gt; to ignite it.&lt;br&gt;&#xA;The only place where we have working fusion at a small scale is inside thermonuclear weapons. These use a fission bomb to initiate the fusion reaction. If there were a way to ignite fusion using conventional explosives, I suspect we'd have seen that in bombs by now. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-24T07:53:07.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14560" PostTypeId="1" AcceptedAnswerId="14573" CreationDate="2016-03-24T08:25:10.217" Score="9" ViewCount="883" Body="&lt;p&gt;How long does it take to the space shuttle to change its orientation while operating out of the Earth's atmosphere, either by turning or rolling?&#xA;I.e. what is the average/maximum rotation speed?&#xA;What is its maximum acceleration while performing these maneuvers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the technical limit given by mass and thrusters' power? What about limits dictated by crew safety or other similar concerns?&lt;/p&gt;&#xA;" OwnerUserId="13875" LastEditorUserId="13875" LastEditDate="2016-03-24T18:44:22.153" LastActivityDate="2024-03-21T15:13:28.207" Title="How fast can the space shuttle change its attitude in space?" Tags="|space-shuttle|attitude|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14561" PostTypeId="2" ParentId="14551" CreationDate="2016-03-24T10:11:56.200" Score="6" Body="&lt;p&gt;According to the &lt;a href=&quot;http://www.stsci.edu/jwst/doc-archive/technical-reports/JWST-MOCD.pdf&quot; rel=&quot;noreferrer&quot;&gt;JWST Mission Operations Concept Document&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The field of regard (FOR) is the region of the sky in which observations can be conducted safely at any time. For JWST, the FOR is a large annulus that is centered on the position of the Sun. The FOR, as is shown in Figure 4-14, allows one to observe targets between 85° and 135° off the Sun line (MR-103, MR-104, MR-105).&#xA;  Most astronomical targets are observable for two periods separated by 6 months during each year. The length of the observing window varies with ecliptic latitude, and targets within 5 ° of the ecliptic poles are visible continuously (MR-106). This continuous viewing zone is important both for some science programs that involve monitoring throughout the year and will also be useful for calibration purposes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xxE0f.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xxE0f.png&quot; alt=&quot;JWST FOR&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The sunshield for JWST will provide a 39.7% celestial field-of- regard (FOR) that is greater than the sky coverage requirement of 35% (MR-104). This large FOR is required to provide the scheduling flexibility to allow JWST to conduct an efficient scientific program and simplifies orbit station-keeping design since it permits a wide range of Sun orientations for thruster firing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-03-24T10:11:56.200" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14562" PostTypeId="2" ParentId="14551" CreationDate="2016-03-24T12:22:50.870" Score="8" Body="&lt;p&gt;The allowable-field-of-regard @Hobbes has shown can also be compiled into an all-sky map of object visibility, as done by P. Feirruit (ESA):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kf2TP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kf2TP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this map is in galactic coordinates, not in ecliptic ones. The sharp decrease in object visibility from blue to teal comes from the structure of the green ring in Hobbes' image:&lt;br&gt;&#xA;At any particular location JWST will always be able to tilt sideways clock or counter-clockwise. It however cannot tilt backwards, due to the constraints of the sunshield facing the sun.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the field-of-view is essentially all-sky on average JWST will be used as follow-up for many surveys, in particular to characterize non-transient phenomena in detail. Transient phenomena like Gamma-ray bursts or transiting Exoplanets will be hard to discover by JWST if they're not luckily in the viewing horizon at any given moment.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A last piece of knowledge that is necessary to understand this map, is that the map only represents the possibility of looking at things for the given amount of time. JWST's instruments have fields of view of $2'\times 2'$ and smaller, so the time allocation committee has additionally to actually decide to look at an object when being in the viewing horizon.&lt;br&gt;&#xA;This again emphasizes JWST's character as follow-up rather than discovery-machine: The coolant will run out after approx. 5 years with no means of refuelling at L2, so in any particular moment the viewing horizon observation time is expected to be heavily overbooked with known objects, leaving little time for 'just poking around in the darkness'.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastActivityDate="2016-03-24T12:22:50.870" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14563" PostTypeId="2" ParentId="14526" CreationDate="2016-03-24T13:44:25.027" Score="4" Body="&lt;p&gt;Let me add one thing both Mark and Kaine omitted in their answers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With &quot;optimal&quot; defined as &quot;minimizing fuel&quot; you use engines of extreme Specific Impulse, and these tend to have minimal thrust. They can't just perform a simple &quot;transfer burn&quot; - a couple of minutes of acceleration. They need to accelerate for hours or even months. So, instead of just entering the transfer orbit they need to spiral out of Earth's gravity well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK there are no simple porkchop plots for these transfers - they are calculated individually, through numerical optimization of trajectories created through computer simulation. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-03-24T13:44:25.027" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14565" PostTypeId="2" ParentId="14556" CreationDate="2016-03-24T14:50:03.327" Score="2" Body="&lt;p&gt;In theory if you used aerobraking when you arrived at Earth then you could just about get there with less fuel (or more payload) than you would with launching from Earth (I haven given my calculations and source of figures below). Of course this is assuming you had the exact same infrastructure on Mars as you do on Earth, for example SpaceX employs around 5,000 people, the satellite company would be another thousand or so. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Youve also got to have all the supporting industry on Mars before you start launching satellites, from the rocket fuel to micro-chip manufacturers and farmers to cooks, you cant build a rocket without fuel, and you cant use employees without fuel (food) either. So I'd say you need at least 10'000 people to start your colony, but that's even neglecting tertiary support sectors like police, teachers, Martian bus drivers etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you didn't achieve all of this and were to import any supplies from Earth it would make the whole proposal unavailable due to the huge cost of launching supplies from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note: I based my Delta-V calculations on the below map, taken from Wikipedia. I assumed the Mars Transfer -GTO stage could be made with aerobraking, hence why I got the value of 8 for the Mars-GEO Delta-V, lower than that required from Earth (~12-14).&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/68UWE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/68UWE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13176" LastEditorUserId="25" LastEditDate="2017-12-19T13:30:57.333" LastActivityDate="2017-12-19T13:30:57.333" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14568" PostTypeId="1" AcceptedAnswerId="14569" CreationDate="2016-03-24T18:59:26.110" Score="8" ViewCount="763" Body="&lt;p&gt;From Wikipedia, it seems the objective is to gain experience in Mars missions and taking some scientific observations. My question is, did ISRO scientists discover anything new from their observations?&lt;/p&gt;&#xA;" OwnerUserId="13882" LastActivityDate="2016-03-24T19:26:35.547" Title="What have been India's Mars mission's benefits to science?" Tags="|mars|isro|mars-orbiter-mission|mars-orbiters|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14569" PostTypeId="2" ParentId="14568" CreationDate="2016-03-24T19:26:35.547" Score="9" Body="&lt;p&gt;Other than taking images of Mars' moon Deimos far side (something not done in decades) not much has been attributed to MOM yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key here is to remember that the the orbiter likely won't discover anything per se, but the science data it produces can be used (probably in conjunction with other data) to help us understand the history of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an article you might find interesting: &lt;a href=&quot;http://spaceflight101.com/mom/indias-mars-orbiter-mission-delivers-first-science-results-looks-at-future-challenges/&quot;&gt;http://spaceflight101.com/mom/indias-mars-orbiter-mission-delivers-first-science-results-looks-at-future-challenges/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2016-03-24T19:26:35.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14570" PostTypeId="1" CreationDate="2016-03-24T23:49:45.907" Score="2" ViewCount="184" Body="&lt;p&gt;On January 14, 2005, the Huygens lander touched down on the surface of Titan.  It sent back several images and plenty of other telemetry data, relayed by the Cassini orbiter overhead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When did we stop receiving information from Huygens?&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2016-03-25T00:00:01.017" Title="When did we stop getting transmissions from Huygens?" Tags="|cassini|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14571" PostTypeId="2" ParentId="14570" CreationDate="2016-03-24T23:53:29.817" Score="5" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Huygens_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, the probe lasted only a short time beyond touchdown:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The probe was designed to gather data for a few hours in the atmosphere, and possibly a short time at the surface. It continued to send data for about 90 minutes after touchdown. It remains the most distant landing of any human-made craft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This was expected:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The spacecraft had no more than three hours of battery life, most of which was planned to be used during the descent. Engineers expected to get at most only 30 minutes of data from the surface.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And in fact, the Cassini orbiter only attempted to listen to Huygens and relay its transmissions for that 30 minute period.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-03-25T00:00:01.017" LastActivityDate="2016-03-25T00:00:01.017" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14573" PostTypeId="2" ParentId="14560" CreationDate="2016-03-25T03:07:51.237" Score="15" Body="&lt;p&gt;I can answer the &amp;quot;average&amp;quot; part.  Here's a table from my usual go-to source for openly available Shuttle details, the &lt;a href=&quot;https://gandalfddi.z19.web.core.windows.net/Shuttle/USA007587%20Rev%20A%20-%20Shuttle%20Crew%20Operations%20Manual%2020081215.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt;, page 1009.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eTkUn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eTkUn.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In this table, &amp;quot;PRCS&amp;quot; refers to the large Primary Reaction System jets, and &amp;quot;YRCS&amp;quot; is a typo for the smaller Vernier Reaction System jets. &amp;quot;NORM&amp;quot; and &amp;quot;LOW&amp;quot; are different settings on the Digital Autopilot. &amp;quot;FWD&amp;quot; and &amp;quot;AFT&amp;quot; refer to the different RCS pods in the Orbiter.&lt;/p&gt;&#xA;&lt;p&gt;(The &lt;a href=&quot;https://www.ibiblio.org/apollo/Shuttle/Crew%20Training/RCS%20Workbook.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;RCS system&lt;/a&gt; was divided up in two ways - a forward pod in the nose in front of the crew cabin, and aft pods located to the left and right of the vertical tail.  Within each system, there were Primary thrusters (~900 lbs thrust) and Vernier thrusters (~25 lbs thrust).  Each pod had an independent propellant system.  The two aft pods could be interconnected, or cross-fed to the Orbital Manuevering System engines.  There was no interconnect to the forward pod.)&lt;/p&gt;&#xA;&lt;p&gt;So much for accelerations.  For rates, everything depended on the Digital Autopilot settings, which were extremely flexible and reconfigurable, and therefore highly complex. The settings could be pre-loaded or changed by the crew in flight.  Rotation rates and deadbands varied widely depending on mission phase, payload operations etc. Here are a few examples from the same source:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A7/VERN – Used for attitude hold (1 deg attitude deadband, 0.016&#xA;deg/sec rotation rate)&lt;/p&gt;&#xA;&lt;p&gt;B7/PRI – Used for maneuvers (2 deg attitude deadband, 0.5 deg/sec&#xA;rotation rate)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(page 1144)&lt;/p&gt;&#xA;&lt;p&gt;Re: the A7 and B7 nomenclature:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Each planned DAP configuration is given a reference number. In&#xA;general, the A configurations have larger deadbands and slower&#xA;maneuvering rates than the B configurations. The wider deadbands of&#xA;the A configurations are used to minimize fuel usage, while the&#xA;tighter deadbands of the B configurations allow greater precision in&#xA;executing maneuvers or holding attitude.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(page 520)&lt;/p&gt;&#xA;&lt;p&gt;For some more detailed DAP settings, here are the tables for the last Shuttle mission, from the &lt;a href=&quot;https://archive.org/download/flight-maps-and-charts/ORB_OPS_135.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-135 Orbit Ops Flight Supplement&lt;/a&gt;. (starts on page 34)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q8rdN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q8rdN.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/rAUF5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rAUF5.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/mk9sl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mk9sl.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/xgtmP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xgtmP.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here's a brief writeup on the Orbital DAP. There's much more at the &lt;a href=&quot;https://archive.org/download/space-shuttle-news-reference-1988/SPACE%20SHUTTLE%20NEWS%20REFERENCE_1988.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt; (pdf page 779, document page 763).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The rotation rates and dead bands, translation rate and certain other&#xA;DAP options can be changed by the flight crew during the orbit phase&#xA;using the DAP CRT display. The flight crew can load the DAP with these&#xA;options in two ways: one option set may be accessed by depressing the&#xA;DAP A push button on the orbital DAP panel, the other by depressing&#xA;the DAP B push button. For convenience, each planned DAP configuration&#xA;is given a number and is referred to by that number and the DAP used&#xA;to access it. Typically, the DAP A configurations will have larger&#xA;dead bands and higher rates than the DAP B configurations. The wide&#xA;dead bands are used to minimize fuel usage, while the tight dead bands&#xA;allow greater precision in executing maneuvers or in holding attitude.&lt;/p&gt;&#xA;&lt;p&gt;The RCS DAP can operate in both an automatic and a manual rotation&#xA;mode, depending on whether the flight crew selects the auto or man&#xA;push button light indicators on the orbital DAP panel. The manual mode&#xA;is also accessed when the RHC is moved out of its detent (neutral)&#xA;position. In both the automatic and manual modes, the rotation rate is&#xA;controlled by the selection of DAP A or B and the information loaded&#xA;in the DAP config display. In addition, in automatic, the DAP&#xA;determines the required attitude to be achieved from universal&#xA;pointing and then computes the RCS jet fire commands necessary to&#xA;achieve these requirements within the current set of dead bands. In&#xA;the manual rotation mode, the RCS DAP converts flight crew inputs with&#xA;any of the three RHCs to RCS jet fire commands, depending on whether&#xA;pulse, disc rate or accel is selected on the orbital DAP panel.&#xA;Simply, when pulse is selected, a single burst of jet fire is produced&#xA;with each RHC deflection. The resultant rotational rate is specified&#xA;on the DAP config display. When disc rate is selected, jet firings&#xA;continue to be made as long as the RHC is out of detent in order to&#xA;maintain the rotational rate specified on the DAP config display. When&#xA;accel is selected, continuous jet firings are made as long as the RHC&#xA;is out of detent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2024-03-21T15:13:28.207" LastActivityDate="2024-03-21T15:13:28.207" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14574" PostTypeId="1" AcceptedAnswerId="14575" CreationDate="2016-03-25T11:04:51.710" Score="27" ViewCount="3867" Body="&lt;p&gt;I was just wondering if it were possible to A) Orbit the moon and earth B) To orbit any two celestial bodies really?&lt;/p&gt;&#xA;" OwnerUserId="13888" LastActivityDate="2019-09-18T13:39:36.823" Title="Is it possible to have an orbit around the earth and the moon" Tags="|orbital-mechanics|the-moon|earth|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14575" PostTypeId="2" ParentId="14574" CreationDate="2016-03-25T13:11:27.817" Score="37" Body="&lt;p&gt;The simple answer is yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The more complicated answer starts to look at types of orbit. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You could have a figure-8 orbit, which has centres around both.  &lt;/li&gt;&#xA;&lt;li&gt;You could have an orbit that goes around the centre of mass of the two bodies (in the case of Earth - Moon this is a point within the Earth, but for Pluto-Charon it lies between the two bodies) - this is likely to be a much wider orbit &lt;/li&gt;&#xA;&lt;li&gt;You could have a Lagrange orbit, where you are orbiting a gravitational point because of the two bodies, but not actually going round them (odd edge case of the word 'orbit')&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some may be stable, others not. In fact the free-return trajectory for Apollo 8, 10 and 11 is an unstable version of the first one. It is a figure-8 that goes out past the moon and comes back. But only once.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have a read of @TildalWave's &lt;a href=&quot;https://space.stackexchange.com/a/4205/38&quot;&gt;post for more info on resonant orbits&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And an example I love - the &lt;a href=&quot;https://en.wikipedia.org/wiki/J002E3&quot; rel=&quot;noreferrer&quot;&gt;Apollo 12 fragment (from Wikipedia)&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lYwge.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lYwge.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-03-25T17:18:31.497" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="14577" PostTypeId="1" AcceptedAnswerId="18118" CreationDate="2016-03-25T16:36:15.057" Score="4" ViewCount="510" Body="&lt;p&gt;In some turbopump designs, propellant is burned and used to power the pumps; then it is dumped overboard. This causes a reduced specific impulse, because not all all the propellant is expelled through the nozzle in an efficient way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, some designs don't dump any propellant overboard; for example, in a closed &lt;a href=&quot;https://en.wikipedia.org/wiki/Expander_cycle&quot; rel=&quot;nofollow&quot;&gt;expander cycle&lt;/a&gt;, fuel is pumped in at an extremely high pressure, then expanded using heat from the combustion chamber / nozzle (which powers the pumps), and then finally enters the combustion chamber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I don't understand is how this could achieve less than a perfect effiency, since all the energy consumed eventually ends up going back to the fuel; an inefficient turbopump simply heat up the fuel slightly, and an inefficent turbine powering the pump simply means that less energy is extracted from the fuel in the first place. In the end, all that energy ends up in the combustion chamber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, this can't be right, so what am I missing?&lt;/p&gt;&#xA;" OwnerUserId="11967" LastActivityDate="2016-09-09T11:43:50.203" Title="In turbopump designs where no propellant is dumped overboard what causes efficiency loss in engines?" Tags="|engine-design|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14578" PostTypeId="1" AcceptedAnswerId="14579" CreationDate="2016-03-25T17:31:02.497" Score="13" ViewCount="1114" Body="&lt;p&gt;You can see those wires spanning between four towers directly around the rocket in the following screenshot (taken from &lt;a href=&quot;https://www.youtube.com/watch?v=hrKzwYUY1E8&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt;). I assume they are some kind of antenna or for some kind of measurement. I have also seen this setup in at other rocket launches. Can anyone tell what it is there for?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7DBGV.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7DBGV.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3200" LastEditorUserId="6816" LastEditDate="2016-03-25T17:39:19.303" LastActivityDate="2016-03-25T18:21:19.663" Title="What are the rectangular wires around Atlas V launch site?" Tags="|launch-site|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14579" PostTypeId="2" ParentId="14578" CreationDate="2016-03-25T18:21:19.663" Score="13" Body="&lt;p&gt;These wires are part of the lightning protection. As a rocket contains flammable [citation needed] components as well as sensitive electronics, it needs to be protected from lightning strikes. The main protective measure are the huge towers surrounding the rocket. If theses towers are high enough and closely located around the rocket they might provide sufficient protection. In your picture an additional grid of wires is used to better shield the central part of the start complex. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/2iQ6P.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2iQ6P.png&quot; alt=&quot;Lightning Protection&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To determine whether protection is sufficient, one uses a geometry based method: If a ball of a given size (blue, typically 20 to 60 meter radius, depending on how much protection is needed) can not reach the object, then it is safe. In the drawing, the two black masts are sufficient to protect the green rocket but not tall enough to protect the red one. In the later case the towers need to be taller, closer together or an additional mesh is needed. Note that protection from sideways is not needed as much as protection from top.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-03-25T18:21:19.663" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14580" PostTypeId="1" CreationDate="2016-03-27T22:19:22.257" Score="6" ViewCount="1115" Body="&lt;p&gt;I'm interested in Earth-Mars transfers that are optimized for speed rather than fuel savings. For standard elliptical, parabolic, and hyperbolic transfers, it appears that the Earth departure flight path angle (fpa) is zero. However, I have data for several hyperbolic transfers where the fpa ranges from 25 to 45 degrees. Can someone shed any light on this? Also where in LEO would the burn actually be initiated? Assume the initial burn will take place in LEO, i.e., disregard launch from Earth's surface. Also assume endpoint at Mars will be LMO of around 300 km altitude. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/XrCec.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XrCec.jpg&quot; alt=&quot;Three basic transfer types and yes the parabolic path is mostly theoretical.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/G6ASa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/G6ASa.jpg&quot; alt=&quot;Summary Table for Mars provided by Microcosm&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13903" LastEditorUserId="13903" LastEditDate="2016-03-28T17:02:53.707" LastActivityDate="2016-04-01T06:53:26.837" Title="Earth departure flight path angle for fast Earth-Mars transfers" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14582" PostTypeId="1" AcceptedAnswerId="14600" CreationDate="2016-03-28T15:07:17.417" Score="5" ViewCount="496" Body="&lt;p&gt;In most of chemical propulsion engines the product of burning fuel and oxidizer is the propellant; in some cases the ratio between the two is non-ideal, in which case some unburnt fuel acts as propellant as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTOH, in concepts like NTR, or with RTG-powered ion drives, the two concepts - fuel and propellant - are entirely distinct. The nuclear fuel provides energy; the inert propellant is the reaction mass. Another trivial example is from amateur rocketry - the compressed air rocket, where compressed air is the energy carrier, but water expelled by air pressure is the main propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any designs of chemical propulsion though, that would use some low (-to-none) energy density propellant of a very high thermal expansion ratio, together with fuel and oxidizer that when combined provide a lot of thermal energy but not nearly as much exhaust velocity and/or thrust?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2018-03-25T22:54:16.507" Title="chemical propulsion with separate fuel and propellant?" Tags="|propulsion|fuel|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="14583" PostTypeId="2" ParentId="10669" CreationDate="2016-03-28T15:45:36.493" Score="2" Body="&lt;p&gt;There are multiple different designs, or wildly varying complexity, so to answer your question in one phrase: it varies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the most common case there's no gas generator as such. Fuel and oxidizer are injected into the combustion chamber from pressurized tanks, ignited, and the burning process takes care of both turning them from liquid to gas, and then of the propulsive combustion. No gas generator, no turbopumps, just some regulated valves, an igniter and the combustion chamber with nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there are designs of increasing complexity: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;channeling the fuel and/or the oxidizer through the bell nozzle, both cooling it and turning the two into gas for better mixing.&lt;/li&gt;&#xA;&lt;li&gt;driving the turbopumps with an entirely separate propellant, like hydrazine.&lt;/li&gt;&#xA;&lt;li&gt;driving some exhaust gas off the combustion chamber to propel the turbopumps and/or evaporate the cryofuels.&lt;/li&gt;&#xA;&lt;li&gt;using the channeled, heated gasses to propel a turbopump that pumps cryo-fuels. (which is pretty much the case that you doubted. Actually, quite viable as the fuel/oxidizer after boiling off in the walls of combustion chamber has much more energy, and is well capable of driving the turbopumps.)&lt;/li&gt;&#xA;&lt;li&gt;using pre-burners, where fuel and oxidizer are mixed in very different than optimal ratio, both evaporating the liquids (for more smooth combustion) and providing energy to drive turbopumps.&lt;/li&gt;&#xA;&lt;li&gt;using separate gas generators, where hot combustion products (be it from the nozzle or preburners) boils the cryofuels, and separate pre-burners that provide heat and energy for the turbopumps.&lt;/li&gt;&#xA;&lt;li&gt;...and any wild combination of the above.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This pretty much culminates with the design of the Space Shuttle Main Engine. The &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;nofollow&quot;&gt;public-domain presentation&lt;/a&gt;, page 19, shows its gory details: four turbopumps (each with distinct turbines for propelling them and for pumping), two preburners, a gas generator that evaporates LOX, a bell nozzle cooling grid that boils off LH2, and a byzantine grid of connections that drive the fuel and oxidizer heated, hot partially burned oxygen-rich, cool but evaporated. re-cooled after passing the turbopump, heated but not evaporated after being pushed through turbopump, superheated through pre-burning fuel-rich, re-cooled by passing through another turbopump, injected back into tanks to provide pressure, back and forth through the grid, in a way that makes sense if you really put your mind to it, but is hard on one's sanity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So... your understanding is incomplete ;) The bottom line is there's a lot of designs. The one in the picture you linked is pretty straightforward: some propellant and oxidizer diverted from the main lines gets burned in a preburner (the &quot;gas generator&quot;) providing work for propelling the turbine. It may or may not be used to ignite the proper fuel/oxidizer mix. It may or may not be used to heat the oxidizer. The schematics depicts a generic, pretty straightforward, example middle-range rocket engine, and the exact details in reality may vary wildly.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-03-28T15:45:36.493" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14584" PostTypeId="1" CreationDate="2016-03-28T16:31:03.203" Score="2" ViewCount="188" Body="&lt;p&gt;I couldn't find a YouTube video of the welcome ceremony of the newly arrived crew of Soyuz TMA-20M.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was there no broadcast on NASA TV of this event?&lt;/p&gt;&#xA;" OwnerUserId="13909" LastEditorUserId="58" LastEditDate="2016-03-30T23:47:36.060" LastActivityDate="2016-04-03T03:49:21.797" Title="Was footage released of the Soyuz TMA-20M crew arrival ceremony?" Tags="|crewed-spaceflight|soyuz-spacecraft|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14586" PostTypeId="2" ParentId="14580" CreationDate="2016-03-29T00:26:13.403" Score="8" Body="&lt;p&gt;It is not clear what you want light shed upon. In any case, a Hohmann transfer, or something close to it in real-world trajectories, has close to a 0° flight path angle on departure as well as arrival. That uses the least impulsive $\Delta V$ at Earth to get to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you don't care about efficiency, and the table you reference makes it quite clear that you don't, then any faster trajectory will have to aim outwards in order to intercept Mars sooner. The extreme case is aiming a light beam at Mars during closest approach, in which case the flight path angle is 90°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, sure, for faster than optimal flight times, you'll get a flight path angle somewhere between 0° and 90°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that your image is of Earth departure, for which the trajectory relative to Earth must be hyperbolic to leave. This is the definition of &quot;escape&quot;. However the word &quot;hyperbolic&quot; in the table is likely referring to the energy of the trajectory relative to the Sun. Those are &lt;em&gt;solar system&lt;/em&gt; escape trajectories. If you don't stop at Mars, you leave the solar system forever. Efficient trajectories from Earth to Mars (or back) are all solar orbits, and are elliptical relative to the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for where in LEO you do one of these amazing 30 to 40 km/s burns, it's when the resulting hyperbola relative to Earth lines up your departure vector with the transit trajectory hyperbola relative to the Sun. Normally the plane of your low-Earth orbit would already have been chosen to facilitate this, so the Earth launch does in fact matter. However with these incredible $\Delta V$'s, an additional plane change is probably not all that significant.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-03-29T00:26:13.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14587" PostTypeId="2" ParentId="12644" CreationDate="2016-03-29T12:03:18.027" Score="1" Body="&lt;p&gt;To make an answer out of my comment: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://journals.aps.org/pra/abstract/10.1103/PhysRevA.61.013405&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; talks about a flow of about 20 billion Caesium atoms per second and characterizes that as a &quot;high flow&quot;. So let us use that as an example...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;20*10&lt;sup&gt;9&lt;/sup&gt; atom/s, cooled, say 273 Kelvin (from 0°C to 0K), at 32.2 J/(mol K) is a heat flow of...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;29*10&lt;sup&gt;-6&lt;/sup&gt; mW. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words: not even &lt;strong&gt;one part in a million compared to the JWST cooling system&lt;/strong&gt;, and that is with extreme assumptions regarding the temperature of what this system is cooling. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So just forget it... the heat flow is much too small. &lt;/p&gt;&#xA;" OwnerUserId="10493" LastActivityDate="2016-03-29T12:03:18.027" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14588" PostTypeId="2" ParentId="14580" CreationDate="2016-03-29T12:49:28.177" Score="5" Body="&lt;p&gt;Barrycarter is right: the fastest path is straight from Earth, almost perpendicular to its orbit, only angled a little bit to account for matching orbital velocities, with launch at the point of the planets' closest approach, to a point in Mars orbit when Mars will be at the time of arrival, assuming accelerating at full thrust for half the trip and decelerating for the other half (or just accelerating all the way through if you're trying for a fly-by or an impactor.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And of course that path will have ludicruously prohibitive delta-V requirements making it impossible with anything short of whole Earth diverting all funds and labor available towards building the absolutely enormous craft. Any path with a semblance of plausibility will be a kind of trade-off between the shortest (fastest) and a fuel-saving one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the tables you have assume high, but still realistic delta-V. They assume a departure burn and arrival burn, not a continuous full-thrust burn all the way through.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer for all &quot;optimize for speed&quot; question boils down to &quot;How much money are you willing to throw at it?&quot;. You can shed a couple days or even weeks if you throw a couple million dollars at a typical (Hohmann) transfer. The more you straighten the path, the more you save, but as your delta-V requirements grow linearly, with travel time scaling down linearly, the size of the rocket - and the cost of building it - grows exponentially.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The realistic answer to optimizing for speed is that you first find the standard fuel-saving transfer (Hohmann, possibly with gravity assists/Oberth maneuvers) and then you modify it, shortening the travel time at cost of extra delta-V, up to your financial budget allowance.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-04-01T06:53:26.837" LastActivityDate="2016-04-01T06:53:26.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14589" PostTypeId="1" AcceptedAnswerId="14597" CreationDate="2016-03-29T16:57:30.240" Score="8" ViewCount="1753" Body="&lt;p&gt;I realize that there are different orbital maneuvers for different goals (e.g. optimizing time, efficiency, cost, etc.). However, which variables are currently considered as the most important to be optimized? This leads to the question, which orbital maneuver is currently regarded as the most viable for a human mission to mars? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm aware that there is no &lt;em&gt;best&lt;/em&gt; way to get there, just the most accepted way. Each maneuver will have its pros and cons. I'm just asking what is the current consensus on which orbital maneuver to perform?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: In order to clarify entirely what I mean, I am only concerned with transfer orbits such as Hohmann, bi-elliptical, etc. I am doing a school project that requires me to calculate the math behind an orbital transfer to mars for a human mission. It is nothing TOO technical but I still want it to be correct nonetheless, and I was curious to know which orbital transfer I should choose for the calculations. &lt;/p&gt;&#xA;" OwnerUserId="13835" LastEditorUserId="3185" LastEditDate="2016-03-31T11:33:09.933" LastActivityDate="2021-03-06T15:12:25.603" Title="Which orbital maneuver is currently regarded as the most viable for a human mission to mars?" Tags="|mars|orbital-mechanics|nasa|orbital-maneuver|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14590" PostTypeId="2" ParentId="14589" CreationDate="2016-03-29T18:32:59.947" Score="3" Body="&lt;p&gt;The best from my understanding is a 2 year free return trajectory, with launch windows fairly close to the time of a Hohmann transfer. It involves a departure velocity of 5.08 km/s, 2 years to return to Earth, and 180 days to transit to Mars. Source is &quot;The Case for Mars&quot;. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-03-30T17:03:39.137" LastActivityDate="2016-03-30T17:03:39.137" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14591" PostTypeId="2" ParentId="14589" CreationDate="2016-03-30T00:12:31.377" Score="5" Body="&lt;p&gt;The best trajectory depends on many factors, most importantly things like: when you want to get there, how long you want to stay, and whether you want to optimize for time or for ∆v expenditure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Optimizing for time reduces radiation exposure and consumables requirements for manned missions; optimizing for ∆v gives you more payload for a given size of launcher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has a &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php&quot; rel=&quot;noreferrer&quot;&gt;sweet trajectory browser&lt;/a&gt; that lets you explore your options with given constraints. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-03-30T00:12:31.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14592" PostTypeId="2" ParentId="5" CreationDate="2016-03-30T00:30:52.640" Score="15" Body="&lt;p&gt;I did research on wild hogs in the late 1970's.  The shuttle runway was right down the middle of my study area.  While building the runway, the hogs rooted the area up faster than they could build it.  Fencing did not keep the hogs out as they rooted right under the fence.  The solution... dig canals, fill the canals with gators to keep the hogs out of the construction site long enough to finish construction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a link to my M.S. thesis on wild hogs at KSC &#xA;&lt;a href=&quot;https://www.researchgate.net/publication/35615093_An_investigation_of_the_movements_of_feral_swine_Sus_scrofa_in_East_Central_Florida&quot; rel=&quot;noreferrer&quot;&gt;https://www.researchgate.net/publication/35615093_An_investigation_of_the_movements_of_feral_swine_Sus_scrofa_in_East_Central_Florida&lt;/a&gt;&#xA;Here are a couple of other links on wild hogs and gator issues around the shuttle runway.&#xA;&lt;a href=&quot;https://www.csmonitor.com/1983/1012/101225.html&quot; rel=&quot;noreferrer&quot;&gt;https://www.csmonitor.com/1983/1012/101225.html&lt;/a&gt;&#xA;&lt;a href=&quot;https://www.nasa.gov/feature/our-refuge-feral-hogs&quot; rel=&quot;noreferrer&quot;&gt;https://www.nasa.gov/feature/our-refuge-feral-hogs&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13917" LastEditorUserId="8693" LastEditDate="2019-03-11T00:53:59.677" LastActivityDate="2019-03-11T00:53:59.677" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14594" PostTypeId="2" ParentId="6109" CreationDate="2016-03-30T11:45:03.057" Score="4" Body="&lt;p&gt;As far as I've been able to find, the cause is not known yet.&lt;/p&gt;&#xA;&lt;p&gt;They didn't do brightness measurements before launch, and brightness can vary dramatically depending on e.g. the viewing angle. So the expectation was never more than a rough estimate.&lt;/p&gt;&#xA;&lt;p&gt;From a paper published in Q1 2014 (a few months after launch), &amp;quot;&lt;a href=&quot;http://gbot.obspm.fr/gbotpipeline/spie9149-25.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;GBOT - Ground Based Optical Tracking of the Gaia satellite&lt;/a&gt;&amp;quot;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The brightness of the main target of a campaign such as Gaia is probably the single most important piece of information. Unfortunately it is also one of the hardest to obtain, since in most cases the satellite is being assembled on the ground while the campaign is set up.&lt;/p&gt;&#xA;&lt;p&gt;While it looks rather straightforward at first to draw some conclusion based on basic principles, such as area, reflectivity, etc. in practice this is a very complicated process, since it depends on the reflective quantities of the material, which are not always available because of industrial secrets, and may furthermore change over time, due to effects like radiation damage - the L2 is a high radiation environment.&lt;/p&gt;&#xA;&lt;p&gt;Other factors are the aspect angle, the azimuth angle (which is constantly changing for a spinning object like Gaia), and many other issues. In order to get a strong estimate for the predicted brightness of Gaia, the GBOT group did apply to perform measurements, however this proposal was retracted after it was found to be too costly to perform.&lt;/p&gt;&#xA;&lt;p&gt;Therefore we had to rely on our experience with other space craft, namely those we were observing during our tests, i.e. mainly WMAP and Planck; While Planck has a widely different shape and probably also reflective characteristics, see Fig. 5, WMAP was essentially a smaller version of Gaia inclined by half the angle of Gaia, i.e. 22.5◦ instead of 45◦.&lt;/p&gt;&#xA;&lt;p&gt;WMAP in general had a magnitude in R of 18-18.5, Planck about 18 mag. Therefore it seemed safe to assume that conservatively speaking, Gaia has roughly the same magnitude. This turned out to be not the case. For reasons not yet entirely understood (exposed structures, etc.?) Gaia turned out to be more than 2 mags fainter than Planck, leading to the need to reevaluate the whole GBOT program, as described in Sect. 6.&lt;/p&gt;&#xA;&lt;p&gt;This still ongoing process formed the main activities of the GBOT&#xA;team during the opening months of 2014. Now, with several months having elapsed since the launch, we have a somewhat better grasp of the magnitude range of Gaia; it was found to be between 20 and 21.2, depending on the distance, the Earth aspect angle k , and other factors. It was known before that spacecraft can show rapid and long term variability of unknown origin, e.g. the unusual faintness followed by unusual brightness of Planck during the OR3 (see Sect. 4.)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Gaia was launched on 2013-12-19, observations listed in the paper were 2014-01-07 to 2014-02-26, i.e. from 19 days after launch. That makes radiation blackening unlikely as a cause for the less-than-expected brightness.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-03-30T20:15:44.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14595" PostTypeId="2" ParentId="14163" CreationDate="2016-03-30T13:58:13.737" Score="6" Body="&lt;p&gt;I'm unable to give you precise numbers, but for orders of 10kV this will be a sub-millimeter distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mechanism of arcing in vacuum is significantly different than in air. While in air the air particles, excited, become the plasma and start conducting electricity, there is no such medium in vacuum. There's &lt;a href=&quot;https://en.wikipedia.org/wiki/Field_electron_emission&quot; rel=&quot;nofollow&quot;&gt;Field Electron Emission&lt;/a&gt;, especially from any sharp points; either into empty space, or towards a positively charged target (the other spaceship). Now this emission must achieve such intensity as to heat up the electrode until its own material evaporating is emitted into space and carried by the electrons reaches the target - creates a plasma bridge. Only then arcing can occur, as it excites more of the electrode material producing more arc-sustaining plasma.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This largely depends on ability to emit enough electrons in an area small enough so that the current vaporizes the electrode surface. And here the actual calculation problems begin: what is the evaporation temperature of the material? What is its resistance - corresponding to wattage of the current being emitted. What is its shape? Sharp blades produce much higher field emission than flat surfaces. How fast and how far will the emitted material fly before it cools down enough to become resistive? It ceases to be an electrical problem, and becomes a problem of calculating behavior of mechanics of production and travel of evaporated metal plasma in vacuum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit:&#xA;Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Dielectric_strength&quot; rel=&quot;nofollow&quot;&gt;gives&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;High vacuum (field emission limited)  20 - 40MV/m (depends on electrode shape)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;so, 20-40kV/mm.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-04-04T12:31:12.493" LastActivityDate="2016-04-04T12:31:12.493" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14596" PostTypeId="2" ParentId="6109" CreationDate="2016-03-30T14:07:02.100" Score="1" Body="&lt;p&gt;There are a number of reasons why it might be especially hard to detect. Let me give a few of the most common ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, materials change with respect to time in space. The first, and usually largest, process happens during the out-gassing phase, which essentially happens in the first few days of a spacecraft, where any gasses trapped in materials escapes. There are rumors in the space industry of companies using non space approved materials to clean optical surfaces, and those optical surfaces (Or solar panels) ended up darkening over the course of several months to a year later, ending in a loss of the satellite. Outgassing is a serious issue!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another possibility is that radiation, solar wind, etc could have damaged some of the visible components, perhaps leading to a lower visibility. For instance, I understand small plastic components, which often hold antennas in place for launch, are degraded with time in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly is the orientation of the spacecraft. If the spacecraft was pointed differently than expected, it would appear to be dimmer than expected. Some spacecraft have a huge profile of how they are seen, depending on their orientation! If something is reflected in a specular manner, there can be a huge glint if it reflects light directly from the Sun, or even the Earth. This could even be the result from a fairly small component. These things are quite difficult to predict, leading to a fair amount of uncertainty. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-03-30T14:07:02.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14597" PostTypeId="2" ParentId="14589" CreationDate="2016-03-30T21:02:22.073" Score="12" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;In order to clarify entirely what I mean, I am only concerned with transfer orbits such as Hohmann, bi-elliptical, etc.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's none of those. Both are for transferring from one circular orbit to another, and with both orbits on the same orbital plane. Neither Earth's nor Mars's orbit is circular. Mars's orbit is notably non-circular, with an eccentricity of 0.0934. Mars's orbit is inclined with respect to Earth's orbit by 1.85 degrees. This is more than enough to make a Hohmann transfer a useless fiction. (Aside: Even if the orbits were circular and on the same plane, a bi-elliptical transfer wouldn't make sense for transfers from Earth to Mars. The orbits are too close to one another.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is done instead is to solve &lt;a href=&quot;https://en.wikipedia.org/wiki/Lambert%27s_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lambert's problem&lt;/a&gt;, over and over and over. Given a central mass, a pair of distinct points in space $\vec r_1$ and $\vec r_2$ relative to that central mass, and a pair of points in time $t_1$ and $t_2&amp;gt;t_1$, Lambert's problem involves solving for the conic section that starts at $vec r_1$ at time $t_1$ and intersects $\vec r_2$ at time $t_2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Except for diametrically opposed points and radially aligned points, there are two such solutions to Lambert's problem. One, &quot;the short way&quot; (or sometimes, a Type 1 transfer), involves a change in true anomaly that is less than 180 degrees. The other solution, &quot;the long way&quot; (or sometimes, a Type 2 transfer), involves a change in true anomaly that is more than 180 degrees. Note: Some of these solutions might involve going faster than the speed of light. That's not a problem in Newtonian mechanics. It is a problem with regard to delta-V. Regarding those diametrically opposed points and radially aligned points: These are problematic with regard to Lambert's problem. The solutions are singular. The general approach is to ignore those points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose the point $\vec r_1$ represents a point on some initial orbit at time $t_1$, and point $\vec r_2$ represents a point on some target orbit at time $t_2$. With this, one can calculate the delta V needed at time $t_1$ to transfer from the initial orbit to one of those Lambert trajectories and the delta V needed at time $t_2$ to transfer from that Lambert trajectory to the target orbit. This is the delta-V cost for that particular trajectory. One of the two solutions will have a higher cost (typically much higher) than the other. We'll discard the high cost solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now do this over and over and over again, but with different departure times $t_1$ and arrival times $t_2$. Generate a contour plot with departure dates and on one axis and arrival dates on the other, and eventually you'll come up with something like this, from &lt;a href=&quot;https://mars.jpl.nasa.gov/spotlight/porkchopAll.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;br&gt;&#xA;&quot;Porkchop&quot; is the First Menu Item on a Trip to Mars&lt;/a&gt;:&#xA;&lt;img src=&quot;https://mars.jpl.nasa.gov/spotlight/images/porkchop_lg.gif&quot; alt=&quot;Porkchop plot for transfer from Earth to Mars in 2005&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a porkchop plot. The blue contour lines show the energy (here in terms of C3) needed to transfer from Earth to Mars in 2005. (Note: Since we're interested in getting from Earth to Mars (or back), it makes more sense to use C3 rather than delta V.) Note that the transfer orbits are markedly non-Hohmann. A Hohmann transfer would take about 275 days. The optimal transfers instead take about 200 days (short way) and 400 days (long way).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I am doing a school project that requires me to calculate the math behind an orbital transfer to mars for a human mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You didn't mention what level of school this project is for. If the above is over your head, you can ignore all that and use the fiction of Hohmann transfers. If it's not over your head, this is exactly what is done to calculate transfer orbits to Mars at JPL.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.773" LastActivityDate="2016-03-30T21:02:22.073" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14598" PostTypeId="2" ParentId="13454" CreationDate="2016-03-30T22:46:40.957" Score="7" Body="&lt;p&gt;I suggest you look into the chemistry of clay slip and slurries. For example, porcelain slip is very fluid and can pour through distribution channels in molds to form fine features, then sets up as a weak solid after losing only a small amount of water to the porous mold. The percentage of water is quite low, with the liquid properties coming from carefully controlling the properties of the porcelain clay and a small percentage of surfactants and lubricity compounds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in a word, yes, you can have a room-temperature liquid slurry comprised of a particulate solid, lubricity compounds, and a pretty small percentage of water or other carrier liquid. HOWEVER you face problems with settling and clogging. Since the water content is so low, any place you drive a density gradient, such as in a pipe elbow, will cause solids to settle out forming a clog. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can consider liquid metals rather than a particular slurry, there are some Lead-Bismuth and Tin-Bismuth alloys that melt at or below 170°C. Zinc casting alloys melt around 720°F.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought you were crazy pushing the density impulse idea until I remembered that Aerojet demonstrated boosting 1st stage thrust in a (simulated) NTR using LOX injection at the nozzle inlet. Great thrust/weight for the first portion of the flight, then switching to a high-ISP, low-thrust/weight mode once the heavy mass-additive is expended, with very little parasitic tankage weight remaining.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-03-30T22:46:40.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14599" PostTypeId="1" AcceptedAnswerId="14668" CreationDate="2016-03-30T23:45:57.007" Score="3" ViewCount="248" Body="&lt;p&gt;Van Allen belts contain high energy electrons and protons, but what about the area between the belts?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it &quot;regular&quot; space, just like it is past the belts? Anything interesting going on there? &quot;Just&quot; exosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that it is more protected by the Earth's magnetic field than the space past the belts -- when compared to the side facing the sun, correct?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google searches for &quot;between Van Allen belts&quot; and similar don't come up with much useful data.&lt;/p&gt;&#xA;" OwnerUserId="13925" LastActivityDate="2016-04-05T17:02:57.223" Title="What is the space environment between Van Allen belts like?" Tags="|environment|exosphere|van-allen-belt|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14600" PostTypeId="2" ParentId="14582" CreationDate="2016-03-31T00:20:22.920" Score="7" Body="&lt;p&gt;The V2 rocket used 75% ethanol / 25% water for fuel. Purer ethanol was available but led to combustion temperatures that were too high; diluting the fuel limited the temperature so the engine could survive the burn but also changed the rocket's thrust properties. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerojet has demonstrated thrust augmentation of a (simulated) NTR by injecting liquid oxygen into the hot H2 stream in the expansion nozzle. ISP drops but thrust/weight increases; varying the LOX mass ratio allows throttling of the ISP vs thrust until the LOX is expended.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-03-31T00:20:22.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14602" PostTypeId="1" AcceptedAnswerId="14604" CreationDate="2016-03-31T08:04:36.083" Score="10" ViewCount="1567" Body="&lt;p&gt;I assume a nuclear thermal rocket with a solid core first. The principle of operation seems simple enough: Bring your propellant close to your nuclear fuel, start a reaction and wait for some heat/pressure to build up due to neutron (and other?) absorption. Once you have enough heat, you release the propellant through your nozzle and get acceleration, right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when you release propellant, the pressure inside the reaction chamber drops, so you will have to refill it with some fresh propellant from your tanks. I consider this a severe practical problem, since the pressure inside the chamber is still quite high (ideally, if you want to achieve constant thrust, you need to work against your target pressure). From my limited understanding, it seems that you need to bring the propellant to an even higher pressure to press it into the reaction chamber, which would be absurd since then you could pump it through the nozzle directly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what am I missing here? How do you get the propellant into the reaction chamber? Bonus points for answering the same for the way more complicated liquid or gas core engines.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="4660" LastEditDate="2016-03-31T17:52:38.393" LastActivityDate="2023-04-27T06:59:06.913" Title="How does propellant flow work in a nuclear thermal rocket?" Tags="|rockets|propulsion|engines|nuclear|ntr|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14603" PostTypeId="2" ParentId="5040" CreationDate="2016-03-31T08:20:18.137" Score="3" Body="&lt;p&gt;If we ignore the atmospheric effects for a moment, let's see what gravity does as you descend into a planet (and this goes for all planets, rocky or gaseous). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Newton's &lt;a href=&quot;https://en.wikipedia.org/wiki/Shell_theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shell theorem&lt;/a&gt;, inside a sphere of uniform density, gravity is proportional to your &lt;a href=&quot;https://www.quora.com/How-does-gravity-behave-as-you-move-toward-the-center-of-a-large-mass-such-as-the-earth-If-there-is-a-gravity-gradient-inside-the-earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;distance to the center&lt;/a&gt;. Gravity is highest when you're on the surface, with all of the planet's mass below you. When you're at the center of the planet, gravity is 0 because the pull from different directions cancels each other out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter isn't uniform, so the equation becomes more complicated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You get F = gM/r&lt;sup&gt;2&lt;/sup&gt;, where g is the gravitational constant. M is the mass of the sphere with radius r, this depends on the average density of the sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Earth, the gravity profile looks like this: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jO1jj.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jO1jj.jpg&quot; alt=&quot;Earth gravity profile. Gravity stays more or less constant from the surface down to 0.5 times the planet radius. From there to the center of the planet, gravity drops linearly to 0.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Jupiter you get a profile that's more pronounced because the difference in density between the outer layers and the core is more extreme. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-02T12:36:25.217" LastActivityDate="2016-06-02T12:36:25.217" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14604" PostTypeId="2" ParentId="14602" CreationDate="2016-03-31T11:22:34.933" Score="16" Body="&lt;p&gt;You have the same problem in any rocket. The pressure at the pump outlet must be higher than the chamber pressure. What you are missing, is that the density of your propellant drops by heating, both in chemical reactions, as well as by being fed through a nuclear core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a nuclear thermal rocket some of the heated (and thus expanded) hydrogen is tapped off to drive a turbine, which provides the energy to power a fuel pump.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QCqgm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QCqgm.png&quot; alt=&quot;Diagram of a nuclear thermal rocket&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Image credit: &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Nuclear_thermal_rocket_en.png&quot; rel=&quot;noreferrer&quot;&gt;Original drawing&lt;/a&gt; by &lt;a href=&quot;https://commons.wikimedia.org/wiki/User:Tokino&quot; rel=&quot;noreferrer&quot;&gt;Tokino&lt;/a&gt; at Wikimedia Commons, &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Nuclear_thermal_rocket_en.svg&quot; rel=&quot;noreferrer&quot;&gt;vectorized&lt;/a&gt; by &lt;a href=&quot;https://en.wikipedia.org/wiki/User:CommiM&quot; rel=&quot;noreferrer&quot;&gt;CommiM&lt;/a&gt; at Wikipedia; licensed under &lt;a href=&quot;https://en.wikipedia.org/wiki/en:GNU_Free_Documentation_License&quot; rel=&quot;noreferrer&quot;&gt;GFDL&lt;/a&gt; and &lt;a href=&quot;https://creativecommons.org/licenses/by-sa/3.0/deed.en&quot; rel=&quot;noreferrer&quot;&gt;CC-By-SA 3.0&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="189" LastEditDate="2016-03-31T18:02:42.830" LastActivityDate="2016-03-31T18:02:42.830" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14605" PostTypeId="1" AcceptedAnswerId="14609" CreationDate="2016-03-31T16:27:59.373" Score="19" ViewCount="2296" Body="&lt;p&gt;The answer to this &lt;a href=&quot;https://space.stackexchange.com/questions/13539/which-operating-systems-is-the-international-space-station-running/13569#13569&quot;&gt;question&lt;/a&gt; mentions the use of Ada-programmed applications on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ada was developed for the US DoD to replace a plethora of computer languages (some 465), for mission-critical applications. It has a strong ability for fault correction and bug identification, and hence is ideally suited to space applications as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What other space applications, projects and agencies are using the Ada programming language?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-03-31T18:54:03.383" Title="What space applications, projects and agencies are using the Ada programming language?" Tags="|flight-computer|computing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14607" PostTypeId="2" ParentId="14248" CreationDate="2016-03-31T17:55:42.157" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I understand there is a propulsion system on each of Zvezda and Zarya.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There was a propulsion system on Zarya. The attitude jets and main engines on that module have been permanently disabled. The propellant tanks are still used, but only to supply the jets and engines on Zvezda.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Do any of the other permanent ISS space station modules have a propulsion system and what type is it?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There were plans for a ISS Propulsion Module on the US side of the Station, but that was cancelled long ago. It was never built. There is also the Interim Control Module, intended as a backup for Zvesda, and this was built. It currently is located in some warehouse in Washington, D.C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Bottom line: No permanent ISS space station modules other than Zveda have a propulsion system. Zvesda and visiting vehicles are the only options.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-03-31T17:55:42.157" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14608" PostTypeId="2" ParentId="5527" CreationDate="2016-03-31T18:34:58.853" Score="11" Body="&lt;p&gt;The answer is yes, it can used to re-boost the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the information in &lt;a href=&quot;http://www.spaceflightinsider.com/space-flight-news/aerojet-rocketdyne-inspires-new-stem-video/&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflightinsider.com&lt;/a&gt; and in &lt;a href=&quot;https://www.rocket.com/article/aerojet-rocketdyne-completes-cst-100-work-commercial-crew-integrated-capability-contract&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket.com&lt;/a&gt; Aerojet Rockedyne site in the RCS engines paragraph, in a declaration of Terry Lorier, Aerojet Rocketdyne’s CST-100 service module propulsion system program manager, are confirmed the capabilities of Boeing CST-100 Starliner spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Propulsion system of CST-100 includes different types of engine thrusters with different roles. In &lt;a href=&quot;http://spaceflightnow.com/2015/11/27/aerojet-rocketdyne-wins-propulsion-contracts-worth-nearly-1-4-billion/&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflightnow.com&lt;/a&gt; are explained what includes the propulsion system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CST-100 has four 39700-pound thrust (176.6 kN)  launch abort engine (LAE) for the CST-100’s ; 24 orbital maneuvering and attitude control thrusters (OMAC)  1500 pounds of thrust (6.67kN) each, for launch abort attitude control, stage separation functions, large orbital maneuvers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also are included 28 reaction control system engines (RCS), each of them with 100 pound thrust (0.44 kN) using similar technology to other engines in CST-100. RCS role is to re-boost international space station (ISS). Let say that even the above engines the bigger ones, could be used sometimes to help for re-boosting faster the ISS if there is enough fuel in their respective tanks. But with or without them, the RCS can accomplish the re-boosting procedure for ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This hasn’t to do with your question but I see that you have written Dream Chaser would not be able to re-boost. Actually in &lt;a href=&quot;http://www.sncspace.com/Mediakit/FAQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;sncspace.com&lt;/a&gt; Sierra Nevada Corporation (SNC) site and in this article of &lt;a href=&quot;http://www.spaceflightinsider.com/missions/iss/nasa-announces-winners-of-crs-2-contract/&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflightinsider.com&lt;/a&gt;, is confirmed this capability. In the official site of SNC is included also the info for cargo and crew Dream Chaser similarities and differences that you  have asked in another question. Anyway for the part “engines are blocked”, it could be if we are speaking for the crew version. But at least for cargo version, now I am just supposing that since info said that can re-boost, probably this could happen because in spacecraft is attached the cargo module, and when will be attached to ISS distance between Dream Chaser and ISS module will be considerably bigger and its engines could be lower as level. Probably Dream Chaser cargo and crew roles could be the same as Progress and Soyuz spacecraft situation for re-boosting procedure part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway current NASA’s request for the role of this spacecrafts CST-100 and Dream Chaser, is to accomplish crew and cargo missions respectively, but if sometimes it will be necessary to re-boost ISS each company of this two spacecrafts has confirmed this capability.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-11-01T17:50:31.830" LastActivityDate="2016-11-01T17:50:31.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14609" PostTypeId="2" ParentId="14605" CreationDate="2016-03-31T18:54:03.383" Score="24" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What other space applications, projects and agencies are using the Ada programming language?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the US, old stuff such as the Delta rockets, the Atlas rockets, and the TDRSS ground terminal (but this is being switched to C, C++, C#, and Java). There might be new development, but it's mostly stuff you (and I) cannot know about because it's classified. New development is mostly in mainline languages such as C, C++, C#, and Java, and also specialized tools such as Matlab Simulink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One reason for the shift away from Ada was that the Ada mandate came out too soon. Some projects that foundered because of lack of tools switched to C. The managers of these projects remember that Ada experience. The memories are not fond.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason was demographics. Finding Ada programmers, good or bad, has always been a bit tough. It's much easier finding C or C++ programmers, and this gives managers the opportunity to select for quality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet another is that many of the developers of tools that initially targeted Ada (e.g. Rational) found that their tools worked quite nicely with other languages. Because of the demographics, improvements and new features were first made to the mainline language versions before extending them to Ada. This added even more impetus to switch from Ada.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ada appears to be more widely used in Europe than in the U.S. One reason is that the chief architect of Ada was Dr. Jean Ichbiah at CII-Honeywell-Bull in France. The Ada mandate was applied in Europe as well as in the U.S., but the timing was a bit better there. The tools had matured, and many of the tools were developed in Europe by the company founded by Dr. Ichbiah. A recent example is the flight software for the GOCE satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;A not so good example is the flight software for the maiden flight of the Ariane 5 rocket. This is perhaps the most infamous of all software errors. Ada has a number of safety features built into the language. (Compare with C, where the standard has twelve pages that summarize the undefined behaviors built into the language. Compare with C++, where the undefined behaviors are so numerous that they don't even list them.) Those Ada safety features can have a significant performance cost, and because of this, Ada also supplies the ability to selectively disable those safety features.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of Ariane 5 flight 501, the software engineers reused Ariane 4 flight software, where safety features relating to overflow in some of the GNC software was disabled. The Ariane 5 had considerably more thrust than did the Ariane 4. This increased acceleration made the accelerometer data overflow on conversion to an integer. This in turn resulted in hardware exceptions, which in turn made the GNC software come to a halt, which in turn resulted in the destruction of the vehicle.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-03-31T18:54:03.383" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14610" PostTypeId="2" ParentId="12615" CreationDate="2016-03-31T20:28:44.243" Score="3" Body="&lt;p&gt;To carry the answer from &lt;a href=&quot;https://space.stackexchange.com/questions/12644/could-ir-space-telescopes-and-other-very-cold-devices-use-zeeman-slowers-effecti/14587&quot;&gt;the other thread&lt;/a&gt; over here as well: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;No, Zeeman slowers cannot be used as space station coolers&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason is that the heat flow they provide is in the nanoWatt range. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, they are not &quot;close&quot; to being used as general cooling systems in space, for the same reason that placing a home-made water wheel under your kitchen faucet is not &quot;close&quot; to providing you free energy for your home. Sure, you might get a LED to shine faintly but that's about it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Question was asked &quot;Why can't this be scaled up&quot;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because there are very narrow tolerances regarding velocity (speed and direction) as the atoms enter the tube. If you have a low density &lt;strong&gt;beam&lt;/strong&gt; then this is fairly easy to maintain, because the atoms are not interacting. But once you scale it up to have a mass flow that is significant enough to provide the amount of cooling that is needed, then we no longer have a particle beam of atoms but a &lt;strong&gt;flow&lt;/strong&gt; of gas or liquid. This means the atoms are bumping into each other and Browninan motion destroys any chance of meeting the narrow tolerances regarding the entry velocity. &lt;/p&gt;&#xA;" OwnerUserId="10493" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-04-01T10:23:33.693" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14611" PostTypeId="2" ParentId="12615" CreationDate="2016-04-01T00:16:53.417" Score="1" Body="&lt;p&gt;Too cool down the station, we must transport energy away from the station. Period. That can be done through radiation, or by letting some substance boil of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Zeeman slower does neither of that, and is then strictly not usable for &quot;general cooling&quot;. In fact, it produces more heat as it uses electricity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;However:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Changing what parts of the spacecraft are hot or not might still have an effect. For instance, if the radiators are hotter, they radiate more energy. The Zeeman slower only makes the surroundings a little hotter, so traditional expansion-compression systems do that part a lot more efficient. The main advantage of the Zeeman slower is to cool a tiny amount of atoms to a really low temperature (just a few Kelvin), so it can be used to cool components that needs to stay really cool. (like super conductors or space-telescope optics). Note that the output of cool atoms from the beam is very small, so any components cooled by it have to be extremely well thermally isolated.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-01T00:16:53.417" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14614" PostTypeId="1" AcceptedAnswerId="14620" CreationDate="2016-04-01T10:44:03.073" Score="7" ViewCount="531" Body="&lt;p&gt;It is well-known that when we are in space for a long time, our bodies lose some mass. My question is how much time would be needed for a person to lose 10% of their body mass?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tortoise inside the &lt;a href=&quot;https://en.wikipedia.org/wiki/Zond_5&quot; rel=&quot;noreferrer&quot;&gt;Zond 5 spacecraft&lt;/a&gt; seems to have lost the 10% in just a lunar flyby. Is it the same for humans, or is it just because tortoises are smaller than us?&lt;/p&gt;&#xA;" OwnerUserId="13853" LastEditorUserId="34345" LastEditDate="2020-02-26T20:34:00.073" LastActivityDate="2020-12-30T22:53:01.033" Title="Why did the Zond 5 tortoises lose body mass so quickly?" Tags="|health|time|experiment|animals|zond|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="14615" PostTypeId="1" AcceptedAnswerId="14616" CreationDate="2016-04-01T12:01:34.800" Score="11" ViewCount="1355" Body="&lt;p&gt;One of the biggest issues that I've heard of in building a space elevator is the high tensile strength of the cord used to tether the the elevator to the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there's a structure that's at geostationary orbit, shouldn't the tension of the cord be theoretically zero (seeing as from the point of view of the tether point on the ground, the satellite would remain at the same distance above it)? If so, could other, weaker materials be used?&lt;/p&gt;&#xA;" OwnerUserId="410" LastActivityDate="2016-04-01T14:15:19.403" Title="Tethered geostationary orbit" Tags="|geostationary|space-elevator|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14616" PostTypeId="2" ParentId="14615" CreationDate="2016-04-01T12:40:45.053" Score="20" Body="&lt;p&gt;&lt;strong&gt;At&lt;/strong&gt; the geostationary orbit, the &lt;strong&gt;growth&lt;/strong&gt; (gradient) of tension will be practically zero, but the value will be far, far from zero. Actually, it will be the highest in the whole tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After all, the whole length of the tether, that goes all the way down to Earth surface, is moving progressively slower than what orbital speed at corresponding altitude is - and as result, is pulled down by Earth's gravity. Any part of the tether, therefore, must carry the weight of the tether below it, reduced only as much as gravity drag is reduced by the velocity of the tether so far. This all cumulates to the maximum at GEO, and then drops off again, with the pull in the opposite direction (away from Earth) as the tether extends farther, to a counter-balance at its end, as its velocity exceeds local orbital velocity, creating an outwards pull - that keeps the tether taut and prevents it from falling all the way down to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That also means there is no limit (well, within reason) to the size of an orbital station placed at GEO on the tether, as it doesn't contribute to the strain. Any other station creates an extra strain either way - although in case of one near the far end of the tether this is actually welcome as it contributes to the pull that keeps the tether taut.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, the least strained point of the tether is right at Earth surface, where there is no more mass hanging down from the tether - only as much strain as needed by surplus counter-balance necessary to allow the extra mass of traveling payload, improve rigidity of the tether and combat winds and rotation of Earth that could otherwise wrap it around Earth surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, one practical way of building the space tower is to deliver the (rolled) tether to GEO and unroll it there, both directions simultaneously, up and down, so that the &quot;downwards&quot; part's pull evens up with the &quot;upwards&quot; one. The net pull remains zero, but as the two ends travel apart, the strain grows immensely.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-04-01T14:15:19.403" LastActivityDate="2016-04-01T14:15:19.403" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14618" PostTypeId="1" AcceptedAnswerId="14623" CreationDate="2016-04-01T13:02:38.170" Score="5" ViewCount="606" Body="&lt;p&gt;I know this is a bit of apples-to-oranges comparison, since bi-elliptic transfer is a purely mathematical construct in a two-body problem, while ITN is a great &quot;maze of opportunities&quot; based on the immense complexity of the solar system, that is largely time-dependent, and finding an &lt;em&gt;optimal&lt;/em&gt; trajectory (as opposed to just a &lt;em&gt;good&lt;/em&gt; one) may be an NP-hard problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, a rough estimate? Say, how much delta-V would one save/lose going to Saturn through a bi-elliptic transfer vs ITN, willing to spend, say, 30 years? Or some similar figure, that gives a rough hunch which one may be preferable in what conditions?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-04-02T15:20:53.007" Title="How does bi-elliptic transfer compare to Low Energy transfers / Interplanetary Transfer Network?" Tags="|orbital-maneuver|low-energy-transfer|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14619" PostTypeId="1" AcceptedAnswerId="14644" CreationDate="2016-04-01T14:18:12.933" Score="4" ViewCount="196" Body="&lt;p&gt;There are some concepts of providing propulsion (or at least preventing orbital decay) by driving current through extended rods from a satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK currently they are used only to provide torque (magnetorquers) to stabilize spin of satellites, but some works - like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Skyhook_%28structure%29&quot;&gt;non-rotating skyhook&lt;/a&gt; foresee using them for acceleration - boosting the orbit of the vessel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know current moving parallel to lines of a magnetic field doesn't exert any force on the wire it travels through, so certainly there is a &quot;dead direction&quot; where a magnetic rod would provide no propulsion/force whatsoever. I'm not sure about shape of Earth's magnetic field though, and as result &lt;em&gt;what&lt;/em&gt; direction it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - if we take a satellite in equatorial, or other not heavily inclined orbit, what translation maneuvers can we perform using just magnetic rods?&#xA;How do the rods need to be oriented for them?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-04-04T06:55:52.243" Title="Which acceleration vectors can magnetic rods provide?" Tags="|propulsion|" AnswerCount="1" CommentCount="3" ClosedDate="2016-09-27T18:22:43.537" ContentLicense="CC BY-SA 3.0" />
  <row Id="14620" PostTypeId="2" ParentId="14614" CreationDate="2016-04-01T14:49:06.107" Score="9" Body="&lt;p&gt;It's important to note that the tortoises carried on the Zond flight were probably not given any food or water, and I suspect they were selected because they could survive that for a long period. So weight loss would be inevitable.  (We know that Apollo 17 carried pocket mice in the Command Module to study radiation exposure, and that the species was chosen for their ability to survive on very small amounts of water.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For people it's completely different as of course they are well supplied.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Muscle mass through atrophy is common if exercise is not taken. There is a good summary on the Wikipedia page &lt;a href=&quot;https://en.wikipedia.org/wiki/Effect_of_spaceflight_on_the_human_body&quot;&gt;Effect of spaceflight on the human body&lt;/a&gt;, and the headline claim there is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;without regular exercise astronauts can lose up to 20% of their muscle mass in just 5 to 11 days&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also see &lt;a href=&quot;http://www.nasa.gov/pdf/64249main_ffs_factsheets_hbp_atrophy.pdf&quot;&gt;a NASA factsheet on muscle atrophy&lt;/a&gt; which is the source of this figure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We should remember the &quot;up to 20%&quot; figure is for muscle loss, so the overall body mass loss will be much less and anyway will depend on the calorie intake...&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-04-01T14:57:31.220" LastActivityDate="2016-04-01T14:57:31.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14622" PostTypeId="2" ParentId="5075" CreationDate="2016-04-01T19:11:44.977" Score="10" Body="&lt;p&gt;A four-planet lineup is rare, as @Nickolai said. 3-planet lineups are &lt;a href=&quot;https://space.stackexchange.com/questions/8643/when-will-3-outer-planets-line-up-for-a-flyby-mission-next-time/8859#8859&quot;&gt;a bit more common&lt;/a&gt;. New Horizons used a single gravity assist (Jupiter) to get to Pluto.&lt;br&gt;&#xA;Grand Tour-type missions are going to be rare: the focus has shifted to missions that study a single planet in more detail. Many of these missions were designed to answer the questions raised by the Voyager flybys.&lt;br&gt;&#xA;You could argue that Dawn is conducting a tour of the asteroid belt, and there's a mission plan for another probe that will visit 6 asteroids. Not as grand as the 4 outer planets, but the same basic idea: initial exploration that will be followed up by more detailed investigations. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-04-01T19:11:44.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14623" PostTypeId="2" ParentId="14618" CreationDate="2016-04-01T19:48:26.767" Score="5" Body="&lt;p&gt;To enjoy a bi-elliptic savings, destination needs to be greater than destination orbit by a factor of 11.94 or greater.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going from LEO to the moon certainly qualifies: 384,400/6678 = ~58.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll pull a bi-elliptic path out of the hat, don't know if it's optimal:&#xA;300 km altitude circular orbit to a 900,000 km altitude apogee: 3.16 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apogee burn at 900,000 km to raise perihelion to 384,400 km: .43 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Circularize burn at 384400 perihelion: .18 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a total of about 3.8 km/s. It's noteworthy that Farquhar's more direct route reaches EML2 for around 3.4 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you time your burn from LEO to land near a certain region of earth's Hill Sphere, the sun's tidal influence can raise your perihelion to 443,000 km (the neighborhood of EML2). From there the ship can do a ballistic slide into EML2. I describe such a path in &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;EML2 blog post&lt;/a&gt;. This can take as little as 3.1 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for LEO to EML2 a .7 km/s savings can be enjoyed over bi-elliptic and a .3 km/s savings over the Farquhar route.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saturn's orbit is less than 11.94 A.U., so no bi-elliptic savings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is no ITN route from earth to Saturn or even from earth to Mars or Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;What?&quot; you may ask. &quot;How come the earth-moon-spaceship 3-body system confers savings while the sun-rocky planet-spaceship 3-body doesn't&quot; The difference is in the &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/06/mass-parameter-and-itn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;3 body mass parameter&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BklfR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BklfR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All kinds of interesting trajectories emanate from the Pluto/Charon L1 and L2. Ditto Earth/Moon L1 and L2 as well as gas giant/big moons L1 and L2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the sun/small rocky planets mass parameters are in the millionths to hundred millionths. I don't believe there are any interesting WSBs leading from the sun earth L2 to Mars or any other planet. At least none that take less then centuries to arrive at the destination.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-04-02T15:20:53.007" LastActivityDate="2016-04-02T15:20:53.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14625" PostTypeId="2" ParentId="14163" CreationDate="2016-04-02T12:30:24.400" Score="2" Body="&lt;p&gt;It is going to depend upon the orbital region and so this is a partial answer relating specifically to the ISS.&lt;/p&gt;&#xA;&lt;p&gt;[The space-flight environment: the International Space Station and beyond] &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2691437/&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt; provides this description:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Ionospheric plasma&lt;/p&gt;&#xA;&lt;p&gt;Spacecraft in low orbit around the earth have a complex interaction&#xA;with ionospheric plasma. The solar arrays on the International Space&#xA;Station operate at 160V, and the distribution system is at 120V DC.&#xA;The negative side of the power system is grounded to the structure of&#xA;the space station, resulting in a large amount of energy stored in the&#xA;structure at –140V. High voltage solar arrays, coupled with the design&#xA;and material properties of the International Space Station, can lead&#xA;to detrimental interactions with the ionospheric plasma.&lt;/p&gt;&#xA;&lt;p&gt;Two plasma contactor units have been placed on the International Space&#xA;Station to provide a “ground wire” to prevent arc discharging. These&#xA;devices emit a low-energy stream of electrons during spacewalks that&#xA;reduces the buildup of electrical charge.[13] As long as the plasma&#xA;contactor units are functional, an astronaut floating freely during a&#xA;spacewalk has no risk of exposure to arcing. However, the steel&#xA;tethers used by astronauts to attach themselves to the structure of&#xA;the space station and the exposed metallic surfaces of the spacesuit&#xA;or tools used during the spacewalk are potential sources for arcing if&#xA;both of the plasma contactor units were to fail during a spacewalk.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;For full credit please note that the reference &amp;quot;[13]&amp;quot; in the quote refers to this book:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Tribble AC. The space environment and its impact on spacecraft design.&#xA;31st American Institute of Aeronautics and Astronautics (AIAA),&#xA;Aerospace Sciences Meeting and Exhibit; 1993 Jan 11–14; Reno (NV).&#xA;Webster (TX): American Institute of Aeronautics and Astronautics;&#xA;1993. p. 491.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;To address &lt;strong&gt;the question&lt;/strong&gt;:  its not clear what distance can be inferred here. The risk of metal hand tools appears to be that if left &amp;quot;floating&amp;quot; in electrical terms then at some point in their own charge build-up a discharge event can bridge insulating materials such as in a space suit. It seems reasonable that an arc could also occur when such a tool is returned to close proximity with the space station.&lt;/p&gt;&#xA;&lt;p&gt;Its not clear what the role of the steel tether would be. Superficially it would appear to be a good opportunity to reduce the risks if each loose item could be connected back to the space station 0 volt reference, but it appears that a different design philosophy applies from this context.&lt;/p&gt;&#xA;&lt;p&gt;To get more on the distance answer I'd suggest, as a next stop, reading more into the plasma contactor unit purpose and design.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2020-09-18T19:09:08.493" LastActivityDate="2020-09-18T19:09:08.493" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14627" PostTypeId="1" AcceptedAnswerId="14630" CreationDate="2016-04-02T14:42:53.897" Score="12" ViewCount="1713" Body="&lt;p&gt;Consider the following hypothetical event:&#xA; a Voyager spacecraft (1 or 2) changes its planned interstellar trajectory quite suddenly (I'm well aware of motion and gravity laws and I know that this shouldn't occur, but let's just pretend they are flying by an unobserved massive object).&#xA;Would the engineer team here on Earth detect it? How?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've read a lot through &lt;a href=&quot;http://voyager.jpl.nasa.gov/index.html&quot;&gt;Voyager Interstellar Mission website&lt;/a&gt;, but I couldn't find any detailed information on how spacecraft position and speed is determined.&lt;/p&gt;&#xA;" OwnerUserId="13945" LastEditorUserId="6278" LastEditDate="2016-04-03T02:17:56.060" LastActivityDate="2016-04-04T04:12:15.157" Title="Would a Voyager spacecraft trajectory change be recognized?" Tags="|probe|voyager|tracking|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14628" PostTypeId="2" ParentId="14627" CreationDate="2016-04-02T15:23:19.487" Score="4" Body="&lt;p&gt;Spacecraft position and speed are determined from Earth: we point an antenna at the point where we expect the spacecraft to be, and we try to receive its radio signal. The antenna position that gives the strongest signal gives a heading. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To determine distance, we send a signal to the spacecraft and we wait for the response.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You see the problem: an unexpected change in trajectory means the spacecraft is no longer in the position we expect. We point the antenna in the wrong direction, and we don't find the spacecraft.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A search is possible (gradually sweep the antenna across the sky), but would take a long time. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-02T15:23:19.487" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14629" PostTypeId="2" ParentId="14627" CreationDate="2016-04-02T16:53:31.647" Score="3" Body="&lt;p&gt;There are several different techniques to measure the speed and position of a spacecraft. &#xA;First, distance from Earth can be measured by the round-trip time of a signal. If the time the spacecraft needs between receiving a signal and sending it back is well known, distance can be determined very precisely.&#xA;From two measurements of distance, one can calculate the speed of the vehicle This is, the rate it moves away from Earth, not the total speed. &#xA;There is a second method for calculating this speed by observing the frequency of the received signal: The faster the spacecraft moves away from Earth, the lower the frequency gets (see Doppler shift). These measurements can have almost arbitrary precision, only limited by the length of observation. A good example is the observation of probes doing a fly-by of Earth whose speed has been measured to &lt;a href=&quot;https://en.wikipedia.org/wiki/Flyby_anomaly&quot; rel=&quot;nofollow&quot;&gt;less than 1 mm/s precision&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The precise position in space, on the other hand, is difficult to measure. As Hobbes wrote, one can use a directional antenna and do a scan to find the best position. But, even the 70 meter antennas of the Deep Space Network have a beam width of 0.1 degree. We can assume that we can, through repeated measurements and interpolation with this antenna, determine the direction of the spacecraft with a precision of 0.01 degrees. In the case of Voyager one, 130 AU from Earth this results in a position error of 3 million kilometers. &#xA;But, this large opening angle of the beam sent by the antenna also has the advantage that the spacecraft can not easily vanish from the space covered by the transmission due to some unexpected change in trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, changes in speed and distance from Earth can be determined very precisely, the absolute position in lateral coordinates is a much trickier task and difficult to find changes here.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-04-02T16:53:31.647" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14630" PostTypeId="2" ParentId="14627" CreationDate="2016-04-02T19:46:54.967" Score="22" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Would the engineer team here on Earth detect it? How?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are two issues here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose Voyager 2 (Voyager 1 is moving faster and is further away, so the effect will be lesser on Voyager 1 than on Voyager 2) made a ridiculously close flyby of an object with a similar size and mass of Neptune. Such a flyby would result in a delta-V of about &lt;a href=&quot;http://www.wolframalpha.com/input/?i=&amp;#40;15.4km%2Fs&amp;#41;%2F&amp;#40;1%2B&amp;#40;15.4km%2Fs&amp;#41;%5E2*&amp;#40;26e3km&amp;#41;%2F&amp;#40;7e15m%5E3%2Fs%5E2&amp;#41;&amp;#41;&quot;&gt;8.2 km/sec&lt;/a&gt;. Suppose this delta-V is orthogonal to the line of sight to Earth (worst case). Even then, it would be almost &lt;a href=&quot;http://www.wolframalpha.com/input/?i=&amp;#40;111+AU&amp;#41;*sin&amp;#40;0.1+degrees&amp;#41;%2F&amp;#40;8.2+km%2Fs&amp;#41;&quot;&gt;41 days&lt;/a&gt; to fly out of the beam width of the Deep Space Network antennae. To this day, JPL still receives data from the Voyagers on a daily basis. That gives plenty of time to notice the change in velocity. It appears that continued contact with the Voyagers could continue even in the face of a ridiculously close flyby of a ridiculously large object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, such a ridiculously close flyby would have another effect. We don't &quot;see&quot; the Voyager satellites. We instead receive their transmissions. The vehicles are invisible if we can't receive those transmissions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that a flyby not only changes the satellite's velocity, it also changes the satellite's orientation and rotation rate thanks to gravity gradient torque. That's fine in the case of a planned flyby. It's no so fine in the case of an unplanned flyby when the vehicle has limited fuel to correct for an unplanned rotation and limited intelligence to orient itself so the antenna points back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only downlink capability currently available to Voyagers is the X-band transmission, with a beam width of 0.5 degrees. Even a small unplanned change in Voyager attitude means we on Earth couldn't see that Voyager, at least not until the Voyager has found out where it is pointing and corrected for that. A change in attitude rate is an even bigger problem. The Voyagers have a very limited amount of remaining fuel for attitude control. While that limited amount of fuel is more than enough to last until 2025 assuming that the vehicles are flying through empty space, whether it's enough to correct for an unexpected large gravity gradient torque is a very different matter.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-04-02T19:52:23.617" LastActivityDate="2016-04-02T19:52:23.617" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14631" PostTypeId="1" AcceptedAnswerId="14832" CreationDate="2016-04-03T01:30:07.450" Score="4" ViewCount="650" Body="&lt;p&gt;I am reading this paper from the NTRS, &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=19750009325&quot; rel=&quot;nofollow&quot;&gt;Passive stabilization of the Long Duration Exposure Facility (LDEF)&lt;/a&gt;, about (not surprisingly) gravity-gradient stabilization of the LDEF satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like it says that gravity-gradient torque was used to stabilize the spacecraft in &lt;em&gt;all three&lt;/em&gt; axes.  It was my understanding that gravity-gradient torque tends to align the &lt;em&gt;minor&lt;/em&gt; axis of inertia with the radius vector (perpendicular to the local horizon), giving only two axes of control.  &lt;strong&gt;So how is 3-axis control possible?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper mentions that the third control torque is related to the difference between the two smaller axes of inertia, so it seems to be gravitational in nature.  The stabilizing torque is also &lt;em&gt;not&lt;/em&gt; magnetic or aerodynamic, since those torques are also discussed in the paper but only treated as perturbative.  What causes the third component of torque?  Is it related to the nonsphericity of the Earth?&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="8996" LastEditDate="2016-04-14T04:01:09.593" LastActivityDate="2016-04-14T04:09:55.590" Title="3-axis gravity gradient stabilization?" Tags="|orbital-mechanics|gravity|attitude|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14633" PostTypeId="2" ParentId="14584" CreationDate="2016-04-03T03:49:21.797" Score="3" Body="&lt;p&gt;The video of the welcome ceremony for Expedition 47, the crew of the Soyuz TMA-20M, was in fact released publicly. I found one copy of it on Daily Motion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dailymotion.com/video/x3zh9jo&quot; rel=&quot;nofollow&quot;&gt;Expedition 47 Welcome Ceremony&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The footage does contain the hatch opening as well and is a little over 18 minutes long, so you may want to grab some popcorn.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-04-03T03:49:21.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14635" PostTypeId="2" ParentId="14627" CreationDate="2016-04-03T18:11:17.893" Score="0" Body="&lt;p&gt;On top of the mentioned &quot;roundtrip&quot; and &quot;antena aim&quot; data, which is good for rough measurements (not &lt;em&gt;very&lt;/em&gt; rough, but not exactly a sub-meter precision), there's a way to triangulate the position of any broadcasting object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Point three antennas at different locations of the world, with precisely synchronized timers, at Voyager. Record the incoming wave, precisely recording the timing of some significant point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The X-band wavelength is about 3cm. Measuring speed is quite simple, calculate deviation from that wavelength, caused by &quot;red shift&quot;. But moreover, if you compare phase of the wave - timings of arrival of a specific point of it - you can triangulate the craft's position with extreme accuracy, way better than &quot;signal strength&quot; estimate. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-04-04T04:12:15.157" LastActivityDate="2016-04-04T04:12:15.157" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14636" PostTypeId="1" AcceptedAnswerId="14641" CreationDate="2016-04-03T18:59:33.037" Score="10" ViewCount="1620" Body="&lt;p&gt;In a tap-off cycle, combusted gases are extracted from the combustion chamber and routed to the turbines, which then drive fuel/oxidizer pumps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a pressure-feed cycle, fuel/oxidizer is fed into the combustion chamber by high tank pressures.  A pressurant gas (e.g. helium) is also fed into the fuel/oxidizer tanks to keep the tank pressure constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be possible to combine the concepts of these cycles, by using the tap-off gas directly as pressurant gas? On the face of it, this would lead to a very simple cycle, but I can't find any material on it. Why is this not done/possible?&lt;/p&gt;&#xA;" OwnerUserId="13956" LastEditorUserId="13956" LastEditDate="2016-04-03T19:04:50.590" LastActivityDate="2016-12-22T21:37:29.310" Title="Why not use tap-off gas for repressurization for a simple rocket engine cycle?" Tags="|engine-design|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14638" PostTypeId="1" CreationDate="2016-04-03T19:32:31.953" Score="1" ViewCount="379" Body="&lt;p&gt;How does NASA's funding compare to other space agencies' funding? How many papers are they publishing and how many US patents do they have? Compare different space agencies in terms of &#xA;number of patents per dollar funded&lt;br&gt;&#xA;and&lt;br&gt;&#xA;number of research papers published per dollar funded&lt;br&gt;&#xA;and&lt;br&gt;&#xA;number of papers published in Science or Nature (or an equally reputed space studies journal) per dollar funded&lt;/p&gt;&#xA;" OwnerUserId="13882" LastActivityDate="2016-04-03T20:32:23.460" Title="How does NASA compare with other space agencies in terms of RoI (in terms of science: papers published per $ invested)" Tags="|nasa|research|cost|esa|jaxa|" AnswerCount="0" CommentCount="6" ClosedDate="2016-04-04T21:57:57.663" ContentLicense="CC BY-SA 3.0" />
  <row Id="14639" PostTypeId="1" CreationDate="2016-04-03T19:52:11.540" Score="8" ViewCount="169" Body="&lt;p&gt;My thinking is that in an atmosphere rich in hot sulfuric acid they couldn't stand a chance. Although I am unsure of what exactly would be going on chemically at the interface between the venerean litho and atmo spheres.&lt;/p&gt;&#xA;" OwnerUserId="13167" LastEditorUserId="1054" LastEditDate="2020-12-15T23:57:55.023" LastActivityDate="2020-12-15T23:57:55.023" Title="Should we expect the surface rocks of Venus to be oxide poor?" Tags="|planetary-science|venus|chemistry|geology|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14640" PostTypeId="2" ParentId="14636" CreationDate="2016-04-03T20:28:22.807" Score="10" Body="&lt;p&gt;Most engines are run with an imperfect &lt;a href=&quot;https://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio&quot;&gt;(non-stoichiometric)&lt;/a&gt; mix of oxygen and propellant, so the exhaust will have either oxygen or propellant in it.&lt;br&gt;&#xA;Running propellant-rich, hot exhaust into a LOX tank can lead to ignition of the remaining propellant, so you'd have a fire inside your tank. The same goes for oxygen-rich exhaust being run into a propellant tank. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-03T20:28:22.807" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14641" PostTypeId="2" ParentId="14636" CreationDate="2016-04-03T21:40:56.680" Score="12" Body="&lt;p&gt;Besides the safety concerns mentioned by Hobbes, there are other things to consider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gases at the combustion chamber are at a very high pressure and temperature and as such are excellent at driving turbopumps. They are exhausted at a much lower temperature and pressure, having given up a considerable amount of useful energy in an efficient expansion cycle. More importantly, the turbopump is able to achieve a higher pressure (albeit at lower volume flow) than the combustion chamber pressure, as it is effectively 2 independent machines connected by a shaft: a turbine and a pump.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a pressure fed cycle, gas from the combustion chamber, besides being very hot, is necessarily at a slightly lower pressure than tank pressure. It is difficult to make a lower pressure gas pressurise a higher pressure tank. Surprisingly, it is possible if the low pressure gas has greater volume and here are some tricks employed in steam engines to make this happen, but it requires a considerable waste of fluid. &lt;a href=&quot;https://en.wikipedia.org/wiki/Injector&quot;&gt;https://en.wikipedia.org/wiki/Injector&lt;/a&gt; (see section on feedwater injectors.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other issue is the nature of the combustion gases. They are mainly H2O and CO2. They need to be cooled before entering the tank, or, if not, they will reach the temperature of the tank contents as soon as they enter it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Oxygen or Hydrogen, these substances will instantly freeze and be useless at maintaining pressure, other than by the heating effect they have on the tank medium. In a kerosene tank, the H2O would condense and sink to the bottom, which might not cause problems if the percentage was small. Only the CO2 would go to pressurizing the tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all, it's more complicated than you make it sound. A separate helium tank is a better solution. &lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="6117" LastEditDate="2016-04-03T23:38:24.070" LastActivityDate="2016-04-03T23:38:24.070" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14644" PostTypeId="2" ParentId="14619" CreationDate="2016-04-04T06:44:03.697" Score="2" Body="&lt;p&gt;The earth's magnetic field is &lt;em&gt;roughly&lt;/em&gt; that of a dipole, tilted from the rotational axis by about 10 degrees at the moment, but it's actually somewhat lumpy/bumpy. I plotted the three components (x, y, z = North, East, Down) as a function of longitude, for a LEO at 300km above the equator using the &lt;a href=&quot;https://en.wikipedia.org/wiki/World_Magnetic_Model&quot; rel=&quot;nofollow noreferrer&quot;&gt;World Magnetic Model&lt;/a&gt; WMM2015 from &lt;a href=&quot;http://www.ngdc.noaa.gov/geomag-web/#igrfgrid&quot; rel=&quot;nofollow noreferrer&quot;&gt;NOAA &gt; NESDIS &gt; NCEI (formerly NGDC) &gt; Geomagnetism&lt;/a&gt; using the menu settings and script shown below. The field is roughly 0.3 Gauss pointing North, but there are &quot;bumps&quot; available in the other two directions of about +/- 0.1 Gauss. The full description of the model is &lt;a href=&quot;http://www.ngdc.noaa.gov/geomag/WMM/data/WMM2015/WMM2015_Report.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. I also plotted the fields from a &lt;a href=&quot;https://en.wikipedia.org/wiki/Dipole_model_of_the_Earth%27s_magnetic_field&quot; rel=&quot;nofollow noreferrer&quot;&gt;dipole approximation&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read more about the Earth's magnetic field &lt;a href=&quot;http://core2.gsfc.nasa.gov/geomag.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QaSvA.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QaSvA.gif&quot; alt=&quot;Earth&amp;#39;s lumpy magnetic field&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The force on a current in a field follows the &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Regla_mano_derecha_Laplace.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laplace right hand rule&lt;/a&gt; (I never knew he had one!) so for a satellite in orbit, using North, East, and Down as the three axes, to boost an equatorial LEO satellite higher, I think we need an East-West force (accelerate the speed), which means either an Up-Down rod using the strongest North-South field, or - as you mention in the question - a North-South rod using the weaker Up-Down field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnitude of the force (Newtons) is equal to the length of the rod (meters) times the field strength (Tesla). So 1 Amp in 0.3 gauss gives 3E-5 Newtons. I'm not sure how the ends of the rod work - but if you assume a 1.5 eV work function for the electron and ion guns (and otherwise no losses), that's 1E-05 Newtons per Watt, or 100kW per Newton - very roughly the same scale as plasma thrusters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in order to carry a current, the rods must emit negative charge (electrons or ions) from one end and positive charge (ions) from the other. But I have no idea where an Amp of ions can come from! If this technology is supposed to take advantage of gas that it encounters in its orbit, it will probably be microamps or nanoamps. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a sneaking suspicion that the &lt;em&gt;drag&lt;/em&gt; from a given gas pressure may be worse than the &lt;em&gt;thrust&lt;/em&gt; from a rod carrying a current produced by ionizing the gas at that pressure, but that's a different question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If instead it carries it's own gas to ionize, then I wonder if maybe a conventional ion thruster is a better choice?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me know if this is helpful or if I've missed the point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YUBaD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YUBaD.png&quot; alt=&quot;Earth magnetic field 300km above equator&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hF3rh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hF3rh.png&quot; alt=&quot;Right Hand Rule of Laplace&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8hiK6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8hiK6.png&quot; alt=&quot;Field coordinates used in WMM2015&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gYvEX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gYvEX.png&quot; alt=&quot;Input settings&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some scrappy Python to give an idea what I did:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;fnames     = [&quot;equator_X.csv&quot;, &quot;equator_Y.csv&quot;, &quot;equator_Z.csv&quot;]&#xA;directions = [&quot;North&quot;,         &quot;East&quot;,          &quot;Down&quot;]&#xA;&#xA;# http://www.ngdc.noaa.gov/geomag-web/#igrfgrid&#xA;&#xA;fields = []&#xA;for fname in fnames:&#xA;    with open(fname, &quot;r&quot;) as infile:&#xA;        lines = [line.rstrip('\n') for line in infile]&#xA;    print lines[13]&#xA;&#xA;    lon, hkm, nT = np.array([[float(x) for x in line.split(',')]&#xA;                             for line in lines if &quot;#&quot; not in line]).T[2:5]&#xA;    fields.append(nT * 1E-09 * 1E+04) # nT to T to gauss&#xA;&#xA;fields = np.vstack(tuple(fields))&#xA;total  = np.sqrt((fields**2).sum(axis=0))&#xA;&#xA;fields = np.vstack((fields, total))&#xA;directions += [&quot;Total&quot;]&#xA;&#xA;lon = np.array(lon)&#xA;zlon = np.zeros_like(lon)&#xA;&#xA;&#xA;# Now get simple dipole approximation&#xA;# Geomagnetic poles - axis of &quot;best dipole approximation&quot; to earth's real field&#xA;&#xA;gmlat, gmlon = 80.3, -72.6&#xA;&#xA;rads, degs = np.pi/180., 180./np.pi&#xA;&#xA;mlon = lon - gmlon - 90&#xA;sinmlat = -np.sin(rads*mlon)*np.sin(rads*(90-gmlat))&#xA;mlat = np.arcsin(sinmlat)&#xA;&#xA;plt.figure()&#xA;plt.plot(lon, degs*mlat)&#xA;plt.show()&#xA;&#xA;def earth_dipole(r_km, maglat):&#xA;    &quot;&quot;&quot;from: https://en.wikipedia.org/wiki/Dipole_model_of_the_Earth%27s_magnetic_field&quot;&quot;&quot;&#xA;&#xA;    a_km = 6371. # km&#xA;    B0 = 3.12E-5 # T&#xA;&#xA;    R = r_km / a_km  # in earth radii&#xA;&#xA;    Br  = -2. * (B0 / R**3) * np.sin(maglat)&#xA;    Bth =       (B0 / R**3) * np.cos(maglat)&#xA;&#xA;    return Br, Bth&#xA;&#xA;r_km = (6371. + 300)*np.ones_like(mlat)&#xA;&#xA;Bz_dip, Bx_dip = earth_dipole(r_km, mlat)&#xA;&#xA;fields = np.vstack((fields, 1E+04*Bx_dip, -1E+04*Bz_dip)) # Tesla to Gauss&#xA;directions += [&quot;North(dipole)&quot;, &quot;Down(dipole)&quot;]&#xA;&#xA;&#xA;plt.figure()&#xA;&#xA;for field, label in zip(fields, directions):&#xA;    plt.plot(lon, field, label=label)&#xA;&#xA;plt.plot(lon, zlon, '-k')&#xA;plt.legend(loc='center right', shadow=False, fontsize = 'large')&#xA;plt.xlim(lon.min(), lon.max())&#xA;plt.title(&quot;Magnetic field 300km above Earth Equator&quot;, fontsize=18)&#xA;&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-04-04T06:55:52.243" LastActivityDate="2016-04-04T06:55:52.243" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14645" PostTypeId="1" CreationDate="2016-04-04T15:48:01.237" Score="46" ViewCount="10182" Body="&lt;p&gt;This is a question about early planetary missions. It looks like there was only one early (unsuccessful) landing mission to Mars and that subsequently Venus became the target for interplanetary landings. Here's my question: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why was Venus prioritized when (at least with modern technology) the surface of Mars seems way easier to explore, and e.g. send back amazing images? Does this have something to do with the difficulty of getting there? Something with the fact that we didn't know about the extremeness of Venus' atmosphere? Or was there a non-technology related reason that deemed Venus as the more exciting destination?&lt;/p&gt;&#xA;" OwnerUserId="11397" LastEditorUserId="6667" LastEditDate="2016-04-05T23:07:22.700" LastActivityDate="2020-09-16T20:24:20.687" Title="Why was Venus rather than Mars targeted for the first interplanetary landings?" Tags="|spacecraft|landing|history|interplanetary|venus|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14646" PostTypeId="2" ParentId="14645" CreationDate="2016-04-04T15:55:35.647" Score="37" Body="&lt;p&gt;We didn't know how hostile Venus's surface was, until we had landed there. The atmosphere of Venus makes it easier to land there than Mars. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Observations_and_explorations_of_Venus&quot;&gt;Wikipedia&lt;/a&gt;, we learn:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Before radio observations in the 1960s, many believed that Venus contained a lush, Earth-like environment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While there was some concept that Venus was hot, and had a high pressure, the exact amount as to how hostile it was wasn't really known until we had sent a lander there.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-05T12:04:56.770" LastActivityDate="2016-04-05T12:04:56.770" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="14648" PostTypeId="1" CreationDate="2016-04-04T18:38:53.913" Score="4" ViewCount="400" Body="&lt;p&gt;At orbital speeds around the Earth, things tend to burn up on striking the atmosphere, since they're going so fast.  To get an astronaut (for example) home safely from those speeds, they need to have some kind of heat shield to protect them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if they were landing on Titan instead of Earth?  At orbital speeds around Titan, how much heat shielding would you need to enter Titan's atmosphere without burning up? &lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="25" LastEditDate="2016-04-04T19:45:43.253" LastActivityDate="2016-04-04T20:21:53.137" Title="Would astronauts need a heat shield to land on Titan from orbital speeds?" Tags="|spacecraft|reentry|heat-shield|titan|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14649" PostTypeId="2" ParentId="14648" CreationDate="2016-04-04T18:44:08.597" Score="1" Body="&lt;p&gt;from &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Titan&quot; rel=&quot;nofollow&quot;&gt;wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Observations from the Voyager space probes have shown that the Titanean atmosphere is denser than Earth's, with a &lt;strong&gt;surface pressure about 1.45 times that of Earth's&lt;/strong&gt;. Titan's atmosphere is about &lt;strong&gt;1.19 times as massive as Earth's&lt;/strong&gt; overall,[6] or about 7.3 times more massive on a per surface area basis.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Just based on those two facts, I would assume that heat shielding would absolutely be needed, comparable or more massive than those for Earth reentry. Of course, the size of the TPS would depend on the relative speed between the craft and Titan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the object were in strict orbit around Titan (I assume 1000 km altitude for a stable orbit), the orbital speed would be around 1.6 km/s, which would not require &lt;em&gt;that&lt;/em&gt; much heat shielding (&quot;only&quot; comparable to a hypersonic aircraft, not e.g. a Shuttle)&lt;/p&gt;&#xA;" OwnerUserId="11653" LastActivityDate="2016-04-04T18:44:08.597" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14650" PostTypeId="2" ParentId="14645" CreationDate="2016-04-04T19:36:59.837" Score="59" Body="&lt;p&gt;The reason is delta-v, which is a crucial concept in Spaceflight. It means change in velocity, and is the primary 'currency' that space mission have to expend in order to reach places in the solar system. On earth, if you want to go anywhere, you can get there at any speed, it just takes longer. Unfortunately, that is not how it works in space, because the body you come from and the body you want to reach both cycle the sun at very large speeds. After getting into orbit, you need to accelerate by another 3.21 kilometers per second to escape the gravity influence of the earth completely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to get to Mars from this point, you would need to accelerate another 1.06 km/s, so that the orbit of your spacecraft crosses the orbit of Mars. Alternatively, to achieve the same with Venus you would need to accelerate by only 640 meters per second. Already, it is cheaper to go to Venus than Mars, but it gets better:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I said above, your spacecraft's orbit now crosses the orbit of your target planet in one point. However, the orbital speed is still quite different. With Mars, the spacecraft would crash right into the surface, and you need to accelerate another 4.28 kilometers per second in order to reach the surface safely*. If you went to Venus, you can enter the planet's dense atmosphere to cushion the impact. As a result, you don't need to carry any rocket fuel to make a landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This makes Venus a low hanging fruit. At a favorable constellation, Venus is the planetary body that takes the least delta-v to reach. That even includes our moon. Heck, it takes less delta-v to reach than geostationary orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, the Russian Venera Probes slammed right into the Venerean atmosphere from an interplanetary trajectory (11km/s). This caused a deceleration exceeding 300G and a heat shield temperature of 11,000 °C, but that didn't destroy those probes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The diagram below shows different places in the solar system and rough estimates of how much delta-v you need to reach them. It all still depends on the exact constellation and the maneuver you use, so use these as ballpark estimates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/piNqI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/piNqI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Copyright as noted in the image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Actually a bit less, because Mars also has some atmosphere that can help you with the landing.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2017-04-23T20:06:22.193" LastActivityDate="2017-04-23T20:06:22.193" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14651" PostTypeId="2" ParentId="14648" CreationDate="2016-04-04T19:45:17.550" Score="4" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/questions/8026/do-you-need-a-heat-shield-to-enter-the-atmosphere-from-non-orbital-speeds&quot;&gt;this answer&lt;/a&gt;, we learn that sounding rockets typically don't have a heat shield, despite entering very quickly and at high angles (Although not orbital speeds). Huygens, which orbited Saturn, entered Titan's atmosphere at about 6 km/s. The increased atmosphere will allow for a more gradual entry into Titan's atmosphere. Bottom line, it should be possible to make a spacecraft that can enter Titan's atmosphere without a heat shield, if one does so carefully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for a human rated one, it would be a bit more difficult, but not terribly so. Heat is related to the speed at which the spacecraft slows down. One can slow down very carefully on Titan, and could probably glide for considerable range, which would allow one to build a spacecraft that could enter fairly easily without an ablative heat shield.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-04-04T20:21:53.137" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14652" PostTypeId="1" CreationDate="2016-04-04T20:06:40.823" Score="1" ViewCount="57" Body="&lt;p&gt;I have wanted to ask this question for ever. I realize that the numbers are still ticking up but i just want a number from last year or something.&lt;/p&gt;&#xA;" OwnerUserId="13961" LastActivityDate="2016-04-04T20:06:40.823" Title="how many stars have been recognized?" Tags="|stars|" AnswerCount="0" CommentCount="4" ClosedDate="2016-04-04T20:07:59.177" ContentLicense="CC BY-SA 3.0" />
  <row Id="14653" PostTypeId="1" CreationDate="2016-04-04T22:35:56.723" Score="15" ViewCount="2719" Body="&lt;p&gt;The following diagram was included in Rikki-Tiki-Tavis' answer to &lt;a href=&quot;https://space.stackexchange.com/questions/14645/why-was-venus-rather-than-mars-targeted-for-the-first-interplanetary-landings/14650&quot;&gt;this fine question&lt;/a&gt; of a few hours ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am hoping somebody can explain some of the questions I have about this data. It shouldn't matter which direction you are travelling, the numbers are supposed to indicate delta-v in transferring from one spot or orbit to another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some strange things I want to observe about this data are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) The value between Venus and Venus Low orbit is 27000 while the value between Earth and earth low orbit is 9400. That's almost 3 factors difference. The escape velocities of the two planets however, are nearly identical (Earth's is roughly 11,200 m/sec and Venus's is roughly 10,200 m/sec). How can the delta-vs be so much farther apart?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) The arrows and their explicit direction are supposed to indicate that &quot;inbound aerobraking is available&quot;. Yet these arrows are placed 250km above Earth and 400km above Venus. Those would be the upper limits of the braking as I understand it (the top of the atmosphere) and any potential braking would drop off sharply above that. Yet, several people have contradicted my interpretation of the data as though the transition from intercept to low orbit is going to provide much braking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) other users also seem convinced that when the delta-v is associated with aerobraking, that 100% of it can necessarily be applied as braking. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am hoping that these issues can be better clarified, specifically with regards to how the numbers and arrows in the chart are to be interpretted, maybe with an example fly-thru from Earth to another planet and back, ignoring the obvious problem of fuel capacity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps if the actual source of this data was identified, I would be less confused.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ocnxB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ocnxB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6392" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-04-05T01:30:57.787" Title="How can delta-v and aerobraking as shown in this chart be explained?" Tags="|delta-v|aerobraking|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14655" PostTypeId="2" ParentId="14653" CreationDate="2016-04-04T22:45:23.773" Score="13" Body="&lt;p&gt;Here's the original source of the diagram:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/space/comments/29cxi6/i_made_a_deltav_subway_map_of_the_solar_system/&quot;&gt;https://www.reddit.com/r/space/comments/29cxi6/i_made_a_deltav_subway_map_of_the_solar_system/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your concerns are also discussed there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;27000m/s is a very theoretical value, that takes into account the losses of a chemical rocket escaping the thick atmosphere of Venus. This makes it clear that launching a rocket from Venus is impractical to say the least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the precise example of the question why early planetary exploration (especially in the Soviet Union) had such a focus on Venus, it is only relevant to calculate the delta-v required to intercept Venus. Unless you want to orbit Venus, it is not necessary to expend any further fuel. A robotic probe can just slam into the atmosphere with interplanetary speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the Russian Wikipedia says that the Venera Probes did just that and entered the atmosphere with 11km/s. This caused a deceleration exceeding 300G and a heat shield temperature of 11,000 °C.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-04-04T23:01:52.747" LastActivityDate="2016-04-04T23:01:52.747" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14656" PostTypeId="2" ParentId="14653" CreationDate="2016-04-04T23:11:34.893" Score="10" Body="&lt;p&gt;The calculations for Ulysse Carion's chart were done by Curious Metaphor. If you go the &lt;a href=&quot;https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of_the_solar_system/&quot; rel=&quot;noreferrer&quot;&gt;this Reddit thread&lt;/a&gt;, you'll see a conversation between Curious Metaphor and I. For years I made delta V charts where it took about the same to escape earth and Venus. But Metaphor pointed out that Venus has a much thicker atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But who would go to the surface of Venus? My earlier delta V maps assumed Venusians were arriving at and departing from the strata of Venus' atmosphere where temperature and pressure are hospitable to humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So acknowledging Metaphor's point, I redid my delta V map with &quot;Landis Land&quot; included enroute to Venus' surface. I call the hospitable atmosphere layers &quot;Landis Land&quot; since it was Geoffrey Landis that popularized the notion of humans inhabiting the floating cloud cities on Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XDy51.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XDy51.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding aerobraking, on each branch with a red vine there's a smaller number. For example look at the Phobos to Mars branch. See a .6 near where the red vine exits? It would take a .6 km/s deceleration to drop the periaerion low enough to feel substantial friction from aerobraking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This delta V map is described in more detail at my blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/04/cartoon-delta-v-map.html&quot; rel=&quot;noreferrer&quot;&gt;Cartoon Delta V Map&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What Metaphor and Carion call &quot;Intercept&quot;, I call &quot;Capture&quot; (I believe). My capture orbits have peri apsis 300 km above the planet's surface and a apoapsis just within the planet's sphere of influence. This is an elliptical orbit that would only take a tiny nudge to achieve an escape orbit.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-04-05T01:30:57.787" LastActivityDate="2016-04-05T01:30:57.787" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14657" PostTypeId="2" ParentId="14653" CreationDate="2016-04-04T23:13:25.380" Score="7" Body="&lt;p&gt;Adding to @Rikki-Tikki-Tavi answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For destinations where aerobraking is possible, and you have a sufficiently heat and acceleration hardened vehicle, it's possible to cheat. The map shows how much delta-v must be applied, not where it has to come from. The single most fuel efficient way of shedding speed is to smack into an atmosphere blunt end first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To reach venus surface with a well hardened vehicle, you don't enter orbit at all. Go for a direct re-entry from an interplanetary transfer orbit. This saves you the costs of capture, transfer to low orbit, and re-entry, leaving only the cost to reach a venus intercept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To achieve this, you have to hit a fairly narrow window between skipping out of the atmosphere without slowing down enough, and burning up.&lt;/p&gt;&#xA;" OwnerUserId="13974" LastActivityDate="2016-04-04T23:13:25.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14658" PostTypeId="2" ParentId="14653" CreationDate="2016-04-04T23:24:22.360" Score="7" Body="&lt;p&gt;A 250km periapsis Earth orbit won't produce significant atmospheric drag. The inbound-aerobraking-possible notations for orbits are assuming that you dip lower into the atmosphere in order to slow down into an eccentric orbit, which you then circularize once you've come back up out of the atmosphere. This is tricky; if you hit the atmosphere too high, you won't slow down enough to close your orbit, but if you hit the atmosphere too low, you'll lose too much speed and fall to the surface.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-04-04T23:24:22.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14659" PostTypeId="2" ParentId="14645" CreationDate="2016-04-05T00:15:16.657" Score="21" Body="&lt;p&gt;&lt;strong&gt;&lt;em&gt;One picture is worth all your base to us.&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before we sent probes to Venus we had no pictures of its surface. Whether it lands or not you have to penetrate the atmosphere of Venus to take pictures of it, whereas the surface of Mars can be easily seen through its weak atmosphere from afar.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Most of the information about Venus has been derived from the intensive Soviet study of the planet. &lt;strong&gt;The only existing images from the surface were returned from four of their landing craft.&lt;/strong&gt; –&lt;a href=&quot;http://www.unbelievable-facts.com/2013/08/the-soviets-landed-spacecraft-on-venus.html&quot; rel=&quot;noreferrer&quot;&gt;unbelievable-facts.com&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It should be noted that the explorations of &lt;a href=&quot;https://en.wikipedia.org/wiki/Observations_and_explorations_of_Venus&quot; rel=&quot;noreferrer&quot;&gt;Venus&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_of_Mars&quot; rel=&quot;noreferrer&quot;&gt;Mars&lt;/a&gt; both began ca. 1960. Venus is just the one that you &lt;em&gt;have&lt;/em&gt; to land on (soft or hard) to gather any meaningful data*: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/C9ajc.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C9ajc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;** &lt;em&gt;Pics, or it didn't happen.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I can't rule out militaristic reasoning, I think it's more about the &lt;em&gt;Space Race&lt;/em&gt; as a whole. During which, other than landing twelve people on the moon, the Soviet space program (&lt;a href=&quot;https://en.wikipedia.org/wiki/Soviet_space_program#Notable_firsts&quot; rel=&quot;noreferrer&quot;&gt;notable firsts&lt;/a&gt;) beat the Americans' at nearly every conceivable turn. Including as a shining example: to this date theirs is the only country on Earth to ever take pictures of Venus' surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We hadn't seen it before nor have we seen it since. That's pretty exciting to me. CCCP-1, USA-0.&lt;/p&gt;&#xA;" OwnerUserId="6667" LastEditorUserId="6667" LastEditDate="2016-04-07T23:21:28.220" LastActivityDate="2016-04-07T23:21:28.220" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="14660" PostTypeId="1" CreationDate="2016-04-05T02:44:02.077" Score="7" ViewCount="1121" Body="&lt;p&gt;There have been a few attempts to transmit messages to the stars with the assumption that there may be extraterrestrial civilizations somewhere out there to receive them. These messages (such as the infamous &lt;a href=&quot;https://en.wikipedia.org/wiki/Arecibo_message&quot; rel=&quot;nofollow noreferrer&quot;&gt;Arecibo message&lt;/a&gt;) were constructed based on certain assumptions about the way a message could be structured so that it could be decoded and understood by intelligent beings on a technological par with us. The Arecibo message, for example, has been subsequently deemed flawed due to the inability of test subjects to decipher it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Carl Sagan's novel &lt;a href=&quot;https://en.wikipedia.org/wiki/Contact_(novel)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Contact&lt;/a&gt; (and its movie adaptation), there is a depiction of a message received from space, the structure of the message, and the effort to interpret it. It highlights some of the challenges of both constructing and interpreting a message where neither sender nor recipient knows anything about the other's language, culture, or level of science and technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is: if you are going to send a &quot;message in a bottle&quot; to the stars as a radio transmission, and you just want it to be the equivalent of &quot;hello, we're here and we want to be friends and get to know you&quot;, how do you design and encode such a message so that a recipient on a technological par with us has a good chance at understanding it? What technical choices do you make? How do you construct it so that it can say something meaningful without requiring the understanding of any human language? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Addendum&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seems I need to add some further clarification to my question. I am not asking about choice of medium or spectrum (radio vs light, gamma pulse, whatever) but instead, given a suitable medium with which we can transmit a signal within which we have placed a constructed message, how do we arrange the information in that message so that a civilization with no prior knowledge of human language or culture could correctly interpret it? Further assuming it will be a stream of bits, how do we encode our message into bits? Given that the recipient has received our bit stream from their receiving apparatus, what steps would they have to perform to recover the intended meaning from that bit stream?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-05-14T17:25:20.297" LastActivityDate="2017-05-14T17:25:20.297" Title="How to construct a message to send to the stars?" Tags="|seti|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14661" PostTypeId="2" ParentId="14660" CreationDate="2016-04-05T04:10:47.443" Score="4" Body="&lt;p&gt;I think this problem is often approached too explicitly, with the assumption that we have to explain all the basic concepts before someone can decode a complex message.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, research into unsupervised deep learning of embedded features in images and text shows that an explicit set of concept definitions is not required to capture the meaning of the source material.  Once you have machine-level representations of meaning, you can then translate to different formats, like image to text, or English to Spanish (&lt;a href=&quot;http://arxiv.org/pdf/1309.4168.pdf&quot; rel=&quot;nofollow&quot;&gt;http://arxiv.org/pdf/1309.4168.pdf&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this in mind, deep neural networks are often trained on large corpuses of text, at minimum the contents of Wikipedia.  So if you wanted teach aliens about us without requiring them to understand human language, you could transmit Wikipedia and be done with it, assuming that they have good computers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you only needed to say &quot;we are here,&quot; then human language is not necessary, as shown in the Pioneer and Voyager line drawings.  Perhaps someone else here can take a whack at helping aliens visualize a transmission containing a bitmap.&lt;/p&gt;&#xA;" OwnerUserId="13978" LastActivityDate="2016-04-05T04:10:47.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14662" PostTypeId="1" CreationDate="2016-04-05T06:09:06.563" Score="16" ViewCount="801" Body="&lt;p&gt;I read about the Skylab 1 mission in which SA-513 was used to lob the Skylab OWS into orbit. One source (&lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=12519.20&quot;&gt;http://forum.nasaspaceflight.com/index.php?topic=12519.20&lt;/a&gt;) gave some payload masses:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Total mass to LEO = 147,531kg&lt;/li&gt;&#xA;&lt;li&gt;OWS mass = 88,474kg&lt;/li&gt;&#xA;&lt;li&gt;S-II dry mass = 36,697kg&lt;/li&gt;&#xA;&lt;li&gt;S-II/OWS interstage = 3453kg&lt;/li&gt;&#xA;&lt;li&gt;Payload shroud = 11,630kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The extra 7,000+ kg is redisual S-II propellant, and also the S-IC/S-II interstage that failed to separate after being damaged by debris from the OWS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Excluding the S-II and its roughly 3t of propellant, but including the 4.1t S-IC/S-II interstage, the potential for usable payload was between 107t and 108t.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This payload was delivered to 50 degrees inclination at 434km in altitude. So, &lt;strong&gt;what could it have thrown to 185km and 28.5 degrees?&lt;/strong&gt; Studies (From 1965, I believe) on the very similar Saturn INT-21 concept give that launcher a maximum payload of 115.7t (not including S-II) to 185km and 34.5 degrees. The date of those studies means that this may not factor in late-model weight savings and engine upratings made to SA-513, which went up in 1973. &lt;strong&gt;Could it have lifted more than 115.7t?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some other sources give the Skylab OWS mass as being 75,000kg or even around 77t. I'm not sure of what figures are best.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, how tall was the Skylab/Saturn V combo anyway? I've found 333.7ft, 341.0ft and 343.8ft, but can't verify any of them.&lt;/p&gt;&#xA;" OwnerUserId="12678" LastActivityDate="2021-01-06T01:40:28.353" Title="What would have been the absolute maximum LEO payload of a two-stage Saturn V?" Tags="|apollo-program|low-earth-orbit|payload|saturn-v|skylab|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14663" PostTypeId="1" CreationDate="2016-04-05T10:04:52.217" Score="5" ViewCount="1148" Body="&lt;p&gt;I just read &lt;a href=&quot;http://www.popsci.com/we-may-need-fusion-powered-rockets-to-stop-comets-from-destroying-earth&quot; rel=&quot;nofollow&quot;&gt;an article&lt;/a&gt; arguing that we need very fast rockets for the deflection of comets, since we will probably detect them just several months before impact. So we need to hurry in order to pick them up as far away as possible. It explicitly motivates fusion rockets, since comets might be detected only 6 to 18months prior to impact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That got me thinking. For the sake of argument, assume we have several ICBMs in orbit (political likelihood left out of this question). Assume they are armed with 500kt warheads. Furthermore, assume they are able to meet the comet 1 month away from earth and we want it to be deflected by 5 times the diameter of earth (60 000 km). So by my rough calculation we want to achieve a 23m/s acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scary &lt;a href=&quot;http://www.5596.org/cgi-bin/nuke.php&quot; rel=&quot;nofollow&quot;&gt;nuclear weapons calculator&lt;/a&gt; gives a vaporization of 140mm of (carbon) surface material for a detonation as close as 250m. That value more than doubles (330mm) in case of water ice. That seems to be quite a lot. That means the payload of a single Trident II missile could vaporize between 1.7m and 4.5m of surface material from a given comet. So for a water ice surface of roughly 4km x 4km that gives us a &quot;propellant&quot; of 72 000 000t of fuel. For your typical $10^{13}$ kg comet that should be 0.5/1000 of your mass (give or take). Assuming that our vaporization is at least as effective as a rocket engine (ISP 500 seconds) &lt;a href=&quot;http://www.strout.net/info/science/delta-v/&quot; rel=&quot;nofollow&quot;&gt;and crunching the numbers using a delta-v calculator&lt;/a&gt; that gives 2.5m/s delta-v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So according to my rough calculations, there is no extreme advantage of a faster rocket here. How would a fusion rocket change the above principles? Why can't we simply put some 20+ Trident II like missiles somewhere in orbit (as I said, politics aside) and wait for a target to pop up? Where is my error? And if my calculations are totally wrong (which might well be), what makes it better to have a very fast rocket and get things done 6 months away from earth (couldn't we as well send 6 times the number of nuclear warheads with conventional slow rockets)? &lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="11256" LastEditDate="2016-04-05T15:02:39.423" LastActivityDate="2017-03-01T12:01:52.820" Title="Why is comet deflection so hard (and how does a fusion rocket help)?" Tags="|comet|nuclear|planetary-defense|" AnswerCount="4" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="14664" PostTypeId="2" ParentId="14663" CreationDate="2016-04-05T10:38:09.093" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Assuming that our vaporization is at least as effective as a rocket engine (ISP 500) that gives 2.5m/s delta v&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Unlikely - the rocket engine uses a shaped engine bell tuned to give the best thrust (put simply) over a several minute burn.  Whereas vaporised material from the comet itself would consist mainly of low energy expanding gas and its &quot;exhaust angle&quot; (don't know if there is a proper term) would be spread out over 180 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short I think the question overestimates the available thrust from vaporising cometary ice. There would be some, but I suspect it would be far, far less than is needed.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Another point:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question's calculation is based on vaporising a given depth of material from a range of 250 meters, but assuming this depth is taken over an area of 4x4 kilometres. That looks like another overestimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could perhaps take the depth equation over the 4x4 kilometre area, work out the ranges and depths for each point and integrate to get the volume vaporised. This still wouldn't give a reliable exhaust velocity for the material though.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-04-05T13:03:59.740" LastActivityDate="2016-04-05T13:03:59.740" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14665" PostTypeId="2" ParentId="14663" CreationDate="2016-04-05T13:39:38.393" Score="4" Body="&lt;p&gt;The simple fact is that we don't know what the effects of a nuclear blast on a comet would be as it's never been done, and each comet is different. You're thinking of a nuclear explosion next to a comet in terms of a hammer, but it's more like a gentle nudge at best. Whatever thrust ratio you get is likely to be very low, what if only .1% of the energy gets translated into movement? If all you get is a .05 m/s change then that adds up to 4 km per day - good enough if it's done soon enough as that change will add up quite a lot over time. Close to the earth it's not likely to help. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You assume that a nuke is going to just move the comet, but there's other things that might happen. The comet may break into chunks, and those pieces each would need to be dealt with separately.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you let the comet get into earth orbit then you have to fight the earth's gravity as well. This gets exponentially greater the closer the comet gets to the earth. If you let the comet get into a low earth orbit you will have to impart it with around 9m/s acceleration just to counteract gravity, meaning more nukes are needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting nuclear weapons into space and be able to maneuver them to where they need to go will require powerful rockets - it's not just the nuclear charge that needs to be lifted, it needs to be on a rocket of its own - and these are expensive. It won't cost that much more to make a rocket that can reach a comet millions of miles away than one that can get to geosynchronous orbit. If you wait until it's close you will need many of these and that adds huge cost. It's much cheaper to hit the comet farther away using fewer charges. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would you want to take the risk of waiting until you see the whites of its craters anyway? Hitting a comet far away gives you options, if you wait until it's close and it doesn't work you are totally screwed. Hitting it far away gives you time to evaluate changes in its orbit, and take other steps. You can nudge it again - I wouldn't send just one nuke! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: A fusion rocket helps because it gets your nukes to the comet sooner, so you will have less time between detection and intercept. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think you have the right idea that you want this already in orbit, I wouldn't want the delays in prepping a rocket on short notice, even if there's one available. As for problems with nukes in orbit the best way to appease people would be to design the charges so they would not be able to survive re-entry. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2016-04-05T15:53:20.440" LastActivityDate="2016-04-05T15:53:20.440" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14666" PostTypeId="2" ParentId="13574" CreationDate="2016-04-05T15:13:53.250" Score="11" Body="&lt;p&gt;It seems that they initially planned to use the landing legs as air brakes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk &lt;a href=&quot;http://shitelonsays.com/transcript/elon-musk-at-mits-aeroastro-centennial-part-1-of-6-2014-10-24&quot; rel=&quot;nofollow&quot;&gt;stated&lt;/a&gt; that: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If you then do some interesting things, like look at our landing gear,&#xA;  they're essentially like giant body flaps, so the drag - when we&#xA;  deploy the landing gear, the drag massively increases, so we have dual&#xA;  use of the landing gear as giant body flaps and as landing gear. That&#xA;  actually cuts the terminal velocity in half and therefore the fuel -&#xA;  the propellant we need to stop the vehicle in half, and actually it's&#xA;  quite an efficient method of landing precisely.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This maybe just an earlier idea which didn't materialized.   Looking at the &lt;a href=&quot;https://www.youtube.com/watch?v=sSF81yjVbJE&amp;amp;t=0m51s&quot; rel=&quot;nofollow&quot;&gt;earlier reusability video&lt;/a&gt;, you see that legs are partially deployed as air brakes.  But with the &lt;a href=&quot;https://www.youtube.com/watch?v=4Ca6x4QbpoM#t=1m14s&quot; rel=&quot;nofollow&quot;&gt;latest video&lt;/a&gt;, legs are no longer used in that manner so it is likely they gave up on the idea.  My speculation is that they indeed intended to use landing legs for attitude control during the unpowered decent, but didn't work well during the initial test landings out in the ocean and switched to the idea of grid fin higher up on the body. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also came a cross a picture that they were &lt;a href=&quot;https://www.instagram.com/p/4NMIBSl8a0/?hl=en&quot; rel=&quot;nofollow&quot;&gt;wind tunnel testing&lt;/a&gt; the deployed legs at one point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To give a little more context, I like to add that the above comment was made on 10/24/2014, after two landing test flight had been conducted with the landing legs, flight 9 and 10 on 2/18/2014 and 7/14/2014.  But also, on 6/17/2014, they already tested the steering fins for the first time on &lt;a href=&quot;https://www.youtube.com/watch?v=DgLBIdVg3EM&quot; rel=&quot;nofollow&quot;&gt;F9R test flight&lt;/a&gt; and only 3 months after the comment, flight 14 flew with grid fins on 1/10/15.  So it is likely that by the time he made the comment, it was already decided that the air braking with legs is no go. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was also &lt;a href=&quot;https://twitter.com/elonmusk/status/536268250620125185&quot; rel=&quot;nofollow&quot;&gt;a twitter exchange&lt;/a&gt; on this subject on 11/22/14 and he stated: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Using legs as air brakes to drop terminal velocity in half requires&#xA;  slight redesign &amp;amp; more data. Maybe flight 21.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;SpaceX &lt;a href=&quot;http://www.spacex.com/news/2015/12/21/background-tonights-launch&quot; rel=&quot;nofollow&quot;&gt;news article on 12/21/15&lt;/a&gt; before the Orbcomm-OG2 launch has a conflicting information regarding grid fins. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first attempt to touch down softly on water failed, as we tried to&#xA;  control the rocket with small attitude thrusters alone. While it works&#xA;  well for a smooth, blunt body shape like Dragon, that turns out to be&#xA;  a hopeless proposition for something the shape of a rocket booster.&#xA;  Falcon spun out of control and smashed into the water at high speed.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;We then added four grid fins in an X-wing configuration to give us the&#xA;  necessary three axis control under high dynamic atmospheric pressure,&#xA;  which peaks at 1.5 tons per square foot.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This solved the control problem and we were able to do two successful&#xA;  soft landings in the water. Max altitude of the rocket stage was 210&#xA;  km, which doesn't matter a lot, and max transfer kinetic energy was&#xA;  200 GJ.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The first attempt&lt;/strong&gt; must be referring to flight 6, and &lt;strong&gt;two successful&#xA;soft landings in the water&lt;/strong&gt; can only be referring to flight 9 and 10.  But the grid fin were not flown until flight 14, so the success of flight 9 and 10 cannot be attributed to the grid fin as mentioned in this article.  Well, obviously, that article was written in a hurry as stated at the bottom: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;T-zero in 15 minutes, so have to sign off. Apologies for any typos in&#xA;  the above.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;-- Elon&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="13988" LastEditorUserId="13988" LastEditDate="2016-04-07T13:35:11.007" LastActivityDate="2016-04-07T13:35:11.007" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14667" PostTypeId="2" ParentId="14639" CreationDate="2016-04-05T15:29:13.217" Score="6" Body="&lt;p&gt;As best as I can understand from these papers, it would seem that the precise levels are not known, but that noticeable weathering on Venus implies significant(?) levels of oxidation on the surface. The papers primarily address the oxidation of &lt;strong&gt;Fe&lt;/strong&gt; (iron) mineral on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the material in the papers is derived from modeling of the surface and atmospheric interactions from what little data we have, as well as spectroscopy and other observations of the character of the surface material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://adsabs.harvard.edu/full/1982LPSC...13....3B&quot; rel=&quot;noreferrer&quot;&gt;The crust of Venus: Theoretical models of chemical and mineral composition - Barsukov, V. L., Volkov, V. P., &amp;amp; Khodakovskii, I. L.&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://adsabs.harvard.edu/full/1996LPI....27.1451W&quot; rel=&quot;noreferrer&quot;&gt;Must the Venus Surface Material Contain Hematite? - Wood, J. A.&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-04-05T15:29:13.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14668" PostTypeId="2" ParentId="14599" CreationDate="2016-04-05T17:02:57.223" Score="0" Body="&lt;p&gt;I simulated a highly elliptical orbit in &lt;a href=&quot;https://www.spenvis.oma.be/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SPENVIS&lt;/a&gt; with a low inclination of 8 degrees, that went past the outer belt. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/SoHKJ.jpg&quot; alt=&quot;Orbit in fixed reference system&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/A1WyO.png&quot; alt=&quot;Altitude vs time&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Judging by the graphs created in SPENVIS (AP-8 and AE-8 models with maximum solar activity) and the link in 2012crampion's comment, the area between the belts still has a relatively high electron and proton flux, just a few orders of magnitude lower, but much higher than outside of the belts proper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/eR1QL.png&quot; alt=&quot;Electron flux vs time&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/85EnN.png&quot; alt=&quot;Proton flux vs time&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, Earth's magnetic field seems pretty strong, so my guess is that the area doesn't see much cosmic rays and solar particles, compared to the apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/4SiLp.png&quot; alt=&quot;Magnetic field vs time&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="13925" LastActivityDate="2016-04-05T17:02:57.223" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14669" PostTypeId="1" AcceptedAnswerId="14671" CreationDate="2016-04-05T18:09:41.990" Score="3" ViewCount="271" Body="&lt;p&gt;So, we've got this huge, hermetically sealed tube of metal nearly to orbit, just to push another huge hermetically sealed tube into orbit, and then we let the first tube fall and burn in the atmosphere... seems like a big waste to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course all the &quot;furniture&quot; would need to fly in another launch, and the project would need to foresee the dual use - equip the rocket with at least stubs for all the necessary infrastructure. But the heaviest part - the strong walls, the structural reinforcements, the airtight sealing, plus a lot of tubing seems to be viable for reuse once in orbit, as actual structure of the station - habitats, greenhouses, even dumb structural parts like trusses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be possible - is there any research or projects - of reusing fuel tanks of rockets as structural parts of a space station?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-04-05T19:00:49.493" Title="Reuse of fuel tanks as habitats?" Tags="|space-station|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14671" PostTypeId="2" ParentId="14669" CreationDate="2016-04-05T19:00:49.493" Score="4" Body="&lt;p&gt;This is known as a &quot;wet workshop&quot;. This was o&lt;a href=&quot;https://en.wikipedia.org/wiki/Wet_workshop&quot; rel=&quot;nofollow&quot;&gt;ne of the options looked at for Skylab&lt;/a&gt;: the S-II second stage would place itself plus a payload into orbit.&lt;br&gt;&#xA;Once in orbit, the payload (containing all the equipment for the station: life support, experiments etc.) would be inserted into the S-II hydrogen tank once piece through a large hatch.   &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-05T19:00:49.493" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14673" PostTypeId="2" ParentId="14660" CreationDate="2016-04-05T19:33:11.520" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://mnras.oxfordjournals.org/content/early/2016/03/30/mnras.stw672&quot; rel=&quot;nofollow&quot;&gt;With lasers&lt;/a&gt; we could alter Earth's light curve to an artificial shape (maybe an &lt;a href=&quot;https://en.wikipedia.org/wiki/Ulam_spiral&quot; rel=&quot;nofollow&quot;&gt;Ulam spiral&lt;/a&gt; portion?), so that we would be visible to whatever star system we pointed our lasers at. The star system would have to be able to view our transit of the Sun. This is from a recent paper &quot;A Cloaking Device for Transiting Planets&quot; by David M. Kipping and Alex Teachey in the &lt;em&gt;Monthly Notices of the Royal Astronomical Society&lt;/em&gt;. The authors propose cloaking the ingress and egress of the transit and leaving the main transit undistorted, since this is not produced by any known natural process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is actually cheaper to broadcast a signal this way than it is to cloak a planet, which was the main focus of the paper.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-04-05T19:33:11.520" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14674" PostTypeId="2" ParentId="14663" CreationDate="2016-04-05T22:13:32.507" Score="4" Body="&lt;p&gt;I don't have an exact answer but some relevant bits of information:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're looking at how much material is vaporized.  That's irrelevant, vaporizing material gets you nothing.  To move the comet you need to expel material from the surface.  Your thrust is based on mass * final velocity (after climbing the comet's admittedly minuscule gravity well), whether it's solid, liquid or gas gains you nothing.  While vaporization is pretty much necessary to expel it it's actually lost energy when it comes to pushing the comet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of trying to figure the amount vaporized we should look for data we already have:  We are describing an Orion drive.  A NASA page I found some time back gave efficiencies of 10-50% for Orion--since we don't get to carefully build our engine I would take the lowest of their numbers as the best we could hope for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Expecting a result similar to rocket ISP is completely unjustified.  Much of the energy is wasted in material being blown sideways as you have no bell on your engine to direct the force in the direction you want.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite all this my attempts to work out the math show practical nuclear weapons can deliver quite a shove.  However, that's not the whole picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, what is local escape velocity on the comet?  If you administer a shove that exceeds this you're just asking to break the comet and now you have two (or more) parts to deal with, unless you have plenty more boom to use you're worse off than if you didn't shoot in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, you're proposing an intercept one month out.  Lets consider the recent near miss--Siding Spring.  It flew past Mars at 56 km/sec.  I'm not finding the delta-V of a Titan missile but it's probably in the ballpark of 10 km/sec.  Supposing it's in low Earth orbit--it's going to use nearly half of that to escape Earth, it's going to be heading out at about 6 km/sec.  Thus to get that intercept one month out you need to fire almost 10 months out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice it's going to be worse because you won't have precise targeting that far out, you'll need a maneuvering engine.  You'll also need a separate probe to refine the targeting as you need to place the warhead very accurately (against a target like Siding Spring a miss by a single kilometer renders it entirely useless and if the threat is from ahead or behind you'll have to shoot in passing and your accuracy will have to be in the tens of meters.)  All that takes mass that will reduce your delta-V and require a launch even farther out.  (If all the extras weigh as much as the warhead you have no way to get your one month intercept against Siding Spring.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, lifting a single Titan missile into orbit will take multiple launches of the heaviest rockets we have.  (Yes, we can do it with multiple launches as most of that weight is fuel.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-04-05T22:13:32.507" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14678" PostTypeId="1" CreationDate="2016-04-06T14:51:46.470" Score="25" ViewCount="4236" Body="&lt;p&gt;What were key factors that made the N1 the recipe for disaster it became for the USSR? Especially when the USSR was the Space King at the time?&lt;/p&gt;&#xA;" OwnerUserId="13960" LastEditorUserId="13960" LastEditDate="2016-04-06T15:18:47.620" LastActivityDate="2016-04-07T15:28:36.333" Title="What caused the N1 to become a failure?" Tags="|history|soviet-union|n-1|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14679" PostTypeId="2" ParentId="14678" CreationDate="2016-04-06T15:18:31.793" Score="25" Body="&lt;p&gt;Generically the program was underfunded, and the main issue was lack of full up testing of each stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With 30 engines, the interactions in the plumbing were very complex and caused imperfect fuel/oxidizer flow that affected the engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Engines that were sensitive to fuel/oxidizer flow issues with lots of them, lots of plumbing, huge volumes, needs lots of testing to get just right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible that quality control to achieve whatever solution was needed might have been beyond the Soviets. (Imagine rough welds inside pipes, and issues that might cause in this context. Then imagine the thousands of welds it likely needed). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With more time and money, they likely would have gotten much closer or even succeeded.  Hard to know. But without the testing, failure was almost certainly guaranteed without great amounts of dumb luck.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-04-06T15:18:31.793" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14680" PostTypeId="2" ParentId="14678" CreationDate="2016-04-06T16:43:35.613" Score="21" Body="&lt;p&gt;To add to geoffc's &lt;a href=&quot;https://space.stackexchange.com/a/14679/13208&quot;&gt;excellent answer&lt;/a&gt;, another issue that might have been worked out with more testing was the KORD, a computer system for controlling the 30 different engines. Ideally, the KORD would handle the failure of one engine by stopping the engine on the opposite side of the rocket, maintaining symmetry of thrust. However, during the first and second test flights, the KORD ended up shutting down &lt;em&gt;all&lt;/em&gt; the engines after detecting a failure of one engine. The second failure happened just a few seconds after liftoff; the rocket fell back to the pad and exploded, destroying the main pad and forcing the Soviets to spend a year and a half rebuilding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, a lot of the N1's problems stem from trying to eke an extra 20 tons out of it. The 7K-LOK/LK lunar spacecraft ended up substantially overweight, requiring the N1 to lift 95 metric tons instead of 75. This forced the designers to add 6 extra engines, which can't have helped things (although 24 engines was already pushing it). The N1 (and the Soviet program in general) also suffered from a lack of commitment and leadership, especially after Korolev's death in 1966. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources/further reading:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://falsesteps.wordpress.com/2012/07/14/n1/&quot; rel=&quot;noreferrer&quot;&gt;False Steps: N1: The Soviet Moon Rocket&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.russianspaceweb.com/n1_5l.html&quot; rel=&quot;noreferrer&quot;&gt;RussianSpaceWeb: The second launch of the N1 rocket&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13208" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-04-07T15:28:36.333" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="14681" PostTypeId="1" AcceptedAnswerId="14683" CreationDate="2016-04-07T02:17:39.270" Score="6" ViewCount="575" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/q/14627/12102&quot;&gt;question&lt;/a&gt; asks if Voyager suddenly encountered unexpected gravity from an unseen object, how would it be detected, and there are several answers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the farthest probes - those now farther from the sun than Neptune - how are (or have)  precision trajectory measurements &lt;s&gt;actually&lt;/s&gt; been made? I'm looking for a concise answer with links. Of course more info is always welcome, but I need one or a few &quot;punch lines.&quot; For example &quot;ranging by means of timing of a coded pulse train transmitted from earth, received by spacecraft and rebroadcast on a different frequency&quot; (&lt;a href=&quot;https://space.stackexchange.com/q/7953/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/7964/12102&quot;&gt;answer&lt;/a&gt;) or &quot;doppler shift by means of same&quot; or &quot;apparent position by VLBI&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not looking for &quot;well for example you could...&quot; type of answers in this case, just how &lt;s&gt;actually&lt;/s&gt; it is, or has, been done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With velocity anomalies measured via doppler on the scale of &lt;a href=&quot;https://en.wikipedia.org/wiki/Flyby_anomaly#Observations&quot; rel=&quot;nofollow noreferrer&quot;&gt;milli-Hz&lt;/a&gt; mentioned &lt;a href=&quot;https://space.stackexchange.com/questions/14627/would-a-voyager-spacecraft-trajectory-change-be-recognized#comment38383_14627&quot;&gt;here&lt;/a&gt; by @JohannesD, I'm guessing the doppler measurements &lt;em&gt;are not&lt;/em&gt; based on an internal, high-precision clock in the spacecraft because the remote timebase stability could always be called into question, but instead rely on retransmission by each spacecraft of a ground-based signal, with some clever way possibly to change frequency/band in a precise way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference only, here are some related hypothetical items: &lt;a href=&quot;https://space.stackexchange.com/a/3586/12102&quot;&gt;visible imaging&lt;/a&gt;, and &lt;a href=&quot;https://space.stackexchange.com/a/3630/12102&quot;&gt;infrared thermal imaging&lt;/a&gt; are ruled out quantitatively for &lt;a href=&quot;https://space.stackexchange.com/q/3585/12102&quot;&gt;this question&lt;/a&gt;. Ranging is mentioned &lt;a href=&quot;https://space.stackexchange.com/a/14286/12102&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/14287/12102&quot;&gt;here&lt;/a&gt; as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the image below (from &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_active_Solar_System_probes&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;) I count five spacecraft now past Neptune, four of which are using 1970's technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/t8zNq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t8zNq.png&quot; alt=&quot;solar system probe spacecraft&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-26T07:55:58.623" LastActivityDate="2017-04-26T07:55:58.623" Title="How are precision trajectory measurements made of trans-Neptunian spacecraft?" Tags="|probe|new-horizons|voyager|measurement|pioneer|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14682" PostTypeId="1" AcceptedAnswerId="14893" CreationDate="2016-04-07T03:35:19.773" Score="8" ViewCount="491" Body="&lt;p&gt;The various flavors of GPS satellite constellations use atomic clocks on each satellite. As far as I know, all of the constellations are &quot;shells&quot; of circular MEO orbits in 3 to 6 planes, 20,000 to 25,000 km above Earth's surface, with a few in GEO (Compass). There may be other satellites in GEO that have atomic clocks as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How far from earth have atomic clocks been placed and monitored? Could be for any purpose. I'm looking for the farthest distance at which the clock was working and data obtained in some way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was thinking about timebases for deep space probes &lt;a href=&quot;https://space.stackexchange.com/q/14681/12102&quot;&gt;here&lt;/a&gt;, but @TildalWave's &lt;a href=&quot;https://space.stackexchange.com/questions/13975/would-a-gps-type-system-on-mars-require-ground-stations#comment36503_13975&quot;&gt;comment&lt;/a&gt; linking to a &lt;a href=&quot;http://www.jpl.nasa.gov/events/lectures_archive.php?year=2016&amp;amp;month=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Von Kármán lecture&lt;/a&gt; lead me to read about the &lt;a href=&quot;http://www.nasa.gov/mission_pages/tdm/clock/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Atomic Clock project&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-07-09T22:24:10.373" LastActivityDate="2019-07-09T22:24:10.373" Title="How far from earth have atomic clocks (or ultra-stable oscillators) been placed and monitored?" Tags="|spacecraft|time|gps|record|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14683" PostTypeId="2" ParentId="14681" CreationDate="2016-04-07T04:45:39.113" Score="7" Body="&lt;p&gt;Doppler and ranging are used routinely. They are both two-way, with the Doppler turning the frequency around and the ranging turning around a pseudo-noise signal. This is complicated, only for these spacecraft, due to the transmitting station on Earth rotating out of view (and in, and out again!) by the time the return signal hits Earth, being then received by an antenna at a different complex. &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsndocs/810-005/210/210A.pdf&quot; rel=&quot;nofollow&quot;&gt;$\Delta$DOR (delta-differential one-way ranging)&lt;/a&gt; has been used to get plane-of-sky position measurements (in one dimension). Optical navigation was used by New Horizons, using images of the targets on approach to pin down their location relative to the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-04-07T15:17:44.873" LastActivityDate="2016-04-07T15:17:44.873" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14684" PostTypeId="1" AcceptedAnswerId="14685" CreationDate="2016-04-07T11:56:50.463" Score="4" ViewCount="2671" Body="&lt;p&gt;Here on earth, we have four directions(East, West, North, South). We have earth maps which shows our exact location from any other location in 2D plane.&#xA;But in space, planets and stars are in three dimensions, so how can we say that a certain planet is at &quot;this&quot; location in space. If we are using 3D geometry consisting of X,Y,Z axes, then where is the assumed center of that model?&#xA;Here on earth we can say that Canada is at &quot;NORTH&quot; of USA, how do we say that Earth is at &quot;****&quot; of Mars?&lt;/p&gt;&#xA;" OwnerUserId="14015" LastEditorUserId="288" LastEditDate="2016-04-09T09:27:02.167" LastActivityDate="2016-04-09T09:27:02.167" Title="How is space mapped out?" Tags="|navigation|mapping|coordinates|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14685" PostTypeId="2" ParentId="14684" CreationDate="2016-04-07T12:19:52.093" Score="10" Body="&lt;p&gt;There are many different coordinate systems (X, Y, Z axes as you refer to them).  The center of the Earth is often used as the origin &quot;center&quot; of the coordinate system, at least for calculations concerning the space in the vicinity of Earth.  There are many such Earth-centered coordinate systems as well.  In the Space Shuttle Program we used the &quot;Mean of 50&quot; and the &quot;J2000&quot; inertial systems often. They are well described in this &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth-centered_inertial&quot; rel=&quot;nofollow&quot;&gt;article&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I have no personal experience with it, I imagine for planetary calculations, there are Sun-centered coordinate systems, and ones for each planet that people care to calculate such things for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer the last part of your question, such terminology as &quot;Earth being North of Mars&quot; is used only informally and would require some up-front definitions to be meaningful.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-04-07T18:09:07.497" LastActivityDate="2016-04-07T18:09:07.497" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14686" PostTypeId="1" CreationDate="2016-04-07T13:20:22.267" Score="6" ViewCount="492" Body="&lt;p&gt;After the failure of the N1 program, what impact did this have on the budget for the space program of the Soviet Union?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What technical impacts did it have? For example, did other missions suffer because of this failure? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Yes, this is a follow-up to this one: &lt;a href=&quot;https://space.stackexchange.com/questions/14678/what-caused-the-n1-to-become-a-failure&quot;&gt;What caused the N1 to become a failure?&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13960" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-04-07T15:17:12.327" Title="What budgetary and technical impact did the N1 program's failure have on the Soviet Union's space program?" Tags="|history|soviet-union|n-1|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14687" PostTypeId="1" AcceptedAnswerId="14695" CreationDate="2016-04-07T14:09:43.500" Score="16" ViewCount="9566" Body="&lt;p&gt;I wanted to ask this at aviation SE because they are quite rules-and-regulations oriented there, but this question is not (really) about aviation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm trying to understand the &lt;em&gt;current situation&lt;/em&gt; for obtaining, and using GPS receivers that continue to work at very high altitude, or high speed, or both. Due to the existence of &lt;a href=&quot;https://en.wikipedia.org/wiki/CoCom#Legacy&quot;&gt;CoCom reguations&lt;/a&gt; I believe all GPS module manufacturers added limits to their products - probably in firmware - so that they would stop working if some combination of two limits had been reached. The limits are (I believe) &lt;code&gt;altitude &amp;gt; 18km&lt;/code&gt; and &lt;code&gt;speed &amp;gt; 1000 knots&lt;/code&gt; (about 0.5 kilometer per second). &lt;em&gt;I think&lt;/em&gt;, although I am not sure, that these were implemented across the board by all manufacturers to avoid any possibility of sanctions or falllout similar to the &lt;a href=&quot;https://en.wikipedia.org/wiki/CoCom#Violations&quot;&gt;CNC machines sold and potential use for quiet sub propellors and subsequent sanctions&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One subtlety is the question if the two limits (altitude, speed) are &lt;a href=&quot;http://blog.jgc.org/2010/11/gaga-1-cocom-limit-for-gps.html&quot;&gt;applied in an OR or AND configuration&lt;/a&gt;, which matters if your application is balloon borne or even testing cubesat subsystems via balloon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This paragraph in &lt;a href=&quot;http://www.wired.com/2013/09/bypassin-us-gps-limits-for-active-guided-rockets/&quot;&gt;Wired&lt;/a&gt; is intriguing, but it doesn't actually tell what the &quot;more relaxed rules&quot; are!&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;&lt;em&gt;Due to more relaxed rules, we now have the possibility to use GPS signals at higher altitudes and speeds. The problem is people normally don’t know how to either construct their own receivers or disable/modify the limits inside ready available modules (until now).&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One can find references to changing/updating/hacking the firmware to remove limits &lt;a href=&quot;https://saraspangelo.files.wordpress.com/2012/09/rax_gps_design_combined.pdf&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;http://copenhagensuborbitals.com/gps-without-limits/&quot;&gt;here&lt;/a&gt;, and as a product &lt;a href=&quot;http://www.cubesatkit.com/docs/datasheet/DS_CSK_GPSRM_1_710-00908-C.pdf&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My Question is:&lt;/strong&gt; What exactly &lt;em&gt;are the rules now&lt;/em&gt;? Is it &quot;&lt;em&gt;OK&lt;/em&gt;&quot; to change firmware or otherwise remove limits on ones own module? Can manufacturers of GPS modules ship them without the limits in certain cases?  What are the conditions? &lt;em&gt;Where&lt;/em&gt; are the rules written so we can read them?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-04-07T14:15:27.173" LastActivityDate="2017-01-21T03:08:50.103" Title="Current situation with CoCom regulations and GPS receivers for balloons and cubesats" Tags="|cubesat|gps|balloons|regulatory|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14688" PostTypeId="2" ParentId="14686" CreationDate="2016-04-07T15:17:12.327" Score="8" Body="&lt;p&gt;The most obvious effect is the lack of a suitable rocket to lift heavy cargo into orbit. That set a constraint, especially on the manned space program. For example, the N1 was basically the backbone of the Soviet effort for a manned mission to the Moon, and a termination of the program meant that the most basic requirement for that kind of mission was gone. Similarly, even more distant goals, like a Mars mission, which have had some possible mission architectures sketched,needed a N1 or a rocket with similar payload capacity in order to be possible at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The direction the Soviet space program went instead was therefore stand alone, and later modular, space stations in low Earth orbit, with interplanetary missions limited to small robotic spacecraft. That is pretty much how space travel is today, and how it has been for approximately 40 years both in Russia and in the US. The brief appearance of a new heavy lift launcher, Energija, did not change that.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-07T15:17:12.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14689" PostTypeId="2" ParentId="14684" CreationDate="2016-04-07T15:18:34.690" Score="2" Body="&lt;p&gt;The answer by @OrganicMarble really sums up the situation well! I'm going to add this supplemental information here only because it won't fit into a comment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The documentation of &lt;a href=&quot;http://rhodesmill.org/skyfield/positions.html#barycentric-position&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt; discusses Barycentric position - i.e. position of things with respect to the center of mass of the solar system, which is &lt;em&gt;approximately&lt;/em&gt; at the sun, although because Jupiter and to a lesser extent Saturn are so heavy, the sun moves around a bit as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I like this software very much, because it gives you a very simple Python interface to access some very complicated things. In a few lines we can get the (X, Y, Z) position of Earth and Mars, at the exact moment @OrganicMarble posted the answer, and then some other useful things as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/23d6i.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/23d6i.png&quot; alt=&quot;Answer Timestamp&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But first (before the actual X, Y, Z positions) here is a cartoon of the Barycentric coordinates. The origin is at the center of mass of the solar system, and the Z axis points where the axis of the earth pointed at one moment (it slowly drifts, so they froze it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YPyTr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YPyTr.png&quot; alt=&quot;cartoon of Barycentric coordinates&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from skyfield.api import load, JulianDate&#xA;&#xA;data = load('de421.bsp')&#xA;&#xA;earth = data['earth']&#xA;mars  = data['mars']&#xA;&#xA;jd = JulianDate(utc=(2016, 4, 7, 12, 19, 52))  # Timestamp of Organic Marble's answer&#xA;&#xA;ep = earth.at(jd).position.km&#xA;mp = mars.at(jd).position.km&#xA;&#xA;print &quot;Earth: &quot;, ep   # (X, Y, Z) of Earth at the moment Organic Marble posted the answer!&#xA;print &quot;Mars:  &quot;, mp   # ditto for Mars&#xA;&#xA;    Earth:  array([ -1.41969644e+08,  -4.19228367e+07,  -1.81995394e+07])&#xA;    Mars:   array([ -1.83099445e+08,  -1.36301780e+08,  -5.75942828e+07])&#xA;&#xA;&#xA;print earth.at(jd).observe(mars).radec()  # corrected for 6 minute delay of the light from Mars&#xA;    (&amp;lt;Angle 16h 25m 47.63s&amp;gt;, &amp;lt;Angle -20deg 56' 19.3&quot;&amp;gt;, &amp;lt;Distance 0.736829 au&amp;gt;)&#xA;&#xA;print earth.at(jd).observe(mars).apparent().radec()  # corrected abberation and gravitational delfection of light&#xA;    (&amp;lt;Angle 16h 25m 48.53s&amp;gt;, &amp;lt;Angle -20deg 56' 21.5&quot;&amp;gt;, &amp;lt;Distance 0.736829 au&amp;gt;)&#xA;&#xA;&#xA;Houston = earth.topos(29.76, -95.38, elevation_m = 13)&#xA;&#xA;&#xA;print Houston.at(jd).observe(mars).apparent().radec() # now R.A. and Dec from Houston, not center of Earth&#xA;    (&amp;lt;Angle 16h 25m 48.08s&amp;gt;, &amp;lt;Angle -20deg 56' 29.9&quot;&amp;gt;, &amp;lt;Distance 0.73681 au&amp;gt;)&#xA;&#xA;print Houston.at(jd).observe(mars).apparent().altaz() # almost 27 degrees above the horizon&#xA;    (&amp;lt;Angle 26deg 49' 51.8&quot;&amp;gt;, &amp;lt;Angle 221deg 17' 44.6&quot;&amp;gt;, &amp;lt;Distance 0.73681 au&amp;gt;)&#xA;&#xA;print Houston.at(jd).observe(mars).apparent().altaz('standard') # corrected for refraction - standard atmosphere&#xA;    (&amp;lt;Angle 26deg 51' 48.9&quot;&amp;gt;, &amp;lt;Angle 221deg 17' 44.6&quot;&amp;gt;, &amp;lt;Distance 0.73681 au&amp;gt;)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-04-07T15:23:46.067" LastActivityDate="2016-04-07T15:23:46.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14690" PostTypeId="1" CreationDate="2016-04-07T17:28:39.143" Score="10" ViewCount="4163" Body="&lt;p&gt;In Project Gemini, the Titan II GLV space rocket used as fuel hydrazine and nitrogen tetroxide, so safety concerns over its toxic propellants were present. But along with the special care that was needed due to its toxic fuel, something else that should guarantee safety was the launch escape system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually there are two types of escape systems: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;-First, the capsule abort system with rockets mounted on a tower above the capsule. (Dragon V2 uses a different concept, but still the role is to “push” the capsule away from the rocket.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;-Second, ejection seats as used in military aircraft: each crewmember returns to Earth with an individual parachute. But such systems are effective in a limited range of altitudes and speeds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In human spaceflight programs, a possible failure would mean crew loss and a possible termination of the program or, in the best case, postponing it for years until everything would be 100% safe. But Project Gemini was like a stepping stone for the manned moon program. Time was very important because it was a space race in those years, and to achieve the goals, everything should be successful, at the right time, and safe. Of course, taking the risk could lead to success, but why did NASA choose that kind of technology, the ejection seat, instead of a system with rockets mounted on a tower above the capsule, which was tested previously in Project Mercury and used in later capsules like Apollo and Soyuz. The tower escaping system is used even in this days in Soyuz spacecraft and will be used in the future missions of Orion spacecraft. Since the ejection seat system is effective and safer in a limited range of altitudes and speeds, the risk is higher by using this system because the failure of the rocket could happen at any time in different altitudes and speeds. The tower escape system had been developed and tested as a technology before Project Gemini, why wasn’t it chosen?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="1142" LastEditDate="2016-04-08T02:43:56.400" LastActivityDate="2023-08-10T20:44:48.707" Title="Why were ejection seats used in Project Gemini instead of a tower escape system?" Tags="|safety|crewed-spaceflight|launch-escape-system|project-gemini|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14691" PostTypeId="1" CreationDate="2016-04-07T17:39:22.467" Score="3" ViewCount="416" Body="&lt;p&gt;UPDATE: 5/12: I'm making progress on this project and I can now ask a few, more specific questions. However, to expedite things, would anyone be willing to allow me to contact them directly? It would speed things up considerably.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until then, (and please excuse my inability to calculate variations of the formulas offered below) my current question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Traveling vertically straight up at an acceleration of 3Gs, how long would it take to reach an altitude of 250 miles? And how fast would you be traveling upon arrival (no need to decelerate)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ORIGINAL QUESTIONS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm a screenwriter developing a screenplay for a possible movie and I need some help calculating acceleration so the facts in the film are accurate. I also need solutions with different variables (described below) so I can build the story properly (I tried doing the math myself, but my algebra skills have atrophied).&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Accelerating vertically at a constant 1G, how long would it take to reach an altitude of 250 miles? (Please note, this is not into orbit, just altitude)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How about to an altitude of 22,236 miles?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How fast would you be traveling when you reached 250 miles and 22,236 miles respectively?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I also need answers for acceleration at 3Gs, 6Gs and 9Gs.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If I only had 30 minutes to reach 250 miles altitude, how many Gs would I experience and how fast would I be traveling upon arrival?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What if I only had 20 minutes?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="14018" LastEditorUserId="14018" LastEditDate="2016-05-12T18:06:06.460" LastActivityDate="2016-05-12T18:06:06.460" Title="Calculating acceleration to various altitudes straight up" Tags="|launch|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14692" PostTypeId="1" AcceptedAnswerId="14701" CreationDate="2016-04-07T18:04:10.603" Score="3" ViewCount="1520" Body="&lt;p&gt;I'm doing research for a movie. If you were in a space elevator/tower at an altitude of 250 miles, how much gravity would you experience? (I've heard various estimates of .9Gz and .65Gz)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How about if you were at an altitude of 22,236 miles?&lt;/p&gt;&#xA;" OwnerUserId="14018" LastEditorUserId="415" LastEditDate="2016-04-07T20:13:08.543" LastActivityDate="2016-04-07T22:37:14.990" Title="How does the gravity of Earth change as you climb a space elevator?" Tags="|gravity|space-elevator|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14693" PostTypeId="2" ParentId="14692" CreationDate="2016-04-07T18:15:32.920" Score="2" Body="&lt;p&gt;I could calculate it for you, but using the &quot;Teach a Man to Fish&quot; principle, here's a simple equation (lifted from &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_of_Earth&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ g_h = g_0 \left( \frac{r_e}{r_e + h} \right) ^2 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$h$ = height above sea level&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$r_e$ = radius of Earth&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$g_0$ = sea level gravity&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Use the units of your choice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: this accounts for altitude effects only and not centripetal force.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="415" LastEditDate="2016-04-07T20:14:32.333" LastActivityDate="2016-04-07T20:14:32.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14694" PostTypeId="2" ParentId="14691" CreationDate="2016-04-07T18:25:35.917" Score="3" Body="&lt;p&gt;Distance travelled ($h$) after a given time ($t$) at a given acceleration ($a$), is calculated by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$h=\frac{t^2a}{2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Re-arranging that into:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$t=\sqrt{\frac{2h}{a}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should answer your first and second question, giving $286.5s$ for 250 miles, and $2702s$ for 22,236 miles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That velocity is just: $v=ta$, is what you need for your third question, $2,809 m/s$ and $26,493 m/s$ respectively. Insert any other acceleration into the previous formula to get the new time, and use that together with the acceleration value in this formula to get the required numbers for your fourth question, namely:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v=\sqrt{2ha}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, for number five and six, the required acceleration given altitude and time is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a=\frac{2h}{t^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That gives you $0.02533 g$ and $0.05698 g$, for 30 minutes and 20 minutes respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is all of course assuming you mean change in velocity with your acceleration values, not the felt acceleration. (e.g. you feel 1g by just standing).&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-07T18:25:35.917" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14695" PostTypeId="2" ParentId="14687" CreationDate="2016-04-07T18:32:45.370" Score="9" Body="&lt;p&gt;There is a standard clause included in all GPS receiver manuals regarding COCOM Limits. I cannot find the source of this clause, but since it is worded exactly the same in all the manuals I could find, I assumed it was probably derived directly from the regulation at some point:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;COCOM Limits&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The U.S. Department of Commerce requires that all exportable GPS&#xA;  products contain performance limitations so that they cannot be used&#xA;  in a manner that could threaten the security of the United States. The&#xA;  following limitations are implemented on the [named] receiver.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Immediate access to satellite measurements and navigation results is&#xA;  disabled when the receiver’s velocity is computed to be greater than&#xA;  1000 knots, &lt;strong&gt;or&lt;/strong&gt; its altitude is computed to be above 18,000 meters. The&#xA;  receiver continuously resets until the COCOM situation is cleared.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;(emphasis mine)&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would seem based on this wording that the actual rule was OR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this rule appears to no longer be enforced. I am not a lawyer, but it appears that the current rule for those states which accept the Missile Technology Control Regime's controls in its Technical Annex is as follows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;11.A.3. Receiving equipment for Global Navigation Satellite Systems (GNSS; e.g. GPS, GLONASS or Galileo), having any of the following characteristics, and specially designed components therefor:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;a. Designed or modified for use in systems specified in 1.A.; or&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;b. Designed or modified for airborne applications and having any of the following:&lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;Capable of providing navigation information at speeds in excess of 600 m/s;&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If your GPS reciever is capable of providing navigation information at speeds greater than 600 m/s, then you must have a special license, it would seem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bottom line:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you wish to alter a GPS receiver to remove COCOM limits or purchase a special GPS receiver without COCOM limits, it would appear that this is acceptable, but there may be licensing that you have to apply for. &lt;em&gt;I highly recommend consulting a lawyer before using such a GPS receiver in a COCOM situation.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.ngs.noaa.gov/corbin/class_description/5700-5800V2UserGuide.pdf&quot; rel=&quot;noreferrer&quot;&gt;5700-5800 V2 User Guide - NOAA&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.mtcr.info/english/MTCR-TEM-Technical_Annex_2015-10-08_Current_version.pdf&quot; rel=&quot;noreferrer&quot;&gt;Missile Technology Control Regime Equipment, Software, and Technology Annex - MTCR&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-04-07T18:32:45.370" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14696" PostTypeId="2" ParentId="14692" CreationDate="2016-04-07T18:35:50.460" Score="8" Body="&lt;p&gt;If you &lt;strong&gt;want&lt;/strong&gt; to consider the centripetal force too, you can do that by just adding a little more to the equation &lt;a href=&quot;https://space.stackexchange.com/a/14693/415&quot;&gt;already suggested by Organic Marble&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$g_h=g_0 \left(\frac{r_e}{r_e + h}\right)^2 - \frac{4\pi^2 (r_e+h)}{t^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $h$ = height above sea level, $r_e$ = radius of Earth, $g_0$ = sea level gravity and the additional $t$ is the length of the Earth's rotational period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The acceleration profile looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SaMd4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SaMd4.png&quot; alt=&quot;acceleration per altitude&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the two values you have chosen, the acceleration is 0.88g for 250 miles, and 0g for 22236 miles, usually known as the geostationary altitude,&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-07T22:16:43.063" LastActivityDate="2016-04-07T22:16:43.063" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14697" PostTypeId="2" ParentId="14690" CreationDate="2016-04-07T19:12:13.200" Score="17" Body="&lt;p&gt;While a number of sources say the motivation for using ejection seats instead of LES is to save weight, a tower launch escape system can be jettisoned relatively early in flight, while ejection seats have to be carried all the way into orbit. Depending on the exact masses and time of tower jettison, this can be a wash or even slightly favorable to a tower LES.&lt;/p&gt;&#xA;&lt;p&gt;A down side to ejection seats is that they offer less protection to the crew members and are more likely to cause injuries (if a crew member has an arm outstretched in a way that doesn't clear the hatch during the ejection sequence, for instance). Thus LES is generally preferable.&lt;/p&gt;&#xA;&lt;p&gt;For Gemini, &lt;a href=&quot;https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/FagetMA/MAF_6-18-97.pdf#page=38&quot; rel=&quot;nofollow noreferrer&quot;&gt;according to Max Faget (page 38)&lt;/a&gt;, there were two major considerations that made ejection seats more attractive.&lt;/p&gt;&#xA;&lt;p&gt;One, at one point in development, Gemini was going to land as a controlled glider, touching down on a runway under a parasail instead of splashing down at sea under a parachute. If something went wrong during the glide approach, having ejection seats would be nice, but a LES would be overkill (and would require another parachute besides), and carrying the LES through the entire flight would be too large a mass penalty.&lt;/p&gt;&#xA;&lt;p&gt;Two, unlike the launchers for the Mercury and Apollo programs, the Titan used hypergolic propellants which burn on contact; while that sounds dangerous, in a major failure situation where the propellant tanks ruptured, they would start burning as soon as mixing began, which would tend to disperse the remaining propellants and give an overall less energy-dense, slower-burning fireball. With the non-hypergolics in the Mercury and Apollo launchers, it would be possible for there to be extensive mixing of the propellants before ignition, which would yield an explosion, in which case the extra protection afforded by the capsule would be very important.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="14637" LastEditDate="2023-08-10T20:44:48.707" LastActivityDate="2023-08-10T20:44:48.707" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="14698" PostTypeId="1" AcceptedAnswerId="14699" CreationDate="2016-04-07T20:34:55.983" Score="4" ViewCount="534" Body="&lt;p&gt;It's easy to say in retrospect that the USSR's Sputnik 1 was just a dumb radio transmitter that didn't actually do anything useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it also stands to reason that putting that same &quot;dumb radio transmitter&quot; into Earth orbit and keeping it there for a while demonstrated a number of technological feats that had not been done previously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What scientific and technological advances occured as a &lt;em&gt;direct result of the work that culminated in the launch and operation&lt;/em&gt; of Sputnik 1? How did specifically Sputnik 1 contribute to each of those?&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2016-04-07T20:42:03.803" LastActivityDate="2016-04-08T05:05:15.670" Title="What scientific and technological advances were prompted directly by Sputnik 1?" Tags="|history|sputnik|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14699" PostTypeId="2" ParentId="14698" CreationDate="2016-04-07T20:58:07.540" Score="4" Body="&lt;p&gt;First off, Sputnik 1 was when space travel went from being &quot;just a theory&quot; to being an actual thing. The difference is huge, do you not see a lot of theories about future technologies here and there? Can all of them be true? No matter people where sceptical to the concept of space travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, the successful launch saved &lt;a href=&quot;https://en.wikipedia.org/wiki/Sergei_Korolev&quot; rel=&quot;nofollow&quot;&gt;Korolev's&lt;/a&gt; ass. The head designer og the Soviet Rocket program had great visions for space. However, the military wanted missiles, and nothing but that. (even ICBMs where questioned for their actual usefulness as weapons). One can truly say that he went quite far, launching the last attempt to get Sputnik into orbit &lt;strong&gt;after&lt;/strong&gt; he got an order from Moscow to &quot;stop it&quot;. When the satellite was successful, everything was of course forgiven. Finally getting the space program a budget in the Soviet Union, can indeed be said to have been the start of the amazing technological progression that was the spacerace.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Nothing is guaranteed to work until you test it!&lt;/strong&gt;&lt;br&gt;&#xA;A for-real test of an orbital rocket is an experience that can not be replaced. If that is not what you are interested in, and strictly mean the probe itself, it was indeed a simple design, featuring very limited instrumentation. Apart from &quot;the components used did not break&quot;, the role of the Sputnik 1 was merely to pave the way for the following series of much more sophisticated Sputnik satellites.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-07T20:58:07.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14701" PostTypeId="2" ParentId="14692" CreationDate="2016-04-07T22:31:56.380" Score="2" Body="&lt;p&gt;Organic Marble gives a good answer. Hohmannfan gives a better answer if you want the net acceleration someone at that point of the elevator would feel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using both gravity and the so called centrifugal force, someone would feel .88 g's 250 miles up the elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 22237 miles altitude I get -7.23 millionths of a g which is darn near zero. Just a slight tug upward so that altitude is just a tad higher than geosynchronous orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a spreadsheet &lt;a href=&quot;http://clowder.net/hop/NetAccelOnElevator.xls&quot; rel=&quot;nofollow&quot;&gt;NetAccelOnElevator.xls&lt;/a&gt;. User inputs altitude in miles to the colored cell at the top. Below will appear the net acceleration in g's.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-04-07T22:37:14.990" LastActivityDate="2016-04-07T22:37:14.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14702" PostTypeId="2" ParentId="5947" CreationDate="2016-04-07T22:49:10.803" Score="19" Body="&lt;p&gt;Both are correct, it just depends on who/what is doing the verb (sucking/blowing). In english we would call this the subject of the sentence.&lt;/p&gt;&#xA;&lt;p&gt;Consider the following scenario. You have two large tanks, sitting in your garage (presumably on planet earth). One has a vacuum in it at 0.01 ATM, the other has some pressurized air, lets say at 10 ATM. The two tanks are connected by a sealed pipe with a valve in the middle.&lt;/p&gt;&#xA;&lt;p&gt;Now lets say you open this valve so air now starts rushing from the pressurized tank into the vacuum tank. Is the air being sucked or blown? Well the answer is, both, just depends how you construct the sentence. You could say &amp;quot;The low pressure tank sucked the air out of the high pressure tank&amp;quot;. Alternatively, you could also say &amp;quot;The high pressure tank blew the air into the low pressure tank&amp;quot;. Both statements are true, the verb just changes depending on what is doing the action (the subject).&lt;/p&gt;&#xA;&lt;p&gt;So back to your original Star Trek quote. Both are right. The people were sucked out by the vacuum of space. But it is also true that the people were blown out into space by the space station.&lt;/p&gt;&#xA;&lt;h1&gt;Technical Physics Explanation&lt;/h1&gt;&#xA;&lt;p&gt;The technical definition of &lt;em&gt;suck&lt;/em&gt;, or rather &lt;em&gt;suction&lt;/em&gt;, is according to wikipedia:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Suction is the colloquial term to describe the air pressure differential between areas... When the pressure in one part of a system is reduced relative to another. [&lt;a href=&quot;https://en.wikipedia.org/wiki/Suction&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Similarly the dictionary defines it as such:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The force that, by a pressure differential, attracts a substance or object to the region of lower pressure. [&lt;a href=&quot;https://www.dictionary.com/browse/suction&quot; rel=&quot;nofollow noreferrer&quot;&gt;2&lt;/a&gt;]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In other words, suck, or suction, does not suggest that the force comes from the low pressure side, only that the force exerted on the object results in an attractive force towards the side with the lower pressure.&lt;/p&gt;&#xA;&lt;p&gt;In the case of people, or a space door hatch, it satisfies these definitions, there is a pressure differential across them and from the perspective of the low pressure side, since objects are attracted to it by the produced force, there is suction.&lt;/p&gt;&#xA;&lt;p&gt;Most of the answers which suggest the &amp;quot;motive force&amp;quot; is coming from the high pressure side is true, but this ignores the definition of the word suction, which does not in anyway rely on where the force comes from but only its direction.&lt;/p&gt;&#xA;&lt;p&gt;In fact, for that reason, the word &amp;quot;suck&amp;quot; is often used in scientific literature and is quite acceptable, despite Mr. Data's objections.&lt;/p&gt;&#xA;&lt;h1&gt;Examples in Scientific Literature&lt;/h1&gt;&#xA;&lt;h2&gt;An Elementary Treatment of the Reverse Sprinkler&lt;/h2&gt;&#xA;&lt;p&gt;In an attempt to back up my claim that both &lt;em&gt;suck&lt;/em&gt; and &lt;em&gt;blow&lt;/em&gt; are equally valid I tried to find some scholarly source on the matter. Ultimately this is an English question, and I could not find a scholarly English source. I was however able to find a peer-reviewed physics paper published on arXiv, a well respected journal, that used similar language.&lt;/p&gt;&#xA;&lt;p&gt;Here are some quotes from the paper:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;the box will turn in the direction of the long arrow when blown into and in the direction of the short arrow when sucked on.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Steam bubbles cause water to be alternately blown out of and sucked into the tank.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;If, therefore, an elastic ball, which has one escape-tube, be attached to the reaction-wheel, in the manner represented in [Fig. 8(a)], and be alternately squeezed so that the same quantity of air is by turns blown out and sucked in, the wheel will continue to revolve rapidly in the same direction as it did in the case in which we blew into it&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;All of the above quotes can be found in the following physics paper:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://arxiv.org/abs/physics/0312087v1&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://arxiv.org/abs/physics/0312087v1&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h2&gt;Study of Combined Rice Husk Gasifier Thermoelectric Generator&lt;/h2&gt;&#xA;&lt;p&gt;Similarly here is another separate paper, also peer-reviewed, using &lt;em&gt;suck&lt;/em&gt; and &lt;em&gt;blow&lt;/em&gt; as equally valid terms in the paper.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A blower was used to suck the ambient air to cool the heat sink and blow the air from the heat sink to the reactor of the gasifier.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Which can be found in this paper:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.sciencedirect.com/science/article/pii/S187661021400928X&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.sciencedirect.com/science/article/pii/S187661021400928X&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;As we can see in the above again &lt;em&gt;suck&lt;/em&gt; and &lt;em&gt;blow&lt;/em&gt; are both used in a scholarly scientific context with peer review. So clearly both terms are considered acceptable and correct within the scientific community.&lt;/p&gt;&#xA;&lt;h1&gt;Conclusion&lt;/h1&gt;&#xA;&lt;p&gt;I think its a safe bet that if multiple research scientists and physicists can publish papers and have it pass peer-review using &lt;em&gt;blow&lt;/em&gt; and &lt;em&gt;suck&lt;/em&gt;, then either word, when in agreement with the subject (as described above), is acceptable to use. Clearly the scientific community finds both words equally valid and I think it's common sense that these are valid English usages as well.&lt;/p&gt;&#xA;" OwnerUserId="14022" LastEditorUserId="14022" LastEditDate="2021-07-02T18:03:30.237" LastActivityDate="2021-07-02T18:03:30.237" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="14703" PostTypeId="2" ParentId="5947" CreationDate="2016-04-08T03:12:41.353" Score="1" Body="&lt;p&gt;Actually, Data is also wrong - possibly!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A pressure due to incident wind (blowing) and a pressure difference across an object due to a sudden pressure gradient &lt;em&gt;are two separate things!&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The air pressure pushes on the hatch, the hatch exerts a pressure back on the gas, and the hatch-latch in turn pushes on the hatch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the hatch-latch stops holding, the hatch pressure drops to zero, and the air pressure accelerates the hatch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now there is a pressure gradient in the air. The air closest to the opening can not push back on the air farther inside. The air deeper in the ship pushes and accelerates the air closer to the opening. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Blowing&quot; usually refers to being pushed by a wind or velocity. You might have used pressure to create the wind, but &quot;wind pressure&quot; is not the same as a pressure gradient. For example, wind pressure depends on the mass density of the fluid and the velocity of the fluid. A pressure gradient is just about pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The air &quot;Pushes&quot; itself out, it doesn't really get &quot;Blown&quot; out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same for the people. While there will be a force associated with the moving air blowing on them, there is a much much BIGGER force associated with the pressure gradient. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I think that Data is also wrong. They were &quot;Pushed&quot; out by the sudden, transient pressure gradient. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-04-08T04:56:28.950" LastActivityDate="2016-04-08T04:56:28.950" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14704" PostTypeId="1" CreationDate="2016-04-07T23:34:24.057" Score="0" ViewCount="3233" Body="&lt;p&gt;My understanding is that space has powerful radiations in it, such as cosmic rays and high-speed charged particles. The earth's magnetic field and atmosphere largely protect earth dwellers from these emanations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, obviously anyone in space would be exposed to them because many such particles will pass through the thin steel of a space vehicle. In fact, the ISS is now testing a balloon room that would offer no protection at all to radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I heard a rumor that when astronauts are selected, they deliberately pick those who have children and are not planning to have any more children for this reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that true? Do you become sterile if you go into space?&lt;/p&gt;&#xA;" OwnerUserId="6871" OwnerDisplayName="Tyler Durden" LastEditorUserId="10791" LastEditDate="2016-04-08T11:06:11.087" LastActivityDate="2017-09-03T23:06:59.967" Title="Do astronauts become sterile from being in space, like on the ISS?" Tags="|radiation|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14707" PostTypeId="2" ParentId="1904" CreationDate="2016-04-08T06:50:18.020" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://pythonhosted.org/OrbitalPy/&quot; rel=&quot;noreferrer&quot;&gt;OrbitalPy&lt;/a&gt; has a handy &lt;code&gt;elements_from_state_vector&lt;/code&gt; function that does just that:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://github.com/RazerM/orbital/blob/0.7.0/orbital/utilities.py#L252&quot; rel=&quot;noreferrer&quot;&gt;https://github.com/RazerM/orbital/blob/0.7.0/orbital/utilities.py#L252&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can check that the math is the same as in user29 answer.&lt;/p&gt;&#xA;" OwnerUserId="13959" LastActivityDate="2016-04-08T06:50:18.020" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14708" PostTypeId="2" ParentId="14704" CreationDate="2016-04-08T08:31:54.327" Score="9" Body="&lt;p&gt;Astronauts are not sterilized by radiation in space. &lt;a href=&quot;http://phys.org/news/2011-01-sex-space-houston-problem.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;According to a 2005 study&lt;/a&gt;, at least 17 children were born from female astronauts after they had been in space. Male astronauts were able to conceive children soon after returning from spaceflight.&lt;br&gt;&#xA;That also suggests NASA has no policy in place of only picking astronauts who don't want to have more children.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Animal studies showed a decrease in fertility: sperm counts declined, ovaries shut down in mice. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Jennings has been involved in space medicine since 1987. He hasn't seen evidence that floating in a spacecraft affects astronauts' fertility, or their sex lives.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;We've had astronauts - after a two-week flight, they were able to get their spouses pregnant a day or two after landing,&quot; Jennings said. A 2005 study Jennings co-authored counted 17 babies born to female astronauts after they had been in space. The astronauts had a high miscarriage rate, but women who've gained the seniority to go into space tend to be near the end of their child-bearing years, Jennings said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-09-03T15:36:43.043" LastActivityDate="2017-09-03T15:36:43.043" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14709" PostTypeId="1" AcceptedAnswerId="14713" CreationDate="2016-04-08T10:32:29.037" Score="13" ViewCount="3247" Body="&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/questions/1034/how-are-the-astronauts-in-the-iss-protected-from-solar-flares&quot;&gt;related question&lt;/a&gt;'s answer tells about the mitigation procedures for the duration of a 'proton storm'. In particular,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ISS crew did receive a Solar weather warning several times and were advised to enter the more protected areas of the ISS, such as the US built Destiny laboratory, or the Russian built service module Zvezda&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How are these modules protected - what shielding or other measures make them safer than the rest of the station?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-04-08T17:35:16.047" Title="How does ISS protect astronauts from Coronal Mass Ejections?" Tags="|iss|radiation|space-weather|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14710" PostTypeId="2" ParentId="14709" CreationDate="2016-04-08T11:06:51.117" Score="0" Body="&lt;p&gt;According to the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#/media/File:ISS_configuration_2015-05_en.svg&quot; rel=&quot;nofollow&quot;&gt;ISS schematics on wikipedia&lt;/a&gt;, Destiny is quite in the middle of the station. That should provide some shielding. Zvezda seems to be more exposed, so that might not be the only reason.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-04-08T11:06:51.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14711" PostTypeId="1" AcceptedAnswerId="14721" CreationDate="2016-04-08T13:48:33.020" Score="12" ViewCount="2304" Body="&lt;p&gt;I have heard that the tiles on the space shuttle were sensitive to being wet, in that they might be damaged. I recently saw this photo, ported to be of the Space Shuttle launching. It struck me as weird, having flown through clouds, which contain significant amounts of liquid water. What were the flight rules for flying through clouds with the Shuttle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yM5yB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yM5yB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="10791" LastEditDate="2016-04-09T10:04:57.133" LastActivityDate="2016-04-09T10:04:57.133" Title="What were the Space Shuttle's rules about flying through clouds?" Tags="|launch|space-shuttle|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14713" PostTypeId="2" ParentId="14709" CreationDate="2016-04-08T14:42:03.507" Score="17" Body="&lt;p&gt;The two specific modules are protected by two mechanisms: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;TeSS Polyethylene radiation protection tiles and bricks&lt;/li&gt;&#xA;&lt;li&gt;Water storage bags attached to the walls making a &quot;water wall&quot;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;High densities of hydrogen are good at radiation protection, and water is a good hydrogen source that needs to be stored on the ISS. Another good source of dense hydrogen is polyethylene, which is nice and light. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Zvezda module in particular was lined with Polyethylene in its construction to enable it to play the role of a radiation protection shelter. The radiation protection was modelled as part of the design:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3YxMG.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3YxMG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Future deep space missions propose to utilise water storage walls as part of their design following from experiences on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(1) &lt;a href=&quot;http://www.iss-casis.org/files/CASIS_RFP_2013-3_Radiation_Measurements_and_Shielding_Analysis_for_ISS.pdf&quot; rel=&quot;noreferrer&quot;&gt;Shavers et.al., 2004,  &lt;em&gt;&quot;Radiation Measurements and Shielding Analysis for ISS&quot;&lt;/em&gt;, Workshop on Radiation Analysis for the International Space Station, Vienna&lt;/a&gt;.&lt;br&gt;&#xA;(2) &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2000/ast02nov_1/&quot; rel=&quot;noreferrer&quot;&gt;Barry &amp;amp; Phillips, 2000, &lt;em&gt;&quot;Water on the Space Station&quot;&lt;/em&gt;, Science@NASA, November 2000&lt;/a&gt;&lt;br&gt;&#xA;(3) &lt;a href=&quot;http://www.spacesafetymagazine.com/aerospace-engineering/spacecraft-design/massively-redundant-water-walls-spacecraft-water/&quot; rel=&quot;noreferrer&quot;&gt;Azriel, 2012, &lt;em&gt;&quot;“Massively Redundant” Water Walls Spacecraft To Use Water for Everything&quot;&lt;/em&gt;, Space Safety Magazine, 10 Sep 2012&lt;/a&gt;  &lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="746" LastEditDate="2016-04-08T17:35:16.047" LastActivityDate="2016-04-08T17:35:16.047" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14714" PostTypeId="1" AcceptedAnswerId="14716" CreationDate="2016-04-08T17:56:00.760" Score="8" ViewCount="513" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9&quot; rel=&quot;noreferrer&quot;&gt;wikipedia&lt;/a&gt;, the Falcon 9 1.0 had ca. 10t payload capacity to LEO. &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;The Dragon&lt;/a&gt; has a 3.3t payload capacity to the ISS with a dry mass of 4.2t that leaves 2.5t unused lift capacity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this all used for fuel and propellant? That seems like a lot (that would mean more than 800m/s delta-v assuming the Draco has 300s isp, as it is &quot;comparable&quot; to the R-4D). The difference grows even larger with the Falcon 9 1.1 and (presumably) the full thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why is there this gap between the Falcon 9's performance and the Dragon's? &lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="4660" LastEditDate="2016-04-08T18:28:02.733" LastActivityDate="2016-04-08T18:57:47.913" Title="Why is the Dragon small compared with the Falcon 9 LEO payload" Tags="|spacex|falcon-9|dragon|payload|mass|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14716" PostTypeId="2" ParentId="14714" CreationDate="2016-04-08T18:57:47.913" Score="9" Body="&lt;p&gt;First of all, the difference in capacity is actually greater than you included. Dragon launches without a fairing, thus there is an increased capacity as a result of launching without the fairing. The exact mass isn't known.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what else might be different in the cargo capacity? Here's a few things:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Dragon might not be structurally capable of carrying more than that weight given the vibration requirements imposed on launch. &lt;/li&gt;&#xA;&lt;li&gt;Fuel mass (Unlikely, however, to be 2.5 tons)&lt;/li&gt;&#xA;&lt;li&gt;The ISS orbit requires more fuel to achieve than the referenced LEO orbit. This is quite likely to be a major contributor, accounting for over a ton reduction in mass to LEO per the &lt;a href=&quot;https://www.spaceflightnow.com/falcon9/001/f9guide.pdf&quot;&gt;old Falcon 9 user's guide&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I think the loss in mass is partially due to the inclination, partially fuel, possible some other consumables (Port covers, etc), and the remaining portion being deliberately keeping the capacity lower to ensure there is sufficient margin in case of sub-optimal performance.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-08T18:57:47.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14717" PostTypeId="1" AcceptedAnswerId="34153" CreationDate="2016-04-08T19:07:56.663" Score="9" ViewCount="730" Body="&lt;p&gt;I know it's hard to quantify, but are rocket designers and astronauts more happy about the possibility of cold welding, or more negative about it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean: two &quot;bare metal&quot; surfaces in vacuum weld together in the ambient temperature of space when they come in contact. In one hand, it could be invaluable in construction work, saving the whole bother with welding equipment - a good smack of a hammer will take care of bringing the elements in contact firm enough to form a weld. On the other hand, everything that isn't supposed to weld itself to anything else (like the aforementioned hammer) must be coated with, or made of non-welding substances. And if the protective layer is scratched, that still can &quot;catch&quot;. It must add quite a bit of headaches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - in the absolutely subjective opinion of space equipment makers - do the headaches outweigh the benefits or vice versa?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="4660" LastEditDate="2016-04-08T19:47:33.230" LastActivityDate="2019-02-11T23:18:33.383" Title="How much of a mixed blessing is cold welding?" Tags="|construction|cold-welding|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14718" PostTypeId="1" AcceptedAnswerId="14720" CreationDate="2016-04-08T20:04:10.487" Score="16" ViewCount="2558" Body="&lt;p&gt;Jerard Puckett mentioned in &lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/the-pod-bay&quot;&gt;The Pod Bay&lt;/a&gt; yesterday that Shuttles did S-Curve maneuvers back and forth across their reentry trajectory to shed speed. After a bit of looking, I found some reference to this, but not much. In the Kerbal Space Program Forum &lt;a href=&quot;http://forum.kerbalspaceprogram.com/index.php?/topic/78521-orbiter-reentry/&quot; rel=&quot;noreferrer&quot;&gt;this thread&lt;/a&gt; mentioned it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Also, space shuttles would re-enter in a rolled attitude to steepen their trajectory (i.e. the wing's lift was directed sideways not up). The roll would result in a cross-track velocity component but roll reversal maneuvers were used to &quot;sway&quot; the re-entry trajectory back and forth across the target landing site.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And they were discussing the photo below, of Atlantis starting reentry, I guess taken from the ISS, which shows how the maneuver looks, seen from behind and above. Atlantis is the bright spot, engulfed in plasma, doing a turn:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/c41YD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/c41YD.jpg&quot; alt=&quot;Shuttle Atlantis reentry trajectory as seen from ISS&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much of their velocity did the Shuttles shed this way? Was it a critical part of the reentry process?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2023-08-02T14:09:35.580" Title="How much reentry speed did the Shuttles shed by doing a series of sharp turns?" Tags="|reentry|space-shuttle|trajectory|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14719" PostTypeId="1" AcceptedAnswerId="14757" CreationDate="2016-04-08T20:58:21.407" Score="7" ViewCount="371" Body="&lt;p&gt;In SpaceX's successful landing on the barge today, April 8th, we see there is almost no room for error, at least from the &lt;a href=&quot;https://www.youtube.com/watch?v=7pUAydjne5M&amp;amp;feature=youtu.be&amp;amp;t=2140&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hD6nz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hD6nz.png&quot; alt=&quot;still of Falcon Stage 1 on OCISLU&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that it appears that the width of the drone ship is only about twice the length of the square formed by the landing legs. What is the maximum permissible error to achieve the landing? In particular, if it was diagonal, there would be even less room, it seems there is only an error margin at most of the landing leg spread, but I'm curious also as to if that is an issue.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="51" LastEditDate="2016-04-12T12:51:09.887" LastActivityDate="2016-04-12T18:06:54.600" Title="What is the maximum error to land on the barge?" Tags="|launch|spacex|falcon-9|landing|booster|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14720" PostTypeId="2" ParentId="14718" CreationDate="2016-04-09T02:57:23.680" Score="24" Body="&lt;p&gt;It's a bit of a misconception that the shuttle used S-turns to slow down. To quote the &lt;em&gt;&lt;a href=&quot;https://gandalfddi.z19.web.core.windows.net/Shuttle/USA005512%20-%20Entry,%20TAEM%20and%20Approach%2021002%20Basic%2020060123.pdf&quot; rel=&quot;noreferrer&quot;&gt;Entry, TAEM, and Approach/Landing Guidance Workbook&lt;/a&gt;&lt;/em&gt; by the United Space Alliance:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The next time you hear someone talk about the shuttle doing roll reversals to bleed off energy, do not listen. The shuttle does roll reversals because it has a very small alpha envelope.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What it's saying is that the &lt;em&gt;ideal&lt;/em&gt; reentry strategy is to simply point the nose at the landing site and control how fast you're dissipating energy by controlling &lt;a href=&quot;https://en.wikipedia.org/wiki/Angle_of_attack&quot; rel=&quot;noreferrer&quot;&gt;angle of attack&lt;/a&gt; (alpha). A higher alpha will result in more drag, and it's easy to make quick adjustments to get the level of drag you need.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in order keep the orbiter both controllable and protected by the thermal protection system, it had to be flown at a particular alpha (40° for most of entry) with no more than 3° of variation. This meant they had to use different means to control drag, and the way they did it was by controlling the lift vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A steeper bank angle meant that the lift vector would be more sideways than up, which would cause the shuttle to descend quicker, and, as you descend, the air gets thicker, which causes more drag. A shallower bank angle will slow your descent and keep you in thinner air for longer, which minimizes your drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there's a problem with using bank... it starts to turn you off course. So the solution is to use roll reversals (aka s-turns) to keep you pointed towards the landing site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, to sort of answer the literal question you asked (how much entry speed was bled off during the s-turns): most of it. The first roll would occur at nearly orbital velocity, and entry guidance ended at 2500 feet per second (Earth-relative velocity). After entry guidance ended, TAEM (Terminal Area Energy Management) guidance began, which primarily used alpha for energy management. Once the orbiter was subsonic, the speedbrake was also used for energy management.&lt;/p&gt;&#xA;" OwnerUserId="1342" LastEditorUserId="6944" LastEditDate="2020-05-20T17:25:38.373" LastActivityDate="2020-05-20T17:25:38.373" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14721" PostTypeId="2" ParentId="14711" CreationDate="2016-04-09T04:12:53.727" Score="11" Body="&lt;p&gt;Based on the picture I assume you are asking about launch constraints due to clouds (versus cloud rules relating to shuttle entry and landing).  Fortunately there is a nice human readable summary &lt;a href=&quot;https://www.nasa.gov/centers/kennedy/pdf/531435main_weather-rules-march2011.pdf&quot;&gt;here&lt;/a&gt;, and the cloud rules are&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Cumulus Clouds: &#xA;       Do not launch through cumulus- &#xA;      type clouds with tops higher than the 41 degree F &#xA;      temperature level.  Launch may occur through clouds &#xA;      with tops as cold as 23 degrees F if the cloud is not &#xA;      producing precipitation, there is a field mill within 2 &#xA;      nautical miles of the cloud, and this field mill and all &#xA;      field mills within 5 nautical miles of the flight path read &#xA;      between -100 volts per meter and +500 volts per meter &#xA;      for the past 15 minutes.&#xA;      - Do not launch through or within 5 nautical miles &#xA;      of the nearest edge of cumulus-type clouds with tops &#xA;      higher than the 14 degree F level.&#xA;       - Do not launch through or within 10 nautical miles &#xA;      of the nearest edge of cumulus clouds with tops higher &#xA;      than the -4 degrees F level.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Disturbed Weather:   Do not launch if the flight path  is through any&#xA;  non-transparent clouds that extend to  altitudes at or above the 32&#xA;  degrees F level, which are  associated with disturbed weather&#xA;  producing moderate  or greater precipitation, or melting&#xA;  precipitation, within  5 nautical miles of the flight path.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Thick&#xA;  Clouds:   Do not launch if any part of the  planned flight path is&#xA;  through a layer of clouds within  5 nautical miles, is 4,500 feet&#xA;  thick or greater, and the  temperature is between 32 degrees F and -4&#xA;  degrees  F.  Launch may occur if the cloud layer is a cirrus-like &#xA;  cloud that never has been associated with convective  clouds, is&#xA;  located entirely at temperatures of 5 degrees  F or colder, and shows&#xA;  no evidence of containing water  droplets. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Anvil Cloud:   Do not&#xA;  launch through an attached  or detached anvil cloud if it is within 10&#xA;  nautical miles  of the launch pad if  it is determined to be&#xA;  electrified  and could trigger a lightning strike by flight through &#xA;  the cloud. Launch can be permitted if the conditions  in the launch&#xA;  criteria directives for Volume-Averaged  Height-Integrated Radar&#xA;  Reflectivity (VAHIRR) are  met. This specifies the distance, time,&#xA;  radar and field  mill measurements required to ensure safe flight. The&#xA;  condition of an anvil cloud must be evaluated for these  criteria when&#xA;  it is within 10 nautical miles of the launch  pad. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Debris Cloud:  Unless VAHIRR launch criteria  directives permit, do not launch if the&#xA;  flight path will  carry the vehicle through a debris cloud which is&#xA;  not  transparent and less than three hours old.  Launch  may not occur&#xA;  within five nautical miles of these debris  clouds unless: 1) for 15&#xA;  minutes preceding launch there  is at least one working field mill&#xA;  within five nautical miles  of the debris cloud; 2) all electric field&#xA;  mill readings are  between -1 kilovolt and + 1 kilovolt per meter&#xA;  within  5 nautical miles of the flight path for the past 15 minutes; &#xA;  3) weather radar has detected less than 10 dbz of  reflectivity in the&#xA;  debris cloud within 5 nautical miles of  the flight path for 15&#xA;  minutes. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Smoke Plume:   Do not launch if the flight path  will carry&#xA;  the vehicle through any cumulus cloud that  has developed from a smoke&#xA;  plume while the cloud is  attached to the plume, or for the first 60&#xA;  minutes after  the cumulus cloud detaches from the smoke plume.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Eastern Test Range also had range safety cloud constraints, to wit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Direct visual observation of the shuttle is required  through 8,000&#xA;  feet.  This requirement may be satisfied  using optical tracking sites&#xA;  or a forward observer. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;– A cloud ceiling of 6,000 feet is permitted&#xA;  for short- duration launch windows if all required range safety &#xA;  instrumentation systems are functioning. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;– A cloud ceiling of 4,000&#xA;  feet is permitted if:  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;a)  The cloud layers between 4,000 and 8,000 &#xA;  feet are not more than 500 feet thick.   &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;b)  The vehicle can be seen&#xA;  by the Eastern  Range airborne and/or the ground forward  observers&#xA;  through 8,000 feet and they can  communicate with the flight control&#xA;  officer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There were other weather related constraints at the link provided earlier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were weather constraints for the abort landing sites as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, there were many and different weather rules for landing.  Also summarized at the link.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-04-09T04:21:35.770" LastActivityDate="2016-04-09T04:21:35.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14723" PostTypeId="2" ParentId="3569" CreationDate="2016-04-09T09:20:03.400" Score="1" Body="&lt;p&gt;The practical upper velocity that can be gained by gravitational assists is limited by the number of times a spacecraft can reach another gravitational body to increase its velocity as well as the strength of the gravity well of the body. If the spacecraft is moving slowly it can alternate between two planets, increasing its velocity during each pass (and decreasing ever so slightly the orbital momentum of the planets it passes). As it moves faster and faster it will need to make passes closer closer to the planets to allow a return to the other planet. Eventually, passing closer to the planets leads to passing through an atmosphere (slowing the craft), grazing the planet (destroying the craft), or flying off away from both planets (the last pass causes a minor path deflection and not a half-orbit return to the other planet).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The limits of gravitational assists is also discussed at &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist#Limits_to_slingshot_use&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Gravity_assist#Limits_to_slingshot_use&lt;/a&gt;, where it mentions the &quot;Grand Tour&quot; made by Voyager, where the planets Jupiter, Saturn, Uranus and Neptune were lined up in such a way that allowed Voyager to increase its speed during the flyby of each planetary mass. If the solar system had another dozen planets that Voyager could have passed close to during its journey, it might have used them all to end up going even faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you had two black holes orbiting each other, due to their extreme gravity you might, in theory, be able alternate back and forth between the two for gravitational assists until you approached the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another real-life limit to unlimited numbers of gravitational assists is that in order to make a pass occur along the precise path required to reach the next gravity well some reaction mass will be expended for course correction. The catch-22 here is that if you had unlimited on-board thrust you wouldn't need gravitational assists, but if you have limited thrust, after you run out you can't course correct to do the next gravitational assist.&lt;/p&gt;&#xA;" OwnerUserId="14036" LastEditorUserId="14036" LastEditDate="2016-04-09T09:41:50.367" LastActivityDate="2016-04-09T09:41:50.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14724" PostTypeId="1" CreationDate="2016-04-09T09:48:08.860" Score="5" ViewCount="136" Body="&lt;p&gt;I am currently reading the &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Expanse_%28novel_series%29&quot; rel=&quot;noreferrer&quot;&gt;The Expanse&lt;/a&gt; novel series. In the books there are several space colonies that are basically asteroids or planetoids (in particular Eros and Ceres) that have been spun up to act as huge centrifuges for the people living &quot;upside-down&quot; in tunnels inside of them. The artificial gravity on the lowest levels near the surface is stated as (minus) one third of a g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this feasible? Obviously, as soon as the &lt;a href=&quot;https://xkcd.com/123/&quot; rel=&quot;noreferrer&quot;&gt;centrifugal force&lt;/a&gt; exceeds gravity everything on the surface (or &lt;em&gt;the&lt;/em&gt; surface for that matter) wants to fly off. So you need to have some inherent stability to hold things together. Are asteroids and planetoids stable enough to do that? Intuitively I would assume that bigger bodies are more solid that smaller ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Roche_limit&quot; rel=&quot;noreferrer&quot;&gt;roche limit&lt;/a&gt; but that is about tidal forces between two bodies not centrifugal forces acting on one body but I assume the same concepts apply here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;(I thought about asking this to the astronomy stack exchange instead but the question is ultimately about space colonization so I put it here. Feel free to move it if you disagree.)&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="10488" LastActivityDate="2016-04-09T09:48:08.860" Title="How fast could the rotation of asteroids and planetoids be sped up before they disintegrate?" Tags="|colonization|asteroid|asteroid-belt|ceres|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14726" PostTypeId="1" CreationDate="2016-04-09T16:12:44.883" Score="2" ViewCount="1059" Body="&lt;p&gt;How was SpaceX able able to keep the rocket from tipping over on the barge once it landed?  The barge looked like it was rocking quite a bit.&lt;/p&gt;&#xA;" OwnerUserId="14042" LastEditorUserId="51" LastEditDate="2016-04-12T12:50:42.087" LastActivityDate="2016-04-12T12:50:42.087" Title="How was SpaceX able to keep the rocket from tipping over on the barge?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="1" CommentCount="0" ClosedDate="2016-04-09T16:36:12.263" ContentLicense="CC BY-SA 3.0" />
  <row Id="14727" PostTypeId="2" ParentId="14726" CreationDate="2016-04-09T16:29:31.820" Score="7" Body="&lt;p&gt;The center of gravity of the first stage is much lower than it would appear at a glance. The stage is mostly empty fuel tanks, which are very light; the engines and landing legs are the heavier components, all very low on the stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you mentally visualize the rocket body as being only about half its actual height, you'll have a much more accurate picture of the weight balance of the stage, and you'll see that the rocket is unlikely to tip over.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the landing, my understanding is that another manned sea vessel will meet up with the barge, and workers will either install support chains in a &quot;guy wire&quot; configuration, or weld &quot;shoes&quot; onto the deck over the landing legs, or both, which will keep the stage stable in case of rougher seas on the trip home. That support vessel keeps its distance during the landing itself for safety reasons.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-04-09T16:36:51.903" LastActivityDate="2016-04-09T16:36:51.903" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14728" PostTypeId="1" CreationDate="2016-04-09T17:53:15.310" Score="2" ViewCount="190" Body="&lt;p&gt;Apparently, I am a bit indecisive regarding an event that could possibly occur within the context of reality regarding nuclear thermal rockets, and I was thinking: is this space disaster scenario plausible: &quot;The first manned Mars mission is launched under the Ares 1 rendezvous with the IMIS Endeavour. Shortly after docking with the Mission Module, the craft leaves Earth-Moon L2 for Mars. However, the mission ends in failure following a rupture in the reactor caused by micrometeorites punctures during Mars orbital insertion, rendering the crew stranded.  This results in a huge national tragedy for the States as the crew is declared KIA due to extreme radiation poisoning several days later, thus delaying the launch of the Ares 2 mission by two years.&quot; I will be asking a separate question in a different site regarding the political and economic consequences. :P&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are wondering: Integrated Manned Interplanetary Spacecraft (IMIS) was a Boeing 1968 study that was designed for a Mars mission around the 1980s - 1990s span of time, that never made it out of the drawing board. In other words, this: &lt;a href=&quot;http://www.astronautix.com/craft/imis1968.htm&quot; rel=&quot;nofollow&quot;&gt;IMIS 1968 Encyclopedia Astronautica Article&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13960" LastEditorUserId="415" LastEditDate="2016-04-09T22:24:04.027" LastActivityDate="2016-04-10T10:39:52.937" Title="Could a micrometeorite bombardment in an IMIS craft result in reactor rupture leading to the crew becoming stranded?" Tags="|mars|nuclear|failure|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="14729" PostTypeId="2" ParentId="5103" CreationDate="2016-04-09T19:33:37.733" Score="2" Body="&lt;p&gt;Rockets are cylindrical similar to airliners, etc. but the structural design is geared to vertical flight (and as you know; now vertical landing). &lt;a href=&quot;https://aviation.stackexchange.com/a/26778/11482&quot;&gt;A contemporary answer on Aviation.SE&lt;/a&gt; shows the structural design of a horizontal cylinder, in this case a 777 airliner. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless orbital vehicles develop from a horizontal takeoff-to-orbit design philosophy, a given vehicle would need to be designed to stand up to both vertical take off and horizontal landing. This adds weight and complexity, breaking some pretty fundamental economic rules of space flight. &lt;/p&gt;&#xA;" OwnerUserId="11145" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:54.153" LastActivityDate="2016-04-09T19:33:37.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14730" PostTypeId="1" AcceptedAnswerId="14822" CreationDate="2016-04-09T21:51:12.697" Score="6" ViewCount="862" Body="&lt;p&gt;Are orbital elements in TLEs valid at the epoch? I want to predict an orbit without SGP, but I need some initial orbital conditions.&lt;/p&gt;&#xA;" OwnerUserId="14047" LastActivityDate="2016-04-17T16:22:20.597" Title="Can Keplerian elements from TLEs be used without SGP?" Tags="|orbit|orbit-determination|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14731" PostTypeId="1" AcceptedAnswerId="14752" CreationDate="2016-04-09T22:15:25.987" Score="22" ViewCount="5294" Body="&lt;p&gt;I've been assuming that osculating, Keplerian, and orbital elements are all synonyms, with mean orbital elements just being these averaged over some time. However occasional comments make me suspect that there are some subtleties I'm missing. Could someone clarify the differences (if any) between these terms:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Orbital elements&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Keplerian elements&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Osculating elements&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mean orbital elements&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="14047" LastEditorUserId="1142" LastEditDate="2016-04-10T12:27:01.200" LastActivityDate="2022-05-12T04:42:41.230" Title="Nuances of the terms (mean / osculating / Keplerian / orbital) elements" Tags="|terminology|orbital-elements|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14732" PostTypeId="1" AcceptedAnswerId="14734" CreationDate="2016-04-09T22:26:16.297" Score="5" ViewCount="155" Body="&lt;p&gt;I understand that TLEs are used with SGP to predict future positions/velocities of a Earth-orbiting satellite. However, how are these sets obtained in the first place? Is it based on ground observations?&lt;/p&gt;&#xA;" OwnerUserId="14047" LastActivityDate="2016-04-10T03:56:41.810" Title="How are TLEs updated?" Tags="|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14734" PostTypeId="2" ParentId="14732" CreationDate="2016-04-10T03:44:07.677" Score="4" Body="&lt;p&gt;In short, yes. The U.S. Air Force uses the &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_Space_Surveillance_Network&quot; rel=&quot;nofollow&quot;&gt;Space Surveillance Network&lt;/a&gt; to track all observable objects in space. They have a system in place which attempts to correlate tracks with existing objects. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the system encounters tracks which do not match up with an existing object, they create a new object to track. The system then continues to correlate tracks to this new object. After collecting multiple tracks (which could take a few days depending on the geometry of the orbit), the object is assigned a static catalog ID number, and finally released to the public (assuming they designate the object as unclassified).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I simplified the process somewhat, but that's the general idea.&lt;/p&gt;&#xA;" OwnerUserId="14049" LastEditorUserId="6816" LastEditDate="2016-04-10T03:56:41.810" LastActivityDate="2016-04-10T03:56:41.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14735" PostTypeId="2" ParentId="14150" CreationDate="2016-04-10T07:40:07.203" Score="6" Body="&lt;p&gt;Flame color should not be directly related to hypergolic...ity(?). Rather, it's dependent on the chemical properties of the particular fuel and oxidizer, and their products.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/LGM-25C_Titan_II#Development&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia says&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Titan II also used storable propellants: &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerozine_50&quot; rel=&quot;noreferrer&quot;&gt;Aerozine 50&lt;/a&gt;, which is a 1:1 mixture of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrazine&quot; rel=&quot;noreferrer&quot;&gt;hydrazine&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Unsymmetrical_dimethylhydrazine&quot; rel=&quot;noreferrer&quot;&gt;unsymmetrical dimethylhydrazine (UDMH)&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Dinitrogen_tetroxide&quot; rel=&quot;noreferrer&quot;&gt;dinitrogen tetroxide&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As to why those chemicals burn with a colorless flame, I can tell you that it has to do with electron configurations and excitation states, but for any more detail, you might want to ask on the chemistry site.&lt;/p&gt;&#xA;" OwnerUserId="13871" LastActivityDate="2016-04-10T07:40:07.203" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14736" PostTypeId="1" AcceptedAnswerId="14742" CreationDate="2016-04-10T08:11:57.863" Score="6" ViewCount="610" Body="&lt;p&gt;If I understand correctly, Falcon 9 may be launched with no intent to land the first stage, when carrying exceptionally heavy payloads - in this scenario the fuel meant for landing is then used during the launch/acceleration stage and the stage just crashes into the sea at full speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case the landing legs, extra RCS propellant etc are just a dead weight and a useless extra cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does SpaceX have a variant of the launch stage without these, or does it just go with &quot;economy of scale&quot;, including them even when they are not needed?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-01T19:36:59.960" Title="Does SpaceX have a disposable variant of Falcon?" Tags="|spacex|falcon-9|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14737" PostTypeId="1" CreationDate="2016-04-10T08:43:17.400" Score="1" ViewCount="128" Body="&lt;p&gt;When researching &lt;a href=&quot;https://space.stackexchange.com/q/14687/12102&quot;&gt;this question&lt;/a&gt; I came upon a presentation entitled &lt;a href=&quot;http://www.gps.gov/multimedia/presentations/2011/09/ICG/miller.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Enabling a Fully Interoperable GNSS Space Service Volume&lt;/a&gt;; slides 12, 24, 25, 26 are also shown below. The idea is that while GPS satellites concentrate most of their radiated power at the earth, there is some &quot;spillage&quot; past the edge of the earth and out into space, from both the main lobe and side lobes, depending on the particular GPS satellite configuration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Signals may be weaker and good fixes may be intermittent, but for many space applications this can still be extremely useful. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/12863/12102&quot;&gt;This answer&lt;/a&gt; has a great discussion (and nice plots!) of radiation patterns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last slide suggests the example of a mission returning from the moon, which I assume is a potential &lt;em&gt;future&lt;/em&gt; mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apart from the signal strength data from a piggy-backed experiment aboard &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/amsat-p3d.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;AO-40&lt;/a&gt; (and &lt;a href=&quot;https://en.wikipedia.org/wiki/AO-40&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;), has GPS ever been actually &lt;em&gt;used&lt;/em&gt; or even &lt;em&gt;validated or tested&lt;/em&gt; as a potential source of supplemental navigational information &lt;em&gt;beyond&lt;/em&gt; Geostationary orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GHq56.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GHq56.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j6frw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j6frw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mXzKt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mXzKt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qadLS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qadLS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/21xde.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/21xde.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-04-10T20:58:02.000" Title="Has GPS been used beyond GEO?" Tags="|artificial-satellite|navigation|gps|" AnswerCount="0" CommentCount="5" ClosedDate="2016-04-10T20:58:25.497" ContentLicense="CC BY-SA 3.0" />
  <row Id="14738" PostTypeId="2" ParentId="14736" CreationDate="2016-04-10T08:57:37.517" Score="6" Body="&lt;p&gt;SpaceX upgraded to the Full Thrust variant specifically to allow the recovery even for those heavy  satellites currently in common use. But the &quot;expendable&quot; variant is still often discussed online.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing legs are only added to the stage at the Cape (or Vandy) probably because with them the core dimensions would be over the limit for truck transportation. So it should be simple enough to just not add them. Similarly for the grid fins and RCS propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other more integrated parts would probably just stay there as SpaceX declared they want to mass produce things with as few differences as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did not check the numbers but it would seem that all these would just add to a bit better margin for error (engine out capability, bad upper level winds etc.) and not significantly change the safe maximal payload mass because all these savings would only happen on the first stage where the effect is not as big. &lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-04-10T08:57:37.517" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14739" PostTypeId="1" AcceptedAnswerId="18834" CreationDate="2016-04-10T09:12:53.260" Score="13" ViewCount="848" Body="&lt;p&gt;&lt;strong&gt;My question is:&lt;/strong&gt; Has there been any effort to &lt;em&gt;improve or optimize&lt;/em&gt; the radiation patterns on the subsequent satellite navigation constellations (e.g. Glonass, Galileo, BeiDou-2) to facilitate trans-GEO navigation? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a presentation entitled &lt;a href=&quot;http://www.gps.gov/multimedia/presentations/2011/09/ICG/miller.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Enabling a Fully Interoperable GNSS Space Service Volume&lt;/a&gt; (slides 12, 24, 25, 26 are also shown below) some aspects of the possible use of GPS signals for navigation beyond Geosynchronous orbit are discussed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I have &lt;a href=&quot;https://space.stackexchange.com/q/14737/12102&quot;&gt;asked elsewhere&lt;/a&gt; about any actual navigation uses or tests of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;GPS&lt;/a&gt; system, I am wondering now if this possibility was considered for other navigation constellations, e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/GLONASS&quot; rel=&quot;nofollow noreferrer&quot;&gt;Glonass&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo&lt;/a&gt;, or &lt;a href=&quot;https://en.wikipedia.org/wiki/BeiDou_Navigation_Satellite_System#Global_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;BeiDou-2&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Have the radiation patterns of the broadcast antennae on any of these satellite constellations been designed to provide additional power&lt;/em&gt; &quot;over-the-limb&quot; &lt;em&gt;of the earth (see below)&lt;/em&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/12863/12102&quot;&gt;This answer&lt;/a&gt; has a great discussion (and nice plots!) of radiation patterns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The AO-40 data show that the signals from the IIR Block satellites at large off-nadir angles are much larger than expected, and even at small angles are distinctly different than those of II and IIA Block satellites. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That suggests the possibility at least that the design was somehow modified to enhance the over-the-limb signal and improve the possibility of navigational use far from LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GHq56.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GHq56.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j6frw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j6frw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mXzKt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mXzKt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qadLS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qadLS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/21xde.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/21xde.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-10-29T00:44:58.567" Title="Will Glonass, Galileo, or BeiDou-2 satellites provide better cis-lunar navigation than GPS?" Tags="|artificial-satellite|navigation|gps|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14740" PostTypeId="2" ParentId="14728" CreationDate="2016-04-10T10:39:52.937" Score="3" Body="&lt;p&gt;The circumstances would have to be pretty unusual for the crew to die of radiation poisoning at that point in the mission. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The micrometeoroid would have to be pretty big: it has to disable the NERVA engine of the Mars Capture Stage (MCS) completely. If there's any residual thrust, the spacecraft will move away from the radioactive debris cloud. Only if thrust falls to 0 will the spacecraft stay with the debris and expose the astronauts to lots of radiation.  &lt;/li&gt;&#xA;&lt;li&gt;The micrometeoroid would have to be big enough to break off parts of the reactor. Punching a small hole in the wall is not enough, you need reactor parts or the fuel itself to be part of the debris cloud. &lt;/li&gt;&#xA;&lt;li&gt;At Mars orbit insertion, the spacecraft still contains another unused NERVA stage, the Mars depart stage (MDS). If the MCS fails, they can separate it, and fire the MDS to get on their way back to Earth.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-10T10:39:52.937" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14742" PostTypeId="2" ParentId="14736" CreationDate="2016-04-10T11:16:54.483" Score="8" Body="&lt;p&gt;The Falcon 9 first stage booster is meant to be fairly flexible. Reusability is meant to reduce cost. There is nothing stopping them from launching an expendable version in order to maximize payload, if someone was willing to pay for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 first stage is the basis for the Falcon Heavy, where there will be two variants of the core. The two side boosters will be the same, but differ slightly from the center core booster. This will keep manufacturing costs down. The legs are attached later, and the grid fin control system is part of the interstage, so it seems likely they could build a lighter interstage without the grid fins and N2 cold gas thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to offer maximum payload to LEO, should a customer wish to pay for it, SpaceX has some flexibility. For minimal cost of launch, they intend to return all three cores to the launch site (RTLS). But that has the highest cost in terms of payload launched. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For higher payload masses, they intend to return the side cores to the launch site, but recover the center core downrange on a barge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Potentially for even higher payload masses they could expend the center core and only recover the side boosters. There is only one ASDS in the Atlantic, so it is unlikely they would be doing down range recovery of the side boosters, but that can change with time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the main issue in expending the core or not comes down to, is it worth it? On a standalone Falcon 9, maximum payload will be less than a fully recoverable Falcon Heavy, so it may just be cheaper to launch on a Falcon Heavy recoverable, than on an expendable Falcon 9.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once they started landing first stages, SpaceX made at least some kind of landing attempt on most missions.  Sometimes that meant guiding it down to the ocean and letting it fall in (when the ASDS was unavailable, or due to weather). After that point, it seemed like they got it nailed down and all launches would be recovery attempts after that point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Echostar 23 mission (targeted at Mar 2017) turned out to be too heavy to launch and recover, even on a downrange ASDS, so will be launched expendable. Musk said that this would be the last expendable launch. Further heavy payloads would launch on a Falcon Heavy instead. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-03-01T19:36:59.960" LastActivityDate="2017-03-01T19:36:59.960" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14743" PostTypeId="2" ParentId="14730" CreationDate="2016-04-10T12:59:17.257" Score="4" Body="&lt;p&gt;The &lt;strong&gt;short&lt;/strong&gt; answer is: its not clear**, however, if you have no independent measurements of your own then TLEs will have to do.&lt;/p&gt;&#xA;&lt;p&gt;If you have some comparable case where you have both your own measurements and the TLEs then you could make a meaningful comparison. Planet labs used to provide both on their website though I can't immediately find the page ++.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;strong&gt;slightly longer&lt;/strong&gt; answer: its not clear because despite it being intuitively appealing that at the epoch the TLE could effectively rank with a cartesian pos-vel and thus be good for any other propagator model (not withstanding the positional uncertainty of course), there are good reasons to see how the TLE solution has already been subject to propagation with the SGP model, and thus be degraded to an extent.&lt;/p&gt;&#xA;&lt;p&gt;I have the impression that the published epoch does not necessarily relate to  the period of data collection in an obvious way. The following examples are a starting point.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Normal business:&lt;/strong&gt;&#xA;If the orbit for an object has been constructed from a set of ranges from one radar or telescope then its easy to imagine that the solved orbit could be published with an epoch somewhere along the arc, or track, of observations, perhaps best where the residuals were least. It could equally simply be the mid-point of the track.  The same applies if the orbit had been resolved from two sensors.  In any such case the published orbit necessarily involves some use of the SGP propagation between each data point and the epoch.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Privacy:&lt;/strong&gt;&#xA;If the sensor's capabilities or operational status are sensitive then I would expect the operator to be a little coy about allowing easy correlation between sensors and resolved orbits. That would be enough to expect some random propagation from the mid-point of a track to the published epoch.&lt;/p&gt;&#xA;&lt;p&gt;** = &lt;strong&gt;Obviously&lt;/strong&gt;, &amp;quot;validity&amp;quot; depends upon the purpose, which isn't stated in the question. e.g. TLE's are ok for pointing a ground antenna but not for collision avoidance.&lt;/p&gt;&#xA;&lt;p&gt;EDIT - I found a helpful &lt;a href=&quot;https://celestrak.org/columns/v04n05/&quot; rel=&quot;nofollow noreferrer&quot;&gt;quote&lt;/a&gt; towards this end:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The elements in the two-line element sets are mean elements calculated to fit a set of observations using a specific model—the SGP4/SDP4 orbital model. Just as you shouldn't expect the arithmetic and geometric means of a set of data to have the same value, you shouldn't expect mean elements from different element sets—calculated using different orbital models—to have the same value.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;EDIT 2&lt;/p&gt;&#xA;&lt;p&gt;++ Here is the link to the PlanetLabs comparison that I mentioned in my second paragraph: &lt;a href=&quot;http://ephemerides.planet-labs.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ephemerides.planet-labs.com&lt;/a&gt;  it provides further links to files giving their own UHF ranging of their satellites and the corresponding TLEs.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2016-04-17T16:22:20.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14744" PostTypeId="1" CreationDate="2016-04-10T15:03:50.987" Score="4" ViewCount="2490" Body="&lt;p&gt;I have never seen a launch in person.  It always looks like (especially old Saturn V launches) as if the rocket becomes somewhat horizontal early in the launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simple-minded engineer in me says the most fuel-efficient launch would be straight up.  I realize that at some point the velocity needs to shift so that (for example) one ends up in a low earth orbit at the right speed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the optimization that drives the flight path, and how quickly does the launch vehicle start flying away from pure vertical - and how does it stay aloft?&lt;/p&gt;&#xA;" OwnerUserId="14053" LastActivityDate="2017-08-14T04:26:20.287" Title="What is the basic optimal trajectory for a launch vehicle?" Tags="|launch|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14745" PostTypeId="1" AcceptedAnswerId="14756" CreationDate="2016-04-10T16:44:58.680" Score="18" ViewCount="5651" Body="&lt;p&gt;I read that SpaceX is starting to work on fairings reusability. I couldn't find any information, so I would like to know more about them. In particular, I remember a quote from Musk saying that fairings cost up to several millions of dollars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much does a fairing system cost? How can they be recovered? Would they float on water or would they need floatation devices?&lt;/p&gt;&#xA;" OwnerUserId="8925" LastEditorUserId="1142" LastEditDate="2017-02-21T23:59:18.200" LastActivityDate="2018-02-23T06:55:15.010" Title="Falcon 9 fairings cost and reusability" Tags="|spacex|falcon-9|reuse|fairings|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14748" PostTypeId="1" CreationDate="2016-04-10T19:45:10.253" Score="4" ViewCount="297" Body="&lt;p&gt;In the Apollo 13 movie extra material, there are several documentary shots interspersed with commentary about the makings of the movie.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am interested in the sequence beginning at about 29 minutes (DVD title 1, chapter 7, approximately 2 min 20 sec into the chapter) with Gene Kranz where he talks about the initial problems being referred to as a &quot;funny&quot; and there is a cut to a significantly lower-quality shot showing (him?) in likely Mission Control during the actual mission. There's also a minute later a sequence showing Sy Liebergot discussing the problem of loss of electrical power and oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know I have seen these sequences before, and I am certain that I've seen a bit more of both of them; alternatively, something where Kranz and Liebergot said something &lt;em&gt;extremely&lt;/em&gt; similar almost down to the specific wording.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Where are those sequences from?&lt;/strong&gt; Is that NASA footage, some third-party documentary footage that was reused for the extra material, or Howard original footage? &lt;strong&gt;Where can I find it&lt;/strong&gt; (like e.g. documentary title, original broadcast network, archive number with NASA, ...)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a bonus question (not required for a valid answer to this question), in what other documentaries has the material been used?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The relevant portions of the ending credits appear to be (which doesn't really help me very much):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;ARCHIVAL FOOTAGE&#xA;&#xA;NASA&#xA;&#xA;Courtesy of the&#xA;NATIONAL BROADCASTING COMPANY, INC.&#xA;&#xA;CARSON PRODUCTIONS&#xA;&#xA;&#xA;ADDITIONAL FOOTAGE&#xA;&#xA;THE BERKELEY GROUP&#xA;&#xA;GRB ENTERTAINMENT&#xA;&#xA;DIGITAL DOMAIN&#xA;&#xA;BILL PAXTON&#xA;&#xA;Courtesy of WGBH-TV/BOSTON&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2016-04-14T18:14:52.647" LastActivityDate="2016-04-14T18:14:52.647" Title="From where is this sequence in the Apollo 13 (movie) extra material showing Gene Kranz?" Tags="|nasa|history|apollo-13|" AnswerCount="0" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14750" PostTypeId="1" CreationDate="2016-04-10T20:26:48.327" Score="6" ViewCount="3195" Body="&lt;p&gt;In Germany, there is a very well known sci-fi novel series in which some of the spaceships are spheres with a toroidal bulge around the equator in which the (downward pointing) engines are situated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the spacecraft are used for flights inside the solar system including starting and landing on Earth and other planets and moons of the solar system (where this is reasonably possible), would such a spherical form with an equatorial ring of propulsion engines be a viable alternative to the needle-like rockets of today (in reality, not in science-fiction)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least it should be much easier to land such a spacecraft. Also, the payload space could be designed more generously. Of course, the more energy can be provided from the power source of the propulsion engine, the easier it should possibly be to change to a spherical form with higher air resistance than the current needle like forms. So another question is: Can a chemical energy powered spacecraft be built (and used, of course) in spherical form and if not would fission power suffice, or if not, then fusion energy power?&lt;/p&gt;&#xA;" OwnerUserId="14059" LastEditorUserId="6537" LastEditDate="2018-01-11T09:25:01.373" LastActivityDate="2020-05-27T06:38:04.587" Title="Is a spherical rocket design a plausible replacement for current designs today?" Tags="|spacecraft|" AnswerCount="6" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14751" PostTypeId="2" ParentId="14750" CreationDate="2016-04-10T20:39:06.520" Score="9" Body="&lt;p&gt;Spherical chemical-energy powered spacecraft can be built. I know this because they have been (at least roughly spherical). To wit, the &lt;a href=&quot;http://www.npointercos.jp/Energiamuseum.html&quot; rel=&quot;noreferrer&quot;&gt;Soyuz orbital module&lt;/a&gt; (seen here at the Energia Museum).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GB6fY.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GB6fY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and Vostok&#xA;&lt;a href=&quot;https://i.stack.imgur.com/tVuC2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tVuC2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as spherical boosters, none have been built to my knowledge.  The drag penalty would be prohibitive.  However, there was a &lt;a href=&quot;http://www.pmview.com/spaceodysseytwo/spacelvs/sld034.htm&quot; rel=&quot;noreferrer&quot;&gt;serious proposal by Chrysler&lt;/a&gt; in the Space Shuttle Phase A design studies to build a squatty booster that approaches the spherical as closely as anything I know of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H8tUi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H8tUi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-04-10T21:12:01.970" LastActivityDate="2016-04-10T21:12:01.970" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14752" PostTypeId="2" ParentId="14731" CreationDate="2016-04-10T21:52:08.333" Score="21" Body="&lt;p&gt;Your assumption is a good starting place and its good to be cautious about it. Many of us are guilty of abbreviation or outright misuse of the terms for convenience. Here's my rough guide, not meant to be precise but rather to address the overlapping usage. I've spent a bit more time on the basic definitions as this is probably where variations in usage arises.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Orbital elements:&lt;/strong&gt; the 6 parameters: semi-major axis, eccentricity, inclination, RAAN, argument of perigee, and mean anomaly. This describes the shape of an orbit and the position of an object in it, plus a 7th parameter, the time (epoch) at which this situation exists.  This is the most general term, all the others in the OP's question are subsets of this term.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Keplerian elements:&lt;/strong&gt; This specifically refers to the same 6 elements + time that apply to a Keplerian orbit. This means different things according to the context.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Limited context: orbits that are ideal ellipses. The term thus separates such hypothetical orbits from reality.&lt;/li&gt;&#xA;&lt;li&gt;Common usage: the idea of using the six orbital parameters to describe a real orbit, see Osculating below. In this context using the term &amp;quot;Keplerian&amp;quot; could be simply to distinguish an element set as a way of presenting the information rather than presenting the instantaneous position and velocity.&lt;/li&gt;&#xA;&lt;li&gt;Much broader definition: referring to item 2 above and distinguishing from a &amp;quot;non-keplerian&amp;quot; orbit, where the latter could refer to an orbit in a 3 body problem or continually under the influence of propulsion or a solar sail.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;strong&gt;Osculating elements:&lt;/strong&gt;  the same 6 elements + time but referring to a real orbit, where all the rest of the features of a real orbit such as the non-spherical Earth, atmosphere, solar radiation pressure and the effects of the Sun and the Moon are acknowledged implicitly. Such an orbit is not elliptical, it is lumpy and evolves continually so that the satellite path is different for every revolution.&lt;/p&gt;&#xA;&lt;p&gt;For this concept to make sense the context has to be that these features are modelled, just not explicitly announced at this moment.  Thus the element set might be numerically indistinguishable from a keplerian set (ellipse definition) but would describe a different orbit by virtue of the propagator that is implicitly referenced.&lt;/p&gt;&#xA;&lt;p&gt;In practice osculating elements are likely to include some additional parameters reference to an object's area/mass ratio in order to support calculation of the effects of atmospheric drag or solar radiation pressure, where the latter will probably roll-in the effects of surface reflectance.&lt;/p&gt;&#xA;&lt;p&gt;Its a feature of the community that people rarely go out of their way to describe the ins and outs of their model as its assumed as a given. If you wanted to use osculating elements solved by a different satellite operator, for perhaps for co-operation in collision avoidance, it would be up to you to learn about their orbit determination propagator and do comparisons on a mutually known object. I suspect many don't have the budget for such things.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Mean orbital elements:&lt;/strong&gt;  The 6 elements plus time. I have only seen this term in the context of SGP4 TLEs though in principle it could apply to other approaches. There is a public definition of this system and the additional parameters it requires such as for addressing atmospheric drag.&lt;/p&gt;&#xA;&lt;p&gt;I understand that the lumpiness of the orbit developed from osculating elements is smoothed out. Whilst the name &amp;quot;mean&amp;quot; suggests that this smoothed orbit could be an ideal ellipse, as per Keplerian definition 1 above, the SGP 4 model propagates orbits that are not elliptical, they just have less bumps than modern osculating orbits.&lt;/p&gt;&#xA;&lt;p&gt;Edit: I found this &lt;a href=&quot;https://celestrak.org/columns/v04n05/&quot; rel=&quot;nofollow noreferrer&quot;&gt;quote&lt;/a&gt; which helps with the &amp;quot;mean elements&amp;quot; concept.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The elements in the two-line element sets are mean elements calculated to fit a set of observations using a specific model—the SGP4/SDP4 orbital model. Just as you shouldn't expect the arithmetic and geometric means of a set of data to have the same value, you shouldn't expect mean elements from different element sets—calculated using different orbital models—to have the same value. The short answer is that you cannot simply reformat the data unless you are willing to accept predictions with unpredictable errors.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2022-05-12T04:42:41.230" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14754" PostTypeId="2" ParentId="14750" CreationDate="2016-04-10T22:14:08.730" Score="2" Body="&lt;p&gt;With current technology, spherical spacecraft are non-practical for two reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Heat dissipation. A true (in your sense) spacecraft, AKA a &lt;a href=&quot;http://www.sarna.net/wiki/DropShip&quot; rel=&quot;nofollow&quot;&gt;Dropship&lt;/a&gt; would have to dssipate loads of heat into space in order not to fry its crew. This requires huge radiators, so the form would more look like a shuttlecock.&lt;/li&gt;&#xA;&lt;li&gt;Radiation exposure. If you put your power source in the middle of your space craft, you expose all parts of it equally to the deadly radiation. Since you need a form of nuclear propulsion for any reasonably advanced spacecraft design that renders a spherical craft impractical. The reactor needs to be as far as possible from the crew and shielded by as much mass as possible.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The bottomline is: You might end up with a spherical head section (living quarters, command and control etc.) but the whole spacecraft will look quite different from your typical STARDUST class ship.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-04-10T22:14:08.730" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14755" PostTypeId="2" ParentId="14750" CreationDate="2016-04-11T07:27:56.103" Score="15" Body="&lt;p&gt;The shapes of rockets today are based on the fact that fuel mass (and fuel volume) dominate any current launch vehicle.  Inefficiencies in reaching orbit mean non-linear losses in payload.  An extra 0.1% drag could require burning 5% more fuel to compensate.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long, thin rockets are much more efficient in the atmosphere, both from direct drag losses as well as weight penalties on a sufficiently strong structure.  As long as we have to start on the surface of the earth with fuel mass limited machines, almost any other shape seems useless.  Once you're out of the atmosphere though, you can choose shapes based on other constraints.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fission and fusion power have the theoretical potential to be more efficient than today's chemical rockets.  But that doesn't necessarily mean they would be suitable for a launch vehicle.  Almost any work on these fuels today is for power generation, not for thrust.  In a spacecraft, this gives you the means to drive efficient but low-thrust engines for a long period of time out of the atmosphere.  But it doesn't give a way to more easily reach orbit.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2016-04-11T07:27:56.103" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14756" PostTypeId="2" ParentId="14745" CreationDate="2016-04-11T08:08:09.943" Score="15" Body="&lt;p&gt;Apparently this was considered by SpaceX in Q1 2015, to work around a production bottleneck. Due to delays in 2015 after the June 2015 loss of CRS-7 that bottleneck may not have occurred.&#xA;As of &lt;a href=&quot;https://www.youtube.com/watch?v=Nz60GcmKOvc#t=33m28s&quot; rel=&quot;noreferrer&quot;&gt;CRS-8&lt;/a&gt; (1 year later) we haven't seen a recovery attempt but it's still something SpaceX wants to do.&lt;/p&gt;&#xA;&lt;p&gt;This slide is from an &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/2zkxi5/spacex_design_and_operations_overview_of_fairing/&quot; rel=&quot;noreferrer&quot;&gt;internal SpaceX presentation&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ymf9C.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ymf9C.png&quot; alt=&quot;Fairing recovery: the fairings are passively stable during reentry, presenting their outside surface to the airstream. After reentry, each fairing half deploys a parafoil and is picked up mid-air by a helicopter.&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SpaceX note that by the the end of 2015, fairing production will not be able to keep up with desired fairing launch frequency.&lt;/p&gt;&#xA;&lt;p&gt;Incremental improvements in production are occurring to resolve this, but a redesign is needed to drastically improve fairing production.&lt;/p&gt;&#xA;&lt;p&gt;This will be accomplished by decreasing the number of piece parts, reducing the number of structural bonds, and component redesigns with production hours in mind.&lt;/p&gt;&#xA;&lt;p&gt;Falcon heavy launches will have higher loads, environments, and thermal. Many fairing components will require redesign to meet these requirements. The new fairing will be designed from the ground up to meet these requirements.&lt;/p&gt;&#xA;&lt;p&gt;The new fairing will be designed from the beginning with reusability in mind. As reentry load cases mature, parts will be designed to these loads. Also, an experimental cold gas ACS system is being added to the current fairing and fairing 2.0 will include a more production ready system.&lt;/p&gt;&#xA;&lt;p&gt;Assuming the ACS system is successful in making the fairing survive reentry, a parachute system will be added to each fairing half as well - with helicopter recovery shown in slide.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-05-26T11:26:17.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14757" PostTypeId="2" ParentId="14719" CreationDate="2016-04-11T10:09:30.607" Score="5" Body="&lt;p&gt;The landing platform of the upper deck of the initial &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;noreferrer&quot;&gt;Just Read the Instructions&lt;/a&gt; (JRtI) vessel was 52 m × 91 m (170 ft × 300 ft) while the span of the Falcon 9 landing legs is 18 m (60 ft). The other barges used by SpaceX have similar dimensions.&lt;br&gt;&#xA;So in the most constrained direction, they can land (52/2 - 18/2) = 17 m off-center before one of the legs goes over the edge. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-11T10:09:30.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14758" PostTypeId="2" ParentId="14075" CreationDate="2016-04-11T13:10:53.547" Score="11" Body="&lt;p&gt;For the &lt;a href=&quot;https://zerog.jsc.nasa.gov/home.html&quot;&gt;NASA KC-135&lt;/a&gt;, this is the trajectory they used to fly, for 25 seconds of 0 G: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0tolk.gif&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0tolk.gif&quot; alt=&quot;parabolic flight&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.gozerog.com/index.cfm?fuseaction=Experience.How_it_Works&quot;&gt;Zero-G corporation&lt;/a&gt; uses a similar flight plan: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Before starting a parabola, G-FORCE ONE flies level to the horizon at an altitude of 24,000 feet. The pilots then begins to pull up, gradually increasing the angle of the aircraft to about 45° to the horizon reaching an altitude of 32,000 feet. During this pull-up, passengers will feel the pull of 1.8 Gs. Next the plane is “pushed over” to create the zero gravity segment of the parabola. For the next 20-30 seconds everything in the plane is weightless. Next a gentle pull-out is started which allows the flyers to stabilize on the aircraft floor. This maneuver is repeated 15 times, each taking about ten miles of airspace to perform.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Research/Experience_weightlessness_on_board_the_Zero-G_Airbus&quot;&gt;ESA&lt;/a&gt; uses the same approach. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;50 seconds is probably a mistake, you'd need a Concorde or be willing to go to much lower altitudes to get that much. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The '10 pilots' quote is highly suspect too. There's not enough room in the cockpit to fit 10 pilots. There's maybe one observer seat you could use. There's no way the operator would allow anyone to stand in the cockpit during 0 G maneuvers, someone flailing around would be too big a risk for the cockpit instruments.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-04-11T13:16:23.723" LastActivityDate="2016-04-11T13:16:23.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14759" PostTypeId="1" AcceptedAnswerId="14760" CreationDate="2016-04-11T13:46:38.487" Score="3" ViewCount="1285" Body="&lt;p&gt;I've read a gravity capture of an object requires the captured object to lose velocity by some means (impact, third body tidal force, etc).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there not a possible encounter that happens at a low enough speed to be below escape velocity?   Even for objects on nearly identical orbits?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="25" LastEditDate="2016-04-11T13:47:51.957" LastActivityDate="2018-12-10T18:36:59.057" Title="Why does a gravity capture require a loss of velocity?" Tags="|orbital-mechanics|planetary-science|gravity|asteroid|capture|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14760" PostTypeId="2" ParentId="14759" CreationDate="2016-04-11T13:49:17.450" Score="4" Body="&lt;p&gt;No, actually. Imagine that you launched a rocket straight up, to the point where it wasn't moving at all just where the Earth's gravity was essentially 0. Dropping it, it would gain the Escape Velocity of Earth, and in fact it would always have that escape velocity. It would have to somehow slow down for it to not eventually leave Earth's gravitational field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, a rocket that goes straight up and just barely hits escape velocity will slowly hit 0 velocity (Asymptotically) If when it was approaching that 0, something nudged it back, that would be the slowest an object could intersect, which would be just over escape velocity.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-11T14:33:05.177" LastActivityDate="2016-04-11T14:33:05.177" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14761" PostTypeId="1" CreationDate="2016-04-11T14:16:05.577" Score="2" ViewCount="93" Body="&lt;p&gt;I am trying to create a graphic with the sun, moon, and earth in the same frame as if looking from the end of the Apex node, assuming a 100,000 km tether, but with the correct sizes, placement, axis angle and color.  Something like this.....&#xA;&lt;a href=&quot;https://i.stack.imgur.com/p8ks1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p8ks1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This graphic has issues....&lt;br&gt;&#xA;1) It is not from the point of view of the Apex node at 100,000.&lt;br&gt;&#xA;2) I have no idea if there is ever a time in the orbits of the earth, moon and sun where they would close enough to preserve the scale.&lt;br&gt;&#xA;3) I have no idea if the colors are correct.&lt;br&gt;&#xA;4) In the real world the tether would loose visibility quickly as it get farther away from the view point.  But how soon? 100km out?&lt;br&gt;&#xA;5) It is not to scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question 1) Where would a non-expert go to get answers?&lt;br&gt;&#xA;Question 2) Is there software that can do this?&lt;br&gt;&lt;/p&gt;&#xA;" OwnerUserId="6645" LastEditorUserId="4660" LastEditDate="2016-04-18T14:39:15.990" LastActivityDate="2016-04-18T14:39:15.990" Title="Getting the scale, placement and color correct" Tags="|simulation|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14762" PostTypeId="2" ParentId="14745" CreationDate="2016-04-11T15:44:50.357" Score="12" Body="&lt;p&gt;The fairings cost several million dollars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the &lt;a href=&quot;https://www.youtube.com/watch?v=Nz60GcmKOvc#t=33m28s&quot; rel=&quot;nofollow noreferrer&quot;&gt;post launch briefing of CRS-8&lt;/a&gt;, Elon Musk mentioned that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A few more things we want to do...  we want to try to bring back the&#xA;  fairing - the big nose come back.  That will certainly help because&#xA;  each of those cost several millions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Midair helicopter capture would be tricky.  Is there such a system currently in use for any use case?  Only thing I could find with a quick search is &lt;a href=&quot;http://www.criticalpast.com/video/65675067712_CH-53-Sea-Stallion_BQM-34-B-Firebee_parachute-deployed_drone-descends-in-air&quot; rel=&quot;nofollow noreferrer&quot;&gt;this system&lt;/a&gt; developed in the 70s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Correcting myself.  There is &lt;a href=&quot;http://genesismission.jpl.nasa.gov/gm2/mission/mid_air.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;a system developed by NASA&lt;/a&gt; for space probe recovery. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also came across &lt;a href=&quot;https://www.youtube.com/watch?v=I8TsdYxugn0&quot; rel=&quot;nofollow noreferrer&quot;&gt;a youtube video&lt;/a&gt; which is obviously unofficial and must be based on the reddit post mentioned in the reply by Hobbes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing I disagree with that picture is that the parachute has to be &lt;a href=&quot;https://en.wikipedia.org/wiki/Parafoil&quot; rel=&quot;nofollow noreferrer&quot;&gt;parafoil&lt;/a&gt; rather than round parachute.  Capturing something coming down straight is quite difficult and dangerous.  It has to be under parafoil with good forward speed so the helicopter can come up from behind, match the glide and then capture with more ease. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: Shotwell &lt;a href=&quot;http://www.spaceflightinsider.com/organizations/space-exploration-technologies/shotwell-cabana-discuss-spacexs-future/&quot; rel=&quot;nofollow noreferrer&quot;&gt;mentioned&lt;/a&gt; fairing recovery on 2/18/2017: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Though confirming that the Falcon upper stage would not be reused,&#xA;  Shotwell was optimistic about a redesign for reuse of the payload&#xA;  fairing.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;“We’re working on that,” she said. “It’s such a shame. This hardware&#xA;  is so expensive, you want to save everything.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="13988" LastEditorUserId="13988" LastEditDate="2018-02-19T14:20:28.320" LastActivityDate="2018-02-19T14:20:28.320" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14763" PostTypeId="2" ParentId="14759" CreationDate="2016-04-11T17:18:50.620" Score="4" Body="&lt;p&gt;The speed of something in a hyperbolic orbit is $\sqrt{V_{esc}^{2} + V_{inf}^{2}}$ where $V_{esc}$ is escape velocity and $V_{inf}$ is the object's speed when it's an infinite distance from the gravitating body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RfeFf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RfeFf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At infinity incoming and outgoing speeds are both the same on the arms of a hyperbola.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you drop $V_{inf}$ to zero, you have a parabola. In which case &#xA; $\sqrt{V_{esc}^{2} + 0^{2}}$ reduces to simply&#xA; $V_{esc}$. At every point of a parabolic orbit, the object is traveling escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The parabola is sort of a boundary conic between hyperbolas and ellipses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OkHt9.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OkHt9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be in an elliptical orbit (in other words, a capture orbit), the velocity must be less than $V_{esc}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Illustrations are from my coloring book &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1936037106&quot; rel=&quot;nofollow noreferrer&quot;&gt;Conic Sections and Celestial Mechanics&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-04-11T17:18:50.620" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14764" PostTypeId="1" AcceptedAnswerId="14765" CreationDate="2016-04-11T18:33:56.787" Score="10" ViewCount="843" Body="&lt;p&gt;Reading about the North Korean space program, and its recent launch of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kwangmy%C5%8Fngs%C5%8Fng-4&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kwangmyŏngsŏng-4&lt;/a&gt; satellite, I stumbled upon this logo:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YKuGY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YKuGY.jpg&quot; alt=&quot;North Korea Space Program Logo, saying &amp;#39;NADA&amp;#39;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is referenced in quite a few news stories, like &lt;a href=&quot;http://www.collectspace.com/news/news-040214a-north-korea-nada-space-agency.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; or &lt;a href=&quot;http://www.cnet.com/news/north-korea-names-space-agency-nada-mimics-nasa-logo/#!&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, or &lt;a href=&quot;http://www.foxnews.com/science/2014/04/03/north-korea-nada-space-agency-logo-are-anything-but-nothing.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am, however, a little in doubt about this one. The main reason is the acronym &quot;NADA&quot; (National Aerospace Development Administration). That really does sound like a pun, meaning &quot;nothing&quot; in Spanish, and the word is used about the same concept in several other languages. Also, the agency is much more often referred to as &quot;Korean Committee of Space Technology&quot; (KCST), so why the sudden change of name? However, some sites, like &lt;a href=&quot;http://www.northkoreatech.org/tag/national-aerospace-development-administration/&quot; rel=&quot;nofollow noreferrer&quot;&gt;North Korea Tech&lt;/a&gt; uses that name, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/National_Aerospace_Development_Administration&quot; rel=&quot;nofollow noreferrer&quot;&gt;wiki page&lt;/a&gt; exists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Honestly, I can not tell if this is a joke or not, as you never know when talking about North Korea. &lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="26446" LastEditDate="2020-09-23T04:08:12.427" LastActivityDate="2020-09-23T04:08:12.427" Title="Is this really the logo of the North Korean space program?" Tags="|terminology|reference-request|north-korea|insignia|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14765" PostTypeId="2" ParentId="14764" CreationDate="2016-04-11T20:23:47.227" Score="7" Body="&lt;p&gt;Among others, the Wikipedia page cites &lt;a href=&quot;http://www.cnet.com/news/north-korea-names-space-agency-nada-mimics-nasa-logo/&quot; rel=&quot;noreferrer&quot;&gt;this CNET article&lt;/a&gt;, which in turn links to a copy of the original KCNA statement; this is also linked from the WP page. It's now offline, but &lt;a href=&quot;https://web.archive.org/web/20140406054614/http://www.kcna.co.jp/item/2014/201403/news31/20140331-24ee.html&quot; rel=&quot;noreferrer&quot;&gt;here is an archived copy&lt;/a&gt;. No image (possibly didn't archive) but the text confirms a) the name and b) the logo.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The National Aerospace Development Administration (NADA) is the country's central guidance institution organizing all the space development projects.&lt;/p&gt;&#xA;&lt;p&gt;Its mission is to put into practice the idea and principle of the DPRK government to develop the space for peaceful purpose.&lt;/p&gt;&#xA;&lt;p&gt;The emblem of the NADA was recently instituted, which represents its character, mission, position and development prospects.&lt;/p&gt;&#xA;&lt;p&gt;Seen in the lower part of the globe-shaped dark blue emblem are white-colored letters &amp;quot;Kukgaujugaebalkuk&amp;quot; (National Aerospace Development Administration) in Korean and in its upper part light blue-colored letters &amp;quot;DPRK&amp;quot; with the Great Bear above them. Printed in its middle are white-colored letters &amp;quot;NADA&amp;quot; in English.&lt;/p&gt;&#xA;&lt;p&gt;Two light blue-colored rings intercrossing the emblem symbolize satellite orbits.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;KCST seems to have been the previous body which carried out space roles, now closed and rolled into NADA.&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-04-11T20:23:47.227" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14768" PostTypeId="1" AcceptedAnswerId="14778" CreationDate="2016-04-11T22:34:42.323" Score="6" ViewCount="2324" Body="&lt;p&gt;How effective can gravity assists from the Moon be at getting a vehicle from near earth to Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this question, let's assume that a transit time of up to ten years is acceptable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say we've put together a vehicle at Earth-Moon L1.  Could we get to Mars with minimal delta-V by entering an elliptical orbit retrograde vs. the moon, performing a gravity assist resulting in a larger ellipse which returns near the moon also in retrograde, performing another gravity assist, and repeat until sufficient velocity is established to reach Earth escape?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, if we don't have enough to reach a Mars orbit, could a gravity assist with Earth, Mars, or Venus get the vehicle out to Mars where aerobraking could be used to capture the vehicle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much savings is possible this way compared to a simple Hohmann transfer?&lt;/p&gt;&#xA;" OwnerUserId="11479" LastActivityDate="2016-04-12T12:21:50.937" Title="Multiple Gravity Assist from Moon to reach Mars" Tags="|orbital-mechanics|mars|the-moon|orbit|gravity-assist|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14769" PostTypeId="1" CreationDate="2016-04-12T04:03:50.133" Score="2" ViewCount="2443" Body="&lt;p&gt;Given the dangers of the massive heat generated during re-entry, why can't spacecraft simply slow down using thrusters (or whatever) and just come in gently?&lt;/p&gt;&#xA;" OwnerUserId="14075" LastActivityDate="2016-04-12T06:25:01.013" Title="Why can't spacecraft slow down before entering Earth's atmosphere?" Tags="|reentry|" AnswerCount="1" CommentCount="1" ClosedDate="2016-04-12T09:11:02.920" ContentLicense="CC BY-SA 3.0" />
  <row Id="14770" PostTypeId="5" CreationDate="2016-04-12T06:12:01.960" Score="0" Body="&lt;p&gt;North Korea is the 10th country to manage to launch a satellite, the Kwangmyŏngsŏng-3. The current name of the DPRK space agency is NADA (Nation Aerospace Development Administration).&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-12T06:12:01.960" LastActivityDate="2016-04-12T06:12:01.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14771" PostTypeId="4" CreationDate="2016-04-12T06:12:01.960" Score="0" Body="Questions related to the North Korean space program." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-12T06:12:01.960" LastActivityDate="2016-04-12T06:12:01.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14772" PostTypeId="2" ParentId="14769" CreationDate="2016-04-12T06:25:01.013" Score="3" Body="&lt;p&gt;Because &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;propellant is extremely costly&lt;/a&gt; in orbit (as of now). As long as you have to bring up every gram from earths gravity well, you &lt;strong&gt;want&lt;/strong&gt; your tanks to be as empty as possible before re-entry. Aerobraking is currently the only way to get rid of speed without relying on costly propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, things might change if propellant would already exist in-orbit and was relatively cheap to acquire. In that case it might be more sensible to re-fuel (re-propell?) before descent.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-04-12T06:25:01.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14773" PostTypeId="5" CreationDate="2016-04-12T06:40:33.340" Score="0" Body="&lt;p&gt;Kerbal Space Program, often abbreviated to KSP, is a simulator game, aimed to give a realistic representation of orbital mechanics and spacecraft design. It uses a patched conics approach to simulate orbits, and operates in a scaled down solar system, however a great variety of mods are available to change that behaviour.&lt;/p&gt;&#xA;&lt;p&gt;Game play consist of designing spacecraft from a set of parts, and launching them and navigate them through space. The game has no progressing storyline, and is mostly a sandbox, though there are modes where you visits different locations in order to gain &amp;quot;science&amp;quot;, which is used to get access to more parts. Playing with a budget and contracts is also possible.&lt;/p&gt;&#xA;&lt;p&gt;This game is relevant for this community, because it can often spark interesting questions about unusual kinds of space navigation, typically in the form of: &amp;quot;This is possible in KSP, can it be done in reality too?&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;For general game-related questions outside the scope of Space Exploration, you may want to ask on our sister site &lt;a href=&quot;https://gaming.stackexchange.com/questions/tagged/kerbal-space-program&quot;&gt;Arqade&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;xkcd reference: &lt;a href=&quot;https://xkcd.com/1244/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://xkcd.com/1244/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="43908" LastEditDate="2021-09-10T07:31:09.590" LastActivityDate="2021-09-10T07:31:09.590" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14774" PostTypeId="4" CreationDate="2016-04-12T06:40:33.340" Score="0" Body="Questions related to a popular space game. USE WITH CARE! Try to ask questions that can be applied to reality and not be game-specific.  Game-specific questions may be appropriate on the Gaming Stack Exchange site Arqade." OwnerUserId="8693" LastEditorUserId="43908" LastEditDate="2021-09-10T07:31:48.993" LastActivityDate="2021-09-10T07:31:48.993" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14775" PostTypeId="1" CreationDate="2016-04-12T09:15:53.110" Score="5" ViewCount="15221" Body="&lt;p&gt;I want to get an idea of how long the falcon 9 rocket accelerates and at what g level.  Ideally a plot of gs vs time.  I see various statistics like specific impulse and thrust and weight, is there a way to calculate average accel from the basic rocket data?  I suppose average thrust and weight gives you average accel...&lt;/p&gt;&#xA;" OwnerUserId="14077" LastEditorUserId="6816" LastEditDate="2017-09-30T23:00:40.583" LastActivityDate="2023-03-09T18:45:48.743" Title="Falcon 9 g-level/acceleration profile" Tags="|orbital-mechanics|spacex|falcon-9|orbital|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14776" PostTypeId="2" ParentId="5250" CreationDate="2016-04-12T10:59:49.603" Score="3" Body="&lt;p&gt;For the 4/8/2016 landing of the first stage, someone made a Google Map of the location of the barge: &lt;a href=&quot;https://www.google.com/maps/d/viewer?mid=zeQeb73wSTuU.kjrfK2rN8ICk&quot; rel=&quot;nofollow&quot;&gt;https://www.google.com/maps/d/viewer?mid=zeQeb73wSTuU.kjrfK2rN8ICk&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first point looks a little less than a day after the landing, so not sure if truly accurate. But I &lt;a href=&quot;http://www.movable-type.co.uk/scripts/latlong.html&quot; rel=&quot;nofollow&quot;&gt;calculated the distance&lt;/a&gt; between that and the Cape. I got &lt;strong&gt;221.4 km&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: but &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-8&quot; rel=&quot;nofollow&quot;&gt;Wikipedia is giving&lt;/a&gt; 300 km.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2016-04-12T10:59:49.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14777" PostTypeId="2" ParentId="5250" CreationDate="2016-04-12T11:14:34.370" Score="3" Body="&lt;p&gt;The locations for landing were some 600-700km from launch site for DSCOVR and SES-9 and those two did not do the boostback burn so they are as close to &amp;quot;free&amp;quot; flight as possible (the re-entry burn still sheds some velocity otherwise the stage would burn up).&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2021-08-11T09:58:21.407" LastActivityDate="2021-08-11T09:58:21.407" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14778" PostTypeId="2" ParentId="14768" CreationDate="2016-04-12T12:21:50.937" Score="11" Body="&lt;p&gt;It does not offer that much of a saving, as the $\Delta v$ needed for a Moon transfer is 3120 m/s, which is not a lot less than a Mars transfer at 3600 m/s. Timing the Moon to be in the right place may even give you a slightly less optimal launch window for the Mars transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, such manoeuvres are entirely possible. A single Moon flyby can save you a little over 100 m/s. But, you also have the possibility of a tipple Moon flyby:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Spend the first 3120 m/s of $\Delta v$ in LEO, to get a minimal Moon transfer orbit. Doing a flyby will not get you up to escape velocity, as the Moon can not bend the orbit 180 degrees (relative to the Moon). The new orbit can be targeted to have an orbital period of exactly 1 month.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) After one month spent in the new orbit, the spacecraft encounters the Moon at the same location, and completes the first manoeuvre. The target velocity is just at Earth's escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) At the border of the Earth's Hill sphere, perform a very small burn (some 10-20 m/s). Fall back towards the Earth, and do a little extra burn at periapsis. On the outbound leg, the last Moon flyby is performed, and the spacecraft does now have enough velocity to achieve a Mars transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all, saving approximately 300 m/s of those 480 m/s gives you a lot more trouble than it is worth, adding more things that can go wrong with the mission, and adding several extra months to the trip time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Out of pure theoretical obscurity, you can actually skip the periapsis burn at stage 3) of the tripple Moon flyby. The spacecraft does then still have a little $v_{inf}$ after escaping the system, but not enough for a Mars transfer. An &quot;Earth flyby&quot; is not possible in this context, as what we are trying to achieve is a higher Earth $v_{inf}$. However, we can take advantage of Earth not being just a planet, but a system including the Moon. Entering a two-body system, you can actually gain some $v_{inf}$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3X4Get.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3X4Get.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The synodic period to synch up with the Earth-Moon system is of course several years long, and one such flyby is not enough to gain the remaining velocity for a Mars transfer. So after a few decades, you have eventually gained all of the 480 m/s for &quot;free&quot;. (Easily lost in station keeping during the extremely long and precision demanding mission.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-12T12:21:50.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14779" PostTypeId="2" ParentId="14775" CreationDate="2016-04-12T12:40:17.403" Score="10" Body="&lt;p&gt;From the telemetry link mentioned by @jkavalik, someone posted an interesting graph&lt;a href=&quot;https://i.stack.imgur.com/LMEJV.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LMEJV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The blue line here is the acceleration of the rocket. It ramps up as fuel is consumed. There's a dip from about 55 to 85 seconds where I believe the engines are throttled back in anticipation of &quot;max Q&quot;, the point of highest aerodynamic stress. The acceleration then increases continuously until the first stage shuts down and separates. There's a few seconds of free fall before the second stage ignites, and it likewise ramps its acceleration up over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what the units are supposed to be; the data the creator was working from was given in kph rather than m/s. The Falcon 9 Manual says that full-weight payloads will take a maximum of 6g axial acceleration. so the peak of the blue line should be something under that, probably in the ballpark of 5g. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-04-12T12:40:17.403" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14780" PostTypeId="1" AcceptedAnswerId="14793" CreationDate="2016-04-12T14:20:05.137" Score="16" ViewCount="3796" Body="&lt;p&gt;According to &lt;a href=&quot;https://space.stackexchange.com/questions/14745/falcon-9-fairings-cost-and-reusability&quot;&gt;this question&lt;/a&gt;, a payload fairing costs 'several million' dollars.&lt;br&gt;&#xA;From the outside, the fairing seems like a simple structure: aluminium honeycomb sandwiched between 2 sheets of carbon fiber composite. What makes such a simple structure so expensive to build? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2018-06-17T22:19:42.593" Title="What makes a payload fairing so expensive?" Tags="|manufacturing|fairings|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14781" PostTypeId="1" CreationDate="2016-04-12T14:51:39.177" Score="4" ViewCount="302" Body="&lt;p&gt;Elon Musk discussed in a &lt;a href=&quot;http://spaceflightnow.com/2016/04/09/video-press-conference-following-falcon-launch-and-landing/&quot; rel=&quot;nofollow&quot;&gt;question/answer session&lt;/a&gt; after the recent Falcon 9 launch that interplanetary missions are likely to use a landing barge, due to high horizontal launch speeds required for such launches. However, for an interplanetary mission, it seems that firing straight up might be preferred, or at least near to it, thus increasing the speed of the rocket, but still essentially being vertical. High horizontal speed isn't necessarily needed for interplanetary missions, as they don't need to orbit Earth, however, a high horizontal speed isn't precluded. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is typically done for interplanetary missions as far as the launch profile goes, and will the Falcon 9 change this launch profile to make it easier to recover the rocket?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-12T15:53:32.900" LastActivityDate="2016-04-12T15:53:32.900" Title="What is the launch profile for interplanetary missions typically like?" Tags="|launch|rockets|falcon-9|trajectory|interplanetary|" AnswerCount="1" CommentCount="1" ClosedDate="2016-04-12T18:56:04.167" ContentLicense="CC BY-SA 3.0" />
  <row Id="14782" PostTypeId="2" ParentId="14719" CreationDate="2016-04-12T15:14:34.130" Score="1" Body="&lt;p&gt;According to &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt;, the total error budget is 30 m to have an acceptable landing. So that means at most, 15m error from dead center is acceptable to land perfectly, and that 3 rockets couldn't fit on the barge at the same time, side by side.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-12T18:06:54.600" LastActivityDate="2016-04-12T18:06:54.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14784" PostTypeId="2" ParentId="14781" CreationDate="2016-04-12T15:31:58.190" Score="1" Body="&lt;p&gt;Most interplanetary flights are put into a parking orbit first, not &quot;straight up&quot; direct ascents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, assuming a low energy Hohmann transfer is required, you'd want to depart Earth's orbit at a tangent. This would mean starting with a high horizontal velocity, compared to Earth's surface. So there wouldn't be anything to gain by leaving vertically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an alternative way of looking at this: a near-vertical launch doesn't require less velocity/energy to escape Earth. In fact it may need more, as you would lose the usual benefit of launching East. (Several hundred metres per second.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-04-12T15:31:58.190" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14785" PostTypeId="1" AcceptedAnswerId="16887" CreationDate="2016-04-12T16:12:46.203" Score="9" ViewCount="1352" Body="&lt;p&gt;For every Falcon 9 launch to date, the process has been:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each Merlin is test fired/acceptance fired at McGregor by themselves.&#xA;The entire stage is test fired at McGregor.&#xA;The entire stage is hot fired before launch.&#xA;Launch&#xA;Re-entry, 3 engines, then 1 engine, then 1 engine burn to land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the recovery of the F9 S1 from the CRS-8 mission, Elon Musk said at the post-launc press conference, (Not on shitelonsays.com yet to quote the transcript) they will take the stage to LC-39A and test fire it ten times. Now he was not clear if that was 10 hot fires, of a second or three each, or if it was a full 2 minute mission duration burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless, that is quite the number of firings. In the past we have heard numbers like 40 firings as the design limit for a Merlin. Has this changed? Will this be a limitation in reuse, if they need to do so many test burns, before launch, and before each re-lauch?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2019-07-01T17:56:38.790" Title="How many engine firings can a SpaceX Falcon handle?" Tags="|spacex|falcon-9|reuse|merlin-1d|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14786" PostTypeId="1" CreationDate="2016-04-12T17:02:10.287" Score="5" ViewCount="605" Body="&lt;p&gt;Every time we've seen a &lt;a href=&quot;https://www.instagram.com/p/_m645Yl8fu/&quot;&gt;falcon 9&lt;/a&gt; &lt;a href=&quot;https://www.instagram.com/p/BD_4mrtl8RT/&quot;&gt;land&lt;/a&gt; or &lt;a href=&quot;https://www.instagram.com/p/BAqcRazF8Y5/&quot;&gt;attempt to land&lt;/a&gt;, it's had a start change in coloration on the outside, just above the spaceX logo. It seems that the charring/exhaust smoke/residue suddenly stops and then reappears jaggedly roughly another third of the way up the rocket. Is this caused by a difference in material or some other effect?&lt;/p&gt;&#xA;" OwnerUserId="10856" LastActivityDate="2016-04-12T17:23:16.717" Title="Why is the Falcon 9 charred like this?" Tags="|spacex|landing|materials|falcon-9|" AnswerCount="1" CommentCount="0" ClosedDate="2016-04-12T19:59:18.503" ContentLicense="CC BY-SA 3.0" />
  <row Id="14787" PostTypeId="2" ParentId="14786" CreationDate="2016-04-12T17:15:13.627" Score="7" Body="&lt;p&gt;Realize that Kerosene (RP1) is not the cleanest burning fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, this stage has not just ascended up through the atmosphere, it has re-entered, taking in a fair bit of heat along the way. So some of that is likely burnt paint.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as it does this, it is flying tail first, through its own exhaust plume. This is mostly just burnt kerosene residue that comes from flying through 3, then 1, then 1 engines thrust (175 klbs of thrust each engine) which burns ALOT of kerosene when running.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see where the outlines of the legs go, still being pristine and white. They stay closed till about the last 8-10 seconds of flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, there is suggestions that the LOX tank forms ice on the outside, that persists and protects some of the paint till it melts or falls off. Which can account for some of the odd patterning.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-04-12T17:23:16.717" LastActivityDate="2016-04-12T17:23:16.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14788" PostTypeId="2" ParentId="8779" CreationDate="2016-04-12T17:31:10.053" Score="11" Body="&lt;p&gt;According to Musk at &lt;a href=&quot;https://www.youtube.com/watch?v=Nz60GcmKOvc#t=16m20s&quot; rel=&quot;noreferrer&quot;&gt;the post CRS-8 briefing&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Both (ship and rocket) is going to an absolute position.  So the ship is holding to the absolute GPS position, with relative GPS and today it was holding to the accuracy of below a meter.  It has four engines all of which can rotate 360 degrees and operating continuously to hold the position, to hold both attitude and position in the ocean.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;del&gt;I assume that means that there is no communication between the drone and the rocket and they simply meet at a set GPS location.&lt;/del&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I caught following during the &lt;a href=&quot;https://www.youtube.com/watch?v=zBYC4f79iXc#t=31m58s&quot; rel=&quot;noreferrer&quot;&gt;THAICOM 8 launch&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Recovery vessel has AOS.   AOS means Acquisition Of Signal.  That means that the recovery.. the drone ship has reached communication contact with the rocket.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So they do communicate with each other, and that seems to confirm the discussion below that the drone goes to absolute GPS location and that rocket goes to the relative GPS location of the drone by communicating with the drone. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So not sure if they do bother to have relative GPS communication with ground station, since ground doesn't move, rocket may go straight to absolute GPS coordinate.   &lt;/p&gt;&#xA;" OwnerUserId="13988" LastEditorUserId="13988" LastEditDate="2016-05-27T23:22:32.940" LastActivityDate="2016-05-27T23:22:32.940" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14789" PostTypeId="2" ParentId="6403" CreationDate="2016-04-12T17:43:22.310" Score="3" Body="&lt;p&gt;Musk &lt;a href=&quot;https://www.youtube.com/watch?v=Nz60GcmKOvc#t=17m20s&quot; rel=&quot;nofollow&quot;&gt;confirmed in the CRS-8 briefing&lt;/a&gt; that they are putting steel shoes over the landing feet and welding them down. &lt;/p&gt;&#xA;" OwnerUserId="13988" LastActivityDate="2016-04-12T17:43:22.310" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14790" PostTypeId="2" ParentId="14780" CreationDate="2016-04-12T17:52:04.953" Score="1" Body="&lt;p&gt;The structure needs to be designed and built to a standard where it is able to withstand the extreme heat and vibrational environment of the launch process, while protecting the internal payload as best possible. But still be able to separate quickly and efficiently once in orbit so it doesn't interfere with the trajectory of the spacecraft, thrusters are often used which require propellant which is expensive. And the structure can only be used once (so far).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bear in mind though that the total of a space mission usually ranges from hundreds of millions to a few billion dollars so just a few million doesn't make that much of a difference.&lt;/p&gt;&#xA;" OwnerUserId="14065" LastActivityDate="2016-04-12T17:52:04.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14791" PostTypeId="2" ParentId="14780" CreationDate="2016-04-12T18:22:50.653" Score="2" Body="&lt;p&gt;&lt;strong&gt;Fairings are hand crafted.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not in the sense of old tools used, but because the production totally lacks the mass production principle. Each and every fairing is pretty much unique, highly influenced in design by that particular payload. The structure is indeed just 'simple sandwiched aluminium honeycomb and carbon fibre', but as the forces and temperature loads on this structure are not insignificant, and a fairing payload will result in mission failure, you pretty much have to run custom tests and analysis on all the joints and areas on an unique design for every single mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also vital for the fairing to be able to separate from the launcher cleanly, without causing unexpected disturbing thudding on the rocket or the payload. Add that extra hardware, and the new battery of test and analysis to the list.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all, the costs required to make those highly specific fairings may easily cost several million dollars, and that is money you want to spend, as the cost is relatively small compared to the whole launcher, and getting it done right is essential. Better not screw up an o-ring or something.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-12T18:22:50.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14792" PostTypeId="2" ParentId="14371" CreationDate="2016-04-12T19:39:06.703" Score="5" Body="&lt;p&gt;Yes. Thermal expansion/contraction &lt;a href=&quot;https://www.fastcodesign.com/3056884/the-untold-story-of-the-iss-the-most-complex-structure-youll-never-visit&quot; rel=&quot;noreferrer&quot;&gt;does cause audible creaking.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For example, astronaut Nicole Stott explains that because of the changes in temperature on the surface of the modules, the spaces often echo with a startling creaking (&amp;quot;at first it is slightly alarming, but soon becomes familiar,&amp;quot; she writes).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.quora.com/Does-the-ISSs-structure-ever-creak-or-groan&quot; rel=&quot;noreferrer&quot;&gt;Astronaut Clayton Anderson&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Yes, the ISS makes noise all the time --those noises may include creaks, groans, hums, buzzes, beeps, clicks, squawks (from the various pumps, fans, switching valves and computers), and loud bangs (when spacewalking astronauts are crawling around on her outside, we hear it on the inside... it can be quite loud, but it indicates they are working and alive outside!)...&lt;/p&gt;&#xA;&lt;p&gt;Ambient noise inside the station is comparable to that experienced from the lower decks of an aircraft carrier; somewhere in the 50 dB range if my memory serves me correctly.  Essentially it is &amp;quot;white noise,&amp;quot; which can be offset if needed by foam earplugs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;On BBC Stargazing 2016 episode 4 around 36:10, Chris Hadfield describes he could hear impacts from micrometeoroids, if he was near enough the metal of the hull.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-04-12T19:39:06.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14793" PostTypeId="2" ParentId="14780" CreationDate="2016-04-12T19:53:21.877" Score="6" Body="&lt;p&gt;They are expensive because the manufacturing process is complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this video, there is a good section explaining how those structures are built: &lt;a href=&quot;https://www.youtube.com/watch?v=tZhH2B-EI1I#t=3m51s&quot; rel=&quot;noreferrer&quot;&gt;NASA 360 - Composite Materials&lt;/a&gt;.  Very complex and labor intensive process that can hardly be called &quot;simple&quot;.  I doubt they have invested in something like &lt;a href=&quot;https://www.youtube.com/watch?v=NJGOi_2J2bY&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; to highly automate the process. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I disagree with Hohmannfan that it is expensive because each is unique.  It seems to be rather standardized as described in &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;noreferrer&quot;&gt;Falcon 9 Launch Vehicle PAYLOAD USER’S GUIDE&lt;/a&gt;.  It describe what comes standard with the list prices and what kind of customization is available with extra cost.  I doubt SpaceX would accept a customer who require completely custom fairing to be built. &lt;/p&gt;&#xA;" OwnerUserId="13988" LastActivityDate="2016-04-12T19:53:21.877" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14794" PostTypeId="1" CreationDate="2016-04-12T20:52:37.953" Score="1" ViewCount="362" Body="&lt;p&gt;Simply put, would it make sense to place laser in space to propel a solar sail?  Thinking about solar sails, and the inefficiencies of sending a laser through the atmosphere, I wondered if it made sense to position the laser outside the atmosphere.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Power would be an issue, but since ideally we would not be firing this continuously (just while a craft was accelerating), we could collect solar energy and store it for use during the acceleration phase.  &lt;/p&gt;&#xA;" OwnerUserId="14087" LastActivityDate="2016-04-12T21:10:09.107" Title="laser propulsion from a lagrange point?" Tags="|propulsion|laser|" AnswerCount="1" CommentCount="1" ClosedDate="2016-04-13T11:15:25.123" ContentLicense="CC BY-SA 3.0" />
  <row Id="14795" PostTypeId="1" CreationDate="2016-04-12T20:55:10.447" Score="-2" ViewCount="386" Body="&lt;p&gt;I understand we may not have the technology for it as of now. But I have also heard it's because we don't have enough energy, and traveling at or near the distance of light speed will require an almost infinite amount. My question is this the only thing stop us from traveling at the speed of light?&lt;/p&gt;&#xA;" OwnerUserId="14088" LastEditorUserId="4660" LastEditDate="2016-04-13T00:57:31.220" LastActivityDate="2016-04-18T09:54:24.260" Title="Why can't we travel at the speed of light?" Tags="|deep-space|physics|spacecraft-development|" AnswerCount="2" CommentCount="4" ClosedDate="2016-04-13T13:54:36.137" ContentLicense="CC BY-SA 3.0" />
  <row Id="14796" PostTypeId="1" AcceptedAnswerId="14798" CreationDate="2016-04-12T21:09:52.817" Score="3" ViewCount="166" Body="&lt;p&gt;I've been reading over &lt;a href=&quot;http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf&quot; rel=&quot;nofollow&quot;&gt;NASA's Human Exploration of Mars Design Reference Architecture 5.0&lt;/a&gt;. In section 3.1.2, &quot;Scientific objectives for Mars: present and future&quot;, when discussing future science missions to Mars, the document mentions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Although other science missions will certainly&#xA;  be considered (most importantly, a &lt;strong&gt;network science mission&lt;/strong&gt;)... [Highlighting mine]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What exactly is meant by a &quot;network science mission&quot;? Googling turns up mentions of a potential ESA mission called INSPIRE, most notably described in &lt;a href=&quot;http://www.hou.usra.edu/meetings/ippw2014/pdf/8040.pdf&quot; rel=&quot;nofollow&quot;&gt;this paper&lt;/a&gt;, but I don't quite understand what is meant by network science. Is this referring to the ability to have multiple networked orbiters/landers/rovers on Mars? If so, how would this be different from the existing networks between the various rovers and Mars Odyssey/Mars Reconnaissance Orbiter/Mars Express?&lt;/p&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2016-04-12T22:38:51.460" Title="What is the Mars &quot;network science mission&quot; referred to in NASA's Mars Design Reference Architecture?" Tags="|mars|communication|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14797" PostTypeId="2" ParentId="14794" CreationDate="2016-04-12T21:10:09.107" Score="1" Body="&lt;p&gt;What I am concerned about is how you are planning to store that energy. Just for reference, 1kWh of energy is only going to push the spacecraft an extra 0.024 Ns. That is assuming perfect conversion to laser, the sail being perfectly reflecting, and all the beamed power hitting the sail head on. Storing the amount of energy required to do significant propulsion is difficult, your best options being chemical batteries and fuel cells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a sample &lt;a href=&quot;https://en.wikipedia.org/wiki/Energy_density&quot; rel=&quot;nofollow&quot;&gt;energy density&lt;/a&gt; for Lithium ion batteries of 1.8 MJ/kg, you are going to need 40 kg of batteries per kg of spacecraft you want to accelerate by 1 m/s. (In practice, at least 100 kg including some energy losses).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generating the energy on-the-fly may be easier, but there are still issues. If for example solar power is being used, you are still relying on the energy flux from the Sun like the solar sail, but are effectively losing 95% of the energy from converting it first to electricity, then back to light again. The laser station will in any case massively out weight the sail craft.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-12T21:10:09.107" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14798" PostTypeId="2" ParentId="14796" CreationDate="2016-04-12T21:40:26.923" Score="4" Body="&lt;p&gt;It refers to science objectives that are met by a large number of small, identical landers dispersed over the planet in latitude and longitude. These include seismological and weather observations, conducted over at least one Mars year. (&quot;Network&quot; doesn't mean that they communicate with each other, but rather simply that there are a lot of them.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, this was the original purpose of Mars Pathfinder. What that mission was intended to be the pathfinder &lt;em&gt;for&lt;/em&gt; was &lt;a href=&quot;http://commons.erau.edu/cgi/viewcontent.cgi?article=1740&amp;amp;context=space-congress-proceedings&quot; rel=&quot;nofollow&quot;&gt;&quot;MESUR&quot;, Mars Environmental Survey&lt;/a&gt;, planned to be a network of 16 low-cost landers on Mars. (It ended up instead being a pathfinder for the Mars Exploration Rovers.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-04-12T22:38:51.460" LastActivityDate="2016-04-12T22:38:51.460" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14799" PostTypeId="1" AcceptedAnswerId="14801" CreationDate="2016-04-13T00:23:41.503" Score="7" ViewCount="328" Body="&lt;p&gt;Re-entry vehicles tend to be either mono-stable (eg soyuz) or bi-stable (apollo) which refers to the orientations in which they're aerodynamically stable. &lt;a href=&quot;http://www.spaceflightnow.com/station/exp16/080502peggywhitson.html&quot;&gt;In at least one case, this has proved very useful&lt;/a&gt; on the soyuz where the capsule corrected orientation when guidance/control systems failed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In how many modes are the SpaceX Dragon V1 and V2 stable?&lt;/p&gt;&#xA;" OwnerUserId="10856" LastActivityDate="2016-04-13T01:28:22.770" Title="Is the Dragon Mono-Stable?" Tags="|spacex|reentry|dragon|dragon-v2|aerodynamics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14800" PostTypeId="2" ParentId="14795" CreationDate="2016-04-13T00:26:37.287" Score="5" Body="&lt;p&gt;We simply have to look at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kinetic_energy#Relativistic_kinetic_energy_of_rigid_bodies&quot;&gt;formula relativistic kinetic energy&lt;/a&gt; to see why we can not travel at the speed of light:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_k = \frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}-mc^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $m$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Invariant_mass&quot;&gt;rest mass&lt;/a&gt;, $v$ is the velocity, and $c$ the speed of light. If we try to set $v=c$, we get the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_k = \frac{mc^2}{0}-mc^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can not divide by zero. Nope.&lt;br&gt;&#xA;It is not just an &quot;almost infinite amount&quot; of energy, it &lt;strong&gt;&lt;em&gt;is&lt;/em&gt;&lt;/strong&gt; infinite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note: Things like photons that travels at the speed of light elegantly avoids this problem by not having any mass at all.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-13T00:26:37.287" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14801" PostTypeId="2" ParentId="14799" CreationDate="2016-04-13T01:28:22.770" Score="7" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/a/9063/25&quot;&gt;this answer to this question&lt;/a&gt;, we can see that there are two stability modes used. The capsule by itself is mono-stable, with the heat shield pointing towards the ground. The second mode is with the trunk included, and wings extended, which is stable with the nose in the direction of movement. The second method is used in rocket abort systems, keeping the spacecraft moving with the nosecone in the direction of motion.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-04-13T01:28:22.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14802" PostTypeId="2" ParentId="8771" CreationDate="2016-04-13T02:45:32.877" Score="1" Body="&lt;p&gt;Using &lt;a href=&quot;https://en.wikipedia.org/wiki/Spherical_cow&quot; rel=&quot;nofollow noreferrer&quot;&gt;spherical cow&lt;/a&gt; physics, ignoring momentum and sloshing (because the time scales are so different) I just looked at where the cm is for an empty F9 1st stage of uniform density, and one with 5% remaining propellants.  I get static tip-over angles of 15° and 18°, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional remaining propellants bring the center of mass even lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Counting localized masses like bulkheads and engines would further lower the center of mass.  See &lt;a href=&quot;https://space.stackexchange.com/a/9199/12102&quot;&gt;this answer&lt;/a&gt; above!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's not inconsistent with &lt;a href=&quot;https://youtu.be/Nz60GcmKOvc?t=890&quot; rel=&quot;nofollow noreferrer&quot;&gt;Musk's comments&lt;/a&gt; that 8-9° would be possible, which probably includes various non-spherical-horse factors like dynamics, leg strength (especially if there turned out to be &lt;em&gt;more fuel than expected&lt;/em&gt;) and others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NOTE: All numbers are the ones I &lt;em&gt;used&lt;/em&gt; for this quick calculation - they are not necessarily &lt;em&gt;correct&lt;/em&gt; so don't use them!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/o6ylP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/o6ylP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-07-19T00:11:07.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14803" PostTypeId="5" CreationDate="2016-04-13T08:42:32.763" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:42:32.763" LastActivityDate="2016-04-13T08:42:32.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14804" PostTypeId="4" CreationDate="2016-04-13T08:42:32.763" Score="0" Body="Questions regarding the hardware, development, use and cost of rocket fairings and protective shells." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:42:32.763" LastActivityDate="2016-04-13T08:42:32.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14805" PostTypeId="5" CreationDate="2016-04-13T08:44:13.600" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:44:13.600" LastActivityDate="2016-04-13T08:44:13.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14806" PostTypeId="4" CreationDate="2016-04-13T08:44:13.600" Score="0" Body="Questions regarding the application of chemistry in space exploration, for example the composition of rocket propellants." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:44:13.600" LastActivityDate="2016-04-13T08:44:13.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14807" PostTypeId="5" CreationDate="2016-04-13T08:45:38.657" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:45:38.657" LastActivityDate="2016-04-13T08:45:38.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14808" PostTypeId="4" CreationDate="2016-04-13T08:45:38.657" Score="0" Body="Questions about the SpaceX produced Merlin-1D engine, used by the Falcon rocket." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:45:38.657" LastActivityDate="2016-04-13T08:45:38.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14809" PostTypeId="5" CreationDate="2016-04-13T08:47:00.630" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:47:00.630" LastActivityDate="2016-04-13T08:47:00.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14810" PostTypeId="4" CreationDate="2016-04-13T08:47:00.630" Score="0" Body="Questions about the methodology and results of collecting data." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:47:00.630" LastActivityDate="2016-04-13T08:47:00.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14811" PostTypeId="5" CreationDate="2016-04-13T08:50:51.510" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:50:51.510" LastActivityDate="2016-04-13T08:50:51.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14812" PostTypeId="4" CreationDate="2016-04-13T08:50:51.510" Score="0" Body="Questions regarding rocket propellant in liquid form, the main type used for chemical rockets." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-13T08:50:51.510" LastActivityDate="2016-04-13T08:50:51.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14813" PostTypeId="1" AcceptedAnswerId="14815" CreationDate="2016-04-13T10:22:35.117" Score="43" ViewCount="4663" Body="&lt;p&gt;Watching the CRS-8 Berthing live broadcast on &lt;a href=&quot;https://www.nasa.gov/multimedia/nasatv/index.html&quot; rel=&quot;noreferrer&quot;&gt;NASA TV&lt;/a&gt; I noticed a baseball bat resting on top of a console in the front left part of the room, so I took some screenshots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking closer, it seems to be about a meter or two away from a large, delicate-looking model of the International Space Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this part of NASA internal humor?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jEyYG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jEyYG.jpg&quot; alt=&quot;NASA CRS-8 berthing baseball bat larger&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1JOgr.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1JOgr.png&quot; alt=&quot;NASA CRS-8 berthing baseball bat full screen&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2019-04-08T11:49:19.183" Title="What's the baseball bat for? (seen in CRS-8 berthing NASA TV broadcast)" Tags="|iss|nasa|berthing|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14814" PostTypeId="2" ParentId="14795" CreationDate="2016-04-13T10:28:27.310" Score="3" Body="&lt;p&gt;Hohmannfan neatly answered the &quot;AT&quot; speed of light part. Let me bite the &quot;Near&quot; one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's pick a target speed of 225,000 kilometers per second, or 0.75c. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a kinda neat number which I like to use because had Newton been right and Einstein wrong, the same kinetic energy of a body moving currently at 0.75c, would give it just a notch above 1c in Newtonian mechanics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's build an ideal rocket engine, that uses matter-antimatter annihilation (the absolutely most energetic reaction currently known to man, and unlikely to be topped by any other, ever.) and ejects the reaction mass (or in this particular case &lt;em&gt;reaction energy&lt;/em&gt; at speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Relativistic_rocket#Specific_impulse&quot; rel=&quot;nofollow&quot;&gt;Specific Impulse&lt;/a&gt; would be c.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(all units are off by the $g0$ factor, a constant of 9.8m/s^2 of Earth's gravitational acceleration; it doesn't really matter here as we obtain a dimensionless factor in the end anyway)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same article gives relativistic $\Delta v = c \cdot \tanh \left(\frac {I_{sp}}{c} \ln \frac{m_0}{m_1} \right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's get the mass fraction, $M = \frac{m_0}{m_1}$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$M = e^{{c \over I_{sp} }{\tanh^{-1}( {\Delta v \over c} )}} $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${c/I_{sp}}$ is conveniently 1, and $0.75c \over c$ is 0.75. So, $M = e^{\tanh^{-1}(0.75)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our mass fraction comes down to about $m_0 = 2.65 m_1$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means for 1 ton of rocket, you need 1.65 ton of propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's &lt;em&gt;very&lt;/em&gt; good given rocket standards. For chemical rockets this comes up as something of order of 95-98 tons of propellant for a ton of cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And still, that means a rocket of 1 ton (a very conservative number) which contains everything, including generators of magnetic field to contain the antimatter, and the annihilation-engine, on top of whatever else it needs (guidance? RCS? Any actual payload?) needs 825kg of propellant matter, and another 825kg of propellant antimatter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current estimate is \$1bln per 40 milligrams of positrons&lt;a href=&quot;https://en.wikipedia.org/wiki/Antimatter#Cost&quot; rel=&quot;nofollow&quot;&gt;*&lt;/a&gt;. So, for the needed 825kg... \$20,625,000,000,000,000 or 20 quadrillion dollars? The world economy is about two orders of magnitude short; if you sold everything in the world that can be sold, you'd still come up way short.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Never mind the engine will NOT be 100% efficient. Never mind we have absolutely no clue how to store such amounts of antimatter, especially in anything that weights less than the stored antimatter (or even less than a million times more than the stored antimatter...).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's the ONLY engine that can get us near relativistic speeds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take the next big candidate: Nuclear fusion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same source as before, ISp = 0.119c. Again, but ${c \over I_{sp}}$ is no longer 1. It is now 8.4.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$M = e^{{c \over I_{sp} }{\tanh^{-1}( {\Delta v \over c} )}} = e^{{c \over 0.119c }{\tanh^{-1}( 0.75 )}} = e^{8.4 \cdot 0.973} = e^{8.17} = 3533$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And so, with 8.17 instead of 0.97 we had before, our neat exponent blows up to over 3500. We are below a 0.03% dry mass fraction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means for every ton of equipment, fusion reactor, engine that exploits the products of fusion, even wall of a tank - you need to pack 3500 tons of propellant. Can you even store 3500 tons of hydrogen in a tank that weighs less than a ton?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's not something that scientists and engineers can improve upon. That's the end of the project, a simple, flat &quot;impossible&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what is known as &quot;a victim of the tyranny of the rocket equation&quot;, a design that is completely and irrevocably impossible from practical point of view, because we don't foresee any storage technology of hydrogen that would give us a 1:3500 weight ratio between the tank and the stored hydrogen, never mind giving any wiggle room for the actual payload, engines, reactor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be easier to design an annihilation-based rocket from scratch, than one that would keep this mass ratio viable - never mind developing nuclear fusion technology that squeezes every last possible joule off the fusion, without losses. And the losses would likely drive our payload fraction another order of magnitude down.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-04-18T09:54:24.260" LastActivityDate="2016-04-18T09:54:24.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14815" PostTypeId="2" ParentId="14813" CreationDate="2016-04-13T12:16:57.153" Score="57" Body="&lt;p&gt;The baseball bat is for &quot;Attitude adjustment&quot;, apparently :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://cdn.arstechnica.net/wp-content/uploads/2013/06/topo-adco.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://arstechnica.com/science/2013/07/how-nasa-steers-the-international-space-station-around-space-junk/&quot; rel=&quot;noreferrer&quot;&gt;Arstechnica photos of ISS control room&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(ADCO=Attitude Determination and Control officer)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a better pic of the bat from the Twitter:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Dmbxo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Dmbxo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.793" LastActivityDate="2016-04-13T19:06:57.533" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="14816" PostTypeId="1" CreationDate="2016-04-13T15:06:05.197" Score="3" ViewCount="153" Body="&lt;p&gt;I know that a lot of brilliant engineering has gone into developing the space technology that we have. So, what engineering lessons about dealing with extreme cold and portable power sources do you think we can take from space exploration and apply to an Antarctic context?&lt;/p&gt;&#xA;" OwnerUserId="14097" LastEditorUserId="14097" LastEditDate="2016-04-14T13:15:25.610" LastActivityDate="2016-05-08T14:12:54.080" Title="Lessons Learned - What can space power system design teach Antarctic remote sensing systems" Tags="|spacecraft|design|technology|power|engineering|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14817" PostTypeId="2" ParentId="8771" CreationDate="2016-04-13T16:24:44.897" Score="1" Body="&lt;p&gt;&lt;a href=&quot;https://spaceflightnow.com/2016/04/09/video-press-conference-following-falcon-launch-and-landing/&quot; rel=&quot;nofollow&quot;&gt;Elon Musk&lt;/a&gt; gave some interesting information about it relating to the CRS-8 booster landing. When it was on the ground, it suffered 50 miles per hour winds, which tilted it about 2-3 degrees. He said that the maximum tilt the booster could support would be about 3 times that, so somewhere between 6-9 degrees. Presumably it could also take more than 50 miles per hour winds to do so as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next step is to weld special containers around the feet to the drone ship, known as &lt;a href=&quot;http://www.inc.com/oscar-raymundo/elon-musk-to-unveil-spacex-mars-ship-spacesuit-2015.html&quot; rel=&quot;nofollow&quot;&gt;&quot;Steel Shoes&quot;&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-13T16:24:44.897" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14818" PostTypeId="1" CreationDate="2016-04-13T18:26:20.337" Score="5" ViewCount="848" Body="&lt;p&gt;This could be done by leading tapped-off exhaust gases through heat-exchangers in the tanks (being careful that the exhaust gases don't freeze and block the pipe). &#xA;I'm imagining this as an alternative to carrying separate pressurization gases in pressure-fed engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will clarify a bit. I'm aware both of expander cycles, and of leading heated propellants back into the tank. This is similar, but I'm interested in the specific case of leading exhaust gases through fuel tanks. I couldn't find any previous description of this, so that's why I'm asking. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being more specific, there are at least two options:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;In a first variant, some amount of exhaust gas is tapped from the combustion chamber and driven through heat exchangers in the propellant tanks. The cooled gas could be injected along the nozzle walls to protect them (ala F-1), or otherwise expelled.&lt;/li&gt;&#xA;&lt;li&gt;In a second variant, one could have the combustion chamber at the top of the rocket, and lead the exhaust gas through the propellant tanks, before expelling it through a diverging nozzle at the bottom.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I expect that there is there some reason for a considerable loss of energy here compared to other (turbomachinery-free) engine cycles, but I am too inexperienced to realize where. Hopefully someone thinks this is interesting and can offer an educational take on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT 2: Tightened the wording in the heading.&lt;/p&gt;&#xA;" OwnerUserId="13956" LastEditorUserId="13956" LastEditDate="2016-04-13T22:11:27.660" LastActivityDate="2020-06-27T14:03:54.863" Title="Is it possible to keep fuel and oxidizer tanks pressurized during flight by heating up their contents using exhaust gases?" Tags="|engine-design|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14820" PostTypeId="1" CreationDate="2016-04-13T19:54:55.783" Score="3" ViewCount="2172" Body="&lt;p&gt;I understand that underexpanded nozzles lose efficiency, due to untapped potential in the gas before exiting the nozzle, but what happens to overexpanded nozzles? I understand that the pressure of the gas is below the pressure of the ambient atmosphere, but what does this do to the overall engine performance?&lt;/p&gt;&#xA;" OwnerUserId="14100" LastEditorUserId="6816" LastEditDate="2016-04-13T20:02:56.533" LastActivityDate="2016-04-21T04:20:32.037" Title="Can someone tell me the effect on performance of overexpanded nozzles?" Tags="|rockets|engine-design|nozzle|performance|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14822" PostTypeId="2" ParentId="14730" CreationDate="2016-04-13T20:46:32.793" Score="8" Body="&lt;p&gt;Not without a lot of error.  The elements in the TLEs are &quot;mean elements&quot;, not &quot;osculating elements&quot;.  i.e. although they have the same names as the Keplerian elements, they are not really measuring the same things.  What you can do is evaluate the TLE at epoch using SGP4, and take the resulting Cartesian state vector and transform that via the usual closed-form equations into Keplerian elements.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2016-04-13T20:46:32.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14823" PostTypeId="2" ParentId="4826" CreationDate="2016-04-13T20:55:39.663" Score="-4" Body="&lt;p&gt;What most of you seem to miss is the ssto advantage of useing extremely efficient air breathing jet engines to get to upper atmosphere and loads of horizontal speed, so it takes only a small fraction of expensive rocket fuel to boost it up to lower orbit. After all its the fuel that costs most i belive.&lt;/p&gt;&#xA;" OwnerUserId="14102" LastActivityDate="2016-04-13T20:55:39.663" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14824" PostTypeId="1" CreationDate="2016-04-13T21:52:02.910" Score="3" ViewCount="1151" Body="&lt;p&gt;Which is the range of thrust (minimum and maximum) that a nuclear thermal rocket is estimated to produce?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Knowing these values, which thrust to weight ratio would be able to produce?&lt;/p&gt;&#xA;" OwnerUserId="13854" LastActivityDate="2016-04-13T22:31:10.317" Title="How much thrust can a nuclear thermal rocket provide?" Tags="|rockets|propulsion|nuclear|ntr|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14825" PostTypeId="2" ParentId="12869" CreationDate="2016-04-13T22:23:34.880" Score="3" Body="&lt;p&gt;I believe you are describing the &lt;a href=&quot;https://en.wikipedia.org/wiki/Combustion_tap-off_cycle&quot; rel=&quot;nofollow&quot;&gt;combustion tap-off cycle&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The J-2S engine from the Apollo era did this (but was never used), as well as the BE-3 mentioned earlier.&lt;/p&gt;&#xA;" OwnerUserId="13956" LastEditorUserId="6816" LastEditDate="2016-04-13T23:00:58.240" LastActivityDate="2016-04-13T23:00:58.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14826" PostTypeId="2" ParentId="14824" CreationDate="2016-04-13T22:24:48.913" Score="8" Body="&lt;p&gt;The American &lt;a href=&quot;https://en.wikipedia.org/wiki/NERVA&quot;&gt;NERVA&lt;/a&gt; program developed a design expected to produce up to 333kN (75klbf) of thrust in a 6.8 ton package, for a thrust to weight ratio of about 5:1. It was never completed or flown. NERVA 2 was supposed to produce 867kN (195klbf) from 11.9 tons, a thrust to weight ratio of 7.5:1. Those are figures for thrust in vacuum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian &lt;a href=&quot;http://www.astronautix.com/engines/rd0410.htm&quot;&gt;RD-0410&lt;/a&gt; was similar in concept, but smaller, producing 35kN and weighing 2 tons, a 1.8:1 TWR. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those TWRs are for the engine itself, not including any fuel tankage. Generally these engines weren't considered for first-stage launch from Earth's surface, both because of environmental concerns and because their TWR is fairly low; many such engines would need to be clustered to achieve 1.2 TWR off the pad with any substantial fuel tankage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe nuclear thermal rockets are simpler to throttle than bipropellant chemical rockets. I don't know how far down they can throttle but I would guess they can go to quite low thrust values with good specific impulse across their whole range. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-04-13T22:31:10.317" LastActivityDate="2016-04-13T22:31:10.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14827" PostTypeId="2" ParentId="14820" CreationDate="2016-04-13T22:42:01.747" Score="4" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.aerospaceweb.org/question/propulsion/q0220.shtml&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt;, in an overexpanded nozzle, the loss of efficiency is caused by the &quot;pinching&quot; of the exhaust plume by the ambient air pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In grossly overexpanded nozzles, there's another, more serious problem, where the exhaust flow separates from one side of the nozzle, adhering to the opposite side, which causes very uneven heating and wear on the nozzle, and directs the thrust off the central axis of the engine.  &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-04-13T22:42:01.747" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14828" PostTypeId="1" AcceptedAnswerId="14831" CreationDate="2016-04-13T22:51:16.520" Score="5" ViewCount="392" Body="&lt;p&gt;We've seen a lot lately about light sails, but &lt;a href=&quot;http://www.gizmag.com/nasa-technology-sail-deep-space/42760/&quot; rel=&quot;noreferrer&quot;&gt;this article talks about an &quot;e-sail&quot; that gets pushed along by radio waves&lt;/a&gt;.  The article says that the idea is for a &quot;sail&quot; that is an electric field generated by the spacecraft itself.  Author Michael Flynn talked about his concept in &lt;em&gt;The Wreck of the River of Stars&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would such a spacecraft keep its sails &quot;aloft&quot;?  Does it need to constantly generate power, or can it pick up enough power from the radiation it's sailing on to keep the sail going?&lt;/p&gt;&#xA;" OwnerUserId="14104" OwnerDisplayName="J.D. Ray" LastActivityDate="2016-09-23T17:15:20.607" Title="What powers an e-sail?" Tags="|physics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14829" PostTypeId="2" ParentId="9408" CreationDate="2016-04-13T23:54:18.437" Score="0" Body="&lt;p&gt;Let's say you catch the hazardous object one orbit before it's going to hit earth. That means it's got about a billion kilometers left to go, and you want to deflect it by say 10,000 kilometers (almost an earth-diameter). That's a (very) roughly speaking ten parts per million adjustment of the momentum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case you'll need it to intercept a cloud of &quot;stuff&quot; with an (also very) roughly speaking mass of ten parts per million of the hazard. Assuming similar densities, that means your stuff - in the rocket - before expansion - will have a size of 1/100,000^0.33  or about 1/50th of the size, or the mass, of the hazzard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your hazard is 1km in diameter with a density of 3 g/cm^3 and your liquid Xenon has the same density (it does) then you need a 20 meter diameter ball of liquid xenon to boil, turn into a gas, and then stay put. Of course if it's got much much farther to go, then you can use less mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your gravel (&quot;&lt;em&gt;micro&lt;/em&gt;-impactor&quot;) cloud, or particulate (&quot;&lt;em&gt;nano&lt;/em&gt;-impactor&quot;) cloud, or molecule (&quot;&lt;em&gt;pico&lt;/em&gt;-impactor&quot;) cloud is released in space, then it is in orbit - in &lt;em&gt;an&lt;/em&gt; orbit - whether you like it or not - around the sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means if you want it to stay in front of the hazardous object for a long time, it would have to be in the same or similar orbit - with the easter egg that it could be moving in either direction. So you could put it in the &quot;backwards&quot; orbit and maximize the momentum loss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the impactor clouds mentioned above work similarly - whether it's molecular collisions or gravel collisions, the collisions &quot;remove&quot; momentum from the hazardous body loosely speaking. They will all have some &lt;em&gt;sticking fraction&lt;/em&gt; - sticking is half as effective as rebounding for momentum transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The term &quot;fluid&quot; refers to an approximation - temporarily forgetting that molecules exist and pretending you have a uniform material. It's and &lt;em&gt;extremely useful&lt;/em&gt; approximation, but here we should just stick to the somewhat more realistic view of the gas as individual impactors and momentum transfer.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference between the cloud of molecular particles, and the clouds of larger particles is the relative speeds, and effect of collisions. If you think of temperature as kinetic energy per particle, then the lighter the particle, the faster velocity for a given temperature. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, by the time the gas expands from 1 meter to 10 kilometers in size, it's really ultra high vacuum (UHV). The mean free path is as big as the cloud itself. It is now &quot;cold&quot; after expansion - in the sense that the random motion is low, but the velocities are now &quot;ordered&quot;. The molecules are still going fast, but radially expanding. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a gas cools by expansion in space, the molecules don't slow down and stop. &quot;Cold&quot; just refers to the random part of the motion. The ordered part (expansion) will continue at roughly the same average speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Punch line:&lt;/strong&gt; So if you want to keep your particulates in place (meaning - all in one orbit) longer, then possibly molecules are not the best bet because they will have the fastest expansion rate.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-04-14T04:38:54.243" LastActivityDate="2016-04-14T04:38:54.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14830" PostTypeId="2" ParentId="14820" CreationDate="2016-04-14T00:12:09.723" Score="4" Body="&lt;p&gt;Ignoring the complicated separation issue, there is a simple relation to calculate the effect on thrust based on mismatch of the exit plane pressure and ambient pressure, namely:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$F = q V_e + (P_e-P_a) A_e$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Where &lt;span class=&quot;math-container&quot;&gt;$P_e$&lt;/span&gt; is the exit plane pressure, &lt;span class=&quot;math-container&quot;&gt;$P_a$&lt;/span&gt; is ambient pressure, and &lt;span class=&quot;math-container&quot;&gt;$A_e$&lt;/span&gt; is the exit plane area.  &lt;span class=&quot;math-container&quot;&gt;$qV_e$&lt;/span&gt; without the correction term gives the thrust when the exit plane pressure matches ambient.  Here &lt;span class=&quot;math-container&quot;&gt;$q$&lt;/span&gt; is mass flow and &lt;span class=&quot;math-container&quot;&gt;$V_e$&lt;/span&gt; is exit velocity.&lt;/p&gt;&#xA;&lt;p&gt;Taken from &lt;a href=&quot;http://www.braeunig.us/space/propuls.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robert A. Braeunig's page on Rocket Propulsion&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;As Russell Borogove mentions in his answer, real world effects make the thrust degradation worse (and can even physically damage the nozzle).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-04-21T04:20:32.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14831" PostTypeId="2" ParentId="14828" CreationDate="2016-04-14T02:13:12.383" Score="6" Body="&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/8UpdnhOTlG4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/centers/marshall/news/news/releases/2016/nasa-begins-testing-of-revolutionary-e-sail-technology.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/centers/marshall/news/news/releases/2016/nasa-begins-testing-of-revolutionary-e-sail-technology.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spectrum.ieee.org/tech-talk/aerospace/space-flight/electrostatic-glider-can-maneuver-around-asteroids-without-expending-fuel&quot; rel=&quot;noreferrer&quot;&gt;http://spectrum.ieee.org/tech-talk/aerospace/space-flight/electrostatic-glider-can-maneuver-around-asteroids-without-expending-fuel&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Specific answer first: The &amp;quot;power&amp;quot; or actually the propulsive momentum comes from the protons in the solar wind itself. The spacecraft uses some electrical power to charge up, and maintain a positive bias, but then is passively pushed by the electrostatic fields from the larger number of protons piled up behind the craft (sunward) compared to the lower number of protons expected in the wake at the front of the craft (forward).&lt;/p&gt;&#xA;&lt;p&gt;The article in your link (&lt;a href=&quot;http://www.gizmag.com/nasa-technology-sail-deep-space/42760/&quot; rel=&quot;noreferrer&quot;&gt;Gizmag&lt;/a&gt;) actually talks about electostatic repulsion, not radio waves. If you look at the bottom of the article you can get out of Gizmag's self-referencing environment and to something more reliable.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cArVT.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cArVT.png&quot; alt=&quot;Source NASA&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;That takes you to &lt;a href=&quot;http://www.nasa.gov/centers/marshall/news/news/releases/2016/nasa-begins-testing-of-revolutionary-e-sail-technology.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Begins Testing of Revolutionary E-Sail Technology&lt;/a&gt;, where they do NOT claim that a 20km long, 1mm metal wire &lt;a href=&quot;http://www.gizmag.com/nasa-technology-sail-deep-space/42760/&quot; rel=&quot;noreferrer&quot;&gt;&amp;quot;weighs only a few grams&amp;quot;&lt;/a&gt; the way Gizmag does.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Extending outward from the center of the spacecraft, 10 to 20 electrically charged, bare aluminum wires would produce a large, circular E-Sail that would electrostatically repel the fast moving protons of the solar wind. &lt;strong&gt;The momentum exchange produced as the protons are repelled by the positively charged wires would create the spacecraft’s thrust.&lt;/strong&gt; Each tether is extremely thin, only 1 millimeter -- the width of a standard paperclip -- and very long, nearly 12 and a half miles -- almost 219 football fields. As the spacecraft slowly rotates at one revolution per hour, centrifugal forces will stretch the tethers into position.&lt;/p&gt;&#xA;&lt;p&gt;The testing, which is taking place in the High Intensity Solar Environment Test system, is designed to examine the rate of proton and electron collisions with a positively charged wire. Within a controlled plasma chamber simulating plasma in a space, the team is using a stainless steel wire as an analog for the lightweight aluminum wire. Though denser than aluminum, stainless steel’s non-corrosive properties will mimic that of aluminum in space and allow more testing with no degradation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The spacecraft uses an electron gun to loose electrons - thereby charging itself positive. The mutual repulsion moves all the excess charge to the wires.&lt;/p&gt;&#xA;&lt;p&gt;I get &lt;strong&gt;42 kilograms per wire&lt;/strong&gt; for Aluminum 1mm diameter, 20km long. They may be using solid metal wire for testing, and planning to use something more clever and light weight later on.&lt;/p&gt;&#xA;&lt;p&gt;What is necessary is&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A modest{1} amount of conductivity - doesn't have to be metal, seems current should be low&lt;/li&gt;&#xA;&lt;li&gt;Good tensile strength to weight ratio - doesn't break under own centrifugal force&lt;/li&gt;&#xA;&lt;li&gt;Deployability = flexible and stowable&lt;/li&gt;&#xA;&lt;li&gt;Durability in space environment - ergo &lt;em&gt;the High Intensity Solar Environment Test system&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;{1} if the space plasma has a lot of high frequency noise, then you do need low resistitivity (short RC time constant).&lt;/p&gt;&#xA;&lt;p&gt;I think the radio wave-driven sail you are thinking of must be another technology. See if you can find it, and ask a separate question.&lt;/p&gt;&#xA;&lt;p&gt;By the way, in the Michael Flynn story, the propulsion seems to involve &lt;a href=&quot;https://sevencircumstances.com/the-wreck-of-the-river-of-stars-by-michael-flynn/&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;magnetic fields and superconductors&lt;/em&gt;&lt;/a&gt; not electric fields, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Fusor&quot; rel=&quot;noreferrer&quot;&gt;Farnsworth-Hirsch Fusor&lt;/a&gt; which is a real thing and &lt;a href=&quot;http://ssl.mit.edu/files/website/theses/PhD-2007-DietrichCarl.pdf&quot; rel=&quot;noreferrer&quot;&gt;still actively discussed&lt;/a&gt; - see Section 1.1. The magnetic drive use in that story would be a good question for &lt;code&gt;scifi.stackexchange.com&lt;/code&gt; if it's not there already.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Philo_Farnsworth&quot; rel=&quot;noreferrer&quot;&gt;Philo T. Farnsworth&lt;/a&gt; is sometimes called &amp;quot;inventor of the television&amp;quot; although it was more of a collective effort. His likeness can be seen in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Philo_Farnsworth#/media/File:Farnsworth_ldac.jpg&quot; rel=&quot;noreferrer&quot;&gt;Letterman Digital Arts Center&lt;/a&gt; as well as the &lt;a href=&quot;http://futurama.wikia.com/wiki/Philo_Farnsworth&quot; rel=&quot;noreferrer&quot;&gt;Futurama Wiki&lt;/a&gt;. Now you know why the name sounds familliar!&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0N6dF.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0N6dF.jpg&quot; alt=&quot;Philo T. Farnsworth CROPPED from Wikipedia&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Philo T. Farnsworth&lt;/strong&gt; (Right):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/IEwFg.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IEwFg.png&quot; alt=&quot;Philo T. Farnsworth - on the right&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-23T17:15:20.607" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14832" PostTypeId="2" ParentId="14631" CreationDate="2016-04-14T04:09:55.590" Score="0" Body="&lt;p&gt;Reading the paper more carefully has provided the answer. The beginning of section 4.1 states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Attitude stabilization is provided directly by gravity gradient on the pitch and roll axes, and by &quot;dynamic torques&quot; on the yaw axis. The &quot;torques&quot; result from the fact that &lt;strong&gt;the spacecraft prefers to rotate about its maximum moment of inertia, and will move toward that orientation when forced to rotate at orbital rate by gravity gradient torques.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(emphasis mine)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The analysis of gravity-gradient torque I saw in school was done in an LVLH (local vertical, local horizontal) reference frame in which the direction of the gravity gradient is constant.  However, it was treated as an inertial frame, when it is actually a rotating frame (making one rotation per orbit), which is where the &quot;dynamic torques&quot; come from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Put another way, the gravity-gradient torque spins the spacecraft, and then the third axis is &lt;strong&gt;spin-stabilized&lt;/strong&gt; (at a &lt;em&gt;very&lt;/em&gt; low spin rate).  Quite a clever idea!&lt;/p&gt;&#xA;" OwnerUserId="8996" LastActivityDate="2016-04-14T04:09:55.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14833" PostTypeId="1" AcceptedAnswerId="14834" CreationDate="2016-04-14T07:42:19.317" Score="9" ViewCount="493" Body="&lt;p&gt;Does anyone know what exactly is &lt;a href=&quot;https://www.facebook.com/photo.php?fbid=10102777497843851&amp;amp;set=a.612287952871.2204760.4&amp;amp;type=3&amp;amp;theater&quot; rel=&quot;nofollow&quot;&gt;their plan&lt;/a&gt;, once they get there? How is that nano-thing going to slow down / take images / send data back to us? I couldn't find any information on this.&lt;/p&gt;&#xA;" OwnerUserId="14122" OwnerDisplayName="Geek Dunkman" LastEditorUserId="1235" LastEditDate="2019-10-26T19:38:52.327" LastActivityDate="2019-10-26T19:38:52.327" Title="How will Yuri Milner and Stephen Hawking's nanobots decelerate and transmit data upon arrival at Alpha Centauri?" Tags="|communication|probe|interstellar-travel|light-sail|breakthrough-starshot|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14834" PostTypeId="2" ParentId="14833" CreationDate="2016-04-14T09:17:37.747" Score="6" Body="&lt;p&gt;There seems to be very little published information at this stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a &lt;a href=&quot;http://breakthroughinitiatives.org/Initiative/3&quot;&gt;main page&lt;/a&gt; that has been set up for the initiative (Yuri Milner's Breakthrough Foundation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most interesting bits seem to be a &lt;a href=&quot;http://breakthroughinitiatives.org/Challenges/3&quot;&gt;list of challenges&lt;/a&gt; yet to be solved and a long &lt;a href=&quot;http://breakthroughinitiatives.org/Research/3&quot;&gt;page of research papers&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's an interesting &lt;a href=&quot;http://www.deepspace.ucsb.edu/wp-content/uploads/2015/04/A-Roadmap-to-Interstellar-Flight-15-d.pdf&quot;&gt;roadmap document&lt;/a&gt; too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for slowing down - no chance. As a page linked from the Challenges list says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nanocraft would traverse a distance of 1 AU in about 2500 seconds. To stabilize the image, the camera would need to “slew” at this rate.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8756" OwnerDisplayName="Andy" LastActivityDate="2016-04-14T09:17:37.747" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14835" PostTypeId="2" ParentId="14833" CreationDate="2016-04-14T16:24:24.840" Score="1" Body="&lt;p&gt;Solar sails often suggest a couple of modes for slowing down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Have a launch laser at the target location to slow it down the same way.  Assumes much infrastructure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Use the solar energy of the target sun to slow the sail down. I.e. Turn it around and spend a longer time decelerating as you get closer and closer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) There was a model, where the main sail had a second larger sail that would fly with it, during acceleration, but upon cruise would separate, and advance in front of the main sail. Then the launch laser would target the larger sail, to reflect the energy back on the smaller sail to slow it down. (Lots of tricky bits in that approach).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-04-14T16:24:24.840" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14836" PostTypeId="2" ParentId="14833" CreationDate="2016-04-14T16:45:24.390" Score="3" Body="&lt;p&gt;They don't propose slowing down, which is part of why they don't expect high resolution. In fact, they seem to expect most of the acceleration will occur from the beginning, by the time they hit Pluto's orbit, they expect them to be going at a very high speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The communication is expected to be done via lasers. Maybe they can re-convert their reflective shield to assist in improving the laser focus, to increase the power received.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last interesting bit is they are expected to remain in hibernation until they make their final approach, making communication during the deep void virtually impossible.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-14T16:45:24.390" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14837" PostTypeId="1" AcceptedAnswerId="14839" CreationDate="2016-04-14T16:51:04.617" Score="6" ViewCount="1868" Body="&lt;p&gt;I had &lt;a href=&quot;https://van.physics.illinois.edu/qa/listing.php?id=184&quot; rel=&quot;nofollow&quot;&gt;read&lt;/a&gt; that the Sun's gravity at the distance of the Earth's orbit was 0.0006 times that of Earth's gravity. So, I was wondering what it would be further out, at the distance of Jupiter's orbit.&lt;/p&gt;&#xA;" OwnerUserId="14110" LastEditorUserId="6816" LastEditDate="2016-04-14T17:51:12.773" LastActivityDate="2016-04-14T17:51:12.773" Title="How strong is the Sun's gravity at the distance of Jupiter's orbit?" Tags="|gravity|jupiter|the-sun|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14838" PostTypeId="1" AcceptedAnswerId="14846" CreationDate="2016-04-14T16:52:07.480" Score="6" ViewCount="893" Body="&lt;p&gt;I can't follow exactly what happened from this &lt;a href=&quot;https://twitter.com/ROGAstronomers/status/720254837095575552&quot; rel=&quot;nofollow noreferrer&quot;&gt;string of tweets&lt;/a&gt; - I'm not really twitter-literate. It seems like there was a predicted conjunction of the ISS with the moon, and an actual video of what &lt;em&gt;could be&lt;/em&gt; a satellite crossing in front of the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it looks like the trajectory is roughly parallel to the moon's terminator?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Can anyone reconstruct this?&lt;/em&gt; We have a timestamp, a distinct crater &lt;a href=&quot;https://twitter.com/ROGAstronomers/status/720541433837568000&quot; rel=&quot;nofollow noreferrer&quot;&gt;Theophilus&lt;/a&gt; on the terminator, and possible IDs both as a Russian satellite and a Ferrari. Are there &quot;reverse-satellite-lookups?&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/ROGAstronomers/status/720254837095575552&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://twitter.com/ROGAstronomers/status/720254837095575552&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Manual Screenshot of the tweet, showing (presumably) a speeding satellite as a little blurred white thing moving roughly parallel to the moon's terminator. The tweeted image says it's a GIF, &lt;a href=&quot;https://space.meta.stackexchange.com/a/742/12102&quot;&gt;but it's not!&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZRQe2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZRQe2.png&quot; alt=&quot;screen shot of tweet&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:04.403" LastActivityDate="2016-04-15T09:03:18.830" Title="What actually happened here with a satellite, the ISS and the moon?" Tags="|iss|the-moon|artificial-satellite|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14839" PostTypeId="2" ParentId="14837" CreationDate="2016-04-14T17:12:59.107" Score="12" Body="&lt;p&gt;Acceleration due to gravity is given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ a = \frac{GM}{r^2} $$ where G is the universal gravitational constant, M is the mass of the central body and r is the distance between the bodies' centers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Jupiter at ~5.2 AU average orbital radius, this works out to 0.22 mm/s$^2$.  This is $2.2\times10^{-5}$ (or 0.000022) times the gravitational acceleration due to the Earth at its surface.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2016-04-14T17:12:59.107" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14840" PostTypeId="2" ParentId="14838" CreationDate="2016-04-14T18:34:37.507" Score="3" Body="&lt;p&gt;I just used the &lt;a href=&quot;http://www.calsky.com/cs.cgi&quot; rel=&quot;nofollow&quot;&gt;CalSky web tool&lt;/a&gt;.  When set up for a location and a timeframe, this should be able to predict close ISS passes to the sun or moon.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I set it up for London, there are no ISS passes that close to the moon for that date.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But trying the Resurs DK-1 satellite shows a good match.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.calsky.com/csrender.cgi?object=Satellite&amp;amp;number=11&amp;amp;obs=41347974266773&amp;amp;tdt=2457491.34089252&amp;amp;sat=29228&amp;amp;skymap=&quot; rel=&quot;nofollow&quot;&gt;This link&lt;/a&gt; should show the pass from london at that time.  You can see it shows a pass near the moon along the general direction of the terminator.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2016-04-14T18:34:37.507" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14841" PostTypeId="2" ParentId="14816" CreationDate="2016-04-14T20:46:17.760" Score="1" Body="&lt;p&gt;I don't think there is much overlap at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 2 types of power system used for space exploration - solar and RTGs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar panels will not fare well in the Antarctic where clouds and whiteouts are common. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;RTGs produce relatively little power and are expensive thus not practical for terrestrial applications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The environments are too different for any sort of direct parallels. &lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2016-04-14T20:46:17.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14842" PostTypeId="1" AcceptedAnswerId="14847" CreationDate="2016-04-14T23:58:02.977" Score="4" ViewCount="462" Body="&lt;p&gt;What are the plans for the New Shepard capsule if the engine fails near the start of its flight? Are there plans for any sort of ballistic escape like the SpaceX capsule?&lt;/p&gt;&#xA;" OwnerUserId="10530" LastEditorUserId="6537" LastEditDate="2018-01-23T23:21:02.067" LastActivityDate="2018-01-24T03:06:19.500" Title="What happens to Blue Origin's New Shepard capsule if their one engine fails?" Tags="|rockets|commercial|blue-origin|new-shepard|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14843" PostTypeId="2" ParentId="13388" CreationDate="2016-04-15T00:02:05.853" Score="4" Body="&lt;p&gt;ULA cannot really make it much cheaper because of the Solid Rocket Booster price. The Delta II-7920 (the most commonly used configuration) uses 9 &lt;a href=&quot;https://en.wikipedia.org/wiki/Graphite-Epoxy_Motor&quot; rel=&quot;noreferrer&quot;&gt;GEM-40&lt;/a&gt;/46 boosters, \$2.5M (&lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=19363.25;wap2&quot; rel=&quot;noreferrer&quot;&gt;one&lt;/a&gt;) each, produced by an &lt;a href=&quot;http://www.orbitalatk.com/&quot; rel=&quot;noreferrer&quot;&gt;external manufacturer Orbital ATK (former Alliant Techsystems, former ATK Thiokol)&lt;/a&gt;. This makes it total about \$22.5M for the boosters alone + at least $30M for the rest of the rocket, including 1st, 2nd and, sometimes, 3rd stage, which is a normal price, mostly on par with SpaceX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While SpaceX offers 2X heavier payload for the lower price tag of ~\$40M. SpaceX builds fewer Merlin engines in-house and at a lower manufacturing cost.&lt;/p&gt;&#xA;" OwnerUserId="14118" LastEditorUserId="14118" LastEditDate="2016-04-15T08:55:25.783" LastActivityDate="2016-04-15T08:55:25.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14844" PostTypeId="2" ParentId="12708" CreationDate="2016-04-15T00:23:20.163" Score="6" Body="&lt;p&gt;The original wording above says &quot;Do not stand in front of the bigger head&quot;.&#xA;Two reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;There is a &lt;a href=&quot;http://www.spacesafetymagazine.com/spaceflight/reentry-descent-landing/soft-landing-on-kaktus/&quot;&gt;Kaktus precision gamma-ray altimeter&lt;/a&gt; (Containing &lt;sup&gt;60&lt;/sup&gt;Со or  &lt;sup&gt;137&lt;/sup&gt;Сs).&lt;/li&gt;&#xA;&lt;li&gt;There is a chance of one of the soft landing engines firing late due to malfunction.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="14118" LastEditorUserId="14118" LastEditDate="2016-04-15T08:18:33.917" LastActivityDate="2016-04-15T08:18:33.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14846" PostTypeId="2" ParentId="14838" CreationDate="2016-04-15T02:52:02.993" Score="14" Body="&lt;p&gt;I had difficulty with the site in the other answer, so I tried &lt;a href=&quot;https://in-the-sky.org/&quot; rel=&quot;noreferrer&quot;&gt;a newer website&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It turns out the ISS did indeed pass by the moon about a minute later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Home-made GIF from screenshots:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;strong&gt;Moon&lt;/strong&gt; is the &lt;strong&gt;Purple Dot&lt;/strong&gt; and &lt;strong&gt;CRS-8&lt;/strong&gt; is shown together with &lt;strong&gt;ISS&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TIME in UTC&lt;/strong&gt; is shown just above the image on the left.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oIL0N.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oIL0N.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite that transited the moon seen from the Royal Observatory is &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/resurs-dk.htm&quot; rel=&quot;noreferrer&quot;&gt;Reseurs DK-1&lt;/a&gt; a Russian civilian remote-sensing spacecraft (i.e. &lt;a href=&quot;https://en.wikipedia.org/wiki/Resurs-DK_No.1&quot; rel=&quot;noreferrer&quot;&gt;0.9m resolution&lt;/a&gt; &lt;em&gt;earth-pointing telescope&lt;/em&gt;) that looks &lt;a href=&quot;http://www.russianspaceweb.com/resurs_dk.html&quot; rel=&quot;noreferrer&quot;&gt;pretty amazing&lt;/a&gt;! .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nZfeR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nZfeR.jpg&quot; alt=&quot;russianspaceweb.com/resurs_dk.html&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the screenshot again from the original &lt;a href=&quot;https://twitter.com/ROGAstronomers/status/720254837095575552&quot; rel=&quot;noreferrer&quot;&gt;tweeted GIF&lt;/a&gt; which is really a &lt;a href=&quot;https://space.meta.stackexchange.com/a/742/12102&quot;&gt;faux-GIF&lt;/a&gt;. The short white streak in the lower-left is probably the DK-1 satellite moving fast in LEO. Since this is just after sunset locally, the satellite, at an altitude of 572km here, is probably still in full sunlight and therefore very bright!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZRQe2.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZRQe2.png&quot; alt=&quot;screenshot twitter.com/ROGAstronomers/status/720254837095575552&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crater is &lt;a href=&quot;https://twitter.com/ROGAstronomers/status/720541433837568000&quot; rel=&quot;noreferrer&quot;&gt;Theophilus&lt;/a&gt; and this image from the linked tweet shows the sun rising on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_craters#Characteristics&quot; rel=&quot;noreferrer&quot;&gt;central peak&lt;/a&gt; of the crater, taken with the &lt;a href=&quot;http://www.royalobservatorygreenwich.org/articles.php?article=1174&quot; rel=&quot;noreferrer&quot;&gt;28 inch Refractor at the Royal Observatory Grenwich&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jCpPL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jCpPL.jpg&quot; alt=&quot;from twitter.com/ROGAstronomers/status/720541433837568000&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crater is nearest the Apollo 16 landing site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wSJtE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wSJtE.png&quot; alt=&quot;Theophilus crater in google.com/moon &quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite trajectory data was obtained &lt;a href=&quot;https://in-the-sky.org/&quot; rel=&quot;noreferrer&quot;&gt;like this&lt;/a&gt;. I used user-specified location instead of the &lt;strong&gt;London&lt;/strong&gt; preset, to make sure the time was UTC.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L8uvO.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L8uvO.png&quot; alt=&quot;in-the-sky.org setup&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-03-16T15:52:04.403" LastActivityDate="2016-04-15T04:10:41.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14847" PostTypeId="2" ParentId="14842" CreationDate="2016-04-15T07:30:13.233" Score="10" Body="&lt;p&gt;The capsule is equipped with a Launch Escape System capable of aborting from pad or in-flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pad abort was already tested in 2012:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/SuR6sDMAdXs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/exploration/commercial/crew/blue-origin-padescapetest.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nasa.gov/exploration/commercial/crew/blue-origin-padescapetest.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the in flight escape was tested in October 2016:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/0K3R0tiClLg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="3044" LastEditDate="2018-01-24T03:06:19.500" LastActivityDate="2018-01-24T03:06:19.500" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14848" PostTypeId="1" AcceptedAnswerId="14849" CreationDate="2016-04-15T08:46:40.703" Score="10" ViewCount="831" Body="&lt;p&gt;Specifically: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Using some high pressure gas (before or after combustion) as a fluid bearing.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A superconducting maglev bearing, possibly using fuel as coolant.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Any related work is also welcome.&lt;/p&gt;&#xA;" OwnerUserId="14123" LastEditorUserId="3185" LastEditDate="2016-05-17T12:24:21.290" LastActivityDate="2016-05-17T12:24:21.290" Title="Has any work been done on alternatives to rolling element bearings in rocket turbopumps?" Tags="|rockets|engine-design|engines|engineering|turbomachinery|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14849" PostTypeId="2" ParentId="14848" CreationDate="2016-04-15T09:44:33.610" Score="9" Body="&lt;p&gt;Yes, plenty of work has been done in that area. The best primer is probably the paper that kicked off the whole hydrostatic bearing discussion (this is what you mean by &quot;some high pressure gas&quot;), &quot;&lt;a href=&quot;http://tribology.asmedigitalcollection.asme.org/article.aspx?articleid=1461666&quot; rel=&quot;noreferrer&quot;&gt;Reddecliff and Vohr, 1969&lt;/a&gt;&quot;. A lot of work has been done since then. The French had a hydrostatic bearing test stand for the never-completed European staged combustion engine (TPX/TPTech). Pratt &amp;amp; Whitney wanted to replace the pump side bearing in one of the Space Shuttle turbopumps with a hydrostatic one, but nothing came of this either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IHI, the Japanese rocket turbopump manufacturer, has released information about a turbopump that they want to sell overseas that uses hydrostatic bearings, so they are probably the furthest along. Unless of course SpaceX uses hydrostatic, but for all we know their bearings run on fairy dust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/0964180793904355&quot; rel=&quot;noreferrer&quot;&gt;Superconducting magnetic bearings&lt;/a&gt; have been studied as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the three largest problems for these bearings:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The stiffness is tiny compared to rolling element bearings. Hydrostatic bearings have at most a tenth the stiffness, magnetic bearings maybe only a hundredth or a thousandth. This means that rotordynamic instability is a huge development risk, since it can't be well predicted.&lt;/li&gt;&#xA;&lt;li&gt;They require an additional supply of propellant, which lowers the efficiency of the turbopump and increases the complexity.&lt;/li&gt;&#xA;&lt;li&gt;Rolling element bearings have made huge strides, so expendable and even some reusable rockets have really no need. Silicon nitride bearing balls together with induction-harded Cronidur steel races have made some other components in the Space Shuttle look really bad in terms of life time.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-04-15T15:24:09.537" LastActivityDate="2016-04-15T15:24:09.537" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14850" PostTypeId="2" ParentId="14242" CreationDate="2016-04-15T16:28:18.953" Score="4" Body="&lt;p&gt;I found the answer from a &lt;a href=&quot;http://spaceflightnow.com/2015/02/17/spacex-leases-property-for-landing-pads-at-cape-canaveral-vandenberg/&quot; rel=&quot;nofollow&quot;&gt;SpaceFlightNow article&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The contingency pads would only be utilized in order to enable the safe landing of a single vehicle should last-second navigation and landing diversion be required,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line, there are 4 because they are only to be used if for some reason the booster is off target, and one of the contingency pads is closer. You need 4 to spread them far enough apart to make it likely to always have one near.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-15T16:28:18.953" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14853" PostTypeId="2" ParentId="6026" CreationDate="2016-04-15T17:36:21.730" Score="1" Body="&lt;p&gt;The next lander beyond Mars will be the Osiris-Rex lander, expected to launch in 2018, which will land on an asteroid. The next mission to Saturn or Jupiter will likely be the &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/van-kane/20160105-nasa-europa-lander.html&quot; rel=&quot;nofollow&quot;&gt;Europa Lander&lt;/a&gt;, which is tentatively planned for 2022.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-15T17:36:21.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14854" PostTypeId="2" ParentId="14422" CreationDate="2016-04-15T17:55:41.350" Score="4" Body="&lt;p&gt;Assume for a moment your 1 degree/meter change, and extrapolate it to the center of the Moon. The radius of the moon is 1737 km, so that would be over a million degrees, which is plainly wrong! The core temperature of the Moon, per &lt;a href=&quot;https://space.stackexchange.com/a/3389/25&quot;&gt;this question,&lt;/a&gt; is estimated to be around 1200-1800K. Thus, I suspect 1 degree/km would be a much more realistic increase in temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the article indicating a constant temperature, I suspect that is simply in comparison to the surface, which varies wildly with the time of the day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to why the difference with the Apollo measurements, I'm really not sure. I would expect the temperature to go down when digging, although there might be a period of some increase. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the Apollo missions landed in the early morning local time. I suspect that deep inside the temperature was fairly constant. The surface, however, had lost it's heat, and was only just starting to warm up. I believe the diagram was intending to show just the gradient, and thus did not show the portion of quick change near the surface, nor the flattening out below the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that the Apollo data shows the temperature is more or less constant at those depths, across many different regions, and is in the range that the Science@NASA article indicated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: After thinking about this more, I think I understand what is going on. The top 50 cm is largely dominated by the day/night cycle. The next few meters below will tend toward the natural internal temperature of, say, 100m below the surface (Unknown from present data). The temperature cools as you rise from there, but I suspect it will follow an exponential decay, the start of which is seen in the Apollo 17 data. Bottom line, I suspect that the bottom of a 100m lava tube would be warmer than the temperatures listed, but still likely be below the freezing point of water.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-04-15T19:08:40.803" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14855" PostTypeId="2" ParentId="9387" CreationDate="2016-04-15T18:56:00.717" Score="4" Body="&lt;p&gt;Yes. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Hope&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, the science team will be advised from scientists at the University of Colorado, Arizona State University, and the University of California, Berkley.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-15T18:56:00.717" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14856" PostTypeId="2" ParentId="13947" CreationDate="2016-04-15T19:16:55.060" Score="3" Body="&lt;p&gt;With the 5 Lagrange points, there is a local 0 gravitational field. Of them, L4/L5 are &quot;hills&quot;, such that a small deviation from perfect will tend to put them back on course. L1/L2/L3 are &quot;Saddles&quot;, which means if they can stay perfectly on point, they will remain, otherwise they will slowly start to drift. This can be seen in this illustration of the Lagrange points:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J8hJs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J8hJs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;L3 will tend to remain at the right distance, but not the right relative spot in the orbit. This will eventually cause it to fall out of that alignment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, a current theory regarding the formation of the moon is that a Mars sized planet formed at the Earth-Sun L4 point. This object has been nicknamed &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Theia_%28planet%29#Theia&quot; rel=&quot;nofollow noreferrer&quot;&gt;Theia&lt;/a&gt;&quot;. It eventually was nudged out, after millions of years. Even such a relatively stable location long term isn't stable, due to the other planets in our Solar System, which tend to gently nudge objects periodically. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-15T19:16:55.060" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14858" PostTypeId="2" ParentId="10181" CreationDate="2016-04-15T20:30:52.447" Score="14" Body="&lt;ul&gt;&#xA;&lt;li&gt;The &quot;A&quot; blocks (i.e. the side boosters, there are 4 of them) are offset over the center of mass in the direction opposite to the payload (&lt;a href=&quot;https://translate.google.com/translate?sl=ru&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=en&amp;amp;ie=UTF-8&amp;amp;u=http%3A%2F%2Fwww.buran.ru%2Fhtm%2Fmodelist.htm&amp;amp;edit-text=&amp;amp;act=url&quot; rel=&quot;nofollow noreferrer&quot;&gt;rus1&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;The engines of the &quot;C&quot; block (the main body) are also offset a bit.&lt;/li&gt;&#xA;&lt;li&gt;A quite advanced control system for the time, that calculates and adjusts the mathematical model of the whole system during the flight and controls the engines in order to compensate for the asymmetry (&lt;a href=&quot;https://translate.google.com/translate?sl=ru&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=en&amp;amp;ie=UTF-8&amp;amp;u=http%3A%2F%2Fwww.buran.ru%2Fhtm%2F20-3.htm&amp;amp;edit-text=&amp;amp;act=url&quot; rel=&quot;nofollow noreferrer&quot;&gt;rus2&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=m4S25xcWQSc&quot; rel=&quot;nofollow noreferrer&quot;&gt;At the 05/15/1987 &quot;polyus&quot; (w/o the buran shuttle) video&lt;/a&gt;, you can see that the rocket pitched at 3 degrees at the initial takeoff seconds before the control system took control. However the computer managed to compensate the asymmetry in the end.&lt;/p&gt;&#xA;" OwnerUserId="14118" LastEditorUserId="23891" LastEditDate="2019-06-25T02:59:07.877" LastActivityDate="2019-06-25T02:59:07.877" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14859" PostTypeId="1" CreationDate="2016-04-16T06:02:32.793" Score="1" ViewCount="80" Body="&lt;p&gt;Can you name some of the commercially and non-commercially available tools to evaluate the link budget for a space mission?&#xA;Aside Satellite Tool Kit, of course ;)&lt;/p&gt;&#xA;" OwnerUserId="12130" LastActivityDate="2016-04-16T06:02:32.793" Title="Tools to evaluate telecommunications link budget" Tags="|communication|communication-satellite|simulation|" AnswerCount="0" CommentCount="8" ClosedDate="2016-04-16T14:47:09.657" ContentLicense="CC BY-SA 3.0" />
  <row Id="14860" PostTypeId="2" ParentId="8465" CreationDate="2016-04-16T11:00:38.187" Score="6" Body="&lt;p&gt;I think the closest answer to your question is &lt;a href=&quot;https://satnogs.org/&quot; rel=&quot;noreferrer&quot;&gt;Satnogs&lt;/a&gt;.&#xA;Other similar projects are&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Educational_Network_for_Satellite_Operations&quot; rel=&quot;noreferrer&quot;&gt;Genso&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.leafspace.eu/leaf-line/&quot; rel=&quot;noreferrer&quot;&gt;Leaf Line&lt;/a&gt; (commercial, I don't think they'll release an API in the short term)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12130" LastActivityDate="2016-04-16T11:00:38.187" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14861" PostTypeId="2" ParentId="9238" CreationDate="2016-04-16T11:49:04.363" Score="6" Body="&lt;p&gt;There is a decent &lt;a href=&quot;https://wikileaks.org/gifiles/attach/101/101944_ISS%20RESEARCH.doc&quot;&gt;compilation at wikileaks&lt;/a&gt; (references a few now-missing resources and the &lt;a href=&quot;http://www.astronautix.com/articles/costhing.htm&quot;&gt;astronautix page&lt;/a&gt;). Also, the &lt;a href=&quot;https://translate.google.com/translate?sl=ru&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=en&amp;amp;ie=UTF-8&amp;amp;u=https%3A%2F%2Fru.wikipedia.org%2Fwiki%2F%D0%9F%D0%BE%D0%BB%D0%B5%D0%B7%D0%BD%D0%B0%D1%8F_%D0%BD%D0%B0%D0%B3%D1%80%D1%83%D0%B7%D0%BA%D0%B0_%D0%BA%D0%BE%D1%81%D0%BC%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%BE%D0%B3%D0%BE_%D0%B0%D0%BF%D0%BF%D0%B0%D1%80%D0%B0%D1%82%D0%B0&amp;amp;edit-text=&amp;amp;act=url&quot;&gt;russian wiki&lt;/a&gt; page has the prices table, the numbers are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz U:    \$48M&lt;br&gt;&#xA;Soyuz ST:   \$61M&lt;br&gt;&#xA;Proton: \$68M&lt;br&gt;&#xA;Proton/Block DM:    \$95M&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It worth mentioning that, since the most of the rockets are produced at the state-owned enterprises, and launches are made by the state-owned space forces, there is a different price tag for the own government programs w/o any margin profit, for example the price tag for &lt;a href=&quot;https://translate.google.com/translate?sl=ru&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=en&amp;amp;ie=UTF-8&amp;amp;u=http%3A%2F%2Fecoruspace.me%2F%25D0%25A1%25D0%25BE%25D1%258E%25D0%25B7-2.1%25D0%25B0.html&amp;amp;edit-text=&amp;amp;act=url&quot;&gt;Soyuz 2.1a&lt;/a&gt; is just \$24M.&lt;/p&gt;&#xA;" OwnerUserId="14118" LastActivityDate="2016-04-16T11:49:04.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14862" PostTypeId="2" ParentId="8751" CreationDate="2016-04-16T12:30:51.650" Score="2" Body="&lt;p&gt;Since the fuel performance depends heavily on the oxidizer, engine, open/closed cycle, temperature, pressure, shape of the nozzle and many other things, just list of fuels is us not always enough, you need more like a set of formulas for each propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best calculator I saw in public is the &lt;a href=&quot;http://www.propulsion-analysis.com/&quot; rel=&quot;nofollow&quot;&gt;Rocket Propulsion Analysis App&lt;/a&gt; (Windows / Mac / Linux). There is a properties.inp propellant database inside the app directory which is just a text spreadsheet that you can use.&lt;/p&gt;&#xA;" OwnerUserId="14118" LastActivityDate="2016-04-16T12:30:51.650" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14864" PostTypeId="2" ParentId="12973" CreationDate="2016-04-16T22:53:27.283" Score="2" Body="&lt;p&gt;As for now (April of 2016) the &lt;a href=&quot;http://kosmolenta.com/index.php/new-tech/ptknp&quot; rel=&quot;nofollow&quot;&gt;plans&lt;/a&gt; are to build the first ship in 2019, launch in 2021.&#xA;However, the country's economy is declining with negative forecast, so I highly doubt anything will be launched before 2026.&lt;/p&gt;&#xA;" OwnerUserId="14118" LastActivityDate="2016-04-16T22:53:27.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14866" PostTypeId="1" AcceptedAnswerId="14877" CreationDate="2016-04-17T00:51:20.463" Score="18" ViewCount="3793" Body="&lt;p&gt;As I understand it, the moon makes for an ideal location for a telescope because it offers environmental factors that are unlike anything achievable on earth: A lack of atmosphere, light pollution, and electromagnetic sources all make for ideal conditions. Is this accurate? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretically, would there be a significant improvement in observability?&lt;/p&gt;&#xA;" OwnerUserId="14137" LastEditorUserId="195" LastEditDate="2016-04-17T04:23:18.780" LastActivityDate="2016-10-24T22:40:05.060" Title="Would a lunar telescope provide a significant improvement over terrestrial based equipment?" Tags="|the-moon|astronomy|telescope|space-telescope|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14867" PostTypeId="2" ParentId="4826" CreationDate="2016-04-17T01:50:39.880" Score="4" Body="&lt;p&gt;In addition to what's been posted earlier:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Separation events are risky. Every time a multi-stage craft experiences stage separation, there's a chance for things to go wrong. If the stages fail to separate, you won't make it to orbit. If they only partially separate, you might tear the craft apart. If they separate late, the upper-stage engine might take damage from overpressure. If they separate early, the lower stage might not have finished boosting and will crash into the upper stage, damaging it. If they separate unevenly, the upper-stage engine might be damaged or the upper stage may be put into a spin that damages its payload or centrifuges its fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Falcon_1_launches&quot; rel=&quot;nofollow&quot;&gt;Falcon 1 launch history&lt;/a&gt;, where launch #2 experienced an unplanned contact between the interstage and the second-stage engine bell (first-stage rotation during separation), and launch #3 failed because residual thrust after MECO collided the first stage into the second (insufficient delay before separation). Other rockets have experienced similar problems. Using fewer stages means having fewer separation events, with SSTO obviously having the fewest possible.&lt;/p&gt;&#xA;" OwnerUserId="11532" LastActivityDate="2016-04-17T01:50:39.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14868" PostTypeId="2" ParentId="2971" CreationDate="2016-04-17T04:18:19.303" Score="10" Body="&lt;p&gt;The current launcher with the highest overall system specific impulse is almost certainly the &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_IV_Heavy&quot; rel=&quot;noreferrer&quot;&gt;Delta IV Heavy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I can tell from the list of &lt;a href=&quot;http://spaceflight101.com/spacerockets/&quot; rel=&quot;noreferrer&quot;&gt;current orbital launchers&lt;/a&gt; on Spaceflight 101, it is the only one that uses hydrogen-fueled engines on all stages. All the others use hypergolics or kerosene or solids somewhere in the stack, which all have far lower specific impulse than hydrogen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Mark Adler notes, exact figures for system specific impulse are hard to come by, because they require knowledge of the Isp versus altitude curve of the engines and the throttle schedule of the launcher. The complexity of throttle scheduling in particular means that the calculation is best done as a discrete-time iterative simulation rather than analytically. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At each time step of simulation:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The engine specific impulse is determined based on the current altitude&lt;/li&gt;&#xA;&lt;li&gt;The current thrust is computed from the specific impulse and throttled propellant flow rate&lt;/li&gt;&#xA;&lt;li&gt;Momentary impulse is computed by integrating thrust over the duration of the time step and summed to total impulse.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The final total impulse divided by non-payload launch mass gets you system specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have been developing such a simulation. I've made educated guesses about throttling and done my best to research the other issues. The simulation isn't completely accurate, but the results it gives are reasonably aligned with reality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, the system specific impulse contest isn't even close, with Delta 4 Heavy (RS-68A Upgrade) 20% better than anything else I've looked at, because the Isp of hydrogen is so much better than other common propellants. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My simulations yield the following for max-payload launches to 200km LEO:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Launcher         Issp      Comments &#xA;Falcon 9 FT      259 s     all kerosene&#xA;Ariane 5 ECA     262 s     solids + hydrogen&#xA;Saturn V/INT21   293 s     kerosene + hydrogen&#xA;Delta IV Heavy   352 s     all hydrogen&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Calculating stage specific impulse for a single &lt;em&gt;upper&lt;/em&gt; stage is simpler; the burn is assumed to take place in vacuum, so it's just the propellant fraction of the stage times the specific impulse of the engine. The winner appears to be the Delta IV's 5-meter &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_Cryogenic_Second_Stage&quot; rel=&quot;noreferrer&quot;&gt;DCSS&lt;/a&gt; upper stage, with a stage specific impulse of 409.5 s. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="12865" LastEditDate="2016-08-24T13:20:27.213" LastActivityDate="2016-08-24T13:20:27.213" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14870" PostTypeId="2" ParentId="14866" CreationDate="2016-04-17T05:39:33.837" Score="4" Body="&lt;p&gt;That depends on what kind of telescope we are talking about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Disclaimer: I am NOT an astronomer.  I'm just a nerd who happens to provide computer security help for some awesome telescopes, and has seen them up close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a radiotelescope, I believe that there would probably be an advantage to running it on the moon.  There would also be a number of logistical disadvantages (getting someone out there to repair the thing is suddenly expensive and time-consuming, for example).  However, the radiotelescopes here on earth do a lot of work filtering out terrestrial signals, and getting them even a little further away from those signals (it's really only a little in astronomical scale) could make that easier.  Worth all the money and logistical overhead?  Maybe not...astronomers have gotten good enough at this that the optical telescopes around the radio telescope installations on Mauna Kea only get a modicum of grumbling over their wifi access points, and the dining hall shared by the 13 observatories is equipped with a microwave.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an optical telescope, we'd have to be building the thing at a MUCH higher focal length, resolution, and sensitivity than we are doing now to get an advantage by placing it on the moon.  I recently got to play around (well, not play, I was working) on Mauna Kea in Hawaii, and got an up-close look at the Gemini Observatory there.  At about 14,000 feet above sea level, the air is pretty thin, and they have a system for sampling atmospheric disturbance between the telescope and bright objects so that that distortion can be compensated for when viewing less-bright objects near that field of view.  Light pollution really isn't an issue atop Mauna Kea.  It's high above and miles away from the nearest homes and businesses.  The visitor station is about 7k feet below the summit if I recall correctly, and closed at night.  I believe that the area around and over the observatories is a no-fly zone as well.  Looking at blown-up photos of the same deep-space objects taken by Gemini and by Hubble side by side, I couldn't find a difference in clarity.&lt;/p&gt;&#xA;" OwnerUserId="14139" LastActivityDate="2016-04-17T05:39:33.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14871" PostTypeId="2" ParentId="14866" CreationDate="2016-04-17T05:48:22.900" Score="1" Body="&lt;p&gt;Theoretically, yes it would improve our scope of observation if we use a lunar telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I believe, the cost of doing so would significantly outweigh the advantages it will have than launching telescopes in low Earth orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plus, moon is a heavily cratered body. If we were to position a telescope on the dark side of the moon ( the face we do not see ), it will increase the chances of the telescope being damaged.&#xA;If we position it to the other part of the moon, we would not have a bigger horizon on our plate, thus reducing the productivity of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, we have the problems with managing the telescope remotely. The Hubble telescope itself had to be repaired/upgraded on several occasions and these maintenance missions are expensive and dangerous, to both the telescope and astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And again, having a telescope orbiting around does not really hinder our observations since they are out of most of Earth's atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="1231" LastActivityDate="2016-04-17T05:48:22.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14872" PostTypeId="1" AcceptedAnswerId="14875" CreationDate="2016-04-17T08:50:55.530" Score="5" ViewCount="246" Body="&lt;p&gt;Rough estimates shows that having around one hundred satellites would be enough to observe a single spot on Earth continuously and via peer to peer communication deliver that information to an earth-based single station in real-time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But is it possible to deliver those satellites to the right orbits in one shot?&lt;/p&gt;&#xA;" OwnerUserId="14140" LastEditorUserId="6816" LastEditDate="2016-04-18T20:00:03.570" LastActivityDate="2016-04-18T20:00:03.570" Title="Is it possible to deliver enough LEO satellites with one launch to substitute for one in GEO?" Tags="|low-earth-orbit|geosynchronous|earth-observation|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14874" PostTypeId="2" ParentId="9543" CreationDate="2016-04-17T09:39:04.273" Score="3" Body="&lt;p&gt;Taking the question literary; If you have a &quot;thrust&quot; pushing the ball, the only limiting factor is to manage to get off the ground. g ($9.81 m/s^2$) times 420-450grams is 4.16 to 4.46 Newtons. Why there is such a force acting on the ball in the first place seems to be outside the scope of the question.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-17T16:08:36.453" LastActivityDate="2016-04-17T16:08:36.453" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14875" PostTypeId="2" ParentId="14872" CreationDate="2016-04-17T10:42:12.397" Score="2" Body="&lt;p&gt;In principle yes, some caveats.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Possibility 1: Let us imagine that the target orbit has zero&#xA;inclination, in this case all the satellites are launched to that&#xA;orbit and must spread themselves out, by being in slightly higher or&#xA;lower orbits until they reach equidistant spacing.&lt;/li&gt;&#xA;&lt;li&gt;Possibility 2: The target orbit has a non zero inclination, perhaps&#xA;because the &quot;single spot&quot; on the Earth has a latitude sufficiently&#xA;far from the equator to be reached from a zero degree orbit. Now the&#xA;100 satellites will need to be distributed over different planes,&#xA;possibly 100 different planes. The inclination of the planes could be&#xA;the same for all, but they would each have a different Right&#xA;Ascension of the Ascending Node, RAAN. By launching in &quot;one shot&quot; we&#xA;are now causing most of the satellites to have to perform a plane&#xA;change. This isn't a show stopper in principle but it can be&#xA;extremely expensive in delta-V and thus propellant terms. It may well&#xA;put such a demand on the satellites mass in propellant capacity that they&#xA;can no longer all be launched together.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In both of the foregoing examples I have assumed that the phasing along the orbit, or between planes, is carried out by the individual satellites after separation. This isn't at all necessary as the launch vehicle could perform this instead. The energy requirements would be about the same though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an &lt;strong&gt;aside&lt;/strong&gt; the starting premise that one would need 100 satellites covers many assumptions itself - such as the beam width from ground terminals looking up, the beam width from the satellites looking down etc.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-04-17T23:30:19.820" LastActivityDate="2016-04-17T23:30:19.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14876" PostTypeId="2" ParentId="14872" CreationDate="2016-04-17T12:27:19.713" Score="5" Body="&lt;p&gt;In one launch? That depends on the mass of the satellites. Most rockets can deliver (roughly) 2x as much payload to LEO as they can to GEO. So your LEO satellites would need to be 1/50 the weight of your original GEO satellite. So you'd go from 5 tons to 100 kg per satellite.&lt;br&gt;&#xA;For Earth observation, being 100x closer (360 km vs. 36,000 km) also means your optics get a lot smaller (100x?). But other components of the satellite won't get smaller by that much.&lt;br&gt;&#xA;Having 100 times more satellites increases redundancy, but it also increases the number of failures. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-17T12:27:19.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14877" PostTypeId="2" ParentId="14866" CreationDate="2016-04-17T14:52:45.533" Score="28" Body="&lt;p&gt;As stated, the answer to the question has to be yes, a telescope on the Moon can have significant advantages over a telescope on Earth, because of Earth's atmosphere. That's why we have space telescopes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However that is the wrong question. The real question is what advantages, as well as disadvantages, does a telescope on the Moon have over a telescope &lt;em&gt;somewhere else in space&lt;/em&gt;, e.g. in Earth orbit or at a Lagrange point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall, a telescope on the Moon is a terrible idea. First off, thermal control would be a nightmare. There are two week days and two-week nights, misaligning everything you might try to align in a telescope and with the swings making it extremely difficult to keep infrared sensors at cryogenic temperatures. Second, for the same reason, you only have solar power half the time. So you need to have enough batteries to stay alive for two weeks, or nuclear energy! If on batteries, you will want to conserve power exactly when you'd like to be observing more. Which brings up visibility. You can only see half the sky, and your telescope keeps getting turned towards the Sun every month whether you want it to or not. Which brings up pointing. You now have the Moon's gravity to deal with when turning your telescope. In space you have only your own inertia to deal with. Furthermore it will need to deploy itself in lunar gravity as opposed to zero gravity. &lt;a href=&quot;http://www.airspacemag.com/daily-planet/dust-cloud-around-moon-180955624/&quot;&gt;And then there's the dust&lt;/a&gt;. Dust that gets kicked up by impacts and electrostatically, eventually covering your optics with a layer of crap.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You mentioned in a comment excluding considerations of cost, but you can never exclude that. Landing something softly on the Moon is hugely more expensive, in terms of mass and money, than putting it at a Lagrange point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should I go on?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only conceivable advantages I can come up with are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A. You have a stable surface in a vacuum on which to build a large interferometer,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;B. Radio silence on the far side (but adding the complexity of requiring a relay orbiter to get the data back), and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;C. Availability of raw materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For A, we can do the same thing in free space with precision formation flying, and then the horizon of the Moon doesn't limit your baseline distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For B, a radio telescope would get excellent isolation from Earth radio transmissions. However it seems that just putting your radio telescope on Earth in a valley with some mountains around you, and maybe &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_National_Radio_Quiet_Zone&quot;&gt;some radio silence laws&lt;/a&gt;, works just dandy. You don't need the expense of a space-borne radio telescope (where such telescopes need to be ginormous.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For C, you could extract materials from lunar regolith to make your mirrors. You could do the same from an asteroid more easily, with a wider range of asteroid types to choose from, and in lower gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, it would be better than Earth (for optical), but much, much worse than simply putting your telescope in space somewhere which is cheaper and easier.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-06T19:18:18.913" LastActivityDate="2016-05-06T19:18:18.913" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14879" PostTypeId="2" ParentId="14761" CreationDate="2016-04-18T12:21:22.193" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YZfWk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YZfWk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to thanks Andy for giving me the link to the &lt;a href=&quot;http://www.shatters.net/celestia/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestia software&lt;/a&gt; that got me the answer.&lt;/p&gt;&#xA;" OwnerUserId="6645" LastEditorUserId="4660" LastEditDate="2016-04-18T14:37:45.803" LastActivityDate="2016-04-18T14:37:45.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14880" PostTypeId="2" ParentId="14866" CreationDate="2016-04-18T13:53:28.540" Score="3" Body="&lt;p&gt;Mark asked the correct question, which is how would it compare to a satellite around Earth, or other location in space. I'm going to offer the only 2 suggestions that I've heard of that make sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radio Telescope- On Earth is is virtually impossible to obtain pure radio silence. Being an Amateur Radio operator, I can attest to that. The best that one can do is to have an area very isolated, preferably with mountains around it, but even that is difficult, and doesn't work for some bands. For RF bands that are affected by the atmosphere it is virtually impossible to obtain complete radio silence on Earth. The far side of the Moon does offer an attractive proposition for such an observatory, although as noted, it does have some serious difficulties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Infra-red. There are a &lt;a href=&quot;https://en.wikipedia.org/wiki/Shackleton_%28crater%29&quot; rel=&quot;nofollow&quot;&gt;few craters&lt;/a&gt; near the poles which are in perpetual shadow. These could make for an excellent infra-red observatory. The biggest problems are power, and the fact that you are limited to only the southern half of the sky (Really less than that) by such a telescope. Still, it could work conceivably. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-18T13:53:28.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14881" PostTypeId="2" ParentId="14549" CreationDate="2016-04-18T14:23:10.710" Score="2" Body="&lt;p&gt;I found these in the DSN's &lt;a href=&quot;https://deepspace.jpl.nasa.gov/files/dsn/DSN%20Mission%20Contact%20List%2012_16_15.pdf&quot; rel=&quot;nofollow&quot;&gt;contact list&lt;/a&gt;:&lt;br&gt;&#xA;SGP: NASA's &lt;a href=&quot;http://space-geodesy.nasa.gov&quot; rel=&quot;nofollow&quot;&gt;Space Geodesy Programme&lt;/a&gt;. From the &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20000074740&amp;amp;hterms=Space%20Geodesy%20Project%20DSN&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3DSpace%2520Geodesy%2520Project%2520DSN%26Ntx%3Dmode%2520matchallpartial&quot; rel=&quot;nofollow&quot;&gt;NTRS&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Although the prime purpose of the DSN is to support spacecraft operations and space research in deep space, this unique facility is also used on a noninterference basis with flight projects to support ground-based science experiments. The DSN VLBI capabilities are an integral part of a number of space- and ground-based projects. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;MCOA and MCOB: &lt;a href=&quot;http://www.jpl.nasa.gov/cubesat/missions/marco.php&quot; rel=&quot;nofollow&quot;&gt;Mars Cubesat One&lt;/a&gt;-A,B. I've no idea what a mission that hasn't been launched yet would do with DSN time, though.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-18T14:23:10.710" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14882" PostTypeId="1" AcceptedAnswerId="14885" CreationDate="2016-04-18T14:25:27.727" Score="3" ViewCount="478" Body="&lt;p&gt;Is there any existing technology for attaching fueled boosters to a spacecraft?  (not for a space station)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space assets (e.g. module, spacecraft or satellite orbiter) could be boosted to place in earth orbit first, then additional fueled boosters (from a second launch) could be docked to this space asset for a trip to the moon or Mars, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To reiterate, I'm interested in launching fuel vessel boosters in a second launch and docking these fueled boosters onto a spacecraft orbiting Earth.&lt;/p&gt;&#xA;" OwnerUserId="14112" LastEditorUserId="58" LastEditDate="2016-04-18T15:27:12.127" LastActivityDate="2016-04-18T19:33:09.977" Title="Could fueled boosters be launched separately from the primary payload?" Tags="|spacecraft|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14883" PostTypeId="1" AcceptedAnswerId="14884" CreationDate="2016-04-18T15:07:37.080" Score="0" ViewCount="1016" Body="&lt;p&gt;And I also wanted to ask if I constructed a wall made out of it, with GCRs and SEPs and Neutron radiation (basically every radiation in space) from the other side, how many kgs per m^2 of H-BNNT would be required to bring the radiation levels below the maximum permissible limit? For example, if I constructed a wall out of polyethylene, then it would be 7 metric tonnes per m^2, according to &lt;a href=&quot;http://space.alglobus.net/papers/Easy.pdf&quot; rel=&quot;nofollow&quot;&gt;http://space.alglobus.net/papers/Easy.pdf&lt;/a&gt;. I want to know this value, along with the density, for H-BNNT. Please cite your sources as well, since I need to reference this.&lt;/p&gt;&#xA;" OwnerUserId="14148" LastActivityDate="2018-10-25T13:29:58.343" Title="What is the density of Hydrogenated Boron Nitride Nanotubes (H-BNNT)?" Tags="|spacecraft|radiation|cosmic-radiation|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14884" PostTypeId="2" ParentId="14883" CreationDate="2016-04-18T15:37:28.110" Score="2" Body="&lt;p&gt;You might be interested in this paper: &lt;a href=&quot;http://waset.org/publications/9997248/simulation-of-hydrogenated-boron-nitride-nanotube-s-mechanical-properties-for-radiation-shielding-applications&quot; rel=&quot;nofollow&quot;&gt;Properties of H-BNNT for radiation shielding applications&lt;/a&gt;. It briefly mentions how the neutron absorbing properties of the material can be used for radiation shielding in space, but the main purpose is to find a value for the density of it. A simulation approach is used (it is not clear if someone has actually managed to produce the material yet), and the paper concludes with a density of &lt;code&gt;2.62&lt;/code&gt; g/cm³&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A definitive value for how many kg/m² of shielding material is very difficult to give any measure for. Firstly, the amount of radiation received at various locations in space varies &lt;em&gt;extremely&lt;/em&gt;, secondly, tolerable radiation doses is dependent on mission length, and the properties for H-BNNT for non-neutron radiation seems hard to find.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, bringing mass just to act as shielding is a last resort in space exploration as every gram counts. A more common approach is to use for example tanks of waters or the fuel tanks for that purpose.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-18T15:43:17.627" LastActivityDate="2016-04-18T15:43:17.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14885" PostTypeId="2" ParentId="14882" CreationDate="2016-04-18T15:58:02.520" Score="8" Body="&lt;p&gt;Does the technology exist? Yes, and it has since it was clear that docking was reliable enough to be used in the Apollo project. When the Lunar Module ascended from the Moon, it docked with the CSM. That is by most common measures to dock with a booster. After all, the main purpose of the SM was to insert the Apollo stack into lunar orbit, and then depart towards Earth. That is very much a booster role. Through hundreds of visits to space stations, docking is made an everyday and safe operation. We have the technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the spacecraft exist? No, not at the moment. While for example space station resupply craft, like &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Progress&lt;/a&gt;, have engines, and can be used for station re-boost, they are not dedicated boosters by themselves. Apart from space stations, docking is not much used. Today, we have no such boosters. Building them is however not an issue if needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scenario you describe is very close to the concept of the &lt;a href=&quot;http://www.russianspaceweb.com/parom.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Parom space tug&lt;/a&gt;, a planned component for Clipper, but may also be used for in-orbit cargo transportation. We have yet to see how many of those plans will be realized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7nq5j.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7nq5j.jpg&quot; alt=&quot;Clipper and Parom&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a historical reference, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Agena_target_vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Agena target vehicle&lt;/a&gt; used by Gemini, was among other things also used for propulsion, for example for setting the at-the-time record for being farthest away from Earth.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="6816" LastEditDate="2016-04-18T19:33:09.977" LastActivityDate="2016-04-18T19:33:09.977" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14886" PostTypeId="1" AcceptedAnswerId="14892" CreationDate="2016-04-18T17:01:38.327" Score="5" ViewCount="941" Body="&lt;p&gt;HST is in Low Earth Orbit. It can of course not look through Earth which at that low altitude covers almost half of the sky. As far as I know it is also never pointed even remotely close to the direction of the Sun for safety reasons. When the HST is in between the Sun and the Earth, once every 90 minutes, what can it look at then?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="4660" LastEditDate="2016-04-18T18:17:52.923" LastActivityDate="2016-04-19T04:43:32.313" Title="What viewing angles does the Hubble Space Telescope use during its orbit in LEO, avoiding the Sun and Earth?" Tags="|observation|hubble|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14887" PostTypeId="2" ParentId="14886" CreationDate="2016-04-18T17:22:47.733" Score="2" Body="&lt;p&gt;180 degrees of the sky is occupied by the Earth. 180 degrees of the field of view will cause sunlight to reflect in the optics of Hubble, causing unrecoverable damage. Nothing left then. The &quot;downtime&quot; is usually spent re-targeting and changing the observation setup. Even though non of the angles are as great as 180 degrees, the optics still requires a &quot;shadow zone&quot; margin of significant width, making the avoidance zone hemispherical in practise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fq5cu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fq5cu.png&quot; alt=&quot;Hubble&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NOTE: This answer is actually wrong in three different ways.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-19T04:43:32.313" LastActivityDate="2016-04-19T04:43:32.313" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14888" PostTypeId="1" CreationDate="2016-04-18T18:18:17.483" Score="4" ViewCount="155" Body="&lt;p&gt;Taper ratio is a central concept for &lt;a href=&quot;http://hopsblog-hop.blogspot.no/2012/09/beanstalks-elevators-clarke-towers.html&quot; rel=&quot;noreferrer&quot;&gt;space elevators and tethers&lt;/a&gt;. It tells how much larger the cross section of the tether has to be at one end of the tether, compared to the other. Being a dimensionless quantity, it is simply found by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R_{taper}=e^{\frac{Y}{S}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $S$ is the tensile strength of the material divided by its density, giving the unit $m^2/s^2$, and $Y$ is the acceleration experienced by the tether, integrated over its length. (again $m^2/s^2$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This works nice and well for a tether, which is stretched, and that is also the most common application, as materials tends to have a greater tensile strength than compression strength. For instance, &lt;a href=&quot;https://en.wikipedia.org/wiki/Zylon&quot; rel=&quot;noreferrer&quot;&gt;Zylon&lt;/a&gt; has a tensile strength of $5.8GPa$, which together with a density of $1560kg/m^3$ gives it an $S$ of $3.72Mm^2/s^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By that reason, space elevators are supported from the top. However, by the exponential nature of the mechanics, supporting half the elevator (&quot;half&quot; is here referring to the acceleration-distance integrated quantity, not mass or distance.) from the bottom should give a taper ratio equivalent to the square root of a top-only design. That is only the case when the $S$ of the ground-supported structure is equal to the $S$ of the tether. That is not the case. For finding actual taper ratio improvements for an elevator with dual support, one has to know a reasonable value for $S_{ground}$. An updated formula for taper ratio is surprisingly then just:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R_{taper}=e^{\frac{Y}{S_{tether}+S_{ground}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(yes, you can simply add them!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Data for compression strength of various materials is easily available, however, it is insufficient as the design of the structure is an important factor when it is no longer a tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is a reasonable value for $S_{ground}$, in $m^2/s^2$, for a supporting, compressing strength limited structure?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that this quantity is independent of surface gravity.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-18T18:28:07.247" LastActivityDate="2016-04-18T18:28:07.247" Title="What is the specific compression strength of an orbital tower structure?" Tags="|design|materials|space-elevator|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14891" PostTypeId="1" AcceptedAnswerId="14897" CreationDate="2016-04-18T22:52:41.397" Score="3" ViewCount="245" Body="&lt;p&gt;If you don't have access to Earth(but still the moon) or the Sun, where could you obtain enough hydrogen, oxygen, and nitrogen to create enough water and an N&lt;sub&gt;2&lt;/sub&gt; and O&lt;sub&gt;2&lt;/sub&gt; atmosphere for, say a population of 2 billion people? And you wouldn't be able to get materials from anywhere past Uranus.&#xA;Would you be able to artificially mass produce these elements relatively easily?&lt;/p&gt;&#xA;" OwnerUserId="14153" LastEditorUserId="25" LastEditDate="2016-04-19T14:00:04.843" LastActivityDate="2016-04-19T20:36:24.720" Title="How could you create water and air from materials in the Solar System?" Tags="|planetary-science|colonization|water|natural-resources|chemistry|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14892" PostTypeId="2" ParentId="14886" CreationDate="2016-04-19T00:48:19.770" Score="6" Body="&lt;p&gt;Wikipedia lists several angles in &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot;&gt;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Observations cannot take place when the telescope passes through the South Atlantic Anomaly due to elevated radiation levels, and there are also sizable exclusion zones around the Sun (precluding observations of Mercury), Moon and Earth. &lt;em&gt;The solar avoidance angle is about 50°&lt;/em&gt;, to keep sunlight from illuminating any part of the OTA. Earth and Moon avoidance keeps bright light out of the FGSs, and keeps scattered light from entering the instruments.  ... There is a so-called continuous viewing zone (CVZ), at roughly 90° to the plane of Hubble's orbit, in which targets are not occulted for long periods.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, +-50° away from sun (not ~180° as in Hohmannfan's answer) and some angle from Earth (and its limb) gives a lot of usable space for any moment of time...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some older HST Primers have an exact value of prohibited angles (but they are not listed for &lt;a href=&quot;http://www.stsci.edu/hst/proposing/documents/primer/Ch_2_Systemoverview4.html&quot;&gt;cycle 24&lt;/a&gt;): &lt;a href=&quot;ftp://ftp.stsci.edu/cdbs/perigee/jyounger/cp_primer/Ch_2_Systemoverview5.html#1876818&quot;&gt;ftp://ftp.stsci.edu/cdbs/perigee/jyounger/cp_primer/Ch_2_Systemoverview5.html#1876818&lt;/a&gt; - Hubble Space Telescope Primer for Cycle 11, 2.4    Pointing Constraints&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the telescope does not observe targets that are&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;within 50 degrees of the Sun;&lt;/li&gt;&#xA;  &lt;li&gt;within 15.5 degrees of any illuminated portion of the Earth;&lt;/li&gt;&#xA;  &lt;li&gt;within 7.6 degrees of the dark limb of the Earth; or&lt;/li&gt;&#xA;  &lt;li&gt;within 9 degrees of the Moon.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2843" LastActivityDate="2016-04-19T00:48:19.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14893" PostTypeId="2" ParentId="14682" CreationDate="2016-04-19T01:27:23.813" Score="13" Body="&lt;p&gt;There was Rb-based Ultra Stable Oscillator in outer space, delivered to Titan (VI moon of Saturn) inside the &lt;a href=&quot;https://en.wikipedia.org/wiki/Huygens_(spacecraft)&quot;&gt;Huygens&lt;/a&gt;, brought there by &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot;&gt;Cassini&lt;/a&gt;. (DWE RUSO in &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/image/spacecraft/huygens_cutaway.jpg&quot;&gt;http://nssdc.gsfc.nasa.gov/image/spacecraft/huygens_cutaway.jpg&lt;/a&gt;). Schemes and paper - &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.32.9215&amp;amp;rep=rep1&amp;amp;type=pdf&quot;&gt;RUBIDIUM ULTRA-STABLE OSCILLATORS AT TITAN: THE HUYGENS DOPPLER WIND EXPERIMENT&lt;/a&gt;, 1997. &lt;a href=&quot;http://lasp.colorado.edu/~horanyi/graduate_seminar/Doppler_wind.pdf?fobjectid=31193&amp;amp;fbodylongid=735&quot;&gt;THE HUYGENS DOPPLER WIND EXPERIMENT. Titan Winds Derived from Probe Radio Frequency Measurements&lt;/a&gt;, 2002&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were some stable non-atomic clocks on several other probes. The Cassini itself was listed to have on-board Ultra Stable Oscillator (quartz-based, &lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/navigation/missionnavigationuso/&quot;&gt;for navigation&lt;/a&gt; with Doppler effect), similar were used on some Mars missions: &lt;a href=&quot;https://solarsystem.nasa.gov/docs/Asmar.pdf&quot;&gt;https://solarsystem.nasa.gov/docs/Asmar.pdf&lt;/a&gt; &quot;ULTRA-STABLE OSCILLATORS FOR PLANETARY ENTRY PROBES&quot;, S.W. Asmar et al. NASA JPL (possibly from 2004)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Oscillators on NASA Deep Space Missions &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Voyager 2 USO is still working and the Voyager 1 USO ceased operations nearly 20 years after its procurement, exceeding specifications.  ...&#xA;  USO serial #4 was later flown on the Galileo spacecraft. ...&#xA;  several deep space missions have carried USOs for Radio Science experiments (e.g. Mars Global Surveyor, Cassini). These oscillators were single-oven, SC-cut crystals with a typical Allan deviation of 1x10-13 at τ = 100 s. More recently, a flight-qualified USO is being prepared for proximity communications and navigation on the upcoming Mars Reconnaissance Orbiter mission&lt;/p&gt;&#xA;  &#xA;  &lt;ol start=&quot;3&quot;&gt;&#xA;  &lt;li&gt;The Huygens Ultra-Stable Oscillator &#xA;  The ESA Huygens probe to Titan, carried by the Cassini spacecraft, has a Doppler Wind Experiment (DWE), a Radio Science experiment with requirements that were met with a rubidium (Rb) atomic standard &lt;a href=&quot;http://saturn.jpl.nasa.gov/mission/navigation/missionnavigationuso/&quot;&gt;5&lt;/a&gt;.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Check also table from the paper - &quot;Table 1: Representative data of parameters characterizing ultra-stable oscillators on deep space missions collected from various sources, in some cases published specifications and, in some cases, in flight data.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found presentations about the topic and future experiments: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://exphy.uni-duesseldorf.de/Opt_clocks_workshop/Talks_Workshop/Presentations%20Thursday%20morning/Presentation%20Schiller%20Gravitational%20Physics%20with%20Optical%20Clocks.pdf&quot;&gt;http://exphy.uni-duesseldorf.de/Opt_clocks_workshop/Talks_Workshop/Presentations%20Thursday%20morning/Presentation%20Schiller%20Gravitational%20Physics%20with%20Optical%20Clocks.pdf&lt;/a&gt;  (future missions)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.quest-lfs.uni-hannover.de/fileadmin/quest-lsf/pdf/QUEST_lecture/2013_10_21_-_Riehle_Time_and_Frequency_Metrology_Using_Satellites_I_II.pdf&quot;&gt;https://www.quest-lfs.uni-hannover.de/fileadmin/quest-lsf/pdf/QUEST_lecture/2013_10_21_-_Riehle_Time_and_Frequency_Metrology_Using_Satellites_I_II.pdf&lt;/a&gt; (lists USO of Cassini)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The best overview about USOs is again from Sami Asmar, from 2012:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://websites.isae.fr/IMG/pdf/uso-toulouse.pdf&quot;&gt;http://websites.isae.fr/IMG/pdf/uso-toulouse.pdf&lt;/a&gt; - Ultra-Stable Oscillators&#xA;For Probe Radio Science Investigations, 9th International Planetary Probe  Workshop, 16 June 2012 &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2843" LastActivityDate="2016-04-19T01:27:23.813" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14896" PostTypeId="2" ParentId="792" CreationDate="2016-04-19T08:00:38.267" Score="1" Body="&lt;p&gt;If you used a laser connected to your &quot;lightship&quot; to push against the sail then only that portion of reflected light from the sail would propel your &quot;lightship&quot; forward. Any light that was absorbed by the sail or reflected by the sail back to the ship would be wasted. Pointing the laser out the back of the ship would give the best propulsion with the same laser.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A ground based laser pushing against your sail would use all the light hitting it to propel your ship, but if you used a sail that reflected that laser light then you would almost double the amount of acceleration from the same laser. You would get the transferred momentum from the light hitting the sail and the added thrust of the light reflecting off the sail being bounced back toward its source. In low volume high velocity water turbines they use a blade shaped like two bowls and direct the jet of water to the middle of the two bowls. That momentum is absorbed and turns the wheel but the water continues to travel across the bowls inner surface and exits in nearly the same direction from which it came adding that reaction to push the turbine even faster. A mirrored light sail would be nearly twice as effective as an absorbent sail. &lt;/p&gt;&#xA;" OwnerUserId="14155" LastActivityDate="2016-04-19T08:00:38.267" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14897" PostTypeId="2" ParentId="14891" CreationDate="2016-04-19T13:59:40.780" Score="5" Body="&lt;p&gt;Water- Can be obtained from pretty much any non-gas giant object beyond the middle of the asteroid belt. This includes comets, moons of outer solar system planets, even objects like Pluto, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nitrogen- The best source is Titan, although it can be found in a number of places.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oxygen- Can be found where there is water, and is in quite a few other locations, if one has the energy to separate. For instance, it can be found on the Moon, Mars, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The materials for life are quite abundant in the Solar System, one just has to look. The hardest is Nitrogen, but that is still available in the right places.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="8162" LastEditDate="2016-04-19T20:36:24.720" LastActivityDate="2016-04-19T20:36:24.720" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14899" PostTypeId="2" ParentId="1232" CreationDate="2016-04-19T14:47:32.527" Score="6" Body="&lt;p&gt;There are 48 total MDMs on ISS (not including spares) of two major varieties placed both internal (24) and externally(24) on  the station.  They are Standard (33)  and Enhanced (15).  The Standard MDMs all utilize an Intel military 386 processor and perform the sensor and effector type IO.  There have been two standard MDM and one enhanced failure on orbit resulting in replacement of either the  box or the specific card.  The Enhanced MDMs have either a higher speed 386 or a Pentium depending on the processing requirement of that MDMs function and perform computational and communication duties rather than the sensor and effector type IO of the standards.  The Pentium was the most recent upgrade to the MDMs with 7 internal Enhanced having been upgraded and 2 external upgrades yet to be performed.  The other upgrade to the enhanced MDMs was the 2001 replacement of the rotating mass storage device with a solid state unit.  There were several failures of the rotating media on orbit prior to the solid state upgrade.  Five of the MDMs utilize the mass storage option.  In addition to the actual failures, there have been several anomalous behavior incidents which were self recovered or recovered by power cycle.  These are mostly attributable to radiation upsets (the design is rad tolerant, not fully rad hard) or power disruptions.&lt;/p&gt;&#xA;" OwnerUserId="14158" LastActivityDate="2016-04-19T14:47:32.527" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14900" PostTypeId="2" ParentId="14818" CreationDate="2016-04-19T18:57:40.697" Score="4" Body="&lt;p&gt;The first stage (Emeraude) of the 1960s French launch vehicle &lt;a href=&quot;https://en.wikipedia.org/wiki/Diamant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Diamant&lt;/a&gt; used a cycle somewhat similar to what you're describing. It had a &lt;a href=&quot;https://www.flightglobal.com/FlightPDFArchive/1962/1962%20-%201082.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;gas generator that burnt a powdered solid fuel&lt;/a&gt;. Its exhaust gases were cooled with water and injected into the first stage propellant tanks (the stage ran on nitric acid and turpentine oil).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.capcomespace.net/dossiers/espace_europeen/espace_francais/diamant_A.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Another source,&lt;/a&gt; in French though.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2020-06-27T14:03:54.863" LastActivityDate="2020-06-27T14:03:54.863" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14901" PostTypeId="1" AcceptedAnswerId="14902" CreationDate="2016-04-19T19:45:37.760" Score="10" ViewCount="342" Body="&lt;p&gt;...or whatever is the closest thing to it. Probably some laptop workstation, I guess.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words - where, inside the station is the console that is used to control ISS when it performs maneuvers - reboosts, rotation in case of incoming coronal mass ejection, rotation before separation of a craft, and so on. Is there a &quot;traditionally dedicated&quot; console for that, or is &quot;whatever laptop is at hand currently&quot; used for pulling up the interface and performing the maneuvers?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-04-20T04:00:44.703" Title="Where's the helm of the ISS?" Tags="|iss|attitude|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14902" PostTypeId="2" ParentId="14901" CreationDate="2016-04-20T04:00:44.703" Score="6" Body="&lt;p&gt;Essentially, there isn't one. Most of the planning and monitoring is done on the ground, and the execution is for the most part performed by automated guidance, control, and navigation systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of reboosts, cosmonauts and astronauts monitor the operation, but this is as a contingency backup to a contingency backup. In the case of proximity operations, astronauts and cosmonauts do again monitor the operations visually and via displays. In the case of automated rendezvous, they can command the rendezvousing spacecraft to abort the rendezvous. This should never happen; rendezvousing spacecraft are supposed to detect problems before they become visible. The cosmonauts and astronauts are backup contingencies in case all of the other backups have already failed.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-04-20T04:00:44.703" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14903" PostTypeId="1" AcceptedAnswerId="14924" CreationDate="2016-04-20T06:38:21.517" Score="1" ViewCount="842" Body="&lt;p&gt;I have read that it is better to use materials like polyethylene for protection against space radiation as compared to lead. But I need transparent materials, so lead glass seemed like a good option. I will have a layers of other transparent materials over it, but will lead glass work or will it produced secondary radiation?&lt;/p&gt;&#xA;" OwnerUserId="14148" LastActivityDate="2016-04-21T08:40:25.250" Title="Is lead glass helpful in protecting against space radiation?" Tags="|radiation|materials|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14904" PostTypeId="1" CreationDate="2016-04-20T09:43:10.437" Score="10" ViewCount="543" Body="&lt;p&gt;I noticed that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLEs&lt;/a&gt; for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt; (HST) are posted almost twice a day. Then I looked back in time and saw that this rate varies quite a lot! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plot &lt;strong&gt;shows the time interval between consecutive TLEs (in days)&lt;/strong&gt; for the HST in &lt;a href=&quot;https://www.space-track.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;space-track.org&lt;/a&gt;. The blue line is all points and the black line is a rolling average of 100 points. It seems to suddenly jump from about 0.7 day to 2 days between TLEs in May 2009 (JD ~ 2,454,960) , then slowly increase in frequency, until August 2015 when it drops back down to about 0.7 days (JD ~ 2,457,285).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do these changes (and this pattern) reflect anything of significance? Budget cuts? Shifts in Observing Program requirements? &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_Strategic_Command&quot; rel=&quot;nofollow noreferrer&quot;&gt;USSTRATCOM&lt;/a&gt; busy?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Data from 2000-01-01 to present:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;These plots are intervals (days) between successive TLEs, &lt;em&gt;not TLEs/day&lt;/em&gt;&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kxRHu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kxRHu.png&quot; alt=&quot;HST mean time between TLEs&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;CLICK TO EXPAND&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gL79g.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gL79g.png&quot; alt=&quot;HST mean time between TLEs by year&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-08-01T23:12:47.240" LastActivityDate="2020-04-24T06:54:37.657" Title="Why did the frequency of TLEs for the Hubble Space Telescope drop by a factor of 3 in 2009?" Tags="|hubble|two-line-elements|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="14906" PostTypeId="1" AcceptedAnswerId="14907" CreationDate="2016-04-20T08:05:43.803" Score="1" ViewCount="529" Body="&lt;p&gt;Refer to &lt;a href=&quot;http://www.theatlantic.com/science/archive/2016/04/yuri-milner-zuckerberg-starshot-interstellar-centauri/477669/&quot; rel=&quot;nofollow&quot;&gt;the ambitious probe sent by Yuri Milner, Stephen Hawking and Mark Zuckerberg to Alpha Centauri&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Yuri Milner is spending $100 million on a probe that could travel to&#xA;  Alpha Centauri within a generation—and he's recruited Mark Zuckerberg&#xA;  and Stephen Hawking to help. In an interview with ​The Atlantic,&#xA;  Milner makes his case for star travel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are so many unknown gravitational sources along the way, and human cannot hope to steer the direction of the space probe in real time in order to avoid them all. Assuming that you want to change the course of the probe when it's halfway through ( about 2 light years from the earth), your signal will have to travel at least 2 years before reaching the probe, but by then the probe might already have collided with a planet. It's too late. The remote control cannot be done in real time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how can scientists and engineers ensure that this probe actually reaches Alpha Centauri-- a galaxy that is a few light years away from earth-- instead of colliding with or being sucked into stars, planets, comets and other unknown gravity sources ( like dark matter perhaps)?&lt;/p&gt;&#xA;" OwnerUserId="10798" OwnerDisplayName="Graviton" LastEditorUserId="4660" LastEditDate="2016-04-20T14:07:27.427" LastActivityDate="2016-04-20T16:11:22.653" Title="How would an interstellar probe navigate the pull of gravity from stars and other large objects on the way?" Tags="|interstellar-travel|navigation|collision-avoidance|interstellar-medium|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14907" PostTypeId="2" ParentId="14906" CreationDate="2016-04-20T09:11:30.397" Score="8" Body="&lt;p&gt;There are a number of reasons why the probe does not stray off course on its journey. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;There are no known large objects between the solar system and Alpha&#xA;Centauri. Alpha Centauri is the closest star system to our own which&#xA;means that the only objects in the probe's path will be in the&#xA;interstellar medium.&lt;/li&gt;&#xA;&lt;li&gt;Interstellar space is extremely rarefied. There is hardly any matter&#xA;in the space between the large centres of gravity (stars). The&#xA;probability of the probe passing a stellar object at a distance that&#xA;affects its path is highly unlikely.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So it is simply very unlikely that that the probe will encounter any significant gravitational influence until it reaches the Centauri system&lt;/p&gt;&#xA;" OwnerDisplayName="Jaywalker" LastActivityDate="2016-04-20T09:11:30.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14908" PostTypeId="1" CreationDate="2016-04-20T13:02:21.737" Score="2" ViewCount="848" Body="&lt;p&gt;It is sometimes suggested that space ports, or at least launch pads, should be constructed on the Moon to support water exports and on Mars to support large scale exploration and settlement. I get the impression that what is proposed is to have a rover flatten the surface, maybe using additive manufacturing or sintering of the regolith. Space ports on Earth are not constructed that easily. SpaceX's new Texan space port requires a pretty huge operation. 300,000 cubic feet of soil will be moved, twice. The alternative would be to put many steel reinforced concrete pillars 200 feet deep.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it that much harder to build a space port on Earth because of the active erosion here? Or are space ports in space futuristic mega projects which cannot even begin until Mars has already been settled by humans?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.parabolicarc.com/2016/04/18/giant-pile-dirt-marks-spacexs-texas-launch-site/&quot; rel=&quot;nofollow&quot;&gt;http://www.parabolicarc.com/2016/04/18/giant-pile-dirt-marks-spacexs-texas-launch-site/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="4660" LastEditDate="2016-04-20T15:36:10.763" LastActivityDate="2021-04-19T16:26:56.357" Title="What's required to construct a space port on the Moon or Mars?" Tags="|mars|the-moon|construction|launchpad|spaceport|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14909" PostTypeId="1" CreationDate="2016-04-20T13:14:01.087" Score="13" ViewCount="3008" Body="&lt;p&gt;As Voyager 1 and 2 proceed further away do the radio waves red shift (change frequency)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, then don't we have to compensate for that red shift by modulating the frequency so that it arrives at the expected frequency at the destination?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretical Example: if the spacecraft is expecting 5.0 GHz then doesn't Earth have to transmit at 5.2 GHz to allow for the red shift?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or am I confusing issues?&lt;/p&gt;&#xA;" OwnerUserId="8808" LastEditorUserId="1142" LastEditDate="2016-04-20T14:04:28.377" LastActivityDate="2018-09-21T21:45:16.990" Title="Does communications with spacecraft red shift the further away they are?" Tags="|communication|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14910" PostTypeId="2" ParentId="14906" CreationDate="2016-04-20T13:56:16.740" Score="4" Body="&lt;p&gt;The effects of gravity falls off very quickly (&lt;a href=&quot;https://en.wikipedia.org/wiki/Inverse-square_law#Gravitation&quot; rel=&quot;nofollow&quot;&gt;following the inverse square law&lt;/a&gt;) at large distances, so the pull of other stars would be negligible on the probe.  This is even more so for planets in other systems, or comets/asteroids.  As they are so far away, stars and planets would not be an issue. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Encountering an ejected planet, asteroid, comet, etc. along the route between the stars is always possible. However, the probe would have to come very close to the object or even impact the object to be disturbed from its trajectory.  There's a very low chance of this happening, as interstellar space has &lt;a href=&quot;http://www-ssg.sr.unh.edu/ism/LISM.html&quot; rel=&quot;nofollow&quot;&gt;extremely low densities&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="1142" LastEditDate="2016-04-20T14:10:00.240" LastActivityDate="2016-04-20T14:10:00.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14911" PostTypeId="2" ParentId="14906" CreationDate="2016-04-20T14:47:51.967" Score="6" Body="&lt;p&gt;Others have already clarified that the chance of encountering another massive body in interstellar space is astronomically small.  However, there is still something to consider for course corrections, which is that a very small error in velocity early in your transfer orbit to Alpha Centauri can result in a huge error in your intercept with the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Think of it like somebody firing a single atom while in orbit around Earth, trying to set up the trajectory so that hundreds of years later, it precisely hits a microscopic target that was traveling in a different orbit.  For extra fun, also try to take into consideration miniscule perturbing effects that cause the orbits to deviate from their idealized model over that period of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, the above means that you do need some way to perform course corrections if you're going to intercept your target.  As you say, 'real-time' remote control is not feasible, so what this means is that the probe would need some automated control systems to gather information and autonomously correct course along the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a trade-off in when you make the corrections.  The earlier you make them, the less delta-V (and hence fuel) required.  But, due to the mindbogglingly huge distances involved, the more precise/accurate you need to be.  In practice, you probably schedule a number of short burns along the way, so that you keep the delta-V required for your 'late' adjustments to a minimum.  How you schedule those adjustments is likely a matter of careful optimization based on estimated error magnitudes.&lt;/p&gt;&#xA;" OwnerUserId="12168" LastEditorUserId="12168" LastEditDate="2016-04-20T16:11:22.653" LastActivityDate="2016-04-20T16:11:22.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14912" PostTypeId="2" ParentId="14909" CreationDate="2016-04-20T15:30:23.137" Score="17" Body="&lt;p&gt;&lt;strong&gt;No, at the distances we have sent any spacecraft or probe to, there is no need to compensate for redshift (the &lt;a href=&quot;https://en.wikipedia.org/wiki/Doppler_effect&quot;&gt;Doppler effect&lt;/a&gt;) because a spacecraft is far away. Doppler shift is an effect of relative velocity, not distance.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Doppler effect is defined such that the frequency $f$ as observed by a receiver moving at velocity $v_r$ relative to a source moving at velocity $v_s$ and where the source is transmitting at frequency $f_0$ is $$ f = \left( \frac{c + v_r}{c + v_s} \right) f_0 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of radio transmissions, $c$ is the speed of light. $v_r$ and $v_s$ are measured in some inertial (that's basically a fancy word for &quot;non-moving&quot;) reference frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a stationary sender or receiver, simply set $v_s = 0$ or $v_r = 0$ respectively. From this it is obvious how, when both sender and receiver are stationary (in an inertial reference frame), the transmitted frequency equals the received frequency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, for a given type of transmission (such as a radio transmission), only the relative velocities of the sender and receiver and the transmission frequency affect the received frequency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This effect is caused by the fact that when the sender is moving toward the receiver, each wave front is closer to its neighboring wave fronts than merely its frequency would warrant. Likewise, when the two are moving away from each other, each wave front is farther away from its neighbors. Colloquially, the transmission is being &quot;pushed together&quot; or &quot;dragged apart&quot; by the relative movement of the sender and receiver. This is &lt;em&gt;observed&lt;/em&gt; as a change in frequency at the receiver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The distance between the two do not play a role in this effect, and thus we do not need to compensate for Doppler shift merely because an object is far away. We do however have to compensate for Doppler shift because the object is moving relative to us. Of course, because spacecraft generally move at relatively high speeds relative to an Earth ground station, &lt;strong&gt;in spacecraft communications we often &lt;em&gt;do&lt;/em&gt; need to pay attention to Doppler shift,&lt;/strong&gt; regardless of whether they are close to or far from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;For example,&lt;/strong&gt; if the spacecraft is moving away from the Sun at 15 km/s (not unreasonable at all for interplanetary missions) and Earth happens to be moving in the opposite direction at 30 km/s (which it does once a year), and the spacecraft is transmitting on 5 GHz, the frequency as received on Earth will be $$ f = \left( \frac{300~000~000 - 15~000}{300~000~000 + 30~000} \right) \times 5~000~000~000 \approx 4~999~250~075 $$ or approximately 4999.25 MHz instead of 5000.00 MHz. Conversely, if we want the spacecraft to receive our transmission on 5000.00 MHz, then we need to transmit at approximately 5000.75 MHz. If we didn't compensate for Doppler shift, we would need a receiver bandpass filter bandwidth of nearly a megahertz; enough to cram in two or three broadcast FM stations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also worth keeping in mind that this is an issue not just for deep space missions. The math is a bit more involved because of the trigonometry to calculate the absolute value of the velocity and if you want an absolute value also compensating for the planet's rotation, but &lt;strong&gt;the exact same principles apply when a low orbit spacecraft is communicating with a ground station&lt;/strong&gt; or for that matter with a spacecraft in a different orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If we were to send spacecraft to other galaxies or even far-away stars,&lt;/strong&gt; then Doppler shift due to the expansion of the universe might start to actually matter. However, for as long as it can be argued whether we have even &lt;em&gt;exited our own solar system,&lt;/em&gt; the relative velocities caused by expansion of the universe are of magnitudes that are completely irrelevant for practical radio communications. The expansion of the universe is given as &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble%27s_law#Observed_values&quot;&gt;Hubble's constant&lt;/a&gt; $H_0$, and the most recent estimate for its value is $67~800 \pm 770$ m/s per megaparsec. The Voyagers are about 134 and 110 AU away, respectively, which works out to $6.497 \times 10^{-10}$ and $5.333 \times 10^{-10}$ megaparsec. If my math is right, taking Voyager 1 because it is farther away, this means an observed expansion rate of about 0.00004405 m/s (44 µm/s) corresponding to a received frequency of $$ \left( \frac{300~000~000}{300~000~000 + 0.00004405} \right) \times 5~000~000~000 = 4~999~999~999.999265833 $$ or a Doppler shift of about 0.001 Hz when the transmitted frequency is exactly 5 GHz. In other words, valid in theory but &lt;strong&gt;completely negligible in practice.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2016-04-20T19:52:37.060" LastActivityDate="2016-04-20T19:52:37.060" CommentCount="19" ContentLicense="CC BY-SA 3.0" />
  <row Id="14913" PostTypeId="1" CreationDate="2016-04-20T16:13:50.407" Score="7" ViewCount="229" Body="&lt;p&gt;The Starshot Breakthrough Initiative, sponsored by Yuri Milner, now finances the attempt to use a huge laser on Earth to propell a tiny spacecraft to huge velocity. Not having to carry the energy for its own propulsion, the rocket equation is bypassed. But could such an energy beam from Earth also solve some other of the huge challenges of interstellar travel?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Power&lt;/strong&gt;. Could a spacecraft transform the energy beam to electric power for use onboard?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Communication&lt;/strong&gt;. Could the energy beam be modulated to communicate data to the spacecraft and be reflected in a modulating way by the spacecraft's sail to send data back?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Impact prevention&lt;/strong&gt;. Could parts of the energy beam be focused by parts of the sail to first detect impacting dust grains ahead from their reflections, and the refocused to evaporate them?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Beam reinforcement&lt;/strong&gt;. Could a convoy of similar sails (without payload) be launched regularly to refocus the energy beam to the next sail ahead?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Deceleration&lt;/strong&gt;. Could such a convoy or the spacecraft itself, use the energy beam to decellerate the flyby speed?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Teleacope&lt;/strong&gt;. Could the sail be reconfigured to work as a teleacopic mirror to observe the target star system as it approaches?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Could multiple such functions be performed by a single low mass large area thin sail/photovoltaic/reflector/lens component? I understand that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Technology_readiness_level&quot; rel=&quot;nofollow&quot;&gt;TRL&lt;/a&gt; for this is low, but basic physical challenges and opportunities maybe can be identified anyway.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-04-21T01:05:28.083" LastActivityDate="2016-04-21T01:05:28.083" Title="How could the laser from Earth to a spacecraft be used other than to propel its light sail?" Tags="|interstellar-travel|mission-design|optical|beamed-propulsion|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14914" PostTypeId="2" ParentId="14909" CreationDate="2016-04-20T16:32:30.793" Score="4" Body="&lt;p&gt;Let me add to @Michael's answer by saying that this is also very much a factor when communicating with earth-orbiting satellites. For instance, the ISS signal bounces off a geostationary communications satellite before it (the signal) gets to the ground. If I recall correctly, these two spacecraft can differ in velocity up to 7 km/s. Not only can this cause either red or blue shift, but the Doppler shift varies dynamically during a single communication event. So Doppler handling is an important feature in the modems used to process these communications.&lt;/p&gt;&#xA;" OwnerUserId="14169" LastActivityDate="2016-04-20T16:32:30.793" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14915" PostTypeId="2" ParentId="624" CreationDate="2016-04-20T16:47:53.420" Score="2" Body="&lt;p&gt;I think that Earth - Moon communications with a CubeSat in lunar orbit are feasible, even though it would require a lot of effort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Closing the link is a trade off between operational frequency, the available power on the satellite, and the volume on the satellite available for the antenna (the higher the frequency, the higher the path losses, the smaller the antenna).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How is the communication supposed to work with a cubesat over such long distances?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Mostly in the same way, except for the very long path, which means very high losses.&#xA;The spacecraft tracking would be less demanding in terms of speed, but more demanding in terms of pointing accuracy.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Has it been researched or proposed for 1U cubesats?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't think so, only 3U or (preferably) 6U cubesats have been considered so far for beyond-LEO missions.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Are there any special requirements on frequencies, antennas, power consumption ... ?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Basically, you would require a very good pointing mechanism, a very directive and high gain antenna, and a very good low noise amplifier.&#xA;Which is something uncommon for the usual backyard ground station.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What needs to be taken care of if you want to use your hobby ground station for something like this, if it is already working for cubesats in LEO?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Answers to the previous questions are all points that require some effort.&lt;/p&gt;&#xA;" OwnerUserId="12130" LastActivityDate="2016-04-20T16:47:53.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14916" PostTypeId="2" ParentId="14909" CreationDate="2016-04-20T18:16:54.870" Score="7" Body="&lt;p&gt;Contrary to the other answers, distance from the earth does does cause/influence red-shift (albeit indirectly).  The universe is expanding away from us, and the further away a point in space is, the faster the expansion is at that point.  The expansion rate is rather regular and predictable at macroscopic scales, so scientists actually use red-shifts of hydrogen emission spectra for distant stars to gauge how far stars and galaxies are from us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you had a spacecraft very, very far away, the universe's relative expansion to us would cause signals between us and the craft to be redshifted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The expansion rate of our universe is usually given as Hubble's constant -- $H_0 = 67.15 (km/s)/Mpc$.  We can calculate the velocity of the universe's expansion at a given distance as $H_0$ * the distance away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in a distance like the distance to the closest star in &lt;a href=&quot;http://www.wolframalpha.com/input/?i=67.15+(km%2Fs)%2FMpc+*+distance+to+alpha+centauri&quot; rel=&quot;nofollow&quot;&gt;wolfram alpha&lt;/a&gt;, we get that between us and Alpha Centauri, the universe is expanding at a rate of 0.2 miles per hour, or 0.3 kilometers per hour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is probably much lower than the actual relative velocity between us and Alpha Centurai just due to the fact that we're moving in different directions!  So redshift will not be a significant factor if we ever decide to communicate to Alpha Centauri -- our actual relative velocities due to our relative motion will be more significant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At intergalactic differences things might start getting a bit more noticeable.  Between us and andromeda, the universe is expanding at a rate of about 53 km/s, about 120 million miles per hour, or about one ten thousandths of the speed of light.  This is still not really enough to be terribly significant, I believe, but it does give a good scale as to how distance necessarily affects relative velocity and, indirectly, redshift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt; A comment was made that reminded me that while expansion is homogeneous at the macroscopic level, the metric expansion of space might behave as homogeneously at sub-lightyear levels.  I'm not familiar enough with this topic to give a definitive answer as to what models could be useful here.&lt;/p&gt;&#xA;" OwnerUserId="14172" LastEditorUserId="14172" LastEditDate="2016-04-20T20:39:03.833" LastActivityDate="2016-04-20T20:39:03.833" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="14917" PostTypeId="2" ParentId="14913" CreationDate="2016-04-20T22:09:54.903" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Power. Could a spacecraft transform the energy beam to electric power&#xA;  for use onboard?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, a spacecraft can definitely turn a steady flux of light into electrical energy. A device that does exactly that is pictured below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BDpNUm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BDpNUm.jpg&quot; alt=&quot;solar panel&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar are more effective at some wavelengths than others, so the monochromatic light of a laser is especially useful for energy transmission.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Communication. Could the energy beam be modulated to communicate data&#xA;  to the spacecraft and be reflected in a modulating way by the&#xA;  spacecraft's sail to send data back?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At large distances, the targeting of the spacecraft is mostly about hitting it with at least a part of your light cone (yes, &lt;a href=&quot;https://physics.stackexchange.com/questions/79417/why-does-a-laser-beam-diverge&quot;&gt;lasers diverge too&lt;/a&gt;). You then have the opportunity to use single-blinking-light communication, such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Morse_code&quot; rel=&quot;nofollow noreferrer&quot;&gt;Morse code&lt;/a&gt; or plain binary bits. Variations in the intensity of the beam can also be used to transfer the information in a analogue manner. There are limitations to this form of communications though. Big lasers are not easy to just turn on and off quickly, and an occultation disc is going to experience the most serious overheating problems ever heard of. A better solution might be to slightly tilt the beam a tiny bit back and forth, but getting it pointed right on target again is perhaps difficult. In short, it can be used for communications, but the bandwidth is surely limited.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Impact prevention. Could parts of the energy beam be focused by parts&#xA;  of the sail to first detect impacting dust grains ahead from their&#xA;  reflections, and the refocused to evaporate them?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I am more in doubt about this one, first of all, looking ahead is simply not enough. This situation, for example, is not going to end well:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oi4lgm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oi4lgm.png&quot; alt=&quot;collision&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As velocities in space are relative, impacting objects may come from every direction. The difficulties with this method is more in the ways of actually being able to discover very small objects at a fairly large distance, and having the ability to illuminate them is not helping that much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for combining multiple functions into one, say sail/photovoltaic, I can not see many situations where this is useful. The reason here is the enormous difference in pushing force and energy content of the beam. $1N$ to $150,000,000W$. That means that the area you are going to need for power production is far smaller than the propulsive part of the sail. That is convenient as well, as photovoltaics are much thicker and heavier than the currently used sail materials. Other things, like sail/reflector is much more reasonable, as the sail &lt;em&gt;is&lt;/em&gt; a reflector. Throwing optical lensing properties into the mix should certainly possible as well, at the cost of more structural mass for calibration and accuracy.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2016-04-20T22:09:54.903" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14918" PostTypeId="2" ParentId="14908" CreationDate="2016-04-20T22:22:43.107" Score="5" Body="&lt;p&gt;You need a flat surface, that is it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo lunar landers landed at the Moon, without any present infrastructure. The descent stage then served as a space port pocket edition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a zero-infrastructure solution is possible, improving on that is trivial. Any action that makes the landing zone flatter or larger, and removes disturbing rocks and cliffs is helping, improving the site's quality as a space port. Additionally, a few pieces of navigation helping equipment can be deployed to make operations more accurate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This may sound like a cheap argument, and it is. But the actual difficult and labour requiring infrastructure is not really directly tied to the space port itself. It is the production of space craft, components, materials and fuel in-situ that is the difficult part. The space port itself can be as simple as you desire.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-20T22:22:43.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14919" PostTypeId="2" ParentId="13799" CreationDate="2016-04-20T22:47:48.963" Score="12" Body="&lt;p&gt;There are several kinds of hatches on the ISS to permit many different kinds of vessels and modules from different sources to connect to the station. CBM is one of those, and as you note is an asymmetric gendered connection system with active and passive nodes. The ISS is equipped with both active and passive CBM nodes. The full range of connectors available and their locations can be illustrated by this diagram:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/X3loB.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/X3loB.png&quot; alt=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110010964.pdf&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Source: &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110010964.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110010964.pdf&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CBMs connect in pairs, one passive and one active, each of which has a hatch. Both hatches can be opened from either side, but are normally opened from the active port side. The volume beyond each hatch can be (and is) pressurised and remains pressurised. When a volume berths it is pressurised and when it unberths it remains pressurised. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a small space between the two hatches which is called is &lt;em&gt;vestibule&lt;/em&gt; which becomes exposed to the vacuum of space. The pressure within this vestibule must be managed as part of the berthing and unberthing procedures. The temperature gradient between the two modules must also be equalised as part of the berthing procedures as unequal thermal expansion can affect the firm and safe connection of the two module mechanisms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hatches of each CBM open inwards and upwards at 90&amp;deg; to the connection and have a latch to hold them in the open position. You also asked about the cupola. Unusually this does not have its own hatch, but it uses a passive CBM. This is because it is so short and there is no room for an opening hatch. Only the main node can close it off with a hatch. This is shown by the two illustrations of the Cupola before installation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/3/3e/Cupola_being_aligned_with_the_Tranquility.jpg&quot; alt=&quot;Copola shown no hatch&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Cupola_(ISS_module)&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VlUqy.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VlUqy.png&quot; alt=&quot;ISS Cupola diagram&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Source: &lt;a href=&quot;http://www.bbc.co.uk/blogs/thereporters/jonathanamos/2010/02/the-cupola-the-window-on-the-w.shtml&quot; rel=&quot;noreferrer&quot;&gt;BBC News&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much of the illustration of the CBM hatches was shown when when the MPM (Multipurpose Module) Leonardo was moved on the ISS. It was originally positioned on the Unity Module; the hatches were closed and it was relocated (using the Canadarm) to the Tranquility module whilst still pressurised. There are videos of the hatches opening and closing as well as its original installation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=3e3woEMPboY&quot; rel=&quot;noreferrer&quot;&gt;STS-128: HATCH OPENING TO LEONARDO&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=PYLdkyVJrEw&quot; rel=&quot;noreferrer&quot;&gt;NASA: Hatch Closing: International space station moved to higher orbit&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=CwGeZENfkWw&amp;amp;list=PLACeE0JtEtY8JBxhoXiyq36Kl19swLFfx&amp;amp;index=1&quot; rel=&quot;noreferrer&quot;&gt;PMM Leonardo Relocation on ISS International Space Station 2015 NASA&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(1) Cook, Aksamentov, Hoffman &amp;amp;  Bruner (2011), &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110010964.pdf&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;ISS Interface Mechanisms and their Heritage&lt;/em&gt;&lt;/a&gt;&lt;br&gt;&#xA;(2) Newby, Mark A (2000), &lt;a href=&quot;http://pages.erau.edu/~ericksol/projects/issa/s_m.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;International Space Station Structures &amp;amp; Mechanisms&lt;/em&gt;&lt;/a&gt;&lt;br&gt;&#xA;(3) McLaughlin &amp;amp; Warr (2001), &lt;a href=&quot;http://spacecraft.ssl.umd.edu/design_lib/ICES01-2435.ISS_CBM.pdf&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;The Common Berthing Mechanism (CBM) for International&#xA;Space Station&lt;/em&gt;&lt;/a&gt;&lt;br&gt;&#xA;(4) Harding, Pete (2013), &lt;a href=&quot;https://www.nasaspaceflight.com/2013/07/nasa-planning-module-relocations-future-vehicles/&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;NASA planning ISS module relocations to support future crew vehicles&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="8162" LastEditDate="2016-04-21T22:36:28.897" LastActivityDate="2016-04-21T22:36:28.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14921" PostTypeId="1" AcceptedAnswerId="17319" CreationDate="2016-04-21T04:40:50.533" Score="0" ViewCount="310" Body="&lt;p&gt;All of the discussion around this &lt;a href=&quot;https://space.stackexchange.com/q/14909/12102&quot;&gt;question&lt;/a&gt; and in the &lt;em&gt;comments below this&lt;/em&gt; &lt;a href=&quot;https://space.stackexchange.com/a/14912/12102&quot;&gt;&lt;em&gt;answer&lt;/em&gt;&lt;/a&gt;, about the local effects of &quot;the expansion of space&quot; (&lt;a href=&quot;https://en.wikipedia.org/wiki/Metric_expansion_of_space&quot; rel=&quot;nofollow noreferrer&quot;&gt;Metric expansion of space&lt;/a&gt;) or &lt;a href=&quot;http://astronomy.swin.edu.au/cosmos/c/cosmological+redshift&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cosmological redshift&lt;/a&gt; or just simply the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble%27s_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hubble constant&lt;/a&gt; has got me wondering about the feasibility of detecting or even measuring the effect directly using spacecraft as a probe of distance.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most (&lt;s&gt;if not all&lt;/s&gt; see edit below) of the data so far come from interpreting measured shifts of spectral features. These include atomic and molecular emission and absorption lines, as well as broader features including Blackbody radiation and ad-hoc averages of stellar populations. They all have one thing in common - &lt;em&gt;they are interpreted measurements of one-way light from things really really far away&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we can now actually put complex instruments on the order of ten billion kilometers away and still exchange data with them and &lt;em&gt;perform measurements on them&lt;/em&gt;, it's not unreasonable to start thinking of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Scientific_control&quot; rel=&quot;nofollow noreferrer&quot;&gt;controlled experimental measurement&lt;/a&gt; of the effect. For example this might be done by &lt;a href=&quot;https://space.stackexchange.com/a/14683/12102&quot;&gt;Doppler ranging&lt;/a&gt; using reflection, or better yet reception and simultaneous rebroadcast by a spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is - have there been institutionally proposed, or peer-reviewed and published discussions of the detection or measurement of the Metric expansion of space using spacecraft as a probe of distance?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I've changed the title to be plural - I don't want to exclude things like (just for example) compensation for the gradient in local gravity field for want of an &quot;s&quot;. (imagine six spacecraft exploring +/- x, y, z)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note - I am &lt;strong&gt;not&lt;/strong&gt; asking if &lt;em&gt;you&lt;/em&gt; think it can or can't be done. I'd like to read serious discussions or proposals on the subject that quantitatively address feasibility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; &lt;a href=&quot;http://arxiv.org/pdf/gr-qc/0411113v2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This&lt;/a&gt; was shared with me in &lt;a href=&quot;https://physics.stackexchange.com/a/251132/83380&quot;&gt;this answer&lt;/a&gt;, and it's a good example of something quantitative, though it's data rather than explanation. The main focus of the paper is experimental testing of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Equivalence_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Equivalence Principle&lt;/a&gt; (EP; gravitational mass vs inertial mass) using the Sun, Earth and Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to be in the negative as far as the present question is concerned. Expansion seems to be quite readily &quot;measureable&quot;, and their results seem to show it to be very small and consistent with zero locally. It reports on &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laser ranging of the retro-reflector arrays on the moon&lt;/a&gt;. In 34 years, the distance between the earth and moon has followed the best predictions to within a scatter of about 2cm. If my arithmetic is correct, expansion of space at the Hubble constant rate would be about 82cm over 34 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While this doesn't help me understand why, it certainly suggests in a peer-reviewed, quantitative way that the expansion &lt;em&gt;might also not be seen&lt;/em&gt; by ranging spacecraft in distant orbit around the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;APOLLO 11:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gukac.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gukac.jpg&quot; alt=&quot;APOLLO 11 retroreflector array&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;APOLLO 14:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UAwYl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UAwYl.jpg&quot; alt=&quot;APOLLO 14 retroreflector array&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;APOLLO 15:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0CxL1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0CxL1.jpg&quot; alt=&quot;APOLLO 15 retroreflector array&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;GAO Laser Ranging Facility at the Goddard Spaceflight Center:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wHSaS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wHSaS.jpg&quot; alt=&quot;GAO Laser Ranging Facility at the Goddard Spaceflight Center&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-07-19T18:36:55.353" Title="Local expansion measured, near zero via Lunar Ranging - what about deep space probes?" Tags="|spacecraft|experiment|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="14922" PostTypeId="1" AcceptedAnswerId="14933" CreationDate="2016-04-21T05:02:01.183" Score="12" ViewCount="6156" Body="&lt;p&gt;With the recovery of the first stage on their most recent launch, I was excited and congratulatory, of course. Then my thoughts went forward to the re-use of this launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would seem that the most clear-eyed assessment of the risk for the re-launch would come from the insurers of that launch.   Thus the question: who's been providing the insurance for SpaceX?   Are they self-insured?   I would think not.&lt;/p&gt;&#xA;" OwnerUserId="14179" LastEditorUserId="8693" LastEditDate="2016-05-08T21:19:53.480" LastActivityDate="2016-05-08T21:19:53.480" Title="Who provides the insurance for SpaceX launches?" Tags="|launch|rockets|spacex|economics|insurance|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14923" PostTypeId="1" CreationDate="2016-04-21T08:09:00.627" Score="11" ViewCount="1922" Body="&lt;p&gt;I'm writing a sci-fi novel and I need some clarification on some technical issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An HEMP is an &lt;a href=&quot;https://en.wikipedia.org/wiki/High-altitude_nuclear_explosion&quot;&gt;high-altitude EMP&lt;/a&gt;. A 400km high, high-yield nuclear explosion over Kansas could &quot;switch off&quot; all the continental US.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, we could use a missile to take the warhead in the right position. But &quot;400km&quot; is also the orbital height of the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So: since that such a launch could be easily detected (I suppose), is it possible to put a nuclear warhead in that position using a &quot;peaceful&quot; space launch as a &quot;camouflage&quot;? What's the best way to do it? &lt;/p&gt;&#xA;" OwnerUserId="14181" LastEditorUserId="415" LastEditDate="2016-04-21T14:57:26.330" LastActivityDate="2016-04-21T14:57:26.330" Title="Is it possible to covertly put an EMP weapon into an ISS-like orbit by masquerading it as a legitimate launch?" Tags="|artificial-satellite|low-earth-orbit|nuclear|millitary|weapon|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="14924" PostTypeId="2" ParentId="14903" CreationDate="2016-04-21T08:40:25.250" Score="1" Body="&lt;p&gt;I haven't found specific research into lead glass for space applications. But the &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20040086568&amp;amp;hterms=shielding%20performance%20index%20Wilson&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3Dshielding%2520performance%2520index%2520Wilson%26Ntx%3Dmode%2520matchallpartial&quot; rel=&quot;nofollow&quot;&gt;general principle&lt;/a&gt; that light atoms are better than heavy ones for shielding against space radiation also applies to lead glass. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-21T08:40:25.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14925" PostTypeId="2" ParentId="14923" CreationDate="2016-04-21T09:22:57.690" Score="18" Body="&lt;p&gt;Classified satellites are launched all the time. For example, fourteen &lt;a href=&quot;https://en.wikipedia.org/wiki/KH-11_Kennan&quot;&gt;KH-11&lt;/a&gt; espionage satellites were launched between 1976 and 2013, and we still have no idea what they look like. So you can launch pretty much what you want and nobody will know for sure what it is. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A satellite that does nothing would be suspicious, so you'd have to stick your warhead inside an operational satellite: one that generates enough data traffic to give a plausible explanation for its existence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long-range detection of a bomb is &lt;a href=&quot;https://www.fas.org/sgp/crs/nuke/R40154.pdf&quot;&gt;very difficult&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Defense Threat Reduction Agency (DTRA), a unit of DOD, is pursuing capabilities to increase the distance at which SNM (Special Nuclear Material, i.e. uranium or plutonium) can be detected, with the goal of increasing it to &lt;em&gt;1 km&lt;/em&gt; or more.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(emphasis mine)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But all that doesn't matter. Space launches are easy to monitor, so everyone would know who launched this satellite. That makes it a one-shot weapon. When it's used, you can expect massive retaliation as soon as the victim nation is able. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-21T09:22:57.690" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14926" PostTypeId="2" ParentId="14908" CreationDate="2016-04-21T09:58:00.057" Score="4" Body="&lt;p&gt;One big factor is Earth's gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To reach space from Earth, any spacecraft needs to be huge. It requires a huge crawler and good crawlerway. It requires a solid, big launchpad to support the weight of the craft and infrastructure to keep it upright, fueling it with hundreds or thousands of tons of fuel, keeping it powered, and safe against the environment (including undisciplined bystanders). The mighty engines require flame trenches to contain the blast of the exhaust and protect the craft from it. There's a big water tower and a massive sprinkling (more like flooding) system to extinguish the sound waves from the launch which are enough to destroy the craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This all weighs a lot and needs structural infrastructure to support it and keep it from simply sinking into the soil or breaking under own weight. That's why Earth spaceports are so huge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Moon, the delta-V needed to reach Moon orbit is way, way smaller, and taking the Rocket Equation into account, the needed size of the craft is exponentially smaller. Smaller engines, as not nearly as much thrust is needed. Way smaller tanks. Supply pipes and cables that will not snap under own weight. That means far less - exponentially less - infrastructure needed to support the launch. Your craft will be maybe 20 ton instead of 20,000, for similar payload to the orbit. Even completely independently of the same structural construction being able to support six times the mass it could support on Earth, simply due to everything being lighter!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars lies somewhere in the middle mass-wise, but again, the rocket equation with its exponent is on our side, reducing the mass exponentially in relation to the gravity. You might need more infrastucture, but really, a simple slab of concrete (or something similar) will suffice where the launchpad with flame trenches and water dump system was required.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-04-21T09:58:00.057" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14927" PostTypeId="2" ParentId="14908" CreationDate="2016-04-21T10:32:20.437" Score="2" Body="&lt;p&gt;The spaceport SpaceX is building in Texas consists of at least the following:   &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;launch pad capable of supporting the Falcon Heavy, FH weight is in the region of 1500 tons.&lt;/li&gt;&#xA;&lt;li&gt;a building for horizontal assembly of the FH, i.e. 75+ m long, and capable of supporting 30-ton capacity gantry cranes that have to be in precise positions to assemble the rocket. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Both of these require good foundations, because the terrain consists of soil: loose, compressible material. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For now, all we need on Mars is a launch pad that can support an ascent stage that can reach Mars orbit, and that can weigh as little as &lt;a href=&quot;https://space.stackexchange.com/questions/2820/how-big-would-a-manned-ascent-stage-for-mars-need-to-be&quot;&gt;22&lt;/a&gt; tons. Maybe a bit more to accommodate the larger lander that brings the ascent stage down. &#xA;Clearing away the loose regolith until you encounter solid rock would be an easy way to construct such a launch pad. Add a few blast barriers and you're done.&lt;br&gt;&#xA;It's not until you start building rockets on Mars that you need a Texas-scale spaceport. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-04-21T10:32:20.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14928" PostTypeId="1" CreationDate="2016-04-21T11:00:45.253" Score="11" ViewCount="585" Body="&lt;p&gt;It has been proposed that the US military should repurpose suborbital, nuclear weapons carrying, intercontinental ballistic missiles for civil use, like Russia and Ukraine has done since decades. How many ICBM's would this concern? And what kinds of civil mission profiles would they be capable of? Aren't they overkill for use as sounding rockets? What other suborbital use could come in question? What payload could they launch to LEO? Could they resupply the Space Station? How much could they put in Lunar orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are they more valuable for civil purposes if they are reconstructed, maybe even used as solid strap on boostes on a liquid fuel orbital launcher?&#xA;And why do they become &quot;surplus&quot;? Disarmament or aging?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2016-04-21T14:15:36.957" LastActivityDate="2016-04-22T20:39:11.477" Title="What kind of missions are US surplus ICBM's capable of launching?" Tags="|launch|solid-fuel|launch-vehicle|orbital|icbm|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14929" PostTypeId="2" ParentId="624" CreationDate="2016-04-21T11:36:39.183" Score="4" Body="&lt;p&gt;Yes, it's being researched. For instance, &lt;a href=&quot;https://icubesat.org/papers/2014-2/2014-b-1-4-ultra-compact-ka-band-parabolic-deployable-antenna-for-cubesats/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this JPL paper&lt;/a&gt; describes a design for a parabolic antenna with a diameter of 50 cm that can be folded and stowed in 1.5 U, and provides a gain of 42 dB.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cFeUB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cFeUB.jpg&quot; alt=&quot;JPL parabolic antenna&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another approach is the reflectarray, an antenna that uses a flat panel instead of a dish, with structures on the panel that look like a Fresnel lens to focus the radio waves.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;JPL &lt;a href=&quot;http://www.jpl.nasa.gov/cubesat/isara.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;is looking into this&lt;/a&gt; as well.&lt;br&gt;&#xA;MMA Design is working on a &lt;a href=&quot;https://marscubesatworkshop.jpl.nasa.gov/static/files/presentation/Harvey-Thomas/deployable.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;reflectarray&lt;/a&gt; of 1 m diameter, packs into 1U, also gain in the region of 40 dB. This design combines the antenna with the solar panel:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GLAwD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GLAwD.png&quot; alt=&quot;MMA reflectarray&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-04-21T19:13:23.000" LastActivityDate="2016-04-21T19:13:23.000" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14930" PostTypeId="2" ParentId="14928" CreationDate="2016-04-21T13:01:56.393" Score="9" Body="&lt;p&gt;This is actually a bit tricky, and relates to one of the basic tenants of the space race. The United States made small, accurate weapons, the USSR made large, less accurate weapons. Because the USSRs weapons were larger, and therefore heavier, it required heavier missiles to launch them, and thus they have the mass to achieve orbit if required, while the US's fell short. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is exactly 1 long range US ballistic missile that is currently operational, the &lt;a href=&quot;https://en.wikipedia.org/wiki/LGM-30_Minuteman&quot;&gt;Minuteman&lt;/a&gt;. The speed which Wikipedia lists is 7 km/s. Orbital speed is about 7.8 km/s, so the speed is very close to the minimum required to achieve orbit. Estimated payload mass is ~350-400 kg. Thus, it might be possible to get a very small payload, on the order of 200 kg, in to orbit, especially if a fourth stage was introduced to give the required ~1 km/s delta v required to obtain orbit (Including the fact that a very low orbit isn't desired typically). That would be in LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It turns out that modified Peacekeeper missiles have been retrofitted for orbital missions, by the program of &lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_%28rocket_family%29#Minotaur_VI&quot;&gt;Minotaur&lt;/a&gt;, which can carry up to 1700 kg, by adding additional stages and other modifications. These are highly modified, however, the most significant component of reuse being the solid rocket motors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dnepr-1&quot;&gt;DNEPR-1&lt;/a&gt; rocket, the most commonly used Russian ICBM turned to rocket, has a payload of about 4500 kg in to LEO. It has some uses, but is still a very small payload, it could lift a very small ISS supply mission, but not much beyond that. The ISS payload is listed at only 3200 kg.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-21T15:04:05.047" LastActivityDate="2016-04-21T15:04:05.047" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14932" PostTypeId="2" ParentId="14923" CreationDate="2016-04-21T13:54:31.147" Score="4" Body="&lt;p&gt;First of all, yes, a payload can be launched in to space that would remain at the key altitude for a HEMP, and stay there for some period of time. This has been discussed in terms of North Korea, for instance, which has launched satellites into an orbit that a HEMP attack would be possible, although they likely don't have the payload capacity to launch a weapon in to that orbit today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so how would one hide who did this? Hiding the launch is all but impossible, the United States monitors for launches continually across the world, due in large part to the ICBM threat. So one would have to launch a satellite that contained this as a secondary payload. Once the payload was in orbit, the HEMP payload would have to remain, something that large is essentially impossible to hide in LEO today. The track for the satellite would be well established for a long period of time in the future. That track would be tied to the country of origin. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it then possible to confuse who launched the attack? Well, maybe. The best way that I can think of would be to explode the weapon when it was physically close to another satellite. I don't know the accuracy of nuclear weapon detection systems, but if two or even 3 satellites were physically close, say, withing 10 km, it might be difficult to determine which of the 3 detonated the weapon. In particular, it might be an interesting scenario if there was both a US and a Russian satellite close together, along with this hidden device. It might be possible to track exactly what happened, but it would take some time. The primary satellite would be completely destroyed, the other ones would likely be in pieces, and non-functional, and even in different orbits, but given some analysis it would likely be possible to determine which satellite of the 3 was the culprit, although it likely would take hours to figure out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, a few more tidbits. The shape of the satellite might be known from the ground, due to advanced radar systems, which no doubt would be employed to their max for any &quot;hostile&quot; country's launch. Any such item would have to be disguised to prevent that, which might be met with varying success. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-21T14:09:27.517" LastActivityDate="2016-04-21T14:09:27.517" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14933" PostTypeId="2" ParentId="14922" CreationDate="2016-04-21T14:04:47.670" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www.aon.com/singapore/risk-services/products-services/isb.jsp&quot;&gt;Aon International Space Brokers&lt;/a&gt; does, according to &lt;a href=&quot;http://www.businessinsurance.com/article/20131229/NEWS07/312299982&quot;&gt;this article&lt;/a&gt;. Although they likely only cover liability type losses from SpaceX, not the cost of the rocket themselves. In fact, this &lt;a href=&quot;http://apps.fcc.gov/ecfs/document/view;jsessionid=ZP1cQlzCln1zHsJtqGPSgRG6rL32b26G1vBZdRFP3GNC4PJ9LwCR!-224088840!NONE?id=7022022886&quot;&gt;2012 FCC document&lt;/a&gt; states that SpaceX has 3rd party liability insurance for the first 45 days after a launch, afterward which they are self-insured.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each payload that is on any rocket tends to be insured to some extent by the payload owner. They may use a variety of insurance providers. These insurance providers will typically review the tests of the satellites and rockets to ensure they are satisfied before agreeing to the launch, and are thus given a seat in making launch decisions. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-21T14:04:47.670" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14936" PostTypeId="2" ParentId="13340" CreationDate="2016-04-21T19:16:30.793" Score="7" Body="&lt;p&gt;The earliest communication constellation, as you noted, is likely the &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_%28satellite%29&quot;&gt;Moylniya satellites&lt;/a&gt;, which have been active since 1965, and as a constellation since 1968.   There were a number of Spy satellites which might be considered constellations in the early 60s, most notably the &lt;a href=&quot;https://en.wikipedia.org/wiki/Corona_%28satellite%29&quot;&gt;Corona&lt;/a&gt;. Depending on your definition, I would choose one of those two.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-04-21T19:16:30.793" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14939" PostTypeId="1" AcceptedAnswerId="14942" CreationDate="2016-04-22T01:06:44.713" Score="3" ViewCount="457" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/14936/12102&quot;&gt;This answer&lt;/a&gt; links to an &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_%28satellite%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; on the Molniya satellites, which shows the image below of Molniya 1, and what looks like a large structure behind it. The structure appears to be long and slightly concave rather than flat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is that structure, or what does it represent?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Molniya-1_satellite.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gV8rd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gV8rd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-05-12T01:45:16.227" LastActivityDate="2021-05-12T13:19:09.440" Title="What is the large structure in the background of this picture of Molniya 1?" Tags="|artificial-satellite|museum-exhibits|molniya|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14940" PostTypeId="2" ParentId="14939" CreationDate="2016-04-22T01:42:14.053" Score="3" Body="&lt;p&gt;The image caption says &quot;Molniya-1 satellite in a museum&quot;, so that's the museum roof.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A large structure indeed!&lt;/p&gt;&#xA;" OwnerUserId="1348" LastActivityDate="2016-04-22T01:42:14.053" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14941" PostTypeId="1" CreationDate="2016-04-22T01:44:54.937" Score="2" ViewCount="189" Body="&lt;p&gt;Falcon 9 rocket consumes approximately 30 tons of fuel for the first 15 seconds of flight while it's still less than 350 meters in altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to build a giant tower near the launchpad, so the rocket can be refueled during the first seconds of flight? If so, what would be the main technical challenge of that idea?&lt;/p&gt;&#xA;" OwnerUserId="14191" LastEditorUserId="14191" LastEditDate="2016-04-22T04:13:18.647" LastActivityDate="2016-04-22T08:03:43.927" Title="Is it possible to load propellant during the lift off?" Tags="|launch|fuel|" AnswerCount="2" CommentCount="4" ClosedDate="2016-04-22T08:11:48.507" ContentLicense="CC BY-SA 3.0" />
  <row Id="14942" PostTypeId="2" ParentId="14939" CreationDate="2016-04-22T04:19:25.983" Score="8" Body="&lt;p&gt;It seems to be the roof of &lt;a href=&quot;https://www.google.ru/search?q=%D0%BF%D0%B0%D0%B2%D0%B8%D0%BB%D1%8C%D0%BE%D0%BD%20%D0%BA%D0%BE%D1%81%D0%BC%D0%BE%D1%81&amp;amp;tbm=isch&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cosmos Pavilion&lt;/a&gt; at &lt;a href=&quot;https://en.wikipedia.org/wiki/VDNKh_%28Russia%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;VDNKh&lt;/a&gt; in Moscow.&lt;/p&gt;&#xA;&lt;p&gt;For many years (~1967-1994) the Pavilion hosted the large exhibition of spacecrafts and space related artifacts.&lt;/p&gt;&#xA;&lt;p&gt;There are much better quality photos of the same scene (if not the same angle) available online. See for example &lt;a href=&quot;http://ba-kground.com/moscow-1969-part-1/4/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the 1969 color slides by David C. Cook&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IzPYm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IzPYm.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;em&gt;The work by David C. Cook. Reproduced verbatim under the terms of &lt;a href=&quot;https://creativecommons.org/licenses/by-nc-sa/2.0/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CC BY-NC-SA&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://vdnh.ru/en/places/pavilon-34-kosmos-byvshiy-mekhanizatsiya-/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space and Aviation Pavilion has reopened in 2018&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2021-05-12T13:19:09.440" LastActivityDate="2021-05-12T13:19:09.440" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="14943" PostTypeId="1" AcceptedAnswerId="14946" CreationDate="2016-04-22T05:03:54.033" Score="8" ViewCount="1756" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/q/14921/12102&quot;&gt;Local expansion measured, near zero via Lunar Ranging - what about deep space probes?&lt;/a&gt; I used images of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Retroreflector&quot; rel=&quot;nofollow noreferrer&quot;&gt;retro-reflector&lt;/a&gt; arrays that were left on the moon by Apollo 11, 14, and 15. The Lunar Ranging Retro Reflectors (LRRRs) are there for &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Laser Ranging&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The arrays and their supporting structures appear to be significantly different from each other, suggesting design improvements or slight repurposing. Looking closer at the images, I see auxiliary structures on each one. What are all these things and how were they used?&lt;/p&gt;&#xA;&lt;p&gt;Some appear to be sun-dial-like (for want of a magnetic field), and at least one looks a little like a bubble level with a vertical tab next to it. I'm pretty sure &lt;strong&gt;B&lt;/strong&gt; is not there to blow moon dust off of the array :) - it could maybe be an infrared thermometer, but was there telemetry? See &lt;a href=&quot;http://arxiv.org/pdf/1203.2150v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Laser Ranging Tests of the Equivalence Principle&lt;/a&gt;, page 8 for a discussion of the relevance of the widely cycling temperature of the array - for &lt;a href=&quot;https://en.wikipedia.org/wiki/Equivalence_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Equivalence Principe&lt;/a&gt; (EP) experiments it's necessary to measure throughout the entire 27 day long &lt;em&gt;lunar day-night cycle&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Only one shows the faces of the reflectors directly, and they appear to be &lt;a href=&quot;https://en.wikipedia.org/wiki/Corner_reflector&quot; rel=&quot;nofollow noreferrer&quot;&gt;corner-cubes&lt;/a&gt;, which, &lt;em&gt;individually&lt;/em&gt; at least, don't necessarily have to be pointed very accurately at all in every-day use, although I imagine an &lt;a href=&quot;https://en.wikipedia.org/wiki/Anti-reflective_coating&quot; rel=&quot;nofollow noreferrer&quot;&gt;antireflection coating&lt;/a&gt; might have a cone of best performance.&lt;/p&gt;&#xA;&lt;p&gt;Since the orientation of the arrays is always changing due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Libration&quot; rel=&quot;nofollow noreferrer&quot;&gt;Libration&lt;/a&gt; - albeit in a very well determined way - if careful orientation was necessary, or at least careful documentation of the orientation, why?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Libration&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Libration&lt;/a&gt;&lt;/strong&gt; image from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase2.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ncBW3.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ncBW3.gif&quot; alt=&quot;Lunar Libration GIF&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below: Apollo 11 LRRR&lt;/strong&gt; annotated detail, original image &lt;a href=&quot;https://i.stack.imgur.com/Ne7Ak.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (0.7°N 23.5°E)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ne7Ak.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ne7Ak.jpg&quot; alt=&quot;Apollo 11 retroreflector array detail&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below: Apollo 14 LRRR&lt;/strong&gt; annotated detail, original image &lt;a href=&quot;https://en.wikipedia.org/wiki/File:ALSEP_AS14-67-9386.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (3.6°S 17.5°W)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FysDp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FysDp.jpg&quot; alt=&quot;Apollo 14 retroreflector array detail&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below: Apollo 15 LRRR&lt;/strong&gt; annotated detail, original image &lt;a href=&quot;https://en.wikipedia.org/wiki/File:ALSEP_AS15-85-11468.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (26.1°N 3.6°E)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AWFAA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AWFAA.jpg&quot; alt=&quot;Apollo 15 retroreflector array detail&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;After reading the &lt;a href=&quot;https://space.stackexchange.com/a/14946/12102&quot;&gt;answer&lt;/a&gt; by @Andy and the linked &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11EASEPHandbkCrew.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;EASEP Handbook For Apollo ll Flight Crew&lt;/a&gt;, I've included Table 4.1-2 here as well.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kYBkI.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kYBkI.png&quot; alt=&quot;Apollo 11 EASEP handbook screenshot annotated&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-07-23T00:51:52.730" LastActivityDate="2021-07-23T00:51:52.730" Title="What are these structures on the Lunar Ranging Retro Reflector (LRRR) arrays for?" Tags="|apollo-program|lunar-landing|measurement|laser|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14944" PostTypeId="2" ParentId="14941" CreationDate="2016-04-22T06:57:29.013" Score="3" Body="&lt;p&gt;The main technical challenges with this idea: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;You've got a giant hose (2 tons/s capacity) hanging off one side of the rocket, making the weight distribution very unsymmetrical. You could relieve the weight by suspending the hose from a winch, but then you'd have to synchronize that exactly with the movement of the rocket.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You'll need to keep that hose out of the exhaust, so you need to reel it in/out during liftoff. That's not easy to do while the hose is full of pressurized liquids, one of which is cryogenic. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Clean separation gets difficult. You have no time to properly drain the hose, so you have to eject the (heavy) hose from the rocket while making sure it doesn't bang into the rocket after separation.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-22T06:57:29.013" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14945" PostTypeId="2" ParentId="14941" CreationDate="2016-04-22T07:51:16.323" Score="1" Body="&lt;p&gt;Your main challenge is Newton's 3rd Law.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are delivering 2ton/s of fuel to the side of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the velocity of flow (which is inversely proportional to the size of the hose, and proportional to the ratio of density of the exhaust and propellant). You will be thrusting the rocket side ways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming that your hose is the same diameter as your rocket nozzle. Then the velocity of flow will be a percentage of the exhaust velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This the thrust exerted sideways will be that same percentage of the exhaust velocity...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The result is the rocket will go sideways. Not good.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a smaller hose will result in worse outcomes (fyi the rocket nozzle is often a large proportion of the diameter of the rocket)...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the same reason that KSP style asparagus launch systems aren't used IRL.&lt;/p&gt;&#xA;" OwnerUserId="6157" LastEditorUserId="6157" LastEditDate="2016-04-22T08:03:43.927" LastActivityDate="2016-04-22T08:03:43.927" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14946" PostTypeId="2" ParentId="14943" CreationDate="2016-04-22T11:04:02.377" Score="8" Body="&lt;p&gt;The Apollo 11 EASEP &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11EASEPHandbkCrew.pdf&quot; rel=&quot;noreferrer&quot;&gt;handbook&lt;/a&gt; gives basic information on the first version. Of the LRRR, it has just two pages of text...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some (poorly reproduced) images labelling the main parts. It's correct there is a sun &quot;compass&quot; and a level. The large part labelled &quot;B&quot; in the question is a rear support, presumably so the folded device could be stood on the soil during the deployment process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The part labelled &quot;A&quot; in the question is a handle the crew may grab while aligning and setting up the device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There wasn't any telemetry by the way, this handbook states the experiment was totally passive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for placement accuracy, an &lt;a href=&quot;http://www.lpi.usra.edu/lunar/ALSEP/pdf/31111000674083.pdf&quot; rel=&quot;noreferrer&quot;&gt;Interface Control Specification&lt;/a&gt; was written, which covers all of the many considerations which &lt;em&gt;could&lt;/em&gt; matter when astronauts are handling the device. Of accuracy, it states only that levelling was to be done within 5 degrees by a bubble, and alignment of the sun compass was to be done within 5 degrees too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo 15 version was larger (and has a better &lt;a href=&quot;http://www.lpi.usra.edu/lunar/ALSEP/pdf/ALSEP%20%2323%20-%20LRRR%20Familiarization%20Manual_RevA_050171.pdf&quot; rel=&quot;noreferrer&quot;&gt;manual&lt;/a&gt;). Its level indicator has rings for 5, 2 and 1 degrees. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/GNmDP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GNmDP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note nothing I've described here gives the &lt;strong&gt;actual&lt;/strong&gt; accuracy of the reflector prisms, so this is only a half-answer.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(And a note: various documents mention a &quot;bubble&quot;, but I gather at least some lunar equipment in fact used a small ball in a cup. This might have been to avoid having breakables, or just for better visibility. It's not clear what type was used in the LRRR.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-04-22T11:27:37.293" LastActivityDate="2016-04-22T11:27:37.293" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14947" PostTypeId="1" AcceptedAnswerId="14948" CreationDate="2016-04-22T13:11:15.857" Score="19" ViewCount="5621" Body="&lt;p&gt;For years the theoretical calculated gravity of Mars has proliferated science texts and Internet documentation - but did the rover actually do a test to confirm the theoretical gravity is truly equal to the actual measured gravity?&lt;/p&gt;&#xA;" OwnerUserId="14197" LastEditorUserId="1142" LastEditDate="2016-04-23T23:20:14.443" LastActivityDate="2022-01-27T14:32:36.927" Title="Did the Mars rovers actually confirm the gravity of Mars?" Tags="|mars|planetary-science|gravity|mass|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14948" PostTypeId="2" ParentId="14947" CreationDate="2016-04-22T13:25:00.847" Score="57" Body="&lt;p&gt;The &quot;gravity of Mars&quot; is not a number but rather a complex field. &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103516001287&quot; rel=&quot;noreferrer&quot;&gt;The most recent is remarkably detailed, made up to spherical harmonics degree and order 120&lt;/a&gt;, described by &lt;a href=&quot;http://pds-geosciences.wustl.edu/mro/mro-m-rss-5-sdp-v1/mrors_1xxx/data/shadr/gmm3_120_sha.tab&quot; rel=&quot;noreferrer&quot;&gt;29,512 coefficients&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8QPIL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8QPIL.jpg&quot; alt=&quot;gravity map showing Tharsis region&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These maps are made using orbiters (three orbiters in this case), not landers. A lander/rover can give just one local gravitational acceleration and direction, which will never be a textbook value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbiter data also show how the gravity field varies with time as mass moves between the atmosphere and the ice deposits at the poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, no, the Mars rovers can do little to &quot;confirm&quot; the gravity of Mars. Unless you just want to check that it's somewhere around $3.7\,\mathrm{m/s^2}$. Our Mars &lt;em&gt;orbiters&lt;/em&gt; have &quot;confirmed&quot; the gravity of Mars, or really measured it, to very high accuracy, far above our rovers' poor power to add or detract.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-04-22T22:46:14.407" LastActivityDate="2016-04-22T22:46:14.407" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14949" PostTypeId="1" CreationDate="2016-04-22T13:26:54.283" Score="2" ViewCount="778" Body="&lt;p&gt;Ok, this is a question from a student in my class that was inspired from us watching the ISS Live Earth Feed.  He wanted to know if there was a point in space when you would start falling.  He likened it to a swimming pool.  He said if you were swimming in space was there a point when if you stepped over an imaginary barrier you would suddenly start falling towards Earth.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was not able to answer him or explain what the transition would be like but thought I could get an answer here so over to you and thanks for anyone who takes the time to respond to an 14/15 year old student.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chris&lt;/p&gt;&#xA;" OwnerUserId="14199" LastActivityDate="2016-04-26T14:58:12.273" Title="If you swam towards Earth from Space would there be a point when you started falling?" Tags="|reentry|gravity|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="14950" PostTypeId="2" ParentId="14949" CreationDate="2016-04-22T13:40:48.607" Score="8" Body="&lt;p&gt;Let's assume an astronaut on the ISS goes out for a swim.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When will he start falling ? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually he is already falling to earth. But he is also moving at the same speed as the ISS relative to earth (7.66km/s). To be specific, this is not called &quot;falling&quot; but orbiting. He never reaches the ground but just keeps turning around the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, our astronaut is very slowly losing speed, because of the very rarefied atmosphere. After months/years, he will reenter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As our astronaut is moving slower and slower, he is also losing altitude.&#xA;This is a very gradual process... Until he reaches the reentry interface, which varies by atmospheric conditions (meteo) but is about 128 km high.&#xA;At this point, he starts losing speed MUCH more quickly, and also loses height much more quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So from this point, yes, you can say he is falling back to earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS: Of course, our astronaut will die from suffocation in his space suit and burn up during the reentry process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR: Yes, at around 128 km high&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="4660" LastEditDate="2016-04-23T01:28:39.410" LastActivityDate="2016-04-23T01:28:39.410" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14952" PostTypeId="1" AcceptedAnswerId="14953" CreationDate="2016-04-22T16:50:58.843" Score="7" ViewCount="783" Body="&lt;p&gt;Say for instance if we were to build a base on the Moon, how could we possibly reduce the cost to send the parts and materials into space and off to the Moon.&lt;/p&gt;&#xA;" OwnerUserId="14189" LastEditorUserId="51" LastEditDate="2016-04-22T17:19:00.597" LastActivityDate="2019-07-18T07:23:00.473" Title="Is there any possible way to reduce the cost of a manned mission to the Moon?" Tags="|launch|the-moon|reuse|cost-to-orbit|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14953" PostTypeId="2" ParentId="14952" CreationDate="2016-04-22T17:18:43.737" Score="10" Body="&lt;p&gt;Your question contains the answer, and sort of suggests a different question at its heart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The major cost to anything in space is launch costs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Answer: Reduce launch costs.  Thus a better question is, what can we do to reduce launch costs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One example question with answers would be this one:&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/11974/how-can-new-leo-manned-launch-vehicle-development-costs-be-reduced&quot;&gt;How can new LEO manned launch vehicle development costs be reduced?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next answer would be, stop launching entirely and move to a totally different method, like a space elevator, and here is a good question with some answers:&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/2699/what-is-a-space-elevator&quot;&gt;What is a &amp;quot;space elevator&amp;quot;?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For practical examples of how launch costs are being reduced, investigate more about SpaceX and their attempts to recover and reuse the first stage of their Falcon 9 booster. (Short of the Shuttle SRB's, no one has ever tried this before on real boosters). Whether SpaceX will succeed is something time will tell. But they have already forced the industry to reduce prices across the board to compete. Ariane (ABILENE) and ULA (Vulcan) have discussed approaches to reusable boosters. They may or may not actually build them, but they are talking about it, which is a credit to SpaceX. (Similar to how Tesla's work has pushed every other vendor into making or seriously considering electric cars. Disruptive technology means others compete or die.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you have reduced launch costs, then it is much easier and cheaper. But still everything will cost a lot. So next you can look at lighter, simpler structures.  Perhaps insitu resource usage as best as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is work on making cement from lunar materials, which could mean building structures heavy bits from native materials, with perhaps only an inner airtight lining needing to be launched? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-04-22T17:31:15.227" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="14954" PostTypeId="2" ParentId="14947" CreationDate="2016-04-22T17:49:56.380" Score="15" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Did the rover actually do a test to confirm the theoretical gravity is truly equal to the actual measured gravity?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;First off, what would be the point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, of course they did, but not to perform that test. The Mars rovers were/are autonomous. They needed to know where they were without outside help. To accomplish this, Mars rovers are equipped with inertial measurement units (IMUs). One of the key components of an IMU is an accelerometer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At periods when a rover was stationary, the accelerometer &quot;measured&quot;&lt;sup&gt;1&lt;/sup&gt; gravity. This question asks for the boring part of that reading, the magnitude of that vector. The important part of those stationary readings was the direction of the measured acceleration. Ignoring the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vertical_deflection&quot;&gt;deflection of the vertical&lt;/a&gt;, those steady-state measurements gave an indication of the rover's orientation&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The accelerometers were also used while the rovers were in motion. Integrating sensed acceleration, less gravity, is a crucial part of inertial navigation. For example, see &lt;a href=&quot;https://www-robotics.jpl.nasa.gov/publications/Khaled_Ali/SAPP_paper.pdf&quot;&gt;Attitude and Position Estimation on the Mars Exploration Rovers&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; I used &quot;measured&quot; in quotes because you cannot &quot;measure&quot; gravity directly. This is one of the consequences of Einstein's equivalence principle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt; Deflection of the vertical on Mars can be huge by Earthly standards (0.3 degrees near the peak of Olympus Mons). However, even that extreme value is rather small, and the value on terrain navigable by a rover is tiny. It was safe to ignore deflection of the vertical.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-04-22T17:49:56.380" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14956" PostTypeId="2" ParentId="14949" CreationDate="2016-04-22T19:47:44.230" Score="3" Body="&lt;p&gt;For a far more theoretical perspective, I'll interpret the question as &quot;can you swim through space (perfect vacuum, without reaction mass, i.e. rockets)?&quot; and answer it like they might at Physics.SE&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In flat space, no. If you were to move your arms forward, most of the rest of you would move backwards to keep your center of mass in the exact same point. Space, however, is not flat in the vicinity of things with mass (i.e. Earth), so you can swim. The math is beyond me to calculate how much an average human could move themselves in Earth's G field, but check out:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Avron, Kenneth (2008). &lt;a href=&quot;http://arxiv.org/pdf/math-ph/0602053.pdf&quot; rel=&quot;nofollow&quot;&gt;Swimming in curved space or The Baron and the cat.&lt;/a&gt; on arXiv&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=deSPB9lq8vI&quot; rel=&quot;nofollow&quot;&gt;Animation by one of the above authors&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Dixon (1970). &lt;a href=&quot;http://www.jstor.org/stable/2416466?seq=1#page_scan_tab_contents&quot; rel=&quot;nofollow&quot;&gt;Dynamics of extended bodies in General Relativity&lt;/a&gt; Proc. R. Soc. London&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="881" LastActivityDate="2016-04-22T19:47:44.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14957" PostTypeId="2" ParentId="14928" CreationDate="2016-04-22T20:25:42.573" Score="4" Body="&lt;p&gt;The Trident D5 appears to have just enough impulse to reach orbit without its normal payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://astronautix.com/lvs/trientd5.htm#more&quot; rel=&quot;nofollow&quot;&gt;summary&lt;/a&gt;, and its daughter pages for each stage provide enough information to determine that whilst the whole missile is 59 tonnes the mass devoted to items other than the three solid stages is nearly 6 tonnes. Whilst the payload is given as 1.6 tonnes I suspect that is a narrow definition of payload. This &lt;a href=&quot;http://fas.org/nuke/guide/usa/slbm/d-5.htm&quot; rel=&quot;nofollow&quot;&gt;link&lt;/a&gt; describes a &quot;mission dedicated&quot; Equipment Section that could well account for the difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Subjective moment: I don't believe that the whole of that 6 tonnes would be necessary for a small launch vehicle for the normal necessities of modern avionics and a fairing. If you assume 500kg instead and a very poor Isp of 250s for the solids then the delta-V comes up as 9.8km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, a very small microsat class payload might be possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The data I used:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://astronautix.com/stages/trintd51.htm&quot; rel=&quot;nofollow&quot;&gt;First&lt;/a&gt; stage:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Gross mass: 39,100 kg. Unfuelled mass: 2,242 kg.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://astronautix.com/stages/trintd52.htm&quot; rel=&quot;nofollow&quot;&gt;Second&lt;/a&gt; stage:   &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Gross mass: 11,800 kg. Unfuelled mass: 790 kg.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://astronautix.com/stages/trintd53.htm&quot; rel=&quot;nofollow&quot;&gt;Third&lt;/a&gt; stage:   &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Gross mass: 2,200 kg. Unfuelled mass: 159 kg.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Assumed Isp = 250s, a browse through wiki suggest it could be much better.&#xA;Assumed other deadweight - 500kg. &lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-04-22T20:39:11.477" LastActivityDate="2016-04-22T20:39:11.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14958" PostTypeId="2" ParentId="14922" CreationDate="2016-04-22T21:02:11.117" Score="4" Body="&lt;p&gt;There are regular insurance markets for satellite launches (i.e. successful delivery to separation), in-orbit satellite operation and third party liability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of insurance &lt;strong&gt;brokers&lt;/strong&gt;. Aon, mentioned in PearsonArtPhoto's answer, are one of the larger brokers. They don't take any of the risk but allocate it competitively amongst &lt;strong&gt;underwriters&lt;/strong&gt;. There are about twenty or thirty major underwriters who participate in the asset related insurance for launch and in-orbit and a smaller number that provide third party liability cover. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Almost always the insurance for any one launch is spread over most of the market.  The chain of participants does not stop there. For some of the underwriters, their capacity is actually only managed and the risk is borne by other insurers who have placed their trust in them. Furthermore, some underwriters will &lt;strong&gt;re-insure&lt;/strong&gt; a part of their share of the risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The insured party in respect of a launch is normally the prospective satellite owner, though it can also be the satellite manufacturer. The insured amount is based on the cost of replacing the satellite and launch.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;It would seem that the most clear-eyed assessment of the risk for the&#xA;  re-launch would come from the insurers of that launch.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It makes sense though insurers are falible too and companies occasionally exit the market due to losses or low returns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;strong&gt;third party liability&lt;/strong&gt; market is driven to an extent by regulation. There are varying national obligations on launch service providers and satellite operators, either of whom can be considered the sponsoring party when it comes to third party liability and, correspondingly, can determine which countries are implicated in liability through the Outer Space Treaty and the Liability Convention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In principle &lt;strong&gt;reused&lt;/strong&gt; launch vehicle stages are little different from new ones in both the launch and liability insurance contexts, they just have a different risk and perceived risk. We'll have to see what happens with the first few reused Falcon 9 first stages though it would make sense that they may be marked up in risk terms to start with until the market gains confidence.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-04-22T21:09:35.893" LastActivityDate="2016-04-22T21:09:35.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14959" PostTypeId="1" AcceptedAnswerId="14960" CreationDate="2016-04-22T21:56:19.213" Score="10" ViewCount="215" Body="&lt;p&gt;Many spacecraft have &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandra_X-ray_Observatory&quot;&gt;twofold symmetry&lt;/a&gt;, some have &lt;a href=&quot;https://en.wikipedia.org/wiki/Mariner_5&quot;&gt;fourfold symmetry&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have any spacecraft had &lt;em&gt;threefold&lt;/em&gt; symmetry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm specifically looking at ones that don't land on the surface of a planet/moon/whatever.&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2016-04-23T00:55:15.987" Title="Threefold-symmetry in non-landing spacecraft?" Tags="|design|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14960" PostTypeId="2" ParentId="14959" CreationDate="2016-04-22T22:14:02.077" Score="11" Body="&lt;p&gt;There is &lt;a href=&quot;http://lambda.gsfc.nasa.gov/product/cobe/&quot; rel=&quot;noreferrer&quot;&gt;COBE&lt;/a&gt; the Cosmic Background Explorer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5wlot.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5wlot.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the &lt;a href=&quot;http://www.lgarde.com/assets/content/files/publications/96-1367.pdf&quot; rel=&quot;noreferrer&quot;&gt;Inflatable Antenna Experiment&lt;/a&gt; launched on STS-77&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aofM4.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aofM4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least until it got all twisted up.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-04-23T00:55:15.987" LastActivityDate="2016-04-23T00:55:15.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14962" PostTypeId="2" ParentId="10048" CreationDate="2016-04-23T01:34:58.960" Score="-3" Body="&lt;p&gt;The law of supply and demand has been proven true, that is why they call it a law.&#xA;Demand for tulips caused the prices to rise to unbelieveable prices, one of the commonly mentioned &quot;Bubbles&quot; in economic history.&#xA;The reason air is free is due to it being everywhere, &#xA;if air was rare it would be something to pay for.&#xA;Metals are higly susceptible to the laws of supply and demand, just as we are seeing with oil currently. &#xA;Oil glut causes price drops. &#xA;Steel is relatively cheap due to availabilty of iron ore. &#xA;Diamonds command high prices because they are mostly controlled by a cartel. &#xA;The uneducated masses who graduate our public schools are mostly unaware of simple economics.&#xA;These ridiculous claims of value and quantity are made to part fools from their money. I see a public offering down the road.&lt;/p&gt;&#xA;" OwnerUserId="14207" LastActivityDate="2016-04-23T01:34:58.960" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14964" PostTypeId="1" AcceptedAnswerId="17155" CreationDate="2016-04-23T09:29:55.263" Score="4" ViewCount="212" Body="&lt;p&gt;The Dawn mission was supposed to end after its rendezvous with Ceres, but now, reports say that &lt;a href=&quot;http://www.universetoday.com/128547/dawn-just-wants-make-probes-look-bad/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Dawn team has asked NASA to extend the mission to visit a third asteroid&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There’s only a small amount of propellant left, so there’s only a limited selection of possible destinations for Dawn at this point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iVnk9m.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iVnk9m.jpg&quot; alt=&quot;Dawn probe&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chris Russell, of the University of California, Los Angeles, is the principal investigator for the Dawn mission. &lt;a href=&quot;https://www.newscientist.com/article/2085288-nasas-dawn-probe-may-visit-third-asteroid-after-ceres-and-vesta/&quot; rel=&quot;nofollow noreferrer&quot;&gt;He told New Scientist,&lt;/a&gt; “As long as the mission extension has not been approved by NASA, I’m not going to tell you which asteroid we plan to visit,” he says. “I hope a decision won’t take months.”&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I ask this question to the stackexchange community: can you make any educated guesses about this &quot;third asteroid&quot;?&lt;/p&gt;&#xA;" OwnerUserId="13643" LastEditorUserId="58" LastEditDate="2016-04-25T15:03:27.440" LastActivityDate="2016-07-12T10:16:41.703" Title="If the Dawn probe visits a third asteroid after Ceres and Vesta, which asteroid could it visit?" Tags="|asteroid|dawn-mission|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14966" PostTypeId="1" CreationDate="2016-04-23T15:18:28.220" Score="3" ViewCount="951" Body="&lt;p&gt;I was reading this &lt;a href=&quot;https://www.technologyreview.com/s/601299/the-curious-link-between-the-fly-by-anomaly-and-the-impossible-emdrive-thruster/&quot; rel=&quot;nofollow&quot;&gt;article about an &quot;EmDrive&quot;&lt;/a&gt; recently.  The article suggests that we've just been looking at the problem the wrong way all along, which sounds either like genius or lunacy.  What would it take for someone (a space organization like NASA or the ESA) to test the viability of such a device to the extent that there is no doubt it works?&lt;/p&gt;&#xA;" OwnerUserId="14104" LastEditorUserId="6816" LastEditDate="2016-11-23T19:45:02.717" LastActivityDate="2016-11-23T19:45:02.717" Title="Inertialess Drive?" Tags="|advanced-propulsion|emdrive|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14968" PostTypeId="2" ParentId="14966" CreationDate="2016-04-23T16:48:02.830" Score="7" Body="&lt;p&gt;NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster&quot;&gt;already testing the EmDrive&lt;/a&gt;, at the NASA/JSC Advanced Propulsion Physics Laboratory.&lt;br&gt;&#xA;To have no doubt that the device works, we'd need to see the test results replicated consistently by multiple laboratories. As of early 2016, 3 laboratories have done tests with inconsistent results (different levels of thrust/input power).&lt;br&gt;&#xA;It'd also be nice to understand how the thing works. As the article linked in the question says, there are some &lt;a href=&quot;https://www.technologyreview.com/s/601299/the-curious-link-between-the-fly-by-anomaly-and-the-impossible-emdrive-thruster/&quot;&gt;interesting hypotheses&lt;/a&gt; which would revolutionize physics if they can be confirmed.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-23T16:48:02.830" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="14970" PostTypeId="1" CreationDate="2016-04-24T06:07:22.777" Score="3" ViewCount="208" Body="&lt;p&gt;I often work with the programmatic manipulation of images and 3d environments in my work at an engineering firm. Out of pure curiosity I was wondering about the Venera mission data format. I read that the landers sent data transmissions and that images were included. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wanted to understand the data format but can find very little information. Was the image sent as a mosaic of pixels in some way? Was it effectively a television image broadcast on a different frequency (as with TIROS I)? Searching for answers has me trying to understand how magnetic tape is used to store image data as a possible way to understand what the thinking would have been.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I came across this, &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc1989/pdf/1024.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.lpi.usra.edu/meetings/lpsc1989/pdf/1024.pdf&lt;/a&gt; but it says very little.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sending images over the local phone network, once slow and pixelated, seemed to take a decade before it worked right on our phones. How did the Russians manage this from Venus? &lt;/p&gt;&#xA;" OwnerUserId="14220" LastActivityDate="2016-04-24T07:57:20.483" Title="Venera mission image data format" Tags="|probe|history|venus|russia|venera|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14971" PostTypeId="2" ParentId="14970" CreationDate="2016-04-24T07:57:20.483" Score="4" Body="&lt;p&gt;After a lot of looking, I found &lt;a href=&quot;http://mentallandscape.com/C_CatalogVenus.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mentallandscape.com/C_CatalogVenus.htm&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;A digital video signal was transmitted from the lander to the orbiting main spacecraft and recorded on tape. It consisted of 6 bits per pixel (plus a 7th parity bit) encoding a logarithmic brightness value. Each scanline consists of 128 pixels, 11 of which are calibration and synchronization sent during the return stroke of the scanner. This was relayed in real time and retransmitted a few times from the tape recording.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pixelation is evident in the early images: &lt;a href=&quot;http://mentallandscape.com/C_Venera09_Processed.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mentallandscape.com/C_Venera09_Processed.jpg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The site has some interesting photos (scans) from the orbiter as well.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/qYA9E.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qYA9E.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14220" LastActivityDate="2016-04-24T07:57:20.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14972" PostTypeId="1" CreationDate="2016-04-24T08:13:52.383" Score="2" ViewCount="160" Body="&lt;p&gt;When I was looking into electric rockets I found out that the propellants used were Helium, Nitrogen, Argon, Krypton etc. The above elements are noble gases (with the exception of Nitrogen) but I also found that Lithium, Hydrazine and Hydrogen were also being used as propellants which aren't inert per se. So what all can be described as the ideal qualities of a propellant in electric rockets?&lt;/p&gt;&#xA;" OwnerDisplayName="user12889" LastEditorUserId="58" LastEditDate="2016-04-25T14:59:40.343" LastActivityDate="2016-04-25T14:59:40.343" Title="Desirable qualities of a propellant used in Electric Rockets" Tags="|fuel|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14973" PostTypeId="1" AcceptedAnswerId="14989" CreationDate="2016-04-24T08:28:08.727" Score="6" ViewCount="719" Body="&lt;p&gt;What are the joint systems used for mounting mobile manipulators to spacecraft?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking answers for questions like: How SSRMS (Canadarm2) is mounted to space station? What joint system does it use for relocation? How are power and control wires connected at joints?&lt;/p&gt;&#xA;" OwnerUserId="12449" LastEditorUserId="6944" LastEditDate="2020-09-04T13:38:16.303" LastActivityDate="2021-11-12T02:05:01.613" Title="What are joint systems used for mounting manipulators to spacecrafts?" Tags="|iss|robotics|mechanisms|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14974" PostTypeId="2" ParentId="14972" CreationDate="2016-04-24T09:33:02.140" Score="2" Body="&lt;p&gt;Being inert is one of the most important properties you are looking for, especially in the electrostatic thrusters, where corrosion of the grid is what limits their operational life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make the propellant easy to store, it should ideally be a liquid. Hydrogen and helium, however, require cryogenic cooling to achieve that. Smaller tanks usually means a lower dry mass, so high density is preferable too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the tyranny of the rocket equation, a high exhaust velocity is desirable. That favours a low atomic mass. However, there are a few other things to consider in electrical engines that change this picture a little:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Low thrust is the main drawback of electrical rocket engines, even though they have a very good exhaust velocity. For that reason, trading propellant efficiency for thrust is often desirable. That means using propellants with a higher atomic mass.&lt;/li&gt;&#xA;&lt;li&gt;Available electrical energy is also a limit. Preferably, the energy required to ionize the propellant should be low, so more energy can be used for the acceleration of the propellant instead. When a low ionization energy is required, the engine is also more easy to build overall, and can sometimes also be quite a bit lighter. A low ionization energy is the reason why xenon is a popular propellant.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-04-24T13:35:32.443" LastActivityDate="2016-04-24T13:35:32.443" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14975" PostTypeId="1" CreationDate="2016-04-24T13:07:23.807" Score="3" ViewCount="5107" Body="&lt;p&gt;So I've been making small steam rockets and I was just wondering if I had a huge power plant connected to an electrical heater inside the rocket and heated enough to bring the pressure and temperature to a super heated state or even plasma will I be able to shoot it out into space ?(suppose we have a super indestructible material that can hold the water it). How much energy do we need?&lt;/p&gt;&#xA;" OwnerUserId="14224" LastActivityDate="2022-09-10T06:46:11.733" Title="Is it possible to energize a steam rocket enough to go to space?" Tags="|rockets|water|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14976" PostTypeId="2" ParentId="14952" CreationDate="2016-04-24T13:32:48.423" Score="9" Body="&lt;p&gt;Referring the answer of geoffc who has included reasons and the key process for reducing launch cost, I will include something that has to do with Moon bases since you have mentioned it in your question. Also geoffc has mentioned option for new materials lighter and simpler structures. I would be more specific by linking Moon base cost not only with the technology that can be used to build them, but also with the cost of launching them beyond low Earth orbit (LEO) to the Moon. For the Moon bases could be considered as option expandable space habitats which has been studied for years, but now in this days more seriously has been studied and developed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Bigelow_Aerospace&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bigelow Aerospace Company&lt;/a&gt; which has invested a lot in this field. The company is testing Bigelow Expandable Activity Module (BEAM) at International Space Station (ISS) and if it is successful they will continue developing this technology for future projects such as new space stations, Moon and Mars bases. &lt;a href=&quot;https://en.wikipedia.org/wiki/B330&quot; rel=&quot;nofollow noreferrer&quot;&gt;B330&lt;/a&gt;-MDS module ( 330 m3 habitable volume ) is proposed as an option for Moon base which would require three of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.space.com/images/i/000/049/542/i02/bigelow-moon-base-1.jpg?1439834200?interpolation=lanczos-none&amp;amp;downsize=640:*&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://www.space.com/images/i/000/049/542/i02/bigelow-moon-base-1.jpg?1439834200?interpolation=lanczos-none&amp;amp;downsize=640:*&quot; alt=&quot;Bigelow Moon Base&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://www.space.com/32480-beam-inflatable-module-private-moon-bases.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bigelow space habitats&lt;/a&gt; are lighter, cheaper (less $ per cubic meter) and with much more living space than traditional metal space habitats. Also they are considered better for radiation and space debris protection which are very important for long duration missions in Moon bases, but it remains to be tested. In this way we don’t need to invest in this traditional types of space modules by making them even more expensive that they are, to achieve the Bigelow space habitat standards, but to concentrate more in this new technology if it is successful. Since expandable space habitats give more living space we can put on the Moon less number of this space habitat modules compared to traditional metal modules. So for example just for comparison, instead of 3 traditional modules with capacity of a Destiny class module ( ISS module ) with 106 m3 habitable volume and 15 tons mass each of them ( 45 tons in total), we can use only 1 B330-MDS module (20 tons, less than half of 3 Destiny modules mass) which means less number of modules so less number of launches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the expandable modules are also lighter and tightly packed to take less volume in the payload fairings of the rockets before expanding in space. So we could say that in the future developments of the expandable modules concept, in 1 rocket launch we can pack inside the payload fairing more than one of this types of modules instead of traditional modules which can’t change their form.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.airlinereporter.com/wp-content/uploads/2014/05/BA330.jpg&quot; alt=&quot;Bigelow B330 module and ISS Destiny module&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conclusion is that we can put in LEO and Moon surface less number of expandable space habitats (because has more living space) with less number of launches by the same rocket (because are lighter and can be packed tightly) instead of launching current metal space habitats. Less launches of the same rocket means less cost and the total cost of the project will be lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The expandable space habitats concept it is not yet proved as successful and certified operational but it is a good concept that could make space exploration easier, cheaper and safer, so in to the future if it successful could be very important to invest more in this technology.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="10273" LastEditDate="2019-07-18T07:23:00.473" LastActivityDate="2019-07-18T07:23:00.473" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="14977" PostTypeId="2" ParentId="14975" CreationDate="2016-04-24T13:57:30.933" Score="5" Body="&lt;p&gt;Interesting thought. You'd need to store the steam at high pressure. &#xA;The highest pressure used for (helium) gas cylinders is 1000 bar, so if we use that as the limit...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We need the &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Phase_diagram_of_water.svg&quot; rel=&quot;noreferrer&quot;&gt;phase diagram for water&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PKwXU.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PKwXU.png&quot; alt=&quot;Phase diagram&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 1000 bar, water exists as a liquid. At the nozzle, the water would flash to steam. So far so good. But how much energy can we get from this? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Steam_rocket&quot; rel=&quot;noreferrer&quot;&gt;Steam rockets&lt;/a&gt; have been considered. At reasonable temperatures, Isp is very low so it'd be an inefficient engine. At very high temperatures, Isp gets better but water at high temperatures is &lt;a href=&quot;http://forum.kerbalspaceprogram.com/index.php?/topic/130433-why-hasnt-anybody-used-superheated-water-as-rocket-fuel/&quot; rel=&quot;noreferrer&quot;&gt;corrosive&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The specific impulse will be very low because the specific heat of water is much much lower than the heat of combustion of, say, kerosene. Hot liquid water can hold something like 5J/g/K of energy (so cooling it from say 300 C to 0C as it expands in the nozzle could give you, say, 1500J per gram of water. The heat of combustion of kerosene is about 50000 J per gram! &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-04-24T14:09:10.993" LastActivityDate="2016-04-24T14:09:10.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14978" PostTypeId="2" ParentId="14975" CreationDate="2016-04-24T16:02:43.970" Score="4" Body="&lt;p&gt;The difficulty of doing this by storing steam in a pressure container is already covered in the &lt;a href=&quot;https://space.stackexchange.com/a/14977/8693&quot;&gt;answer by Hobbes&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it is quite a bit easier if we can boil the water and generate steam in-flight. Apart from the boring way to do this, to burn hydrogen and oxygen to get water, as in a regular chemical rocket, there is another interesting possibility:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chemical reactions have a very high energy density, so finding something else that can outperform regular rockets is difficult. But there is one other option, &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nuclear thermal rockets&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Passing a stream of propellant through a reactor to heat it is not something revolutionary. In fact, working prototypes of nuclear rocket engines where built 50 years ago, by both the US (&lt;a href=&quot;https://en.wikipedia.org/wiki/NERVA&quot; rel=&quot;nofollow noreferrer&quot;&gt;NERVA&lt;/a&gt;), and the Soviet Union (&lt;a href=&quot;http://www.astronautix.com/engines/rd0410.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-0410)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although hydrogen is the most commonly discussed propellants for an NTR, water is perfectly fine. Water has an $I_{sp}$ of 420s, a bit less than a LOX/LH2 engine, but better than LOX/methane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nuclear rocket engines are normally not considered for the first stage of a launcher, because they have a relatively low thrust to weight ratio. The thrust is in fact improved by using water, as the approximate relationship between thrust and $I_{sp}$ at constant reactor power is that $I_{sp} \cdot Thrust = constant$.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-04-24T16:13:09.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14979" PostTypeId="1" CreationDate="2016-04-24T16:03:40.233" Score="4" ViewCount="109" Body="&lt;p&gt;What would be gained from placing a powerplant with a photon beam canon and a light sail on opposite sides of the Sun, perfectly alligned, both at &gt;550 AU from the Sun? That'swhere the lens starts, and it goes on along a line.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-04-24T17:38:18.840" Title="What are the benefits from using the Sun as a gravitational lens for beamed interstellar propulsion?" Tags="|interstellar-travel|beamed-propulsion|light-sail|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14980" PostTypeId="2" ParentId="14979" CreationDate="2016-04-24T17:38:18.840" Score="2" Body="&lt;p&gt;The accuracy of optics is limited, so to get a close perfectly parallel beam of light does require a lens. The larger the distance between the light source and the lens, the better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic concept here is not flawed. Close to parallel light from distant objects gets focused in the focal point of the Sun's gravitational lens. Because optics are &lt;strong&gt;ALWAYS&lt;/strong&gt; reversible, the opposite is also true, light beamed out from the focal point, gets close to parallel. (That optics are reversible can not be stressed enough, confusion about optics causes almost as many perpetual motion machines annually as magnets).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5vl8em.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5vl8em.png&quot; alt=&quot;lensing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is a problem. Although the beam is now close to parallel, it is now extremely wide. Just think about it for a moment, the cross section has a hole in the middle, with the same size as the Sun. That is a wide beam. If your sail id not actually that big, and it is likely not, only a tiny fraction of the light hits the sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But wait, can you not straighten out the beam &lt;em&gt;and&lt;/em&gt; keep it narrow? &lt;a href=&quot;https://what-if.xkcd.com/145/&quot; rel=&quot;nofollow noreferrer&quot;&gt;No, you can not&lt;/a&gt;. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Etendue&quot; rel=&quot;nofollow noreferrer&quot;&gt;conservation of Etendue&lt;/a&gt; prevents that.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-24T17:38:18.840" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14981" PostTypeId="2" ParentId="14975" CreationDate="2016-04-24T18:40:20.863" Score="2" Body="&lt;p&gt;Take a look at hydrogen peroxide rockets.  H2O2 decomposes in the presence of a catalyst into water (H2O) and free oxygen (O2) at very high temperatures (300-600 C).  Water at that temperature is steam, &lt;a href=&quot;https://honchemistry.wikispaces.com/Hydrogen+Peroxide+as+Rocket+Fuel&quot; rel=&quot;nofollow&quot;&gt;and acts as a reasonably good propellant&lt;/a&gt; by itself, though has a low ISP of about 161.  As an example, it was the propellant for the Bell Rocket Belt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most successful H2O2 rockets use a &quot;bi-propellant&quot; blend of some sort of hydrocarbon (kerosene, alcohol, etc.) mixed in some proportion with the peroxide.  This cocktail is passed over or mixed with a catalyst (depending on if its solid, such as silver, or liquid, for which I can't think of an example; potassium permanganate, perhaps), the hydrogen peroxide decomposes in to steam and free oxygen, the free oxygen mixes with the hydrocarbon at the high temperature and ignites, producing a secondary reaction of heat, and it all gets directed out the back of the rocket nozzle to produce thrust (&lt;strong&gt;&lt;em&gt;phew!&lt;/em&gt;&lt;/strong&gt;).  According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Bristol_Siddeley_Gamma&quot; rel=&quot;nofollow&quot;&gt;the Wikipedia article on one of these engines&lt;/a&gt;, the decomposition with kerosene goes like this: &lt;strong&gt;CH2 + 3H2O2 → CO2 + 4H2O&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="14104" LastActivityDate="2016-04-24T18:40:20.863" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14982" PostTypeId="2" ParentId="14750" CreationDate="2016-04-24T23:56:47.293" Score="2" Body="&lt;p&gt;&lt;strong&gt;Sphere&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Needs to figure out, what spherical shape good for.&lt;br&gt;&#xA;Atmosphere landing - not so good, but less surface to cool, bigger volume for coolant, less material(mass) of craft, ability to rotate surface for even heat distribution, there is something to think about.&lt;br&gt;&#xA;One of good things is: Maximum volume with minimum surface(and mass as result).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, you have to need that volume, for some reason.&lt;br&gt;&#xA;Less surface, means less mass of construction, which is always good for spaceship, if it uses reactive propulsion. And this is pretty universal reason to have spherical form, but how much it affects on decision of choose spherical form, depends on: our goals, needs, which possibility's we have to implement our wishes, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So talking about form, prior to define all of that, is bad idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spherical form is good for space and space ship, not so good for landing on Earth or planets with atmosphere.&lt;br&gt;&#xA;Let assume we have plenty activities to do in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Good&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Less surface&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;less craft mass&lt;/li&gt;&#xA;&lt;li&gt;less surface repairs&lt;/li&gt;&#xA;&lt;li&gt;less material for radiation protection.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Spherical form&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;there no defined axis of symmetry (indefinite numbers of them). It means no preferred direction for micrometeorites to hit from, all directions kinda equal, in therms of impact results. That good in case if there no preferred direction for them to come from, if they intended to destroy ship. Projection is same from different angles.&lt;/li&gt;&#xA;&lt;li&gt;less inertia momentum, good for maneuvers&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Volume&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;recreation&lt;/li&gt;&#xA;&lt;li&gt;industry&lt;/li&gt;&#xA;&lt;li&gt;ship repair capability (may place all needed equipment, just in case, and use if needed)&lt;/li&gt;&#xA;&lt;li&gt;volume to place reactors or energy sources, shields if needed.&lt;/li&gt;&#xA;&lt;li&gt;food production&lt;/li&gt;&#xA;&lt;li&gt;waste utilization&lt;/li&gt;&#xA;&lt;li&gt;size of crew personal volume&lt;/li&gt;&#xA;&lt;li&gt;artificial gravity (cylinder is better though, less tech needed)&lt;/li&gt;&#xA;&lt;li&gt;more equipment for mission, which may be used at arrival.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Propulsion system + Spherical form + maneuvering&lt;br&gt;&#xA;Main propulsion system may be attached on surface of that sphere and may be movable. It means that we don't need to have extra engines to maneuver and no need to spend much energy or reactive mass to fight inertia momentum of the main part of the ship. Which may be big, huge, in case if it transports much hydrogen, for mars terraforming(as example)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Construction&lt;/strong&gt;&lt;br&gt;&#xA;You may or may not to be able build ship in space.&lt;br&gt;&#xA;If not, if you can't, there is not much to choose. Almost everything depends on your capabilities to carry payload to orbit. You fit that construction to satisfy payload requirements. Is that &lt;a href=&quot;https://en.wikipedia.org/wiki/Bigelow_Expandable_Activity_Module&quot; rel=&quot;nofollow&quot;&gt;BEAM&lt;/a&gt; or anything else.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can build construction in space, there is not much difference between cube, sphere, cylinder. In therms of building, at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Heat dissipation&lt;/strong&gt;&#xA;One thing people constantly mention, and usually they mention it as stop factor, without defining some circumstances, which are important in that topic.&lt;br&gt;&#xA;$$&#xA;\frac{Radiative Power}{Surface Area}=j=\sigma T^{4}&#xA;$$&#xA;$$&#xA;\sigma=\frac{2\pi^5 k^4}{15c^2h^3}= 5.670373 \times 10^{-8}\, \mathrm{W\, m^{-2}K^{-4}}&#xA;$$&#xA;$T=100K, j \approx 5.7W/m^2$&lt;br&gt;&#xA;$T=200K, j \approx 91W/m^2$ ISS uses NH3, near freezing for it, with normal pressure&lt;br&gt;&#xA;$T=240K, j \approx 188W/m^2$ ISS NH3 boiling point, with normal pressure&lt;br&gt;&#xA;$T=300K, j \approx 460W/m^2$ Kinda room temperature&lt;br&gt;&#xA;$T=336K, j \approx 726W/m^2$ Temperature of thin sheet heated by Sun on earth orbit&#xA;$T=400K, j \approx 1452W/m^2$ Something thick with low heat conductivity, surface temperature&lt;br&gt;&#xA;$T=823K, j \approx 26014W/m^2$ 550 Celsius, Steam in steam turbines&#xA;$T=1273K, j \approx 149kW/m^2$ 1000 Celsius&lt;br&gt;&#xA;$T=1773K, j \approx 560kW/m^2$ 1500 Celsius, Some cobalt based cutting tools work even that hot  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is Very important to understand what $T^4$ means, to have grasp how fast it grows, it makes big difference in efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also important, how energy is generated. Because of $T^4$ it is even more important then heat dissipation in space.&lt;br&gt;&#xA;As example if solar will work, when all it part are at temperature 400K (is doable) then using that energy, you pretty easy may have $823K, 26014W/m^2$ dissipation from surface of the ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Living volume as heat source&lt;/strong&gt;&lt;br&gt;&#xA;Let say there is 50kW heat sources per capita, volume 1000 $m^3$ per capita(food growing, living rooms, storages, service rooms etc), and heat dissipation $5kW/m^2$ (ca 270 Celsius ship surface temperature, not big problem, if there is energy), with 50% efficiency of cooling system - this ship may be 250 meters in diameter.&lt;br&gt;&#xA;10 by 10 by 10 meters minus all service etc stuff - not to much room to live, not so pleasant.&lt;br&gt;&#xA;Same as above and 10000 $m^3$ per capita - ship may be 2500 meters in diameter.&lt;br&gt;&#xA;In such cases there will be equilibrium between Radiative heat dissipation and needs in that dissipation.(less diameter and it will loose heat faster then it is generated)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So ship may be pretty big.&lt;br&gt;&#xA;If that ship have to transport some goods, resources etc - living volume would be fraction of it's volume, so even ISS approach may work well, just using surface of ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Engines as heat source&lt;/strong&gt;&lt;br&gt;&#xA;Depends on engines, on their efficiency. Current engines handle that issue, and no reasons to think oppositely. Current engines are pretty efficient. But that is not much connected with ship shape. Good thing there is no need to place there engines inside ship, with spherical ship it even better to be outside.&lt;/p&gt;&#xA;" OwnerUserId="3039" LastEditorUserId="3039" LastEditDate="2016-07-10T19:01:44.247" LastActivityDate="2016-07-10T19:01:44.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14983" PostTypeId="1" CreationDate="2016-04-25T00:50:11.200" Score="9" ViewCount="3331" Body="&lt;p&gt;Given certain simplifying assumptions, a &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/643209.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;linear tangent&quot; steering law&lt;/a&gt; is optimal for orbital insertion of a launch vehicle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\tan \psi = a\cdot t + b$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;i.e. the tangent of the flight path angle changes linearly from the point at which insertion guidance starts until circular orbit is achieved. My question is then how to compute the correct values for $a$ and $b$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming that an open-loop pitch guidance program is used for the initial ascent, linear-tangent guidance will take over from a given velocity vector and position, and our goal is to reach a given altitude with the correct horizontal velocity for circular orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a more direct way to determine the $a$ and $b$ coefficients than by a trial-and-error search algorithm? My application is for a generalized launch-to-LEO simulation tool.  &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="17912" LastEditDate="2018-07-09T09:03:16.907" LastActivityDate="2018-07-09T09:03:16.907" Title="Coefficients for linear tangent steering law" Tags="|launch|low-earth-orbit|guidance|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="14985" PostTypeId="1" AcceptedAnswerId="14986" CreationDate="2016-04-25T06:12:07.233" Score="19" ViewCount="14900" Body="&lt;p&gt;I just read &lt;a href=&quot;https://astronomy.stackexchange.com/a/14655/7982&quot;&gt;this answer&lt;/a&gt; in astronomy.stackexchange, where a sobering point was made that the overall to-date chance of losing ones life in a spacecraft is about 4%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I don't want to dwell on that, is it roughly correct? What are the numerator and denominator in that ratio, and is it improving over the years? It seems to me that most, if not all, incidents have been within the atmosphere, and so far none actually &lt;em&gt;in space&lt;/em&gt;?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2016-04-25T19:47:13.053" Title="Is the overall mortality rate for being in a spacecraft in space or bound for space about 4%?" Tags="|spacecraft|safety|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14986" PostTypeId="2" ParentId="14985" CreationDate="2016-04-25T07:18:24.327" Score="33" Body="&lt;p&gt;It depends on how you count.  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Flights or individual astronauts:  &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;302 manned spaceflights, &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spaceflight-related_accidents_and_incidents&quot;&gt;4 of which&lt;/a&gt; ended with a total of 18 fatalities: 1.3% of flights.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.worldspaceflight.com/bios/stats.php&quot;&gt;544 people&lt;/a&gt; have been in Earth orbit, 18 fatalities: 3.3% of astronauts have died during spaceflight. But many astronauts have made more than one flight. &#xA;Thanks to @Eoin: the number of &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spaceflight_records#Total_human_spaceflight_time_by_country&quot;&gt;person-flights&lt;/a&gt; is 1228, for a fatality rate of 1.4%. &lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;When do you take a sample: &#xA;Just after the Columbia disaster, the number of astronaut fatalities could well have been 4%. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Who do you include? &#xA;In my view, a spaceflight includes everything from the countdown to the landing. Everything before that is a training accident. Apollo 1 is an unusual case: a training accident that took place in the spacecraft itself. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-04-25T14:08:44.917" LastActivityDate="2016-04-25T14:08:44.917" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="14987" PostTypeId="1" AcceptedAnswerId="14988" CreationDate="2016-04-25T08:29:08.900" Score="4" ViewCount="852" Body="&lt;p&gt;The proposed &lt;a href=&quot;https://www.nasa.gov/mission_pages/asteroids/initiative/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asteroid Redirect Mission&lt;/a&gt; would capture a small astroid, then tow it back to the moon and place it in orbit - in a bag - if I understand correctly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm all for creative thinking applied to finding a justification for a (manned?) NASA mission to deep(-ish) space that's easier than anything involving humans and the surface of Mars, because more and more people are realizing how difficult (and expensive) that would really be in the short term. But this one worries me for the following reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the asteroid somehow fragmented, through an unanticipated impact or explosion (I said &quot;if&quot;) or even through mechanical instability within the asteroid itself, it could possibly put many small particles, pebbles etc. into orbit around the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are meteorites from far, and suborbital debris released from impacts on the surface, but this scenario could put many small objects in a &lt;em&gt;relatively stable&lt;/em&gt; orbits around the moon, since the event is presumably starting from a stable orbit. By relatively stable, I mean 10 or 100 years at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without a substantial atmosphere like the Earth, objects in lunar orbit wouldn't safely &quot;burn up&quot; like the &lt;a href=&quot;https://space.stackexchange.com/questions/13340/what-was-the-first-practical-use-of-a-satellite-constellation#comment34854_13340&quot;&gt;space needles&lt;/a&gt; are &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_West_Ford&quot; rel=&quot;nofollow noreferrer&quot;&gt;said&lt;/a&gt; to have mostly done. (see also &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/newsletter/pdfs/ODQNv17i4.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Debris Quarterly News&lt;/a&gt;). Forces in space like photon pressure and perturbations from Earth will cause these orbits to evolve over time, but without an atmosphere, I'm guessing that either transport into space via open-ended orbits induced by perturbations, or collision with the lunar surface will have to be the only removal mechanisms. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you happen to be standing on the moon, an orbiting mote at it's apoapsis could be cruising across the lunar surface and hit you in the head at 6000 kph!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unlikely of course! And standing on the moon implies many risks already, but &lt;em&gt;has this been worked out somewhere?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking either for a review or calculation that addresses this technically, or if that's not forthcoming, some quantitative argument why it is more, or less dangerous than I'm suggesting here. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-04-25T09:52:19.537" Title="Could &quot;Asteroid Redirect&quot; make the moon more dangerous for astronauts for a long time?" Tags="|the-moon|moonwalking|asteroid-redirect-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14988" PostTypeId="2" ParentId="14987" CreationDate="2016-04-25T09:52:19.537" Score="9" Body="&lt;p&gt;The &lt;a href=&quot;https://www.nasa.gov/pdf/756122main_Asteroid%20Redirect%20Mission%20Reference%20Concept%20Description.pdf&quot;&gt;Asteroid Redirect Mission concept&lt;/a&gt; shows two strategies that should minimize problems: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;the asteroid will be transported inside a bag.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;the asteroid will be placed in a '&lt;a href=&quot;http://www.planetary.org/blogs/jason-davis/2014/the-latest-on-nasas-asteroid.html&quot;&gt;distant retrograde orbit&lt;/a&gt;', at an altitude of 70,000 km. These orbits are stable, and far away from any orbit you'd want to use for landing on the Moon. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In both cases, the spacecraft and captured asteroid are inserted into what’s called a Distant Retrograde Orbit, or DRO, around the moon. DROs are very stable over long periods of time. “The orbit that we currently have in mind is about 75,000 kilometers [47,000 miles] above the surface of the moon,” said ARM Program Director Michelle Gates during the June 19 briefing. She added the spacecraft and asteroid would be stable for more than a hundred years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-04-25T09:52:19.537" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14989" PostTypeId="2" ParentId="14973" CreationDate="2016-04-25T11:39:47.263" Score="9" Body="&lt;p&gt;The Space Station Remote Manipulator System (SSRMS) is a seven-jointed teleoperated manipulator used on the International Space Station (ISS).  Each end of the manipulator is composed of a Latching End Effector (LEE) which serves a dual purpose: to grapple payloads, and to mount the manipulator on the ISS.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dyEgb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dyEgb.jpg&quot; alt=&quot;photograph of the SSRMS showing joints. Earth is in the background&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The LEE can only attach (&amp;quot;grapple&amp;quot;) to specially provided points on the ISS and payloads (&amp;quot;grapple fixtures&amp;quot;).  The grapple fixtures come in several different flavors.  Simple payloads might have a Flight Releasable Grapple Fixture (FRGF), the simplest flavor.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/c2GxZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/c2GxZ.jpg&quot; alt=&quot;Closeup of a Flight Releasable Grapple Fixture &quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This grapple fixture provides only a mechanical interface for the LEE.  The LEE incorporates a rotating, retractable carriage fitted with snare wires.  During a grapple, the SSRMS is positioned so that the grapple pin at the center of the fixture enters the LEE.  When a capture is commanded, the carriage rotates to wrap the snare wires around the shaft, then retracts, pulling the wires against the knob at the end of the shaft to provide tension to hold the interface tight.  The three curved metal bars around the grapple shaft provide rotational alignment and stability by interfacing with the semi-circular cutouts in the outer ring of the LEE.  The black and white object at the top of the grapple fixture is a camera target which aligns with a camera on the LEE.  Here is a schematic of this grapple sequence, from &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950020841.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/OcArr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OcArr.png&quot; alt=&quot;Schematic drawing showing the stages of the grapple as written above&quot; /&gt;&lt;/a&gt;&#xA;To base the manipulator on the ISS, this simple grapple fixture is insufficient, because it does not provide any power or data connections.  In this case, a Power Data Grapple Fixture must be provided.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/8lzbE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8lzbE.jpg&quot; alt=&quot;Photograph of a PDGF with the insulating blankets removed&quot; /&gt;&lt;/a&gt;&#xA;While similar to the FRGF, it provides in addition, the four rectangular interfaces around the circular feature.  Behind the doors of these interfaces are provisions to receive latching umbilicals which can extend from the LEE.  These umbilicals provide a stronger mechanical connection than the snare wire/grapple shaft interface, and also contain connectors which allow for the flow of power and data into and out of the interface. You can see the umbilicals mounted around the LEE in this picture (which was taken with the insulating blankets removed).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9MPi3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9MPi3.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The PDGF also has a &amp;quot;curvilinear coupling&amp;quot; - the circle of square black teeth - that interfaces with the similar coupling on the LEE to provide additional resistance to rotational forces when grappled.&lt;/p&gt;&#xA;&lt;p&gt;To grapple a PDGF, the sequence proceeds as described above.  Additionally, after the carriage is ridigized, the umbilicals extend through the flipper doors on the PDGF, attach mechanically, and connect up the power, data, and video lines.  The tension can then be removed from the snare wires to reduce the strain on the LEE mechanisms.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-11-12T02:05:01.613" LastActivityDate="2021-11-12T02:05:01.613" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14990" PostTypeId="2" ParentId="12495" CreationDate="2016-04-25T13:26:25.170" Score="5" Body="&lt;p&gt;A couple of things:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reference OP provided for cosmic rays states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Cosmic Ray Composition: Cosmic rays include essentially all of the elements in the periodic table; about 89% of the nuclei are hydrogen (protons), 10% helium, and about 1% heavier elements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This implies that neutrons are not a significant constituent of cosmic rays.  Since fission is driven by neutron absorption, reactor control is unlikely to be affected by non-neutron radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, discussing fission from fast neutrons OP states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I haven't found a reference for uranium but I suppose it must be similar.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's not.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A search for &quot;neutron absorption cross section,&quot; will yield more detailed explanations, but basically the likelihood of neutron absorption is highly dependent on the energy of the neutron, and for uranium, LOW energy neutrons are much more likely to cause fission.  These neutrons are called Thermal Neutrons, or Slow Neutrons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speaking of Slow Neutrons:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Multiple control drums, located in the reflector region surrounding the reactor core, regulate the neutron population and reactor power level&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;These reflectors keep neutrons IN rather than provide protection from external neutrons.  Neutrons born from fission are born Fast.  If they slow down before the leave the core, they are likely to cause fission.  If they don't slow down, they leak out of the core and do not cause fission.  Neutron reflectors allow the system to control that leakage rate: if the leakage rate is too too high, reflect more neutrons back into the core so that they have a second chance to cause fission.  If leakage rate is too low, reflect fewer neutrons back into the core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, controlling this leakage rate is done to control core temperature: the reactor remains critical (steady state) by design, because a change power causes a change in temperature causes a physical response like thermal expansion/contraction to change the leakage rate, which means the system returns itself to steady state.   But the new steady state may not be at the optimum temperature, so an operator adjusts the reflectors / absorbers in the system to return the system to steady state at the temperature she desires.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Nathan points out, this means the system is dynamically stable.  If cosmic radiation does produce some Thermal Neutrons, the system will remain in steady state so long as the neutron production rate is low  / constant compared to other sources of Thermal Neutrons. &lt;/p&gt;&#xA;" OwnerUserId="11115" LastActivityDate="2016-04-25T13:26:25.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14991" PostTypeId="2" ParentId="14985" CreationDate="2016-04-25T14:19:09.560" Score="6" Body="&lt;p&gt;(this post assumes that by going to space you mean going to orbit, if you also include high alititude suborbital missions things get more vauge).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What are the numerator and denominator in that ratio,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The numerator is pretty small, especially if you count by missions rather than by people (which makes sense because generally when a spacecraft fails it kills everyone onboard). At another answer says if you go from &quot;countdown to landing&quot; you get four accidents. If you also include testing on the pad prior to countdown you get five.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;and is it improving over the years?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's hard to say because the numbers are so small. 4-5 fatal accidents total on orbital missions is about one per decade on average. The last fatal accident was in 2003 but whether that means safety is improving or is just good luck is difficult to say.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Footnote on suborbital missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm aware of two suborbital craft that were considered spacecraft by their promotors and broke up killing thier pilots. I'm not aware of any comprehensive list of such craft though. I'm also not positive if the virgin galactic craft was actually on a suborbital mission at the time (my memory of the news articles at the time is it was but I may be misremebering and the wikipedia article on the crash doesn't say)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/X-15_Flight_3-65-97&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/X-15_Flight_3-65-97&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/VSS_Enterprise_crash&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/VSS_Enterprise_crash&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12039" LastEditorUserId="12039" LastEditDate="2016-04-25T14:54:33.107" LastActivityDate="2016-04-25T14:54:33.107" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="14992" PostTypeId="1" AcceptedAnswerId="14996" CreationDate="2016-04-25T15:39:03.470" Score="6" ViewCount="1980" Body="&lt;p&gt;Let me explain. The term &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid_rocket_booster&quot; rel=&quot;nofollow noreferrer&quot;&gt;SRB&lt;/a&gt; (Solid (fuel) Rocket Booster) usually refers to a solid (fuel) rocket which is attached to something else in order to &quot;boost&quot; it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I just read &lt;a href=&quot;https://space.stackexchange.com/a/12798/12102&quot;&gt;this answer&lt;/a&gt; by @dotancohen about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I-X&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aries 1-X&lt;/a&gt; which seems to be a one-off test of an existing SRB from &quot;Space Shuttle Inventory&quot; used as a stand-alone rocket, in this case with a 2nd stage simulator. According to that article, a guidance system was assembled and used to vector the thrust of the SRB, with roll implemented via thrusters. So it got me thinking, is this really unusual for an SRB? In general are solid rocket boosters ever thrust vectored?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess a related question would be - are solid fuel rockets of this or similar size ever thrust vectored?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now &lt;a href=&quot;https://www.youtube.com/watch?v=H0ZHzAvFuYc&quot; rel=&quot;nofollow noreferrer&quot;&gt;enjoy the video&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/H0ZHzAvFuYc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-04-25T16:45:30.063" Title="Is it unusual to vector the thrust from an SRB?" Tags="|solid-fuel|srb|thrust-vectoring|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14993" PostTypeId="1" AcceptedAnswerId="15000" CreationDate="2016-04-25T15:53:04.063" Score="8" ViewCount="950" Body="&lt;p&gt;In this &lt;a href=&quot;https://space.stackexchange.com/q/14992/12102&quot;&gt;question&lt;/a&gt; I linked to a &lt;a href=&quot;https://www.youtube.com/watch?v=H0ZHzAvFuYc&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt; of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I-X#First_stage&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ares X-1&lt;/a&gt; launch. You can start listening at &lt;code&gt;02:00&lt;/code&gt; for the point where these are mentioned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;&lt;em&gt;Supersonic large amplitude ID maneuver PTI&quot; and &quot;a tumble motor ignition&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What do these mean?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;cued at &lt;code&gt;02:00&lt;/code&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/H0ZHzAvFuYc?start=120&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-12-08T04:22:58.283" LastActivityDate="2021-03-15T14:49:18.183" Title="What does “supersonic large amplitude ID maneuver PTI” mean, and what does a tumble motor do?" Tags="|terminology|solid-fuel|srb|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="14994" PostTypeId="1" CreationDate="2016-04-25T16:37:54.413" Score="5" ViewCount="679" Body="&lt;p&gt;For a project a professor assigned, he described an elliptical orbit by using the notation: (3000km x 40000km). Ive never seen this notation before and couldn't find much help online. I just assumed that... rp = 3000km &amp;amp; ra = 40000km, or am I wrong? Thanks!&lt;/p&gt;&#xA;" OwnerUserId="13183" LastActivityDate="2017-03-14T22:51:39.617" Title="notation to describe an elliptical orbit? (3000km x 40000km)" Tags="|orbital-mechanics|engineering|astrodynamics|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="14995" PostTypeId="2" ParentId="14992" CreationDate="2016-04-25T16:40:12.647" Score="5" Body="&lt;p&gt;It's not unusual. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#Thrust_vector_control&quot;&gt;Shuttle SRBs&lt;/a&gt; and the &lt;a href=&quot;http://www.esa.int/Our_Activities/Launchers/Launch_vehicles/Boosters_EAP&quot;&gt;Ariane 5 SRBs&lt;/a&gt; have thrust vectoring via steerable nozzles.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Ares 1-X needed extras relative to the Shuttle SRB: roll control of the stack (and guidance) is normally handled by the Shuttle orbiter. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-04-25T16:45:30.063" LastActivityDate="2016-04-25T16:45:30.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="14996" PostTypeId="2" ParentId="14992" CreationDate="2016-04-25T16:42:37.300" Score="8" Body="&lt;p&gt;It's not uncommon. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#Thrust_vector_control&quot;&gt;space shuttle SRBs had movable nozzles&lt;/a&gt;, as do many other SRBs. Some solid rockets such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_IIIC#History&quot;&gt;Titan III-C&lt;/a&gt;  boosters or &lt;a href=&quot;https://en.wikipedia.org/wiki/LGM-30_Minuteman#Minuteman-II_.28LGM-30F.29&quot;&gt;Minuteman II&lt;/a&gt; use a secondary liquid propellant injected into the side of the nozzle in order to do thrust vectoring as well.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-04-25T16:42:37.300" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="14997" PostTypeId="2" ParentId="14994" CreationDate="2016-04-25T16:47:58.370" Score="7" Body="&lt;p&gt;That's a common convention. The smaller number is the periapsis altitude, the larger is the apoapsis altitude, normally both given as height above a planetary surface rather than radius from barycenter.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-04-25T16:47:58.370" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="14998" PostTypeId="2" ParentId="14994" CreationDate="2016-04-25T19:07:46.627" Score="5" Body="&lt;p&gt;Edit -- I was wrong. I've asked around. John Schilling and Henry Spencer tell me altitude of apoapsis and periapsis is what's usually used to describe elliptical orbits about a planet. These guys know a lot more about space than I do. Please disregard what I said earlier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;s&gt;I would chastise your prof for ambiguous notation. Most instances I've seen the first number is periapsis distance from body center and second number is apoapsis distance to body center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it could also be periapsis and apoapsis altitudes, the distances from body surface rather than center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the 3000 km x 40000 km ellipse is an earth orbit, it would seem to be using altitude -- since a 3000 km distance from body center would put the periapsis below earth's surface.&lt;/s&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-05-28T04:01:31.950" LastActivityDate="2016-05-28T04:01:31.950" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="14999" PostTypeId="1" CreationDate="2016-04-25T20:53:20.467" Score="6" ViewCount="548" Body="&lt;p&gt;The new version of the Falcon 9 v1.1 Full Thrust, or as SpaceX would like everyone to start calling it &quot;Falcon 9&quot;, uses super cooled LOX and RP1 to improve the density of the fuel for better performance in the engines, but also the ability to store more fuel in the same size tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this introduces a number of operational issues. For one, there has been a limited amount of LOX and RP1 available at the LC-40 launch pad. Now that LOX and RP1 needs to be cooled, which takes time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much time does it take to cool a load of LOX and RP1, enough for a single launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has consequences to how far apart a hot fire (that requires a full tank load) can be from a real launch. It also means recycling after an abort might have time constraints. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-04-25T20:53:20.467" Title="How long does it take to chill the LOX and RP1 for Falcon 9 V1.1 Full Thrust?" Tags="|launch|falcon-9|fuel|merlin-1d|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15000" PostTypeId="2" ParentId="14993" CreationDate="2016-04-25T20:53:21.527" Score="9" Body="&lt;p&gt;&amp;quot;tumble motors&amp;quot; were solid rocket motors similar to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Booster_separation_motor&quot; rel=&quot;nofollow noreferrer&quot;&gt;Booster Separation Motors&lt;/a&gt; used on regular shuttle SRBs, but mounted in such a way to cause the booster to tumble in an attempt to prevent it from stabilizing aerodynamically on the downward leg of the trajectory. (from the &amp;quot;first stage&amp;quot; part of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I-X&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;) Normal shuttle SRBs weren't fitted with tumble motors, but early shuttle External Tanks (ETs) had a tumble valve which was supposed to serve the same purpose for the ET by venting residual gas in a radial direction.  The valves were problematic and were deleted after a few flights&lt;sup&gt;1&lt;/sup&gt;, apparently without much effect.  Tumbling is desired during reentry to help ensure that the entry trajectory is predictable &amp;quot;since satellites during a tumbling re-entry generally do not produce significant average lift forces acting against gravity [and therefore] a simplified ballistic re-entry approach can be used to model the trajectory and flight dynamics.&amp;quot; (from &lt;a href=&quot;https://books.google.com/books?id=qjDPCwAAQBAJ&amp;amp;pg=PT55&amp;amp;lpg=PT55&amp;amp;dq=benefit%20of%20tumbling%20on%20reentry%20from%20space&amp;amp;source=bl&amp;amp;ots=zg_jLfSiad&amp;amp;sig=bfV8iGtewX1UXqHB5jAa8GnqPgY&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwiX-vetp6vMAhXFvYMKHT6UAm0Q6AEITzAH#v=onepage&amp;amp;q=benefit%20of%20tumbling%20on%20reentry%20from%20space&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Safety Design for Space Operations, Chapter 9, Re-entry Operations Safety&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;A PTI is a Programmed Test Input, used throughout the Shuttle program to introduce small preplanned deviations into the control system or trajectory and take data on the resulting response of the system.  The specifics of the PTIs on this flight are described in &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080040163.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt;.  There were 3 called out on the sound track, all are in the paper.  The one you mention is described as&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;... a  large  amplitude   (nominal peak 0.35 degree)&#xA;sweep from 75-85 seconds and is intended to identify aerodynamic&#xA;response as well as  control  stability  margins.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Much, much more detail is available in the paper if you are interested. I've excerpted one graph from the paper showing some actual trajectory data versus&#xA;what it would have been if the PTI hadn't been incorporated.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y7EV1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y7EV1.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; &lt;a href=&quot;https://ntrs.nasa.gov/api/citations/20110001406/downloads/20110001406.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Missions Summary&lt;/a&gt; page 2-38&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-03-15T14:49:18.183" LastActivityDate="2021-03-15T14:49:18.183" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="15001" PostTypeId="1" AcceptedAnswerId="15009" CreationDate="2016-04-26T04:56:24.830" Score="5" ViewCount="860" Body="&lt;p&gt;Atmospheric reentry is notoriously a difficult and HOT endevour. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could this thermal energy be used as an alternative energy source on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could this be used through some sort of orbital charging station?&lt;/p&gt;&#xA;" OwnerUserId="14137" LastEditorUserId="14137" LastEditDate="2016-04-26T21:09:18.140" LastActivityDate="2019-07-18T05:58:38.730" Title="Could thermal energy be collected by dragging a thermoconductive device against the outer atmosphere" Tags="|reentry|energy|atmospheric-drag|thermodynamics|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15002" PostTypeId="2" ParentId="15001" CreationDate="2016-04-26T05:37:04.677" Score="12" Body="&lt;p&gt;No: The heat produced by atmospheric reentry isn't a happy side effect of returning to the earth, it's a byproduct of the fact that your satellite/orbiter has enough kinetic energy to be circling the earth every 90 minutes and you want it to stop doing that and come down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To have something you've made for the purpose of harvesting energy reenter the atmosphere it must first leave it. Due to drag (gravity, atmospheric) and inefficiencies in however you're lifting it you're always going to spend more energy launching your device out of the atmosphere than it can possibly encounter on the way back in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, there's nothing special about the outer atmosphere: if you got something moving that fast at sea level you'd produce way more heat due to the density of the atmosphere, but you'd also be pumping even more energy into achieving/maintaining that speed.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2016-04-26T05:37:04.677" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15003" PostTypeId="1" CreationDate="2016-04-26T11:29:59.030" Score="1" ViewCount="138" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lo87r.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lo87r.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Excuse me, friends. In paper , for passive control with eq.4, how to compute beta?&#xA;beta is angular velocity of fluid in fluid. the satellite has initial angular velocity, but with filled fluid ring we want to damp it, until the velocity of satellite in space tends to zero!&#xA;Kind regards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;another link:&lt;a href=&quot;http://link.springer.com/article/10.1007%2Fs00707-008-0132-5#/page-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;paper&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12853" LastEditorUserId="512" LastEditDate="2016-04-28T14:42:15.523" LastActivityDate="2016-04-28T14:42:15.523" Title="How to compute angular velocity of fluid ring for passive control?" Tags="|artificial-satellite|spacecraft|attitude|engineering|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15004" PostTypeId="2" ParentId="13212" CreationDate="2016-04-26T11:44:44.430" Score="3" Body="&lt;p&gt;From &lt;a href=&quot;https://www.youtube.com/watch?v=dIzLMjiyBog&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; of LC1, the answer is revealed. 10 feet (3m) away from dead center is the answer.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6642" LastEditDate="2017-08-28T09:06:22.853" LastActivityDate="2017-08-28T09:06:22.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15006" PostTypeId="1" CreationDate="2016-04-26T14:37:34.343" Score="-2" ViewCount="162" Body="&lt;p&gt;From the gravitation equation it is clear the earth gets pulled but we fall due to negligible mass. But, if each and every person on the earth takes a jet-pack and jumps to a height equivalent to 20 to 30 meters at the same instant of time, will the earth have any change in its motion? If yes then what could be the outcomes?   &lt;/p&gt;&#xA;" OwnerUserId="14237" LastEditorUserId="14237" LastEditDate="2016-04-26T15:16:44.303" LastActivityDate="2016-04-28T14:50:29.103" Title="Can we displace earth?" Tags="|gravity|future-missions|time|rotation|" AnswerCount="1" CommentCount="2" ClosedDate="2016-04-26T15:24:46.503" ContentLicense="CC BY-SA 3.0" />
  <row Id="15007" PostTypeId="2" ParentId="14949" CreationDate="2016-04-26T14:58:12.273" Score="1" Body="&lt;p&gt;There is no such point where the gravity is incredibly high and having low gravity areas beside it. The gravitational field of the earth (or any object) at outside points decrease with a parabolic path (in graph). And there is no such discontinuity. And hence, such points aren't possible. If the gravity free area is any spaceship , then the radial force component balances the gravity and in such case also, there is no such point. But there can be the point if you can deflect your velocity from 8kph approx. (satellite velocity) by large magnitudes {only in case of satellite}. &lt;/p&gt;&#xA;" OwnerUserId="14237" LastActivityDate="2016-04-26T14:58:12.273" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15008" PostTypeId="2" ParentId="15006" CreationDate="2016-04-26T15:22:54.840" Score="2" Body="&lt;p&gt;Unlikely any meaningful effect.  Some napkin math...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Weight of average human: &lt;a href=&quot;http://www.biomedcentral.com/content/pdf/1471-2458-12-439.pd&quot; rel=&quot;nofollow&quot;&gt;$62 kg$&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Population Estimate:  &lt;a href=&quot;http://www.worldometers.info/world-population/&quot; rel=&quot;nofollow&quot;&gt;$7.4 \times 10^9$&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass of combined population: $4.5 \times 10^{11} kg$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass of Earth is &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth_mass&quot; rel=&quot;nofollow&quot;&gt;estimated&lt;/a&gt; $5.972 \times 10^{24} kg$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's a huge order of magnitude in difference, and as such any gravity effects should be barely noticeable.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="3101" LastEditDate="2016-04-28T14:50:29.103" LastActivityDate="2016-04-28T14:50:29.103" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15009" PostTypeId="2" ParentId="15001" CreationDate="2016-04-26T21:48:24.173" Score="6" Body="&lt;p&gt;Reentry heat is not some magically created energy - it's all dissipation of energy that was painstakingly pumped into the orbital vehicle on ascent, so you will never break even with what you spent on propelling it to the orbit (plus propelling its second stage and its fuel to suborbital speed, plus propelling that fuel to nearly orbital speed, plus all that energy spent on launch and dissipated with the reaction mass). Never mind delivering it to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that doesn't mean you can't tap into that energy. You won't recover nearly as much as you'd spent, but you can still recover aplenty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On descent, on suborbital trajectory it would be extremely difficult due to sheer amount of energy created, and a very short time it takes to land. When moving in LEO, dissipating/collecting even a little of energy will send you into suborbital trajectory, and the above situation. But if you're approaching the body &quot;from outside&quot; - on a very elongated, elliptical trajectory, like most Martian probes - there's a lot of energy to be dissipated during the periapsis pass, and it lowers the apoapsis and orbital speed (while hardly affecting periapsis), meaning much more gentle reentry, or much less delta-V for circularization into a low orbit. It's normally always wasted - multiple aerobraking passes through upper Mars atmosphere, heating the probe somewhat, but not enough to mean damage or tap into the ablator supply, dissipated during the flight through apoapsis, repeatedly, often for many months, until the probe is in low orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I don't have any good ideas just &lt;em&gt;how&lt;/em&gt; to tap into that energy, but it should be perfectly possible - each pass through periapsis heating the craft, then the craft cooling when it leaves the atmosphere. Possibly a thermoelectric layer, or such. This energy is not &quot;free&quot; - it was bought by burning a lot of fuel to get there - but it's up for taking; it can be recuperated/recovered, and e.g. supply the craft's own needs and (considering extended solar panels don't take well to aerobraking), it could be a good alternative to solar, especially for missions to distant planets where sunlight is too weak to sustain a probe.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-04-26T21:48:24.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15010" PostTypeId="1" AcceptedAnswerId="18290" CreationDate="2016-04-26T22:34:28.433" Score="5" ViewCount="419" Body="&lt;p&gt;What fraction of launch payload mass for a manned (or animal'd) mission is attributable to life support equipment?&lt;/p&gt;&#xA;" OwnerUserId="14243" LastEditorUserId="14243" LastEditDate="2016-04-27T20:21:28.387" LastActivityDate="2016-09-20T05:01:30.620" Title="Life support: fraction of launch weight?" Tags="|launch|life-support|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15012" PostTypeId="1" AcceptedAnswerId="21526" CreationDate="2016-04-26T22:45:52.187" Score="10" ViewCount="1056" Body="&lt;p&gt;There are some conflicting accounts on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nedelin_catastrophe&quot;&gt;Nedelin catastrophe&lt;/a&gt;. &quot;A significant number&quot;, &quot;Hundreds&quot;, &quot;Scores of engineers&quot;. The closest to a concrete number I found is 74 deaths on site, but the number of these, who died in hospitals due to injuries is not available, estimates of total death toll ranging from 101 to 180. &lt;a href=&quot;http://history.nasa.gov/SP-4110/vol2.pdf&quot;&gt;&lt;em&gt;Rockets and People:  Creating a Rocket Industry&lt;/em&gt; vol 2, page 598&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, these sources are rather dated, and many secret documents of Soviet Union have been revealed since. Are there any more precise estimates on the number of victims of the disaster?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-15T18:09:15.563" Title="What was the death toll of the Nedelin catastrophe?" Tags="|failure|icbm|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15013" PostTypeId="1" AcceptedAnswerId="15014" CreationDate="2016-04-26T23:02:35.720" Score="9" ViewCount="4583" Body="&lt;p&gt;What is(are) the root cause(s) of the heat and friction experienced during atmospheric reentry (or initial entry)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that as items descend to earth they experience a force that can annihilate debris and meteorites. Can someone explain what happens in a more in depth way?&lt;/p&gt;&#xA;" OwnerUserId="14137" LastEditorUserId="12978" LastEditDate="2016-04-27T13:35:55.183" LastActivityDate="2016-04-27T15:56:48.683" Title="Requesting an in depth explanation of heat created during atmospheric reentry," Tags="|reentry|physics|atmospheric-drag|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15014" PostTypeId="2" ParentId="15013" CreationDate="2016-04-26T23:40:30.707" Score="14" Body="&lt;p&gt;The term &quot;friction&quot; is a misnomer. The source of heat is &lt;a href=&quot;https://en.wikipedia.org/wiki/Adiabatic_process#Adiabatic_heating_and_cooling&quot;&gt;adiabatic compression&lt;/a&gt; - gas on trajectory of the reentering object is compressed against its leading surface, and as result heats up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On molecular level you can think of it as number of molecules rising in given volume (compressed) and additionally speeding up (by elastic collisions against the fast-moving surface). This all converts to massive temperature rise; also, these collisions constitute air drag, causing slowing down of the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to inertia of the gas, it takes some time before it moves sideways off the leading surface (giving away some of its heat to the object it touches at the time), and flies free off the edges, the following decompression (and resulting cooling) occurring far beyond the surface of the object, and so unable to cool it back down. There's also an adiabatic decompression on the trailing side, which would cool it down - except while the pressure there can drop only by 1 bar (from atmospheric to zero) the pressure can rise much more on the leading side, causing much more heating than cooling effect on the object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At a certain point, the amount of heat is enough to turn any material of that temperature to plasma, thus the &quot;flaming trail&quot; as both the excited air and material of the body (be it rock of a meteorite, or ablator on reentering capsule) gets excited to the point of turning to &lt;a href=&quot;https://en.wikipedia.org/wiki/Plasma_%28physics%29&quot;&gt;plasma&lt;/a&gt; and blown away, leaving a blazing trail in the object's wake. Ablator - material that burns away carrying the heat with it, being a very poor heat conductor - is used on spacecraft to protect the inside of the craft from overheating through heat transfer from the superheated leading surface.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-04-26T23:40:30.707" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15015" PostTypeId="2" ParentId="15013" CreationDate="2016-04-27T01:32:22.300" Score="2" Body="&lt;p&gt;The root cause is the high speed of reentry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a typical craft that ascends into low Earth orbit then returns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During ascent, the acceleration is provided by rocket thrust, spread over several minutes of flight, and the highest-speed portion of the flight is done above the dense part of Earth's atmosphere. Some heat is produced by compression (as SF's answer explains) and friction between the atmosphere and the spacecraft skin, but not a dramatic amount. At 45km altitude, the rocket may be going &quot;only&quot; mach 4 or 5, and air pressure is about 0.2% of sea level. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On reentry, no rocket thrust is slowing the craft -- all the deceleration will be done by atmospheric drag. When it first begins to enter the atmosphere, it will be going about mach 20; by the time it falls to 45km altitude, it will still be going around mach 10. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-04-27T01:32:22.300" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="15016" PostTypeId="1" CreationDate="2016-04-27T08:16:46.427" Score="1" ViewCount="2172" Body="&lt;p&gt;I would like to write some AR mobile app that will show Earth velocity vector, but I have no any experience with astrodynamics. So how to calculate Earth's velocity vector in ECEF, having date and time as input data?&lt;/p&gt;&#xA;" OwnerUserId="14247" LastActivityDate="2016-04-27T15:37:05.293" Title="How to calculate Earth's velocity vector in ECEF?" Tags="|orbit|earth|mathematics|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15017" PostTypeId="1" CreationDate="2016-04-27T10:09:02.040" Score="2" ViewCount="164" Body="&lt;p&gt;I am asking this question in a context of feasability of orbital fuel depots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that cost/kg estimates exist (2-14 k$/kg): &lt;a href=&quot;https://space.stackexchange.com/questions/1989/what-is-the-current-cost-per-pound-to-send-something-into-leo&quot;&gt;What is the current cost-per-pound to send something into LEO?&lt;/a&gt; ]), but the post dates back to 2013 and asked for an update on the values.&#xA;But what about larger masses ie 1 metric ton and more? What would the launching cost be, considering the structural requirements for storing the LOX/LH2?&lt;/p&gt;&#xA;" OwnerUserId="14249" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-04-27T18:22:29.140" Title="What would be the incremental cost of launching 1 ton of LOX/LH2 into LEO?" Tags="|low-earth-orbit|fuel|cost-to-orbit|fuel-depots|" AnswerCount="1" CommentCount="5" ClosedDate="2016-04-27T14:24:13.843" ContentLicense="CC BY-SA 3.0" />
  <row Id="15018" PostTypeId="2" ParentId="15013" CreationDate="2016-04-27T12:21:09.193" Score="5" Body="&lt;p&gt;While @SF's answer is great in detailing how re-entry heating works, it doesn't directly address the question to the root causes, thus I have taken the time to answer the question you clarified in the comments: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why do we need such a high re-entry velocity?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital velocity in Low Earth Orbit (LEO) is ~7.8km/s. In order to re-enter, a short burn is performed at apogee to lower the perigee deep enough into the atmosphere, usually in the amount of a few dozen m/s up to a few hundred m/s. Still, the re-entry velocity will be higher than 7km/s. The Apollo capsules had more than 11km/s of velocity upon beginning of re-entry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can not slow down further, because of the so-called &quot;Tyranny of the rocket equation&quot;. In order to slow down from 7.8 km/s to 0 m/s, you need a rocket that is roughly the same size as the rocket that launched you into orbit. And in order to launch &lt;em&gt;that&lt;/em&gt; rocket into space you would need a rocket that is exponentially bigger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you could, having an object drop dead from 400km height isn't desirable. If you came to a full stop at ISS altitude somehow, you would be going ~2500m/s once passing the Karmann line, and ~3000m/s once at 50km height. but instead of having a rather long path through the atmosphere that slows the vessel down to terminal velocity, it would take the shortest possible path, and will likely not be able to deploy its chutes to slow down, resulting in lithobraking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During SpaceX launches, the first stage reaches space and returns to earth without a heat shield. This is because the first stage does not achieve orbit, it stays on a suborbital trajectory and is &lt;em&gt;much&lt;/em&gt; slower (from a quick web search, it seems to go about 2km/s at seperation), and thus re-entry heating is not a major concern. Add to that the fact that it is a powered landing, going down on quite a steep path is not that big of a problem - you don't need to be able to deploy chutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally re-entry heating is better than alternatives. When Apollo returned from the moon, they re-entered with more than 11km/s. This velocity could have been reduced by circularizing in LEO first, then re-entering. But the problem is again with orbital mechanics: You need the same amount of fuel for the trans-lunar-injection than you would need to circularize back around earth when coming back from the moon. This is quite a big amount of fuel, and not taking that fuel but instead a bigger heat shield is by far easier (again, tyranny of the rocket equation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Returning interplanetary (e.g. from mars) will again have &lt;em&gt;much&lt;/em&gt; higher re-entry speeds, and circularizing around earth won't be an option, either. You'd need as much delta-v to circularize around as you needed to leave earth to get to mars. It is simply not feasible to take such amounts of fuel on that journey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, you need to have high velocities during re-entry. And with those velocities, SF's answer sums up quite good what happens.&lt;/p&gt;&#xA;" OwnerUserId="13867" LastEditorUserId="-1" LastEditDate="2016-04-27T15:56:48.683" LastActivityDate="2016-04-27T15:56:48.683" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15019" PostTypeId="2" ParentId="15017" CreationDate="2016-04-27T12:28:40.123" Score="2" Body="&lt;p&gt;Assuming that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;regarding the storage system, we are considering an aluminium-lithium SLWT Super Lightweight external Tank, which was used for the space shuttle launches ;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;the mass ratio of LOX to LH2 is approximately 1 to 2.7 ;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;that the mass of the storage tank is 0.03% of the total fuel mass (based on the ratio for the space shuttle, which is not entirely accurate) ;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Then for  1,000kg of fuel (730kg LOX/270kg LH2), the total payload is 1,030kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering &lt;a href=&quot;https://space.stackexchange.com/questions/1989/what-is-the-current-cost-per-pound-to-send-something-into-leo&quot;&gt;this 2013 cost per kg&lt;/a&gt; to launch in LEO, the average cost of launching 1 ton of fuel is \$7,862,000 (\$2,266,000 to \$13,464,000).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no idea on how scaling the cryogenics or other systems would work though, but this seems like a decent enough estimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Source : &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_external_tank&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Space_Shuttle_external_tank&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14249" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-04-27T18:22:29.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15020" PostTypeId="1" CreationDate="2016-04-27T13:28:12.980" Score="6" ViewCount="137" Body="&lt;p&gt;Which system did spacex rely to? &#xA;Falcon, not dragon&lt;/p&gt;&#xA;" OwnerUserId="14251" LastActivityDate="2016-04-27T13:28:12.980" Title="Did SpaceX use TDRSS or USAF ground station to track Falcon 9 stage 2?" Tags="|spacex|tdrss|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15021" PostTypeId="2" ParentId="15016" CreationDate="2016-04-27T15:37:05.293" Score="2" Body="&lt;p&gt;To convert my comment into an answer now that I'm back to my desk:  The origin of the &lt;a href=&quot;https://en.wikipedia.org/wiki/ECEF&quot; rel=&quot;nofollow&quot;&gt;ECEF reference frame&lt;/a&gt; is the center of the earth, and the X axis goes through the center of the earth and the point on the surface at lat/lon 0/0.  Therefore, by definition the earth neither moves nor rotates in the ECEF coordinate space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To try to provide what you're looking for, I'll offer the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow&quot;&gt;JPL Horizons interface&lt;/a&gt;.  Specify ephemeris type &quot;VECTORS&quot;, target body &quot;Sun [Sol] [10]&quot;, coordinate origin set to your current lat/lon location, and a time span starting at the current time with a step size in minutes.  That should give you a table of vector positions of the sun relative to your current location.  Take the delta between positions (or possibly just use a velocity vector instead of multiple position vectors) and reverse it and you should get your velocity vector relative to the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously this doesn't give you the math to do the calculations yourself, but it should give you the answers and maybe someone else can chime in with the math.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2016-04-27T15:37:05.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15022" PostTypeId="1" CreationDate="2016-04-27T15:42:11.787" Score="2" ViewCount="96" Body="&lt;p&gt;Everyone is constantly focusing on the terraforming of Mars. However, how could one terraform Venus?&lt;/p&gt;&#xA;&lt;p&gt;The most obvious problems are the high pressure CO&lt;sub&gt;2&lt;/sub&gt; atmosphere, sulphuric rain, lack of water, lack of magnetosphere, slow rotation, lack of nitrogen, high temperature and an inadequate ozone layer. However, when you try and solve these problems more problems seem to arise.&lt;/p&gt;&#xA;&lt;p&gt;The high pressure CO&lt;sub&gt;2&lt;/sub&gt; atmosphere may be able to be condensed and separated leaving you with more oxygen in the atmosphere and hardened carbon on the surface. This would lower the heat because of a lower greenhouse effect and simultaneously lower the pressure. This could possibly be done with thermophilic bacteria already present on earth that do similar separations. The high pressure would increase their heat resistance.  However, with less pressure, Venus’s entire atmosphere would be stripped by solar wind when a magnetosphere is not present. The obvious course of action would be jumpstarting Venus’s core and returning it to liquid but you still need geodynamo forces to actually create a magnetosphere.&lt;/p&gt;&#xA;&lt;p&gt;This brings us back to the slow rotation, which, if sped up, could possibly aid in the creation of a magnetosphere. That seems like an obvious solution, but it is hard to actually accomplish. So, in order to even start reducing pressure and separating the oxygen and water vapor from the atmosphere, you need a magnetosphere or else the hydrogen and such would get stripped away. Solar wind already strips away &lt;span class=&quot;math-container&quot;&gt;$2 \cdot 10^{24}$&lt;/span&gt; hydrogen atoms every second. But, from there, more problems arise.&lt;/p&gt;&#xA;&lt;p&gt;You would have a huge excess of oxygen and practically no nitrogen which only accounts for 3% of the atmosphere. Extreme athletes can only survive with about 50% oxygen in the atmosphere and venus would have way over that. Also with a reduced bond albedo and greenhouse effect the planet could possibly get way too cold.&lt;/p&gt;&#xA;&lt;p&gt;The whole thing is all very problematic and we haven't even tried to introduce hydrogen. Does anyone have a way of solving any of these?&lt;/p&gt;&#xA;" OwnerUserId="14253" LastEditorUserId="15685" LastEditDate="2021-03-31T03:22:29.427" LastActivityDate="2021-03-31T03:22:29.427" Title="Terraforming of Venus" Tags="|venus|terraforming|magnetosphere|" AnswerCount="0" CommentCount="3" ClosedDate="2016-04-27T15:52:44.437" ContentLicense="CC BY-SA 4.0" />
  <row Id="15023" PostTypeId="1" AcceptedAnswerId="15026" CreationDate="2016-04-27T16:07:55.757" Score="6" ViewCount="755" Body="&lt;p&gt;I've been reading about various components that are attached to the ISS and occasionally see references to &quot;remotely controlled bolts&quot;. That makes sense, but I've barely been able to find more than a reference to them with a basic Google search.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What do these bolts look like? Is it basically a standard bolt with a servo wrapped around it?&lt;/p&gt;&#xA;" OwnerUserId="14254" LastEditorDisplayName="user20636" LastEditDate="2020-09-04T14:49:04.537" LastActivityDate="2020-09-04T14:49:04.537" Title="What do remotely-controlled bolts look like?" Tags="|iss|terminology|docking-port|mechanisms|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15024" PostTypeId="2" ParentId="15023" CreationDate="2016-04-27T16:37:31.200" Score="6" Body="&lt;p&gt;Their name seems to be &quot;&lt;a href=&quot;http://web.mscsoftware.com/events/vpd2003/na/proceedings/Tues_Marquis_1345_Multibody_Dynamics/2003-28_Favero.pdf&quot; rel=&quot;noreferrer&quot;&gt;Motorized Bolt Assembly&lt;/a&gt;&quot;, this is what they look like: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/chakG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/chakG.jpg&quot; alt=&quot;MBA&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The servo is in the rectangular box marked BMA, the bolt extends through the fine guide cone. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://hdl.handle.net/2060/20000048380&quot; rel=&quot;noreferrer&quot;&gt;More about the BMA&lt;/a&gt; (PDF page 89-102):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Moog Space Station Bolt Motor Actuator (BMA) is a rotary actuator comprised of a brushless DC motor, controller, and 120V DC-DC power supply coupled to a four-pass planetary gear system and containing a manual override (MO) system. This paper discusses the new technologies developed and lessons learned during the BMA development. These include the use of a relatively new material,&#xA;  Pyrowear 675, a new Vapor Deposition coating, WC/C, and a hard-on-hard (HOH) worm drive system delivering 135.6 N,m (1200 in.lb) of torque (the primary focus of this paper).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The BMA is a rotary actuator comprised of a brushless DC motor, controller, and 120V DC-DC power supply coupled to a four-pass planetary gear system. The motor runs at approximately 3400 rpm, which results in 4.1 rpm at the BMA output (a 25.4-mm 16/32 female spline) at the end of the 833:1 gear train.&#xA;  Dual input electrical connectors allow powered operation from either of two independent Bolt Bus Controllers (BBC's). The electrical connectors supply power (120VDC), direction logic signal, redundant heater current, redundant RTD based temperature feedback, and limit switch pigtail pass-through lines. The limit switches are part of a customer-controlled mating assembly. The RTD's are customer-supplied (Boeing Part Number 1F08041-1) and the heaters are Kapton® 1 encased, inconel element, thermal foil heaters supplying approximately 28 watts per circuit. These devices were needed to maintain the electronics at or above -42.8°C (-45°F) while on orbit.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The brushless DC controller and DC-DC power supply were totally contained inside the unit on two independent multi-layered circuit boards. The dual power line inputs were Y'd internal to the unit and opto isolated at the board level. All electronics were designated &quot;S&quot; class or equivalent and rated for -53.9°C (-65°F) operation or colder. All precautions were taken to minimize ESD susceptibility, including the incorporation of soft EMI gaskets at the side cover interfaces. The brushless motor was designed and&#xA;  built by Moog Inc. and had a 51mm (2.0 in) OD by ~15mm (.6 in) stack length. The motor is capable of providing more than 3 times the required torque, but is current clipped by the controller electronics at about 1.5 times the required torque.&#xA;  Manual Override capability is supplied in the event of a power failure via a dual input, 10-mm (3/8 in), recess square drives. The dual input allows equal operation from two opposite sides of the BMA, meeting the requirement of a single unit to fit into one of four possible positions at the final level assembly in different orientations. The basic MO input of 50 rpm and up to 30.5 N-m (270 in°lb) of torque supply the same functionality at the BMA output as the powered main drive system. The effective gear ratio of the MO mechanism is 12.17:1. The MO utilized a unique worm drive arrangement to be able to function even with the motor through the third stage gear set jammed, and to meet the overall package size constraints.&#xA;  While the maindrive is running, the MO drive is held stationary via a &quot;no-back-drive&quot; device. Similarly, while the MO runs, the main drive must remain stationary from the third stage planetary output (fourth stage sun gear) to the first stage input (motor shaft). There was insufficient cogging torque in the base motor design to guarantee no rotation would be allowed during MO operation. This deficiency was overcome by adding a passive magnetic &quot;cogging&quot; unit onto the top of the motor. The cogging unit supplies -170 mN°m (24 in°oz) of torque statically, while only requiring 40 mA of driving force (or loss) during motor operation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The PDF also contains photos, but unfortunately the PDF is a low-quality scan and the photos are nothing more than black smudges. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-04-28T07:11:50.853" LastActivityDate="2016-04-28T07:11:50.853" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15025" PostTypeId="1" AcceptedAnswerId="15296" CreationDate="2016-04-27T17:53:24.467" Score="19" ViewCount="1441" Body="&lt;p&gt;SpaceX has long been planning to use Dragon capsules to carry objects anywhere in the Solar System. &lt;a href=&quot;http://arstechnica.com/science/2016/04/spacex-plans-to-send-its-dragon-spacecraft-to-mars/&quot;&gt;They have officially announced&lt;/a&gt; a 2018 launch to Mars. Specifically, the launch window is April-May time frame, about 2 years from how. What, if any, payload are they expecting to launch, given only 2 years of preparation?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-02-16T23:31:37.653" LastActivityDate="2019-03-14T08:46:13.270" Title="What payload was expected for the 2018 Mars Dragon flight (&quot;Red Dragon&quot;)" Tags="|mars|spacex|dragon|payload|red-dragon|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15026" PostTypeId="2" ParentId="15023" CreationDate="2016-04-27T17:56:26.607" Score="7" Body="&lt;p&gt;Maybe this cross section is more what you are looking for?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UJEOL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UJEOL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://pages.erau.edu/~ericksol/projects/issa/s_m.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-04-27T17:56:26.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15027" PostTypeId="2" ParentId="15025" CreationDate="2016-04-27T18:10:08.353" Score="6" Body="&lt;p&gt;The article you linked makes it pretty clear that the goal is to test powered landing of Dragon spacecraft. Hence, I expect little to no payload at all. Of course there might be some token experiment, but it really does not need to be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I do expect is some magnificent PR coverage of the whole event. So there will be some sort of moving pictures from the landing site. So probably small rovers or drones mainly equipped with decent camcorders. Maybe even some kind of coverage of the landing itself (by a capsule jettisoned during descent).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second guess would be everything remotely usable for future missions. Any kind of equipment that would be usable for future colonists/explorers: Life support, basic tools, basic materials (water, oxygen, soil) even a small greenhouse. All this stuff is readily available and could be taken off the shelf. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: Whatever you put into the Dragon, the capsule remains a spacecraft intended for docking or manned landing. In order to unload bulkier experiments, a major structural refactoring would be in order, so I definitely do not expect large rovers, excavation equipment and the like.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="11256" LastEditDate="2016-04-27T19:49:32.223" LastActivityDate="2016-04-27T19:49:32.223" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15028" PostTypeId="1" AcceptedAnswerId="15035" CreationDate="2016-04-27T20:19:05.623" Score="4" ViewCount="298" Body="&lt;p&gt;In one hand, ICBMs used fancy, dangerous, poisonous chemicals as propellant. That must be more expensive as fuel, and might necessitate more expensive infrastructure than liquid hydrogen or kerosene. Plus the old problem: &quot;a hammer from a hardware store is \$5; a hammer made by military contractor for the army is \$5000&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, unlike with most civilian rockets, manufacture of ICBMs was &lt;em&gt;streamlined&lt;/em&gt;. They were produced in thousands. That's a massive economy of scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course ICBMs being &lt;em&gt;suborbital&lt;/em&gt; don't have a &quot;natural&quot; payload-to-orbit value. OTOH they carry quite heavy payloads (warheads), so either replacing these payloads with yet another stage, or even just reducing the payload, most of them could be used as space launch vehicles. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Start-1&quot; rel=&quot;nofollow&quot;&gt;START-1&lt;/a&gt; system is an example of such a conversion (but of course the actual cost is hidden in the fact military surplus rockets were used, which would have to be scrapped otherwise, so the launch costs doesn't really reflect the manufacturing costs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder how they compare cost-wise in that case. Do the military prerequisites drive the costs up more than the bulk production drives them down - comparing to civilian rockets?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-04-28T12:12:42.570" Title="How do ICBMs compare to civilian rockets in terms of $/kg to orbit?" Tags="|cost-to-orbit|icbm|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15029" PostTypeId="1" AcceptedAnswerId="15031" CreationDate="2016-04-27T20:34:42.807" Score="2" ViewCount="176" Body="&lt;p&gt;I'm visualizing a vehicle with a tube that travels the length of the vehicle, top to bottom, going through both propellant tanks. Is this feasible? What are the issues with such a design?&lt;/p&gt;&#xA;" OwnerUserId="6423" LastEditorUserId="6816" LastEditDate="2018-07-22T22:21:19.690" LastActivityDate="2018-07-22T22:21:19.690" Title="What are the issues encountered when placing a ladder up the middle of propellant tanks?" Tags="|fuel|design|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15031" PostTypeId="2" ParentId="15029" CreationDate="2016-04-27T21:09:26.103" Score="5" Body="&lt;p&gt;It should be feasible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary issue is going to be temperature; if the tunnel is air-pressurized, going through cryogenic propellant tanks, water will condense and freeze on the inside of the tunnel, and if the tunnel is kept warm enough for crew to move through, it'll be heating the cryo propellants. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're not planning to use the tunnel until the propellants are depleted, there's no problem; depressurize the tunnel and it shouldn't have substantial impact on the thermal situation in the tanks. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V first stage stacked the liquid oxygen tank above the kerosene tank; the flow rate required by the engines was too large to allow for routing the LOX lines around the outside of the fuel tank, so they had to go straight through. The lines required substantial insulation to keep kerosene from freezing on them, and the routing holes in the fuel tank complicated manufacturing. This is more or less the inverse of the situation with a warm pressurized tunnel going through a cryo fuel tank. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Liquid hydrogen being colder than liquid oxygen, a hydrogen-fueled stage will be more of a thermal engineering challenge than a kerosene-fueled one. There are, naturally, no thermal issues if your propellants are room-temperature storable (e.g. MMH/UDMH/Aerozine-50 with NTO oxidizer). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the design and purpose of the stage, it might be simpler to put the tunnel alongside the tankage, within an outer structure large enough to contain both, rather than straight through. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-04-27T21:20:00.937" LastActivityDate="2016-04-27T21:20:00.937" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15032" PostTypeId="1" CreationDate="2016-04-27T21:13:38.477" Score="7" ViewCount="1222" Body="&lt;p&gt;Reusability of the rocket engines, which are the most expensive part of the rocket, is very important to keep low cost of the space rockets and in this way making reality the space programs considered expensive. But does this reusability affect the performance of the rocket engine? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am interested to know the performances of the same type of rocket engines but in different versions, one reusable and the other single launch. Probably reusability affects the weight (in this way even thrust to weight ratio) and the cost of the engine but what about other elements that have to do with performance such as Thrust it's self, Specific Impulse, Chamber Pressure. If performances are different how much could be different. It is about something significant let say a considerable different performance, or a minor influence in performance almost identical. Illustrating with values or examples would be better to understand how much are the differences in case that differences exist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: &#xA;As Russell has mentioned a reusable rocket engine has to be more robustly engineered in general.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the problems at rocket engines is heating up from burning the propellants. By forcing more the engine the faster it's heating up and in a higher level. For a reusable engine if the temperature remains at those levels that are not dangerous, I guess it could operate for a longer time than single use engine. In the safe temperature level where the engine remains reusable, it could operate “indefinitely” (for a very long time). But I was wondering if the performance could be higher by forcing more this reusable engine, forcing it in those temperature levels that doesn’t destroy the entire engine but destroy essential parts and making its durability in that situation that can't survive two launches, as in the single use engines, or it is not so simple that you can force the engine as you wish and rise the Thrust, Specific Impulse or Chamber Pressure as you want. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So does it mean this that reusable engine has more &quot; space &quot; to force it at its limits in maximum, maybe with the cost that can’t survive another launch such as the single use engine design, but with the “profit” that the performance will be higher?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-07-26T13:07:53.463" LastActivityDate="2019-06-02T17:54:09.240" Title="Would reusability affect the performance of a rocket engine compared to a single launch version of that rocket engine" Tags="|reuse|engine-design|engines|" AnswerCount="5" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15033" PostTypeId="1" CreationDate="2016-04-27T21:32:59.650" Score="8" ViewCount="1256" Body="&lt;p&gt;Today &lt;a href=&quot;https://www.nasaspaceflight.com/2016/04/spacex-debut-red-dragon-2018-mars-mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;it was revealed&lt;/a&gt; that SpaceX has a deal with NASA to send a Red Dragon lander to Mars, launched on a Falcon Heavy, in 2018. The plan is that it will use supersonic retro-propulsion during the final phase of descent to slow the vehicle from about Mach 4 to 0. SpaceX's reusable first stage is the only vehicle to have ever attempted the maneuver. There have been 2 successes, and several of the failures might be survivable by a blunt body lander that can roll. But this isn't Mars, and those weren't landers. &#xA;&lt;hr&gt;&#xA;Edit: This question is mostly about supersonic retro-propulsion. I think the crux of it is does a Falcon 9 first stage re-entering on Earth that way provide adequate data to confidently predict the same process for a blunt-body lander on Mars will succeed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several people have pointed out that SpaceX wouldn't do it if there wasn't a reasonable chance of success, however I think it is quite possible they would send it even if they felt it would probably crash, given that they have a large-scale, long-term business plan to settle Mars. They need the data to continue to develop the vehicle. I don't like to move the goalposts when there are already answers, but I'm changing the title to 'Is it probable', as it seems the choice of 'reasonably likely' was too open and has led to very general answers.&#xA;&lt;hr&gt;&#xA;&lt;a href=&quot;http://www.ssdl.gatech.edu/papers/conferencePapers/AIAA-2016-0219.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This paper by JPL staff&lt;/a&gt; seems pretty upbeat about supersonic retro-propulsion:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;From 2013 to 2015, through a partnership with SpaceX, NASA received&#xA;  its first insight into the performance of a flight-qualified&#xA;  propulsion system operated into an opposing supersonic freestream.&#xA;  These efforts focused on analysis of SpaceX first stage recovery&#xA;  flight data. To return this launch stage safely to Earth, operation of&#xA;  its propulsion system in the supersonic regime at the right altitudes&#xA;  on Earth to yield Mars-relevant conditions is required. To date, SpaceX has performed SRP maneuvers during recovery operations of seven&#xA;  Falcon 9 first-stage systems. NASA personnel have independently&#xA;  reviewed these data sets. Multiple flights are in the specific Mach&#xA;  and dynamic pressure regime required by the present Mars EDL system.&#xA;  While &lt;strong&gt;the SpaceX first stage is not Mars-like in configuration, no&#xA;  showstoppers have been identified&lt;/strong&gt; for this technology. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;SRP&#xA;  computational, ground-based, and flight data have demonstrated that&#xA;  aerodynamic force and moment modeling uncertainty in the SRP phase is&#xA;  low for steady state, Mars-relevant conditions. Some uncertainty&#xA;  remains for SRP operation during startup and transition to&#xA;  steady-state operation, but performance uncertainty during this small&#xA;  time period may be mitigated by robust control system design. Combined&#xA;  with ground-based test data, the SpaceX flight data bounds the range&#xA;  of SRP thrust coefficients needed for human Mars EDL. Taken in total,&#xA;  these computational, ground-based, and flight test efforts&#xA;  significantly reduce the SRP flight system development risks for Mars&#xA;  EDL.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Emphasis mine. 'No showstoppers' doesn't ring with confidence. And they say data sets have been 'Mars-relevant', which isn't to say they are the same as what a vehicle entering Mars' atmosphere would experience. Based on the kind of risk assessment done for missions like this, is it genuinely probable that Red Dragon will successfully land on the first try?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="25" LastEditDate="2017-12-19T13:17:50.043" LastActivityDate="2017-12-19T13:17:50.043" Title="Is it probable the Red Dragon will successfully land on Mars on the first try?" Tags="|mars|spacex|landing|dragon-v2|red-dragon|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="15034" PostTypeId="2" ParentId="15032" CreationDate="2016-04-27T21:34:45.153" Score="6" Body="&lt;p&gt;A reusable engine has to be more robustly engineered in general, but as evidenced by SpaceX's Merlin engine, high thrust-to-weight ratios are certainly achievable in reusable engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some engines, like the &lt;a href=&quot;https://en.wikipedia.org/wiki/RS-68&quot; rel=&quot;nofollow&quot;&gt;RS-68&lt;/a&gt;, use an ablative coating in the nozzle to dissipate heat, rather than active cooling in the nozzle walls. In the RS-68, this is actually heavier than the alternative -- thus yielding lower thrust-to-weight ratio -- but much cheaper to build. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think a sufficient engineering and manufacturing budget can overcome any given performance limitation of a reusable engine design. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-26T14:06:01.720" LastActivityDate="2016-07-26T14:06:01.720" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="15035" PostTypeId="2" ParentId="15028" CreationDate="2016-04-27T21:54:34.067" Score="6" Body="&lt;p&gt;Nuclear warheads are relatively &lt;em&gt;light&lt;/em&gt; payloads by modern standards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A single US W88 warhead and its reentry vehicle are probably in the ballpark of 250-300kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Minuteman III carries 3 such, i.e. less than 1 ton of suborbital payload, with a unit cost of \$7 million. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trident II/D-5 carries up to 14 of them, less than 4.2 tons of suborbital payload -- possibly much less -- with a unit cost of \$37 million. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know what the exact ∆v yield of a long-range ICBM is, but I believe it's fairly close to orbital. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those costs are hardware only, not launch logistics cost. The figures seem broadly in line with general orbital launch costs, possibly somewhat more expensive.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-04-28T12:12:42.570" LastActivityDate="2016-04-28T12:12:42.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15036" PostTypeId="2" ParentId="15033" CreationDate="2016-04-27T22:36:48.527" Score="1" Body="&lt;p&gt;First of all, let's take a look at what SpaceX should have done prior to this point in time with the Dragon 2. The first uncrewed test is expected for May 2017. Before that, the Max Q abort test will have a limited test of landing using the propulsion (The primary will be a parachute, but some testing can be made of the landing from the landing propulsion system). These tests will be sufficient that NASA will agree to send men on the spacecraft, which seems that they have confidence in the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so how does that translate to Mars? The Viking landers were moving at about 250 m/s when the parachutes deployed. It is estimated that the Dragon 2 has about 1200 m/s of delta v. If that holds, there is an excess of capacity to make the landing happen. The spacecraft is rated for a 9 month stay on the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA Ames has done a study of the &lt;a href=&quot;http://science.nasa.gov/media/medialibrary/2012/01/23/NAC_Science_Meeting_ReportOctober_31-November_1_2011-finalTAGGED.pdf&quot; rel=&quot;nofollow&quot;&gt;&quot;Red Dragon&quot;&lt;/a&gt; that it believes it would be possible to achieve. There is still a fair amount of uncertainty, of course, but it seems possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to what the odds of success, I suspect the challenges will be with the Falcon Heavy, and not with the Dragon. The cost of such a mission will be just under $200 million. At that price tag, I am confident that SpaceX will only attempt it if they have a reasonable chance of success. So I say if they launch, they will have at least a 50% chance of success, more likely in the 80% range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT One thought I read from an article online. SpaceX has more experience firing retograde supersonic rockets than any other entity on Earth. Every flight since Falcon 9 v1.1 has tested a supersonic retrograde thrust in the high atmosphere of the Earth, which is very similar to Mars. Bottom line is, they have proven that can work, it will also probably work on Mars. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-29T22:14:57.850" LastActivityDate="2016-04-29T22:14:57.850" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15037" PostTypeId="2" ParentId="15033" CreationDate="2016-04-28T03:25:12.150" Score="7" Body="&lt;p&gt;If the question is really &quot;&lt;em&gt;Is it reasonably likely the Red Dragon will successfully land on Mars on the first try?&lt;/em&gt;&quot; I'd say 'probably yes' because they wouldn't accept that level of exposure and cost if it wasn't (reasonably likely).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If SpaceX then says it is, then I'd just shorten that to 'yes'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if they say &quot;it's probably not going to work the first time&quot; the way they did about landing on ships (&quot;barges&quot;) at sea at the beginning of those tests, then I'd say 'probably not'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So basically, I'd just defer to my best understanding of what their assessment of risk is. They seem to be doing a pretty good job of carrying on NASA's tradition of methodical and exhaustive risk assessment.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-04-28T05:57:43.560" LastActivityDate="2016-04-28T05:57:43.560" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15038" PostTypeId="1" AcceptedAnswerId="15043" CreationDate="2016-04-28T10:15:07.270" Score="15" ViewCount="41831" Body="&lt;p&gt;We all know spacecraft reentry causes extreme heat - plasma, ablator, flaming trail, all that jazz. I'd like to know just &lt;em&gt;what&lt;/em&gt; level of heat are we dealing with - could someone throw some numbers, like what's the maximum temperature occurring in the air or on the heatshield surface, or in the hottest place during reentry generally? Just how many Celcius degrees are we dealing with?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2018-12-31T15:22:58.850" Title="What are the top temperatures occurring during reentry?" Tags="|reentry|" AnswerCount="5" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="15039" PostTypeId="2" ParentId="15033" CreationDate="2016-04-28T11:14:22.310" Score="3" Body="&lt;p&gt;SpaceX has been taking a very methodical approach to development. They started with a single engine vehicle (Falcon-1) and before they even had a successful first launch upgraded the engine. (Merlin-1A to Merlin-1B). Then they upgraded to a 9 engine version. Several inline upgrades along the way (1.0 to 1.1 to 1.1 Full Thrust, with engines going from 1C to 1D versions). Then going to a three core version with 27 engines (Falcon Heavy). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For their capsule, they have been upgrading with information learned from each flight. Their manned capsule, the basis for the Red Dragon will have been flying by 2018 if all goes according to plan, landing humans on Earth to and from the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have been experimenting on flights with hypersonic reentry of the first stage. It took a number of tries, but they have now landed at least twice (I personally would count the Jason-3 attempt as a landing, since it landed, but one leg did not lock which caused the topple and explosion, but that baby landed! But will stick with 2 at this time for proprieties sake.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have data from Dragon cargo reentries. They have data from Falcon 9 first stage reentries. They will have data on Dragon V2 reentries by the time they get to 2018.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have access to the data NASA has on the atmospheric makeup of Mars from its previous landing missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have demonstrated that they can deliver, and they seem to get more and more experience along the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are no certainties, but they do look like they have taken the proper approach to the problem. Small steps, one after another, always learning something. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-04-28T11:14:22.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15040" PostTypeId="2" ParentId="15032" CreationDate="2016-04-28T11:18:30.923" Score="2" Body="&lt;p&gt;The dataset of rocket engines designed for reuse is possibly three or four?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The SSME as others have mentioned.&lt;/li&gt;&#xA;&lt;li&gt;Merlin from SpaceX&lt;/li&gt;&#xA;&lt;li&gt;BE-3 from Blue Origin&lt;/li&gt;&#xA;&lt;li&gt;X-15's engine&lt;/li&gt;&#xA;&lt;li&gt;RL-10 which was reused in the DC-X&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Pretty much every other engine has been designed for expendability. But rarely in the true sense, which would be make it cheap and cluster them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME is considered very high performing and very expensive. Many will contest that it was even designed for reuse based on its actual history. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The information from the Merlin engine is that the same basic design has been used for both reuse and expendability. Beyond the need to be able to air start (Something the SSME lacks, and some initial Constellation designs considered) there does not seem to have been much that needed to be changed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously if you use an ablative nozzle design it does not make much sense in the reuse world.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of performance, SpaceX has never indicated that there was any penalty for reuse of the Merlin engine. It was designed that way from the start. You want to be able to test your engine, again and again before flight, so at least some basic, marginal level of reuse is pretty much required. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME were test fired before launches and then prepped for launch, more extensively then say a Merlin would require. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some liquid boosters do hot fire testing, so even the expendable ones would need to be marginally reusable to accomplish this task.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So at some level, liquid fueled engines are usually built for marginal reusability. (Obviously solid engines, not so much). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, it may be that the question is somewhat moot, since any engine that does hot fire testing, or acceptance testing of the engine, is likely close to reusable to start with, even if used expendable.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-04-28T14:19:13.403" LastActivityDate="2016-04-28T14:19:13.403" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15041" PostTypeId="2" ParentId="15038" CreationDate="2016-04-28T11:21:50.703" Score="16" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle thermal protection system&lt;/a&gt; is rated for temperatures of up to 1510 °C.&lt;br&gt;&#xA;There's a &lt;a href=&quot;https://www.technologyreview.com/s/415476/hot-space-shuttle-images/&quot; rel=&quot;noreferrer&quot;&gt;boundary layer&lt;/a&gt; of air just above the TPS, outside that temperatures can reach 5500 °C. &#xA;NASA used &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100000137.pdf&quot; rel=&quot;noreferrer&quot;&gt;HYTHIRM&lt;/a&gt; to make thermal images of the orbiter during reentry: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hWx86.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hWx86.png&quot; alt=&quot;thermal image of STS, showing temperatures of up to 1650 °C on the nose and wing leading edges&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-05-25T07:37:32.857" LastActivityDate="2016-05-25T07:37:32.857" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15042" PostTypeId="2" ParentId="15038" CreationDate="2016-04-28T13:12:34.560" Score="15" Body="&lt;p&gt;&lt;a href=&quot;http://www.universetoday.com/119392/the-orions-heat-shield-gets-a-scorching-on-re-entry/&quot;&gt;This article&lt;/a&gt; says the recent Orion test experienced 2200 deg C, and this old &lt;a href=&quot;https://www.hq.nasa.gov/alsj/CSM05_Apollo_Flt_Tests_pp33-38.pdf&quot;&gt;Apollo fact sheet&lt;/a&gt; says 5000 deg F (2760 deg C) on Apollo 4 (a test at lunar return speed).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-04-28T13:12:34.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15043" PostTypeId="2" ParentId="15038" CreationDate="2016-04-28T14:23:54.733" Score="18" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_(spacecraft)&quot;&gt;Stardust&lt;/a&gt; sample return probe had an interesting re-entry to Earth's atmosphere. Returning from a solar orbit the maximum deceleration has been &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2007.tb00232.x/pdf&quot;&gt;reported as 34g&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maximum temperatures are &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100021412.pdf&quot;&gt;estimated at around 3,200 Kelvin&lt;/a&gt; or 2900 degrees C at the surface. It should be noted that the entry probe had no re-entry data recording so this measurement was estimated from spectroscopic examination of the heat shield as it descended, which must have been an interesting day's work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spectroscopic measurement was taken through the glowing plasma surrounding it, and the range of the measurement will mean that the temperature is an average over the whole shield.  As a result this doesn't represent a direct measurement from the hottest point on the heat shield, but it's interesting reading nonetheless.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-04-28T15:27:54.590" LastActivityDate="2016-04-28T15:27:54.590" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15044" PostTypeId="1" AcceptedAnswerId="15046" CreationDate="2016-04-28T14:31:54.577" Score="1" ViewCount="362" Body="&lt;p&gt;I've been trying to get a growth rate for commercial space flight, as it may be amenable to something like Moore's law. Wheras government funded things are prone to politics so can't really be rellied upon. Having such a number will make it possible to have more realistic timelines for things like colonizing moon/mars, mining asteroids etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a previous post about general statistics for space launches, which included both government and commercial launches:&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/8566/what-is-the-success-failure-ratio-of-space-bound-rockets&quot;&gt;What is the success/failure ratio of space bound rockets?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is obvious from the chart that the &quot;space wars&quot; drama from the 60's to 80's artificially inflated the amount of orbital launches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only other metric I was able to get is &quot;space vehicle and missle manufacturing&quot; which is also heavily interlaced with government funded military operations: &lt;a href=&quot;http://www.ibisworld.com/industry/default.aspx?indid=843&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ibisworld.com/industry/default.aspx?indid=843&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'm wondering if anyone knows or can come up with statistics for just the commercial space industry,  unmanned such as satellites has been ongoing for a while.  Though manned (commercial) space flight is relatively recent, I think the first was when someone paid multiple millions of dollars to visit the ISS, and more recently the Virgin Galactic. &#xA;SpaceX while a great innovator, many of their launches are still government contracts, so not all of their launches can be attributed to purely commercial purposes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also correct me if I'm wrong, but I think the first &quot;commercial&quot; or non-government funded space-probe is light-sail ( &lt;a href=&quot;http://sail.planetary.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://sail.planetary.org&lt;/a&gt; ), which has yet to launch. The closest other thing I know to a &quot;space-probe&quot; is space burial, Celestis offers sending cremated remains to the moon and deep space. &lt;/p&gt;&#xA;" OwnerUserId="14264" LastEditorUserId="25" LastEditDate="2017-12-19T13:32:13.430" LastActivityDate="2017-12-19T13:32:13.430" Title="commercial launches - what is the growth rate of commercial space flight?" Tags="|launch|artificial-satellite|probe|commercial|statistics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15046" PostTypeId="2" ParentId="15044" CreationDate="2016-04-28T15:00:47.107" Score="3" Body="&lt;p&gt;The United States Federal Aviation Administration (FAA) prepared a 2015 forecast document which contained global historical commercial launches and launch forecasts. Here is the chart they provided:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vhL8Y.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vhL8Y.png&quot; alt=&quot;Combined 2015 GSO and NGSO Historical Launches and Launch Forecasts&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GSO is geostationary orbit and NGSO is non-geostationary orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't been able to find the raw data set used for this chart yet, but it should give you an idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.faa.gov/about/office_org/headquarters_offices/ast/media/Commercial_Space_Transportation_Forecasts_2015.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here's&lt;/a&gt; the full FAA report.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-04-28T15:00:47.107" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15047" PostTypeId="1" AcceptedAnswerId="15049" CreationDate="2016-04-28T16:14:58.403" Score="6" ViewCount="1494" Body="&lt;p&gt;I know that Single-Stage-to-Orbit is quite challenging from the surface of Earth, but it's relatively straightforward from the moon (as demonstrated by the Apollo LM). What about Mars? Let's assume in-situ fueling of methane-LOX. &lt;/p&gt;&#xA;" OwnerUserId="13179" LastActivityDate="2020-10-26T14:41:06.077" Title="Is SSTO viable on Mars?" Tags="|mars|ssto|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15048" PostTypeId="2" ParentId="15047" CreationDate="2016-04-28T19:19:30.800" Score="3" Body="&lt;p&gt;Yes. Though one vs. two stages is always a trade. From what I've seen from Mars Sample Return studies anyway, two stages is usually the winner, mainly due to mass and volume constraints.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-04-28T19:19:30.800" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15049" PostTypeId="2" ParentId="15047" CreationDate="2016-04-28T19:28:04.400" Score="5" Body="&lt;p&gt;It's definitely viable. By the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;noreferrer&quot;&gt;rocket equation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$∆v = v_e \ln \frac {m_0} {m_f}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methane-LOX gives an exhaust velocity around 3500m/s. Mars surface to orbit requires the expenditure of about 3800 m/s of ∆v. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the ratio of initial (fully fueled) mass to final (fuel spent) mass need only be around:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$e^{\frac {3800} {3500}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$e^{1.09}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or about 3:1 -- 2 tons of propellant per 1 ton of empty ship/payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is as compared to an Earth-launched SSTO, which needs about 9500 m/s, about a 15:1 mass ratio for methalox!&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-04-28T19:34:17.407" LastActivityDate="2016-04-28T19:34:17.407" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15050" PostTypeId="2" ParentId="721" CreationDate="2016-04-28T22:03:26.293" Score="3" Body="&lt;p&gt;Searching for radio signals is probably the most mainstream method. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Transit Photometry is another method that has been in the news lately. Theoretically, it should be able to detect alien megastructures around stars. &lt;a href=&quot;http://www.popularmechanics.com/space/telescopes/a19346/james-webb-telescope-alien-megastructure/&quot; rel=&quot;nofollow&quot;&gt;One candidate has already been identified.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Optical searches for laser communications has also been attempted. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spectroscopy is a method for which I hold high hopes. When perfected it will able to tell us the composition of the atmosphere of other planets and thus could reveal signs of life. While this is not exactly SETI, we might be able to detect the presence of pollution which could indirectly point to intelligence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More info on the different methods can be found &lt;a href=&quot;http://www.seti-setr.org/SETI/Methods/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-04-28T22:12:07.650" LastActivityDate="2016-04-28T22:12:07.650" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15051" PostTypeId="1" CreationDate="2016-04-28T23:27:15.270" Score="12" ViewCount="947" Body="&lt;p&gt;Does anyone know of any modern (1990's - present) computer modeling of the Orion pusher plate and its reaction/survivability to repeated plasma impacts?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding the 1950s/1960s Orion Project using nuclear pulse propulsion: I see a lot of statements that &quot;Orion would have worked. The only thing stopping us is the political will.&quot; I am skeptical of this claim.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0805072845&quot; rel=&quot;noreferrer&quot;&gt;George Dyson's book&lt;/a&gt; included quotes from project veterans and modern experts who pointed out that some key questions remained about basic feasibility. Specifically, could the pusher plate survive the repeated plasma impacts? To my knowledge, two key questions remain unanswered in the public domain. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first is regarding the erosion of the pusher plate and hinged on questions of the turbulence of the plasma stagnation zone at the plate surface and its opaqueness or transparency to UV. The interviewee claimed that this could be modeled by modern nuclear bomb software, but either nobody had done the work or they couldn't talk about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second question regarded spalling of the pusher plate. It's possible that the shock wave would traverse the pressure plate and blow material off of the upper side. Orion was before HESH warheads were developed; I believe its designers were unaware of this physical process. Again, modern computer codes run by experts in armor, ballistics, and metallurgy could probably nail down if the pusher plate would spall, and if so would know if there are any modern materials or construction methods that could prevent it. Again, this may be &quot;military grade&quot; expertise and either the work hasn't been done or can't legally be divulged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do any of you space experts have any substantial inputs on this?&lt;/p&gt;&#xA;" OwnerUserId="8247" LastEditorUserId="4660" LastEditDate="2016-08-22T15:05:01.147" LastActivityDate="2022-12-14T12:18:56.583" Title="Orion Project (NPP) Pusher Plate computer models and survivability?" Tags="|interstellar-travel|nuclear|shielding|nuclear-pulse-propulsion|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15052" PostTypeId="1" CreationDate="2016-04-28T23:43:24.663" Score="6" ViewCount="2416" Body="&lt;p&gt;Lately I've been exploring the idea of cold gas thrusters, or in this case cold gas rocket engines, to be used in model rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've read that nitrogen gas can get &lt;a href=&quot;https://en.wikipedia.org/wiki/Cold_gas_thruster&quot; rel=&quot;noreferrer&quot;&gt;73 seconds&lt;/a&gt; and helium can get &lt;a href=&quot;http://www.lr.tudelft.nl/en/organisation/departments/space-engineering/space-systems-engineering/expertise-areas/space-propulsion/propulsion-options/chemical-rockets/cold-gas/&quot; rel=&quot;noreferrer&quot;&gt;2.5 times nitrogen&lt;/a&gt;, which would be 182.5 seconds. &lt;strong&gt;That would be pretty darn good as far as model rockets go.&lt;/strong&gt; (However, those were for vacuum performances and idk what they might be at sea level.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been using &lt;a href=&quot;http://www.braeunig.us/space/propuls.htm#combustion&quot; rel=&quot;noreferrer&quot;&gt;equation 1.22&lt;/a&gt; and finally got it making sense after some difficulty with R (the ideal gas constant).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's something else I want to know, but haven't found. &lt;strong&gt;How noisy would a helium rocket engine be?&lt;/strong&gt; I'm thinking for an engine on the order of 10 to 50 N. I'm really hoping it would be less noise than a solid rocket motor of equivalent thrust, since it would have no combustion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tank weight is probably going to be the biggest drawback. I've handled those large cylinders of helium at party stores, and they are heavy. However, let's just consider the state of the art: a very small spherical tank of highly compressed helium. I don't know how to calculate tank weight based on max pressure and volume, but I'm hoping it won't be too heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know nitrogen is cheaper, but honestly, we fill up party balloons with helium all the time so how much more expensive could it be? I'm guessing the &quot;fuel&quot; costs would be not much different from a solid rocket. Besides, I prefer to take the ideal case first, and then see how close we can get. (Helium is more ideal because of its lower molecular mass compared to N2.) I was even considering preheating the helium to like 2000 K or something in the tank, to get better thrust, but one thing at a time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: Okay, in bullet points, here are the equations I'm looking for:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;an equation to tell me how noisy a cold gas engine would be for a given thrust (at sea level).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;an equation to tell me how heavy a spherical gas tank would be, given internal pressure of helium (500 atmospheres).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;an equation to tell me how thrust would vary with time.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2016-04-29T00:14:12.423" LastActivityDate="2023-09-12T20:06:23.700" Title="Are cold gas thrusters viable for model rockets?" Tags="|rockets|engines|model-rocketry|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="15053" PostTypeId="1" AcceptedAnswerId="15059" CreationDate="2016-04-28T23:45:56.373" Score="7" ViewCount="367" Body="&lt;p&gt;I was excited to read several articles about experiments generating anti-matter (&lt;a href=&quot;https://www.sciencedaily.com/releases/2008/11/081117193019.htm&quot;&gt;production from gold&lt;/a&gt; or &lt;a href=&quot;http://phys.org/news/2013-06-physicists-tabletop-antimatter-gun.html&quot;&gt;helium&lt;/a&gt;) at what seemed like a rather low energy cost. I thought anti-matter generation was the biggest obstacle to the development of things like pion rockets or other anti-matter-dependent propulsion/power generation devices, however, I didn't see any resurging interest in designing such things and no one seemed particularly excited. Did I misunderstand the importance of these findings?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've considered the following reasons why these experimental methods might not be useful, and I'd appreciate your input:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Positrons aren't useful, we need antimatter with mass (antiprotons), and these methods cannot be used to generate these &lt;/li&gt;&#xA;&lt;li&gt;Energy cost is still significantly higher than potential energy yield&lt;/li&gt;&#xA;&lt;li&gt;Supporting system requirements are still beyond our technical capabilities &lt;/li&gt;&#xA;&lt;li&gt;These methods still generate amounts far smaller than what would be required to propel theoretical antimatter drives&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="14271" LastEditorUserId="6816" LastEditDate="2016-04-29T01:07:35.440" LastActivityDate="2016-04-29T08:41:52.970" Title="Recent advances in anti-matter generation" Tags="|rockets|propulsion|antimatter|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15054" PostTypeId="2" ParentId="15052" CreationDate="2016-04-29T00:01:14.793" Score="4" Body="&lt;p&gt;I think you're doing something wrong in the calculation; with those figures I get Ve = 1694.6 m/s, or 172.7 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used this python code:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import math&#xA;&#xA;k = 1.66&#xA;M = 4.0&#xA;Tc = 300.0&#xA;Pe = 1.0&#xA;Pc = 500.0 &#xA;&#xA;R = 8314.4621&#xA;&#xA;T1 = 2.0*k/(k-1.0)&#xA;T2 = (R*Tc)/M&#xA;x = (k-1.0)/k&#xA;T3 = 1.0-math.pow((Pe/Pc),x)&#xA;&#xA;Ve = math.sqrt( T1*T2*T3 )&#xA;&#xA;print &quot;Ve %.1f m/s or %.1f s&quot;%(Ve,Ve/9.81)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-04-29T00:01:14.793" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15055" PostTypeId="2" ParentId="15053" CreationDate="2016-04-29T02:32:59.937" Score="0" Body="&lt;p&gt;While it's theoretically possible to use anti-matter, the engineering is just not there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recently attended a lecture on nuclear fusion, and the gentleman giving it stressed more than once how difficult is to take something from theoretically possible to real world possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've known fusion to be possible since the 1920s and we still don't have fusion reactors. It's the same with anti-matter. It has no practical applications and it will probably be another 100 or more years before we are anywhere near using it in any practical sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe that could change if we make science a priority, but somehow I don't see that happening any time soon.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="6816" LastEditDate="2016-04-29T05:17:24.797" LastActivityDate="2016-04-29T05:17:24.797" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15056" PostTypeId="1" AcceptedAnswerId="15065" CreationDate="2016-04-29T03:01:58.163" Score="9" ViewCount="1058" Body="&lt;p&gt;Musk has &lt;a href=&quot;https://twitter.com/elonmusk/status/725364699303301120&quot;&gt;said that the capsule was designed to land anywhere in the solar system&lt;/a&gt;. As doubtful as that sound, I can't think of any reasons preventing them from landing on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it make more sense to test the capsule by landing it on the Moon (no travel time, no need to wait for transit window, etc.) instead of Mars? Is there any fundamental difference between the two that would prevent flying essentially the same spacecraft to both?&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="25" LastEditDate="2016-04-29T22:17:24.820" LastActivityDate="2016-10-26T12:42:35.280" Title="Why not land Red Dragon on the Moon?" Tags="|mars|spacex|the-moon|dragon-v2|red-dragon|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15058" PostTypeId="1" CreationDate="2016-04-29T04:18:19.180" Score="5" ViewCount="446" Body="&lt;p&gt;Red Dragon will presumably have to shed or re-stow solar arrays before any landing. What will be the source of electrical power after landing?&lt;/p&gt;&#xA;" OwnerUserId="14276" LastEditorUserId="29286" LastEditDate="2020-08-04T08:39:08.587" LastActivityDate="2020-08-04T08:39:08.587" Title="Can Red Dragon generate electrical power after a Mars landing, or will it rely only on battery power?" Tags="|spacex|mars|power|future-missions|red-dragon|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15059" PostTypeId="2" ParentId="15053" CreationDate="2016-04-29T08:41:52.970" Score="2" Body="&lt;p&gt;There are &lt;a href=&quot;https://en.wikipedia.org/wiki/Antimatter_rocket#Thermal_antimatter_rocket:_heating_of_a_propellant&quot; rel=&quot;nofollow&quot;&gt;proposed thermal antimatter rocket designs&lt;/a&gt; which could work with positrons. I am focusing on these to answer your questions (in reverse order):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The amount of energy generated is (more or less) an engineering problem. Assuming that asteroid mining is feasible, there is no principle limit to scale these operations. Since antimatter is the fuel for a rocket (think of hydrogen in electrical cars), conversion efficiency is &quot;merely&quot; a cost factor.&lt;/li&gt;&#xA;&lt;li&gt;Indeed this is the major obstacle IMO. The more efficient (liquid or gaseous core) designs of a &lt;strong&gt;nuclear fission&lt;/strong&gt; rocket are already quite beyond our current capabilities. Not to speak of the ambitious plasma core design of an antimatter rocket. Simpler designs yield considerably less ISP and are probably just not worth the effort (of generating the antimatter).&lt;/li&gt;&#xA;&lt;li&gt;See answer to your last question.&lt;/li&gt;&#xA;&lt;li&gt;In fact, positrons can be useful. There is no principle reason you could not heat your propellant with them as well. However, positrons are &lt;a href=&quot;http://www.icarusinterstellar.org/antimatter-propulsion-storage-antiparticles/&quot; rel=&quot;nofollow&quot;&gt;hard to store&lt;/a&gt;. Current research is ongoing, but with the cited capacity of 10¹² positrons per trap, you'd end up (with 1.6MeV per annihilation pair) with (roughly) 0.16J per cm³ trap volume. That means you can store the energy to heat 0.16g of water by 0.24K. Not quite a rocket. If you store antihydrogen however, you can not only increase your storage capacity by a factor of 10³, but also relase 10³ times the energy by annihilation (1.8GeV, not counting secondary reactions). This gives you a potential energy of 160J per cm³ trap volume. Bringing a ship to orbit requires (just an estimate) 10¹²J. So you'd end up with 1000m³ volume for fuel!&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The bottomline is: You need too much volume to store the antimatter in your spaceship (at least with current storage capacities).&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-04-29T08:41:52.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15060" PostTypeId="2" ParentId="15038" CreationDate="2016-04-29T11:07:03.010" Score="11" Body="&lt;p&gt;&lt;a href=&quot;http://ngpdlab.engin.umich.edu/ablation/surface-chemical-processes&quot; rel=&quot;nofollow noreferrer&quot;&gt;CFD simulations&lt;/a&gt; show the air in the bow shock of the stardust probe reached temperatures of around 50,000°K at 71km, falling to 10,000°K at 51km (thin red line). It must be remembered that the air is extremely thin at these altitudes, fortunately resulting in poor heat transfer to the craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The surface temperature was much lower, as mentioned in Andy's answer, due to ablative cooling. The surface is designed to burn away, so the surface temperature largely depends on the decomposition temperature of the ablative material.&lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="12102" LastEditDate="2017-08-26T05:41:22.447" LastActivityDate="2017-08-26T05:41:22.447" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15061" PostTypeId="1" CreationDate="2016-04-29T12:34:52.957" Score="2" ViewCount="185" Body="&lt;p&gt;What are the maintenance benefits of the &lt;a href=&quot;https://en.wikipedia.org/wiki/SABRE_(rocket_engine)&quot; rel=&quot;nofollow&quot;&gt;SABRE&lt;/a&gt; engine over all the other engines being considered for the propulsion concepts for SSTO Reusable launchers? &lt;/p&gt;&#xA;" OwnerUserId="14278" LastEditorUserId="12102" LastEditDate="2021-01-01T10:52:53.903" LastActivityDate="2021-01-01T10:52:53.903" Title="What are the maintenance benefits of SABRE engine technology for SSTO launchers?" Tags="|rockets|engines|sabre-engine|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15062" PostTypeId="1" CreationDate="2016-04-29T18:52:33.717" Score="6" ViewCount="7692" Body="&lt;p&gt;Which planets and moons have a surface where you could survive with a conventional or near-future spacesuit? Assume oxygen, water, and food supplies are accounted for, but roughly current limitations on handling heat, pressure, gravity, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, you might be able to survive on Mercury if you avoid going to the side facing the sun and stay on its cold side with your spacesuit on. Similarly a lot of moons of the gas giants like Ganymede might be survivable with technological aid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would other such heavenly bodies be in our solar system?&lt;/p&gt;&#xA;" OwnerUserId="14280" LastEditorUserId="6816" LastEditDate="2016-04-29T19:07:42.183" LastActivityDate="2020-06-17T07:39:17.003" Title="Which planets and moons can you walk on?" Tags="|planet|moons|moonwalking|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15063" PostTypeId="2" ParentId="15051" CreationDate="2016-04-29T21:18:43.037" Score="2" Body="&lt;p&gt;Have a read &lt;a href=&quot;http://www.spacedaily.com/news/nuclearspace-03h.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. 'Evidence of this obtained from nuclear tests was the foundation for further research into the feasibility of a pusher plate. It was discovered that ablation (erosion) of the surface of a pusher plate could also be reduced by coating it with graphite. Coating the launch pad in similar fashion would minimize ablation of the surface and therefore create very little fallout indeed.'&lt;/p&gt;&#xA;&lt;p&gt;I have been studying this project for a year or so and it looks quite promising if an exception can be made from the Nuclear test treaty for a ground or space launch.&lt;/p&gt;&#xA;" OwnerUserId="11671" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-04-29T21:18:43.037" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15064" PostTypeId="2" ParentId="15062" CreationDate="2016-04-29T22:13:08.513" Score="2" Body="&lt;p&gt;Let's split the question into parts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;where can you walk&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The celestial body has to have enough mass, so you can walk/stand on it, without drifting away after the slightest movement. Should have landmass (no gas planets) / hard surface -&gt; ice surface&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;time&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest issue is time, the acid rains on Venus are not that much of a problem, if you stay there only for a short period of time. The same goes for Pluto, you would need a lot of energy for keeping your body warm. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even Mercury's hot side could be a destination, if you don't extend the visit too long, but landing there would probably be the bigger issue (gas eruptions and so on).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;physics&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Celestial bodies that are so close to other objects, that they interact in a bad way for things you don't want to be broken -&gt; solar flares, quakes, gas eruptions and so on.&lt;/p&gt;&#xA;" OwnerUserId="14245" LastEditorUserId="10273" LastEditDate="2017-11-27T11:00:35.487" LastActivityDate="2017-11-27T11:00:35.487" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15065" PostTypeId="2" ParentId="15056" CreationDate="2016-04-29T22:17:07.600" Score="5" Body="&lt;p&gt;No, not really. SpaceX has been focused on Mars for a long time. It would gain nothing by landing on the Moon towards that objective. The only advantage as compared to Mars is the timeline is considerably less, taking only a few days vs a few months. The hardest piece isn't landing on an object, it's landing it in the thin atmosphere of Mars. Bottom line is, SpaceX wants to land it on Mars because that is their goal.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-04-29T22:23:47.260" LastActivityDate="2016-04-29T22:23:47.260" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15066" PostTypeId="2" ParentId="15062" CreationDate="2016-04-29T22:57:20.943" Score="2" Body="&lt;p&gt;Well, let's start with the minimum size of an object. The formula for escape velocity is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{escape} = \sqrt{2 G M/R}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, let's say that you don't want to accidentally jump off. Let's say that you can do that with about a 3 m/s jump. Let's give a density of $3 g/cm^3$, and let's just say a uniform sphere. Putting all of that gives:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$3=\sqrt{2*3000*G*(4/3*\pi*r^2)}=8000*G*\pi*r^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solving for r gives about 2.3 km. So that's your absolute lower limit. Of course, you might want something that you could walk more upright. Let's say you wanted at least 50 m/s for an escape velocity, as that you would come down relatively quickly. That would come to about 38 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no object with a solid surface that is too massive to walk on, so that's okay. It must have a solid surface, so that takes out the gas giants. Aside from that, the only objects to be wary of are those with a toxic atmosphere, which would be Venus, and Titan. Venus would be difficult with any conceivable technology. Titan might be doable, but it would require some special work, and likely wouldn't work for long. The main problem with Titan is the atmosphere is flammable, which could cause some interesting complications if there was any kind of a leak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, let's look at heat and cold. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_suit&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, a space suit can operate between -156 °C (−249 °F) to 121 °C (250 °F). Saturn is about -178, so you'd have to be inside of Saturn's orbit. The upper temperature range would be reached for an object at Venus's orbit, probably not that far.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very last thing is extra hazards. The main hazard would be radiation, which only really affects Jupiter's closest moons, Io and Europa. The further ones should be okay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, that leaves hundreds of asteroids and NEOs, the moons of Mars, Mars, the Moon, Earth, and the further large moons of Jupiter, and the trojan asteroids of Jupiter, and probably some other stuff I'm missing. With some work, we could probably get to Mercury and the outer solar system. Venus, and the gas giants, well, not likely. Titan might be tricky, as it's atmosphere is literally flammable. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-05-02T01:24:57.527" LastActivityDate="2016-05-02T01:24:57.527" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15067" PostTypeId="2" ParentId="15056" CreationDate="2016-04-29T23:00:56.690" Score="4" Body="&lt;p&gt;For a mission to Mars test should be done in Mars and for a Moon mission test should be done at Moon, because we have here different situation. Concentration for manned space exploration now is to Mars, for something new not achieved yet, so the interest it is more for that destination. In this way to fulfill their purposes and what they want to achieve with their Red Dragon, tests should be done to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As per info in Wikipedia you can find for Dragon V2 that according to Musk launching the Dragon spacecraft with Falcon Heavy could transport to Mars 2-4 tons payload and has capability even for a mission to Europa. For beyond LEO missions I guess probably they will do upgrades. But to be sure that for different missions beyond LEO we will have different configurations. For destinations with atmosphere as Mars it would have a heat shield and parachutes, for destinations without atmosphere as Moon in place of the heat shield and parachutes system mass, it would have more propellant available to descent and approach to surface slowly and touchdown safely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moon landings test would not help too much because Mars landings are different case with different condition. There is atmosphere, different gravity and orbital speeds. So in this way we have different procedures and spacecraft configuration. Landing on the Moon it would be just to spend time and moneys for something proved before because they will not get useful data and information for Mars landings. In other words “calculations” are different because we have different situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something else that I want to write about the Mars landing attempt it has to do with reasons that SpaceX is focusing to Mars and concentrating energies to this missions. This reasons make another difference between Moon and Mars mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let see the situation for a Moon landing and a Mars landing. For the first one we are speaking to prove a technology already done so it hasn’t too much interest. It will look like landing for show since the focus in space exploration as I said above is for something new, different, not done yet as landing a human-rated spacecraft to Mars. Landing for show are trying some companies for the Google Lunar X Prize, putting a Lander with a rover in the surface of the Moon. For the SpaceX in the future for commercial reasons could be as an option for tourists to make landings in the Moon or maybe for Moon bases and laboratories. But this is not so near, a lot of tests should be done to make perfectly safety with 100% success for manned missions and for this tests a lot of money must be spent by SpaceX himself, alone without any help, because for this NASA would not fund since is focused to prove Mars landings and doing improvements for success there. Also for a supposed tourist option, SpaceX needs to do new developments. So in these types of Moon missions, it should make possible not just to land but even to get back home safely. In the Mars case it is important for now, just to prove landing a human-rated spacecraft or Lander, safely which has interest. Remember that NASA is trying with flying saucer which is partial successful and without warranty that could work. SpaceX has proved to land something heavy ( Falcon 9 first stage) and knows how to deal with gravity to land softly heavy vehicle. The very useful data taken in this landings are very important for a place with more gravity than Moon and with atmosphere, the Mars. SpaceX is proving capabilities for a Mars landing, or at least they are almost at the finish line and trying to prove it by testing as fast as they can in the first mission of Dragon spacecraft beyond LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would give SpaceX the chance being part in one of the biggest manned space exploration projects and it would have the most important role, landing a manned spacecraft safely, making it a key element. Interest for Mars it is even for NASA even for SpaceX. Also NASA would be interested for SpaceX which achieves success even with low cost, so for a program to Mars would be better even for NASA. Although Musk has shown ambition to have their own big Mars rockets (Interplanetary Transport System project), their own Mars missions and colonizing it, would agree a collaboration with NASA which has interest and can fund it. All the private companies would like this. Remember in the Moon program in 60s, private companies were in race with each other in competitions to win the contracts and the patent for a technology, without even knowing too much for space because of course it was the beginning of the space age. And they offered themselves with their projects to receive one part of the budged for the development of new technologies for example a human-rated Lunar Lander, without knowing too much for space. SpaceX already have the capabilities to achieve a Mars landing of the human-rated spacecraft. It is ahead and gone even further than NASA has done for this part. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So firstly choosing Moon landings would not give the useful data needed for Mars landing since between them are different conditions, and also SpaceX would be more interested to have collaboration with NASA for a Mars program, staying at the first places for private companies because of their achievements.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-10-26T12:42:35.280" LastActivityDate="2016-10-26T12:42:35.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15068" PostTypeId="2" ParentId="4765" CreationDate="2016-04-30T00:58:05.360" Score="9" Body="&lt;p&gt;A rocket engine type could use different propellants but it needs modifications. I can't say if astronauts could do such modifications in space for your scenario and if they can how easy it is, but maybe theoretically it could be done. Using different propellants in a rocket engine type has been proved by Aerojet &lt;a href=&quot;https://en.wikipedia.org/wiki/LR-87&quot; rel=&quot;nofollow noreferrer&quot;&gt;LR-87&lt;/a&gt; rocket engine. It was the only rocket engine in the world that with modifications has tested and operated using all three major propellant combinations: LOX/RP-1, LOX/LH2, N2O4 /Aerozine-50. It tested even an N2O4/Alumizine version. For the LOX/CH4 which was not popular at that time and not studied or invested for LOX/CH4 rocket engine projects, I don't know if they could, maybe yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LR-87 rocket engine versions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/l/lr87-3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;LR-87-3&lt;/a&gt;, LOX/RP-1 was used in the Titan I missile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/l/lr87-5.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;LR-87-5&lt;/a&gt;, N2O4 /Aerozine-50 used in Titan II. Was lighter and simpler than its predecessor and did not need independent ignition system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According astronautix:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/l/lr87alumizine.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;LR-87 Alumizine&lt;/a&gt;, N2O4/ Alumizine tested in laboratories using a metallized fuel (for greater impulse density) and gelled propellants (to facilitate in-space starts after a period of coasting). The Aerozine 50 was slurried with aluminum powder (using Carbopol 904 gelling agent), and the engine was operated without any modifications, but could not attain stable long duration operation in that configuration. This was the first time a liquid rocket booster engine had ever been run on aluminized propellant. Many years later Aerojet operated small thrusters on metallized storable propellants and achieved satisfactory results.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/l/lr87lh2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;LR-87 LH2&lt;/a&gt;, LOX/LH2 was in competition with J-2 for Saturn V. It was the first large Lox/LH2 engine in the world an LR-87 regeneratively cooled thrust chamber and nozzle, with a modified injector, and redesigned fuel pump. In comparison with LR87 Lox/RP-1, major changes were made on the injectors, and the RP-1 fuel pump was replaced by a single stage hydrogen pump specially designed for the purpose. The oxygen pump and its gearbox were the same.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also are the tripropellant rocket engines that probably would be more practical since you don't have to do major modifications by changing parts. Considered two engines in one, with a common engine core with the engine bell, combustion chamber and oxidizer pump, but two fuel pumps and feed lines.&#xA;In the 1960s, Rocketdyne fired an engine which mixed three separate streams of propellants, using a mixture of liquid lithium, gaseous hydrogen, and liquid fluorine to achieve highest specific impulse for a chemical rocket motor by 542 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/r/rd-701.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-701&lt;/a&gt; lox/lh2/kerosene. Uses one oxidizer and two fuels, switching them in mid-flight. Proposed for reusable MAKS space plane project. Built but cancelled later due to lack of funding. The mixing block of the main combustion chamber has three groups of injectors, one for each of three components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other versions  &lt;a href=&quot;http://www.astronautix.com/r/rd-704.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-704&lt;/a&gt;, &lt;a href=&quot;http://www.astronautix.com/r/rd-0120td.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-0120TD&lt;/a&gt;, &lt;a href=&quot;http://www.astronautix.com/r/rd-0750.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-0750&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-11-01T16:32:28.443" LastActivityDate="2016-11-01T16:32:28.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15069" PostTypeId="2" ParentId="15058" CreationDate="2016-04-30T01:54:16.090" Score="4" Body="&lt;p&gt;Solar panels for both Dragon versions are on the trunk of the craft, the unpressurized external (relative to the capsule) component that separates the capsule from the upper stage of the rocket that launches it. There's a bunch of reasons that the trunk must be jettisoned before landing:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The trunk is aerodynamic for takeoff, though only with the panels stowed (on Dragon 1, they are stowed folded inside fairings on each side; I'm not sure about Dragon 2). It is not aerodynamic for landing, nor is it heat-shielded (it actually covers the capsule's heat shield) so it certainly can't survive (re-)entry anywhere with an atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;The small Draco thrusters used for maneuvering and orbital adjustments can be used with the trunk attached - otherwise, the Dragon 1 wouldn't be able to reach the ISS safely - but I'm not sure if the large SuperDraco thrusters that Dragon 2 uses to land propulsively can be used while the trunk is attached (without damaging it).&lt;/li&gt;&#xA;&lt;li&gt;While the trunk can withstand considerable vertical compression force (it certainly does so during liftoff), it has no landing legs or similar. Dragon 2's landing legs are behind its heat shield, which is behind the trunk, and they aren't long enough to land on even if they could be deployed with the trunk still attached.&lt;/li&gt;&#xA;&lt;li&gt;The trunk adds weight (equipment and structure) that will probably not be needed after landing. Of course, part of that equipment is the solar panels, but other parts are just dead weight. Jettisoning it means there's that much less work for the heat shield, parachutes, and/or SuperDracos to do.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, altogether, the trunk definitely has to go before landing (unless Dragon 2 gets massively redesigned to permit the trunk to survive landing). However, that doesn't necessarily mean that the capsule will only be on battery power. Dragon 1 has 11 cubic meters (388 cubic feet) of internal capacity (couldn't find the numbers for Dragon 2, but Elon Musk has described it as &quot;about the size of an SUV&quot;), and Falcon Heavy can get the capsule to a Mars orbit with several (numbers I've seen have said 2-4) tons of payload. That's plenty of space and mass to carry solar panels, a radioisotope thermal generator, or even a &lt;a href=&quot;https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor&quot; rel=&quot;nofollow&quot;&gt;small nuclear fission reactor&lt;/a&gt;, probably with volume and mass left over for a scientific payload.&lt;/p&gt;&#xA;" OwnerUserId="11532" LastActivityDate="2016-04-30T01:54:16.090" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15070" PostTypeId="1" CreationDate="2016-04-30T02:19:46.930" Score="0" ViewCount="788" Body="&lt;p&gt;What is the distance between Jupiter and the L4 and L5 points of the Sun-Jupiter system? I need it to attempt to calculate the $\Delta v$ needed to reach Jupiter from there.&lt;/p&gt;&#xA;" OwnerUserId="13960" LastEditorUserId="10335" LastEditDate="2016-05-05T14:17:55.957" LastActivityDate="2016-05-05T14:17:55.957" Title="What is the distance of the Sun-Jupiter L4 and L5 points relative to Jupiter?" Tags="|orbital-mechanics|crewed-spaceflight|mission-design|delta-v|deep-space|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15071" PostTypeId="2" ParentId="15070" CreationDate="2016-04-30T03:23:40.040" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#Lagrange_points&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt;, L4 and L5 for Sun-Jupiter will each be at the same distance from Jupiter as Jupiter is from the Sun (something like 780,000,000 km).&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-04-30T03:23:40.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15072" PostTypeId="2" ParentId="14146" CreationDate="2016-04-30T05:24:04.540" Score="3" Body="&lt;p&gt;Wikipedia article &quot;Pioneer anomaly&quot; seems to address this. The accepted answer to this slowing down of the spacecraft slightly more than predicted is that the deceleration was caused by &quot;an anisotropic radiation pressure caused by the spacecraft's heat loss.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the anomaly was caused by radiation pressure caused by heat being emitted from the spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The is also an article here at &lt;a href=&quot;http://www.nature.com/nphys/journal/v8/n9/full/nphys2428.html&quot; rel=&quot;nofollow&quot;&gt;Nature Physics&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14276" LastActivityDate="2016-04-30T05:24:04.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15075" PostTypeId="1" AcceptedAnswerId="53943" CreationDate="2016-04-30T12:46:53.333" Score="13" ViewCount="1638" Body="&lt;p&gt;In this short but very &lt;a href=&quot;https://space.stackexchange.com/a/15072/12102&quot;&gt;helpful answer&lt;/a&gt; @MikeH provides a link to this short item published in Nature:&lt;a href=&quot;http://www.nature.com/nphys/journal/v8/n9/pdf/nphys2428.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&amp;quot;...&lt;em&gt;and farewell to the Pioneer Anomaly&lt;/em&gt;&amp;quot;&lt;/a&gt; which is well worth reading! It references a Physical Review Letter which can also be read &lt;a href=&quot;https://arxiv.org/abs/1204.2507&quot; rel=&quot;nofollow noreferrer&quot;&gt;via ArXiv&lt;/a&gt; for the specifics of the whole thing, including things like the screenshot below.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pioneer Anomaly&lt;/a&gt; is related to anisotropic thermal radiation. If the spacecraft is spin-stabilized, it's orientation is carefully maintained.&lt;/p&gt;&#xA;&lt;p&gt;The Nature item (published in 2012) contains this forward-thinking paragraph:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;For decades, the anomaly remained. The Pioneers were realized to be uniquely sensitive to the decelerating force, cruising smoothly over huge distances because they were spin stabilized: other craft, including the Voyagers, were not, and their regular firing of thrusters to set or maintain course overwhelmed any sensitivity they might have had to the tiny Pioneer anomaly. &lt;strong&gt;There is some hope&lt;/strong&gt; that NASA’s New Horizons mission, launched in 2006 and set to reach Pluto in 2015, &lt;strong&gt;might display a measurable anomalous deceleration, as it too is spin-stabilized for a chunk of its voyage&lt;/strong&gt;; however, the design of the craft might result in some degree of unpredictable thermal-radiation pressure that would again overwhelm the effect.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;How did that all work out? Did New Horizons &amp;quot;&lt;em&gt;display a measurable anomalous deceleration&lt;/em&gt;&amp;quot; due to the same, now very well understood effect? And if so, was it during the spin-stabilized chuck of it's voyage?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZnwEn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZnwEn.jpg&quot; alt=&quot;Pioneer Anomaly screenshot&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Fig. 1: Illustrative representation of the thermal model of the Pioneer 10 spacecraft evaluated at 40 AU. Top left: spacecraft body interior (temperature range: blue -16° C, red +10° C); Bottom left: spacecraft exterior (blue -155° C, red -108° C); Right: entire spacecraft (blue -213° C, red + 136° C). Un-modeled struts that connect the RTGs to the spacecraft body are indicated with yellow-black dashed lines.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://arxiv.org/abs/1204.2507&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt; (click for larger)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-06-30T14:11:02.147" LastActivityDate="2021-07-01T22:09:46.403" Title="Did New Horizons also demonstrate the &quot;Pioneer Anomaly&quot;?" Tags="|new-horizons|pioneer|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="15076" PostTypeId="1" AcceptedAnswerId="15085" CreationDate="2016-04-30T13:46:33.397" Score="3" ViewCount="306" Body="&lt;p&gt;In the recently proposed entry of Red Dragon into the Mars atmosphere, the supersonic retro-propulsion (or SRP, roughly: slowing down using thrusters while still moving against the atmosphere at supersonic speed) data collection will be extremely valuable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But &lt;a href=&quot;https://space.stackexchange.com/questions/15033/is-it-probable-the-red-dragon-will-successfully-land-on-mars-on-the-first-try/15037?noredirect=1#comment39603_15037&quot;&gt;this comment&lt;/a&gt; got me thinking...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am sure a huge amount of data will be collected and stored - to be broadcast later when things have settled down simply due to the volume of data collected vs bandwidth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If for some strange reason there is an unanticipated catastrophic event, will there be &lt;em&gt;at least some&lt;/em&gt; real-time radio broadcast of telemetry (to anywhere - Mars satellite, earth, etc.) during the SRP? If not, is it because there is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Communications_blackout#Spacecraft_reentry&quot; rel=&quot;nofollow noreferrer&quot;&gt;(radio) Communications Blackout&lt;/a&gt; due to heating and ionization or other similar phenomenon? &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-05-01T14:24:59.913" Title="Will Red Dragon experience a &quot;Communications Blackout&quot; during SRP?" Tags="|spacex|data-transmission|red-dragon|telemetry|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15077" PostTypeId="2" ParentId="15070" CreationDate="2016-04-30T15:40:16.597" Score="5" Body="&lt;p&gt;L4 and L5 are the same distance from Jupiter as Jupiter is from the sun: about 5.2 A.U. L4, the sun and Jupiter form an equilateral triangle. Ditto L5, sun, Jupiter. All together, L4, Jupiter, L5, sun form a 120º/60º rhombus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nVI2w.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nVI2w.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Mark Adler says, distance and delta V are different things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Describe your scenario in more detail and I might be able to help with the delta V. Are you thinking of going from a Trojan asteroid to one of Galilean moons? What trip time do you have in mind?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-04-30T15:40:16.597" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15078" PostTypeId="1" CreationDate="2016-04-30T16:03:07.310" Score="4" ViewCount="419" Body="&lt;p&gt;Would there be any interest in a manned mission to Mars without landing?&#xA;This could consists of none (just a flyby) to many revolution(s) around the planet.&#xA;Would it cut a consequent part of the cost compared to a landing and return mission?&lt;/p&gt;&#xA;" OwnerDisplayName="user14286" LastActivityDate="2020-06-11T16:40:36.417" Title="Any interest in crewed mission to Mars without landing?" Tags="|mars|flyby|cost|crewed-spaceflight|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15079" PostTypeId="2" ParentId="15078" CreationDate="2016-04-30T18:00:47.573" Score="2" Body="&lt;p&gt;Do you know the names of the first humans who landed on the Moon? Yes! Armstrong and Aldrin, of course. (and if you scratch your head a little, you remember Collins in orbit too).&#xA;Do you know the names of the first who orbited the the Moon? (Borman, Lovell and Anders). Less likely. My point is: If you later land on Mars, people forget about the first mission. And if you do not follow up with a landing, people feel like you have cheated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a technical point of view, such a mission is just a long stay on a single use space station, without the possibility of a return in case of an emergency. On the other side, the technology is pretty much off-the-shelf. The much lower $\Delta v$ requirements and lower complexity are undoubtely cutting a large part of the cost too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, a moderate risk, high cost voyage, with only a small reward.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-04-30T18:00:47.573" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15080" PostTypeId="1" CreationDate="2016-04-30T18:21:46.903" Score="4" ViewCount="591" Body="&lt;p&gt;I'm trying to calculate the perturbations of a satellite due to the solar radiation pressure, but I'm not sure of which reflection factor to use. Do you know which are the common values of this factor?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pd: I'm studying specifically the Scisat 1 satellite, but the reflection factor does not have to be the exact one, just an approximation.&lt;/p&gt;&#xA;" OwnerUserId="13854" LastActivityDate="2016-06-23T12:12:47.510" Title="Which is the reflection factor of a satellite?" Tags="|artificial-satellite|radiation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15081" PostTypeId="1" CreationDate="2016-04-30T21:28:08.103" Score="4" ViewCount="546" Body="&lt;p&gt;The timing is supposedly pretty interesting also, but that is not the question. I'm asking about the lunar lift-off. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in knowing if it was also a Hasselblad video camera, if that even exists, I'm not sure.&lt;/p&gt;&#xA;" OwnerUserId="14288" LastEditorUserId="38" LastEditDate="2016-04-30T22:29:15.257" LastActivityDate="2016-05-01T00:39:38.137" Title="What is the model name of video camera that filmed the lunar lift-off?" Tags="|the-moon|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15082" PostTypeId="2" ParentId="15081" CreationDate="2016-04-30T21:33:12.897" Score="10" Body="&lt;p&gt;The liftoffs were viewed using the TV cameras mounted on the rovers.  These cameras were &lt;a href=&quot;http://airandspace.si.edu/collections/artifact.cfm?object=nasm_A19850021000&quot; rel=&quot;noreferrer&quot;&gt;made by RCA&lt;/a&gt;.  The &quot;model name&quot; was &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_TV_camera&quot; rel=&quot;noreferrer&quot;&gt;RCA J-Series Ground-Commanded Television Assembly.&lt;/a&gt;  The operations and checkout manual for the camera can be read &lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/alsj/GCTA-Manual.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.  This is an image of the cover of the manual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gHNBt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gHNBt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-05-01T00:39:38.137" LastActivityDate="2016-05-01T00:39:38.137" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15083" PostTypeId="2" ParentId="15078" CreationDate="2016-05-01T02:51:44.753" Score="4" Body="&lt;p&gt;Yes, there is interest, at least from some quarters. The Planetary Society proposes an &lt;a href=&quot;http://hom.planetary.org/&quot; rel=&quot;noreferrer&quot;&gt;orbit-first strategy&lt;/a&gt; for humans at Mars as part of an &lt;em&gt;affordable&lt;/em&gt; plan to make progress and build capability. Yes, putting humans into orbit around Mars and returning from there would be much, much cheaper than landing them and getting them back off the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While there, the crew could teleoperate rovers on Mars far, far more capably than we can do from Earth due to the light-time difference. The rovers themselves would be much more advanced than what we have today, and would be able to explore wider ranging, diverse terrains across the planet, more than the humans on a single landed mission could. The crew can also land on the moons of Mars and explore more directly there, both for science and for resource prospecting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see here heartfelt concerns for the astronauts taking all that risk without sufficient reward. You can set your concerns aside. I am confident that you will find many extremely highly qualified candidates who are willing to go on a Mars orbit and Mars moon rendezvous mission.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-05-01T02:51:44.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15084" PostTypeId="2" ParentId="15078" CreationDate="2016-05-01T06:27:02.540" Score="1" Body="&lt;p&gt;I think if a mission was to be sent to Mars without the intention of landing, there could be a great deal of science and hardware development that could be done by landing on Phobos (escape velocity ~41km/hr).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to one &lt;a href=&quot;https://www.nasaspaceflight.com/2015/07/mission-phobos-precursor-human-mars-landing/&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt; proposal:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Precursor missions to the surface of Phobos prior to undertaking human Martian surface operations would allow for a continuation in the phased approach of development, implementation, and in-situ testing/experience of the necessary hardware for an eventual Mars surface mission.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This plan calls for development of surface habitat/landing vehicle and a rover vehicle which could later be used on future missions to the Martian surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the &lt;a href=&quot;http://www.space.com/20500-mars-moon-mission-student-challenge.html&quot; rel=&quot;nofollow&quot;&gt;Caltech Space Challenge&lt;/a&gt; in 2013 did some interesting development work on a Phobos mission. They devised a plan using 11 Spacex launches to assemble the mission hardware in LEO, with a total mission duration of ~450 days. However this plan does involve a &quot;nuclear propulsion unit&quot; which may need a bit of development work first.&lt;/p&gt;&#xA;" OwnerUserId="14276" LastActivityDate="2016-05-01T06:27:02.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15085" PostTypeId="2" ParentId="15076" CreationDate="2016-05-01T08:41:36.833" Score="4" Body="&lt;p&gt;Not much is known about Red Dragon yet. There will be a brief communications blackout. For Mars the &lt;a href=&quot;https://www.nasa.gov/mission_pages/tdm/medli/medli_success.html&quot; rel=&quot;nofollow&quot;&gt;blackout period&lt;/a&gt; is around 30 seconds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Detailed study of &lt;a href=&quot;http://ipnpr.jpl.nasa.gov/progress_report/42-197/197A.pdf&quot; rel=&quot;nofollow&quot;&gt;comms blackout for Mars&lt;/a&gt;. Blackout is related to electron density, which depends on the speed/altitude profile. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This article discusses the analysis of the UHF communications blackout (and brownout) experienced by the Mars Science Laboratory (MSL) during the period around peak heating of its entry, descent, and landing (EDL) phase into the Martian atmosphere on August 6, 2012. The UHF relay links from MSL to the Mars Reconnaissance Orbiter (MRO) and to the Mars Express (MEX) suffered a period of ~70 s of degradation, consisting of a combination of brownout (signal fades) and blackout (complete loss of signal) that coin- cided with the predicted period of signal degradation from preflight analyses.&lt;br&gt;&#xA;  Earlier work involving an analysis of entry vehicles into the Martian atmosphere included a postflight study of the 30-s signal outage suffered by the Mars Pathfinder spacecraft in 1997 at X-band on a direct-to-Earth (DTE) link, attributed mostly to charged particles &lt;a href=&quot;http://ipnpr.jpl.nasa.gov/progress_report/42-197/197A.pdf&quot; rel=&quot;nofollow&quot;&gt;2&lt;/a&gt;. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-05-01T11:09:33.280" LastActivityDate="2016-05-01T11:09:33.280" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15086" PostTypeId="2" ParentId="15075" CreationDate="2016-05-01T09:04:32.030" Score="12" Body="&lt;p&gt;I found this reference in a &lt;a href=&quot;http://interviewly.com/i/nasas-new-horizons-feb-2015-reddit&quot; rel=&quot;noreferrer&quot;&gt;2015 Reddit interview of the New Horizons team&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Has the team had any success looking for Pioneer Anomaly type effects in New Horizon's position and trajectory data? Thanks for doing this AMA!&lt;/p&gt;&#xA;&lt;p&gt;Michael Vincent: We did look at that. It was attributed to the thermal radiation from the RTG pushing on the spacecraft asymmetrically on the Pioneer spacecraft. I believe that was finally determined in 2008 or so.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Unfortunately that's an ambiguous answer: Vincent seems to be referring to NASA's work on resolving the cause of the Pioneer anomaly, and not about observing this same effect on New Horizons.&lt;/p&gt;&#xA;&lt;p&gt;From a &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.339.8972&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;noreferrer&quot;&gt;study published around 2006&lt;/a&gt;, there was a plan to study this:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The thermal output of the single RTG on New Horizons is nearly twice the combined thermal output of the four SNAP-19 RTGs that powered one Pioneer spacecraft. The New Horizons RTG generates more electricity, which ultimately is converted to heat inside the spacecraft and then preferentially radiated away in a direction opposite the HGA. For these reasons, the contribution of on-board heat to the unmodeled acceleration of New Horizons will be larger and will somewhat degrade the navigational accuracy for this mission. In fact, it was recently realized that a number of on-board sources of systematic effects, most notably the radiant heat emitted by the RTGs, will limit New Horizons’ acceleration sensitivity at the level of ∼ 4 × 10−9 m/s2, affecting some mission objectives.&lt;/p&gt;&#xA;&lt;p&gt;Fortunately, relying on our recent experience of working with the Pioneer spacecraft, we can demonstrate that, by using the on-board telemetry, New Horizons can still accomplish all its radio-science objectives. To achieve this goal, one needs to build a high accuracy thermal-electric-dynamical model for the New Horizons spacecraft. Because of the availability of the relevant data and for the purposes of proving the feasibility of the approach, we first embarked on the development of such a model for the Pioneer spacecraft, as discussed in [8]. However, once the appropriate data (radiometric and telemetry) is available from New Horizons, a similar model will be developed for this mission to allow a “real-time” calibration of navigational data with respect to the set of on-board systematics. As a quantitative objective, we hope to improve acceleration sensitivity on this spacecraft to the level below 2×10−10 m/s2. In return, this will lead to an improvement of the spacecraft’s navigation accuracy, increasing the mission’s science return. A detailed analysis of the impact of this effect on the trajectory of the New Horizons spacecraft has been initiated and will be reported elsewhere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;But from this 2012 presentation, it looks like NASA &lt;a href=&quot;https://www.vttoth.com/DOCUMENTS/201209_Carleton-nobak.pdf&quot; rel=&quot;noreferrer&quot;&gt;didn't look for the Pioneer Anomaly in the NH data:&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;New Horizons: no funding for Doppler tracking; opportunity to confirm “onset” lost&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-05-01T09:04:32.030" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15088" PostTypeId="2" ParentId="886" CreationDate="2016-05-01T18:36:30.893" Score="5" Body="&lt;p&gt;&quot;How feasible&quot; is a very large question.&#xA;Technically feasible? They had found no show-stoppers by the time the project was cancelled, but acknowledged that it could only be proven by taking the next step and testing hardware. Testing might have (or probably would have, depending on who you ask) reveal that the pusher plate would be destroyed, even with the oil spray between blasts. What makes Orion so heartbreaking is that it was cancelled at the peak of its promise, before we had a chance to see a pusher plate in an underground cavern get torn to shreds by Test Pulse Number Five. Or survive. We may never know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Environmentally feasible? Fission bombs can be made cleaner, but not actually clean. There would be fallout. Even if the launch scenario minimizes ground burst effects, and even if the bombs were engineered with boron to &quot;eat&quot; a lot of their own radiation, fallout would still be orders of magnitude above what society is prepared to accept. Any hint of &quot;fourth generation nukes&quot; that initiate fusion without a fission trigger is empty hype. Design a &quot;low-fallout&quot; Orion and launch it, and within 48 hours radiation monitors worldwide would know it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Geo-politically, the only entities who could build and test Orion are the ones most invested in nuclear non-proliferation. Even if it's launching a peaceful scientific payload, Orion carries nukes into space, thereby &quot;militarizing&quot; it. It requires treaty exceptions or violations, and those treaties work as well as they do because there &lt;em&gt;are&lt;/em&gt; no exceptions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as I see it (sorry, speculating here) the only way Orion could move forward would be in response to an extinction-level threat. Asteroid collision is the most commonly cited one, but global warming might be more realistic. Sea levels rise and we start losing cities, and the global perception of risk changes. Suddenly we want to move hundreds of tons of particulates into the upper atmosphere, or deploy a hundred massive solar sails at the earth-sun L1 point to reduce solar heating by 2% worldwide. Then, and only then, do treaty exceptions get made and the project gets authorization to build a pusher plate test article and begin underground testing with real nukes. Sadly, that's probably when some nice people at Las Alamos come forward and say &quot;Just for kicks we computer modeled this in 2008. The pusher plate definitely does not survive. Don't bother building it.&quot;&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-05-01T18:36:30.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15089" PostTypeId="1" AcceptedAnswerId="15090" CreationDate="2016-05-01T19:07:14.430" Score="8" ViewCount="4859" Body="&lt;p&gt;I'm really impressed with these two spaceplanes, so I'm thinking is there any chance to fly again, or some similar spaceplanes (similar design, size...)  to be built?&lt;/p&gt;&#xA;" OwnerUserId="13559" LastEditorUserId="415" LastEditDate="2016-05-04T13:18:20.627" LastActivityDate="2022-07-22T18:40:47.233" Title="Might either the Buran or the Space Shuttle fly again, or similar spacecraft be built in the future?" Tags="|space-shuttle|spaceplane|buran|" AnswerCount="8" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15090" PostTypeId="2" ParentId="15089" CreationDate="2016-05-01T19:36:55.890" Score="22" Body="&lt;p&gt;Buran and the Space Shuttle will not fly again.  Both projects have ended, and the orbiters have gone to museums (or have been destroyed). Reactivating these programs would be enormously expensive at this point.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle showed that a spaceplane has some disadvantages. It turned out to be really expensive to refurbish the spacecraft for each flight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle was designed for things NASA rarely or never used: the ability to launch, do something and then land one orbit later was never used. This ability drove the size of the wings, for example. The ability to retrieve satellites from orbit was used a few times, but not as often as envisioned.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a company working on a space plane right now (&lt;a href=&quot;http://www.sncspace.com/ProductLines/SpaceExplorationSystems&quot; rel=&quot;noreferrer&quot;&gt;Dream Chaser&lt;/a&gt;), but it's much smaller than the Shuttle. If &lt;a href=&quot;http://www.reactionengines.co.uk/space_skylon.html&quot; rel=&quot;noreferrer&quot;&gt;Skylon&lt;/a&gt; ever becomes reality, we could see a large space plane again. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="1142" LastEditDate="2016-05-01T23:44:11.337" LastActivityDate="2016-05-01T23:44:11.337" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15091" PostTypeId="1" CreationDate="2016-05-01T22:25:34.620" Score="4" ViewCount="1419" Body="&lt;p&gt;The &lt;a href=&quot;http://sci.esa.int/Conferences/ILC2005/Manuscripts/JablonskA-01-DOC.pdf&quot; rel=&quot;nofollow&quot;&gt;current estimate&lt;/a&gt; for how much shielding is needed for a long-term habitat on the Moon is a minimum of 700 g/cm&lt;sup&gt;2&lt;/sup&gt; of regolith, and 1000 g/cm&lt;sup&gt;2&lt;/sup&gt; for the levels at sea level on Earth. (&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0032063312002085&quot; rel=&quot;nofollow&quot;&gt;The radiation dose on the lunar surface&lt;/a&gt; is not really known, so this is an estimate based on highly complex modelling.) That same estimate is mentioned in this video on civil engineering on the Moon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/DI41N-eC7VA?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been designing such habitats for a project to create a &lt;a href=&quot;http://moonwards.com/index.html#colony&quot; rel=&quot;nofollow&quot;&gt;virtual lunar colony online&lt;/a&gt;. One area &lt;a href=&quot;http://moonwards.com/forum/index.php?id=63&quot; rel=&quot;nofollow&quot;&gt;uses a configuration&lt;/a&gt; designed to allow in lots of natural light from above while still keeping the overall radiation low enough that a person can spend lots of time there. In that process I've been using the assumption that what they mean is that at least 700 g/cm&lt;sup&gt;2&lt;/sup&gt; in all directions yields this result. Today it occurred to me that it could instead mean that case over a hemisphere. At any rate, the only scenario where exactly the same amount of material stands between you and the outside environment is when you are standing dead center in an empty sphere with walls of uniform thickness, all of the same material (and you are also a dimensionless point, if one is especially a stickler about it). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Up to now I have been subtracting directions in which there is material of greater than 1000 g/cm&lt;sup&gt;2&lt;/sup&gt; from the total radiation dose, and I have been adding on higher radiation doses for the directions where there is less, as a proportion of all directions. It seems to be the best way to estimate dose within a given design. I also consider the proportion of time that a person might spend in an area with a given dose. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If these estimates aren't based on that simplification of a sphere, those results are off. If it is a model of a half-sphere, then I need to double the numbers. Or maybe it is modeled on something much more complex, like a point under a plane of that much material. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know what assumption is used?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-06-02T01:27:53.800" LastActivityDate="2016-06-02T01:27:53.800" Title="When estimating radiation doses inside a lunar habitat, what assumptions are made as to shape?" Tags="|the-moon|colonization|radiation|habitat|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="15092" PostTypeId="2" ParentId="15025" CreationDate="2016-05-01T22:30:10.620" Score="0" Body="&lt;p&gt;SpaceX/Elon Musk will be 'debugging' the process of delivering payloads and/or humans to the surface of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my &lt;strong&gt;guess&lt;/strong&gt; is that they would have a payload that would be equivalent to 2-7 astronauts + minimal life support (enough for descent + getting to their next objective/base) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I say equivalent I mean in mass and weight balance. Some of that would be sensors to measure the vibrations etc that a 'real' astronaut (marsonaut?) would experience. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That would be a 'primary' mission. Something like: &lt;em&gt;'Can we ride marsonauts down to mars on a red dragon/discover what we need to do, to be able to?'&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A secondary mission might be to deliver a small (i.e., cheap incase dragon fails) science payload from NASA or the European Space Agency etc. and/or several 'cubesat' like payloads from universities etc (presumably piggybacking on the dragons communications, to relay data back to earth).&lt;/p&gt;&#xA;" OwnerUserId="3460" LastEditorUserId="3460" LastEditDate="2016-05-02T05:20:19.630" LastActivityDate="2016-05-02T05:20:19.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15093" PostTypeId="2" ParentId="15089" CreationDate="2016-05-01T23:46:30.387" Score="16" Body="&lt;p&gt;The US Shuttle program is shut down and &lt;a href=&quot;https://blogs.nasa.gov/waynehalesblog/2008/08/28/post_1219932905350/&quot; rel=&quot;noreferrer&quot;&gt;can not be restarted&lt;/a&gt; without a-never-gonna-happen expediture of funds.  The Orbiters are in museums, the tooling to build External Tanks is gone or repurposed, the work force is dispersed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soviet-era Buran program is, if possible, even deader.  This program only flew one mission, for two orbits, with an incomplete orbiter, uncrewed. This vehicle was &lt;a href=&quot;https://en.wikipedia.org/wiki/Buran_%28spacecraft%29#Destruction&quot; rel=&quot;noreferrer&quot;&gt;destroyed&lt;/a&gt; when its hangar collapsed in 2002.  Other uncompleted and unflown orbiters may exist in museums; the last time I tried to get the full story on this, it was complicated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only other operational resuable spaceplane is the hush-hush &lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_X-37&quot; rel=&quot;noreferrer&quot;&gt;Boeing X-37&lt;/a&gt;.  It has flown several uncrewed missions and in fact is in space as I type this.  There are no plans to fly people in it as far as I know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sierra Nevada has &lt;a href=&quot;http://www.sncorp.com/AboutUs/NewsDetails/2754&quot; rel=&quot;noreferrer&quot;&gt;some funding&lt;/a&gt; to fly their Dream Chaser uncrewed.  It will be interesting to see if this happens.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-05-01T23:46:30.387" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15094" PostTypeId="1" AcceptedAnswerId="15096" CreationDate="2016-05-02T04:07:47.507" Score="25" ViewCount="5056" Body="&lt;p&gt;Hobbes &lt;a href=&quot;https://space.stackexchange.com/a/15090/103&quot;&gt;wrote&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Shuttle was designed for things NASA rarely or never used: [...] The ability to retrieve satellites from orbit was used a few times, but not as often as envisioned. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wasn't aware that ability was &lt;em&gt;ever&lt;/em&gt; used. I know the Shuttle was used in a few repair missions, and possibly it could deorbit (burn) some satellites, but I never heard about it picking a satellite from the orbit and landing it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What satellites were retrieved and why?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-10-23T22:55:55.463" Title="What satellites did the Shuttle retrieve from orbit?" Tags="|artificial-satellite|space-shuttle|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15095" PostTypeId="2" ParentId="13918" CreationDate="2016-05-02T04:48:31.440" Score="3" Body="&lt;p&gt;My take: You could use an Arduino board, but you would have to remanufacture it to be non-RoHS compliant, i.e. using solder made of lead-tin alloy, using conformal coatings and potting it. As it stands, space electronics have an exemption from RoHS compliance anyway.&lt;/p&gt;&#xA;&lt;p&gt;The Arduino boards you buy off the shelf are RoHS compliant and use lead-free solders, whose only main structural metal is tin, alloyed with small amount of silver. One problem with that in a space environment is the high susceptibility to growth of tin whiskers from the solder joints, which can cause short circuits by bridging to adjacent tracks. Tin whiskers have been asked about as a question here before. Lead content slows down the growth of the whiskers and inhibits the tin pest.&lt;/p&gt;&#xA;&lt;p&gt;Unfortunately, the growth and causes of these whiskers is not well understood, and the mission duration isn't a factor - they can occur at any time.&lt;/p&gt;&#xA;&lt;p&gt;So your COTS Arduino could go on the fritz at any time.&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="47149" LastEditDate="2022-05-13T21:32:47.450" LastActivityDate="2022-05-13T21:32:47.450" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="15096" PostTypeId="2" ParentId="15094" CreationDate="2016-05-02T07:35:54.027" Score="24" Body="&lt;p&gt;From &lt;a href=&quot;http://rammb.cira.colostate.edu/dev/hillger/Shuttle-related.htm&quot; rel=&quot;noreferrer&quot;&gt;this list:&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;STS-41-C (launch) / STS-32R (retrieve): LDEF&lt;br&gt;&#xA;STS-41-B (launch) / STS-51-A (retrieve): Palapa B-2 and Westar 6&lt;br&gt;&#xA;STS-46 (launch) / STS-57 (retrieve): EURECA&lt;br&gt;&#xA;H-II Test Vehicle 3 (launch) / STS-72 (retrieve): Space Flyer Unit  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's fewer than I expected. There were some on-orbit repairs where the satellite was redeployed after the repair. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Let me supply the &quot;why&quot; part from what I found --SF.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Long_Duration_Exposure_Facility&quot; rel=&quot;noreferrer&quot;&gt;LDEF&lt;/a&gt; from NASA was more of a container than a satellite in the classic sense; its purpose was to expose a multitude of material samples to space for a long time; recovery was necessary to perform analysis of the samples and how they fared in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/palapa-b.htm&quot; rel=&quot;noreferrer&quot;&gt;Palapa B-2&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Westar&quot; rel=&quot;noreferrer&quot;&gt;Westar 6&lt;/a&gt; were commercial communication satellites, that ended up in wrong, useless orbits through their boosters misfiring. They were recovered for refurbishing, resold to other parties and put in orbit again, where they serve just fine. (in case of Palapa insurance reasons might have played a role, but I'm not exactly clear as to, &lt;em&gt;how&lt;/em&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/European_Retrievable_Carrier&quot; rel=&quot;noreferrer&quot;&gt;EURECA&lt;/a&gt; was an European unmanned laboratory, many of its experiments involving material engineering (also, biology). It was recovered to collect the results (products) of the experiments and to be refurbished and resupplied with a new batch of experiments for relaunch (which never happened).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Flyer_Unit&quot; rel=&quot;noreferrer&quot;&gt;Space Flyer Unit&lt;/a&gt; was another laboratory, this time launched by Japan, but containing experiments valuable to NASA and American organizations and corporations. It contained some material engineering related experiments, and one biology-related (hatching an egg). Retrieval of products of the experiments was required.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6944" LastEditDate="2016-10-23T22:55:55.463" LastActivityDate="2016-10-23T22:55:55.463" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15097" PostTypeId="2" ParentId="15080" CreationDate="2016-05-02T09:45:19.987" Score="1" Body="&lt;p&gt;Handwaving a bit, I expect this to be dominated by the &lt;a href=&quot;http://www.appropedia.org/The_effect_of_spectral_albedo_on_amorphous_silicon_and_crystalline_silicon_solar_photovoltaic_device_performance&quot; rel=&quot;nofollow noreferrer&quot;&gt;albedo of the solar panels&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WC25I.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WC25I.png&quot; alt=&quot;Solar panel albedo peaks at 0.2-0.5 depending on the type of panel, the peak occurs at different wavelengths depending on the type of panel&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So albedo depends on the composition of the panel, and varies from 0.2-0.5. The contribution of the solar panels depends on their size relative to the rest of the satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For SCISAT 1, this is easy: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uMowS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uMowS.jpg&quot; alt=&quot;SICSAT is covered in solar panels&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pretty much the entire sun-facing surface is solar panel. &#xA;So all you need to know is the composition of the solar panels.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-05-24T11:40:27.917" LastActivityDate="2016-05-24T11:40:27.917" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15098" PostTypeId="2" ParentId="15091" CreationDate="2016-05-02T15:33:45.167" Score="1" Body="&lt;p&gt;From the PDF you linked: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Large-scale radiation shielding will be required, beginning in phase two, to protect humans spending time in lunar structures. The most commonly suggested type of radiation shielding is a regolith cover because it includes the advantages of in-situ resource utilization, as well as provides meteoroid and thermal shielding.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That sounds to me like they're assuming you're building on the Moon's surface, with a layer of regolith around and on top of your structure. A half-sphere would satisfy that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the nature of light on the Moon (no atmosphere, so very little indirect light), you'll have to treat windows differently than you would on Earth. Direct sunlight will be much too bright (and full of UV) so would have to be shielded, and if your structure has no sun-facing window it'll be dark inside. To have usable illumination from the Sun you'll need light pipes and filters. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-05-02T15:33:45.167" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15099" PostTypeId="1" AcceptedAnswerId="15100" CreationDate="2016-05-02T13:46:03.687" Score="24" ViewCount="7295" Body="&lt;p&gt;Recently, there has been growing interest in &lt;a href=&quot;https://www.raspberrypi.org/blog/compute-module-cubesats/&quot;&gt;using Raspberry Pis in CubeSats&lt;/a&gt;. &lt;a href=&quot;http://spaceref.com/nasa-hack-space/nasas-raspberry-pi-based-pi-sat-cubesat.html&quot;&gt;NASA is even in on the trend&lt;/a&gt;. But, considering that outer space is pretty harsh on electronics (i.e. radiation and cosmic rays, wild temperature swings, space-dust and micro-meteors), &lt;em&gt;is the Pi really viable as a space satellite?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on what I remember, space-worthy electronics usually feature a way to recover from a disaster, ECC memory, redundant systems, redundant communications, some form of radiation hardening, and could handle temperature swings. Basically, as reliable as they can make it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I know, the Pi has &lt;em&gt;none&lt;/em&gt; of those. Yes, the Pi can probably handle the temperature part, but one wrong bit-flip can crash whatever is running on it and render it unusable by the lack of recovery. They can probably use the &lt;code&gt;watchdog&lt;/code&gt; service in that case. But is it enough?&lt;/p&gt;&#xA;" OwnerUserId="11846" OwnerDisplayName="PandaLion98" LastEditorUserId="25" LastEditDate="2016-05-02T15:51:12.537" LastActivityDate="2016-05-03T14:12:46.823" Title="Is the Pi really fit for outer space?" Tags="|artificial-satellite|design|cubesat|flight-computer|reliability|" AnswerCount="4" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="15100" PostTypeId="2" ParentId="15099" CreationDate="2016-05-02T15:53:04.663" Score="19" Body="&lt;p&gt;Most cubesats are student projects that they want to have for only a matter of a few months at most. That being said, the Pi would likely be very susceptible to Single Event Upsets, SEUs, and thus might have some serious issues. It is low cost, and would fit inside of the spacecraft, with a bit of shielding it might be okay, but only for a few months for a low importance build. Bottom line, it might work for a student project, but I wouldn't have it be the flight computer of anything important!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is worth noting that a cell phone has been used as a flight computer for a cubesat. This sat was known as PhoneSat. These are comparable to the Pi in terms of SEU and radiation danger. They did in fact work for some period of time. I haven't found a great source, but the lifetime was about a week for every one of the 5 PhoneSat launched. The first 2 only had batteries, the last 3 had solar power as well. They were deliberately placed in orbits that would keep them up only for a few weeks, but I can't find any reported issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Small note, it might be better to use an Arduino than a Pi for spaceflight, as the Arduino is somewhat better for integrated electronics. Pi works great if you have access to a video port, but space is a long ways away, the odds of getting an HDMI cable all that way are pretty slim;-) But there might be other advantages to the Pi. At least keep that in mind when making the decision.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-05-03T14:12:46.823" LastActivityDate="2016-05-03T14:12:46.823" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="15101" PostTypeId="2" ParentId="15089" CreationDate="2016-05-02T16:27:26.797" Score="4" Body="&lt;p&gt;As others have noted, the concept of a winged spaceplane at the scale of the Shuttle is possible (Consider Skylon, Dream Chaser, X-37B (much smaller). But the specific examples of the Shuttle and Buran have proven that there were structural defects in their design process that made them untenable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is nothing fundamentally wrong with wings and reuse, but it may not always be the right answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle was overdesigned as mentioned in another answer. The Russian infrastructure is gone, more so than the US side, and neither will ever be resurrected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it looks like the future is smaller spaceplanes, if any at all.  Dream Chaser and X-37B are pretty small in comparison to the Shuttle.  Skylon is closer in size to the shuttle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But then again, it is clear SpaceX plans a very large vehicle for Mars missions, tentatively known as the MCT (Mars Colonial Transport) with the assumption it will be winged for reentry on Earth and then reuse. This is described as the upper stage of the BFR (Big F-ing Rocket) and Mars transport and Mars lander, and Earth reentry vehicle. For Mars they will need a large aero-surface to brake so it is likely they will try wings, for Earth reentry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The future is unclear, but the past is not coming back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is worth considered examples of previous production lines brought back into service to consider costs and utility. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The B-1 was started, a number built, cancelled by Carter, brought back as the B-1B by Reagan. It would be interesting to know costs involved there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some discussion currently about reopening the recently closed F-22 line, but the costs are so high as to be daunting. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-02T16:27:26.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15102" PostTypeId="2" ParentId="15099" CreationDate="2016-05-02T17:01:15.890" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Based on what I remember, space-worthy electronics usually feature a way to recover from a disaster, ECC memory, redundant systems, redundant communications, some form of radiation hardening, and could handle temperature swings. Basically, as reliable as they can make it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Using radiation hardened computer processors is the traditional approach. This results in an avionics system that is rather massive and bulky, very expensive, and very, very slow (by modern standards). Their use is not mandated, even for human-rated systems. SpaceX, for example, does not use radiation-hardened processors in their Dragon vehicle. What is mandated, for human-rated systems, is being able to show that the avionics system is radiation tolerant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CubeSats typically are low cost school projects that aren't intended to last for long. A \$250000 radiation hardened processor is about twice the budget for a typical CubeSat project budget, including launch. Interconnecting a couple of \$35 Pis for redundancy would be well within budget and would be a great student project.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-05-02T17:01:15.890" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15103" PostTypeId="2" ParentId="15099" CreationDate="2016-05-02T19:48:14.950" Score="2" Body="&lt;p&gt;Silicon valley has been moving into the space industry and it's bringing a new philosophy to the field. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The emphasis is being put on quick development time, not reliability. The cost of replacing a relatively cheap cube sat is seen as well worth it the ability to quickly prototype, test and move onto the next iteration of the product.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mentality is very much &quot;Build it, fly it, test it, repeat&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most companies that work in this space expect their satellites to last only a couple of years and plan to make up for it in numbers.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2016-05-02T19:48:14.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15106" PostTypeId="2" ParentId="15099" CreationDate="2016-05-03T00:57:04.673" Score="3" Body="&lt;p&gt;In low orbit the Earth's magnetic field is an effective shield against solar radiation. It sweeps up the radiation and traps it in areas known as the van Allen belts. We avoid putting spacecraft or people there... One part of the belt does dip into low orbit altitudes and it sometimes causes problems. That's most of the radiation flux you have to worry about and you can shield against it as an extra precaution. Don't picture lead. Plastic is used. Lead doesn't work for this type of radiation (or rather, it actually make it worse).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cosmic rays are more troublesome. They are a much smaller portion of the radiation in space. This includes all the radiation that does not come from our Sun. The Earth's magnetic field offers no protection against most of it. The 50 miles of our atmosphere shields us from about 90% of it. In spacecraft there is no practical shielding against it. Electronic devices are designed to accept or correct for the erratic behavior cosmic rays can cause. But even a device like the Raspberry Pi, built without much fault tolerance can run for a few days or weeks before a critical bit gets flipped that causes the CPU to lock up. I presume that the mission controller have the ability to remotely reset the device. Another simple solution would be to have a separate watchdog circuit simply reset the board once an hour unless it gets a signal to prevent reset.&lt;/p&gt;&#xA;" OwnerUserId="10365" LastActivityDate="2016-05-03T00:57:04.673" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15107" PostTypeId="1" AcceptedAnswerId="15111" CreationDate="2016-05-03T03:09:12.577" Score="4" ViewCount="1176" Body="&lt;p&gt;I'm looking for a simple machine-readable table with basic parameters for all the known moons of the planets in the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stumbling around the internet, I can find plenty of &lt;a href=&quot;http://www.windows2universe.org/our_solar_system/moons_table.html&quot;&gt;sources like this one&lt;/a&gt; that have a listing with orbital parameters, but they leave out mass. If you're trying to automate some Delta V calculations, then you're out of luck. Even Wikipedia does this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would be happy to grab these from any larger (more arbitrary) database, but the JPL tools I've found don't seem to include natural satellites of planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even though this question sounds absurdly simple, it's actually quite non-trivial and I would hate to write a script to crawl individual links for the data. Any suggestions to hasten this process would be appreciated, thanks.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="6816" LastEditDate="2016-07-23T08:18:03.363" LastActivityDate="2016-07-23T08:18:03.363" Title="Data for Moons in the Solar System with Masses" Tags="|moons|mass|raw-data|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15108" PostTypeId="2" ParentId="15107" CreationDate="2016-05-03T04:24:47.423" Score="2" Body="&lt;p&gt;There is one source on Wikipedia, &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_gravitationally_rounded_objects_of_the_Solar_System&quot; rel=&quot;nofollow&quot;&gt;List of gravitationally rounded objects of the Solar System&lt;/a&gt; which lists 6 solar system dwarf planets and 10 trans Neptunian dwarf planets, together with 19 moons. All include their mass. Small moons, too small to be spherical, are not listed here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However there is another Wikipedia page (&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size&quot; rel=&quot;nofollow&quot;&gt;List of Solar System objects by size&lt;/a&gt;) which gives masses (where known) of remaining objects down to 1km size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So between the two lists everything seems to be covered. I'm not sure what is needed to be &quot;machine readable&quot; though.&lt;/p&gt;&#xA;" OwnerUserId="14276" LastEditorUserId="14276" LastEditDate="2016-05-03T11:57:51.473" LastActivityDate="2016-05-03T11:57:51.473" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15109" PostTypeId="1" AcceptedAnswerId="15110" CreationDate="2016-05-03T04:33:23.767" Score="6" ViewCount="1655" Body="&lt;p&gt;I was reading this article about NASA's &lt;a href=&quot;http://www.nasa.gov/offices/oct/crosscutting_capability/edison/phonesat_prt.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;PhoneSat Flight Demonstrations&lt;/a&gt; and came across this sentence.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;To achieve this, NASA's PhoneSat design makes extensive use of commercial-off-the-shelf components, including an unmodified, consumer-grade smartphone. Out of the box smartphones already offer a wealth of capabilities needed for satellite systems, including fast processors, versatile operating systems, multiple miniature sensors, high-resolution cameras, GPS receivers, and several radios.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My Question:&lt;/strong&gt; Is this just enthusiastic writing, or can &quot;Out of the box smartphones ... offer ... capabilities needed for satellite systems, including ... GPS receivers...&quot;?  Is it known if they (or anyone) has tried it? Possibly with firmware modification? (see below)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Background:&lt;/strong&gt; (phones in sats)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/PhoneSat&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's PhoneSats&lt;/a&gt; use ordinary smart phones as the main control system for the nanosatellite. &lt;a href=&quot;https://en.wikipedia.org/wiki/STRaND-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;University of Surrey's STRaND-1&lt;/a&gt; uses a smart phone to control some functions, but with a more standard computer to share in the satellite's operation. It was the first 'smart phone satellite' in space. &lt;a href=&quot;https://amsat-uk.org/satellites/telemetry/strand-1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;From here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;&lt;em&gt;During phase two, the STRaND-1 team plan to switch the satellite’s in-orbit operations to the smartphone, thereby testing the capabilities of a number of standard smartphone components for a space environment. The satellite is operated from the Surrey Space Centre’s ground station at the University of Surrey&lt;/em&gt;.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Based on all of the information and comments in this &lt;a href=&quot;https://space.stackexchange.com/q/14687/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/14695/12102&quot;&gt;answer&lt;/a&gt; pair, I'm guessing that the GPS chips inside off-the-shelf smart phones would never provide data when in orbit, either because they are moving too fast, or the altitude is too high, or both. (I think that some manufacturers apply the 'OR', others the 'AND' of the two conditions) The block is usually applied in Firmware, so could conceivably be circumvented by clever &lt;s&gt;hacking&lt;/s&gt; programming of the phone. &lt;a href=&quot;https://space.stackexchange.com/questions/14687/current-situation-with-cocom-regulations-and-gps-receivers-for-balloons-and-cube#comment38674_14687&quot;&gt;This comment&lt;/a&gt; mentions this &lt;a href=&quot;https://www.youtube.com/watch?v=6A99O-mCZSM&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt; from Adafruit's founder Limor  &quot;Ladyada&quot; Fried (&lt;a href=&quot;https://www.adafruit.com/products/746&quot; rel=&quot;nofollow noreferrer&quot;&gt;read more&lt;/a&gt;) where the firmware has been modified to extend the altitude to at least 50km as an example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article says further on:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;NASA's &lt;strong&gt;PhoneSat 1.0&lt;/strong&gt; satellite has a basic mission goal–to stay alive in space for a short period of time, sending back digital imagery of Earth and space via its camera, while also sending back information about the satellite's health.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and then:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;&lt;strong&gt;PhoneSat 2.0&lt;/strong&gt; also will supplement the capabilities of PhoneSat 1.0 by adding a two-way S-band radio to allow engineers to command the satellite from Earth, solar panels to enable longer-duration missions, and a GPS receiver. In addition, PhoneSat 2.0 will add magnetorquer coils – electro-magnets that interact with Earth's magnetic field – and reaction wheels to actively control the satellite's orientation in space. &quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Which suggests that the &quot;GPS and several radios&quot; of the smartphones may not have been used. I'm curious if it's ever been attempted and then written about.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-05-03T11:42:55.127" Title="Did the NASA PhoneSat actually try to use the GPS from the phone itself?" Tags="|cubesat|gps|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15110" PostTypeId="2" ParentId="15109" CreationDate="2016-05-03T05:44:28.770" Score="4" Body="&lt;p&gt;No, they didn't. (personal knowledge)&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2016-05-03T05:44:28.770" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15111" PostTypeId="2" ParentId="15107" CreationDate="2016-05-03T09:56:57.990" Score="4" Body="&lt;p&gt;JPL have a &lt;a href=&quot;http://ssd.jpl.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;Solar System Dynamics&lt;/a&gt; site that looks like it has useful information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://ssd.jpl.nasa.gov/?sat_phys_par&quot; rel=&quot;nofollow&quot;&gt;Satellite physical parameters&lt;/a&gt; page has useful information, though note there isn't a raw mass. Instead it gives a G.M product which one assumes you're meant to use directly for calculations. (It also gives references for most of the figures.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a &lt;a href=&quot;http://ssd.jpl.nasa.gov/sbdb_query.cgi&quot; rel=&quot;nofollow&quot;&gt;Search engine&lt;/a&gt; page that looks like it will return information for smaller bodies (asteroids, comets etc. not satellites.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-05-03T09:56:57.990" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15112" PostTypeId="1" AcceptedAnswerId="15115" CreationDate="2016-05-03T11:24:08.153" Score="6" ViewCount="1086" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; currently gives the standard gravitational parameter of Mars as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ GM_{Mars} = 4.2828(9) *10^{13}  (m^3/s^2)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;(9)&lt;/em&gt; is the uncertainty of the last digit, so it works out to about 210ppm (parts per million) which is astronomically large (pardon the pun). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;note: the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Standard Gravitational Parameter&lt;/a&gt; of a body is the product of the gravitational constant G and the mass M of the body. Orbital data of satellites (natural and artificial) depends on the product of the two, so it can both be measured to higher precision (accuracy?) and it's actually just the thing you need to calculate the next mission's trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With all of the orbiting satellites and landers, I'm guessing there are updated values with more digits. &lt;a href=&quot;https://space.stackexchange.com/a/15111/12102&quot;&gt;This answer&lt;/a&gt; is helpful, but while the linked page for solar system satellites &lt;a href=&quot;http://ssd.jpl.nasa.gov/?sat_phys_par&quot; rel=&quot;nofollow noreferrer&quot;&gt;ssd.jpl.nasa.gov/?sat_phys_par&lt;/a&gt; lists GM, a similar page for planets &lt;a href=&quot;http://ssd.jpl.nasa.gov/?planet_phys_par&quot; rel=&quot;nofollow noreferrer&quot;&gt;ssd.jpl.nasa.gov/?planet_phys_par&lt;/a&gt; lists M in kg:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ M_{Mars} = 0.641693 ± 0.000064 *10^{24} (kg)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That value still has an uncertainty of 100ppm, and if one tries to recover GM by multiplying by &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravitational constant&lt;/a&gt; G, the uncertainty there is still also high - about 47ppm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question:&lt;/strong&gt; What is a good source for the most up-to-date value of Mars' standard gravitational parameter which also gives the experimental uncertainty of the value?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bonus:&lt;/strong&gt; The same &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; gives a value for pluto of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ GM_{Pluto} = 8.71(9) *10^{11} (m^3/s^2)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which has an uncertainty of 1%! (10,000ppm). Has the recent New Horizons fly-by trajectory and extended imaging been enough to improve on this value?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-05-03T15:35:28.850" Title="Source for up-to-date values of Mars' standard gravitational parameter" Tags="|mars|orbital-mechanics|planetary-science|gravity|celestial-mechanics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15113" PostTypeId="2" ParentId="15109" CreationDate="2016-05-03T11:42:55.127" Score="7" Body="&lt;p&gt;They could not have used it without some serious modifications. Most GPS systems fall under what is known as the &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/CoCom#Legacy&quot; rel=&quot;noreferrer&quot;&gt;CoCom&lt;/a&gt;&quot;, which prohibits GPS from working over 60,000 feet or 1200 miles per hour. Thus it could not have functioned without some significant improvements in the firmware. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-03T11:42:55.127" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="15114" PostTypeId="2" ParentId="15112" CreationDate="2016-05-03T12:22:36.203" Score="2" Body="&lt;p&gt;Mars has had a number of studies to determine the gravitational constant. The best paper that I was able to find dates to &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1029/2000JE001426/pdf&quot; rel=&quot;nofollow&quot;&gt;2001&lt;/a&gt;, using MGS data, and estimates Mars's GM to be 42828.371901 uncertainty .000074 km&lt;sup&gt;3&lt;/sup&gt;/s&lt;sup&gt;2&lt;/sup&gt;, which was calculated by two independent papers at the same time. I show that to be about 6 ppb, much closer to the Standard Gravitational Parameter's accuracy from Earth. This is about what the accuracy of the fraction of Mars/Sun's mass is shown from &lt;a href=&quot;http://ssd.jpl.nasa.gov/?constants&quot; rel=&quot;nofollow&quot;&gt;this page from NASA&lt;/a&gt;, which shows an accuracy of about 30 ppb. So I would use this number, with an assumed accuracy on the order of 10 ppb. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found an even better source, using &lt;a href=&quot;http://geodesy.curtin.edu.au/research/models/mgm2011/&quot; rel=&quot;nofollow&quot;&gt;2011 data&lt;/a&gt;, that lists the GM as GM =4.2828372 x10&lt;sup&gt;13&lt;/sup&gt; m&lt;sup&gt;3&lt;/sup&gt; s&lt;sup&gt;-2&lt;/sup&gt;. Assuming the last digit is the uncertainty, the accuracy is about 20 ppb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for Pluto, I suspect a small improvement will be made from New Horizons but it really takes orbital data to get accurate GM calculations. The flybys of Mars gave an accuracy on the order of 10 ppm, which I suspect will not be achievable with Pluto due to the already extreme distance and very high speed flyby, but a small improvement is likely. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="897" LastEditDate="2016-05-03T13:49:43.357" LastActivityDate="2016-05-03T13:49:43.357" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15115" PostTypeId="2" ParentId="15112" CreationDate="2016-05-03T15:09:38.730" Score="8" Body="&lt;p&gt;The most up-to-date is from the work in &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103516001287&quot;&gt;this paper&lt;/a&gt;, due to be published in July 2016. The actual numbers can be found in this &lt;a href=&quot;http://pds-geosciences.wustl.edu/mro/mro-m-rss-5-sdp-v1/mrors_1xxx/data/shadr/gmm3_120_sha.tab&quot;&gt;large file&lt;/a&gt;, for which the number you seek is the second number in the file, and its uncertainty is the third number in the file. (The rest of the numbers are the higher order terms of the gravity field. You can find a detailed description &lt;a href=&quot;http://pds-geosciences.wustl.edu/mro/mro-m-rss-5-sdp-v1/mrors_1xxx/data/shadr/gmm3_120_sha.lbl&quot;&gt;here&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are those numbers, copied and pasted from the file, in units of $\mathrm{{km}^3/s^2}$:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;4.2828372854187757e+04, 1.6202815226760665e-05&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I would write that a little more accessibly as $42828.372854(16)\,\mathrm{{km}^3/s^2}$, where the two digits in parentheses is the $1\sigma$ uncertainty in the last two digits of the value. Note that if you need that many digits in GM, then you also need a higher order gravity field to get the real answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GM of Pluto is best determined by observations of it and its moons over long periods of time. The value I found in &lt;a href=&quot;http://www.boulder.swri.edu/~buie/biblio/pub097.pdf&quot;&gt;this recent paper&lt;/a&gt; is $869.6\pm 1.8\,\mathrm{{km}^3/s^2}$ (where the error is about $0.2\%$). New Horizons may have improved that some, by virtue of better locating Pluto and its moons. But I suspect not by much.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-03T15:35:28.850" LastActivityDate="2016-05-03T15:35:28.850" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15116" PostTypeId="1" AcceptedAnswerId="15117" CreationDate="2016-05-03T16:27:54.507" Score="5" ViewCount="679" Body="&lt;p&gt;The SLS development budget is &lt;a href=&quot;http://www.spacepolicyonline.com/news/new-nasa-crew-transportation-system-to-cost-18-billion-through-2017&quot; rel=&quot;nofollow&quot;&gt;\$18 billion&lt;/a&gt; (currently \$10 billion are spent for SLS rocket), although &lt;a href=&quot;http://www.nbcnews.com/id/44517102/ns/technology_and_science-space/t/nasa-unveils-giant-rocket-design-future-odysseys/#.VrBowCaLe1E&quot; rel=&quot;nofollow&quot;&gt;another site&lt;/a&gt; shows even \$35B.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I am not mistaken SLS is a new &amp;amp; improved version of Saturn V, so most of the research should exist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest thing to SLS is Falcon Heavy. I know SLS is more powerful and can lift heavier payloads, but they are relatively similar.&#xA;Since \$18B (or \$35B) is an amount that SpaceX can't afford, SpaceX must have spent less for Falcon Heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what makes SLS too expensive? &lt;/p&gt;&#xA;" OwnerUserId="688" LastEditorUserId="6816" LastEditDate="2016-05-04T04:50:18.697" LastActivityDate="2016-05-04T04:50:18.697" Title="What makes SLS (Space Launch System) \$10 Billion (or \$36 Billion) project?" Tags="|cost|sls|falcon-heavy|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15117" PostTypeId="2" ParentId="15116" CreationDate="2016-05-03T16:40:29.257" Score="12" Body="&lt;p&gt;Pork. It is a jobs program not a technology program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRB's are built by ATK, but the design ensured they needed to be reworked, so they could not just the existing Shuttle ones.  Need an extra segment, which in SRB land means redoing the inner contour to control the burn rate to control thrust profile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The switch between SSME to RS-68 to SSME (Yes, some of that was Ares-V, and some was SLS but basically same ideas) wasted a lot of time and money, but also meant that the SSME's could not simply be used. They needed a new digital controller upgrade. Which is probably good, but costs more in development costs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main barrel section is designed based on the ET from the Shuttle, but of course, needed new tools to build it since it is apparently just enough different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper stage engine has been a total goat rodeo of indecision. J-2X, RL-10's, SSME, can someone please decide and stop switching.  Each time money is spent, time is wasted, and little comes out of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality the true cost is in salary. So many people are employed in working on this, in by design inefficient fashion, so as to keep as many jobs as possible every time delay costs millions if not billions in salaries. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason other companies can launch for cheaper is that they employ fewer people. Raw material costs for launcher and fuel costs are a pittance. They are not manufactured out of solid gold. (By weight, simply building an SLS out of literal solid gold, might be cheaper than the current project. Anyone want to do the math?).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-03T16:40:29.257" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="15118" PostTypeId="1" CreationDate="2016-05-03T18:56:50.637" Score="7" ViewCount="1422" Body="&lt;p&gt;The recent discussions about the SpaceX SuperDraco thrusters got me to thinking:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you restart a turbopump rocket engine in zero-g, you have to perform a &quot;settling&quot; or &quot;ullage&quot; burn to get the propellant that's floating around in the tanks to settle down near the pump inlets, or the pumps ingest the tank pressurization gas and bad things happen.  Typically, you do this by firing your RCS thrusters just enough to put some force on the tank, causing liquid to go to the &quot;bottom&quot; of the tank and the pressurization gas to be buoyed to the &quot;top&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But here's where I got confused:  Why don't the RCS thrusters &lt;i&gt;themselves&lt;/i&gt; need settling?  If they're cold gas thrusters, there's no problem, but why don't hypergolic thrusters have the same problem as turbopumped engines?  How does this problem get solved?&lt;/p&gt;&#xA;" OwnerUserId="14308" LastActivityDate="2017-03-03T14:44:39.940" Title="Propellant Settling for RCS Thrusters" Tags="|propulsion|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15119" PostTypeId="1" CreationDate="2016-05-03T20:15:10.230" Score="12" ViewCount="1522" Body="&lt;p&gt;There have been many documented cases of astronauts not following direct orders during the Skylab and Apollo missions. Some of these have been publicly reprimanded but most have not. Most of the issues have been about relatively minor things - things which do not endanger the safety of anyone, but never the less have been disobedience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about the astronauts currently aboard the ISS? Do the astronauts exercise their own volition and flat out refuse to do some things, or do things they have been forbidden to do, or do the orders of the ground crew always come first? Is the fear of being reprimanded and thrown out of the space program big enough to ensure there are no deviations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is unlikely any sort an official answer could admit to any disobedience by the astronauts because of public image, but are there any 3rd party accounts or external sources that have revealed any problems?&lt;/p&gt;&#xA;" OwnerUserId="2631" LastEditorUserId="6944" LastEditDate="2017-07-06T14:40:02.463" LastActivityDate="2024-02-09T17:30:35.453" Title="Do astronauts aboard ISS always follow orders?" Tags="|iss|crewed-spaceflight|astronauts|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="15120" PostTypeId="2" ParentId="15119" CreationDate="2016-05-03T23:22:11.030" Score="14" Body="&lt;p&gt;One ISS astronaut, Expedition 15/16 crew member &lt;a href=&quot;http://www.jsc.nasa.gov/Bios/htmlbios/anderson-c.html&quot; rel=&quot;noreferrer&quot;&gt;Clayton Anderson&lt;/a&gt;, has written frankly about his conflicts with Mission Control while serving aboard the station.  Chapters 13 and 14 of his very readable memoir &lt;a href=&quot;http://www.space.com/30901-the-ordinary-spaceman-book-excerpt.html&quot; rel=&quot;noreferrer&quot;&gt;&quot;The Ordinary Spaceman&quot;&lt;/a&gt; detail his sparring with the ground and the subsequent effect on his career.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, at one point he made a call to the ground &quot;Houston, Station on Space-to-Ground 2 for inefficiency&quot; and proceeded to list out how foolish he felt it was that Houston had had the crew open up the same panel for maintenance three separate times in a week, instead of planning the activities to access the panel once, then do all three activities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The total number of weightless turds I launched from the ISS escapes&#xA;  me, but no doubt it was substantial.  Safely back on the ground after&#xA;  a sometimes combative five months, I was sentenced to what I would&#xA;  call the astronaut version of &quot;community service&quot;, otherwise known as&#xA;  the astronaut penalty box.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On his return from his long duration mission, he states that his flight status was &quot;conditionally eligible.&quot;  He did end up flying again on STS-131.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Clayton Anderson made a post on this subject on Quora.&#xA;&lt;a href=&quot;https://www.quora.com/Has-NASA-ever-deemed-an-astronauts-performance-on-a-space-mission-unsatisfactory&quot; rel=&quot;noreferrer&quot;&gt;https://www.quora.com/Has-NASA-ever-deemed-an-astronauts-performance-on-a-space-mission-unsatisfactory&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-08-18T02:06:09.083" LastActivityDate="2016-08-18T02:06:09.083" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15121" PostTypeId="1" CreationDate="2016-05-04T00:07:06.777" Score="1" ViewCount="133" Body="&lt;p&gt;Astronauts experience increased blood to the upper body as well as optical problems while in a state of free fall being in a micro gravity environment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is whether they would encounter the same biologic effects if orbiting at L1 or in interstellar space with the absence of the gravitational pull of a large mass? Has NASA ever considered this or are they going to keep trying to figure out why there blood moves to their heads while endlessly riding a roller coaster?&lt;/p&gt;&#xA;" OwnerUserId="14219" LastEditorUserId="10335" LastEditDate="2016-05-05T14:26:25.740" LastActivityDate="2016-05-05T15:34:56.190" Title="Physical effects of microgravity" Tags="|iss|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15122" PostTypeId="1" AcceptedAnswerId="15123" CreationDate="2016-05-04T05:46:56.310" Score="4" ViewCount="196" Body="&lt;p&gt;I randomly stumbled across &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/GT03_TEC.PDF&quot; rel=&quot;nofollow&quot;&gt;this old Gemini 3 mission transcript online&lt;/a&gt;. On the last page, after splash-down, the crew begins corresponding with &quot;Big Box 15&quot; and &quot;Big Box 14&quot; — I'm assuming these are probably references to naval ships, such as the Intrepid, which ultimately retrieved the capsule and crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that correct — does &quot;Big Box&quot; refer to the recovery vehicles? What's the origin of the name?&lt;/p&gt;&#xA;" OwnerUserId="14312" LastEditorUserId="8162" LastEditDate="2016-05-04T09:13:20.740" LastActivityDate="2022-07-21T04:05:28.800" Title="What is a &quot;Big Box&quot; (referenced in GT-3 mission transcripts)?" Tags="|history|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15123" PostTypeId="2" ParentId="15122" CreationDate="2016-05-04T09:20:45.183" Score="6" Body="&lt;p&gt;The key letters &quot;RA&quot; adjacent to the lines make it clear that these are two recovery aircraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The transcript is also shown &lt;a href=&quot;http://gemini3.spacelog.org/page/00:04:51:40/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not know the origin of the name, but I assume the aircraft are the helicopters that came from the carrier &lt;a href=&quot;http://www.intrepidmuseum.org/LatestNews/March-2014/The-Gemini-3-Recovery--49-Years-Ago-This-Week&quot; rel=&quot;nofollow noreferrer&quot;&gt;USS Intrepid&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.honeysucklecreek.net/images/other_stations/carnarvon/missions/GT_3_Recovery.jpg&quot; alt=&quot;picture of helicopter during a recovery mission&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="10273" LastEditDate="2019-01-19T13:18:36.020" LastActivityDate="2019-01-19T13:18:36.020" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="15124" PostTypeId="2" ParentId="15121" CreationDate="2016-05-04T13:04:57.150" Score="1" Body="&lt;p&gt;&quot;endlessly riding in a rollercoaster&quot; implies that you think the astronauts' blood is forced to their head by negative G. That's not the case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a simple test that can answer this. If blood flow to the head is caused by being in orbit of a planet, you'd find a measurable difference between astronauts positioned with their feet pointing towards Earth or away from it. You'd be able to solve the blood flow problem by declaring the zenith side of the ISS to be the floor, and have everybody work with their feet pointed away from Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;No such effect has been found. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say an astronaut's blood is pushed somewhere by an outside force (gravity). That same force acts on the whole astronaut. Since the astronaut is not restrained (unlike when you're in a roller coaster), that force would eventually move the astronaut to one of the walls of the spacecraft. That doesn't happen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronauts wind up with increased blood flow to the head because the body, including the heart, is designed to function in 1G. The heart pushes the blood &quot;uphill&quot; from the feet to the head against the force of gravity. Blood vessels have the right size so every body part gets enough blood in 1G even though blood flows more easily to the feet than the head. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In freefall, your blood is as weightless as the rest of the body, so the heart has much less resistance to work against. In the first few weeks after launch, the heart keeps working at the rate it's used to. Without gravity, this means increased blood flow, especially to the areas that are 'hard to reach' under normal gravity, i.e. the head. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-05-05T15:34:56.190" LastActivityDate="2016-05-05T15:34:56.190" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15125" PostTypeId="1" CreationDate="2016-05-04T13:45:11.150" Score="5" ViewCount="217" Body="&lt;p&gt;I would like to know if composite materials were used on Soyuz and Atlas launchers but I can't find a lot on the web.&#xA;I also would like to know which components of other launchers are made of composites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can you show me some pics or links or just what you know about composites used in launchers and why are them used instead of other materials?&#xA;Do you know if and where composite materials are used in space launchpads?&lt;/p&gt;&#xA;" OwnerUserId="14314" LastEditorUserId="13208" LastEditDate="2017-02-27T21:47:28.490" LastActivityDate="2017-02-27T21:47:28.490" Title="Were composite materials used on Soyuz and on Atlas launchers?" Tags="|launch|launchpad|atlas-v|soyuz-launcher|composite-materials|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15126" PostTypeId="2" ParentId="15121" CreationDate="2016-05-04T16:09:27.380" Score="2" Body="&lt;p&gt;There are lot of differences between a space station in LEO versus one in a pseudo-orbit about L1, cost and radiation hazard being two of the biggest. However, gravity is a non-factor as far as humans are concerned. People don't feel gravity. (No local experiment can, per Einstein's equivalence principle.) People and accelerometers instead feel everything but gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of people on the surface of the Earth, what they feel as &quot;weight&quot; is instead the normal force exerted by the ground on them. Our bodies depend in many ways on this sensible force being present. This normal force is not present in a non-rotating space station, whether that space station is in low Earth orbit, in a pseudo-orbit about the Earth-Moon L1 point, or on the way to Alpha Centauri.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-05-04T16:09:27.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15129" PostTypeId="1" CreationDate="2016-05-05T12:27:43.713" Score="1" ViewCount="214" Body="&lt;p&gt;I am trying to build a cubesat using a Raspberry Pi. I am now figuring out how to communicate with it. As far as I know, I understand I will have to use VHF or UHF to do so.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do I decide on the frequency to use? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How will frquency affect the cost?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do I build an antenna to receive the signals?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where can I buy its components for the Pi? &lt;/p&gt;&#xA;" OwnerUserId="14322" LastEditorUserId="1142" LastEditDate="2016-05-05T14:37:26.547" LastActivityDate="2016-05-05T14:37:26.547" Title="How to build an antenna to receive VHF or UHF?" Tags="|design|cubesat|radio-communication|" AnswerCount="0" CommentCount="8" ClosedDate="2016-05-06T18:41:22.250" ContentLicense="CC BY-SA 3.0" />
  <row Id="15130" PostTypeId="1" CreationDate="2016-05-05T15:31:00.240" Score="6" ViewCount="482" Body="&lt;p&gt;SpaceX has said they will take the recovered CRS-8 Falcon 9 first stage and test fire it 10 times at LC-39A.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is lots of discussion as to what that means.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;10 full 2+ minute duration fires?&lt;/li&gt;&#xA;&lt;li&gt;10 3 second test fires?&lt;/li&gt;&#xA;&lt;li&gt;One after another?&lt;/li&gt;&#xA;&lt;li&gt;Days apart?&lt;/li&gt;&#xA;&lt;li&gt;NASA Style, and take 3 years to get all 10 tests done?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These things may not be knowable in advance, due to confidentiality within SpaceX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But one hint as the rapidity of the testing might be hinted at by the capacity of the deluge system used in each test fire. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the capacity of the deluge system, and how fast can it be refilled?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the entire system is dumped on each test, and requires 2 days to refill from its water source, then we now they cannot do rapid tests in a row. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the deluge system water is collected in settling pools that need time to be absorbed into the ground, then that may be a limit as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the question is, what is the timeline for resetting the LC-39A deluge system between uses?  If you happen to have data for any other launch pad, ask the question standalone, answer, and link back and forth to this please.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-05-05T15:54:27.043" LastActivityDate="2016-09-21T13:18:28.517" Title="Deluge system refresh rates" Tags="|launch|reuse|testing|launchpad|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15131" PostTypeId="1" CreationDate="2016-05-05T15:32:03.700" Score="0" ViewCount="139" Body="&lt;p&gt;I'm a game developer, and I'm working a space game, where I have to simulate a spacecraft's orbit, traveling from the earth to the moon. I would need the apoapsis and periapsis of the orbit. I have the mass, position, and velocity of the objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only problem is, I absolutely don't understand astrophysics, so I couldn't understand the equations I found on the internet. So I would be very grateful, if someone could explain me this.&lt;/p&gt;&#xA;" OwnerUserId="14349" OwnerDisplayName="mbalint1" LastEditorUserId="10335" LastEditDate="2016-05-05T17:03:42.847" LastActivityDate="2016-05-05T17:03:42.847" Title="Calculating orbits for beginners" Tags="|orbit|" AnswerCount="0" CommentCount="6" ClosedDate="2016-05-05T18:05:54.423" ContentLicense="CC BY-SA 3.0" />
  <row Id="15132" PostTypeId="2" ParentId="15130" CreationDate="2016-05-05T15:57:51.607" Score="6" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: SpaceX had several testfires of the recovered CRS-8 Falcon 9 first stage at one-day intervals. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;old: &#xA;&lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-lc39.html&quot; rel=&quot;nofollow&quot;&gt;Capacity&lt;/a&gt; is 300k gallons (1135 tons), flowrate of up to 900 kgallon/min. For a Shuttle launch, those numbers suggest they emptied the reservoir for every launch, in 20-30 seconds (T-10 to T+20, give or take a few). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;70% of the water evaporates. &lt;a href=&quot;http://www.capcomespace.net/dossiers/espace_US/shuttle/ksc/SSWS/LC39_SSWS.htm&quot; rel=&quot;nofollow&quot;&gt;The remaining 30% is caught in two ponds&lt;/a&gt;, where it has to be treated before it can be pumped out, depending on the launcher: the ponds contain combustion products. The Shuttle SRB exhaust was toxic. IDK if that's an issue with kerosene. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Falcon 9 FT, thrust is about 1/5 of the Shuttle's thrust. I expect this means they can reduce the flowrate by 80%.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Another approach:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;For the &lt;a href=&quot;https://www.nasaspaceflight.com/2016/05/falcon-9-jcsat-14-launch/&quot; rel=&quot;nofollow&quot;&gt;JCSAT-14 launch&lt;/a&gt;, SpaceX had a static test fire on May 1, followed by a launch on May 5. Both on SLC-40. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-09-21T13:18:28.517" LastActivityDate="2016-09-21T13:18:28.517" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15134" PostTypeId="1" AcceptedAnswerId="15145" CreationDate="2016-05-05T18:14:13.280" Score="10" ViewCount="1501" Body="&lt;p&gt;What launch vehicle, spacecraft and target orbit parameters decide the time of launch to GTO for satellites destined for GEO?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The target orbital slot is always in the same relative position to the launch site so it seems like the launch should be possible anytime?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some example launches and their windows (all from the Cape):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SpaceX - SES-9 - 23:35 to 01:06 UTC&lt;/li&gt;&#xA;&lt;li&gt;SpaceX - JCSAT-14 - 05:21 to 07:21 UTC&lt;/li&gt;&#xA;&lt;li&gt;ULA - MUOS-4 - 09:59 - 10:43 UTC&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I know there has to be a window - range, people at work, exclusion zones for ships and aircraft - nothing of these can stay ready indefinitely.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-05-06T09:34:55.933" Title="How is the launch window decided for GTO launches?" Tags="|launch|orbit|geostationary|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15135" PostTypeId="2" ParentId="15130" CreationDate="2016-05-05T18:28:17.293" Score="1" Body="&lt;p&gt;Watching the &lt;a href=&quot;https://youtu.be/muDPSyO7-A0?t=9m50s&quot; rel=&quot;nofollow&quot;&gt;SpaceX launch of SES-9&lt;/a&gt;, I noticed the water system is activated at T-8s, if not earlier. SpaceX has the capability of doing a launch after &lt;a href=&quot;https://www.nasaspaceflight.com/2014/08/spacex-falcon-9-v1-1-asiasat-8-launch/&quot; rel=&quot;nofollow&quot;&gt;a last minute scrub&lt;/a&gt;. I would say given that fact, they can do at least 2 launches at once. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I don't think it will take months between test fires. In fact, I'm quite confident when they are ready they can do at least 2 a day, maybe even 3. The timeline is less sure, but it should be ready for a full launch anywhere from &lt;a href=&quot;https://www.nasaspaceflight.com/2016/04/falcon-9-booster-reuse-testing-ksc/&quot; rel=&quot;nofollow&quot;&gt;June to August&lt;/a&gt;. They might even wait for the SES launch scheduled for September, but there's no guarantee of that.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-05T18:28:17.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15136" PostTypeId="1" AcceptedAnswerId="15139" CreationDate="2016-05-05T18:49:01.450" Score="4" ViewCount="574" Body="&lt;p&gt;I've been thinking lately about bubble helmets used by many cosmonauts and astronauts. Almost every space suit I've seen features &lt;strong&gt;a curved helmet&lt;/strong&gt;, even the Gemini astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But a curved piece of glass/plastic &lt;strong&gt;should focus sunlight&lt;/strong&gt; somewhere beyond the convex side, maybe with a little spherical aberration, but potentially still dangerous. We all know what a magnifying glass does when held at the right distance in open sunlight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now obviously, no space traveler has suffered this terrible fate, so I will phrase the question these ways: Why doesn't focused heat happen through the curved visors of spacesuit? Why aren't bubble helmets dangerous in direct sunlight?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2016-05-05T21:15:06.560" Title="Why aren't bubble helmets dangerous in direct sunlight?" Tags="|spacesuits|safety|sunlight|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15137" PostTypeId="1" AcceptedAnswerId="15141" CreationDate="2016-05-05T18:51:53.737" Score="10" ViewCount="1554" Body="&lt;p&gt;In the landing procedure for the first stage of the Falcon 9 one concern is that : &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Merlin 1D throttling at its lowest level still generates more thrust than the empty mass of the first stage. So what are the challenges for throttling lower. For example RD-191 has a throttle range 27%-105% according to Wikipedia. But I am interested only for Merlin 1D engine if it is possible to have this capabilities and what is the work that should be done for this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If SpaceX have problems for throttling low could be as an option that only the engine at the center of the rocket to be a different Merlin engine (different Merlin 1D version or different new engine at all ) that is produced with the primary purpose to be very low throttle (no matter what is the thrust) ? Even if by achieving the necessary throttling could affect the thrust or Isp of the engine (by not having the good performance of the Merlin 1D FT), the overall performance of Falcon 9 still will be ok. They win in this way a higher possibility to landing successfully the first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Production of a new rocket engine of course has a cost and maybe in economies of scale for the production of Merlin 1D it is not economical because you should produce a different type of engine (in fact this would not influence the cost of the rocket),  but it would be very good for SpaceX economy if it helps to lands successfully (slowly and safely). According to SpaceX, Falcon 9 FT now could have a higher total thrust at sea level by 7607 kN (1.71 million pound thrust) compared to previous 6806 kN (1.53 million pound thrust). So even if one of the engines will have a lower thrust or even shut down it, Falcon 9 FT again would have the same performance that was at its previous missions at least around 6800 kN or even more.  Probably for the center engine some options could be the previous versions of Merlin engine. So they don’t need to modify one of the Merlin 1D or developing a new version (naming Merlin 1E) for low throttling since they could have difficulties for this, they could use the previous Merlin 1A, 1B, 1C. They have lower thrust Merlin 1A (340 kN) , Merlin 1B (380 kN) , Merlin 1C (350 kN at the beginning or 560 kN the upgraded version) and including even their throttling capability would make possible to thrust in lower levels, in this way making a perfect speed deceleration for the first stage. Merlin 1C is a better option than A and B for reliability, because uses a regeneratively cooled nozzle and combustion chamber. Of course this engines could need some modification to fit properly with Falcon 9 FT but their thrust which is appropriate will be helpful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to achieve low throttling of Merlin 1D what could be done, and if this faces difficulties could it be as an option to use a new Merlin engine that would have a low (maybe very low) thrust with/or very low throttling capabilities at the value that is necessary for comfortable landings (no matter what thrust will have), or maybe using the previous Merlin engines with modifications if are necessary to do ?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-05-07T16:00:53.493" LastActivityDate="2016-05-07T16:33:37.310" Title="If it is difficult for Merlin engine to have deeper throttling, could it be as an option a different engine at the center of the rocket?" Tags="|spacex|falcon-9|engine-design|engines|merlin-1d|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15138" PostTypeId="2" ParentId="15134" CreationDate="2016-05-05T18:54:20.633" Score="1" Body="&lt;p&gt;It primarily depends on where they want to end up, relative to where they are launching from. It also depends something on the launch profile, so it helps to compare like launching companies. Here's a few other interesting bits, about the desired location.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SES-9 108.2 E&lt;/li&gt;&#xA;&lt;li&gt;JCSAT- 154 E&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, so there's a 46 degree difference, which corresponds to about 2.5 hours, which is only about half the difference. There's a lot that goes in to these GTO orbits to get them quickly to the desired location, and that affects the target orbit. It's really hard to know exactly why without knowing the exact flight trajectories, but I can tell you that the launch for a GTO satellite will be at the same time of the day, but will vary depending on the eventual destination. This is because the initial orbit isn't a 24 hour orbit, but rather something intended to get the satellite in to the correct orbit quickly.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-05T18:54:20.633" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15139" PostTypeId="2" ParentId="15136" CreationDate="2016-05-05T21:15:06.560" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;A curved piece of glass/plastic should focus sunlight somewhere beyond the convex side&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is true;  however it requires that the curvature of the two sides of the piece of glass be &lt;strong&gt;&lt;em&gt;different&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the Lens Maker's Formula &#xA;&lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenmak.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenmak.html&lt;/a&gt; is applied to acurved piece of glass with the two identical radii of curvature, the focal length is infinite...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sKFKl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sKFKl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;shows the various categories of converging or diverging lenses, based on the radius of curvature of the two surfaces.  Note that a lens with the two surfaces curved &lt;strong&gt;&lt;em&gt;and&lt;/em&gt;&lt;/strong&gt; parallel is notable missing...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curved surface &lt;strong&gt;&lt;em&gt;mirrors&lt;/em&gt;&lt;/strong&gt; are of course a different matter.  This hotel &lt;a href=&quot;http://www.dailymail.co.uk/news/article-1315978/Las-Vegas-hotel-death-ray-leaves-guests-severe-burns.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.dailymail.co.uk/news/article-1315978/Las-Vegas-hotel-death-ray-leaves-guests-severe-burns.html&lt;/a&gt; found out the danger of a south facing curved glass wall...&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2016-05-05T21:15:06.560" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15141" PostTypeId="2" ParentId="15137" CreationDate="2016-05-06T03:50:40.297" Score="14" Body="&lt;p&gt;The question presumes that lower landing thrust for Falcon 9 is desirable, which I believe is false.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing thrust is a compromise between opposing requirements. With lower thrust, the rocket could make a slower approach, giving more time to correct its trajectory and land closer to the target. But a slower approach means spending more time and thus more propellant fighting gravity. Higher thrust allows a very fast approach, and very short terminal burn, saving fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears that the landing attempt on the SES-9 Mission (Flight 22), having very little reserve fuel after the primary mission, attempted to do a 3-engine terminal burn. Video of that landing attempt seemed to show the rocket approaching the edge of the ASDS rather than the center, possibly indicating that the fast approach didn't allow enough time to correct the trajectory. So 3 engines might be too much thrust, although flight 24 managed a similar 3-engine ASDS landing successfully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9's current single-engine throttle limits subsequent to the CRS-6 mission (Flight 17) have proven adequate to land on target; flights 20, 21, and 23 all hit the mark. A lower throttle would spend more fuel and solve no problem. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-05-07T16:33:37.310" LastActivityDate="2016-05-07T16:33:37.310" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15142" PostTypeId="1" AcceptedAnswerId="15143" CreationDate="2016-05-06T05:57:11.753" Score="3" ViewCount="296" Body="&lt;p&gt;I was watching the jcsat-14 launch, and I noticed the vehicle was slowly slowing down when 2nd stage engine was off even at 250+km. Is it because of atmospheric drag or pulling force of gravity?&lt;/p&gt;&#xA;" OwnerUserId="688" LastActivityDate="2016-05-06T06:51:50.577" Title="Atmospheric drag at 250 km" Tags="|orbit|atmospheric-drag|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15143" PostTypeId="2" ParentId="15142" CreationDate="2016-05-06T06:51:50.577" Score="7" Body="&lt;p&gt;Thats because when the burn ended, the stage was near the lowest point (&lt;em&gt;perigee&lt;/em&gt;) of its very elliptical orbit&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ceb5o.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ceb5o.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://sci.esa.int/giotto/36672-geostationary-transfer-orbit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and going &quot;up&quot;. So gravity is slowing it down until it reaches the highest point (&lt;em&gt;apogee&lt;/em&gt;). Then it will start gaining speed again as it &quot;falls&quot; closer to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For elliptical orbits the speed is not constant, but it is lowest at the highest point and highest near the lowest point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine sending a ball rolling uphill - it will slow down while going up and then when it goes over the top, it accelerates again.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-05-06T06:51:50.577" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15144" PostTypeId="1" CreationDate="2016-05-06T09:31:22.633" Score="9" ViewCount="893" Body="&lt;p&gt;Towards the end of the &lt;a href=&quot;https://www.youtube.com/watch?v=L0bMeDj76ig&quot; rel=&quot;noreferrer&quot;&gt;JCSAT-14 live webcast&lt;/a&gt;, after the satellite was deployed, we could see small thrusters firing on the second stage, starting to rotate it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Uk3GP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Uk3GP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The webcast ended shortly afterwards, with (I believe) no explanation of what was happening&lt;sup&gt;1&lt;/sup&gt;, so I'm wondering what the purpose of this was.  Perhaps to get ready for an engine relight to re-test that capability / deorbit the second stage more quickly?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;Note that before the deployment, they did mention rotating the stage, but that was to stabilise it before deployment.  I believe this later rotation is unrelated.&lt;/p&gt;&#xA;" OwnerUserId="10686" LastEditorUserId="25" LastEditDate="2016-05-06T11:56:50.183" LastActivityDate="2018-05-23T14:11:55.997" Title="Why rotate the Falcon 9 2nd stage after deployment?" Tags="|rockets|spacex|falcon-9|orbital-maneuver|upper-stage|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15145" PostTypeId="2" ParentId="15134" CreationDate="2016-05-06T09:34:55.933" Score="10" Body="&lt;p&gt;When I was a bit more active supporting GTO launches, 15 years ago, the following constraints helped shape the launch window (not in any order and I may have forgotten some important points completely):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Sensor viewing angle to ensure attitude acquisition coming off the&#xA;launch vehicle (Sun and Earth sensors, may have been superseded by direct use of a star-tracker in the most advance cases these days)&lt;/li&gt;&#xA;&lt;li&gt;A period for orbit determination before and after each apogee manoeuvre, at least a full orbit for the first one&lt;/li&gt;&#xA;&lt;li&gt;Thermal constraints on parts of the satellite for the time spent in GTO (its more common to re-orient the satellite for each apogee burn now so this may be less significant)&lt;/li&gt;&#xA;&lt;li&gt;Sensor viewing constraints for guidance prior to and during the apogee manoeuvres&lt;/li&gt;&#xA;&lt;li&gt;A policy decision to have the opportunity to launch on any day of the year, i.e. helps with managing the effects of short launch delays&lt;/li&gt;&#xA;&lt;li&gt;The target orbit: the delivery orbit for each launch vehicle is different, compounded by the launch site latitude as this drives the injection inclination, whether a supersynchronous or plain GTO is required, whether the satellite is eventually aiming for pure GEO or an initial non-zero combination of inclination and right ascension.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-05-06T09:34:55.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15146" PostTypeId="1" AcceptedAnswerId="15154" CreationDate="2016-05-06T10:39:46.173" Score="4" ViewCount="631" Body="&lt;p&gt;When plotting orbits using the Python package &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt;, the coordinates are &lt;a href=&quot;http://rhodesmill.org/skyfield/positions.html#barycentric-position&quot; rel=&quot;nofollow noreferrer&quot;&gt;barycentric ICRS&lt;/a&gt;. This puts the center of mass of the solar system at the origin, but the $\hat{z}$ axis is tilted about 23° with respect to the normal to the ecliptic. I &lt;em&gt;believe&lt;/em&gt; the ICRS $\hat{z}$ axis is coincident with declination of +90°, but not even sure of that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the most appropriate way to rotate the ICRS data so that the plane of the ecliptic (whatever that &lt;em&gt;exactly&lt;/em&gt; means) coincides with the xy plane?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7lFYB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7lFYB.png&quot; alt=&quot;orbits&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&#xA;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from skyfield.api import load&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from mpl_toolkits.mplot3d import Axes3D&#xA;&#xA;eph = load('de421.bsp')&#xA;&#xA;venus = eph['venus']&#xA;earth = eph['earth']&#xA;mars  = eph['mars barycenter']&#xA;&#xA;ts = load.timescale()&#xA;&#xA;t  = ts.utc(2016, 1, range(700))&#xA;&#xA;vp = venus.at(t).position.km&#xA;ep = earth.at(t).position.km&#xA;mp = mars.at(t).position.km&#xA;&#xA;ps = [vp, ep, mp]&#xA;hw = max([np.abs(p).max() for p in ps])&#xA;size = [-hw, hw]&#xA;&#xA;fig = plt.figure()&#xA;ax = fig.add_subplot(1, 1, 1, projection ='3d')&#xA;for p in ps:&#xA;    x, y, z = p&#xA;    ax.plot(x, y, z)&#xA;ax.set_xlim(size), ax.set_ylim(size), ax.set_zlim(size)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="8162" LastEditDate="2016-05-06T14:13:25.833" LastActivityDate="2016-05-06T14:16:18.370" Title="How to (correctly) rotate this data so the Ecliptic is in the XY plane?" Tags="|skyfield|coordinates|ecliptic-plane|python|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15147" PostTypeId="2" ParentId="15144" CreationDate="2016-05-06T11:12:19.080" Score="2" Body="&lt;p&gt;Likely it was to aim the stage for an orbital change either to reenter sooner, or a parking orbit much higher and out of the way. Do not want to leave clutter around GEO/GTO orbits.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-06T11:12:19.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15148" PostTypeId="1" CreationDate="2016-05-06T11:28:42.570" Score="2" ViewCount="189" Body="&lt;p&gt;As one way missions astronauts would never come back (because it's a real one way mission with a many years trip), is it planed to pay them or giving them enough hardware, tools, food is their salary ? Has this subject ever been considered in plans to send a one way mission to any planet without return ?&lt;/p&gt;&#xA;" OwnerUserId="14331" LastEditorUserId="14331" LastEditDate="2016-05-07T18:09:29.950" LastActivityDate="2016-05-07T18:09:29.950" Title="For one way missions to any planet is it planned to pay astronauts?" Tags="|colonization|future-missions|exoplanet|" AnswerCount="1" CommentCount="5" ClosedDate="2016-05-07T15:11:29.247" ContentLicense="CC BY-SA 3.0" />
  <row Id="15149" PostTypeId="1" AcceptedAnswerId="15161" CreationDate="2016-05-06T11:40:15.873" Score="6" ViewCount="808" Body="&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/728459808270000128&quot;&gt;Elon Musk tweeted this&lt;/a&gt; after the early morning successful landing of another booster on a barge:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;May need to increase size of rocket storage hangar&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How big is their rocket storage hanger at the Cape? There are 3 boosters there currently to my knowledge, plus 1-2 being prepared for launch, but I'm curious what the full capacity actually is.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="51" LastEditDate="2016-05-06T11:55:43.243" LastActivityDate="2016-05-18T00:00:45.093" Title="How many booster stages can SpaceX store at Cape Canaveral?" Tags="|rockets|spacex|falcon-9|storage|cape-canaveral|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15150" PostTypeId="2" ParentId="15148" CreationDate="2016-05-06T11:46:23.313" Score="4" Body="&lt;p&gt;Yes, they would, for a couple of reasons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The money might be little good to them most of the time, but they might have family on Earth that could benefit from it. &lt;/li&gt;&#xA;&lt;li&gt;If they have a successful long term colony, they would need the money to purchase goods back on Earth. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I suspect it would be similar to how the first colonies in the America's were, which was more of a barter type system, where everyone earned a portion of the goods. Wikipedia talks a great deal about how money functioned in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Early_American_currency&quot; rel=&quot;nofollow&quot;&gt;early American settlements&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There were three general types of money in the colonies of British America: specie (coins), paper money and commodity money.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Commodity money, it goes on to explain, was using commodities for money. They had their own coins, distinct from the British coins, but they could be used somewhat interchangeably. However, this might not be required for Mars, as they will likely be more concerned about digital money, and might have very little need of physical cash. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://amzn.to/23utJit&quot; rel=&quot;nofollow&quot;&gt;The Case for Mars&lt;/a&gt; speculates that people will move to Mars because they will earn higher salaries there, which will allow them to bring their families over, similar to what was done in the early colonization days.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-05-06T11:58:59.840" LastActivityDate="2016-05-06T11:58:59.840" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15151" PostTypeId="2" ParentId="15144" CreationDate="2016-05-06T11:56:26.463" Score="6" Body="&lt;p&gt;At the end of the webcast, they mention the last step is to safe the spacecraft. I suspect all they were doing was getting rid of the remaining fuel on the spacecraft. Rotating it might lead to a slightly quicker reentry as it will be more likely to be broadside for longer to the atmosphere, increasing drag, but I think that is a secondary concern.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-06T11:56:26.463" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15152" PostTypeId="1" AcceptedAnswerId="27415" CreationDate="2016-05-06T12:12:04.427" Score="11" ViewCount="1558" Body="&lt;p&gt;Towards the end of the &lt;a href=&quot;https://www.youtube.com/watch?v=L0bMeDj76ig#t=53m58&quot; rel=&quot;noreferrer&quot;&gt;JSSAT-14 webcast&lt;/a&gt; (around T+32:58), we see a thruster firing.  Over the space of around 20 seconds, you can see it &quot;icing up&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5iYw5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5iYw5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is this that's forming?  Is this a normal process for thrusters on spacecraft, or is it related to this being an upper stage that doesn't have much of a use beyond getting the payload to orbit - I would expect thrusters used in the longer term to not do this (especially since presumably anything that forms is not contributing to the actual thrust, reducing overally efficiency).  What makes this different?&lt;/p&gt;&#xA;" OwnerUserId="10686" LastEditorUserId="10686" LastEditDate="2016-06-20T07:45:29.220" LastActivityDate="2018-05-24T07:16:23.253" Title="Is it normal for thrusters to &quot;ice up&quot;?" Tags="|rockets|spacex|falcon-9|upper-stage|thermal|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15153" PostTypeId="2" ParentId="15152" CreationDate="2016-05-06T12:20:54.170" Score="11" Body="&lt;p&gt;When this was captured, it was at the end of the rocket flight. Looking carefully at this, I don't see any other similar nozzles. Furthermore, I didn't see any evidence of this being used in flight. I'm going to assume from all of this that it must be the vent value for the LOX, which was mentioned in the video recording immediately before. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens with such a release is that the temperature tends to drop when a compressed gas is released. Being in a vacuum, the material cannot be atmospheric, such as ice, but must be a frozen variety of whatever is being leaked. We know that the LOX tank is the lower of the two tanks. A &lt;a href=&quot;http://www.jaea.go.jp/jaeri/english/press/980624/ref02.html&quot;&gt;phase diagram&lt;/a&gt; indicates that LOX would be solid if the temperature is around 80K, which is entirely possible. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-06T12:20:54.170" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15154" PostTypeId="2" ParentId="15146" CreationDate="2016-05-06T14:16:18.370" Score="1" Body="&lt;p&gt;You should be able to fetch ecliptic coordinates with:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;x, y, z = venus.at(t).ecliptic_position().km&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Here is the current documentation — it is a bit brief, so I will try adding an example to them this weekend. You will see that there is also a method, in case you ever need it, for ecliptic latitude and longitude:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://rhodesmill.org/skyfield/api-position.html#skyfield.positionlib.ICRF.ecliptic_position&quot; rel=&quot;nofollow&quot;&gt;http://rhodesmill.org/skyfield/api-position.html#skyfield.positionlib.ICRF.ecliptic_position&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8590" LastActivityDate="2016-05-06T14:16:18.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15155" PostTypeId="2" ParentId="8868" CreationDate="2016-05-06T15:09:52.987" Score="-3" Body="&lt;p&gt;I don't believe that SpaceX needs practice at anything that has to do with landing at sea or on a barge. They need to fully understand the forces on the rocket at and near the point of landing. The barge is responsible for positioning and vertical movement entirely independently of the rocket. Windage and velocity control as well as vertical control of the angle of incidence is a separate but related issue again having nothing to do with landing at sea. From what I could see, the dry landing zone was no larger than the barges. &lt;/p&gt;&#xA;" OwnerUserId="11011" LastActivityDate="2016-05-06T15:09:52.987" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15159" PostTypeId="1" AcceptedAnswerId="15162" CreationDate="2016-05-06T20:16:55.730" Score="2" ViewCount="1564" Body="&lt;p&gt;The really fun part about watching SpaceX land their first stage on either their ASDS Barge (JRTI or OCISLY) or Landing Zone 1 on land, is they drew a nice target on the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a literal X that marks the spot for the first stage to hit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reports have been that the stage has landed within 4 feet of the center of the target. If so, which has a greater CEP (Circular Error Probability)?  The Minuteman ICBM missile or the first stage of a Falcon 9 landing?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="25" LastEditDate="2016-05-06T20:59:48.113" LastActivityDate="2016-05-09T11:21:26.520" Title="Which has greater accuracy? Minuteman ICBM or Falcon 9 first stage?" Tags="|spacex|falcon-9|landing|millitary|accuracy|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15160" PostTypeId="2" ParentId="15159" CreationDate="2016-05-06T20:30:07.253" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://nuclearweaponarchive.org/Usa/Weapons/Mmiii.html&quot; rel=&quot;nofollow&quot;&gt;Open sources&lt;/a&gt; have the Minuteman III CEP on the order of 100 m. Falcon 9 needs to be on the order of ~20 m to not miss the deck of the barge.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are big differences in targeting: The Minuteman gets its destination programmed in before launch. You usually can't send in a surveying team, so there may be some uncertainty of the target's position. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon has a cooperative target: it can get live updates of the barge's position during flight. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-05-09T11:21:26.520" LastActivityDate="2016-05-09T11:21:26.520" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15161" PostTypeId="2" ParentId="15149" CreationDate="2016-05-06T20:41:16.383" Score="5" Body="&lt;p&gt;The storage hangar at Launch Complex 39A at the Cape according &lt;a href=&quot;http://www.theverge.com/2016/5/6/11607828/spacex-landed-falcon-rockets-stored-where-cape-canaveral-spaceport&quot; rel=&quot;nofollow noreferrer&quot;&gt;theverge.com&lt;/a&gt;, can store in total up to five boosters at once. Looking to the SpaceX successful landings, it rises the possibilities to have more consecutive 1st stage landings sooner then they thought. Imagining the time when Falcon Heavy will be operational, then a new storage hangar or an increased size of the current one will be needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.nasa.gov/sites/default/files/thumbnails/image/39a_aerial1.jpg&quot; alt=&quot;Spacex storage hangar&quot;&gt;&#xA;&lt;sup&gt;SpaceX storage hangar at Launch Complex 39A in Cape Canaveral.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/OIWhJ.jpg&quot; alt=&quot;Spacex storage hangar inside view&quot;&gt;&#xA;&lt;sup&gt;Inside view of the SpaceX storage hangar at Launch Complex 39A in Cape Canaveral.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-05-18T00:00:45.093" LastActivityDate="2016-05-18T00:00:45.093" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15162" PostTypeId="2" ParentId="15159" CreationDate="2016-05-06T20:57:11.427" Score="2" Body="&lt;p&gt;There are a few differences. The most notable difference is the much larger distance that a Falcon 9 must go to achieve it's orbit. Also, the fact that Falcon 9 does a soft landing as compared to the fast landing required by a BM. Also, the Falcon 9 relies on GPS navigation, whereas a military system cannot be solely reliant. Lastly, these systems won't actually give their real accuracy, just vague terms like &quot;pinpoint precision&quot;, but an &lt;a href=&quot;http://www.navweaps.com/Weapons/WMUS_Tomahawk.php&quot; rel=&quot;nofollow&quot;&gt;open source&lt;/a&gt; claims the Tomahawk missile has an accuracy of about 30m. This is the most comparable weapon I know of to the Falcon 9.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As was noted, the accuracy of the Minuteman is estimated to be around 100m. Given an estimated 6 km/s, that means that it had to explode within 1/60th of a second, assuming pinpoint precision. Furthermore consider that a Minuteman doesn't have to be more accurate, 100m won't make much of a difference, so they don't have the required precision that a Falcon 9 landing requires.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most directly comparable landing would be for the Dragon's landing back on the ground, but again, the speed at landing will be quite different. I suspect still that SpaceX will be more accurate, but as declared above, that doesn't matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another compatible value is the &lt;a href=&quot;http://spaceflightnow.com/falcon9/010/140723orbcomm/#.Vy0HNL7kVJg&quot; rel=&quot;nofollow&quot;&gt;orbital insertion error&lt;/a&gt;, which is estimated at 500m for the ORBCOMM mission in 2014. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-05-06T21:06:44.243" LastActivityDate="2016-05-06T21:06:44.243" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15163" PostTypeId="1" CreationDate="2016-05-07T00:00:42.430" Score="21" ViewCount="2356" Body="&lt;p&gt;Libido can affect behaviour, judgement, group dynamics etc, which could pose a risk for the effectiveness of a mission. Is this somehow addressed by NASA, for example in screening during recruitment, in training, in space-lifestyle design or in medication?&lt;/p&gt;&#xA;" OwnerUserId="8546" LastEditorUserId="8546" LastEditDate="2016-05-09T02:18:04.970" LastActivityDate="2016-10-04T14:54:25.573" Title="Does NASA address libido of astronauts that are in space for prolonged periods of time?" Tags="|nasa|astronaut-lifestyle|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15164" PostTypeId="2" ParentId="15152" CreationDate="2016-05-07T10:09:55.170" Score="1" Body="&lt;p&gt;That LOX was vented before the final burn to chill in the engine. The final burn happens after payload separation to park Stage 2 away from the satellite.&lt;/p&gt;&#xA;" OwnerUserId="12801" LastActivityDate="2016-05-07T10:09:55.170" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15166" PostTypeId="1" CreationDate="2016-05-07T14:05:34.723" Score="4" ViewCount="721" Body="&lt;p&gt;I would like to track the all the Falcon 9-r boosters that are landing or are attempting to land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that the third booster which landed successfully is the F9-0024-S1 but I don't know the version.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where can I find this information for the previous ones and all future flights as they append?&lt;/p&gt;&#xA;" OwnerUserId="14351" LastEditorUserId="51" LastEditDate="2016-05-08T01:53:58.477" LastActivityDate="2017-06-09T20:52:21.730" Title="What are the version number and identification names for each Falcon 9-r boosters?" Tags="|launch|spacex|falcon-9|recovery|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15167" PostTypeId="1" CreationDate="2016-05-08T01:51:17.547" Score="11" ViewCount="1606" Body="&lt;p&gt;There have been many examples of the placement of satellites in orbits around Lagrange points, most have been sun-earth and earth-moon &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$L_2$&lt;/span&gt; due to their proximity to earth. In each case both bodies and both points fall on a straight line, so to have the right combination of non-blocked line-of-sight to earth for communications, safe angular separation from the sun, and non-blocked line-of-sight to the sun for solar power, the size and shape orbits about the Lagrange points are carefully optimized. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Besides those reasons&lt;/em&gt;, are there orbital mechanical or other reasons why placement in orbit around  L₁ and L₂ would be preferred over placement near the points themselves? Does it require significantly more delta-v to get there (shortening station keeping lifetime)? Would station keeping be more difficult or costly (delta-v) near the point than it would be in a larger orbit about the point (excluding communications issues)?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-10-22T03:29:52.580" LastActivityDate="2018-10-22T03:29:52.580" Title="Are large halo orbits around L₁'s and L₂'s preferred over small orbits for reasons other than geometry?" Tags="|orbital-mechanics|artificial-satellite|lagrangian-points|halo-orbit|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="15168" PostTypeId="2" ParentId="15166" CreationDate="2016-05-08T01:53:32.850" Score="1" Body="&lt;p&gt;The first to land was the Orbcomm OG2 mission, Dec 21, 2015 which was a F9 1.1 Full Thrust model. It landed at LZ-1 in Florida.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last F9 1.1 (non-Full thrust version) carried the Jason-3 mission to orbit (Jan 17, 2016), landed on the JRTI barge in the Pacific, but one of the four legs did not lock, so it fell over and expoded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SES-9 on Mar 4, 2016 failed by running out of fuel, also a F.9 1.1 Full Thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apr 8, 2016 the CRS-8 mission landed on the OCISLY ASDS a F9 1.1 Full Thrust. F9-023&lt;/p&gt;&#xA;&#xA;&lt;p&gt;May 6, 2016 the JCSat-14 mission landed on OCISLY ASDS and was a F91.1 Full Thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a good Reddit page that is &lt;a href=&quot;https://www.reddit.com/r/spacex/wiki/cores&quot; rel=&quot;nofollow noreferrer&quot;&gt;tracking Falcon 9 missions&lt;/a&gt; with the info you want.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has a &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Falcon_9_and_Falcon_Heavy_launches&quot; rel=&quot;nofollow noreferrer&quot;&gt;tracking page of Falcon 9 Missions&lt;/a&gt; as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Someone is generating a PNG from a data file trying to capture all the flight info and I will insert the latest version as of this writing.  I have yet to find a dynamic link to an updated copy alas. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mkFIC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mkFIC.png&quot; alt=&quot;Infographic of SpaceX launches&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-06-09T20:52:21.730" LastActivityDate="2017-06-09T20:52:21.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15169" PostTypeId="1" AcceptedAnswerId="15172" CreationDate="2016-05-08T08:38:17.617" Score="10" ViewCount="505" Body="&lt;p&gt;I'm watching CSA videos, and have watched numerous documentaries before, and have noticed that both Astronauts and Cosmonauts alike aboard the ISS tend to refer to something as either Russian, or American.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a Chris Hadfield video, he refers to a towel as being a &quot;Russian Towel&quot;, and numerous documentaries, other mundane objects as either being a &quot;Russian object&quot; or &quot;American object&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why bother noting the origin of the objects used upon the ISS? Is it because of international differences or is there some major cause of concern if not mentioned?&lt;/p&gt;&#xA;" OwnerDisplayName="user14355" LastEditorUserId="6944" LastEditDate="2017-07-06T14:33:06.483" LastActivityDate="2017-07-06T14:33:06.483" Title="Why do Astro/Cosmonauts refer to things as Russian or American?" Tags="|iss|crewed-spaceflight|russia|united-states|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15170" PostTypeId="2" ParentId="14816" CreationDate="2016-05-08T10:25:09.563" Score="7" Body="&lt;p&gt;(disclaimer: I am not a cold-environments engineer, but I work around them)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's not as much scope for transfer as might initially be assumed. Let's look at @ventsyv's comments in more detail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://scitation.aip.org/content/aip/journal/rsi/85/4/10.1063/1.4871555&quot; rel=&quot;noreferrer&quot;&gt;recent study&lt;/a&gt; looked at power options for remote sites in Antarctica (ie, those only visited occasionally by people, and which may go one or two years without maintenance). They concluded that solar is great when it works, but is only useful for around a third of the year. Remember that the worst a solar-powered space probe usually has to contend with is brief eclipses, or, in the case of a lander, several-hour nighttime shutdowns. By comparison, an Antarctic site will suffer a blackout period which might be several months long, with very long night cycles and twilight periods on either side. The closest analogy here is a long-duration Mars lander; however, Spirit/Opportunity had trouble enough with winter daylight at &lt;em&gt;equatorial&lt;/em&gt; locations, and Mars Polar Lander/Phoenix were not intended to survive the winter at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RTGs are in theory a solution to this problem, but are no longer used in Antarctica. The Americans deployed them as early as the 1960s for remote weather stations, but &lt;a href=&quot;http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-11-00015.1&quot; rel=&quot;noreferrer&quot;&gt;removed the last ones in the 1990s&lt;/a&gt;. The Soviets used them widely, and the last four were recovered &lt;a href=&quot;http://www.osti.gov/scitech/biblio/1241969&quot; rel=&quot;noreferrer&quot;&gt;only a couple of years ago&lt;/a&gt;. Leaving aside the questions of cost and security (unattended radiological materials are very, very unpopular these days!), there's also something of an open question about the regulatory context. The Antarctic Treaty does not prohibit RTGs (or indeed nuclear power in general - the US used to operate a 'real' reactor at McMurdo), but it does require fairly thorough environmental impact studies and prohibit leaving any nuclear waste. I can imagine that unless you have a very clear plan for removing the RTG after use, you would have problems getting its deployment approved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside of the space environment, there is more flexibility for alternative power sources; the study mentions conventional diesel generators and propane thermo-electric generators, neither of which would be practical without an oxygen atmosphere. Wind turbines can also be used; again, you need an atmosphere of some kind for these! The AGO stations use solar power supplemented by wind during the winter, with a two-day battery in case of calm weather, and the operators simply accept the fact that the system will shut down sometimes due to prolonged unfavourable weather conditions. But it'll probably come back online. If the whole thing fails to start up again, which is unlikely but possible, you can say &quot;oh well&quot; and fly someone in to start it back up in the summer. This is of course completely impractical for spacecraft - both the restart and the tolerant approach to downtime - but it simplifies the engineering requirements immensely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For smaller &quot;place and forget&quot; devices, with minimal power requirements, it is simplest to just use conventional batteries. &lt;a href=&quot;http://nora.nerc.ac.uk/503748/&quot; rel=&quot;noreferrer&quot;&gt;These probes&lt;/a&gt;, for example, have eight D-cell batteries with a two-year lifespan - they may even be acquired off the shelf - and as the unit is expected to be buried by snow before then, two years is seen as fine. Alternatively, if the units are accessible, a two- or three-year lifespan for a remote installation would make it quite practical to schedule a battery change as needed. Most batteries are good down to around -40C, which would rule out very deep-field operations (which might be -70C) but is reasonable for coastal work. Likewise, electronics good to -40C can be obtained &lt;a href=&quot;http://www.geosci-instrum-method-data-syst.net/5/65/2016/gi-5-65-2016.pdf&quot; rel=&quot;noreferrer&quot;&gt;more or less off the shelf&lt;/a&gt;, or rebuilt with fairly simple equipment. This highlights that specialist batteries and electronics are not necessarily needed. The AGO paper notes that &quot;industrial&quot; grade electrical equipment is usually fine to -40C, might cope with worse, and can happily be left unattended at much lower temperatures if turned off and gradually cooled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, though, the disposable probes highlight an unexpected bit of technology transfer - the parachute used was the same type used for Viking!&lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="11259" LastEditDate="2016-05-08T14:12:54.080" LastActivityDate="2016-05-08T14:12:54.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15172" PostTypeId="2" ParentId="15169" CreationDate="2016-05-08T11:35:55.087" Score="13" Body="&lt;p&gt;It is because the two programs are separately managed and tracked to an extent that would astound the outsider. USOS crewmembers theoretically would have to ask for permission to use a Russian screwdriver; consumables from the two sides are even more obsessively managed. Casual sharing does go on but this produces headaches for the legion of logistics analysts on the ground who track such things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, whether an object is owned by the Russian or US program is a matter of deep concern to all, including the crew, and this concern becomes apparent in the way they think of and refer to things, as you noticed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider the fundamental engineering differences between the two sides of the ISS: they do not even use the same voltage in their power supplies!&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-05-08T11:41:10.127" LastActivityDate="2016-05-08T11:41:10.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15175" PostTypeId="2" ParentId="15167" CreationDate="2016-05-08T18:50:45.540" Score="4" Body="&lt;p&gt;One thought is traffic collisions. Once you put the first satellite exactly at the L point in question no other satellites can occupy that position. And because that are gravitationally stable points shifting them out again at the end of their life time is relatively expensive. So to be good neighbors to the other missions in the neighborhood you will prefer to orbit the L point which increases the number of missions that can go on simultaneously. &lt;/p&gt;&#xA;" OwnerUserId="11169" LastActivityDate="2016-05-08T18:50:45.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15176" PostTypeId="5" CreationDate="2016-05-08T21:03:33.280" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:03:33.280" LastActivityDate="2016-05-08T21:03:33.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15177" PostTypeId="4" CreationDate="2016-05-08T21:03:33.280" Score="0" Body="questions regarding an engine's capability to control the direction of its thrust." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:03:33.280" LastActivityDate="2016-05-08T21:03:33.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15178" PostTypeId="5" CreationDate="2016-05-08T21:07:52.617" Score="0" Body="&lt;p&gt;Common ways of capture includes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Purely gravitational capture, that relies on moon fly bys and weak stability manifolds.&lt;/li&gt;&#xA;&lt;li&gt;Aero capture, that uses the atmosphere of a planet to slow the spacecraft down.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Both methods can be combined with propulsive braking and manoeuvring.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:07:52.617" LastActivityDate="2016-05-08T21:07:52.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15179" PostTypeId="4" CreationDate="2016-05-08T21:07:52.617" Score="0" Body="questions about the process of capturing a spacecraft into orbit around a planet or moon." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:07:52.617" LastActivityDate="2016-05-08T21:07:52.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15180" PostTypeId="5" CreationDate="2016-05-08T21:11:15.087" Score="0" Body="&lt;p&gt;Light sails are capable of producing thrust without loosing mass, but the resulting force is extremely small, requiring large areas of thin and light material in order to be feasible.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:11:15.087" LastActivityDate="2016-05-08T21:11:15.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15181" PostTypeId="4" CreationDate="2016-05-08T21:11:15.087" Score="0" Body="questions about spacecraft propelled by the reflection of light, either from the Sun, or an artificial source." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:11:15.087" LastActivityDate="2016-05-08T21:11:15.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15182" PostTypeId="5" CreationDate="2016-05-08T21:14:53.170" Score="0" Body="&lt;p&gt;Current plans is an unmanned launch in 2018.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:14:53.170" LastActivityDate="2016-05-08T21:14:53.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15183" PostTypeId="4" CreationDate="2016-05-08T21:14:53.170" Score="0" Body="Questions about the version of the SpaceX Dragon spacecraft designed for a voyage to Mars." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:20:57.393" LastActivityDate="2016-05-08T21:20:57.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15184" PostTypeId="5" CreationDate="2016-05-08T21:17:42.343" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:17:42.343" LastActivityDate="2016-05-08T21:17:42.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15185" PostTypeId="4" CreationDate="2016-05-08T21:17:42.343" Score="0" Body="questions about the Molniya series of Soviet military communication satellites." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:17:42.343" LastActivityDate="2016-05-08T21:17:42.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15186" PostTypeId="5" CreationDate="2016-05-08T21:23:37.470" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:23:37.470" LastActivityDate="2016-05-08T21:23:37.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15187" PostTypeId="4" CreationDate="2016-05-08T21:23:37.470" Score="0" Body="Questions regarding model and/or high-powered rocketry." OwnerUserId="8693" LastEditorUserId="40257" LastEditDate="2022-05-16T13:49:21.773" LastActivityDate="2022-05-16T13:49:21.773" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="15188" PostTypeId="5" CreationDate="2016-05-08T21:26:30.033" Score="0" Body="&lt;p&gt;Bigelow has delivered the &lt;a href=&quot;https://en.wikipedia.org/wiki/Bigelow_Expandable_Activity_Module&quot; rel=&quot;nofollow&quot;&gt;Bigelow Expandable Activity Module&lt;/a&gt; to the ISS, launched 10 April 2016.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:26:30.033" LastActivityDate="2016-05-08T21:26:30.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15189" PostTypeId="4" CreationDate="2016-05-08T21:26:30.033" Score="0" Body="Questions regarding the Bigelow company." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-05-08T21:26:30.033" LastActivityDate="2016-05-08T21:26:30.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15190" PostTypeId="1" AcceptedAnswerId="15226" CreationDate="2016-05-09T00:54:37.270" Score="6" ViewCount="1032" Body="&lt;p&gt;My goal is to get a graph &quot;spaceport latitude&quot;&amp;lt;&gt;&quot;delta-v to reach mars&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using &quot;Trajectory Optimization Tool&quot; (Matlab one) I have successfully generated porkchop graph for Earth-&gt;Mars transfer. It does not take into account orbital parameters around Earth i.e. it is the trajectory we are going to use regardless of space port location. Is that correct to assume that spaceport latitude would not affect optimal Earth-&gt;Mars trajectory?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get to this specific trajectory, I do departure burn calculation in the same tool for initial circular 200km LEO with different inclinations of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found that lowest delta-v is needed to depart from 21° orbit inclination, although I assume achieving this orbit will require more energy than 0° one from equator. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My further thinking is that I need to record delta-v requirements for all orbital inclinations, calculate all possible delta-v's for all spaceport latitudes to reach initial orbit with all inclinations, try all orbital plane changes on LEO and by combining all this (i.e. finding optimal combinations of initial inclination and plane change for each spaceport location) with delta-v I got for departure burn to Mars for all inclinations I might get the graph I want. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a more straightforward way of doing so? Maybe there was already some research done on this topic which I am overlooking?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HHy7E.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HHy7E.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11701" LastEditorUserId="11701" LastEditDate="2016-05-11T01:54:04.737" LastActivityDate="2016-05-11T03:45:22.973" Title="Earth-&gt;Mars: Porkchop, departure burn and orbit inclination" Tags="|mars|orbital-mechanics|orbital-maneuver|mars-orbiter-mission|mars-orbiters|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15191" PostTypeId="1" AcceptedAnswerId="15193" CreationDate="2016-05-09T02:46:19.937" Score="11" ViewCount="5501" Body="&lt;p&gt;I never thought about this until reading the comments below &lt;a href=&quot;https://space.stackexchange.com/a/6584/12102&quot;&gt;this answer&lt;/a&gt;. To radiate something like 150W (a very conservative lower limit of human + suit systems heat production - see @Uwe 's comment below) I think a radiator would have to be conspicuously visible, wouldn't it? I don't remember seeing heat sink fins sticking out of the astronauts packs while walking on the moon or doing ISS maintenance though. In fact I think everything was/is pretty much white all around - which minimizes heating from the sun but for essentially the same reason will not radiate efficiently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is the heat actually dissipated? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; see the answer - the &lt;em&gt;total&lt;/em&gt; heat is probably much larger than 150W I mention here.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-06-13T15:36:30.267" LastActivityDate="2021-06-13T15:36:30.267" Title="How have space suits dissipated the heat removed from astronauts?" Tags="|astronauts|spacesuits|thermal|life-support|thermal-control|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="15192" PostTypeId="2" ParentId="15190" CreationDate="2016-05-09T05:37:19.463" Score="-1" Body="&lt;p&gt;I don't know how that Matlab tool works, but if it requires you to start from some specific orbit about Earth, it is doing it wrong. You are addressing the first phase of trajectory planning. The best thing to do in this phase is to ignore that the spacecraft starts from some LEO orbit. That's a detail, something that should be ignored in up-front planning. If the subsequent detailed plan involves launching into LEO, and from there to Mars, the optimal trajectory plan should instead inform you of the inclination of the optimal LEO orbit.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-05-09T05:37:19.463" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15193" PostTypeId="2" ParentId="15191" CreationDate="2016-05-09T07:47:48.923" Score="11" Body="&lt;p&gt;For Apollo, space suits used the &lt;a href=&quot;https://en.wikipedia.org/wiki/Primary_Life_Support_System&quot; rel=&quot;noreferrer&quot;&gt;Primary/Portable/Personal Life Support System&lt;/a&gt; during EVAs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a schematic of the PLSS: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v3Q7v.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v3Q7v.png&quot; alt=&quot;PLSS showing a sublimator that vents steam to the vacuum of space&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The PLSS used &lt;a href=&quot;https://www.hq.nasa.gov/alsj/LM15_Portable_Life_Support_System_ppP1-5.pdf&quot; rel=&quot;noreferrer&quot;&gt;sublimation cooling&lt;/a&gt;. Water was run over a heat exchanger, the heated water was then allowed to vent into the vacuum of space where it would sublimate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eclss/emu.html&quot; rel=&quot;noreferrer&quot;&gt;Shuttle/ISS EMU&lt;/a&gt; suit uses the same principle: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the primary purpose of the water tanks is to feed water to the sublimator. The sublimator works on the principle of sublimation, that is, the process by which a solid turns directly into a vapor, bypassing the liquid phase. In this case, ice is formed on the sublimator evaporator sieve and is allowed to vaporize to space, removing heat with it. Air and cooling water are passed through fins in the sublimator, which extracts heat from each system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-05-24T10:37:44.137" LastActivityDate="2016-05-24T10:37:44.137" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15194" PostTypeId="1" AcceptedAnswerId="15196" CreationDate="2016-05-09T10:28:13.210" Score="6" ViewCount="483" Body="&lt;p&gt;EVA suits use an array of consumable resources: oxygen, CO2 filter, nitrogen, electricity, sublimator/coolant water, drink water, maybe something else I missed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if an astronaut on EVA in LEO was unable to return to the craft/station, floating away into space - how would they die? What's the first resource(s) they'd run out of that are necessary to survival?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-05-09T13:52:33.100" LastActivityDate="2016-05-09T13:52:33.100" Title="Which consumable resource puts the hard limit on EVA time?" Tags="|spacesuits|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15195" PostTypeId="1" CreationDate="2016-05-09T11:41:06.617" Score="4" ViewCount="784" Body="&lt;p&gt;Even when the Falcon 9 &quot;punched the barge&quot;, it did so with extreme precision. SpaceX has now had several successful landings, many of them extremely spot-on. Two questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;There are obvious visual navigation markers on the barges. Is that only for show, or does the Falcon 9 use visual navigation?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What other techniques (if any) does the Falcon 9 use to enable those precision landings?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To be specific, I'm asking about terminal guidance. GPS alone will not do the trick. Relative GPS might do the trick.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="3185" LastEditDate="2016-05-09T13:05:00.360" LastActivityDate="2016-05-09T18:09:07.033" Title="Technologies use by SpaceX to enable landing on a drone ship" Tags="|spacex|falcon-9|landing|" AnswerCount="1" CommentCount="8" ClosedDate="2016-05-09T20:12:50.953" ContentLicense="CC BY-SA 3.0" />
  <row Id="15196" PostTypeId="2" ParentId="15194" CreationDate="2016-05-09T13:17:22.180" Score="6" Body="&lt;p&gt;From what I remember it varies between oxygen and battery.  &lt;a href=&quot;http://www.spaceflightnow.com/shuttle/sts127/status15.html&quot; rel=&quot;noreferrer&quot;&gt;Here's&lt;/a&gt; an example from STS-127 where the answer was oxygen.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Both spacesuits are working well today. Mission Control projects the&#xA;  EVA-limiting consumable is oxygen that should last a total duration of&#xA;  seven hours and 30 minutes. The spacewalk just passed the&#xA;  four-and-a-half hour mark. If the crew needs more time to finish the&#xA;  battery task, they could return to the airlock for a quick recharge of&#xA;  the suits' oxygen supply.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;ISTR that battery life limited some EVAs, but perhaps battery upgrades have made that obsolete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For your case of a stranded EVA crewmember, it might depend on what they did.  If they just lay quietly, battery would probably be the limit.  If they thrashed about in panic, probably O2 or cooling water.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-05-09T13:17:22.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15197" PostTypeId="1" CreationDate="2016-05-09T14:05:27.380" Score="3" ViewCount="609" Body="&lt;p&gt;This is a questions which has been asked on &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/4idcq7/why_did_the_landing_legs_of_the_jcsat14_booster/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reddit&lt;/a&gt; after the landing at night of the booster used in JCSAT-14 mission, however no one has been able to provide a valid reason. The lights in question can be seen &lt;a href=&quot;https://i.stack.imgur.com/yeWm6.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; or in the &lt;a href=&quot;https://youtu.be/L0bMeDj76ig?t=29m42s&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt;, right after the landing. They appear to be steady-white for about 30 seconds, after which they began to blink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some possible explanations included:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;FAA regulations (but the rocket operates in a no-fly zone, and the legs are opened just seconds before touchdown)&lt;/li&gt;&#xA;&lt;li&gt;Naval regulations (but they could have attached lights after the landing and avoid the weights - if the lights from the barge were not enough)&lt;/li&gt;&#xA;&lt;li&gt;Safety lights in case the rocket would splash down, but it's highly probable that it would sink in this case&lt;/li&gt;&#xA;&lt;li&gt;Safety signaling for the crew, but the use of telemetry data makes more sense for that&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="14363" LastEditorUserId="51" LastEditDate="2016-05-09T14:14:33.047" LastActivityDate="2016-05-09T15:20:57.730" Title="What's the purpose of the white flashing lights on the Falcon 9 legs?" Tags="|spacex|falcon-9|landing|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15198" PostTypeId="1" AcceptedAnswerId="15199" CreationDate="2016-05-09T14:29:44.867" Score="7" ViewCount="1906" Body="&lt;p&gt;Let's say the guidance system fails, and Falcon 9 first stage &lt;em&gt;gently&lt;/em&gt; touches down on surface of water before switching the engine off (and plummeting into the water).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will it sink or will it float?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At that moment it's a huge, mostly empty tube, although rather heavy on itself, I'm not sure if it's durable enough to survive a splashdown from 0 altitude, and how much fuel is left after the landing. And while I could find the landing mass, I'm really unsure how to discover the landing volume.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="51" LastEditDate="2016-05-09T14:34:42.477" LastActivityDate="2016-05-09T20:35:38.010" Title="Would Falcon 9 sink on gentle splashdown?" Tags="|spacex|falcon-9|" AnswerCount="4" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15199" PostTypeId="2" ParentId="15198" CreationDate="2016-05-09T14:35:17.687" Score="14" Body="&lt;p&gt;Past events demonstrate that it will sink. Following the ORBCOMM launch in 2014, the first stage performed a soft landing in the Atlantic Ocean, and the water impact caused loss of hull integrity:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After landing, the vehicle tipped sideways as planned to its final water safing state in a nearly horizontal position. The water impact caused loss of hull integrity, but we received all the necessary data to achieve a successful landing on a future flight. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;http://www.spacex.com/news/2014/07/22/spacex-soft-lands-falcon-9-rocket-first-stage&quot;&gt;Source: SpaceX press release&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="14363" LastEditorUserId="6816" LastEditDate="2016-05-09T20:35:38.010" LastActivityDate="2016-05-09T20:35:38.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15200" PostTypeId="2" ParentId="15198" CreationDate="2016-05-09T14:37:30.223" Score="6" Body="&lt;p&gt;The SRB's, made of much heavier material than the Falcon 9 first stage, would hit the water, tail end first, with the heavy motor casing at the bottom trapping air inside it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted, the SRB's did NOT impact gently, they hit at terminal velocity under their parachutes, and usually would bend the SRB segements some amount out of round that had to be fixed in the factory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This air inside would suffice to keep it floating mostly vertical until the recovery ships could find it. Only about 20 feet or so stuck up out of the water. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To recover it, they would have divers swim down with an inflatable plug, to close the hole the engine makes, and pump air into it until it was floating on it's side, then they could tow it back to Port Canaveral for processing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can watch this in a great video &lt;a href=&quot;https://www.youtube.com/watch?v=Gbtulv0mnlU&quot; rel=&quot;nofollow&quot;&gt;SRB Recovery&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 first stage is much lighter and less structurally sound. Additionally, it would come to a 'stop' relative to the surface and then fall down, somehow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the soft landing attempts SpaceX made before they had an ASDS they reported that the stage survived landing and was destroyed by wave action once on the surface, which indicates it likely was floating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does seem that the fall from standing tall, on the surface, down to the surface likely did much of the damage as well.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-05-09T15:51:58.000" LastActivityDate="2016-05-09T15:51:58.000" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15201" PostTypeId="2" ParentId="15197" CreationDate="2016-05-09T14:41:00.853" Score="1" Body="&lt;p&gt;It is likely, that as they traverse from the realm of totally experimental rocket techniques to more regular operations that FAA may actually want them to have lights.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You know, in case an airplane is flying by, and cannot see the 70,000 lb thrust engine firing. It can be mighty inconspicuous, to those who wish to ignore it. After all, not like any light can stand out next to a rocket engine exhaust light.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-09T14:41:00.853" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15202" PostTypeId="2" ParentId="15198" CreationDate="2016-05-09T15:10:20.210" Score="1" Body="&lt;p&gt;The Falcon 9 would survive partially, but would be in pieces. I have heard rumor that the first ocean soft touchdowns of the Falcon 9 that parts of the booster survived, which were later tested when various issues arose with the rocket at a later date. However, the tanks themselves were gone, and I'm sure the engine was rusted to the point of not being easily usable. But it could survive, at least to some extent, as has been shown.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-09T15:10:20.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15203" PostTypeId="2" ParentId="15197" CreationDate="2016-05-09T15:20:57.730" Score="1" Body="&lt;p&gt;I suspect the lights are not for when the engine is firing, but when it is not. Keep in mind that the rocket booster isn't being lit the entire fall, but only for the critical portions of said fall. The rocket falls through the atmosphere for some time without having the rocket firing, a blinking light would make a lot of sense for that situation. From the photo below, I think the light can actually be seen without the legs being deployed, making it all the more likely that it is the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/q3MOZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/q3MOZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-09T15:20:57.730" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15204" PostTypeId="1" CreationDate="2016-05-09T18:06:12.967" Score="3" ViewCount="158" Body="&lt;p&gt;Could one use a space probe to orbit the sun close enough to take measurements of angular / spherical gravitational field variations similar to what Juno plans to do for Jupiter? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the sun rotates on an axis, there would be some bulging of mass at the equator. And since density varies with the distance from the center of the sun, the effect of rotation might have varying effects depending on this distance which might affect the speed of a circumpolar orbiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much insight into this density versus radius function can one obtain from gravitational measurements alone taken by such a circumpolar orbiter? I ask knowing that if the sun were not rotating and spherically symmetrical, then there would be no angular gravity field variations even though the density would still vary versus radius - at least from a Newtonian viewpoint (not sure if this would still be true from a GR standpoint).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, as a trivia question which space probe has the distinction of passing closest to the sun and survive? Messenger?&lt;/p&gt;&#xA;" OwnerUserId="11095" LastEditorUserId="11095" LastEditDate="2016-05-09T18:14:01.180" LastActivityDate="2016-05-10T22:36:22.723" Title="Using a probe to take gravitational measurements of the interior of the sun" Tags="|probe|gravity|the-sun|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15205" PostTypeId="2" ParentId="15195" CreationDate="2016-05-09T18:09:07.033" Score="1" Body="&lt;p&gt;The rocket and ASDS use absolute GPS to share a joint location. The ASDS stays where it is supposed to be, and the rocket lands where the ASDS is supposed to be.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-09T18:09:07.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15206" PostTypeId="2" ParentId="15198" CreationDate="2016-05-09T19:39:49.267" Score="0" Body="&lt;p&gt;Even if it stays afloat the engines will be wrecked by the salt water.  The recovery value would be pretty low.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-05-09T19:39:49.267" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15207" PostTypeId="2" ParentId="15204" CreationDate="2016-05-09T22:22:11.480" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://solarsystem.nasa.gov/missions/helios_02/indepth&quot; rel=&quot;nofollow&quot;&gt;Helios 2&lt;/a&gt; got the closest so far, at 0.29 AU. &lt;a href=&quot;http://solarprobe.jhuapl.edu/mission/index.php&quot; rel=&quot;nofollow&quot;&gt;Solar Probe Plus&lt;/a&gt; will get much closer, if all goes well, down to 0.04 AU. Though only for a short time at each perihelion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that the Sun is extremely close to spherical, combined with the fact that the effect of even the first non-spherical gravitational parameter, $J_2$, falls off with the fourth power of distance, combined with the fact that you get fried when you get close, makes such measurements extremely difficult. This is further complicated, in a good way, by the fact that general relativity enters into the picture this deep in the Sun's gravitational field, so you need to measure other hard-to-measure parameters as well. Your experiment ends up being a General Relativity test.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tracking of Mercury while MESSENGER was there &lt;a href=&quot;http://arxiv.org/pdf/1306.5569.pdf&quot; rel=&quot;nofollow&quot;&gt;improved our knowledge of the Sun's $J_2$ and put more stringent limits on the General Relativity parameters $\gamma$ and $\beta$&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-10T04:12:37.970" LastActivityDate="2016-05-10T04:12:37.970" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15208" PostTypeId="2" ParentId="15107" CreationDate="2016-05-10T03:18:15.850" Score="1" Body="&lt;p&gt;It was a little bit like pulling teeth, but I have data now. I started &lt;a href=&quot;https://github.com/AlanCoding/Spheres-of-Influence&quot; rel=&quot;nofollow&quot;&gt;a github repo&lt;/a&gt; for this data sanitizer and for what I'm planning on doing with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are certainly caveats to this data. Obviously not all orbits are circular, and not all moons are very sphere-like. But sometimes you just want to strangle your computer yelling &lt;em&gt;I don't care if it's right, just make it machine-readable already!&lt;/em&gt; In this list, you got the moon name in a row and the planet it orbits. Now that I look at it, I really should not have bothered including the numbering of the moon. If you wanted the order, you would just sort the list by &lt;em&gt;a&lt;/em&gt; for that planet. I guess I forgot to add the planet names too, but that shouldn't be difficult to figure out. Determining what object each planet is orbiting is also an exercise left to the reader.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&#xA; --planet list--&#xA;Diameter (km)   Orbital Inclination (degrees)   Distance from Sun (106 km)  Mass (1024kg)&#xA;4879.0  7.0 57.9    0.33&#xA;12104.0 3.4 108.2   4.87&#xA;12756.0 0.0 149.6   5.97&#xA;3475.0  5.1 0.384   0.073&#xA;6792.0  1.9 227.9   0.642&#xA;142984.0    1.3 778.6   1898.0&#xA;120536.0    2.5 1433.5  568.0&#xA;51118.0 0.8 2872.5  86.8&#xA;49528.0 1.8 4495.1  102.0&#xA;2370.0  17.2    5906.4  0.0146&#xA;&#xA;--moon data--&#xA;moon_name   planet_name a   dia number  r   rho GM  albedo&#xA;Moon    Earth   3476    27.322  0   1737.5  3.344   4902.801    0.12&#xA;Phobos  Mars    24.0    0.319   1   11.1    1.872   0.0007112   0.071&#xA;Deimos  Mars    8   1.263   0   6.2 1.471   0.0000985   0.068&#xA;Hegemone    Jupiter 3   781.6   22  1.5 2.6 0.000003    0.04&#xA;Adrastea    Jupiter 21.0    0.298   0   8.2 3   0.0005  0.1&#xA;S/2003  Jupiter 2   732.44  57  1   2.6 0.000001    0.04&#xA;S/2000  Jupiter 4   284.3   44  1   2.6 0.000001    0.04&#xA;Thyone  Jupiter 4   615 62  2   2.6 0.000006    0.04&#xA;Pasiphae    Jupiter 36  735 39  30  2.6 0.02    0.04&#xA;Eurydome    Jupiter 3   713 19  1.5 2.6 0.000003    0.04&#xA;Themisto    Jupiter 8   130.07  61  4   2.6 0.000046    0.04&#xA;Orthosie    Jupiter 2   617 38  1   2.6 0.000001    0.04&#xA;Hermippe    Jupiter 4   630 24  2   2.6 0.000006    0.04&#xA;Cyllene Jupiter 2   737.8   12  1   2.6 0.000001    0.04&#xA;Autonoe Jupiter 4   753 6   2   2.6 0.000006    0.04&#xA;Thelxinoe   Jupiter 2   628.09  60  1   2.6 0.000001    0.04&#xA;Europa  Jupiter 3126    3.551   18  1560.8  3.013   3202.739    0.67&#xA;Kore    Jupiter 2   779.2   32  1   2.6 0.000001    0.04&#xA;Helike  Jupiter 4   233.8   23  2   2.6 0.000006    0.04&#xA;Sinope  Jupiter 28  758 42  19  2.6 0.005   0.04&#xA;Sponde  Jupiter 2   732 43  1   2.6 0.000001    0.04&#xA;Carme   Jupiter 30  692 8   23  2.6 0.0088  0.04&#xA;Himalia Jupiter 170 250.57  25  85  2.6 0.45    0.04&#xA;Praxidike   Jupiter 6.8 632 41  3.4 2.6 0.000029    0.04&#xA;Chaldene    Jupiter 3.8 741 11  1.9 2.6 0.000005    0.04&#xA;Metis   Jupiter 40  0.295   36  21.5    3   0.008   0.061&#xA;Arche   Jupiter 3   715.6   5   1.5 2.6 0.000003    0.04&#xA;Amalthea    Jupiter 198.0   0.498   2   83.45   0.849   0.138   0.090&#xA;Eukelade    Jupiter 4   746.4   16  2   2.6 0.000006    0.04&#xA;Leda    Jupiter 10  238.72  33  10  2.6 0.00073 0.04&#xA;Taygete Jupiter 5   687 58  2.5 2.6 0.000011    0.04&#xA;Elara   Jupiter 80  259.65  13  43  2.6 0.058   0.04&#xA;Thebe   Jupiter 100 0.675   59  49.3    3   0.1 0.047&#xA;Ananke  Jupiter 20  631 3   14  2.6 0.002   0.04&#xA;Mneme   Jupiter 2   620.04  37  1   2.6 0.000001    0.04&#xA;Carpo   Jupiter 3   456.5   10  1.5 2.6 0.000003    0.04&#xA;Iocaste Jupiter 5.2 657 27  2.6 2.6 0.000013    0.04&#xA;Lysithea    Jupiter 24  259.22  34  18  2.6 0.0042  0.04&#xA;Kale    Jupiter 2   609 29  1   2.6 0.000001    0.04&#xA;Harpalyke   Jupiter 4.3 595 21  2.2 2.6 0.000008    0.04&#xA;Ganymede    Jupiter 5276    7.155   20  2631.2  1.942   9887.834    0.43&#xA;Aitne   Jupiter 3   736 1   1.5 2.6 0.000003    0.04&#xA;Euporie Jupiter 2   534 17  1   2.6 0.000001    0.04&#xA;Callisto    Jupiter 4800    16.689  7   2410.3  1.834   7179.289    0.17&#xA;Pasithee    Jupiter 2   715 40  1   2.6 0.000001    0.04&#xA;Aoede   Jupiter 4   748.8   4   2   2.6 0.000006    0.04&#xA;Io  Jupiter 3629    1.769   26  1821.6  3.528   5959.916    0.63&#xA;Kalyke  Jupiter 5.2 760 31  2.6 2.6 0.000013    0.04&#xA;Kallichore  Jupiter 2   683 30  1   2.6 0.000001    0.04&#xA;Callirrhoe  Jupiter 10  774 9   4.3 2.6 0.000058    0.04&#xA;Erinome Jupiter 3.2 672 14  1.6 2.6 0.000003    0.04&#xA;Euanthe Jupiter 3   622 15  1.5 2.6 0.000003    0.04&#xA;S/2006  Saturn  6   1142    50  2.5 2.3 0   0.04&#xA;Narvi   Saturn  8   956.2   31  3.5 2.3 0.000023    0.04&#xA;Suttungr    Saturn  7   1016.8  44  3.5 2.3 0.000014    0.06&#xA;Titan   Saturn  5150    15.945  58  2574.73 1.882   8978.1382   0.2&#xA;Siarnaq Saturn  45  893.07  40  20  2.3 0.0026  0.06&#xA;Pandora Saturn  86.0    0.629   35  40.7    0.490   0.00924 0.5&#xA;S/2007  Saturn  5   1100    52  2   2.3 0   0.04&#xA;Mundilfari  Saturn  7   951.38  30  3.5 2.3 0.000014    0.06&#xA;Bergelmir   Saturn  6   1005.90 5   3   2.3 0   0.04&#xA;Epimetheus  Saturn  124.0   0.694   11  58.1    0.640   0.0351  0.73&#xA;S/2004  Saturn  4   986 48  2   2.3 0   0.04&#xA;Methone Saturn  3   1.01    28  1.6 0.5 0.0000006   ?&#xA;Tarvos  Saturn  16  925.7   54  7.5 2.3 0.00018 0.06&#xA;Atlas   Saturn  32.0    0.602   3   15.1    0.460   0.00044 0.4&#xA;Skoll   Saturn  6   878.3   42  3   2.3 0   0.04&#xA;Pan Saturn  19.32   0.575   34  14.1    0.420   0.00033 0.5&#xA;Kiviuq  Saturn  17  449.22  26  8   2.3 0.00022 0.06&#xA;Loge    Saturn  6   1312.00 27  3   2.3 0   0.04&#xA;Ijiraq  Saturn  14  451.48  22  6   2.3 0.00008 0.06&#xA;Bebhionn    Saturn  6   834.8   4   3   2.3 0   0.04&#xA;Fornjot Saturn  6   1490.90 15  3   2.3 0   0.04&#xA;Polydeuces  Saturn  4   2.74    37  1.3 0.5 0.0000003   ?&#xA;Prometheus  Saturn  108.0   0.613   38  43.1    0.480   0.01074 0.6&#xA;Anthe   Saturn  1   1.04    2   0.9 0.5 0.0000001   ?&#xA;Farbauti    Saturn  5   1086.10 13  2.5 2.3 0   0.04&#xA;Tarqeq  Saturn  7   887.5   53  3   2.3 0   0.04&#xA;Greip   Saturn  6   921.2   16  3   2.3 0   0.04&#xA;Helene  Saturn  32.0    2.737   18  17.6    0.5 0.00076 1.67&#xA;Calypso Saturn  23.0    1.888   7   10.7    0.5 0.00017 1.34&#xA;Fenrir  Saturn  4   1260.30 14  2   2.3 0   0.04&#xA;Pallene Saturn  4   1.14    33  2.5 0.5 0.0000022   ?&#xA;Thrymr  Saturn  7   1088.89 57  3.5 2.3 0.000014    0.06&#xA;Paaliaq Saturn  25  686.92  32  11  2.3 0.00055 0.06&#xA;Rhea    Saturn  1528    4.518   39  764.30  1.233   153.9426    0.949&#xA;Ymir    Saturn  20  1312.37 59  9   2.3 0.00033 0.06&#xA;Janus   Saturn  172.0   0.695   23  89.5    0.630   0.1263  0.71&#xA;Phoebe  Saturn  220 550.48  36  106.50  1.638   0.5532  0.081&#xA;Jarnsaxa    Saturn  6   964.7   24  3   2.3 0   0.04&#xA;Enceladus   Saturn  498 1.37    10  252.10  1.608   7.2027  1.375&#xA;Hyperion    Saturn  293.0   21.277  19  135.00  0.544   0.3727  0.3&#xA;Skathi  Saturn  8   728.93  41  4   2.3 0.000021    0.06&#xA;Telesto Saturn  23.0    1.888   55  12.4    0.5 0.00027 1&#xA;Bestla  Saturn  7   1083.60 6   3.5 2.3 0   0.04&#xA;Daphnis Saturn  7   0.594   8   3.8 0.340   0.0000052   ?&#xA;Dione   Saturn  1120    2.737   9   561.70  1.476   73.1146 0.998&#xA;Tethys  Saturn  1060    1.888   56  533.00  0.973   41.2067 1.229&#xA;Surtur  Saturn  6   1297.70 43  3   2.3 0   0.04&#xA;Hati    Saturn  6   1038.70 17  3   2.3 0   0.04&#xA;Aegir   Saturn  6   1116.50 0   3   2.3 0   0.04&#xA;Albiorix    Saturn  30  783 1   16  2.3 0.0014  0.06&#xA;Kari    Saturn  7   1233.60 25  3   2.3 0   0.04&#xA;Mimas   Saturn  398 0.942   29  198.20  1.150   2.5026  0.962&#xA;Iapetus Saturn  1436    79.3215 21  735.60  1.083   120.5038    0.6&#xA;Ariel   Uranus  1160    2.52    0   578.9   1.592   86.4    0.39&#xA;Caliban Uranus  80  579.5   3   36  1.5 0.02    0.04&#xA;Cupid   Uranus  12  0.618   6   9   1.3 0.0002  0.070&#xA;Ferdinand   Uranus  21  2823.40 8   10  1.5 0.00042 0.04&#xA;Oberon  Uranus  1526    13.463  14  761.4   1.559   192.4   0.23&#xA;Cressida    Uranus  62  0.464   5   41  1.3 0.0229  0.069&#xA;Portia  Uranus  108 0.513   17  70  1.3 0.1122  0.069&#xA;Francisco   Uranus  22  266.6   9   11  1.5 0.00056 0.04&#xA;Ophelia Uranus  30.4    0.3764  15  21.4    1.3 0.0036  0.07&#xA;Bianca  Uranus  42  0.433   2   27  1.3 0.0062  0.065&#xA;Margaret    Uranus  11  1694.80 12  10  1.5 0.00042 0.04&#xA;Rosalind    Uranus  54  0.558   20  36  1.3 0.017   0.072&#xA;Mab Uranus  16  0.923   11  12  1.3 0.0006  0.103&#xA;Stephano    Uranus  32  677.4   22  16  1.5 0.0017  0.04&#xA;Belinda Uranus  66  0.624   1   45  1.3 0.0238  0.067&#xA;Miranda Uranus  472 1.414   13  235.8   1.214   4.4 0.32&#xA;Sycorax Uranus  160 1283.39 23  75  1.5 0.18    0.04&#xA;Prospero    Uranus  30  5.346   18  25  1.5 0.0066  0.04&#xA;Setebos Uranus  47  2234.80 21  24  1.5 0.0058  0.04&#xA;Cordelia    Uranus  26  0.335   4   20.1    1.3 0.003   0.07&#xA;Titania Uranus  1578    8.706   24  788.9   1.662   228.2   0.27&#xA;Perdita Uranus  20  0.638   16  13  1.3 0.0012  0.070&#xA;Trinculo    Uranus  10  759 25  9   1.5 0.00031 0.04&#xA;Puck    Uranus  154 0.762   19  81  1.3 0.1931  0.104&#xA;Umbriel Uranus  1190    4.144   26  584.7   1.459   81.5    0.21&#xA;Desdemona   Uranus  54  0.474   7   35  1.3 0.0119  0.084&#xA;Juliet  Uranus  84  0.493   10  53  1.3 0.0372  0.075&#xA;Triton  Neptune 2705    5.877   12  1353.4  2.059   1427.6  0.719&#xA;Nereid  Neptune 340 360.16  6   170.    1.5 2.06    0.155&#xA;Thalassa    Neptune 90  0.31    11  41. 1.3 0.025   0.091&#xA;Sao Neptune 38  2925.60 10  22  1.5 0.0045  0.04&#xA;Halimede    Neptune 60  1874.83 2   31  1.5 0.012   0.04&#xA;Laomedeia   Neptune 38  2980.40 4   21  1.5 0.0039  0.04&#xA;Naiad   Neptune 50  0.3 5   33. 1.3 0.013   0.072&#xA;Psamathe    Neptune 38  9136.11 9   20  1.5 0.0033  0.04&#xA;Despina Neptune 160 0.4 0   75. 1.3 0.14    0.09&#xA;Proteus Neptune 420 1.12    8   210.    1.3 3.36    0.096&#xA;Galatea Neptune 140 0.33    1   88. 1.3 0.25    0.079&#xA;Neso    Neptune 60  9007.10 7   30  1.5 0.011   0.04&#xA;Larissa Neptune 200 0.56    3   97. 1.3 0.33    0.091&#xA;Charon  Pluto   1207    6.387   0   603.6   1.664   102.3   0.372&#xA;Nix Pluto   87.0    24.856  1   23.0    2.1 0.0013  0.35&#xA;Hydra   Pluto   87.0    38.206  2   30.5    0.8 0.0065  0.35&#xA;&lt;/pre&gt;&#xA;" OwnerUserId="26" LastActivityDate="2016-05-10T03:18:15.850" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15209" PostTypeId="1" AcceptedAnswerId="15211" CreationDate="2016-05-10T09:22:29.670" Score="16" ViewCount="2341" Body="&lt;p&gt;Given the fact that the Boosters of the Falcon Heavy will separate much earlier during ascent (and hence at much lower speeds), there should be a notable difference in the complexity (and thus chance of success) of landing one or both of the boosters successfully than the actual first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the first stage shall be reused as well. So I wonder how the difficulties of both separated parts compare to the (seemingly somewhat working) Falcon 9 landing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the Falcon Heavy booster so much simpler to land than the first stage of a Falcon 9 or is the Falcon Heavy's first stage much harder to land than the one from its smaller cousin?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words: Where do you put the chance of successful landings of one rocket when p is the probability to land one Falcon 9 stage successfully: More, equal to or less than p?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="51" LastEditDate="2016-06-03T19:17:23.980" LastActivityDate="2018-02-13T20:59:06.773" Title="Is the Falcon Heavy booster much simpler to land than the Falcon 9 first stage?" Tags="|launch|spacex|falcon-9|falcon-heavy|recovery|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15210" PostTypeId="2" ParentId="15209" CreationDate="2016-05-10T10:13:19.340" Score="9" Body="&lt;p&gt;SpaceX is planning on the side boosters mostly doing a Return to Launch Site (RTLS) landing unless the payload is really heavy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The penalty for an RTLS for the center core is very high, so most missions will land the center core downrange on an ASDS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what is nice about the Falcon Heavy design is that they have a number of ways of scaling a launch, not based on strap on boosters (like an Atlas 5 or Delta) but by how they recover the boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the highest payloads they may have to expend the center core at the extreme end of the scale &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-10T10:13:19.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15211" PostTypeId="2" ParentId="15209" CreationDate="2016-05-10T10:14:06.523" Score="15" Body="&lt;p&gt;The FH side boosters should be almost equivalent to the F9 first stage (at least the &lt;em&gt;Block 5&lt;/em&gt; ones) - the difference will be mostly in mount points and the nosecone instead of the interstage. The center core is a bit more different with reinforcements to handle the forces.  So from hardware standpoint the landings are the same thing. (One of the probable reasons for FH delay is to verify the landings before trying to land 3 boosters at once.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The side boosters landing should be quite comparable to small-payload-to-LEO RTLS landing - the one done in December (Orbcomm). The booster has the same fuel capacity so the burn time will be similar and &lt;a href=&quot;https://en.wikipedia.org/wiki/Booster_%28rocketry%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;BECO&lt;/a&gt; will therefore probably happen somewhere around T+2:30 mark depending on booster throttling needed at MaxQ and the fuel reserves for the RTLS. But because of the heavy payload of throttled-down core the velocity and distance from the launch site will be probably lower so lower margins are needed for the &lt;em&gt;boostback&lt;/em&gt; burn. In the end the landing should be quite similar to the one RTLS which was tried and successful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The center core on the other hand will be going farther away and faster at MECO so possibly even with a boostback it will go through similar profile as current GEO/GTO launches which currently have 50% success from the 2 tries (SES-9 and JCAST-14).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I would say that landing the side boosters has probability at least of the (currently 1/1) F9 RTLS and the core booster similar (slightly smaller than) the probability of GTO barge landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Edit now when FH exists and should launch soon:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the center core - there were more succesful landings from GTO using 1-3-1 landing burns and there was another (most probably succesful - &lt;a href=&quot;https://twitter.com/elonmusk/status/958847818583584768&quot; rel=&quot;nofollow noreferrer&quot;&gt;expendable booster survived&lt;/a&gt;) test of the 3-engine landing burn all the way down (the previous known test on SES-9 made a hole to the ASDS, lateer succesful GTO landings possibly used 3-1 or 1-3-1 landing burns? not sure if there was any other full 3-engines landing). So I would say the probability of the center core landing is quite high as SpaceX is testing and polishing these profiles. For the first launch even more so as the ASDS position seems to be quite a bit closer to the land than on standard GTO launches, suggesting somehow lofted trajectory for the interplanetary launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About the side boosters - for the test flight they got the titanium grid fins where the (expected to be faster and &quot;hotter&quot;) center core has aluminum ones - a suggested reason being &lt;a href=&quot;https://space.stackexchange.com/a/24110&quot;&gt;the nose cone being shorter and aerodynamically different than the interstage making the grid fins less effective&lt;/a&gt;. So the boosters will be a bit more different from normal F9 than I previously expected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;After the first FH launch:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, statistics and reality.. The side boosters are fine, the center core is not, somehow &lt;a href=&quot;https://twitter.com/elonmusk/status/963107229523038211&quot; rel=&quot;nofollow noreferrer&quot;&gt;the igniter fluid reserve was not enough this time&lt;/a&gt; - so a new failure mode after all these tests. Currently not publicly known if bad valve, miscalculation or something FH-center-core specific.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2018-02-13T20:59:06.773" LastActivityDate="2018-02-13T20:59:06.773" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15212" PostTypeId="1" AcceptedAnswerId="15213" CreationDate="2016-05-10T13:44:10.500" Score="12" ViewCount="1589" Body="&lt;p&gt;I found this &lt;a href=&quot;http://video8.ru/ntv-luna.php&quot;&gt;page&lt;/a&gt; where Evgeny Molotov, former head of the Research institute of Space instruments, plays supposed &lt;a href=&quot;https://yadi.sk/d/tVjcJEfcXJZVm&quot;&gt;intercepted communication of unknown Apollo mission&lt;/a&gt;. Can anyone identify from which Apollo mission that audio sample is?&lt;/p&gt;&#xA;" OwnerUserId="6179" LastEditorUserId="415" LastEditDate="2016-05-10T21:25:21.613" LastActivityDate="2016-05-11T04:22:04.870" Title="From which Apollo mission is this audio sample?" Tags="|the-moon|apollo-program|communication|russia|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15213" PostTypeId="2" ParentId="15212" CreationDate="2016-05-10T14:02:43.523" Score="16" Body="&lt;p&gt;NASA's Lunar Surface Journal collection has annotated transcripts of all the Apollo landing missions, so I just picked out a couple of key phrases from the recording and googled for: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;apollo surface transcript &quot;nothing flat&quot; &quot;reasonable to go&quot;&lt;/code&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, I believe that's &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a12/a12.posteva1.html&quot; rel=&quot;nofollow&quot;&gt;Apollo 12&lt;/a&gt;, starting at 121:27:45:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;121:27:45 Conrad: Okay. Now, in looking at the map, we got all the way over to... If you go to, what is it, the general map, map 5, or whatever you want to call them, we got over in that Shelf Crater (Middle Crescent Crater). That's where you sent us, and we got to that fellow, so some of that stuff we picked up might be of that Copernican ray material. We also had photographs down there of that shelf, which everybody thought was interesting. I took a set of stereos in that thing, all the way around that big crater. Now, we made it over there with no strain. Matter of fact, we ran over and ran back in nothing flat. So, I think it's reasonable to go as you have indicated. Which would be one, starting at F, which is right in front of the spacecraft, then going to Sharp, then going to Bench, then to Halo, then to the Surveyor crater, then to Block, and back to the spacecraft. How's that sound?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The recording is of the moon-to-Earth side only (as you would expect in this case); in the transcript, Gibson is the Earth-side CAPCOM, not heard in the recording. (Pete) Conrad is the commander and (Alan) Bean is the LM pilot. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point in the mission, they've completed their first EVA on the surface of the moon, and are planning the route for their second EVA and discussing various complications, like water leaking into Conrad's suit and pooling in his boots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even without searchable transcripts, the mention of &quot;the Surveyor crater&quot; points to Apollo 12 (they landed a few hundred meters from the site of the unmanned Surveyor 3 landing). &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-05-11T04:22:04.870" LastActivityDate="2016-05-11T04:22:04.870" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15214" PostTypeId="2" ParentId="6403" CreationDate="2016-05-10T18:46:58.403" Score="3" Body="&lt;p&gt;The recent landing (JCSAT-14) shows the Falcon 9 held on jack stands and chained to deck:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k3494.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k3494.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2016-05-10T18:46:58.403" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15215" PostTypeId="1" AcceptedAnswerId="15234" CreationDate="2016-05-10T19:03:13.617" Score="5" ViewCount="323" Body="&lt;p&gt;Would existing or upcoming telescopes (such as James Webb) be powerful enough to perform spectroscopy on an extra solar planet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or would this require technology that is far beyond our current or imminent technological capabilities e.g. optical interferometry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To constrain the answer further imagine we find an earth sized planet around alpha centauri b in the habitable zone - what would we need to see if free oxygen is present on the surface of the planet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for your time!&lt;/p&gt;&#xA;" OwnerUserId="13439" LastActivityDate="2016-05-11T18:40:13.093" Title="What kind of space telescope would be needed to detect oxygen in the atmosphere of a nearby extrasolar planet?" Tags="|space-telescope|extra-solar-object|kepler-telescope|james-webb-telescope|" AnswerCount="2" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15216" PostTypeId="2" ParentId="4435" CreationDate="2016-05-10T19:55:00.177" Score="0" Body="&lt;p&gt;Long range communication using HF (shortwave) probably not using ionospheric methods like on earth, however ground wave propagation would be possible if the antenna was aimed at the lunar horizon and the transmitter antenna was high enough to maximize line of sight. Using FM mode or digital mode would probably cut down on solar wind noise on point to point operations while SSB or AM would be used on communications with earth as those modes are not affected by Doppler shift. FM would be affected by Doppler shift. The preferred bands would be VHF and UHF because of antenna size. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the Moon ever had colonies on it they would communicate by using linked lunar-stationary satellites. What would be interesting is trying to listen to are AM/FM broadcast signals from Earth.&lt;/p&gt;&#xA;" OwnerUserId="14377" LastActivityDate="2016-05-10T19:55:00.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15217" PostTypeId="2" ParentId="7860" CreationDate="2016-05-10T20:19:39.437" Score="1" Body="&lt;p&gt;In this &lt;a href=&quot;https://twitter.com/SpaceX/status/718605741288894464/video/1&quot; rel=&quot;nofollow&quot;&gt;video&lt;/a&gt;, claiming a high-wind landing, the grid fins can be seen as they are moving until touchdown - probably they influence the orientation, albeit slightly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides that, the orientation is controlled by the motor vectoring and some horizontal thrusters, best seen &lt;a href=&quot;https://www.flickr.com/photos/spacex/23985921532/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; and in action in &lt;a href=&quot;https://www.youtube.com/watch?v=vmJK_v5wRZw&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combined, they provide enough control to counteract the effect of the wind.&lt;/p&gt;&#xA;" OwnerUserId="14363" LastEditorUserId="14363" LastEditDate="2016-05-11T08:18:41.470" LastActivityDate="2016-05-11T08:18:41.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15218" PostTypeId="1" CreationDate="2016-05-10T21:08:19.880" Score="16" ViewCount="1176" Body="&lt;p&gt;In March 2016 China's Manned Space Engineering Office announced that all Tiangong-1 telemetry had failed leaving no ability to safely control the space station's decent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With what we know about the station, and the last known telemetry data, is it possible to calculate the station's orbital decay? Do we have &lt;strong&gt;any idea&lt;/strong&gt; when Tiangong-1 will deorbit?  &lt;/p&gt;&#xA;" OwnerUserId="14380" LastEditorUserId="12102" LastEditDate="2017-07-19T01:41:09.060" LastActivityDate="2017-07-19T01:41:09.060" Title="Do we have any idea when Tiangong-1 will deorbit?" Tags="|spacecraft|reentry|space-station|tiangong-1|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15219" PostTypeId="2" ParentId="15218" CreationDate="2016-05-10T21:23:39.273" Score="15" Body="&lt;p&gt;This issue was discussed recently in &lt;a href=&quot;http://www.spacedaily.com/reports/Has_Tiangong_1_gone_rogue_999.html&quot;&gt;SpaceDaily&lt;/a&gt; where it explains why this is a known unknown:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Right now, it's impossible to make an educated guess on even a rough &quot;window&quot; for Tiangong's return. The orbit is still high.  Unpredictable factors such as solar activity will influence its orbital decay. We won't have an estimate until it starts to fall much lower.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2016-05-10T21:23:39.273" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15220" PostTypeId="2" ParentId="7860" CreationDate="2016-05-10T21:49:09.740" Score="4" Body="&lt;p&gt;First, its descent speed is very rapid, the deceleration very strong - only the last meter or so is probably below 10m/s. Until then the grid fins work fine. And for the remainder, there are the RCS thrusters. Plus the engine vectoring can keep it angled against the wind and only let RCS and the legs straighten it after touchdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTOH it's a 40-meter mostly empty thin tube on pretty short legs, and possibly on a swaying platform. Once it touches down and RCS is off, it can very much be tipped over by the wind. But if it can keep standing unpowered in the wind, it can stick the landing. It's not a very strong wind it can withstand, certainly not a storm, but the RCS thrusters can help it quite a bit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And of course if the wind is too strong, the launch will be delayed. It's not like in the several minutes since launch a totally unexpected storm will appear.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-05-10T21:49:09.740" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15221" PostTypeId="1" CreationDate="2016-05-10T21:53:35.563" Score="3" ViewCount="443" Body="&lt;p&gt;I'd like to understand how &lt;a href=&quot;https://en.wikipedia.org/wiki/Composite_material&quot; rel=&quot;nofollow noreferrer&quot;&gt;composite materials&lt;/a&gt; (CMs) are used on satellites and rovers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which parts are made of CMs? If there are numerous examples, then the ones which are particularly large, or where CM achieved some particularly important improvement would be the of interest.&lt;/p&gt;&#xA;" OwnerUserId="14314" LastEditorUserId="12102" LastEditDate="2018-10-10T03:33:53.797" LastActivityDate="2018-10-10T03:33:53.797" Title="How are composite materials most frequently or most prominently used in satellites and rovers?" Tags="|artificial-satellite|materials|rovers|composite-materials|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="15222" PostTypeId="2" ParentId="15204" CreationDate="2016-05-10T22:36:22.723" Score="3" Body="&lt;p&gt;To have such a probe is completeley unnecessary, since we have &lt;a href=&quot;https://en.wikipedia.org/wiki/Helioseismology&quot; rel=&quot;nofollow noreferrer&quot;&gt;Helioseismology&lt;/a&gt;.&lt;br&gt;&#xA;By observing the surface features of pressure waves that have a wavelength-dependent penetration depth into the sun itself, we can measure all the quantities you've just asked for.&lt;br&gt;&#xA;Oblatenes, rotation (the one you've mentioned in your question), differential rotation, density and temperature profiles have all been obtained using this technique.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A famous result is the rotational profile of the sun, that I'm just gonna quote from Wikipedia:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/C6MLR.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C6MLR.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;Here the rotational frequency $\Omega$ is plotted vs. discance from the stellar center $r$ in units of the solar radius $R$. The curves vary with the latitude as parameter and all join at $r/R \approx 0.7$ indicating that interior of this radius the sun rotates as solid body.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastActivityDate="2016-05-10T22:36:22.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15223" PostTypeId="1" CreationDate="2016-05-11T03:10:53.153" Score="2" ViewCount="311" Body="&lt;p&gt;Instead of deorbiting the second stage (particularly with Falcon Heavy) can they be sent into a common parking orbit, there must be a lot of useful materials, not to mention engines, that can be used in the construction of space structure. Modern fabrication techniques would aid here, 3D print etc.&#xA;The second stage could be boosted into a more stable orbit if it could be partially refuelled.   &lt;/p&gt;&#xA;" OwnerUserId="14383" LastEditorUserId="51" LastEditDate="2016-05-11T03:23:59.883" LastActivityDate="2016-05-12T08:55:53.040" Title="Sending the second stage into a parking orbit" Tags="|orbit|reuse|upper-stage|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15224" PostTypeId="1" CreationDate="2016-05-11T03:16:45.107" Score="17" ViewCount="1103" Body="&lt;p&gt;In the past I had encountered a few mentions of orbits being patented (or at least applications filed) but didn't take much notice. Then I did a simple search and was blown away by the sheer volume of activity - mostly USPTO but EPO as well.&lt;/p&gt;&#xA;&lt;p&gt;Is &lt;em&gt;spacecraft maneuver intellectual property&lt;/em&gt; actively traded &lt;em&gt;by entities actually involved in the space industry&lt;/em&gt;? Are orbits, or orbital maneuvers ever avoided - with deference to less optimal ones - in order to avoid paying royalties?  (...or are royalties ever paid?)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Bonus points:&lt;/strong&gt; If I patent a bunch of orbits or maneuvers, can I convince (leverage) entities to hire me as a consultant in exchange for not suing them? (humor - mostly)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-09-02T19:01:36.750" LastActivityDate="2022-12-08T16:32:45.300" Title="Spacecraft Maneuvers as Intellectual Property? Wow!" Tags="|orbital-mechanics|spacecraft|law|business|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="15225" PostTypeId="2" ParentId="15223" CreationDate="2016-05-11T03:23:42.750" Score="0" Body="&lt;p&gt;There is a parking orbit that GTO launches often send their upper stages, that avoids the GEO belt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However it is possible that you underestimate how big space is and how far apart everything is. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-11T03:23:42.750" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15226" PostTypeId="2" ParentId="15190" CreationDate="2016-05-11T03:45:22.973" Score="1" Body="&lt;p&gt;In addition to the C&lt;sub&gt;3&lt;/sub&gt; for any given pair of departure and arrival dates, you will also need to know the departure declination for that pair. (The Lambert solver you are using must be generating that as well.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You then want to optimize mass delivered to the interplanetary trajectory. If the departure declination is less than the latitude of the site, then the inclination of the parking orbit is the latitude of the site. There is no penalty due to the declination, so you can pick the minimum C&lt;sub&gt;3&lt;/sub&gt;. If the departure declination is greater than the latitude, then there is a mass penalty since you now have to go to an orbit inclination equal to the departure declination. You will need some way of estimating the mass penalty on the launch vehicle. Given that, you can optimize for mass. You may find that a higher C&lt;sub&gt;3&lt;/sub&gt; with a lower declination delivers more mass to the interplanetary trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the selected the departure conditions, the parking orbit velocity and radius can be used to easily compute the $\Delta V$ from that orbit to the departure C&lt;sub&gt;3&lt;/sub&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19840010158.pdf&quot; rel=&quot;nofollow&quot;&gt;This handbook&lt;/a&gt; provides more information on the departure geometry.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-05-11T03:45:22.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15227" PostTypeId="1" CreationDate="2016-05-11T05:52:43.370" Score="1" ViewCount="66" Body="&lt;p&gt;What is their underlying technology, and is it going to be cheaper like their Mars orbital mission?&lt;/p&gt;&#xA;" OwnerUserId="14384" LastEditorUserId="8693" LastEditDate="2016-05-11T05:59:50.570" LastActivityDate="2016-05-11T05:59:50.570" Title="What is the current status of India's RLV craft?" Tags="|space-shuttle|isro|" AnswerCount="0" CommentCount="2" ClosedDate="2016-05-11T11:08:15.950" ContentLicense="CC BY-SA 3.0" />
  <row Id="15228" PostTypeId="2" ParentId="15167" CreationDate="2016-05-11T06:08:52.793" Score="13" Body="&lt;p&gt;There are several reasons why spacecraft are sent into pseudo-orbits (they aren't actually &quot;orbits&quot;) about the unstable Lagrange points. The least important reason is that only one spacecraft can be at one of those Lagrange points. Wide pseudo-orbits allow multiple spacecraft to simultaneously operate in the vicinity of one of those Lagrange points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More importantly, those Lagrange points are a bit of a fiction, in a spherical cow sense. The Lagrange points are stationary only in the context of one body in a circular orbit about another more massive body. Add in elliptical orbits and the points move. Add in perturbations from other bodies such as the Moon in the case of the Sun-Earth Lagrange points, the Sun in the case of the Earth-Moon Lagrange points, plus Jupiter, Venus, and the other planets, and the exact location of the Lagrange points becomes a bit fuzzy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, suppose some organization wanted to have a spacecraft be at exactly one of those fuzzy points. That's not possible, for a number of reasons. The spacecraft's inertial measurement unit cannot sense that it's at a Lagrange point. It would instead have to be calculated. This calculated position (the guidance state) will inherently be in error. To make matters worse, where the spacecraft thinks it is (the navigated state) will also inherently be in error. To make matters worse yet, the propulsion and control systems are also imperfect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best that could be done would be to keep the spacecraft within some control volume surrounding the point. The control algorithm would necessarily involve deadbands, regions where the thrusters don't fire, for a couple of reasons. One is the nature of the thrusters on those vehicles, which are either on or off (they are not throttleable). This suggests using what is called bang-bang control. More importantly, the cameras or other instrumentation typically can't be used during thruster firings. Bang-bang control is optimal from both a control theory and operational perspective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Making the control volume too small will result in infeasibility. (It can't be done.) A small feasible control volume will result in a system that involves frequent thruster firings that consume large amounts of fuel and that make the science part of the mission worthless. A  somewhat larger control volume is what's needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This however misses the goal of being close to the Lagrange point. This approach also doesn't take advantage of orbital mechanics. A spacecraft in a pseudo-orbit about one of those Lagrange points explicitly does take advantage of orbital mechanics. While these pseudo-orbits are not stable, the stationkeeping requirements for them are vastly reduced compared those for a comparably sized control volume approach.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-05-12T11:22:32.310" LastActivityDate="2016-05-12T11:22:32.310" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15229" PostTypeId="1" CreationDate="2016-05-11T07:08:24.827" Score="10" ViewCount="655" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/questions/8779/how-does-the-falcon-9-know-where-the-landing-barge-pad-is&quot;&gt;this question&lt;/a&gt; we know the barge can be held at a set position with an accuracy of one meter or so. So the barge doesn't really move in the X and Y direction. This takes care of yaw as well. But that leaves 3 degrees of freedom: moving in the Z direction (up/down), pitch and roll.&lt;br&gt;&#xA;None of these can be easily compensated for with the current barge design. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the stage have features that would help compensate for this motion? As the deck can't be counted on to be horizontal, you run the risk of having one leg touch down first. For the Jason-3 mission, the landing attempt failed because the lockout on one of the legs failed: would this lockout be applied before landing, or after?&lt;br&gt;&#xA;Doing it before landing gives the opportunity to use the hydraulics to adjust the angle of the legs, at the cost of making the hydraulics more complicated.&lt;br&gt;&#xA;Compensating in the Z direction would require a landing altimeter on the stage to get accurate height readings just before landing (GPS being less accurate in the Z direction).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-06-03T19:17:11.900" Title="How does the Falcon 9 first stage land on the moving surface of the barge?" Tags="|spacex|falcon-9|landing|first-stage|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15231" PostTypeId="1" AcceptedAnswerId="64465" CreationDate="2016-05-11T08:57:42.070" Score="10" ViewCount="1162" Body="&lt;p&gt;The subsection &lt;a href=&quot;https://en.wikipedia.org/wiki/Stability_of_the_Solar_System#Mercury.E2.80.93Jupiter_1:1_perihelion-precession_resonance&quot; rel=&quot;noreferrer&quot;&gt;Mercury–Jupiter 1:1 perihelion-precession resonance&lt;/a&gt; in the Wikipedia article &lt;a href=&quot;https://en.wikipedia.org/wiki/Stability_of_the_Solar_System&quot; rel=&quot;noreferrer&quot;&gt;Stability of the Solar System&lt;/a&gt; links to the article &lt;a href=&quot;https://www.newscientist.com/article/dn13757-solar-system-could-go-haywire-before-the-sun-dies&quot; rel=&quot;noreferrer&quot;&gt;Solar system could go haywire before the Sun dies&lt;/a&gt; in the NewScientist, which references two articles that were still in press back in 2008 when it was published.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More time (both people-time and CPU-time) has passed. In 2016, is there still thought to be a 2% chance that when Jupiter increases Mercury's eccentricity beyond 0.6, it could get ejected?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I bring this up because the question of weather an orbit is &quot;stable&quot; or &quot;unstable&quot; always dances around the fact that many things are really in the middle ground - stable for months, or eons, but not necessarily actually &lt;strong&gt;stable&lt;/strong&gt; in the sense that two masses orbiting in an otherwise totally empty Euclidian non-relativistic universe would be gravitationally stable. Put a handful (or more) bodies in orbit and chaotic behavior can arise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'm looking for a good example of &quot;stability is an illusion&quot; loosely speaking, and Mercury is the best one I can think of at the minute - only I'm stuck in Wikipedia in 2008. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any further analysis?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="58" LastEditDate="2020-04-10T14:12:46.763" LastActivityDate="2024-01-06T03:16:15.417" Title="Is Mercury's orbit still considered potentially unstable (in the very long term)?" Tags="|orbital-mechanics|jupiter|celestial-mechanics|mercury|stability|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15232" PostTypeId="1" AcceptedAnswerId="15233" CreationDate="2016-05-11T08:59:02.897" Score="12" ViewCount="1557" Body="&lt;p&gt;I watched a few videos &quot;from the boosters&quot; of their suborbital flight with the Shuttle, and what I noticed is that after the boosters were jettisoned there was a phase of flight that was quite chaotic - and they didn't travel far apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can imagine given enough bad luck to enter the atmosphere at just the right angle they could crash into each other. Was that possible or was that apparent chaos just &lt;em&gt;apparent&lt;/em&gt;, but in fact calculated not to create such risk?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(on unrelated note, how did other multi-booster platforms fare in that respect?)&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-05-11T09:21:47.123" Title="Could the Shuttle boosters crash into each other?" Tags="|reentry|srb|failure|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15233" PostTypeId="2" ParentId="15232" CreationDate="2016-05-11T09:21:47.123" Score="13" Body="&lt;p&gt;There was certainly a risk that the boosters could collide with each other, once they separated they were essentially just big pieces of pipe. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The chance of it happening was extremely low though. At separation the boosters were moving at mach 4+, and as they were angled to clear the tank they were angled away from each other. The aerodynamic forces would be pushing them apart strongly at that point, and they would be pretty far apart by the time they slowed down. At some point the boosters would stop flying and start dropping, from then on random chance could conceivably have brought them together as they spun through the sky, but it would have been very unlikely for that to happen given the space they would have occupied. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-05-11T09:21:47.123" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15234" PostTypeId="2" ParentId="15215" CreationDate="2016-05-11T10:00:36.243" Score="2" Body="&lt;p&gt;Ok, I've done some more research and found a good review article by Sara Seager:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.pnas.org/content/111/35/12634.full.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.pnas.org/content/111/35/12634.full.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is my understanding:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Near future:&#xA;The TESS (transiting exoplanet surveying satellite) will discover a large number of exoplanets using the transiting technique (like kepler). Some of these may be good candidates for observation with the James Webb telescope. If we get really really really lucky we might find enough exoplanets that happen to transit their suns in a convenient way for us to image and do atmospheric spectroscopy. Its unlikely that enough/any data will be forthcoming to get any reasonable probability that biosignature gases are present.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many decades from now:&#xA;If this doesn't work (and it probably wont) then some form of direct imaging is required. Two technologies are discussed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Internal coronography - special optics in the space telescope which blank out the light from the star but not the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starshade designs - a shade is flown tens of thousands of km away from a telescope and positioned very accurately to block the light from the star but not the planet.&#xA;If this type of thing sounds difficult, expensive and time consuming its because it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Far future:&#xA;Build a massive telescope that can directly image an exoplanet. Not going to happen any time soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issue with this type of work is that a large number of sample planets are needed to be able to overcome the large number of false positives that are undoubtedly going to be discovered. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to answer my question, yes the JWST can perform exoplanet spectroscopy but its unlikely that it will give us any definitive answers. Optical interferometry will not be needed and the above descriptions of future technology should give us the answers we need. &lt;/p&gt;&#xA;" OwnerUserId="13439" LastActivityDate="2016-05-11T10:00:36.243" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15235" PostTypeId="1" AcceptedAnswerId="15239" CreationDate="2016-05-11T10:31:50.057" Score="10" ViewCount="1918" Body="&lt;p&gt;What is the mechanism of solar flares that causes degradation of spacecraft orbits? The kind that deorbited Skylab?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The proton storm is fairly &lt;em&gt;thin&lt;/em&gt;, its pressure shouldn't affect the kinetic energy. What other effects can cause that? Electrodynamic drag?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-05-11T22:50:30.190" Title="How do solar flares degrade orbits?" Tags="|orbital-decay|cme|solar-flares|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15236" PostTypeId="5" CreationDate="2016-05-11T10:39:37.143" Score="0" Body="" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-05-11T10:39:37.143" LastActivityDate="2016-05-11T10:39:37.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15237" PostTypeId="4" CreationDate="2016-05-11T10:39:37.143" Score="0" Body="Questions pertaining the mission of the Galileo probe studying Jupiter and its moons between 1995 and 2003." OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-05-11T10:39:37.143" LastActivityDate="2016-05-11T10:39:37.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15238" PostTypeId="2" ParentId="15235" CreationDate="2016-05-11T11:17:16.200" Score="8" Body="&lt;p&gt;It is an indirect effect; increased solar activity affects the Earth's atmosphere in such a way to cause a net density increase at a given orbital height. This increases the drag on objects in LEO. See the entry section of &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Skylab&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-05-11T22:50:30.190" LastActivityDate="2016-05-11T22:50:30.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15239" PostTypeId="2" ParentId="15235" CreationDate="2016-05-11T11:29:44.740" Score="12" Body="&lt;p&gt;Density of the proton wind coming from the Sun is few ten particles per cubic centimeter (atmospheric density is orders of magnitude higher in LEO), thus not influencing a satellite by drag. Actually, the proton flow could also add tiny amounts of speed to the satellite when hitting it from the back. &#xA;The solar storm is accompanied by a tiny magnetic field (up to 10 nT, less than Earth magnetic field) which could cause some electrodynamic drag, but not very much. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two possible ways how increased solar activity can degrade a orbit in indirect ways:&#xA;First, the protons (as far as they do penetrate the Earth magnetic field at all) can cause radiation influenced damage to on-board electronics. This could lead to a failure of the guidance system and subsequently to unplanned orbital maneuvers. Fortunately, these systems are usually redundant and therefore save from such errors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you explicitly mention Skylab - its faster reentry was due to increased solar activity over a longer period. The activity heats up the upper atmosphere and increases its density. Some nice plots can be found in this paper: &lt;a href=&quot;http://www.springer.com/cda/content/document/cda_downloaddocument/9783642251283-c2.pdf?SGWID=0-0-45-1269313-p174259871&quot;&gt;Empirical Modeling of the Thermosphere&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, solar activity can change the density of the medium in LEO by more than a factor 10. Any increase in density proportionally increases the drag and decreases orbital lifetime. It seems that in the Skylab case the estimate of atmospheric density was about half of the actual density, therefore halving the time before a reboost of the station was necessary.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-05-11T11:29:44.740" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15240" PostTypeId="1" AcceptedAnswerId="15243" CreationDate="2016-05-11T11:48:07.780" Score="2" ViewCount="179" Body="&lt;p&gt;When thinking about velocity I could imagine that the ISS getting hit by some space junk (let it be just a plate of 1 meter in span) with a velocity of some 100 meters per second could be quite fatal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is this actually handled?&#xA;are there some &quot;defensive&quot; systems installed against this kinds of collisions?&#xA;Or are there just regions that are considered to leave the junk in so it never is interfering with the orbit of the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the latter is the case, how is the launching of space vehicles of states like northkorea handled? I could imagine that northkorea wouldn't care much about such convetions. Not to say they might even be happy about &quot;accidently&quot; hitting the ISS with some space junk.&lt;/p&gt;&#xA;" OwnerUserId="11099" LastEditorUserId="25" LastEditDate="2016-05-11T16:02:43.967" LastActivityDate="2016-05-11T16:02:43.967" Title="What actions are performed to prevent i.e. ISS from getting hit by space junk?" Tags="|iss|debris|collision-avoidance|" AnswerCount="2" CommentCount="0" ClosedDate="2016-05-11T13:54:39.867" ContentLicense="CC BY-SA 3.0" />
  <row Id="15241" PostTypeId="2" ParentId="15229" CreationDate="2016-05-11T11:58:21.343" Score="9" Body="&lt;p&gt;The barge is fairly long (300 ft?) and is held pointed into the waves. As a consequence for it to pitch significantly, it needs a wave longer than itself. It is rare to have waves that large in conditions that are safe enough to land. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus it mostly rides the crests, and apparently stays fairly stable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The legs have some bounce in them, as has been seen in the two landings, and can accommodate the slightly moving deck.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Thaicomm-8 mission we saw the landing come in at something of an angle, and over extend one of the legs. Elon tweeted that there is an aluminum honeycomb crush core, used as a contingency to handle excessive movement. It also means it does not recover back, as the list of the landed stage showed as it came into port.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-06-03T19:17:11.900" LastActivityDate="2016-06-03T19:17:11.900" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15242" PostTypeId="2" ParentId="15240" CreationDate="2016-05-11T13:19:37.283" Score="1" Body="&lt;p&gt;Large pieces of debris are tracked from the ground using radar, and the ISS moves to avoid them. &lt;a href=&quot;http://arstechnica.com/science/2013/07/how-nasa-steers-the-international-space-station-around-space-junk/&quot; rel=&quot;nofollow&quot;&gt;(Ars Technica)&lt;/a&gt; &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/news/orbital_debris.html&quot; rel=&quot;nofollow&quot;&gt;(NASA)&lt;/a&gt;. This isn't automatic or spontaneous - these maneuvers are carefully planned from the ground. Also, your hypothetical 1m plate would be devastating to the ISS - anything bigger than a fist-sized rock can cause major damage if it hits the wrong part of the station. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Smaller pieces of debris can't be tracked, but the ISS is designed to survive impacts with most of them. I imagine (but couldn't find any hard sources) that the ISS is designed with redundant systems, and that parts of it can be sealed off in case of major damage. &lt;/p&gt;&#xA;" OwnerUserId="3063" LastActivityDate="2016-05-11T13:19:37.283" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15243" PostTypeId="2" ParentId="15240" CreationDate="2016-05-11T13:26:55.557" Score="5" Body="&lt;p&gt;The actual speed of impacts can be up to 14 km/s, it is something quite serious, even for small issues. There are a number of procedures taken to prevent such thing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first level is known as the &quot;Big Space&quot; theorem, which is that space is really big, and there isn't that many objects. We are getting to the point where this isn't good enough overall. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second layer is to detect such objects. There are a number of systems that can do this, the best is the system provided by the US military, called JSpoc, or the public outreach called &lt;a href=&quot;https://www.space-track.org/&quot;&gt;Space Track&lt;/a&gt;. They monitor all objects, and will inform parties, both commercial and government, when a predicted impact might occur. These can be predicted days in advance, and allow an object such as the International Space Station to flee a potential collision. This works for objects larger than 10 cm. These sometimes occur too late to be of help, in which case additional actions are taken, see &lt;a href=&quot;http://www.nytimes.com/2011/06/29/science/space/29junk.html?_r=0&quot;&gt;this article&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third layer is to protect the space around the space station. This is done primarily for the objects visiting the space station, which are launched into a non-intersecting orbit and slowly transition to the final orbit, to reduce the debris in the area. NASA also requests coordination with commercial entities operating near the space station to ensure they are not going to impact the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another layer is provided by selecting an altitude where the space station is relatively low, reducing the lifetime of debris in the area to only a year or two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, there is shielding on the space station that will prevent a small item, say a paint flake or grain of sand sized object, from damaging the station. I understand it works until about a 1 cm object. The shielding is called a &quot;Whipple Shield&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this being said, there is still a substantial risk of damage. Take a look at this &lt;a href=&quot;http://www.nasa.gov/externalflash/iss_impact_risk/&quot;&gt;graphic&lt;/a&gt;, which shows the relative chance of impact. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-11T13:26:55.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15244" PostTypeId="2" ParentId="15215" CreationDate="2016-05-11T18:40:13.093" Score="1" Body="&lt;p&gt;JWST is &lt;a href=&quot;http://jwst.nasa.gov/nirspec.html&quot; rel=&quot;nofollow&quot;&gt;designed to do that&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kepler is biased towards large planets close to their star. Those are (thought thought) not suitable for life, thus TESS and WFIRST are designed specifically to look for smaller worlds that in the habitable zone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;JWST and TESS are scheduled to be launched just a year apart (2018 &amp;amp; 2017 respectively) with WFIRST scheduled for mid 2020s which will hopefully overlap JWST for a few years.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2016-05-11T18:40:13.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15246" PostTypeId="2" ParentId="15025" CreationDate="2016-05-11T19:30:41.467" Score="7" Body="&lt;p&gt;In 2018 Red Dragon flight primary mission would be to prove a soft landing for a human rated vehicle, but this mission is also an opportunity to conclude even some others objectives. SpaceX cooperating with NASA has considered before a drilling mission concept searching for signs of water and current or prior life existence. According Wikipedia and this &lt;a href=&quot;http://digitalvideo.8m.net/SpaceX/RedDragon/karcz-red_dragon-nac-2011-10-29-1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt; in the Mission Concept procedures is included even this point:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Deploy surface systems &amp;amp; commence surface operations.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And in the drilling mission concept goals are:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Scientific goals&lt;/li&gt;&#xA;  &lt;li&gt;Search for evidence of life&lt;/li&gt;&#xA;  &lt;li&gt;Assess subsurface habitability&lt;/li&gt;&#xA;  &lt;li&gt;Establish the origin, distribution, and composition of ground ice&lt;/li&gt;&#xA;  &lt;li&gt;Reconstruct climate using ground ice record&lt;/li&gt;&#xA;  &lt;li&gt;Human precursor goals&lt;/li&gt;&#xA;  &lt;li&gt;Conduct human-relevant EDL demonstrations&lt;/li&gt;&#xA;  &lt;li&gt;Assess potential hazards in dust, regolith, and ground ice&lt;/li&gt;&#xA;  &lt;li&gt;Characterize resources&lt;/li&gt;&#xA;  &lt;li&gt;Demonstrate access to subsurface resources&lt;/li&gt;&#xA;  &lt;li&gt;Conduct ISRU demonstration&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://danielmarin.naukas.com/files/2012/10/IM20121014alas143.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://danielmarin.naukas.com/files/2012/10/IM20121014alas143.png&quot; alt=&quot;Possible similar payloads for drilling mission&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Possible similar payloads for drilling mission.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A payload for drilling at the Red Dragon spacecraft is a concept supported by Dr. John Karcz at (Ames Research Center) known as NASA Ames in this &lt;a href=&quot;http://science.nasa.gov/media/medialibrary/2012/01/23/NAC_Science_Meeting_ReportOctober_31-November_1_2011-finalTAGGED.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A possible drilling system is a technology used by Zaptec company, a Norwegian company which uses a method called Plasma Channel drilling, or Electropulse drilling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According &lt;a href=&quot;http://nextbigfuture.com/2015/03/technical-details-on-zaptec-plasma.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;nextbigfuture.com&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On Mars, the proposed Zaptec system will allow a depth of 2 km to be reached with less than 1 metric ton of surface payload housed in a SpaceX Dragon-sized capsule and peak power requirements of less than 2 kW.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://static1.squarespace.com/static/56bb3a18f699bb9426d19755/t/56bba03b04426211a9bb73ff/1455136835017/?format=1000w&quot; alt=&quot;Zaptec drilling system at SpaceX Dragon-class landed capsule&quot;&gt;&#xA;&lt;sup&gt;Zaptec deep drilling system capable of reaching a depth of 2 km, deployed from a SpaceX Dragon-class landed capsule.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This SpaceX mission to Mars is a good opportunity to include this kind of payloads for scientific studies. Probably until 2018 mission, could be difficult developing and fitting the Red Dragon spacecraft type with a Zaptec drilling system, but maybe a less complicated system also smaller and easier to fit with the Red Dragon lander could be as an option.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="10273" LastEditDate="2019-03-14T08:46:13.270" LastActivityDate="2019-03-14T08:46:13.270" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="15247" PostTypeId="2" ParentId="15001" CreationDate="2016-05-11T19:43:19.820" Score="-2" Body="&lt;p&gt;The Space Shuttle remains the best by &quot;harvesting the atmoshere&quot; to act as an air cushion for descent. Once you start applying &quot;ablative cooling&quot; and &quot;drogue chutes&quot; and &quot;parachutes&quot; and &quot;retro rockets&quot; and &quot;fancy seats&quot; you're really failing to harvest all that wonderful energy that Mother Earth is giving to you for free.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words the &quot;air&quot; represents &quot;brake horsepower&quot; and any heat or fire generated and &quot;absorbed&quot; is energy wasted both in form of inefficiency since absorbing the heat does not slow you down and materially since basically your just &quot;burning off&quot; perfectly good and probably expensive  &quot;stuff&quot; that then has to be put right back on only to be burned right back off again.&lt;/p&gt;&#xA;" OwnerUserId="14394" LastActivityDate="2016-05-11T19:43:19.820" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15249" PostTypeId="1" CreationDate="2016-05-12T08:19:21.760" Score="2" ViewCount="390" Body="&lt;p&gt;How does &lt;strong&gt;inclination&lt;/strong&gt; perturbation affect drift in &lt;strong&gt;local time&lt;/strong&gt; of a sun synchronous orbit? e.g orbit with inclination of 98 degrees and height 600km with local time 10:30&lt;/p&gt;&#xA;" OwnerUserId="14400" LastEditorUserId="14402" LastEditDate="2016-05-12T15:41:51.657" LastActivityDate="2016-05-12T15:41:51.657" Title="Local Time drift of Sun synchronous orbit" Tags="|orbit|sun-synchronous|" AnswerCount="0" CommentCount="2" ClosedDate="2016-05-16T15:00:39.913" ContentLicense="CC BY-SA 3.0" />
  <row Id="15250" PostTypeId="2" ParentId="15223" CreationDate="2016-05-12T08:55:53.040" Score="3" Body="&lt;p&gt;That does not make much sense, unfortunately: Most payloads are launched into a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;Geostationary Transfer Orbit&lt;/a&gt;. Such an orbit is highly elliptic. If the second stage could reach the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;nofollow&quot;&gt;Geostationary Orbit&lt;/a&gt;, it would just go there directly and remain there with the payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in order to reuse the second stages you would need to cluster (ideally dock) them in roughly the same spot on a highly elliptical but stable orbit. This will cost you money since parking (and carrying docking capabilities) requires more fuel. Hitting that spot constrains your launch window severely. Unfortunately your launch window for a geostationary satellite is already quite constrained (you want your apogee above your target, if you wait too long before raising your perigee your orbit will decay). So your orbital mechanics would be more complicated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, in order to do something with the spent stages, you would have to climb up to your parking orbit (i.e. a close-to GTO). This requires a lot of fuel and basically &lt;strong&gt;halves your payload&lt;/strong&gt; compared to the Low Earth Orbit. As long as you do not need &lt;strong&gt;many&lt;/strong&gt; used stages, you are better off launching your new engines into the LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;tl;dr: Too complicated, no real benefit&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-05-12T08:55:53.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15251" PostTypeId="1" CreationDate="2016-05-12T09:56:32.070" Score="12" ViewCount="1471" Body="&lt;p&gt;The Dragon capsule landed yesterday, and I saw this picture of it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ktVWI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ktVWI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Historically, capsules had a rather large uncertainty when landing, especially with parachutes. This picture was obviously taken while the capsule was still in the air, but close to ground, from the air, and must have been relatively close. How does the uncertainty of Dragon compare to Soyuz in terms of it's landing accuracy?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="13208" LastEditDate="2017-03-20T15:24:26.983" LastActivityDate="2017-03-20T15:24:26.983" Title="How does the landing accuracy of Dragon (under parachutes) compare to Soyuz?" Tags="|spacex|landing|soyuz-spacecraft|dragon|accuracy|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15253" PostTypeId="1" AcceptedAnswerId="15254" CreationDate="2016-05-12T20:42:12.297" Score="7" ViewCount="616" Body="&lt;p&gt;Assuming the Dragon V2 will be the lander for SpaceX Mars mission launching in 2018, will it be the largest lander (either by weight or by size) that humans have put on Mars to date?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has the Dragon V2 listed as 8.1m tall (~5.1m tall excluding the trunk) with a diameter of 3.7m.  It also has a dry mass of 6,400 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity, the largest rover I'm aware of is 2.9m x 2.7m x 2.2m, so it's much shorter than the Dragon V2, and about a third less wide.  It has a listed dry mass of 900kg so it's much lighter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anything larger been put on the surface of Mars, or will the Dragon V2 be the largest lander to date?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="6773" LastEditDate="2016-06-21T13:46:36.350" LastActivityDate="2016-06-21T13:46:36.350" Title="Will the 2018 Mars Dragon Lander be the largest lander on Mars to date?" Tags="|mars|spacex|dragon|lander|red-dragon|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15254" PostTypeId="2" ParentId="15253" CreationDate="2016-05-12T21:11:22.717" Score="12" Body="&lt;p&gt;Yes, by quite a bit. Right now the heaviest object soft landed on Mars is Curiosity, as you noted. The mass of it is 900 kg. The Red Dragon is 6400 kg, plus payload of up to 2000 kg (Maybe more for later flights)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, when successful, the Dragon will be more than the sum of every other object which attempted to soft landed on Mars. Each Viking massed 600 kg, Curiosity 900, Phoenix 350, Spirit and Opportunity each 530 kg (For the package that landed on the surface, the rovers are much less), Pathfinder/Sojourner 280, Mars 2, 3, and 6 about 360 kg each, Mars Polar Landar was 290 kg, and Beagle 2 35 kg. Total is about 5300 kg, under the mass of the Red Dragon alone.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-05-15T18:22:35.597" LastActivityDate="2016-05-15T18:22:35.597" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15256" PostTypeId="1" AcceptedAnswerId="15257" CreationDate="2016-05-12T23:15:16.333" Score="8" ViewCount="4127" Body="&lt;p&gt;Look at that:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/0T6Rx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/bnMJQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/f673C.jpg&quot; alt=&quot;enter image description here&quot;&gt;&#xA;&lt;img src=&quot;https://i.stack.imgur.com/bUHpB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...and now tell me how comes Opportunity weighs 530kg and Curiosity 900kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks, like by volume, Curiosity is good 4x larger, if not more. Plus it contains heavyweight RTGs, its chassis/frame is much thicker, in short it looks like way more than twice the weight. Meanwhile, it's only 70% heavier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What properties, construction solutions, mass savings allowed such weight savings in a construction so much bigger?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2016-05-13T01:59:11.513" LastActivityDate="2018-10-07T23:58:37.880" Title="Why is Curiosity so light compared to Opportunity?" Tags="|mars|rovers|curiosity|mass|opportunity|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15257" PostTypeId="2" ParentId="15256" CreationDate="2016-05-13T01:32:48.970" Score="15" Body="&lt;p&gt;The number is slightly misleading. NASA provides &lt;a href=&quot;http://mars.nasa.gov/mer/mission/spacecraft.html&quot; rel=&quot;noreferrer&quot;&gt;a complete breakdown&lt;/a&gt; on the mass. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Rover 185 kg&lt;/li&gt;&#xA;&lt;li&gt;Lander 348 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Almost 2/3rds of the mass that soft landed was in the stand and airbags, only about 1/3rd of the mass of the rover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, Curiosity was more effective in terms of the mass of the rover to the mass of the EDL package. The skycrane deployment allowed it to make use of such a high useful mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://youtu.be/-_9BYSDtwRc?t=3m43s&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; for why so much mass of MER was in the non-roving lander portion, or see this picture of the landing platform for Spirit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4tYYi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4tYYi.jpg&quot; alt=&quot;Spirit&amp;#39;s landing platform&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-13T01:32:48.970" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15260" PostTypeId="2" ParentId="15038" CreationDate="2016-05-13T04:14:18.980" Score="7" Body="&lt;p&gt;&quot;The most difficult atmospheric entry ever attempted&quot; was by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_Probe&quot; rel=&quot;noreferrer&quot;&gt;Galileo Probe&lt;/a&gt;. Temperature could refer to the plasma temperature or heat-shield temperature, but the latter generally caps out because a) nothing will remain solid past ~4000 &amp;deg;C, and b) many heat shields are designed to ablate, vaporizing in order to absorb some of the thermal energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyways, the Galileo Probe had to endure a 230-250 &lt;em&gt;g&lt;/em&gt; deceleration. Citations claim that it endured &quot;15,500 &amp;deg;C&quot;, which I don't quite understand given the above, but it did go on a rapid weight-loss program, shedding 80 kg of mass in about 2 minutes. Some other stats visible on the &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/2.3293&quot; rel=&quot;noreferrer&quot;&gt;first page of this pay-walled paper&lt;/a&gt; (full paper available through Marcia McNutt's favorite website, though most of it is technical details about how they measured the ever-decreasing thickness of the heat shield) include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;30 kW/cm&lt;sup&gt;2&lt;/sup&gt; heat flux&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;300,000 suns&quot; (300k &amp;times; the solar insolation at Earth's surface)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;300 kJ/cm&lt;sup&gt;2&lt;/sup&gt; heat load&lt;/li&gt;&#xA;&lt;li&gt;Entry speed of Mach 50 (47.4 km/s)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="881" LastActivityDate="2016-05-13T04:14:18.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15261" PostTypeId="1" CreationDate="2016-05-13T05:32:42.057" Score="0" ViewCount="94" Body="&lt;p&gt;Does anyone know a good source for watching/downloading launch videos (saved webcasts for example of SpaceX and NASA ). Launch videos are extremely informative especially as they are curated for the general public.&lt;/p&gt;&#xA;" OwnerUserId="6753" LastActivityDate="2016-05-13T14:33:13.773" Title="Good source for launch videos" Tags="|launch|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15262" PostTypeId="2" ParentId="15251" CreationDate="2016-05-13T05:48:58.057" Score="8" Body="&lt;p&gt;No values for Soyuz or Dragon on &lt;a href=&quot;https://en.wikipedia.org/wiki/Splashdown#Locations&quot; rel=&quot;nofollow noreferrer&quot;&gt;wiki splashdown page&lt;/a&gt;, but the lake listed for the one Soyuz water landing seems to be like 20x40km. &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/structure/elements/soyuz/landing.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; shows the Soyuz landing zone as 25km wide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a thread on &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=19141.msg1029125#msg1029125&quot; rel=&quot;nofollow noreferrer&quot;&gt;NSF&lt;/a&gt; where the users posted some graphs of landing accuracy, one of them showing the SpaceX COTS-1 and claiming the precision (0.8km) was much better than average Soyuz and even better than the best of Soyuz TMA-04M (2.2km):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2ymhJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2ymhJ.jpg&quot; alt=&quot;Soyuz landing accuracy&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;user &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=19141.msg955175#msg955175&quot; rel=&quot;nofollow noreferrer&quot;&gt;Comga, forum.nasaspaceflight.com&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Dragon landings there is a &lt;a href=&quot;https://www.google.com/maps/d/viewer?mid=1wvgFIPuOmI8da9EIB88tHo9vamo&quot; rel=&quot;nofollow noreferrer&quot;&gt;map&lt;/a&gt; posted by reddit user &lt;a href=&quot;https://www.reddit.com/user/Raul74Cz&quot; rel=&quot;nofollow noreferrer&quot;&gt;Raul74Cz&lt;/a&gt; but no hard numbers in there, only estimates based on reentry areas (which - being the expected worst case - are quite big). I was not yet able to find some exact values for other Dragon landings, articles only mentioning the landings being &quot;precise&quot; with the recovery vessels being &quot;near&quot; the spot and quick to get to the capsule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2017-03-19 this image was &lt;a href=&quot;https://twitter.com/SpaceX/status/843488532035338240&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweeted&lt;/a&gt; by SpaceX&#xA;&lt;a href=&quot;https://i.stack.imgur.com/kFX3p.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kFX3p.jpg&quot; alt=&quot;Dragon CRS-10 descending on her chutes&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It shows the CRS-10 Dragon capsule still in the air being photographed by the recovery crew. I found no data about the focal lenght or similar which would allow to estimate the actual distance but it is visible and does not look too far. Taking into account that the recovery vessels probably stay out of the designated zone, it may be just a lucky coincidence but it being not a first such photo suggests it is not just random chance.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2017-03-19T21:40:27.823" LastActivityDate="2017-03-19T21:40:27.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15263" PostTypeId="2" ParentId="15261" CreationDate="2016-05-13T06:26:39.077" Score="2" Body="&lt;p&gt;Perhaps the ultimate Shuttle launch video is &lt;a href=&quot;https://sites.google.com/site/ascentcommemoratingshuttle/home&quot; rel=&quot;nofollow&quot;&gt;Ascent&lt;/a&gt;: about an hour of footage from dozens of high-speed cameras.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ESA has a &lt;a href=&quot;http://www.esa.int/spaceinvideos/content/search?SearchText=launch&amp;amp;SearchButton=Go&quot; rel=&quot;nofollow&quot;&gt;video section&lt;/a&gt; on their website that contains lots of launch webcasts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX have a &lt;a href=&quot;https://www.youtube.com/user/spacexchannel&quot; rel=&quot;nofollow&quot;&gt;Youtube channel&lt;/a&gt; and a &lt;a href=&quot;http://www.spacex.com/media/all/video&quot; rel=&quot;nofollow&quot;&gt;video section&lt;/a&gt; on their website.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-05-13T06:26:39.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15264" PostTypeId="1" CreationDate="2016-05-13T08:23:15.610" Score="2" ViewCount="945" Body="&lt;p&gt;Our local amateur radio club has been approached by Veikkola junior school with regard to our members participating in the school's proposed High altitude balloon project (Spring 2017); in particular with reference to realizing position reporting (APRS) and possible live stream video imaging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that the target altitude will be approx. 30km, temp -40C(?) and payload capacity 1000g, what sort of power supply should we contemplate, together with any necessary means to keep it warmed?&lt;/p&gt;&#xA;" OwnerUserId="14407" LastActivityDate="2016-05-13T12:57:24.973" Title="Power supply for (30km stratosphere) school's High Altitude Balloon project" Tags="|balloons|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15265" PostTypeId="1" CreationDate="2016-05-13T09:24:25.157" Score="1" ViewCount="153" Body="&lt;p&gt;What is the difference in satellite orbit-sun geometry of  &lt;em&gt;Sun synchronous&lt;/em&gt; orbits of &lt;strong&gt;6am local time&lt;/strong&gt; , &lt;strong&gt;9 am local time&lt;/strong&gt; , &lt;strong&gt;12 pm local time&lt;/strong&gt; ?&lt;/p&gt;&#xA;" OwnerUserId="14400" LastEditorUserId="58" LastEditDate="2016-05-16T15:00:02.440" LastActivityDate="2016-05-16T15:00:02.440" Title="Satellite Orbit-Sun Geometry of Sun synchronous Orbit" Tags="|artificial-satellite|orbit|sun-synchronous|the-sun|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15266" PostTypeId="2" ParentId="15264" CreationDate="2016-05-13T10:14:17.087" Score="6" Body="&lt;p&gt;&quot;Primary&quot; (non-rechargeable) lithium AA batteries are the popular choice for amateur high altitude balloon projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not too expensive, good energy density and they hold up well under low temperature compared to most other kinds of cell (notably rechargeable Li-Ion or LiPo are not good in the cold).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Be aware that depending on how you insulate your &quot;payload compartment&quot; and how much power is being dissipated inside it, the interior may actually end up quite warm due to the low air density and correspondingly reduced convective cooling.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="415" LastEditDate="2016-05-13T12:57:24.973" LastActivityDate="2016-05-13T12:57:24.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15267" PostTypeId="2" ParentId="15261" CreationDate="2016-05-13T14:33:13.773" Score="0" Body="&lt;p&gt;The Internet Archive has a lot of older public domain NASA material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example: footage from the &lt;a href=&quot;https://archive.org/details/APOLLO16MMLAUNCHVIEWS&quot; rel=&quot;nofollow&quot;&gt;Apollo 16mm launch cameras&lt;/a&gt; for various launches, all rolled into one very long slow-motion movie. These show detail of disconnection of the various support umbilicals during the launch sequence. (No sound.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-05-13T14:33:13.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15268" PostTypeId="1" AcceptedAnswerId="21910" CreationDate="2016-05-13T15:08:13.947" Score="17" ViewCount="4007" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; some more sources; the broken site spacecraftforall.com/a-new-orbit used to have an interactive simulation, here's an old screen shot:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rvPuZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rvPuZ.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Hat tip to @NgPh for finding this &lt;a href=&quot;http://spacecollege.org/video-original-isee-3-trajectory-from-1978-1985-1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space College page&lt;/a&gt; linking to the video &lt;em&gt;ISEE_3 Original&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/GMqBRqVFztc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;While finding some good patent examples to add to the question &lt;a href=&quot;https://space.stackexchange.com/q/15224/12102&quot;&gt;Spacecraft Maneuvers as Intellectual Property? Wow!&lt;/a&gt;, I ran across this patent:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://patents.google.com/patent/US7744036B2/en&quot; rel=&quot;nofollow noreferrer&quot;&gt;US7744036B2 Method for designing an orbit of a spacecraft&lt;/a&gt; (J. Kawaguchi and K. Tarao, JAXA)&lt;/p&gt;&#xA;&lt;p&gt;You can click on the PDF icon to download it.&lt;/p&gt;&#xA;&lt;p&gt;My question comes from reading the following parts of these two sections:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h2&gt;BACKGROUND OF THE INVENTION&lt;/h2&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;…By the way, it has been revealed by previous studies that if a Lissajous orbit is enlarged to a much more large scale one to enter into a non-colinear area, the non-linear effect makes the spacecraft trajectory closed. And the orbit of a spacecraft, if the spacecraft is positioned close to point L2 can avoid shadow by the earth and if the spacecraft is positioned close to point L1, can avoid passage in front of the Sun. When viewed from the Earth it looks like a halo around the sun, and thus is called a halo orbit….&lt;/p&gt;&#xA;&lt;p&gt;and then:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h2&gt;SUMMARY OF THE INVENTION&lt;/h2&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As stated above, the natural halo orbit is adventageous on the fact that it is stable and draws a closed locus without any special artificial correction operation maintaining the motion. However, the natural halo orbit disadvantageously leads to an extraordinarily large orbit which must depart from a co-linear line connecting two celestial bodies. The distance may be as large as, for example, about one million km from the Lagrange point referred to here. The resulted natural Halo orbits around point L1 or L2 are too far and very much apart from the aimed points originally. Because of this, the merit associated with the halo orbits, that is, no maintenance of the position of a spacecraft is needed, is inevitably lost, if the trajectory is controlled and maintained artificially...&lt;/p&gt;&#xA;&lt;p&gt;I take this to mean that when a halo orbit around an &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$L_2$&lt;/span&gt; point (let's say Sun-Earth) is sufficiently large, it is (at least within the &lt;a href=&quot;https://en.wikipedia.org/wiki/Three-body_problem#Circular_restricted_three-body_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;CR3BP&lt;/a&gt; model) actually stable to small perturbations, i.e &lt;em&gt;no station keeping is required for sufficiently large halo orbits about &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$L_2$&lt;/span&gt;&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is this true? Are there other stable orbits associate with unstable Lagrange points?&lt;/p&gt;&#xA;&lt;p&gt;I had always thought that all orbits around &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$L_2$&lt;/span&gt; were also similarly exponentially unstable just like the points themselves, (again, within the CR3BP model).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; I am &lt;strong&gt;not&lt;/strong&gt; asking a question about this patent. I am referencing it only because of the clear statement it makes about Halo orbits - this is where I read it. The patent discusses an artificially maintained smaller halo orbit. Halo orbits are a special class of Lissajous orbits where the in-plane and out-of-plane periods are equal so that it stays &amp;quot;open&amp;quot; and away from the central exclusion zone.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Figure 3 from the Patent US7744036B2:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TqPYc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TqPYc.png&quot; alt=&quot;Fig 3 from US7744036B2&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-11-29T23:34:21.070" LastActivityDate="2021-12-02T17:06:32.903" Title="Are some Halo Orbits actually Stable? (stable orbits about unstable Lagrange points)" Tags="|orbital-mechanics|lagrangian-points|three-body|" AnswerCount="4" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="15269" PostTypeId="1" CreationDate="2016-05-13T18:31:35.893" Score="4" ViewCount="491" Body="&lt;p&gt;At the poles the solar wind directly impinges. Suppose we put a solar sail satellite in a weather balloon at one of the poles. When it reaches the maximum altitude (~50km) it opens the solar sail and uses the solar wind to accelerate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible for such a configuration to put something in orbit?&lt;/p&gt;&#xA;" OwnerUserId="14412" LastEditorUserId="6816" LastEditDate="2016-05-13T18:45:20.510" LastActivityDate="2016-06-02T20:15:16.587" Title="Is it possible to launch a satellite into orbit with a solar sail and a weather balloon using the magnetic poles?" Tags="|launch|solar-sail|solar-wind|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15270" PostTypeId="2" ParentId="15269" CreationDate="2016-05-13T18:55:17.457" Score="8" Body="&lt;p&gt;The problem is that you have to go from low-speed regime in the atmosphere (the balloon), to a high-speed regime out of the atmosphere (~7000m/s).  The low forces available from a solar sail don't allow this to happen quickly enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At an intermediate speed it is either destroyed by forces from interacting with the atmosphere or it is insufficiently supported and falls back down.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;My hope was not to have to get the high speed regime. Instead use the solar thrust to nullify and constant help the probe to get out the earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The problem is that solar thrust is very low, so you need a huge surface area to capture it.  Let's say you had a 1kg payload in the upper atmosphere at low speed.  Without a balloon, you need the sail to counteract the weight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the force available from light is low.  Discounting all sail losses, it's about 8.25µN/m^2.  If we imagine a magical solar sail that is massless, then to counter the payload weight it would have to be:&#xA;$$A = \frac{W}{P}$$&#xA;$$A = \frac{9.7\text{N}}{8.25\frac{\text{µN}}{\text{m}^2}}$$&#xA;$$A = 1.1\text{km}^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a perfect, massless sail would still need to be over a square kilometer in size to counteract the weight of a kilogram.  This is simply not practical for such a small payload.  Add in the mass of sail and it becomes impossible.  Adding in ionic pressure doesn't help significantly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only after you reach orbit (when weight no longer is a problem and tiny forces can do useful work) can such a low-thrust device be useful.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastEditorUserId="5979" LastEditDate="2016-06-02T20:15:16.587" LastActivityDate="2016-06-02T20:15:16.587" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15271" PostTypeId="2" ParentId="15269" CreationDate="2016-05-13T20:48:01.723" Score="7" Body="&lt;p&gt;You need 1000km of altitude to break even between air drag and light pressure of sunlight. Below that the solar sail acts as a parachute, dragging against the remains of the atmosphere. You must reach 1000km altitude by other means. Definitely not a balloon, and not an orbital gun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, solar wind (protons and ions) accounts for about 0.5% of &quot;thrust&quot; produced by the solar sail. It's the light (photons) that provides the bulk of its propulsive force, and it's not impeded by Earth's magnetic field, so the launch at a pole doesn't help in any significant way.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-05-13T20:48:01.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15272" PostTypeId="1" AcceptedAnswerId="15273" CreationDate="2016-05-14T05:17:11.980" Score="3" ViewCount="907" Body="&lt;p&gt;How do the following perturbation factors affect a sun-synchronous orbit?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Atmospheric drag&lt;/li&gt;&#xA;&lt;li&gt;Solar gravity&lt;/li&gt;&#xA;&lt;li&gt;Lunar gravity&lt;/li&gt;&#xA;&lt;li&gt;Solar radiation pressure&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="14400" LastEditorUserId="6816" LastEditDate="2016-08-10T02:52:29.203" LastActivityDate="2021-11-20T01:15:08.247" Title="Perturbation effects on sun-synchronous orbit" Tags="|orbit|sun-synchronous|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15273" PostTypeId="2" ParentId="15272" CreationDate="2016-05-14T06:20:56.273" Score="1" Body="&lt;p&gt;Sun-synchronous orbit would have an altitude of between 150 and 900 km. That being said, atmospheric drag would be determined by altitude, with altitudes higher than 600 km experiencing no drag. At lower altitudes, this drag will cause the satellite to slow and lose altitude as a result (call it a change in a).&lt;br&gt;&#xA;Solar radiation pressure would affect the orbit by causing the satellite to torque over time, which in combination with atmospheric drag could cause more or less drag over the surface area. This change takes a lot of time, however, because solar radiation results in a very small force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar radiation perturbation can be calculated by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F=\frac{F_s}{c}\cdot A_s(1+r)\cos(I)$$&#xA;where:&lt;br&gt;&#xA;$F$ is the force in N&lt;br&gt;&#xA;$F_s$ is the solar constant, $1358 \,\mathrm{W/m^2}$&lt;br&gt;&#xA;$c$ is the speed of light, $3\cdot 10^8 \,\mathrm{m/s}$&lt;br&gt;&#xA;$A_s$ is the illuminated surface area&lt;br&gt;&#xA;$r$ is the surface reflectance with $1&amp;gt;r&amp;gt;0$&lt;br&gt;&#xA;$I$ is the incidence angle to the Sun&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This comes from: Sellers, Jerry. Understanding Space, 3rd Ed.(a book I know from my orbital mechanics class)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, I haven't found anything about solar or lunar gravity perturbations for sun-synch satellites. I would consider that these forces would have the minutest of minute perturbations on a satellite so close to the Earth.&lt;/p&gt;&#xA;" OwnerUserId="13563" LastEditorUserId="6816" LastEditDate="2016-08-09T19:38:10.723" LastActivityDate="2016-08-09T19:38:10.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15274" PostTypeId="1" CreationDate="2016-05-14T07:10:43.620" Score="0" ViewCount="469" Body="&lt;p&gt;How To Calculate Local Time Of A Sun Synchronous Orbit With A Given Altitude ( Say 600 km ) , Inclination ( 98 degrees ) ?&lt;/p&gt;&#xA;" OwnerUserId="14400" LastActivityDate="2016-05-14T13:38:07.540" Title="Local Time Calculation" Tags="|time|sun-synchronous|the-sun|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15275" PostTypeId="2" ParentId="998" CreationDate="2016-05-14T07:40:47.480" Score="10" Body="&lt;p&gt;I would like to add my own answer &lt;strong&gt;based purely on math&lt;/strong&gt;, which is not as complex as you may think (but I explain each term and everything else so it &lt;strong&gt;looks&lt;/strong&gt; long). We only need a couple equations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First the &lt;strong&gt;Tsiolkovsky Rocket Equation&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_f = V_e \ln(\displaystyle \frac{m_i}{m_f})$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Soon, we'll need this rearranged for $m_f$, which is $m_f = \displaystyle \frac{m_i}{e^\frac{V_f}{V_e}}$)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_f$ is the final velocity, or delta v, or change in velocity. If you're going from 0 to 10 km/s, then 10 km/s will be your final velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_e$ is the effective exhaust velocity, which is basically how fast the exhaust is propelled out the engine. The faster, the better. $V_e$ is equivalent to the specific impulse (in seconds) multiplied by g (9.80665 m/s/s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_i$ and $m_f$ are the initial and final masses of the whole rocket. These are both &lt;strong&gt;total masses&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The $\ln$ part is a natural logarithm, which is log base e (2.718...). If you don't know what those are, don't worry, just use a calculator. It's noteworthy, though, that this logarithm makes the equation &lt;strong&gt;exponential&lt;/strong&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, we need to know the &lt;strong&gt;tangential velocity of Earth's surface&lt;/strong&gt;, based on latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Equatorial tangential speed will be $\displaystyle \frac{2\pi*6,371,000}{86,164}$ = &lt;strong&gt;464.58 m/s&lt;/strong&gt;. (radius of earth = 6371 km, siderial rotational period = 86164 seconds, which is actually 23 hours, 56 minutes, 4 seconds, not 24 hours even.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For other latitudes, multiply by the cosine of the latitude. At 45 degrees north (or south for that matter), ground tangential speed will be $464.58 * \cos(45°)$ = &lt;strong&gt;328.51 m/s&lt;/strong&gt;. (be careful your calculator is in degree mode, not radian mode).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One more thing we need to know is the orbital speed our satellite needs. This is typically &lt;strong&gt;7.8 km/s, HOWEVER:&lt;/strong&gt; Due to air drag, gravity drag, and some obvious vertical acceleration, the typical carrier rocket needs to send its payload to something like &lt;strong&gt;9.7 km/s&lt;/strong&gt;. This is the figure I will use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for prograde orbits, the actual delta v your rocket needs is &lt;strong&gt;9.7 km/s minus the ground speed.&lt;/strong&gt; Obviously, the larger the ground speed, the less work our rocket needs to do. This is what drives launch sites towards the equator.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Now let's pick some figures for our rocket's final stage. I'll choose some based on the Soyuz' final (3rd) stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say this final stage can take a 7-ton payload up to 4 km/s. Its total mass is 30 tons (including the payload), while its empty mass is 9.3 tons (including the payload). This implies that the empty 3rd stage without the payload is always 2.3 tons, and the effective exhaust velocity is 3.415 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now with a boost from ground speed at the equator, the final stage only need to take this payload 3.535 km/s. Therefore, we can remove some fuel and add some payload mass. This will keep the total mass constant, because we're just trading fuel for payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;This is the benefit of ground speed boost: increasing payload mass by reducing needed delta v.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now it's time for a giant chart:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{array}{|c|c|c|c|c|} \hline&#xA; \text{site} &amp;amp; \text{latitude} &amp;amp; \text{ground speed} &amp;amp; \text{final stage needed speed} &amp;amp; \text{Final Stage Masses}\\ \hline&#xA;\text{Equator} &amp;amp; 0° &amp;amp; \text{465 m/s} &amp;amp; \text{3.535 km/s} &amp;amp; \text{mf = 10.655 t, payload = 8.355 t}\\ \hline&#xA;\text{Korou} &amp;amp; 5° &amp;amp; \text{463 m/s} &amp;amp; \text{3.537 km/s} &amp;amp; \text{mf = 10.649 t, payload = 8.349 t}\\ \hline&#xA;\text{US Virgin Islands} &amp;amp; 10° &amp;amp; \text{458 m/s} &amp;amp; \text{3.542 km/s} &amp;amp; \text{mf = 10.633 t, payload = 8.333 t}\\ \hline&#xA;\text{Cape Canaveral} &amp;amp; 28° &amp;amp; \text{411 m/s} &amp;amp; \text{3.589 km/s} &amp;amp; \text{mf = 10.488 t, payload = 8.188 t}\\ \hline&#xA;\text{Tanegashima} &amp;amp; 30° &amp;amp; \text{403 m/s} &amp;amp; \text{3.597 km/s} &amp;amp; \text{mf = 10.464 t, payload = 8.164 t}\\ \hline&#xA;\text{Baikonur} &amp;amp; 46° &amp;amp; \text{323 m/s} &amp;amp; \text{3.677 km/s} &amp;amp; \text{mf = 10.221 t, payload = 7.921 t}\\ \hline&#xA;\text{Plesetsk} &amp;amp; 62° &amp;amp; \text{218 m/s} &amp;amp; \text{3.782 km/s} &amp;amp; \text{mf = 9.912 t, payload = 7.612 t}\\ \hline&#xA;\text{North Pole} &amp;amp; 90° &amp;amp; \text{0 m/s} &amp;amp; \text{4.000 km/s} &amp;amp; \text{mf = 9.300 t, payload = 7.000 t}\\ \hline&#xA;\end{array}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So comparing your two cases, Korou and Cape Canaveral, we see that we can get up to an additional 161 kg of payload. It may not seem like a lot, but keep in mind, we are paying &lt;strong&gt;tens of thousands of dollars&lt;/strong&gt; for every pound we put into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the same token, a reduction of 52 m/s may not seem like a lot either, but &lt;strong&gt;the exponential nature&lt;/strong&gt; of the Tsiolkovsky Rocket Equation means that this is nothing to sneeze at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case you're wondering about the very small differences at first, but the larger differences later, see &lt;a href=&quot;http://fooplot.com/#W3sidHlwZSI6MCwiZXEiOiJjb3MoeC85MCpwaS8yKSoxMDAiLCJjb2xvciI6IiMwMDAwMDAifSx7InR5cGUiOjEwMDAsIndpbmRvdyI6WyItMTM5LjYxOTczMTk0MzIwODA1IiwiMTU1Ljk2NjA0NjEzMjk5MzI4IiwiLTU4LjM2Mjk3MjM5NDE3NjUxNiIsIjEyMy41MzU5Njc5NjA0MDg5MyJdfV0-&quot; rel=&quot;noreferrer&quot;&gt;this graph&lt;/a&gt;, because &lt;strong&gt;the cosine function is exponential too.&lt;/strong&gt; 45 degrees is halfway between pole and equator (50 percent), but take the cosine of it and you get something closer to 71 percent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But guess what. It's not over:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;This only holds for launches &lt;strong&gt;due east&lt;/strong&gt;. If you're at 46 degrees north and launch due east, your sputnik ends up in an orbit inclined 46 degrees. But Baikonur launches in a 51.6 degree orbit, and used to launch in a 63 degree inclined orbit! This is to keep the sputnik over Russia, not China, for the early phase in case the rocket fails and the sputnik (or Soyuz with people aboard) comes back down. For these other inclinations, you'll need to do a vector subtraction in only one dimension to find the rocket needed speed.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Spy sputniks typically go into polar orbit (about a 90 degree inclination). So large ground speed is actually a bad thing. This actually drives launch sites &lt;strong&gt;towards the poles&lt;/strong&gt;, if it's a launch site intended for spy sputniks, like the Plesetsk Cosmodrome.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There's an alternate way to apply benefits: keeping the payload mass constant, but sending it to a higher orbit.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If you're launching into a retrograde orbit (opposite of the way the Earth turns), then being closer to the equator is a &lt;strong&gt;disadvantage&lt;/strong&gt;. Israel faces this problem as the only safe direction to launch their orbital rockets is to the west. I believe they orbit things in an approximately 120 degree inclination.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Latitude is not the only factor in choosing sites. Other factors include weather, security (USSR chose a central inland area for this), and transportability (Cape Canaveral is transportable by sea barge, so they can ship huge rockets without a problem).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Don't assume that the ratio of masses for any two examples in the chart will be the same ratio for the same latitudes of a different rocket. You really need to look at the rocket's final stage, see what it's Ve is and other parameters. Then use those numbers in the equations and make your own chart.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Earth isn't a perfect sphere. Equatorial radius is actually a bit more than 6371 km. There may also be a mountain or plateau somewhere that would give a decent boost by extra radius (which yields extra ground speed) and extra height above mean sea level (which yields less vertical acceleration needed). Of course, remote and inhospitable areas incur their own expenses if you're really thinking about building there, but these sort of boosts once spawned the concept of balloon-launched rockets, and still today there is talk of rockets launched from airplanes as well as those exotic launch systems like the Launch Loop which can run up a mountainside for the same &quot;Newton's Cannonball&quot; physical boost.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Alright. There's my answer based purely on math. If you dare, you can plug in your own example and figure out the exact benefits for your specific case.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2016-05-14T16:15:59.670" LastActivityDate="2016-05-14T16:15:59.670" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15276" PostTypeId="1" AcceptedAnswerId="15284" CreationDate="2016-05-14T08:56:02.517" Score="3" ViewCount="712" Body="&lt;p&gt;How is a &quot;Star Tracker&quot; mounted on a spacecraft able to compute &quot;absolute attitude&quot;?&lt;/p&gt;&#xA;" OwnerUserId="14400" LastEditorUserId="58" LastEditDate="2016-05-16T14:57:37.817" LastActivityDate="2016-05-16T14:57:37.817" Title="Star Tracker And Absolute Attitude" Tags="|spacecraft|attitude|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15277" PostTypeId="1" AcceptedAnswerId="15278" CreationDate="2016-05-14T11:21:48.207" Score="15" ViewCount="2512" Body="&lt;p&gt;After watching an &lt;a href=&quot;https://youtu.be/XRCIzZHpFtY?t=3m15s&quot;&gt;animation&lt;/a&gt; of how the Spirit rover landed on Mars, I was wondering how the rover managed to end up the right way up after bouncing around?&lt;/p&gt;&#xA;" OwnerUserId="14415" LastActivityDate="2016-05-14T13:46:03.983" Title="How did the spirit rover land right way up after bouncing" Tags="|mars|rovers|spirit|air-bag|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15278" PostTypeId="2" ParentId="15277" CreationDate="2016-05-14T13:24:25.500" Score="23" Body="&lt;p&gt;Mars Pathfinder, Spirit, and Opportunity all could come to rest on any of four sides of the tetrahedral-shaped lander. Just by chance, Mars Pathfinder and Spirit landed with the base petal down. However Opportunity landed on one of the side petals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The system was, of course, designed for that. The three side petals all have motors to drive them open to be co-planar with the base petal. The computer detects which side is down using the accelerometer in the inertial measurement unit. For Opportunity, the side petal that was down was driven open first. The motor had more than enough torque for that side petal to push the whole lander over so that it fell onto the base petal. Then the other two petals opened.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/-_9BYSDtwRc?t=3m46s&quot;&gt;This video&lt;/a&gt; shows a side petal landing at 3:50.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-14T13:46:03.983" LastActivityDate="2016-05-14T13:46:03.983" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15279" PostTypeId="2" ParentId="15274" CreationDate="2016-05-14T13:38:07.540" Score="1" Body="&lt;p&gt;The local time can be whatever you choose. It is not something you calculate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to be able to maintain a fixed local time with very little propellant usage, the inclination has to be a particular value for any given altitude. What you &lt;em&gt;can&lt;/em&gt; calculate is the &lt;em&gt;rate of change&lt;/em&gt; of the local time as a function of the altitude and inclination.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-05-14T13:38:07.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15280" PostTypeId="1" AcceptedAnswerId="15281" CreationDate="2016-05-14T16:13:33.997" Score="5" ViewCount="441" Body="&lt;p&gt;How does the number of observations of spacecraft position and velocity affect the orbit determination accuracy? Suppose in one case we have 1000 instances of position and velocity measurements with a sampling of 1 second and in other case we have 2000 instances of position and velocity measurements with sampling of 500 milliseconds. Why would the accuracy of the orbit be more in the second case?&lt;/p&gt;&#xA;" OwnerUserId="14402" LastEditorUserId="6816" LastEditDate="2016-05-14T18:35:48.957" LastActivityDate="2016-05-19T02:15:35.257" Title="How does the Orbit Determination Accuracy depend on the number of measurements?" Tags="|orbit|orbit-determination|accuracy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15281" PostTypeId="2" ParentId="15280" CreationDate="2016-05-14T17:09:52.750" Score="8" Body="&lt;p&gt;There are several aspects playing a role here: first of all, every measurement has a certain random inaccuracy. Ask ten people to measure the size of an object with the best precision possible and you'll likely get ten slightly different answers. The average of those measurements is usually a much better estimate for the size than any individual measurement. Thus, with twice as many measurements you know the position and velocity of the space craft better. If the error of those measurements is purely random, you expect the total error to be sqrt(2) better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, if you have many measurements close together in time, you need to extrapolate a lot to get the full orbit of your spacecraft. Extrapolation is always a source of errors, as a very small variation in the starting conditions can cause tremendous differences in a later state. You specified measurements 16 minutes apart, which is a significant time of a full orbit in LEO, but not so much in GEO. Hence, a LEO orbit can be determined rather well within these 16 minutes, but for a satellite close to GEO (or a probe even further out in space) you might want to spread measurements further. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, measurement of the position of a satellite can't be done equally well in the three spatial directions. Distance to the satellite and its speed away or towards us can be measured very accurately using signal latencies and the Doppler effect. The other two dimensions are not as easy, e.g. you need to know very precisely where your telescope points, and due to the large distances even minute errors in the pointing angle sum up to large uncertainties in the position and subsequently the speed measurements. At different points in time, the relative movement of the satellite is in different directions. This means these inaccuracies can cancel out to some extent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary: Having more measurements helps by reducing the error due to measuring inaccuracies. Having them spread over a larger time helps by knowing more information on very different points of the orbit and helps by reducing the amount of extrapolation to be done.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="1142" LastEditDate="2016-05-19T02:15:35.257" LastActivityDate="2016-05-19T02:15:35.257" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15282" PostTypeId="2" ParentId="15276" CreationDate="2016-05-14T17:34:24.300" Score="2" Body="&lt;p&gt;At least on the Shuttle orbiter, a predefined, known pair of stars was acquired in the two star trackers (one per tracker).  This defined the vehicle's attitude since the position of the stars was known, and the position of the trackers in the vehicle was known.  This was done because...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Alignment of the IMUs is required approximately every 12 hours to&#xA;  correct IMU drift, within one to two hours before major on-orbit&#xA;  thrusting duration or after a crewman optical alignment sight IMU&#xA;  alignment. IMU alignment is accomplished by using the star trackers to&#xA;  measure the line-of-sight vector to at least two stars. With this&#xA;  information, the GPC calculates the orientation between these stars&#xA;  and the orbiter to define the orbiter's attitude. A comparison of this&#xA;  attitude with the attitude measured by the IMU provides the correction&#xA;  factor necessary to null the IMU error.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;IMU = inertial measurement unit&#xA;GPC = general purpose computer&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-gnnc.html#sts-star-tracker&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. (the references to how often the IMUs require alignment are from early in the program, before IMU upgrades).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-05-14T17:34:24.300" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15284" PostTypeId="2" ParentId="15276" CreationDate="2016-05-14T18:40:38.510" Score="4" Body="&lt;p&gt;Modern star trackers are digital cameras coupled with sophisticated computer equipment that uses pattern matching techniques to determine which stars are in the image taken by the camera. The location of one star in the camera frame yields two pieces of information, which isn't quite enough to determine the orientation of the star tracker. Two stars are needed, at a minimum, providing four pieces of information. In practice, multiple stars are used and orientation is determined via a least squares filter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having the Sun or a bright planet in the field of view would damage the sensitive equipment. Star trackers typically use a sun sensor to close the shutter on the star tracker camera. Having the shutter closed means the star tracker is temporarily off-line. Some vehicles use multiple star trackers or star trackers with multiple heads because of this.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-05-14T18:40:38.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15287" PostTypeId="1" CreationDate="2016-05-14T22:26:08.160" Score="6" ViewCount="699" Body="&lt;p&gt;&lt;a href=&quot;https://movies.stackexchange.com/a/52981&quot;&gt;This answer on Movies.SE&lt;/a&gt; collects some fascinating facts about the sulfur-dominated surface and atmosphere of Jupiter's moon Io. But it made me wonder where all the sulfur comes from in the first place. Volcanoes powered by tidal heating of the interior, it says -- very well, but volcanoes don't &lt;em&gt;make&lt;/em&gt; sulfur.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is Io simply unusually rich in sulfur, right back from when it accreted? Or do the other Galilean moons contain similar amounts of sulfur but just don't wear it on their outsides? Because they lack volcanism? Earth does have volcanism, but that hasn't led its surface being sulfur-dominated. Because we had less sulfur in the first place, or just because our atmosphere retains lighter elements that dominate over the same amount of sulfur that Io also has?&lt;/p&gt;&#xA;" OwnerUserId="10165" LastEditorUserId="-1" LastEditDate="2017-04-13T12:54:05.243" LastActivityDate="2021-02-07T22:42:36.753" Title="Where does Io's sulfur come from?" Tags="|planetary-science|jupiter|moons|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15289" PostTypeId="2" ParentId="15287" CreationDate="2016-05-15T14:14:00.167" Score="5" Body="&lt;p&gt;It's erroneous to think of those volcanos on Io as sulphur volcanos. This is an old idea. Io's volcanos are instead somewhat similar to volcanos on the Earth, where the primary product is mafic rock (possibly ultramafic rock in the case of Io), but with lots of volatiles as secondary products.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of volcanos on Earth, those volatiles include water, carbon dioxide, and various gaseous sulfur compounds. Lots of sulfur compounds. The 1783-1784 eruption of Laki in Iceland and the 1815 eruption of Mount Tambora in the Philipines dumped massive quantities of sulfur dioxide into the atmosphere. Both of these eruptions had very significant (but short-lived) impacts on the Earth's climate. Much more severe volcanic eruptions are now believed to be responsible for many of the extinction events, including the biggest of all, the Permian–Triassic extinction event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of Io, there's no water, and very little carbon dioxide. The volatiles released by Io's volcanos are mostly sulfuric compounds. If Io ever did have water, it lost it long ago. The same is true for carbon dioxide. Water and carbon dioxide are both light and highly volatile. Sulfur dioxide is 45% denser than is CO&lt;sub&gt;2&lt;/sub&gt;, and is significantly less volatile that are either water or CO&lt;sub&gt;2&lt;/sub&gt;. While Io can't quite hold onto its thin atmosphere of sulfur dioxide, the decreased volatility means that a good portion of that vented sulfur dioxide gas condenses and falls to Io's surface, eventually to be recycled into Io's interior.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With regard to where all that sulfur came from, sulphur 32 is the tenth most abundant isotope in the Solar System and in the Galaxy. The reason for this abundance is that &lt;sup&gt;32&lt;/sup&gt;S is on the alpha ladder, a fusion process that eventually results in the production of &lt;sup&gt;56&lt;/sup&gt;Ni (which quickly decays into &lt;sup&gt;56&lt;/sup&gt;Co and then &lt;sup&gt;56&lt;/sup&gt;Fe) in supernovae. The alpha ladder is why we see so much carbon, oxygen, neon, magnesium, silicon, sulfur, and iron in the universe.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-05-15T14:32:51.023" LastActivityDate="2016-05-15T14:32:51.023" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15290" PostTypeId="1" AcceptedAnswerId="15291" CreationDate="2016-05-15T16:41:40.440" Score="31" ViewCount="5298" Body="&lt;p&gt;I'm preparing a talk on data storage for an ASME meeting. Given that it's 20 billion km away, I want to quote Voyager I as the &quot;largest control loop&quot; in existence. I'm guessing the tape recording capability is no longer used, but is it still true that someone here on earth could press a button and 18 hours later the tape would start running and 18 hours after that we could pick up the data with the deep space network. I want a &quot;gee-whiz&quot; start to the talk so appreciate any help on interesting aspects or anecdotes, etc. (I see the discussion on angular momentum, for example) &#xA;- Roger&lt;/p&gt;&#xA;" OwnerUserId="14419" LastActivityDate="2022-01-08T17:54:57.233" Title="Does the tape recorder on Voyager-1 still work?" Tags="|voyager|data-storage|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15291" PostTypeId="2" ParentId="15290" CreationDate="2016-05-15T17:46:50.713" Score="30" Body="&lt;p&gt;Edit: The &lt;a href=&quot;https://voyager.jpl.nasa.gov/mission/status/#sfos&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacecraft operations schedule&lt;/a&gt; contains the answer. On December 18, 2021, we see an item 'PLAYBACK' show up in the schedule for Voyager 1.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6I1Wv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6I1Wv.jpg&quot; alt=&quot;Schedule&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;S/C 31 is Voyager 1. The 'PLAYBACK' label is the data playback itself, this takes about 6 hours at 1.4 kbit/s. Before playback you can see 'X/B HI POWER', which switches the transmitter to high power mode (18 instead of 12 W).&lt;/p&gt;&#xA;&lt;p&gt;'31 DSN Coverage' shows the antennas that are used. '55' is DSS 55, one of the antennas at the Madrid station. They're using 4 antennas (at Madrid) in an array: DSS 63 is the 70-m antenna, the others are 34 m diameter (DA = downlink pass in an array, R is the array reference antenna).&lt;/p&gt;&#xA;&lt;p&gt;At various other times in the schedule you'll see 'TAPPOS', which I'm guessing is a Tape Position command, in preparation for recording PWS data.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;This part was written in 2016. The tape recorder on &lt;a href=&quot;http://voyager.jpl.nasa.gov/science/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1 is still in use:&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Science data are returned to earth in real time at 160 bps. Real time data capture uses 34 meter Deep Space Network (DSN) resources (see below) with the project goal to acquire at least 16 hours per day of real time data per spacecraft. This goal is not always achieved due to the competition for DSN resources with prime mission projects and other extended mission projects.&lt;/p&gt;&#xA;&lt;p&gt;Once a week per spacecraft, 48 seconds of high rate (115.2 kbps) PWS data are recorded onto the Digital Tape Recorder (DTR) for later playback.  An additional 48 seconds are recorded each week on Voyager 1.  These data are played back to Earth once every 6 months per spacecraft and require 70 meter DSN support for data capture.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://deepspace.jpl.nasa.gov/about/#&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; (DSN) is a series of large dish antennas used by NASA to communicate with deep-space missions like Voyager. The DSN includes dishes with a diameter of 34 m and a few with a diameter of 70 m.&#xA;There are only a few antennas this large in the world, so they are shared between various deep-space missions.&lt;/p&gt;&#xA;&lt;p&gt;On Voyager 2, &lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;DTR operation was ended in 2007&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Voyager 2 DTR operations was no longer needed due to a failure on the high waveform receiver on the Plasma Wave Subsystem (PWS) on June 30th, 2002.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The JPL site lists a &lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;cutoff date in approx. 2018&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;limited by ability to capture 1.4 kbit/s data using a 70 m/34 m antenna array. This is the minimum rate at which the DTR can read-out data.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I'd expect it to be possible to play back some data from the DTR into Voyager's RAM, then transmit at a slower rate from RAM. The JPL site doesn't mention this possibility, though.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2022-01-08T17:54:57.233" LastActivityDate="2022-01-08T17:54:57.233" CommentCount="20" ContentLicense="CC BY-SA 4.0" />
  <row Id="15292" PostTypeId="1" CreationDate="2016-05-15T20:22:46.603" Score="4" ViewCount="2345" Body="&lt;p&gt;This question is explicitly not about doing burns at the apoapsis or periapsis in order to raise/lower the other apsis.  Of course those are going to be the most efficient maneuvers, and the burns should be prograde or retrograde to the orbit at those points.  The question is also not about Hohmann transfers, or circular starting orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, given a point which is closer to midway between the apses, what is the optimum burn $\Delta V$ and direction to modify one of the apses?  For a more concrete example, lets say that we are &quot;roughly&quot; halfway on the return from the apoasis to the periapsis and wish to raise or lower the periapsis.  Also this problem could be a hyperbolic entry orbit and we're trying to raise or lower the periapsis.  The change in the final apoapsis is only going to be constrained by the requirement that the $\Delta V$ of the burn is minimized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know of one solution to this problem which assumes that a good enough burn direction will be tangent to the surface we are orbiting (not tangent to the orbit) and then does a binary search to find the burn that produces the right periapsis.  Is there a better closed-form solution to this problem, and one which finds the minimal-$\Delta V$ burn?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Burning prograde or retrograde can also raise or lower the periapsis, but when away from the apoapsis this direction of the burn is less efficient.&lt;/p&gt;&#xA;" OwnerUserId="14420" LastEditorUserId="14420" LastEditDate="2016-05-15T22:18:41.480" LastActivityDate="2020-07-01T05:19:50.617" Title="Optimum delta-v burn to change periapsis or apoapsis at an arbitrary point on an elliptical orbit?" Tags="|orbital-mechanics|orbital-maneuver|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15293" PostTypeId="2" ParentId="15292" CreationDate="2016-05-16T03:17:54.557" Score="3" Body="&lt;p&gt;I am not aware of a closed form solution, and I'm pretty sure there isn't one. You need to optimize the direction for each $\Delta V$, and find the pair that gets you to where you want to be optimally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the apoapsis and the orbit orientation will change as well. So to optimize the periapsis change, you will need to be flexible on exactly &quot;where you want to be&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a simple example, where the direction was optimized for a fixed $\Delta V$ to maximize the periapsis increase:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rOrkn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rOrkn.png&quot; alt=&quot;two orbits&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The initial blue orbit was changed to the orange orbit with an impulsive maneuver at the location and direction of the green arrow, which is on the inbound portion of the orbit halfway across the ellipse. The body is at the origin. It is clear that in addition to raising the periapsis, the apoapsis was also raised and the orbit was rotated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, that optimal direction is 7.2° above a tangent to a circle around the body through that point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can learn more about orbit changes in these locations by looking at infinitesimal maneuvers, and seeing analytically how the orbit elements change. This is useful for low-thrust activities, where you would like to thrust over much or all of the orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-16T04:21:36.193" LastActivityDate="2016-05-16T04:21:36.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15294" PostTypeId="1" CreationDate="2016-05-16T07:49:45.257" Score="4" ViewCount="481" Body="&lt;p&gt;In this &lt;a href=&quot;https://space.stackexchange.com/questions/1551/what-is-the-purpose-of-the-jets-of-water-often-under-rocket-engines-during-launc&quot;&gt;Q and A&lt;/a&gt;, it is explained that at Baikanour rocket launches are suspended over a cliff and vented instead of using a water-based sound suppression system as is commonly used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would seem that this would have significant advantages in terms of not having to build and maintain a water storage, draining and pumping infrastructure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this 'launch over a cliff' system not more widely used?&lt;/p&gt;&#xA;" OwnerUserId="14426" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-05-16T13:47:22.217" Title="What are the advantages of water-based sound suppression vs. cliff-based launching?" Tags="|launch|water|launchpad|baikonur-cosmodrome|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15295" PostTypeId="2" ParentId="15294" CreationDate="2016-05-16T12:51:57.830" Score="4" Body="&lt;p&gt;Weather is one major reason. In Florida, it basically never stays below freezing for longer than a day. So the water stays water. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Baikanour, it can be quite cold for long periods of time.  If you tried a water suppression system in the midst of winter in Baikanour, odds are good you will have to wait for spring to use your launch pad again, as all the ice forms so thick you cannot get it off without destroying the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Florida also is somewhat lacking in natural or even unnatural cliffs. Something of a flat place. Sure you could build one, and in some ways, they did for LC-39A/B where they drive the Shuttle/Saturn V (Soon to be Falcon 9/Falcon Heavy) up a manmade ramp to a man made elevated platform so that the thrust can go down below. While the scale of the platform is quite large, it is nothing quite like what the Baikanour cliffs are like.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-05-16T13:47:22.217" LastActivityDate="2016-05-16T13:47:22.217" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15296" PostTypeId="2" ParentId="15025" CreationDate="2016-05-16T14:55:21.460" Score="10" Body="&lt;p&gt;&lt;a href=&quot;http://aviationweek.com/space/nasa-outlines-mars-red-dragon-deal-spacex&quot; rel=&quot;noreferrer&quot;&gt;Aviation Week&lt;/a&gt; seems to be the best source right now. A few interesting bits: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;NASA is spending up to $30 million in support, mostly to help gain information for EDL on Mars.&lt;/li&gt;&#xA;&lt;li&gt;A photo might be taken using either ground-based sources, or orbital sources, to help the process.&lt;/li&gt;&#xA;&lt;li&gt;The specific plans will be outlined at the International Astronautical Congress in Guadalajara, Mexico in September 2016.&lt;/li&gt;&#xA;&lt;li&gt;The partnership with NASA is only in effect for the 2018 launch, not for a future launch window.&lt;/li&gt;&#xA;&lt;li&gt;EDL data will be gathered by the orbiters in real time, in case the spacecraft doesn't survive the landing.&lt;/li&gt;&#xA;&lt;li&gt;SpaceX will determine the payload, NASA wants to include the following: Mars-weather sensors, instruments to analyze atmospheric dust, and experimental in situ resource utilization gear.&lt;/li&gt;&#xA;&lt;li&gt;On board video will likely be used to image the plumes during decent. This video will be uploaded upon a successful landing.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it seems that most of the payload will be in EDL data, but there will likely be a few small experiments done that haven't been prioritized previously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other point of interest, it seems likely that they want the landing sequence to go over one of the rovers on Mars first, probably in the Supersonic Retropropulsion phase. Not sure what that means for it's eventual landing site, but it likely won't be that far away from one of the existing rovers on Mars.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2016-05-16T22:51:23.370" LastActivityDate="2016-05-16T22:51:23.370" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15297" PostTypeId="2" ParentId="12835" CreationDate="2016-05-16T17:47:58.023" Score="0" Body="&lt;p&gt;The four control-moment-gyroscopes employ a double-gimbal design that allows the spin axis to maintain an appropriate orientation that is not fixed in the station frame of reference.  Although not usually 'free to swing around', this allows electric motors to create torque by acting against the spin axis momentum vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The software controller uses the motors to overcome 'resistance torques' to keep the station stable and rotating to track orbital motion around the earth.  In effect, the station uses energy from the sun to drive the motors and generate the required torques to overcome resistance and maintain the orientation in orbit.  Z-axis of the station points along the radial line to the earth.  Occupants get best view of the planet throughout the orbit.&lt;/p&gt;&#xA;" OwnerUserId="14432" LastActivityDate="2016-05-16T17:47:58.023" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15299" PostTypeId="1" AcceptedAnswerId="15314" CreationDate="2016-05-17T00:25:48.480" Score="9" ViewCount="2227" Body="&lt;p&gt;Would rocks on Mars or the Moon be the same as rocks on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or would the lower gravity mean they would not be as dense or strong?&#xA;Or because you wouldn't have plate tectonics would you not find rocks formed by pressure on the surface?&lt;/p&gt;&#xA;" OwnerUserId="10094" LastEditorUserId="415" LastEditDate="2016-05-17T14:11:26.660" LastActivityDate="2016-05-17T16:41:04.737" Title="Are rocks on Mars or the Moon the same as rocks on Earth?" Tags="|mars|the-moon|regolith|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15300" PostTypeId="1" CreationDate="2016-05-17T01:12:40.583" Score="0" ViewCount="143" Body="&lt;p&gt;I remember reading about an accident where a big, expensive communications satellite actually fell off it's mount and landed on the floor of the room where it was undergoing final assembly and preparation (I believe). It was supported on a tilting mount, and it was raised up without the mounting bolts having being in place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time frame is roughly two years ago (~2013 to 2015).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recently I searched for the news again, and I can't find a trace of it on the internet! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Payloads are built to endure serious vibration and shock in order to reliably survive launch with a wide safety margin. However, when a rigid object without &quot;crush zones&quot; or shock absorbers falls and lands on a hard surface, accelerations can be 100's or 1000's of gees for milliseconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any idea which satellite it was? A link would be great. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea is that even in the 21st century, rigid adherence to detailed written procedures as well as &lt;em&gt;writing those detailed procedures&lt;/em&gt; are still both &lt;em&gt;really important&lt;/em&gt;!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-05-17T01:12:40.583" Title="Which communications satellite &quot;fell down?&quot;" Tags="|artificial-satellite|testing|failure|" AnswerCount="0" CommentCount="3" ClosedDate="2016-05-17T01:17:06.567" ContentLicense="CC BY-SA 3.0" />
  <row Id="15303" PostTypeId="2" ParentId="13095" CreationDate="2016-05-17T06:06:47.090" Score="8" Body="&lt;p&gt;Very late to the party on this one, but I found a really, really, &lt;em&gt;really&lt;/em&gt; detailed description of SSME startup &lt;a href=&quot;http://www.enginehistory.org/Rockets/SSME/SSME3.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (PDF).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, the whole system is started when opening the main fuel valve lets LH2 into the nozzle and main combustion chamber cooling systems, which aren't insulated.  The environmental heating to the LH2 causes it to expand, which generates the inlet pressure for the preburners and also gets the low-pressure fuel pump going.  After that, the oxidizer system has to be primed in the proper sequence, which are carefully timed to synchronize with wild 2 Hz pressure fluctuations in the fuel system, caused by the heated LH2 trying very hard to flow backward into the pump outlet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Get too much back pressure on the fuel pump outlet and the resistance added to the pump/turbine shaft causes the preburner to explode.  Get too little back pressure, and the pump will over-spin and rip itself to pieces.  The whole thing is... &lt;em&gt;tricky&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="14308" LastEditorUserId="511" LastEditDate="2018-06-05T19:22:44.153" LastActivityDate="2018-06-05T19:22:44.153" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="15304" PostTypeId="1" AcceptedAnswerId="15308" CreationDate="2016-05-17T06:29:44.383" Score="4" ViewCount="1070" Body="&lt;p&gt;There's been a lot of discussion about SpaceX's ~30% price reduction from re-use of the first stage, which got me to wondering:  why exactly was the Space Shuttle so expensive to service and prep for re-launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm mostly interested in the actual servicing of the SSMEs themselves.  They're fiendishly complicated, but does anybody know exactly what the main procedures were, and how much they cost?  Or was it the case that other Orbiter processing tasks and low launch tempo were the biggest contributors to the cost?&lt;/p&gt;&#xA;" OwnerUserId="14308" LastEditorUserId="6944" LastEditDate="2016-07-07T12:45:01.373" LastActivityDate="2022-04-04T17:18:21.783" Title="Space Shuttle and SSME Servicing Procedures for Re-Launch" Tags="|launch|spacex|space-shuttle|reuse|ssme|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15305" PostTypeId="1" AcceptedAnswerId="15313" CreationDate="2016-05-17T08:03:53.653" Score="19" ViewCount="3250" Body="&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_13h_Timeline.htm&quot; rel=&quot;noreferrer&quot;&gt;Looking at the Apollo 13 mission timeline&lt;/a&gt; within three hours after the explosion of O2 tank two all oxygen in the SPS was completely depleted. Fuel cells #1 and #3 were off line. CM had even started to cannibalize its reentry batteries, before it was shut down. The damage to the SPS was unknown and deemed unreliable &lt;strike&gt;(&lt;a href=&quot;https://space.stackexchange.com/questions/2140/why-was-the-apollo-cm-main-engine-thought-to-be-unreliable-on-apollo-13/2145#2145?newreg=aea4748c919b4be792db5659c7be4049&quot;&gt;you couldn't fire it with the LEM attached anyway&lt;/a&gt;)&lt;/strike&gt; [see &lt;a href=&quot;https://space.stackexchange.com/questions/15305/why-wasnt-the-apollo-13-service-module-jettisoned-until-hours-before-reentry#comment40181_15305&quot;&gt;comment]&lt;/a&gt;. It seems to me that, at that point, the Service Module had no resources that were of any use anymore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What were the reasons NASA opted to keep the SM attached for the rest of mission? I would expect that attitude and trajectory control would have been much easier without hauling the dead SM around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only explanation I could think of was to protect the CM's heat shield from direct exposure to space?&lt;/p&gt;&#xA;" OwnerUserId="14440" LastEditorUserId="6944" LastEditDate="2018-06-26T16:00:36.040" LastActivityDate="2018-06-26T16:00:36.040" Title="Why wasn't the Apollo 13 Service Module jettisoned until hours before reentry?" Tags="|apollo-program|apollo-13|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15306" PostTypeId="1" AcceptedAnswerId="15307" CreationDate="2016-05-17T10:43:55.810" Score="14" ViewCount="2860" Body="&lt;p&gt;I struck out with my &lt;a href=&quot;https://space.stackexchange.com/q/15300/12102&quot;&gt;previous question&lt;/a&gt; so I'll try again.&lt;/p&gt;&#xA;&lt;p&gt;What are these large circular 4-vane structures on the sides of &lt;a href=&quot;https://en.wikipedia.org/wiki/NOAA-19&quot; rel=&quot;nofollow noreferrer&quot;&gt;NOAA-19&lt;/a&gt; - the satellite that &amp;quot;fell down&amp;quot;?&lt;/p&gt;&#xA;&lt;p&gt;They remind me of adjustable air vents but I have a feeling that's not quite right.&lt;/p&gt;&#xA;&lt;p&gt;You can see them from &lt;a href=&quot;http://www.spaceref.com/news/viewsr.html?pid=10299&quot; rel=&quot;nofollow noreferrer&quot;&gt;another angle here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WsOdl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WsOdl.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Spacecraft rotated for better view:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nm1qV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nm1qV.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/W75Yc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/W75Yc.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2022-01-29T07:01:59.867" LastActivityDate="2022-01-29T07:01:59.867" Title="What are these air-vent-like structures on this satellite?" Tags="|artificial-satellite|identify-this-object|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="15307" PostTypeId="2" ParentId="15306" CreationDate="2016-05-17T10:58:45.423" Score="15" Body="&lt;p&gt;They are thermal control pinwheels.  Reference: figure on page 3 of &lt;a href=&quot;http://www.osd.noaa.gov/Spacecraft%20Systems/Pollar_Orbiting_Sat/NOAA_N_Prime/NOAA_NP_Booklet.pdf&quot; rel=&quot;noreferrer&quot;&gt;this document &lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HDR1R.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HDR1R.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: odd how the diagram shows 4 devices, but the note says 3.  In the photos there are 3 on one side, and 4 on another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;&lt;a href=&quot;https://books.google.com/books?id=gSseFMvSiBoC&amp;amp;pg=PA302&amp;amp;lpg=PA302&amp;amp;dq=thermal%20control%20pinwheels&amp;amp;source=bl&amp;amp;ots=_isUIwZ85y&amp;amp;sig=RGOnpphVmcgFVTFYp6W6aWY48XY&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwj4t5yqj-HMAhUh64MKHdtuALQQ6AEIHTAA#v=onepage&amp;amp;q=thermal%20control%20pinwheels&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Elements of Space Technology for Aerospace Engineers&lt;/a&gt;&quot;, page 302, describes thermal control pinwheels as follows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Pinwheel louvers are similar, but in place of rectangular blades&#xA;  incorporate vanes (typically four) in the form of sectors of a circle&#xA;  that together cover one-half of the area under the pinwheel.  They are&#xA;  also rotated by bimetal actuators.  Depending on the position of the&#xA;  sectors, they will cover (uncover) areas that have high ratios of&#xA;  solar absorptivity to emissivity and uncover (cover) areas that have a&#xA;  low such ratio.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And, there is a cross-section:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/JnCE1.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JnCE1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-05-17T12:38:37.860" LastActivityDate="2016-05-17T12:38:37.860" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="15308" PostTypeId="2" ParentId="15304" CreationDate="2016-05-17T11:04:31.090" Score="6" Body="&lt;p&gt;It varied throughout the program - the SSMEs changed a &lt;em&gt;lot&lt;/em&gt; over its course. Columbia's original FMOF engines were only marginally OK for flight. Until the Pratt turbopump redesign, the high pressure turbopumps had to be overhauled after every mission. By the end, overhaul requirements were somewhat reduced. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is overview information on SSME ground servicing procedures as they were in 1989.  All information is from &lt;a href=&quot;https://archive.org/download/PocketDataRIRD87142SpaceShuttleMainEngine/Pocket_Data_RI-RD87-142_Space_Shuttle_Main_Engine.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SSME Pocket Data, RI/RD87-142&lt;/a&gt;.  The table of contents lists the major procedures performed.  I have included a scan of page 3-3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1SXtK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1SXtK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/cGcH3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cGcH3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/1sBNA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1sBNA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-11-04T19:52:33.307" LastActivityDate="2018-11-04T19:52:33.307" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="15309" PostTypeId="2" ParentId="15304" CreationDate="2016-05-17T11:56:48.530" Score="0" Body="&lt;p&gt;I do not know the answer to the SSME specific thing, but the main cost driver for the shuttle was the standing army of 20,000 some odd folk employed by NASA, ostensibly to support the Shuttle program. If you launch 0 times a year, you cost is almost the same as if you launch 10 times a year, for those 20,000 salaries and benefits, and offices for them to work in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is using a much smaller workforce.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-17T11:56:48.530" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15310" PostTypeId="1" AcceptedAnswerId="15337" CreationDate="2016-05-17T12:12:32.403" Score="1" ViewCount="295" Body="&lt;p&gt;In a &lt;a href=&quot;https://space.stackexchange.com/q/15268/12102&quot;&gt;related question&lt;/a&gt; I'm trying to find some conclusive reference(s) helping explain if some halo orbits around the sun-earth or earth-moon $L_1$ or $L_2$ locations can actually be somewhat stable (say, for half-a-dozen orbits). There I mentioned the quote by Dennis Wingo describing ISEE-3's original Halo orbit. He describes Sun-Earth $L_1$ as &quot;&lt;em&gt;a point about 1.5 million kilometers from earth where a spacecraft can safely orbit without using any fuel&lt;/em&gt;.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The site &lt;a href=&quot;http://spacecraftforall.com/a-new-orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spacecraftforall.com/a-new-orbit&lt;/a&gt; is an interactive thing - if you leave it alone it will usually start the video in about 10 or 15 seconds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here I would &lt;em&gt;just like to know specifically&lt;/em&gt; - did ISEE-3 spend a few years in a halo orbit around sun-earth $L_1$ without using any fuel, or at least without the regular station keeping thrust maneuvers needed in unstable orbits? (e.g. like DSCOVR needs to do)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-05-19T14:56:44.707" Title="Did ISEE-3 spend a few years in a halo orbit around sun-earth $L_1$ without using any fuel?" Tags="|orbital-mechanics|artificial-satellite|lagrangian-points|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15311" PostTypeId="5" CreationDate="2016-05-17T12:29:11.547" Score="0" Body="" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-05-17T12:29:11.547" LastActivityDate="2016-05-17T12:29:11.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15312" PostTypeId="4" CreationDate="2016-05-17T12:29:11.547" Score="0" Body="Auxiliary components to liquid and hybrid propellant rocket engines that use rotating parts to feed propellant into a combustion chamber. Includes turbines, turbopumps, electrical pumps, barrier seals, bearings, and the like." OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-05-17T12:29:11.547" LastActivityDate="2016-05-17T12:29:11.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15313" PostTypeId="2" ParentId="15305" CreationDate="2016-05-17T15:16:44.153" Score="26" Body="&lt;p&gt;It appears that your speculation is correct.  From the Lunar and Planetary Institute's &lt;a href=&quot;http://www.lpi.usra.edu/lunar/missions/apollo/apollo_13/overview/&quot; rel=&quot;noreferrer&quot;&gt;Apollo 13 Mission Overview&lt;/a&gt; (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the Apollo spacecraft, the Service Module (SM) was intended to&#xA;  provide most of the consumables such as oxygen, water, and power for&#xA;  the mission. It was also designed to serve as the primary propulsion&#xA;  and maneuvering system of the spacecraft. The explosion of the oxygen&#xA;  tank however, led to the loss of the consumables and rendered the&#xA;  service module nonfunctional. &lt;strong&gt;The SM was retained until just before&#xA;  reentry to protect the command module heat shield from the possible&#xA;  degrading effects of long exposure to the cold of space.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-05-18T19:55:24.510" LastActivityDate="2016-05-18T19:55:24.510" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15314" PostTypeId="2" ParentId="15299" CreationDate="2016-05-17T16:41:04.737" Score="7" Body="&lt;p&gt;Since rock is classified by formation processes, each type of rock would be about the same no matter what planet you found it on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EeAwQ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EeAwQ.png&quot; alt=&quot;metamorphic facies&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;https://en.wikipedia.org/wiki/Metamorphic_facies&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Metamorphic_facies&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you're right that finding certain types of rock is harder on the surface of the Moon or Mars than it is on Earth, whether it's because the minerals and pressure/temperature ranges are not found or because there are fewer mechanisms allowing deeper rock to rise to the surface.  Also, as far as I know the Moon has not had oceans or lakes, which are a requirement for the formation of limestone and hence marble.  Limestone would also be hard to find on Mars since though there is evidence of past large bodies of water, we haven't found the depositions of shells and other material that typically forms limestone on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not a planetary geologist, though, so hopefully one of them can answer more completely.&lt;/p&gt;&#xA;" OwnerUserId="13978" LastActivityDate="2016-05-17T16:41:04.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15315" PostTypeId="2" ParentId="5583" CreationDate="2016-05-17T22:16:16.187" Score="0" Body="&lt;p&gt;100 years, possibly yes. 200, with high tech plasma generator (currently in research stage) possibly. 5 billions years.. most likely not. Solar energy gives you energy, yes, but then you cannot just point a battery on your rear and get a push. You need to 'throw' something so in order to get thrust. Rocket engine releases gas at very high speed, which gives you thrust. Microthrusters also pushes gas typically, though you can have ion thrusters nowadays. Ion thrusters still have to carry their own gas to ionize and throw out. So you need some material to throw or consummate (think of it as fuel to burn), and so after a while, you will run out of matter to throw, even if your battery is full. Voyager use gravity assist. Staying in orbit around the Earth does not give you that.&lt;/p&gt;&#xA;" OwnerUserId="14446" LastActivityDate="2016-05-17T22:16:16.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15316" PostTypeId="1" CreationDate="2016-05-18T00:47:33.940" Score="5" ViewCount="997" Body="&lt;p&gt;Related to this question, but hopefully more narrow and therefore has an answer: &lt;a href=&quot;https://space.stackexchange.com/questions/12324/are-there-any-other-options-for-human-settlements-in-the-solar-system-other-than&quot;&gt;Are there any other options for human settlements in the solar system other than Mars?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has announced that it has a plan to colonize Mars by 2030: &lt;a href=&quot;http://www.telegraph.co.uk/science/2016/03/14/nasa-planning-earth-independent-mars-colony-by-2030s/#ifrndnloc&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.telegraph.co.uk/science/2016/03/14/nasa-planning-earth-independent-mars-colony-by-2030s/#ifrndnloc&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming that is a success, what is the next most likely target (for NASA or other organizations/countries) to be pursued for independent, permanent, human settlement in space? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sake of argument, let's consider independent to be defined as a state in which getting cut off from Earth for a 5 year period would not be fatal to everyone in the settlement. &lt;/p&gt;&#xA;" OwnerUserId="14448" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-05-18T10:24:46.643" Title="After Mars, what's the next likely location to be permanently settled by humans in space?" Tags="|mars|colonization|" AnswerCount="3" CommentCount="3" ClosedDate="2016-05-18T09:34:23.220" ContentLicense="CC BY-SA 3.0" />
  <row Id="15317" PostTypeId="2" ParentId="15316" CreationDate="2016-05-18T01:33:57.343" Score="3" Body="&lt;p&gt;I think the asteroid belt between Mars and Jupiter could be the next place to be settled by humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I think it could be settled next is because I think asteroid mining could become the next thing to decrease costs in building for space travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the reason I think it would need to be settled is because I don't think you would have the gravity assist that you could for traveling to planets or moons. So you would probably need to use ion propulsion to speed up and slow down and that I think that would take longer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this is probably looking at what is going to be possible in 50 to 100 years, so changes in technology will probably make a big difference in where we end up. &lt;/p&gt;&#xA;" OwnerUserId="10094" LastActivityDate="2016-05-18T01:33:57.343" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15318" PostTypeId="2" ParentId="15316" CreationDate="2016-05-18T05:59:56.770" Score="0" Body="&lt;p&gt;I would put my bet on one of Saturns moons, because &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetosphere_of_Saturn#Radiation_belts&quot; rel=&quot;nofollow&quot;&gt;The saturnian radiation belts are generally much weaker than those of Jupiter&lt;/a&gt;. The infrastructure for colonizing Mars in a way you imagine requires some very strong rocketry (NTR or the like). If we have such an infrastructure, Saturn is not that far away anymore. The reason to go there in the first place might be the abundance of hydrogen, helium and even &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_%28moon%29#Liquids&quot; rel=&quot;nofollow&quot;&gt;hydrocarbons&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-05-18T05:59:56.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15319" PostTypeId="2" ParentId="15316" CreationDate="2016-05-18T09:34:27.157" Score="0" Body="&lt;p&gt;Technically, the most doable would be a solar powered space station which recycles its resources well. It is almost being done already.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To motivate it, maybe near the Sun to get more energy. It could pass by Mercury once a month or two if in retrograde or polar solar orbit, for the view, for the gravity assist options and maybe to catch resources launched robotically from Mercurius' cool shadowed poles. Solar electric propulsion would have seven times more power there to help with the delta v's. Science objectives could be Mercury, heliophysics, disintegrating sun grazing comets, microgravity. It would be a great next step towards interplanetary human spaceflight in general. Maybe it could be helpful for sending huge solar sails very close to the Sun to accelerate small payloads to extreme speeds. Such Krafft Ehricke trajectories are hard to reach from Earth, going via Jupiter to lose speed enough to fall inwards the Sun. Maybe such sails could be manufactured from Mercury's resources and the station could over the years achieve an orbit from where they are easier to launch? Hey, it's hard to motivate any human stay in space, one has to brainstorm it. The ISS is about to be replaced anyway, so why not do it in a big way?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And it could save the world! :-)&#xA;&lt;a href=&quot;https://bokunosekai.files.wordpress.com/2010/03/sunshine.jpg&quot; rel=&quot;nofollow&quot;&gt;https://bokunosekai.files.wordpress.com/2010/03/sunshine.jpg&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-05-18T10:24:46.643" LastActivityDate="2016-05-18T10:24:46.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15320" PostTypeId="1" CreationDate="2016-05-18T13:22:12.497" Score="47" ViewCount="12775" Body="&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt; Bezos' recent &lt;a href=&quot;https://twitter.com/JeffBezos/status/965957024109379602&quot; rel=&quot;noreferrer&quot;&gt;Tweet&lt;/a&gt;ed update on the 10,000 year clock. Also see The Verge's &lt;a href=&quot;https://www.theverge.com/tldr/2018/2/20/17031836/jeff-bezos-clock-10000-year-cost&quot; rel=&quot;noreferrer&quot;&gt;Construction begins on Jeff Bezos’ $42 million 10,000-year clock&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The discussions associated with the question &lt;a href=&quot;https://space.stackexchange.com/q/5583/12102&quot;&gt;Can an artificial satellite stay in orbit forever?&lt;/a&gt; and its answers suddenly reminded me of the &lt;a href=&quot;http://longnow.org/clock/&quot; rel=&quot;noreferrer&quot;&gt;10,000 year clock project&lt;/a&gt;. Take a moment to look there now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After watching the &lt;a href=&quot;https://vimeo.com/56495936&quot; rel=&quot;noreferrer&quot;&gt;VIMEO video&lt;/a&gt; there, here is a &lt;a href=&quot;https://www.youtube.com/watch?v=JLK36ETIuIs&quot; rel=&quot;noreferrer&quot;&gt;YouTube video&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/JLK36ETIuIs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also thought about the passive mechanical structures described in &lt;a href=&quot;https://space.stackexchange.com/a/15307/12102&quot;&gt;this answer&lt;/a&gt; — basically bi-metallic strips that open/close shutters, but their designed behavior reacting to temperature distributions inside is actually pretty complicated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An Earth-orbiting satellite that needs to remain in orbit for a long time should at perigee be high enough to avoid essentially all atmospheric drag, but not &lt;em&gt;too high&lt;/em&gt; in order to at least &lt;em&gt;reduce&lt;/em&gt; the gravitational perturbations from Moon, Sun, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could &lt;em&gt;passively&lt;/em&gt; absorb thermal energy from the thermal difference between Sun and space, and store it through springs and gears and latches. Then, maybe once a year it would have enough power to produce a signal - by radio, or light. 10,000 year electronics might not be impossible if they are low tech (discrete devices) and efficient, and one &quot;beep&quot; per year may not be very demanding. Whatever it does should be fairly limited - not impose itself on those not wanting to see it specifically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a backup, if the active electronic &quot;beep&quot; fails, I suppose it could mechanically just change reflectivity, and &quot;suddenly appear&quot; once a year. Large surface area can be obtained in various ways — unrolling something then rolling it back up just for example. And it doesn't have to launch next month — it could be a development project (like the 10,000 year clock). Once it's up there, it wouldn't require any further management from the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculating the orbital mechanics would be a substantial project, so I'm not asking here for an orbit that can last 10,000 years. If there are calculations showing it can not exist, please post the link or calculations! But otherwise let's avoid the &quot;Orbital Mechanical Opinions&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What &lt;em&gt;else&lt;/em&gt; besides orbital mechanics is there that would pose the most difficult challenges to making an artificial satellite stay in &lt;em&gt;an orbit&lt;/em&gt; around the earth for 10,000 years, and &quot;beep&quot; or change appearance once a year?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-11-28T09:54:39.297" LastActivityDate="2020-11-28T09:54:39.297" Title="What would be the (most difficult) challenge to make a 10,000 year satellite?" Tags="|artificial-satellite|engineering|orbital-lifetime|space-art|" AnswerCount="8" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="15321" PostTypeId="2" ParentId="15320" CreationDate="2016-05-18T15:09:04.423" Score="6" Body="&lt;p&gt;The biggest problem I suspect is not the orbital mechanics, but rather the power. We don't have any method for generating power that would last that long, solar panels will lose power eventually. One could presumably come up with such a system, but it wouldn't be easy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiation is another significant problem. I suspect anything involving electronics would be worn away with time given such conditions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the orbit, I would put it in a sun synchronous orbit with 7 orbits per day, at   5172 km high. Or even lower would be fine, I think anything above, say, 1300 km or so would work for 10,000 years.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-05-18T20:52:19.570" LastActivityDate="2016-05-18T20:52:19.570" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15323" PostTypeId="2" ParentId="15320" CreationDate="2016-05-18T16:49:06.883" Score="24" Body="&lt;p&gt;The problems would be many to transmit a radio signal for 10,000 years. However there is nothing about a 10,000 year lifetime that would violate physics. It would just be extremely difficult engineering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would use a thermally driven &lt;a href=&quot;https://en.wikipedia.org/wiki/Stirling_engine&quot;&gt;Stirling engine&lt;/a&gt; for power, &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetorquer&quot;&gt;magnetic torquer&lt;/a&gt; for attitude control, and vacuum tubes for the electronics (which are much more radiation-resistant than semiconductors). The latter would be all analog for attitude sensing and control with a photoelectric tube and analog computing, and an analog, vacuum tube RF system. The system would not have a battery, and would only operate when in sunlight. (You might be able to come up with long-lived thermal energy storage to make an effective battery as well.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 10,000 year orbit would not a problem. You would let it drift some. Something like a 2000 km orbit should be sufficiently high.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-19T14:12:34.830" LastActivityDate="2016-05-19T14:12:34.830" CommentCount="21" ContentLicense="CC BY-SA 3.0" />
  <row Id="15324" PostTypeId="2" ParentId="15320" CreationDate="2016-05-18T16:50:49.040" Score="15" Body="&lt;p&gt;The biggest challenges are going to be what people have already mentioned - the funding (don't skirt over that comment, you &lt;strong&gt;did&lt;/strong&gt; ask for the challenges), an energy source for 10,000 years, and how to make a flash or radio pulse based on parts that can last that length of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electronics, by the way, is a particular problem. I've heard people boast low power computers can last for centuries, when we know the electrolytic capacitors will fail after a few decades. Amazing how easy some people assume this sort of thing is, when really it's very difficult. That's probably a question for the Electronics stack exchange.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting it in a safe orbit for 10,000 years is probably the easiest bit. (Especially a low orbit around Jupiter, for example. Not many things would have the energy to disturb it there. I admit Jupiter is out of scope though, as Earth orbit was the requirement.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Meaningful testing is an interesting thought experiment. How would we test something to be sure it would last 10,000 years? (Without &lt;strong&gt;actually&lt;/strong&gt; making the test period that long.) I'm not even sure transistors will last that long, due to solid diffusion at the junctions. (Probably why Mark Adler opted to use vacuum tubes.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-05-18T17:16:09.360" LastActivityDate="2016-05-18T17:16:09.360" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="15325" PostTypeId="2" ParentId="15304" CreationDate="2016-05-18T18:06:16.387" Score="-1" Body="&lt;p&gt;The shuttle was built with lots of unknowns, there were so many unknowns that the designers just could not think about unimportant things like the cost of reusing it.    If there was a mark2 and then a mark3, mark4, etc the costs could have come down - but that would have required people admitting that got something wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is a set of changes to rocket designs that were already well understood, with the willingness to learn for each launch.   Just think if the shuttle failed to land in one bit on it first few uses, but SpaceX got paid for doing their landing experiments, as they charge for the loads that they sent up.&lt;/p&gt;&#xA;" OwnerUserId="12645" LastActivityDate="2016-05-18T18:06:16.387" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15326" PostTypeId="2" ParentId="15320" CreationDate="2016-05-18T19:25:51.827" Score="8" Body="&lt;p&gt;I think it should be possible to put a big, dumb sphere into such an orbit around Earth that it (partially) eclipses the Sun (seen from the Earth's surface) exactly once a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This idea is exploiting the fact that the Earth-Sun system is already (per definition) the best time-keeping device for measuring exactly one year. Does that count as an answer? The orbit would lie in the ecliptic and its period would be one year, which would put it at around 2'151'500km...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; ...which is actually outside the Earth's sphere of influence (thanks for pointing that out @hiergiltdiestfu). I guess the idea could still work by putting it in a synchronous orbit around the sun, but that's not what the question asked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit 2:&lt;/strong&gt; What I was going for is a design that doesn't require any electronics or mechanics whatsoever, so there's nothing that can break. To detect the relatively small object passing in front of the Sun (or Moon for that matter), you would need specialized instruments (like a telescope), but that's true for receiving radio signals from the satellite as well.&lt;/p&gt;&#xA;" OwnerUserId="4558" LastEditorUserId="4558" LastEditDate="2016-05-19T08:09:43.407" LastActivityDate="2016-05-19T08:09:43.407" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15327" PostTypeId="2" ParentId="15320" CreationDate="2016-05-18T20:30:16.563" Score="22" Body="&lt;p&gt;To complement, not attempt to replace, the other answers, I would like to propose a difficulty I see nobody having mentioned so far, but which could potentially be very problematic over such long time scales.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Micrometeorite bombardment.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even after only 15 years in low (550 km) Earth orbit, we know that the Wide Field Planetary Camera II (WFPC2) on Hubble has seen &lt;a href=&quot;https://space.stackexchange.com/q/10029/415&quot; title=&quot;How is the Hubble main mirror protected from debris?&quot;&gt;significant micrometeorite bombardment&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, you can build your spacecraft with whipple shields and other protective measures, but particularly against bombardment at &lt;a href=&quot;https://space.stackexchange.com/q/21925/415&quot; title=&quot;What is the typical relative impact velocity of orbital debris in low Earth orbit?&quot;&gt;multiples of km/s relative velocity&lt;/a&gt;, those don't last forever. And they increase the complexity of the spacecraft, especially if you want something like the suggested &quot;become bigger once a year&quot; backup to the primary radio transmission to let people know it's still there and alive.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2017-07-20T14:27:23.990" LastActivityDate="2017-07-20T14:27:23.990" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15328" PostTypeId="2" ParentId="15304" CreationDate="2016-05-18T20:30:45.403" Score="3" Body="&lt;p&gt;Besides the SSME, I've seen estimates that a new external tank cost \$40-\$50 million and each SRB cost \$40 million to refurbish/rebuild. Add to that the cost of inspecting and replacing the TPS tiles, and pretty soon you're talking real money.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-05-19T13:44:28.910" LastActivityDate="2016-05-19T13:44:28.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15329" PostTypeId="2" ParentId="15320" CreationDate="2016-05-18T21:48:02.973" Score="52" Body="&lt;h1&gt;LAGEOS satellites&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;This has, in a way, already been done, with the &lt;a href=&quot;https://en.wikipedia.org/wiki/LAGEOS&quot; rel=&quot;noreferrer&quot;&gt;Laser Geodynamics Satellite (LAGEOS)&lt;/a&gt; satellites.  LAGEOS satellites, (the second of which was launched from the shuttle on mission STS-52), have a projected orbital lifetime of over 8 &lt;em&gt;million&lt;/em&gt; years. They are in a very stable &lt;a href=&quot;https://en.wikipedia.org/wiki/Medium_Earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;medium Earth orbit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GCgQq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GCgQq.jpg&quot; alt=&quot;LAGEOS 2 being launched from Shuttle&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are completely passive, but are illuminated by ground based lasers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/13feE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/13feE.jpg&quot; alt=&quot;LAGEOS&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, they look like disco balls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good info at the Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/LAGEOS&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6816" LastEditDate="2016-05-21T23:58:31.317" LastActivityDate="2016-05-21T23:58:31.317" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15330" PostTypeId="1" AcceptedAnswerId="15388" CreationDate="2016-05-18T22:48:27.637" Score="2" ViewCount="512" Body="&lt;p&gt;Has any actual work been done on the idea of traveling to higher dimensions, as a shortcut through space? Plenty of sci-fi books use this concept, but I want to know if there is any research being conducted on this concept. (I'm not talking about all of the warp drive research.)&lt;/p&gt;&#xA;" OwnerUserId="14456" LastEditorUserId="6816" LastEditDate="2016-05-18T22:59:01.163" LastActivityDate="2016-05-23T17:19:55.850" Title="To Hyperspace and higher dimensions" Tags="|propulsion|research|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15332" PostTypeId="2" ParentId="15058" CreationDate="2016-05-18T23:32:13.973" Score="0" Body="&lt;p&gt;It could use a large radioisotope thermoelectric generator. If it used this along with several rechargeable and non rechargeable batteries, it could possibly remain with power for several months.&lt;/p&gt;&#xA;" OwnerUserId="14456" LastEditorUserId="12102" LastEditDate="2020-08-04T03:35:38.703" LastActivityDate="2020-08-04T03:35:38.703" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="15334" PostTypeId="1" CreationDate="2016-05-19T08:24:06.043" Score="4" ViewCount="436" Body="&lt;p&gt;Is there any software that allows me to experiment with solar sail models?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to try models of sails of different sizes, different positions around the Solar System, and different trajectories.&lt;/p&gt;&#xA;" OwnerUserId="13749" LastEditorDisplayName="user10509" LastEditDate="2016-05-19T14:44:30.523" LastActivityDate="2016-10-18T00:31:47.847" Title="Is there software that allows me to experiment with solar sail parameters?" Tags="|software|solar-sail|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15335" PostTypeId="2" ParentId="15334" CreationDate="2016-05-19T08:57:36.357" Score="4" Body="&lt;p&gt;Martin Schweiger's &lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/download.html&quot; rel=&quot;nofollow&quot;&gt;&quot;Orbiter&quot;&lt;/a&gt; (a freely downloadable simulation/game) has basic radiation pressure simulation and apparently a solar sail scenario which would be the first thing to try. (I haven't tried radiation pressure simulation at all though.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example see a vague mention of this ability &lt;a href=&quot;http://www.orbiter-forum.com/showthread.php?t=33480&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Hint: if the solar sail doesn't have any attitude controls, point it coarsely using the Scenario Editor to alter pitch/roll/yaw, and kill any unwanted angular velocity if necessary.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-05-19T13:26:35.377" LastActivityDate="2016-05-19T13:26:35.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15336" PostTypeId="2" ParentId="15012" CreationDate="2016-05-19T11:22:00.817" Score="0" Body="&lt;p&gt;The death toll was about a hundred people, most websites suggest it was around 90 or 100.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See e.g. James Oberg's account &lt;a href=&quot;http://www.astronautix.com/t/thenedelincatastrophe.html&quot; rel=&quot;nofollow&quot;&gt;Disaster at the Cosmodrome&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But every answer raises another question I might have taken the 54 names listed there as the total death toll had I not noticed that Nedelin's name was not among them. When I asked my guide why, he replied that the commander's body had been sent home for burial. How many others had been sent home? I asked. The guide thought for a moment &quot;About 40,&quot;he suggested tentatively. The death toll, then, was nearly 100 men &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;or the &lt;a href=&quot;http://www.spacesafetymagazine.com/space-disasters/nedelin-catastrophe/historys-launch-padfailures-nedelin-disaster-part-1/&quot; rel=&quot;nofollow&quot;&gt;two&lt;/a&gt; &lt;a href=&quot;http://www.spacesafetymagazine.com/space-disasters/nedelin-catastrophe/failure-launch-pad-nedelin-disaster-part-2/&quot; rel=&quot;nofollow&quot;&gt;articles&lt;/a&gt; in Space Safety Magazine:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Despite at least 150 people killed (the real number remains unknown, as the official death toll was either never counted or revealed), the catastrophe was completely covered up.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sadly, this is only a estimate, and the death toll may be much higher. The number of people killed is something lost in the collapse of the Soviet Union.&lt;/p&gt;&#xA;" OwnerUserId="14456" LastEditorDisplayName="user10509" LastEditDate="2016-08-18T07:34:08.220" LastActivityDate="2016-08-18T07:34:08.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15337" PostTypeId="2" ParentId="15310" CreationDate="2016-05-19T14:51:02.353" Score="1" Body="&lt;p&gt;It is true that in some libration points, and this includes ESL-1 (Earth-Sun-Libration point 1), you can orbit the location with zero fuel.  Think of it as Einstein's visualization of a dimple in space time (which is what gravity is) and that the spacecraft orbits the deepest point of the dimple.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the problem is that in a tight ESL-1 orbit, the spacecraft is in a direct line of sight to the sun.  This creates a communications problem because the sun's radio frequency noise is orders of magnitude higher power than the transmitters on the spacecraft.  Therefore what spacecraft actually do is orbit the libration point near the &quot;top&quot; of the gravitational dimple.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For ISEE-3 this was about 500,000 km from the center of the ESL-1 point.  This is how far away from the ESL-1 point the spacecraft had to be in order to be outside of the sun's radio frequency interference.  This required dV to remain in that orbit.  If my memory serves the order of magnitude of this dV was around 30 meters/sec/year.&lt;/p&gt;&#xA;" OwnerUserId="14469" LastEditorUserId="58" LastEditDate="2016-05-19T14:56:44.707" LastActivityDate="2016-05-19T14:56:44.707" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15338" PostTypeId="1" AcceptedAnswerId="15345" CreationDate="2016-05-19T14:58:11.180" Score="3" ViewCount="186" Body="&lt;p&gt;In any (deep) spacecraft, radiation shielding is paramount. Neutrons and gamma rays can only be stopped by putting mass (or distance) between source and target. It seems that &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/radiation.php#id--Radiation_Shielding--Shielding&quot; rel=&quot;nofollow&quot;&gt;one should put more lightweight elements on the outside of the hull for proper shielding.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder if PV cells could provide this kind of neutron absorption/deflection layer and if so at what cost. My rationale is to generate a secondary usage to the inevitable mass. For a larger spacecraft it might e.g. be viable to partially rely on solar instead of nuclear power in order to reduce radiation exposure to the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does a typical solar cell compare as a neutron shield?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="1142" LastEditDate="2016-05-19T15:02:29.797" LastActivityDate="2016-05-19T20:50:22.793" Title="Could Solar Cells provide Neutron shielding?" Tags="|design|radiation|solar-power|shielding|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15339" PostTypeId="2" ParentId="15338" CreationDate="2016-05-19T15:23:40.623" Score="2" Body="&lt;p&gt;There is practically negligible neutron radiation in the deep space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gamma radiation exists, but it is also very small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This problem happens only if the spacecraft has a nuclear power source (which is mostly &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow&quot;&gt;RTG&lt;/a&gt; in todays practical designs). But if there is an RTG power source, there is no need for PV cells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PV cells have very strict requirements to their chemical composition, and they are thick.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;PV cells need to be semiconductors, thus the feasibility of a material to be used as PV cell has a requirement to the structure of its electron shell. They can be also very thick, only some wavelength of the visible light is enough. The only reason, why the current PV cells are thinner, that it is simpler (= cheaper) to produce some tenths of mm thin silicon layer as some micron.&lt;/li&gt;&#xA;&lt;li&gt;Neutron shielding and gamma shielding depends mostly on the nucleus. Unfortunately, effective neutron shields have mostly small nuclei, while the gamma shielding requires big (for example, lead).&lt;/li&gt;&#xA;&lt;li&gt;In the case of neutron shielding, there are big differences in the actually used isotopes, while the gamma cross section depends mostly simply on the proton number of the nuclei (and it is proportional to its fourth power).&lt;/li&gt;&#xA;&lt;li&gt;Both neutron and gamma shielding requires thick layers. In the nuclear technology, some cm thick lead layers are used.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="1054" LastEditDate="2016-05-19T15:35:54.203" LastActivityDate="2016-05-19T15:35:54.203" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15340" PostTypeId="1" CreationDate="2016-05-19T15:34:08.360" Score="2" ViewCount="307" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/LAGEOS&quot; rel=&quot;nofollow&quot;&gt;LAGEOS&lt;/a&gt; satellites have plaques that try to explain what they are without using any words or decimal numbers or similar 'current' concepts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To me, this suggests we expect the satellites to last their full lifetime, as the technology to find them in space will surely exist in much shorter time-scales (a few thousand if not hundreds of years, when we might expect English to still be understood in some way).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But when they de-orbit I'd expect them to either burn up or be destroyed when when they impacted the earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So: am I wrong assuming that we think they'll last their full lifetime, or am I wrong assuming that they'll be destroyed on impact?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: the lifetime of these satellites is expected to be around 8m years, but for the purposes of this question lets assume that structure of the earth, atmosphere and solar system is pretty similar to now.&lt;/p&gt;&#xA;" OwnerUserId="14471" LastEditorUserId="25" LastEditDate="2016-05-19T17:43:06.140" LastActivityDate="2016-05-29T01:08:38.180" Title="Will LAGEOS burn up or be destroyed on impact?" Tags="|artificial-satellite|reentry|impact|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="15341" PostTypeId="2" ParentId="15340" CreationDate="2016-05-19T17:42:36.967" Score="1" Body="&lt;p&gt;The satellite is made from aluminum and brass, so I expect it will be burned up in the atmosphere when it reenters. The only chance it has of being read is if someone goes and picks it up, or at least gets a really big telescope.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-19T17:42:36.967" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15342" PostTypeId="1" CreationDate="2016-05-19T17:44:44.387" Score="1" ViewCount="56" Body="&lt;p&gt;What is the best introduction for those interested in studying &lt;a href=&quot;http://radhome.gsfc.nasa.gov/radhome/see.htm&quot; rel=&quot;nofollow&quot;&gt;Single Event Effects, Upsets, and Latchups&lt;/a&gt;. I'm mainly interested in introductions for Electrical and Computer Engineers and designing spacecraft to mitigate these effects.&lt;/p&gt;&#xA;" OwnerUserId="5741" LastActivityDate="2016-05-19T17:44:44.387" Title="What is the best introduction for those studying Single Event Upsets in spacecraft?" Tags="|electronics|cosmic-radiation|" AnswerCount="0" CommentCount="1" ClosedDate="2016-05-20T05:54:08.283" ContentLicense="CC BY-SA 3.0" />
  <row Id="15345" PostTypeId="2" ParentId="15338" CreationDate="2016-05-19T20:50:22.793" Score="3" Body="&lt;p&gt;PV cells would not be a terribly good neutron absorber, mainly because PV cells are thin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neutrons penetrate deeply into materials because they are not charged - the main method of reducing neutron radiation is through physical collisions of neutrons.  Atoms with low atomic weights remove the energy from neutrons most efficiently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water is a good neutron absorbing material because it includes a lot of hydrogen, which has a low atomic number.  Water has a tenth value / thickness of 10 inches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means that water reduces the neutron radiation flux by 1 order of magnitude for every 10 inches of water present.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PV cells are going to be less effective at reducing neutron radiation, because they will have a higher average atomic number, and they will only be a few cm thick.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So PV cells will have a negligible neutron shielding effect.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that basic information about the tenth thickness for water is present in the next section of the Project Rho link that OP provided, labeled &quot;Shield Rating.&quot;&lt;/p&gt;&#xA;" OwnerUserId="11115" LastActivityDate="2016-05-19T20:50:22.793" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15346" PostTypeId="2" ParentId="13876" CreationDate="2016-05-20T01:39:40.327" Score="10" Body="&lt;p&gt;From the way the question is phrased, it would be referring to the eclipse of the spacecraft by the Earth (i.e. the sun is blocked by the Earth), not of the spacecraft by the Moon. Eclipse can refer to any body eclipsing any other body, not just the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LTAN is Local Time of the Ascending Node. The spacecraft crosses the equatorial plane of the Earth and, since it is in a sun-synchronous orbit, it will always cross that plane when it is the same time of day below. In this case, that means the spacecraft is always &quot;looking down&quot; at some point on Earth that currently has a time of 11 AM (or 11 PM on the other side of the Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 12 Noon LTAN would indicate that the plane of the spacecraft's orbit is exactly lined up with the direction of the sun. 11 AM is very close to this situation, so all this is telling you is that you should assume approximately the &quot;worst case&quot; maximum eclipse duration as the spacecraft passes behind the Earth each orbit (somewhere around 35 minutes for an 800 km orbit).&lt;/p&gt;&#xA;" OwnerUserId="14477" LastActivityDate="2016-05-20T01:39:40.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15347" PostTypeId="1" CreationDate="2016-05-20T03:29:16.937" Score="2" ViewCount="349" Body="&lt;p&gt;As I understand &lt;code&gt;tone ranging&lt;/code&gt; is one of the technique used to determine &lt;code&gt;satellite range&lt;/code&gt; from the earth station. In this case the position along the &lt;code&gt;line of sight&lt;/code&gt; is being determined. How the position along the other 2 directions are determined ?&lt;/p&gt;&#xA;" OwnerUserId="14400" LastActivityDate="2016-05-21T22:54:41.010" Title="Spacecraft position determination" Tags="|orbit|orbit-determination|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15348" PostTypeId="2" ParentId="15347" CreationDate="2016-05-20T06:47:56.440" Score="2" Body="&lt;p&gt;If you got the position along the line of sight, you can measure the elevation and direction angle of the line of sight too. You have one distance and two angles and using a coordinate transformation from polar coordinates to a cartesian system you can get three distances in x, y and z direction.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-05-20T06:47:56.440" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15349" PostTypeId="1" CreationDate="2016-05-20T11:38:37.183" Score="6" ViewCount="15509" Body="&lt;p&gt;I've seen charts and tables, etc., that list the delta-V to Low Earth Orbit as 9.3-10 km/s. But LEO could be anywhere below 2000 km. So how do I calculate the delta-V to reach a specific altitude in this range, or to any altitude in Earth orbit?&lt;/p&gt;&#xA;" OwnerUserId="14480" LastEditorUserId="25" LastEditDate="2016-05-20T14:15:49.437" LastActivityDate="2016-05-21T13:18:28.250" Title="How can the delta-V to a specific altitude in Earth orbit be calculated?" Tags="|launch|rockets|orbit|orbital-maneuver|delta-v|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15350" PostTypeId="2" ParentId="3461" CreationDate="2016-05-20T13:54:01.657" Score="0" Body="&lt;p&gt;Probably the biggest artificial satellite is a military satellite with classified specs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hubble Space Telescope with 11.000 kg of mass and Envisat with 8.000 kg are probably the on the league of the biggest satellites in orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And responding to the comment above...&#xA;... roseta is not a satellite because it is not technically orbiting anything, so it is a spacecraft but not a satellite.&lt;/p&gt;&#xA;" OwnerUserId="14483" LastActivityDate="2016-05-20T13:54:01.657" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15351" PostTypeId="2" ParentId="15349" CreationDate="2016-05-20T14:09:14.533" Score="4" Body="&lt;p&gt;Without a specific rocket launcher design in mind, the exact ∆v can't be determined because it is not dependent only on the altitude, but on the characteristics of the launcher. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The largest variable factor is what's called &quot;gravity losses&quot;. In the early part of the launch, the rocket is mainly expending ∆v to accelerate vertically. As it does so, it's fighting Earth's gravity. As an extreme case, consider a rocket that produces only 1% more thrust than its weight; it will accelerate very slowly and most of the ∆v it produces will be wasted countering the force of gravity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rocket with a very high thrust-to-weight ratio, on the other hand, spends less time on vertical acceleration and therefore loses less ∆v to gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;nofollow&quot;&gt;Saturn V&lt;/a&gt;, launching into a 185-190km circular orbit, expends ~9200 m/s ∆v, of which ~1750 m/s is lost to gravity and ~50 m/s lost to atmospheric drag. It is a moderate-to-high acceleration launcher.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in low Earth orbit, with gravity losses no longer a concern, you can calculate the ∆v needed to &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow&quot;&gt;transfer&lt;/a&gt; from a low orbit to a higher one. In most cases, the (launcher variable) ∆v needed to launch directly to a high orbit is approximately the same as that needed to launch into a low orbit and then transfer to a high one.   &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-05-20T14:09:14.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15352" PostTypeId="1" AcceptedAnswerId="15355" CreationDate="2016-05-20T14:09:44.450" Score="12" ViewCount="1004" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The telescope has an expected mass about half of Hubble Space Telescope's, but its primary mirror (a 6.5 meter diameter gold-coated beryllium reflector) will have a collecting area about five times as large (25 m2 vs. 4.5 m2). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/opIhC.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/opIhC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble telescope is about 11.000 kg, while J.W. - 6,500 kg.&#xA;It's also larger. Hubble: 13.2 × 4.2 m vs J.W. - 20.1 m × 7.21 m.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why such a big difference? What was so heavy in Hubble that is so light in James Webb?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2016-05-20T16:57:38.130" LastActivityDate="2018-04-04T19:00:57.567" Title="What savings allowed James Webb telescope to be lighter than Hubble?" Tags="|design|telescope|mass|hubble|james-webb-telescope|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15353" PostTypeId="2" ParentId="15349" CreationDate="2016-05-20T14:15:22.263" Score="0" Body="&lt;p&gt;Usually the official papers will site an inclination and an orbit with the specified payload capacity. The orbit will typically be quite a bit lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://knts.tsniimash.ru/ru/src/CenterInfRes/Falcon9UsersGuide_2009.pdf&quot; rel=&quot;nofollow&quot;&gt;old Falcon 9 user's guide&lt;/a&gt; shows plots of payload vs both orbital altitude and inclination. The more current one is available upon request only. You can see some of the factors that lead to LEO. The reported LEO number tends to be in a low orbit at optimal inclination, or in other words, best case scenarios.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-05-20T14:15:22.263" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15355" PostTypeId="2" ParentId="15352" CreationDate="2016-05-20T17:12:07.117" Score="4" Body="&lt;p&gt;Here is a &lt;a href=&quot;http://robustdesignconcepts.com/files/hubble/HST/Hubble_SE_Case_Study.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;weight breakdown&lt;/a&gt; of the HST: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SUeCB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SUeCB.png&quot; alt=&quot;Hubble weight&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For JWST, the &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150000768.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;telescope plus instruments&lt;/a&gt; weigh 8686 lbs (3940 kg), leaving 2500 kg for the spacecraft. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-05-20T17:24:38.640" LastActivityDate="2016-05-20T17:24:38.640" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15358" PostTypeId="2" ParentId="15056" CreationDate="2016-05-20T20:00:55.080" Score="2" Body="&lt;p&gt;From LEO to a lunar soft landing takes about 6 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike a moon lander, a Mars lander can use aerobraking to shed velocity on arrival. I'm not sure how much aerobraking the Red Dragon would do but I'd venture to guess delta V from LEO to soft Mars landing is around 4 km/s.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-05-20T20:00:55.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15359" PostTypeId="2" ParentId="15349" CreationDate="2016-05-20T20:34:20.787" Score="1" Body="&lt;p&gt;Depending on T/W and other factors, delta V to LEO can vary. But once you've reached LEO, gravity loss and atmospheric drag are no longer factors. So I'll give you some delta Vs going from a 300 km altitude circular orbit to higher altitude orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 500 km - .11 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 1000 km - .38 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 2000 km - .83 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 4000 km - 1.51 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 8000 km - 2.37 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 16000 km - 3.22 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 32000 km - 3.83 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 64000 km - 4.1 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 128000 km - 4.13 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 256000 km - 4.02 km/2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 512000 km - 3.87 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;300 km to 1024000 km 3.72 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I got these numbers from my Hohmann spreadsheet which can be downloaded from &lt;a href=&quot;http://clowder.net/hop/railroad/sched.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cosmic Train Schedule&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screen capture for going from 300 to 1000 km. I add the apoapsis and periapsis circularize burns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lAp7o.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lAp7o.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-05-20T20:34:20.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15360" PostTypeId="1" AcceptedAnswerId="15361" CreationDate="2016-05-21T01:59:41.183" Score="2" ViewCount="109" Body="&lt;p&gt;Magnetars are generally accepted as existing. Wikipedia suggests that the magnetic forces are strong enough to kill you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am assuming that virtually any spacecraft would have some magnetic properties vastly in excess of that its inhabitants. A steel hulled spaceship would thus be attracted to the magnetar long before its inhabitants would be threatened. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This brings up the question of avoiding a magnetar. Obviously, a spaceship with propulsion could just use its engines to offset the force from the magnetar. Given that the force between to magnets is proportional to the magnitudes of the magnetic poles, could we use an electromagnet to offset the intrinsic magnetism of the spaceship? The magnetar would essentially behave like a giant point source of magnetism at a distance. The electromagnet would need to be &quot;pointed&quot; at the magnetar to offset the force. Depending on the rotational velocity of the magnetar this could be challenging. As long as the spacecraft was not highly magnetic the electromagnet would not need to be significant in size.&lt;/p&gt;&#xA;" OwnerUserId="14488" LastActivityDate="2016-05-21T07:52:06.050" Title="Can I avoid a magnetar with an electromagnet?" Tags="|interstellar-travel|escape-velocity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15361" PostTypeId="2" ParentId="15360" CreationDate="2016-05-21T06:38:13.000" Score="2" Body="&lt;p&gt;The electromagnet would work if you were still far away enough that you could overcome the force, but you'd probably want to cool superconducting coils sufficiently and insulate them from incident radiation and heat from your ship. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you tried to use copper coils, you'd need a lot of electrical power because of the $I^2R$ losses, and then they would melt because there's no way to cool them sufficiently in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, I'd use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Meissner_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Meissner effect&lt;/a&gt; and just &lt;em&gt;bounce off&lt;/em&gt; the magnetic field of that pesky magnetar, or any sufficiently strong magnetic field gradient. Maybe even use it for propulsion if someone hasn't patented that already.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's the size of the &lt;em&gt;gradient of the field&lt;/em&gt; that you need to pay attention to. There is no net repulsion from a &lt;em&gt;uniform field&lt;/em&gt; if you have become superconducting in a low-field volume of space. But beware - you must cool your superconductors down &lt;em&gt;before&lt;/em&gt; you get into a high field region, otherwise you'll get stuck with a strongly magnetized ship. You could use &lt;em&gt;that&lt;/em&gt; for propulsion too!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose you could use thrusting maneuvers to &lt;em&gt;rotate&lt;/em&gt; from attractive to repulsive orientation if you had very strong attitude thrusters. This sounds like the beginning of a cool video game - especially if you start visualizing the field lines in real time in VR. Wait - shhh... don't tell anyone :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wdeut.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wdeut.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image from &lt;a href=&quot;http://www.wired.com/2015/06/lexus-hoverboard-slide/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wired article&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kwf20.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kwf20.jpg&quot; alt=&quot;Lexus Skateboard&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do I suddenly get the feeling I've been duped into advertising something?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/bvYUq6Ox0Hc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-05-21T07:52:06.050" LastActivityDate="2016-05-21T07:52:06.050" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15362" PostTypeId="2" ParentId="12365" CreationDate="2016-05-21T10:09:20.523" Score="1" Body="&lt;p&gt;Hydrazine isn't difficult to make, and most of it is made in Europe and India.  Russia imports hydrazine from Spain, UK, Holland, and Germany.  France also imports hydrazine from Germany for fueling its Ariane space rockets. The biggest market for hydrazine is the plastics industry.&lt;/p&gt;&#xA;" OwnerUserId="14490" LastActivityDate="2016-05-21T10:09:20.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15363" PostTypeId="2" ParentId="15349" CreationDate="2016-05-21T13:18:28.250" Score="3" Body="&lt;p&gt;$\Delta v$ is just a quantity masking the real effort that needs to be put into a change of orbits:&lt;br&gt;&#xA;The potential Energy $E_p$. What is usually done is that the difference between Earth's potential energy and the potential energy of your target orbit $\Delta E_p = E_p(r_{\oplus}) - E_p(r_{target})$ is equated with the kinetic energy $\frac{1}{2}m \Delta v^2$. This comes simply from energy conservation. So now we get&lt;br&gt;&#xA;\begin{align}&#xA;\Delta v = \sqrt \frac{2 \Delta E_p}{m}&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now to get any further from there, we have to realize that the potenial difference we seek must be coming from the work $W = \int \vec F \cdot d\vec s$ that one needs to invest in order to fight the force $\vec F$ at any given altitude.&lt;br&gt;&#xA;With that we get &#xA;\begin{align}&#xA;\Delta v = \sqrt {\frac{2}{m} \int\limits_{r_{\oplus}}^{r_{target}} F(r) dr}&#xA;\end{align}&#xA;where I already have taken the liberty of simplifying the scalar product in the integral.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now there are two different regimes for this Force:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At &lt;strong&gt;low altitudes&lt;/strong&gt; both gravity and friction play a role. Gravity is well known to be $F_g = \frac{GM_{\oplus}m}{r^2} $ with the usual notation (do tell if you need explanation for this) and subsonic friction can be approximated as $F_f= - c_w \rho v^2 $. Here $c_w$ is a geometrical factor for the rocket quantifying it's coupling to the airflow, and $\rho$ is the air density.&lt;br&gt;&#xA;This formula for the air-friction is not the correct one, as rockets can reach the supersonic friction regime, but it illustrates the point that was only mentioned in textual answers: geometry and density both play a role into the calculation of $\Delta v$.&lt;br&gt;&#xA;Thus we'd have $F = F_g + F_f$ at low altitudes and we'd need to solve the work-integral for this given force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This leads us to the realization that at high altitudes $F_f$ is negligible and thus at &lt;strong&gt;high altitudes&lt;/strong&gt; we have $F = F_g$.&lt;br&gt;&#xA;For simplcity I'll assume now friction to be negligible from the Earth's surface into the orbit, so that we can solve the above integral directly into &lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{align}&#xA;\Delta v = \sqrt {2GM_{\oplus} \left( \frac{1}{r_{\oplus}}- \frac{1}{r_{target}} \right)}&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which is our final result for $\Delta v$ with the gravitational constant $G$, Earth's Mass $M_{\oplus}$, and launch and target distance from Earth's center, $r_{\oplus}$ and $r_{target}$.&lt;br&gt;&#xA;Plugging in some numbers into that (e.e. for the 300-&gt;500km transfer orbit and keeping in mind that we have to add this always to Earth's radius of 6370 km).&lt;/p&gt;&#xA;" OwnerUserId="2839" LastActivityDate="2016-05-21T13:18:28.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15364" PostTypeId="1" AcceptedAnswerId="15369" CreationDate="2016-05-21T15:55:20.707" Score="6" ViewCount="2763" Body="&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; if you up-vote (or even if you don't), don't forget to scroll down and see the excellent answer as well - it's beautiful!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Three-body_problem#Additional_solutions&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pythagorean Three Body Problem&lt;/a&gt; also know as &lt;a href=&quot;http://www.ucolick.org/~laugh/oxide/projects/burrau.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Burrau's problem&lt;/a&gt; is a special case of the general three body problem, where the the three bodies have masses of 3, 4, and 5, and the initial conditions are such that they begin at rest, at the vertices of a 3-4-5 right triangle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've pasted some screenshots from the papers &lt;a href=&quot;http://www.ucolick.org/~laugh/oxide/projects/burrau.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;linked here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see and read more &lt;a href=&quot;http://oklo.org/2012/10/10/the-pythagorean-problem/&quot; rel=&quot;nofollow noreferrer&quot;&gt;in this posting&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And watch this video - it looks like time displayed in the plot in the video is $40\times$ time in the paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Y2pi5Ra-ZFQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea was originally that it might hold some special significance, but it doesn't seem to. However, it does pose a big challenge to numerical integrators because it results in several very close (~$10^{-4}$) passes between pairs, and many common integrators will not respond quick enough with step-size reduction to maintain numerical accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what has happened to me using the standard default ODE integrator in SciPy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some tricks to try within SciPy, and of course other integrators available in python, and actually I can just implement some higher order Runge-Kutta methods and write my own &lt;em&gt;hyper-vigilant step size handler&lt;/em&gt;. It does't have to be fast because fairly soon, one of the three is ejected and the other two settle down to two-body rotation. This is pretty common in three body situations, in computers and in ternary star systems that aren't sufficiently hierarchical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What I do need now&lt;/strong&gt; is to compare results to the correct numerical solution - a table with a selection of some precise coordinates vs time. Comparing to YouTube isn't so accurate, and no guarantees those are right either!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Does anyone know where I can find such numbers&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; The comment points out I should be careful with the word &quot;correct.&quot; I'm looking for results using an ODE solver that works well with &lt;a href=&quot;https://en.wikipedia.org/wiki/Stiff_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;stiff equations&lt;/a&gt; (see &lt;a href=&quot;http://www.scholarpedia.org/article/Stiff_systems&quot; rel=&quot;nofollow noreferrer&quot;&gt;here also&lt;/a&gt;) that may be &lt;a href=&quot;https://en.wikipedia.org/wiki/Numerical_stability&quot; rel=&quot;nofollow noreferrer&quot;&gt;numerically unstable&lt;/a&gt;, and in this case are expected to be accurate to - say - six digits of accuracy by $t=70$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a sample output and script. This is wrong. You can find nice solutions displayed in YouTube and other places, but I can't find the numerical results to help my debugging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to suggest python improvement, you can leave an answer or comment at my &lt;a href=&quot;https://stackoverflow.com/q/34789575/3904031&quot;&gt;question in stackoverflow&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gklY7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gklY7.png&quot; alt=&quot;Wrong Answer&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(X, t):&#xA;&#xA;    Y[:6] = X[6:]&#xA;&#xA;    r34, r35, r45 = X[2:4]-X[0:2], X[4:6]-X[0:2], X[4:6]-X[2:4]&#xA;    thing34 = ((r34**2).sum())**-1.5&#xA;    thing35 = ((r35**2).sum())**-1.5&#xA;    thing45 = ((r45**2).sum())**-1.5&#xA;&#xA;    Y[6:8]   =  r34*thing34*m4 + r35*thing35*m5&#xA;    Y[8:10]  =  r45*thing45*m5 - r34*thing34*m3&#xA;    Y[10:12] = -r35*thing35*m3 - r45*thing45*m4&#xA;&#xA;    return Y&#xA;&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;# Pythagorean Three Body Problem&#xA;# This script WILL NOT solve it yet, just for illustration of the problem&#xA;&#xA;m3, m4, m5 = 3.0, 4.0, 5.0&#xA;&#xA;x0 = [1.0, 3.0] + [-2.0, -1.0] + [1.0, -1.0]&#xA;v0 = [0.0, 0.0] + [ 0.0,  0.0] + [0.0,  0.0] &#xA;X0 = np.array(x0 + v0)&#xA;&#xA;t = np.linspace(0, 60,  50001)&#xA;&#xA;Y = np.zeros_like(X0)&#xA;&#xA;tol  = 1E-9 # with default method higher precision causes failure&#xA;hmax = 1E-04&#xA;answer, info = ODEint(deriv, X0, t, rtol=tol, atol=tol,&#xA;                      hmax=hmax, full_output=True)&#xA;&#xA;xy3, xy4, xy5 = answer.T[:6].reshape(3,2,-1)&#xA;paths         = [xy3, xy4, xy5]&#xA;&#xA;plt.figure()&#xA;plt.subplot(2, 1, 1)&#xA;for x, y in paths:&#xA;    plt.plot(x, y)&#xA;for x, y in paths:&#xA;    plt.plot(x[:1], y[:1], 'ok')&#xA;plt.xlim(-6, 6)&#xA;plt.ylim(-4, 4)&#xA;plt.title(&quot;This result is WRONG!&quot;, fontsize=16)&#xA;plt.subplot(4,1,3)&#xA;for x, y in paths:&#xA;    plt.plot(t, x)&#xA;plt.ylim(-6, 4)&#xA;plt.subplot(4,1,4)&#xA;for x, y in paths:&#xA;    plt.plot(t, y)&#xA;plt.ylim(-6, 4)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pWrWT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pWrWT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/hMVAj.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hMVAj.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/WWKOs.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WWKOs.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Sv6Ij.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sv6Ij.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/QnKZG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QnKZG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/2MW31.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2MW31.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/1KPnf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1KPnf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/yrz5V.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yrz5V.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="25618" LastEditDate="2018-04-29T17:20:00.830" LastActivityDate="2018-04-29T17:20:00.830" Title="&quot;Pythagorean Three Body Problem&quot; - need some points from an accurate solution for comparison" Tags="|orbital-mechanics|celestial-mechanics|calculation|python|three-body|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15366" PostTypeId="1" AcceptedAnswerId="15368" CreationDate="2016-05-21T21:55:29.103" Score="3" ViewCount="1460" Body="&lt;p&gt;I'm programming an orbital simulator and need some help modeling hyperbolic orbits (or trajectories, whatever you want to call them). So far I can model elliptical orbits with the standard orbital elements (perigee, eccentricity, semi-major axis, mean anomaly, etc); I can calculate the mean anomaly in a few seconds from now, and from that derive the craft's position and velocity. Is there a way to do this for hyperbolic orbits? If you start with just the velocity and position vector of a craft can you predict where it will be every second from now while it is in a hyperbolic orbit/trajectory?&lt;/p&gt;&#xA;" OwnerUserId="14494" LastEditorUserId="6816" LastEditDate="2016-05-21T22:29:32.793" LastActivityDate="2016-05-22T02:13:59.730" Title="How do you Model Hyperbolic Orbits?" Tags="|orbital-mechanics|simulation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15367" PostTypeId="2" ParentId="15347" CreationDate="2016-05-21T22:54:41.010" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;As I understand tone ranging is one of the technique used to determine satellite range from the earth station. In this case the position along the line of sight is being determined. How the position along the other 2 directions are determined ?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;By taking lots of readings, spread over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with angle measurements is that even the most exquisitely precise angle measurements aren't that precise when it comes to determining position. Range rate measurements truly are exquisitely precise, as are range measurements, particularly with delta-DOR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Precision orbit determination mandates taking multiple readings over a span of time, a lot more than the six degrees of freedom suggested by orbital elements. A number of filtering techniques have been developed over the last 50+ years to address the fact that the multitude of readings form an over-specified system.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-05-21T22:54:41.010" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15368" PostTypeId="2" ParentId="15366" CreationDate="2016-05-22T00:36:17.217" Score="4" Body="&lt;p&gt;Yes. (It sounds like you are dealing with a two-body problem, which has analytic solutions.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, given the position, $\vec{r}$, the velocity, $\vec{v}$, both in a coordinate system with the central body at the origin at rest, and the GM of the central body, $\mu$, you can readily compute the specific energy (energy per unit mass), which will tell you whether the trajectory is elliptical (negative energy) or hyperbolic (positive energy). Or perhaps parabolic (exactly zero energy), which is however only of mathematical interest. From the energy, you can get the semi-major axis, $a$. Here $v=|\vec{v}|$ and $r=|\vec{r}|$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathcal{E}={v^2\over 2}-{\mu\over r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a={\mu\over 2\mathcal{E}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are probably propagating elliptical orbits with sines and cosines. A hyperbolic trajectory is done in a similar way with hyperbolic sines and hyperbolic cosines. You can compute the specific angular momentum from just the position and velocity, and from that get the eccentricity, $e$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\vec{\mathcal{M}}=\vec{r}\times\vec{v}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$e=\sqrt{1+{\vec{\mathcal{M}}\cdot\vec{\mathcal{M}}\over\mu a}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then a simple form of the trajectory in the $xy$ plane, with closest approach on the $+x$ axis is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$x=a\left(e-\cosh\tau\right)$$&#xA;$$y=a\sqrt{e^2-1}\sinh\tau$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\tau$ is the eccentric anomaly, related to time, where $\tau=0$ and $t=0$ is at closest approach:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$t=\sqrt{a^3\over\mu}\left(e\sinh\tau-\tau\right)$$&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-22T02:13:59.730" LastActivityDate="2016-05-22T02:13:59.730" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15369" PostTypeId="2" ParentId="15364" CreationDate="2016-05-22T02:11:01.787" Score="5" Body="&lt;p&gt;I just ran it, and mine look pretty much like those in the paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See some coordinates at the bottom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wWAkk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wWAkk.png&quot; alt=&quot;0-10&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/76upl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/76upl.png&quot; alt=&quot;10-20&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/uGb1v.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uGb1v.png&quot; alt=&quot;20-30&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/hQQng.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hQQng.png&quot; alt=&quot;30-40&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/1kOsz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1kOsz.png&quot; alt=&quot;40-50&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/raME6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/raME6.png&quot; alt=&quot;50-60&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/NM2LY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NM2LY.png&quot; alt=&quot;60-70&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some {x,y} coordinates at the times in the left column:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;0.      {1.,3.}                 {-2.,-1.}               {1.,-1.}&#xA;5.      {2.46917,-1.22782}      {-2.2782,-0.20545}      {0.34106,0.901049}&#xA;10.     {0.77848,0.141392}      {-2.02509,0.0972194}    {1.15299,-0.162611}&#xA;15.     {1.41845,0.686214}      {-2.00654,0.0599408}    {0.754159,-0.459681}&#xA;20.     {3.00429,0.511925}      {-1.38863,-0.470476}    {-0.691674,0.0692257}&#xA;25.     {2.2699,-0.0832}        {-2.63692,-0.426417}    {0.747596,0.391054}&#xA;30.     {0.85634,2.28709}       {-0.877984,-0.865964}   {0.188583,-0.679485}&#xA;35.     {0.0273748,0.895529}    {0.942553,-1.60223}     {-0.770468,0.744467}&#xA;40.     {-0.622004,1.85832}     {0.173545,-2.36841}     {0.234367,0.779737}&#xA;45.     {-0.657058,2.53557}     {1.61355,-1.23947}      {-0.896608,-0.529771}&#xA;50.     {-2.70146,-3.79723}     {1.50595,0.960811}      {0.416122,1.50969}&#xA;55.     {-2.75171,-4.29907}     {1.72673,0.97731}       {0.269648,1.7976}&#xA;60.     {0.743681,1.93961}      {0.263967,-0.731477}    {-0.657382,-0.578586}&#xA;65.     {4.05348,11.7131}       {-1.0722,-3.92197}      {-1.57432,-3.8903}&#xA;70.     {6.93108,20.2566}       {-1.99418,-6.87252}     {-2.5633,-6.65594}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;That was all with 30 digits working precision. Checking the final total energy and total angular momentum against the initial conditions, with 30 working digits the results are good to 10 digits. With 50 working digits, the results are good to 20 digits. With machine precision (about 15 working digits), the results are good to five to six digits, which is still pretty good considering the close approaches.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-23T16:52:00.380" LastActivityDate="2016-05-23T16:52:00.380" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15370" PostTypeId="1" AcceptedAnswerId="15386" CreationDate="2016-05-22T04:31:40.923" Score="6" ViewCount="292" Body="&lt;p&gt;I was looking at the Galleries in the &lt;a href=&quot;https://svs.gsfc.nasa.gov/Gallery/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Scientific Visualization Studio&lt;/a&gt; and in the banner they show an image from &lt;a href=&quot;http://sdo.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SDO&lt;/a&gt; - the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_Dynamics_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Dynamics Observatory&lt;/a&gt;. I've posted it below along with a cropped, reversed tone of the green channel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see these &quot;echoes&quot; - really just periodic tire-tread-like little blip-artifacts (&lt;em&gt;blipifacts&lt;/em&gt;) from time to time when there are certain bright events. They do seem to have a roughly cartesian distribution, spreading out away from the bright spot along all four diagonals. The vertical &quot;spike&quot; suggests the CCD axis is aligned to (x, y), so maybe it is vane-related? Does the instrument have a secondary supported by vanes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;What are they?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE:&lt;/strong&gt; I found a photomontage of a bright event shown with six different wavelength bands. It's the &lt;em&gt;Cinco de Mayo Flare&lt;/em&gt; of May 5, 2015. Many show the same effect, but different wavelengths have different patterns - strongly suggesting this is interference or diffraction, not electronic. There is always a short-period &quot;tire-tread&quot; combined with a long period envelope - maybe a dozen blips, then nothing, then a dozen blips... It reminds me of the diffraction pattern from a square array of points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qizU1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qizU1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image: &lt;a href=&quot;https://svs.gsfc.nasa.gov/vis/a010000/a010900/a010925/SDO_2012-03-07_171_X5Flare-Closeup.00369.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;svs.gsfc.nasa.gov/vis/a010000/a010900/a010925/SDO_2012-03-07_171_X5Flare-Closeup.00369&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PCNKD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PCNKD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image: The &quot;&lt;em&gt;Cinco de Mayo Flare&lt;/em&gt;&quot; 05-05-2015: &lt;a href=&quot;http://www.nasa.gov/feature/goddard/nasas-sdo-observes-cinco-de-mayo-solar-flare&quot; rel=&quot;nofollow noreferrer&quot;&gt;www.nasa.gov/feature/goddard/nasas-sdo-observes-cinco-de-mayo-solar-flare&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HMI Continumum | 171Å upper transiti | 304Å Chromosph | 193Å Corona/Flare | 131Å Flaring reg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GCl5h.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GCl5h.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yunwb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yunwb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L9UNp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L9UNp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;You can use this guide&lt;/strong&gt; to compare the artificial color-coding to get the actual UV wavelength for each image. &lt;a href=&quot;http://www.nasa.gov/mission_pages/sunearth/news/light-wavelengths.html#.V0KpxJN97_R&quot; rel=&quot;nofollow noreferrer&quot;&gt;From here.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UD4p8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UD4p8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-05-23T07:05:24.473" LastActivityDate="2017-08-21T20:35:37.883" Title="What are these echo-like artifacts in some SDO images?" Tags="|the-sun|space-telescope|solar-dynamic-observatory|image-processing|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15371" PostTypeId="1" AcceptedAnswerId="15372" CreationDate="2016-05-22T07:55:06.380" Score="10" ViewCount="1028" Body="&lt;p&gt;For various reasons, spacecraft are sometimes &quot;put to sleep&quot; for periods of time on long voyages. Many systems are shut down, and radio communications suspended. Tracking of spacecraft at very large distances usually requires the spacecraft to receive the weak signal, amplify it, and rebroadcast it over a narrow beam from a high gain antenna if possible, so I'm guessing that sleeping spacecraft at large distances can't be actively tracked, although their trajectories can be forward-calculated with high precision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/15367/12102&quot;&gt;answer&lt;/a&gt; started me thinking about the whole process, and I remembered the videos of the crowd at ESA going crazy when Rosetta woke up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the video below it looks like the &lt;a href=&quot;http://www.esa.int/spaceinvideos/Videos/2013/12/How_Rosetta_wakes_up_from_deep_space_hibernation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta spacecraft&lt;/a&gt; was hibernating for about 2.5 years. Is this the longest time that a space craft has &quot;gone to sleep&quot; with no tracking, and then &quot;woke up&quot;, on schedule and in the right place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I say &quot;on schedule&quot; is that I am interesting in how long mission planners have been comfortable loosing contact with a spacecraft in an intentional, scheduled manner. Was it really without communication for 2.5 years? Was there also no tracking for the whole time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/3444/12102&quot;&gt;This&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/3447/12102&quot;&gt;this&lt;/a&gt; answer are informative reading about Rosetta waking up for example, and some references on tracking (ranging) can be found in this &lt;a href=&quot;https://space.stackexchange.com/a/5140/12102&quot;&gt;nice answer&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/UqcDtRmJbKY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-03-31T10:15:28.453" LastActivityDate="2024-02-19T15:55:03.417" Title="Is this the longest that a spacecraft &quot;went to sleep&quot; and then woke up on schedule?" Tags="|artificial-satellite|record|tracking|radar|deep-space-hibernation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15372" PostTypeId="2" ParentId="15371" CreationDate="2016-05-22T12:56:32.767" Score="9" Body="&lt;p&gt;First of all, yes, Rosetta was indeed without any contact for 31 months. That is longer than New Horizons, which wasn't in hibernation for more than 202 days&lt;sup&gt;1&lt;/sup&gt;. These are the only two spacecraft that I can find that meet this criteria.&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;&lt;a href=&quot;https://hub.jhu.edu/gazette/2015/january-february/currents-new-horizons-spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;On Pluto's doorstep, New Horizons spacecraft awakens for encounter&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="12102" LastEditDate="2024-02-19T15:55:03.417" LastActivityDate="2024-02-19T15:55:03.417" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="15373" PostTypeId="2" ParentId="15371" CreationDate="2016-05-22T12:59:55.203" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Was there also no tracking for the whole time?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There was no need for tracking. The satellite merely had to be in view of the antenna on the Earth and the Earth had to be in view of the antenna on the satellite. That leaves a very wide range for error in the estimated orbit prior to putting the satellite to sleep.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the video, Rosetta was 45 light minutes from Earth at the time it woke up. The beamwidth of the 34 meter high gain antennas used by NASA's Deep Space Network is 0.077 degrees. (I would assume ESA's high gain antennas have a similar behavior.) At 45 light minutes, that corresponds to a one million kilometer cross track error. Precision orbit determination does much, much better than that (orders of magnitude better than that), even with a 31 month extrapolation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This, by the way, is why angle measurements are pretty much useless for orbit determination of deep space vehicles.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-05-22T14:00:47.257" LastActivityDate="2016-05-22T14:00:47.257" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15374" PostTypeId="1" AcceptedAnswerId="15376" CreationDate="2016-05-22T13:58:24.290" Score="17" ViewCount="5595" Body="&lt;p&gt;It seems the &lt;a href=&quot;http://rosetta.esa.int/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta spacecraft&lt;/a&gt; was put (put itself) into hibernation 08-June-2011. It spun itself up to stabilize its attitude, then shut down navigation, attitude control, communication systems as well as some of the heaters - if I understand correctly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It then remained in hibernation for about &lt;strong&gt;2.5 years&lt;/strong&gt; (by far the &lt;a href=&quot;https://space.stackexchange.com/a/15372/12102&quot;&gt;longest scheduled hibernation/wake-up on record!&lt;/a&gt;), with only the (a) computer heated and powered, but probably without a significant workload. This was powered by the small amount of light that fell &lt;s&gt;intermittently&lt;/s&gt; on the panels &lt;s&gt;at various angles&lt;/s&gt; &lt;em&gt;which were pointed towards the sun&lt;/em&gt;. The rotation allowed for passive maintenance of spacecraft attitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It then  woke itself up 2.5 years later on 20-January-2014, de-spun, re-oriented for better sunlight on the panels, and &quot;phoned home.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The video below is great, and this &lt;a href=&quot;https://space.stackexchange.com/a/3447/12102&quot;&gt;answer&lt;/a&gt; is helpful to read - including  &lt;a href=&quot;http://download.esa.int/esoc/shares/rosetta_wake-up_faq_20_jan_2014.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;rosetta_wake-up_faq_20_jan_2014.pdf&lt;/a&gt; about Rosetta coming out of hibernation. However, it seems to explain that the &quot;special hibernation mode&quot; was necessary to survive the hibernation itself, but I don't see &lt;em&gt;why that in turn&lt;/em&gt; was necessary.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A special ‘hibernation mode’ for the spacecraft was designed by engineers at EADS Astrium, the main industrial prime contractor that built Rosetta, to allow it to survive the large distances from the Sun during the cruise.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Was the spin done to conserve propellants? Lifetime of the reaction wheels? I can't think of anything else that would get &quot;used up&quot; by leaving it active.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;WHY did Rosetta have to go into hibernation for 2.5 years?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/UqcDtRmJbKY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I measured some screen shots of this video - Rosetta is only about 18% farther from the sun at aphelion (middle of hibernation) than it is when it enters and exits hibernation. That means sunlight at perihelion was about 72% as bright as at hibernation enter/exit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ktrtvCvZb28?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-16T23:56:51.863" LastActivityDate="2017-05-16T23:56:51.863" Title="WHY did Rosetta have to go into hibernation for 2.5 years?" Tags="|artificial-satellite|power|rosetta|deep-space-hibernation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15375" PostTypeId="2" ParentId="15374" CreationDate="2016-05-22T14:10:25.303" Score="11" Body="&lt;p&gt;Solar arrays are pretty much worthless beyond 4 AU or so from the Sun (depending on the size of the arrays of course). The Juno spacecraft will be the first mission to Jupiter (about 5 AU from the Sun) that uses solar arrays. In the case of Rosetta, it was easier to design the spacecraft to go to sleep for a while than it was to outfit it with solar arrays sufficiently large for that portion of its journey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rosetta was put into hibernation mode on 2011-06-11 when it was 4.47 AU from the Sun and woke up on 2014-01-20 when it was 4.49 AU from the Sun. It's aphelion distance occurred on 2012-10-02, when it was 5.29 AU. Since solar power is an inverse square law, making the solar arrays large enough to keep Rosetta fully powered would have meant making her solar arrays 40% larger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That in turn would have meant more fuel would have been needed to push that extra mass around. Even more fuel would have been needed to keep Rosetta under three axis control during that hibernation period. More fuel would be needed yet to accommodate the extra mass of that extra fuel. Anyone interested in space exploration via rockets needs to fully understand the tyranny of the rocket equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tyranny of that equation means that shaving mass is an extremely important concern to spacecraft designers. In the case of Rosetta, putting the spacecraft to sleep for the 31 months around aphelion was a nice way to shave a lot of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting Rosetta to sleep involved despinning Rosetta's momentum wheels (which involved fuel use) and then making the entire spacecraft slowly rotate (which involved even more fuel use). The reverse process (also involving fuel use) was applied during the wakeup sequence. This expenditure of fuel would not have been needed if Rosetta's arrays were 40% larger. That expenditure of fuel apparently was small compared to the much larger mass and fuel penalties that would have resulted if Rosetta had been kept fully operational during that 31 month hibernation period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above is public knowledge. Asking why Rosetta was designed to work this way is not, at least as far as I can tell. Partly it's because everyone in the know knows that solar arrays are not a good solution at Jupiter's orbit and beyond. Making Rosetta go into a low power mode just made sense. The details of when and how are hard to uncover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Partly it's how consensus is reached. Many of these decisions are made in meetings. I've been in multiple meetings where someone asked the person taking minutes to stop doing so and then spoke candidly, or where people walked up to the blackboard or whiteboard and put down their ideas. These things are not recorded. The after-the-fact consensus is what's recorded.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-05-22T17:53:26.277" LastActivityDate="2016-05-22T17:53:26.277" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="15376" PostTypeId="2" ParentId="15374" CreationDate="2016-05-22T14:28:11.813" Score="26" Body="&lt;p&gt;From &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions&quot; rel=&quot;noreferrer&quot;&gt;ESA's FAQ page&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Why was it necessary to keep Rosetta in hibernation for 31 months?&lt;/p&gt;&#xA;&lt;p&gt;To limit its consumption of power and fuel, and to minimise operating costs. During hibernation it was spinning once per minute and faced the Sun, so that its solar panels could receive as much sunlight as possible. Almost all of the electrical systems were switched off, with the exception of the radio receivers, command decoders and power supply.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The spin stabilisation made sure Rosetta continued to point at the Sun, receiving the largest amount of solar power possible while not needing fuel for the attitude control thrusters.&lt;/p&gt;&#xA;&lt;p&gt;Rosetta's solar panels are already among the largest ever deployed on a satellite. Making them much larger would have eaten into the mass budget of the probe.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.maia.ub.es/%7Egerard/AstroNet-II/IFC/Vicenc_Companys.pdf&quot; rel=&quot;noreferrer&quot;&gt;This ESA presentation on Rosetta flight operations contains an orbital diagram&lt;/a&gt; that shows where Rosetta entered and exited hibernation.&lt;/p&gt;&#xA;&lt;p&gt;Eyeballing, I'd say Rosetta entered hibernation at a distance from the Sun of 4.5 AU, had aphelion at 5.5 AU and exited hibernation at 4.5 AU.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6arZz.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6arZz.png&quot; alt=&quot;Orbital diagram&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This long hibernation period is unprecedented. But the designers felt they had no other option, &lt;a href=&quot;https://newscientist.atavist.com/rosetta&quot; rel=&quot;noreferrer&quot;&gt;despite the risk&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;One such person is Andrea Accomazzo. He joined the Rosetta mission in 1996 and is now the spacecraft's operation manager. He knew early on that hibernation would be a challenge. &amp;quot;In the development phase of the mission, I tried to convince my manager many, many times not to design a hibernation mode. We had no other choice but to create this,&amp;quot; he says.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Rosetta was near/at the maximum lift capacity of the Ariane 5 rocket (budget: &lt;a href=&quot;http://www.rssd.esa.int/SB/ROSETTA/docs/swt7/PR-3540-JE_Project_status_presented_at_SWT7.pdf&quot; rel=&quot;noreferrer&quot;&gt;3004 kg&lt;/a&gt;, launch weight is given at &amp;quot;&lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/The_Rosetta_orbiter&quot; rel=&quot;noreferrer&quot;&gt;approx. 3000 kg&lt;/a&gt;&amp;quot;). Adding larger solar arrays would have put the spacecraft over budget.&lt;/p&gt;&#xA;&lt;p&gt;I found the power budget in &lt;a href=&quot;http://www.rssd.esa.int/SB/ROSETTA/docs/swt7/PR-3540-JE_Project_status_presented_at_SWT7.pdf&quot; rel=&quot;noreferrer&quot;&gt;this presentation&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Acmse.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Acmse.png&quot; alt=&quot;Rosetta power budget&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;During hibernation, the spacecraft drew 339 W, most of it for heating. During active operations, that rose to 425 or 478 W depending on activity. At aphelion, the solar array produced 405 W.&lt;/p&gt;&#xA;&lt;p&gt;From the ESA presentation:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Some systems had to be kept on. For instance tank heaters to avoid&#xA;propellant freezing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Hibernation also &lt;a href=&quot;http://sci.esa.int/rosetta/35134-engineering/&quot; rel=&quot;noreferrer&quot;&gt;preserved spacecraft lifetime&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;nominal spacecraft lifetime is 11 years in heliocentric trajectory&lt;/p&gt;&#xA;&lt;p&gt;the achievement of this lifetime is helped by the long hibernation periods during cruise, where most of the electrical systems are not operational. This hibernation increases their lifetime by a factor of 10 but requires on-board autonomy to guarantee continued operation in all circumstances&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I suspect the main lifetime limitation is on the reaction wheels (which were spun down during hibernation), not the electronics themselves.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-05-23T09:21:17.853" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15377" PostTypeId="1" AcceptedAnswerId="15379" CreationDate="2016-05-22T16:56:45.513" Score="9" ViewCount="1227" Body="&lt;p&gt;The &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2016/05/21/478992076/a-66-000-pound-space-shuttle-fuel-tank-is-parading-through-the-streets-of-la&quot; rel=&quot;nofollow noreferrer&quot;&gt;last flight-qualified space shuttle external tank in existence&lt;/a&gt;  is now in Los Angeles and moved to its new home over the weekend. I noticed that what appears to be &lt;em&gt;the metal skin under the insulation&lt;/em&gt; is also dark orange/rust colored, very similar to the insulation. I'm guessing it's paint on the metal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As explained in &lt;a href=&quot;https://space.stackexchange.com/a/2450/12102&quot;&gt;this answer&lt;/a&gt; the dark orange/rust color of the foam insulation is it's natural color, and it darkens after exposure to the sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the first photo (found in &lt;a href=&quot;https://twitter.com/pfal/status/734136090039226368&quot; rel=&quot;nofollow noreferrer&quot;&gt;Twitter&lt;/a&gt;) you can see the metal tank exposed through the funny-shaped hole in the insulation. Not identical colors, but very similar. There's a &lt;a href=&quot;https://twitter.com/suesims/status/734073606657843204?ref_src=twsrc%5Etfw&quot; rel=&quot;nofollow noreferrer&quot;&gt;good video&lt;/a&gt; in the article also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering if the metal skin was painted a similar color for aesthetic reasons, or for another reason. If it were white underneath, then &quot;missing foam&quot; would be much more apparent. That could be considered a good thing or a bad thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Photos are circa Saturday, May 21, 2016 in Los Angeles, California. First two &lt;a href=&quot;https://twitter.com/pfal/status/734136090039226368&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XJj2r.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XJj2r.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2jtc5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2jtc5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This one is from the &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2016/05/21/478992076/a-66-000-pound-space-shuttle-fuel-tank-is-parading-through-the-streets-of-la&quot; rel=&quot;nofollow noreferrer&quot;&gt;NPR story&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mRirb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mRirb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="26446" LastEditDate="2020-05-22T21:18:38.697" LastActivityDate="2020-05-22T21:18:38.697" Title="Why is the shuttle's external tank's metal skin painted the same dark color as the foam insulation?" Tags="|space-shuttle|external-tank|insulation|museum-exhibits|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15378" PostTypeId="2" ParentId="441" CreationDate="2016-05-22T19:35:46.563" Score="2" Body="&lt;p&gt;Looks like LocalFluff also talked about centrifuges. Here's my take, which is on similar lines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be limiting for individuals to allow their physiology to weaken from lack of gravity (as long as a return trip to a higher gravity at a later date was possible or desired). It may be ok to let the plants and animals adapt, however, which reduces the scope of the problem to maintaining the health of human beings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Carnival rides on Earth can create higher than earth gravity effects. NASA is already &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2003/07feb_stronggravity/&quot; rel=&quot;nofollow&quot;&gt;looking into applications of this&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One possible solution for a colony on a lower mass planet like Mars, where there is some gravity but it is still weaker than earth, is to provide centrifuge style exercise areas as big as ferris wheels that spin constantly (except for planned maintenance or no-use time periods). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A pod (that traces an identical circular path) could be spun up to speed to match the centrifuge and docked magnetically. The people inside the pod could be spun up to speed, dock, jog for an hour or lift weights or do squats, then get back in the pod and go home. All this, of course, would need to be refined a fair amount for safety and convenience. However, as long as the centrifuge had a ceiling that was no higher than, say, 12 feet, a person could safely fall to the &quot;ceiling&quot; in the event that the centrifuge lost power (due to the lighter gravity making a fall like this safer than it would be on Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would imagine that the exercise chamber, if designed like a ferris wheel, would have highest gs when a person standing still has their feet pointed at the Martian ground and the least gs when their feet point toward the Martian sky. So it would require an adjustment period so that people don't injure themselves by miscalculating the amount of gravity at any one moment (i.e. landing too hard). This assumes the exercise chamber doesn't have smart enough motor programming to speed up and slow down to manage the changing gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That would help solve the problem of the whole-body effects of gravity, unlike wearing ankle weights. You could even choose your amount of gravity to be a little higher than earth if that would reduce the time needed for a person to stay strong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another NASA article on their explorations with &lt;a href=&quot;http://www.nasa.gov/missions/science/hyper.html&quot; rel=&quot;nofollow&quot;&gt;centrifuges&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="14501" LastEditorUserId="14501" LastEditDate="2016-05-22T20:48:23.183" LastActivityDate="2016-05-22T20:48:23.183" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15379" PostTypeId="2" ParentId="15377" CreationDate="2016-05-22T20:07:04.557" Score="14" Body="&lt;p&gt;tl;dr - the parts at the rear of ET-94 where the foam was removed were painted orange for display.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The foam was not dyed but started out a light cream color.  It slowly turned orange when exposed to light.  Here is a picture of foam that was trimmed off during the &lt;a href=&quot;http://www.spaceflightnow.com/shuttle/sts133/101111leak/&quot; rel=&quot;noreferrer&quot;&gt;stringer crack problem on STS-133&lt;/a&gt;. You can see the internal foam is lighter, and the metal is natural color.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DzeRg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DzeRg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some areas of the tank had other thermal protection materials applied, like Super Light Ablator (SLA).  A good picture showing the different materials is found in the &lt;a href=&quot;https://www.nasa.gov/columbia/caib/html/start.html&quot; rel=&quot;noreferrer&quot;&gt;CAIB report&lt;/a&gt; page 51.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/zdskY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zdskY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;  The area in the odd-shaped hole in your picture either had one of these other materials applied, or, most likely, had the foam shaved down for some reason to a lesser thickness, and the remaining foam turned orange.  This tank - ET-94 - &lt;a href=&quot;http://govinfo.library.unt.edu/caib/photos/sub_section0853.html-category=&amp;amp;main=materials_testing&amp;amp;sub=et94_dissection&amp;amp;item=et94&amp;amp;thumbnails=no.htm&quot; rel=&quot;noreferrer&quot;&gt;was used extensively for investigation&lt;/a&gt; by the CAIB, I would guess that is where the odd-shaped hole originated.  However, &lt;a href=&quot;http://www.nasa.gov/sites/default/files/114022main_TPS_FS.pdf&quot; rel=&quot;noreferrer&quot;&gt;this fact sheet&lt;/a&gt; states that the aft dome had SLA applied, so it may be that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the variance in the foam color in this amazing picture of four ETs at Michoud, when production was humming along.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Mgfrg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mgfrg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: In the left background of this image taken at Michoud, you can see an unfinished aft dome with some foam applied.  The metal below is clearly unpainted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SZ8Xv.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SZ8Xv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/edu/news/tag/Space+Shuttle&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; confirms that the &quot;odd shaped hole&quot; was caused by CAIB work, and says &quot;work was done to return it to its original appearance&quot;...I believe this is implying that the hole in ET-94 tank was painted orange for display.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit 2: And the smoking gun: This &lt;a href=&quot;http://corporate.ppg.com/Media/Newsroom/2016/PPG-donates-paint-for-final-space-shuttle-external&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; talks about the paint used to touch up the tank for display and transport.  The paint &quot;was tinted to match the burnt-orange foam by PPG aerospace coatings color experts&quot;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-05-23T03:33:58.953" LastActivityDate="2016-05-23T03:33:58.953" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15381" PostTypeId="2" ParentId="15166" CreationDate="2016-05-23T07:56:24.897" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://www.spacelaunchreport.com/falcon9ft.html&quot; rel=&quot;nofollow&quot;&gt;Space Launch Report&lt;/a&gt; keeps a list of serial numbers and versions.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-05-23T07:56:24.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15382" PostTypeId="1" CreationDate="2016-05-23T08:05:24.880" Score="2" ViewCount="221" Body="&lt;p&gt;As I understand the &lt;code&gt;Angular Momentum&lt;/code&gt; of orbit is directed perpendicular to the orbital plane and is the cross product of &lt;code&gt;position&lt;/code&gt; and &lt;code&gt;velocity&lt;/code&gt; vector.&#xA;In case of &lt;code&gt;nodal regression&lt;/code&gt;, how does the &lt;code&gt;position&lt;/code&gt; and &lt;code&gt;velocity&lt;/code&gt; vary so that the &lt;code&gt;Angular momentum&lt;/code&gt; is conserved ?&lt;/p&gt;&#xA;" OwnerUserId="14400" LastActivityDate="2016-05-23T13:01:37.320" Title="Orbital angular momentum" Tags="|orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15383" PostTypeId="1" CreationDate="2016-05-23T10:30:28.883" Score="4" ViewCount="3063" Body="&lt;p&gt;I am designing a communication system for a small satellite. One of the major constraints is the duration of communication between the satellite and the ground station. I did link budget calculations and the minimum &lt;a href=&quot;http://en.wikipedia.org/wiki/Signal-to-noise_ratio&quot; rel=&quot;nofollow&quot;&gt;SNR&lt;/a&gt; required is 15 dB. I also understand that path loss depends on distance between the transmitter and the receiver, and increasing the SNR will require increasing the power transmitted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get to the question, my constraints are :&lt;br&gt;&#xA;1- Max. data rate 9600 bps.&lt;br&gt;&#xA;2- Amount of data to be sent is 3.9MB.&lt;br&gt;&#xA;3- My SNR is 10 dB.&lt;br&gt;&#xA;4- Maximum transmit power = 1 W&lt;br&gt;&#xA;5- Altitude = 667km  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the above values, how can I calculate the communications window (duration). At what elevation angle should communication start? Is there a way to graph the relationship between SNR, path loss and elevation angle?&lt;/p&gt;&#xA;" OwnerUserId="14507" LastEditorUserId="1142" LastEditDate="2016-05-24T02:01:08.210" LastActivityDate="2016-06-25T18:09:47.600" Title="Calculate communications window for LEO" Tags="|cubesat|communication-satellite|data-transmission|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15384" PostTypeId="2" ParentId="15382" CreationDate="2016-05-23T13:01:37.320" Score="3" Body="&lt;p&gt;Gravity goes both ways - the satellite pulls on the earth's bulge as much as the bulge pulls on the satellite - roughly speaking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the angular momentum vector of the satellite changes, so does the angular momentum of the earth change an equal but opposite amount (but wait!) So the total angular momentum of the satellite + earth is roughly constant - or would be if it was in a universe without any other objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a &lt;em&gt;much larger&lt;/em&gt; amount of angular momentum exchanged between the moon and the earth, and Jupiter, and Venus, and the sun, and everything else.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These darn $\frac{1}{r^2}$ forces never reach zero, so everything is an approximation, and energy, momentum, angular momentum are never perfectly conserved locally.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-05-23T13:01:37.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15385" PostTypeId="2" ParentId="13286" CreationDate="2016-05-23T14:30:36.777" Score="1" Body="&lt;p&gt;For ariane 5 the thickness is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;8mm for the solid boosters &#xA;4mm for the cryogenic main stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However both have an additional thermal insulation layer. I couldn't find data for the booster [see comments], but it's 2cm for the cryogenic one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering this data I wouldn't be surprised if the insulating layer is missing from the other answers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS: the fairing is 3cm thick. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ariane_5&quot; rel=&quot;nofollow&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6642" LastEditDate="2016-05-23T14:43:55.007" LastActivityDate="2016-05-23T14:43:55.007" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15386" PostTypeId="2" ParentId="15370" CreationDate="2016-05-23T14:51:57.657" Score="3" Body="&lt;p&gt;2012rcampion found a nice &lt;a href=&quot;http://arxiv.org/abs/1501.07805&quot; rel=&quot;nofollow&quot;&gt;paper&lt;/a&gt; detailing all the visual effects caused by the optical system of the satellite. According to the paper, the many small artifacts are caused by diffraction:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The optical setup of each AIA telescope is characterized by structures with uniform wire meshes used to support the thin filters  that  create  the  EUV  passbands.   The interaction between the incoming EUV radiation and the grids generates a diffraction effect. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is the same optical effect that produces e.g. the shiny colors you see on the bottom side of a CD or any other microscopic and regular structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The banding of this pattern, i.e. groups of higher and lower intensity of these artifacts is not mentioned in the paper, but I suspect these to be an overlaying diffraction pattern caused by the aperture of the lens system (see &quot;airy discs&quot;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vertical band is, as you wrote, a blooming effect commonly seen on all CCD-based photo sensors and caused by their specific electrical structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the paper describes, these artifacts are not only an error in the final image, but can actually be used to increase the quality of the data. In your example images it is clearly visible that the most central regions are so bright that the camera sensor is saturating and not able to show any detailed structures. The diffraction patterns, on the other hand, are more or less precise copies of this region, but at lower intensity. Hence, they can be used to reconstruct the missing information in the central region. The paper shows some examples of this process, including some of the math involved to re-calculate the original image.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-05-23T14:51:57.657" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15388" PostTypeId="2" ParentId="15330" CreationDate="2016-05-23T17:19:55.850" Score="5" Body="&lt;p&gt;Imagine two parallel lines. You are a line segment on the lower line, and your perception is completely limited to your line. All objects you are aware of are oriented along the line. You theorize there might be a higher spatial dimension and that there might be another line above you in this second dimension. You would like to construct a bridge to the other line, but you have a problem. In order to bridge the two lines, you have to already have a two dimensional object: an angle. Without an angle, you cannot create a bridge between the two lines. No one has ever observed an angle, and no method to create an angle is known.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the problem we are facing with the proposition of hyperspace. No one has ever observed another spatial dimension. We don't know how to access hyperspace, if it exists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See: &lt;a href=&quot;https://en.wikipedia.org/wiki/Brane_cosmology#Empirical_tests&quot; rel=&quot;noreferrer&quot;&gt;Brane cosmology - Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-05-23T17:19:55.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15389" PostTypeId="1" CreationDate="2016-05-23T20:31:52.940" Score="3" ViewCount="239" Body="&lt;p&gt;NASA launched &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Orbiter_program&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbiter Program&lt;/a&gt; in an effort to map Moon's surface, choose landing spots for Apollo missions and acquire images for landing simulator. Did USSR have similar program? &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_12&quot; rel=&quot;noreferrer&quot;&gt;Luna 12&lt;/a&gt; seems to fit the criteria but it was only one mission. Were there any other attempts?&lt;/p&gt;&#xA;" OwnerUserId="6179" LastEditorUserId="6179" LastEditDate="2016-05-23T20:37:52.253" LastActivityDate="2017-09-26T15:14:34.773" Title="Soviet equivalent to Lunar Orbiter Program" Tags="|the-moon|artificial-satellite|orbit|nasa|russia|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15390" PostTypeId="1" CreationDate="2016-05-23T21:02:12.603" Score="15" ViewCount="703" Body="&lt;p&gt;Another run of kerbal got me thinking about what, in reality, makes up the limit for small, but still useful space probes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rationale should be obvious: The rocket equation does not hurt that much when we can make our payloads smaller and smaller. Assume a 10kg dry mass. With 260s ISP you get close to 6km/s delta-v out of a 100kg probe, enough to bring you into Mars orbit (and thus probably also somewhere in the belt). On a Falcon 9 you can start 220 of such probes at once, bringing launch cost down to about 300 000$. Probe production would also benefit from economics of scale. Finally, a certain risk of loss is not big deal anymore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the obvious question is: Are such probes useful? Or rather: What is the reason we do not launch such Microprobes today? Is it:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The engine (is the smallest possible liquid fuel engine much bigger)?&lt;/li&gt;&#xA;&lt;li&gt;Power supply (solar panels, batteries)&lt;/li&gt;&#xA;&lt;li&gt;Controls&lt;/li&gt;&#xA;&lt;li&gt;Scientific instruments&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What is driving the payload weight currently?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; David's answer below focused on optics and communication. So I try to be a little bit more precise: &#xA;A large CMOS chip is roughly 16cm². Does one need that much more for in-orbit photography (because there is so little light)? According communication: Is antenna size really the issue (not wattage)? If so, why shouldn't you fold it?&#xA;Say we put a conventional, lightweight camera, top notch solar cells, a small battery, a raspi and a conventional GSM like sender on a probe and send it to a Phobos polar orbit. This &quot;science package&quot; should weigh under 1kg. That gives us another 9kg for attitude control, shielding etc. what am I missing?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="11256" LastEditDate="2016-05-24T07:51:33.777" LastActivityDate="2022-08-28T21:33:16.143" Title="What limits miniaturization of space probes?" Tags="|probe|mass|" AnswerCount="6" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="15391" PostTypeId="2" ParentId="15390" CreationDate="2016-05-24T00:34:41.320" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What is driving the payload weight currently?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Optics.&lt;br&gt;&#xA;You need a big camera with a big lens if you want pretty, high resolution pictures. There is no way around this. It's physics.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Communications.&lt;br&gt;&#xA;You need a big antenna with all the big stuff that goes along with that if you want to communicate at a high rate. There is no way around this. This too is physics.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The cube-square law.&lt;br&gt;&#xA;In some places, this benefits smaller payloads. But in many other places, this benefits larger rockets, larger payloads. Physics, baby!&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In short, its physics. What drives payloads to be smaller than optimal is economics.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-05-24T00:34:41.320" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15392" PostTypeId="1" CreationDate="2016-05-24T04:30:24.917" Score="0" ViewCount="514" Body="&lt;p&gt;Why does the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apse_line&quot; rel=&quot;nofollow&quot;&gt;line of apsides&lt;/a&gt; rotate in the plane of the orbit ? In case of a sun synchronous orbit does it have any effect on imaging accuracy or quality?&lt;/p&gt;&#xA;" OwnerUserId="14400" LastEditorUserId="9060" LastEditDate="2016-05-24T17:42:11.997" LastActivityDate="2017-02-03T23:51:16.943" Title="Rotation Of Line Of Apsides" Tags="|orbit|sun-synchronous|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15393" PostTypeId="1" AcceptedAnswerId="15399" CreationDate="2016-05-24T08:54:37.940" Score="3" ViewCount="345" Body="&lt;p&gt;Let's assume a simplified 2-body problem, with negligibly small satellite mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that the full state vector $\{x,\dot{x},t\}$ (position, velocity, time) at any point of the orbit uniquely determines the orbit. $\{x,t\}$ (Position+time), given 2 points allows to determine the former. But how many points of Position $x$ alone are needed to determine the orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;extra-hard mode: position of the central body is not known. We know the satellite is in orbit, but not around what exactly. How many position readouts would give us the orbit? (and will the central body's position uniquely determinable)?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-05-24T23:18:12.170" Title="How many position data points define an orbit uniquely?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15395" PostTypeId="2" ParentId="10610" CreationDate="2016-05-24T12:39:58.060" Score="1" Body="&lt;p&gt;In addition to the main reasons in other answers, it's important to note that the cost savings from standard frames, modular parts, and mass production is that each additional unit spreads the research, design, and startup costs around.  This is a huge savings when you are going to make 1million cars each year for the next 5; but when you are only making 2 or 3 units ever year or three, and then launching them on a very expensive rocket, there just isn't that much benefit.&lt;/p&gt;&#xA;" OwnerUserId="11720" LastActivityDate="2016-05-24T12:39:58.060" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15396" PostTypeId="1" CreationDate="2016-05-24T18:59:55.430" Score="14" ViewCount="2826" Body="&lt;p&gt;I am currently working on a small hybrid rocket as a summer project. I've decided on using nitrous oxide as the oxidizer, but large tanks are expensive and I don't have a lot of money (I'm in high school). Would it be possible to use the small nitrous tanks used in &lt;a href=&quot;https://en.wikipedia.org/wiki/Whipped-cream_charger&quot;&gt;whipped cream makers&lt;/a&gt; as a source of oxidizer? You could make some sort of adapter and valve and pipe it into the engine no problem, but would this provide a high enough quality oxidizer for the engine to work (even if just for a few seconds)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I am NOT trying to get to space with this engine, nor anywhere close for that matter. It's merely a proof of concept and a predecessor to what I hope WILL fly.&lt;/p&gt;&#xA;" OwnerUserId="14520" LastEditorUserId="9092" LastEditDate="2021-03-25T12:04:10.647" LastActivityDate="2021-03-25T12:04:10.647" Title="Whipped cream hybrid rocket" Tags="|rockets|amateur-rocketry|hybrid-rocket|nitrous-oxide|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15397" PostTypeId="2" ParentId="15383" CreationDate="2016-05-24T21:59:19.887" Score="3" Body="&lt;p&gt;Thanks for the clarification. Your problem has two parts. 1) What is the spacial relationship between the ground station and the satellite and 2) given a spacial relationship between two radios, how 'good' is the connection between them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Problem 1:&lt;/strong&gt; Taking the &lt;s&gt;Easy&lt;/s&gt; Efficient Route &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacial relationship between a satellite and a receiver can be calculated by hand (a common home work assignment in aerospace classes, and another discussion in itself), but the fastest way to find satellite coverage windows is to use a computer. There is plenty of free software available that can do this, but I'll recommend the free version of STK. &lt;a href=&quot;https://www.agi.com/downloads/products/product-literature/120213_STK_Free_Flyer_Air.pdf&quot; rel=&quot;nofollow&quot;&gt;Here&lt;/a&gt; is what you can do with the free version, and &lt;a href=&quot;http://www.agi.com/products/stk/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; is where you can download it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Problem 2:&lt;/strong&gt; Elevation Angle&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The system could be designed in various ways. Data is often broken into packets, which can be transmitted individually. The satellite needs to know which packets the ground station has received successfully and vice versa. Successful reception of a packet is acknowledged with an 'ACK' transmission. To downlink, the satellite could be commanded to transmit a packet repeatedly until it is received successfully, then commanded to transmit the next packet. To uplink data, the ground station would transmit a command or packet and wait for an ACK, then transmit another packet. These methods are agnostic of communication angle and path loss. You simply start when the satellite is approaching the window.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Problem 3:&lt;/strong&gt; Relating SNR and Path Loss to Elevation Angle&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, Power. &lt;a href=&quot;https://en.wikipedia.org/wiki/Free-space_path_loss&quot; rel=&quot;nofollow&quot;&gt;Free Space Path Loss (FSPL)&lt;/a&gt; is used in conjunction with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Friis_transmission_equation&quot; rel=&quot;nofollow&quot;&gt;Friss Transmission Equation&lt;/a&gt; to compute signal power at the input of the receiver (not the receivers antenna, the actual receiver). Given the location of the radios and some information about the receivers this is straight forward. Including atmospheric and other effects can be done if you want a more realistic model (let me know if you're curious).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, Noise. Noise is a little complicated since it involves the actual noise of the transmitter and receiver, as well as external noise. Let's ignore external noise and look at thermal noise. Electrical noise is often measure in temperature because thermal energy translates to electron motion, which is electrical noise. Noise power density (noise power per unit bandwidth) is found as $N_0=kT$ where $N_0$ is in watts, $k$ is the Boltzmann constant in joules/kelvin, and $T$ is the receiver system noise temperature in kelvin. A simple approximation of $T$ on Earth is 290 K. (This is ignoring a few things, but it's an okay first attempt). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Noise power spectral density is found as $\frac{E_b}{N_0}=\frac{S/R}{N_0}=\frac{S}{kTR}$ (not in dB) where $S$ is signal power, and $R$ is the bit rate. This is the Energy per bit over the noise density, or the SNR per bit. Defining this in terms of bits will help us relate it to data rate. Further, $SNR = \frac{S}{R}=\frac{E_bR}{N_0B}$ where $B$ is the bandwidth of the signal. Changing your encoding will give different relationships between noise and bit error rate.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;increasing the SNR will require increasing the power transmitted&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Remember decreasing system noise also will increase SNR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Final Thought:&lt;/strong&gt; Data Rate&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem&quot; rel=&quot;nofollow&quot;&gt;The Shannon-Hartley theorem&lt;/a&gt; defines the maximum possible information rate over any (AWGN) channel. $I&amp;lt;B \log_2{\left(1+\frac{S}{N}\right)}$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can now relate elevation angle with path loss, SNR, and even maximum data rate. Hurray!&lt;/p&gt;&#xA;" OwnerUserId="12966" LastEditorUserId="415" LastEditDate="2016-05-25T11:26:07.683" LastActivityDate="2016-05-25T11:26:07.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15399" PostTypeId="2" ParentId="15393" CreationDate="2016-05-24T23:09:55.067" Score="8" Body="&lt;p&gt;It takes five points to define an arbitrary ellipse in a plane, not knowing a priori where the center is. The five points, or really any three of them, also define the plane in three dimensions. (You never said the number of dimensions in your problem, so I am assuming three.) You will then know where the foci of the orbit are, but you won't know &lt;em&gt;which one&lt;/em&gt; is where the body is, with no information about time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes three points to define an ellipse if you know where a focus is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In both cases, it is possible to pick points that give you no new information, so not &lt;em&gt;all&lt;/em&gt; sets of five or three points on an ellipse will give you a unique solution. However those points are of measure zero out of all possible points, so if you pick points randomly as opposed to maliciously, five or three will be enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In neither case will you really define the &quot;orbit&quot;, since you don't know anything about time. E.g. what the orbit period is, unless you know the mass of the central body, which you made no mention of, and even if you know that, you don't know when the object is at any given position in the orbit. So you are left with one degree of freedom that cannot be determined, no matter how many positions you have.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-24T23:18:12.170" LastActivityDate="2016-05-24T23:18:12.170" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15400" PostTypeId="1" AcceptedAnswerId="15410" CreationDate="2016-05-25T03:30:45.667" Score="3" ViewCount="2229" Body="&lt;p&gt;Whenever Falcon 9's engines start, there's a green ignition flash. During the latest landing there were similar green flashes right after touchdown:&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;1st stage landing (3 camera angles): &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/LHqLz9ni0Bo?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Why were there ignition flashes AFTER the Falcon actually landed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note: It seems not all landings have this flash. See footage from the OrbComm mission landing: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ANv5UfZsvZQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="4603" LastActivityDate="2023-08-31T18:29:10.717" Title="Why were there green ignition flashes after Falcon 9's landing?" Tags="|falcon-9|landing|restartable-engine|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15401" PostTypeId="1" AcceptedAnswerId="15402" CreationDate="2016-05-25T03:34:03.903" Score="6" ViewCount="4421" Body="&lt;p&gt;Right before Falcon 9's engines start, there's a green flash. I assume it's some kind of chemical ignition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What makes the flash the color green?&lt;/p&gt;&#xA;" OwnerUserId="4603" LastActivityDate="2021-02-02T23:57:16.687" Title="What causes the green flash during Falcon 9's ignition?" Tags="|falcon-9|chemistry|restartable-engine|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15402" PostTypeId="2" ParentId="15401" CreationDate="2016-05-25T04:17:18.273" Score="10" Body="&lt;p&gt;The Merlin engines are ignited with a mixture of &lt;a href=&quot;https://en.wikipedia.org/wiki/Triethylaluminium&quot; rel=&quot;nofollow noreferrer&quot;&gt;triethylaluminium&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Triethylborane&quot; rel=&quot;nofollow noreferrer&quot;&gt;triethylborane&lt;/a&gt; (TEA-TEB); according to Wikipedia:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Triethylborane is strongly pyrophoric, igniting spontaneously in air, burning with an &lt;strong&gt;apple-green&lt;/strong&gt; flame characteristic for boron compounds. Thus, it must be handled and stored in nitrogen or argon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Note that this effect should not be confused with the green flame seen occasionally from damaged Raptor engines that are running &amp;quot;engine-rich&amp;quot;, i.e. starting to destroy themselves, and is the color of burning copper. Raptor is spark-ignited, and doesn't use TEA-TEB.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-02-02T23:57:16.687" LastActivityDate="2021-02-02T23:57:16.687" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="15403" PostTypeId="1" AcceptedAnswerId="15413" CreationDate="2016-05-25T04:43:40.940" Score="20" ViewCount="25706" Body="&lt;p&gt;Both the Saturn V and the Falcon 9 use &lt;a href=&quot;https://en.wikipedia.org/wiki/Triethylborane#Rocket&quot; rel=&quot;noreferrer&quot;&gt;TEA-TEB&lt;/a&gt; to ignite their kerosene-fueled engines. TEA-TEB is pyrophoric, igniting spontaneously on contact with air. This poses handling issues; it must be stored in nitrogen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also an expendable resource which puts limits on the number of times an engine can be started in flight; the Falcon 9 only loads TEA-TEB for restarts on those engines that will be restarted on a given mission, reasonably enough, but this makes it susceptible to engine failures in the reentry and landing burns that could otherwise be recovered from by using different engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some other engines, such as the hydrogen-fueled J-2 and RL-10, use electric spark ignition rather than a chemical starter. Both engines are reliable and capable of multiple restarts after launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can kerosene-LOX engines use electric ignition? If so, why consider a volume-limited and hazardous chemical igniter like TEA-TEB?&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2023-02-27T15:01:56.327" Title="Why is TEA-TEB chemical ignition used instead of spark ignition?" Tags="|fuel|engines|hydrogen|restartable-engine|kerosene|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15404" PostTypeId="1" AcceptedAnswerId="15516" CreationDate="2016-05-25T06:15:38.463" Score="12" ViewCount="859" Body="&lt;p&gt;The first sample return mission from a comet has been achieved by NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stardust&lt;/a&gt; spacecraft, and the first sample return mission from an asteroid has been achieved by JAXA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa&lt;/a&gt; spacecraft. What are the main challenges and difficulties for such missions, and how much do these types of missions differ from each other regarding difficulties encountered (for example spacecraft different velocities, technical or technological features)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://media.gettyimages.com/photos/stardust-timeline-launch-feb-07-1999-1st-interstellar-dust-collection-picture-id1299297&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://media.gettyimages.com/photos/stardust-timeline-launch-feb-07-1999-1st-interstellar-dust-collection-picture-id1299297&quot; alt=&quot;Stardust spacecraft, NASA&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;Stardust spacecraft, NASA.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RCWfY.jpg&quot; alt=&quot;Hayabusa spacecraft, JAXA&quot;&gt;&#xA;&lt;sup&gt;Hayabusa spacecraft, JAXA.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="10273" LastEditDate="2019-07-19T06:26:43.870" LastActivityDate="2019-07-19T06:26:43.870" Title="Which are the main challenges for sample return mission from an asteroid and a comet?" Tags="|asteroid|design|comet|sample-return|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="15405" PostTypeId="2" ParentId="15390" CreationDate="2016-05-25T06:39:31.943" Score="0" Body="&lt;p&gt;One particular problem of putting miniaturised components into space is the effects of cosmic radiation: Miniature components are more affected by this radiation and that can produce unexpected consequences: Computer resets, memory corruption, noise on instrumentation readouts (especially for imaging satellites). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1545891&amp;amp;tag=1&quot; rel=&quot;nofollow&quot;&gt;research paper&lt;/a&gt;, while terrestrial, shows some of the effects of scaling technology:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The author has shown that while the SER of DRAM&#xA;  in a system is relatively unchanged by scaling, SRAM and&#xA;  peripheral logic system SER are increasing rapidly with each&#xA;  new technology node&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="14509" LastEditorUserId="14509" LastEditDate="2016-05-25T16:32:31.517" LastActivityDate="2016-05-25T16:32:31.517" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15407" PostTypeId="1" AcceptedAnswerId="15408" CreationDate="2016-05-25T07:12:10.800" Score="1" ViewCount="109" Body="&lt;p&gt;There was a bit of fanfare for the SAFFIRE-I experiment. I got excited...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1761.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/mission_pages/station/research/experiments/1761.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So they set fire to a Cygnus module before de-orbit. Awesome! Now... why can't I seem to find anything about the actual experiment? Some results?&lt;/p&gt;&#xA;" OwnerUserId="14422" LastActivityDate="2016-05-25T07:30:35.073" Title="Where are the SAFFIRE-I results?" Tags="|iss|spacecraft|space-station|life-support|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15408" PostTypeId="2" ParentId="15407" CreationDate="2016-05-25T07:30:35.073" Score="7" Body="&lt;p&gt;The experiment hasn't been done yet. &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_unmanned_spaceflights_to_the_International_Space_Station&quot;&gt;Cygnus OA-6 is still docked to the ISS&lt;/a&gt;, it's planned to be undocked in June 2016, that's when the experiment will take place. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-05-25T07:30:35.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15410" PostTypeId="2" ParentId="15400" CreationDate="2016-05-25T11:34:01.447" Score="7" Body="&lt;p&gt;The green colour is characteristic of TEB (Triethylborane) combustion, which is used in combination with TEA (Triethylaluminum) to ignite the Merlin 1D engines (those two substances are pyrophoric, and are used to kickstart the RP1/Oxygen combustion).&lt;/p&gt;&#xA;&lt;p&gt;At least three of the nine engines are equipped with an onboard supply of TEA-TEB for in-flight relights. During the processing of the CRS-8 core in Port Canaveral, the remains of those reserves were dumped on the ground to safe the stage for transport. It's possible the first stages are now commanded to dump as much as possible as soon as the landing is confirmed, to reduce the risks &amp;amp; time needed once the stage is brought back to land (some was still left in JCSAT's first stage though, we could see the green flashes during Port Canaveral processing)&lt;/p&gt;&#xA;" OwnerUserId="8841" LastEditorUserId="30164" LastEditDate="2022-06-11T19:20:45.923" LastActivityDate="2022-06-11T19:20:45.923" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="15411" PostTypeId="2" ParentId="15404" CreationDate="2016-05-25T16:04:17.297" Score="1" Body="&lt;p&gt;To get samples from a comet, the spacecraft may fly through the tail to collect some tiny dust parts. To get samples from an asteroid, the spacecraft must land, collect samples and then leave the orbit of the asteroid and fly back to earth. A lot of fuel is needed for landing and accelerating again to get back to earth. High precision navigation is necessary for a smooth landing.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-05-25T16:04:17.297" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15412" PostTypeId="2" ParentId="15390" CreationDate="2016-05-25T16:34:44.427" Score="8" Body="&lt;p&gt;Aside from optics and communications, there are other limitations to miniaturization.&lt;/p&gt;&#xA;&lt;p&gt;For example, smaller objects become harder to track if communication is lost. Our current orbital debris tracking system only tracks objects down to about the size of a marble, and with greater numbers of small items it becomes harder to avoid collisions with larger objects.&lt;/p&gt;&#xA;&lt;p&gt;Electronic components also suffer from reliability issues from radiation. Scaling down many components results in higher component density, and thus higher error rates as shown in &lt;a href=&quot;https://ieeexplore.ieee.org/document/1545891&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radiation-induced soft errors in advanced semiconductor technologies - Baumann, R.C.&lt;/a&gt; and &lt;a href=&quot;https://www.intechopen.com/chapters/39233&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soft-Error Rate of Advanced SRAM Memories: Modeling and Monte Carlo Simulation - Autran, et al.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;As explained in &lt;a href=&quot;https://space.stackexchange.com/a/15405/58&quot;&gt;another answer&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Miniature components are more affected by this radiation and that can produce unexpected consequences: Computer resets, memory corruption, noise on instrumentation readouts (especially for imaging satellites).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Down-scaling amplifies these consequences:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;With the constant downscaling of microelectronic devices, the sensitivity of integrated circuits to natural radiation coming from the space or present in the terrestrial environment has been found to seriously increase&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;sub&gt; (Autran, et al.)&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="10273" LastEditDate="2022-04-30T18:28:55.337" LastActivityDate="2022-04-30T18:28:55.337" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="15413" PostTypeId="2" ParentId="15403" CreationDate="2016-05-25T17:11:32.627" Score="19" Body="&lt;p&gt;The way you ignite a rocket engine depends a lot on the fuel/oxidiser combination. Some combinations, like Liquid Hydrogen/Liquid Oxygen (HydroLox) ignite very easily and are ignited using a glorified spark plug (this is the case of the RL10, used on the Centaur Upper Stage). The RP1 (Kerosene)/Liquid Oxygen mixture is harder to ignite, and requires more energy. You also can't afford to have fuel pool up in the combustion chamber and then ignite all at once (it could be too brutal for the engine to withstand). In this case, you inject TEA-TEB with the oxidiser, which causes it to ignite, and only then is the fuel injected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA article on ignition systems: &lt;a href=&quot;http://blogs.nasa.gov/J2X/tag/ignition/&quot; rel=&quot;noreferrer&quot;&gt;http://blogs.nasa.gov/J2X/tag/ignition/&lt;/a&gt;&lt;br&gt;&#xA;Centaur/RL10: &lt;a href=&quot;http://spaceflight101.com/spacerockets/atlas-v-551/&quot; rel=&quot;noreferrer&quot;&gt;http://spaceflight101.com/spacerockets/atlas-v-551/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8841" LastActivityDate="2016-05-25T17:11:32.627" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15415" PostTypeId="2" ParentId="6461" CreationDate="2016-05-26T03:52:23.737" Score="4" Body="&lt;p&gt;To supplement the other answers (and perhaps of historical interest) &lt;a href=&quot;https://www.nasa.gov/pdf/522589main_AP_ST_Phys_ShuttleLaunch.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here's&lt;/a&gt; a Space Shuttle launch acceleration profile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q7ODR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q7ODR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Segment labels correspond to the following events/phases:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A: liftoff to throttle-down&lt;/p&gt;&#xA;&#xA;&lt;p&gt;B: throttle-down for max dynamic pressure&lt;/p&gt;&#xA;&#xA;&lt;p&gt;C: throttle-up to start of SRB burnout&lt;/p&gt;&#xA;&#xA;&lt;p&gt;D: SRB burnout and sep&lt;/p&gt;&#xA;&#xA;&lt;p&gt;E: constant main engine throttle setting with ET prop burning off&lt;/p&gt;&#xA;&#xA;&lt;p&gt;F: main engines throttling back to maintain 3G design limit&lt;/p&gt;&#xA;&#xA;&lt;p&gt;G: main engine cutoff and ET sep&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Re: vibration, all accounts say that the vibration from the SRBs was significant enough to make it challenging to focus on the instruments.  After SRB sep the ride was much smoother.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-05-26T03:52:23.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15417" PostTypeId="2" ParentId="15383" CreationDate="2016-05-26T09:10:49.780" Score="1" Body="&lt;p&gt;I'm not sure I understand, but I'm going to try to explain. I'm an astronautical engineer, so I'm gonna explain this in orbital mechanics instead of electronics. Hope it helps!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, in orbital mechanics we assume that you can communicate with your sat between the time that it enters to G/S's horizon and the time when it leaves it. You know your altitude and Earth's radius so you can evaluate the central angle related to your orbit using basic trigonometric calculations. Then you can calculate your period in that altitude. Now you know your communication time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0mhQW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0mhQW.png&quot; alt=&quot;Diagram of altitude and Earth&amp;#39;s radius&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For further analysis, communication time depends on satellite's attitude capability. Also you need to perform an orbital analysis to know that satellite passes directly above GS or at an angle, and when it will pass above GS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck.&lt;/p&gt;&#xA;" OwnerUserId="14535" LastEditorUserId="58" LastEditDate="2016-05-26T18:07:10.113" LastActivityDate="2016-05-26T18:07:10.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15419" PostTypeId="1" AcceptedAnswerId="15511" CreationDate="2016-05-26T16:16:32.090" Score="4" ViewCount="197" Body="&lt;p&gt;The Indian Space Research Organization (ISRO) is currently testing a &lt;a href=&quot;https://en.wikipedia.org/wiki/RLV_Technology_Demonstration_Programme&quot; rel=&quot;nofollow&quot;&gt;Reusable Launch Vehicle (RLV)&lt;/a&gt; in a technology demonstration program. Does this program have a future manned vehicle in mind? Or is it currently planned to stay unmanned?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe there's no clear future goal in mind aside from having a reusable launch vehicle, but I have a hard time imagining that ISRO doesn't have some sort of (tenative, at least) goal in mind for the fruits of this RLV program.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-05-26T16:32:08.890" LastActivityDate="2016-06-02T03:01:20.973" Title="Does ISRO's RLV have a future manned spacecraft in mind?" Tags="|reuse|isro|launch-vehicle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15420" PostTypeId="1" CreationDate="2016-05-26T17:46:22.727" Score="6" ViewCount="1235" Body="&lt;p&gt;Typically cars are rated on their 0-60 mph times. (Tesla's currently kicking some serious gas car butt, where the P90D with Ludicrous mode hitting 2.8s which is literally ludicrous. As in you can hurt yourself just hitting the gas pedal). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a faster vehicle, I am thinking 0-Mach 1 is more fun time to consider. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the &lt;a href=&quot;https://spaceflightnow.com/2016/05/25/timeline-for-falcon-9s-launch-of-thaicom-8/&quot;&gt;Thaicomm-8 mission timeline at Spaceflightnow.com&lt;/a&gt; it suggests from a standing start, to Mach 1 at 1 minute 10 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is pretty darn good. 70 seconds to Mach 1. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does that time compare for:&#xA;a) Other boosters.&#xA;b) Fighter jets&#xA;c) What else can exceed Mach 1 under power?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Man, the drag racing videos of a Tesla Model S against various gas powered 'super cars' are fun to watch.  Can you imagine a Falcon 9 vs F-22 race? That would be cool. Impossible, but cool!&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-27T06:56:28.160" Title="How does Rocket performance compare to other things?" Tags="|spacex|falcon-9|performance|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15421" PostTypeId="2" ParentId="15420" CreationDate="2016-05-26T18:33:58.473" Score="4" Body="&lt;p&gt;Not sure you'll get much useful data, but here's a useful datum point:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bloodhoundssc.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bloodhound SSC&lt;/a&gt; - the worlds fastest car is expected to reach 1000mph in 55 seconds from rest (42 seconds would be a perfect run, but 55 is the expected run), with a peak acceleration of 2g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0 - 300mph (Point a) using the EJ-200 jet engine (generating around 0.5g), where the rocket will be ignited (total acceleration from the two hitting 2g) until maximum speed of around 1050mph, where the rocket will be cut off (Point b)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly deceleration will be around 3g from aerodynamic drag alone! Brakes will be used at Point c - for about 0.25g deceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/O9wuj.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/O9wuj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2016-05-26T18:33:58.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15422" PostTypeId="1" CreationDate="2016-05-26T20:51:32.330" Score="13" ViewCount="3559" Body="&lt;p&gt;Pretty much all model rockets have fins for stability, but when I think about it, I can't seem to come up with a full scale, commercial or government rocket that has fins. I'm probably forgetting one really obvious, but does anyone have an example?&lt;/p&gt;&#xA;" OwnerUserId="14520" LastEditorUserId="1142" LastEditDate="2016-05-27T03:20:33.307" LastActivityDate="2018-02-14T21:21:45.303" Title="Are there commercial rockets with fins?" Tags="|rockets|design|model-rocketry|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15423" PostTypeId="2" ParentId="15422" CreationDate="2016-05-26T21:26:15.523" Score="21" Body="&lt;p&gt;Sure, lots of sounding rockets, like these from NASA's Wallops Flight Facility:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hjau0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hjau0.jpg&quot; alt=&quot;stable of sounding rockets&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This one had pretty darned big fins:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5ugDl.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5ugDl.jpg&quot; alt=&quot;space shuttle&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similar fins on this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cehxy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cehxy.jpg&quot; alt=&quot;pegasus&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This one had small fins, but it really didn't need them:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WBRAo.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WBRAo.jpg&quot; alt=&quot;saturn v&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The old Scout rocket had small fins too:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zZYfy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zZYfy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-05-26T21:32:34.960" LastActivityDate="2016-05-26T21:32:34.960" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="15424" PostTypeId="1" AcceptedAnswerId="15434" CreationDate="2016-05-26T21:34:00.533" Score="15" ViewCount="3128" Body="&lt;p&gt;In this &lt;a href=&quot;http://www.meaus.com/whyExplore.html&quot;&gt;article&lt;/a&gt; there is following statement by Dr. Ernst Stuhlinger:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Let me only remind you of the recent near-tragedy of Apollo 13. When the time of the crucial reentry of the astronauts approached, the Soviet Union discontinued all Russian radio transmissions in the frequency bands used by the Apollo Project in order to avoid any possible interference, and Russian ships stationed themselves in the Pacific and the Atlantic oceans in case an emergency rescue would become necessary.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Any other sources for that claim?&lt;/p&gt;&#xA;" OwnerUserId="6179" LastEditorUserId="415" LastEditDate="2016-12-01T15:06:55.353" LastActivityDate="2016-12-01T15:06:55.353" Title="Did USSR discontinue radio transmissions, relocate ships, to aid the US in response to Apollo 13?" Tags="|crewed-spaceflight|communication|russia|radio-communication|apollo-13|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15425" PostTypeId="2" ParentId="15422" CreationDate="2016-05-26T22:06:31.333" Score="21" Body="&lt;p&gt;As for the reason you see fins on model rockets but not on most large launchers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets that require precision guidance to a specific trajectory have to be actively steered; the two obvious ways to do that are by moving fins or vectored thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vectored thrust has two big advantages: one, it works outside of the atmosphere, two, fins produce drag. So big guided orbital launchers go with vectored thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Passive fixed fins like those on model rockets help stabilize, but not guide, the rocket's flight. If you have active steering, that gives you stabilization as well, and unless the aerodynamics of the rocket are very poor, you don't need fins at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mark Adler's exceptions fall into two main categories:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Sounding rockets -- generally unguided, fin or spin stabilized&lt;/li&gt;&#xA;&lt;li&gt;Vehicles that need aerodynamic surfaces for lift at some point in their flight (shuttle, Pegasus). &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Saturn I and V are outliers; as Mark notes, the fins weren't needed in normal flight because the engines were gimbaled, but they would improve stability in the event of a catastrophic first-stage engine problem, providing a longer time window in which to abort safely.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-02-14T21:21:45.303" LastActivityDate="2018-02-14T21:21:45.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15426" PostTypeId="2" ParentId="15396" CreationDate="2016-05-26T23:21:02.763" Score="16" Body="&lt;p&gt;What a fantastic question! Absolutely it's possible! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$N_2O$ is $N_2O$, whether you get it from an 8g whipped cream charger or a massive industrial tank, it oxidises just the same. The valve you'll need to buy/make for 8g canisters is called a pin valve. &lt;a href=&quot;http://www.homebrewcentre.co.uk/brass-c02-injector-valve-with-stainless-steel-piercing-pin&quot; rel=&quot;noreferrer&quot;&gt;Here's an example&lt;/a&gt;; basically it is just a hollow needle that breaks the seal when the canister is pressed or screwed into it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're not the first to think of using 8g and 16g nitrous canisters in hybrids.  There's a &lt;a href=&quot;https://www.youtube.com/watch?v=mwAhk1Vws6U&quot; rel=&quot;noreferrer&quot;&gt;test fire of one motor on YouTube&lt;/a&gt;. Unfortunately there's very little detail on the design. &#xA;However, there are &lt;a href=&quot;http://www.aeroconsystems.com/motors/Screamer_folder/screamer.htm&quot; rel=&quot;noreferrer&quot;&gt;complete motor kits&lt;/a&gt; (with cutaway diagrams) if you want to really cut corners.  Apparently these motors usually get around 11Ns of impulse, the same as a class D solid motor, so a respectable amount of power for their size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So go out and do it! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, always exercise extreme caution when dealing with pressurised gasses. 8g of gas may not sound like much, but one of those little cylinders has enough energy to fire a needle-like pin valve through your eye and into your brain. Always wear eye protection when you're working with pressurised gasses, avoid situations where the gas could propel any object (including its own cylinder), and keep your work area well ventilated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$N_2O$ is a particularly unpleasant gas to deal with, and has killed many people who thought they knew what they were doing. See &lt;a href=&quot;http://www.spl.ch/old/publication/SPL_Papers/N2O_safety_e.pdf&quot; rel=&quot;noreferrer&quot;&gt;SPL's safety PDF&lt;/a&gt; for a good summary of some of the often overlooked dangers of $N_2O$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, any rocket is always just one mistake or failure away from turning into a bomb, and amateur rockets are definitely no exception. Follow the same (or greater) safety precautions you would for a standard solid-fuel rocket when handling, firing, or launching.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck. Fly safe.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6816" LastEditDate="2017-09-28T02:08:55.280" LastActivityDate="2017-09-28T02:08:55.280" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15427" PostTypeId="1" AcceptedAnswerId="15429" CreationDate="2016-05-27T04:30:36.643" Score="7" ViewCount="812" Body="&lt;p&gt;In the SOHO images for today, there was a white dot at the bottom right of the image. There is also a strange white  line parallel to the top and bottom of the image that crosses most of the image. From my amount of research, I've assumed that the dot is either Mercury or Venus, most likely the latter though. But what about the line across tbe image that crosses the planet? Did I even guess right that the dot is a planet&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the image:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/hYMPS.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hYMPS.jpg&quot; alt=&quot;The image&quot;&gt;&lt;/a&gt;&#xA;The line seems to get thicker around the dot, and seems to be coming from the planet, but thst doesn't make sense. Also, none of the other images have the dot or line, as they are zoomed in too far.&lt;/p&gt;&#xA;" OwnerUserId="14305" LastActivityDate="2016-05-29T05:25:05.293" Title="What is this white dot and strange line in SOHO image?" Tags="|venus|the-sun|mercury|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15428" PostTypeId="1" CreationDate="2016-05-27T05:23:16.227" Score="3" ViewCount="3010" Body="&lt;p&gt;I am wondering what are the big advantages and disadvantages using QPSK, FSK,and ASK in satellite communication. In our project, which is in phase 3, are we supposed to work with QPSK modulation for our cubesat? I don't really know why they choose to work with QPSK instead of FSK. Can anybody state the big difference between these two type of modulation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second question, in a cubesat since the link is very costly we should use FEC instead of retransmitting the data. In our case we used convolution encoding. I would like to know what the common code rate for this encoding is ( 1/2,3/2,..).&lt;/p&gt;&#xA;" OwnerUserId="14546" LastEditorUserId="4660" LastEditDate="2016-05-27T23:04:35.243" LastActivityDate="2016-05-28T02:19:19.933" Title="What is the advantage of QPSK over FSK or ASK for satellite data transmission?" Tags="|artificial-satellite|cubesat|data-transmission|radio-communication|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15429" PostTypeId="2" ParentId="15427" CreationDate="2016-05-27T05:34:08.507" Score="11" Body="&lt;p&gt;&lt;s&gt;It might be Mercury - thanks for the tags.&lt;/s&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;s&gt;It might be...&lt;/s&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It &lt;strong&gt;is&lt;/strong&gt; Venus - thanks for the tags. :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the mean time enjoy seeing the Pleiades pass closer to the sun than you'd normally see them!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These &lt;a href=&quot;http://lasco-www.nrl.navy.mil/index.php?p=content/handbook/hndbk_3&quot; rel=&quot;noreferrer&quot;&gt;LASCO C3&lt;/a&gt; images from &lt;a href=&quot;http://sohowww.nascom.nasa.gov/home.html&quot; rel=&quot;noreferrer&quot;&gt;SOHO&lt;/a&gt; were downloaded &lt;a href=&quot;http://sohodata.nascom.nasa.gov/cgi-bin/data_query&quot; rel=&quot;noreferrer&quot;&gt;sohodata.nascom.nasa.gov/cgi-bin/data_query&lt;/a&gt;. &lt;em&gt;The square frame is about 15.9 degrees wide&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LgrIs.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LgrIs.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I like to use the NIH free software &lt;a href=&quot;https://imagej.nih.gov/ij/&quot; rel=&quot;noreferrer&quot;&gt;ImageJ&lt;/a&gt; for image manipulation, including making GIFs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; SOHO is in orbit around Sun-Earth L1, which means it's within about a half-million km near the Sun-Earth line, but about 1.5 million km closer. While Mercury's apparent position relative to the sun will be slightly different as seen from SOHO compared to Earth, it's a small effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lehK2.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lehK2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Apparent angular separation of Mercury and Venus from the Sun for the month of May 2016.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Coordinates for SOHO are from JPL's Horizons web interface at &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;ssd.jpl.nasa.gov/horizons.cgi&lt;/a&gt;. I pasted the coordinates into this Python script and used &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;noreferrer&quot;&gt;Skyfield&lt;/a&gt; to get the positions of the sun and planets.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def angsep(a, b):&#xA;&#xA;    top = (a*b).sum(axis=0)&#xA;    bot = np.sqrt( (a**2).sum(axis=0) * (b**2).sum(axis=0) )&#xA;&#xA;    return np.arccos(top/bot)&#xA;&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from skyfield.api import load&#xA;from skyfield.positionlib import ICRF&#xA;&#xA;data = load('de421.bsp')&#xA;&#xA;sun     = data['sun']&#xA;mercury = data['mercury']&#xA;venus   = data['venus']&#xA;earth   = data['earth']&#xA;&#xA;days = range(1, 33)&#xA;ts   = load.timescale()&#xA;t    = ts.utc(2016, 5, days, 0, 0, 0)&#xA;&#xA;# SOHO_pos downloaded from http://ssd.jpl.nasa.gov/horizons.cgi&#xA;&#xA;sun_pos     = sun.at(t).ecliptic_position().km&#xA;mercury_pos = mercury.at(t).ecliptic_position().km&#xA;venus_pos   = venus.at(t).ecliptic_position().km&#xA;earth_pos   = earth.at(t).ecliptic_position().km&#xA;&#xA;r_earth_sun     = sun_pos     - earth_pos&#xA;r_earth_mercury = mercury_pos - earth_pos&#xA;r_earth_venus   = venus_pos   - earth_pos&#xA;&#xA;r_SOHO_sun      = sun_pos     - SOHO_pos&#xA;r_SOHO_mercury  = mercury_pos - SOHO_pos&#xA;r_SOHO_venus    = venus_pos   - SOHO_pos&#xA;&#xA;mercury_sep_from_earth = angsep(r_earth_mercury, r_earth_sun)&#xA;venus_sep_from_earth   = angsep(r_earth_venus,   r_earth_sun)&#xA;&#xA;mercury_sep_from_SOHO  = angsep(r_SOHO_mercury,  r_SOHO_sun)&#xA;venus_sep_from_SOHO    = angsep(r_SOHO_venus,    r_SOHO_sun)&#xA;&#xA;degs = 180. / np.pi&#xA;&#xA;plt.figure()&#xA;&#xA;plt.subplot(1, 2, 1)&#xA;plt.plot(days, degs*mercury_sep_from_earth, '--g')&#xA;plt.plot(days, degs*venus_sep_from_earth, '-b')&#xA;plt.plot(days[25:26], degs*venus_sep_from_earth[25:26], 'ok')&#xA;plt.text(22, 5, 'Venus', fontsize=16)&#xA;plt.text(22, 15, 'Mercury', fontsize=16)&#xA;plt.title('Seen from Earth')&#xA;&#xA;plt.subplot(1, 2, 2)&#xA;plt.plot(days, degs*mercury_sep_from_SOHO, '--g')&#xA;plt.plot(days, degs*venus_sep_from_SOHO, '-b')&#xA;plt.plot(days[25:26], degs*venus_sep_from_SOHO[25:26], 'ok')&#xA;plt.text(22, 5, 'Venus', fontsize=16)&#xA;plt.text(22, 15, 'Mercury', fontsize=16)&#xA;plt.title('Seen from SOHO')&#xA;&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Here is another image from &lt;a href=&quot;http://sungrazer.nrl.navy.mil/index.php?p=transits/transits&quot; rel=&quot;noreferrer&quot;&gt;sungrazer.nrl.navy.mil/index.php?p=transits/transits&lt;/a&gt; that sheds some light on the subject. It was taken in May, 2000. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/otxa1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/otxa1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this one is &lt;a href=&quot;http://sungrazer.nrl.navy.mil/index.php?p=transits/transits_2016&quot; rel=&quot;noreferrer&quot;&gt;from here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/01OYE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/01OYE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-05-28T13:29:39.497" LastActivityDate="2016-05-28T13:29:39.497" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15430" PostTypeId="1" AcceptedAnswerId="15436" CreationDate="2016-05-27T05:46:59.813" Score="6" ViewCount="502" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/15429/12102&quot;&gt;this answer&lt;/a&gt; I calculated the approximate angular separation between Mercury and the Sun as seen by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHO satellite&lt;/a&gt; using &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt;'s method of &lt;code&gt;.ecliptic_position()&lt;/code&gt; because I couldn't get true ICRF coordinates for SOHO from the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;Horizons web interface&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Is there a way I can get ICRF coordinates for SOHO that haven't been rotated into the ecliptic?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Coordinate system description:&#xA;Ecliptic and Mean Equinox of Reference Epoch&lt;/p&gt;&#xA;&lt;p&gt;Reference epoch: J2000.0&lt;/p&gt;&#xA;&lt;p&gt;xy-plane: plane of the Earth's orbit at the reference epoch&lt;/p&gt;&#xA;&lt;p&gt;x-axis  : out along ascending node of instantaneous plane of the Earth's orbit and the Earth's mean equator at the reference epoch&lt;/p&gt;&#xA;&lt;p&gt;z-axis  : perpendicular to the xy-plane in the directional (+ or -) sense of Earth's north pole at the reference epoch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;You can see that x,y are indeed a circle, but z is just +/- a tiny fraction of an au - basically the vertical component of the halo orbit. That's confirmed by the 6-month periodicity.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;So how can I get true ICRF coordinates instead of these coordinates flattened to the particular plane the earth moves in at epoch&lt;/em&gt;?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Horizons web interface setup:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/E6KEh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/E6KEh.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hp1Fu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hp1Fu.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LCnEp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LCnEp.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-05-30T02:35:46.410" Title="Need help getting the true ICRF coordinates of SOHO using Horizons" Tags="|orbital-mechanics|artificial-satellite|skyfield|python|soho|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15431" PostTypeId="2" ParentId="15420" CreationDate="2016-05-27T06:56:28.160" Score="1" Body="&lt;p&gt;The fastest road cars in the world accelerate at around 1 G, and acceleration drops off rapidly as speed increases. Rockets reach accelerations of 3-5 G just before burnout of the first stage. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-05-27T06:56:28.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15434" PostTypeId="2" ParentId="15424" CreationDate="2016-05-27T20:15:26.243" Score="28" Body="&lt;p&gt;According to &lt;a href=&quot;http://er.jsc.nasa.gov/seh/pg13.htm&quot; rel=&quot;noreferrer&quot;&gt;NASA's Space Educator's Handbook&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As the men in Apollo 13 experienced what no men had undergone before, millions followed the developing drama by radio and television in public squares, private homes, schools, offices and factories. Pope Paul, at an audience in St. Peter's Basilica for 10,000 Romans and tourists, said &amp;quot;We cannot forget at this moment the lot of the astronauts of Apollo 13. We hope that at least their lives can be saved.&amp;quot; Prolonged applause followed. Prayers were said at Jerusalem's Wailing Wall and on the floor of Chicago's Board of Trade.&lt;/p&gt;&#xA;&lt;p&gt;On Tuesday, April 14, the U.S. Senate adopted a resolution which urged all businesses and communications media to pause at 9 p.m., their local time, to &amp;quot;permit persons to join in prayer for the safety of the astronauts.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Offers of assistance with ships to aid in the recovery came from many nations. The Associated Press quoted the Russian news agency Tass as saying that four Soviet ships were steaming toward the splashdown area, one of them the Chumikan, a missile tracker equipped with a helicopter. Tass said the Chumikan and fishing trawler No. 8452 were ordered to join the cargo carriers Academician Rykachev and Novopolotsk converging on the Pacific target area.&lt;/p&gt;&#xA;&lt;p&gt;Premier Aleksei N. Kosygin sent a message saying: &amp;quot;I want to inform you (U.S. Government) the Soviet Government has given orders to all citizens and members of the armed forces to use all necessary means to render assistance in the rescue of the American (Apollo 13) astronauts.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;And what about the radio transmissions? A contemporary &lt;a href=&quot;https://news.google.com/newspapers?nid=1955&amp;amp;dat=19700417&amp;amp;id=pR8rAAAAIBAJ&amp;amp;sjid=CqAFAAAAIBAJ&amp;amp;pg=5270,4050234&amp;amp;hl=en&quot; rel=&quot;noreferrer&quot;&gt;newspaper article in the Pennsylvania &lt;em&gt;Reading Eagle&lt;/em&gt;&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Soviet government ordered all Soviet radio transmitters using frequencies close to those of Apollo 13 to maintain silence from the spacecraft's entry into the earth's atmosphere to its splashdown in the Pacific today.&lt;/p&gt;&#xA;&lt;p&gt;An official announcement said the precaution was being taken in response to the &amp;quot;request of the U.S.A. administration and bearing in mind the exceptional importance of safeguarding stable, continuous radio communications with the Apollo 13 spacecraft.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;The government also said a whaling expedition of several Soviet ships in the Pacific had been alerted to aid in the recovery of the astronauts if needed. The Soviets announced earlier that four other ships were heading for the splashdown target area to render any assistance needed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-05-27T20:15:26.243" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15435" PostTypeId="2" ParentId="15428" CreationDate="2016-05-27T21:12:59.397" Score="6" Body="&lt;p&gt;&lt;strong&gt;Question 1 :&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The three modulation schemes you mention are all forms of digital modulation. As you allude to, information can be encode in the amplitude, frequency, or phase of a signal (frequency is the time derivative of phase). For &lt;em&gt;digital&lt;/em&gt; communication in general: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ASK is very inefficient and susceptible to interference for wireless (but can be useful over fiber optics or copper). &lt;/li&gt;&#xA;&lt;li&gt;FSK is less susceptible to errors and interference than ASK because interference is often confined to a specific frequency.&lt;/li&gt;&#xA;&lt;li&gt;PSK is the least susceptible to error of these three. In terms of bit error rate (BER), binary PSK is about twice as good as ASK and FSK in the presence of noise, and the BER can be significantly improved through other techniques.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;QPSK has the very significant advantage that it can be used to double the data rate over the same bandwidth, or half the bandwidth for the same data rate. Further, as the number of signal states increases from 4 (quadrature) to N-ary, the trade-off can be furthered. As N increases the complexity of the receiver increases. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question 2:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will want to use a combination of FEC &lt;em&gt;and&lt;/em&gt; re-transmission. FEC, or at the least error detection, should be used to know if a packet of data has been received correctly. Re-transmission WILL be required because it is inevitable that data is lost at some point, so the system should be able to request a packet be resent if it is incorrectly received.&lt;/p&gt;&#xA;" OwnerUserId="12966" LastEditorUserId="1142" LastEditDate="2016-05-28T02:19:19.933" LastActivityDate="2016-05-28T02:19:19.933" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15436" PostTypeId="2" ParentId="15430" CreationDate="2016-05-28T07:18:24.730" Score="4" Body="&lt;p&gt;Change the &quot;Table Settings&quot;. You want to select&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;reference plane: Earth mean equator and equinox of reference epoch&#xA;reference system: ICRF/J2000.0&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-05-28T07:18:24.730" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15437" PostTypeId="2" ParentId="8779" CreationDate="2016-05-28T12:05:22.010" Score="2" Body="&lt;p&gt;During the Thaicom launch they definitely announced AOS. But that doesn't necessarily mean there is two way comm between the booster and the ship. It could simply mean that the drone ship is receiving rocket telemetry for relay to Hawthorne. This makes sense because the booster is over the horizon from Florida and the receivers there will lose signal as the rocket descends.&lt;/p&gt;&#xA;" OwnerUserId="14560" LastActivityDate="2016-05-28T12:05:22.010" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15440" PostTypeId="1" AcceptedAnswerId="15444" CreationDate="2016-05-28T16:36:03.217" Score="0" ViewCount="306" Body="&lt;p&gt;What formula could be used to describe the minimums required for an unprotected human body to burn up completely in reentry to Earth's gravity well?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a hypothetical, a burial of the deceased in orbit, who's last wish was to be cremated in such a way, say from the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second hypothetical; an Earth company that wanted to offer reentry cremation for the deceased.  What would be the minimum requirements (the least expensive) to fulfill the contract?&lt;/p&gt;&#xA;" OwnerUserId="10198" LastActivityDate="2019-11-10T01:15:13.320" Title="Burial in space with reentry cremation" Tags="|reentry|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15441" PostTypeId="2" ParentId="15390" CreationDate="2016-05-28T16:50:49.383" Score="5" Body="&lt;p&gt;For interplanetary probes, the drivers are probably economic as much as physical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even for a tiny probe, your mission is going to start with an orbital launcher, likely costing upwards of \$100 million for the launch. It doesn't make sense to send less than several million dollars worth of scientific instruments in that case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your suggestion of sending a large number of small probes to a single destination has a big drawback; all the control, guidance, and communication systems must be replicated on each one. Ten microprobes each carrying 3kg of instruments is going to wind up heavier than one miniprobe carrying 30kg of instruments. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;New Horizons&lt;/a&gt; is a good design example to study. 400kg dry, 30kg of scientific payload. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-05-28T16:50:49.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15443" PostTypeId="2" ParentId="15422" CreationDate="2016-05-28T16:59:27.570" Score="6" Body="&lt;p&gt;SpaceX's Falcon 9 uses &lt;em&gt;grid fins&lt;/em&gt; to help stabilize the rocket while returning back to land.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Grid fins perform very well at subsonic and supersonic speeds, but poorly at transonic speeds; the flow causes a normal shockwave to form within the lattice, causing much of the airflow to pass completely around the fin instead of through it and generating significant wave drag. At high Mach numbers, grid fins flow fully supersonic and can provide lower drag and greater maneuverability than planar fins.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rnHbB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rnHbB.jpg&quot; alt=&quot;Falcon 9 grid fins&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0mfCN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0mfCN.jpg&quot; alt=&quot;Falcon 9 grid fins&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14563" LastActivityDate="2016-05-28T16:59:27.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15444" PostTypeId="2" ParentId="15440" CreationDate="2016-05-28T17:04:46.267" Score="5" Body="&lt;p&gt;Build a 20-ton orbital corpse deployment pod carrying 75 bodies, able to eject them one at a time into a reentry trajectory. Each body is in an individual shroud, constructed and weighted in such a way to optimize incineration (e.g. streamlined to avoid decelerating too early in the reentry). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch it on a Falcon 9. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you charge US\$2 million per body you bring in \$150M revenue per mission. Launch costs \$62M, deployment pod costs \$3M. Assume that permits, lawsuits, mission operations, recording the deployments and distributing videos to the families, etc. eat another \$10M, and you're left with \$75M profit.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-05-28T17:04:46.267" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15446" PostTypeId="1" AcceptedAnswerId="15447" CreationDate="2016-05-28T19:28:24.567" Score="3" ViewCount="1039" Body="&lt;p&gt;A tiny model rocket can't get up very high. Massive rockets can make it to GEO.&lt;br&gt;&#xA;There must be some theoretical mass threshold where the rocket is &quot;big enough&quot; to make it to GEO - what is that mass?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put some parameters around the question, assume:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;no atmosphere (simple physics, no air resistance, like launching from moon)&lt;/li&gt;&#xA;&lt;li&gt;negligible payload mass&lt;/li&gt;&#xA;&lt;li&gt;some solid rocket boosters (with currently available propellant and well designed nozzle) and liquid propellant main engine (like shuttle but without most/all of the vehicle)&lt;/li&gt;&#xA;&lt;li&gt;stationary launch (no rail guns, cannons, trebuchets, slingshots, etc)&lt;/li&gt;&#xA;&lt;li&gt;&quot;near&quot; the equator, say &lt;code&gt;±30°&lt;/code&gt; latitude&lt;/li&gt;&#xA;&lt;li&gt;rocket motor is the only source of lifting thrust&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10584" LastEditorUserId="10584" LastEditDate="2016-05-29T04:43:37.843" LastActivityDate="2016-05-30T22:42:53.847" Title="What is the minimum rocket mass that can achieve GEO" Tags="|rockets|geostationary|mass|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15447" PostTypeId="2" ParentId="15446" CreationDate="2016-05-28T22:07:25.003" Score="7" Body="&lt;p&gt;It depends entirely on what assumptions you make about the fixed mass portions of the launcher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The needed velocity increment, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;nofollow noreferrer&quot;&gt;delta-v&lt;/a&gt;, to GEO, depending on where you launch from, is about 13800 m/s (atmospheric drag makes only about a 1% difference to this value; the exact acceleration profile of the launcher makes a much larger difference). This is the only &quot;theoretical threshold&quot; figure involved; everything else is a practical limit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket equation&lt;/a&gt; relates delta-v to rocket exhaust velocity (or &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific impulse&lt;/a&gt;) and the ratio of fueled mass to empty mass. To maximize that ratio, and thus delta-v, minimize the empty mass of each stage -- this is structural weight, weight of empty fuel tanks, weight of guidance and control systems, and weight of the rocket engines. These are engineering problems, not theoretical ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With off-the-shelf engines using hypergolic propellants, it seems feasible to make a rocket of around 3 tons capable of reaching GEO with no payload*. If you had unlimited budget to design all-new, ultra-small engines specifically for this purpose, it might be possible to get a lot smaller. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At very small sizes, as discussed &lt;a href=&quot;https://space.stackexchange.com/questions/13729/can-a-miniature-saturn-v-get-to-the-moon-and-back&quot;&gt;in another question&lt;/a&gt;, the effect of atmospheric drag eventually comes to dominate, so there's a fairly hard practical minimum limit somewhere.  &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note: I am not a real rocket scientist, so take my estimates with a big grain of salt.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;* I based this estimate on a back-of-the-envelope design using an &lt;a href=&quot;http://www.rocket.com/propulsion-systems/bipropellant-rockets&quot; rel=&quot;nofollow noreferrer&quot;&gt;R4D&lt;/a&gt; thruster as the upper stage engine with a few smaller ones for steering; I figured anything smaller was impractical because you'd need some fairly robust guidance and control hardware on it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust of the R4D determined the overall mass limit of the upper stage. I assumed 5% tank mass to fuel mass ratio (which might not be appropriate for 55kg of pressurized MMH/NTO). The mass ratio and engine specific impulse gives the ∆v contribution of the upper stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I then iterated that process for two more stages, choosing bigger engines for each -- Aerojet Rocketdyne R40 on the middle stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had assumed that pressure-fed engines would be the only way to go for a very small rocket, turbopump mass being prohibitive, but I ran across the interesting &lt;a href=&quot;https://rocketlabusa.com/about-us/propulsion/rutherford/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rutherford&lt;/a&gt; from Rocket Lab -- a tiny kerosene engine that uses battery power to drive a pump, giving decent specific impulse, which might work for the bottom stage. I had to guess at the engine mass, though, as Rocket Lab doesn't publish that figure. If it's not a feasible choice due to mass, I'd choose something like a pair of &lt;a href=&quot;https://en.wikipedia.org/wiki/Aestus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aestus&lt;/a&gt; engines (pressure-fed, hypergolic-fueled) with their nozzles shortened for sea level optimization. In any case, it took three stages totaling almost 3 tons to reach the required ∆v. I've got a spreadsheet I use for this kind of napkin-sketch design, so it came out like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jz6ir.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jz6ir.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-05-30T22:42:53.847" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15448" PostTypeId="2" ParentId="15340" CreationDate="2016-05-29T01:08:38.180" Score="0" Body="&lt;p&gt;This video had been posted recently by &lt;a href=&quot;https://www.youtube.com/channel/UCYKfAzPEXMQsGtNfBCNa_BA&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Marshall Center&lt;/a&gt;, I'm posting this just as supplementary information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/zWphd6HAaHA?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few screenshots:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2ro8Y.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2ro8Y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gZOhS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gZOhS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wLKhk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wLKhk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-05-29T01:08:38.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15449" PostTypeId="2" ParentId="15427" CreationDate="2016-05-29T05:12:19.860" Score="7" Body="&lt;p&gt;The white line is known as a &quot;saturation trail&quot; and is definitely a CCD artifact. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each pixel can store only a certain amount of electrons (of the order of 100,000). If a pixel is illuminated by a bright star and/or if the exposure time is long enough, that pixel will fill up, and the electrons will start to fill the neighboring pixels: the CCD is saturated. When the image is read, all the extra electrons will be spread over the column containing the saturated pixels, making a saturation trail&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;https://www.eso.org/~ohainaut/ccd/CCD_artifacts.html&quot; rel=&quot;noreferrer&quot;&gt;Source: Oliver Hainaut at the European Southern Observatory&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="14567" LastEditorUserId="6816" LastEditDate="2016-05-29T05:25:05.293" LastActivityDate="2016-05-29T05:25:05.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15450" PostTypeId="2" ParentId="15422" CreationDate="2016-05-29T05:27:33.757" Score="2" Body="&lt;p&gt;The V2 definitely couldn't gimbal and used the fins and control vanes in the rocket exhaust for stability and little spoilers on the wingtips for guidance too&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2XSPG.jpg&quot; alt=&quot;V2 rocket in &quot; title=&quot;tooltip&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I took this photo in the imperial war museum in London&lt;/p&gt;&#xA;" OwnerUserId="14567" LastEditorUserId="14567" LastEditDate="2016-05-31T03:39:17.217" LastActivityDate="2016-05-31T03:39:17.217" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15451" PostTypeId="1" CreationDate="2016-05-29T09:26:01.067" Score="1" ViewCount="50" Body="&lt;p&gt;An example is the &lt;a href=&quot;http://www.isi.edu/centers/serc/home&quot; rel=&quot;nofollow&quot;&gt;Space Engineering Research Center&lt;/a&gt; at University of Southern California Viterbi School of Engineering. What other colleges in the US are contributing to research on these topics?&lt;/p&gt;&#xA;" OwnerUserId="14568" LastEditorUserId="1142" LastEditDate="2016-05-29T12:09:03.330" LastActivityDate="2016-05-29T12:09:03.330" Title="Which colleges in the US are researching Astrionics/ Space communication systems?" Tags="|communication|research|avionics|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15452" PostTypeId="2" ParentId="15422" CreationDate="2016-05-29T14:28:54.437" Score="3" Body="&lt;p&gt;Ariane 1-4 had fins. Ariane 4 is an odd case, having fins on some versions and no fins on others: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/d35yL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d35yL.jpg&quot; alt=&quot;Ariane 4 with fins on the 40, 42L and 44L variants, no fins on the 42P and 44LP variants&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-05-29T14:28:54.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15453" PostTypeId="1" AcceptedAnswerId="15454" CreationDate="2016-05-29T14:50:01.710" Score="1" ViewCount="670" Body="&lt;p&gt;How was software developed during apollo?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From various sources, it can be gleaned that the computer was a ibm system 360, and the language was basically a fortran dialect or fortran.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I'd like to know, is how did they do it? Did they just write away or plan? Were there reviews? What did test procedures look like back then? What editor was used? Was there a version control system in place? How was change management handled?&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="415" LastEditDate="2016-05-29T14:52:14.323" LastActivityDate="2016-05-29T16:27:06.883" Title="Software development for Apollo" Tags="|apollo-program|software|flight-computer|" AnswerCount="1" CommentCount="9" ClosedDate="2016-05-29T18:36:22.543" ContentLicense="CC BY-SA 3.0" />
  <row Id="15454" PostTypeId="2" ParentId="15453" CreationDate="2016-05-29T16:27:06.883" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/computers/Ch2-6.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's history&lt;/a&gt; site gives a lot of the answers to your questions:&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;What I'd like to know, is how did they do it? Did they just write away or plan? Were there reviews?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In the Apollo program, as well as other space programs with multiple missions, system software and some subordinate computer programs are only written once, with some modifications to help integrate new software. However, each mission generates new operational requirements for software, necessitating a design that allows for change. Since 1968, when designers first used the term software engineering, consciousness of a software life cycle that includes an extended operational maintenance period has been an integral part of proper software development.&lt;/p&gt;&#xA;&lt;p&gt;Even during the early 1960s, the cycle of requirements definition, design, coding, testing, and maintenance was followed ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;What did test procedures look like back then? Was there a version control system in place? How was change management handled?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Three boards contributed directly to the control of the Apollo software and hardware development. The Apollo Spacecraft Configuration Control Board monitored and evaluated changes requested in the design and construction of the spacecraft itself, including the guidance and control system, of which the computer was a part. The Procedures Change Control Board, chaired by Chief Astronaut Donald K. Slayton, inspected items that would affect the design of the user interfaces. Most important was the Software Configuration Control Board, established in 1967 in response to continuing problems and chaired for a long period by Christopher Kraft. It controlled the modifications made to the on-board software. All changes in the existing specification had to be routed through this board for resolution. NASA's Stan Mann commented that MIT &amp;quot;could not change a single bit without permission&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;NASA also developed a specific set of review points that paralleled the software development cycle. ...&lt;/p&gt;&#xA;&lt;p&gt;Designers developed the programs using a Honeywell 1800 computer and later an IBM 36O, but never with the actual flight hardware. The development computers generated binary object code and a listing. The tape [44] containing the object code would be tested and eventually released for core rope manufacture. The listing served as documentation of the code.&lt;/p&gt;&#xA;&lt;p&gt;For Apollo, MIT developed a special higher order language that translated programs into a series of subroutine linkages, which were interpreted at execution time. This was slower than a comparable assembly language program, but the language required less storage to do the same job. The average instruction required two machine cycle-about 24 milliseconds-to execute.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="11962" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-05-29T16:27:06.883" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15456" PostTypeId="1" AcceptedAnswerId="15461" CreationDate="2016-05-29T20:09:40.763" Score="12" ViewCount="4301" Body="&lt;p&gt;When watching rocket launches, the commentary or status reports often mention that the vehicle is &lt;em&gt;&quot;passing through Max-Q&quot;&lt;/em&gt; - how long does the &quot;passing&quot; take? Is the event instantaneous or is there some time interval while the &lt;em&gt;dynamic pressure&lt;/em&gt; stays maximal?&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2018-12-30T06:38:29.110" Title="How long does Max-Q last?" Tags="|launch|aerodynamics|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15457" PostTypeId="1" CreationDate="2016-05-29T20:36:30.987" Score="2" ViewCount="127" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=yn3FWb-vQQ4&quot; rel=&quot;nofollow&quot;&gt;Morphex&lt;/a&gt; is a hexapod robot which can walk, transform into a ball, roll in a controlled manner... To see it on another world like Mars several issues need to be solved. First it needs a power source, probably RTG, electronics that are radiation hardened, it needs to be scaled up to accommodate all the scientific instruments and the power source. What other obstacles need to overcome in order to see Morphex exploring other worlds?&lt;/p&gt;&#xA;" OwnerUserId="6179" LastEditorUserId="6179" LastEditDate="2016-05-29T20:46:46.853" LastActivityDate="2016-05-29T22:05:16.073" Title="What problems exploration robot needs to overcome?" Tags="|robotic-missions|robotics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15458" PostTypeId="2" ParentId="15457" CreationDate="2016-05-29T22:05:16.073" Score="2" Body="&lt;p&gt;Thermal control, especially for the actuators. Communication. Vision. Elimination of captured/stuck foreign objects.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-05-29T22:05:16.073" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15460" PostTypeId="2" ParentId="15456" CreationDate="2016-05-29T22:27:43.910" Score="14" Body="&lt;p&gt;As the other answers say, the actual peak of dynamic pressure is an instaneous point, but actions taken around this time are often described in terms of Max Q.  For example, Space Shuttles would throttle down their main engines for 30 seconds or so while Q peaked.  (The SRB propellant grain was also shaped to lower thrust during this time, but that was pretty invisible.)  Typical throttle command times for this &quot;thrust bucket&quot; were:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Throttle down from 104.5% to 74% --&gt; 32.8 seconds Mission Elapsed Time&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Throttle up from 74% to 104.5% ----&gt; 54.4 seconds Mission Elapsed Time&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Numbers are from STS-118, but are representative.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-05-29T22:27:43.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15461" PostTypeId="2" ParentId="15456" CreationDate="2016-05-29T23:14:33.860" Score="20" Body="&lt;p&gt;&quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Max_Q&quot; rel=&quot;noreferrer&quot;&gt;Maximum Q&lt;/a&gt;&quot; is technically instantaneous, but less formally it can refer to a period of time where pressures are nearly at maximum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a graph of dynamic pressure for a &lt;a href=&quot;http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;noreferrer&quot;&gt;simulation of the Saturn V&lt;/a&gt;;  (gone missing; &lt;a href=&quot;https://web.archive.org/web/20170313142729/http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;noreferrer&quot;&gt;archive.org link&lt;/a&gt;) most launchers will have an essentially similar curve. You can see that it peaks close to 34k&lt;a href=&quot;https://en.wikipedia.org/wiki/Pascal_(unit)&quot; rel=&quot;noreferrer&quot;&gt;Pa&lt;/a&gt;, but it's above 95% of that value for a period of at least 10 seconds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/A11MissRules.pdf&quot; rel=&quot;noreferrer&quot;&gt;mission abort criteria for the Saturn-Apollo flights&lt;/a&gt; use the term &quot;max Q region&quot; to refer to the period of flight time from 50 seconds to 2 minutes elapsed -- this span of time includes dynamic pressures only 1/4 that of the actual &quot;max Q&quot;!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RWzJt.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RWzJt.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-09-16T02:51:25.743" LastActivityDate="2018-09-16T02:51:25.743" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="15462" PostTypeId="1" AcceptedAnswerId="30654" CreationDate="2016-05-30T02:35:26.110" Score="7" ViewCount="3952" Body="&lt;p&gt;I found the python package &lt;a href=&quot;https://pypi.python.org/pypi/HorizonJPL/&quot; rel=&quot;noreferrer&quot;&gt;HorizonJPL&lt;/a&gt; in the &lt;a href=&quot;https://pypi.python.org/pypi&quot; rel=&quot;noreferrer&quot;&gt;Python Package Index&lt;/a&gt;, but it looks like it's limited to activity in 2013. When I go to the linked documentation page &lt;a href=&quot;https://docs.google.com/document/d/1g9q3ln9LVAATOZ15986HLTCaqcAj_Jd8e_jOGS3YWrE/pub&quot; rel=&quot;noreferrer&quot;&gt;https://docs.google.com/document/d/1g9q3ln9LVAATOZ15986HLTCaqcAj_Jd8e_jOGS3YWrE/pub&lt;/a&gt; the info is fairly sparse, and the two links there lead to a dead end in Japanese (below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a way I can access &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; for tables of data (Like I am &lt;a href=&quot;https://space.stackexchange.com/q/15430/12102&quot;&gt;doing here&lt;/a&gt;) from within a Python script? In other words, I want to build a url query and receive a json of text with state vectors in return using &lt;code&gt;urllib&lt;/code&gt; or &lt;code&gt;urllib2&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt; This is a typical example of what I want to do:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XfcWc.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XfcWc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This is what is displayed on the pypi site - there seems to be names/handles but I don't know how to query them.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;h2&gt;API Documentation&lt;/h2&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://docs.google.com/document/d/1g9q3ln9LVAATOZ15986HLTCaqcAj_Jd8e_jOGS3YWrE/pub&quot; rel=&quot;noreferrer&quot;&gt;https://docs.google.com/document/d/1g9q3ln9LVAATOZ15986HLTCaqcAj_Jd8e_jOGS3YWrE/pub&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;h2&gt;Resources&lt;/h2&gt;&#xA;  &#xA;  &lt;p&gt;Planetary Data System: &lt;a href=&quot;http://pds.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;http://pds.nasa.gov/&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Jet Propulsion Labs: &lt;a href=&quot;http://www.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;http://www.jpl.nasa.gov/&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;HORIZON User Manual: &lt;a href=&quot;http://ssd.jpl.nasa.gov/?horizons_doc&quot; rel=&quot;noreferrer&quot;&gt;http://ssd.jpl.nasa.gov/?horizons_doc&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Contributors&lt;/p&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;Matthew Mihok (@mattmattmatt)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Dexter Jagula (@djagula)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Siddarth Kalra (@SiddarthKalra)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Tiago Moreira (@Kamots)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Here is what happens when I try the links in the google doc&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Gih89.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Gih89.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4eS0w.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4eS0w.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2018-10-30T13:28:15.573" Title="Python API for JPL Horizons?" Tags="|orbital-mechanics|artificial-satellite|celestial-mechanics|python|jpl-horizons|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15463" PostTypeId="1" AcceptedAnswerId="18433" CreationDate="2016-05-30T03:19:26.923" Score="13" ViewCount="2889" Body="&lt;p&gt;With a million pounds of thrust and many g of acceleration and a significant fraction of a g of vibration, connecting the engine to the rocket can't be trivial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At launch the mass is very high and maybe inertia keeps the low frequency vibrations down, but as propellants run out, maybe that becomes less effective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the thrust from the engine transmitted through the nozzle directly to the rocket frame? Is there any attempt to dampen the vibration?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just found this video after watching the one in the &lt;a href=&quot;https://space.stackexchange.com/a/15485/12102&quot;&gt;answer by @jlansey&lt;/a&gt; below. You can see the engine move vertically but the test frame doesn't. At least in this test set-up, &lt;em&gt;there are shock-abosrbers&lt;/em&gt; somewhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/UEhHRXDFkeU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In those old-style gasoline-burning automobiles of the late 19th, 20th and early 21st century, motors had &lt;em&gt;motor-mounts&lt;/em&gt; - rubberized bushings or brackets or something similar that allowed the motor some degree of motion/vibration. Do rocket motors on crewed vehicles have something analogous? I'm guessing that vibration limitations for crewed launches are tighter than for non-crewed missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found &lt;a href=&quot;https://space.stackexchange.com/a/1632/12102&quot;&gt;this answer&lt;/a&gt; about vibration isolation of SRB's to be potentially used as crewed launch vehicles interesting and related, but it's focus is certain characteristic vibration modes of SRBs that are nearly finished and basically hollow tubes.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2018-11-04T19:53:37.937" Title="Do rocket engines have shock absorbers?" Tags="|rockets|propulsion|engine-design|vibration|" AnswerCount="4" CommentCount="14" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15464" PostTypeId="1" AcceptedAnswerId="15467" CreationDate="2016-05-30T13:35:59.640" Score="23" ViewCount="2678" Body="&lt;p&gt;As can be seen in close-up footage of SSME ignition, such as &lt;a href=&quot;http://www.youtube.com/watch?v=NjZ9ly7c08s&quot; rel=&quot;nofollow noreferrer&quot;&gt;the video below&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/2qUCRbVm5xg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;there are small jets of an unknown gas/vapor emanating from specific points around the rim of each engine bell. I've been unable to dig up a schematic detailed enough to figure this out. What are these jets?&lt;/p&gt;&#xA;" OwnerUserId="1419" LastEditorUserId="6944" LastEditDate="2020-06-16T20:07:26.293" LastActivityDate="2020-06-16T20:07:26.293" Title="What are the small gas jets at the rims of the Space Shuttle Main Engines?" Tags="|launch|space-shuttle|engines|ssme|identify-this-object|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="15465" PostTypeId="2" ParentId="7751" CreationDate="2016-05-30T14:46:35.743" Score="3" Body="&lt;p&gt;Flames are complicated and interesting. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They can emit light from atomic (and molecular) excited states recombining (&quot;blue part&quot; of flame), and, especially if there are particulates from incomplete combustion, they can glow white, yellow, or red from something similar to blackbody radiation if the incomplete combustion products agglomerate to make particulates (soot).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the flame is in air, those can continue to combust and stay hot and glow. In a vacuum, the un-combusted material cools quickly and there is no more thermal glow. That's why the LOX/Kerosene engines make bright flames near sea level, but you don't see much of anything once they are above most of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;BUT&lt;/em&gt;: &lt;strong&gt;Bright is not the same as Opaque!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Transparency for a wide band of visible light (our eyes and our cameras) is not really reduced much by the atomic and molecular emission! It may be so bright that it's hard to notice the transmitted light, but that's not the same as &lt;em&gt;opacity&lt;/em&gt;. Materials or regions of gas can be glowing AND nearly transparent at the same time! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Think about bioluminescent algae in water, or red-hot glass during glass blowing, or even Cherenkov radiation in water!  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, here is a sequence of screenshots from the recent SpaceX launch of Thaicom-8 on &lt;a href=&quot;https://www.youtube.com/watch?v=zBYC4f79iXc&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube&lt;/a&gt;. That's probably something between smoke and soot. LOX/Kerosene engines usually get the most thrust by running rich - more Fuel than the LOX can oxidize. That leaves a lot of carbon and hydrocarbons to absorb and scatter light. While these are not opaque - you can still see through them - they are genuinely darker due to light absorption and scattering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0XZSn.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0XZSn.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/zBYC4f79iXc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mCWBQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mCWBQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ztrX3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ztrX3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZnUig.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZnUig.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/W32vD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/W32vD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-05-30T15:06:14.580" LastActivityDate="2016-05-30T15:06:14.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15467" PostTypeId="2" ParentId="15464" CreationDate="2016-05-30T15:45:46.443" Score="28" Body="&lt;p&gt;The lines that exited at the end of the nozzle were drain lines carrying leakage from seals, output of hydraulic actuator drain lines, etc.  The following schematic shows the various systems attached to these drain lines.  Source: &lt;a href=&quot;https://archive.org/download/PocketDataRIRD87142SpaceShuttleMainEngine/Pocket_Data_RI-RD87-142_Space_Shuttle_Main_Engine.pdf&quot; rel=&quot;noreferrer&quot;&gt;Rockwell SSME Pocket Data Book, R/RD87-142&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uHOyt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uHOyt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This graphic differentiates between the transfer ducts (which carried the hydrogen which cooled the nozzle) and the drain lines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oL3Ct.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oL3Ct.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, this graphic shows a &quot;looking down the length of the nozzle&quot; view with all the drain lines annotated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GmVQc.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GmVQc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: this answer represents a 1989 version of the SSME and may not be totally accurate for the versions flying at the end of the program; the important take-away is that what you see is only leakage, drainage, purge gas, etc, and not part of the propellant loops.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-11-04T19:52:57.190" LastActivityDate="2018-11-04T19:52:57.190" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="15469" PostTypeId="1" AcceptedAnswerId="15475" CreationDate="2016-05-30T15:57:18.917" Score="2" ViewCount="1005" Body="&lt;p&gt;How much delta v does it take to get to the Sun-Earth Lagrange 3 point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also,&#xA;Would higher delta-v allow a craft to get there much quicker?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would two hohmann transfer orbits be the most efficient path from Earth to L3?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much delta-v per year would be needed for station keeping?&lt;/p&gt;&#xA;" OwnerUserId="14584" LastActivityDate="2016-05-30T21:51:28.373" Title="How much delta v does it take to get to the Sun-Earth Lagrange 3 point?" Tags="|orbital-mechanics|orbital-maneuver|lagrangian-points|delta-v|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15470" PostTypeId="1" CreationDate="2016-05-30T16:18:23.683" Score="1" ViewCount="570" Body="&lt;p&gt;After partaking in a GTO delivery, the successful re-entry and landing of a Falcon 9 booster was described by Elon Musk &lt;a href=&quot;https://mobile.twitter.com/elonmusk/status/731984739012251648&quot; rel=&quot;nofollow&quot;&gt;as follows&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Most recent rocket took max damage, due to v high entry velocity. Will&#xA;  be our life leader for ground tests to confirm others are good.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, I would expect the center core of a Falcon 9 Heavy configuration to have even higher velocity at re-entry, and therefore sustain even more damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I mistaken, or did Mr. Musk speak exclusively of Falcon 9 &lt;em&gt;non&lt;/em&gt;-Heavy?&lt;/p&gt;&#xA;" OwnerUserId="14585" LastEditorUserId="51" LastEditDate="2016-05-30T16:55:10.123" LastActivityDate="2016-05-30T17:06:25.203" Title="Will F9H's middle core receive more than max damage?" Tags="|spacex|falcon-9|reentry|reuse|falcon-heavy|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15471" PostTypeId="2" ParentId="15470" CreationDate="2016-05-30T16:28:19.057" Score="2" Body="&lt;p&gt;The core will have higher velocity at stage separation, but it should have more fuel left (compared to a F9 core for GTO launch) to use for boostback and re-entry burns to slow down to more survivable velocities.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-05-30T16:28:19.057" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15472" PostTypeId="2" ParentId="15470" CreationDate="2016-05-30T17:06:25.203" Score="2" Body="&lt;p&gt;The interesting thing about the Falcon Heavy is that it has a range of possible modes to launch in. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For maximum payload, if you needed it, and would pay for it, it could expend all three cores to get maximum payload to orbit. Each step along the way to complete recovery eats up margin, in terms of fuel/oxidizer required for recovery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal for SpaceX is to be able to recover all three stages on each launch to minimize costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lowest payload/performance missions will try and land all three cores back in Florida. (Whether LZ-1 gets expanded (per the plans, 4 smaller pads around a middle larger one, only middle larger one appears to be built yet, or if they build more LZ sites.).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Somewhere in between is recover the two side boosters back in Florida, and then land the middle core on an ASDS downrange.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said all that, the middle core will always be going higher and faster than the side cores, else it is not doing a very good job as an extra stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it is not per se the final speed/height of the stage before recovery that causes the extra damage/heating, rather it is how much fuel/oxidizer is set aside to slow it down as it transits back through the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus it all depends on the mission, the mass of the payload, the orbit being targeted and how that mission plans out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then all remaining fuel can be dedicated to recovery. With enough fuel for recovery it can fly itself back to Florida. With less fuel available, it slows itself down enough for an ASDS landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are three (at least) possible burns that a Falcon core uses during recovery.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First is to kill forward velocity and head back to base. This is minimized on ASDS missions as the ASDS is placed far enough downrange so much of this is not needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second is the reentry burn, designed to slow the stage down, as it hits the thick parts of the atmosphere to minimize friction heating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third finally is the landing burn. This is where they let the atmosphere slow the stage down a do their famous hover slam to land. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each of these burns can be modified, but even more so, by the number of engines.  Typically the first burn is a 3 engine burn, with the next two being a single engine burn.  But on the Thaicomm-8 launch, which was heavy, but not quite as heavy as SES-9 and not quite as performant an orbit, they apparently used a 1-3-1 engine burn pattern on landing. By using 3 engines they use less fuel, by not suffering gravity losses. (Decelerate faster but closer to target).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have a lot of room to play around with variables on each mission. Finding all the edge cases that each variable has is going to be a ton of fun to watch. (SES-9 being a great example, as the hole it left in OCISLY was pretty awesome. Even more awesome was how fast they fixed it and caught the next landing stage, a matter of weeks).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SES-9 was a very heavy payload (About heaviest reusable F-9 can handle) and a very high performance orbit. In order to deliver the second stage to an orbit that it could make the target from, it needed to be going faster and higher than usual. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it came time to land, they ran out of fuel 3 seconds left on the landing burn. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-30T17:06:25.203" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15475" PostTypeId="2" ParentId="15469" CreationDate="2016-05-30T21:51:28.373" Score="4" Body="&lt;p&gt;For a transfer using a minimum of propellant, the required $\Delta v$ is very close to Earth's escape velocity. At only a very small $v_{inf}$, you can still have a solar orbit with an orbital period slightly less than a year, making you slowly crawl ahead of Earth, eventually reaching L3, where only a close to zero $\Delta v$ burn is needed to halt. This route however is very time consuming.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are willing to spend more $\Delta v$, the transfer can indeed be made quicker. For non-brachistocrone transfers, the fastest route is to enter an orbit with a period of 1/2 year and a periapse of 0.26 AU, entering L3 after only one revolution. This requires a $v_{inf}$ of $10.65 km/s$, that must be paid in full at L3, and also costing a $\Delta v$ of a mere $4.26km/s$ on top of Earth escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Total cost&lt;/strong&gt;: $14.9 km/s$ + escape velocity&lt;br&gt;&#xA;&lt;strong&gt;Transfer time&lt;/strong&gt;: 1/2 year&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A little less costly is entering an orbit with a period of one and a half year, reaching an apoapsis of 1.62 AU. That increases the transfer time three times, but reduces the $v_{inf}$ to $3.34km/s$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Total cost&lt;/strong&gt;: $3.8 km/s$ + escape velocity&lt;br&gt;&#xA;&lt;strong&gt;Transfer time&lt;/strong&gt;: 1 + 1/2 year&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Add more revolutions to taste, trading a $\Delta v$ reduction for a longer transfer time. For instance, two revolutions in a 1.25 year period orbit further reduces the $v_{vinf}$ to $2.0km/s$, and the total $\Delta v$ to $2.18km/s$ + escape velocity, at a transfer time of 2 + 1/2 year.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-05-30T21:51:28.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15477" PostTypeId="1" CreationDate="2016-05-30T22:17:31.887" Score="4" ViewCount="269" Body="&lt;p&gt;According to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Station_Freedom#Dual-keel_design_.281986.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;space station Freedom wikipedia page&lt;/a&gt; and &lt;a href=&quot;https://www.reddit.com/r/askscience/comments/1wncfp/why_does_the_iss_point_the_same_direction/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this reddit thread&lt;/a&gt;, a dual keel structure would have been more stable than the final ISS design. Can someone explain why?  And if a gravity gradient is the answer, then why is the ISS truss not oriented along the nadir/zenith axis?&lt;/p&gt;&#xA;" OwnerUserId="13978" LastActivityDate="2016-05-30T22:17:31.887" Title="Why would the dual keel space station design have been more stable?" Tags="|iss|gravity|attitude|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15478" PostTypeId="1" AcceptedAnswerId="15483" CreationDate="2016-05-30T23:00:05.547" Score="4" ViewCount="3173" Body="&lt;p&gt;Can we not escape the expansion of space to get outside the local group? I was doing some math, and the closest Group to us besides the Local Group is the M81 Group which is 11.4 Million light years away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Math: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space expands at roughly &lt;a href=&quot;http://phys.org/news/2015-02-fast-universe.html&quot; rel=&quot;nofollow&quot;&gt;68 km/s/Mpc&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Local Group is &lt;a href=&quot;http://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters&quot; rel=&quot;nofollow&quot;&gt;11.4 Million light years&lt;/a&gt; away from the M81 Group&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;11.4 Million Lightyears = 3.5 Mpc &#xA;&#xA;68 km/s/3.5&#xA;&#xA;3.5*68 km/s = 238 km/s&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The fastest space probe is the New Horizons probe which is currently flying at &lt;a href=&quot;http://io9.gizmodo.com/5786083/what-are-the-fastest-spacecrafts-ever-built&quot; rel=&quot;nofollow&quot;&gt;15.73 km/s&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it true that it is impossible to escape from the Local group? Please correct any math that is wrong.&lt;/p&gt;&#xA;" OwnerUserId="14588" LastEditorUserId="4660" LastEditDate="2016-05-31T01:23:18.873" LastActivityDate="2019-10-16T08:28:40.790" Title="Can we not get outside the Local Galactic group?" Tags="|probe|interstellar-travel|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15479" PostTypeId="1" CreationDate="2016-05-30T23:30:11.390" Score="0" ViewCount="38" Body="&lt;p&gt;So far, what is the furthest space exploration machines have been able to reach in the universe? Thanks.&lt;/p&gt;&#xA;" OwnerUserId="14589" LastActivityDate="2016-05-30T23:30:11.390" Title="Most far away location reached thus far" Tags="|spacecraft|space-shuttle|deep-space|" AnswerCount="0" CommentCount="2" ClosedDate="2016-05-30T23:42:17.613" ContentLicense="CC BY-SA 3.0" />
  <row Id="15481" PostTypeId="1" AcceptedAnswerId="15482" CreationDate="2016-05-30T15:29:39.667" Score="8" ViewCount="4968" Body="&lt;p&gt;&lt;strong&gt;&lt;em&gt;Luna 1&lt;/em&gt;&lt;/strong&gt; was the first attempt by the Soviet Union to crash land into the surface of the moon but narrowly missed after a programming error becoming &lt;strong&gt;the first man-made object to escape Earth's gravitational well.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is &lt;em&gt;Luna 1&lt;/em&gt; still operational? If so, where is it? Are there any sightings?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="19097" OwnerDisplayName="The Code Man" LastActivityDate="2019-12-19T15:54:20.310" Title="Where is Luna 1?" Tags="|orbit|the-moon|gravity|artificial-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15482" PostTypeId="2" ParentId="15481" CreationDate="2016-05-30T18:05:14.677" Score="10" Body="&lt;p&gt;It currently is orbiting the sun, on an orbit that takes 450 days. It is an orbit that takes it close to Earth's orbit, and so is not stable in the long term. It may eventually collide with the Earth, but perhaps only after many thousands of years. It is not functioning (it doesn't have any way of generating power) and is pretty small, much to small to be picked up in Earth telescopes. We don't know exactly where it is now.&lt;/p&gt;&#xA;" OwnerUserId="11966" OwnerDisplayName="James Kilfiger" LastActivityDate="2016-05-30T18:05:14.677" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15483" PostTypeId="2" ParentId="15478" CreationDate="2016-05-31T01:38:24.670" Score="11" Body="&lt;p&gt;Your math is fine, but the speed of the fastest probe we have now is no indication of what is possible. 238 km/s is less than a thousandth of the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Laser- or maser-powered &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight&quot; rel=&quot;noreferrer&quot;&gt;light sails&lt;/a&gt; have been modeled as attaining perhaps 30% of the speed of light on interstellar missions. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Breakthrough_Starshot&quot; rel=&quot;noreferrer&quot;&gt;Breakthrough Starshot&lt;/a&gt; project is hoping to use this technology to &lt;a href=&quot;http://www.nytimes.com/2016/04/13/science/alpha-centauri-breakthrough-starshot-yuri-milner-stephen-hawking.html?_r=0&quot; rel=&quot;noreferrer&quot;&gt;get to Alpha Centauri&lt;/a&gt; sometime later in this century. Many technologies would need to be developed to make this possible, but the physics of it are well understood and sails propelled by sunlight have already been flown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An earlier project, &lt;a href=&quot;http://large.stanford.edu/courses/2012/ph241/klein2/docs/19890007533_1989007533.pdf&quot; rel=&quot;noreferrer&quot;&gt;Project Longshot&lt;/a&gt;, looked at doing the same thing using pulsed fusion propulsion. Controlled fusion would need to be developed in order for this to work, so it was more speculative, but again it relied on physical phenomena we know a lot about, though we can't reproduce them yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are several other options for such things. The issue is that it would take tremendous infrastructure and technology development to launch them. But we have good ideas as to how it might be done. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, if it is going to take 30 million years for your craft to get outside the Local Group, you have other things to worry about besides how fast it can go. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-05-31T14:33:20.207" LastActivityDate="2016-05-31T14:33:20.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15484" PostTypeId="1" AcceptedAnswerId="29827" CreationDate="2016-05-31T04:11:36.007" Score="8" ViewCount="450" Body="&lt;p&gt;Aluminum/Magnesium alloys can burn quite nicely. The stoichiometry is favoriable. Instead of $CO_2$ which is mostly oxidizer by mass, you might end up with $Al_2O_3$ and $MgO$ as exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/13222/12102&quot;&gt;Using lower stage as reaction mass&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/13227/12102&quot;&gt;answer&lt;/a&gt; got me thinking - what if &lt;em&gt;reaction mass&lt;/em&gt; referred to a &lt;em&gt;chemical reaction&lt;/em&gt; and not just a &lt;em&gt;kinematic reaction&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However I can't imagine any possible way to actually cannibalize the structural material of a spent stage and then burn it. One problem is that the metal oxides of the exhaust are basically &quot;ceramics&quot; and even at very high temperatures they might agglomerate into nanoparticles, basically &quot;glass soot&quot;. They'd be hot, but there may be little gas or volume and therefore little pressure and therefore little thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has this every been considered &amp;amp; written about in any semi-scientific way?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2019-03-13T01:52:28.130" Title="Burn 1st stage structural material as fuel?" Tags="|propulsion|advanced-propulsion|solid-fuel|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15485" PostTypeId="2" ParentId="15463" CreationDate="2016-05-31T04:19:21.150" Score="4" Body="&lt;p&gt;The process of starting a rocket engine is highly controlled and choreographed. It doesn't start up from 0 to several g's in an instant. Usually it takes a second or two. The burning itself is also very optimized to stop any instabilities from building up – so minimizing the vibration. Here is a video from the Space Shuttle where they also start each of the 3 engines at a slightly different time (don't know if this is common.&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/urxrOI6-RlE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&#xA;Another final reason they probably don't have shocks is efficiency, there is no way to absorb the force without a loss of efficiency usually as heat. Same reason high-performance bicycles don't have shocks.&lt;/p&gt;&#xA;" OwnerUserId="14567" LastEditorUserId="14567" LastEditDate="2016-05-31T21:19:48.297" LastActivityDate="2016-05-31T21:19:48.297" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15486" PostTypeId="1" CreationDate="2016-05-31T07:59:45.580" Score="50" ViewCount="12378" Body="&lt;p&gt;I just saw this video of supposed &amp;quot;&lt;a href=&quot;https://www.youtube.com/watch?v=WLE-ocDoXrs&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Rocket Camera Landing Footage Fakery&lt;/a&gt;&amp;quot; with over a million views.&lt;/p&gt;&#xA;&lt;p&gt;The narrator repeatedly says it's complete garbage because he doesn't understand and would like to know:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Now, you got a rocket at the bottom, how does this pencil, basically a flying pencil, regain itself and land straight down like this? ...&lt;/p&gt;&#xA;&lt;p&gt;[How does it] go from that position, I understand the rocket [at the bottom] but what is the mechanism to straighten it out after being basically parallel, and coming straight down? Someone wanna explain that?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="14595" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-19T13:25:27.030" Title="How does the SpaceX Falcon 9 first-stage straighten for landing?" Tags="|spacex|falcon-9|landing|first-stage|droneship|" AnswerCount="2" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="15487" PostTypeId="1" AcceptedAnswerId="15492" CreationDate="2016-05-31T08:35:06.373" Score="6" ViewCount="618" Body="&lt;p&gt;Every now and then you can see crewmembers carrying paper manuals or notes around. Is this all pre-printed on earth / handwritten or is there actually a printer on the ISS?&lt;/p&gt;&#xA;" OwnerUserId="3200" LastEditorUserId="14609" LastEditDate="2016-05-31T21:59:19.577" LastActivityDate="2016-05-31T21:59:19.577" Title="Printer on board of the ISS?" Tags="|iss|equipment|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15488" PostTypeId="2" ParentId="15486" CreationDate="2016-05-31T08:53:24.683" Score="92" Body="&lt;p&gt;Ironically, the answer is in his own (or rather SpaceX's) video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/T9ruZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/T9ruZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt; Still from 0:49 of the video showing cold gas thruster firing&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first stage of the Falcon 9 uses a set of nitrogen &lt;a href=&quot;https://en.wikipedia.org/wiki/Cold_gas_thruster&quot; rel=&quot;noreferrer&quot;&gt;cold gas thrusters&lt;/a&gt; to perform its flip after separation, and you can see them repeatedly firing in the video. As the compressed gas leaves the thruster its pressure drops very quickly, it condenses, and shows up as white puffs as the thrusters fire.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrusters are mounted on the interstage at the top of the first stage to provide the highest torque to orient the stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AMiHX.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AMiHX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;Thruster group highlighted with red circle &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton.27s_third_law&quot; rel=&quot;noreferrer&quot;&gt;Newton's Third Law&lt;/a&gt; is one of those physical laws that is so blatantly obvious that everyone knows its result, but many people can't name it. If you've ever used a CO&lt;sub&gt;2&lt;/sub&gt; fire extinguisher, you're familiar with the kick as the contents are released, this force can even &lt;a href=&quot;https://www.youtube.com/watch?v=3k_TagfAJFY&quot; rel=&quot;noreferrer&quot;&gt;propel a tricycle&lt;/a&gt; or office chair. It is exactly that force from releasing compressed gas that allows the thrusters to operate and flip the stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=vdDFxr_1M8o&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Gmjko.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; OSHA violation &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first stage has an apogee of around 100km (depending on flight profile). At that altitude there is essentially no atmosphere (0.00003% of ground level ambient pressure), so aerodynamic control surfaces are completely useless for quickly flipping the stage.  However, after the entry burn (about 0:53 in the video) the stage rapidly encounters enough atmosphere to start using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Grid_fin&quot; rel=&quot;noreferrer&quot;&gt;grid fins&lt;/a&gt; for control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/E72MG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/E72MG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Grid fins highlighted with red circles&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The grid fins operate like any other aerodynamic control surface; they push air one direction, and good ol' Newton provides a force the other direction. For a practical demonstration get hold of a children's swing tennis set.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ButnE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ButnE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt; Future rocket scientists &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Often the rackets are a good 20-30mm thick, so if you angle one as you swing you feel a force as the racket moves through the air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, as the stage performs the landing burn (about 1:00 in the video) we add in the main engines. The stage now becomes essentially an inverted pendulum supported by the firing engine/s. The inverted pendulum is a control problem that humanity has &lt;a href=&quot;https://www.youtube.com/watch?v=cyN-CRNrb3E&quot; rel=&quot;noreferrer&quot;&gt;Very&lt;/a&gt;. &lt;a href=&quot;https://www.youtube.com/watch?v=15DIidigArA&quot; rel=&quot;noreferrer&quot;&gt;Much&lt;/a&gt;. &lt;a href=&quot;https://scholar.google.co.uk/scholar?hl=en&amp;amp;q=inverted%20pendulum&amp;amp;btnG=&amp;amp;as_sdt=1%2C5&amp;amp;as_sdtp=&quot; rel=&quot;noreferrer&quot;&gt;Solved&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(see also &lt;a href=&quot;https://space.stackexchange.com/questions/12093/&quot;&gt;How is a rocket stabilized during the initial, slow speed, portion of launch?&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GPNpV.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GPNpV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt; Stooping Falcon &lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the landing burn all three systems are in use, the engine/s are gimbaling, grid fins rotating, and thrusters firing (not visible in this video, but very obvious in the &lt;a href=&quot;https://www.youtube.com/watch?v=BhMSzC1crr0&quot; rel=&quot;noreferrer&quot;&gt;CRS-6 landing video&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-06-01T17:59:20.450" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="15489" PostTypeId="1" AcceptedAnswerId="15490" CreationDate="2016-05-31T08:53:47.430" Score="1" ViewCount="124" Body="&lt;p&gt;There are some calendars for upcoming launches, like the &lt;a href=&quot;https://calendar.google.com/calendar/embed?src=space.reddit@gmail.com&amp;amp;mode=AGENDA&amp;amp;pli=1&quot; rel=&quot;nofollow&quot;&gt;Reddit one&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there some corresponding calendar for arrivals and other planned events of all ongoing deep space missions? So that events like Juno arrival into Jupiter orbit in 35 days don't come as a surprise to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(since links these resources go outdated after a while, it would be nice to paste the current schedule for as far as it reaches... probably a good 15 years since today would be covered that way.)&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-05-31T09:01:14.000" Title="Calendar of deep space arrivals?" Tags="|probe|deep-space|research|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15490" PostTypeId="2" ParentId="15489" CreationDate="2016-05-31T09:01:14.000" Score="2" Body="&lt;p&gt;The best I've seen is Emily Lakdawalla's blog at the Planetary Society. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;She has a &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2016/05030906-whats-up-may-2016.html&quot; rel=&quot;nofollow&quot;&gt;monthly article&lt;/a&gt; called 'What's up in the solar system' which gives a summary of all deep space missions. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-05-31T09:01:14.000" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15491" PostTypeId="2" ParentId="15486" CreationDate="2016-05-31T09:14:00.367" Score="30" Body="&lt;p&gt;It's actually a combination of several systems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Nitrogen gas thrusters for attitude control in vacuum (one can see them firing in the video)&lt;/li&gt;&#xA;&lt;li&gt;4 grid fins (two of them visible in the video) for attitude control during the atmospheric descent &lt;/li&gt;&#xA;&lt;li&gt;Gimbaled engines for attitude control during powered flight &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot;&gt;Falcon 9 User's Guide&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2016-05-31T09:14:00.367" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15492" PostTypeId="2" ParentId="15487" CreationDate="2016-05-31T10:15:48.083" Score="5" Body="&lt;p&gt;Yes, according to &lt;a href=&quot;https://www.quora.com/Is-there-a-laser-or-an-inkjet-printer-on-the-International-Space-Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robert Frost&lt;/a&gt; , NASA Instructor and Flight Controller, the ISS has a Epson Stylus Color 800 mounted on the aft wall of Destiny.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ge8wL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ge8wL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Epson Stylus Color 800 from 1996&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The printer is also visible in &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/International_Space_Station/Highlights/International_Space_Station_panoramic_tour&quot; rel=&quot;nofollow noreferrer&quot;&gt;the International Space Station Panoramic Tour&lt;/a&gt; provided by ESA.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ZjgG8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZjgG8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2016-05-31T13:25:15.723" LastActivityDate="2016-05-31T13:25:15.723" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15494" PostTypeId="1" AcceptedAnswerId="15497" CreationDate="2016-05-31T13:24:01.467" Score="8" ViewCount="1998" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/RD_Amross&quot;&gt;Wikipedia:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Under RD AMROSS, Pratt &amp;amp; Whitney is licensed to produce the RD-180 in the United States. Originally, production of the RD-180 in the US was scheduled to begin in 2008, but this did not happen. According to a 2005 GAO Assessment of Selected Major Weapon Programs, Pratt &amp;amp; Whitney planned to start building the engine in the United States with a first military launch by 2012.[3] This, too, did not happen. In 2014, the Defense Department estimated that it would require approximately $1 billion and five years to begin US domestic manufacture of the RD-180 engine.[4]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From my understanding, Prat &amp;amp; Whitney have technical plans and documentation, why they still can't manage to produce engines independently?&lt;/p&gt;&#xA;" OwnerUserId="6179" LastEditorUserId="6179" LastEditDate="2016-05-31T13:29:56.557" LastActivityDate="2016-05-31T13:52:04.653" Title="Why can't Pratt &amp; Whitney make RD-180 engines on their own?" Tags="|engines|rd-180|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15495" PostTypeId="2" ParentId="15484" CreationDate="2016-05-31T13:32:50.910" Score="4" Body="&lt;p&gt;The main problem is mechanical in nature. Getting the structural into a form that can be burned and getting that material funneled through a nozzle while burning. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any thrust that can't be directed ranges from useless to harmful to a spaceship. Since when you burn the structural material, you are currently burning up what could direct the gas flow, you have a slight problem at your hands...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than that, Aluminum and Magnesium ARE used already as fuel. Solid booster rockets usually use Ammonimum perchlorate composite propellant. One portion of this composite is a high energy fuel like Aluminum, Magnesium or Zinc &lt;a href=&quot;https://en.wikipedia.org/wiki/Ammonium_perchlorate_composite_propellant&quot; rel=&quot;nofollow&quot;&gt;see Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12555" LastActivityDate="2016-05-31T13:32:50.910" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15496" PostTypeId="2" ParentId="17" CreationDate="2016-05-31T13:40:06.033" Score="6" Body="&lt;p&gt;Stored Chemical Energy Power Systems (SCEPS) is one of the possible alternatives. From &lt;a href=&quot;https://www.nasa.gov/feature/sceps-in-space-non-radioisotope-power-systems-for-sunless-solar-system-exploration-missions&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Stored Chemical Energy Power Systems (SCEPS) have been used in U.S. Navy torpedoes for decades. This high-energy-density, high-power technology can be reliably stored for years. In Phase I we analyzed the applicability of SCEPS to in situ solar system exploration, looking to see if it could be adapted to power a lander sent to a target with no usable sunlight as an energy source. We developed a candidate mission to the surface of Venus, showing that SCEPS could be used for powering spacecraft and landers. The team compared it to conventional battery and Plutonium powered systems, both of which have deficiencies that are overcome by SCEPS. Our concept holds the promise of a power solution that could far exceed the operational capacity of existing batteries, allowing exciting exploration to continue despite the lack of available Plutonium. We propose to continue the research into applying SCEPS to exploration missions that can't be powered by sunlight. In this study we will mature the Venus mission studied in Phase I. We will also expand our understanding of the usefulness of SCEPS to exploration of moons, comets, asteroids and other targets where sunlight is not sufficient to power the mission. We will engage with the leaders in science planning for small bodies, outer planets, and robotic missions to our own Moon and make a determination of the first, most high-impact use of SCEPS in space. An experiment will be performed to determine SCEPS performance when using CO2 as an oxidizer, approximating the in situ resource utilization of the Venusian atmosphere. Venus science goals will be revisited to prepare the Venus concept for the next level of study. Two key risks stand out. The first is our ability to scale down the power from current SCEPS implementation to levels more in family with spacecraft. Landed systems on Mars, for example, have had power levels on the order of hundreds of watts, far less than the many thousands of kilowatts that SCEPS provides for a U.S. Navy torpedo. The work proposed here would lead to better understanding of SCEPS operations at power levels appropriate to space exploration. The second risk is combustion with in situ resources. In the case of the ALIVE mission, the atmospheric CO2 is proposed as the oxidizer. The analysis performed in Phase I indicates that the reaction would give of the necessary heat to power the lander. The use of in situ resources has its benefits: in the case of the ALIVE mission it reduces the mass of consumables that would otherwise have to be included on launch day by hundreds of kilograms. In Phase II we seek experimental confirmation that this reaction can be initiated and sustained at the power levels required for such a lander. We see an opportunity to expand our understanding of the impact that SCEPS could have on solar system exploration. The sunless environment of Venus may indeed be explored through the use of SCEPS, but many cold, sunless regions may also benefit. Sending a SCEPS system to power a lander on the surface of Europa or the lakes or dunes of Titan may return substantial science that would be otherwise left unknown, or at least greatly delayed as the community works to solve the Plutonium-availability problem. We will develop a multi-variable model for SCEPS function and performance using advanced trade space visualization and exploration tools and techniques. The trade space will include the information gleaned from the stakeholders. The trade space tools will allow us to see the intersection of SCEPS capability and mission utility. The collective results of the study will be used to create a roadmap for further maturation of SCEPS for use in space. In Phase II we seek to expand the understanding of how best to target this technology and plan a path for development by developing a roadmap for TRL advancement of SCEPS in space that mirrors NASA’s solar system science goals in this decade.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6179" LastActivityDate="2016-05-31T13:40:06.033" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15497" PostTypeId="2" ParentId="15494" CreationDate="2016-05-31T13:52:04.653" Score="11" Body="&lt;p&gt;In something as complex as a rocket engine, there are specs and docs, and then there are things that only the people on the production line seem to know. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recall an anecdote for the center body of some fighter jet, where they were having a problem separating the product from the mold. They called in an old timer who used to work on the line, and he explained that they had applied a thin coat of ranch dressing before starting as a lubricant.  But it had never been documented, it was just how it was done. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket engines are tricky beasts. The Russians have coatings and materials that are not available in the United States, which is one of the main complaints I have heard. Is it really a US based rocket engine if you buy all the components from Russia?  So how many do you have to make inhouse?  How far down the chain do you need to go?  100% US based?  If so, then you have to start with making tools to make tools to make tools sort of things.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-05-31T13:52:04.653" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="15498" PostTypeId="1" AcceptedAnswerId="15499" CreationDate="2016-05-31T13:59:12.233" Score="15" ViewCount="2931" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/15487/12102&quot;&gt;Printer on board of the ISS?&lt;/a&gt; was &lt;a href=&quot;https://space.stackexchange.com/a/15492/12102&quot;&gt;answered&lt;/a&gt; in the affirmative. This &lt;em&gt;seems&lt;/em&gt; to be a printer with a special paper cassette on &quot;top&quot; to feed the paper, since gravity feed is not going to work. But is it really? Or is that the &quot;receiver&quot; and somebody hid the blank paper cassette which seems to be missing from the bottom?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has this printer been specially built to work in microgravity? If so, by whom? What &lt;em&gt;kind&lt;/em&gt; of printer is it (bubble jet? laser? etc.) Does the ink or toner feed also need special considerations to work in microgravity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; For any piece of equipment to be shipped to the ISS and installed on board, it must go through a significant amount of testing for safety and reliability. So if there is a printer there, then there must be some documented evidence (beyond personal opinion) that it's going to really work reliably - and is not expected to cause problems - in the ISS environment. I think it would be remarkable that an un-modified, off-the-shelf 99 or 49 dollar printer was just put in a launch vehicle, subjected to all sorts of vibrations, then operated in microgravity and reduced pressure in close proximity to humans without any modification. &lt;em&gt;Was it&lt;/em&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screenshot from the &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/International_Space_Station/Highlights/International_Space_Station_panoramic_tour&quot; rel=&quot;noreferrer&quot;&gt;3D interactive walkthrough&lt;/a&gt; from photos by record-holding ESA astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Samantha_Cristoforetti&quot; rel=&quot;noreferrer&quot;&gt;Samantha Cristoforetti&lt;/a&gt; taken in June 2015&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OkWcJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OkWcJ.jpg&quot; alt=&quot;printer on the international space station?&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zoomed and rotated from here (printer on lower-left):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TJAxZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TJAxZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Reblogged&quot; from @ForgeMonkey 's &lt;a href=&quot;https://space.stackexchange.com/a/15492/12102&quot;&gt;answer&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZjgG8.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZjgG8.jpg&quot; alt=&quot;reblogged&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2024-02-06T00:54:21.180" Title="Was this printer shown the ISS in 2015 built specially to work in microgravity?" Tags="|iss|microgravity|equipment|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15499" PostTypeId="2" ParentId="15498" CreationDate="2016-05-31T14:11:24.683" Score="18" Body="&lt;p&gt;It's an Epson Stylus Color 800 - a pretty standard inkjet printer (a laser printer would be a nightmare - all that fine toner dust!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an example of an Earthly one from a 1998 page of the &lt;a href=&quot;http://www.wap.org/journal/epson/epson.html&quot; rel=&quot;noreferrer&quot;&gt;Washington Apple Pi Journal&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1SoTI.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1SoTI.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ink cartridges don't care about gravity - they work by capillary attraction from a storage sponge in the cartridge, and then vaporisation at the print head. You can run print heads at whatever angle you like here on Earth, and they work just fine in microgravity, but some optimisation &lt;strong&gt;could&lt;/strong&gt; have been carried out, as per &lt;a href=&quot;http://taga.rit.edu/2010/nasa.html&quot; rel=&quot;noreferrer&quot;&gt;this paper from RIT.edu&lt;/a&gt;. That optimisation is for drop speed, size and shape, and not essential for operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only challenging bit is the paper feed, as it is usually gravity fed from the top, letting the paper sit in place until the pickup and feed rollers are in use. The roller friction itself is sufficient once the paper is pulled into the feed path, but without gravity this may not work - compare the two pictures to see the additional section which feeds the paper in the absence of gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To answer the actual question:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The printer was not built specifically for space, but there are modifications to ensure it works in space. The obvious one being the paper feed unit, but others are likely, such as optimisation of print heads and strengthening of ribbon cables etc to cope with the vibration of launch.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2016-06-04T13:21:36.347" LastActivityDate="2016-06-04T13:21:36.347" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15501" PostTypeId="1" AcceptedAnswerId="15503" CreationDate="2016-05-31T17:17:12.603" Score="6" ViewCount="3804" Body="&lt;p&gt;How would one go about calculating ISp of a rocket with combined propulsion? E.g. the Space Shuttle during the phase of launch, with both SRBs and SSME active?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course there's the simple &quot;experimental&quot; method, taking wet and dry mass, and speed at the beginning and end of acceleration, but that is not very good if we don't have the complete, working rocket at hand. For the planning stage, we'd know the exhaust speed of different engines, their thrust and mass flow - how then would we go about finding the ISp of the whole?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that various engine contribute a different amount of thrust, how to assign weights to their ISp to calculate the value for the whole craft?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-05-31T19:56:21.687" Title="Averaging Specific Impulse for combined propulsion" Tags="|specific-impulse|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15502" PostTypeId="2" ParentId="15501" CreationDate="2016-05-31T19:01:01.953" Score="3" Body="&lt;p&gt;Well, what can I say - &lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Specific_impulse#Multiple_engines&quot; rel=&quot;nofollow&quot;&gt;the Kerbal Space Program Wiki&lt;/a&gt; has a good answer to this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$g_n I_{sp}=\frac{\sum\limits_{i}F_{T_i}}{\sum_\limits{i}\overset{.}{m}}=\frac{{\sum\limits_{i}}F_{T_i}}{\sum\limits_{i}{\frac{F_{T_i}}{g_n I_{sp_i}}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$I_{sp}$ is the specific impulse in seconds&lt;/li&gt;&#xA;&lt;li&gt;$I_{sp_i}$ is the specific impulse of each engine in seconds&lt;/li&gt;&#xA;&lt;li&gt;$F_{T_i}$ is the thrust of each engine in newtons&lt;/li&gt;&#xA;&lt;li&gt;$\overset{.}{m}$ is the fuel consumption in kilograms per second&lt;/li&gt;&#xA;&lt;li&gt;$g_n$ is the standard acceleration of gravity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When the fuel consumption is not used in this formula, it is only&#xA;  important that all thrust values have the same unit (e.g. kilonewtons)&#xA;  and the specific impulse have all the same unit (e.g. seconds). The&#xA;  result is then in the same unit as the specific impulses of the&#xA;  engines. If all engines have the same specific impulse the resulting&#xA;  specific impulse will be the same.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The result is equivalent to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Harmonic_mean#Weighted_harmonic_mean&quot; rel=&quot;nofollow&quot;&gt;weighted harmonic mean&lt;/a&gt; of the engines'&#xA;  specific impulses, weighted by each engine's thrust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="265" LastEditDate="2016-05-31T19:56:21.687" LastActivityDate="2016-05-31T19:56:21.687" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15503" PostTypeId="2" ParentId="15501" CreationDate="2016-05-31T19:05:34.610" Score="9" Body="&lt;p&gt;$$I_{sp}=\dfrac{I_{sp1}\dot{m}_1+I_{sp2}\dot{m}_2+{...}}{\dot{m}_1+\dot{m}_2+{...}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So each $I_{sp}$ is simply weighted by its fraction of the total mass flow rate. This extends to any number of $I_{sp}$'s.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="58" LastEditDate="2016-05-31T19:28:35.540" LastActivityDate="2016-05-31T19:28:35.540" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15504" PostTypeId="2" ParentId="15498" CreationDate="2016-05-31T21:16:17.727" Score="5" Body="&lt;p&gt;There are no modern printers with gravity fed paper input.  The paper is always &quot;picked&quot; by a roller on a bar that is mechanically pressed against the paper.  Output is the problem. Printers rely on gravity to hold the paper in the output tray.  The ISS printer is modified to enclose the output trays.  The input tray is also enclosed because the roller pressure is intentionally weak on top loaders until the printer performs a paper feed. This allows users to add paper easily.&#xA;Older inkjet cartridges used sponges, newer ones use fiber. The cartridge fiber is saturated with ink when new, there is no ink sloshing around in the cartridge. Canon and HP inkjets use heat to shoot out ink drops, Epsons use piezoelectric squirt guns. &lt;/p&gt;&#xA;" OwnerUserId="14610" LastActivityDate="2016-05-31T21:16:17.727" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15505" PostTypeId="1" CreationDate="2016-06-01T03:19:34.000" Score="0" ViewCount="886" Body="&lt;p&gt;What is the easiest (and cheapest) way to launch a small object into orbit? I know this is not anywhere near a simple task, but I want to put something into orbit for a small amount of money.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&#xA;Holden&lt;/p&gt;&#xA;" OwnerUserId="14614" LastActivityDate="2016-06-01T03:57:56.593" Title="Easiest way to put something into orbit" Tags="|orbit|" AnswerCount="1" CommentCount="1" ClosedDate="2016-06-01T07:30:48.563" ContentLicense="CC BY-SA 3.0" />
  <row Id="15506" PostTypeId="1" CreationDate="2016-06-01T03:28:39.813" Score="0" ViewCount="51" Body="&lt;p&gt;For a Senior Design Project (Which has been completed and submitted) I (and another teammate) made a numerical simulation for a single stage launch vehicle from mars that included a lot of various effects like gravity varying as a function of height, atmospheric drag (with the $C_d $ varying as a function of mach number) and the gravity turn.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We never really figure out how to calculate the orbit at each and every time step which would have been very useful.  We did have the altitude and velocity at every time point.  I was wondering how one would go about calculating the orbital parameters at every point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was thinking that because the launch location was known and the altitude, velocity and down range distance were all calculated at each time step, could I find the position and velocity vectors for each time step and work from the vectors to find the orbital parameters? or would some other method need to be be used?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition what is the radial vector aligned to?  As in if the launch site is Eberswadale Crater (24°S, 33°W) with a altitude of 0 m (just using this as an example altitude), what would the starting radial vector and velocity vector be?&lt;/p&gt;&#xA;" OwnerUserId="14613" LastEditorUserId="14613" LastEditDate="2016-06-01T04:29:27.747" LastActivityDate="2016-06-01T04:29:27.747" Title="Orbit Determination Question for Numerical Launcher Simulation" Tags="|mars|simulation|" AnswerCount="0" CommentCount="5" ClosedDate="2016-06-01T07:31:40.427" ContentLicense="CC BY-SA 3.0" />
  <row Id="15507" PostTypeId="1" AcceptedAnswerId="17893" CreationDate="2016-06-01T03:48:54.160" Score="16" ViewCount="6593" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/15488/12102&quot;&gt;answer&lt;/a&gt; discusses the three methods that the Falcon 9 1st stage uses for attitude control during re-entry and landing. They are: cold nitrogen jets, &quot;hypersonic&quot; grid fins, and thrust vectoring of the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is pointed out there that the grid fins are moving even in the last seconds of landing. I &lt;a href=&quot;https://space.stackexchange.com/questions/15486/how-does-the-spacex-falcon-9-first-stage-straighten-for-landing#comment40762_15491&quot;&gt;suggested&lt;/a&gt; that like the front arms of the Tyrannosaurus Rex - they may move when it moves, but that doesn't mean that they are necessarily doing anything useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following questions and their answers are relevant:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/6174/12102&quot;&gt;What is the function of the hypersonic grid fins on SpaceX's booster?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/13211/12102&quot;&gt;How long does the F9 first stage remain hypersonic under control of its grid fins?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/8817/12102&quot;&gt;Did CRS-6 landing fail because the steering fins are ineffective at low speed?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://twitter.com/elonmusk/status/536258543675252739/photo/1&quot; rel=&quot;noreferrer&quot;&gt;this Tweet&lt;/a&gt; and image, Elon Musk calls them &quot;&lt;strong&gt;hypersonic grid fins&lt;/strong&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3TdA2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3TdA2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is &lt;em&gt;How effective are Falcon 9 1st stage grid fins in the last few seconds before landing&lt;/em&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for something quantitative, and with some kind of supporting information, rather than opinions. Thanks!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speeds are at most a few hundred kph and of course down to zero. My feeling is that since they are actually pretty small and mostly holes, &lt;strong&gt;at these speeds&lt;/strong&gt; they are not significantly effective, but they keep moving anyway because it is simply safer to not have anything in the control code that turns them off, thereby reducing one possible failure mode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also believe that in the last few seconds before landing, where velocity is low and fast attitude correction time is crucial, &lt;em&gt;only thrust vectoring&lt;/em&gt; is useful for attitude control of the Falcon 9 1st stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So &lt;em&gt;if&lt;/em&gt; they are indeed ineffective at the end, bonus points for an explanation why they keep moving! :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a video of an early test of the articulation (no cows were harmed). &lt;strong&gt;At around 01:20 they rotate, and the rocket begins to roll as well.&lt;/strong&gt; Is this cause and effect? Is it the correct direction?:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/DgLBIdVg3EM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screen shots from the video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RuwDC.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RuwDC.png&quot; alt=&quot;Falcon 9 Grid Fins&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P5vLu.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P5vLu.png&quot; alt=&quot;Falcon 9 Grid Fins&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;T. Rex:&lt;/strong&gt; (model thereof) and annotation to help illustrate the question. (from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Feathered_Tyrannosaurus_model.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HTLBt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HTLBt.jpg&quot; alt=&quot;T. Rex&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2023-08-02T13:13:23.250" Title="How effective are Falcon 9 1st stage grid fins in the last few seconds before landing?" Tags="|spacex|falcon-9|landing|supersonic|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="15508" PostTypeId="2" ParentId="15505" CreationDate="2016-06-01T03:57:56.593" Score="1" Body="&lt;p&gt;The Easiest and cheapest way would probably be as a secondary payload on a already scheduled launch as a cubesat or micro sat or pico sat.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also Cheapest here is a very misleading term as most cubesat launches are in the tens and hundred of thousands of dollars.  There are companies that are trying to bring the price down like &lt;a href=&quot;http://www.interorbital.com/&quot; rel=&quot;nofollow&quot;&gt;http://www.interorbital.com/&lt;/a&gt; and &lt;a href=&quot;https://www.rocketlabusa.com/&quot; rel=&quot;nofollow&quot;&gt;https://www.rocketlabusa.com/&lt;/a&gt; but they are still year(s) away from launching orbital missions.&lt;/p&gt;&#xA;" OwnerUserId="14613" LastActivityDate="2016-06-01T03:57:56.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15509" PostTypeId="1" AcceptedAnswerId="15510" CreationDate="2016-06-01T04:55:23.960" Score="2" ViewCount="2110" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt; is derived for a rocket under no external forces. No drag, no gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\Delta v}{v_{exhaust}}=ln\frac{m_{initial}}{m_{final}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While sometimes g appears, that only happens when someone is using specific impulse instead of exhaust velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember seeing &lt;em&gt;the derivation&lt;/em&gt; of a variant of this equation where a constant external force is included. That could be for example gravity near the surface of a body during a vertical take-off (ok ok... at the north pole).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't want to debate the usefulness of this variant, I would just like to see the derivation and possibly a link to its use. I know it was closed form, and required $t_{burn}$ explicitly.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-06-01T05:00:58.900" LastActivityDate="2016-06-01T07:45:43.763" Title="Derivation of a variant of the Tsiolkovsky rocket equation which includes gravity" Tags="|orbital-mechanics|rocket-equation|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15510" PostTypeId="2" ParentId="15509" CreationDate="2016-06-01T07:23:43.280" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;where a &lt;strong&gt;&lt;em&gt;constant&lt;/em&gt;&lt;/strong&gt; external force is included&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the vertical take off case, the external force is not constant, as the gravitational force decreases somewhat with altitude. But as you only require it to be valid near the surface, I am going to assume you actually want the force to be constant&lt;sup&gt;1&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have to sources of change in velocity here.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The rocket engine, given by the Tsiolkovsky equation and&lt;/li&gt;&#xA;&lt;li&gt;The acceleration due to the external acceleration, that is simply acceleration times duration, in this case $g \cdot t_{burn}$&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So, if the direction of the external force is exactly opposite of the direction we thrust, like gravity, it is not more complicated than subtracting:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v=\left|ln\left(\frac{m_{initial}}{m_{final}}\right) \cdot v_{exhaust} -g \cdot t_{burn}\right|$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We need to take the absolute value, as the external force can be stronger than our own rocket's acceleration, but $\Delta v$ is a scalar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, if you have an angle $u$ between the two forces, you can express it as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v=\sqrt{\left(ln\left(\frac{m_{initial}}{m_{final}}\right) \cdot v_{exhaust}+cos(u) \cdot g \cdot t_{burn}\right)^2+(sin(u)\cdot g \cdot t_{burn})^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The derivation should be pretty self explanatory, as this is only adding two vectors by decomposing the $g \cdot t_{brun}$ component, and get the length of the resulting vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I am still representing the acceleration of the outside force as $g$)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not sure how this is going to be useful though, as launch analysis usually requires numerical simulation anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;Err, acceleration really, as the system has a changing mass.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-06-01T07:45:43.763" LastActivityDate="2016-06-01T07:45:43.763" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15511" PostTypeId="2" ParentId="15419" CreationDate="2016-06-01T08:53:36.560" Score="2" Body="&lt;p&gt;Yes, ISRO has ambition for a full fledged manned space program, but is actually awaiting the green light from the government.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISRO's director has clarify their planning on how they would achieve it but in India according to five year planning granted by planning commission they have to wait for year 2017 for the complete clarification of government and granting of budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.indianspacestation.com/iss/35-articles/194-indian-space-flight-are-we-there-yet?showall=1&amp;amp;limitstart=&quot; rel=&quot;nofollow&quot;&gt;http://www.indianspacestation.com/iss/35-articles/194-indian-space-flight-are-we-there-yet?showall=1&amp;amp;limitstart=&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISRO has manned space exploration planning which are executing in parallel. some are in planning some are in development some are in testing phase.&#xA;some of these are&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;CARE&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Crew module Atmospheric Reentry Experiment which tested a capsule type vehicle similar to Soyuz, which is I think &lt;a href=&quot;https://en.wikipedia.org/wiki/ISRO_Orbital_Vehicle&quot; rel=&quot;nofollow&quot;&gt;what will send the first Indian astronauts from Indian soil&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;GSLV MK3&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And GSLV MK3 is the workforce which is going to send the capsule up to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight101.com/spacerockets/gslv-mk-iii/&quot; rel=&quot;nofollow&quot;&gt;http://spaceflight101.com/spacerockets/gslv-mk-iii/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Astronauts&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an astronaut programme is also in planning awaiting the government decision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dnaindia.com/india/report-indian-space-research-organisation-hunt-on-for-vyomanauts-desi-astronauts-1329928&quot; rel=&quot;nofollow&quot;&gt;http://www.dnaindia.com/india/report-indian-space-research-organisation-hunt-on-for-vyomanauts-desi-astronauts-1329928&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Avatar&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is going to be final phase of RLV experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://aermech.in/avatar-spacecraftreusable-launch-vehiclefuture-spaceplanedrdoisroindia/&quot; rel=&quot;nofollow&quot;&gt;http://aermech.in/avatar-spacecraftreusable-launch-vehiclefuture-spaceplanedrdoisroindia/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all ISRO has planned a manned space program which depends on the political will and need of government.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although ISRO's motto is different from other space agencies out there.&lt;/p&gt;&#xA;" OwnerUserId="14616" LastEditorUserId="14616" LastEditDate="2016-06-02T03:01:20.973" LastActivityDate="2016-06-02T03:01:20.973" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15513" PostTypeId="1" CreationDate="2016-06-01T11:07:15.030" Score="3" ViewCount="3136" Body="&lt;p&gt;I mean detailed pics like this: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/mCXSi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mCXSi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's impossible to get this information please try to describe an approximate structure. &#xA;Also provide additional info if you can. Which metal are used? What extra features you know like something for safety? Which details were used as part of it? What is the detailed structure of Dragon?&lt;/p&gt;&#xA;" OwnerUserId="14620" LastEditorUserId="51" LastEditDate="2016-06-01T11:54:34.870" LastActivityDate="2017-11-05T13:11:10.750" Title="What is the structure of Falcon 9 and Dragon?" Tags="|spacex|falcon-9|design|dragon|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15514" PostTypeId="2" ParentId="1059" CreationDate="2016-06-01T16:19:31.140" Score="11" Body="&lt;p&gt;In 2014, the record for the maximum number of satellites to be launched in a single mission was set by the DNEPR rocket from Russia, releasing 37 satellites, 34 of them directly.&#xA;&lt;a href=&quot;https://www.nasaspaceflight.com/2014/06/russian-dnepr-rocket-record-launch-37-satellites/&quot;&gt;Source: NASA News&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14621" LastEditorUserId="6816" LastEditDate="2016-06-01T16:30:54.787" LastActivityDate="2016-06-01T16:30:54.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15515" PostTypeId="2" ParentId="3556" CreationDate="2016-06-01T21:42:18.887" Score="4" Body="&lt;p&gt;I will try to answer your question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Best flying opportunity&lt;/strong&gt;. Sedna is now (May 2016) at 85.7 AU from Sol. It will be at 76 AU in 2076 (in 60 years from now). Then, again, after other 60 years (in 2136), Sedna will be again at 86 AU. So, there really is no hurry to send a mission towards it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At every 12 years, Jupiter can be used for a gravity assist. A second gravity assist can, at current position, be performed by a Neptune flyby. All spacecrafts that are now escaping from Sun's gravity, have used a Jupiter gravity assist. This includes the Pioneer 10, Pioneer 11, Voyager 1, Voyager 2 and New Horizons. All 5 spaceships had to travel (or will have to) about 30 years to get to Sedna's current distance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With current technology, we can send a space probe towards Sedna, similar to New Horizons. We will need to wait for about 30 years to conduct a flyby and see what is there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Propulsion&lt;/strong&gt;. A space probe to Sedna can be very similar to New Horizons. It can be launched with an Atlas V rocket and travel with a similar speed. It might be possible to use a solar sail or an ion engine, but nobody has tried this on such a scale. So, I will only talk about classic, chemical engines, that have been used for previous ships. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sending a probe for a Sedna flyby is probably as difficult as sending New Horizons towards Pluto. If the probe weights the same as New Horizons, the amount of fuel needed will be almost the same. However, if you want to build an orbiter or even a lander, everything is more complicated. Sedna is smaller then Pluto and so it has little gravity. If a spaceship approaches with similar speed to New Horizons (14.5 km/s), there is nothing that can stop it. There are two ways to insert an object on orbit around Sedna: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Make a huge spacecraft, with enough reserves of hydrazine, to slow down. This means that you will have to increase ship's weight too much, which is not feasible. &lt;/li&gt;&#xA;&lt;li&gt;Send the probe with lower speed, so that Sedna's gravity can capture it. Doing so, the voyage will take much longer, probably over 100 years. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If we want to touch Sedna's surface, we should better build an impactor, like the one used by &lt;strong&gt;&lt;em&gt;Deep Impact&lt;/em&gt;&lt;/strong&gt; probe. The impactor will provide us with images from near the surface. The impact will expose materials below the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Technical challenges&lt;/strong&gt;. The most important problem is that the radioisotope thermoelectric generator (RTG) has a limited lifetime. The plutonium isotope used has a half life of 87 years, so it should be enough. However, the thermocouples used in an RTG also have a limited lifetime. The Pioneers have lost contact roughly at the same distance where Sedna is. The Voyagers have survived further away in space, but they hardly can power up their antennas. New Horizons is expected to stop functioning at Sedna's orbit. So, our spacecraft will have to use a much larger and more powerful RTG. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Communications to Earth are far more difficult at high distance. New Horizons needed less time to send data after its Jupiter encounter then it needs now, after Pluto encounter, even if the Jupiter encounter resulted in acquiring far more data. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Conclusion&lt;/strong&gt;. With our current technology, we can build a space probe similar to New Horizons. The probe, with the help of a Jupiter gravity assist, will travel 30 years to Sedna and will conduct a flyby. The RTG must be larger then the ones used for previous probes (probably it will carry 50 kg of plutonium). Data from the encounter will need a few years to be downloaded to Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sending an orbiter or a lander to Sedna is, with current technology, not feasible. &lt;/p&gt;&#xA;" OwnerUserId="14629" LastActivityDate="2016-06-01T21:42:18.887" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15516" PostTypeId="2" ParentId="15404" CreationDate="2016-06-01T21:44:31.413" Score="11" Body="&lt;p&gt;There is a lot that the two missions you mentioned--&lt;em&gt;Stardust&lt;/em&gt; and &lt;em&gt;Hayabusa&lt;/em&gt;--have taught us about sample return.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Sample Contamination: Serious measures were taken with &lt;em&gt;Stardust&lt;/em&gt; to control for the possibility of sample contamination, but notable problems were still encountered:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, despite these precautions the&#xA;  Stardust spacecraft outgassing was sufficient to degrade&#xA;  camera operations, and the aerogel capture media&#xA;  was significantly contaminated during manufacture.&#xA;  We also never completely solved the problem of&#xA;  defining useful limits for organic contaminants of&#xA;  spacecraft hardware, which haunts us as we rather unexpectedly&#xA;  captured primitive cometary organics.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Landing Conditions and Sample Recovery: The landing site for &lt;em&gt;Stardust&lt;/em&gt; in the Utah Test and Training Range was unexpectedly wet and the lack of a canister seal for the samples became an issue.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Sample Curation: The necessary preparations for managing the returned sample take a long time--longer than expected:&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;More than two full years&#xA;  were required to prepare curation facilities for Stardust.&#xA;  Despite this seemingly adequate lead time, major&#xA;  changes to curation procedures were required once the&#xA;  actual state of the returned samples became apparent.&#xA;  Two years of Curation preparation are insufficient.&#xA;  The Stardust sample database was not fully implemented&#xA;  before sample return –we did not adequately&#xA;  think through all of the possible sub-sampling and&#xA;  analytical activities before settling on a database design.&#xA;  Remote storage of a sample subset is critical, for&#xA;  Stardust the remote samples are in a vault in New&#xA;  Mexico.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ol start=&quot;4&quot;&gt;&#xA;&lt;li&gt;Unknown Surface Conditions: &lt;em&gt;Hayabusa&lt;/em&gt; had to be designed without knowing what the surface of its target was like:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We didn't know what Itokawa's surface would be like before the spacecraft actually arrived there. Indeed, the assignment given to us was quite broad. We were to build a device that, no matter what the surface was like, would grab samples, ranging from monolithic bedrocks to powdery regolith, in the brief instant of each of the spacecrfat's [sic] touchdowns.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ol start=&quot;5&quot;&gt;&#xA;&lt;li&gt;Touchdown Time: &lt;em&gt;Hayabusa&lt;/em&gt; had to ascend immediately after touchdown, forcing the retrieval to occur quickly:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Because the sampler horn is attached to the edge of the spacecraft, if the horn were to contact the surface of the asteroid for too long, the spacecraft would start tumbling on its center of mass. In other words, if the spacecraft did not ascend right after the touchdown, it would lose its balance and fall on its rear. Thus samples would have to be collected in less than one second, by firing a bullet at a speed of 300 meters per second.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140001406.pdf&quot;&gt;LESSONS LEARNED FROM THE STARDUST SAMPLE RETURN MISSION - Zolensky, M.E. @ NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://global.jaxa.jp/article/special/hayabusa/yano_e.html&quot;&gt;High Hopes for Successful Sample Collection - Yano, Hajime @ JAXA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-06-01T21:44:31.413" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15517" PostTypeId="2" ParentId="15513" CreationDate="2016-06-01T22:20:30.217" Score="4" Body="&lt;p&gt;The best that I can find available is the one that is released by SpaceX, as a &lt;a href=&quot;http://www.spacex.com/falcon9&quot; rel=&quot;nofollow noreferrer&quot;&gt;somewhat interactive graphic&lt;/a&gt; on the Falcon 9 website. Here's a screen grab: &lt;a href=&quot;https://i.stack.imgur.com/AIg2G.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AIg2G.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;Click on the blue plus signs when you're on the actual SpaceX page for additional details.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is there not a better diagram? Well, for a number of reasons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, SpaceX is known to change its configuration frequently for flight, meaning that the rocket breakdown might be flight specific to some extent. Granted, some things won't be changed on such a regular basis, but still, they are known for design iteration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, there isn't a reason to release that specific of a diagram. The shuttle had its configuration released for a number of reasons, from attracting interest, to making sure all of the parts fit together as specified from a number of different contractors. Tax payers were paying for it, and wanted to know. SpaceX doesn't fit under those parameters, and as a commercial company, it is in their interest to not disclose the entire design to the public, as it could aid their competition. Also, such items must be released according to ITAR regulations, and there is cost associated with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, we know of quite a few more things than are listed. There are thruster nozzles clearly seen. We know there are helium tanks inside. There are two tanks, one for LOX and one for RP1, the RP1 tank is the lower of the two. If you have specific questions, it's likely they can be answered as well.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="1142" LastEditDate="2016-06-05T13:01:18.867" LastActivityDate="2016-06-05T13:01:18.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15518" PostTypeId="1" AcceptedAnswerId="56094" CreationDate="2016-06-02T00:33:15.113" Score="7" ViewCount="850" Body="&lt;p&gt;Is it possible to calculate $B^*$ term in TLE data using only a few or more velocity and position vectors for a satellite?&lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="23934" LastEditDate="2019-02-15T13:43:30.567" LastActivityDate="2021-12-07T18:59:26.727" Title="How to construct $B^*$ drag term in TLE?" Tags="|orbital-mechanics|two-line-elements|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15519" PostTypeId="2" ParentId="15518" CreationDate="2016-06-02T07:22:39.350" Score="6" Body="&lt;p&gt;Dwight E. Andersen claims that its an impossible task in his thesis &quot;Computing Norad Mean Elements From a State Vector&quot; (1994)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An extension of this [calculating other TLE elements than drag and mean motion] would be to compute, from just the state, the mean elements and the drag terms and B* in SGP4, SDP4, SGP8, and SDP8. Future research on satellite drag and methods of estimating the drag would be valuable. Since the drag term is a function of the physical geometry as well as atmospheric conditions, some aspects of the satellites' physical characteristics would be needed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8902" LastActivityDate="2016-06-02T07:22:39.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15520" PostTypeId="2" ParentId="1059" CreationDate="2016-06-02T11:26:20.723" Score="4" Body="&lt;p&gt;Depending on how fine (or creative!) your definitions are, the answer may be &quot;about a hundred, and either 2014 or later this year&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-3&quot; rel=&quot;nofollow&quot;&gt;SpaceX CRS-3 flight&lt;/a&gt; in 2014 carried the main payload, a Dragon resupply flight for the space station, and five cubesats. One of these, KickSat, was a container designed to release &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/k/kicksat&quot; rel=&quot;nofollow&quot;&gt;104 independently orbiting microsatellites&lt;/a&gt;. Each would have a small radio, and is apparently capable of independent ground communication. However, the timer on the satellite failed, and it did not release the microsatellites before re-entering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second attempt is &lt;a href=&quot;http://www.nature.com/news/tiny-chipsat-spacecraft-set-for-first-flight-1.20006&quot; rel=&quot;nofollow&quot;&gt;due to fly in July 2016&lt;/a&gt;; it's not clear exactly how many microsatellites are involved this time around, or how many other secondary payloads are on the launch, but &quot;around a hundred&quot; seems a good estimate. &lt;/p&gt;&#xA;" OwnerUserId="11259" LastEditorUserId="11259" LastEditDate="2016-06-02T11:57:45.153" LastActivityDate="2016-06-02T11:57:45.153" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15521" PostTypeId="1" CreationDate="2016-06-02T13:22:32.687" Score="2" ViewCount="4552" Body="&lt;p&gt;The X-15 has a maximum velocity of 7.274 km/s.  As the velocity to achieve orbit is 8 km/s, why did they not fly horizontal/diagonal until they achieved around  7km/s and then go vertical to get to space and reach orbit?&lt;/p&gt;&#xA;" OwnerUserId="14412" LastEditorUserId="12102" LastEditDate="2017-08-22T12:55:01.797" LastActivityDate="2019-02-14T16:03:36.763" Title="Why did the X-15 not achieve orbit?" Tags="|launch|orbit|air-launch|x-15|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="15522" PostTypeId="2" ParentId="15521" CreationDate="2016-06-02T13:31:03.217" Score="32" Body="&lt;p&gt;You're confusing units. The maximum speed of the X-15 was 7274 km/h, or about 2 km/s. Orbital speed is around 8 km/s. The X-15 didn't carry enough fuel to reach orbital speed. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-02T13:31:03.217" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15523" PostTypeId="2" ParentId="15521" CreationDate="2016-06-02T13:42:22.360" Score="4" Body="&lt;p&gt;The X-15 didn't have the capability to get to orbit, and it was never intended to. the X-15's purpose was not space flight but to test aerodynamic heating at high altitude and high speed. It's skin was designed to tolerate a great deal of heat, but nowhere near what would be required for re-entry from orbital velocity. If it went to orbit it would burn up on return. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-06-02T13:42:22.360" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15524" PostTypeId="1" AcceptedAnswerId="15526" CreationDate="2016-06-02T15:12:45.243" Score="1" ViewCount="3520" Body="&lt;p&gt;I'm trying to use this &lt;a href=&quot;http://www.braeunig.us/space/interpl.htm&quot; rel=&quot;nofollow&quot;&gt;web site&lt;/a&gt; to make a DeltaV calculator but i've got stuck into calculating the velocity vector of the origin planet.&#xA;Particulary i'm refering to the problem 5.6 described &lt;a href=&quot;http://www.braeunig.us/space/problem.htm#5.6&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; where the velocity vector is given as VP = 25876.6X + 13759.5Y m/s.&#xA;The problem is, how do i calculate it? &#xA;I only know when i'm going to launch my spacecraft and from which planet.&#xA;Of that planet i've already calculated the position vector. &#xA;I haven't studied physics so if you can, keep it simple and, if you introduce new values, explain how to calculate them. Thanks!&lt;/p&gt;&#xA;" OwnerUserId="14640" LastActivityDate="2016-06-02T21:16:25.533" Title="How to calculate the velocity vector components of a planet?" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15525" PostTypeId="1" CreationDate="2016-06-02T15:30:32.170" Score="7" ViewCount="213" Body="&lt;p&gt;Where it comes to fuels, &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_energy&quot;&gt;Specific Energy&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Energy_density&quot;&gt;Energy Density&lt;/a&gt; are two major quantities deciding &quot;how good&quot; given fuel is. These are either given neglecting the oxidizer mass, or as a value combined with the oxidizer, as bipropellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the corresponding quantity measuring efficiency of oxidizers? E.g. if the rocket uses the same amount of the same fuel, what measure would tell how its energy output to mass ratio changes when switching between LOX, nitrous oxide or peroxide - each in stochiometric ratio to the fixed amount of fuel? (...or some other method of measurement that yields a similar knowledge).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some table with example values for various oxidizers would be nice too.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-06-02T17:00:12.753" LastActivityDate="2016-06-05T02:16:35.023" Title="Specific Energy counterpart for oxidizers" Tags="|oxidizer|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15526" PostTypeId="2" ParentId="15524" CreationDate="2016-06-02T15:40:00.490" Score="4" Body="&lt;p&gt;If you're going between a planet and a moon of that planet, it's easiest to treat the planet as stationary. For planet to planet (or moon to moon) transfers within a system, if you don't need great accuracy, you can use circular orbit velocity (&lt;a href=&quot;https://en.wikipedia.org/wiki/Circular_orbit#Velocity&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Circular_orbit#Velocity&lt;/a&gt;) as an approximation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Complication increases rapidly beyond that. This paper is the best walk-through I've found for converting from Keplerian orbital elements to Cartesian position and velocity coordinates: &lt;a href=&quot;https://downloads.rene-schwarz.com/download/M001-Keplerian_Orbit_Elements_to_Cartesian_State_Vectors.pdf&quot; rel=&quot;nofollow&quot;&gt;https://downloads.rene-schwarz.com/download/M001-Keplerian_Orbit_Elements_to_Cartesian_State_Vectors.pdf&lt;/a&gt; -- it explains a great deal, but there are still some tricky bits.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-06-02T15:47:39.153" LastActivityDate="2016-06-02T15:47:39.153" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15527" PostTypeId="1" CreationDate="2016-06-02T16:44:10.170" Score="13" ViewCount="241" Body="&lt;p&gt;Most orbital debris eventually re-enters the atmosphere. The vast majority of this burns up on re-entry, and only a small proportion of what's left lands on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of that small proportion, the majority will land in the ocean (there is a lot of ocean!), and a fair chunk of the rest is probably never found, or just never identified. Nonetheless, every year or two something is turned up, and occasionally it's &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#Re-entry&quot;&gt;very dramatic&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When was the &lt;em&gt;first&lt;/em&gt; time an object was found and identified as re-entering debris? I've been able to find a reference to a probable case in 1964 - Cosmos 50, which apparently had debris recovered in Malawi, discussed &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/library/satellitefraghistory/13theditionofbreakupbook.pdf&quot;&gt;on p. 30 here&lt;/a&gt; - but nothing as yet from earlier. There were a number of cases as early as 1960 when debris from sub-orbital second stages were found in &lt;a href=&quot;https://history.state.gov/historicaldocuments/frus1961-63v25/d360&quot;&gt;unusual places&lt;/a&gt;, but that's not quite the same question...&lt;/p&gt;&#xA;" OwnerUserId="11259" LastActivityDate="2016-06-03T00:08:39.320" Title="What was the earliest piece of debris to survive reentry?" Tags="|reentry|history|debris|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15528" PostTypeId="2" ParentId="15521" CreationDate="2016-06-02T18:01:52.977" Score="3" Body="&lt;p&gt;The X-15 was undoubtedly fast enough to reach altitudes at which the pilot could have looked into the blackness of the heavens, but high altitudes are not enough for achieving an orbital trajectory.  I know this doesn't exactly answer your question (and others have already explained the difference in units), but there's also a misunderstanding about achieving orbit.  Let's assume the X-15 did have the juice to make it into orbit.  As it climbed, it wouldn't go vertical; rather, it needs to flatten out and make its velocity more and more parallel with the earth's surface below it.  Going vertical would have actually guaranteed that it wouldn't go into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a very common misconception about things in high orbit.  Satellites, for example, have the appearance of simply floating at a certain distance from the earth, when in actuality, all of them are in free fall.  Yes, you read that right.  They are always falling.  How, then, do they maintain their distance from the earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's use a hypothetical example (with a healthy serving of exaggeration).  Let's say you can jump with extraordinary force, such that you can jump many miles away.  Of course, you can also jump just a few feet, when you like.  If you were to draw the path of a small jump on a large circle, which represents the earth, then you'd have small arc that goes up and then down very quickly.  Let's jump farther -- say, a quarter of the way around the earth -- and draw that arc.  The arc is longer, of course, and now it's becoming more obvious that the shape of the earth (its roundness) is influencing how far you can jump &lt;em&gt;and&lt;/em&gt; how long you're staying in the air.  Truly, you're jumping part way around the earth every time you jump.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's jump really far -- halfway around the earth.  What it will take is a lot more force giving you a lot more speed.  When you draw that arc, it's now very clear that you are, indeed, jumping &lt;em&gt;around&lt;/em&gt; the earth.  And what was the ground doing, below your feet?  The circle of the earth was, in a sense, &quot;moving away&quot; from you, was it not?  When you land, you're actually standing upside down relative to your position when you first jumped.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What will it take to jump so fast that you go all the way around the earth once and land back where you started?  This is something you can actually calculate, although I'll leave that to you.  Looking at your arcs, though, and considering what you know about jumping (that you're always falling), what matters for longer jumps is more speed (assuming you always jump at an ideal angle).  When you jumped halfway around the earth, weren't you still falling the whole time?  You just jumped with enough force so that you could go farther before you landed again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the clincher.  Is it possible to jump so fast that, as you fall, the circle of the earth moves away from your feet as fast as you fall towards it?  You'd never stop falling.  I hope that helps explain why going vertical is what you don't want to do for achieving orbit.  For fun, consider if you were in the space shuttle and you wanted to come back down to earth, what do you have to do?  You can also now explain why, in the movies, they say that going too fast will make you &quot;bounce off the atmosphere.&quot;  It's just a cheap way of saying that the speed will send the craft into an elliptical orbit, but that's not nearly as fun as bouncing.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This isn't a dead-on accurate description, of course.  It would take careful planning to achieve a circular orbit, even if you had the right speed, because you could easily go into an elliptical orbit and even crash back into the earth if your angle was wrong.&lt;/p&gt;&#xA;" OwnerUserId="14641" LastActivityDate="2016-06-02T18:01:52.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15529" PostTypeId="1" CreationDate="2016-06-02T19:38:52.830" Score="2" ViewCount="177" Body="&lt;p&gt;The sheer speed of an object at orbital velocity (8 km/s) is like the kinetic energy of 7.5 kg of TNT for every kg of mass. At the speed of an object falling from near the Moon, it's like 14.5 kg of TNT for every kg of mass. Meteors normally enter the atmosphere at even higher speeds, and are famously destructive if they don't burn up in the atmosphere enough for their energy to be dissipated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of turning that into a weapon &lt;a href=&quot;https://en.wikipedia.org/wiki/Kinetic_bombardment&quot; rel=&quot;nofollow&quot;&gt;has been pondered&lt;/a&gt;, but at the moment is not practical and so plays little role in strategic thinking. The near-term idea was to use tungsten rods dropped from orbit. That would seem to be designed to penetrate hardened defenses, not to do general damage, so a bit specialized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some point operations in space will make it possible to take material being mined from an asteroid or on the Moon and do something like this. It's still pretty far off, but considering the potential destruction, it seems to me it would be good for people to be aware of what could be done and what would be involved to do it. Surely our analysis of the value of space exploration, and how best to do it, needs to include this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you could aim objects to enter at a near-vertical angle you could minimize how much the atmosphere slows them down and burns them away. So maybe the shortest path involves determining how to select appropriate boulders and shape them enough to be missiles. Is this an appropriate short list of necessary qualities?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a conical shape, &lt;/li&gt;&#xA;&lt;li&gt;Solid enough to not break up before being close to the ground, but fractured enough to air burst once it is&lt;/li&gt;&#xA;&lt;li&gt;Perhaps coated with an &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry#Ablative&quot; rel=&quot;nofollow&quot;&gt;ablative material&lt;/a&gt;, also to help it get close to the ground&lt;/li&gt;&#xA;&lt;li&gt;As &lt;a href=&quot;http://meteorites.wustl.edu/id/density.htm&quot; rel=&quot;nofollow&quot;&gt;dense as possible&lt;/a&gt;, so a bit of an iron meteor maybe, or a chunk of ilmenite if you are doing it from the Moon. If you have managed to &lt;a href=&quot;http://www.philipmetzger.com/blog/type-of-asteroid-to-mine-part-3/&quot; rel=&quot;nofollow&quot;&gt;start mining platinum group metals&lt;/a&gt;, they would work wonderfully but you probably wouldn't want to waste them.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And then there is the question of how much stuff would be needed to do big damage, say city-destroying level. The kinetic energy of a ton of material in the vicinity of the Moon is equal to the Hiroshima bomb, but much of that would be lost during atmospheric entry. If the material was prepared as in the list above, about how much of it would result in an explosion like Hiroshima?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notes: &lt;a href=&quot;https://en.wikipedia.org/wiki/TNT_equivalent&quot; rel=&quot;nofollow&quot;&gt;TNT equivalent&lt;/a&gt; considers a ton of TNT to be 4.2 x 10&lt;sup&gt;9&lt;/sup&gt; J of energy, and speed can be considered as kinetic energy with the formula $E_k=0.5mv^2$. The Hiroshima bomb exploded at a height of about 600 m.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-06-02T20:17:12.123" LastActivityDate="2016-06-02T20:17:12.123" Title="Is this an efficient way to turn bulk minerals in space into kinetic weapons?" Tags="|reentry|mining|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15530" PostTypeId="2" ParentId="15529" CreationDate="2016-06-02T19:57:20.830" Score="2" Body="&lt;p&gt;&quot;Solid enough not to break up&quot; is what you need. Elongated shape (possibly spin-stabilized) to minimize the attack surface would be a boon, but not essetial. They are their own ablator, and the solid enough ones will be dense enough too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Breaking up is the demise of great most meteoroids as turning into swarm of rocks and pebbles increases the attack surface dramatically, square-cube law making their area much larger in proportion to mass and allowing heat to penetrate the cross-section either leading to evaporation, breaking up into harmless gravel or slowing down enough to be harmless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides hardening them though, your number one enemy will be delta-V. You need to accelerate them into a collision trajectory, by ejecting enough reaction mass (propellant) - likely amount comparable to the mass of the asteroid, maybe an order or two smaller - but with these things weighing thousands tons, that's still aplenty - and you need to bring that propellant there somehow. Probably by burning some 30x as much or more propellant on launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Economically speaking, you could nuke Earth to oblivion ten times over for the same price.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-06-02T19:57:20.830" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="15531" PostTypeId="1" CreationDate="2016-06-02T20:49:15.997" Score="5" ViewCount="1957" Body="&lt;p&gt;The SpaceX Falcon 9 (F9-0025) mission that launched the Thaicomm-8 satellite to GTO orbit recovered its first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at pictures of it coming into Port Canaveral we see it listing to the side, the Leaning Tower of SpaceX. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is it leaning?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-02T20:58:47.593" Title="Why did the Thaicomm-8 SpaceX First stage come into port leaning to the side?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15532" PostTypeId="2" ParentId="15531" CreationDate="2016-06-02T20:58:47.593" Score="8" Body="&lt;p&gt;Landing looks like it came in hard on one leg, as it corrected at the last moment to get right on target (since the center engines gimbal is the main control mechanism at that point in flight).  You can see this in the SpaceX &lt;a href=&quot;https://www.flickr.com/photos/spacex/26812792104/&quot; rel=&quot;noreferrer&quot;&gt;Flickr feed&lt;/a&gt; like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GTepb.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GTepb.jpg&quot; alt=&quot;Landing hard&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So one leg appears to have taken more load than others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon tweeted that the aluminum honeycomb crush core worked to take the load:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/736328917317910528&quot; rel=&quot;noreferrer&quot;&gt;Tweet about crush core&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/SpaceX/status/738453366649430016&quot; rel=&quot;noreferrer&quot;&gt;Tweet on how Apollo used crush cores in lunar lander &lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears that once you start crushing this way, it does not bounce back, which makes sense. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see this, when OCISLY came into Port Canaveral (Jul 2, 2016), carrying the first stage, that the three visible landing legs are at different levels of extension. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here you can see the leg that is mostly extended, least collapsed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OgesM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OgesM.jpg&quot; alt=&quot;Extended landing leg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here you can see a half collapsed, half extended leg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0a4Cs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0a4Cs.jpg&quot; alt=&quot;Half extended&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is probably the leg that was mostly collpased, as you can below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6LNMN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6LNMN.jpg&quot; alt=&quot;Mostly collapsed&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As they came into port, they used high tech materials to prop the leg up:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0oZHe.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0oZHe.png&quot; alt=&quot;High tech shimmery&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-02T20:58:47.593" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15533" PostTypeId="1" CreationDate="2016-06-02T21:00:50.717" Score="5" ViewCount="635" Body="&lt;p&gt;The SpaceX Falcon 9 mission that launched the Thaicomm-8 satellite to GTO orbit, landed hard on the OCISLY ASDS barge. One of the legs over extended into the aluminum crush core and did not retract. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the stage stay in place while on three legs?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-03T20:14:56.160" Title="Did the Thaicomm-8 SpaceX landed first stage move after landing?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15534" PostTypeId="2" ParentId="15533" CreationDate="2016-06-02T21:06:33.957" Score="7" Body="&lt;p&gt;The landing from the video looked pretty X marks the spot on target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The images on the &lt;a href=&quot;https://www.flickr.com/photos/spacex/26812794034/in/photostream/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX Flickr feed&lt;/a&gt;, it looks like it nailed the X on landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/en2jq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/en2jq.jpg&quot; alt=&quot;Standing Proud on the X&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when it came into port it was pretty clear how far it had moved around on the ASDS barge:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RNi81.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RNi81.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of that is the angle of the image, you can see the right most leg is almost on the outer circle, but the leg facing us may have hit the yellow railing or really close to it.  But you can see that the landing in the first photo was well inside the circle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looks like three legged, peg-legged first stages have a tendency to wander a bit until they get tied down as in this closeup.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WJU19.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WJU19.jpg&quot; alt=&quot;Focus on Engines&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the airplane jackstands they attach to the stage (And these apparently get welded to the deck) and then cables with strain gauges tieing it down even further.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;https://www.flickr.com/photos/mseeley1/27145081830/sizes/h/&quot; rel=&quot;nofollow noreferrer&quot;&gt;picture at Flickr, not allowing downloads&lt;/a&gt;, shows that one of the legs is on top of the barge name writing, showing just how far it really moved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some super high res photos of the stage shows some yellow paint on the leg closest to the rail, which is making people think that although it 'walked back' from the edge, it may have actually hit the railing.  See what you think here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oDgTt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oDgTt.jpg&quot; alt=&quot;Landing Leg with Yellow paint&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-06-03T20:14:56.160" LastActivityDate="2016-06-03T20:14:56.160" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15535" PostTypeId="2" ParentId="15524" CreationDate="2016-06-02T21:16:25.533" Score="0" Body="&lt;p&gt;To get magnitude of velocity vector use the Pythagorean theorem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PiaJ8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PiaJ8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$c=\sqrt{a^2 + b^2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case the speed is 29307.38 meters per second or 29.3 km/s. (sounds like earth).&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-06-02T21:16:25.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15537" PostTypeId="2" ParentId="15527" CreationDate="2016-06-03T00:08:39.320" Score="5" Body="&lt;p&gt;According &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_reentering_space_debris&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, the earliest space debris that survived re-entry, were pieces and components from &lt;a href=&quot;https://en.wikipedia.org/wiki/AS-101_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Apollo SA-6 CSM BP-13 called AS-101&lt;/a&gt; which was a testing spacecraft completing 54 orbits before re-entering in atmosphere and crashing in east of Kanton Island in the Pacific Ocean on June 1, 1964. These space debris are listed as the earliest to survive re-entry in atmosphere but i guess that were not recovered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The re-entry date of Cosmos 50 was later on November 5, 1964 according the source you have found.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastActivityDate="2016-06-03T00:08:39.320" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15539" PostTypeId="1" AcceptedAnswerId="15541" CreationDate="2016-06-03T06:07:07.573" Score="4" ViewCount="344" Body="&lt;p&gt;Many years ago I remember reading about the upcoming implementation of the &lt;a href=&quot;https://en.wikipedia.org/wiki/IPv6_address&quot; rel=&quot;nofollow&quot;&gt;internet protocol IPv6&lt;/a&gt; - which is now in place. In that article it mentioned that there was some discussion about space - I mean &lt;em&gt;outer space&lt;/em&gt;, not namespace. At the time, it seemed that the author was not just speculating, but that they were actually referring to some plans or at least calculations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not asking about private networks, or ESA or NASA networks. I'm asking about the &quot;normal internet&quot; that we all (well, most of us) can use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; This question is about the internet, not internet access. Put in terrestrial terms, I'm not asking about the WiFi vs 4G access to a router, I'm asking about the router's connection to the rest of the routers - &lt;em&gt;the internet&lt;/em&gt;. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-06-03T06:26:19.987" LastActivityDate="2016-06-03T12:58:53.937" Title="Are there discussions or plans for extending the internet into space beyond Earth?" Tags="|communication|internet|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="15540" PostTypeId="1" AcceptedAnswerId="15544" CreationDate="2016-06-03T09:14:47.977" Score="6" ViewCount="408" Body="&lt;p&gt;I remember there was a test of optical communication and some data transfer from lunar orbit to earth, and I think the ISS uploaded (or &lt;em&gt;downloaded-up&lt;/em&gt;? which is correct here?) a movie from earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to have the &lt;a href=&quot;https://space.stackexchange.com/q/15539/12102&quot;&gt;internet on Mars&lt;/a&gt; not end up to be &lt;a href=&quot;https://space.stackexchange.com/questions/15539/are-there-discussions-or-plans-for-extending-the-internet-into-space-beyond-eart?noredirect=1#comment40973_15539&quot;&gt;too expensive&lt;/a&gt; and to extend the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; (and &lt;a href=&quot;http://deepspace.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSNnow here&lt;/a&gt;) capability, optical seems to be a very important technology for the not-so-distant future.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Free space optical communications (no optical fiber) seems to be just made for space. With a wavelength of the order of 1 micron instead of  10 centimeters, transmission antenna can be smaller in diameter for the same gain. Receiving antennae can possibly be smaller - basically &lt;em&gt;something like&lt;/em&gt; the kind of optical telescopes they use now for astrophotography - but that depends on nearby bright sources of light and data rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Were these tests successful? Have there been other tests? What's coming next&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screenshot(s) of &lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network Now&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aknlF.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aknlF.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="58" LastEditDate="2020-02-26T14:02:31.340" LastActivityDate="2020-02-26T14:02:31.340" Title="How is long-distance optical communications coming along in space?" Tags="|deep-space-network|optical-communication|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15541" PostTypeId="2" ParentId="15539" CreationDate="2016-06-03T12:58:53.937" Score="2" Body="&lt;p&gt;As Michael pointed out, there is a protocol specified for &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Internet&quot; rel=&quot;nofollow&quot;&gt;Interplanetary Internet&lt;/a&gt;, which is designed to cope with the much larger (and varying) delays that are incurred over such distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently there is &lt;a href=&quot;http://www.theatlantic.com/technology/archive/2015/06/the-internet-in-space-slow-dial-up-lasers-satellites/395618/&quot; rel=&quot;nofollow&quot;&gt;Internet in space&lt;/a&gt;, and for its end users it works exactly the same way yours does on the ground. But the data stream is then piggy-backed onto the existing comms that are used between the ISS and ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you seem to be asking for is ubiquitous connectivity - to be honest, it is always going to be directional comms links that are used, and any Internet traffic will be piggy-backed over them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is actually what happens on Earth anyway - your OSI model network stack includes a physical layer which can be fibre, or copper, or wireless, etc. For space that bottom layer will be optical or wireless (an EM wave of some kind, anyway) and rather than trying to run enough power for an omnidirectional antenna, planet to planet links would be directional (similar to point to point links here on Earth)&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2016-06-03T12:58:53.937" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15542" PostTypeId="2" ParentId="15540" CreationDate="2016-06-03T13:23:37.880" Score="5" Body="&lt;p&gt;The lunar mission which tested optical communication was &lt;a href=&quot;http://esc.gsfc.nasa.gov/267/271.html&quot;&gt;LADEE&lt;/a&gt;.  The ISS program you're thinking of is &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/opals_data_rates_soar&quot;&gt;OPALS&lt;/a&gt;.  Both tests succeeded, but neither was meant for ongoing operation.  LADEE intentionally impacted the moon, and the US orbital segment of ISS currently uses S and Ku band radio communication through TDRSS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next proposed study on laser communication is &lt;a href=&quot;http://www.nasa.gov/mission_pages/tdm/lcrd/overview.html&quot;&gt;LCRD&lt;/a&gt;, which hopes to fly on a commercial payload in 2019.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a high probability that the next Mars orbiter will use optical communication.  NASA has been interested in optical communications for  Mars since at least 2008, as shown in &lt;a href=&quot;http://slideplayer.com/slide/6216963/&quot;&gt;this presentation&lt;/a&gt; from Jim Schier, Chief Architect for NASA’s Space Communications and Navigation Program at NASA Headquarters.  Most recently, in April 2016 a request for information went out concerning this upcoming Mars orbiter, and the awards are due to come out this June, so stay tuned.  Look for lasercomm, solar electric propulsion, a high-res camera, and ground-penetrating radar in the winning proposals.&lt;/p&gt;&#xA;" OwnerUserId="13978" LastActivityDate="2016-06-03T13:23:37.880" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15543" PostTypeId="1" AcceptedAnswerId="15547" CreationDate="2016-06-03T14:10:31.823" Score="12" ViewCount="1456" Body="&lt;p&gt;Since the retirement of the Space Shuttle, NASA has depended on contracts with Russia to launch manned vehicles to the International Space Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The specifics for other manned spacecraft must be well understood by engineers at NASA.  Why do they not &quot;simply&quot; build their own Soyuz, or other similar rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it due to some form of copyright on the craft, efficiency/economy of scale, or ease of logistics?  Does NASA simply not have the research and understanding to make a similar craft at this time without lengthy R&amp;amp;D?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should clarify; I'm talking &quot;medium&quot; payload boosters.  Not super lifters like the Saturn V/STS/SLS, but specifically existing boosters like the Soyuz-2&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="6773" LastEditDate="2016-06-06T03:07:55.150" LastActivityDate="2016-06-06T10:46:58.097" Title="Why does NASA not replicate existing manned launch systems?" Tags="|crewed-spaceflight|nasa|launch-vehicle|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15544" PostTypeId="2" ParentId="15540" CreationDate="2016-06-03T14:51:14.050" Score="6" Body="&lt;p&gt;The &lt;a href=&quot;http://m.esa.int/Our_Activities/Telecommunications_Integrated_Applications/EDRS/Overview&quot; rel=&quot;noreferrer&quot;&gt;European Data Relay System&lt;/a&gt; has recently begun testing. It's for getting data from Earth orbiting satellites down to the ground quickly, without waiting for the satellite to pass over a base station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Claimed data rate: &quot;Up to 1800 Mbit/s&quot; and &quot;At least 50 TBytes/day&quot;. (Note: I don't see how these two figures equate.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the first big test: &lt;a href=&quot;http://m.esa.int/Our_Activities/Telecommunications_Integrated_Applications/EDRS/Europe_s_SpaceDataHighway_relays_first_Sentinel-1_images_via_laser&quot; rel=&quot;noreferrer&quot;&gt;an image&lt;/a&gt; taken by Sentinel 1A, sent by laser to an EDRS satellite (in geostationary orbit) and beamed down direct to the base station in Germany. (&lt;a href=&quot;http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2016/06/la_reunion_island_via_laser/16013249-1-eng-GB/La_Reunion_Island_via_laser.jpg&quot; rel=&quot;noreferrer&quot;&gt;Hi-res version&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest &lt;a href=&quot;http://m.esa.int/Our_Activities/Telecommunications_Integrated_Applications/EDRS/Applications&quot; rel=&quot;noreferrer&quot;&gt;advantage&lt;/a&gt; of all this is a Sentinel satellite in low earth orbit can pass its information to base with the minimum of delay, even when out of sight of a base station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a picture of one of the satellite-mounted laser communication terminals:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4pKKN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4pKKN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Bigger version of the picture available &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2015/06/Laser_communication_terminal&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-06-03T16:32:25.253" LastActivityDate="2016-06-03T16:32:25.253" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15545" PostTypeId="2" ParentId="15543" CreationDate="2016-06-03T15:32:41.900" Score="12" Body="&lt;p&gt;We can't even directly rebuild a Saturn V at this point (without tons of new R&amp;amp;D) because the tools and manufacturing processes no longer exist and we've lost the institutional knowledge of the people who built it that have since retired: &lt;a href=&quot;https://space.stackexchange.com/questions/6281/why-not-build-saturn-vs-again&quot;&gt;Why not build Saturn V&amp;#39;s again?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Copying a design that we've never built is way more complex than trying to rebuild something we actually have all the plans for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to why we don't &quot;simply&quot; build a similar rocket we pretty much are (&lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;SLS&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Commercial_Crew_Development&quot; rel=&quot;nofollow noreferrer&quot;&gt;Commercial Crew Program&lt;/a&gt;), but to build spacecraft we have to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;develop and build the tools/manufacturing line&lt;/li&gt;&#xA;&lt;li&gt;certify everything as man-rated because we don't currently have a launch system that is man-rated and the vehicle will be new (requirements are &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8705_002B_&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;test everything extensively to make sure we don't kill any astronauts (because space is an amazingly unfriendly place if anything goes wrong)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All of that takes lots of funding and time. Throwing more money at it could speed things up to some degree, but there's not enough political interest for that to happen.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-06-03T15:40:15.933" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15547" PostTypeId="2" ParentId="15543" CreationDate="2016-06-03T16:56:16.360" Score="6" Body="&lt;p&gt;The US chose to rely on the commercial sector for manned launch to LEO instead of developing its own launch vehicle. Sure, it's taking a few years but several companies are getting close now.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an alternative, NASA could have... &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;restarted production of the Titan variant used for Gemini or the Saturn 1B used for Apollo. Both are 50 year-old designs that would cost a lot of money to redevelop (see 1337joe's answer). &lt;/li&gt;&#xA;&lt;li&gt;designed a new rocket. This is politically difficult and takes a long time. Since the demise of the Shuttle, we've seen 3 different heavy-lift architectures, one of which included a lightweight variant for LEO crew access (Ares I). Their history (Constellation, Ares, SLS) shows the pitfalls of NASA developing anything. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-03T16:56:16.360" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15548" PostTypeId="5" CreationDate="2016-06-03T17:25:31.543" Score="0" Body="&lt;p&gt;JPL Horizons web interface: &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow&quot;&gt;http://ssd.jpl.nasa.gov/horizons.cgi&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-06-03T17:25:31.543" LastActivityDate="2016-06-03T17:25:31.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15549" PostTypeId="4" CreationDate="2016-06-03T17:25:31.543" Score="0" Body="Questions relating to the JPL Horizons solar system data and ephemeris computation service" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-06-03T17:25:31.543" LastActivityDate="2016-06-03T17:25:31.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15550" PostTypeId="5" CreationDate="2016-06-03T17:34:48.780" Score="0" Body="&lt;p&gt;Useful links:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/soho/index.html&quot; rel=&quot;nofollow&quot;&gt;NASA Mission Page&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://sci.esa.int/soho/&quot; rel=&quot;nofollow&quot;&gt;ESA Mission Page&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-06-03T17:34:48.780" LastActivityDate="2016-06-03T17:34:48.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15551" PostTypeId="4" CreationDate="2016-06-03T17:34:48.780" Score="0" Body="Questions relating to the Solar and Heliospheric Observatory" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-06-03T17:34:48.780" LastActivityDate="2016-06-03T17:34:48.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15552" PostTypeId="5" CreationDate="2016-06-03T17:39:27.830" Score="0" Body="" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-06-03T17:39:27.830" LastActivityDate="2016-06-03T17:39:27.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15553" PostTypeId="4" CreationDate="2016-06-03T17:39:27.830" Score="0" Body="Questions relating to the circular restricted three-body problem, where two large bodies orbit their center of mass and the third mass is negligible compared to the larger ones." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-06-03T17:39:27.830" LastActivityDate="2016-06-03T17:39:27.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15554" PostTypeId="5" CreationDate="2016-06-03T17:41:16.143" Score="0" Body="&lt;p&gt;&lt;strong&gt;Useful Links&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-06-03T17:41:16.143" LastActivityDate="2016-06-03T17:41:16.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15555" PostTypeId="4" CreationDate="2016-06-03T17:41:16.143" Score="0" Body="Questions relating to the SpaceX Autonomous Spaceport Drone Ship, or ASDS." OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-06-03T17:41:16.143" LastActivityDate="2016-06-03T17:41:16.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15557" PostTypeId="1" CreationDate="2016-06-03T20:12:05.220" Score="7" ViewCount="310" Body="&lt;p&gt;Astronauts in microgravity for extended periods experience a number of maladaptations, including bone loss, muscle atrophy and muscle mass loss, redistribution of fluids, and reduction in immune function. Many proposals have been put forward to use centripetal acceleration to simulate gravity in long duration flights, thereby reducing the astronauts' physical deterioration. Most of these proposals are built around the assumption of simulating 1g, earth surface gravity. The resulting designs are heavy and large enough to eliminate them from serious contention for near-term human spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This thinking seems oversimplified to me. Why a full 1.0g? My intuition tells me there may be a step function involved. i.e., bone loss is minimal from 1g down to a threshold value, below which some mal-adaptation trigger point is reached and bone loss jumps up to the high levels we see from 0g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is two-fold. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) Do we have any data (I'm thinking from mice on a centrifuge in space) correlating mal-adaptation versus artificial gravity in values below 1.0g? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) How could we most cheaply collect this data, if it does not exist?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Expanding on my second question, my first thought is to put mice into a centrifuge at 0.5g and monitor their urine for calcium. Are there protein biomarkers for space mal-adaptation? Could you collect a sample of urine or blood from a mouse and tell quickly if that mouse is losing bone, losing muscle mass, redistributing fluids, or suffering a loss of immune function? &#xA;I would love to see a &quot;cheap&quot; research mission with sealed cages of mice arranged along a long rotating tether like a string of pearls, the habitats closest to the counterweight experiencing the least artificial gravity but all other parameters identical, with their collected urine telling a story of how much gravity it takes to keep them healthy. Or have these questions been answered already to our satisfaction?&lt;/p&gt;&#xA;" OwnerUserId="8247" LastEditorUserId="12102" LastEditDate="2021-08-18T01:07:26.780" LastActivityDate="2021-08-18T01:07:26.780" Title="How might artificial gravity experiments be conducted to determine adaptation in mice?" Tags="|astronauts|microgravity|health|artificial-gravity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="15558" PostTypeId="2" ParentId="110" CreationDate="2016-06-03T20:50:09.660" Score="0" Body="&lt;p&gt;The Deep Space 1 mission in 1998 had SCARLET solar concentrating arrays.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-06-03T20:50:09.660" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="15559" PostTypeId="1" CreationDate="2016-06-03T22:51:34.073" Score="4" ViewCount="579" Body="&lt;p&gt;Because stars kill stuff with radiation, flares, etc, then die taking planets with them, might it not be easier for living things to live on a rogue planet's moon?  Say you had a rogue Jupiter-like planet with tidally heated moons like Europa orbiting it.  Europa stays warm inside just from gravity/going around Jupiter.  Wouldn't that end up being a safer and longer-lived environment for life or would that not work due to lack of energy?&lt;/p&gt;&#xA;" OwnerUserId="14659" LastEditorUserId="4660" LastEditDate="2016-06-04T00:37:17.083" LastActivityDate="2016-06-08T16:47:11.073" Title="Could a rogue planet's moons harbor life?" Tags="|life|moons|exoplanet|gas-giant|rogue-planet|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="15560" PostTypeId="2" ParentId="15557" CreationDate="2016-06-03T23:18:54.727" Score="4" Body="&lt;p&gt;The formula for artificial &quot;gravity&quot; due to centrifugal action is simple:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;g = rω²&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where &lt;code&gt;g&lt;/code&gt; is the gravitational acceleration, &lt;code&gt;r&lt;/code&gt; is the radius of the centrifuge, and &lt;code&gt;ω&lt;/code&gt; is the angular velocity.  So the &quot;gravity&quot; is proportional to the radius.  This is a problem - attempting to scale the centrifuge down for lower-mass mice is not helpful - mass is not a variable in the above equation.  So regardless of the species of meat popsicle that you want to run this experiment on, you'll need a pretty big centrifuge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How big?  &lt;a href=&quot;https://www.youtube.com/watch?v=EHKQIC5p8MU&quot; rel=&quot;nofollow&quot;&gt;This youtube clip&lt;/a&gt; attempts to answer this by comparing several sci-fi centrifugal gravity schemes.  It turns out that there is a trade-off:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If the centrifuge is too small (e.g. &lt;code&gt;r=8m&lt;/code&gt; as per Space Odyssey), the coriolis effects will be huge and make it extremely uncomfortable (nauseating) to live on such a centrifuge for any period of time.&lt;/li&gt;&#xA;&lt;li&gt;Conversely, if the centrifuge is too big (e.g. &lt;code&gt;r=93 million miles&lt;/code&gt; as per Ringworld), then the structural integrity of the centrifuge is impractical.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The clip posits that Babylon 5 strikes a pretty good balance of these two at &lt;code&gt;r=8km&lt;/code&gt; for 1g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So even if we only want to produce 0.5g, we'd need a 4km centrifuge for humans.  Perhaps mice can handle the nausea due to coriolis effect better than humans can.  The largest man-made structure in space is the ISS, with a length of 108.5m.  In order to use the ISS for this experiment, we'd have to:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;assume mice can handle 40x the coriolis effect that humans can&lt;/li&gt;&#xA;&lt;li&gt;vacate it of all humans&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;At this point, this seems like an unlikely set of assumptions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;How does this answer the OP questions?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Given the analysis above, I think we can be fairly confident that there currently is no such data for gravitational acceleration between 0 and 1g.  Given the scale of centrifuge required, it might be technically possible today, but there is no evidence I'm aware of of an orbital centrifuge on this very large scale.&lt;/li&gt;&#xA;&lt;li&gt;I can only conjecture the answer to the second part of the question.  I would guess that spinning the existing ISS might possibly be the cheapest way to do this, despite the need to vacate all human crew.  I don't know if the strength of the structure would be up for this sort of motion though.  Alternatively, it might be possible to have two crew modules connected by a several km cable and spin it.  I have no idea how this would look cost-wise, though.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="758" LastEditorUserId="758" LastEditDate="2016-06-06T02:21:50.417" LastActivityDate="2016-06-06T02:21:50.417" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="15561" PostTypeId="5" CreationDate="2016-06-04T00:39:57.647" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-06-04T00:39:57.647" LastActivityDate="2016-06-04T00:39:57.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15562" PostTypeId="4" CreationDate="2016-06-04T00:39:57.647" Score="0" Body="A planet not orbiting a star, but moving through interstellar space according to the pull of various nearby stars and ultimately orbiting the center of the galaxy." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-06-04T00:39:57.647" LastActivityDate="2016-06-04T00:39:57.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15563" PostTypeId="1" CreationDate="2016-06-04T10:49:55.957" Score="2" ViewCount="347" Body="&lt;p&gt;The year is 2036 and humanity has established a colony on Mars. There's hundreds of people there, more coming every two years, and now it's time to make the colony self-sustaining.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the question is: what's the availability of construction materials and other resources on Mars? A lot of people asked about production of methane and oxygen from CO2 and water to refuel rockets coming to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about other things, say, how would one go about making steel or aluminium or some other construction material out of available resources? How does that compare to availability of ores on Earth or the Moon?&lt;/p&gt;&#xA;" OwnerUserId="10274" LastActivityDate="2016-06-18T00:36:38.343" Title="Mining construction materials and other resources on Mars" Tags="|mars|colonization|isru|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="15564" PostTypeId="2" ParentId="15563" CreationDate="2016-06-04T11:25:14.727" Score="4" Body="&lt;p&gt;You can make &lt;a href=&quot;https://www.technologyreview.com/s/545216/materials-scientists-make-martian-concrete/&quot; rel=&quot;nofollow&quot;&gt;a kind of concrete&lt;/a&gt; from sulphur, although the idea of living in buildings made of a potentially poisonous material might not sound very attractive. There is also water which should provide you with rocket fuel. However, the actual building materials requirements will be quite low initially. What is really needed is solar cells, wind turbines and green houses. Also a lot of tools for plumbing, digging and wiring. Making these locally is probably not an option, since a factory/workshop weighs a lot more than the tools themselves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: You can probably find &lt;a href=&quot;https://en.wikipedia.org/wiki/Ore_resources_on_Mars&quot; rel=&quot;nofollow&quot;&gt;some easy to reach usefuel ores &lt;/a&gt; on Mars, but you'd still need to &lt;a href=&quot;https://en.wikipedia.org/wiki/Ore_resources_on_Mars&quot; rel=&quot;nofollow&quot;&gt;separate&lt;/a&gt; the ore from the gravel and a furnace to process it.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="11256" LastEditDate="2016-06-04T11:31:29.923" LastActivityDate="2016-06-04T11:31:29.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="15568" PostTypeId="2" ParentId="15543" CreationDate="2016-06-05T00:16:04.507" Score="1" Body="&lt;p&gt;Anytime a complex machine is produced for a long time, there exists a problem where some of the parts no longer are manufactured after a period of time. This can happen for a variety of reasons, and is most common with electronics. For instance, buying a VME chassis today is difficult, and much more expensive than standard computer electronics today. Standards change over time to ones that are better, cheaper, faster, etc. The old goods become increasingly difficult to maintain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've worked in the aerospace industry for some time. Sometimes certain key parts suddenly are no longer available. There is a couple of different ways of addressing these issues, but they basically come down to some kind of a re-design. The objects that I have seen were more mass produced than the Saturn 5, and were about 15 years old, and some of them were no longer available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Re-building the Saturn 5 would require tremendous work, almost as much as building a new similar rocket would be today. Modern techniques could make it lighter, stronger, more reliable, and cheaper than simply rebuilding the existing design would be. The Apollo spacecraft would be even more difficult, as it relied heavily on very dated technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, a re-design of the electronics is required to make it work, it'd be a vast improvement. The general physics, however, remain the same, and so there are some principals from the existing designs which are still applicable today. Saturn 5 lead to the Space Shuttle, and it is leading to SLS. And those technologies are inspiring the rockets we have today.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-05T00:16:04.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16567" PostTypeId="2" ParentId="15525" CreationDate="2016-06-05T02:16:35.023" Score="4" Body="&lt;p&gt;A good way to get a feel for the relative strengths of various oxidizers is to compare their &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)&quot; rel=&quot;nofollow&quot;&gt;standard reduction potentials&lt;/a&gt;.  Fluorine is one of the most potent oxidizers known, with a reduction potential of +2.87 V. Here are some others that you might find relevant:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;S$_2$O$_8$$^2$$^-$,                             +2.01 V&lt;br&gt;&#xA;H$_2$O$_2$,                                     +1.78 V&lt;br&gt;&#xA;MnO$_4$$^2$$^-$,                                +1.70 V&lt;br&gt;&#xA;Cl$_2$,                                         +1.36 V&lt;br&gt;&#xA;O$_2$,                                          +1.22 V&lt;br&gt;&#xA;NO$_3$$^-$,                                     +0.96 V     &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bear in mind that these potentials are really showing the oxidizer's ability to oxidize water (or protons), but their order should remain the same regardless of what molecule is being oxidized.  Proper choice of an oxidizer for a rocket propellant would take into consideration both the oxidizer's strength and the mass required to combust a given amount of fuel.  &lt;/p&gt;&#xA;" OwnerUserId="14510" LastActivityDate="2016-06-05T02:16:35.023" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16568" PostTypeId="1" CreationDate="2016-06-05T05:36:11.700" Score="4" ViewCount="179" Body="&lt;p&gt;This is a comparison of a visible light image and a radio image of the area around Jupiter's great red spot from an article in &lt;a href=&quot;https://www.sciencenews.org/article/jupiter%E2%80%99s-stormy-weather-no-tempest-teapot&quot; rel=&quot;nofollow noreferrer&quot;&gt;Science News&lt;/a&gt;. The radio data is from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Karl_G._Jansky_Very_Large_Array&quot; rel=&quot;nofollow noreferrer&quot;&gt;VLA&lt;/a&gt;, and the frequency is chosen to image Ammonia radio emission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gOB6D.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gOB6D.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a radio image of both jupiter (smeared over 10 hours of observing) and its surrounding environment, colors represent radio wavelengths of 2cm (blue) 3cm (gold) and 6cm (red). Trapped charged particles in Jupiter's magnetic field produce very strong emission as well. Taken from an article in &lt;a href=&quot;http://gizmodo.com/jupiters-mysterious-interior-is-coming-into-focus-1779754843&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gizmodo&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CNqFY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CNqFY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Juno spacecraft will &quot;scoot&quot; under this emission so its antennae can see more clearly radio structure from the planet, reducing interference, but will only see a small area of the planet at a time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During Juno's mission, will there be any &lt;em&gt;simultaneous&lt;/em&gt; or near-simultaneous measurements of Jupiter from earth to complement data from Juno?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional source: &lt;a href=&quot;http://science.sciencemag.org/content/352/6290/1198&quot; rel=&quot;nofollow noreferrer&quot;&gt;Science&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-06-05T13:31:32.933" LastActivityDate="2016-06-14T14:06:49.477" Title="Will there be joint observations from earth and the Juno spacecraft when it reaches Jupiter?" Tags="|jupiter|juno|radio-telescope|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16569" PostTypeId="1" AcceptedAnswerId="16570" CreationDate="2016-06-05T06:01:40.270" Score="11" ViewCount="2412" Body="&lt;p&gt;In this &lt;a href=&quot;https://space.stackexchange.com/q/12296/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/12301/12102&quot;&gt;answer&lt;/a&gt; pair I see that the ISS can maintain data links to earth through direct access to ground stations as well as through the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_Data_Relay_Satellite_System&quot; rel=&quot;noreferrer&quot;&gt;TDRSS&lt;/a&gt; geostationary satellite network. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of antennae are used on the ISS for these links? Are they directional? Do they have to continuously move to maintain a live link - for example in the case of the live streaming &lt;a href=&quot;http://www.ustream.tv/channel/live-iss-stream&quot; rel=&quot;noreferrer&quot;&gt;HDEV&lt;/a&gt; views? I'm wondering if they are phased arrays which can be steered (potentially very quickly) electronically, or a more conventional gimbaled dish, &lt;em&gt;a little like&lt;/em&gt; the picture below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I got excited when I found this image through a google search for space antenna GIFs, but it turns out to be &lt;a href=&quot;https://space.stackexchange.com/a/8742/12102&quot;&gt;something completely different&lt;/a&gt;. (also &lt;a href=&quot;https://www.youtube.com/watch?v=FGK8IC-bGnU&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cdkOW.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cdkOW.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2020-11-14T20:31:11.240" Title="Do antennae on the ISS have to constantly move to maintain data links?" Tags="|iss|communication|communication-satellite|tdrss|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16570" PostTypeId="2" ParentId="16569" CreationDate="2016-06-05T11:49:08.527" Score="16" Body="&lt;p&gt;The US side of the ISS has a number of antennas to support its rather complicated comm system.  Most visible are the two Ku-band High Gain Antennas which are 6 foot diameter azimuth/elevation gimbaled dish antennas mounted on the Z1 truss segment (near the center of the ISS truss).  These antennas are sometimes referred to as SGANTs (Space-to-Ground ANTennas).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GnKb2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GnKb2.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;For most of its life, the ISS only had one SGANT.  &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110004354.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;A second one was carried up on one of the later shuttle missions&lt;/a&gt;.  This system carries the video and experiment data to the ground and would normally be gimbaling continuously to point at the selected geosynchronous comsat (i.e. TDRS).&lt;/p&gt;&#xA;&lt;p&gt;You can see one tracking a TDRS satellite in this video (ignore the ammonia venting!)&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/PJzjs4EI22k?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;The S-band system uses S-band Antenna Support Assemblies (SASAs) which contain a conical high gain antenna and an omnidirectional antenna.  The SASAs can move in elevation and azimuth as well to point at the TDRS in use. They are located on the S1 and P1 truss sections, near the center of the truss. S-band is used for voice and data communications.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S3pPp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S3pPp.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;(NASA photo ISS021E033057, doesn't appear to be online at present)&lt;/p&gt;&#xA;&lt;p&gt;There are also two Ultra High Frequency antennas on the ISS, primarily used to communicate with extravehicular activity crewmembers, formerly used to communicate with the space shuttle orbiter.  They are non-gimbaling omni antennas.&lt;/p&gt;&#xA;&lt;p&gt;My information on the Russian ISS comm system is very limited and possibly outdated but they have (or had) a system analogous to the S-band system called Regul that communicates with Russian ground stations and the former Luch satellite system. There is also a Russian UHF system used for space-to-space comm.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-11-14T20:31:11.240" LastActivityDate="2020-11-14T20:31:11.240" CommentCount="15" ContentLicense="CC BY-SA 4.0" />
  <row Id="16571" PostTypeId="1" CreationDate="2016-06-05T14:37:09.913" Score="12" ViewCount="418" Body="&lt;p&gt;What is the least gravity needed for human health on a one year space trip?&lt;/p&gt;&#xA;" OwnerUserId="15664" LastEditorUserId="8162" LastEditDate="2016-06-05T14:43:55.590" LastActivityDate="2022-08-25T13:46:59.200" Title="Least artificial gravity needed to maintain health on a one year space trip?" Tags="|mars|gravity|microgravity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16572" PostTypeId="2" ParentId="16571" CreationDate="2016-06-05T16:25:38.063" Score="7" Body="&lt;p&gt;An excellent question. Finding minimum gravity to keep humans healthy should be a high priority. Maybe lunar or Mars gravity isn't sufficient. In which case efforts to colonize these places would be unproductive. If it turns out only a small amount is needed, it becomes much more doable to build orbital habs and workplaces for asteroid mines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this time it remains an open question. We really have only two data points: the weightlessness of someone in orbit and the full g on earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russian astronaut &lt;a href=&quot;http://www.rocketstem.org/2015/02/09/russian-cosmonaut-valeri-polyakov-spent-record-breaking-14-months-aboard-mir-space-station-in-1990s/&quot;&gt;Valeri Polyakov&lt;/a&gt; spent 438 days in orbit without excessive bone loss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However Valeri had a great deal of self discipline and had a very rigorous exercise regimen. We can't expect everyone to do as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I look at this question in my blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/12/whats-minimum-spin-hab.html&quot;&gt;What is Minimum Spin Hab?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-06-05T16:25:38.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16575" PostTypeId="2" ParentId="3350" CreationDate="2016-06-05T21:34:28.337" Score="2" Body="&lt;p&gt;Yes. Any system beaming power from space to the earth will add heat to the earth. &#xA;Most current plans call for solar power satellites (&quot;powersats&quot;) located in geostationary orbit, about 36,000 km up. The sunlight they intercept would have otherwise missed earth, so the energy they beam down would not otherwise have been part of earth's heat balance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are evaluating electrical generation systems, then the question becomes one of scale and comparison with other energy systems, both of the heat delivered and also the overall effect on the environment. In spite of the heat they add, powersats compare very favorably with other power generation systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll have to veer into speculation and opinions about energy policy, but I can pull in some specific backing details if they are requested. Since no powersat systems have been tested, everything about them is speculation to some degree.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As noted earlier, JAXA's beamed power demonstration is projected to have an end-to-end efficiency of only 22%, but researchers hope to achieve 80% at each end eventually. So somewhere between 20% and 50% of the microwave energy reaching earth will not generate electrical power but instead will create waste heat, primarily at the rectenna. The atmosphere is very transparent to the microwave frequencies being used, so very little of the energy will be lost directly to the air as it traverses the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This compares favorably with most power generation concepts currently in use. Coal, natural gas, and nuclear plants all generate more waste heat than electricity. A good reference is this: &lt;a href=&quot;http://www.mpoweruk.com/energy_efficiency.htm&quot; rel=&quot;nofollow&quot;&gt;http://www.mpoweruk.com/energy_efficiency.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More importantly, the fossil-fueled plants also release carbon dioxide, a greenhouse gas that causes earth's atmosphere to retain heat rather than radiating it back into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in decreasing order of environmental damage, you have coal (roughly 50% waste heat, plus CO2 release), natural gas (roughly 60% waste heat but less CO2), Nuclear (roughly 60% waste heat but zero CO2), Space Solar Power (20% - 50% waste heat and zero CO2), Wind (no waste heat) and terrestrial solar (converts roughly 25% of incoming sunlight to electricity, not heat). Hydropower is its own special case because it does not generate waste heat or CO2 in the conventional sense but does cause serious environmental damage through other mechanisms. &#xA;Wind and terrestrial solar are great from a waste heat perspective but it will be impossible to meet our energy needs with just those in the coming decades. (That's digressing from your question and into energy policy. I can discuss further in comments or PM if you like.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Summing up, a utility-scale powersat system would add less waste heat per megawatt-hour of electricity delivered to the grid than most current grid-scale generation systems and zero greenhouse gases. If we can overcome the engineering challenges and get the cost comparable to nuclear, then it would be the hands-down winner for &quot;green&quot; energy on large scales.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar Power Satellites &quot;SPS&quot; or &quot;Powersats&quot;,  remains an active area of research and development. Check out Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Space-based_solar_power&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Space-based_solar_power&lt;/a&gt;, recent work by JAXA &lt;a href=&quot;http://spectrum.ieee.org/green-tech/solar/how-japan-plans-to-build-an-orbital-solar-farm&quot; rel=&quot;nofollow&quot;&gt;http://spectrum.ieee.org/green-tech/solar/how-japan-plans-to-build-an-orbital-solar-farm&lt;/a&gt;, and many other sources.&#xA;The best configuration for a powersat system remains under evaluation and debate. Most proposed systems use microwaves to beam power from a powersat located in geostationary orbit. System proposals also exist which beam the power from a lower orbit, decreasing transmitter size and improving efficiency but with other system trade-offs.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-06-05T21:34:28.337" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16576" PostTypeId="2" ParentId="14100" CreationDate="2016-06-05T22:10:41.437" Score="5" Body="&lt;p&gt;There are space radiators, and then there are space radiators: &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_droplet_radiator&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Liquid_droplet_radiator&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spraying droplets of your thermal fluid through space before collecting and recirculating them effectively creates a radiator of large surface area without the mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plans were drawn up to fly a Liquid Droplet radiator demonstration unit on an STS mission but never reached fruition. &lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-06-05T22:10:41.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16579" PostTypeId="1" AcceptedAnswerId="16580" CreationDate="2016-06-05T22:44:09.923" Score="2" ViewCount="500" Body="&lt;p&gt;I want to model a slingshot to space. Ignoring all the impossibility of this, and assuming I could launch anything at any speed, can the projectile enter orbit without any propulsion system?&lt;/p&gt;&#xA;" OwnerUserId="15669" LastEditorUserId="6816" LastEditDate="2016-06-05T22:52:54.920" LastActivityDate="2016-06-05T22:52:54.920" Title="Can a surface impulse be enough to allow entering orbit with no further propulsion?" Tags="|launch|orbital-mechanics|orbit|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16580" PostTypeId="2" ParentId="16579" CreationDate="2016-06-05T22:51:22.517" Score="5" Body="&lt;p&gt;Not without &lt;em&gt;a little&lt;/em&gt; propulsion to circularize the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When giving an initial boost, to orbital speed, you're establishing the new orbit - that passes through the point of launch; arriving &quot;from behind&quot; and &quot;from below&quot;. In other words, if you launch something into Earth orbit from the surface, the orbit crosses the surface - your satellite will crash; the periapsis of the orbit is located below the planet's surface (or at best at it, if you launch horizontally).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your satellite needs to perform a prograde burn when at apoapsis, to raise the periapsis above the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course this all is pretty much moot considering your satellite will burn up on ascent if you ever manage to give it enough initial propulsion to reach orbital speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Caveat: The little propulsion could come from a gravity assist of the Moon. But then, you need to launch it not just into LEO; you'll need to launch it into a lunar flyby trajectory.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-06-05T22:51:22.517" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="16581" PostTypeId="1" CreationDate="2016-06-05T23:26:22.990" Score="1" ViewCount="1224" Body="&lt;p&gt;I'm trying to model a slingshot launch to space, literally, and I'm using 2 basic formulas. Simplifying my model, a object with a (really big) initial velocity is launched from Earth's surface on the equator, and it is affected by air resisance and gravity. I am using the following formulas:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air resistance:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;F=\frac{C_D}2\rho A v^2&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A = Reference area&#xA;C = Drag coefficient&#xA;F = Drag force, N.&#xA;V = Velocity, m/s.&#xA;ρ = Density of fluid (liquid or gas), kg/m3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And :&#xA;universal gravitational force:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F = G \frac{m_1 m_2}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but I'm getting some wierd results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to know if i'm using the right formulas for a object that is really low mass in comparison to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More info:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm using a Python program to plot a graphic that would be the X and Y position. For the method I'm using (Odeint) I need the differential equation (don't know if the name is correct in english) for it, as F = ma:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F/m_1 = G \frac{m_2}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;if $m_1$ = my object mass&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so my Speed Variation/Time Variation = $G*M2 / r^2$ (being M2 = earth mass)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I think it makes sense because a object's gravity acceleration does not depend on that object's mass.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm almost sure I'm doing a basic and terrible mistake here, but I'm in my first year in college and I really need help, thanks.&lt;/p&gt;&#xA;" OwnerUserId="15669" LastEditorUserId="4660" LastEditDate="2016-06-06T15:00:58.707" LastActivityDate="2016-06-06T15:00:58.707" Title="What formulas do I use to calculate the gravity and drag forces on an object ascending from Earth's surface?" Tags="|orbital-mechanics|calculation|gravity-fields|python|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16582" PostTypeId="2" ParentId="16581" CreationDate="2016-06-06T00:13:02.490" Score="6" Body="&lt;p&gt;One thing that may be tripping you up is that the &lt;code&gt;d&lt;/code&gt; term in the gravitational formula is the distance between the centers of mass of the objects, not the altitude above Earth's surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other thing to keep an eye on is your units. The big G gravitational constant is ~6.67 x 10&lt;sup&gt;-11&lt;/sup&gt; m&lt;sup&gt;3&lt;/sup&gt; kg&lt;sup&gt;-1&lt;/sup&gt; s&lt;sup&gt;-2&lt;/sup&gt;; if you're using that value, make sure you're consistently using kg and m rather than grams and cm, for instance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're correct that the mass of an object factors out when you consider its acceleration under gravity. Near Earth's surface you should be getting acceleration of ~9.8 m s&lt;sup&gt;-2&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An unpowered ballistic trajectory that can reach space from Earth's surface is going to incur a terrifying amount of drag. I know maximum dynamic pressure for a launching rocket is usually in the 20-30KPa range, but your case will be much higher, because maximum speed -- the moment of launch) -- will coincide with maximum air density, which is not the case for rockets. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-06-06T00:28:33.703" LastActivityDate="2016-06-06T00:28:33.703" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16583" PostTypeId="2" ParentId="15543" CreationDate="2016-06-06T04:30:30.043" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I should clarify; I'm talking &quot;medium&quot; payload boosters. Not super lifters like the Saturn V/STS/SLS, but specifically existing boosters like the Soyuz-2&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Delta IV, Atlas V, and Falcon 9 each have more LEO payload capacity than Soyuz-2. Man-rating any of those launchers would be vastly cheaper than developing the capability to build a copy of the Soyuz launcher. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That would still leave us without a spacecraft comparable to the manned Soyuz craft, to rendezvous and dock with ISS, of course, but getting Dragon V2 into service is again more likely than making copies of a 50-year-old Russian design.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-06-06T04:30:30.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16584" PostTypeId="2" ParentId="14100" CreationDate="2016-06-06T04:45:23.500" Score="10" Body="&lt;p&gt;Don't convert it into heat in the first place, and then don't dissipate it fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MSL used an electromagnetic brake to slow the descent of the rover from the skycrane. It dissipated the energy into a resistor. Here, instead &lt;a href=&quot;https://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;store&lt;/em&gt; the energy in a superconducting magnet&lt;/a&gt;. It can remain there as long as you can keep the magnet cold, and you can then either use the energy, or dissipate it over a long period of time through small radiators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The conversion of mechanical energy into electrical energy won't be 100% efficient, so you will inevitably be left with some heat energy to dissipate. However such conversion efficiencies can be 98% or better, so you have reduced your immediate problem by a factor of 50.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-06-06T04:45:23.500" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16585" PostTypeId="2" ParentId="16571" CreationDate="2016-06-06T06:39:49.967" Score="1" Body="&lt;p&gt;Zero. None. Nada. Zip.  0.0&lt;/p&gt;&#xA;&lt;p&gt;Artificial gravity is not necessary to stay healthy for one year.&lt;/p&gt;&#xA;&lt;p&gt;From: &lt;a href=&quot;http://arstechnica.com/science/2016/03/after-his-picture-perfect-landing-last-night-scott-kelly-seemed-just-fine/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://arstechnica.com/science/2016/03/after-his-picture-perfect-landing-last-night-scott-kelly-seemed-just-fine/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;OK it's 340 days, but it's probably safe to assume another 25 days wouldn't change the outcome too much.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4mS2F.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4mS2F.png&quot; alt=&quot;Scott Kelly seems just fine&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2022-08-25T13:46:59.200" LastActivityDate="2022-08-25T13:46:59.200" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="16586" PostTypeId="1" AcceptedAnswerId="16592" CreationDate="2016-06-06T07:09:34.093" Score="1" ViewCount="782" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/15560/12102&quot;&gt;answer&lt;/a&gt; and the linked &lt;a href=&quot;https://space.stackexchange.com/a/15560/12102&quot;&gt;YouTube video&lt;/a&gt; discuss some difficulties with the experience of artificial &quot;gravity&quot; generated by small rotating craft (like one that could be launched to Mars in the near future). The &lt;em&gt;gravity gradient&lt;/em&gt; from head to foot and especially the &lt;a href=&quot;https://en.wikipedia.org/wiki/Coriolis_effect_(perception)&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Coriolis effect&lt;/em&gt;&lt;/a&gt; would be quite strong for a small craft, and could conceivably be more uncomfortable than no gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IqZfA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IqZfA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/EHKQIC5p8MU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; One could use an external mass and tether for a more uniform artificial gravity, but that's a separate issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After a flight to Mars with no artificial gravity, what would the initial experience be like after landing? With no ground crew to assist, would the crew simply sit there for a while, then slowly start moving and assisting each other? Would robotic assistance be particularly helpful in this case? I can imagine without a ground crew, I'd appreciate some help from a robotic arm, bringing me things I ask for, helping me stand, and possibly get out of my bulky suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would Mars' lower gravity compared to earth reduce the risk of breaking a bone (considering the bone loss associated with the trip) or would that still be a major concern?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GsASN.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GsASN.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OwyCo.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OwyCo.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a prototype of the robotic arc welder that will tack-weld the F9 legs down after landing at sea. (humor) A similar device could assist the astronauts, but at lower voltage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GlGwI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://i.stack.imgur.com/GlGwI.jpg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2020-06-12T13:33:04.223" Title="How would travel to Mars without artificial gravity affect a crew's initial experience in Mars gravity?" Tags="|mars|astronauts|health|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16587" PostTypeId="2" ParentId="16586" CreationDate="2016-06-06T08:16:53.237" Score="3" Body="&lt;p&gt;Honestly I don't see how can we provide you with a good answer.&#xA;Most of it depends on many parameters (nutrition, training, ...) that are currently heavily researched by NASA and other space agencies.&#xA;I guess the correct answer should be &quot;We don't know yet, we're working on making it as negligible as possible&quot;.&#xA;Also, how well the crew is trained during trip will depend on compromises too: diminished workload/leisure time, space needed for training machines, ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, considering the results of the latest NASA's twin study, you can expect that the crew will arrive in good shape, especially since mars gravity is lower than that of earth.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-06-06T08:16:53.237" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16588" PostTypeId="1" CreationDate="2016-06-06T09:40:28.057" Score="8" ViewCount="821" Body="&lt;p&gt;SpaceX launched two GEO comsats, and is about to launch two more, that use all-electric ion propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I'm wondering, how much time would it take to actually make a satellite like that operational. After all, while ion propulsion is cool and lightweight, it has a very, very low thrust. (&lt;a href=&quot;https://en.wikipedia.org/wiki/Eutelsat_115_West_B&quot; rel=&quot;noreferrer&quot;&gt;Eutelsat 117 West B&lt;/a&gt; apparently has only about 200 mN — that's &lt;em&gt;millinewtons&lt;/em&gt; — of thrust)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously I can just look it up for an operational satellite, but would be interested in calculating the answer for any arbitrary satellite.&lt;/p&gt;&#xA;" OwnerUserId="10274" LastActivityDate="2020-04-09T12:54:31.660" Title="How much time does it take to circularize a GTO orbit using ion propulsion?" Tags="|spacex|falcon-9|propulsion|communication-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16589" PostTypeId="2" ParentId="16588" CreationDate="2016-06-06T09:40:28.057" Score="10" Body="&lt;p&gt;I'm taking Eutelsat 117 W B as an example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simple, obvious answer is &quot;about half a year&quot; — this satellite was launched in March and entered service in October 2015.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I wanted to figure out the answer with physics and come up with roughly the same answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite has a mass of &lt;code&gt;m = 2200 kg&lt;/code&gt; and thrust of &lt;code&gt;T = 0.2N&lt;/code&gt;. It takes about &lt;code&gt;dv = 1.6 km/s&lt;/code&gt; to circularize the orbit from GTO to GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, we need a change of momentum of:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;dp = m * dv = 1.6 km/s * 2200kg = 3.52 * 10^6 kg m/s&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;to achieve our final orbit. Since change in momentum is equivalent to impulse:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;dp = J = T * t&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;we can calculate the time it takes, with our tiny thrust, to achieve that impulse:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;t = dp / T = 3.52 * 10^6 / 0.2 = 1.76 * 10^7 s = 204 days&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;204 days is about 6 months and 3 weeks.&lt;/p&gt;&#xA;" OwnerUserId="10274" LastActivityDate="2016-06-06T09:40:28.057" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16590" PostTypeId="2" ParentId="15543" CreationDate="2016-06-06T10:46:58.097" Score="-1" Body="&lt;p&gt;It is not useful to rebuild a Saturn the same way as decades ago. All electronics require a new development using today available parts. The rocket engines can not be build the same way today. The combustion chamber and the nozzle were built by welding a lot of thin tubes together. Nowadays the cooling of the walls is done with channels machined into copper and closed by galvanic deposition of nickel. The art of welding thin tubes is to difficult and expensive today when we have a cheaper, easier and more reliant method now. Therefore a completely new design of rocket engines is necessary. The new design requires a new series of tests.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-06-06T10:46:58.097" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16591" PostTypeId="2" ParentId="16581" CreationDate="2016-06-06T11:18:31.363" Score="3" Body="&lt;p&gt;This might help to &lt;em&gt;get you started&lt;/em&gt;. It's just a &lt;s&gt;1D&lt;/s&gt; &lt;em&gt;radial&lt;/em&gt; solver, but you can play with the math. I included a 3D version of the derivative function to show one way to make the acceleration a vector in NumPy. Don't forget to add the rotation of the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea about &lt;a href=&quot;https://space.stackexchange.com/questions/16581/what-formulas-do-i-use-to-calculate-the-gravity-and-drag-forces-on-an-object-asc#comment42062_16581&quot;&gt;starting on top of a mountain&lt;/a&gt; like &lt;a href=&quot;https://space.stackexchange.com/questions/16581/what-formulas-do-i-use-to-calculate-the-gravity-and-drag-forces-on-an-object-asc#comment42074_16581&quot;&gt;Mt. Kilimenjaro&lt;/a&gt; is worth exploring for sure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just chose a value for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Scale_height&quot; rel=&quot;nofollow noreferrer&quot;&gt;scale height&lt;/a&gt;, the 1/e length of the decrease in density with altitude. It's not really that simple, and of course &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_(physics)&quot; rel=&quot;nofollow noreferrer&quot;&gt;drag&lt;/a&gt; will be much more complicated and &lt;a href=&quot;https://space.stackexchange.com/questions/16581/what-formulas-do-i-use-to-calculate-the-gravity-and-drag-forces-on-an-object-asc#comment42075_16581&quot;&gt;much worse!&lt;/a&gt; than this simple equation, but it's at least a start, and you can add/modify math as you learn more about supersonic drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the dramatic loss of velocity while still in the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BVYyH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BVYyH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;# FROM: https://space.stackexchange.com/questions/16581/what-formulas-do-i-use-to-calculate-the-gravity-and-drag-forces-on-an-object-asc&#xA;&#xA;def deriv(X, t):&#xA;&#xA;    r, v = X&#xA;    acc_grav = -GMe / r**2&#xA;    acc_drag = -0.5 * rhocalc(r) * v**2 * Cd * A / mass&#xA;&#xA;    return [v, acc_grav + acc_drag]&#xA;&#xA;def rhocalc(r):&#xA;&#xA;    altitude = r - re&#xA;    rho      = rho_0 * np.exp(-altitude / h_scale)&#xA;&#xA;    return rho&#xA;&#xA;def deriv3D(X, t):&#xA;&#xA;    r, v = X[:3], X[3:] # 3D state vector np.array([x, y, z, vx, vy vz])&#xA;&#xA;    acc_grav = -GMe * r * ((r**2).sum(axis=0))**-1.5&#xA;    acc_drag = -0.5 * rhocalc(r) * v*np.sqrt((v**2).sum()) * Cd * A / mass&#xA;&#xA;    return np.hstack((v, acc_grav + acc_drag))&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;GMe = 3.986E+14   # m^3/s^2&#xA;&#xA;mass = 1000       # kg&#xA;Cd   = 1.0&#xA;A    = 0.1        # m^2&#xA;rho_0   = 1.3     # kg/m^3  or  mg/cm^3&#xA;h_scale = 7600    # m   scale height &#xA;&#xA;re  = 6371000.    #m     radius of earth&#xA;alt = 0.          #m     launch altitude&#xA;v0  = 3000.       #m/s   launch velocity&#xA;&#xA;X0 = [re+alt, v0]  # 1D state vector&#xA;&#xA;print deriv(X0, 0)&#xA;&#xA;t = np.linspace(0, 90, 1000)&#xA;&#xA;tol = 1E-9&#xA;&#xA;answer, blob = ODEint(deriv, X0, t, rtol=tol, atol=tol, full_output=True)&#xA;&#xA;plt.figure()&#xA;&#xA;r, v = answer.T&#xA;&#xA;plt.subplot(4, 1, 1)&#xA;plt.plot(t, r-re)&#xA;plt.title('altitude (m)')&#xA;plt.subplot(4, 1, 2)&#xA;plt.plot(t, v)&#xA;plt.title('velocity (m/s)')&#xA;plt.subplot(4, 1, 3)&#xA;plt.plot(t, rhocalc(r))&#xA;plt.title('density (kg/m^3)')&#xA;plt.subplot(4, 1, 4)&#xA;plt.plot(t[:-1], blob['hu'])&#xA;plt.title('step size (sec)')&#xA;plt.yscale('log')&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-06-06T13:43:19.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16592" PostTypeId="2" ParentId="16586" CreationDate="2016-06-06T11:49:56.900" Score="2" Body="&lt;p&gt;It's really difficult to say how this might be. At the very least, I'm confident that a Mars crew would have access to the same kind of equipment used on the ISS to help strengthen muscles. The trip to Mars is about 6 months. The gravity is significantly less than on Earth. Bottom line, I suspect that it might take a little time to adjust, but the 1/3rd gravity would be relatively easy to overcome after the 6 month stay. They no doubt would need to take it a bit easy for the first few days, but they would be quickly back into the swing of things. On Earth, it takes up to 3 weeks to get back to normal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, we'll only really know once it's happened. It's impossible to simulate that level of gravity on Earth. And if we could create a Martian gravity in space, we'd just take that system to Mars. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-06T11:49:56.900" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16593" PostTypeId="1" AcceptedAnswerId="16594" CreationDate="2016-06-06T13:37:31.160" Score="5" ViewCount="933" Body="&lt;p&gt;It's my perception that discussions of the use of &lt;a href=&quot;https://space.stackexchange.com/q/16586/12102&quot;&gt;artificial gravity&lt;/a&gt; for manned deep-space travel missions are primarily centered on avoiding bone loss. Am I right so far? Microgravity seems to be fairly agreeable to most of the popular reports and YouTube videos, except possibly for issues of sleep difficulty, loss of some sensation of taste of food, and potential eyesight degradation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the idea is that bones that do not experience &quot;normal&quot; regiments of strain will start to loose calcium. Is the idea behind the discussions of artificial gravity for long missions that it will help reproduce this type of strain to the bones?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does each bone need to experience strain separately - is it localized, or is it more systemic. Put bluntly, will straining my tibia reduce decalcification of my ulna?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-30T12:56:53.440" LastActivityDate="2017-07-30T12:56:53.440" Title="In what way is artificial gravity expected to avoid/reduce bone loss?" Tags="|astronauts|microgravity|health|artificial-gravity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16594" PostTypeId="2" ParentId="16593" CreationDate="2016-06-06T16:58:06.807" Score="5" Body="&lt;p&gt;Just a partial answer, limited to this part of the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Does each bone need to experience strain separately - is it localized, or is it more systemic.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bone growth is definitely controlled locally, in response to the stress applied to each part of the bone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://biomechanics.stanford.edu/me337/projects/jones77.pdf&quot; rel=&quot;noreferrer&quot;&gt;Humeral hypertrophy in response to exercise (Jones 1977)&lt;/a&gt; described thickened bone in the dominant arms of tennis players.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also a more modern paper with a more modern looking density plot &lt;a href=&quot;http://biomechanics.stanford.edu/paper/CMBBE08.pdf&quot; rel=&quot;noreferrer&quot;&gt;(Taylor et al 2008)&lt;/a&gt; describes bone density in the arms of one tennis player as well as attempts to match these with a simplified mechanical analysis.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The X-ray images revealed a significant increase of bone&#xA;  density in the right upper arm. The bone mass density index&#xA;  of the humeral shaft was found to be 24% higher in the&#xA;  dominant right arm as compared to non-dominant left arm.&#xA;  These values represent the average bone mass density of the&#xA;  entire humerus shaft. Since bone adaptation is an extremely&#xA;  local phenomenon that primarily affects selected regions of&#xA;  bone, we conclude that the local density increase is&#xA;  significantly higher than just one fourth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Taylor et al, &quot;The phenomenon of twisted growth: humeral torsion in dominant arms of high performance tennis players&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bear in mind these high bone densities have built up over many years as these are long-term players. So these should probably be seen as relatively extreme examples, but they prove the principle.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-06-06T16:58:06.807" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16595" PostTypeId="1" AcceptedAnswerId="16597" CreationDate="2016-06-06T22:33:25.563" Score="28" ViewCount="2051" Body="&lt;p&gt;In this &lt;a href=&quot;http://www.theregister.co.uk/2016/04/09/falcon_9_rocket_reused_in_two_months/&quot;&gt;article&lt;/a&gt; Elon Musk said that first stage of Falcon 9 which landed on the barge in April will be reused in June. Launching and landing wears material down, what parts need to be replaced and what other work must be done on the engines and the rocket body?&lt;/p&gt;&#xA;" OwnerUserId="6179" LastEditorUserId="51" LastEditDate="2016-06-07T02:35:16.567" LastActivityDate="2016-06-07T13:05:54.570" Title="What work needs to be done before reusing Falcon 9?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16596" PostTypeId="1" AcceptedAnswerId="16618" CreationDate="2016-06-06T23:52:42.703" Score="2" ViewCount="447" Body="&lt;p&gt;It doesn't have to be an exact answer, but something close to reality would be nice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically I want to know what the travel time to Mars would be using constant firing low-thrust engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the travel times be using something like 0.0004, 0.0008, 0.0016 or 0.0032 m/s²?&lt;/p&gt;&#xA;" OwnerUserId="6350" LastActivityDate="2016-06-09T02:22:04.590" Title="What are the travel times associated with different low-thrust accelerations going to Mars?" Tags="|mars|propulsion|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="16597" PostTypeId="2" ParentId="16595" CreationDate="2016-06-07T02:35:01.410" Score="33" Body="&lt;p&gt;At the moment no one knows, since no one has done quite this task before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is learning alot with each recovered core. In this image of three of the recovered cores in the LC-39A HIF building you can see different levels of wear on each stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YDPv5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YDPv5.jpg&quot; alt=&quot;Three sets of engines post landing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that some of the engines have been removed for likely testing on the initial Orbcomm mission stage (far left). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just for fun lets contrast that with a brand new stage, all shiny, pretty, and new:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k951n.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k951n.jpg&quot; alt=&quot;Fresh off the lot first stage&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is 'lucky' in that they have landed 4 stages as I write this (Orbcomm OG2, CRS-8, JCSat 14, and Thaicomm 8 missions) in a variety of different states. One landed on land (Orbcomm OG2) in a low velocity mission, so no salt water exposure for a week coming back to port. Three landed on the OCISLY ASDS but from three fairly different mission profiles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The JCSat 14 stage is said to have come in, right at the edge of survivability, and thus has taken the most 'damage' along the way. SpaceX plans to stress that stage to see when it fails and what fails and in what order it fails. Elon Musk has said they will test fire it 10 times to see what work is needed to allow that. Testing the most stressed stage, will give more confidence in reuse to SpaceX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the answer to your question is not yet known, but SpaceX is conducting a large scale experiment to find out, and fix the issues as they come up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;People have observed on the GTO missions (Thaicomm 8, JCSat 14) that the 'boots' get burned up pretty badly on the engines. These are covers that look like Kevlar like bags, covering the gimbals and some of the engine externals, that could do with a little thermal protection.  It is expected that a more temperature resistant material will be coming off the line soon enough for those. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Thaicomm 8 landing, it hit hard, and over extended a leg, such that the leg used up the contigency aluminum crush core. Clearly that will need to be replaced, but that is what it was designed for, so not that big a deal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With each stage recovered SpaceX is learning new things, and likely will integrate fixes and tweaks into new stages as they come off the assembly line, possibly even retrofitting some onto already built stages for coming missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is also in a unique position where the landing is totally a bonus. They are paid to throw the stage away. If they manage to recover it, excellent. If not, no actual financial loss yet. (Once they reduce costs to account for reuse that changes of course). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have the ability to test ideas, see what works, what does not, and narrow down the optimum solution. They have in theory 18 launches planned for 2016 and all are meant to be recovered.  It is unlikely they will get 18 launches down in 2016 but 10 or more seems very likely, so expect lots of learning and minor tweaks to make everything last better based on that experience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that Thaicomm-8 has been moved to the LC-39A HIF you can see all four cores in this image, and you can see how shiny the far right core (Thought to be Orbcomm OG2, getting ready to ship to Hawthorne for static display outside the factory) is, which shows that likely washing was all that was needed to clean up the stage. Though the interstage is still pretty dirty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4bGSe.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4bGSe.jpg&quot; alt=&quot;Four in the barn&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-06-07T13:05:54.570" LastActivityDate="2016-06-07T13:05:54.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16601" PostTypeId="1" AcceptedAnswerId="16603" CreationDate="2016-06-07T07:42:13.173" Score="0" ViewCount="566" Body="&lt;p&gt;How likely is it, that we see Faster Than Light ships in our life-time, Nasa is currently &lt;code&gt;working&lt;/code&gt; (more like figuring the basics out) of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Alcubierre_drive&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alcubierre Drive&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;What's the current probability that we / I (currently 20yrs old) see a ship capable of FTL in our / my life time?&lt;/p&gt;&#xA;&lt;p&gt;sources - 1. &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110015936.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;nasa pdf file&lt;/a&gt;&#xA;2. &lt;a href=&quot;http://www.collective-evolution.com/2014/06/21/nasa-builds-design-for-the-first-faster-than-light-speed-spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;collective evolution&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15683" LastEditorUserId="11902" LastEditDate="2022-07-18T04:54:55.183" LastActivityDate="2022-07-18T04:54:55.183" Title="Faster than Light in our lifetime?" Tags="|spacecraft|alcubierre-drive|ftl|" AnswerCount="1" CommentCount="4" ClosedDate="2016-06-07T14:23:02.550" ContentLicense="CC BY-SA 4.0" />
  <row Id="16602" PostTypeId="1" CreationDate="2016-06-07T08:30:02.330" Score="2" ViewCount="147" Body="&lt;p&gt;The launch of the next crew to ISS with Soyuz MS 1 has been delayed.&#xA;Why the return date of the current crew  on Soyuz TMA 19M remains unchanged?&#xA;An extension of their mission for ca. 2 weeks would have been good for science.&lt;/p&gt;&#xA;" OwnerUserId="13909" LastEditorUserId="51" LastEditDate="2016-06-07T08:48:28.137" LastActivityDate="2016-06-07T12:10:57.490" Title="Soyuz TMA-19M return" Tags="|iss|crewed-spaceflight|soyuz-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16603" PostTypeId="2" ParentId="16601" CreationDate="2016-06-07T08:36:55.643" Score="7" Body="&lt;p&gt;Up to now, everything related to faster-than-light travel is pure speculations. You can do some magic with mathematics, insert negative densities for matter or assume the existence of tachyons traveling faster than light. In theory everything works out, but nobody has ever found such materials. Not even a hint that they might exist has been found besides finding &quot;strange&quot; solutions to equations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole idea is currently not a problem of when it will be done, the major issue is if actual physics is able to support it at all. &lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-06-07T08:36:55.643" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16604" PostTypeId="1" CreationDate="2016-06-07T11:46:25.313" Score="10" ViewCount="1686" Body="&lt;p&gt;The previous question &lt;a href=&quot;https://space.stackexchange.com/q/16588/12102&quot;&gt;How much time does it take to circularize a GTO orbit using ion propulsion?&lt;/a&gt; has a good, &lt;a href=&quot;https://space.stackexchange.com/a/16589/12102&quot;&gt;quantitative answer&lt;/a&gt; based on a known delta-v. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I'm wondering how would you actually aim a continuous ion thrust as you move around in a highly elliptical orbit at the beginning? Is there a way to do this without wasting a lot of propellant? Or do you use the thrust only during a fraction of the orbit (in the beginning) when near periapsis, and therefore it takes longer to accumulate the delta-v because the thruster is actually turned off a lot of the time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a quick, silly calculation showing that for 0.2N and 2200kg, starting at 300km LEO, it takes about 600 days to slowly spiral out to GEO. The satellite rotates once per revolution around the earth, so I have to slow down the rotation rate a teeny tiny bit once in a while to keep them synchronized so the thrust is always tangent to the circular orbit.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Now suppose I want to start from a standard Hohman transfer ellipse (GTO), and run my program again. Periapsis altitude 300km, apoapsis at GEO. Let's assume there's nothing else at GEO to collide with to make it simple.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Q1:&lt;/strong&gt; In order to use the least amount of ion propellant, should the spacecraft be slowly rotating, or have a fixed attitude, perhaps tangent to apoapsis? If rotating, at a constant, or ramped rate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Q2:&lt;/strong&gt; How would one modulate the thrust and attitude to circularize a standard elliptical GTO using the 0.2N ion propulsion in the least amount of time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tvt7z.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tvt7z.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(X, t):&#xA;&#xA;    r, v  = X.reshape(2, -1)&#xA;    vhat  = v / np.sqrt((v**2).sum())&#xA;    rhat  = r / np.sqrt((r**2).sum())&#xA;    rsq   = (r**2).sum()&#xA;&#xA;    acc_grav   = -GMe * rhat  / rsq&#xA;    acc_thrust = vhat * acc   # right now, thrust is just in vhat direction&#xA;&#xA;    return np.hstack((v, acc_grav + acc_thrust))&#xA;&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;GMe = 3.986004418E+14  # m^3/s^2&#xA;&#xA;re  = 6371.E+03   # m&#xA;alt = 300.E+03    # m&#xA;a   = re + alt    # m&#xA;v0  = np.sqrt(GMe / a)  # vis-viva equation for circular&#xA;&#xA;T = 2. * np.pi * a / v0&#xA;print &quot;period (min) &quot;, T/60., &quot; at &quot;, alt, &quot; km altitude.&quot;&#xA;&#xA;acc  = 0.2/2200.  # m/s^2  0.2N and 2200kg&#xA;&#xA;X0   = np.array([a, 0, 0, 0, v0, 0])  # initial state vector&#xA;&#xA;days = np.linspace(0, 600, 100000)&#xA;t    = days * 24. * 3600.&#xA;&#xA;tol  = 1E-10  # good enough for rough answer but not an accurate trajectory&#xA;answer, blob = ODEint(deriv, X0, t, rtol=tol, atol=tol, full_output=True)&#xA;&#xA;xp, yp  = 0.001*answer.T[:2]&#xA;alti_km = np.sqrt(xp**2 + yp**2) - 0.001*re&#xA;&#xA;if 1 == 1:&#xA;    plt.figure()&#xA;    plt.subplot(3, 1, 1)&#xA;    plt.plot(days, xp)&#xA;    plt.plot(days, yp)&#xA;    plt.title('x, y (km)')&#xA;    plt.subplot(3, 1, 2)&#xA;    plt.plot(xp, yp)&#xA;    plt.title('xy (km)')&#xA;    plt.subplot(3, 1, 3)&#xA;    plt.plot(days, alti_km)&#xA;    plt.title('altitude (km)')&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-13T12:53:53.263" LastActivityDate="2021-03-23T01:59:55.507" Title="Thrust and rotation strategy to circularize a standard GTO orbit using ion propulsion?" Tags="|orbital-mechanics|geostationary|hohmann-transfer|python|ion-thruster|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="16605" PostTypeId="2" ParentId="16602" CreationDate="2016-06-07T12:10:57.490" Score="2" Body="&lt;p&gt;Mission was already &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/Principia/New_landing_date_for_ESA_astronaut_Tim_Peake&quot; rel=&quot;nofollow&quot;&gt;extended&lt;/a&gt; for almost two more weeks. If their presence is needed, maybe mission will be extended once more, we don't know yet for sure.&lt;/p&gt;&#xA;" OwnerUserId="6179" LastActivityDate="2016-06-07T12:10:57.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16606" PostTypeId="1" CreationDate="2016-06-07T13:33:14.130" Score="6" ViewCount="1186" Body="&lt;p&gt;Is it possible to have some sort of a spaceship that basically runs on electricity and &quot;butt kicks&quot; itself forward? Or would it stand still because every force has an equal force in the opposite direction?&lt;/p&gt;&#xA;" OwnerUserId="15688" LastEditorUserId="58" LastEditDate="2016-06-07T14:15:57.870" LastActivityDate="2016-06-08T05:17:33.277" Title="Mechanically propelled spaceship?" Tags="|physics|travel|movement|" AnswerCount="3" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="16607" PostTypeId="2" ParentId="16606" CreationDate="2016-06-07T15:15:04.180" Score="12" Body="&lt;p&gt;It would stand still, because of the equal and opposite force rule (aka conservation of momentum). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On earth's surface it's possible to have a self-propelled butt-kicking machine by having a slow &quot;wind-up&quot; phase alternating with an abrupt &quot;kick&quot;. During the wind-up, static friction with a ground surface holds the vehicle in place, but the kick phase can break the frictional threshold. However, the momentum of the system of (ground + vehicle) remains conserved, ultimately using the Earth's vast mass as a &quot;momentum sink&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, with no ground to brace against, the butt-kicking machine goes nowhere.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-06-07T15:15:04.180" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="16608" PostTypeId="1" AcceptedAnswerId="16609" CreationDate="2016-06-07T15:48:35.907" Score="15" ViewCount="1634" Body="&lt;p&gt;I was looking at &lt;a href=&quot;https://space.stackexchange.com/a/632/4660&quot;&gt;this answer&lt;/a&gt; that talks about how a good alternative to &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;Radioisotope Thermoelectric Generators&lt;/a&gt; would be &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_power_in_space#Fission_systems&quot; rel=&quot;noreferrer&quot;&gt;fission reactors&lt;/a&gt;. It makes a number of good points, but in the comment thread there was a discussion of the particular issue that in both cases if there is a worst-case-scenario accident and the launcher burns up in the atmosphere, radioactive material is widely dispersed. From &lt;a href=&quot;http://science.nasa.gov/media/medialibrary/2010/11/05/MSL-FEIS_Vol1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Final Environmental Impact Statement for the Mars Science Laboratory Mission&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The predicted mean radiological dose to the maximally exposed&#xA;  individual from an unlikely launch area accident would be about 0.1&#xA;  rem. No short-term radiological effects would be expected from such an&#xA;  exposure. Each exposure would, however, yield an increase in the&#xA;  statistical likelihood of a latent cancer fatality over the long&#xA;  term... Less likely launch area accidents include explosion on the&#xA;  pad, situations where the spacecraft is detached from the launch&#xA;  vehicle, and accidents where the vehicle safety systems are assumed to&#xA;  fail. The probabilities of these types of accidents range from&#xA;  approximately 1 in 8,000 to 1 in 800,000, and could result in higher&#xA;  mean releases of plutonium dioxide (up to 2 percent of the MMRTG&#xA;  inventory) with the corresponding potential for higher consequences.&#xA;  The maximally exposed individual could receive a dose ranging from a&#xA;  fraction of one rem up to about 30 rem following the more severe types&#xA;  of very unlikely accidents, such as ground impact of the entire launch&#xA;  vehicle.It should be noted that there are large&#xA;  variations and uncertainties in the prediction of close-in dose&#xA;  modeling for such complicated accident situations. Assuming no&#xA;  mitigation actions, such as sheltering and exclusion of people from&#xA;  contaminated land areas, radiation doses to the potentially exposed&#xA;  members of the population from a very unlikely launch accident could&#xA;  result in up to 60 mean additional cancer fatalities over the long&#xA;  term. Results of the risk assessment also indicate that for the very&#xA;  unlikely accident that involves ground impact of the entire launch&#xA;  vehicle, roughly 90 square kilometers (about 35 square miles) of land&#xA;  area could be contaminated above the 0.2 μCi/m 2 screening level.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I asked about this as it wasn't clear to me how the two systems compare, RTGs releasing plutonium and micro fission reactors releasing uranium. The response was that the fresh U-235 fuel in a launched fission reactor is 'a trivial biological risk compared to either RTGs or reactors that have been operating and producing fission products'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How exactly does it compare quantitatively? I accept the statement but it would be nice to know the details, as &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=4806&quot; rel=&quot;noreferrer&quot;&gt;plutonium production for RTGs&lt;/a&gt; has started up again, while fission reactor development languishes.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-06-07T19:17:00.173" Title="How does the launch risk for a plutonium RTG and a uranium fission reactor compare?" Tags="|launch|safety|nuclear|rtg|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16609" PostTypeId="2" ParentId="16608" CreationDate="2016-06-07T17:11:42.993" Score="19" Body="&lt;p&gt;The probability and consequences of a release of Pu-238 from an RTG in a launch accident &lt;a href=&quot;http://science.nasa.gov/media/medialibrary/2010/11/05/MSL-FEIS_Vol1.pdf&quot;&gt;are very low&lt;/a&gt;, due to the protections in place for such an incident. It's not like they never thought of that. The radioactive material is not &quot;widely dispersed&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the numbers, the rate of decay is inversely proportional to the half-life. The half-life of U-235 is 700 million years, and that of the remainder of the fuel, U-238, is 4.6 billion years. The half-life of Pu-238, RTG fuel, is 87 years. So Pu-238 is about eight million times as radioactive as unused fission fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You don't really need to worry about unused fission fuel. The usual ceramic form of the fuel could be used for a dinner plate. A really heavy dinner plate. Even the metal toxicity is not a problem in that case, since the fuel is in its oxide form. Though be careful when stacking a lot of those plates together!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(By the way, I have some uranium kitchen ware -- not fission fuel and not consisting only of uranium oxide, but rather glass tinted green with uranium.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;unused&quot; is important. Once you start up a fission reactor, the fission products are highly radioactive. So don't ever start up a space fission reactor until it's well out in space and not coming back.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-06-07T19:17:00.173" LastActivityDate="2016-06-07T19:17:00.173" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16610" PostTypeId="1" AcceptedAnswerId="16633" CreationDate="2016-06-07T18:08:36.763" Score="9" ViewCount="1688" Body="&lt;p&gt;Specifically: not a &amp;quot;space gun&amp;quot; for launching payloads to space. This subject has been discussed a lot, and I know of the slew of problems and their partial solutions enough.&lt;/p&gt;&#xA;&lt;p&gt;I mean a device mounted on a craft, using electricity for launching macroscopic pieces of metallic reaction mass at hypervelocity, in the opposite direction the craft is meant to move.&lt;/p&gt;&#xA;&lt;p&gt;Railguns operating in atmosphere have limited applications and their own slew of problems. Some of them should vanish in void, while others might remain or get exacerbated. Nevertheless, if &lt;a href=&quot;https://en.wikipedia.org/wiki/Railgun#Tests&quot; rel=&quot;nofollow noreferrer&quot;&gt;EDO-1&lt;/a&gt; ever reached the planned 7000&lt;span class=&quot;math-container&quot;&gt;$\frac{\text{m}}{\text{s}}$&lt;/span&gt; exit velocity, it would constitute an engine of over 700s of &lt;span class=&quot;math-container&quot;&gt;$\text{I}_{sp}$&lt;/span&gt;, and that's surely not the end of it - if we don't strive for heavy projectiles of aerodynamic shape and armor piercing properties, just focus on getting whatever shape ejected as fast as possible, this could see even further improvements.&lt;/p&gt;&#xA;&lt;p&gt;Of course there would be problems. Pulsed propulsion stress, energy supply, heat dissipation and so on. But you have to admit 700s of &lt;span class=&quot;math-container&quot;&gt;$\text{I}_{sp}$&lt;/span&gt; sounds very tempting.&lt;/p&gt;&#xA;&lt;p&gt;And of course I'd be very surprised if I was the first to think of it. So - has this been attempted, developed, researched? Some unforeseen problems that make it totally impossible or otherwise a misguided endeavor?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="47149" LastEditDate="2022-06-01T20:19:20.470" LastActivityDate="2022-06-01T20:19:20.470" Title="Is railgun propulsion being researched?" Tags="|future-missions|advanced-propulsion|research|rail-gun|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="16611" PostTypeId="2" ParentId="16606" CreationDate="2016-06-07T20:00:32.330" Score="0" Body="&lt;p&gt;If you used 1.000.000 cubic meters of steel to create a 1 mm² wire spanning a large distance of 1000 million km you might be able to tug spacecraft along it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not very practical though and the wire might be unstable or snap.&#xA;IF it worked and you could get something like 70 000 N out of it (~70 kg on Earth) that would be quite good though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would always need to have spacecraft going both ways to keep it stable and each craft would have to share the max load with all the others.&lt;/p&gt;&#xA;" OwnerUserId="6350" LastActivityDate="2016-06-07T20:00:32.330" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16612" PostTypeId="2" ParentId="16610" CreationDate="2016-06-07T20:07:32.437" Score="5" Body="&lt;p&gt;I suspect that if you have the electric power available to fire macroscopic projectiles from a railgun, you're better off accelerating Xenon gas with it. Current ion thrusters yield exhaust velocities above 30,000m/s. &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Ion_thruster&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-06-07T20:07:32.437" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16613" PostTypeId="1" CreationDate="2016-06-07T20:16:59.913" Score="2" ViewCount="372" Body="&lt;p&gt;There's enough atmosphere to produce 1 bar of pressure at sea level, and I believe from playing Universe Sandbox, a .320 Gauss magnetic field on the Earth.  Maybe some portion of that would be enough to really protect space travelers well.  How much material would you need to equal the atmosphere and how much power/electricity would you need to produce a similar magnetic field for space travel?  &lt;/p&gt;&#xA;" OwnerUserId="14659" LastActivityDate="2016-06-10T12:39:37.030" Title="How much material AND energy are needed to protect a spacecraft from radiation as well as the Earth does?" Tags="|radiation|magnetic-field|shielding|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16614" PostTypeId="2" ParentId="16613" CreationDate="2016-06-07T22:31:25.047" Score="2" Body="&lt;p&gt;As well as Earth at sea level would be 14.7 lbs per square inch of a low-Z material. (You don't need the magnetic field.) E.g. a 34-foot thick shield of plastic.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-06-07T22:31:25.047" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16615" PostTypeId="1" AcceptedAnswerId="16624" CreationDate="2016-06-08T00:48:19.470" Score="14" ViewCount="944" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=5kZZdp727ek&quot; rel=&quot;noreferrer&quot;&gt;This video from NASA&lt;/a&gt; shows Jeff Williams entering BEAM for the first time. He's wearing a simple dust mask. Why is that? If there is concern about floating particles, wouldn't there be a concern with leaving the hatch ajar?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Qz3ea.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qz3ea.png&quot; alt=&quot;Screenshot from video of Jeff Williams entering BEAM for the first time. Federal government publications are public domain.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14254" LastEditorUserId="4660" LastEditDate="2016-06-08T18:31:06.380" LastActivityDate="2016-06-08T18:31:06.380" Title="Why does the ISS crew wear a dust mask in BEAM?" Tags="|iss|bigelow|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16616" PostTypeId="1" AcceptedAnswerId="16687" CreationDate="2016-06-08T02:02:19.657" Score="2" ViewCount="576" Body="&lt;p&gt;Trying to understand ion thrusters a little, I started looking at &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster#Propellants&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Ion_thruster#Propellants&lt;/a&gt; where I saw:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The CubeSat Ambipolar Thruster (CAT) used on the Mars Array of Ionospheric Research Satellites Using the CubeSat Ambipolar Thruster (MARS-CAT) mission proposes to use solid Iodine as the propellant to minimize storage volume.[42]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Reference [42] says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;MARS-CAT Mission Implementation&quot;. www.marscat.space. University of Houston College of Natural Sciences and Mathematics. Retrieved June 25, 2015.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But that link doesn't go anywhere for me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is a Cubesat Ambipolar Thruster and how does it work? Is this thrust to get the cubesat &lt;em&gt;to Mars&lt;/em&gt;, or &lt;em&gt;to maneuver&lt;/em&gt; in Mars' ionosphere after getting there, or both?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; CAT is definitely written about, a quick search already gives&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pepl.engin.umich.edu/thrusters/CAT.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://pepl.engin.umich.edu/thrusters/CAT.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pepl.engin.umich.edu/pdf/IEPC-2015-243.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://pepl.engin.umich.edu/pdf/IEPC-2015-243.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.phasefour.io/cat-engine.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.phasefour.io/cat-engine.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how close is it to seeing a real cubesat mission to Mars?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-06-22T15:09:05.067" LastActivityDate="2018-06-22T15:09:05.067" Title="MARS-CAT; What is a Cubesat Ambipolar Thruster and how does it work?" Tags="|propulsion|cubesat|ion-thruster|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="16617" PostTypeId="2" ParentId="16606" CreationDate="2016-06-08T04:13:08.810" Score="0" Body="&lt;p&gt;Depending on what you count as a 'mechanical' you could consider Project Orion. &#xA;(Technically a nuke uses nuclear energy, not mechanical but it would be 'pushed' by the explosion)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, detonate a nuke and use it to propel you. So a very hot kick in the butt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://99percentinvisible.org/article/retro-rockets-nuclear-explosion-powered-spaceships-atomic-age/&quot; rel=&quot;nofollow&quot;&gt;http://99percentinvisible.org/article/retro-rockets-nuclear-explosion-powered-spaceships-atomic-age/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15697" LastActivityDate="2016-06-08T04:13:08.810" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16618" PostTypeId="2" ParentId="16596" CreationDate="2016-06-08T05:17:22.113" Score="5" Body="&lt;p&gt;To calculate delta V of low thrust ion spirals, you subtract speed of departure orbit from speed of destination orbit. See &lt;a href=&quot;https://space.stackexchange.com/questions/8420/general-guidelines-for-modeling-a-low-thrust-ion-spiral&quot;&gt;Mark Adler's explanation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time spent is delta V/acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEO is ~7.7 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the edge of earth's Hill sphere, escape velocity is about .7 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So 7 km/s to climb out of earth's gravity well from LEO&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth heliocentric orbit is about 30 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars heliocentric orbit is 24 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So 6 km/s to get from earth to Mars heliocentric orbits&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Escape velocity at the edge of Mars Hill Sphere is .3 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low Mars Orbit velocity is 3.4 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So about 3 km/s to climb down Mars gravity well.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;7 + 6 + 3 is 16 km/s. 16 km/s to get from LEO to LMO via ion engines. In meters, that's about 16,000 meters/sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(16,000 m/s) / (.0004 m/s^2) = 40 million seconds = 463 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the other accelerations, the 1 A.U. to 1.52 A.U. heliocentric trip takes less time than Hohmann and delta V will be higher than 6 km/s. I can't give you the times for the other accelerations without investing more time and effort than I can afford at the moment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, climbing in and out of planetary gravity wells takes more delta V (and therefore time) than doing the heliocentric transfer orbit.  Which is why I advocate berthing a &lt;a href=&quot;https://www.youtube.com/watch?v=khIHZp_GTEI&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hermes&lt;/a&gt; like craft at &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;EML2&lt;/a&gt; between trips. At the Mars end of the trip, &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2016/01/deimos-tether.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deimos&lt;/a&gt; might be good place to berth an ion propelled craft.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-06-08T05:17:22.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16620" PostTypeId="1" CreationDate="2016-06-08T08:33:47.327" Score="1" ViewCount="82" Body="&lt;p&gt;Xenon is the most common, most popular propellant for ion engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But AFAIK, any ionizable gas could do - the ion engine accelerates charged particles, it doesn't really care what these particles are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, Xenon is way down below the periodic table; its atoms quite heavy compared to &quot;competitors&quot;, and that means reduction of specific impulse - applying the same energy to a lighter particle you're attaining higher exhaust velocity (at cost of thrust) - and ion engines are all about extreme ISp, so choice of a gas with so heavy particles seems quite counter-intuitive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What properties of Xenon make it the propellant of choice for ion engines?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-06-08T08:33:47.327" Title="What's so special about Xenon?" Tags="|ion-thruster|" AnswerCount="0" CommentCount="0" ClosedDate="2016-06-08T09:35:52.997" ContentLicense="CC BY-SA 3.0" />
  <row Id="16622" PostTypeId="1" AcceptedAnswerId="16639" CreationDate="2016-06-08T11:30:12.403" Score="6" ViewCount="1855" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; At the &lt;a href=&quot;http://sdo.gsfc.nasa.gov/data/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Dynamics Observatory (SDO)&lt;/a&gt; website, I just found the image &lt;a href=&quot;http://sdo.gsfc.nasa.gov/assets/img/latest/latest_1024_HMIIC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;sdo.gsfc.nasa.gov/assets/img/latest/latest_1024_HMIIC.jpg&lt;/a&gt;. The color gradient of the limb darkening seems very similar to the Wikimedia image below. I've discovered that it is called a &quot;colorized intensitygram&quot; and the color gradient is purely artificial - the data is single channel intensity. The limb darkening is certainly real (compare to the artificially &quot;flattented&quot; display!)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.solarham.net/latest_imagery/hmi1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.solarham.net/latest_imagery/hmi1.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/a8KdC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/a8KdC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to &lt;em&gt;try to start to&lt;/em&gt; understand what the sun actually &quot;looks like&quot;, with human vision, assuming the brightness has been reduced. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like the gas giant planets, the sun doesn't have an abrupt surface, it just gets denser and hotter and denser and hotter as you go deeper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One consequence of this is &lt;a href=&quot;https://en.wikipedia.org/wiki/Limb_darkening&quot; rel=&quot;nofollow noreferrer&quot;&gt;limb-darkening&lt;/a&gt;. As the material becomes denser the deeper you go, it becomes more opaque, so - roughly speaking - the light you see, including the color and brightness, is determined by the layers above that point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at center of the solar disk, you can see deeper and hotter parts. If you look near the edge of the sun, or solar limb, the incidence is oblique and you are not seeing as deep or as hot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar physics and photon transport theory is complicated, but in the visible part of the spectrum it may be OK to think of blue light scattering much more strongly than red light. In the thick, dense solar atmosphere, it scatters enough to attenuate. So in blue light you are seeing even shallower, which is even colder, and therefore dimmer in blue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Right now I'd just like some good analytical approximation of the wavelength dependent limb darkening of the sun, or some actual linear images (before web-processing) of the sun at various visible wavelengths so I can make my own approximation&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an image from Wiki&lt;em&gt;media&lt;/em&gt; titled &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/2012_Transit_of_Venus_from_SF.jpg/600px-2012_Transit_of_Venus_from_SF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;2012_Transit_of_Venus_from_SF&lt;/a&gt; which I've separated into RGB channels and plotted brightness across horizontal (solid) and vertical (dashed) diameters, 20 pixels wide. You can see the dramatic difference in behavior of different wavelengths. Since the image is &quot;from the internet&quot; I've no information about the linearity, so it's an illustrative example, but not what you'd think of as data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FrDdg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FrDdg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;img = plt.imread(&quot;sun limb darkening.png&quot;) &#xA;# FROM: https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/2012_Transit_of_Venus_from_SF.jpg/600px-2012_Transit_of_Venus_from_SF.jpg&#xA;&#xA;w, c, hw  = 600, 300, 10   # image happens to be 600x600 pixels square!&#xA;&#xA;hor = img[    c-hw:c+hw].sum(axis=0) / (2*hw)&#xA;ver = img[ :, c-hw:c+hw].sum(axis=1) / (2*hw)&#xA;&#xA;plt.figure()&#xA;&#xA;plt.imshow(img)&#xA;&#xA;r, g, b = c - 0.67*c*hor.T&#xA;&#xA;plt.plot(r, '-r')    &#xA;plt.plot(g, '-g')    &#xA;plt.plot(b, '-b')    &#xA;&#xA;r, g, b = c - 0.67*c*ver.T&#xA;&#xA;plt.plot(r, '--r')    &#xA;plt.plot(g, '--g')    &#xA;plt.plot(b, '--b')&#xA;&#xA;plt.plot([0, w], [c, c], '-k')&#xA;plt.xlim(0, 599)&#xA;plt.ylim(599, 0)&#xA;&#xA;plt.text(150, 137, 'R', fontsize=18)&#xA;plt.text(156, 192, 'G', fontsize=18)&#xA;plt.text(170, 272, 'B', fontsize=18)&#xA;&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-11T08:47:53.820" LastActivityDate="2017-04-11T08:47:53.820" Title="Need help simulating solar limb darkening" Tags="|the-sun|python|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="16623" PostTypeId="2" ParentId="15559" CreationDate="2016-06-08T11:57:56.943" Score="0" Body="&lt;p&gt;It wouldn't work. Heat is a form of energy, but thermodynamics means that not all forms of energy are equally valuable. Any form of energy can be converted to heat, but you need temperature &lt;em&gt;differences&lt;/em&gt; to turn a fraction of the heat into usable energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hypothetical moon has only a limited amount of heat generation, which also means there are only very small heat differences. Life needs energy in other forms: light for plants, chemical for animals. Converting a very small fraction of a little bit of heat simply can't provide that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tidal heat generation occurs most strongly in liquids (oceans), but those are also the environments in which temperature differences are hardest to obtain. The combination of tidal forces and temperature/density differences drive currents which causes mixing, and thus a decrease in temperature differences.   &lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2016-06-08T11:57:56.943" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="16624" PostTypeId="2" ParentId="16615" CreationDate="2016-06-08T13:14:03.103" Score="9" Body="&lt;p&gt;The planetary society had a nice article on this: &lt;a href=&quot;http://www.planetary.org/blogs/jason-davis/2016/20160606-first-look-in-beam.html&quot;&gt;http://www.planetary.org/blogs/jason-davis/2016/20160606-first-look-in-beam.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Williams and Skripochka wore face masks as they entered, which is standard procedure for initial inspections of cargo ships visiting the station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="15702" LastEditorUserId="3185" LastEditDate="2016-06-08T14:12:06.047" LastActivityDate="2016-06-08T14:12:06.047" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16626" PostTypeId="2" ParentId="15559" CreationDate="2016-06-08T15:41:51.127" Score="3" Body="&lt;p&gt;If the right chemical bath spews from hydrothermal vents on the floor of an ocean on this moon, then perhaps it could host creatures like the &lt;a href=&quot;http://ocean.si.edu/ocean-videos/hydrothermal-vent-creatures&quot; rel=&quot;nofollow&quot;&gt;ones found around such vents on Earth&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a quote from &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2001/ast13apr_1/&quot; rel=&quot;nofollow&quot;&gt;an article on the subject&lt;/a&gt; in NASA Science News:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Instead of photosynthesis, vent ecosystems derive their energy from&#xA;  chemicals in a process called &quot;chemosynthesis.&quot; Both methods involve&#xA;  an energy source, carbon dioxide, and water to produce sugars.   Photosynthesis gives off oxygen gas as a byproduct, while&#xA;  chemosynthesis produces sulfur... Because they offer an alternative way for&#xA;  life to meet its fundamental need for energy, these vent ecosystems&#xA;  have piqued the interest of astrobiologists -- scientists who study&#xA;  the plausibility of life starting elsewhere in the universe.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The level of tidal heating required for &lt;a href=&quot;https://en.wikipedia.org/wiki/Io_(moon)#Tidal_heating&quot; rel=&quot;nofollow&quot;&gt;volcanic activity on that level&lt;/a&gt; is seen in our solar system on Io, Jupiter's inner moon. In Io's case, it would not remain in an orbit eccentric enough for these forces to act on it in this way if the other large moons of Jupiter weren't preventing its orbit from circularizing. So for such a model there would need to be several moons around this rogue planet, orbiting in a particular way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may also be conditions under which rogue planets could support life without any moons involved, as investigated in the article &lt;a href=&quot;http://iopscience.iop.org/article/10.1088/2041-8205/735/2/L27/meta&quot; rel=&quot;nofollow&quot;&gt;The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We find that a rogue planet of Earth-like composition and age could&#xA;  maintain a subglacial liquid ocean if it were ≈3.5 times more massive&#xA;  than Earth, corresponding to ≈8 km of ice. Suppression of the melting&#xA;  point by contaminants, a layer of frozen gas, or a larger complement&#xA;  of water could significantly reduce the planetary mass that is&#xA;  required to maintain a liquid ocean.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-06-08T16:47:11.073" LastActivityDate="2016-06-08T16:47:11.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16627" PostTypeId="2" ParentId="16610" CreationDate="2016-06-08T17:24:31.867" Score="2" Body="&lt;p&gt;From a science fiction perspective, there is a book &lt;a href=&quot;https://en.wikipedia.org/wiki/Heart_of_the_Comet&quot; rel=&quot;nofollow&quot;&gt;Heart of the Comet&lt;/a&gt; by Gregory Benford and David Brin about an attempt to corral Halleys comet into Earth orbit. At apogee, they use mass-slingers, basically railguns firing mined mass from the comet to transfer momentum and 'fly the comet'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Great story regardless, but Brin and Benford are actual scientists, who often pay at least superficial care to the science in the story.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-08T17:24:31.867" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16628" PostTypeId="2" ParentId="16615" CreationDate="2016-06-08T17:37:30.750" Score="11" Body="&lt;p&gt;They also wear the masks when they add a new module to the station.  Less so now, with assembly mostly complete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, when they are packed, all sorts of stuff settles, no matter how clean-room an approach is taken. During launch lots of disturbances in the Force (gravity forces) occur, and stuff gets shaken around a lot, and they are unsure what will be in their faces when they open it. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="4660" LastEditDate="2016-06-08T18:30:31.003" LastActivityDate="2016-06-08T18:30:31.003" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16629" PostTypeId="1" AcceptedAnswerId="16630" CreationDate="2016-06-08T21:41:35.303" Score="4" ViewCount="92" Body="&lt;p&gt;When was the first &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravimetry&quot; rel=&quot;nofollow&quot;&gt;gravitational map&lt;/a&gt; of a planet or moon made by measurements from space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm guessing it was a map of Earth, since that's the closest planet to us,&lt;sup&gt;[citation needed]&lt;/sup&gt; but for all I've been able to learn, we might have already mapped it sufficiently from the ground.&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="4660" LastEditDate="2016-06-08T22:24:31.843" LastActivityDate="2016-06-09T00:37:19.353" Title="Earliest gravitational map of a planet/moon?" Tags="|gravity|history|measurement|geology|mapping|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16630" PostTypeId="2" ParentId="16629" CreationDate="2016-06-09T00:37:19.353" Score="3" Body="&lt;p&gt;It was the Earth, of course. As early as 1964, scientists realized that the non-Keplerian orbits satellites gave a picture of the Earth's interior. For example, see S. K. Runcorn, &quot;Satellite gravity measurements and a laminar viscous flow model of the earth's mantle,&quot; &lt;em&gt;Journal of Geophysical Research&lt;/em&gt; 69.20 (1964): 4389-4394.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-06-09T00:37:19.353" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16631" PostTypeId="2" ParentId="16596" CreationDate="2016-06-09T02:14:22.207" Score="1" Body="&lt;p&gt;Maybe you watched the movie &quot;The Martian&quot;? Well, most of that was pretty accurate (most egregious silliness was the duct tape and tarp hole repair, but I digress). The point mentioning that is that they were using a &lt;em&gt;lot&lt;/em&gt; of computer time to come up with orbital solutions, and IIRC those were ballistic. (Ballistics is much much much easier than non-ballistic dynamics.) So, you are asking a really difficult question. What you want to do is to arrive at Mars' orbit exactly when Mars is there and with exactly zero velocity, right? Can you see how that depends on when you start? (Earth is orbiting the Sun, so when you start will define what time you need to get to that zero velocity just when Mars passes by. You can't just pick &quot;any old time&quot; of the year because depending on what year, Mars could be anywhere in its orbit. It would be a shame if you missed Mars by a day, and had to wait another whole Martian year &amp;mdash; 686 days.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK now that I've quashed your dreams of an easy answer, have you considered &lt;a href=&quot;http://orbit.medphys.ucl.ac.uk&quot; rel=&quot;nofollow&quot;&gt;Orbiter&lt;/a&gt;, a (free) software simulator? It's not a game, but you can pilot your own spaceship to Mars.&lt;/p&gt;&#xA;" OwnerUserId="15706" LastEditorUserId="6816" LastEditDate="2016-06-09T02:22:04.590" LastActivityDate="2016-06-09T02:22:04.590" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16633" PostTypeId="2" ParentId="16610" CreationDate="2016-06-09T07:55:03.987" Score="5" Body="&lt;p&gt;Yes, Made on Space is researching a similar idea.   &lt;a href=&quot;http://www.space.com/33079-turning-asteroids-into-spaceships-made-in-space.html&quot; rel=&quot;noreferrer&quot;&gt;Plan to Turn Asteroids Into Spaceships&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the propulsion system might be some sort of catapult that launches boulders or other material off the asteroid in a controlled way, thereby pushing the space rock in the opposite direction (as described by Newton's Third Law of Motion)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While the Made in Space plan involves much simpler forms of object expulsion then a rail gun to create propulsion, the basic principles still apply.&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2016-06-09T07:55:03.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16634" PostTypeId="1" AcceptedAnswerId="16638" CreationDate="2016-06-09T10:45:56.050" Score="6" ViewCount="597" Body="&lt;p&gt;I'm not a physicist or an engineer, so my apologies if I messed up any technical details. I've looked through the SE and found a few questions that touch on this issue (&lt;a href=&quot;https://space.stackexchange.com/questions/6585/orbital-altitudes-are-some-better-than-others-and-why/6591#6591&quot;&gt;this one&lt;/a&gt; gets at the benefits of altitudes; &lt;a href=&quot;https://space.stackexchange.com/questions/8257/launch-altitude-and-fuel-cost&quot;&gt;this one&lt;/a&gt; at the fuel costs associated with launching &lt;em&gt;from&lt;/em&gt; different altitudes; &lt;a href=&quot;https://space.stackexchange.com/questions/10837/why-are-the-gps-constellation-satellites-in-such-a-high-orbit/10840#10840&quot;&gt;this one&lt;/a&gt; is again about the benefits of different altitudes; &lt;a href=&quot;https://space.stackexchange.com/questions/11981/is-a-fluorine-hydrogen-engine-feasible-for-use-in-space/11982#11982&quot;&gt;this one&lt;/a&gt; gets at stationkeeping fuel costs), but I haven't been able to put an answer together from them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a firm trying to launch a payload into orbit. The firm is trying to choose between two identically-shaped orbits at different altitudes, H and L (H&gt;L). The orbits are not &quot;too far&quot; from each other, maybe H=L+150km. There are two fuel costs associated with H and L. One is the launch fuel cost, l(), where l(H) &gt; l(L). The other is the stationkeeping fuel cost, where s(H) &amp;lt; s(L). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(My understanding is that the coverage area g() of orbit H will be greater than the coverage area of orbit L, i.e. g(H)&gt;g(L), but I'm mainly interested in the fuel costs for now.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question:&lt;/strong&gt; Is it possible to unambiguously rank the fuel costs associated with these orbits? That is, can I make a statement like, &quot;l(H) + s(H) &gt; l(L) + s(L)&quot;? From what I've read, it seems like the fuel mass required for launch is orders of magnitude greater than the fuel mass required for stationkeeping, but I haven't been able to figure out the associated costs per unit of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine this will depend on the altitudes H and L. I'm thinking about relatively low orbits, maybe L=700km. I've used the transfer orbit calculator &lt;a href=&quot;http://www.satsig.net/orbit-research/delta-v-geo-injection-calculator.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; to get an idea of the magnitudes involved, but I'm not sure how to convert the delta-V difference (~40m/s for 700km to 850km) into $ cost, and I haven't been able to figure out the stationkeeping fuel requirement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any thoughts or suggestions would be much appreciated. Thank you!&lt;/p&gt;&#xA;" OwnerUserId="15707" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-08-06T02:19:37.693" Title="How do launch and stationkeeping fuel costs vary with altitude?" Tags="|launch|low-earth-orbit|cost-to-orbit|station-keeping|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16635" PostTypeId="1" CreationDate="2016-06-09T12:28:45.943" Score="1" ViewCount="354" Body="&lt;p&gt;Take a very light object, propel it with a laser source on Earth at a constant acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can this object reach the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could this be more reliable than a rocket? (supposing solar panel energy to power the laser.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yea the idea its use a constant thrust to get out the LEO and go on the the nearest corpus, and not accelerate to get escape velocity...&lt;/p&gt;&#xA;" OwnerUserId="14412" LastEditorUserId="14412" LastEditDate="2016-06-13T14:24:32.753" LastActivityDate="2016-06-13T14:24:32.753" Title="Can you launch a probe to the Moon using a laser propelled craft?" Tags="|launch|the-moon|laser|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="16638" PostTypeId="2" ParentId="16634" CreationDate="2016-06-09T14:34:10.490" Score="7" Body="&lt;p&gt;Your question is hard to answer because it's the reverse of how things are normally done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typically satellite missions are driven first by the orbital and lifetime requirements, which determines the needed amount of stationkeeping fuel, which determines launch mass of the satellite, which constrains the selection of launcher and thus provider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Selection of the launcher in conjunction with payload mass and destination orbit determines how much fuel will actually be loaded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what really sets the cost is negotiation between the satellite owner and the launch provider. The number of future launches the satellite owner can offer to the provider has a much larger impact on the launch cost than the altitude of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, a 40 m/s difference in ∆v is about 0.4% of the total ∆v budget for orbital launch. ∆v changes aren't linear with fuel mass; at the margin the slope might be 3-5x linear, so you're looking at 1%-2% marginal increase in fuel load. That should let you make an estimate, but fuel costs are rarely given to more than 2 places of precision, and &quot;around \$400,000&quot; times 1.02 is still &quot;around \$400,000&quot;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-08-06T02:19:37.693" LastActivityDate="2016-08-06T02:19:37.693" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16639" PostTypeId="2" ParentId="16622" CreationDate="2016-06-10T02:51:11.163" Score="4" Body="&lt;p&gt;I used the paper &lt;a href=&quot;http://www.physics.hmc.edu/faculty/esin/a101/limbdarkening.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Wavelength dependency of the Solar limb darkening&lt;/em&gt;&lt;/a&gt; for solar limb darkening data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It uses the following model for the normalized brightness distribution across the disk of the Sun:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;I(\mu)=1-u(1-\mu^\alpha)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, $\mu$ is the normalized distance from the limb; when far from the Sun, it can be expressed in terms of the normalized distance from the center of the disk, $r$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mu = \sqrt{1-r^2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$u$ and $\alpha$ are parameters.  The paper is not very clear about the value of $u$, but it seems like they used $u=0.85$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This formula tells us how the brightness varies from the center of the solar disk to the limb for a given wavelength.  In order to properly represent the color of the Sun, we also need to know the relative brightness of different wavelength bands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this we need data on the solar spectrum, which I obtained from &lt;a href=&quot;http://rredc.nrel.gov/solar/spectra/am0/ASTM2000.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (this data is for &quot;air mass zero,&quot; meaning our result will be what the Sun would look like outside the Earth's atmosphere).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume that this spectrum is averaged over the whole disk, in which case we need to be able to scale the limb darkening model by average instead of peak (center) brightness.  The formula given in the paper is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;I_\text{avg} \propto \int_0^1 I(\mu)\mu\ d\mu \propto \frac{2 + \alpha (1 - u)}{2 + \alpha}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last piece of the puzzle is to convert the spectrum to a color.  In this case I first compute the color in the CIE XYZ color space, then transform to linear RGB and then sRGB.  (See &lt;a href=&quot;https://photo.stackexchange.com/a/75590/39450&quot;&gt;this answer&lt;/a&gt; I posted on photography.SE for more details about computing the XYZ color of a spectrum, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/SRGB#The_forward_transformation_.28CIE_xyY_or_CIE_XYZ_to_sRGB.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;sRGB Wikipedia page&lt;/a&gt; for more detail about the conversion to sRGB.)  I used the standard colorimetric observer tables from &lt;a href=&quot;http://www.cis.rit.edu/research/mcsl2/online/CIE/StdObsFuncs.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here is the &lt;em&gt;Mathematica&lt;/em&gt; code that I used:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;alphaRaw = ImportString[(* copy and paste from the PDF table *), &quot;Table&quot;];&#xA;(* remove headers *)&#xA;Select[ArrayQ] @ SplitBy[alphaRaw, Head@*First];&#xA;(* join corresponding columns *)&#xA;Flatten[Partition[%, 2], {{2}, {1, 3}}];&#xA;(* resample to 1 nm *)&#xA;{alphaPS, alphaNL} = (Interpolation[#] /@ Range[360, 830]) &amp;amp; /@ %;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Nh2eT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;spectrumRaw = Import[&quot;http://rredc.nrel.gov/solar/spectra/am0/E490_00a_AM0.xls&quot;];&#xA;(* resample to 1 nm (inputs in um!) *)&#xA;spectrum = Interpolation[spectrumRaw[[1, 2 ;;, ;; 2]]] /@ (Range[360, 830] / 1000);&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2KGXL.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;cieRaw = Import[&quot;http://www.cis.rit.edu/research/mcsl2/online/CIE/StdObsFuncs.xls&quot;];&#xA;(* pair each color function column with the wavelength column *)&#xA;Transpose[{cieRaw[[1, 6 ;;, 2 ;; 4]]], #}] &amp;amp; /@ cieRaw[[1, 6 ;;, 2 ;; 4]]];&#xA;(* resample to 1 nm *)&#xA;cieXYZ = (Interpolation[#] /@ Range[360, 830]) &amp;amp; /@ %;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xKNkk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;With[{u = 0.85},&#xA;  (* compute color at 101 points evenly spaced in mu *)&#xA;  colorXYZ = Transpose @ Table[&#xA;               Total[ (* integration -&amp;gt; summation *)&#xA;                 Transpose @ cieXYZ&#xA;                 * spectrum&#xA;                 * (1 - u (1 - mu^alphaPS))&#xA;                 / ((2 + alphaPS (1 - u)) / (2 + alphaPS))&#xA;               ],&#xA;               {mu, 0, 1, 0.01}&#xA;             ];&#xA;  (* scale so maximum = 1 *)&#xA;  colorXYZ /= Max[colorXYZ / {0.97, 1, 0.83}];&#xA;  (* dim by 15% *)&#xA;  colorXYZ *= 0.85;&#xA;]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/hFf8y.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I'm not sure why &lt;em&gt;Mathematica&lt;/em&gt; renders the XYZ colors with a blue tint.)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;colorRGBlinear = {{3.2406, -1.5372, -0.4986}, {-0.9689, 1.8758, 0.0415}, {0.0557, -0.2040, 1.0570}}.colorXYZ;&#xA;colorRGBlinear /= Max[colorRGBlinear];&#xA;colorRGBlinear *= 0.85;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/PiksP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;gamma = With[{a = 0.055}, If[# &amp;lt;= 0.0031308, 12.92 #, (1 + a) #^(1/2.4) - a] &amp;amp;];&#xA;colorSRGB = Map[gamma, colorRGBlinear, {2}];&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MwI2y.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plot of the color difference (in CIE xy coordinates) from the limb to center (small black arrow).  The ticks on the spectral locus are wavelength in nm, and the ticks on the Planckian locus are temperature in K.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/X6NDk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="-1" LastEditDate="2017-04-13T12:44:06.683" LastActivityDate="2016-06-10T17:40:02.067" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="16641" PostTypeId="2" ParentId="16635" CreationDate="2016-06-10T08:09:46.193" Score="3" Body="&lt;p&gt;Theoretically it is possible to launch a spacecraft using a laser. In practice this is very hard to do and the technology is nowhere near being able to launch anything. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem you will have getting a very light spacecraft to the moon is gravity. You have to escape Earth's gravity and then negate the effects of the moon's. When you go to the moon from the Earth there is a point where the gravitational forces are exactly the same, called the neutral point. Once past the neutral point the moon's gravity is stronger and your probe will get pulled into the moon, and if you don't have some sort of propulsion to slow it down it will be drawn in faster and faster and impact on the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a very light spacecraft means no propulsion system then it's going to and make its own crater on the surface.&lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-06-10T08:09:46.193" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16643" PostTypeId="2" ParentId="16635" CreationDate="2016-06-10T11:20:07.520" Score="2" Body="&lt;p&gt;I think by &quot;Can this object reach the Moon?&quot; you are asking the wrong question. In order for something to reach a given destination the only parameters are its velocity and the distance. Even something traveling only at 1mm/s could reach the moon eventually (or any other target in that matter).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you look for example at the &lt;a href=&quot;http://sail.planetary.org/&quot; rel=&quot;nofollow&quot;&gt;LightSail&lt;/a&gt; project from the Planetary Society, they are using the momentum of the photons that are coming from the sun. This gives them a rather &quot;low&quot; acceleration of a few mm*s^(-2) but since they don't need any propelant (or fuel) they can accelerate basically for an infinite amount of time, allowing them to reach much higher velociities (given enough time) than rockets relaying on some kind of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This makes something like the LightSail a very good alternative to rockets when you try to reach object further away, since a rocket would need a lot of fuel, while the light sail does not need any. However, for close targets like the moon, a rocket would be much faster and probably also more ecnomic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a laser stationed on earth you can increase the acceleration of the satelite that has a light sail by quite a bunch and therefor make it even a more viable alternative to classic rocket propulsion. But still for close targets like the moon rockets will probably be the best tech to go for in the near future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that also the Starshot project plans on using lightsails with a stationary laser station on earth to accelerate their light sails to around 20% of light speed, with that we could reach alpha centauri in around 20 years, or pluto in only around a day!&lt;/p&gt;&#xA;" OwnerUserId="13141" LastActivityDate="2016-06-10T11:20:07.520" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16644" PostTypeId="1" AcceptedAnswerId="16645" CreationDate="2016-06-10T11:29:17.877" Score="18" ViewCount="1941" Body="&lt;p&gt;I've been enjoying the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizons web interface&lt;/a&gt; and after I discovered the incredibly extensive database associated with SOHO (&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory&quot; rel=&quot;noreferrer&quot;&gt;Solar and Heliospheric Observatory&lt;/a&gt;, also see &lt;a href=&quot;http://sohowww.nascom.nasa.gov/about/about.html&quot; rel=&quot;noreferrer&quot;&gt;sohowww.nascom.nasa.gov&lt;/a&gt;) for &lt;a href=&quot;https://space.stackexchange.com/a/15429/12102&quot;&gt;this answer&lt;/a&gt; I decided to learn more about the satellite's orbit.&lt;/p&gt;&#xA;&lt;p&gt;I exported a dataset of historical (and projected) orbit data for &lt;code&gt;SOHO Spacecraft&lt;/code&gt; in 1 day steps using the following setup:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wp2gM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wp2gM.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I also exported the positions of &lt;code&gt;the Sun&lt;/code&gt;, and of &lt;code&gt;the Earth-Moon Barycenter&lt;/code&gt; the same way.&lt;/p&gt;&#xA;&lt;p&gt;Using the reduced mass of the Sun vs Earth-Moon Barycenter (EMB) system as:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mu = \frac{m_{earth}+m_{moon}}{m_{sun}+m_{earth}+m_{moon}}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;I used the equations of motion in a rotating frame, and solved numerically for zero acceleration along the EMB-sun line (y, z set to zero):&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ r_1=\sqrt{(x+\mu)^2} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ r_2=\sqrt{(x-1+\mu)^2} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \ddot x=x-\frac{(1-\mu)(x+\mu)}{{r_1}^3}-\frac{\mu(x-1+\mu)}{{r_2}^3} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;I found a zero at &lt;code&gt;x = 0.989986&lt;/code&gt; which is about 99% of the distance from the Sun-EMB Barycenter (basically the Sun) to the  EMB, or about 1.5 million km from the earth towards the sun.&lt;/p&gt;&#xA;&lt;p&gt;So I made a point and called it &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; and defined it as this location, so as the EMB slowly moves closer and farther from the sun over the year, my &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; 'origin' will breath with it.&lt;/p&gt;&#xA;&lt;p&gt;Finally, I un-rotated the position so that the Sun-EMB line stayed fixed with the sun on the left, so that it looks like the standard configuration for CR3BP problems.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OL8gz.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OL8gz.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I got a pretty nice looking trajectory! The plot isn't breathtaking - limitations of Matplotlib and the (overly-)restrictive SE 2MB limit for GIFs.&lt;/p&gt;&#xA;&lt;p&gt;Note, x, y, and z scales are all different - this is plotted at the moment just to fit into a cube. The blob (red) on the left in the middle of the orbit is &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; as defined above, and the little cigar-shaped blob (blue) on the right represents the motion of the EMB in this particular frame. I did look at a plot with EMB-fixed instead of &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt;, but the orbit became thicker. This way is really better looking, as you'd expect. The little black dots are the first and last 20 days (data points) in the extracted dataset.&lt;/p&gt;&#xA;&lt;p&gt;If you look closely at the &amp;quot;top&amp;quot; there's a little blip, and excursion toward the sun. I think this is the famous, frightening episode in June to November 1998 when SOHO was almost lost. There is a &lt;a href=&quot;http://sohowww.nascom.nasa.gov/about/Recovery/docs/index.html&quot; rel=&quot;noreferrer&quot;&gt;page of recovery docs&lt;/a&gt; or you can read about it in Aerospace America May 1999: &lt;a href=&quot;http://sohowww.nascom.nasa.gov/operations/Recovery/aerospace.pdf&quot; rel=&quot;noreferrer&quot;&gt;Saving SOHO&lt;/a&gt; or the article by ESA's F.C. Vandenbussche &lt;a href=&quot;http://sohowww.nascom.nasa.gov/operations/Recovery/vandenbu.pdf&quot; rel=&quot;noreferrer&quot;&gt;SOHO’s Recovery – An Unprecedented Success Story&lt;/a&gt; or for more of the technical details; Roberts 2002 &lt;a href=&quot;http://www.ieec.cat/hosted/web-libpoint/papers/roberts.pdf&quot; rel=&quot;noreferrer&quot;&gt;The SOHO Mission L1 Halo Orbit Recovery&#xA;From the Attitude Control Anomalies of 1998&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;I wanted to see if I could find the event, and other stationkeeping maneuvers in the raw data. So, just to see what happened, I plotted the variation of x, y and z in my coordinates over the interval 3-dec-1996 to 4-jul-2016, which is all Horizons would give me on the day I downloaded it. Apparently it propagates the state vector a few weeks into the future.&lt;/p&gt;&#xA;&lt;p&gt;To look for maneuvers, I plotted the first, second and third finite difference - a numerical way of approximating derivatives. Here's what I got:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lW2oc.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lW2oc.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b5fYU.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b5fYU.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the 2nd and 3rd difference plots, the approximately harmonic motion nearly disappears, and discontinuities become prominent. I was assuming the data was showing me station keeping (SK) maneuvers. I noticed a big zig-zag around mid-October 2014. I found &lt;a href=&quot;http://sohowww.nascom.nasa.gov/soc/soho_events/SOHO-Spacecraft-Events.pdf&quot; rel=&quot;noreferrer&quot;&gt;SOHO-Spacecraft-Events.pdf&lt;/a&gt; but strangely there are no propulsive maneuvers listed around that time.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;click to enlarge:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fEvFU.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fEvFU.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;So my question is: are these zig-zags (and in particular, the one around October 15, 2014) just stitching errors in the reconstructed data, or did a particle of dark matter collide with a Higgs boson and SOHO can report the first observation&lt;/em&gt;? (humor).&lt;/p&gt;&#xA;&lt;p&gt;These are also from the &lt;a href=&quot;http://sohowww.nascom.nasa.gov/about/about.html&quot; rel=&quot;noreferrer&quot;&gt;sohowww.nascom.nasa.gov&lt;/a&gt; site:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YD6c6.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YD6c6.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JgjEA.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JgjEA.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-07-06T05:33:08.517" LastActivityDate="2020-07-06T05:33:08.517" Title="Is this what station keeping maneuvers look like, or just glitches in data? (SOHO via Horizons)" Tags="|lagrangian-points|station-keeping|jpl-horizons|halo-orbit|soho|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="16645" PostTypeId="2" ParentId="16644" CreationDate="2016-06-10T11:51:46.360" Score="22" Body="&lt;p&gt;You are most likely seeing an artifact of how JPL represents its ephemerides for fast numerical computation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;JPL integrates the equations of motion over time. This inevitably results in mismatches between the integrated state and observations. These errors are used to adjust initial states and the integration is then re-performed. The cycle stops when the errors drop below some threshold. JPL then computes sets of Chebyshev polynomial coefficients that approximately yield the integrated state. Each set of coefficients pertains to some specific span of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Multiple sets are used to enable coverage of longer spans of time. (An alternative would be to use higher order polynomials, but that will almost inevitably run into serious numerical computation issues. Higher order does not necessarily mean greater accuracy.) JPL tries to make the transitions from one set to another be continuous, definitely with regard to position, possibly with regard to velocity, but not to second and third derivatives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your third differences are magnifying the discontinuities in those transitions from one set of coefficients to another. I doubt there's any physical significance to those spikes.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-06-10T11:57:52.633" LastActivityDate="2016-06-10T11:57:52.633" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16647" PostTypeId="1" CreationDate="2016-06-10T16:41:03.033" Score="3" ViewCount="285" Body="&lt;p&gt;Current answers at the new question &lt;a href=&quot;https://space.stackexchange.com/questions/16635/can-you-launch-a-probe-to-the-moon-using-a-laser-propelled-craft&quot;&gt;Can you launch a probe to the Moon using a laser propelled craft?&lt;/a&gt; points out getting to the moon is not the problem, landing on it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2741/could-a-human-reach-escape-velocity-by-jumping-from-the-surface-of-ceres-a-dwar/2777#2777&quot;&gt;This answer&lt;/a&gt; suggests that you can achieve escape velocity from Deimos using a bike and a ramp.  If you can launch a bike presumably you can land it safely.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In theory you could use a light sail to put yourself in orbit around Deimos. You should also then be able to use it to lower your orbital speed so you touch down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to use a light sail to achieve a touch down survivable for a human? (Not sure what this would be, I saw several limits, but not sure how to translate that into something here)&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-06-18T20:42:05.270" Title="Can you use a light sail to achieve a survivable landing anyplace in our solar system?" Tags="|landing|solar-sail|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16648" PostTypeId="2" ParentId="16647" CreationDate="2016-06-10T17:45:48.613" Score="4" Body="&lt;p&gt;A light sail in orbit around the sun can reflect light in a way to increase or decrease the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A light sail in orbit around another body is much more constrained.  I haven't seen that you could easily manipulate such an orbit in a way that keeps it roughly circular.  But if so, it would be the same as any other low-thrust engine.  It would give you the ability to change the orbit over time, but not provide any significant landing thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the minimum speed you would have for impact with Deimos would be the same as the speed of a circular orbit at the surface, with higher points better.  Lets be optimistic and say that above the mean radius of $6.2\text{km}$, there's a hill with an altitude of $0.4\text{km}$ (wild overoptimistic guess).  The mass of the moon is more than $1.4 \times 10^{15}\text{kg}$.  That makes the landing speed: &#xA;$$v = \sqrt{\frac{GM}{r}}$$&#xA;$$v = \sqrt{\frac{(6.67\times 10^{-11} \text{N m}^2\text{kg}^{-2})(1.4\times10^{15}\text{kg})}{(6.6\text{km})}}$$&#xA;$$v = 3.8\text{m/s}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the craft were small enough, you could probably design a landing/impact system that could cope with such a speed.  But if the only propulsion is a sail, it will have virtually no maneuverability.  Hitting a sandy plain at that speed and smacking the face of a boulder at that speed will probably have different outcomes.  &lt;/p&gt;&#xA;" OwnerUserId="5979" LastEditorUserId="20089" LastEditDate="2017-06-18T20:42:05.270" LastActivityDate="2017-06-18T20:42:05.270" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16649" PostTypeId="1" CreationDate="2016-06-10T21:08:00.920" Score="5" ViewCount="216" Body="&lt;p&gt;I recently started reading an interesting piece of history called the &lt;a href=&quot;http://history.nasa.gov/conghand/spcover.htm&quot;&gt;Space Handbook&lt;/a&gt; and I found something that surprised me a bit.  I learned that the Oberth Effect is named after one of the early pioneers of rocketry.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In his &lt;strong&gt;1927&lt;/strong&gt; book &quot;Ways to Spaceflight&quot; Oberth reportedly &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect#cite_note-ways-3&quot;&gt;described the effect&lt;/a&gt; in detail.  According to the Space Handbook: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The principal early work in the technological field of space flight&#xA;  was done in Russia, Germany, and the United States. The chief United&#xA;  States effort was that of Goddard. The early German work was done by&#xA;  H. Oberth, beginning in the 1920's. Russian efforts commenced at a&#xA;  substantially earlier date, giving them a clear and valid claim to a&#xA;  &quot;first.&quot; Russian activity began with the work of Mescherskii and&#xA;  Tsiolkovskii near the end of the 19th century.  Tsiolkovskii is&#xA;  generally recognized as the father of astronautics. Considerable work,&#xA;  both theoretical and experimental, was accomplished in the U. S. S. R.&#xA;  in the 1920's and 1930's.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Serious and substantial Government-sponsored rocket-research programs&#xA;  were established in Germany &lt;strong&gt;in about 1930,&lt;/strong&gt; in the Soviet Union&#xA;  sometime in or before 1934, and in the United States in 1942. The&#xA;  astronautical activities of the United States and the U. S. S. R. will&#xA;  be discussed in greater detail below.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In flipping through Herr Oberth's book I found the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The rocketry theorist must always be aware of the entirely different &#xA;  conditions under which the rocket works and cannot adopt any models &#xA;  that have proved theuselves in the calculation of other propulsion &#xA;  devices &lt;strong&gt;without first testing them.&lt;/strong&gt; It took me, for exauple, over  10&#xA;  yesrs to work out the theory of rocketry. Not every one is suited  for&#xA;  such a method of work, which is one reason wliy, until 15 years  ago,&#xA;  no theory of rocketrj* existed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Did Oberth himself use scientific experimentation or the scientific theory to validate his assertion?  If not Oberth himself, has anyone used scientific theory to prove the Oberth Effect is real?  &lt;/p&gt;&#xA;" OwnerUserId="14380" LastEditorUserId="14380" LastEditDate="2016-06-10T21:22:54.510" LastActivityDate="2016-06-11T13:48:09.223" Title="Did H. Oberth use scientific experimentation or the scientific theory to prove the validity of the Oberth effect?" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16650" PostTypeId="2" ParentId="9288" CreationDate="2016-06-10T21:46:01.923" Score="1" Body="&lt;p&gt;Yes, these are composites of WAC and NAC images. The camera operation planners do their best to cover a target sufficient for good science, but sometimes time or data volume constraints prevent catching every part of an object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.ciclops.org/view.php?id=8030&quot; rel=&quot;nofollow&quot;&gt;image caption&lt;/a&gt; on the CICLOPS site says &quot;The mosaics each consist of multiple narrow-angle camera (NAC) images with data from the wide-angle camera used to fill in areas where NAC data was not available.&quot;   &lt;/p&gt;&#xA;" OwnerUserId="212" LastActivityDate="2016-06-10T21:46:01.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16652" PostTypeId="2" ParentId="16649" CreationDate="2016-06-10T23:58:28.420" Score="2" Body="&lt;p&gt;I can't source this, but if with a couple years of messing around I can understand and explain the basis, mr. Oberth, with 10 years of thorough study would have no problems developing the maths that prove it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Experimental confirmation is costly and difficult here, but the theory is sound and fairly simple (if a little counter-intuitive).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Change of speed as propellant is ejected, in the rocket-propellant frame of reference (say, bound to center of mass of rocket and ejected propellant) is directly derived from conservation of momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v_{ship} \cdot m_{ship} = \Delta v_{propellant} \cdot m_{propellant} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note how this completely disregards initial/external conditions like speed of the rocket, its altitude, direction and so on - it's an inertial frame of reference, where initially the speed of both is zero (relative to the frame of reference) and no external conditions influence it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Burning a fixed amount of fuel, as result ejecting a fixed amount of propellant, at a fixed speed &lt;em&gt;relative to the rocket&lt;/em&gt;, we obtain a fixed change of velocity, completely regardless of external conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, &lt;em&gt;Energy&lt;/em&gt; of the rocket can't be taken in such a willy-nilly frame of reference. We must take it relative to a planet. Potential energy - altitude, and kinetic - speed relative to the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does the kinetic energy change if we perform a burn - as before, eject a fixed amount of mass, at fixed speed relative to the rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta E_k = {1\over 2} m (v + \Delta v)^2 - {1\over 2} m v^2  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The square here is the key. We're turning a fixed amount of chemical energy of the fuel into kinetic energy that scales with square of speed. If initial speed is 0 and we accelerate a mass of 2kg by 1m/s, we have &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta E_k =  1^2 - 0^2 = 1-0 = 1J$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we apply exactly the same acceleration to the same mass but with initial speed of 100m/s, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta E_k = 101^2 - 100^2 = 10201 - 10000 = 201 J$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've just gained 200 Joules by the mere fact our mass was moving fast when the acceleration occurred; we used the same amount of fuel/propellant though, and all the rest of the parameters remained unchanged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an immense gain of kinetic energy. And that, in order converts to potential energy - our ability to escape gravity wells. And in our favor, this one scales with inversely to distance from the planet. So, linear &quot;burn size&quot; causes geometric growth of kinetic energy, and so, geometric growth of our &quot;range&quot; - ability to reach higher orbits, or depart the system. Burn the same fuel at twice the speed, extend your apoapsis altitude four times as much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is all pretty basic physics, and it really doesn't take much &lt;em&gt;guessing&lt;/em&gt;. So, even without experimental confirmation, mr. Oberth would have no problems just deriving this mathematically. It feels counter-intuitive because the energy gain &lt;em&gt;seems&lt;/em&gt; free, and we all know &lt;a href=&quot;https://en.wikipedia.org/wiki/There_ain&amp;#39;t_no_such_thing_as_a_free_lunch&quot; rel=&quot;nofollow&quot;&gt;TANSTAAFL&lt;/a&gt;. But it only &lt;em&gt;seems&lt;/em&gt; so. We first need to spend energy to accelerate the rocket and the propellant to these speeds, and &lt;em&gt;that&lt;/em&gt; costs.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This is an extreme simplification. It proves &lt;em&gt;this works&lt;/em&gt;, it's enough to get a funding for further research if you're in mr. Oberth's position, and it promises immense savings. But the reality is this is based all on a &lt;em&gt;point burn&lt;/em&gt;. Which would be equivalent to blowing up an explosive charge to separate some reaction mass. This is not how it works in reality. The additions and substractions need to be replaced with integrals over mass, velocity, energy which change over time themselves. (as the ship accelerates, so does the fuel on it, contributing more to the efect...). This also gives the rough idea how much the orbit extends, but none &lt;em&gt;where to&lt;/em&gt;. We don't just want to fling the spaceship out into a random direction in space, we want it to head towards a specific destination - so the burn must be performed in precise conditions that give the craft the proper trajectory. We're treating this as a two-body problem, the craft and the planet. We must take into account the rest of the solar system too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's how this simple page of explanation turns into ten years of research, and &lt;em&gt;this&lt;/em&gt; is the source of mr. Oberth's doubts. The fact &lt;em&gt;the effect works&lt;/em&gt; was beyond doubt. It was the calculations to make it practically useful, that required extensive testing.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-06-11T00:57:46.427" LastActivityDate="2016-06-11T00:57:46.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16653" PostTypeId="1" CreationDate="2016-06-11T01:29:40.007" Score="4" ViewCount="757" Body="&lt;p&gt;Today, our highest-resolution &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravimetry&quot; rel=&quot;nofollow&quot;&gt;gravitational maps&lt;/a&gt; of the Earth are from the &lt;a href=&quot;http://www.csr.utexas.edu/grace/&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;Gravity Recovery and Climate Experiment&lt;/em&gt;&lt;/a&gt; (GRACE) project, launched in 2002.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before GRACE, how fine was the resolution of gravitational maps of the Earth (produced using spacecraft)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(And is resolution of gravitational maps best described in area (like square miles per datapoint) or in the solid angle subtended (like square degrees per datapoint)?)&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="552" LastEditDate="2016-06-11T01:36:17.367" LastActivityDate="2016-11-26T20:04:26.463" Title="How much resolution did gravitational maps of Earth have before GRACE?" Tags="|gravity|history|earth-observation|mapping|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16654" PostTypeId="1" CreationDate="2016-06-11T03:45:00.450" Score="9" ViewCount="424" Body="&lt;p&gt;In periods before human flight, did scientists, philosophers, or other thinkers seriously consider space travel and exploration a possibility?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have come across nineteenth-century discussions of probable civilizations on other planets and I am curious if anyone seriously thought about how we might reach or contact them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am familiar with early attempts (i.e. Wan Hu) but I am particularly interested if there was any theoretical activity in this field before it was even remotely practical (by our standards).&lt;/p&gt;&#xA;" OwnerUserId="6398" LastEditorUserId="1142" LastEditDate="2016-06-12T18:04:15.773" LastActivityDate="2016-12-15T17:38:38.550" Title="Pre-flight concepts of space travel" Tags="|rockets|history|reference-request|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="16655" PostTypeId="1" AcceptedAnswerId="16659" CreationDate="2016-06-11T05:58:44.913" Score="2" ViewCount="624" Body="&lt;p&gt;I am using Unity engine and C#, and am trying to work out a way to plot future 2D orbital trajectories. I currently have a two-body setup. The planet has a mass of 1000000, the satellite a mass of 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4JpAi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4JpAi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the image above you can see a pink line indicating the orbital path (PAST, NOT FUTURE) created by a trail renderer, which indicates the math is wrong since the eccentricity is not anywhere between 0 and 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have also noticed that the AP and PE are incorrect since the PE is showing up as -AP. I know my math is wrong somewhere but have no idea where.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If someone much smarter than I is able to figure it out/fix this, it would be great if they could also point me in the direction of how I would go about using this to plot a future trajectory in real time.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;void JustKeplerThings(Vector3 curPos, Vector3 shipCurVel, GameObject orbiting) {        &#xA;    //Gravitational Constant&#xA;    float G = 6.67408f * Mathf.Pow(10, -11);&#xA;&#xA;    //Mass of ship&#xA;    float m1 = thisShip.mass;&#xA;&#xA;    //Mass of the larger body&#xA;    float m2 = -orbiting.GetComponent&amp;lt;PointEffector2D&amp;gt;().forceMagnitude;&#xA;    float M = m2;&#xA;&#xA;    //Standard Gravitational Parameter&#xA;    float µ = G * (m1 + m2);&#xA;    //float µ = G * M;&#xA;&#xA;    //Relative Position Vector&#xA;    Vector3 r = FindRelativePosition(transform, orbitee.transform.position);&#xA;&#xA;    //Relative Velocity Vector&#xA;    Vector3 v = shipCurVel - new Vector3(orbitee.GetComponent&amp;lt;Rigidbody2D&amp;gt;().velocity.x, orbitee.GetComponent&amp;lt;Rigidbody2D&amp;gt;().velocity.y, 0);&#xA;&#xA;    //Specific Angular Momentum&#xA;    Vector3 h = Vector3.Cross(r, v);&#xA;&#xA;    //Eccentricity Vector&#xA;    Vector3 evec = (Vector3.Cross(v, h) / µ) - (r / Vector3.Magnitude(r));&#xA;&#xA;    //Eccentricity&#xA;    float e = Vector3.Magnitude(evec);&#xA;&#xA;    //Mechanical Energy&#xA;    float E = (Mathf.Pow(Vector3.Magnitude(v), 2) / 2) - (µ / Vector3.Magnitude(r));&#xA;&#xA;    //Semi-Major Axis&#xA;    float a = µ / (2 * E);&#xA;    if (Mathf.Abs(e - 1.0f) &amp;lt;= Mathf.Epsilon)&#xA;        a = Mathf.Infinity;&#xA;&#xA;    //Semi-Latus Rectum Check&#xA;    float p = a * (1 - Mathf.Pow(e, 2));&#xA;    if (Mathf.Abs(e - 1.0f) &amp;lt;= Mathf.Epsilon)&#xA;        p = Mathf.Pow(Vector3.Magnitude(h), 2) / µ;&#xA;&#xA;    //Inclination (2D)&#xA;    float i = 0;&#xA;&#xA;    //Longitude of Ascending Node (2D)&#xA;    float Ω = 0;&#xA;&#xA;    //Argument of Periapsis&#xA;    float ω = Mathf.Atan2(evec.y, evec.x);&#xA;    //ω = (2 * Mathf.PI) - ω;&#xA;&#xA;    //True Anomaly&#xA;    float t = Mathf.Acos((Vector3.Dot(evec, r)) / (e * Vector3.Magnitude(r)));&#xA;    if (Vector3.Dot(r, v) &amp;lt; 0)&#xA;        t = (2 * Mathf.PI) - t;&#xA;&#xA;    //Apoapsis&#xA;    float ap = a * (1 + e);&#xA;&#xA;    //Periapsis&#xA;    float pe = a * (1 - e);&#xA;&#xA;    //Orbital Period&#xA;    float T = (2 * Mathf.PI) * Mathf.Sqrt(Mathf.Pow(a, 3) / µ);&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;-&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;public static Vector3 FindRelativePosition(Transform origin, Vector3 position) {&#xA;    Vector3 distance = position - origin.position;&#xA;    Vector3 relativePosition = Vector3.zero;&#xA;    relativePosition.x = Vector3.Dot(distance, origin.right.normalized);&#xA;    relativePosition.y = Vector3.Dot(distance, origin.up.normalized);&#xA;    relativePosition.z = Vector3.Dot(distance, origin.forward.normalized);&#xA;    return relativePosition;&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="15714" LastEditorUserId="15714" LastEditDate="2016-06-12T02:06:16.400" LastActivityDate="2016-06-12T02:58:41.930" Title="I am having trouble calculating my classic orbital elements and am at a loss on where to look" Tags="|orbital-mechanics|gravity|orbital-elements|" AnswerCount="1" CommentCount="13" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16657" PostTypeId="2" ParentId="16649" CreationDate="2016-06-11T13:48:09.223" Score="3" Body="&lt;p&gt;It seems to me Oberth is referring to design work on rocket engines. The behavior of an engine couldn't be modeled comprehensively at the time, so you need experiments to verify your design works.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Oberth Effect, on the other hand is basic physics. There's no complex modeling involved, so he could be confident it'd work without experimentation. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-11T13:48:09.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16658" PostTypeId="2" ParentId="16653" CreationDate="2016-06-11T22:02:17.877" Score="8" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Today, our highest-resolution gravitational maps of the Earth are from the Gravity Recovery and Climate Experiment (GRACE) project, launched in 2002.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's not the case. The latest GRACE gravity model, which combines results from the GRACE and GOCE gravity missions and DTU13 is a 360x360 (degree and order) gravity model. On the other hand, the &lt;a href=&quot;http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/&quot; rel=&quot;noreferrer&quot;&gt;EGM2008 model&lt;/a&gt; &quot;is complete to spherical harmonic degree and order 2159, and contains additional coefficients extending to degree 2190 and order 2159.&quot; How much of that is pure fiction is a matter of debate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An alternative is to combine satellite-based observations with terrain. The images below depicts radial gravity anomalies over a 400x400 km area about Mount Everest (circled). The spatial resolution of a satellite-only model based on combined GRACE and GOCE data is about 100 km. Adding EGM2008 improves the resolution to about 10 km. Incorporating terrain knowledge improves the resolution even further, to about 200 meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PW1Sv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PW1Sv.jpg&quot; alt=&quot;see above caption&quot;&gt;&lt;/a&gt;&#xA;Source: &lt;a href=&quot;http://ddfe.curtin.edu.au/gravitymodels/GGMplus/hirt2013_ultrahighres_gravity.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://ddfe.curtin.edu.au/gravitymodels/GGMplus/hirt2013_ultrahighres_gravity.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Combining satellite-based gravity experiments with terrestrial observations has pluses and minuses. On the plus side, satellite-based observations are rather coarse but are global in nature, while terrestrial-based observations can be extremely fine but are local in nature. On the minus side, satellite-based observations are sensitive to the mass of the atmosphere while surface-based observations are not, and satellite-based and surface-based observations measure different things. There's always a potential for injected errors when combining measurements different things.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-06-11T22:02:17.877" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16659" PostTypeId="2" ParentId="16655" CreationDate="2016-06-12T02:35:31.357" Score="6" Body="&lt;p&gt;The computation you're doing is for a 1 million kg body with a satellite 150 meters from its center moving past it at 83 meters a second. 1 million kg is almost nothing in gravitational terms; Earth masses 6e24 kg. So the satellite is moving faster than escape velocity. This means orbital energy is by convention positive, so semi-major axis &quot;appears&quot; negative, and there's no solution possible for orbital period, because it ain't coming back. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unity's physics simulation is using a force (maybe expressed in newtons if you assume Unity distance units are meters -- but maybe not, because it's Unity), not a mass; you're treating it as a mass, which doesn't make sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Unity caps 2D movement speeds in its physics system, and doesn't document the basis distance at which PointEffector2D force is equal to the value specified, it'll be problematic to get your computations to line up with the output of Unity's simulation, so I recommend writing your own gravitational simulation -- it's fairly straightforward. You'll have to be conscious of the scale of your simulation, i.e. decide how many meters are represented by one Unity distance unit and stick to it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding plotting the future path of a body, you have a couple of options. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One, once you've got your own gravity sim working, is to simply remember the last N positions of the sim, and put the orbiting body at the location of the N-step-old results, and draw future-path lines up to the latest position. You'll have to run the simulation N times in advance before displaying the body, of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Option two, if you only have one or two massive bodies in play, is to tackle the equations &lt;a href=&quot;https://space.stackexchange.com/questions/14095/what-is-this-algorithm-to-calculate-orbital-state-vectors&quot;&gt;here&lt;/a&gt; or &lt;a href=&quot;https://downloads.rene-schwarz.com/download/M001-Keplerian_Orbit_Elements_to_Cartesian_State_Vectors.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; to translate between your orbital state vectors and future-time coordinates. With more than two bodies, you can't solve it this way. (You can have one massive body and any number of infinitesimal ones that don't affect each other gravitationally, of course.) &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-06-12T02:58:41.930" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16661" PostTypeId="1" AcceptedAnswerId="18744" CreationDate="2016-06-12T10:34:32.770" Score="12" ViewCount="1671" Body="&lt;p&gt;Deimos is the small moon of Mars. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Deimos_(moon)&quot; rel=&quot;noreferrer&quot;&gt;surface gravity is 0.003m/s2&lt;/a&gt; (compare to &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth&quot; rel=&quot;noreferrer&quot;&gt;Earth @ 9.807 m/s2&lt;/a&gt;).  In theory a person with a bicycle could &lt;a href=&quot;https://space.stackexchange.com/questions/2741/could-a-human-reach-escape-velocity-by-jumping-from-the-surface-of-ceres-a-dwar/2777#2777&quot;&gt;launch from&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/16647/can-you-use-a-light-sail-to-achieve-a-survivable-landing-anyplace-in-our-solar-s&quot;&gt;land on&lt;/a&gt; Deimos with a bicycle and a ramp.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A variation of the loop below, could in theory, be used increase traction on Deimos for take off and landing.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/coFfs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/coFfs.jpg&quot; alt=&quot;Bicycle daredevil Diavolo loops the loop&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But would you actually be able to ride a bicycle on Deimos? With only human power would there be enough friction to move forward or turn? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2020-06-22T16:25:32.813" Title="Can you ride a bicycle on Deimos?" Tags="|propulsion|gravity|reduced-gravity-sports|deimos|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16662" PostTypeId="1" AcceptedAnswerId="16678" CreationDate="2016-06-12T11:33:13.750" Score="5" ViewCount="797" Body="&lt;p&gt;While most applications of ion thrust involves long &quot;burns&quot; with either a tiny, slow rotation or none, in the case of this question: &lt;a href=&quot;https://space.stackexchange.com/q/16604/12102&quot;&gt;Thrust strategy to circularize a standard GTO orbit using ion propulsion?&lt;/a&gt; you might want to rock the satellite back and forth every orbit (for a fixed ion engine) to optimize for least time to circularization with a fixed solar panel area. (ion engines are typically powered in real time by the solar panels).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;edit:&lt;/em&gt; You might be in a hurry to get away from low orbiting debris for example - and depending on space weather - trapped radiation like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/File:Van_Allen_Belts.ogv&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/File:Van_Allen_Belts.ogv&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you orient a satellite with fixed panels for maximum power, you may wish you could vector the ion thrust. This is just an example of a situation where you might want to do this, it's not the subject of the question, nor the only possible reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Charged particles are deflected all the time, naturally and artificially, by either (or both) magnetic and electric fields. One could vary a DC bias on electrostatic deflector plates or even adjust positions of some permanent magnets to steer the thrust without doing (too much) work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, one could just gimbal the entire engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of these are simple, and they'll surely have design, cost, complexity, risk and performance impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: are there any stated plans or research to vector ion thrust?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-06-14T11:58:42.810" Title="Thrust vectoring for ion propulsion - any plans or current research?" Tags="|propulsion|attitude|ion-thruster|thrust-vectoring|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16664" PostTypeId="2" ParentId="16662" CreationDate="2016-06-12T13:51:51.837" Score="3" Body="&lt;p&gt;I don't know if there are any plans or developments in vectoring thrust from a single ion thruster, be that by gimballing or deflecting the ion stream, but &lt;a href=&quot;https://en.wikipedia.org/wiki/LISA_Pathfinder&quot; rel=&quot;nofollow&quot;&gt;LISA Pathfinder&lt;/a&gt; (a prototype for planned &lt;a href=&quot;https://en.wikipedia.org/wiki/Evolved_Laser_Interferometer_Space_Antenna&quot; rel=&quot;nofollow&quot;&gt;eLISA&lt;/a&gt; mission) is testing two sets of ion thrusters - colloid and field emission - for ultra-precise attitude control. These are installed and act as &lt;a href=&quot;http://en.wikipedia.org/wiki/Reaction_control_system&quot; rel=&quot;nofollow&quot;&gt;RCS&lt;/a&gt; thruster blocks - sets of multiple tiny thrusters of minuscule thrust aimed in various directions, able to provide both arbitrary rotation and translation to the craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are extremely important in the interferometer experiment, where the distance and orientation between the probes located millions of kilometers apart must be maintained with sub-micron precision. The Pathfinder mission is to determine which ones fulfill the task better.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-06-12T14:29:05.780" LastActivityDate="2016-06-12T14:29:05.780" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16665" PostTypeId="1" AcceptedAnswerId="16666" CreationDate="2016-06-12T15:15:21.697" Score="2" ViewCount="952" Body="&lt;p&gt;I was reading the comments in &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/4bqrih/spaceflightnow_now_reports_sherpa_to_fly_together/&quot; rel=&quot;nofollow&quot;&gt;this Reddit&lt;/a&gt; and I'm confused. How will Sherpa and Formosat-5 be configured inside the Fairing, and then deployed. One comment seems to suggest that Formosat-5 is one of the objects that Sherpa will deploy, but it's hard for me to imagine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will they be deployed together or separately? Are there any drawings or descriptions? &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="51" LastEditDate="2016-06-14T01:49:16.623" LastActivityDate="2020-07-22T09:02:59.857" Title="How will Sherpa and Formosat-5 be configured as payload and launched?" Tags="|spacex|falcon-9|payload-deployment|sherpa|formosat|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16666" PostTypeId="2" ParentId="16665" CreationDate="2016-06-12T15:46:12.317" Score="4" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;:&lt;/p&gt;&#xA;&lt;p&gt;Plans for the launch were changed, and the &lt;a href=&quot;https://www.nasaspaceflight.com/2017/08/falcon-9-vandenberg-launch-formosat-5/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Formosat-5&lt;/a&gt; launch didn't have any secondary payloads.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A Spaceflight Incorporated rideshare payload, consisting of a SHERPA adaptor with up to ninety small satellites, had been scheduled to fly as a secondary payload however this was removed from the launch due to uncertainty over the launch date, after Falcon 9 was grounded towards the end of last year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;I'm leaving the original text here as it contains useful info on Sherpa.&lt;/p&gt;&#xA;&lt;p&gt;I don't think Formosat-5 will be deployed by Sherpa. Formosat 5 weighs 525 kg, too heavy to be handled by Sherpa. &lt;a href=&quot;http://spacenews.com/darpa-satlets-to-fly-on-sherpas-debut-mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sherpa&lt;/a&gt; is made to deploy up to 5 satellites that weigh max. 300 kg each. It has 5 spots on its side where you can install small satellites with a separation mechanism, or a cubesat dispenser, each spot has a load limit of 300 kg. Because the 5 spots are in a radial position, you also have a size limit (the space between the Sherpa and the fairing).&lt;/p&gt;&#xA;&lt;p&gt;The Sherpa contains a battery, computer/sequencer, GPS and transmitter. It provides power to the satellites until separation, and it controls the separation of each satellite. The Sherpa separates from the second stage before it starts dispensing its payloads.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EbeYa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EbeYa.jpg&quot; alt=&quot;Sherpa minisatellite dispenser with 5 satellites around its perimeter&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In this image, we're looking at the top of the Sherpa. The blocks around the side of the cylinder are the 5 smaller payloads or cubesat dispensers. This launch will carry 3 cubesat dispensers with a capacity of 84U each (7x12U), plus 2 small satellites.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=3171&amp;amp;context=smallsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;This paper&lt;/a&gt; (by the company that built Sherpa) says the primary payload can be placed on top of the Sherpa. This apparently has a larger load limit than the 300 kg of the side spots.&#xA;The Sherpa goes on top of the payload adapter of the rocket.&lt;/p&gt;&#xA;&lt;p&gt;The Sherpa is based on the &lt;a href=&quot;http://www.csaengineering.com/products-services/espa/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moog ESPA Grande&lt;/a&gt; ring structure (the big cylinder in the image).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The ESPA ring, or the Evolved Expendable Launch Vehicle (EELV) Secondary Payload Adapter was developed to utilize excess launch capacity by mounting additional payloads below the primary spacecraft. This reduces launch costs for the primary mission and enables secondary and even tertiary missions with minimal impact to the original mission.&lt;/p&gt;&#xA;&lt;p&gt;ESPA can accommodate up to a 15,000-pound primary satellite using the standard EELV interface plane bolt pattern, which is a drop-in component in the launch stack.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So the ring on top of Sherpa is a duplicate of the ring on top of the Falcon's payload adapter.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/sherpa&quot; rel=&quot;nofollow noreferrer&quot;&gt;primary payload separates first&lt;/a&gt;, then the Sherpa launches its secondary payloads.&lt;/p&gt;&#xA;&lt;p&gt;Here's the &lt;a href=&quot;http://www.spaceflight.com/wp-content/uploads/2015/05/SPUG-RevF.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sherpa user's guide&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2020-07-22T09:02:59.857" LastActivityDate="2020-07-22T09:02:59.857" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="16667" PostTypeId="2" ParentId="16654" CreationDate="2016-06-12T16:02:26.710" Score="5" Body="&lt;p&gt;NASA provides a good &lt;a href=&quot;https://spaceflightsystems.grc.nasa.gov/education/rocket/TRCRocket/history_of_rockets.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Brief History of Rockets&lt;/a&gt; which summarizes in concise form many of the historical advances in rocketry and space exploration, including Wan-Hu's unfortunate attempt at a rocket-powered chair.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/DR2tI.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DR2tI.gif&quot; alt=&quot;Wan-Hu&amp;#39;s rocket chair&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simple rocket in the form kids light on the Fourth of July, &lt;a href=&quot;http://en.wikipedia.org/wiki/Rocket#History&quot; rel=&quot;nofollow noreferrer&quot;&gt;invented by the Chinese&lt;/a&gt;, has been generally around by the end of the 13th century, used as a supplement to more traditional armament, as in Francis Scott Keys &quot;&lt;a href=&quot;http://en.wikipedia.org/wiki/The_Star-Spangled_Banner&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket's red glare&lt;/a&gt;&quot; from the bombardment of Fort McHenry during the War of 1812.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the western Renaissance brought in the telescope, the concept that there was space to be explored began to come into focus. Before &lt;a href=&quot;http://en.wikipedia.org/wiki/Galilean_moons#Discovery&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo&lt;/a&gt; could look through a pair of lenses at Jupiter and see its four largest moons orbiting the planet, scientists and philosophers often thought of the distant stars and planets as residing on vast transparent spheres rotating about the Earth. &lt;a href=&quot;http://en.wikipedia.org/wiki/Copernican_Revolution&quot; rel=&quot;nofollow noreferrer&quot;&gt;Copernicus&lt;/a&gt;' sun-centered theory of the universe shattered the awkward and never-quite-right theory of epicycles, by which astronomers had tried to reconcile, in an Earth centered universe, their observations with their mathematics.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early 1600s, &lt;a href=&quot;http://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Johannes Kepler&lt;/a&gt; inherits years of precise astronomical data from the school of Tycho Brahe and, after many false starts, derives his laws of planetary motion:    &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The orbit of a planet is an ellipse with the Sun at one of the two foci.   &lt;/li&gt;&#xA;&lt;li&gt;A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.    &lt;/li&gt;&#xA;&lt;li&gt;The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.    &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;By providing a mathematical solution which finally agreed with the tricky orbit of Mars, Kepler proved Copernicus right. Today Keplerian conic sections are a staple of orbital dynamics and astronavigation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1687. Sir &lt;a href=&quot;http://en.wikipedia.org/wiki/Philosophi%C3%A6_Naturalis_Principia_Mathematica&quot; rel=&quot;nofollow noreferrer&quot;&gt;Issac Newton&lt;/a&gt; provided the science still used for many basic tasks of space exploration:    &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Principia states Newton's &lt;a href=&quot;http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;laws of motion&lt;/a&gt;, forming the foundation of classical mechanics, also Newton's &lt;a href=&quot;http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation&quot; rel=&quot;nofollow noreferrer&quot;&gt;law of universal gravitation&lt;/a&gt;, and a derivation of Kepler's laws of planetary motion (which Kepler first obtained empirically).   &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As to serious consideration of space travel and exploration, one should look to the putative father of space exploration, &lt;a href=&quot;http://en.wikipedia.org/wiki/Konstantin_Tsiolkovsky&quot; rel=&quot;nofollow noreferrer&quot;&gt;Konstantin Tsiolkovsky&lt;/a&gt;. His 1903 publication, &lt;em&gt;Exploration of Outer Space by Means of Rocket Devices&lt;/em&gt; and subsequent writings proved (for the first time) that a rocket could perform space flight, calculated the speed required for a successful Earth orbit and escape velocity, introduced the concept of the multistage rocket and liquid fuel/oxidizers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tsiolkovsky's most important contribution was the formulation of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket equation&lt;/a&gt;:&lt;br&gt;&#xA;$$\Delta v=v_e ln\frac{m_0}{m_f}$$&lt;br&gt;&#xA;where:&lt;br&gt;&#xA;$\Delta v$ is the required change in velocity;&lt;br&gt;&#xA;$v_e$ is the effective exhaust velocity of the combusted propellants;&lt;br&gt;&#xA;$ln$ is the natural logarithm;&lt;br&gt;&#xA;$m_0$ is the starting mass of the rocket including propellant; and&lt;br&gt;&#xA;$m_f$ is the empty mass of the rocket.&lt;br&gt;&#xA;$\frac{m_0}{m_f}$ is the mass ratio of the rocket&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what is being referenced when you hear the phrase, on this site and many others, &quot;the tyranny of the rocket equation.&quot; You can find a good discussion &lt;a href=&quot;https://space.stackexchange.com/questions/13763/is-this-a-correct-understanding-of-tsiolkovskys-rocket-equation&quot;&gt;here&lt;/a&gt;, and I'll use the answers from that question to briefly explicate. Since your exhaust velocity is a constant, in order to achieve linear increases in $\Delta v$, you need exponentially greater mass ratios. The devil is the natural logarithm function. Here's a graph:    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0ZQst.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0ZQst.jpg&quot; alt=&quot;Natural log of x greater than 1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass ratio is always going to be greater than one, so I've ditched lower values. You can see that to get one unit of $\Delta v$ an approximate mass ratio of less than three; to double it you need more than seven; to triple it you jump to over 20. Rather than try to engineer absurdly high mass ratios, rocket research tries the more rational approach of finding engines, such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;ion thrusters&lt;/a&gt;, that substantially increase the exhaust velocity.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-12-13T20:41:03.797" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16668" PostTypeId="1" AcceptedAnswerId="16669" CreationDate="2016-06-12T20:09:21.313" Score="0" ViewCount="237" Body="&lt;p&gt;There are substances with such massive density that a relatively small volume, could contain an immense mass. Given a substance like neutronium, could you conceivably synthesize gravity for a space station by placing the substance at the core and then building a structure that places the &quot;floor&quot; at an appropriate distance from the core?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Force of Gravity equation&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;F&lt;sub&gt;grav&lt;/sub&gt; = (G m1 m2)/d&lt;sup&gt;2&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am asking specifically about whether there are other factor to consider or if this theoretically true. This is not proposing building it.&lt;/p&gt;&#xA;" OwnerUserId="14137" LastEditorUserId="14137" LastEditDate="2016-06-12T23:24:12.360" LastActivityDate="2016-06-13T13:41:17.733" Title="Synthesizing gravity" Tags="|gravity|space-station|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16669" PostTypeId="2" ParentId="16668" CreationDate="2016-06-12T20:21:44.930" Score="9" Body="&lt;p&gt;It's &quot;theoretically true&quot; in that mass is what creates a gravitational pull. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact is impossible to do anything useful with, because you need to hold the mass at a particular distance from the floor, and any structure you built to support the floor above the core mass would be subject to a much greater force (because of the inverse-square law), and so supports, floors, crew, and spacecraft would all pretty instantly turn into a thin plating on the surface of a neutronium sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, to get a significant gravitational force, you'd need so much mass that it would be impractical to try and move the ship.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could consider a thin spherical shell around a massive core, but it would be positionally unstable; when one side moves towards the core even by a millimeter, it will be pulled more and more rapidly to the core. You'd need some sort of active thrust system to monitor and keep it stable -- and how would you assemble such a thing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Neutronium&quot;&gt;Neutronium&lt;/a&gt;&quot; as in real neutron star material wouldn't work for this, by the way; it doesn't remain stably dense at the smallish volumes you'd want for synthesizing gravity. Neutron stars can't exist below about 0.2 solar masses (~4e29 kg). To get 9.81 m/s&lt;sup&gt;2&lt;/sup&gt; (Earth surface gravity) acceleration at the floor, with the floor 100m above the center of the massive core, you'd need about 1.5e15kg, a 19cm diameter sphere of the stuff, which would &lt;a href=&quot;https://arxiv.org/abs/astro-ph/9707230&quot;&gt;instantly explode&lt;/a&gt;, since it isn't heavy enough to hold itself together. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-06-12T21:02:46.803" LastActivityDate="2016-06-12T21:02:46.803" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="16670" PostTypeId="1" AcceptedAnswerId="16671" CreationDate="2016-06-12T22:25:31.273" Score="12" ViewCount="2770" Body="&lt;p&gt;I just read about &lt;a href=&quot;https://en.wikipedia.org/wiki/Yo-yo_de-spin&quot; rel=&quot;noreferrer&quot;&gt;yo-yo de-spin&lt;/a&gt; as a measure to reduce the spin of objects. The basic idea is simple and makes sense, but then I read this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As an example of yo-yo de-spin, on the Dawn Mission […] reduced the initial spin rate of the 1420 kg spacecraft from 36 RPM down to 3 RPM &lt;em&gt;in the opposite direction.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://dawn.jpl.nasa.gov/mission/journal_9_12_07.asp&quot; rel=&quot;noreferrer&quot;&gt;linked source&lt;/a&gt; also mentions this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Even with a 204-kilogram ([…]) third stage […] and a 1218-kilogram ([…]) spacecraft, the small yo-yo system halts the spin &lt;em&gt;and even reverses it&lt;/em&gt;, leaving Dawn rotating at 3 rpm in the opposite direction from its original spin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How does extending the cables / weights actually cause a &lt;em&gt;reversal&lt;/em&gt; of the rotation? What is missing from the explanation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My gut tells me that this technique should only be able to reduce spin to zero as the extension goes towards infinity. If at some point it were to reverse the spin direction, there'd have to be a point of &lt;em&gt;no&lt;/em&gt; spin at all which would violate the conservation of momentum, no?&lt;/p&gt;&#xA;" OwnerUserId="15726" LastEditorUserId="6816" LastEditDate="2016-06-12T22:29:15.317" LastActivityDate="2019-06-24T12:52:44.130" Title="How can a yo-yo de-spin maneuver reverse the rotation?" Tags="|attitude|physics|dawn-mission|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16671" PostTypeId="2" ParentId="16670" CreationDate="2016-06-12T23:30:19.507" Score="5" Body="&lt;p&gt;It would only reduce spin if the weights remained attached.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine this: two ropes wrapped on a spinning spool, with weights on the ends.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the weights are released, they spin outwards, extending the rope. The centrifugal force pulls the rope taut (most of it still wound on the spool). As they continue to spin outwards, the pull of the rope on the spool caused by the centrifugal force creates a torque on the rocket - pulling to unwind more rope faster, as result rotating the spool (and the rocket) in direction opposite to original. This torque makes the rocket spin in the opposite direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now as the rope ends, normally it would start winding back on in reverse, then again unwinding, and so on, the harmonic cycle dying down due to losses in flexibility of the rope, until the whole thing ends up spinning slowly in the original direction with the weights fully extended. But when the rope ends unwinding, it's released and the weights float away, and nothing stops the reverse spin of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the pic below, the whole thing originally turns clockwise. As the pull on the ropes increases, the weights continue to turn clockwise, traveling outwards, but the torque from the ropes causes the hull to start spinning counter-clockwise.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/cWjGo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cWjGo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-06-12T23:30:19.507" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16672" PostTypeId="1" AcceptedAnswerId="16674" CreationDate="2016-06-13T01:28:39.637" Score="12" ViewCount="1998" Body="&lt;p&gt;I'm pretty sure that at least some large satellites are deployed with their batteries charged so that they can initialize themselves, get their own attitude and position, and possibly establish some communication and even change their orbit without needing time to charge their batteries from the solar panels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember reading that most cubesat launch providers have fairly strict rules about &lt;em&gt;stored energy&lt;/em&gt;. Cubesats usually can't contain highly reactive chemicals, very strong and tightly wound springs, or fully charged batteries. Possibly even the real-time clock is not running?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Have I got this basically right&lt;/em&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sherpa is a really interesting system. It carries a substantial supply of charged batteries. A good discussion of it's structure and operation can be found &lt;a href=&quot;https://space.stackexchange.com/q/16665/12102&quot;&gt;in this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is Sherpa's sequence - &lt;em&gt;I have paraphrased&lt;/em&gt; it from &lt;a href=&quot;https://space.stackexchange.com/q/16665/12102&quot;&gt;that answer&lt;/a&gt; and from the &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/sherpa&quot; rel=&quot;nofollow noreferrer&quot;&gt;linked document&lt;/a&gt; therein:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Separation from the upper stage&lt;/li&gt;&#xA;  &lt;li&gt;Power-up; avionic system initiated &amp;amp; start mission sequence &lt;/li&gt;&#xA;  &lt;li&gt;30 minute coast – no SHERPA activity&lt;/li&gt;&#xA;  &lt;li&gt;Start payload deployment sequence&lt;/li&gt;&#xA;  &lt;li&gt;45 minute payload deployment time - detailed position/timing information of each event stored and relayed to ground&lt;/li&gt;&#xA;  &lt;li&gt;Continue telemetry to ground until end of battery life approximately 10 hours post SHERPA separation.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Conceivably, Sherpa &lt;em&gt;could&lt;/em&gt; pass a little bit of juice (electrical power, not Tang) to some of the cubesats, and even the correct time, position, and attitude of each deployment (but that's really a separate question). However I don't know if the cubesat standard even provides for the necessary electrical connections to do this, or if Sherpa can afford to give away much power. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, &lt;em&gt;Are cubesats deployed with fully discharged batteries? Even those on Sherpa&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;note: Sherpa can host a variety of small satellite shapes and sizes. I'm primarily interested in the batteries of those that meet the cubesat standard, or are at least considered cubesats.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-06-13T04:25:48.563" Title="Are cubesats deployed with fully discharged batteries? Even those on Sherpa?" Tags="|power|cubesat|payload-deployment|sherpa|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16673" PostTypeId="2" ParentId="16672" CreationDate="2016-06-13T01:46:12.227" Score="5" Body="&lt;p&gt;Every cubesat I've seen is completely disconnected from the rocket electrically, unlike most satellites. They typically reside inside of a pod of some kind, such as the P-POD. This means there is no method to charge the satellites. In addition, as such a low priority user, they are often loaded into the satellite weeks, or occasionally months ahead of time (For classified launches). Bottom line, it seems unlikely anytime soon that they would be charged at launch. The cubesats that have been launched thus far seem to be able to compensate for this. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-13T01:46:12.227" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16674" PostTypeId="2" ParentId="16672" CreationDate="2016-06-13T02:22:25.400" Score="15" Body="&lt;p&gt;All cubesats that I have personal knowledge of (including the &lt;a href=&quot;http://www.space.com/24546-cubesat-record-space-station-launch.html&quot;&gt;Planet Labs fleet&lt;/a&gt;) were launched with partially charged batteries, typically at around the 50% level that minimizes degradation in storage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch providers do typically impose fairly strict rules requiring redundant physical interlock switches to prevent any electronics from being energized before deployment, especially for sats going via the ISS.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="1142" LastEditDate="2016-06-13T02:31:08.453" LastActivityDate="2016-06-13T02:31:08.453" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16675" PostTypeId="2" ParentId="16670" CreationDate="2016-06-13T03:14:17.810" Score="5" Body="&lt;p&gt;NASA - and math - to the rescue!&lt;/p&gt;&#xA;&lt;p&gt;2nd edit: &lt;em&gt;Shorter answer even &amp;quot;firster&amp;quot;. Try a &lt;a href=&quot;https://en.wikipedia.org/wiki/Thought_experiment&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gedankenexperiment&lt;/a&gt;. If you are happy with it stopping, then just image the satellite is suddenly much lighter. The ropes keep pulling and yo-yo's keep moving about the same, but the satellite slows down much faster. It stops and starts turning&lt;/em&gt; the same way the ropes are pulling it.&lt;/p&gt;&#xA;&lt;p&gt;edit: &lt;em&gt;Short answer first. Just like in linear momentum recoil, you conserve energy and angular momentum here. Whatever the objects need to do to conserve them&lt;/em&gt;, that's what happens. &lt;em&gt;There isn't a different &amp;quot;why&amp;quot; for each new conservation problem. However, there may be an &amp;quot;aha!&amp;quot; specific to each person, after they solve enough conservation problems in a row&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Yep, at least according to Equation 3 (shown below) in &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19620006811.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA technical note D-1420 (1962)&lt;/a&gt;, you can go beyond zero and reverse the spin as much as you want, provided you can manage the logistics &lt;em&gt;increasingly heavy yo-yos, long wires, large yo-yo-deploy radius, or increasingly small satellite moment of inertia&lt;/em&gt;. &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19630004528.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA technical note D-1676(1963)&lt;/a&gt; develops the math further for &lt;em&gt;stretch yo-yo&lt;/em&gt; techniques.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{1+r}{1-r}=\frac{I}{m(d+a)^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$r=\text{spin reduction ratio (final spin rate / initial spin rate),}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$I=\text{spacecraft moment of inertia about the spin axis,}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$m=\text{mass of weights plus 1/3 mass of wires,}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$d=\text{cord length}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$a=\text{spacecraft radius}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I like it better upside down and reversed:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{m(d+a)^2}{I}=\frac{1-r}{1+r}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I plotted the equation down to a ratio of &lt;span class=&quot;math-container&quot;&gt;$r=-0.2$&lt;/span&gt;. Your example of +36RPM down to -3RPM would be &lt;span class=&quot;math-container&quot;&gt;$r=\frac{-3}{36}\approx-0.083$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HxCPt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HxCPt.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;@OrganicMarble mentions this &lt;a href=&quot;https://www.youtube.com/watch?v=HCtNqD-jlPE&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt; in his comment. It sure looks to me like the yo-yo's are getting a slight propulsive kick beyond their inertia, but haven't analyzed frame-by-frame yet.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/HCtNqD-jlPE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zKABG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zKABG.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QiR2s.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QiR2s.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-06-13T06:08:47.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16676" PostTypeId="1" AcceptedAnswerId="16677" CreationDate="2016-06-13T05:27:44.127" Score="2" ViewCount="875" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/4449/visualization-of-non-keplerian-orbits-for-solar-sails#comment9182_4449&quot;&gt;this comment&lt;/a&gt; below the question &lt;a href=&quot;https://space.stackexchange.com/q/4449/12102&quot;&gt;Visualization of non-Keplerian orbits for solar sails&lt;/a&gt;, NASA's open source General Mission Analysis Tool is suggested. Sounds great!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My google search found this &lt;a href=&quot;http://gmat.sourceforge.net/blog/&quot; rel=&quot;nofollow noreferrer&quot;&gt;blog from 2012&lt;/a&gt; and this big PDF &lt;a href=&quot;http://gmat.sourceforge.net/doc/R2012a/help-letter.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Users Guide&lt;/a&gt; also dated 2012, both on SourceForge. But the links I was aiming for didn't help much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The link &lt;a href=&quot;https://gmat.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://gmat.gsfc.nasa.gov/&lt;/a&gt; only gives me a &quot;your connection is not private&quot; error (shown below) even when I delete the &lt;code&gt;s&lt;/code&gt; and change it back to &lt;code&gt;http://&lt;/code&gt; , and  &lt;a href=&quot;http://opensource.gsfc.nasa.gov/projects/GMAT/index.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://opensource.gsfc.nasa.gov/projects/GMAT/index.php&lt;/a&gt; seems to be a dead end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where can I find this software? Is it something I have to compile on my computer, or is it hosted somewhere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QYpRw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QYpRw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-06-13T07:53:32.613" Title="Is GMAT still available? Where? How? (NASA's General Mission Analysis Tool)" Tags="|nasa|mission-design|software|visualization|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16677" PostTypeId="2" ParentId="16676" CreationDate="2016-06-13T07:53:32.613" Score="8" Body="&lt;p&gt;If you go beyond the expired certificate, you get a page telling you it is now supported on an &lt;a href=&quot;http://gmatcentral.org/&quot; rel=&quot;noreferrer&quot;&gt;off-NASA site&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j9pqL.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j9pqL.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2016-06-13T07:53:32.613" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16678" PostTypeId="2" ParentId="16662" CreationDate="2016-06-13T10:20:13.200" Score="11" Body="&lt;p&gt;The ion engines on Deep Space 1 and Dawn were gimbaled. If you are going to operate one engine, it pretty much has to be gimbaled or vectored to direct the thrust, on average, through the CG (center of gravity). That seems unavoidable to me. If you didn't have a way to direct the thrust, then the thrust could not be assured to be through the CG as, for example, propellant is expended, moving the CG. Then the ion engine would act as an extremely efficient attitude control thruster applying torque to the spacecraft, that the hydrazine attitude control thrusters would have no hope of countering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose you could try to electromagnetically vector the thrust, but plasma physics is complicated whereas gimbals are pretty darned simple. I think you would just reduce the efficiency of the engine with electromagnetic vectoring, not getting all the ions to go in the same direction. A gimbal doesn't have that problem.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-06-14T11:58:42.810" LastActivityDate="2016-06-14T11:58:42.810" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16680" PostTypeId="2" ParentId="16668" CreationDate="2016-06-13T12:03:46.417" Score="0" Body="&lt;p&gt;Lets calculate the necessary mass for a distance of 10 m and a &quot;gravity&quot; of 9.81 m/s*s. It is only 14.7E12 kg, that is many orders of magnitude too much.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-06-13T12:03:46.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16681" PostTypeId="1" AcceptedAnswerId="16682" CreationDate="2016-06-12T13:47:36.327" Score="5" ViewCount="346" Body="&lt;p&gt;In order to aim an antenna at Earth, how do unmanned spacecraft find their own orientation (xyz position in space, what direction it is flying and which direction the antenna is pointing)? Are they using a 'camera' to look at stars? Do they need a telescope lens and long tubes to get good image magnification and angular resolution?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can they move/steer/rotate themselves? Are they using reaction wheels, which run on un-limited electrical power or using limited-supply gas jet thrusters? &lt;/p&gt;&#xA;" OwnerUserId="15733" OwnerDisplayName="EE developer" LastEditorUserId="1142" LastEditDate="2016-06-13T22:42:56.557" LastActivityDate="2016-06-14T06:08:24.580" Title="Deep space craft antenna aiming" Tags="|navigation|uncrewed-spaceflight|instrument|antenna|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16682" PostTypeId="2" ParentId="16681" CreationDate="2016-06-13T13:32:41.423" Score="7" Body="&lt;p&gt;Spacecraft use a star tracker to find their orientation (attitude). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a &lt;a href=&quot;http://www.terma.com/space/space-segment/star-trackers/&quot; rel=&quot;nofollow noreferrer&quot;&gt;random example&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p6S3y.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p6S3y.gif&quot; alt=&quot;Terma star tracker&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dimensions and Mass:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Camera 120 by 120 by 33 mm, 1.0 kg&lt;br&gt;&#xA;(note: optics protrude 58 mm inside baffle)  &lt;/li&gt;&#xA;&lt;li&gt;Processor 245 by 165 by 29 mm, 1.2 kg  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Baffle Examples:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;30º (sun exclusion), Ø234 by 346 mm, 800 g  &lt;/li&gt;&#xA;&lt;li&gt;45º (sun exclusion), Ø167 by 203 mm, 470 g  &lt;/li&gt;&#xA;&lt;li&gt;60º (sun exclusion), Ø160 by 211 mm, 530 g  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So the camera and lens are tiny, they just need a small sunshield (baffle) to work correctly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Accuracy:&lt;br&gt;&#xA; - &amp;lt;1 arc-seconds RMS pitch, yaw&lt;br&gt;&#xA; -  &amp;lt;5 arc-seconds RMS roll  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft use thrusters, reaction wheels, or both to control their attitude. Thrusters have a finite amount of propellant, but can be more reliable than reaction wheels. For long-term missions (including deep-space missions like New Horizons), thrusters are commonly chosen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: Accuracy is much better than is needed for pointing the antenna. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-14T06:08:24.580" LastActivityDate="2016-06-14T06:08:24.580" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16683" PostTypeId="2" ParentId="16668" CreationDate="2016-06-13T13:41:17.733" Score="3" Body="&lt;p&gt;Hmmm, why do people always insist on a spherical mass and the $\frac{1}{r^2}$ law?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity and electrostatics are very very similar. For electrostatics, Maxwell's first equation is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{Q}{\varepsilon_0}=\int_A\vec{E}d\vec{A}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which means: If you have a closed surface and sum up the product of surface times perpendicular E-field, you get the charge enclosed divided by $\varepsilon_0$. In case of a spherical charge distribution, you can put a sphere of radius r and surface $4\pi r^2$ around it. The field is constant and perpendicular at each point of the surface, so the equation becomes simple and gives a well known result:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{Q}{\varepsilon_0}=4\pi r^2 E \quad \Rightarrow\quad E=\frac{Q}{4\pi\varepsilon_0 r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if there's an infinite plate with charge Q, you would place two planes on both sides of it as surface. This time, you get&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{Q}{\varepsilon_0}=2A E \quad \Rightarrow\quad E=\frac{Q}{2A \varepsilon_0}=\frac{\rho}{2 \varepsilon_0}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, $\rho$ ist the charge per surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The result is remarkable because first, the field is parallel everywhere and points to the plate and second, it does not depend on distance!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back to gravity, we know a sphere gives&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a=G\frac{M}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparing this to the formula for a spherical charge gives&#xA;$$G\equiv\frac{1}{4\pi\varepsilon_0} \quad\Rightarrow\quad2\pi G\equiv\frac{1}{2\varepsilon_0}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which can be used for the formula of the plate. So, a plate with given weight per surface $\rho$ generates a constant (!) gravitational field of&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a=2\pi G\rho$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a=9.81m/s², one gets $\rho=23.4\cdot10^9kg/m^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Osmium has a density of $22.6\cdot10^3kg/m^3$, so the plate would have a thickness of 1000km.&#xA;Neutronium, having a density of $3\cdot10^{17}kg/m^3$ would only require a layer of 0.08µm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;BUT:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The command module of the apollo missions had a diameter of 3.9m, or a base area of 12m². This would require a &quot;neutronium carpet&quot; with a weight of 280,800,000t (For comparison: Entire Saturn V: 3,000t; cheops pyramid: 6,500,000t). So, no way to lift/move this, and even no way to support it while the rocket is on the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further more, the field of this plane can't be considered constant,since the dimension of the module is large compared to the diameter. A space ship shaped more like a flying saucer would be better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as said in the other answers: Neutronium only exists under high pressure. Under &quot;normal&quot; conditions, it would for sure expand (explode?)to a more common type of matter. &lt;/p&gt;&#xA;" OwnerUserId="11749" LastActivityDate="2016-06-13T13:41:17.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16684" PostTypeId="1" AcceptedAnswerId="16686" CreationDate="2016-06-13T15:19:50.667" Score="7" ViewCount="672" Body="&lt;p&gt;Most of space-based hardware operates on solar panels composed of lots of photovoltaic cells connected in short series that produce around &lt;a href=&quot;https://space.stackexchange.com/questions/8709/why-is-28-v-a-common-nominal-spacecraft-bus-voltage&quot;&gt;28V&lt;/a&gt;; lots of these are connected in parallel to produce desired amperage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, many devices - in particular, ion propulsion - requires very high voltages to operate. They use &lt;a href=&quot;https://space.stackexchange.com/a/1690/103&quot;&gt;massive Power Processing Units&lt;/a&gt; to provide the desired voltages, producing a lot of heat and losing power in the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be advantageous to use solar panels that connect lots and lots of solar cells in series, generating high voltage directly? (and similarly, connect thermocouples in RTG in series)? Stepping the voltage down for various electronics use should be at the very least similarly hard as stepping it up, and the devices that require high voltage could be supplied through a simple regulator, without need to step up the voltage by a factor of a couple thousands.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-11-27T18:23:27.183" Title="Solar cells in series" Tags="|power|solar-power|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16685" PostTypeId="2" ParentId="9240" CreationDate="2016-06-13T16:22:56.710" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910006728.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;After each STS launch&lt;/a&gt;, the pad including the fixed service structure was washed down. There's a video in the &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=13476.0&quot; rel=&quot;nofollow noreferrer&quot;&gt;L2 subscriber section&lt;/a&gt; of Nasaspaceflight.com. The video resolution is too low to see if the crew access white room is washed down.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Post Launch Pad Debris Inspection&lt;/p&gt;&#xA;&lt;p&gt;Objectives: Locate and identify debris that could have damaged the Shuttle vehicle during launch.&lt;br /&gt;&#xA;Areas: MLP deck, flame exhaust holes and trenches, FSS, pad surfaces and slopes, extension of trenches to perimeter fence, walkdown of the beach from Playalinda to Complex 40, aerial overview of inaccessible areas.&lt;br /&gt;&#xA;Time: Launch + 3 hours (after pad safing, before washdown)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-06-13T16:22:56.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16686" PostTypeId="2" ParentId="16684" CreationDate="2016-06-13T16:57:30.253" Score="10" Body="&lt;p&gt;No, it doesn't make sense, for two key reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, it is a loss of redundancy. If one of the cells fails, then the entire string fails. You really don't want that to happen... If you build in a shunting circuit of some kind to manage a failed cell, then the voltage could fluctuate, causing additional issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, solar cells placed in series need to match each other. It's harder to match the amount required for a really high voltage than is required to match the 28V power. Badly matched cells results in lost power inside the cells, and a really bad match could damage the cells. They are matched according to their IV curves. Basically, you want a set of cells to have the same amount of current running through them given the same voltage. Otherwise, some of the energy must be lost.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-06-13T22:00:39.167" LastActivityDate="2016-06-13T22:00:39.167" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16687" PostTypeId="2" ParentId="16616" CreationDate="2016-06-13T22:53:52.257" Score="3" Body="&lt;p&gt;The Cubesat Ambipolar Thruster (CAT) is currently being designed and tested by Phase Four, a company that formed from the University of Michigan team that originally came up with the idea. Here's the website page describing CAT: &lt;a href=&quot;http://www.phasefour.io/cat-engine.html&quot; rel=&quot;nofollow&quot;&gt;http://www.phasefour.io/cat-engine.html&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The operational concept is explained really well in the technology section of their website: A radio helix antenna is used to pump RF energy into the equivalent of a combustion chamber on a chemical rocket engine. The resultant plasma is then accelerated out the back by focusing the beam with a &quot;magnetic nozzle&quot; created by a permanent magnet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know from talking with people in the Cubesat community that Phase Four is working with multiple entities with the intent of getting CAT to fly on a Cubesat within the next two years. I haven't heard of any concrete plans to use CAT on a Mars mission yet, but a LEO demonstration mission is definitely the first step.&lt;/p&gt;&#xA;" OwnerUserId="14477" LastActivityDate="2016-06-13T22:53:52.257" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16688" PostTypeId="1" CreationDate="2016-06-14T00:58:39.947" Score="3" ViewCount="149" Body="&lt;p&gt;Awkwardly phrased title aside, the question is as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you wished to launch from Earth to head to Mars at the moment when the two planets are furthest apart, would you arrive sooner if you left immediately or if you waited until the planets were closer together?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put this another way, what closes distance faster, a rocket-propelled spacecraft, or the planets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus: what is the minimum speed a spacecraft would have to travel at in order to make it win this &quot;contest&quot;?&lt;/p&gt;&#xA;" OwnerUserId="13965" LastEditorUserId="84" LastEditDate="2016-06-14T05:19:28.210" LastActivityDate="2016-06-14T05:19:28.210" Title="Would you arrive sooner on Mars from Earth if you wait or if you leave immediately, when it's furthest from Earth?" Tags="|orbital-mechanics|trajectory|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16689" PostTypeId="1" CreationDate="2016-06-14T04:26:18.133" Score="1" ViewCount="247" Body="&lt;p&gt;Simply inject heat into the core of Mars should do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recently NASA said that there was possibly ocean on Mars, if that's true, then the loss of water couldn't be related to it being lost in space. The only other explanation is that the water sank into the mantle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why didn't water on Earth sank into the mantle? Because of the internal heat from Earth's core. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how much energy is required to reheat the core of Mars? Now earth's internal heat budget is only 47 terawatts, while the solar radiation falling on earth's surface is 173,000 terawatts. Then it looks like it's fairly easy to meet the energy requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the core is heated up, we'll possibly get the geomagnet working too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the question is whether Earth's water is held at the surface because of the internal heat, because I've heard people saying the reason is that rock and iron is heavier, of which I'm skeptical.&lt;/p&gt;&#xA;" OwnerUserId="12507" LastEditorUserId="12507" LastEditDate="2016-06-14T15:57:59.737" LastActivityDate="2016-06-14T15:57:59.737" Title="A method to restore the oceans on Mars" Tags="|terraforming|" AnswerCount="1" CommentCount="3" ClosedDate="2016-06-14T08:27:05.160" ContentLicense="CC BY-SA 3.0" />
  <row Id="16690" PostTypeId="2" ParentId="16689" CreationDate="2016-06-14T08:02:05.853" Score="4" Body="&lt;p&gt;There's a problem with your idea:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This 47TW is not a measure of the heat stored in the earth, it's just the loss. Since the earth did dissipate heat for billions of years and will continue to do so for billions of years, the total amount of heat energy in the earth is tremendous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's estimate how much time it takes to re-heat the earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core of the earth has a radius of 3500km. Given an average density of 11g/cm³, it weights about $2\cdot10^{24}kg$ (Entire earth: $6\cdot10^{24}kg$). Assuming a heat capacity of 400J/(kg K) and a temperature of 4000K, the heat energy inside the earth is $3.2\cdot10^{30}J$. Divide this by your power from the sun, and you get 590,000 years. This is roughly the time it takes to heat up earth's core by sunlight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this is a very low estimation. The inner core has a temperature of 6000K and consists mainly of iron with higher heat capacity. And the mantle of the earth also contains lots of heat. I would not wonder if the real value is several times larger, but I don't want to do the math for a more precise estimation now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's assume Mars is a little (shrinked) earth. It's about 50% farther away from sun (reduces sunlight power per surface to 1/(1.5²)=44%) and has a radius of 3400km (reduces cross section to 3400²/6370²=28%. So the entire sunlight power is 12.7%. The volume of mars is 3400³/6370³=15.2% of that of the earth, and so the heat energy is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would take about 15.2/12.7=20% more time, i.e. 708,000 years to heat up mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isn't feasible. Even if, how could the entire sunlight be used to heat the core? Painting the entire planet black would heat up the surface, and the heat would rather dissipate to space than into the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note again: My calculations are in no way precise. They use lots of assumptions, which are chosen so that the numbers are better for your idea, and neglect effects that would make the numbers worse.&lt;/p&gt;&#xA;" OwnerUserId="11749" LastActivityDate="2016-06-14T08:02:05.853" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16691" PostTypeId="1" CreationDate="2016-06-14T08:31:43.587" Score="4" ViewCount="220" Body="&lt;p&gt;I was thinking about the Hall Effect Thrusters and how they are able to produce thrust. There's something that I don't understand: when the hollow cathodes emit electrons to neutralize the flow at the exit, there are some electrons going back to the exit of the channel. On the other hand, there are some electrons trapped in the magnetic field (radial direction) along the channel and it works as a magnetic resistance. Do you know if there's an accumulation of electrons over time in the magnetic field? If yes, can problems appear when ionizing the fluid?&lt;/p&gt;&#xA;" OwnerUserId="15736" LastEditorUserId="6816" LastEditDate="2016-06-14T08:40:52.990" LastActivityDate="2022-09-08T11:38:43.257" Title="Electron buildup in Hall effect thrusters?" Tags="|propulsion|advanced-propulsion|ion-thruster|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16692" PostTypeId="1" CreationDate="2016-06-14T13:10:03.313" Score="2" ViewCount="74" Body="&lt;p&gt;Talking about Hall Effect Thrusters (HET), we can say that the presence of a hollow cathode is relevant to reach a good operation of the thruster. We also can plot the potential-x (trajectory of electrons) graph to see how they behave when approaching to the channel. &lt;strong&gt;How would differ the potential-x plot if we switch off the magnetic coils?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="15736" LastEditorUserId="1142" LastEditDate="2016-06-14T15:21:53.283" LastActivityDate="2016-06-14T15:21:53.283" Title="Switching off the magnetic coils in a Hall Effect Thruster" Tags="|propulsion|advanced-propulsion|ion-thruster|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16693" PostTypeId="2" ParentId="16568" CreationDate="2016-06-14T14:06:49.477" Score="4" Body="&lt;p&gt;I've only been able to find one reference to joint observations thus far:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Juno produces five pole-to-pole latitudinal maps of microwave opacity as a function of&#xA;  altitude to depths greater than 100 bars (Figure 2). The independent swaths at different&#xA;  longitudes provide the ability to understand large scale features such as the Great Red&#xA;  Spot. The 0.1% precision of the radiometer allows us to measure small variations in radiance&#xA;  with respect to horizontal position and emission angle. With these measurements,&#xA;  we determine the global O/H and N/H ratios; we correlate the patterns of ammonia and&#xA;  water abundance below the clouds with the principal dynamical features at cloud-top&#xA;  level; we examine the deep roots of features like the Great Red Spot, the belts and zones,&#xA;  and potentially the 5- ?m hot spots. Context for these features is provided through &lt;strong&gt;coordinated&#xA;  Earth-based images&lt;/strong&gt; and comparison with data from the E/PO imager, JunoCam.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-- &lt;a href=&quot;http://journals.cambridge.org/download.php?file=%2FIAU%2FIAU6_S269%2FS1743921310007313a.pdf&amp;amp;code=5bf61c03bd538e669ea5e022a6df2e32&quot; rel=&quot;nofollow&quot;&gt;The Juno Mission, S. J. Bolton and the Juno Science Team&lt;/a&gt; (emphasis added)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This pretty much amounts to a mission statement though. It is in a paper titled &quot;The Juno Mission&quot; put together by the Juno Science Team, and it says they will be providing context for Jupiter features with &quot;coordinated Earth-based images&quot;. So it sounds like the answer to your question is a simple yes.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-06-14T14:06:49.477" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16694" PostTypeId="1" AcceptedAnswerId="16711" CreationDate="2016-06-14T20:14:40.913" Score="16" ViewCount="869" Body="&lt;p&gt;I'm really interested about the Juno mission launched in 2011. The spacecraft will reach Jupiter on July 4, 2016, in couple days from when I write this question. It will orbit Jupiter 37 times collecting all different sorts of data, and at the end of the mission, it will deorbit, or just fly into Jupiter's atmosphere and burn up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a visible-light camera/telescope onboard of Juno, so it will be able to take pictures. &lt;strong&gt;Since it is able to take pictures, would it be able to record and send the video of how does travel through Jupiter's atmosphere look like before it burns up?&lt;/strong&gt; I couldn't find the answer anywhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be awesome to have a real footage of flying through Jupiter's clouds, wouldn't it?&lt;/p&gt;&#xA;" OwnerUserId="15742" LastEditorUserId="15742" LastEditDate="2016-06-15T15:12:30.507" LastActivityDate="2016-06-15T19:09:46.033" Title="Will Juno record its deorbit?" Tags="|orbit|nasa|spacecraft|jupiter|juno|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16695" PostTypeId="2" ParentId="13947" CreationDate="2016-06-14T20:40:25.710" Score="2" Body="&lt;p&gt;Because Earth and the Sun are not alone in this dance. Jupiter and Venus tug on us too. They have unsynochrized periods and slightly different inclinations. If a copy of Earth were suddenly put at L3, it would soon gradually spiral into another orbit. And unavoidably collide with us. Such a copycat planet 180 degrees away would have to have a Moon like ours with identical mass and orbital parameters too, because it influences Earth's orbit around the Sun. It irregularly sometimes pulls us ahead or away when we are slightly closer to the Sun, because of the eccentricities of both Earth and the Moon. For the same reason that there is not an even number of months per year. I have not done the math, this is a simple (and rare) case where sound space intuition knows why it is impossible.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-06-14T20:55:59.170" LastActivityDate="2016-06-14T20:55:59.170" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16696" PostTypeId="1" CreationDate="2016-06-14T20:41:21.763" Score="11" ViewCount="718" Body="&lt;p&gt;On the ISS each astronaut gets &lt;a href=&quot;https://www.quora.com/Do-astronauts-use-disposable-towels-or-do-they-wash-them-What-else-do-they-use-to-clean-themselves&quot; rel=&quot;noreferrer&quot;&gt;a packaged wet towel every two days, two dry towels and two washcloths a week, and 3 small packets of moist napkins every two days&lt;/a&gt; with which to wash themselves. At the end of the week these towels are discarded because there is no way to wash them. On earlier, shorter ISS missions the method was mostly to use &lt;a href=&quot;https://youtu.be/yq0Kzx2uDxs&quot; rel=&quot;noreferrer&quot;&gt;wet wipes&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The method currently used also includes using a pouch of special &lt;a href=&quot;https://www.youtube.com/watch?v=tDbbJWKKQu0&quot; rel=&quot;noreferrer&quot;&gt;no rinse body bath fluid&lt;/a&gt; that comes in pouches. (Or perhaps the fluid is sent in a bulk pouch and  astronauts dispense it into the smaller pouches.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Altogether this is a fair amount of mass and volume taken up on cargo missions so that astronauts can bathe. Could this be more optimal? The towels are standard terry cloth and the pouches are the same ones used for most fluids on the ISS.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-06-24T15:08:22.520" LastActivityDate="2016-12-05T01:55:57.053" Title="Is there a better way for astronauts to bathe?" Tags="|iss|health|astronaut-lifestyle|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="16697" PostTypeId="2" ParentId="16696" CreationDate="2016-06-14T20:42:02.857" Score="8" Body="&lt;p&gt;Maybe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.gizmag.com/epic-wipes/43839/&quot; rel=&quot;noreferrer&quot;&gt;giant wet wipe&lt;/a&gt;  for use on Earth when people want to shower but can't (camping, sweaty sports in the country) has been developed. It measures 50 x 80 cm, comes in individual packages, pre-moistened with a cleansing solution that doesn't need to be dried off afterwards. Thus it has all the characteristics that would also be convenient for astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass of them isn't listed but when compared with the packaged towels currently sent, they look lighter, and also eliminate the need to send pouches of body bath separately. They seem like they would optimize the amount of body bath needed, and be easier to dry out so their water can be recycled, as they are quite thin. That is just an eyeball assessment, of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TKapB.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TKapB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2016-06-14T20:42:02.857" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16698" PostTypeId="2" ParentId="16696" CreationDate="2016-06-15T00:46:07.417" Score="8" Body="&lt;p&gt;Skylab had a &lt;a href=&quot;http://www.space.com/24927-showering-on-skylab.html&quot; rel=&quot;noreferrer&quot;&gt;shower&lt;/a&gt; but I seem to recall it was a huge pain to clean up after use since the water clung to the sides.  So much trouble that they used towels / wipes instead or did without.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ceNfM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ceNfM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So not sure it was better, just listing it for completeness.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-06-15T00:46:07.417" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16699" PostTypeId="1" CreationDate="2016-06-15T09:52:03.220" Score="1" ViewCount="2370" Body="&lt;p&gt;Can I calculate the two-line element for a new satellite using the orbital parameters? How do I calculate mean motion for a satellite?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is BSTAR drag term in TLE same for all satellites? If not, then how do I calculate BSTAR drag term? &lt;/p&gt;&#xA;" OwnerUserId="15747" LastEditorUserId="58" LastEditDate="2016-06-15T16:41:44.070" LastActivityDate="2016-06-16T00:31:28.847" Title="How to find the TLE (two-line element) for a satellite using orbital parameters?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16700" PostTypeId="1" AcceptedAnswerId="16702" CreationDate="2016-06-15T13:37:03.923" Score="20" ViewCount="3541" Body="&lt;p&gt;If my memories are accurate, then there's only one celestial body beyond Mars, on which any man-made device was landed. &lt;a href=&quot;https://www.google.hu/search?q=titan+surface&amp;amp;biw=1440&amp;amp;bih=775&amp;amp;source=lnms&amp;amp;tbm=isch&amp;amp;sa=X&amp;amp;ved=0ahUKEwiU4_OIk6rNAhUD3SwKHUA9BsUQ_AUIBigB&quot;&gt;Pictures made there&lt;/a&gt; are, in fact, quite popular among space fans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;But why Titan?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One would assume Europa is much more of an object of interest, because of the presumed ice layer.&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="4660" LastEditDate="2016-06-15T14:47:20.877" LastActivityDate="2016-06-16T14:34:08.690" Title="Why was Titan the first celestial body beyond Mars to be landed on?" Tags="|landing|planetary-science|history|europa|titan|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16701" PostTypeId="2" ParentId="16700" CreationDate="2016-06-15T13:45:54.957" Score="4" Body="&lt;p&gt;They landed on Titan because Titan is the only natural satellite in the Solar System that has a dense atmosphere. And it is the only object other than Earth where surface liquid has been found. Source: &lt;a href=&quot;http://en.wikipedia.org/wiki/Titan_(moon)&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13559" LastEditorUserId="1142" LastEditDate="2016-06-15T13:55:15.020" LastActivityDate="2016-06-15T13:55:15.020" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16702" PostTypeId="2" ParentId="16700" CreationDate="2016-06-15T13:46:45.713" Score="31" Body="&lt;p&gt;Simple. It was the easiest to land on. Titan has an atmosphere, which makes landing there quite a bit easier than landing on Europa, which does not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, Europa has only been known as an object of interest since Galileo, which was the last mission that even had a chance of sending a lander there. It was suspected as an object in the Voyager flyby, but it was much less sure until recently. Cassini was under works, along with the mission to Titan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Titan is as interesting of an object as Europa, and has been known to be an object of such interest even well before the space age, due to it's atmosphere of hydrocarbons. Titan, Europa, Enceladus, Triton, and slowly Pluto are emerging as the most interesting objects in the outer Solar System, from the perspective of potentially having life, or at least the building blocks. While Europa is now known to be the most promising candidate, it took scientists a while to realize that, and there hasn't been time to send a mission there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, there is the issue of radiation. Europa, being so close to Jupiter, is the subject of powerful radiation, similar to the Van Allen belts. That makes it even more difficult to manage a Europa lander.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-06-15T14:18:21.400" LastActivityDate="2016-06-15T14:18:21.400" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16703" PostTypeId="1" AcceptedAnswerId="16705" CreationDate="2016-06-15T13:56:06.020" Score="2" ViewCount="1165" Body="&lt;p&gt;We have never landed on Europa, so the assumptions made on a possible ice layer on its surface are made on the data sent by various objects approaching it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure these assumptions were accurately and carefully made, but still, I'm interested: is it possible that we're wrong and what we think ice is something else?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="25" LastEditDate="2016-06-15T14:02:46.520" LastActivityDate="2016-06-15T16:30:08.077" Title="How likely is that Europa actually has no ice layer?" Tags="|planetary-science|water|europa|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16704" PostTypeId="2" ParentId="16703" CreationDate="2016-06-15T14:02:32.363" Score="3" Body="&lt;p&gt;The ice layer is almost certain, in fact, it's not all that rare. Most outer solar system objects have an ice layer. The real interesting question is the potential water layer on Europa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Europa_%28moon%29#Subsurface_ocean&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, we get a better overall picture of what's going on. It is almost certain that there is a layer of water underneath the ice. The size of that layer is the subject of considerable debate. Ranges estimate from less than a km to a layer of around 10-30 km thick. Scientists are pretty sure that there is in fact a layer of liquid water underneath Europa, however, so it is very likely that is the case.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-15T14:02:32.363" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16705" PostTypeId="2" ParentId="16703" CreationDate="2016-06-15T14:11:10.537" Score="6" Body="&lt;p&gt;The Hubble Space Telescope did an ultraviolet emission study of vapor plumes from Europa's southern hemisphere in 2012. It found that the emissions were consistent with $H_2O$.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;H2O freezes (19), and any enhancement must be localized to its source. In addition, electron impact on H2O (e + H2O) yields HI 121.6 nm and OI 130.4 nm, but has a lower cross section at 135.6 nm (20). Hence, the emission surplus at HI 121.6 nm and OI 130.4 nm in an area, where no remarkable enhancement of OI 135.6 nm is found, suggests a local atmospheric H2O enhancement.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now if liquid water is spraying from below the surface, there can be no doubt that there is water ice on the surface. This also bears significant support to the subsurface ocean theory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science.sciencemag.org/content/343/6167/171.full&quot;&gt;Transient Water Vapor at Europa’s South Pole - Roth, et al.&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-06-15T14:11:10.537" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16708" PostTypeId="2" ParentId="16703" CreationDate="2016-06-15T16:30:08.077" Score="2" Body="&lt;p&gt;Europa's composition has been studied by earth based telescopes, space based telescopes and by probes from close range. Observations have been made using visible light, spectroscopes, and a variety of other instruments. All of these agree that the surface of Europa is ice, and there doesn't seem to be any scientist challenging these results. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it is theoretically possible that all this could be wrong it's extremely unlikely. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-06-15T16:30:08.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16711" PostTypeId="2" ParentId="16694" CreationDate="2016-06-15T17:46:39.913" Score="12" Body="&lt;p&gt;You won't see Jupiter's atmosphere from the &quot;inside&quot;. The entry and destruction of the vehicle is very fast, and occurs relatively high in the atmosphere. Way, way above any clouds. There would not be time to take an image, process it, and get it out the antenna, even if you happened to have Earth in view for the entry. Even if you did have a picture looking up or sideways just before the moment of destruction, it would be like a high-altitude image in Earth's atmosphere, which looks for all purposes like you're in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can of course look down and see the atmosphere, so it would be nice if they did that on the way in and transmitted images for as long as possible. I don't know their intent. In any case, their focus right now is to get into orbit and conduct the science mission.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-06-15T19:09:46.033" LastActivityDate="2016-06-15T19:09:46.033" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16712" PostTypeId="1" AcceptedAnswerId="16713" CreationDate="2016-06-15T20:44:35.980" Score="23" ViewCount="1217" Body="&lt;p&gt;Recent years have brought us some interesting landings: Titan, the comet, the Chinese landed on the Moon, we got a lab the size of a SUV on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, Mars is in the focus of the West, while India and China are targeting the Moon. Are there any landers scheduled (developed, underway, or at least fleshed out in plans) to any other bodies though?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Budget-approved missions are preferred, but others may be included as well as long as their non-approved status is clearly indicated.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-09-13T09:06:15.837" LastActivityDate="2021-05-10T14:21:52.040" Title="What landings beyond Mars and the Moon are scheduled?" Tags="|future-missions|lander|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16713" PostTypeId="2" ParentId="16712" CreationDate="2016-06-15T21:42:26.593" Score="24" Body="&lt;p&gt;Feel free to edit this answer to add more or adjust formatting. List may be incomplete.&lt;/p&gt;&#xA;&lt;p&gt;Ordered by launch date:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa 2 and MASCOT to 162173 Ryugu&lt;/a&gt;, Japan/Germany/France, launched 2014, landed three rovers in 2018 (fourth rover failed but was released to briefly orbit the asteroid in 2019).&lt;br /&gt;&#xA;Hayabusa 2 departed November 2019 and returned to do an Earth flyby&#xA;and sample return December 2020.&lt;br /&gt;&#xA;The mission has been extended, and Hayabusa 2 will now make additional flybys (but no landings) of other asteroids in 2026 and 2031.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-REx to 101955 Bennu&lt;/a&gt;, USA, launched 2016, contacted April 2020 as part of a sample collection rehearsal, and after further contacts for collection, closed its sample return capsule October 2020, return capsule will reenter Earth's atmosphere September 2023&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/1866/58&quot;&gt;ASTER probe to 2001 SN263&lt;/a&gt;, Brazil, launch 2022, arrive 2025, awaiting funding&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/AIDA_%28mission%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;DART to the moon of 65803 Didymos&lt;/a&gt;, NASA, launch between November 2021 and February 2022, impact October 2022 (ESA's simultaneous orbiter Hera is scheduled to arrive later, possibly delaying also DART)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://science.slashdot.org/story/17/01/05/0152213/nasa-unveils-two-new-missions-to-study-truly-strange-asteroids&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lucy to Trojans sharing orbit with Jupiter&lt;/a&gt; NASA Launch oct. 2021, arrival 2027-2033.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter_Icy_Moon_Explorer#Ganymede_Lander&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter Icy Moon Explorer (JUICE)&lt;/a&gt;, ESA/Russia, launch 2022, l̶a̶n̶d̶i̶n̶g̶ ̶2̶0̶3̶3̶, originally proposed to include a Russian lander on Ganymede (the mission was selected, the lander portion will likely be spun off as a separate mission, see Laplace-P)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://science.slashdot.org/story/17/01/05/0152213/nasa-unveils-two-new-missions-to-study-truly-strange-asteroids&quot; rel=&quot;nofollow noreferrer&quot;&gt;Psyche to Psyche (210km metallic asteroid in the Asteroid Belt)&lt;/a&gt; NASA, Launch 2022, arrival 2026&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Europa_Multiple-Flyby_Mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Europa Clipper&lt;/a&gt;, previously Europa Multiple-Flyby Mission, USA, launch around 2024, arrive in a few years, proposed lander split off to separate mission (see below)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Fobos-Grunt#Repeat_mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fobos-Grunt 2 to Phobos&lt;/a&gt;, Russia, launch 2024, awaiting reapproval&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Europa_Lander_(NASA)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Europa Lander&lt;/a&gt;, NASA, launch 2025, arrive and landing about 2030, awaiting funding&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venera-D&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venera-D to Venus&lt;/a&gt;, Russia, launch around 2026, in preliminary design but implementation has been delayed&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Laplace-P&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laplace-P to Ganymede&lt;/a&gt;, Russia, proposed launch 2026, proposed landing 2030, awaiting funding&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/feature/nasa-completes-study-of-future-ice-giant-mission-concepts&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mission to Uranus and or Neptune about 2030&lt;/a&gt; is under study by NASA (full report pdf is &lt;a href=&quot;https://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29#Exploration&quot; rel=&quot;nofollow noreferrer&quot;&gt;CNSA Ceres probe&lt;/a&gt;, China, launch around 2020s, not yet approved&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.spaceref.com/news/viewpr.html?pid=22554&quot; rel=&quot;nofollow noreferrer&quot;&gt;PRIME to Phobos&lt;/a&gt;, Canada, not yet approved&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_Lake_In-situ_Sampling_Propelled_Explorer&quot; rel=&quot;nofollow noreferrer&quot;&gt;TALISE to Titan&lt;/a&gt;, Spain, not yet approved&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Stretching the definition of &amp;quot;landing&amp;quot; a little bit, there could also be our first &amp;quot;landing&amp;quot; (in the sense of arrival) at another star system:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Breakthrough_Starshot&quot; rel=&quot;nofollow noreferrer&quot;&gt;Breakthrough Starshot to Alpha Centauri&lt;/a&gt;, international effort by Breakthrough Initiatives, launch around 2036, arrive between 2056-2066 (communication of successful arrival would take another 4 years), in research phase&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2021-05-10T14:21:52.040" LastActivityDate="2021-05-10T14:21:52.040" CommentCount="2" CommunityOwnedDate="2016-06-15T21:42:33.203" ContentLicense="CC BY-SA 4.0" />
  <row Id="16714" PostTypeId="1" AcceptedAnswerId="16744" CreationDate="2016-06-15T22:35:38.953" Score="5" ViewCount="437" Body="&lt;p&gt;The NOAA Application for Earth Remote Sensing asks &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;If the satellite disposition involves an atmospheric re-entry the applicant must provide an estimate of the total debris casualty area of the system’s components and structure likely to survive re-entry.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume this has a lot to do with the materials. Is there a database that I can turn to to find out which materials will and won't burn up? I've heard Titanium does not burn up well, but I'm not sure about others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there is not a database, how do I calculate based on material properties and structure geometry?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought maybe calculation of debris casualty area could be a separate question.&lt;/p&gt;&#xA;" OwnerUserId="6570" LastEditorUserId="4660" LastEditDate="2016-06-17T15:26:10.363" LastActivityDate="2016-06-17T15:26:10.363" Title="How do I determine what materials will burn up during reentry?" Tags="|reentry|materials|reference-request|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16715" PostTypeId="1" CreationDate="2016-06-15T23:05:15.053" Score="4" ViewCount="99" Body="&lt;p&gt;I've sometimes seen icy formations form and break off of spacecraft.  In &lt;a href=&quot;https://www.youtube.com/watch?v=gLNmtUEvI5A&quot; rel=&quot;nofollow noreferrer&quot;&gt;today's SpaceX launch&lt;/a&gt;, there are a couple examples (times based on video) at &lt;a href=&quot;https://youtu.be/gLNmtUEvI5A?t=51m55s&quot; rel=&quot;nofollow noreferrer&quot;&gt;51:55&lt;/a&gt; where you can see it growing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/gLNmtUEvI5A?t=51m55s&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TdphM.png&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two similar structures are visible at &lt;a href=&quot;https://youtu.be/gLNmtUEvI5A?t=49m53s&quot; rel=&quot;nofollow noreferrer&quot;&gt;49:53&lt;/a&gt; (you can see the larger one break off and more gas or something escape a few seconds later), helping clarify:&#xA;&lt;a href=&quot;https://youtu.be/gLNmtUEvI5A?t=49m53s&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wrFMH.jpg&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or at &lt;a href=&quot;https://youtu.be/gLNmtUEvI5A?t=50m26s&quot; rel=&quot;nofollow noreferrer&quot;&gt;50:26&lt;/a&gt;, which seems to be on the other side:&#xA;&lt;a href=&quot;https://youtu.be/gLNmtUEvI5A?t=50m26s&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EPSiE.jpg&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My best guess is that this is frozen propellant for whatever small thrusters keep the rocket on course, though the first time I saw the first example it seemed like something leaking out of the large ring.  If that's it, does this icing cap/clog the thruster and make it less effective, or risk cutting off the course correction capability altogether? &lt;/p&gt;&#xA;" OwnerUserId="11181" LastEditorUserId="58" LastEditDate="2019-09-16T15:19:10.157" LastActivityDate="2019-09-16T15:19:10.157" Title="What is the icy formation on these rockets?" Tags="|spacex|falcon-9|fuel|attitude|" AnswerCount="0" CommentCount="0" ClosedDate="2016-06-16T12:35:28.237" ContentLicense="CC BY-SA 3.0" />
  <row Id="16716" PostTypeId="2" ParentId="16699" CreationDate="2016-06-16T00:31:28.847" Score="1" Body="&lt;p&gt;The mean motion is inversely proportional to the period:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$n = \frac{1}{P}$$&lt;/span&gt;&#xA;This gives &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; in as a revolutions per unit time.&lt;/p&gt;&#xA;&lt;p&gt;The b-star is essentially empirical. One could calculate it as&#xA;&lt;span class=&quot;math-container&quot;&gt;$$B = \frac{C_d A}{m} $$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$B^* = B \rho / 2$$&lt;/span&gt;&#xA;with &lt;span class=&quot;math-container&quot;&gt;$m$&lt;/span&gt; as mass, &lt;span class=&quot;math-container&quot;&gt;$C_d$&lt;/span&gt; a drag coefficient one might get from CFD, &lt;span class=&quot;math-container&quot;&gt;$A$&lt;/span&gt; the cross-sectional area, &lt;span class=&quot;math-container&quot;&gt;$\rho$&lt;/span&gt; density of atmosphere. In practice this is determined from curve fitting with multiple observations, so it can end up incorporating multiple pertubative effects, not just drag. (In fact, it can be negative!) For a small near-earth satellite setting it to zero often suffices.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://celestrak.org/columns/v04n03/%20%22FAQs:%20Two-Line%20Elements&quot; rel=&quot;nofollow noreferrer&quot;&gt;FAQ from CelesTrak&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://celestrak.org/NORAD/documentation/spacetrk.pdf&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;The complete details&quot;&gt;The complete details&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10477" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2016-06-16T00:31:28.847" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16717" PostTypeId="1" AcceptedAnswerId="16719" CreationDate="2016-06-16T03:01:55.707" Score="2" ViewCount="737" Body="&lt;p&gt;I am using Unity3D and writing C#&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Earth Mass: 5.972e+22&lt;/li&gt;&#xA;&lt;li&gt;Earth Velocity: [0,0,0]&lt;/li&gt;&#xA;&lt;li&gt;Earth Radius: 63.71&lt;/li&gt;&#xA;&lt;li&gt;ISS Mass: 4194.55&lt;/li&gt;&#xA;&lt;li&gt;Starting ISS Velocity: [76.50, 0, 0]&lt;/li&gt;&#xA;&lt;li&gt;ISS Altitude: 4&lt;/li&gt;&#xA;&lt;li&gt;Gravitational Constant: 6.67408e-15&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All values are the real life values / 100, as I am scaling things down. (ex. Gravitational Constant was 6.67408e-11 / 11 = 6.67408e-15.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the code...&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;void Gravity(Gravity body) {&#xA;    float dSquared = (body.transform.position - transform.position).sqrMagnitude;&#xA;    float force = (body.G * shipMass * body.planetMass) / dSquared;&#xA;    Vector3 forceDirection = (body.transform.position - transform.position).normalized;&#xA;    Vector3 forceVector = (forceDirection * force);&#xA;    shipVelocity += forceVector * Time.deltaTime;&#xA;&#xA;    transform.position += shipVelocity * Time.deltaTime;&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;In order to make this work I need to set the gravitational constant to: 1.59508e-21&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an image of the orbit, the pink line is past not future path (the calculated orbital elements shown are most likely wrong):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/mbsuV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mbsuV.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an image of the orbit after I speed up time:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/UteaX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UteaX.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit 1:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using &lt;a href=&quot;http://formulas.tutorvista.com/physics/acceleration-due-to-gravity-formula.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this formula for acceleration&lt;/a&gt; and the first part of Russell Borogove's answer I was able to achieve a circular orbit using these values: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Earth Mass: 5.972e+22&lt;/li&gt;&#xA;&lt;li&gt;Earth Velocity: [0,0,0]&lt;/li&gt;&#xA;&lt;li&gt;Earth Radius: 63.71&lt;/li&gt;&#xA;&lt;li&gt;ISS Mass: 4194.55&lt;/li&gt;&#xA;&lt;li&gt;Starting ISS Velocity: [0.0765, 0, 0]&lt;/li&gt;&#xA;&lt;li&gt;ISS Altitude: 4&lt;/li&gt;&#xA;&lt;li&gt;Gravitational Constant: 6.67408e-24 (Shifted -15 then +2 orders of magnitude)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;and this code:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;void Gravity(Gravity body) {&#xA;    float dSquared = (body.transform.position - transform.position).sqrMagnitude;&#xA;    float M = body.planetMass;&#xA;    Vector3 forceDirection = (body.transform.position - transform.position).normalized;&#xA;    float g = G * M / dSquared;&#xA;    Vector3 forceVector = (forceDirection * g);&#xA;    shipVelocity += forceVector * Time.deltaTime;&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Edit 2: Added gravity calculations using Russell Borogove's accumulator script below...&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;void Update() {&#xA;    Gravity(orbitee);&#xA;    ApplyForce();&#xA;}&#xA;&#xA;void ApplyForce() {&#xA;    transform.position += shipVelocity * Time.deltaTime;&#xA;    transform.Rotate(transform.forward * shipTorque * Time.deltaTime);&#xA;}&#xA;&#xA;void Gravity(Gravity body) {&#xA;    accumulator += Time.deltaTime;&#xA;    while (accumulator &amp;gt; 0.0f) {&#xA;        float dSquared = (body.transform.position - transform.position).sqrMagnitude;&#xA;        float M = body.planetMass;&#xA;        Vector3 forceDirection = (body.transform.position - transform.position).normalized;&#xA;        float g = G * M / dSquared;&#xA;        Vector3 forceVector = (forceDirection * g);&#xA;        shipVelocity += forceVector * simtime;&#xA;        accumulator -= simtime;&#xA;    }&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Edit 3: Updated Orbital Elements Script (Probably not 100% correct)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;    //Mass of satellite&#xA;    float m1 = shipMass;&#xA;&#xA;    //Mass of planet&#xA;    float m2 = orbiting.planetMass;&#xA;    float M = m2;&#xA;&#xA;    //Relative Position Vector&#xA;    Vector3 r = FindRelativePosition(orbitee.transform.position, transform);&#xA;&#xA;    //Relative Velocity Vector&#xA;    Vector3 v = shipCurVel;&#xA;    //#!# add relative to planet velocity&#xA;&#xA;    //Specific Angular Momentum&#xA;    Vector3 h = Vector3.Cross(r, v);&#xA;&#xA;    //Standard Gravitational Parameter&#xA;    float µ = G * (m1 + m2);&#xA;    //float µ = G * M;&#xA;&#xA;    //Eccentricity Vector&#xA;    Vector3 evec = (Vector3.Cross(v, h) / µ) - (r / Vector3.Magnitude(r));&#xA;&#xA;    //Eccentricity&#xA;    float e = Vector3.Magnitude(evec);&#xA;&#xA;    //Vector to Ascending Node #!#&#xA;    Vector3 n = Vector3.Cross(new Vector3(0, 0, 1), h);&#xA;&#xA;    //True Anomaly&#xA;    float t = Mathf.Acos((Vector3.Dot(evec, r)) / (e * Vector3.Magnitude(r)));&#xA;    if (Vector3.Dot(r, v) &amp;lt; 0)&#xA;        t = (2 * Mathf.PI) - t;&#xA;&#xA;    //Eccentric Anomaly&#xA;    float E = 2 * Mathf.Atan(Mathf.Tan(t / 2) / Mathf.Sqrt((1 + e) / (1 - e)));&#xA;&#xA;    //Longitude of Ascending Node (2D)&#xA;    float Ω = 0;&#xA;&#xA;    //Inclination (2D)&#xA;    float i = 0;&#xA;&#xA;    //Argument of Periapsis&#xA;    float ω = Mathf.Atan2(evec.y, evec.x);&#xA;    float ωdegrees = ω * (180 / Mathf.PI);&#xA;    //float ω = Mathf.Acos((Vector3.Dot(n, evec)) / (e * Vector3.Magnitude(n)));&#xA;&#xA;    //Mean Anomaly &#xA;    float MA = E - (e * Mathf.Sin(E));&#xA;&#xA;    //Semi-Major Axis&#xA;    float a = 1 / ((2 / Vector3.Magnitude(r)) - (Mathf.Pow(Vector3.Magnitude(v), 2) / µ));&#xA;&#xA;    //Apoapsis&#xA;    float ap = a * (1 + e);&#xA;&#xA;    //Periapsis&#xA;    float pe = a * (1 - e);&#xA;&#xA;    //Orbital Period&#xA;    float T = (2 * Mathf.PI) * Mathf.Sqrt(Mathf.Pow(a, 3) / µ)/60;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Most recent edit: fixed issue I was having with eccentricity being incorrect - was calculating relative position wrong.&lt;/p&gt;&#xA;" OwnerUserId="15714" LastEditorUserId="15714" LastEditDate="2016-06-29T17:41:39.227" LastActivityDate="2016-06-29T17:41:39.227" Title="Am I supposed to modify the gravitational constant with scale and why do fps &amp; time scale changes cause my orbit to break?" Tags="|orbital-mechanics|gravity|orbital-elements|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16719" PostTypeId="2" ParentId="16717" CreationDate="2016-06-16T03:48:00.993" Score="7" Body="&lt;p&gt;Big G is 6.67408e-11 m&lt;sup&gt;&lt;strong&gt;3&lt;/strong&gt;&lt;/sup&gt; kg&lt;sup&gt;&lt;strong&gt;-1&lt;/strong&gt;&lt;/sup&gt; sec&lt;sup&gt;-&lt;strong&gt;2&lt;/strong&gt;&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you uniformly use 100km units instead of m units, that's 5 magnitudes different, so G has to change by &lt;em&gt;15&lt;/em&gt; magnitudes because the linear dimension is cubed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass units are 100kg, G uses -1 of those, so G goes the other way by 2 magnitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no idea what you're doing with seconds. If seconds are seconds (and it takes 90 minutes to complete an orbit), then there's no change to G. If you're using some other conversion factor, then double the number of zeros you're shifting seconds by to change G.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the instability, that's inevitable when you do time-step simulations of continuous functions. You can split your Unity Update time slice into some small fixed quantum and run your simulation function several times for each game frame:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;float accumulator = 0.0f;&#xA;float simtime = 0.001f;&#xA;&#xA;void Update( void )&#xA;{&#xA;    accumulator += Time.deltaTime;&#xA;    while (accumulator &amp;gt; 0.0f)&#xA;    {&#xA;         SimulateGravityForTime( simtime );&#xA;         accumulator -= simtime;&#xA;    }&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;This runs your sim once for each millisecond of time elapsed in the frame. That should keep things pretty stable. The accumulator is there to retain fractional milliseconds from frame to frame. You'll spend more CPU time doing repeated computations, but a modern computer should be able to afford it easily. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Regarding the units, if I were in your shoes I would separate the &lt;em&gt;view&lt;/em&gt; (based on Unity transforms) from the &lt;em&gt;model&lt;/em&gt; (the physics simulation). This would allow you to run the simulation entirely in standard units, eliminating much confusion and reducing the likelihood of errors. Inside the physics model you'd always use m/kg/s units and keep track of positions in vectors that are not part of Unity transforms. Once per frame you'd carefully convert to Unity units, and set the positions of the transforms.  &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-06-16T13:27:06.767" LastActivityDate="2016-06-16T13:27:06.767" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="16720" PostTypeId="1" CreationDate="2016-06-16T04:08:53.390" Score="4" ViewCount="938" Body="&lt;p&gt;How much iron would a magnet pick up on the surface of Mars?  Would it be rich in iron particles like going to a playground on earth?  Would it be clean enough to take to a refinery and melt down, or would it be too rusty, aka oxidized? How would it have gotten so oxidized, given the currently low levels of oxygen?&lt;/p&gt;&#xA;" OwnerUserId="15761" LastEditorUserId="58" LastEditDate="2016-06-16T15:14:34.097" LastActivityDate="2016-07-11T14:04:54.797" Title="Is Mars iron too oxidized to pick up with a magnet?" Tags="|mars|mining|isru|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16721" PostTypeId="1" AcceptedAnswerId="16725" CreationDate="2016-06-16T06:29:59.427" Score="3" ViewCount="1349" Body="&lt;p&gt;This is similar to &lt;a href=&quot;https://space.stackexchange.com/questions/15292/optimum-delta-v-burn-to-change-periapsis-or-apoapsis-at-an-arbitrary-point-on-an&quot;&gt;another StackExchange post&lt;/a&gt; (right down to the Kerbal Space Program mentions), but with a different approach.  The question I'm trying to solve is thus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given current orbital state vectors for position ($\overrightarrow{r}$) and velocity ($\overrightarrow{v}$) (and it's safe to assume all of the current orbit's Keplerian elements are known as well), I am attempting to determine the $\Delta v $ required to reach a target apoapsis $R_{a}$.  The location in the orbit is arbitrary; it might not be at apoapsis or periapsis; thus the target semi-major axis $a$ and eccentricity $e$ are not known and using the Vis-viva equation for the target semi-major axis isn't an option by itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my particular case, I've added a constraint that the &lt;em&gt;direction&lt;/em&gt; of $\overrightarrow{v}$ will not change -- only its magnitude.  In other words, the resulting burn will be along the prograde/retrograde vector, and I'm essentially only trying to determine the magnitude of said vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My idea was to use a small portion of the &lt;a href=&quot;https://space.stackexchange.com/questions/1904/how-to-programmatically-calculate-orbital-elements-using-position-velocity-vecto&quot;&gt;formulas for programatically calculating orbital elements using position/velocity vectors&lt;/a&gt; in reverse -- namely those involving eccentricity and semi-major axis -- and then solve the resulting equation for the magnitude of $\overrightarrow{v}$.  To do this, I replaced $\overrightarrow{v}$ with $\overrightarrow{d}m$, where $d$ is the normalized current velocity vector and m its magnitude.  The resulting formula, after some simplification, looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;R_{a} = \frac{\mu + 1+|(m-g)\overrightarrow{r} - \overrightarrow{d}(\overrightarrow{r}\cdot\overrightarrow{d})m|}{2g-m}&#xA;$$&#xA;where $m$ is the magnitude of $\overrightarrow{v}$ squared and $g = \frac{\mu}{|\overrightarrow{r}|}$.  This definition of $g$ is technically wrong but works for this equation -- more on that in a minute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This equation gives me a formula that correctly determines my current apoapsis when I toss in the other parameters.  However, I couldn't figure out how to solve it for $m$ on my own... &lt;a href=&quot;https://www.wolframalpha.com/input/?i=NSOLVE%5B%7Ba%20%3D%3D%20(u%20%2B%201%2B(Norm%5B(m-g)%20*%20r%20-%20Dot%5Br,d%5D*d*m%5D))%2F(2*g-m)%7D,%20%7Bm%7D%5D.&quot; rel=&quot;nofollow noreferrer&quot;&gt;so I asked Wolfram Alpha&lt;/a&gt;, which gives me the following:&#xA;$$&#xA;m = \frac{2ag+gr-\mu-1}{a-dr\cdot r + r}&#xA;$$&#xA;or $$&#xA;m = \frac{2ag-gr-\mu-1}{a+dr\cdot r + r}&#xA;$$&#xA;(I couldn't find a way to convince it that $d$ and $r$ were vectors; thus they are not shown as such here.  $r$ is $R_{a}$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging the numbers back into this formula, however, yielded incorrect results.  But!  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found that if I used the &quot;correct&quot; formula for $g$ ($g = \frac{\mu/}{|\overrightarrow{r}|^2}$), it would yield completely wrong values for the apoapsis given other known variables... but plugging that 'wrong' apoapsis in to the other formula ended up yielding the correct value for $m$ for the 'correct' apoapsis.  That said, I have no idea how to take advantage of that fact -- and I'm at a loss at where to continue from here after poking at it for several days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, a few questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;What am I missing here?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is there some other method to solving this that I'm not missing?  I considered something involving conservation of angular momentum, but ran into the same problem where the formulas I could find require knowing the semi-major axis value.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="15717" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-06-16T18:59:24.413" Title="Calculating dV to raise apoapsis at an arbitrary point of an orbit" Tags="|orbital-mechanics|orbital-maneuver|mathematics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16722" PostTypeId="1" CreationDate="2016-06-16T09:28:29.513" Score="2" ViewCount="242" Body="&lt;p&gt;It is common knowledge know that SpaceX uses the landing barges when the requirements for the first stages are so high that a return-to-base is not achievable. This seems to occur both in case of GTO and LEO missions. This is clearly not ideal, since it involves transportation delay, cost and risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That makes me wonder: Are the trajectories between different large mass LEO and GTO missions that much different? Could SpaceX setup a plant somewhere closer to the expected landing zones to minimize transportation time and cost (e.g. airlift recovered boosters). Preferably some island, but also a large swimming platform might suffice.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-06-16T18:53:10.307" Title="How variable is the landing spot of a reusable Falcon 9 with a GTO payload" Tags="|spacex|falcon-9|recovery|facility|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16723" PostTypeId="1" AcceptedAnswerId="16724" CreationDate="2016-06-16T10:04:19.640" Score="5" ViewCount="1604" Body="&lt;p&gt;I have watched last Falcon 9 landing and launch, but during landing screen froze. I have heard that it didn't landed successfully, so what happened with it, does anyone know?&lt;/p&gt;&#xA;" OwnerUserId="13559" LastEditorUserId="25" LastEditDate="2016-06-17T16:05:14.877" LastActivityDate="2017-12-19T13:20:23.953" Title="Falcon 9 landing on drone ship 15/06/2016" Tags="|spacex|rockets|falcon-9|landing|reentry|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16724" PostTypeId="2" ParentId="16723" CreationDate="2016-06-16T10:14:54.960" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/743096769001578498&quot; rel=&quot;nofollow noreferrer&quot;&gt;The craft experienced a RUD&lt;/a&gt;, which is &lt;a href=&quot;https://twitter.com/elonmusk/status/743097668725940225&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elon Musk's technical term for a &lt;em&gt;Rapid Unscheduled Disassembly&lt;/em&gt;&lt;/a&gt;. It thus became a pile of spacecraft parts on the deck of the landing craft, and also burnt quite a bit. &lt;a href=&quot;https://twitter.com/elonmusk/status/743102502225076227&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX will shortly release images of the landed craft&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://pbs.twimg.com/media/ClA8d8xWEAAE7uW.jpg:large&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deimos has tweeted some low resolution images of the debris&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://pbs.twimg.com/media/ClA8d8xWEAAE7uW.jpg:large&quot; alt=&quot;https://pbs.twimg.com/media/ClA8d8xWEAAE7uW.jpg:large&quot;&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The debris will be taken back to Canaveral for forensic analysis to improve future landings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Images of the debris back in port are shown here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;kbd&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=_FdS4osHmn4&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xQy7S.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/kbd&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="25" LastEditDate="2017-12-19T13:20:23.953" LastActivityDate="2017-12-19T13:20:23.953" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16725" PostTypeId="2" ParentId="16721" CreationDate="2016-06-16T12:12:33.623" Score="1" Body="&lt;p&gt;We have the specific energy and specific angular momentum:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathcal{E}=-{\mu\over 2a}={v^2\over 2}-{\mu\over r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathcal{M}^2=\mu a\left(1-e^2\right)=r^2 v^2\cos^2\gamma$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mu$ is the $GM$ of the central body, $v$ and $r$ are the magnitudes of the respective vectors and $\gamma$ is the flight path angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can get $\cos\gamma$ given $\vec{r}$ and your initial $\vec{v_i}$ directly from the definition of angular momentum — a vector without the arrow is its magnitude (we don't need to worry about the sign of $\gamma$ here):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\cos\gamma={\left|\vec{r}\times\vec{v_i}\right|\over r\,v_i}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We'll call the desired final apoapsis $z$, which is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$z=a\left(1+e\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key is that your position and flight path angle will be unchanged before and after an instantaneous maneuver in the direction of the velocity. You can then solve for the final velocity magnitude as a function of the final apoapsis at the fixed $r$ and $\gamma$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_f=\sqrt{2\mu z(z-r)\over r z^2-r^3\cos^2\gamma}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference between that and your initial velocity magnitude, $v_f-v_i$, is your $\Delta V$ in the velocity direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You must have $z\ge r$. Since orbits are closed it will return to $r$. If $r$ is not less than $z$, then $z$ is not the apoapsis. When $z=r$ and $\gamma\ne 0$, you have a degenerate case where $v_f=0$, so the body then falls in a straight line to the center of the body, at infinite velocity at the body, and then back up to $r$. So really you better have $z&amp;gt;r$. If $z=r$ and $\gamma=0$, then you are at apoapsis or periapsis. If you're at apoapsis, then done. Do nothing. If you're at periapsis, then there are an infinite number of solutions. The minimum $\Delta V$ would be to lower the current apoapsis to the current periapsis, making the orbit circular.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that in general the maneuver will also change the periapsis and argument of periapsis. There is no assurance that the new periapsis will be above the surface or atmosphere of the body, so be careful.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-06-16T18:59:24.413" LastActivityDate="2016-06-16T18:59:24.413" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="16726" PostTypeId="2" ParentId="16723" CreationDate="2016-06-16T12:59:05.463" Score="1" Body="&lt;p&gt;The Falcon 9 uses 3 of its engines to land. In this case one of the engines didn't work properly and did not give enough thrust and the other 2 engines did not compensate, so it hit the platform at too high a speed, broke up and exploded. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is working on a system which will compensate for a weak engine, they say they want to test it later this year. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="25" LastEditDate="2017-12-19T13:18:53.350" LastActivityDate="2017-12-19T13:18:53.350" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16727" PostTypeId="2" ParentId="16723" CreationDate="2016-06-16T13:13:38.990" Score="5" Body="&lt;p&gt;SpaceX has an interesting product in the Falcon 9 recoverable, since it has a suite of option. Lowest performance missions, can land the first stage back at the launch site (RTLS - Return to Launch Site). On missions that need more performance, typically GTO/GEO missions, they can land down range on the ASDS ship.  Finally, if you needed max performance and wanted to pay for it, there is an option of an expendable first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was a dual launch GTO mission, so not quite at the absolute max of performance but pretty close. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have said that they are working on tricks to eke out more performance for landing, by trying a three engine landing burn instead of the previous single engine burn. This saves on gravity losses by burning harder and faster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The longer you burn, the more you are fighting gravity, and they are close enough to the edge that this matters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a further twist, they have tried a 1-3-1 burn pattern. First one engine starts slowing it down, two more engines spin up and slow it down hard, and then shut down and only one engine is finally used to land. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, one of the engines either had a slow start, or did not properly get up to full thrust and they were probably unbalanced. Or it could be they ran out of fuel and one engine was not getting fed properly. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-16T13:13:38.990" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16728" PostTypeId="2" ParentId="16722" CreationDate="2016-06-16T13:18:54.407" Score="3" Body="&lt;p&gt;The last few missions doing GTO boosts have had the ASDS in reasonably close to the same location in the ocean. The lower energy CRS-8 mission was further in shore, since it had the performance to come back that much closer to shore. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a fair bit of variability possible, but it does appear that there is little benefit in going much further east, since if there was, for the SES-9 (That crashed hard), and the other high performance missions they would have taken advantage of that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There might be a limit on how far they want to tow an ASDS out to sea, manage it for a day or three and bring it back. It does take about 4-5 days to get to the current GTO landing spot. The support ships have been shown to have fuel supplies sufficient, but there might be a crew usage issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, missions to GEO like orbits have some options.  GEO is harder than GTO both due to performance, but also due to restart and coast issues.. GTO requires that the upper stage/payload do more of the work to get to GEO than a direct injection into GEO. SpaceX usually does GTO not GEO launches, because their second stage cannot coast long enough on batteries and make it GEO, thus they quote GTO, in which there are several standards for delivery. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the orbit, the payload itself needs to expend more energy to get to full GEO, which either takes a long time (Electric thrusters) delaying entry into service by months, or else reduces its on orbit life span by using up the fuel to get there.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-06-16T18:53:10.307" LastActivityDate="2016-06-16T18:53:10.307" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16729" PostTypeId="2" ParentId="16722" CreationDate="2016-06-16T13:23:37.570" Score="1" Body="&lt;p&gt;For now, no. The landing zone is in the middle of the ocean. Setting up a factory there is prohibitively expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IIRC, the long-term plan is to launch from somewhere further West so they can land at Cape Canaveral. That would require flying over land, which requires FAA permission which isn't easy to obtain, This probably requires lots more flights before the FAA considers the rocket to be proven enough to allow it. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-16T13:23:37.570" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16730" PostTypeId="2" ParentId="16722" CreationDate="2016-06-16T13:34:30.780" Score="4" Body="&lt;p&gt;Once SpaceX has a continuous pipeline of rockets being launched, refurbished, and reused, the &lt;em&gt;latency&lt;/em&gt; -- the time it takes for a single stage to go from launch to launch-ready -- won't matter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the turnaround time is, say, 70 days, and SpaceX has 10 first stages in their stable, they can maintain a launch cadence of 1 per week. 4 ASDS would probably be sufficient to service the fleet at that time - at a given time there might be one on station, one unloading a stage, one being refurbished, one on the way out to sea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speeding up the barge trip is solving a non-problem.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-06-16T15:00:54.617" LastActivityDate="2016-06-16T15:00:54.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16731" PostTypeId="2" ParentId="16700" CreationDate="2016-06-16T14:34:08.690" Score="3" Body="&lt;p&gt;It's possible to argue that the &lt;a href=&quot;https://en.wikipedia.org/wiki/NEAR_Shoemaker&quot; rel=&quot;nofollow&quot;&gt;NEAR Shoemaker&lt;/a&gt; probe was the first landing beyond Mars.  The probe landed on near-Earth asteroid Eros on 12 February 2001.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On first encounter in February 2000, &lt;a href=&quot;http://www.spacetoday.org/SolSys/Asteroids/NEAR.html&quot; rel=&quot;nofollow&quot;&gt;Eros was 1.7 AU from the Sun&lt;/a&gt;, outside of Mars' orbit.  However, by the time of landing Eros was only 1.43 AU from the Sun; while Mars was &lt;a href=&quot;http://www.planetary.org/explore/space-topics/mars/mars-calendar.html&quot; rel=&quot;nofollow&quot;&gt;near aphelion&lt;/a&gt;, so Eros was no longer 'beyond Mars' in the sense of being further from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 'ease of landing' argument is much stronger for an asteroid than a moon; really the only trick to landing on an asteroid is not bouncing off :-)&lt;/p&gt;&#xA;" OwnerUserId="15767" LastActivityDate="2016-06-16T14:34:08.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16732" PostTypeId="2" ParentId="16720" CreationDate="2016-06-16T15:22:52.117" Score="2" Body="&lt;p&gt;Rust is usable as a starting point. You can &lt;a href=&quot;https://www.reddit.com/r/askscience/comments/27ko4n/can_rust_be_melted_back_into_its_original_metal/&quot; rel=&quot;nofollow&quot;&gt;smelt it with carbon&lt;/a&gt; to convert the iron oxide into iron.&lt;br&gt;&#xA;The common iron ores found on Earth &lt;a href=&quot;http://www.chemguide.co.uk/inorganic/extraction/iron.html&quot; rel=&quot;nofollow&quot;&gt;are iron oxides&lt;/a&gt;.&lt;br&gt;&#xA;Rust is less ferromagnetic than iron, but &lt;a href=&quot;https://van.physics.illinois.edu/qa/listing.php?id=374&quot; rel=&quot;nofollow&quot;&gt;can be picked up with a magnet&lt;/a&gt;. &#xA;You have to gather a lot though, the dust layer is &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_surface_color&quot; rel=&quot;nofollow&quot;&gt;only a few mm thick&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-16T15:29:13.677" LastActivityDate="2016-06-16T15:29:13.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16733" PostTypeId="2" ParentId="16720" CreationDate="2016-06-16T15:23:33.967" Score="2" Body="&lt;p&gt;Mars' atmosphere is very thin, but scientists think it was once much thicker, so the iron on Mars oxidized millions or even billions of years ago. Any new iron sources exposed by wind, meteor impacts or from meteors themselves will over time still oxidize as there is a small amount of oxygen in the atmosphere. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-06-16T15:23:33.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16734" PostTypeId="1" CreationDate="2016-06-16T16:13:08.237" Score="4" ViewCount="337" Body="&lt;p&gt;The failure of the Falcon 9 first stage landing during the &lt;a href=&quot;https://www.youtube.com/watch?v=gLNmtUEvI5A&quot; rel=&quot;nofollow&quot;&gt;Eutelsat/ABS mission&lt;/a&gt; was attributed to: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/743097337782763521&quot; rel=&quot;nofollow&quot;&gt;thrust was low on 1 of 3 landing engines&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;which indicate that engine thrusts are part of the telemetry data.&lt;br&gt;&#xA;I can see how the thrust can be measured on a test stand.  But how can they be monitored during the flight?   &lt;/p&gt;&#xA;" OwnerUserId="13988" LastEditorUserId="4660" LastEditDate="2016-06-16T16:20:59.613" LastActivityDate="2016-06-16T18:53:01.760" Title="How is engine thrust monitored during flight?" Tags="|launch|spacex|falcon-9|engines|telemetry|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16735" PostTypeId="2" ParentId="16734" CreationDate="2016-06-16T16:22:18.853" Score="0" Body="&lt;p&gt;They get a fair bit of telemetry from the engines. Over time, SpaceX has indicated they have instrumented their engines to greater and greater degrees. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recall that a rocket engines thrust is mostly determined by the fuel (and oxidizer) flow. These are pumped by turbopumps, which SpaceX has innovated on in their engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A turbopump spins and moves fuel. When the fuel flow is lower than expected the turbopump is running lighter and can detect that difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondarily in this case, it is likely that the low thrust engine was one of the two side engines, in which case they would have gotten feedback that the stage was flying off-kilter due to unbalanced thrust.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-16T16:22:18.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16738" PostTypeId="1" AcceptedAnswerId="16742" CreationDate="2016-06-16T16:59:18.517" Score="9" ViewCount="1140" Body="&lt;p&gt;The &lt;a href=&quot;http://spirit.as.utexas.edu/~fiso/telecon/Bottke_1-15-14/&quot; rel=&quot;nofollow noreferrer&quot;&gt;conference call&lt;/a&gt; with Bill Bottke (MP3 plus other documents) about &quot;minimoons&quot; presents an alternative to Asteroid Redirect first got me interested in orbital mechanics. The idea is that there are enough small asteroids that approach the earth-moon barycenter (EMB) that it seems likely there are frequent temporary captures of small asteroids into chaotic 3-body orbits in EMB system, that last for a few months to a few years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an example of one theoretical orbit &lt;a href=&quot;http://www.ifa.hawaii.edu/info/press-releases/minimoons/&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt; or &lt;a href=&quot;http://sservi.nasa.gov/articles/simulations-show-mini-moons-orbiting-earth/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4ev44.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4ev44.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One can read more in &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103511004684&quot; rel=&quot;nofollow noreferrer&quot;&gt;Granvik, Vaubaillon and Jedicke 2012&lt;/a&gt; (&lt;em&gt;The population of natural Earth satellites&lt;/em&gt; Icarus, 218 (1) March 2012, pp262-277) available without paywall in &lt;a href=&quot;https://arxiv.org/abs/1112.3781&quot; rel=&quot;nofollow noreferrer&quot;&gt;ArXiv&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only one &quot;minimoon&quot; &lt;a href=&quot;https://en.wikipedia.org/wiki/2006_RH120&quot; rel=&quot;nofollow noreferrer&quot;&gt;2006 RH120&lt;/a&gt; was known &lt;a href=&quot;http://www.aanda.org/articles/aa/abs/2009/09/aa10965-08/aa10965-08.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;by 2012&lt;/a&gt;, but it seems there should be new ones regularly, according to Granvik et al. 2012.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Have there been others that were detected and reported?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HS4fw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HS4fw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-09-22T02:14:30.713" LastActivityDate="2020-02-28T13:08:23.093" Title="Have there been any documented mini-moons since 2006 RH120?" Tags="|asteroid|near-earth-object|asteroid-redirect-mission|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="16739" PostTypeId="2" ParentId="16734" CreationDate="2016-06-16T17:02:29.343" Score="2" Body="&lt;p&gt;In the SSME the &quot;thrust control loop's&quot; independent variable was chamber pressure.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During the engine run phase, the MOV, MFV, and CCV are switched to run&#xA;  schedules, while the OPOV and  FPOV  are  switched  to  closed-loop &#xA;  operations.    The  run  schedules  for  the  MOV  and  MFV  cause&#xA;  them to simply remain fully open, whereas the run schedule for the CCV&#xA;  drives it between half open at 67 percent thrust (MPL) and fully open&#xA;  at 100 percent thrust (and above).  This action maintains the&#xA;  appropriate  flow  relationships  among  the  several  parallel  fuel &#xA;  flow  paths  as  the  HPFTP  output pressure varies with thrust. &#xA;  During engine run, the OPOV and FPOV are used as control devices for&#xA;  thrust and mixture ratio.  Manipulating these valves affects the&#xA;  output of the preburners, the speed of the  turbopumps,  and, &#xA;  therefore,  the  propellant  flowrates.    &lt;strong&gt;The  FPOV  is  driven &#xA;  alone  to  maintain mixture ratio in the MCC, while the OPOV is driven&#xA;  with the FPOV to increase or decrease thrust while maintaining  the &#xA;  mixture  ratio.&lt;/strong&gt;    &lt;strong&gt;The  control  loops  include  the  controller,  the&#xA;  valve  actuators,  and  the transducers that sense flowrates and MCC&#xA;  pressure; i.e., thrust.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, emphasis mine&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this which is a bit more explicit&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If no malfunctions are discovered, the closed loop thrust control&#xA;  system is activated at 2.4 seconds. The MEC compares the measured MCC&#xA;  chamber pressure to a preprogramed chamber pressure ramp to RPL and&#xA;  modulates the OPOV in an attempt to zero out any differences.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://www.enginehistory.org/SSME/SSME3.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acronymology:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MOV Main Oxidizer Valve&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MFV Main Fuel Valve&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CCV Chamber Coolant Valve aka Coolant Control Valve&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OPOV Oxidizer Preburner Oxidizer Valve&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FPOV Fuel Preburner Oxidizer Valve&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MPL Minimum Power Level&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HPFTP High Pressure Fuel TurboPump&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MCC Main Combustion Chamber&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MEC Main Engine Controller (SSMEC preferred because MEC also is Master Events Controller)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RPL Rated Power Level&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-06-16T18:53:01.760" LastActivityDate="2016-06-16T18:53:01.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16741" PostTypeId="1" AcceptedAnswerId="16777" CreationDate="2016-06-16T20:15:39.713" Score="6" ViewCount="6439" Body="&lt;p&gt;I understand the basic principle of how a rocket engine works, but what I cannot find any info about is how do they initiate the whole process. Can someone please explain to me what gives the turbo-pump the initial &quot;kick&quot; to start the process of pumping the fuel and oxidizer? &lt;/p&gt;&#xA;" OwnerUserId="12158" LastEditorUserId="4660" LastEditDate="2016-06-17T03:31:49.667" LastActivityDate="2016-06-29T19:16:48.407" Title="How is a rocket engine started?" Tags="|launch|rockets|engine-design|fuel-system|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="16742" PostTypeId="2" ParentId="16738" CreationDate="2016-06-16T21:12:45.460" Score="6" Body="&lt;p&gt;Possibly the earliest documented mini-moon was the one associated with the &lt;a href=&quot;https://en.wikipedia.org/wiki/1913_Great_Meteor_Procession&quot; rel=&quot;nofollow noreferrer&quot;&gt;1913 Great Meteor Procession&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After 2006 RH&lt;sub&gt;120&lt;/sub&gt;, that you mentioned, another was identified on October 3, 2015 and designated &lt;a href=&quot;https://en.wikipedia.org/wiki/WT1190F&quot; rel=&quot;nofollow noreferrer&quot;&gt;WT1190F&lt;/a&gt;. It impacted Earth on November 13, 2015. It was probably space debris, and not natural.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's only one other potential candidate that I know of so far: &lt;a href=&quot;https://en.wikipedia.org/wiki/Claimed_moons_of_Earth#Temporary_satellites&quot; rel=&quot;nofollow noreferrer&quot;&gt;S509356&lt;/a&gt;. It was identified on April 8, 2016. It may yet be an artificial satellite, but there are none known that match its orbital period, and it has a color associated with S-type asteroids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update 5/16/2018:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;S509356 turns out to be 2015-019C, 41929, upper stage from a Long March carrying a BeiDou satellite.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update 2/27/2020:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we have an official confirmed mini-moon (temporary natural satellite): &lt;a href=&quot;https://en.wikipedia.org/wiki/2020_CD3&quot; rel=&quot;nofollow noreferrer&quot;&gt;2020 CD&lt;sub&gt;3&lt;/sub&gt;&lt;/a&gt;. It was discovered February 15, 2020, and announced February 25. It is estimated to have been orbiting Earth since around 2017-2018, and it is expected to depart April 2020.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2020-02-28T13:08:23.093" LastActivityDate="2020-02-28T13:08:23.093" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="16743" PostTypeId="2" ParentId="10025" CreationDate="2016-06-16T23:15:38.467" Score="7" Body="&lt;p&gt;For an upper stage, look to the Long March rocket series as a case history. For Long March 2-4, the boosters use hypergolic fuels while the upper stage burns LOX/LH2. Given that they have decades of experience with the hypergolic engines, that suggests LH2 is cheaper. Essentially they made the decision between building a bigger booster or adding strap-ons or going multi-core to increase payload, or else make the final stage lighter by using a higher ISP fuel. This also points out that you can't design a rocket in a (logical) vacuum. An upper stage capable of giving 3 km/s to a 10-ton payload may be cheaper to build, fuel, and operate using hypergolics, but the overall weight will be heavier than a LH2 fueled stage of the same performance, and that means boost stages must be larger and heavier and therefore cost more. A dollar spent on your upper stage may save five dollars getting it to altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also look to the J-2. Hypergolics are easy to restart. However the J-2 was restartable. Most importantly, the J-2 doesn't seem to have suffered from the same operational curses that boosters do when carrying LH2. From a purely empirical standpoint, if we flew the J-2 LOX/LH2 engines in Apollo upper stages without a lot of delays, there doesn't seem to be a compelling reason to spend more $ on a highly poisonous, heavier, corrosive replacement fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, Long March 1 used a solid rocket upper stage. Hydrogen is tricky; development times are long and expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&#xA;Are you factoring system capital costs into it? R&amp;amp;D and building your hardware? Or is your question based on an existing sunk cost?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am afraid that you are asking the same question that every rocket designer since Goddard has asked when they start a project with a clean sheet of paper. There is no definitive answer, only the answer which appears best, given all known factors and best modeling practices, for any specific design at a specific time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hypergolics are expensive and difficult to handle, but the engineering of a hypergolic engine is much simpler than for a LH2 cryogenic. So if you are looking for the fastest, cheapest system to develop and implement for a small number of launches, hypergolic is probably the better of the two. If you have more time and development money, and plan a longer service life for your system, you find yourself driven toward LOX/LH2. LOX doesn't actually appear that difficult to handle; it's the LH2 that kills you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per SF, $/kg of payload is your final engineering metric.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look to history to inform your answer. Goddard and the V2 used LOX with gasoline and alcohol/water, respectively. The Titan 1 used LOX/RP1. For the Titan II, they modified the LR-87 engine into the hypergolic-fueled LR-87-5 so their ICBM could be stored with room-temperature fuel. So the decision was based on storage, not performance, and the engineering challenges were similar enough to modify a LOX/RP-1 engine rather than design something new. From this we can see that R&amp;amp;D and fabrication of a hypergolic engine is on par with that for LOX/RP-1 engines, which is about as cheap as liquid fueled rockets get. Hypergolic fuels are super expensive, but if your launcher has a short development cycle and limited R&amp;amp;D budget and you plan on a small number of launches, hypergolic wins. Actually, LOX/Kerosene wins, but that's not your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have 30 years and billions of $ to iterate your design, then LOX/LH2 wins. The proof is the Delta IV and its RS-68. If decades of engineering experience pointed to a hypergolic booster as getting payloads up more cheaply (per kg of payload to orbit), ULA would be putting money into hypergolic, or pushing the government to fund a new development effort. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a bias. I hate LOX/LH2 systems. LH2 is simply evil. It seeps through &quot;cracks&quot; in welds which any other material would consider perfectly impermeable. Hot hydrogen makes metal BLISTER. It's so cold that the foam insulation on the shuttle tanks had to be foamed with helium; foaming with air leads to the air condensing and the foam collapsing. I feel that if the shuttle program had had fewer delays due to tracking tiny hydrogen leaks, they may have been more willing to address real concerns like the SRB O-rings. I consider it an engineering miracle that they have managed to &quot;tame&quot; LH2 and launch the Delta IV's on schedule. Considering they are building on SSME technology, it's a miracle about 45 years in the making. Also, note that ULA only uses the Delta IV when it can't fit the payload onto a LOX/RP-1 fueled Titan, and the Delta is planned for phase-out once they get a LOX/Methane booster working.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's why LOX/RP-1 has regained popularity, especially in boosters. The lower ISP doesn't hurt performance nearly as badly as it does in an upper stage. Sure, it's &quot;1950's technology&quot;, but as such it has 70 years of engineering refinement and leads to a $/kg of payload much lower than for competing LH2 systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given my bias, my answer is &quot;Neither.&quot; For a first stage, unless you have a nearly unlimited development budget and schedule, go with LOX/RP-1 or LOX/LMethane for your cheapest $/kg payload. That seems to hold true for the smallest to the largest launch systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second stage? More engineering decisions, but LH2 is probably your winner. Look to the J-2 as your case history. The poor ISP of hypergolics will hurt your overall system performance more than on a first stage. Hypergolics are easy to restart. However the J-2 was restartable. Most importantly, the J-2 doesn't seem to have suffered from the same operational curses that boosters do when carrying LH2. From a purely empirical standpoint, if we flew the J-2 LOX/LH2 engines in Apollo upper stages without a lot of delays, there doesn't seem to be a compelling reason to spend more $ on a highly poisonous, heavier, corrosive replacement fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And have you read up on Nitrogen Tetroxide? That stuff is evil. As far as I can tell, if you can smell it, then you are going to die. &lt;/p&gt;&#xA;" OwnerUserId="8247" LastEditorUserId="8247" LastEditDate="2016-06-17T18:36:10.010" LastActivityDate="2016-06-17T18:36:10.010" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16744" PostTypeId="2" ParentId="16714" CreationDate="2016-06-17T01:36:03.500" Score="5" Body="&lt;p&gt;NASA provides open source software for just this purpose. It's called DAS (Debris Assessment Software) and you can find it here: &lt;a href=&quot;http://orbitaldebris.jsc.nasa.gov/mitigate/das.html&quot; rel=&quot;noreferrer&quot;&gt;http://orbitaldebris.jsc.nasa.gov/mitigate/das.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a lot of options and knobs, but you could start off by opening the program, clicking &quot;Requirements Assessment&quot;, clicking &quot;Casualty Risk from Reentry Debris&quot;, and entering in the specific materials that you're concerned about. There's a fairly large database built in already.&lt;/p&gt;&#xA;" OwnerUserId="14477" LastActivityDate="2016-06-17T01:36:03.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16745" PostTypeId="1" CreationDate="2016-06-17T01:37:24.167" Score="0" ViewCount="124" Body="&lt;p&gt;I am making the sort of rhetorical parallel between Oberth effect that I conclude&#xA;applies even to light because light is actually solid of some sort photons or gravity well of Earth/Sun/galaxy, black holes effect. If I am making any&#xA;sense. Oberth effect may cause space-time distortions.&lt;/p&gt;&#xA;" OwnerUserId="15782" LastEditorUserId="238" LastEditDate="2016-06-17T01:46:35.860" LastActivityDate="2016-06-17T02:07:34.763" Title="Can the Oberth effect cause space-time distortions?" Tags="|oberth-maneuver|orbital|spacetime|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16746" PostTypeId="2" ParentId="16745" CreationDate="2016-06-17T02:07:34.763" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Can the Oberth effect cause space-time distortions? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, but the mass(es) used for the maneuver might.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the &lt;em&gt;velocity&lt;/em&gt; of the spacecraft performing the maneuver might cause a slowing of it's time relative to an observer, but:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;At anything less than extreme speeds (a significant percent of the speed of light) it would be almost immeasurable.&lt;/li&gt;&#xA;&lt;li&gt;As far as I understand, time dilation due to a relative velocity is not, technically, part of a space-time distortion - which is due to gravitational effects.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="238" LastActivityDate="2016-06-17T02:07:34.763" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16748" PostTypeId="2" ParentId="7701" CreationDate="2016-06-17T04:22:39.863" Score="4" Body="&lt;p&gt;Supplemental information (NASA sez):&lt;/p&gt;&#xA;&lt;p&gt;I just happened to have run across this, and remembered your question. In a database that contains A LOT of moons (in the solar system), They have added &amp;quot;Luna&amp;quot; for potential disambiguation - just to be sure I think, since they have dropped the pronoun &amp;quot;The&amp;quot; here.&lt;/p&gt;&#xA;&lt;p&gt;From JPL Horizons database web interface at &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ssd.jpl.nasa.gov/horizons.cgi&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1mSpx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1mSpx.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hScY8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hScY8.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-06-17T04:22:39.863" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16749" PostTypeId="1" AcceptedAnswerId="16752" CreationDate="2016-06-17T06:50:38.723" Score="3" ViewCount="1067" Body="&lt;p&gt;I would have thought that you would want to conserve as much energy as possible - or is energy/electricity not a problem?&lt;/p&gt;&#xA;" OwnerUserId="15783" LastActivityDate="2016-06-17T16:11:33.850" Title="Why do some spacecraft have heat radiators? Why not repurpose the energy?" Tags="|spacecraft|heat|thermal-control|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16750" PostTypeId="1" AcceptedAnswerId="16751" CreationDate="2016-06-17T09:08:00.403" Score="14" ViewCount="1955" Body="&lt;p&gt;In &lt;a href=&quot;http://www.dumpert.nl/mediabase/6806207/34b42154/raketmotor_testen.html&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; of a Korea Aerospace Research Institute (KARI) rocket engine test, you can clearly see a second exhaust where thick smoke comes out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2Safi.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Safi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this an essential part of the engine, or just an extra attribute required when testing? In both cases, why is a separate exhaust necessary?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;https://www.youtube.com/watch?v=Ejx3jTRD_ME&quot; rel=&quot;noreferrer&quot;&gt;second video&lt;/a&gt; from a SpaceX tests of the Merlin 1D rocket engine with the same feature.&lt;/p&gt;&#xA;" OwnerDisplayName="user10509" LastActivityDate="2016-06-17T10:14:54.097" Title="What is that second (smoke) exhaust on a rocket engine for?" Tags="|engine-design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16751" PostTypeId="2" ParentId="16750" CreationDate="2016-06-17T09:20:20.850" Score="25" Body="&lt;p&gt;That is the exhaust of the turbopump drive. They burn a small amount of propellant, those exhaust gases are used to drive the turbopump that pumps the propellant and oxygen to the engine.&lt;br&gt;&#xA;There are engines where this exhaust is fed into the main combustion chamber (&lt;a href=&quot;https://en.wikipedia.org/wiki/Staged_combustion_cycle&quot;&gt;staged combustion&lt;/a&gt;), but this is expensive to get right so many engines use the cheaper system with a separate exhaust (&lt;a href=&quot;https://en.wikipedia.org/wiki/Gas-generator_cycle&quot;&gt;gas-generator cycle&lt;/a&gt;) and slightly lower performance. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorDisplayName="user10509" LastEditDate="2016-06-17T10:14:54.097" LastActivityDate="2016-06-17T10:14:54.097" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16752" PostTypeId="2" ParentId="16749" CreationDate="2016-06-17T09:51:56.860" Score="16" Body="&lt;p&gt;Yes, you would like to conserve the energy, but sadly that is not possible. The problem is: The energy you have is heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Second_law_of_thermodynamics&quot;&gt;second law of thermodynamics&lt;/a&gt; states that entropy always increases, and that means that while every process in the spacecraft produces heat, going in the opposite way is impossible. You can produce energy from heat if you have a temperature gradient, but if you have that on board, you would only be moving the heat around in any case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, everything going on produces heat, and you can not magically transfer that back to useful energy. The only solution is to dump it.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-06-17T09:51:56.860" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16753" PostTypeId="2" ParentId="16749" CreationDate="2016-06-17T11:40:18.657" Score="4" Body="&lt;p&gt;Although there is technology which can convert waste heat to electricity you cannot do it economically. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Energy is precious in a spacecraft, the more you need the bigger solar panels you need to generate it, the bigger batteries you need to store it, or the bigger nuclear thermal generator you need. Bigger means heavier, and heavier means more expensive to launch. When electricity is converted to motion, or used in computers/sensors waste heat is generated, the amount of energy lost can be limited by using more efficient materials. There's a limitation to how efficient you can get, so you will always have waste heat. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that you want some heat generation in most spacecraft so it doesn't get too cold to operate. Waste heat is one way, some spacecraft have heaters to convert electricity into heat when in shadow. So waste heat is actually useful. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are devices that can convert heat into electricity, called &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermoelectric_generator&quot; rel=&quot;nofollow&quot;&gt;thermo-electric generators&lt;/a&gt;. These are used in spacecraft already in nuclear thermal generators, aka &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow&quot;&gt;radioisotope thermoelectric generators&lt;/a&gt; which are used in spacecraft going to places where there isn't enough sunlight for solar panels. As radioisotopes break down they produce heat which is converted into electricity to power the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermoelectric generators could in theory be used to convert waste heat back into electricity, but you'd lose more than you'd gain as the weight you'd gain from them would not be offset by less solar panels and batteries - they are heavy and inefficient, with 5-8% typical energy conversion. This means you'd still have 92% of the heat to radiate. Newer materials are being developed that may bring this to around 20% efficiency but that would still not be enough to make them worthwhile.  &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2016-06-17T16:11:33.850" LastActivityDate="2016-06-17T16:11:33.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16754" PostTypeId="2" ParentId="16720" CreationDate="2016-06-17T13:34:49.393" Score="5" Body="&lt;p&gt;No, at least some of it will stick to a magnet. Here is an image of dust on the capture and filter magnets on the Opportunity rover on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GSTWC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GSTWC.jpg&quot; alt=&quot;dust on magnets at Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-06-17T13:34:49.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16755" PostTypeId="2" ParentId="16723" CreationDate="2016-06-17T15:43:16.823" Score="3" Body="&lt;p&gt;We know a few bits of information:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://twitter.com/elonmusk/status/743102502225076227&quot; rel=&quot;nofollow&quot;&gt;landing was initially thought to be hard&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://twitter.com/elonmusk/status/743602894226653184&quot; rel=&quot;nofollow&quot;&gt;landing video shows it at a near hover&lt;/a&gt; for ~2 seconds.&lt;/li&gt;&#xA;&lt;li&gt;It seems it ran out of fuel (Well, oxidizer). &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Putting all of this together, I imagine some kind of a sensor issue. Either the rocket thought it was going faster than it actually was, and as a result hovered and ran out of fuel, or it didn't know how far away the ground was, and a similar issue happened. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-17T15:43:16.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16756" PostTypeId="1" CreationDate="2016-06-17T15:45:28.090" Score="10" ViewCount="703" Body="&lt;p&gt;In the &lt;a href=&quot;https://twitter.com/elonmusk/status/743602894226653184&quot;&gt;latest landing video&lt;/a&gt;, it seems the Falcon 9 just hovered for a moment above the ship. That being said, it begs the question, how does the booster know where the ground is? Does it purely work off of GPS, a 1 way radar/beacon type system from the landing zone, radar from the rocket, or some means that I'm not aware of? &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-18T13:14:26.177" Title="How does a Falcon 9 booster know how far away the ground is?" Tags="|spacex|rockets|falcon-9|landing|reentry|" AnswerCount="1" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="16757" PostTypeId="2" ParentId="13975" CreationDate="2016-06-17T19:43:16.447" Score="4" Body="&lt;p&gt;In the end it always boils down to inertial frames of reference. Every object in free fall has it's own inertial frame of reference and any motion outside of that frame will be a relative measurement. For a system based on the GPS principle to work at least 4 satellites are required (details on how GPS works go to a different question please).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each of these satellites carries its own inertial frame but you want to know your position in a frame of reference that's different from each of the satellites' ones. The first step to do this is to define an &quot;global&quot; frame of reference all the satellites can be referenced against. There are different ways to do this. A ground station would be the most straightforward one. But there are other ways too, and for a mission to an not yet inhabited planet these may be more interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One possible method would be, that the satellites are communicating with each other (the GPS satellites currently in operations don't do this) establishing a frame of reference between them (let's call that &lt;em&gt;Satellite Established Frame&lt;/em&gt; (SEF)), that's not strictly aligned to anything on the planet below. All location would then happen in that frame of reference. Pivot stations on the planet would constantly sample their position and from that derive a SEF to planetary coordinates mapping, which in turn would be distributed to mobile navigation receiver stations. Why do it that way? Because it gives you a working navigation system right at the moment you arrive at the planet's surface. It may be off by a few arcminutes until the exact location of the pivot station could be determined from astronomic measurements. With a system like GPS that depends on a working ground station you get reliable navigation only after the ground station has been established. With a self-establishing frame of reference, that is allowed drift you can use plain old, reliable Rubidium clocks on the satellites and nothing more. If a ground station is required, then a Cesium fountain clock or better has to be put into operation for the whole thing to be accurate.&lt;/p&gt;&#xA;" OwnerUserId="15791" LastActivityDate="2016-06-17T19:43:16.447" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="16758" PostTypeId="2" ParentId="14687" CreationDate="2016-06-17T20:14:50.510" Score="3" Body="&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; here is the most recent document as of Jan 2017: &lt;a href=&quot;http://mtcr.info/wordpress/wp-content/uploads/2016/10/MTCR-TEM-Technical_Annex_2016-10-20.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mtcr.info/wordpress/wp-content/uploads/2016/10/MTCR-TEM-Technical_Annex_2016-10-20.pdf&lt;/a&gt; If the link fails, then go here: &lt;a href=&quot;http://mtcr.info/mtcr-annex/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mtcr.info/mtcr-annex/&lt;/a&gt; and look for the most recent version.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The MTCR has recently been updated (OCT 2015).  The new limit is 600 m/sec.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;CATEGORY II - ITEM 11&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;AVIONICS&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;11.A.3. Receiving equipment GNSS e.g. GPS, GLONASS or Galileo&lt;br&gt;&#xA;  11.A.3. Receiving equipment for Global Navigation Satellite Systems (GNSS; e.g. GPS, GLONASS or Galileo), having any of the following characteristics, and specially designed components therefor:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;a. Designed or modified for use in systems specified in 1.A.; or&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;b. Designed or modified for airborne applications and having any of the following:&lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;Capable of providing navigation information at speeds in excess of 600 m/s;&lt;/li&gt;&#xA;  &lt;li&gt;Employing decryption, designed or modified for military or governmental services, to gain access to GNSS secure signal/data; or&lt;/li&gt;&#xA;  &lt;li&gt;Being specially designed to employ anti-jam features (e.g. null steering antenna or electronically steerable antenna) to function in an environment of active or passive countermeasures.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;  &#xA;  &lt;p&gt;Note:&#xA;  11.A.3.b.2. and 11.A.3.b.3. do not control equipment designed for commercial, civil or 'Safety of Life' (e.g. data integrity, flight safety) GNSS services.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="15793" LastEditorUserId="12102" LastEditDate="2017-01-21T03:08:50.103" LastActivityDate="2017-01-21T03:08:50.103" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16759" PostTypeId="1" CreationDate="2016-06-17T20:26:34.677" Score="3" ViewCount="79" Body="&lt;p&gt;Let's say we had a probe in orbit around the moon mapping out &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitation_of_the_Moon&quot; rel=&quot;nofollow&quot;&gt;mass concentrations&lt;/a&gt; (mascons), as we've done in the past.  Unexpectedly one day, a massive object lands on the surface of the moon, an object as big as a mountain range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How long would it take us to detect it using only the variations in the orbit of our probe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Basically I'm asking about the temporal resolution of gravitational mapping.)&lt;/p&gt;&#xA;" OwnerUserId="552" LastActivityDate="2016-06-17T21:09:27.143" Title="How quickly could we detect a change in a mascon?" Tags="|gravity|mapping|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16761" PostTypeId="2" ParentId="16759" CreationDate="2016-06-17T21:09:27.143" Score="4" Body="&lt;p&gt;It depends on where the masscon is in relation to the orbit, and just how big the masscon is, as well as how carefully the orbit is measured, and whether or not anyone is actually looking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's a polar impact and an equatorial orbit, it would need to move the center of mass appreciably to be noticed at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's an equatorial impact and a polar orbit, it might be noticed in as few as a single orbital path as the path changes during a direct flyover.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, it requires someone be actually looking for the changes in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Technically, it also propagates the gravitational waves at C, but the distances involved in this case are small enough that it is (if you'll pardon the pun) immaterial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need to be close enough to it for long enough to alter the orbit enough that it's detectable. And it is particularly helpful to make multiple passes; the current systems need several orbits and look at the statistics to generate the topological mapping which is turned into the density maps.&lt;/p&gt;&#xA;" OwnerUserId="158" LastActivityDate="2016-06-17T21:09:27.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16763" PostTypeId="1" CreationDate="2016-06-17T23:59:16.573" Score="7" ViewCount="259" Body="&lt;p&gt;Back in the 1970s, how did NASA describe when an event happened during a mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, when Mariner 10 did its &lt;a href=&quot;https://en.wikipedia.org/wiki/Mariner_10#Venus_flyby&quot; rel=&quot;noreferrer&quot;&gt;fly-by of Venus&lt;/a&gt;, how did the Mariner team refer to that moment in time?  Did they use the ordinary date and time from a particular timezone on Earth?  Did they count some number of elapsed seconds since the start of the mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Apollo 11, in 1969, appeared to use elapsed days/hours/minutes/seconds from the start of the mission, at least according to &lt;a href=&quot;http://www.jsc.nasa.gov/history/mission_trans/AS11_CM.PDF&quot; rel=&quot;noreferrer&quot;&gt;transcripts&lt;/a&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="12102" LastEditDate="2021-08-04T00:10:19.140" LastActivityDate="2021-08-04T00:10:19.140" Title="How did NASA refer to dates and times of a mission in the 1970s?" Tags="|nasa|history|mission-design|deep-space|time|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16767" PostTypeId="1" CreationDate="2016-06-18T09:22:34.183" Score="6" ViewCount="200" Body="&lt;p&gt;It is my understanding that asteroids vaporize on impact and spread evenly over some area. An iron rich asteroid does not deposit an iron ore and there are no real ore bodies on the Moon. But had they time to form before Mars cooled billions of years ago and if so, should they still be there, as accessible as on Earth? It would be useful for the Mars mining industry next century or so.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-06-20T13:28:03.647" Title="Are there proper ore bodies in Mars' crust?" Tags="|mars|planetary-science|isru|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16769" PostTypeId="2" ParentId="16756" CreationDate="2016-06-18T13:14:26.177" Score="7" Body="&lt;p&gt;When commenting on one of the early water landings, Elon Musk mentioned there's a radar altimeter (which was calibrated for flat, hard surfaces and might have trouble detecting the water surface as a result).&lt;br&gt;&#xA;Can't find a primary source for this, though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-18T13:14:26.177" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16770" PostTypeId="1" CreationDate="2016-06-18T20:20:30.610" Score="2" ViewCount="317" Body="&lt;p&gt;Is it known why Roman Romanenko and Dmitry Kondratyev retired early in their careers as cosmonauts?&lt;/p&gt;&#xA;" OwnerUserId="13909" LastEditorUserId="4660" LastEditDate="2016-06-18T20:37:21.950" LastActivityDate="2016-06-21T21:41:17.963" Title="Why did these cosmonauts retire early in their careers?" Tags="|history|astronauts|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16772" PostTypeId="1" CreationDate="2016-06-19T02:43:53.417" Score="4" ViewCount="1173" Body="&lt;p&gt;In my question &lt;a href=&quot;https://space.stackexchange.com/q/16771/12102&quot;&gt;How was Earth's &amp;quot;quasi-satellite&amp;quot; 2016 HO3 &amp;quot;first spotted&amp;quot; and it's orbit determined?&lt;/a&gt; I link to two videos of simulations of views of 2016 HO3's orbit seen in two different frames.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/SbbAnVU4rmY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;In this &lt;a href=&quot;https://www.youtube.com/watch?v=SbbAnVU4rmY&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA JPL video&lt;/a&gt; (above) the view is rotating around the sun following Earth. You can see the earth move slightly closer and farther from the sun since the Earth's orbit is not quite circular.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/2mVfE_qmQAg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=2mVfE_qmQAg&quot; rel=&quot;nofollow noreferrer&quot;&gt;This video&lt;/a&gt; (above) from &lt;a href=&quot;http://arksky.org/calendar/alerts/714-what-is-it-the-strange-new-object-2016-ho3&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://arksky.org/calendar/alerts/714-what-is-it-the-strange-new-object-2016-ho3&lt;/a&gt; shows a projection of 2016 HO3's motion against the stars as seen from Earth's location, but in a fixed direction. You can see the the sun and planets tend to follow the ecliptic, while 2016 HO3 does a figure-eight every year.&lt;/p&gt;&#xA;&lt;p&gt;The NASA JPL &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=6537&quot; rel=&quot;nofollow noreferrer&quot;&gt;news brief&lt;/a&gt; announcing the discovery of 2016 HO3 states:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The asteroid's orbit also undergoes a slow, back-and-forth twist over multiple decades. &amp;quot;The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon,&amp;quot; said Chodas. &amp;quot;The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the moon. In effect, this small asteroid is caught in a little dance with Earth.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;note: &lt;em&gt;Paul Chodas is the manager of NASA's Center for Near-Earth Object (NEO) Studies&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I &lt;em&gt;think&lt;/em&gt; that the fact that it's orbit slowly oscillates on the order of decades (tens of orbital periods) with respect to Earth's orbit means it is in a 1:1 resonance with Earth. But I'm using the term orbital resonance without knowing it's exact definition - if there is one. Perhaps it's a &lt;em&gt;soft term&lt;/em&gt; - some orbits may be clearly in resonance, others only roughly in resonance.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Is there a good working definition of orbital resonance, and is 2016 OH3's orbit in 1:1 resonance with Earth's orbit?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;bonus: Are Trojan asteroids (at a planet's &lt;span class=&quot;math-container&quot;&gt;$L_4$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$L_5$&lt;/span&gt; triangular libration points) also considered to be in a 1:1 resonance orbit?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-08-26T13:15:51.643" Title="What (actually) is a 1:1 resonance, and is 2016 HO3 in one with the Earth?" Tags="|orbital-mechanics|terminology|near-earth-asteroid|orbital-resonance|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16773" PostTypeId="2" ParentId="16772" CreationDate="2016-06-19T11:58:53.850" Score="3" Body="&lt;p&gt;1:1 resonance means that each year the asteroid will be in approximately the same place as viewed from Earth as it was the previous year. This as compared to Pluto, which is in a 3:2 resonance with Neptune, meaning for every 3 Neptune years, Pluto will make 2 years. These should be a result of gravitational interaction between the two bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yes, Trojans are considered 1:1 resonance, see &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_resonance&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-06-19T12:57:45.307" LastActivityDate="2016-06-19T12:57:45.307" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16774" PostTypeId="1" CreationDate="2016-06-19T12:11:59.717" Score="4" ViewCount="162" Body="&lt;p&gt;MagnetoPlasmaDynamic Thrusters are plasma thrusters in which the acceleration of the fluid (plasma) is carried in a electromagnetic way. The electric field and the lorentz force are the primary causes of this acceleration. Otherwise, because of the presence of an electric and magnetic field there's a perpendicular component causing a flow of electrons (externally seen as current) which is called Hall effect. What is the main implication of a large Hall effect parameter in MPD?. Remember that the hall parameter is the angle between the j/sigma and the total electric field (E*=E+uxB).&lt;/p&gt;&#xA;" OwnerUserId="15736" LastEditorUserId="1142" LastEditDate="2016-06-19T13:50:02.820" LastActivityDate="2016-06-19T13:50:02.820" Title="What does a high Hall Effect Parameter mean in MPD thrusters?" Tags="|advanced-propulsion|ion-thruster|thrust|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16775" PostTypeId="2" ParentId="16770" CreationDate="2016-06-19T15:00:17.287" Score="4" Body="&lt;p&gt;Romanenko retired &lt;a href=&quot;http://www.spacefacts.de/bios/cosmonauts/english/romanenko_roman.htm&quot; rel=&quot;nofollow&quot;&gt;for health reasons&lt;/a&gt;. I haven't found data on Kondratyev. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-19T15:00:17.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16776" PostTypeId="1" AcceptedAnswerId="17510" CreationDate="2016-06-20T09:03:23.030" Score="12" ViewCount="1634" Body="&lt;p&gt;Here are two cases where links to a specific time code in SpaceX YouTube videos stop showing the intended moment because apparently SpaceX has gone in within hours or perhaps a day and shortened the videos. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In both cases the times are off by roughly ten minutes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My first guess is that they are lopping off some of the pre-launch because its not interesting, but given the amazing interest the videos have generated and number of people including myself (e.g. &lt;a href=&quot;https://space.stackexchange.com/q/13595/12102&quot;&gt;What caused this shadow on Falcon 9 stage 2 w/Jason-3 before circularization burn?&lt;/a&gt; ) who comb them for educational goodies and &quot;aha!&quot;s, the idea that the videos &lt;em&gt;are changing over time&lt;/em&gt; moves them farther from the &lt;em&gt;data&lt;/em&gt; category and closer to the &lt;em&gt;infotainment&lt;/em&gt; category.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be great to know what's gone. I'm not particularly looking for opinions or guesses - is there any factual accounts of what is removed, or are the original broadcasts (&lt;em&gt;raw data&lt;/em&gt;) still available somehow?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;JCSAT-14 Hosted Webcast&lt;/strong&gt; Question: &lt;a href=&quot;https://space.stackexchange.com/q/15152/12102&quot;&gt;Is it normal for thrusters to &amp;quot;ice up&amp;quot;?&lt;/a&gt;&#xA;noted: &lt;a href=&quot;https://space.stackexchange.com/questions/15152/#comment42692_15152&quot;&gt;here&lt;/a&gt;.&#xA;&lt;code&gt;T+ 00:32:58&lt;/code&gt; was at &lt;code&gt;01:02:50&lt;/code&gt;, now at &lt;code&gt;00:53:58&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;SES-9 Technical Webcast&lt;/strong&gt; Question: &lt;a href=&quot;https://space.stackexchange.com/q/14333/12102&quot;&gt;What was all of the junk in the SES satellite deployment?&lt;/a&gt; noted: &lt;a href=&quot;https://space.stackexchange.com/questions/14333/#comment41004_14333&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/14333/#comment41005_14334&quot;&gt;here&lt;/a&gt;.&#xA;&lt;code&gt;T+ 00:32:08&lt;/code&gt; was at &lt;code&gt;00:51:23&lt;/code&gt;, now at &lt;code&gt;00:42:11&lt;/code&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2018-02-20T02:55:08.547" Title="What is it that SpaceX is editing out of their videos after posting them on YouTube?" Tags="|launch|spacex|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16777" PostTypeId="2" ParentId="16741" CreationDate="2016-06-20T09:19:07.480" Score="7" Body="&lt;ul&gt;&#xA;&lt;li&gt;Ariane 5: the Vulcain first stage engine is started using a &lt;a href=&quot;http://www.appbv.nl/index.asp?grp_id=3&amp;amp;sub_id=30&quot;&gt;small solid rocket motor&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Shuttle SSME: after chilldown, the propellant valve is opened, allowing LH into the engine. This &lt;a href=&quot;http://www.enginehistory.org/SSME/SSME3.pdf&quot;&gt;expands, driving the turbopump&lt;/a&gt; enough to fill the gas generators. The mixture in the gas generator is ignited and this drives the turbopump to operational speed.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When a start command is received, the MFV is immediately ramped to its full open position in two-thirds of a second (see Figure 9). This enables the LH2 to fill the downstream system and begin to power the high pressure turbines. The latent heat of the hardware imparts enough energy to the hydrogen to operate as an “expander-cycle” engine for the early part of the start sequence. This eliminates the need for any auxiliary power to initiate the start sequence,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.awesomestories.com/asset/view/F-1-Engine-Propulsion-of-the-Saturn-V-Moon-Rocket&quot;&gt;Saturn V F-1&lt;/a&gt;: The LOX valve opens, allowing LOX to flow through the turbopump, this starts it spinning. Some of the LOX flows to the gas generator. Then the propellant valve opens, RP-1 flows through the pump to the gas generator, the mixture in the gas generator is ignited and this drives the turbopump to operational speed. So basically this is gravity-fed until the gas generator ignites. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-20T09:19:07.480" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16779" PostTypeId="2" ParentId="16767" CreationDate="2016-06-20T12:42:06.117" Score="5" Body="&lt;p&gt;It is not a given that all lunar meteorites are as scattered as the impactor of Barringer crater. Some meteorites strike as slowly as 3 to 4 km/s. And magnetic anomalies suggest rich metal deposits from meteorites. See figure 8 of &lt;a href=&quot;https://arxiv.org/ftp/arxiv/papers/1410/1410.6865.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Resources: A Review&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Near Earth Asteroid with an average Vinf with regard to earth would strike the moon at 10 to 11 km/s but there are some that hit with an impact velocity as low as 3 to 4 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bCvCC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bCvCC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Escape velocity at Mars surface is 3.7 km/s. If Mars had no atmosphere, this would be the minimum impact velocity. For the smaller impactors, aerobraking can shed a lot of the incoming velocity. But the larger impactors don't have a ballistic coefficient amenable to aerobraking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like Mars had hydrologic cycles in the past. So the same hydrologic processes that concentrated ore on earth may have also taken place on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has also been extensive Mars volcanic activity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My bet is that there are some rich ore bodies on Mars' surface.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-06-20T13:28:03.647" LastActivityDate="2016-06-20T13:28:03.647" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16780" PostTypeId="1" CreationDate="2016-06-20T13:22:38.893" Score="3" ViewCount="424" Body="&lt;p&gt;...from common observation, I can easily conclude there is a lot of infrared (from burning, superheated fuel), ending somewhere near yellow of visible spectrum; there should be next to none of UV and higher. But how far up does it reach? Do the engines emit a considerable amount of microwaves?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question is motivated by &lt;a href=&quot;https://space.stackexchange.com/questions/16756/how-does-a-falcon-9-booster-know-how-far-away-the-ground-is#comment42702_16756&quot;&gt;&quot;How does a Falcon 9 booster know how far away the ground is?&quot;&lt;/a&gt;, and relevant available options. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious solution is &quot;reflected waves&quot;, but they must be in a spectrum not obscured by the engines. Ultrasound, or anything audio is right off, with the enormous level of audible noise. A laser (visible or IR) distance measurement will have the dot lost in the bright flame. I wonder how viable microwaves are, though - essentially, a radar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, during a spacecraft reentry, the communicational blackout is caused by the plasma of superheated air and ablator, radiating on a spectrum broad enough to flood both long waves and x-rays with enough noisy energy to make them not viable for communication. Rocket exhaust is much less energetic, but it's still one of most energetic large-scale emissions achieved by technology and I wouldn't be surprised if it exceeds the standard &quot;visible+infrared&quot; range of &quot;burning stuff&quot;.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2023-08-26T22:02:40.343" Title="What is the electromagnetic spectrum profile of rocket flame/exhaust?" Tags="|radiation|measurement|exhaust|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16781" PostTypeId="1" AcceptedAnswerId="16805" CreationDate="2016-06-20T15:10:13.537" Score="10" ViewCount="1066" Body="&lt;p&gt;At the end of December 2015, a giant &quot;painting&quot; made of 4 million color-sorted recycled bottles &lt;a href=&quot;http://www.taipeitimes.com/News/taiwan/archives/2015/12/23/2003635477&quot; rel=&quot;nofollow noreferrer&quot;&gt;was put on display&lt;/a&gt; in Keelung, Taiwan. The &lt;a href=&quot;http://indianexpress.com/article/lifestyle/art-and-culture/van-gogh-recycled-four-million-bottles-used-in-taiwan-replica-2793673/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Indian Express&lt;/a&gt; says it's 53 hectares, which is 530,000 square meters, so it's probably roughly 0.5 x 1 km in size. At 400km, that makes it roughly a quarter of the diameter of the moon visually - if I've done my arithmetic correctly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a reference, the 3U Cubesat PlanetLabs Doves have a resolution of 3 to 5 meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they knew about it, could an astronaut on the ISS have been able to photograph some artificial, artistic pattern &lt;em&gt;similar to this&lt;/em&gt; either with a hand-held camera (with a long lens) or with the &lt;a href=&quot;https://space.stackexchange.com/a/13770/12102&quot;&gt;downward-pointing telescope&lt;/a&gt; on the station? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that either the hand-held camera or the telescope &lt;a href=&quot;https://space.stackexchange.com/questions/13769/#comment35989_13770&quot;&gt;might need to be moving&lt;/a&gt; to compensate for the relative motion between the ISS and the surface. Besides various forms of architecture and civil engineering, has any art on earth been photographed from the ISS? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image from &lt;a href=&quot;http://indianexpress.com/article/lifestyle/art-and-culture/van-gogh-recycled-four-million-bottles-used-in-taiwan-replica-2793673/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Indian Express&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Tp7Z.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Tp7Z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-06-22T16:43:29.983" Title="Could an ISS astronaut photograph something like this 1km &quot;Van Gogh&quot; if they knew it was there?" Tags="|iss|imaging|earth-observation|space-art|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16782" PostTypeId="5" CreationDate="2016-06-20T15:16:10.987" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-06-20T15:16:10.987" LastActivityDate="2016-06-20T15:16:10.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16783" PostTypeId="4" CreationDate="2016-06-20T15:16:10.987" Score="0" Body="use of space exploration or space-access technology for primarily artistic purposes" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-06-20T17:09:26.657" LastActivityDate="2016-06-20T17:09:26.657" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16784" PostTypeId="2" ParentId="16781" CreationDate="2016-06-20T15:50:29.423" Score="6" Body="&lt;p&gt;The ISS, at it's closest, is, say, 500 km (Can be a bit closer, but let's not stretch it). &lt;a href=&quot;http://www.tawbaware.com/maxlyons/calc.htm&quot; rel=&quot;noreferrer&quot;&gt;That would require a FOV&lt;/a&gt; of 1/500, or about 0.1 degrees. That would require a camera with a lens of about 20000 mm to get a full view, given a standard sensor. That isn't going to happen! But still, a low resolution view of the image is entirely possible. The &lt;a href=&quot;http://www.diyphotography.net/camera-gear-nasa-use-international-space-station/&quot; rel=&quot;noreferrer&quot;&gt;ISS's best telephoto lens&lt;/a&gt; is an 800 mm lens with a teleconverter to 1120mm, and with a crop factor, it's FOV is about 1.2 degrees apart. So if you have a, say, 20 megapixel image, about 200,000 pixels will cover the image, which is manageable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so how fast would such an image need to be taken to minimize distortion? Let's try to limit distortion to 10m, which is probably a bit high for such an image. The space station moves at 7667 m/s. So you'd have to get the image in around 1/1500th of a second. Most cameras have something called a shutter sync. I've checked and some of the ISS cameras are at 1/8000th of a second, so a high shutter speed image could be taken that would image it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my knowledge, no such thing has been photographed from the ISS. They simply don't have the resolution to really do it justice. But there are several art worthy photos taken, if you look around, of places on Earth. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-20T15:50:29.423" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="16786" PostTypeId="2" ParentId="2958" CreationDate="2016-06-20T20:49:35.023" Score="13" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fSraj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fSraj.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first piece of Pad 39B to be removed, the 80-foot lightning mast that topped the fixed service structure (FSS), was lowered on March 3, 2009. NASA's first launch pad lightning protection mast, it was used for the 1975 Apollo-Soyuz Test Project (ASTP) before being adapted for the space shuttle program.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p4SfQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p4SfQ.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;http://www.collectspace.com/news/news-091611a.html&quot; rel=&quot;noreferrer&quot;&gt;Source for quote/images: collectSPACE&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Construction of three lightning towers continues at Kennedy Space Center's Launch Pad 39B in January 2009&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;http://www.nasa.gov/centers/kennedy/news/LC39B_Weather_System.html&quot; rel=&quot;noreferrer&quot;&gt;Source: NASA&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="15819" LastEditorUserId="6816" LastEditDate="2016-06-20T23:26:11.860" LastActivityDate="2016-06-20T23:26:11.860" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16788" PostTypeId="1" AcceptedAnswerId="16789" CreationDate="2016-06-21T00:17:31.127" Score="8" ViewCount="855" Body="&lt;p&gt;What difficulties will be faced making modifications for the Falcon 9 FT rocket, so in this way to make smoother, better and easier landings of the first stage from a GTO mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example increasing a little the rocket size in height or diameter so to have a little bit more fuel to burn up 5 more seconds longer or even less than 5 seconds, just  to give a better chance for successful landings. Of course modifying in that way that the aerodynamic pressure will be still ok. Another option could be a better engine performance so how much should be increased the performance, the thrust for example since the specific impulse usually is difficult to increase. This could increase the weight of the engines but engines weight is to small compared to the entire fueled rocket weight. So increasing rocket engines thrust  in that mass that the new performance gives the rocket 5 more seconds thrust ( 5 seconds is taken as reference just for example ) at the final burn before landing. Another option could be a 4 or 5 engine burn more shortly than 3 engine burn before the final burn with 1 engine at the center of octaweb. Maybe this would not be a good choice but just i am supposing as option that could be better in the same way that SpaceX changed for GTO mission from a single engine burn to a 3 engine burn technique. What difficulties will be for this options which i took as examples or even for other options that could be as possible variants. These modifications or tests will have a cost and probably increase a little bit the cost of Falcon 9 FT but the SpaceX prices are still the lowest in the market and will be lower if a first stage lands successfully much more often and easier going toward perfection and 100% successful landings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this description was as an idea or possible suggestions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in a simple question what would be the challenges for SpaceX team to make some kind of modifications which will give more seconds to burn up longer the engines and landing easier and safer the first stage?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-08-07T10:41:06.667" LastActivityDate="2016-08-24T17:41:19.777" Title="What difficulties will be faced modifying the Falcon 9 FT, so to have better and easier landings of the 1st stage from a GTO mission?" Tags="|spacex|falcon-9|landing|reentry|reuse|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16789" PostTypeId="2" ParentId="16788" CreationDate="2016-06-21T03:32:18.917" Score="13" Body="&lt;p&gt;If you think about it, if it was easy, SpaceX would be doing it. After all, a used first stage is potentially worth $40 million dollars, so not a minor thing to recover vs lose a stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Better engine performance would help, but of course, the Merlin engine has been under constant upgrades throughout it's life time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Merlin 1A was used on the Falcon 1. That was actually upgraded to the Merlin 1B on the last two Falcon 1 flights with better performance. The Merlin 1C started the first five Falcon 9 flights, then on to the Merlin 1D for the remainders on the Falcon 9 v1.1 and Full Thrust. Without changing the name, they upgraded the Merlin 1D from 85% to full thrust, and are rumoured to have yet ANOTHER thrust upgrade coming.  So they are basically already doing this in the real world, and have been all along.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of fuel, they have already stretched the first and second stage Falcon 1.0 to 1.1. Then they stretched the second stage a fair bit and the first stage a little bit in the F9 Full Thrust (since SpaceX refuses to call it 1.2). In addition they are sub-cooling the fuel and oxidizer to increase the density to stuff more in the same tanks. They cannot increase the girth of the stage, since it needs to be roadable, and they are limited by standard bridge heights. (Have to drive Hawthorne, CA -&gt; McGregor, TX -&gt; CCAFS, FL, and possibly Boca Chica, TX in the future). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are limited in stretching the first stage length much more since they are approaching bending limits. If it is too long, you might bend the tube (ever done that with a paper towel roll?) when it is in flight and under stress. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one thing they could really do to help is upgrade to a better engine on the second stage. The first stage's job is to get the second stage high enough and fast enough that the second stage can get the payload where it needs to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus a better second stage, perhaps using a Raptor which has better Isp (Measure of performance) since the Merlin 1D-Vac is an ok engine, but not a great upper stage performer. But that is anything but a cheap/easy change. Not just from a new stage design issue, but also a switch to Liquid Methane as a fuel, which means new stuff at the launch site, on the TEL, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the nice things about the approach SpaceX is taking is that everyone is paying for an expendable launch right now. So they are selling the first stage at a cost they can live on. Then they get a free experiment, full scale, where they can try the variants of ideas their engineers have. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We saw this when they changed from a single engine landing burn, to a three-one engine burn, and then to a 1-3-1 landing burn approach. They are probing around the edges, seeing what they can do to both find where the edges are, and how close they can get to them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point in the year, they potentially have 8 to 13 launches remaining in 2016, each of which they will try to recover the stage. Musk has said, he expects only 70% recovery rate as they try to find those edges in their test program. That they had 4 successes was amazing. Perfection is down the road, but first they need some experiments to get there.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-21T03:32:18.917" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="16790" PostTypeId="1" AcceptedAnswerId="16791" CreationDate="2016-06-21T09:15:42.427" Score="14" ViewCount="2812" Body="&lt;p&gt;The usual fare for ISS has very long expiration dates, to last between deliveries, months apart. That, combined with other requirements (no crumbs/powders, no cooking, flatulence-safe etc) considerably limits the choice and surely impacts the taste.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But most delivery missions take below 3 days from launch to docking. Some take only 5-6 hours. Surely the astronauts would welcome food that is different from the usual preserved concentrates. Loading up a couple meals that are prepared shortly before launch, would expire in a couple days, but contain fresh produce shouldn't be that much of a deal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it practiced? Or do astronauts eat the same stuff on the day of delivery as on the day before the next delivery?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-06-21T11:16:40.423" Title="Do astronauts on ISS get fresh perishable goods with deliveries?" Tags="|iss|food|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16791" PostTypeId="2" ParentId="16790" CreationDate="2016-06-21T11:07:04.987" Score="18" Body="&lt;p&gt;Yes they do. Here are some assorted quotes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://observer.com/2015/11/an-inside-look-at-how-nasa-resupplies-the-international-space-station/&quot;&gt;An Inside Look At How NASA Resupplies The International Space Station&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...the menu includes whole fruit...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.quora.com/How-does-NASA-transport-food-to-the-International-Space-Station-How-often-do-they-do-it&quot;&gt;How does NASA transport food to the International Space Station? How often do they do it?&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The fresh food locker contains tortillas, fresh bread, breakfast rolls, and fresh fruits and vegetables such as apples, bananas, oranges, and carrot and celery sticks&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://books.google.nl/books?id=Q-45xmPeimcC&amp;amp;pg=PA28&amp;amp;lpg=PA28&amp;amp;dq=international+space+station+resupply+food+list+contents&amp;amp;source=bl&amp;amp;ots=1VdGdGveyh&amp;amp;sig=TY2sE8Hf3ntHDcOw8pM3rhSQdPA&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwih48CQ_rjNAhUmK8AKHckCAKcQ6AEIWDAJ#v=onepage&amp;amp;q=international%20space%20station%20resupply%20food%20list%20contents&amp;amp;f=false&quot;&gt;Nutrition in Spaceflight and Weightlessness Models&lt;/a&gt; (Helen W. Lane, Dale A. Schoeller) section 2.2.5 &lt;em&gt;International Space Station food&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Refrigerated items include fresh and fresh-treated fruits and vegetables...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user10509" LastEditorDisplayName="user10509" LastEditDate="2016-06-21T11:16:40.423" LastActivityDate="2016-06-21T11:16:40.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16792" PostTypeId="1" CreationDate="2016-06-21T11:22:34.373" Score="3" ViewCount="442" Body="&lt;p&gt;What was the first company that came up with the business plan of taking tourists to space? In which year was the company launched or the outline of the business plan known to public? What was the first spaceflight company to have its own launch vehicle? &lt;/p&gt;&#xA;" OwnerUserId="13882" LastEditorUserId="12102" LastEditDate="2021-09-05T23:16:44.357" LastActivityDate="2021-09-05T23:16:44.357" Title="What was the first space tourism company?" Tags="|history|commercial|spaceflight-firsts|tourism|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16793" PostTypeId="1" AcceptedAnswerId="16794" CreationDate="2016-06-21T11:58:11.723" Score="20" ViewCount="3322" Body="&lt;p&gt;During the recent return of Expedition 47 from the ISS, I noticed the main parachute of their Soyuz capsule pulsating.  You can see it in &lt;a href=&quot;https://www.youtube.com/watch?v=Mur7l2gxb2E#t=68m50&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Flat&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QurXS.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QurXS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Domed&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7KpVN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7KpVN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My wife noted that it looks a little like a jellyfish, moments later I saw this &lt;a href=&quot;https://twitter.com/Astro_Alex/status/744095384641867776&quot; rel=&quot;noreferrer&quot;&gt;tweet by Alexander Gerst&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &quot;jellyfish breathing&quot; of the parachute feels like sitting in a small boat at sea. Not indicative of what is to come...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Is the parachute specifically designed to do this, if so - why?  What benefits does it provide?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's not specifically designed to do this, and it's just a side effect of some other design constraint, what causes it to happen, and is it detrimental to the overall performance of the parachute at all?  Perhaps all parachutes of this design/scale behave this way, and I've just never noticed it before?&lt;/p&gt;&#xA;" OwnerUserId="10686" LastActivityDate="2016-06-21T18:49:01.350" Title="Why does the Soyuz parachute pulsate?" Tags="|soyuz-spacecraft|parachute|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16794" PostTypeId="2" ParentId="16793" CreationDate="2016-06-21T15:13:30.453" Score="19" Body="&lt;p&gt;Thanks to &lt;a href=&quot;http://link.springer.com/article/10.1007/BF01105293#page-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;the paper&lt;/a&gt; linked by Jan Doggen in the comments we can say that it is a natural outcome of the shape of the parachute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, the shape, together with the near absence of wind and the fact that the craft is descending near vertically determines that pulsating motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the words of the paper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The studies of [1, 2] demonstrated that during the motion of an object with a parachute the latter's canopy changes its shape and geometric dimensions periodically - it pulsates. In what follows, the effect of this phenomenon on the dynamics of the object-parachute system (OPS) is examined.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The paper then proceeds to derive the pulsating frequencies and the other characteristics of the motion.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So to directly answer your own question&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is the parachute specifically designed to do this?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, it is not.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What causes it to happen?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You'll find the detailed explanation in the references of the paper mentioned above (that apparently google scholar fails to find).&#xA;But to keep it simple, it is the flow of air around the canopy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The canopy has a hole on the top to allow &lt;em&gt;some&lt;/em&gt; air to escape in a controlled fashion, this provides stability, as mentioned on &lt;a href=&quot;https://aviation.stackexchange.com/q/26061/1467&quot;&gt;another SE&lt;/a&gt;.&#xA;The crucial point is that the hole is not sufficient for &lt;em&gt;all&lt;/em&gt; the air below the canopy to escape (and for good reason, the parachute would offer no resistance, failing at its job!), the air then escapes laterally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under the conditions discussed in the linked paper (vertical descent at constant/terminal velocity) this results in a natural quasi-harmonic oscillatory movement.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Perhaps all parachutes of this design/scale behave this way, and I've just never noticed it before?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Given the paper, yes they do.&lt;/p&gt;&#xA;" OwnerUserId="5862" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:54.153" LastActivityDate="2016-06-21T18:49:01.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16795" PostTypeId="2" ParentId="16793" CreationDate="2016-06-21T15:13:57.580" Score="15" Body="&lt;p&gt;It is called parachute &quot;breathing&quot;, and is quite common. It is a natural consequence of the flexibility of the parachute shape.  It is initiated in the initial inflation, where the parachute will always over-inflate, and then oscillate around a mean inflation state. Hopefully with decreasing amplitude. If the breathing amplitude would be too large or growing, e.g. threatening inversion or damage, then measures are taken to damp the oscillations. Otherwise it is relatively harmless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At supersonic speeds, &quot;Heavy canopy breathing and high-speed flutter frequently result in material disintegration, seam separation. and canopy damage.&quot; (Knacke)&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-06-21T15:13:57.580" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16796" PostTypeId="1" CreationDate="2016-06-21T16:35:27.990" Score="10" ViewCount="719" Body="&lt;p&gt;Presumable it is &lt;em&gt;largely&lt;/em&gt; a coincidence that Juno arrives at the perfect moment for American pride in NASA's achievements, but not &lt;em&gt;entirely&lt;/em&gt;. The video below of the trajectory, released in July 2011 just before it launched, only states it will arrive at Jupiter in July 2016:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/sYp5p2oL51g?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How were the small adjustments made so that the craft would arrive in such a timely fashion? Was it due to the juggling always involved in launch dates, or was more involved?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: There are a range of possible launch dates and possible trajectories on missions, there is margin built in to missions, and a number of precedents for things being done for public relations reasons. July 4th is nearing and this question will occur to others, and at the moment there is no answer out there. If the answer is it is entirely coincidence, it seems that should be possible to demonstrate, and it would be especially nice to have a sense of how much play there is in launch dates, and how much fuel would be involved in making a minor adjustment in deep space to change arrival time by a day or two, and how much that might affect a mission of this kind (a polar orbit of Jupiter). Or some reference point for the decision process. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-06-22T16:05:58.803" LastActivityDate="2016-07-05T15:26:51.747" Title="How was Juno's arrival set up to be on the evening of July 4th?" Tags="|orbital-mechanics|trajectory|jupiter|juno|deep-space-maneuver|" AnswerCount="6" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16797" PostTypeId="2" ParentId="16796" CreationDate="2016-06-21T17:00:23.897" Score="5" Body="&lt;p&gt;I can almost guarantee that it was a coincidence. Essentially they have a launch window, with an entry for each day of the launch window. Depending on the launch date, they will arrive at the given day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can almost guarantee if it didn't make the most sense from a spacecraft point of view, they would have adjusted the arrival to July 5th, to make it easier for the team working on the spacecraft. But as Mark joked, major mission events frequently happen on or near major holidays.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-21T17:00:23.897" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="16798" PostTypeId="2" ParentId="16792" CreationDate="2016-06-21T19:19:46.070" Score="7" Body="&lt;p&gt;Pan Am &lt;a href=&quot;http://blog.nasm.si.edu/behind-the-scenes/first-moon-flights-club/&quot; rel=&quot;nofollow&quot;&gt;accepted the first reservation for a flight to the Moon in 1964&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Between 1968 and 1971, Pan Am issued over 93,000 “First Moon Flights” Club cards to space enthusiasts eager to make a reservation for the first commercial flight to the Moon. Issued at no cost, the membership cards were numbered according to priority. The Club originated from a waiting list that is said to have started in 1964, when Gerhard Pistor, an Austrian journalist, went to a Viennese travel agency requesting a flight to the Moon. The agency forwarded his request to Pan Am, which accepted the reservation two weeks later and replied that the first flight was expected to depart in 2000.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;TWA &lt;a href=&quot;http://www.airspacemag.com/not-categorized/space-tourisms-first-small-steps-20428780/?no-ist&quot; rel=&quot;nofollow&quot;&gt;also got in on the fun&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They seem to have been content to take reservations without investing in a vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian space agency has long been amenable to paying passengers. Initially these were all professionals (through the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_cosmonauts&quot; rel=&quot;nofollow&quot;&gt;Intercosmos&lt;/a&gt; program), but from 2001, &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Adventures&quot; rel=&quot;nofollow&quot;&gt;Space Adventures&lt;/a&gt; sold seats on Soyuz flights to tourists. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1995, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ansari_X_Prize&quot; rel=&quot;nofollow&quot;&gt;X-prize&lt;/a&gt; was announced (later renamed to Ansari X-prize), awarding $10M:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;for the first non-government organization to launch a reusable manned spacecraft into space twice within two weeks.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This served as a catalyst. 26 companies competed, with Scaled Composites and their Spaceship One becoming the winner in 2004. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few notables among &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_private_spaceflight_companies&quot; rel=&quot;nofollow&quot;&gt;spaceflight companies&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Bigelow_Aerospace&quot; rel=&quot;nofollow&quot;&gt;Bigelow&lt;/a&gt; was founded in 1998. Its long-term goal is a hotel in space. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In 2000, &lt;a href=&quot;https://en.wikipedia.org/wiki/Blue_Origin&quot; rel=&quot;nofollow&quot;&gt;Blue Origin&lt;/a&gt; was founded. They have made 4 test flights with their intended tourist vehicle so far, first flight was in 2015.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In 2004, &lt;a href=&quot;https://en.wikipedia.org/wiki/Virgin_Galactic&quot; rel=&quot;nofollow&quot;&gt;Virgin Galactic&lt;/a&gt; was founded, they partnered with Scaled Composites to develop Spaceship One into a commercial venture. First flight of Spaceship Two was in 2013. In 2014 they suffered a crash with one fatality while testing their intended tourist vehicle (Spaceship Two).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX&quot; rel=&quot;nofollow&quot;&gt;SpaceX&lt;/a&gt; was founded in 2002, with a goal of transporting people to Mars. They haven't made a manned flight yet.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, 'first to have its own launch vehicle' is Virgin Galactic. Launch year of the first company will be around 2000. Most of the Ansari X-prize competitors have folded and some weren't in a position to win to begin with, so I'm not sure how seriously to take all of them. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-22T08:01:43.013" LastActivityDate="2016-06-22T08:01:43.013" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16800" PostTypeId="2" ParentId="16770" CreationDate="2016-06-21T19:45:02.423" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.newsru.com/russia/06sep2012/mks.html&quot; rel=&quot;nofollow&quot;&gt;a newsru.com article (in Russian)&lt;/a&gt; , Kondratiev quit cosmonauts corps and started to work for a private company, not related to space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another &lt;a href=&quot;http://m.kp.ru/daily/26129/3021020/&quot; rel=&quot;nofollow&quot;&gt;article (also in Russian)&lt;/a&gt; contains an interview about Lonchakov's quitting the cosmonauts corps and also mentions that Kondratiev quit in protest against transferring &lt;a href=&quot;http://www.gctc.su/&quot; rel=&quot;nofollow&quot;&gt;the cosmonaut training center&lt;/a&gt; from military to civilians (Kondratiev is a colonel of RF Air Force). &lt;/p&gt;&#xA;" OwnerUserId="6491" LastEditorUserId="6491" LastEditDate="2016-06-21T21:41:17.963" LastActivityDate="2016-06-21T21:41:17.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16801" PostTypeId="1" CreationDate="2016-06-21T09:11:16.143" Score="11" ViewCount="434" Body="&lt;p&gt;Looking at time lapses taken from the International Space Station, I discovered that the Earth's atmosphere can glow at night, even when it seems that those pictures were taken far away from auroras.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are a few examples :&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/EIeST.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EIeST.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/z0M6Y.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/z0M6Y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the origin of airglow, according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Airglow&quot; rel=&quot;noreferrer&quot;&gt;this Wikipedia article&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Airglow (also called nightglow) is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diffused sunlight from the far side are removed.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, my question is about whether or not airglow should be visible (with the naked eye) when looking at Earth's atmosphere from space at night. If not always visible or if the intensity of it varies a lot, is there a way to compute how intense it would be ?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you.  &lt;/p&gt;&#xA;" OwnerUserId="15857" OwnerDisplayName="Trevör Anne Denise" LastEditorUserId="15857" LastEditDate="2016-06-30T13:56:49.640" LastActivityDate="2016-07-31T10:31:13.787" Title="Is airglow always visible from space?" Tags="|atmosphere|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="16803" PostTypeId="1" AcceptedAnswerId="16806" CreationDate="2016-06-22T08:11:11.217" Score="3" ViewCount="309" Body="&lt;p&gt;If a probe, equipped with a current state-of-the-art antenna, were to send data to Earth from Proxima Centauri orbit, what power would the antenna need to emit for the signal to be usefully readable on Earth, using our best reception antennas / radiotelescopes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know the power requirements scale quadratically with distance, but I don't have any good data points to extrapolate - e.g. we still have no problems receiving from Voyager 2, far as it is, so its antenna is well above the required minimal output level (and far from state-of-the art).&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-06-22T11:53:45.473" Title="How powerful an antenna needs to be to signal Earth from other solar systems?" Tags="|communication|interstellar-travel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16804" PostTypeId="2" ParentId="1059" CreationDate="2016-06-22T08:53:51.077" Score="3" Body="&lt;p&gt;Now the previous answer might be outdated. &lt;strong&gt;ISRO&lt;/strong&gt; sets the record with 20 satellites launched at once.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Indian Space Research Organisation (ISRO)’s big mission 22nd June 2016 has seen 20 satellites launched successfully in an orbit. The PSLV-C34 mission is being called as the biggest ever mission for India’s space agency.&lt;/p&gt;&#xA;" OwnerUserId="15835" LastActivityDate="2016-06-22T08:53:51.077" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16805" PostTypeId="2" ParentId="16781" CreationDate="2016-06-22T09:17:57.750" Score="11" Body="&lt;p&gt;I like @PearsonArtPhoto's answer, though the considerations about exposure time may not be correct. I was looking for something more visual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The website states an area of 0,53m² of the Van Gogh image, and since the original is 92.1 × 73.7 cm², the large version must be about 0.817km wide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During his &lt;em&gt;year in space&lt;/em&gt;, Scott Kelly took tons of photos, one of New York from straight above at night. And Central Park is about 4.07 × 0,86 km² in size, which means its small side is roughly as long as the long side of the Van Gogh.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here is the picture from Manhattan with the Van Gogh above Central Park. The Van Gogh was scaled by  a factor derived from the size of the original, of Central Park in reality, and the pixel size of Central Park on the photo, so it's quite exact. (Width: 105px)&#xA;I blurred the right one  until I felt that its sharpness is about the same as that of the entire image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, I cropped the picture to the interesting area, but didn't scale it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S2Gvz.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S2Gvz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;(Source of original: &lt;a href=&quot;https://www.nasa.gov/content/best-year-in-space-photographs-from-scott-kelly?id=380150&quot; rel=&quot;noreferrer&quot;&gt;https://www.nasa.gov/content/best-year-in-space-photographs-from-scott-kelly&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's quite interesting now is the EXIF data of the photo:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It was taken with a Nikon D4, making photos of 4.928 × 3.280 px² (16.4MP) with its full frame (36mm) sensor. The size of the original photo is the same, so it was not cropped or scaled.&lt;/li&gt;&#xA;&lt;li&gt;The lens was a fixed-length 400mm f/2.8, and the photo was taken at f/2.8 to collect as much light as possible. Note: A higher aperture of may be f/5.6 collects less light, but would give a sharper image. (Though this effect should be low for this lens).&lt;br&gt;&#xA;&lt;em&gt;By the way: Assuming an altitude of 400km, the width of the Van Gogh should be 112px, which fits my 105px quite well. So, no teleconverter was used!&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Exposure time was 1/8s&lt;/strong&gt;, during which the ISS moves about 1km. But there's absolutely no in-motion unsharpness in the Photo, so the camera tracked the motive very well.&lt;br&gt;&#xA;I'm wondering, because usually it's very difficult to shoot a sharp 1/8s photo at 400mm when holding the camera in your shaking hands. Such lenses have image stabilization, but this doesn't work nicely when you have to track the motive. May be, zero gravity and the weight of the lens helps a lot. Finally, the photo was processed with Photoshop, and some unsharpness could have been removed.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, the Van Gogh is about 100px wide when photographed with a 400mm lens on a 16.4MP full frame camera. If the ISS has a 800mm and a converter to get 1120mm, it would be 200px or 280px wide. If a camera with APS-C sensor is used, the width increases to 448px, and if this has 24MP instead of 16MP, the width would be 546px.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, sharpness of a lens is limited. You can't increase sharpness by using smaller and smaller pixels. While I think you would get a sharper image with the 800mm plus teleconverter, you would not gain more sharpness with an other camera.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Conclusion:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Playing with the numbers, the Van Gogh would still be small compared to the entire photo size, but large enough to be noticed. Sharpness is limited, but looking at the level of detail of the photo, the Van Gogh should be recognizable even when photographed with a 400mm lens. &lt;/p&gt;&#xA;" OwnerUserId="11749" LastEditorUserId="11749" LastEditDate="2016-06-22T16:43:29.983" LastActivityDate="2016-06-22T16:43:29.983" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16806" PostTypeId="2" ParentId="16803" CreationDate="2016-06-22T11:53:45.473" Score="4" Body="&lt;p&gt;Let's start with the basics, and work our way out to an answer. The distance to Proxima Centauri is about 40 trillion km. That leads to a power loss of 664 dB, an incredible amount. Compared to Voyager 1, which is currently about 20 billion km away, an increase in the channel power of 172 dB is required. That means making the signal 10^8 times more powerful to maintain the same level of power. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what is the best that we could do? DSN uses a 70m antenna, Arecibo is a 300m antenna. That leads to about a 16 times increase in power. That covers a very small portion of the required power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume somehow we had a 300m antenna on the spacecraft itself. Voyager's antenna is 3.7m. This is a 6500 increase in power, quite significant. Combined with the use of Arecibo, that would lead to a roughly 100,000 increase in power, or 100 dB. Of course, launching a 300m antenna is virtually impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, this would be extremely difficult using RF power. Using visible is much simpler, as the required dish sizes are much smaller.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it would take an enormous increase in power to make interstellar communication possible, although &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-22T11:53:45.473" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16807" PostTypeId="1" CreationDate="2016-06-22T13:57:40.553" Score="9" ViewCount="637" Body="&lt;p&gt;The ESA recently announced plans to develop a station halfway to the moon.  This station would, I presume, serve as the next international station replacing the ISS.  While I understand that the ISS is aging and some modules are at the end of a useful life (see &lt;a href=&quot;https://space.stackexchange.com/questions/237/why-decommission-the-iss&quot;&gt;this question&lt;/a&gt;), there are still sections that have lots of life left such as newer modules and the truss assembly.  What are the obstacles to keeping these sections by pushing them to higher orbits and utilizing them in a new larger station. Considering the cost of getting anything up there it would seem appropriate to re-use whatever we can. So what are the obstacles in reusing portions of the station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A corollary question:  What parts/sections of the ISS would be salvagable?&lt;/p&gt;&#xA;" OwnerUserId="15744" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2022-04-14T22:05:51.147" Title="Would it make sense to salvage parts of the ISS as it is decommissioned?" Tags="|iss|esa|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16808" PostTypeId="2" ParentId="16781" CreationDate="2016-06-22T14:26:58.990" Score="4" Body="&lt;p&gt;Loosely on the theme of art photographed from space:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A set of temporary &lt;a href=&quot;http://www.thefloatingpiers.com/the-project/&quot; rel=&quot;nofollow noreferrer&quot;&gt;floating piers&lt;/a&gt; have recently been set up on Lake Iseo, Italy as a piece of interactive art.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An animated picture from the &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2016/06/Floating_Piers_on_Lake_Iseo&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sentinel 2A satellite&lt;/a&gt; has just been released. This shows the bright yellow piers (and a few stages of construction).  The piers are 16 metres wide and covered with bright yellow fabric.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eGJDl.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eGJDl.gif&quot; alt=&quot;Floating piers on lake Iseo, by Sentinal-2A&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note - this isn't from the ISS though. A version taken by hand might not match the resolution shown here.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-06-22T14:34:23.767" LastActivityDate="2016-06-22T14:34:23.767" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16809" PostTypeId="2" ParentId="16796" CreationDate="2016-06-22T17:22:45.370" Score="9" Body="&lt;p&gt;There have been four major US planetary probe events on (or scheduled for) 4th July. This compares to &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Solar_System_probes&quot;&gt;approximately a hundred&lt;/a&gt; &quot;major events&quot; for US planetary probes. At least one (Viking) was somewhat intentional - it had to be around early July and the 4th was deliberately picked - while others may fall into this category.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Viking 1 landing, 1976 (delayed to July 20, itself nicely symbolic...)&lt;/li&gt;&#xA;&lt;li&gt;Mars Pathfinder landing, 1997&lt;/li&gt;&#xA;&lt;li&gt;Deep Impact flyby/impact, 2005&lt;/li&gt;&#xA;&lt;li&gt;Juno arrival, 2016.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Of these, the first was partly dictated by celestial mechanics and partly by date preference. Launch windows dictated an arrival at Mars around early July, and when the Viking mission planners sat down in 1970, they made the deliberate choice of 4th July out of a roughly one-week window:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Martin remarked that even if Viking had safely touched down on the Fourth of July, the landing &quot;would have been lost among the Tall Ships,&quot; a reference to the publicity given the bicentennial parade of ships in New York harbor. That historic date had been chosen in 1970 after a preliminary trajectory analysis singled out the first week in July 1976 for a landing, but the Red Planet had not cooperated. The bicentennial was celebrated without a Viking on Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(&lt;em&gt;&lt;a href=&quot;http://history.nasa.gov/SP-4212/ch10.html&quot;&gt;On Mars: Exploration of the Red Planet&lt;/a&gt;&lt;/em&gt;, ch. 10)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pathfinder did not hang around in Mars orbit, and so the landing date was entirely dependent on the trajectory. Again, this would mean that it was &lt;em&gt;probably&lt;/em&gt; an arbitrary selection of a target date in a longer window. I have not as yet been able to confirm that they did pick 4th July as a nicer date than 3rd or 5th, but this seems to have been &lt;a href=&quot;https://groups.google.com/forum/#!msg/sci.space.science/WH_DyJ8hgDI/JvK9y4Y_LT8J&quot;&gt;generally assumed at the time&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Deep Impact - I've not been able to find anything on this one way or another. I suspect it was mostly coincidence - there cannot be much flexibility in the rendezvous window with a comet! The mission was originally planned to launch at the end of December 2004, and delayed two weeks until 12 January; a &lt;a href=&quot;http://www.jpl.nasa.gov/news/press_kits/deep-impact-launch.pdf&quot;&gt;press kit&lt;/a&gt; for a proposed intermediate launch date of 8 January still expected a 4th July impact, but &lt;a href=&quot;https://books.google.co.uk/books?id=CJWLBAAAQBAJ&amp;amp;pg=PA527&amp;amp;lpg=PA527&quot;&gt;it seems&lt;/a&gt; this was driven by orbital and observation opportunities more than symbolism:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An impact on 4 July would not only be one day before the comet's perihelion, but would also mark an American national holiday. This solution roughly optimised several parameters, including lighting conditions and launcher performance, and the impact was scheduled for a one-hour window where it could be tracked by Deep Space Network antennas in both Australia and California, and the effect of the strike could be observed by the large telescopes in Hawaii and in space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Juno - I've not been able to find anything either way as yet. I suspect it may fall into the category of &quot;we have a broad window, pick an arbitrary date in the middle&quot;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;These are all American dates. It's worth considering the experience of other countries. The most obvious example in recent years is North Korea; more or less every launch attempt there is near an anniversary, if not on the specific day. Most prominently, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kwangmy%C5%8Fngs%C5%8Fng-3&quot;&gt;Kwangmyŏngsŏng-3&lt;/a&gt; launch attempt was timed for what would have been Kim Il-Sung's hundredth birthday.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the USSR, Soyuz 1 was scheduled to &lt;a href=&quot;http://history.nasa.gov/SP-4225/documentation/mhh/mirhh-part1.pdf&quot;&gt;coincide with Lenin's birthday&lt;/a&gt;, and it's possible the resulting launch pressures may have contributed to the failure of that flight. I can't immediately find a reference to any other launches being timed around major anniversaries, but I'm sure there were some!&lt;/p&gt;&#xA;" OwnerUserId="11259" LastActivityDate="2016-06-22T17:22:45.370" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16810" PostTypeId="2" ParentId="16796" CreationDate="2016-06-23T05:08:03.213" Score="5" Body="&lt;p&gt;At one of the recent talks at JPL, this exact question was raised. Basically, if there's a significant anniversary date that falls within a launch or arrival window, it's possible for the MDNAV team to push the launch/landing/orbital insertion/rendezvous/etc. towards the significant date by making fairly minor changes to the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;E.g. to advance or delay arrival by a few days, you can make fairly small trajectory adjustments early on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can even make pretty significant changes well after launch depending on the delta-v budget you have. Sure, there's an element of luck involved to have the option to make the dates line up with significant events, but mission designers often do have the ability to nudge towards those dates if it's within a few days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean, think about it. Juno's launch window was 22 days. And this was its trajectory: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/sYp5p2oL51g?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How far does Jupiter move in a couple of days? How much is that trajectory going to change if its arrival were delayed or advanced by a week?&lt;/p&gt;&#xA;" OwnerUserId="403" LastActivityDate="2016-06-23T05:08:03.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16811" PostTypeId="1" AcceptedAnswerId="16812" CreationDate="2016-06-23T08:34:18.353" Score="2" ViewCount="146" Body="&lt;p&gt;I understand launching the module to the ISS has several benefits:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Power supply is provided&lt;/li&gt;&#xA;&lt;li&gt;Servicing capacity&lt;/li&gt;&#xA;&lt;li&gt;Orbital control is provided &lt;/li&gt;&#xA;&lt;li&gt;Thermal control might be provided ?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;But what are the key factors that made it a module instead of a simple satellite &lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-06-23T09:20:58.353" Title="Why was AMS-02 launched to the ISS instead on being standalone?" Tags="|iss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16812" PostTypeId="2" ParentId="16811" CreationDate="2016-06-23T09:20:58.353" Score="2" Body="&lt;p&gt;From the &lt;a href=&quot;https://en.wikipedia.org/wiki/Alpha_Magnetic_Spectrometer&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; article on the AMS: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The power requirements for AMS-02 were thought to be too great for a practical independent spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The AMS needs up to 2.5 kW, so as an independent satellite it would need rather large solar panels.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-23T09:20:58.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16813" PostTypeId="1" AcceptedAnswerId="16814" CreationDate="2016-06-23T12:31:18.790" Score="6" ViewCount="1960" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_2&quot; rel=&quot;nofollow&quot;&gt;its wikipedia article&lt;/a&gt;, the Dragon 2 is powered by 8 SuperDraco engines. And according to &lt;a href=&quot;https://en.wikipedia.org/wiki/SuperDraco&quot; rel=&quot;nofollow&quot;&gt;the SuperDraco's article&lt;/a&gt;, each one of them can store up to 1,388kg of propellant at an Isp of 240s. Does that mean &lt;em&gt;each&lt;/em&gt; engine has that amount of propellant? Even if I generously give the Dragon 2 a dry mass of 10t (e.g. including crew plus cargo), that would amount up to 1,7km/s delta-v. That seems to be quite a lot for a LEO-ferry. On the other hand, it is not enough for a moon landing. So what motivates this budget?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="51" LastEditDate="2016-06-23T14:00:08.367" LastActivityDate="2016-06-23T14:03:19.490" Title="Dragon 2 Propellant Capacity" Tags="|spacex|delta-v|dragon-v2|super-draco|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16814" PostTypeId="2" ParentId="16813" CreationDate="2016-06-23T12:38:42.373" Score="9" Body="&lt;p&gt;That's 1,388 kg of propellant for the whole vehicle, not per engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reading &lt;a href=&quot;http://www.faa.gov/about/office_org/headquarters_offices/ast/environmental/nepa_docs/review/launch/media/fonsi_dragon_pad_abort.pdf&quot;&gt;the original document&lt;/a&gt; referred to by Wikipedia, it says 3,060 pounds of propellant for the Crew Dragon capsule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(It's on page 8. The fuels are NTO and MMH and the combined weight of both is given.)&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-06-23T12:44:40.000" LastActivityDate="2016-06-23T12:44:40.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16815" PostTypeId="2" ParentId="16813" CreationDate="2016-06-23T14:03:19.490" Score="1" Body="&lt;p&gt;The Super Dracos are used for two purposes in the initial design. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch Abort System, and powered landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For LAS usage, they need to get away from an exploding booster, faster than the explosion wavefront. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For powered landing the ultimate goal is to let the capsule hits its aerodynamic terminal velocity and then finally brake in the last few seconds for a powered landing. Initial attempts (at least of DragonFly test vehicle) will be parachutes to reduce terminal velocity and then powered landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their budget of thrust and time is designed around those two criteria. Using this as a Red Dragon (to Mars) is an interesting question as to how they will improve the fuel load for that mission, if at all. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-23T14:03:19.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16816" PostTypeId="1" AcceptedAnswerId="16817" CreationDate="2016-06-23T15:00:44.383" Score="28" ViewCount="3757" Body="&lt;p&gt;Just learned of the Sea Dragon proposal from back in 1962.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Sea_Dragon_(rocket)#Design&quot;&gt;From the Wikipedia entry&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first stage was to be powered by a single enormous 80,000,000 pounds-force (360 MN) thrust engine burning RP-1 and LOX (liquid oxygen). The fuels were pushed into the engine by liquid nitrogen, which provided a pressure of 32 atm for the RP-1 and 17 atm for the LOX, providing a total pressure in the engine of 20 atm (~300 psi) at takeoff.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's a huge amount of thrust, especially for a single engine.  Although the design proposal was never further investigated, would an engine this large be possible to fabricate today?  Has any research into actually producing anything larger than the F1 been seriously carried out?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="195" LastEditDate="2016-06-24T21:11:10.480" LastActivityDate="2020-02-24T02:45:39.383" Title="How realistic would the Sea Dragon engine be to produce given today's technology?" Tags="|engine-design|engineering|sea-dragon|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16817" PostTypeId="2" ParentId="16816" CreationDate="2016-06-23T15:29:07.443" Score="26" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Has any research into actually producing anything larger than the F1 been seriously carried out?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/M-1_(rocket_engine)&quot; rel=&quot;nofollow noreferrer&quot;&gt;M-1&lt;/a&gt; was a hydrogen engine just a little larger than the F-1. Parts of it were built and tested and the engine would likely have worked just fine if completed and flown. Lack of need for a super-heavy lift vehicle larger than a Saturn V prevented it from being developed further. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-170&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-170/171&lt;/a&gt; is comparable to the F-1 -- heavier but more compact, and just a bit more powerful.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Although the [Sea Dragon] proposal was never further investigated, would [Sea Dragon's first-stage engine] be possible to fabricate today? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Probably. The design was fairly conservative for all its size, pressure-fed rather than pump-fed, with quite low chamber pressure. As mentioned in the Wikipedia article, the general design was reviewed and considered sound by TRW; I assume that includes the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main problem I'd expect to see would be combustion instability -- conventional wisdom says that's a bigger problem in large chambers than small ones, and this is certainly a large chamber. &lt;a href=&quot;https://space.stackexchange.com/questions/955/why-do-pressure-fed-engines-have-combustion-instabilities&quot;&gt;This Q/A&lt;/a&gt; discusses that problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the original Aerojet-General proposal suggests that the resonant frequencies of such a large chamber would be so low that feedback instability wouldn't be sustained:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With regard to combustion stability, an analysis on &#xA;  the basis of sensitive time lag theory (perhaps the best theory so far &#xA;  developed) indicates that the Sea Dragon thrust chamber will operate &#xA;  well outside the region of combustion instability. One of the primary &#xA;  advantages of sea based development testing is that it permits early &#xA;  experimental evaluation of combustion stability on a full scale basis &#xA;  without an exorbitant outlay for facilities. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In other words &quot;we don't think it's a problem, but, hey, at least if one of those engines blows up in the ocean it won't hurt anything.&quot; &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2020-02-24T02:45:39.383" LastActivityDate="2020-02-24T02:45:39.383" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="16818" PostTypeId="1" CreationDate="2016-06-23T19:48:21.617" Score="7" ViewCount="193" Body="&lt;p&gt;A friend asked me what the numbers in the following image represent:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://cdn.calref.net/files.calref/12/bd023dc0_13528487_10153710377918527_4676208291139157516_o.jpg&quot; alt=&quot;image&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Mercury   8&#xA;Venus    15&#xA;Earth    61&#xA;Mars     33&#xA;Ceres    42&#xA;Jupiter  42&#xA;Saturn  100&#xA;Uranus   22&#xA;Neptune  57&#xA;Pluto    85&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;My (limited) guesses on what the figures could be turned up nothing. I'm guessing since Saturn is 100, that the figures are expressed as a % of whatever the figure is for saturn? I'm pretty stumped for this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any idea what the numbers stand for?&lt;/p&gt;&#xA;" OwnerUserId="15847" LastActivityDate="2016-06-23T21:33:40.280" Title="What do the enclosed numbers represent?" Tags="|planetary-science|planet|statistics|" AnswerCount="0" CommentCount="8" ClosedDate="2016-06-27T16:06:23.920" ContentLicense="CC BY-SA 3.0" />
  <row Id="16821" PostTypeId="1" AcceptedAnswerId="17115" CreationDate="2016-06-24T03:09:55.313" Score="10" ViewCount="5010" Body="&lt;p&gt;Below are some screen shots from circa June 2016 launch (or re-launch) videos by Blue Origin (&lt;a href=&quot;https://www.youtube.com/watch?v=xYYTuZCjZcE&quot; rel=&quot;nofollow noreferrer&quot;&gt;video link&lt;/a&gt;) and SpaceX (&lt;a href=&quot;https://www.youtube.com/watch?v=gLNmtUEvI5A&quot; rel=&quot;nofollow noreferrer&quot;&gt;video link&lt;/a&gt;). The New Shepard burns $\text{H}_2$/$\text{O}_2$, while the Falcon 9 burns Kerosene/$\text{O}_2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe that at least part of the intense white/yellow light from the Kerosene/$\text{O}_2$ exhaust comes from thermal emission of Carbon particulates (soot) similar to the emission from the white/yellow part of a candle flame. There is very little luminescence from the $\text{H}_2$/$\text{O}_2$ exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://i.stack.imgur.com/ChA3P.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;first image&lt;/a&gt; in the question &lt;a href=&quot;https://space.stackexchange.com/q/2017/12102&quot;&gt;Why do the exhaust flames from cryogenic stage engines appear to be separated from the nozzle?&lt;/a&gt; is quite striking, showing a very sudden appearance of brilliant light when the exhaust is compressed by a static shock wave. Presumably this is $\text{H}_2$/$\text{O}_2$ but I couldn't see a link or citation for the image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Left&lt;/strong&gt; (below) While candle flames also emit blue light - this is said to be mostly from &lt;a href=&quot;https://en.wikipedia.org/wiki/Swan_band&quot; rel=&quot;nofollow noreferrer&quot;&gt;Swan band&lt;/a&gt; emission - vibrational transitions in molecular Carbon as $\text{C}_2$. These do not occur in $\text{H}_2$/$\text{O}_2$ combustion. You can see Swan band emission stimulated by solar radiation in molecular carbon from comets as well - here is comet &lt;a href=&quot;https://en.wikipedia.org/wiki/C/2014_Q2_(Lovejoy)&quot; rel=&quot;nofollow noreferrer&quot;&gt;C/2014 Q2 (Lovejoy)&lt;/a&gt; (larger size available &lt;a href=&quot;https://en.wikipedia.org/wiki/File:C2014_Q2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Right&lt;/strong&gt; (below) Image of Space Shuttle Main Engine test firing from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Shuttle_Main_Engine_Test_Firing.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (larger size available &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Shuttle_Main_Engine_Test_Firing.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;). Note the sudden appearance of bright light as the exhaust reaches the shock wave near the bottom of the image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3ERj6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3ERj6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/nXkXh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nXkXh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/a/6417/12102&quot;&gt;answer there&lt;/a&gt; that I found the most helpful discusses the shock wave and the blue emission from $\text{H}_2$/$\text{O}_2$ exhaust. So I went back to the New Shepard launch video and sure enough - you can see what looks like blue light from the nozzle when viewed from below. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What actually produces this blue light? Is it $\text{H}_2 \text{O}$ emission, $\text{OH}^-$ emission, or something else? And why does it only appear at higher densities?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;side note:&lt;/em&gt; see &lt;a href=&quot;https://www.youtube.com/watch?v=Ir1BddDKj8Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;$\text{O}_2$ flame burning in $\text{H}_2$&lt;/a&gt; and &lt;a href=&quot;https://www.youtube.com/watch?v=UgZOwxn6NNM&quot; rel=&quot;nofollow noreferrer&quot;&gt;mixture ratio optimization&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/waaiT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/waaiT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IbogU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IbogU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9CjY3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9CjY3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-03-25T04:12:35.987" LastActivityDate="2021-04-17T09:50:55.500" Title="What is the cause of the blue light from LH2/LOX rocket engines?" Tags="|spacex|engines|chemistry|exhaust|blue-origin|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="16822" PostTypeId="2" ParentId="16807" CreationDate="2016-06-24T10:07:07.233" Score="5" Body="&lt;p&gt;Short Answer: No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that the ISS is a HUGE satellite. The biggest thing in the Earth's orbit besides the Moon.  Even if you remove sections, you would need to boost those sections up to a higher orbit. The fuel and engine to boost the sections would need to launched and that is not cheap.  Also logistically to start the salvage process of the ISS is difficult. So many manned space flight hours. I doubt when designing the sections they thought about dismantling so those pieces are most likely staying together.  Also there is a power issue. The power onboard the ISS is specific to the ISS. Everything is designed for that wattage/amperage/voltage (I am not an EE), so the new space station would need to follow the old ISS standards, which is highly unlikely.  I am sorry to say that we might see a complete end to the ISS in our lifetime.&lt;/p&gt;&#xA;" OwnerUserId="15856" LastActivityDate="2016-06-24T10:07:07.233" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16823" PostTypeId="1" CreationDate="2016-06-24T13:08:46.403" Score="4" ViewCount="4069" Body="&lt;p&gt;I would assume the planet Mercury represents a 'worst case' for radiation risk to people.  But how bad is it as compared to, say, radiation workers annual dose limit? &lt;/p&gt;&#xA;" OwnerUserId="3227" LastEditorUserId="6186" LastEditDate="2020-07-15T04:06:49.053" LastActivityDate="2020-07-15T04:06:49.053" Title="Radiation risk on Mercury?" Tags="|radiation|mercury|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="16824" PostTypeId="1" CreationDate="2016-06-24T13:40:55.340" Score="3" ViewCount="231" Body="&lt;p&gt;I just created a &lt;a href=&quot;http://pastebin.com/qwXLu3Ta&quot; rel=&quot;nofollow&quot;&gt;Modelica Model&lt;/a&gt; of a rocket landing in vacuum. Now I attempt to verify it against reality. However, I have a hard time to figure out some plausible initial parameters. Apollo 11's Eagle had a Periapsis of 15km. But what was its initial speed? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume that everyone does one deorbit burn and later attempts to get as close to a suicide burn as reasonable. So I modeled my experiment after a falling mass under retrograde thrust in a cartesian coordinate system. Obviosuly, this model has a hard time when it comes to very long landing approaches (since the cartesian coordinates to not take into account the curvature of the target body), so lowering the periapsis to 0 is not an option right now (although probably the most efficient approach). I arbitrarily choose an initial surface velocity of 1km/s which gave me a landing at 140km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is a plausible surface speed after the deorbit burn?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-06-24T14:55:01.897" Title="Plausible starting parameters for a Moon suicide burn" Tags="|rockets|landing|lunar-landing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16825" PostTypeId="5" CreationDate="2016-06-24T14:19:24.473" Score="0" Body="&lt;p&gt;This tag is a misspelling of radiation &amp;amp; needs to be removed. The radiation tag already exists. I do not have enough privileges to do this.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="7062" LastEditDate="2016-06-24T14:30:05.493" LastActivityDate="2016-06-24T14:30:05.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16826" PostTypeId="4" CreationDate="2016-06-24T14:19:24.473" Score="0" Body="This tag is a misspelling of radiation" OwnerUserId="7062" LastEditorUserId="7062" LastEditDate="2016-06-24T14:29:56.243" LastActivityDate="2016-06-24T14:29:56.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16827" PostTypeId="2" ParentId="16823" CreationDate="2016-06-24T14:19:35.997" Score="5" Body="&lt;p&gt;According to this &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Mercury Facts Sheet&lt;/a&gt;, Mercury has almost 7 times the Solar Irradiance that Earth receives (9082.7 W/m^2 versus 1361 W/m^2).  I'm not certain how this value translates to Sievert Units.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mercury does have a large magnetosphere though, which should protect anyone on the planet from a large amount of the harmful radiation the Sun puts out (similar to Earth.)  I'd wager you'd receive less radiation on Mercury than on the Moon because of the magnetosphere.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastActivityDate="2016-06-24T14:19:35.997" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16828" PostTypeId="2" ParentId="381" CreationDate="2016-06-24T14:33:36.823" Score="0" Body="&lt;p&gt;There is also the use of Hydrogenated Polyethylene (HPE) which is a lot like a saran wrap layered many tines to create 'bricks' of shielding.  Still other concepts involve using water-filled pillows or tubes in the walls to absorb radiation.  In one variant of this idea, the 'water' is actually the water from the waste management system, which has a high organics content (!) and so absorbs more particles.  Very likely a spacecraft will need a combination of all the above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Crewmembers can reduce their absorbed dose by wearing Demron fabric, which is used by nuclear workers. . .  &lt;/p&gt;&#xA;" OwnerUserId="3227" LastActivityDate="2016-06-24T14:33:36.823" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16829" PostTypeId="2" ParentId="16824" CreationDate="2016-06-24T14:55:01.897" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_11g_Lunar_Orbit_Phase.htm&quot;&gt;This document&lt;/a&gt; provides the Apollo 11 LM velocity at the start of powered descent as 5564.9 ft/s.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-06-24T14:55:01.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16830" PostTypeId="2" ParentId="16696" CreationDate="2016-06-24T15:05:40.077" Score="0" Body="&lt;p&gt;A temporary return to the womb would solve this problem.  The 'womb' here is an enclosed tube with a sealed hatch.  The occupant breaths through a diving-type mask.  The tube fills with a sanitizing solution which is easily volatized out of the water.  After washing, warm air circulated through the tube to dry both the occupant and the tube interior.  Towels and washcloths would not be needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we really wanted to be clever, a unit could be inserted in the shower to utilize it as a clothes washer/drier.  I have seen something like this, but cannot provide a pic or source.  Sorry.        &lt;/p&gt;&#xA;" OwnerUserId="3227" LastEditorUserId="3227" LastEditDate="2016-06-24T15:12:04.737" LastActivityDate="2016-06-24T15:12:04.737" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16833" PostTypeId="1" AcceptedAnswerId="16834" CreationDate="2016-06-25T12:25:21.013" Score="12" ViewCount="953" Body="&lt;p&gt;My question is which of the outer planets you could see with the naked eye if you were in close proximity to them. It's pretty obvious the human eye can see the inner planets through to Mars, but I know that space probes to the outer planets need sensors designed for extremely low light conditions, and the pictures we see are often enhanced to make them brighter than they would actually appear. So:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Could you tell if a planet was there against the blackness of space?&lt;/li&gt;&#xA;&lt;li&gt;Could you see details? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="58" LastEditDate="2020-02-24T20:42:38.027" LastActivityDate="2020-02-25T13:58:18.783" Title="Which of the outer planets could you see with the naked eye if you were in close proximity?" Tags="|planet|sensors|naked-eye|outer-planets|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16834" PostTypeId="2" ParentId="16833" CreationDate="2016-06-25T12:36:57.950" Score="17" Body="&lt;p&gt;You can &lt;a href=&quot;https://solarsystem.nasa.gov/planets/pluto/plutotime&quot;&gt;try for yourself how bright it is on Pluto&lt;/a&gt;. High noon on Pluto is as bright as a certain point during dusk/dawn on Earth. I tried this, at that point it's bright enough to make out details. My conclusion is that you can see the daylight side of every planet incl. Pluto with the naked eye if you're close enough. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This does depend on the planet's albedo. At 14.5%, Pluto isn't very bright, but some bodies have lower albedos (comets like 99P Chury are around 5%) so you might not be able to see some of the smaller bodies in the solar system. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-06-25T12:36:57.950" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16835" PostTypeId="1" CreationDate="2016-06-25T18:24:38.293" Score="9" ViewCount="2011" Body="&lt;p&gt;It has been my opinion that running out of station keeping propellent is what usually ends a satellite's useful life. Is this true?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much do different factors play in a satellite's lifespan? Some of the factors that occur to me: battery life, solar panel life, station keeping propellent supply, damage from radiation, damage from impact with debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been a supporter of &lt;a href=&quot;https://www.engadget.com/2016/06/24/nasa-restore-l-mission/&quot;&gt;NASA&lt;/a&gt;'s or &lt;a href=&quot;https://www.youtube.com/watch?v=uvkhWIlmHEg&quot;&gt;DARPA&lt;/a&gt;'s efforts to robotically refuel and maintain satellites. However my enthusiasm may be based on erroneous assumptions.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-06-25T18:56:03.357" LastActivityDate="2018-04-23T21:25:51.660" Title="What typically ends a satellite's life?" Tags="|artificial-satellite|communication|fuel|robotic-missions|end-of-life|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16837" PostTypeId="1" AcceptedAnswerId="16838" CreationDate="2016-06-25T22:03:09.953" Score="6" ViewCount="391" Body="&lt;p&gt;Getting rocket propellant from other sources than Earth is by many viewed as an important step towards further exploration of space. One such source could be our closest neighbour, the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is, almost all the surface of the Moon is just this endless ocean of regolith, a mixed blend of minerals where ores are scattered all around the place. That means it does mostly contain iron and aluminium in addition to oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it is easy to find the performance details of powdered aluminium and liquid oxygen, data on how iron performs is more difficult to come by.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are lots of practical problems, I know, like that both the fuel and the exhaust are solids, but from a theoretical point of view, it would be nice to know:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is the $I_{sp}$ of powdered iron/LOX?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="4660" LastEditDate="2016-06-25T22:05:30.437" LastActivityDate="2016-06-25T23:42:35.103" Title="What specific impulse can I expect from iron powder produced from lunar regolith?" Tags="|the-moon|propulsion|isru|specific-impulse|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16838" PostTypeId="2" ParentId="16837" CreationDate="2016-06-25T23:34:52.910" Score="7" Body="&lt;p&gt;Figure 5 on page 23 of &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870013373.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt; shows the Isp of tri-propellants including iron, where the curve goes out to 95% Fe and 5% H2. Just eyeballing it, the curves might converge at around 190 seconds. On the other hand, the curve might go to hell at 100%, with no light hydrogen for the heat of the burning iron to expel. Note that even at 5% H2 by weight, there are still three hydrogen atoms for each iron atom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PGY9b.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PGY9b.png&quot; alt=&quot;Isp as a function of Fe %&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason the curve might go to hell is that the Isp is proportional to the inverse square root of the molecular weight.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-06-25T23:42:35.103" LastActivityDate="2016-06-25T23:42:35.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16839" PostTypeId="1" AcceptedAnswerId="16841" CreationDate="2016-06-26T02:35:03.923" Score="10" ViewCount="429" Body="&lt;p&gt;The NY Times article &lt;a href=&quot;http://www.nytimes.com/2016/06/22/science/cassini-saturn-titan-rings.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini’s Final Mission: Obliteration&lt;/a&gt; says that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini spacecraft&lt;/a&gt; will end it's more than a decade long exploration of the Saturn system by raising the inclination of its orbit around the planet to about 64°, then orbiting between the rings and the planet 22 times before entering Saturn's atmosphere.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To protect Titan and other potentially life-bearing places, Cassini’s final mission calls for its obliteration. Researchers have dubbed the spacecraft’s swan song the “Cassini Grand Finale” because it includes an ambitious maneuver: 22 loops through the gap between Saturn’s surface and its innermost rings.&lt;/p&gt;&#xA;&lt;p&gt;Before launching Cassini, NASA drew up plans to dispose of it in Saturn’s upper atmosphere, but they only developed its elaborate end-of-mission maneuver in the past couple of years. Once they found that some of Saturn’s moons may be potentially habitable to life, keeping the probe clear of them as it ran out of fuel became crucial.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Before Cassini can thread the needle through the planet’s gap, it must first reach the proper angle in its orbit. That means climbing to an inclination of approximately 64 degrees.&lt;/strong&gt; The problem is that Cassini doesn’t have the power to propel itself to that position.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jFXwG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jFXwG.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;em&gt;An artist’s rendering of Cassini’s final mission, a series of 22 loops over a few months. Credit NASA&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;“Each of the six (Titan flybys) will raise the tilt of the orbit a little bit,” said Dr. Spilker, “until we get the &lt;strong&gt;right orbit to go in between&lt;/strong&gt; the rings and the planet.”&lt;/p&gt;&#xA;&lt;p&gt;...&lt;a href=&quot;https://science.jpl.nasa.gov/people/Spilker/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Linda Spilker&lt;/a&gt; (is) the Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(my emphasis)&lt;/p&gt;&#xA;&lt;p&gt;After reading the NASA news item &lt;a href=&quot;https://www.nasa.gov/feature/jpl/cassini-heads-for-higher-ground-at-saturn&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini Heads for 'Higher Ground' at Saturn&lt;/a&gt;, I still don't understand:&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Why&lt;/em&gt; &lt;strong&gt;must&lt;/strong&gt; &lt;em&gt;Cassini reach an orbital inclination of about 64°&lt;/em&gt; &lt;strong&gt;before&lt;/strong&gt;  &lt;em&gt;&amp;quot;threading the needle&amp;quot; between Saturn and its rings? (why not 40°, 50°, 60°...)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-10T12:58:33.587" LastActivityDate="2021-08-10T12:58:33.587" Title="Why would Cassini need to reach 64 degrees inclination before &quot;threading the needle&quot;?" Tags="|orbital-maneuver|flyby|saturn|cassini|end-of-life|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="16840" PostTypeId="1" CreationDate="2016-06-26T04:52:21.690" Score="10" ViewCount="1002" Body="&lt;p&gt;&lt;strong&gt;update&lt;/strong&gt; 2020-11-20: &lt;a href=&quot;https://blog.maxar.com/space-infrastructure/2020/osam-1-formerly-restore-l-continues-to-make-progress-fuel-tank-installed&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSAM-1 (Formerly Restore-L) Continues to Make Progress, Fuel Tank Installed&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; the recent 2019 news &lt;a href=&quot;https://www.engadget.com/2019/10/06/mission-extension-vehicle-launch-october-9/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Northrop's satellite refueling spacecraft launches on October 9th&lt;/a&gt; which is sourced from &lt;a href=&quot;http://www.parabolicarc.com/2019/10/05/mission-extension-vehicle-headed-for-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mission Extension Vehicle Headed for Space&#xA;&lt;/a&gt; reminded me that there may be some closure on this 2016 question, so I've un-accepted the old answer for now.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;I'd just in PC Magazine's &lt;a href=&quot;http://www.pcmag.com/news/345596/nasa-extends-hubble-contract-develops-robotic-spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Extends Hubble Contract, Develops Robotic Spacecraft&lt;/a&gt; that:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Also this week, the agency &lt;a href=&quot;http://www.nasa.gov/feature/nasa-s-restore-l-mission-to-refuel-landsat-7-demonstrate-crosscutting-technologies&quot; rel=&quot;nofollow noreferrer&quot;&gt;announced&lt;/a&gt; that it is moving ahead with plans to create a robotic spacecraft designed to service satellites. Dubbed Restore-L, the mission has two objectives: refuel a communications satellite, possibly the government-owned Landsat 7, and test autonomous docking and navigation capabilities that could be used in future flights to Mars.&lt;/p&gt;&#xA;&lt;p&gt;NASA also hopes Restore-L will usher in a new era of robotic servicing for commercial and scientific satellites.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Restore-L effectively breaks the paradigm of one-and-done spacecraft,&amp;quot; Frank Cepollina, who led the five-crewed servicing missions to Hubble, said in a statement. Space robots could be valuable to commercial operators like a joint venture between Airbus and the start-up OneWeb, which wants to send more than 900 mass-produced satellites into orbit starting in 2018.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(&lt;em&gt;I'm not sure if the &amp;quot;communications satellite&amp;quot; is a misnomer, or refers to an additional satellite besides &lt;a href=&quot;https://en.wikipedia.org/wiki/Landsat_7&quot; rel=&quot;nofollow noreferrer&quot;&gt;Landsat 7&lt;/a&gt;.&lt;/em&gt;)&lt;/p&gt;&#xA;&lt;p&gt;I followed links to the &lt;a href=&quot;http://ssco.gsfc.nasa.gov/educationandoutreach.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Educational Outreach page&lt;/a&gt; of the NASA &lt;a href=&quot;http://ssco.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satellite Servicing Capabilities Office&lt;/a&gt;, which links to the &lt;a href=&quot;http://www.nasa.gov/feature/nasa-s-restore-l-mission-to-refuel-landsat-7-demonstrate-crosscutting-technologies&quot; rel=&quot;nofollow noreferrer&quot;&gt;announcement&lt;/a&gt; and a description of the &lt;a href=&quot;http://ssco.gsfc.nasa.gov/restore-l.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Restore-L mission&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;On the outreach page there is a YouTube video &lt;em&gt;Virtual Tours of NASA Goddard's Satellite Servicing Capabilities Office&lt;/em&gt; (linked below), and an animation there caught my eye. Here is a GIF reconstructed from frames of the video:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NWM2E.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NWM2E.gif&quot; alt=&quot;screenshots from &amp;quot;Virtual Tours of NASA Goddard's Satellite Servicing Capabilities Office&amp;quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/yZB2YZ-mBu0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;The announcement says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Restore-L technologies include an autonomous relative navigation system with supporting avionics, and dexterous robotic arms and software. The suite is completed by a tool drive that supports a collection of sophisticated robotic tools for robotic spacecraft refueling, and a propellant transfer system that delivers measured amounts of fuel at the proper temperature, rate, and pressure.&lt;/p&gt;&#xA;&lt;p&gt;Future candidate applications for individual Restore-L technologies include on-orbit manufacturing and assembly, propellant depots, observatory servicing, and orbital debris management. NASA is also directly applying several Restore-L technologies to the &lt;a href=&quot;https://www.nasa.gov/content/what-is-nasa-s-asteroid-redirect-mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asteroid Redirect Mission&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;To me, the animation suggests that the satellite (&lt;em&gt;is it Landsat-7?&lt;/em&gt;) might have a refueling port for easy access, and the robotic maintenance craft &lt;em&gt;would not need&lt;/em&gt; to cut open panels, turn valves, bleed and then cut into lines &amp;quot;manually&amp;quot; in order to replenish propellant. (don't forget to reseal and leak-test the lines and close the panel as well!)&lt;/p&gt;&#xA;&lt;p&gt;But, if the port were for loading the satellite with propellant before launch, I would suspect that it be sealed reliably somehow - I don't think it would be as simple as a &lt;a href=&quot;http://www.quickfitting.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;quick connect&lt;/a&gt; type of fitting.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is Landsat-7's propellant resupply port &amp;quot;robot-ready&amp;quot;? (Restore-L mission)&lt;/p&gt;&#xA;&lt;p&gt;How accessible are the couplings and valves for replenishing propellants to Landsat-7?&lt;/p&gt;&#xA;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-12-13T08:00:46.337" LastActivityDate="2021-12-13T08:00:46.337" Title="Is Landsat-7's propellant resupply port &quot;robot-ready&quot;? (Restore-L mission)" Tags="|robotic-missions|monopropellant|supply-mission|restore-l|refueling|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="16841" PostTypeId="2" ParentId="16839" CreationDate="2016-06-26T10:34:15.713" Score="10" Body="&lt;p&gt;This is purely a speculation, as I have not found any official sources to confirm it, but I think they want to be able to view Saturn's pole during the fly by. The trigonometry works out at least:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fTCynm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fTCynm.png&quot; alt=&quot;threading the needle&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to get a close-up of the polar regions with a lower inclination, like 40, 50 or 60 degrees, the altitude required is larger than the gap between Saturn and the innermost ring, and therefore not possible.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-06-26T10:34:15.713" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16843" PostTypeId="2" ParentId="16840" CreationDate="2016-06-26T19:08:53.350" Score="7" Body="&lt;p&gt;My reading of the Aviation Week article on this seemed to suggest that while Landsat-7 was not designed to be refueled, when they looked at its design, they found a fuel line, they could patch into that could be used to fill the tanks again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They need to cut away the insulation to get to the pipe, and then will probably use some variant of a vampire tap to connect to the system.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have used these in air conditioning repair to refuel Freon in sealed systems and they are really simple.  Heck there is even a kind of vampire tap for 10-Base5 coax cable. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-06-26T19:08:53.350" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16845" PostTypeId="1" AcceptedAnswerId="19000" CreationDate="2016-06-26T23:43:29.213" Score="10" ViewCount="861" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; I've just read these intriguing first sentence in the recently published (and paywalled) Aviation Weekly article &lt;a href=&quot;http://aviationweek.com/new-space/no-second-chances-webb-telescope-deployment&quot; rel=&quot;noreferrer&quot;&gt;No Second Chances On Webb Telescope Deployment&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA has no plans to service the $8.7 billion James Webb Space Telescope (JWST), but it has not completely closed the door on the idea as it prepares a rigorous series of tests designed to ensure in-space repairs will not be needed. When the 6.5-m (21-ft.) telescope launches to the Sun-Earth Lagrange point (L2) in 2018, &lt;strong&gt;its launch interface ring will carry optical targets that could be sought out by a servicing robot’s machine vision as it approaches to attach itself in order to make ...*&lt;/strong&gt; (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt; (HST) was robotically deployed from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle&lt;/a&gt; under direct local control by the astronauts on the shuttle. There were &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope#Servicing_missions_and_new_instruments&quot; rel=&quot;noreferrer&quot;&gt;five further service missions&lt;/a&gt; using the shuttle using a combination of work by astronauts in suits and astronaut-controlled robotic maneuvers. The HST was designed (at least in part) for servicing - in some cases modules could be pulled out of racks and updated or replaced, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible the &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;James Webb Space Telescope&lt;/a&gt; (JWST) might have some features like that, but probably not to the same extent as the HST, since 1) it will be much farther away and there is not currently any manned or robotic mission activity to earth-moon  (or any) libration points, and 2) the design is optimized for things like weight, reliability, performance and thermal management, rather than easy access. Since it is designd for infrared wavelengths, a substantial fraction of the systems must be maintained at low temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several answer given to the question &lt;a href=&quot;https://space.stackexchange.com/q/4902/12102&quot;&gt;The JWST - What happens if/when it breaks?&lt;/a&gt;. Interestingly all of the answers only address service by astronauts. Now the NASA &lt;a href=&quot;http://ssco.gsfc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Satellite Servicing Capabilities Office&lt;/a&gt;, has &lt;a href=&quot;http://www.nasa.gov/feature/nasa-s-restore-l-mission-to-refuel-landsat-7-demonstrate-crosscutting-technologies&quot; rel=&quot;noreferrer&quot;&gt;announced&lt;/a&gt; and described the &lt;a href=&quot;http://ssco.gsfc.nasa.gov/restore-l.html&quot; rel=&quot;noreferrer&quot;&gt;Restore-L mission&lt;/a&gt; - a first test and demonstration of completely robotic service and/or repair.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answers and comments associated with question &lt;a href=&quot;https://space.stackexchange.com/q/15094/9060&quot;&gt;What satellites did the Shuttle retrieve from orbit?&lt;/a&gt; contain references to on-orbit repairs to satellites (presumably) in LEO carried out by astronauts using the shuttle's robotic arm &lt;a href=&quot;https://en.wikipedia.org/wiki/Canadarm&quot; rel=&quot;noreferrer&quot;&gt;Canadarm 1&lt;/a&gt;. But I'm not sure if those satellites were serviceable &lt;em&gt;by design&lt;/em&gt; or luck, and &lt;a href=&quot;https://space.stackexchange.com/questions/15094/t#comment39681_15094&quot;&gt;this comment&lt;/a&gt; speaks to that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus my question: &lt;em&gt;Besides HST, JWST and stations, are there any examples of satellites designed for service in space?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's exclude all past/present/future space stations and anything else that has been designed specifically &lt;em&gt;for&lt;/em&gt; astronaut activity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've &lt;em&gt;tried&lt;/em&gt; to use the infinitive form for the question, so the answer can include past, present, and future satellites, but let's keep it near-future and well-planned, and exclude the hypotheticals.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://ssco.gsfc.nasa.gov/educationandoutreach.html&quot; rel=&quot;noreferrer&quot;&gt;Educational Outreach page&lt;/a&gt; of the NASA &lt;a href=&quot;http://ssco.gsfc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Satellite Servicing Capabilities Office&lt;/a&gt; shows a &lt;a href=&quot;https://www.youtube.com/watch?v=yZB2YZ-mBu0&quot; rel=&quot;noreferrer&quot;&gt;YouTube video&lt;/a&gt; which contains an animation of robotic satellite repair. For more discussion see &lt;a href=&quot;https://space.stackexchange.com/q/16840/12102&quot;&gt;Is Landsat-7's propellant resupply port “robot-ready”? (Restore-L mission)&lt;/a&gt; and associated answers. Here is a GIF reconstructed from frames of the video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NWM2E.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NWM2E.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/yZB2YZ-mBu0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2022-09-06T22:28:47.833" Title="Besides HST, JWST and stations, are there any examples of satellites designed for service in space?" Tags="|robotic-missions|in-space-repairs|restore-l|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16846" PostTypeId="2" ParentId="16835" CreationDate="2016-06-27T03:52:28.497" Score="2" Body="&lt;p&gt;I have not heard of satellites that run out of station keeping fuel. This could be because they budget their fuel wisely and launch the necessary fuel for their mission lifetime so fuel is not a concern AFTER they launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for satellites in orbit, there are regulations for designing your mission in regards to End of Life (EOL).&lt;br /&gt;&#xA;One EOL regulation is that GEO satellites are required to keep a certain amount of fuel on-board so that they can migrate their satellite out of the GEO belt before its EOL. For LEO, they must make sure their satellites de-orbits within 10 years after its mission life is over.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The end of life for some satellites that I have worked with or worked for include or have heard about:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Main Payload became non-operational (LEO satellite for remote sensing).&lt;/li&gt;&#xA;&lt;li&gt;Improper Radiation shielding (LEO cube sat)&lt;/li&gt;&#xA;&lt;li&gt;Improper orbit insertion from Launch Provider, it was fully operational but in the wrong orbit so primary mission is over, end of life. (GEO Communications satellite)&lt;/li&gt;&#xA;&lt;li&gt;Satellite Collision &lt;a href=&quot;https://www.youtube.com/watch?v=_o7EKlqCE20&quot; rel=&quot;nofollow&quot;&gt;Link to Collision Simulation video&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Satellite stop communicating (Unknown reasons)&lt;/li&gt;&#xA;&lt;li&gt;Battery stop holding charge&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="15856" LastEditorDisplayName="user10509" LastEditDate="2016-06-27T13:09:06.877" LastActivityDate="2016-06-27T13:09:06.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16847" PostTypeId="2" ParentId="1059" CreationDate="2016-06-27T03:57:48.733" Score="-2" Body="&lt;p&gt;There was a mission to launch hundreds of microsatellites from a canister by some  university program.  The AF was worried about this launch because of their capabilities to track object &amp;lt;10 cm is extremely limited. The canister went up in orbit and failed to deploy the hundreds of satellites to the relief of the USAF. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot remember the University Project. It would be great if someone figured this out. &lt;/p&gt;&#xA;" OwnerUserId="15856" LastActivityDate="2016-06-27T03:57:48.733" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16848" PostTypeId="1" CreationDate="2016-06-27T04:47:06.173" Score="5" ViewCount="4802" Body="&lt;p&gt;Usually, I would consider the dry-mass the mass of a vehicle &lt;a href=&quot;https://en.wikipedia.org/wiki/Dry_weight&quot; rel=&quot;noreferrer&quot;&gt;without any consumables, passengers, or cargo&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1969-059C&quot; rel=&quot;noreferrer&quot;&gt;This NASA article&lt;/a&gt; however describes the Apollo 11 Eagle as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The lunar module was a two-stage vehicle designed for space operations near and on the Moon. The spacecraft mass of 15,065 kg was the mass of the LM including astronauts, propellants and expendables. The dry mass of the ascent stage was 2180 kg and it held 2639 kg of propellant. The descent stage dry mass was 2034 kg and 8212 kg of propellant were onboard initially. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here, dry-mass seems to include crew and expendables (since the dry-mass + propellant = total mass).&#xA;Which makes me wonder: When I read Spacecraft X has a dry-mass of Y, do I consider that including cargo, crew and expendables or not?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2023-02-04T16:53:49.600" Title="Common usage of &quot;dry-mass&quot;" Tags="|terminology|mass|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16849" PostTypeId="2" ParentId="16848" CreationDate="2016-06-27T06:28:59.627" Score="13" Body="&lt;p&gt;In rocketry, the two fundamental parameters of &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot;&gt;the Rocket Equation&lt;/a&gt;, $m_0$ and $m_f$ are referred to as &lt;em&gt;wet mass&lt;/em&gt; and &lt;em&gt;dry mass&lt;/em&gt; respectively. And, as you can guess, physics doesn't care what comprises the dry mass; everything has to be included. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While with other crafts, when writing specs, one can just drop the variable crew/payload and give a neat constant for the curious, in rocketry such liberties can't be taken; the exponential nature of the rocket equation can convert a kilogram of payload into a ton of fuel. This is such an essential parameter, that the common usage (&quot;vehicle without cargo or fuel&quot;) is dropped in favor of &quot;vehicle total mass at the end of burn&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, for purpose of stage calculation, &lt;em&gt;dry mass&lt;/em&gt; refers to mass of the stage without fuel combined with mass of all upper stages &lt;em&gt;with&lt;/em&gt; their fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For other vehicles, their dry mass is a secondary spec of little importance: determining bridge accessibility for trucks, runway accessibility for airplanes, pretty much nothing important except broadly understood &lt;em&gt;size&lt;/em&gt; for ships. However, with spacecraft, dry mass, combined with wet mass and &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot;&gt;ISp&lt;/a&gt;, comprise data necessary to determine $\Delta v$, which fundamentally defines what locations are accessible to the craft, which orbits, satellites, celestial bodies. So, in case of spacecraft you'll see the number that matters, not the number that serves to satisfy curiosity.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="1142" LastEditDate="2016-06-27T14:33:36.303" LastActivityDate="2016-06-27T14:33:36.303" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16850" PostTypeId="1" AcceptedAnswerId="16852" CreationDate="2016-06-27T09:40:57.193" Score="18" ViewCount="4327" Body="&lt;p&gt;The &lt;a href=&quot;http://rammb.cira.colostate.edu/dev/hillger/Shuttle-related.htm&quot;&gt;list&lt;/a&gt; of Shuttle missions contains quite a few of in-orbit repairs of commercial satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Russian_manned_space_missions&quot;&gt;Soyuz&lt;/a&gt; is about exclusively resupply/crew exchange missions to space stations, and this is the ongoing trend.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did the era of in-orbit satellite repairs end with the end of the Shuttle program, or did I miss something? Also, are there plans to bring that era back - be it through manned launches, or some robotic missions?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2020-08-16T19:30:23.527" Title="Are satellite repairs in orbit still being performed nowadays?" Tags="|artificial-satellite|in-space-repairs|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16851" PostTypeId="1" AcceptedAnswerId="19080" CreationDate="2016-06-27T12:19:36.767" Score="12" ViewCount="2357" Body="&lt;p&gt;SRBs and missiles use &lt;a href=&quot;https://space.stackexchange.com/questions/14107/ways-to-obtain-thrust-curves-of-different-grain-geometries&quot;&gt;grain&lt;/a&gt; to regulate thrust over time, as only exposed surface of the propellant burns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what causes propellant to burn only on the surface, and regulates the speed at which the surface burns? Surely not atmospheric oxygen supply (which is what causes similar property of free-burning solids) as it has oxidizer distributed throughout the volume; there's no apparently clear reason why the solid fuel couldn't just burn all at once throughout the volume; something puts it apart from common explosives which are effectively just that, a kind of fuel and oxidizer mix that burns all at once all throughout the volume, in enclosed space, releasing all the combustion products at once.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - what chemical additives or properties set solid propellants apart from explosives?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-11-21T00:54:58.833" Title="What limits burning speed of solid propellant?" Tags="|rockets|fuel|solid-fuel|chemistry|explosives|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16852" PostTypeId="2" ParentId="16850" CreationDate="2016-06-27T12:22:29.557" Score="17" Body="&lt;p&gt;It hasn't ended, it has taken a reprieve. Except for space stations, no docking of any kind is currently occurring (At least, that is publicly admitted), nor has it since the last Hubble Space Telescope servicing mission. But there are a few groups interested in researching this technology. I know that Goddard Space Center has a test setup to do on orbit refueling and battery replacement. I also know that &lt;a href=&quot;http://www.space.com/27128-darpa-robotic-satellite-repair-droids.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;DARPA&lt;/a&gt; is interested in the technology. I suspect this will eventually make a resurgence, although it might take a while.&lt;/p&gt;&#xA;&lt;p&gt;EDIT: Since I wrote this, at least one on orbit servicing mission has happened. &lt;a href=&quot;https://news.northropgrumman.com/news/releases/northrop-grumman-successfully-completes-historic-first-docking-of-mission-extension-vehicle-with-intelsat-901-satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;Northrop Grumman&lt;/a&gt; has docked one spacecraft to another one using the thruster port, allowing the mission to continue for longer than it otherwise would have been able to. It only supplies attitude control, but that is significant for geostationary spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2020-08-16T19:30:23.527" LastActivityDate="2020-08-16T19:30:23.527" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="16853" PostTypeId="2" ParentId="16851" CreationDate="2016-06-27T12:24:27.150" Score="0" Body="&lt;p&gt;Surface area, pure and simple. The more surface area visible, the faster the burning will occur. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to what separates them from explosives, it is that the propellant is so tightly bound that only the surface can burn. If there are cracks, then solid rockets can in fact explode, so they are very carefully manufactured.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-27T12:24:27.150" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="16854" PostTypeId="2" ParentId="16850" CreationDate="2016-06-27T12:35:05.557" Score="19" Body="&lt;p&gt;On-orbit repairs are not being done these days. There are no spacecraft from which repairs can be done on orbit.  &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Why not?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The Shuttle era showed on-orbit repairs to be very expensive (on the order of the cost of a new satellite), making it not worth bothering for a commercial satellite. After several experiments early on in the Shuttle program, repair missions dwindled until only the Hubble service missions were left.  &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;What would it take to run a repair mission?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;There are two major factors you need to make a repair mission possible: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The Shuttle showed that on-orbit repairs become much easier if you can attach the satellite to the spacecraft, and if the spacecraft has a manipulator arm. The arm can be used to move large components around, and it can provide astronauts with a surface to stand on/be attached to. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The satellite also has to be designed for it, with a docking interface, handrails, accessible internals with hatches etc. This makes the satellite more expensive and heavier. The difference was several hundred kg for Hubble. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;No current spacecraft contain a manipulator arm or an airlock, so you can't use them. You could bolt the hardware onto an existing spacecraft, but only in an expendable fashion (attached to the service module). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle had a large cargo bay that contained these items, so it could land with its repair/recovery tools intact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd have to design a new spacecraft that includes these in a way that allows recovery of these expensive parts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lowering launch cost changes the economics, and makes it more feasible to design and launch a heavy capsule with spare parts, an airlock and a manipulator arm. &#xA;At the moment, satellite and launch are in the same cost bracket (\$100-300M each). When launch services become cheaper (say, \$10M) it would become cost-effective to service a satellite rather than replace it.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-27T16:56:09.960" LastActivityDate="2016-06-27T16:56:09.960" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="16855" PostTypeId="1" AcceptedAnswerId="16856" CreationDate="2016-06-27T13:17:29.893" Score="16" ViewCount="11765" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mariana_Trench&quot; rel=&quot;nofollow&quot;&gt;The Mariana Trench is the deepest part of the world's oceans.&lt;/a&gt; To date only three people have reached the bottom and returned to the surface alive.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Moon_landing#Manned_landings&quot; rel=&quot;nofollow&quot;&gt;12 People have traveled to the surface of the Moon.&lt;/a&gt; This is four times as many as have been to the Mariana Trench on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is the Moon more visited than Mariana Trench? Ease of access? Politics? Science? Popularity? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="6773" LastEditDate="2016-06-28T16:57:50.137" LastActivityDate="2016-06-28T16:57:50.137" Title="Is it easier to get to and from the moon than to the bottom of the Mariana Trench?" Tags="|the-moon|lunar-landing|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="16856" PostTypeId="2" ParentId="16855" CreationDate="2016-06-27T13:37:05.160" Score="45" Body="&lt;p&gt;The Moon is much harder to reach.  I'd suggest the biggest reason for more visits is popularity and politics.  The Space Race was about tactical advantage (and to a huge extent posturing) between the Soviet Union and the United States.  Both countries wanted to show the other &lt;em&gt;just how big&lt;/em&gt; of an ICBM each could make.  Deepsea diving does not provide the same advantage so isn't as heavily researched by military.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, more people went to the Moon because of a planned series of missions and continued budget for the Apollo missions.  Had they been cut directly after Apollo 11, we'd likely still have only two humans ever to walk there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm going to use Deepsea Challenger for contrast here as it's the most modern dive vehicle for the trench.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a cost perspective, the Deepsea Challenger dive had a cost about about 5 million &lt;a href=&quot;http://www.deepseanews.com/2012/04/shouldnt-we-be-more-skeptical-of-the-deepchallenger-dive/&quot;&gt;according to this article&lt;/a&gt; while the Saturn V launch vehicle for Apollo cost around $375 Million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a duration of journey perspective, the Challenger Deepsea dive took about two hours to reach the bottom of the trench, versus the three days for Apollo 11.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, the Deepsea Challenger mostly works by simply having enough weight to keep sinking without collapsing from pressure.  The Apollo stack had to be supported with &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/7/7a/ApolloEnergyRequirementsMSC1966.png&quot;&gt;a very large Delta V budget&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="6773" LastEditDate="2016-06-27T13:55:37.013" LastActivityDate="2016-06-27T13:55:37.013" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16857" PostTypeId="2" ParentId="16855" CreationDate="2016-06-27T13:55:06.070" Score="16" Body="&lt;p&gt;It's a somewhat common sentiment when talking about space exploration or deep ocean exploration: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Fewer people have reached the deepest point in the world's oceans than&#xA;  have walked on the moon,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is comparing apples and oranges. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a single small valley that happens to be the deepest part of the ocean, vs.  &lt;/li&gt;&#xA;&lt;li&gt;the entire moon with all its variations in geology. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The surface area alone suggests more science can be done on the Moon than in the Mariana Trench. &#xA;Fairer comparisons would be the Mariana Trench vs. Pluto, or the Moon vs. the entire ocean floor area deeper than 2 km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a small area, a few visits will be enough to get a good science return. When you survey a large area, you need more visits to get a good idea of the variations in that area.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, we've done a lot of geology and biology surveys on Earth already and we're pretty good at predicting what a place will look like based on other, similar places. We've been to the ocean floor many times, just not that often to that particular place. &#xA;The Moon, however, is very different from Earth and presents an environment we've never seen before. That warrants at least a few visits to get data from different regions. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-27T19:10:56.733" LastActivityDate="2016-06-27T19:10:56.733" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16858" PostTypeId="2" ParentId="16796" CreationDate="2016-06-27T13:59:02.460" Score="5" Body="&lt;p&gt;On good authority (Warren Robinson of Lockheed Martin Space Systems, speaking unofficially and not on behalf of Lockheed Martin):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Yes, the July 4th Jupiter arrival was a deliberate choice of the Principal Investigator and Customer team, made way back in the early days of the program.  Given the nature of orbital mechanics, the navigation could have fine tuned the date multiple times to be something different, but have done an excellent job keeping it precisely on the original plan.&lt;/p&gt;&#xA;&lt;p&gt;NASA has a great love for having spacecraft events on July 4th, including Deep Impact hitting comet Tempel-1 on July 4th 2005, and even the Viking 1 Lander was originally scheduled to land on the Bi-centennial (July 4th 1776), though it end up delay as the program concluded the original landing location was too dangerous and a more suitable site was required.  It ultimately touched down on July 20th of that year.&lt;/p&gt;&#xA;&lt;p&gt;The plan with multiple gravity assists to arrive at Jupiter on July 4th was actively maintained by the navigation team.  I would have to go back and play with some numbers, but I suspect it was not the most efficient trajectory, but honestly the difference is down in the noise (mere grams of propellant).  So yes, the trajectory correction burns in deep space were adjusted to maintain this timing.&lt;/p&gt;&#xA;&lt;p&gt;Same was true for the Deep Impact mission.&lt;/p&gt;&#xA;&lt;p&gt;For the Viking missions to Mars, the conop was different in that an orbiter/lander carrier was inserted into Mars orbit the month before and the July 4th planned landing date wasn't dependent on in-flight trajectory maneuvers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-06-27T15:15:57.290" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16859" PostTypeId="2" ParentId="16835" CreationDate="2016-06-27T14:17:41.873" Score="2" Body="&lt;p&gt;This depends on the type of satellite:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Commercial satellites have a planned lifetime and a scheduled replacement. The owner generally has contracts to provide a service, and it's up to him to make sure the satellite is replaced before it reaches EOL. &lt;/li&gt;&#xA;&lt;li&gt;Scientific satellites are used until EOL or the money runs out, so you'll occasionally see a mission ending through catastrophic failure. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.satellitetoday.com/publications/2013/03/01/satellite-life-extension-reaching-for-the-holy-grail/&quot; rel=&quot;nofollow&quot;&gt;Satellites in geostationary orbit&lt;/a&gt; are generally limited by propellant. A refueling mission would increase the lifetime of the satellite.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-28T15:07:58.330" LastActivityDate="2016-06-28T15:07:58.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16860" PostTypeId="1" CreationDate="2016-06-23T22:46:37.020" Score="1" ViewCount="561" Body="&lt;p&gt;I presume it's a bit simplified, but ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if we launch a rocket, the same speed but opposite direction of the orbit of our planet around the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will that rocket fall into the sun?&lt;/p&gt;&#xA;" OwnerDisplayName="Emmanuel Delay" LastActivityDate="2016-06-27T16:02:42.957" Title="Send a rocket against orbit around the sun" Tags="|orbit|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16861" PostTypeId="2" ParentId="16860" CreationDate="2016-06-23T23:31:10.853" Score="9" Body="&lt;p&gt;In a simplified sense, yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Relative to the sun, the Earth moves at approximately 30 km/s.  So if we could launch a rocket in the opposite direction of Earth's rotation at 30 km/s, it would have a velocity of 0 relative to the Sun, and would be accelerated directly towards it by gravity from the Sun (&quot;falling towards it&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This assumes that a rocket could be accelerated to 30 km/s before leaving Earth's orbit however.  &lt;a href=&quot;https://space.stackexchange.com/a/10193/15841&quot;&gt;See this answer to a previous question&lt;/a&gt; if you want to see the upper speed limits on systems that we can launch.  The fastest system we've launched has been New Horizons at 16.26 km/s.&lt;/p&gt;&#xA;" OwnerUserId="15841" OwnerDisplayName="Michael R. T." LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-06-27T16:02:42.957" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16862" PostTypeId="1" CreationDate="2016-06-27T16:25:39.097" Score="4" ViewCount="2147" Body="&lt;p&gt;In what way does the concept of '&lt;a href=&quot;https://space.stackexchange.com/q/4056/12978&quot;&gt;C3&lt;/a&gt;' relate to 'Delta-V' in interplanetary missions?&lt;/p&gt;&#xA;" OwnerUserId="3227" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-06-27T21:43:44.983" Title="Relationship of 'C3' to Delta-V for interplanetary missions" Tags="|interplanetary|trajectory|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16863" PostTypeId="2" ParentId="16862" CreationDate="2016-06-27T16:50:02.890" Score="4" Body="&lt;p&gt;That depends on how and where the $\Delta V$ is applied. It is usually applied as an impulsive burn in low-Earth orbit. In that case (assuming a circular LEO, $\mu$ is GM, $r$ is distance from Earth center):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V=\sqrt{C_3+{2\mu_{Earth}\over r_{LEO}}} - \sqrt{\mu_{Earth}\over r_{LEO}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ah, looks like I've &lt;a href=&quot;https://space.stackexchange.com/a/4057/265&quot;&gt;answered this one before&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-06-27T19:01:11.187" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16864" PostTypeId="1" AcceptedAnswerId="16865" CreationDate="2016-06-27T17:30:16.610" Score="15" ViewCount="3771" Body="&lt;p&gt;The SLS will be using five segment boosters derived from the four segment ones used during the STS program.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Almost all of the STS boosters were recovered and reused, however NASA doesn't plan to try and recover any of the new solid rocket boosters for SLS.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the reasoning behind this?  Technical or limited number of launches?  Is it cheaper to simply make new boosters than try and refurbish recovered ones?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="6773" LastEditDate="2016-06-27T18:59:48.600" LastActivityDate="2022-09-13T23:27:48.337" Title="Why won't the SLS boosters be recovered?" Tags="|space-shuttle|sls|recovery|component-refurbishing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16865" PostTypeId="2" ParentId="16864" CreationDate="2016-06-27T17:44:09.320" Score="24" Body="&lt;p&gt;The usual answer - $.  Booster recovery &lt;a href=&quot;http://yarchive.net/space/shuttle/srb_reuse.html&quot; rel=&quot;noreferrer&quot;&gt;was only marginally worth it&lt;/a&gt; for STS - once you witnessed how every screw and nut was taken apart, cleaned, and reassembled, you began to wonder why not just build a new one.  That said, it is a wonderful thing for problem resolution if you get the hardware back to look at.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some pictures I took at the STS-124 TCDT showing a recently &#xA;recovered aft skirt versus an aft skirt in the process of being refurbed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zYC6x.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zYC6x.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;a href=&quot;https://i.stack.imgur.com/Coh3D.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Coh3D.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the low SLS flight rate and the economics of reuse bad anyway, it doesn't make sense. &lt;a href=&quot;http://www.nasa.gov/sls/space-launch-system-boosters-101.html&quot; rel=&quot;noreferrer&quot;&gt;But don't take my word for it&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;We don't need to recover the boosters because we have saved enough&#xA;  booster hardware to advance them for SLS specifications, resulting in&#xA;  cost savings,&quot; said Bruce Tiller, deputy manager of the SLS Boosters&#xA;  Office at Marshall. &quot;Removing all the parachutes and their associated&#xA;  infrastructure lifts many thousands of pounds off the vehicle and&#xA;  improves performance to carry more payload.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Edit: per request, here's a locater diagram showing where the aft skirt is on the SRB.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/3nVDq.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3nVDq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-06-28T13:15:41.903" LastActivityDate="2016-06-28T13:15:41.903" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="16867" PostTypeId="1" AcceptedAnswerId="16874" CreationDate="2016-06-27T22:02:00.790" Score="6" ViewCount="466" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/The_Integral_Trees&quot; rel=&quot;nofollow&quot;&gt;The Integral Trees is a 1984 science fiction novel by Larry Niven&lt;/a&gt; with an entire ecological system of oxygen breathing people and creatures, in free fall, in a gas torus. Water forms ponds, lakes, and puddles, in spheres as you would expect water to do in free fall.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth when diving &lt;a href=&quot;https://en.wikipedia.org/wiki/Underwater#Constraints_on_non-aquatic_life&quot; rel=&quot;nofollow&quot;&gt;in water the pressure increases by 1 atmosphere about every 10 meters or 33 feet of depth&lt;/a&gt;. One of the complications of this is &lt;a href=&quot;https://en.wikipedia.org/wiki/Decompression_sickness&quot; rel=&quot;nofollow&quot;&gt;the bends&lt;/a&gt;. In fact &lt;a href=&quot;//space.stackexchange.com/questions/5690/why-is-the-breathing-atmosphere-of-the-iss-a-standard-atmosphere-at-1-atm-conta/5694#5694&quot; rel=&quot;nofollow&quot;&gt;the ISS is maintained at 1 atmosphere for similar reasons&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming I have located a large body of water, like a lake or even a small sea, in an actual Goldblatt's World.  It might even be a lake in space station. Would I need to worry about increased pressure as I dove deeper into it? What similarities or differences would I encounter diving in free fall as opposed to Earth?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="6816" LastEditDate="2016-06-28T09:23:50.507" LastActivityDate="2016-06-29T17:19:42.870" Title="Scuba diving in free fall, do I need to worry about the Bends?" Tags="|microgravity|space-station|water|pressure-gradient-loss|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16869" PostTypeId="2" ParentId="16867" CreationDate="2016-06-27T22:08:26.433" Score="3" Body="&lt;p&gt;The increase in pressure with depth is solely due to gravitation (rho x g x h) == (density times gravity times depth).  If the body of water is in free fall, there would be no such increase, since g = 0.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-06-27T22:22:32.567" LastActivityDate="2016-06-27T22:22:32.567" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16871" PostTypeId="1" CreationDate="2016-06-28T00:20:06.963" Score="6" ViewCount="1768" Body="&lt;p&gt;I've been looking into a personal satellite. The CubeSat seems interesting but is upwards of 10k for a satellite and extra for the launch. Not something in my price range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found TubeSat by &lt;a href=&quot;http://www.interorbital.com/&quot;&gt;Interorbital&lt;/a&gt;. They have a $8,000 kit that includes the launch. But the website looks like it's from the 80s with some HTML errors thrown in. Besides the website and a few articles mentioning it I haven't found much information on Interorbital.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is Interorbital legit? Do they really do launches?&lt;/p&gt;&#xA;" OwnerUserId="15889" LastEditorUserId="25" LastEditDate="2016-06-28T00:34:32.307" LastActivityDate="2016-11-19T20:46:30.247" Title="Is TubeSat legit?" Tags="|launch|cubesat|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16872" PostTypeId="2" ParentId="16871" CreationDate="2016-06-28T00:34:16.133" Score="5" Body="&lt;p&gt;They seem to be a paper launch company, but they haven't made any work towards actually launching a rocket. I wouldn't count on them launching you to space anytime soon. See &lt;a href=&quot;http://space.skyrocket.de/doc_lau/neptune.htm&quot; rel=&quot;noreferrer&quot;&gt;Gunter's Space Page on the Neptune rocket&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-28T00:34:16.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16874" PostTypeId="2" ParentId="16867" CreationDate="2016-06-28T03:28:09.310" Score="8" Body="&lt;p&gt;&lt;strong&gt;tl;dr:&lt;/strong&gt; The pressure increase due to self-gravity reaches one atmosphere for a sphere of water with a radius of about 1000 km. Since water is still viscous but the average buoyancy along trip would be a tiny fraction of what you'd experience on Earth, it would take a very long time to travel that distance. If you used a  winch to pull yourself through thousands of kilometers of water, you &lt;em&gt;might&lt;/em&gt; be able to get the bends, but you'd have to work at it. But &lt;a href=&quot;https://en.wikipedia.org/wiki/Decompression_sickness#Ascent_from_depth&quot; rel=&quot;nofollow noreferrer&quot;&gt;the bends&lt;/a&gt; is a complicated function of pressure difference, rate of change, and total time. Here is the math to calculate the pressure profile at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; As an fyi, in &lt;a href=&quot;https://space.stackexchange.com/a/14177/12102&quot;&gt;this answer&lt;/a&gt; for a different low-gravity swimming question, several thresholds for different problems &lt;em&gt;besides&lt;/em&gt; the bends (if I understand correctly) are also listed. The the first falls at about 30m depth in earth gravity, where every 10m is roughly one additional atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If you assume the density of the water $\rho$ is constant, then as you move away from the center of the sphere, the pressure in each shell of water of thickness $dr$ increases by $dP$ as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{dP_g}{dr} = -\rho g(r) . $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Equation (3) in the derivation of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Adams%E2%80%93Williamson_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Adams Williamson equation&lt;/a&gt;, where $g(r)$ is the gravitational acceleration inside the sphere. Again assuming the density is constant:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$g(r)=\frac{4}{3} \pi G r \rho  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which comes from &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_of_Earth#Depth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravity of Earth (depth)&lt;/a&gt; and can be derived from the historically significant &lt;a href=&quot;https://en.wikipedia.org/wiki/Shell_theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shell Theorem&lt;/a&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Isaac_Newton&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Integrating $\frac{dP_g}{dr}$ out from $r=0$ and adding a constant to make it zero at the surface $r=R_0$, the pressure due to self-gravity becomes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P_g(r)=\frac{4}{3} \pi G \rho \frac{{R_0}^2-r^2}{2} , $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the maximum pressure due to gravity in the center:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P_{g0}=\frac{2}{3} \pi G \rho {R_0}^2 . $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a uniform contribution to the pressure due to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Surface_tension&quot; rel=&quot;nofollow noreferrer&quot;&gt;surface tension&lt;/a&gt;, which tends to maintain its spherical shape and therefore minimal surface area. For a sphere, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Young%E2%80%93Laplace_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Young-Laplace&lt;/a&gt; equation gives the pressure difference across an interface as: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta P_s = \gamma \frac{2}{R_0} . $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remembering to include the ambient pressure $P_a$, the total pressure inside the water sphere (maintaining assumption of uniform density) is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P_{tot} = P_g + \Delta P_s + P_a $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P_{tot}(r) =\frac{4}{3} \pi G \rho \frac{{R_0}^2-r^2}{2} + \gamma \frac{2}{R_0} + P_a $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the plot below, I've left out the ambient pressure. &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_pressure&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atmospheric pressure&lt;/a&gt; is about $10^5$ Newtons per square meters (Pascal). The pressure increase at the center of a sphere of water floating in air reaches one atmosphere when the radius reaches &lt;em&gt;one micron&lt;/em&gt; (due to dominant surface tension) and when it reaches &lt;em&gt;one thousand kilometers&lt;/em&gt; (due to self-gravity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RReIx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RReIx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used Python to make the plot:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def g(r, rho):&#xA;&#xA;    return (4./3.) * pi * G * r * rho&#xA;&#xA;def Pgrav(r, R0):&#xA;&#xA;    # dP/dr = -rho*g(r) just integrate &#xA;&#xA;    return (4./3.) * pi * G * rho * (R0**2 - r**2) / 2.&#xA;&#xA;def dPsurf(R0):&#xA;&#xA;    return 2. * gamma / R0&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;pi    = np.pi&#xA;G     = 6.674E-11       # N m^2/kg^2&#xA;rho   = 1000.         # kg/m^3 water roughly&#xA;gamma = 73. *1E-03  #  N/m  against air, at 20C, roughly&#xA;&#xA;R0    = 100000.          # m&#xA;r  = np.linspace(0, R0, 1001)&#xA;&#xA;Pg  = Pgrav(r, R0)&#xA;dPs = dPsurf(R0) * np.ones_like(r)&#xA;Pa  = 1E+05 * np.ones_like(r)    # N/m^2  roughly&#xA;&#xA;Ptot = Pg + dPs + Pa&#xA;Pgs  = Pg + dPs&#xA;&#xA;plt.figure()&#xA;plt.plot(r, Pgs, '-k')&#xA;plt.plot(r, Pg)&#xA;plt.plot(r, dPs)&#xA;plt.show()&#xA;&#xA;R0  = np.logspace(-6, 6, 1001)&#xA;&#xA;Pg  = Pgrav(0, R0)&#xA;dPs = dPsurf(R0)&#xA;&#xA;Pgs = Pg + dPs&#xA;&#xA;plt.figure()&#xA;plt.plot(R0, Pgs, '-k')&#xA;plt.plot(R0, Pg)&#xA;plt.plot(R0, dPs)&#xA;plt.yscale('log')&#xA;plt.xscale('log')&#xA;plt.xlabel('R0 (meters)', fontsize=18)&#xA;plt.ylabel('Pressure (Pa = N/m^2)', fontsize=18)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-06-29T15:43:54.040" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16876" PostTypeId="1" AcceptedAnswerId="16877" CreationDate="2016-06-28T09:35:29.387" Score="5" ViewCount="619" Body="&lt;p&gt;One of the answers to &lt;a href=&quot;https://space.stackexchange.com/questions/16850/are-satellite-repairs-in-orbit-still-being-performed-nowadays&quot;&gt;this question&lt;/a&gt; is generating some discussion about the cost and lifetime of a satellite. &#xA;The question is, will a satellite become cheaper when you can design it for a shorter lifetime?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the moment, &lt;a href=&quot;http://www.satellitetoday.com/publications/2013/03/01/satellite-life-extension-reaching-for-the-holy-grail/&quot; rel=&quot;nofollow noreferrer&quot;&gt;commercial geostationary satellites&lt;/a&gt; are designed for an on-orbit lifetime of 15 years, limited by propellant supply. Satellite operators would like to increase that by using more efficient engines.&lt;br&gt;&#xA;If you were to build a satellite for half that lifetime, would it get much cheaper? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess: it wouldn't get much cheaper; the only savings you have is in fuel (and hence, size of the fuel tank and launch mass). Electronics still have to be rated for space, halving the projected lifetime isn't going to change the fundamentals of producing and qualifying the parts. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2018-04-25T11:17:35.637" Title="What is the correlation between the cost of a satellite and its lifetime?" Tags="|artificial-satellite|economics|orbital-lifetime|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16877" PostTypeId="2" ParentId="16876" CreationDate="2016-06-28T11:05:22.223" Score="4" Body="&lt;p&gt;There is some correlation between cost and lifetime. Higher cost systems tend to have redundant electronics, while lower cost tend not to have such backups. The fuel is actually a fairly significant part of the mass of a satellite, less fuel means less structure, smaller tanks, smaller engines, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The correlation is somewhat weak, but it can be seen, if one looks carefully enough.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-28T11:05:22.223" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="16880" PostTypeId="1" CreationDate="2016-06-29T03:08:57.180" Score="11" ViewCount="980" Body="&lt;p&gt;Please forgive me if I am asking a dumb question. I am a little curious about the orbital parameters of &quot;LEO&quot; used by SpaceX on many occasions specifically those related to payload capability. According to SpaceX's Falcon 9 newest user guide it appears, though I cannot confirm, that it refers to a 200km x 360km orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;noreferrer&quot;&gt;Official User Guide&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but on this website it hints to be a 185km x 185km circular orbit (a little dated):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacelaunchreport.com/falcon9.html#config&quot; rel=&quot;noreferrer&quot;&gt;spacelaunchreport&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So which one is more accurate in terms of payload capability? Thanks.&lt;/p&gt;&#xA;" OwnerUserId="1262" LastEditorUserId="9060" LastEditDate="2016-06-29T14:12:58.140" LastActivityDate="2019-07-26T08:45:16.743" Title="What definition of &quot;LEO&quot; does SpaceX use when specifying payload capability?" Tags="|spacex|falcon-9|low-earth-orbit|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16881" PostTypeId="1" CreationDate="2016-06-29T04:29:31.153" Score="15" ViewCount="3655" Body="&lt;p&gt;Now that the UK has &lt;a href=&quot;https://en.wikipedia.org/wiki/United_Kingdom_withdrawal_from_the_European_Union&quot;&gt;voted to leave the European Union&lt;/a&gt;, how is that going to affect the European Space Agency? How much funding and participation does ESA receive from the UK based on the EU relationship? Are there other potential issues with the UK leaving the EU in regards to ESA?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="10234" LastEditDate="2016-06-29T04:35:34.450" LastActivityDate="2016-06-30T00:58:35.153" Title="How will the UK leaving the EU affect ESA?" Tags="|cost|esa|international-politics|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16882" PostTypeId="2" ParentId="16881" CreationDate="2016-06-29T05:22:12.263" Score="20" Body="&lt;p&gt;The EU and ESA are not related. They have different member states, where neither is a subset of the other. The only potential issue is simply economic. If the British exit from the EU results in a depressed UK economy, then they may elect to participate less in ESA.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="15909" LastEditDate="2016-06-30T00:07:38.323" LastActivityDate="2016-06-30T00:07:38.323" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16883" PostTypeId="1" AcceptedAnswerId="16885" CreationDate="2016-06-29T08:14:30.063" Score="3" ViewCount="6741" Body="&lt;p&gt;While I said just a few hours ago that &lt;a href=&quot;https://space.stackexchange.com/questions/16880/#comment43047_16880&quot;&gt;&amp;quot;There's &lt;em&gt;almost&lt;/em&gt; no such thing as a dumb question!&lt;/a&gt; this one just might sound like one.&lt;/p&gt;&#xA;&lt;p&gt;I'm making some slides about first principles thinking applied to engineering problems, and Elon Musk has several good quotes, one for example at a &lt;a href=&quot;http://www.ted.com/talks/elon_musk_the_mind_behind_tesla_spacex_solarcity&quot; rel=&quot;nofollow noreferrer&quot;&gt;TED talk&lt;/a&gt; where near the end he says something like “...boil things down to their fundamental truths and reason up from there.” I'm using both Tesla model S and Falcon 9/Falcon Heavy as examples.&lt;/p&gt;&#xA;&lt;p&gt;For the Tesla Model S, you can get the approximate required battery size easily from a range and speed (400km, 110kph), cross-sectional area (25 square feet), and drag coefficient of 0.24, plus the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rolling_resistance&quot; rel=&quot;nofollow noreferrer&quot;&gt;rolling resistance&lt;/a&gt; of good tires, 90% overall efficiency and another kilowatt or two for air conditioning, computer&amp;amp;display, and maybe some music.&lt;/p&gt;&#xA;&lt;p&gt;I wanted to add the &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_coefficient&quot; rel=&quot;nofollow noreferrer&quot;&gt;drag coefficient&lt;/a&gt; of a Falcon 9 rocket as a tangential comment only. I don't need an exact number, but something in the ballpark, and I have no idea where to begin.&lt;/p&gt;&#xA;&lt;p&gt;Assume sub-sonic, flying straight, with a reasonably large payload fairing. It doesn't have to be a Falcon 9 - any rocket somewhat representative is fine.&lt;/p&gt;&#xA;&lt;p&gt;If there is a plot of drag coeficient vs speed that extends to supersonic, that would be really very helpful also. This is back-of-the-envelope I'm doing here.&lt;/p&gt;&#xA;&lt;p&gt;Is it better or worse than the &amp;quot;Long Cylinder&amp;quot; below?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EoGoq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EoGoq.png&quot; alt=&quot;Measured Drag Coefficients&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:14ilf1l.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/File:14ilf1l.svg&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-04-12T23:31:58.223" LastActivityDate="2021-04-12T23:31:58.223" Title="What's the atmospheric drag coefficient of a Falcon 9 at launch (sub-sonic, large fairing)" Tags="|spacex|falcon-9|aerodynamics|atmospheric-drag|simulation|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="16884" PostTypeId="2" ParentId="4301" CreationDate="2016-06-29T08:21:25.173" Score="-1" Body="&lt;p&gt;It occurs to me that two of the major obstacles to vacuum smelting presented here are the postulated power requirements to smelt Fe and/or Al as well as the gravitational environment of that smelting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking them in order, it is pretty much a foregone conclusion that the sun generates and emits several orders of magnitude more energy than we could ever use in several lifetimes. The harnessing of that solar energy via a solar collector or mirror refraction would solve that issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, centripetal and/or centrifugal force via rotation of the smelter would solve the issue of gravitational separation of wastes from product.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of iron or, rather steel, smelting, why wouldn't taking a page from the electronics industry work? Use an analogous technique to the doping of Silicon wafers to achieve desired electrical pathways, except instead inject carefully measured amounts of elements and minerals into the molten iron to obtain the grade/quality of steel needed or required.&lt;br&gt;&#xA;Another idea/suggestion would be to have the crucible made from extremely high temperature resistant ceramic blends to prevent carbon absorption inherent in using a graphite crucible.&lt;/p&gt;&#xA;" OwnerUserId="15902" LastEditorUserId="3039" LastEditDate="2016-07-16T02:47:24.480" LastActivityDate="2016-07-16T02:47:24.480" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16885" PostTypeId="2" ParentId="16883" CreationDate="2016-06-29T09:54:02.977" Score="3" Body="&lt;p&gt;It's much better than the long cylinder from your example, because its drag is dominated by the flat face on the front. Count on 0.25 for a &lt;a href=&quot;http://www.aerospaceweb.org/question/aerodynamics/q0231.shtml&quot; rel=&quot;nofollow&quot;&gt;long cylinder terminated by a nose cone&lt;/a&gt; (maybe a bit more due to the reduction in diameter below the fairing).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-06-29T16:44:45.683" LastActivityDate="2016-06-29T16:44:45.683" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16886" PostTypeId="2" ParentId="16881" CreationDate="2016-06-29T09:59:38.660" Score="9" Body="&lt;p&gt;The effect should be small, but possibly non-zero.  There currently exists growing cooperation between EU and ESA.&lt;/p&gt;&#xA;&lt;p&gt;Within the &lt;a href=&quot;http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A52012DC0671&quot; rel=&quot;noreferrer&quot;&gt;EU-ESA membership agreement&lt;/a&gt;, one of the noted obstacles is the membership asymmetry:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;2.2.        Membership asymmetry&lt;/p&gt;&#xA;&lt;p&gt;At present, 17 EU Member States are Members of ESA. ESA's members include Norway and Switzerland, which are not EU members. Canada has a bilateral cooperation agreement with ESA. As the collaboration between the EU and ESA grows, this asymmetry combined with a voting system where each Member State has one vote in the ESA Council and the key decisions within this body are adopted by unanimity gives ESA members, which are not members of the EU, disproportionate leverage over matters that may affect the EU. In addition the asymmetry complicates discussions particularly as regards security and defence matters as indicated below.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;If the UK leaves the EU, the UK would be added to the ranks of Norway and Switzerland.  Currently, the UK influences ESA directly as a member state, and indirectly through the EU-ESA cooperation.  Its influence would likely diminish if it becomes an ESA-only member like Norway and Switzerland.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.publications.parliament.uk/pa/cm201314/cmselect/cmsctech/253/25307.htm&quot; rel=&quot;noreferrer&quot;&gt;This UK House of Commons publication&lt;/a&gt; contains more information.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-06-29T09:59:38.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16887" PostTypeId="2" ParentId="14785" CreationDate="2016-06-29T13:45:53.847" Score="9" Body="&lt;p&gt;After the first few landings of the first stage, Elon Musk said that the first core (landed on land) from the Orbcomm OG2 mission will become a static display outside the SpaceX factory in Hawthorne. (Adding to their collection of the first Dragon to reenter, a Merlin engine hanging in the ceiling.  I guess a first stage is too big to hang indoors, maybe in the Tesla factory? Or the BFR factory?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GTO mission stage from JCSat 14 mission was the most intense landing to date (being one of the heaviest GTO missions to date) and Elon said they would refire the stage 10 more times after the recovery and use it as a class leader for understanding the ability of the engine to be reused.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of August 2016, they had test fired it (full duration, 2+ minutes) three times in three days!!!! Which is amazing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gwynne Shotwell at the Small Sat 2016 conference at Utah State U in Logan, UT was noted to have said that they will try and slip the JCSAT-14 booster onto the test stand in between the need to test stages for launch until they get to 8-14 test firings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus it seems like they do not actually know the number of firings, but they designed for a large number (planning on reuse, and as detailed in the question, taking lots of firings to launch and land) and plan on stress testing a stage to find out the actual answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of practical use, the Block 3 and Block 4 Falcon 9 first stages are being used only 2 times, and then discarded/expended on second launch. The claim is that Block 5 model (due in spring of 2018) will be rapidly and cheaply reusable, up to 10 times before major work. Thus the number of firings per Merlin should be going up quite a bit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it will be the penultimate (for now!) Merlin 1D that can perform in this fashion, and it took iterations through 1A-&gt;1B-&gt;1C-&gt;1D and then many tweaks based on 1D experience to get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of mid 2019, the Block 5 has flown a couple of boosters three times, with no fourth flight yet in sight.  (Rumoured that the Starlink launches will try to forward the reuse of boosters into the higher reuse numbers. After all, if you are building 4400 satellites, losing a booster with 60 is expensive and annoying, but literally noise in the overall project. This way you let Starlink pay for testing the boosters ability to refly).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2019-07-01T17:56:38.790" LastActivityDate="2019-07-01T17:56:38.790" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="16888" PostTypeId="2" ParentId="16881" CreationDate="2016-06-29T14:28:42.833" Score="4" Body="&lt;p&gt;On paper it should make no difference given that there are many non EU members contributing to the ESA budget. Reality, however, is often very different than paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The politics of the current situation are difficult to predict. Certainly, the national governments of France, Germany and Italy will find it politically impossible to support any major collaborative aerospace projects based primarily in the UK. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the UK feels marginalised by the major EU powers within ESA then the UK government may respond by reducing the contribution to the budget, which will exacerbate the problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think its possible to extrapolate any further given the current global political and economic uncertainty following on from last Thursday's referendum result. &lt;/p&gt;&#xA;" OwnerUserId="13439" LastEditorUserId="1142" LastEditDate="2016-06-30T00:58:35.153" LastActivityDate="2016-06-30T00:58:35.153" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16890" PostTypeId="2" ParentId="16848" CreationDate="2016-06-29T17:34:22.463" Score="4" Body="&lt;p&gt;Consumables for the astronauts like breathing oxygen and drinking water are also dry mass. Every mass that is not a propellant is dry mass. Propellants are used to gain speed, consumables dont produce any speed change.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2016-06-29T17:34:22.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16891" PostTypeId="1" AcceptedAnswerId="16894" CreationDate="2016-06-29T17:51:51.560" Score="9" ViewCount="6604" Body="&lt;p&gt;I have the 6 orbital elements (excluding Time of Periapsis Passage, obviously), 2 state vectors, and a lot of other parameters like orbital period. I am looking to use them to calculate the time to periapsis and time to apoapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To give you a better idea of the values I already have, here is my code calculating all the data related to the orbit:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; void CalculateOrbitalData(Vector3 pos1, Vector3 vel1, float m1) {&#xA;    Vector3 pos2 = planet.position;&#xA;    Vector3 vel2 = planet.velocity;&#xA;    float m2 = planet.mass;&#xA;&#xA;    //Relative Position Vector&#xA;    Vector3 r = pos1 - pos2;&#xA;&#xA;    //Distance between bodies&#xA;    float rmag = r.magnitude;&#xA;&#xA;    //Relative Velocity Vector&#xA;    Vector3 v = vel1 - vel2;&#xA;&#xA;    //Specific Angular Momentum&#xA;    Vector3 h = Vector3.Cross(r, v);&#xA;&#xA;    //Standard Gravitational Parameter&#xA;    float µ = World.G * m2;&#xA;&#xA;    //Eccentricity Vector&#xA;    Vector3 evec = (Vector3.Cross(v, h) / µ) - (r / Vector3.Magnitude(r));&#xA;&#xA;    //Eccentricity&#xA;    float e = Vector3.Magnitude(evec);&#xA;&#xA;    //Vector to Ascending Node&#xA;    Vector3 n = new Vector3(-h.x, h.z, 0);&#xA;&#xA;    //True Anomaly&#xA;    float t = Mathf.Acos((Vector3.Dot(evec, r)) / (e * Vector3.Magnitude(r)));&#xA;    if (Vector3.Dot(r, v) &amp;lt; 0)&#xA;        t = (2 * Mathf.PI) - t;&#xA;&#xA;    //Longitude of Ascending Node (2D)&#xA;    float Ω = 0;&#xA;&#xA;    //Inclination (2D)&#xA;    float i = 0;&#xA;&#xA;    //Argument of Periapsis&#xA;    float ω = Mathf.Atan2(evec.y, evec.x);&#xA;    float ωdegrees = ω * (180 / Mathf.PI);&#xA;    if (e == 0) {&#xA;        ω = 0;&#xA;        ωdegrees = 0;&#xA;    }&#xA;&#xA;    //Eccentric Anomaly&#xA;    float E = 2 * Mathf.Atan(Mathf.Tan(t / 2) / Mathf.Sqrt((1 + e) / (1 - e)));&#xA;&#xA;    //Mean Anomaly &#xA;    float M = E - (e * Mathf.Sin(E));&#xA;&#xA;    //Semi-Major Axis&#xA;    float a = 1 / ((2 / Vector3.Magnitude(r)) - (Mathf.Pow(Vector3.Magnitude(v), 2) / µ));&#xA;&#xA;    //Apoapsis&#xA;    float ap = a * (1 + e);&#xA;&#xA;    //Periapsis&#xA;    float pe = a * (1 - e);&#xA;&#xA;    //Orbital Period&#xA;    float T = (2 * Mathf.PI) * Mathf.Sqrt(Mathf.Pow(a, 3) / µ) / 60;&#xA;    //patch orbital period to stay in line with unity timestep&#xA;    float fT = T * 1.205f;&#xA;&#xA;    //Mean motion&#xA;    float m = (2 * Mathf.PI) / fT;&#xA;&#xA;    //Perifocal distance&#xA;    float rp = (Vector3.Dot(h, h) / µ) / e + 1;&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="15714" LastActivityDate="2017-05-30T16:07:40.413" Title="How to calculate the time to apoapsis &amp; periapsis, given the orbital elements?" Tags="|orbital-mechanics|orbit|orbital-elements|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16892" PostTypeId="2" ParentId="16833" CreationDate="2016-06-29T18:09:39.827" Score="8" Body="&lt;p&gt;Bright sun light of a clear sky is about 100,000 Lux, room light at night about 100 Lux, street light at night about 10 Lux. We are thus able to see with 1/10000 of the brigthness of summer day with clear sky. If the distance to the Sun is 10 times greater, brightness is 1/100 smaller. Seeing an outer object in a distance of 100 AU to the sun (1 AU is the distance between sun and earth) is therefore possible if we were in close proximity to them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a maximum distance of 100 AU there is enough light to see Pluto, Haumea, Makemake and Neptune but also Arrokoth, the farthest known Kuiper Belt object. All these known and unknown KBOs are less than 50 AU to the Sun and would be visible to the naked eye.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2020-02-25T13:58:18.783" LastActivityDate="2020-02-25T13:58:18.783" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="16893" PostTypeId="2" ParentId="16741" CreationDate="2016-06-29T19:16:48.407" Score="1" Body="&lt;p&gt;Often there is a starting extension to the gas generator. This extension contains a solid fuel which is ignited. The produced hot gas flows to the turbine and starts rotation. The turbine drives the turbo pumps for fuel and oxidizer. A small part of the fuel and oxidizer is pumped into the gas generator and ignited by the still burning solid fuel. The liquid fuel pumped into the gas generator continues the operation when the solid fuel burns out. The larger part of pumped fuel and oxidizer flows to the combustion chamber. When liquid oxygen and hydrogen or kerosene is used, a precise ignition inside the combustion chamber is necessary. A late ignition may cause a catastrophic destruction of the combustion chamber. &lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2016-06-29T19:16:48.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16894" PostTypeId="2" ParentId="16891" CreationDate="2016-06-29T19:40:34.107" Score="7" Body="&lt;p&gt;You seem to be looking for the time equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$t=\sqrt{a^3\over\mu}\left(\tau-e\sin\tau\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $t$ is time and $\tau$ is the eccentric anomaly. It looks like you have the eccentric anomaly at the &quot;current&quot; time. You can just plug that in to see the time relative to periapsis, which is at $t=0$ and $\tau=0$. For the time at apoapsis, plug in $\tau=\pi$. An orbit period is $\tau$ running over a range of $2\pi$, so the period is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T=2\pi\sqrt{a^3\over\mu}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your time since periapsis is $t$, and your time to the next periapsis is $T-t$ (assuming that $\tau$ is in $0$ to $2\pi$).&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-06-30T18:03:55.237" LastActivityDate="2016-06-30T18:03:55.237" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16895" PostTypeId="1" CreationDate="2016-06-29T23:35:02.573" Score="2" ViewCount="8750" Body="&lt;p&gt;I know they have systems that flow water through there suits to keep them cool. But how do they block out the coldness that is space? What materials do they have for this?&lt;/p&gt;&#xA;" OwnerUserId="14088" LastActivityDate="2016-06-30T09:40:02.297" Title="How Do Astronauts Not Freeze In Space?" Tags="|spacesuits|space-weather|" AnswerCount="2" CommentCount="2" ClosedDate="2016-06-30T18:02:44.867" ContentLicense="CC BY-SA 3.0" />
  <row Id="16896" PostTypeId="2" ParentId="13222" CreationDate="2016-06-30T00:25:08.553" Score="4" Body="&lt;p&gt;The fuel-efficiency of a rocket (the delta-V you get from a given amount of fuel) is directly proportional to the speed that you give to the reaction mass when you eject it. It's true that you've got a lot of mass there, but I suspect that the fuel needed to eject it could be better used to accelerate its own exhaust gases to a much higher speed (almost 4.5km/sec for hydrogen-burning engines, for example), providing maybe a couple of extra seconds of thrust from the main engines, than by accelerating that much larger mass to a much smaller speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if we imagine you've got a 30-ton empty stage and you can push it away at 5m/s (as others have commented, you don't have long to push, and you don't want to jolt your ship too hard), that would give roughly the same impulse as pushing 30kg of rocket exhaust away at 5km/s. So if the mass of the ejection fuel, plus the mass of whatever mechanism actually does the pushing (pistons?) comes to more than 30kg, it's a losing proposition.&lt;/p&gt;&#xA;" OwnerUserId="15910" LastActivityDate="2016-06-30T00:25:08.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16897" PostTypeId="2" ParentId="8678" CreationDate="2016-06-30T00:47:24.187" Score="3" Body="&lt;p&gt;The Oberth effect can be explained this way. Every second you are falling in a gravitational field, it is altering your velocity--perhaps helping you, perhaps hurting you. If you do your burn when you're deeper in the gravity well, you're either increasing the time gravity will help you, or decreasing the time it will hurt you. For example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If you're moving away and you want to slow down, burn now. By slowing&#xA;down, you spend more time near the planet with gravity pulling you&#xA;back. &lt;/li&gt;&#xA;&lt;li&gt;If you're moving toward the planet and you want to speed up, wait&#xA;until you're closer. If you burn now, you spend less time falling&#xA;toward the planet getting accelerated by gravity.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;...and so on. In each case, burning while deeper in the gravity well gets gravity to amplify your efforts. So you can see that it's not the fact that you're lightening your ship that makes it work for you (remember, the Oberth effect also works for slowing down your ship), so it should work for any type of drive.&lt;/p&gt;&#xA;" OwnerUserId="15910" LastEditorUserId="15910" LastEditDate="2016-06-30T14:22:16.300" LastActivityDate="2016-06-30T14:22:16.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16898" PostTypeId="1" CreationDate="2016-06-30T00:59:44.237" Score="4" ViewCount="8834" Body="&lt;p&gt;It seems to me, that if we want to make Mars as Earthlike as possible, we should increase its mass. The long term affects of lower gravity environments on humans is not fully known, but we have enough information to assume we would have lower bone density on Mars and smaller, weaker muscles (including our hearts). In order to create a place where Earth life and Martian life could be exchanged back and forth, again, it seems that we need to increase the mass of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we redirected enough asteroids to collide with Mars and increase its mass, could that affect the orbit and rotation of the planet itself? Could it endanger the orbits of the rest of the solar system? &lt;/p&gt;&#xA;" OwnerUserId="15911" LastEditorUserId="1142" LastEditDate="2016-06-30T03:53:26.793" LastActivityDate="2021-12-05T21:18:44.643" Title="What would be the effect of increasing the mass of Mars?" Tags="|mars|terraforming|celestial-mechanics|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16899" PostTypeId="2" ParentId="16895" CreationDate="2016-06-30T01:16:14.527" Score="2" Body="&lt;p&gt;Space isn't actually hot or cold. Here on Earth, you're radiating heat away all the time, although not very fast. Other things around you are radiating heat at you, as well (usually slower). Those processes are very small compared to the way the air and evaporation are cooling you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, nothing is radiating back at you and you will get cold if the sun isn't shining on you, but it shouldn't take much insulation to make it so the outside of your suit is much cooler than the inside, so your losses by radiation will be much smaller.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being in the sun, absorbing all that heat with no good way to get rid of it, is going to become a problem much faster than cooling off in the shade, I'd imagine. Add a little insulation, so the outside of your suit can get very hot and radiate heat away more quickly without heating up the inside, and that will help as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, if you're perfectly insulated, your own body heat will cook you. I suspect that enough insulation to let you withstand the full sun will also keep you from cooling off fast enough in the shade, so you'll need some sort of cooling mechanism in addition to the insulation. You could bleed off some of the air you exhale, maybe, or carry some water and let that boil off into space.&lt;/p&gt;&#xA;" OwnerUserId="15910" LastActivityDate="2016-06-30T01:16:14.527" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16900" PostTypeId="2" ParentId="16898" CreationDate="2016-06-30T02:27:18.427" Score="20" Body="&lt;p&gt;Planets are much, much bigger than you think. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total mass of all the bodies in the main asteroid belt is about half of 1% of Mars's present mass. There's no realistic way to round up enough loose matter to appreciably increase the mass of a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;38% of Earth's gravity is pretty substantial. Humans have managed to survive a year in microgravity without severe long term ill effects. It would not surprise me to learn that 38% g is physiologically more similar to 100% than to 0%. Mars-born hearts won't develop the same strength as Earth-born, but they may not need to.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-06-30T02:33:49.517" LastActivityDate="2016-06-30T02:33:49.517" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16901" PostTypeId="1" CreationDate="2016-06-30T07:16:38.157" Score="9" ViewCount="1154" Body="&lt;p&gt;In this &lt;a href=&quot;https://space.stackexchange.com/a/15193/12102&quot;&gt;excellent answer to &lt;em&gt;How have space suits dissipated the heat removed from astronauts?&lt;/em&gt;&lt;/a&gt; I learned that space suits from the Apollo missions and even when used in space walks from the ISS today use water sublimation to remove both body heat and other heat generated inside.&lt;/p&gt;&#xA;&lt;p&gt;In the NASA document &lt;a href=&quot;https://www.hq.nasa.gov/alsj/LM15_Portable_Life_Support_System_ppP1-5.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Portable Life Support System (Hamilton Standard, undated)&lt;/a&gt; - one of the sources linked in that answer - it says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Feedwater Loop&lt;/p&gt;&#xA;&lt;p&gt;This subsystem supplies 11.8 pounds of expendable water, stored in a rubber bladder reservoir, to the heat-rejecting porous-plate sublimator. Of this expendable feedwater, 8.5 pounds is stored in the main reservoir; an auxiliary tank holds the remaining 3.3 pounds. Suit pressure against the bladder forces water into passages between the sublimator’s heat transport fluid passages and its metal plates, which are exposed to space vacuum. The ice layer formed on the porous plates during sublimation prevents the slightly pressurized water from flowing through the metal pores.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IEvfr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;screenshot&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The concept of having ten pounds of extra, or &amp;quot;expendable water&amp;quot; to lose every time you go outside to the Martian surface seems a little out of reach, at least for early missions or adventure.&lt;/p&gt;&#xA;&lt;p&gt;The Enthalpy of &lt;a href=&quot;https://en.wikipedia.org/wiki/Sublimation_(phase_transition)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sublimation&lt;/a&gt; of water is about &lt;a href=&quot;http://www1.lsbu.ac.uk/water/water_properties.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;51 kJ / mole&lt;/a&gt; , or about 2.8 million J/kg or 2700 BTU/kg. Suit electronics and mechanics will be much more efficient and therefore cooler than they were in 1970. For a nominal 1000 BTU/hour, assuming 70% overall thermodynamic efficiency of the sublimation system (just a guess), that means expending water to the Martian atmosphere at 0.5 kg per hour.&lt;/p&gt;&#xA;&lt;p&gt;Is this actually what's probably going to happen if/when there are suited people walking around on Mars? Maybe there will be modular ice packs sitting around on the surface that one could swap out regularly, with a water-lossy sublimator as backup only?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Both human bodies, and associated electronics and electromechanics generate heat and will cease to operate fairly quickly if you don't get the heat OUT of the suit or packaging. This question is strictly about how to get rid of heat while on the surface. I'd like an answer that is at least &lt;em&gt;roughly defensible&lt;/em&gt; quantitatively.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-07-05T22:11:40.920" LastActivityDate="2021-07-05T22:11:40.920" Title="Will suits worn on Mars lose kilograms of &quot;expendable water&quot; each time they are used?" Tags="|mars|crewed-spaceflight|spacesuits|water|environment|" AnswerCount="6" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="16902" PostTypeId="2" ParentId="16901" CreationDate="2016-06-30T09:14:43.117" Score="5" Body="&lt;p&gt;Well, of course we don't know what our engineers will choose but:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars has an atmosphere so traditional earth cooling/heating is possible.&#xA;Think about a fancy heatsink running in your MarsSuit.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6816" LastEditDate="2018-04-25T07:55:10.550" LastActivityDate="2018-04-25T07:55:10.550" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16903" PostTypeId="2" ParentId="16901" CreationDate="2016-06-30T09:17:25.823" Score="5" Body="&lt;p&gt;As far as I know, no Mars suit has been finalized yet. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo/Skylab_A7L&quot; rel=&quot;nofollow&quot;&gt;Apollo A7L&lt;/a&gt; suit is really heavy (91 kg), which was acceptable in the Moon's lower gravity. On Mars it'd get uncomfortable pretty soon, so I suspect we'll see new suit designs for use on Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is a different environment to LEO or the Moon, so the suit needs to be different anyway:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it's much colder, so less need for cooling. In LEO or on the Moon, the temperature on the surface of a suit swings between +150 and -100 °C. On Mars, insolation is much less (less energy coming in) and the atmosphere (however thin it is) stabilizes the surface temperatures somewhat. Peak temperatures on Mars are around +20 °C, the lowest temperatures are around -120 °C.  &lt;/li&gt;&#xA;&lt;li&gt;a more benign environment (less need for micrometeoroid protection)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6773" LastEditDate="2016-06-30T13:54:58.933" LastActivityDate="2016-06-30T13:54:58.933" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16904" PostTypeId="2" ParentId="16895" CreationDate="2016-06-30T09:40:02.297" Score="2" Body="&lt;p&gt;There are three ways of losing heat:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Conduction (two bodies touching each other, the warmer transferring heat to the colder until both are equally warm/cold)&lt;/li&gt;&#xA;&lt;li&gt;Convection (warmer gas/liquid rising up, taking heat away)&lt;/li&gt;&#xA;&lt;li&gt;Radiation (giving off heat in the form of electromagnetic radiation, e.g. infrared light)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In space, nothing is touching you, and there is no air or liquid, which means #1 and #2 are out. And #3 is &lt;em&gt;extremely&lt;/em&gt; inefficient. That's why, for example, the ISS needs &lt;em&gt;huge&lt;/em&gt; radiators to get rid of the excess heat, otherwise the astronauts would literally be cooked pretty soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in other words: yes, space is really cold, but the mechanism for losing heat to the cold space is so inefficient that &lt;em&gt;getting rid&lt;/em&gt; of, not preserving, your own body heat will be the &lt;em&gt;actual&lt;/em&gt; problem. You will die of a heatstroke, not hypothermia.&lt;/p&gt;&#xA;" OwnerUserId="6156" LastActivityDate="2016-06-30T09:40:02.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16905" PostTypeId="2" ParentId="16898" CreationDate="2016-06-30T11:21:37.363" Score="6" Body="&lt;p&gt;As Russell Borogove pointed out, that plan is quite impossible because you need to get that mass from somewhere, and all the other mass is already concentrated on even larger planets. And even when we would have enough asteroids, the plan would still not be even remotely feasible due to the huge energy requirements it would take to change the orbits of such huge amounts of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let's assume that this plan would be possible and examine it theoretically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When there is a plastic collision (two bodies crash into each other and turn into one), not only their masses combine, but also their momentum. Depending on how they hit, this will result in a rotation and velocity change. So yes, every asteroid impact changes the planet's speed (and thus its orbit) and its rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when you plan the asteroid strikes properly, you can plan them in a way that they cancel each other out and the resulting body has the same orbit and rotation as the original one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, another question remains: Is the resulting body still Mars? To get a surface gravity of Earth, you would have to increase the mass by factor ten. So just 10% of the mass would be original Mars material, the rest from the asteroids you scooped up. So the composition of the new planet's soil and atmosphere would depend far more on your choice of asteroids than on the original properties of Mars. All the asteroid impacts would also generate a huge amount of heat, so you would end up with a hot, geologically unstable mess of a proto-planet, which will take millions of years to cool down.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2016-06-30T11:26:56.923" LastActivityDate="2016-06-30T11:26:56.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16906" PostTypeId="1" AcceptedAnswerId="16907" CreationDate="2016-06-29T22:45:16.880" Score="4" ViewCount="224" Body="&lt;p&gt;The picture below shows a Kuiper Belt object and a number of stars. It was recently taken by the New Horizons probe and published by NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ceizW.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ceizW.jpg&quot; alt=&quot;Photograph of a Kuiper Belt object taken by New Horizons&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The arrow points at the KBO and the rest of the bright points visible are stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the top-left corner, there's an artefact that looks like a secondary mirror of a telescope. This is common in reflectors when looking at an object that is out of focus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is... what could it be that New Horizons captured that was close enough to produce this out-of-focus image? It seems we're looking at a small light source that, compared to the KPO and obviously the star field, is relatively close to the objective.&lt;/p&gt;&#xA;" OwnerUserId="15919" OwnerDisplayName="toniedzwiedz" LastEditorUserId="15919" LastEditDate="2016-07-03T23:11:25.157" LastActivityDate="2019-01-20T21:05:59.223" Title="What did New Horizons photograph that was close enough to be out of focus?" Tags="|camera|kuiper-belt|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16907" PostTypeId="2" ParentId="16906" CreationDate="2016-06-29T22:45:16.880" Score="12" Body="&lt;p&gt;According to &lt;a href=&quot;https://blogs.nasa.gov/pluto/2016/06/24/new-horizons-getting-to-know-a-kbo/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this blog post&lt;/a&gt;, the artefact was caused by scattered light reflected off some element inside the telescope.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ghostly circular pupil image and the little dots that are moving around in the image (and that aren’t JR1) are scattered light from a nearby bright star. LORRI isn’t that big of a telescope – just a little bit smaller than an 8-inch Schmidt-Cassegrain an amateur astronomer would use – so it’s easy for scattered light to bounce around inside the telescope and cause artifacts like these.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Apparently, it happens occasionally despite the lens being embedded in the probe's structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YMqvX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YMqvX.png&quot; alt=&quot;New Horizons Schematic&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;http://pluto.jhuapl.edu/soc/Pluto-Encounter/lorri_about.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source: the New Horizons mission website&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15919" OwnerDisplayName="toniedzwiedz" LastEditorUserId="10273" LastEditDate="2019-01-20T21:05:59.223" LastActivityDate="2019-01-20T21:05:59.223" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="16908" PostTypeId="2" ParentId="14202" CreationDate="2016-06-30T14:21:55.463" Score="2" Body="&lt;p&gt;I'm not an expert on cubesats either, but the factors to take into account would be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;environmental stability. Definitely the &quot;Industrial&quot; or better versions of the chips, rated for a broad range of temperatures. You will need a thermal control system, but don't count on it to be perfect and provide you with neat 4-60 centigrades, for which &quot;consumer&quot; equipment is rated at all times.&lt;/li&gt;&#xA;&lt;li&gt;power consumption. You have some wiggle room here, but you will strive for power for every subsystem, so a low-power CPU is definitely a boon. Rich power management options like throttling down, sleep etc are quite welcome.&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;reliable&lt;/em&gt; features you need - the set of peripherals you need for your application. Before making the purchase, download the &lt;strong&gt;errata sheet&lt;/strong&gt; for your CPU and check if nothing on your list is bugged. The sheer size of the document for more complex CPUs (like the Cortex family) is really eye-opening...&lt;/li&gt;&#xA;&lt;li&gt;playing well with realtime application. Either RTOS, or working without OS. If you need fancy closed-source drivers not available in RTOS version to use given feature, you won't be happy with it. Alternatively, you may go with a &quot;hybrid&quot; approach: Realtime, low-level for time-critical roles (attitude, radio); and a second CPU for a nice friendly OS that will do protocol conversions, media control (camera?), non-time-critical control (power, thermal) and such.&lt;/li&gt;&#xA;&lt;li&gt;either known to you, or known to be learner-friendly. There are some that when you look at the specs, seem very nice, but once you try to program them, you'll be ripping hair off your head.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What not to worry too much about:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The cost, comparing to other subsystems isn't all that high; pretty much any reasonable board for your needs can be found below \$500; attitude control will cost you more; making the structure to specs won't be cheap (unless you have good contacts), and launch cost will totally overshadow the CPU cost. So concentrate on other properties and your own ability to use it.&lt;/li&gt;&#xA;&lt;li&gt;rad-hardening. If a massive Coronal Mass Ejection comes, your cubesat is a toast unless you spend good \$300,000 on rad hardening, and outside of that event, it will be mostly fine. You will need a watchdog, and while most embedded CPUs come with these on board, you may want an external one as well. Also, if you manage to run under the weight budget, consider embedding in epoxy.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-06-30T14:21:55.463" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16909" PostTypeId="1" CreationDate="2016-06-30T17:27:56.260" Score="5" ViewCount="305" Body="&lt;p&gt;There are &lt;a href=&quot;http://hopsblog-hop.blogspot.mx/2016/04/liftport-lunar-tether.html&quot; rel=&quot;noreferrer&quot;&gt;examinations&lt;/a&gt; of lunar tethers going through the &lt;a href=&quot;http://www.lpi.usra.edu/meetings/leag2011/pdf/2043.pdf&quot; rel=&quot;noreferrer&quot;&gt;EML1 point&lt;/a&gt;, and &lt;a href=&quot;http://www.tethers.com/papers/LDC00Cislunar.pdf&quot; rel=&quot;noreferrer&quot;&gt;rotating tethers&lt;/a&gt; in &lt;a href=&quot;http://www.tethers.com/papers/HOYT_InterimReport.pdf&quot; rel=&quot;noreferrer&quot;&gt;various orbits&lt;/a&gt; of the Moon. But I haven't seen an examination of a tether in orbit that doesn't rotate. Wouldn't that have advantages?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A tether with its center of gravity at 3000 km altitude would orbit at a speed of 1.02 km/s. If the foot of the tether extends to as near the surface as it safely can - say 10 km away - then the foot would travel over the surface at a speed of 370 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is less than a quarter of the speed needed to attain orbit from the surface. Sure, you still need a rocket to get to the foot, but it doesn't need to get very high or go very fast. The same approach using a tether at 10 000 km gives a speed at the foot of 95 m/s.That is still a sixth of the distance needed to go through the L1 Lagrange point, and avoids the issue that that point &lt;a href=&quot;http://orbiter-forum.com/showthread.php?t=10036&quot; rel=&quot;noreferrer&quot;&gt;moves around&lt;/a&gt;. The best length would seem to mostly depend on getting the needed Zylon into place, construction and maintenance issues, and issues of climbing and descending the tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We were considering this as especially attractive as a way to move material from the lunar poles to the lunar equator, as the craft wouldn't need to climb the tether, it just grabs hold of it until it has reached its destination, and then lets go. It only needs enough propulsion to catch up to the foot and then land softly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the issues with such a system?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2016-07-01T02:33:27.610" Title="What would be the challenges of a non-rotating tether orbiting the Moon?" Tags="|the-moon|advanced-propulsion|space-tether|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16910" PostTypeId="2" ParentId="8678" CreationDate="2016-06-30T18:26:17.287" Score="1" Body="&lt;p&gt;The easiest way to understand the Oberth effect is simply to remember your Physics 101.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Work = Force x Distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are moving fast you will cover more distance during the burn and therefore do more work.  Anything that applies a force over time will observe this &quot;effect.&quot;  Cheers!&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2016-06-30T18:26:17.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16911" PostTypeId="2" ParentId="16901" CreationDate="2016-06-30T19:31:21.620" Score="3" Body="&lt;p&gt;It seems plausible to me that a CO2 sublimation system is an option. Perhaps instead of water, a pressurized tank of liquid CO2 is expended.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The advantage would be that a station based compressor could replenish the supply of CO2 from the martian atmosphere over time.&lt;/p&gt;&#xA;" OwnerUserId="15922" LastActivityDate="2016-06-30T19:31:21.620" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16912" PostTypeId="2" ParentId="16909" CreationDate="2016-06-30T20:49:14.003" Score="1" Body="&lt;p&gt;Concern 1: as the ship crawls up the tether, the tether is pushed downward. You can have a counterweight crawling the other way to balance it, but you ultimately need as many kilogram-trips from top to bottom as bottom to top, or spend stationkeeping thrust, or something. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Concern 2: all the spacecraft rendezvous we've done thus far have been in free fall with the two craft in very similar orbits. This can done in a very leisurely way with little fuel expenditure. In your case, though, the bottom of the tether is moving much slower than orbital speed, so from the frame of reference of the arriving spacecraft, it's accelerating constantly at 1/6g. The ship has to match that acceleration on the approach, and every second counts. If the grabber doesn't work, the ship is not left on a safe trajectory.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-06-30T20:49:14.003" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16913" PostTypeId="2" ParentId="16909" CreationDate="2016-06-30T20:50:20.820" Score="2" Body="&lt;p&gt;If you are going specifically for the issues, here are some:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Orbital stability&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Your tether is in reality orbiting in a two-body system, as the Earth's gravity have a significant influence when getting far enough from the Moon. The altitudes you are talking about is well within the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hill_sphere&quot; rel=&quot;nofollow&quot;&gt;Hill sphere&lt;/a&gt; of the Moon though, so this disturbance is pretty minor.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)&quot; rel=&quot;nofollow&quot;&gt;Mass-cons&lt;/a&gt; in the lunar crust causes irregularities in lower orbits, affecting the foot of the tether.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Any of the above factors are possible to correct for, but that requires some station keeping propellant budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Changing distribution of mass&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A craft climbing the tether, either up from the foot, or down for landing changes how the mass is distributed along the tether.&lt;/li&gt;&#xA;&lt;li&gt;Even a craft only grabbing the foot and hanging there would change the centre of mass.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This can have some consequences. Firstly, this applies a torque on the tether system, causing things like pendulum movement, stress, and in some cases a slightly changed orbit. It would be a bad thing if the foot collides with the lunar surface. Secondly, momentum change over time has to be balanced, otherwise the change in momentum necessarily alters the orbit of the tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To reduce the short term problems, having a grater tether mass is a good idea, as all types of oscillations and momentum change would have smaller consequences. To preserve momentum over time, you could either have a strict management of up-mass versus down-mass, or use things like ion-engines to provide momentum at high efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Force on the tether&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this a problem? Let me see..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from 10 km above the surface to orbital velocity at an altitude of 3000 km, the &quot;stress&quot; (acceleration over length) should be $1.32 \cdot 10^6 \frac{m^2}{s^2}$. Considering the density and tension strength of &lt;a href=&quot;https://en.wikipedia.org/wiki/Zylon&quot; rel=&quot;nofollow&quot;&gt;Zylon&lt;/a&gt;, the taper ratio is only 1.4, meaning there should be plenty of margin left for movable counterweights, power cables for an elevator and other things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Docking&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At $1.52 \frac{m}{s^2}$, the acceleration is significant at the tether foot. That means docking a craft is very different from docking in weightlessness. As a minute of hoover time costs you $90 \frac{m}{s}$ of $\Delta v$, landing and docking may as well be as costly as actually getting up to $370 \frac{m}{s}$. The acceleration requires a more powerful RCS system as well. In short, time is expensive, and cutting time may come at the cost of a less gentle and more risky docking. The connection would also have to support the weight of the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-06-30T20:50:20.820" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16914" PostTypeId="2" ParentId="6197" CreationDate="2016-06-30T20:59:52.423" Score="4" Body="&lt;p&gt;I think the Oberth effect is easier to understand without referring to conservation of energy or momentum. I don't know the etiquette of repeating answers here but if anyone objects I'll try to figure out how to post a link to another question where I gave essentially the same answer, and remove this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Oberth effect can be explained this way. Every second you are falling in a gravitational field, it is altering your velocity--perhaps helping you, perhaps hurting you. If you do your burn when you're deeper in the gravity well, you're either increasing the time gravity will help you, or decreasing the time it will hurt you. For example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If you're moving away and you want to slow down, burn now. By slowing&#xA;down, you spend more time near the planet with gravity pulling you&#xA;back.&lt;/li&gt;&#xA;&lt;li&gt;If you're moving toward the planet and you want to speed up, wait&#xA;until you're closer. If you burn now, you spend less time falling&#xA;toward the planet getting accelerated by gravity.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;...and so on for the other cases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equations and laws tell you that the Oberth effect will happen. They don't tell how it works. If your capsule is going faster, something had to push or pull on it. In this case, that thing was gravity. Conservation of energy doesn't speed things up or slow them down, it just tells you that something will do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Related: is there a tag for soapbox? Tired rant, maybe?)&lt;/p&gt;&#xA;" OwnerUserId="15910" LastActivityDate="2016-06-30T20:59:52.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16915" PostTypeId="1" AcceptedAnswerId="16918" CreationDate="2016-06-30T21:01:58.870" Score="8" ViewCount="1334" Body="&lt;p&gt;I understand (I think) that a vehicle on or near a planet with greater mass - such as the Juno probe entering Jupiter's orbit soon- will experience time slightly slower relative to us on Earth. However, I'm having trouble wrapping my head around if and how that impacts the mission. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If the probe is set to take a measurement once per second in &quot;Jupiter Time&quot; and sends them to Earth, what will the interval between measurements be to an Earth observer?&lt;/li&gt;&#xA;&lt;li&gt;How do such missions really operate in practice? For example, when Juno has to do a &quot;30 minute burn&quot; to enter orbit, is that in Jupiter minutes or Earth minutes? What do they set the timer to on the craft?  &lt;/li&gt;&#xA;&lt;li&gt;If you have a 100Mhz processor does it operate slower speed (from our perspective on Earth) when nearer massive objects such as Jupiter or the Sun? &lt;/li&gt;&#xA;&lt;li&gt;Or is the difference so negligible that none of this matters until we send a probe to a black hole?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="1414" LastEditorUserId="51" LastEditDate="2016-07-08T13:37:44.377" LastActivityDate="2016-07-08T13:37:44.377" Title="How is gravitational time dilation experienced by spacecraft near Jupiter such as Juno" Tags="|jupiter|time|juno|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16916" PostTypeId="2" ParentId="16871" CreationDate="2016-06-30T21:21:44.500" Score="1" Body="&lt;p&gt;Interorbital Systems is very much alive and continues to offer its TubeSat and CubeSat Kit-and-Launch packages. The company is completing ground testing of its gimbal and guidance system and is preparing for a guided test flight this summer (2016). The current launch manifest includes 128 orbital payloads (see www.interorbital.com), slated to begin launches in early 2017, after completion of the flight test program and licensing. Commercial suborbital services began in 2014, launching and recovering four payloads; summer 2016 launch will carry 11 small sat payloads for testing before their 2017-2018 scheduled orbital launches. If you would like information about our services, please contact me at ios@interorbital.com. ~ Randa Milliron, CEO/CoFounder, Interorbital Systems.&lt;/p&gt;&#xA;" OwnerUserId="15924" LastActivityDate="2016-06-30T21:21:44.500" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16917" PostTypeId="1" AcceptedAnswerId="16919" CreationDate="2016-06-30T22:22:20.807" Score="6" ViewCount="768" Body="&lt;p&gt;…or, how well is the Earth magnetic field mapped?&lt;/p&gt;&#xA;&lt;p&gt;Three hallotrons (simple, cheap, robust, tiny, low-power devices) aligned in three axis can be used to determine the direction of local vector of Earth magnetic field.&lt;/p&gt;&#xA;&lt;p&gt;GPS can be used to determine the location of the satellite. (This is trickier due to mechanisms that disable all commercial GPS chips at these speeds and altitudes, to prevent military use, but still possible.)&lt;/p&gt;&#xA;&lt;p&gt;Knowing the location, direction of Earth magnetic field at the location, and the measurement from the hall sensors, one should be able to determine attitude of the satellite (excluding spin around precisely the axis of the magnetic field vector) quite accurately.&lt;/p&gt;&#xA;&lt;p&gt;Now the problem lies in that &amp;quot;direction of Earth magnetic field at the location&amp;quot;, as the solar wind bends the field into quite a fancy shape:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/zi9So.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zi9So.jpg&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;How precisely is this shape mapped, and how constant/variable is it over time?&lt;/p&gt;&#xA;&lt;p&gt;…or did I miss something else; is this a viable method of attitude determination at all?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-02T22:19:38.587" Title="Can Earth magnetic field be used for attitude determination in LEO?" Tags="|artificial-satellite|low-earth-orbit|attitude|magnetic-field|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16918" PostTypeId="2" ParentId="16915" CreationDate="2016-06-30T22:25:15.993" Score="2" Body="&lt;p&gt;It's pretty small. This plot shows the example of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_time_dilation#/media/File:Orbit_times.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth's orbit&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LOPUX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LOPUX.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Note that is from the surface of the. The difference is on the order of picoseconds/second, or parts per trillion. That small of a time is far less than the uncertainty in pressure, temperature, and even actuation time causing small amounts of thrust uncertainty.&lt;/p&gt;&#xA;&lt;p&gt;The difference only matters if one needs a very accurate clock. GPS satellites famously take into account the time dialation, otherwise their location would drift considerably with time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-06-30T22:25:15.993" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16919" PostTypeId="2" ParentId="16917" CreationDate="2016-06-30T22:37:12.400" Score="8" Body="&lt;p&gt;Yes, actually, and it is commonly used for LEO satellites. I know of a number of satellites that use magnetic field information to determine their location. It won't provide absolute attitude, but it is sufficient that another point of reference, such as the Earth's curvature or sun's position, is sufficient to determine where the spacecraft is pointed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It tends not to be used by further out spacecraft, which tend to be larger and benefit more from having a star tracker or similar system. Plus, as you mentioned, the magnetic field varies the further you go. It tends to follow the same lines, however, which allows one to determine the direction of the spacecraft relative to the magnetic field. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://en.wikipedia.org/wiki/Spacecraft_magnetometer&quot;&gt;Spacecraft Magnetometer&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-06-30T22:37:12.400" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16920" PostTypeId="2" ParentId="16909" CreationDate="2016-07-01T02:22:09.430" Score="4" Body="&lt;p&gt;Employing Wolfe's spreadsheet I'm getting numbers similar to Hohmann fan's. That's reassuring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;With tether center balance point at 3000 km altitude&lt;/strong&gt; and tether foot at 10 km altitude I get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Foot speed: .375 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration at foot 1.52 m/s^2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zylon taper ratio 1.43&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tether mass/payload mass ratio: 1.63&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;With tether center balance point at 10000 km altitude&lt;/strong&gt; and tether foot at 10 km altitude I get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Foot speed: .096 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration at foot 1.6 m/s^2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zylon taper ratio 1.8&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tether mass/payload mass ratio: 7.11&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;CONCERNS&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Momentum change&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless the tether and anchor mass is substantially more massive than the payload, catching or dropping payloads would substantially change the tether's orbit. Which is why a big anchor mass is desirable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The anchor mass I'd like to see is a small asteroid retrieved with a &lt;a href=&quot;http://www.nss.org/settlement/asteroids/Asteroid_Retrieval_Feasibility_Study_2012.pdf&quot; rel=&quot;nofollow&quot;&gt;Keck style vehicle&lt;/a&gt;. Not only would the Keck vehicle retrieve a massive rock, it also has Hall thrusters and robust solar arrays. Hall thrusters can have 30 km/s exhaust velocity. Yes, thrust is minute but it can build up over time if catches and throws aren't too frequent. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in your scenario, the net momentum change the ion engines have to compensate for is &lt;em&gt;zero&lt;/em&gt;. In the act of catching a payload as it passes over the equator, the tether drops a little. In the act of releasing the same payload over the poles, the tether rises a little. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Making the Catch&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The payload would have to be fired at the right time and speed so that it matches position and velocity with tether foot at it's apolune. But even when it matches position and velocity, the approximately 1/6 g is pulling the payload away. The catch must be done quickly.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-07-01T02:33:27.610" LastActivityDate="2016-07-01T02:33:27.610" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16922" PostTypeId="2" ParentId="8678" CreationDate="2016-07-01T03:02:18.297" Score="1" Body="&lt;p&gt;The faster you're moving, the bigger the Oberth effect. You're moving fast during perigee, so changes in velocity done in the vicinity of perigee enjoy an Oberth benefit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But solar sails have minute acceleration. Only a little change in velocity can be done while the sails are in the neighborhood of perigee. To do an impulsive burn at a 300 km altitude perigee, you'd need an acceleration that's a good fraction of 1 g, better yet -- several g's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If acceleration is on the order of 1 mm/s^2, there will be little Oberth benefit unless many small burns are done over many perigees. &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-07-01T03:02:18.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16923" PostTypeId="2" ParentId="16880" CreationDate="2016-07-01T09:15:40.040" Score="5" Body="&lt;p&gt;The most believable figure here seems to be the 200km x 360km figure, as it is from the most recent official source. They claim ±10km x ±15km is within three standard deviations, so there is a significant spread. The other source you mentioned, the space launch report, references the source of the data from a seven years older user guide, and a four year older data sheet. (The 185km x 185km LEO orbit is often used when comparing different launch vehicles.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One more thing though: While SpaceX offers a standardised orbit as a product, they operate in a very specialised niche market where missions are often custom made. As the $\Delta v$ cost of using a slightly different orbit is only minor, you could easily order another orbit for your payload, eliminating the need for having a company LEO definition. Custom orbits are of course much easier if you do not share the launch with many other satellites though.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-07-01T09:15:40.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16924" PostTypeId="1" AcceptedAnswerId="16951" CreationDate="2016-07-01T09:32:48.847" Score="12" ViewCount="4805" Body="&lt;p&gt;I saw the section in Wikipedia about &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid-fuel_rocket#Electric_Solid_Propellants&quot; rel=&quot;noreferrer&quot;&gt;Electric Solid propellants&lt;/a&gt; but I can't figure out what &lt;em&gt;it really is&lt;/em&gt;, and how it works. It sounds like a solid propellant rocket that you can start and stop multiple times by turning it ON and OFF - a restartable solid fuel rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;How!?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any examples of this type of rocket being put to use? (&lt;em&gt;beyond self-validation&lt;/em&gt;)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-04-17T06:22:12.653" LastActivityDate="2022-12-30T13:46:38.637" Title="How does an Electric Solid Propellant rocket work?" Tags="|fuel|solid-fuel|restartable-engine|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="16925" PostTypeId="2" ParentId="16801" CreationDate="2016-07-01T09:50:52.317" Score="1" Body="&lt;p&gt;The think the glow is not always visible from space. For example: the orbit of the ISS is inclined, but at least twice a year, it should be at the exact opposite side of the Earth relative to the Sun:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QQpV3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QQpV3.png&quot; alt=&quot;ang(le|el)s of light&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmosphere stretch a bit farther out, but not a lot farther than the ISS. The light should have a hard time bending in such a drastic angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for how bright it is, that must depend a lot on other condition, such as if the Sun is in the sky. Images can probably not reliably used to determine that, as a lens sees thing quite different than the eye. I guess you would have to ask a real cosmonaut for that :)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-07-01T09:50:52.317" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16926" PostTypeId="1" AcceptedAnswerId="37710" CreationDate="2016-07-01T10:56:38.320" Score="4" ViewCount="2097" Body="&lt;p&gt;Here is a photo of a test firing of the Space Shuttle main &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;RS-25 engine&lt;/a&gt; from 1981 from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Shuttle_Main_Engine_Test_Firing.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pnY39.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pnY39.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The top of the blue luminous area (a static shock wave?) looks like it has a corner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a screen shot from a &lt;a href=&quot;https://i.stack.imgur.com/Sevnq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube Video&lt;/a&gt; of a July 17, 2015 test of the SLS RS-25 engine:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sevnq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sevnq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below are animated GIFs to outline what looks to me like a rectangular cross-sectional shape. This is not a scientific analysis - it just appears rectangular to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the expansion and nozzle are (presumably) circular in cross-section, I wouldn't expect the profile to appear to have that corner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it just me, or is there a significantly non-circular shape to the cross-section at this boundary, and if there is, &lt;em&gt;why?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; The question &lt;a href=&quot;https://space.stackexchange.com/q/16821/12102&quot;&gt;What (actually) is the cause of the blue light from LH2/LOX rocket engines?&lt;/a&gt; also addresses the abrupt luminescence at the shock wave boundary, as does the question &lt;a href=&quot;https://space.stackexchange.com/q/2017/12102&quot;&gt;Why do the exhaust flames from cryogenic stage engines appear to be separated from the nozzle?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1981:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/14Gls.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/14Gls.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2015:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uHlSI.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uHlSI.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-03-27T04:23:51.863" LastActivityDate="2021-11-17T23:19:26.593" Title="Does the luminous blue shockwave from the RS-25 engine appear to be rectangular?" Tags="|engines|exhaust|shockwave|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="16927" PostTypeId="1" AcceptedAnswerId="16928" CreationDate="2016-07-01T12:59:33.800" Score="6" ViewCount="697" Body="&lt;p&gt;Last night, at 22:51 EDT, I went outside to observe a Tiangong-1 pass (the first Chinese space station). It was expected to peak at a magnitude of 1.5, which is moderately bright. However, I had a big surprise because it actually became very, very bright before it entered the shadow. I'd estimate the magnitude as a negative one (-1 or -2 maybe). There was no expected iridium flare in the area and the space station went eastwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How could it become so much brighter than expected?&lt;/p&gt;&#xA;" OwnerUserId="11347" LastActivityDate="2016-07-01T13:09:26.313" Title="Unexpectably bright Tiangong-1?" Tags="|space-station|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16928" PostTypeId="2" ParentId="16927" CreationDate="2016-07-01T13:09:26.313" Score="7" Body="&lt;p&gt;Almost certainly it was the glint of the Sun, or possible the Earth. Most likely the solar panels were pointed in just a way to reflect the light towards you. Lining this up would require some work, but it still could be done. The fact it happened right before it entered shadow also indicates this was likely the reason as well.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-01T13:09:26.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16929" PostTypeId="2" ParentId="10246" CreationDate="2016-07-01T14:27:48.003" Score="1" Body="&lt;p&gt;If you want to do an orbit correction requiring 30 % of the solid rocket burn time, you cant use the remaining 70 % for a later maneuver. That is too expensive if you need several burns. A good rocket for orbit corrections allows a lot of short and longer burns. The efficency of the fuel mass is much better than.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-07-01T14:27:48.003" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16930" PostTypeId="2" ParentId="10246" CreationDate="2016-07-01T15:16:09.683" Score="6" Body="&lt;p&gt;As noted in some other answers, the Minuteman III ICBM 3rd stage had &quot;ports&quot; that were opened and brought the solid rocket chamber pressure to zero which effectively terminated thrust.  The Peacekeeper (&quot;MX&quot;) ICBM used a more elegant solution and used a 'fuel wasting' manuever to avoid having such ports.  The guidance system lofted the trajectory so that the desired velocity was achieved when the solid rocket fuel in the 3rd stage was depleted.&lt;/p&gt;&#xA;" OwnerUserId="15932" LastActivityDate="2016-07-01T15:16:09.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16931" PostTypeId="1" CreationDate="2016-07-01T16:00:16.650" Score="20" ViewCount="2181" Body="&lt;p&gt;I was reading Hollister David's short &lt;a href=&quot;http://clowder.net/hop/TMI/TangentEllipses.pdf&quot; rel=&quot;noreferrer&quot;&gt;paper&lt;/a&gt; on bi-tangential transfers. The example he uses is a transfer between a planar circular and an elliptical orbit. I am wondering: Is an ideal, that is, the lowest $\Delta v$ cost, transfer between two arbitrary orbits around a single point mass &lt;em&gt;always&lt;/em&gt; an ellipse that is tangential to both the orbits? I am excluding the cases where a bi-elliptical transfer is ideal, as the answer is then always a manoeuvre with infinite apoapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not entirely comfortable with trusting that as a safe assumption, as although paying a $\Delta v$ expense in another direction than prograde or retrograde is expensive, it is by no means horrible. If this assumption is indeed not true, what criteria must to planar arbitrary orbits satisfy in order to make the ideal transfer between them a bi-tangential ellipse?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Clarification:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A transfer orbit being &quot;tangential&quot; to an orbit means that in the transition between the orbit and the transfer orbit, velocity change is only applied in the prograde or retrograde direction. A &quot;bi-tangential transfer&quot; is when the transfer orbit between a pair of orbits is tangential to both of them. As a consequence, this problem is strictly planar. The &quot;bi-elliptical transfer&quot; I am referring to is when the alternative with the lowest cost is to do a burn at periapsis accelerating to infinity, then performing zero-cost manoeuvres &quot;at infinity&quot; before falling back to the other orbit's periapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example of a bi-tangential transfer. Both burns are done tangentially:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0KROw.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0KROw.png&quot; alt=&quot;bi-tangential transfer&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bounty:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had a 100 rep bounty on this question that expired gaining only a partial answer. Because it is a bit unfair to a potential complete answer that another one got the bounty, There is now a 500 rep bounty running.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-11-05T16:11:35.643" LastActivityDate="2023-09-13T00:06:12.250" Title="Is the ideal transfer between two arbitrary planar orbits always a bi-tangential ellipse?" Tags="|orbital-mechanics|orbital-maneuver|delta-v|bi-tangential-transfer|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="16932" PostTypeId="2" ParentId="16901" CreationDate="2016-07-01T16:02:21.643" Score="2" Body="&lt;p&gt;&lt;strong&gt;UPDATE:&lt;/strong&gt; &quot;Ice batteries&quot; are already starting to be commercialized, currently as larger scale devices for homes or buildings. With experienced gained here, a miniatureized version may be straightforward to develop. It may not be cost effective for personal use on Earth, but considering the price of &quot;excpendable water&quot; on Mars may be &quot;astronomical&quot; at the beginning, this seems more and more to be the best solution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other working materials such as salt water or ethylene glycol mixtures are also being explored.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ars Technica: &lt;a href=&quot;https://arstechnica.com/business/2017/05/ice-batteries-commissioned-by-utility-will-cool-california-businesses/&quot; rel=&quot;nofollow noreferrer&quot;&gt;California utility augments 1,800 air conditioning units with “ice battery”&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Green Tech Media: &lt;a href=&quot;https://www.greentechmedia.com/articles/read/axiom-exergy-brings-its-refrigeration-batteries-to-market-in-three-years&quot; rel=&quot;nofollow noreferrer&quot;&gt;Axiom Exergy Brings Its Refrigeration Batteries to Market in 3 Years&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt; Possibly not, at least not in the beginning. One possible alternative  to loosing water via sublimation would be exchangeable ice packs (once per hour or longer) that naturally refreeze on the surface. Later, after recovery of water from geologic sub-surface sources is under way, the loss of a few kilograms per 8 hour spacewalk may be acceptable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per other comments, &quot;air conditioners&quot; or $\text{CO}_2$ sublimators are probably not very feasable thermodynamicaly.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The Apollo astronauts had relatively short missions, and therefore the 10 pounds of &quot;expendable water&quot; to cool their suits by sublimation into space per EVA could be accommodated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the ISS, EVA's are rare, since most of the fun stuff is either on the inside, or robotically accessed on the outside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However a manned mission to Mars is a different matter. Presumably they wouldn't be sent there to just sit inside a &quot;hab&quot; for six months and drive radio controlled robots outside. The term &quot;expendable water&quot; sounds a lot more painful considering it would have to be shipped from Earth, or painstakingly extracted from subsurface soil or possibly the tiny traces in the atmosphere (?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the Enthalpy of Sublimation of water is about &lt;a href=&quot;http://www1.lsbu.ac.uk/water/water_properties.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;51 kJ / mole&lt;/a&gt;, the Enthalpy of Fusion (freezing) is almost ten times lower, about &lt;a href=&quot;http://www1.lsbu.ac.uk/water/water_properties.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;6.0 kJ / mole&lt;/a&gt; However on Mars, if you put a bunch of ice packs outside at night, they'll be well frozen in the morning, and also well below 0 °C. If they reach -20 °C by morning, you pick up an extra 1.5 kJ / mole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;7.5 kJ / mole comes to about 1800 BTU of cooling for 10 pounds of captive, recoverable water. So about once an hour, sometimes longer, you just swap out your ice pack for another that's been sitting outside overnight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These ice packs could have some simple insulation flaps that passively open at night so they can radiate to space, and close when warmed by sunlight, so they can stay cold through the end of the day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A pack might be 30 x 30 x 5 cm, (about ten pounds) could radiate about 250 W if facing space with an emissivity of say $\epsilon$ = 0.8, so in only a few hours, it could be &quot;recharged&quot; with &quot;cold&quot; and ready to go again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you run out of ice packs on the surface (or on the outside of your vehicle) in some kind of unusual situation, then you can bite the bullet and puncture it and sublimate the liquid water like they did in the &quot;Apollo days.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These would be high-tech ice packs, able to exchange heat with the suit, and with space, and then to re-insulate themselves during the day, but I can't think of any fundamental reason why this wouldn't work. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-10T01:38:27.283" LastActivityDate="2017-05-10T01:38:27.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16933" PostTypeId="5" CreationDate="2016-07-01T16:05:07.567" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-01T16:05:07.567" LastActivityDate="2016-07-01T16:05:07.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16934" PostTypeId="4" CreationDate="2016-07-01T16:05:07.567" Score="0" Body="A bi-tangential transfer is an orbit between the departure and target orbit which is tangential to both of them. The Hohmann transfer is a bi-tangential transfer, and this type of transfer can be viewed as a generalisation of a Hohmann transfer." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-01T16:05:07.567" LastActivityDate="2016-07-01T16:05:07.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16935" PostTypeId="1" CreationDate="2016-07-01T16:30:17.647" Score="12" ViewCount="3261" Body="&lt;p&gt;Most discussion I have read about using tethers and rotation in order to simulate gravity on spacecraft, talk about simulating Earth's gravity - 1g or 9.8 m/s&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Baked into the 1g figure is the assumption that humans evolved on Earth where gravity is 1g so it's probably healthiest for us. But is that really necessary? Have there been any studies or research into how much gravity is actually needed in order to minimize the long-term health effects?&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;A spaceship that rotates to generate 1g of gravity would either require a (debatably) impractically long tether, or have to spin so fast that it would cause a disorienting Coriolis effect. Wikipedia states that the human factors of a Coriolis effect would be mostly negligible at 2 rpm. By my calculations, at Earth's gravity, that yields a radius of 224 meters, whereas at Mar's gravity, that is reduced to 84 meters. One could potentially imagine a spacecraft with two manned modules connected by tethers and a 168 meter long inflatable tube to allow crew and supplies to pass between them; however, bring that up to 448 meters for 1g, that's over a quarter of a mile - and you can see that it starts to become impractical.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A spaceship that is enduring 1g of centripetal acceleration would have to be built with the same rigidity and structural properties of a similarly sized structure on earth, meaning that it would require more materials and therefore have more mass. If we assume the entire craft is assembled on Earth and launched as a unit, perhaps this would not be such a problem - since the spacecraft would have to be built under Earth conditions and in fact endure acceleration significantly greater than 1g during launch. Even so, the ship would be in a different configuration during launch that could be designed to be more rigid, or perhaps an aerodynamic fairing could be used to provide extra rigidity during launch, or perhaps the acceleration during launch would be on a different axis than the planned rotation. However, assuming some amount of orbital assembly, which is in fact rather likely, there would have to be at least some additional mass overhead to design for 1g of simulated gravity.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If we're going to send astronauts on multi-month or multi-year missions to Mars, or even start a colony there, they're going to be exposed to Martian gravity which is (&lt;strong&gt;very&lt;/strong&gt; approximately) one third that of Earth's. Similarly, gravity on the Moon is &lt;span class=&quot;math-container&quot;&gt;$\sf\frac{1}{6}$&lt;/span&gt; that of Earth's. If we think that's OK then why bother with 1g on board the spacecraft to take them there?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="15933" LastEditorUserId="7062" LastEditDate="2021-07-07T06:16:24.357" LastActivityDate="2023-01-28T06:24:25.277" Title="How much gravity is actually needed to avoid serious health consequences?" Tags="|mars|spacecraft|humans|artificial-gravity|travel|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="16936" PostTypeId="2" ParentId="16931" CreationDate="2016-07-01T17:00:35.440" Score="12" Body="&lt;p&gt;Thanks for linking to my pdf!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have always assumed bi-tangential transfers took the least delta V. But your question has made me realize my assumption is conjecture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My goal is to find a general equation for delta V, integrate it, and then hope the minima of the manifold will correspond to bi-tangential orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes playing with conics is rewarding. It is a delight when complicated equations reduce to something simple and elegant. But so far I've been frustrated. Poking and prodding at these equations have only caused them swell up like an angry puff fish. I'm sharing my efforts in the hope folks will help me cut a path through this thicket of thorns. I will add to this as I have time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Units&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When using AU (Astronomical Unit) and years, the sun's gravitational parameter GM is easy to describe:&#xA;$\mu = 4 \pi^2 AU^3/year^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Circular orbit velocity is described as&#xA;$V = \sqrt{\mu / (rAU)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For earth's orbit r = 1. Plugging $\mu$ and earth's r into the above, we get earth's velocity is $2 \pi AU/year$ which is reassuring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Finding speed at arbitrary rendezvous points&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Picking an arbitrary point rendezvous $P_1$ sets a quantity $r_1AU$. This quantity $r_1AU$ is distance from $P_1$ to sun. $P_1$ is the rendezvous point where transfer and destination orbits intersect. ($P_0$ will be rendezvous point where transfer and departure orbits intersect.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/brh9B.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/brh9B.png&quot; alt=&quot;Transfer and destination orbits&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the vis viva equation we can find speeds of payload and destination at point P.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V = \sqrt{\mu (2/(r AU) - 1/(a AU))}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where &lt;em&gt;aAU&lt;/em&gt; is length of ellipse's semi major axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recall with our units $\mu = 4\pi^2AU^3/year^2$. So the vis viva equation becomes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V=(2\pi AU/year)* \sqrt{2/r -1/a}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the speed of a body in elliptical orbit is &lt;em&gt;earth's speed times&lt;/em&gt; $\sqrt{2/r-1/a}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{payload} = V_{earth} * \sqrt{2/r_1-1/a_2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{destination} = V_{earth} * \sqrt{2/r_1-1/a_1}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Flight Path Angles&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have speeds of payload and destination at point $P_1$ but we don't have direction. For that we need to find the difference between the payload and destination flight path angles. I will call that angle $\phi$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wqsd3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wqsd3.png&quot; alt=&quot;Angle between velocity vectors&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will try to add to this soon.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-07-13T15:53:59.253" LastActivityDate="2016-07-13T15:53:59.253" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="16937" PostTypeId="2" ParentId="9667" CreationDate="2016-07-01T17:26:44.340" Score="-3" Body="&lt;p&gt;so because of the decreasing mass of the vehicle as you burn fuel the total delta V is given by the rocket equation deltaV=Vexhaust * ln(Minitial/Mfinal).  so even if you break up all the burns into multiple events the total deltaV is still governed by the rocket equation&lt;/p&gt;&#xA;" OwnerUserId="15932" LastActivityDate="2016-07-01T17:26:44.340" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16938" PostTypeId="2" ParentId="16935" CreationDate="2016-07-01T17:35:40.733" Score="9" Body="&lt;p&gt;We do not know yet. The main issue is a lack of empirical data. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are only &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spaceflight_records#Most_time_in_space&quot;&gt;four specially trained volunteers&lt;/a&gt; with more than one year exposure to microgravity. We'd need hundreds of volunteers under different gravities to measure the difference (it is however plausible to assume that the effects are gradually dependent on the level of gravity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conclusion: For any long-term mission we should provide as much (up to 1g of course) gravity as is reasonably possible.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-07-01T17:35:40.733" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16939" PostTypeId="1" AcceptedAnswerId="29561" CreationDate="2016-07-02T00:49:40.623" Score="5" ViewCount="187" Body="&lt;p&gt;There are &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/331.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;mice on ISS&lt;/a&gt;. Not the first non-human mammals and likely not the last. Their health provides insights on influence of microgravity on organisms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very few humans stayed in space for longer than a year; after that period the health impact becomes too serious to allow them remain. What about mice, or other animals though? Have any stayed longer? And if so, what were the effects?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...I'm asking this in relation to the recent &lt;a href=&quot;https://space.stackexchange.com/questions/16935/how-much-gravity-is-actually-needed-to-avoid-serious-health-consequences&quot;&gt;How much gravity is actually needed to avoid serious health consequences?&lt;/a&gt;. Building a centrifugal human habitat and letting the crew remain there well in excess of one year is impractical due to the cost. But building a centrifuge for some small animals? With mouse lifespan being only around two years, that wouldn't be too indicative, but guinea pigs reach age of 10, which should provide a plenty of data on influence of various levels of gravity on health.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2018-07-19T20:13:56.720" Title="Have any mammals been exposed to micogravity longer than humans?" Tags="|artificial-gravity|animals|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16940" PostTypeId="1" CreationDate="2016-07-02T05:18:37.533" Score="6" ViewCount="316" Body="&lt;p&gt;ISS and Hubble have vastly different orbits - different altitudes, different inclinations, requiring a massive delta-V to transfer from one to the other. My question is: has anything ever had to perform such a maneuver (not necessarily between ISS and Hubble specifically)? Or has such a requirement never come up, or just never been feasible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm just asking about precedent for a large inclination change while in orbit.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2020-05-12T01:05:39.703" Title="Has anything ever executed an orbit change such as between ISS and Hubble?" Tags="|orbital-mechanics|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16942" PostTypeId="2" ParentId="16940" CreationDate="2016-07-02T09:47:44.573" Score="5" Body="&lt;p&gt;I am not sure if you want count this one, but the currently ongoing &quot;last mission of &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini&lt;/a&gt;&quot; involves the spacecraft changing its inclination gradually from 35 to 64 degrees when it goes through the gap between Saturn and the innermost part of the rings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Z7HoR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z7HoR.jpg&quot; alt=&quot;NASA image Cassini&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The manoeuvre is not done propulsively as large inclination changes are expensive, and the probe is almost out of fuel. (Just for the record, changing the inclination of a circular orbit by 23.9 degrees is as expensive as reaching escape velocity.) Instead, multiple fly-bys of the moon Titan is used to alter the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A similar approach was used for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ulysses_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ulysses spacecraft&lt;/a&gt; that used a fly-by of Jupiter to enter a solar orbit with an inclination of 80.2 degrees to perform observations of the polar regions of the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can not recall that any purely propulsional large change of inclination has been performed, but there exists a Russian plan to keep the ISS alive a little longer by moving their segment to a higher inclination.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-02T14:24:52.917" LastActivityDate="2016-07-02T14:24:52.917" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16943" PostTypeId="1" AcceptedAnswerId="16954" CreationDate="2016-07-02T15:05:11.543" Score="6" ViewCount="220" Body="&lt;p&gt;Say I want to change the inclination of the ISS by one degree. The ISS orbits at 7660m/s so the manoeuvre will cost me 133.7m/s of $\Delta v$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5ICmP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5ICmP.png&quot; alt=&quot;orbital plane change&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From time to time, the station must be reboosted because of orbital decay. Let us just assume that this is done like a Hohman transfer, with the first burn being 10m/s to change the velocity of the station from 7660m/s to 7670m/s. But what if I do a slight plane change at the same time, say 0.1 degrees?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NMmlI.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NMmlI.png&quot; alt=&quot;orbital plane change at reboost&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, that costs me an additional 6.7m/s on top of the original 10m/s. But wait! If I do this on 10 consecutive reboosts, I have changed the inclination by one degree, but only spent 67m/s of $\Delta v$!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It only gets better. With 100 steps of 0.01 degrees, the total $\Delta v$ spent is only 8.9m/s, at a 1000 0.001 degree steps, only 0.9m/s. It is approaching zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this really a free propulsive inclination change? I think a way to summarize this odd effect is something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iu8So.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iu8So.png&quot; alt=&quot;spent and gained&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-07-03T02:22:35.353" Title="Can I move the ISS for free?" Tags="|orbital-mechanics|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16944" PostTypeId="1" CreationDate="2016-07-02T15:21:49.987" Score="11" ViewCount="975" Body="&lt;p&gt;I can find several descriptions of what the &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-4#SPINSAT&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpinSat satellite&lt;/a&gt; was supposed to do. I can find some nice images of it being deployed from the ISS also. But I haven't been so successful in understanding whether the whole mission worked.&lt;/p&gt;&#xA;&lt;p&gt;Did the Digital Solid State Thrusters successfully spin up the satellite, and then de-spin it again? Was the performance in this demonstration as good as what was anticipated?&lt;/p&gt;&#xA;&lt;p&gt;Were the spin-up, and especially the de-spin maneuvers performed using real-time attitude and rotation measurements? The electric thrusters are pulsed, so thrust can be metered out in controlled amounts, so conceivably the de-spin could be done as a single maneuver, with the thrusters fired until spin=0 when they could be stopped.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/explorer/Investigation.html?#id=991&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's mission pages link for SpinSat&lt;/a&gt; says the following:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Operational Protocols&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;ESP thrusters are physically arranged on the exterior of the satellite to provide two basic maneuvers as spin-up (de-spin) maneuver and a normal thrust maneuver. For the spin-up maneuver, pairs of thrusters are co-aligned 180 degrees apart, and provide a tangential component force on the exterior; for de-spin, a 2nd pair of thrusters provide the opposite force. For the normal thrust maneuver, thrusters are oriented perpendicular to the exterior of the satellite to provide force in the normal direction. Another set of thrusters placed at the opposite pole provide normal force in the opposite direction. The space situational awareness component of the mission is a ground based observational plan to detect, characterize and assess state changes in the spacecraft. These observations are verified by data collected on the spacecraft by MEMS gyroscopes and accelerometers used to determine the spin axis orientation and spin rate of the spacecraft before during and after thruster firings. The spacecraft itself acts as the primary sensor for the third experiment, with a well-determined and characterized ballistic coefficient the routine collection of radar tracking and satellite laser ranging data will provide a high resolution atmospheric drag data set used to deriver thermospheric density.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;How did it all go?&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; There are a few unique features of this kind of solid propellant thruster. You activate it using a low current, high voltage arc to simultaneously vaporize and ignite the propellant mixture. Shut off the arc, and the propellant becomes inert. That means these solid propellant thrusters are considered safe enough to spend at least a short time inside the crew area of the ISS.&lt;/p&gt;&#xA;&lt;p&gt;I've &lt;a href=&quot;https://space.stackexchange.com/q/16924/12102&quot;&gt;asked separately&lt;/a&gt; about the principle with which electric solid thrusters actually operate.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oPx2d.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oPx2d.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/goNdv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/goNdv.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wcBtB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wcBtB.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KK33H.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KK33H.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PiBKt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PiBKt.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6tQlv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6tQlv.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-03-19T00:09:22.313" LastActivityDate="2021-03-19T00:09:22.313" Title="What ever happened to SpinSat - did it work?" Tags="|attitude|solid-fuel|experiment|restartable-engine|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="16945" PostTypeId="1" AcceptedAnswerId="16955" CreationDate="2016-07-02T17:09:16.673" Score="4" ViewCount="323" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/16940/12102&quot;&gt;Has anything ever executed an orbit change such as between ISS and Hubble?&lt;/a&gt; has a in &lt;a href=&quot;https://space.stackexchange.com/a/16942/12102&quot;&gt;interesting answer&lt;/a&gt;. It made me wonder, would something like the (patented) Lunar Flyby technique use less detla-v than a direct, propulsive inclination change of about 23 degrees?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit 1:&lt;/strong&gt; Let's say it's not a delicate communications satellite, so aerobraking is allowed for part of the circularization at the end, and there is no rush, a transfer time of maybe six months would be OK.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit 2:&lt;/strong&gt; Hubble to ISS is OK too. Starting at the lower of the two inclinations is probably going to be necessary if this is at all possible.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; something like this was used to &lt;a href=&quot;http://www.spacedaily.com/news/Book_Reveals_How_Hughes_Saved_ComSat_In_1997.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;correct the orbit&lt;/a&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/PAS-22#Rescue_of_satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;PAS-22&lt;/a&gt;, making it the first commercial spacecraft to reach near-lunar space if I understand the Wikipedia article correctly. It says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Using on-board propellant and lunar gravity, the orbit's apogee was gradually increased with several manoeuvres at perigee until it flew by the Moon (9) at a distance of 6,200 km from its surface in May 1998, becoming in a sense the first commercial lunar spacecraft. Another lunar fly-by was performed later that month at a distance of 34,300 km to further improve the orbital inclination.&lt;/p&gt;&#xA;&lt;p&gt;These operations consumed most of the satellite's propellant, but still much less than it would take to remove the inclination without the Moon assist manoeuvres.&lt;/p&gt;&#xA;&lt;p&gt;(9)Space Daily's  &lt;a href=&quot;http://www.spacedaily.com/news/Book_Reveals_How_Hughes_Saved_ComSat_In_1997.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Book Reveals How Hughes Saved ComSat In 1997&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Would a maneuver involving the moon also be lest costly in delta-v than a direct plane change if transferring from the Hubble to the ISS?&lt;/p&gt;&#xA;&lt;p&gt;For reference, here is Figure 2 of &lt;a href=&quot;https://www.google.com/patents/US6116545&quot; rel=&quot;nofollow noreferrer&quot;&gt;US6116545&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OBHRU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OBHRU.png&quot; alt=&quot;from US6116545&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2020-05-12T01:03:45.070" Title="Would lunar flyby be less costly in delta-v than direct change from ISS's to Hubble's orbit?" Tags="|low-earth-orbit|orbital-maneuver|inclination-change|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="16946" PostTypeId="2" ParentId="16944" CreationDate="2016-07-02T17:10:15.370" Score="4" Body="&lt;p&gt;When the results of the experiment become public, they should become visible on &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/1019.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;the NASA page you linked to&lt;/a&gt;. I haven't found anything in NTRS or on the website of the Naval Research Laboratory that runs the experiment.&lt;/p&gt;&#xA;&lt;p&gt;Spinsat was launched from the ISS on Nov. 28, 2014.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/spinsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;This page&lt;/a&gt; has some details on what happened after launch:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;• Sept. 2015: Already in early 2015, researchers at the AFRL (Air Force Research Laboratory) began their collaboration with the NRL SpinSat team, with the goal of using multiple AMOS (Advanced Maui Optical and Space Surveillance) sensors to observe the satellite firing the ESP micro-thruster thrusters, and turning the LEDs on and off. The observational goals include: 1) obtaining time-resolved, multi-band measurements of the satellite actively firing its micro-thrusters, 2) characterizing the detectability and spatial/temporal morphology of the ESP thruster plumes, 3) measuring the spin rate of the satellite with the LEDs turned on, ideally before and after a spin rate adjustment maneuver, and 4) measuring the spin rate of the satellite in its completely inactive mode, using only passive observations of reflected light and/or thermal emissions. 12)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Summary of AMOS Observations: Multiple sensors from the AMOS observatory's 3.6 m and 1.6 m telescopes were used to observe the three passes shown in Figure 14. Table 2 includes color-coded summaries of the status and quality for the collected data sets. The sensors include two acquisition sensors (LAAT and AATS) which are small, side-mounted, co-aligned telescopes imaging visible-band emissions using silicon-based CCD detectors, each with a relatively wide field-of-view (WFOV) subtending ~30 arcmin. The 3.6 m and 1.6 m telescopes also host one or more narrow field-of-view (NFOV) sensors, subtending ~30 arcsec or less. Three of these NFOV sensors (DI, ARDI, and Flash) also image in the visible and near-infrared spectral bands using Si-based CCDs. The 3.6 m LWIR sensor, however, acquires NFOV imagery in two thermal-infrared bands simultaneously. The analysis concentrates on the data gathered during the second AMOS pass, which had the best observing geometry and employed both near-IR and thermal-IR sensors from the largest-available AEOS (Advanced Electro-Optical System) 3.6 m telescope.&lt;/p&gt;&#xA;&lt;p&gt;Observed Light-Curves: Figure 15 shows three light-curve plots acquired by the AEOS telescope's LAAT, ARDI and LWIR sensors during the second AMOS pass. The brightness units are instrumental magnitudes normalized for range variations, which provide a logarithmic measure of whole-object brightness, with the effects of object-to-observer range variations removed (but otherwise not radiometrically calibrated). The upper panel (blue) shows data from the LAAT sensor (a 0.6 m acquisition telescope coaligned with the main 3.6 m telescope), within the approximate 0.85-1.0 µm spectral range using an 850 nm long-pass spectral filter. The middle panel (red) shows data from the ARDI sensor, which is mounted on the 3.6m telescope itself, measuring emissions in the ~0.7-1.0 µm range using an I-band filter; this sensor provided the best signal-to-noise quality data among all of the light-curve data. The bottom panel (green) shows data from one of the two channels of the narrow-field LWIR sensor, also mounted on the 3.6 m telescope, measuring thermal emissions in its N2 filter band spanning the ~10-13 µm spectral range.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-02T18:26:39.790" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16947" PostTypeId="2" ParentId="16935" CreationDate="2016-07-02T17:15:28.930" Score="6" Body="&lt;p&gt;We don't know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We currently only have good data for how humans are doing in 9.81 m/s² or in 0 m/s² acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only case where humans were ever exposed to anything between 1g and 0g for longer than a few minutes was during the Apollo moon landings (1.62 m/s²). The longest was Apollo 17 with 75 hours. Still not nearly long enough to notice any long-term health effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To find out how exactly different health problems scale with decreasing gravity we would need to put some humans in different gravity environments for several months. Options could be a permanent moon base or a centrifugal space station in Earth orbit. As far as I know, neither is currently in the budget of any space agency.&lt;/p&gt;&#xA;" OwnerUserId="335" LastEditorUserId="335" LastEditDate="2016-07-02T17:26:25.340" LastActivityDate="2016-07-02T17:26:25.340" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16948" PostTypeId="1" AcceptedAnswerId="19489" CreationDate="2016-07-02T18:30:29.107" Score="11" ViewCount="2251" Body="&lt;p&gt;When you look at space ships, capsules and stations, one thing that is lacking in space is… space. As in: pressurized volume a human can freely move in. Most are tiny tin cans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I was wondering, &lt;strong&gt;what is or was the largest single pressurized volume (&quot;room&quot;) in space?&lt;/strong&gt; By this I mean the undivided volume an astronaut/cosmonaut could move around in freely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far, I have found these candidates via Wikipedia, but the data seems incomplete/imprecise sometimes.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The prime candidate is the &lt;em&gt;Orbital Workshop&lt;/em&gt; module of &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#Dry_workshop&quot; rel=&quot;noreferrer&quot;&gt;Skylab&lt;/a&gt;&lt;/em&gt;. Wikipedia lists a habitable volume of 302 m³ (diameter 6.6 m, length 14.7 m). But it seems this was divided up, so the actual volume a human could move around in freely was smaller. How much?&lt;/li&gt;&#xA;&lt;li&gt;It seems no early russian space station came even close.&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;Mir&lt;/em&gt; was a large station by pressurized volume (around 350 m³), but it was divided into small modules and it was very cramped.&lt;/li&gt;&#xA;&lt;li&gt;The &lt;em&gt;ISS&lt;/em&gt; is the largest object ever put in space (AFAIK) and &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia lists&lt;/a&gt; a pressurized volume of 931.57 m³ as of 2016-05-28. But again, it's divided into smaller modules.&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It seems the largest &quot;room&quot; on the ISS is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kibo_(ISS_module)&quot; rel=&quot;noreferrer&quot;&gt;Kibō module&lt;/a&gt; with a pressurized volume of about 155 m³ (diameter 4.39 m, length 11.19 m).&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So what was the volume an astronaut was able to move around in freely on Skylab? Was it the largest volume, is Kibō's volume actually large or did I miss a station/vehicle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Sidenote: I was thinking about the &lt;em&gt;Space Shuttle&lt;/em&gt; at first, but while its cargo bay had a volume of about 260 m³, it wasn't pressurized.)&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2019-03-15T20:54:10.483" Title="Largest inhabitable volume in space" Tags="|spacecraft|history|space-station|life|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16949" PostTypeId="1" CreationDate="2016-07-02T19:16:29.040" Score="0" ViewCount="111" Body="&lt;p&gt;You could think perhaps about sunlight. But a while ago I've read something about astronauts missing some other 'strange' radiation. I forgot what is exactly was. But could it be for example a kind of radiation of the magnetic field of the earth or perhaps a kind of radiation/vibrations of the earth. I thought they really needed that kind of radiation and therefor it was mechanically simulated in their (Russian?) spaceship.&lt;/p&gt;&#xA;" OwnerUserId="13303" LastEditorUserId="4660" LastEditDate="2016-07-04T14:19:48.833" LastActivityDate="2016-07-04T14:19:48.833" Title="Do astronauts also have a lack of certain radiation which they had on Earth but not in space?" Tags="|crewed-spaceflight|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16950" PostTypeId="2" ParentId="4026" CreationDate="2016-07-02T19:34:04.833" Score="11" Body="&lt;p&gt;Short story: your body will swell if not restricted by a suit, so the image does not look unlikely to me (Disclaimer: I'm basing this simply on what I've read about this topic so far, I don't know much about medicine.)&lt;/p&gt;&#xA;&lt;p&gt;There was one fatal accident in space, &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_11&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 11&lt;/a&gt;, which gives us an indication what happens. The three cosmonauts died when explosive bolts fired simultaneous instead of sequential while separating the orbital module and descent module, causing a pressure valve to open while they were still in space.&lt;/p&gt;&#xA;&lt;p&gt;Relevant parts of the Wikipedia article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On opening the hatch, they found all three men in their couches, motionless, with dark-blue patches on their faces and trails of blood from their noses and ears.&lt;/p&gt;&#xA;&lt;p&gt;…&lt;/p&gt;&#xA;&lt;p&gt;An extensive investigation was conducted to study all components and systems of Soyuz 11 that could have caused the accident, although doctors quickly concluded that the cosmonauts had died of asphyxiation.&lt;/p&gt;&#xA;&lt;p&gt;…&lt;/p&gt;&#xA;&lt;p&gt;The autopsies took place at Burdenko Military Hospital and found that the cause of death proper for the cosmonauts was hemorrhaging of the blood vessels in the brain, with lesser amounts of bleeding under their skin, in the inner ear, and in the nasal cavity, all of which occurred as exposure to a vacuum environment caused the oxygen and nitrogen in their bloodstreams to bubble and rupture vessels. Their blood was also found to contain heavy concentrations of lactic acid, a sign of extreme physiologic stress. Although they could have remained conscious for almost a minute after decompression began, less than 20 seconds would have passed before the effects of oxygen starvation made it impossible for them to function.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.todayifoundout.com/index.php/2012/06/you-can-survive-being-exposed-to-the-near-vacuum-of-space-for-about-90-seconds-with-no-longterm-damage/&quot; rel=&quot;noreferrer&quot;&gt;the article &lt;em&gt;&amp;quot;You Can Survive Being Exposed To The Near Vacuum Of Space For About 90 Seconds With No Longterm Damage&amp;quot;&lt;/em&gt;&lt;/a&gt; your body will swell if the swelling isn't restricted by a suit. There were lots of tests done with animals (dogs) to study the effects of vaccum. From the article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Finally, the dogs’ bodies themselves swelled to nearly twice their normal size, so that they looked like “an inflated goatskin bag”.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660005052.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA study about these dogs&lt;/a&gt; mention water vapor effects and gas expansion as the causes for the swelling.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Joseph_Kittinger&quot; rel=&quot;noreferrer&quot;&gt;Joseph Kittinger&lt;/a&gt; made a high altitude jump at about 31km:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The pressurization for his right glove malfunctioned during the ascent and his right hand swelled to twice its normal size.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This incident is also mentioned in the &lt;em&gt;&amp;quot;You Can Survive …&amp;quot;&lt;/em&gt; article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;From my previous experiences, I know that the hand will swell, lose most of its circulation, and cause extreme pain…. I decide to continue the ascent, without notifying ground control of my difficulty… Circulation has almost stopped in my unpressurized right hand, which feels stiff and painful… [Upon landing] Dick looks at the swollen hand with concern. Three hours later the swelling disappeared with no ill effect.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-10-22T10:29:51.757" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="16951" PostTypeId="2" ParentId="16924" CreationDate="2016-07-02T19:37:05.707" Score="12" Body="&lt;p&gt;Basing on two papers I had managed to find:  &lt;a href=&quot;http://dsspropulsion.com/wp-content/uploads/2016/02/ESP_Safe_Micro_To_Macro-Website.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Electrical Solid Propellants:&#xA;A Safe, Micro to Macro Propulsion Technology&lt;/a&gt; and &lt;a href=&quot;http://campus.mst.edu/aplab/index_files/AIAA-2015-4185.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Plasma Plume Characterization of Electric Solid Propellant Pulsed Microthrusters&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;First, the concept isn't new; it's an incremental research on &lt;a href=&quot;https://en.wikipedia.org/wiki/Pulsed_plasma_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pulsed Plasma Thrusters&lt;/a&gt; which were used first on soviet Zond 2 and Zond 3, starting in 1964. In Pulsed Plasma Thrusters, all the combustion energy comes from electric discharge, which evaporates a small portion of inert (non-combustible) solid propellant (usually teflon), the process producing a &amp;quot;puff&amp;quot; of charged plasma (moving at relatively low speed), which is subsequently propelled through ion engine principles.&lt;/p&gt;&#xA;&lt;p&gt;The Electric Solid Propellant (ESP) does away with the ion engine part, but instead uses an &lt;em&gt;almost&lt;/em&gt; combustible (just barely self-extinguishing) propellant which evaporates rapidly enough to produce a reasonable exhaust velocity. It also replaces moving propellant stick and the fixed &amp;quot;spark plug&amp;quot; electrode with a non-movable structure of electrodes, conductive (but resistive enough to create plasma) propellant and ablating insulator, ablation of which (during combustion of the propellant) exposes more of the electrode, allowing the electric pulse to travel with the front of the propellant.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sF9Zs.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sF9Zs.png&quot; alt=&quot;structure, as described above; bisection: concentrically - inner electrode (tube), ESP, insulator, outer electrode.&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There is a number of different ESPs, varying in properties&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ASPEN - an early formulation; response time of order of 0.1s, may be difficult to extinguish after achieving higher pressures&lt;/li&gt;&#xA;&lt;li&gt;HIPEP - minimum smoke, non-metalized, good burning rate and ignitability properties, wide temperature range&lt;/li&gt;&#xA;&lt;li&gt;ESP ANAV - high aluminum content, high insensitivity to hazards (flame, spark, impact, friction), can self-sustain after ignition (though electricity still regulates burn rate).&lt;/li&gt;&#xA;&lt;li&gt;BADB - similarly, can self-sustain and offer better performance, but require additional processing to passivate surface active characteristics to&#xA;retain acceptable storage and service life.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Specific impulse in all these is on par, or somewhat below that found in classic SRBs - of order of 200-230s. The primary differences are in smoke signature, environmental vulnerability and ignitability characteristics.&lt;/p&gt;&#xA;&lt;p&gt;Thanks to the plastisol process, they can be prepared with standard lab equipment and just &amp;quot;cast&amp;quot;  like resins and the likes - simply poured into the prepared thruster, in room temperature. &amp;quot;Curing&amp;quot; occurs in temperatures of order of 35C over 7 days. This way, for example, microthruster modules are prepared. Such modules can be installed directly in nanosatellites.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bkpHw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bkpHw.png&quot; alt=&quot;single microthrusters, and six-microthruster modules&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The microthrusters like pictured above can fire for 1.5s, providing 5-10 millinewtons of thrust (x6 in the sextuple modules). The microthrusters can be activated in very short pulses - of order of 0.02s or less, to provide a fine regulation of impulse.&lt;/p&gt;&#xA;&lt;p&gt;Of course the propellant can also be used in larger assemblies.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/INvHB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/INvHB.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Currently, a &lt;em&gt;test article&lt;/em&gt; microsatellite &amp;quot;SpinSat&amp;quot; utilizing the microthrusters was launched from ISS on 28th November 2014. The results have &lt;a href=&quot;https://space.stackexchange.com/a/16946/103&quot;&gt;not been published yet&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;30-12-2022 edit: Integza has released a video where he talks with creators of ESP and experiments with it:&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/eHRyv7ARb5M?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2022-12-30T13:46:38.637" LastActivityDate="2022-12-30T13:46:38.637" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="16952" PostTypeId="1" AcceptedAnswerId="30820" CreationDate="2016-07-03T00:11:24.653" Score="5" ViewCount="172" Body="&lt;p&gt;I was confused by the displayed launch date for &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-4#SPINSAT&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpinSat&lt;/a&gt; which was carried into space aboard &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-4&quot; rel=&quot;nofollow noreferrer&quot;&gt;CRS-4&lt;/a&gt; in September 2014, since it is listed as November 20, 1998. It &lt;a href=&quot;https://space.stackexchange.com/questions/16944/#comment43217_16946&quot;&gt;was explained&lt;/a&gt; that the date refers to the launch date of &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Zarya&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zarya&lt;/a&gt; - the first module of the ISS, since SpinSat was deployed into space from the ISS (via the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Kib.C5.8D&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kibo module&lt;/a&gt; airlock.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at CRS-4, I see &lt;a href=&quot;http://www.n2yo.com/satellite/?s=40210&quot; rel=&quot;nofollow noreferrer&quot;&gt;40210&lt;/a&gt; which shows a launch date of September 21, 2012, and &lt;a href=&quot;http://www.n2yo.com/satellite/?s=40211&quot; rel=&quot;nofollow noreferrer&quot;&gt;40211&lt;/a&gt; showing a launch date of September 21, 2014. Screenshots below. 40211 seems to be a debris object, and the date is the actual launch date of CRS-4.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; However, why is the launch date of 40210 shown as September 21, &lt;strong&gt;2012&lt;/strong&gt;, and not either September 21, 2014 (CRS-4 actual launch) or November 20, 1998 (ISS &quot;launch date&quot;)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the system for applying launch dates to objects involved in docking maneuvers, transfers, or even captures and re-deploys described somewhere? In the future those could become &lt;a href=&quot;https://space.stackexchange.com/q/16840/12102&quot;&gt;more common&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I could have followed CRS-4 and SpinSat for the past few years, would their displayed launch dates change as their situation changed? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screenshots shown below are from &lt;a href=&quot;http://www.n2yo.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.n2yo.com/&lt;/a&gt; circa July 2016:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/deyZa.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/deyZa.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/jvUpX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jvUpX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5JkIb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5JkIb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-09-19T07:01:04.697" LastActivityDate="2018-09-20T15:43:35.280" Title="What is the convention for &quot;launch date&quot; when dockings and transfers are considered?" Tags="|artificial-satellite|docking|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="16953" PostTypeId="1" AcceptedAnswerId="17431" CreationDate="2016-07-03T00:30:14.250" Score="1" ViewCount="459" Body="&lt;p&gt;The page &lt;a href=&quot;http://www.n2yo.com/satellites/?c=2&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.n2yo.com/satellites/?c=2&lt;/a&gt; which lists &quot;&lt;strong&gt;ISS SATELLITES&lt;/strong&gt;&quot; includes &lt;a href=&quot;https://en.wikipedia.org/wiki/Tiangong-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tiangong 1&lt;/a&gt;. Since it looks to me like all of the other objects are associated with ISS, and Tiangong 1 is &lt;em&gt;a different&lt;/em&gt; space station, is this just an oversight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this group is related to all space stations, then &lt;strong&gt;ISS&lt;/strong&gt; is a misnomer. If it's really for ISS-related objects, then Tiangong 1 is incorrectly included. Or maybe there is something else I don't understand? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;edit:&lt;/em&gt; the answer to the last question is &quot;indeed there is.&quot; Why does &lt;strong&gt;ISS (ZVEZDA)&lt;/strong&gt; have a different orbital period than &lt;strong&gt;SPACE STATION&lt;/strong&gt;?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EO5pX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EO5pX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-19T01:41:47.523" LastActivityDate="2017-07-19T01:41:47.523" Title="Why does N2YO list Tiangong 1 as an ISS satellite? Is it just a mistake? yes/no?" Tags="|iss|artificial-satellite|space-station|tiangong-1|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16954" PostTypeId="2" ParentId="16943" CreationDate="2016-07-03T02:22:35.353" Score="3" Body="&lt;p&gt;Your initial maneuver is changing inclination &lt;strong&gt;and&lt;/strong&gt; slowing down station&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's ok, for staying on the orbit (same altitude), but you are using reactive mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In second case slowing down done by air drag, instead of using reactive mass. So yes, second variant is more efficient then first one, in therms of reactive mass, if you wish to change inclination. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that super cheat, no it's not, it's just friction - sometimes it helps, some times not.&lt;/p&gt;&#xA;" OwnerUserId="3039" LastActivityDate="2016-07-03T02:22:35.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16955" PostTypeId="2" ParentId="16945" CreationDate="2016-07-03T11:57:42.633" Score="3" Body="&lt;p&gt;The trans lunar injection of the Apollo missions took a Δv of about 3000 - 3200 m/s. Let us assume all inclination change is for free (quite reasonable with careful planning and possibly multiple fly-bys) and lowering the orbit to LEO again is free as well due to the possibility of aero-braking. &#xA;Compared to that, the direct inclination change of 23 degrees in LEO takes about 3100 m/s Δv. That means we need about the same Δv for both paths. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither Hubble nor the ISS is in the low 200km LEO of the Apollo missions but significantly above at 600 km and 400 km, respectively. This gives us an advantage of about 200 m/s in favor of the lunar fly-by.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why was the maneuver so successful in case of PAS-22 then? It was already in a GTO with an apogee of more than 36000 km. From there it was much less than 1000 m/s of Δv to get to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing I did not take into account: I don't know if it is actually possible to do the required orbital change using the moon due to orbital geometry.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-07-03T11:57:42.633" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16956" PostTypeId="1" AcceptedAnswerId="16958" CreationDate="2016-07-03T12:02:58.917" Score="2" ViewCount="1006" Body="&lt;p&gt;Halley's Comet comes around every 75 years. From what I can gather &lt;a href=&quot;https://en.wikipedia.org/wiki/Halley%27s_Comet&quot; rel=&quot;nofollow&quot;&gt;on Wikipedia&lt;/a&gt; the comet is not spinning that fast, has a large elliptical orbit, and has been recorded since 240 BC returning on a very regular schedule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why would we NOT use Halley's comet as a probe? We already landed Philae on Churyumov–Gerasimenko going 135,000 km/h. Is it that much of a stretch to land it on Halley going 254,016 km/h? (The Sidereal rotation period is ~4.5 times longer for Halley.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't this rock a super-good candidate for a long-term solar system exploring probe?&lt;/p&gt;&#xA;" OwnerUserId="15946" LastActivityDate="2016-07-03T16:01:53.563" Title="Why not use Halley's Comet as a probe?" Tags="|orbit|probe|technology|comet|solar-system|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16957" PostTypeId="1" CreationDate="2016-07-03T12:30:52.550" Score="6" ViewCount="549" Body="&lt;p&gt;The majority of the Moon's mass is oxygen. With energy it could chemically be liberated from its silicon et cetera compounds and be turned into oxygen gas. Given its low gravity, lack of magnetosphere and vicinity to the Solar wind, could enough oxygen gas be produced on the Moon to keep up with the loss to space, so that a pure oxygen atmosphere breathable to humans could be maintained for some time (thousands or millions of years)? Inert gasses would be a problem, I suppose, because of the lack of nitrogen, carbon and nobel gasses, but oxygen is all that we need. Something like a pure oxygen atmosphere at a fifth, maybe a tenth, of Earth's atmospheric pressure is survivable. Aside from technological, economical and mental sanity reasons, is it also somehow physically impossible? Could an object like the Moon have a thick atmosphere even temporarily? Titan does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would one estimate the kind of energy required to produce enough oxygen gas, as a fraction or multiple of the Solar irradiance on the Moon?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2021-01-03T23:48:13.300" Title="Could the Moon be terraformed by outgassing its oxygen?" Tags="|the-moon|atmosphere|terraforming|oxygen|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16958" PostTypeId="2" ParentId="16956" CreationDate="2016-07-03T15:11:47.607" Score="5" Body="&lt;p&gt;There are a number of reasons: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it's near the Sun (anything less than a few AU) a comet evaporates quickly, so it's not a safe place to be in. If you put the probe in the wrong place, evaporation below it can blast it off the comet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the comet is further out that reduces, of course, but it takes a long time for the comet to become completely dormant. Until it's dormant, any scientific instruments measure the comet's atmosphere and not interplanetary space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's the trajectory. Halley doesn't go anywhere interesting, so there's only a limited amount of science you can do. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are limited by the comet's rotation, which takes the probe out of the Earth's view at least half of the time. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-03T15:11:47.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16959" PostTypeId="2" ParentId="16956" CreationDate="2016-07-03T16:01:53.563" Score="2" Body="&lt;p&gt;While normally landing on a comet does not make sense, because it requires the same energy / delta_v as making the spacecraft enter the same trajectory without a comet, NASA is investigating the &lt;a href=&quot;http://www.space.com/30451-nasa-comet-hitchhiker-concept-mission.html&quot; rel=&quot;nofollow&quot;&gt;&quot;Comet Hitchhiker&quot;&lt;/a&gt; idea - a new space travel concept, where spacecraft grabs the nearby comet or asteroid using its harpoon, gaining kinetic energy (similar to gravity assist).&lt;/p&gt;&#xA;" OwnerUserId="6522" LastActivityDate="2016-07-03T16:01:53.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16960" PostTypeId="2" ParentId="16949" CreationDate="2016-07-03T17:55:33.497" Score="3" Body="&lt;p&gt;Short answer: no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your body uses sunlight (UVB) to synthesize vitamin D, and lack of exposure to sunlight can be a contributing cause of vitamin D deficiency. But this is straightforward to prevent if you simply make sure your astronauts get enough dietary vitamin D.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than the UV in sunlight, ionizing radiation such as alpha, beta, and gamma radiation does exist naturally on earth. The levels vary from place to place. There is some evidence that these low levels of radiation are beneficial on the average. Such an effect is called radiation hormesis. If the effect exists, it is statistical and very small -- much too small to be important on an individual basis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronauts are typically exposed to &lt;em&gt;more&lt;/em&gt; of these types of ionizing radiation than if they had stayed on earth, not less. For astronauts in low-earth orbit, these increases are relatively small, and their health effects, positive or negative, are basically too small to worry about on an individual basis. For astronauts on a hypothetical future mission to Mars, the radiation dose would be extremely large, and this is one of the main technological obstacles to such a mission.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But could it be for example a kind of radiation of the magnetic field of the earth or perhaps a kind of radiation/vibrations of the earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The earth's magnetic field is a static field, so it is not considered a form of radiation. There are known effects of the earth's magnetic field on various organisms, e.g., anaerobic bacteria that use it to avoid going upward where there is oxygen. But magnetic fields are not needed for human health per se -- the earth's field is important because it shields us from radiation.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2016-07-03T18:10:41.743" LastActivityDate="2016-07-03T18:10:41.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16961" PostTypeId="2" ParentId="14301" CreationDate="2016-07-03T20:38:43.933" Score="4" Body="&lt;p&gt;Check out &lt;a href=&quot;https://www.youtube.com/watch?v=bhGydridbEA&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt; in which Astronaut Suni Williams gives a tour of the ISS. Among the items she shows is the TVIS, as already mentioned, but at about 16:30, she shows a different exercise device which appears to be exactly the answer to your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, she explains that the vibration hazard from the TVIS would be to the solar panels (but it would certainly be disruptive to microgravity experiments). One could well imagine that a harmonic resonance from someone cranking away on the bike could easily overwhelm the structures which hold the solar panels together and attach them to the station.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2016-07-03T20:43:58.417" LastActivityDate="2016-07-03T20:43:58.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16962" PostTypeId="1" AcceptedAnswerId="16963" CreationDate="2016-07-03T23:22:13.223" Score="17" ViewCount="4072" Body="&lt;p&gt;I've heard this expression a couple of times before. It seems related to the engines, but I can't find any information on it, except &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_engine_test_facility#Sea_level_tests&quot;&gt;in the context of testing them&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's two videos where it occurs: &lt;a href=&quot;https://youtu.be/xIoRWIgzvbM?t=134&quot;&gt;Atlantis launch&lt;/a&gt;, &lt;a href=&quot;https://youtu.be/O9WjCyWq-iA?t=111&quot;&gt;Columbia launch&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="15949" LastEditorUserId="6944" LastEditDate="2016-07-07T12:43:43.413" LastActivityDate="2016-07-07T12:43:43.413" Title="What does &quot;in the bucket&quot; mean?" Tags="|rockets|terminology|ssme|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16963" PostTypeId="2" ParentId="16962" CreationDate="2016-07-04T00:25:45.727" Score="19" Body="&lt;p&gt;The space shuttle throttled down its main engines from the normal setting of approximately 104% to around 67% as it was passing through the region of max dynamic pressure (&quot;max q&quot;), to make sure that the certified dynamic pressure limit was not exceeded.  Once the threat had passed, the engines throttled back up.  If you plotted throttle level vs time on a graph, the throttle down region looks a bit like a bucket, hence the term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See my answer to &lt;a href=&quot;https://space.stackexchange.com/questions/15456/how-long-does-max-q-last/15460&quot;&gt;this question&lt;/a&gt; for some specific numbers from a particular flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: a graph showing the &quot;bucket&quot;, from &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8SDeO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8SDeO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-07-05T02:46:29.020" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="16964" PostTypeId="1" CreationDate="2016-07-04T13:45:53.573" Score="6" ViewCount="305" Body="&lt;p&gt;According to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Scientific_objectives&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article on Juno&lt;/a&gt;, the data that Juno returns will sift through multiple competing theories for Jupiter's formation.  However the more theories that one has, the more likely one of them will be &quot;confirmed&quot; by chance alone.  Are there any theories that are highly expected to be true based on modelling Jupiter's formation, but previous spacecrafts did not test?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; To back up what I meant by &quot;the more likely one of them will be &quot;confirmed&quot; by chance alone&quot;.  When they were &lt;a href=&quot;https://en.wikipedia.org/wiki/Search_for_the_Higgs_boson#Timeline_of_experimental_evidence&quot; rel=&quot;nofollow&quot;&gt;searching for the Higgs boson&lt;/a&gt;, they had to correct for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Look-elsewhere_effect&quot; rel=&quot;nofollow&quot;&gt;look-elsewhere effect&lt;/a&gt;.  They had to search a specific predicted &lt;a href=&quot;http://cms.web.cern.ch/news/should-you-get-excited-your-data-let-look-elsewhere-effect-decide&quot; rel=&quot;nofollow&quot;&gt;bump in the data&lt;/a&gt;, but they had to look everywhere for that bump because they did not know where it would be.  There was a possibility that said bump would occur by chance alone, and they had to correct for that.  As explained by &lt;a href=&quot;http://cms.web.cern.ch/news/should-you-get-excited-your-data-let-look-elsewhere-effect-decide&quot; rel=&quot;nofollow&quot;&gt;cms&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The likelihood of finding something in the entire region you probed is&#xA;  greater than it would be if you had stated beforehand where the signal&#xA;  would be.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So I am asking is there one leading hypothesis that we are &quot;stating beforehand&quot;.  If not, then a follow up question would be how are they counteracting the &lt;a href=&quot;https://en.wikipedia.org/wiki/Multiple_comparisons_problem&quot; rel=&quot;nofollow&quot;&gt;multiple comparison problem&lt;/a&gt;?  If they cannot counteract it, then: if we model the measurements as random variables with mean and variance values based on what we already know, what is the probability that none of these models will be validated?  (Note I am willing to ask these follow up questions as separate questions).&lt;/p&gt;&#xA;" OwnerUserId="15953" LastEditorUserId="15953" LastEditDate="2016-07-05T12:07:04.547" LastActivityDate="2016-07-15T19:57:59.270" Title="What theories is Juno expected to confirm?" Tags="|jupiter|juno|prediction|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="16966" PostTypeId="1" CreationDate="2016-07-04T15:01:27.440" Score="4" ViewCount="279" Body="&lt;p&gt;Constraining exploration of outer bodies to flybys (such as New Horizons) seems unfortunate in terms of the amount of science that can be done per mission. Using a slower planetary approach has obvious disadvantages and transfering momentum to the body (or a moon) might be impractical, especially for bodies where aerobraking is not an option. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Richochet braking seems likely to be even less practical. Even if a trajectory could be planned to bounce off two moons and the main body and the bouncing could be adequately controlled, the amount of padding required would seem almost certain to be more massive than the fuel and rocket engine for a more conventional insertion. While some of the padding mass might be dual purpose (e.g., small balloons might burst to absorb impact energy spreading microprobes, dyes, or reactive chemicals across the surface), it is hard to imagine such justify much of the mass cost.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously &lt;em&gt;in very abstract theory&lt;/em&gt; a probe's momentum could be transferred from one component to another such that one part of the probe might be inserted into orbit while the other part is flung even faster into deeper space (which could be an attractive side effect). However, I cannot think of any mechanism that seems like it might be even close to practical (compared to alternatives that add expensive fuel/rocket engine mass).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rail gun might be able to shoot one probe away from another and be useful for shooting objects at the body from orbit, but the amount of momentum transfer would seem to be far too small for a practical mass cost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise using a variation on gravitational slingshotting (inspired by how a binary star system can have one star fall into a black hole while the other is accelerated away) seems unlikely to be practical. (A modest-length tether might serve a dual purpose as an antenna, justifying some of its mass, but the necessary length and strength required for momentum transfer. Even if there were components at the midpoint of the tether that could be benefit from controlled distance from the primary probes — both the one flung off and the one inserted into orbit — it seems unlikely that any benefits of controlled distance would sufficiently justify the required mass.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Tethering to a component, which could still do a lot of science, that impacts the body, or a moon, could compensate for the issue of relatively weak gravity but aside from requiring even greater precision in approach the required strength of the tether seems certain to require too much mass even for a spring-like tether that breaks off at the orbiting component.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any practical way to use momentum transfer to allow fast approach (for lower arrival delay) and orbital insertion with reasonable mass costs?&lt;/p&gt;&#xA;" OwnerDisplayName="user56" LastEditorUserId="12102" LastEditDate="2019-08-26T13:17:56.497" LastActivityDate="2019-08-27T09:20:45.427" Title="Could momentum transfer be practical (e.g., for orbital insertion at Pluto)?" Tags="|orbital-mechanics|probe|gravity-assist|design-alternative|feasibility|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16967" PostTypeId="2" ParentId="16851" CreationDate="2016-07-04T15:06:36.293" Score="8" Body="&lt;p&gt;If we have a pressure shock front travelling very fast through the material it is an explosion. The reaction inside the explosive is started by the sudden pressure rise, not by a temperature rise. But when the solid rocket fuel burns, we have a nearly constant pressure inside the rocket and no travelling shock front. The thermal conductivity of the fuel is much slower, the reaction in a deeper level of the propellant only starts when the temperature is high enough there. The outer layers of the propellant stay cool and protect the walls of the rocket from high temperatures. If the solid rocket is reuseable, the walls should not be damaged in the last seconds of the burn when the reaction zone comes close to the walls.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-07-04T15:06:36.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16968" PostTypeId="2" ParentId="3054" CreationDate="2016-07-04T15:22:20.773" Score="1" Body="&lt;p&gt;If the tank was filled with nitrogen gas before filling liquid hydrogen, we have a problem, liquid hydrogen is so cold that nitrogen gas goes solid. Therefore helium gas is better, it is still a gas at the temperature of liquid hydrogen. Solid nitrogen may damage or destroy the turbo pumps and fuel valves.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-07-04T15:22:20.773" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16969" PostTypeId="2" ParentId="16966" CreationDate="2016-07-04T18:19:53.747" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Obviously in very abstract theory a probe's momentum could be transferred from one component to another such that one part of the probe might be inserted into orbit while the other part is flung even faster into deeper space...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's nothing abstract or theoretical about it; orbital insertion is &lt;em&gt;invariably&lt;/em&gt; done by a probe transferring momentum to part of its mass, throwing it faster ahead to slow the rest of the probe into orbit. That's all a chemical rocket engine or an electromagnetic thruster does. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-07-04T18:19:53.747" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16971" PostTypeId="1" AcceptedAnswerId="16993" CreationDate="2016-07-04T21:00:46.313" Score="7" ViewCount="2355" Body="&lt;p&gt;As indirectly stated in &lt;a href=&quot;https://space.stackexchange.com/a/15460/415&quot;&gt;an answer to How long does Max-Q last?&lt;/a&gt;, during later Space Shuttle launches the main engines were operating at above 100% of their nominal maximum thrust. Specifically from that answer, in STS-118, the SSMEs were run at 104.5% thrust during launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping 100% fixed makes very good sense if you have the value of that 100% as a fixed point of reference and improvements allow for larger values. In this case, engine improvements that increase the maximum attainable thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Which mission was the first to run the engines normally at above 100% of nominal maximum thrust, and what change(s) to the engine made that thrust increase possible?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="25" LastEditDate="2016-07-04T21:19:43.393" LastActivityDate="2021-03-15T14:50:50.007" Title="Which STS mission raised the normal engine throttle above 100%, and what change to the SSME made that possible?" Tags="|launch|space-shuttle|engines|ssme|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16972" PostTypeId="2" ParentId="16971" CreationDate="2016-07-04T21:19:29.533" Score="4" Body="&lt;p&gt;From the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle#Upgrades&quot; rel=&quot;nofollow&quot;&gt;Wikipedia Article&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first 5 missions, using the &quot;FMOF&quot; engines, were certified only to 100%. From STS-5 for some time, 104% was the nominal maximum. This was increased over time to 109%. This happened with Block II in 2001, and was a result of &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#Upgrades&quot; rel=&quot;nofollow&quot;&gt;new high-pressure turbopumps&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-05T21:18:04.220" LastActivityDate="2016-07-05T21:18:04.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16974" PostTypeId="2" ParentId="16964" CreationDate="2016-07-05T00:15:40.173" Score="3" Body="&lt;p&gt;Juno is looking for how Jupiter formed, which would say a lot about how all planetary systems form, and all theories regarding that are on the line. First, determining how much water it has would help determine where and how fast it formed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oxygen is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements&quot; rel=&quot;nofollow&quot;&gt;3rd most abundant element&lt;/a&gt; in the universe, including in our solar system, and its most common form is combined with hydrogen in water. &lt;a href=&quot;http://www2.jpl.nasa.gov/sl9/gll38.html&quot; rel=&quot;nofollow&quot;&gt;Observation to date&lt;/a&gt; has indicated a range of possible values for water on Jupiter:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The latest analyses of data from the Voyager spacecraft that flew by&#xA;  Jupiter in 1979 have suggested a water abundance for the planet of&#xA;  twice the solar level (based on the Sun's oxygen content).&#xA;  Observations of the propagation of atmospheric waves across Jupiter's&#xA;  cloud tops from the Comet Shoemaker-Levy 9 impacts implied that&#xA;  Jupiter might have a water content of ten times the solar level.&#xA;  Actual probe measurements, while subject to scientific debate, suggest&#xA;  a level near that of the Sun. Scientists are left to wonder, &quot;where is&#xA;  the oxygen?,&quot; &quot;where is the water?,&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Galileo probe detected &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/8629016&quot; rel=&quot;nofollow&quot;&gt;levels much lower&lt;/a&gt; than the solar level of oxygen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno will measure the water in the atmosphere below the troposphere using a &lt;a href=&quot;https://www.missionjuno.swri.edu/media-gallery/instruments?show=fig_562e2fa248b496f704cf3f9e&amp;amp;m=181&quot; rel=&quot;nofollow&quot;&gt;microwave radiometer&lt;/a&gt;. The clouds are transparent to microwaves, while water absorbs them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much water is in this layer will help determine how Jupiter formed. &lt;a href=&quot;http://arxiv.org/abs/1204.3887&quot; rel=&quot;nofollow&quot;&gt;One possible explanation&lt;/a&gt; for a low oxygen content is that water ice became depleted in the inner region of the cloud of gas our solar system formed from:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;That Jupiter formed in a disk with C/O ∼1 implies that water ice was&#xA;  heterogeneously distributed over several AU beyond the snow line in&#xA;  the primordial nebula and that the fraction of water contained in icy&#xA;  planetesimals was a strong function of their formation location and&#xA;  time. The Jovian oxygen abundance to be measured by NASA's Juno&#xA;  mission en route to Jupiter will provide a direct and strict test of&#xA;  our predictions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Or put another way &lt;a href=&quot;http://phys.org/news/2016-07-nasa-spaceship-barrels-jupiter-planet.html&quot; rel=&quot;nofollow&quot;&gt;on phys.org&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Steve Levin, Juno project scientist from NASA's Jet Propulsion&#xA;  Laboratory, said water figures are the most important ones that Juno&#xA;  is going to bring back. &quot;If Jupiter formed far from the sun, where it&#xA;  is cold, out of blocks of ice... you would get a different amount of&#xA;  water inside Jupiter than if it formed closer to the sun than it is&#xA;  now.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But there are other possibilities, as one would expect for something as complex as the formation of a planet. There is a good summary in the &lt;a href=&quot;https://www.geochemsoc.org/publications/geochemicalnews/gn142jan10/atmosphericchemistryoftheg/&quot; rel=&quot;nofollow&quot;&gt;final two sections of this article&lt;/a&gt; by Katharina Lodders. The two main ideas are first, the core accretion model in which a solid core forms slowly, and starts capturing gasses by its gravity once it is big enough, and second the disk instability model, in which slight differences in matter density in the protoplanetary disk cause matter to clump and quickly densify into a planet. How much water Jupiter has, and how that compares with the exact amounts it has of other elements heavier than hydrogen and helium, will give hints about which of the two it was, and how other mechanisms were involved, such as later capture of other objects by Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put it together, it is also crucial to know &lt;a href=&quot;http://www.astrobio.net/news-exclusive/how-jupiter-got-big/&quot; rel=&quot;nofollow&quot;&gt;how big the core of Jupiter is&lt;/a&gt;, and whether it is solid or liquid. &lt;a href=&quot;http://www.space.com/6164-jupiter-core-big-thought.html&quot; rel=&quot;nofollow&quot;&gt;Some core accretion models&lt;/a&gt; predict a large solid core of metals, like Earth's but many times the mass of Earth. &lt;a href=&quot;http://arxiv.org/abs/1003.4798&quot; rel=&quot;nofollow&quot;&gt;Others propose&lt;/a&gt; a small solid core could still accrete a planet like Jupiter. Those proposing that instead disk instability is the source of planet formation say that accretion can't be the explanation because study of other star systems indicates the protoplanetary disk doesn't last long enough for accretion to happen. They say Jupiter might not have a solid core at all, or have a much smaller one that gathered over a long time. It is also possible both mechanisms played a part, and there is also a completely different idea in which accretion happened through the medium of &lt;a href=&quot;http://iopscience.iop.org/article/10.1086/421970/pdf&quot; rel=&quot;nofollow&quot;&gt;formation around a carbonaceous core&lt;/a&gt;, a tar ball. So knowing the size and density of the core won't immediately settle the matter, but has to be known to get any further.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno has a &lt;a href=&quot;https://www.missionjuno.swri.edu/media-gallery/instruments?show=fig_562e2fa248b496f704cf3f94&amp;amp;m=180&quot; rel=&quot;nofollow&quot;&gt;magnetometer&lt;/a&gt; to measure Jupiter's magnetic field and a nice big antenna so that &lt;a href=&quot;https://www.missionjuno.swri.edu/media-gallery/instruments?show=fig_562e2fa248b496f704cf3f62&amp;amp;m=175&quot; rel=&quot;nofollow&quot;&gt;Goldstone&lt;/a&gt; can map its gravity field, together this will tell us about Jupiter's core.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-07-05T00:24:42.563" LastActivityDate="2016-07-05T00:24:42.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16975" PostTypeId="2" ParentId="16796" CreationDate="2016-07-05T03:01:58.253" Score="7" Body="&lt;p&gt;I had thought that July the 4th was deliberately chosen, as it had been for a few previous missions where the celestial mechanics ended up with a range of dates that included July the 4th. However it turns out that the optimization happened to end up with the arrival right on July 4th (in the US time zones anyway), so they left it there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.foxnews.com/science/2016/07/01/juno-set-to-arrive-at-jupiter-on-july-4-patriotic-coincidence.html&quot;&gt;an interview with Scott Bolton&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“We did not actually select July 4,” Bolton told FoxNews.com during a&#xA;  June interview.  “Celestial mechanics selected it.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I confirmed this with a JPL mission designer. The arrival date was originally in August. That date was changed, not to make it July the 4th, but rather to separate the Juno &lt;em&gt;launch&lt;/em&gt; date from the MSL launch date as much as possible for logistical reasons. When they moved the launch as much as they could while satisfying all the other constraints, like Earth visibility of JOI, the arrival happened to land on July 4th. There they left it.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-07-05T03:01:58.253" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16976" PostTypeId="1" AcceptedAnswerId="17169" CreationDate="2016-07-05T04:18:12.590" Score="3" ViewCount="839" Body="&lt;p&gt;Juno just passed its closest approach to Jupiter, going 58 km/s because it was so deep in Jupiter's gravity well. Ironically, it did this while retrofiring its engines, to &lt;em&gt;slow down&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That sounds like a record... Is it? They mentioned the speed only in passing during the broadcast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But - it was also retrofiring for 21 minutes in the solar system's second deepest gravity well, much closer in than anything else has ever been. And it had to shed the velocity needed to enter orbit during that time, it couldn't come around again and aerobrake like a probe can at Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So did Juno also brake harder than anything ever made by human hands?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing that is interesting to me is it was retrofiring for 21 minutes to get into the right orbit. How much velocity did it bleed off during that process, since it was an Oberth maneuver so deep in a gravity well? &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="1142" LastEditDate="2016-07-05T14:23:09.850" LastActivityDate="2016-07-12T18:37:18.720" Title="Has Juno now gone faster than - and braked harder than - any other human-made object ever?" Tags="|probe|jupiter|juno|oberth-maneuver|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16977" PostTypeId="1" AcceptedAnswerId="16978" CreationDate="2016-07-05T04:20:52.107" Score="89" ViewCount="23601" Body="&lt;p&gt;As follows from the mission timeline:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Launch&lt;/em&gt; - August 5, 2011&lt;br&gt;&#xA;&lt;em&gt;Deep Space Maneuvers&lt;/em&gt; - August/September 2012&lt;br&gt;&#xA;&lt;em&gt;Earth flyby gravity assist&lt;/em&gt; - October 2013&lt;br&gt;&#xA;&lt;em&gt;Jupiter arrival&lt;/em&gt; - July 2016&lt;br&gt;&#xA;&lt;em&gt;Spacecraft will orbit Jupiter for 20 months&lt;/em&gt; (37 orbits)&lt;br&gt;&#xA;&lt;strong&gt;&lt;em&gt;End of mission (deorbit into Jupiter)&lt;/em&gt;&lt;/strong&gt; - February 2018  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is it necessary to destroy the spacecraft? Would it hurt to keep it on the orbit indefinitely or re-purpose it for later use?&lt;/p&gt;&#xA;" OwnerUserId="15959" LastEditorUserId="6816" LastEditDate="2016-07-05T04:36:32.917" LastActivityDate="2016-07-06T19:41:28.347" Title="Why destroy Juno at the end of the mission?" Tags="|juno|end-of-life|orbital-lifetime|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16978" PostTypeId="2" ParentId="16977" CreationDate="2016-07-05T05:19:17.630" Score="130" Body="&lt;p&gt;In order to assure that it cannot crash into Europa or other possible ocean moons and potentially contaminate them with Earth organisms. Juno is qualified to survive the radiation environment up to the end of its mission. After that it could succumb to the radiation at any time and become uncontrollable. Planetary protection then requires the disposal of the spacecraft while it is still controllable.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-07-05T05:19:17.630" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="16979" PostTypeId="2" ParentId="16977" CreationDate="2016-07-05T05:24:40.857" Score="62" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why is it necessary to destroy the spacecraft?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's because life might well exist on some of Jupiter's moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite the best efforts to assemble the spacecraft in extremely clean conditions, and despite exposing the spacecraft to vacuum and to the Sun's ultraviolet and X-ray radiation, there's a chance that Earth-borne life remains intact on the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose the dead spacecraft collides with one of Jupiter's moons, and in the process contaminates that moon with Earth-borne life. If that moon already bears life of its own, no matter how primitive, our dead and useless satellite may have spread the seeds that eventually kill that life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To this end, thirteen years ago, the Galileo probe met the same fiery death that Juno will one day meet:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The spacecraft was purposely put on a collision course with Jupiter because the onboard propellant was nearly depleted and to eliminate any chance of an unwanted impact between the spacecraft and Jupiter's moon Europa, which Galileo discovered is likely to have a subsurface ocean.&lt;br&gt;&#xA;  &lt;sub&gt;(&lt;a href=&quot;http://www.nasa.gov/vision/universe/solarsystem/galileo_final.html&quot;&gt;http://www.nasa.gov/vision/universe/solarsystem/galileo_final.html&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;The concept of planetary protection is ensconced in international treaty. Article IX of &quot;Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies&quot; (aka the Outer Space Treaty, to which the US and every other space-faring nation are parties) says that parties to the treaty&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;shall pursue studies of outer space, including the moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose.&lt;br&gt;&#xA;  &lt;sub&gt;(&lt;a href=&quot;http://disarmament.un.org/treaties/t/outer_space/text&quot;&gt;http://disarmament.un.org/treaties/t/outer_space/text&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA has an Office of Planetary Protection. Two of its key goals are&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Preserving our ability to study other worlds as they exist in their natural states; and&lt;/li&gt;&#xA;  &lt;li&gt;Avoiding the biological contamination of explored environments that may obscure our ability to find life elsewhere – if it exists.&lt;br&gt;&#xA;  &lt;sub&gt;(&lt;a href=&quot;https://planetaryprotection.arc.nasa.gov/about&quot;&gt;https://planetaryprotection.arc.nasa.gov/about&lt;/a&gt;)&lt;/sub&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In addition the scientific concerns cited above, there are even stronger ethical concerns regarding contaminating other solar system bodies with Earth-borne life. To this end, the COSPAR (Committee on Space Research) Planetary Protection Policy subcommittee held a workshop in 2010 on &quot;&lt;a href=&quot;http://online.liebertpub.com/doi/pdf/10.1089/ast.2012.0891&quot;&gt;Ethical Considerations for Planetary Protection in Space Exploration&lt;/a&gt;.&quot; One of the key recommendations from this workshop was to bolster COSPAR's Planetary Protection Policy with a statement that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Inherent in the conduct of scientific, exploration, and other activities—whether by robotic or human missions—is the need to consider and appropriately protect potential extraterrestrial life.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This workshop was instigated in part by NASA, was chaired by NASA's previous planetary protection officer, and attended by NASA's current planetary protection officer. Additional attendees included representatives from the European Space Agency, the French space agency (CNES), and the German space agency (DLR). &lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-07-05T15:04:21.193" LastActivityDate="2016-07-05T15:04:21.193" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16980" PostTypeId="2" ParentId="16976" CreationDate="2016-07-05T05:33:10.307" Score="3" Body="&lt;p&gt;I would count a positron (an anti-electron) as a human-made object. That would be the fastest, with many, many of them having reached 0.999999999997 times the speed of light in the CERN Large Electron-Positron collider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for how hard they braked, yeah, really hard. When a positron hit an electron going the same speed in the other direction, it, umm, I don't even know how to calculate that deceleration.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-07-05T05:38:44.480" LastActivityDate="2016-07-05T05:38:44.480" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16982" PostTypeId="1" CreationDate="2016-07-05T08:31:31.013" Score="2" ViewCount="546" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Juno&lt;/a&gt; got me thinking again about scaling and maximizing the output of space exploration missions. It seems weird that (with the notable exception of Mars rovers) we (as in humanity) always only start each probe once and then more or less throw away the design. Given the large up-front investment (allegedly 1 billion $ for Juno) and the long latency between start and scientific results, why do we not invest a little more (I assume roughly 10%) to launch a second or third probe of the same design? After all, production and launch cannot be as expensive as design and control. By this reasonign we could visit all big outer planets for less than double the cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The advantages would be clear: A second probe could provide redundancy in the case of an unexpected problem (mixing metric/imperial units, radiation, micrometeorites, etc.) a third probe could be launched to a different target (i.e. Saturn) and while possibly not being ideally designed still yield interesting results and given the different travel times and launch windows make better use of earth-born control infrastructure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this really just a funding thing (i.e. the budget being as tight as possible) or is there a more fundamental reason behind this? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: Just to clarify, I am &lt;strong&gt;not&lt;/strong&gt; talking about mass-production. I am talking about sharing the common cost between a few (maybe up to a dozen) variants of the same probe-design. My question is: How much could be saved when a design is reused and why it seems to be not that much.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="11256" LastEditDate="2016-07-05T10:58:52.757" LastActivityDate="2016-07-05T10:58:52.757" Title="Why do we always only launch one probe?" Tags="|probe|cost|" AnswerCount="1" CommentCount="8" ClosedDate="2016-07-05T11:54:20.530" ContentLicense="CC BY-SA 3.0" />
  <row Id="16983" PostTypeId="2" ParentId="16982" CreationDate="2016-07-05T09:10:23.580" Score="4" Body="&lt;p&gt;We definitely don't throw the design away. We improve upon it, keeping the good, improving the weak, discarding the bad and adding the new and promising.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch is expensive. Control is expensive and must be done on per-item basis, not just per-design. Construction isn't exactly peanuts either. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.rand.org/content/dam/rand/pubs/technical_reports/2008/RAND_TR418.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;cost estimation&lt;/a&gt; for scientific and experimental craft is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kxjmk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kxjmk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/waxCw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/waxCw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;abbreviations used: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;TT&amp;amp;C telemetry tracking and command  &lt;/li&gt;&#xA;&lt;li&gt;ADCS attitude determination and control subsystem  &lt;/li&gt;&#xA;&lt;li&gt;EPS electrical power subsystem  &lt;/li&gt;&#xA;&lt;li&gt;IA&amp;amp;T integration, assembly, and test&lt;/li&gt;&#xA;&lt;li&gt;SE/PM systems engineering/program management&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You'd save up much on SE/PM and some on IA&amp;amp;T. The rest, for two probes is &lt;em&gt;almost&lt;/em&gt; doubled (discounts for doubling the purchases or streamlining construction may or may not apply), and that even doesn't include launch cost (pure doubled). Mission upkeep cost would be reduced, but then... we'd be getting mostly duplicates of the science. Missions are tailored per destination; it makes no sense to send the exact copy of a probe meant for Jupiter to Venus, so the benefits wouldn't be doubled; they'd be increased but not nearly by as much as you seem to think.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, unfortunately, your guess of 10% is way off. The figure of 60-80% is much more likely, and the returns wouldn't entirely justify the expense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And with the budget available, unfortunately, &quot;less than doubling the cost&quot; means in fact &quot;almost halving the budget per craft&quot;. It doesn't mean we're getting twice the science per mission; it means we're getting half the science per probe! Splitting the budget will mean less instruments, less delta-V, less everything per craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - definitely not worth it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There &lt;em&gt;is&lt;/em&gt; an option where that makes sense though: a failed mission. &lt;a href=&quot;https://en.wiktionary.org/wiki/rapid_unplanned_disassembly&quot; rel=&quot;nofollow noreferrer&quot;&gt;RUD&lt;/a&gt; on launchpad, probe critical failure, not reaching the destination. Of course new improvements and fixes to detected problems would apply, but building a very similar probe for re-launch totally makes sense... if you can find money for it.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-05T09:10:23.580" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16984" PostTypeId="1" CreationDate="2016-07-05T09:11:58.830" Score="20" ViewCount="33794" Body="&lt;p&gt;How long would it take for Juno's communication to reach the Earth? In other words: what is the time delay between perception by Juno and perception by NASA researchers?&lt;/p&gt;&#xA;" OwnerUserId="15964" LastEditorUserId="51" LastEditDate="2016-07-08T13:38:05.833" LastActivityDate="2017-07-06T15:37:13.070" Title="How long does it take for a signal to travel between Earth, and Juno at Jupiter?" Tags="|communication|jupiter|juno|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="16985" PostTypeId="2" ParentId="16984" CreationDate="2016-07-05T09:53:07.270" Score="20" Body="&lt;p&gt;Using &lt;a href=&quot;https://eyes.nasa.gov/&quot;&gt;NASA's Eyes&lt;/a&gt; measuring the distance from Jupiter to Earth at this moment (5th Jul 2016, 11:50&amp;nbsp;CEST) is 48 light minutes, 21.39 light seconds, and that would be the time Juno's communications take to reach Earth.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2016-07-05T18:57:34.667" LastActivityDate="2016-07-05T18:57:34.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16986" PostTypeId="2" ParentId="16984" CreationDate="2016-07-05T11:04:02.443" Score="17" Body="&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; based on @Beska's comment, I went back and calculated the difference including light time. In other words, you have to use Jupiter's position roughly &lt;em&gt;48 minutes ago&lt;/em&gt; to state the travel time. Using the &lt;code&gt;observe()&lt;/code&gt; method, which does this, there is a difference of 0.02 seconds. This doesn't really matter, considering that Juno is in a large orbit around Jupiter,  &lt;em&gt;not inside Jupiter&lt;/em&gt; - yet! :)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;from skyfield.api import load&#xA;&#xA;data   = load('de421.bsp')&#xA;ts     = load.timescale()&#xA;t      = ts.utc(2016, 7, 5, 9, 50, 0)&#xA;&#xA;jupiter, earth  = data['Jupiter barycenter'], data['Earth']&#xA;jpos, epos      = jupiter.at(t).position.km, earth.at(t).position.km&#xA;d_instantaneous = np.sqrt(((jpos - epos)**2).sum())&#xA;&#xA;d_light = earth.at(t).observe(jupiter).distance().km  # where WAS Jupiter 48 minutes ago?&#xA;&#xA;clight = 299792.458  # km/s&#xA;&#xA;print &quot;d_instantaneous / c = &quot;, d_instantaneous/clight&#xA;print &quot;d_light / c =         &quot;, d_light/clight&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;gives&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;d_instantaneous / c =  2901.39437989  &#xA;d_light / c =          2901.4127772&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So it looks like &lt;a href=&quot;https://eyes.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;NASA Eyes&lt;/a&gt; is using the simpler method of using  instantaneous positions, and not actually back-calculating the position where Jupiter WAS when the signal would have started.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This is a different way to look at it. This is the variation of distance, light time, and also angular separations from the sun for Earth seen from Jupiter, and Jupiter seen from Earth. When they are too close, radio communication can become difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used Python and the Skyfield package. @SF. gives the right answer, I'm just plotting values as a function of time. The way I chose to do this I didn't used Skyfield's methods for light-time corrections, gravitation or astronomical aberration or atmospheric refraction, which are done with the &lt;code&gt;.observe()&lt;/code&gt; and &lt;code&gt;.apparent()&lt;/code&gt; methods. (Not all of those affect light time significantly anyway.) For this kind of rough plot it's not necessary so I used a short-cut.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The black dot is approximately July 4, 2016 for reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eCgXN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eCgXN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;from skyfield.api import load&#xA;&#xA;data = load('de421.bsp')&#xA;&#xA;years = np.linspace(2015, 2020, 1000)&#xA;&#xA;ts     = load.timescale()&#xA;t      = ts.utc(years, 0, 0)&#xA;&#xA;jupiter = data['Jupiter barycenter']&#xA;earth   = data['Earth']&#xA;sun     = data['sun']&#xA;&#xA;jpos = jupiter.at(t).position.km&#xA;epos = earth.at(t).position.km&#xA;spos = sun.at(t).position.km&#xA;&#xA;d_je = np.sqrt(((jpos-epos)**2).sum(axis=0))&#xA;d_js = np.sqrt(((jpos-spos)**2).sum(axis=0))&#xA;d_es = np.sqrt(((epos-spos)**2).sum(axis=0))&#xA;&#xA;clight = 2.9979E+05  # km/sec speed of light&#xA;&#xA;t_je, t_js, t_es = [thing/clight for thing in [d_je, d_js, d_es]]&#xA;&#xA;# dot products for angles&#xA;sep_js = np.arccos( ((jpos-epos)*(spos-epos)).sum(axis=0) / (d_je*d_es))&#xA;sep_es = np.arccos( ((epos-jpos)*(spos-jpos)).sum(axis=0) / (d_je*d_js))&#xA;&#xA;degs = 180. / np.pi&#xA;ttjly4 = ts.utc(2016, 7, 4).tt&#xA;i = np.argmax(t.tt&amp;gt;ttjly4)  # find the index of the first time point after 4 July 2016&#xA;&#xA;fig = plt.figure()&#xA;&#xA;ax = fig.add_subplot(2, 2, 1)&#xA;ax.plot(years, d_je)&#xA;ax.plot(years[i], d_je[i], 'ok')&#xA;ax.set_title(&quot;Jupiter-Earth distance(km)&quot;)&#xA;ax.ticklabel_format(useOffset=False)&#xA;&#xA;ax = fig.add_subplot(2, 2, 2)&#xA;ax.plot(years, t_je/60.)&#xA;ax.plot(years[i], t_je[i]/60., 'ok')&#xA;ax.set_title(&quot;Jupiter-Earth light-time (minutes)&quot;)&#xA;ax.ticklabel_format(useOffset=False)&#xA;&#xA;ax = fig.add_subplot(2, 2, 3)&#xA;ax.plot(years, degs*sep_js )&#xA;ax.plot(years[i], degs*sep_js[i], 'ok' )&#xA;ax.set_title(&quot;Jupiter-Sun separation @Earth (deg)&quot;)&#xA;ax.ticklabel_format(useOffset=False)&#xA;&#xA;ax = fig.add_subplot(2, 2, 4)&#xA;ax.plot(years, degs*sep_es )&#xA;ax.plot(years[i], degs*sep_es[i], 'ok' )&#xA;ax.set_title(&quot;Earth-Sun separation @Jupiter (deg)&quot;)&#xA;ax.ticklabel_format(useOffset=False)&#xA;&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-07-06T03:39:53.260" LastActivityDate="2016-07-06T03:39:53.260" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="16989" PostTypeId="1" AcceptedAnswerId="17002" CreationDate="2016-07-05T11:53:05.820" Score="22" ViewCount="6103" Body="&lt;p&gt;After a 5 year long journey in space, Juno finally reached and started orbiting Jupiter. How does the probe actually know that it is in orbit, so that it can send confirmation message like 'Welcome to Jupiter!' ?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For the purposes of this question, please treat &quot;know&quot; in the usual colloquial sense that we use for computers. That is, the question could be phrased: &quot;What triggers the Juno probe to send the confirmation message?&quot; Consciousness is not required.&lt;/p&gt;&#xA;" OwnerUserId="15974" LastEditorUserId="58" LastEditDate="2016-07-06T20:25:13.093" LastActivityDate="2016-07-07T14:43:55.870" Title="How does a spacecraft know that it is in orbit?" Tags="|orbital-mechanics|spacex|nasa|orbital-maneuver|juno|" AnswerCount="7" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="16990" PostTypeId="2" ParentId="16989" CreationDate="2016-07-05T11:55:45.417" Score="3" Body="&lt;p&gt;The probe doesn't know, really, unless it is told so by Earth. The way it can be tested is by finding its location from Earth, tracking its path, and determine if it is in fact in orbit. This is explained in great detail in the &lt;a href=&quot;https://solarsystem.nasa.gov/basics/bsf13-1.php&quot; rel=&quot;nofollow&quot;&gt;Basics of Space Flight—Navigation&lt;/a&gt;. The short of it is, they use two techniques: ranging, where a pulse is sent and responded to immediately, giving the distance; and doppler shift, showing the relative speed between the spacecraft and the Earth. These two, with enough measurements, will allow one to determine the orbit of an object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted what the spacecraft was able to do by itself. What it can do is tell the change in velocity, assuming the instruments are correctly calibrated to do so on board. That is what the spacecraft was looking for, or a time of the burn, either of which would allow it to know that it had hit the correct burn. However, if the aim was wrong, the instrumentation wrong, or something similar, the burn might not have been successful. Only measurements from Earth could really let the system know that it had been successful in its capture burn.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6471" LastEditDate="2016-07-06T19:28:20.133" LastActivityDate="2016-07-06T19:28:20.133" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16991" PostTypeId="2" ParentId="16989" CreationDate="2016-07-05T12:00:31.873" Score="21" Body="&lt;p&gt;Using &lt;a href=&quot;https://space.stackexchange.com/questions/12808/how-does-a-spacecraft-know-its-orientation-in-orbit/12809#12809&quot;&gt;attitude determination&lt;/a&gt; devices, (including doppler shift of radio signal from Earth), it can determine&lt;sup&gt;*&lt;/sup&gt; its location and velocity relative to Jupiter, and from that data, and knowing Jupiter mass, trajectory can be calculated. If the trajectory forms a loop around Jupiter - it's an orbit!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;* the actual determination is performed on Earth, Juno just bounces a signal from Earth back, and sends telemetry from the rest of own instruments.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-07-05T20:07:41.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="16992" PostTypeId="2" ParentId="16977" CreationDate="2016-07-05T12:17:25.417" Score="13" Body="&lt;p&gt;You can't re-purpose Juno after its mission as:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Its instruments are designed for a specific mission profile, if you sent it elsewhere it wouldn't be able to produce good science. Its solar panels won't produce enough power if you get much further away from the sun&lt;/li&gt;&#xA;&lt;li&gt;Juno is going to be exposed to a huge amount of radiation which will degrade its instruments, power systems, computers and support hardware. It's not going to be much good for anything else&lt;/li&gt;&#xA;&lt;li&gt;It doesn't have enough fuel to go anywhere. Even if it had instruments that would be useful in other places, and hadn't been bathed in radiation for so long it doesn't have enough delta-v to leave orbit and go somewhere else &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are other good reasons for destroying it other than preventing the contamination of Jupiter's moons:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Even dead spacecraft cost money, if it's still out there someone has to try to keep track of it. It's irresponsible not to do so&lt;/li&gt;&#xA;&lt;li&gt;A dead spacecraft is a hazard. Sure, space is big, but having a piece of junk somewhere in the vicinity of a place you want to go is something you want to avoid&lt;/li&gt;&#xA;&lt;li&gt;The destruction of the probe may actually be an opportunity for science. We may possibly get some data as it nears the atmosphere, if not from the probe itself but from observations of its path&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="79" LastEditDate="2016-07-06T19:41:28.347" LastActivityDate="2016-07-06T19:41:28.347" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="16993" PostTypeId="2" ParentId="16971" CreationDate="2016-07-05T12:18:26.630" Score="20" Body="&lt;p&gt;tl;dr&lt;/p&gt;&#xA;&lt;p&gt;The first mission to run the engines normally at above 100% of nominal maximum thrust: &lt;strong&gt;STS-6&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The changes to the engine made that thrust increase possible: &lt;strong&gt;the 147 design changes implemented between the FMOF and FPL versions of the SSME&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Details&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The SSME had five major versions over the life of the program.  The definition of 100% Rated Power Level (RPL) changed with each one of them. The most important definition of what 100% RPL means, and the only one consistent throughout the program, is that &lt;em&gt;it is the power level / dynamic balance point attained at the end of the Start phase when the engine enters Mainstage and is ready for flight.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;Taxonomy of the SSME&lt;/p&gt;&#xA;&lt;p&gt;There were five major versions of the SSME&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;First Manned Orbital Flight (FMOF)&lt;/p&gt;&#xA;&lt;p&gt;First flight: STS-1 (April 1981)&lt;/p&gt;&#xA;&lt;p&gt;Main Combustion Chamber (MCC) pressure at 100% RPL: 2960 psi&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The FMOF SSME was a de-rated version that was not certified to operate above 100% RPL except in emergency situations (i.e. where doing so might prevent loss of the crew).  (For the first mission, even the emergency power level was 100%, but this was raised to 107% for the second and subsequent flights.) Many components had to be replaced after every flight. Used only on the first 5 missions of Columbia.&lt;/p&gt;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Full Power Level (FPL) aka Phase I&lt;/p&gt;&#xA;&lt;p&gt;First flight: STS-6 (April 1983) (first flight to use 104% power level nominally) (First flight of the Orbiter Challenger)&lt;/p&gt;&#xA;&lt;p&gt;MCC pressure at 100% RPL: 3006 psi&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The FPL SSME was certified to operate at 104% nominally. However, the turbopumps still were not reliable enough to allow nominal operation at 109% RPL (which was then considered desireable for some planned payloads) so this power level was only allowed for emergencies (although this last still made people very nervous, and was lowered to 107% on some missions).  Major changes from the FMOF included redesign of the powerhead, main injector, and turbopumps.&lt;/p&gt;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Phase II SSME&lt;/p&gt;&#xA;&lt;p&gt;First flight: STS-26 (September 1988) (return-to-flight after the 51-L accident)&lt;/p&gt;&#xA;&lt;p&gt;MCC pressure at 100% RPL: 3006 psi&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Phase II engine improvements included both items coming out of the major safety reviews conducted after the 51-L accident and items intended to improve turbopump life - more powerhead redesign, new high pressure turbopump blades and bearings, and a new main engine controller.  However, turbopump life was still unsatisfactory and margins of safety when operating at high power levels were very thin.&lt;/p&gt;&#xA;&lt;ol start=&quot;4&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Block I/IA SSME&lt;/p&gt;&#xA;&lt;p&gt;First flight: STS-73 (October 1995)&lt;/p&gt;&#xA;&lt;p&gt;MCC pressure at 100% RPL: 3032 psi&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The major change for Block I was that NASA was unhappy enough with the turbopumps to get Rocketdyne competitor Pratt and Whitney to design a new set of high pressure turbopumps.  Block I SSMEs included Pratt's newly designed High Pressure Oxidizer Turbopump along with a number of other significant improvements.  Block IA SSMEs had a further main injector modification to improve performance.&lt;/p&gt;&#xA;&lt;ol start=&quot;5&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Block II/IIA SSME&lt;/p&gt;&#xA;&lt;p&gt;First flight: STS-104 (July 2001)&lt;/p&gt;&#xA;&lt;p&gt;MCC pressure at 100% RPL: 2760 psi&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Block II SSMEs added the redesigned Pratt High Pressure Fuel Turbopump and incorporated the Large Throat Main Combustion Chamber (MCC).  Block IIA was the Large Throat MCC alone.  The new MCC allowed a significant drop in MCC pressure (and therefore pressures throughout the system) to provide the same engine performance.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sBKoZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sBKoZ.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;(source - handout I picked up at a Boeing PR event)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Normal Throttle Operations&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;About six seconds before liftoff, the engines would be commanded to start in a staggered sequence. Start phase always took the engines to 100%.  For most of the program, a few seconds after liftoff, they were then commanded to 104%.  Around 30 seconds mission elapsed time, they would throttle down to around 67%, and around 30 seconds later, throttle back up to 104%.  (This is the thrust bucket referred to in &lt;a href=&quot;https://space.stackexchange.com/questions/16962/what-does-in-the-bucket-mean&quot;&gt;this question&lt;/a&gt;) Some missions had two-step thrust buckets where the throttle-down was done in two increments.  Late in powered flight, the vehicle would reach its limit of 3g due to propellant depletion, and the engines would start to throttle down to maintain 3gs.  When the orbital insertion performance was attained, the engines would be commanded to 67% (if they had not already reached this power level), and then commanded off.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Addendum&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Despite all the changes and redesigns, the normal operating point would remain at 104% RPL until the Space Station era, when it was tweaked to 104.5% RPL to slightly increase performance.  No SSME ever ran at a power level higher than 104.5% in flight during the history of the Shuttle program.&lt;/p&gt;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/api/citations/20110001406/downloads/20110001406.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Missions Summary&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/SSME_Pursuit_Improvement.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Main Engine, the relentless pursuit of improvement&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.msfc.nasa.gov/shuttle/documents/space_shuttle_main_engine_30_years_innovation.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Main Engine, 30 years of innovation&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Main Engine Orientation&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Personal notes&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-03-15T14:50:50.007" LastActivityDate="2021-03-15T14:50:50.007" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="16994" PostTypeId="1" CreationDate="2016-07-05T12:42:03.153" Score="1" ViewCount="73" Body="&lt;p&gt;So, assume that at one point in the distant future, humanity has a docking facility/military observation post in orbit around the star of Alpha Centauri B for.....well, military purposes, which could range from protecting trade trajectories from criminals, to being a secret testing site to basically anything military-related. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume you have a supply craft that just finished resupplying this facility in orbit around Alpha Centauri B(which unfortunately has no worlds anywhere, along with the rest of the system and basically is reserved for the armed forces of some political entity either back on Earth or in some distant rogue colony, which in this case, I will assume it belongs to a nation back on Earth), and it needs to return to the Solar System to get the next round of supplies. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, assuming a multi-stage antimatter beam core engine-propelled craft (which contains four stages used to reach a speed of 0.75 c, two of them have already been used and the supplies got to them via an orbital transfer vehicle or OTV docked with the main craft, which is in a no-fly zone), what would the escape velocity need to be for our spacecraft to leave the Alpha Centauri system (specifically Alpha Centauri B) and return to Earth? &lt;/p&gt;&#xA;" OwnerUserId="13960" LastEditorUserId="238" LastEditDate="2016-07-05T13:04:33.287" LastActivityDate="2016-07-05T13:04:33.287" Title="Escape velocity of Alpha Centauri B: What is it?" Tags="|spacecraft|interstellar-travel|stars|" AnswerCount="0" CommentCount="1" ClosedDate="2016-07-05T13:07:21.080" ContentLicense="CC BY-SA 3.0" />
  <row Id="16995" PostTypeId="1" CreationDate="2016-07-05T13:37:21.427" Score="8" ViewCount="413" Body="&lt;p&gt;Today it was announced that &lt;a href=&quot;https://astronomynow.com/2016/07/05/mystery-solved-martian-moons-formed-by-a-giant-impact/&quot; rel=&quot;noreferrer&quot;&gt;Phobos was formed in a giant explosion&lt;/a&gt;, similar to how the Moon was formed. Phobos is actually coming closer to Mars every year, and &lt;a href=&quot;http://www.universetoday.com/14258/phobos-might-only-have-10-million-years-to-live/&quot; rel=&quot;noreferrer&quot;&gt;has an expected 10 million year lifetime&lt;/a&gt;. It seems highly unlikely that Phobos could have been formed billions of years ago, and yet we first see it just a few million years before it's death. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, is this even possible? Namely, if an object orbits just inside the geostationary belt of said object, which is the point at which it will come either closer or further away, could it survive in orbit for billions of years?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="4660" LastEditDate="2016-07-05T14:35:05.850" LastActivityDate="2019-02-04T02:54:38.053" Title="How long can a moon orbit a planet?" Tags="|orbital-mechanics|mars|orbit|phobos|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="16996" PostTypeId="1" AcceptedAnswerId="16997" CreationDate="2016-07-03T12:29:31.670" Score="3" ViewCount="136" Body="&lt;p&gt;Resolution in km/pixel at: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(i) the closest approach and (ii) typical when in close to Jupiter part of the eccentric orbit.&lt;/p&gt;&#xA;" OwnerUserId="15989" OwnerDisplayName="Cam_Aust" LastEditorUserId="58" LastEditDate="2016-07-05T15:00:43.203" LastActivityDate="2016-07-05T15:00:43.203" Title="Juno Mission. How better resolution will the optical images be to previous missions?" Tags="|jupiter|juno|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="16997" PostTypeId="2" ParentId="16996" CreationDate="2016-07-03T16:48:12.200" Score="8" Body="&lt;p&gt;Juno-cam is designed to achieve about &lt;a href=&quot;https://en.wikipedia.org/wiki/JunoCam&quot;&gt;15km/pixel resolution&lt;/a&gt;, at 4300 km, and proportionately less at greater distance. At closest approach it may achieve &lt;a href=&quot;https://twitter.com/NASAJuno/status/750068514560495616&quot;&gt;3km/pixel&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This compares with Galileo that had a best resolution of &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA00720&quot;&gt;30km/pixel&lt;/a&gt;. And &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA01512&quot;&gt;80km/pixel&lt;/a&gt; for Voyager &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference for Juno is that Jupiter is the main focus of the mission, whereas Galileo was more about the moons. Juno has more instrumentation for investigating the clouds, magnetosphere and mass distribution of Jupiter. Juno's polar orbit allows close imagery high latitudes that were never imaged by previous missions.&lt;/p&gt;&#xA;" OwnerUserId="11966" OwnerDisplayName="James Kilfiger" LastActivityDate="2016-07-04T21:46:35.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="16999" PostTypeId="2" ParentId="16976" CreationDate="2016-07-05T14:57:57.983" Score="4" Body="&lt;p&gt;Juno's main engine is fairly small. The craft reduced its speed by about 500 m/s with a 2000 second burn, so average acceleration was only 0.25 m/s&lt;sup&gt;2&lt;/sup&gt; -- about a 40th of a g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Man-made objects brake at hundreds or thousands of gees all the time -- think of a bullet hitting a wall, for instance. Less destructively, ICBM warheads re-enter the atmosphere much more steeply than manned capsules, and might achieve around 100g for brief periods.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For deceleration done in space by rocket thrust alone, rather than atmospheric compression and drag, I'm not sure what the record is. Mercury used solid fuel retrorockets to terminate orbital flight, which would have produced about 1g acceleration if fired together. Most other braking burns are a fraction of a gee.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-06T14:01:43.873" LastActivityDate="2016-07-06T14:01:43.873" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17000" PostTypeId="2" ParentId="16796" CreationDate="2016-07-05T15:26:51.747" Score="3" Body="&lt;p&gt;I believe my comments were a bit misunderstood.  There wasn't a grand conspiracy to place the event on July 4th, and thus inconvenience the people flying the spacecraft.  Rather, when based upon the replan a new arrival date had to be selected, this one was what came out balancing a bunch of different parameters.  And then the navigation team did an outstanding job making it work and getting the spacecraft there on the plan, with almost unheard of levels precision in the different events (DSM, Earth-fly-by and JOI).  So July 4th is a great day for Juno!&lt;/p&gt;&#xA;" OwnerUserId="15994" LastActivityDate="2016-07-05T15:26:51.747" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17001" PostTypeId="1" AcceptedAnswerId="17015" CreationDate="2016-07-05T15:30:44.300" Score="3" ViewCount="267" Body="&lt;p&gt;I am trying to find out if there is any reason other than cost that &lt;a href=&quot;http://en.wikipedia.org/wiki/Breakthrough_Starshot&quot; rel=&quot;nofollow&quot;&gt;Breakthrough Starshot&lt;/a&gt; is only looking at Alpha Centauri. It seems to me that once the laser system is in place that creating more nanocraft to go to other, even more interesting, places would be just a matter of mass production. I also wonder if a space-based laser satellite may be better as it may be able to accelerate the ships more slowly for a longer period of time. Is such a laser satellite using nuclear or solar power not feasible or again just too costly? I tried to find answers on their website, but was unsatisfied.&lt;/p&gt;&#xA;" OwnerUserId="15990" LastEditorUserId="1142" LastEditDate="2016-07-06T00:44:08.087" LastActivityDate="2016-07-29T16:49:05.420" Title="Breakthrough Starshot?" Tags="|interstellar-travel|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17002" PostTypeId="2" ParentId="16989" CreationDate="2016-07-05T15:47:38.760" Score="42" Body="&lt;p&gt;The Juno spacecraft has no means to directly measure and compute that it is in orbit. It did not send any such confirmation message. All it sent was an FSK tone indicating that it had completed the activities it was commanded to do. After the spacecraft turned back to Earth, it transmitted all of the recorded engineering data from the event, providing much more information on how it had completed the activities it was commanded to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft can determine its attitude, but that gives no information whatsoever about its trajectory. The only way that, in theory, Juno could determine it was in orbit on its own would be to use the outreach camera to observe Jupiter periodically after JOI and see its position against reference stars, comparing that to a prediction of what it would see if it were in orbit vs. not. However the difference between those is initially small, so it might take a few hours or days to make the determination. That capability has been developed at JPL, called &lt;a href=&quot;http://www.spaceops2012.org/proceedings/documents/id1267135-Paper-001.pdf&quot; rel=&quot;noreferrer&quot;&gt;AutoNav&lt;/a&gt;, but it is not being used by Juno.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno could infer onboard that it is in orbit by integrating the accelerometer readings. But that is not a direct orbit determination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most immediate way that &lt;em&gt;we&lt;/em&gt; knew Juno was in orbit was the two-way Doppler signature. We knew the trajectory of Juno approaching the planet, and from that what the change in the Doppler shift of the X-band signal along the line of sight to Earth would look like during a successful orbit insertion burn. We could then look for that signature in real time. Lo and behold, there it was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Juno trajectory was designed so that the spacecraft would be in view of Earth through the entire burn. (It's quite common for the spacecraft to go behind the planet as seen from Earth for part of an orbit insertion burn.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two-way Doppler works by sending a very precise frequency derived from an atomic clock on Earth to the spacecraft, and having the spacecraft turn that frequency around with coherent phase, where that frequency is multiplied by an exact rational number (usually 880/749) for the downlink. The signal received on Earth is converted appropriately and beat against the same atomic clock to get the Doppler shift. This can measure the velocity component of the spacecraft relative to Earth along the line of sight to Earth to within a few &lt;em&gt;millimeters&lt;/em&gt; per second. Two-way Doppler can be done with just the carrier, so no data needs to be carried on the link. This allows for Doppler tracking support with relatively low signal strength from the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-07-07T05:22:14.647" LastActivityDate="2016-07-07T05:22:14.647" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17003" PostTypeId="1" CreationDate="2016-07-05T15:48:25.767" Score="10" ViewCount="4628" Body="&lt;p&gt;Why haven't we sent telescopes to other planets? Wouldn't we be able to see further? Aren't we interested in seeing further without having to send a probe that will take years to get to where it's going. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a guess so tell me if I'm right. Is the reason because the lenses and apertures would break at the velocity and extremes?&lt;/p&gt;&#xA;" OwnerUserId="15995" LastEditorUserId="-1" LastEditDate="2016-07-06T14:54:08.547" LastActivityDate="2016-07-06T14:54:08.547" Title="Why haven't telescopes been sent to other planets?" Tags="|spacecraft|interplanetary|space-telescope|" AnswerCount="5" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17004" PostTypeId="2" ParentId="17003" CreationDate="2016-07-05T15:56:23.833" Score="30" Body="&lt;p&gt;We have sent telescopes to other planets, almost all the optical sensors on probes are in fact telescopes so they can focus on a specific area in detail. These sensors are to explore the planets they orbit and their moons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We don't send deep space telescopes to other planets because it's pointless, a few million miles closer to even our closest neighbor star system makes no difference, and it would be much harder to manage the telescope and receive the data from it.&lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-07-05T15:56:23.833" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="17005" PostTypeId="1" AcceptedAnswerId="17011" CreationDate="2016-07-05T16:16:54.223" Score="4" ViewCount="4639" Body="&lt;p&gt;In lots of science fiction movies, spaceships are shown stationary, but don't they have to be in an orbit? Is it possible to not be moving and not orbiting anything, or is this totally impossible?&lt;/p&gt;&#xA;" OwnerUserId="14456" LastEditorUserId="415" LastEditDate="2016-07-06T09:00:37.570" LastActivityDate="2020-03-22T15:27:32.920" Title="Is it possible to be totally stationary in space?" Tags="|orbital-mechanics|orbit|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17006" PostTypeId="2" ParentId="16995" CreationDate="2016-07-05T16:21:07.777" Score="9" Body="&lt;p&gt;There is no generic answer to your question of how long a moon can orbit a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a moon is in a prograde orbit that is lower than the geosynchronous distance, it has negative work done on it due to the tidal bulge its gravity raises on the planet below. If it's farther than geosynchronous, it has positive work done on it. In the first case, its orbit decays to lower radii, and in the second case (which applies to earth's moon) its orbit expands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The strength of the effect depend on the details of the planet's physical characteristics and on those of the moon. For the earth-moon system, the effect is enhanced because the earth has liquid oceans and because our moon is very large for a moon. The effect on the Mars-Phobos system will be weakened by the fact that Mars is a solid body, but strengthened because Phobos is so close to Mars.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2016-07-05T16:21:07.777" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17007" PostTypeId="2" ParentId="17005" CreationDate="2016-07-05T16:45:06.147" Score="7" Body="&lt;p&gt;It all depends on how far out you want to look.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, you could in theory have a velocity of zero relative to the Earth, and continuously fire your engines to counteract its gravitational pull. Then you would be stationary above Earth... &lt;em&gt;but&lt;/em&gt; you'd still be orbiting the sun along with Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could do the same thing with the Sun, but the Sun is rotating around the center of the galaxy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in short, you are always in an orbit around &lt;em&gt;something&lt;/em&gt;, but if you set your frame of reference small enough, sure you can be stationary above a planet or other body. It would just take killing off any orbital velocity, and then continuously firing your engines to counteract gravity.&lt;/p&gt;&#xA;" OwnerUserId="15841" LastActivityDate="2016-07-05T16:45:06.147" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17008" PostTypeId="1" AcceptedAnswerId="17014" CreationDate="2016-07-05T17:07:10.403" Score="18" ViewCount="2354" Body="&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/q/17003/12102&quot;&gt;question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/17004/12102&quot;&gt;answer&lt;/a&gt; about telescopes sent to different planets got me wondering. There are so many solar system bodies that have been photographed by deep space spacecraft, planets, dwarf planet(s?) moons and rings thereof, comets, asteroids... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I always imagine that these instruments look roughly like the 6 inch Maksutov or Cassegrain telesopes you see in telescope/binocular stores, or the backs of astronomy magazines. There are &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Catadioptric_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;mirror lenses&lt;/a&gt;&quot; which are rougly similar instruments that you can buy at camera stores as well. So this size instrument could be called &lt;em&gt;both&lt;/em&gt; a telescope, and a camera with a really long focal length lens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For deep space or planetary probes, spacecraft that are no longer in the immediate vicinity of Earth, Sun-Earth $L_1$ or $L_2$ or the Moon or any of it's Lagrange points (or just say more than 2 million km from Earth), what is the &lt;em&gt;largest aperture&lt;/em&gt; telescope (camera) that has been used for science? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the answer different if you exclude &lt;em&gt;everything&lt;/em&gt; roughly 1 AU from the sun (STEREO, SOHO, DSCOVR, KEPLER, etc.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NxvHK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NxvHK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9dXwN.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9dXwN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-09-01T18:39:10.053" LastActivityDate="2018-09-01T18:39:10.053" Title="What's the largest aperture telescope sent beyond the Earth-Moon system?" Tags="|artificial-satellite|telescope|space-telescope|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="17009" PostTypeId="2" ParentId="16989" CreationDate="2016-07-05T17:18:59.620" Score="4" Body="&lt;p&gt;The&amp;nbsp;duration of the orbital insertion burn was controlled by an accelerometer on the spacecraft. Mission control knew the spacecraft's speed at entry and the desired speed needed for the planned orbit. While the engine is running, the spacecraft measures its deceleration rate over time, and when the total change in speed reaches the necessary figure, it shuts off the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The confirmation message in this case was sent when the probe finished the insertion burn, and mission control is able to confirm the speed via Doppler shift as indicated in other answers here.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="6471" LastEditDate="2016-07-06T19:28:47.617" LastActivityDate="2016-07-06T19:28:47.617" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17011" PostTypeId="2" ParentId="17005" CreationDate="2016-07-05T17:36:38.870" Score="5" Body="&lt;p&gt;It all depends on your frame of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For &quot;stationary in space&quot;, I'm going to assume you mean something like The Fleet from Battlestar Galactica hanging out in the middle of nowhere; like so:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FiB9Z.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FiB9Z.jpg&quot; alt=&quot;I had no idea there was a BSG Online until I did the image search for this.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isn't necessarily a terribly bad depiction. While The Fleet is moving relative to galactic centre, they aren't so much moving amongst themselves, except for minor jiggles that their reaction control systems are more than capable of making up for, and given their hundred-million-year orbit, the stars would appear, just, completely stationary (in the time frames portrayed in the show).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even when you have Sci-Fi vessels in orbit around planets, these orbits tend to be rather lofty; consider for instance Prometheus orbiting Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FyCkf.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FyCkf.png&quot; alt=&quot;Prometheus in orbit of Earth&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not expert when it comes to estimating these things, but I'd put the vessel at about 350~450 km; maybe more. At that altitude, they'd be orbiting once every 90 minutes, which isn't a long time, but over the course of a vignette lasting a few seconds, the Earth could very reasonably appear stationary with relation to the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is not to mention that the features of the Earth are pretty indistinct at any altitude, and frame dragging alone means that portraying an Earth seen out a window (windows being a whole other kettle of fish, of course…) as pretty much stationary isn't such a stretch either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure there are examples out there of Sci-Fi shows where neither of these considerations apply, and you're left with just … pants on head type stuff, but due to short time frames, the depiction of space ships as stationary can absolutely make sense … &lt;strong&gt;in context&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="5898" LastActivityDate="2016-07-05T17:36:38.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17012" PostTypeId="2" ParentId="17008" CreationDate="2016-07-05T17:39:21.400" Score="18" Body="&lt;p&gt;After looking through various mission articles on Wikipedia, the Mars Reconnaissance Orbiter's &lt;a href=&quot;https://en.wikipedia.org/wiki/HiRISE&quot; rel=&quot;nofollow noreferrer&quot;&gt;HiRISE camera&lt;/a&gt; has an aperture of 19.7 inches (50 cm), which Wikipedia claims is &quot;the largest so far of any deep space mission&quot;.  This camera allows it to take extremely detailed pictures of the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1EJos.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1EJos.jpg&quot; alt=&quot;HiRISE Camera&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're looking for other large-diameter telescopes which we have sent beyond the Earth-Moon system, they're most likely going to be mapping cameras.  As was stated in the answer to your previous question, we don't have a lot of interest in sending space telescopes out very far from Earth.  But when trying to make detailed surface maps of planetary bodies, we use telescopic cameras.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another large camera is on NASA's New Horizons mission.  The &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Long-Range_Reconnaissance_Imager_.28LORRI.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Long-Range Reconnaissance Imager (LoRRI)&lt;/a&gt; is 8.20 inches (20.8 cm) across.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="33" LastEditDate="2018-02-12T11:14:52.663" LastActivityDate="2018-02-12T11:14:52.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17013" PostTypeId="2" ParentId="16977" CreationDate="2016-07-05T17:43:25.120" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Juno starts its tour of Jupiter in a 53.5-day orbit. The spacecraft saves fuel by executing a burn that places it in a capture orbit with a 53.5-day orbit instead of going directly for the 14-day orbit that will occur during the mission's primary science collection period. The 14-day science orbit phase will begin after the &lt;strong&gt;final burn of the mission&lt;/strong&gt; for Juno’s main engine on October 19.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;http://www.nasa.gov/feature/juno-enters-orbit-around-jupiter&quot; rel=&quot;nofollow&quot;&gt;NASA&lt;/a&gt;; emphasis added)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After that probe will not have sufficient fuel to again burn main engine and will ultimately dive into Jupiter’s atmosphere where it will be crushed and vaporized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also fact that mission is planned in such a way that probe will dive into Jupiter’s atmosphere because it can be possible that some Earth organisms may still be alive aboard Juno. Scientists and NASA officials don't want to take the chance that such microbes could contaminate Jupiter's ocean-harboring moon Europa — one of the solar system's best bets to host alien life.&lt;/p&gt;&#xA;" OwnerUserId="15977" LastEditorUserId="6816" LastEditDate="2016-07-05T17:56:48.840" LastActivityDate="2016-07-05T17:56:48.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17014" PostTypeId="2" ParentId="17008" CreationDate="2016-07-05T17:58:08.823" Score="11" Body="&lt;p&gt;Voyager's &lt;a href=&quot;https://books.google.nl/books?id=8HIrh-9o2CkC&amp;amp;pg=PA311&amp;amp;lpg=PA311&amp;amp;dq=voyager%20IRIS%20aperture&amp;amp;source=bl&amp;amp;ots=S4tqlwLufL&amp;amp;sig=thwmIdwMsFJ8ZRrWWX5NOgb499k&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwiqyuC-8tzNAhVEQBoKHXOEA8EQ6AEIJTAB#v=onepage&amp;amp;q=voyager%20IRIS%20aperture&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Infrared Interferometer Spectrometer&lt;/a&gt; (IRIS) has an aperture of 0.5 m (19.685&quot;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is not an imaging instrument though (resolution=1 pixel). The large aperture was needed to provide enough sensitivity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eKcPV.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eKcPV.jpg&quot; alt=&quot;IRIS mirror&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we take the question literally, Kepler would qualify with its heliocentric orbit at a distance of 1 AU. It has a &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_%28spacecraft%29#Camera&quot; rel=&quot;noreferrer&quot;&gt;1.4 m primary mirror&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vSLkN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vSLkN.jpg&quot; alt=&quot;Kepler primary&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-07-05T18:10:34.587" LastActivityDate="2016-07-05T18:10:34.587" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17015" PostTypeId="2" ParentId="17001" CreationDate="2016-07-05T19:28:18.270" Score="2" Body="&lt;p&gt;The laser needs to be very powerful. The largest powerplant we have in space today is the ISS, with about 200 kW peak power. Breakthrough Starshot are talking about GW of power. We'd have to ramp up the space industry by a factor of 1000 to make a space-based laser that large feasible. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-05T19:28:18.270" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17016" PostTypeId="2" ParentId="17003" CreationDate="2016-07-05T19:32:44.900" Score="6" Body="&lt;p&gt;You're right: on Earth we spend some significant amount of effort to combine large telescopes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Advantages of a telescope on other planets could be&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;less or even no atmospheric effects&lt;/li&gt;&#xA;&lt;li&gt;greater distance for enhanced resolution&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;On the other hand side, a telescope in space comes with several issues:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;how to carry a large telescope (&gt;30m) into space? A small one would not help that much.&lt;/li&gt;&#xA;&lt;li&gt;how to maintain/repair it?&lt;/li&gt;&#xA;&lt;li&gt;how to power it?&lt;/li&gt;&#xA;&lt;li&gt;how to safely land it? Philae showed that this is not trivial.&lt;/li&gt;&#xA;&lt;li&gt;if safely landed, how to position it?&lt;/li&gt;&#xA;&lt;li&gt;if there's no atmosphere, how to protect it from meteors?&lt;/li&gt;&#xA;&lt;li&gt;communication and data transfer is slow&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As you can see, putting it onto a planet is even worse than keeping it in space.&lt;/p&gt;&#xA;" OwnerUserId="2989" LastEditorUserId="2989" LastEditDate="2016-07-05T20:46:50.343" LastActivityDate="2016-07-05T20:46:50.343" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17017" PostTypeId="2" ParentId="16957" CreationDate="2016-07-05T20:44:16.163" Score="6" Body="&lt;p&gt;The Moon is capable of having an Earth-like atmosphere. Although the escape velocity of the Moon is much smaller than Earth's, it is still around Mach 6.(*)&lt;/p&gt;&#xA;&lt;p&gt;If the Moon had an atmosphere, it would lose it in some tens of millions of years(*). This is why any atmosphere it once had is long gone, but this would be enough for us and our shorter time scale, which is only some thousands of years in the most optimistic sense.&lt;/p&gt;&#xA;&lt;p&gt;Nitrogen is rare on the Moon, but it is not really needed. We don't use it for anything. What matters is the partial pressure of the &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt; in the atmosphere. In a 100% &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt; atmosphere with around 0.2 bar pressure (20% of the air is &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt;) we could breathe without any major problem.&lt;/p&gt;&#xA;&lt;p&gt;But the Moon has a much weaker gravity, it is only around 1/6 or the Earth. Therefore, approximately 6 times more per-area mass of the atmosphere would be required to get the same pressure. This means that around of 1.2 kg of &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt; would be needed over every &lt;span class=&quot;math-container&quot;&gt;$cm^2$&lt;/span&gt; to get the required &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt; pressure (on the Earth, it is around 1 kg).&lt;/p&gt;&#xA;&lt;p&gt;The Moon's radius is also around 1/6 of Earth's, so its surface area is 1/36 of Earth's. The surface area of the Earth is &lt;span class=&quot;math-container&quot;&gt;$5.1*10^8 km^2$&lt;/span&gt;, so the Moon's surface area is &lt;span class=&quot;math-container&quot;&gt;$14*10^6 km^2$&lt;/span&gt;, which is &lt;span class=&quot;math-container&quot;&gt;$1.4*10^7*10^{10}=1.4*10^{17} cm^2$&lt;/span&gt;. So around &lt;span class=&quot;math-container&quot;&gt;$1.6*10^{17}$&lt;/span&gt; kg of &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt; would be required.&lt;/p&gt;&#xA;&lt;p&gt;The lunar soil is mainly composed of &lt;em&gt;regolith&lt;/em&gt;, which is essentially a mix of different metal oxides. It melts around at &lt;span class=&quot;math-container&quot;&gt;$1200K$&lt;/span&gt;, which is essentially lava. On the Earth, a similar substance is produced from volcanoes.&lt;/p&gt;&#xA;&lt;p&gt;Well, most of its components have a much higher melting point (for example, &lt;span class=&quot;math-container&quot;&gt;$Al_2 O_3$&lt;/span&gt;'s is over &lt;span class=&quot;math-container&quot;&gt;$2000K$&lt;/span&gt;), but not all of them. The molten salt mixes have a normally much lower melting point, this is why this around &lt;span class=&quot;math-container&quot;&gt;$1200K$&lt;/span&gt; would be enough.&lt;/p&gt;&#xA;&lt;p&gt;These molten salt mixes are poor electrical conductors, but are conductive conductive to be electrolysable, and a temperature of around &lt;span class=&quot;math-container&quot;&gt;$1000K$&lt;/span&gt; is not unfeasible. Although most of the electric energy would simply heat the lava, a significant part would produce &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt;. The electric effectivity of different electrolysis processes are in the order of 10-60%, so we can now calculate with their geometric mean, which is around 25%.&lt;/p&gt;&#xA;&lt;p&gt;The burning heat of the Al is around 22 &lt;span class=&quot;math-container&quot;&gt;$\frac{MJ}{kg}$&lt;/span&gt;, its molar mass is 27. Thus, burning of 1kg of Al gives 22 MJ of energy, it is 37 mol, which consumes 37*1.5 (Al2O3!), so 55 mol of &lt;span class=&quot;math-container&quot;&gt;$O$&lt;/span&gt;. This 55 mol of oxygen is 888 g.&lt;/p&gt;&#xA;&lt;p&gt;Thus, to produce an Earth-like O2 atmosphere on the Moon, we would need to produce around &lt;span class=&quot;math-container&quot;&gt;$1.8*10^{17} kg$&lt;/span&gt; of Al. This would require around &lt;span class=&quot;math-container&quot;&gt;$4*10^{18} MJ$&lt;/span&gt; of energy, or &lt;span class=&quot;math-container&quot;&gt;$4*10^{24} J$&lt;/span&gt;. Considering an effectivity of 20%, it is around &lt;span class=&quot;math-container&quot;&gt;$2*10^{25} J$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The yearly Al production on the Earth is around 40 million tons. To produce this mass of Al we would need 4.5 million years - on the Earth.&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_power_by_country#Global_deployment_figures&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; article, on Earth there was around 184 TWh of solar energy produced in 2015. This is &lt;span class=&quot;math-container&quot;&gt;$6.62*10^{17} J$&lt;/span&gt;. Thus, to produce the required &lt;span class=&quot;math-container&quot;&gt;$2*10^{25} J$&lt;/span&gt; of energy, we would need around 30 million years.&lt;/p&gt;&#xA;&lt;p&gt;But on the other hand, although this project doesn't seem feasible in our lifetime, perhaps in a better world it would be possible to make automated factories staffed by robots, to build the required number of PV cells and electrolysis chemical factories on the Moon. Actually, if we had enough robots we could do anything, and robots could be also produced by robots.&lt;/p&gt;&#xA;&lt;p&gt;(*) I am very happy to google some references if anyone asks.&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="37047" LastEditDate="2021-01-03T23:48:13.300" LastActivityDate="2021-01-03T23:48:13.300" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="17018" PostTypeId="1" CreationDate="2016-07-05T23:09:48.543" Score="-1" ViewCount="194" Body="&lt;p&gt;Would the Juno program be more cost effective if the spacecraft took a direct flight to Jupiter rather than using gravitational pull from planets, which took nearly 5 years instead of say, 2 years?  Yes, there are booster and other hardare costs, but the program would save 3 years of engineers and scientists costs (sorry guys from bean counters). Was that option considered in detail?&lt;/p&gt;&#xA;" OwnerUserId="16010" LastEditorUserId="51" LastEditDate="2016-07-08T13:35:36.087" LastActivityDate="2016-07-08T13:35:36.087" Title="Juno to Jupiter Budget" Tags="|jupiter|time|cost|juno|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17019" PostTypeId="2" ParentId="17018" CreationDate="2016-07-05T23:13:30.183" Score="3" Body="&lt;p&gt;The probe was doing very little during the period it was cruising, and needed relatively little manpower for supervision. I think we can assume that the scientists waiting for it to arrive were not idle in the intervening time.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-07-05T23:13:30.183" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17020" PostTypeId="2" ParentId="17008" CreationDate="2016-07-05T23:14:53.613" Score="6" Body="&lt;p&gt;Just for reference, here are the largest apertures (visible or almost visible) sent to every planet that I can find. Note that some of them are kind of difficult to quantify. (Sorry for the mixed units...)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury- 15 cm- Mariner 10 (Messenger's largest was a 12cm x 12cm square)&lt;/li&gt;&#xA;&lt;li&gt;Venus- 7 inch from Galileo, but if you don't count that, likely only a few inches, from which spacecraft I can't identify, but likely Venus Express. Will be similar to Mercury. In fact, both Mercury cameras are also Venus candidates.&lt;/li&gt;&#xA;&lt;li&gt;Moon- 8.7 inches&lt;/li&gt;&#xA;&lt;li&gt;Mars- HiRISE, 19.7 inches, the largest sent beyond Earth&lt;/li&gt;&#xA;&lt;li&gt;Jupiter- LORRI from New Horizons&lt;/li&gt;&#xA;&lt;li&gt;Saturn- Probably Cassini ISS camera, can't find it's aperture, but the focal length is 2m.&lt;/li&gt;&#xA;&lt;li&gt;Uranus/ Neptune- Voyager 2 camera, of course. At least 17 cm, the exact value I can't find.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user3049" LastEditDate="2016-07-06T09:28:17.333" LastActivityDate="2016-07-06T09:28:17.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17021" PostTypeId="2" ParentId="17003" CreationDate="2016-07-05T23:24:07.117" Score="6" Body="&lt;p&gt;There was a proposal to &lt;a href=&quot;http://www.space.com/21064-nasa-donated-spy-telescope-mars.html&quot; rel=&quot;noreferrer&quot;&gt;send a space telescope to Mars&lt;/a&gt;, known as MOST. It was turned down. Benefits were stated to include:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;MOST can provide a larger baseline for stereoscopic viewing of targets combined with Earth-based telescopes,&quot; they write in the SALSO presentation. &quot;A greater range of viewing angles for targets in the outer solar system are possible from Mars than from Earth. The inner asteroid belt can be viewed from significantly closer range.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This proposal was eventually nixed for a higher priority idea, &lt;a href=&quot;http://www.space.com/31984-nasa-wfirst-telescope-spy-telescope-progress.html&quot; rel=&quot;noreferrer&quot;&gt;Wide Field Infrared Survey Telescope&lt;/a&gt;. That just goes to show that while there is some interest, there are a lot of limitations to putting a space telescope so far out there. The reasons why this is difficult have been mentioned before.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-05T23:24:07.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17023" PostTypeId="1" AcceptedAnswerId="17024" CreationDate="2016-07-06T00:03:41.573" Score="0" ViewCount="674" Body="&lt;p&gt;In Our communication systems we have limitations like signal strength to achieve communication between two points in the same country we have to build towers / etc .. my question is how does NASA communicate with spacecrafts which are billions of miles away ?  &lt;/p&gt;&#xA;" OwnerUserId="16013" LastActivityDate="2016-07-06T00:16:47.270" Title="How does NASA communicate to spacecrafts?" Tags="|spacecraft|communication|radio-communication|" AnswerCount="1" CommentCount="1" ClosedDate="2016-07-06T06:47:31.530" ContentLicense="CC BY-SA 3.0" />
  <row Id="17024" PostTypeId="2" ParentId="17023" CreationDate="2016-07-06T00:16:47.270" Score="0" Body="&lt;p&gt;Fundamentally, they use really big, steerable satellite dishes that can point directly at the location of the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To add more detail to that, they use the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow&quot;&gt;Deep Space Network&lt;/a&gt;, which is a geographically dispersed set of sites where the aforementioned satellite dishes are located. The sites were selected to be spread out such that they can cover the entire sky constantly as the earth rotates (so deep space satellites always have something to talk to if needed) and located in places where the terrain blocks terrestrial radio interference (because the signal strength from satellites is faint by the time it gets back to earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each of the DSN sites have multiple dishes of varying sizes to enable communication with different satellites simultaneously. To show what each site/dish is communicating with NASA provides the &lt;a href=&quot;http://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow&quot;&gt;DSN Now&lt;/a&gt; page. As I'm posting this the Goldstone, USA site is communicating with the Juno spacecraft using antenna DSS 14 at a distance of 870 million km.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2016-07-06T00:16:47.270" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17025" PostTypeId="2" ParentId="16989" CreationDate="2016-07-06T00:18:41.640" Score="8" Body="&lt;p&gt;First a clarification. If one insists that &quot;knowing&quot; requires self awareness and intelligence, then the Juno spacecraft of course doesn't &quot;know&quot; anything. Rather than getting hung up on the silliness of what &quot;knowing&quot; means, it's better to look for an alternative way of answering the question. That alternative: How sophisticated is Juno's onboard computer software? Did the spacecraft software know, in some limited way, that it had indeed achieved orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;I'm making this community wiki because this is not an answer. The correct answer is very hard to find. (As far as I can tell, none of the proferred answers are correct.) Both JPL and Lockheed Martin release very little, if any, technical details on their vehicles' inner workings. The vehicle's guidance, navigation, and control systems and mission managers are apparently stamped as ITAR restricted and as proprietary. I'll look at two cases, one in which the vehicle is not aware that it is in orbit about Jupiter and the other in which it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way that the vehicle could have performed the orbit insertion burn autonomously would have been to orient the vehicle in a predetermined orientation, start the rocket, and stop when delta V to go (the difference between a predetermined desired delta V and accumulated sensed delta V) reached zero. Suppose both the predetermined orientation and predetermined desired delta V had been a part of a command sequence sent to the spacecraft from Earth. If this is the case, the vehicle did not know it was in orbit. All it knew was that the burn was complete. The software to do this is very simple. If simple software is good enough to do the job, simple is best.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good deal more sophistication might be needed if that simple approach would not suffice. For example, the spacecraft might need more sophisticated guidance, navigation, and control software, and more complex mission management software. (It apparently has rather extensive failure detection, isolation, and recovery software). In this more sophisticated version, the mission manager may well have a on-orbit mode, the transition to which is triggered by the GNC software assessing that the desired orbit has been achieved. While this is not self-aware software, it is software that is aware that the vehicle is indeed in orbit about Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Note that Juno is controlled by a 200 megahertz RAD750 computer with 128 megabytes of memory and 256 megabytes of flash storage. That's the equivalent of a low cost personal computer from 1999. There's not much room in that rather limited system for very much sophistication.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-07-06T00:18:41.640" CommentCount="8" CommunityOwnedDate="2016-07-06T00:18:41.640" ContentLicense="CC BY-SA 3.0" />
  <row Id="17026" PostTypeId="2" ParentId="17018" CreationDate="2016-07-06T01:20:12.513" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Was that option considered in detail?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Almost certainly not. This question is based on the false assumption that staffing levels remain more or less constant across all phases of flight. This is not the case. Staffing is very light during passive phases such as coast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Requirements for communications mandate a large antenna. No matter how good we become at miniaturization, a large antenna will always be required; it's a matter of physics. There are miniaturization limits on the scientific instruments. Limitations of launch vehicles restrict how much energy can be applied to a payload. This energy translates to velocity, and the larger the payload mass, the smaller the velocity. Sophisticated missions such as Juno, along with most of missions to the outer planets, would not be possible without using gravity assists.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-07-06T01:20:12.513" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17027" PostTypeId="2" ParentId="16984" CreationDate="2016-07-06T01:37:34.510" Score="0" Body="&lt;p&gt;The space.com folks assert the closest earth and jupiter can get in their elliptical paths is 588 million kilometers. My physics text states the speed of light is 1002 million kilometers per hour.  Division yields decimal hours, then multiplication to get minutes: approximately 35 minutes.   Since the actual distance between Jupiter and the Earth the day after Juno arrived is probably a bit more than the minimum I started with, so radio connect is a minimum 35 minutes.&lt;/p&gt;&#xA;" OwnerUserId="16016" LastActivityDate="2016-07-06T01:37:34.510" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17028" PostTypeId="1" CreationDate="2016-07-06T02:00:38.433" Score="5" ViewCount="206" Body="&lt;p&gt;Earth has (at least) 2 radiation belts called the Van Allen belts. Jupiter has similar radiation belt(s). Are they still called Van Allen belts around other planets?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Edit: The reason I ask is &lt;a href=&quot;http://science.howstuffworks.com/dictionary/astronomy-terms/van-allen-radiation-belts-info.htm&quot; rel=&quot;nofollow&quot;&gt;every&lt;/a&gt; document I &lt;a href=&quot;http://www.nasa.gov/content/goddard/van-allen-probes-spot-impenetrable-barrier-in-space&quot; rel=&quot;nofollow&quot;&gt;find&lt;/a&gt; on the &lt;a href=&quot;http://vanallenprobes.jhuapl.edu/newscenter/glossary.php&quot; rel=&quot;nofollow&quot;&gt;VA belts&lt;/a&gt;, they're &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2013/28feb_thirdbelt/&quot; rel=&quot;nofollow&quot;&gt;talking&lt;/a&gt; about Earth &lt;a href=&quot;http://adsabs.harvard.edu/abs/2005Natur.437..227H&quot; rel=&quot;nofollow&quot;&gt;specifically&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="723" LastEditDate="2016-07-06T02:11:53.957" LastActivityDate="2016-07-06T02:14:55.727" Title="Is it still called a Van Allen radiation belt if it's around another planet?" Tags="|planetary-science|radiation|terminology|jupiter|van-allen-belt|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17029" PostTypeId="2" ParentId="17028" CreationDate="2016-07-06T02:14:55.727" Score="3" Body="&lt;p&gt;According to the Wikipedia article on radiation belts, only Earth's radiation belts are referred to as Van Allen belts:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The discovery of the belts is credited to James Van Allen, and as a result the Earth's [radiation] belts are known as the Van Allen belts. ... The term Van Allen belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Since Van Allen did not discover the belts around other planets, he doesn't get his name on them.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-07-06T02:14:55.727" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17030" PostTypeId="1" AcceptedAnswerId="17032" CreationDate="2016-07-06T04:19:40.700" Score="3" ViewCount="371" Body="&lt;p&gt;The answer would have to be best scientific guess or theory, rather than fact, but has any thought been given to what actually happens to the metallic elements of a spacecraft de-orbited into Jupiter? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for scientific thought, speculation or simulation that I can read about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/q/16977/12102&quot;&gt;&quot;Why destroy Juno at the end of the mission?&quot;&lt;/a&gt; and all answers, &lt;a href=&quot;https://space.stackexchange.com/a/16978/12102&quot;&gt;including this one&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume it is going fast, and &quot;burns-up&quot;, but what might happen to the metallic atoms? Aluminum, Copper, Iron... would they form compounds in the atmosphere that are heavy and slowly (very slowly) sink to the core, or happily remain as atmospheric trace compounds at the (quite high) altitude where the disintegration took place, or something else?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-07-06T06:49:33.503" Title="When Juno finally de-orbits into Jupiter, will metal sink to the core, or remain in the atmosphere?" Tags="|jupiter|juno|end-of-life|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17031" PostTypeId="1" CreationDate="2016-07-06T06:39:48.763" Score="5" ViewCount="333" Body="&lt;p&gt;So I have some payload which is going to be sent to Jupiter's moon Europa.&#xA;While my contract partner who is shipping my payload didn't specify strictly enough what the expected achievements of our collaboration are, I have loaded a quantity of sets of microbes and other lifeforms&lt;sup&gt;1&lt;/sup&gt; onto the ship which will be navigated in its last stage to crash into Europa. Not just if my mission is a success, making it for future missions hard to differentiate between those &quot;immigrants&quot; and the population, but maybe even one of my little soldiers has the fitting weapon set to raid and annihilate the whole population actually living on Europa.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If this would be a real case this would obvious be an psychopathic and raging mission with possibly disastrous outcome for the scientific community and any effort for the search of alien lifeforms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But since space is by international conventions not claimable and so the &quot;damage&quot; is done to no ones property, are there any resolutions that would make me reliable for this or even force me into stopping my plans?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; The kind of organisms which are expected to feel pretty well on Europa.&lt;/p&gt;&#xA;" OwnerUserId="11099" LastEditorUserId="2778" LastEditDate="2016-07-06T08:42:31.750" LastActivityDate="2021-08-18T08:45:18.647" Title="I'm going to invade Europa, any international interferences that could stop me?" Tags="|colonization|life|law|europa|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17032" PostTypeId="2" ParentId="17030" CreationDate="2016-07-06T06:49:33.503" Score="4" Body="&lt;p&gt;Juno will burn up in the atmosphere, and very violently so (over 40km/s of reentry speed vs Earth's 8; effects scale quadratically), so it will dissipate as trace contamination of the atmosphere, spread to four winds through Jupiter's violent weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If (dubiously so) any solid pieces survive, they'd sink to the surface of solid hydrogen layer and rest on it (like things rest on the sea bottom). &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-06T06:49:33.503" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17033" PostTypeId="1" CreationDate="2016-07-06T07:22:28.510" Score="1" ViewCount="167" Body="&lt;p&gt;Wouldn't the Moon be a better place to send people than Mars?  If our goal is to really let humans live on a body other than the Earth, why not use transportation that was already worked-out when most retirees were in highschool or college?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It sounds like on Mars, you would have to live underground to stay safe from radiation anyway, as I'd assume you would on the Moon.  The atmosphere on Mars is also so thin you may as well be in a vacuum anyway and you would never be able to go outside without a full-on space suit from what I've heard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't a nice underground Moon base be cheaper, easier to build, and safer?  You have to bury yourself to protect from radiation, anyway, and with half the gravity the Moon would be easier to do this on.  Also, you would be far closer to Earth to either send things back or get help.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that Mars would be more attractive looking and certainly more interesting, but perhaps more difficult and uncomfortable as just building a base in the vacuum anyway.  &lt;/p&gt;&#xA;" OwnerUserId="14659" LastEditorUserId="14659" LastEditDate="2016-07-06T20:55:19.707" LastActivityDate="2016-07-07T16:36:00.970" Title="Would the Moon be a better place than Mars anyways?" Tags="|mars|crewed-spaceflight|radiation|" AnswerCount="0" CommentCount="10" ClosedDate="2016-07-07T16:36:07.077" ContentLicense="CC BY-SA 3.0" />
  <row Id="17034" PostTypeId="1" AcceptedAnswerId="17463" CreationDate="2016-07-06T08:18:00.267" Score="5" ViewCount="326" Body="&lt;p&gt;I know the role and mechanism of ablator, I know some examples, but I still don't know &quot;what makes a material a good ablator&quot;. Certainly combustion/evaporation temperature below the expected work temperatures, and low thermal conductivity, but I'm fairly sure that's not all. So - what properties make a good ablator?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-25T00:31:31.670" Title="What properties characterize material as a good ablator?" Tags="|materials|ablator|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17035" PostTypeId="1" AcceptedAnswerId="17040" CreationDate="2016-07-06T09:45:19.903" Score="7" ViewCount="808" Body="&lt;p&gt;If a spacecraft was accelerated to just above Earth's escape velocity, it would begin orbiting the sun with an orbit very similar to Earth's. Could the spacecraft then repeatedly perform a gravity assist with the Earth to increase it's apogee over and over until it left the Solar System, while only using a nominal amount of delta-v to perform deep-space maneuvers to fine-tune encounters with the Earth?&lt;/p&gt;&#xA;" OwnerUserId="11820" LastActivityDate="2016-07-06T19:59:15.240" Title="Is it possible to leave the Solar System by repeatedly using gravity assists from the Earth?" Tags="|orbital-mechanics|gravity-assist|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17036" PostTypeId="2" ParentId="16851" CreationDate="2016-07-06T10:10:45.397" Score="0" Body="&lt;p&gt;Looking through the description of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ammonium_perchlorate_composite_propellant&quot; rel=&quot;nofollow noreferrer&quot;&gt;APCP&lt;/a&gt;, I noticed that it contains metal &lt;em&gt;powder&lt;/em&gt;. That means your assumption about the oxidizer isn't correct. At microscopic level, the oxidizer is &lt;em&gt;not&lt;/em&gt; distributed throughout the volume of the metal. The distance between oxidizer molecules and metal atoms is measured in micrometers, not nanometers. Of course, a micrometer still isn't that much, which explains why the reaction is very quick once the propellant is vaporized. The atoms only need to travel those few micrometers before they can react. But that's not instant, and it limits the burn rate.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastEditorUserId="1447" LastEditDate="2016-11-17T13:24:15.323" LastActivityDate="2016-11-17T13:24:15.323" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17037" PostTypeId="2" ParentId="17031" CreationDate="2016-07-06T10:39:54.040" Score="5" Body="&lt;h2&gt;TL;DR:&lt;/h2&gt;&#xA;&lt;p&gt;We've all agreed to not do that, but there's no Intergalactic Police stopping you. But you would likely be stopped by earth police forces.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Relevant Treaties and Organisations&lt;/h2&gt;&#xA;&lt;h3&gt;UN&lt;/h3&gt;&#xA;&lt;p&gt;The UN's &lt;em&gt;&lt;a href=&quot;https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/introouterspacetreaty.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies&lt;/a&gt;&lt;/em&gt; has been signed by 89 countries.&lt;/p&gt;&#xA;&lt;p&gt;I'll quote a few relevant parts.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Article VI&lt;/strong&gt;&lt;br /&gt;&#xA;(...)&lt;br /&gt;&#xA;The activities of non-governmental entities in outer space, including the moon and other celestial bodies, shall require authorization and continuing supervision by the appropriate State Party to the Treaty.&lt;br /&gt;&#xA;(...)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Article IX&lt;/strong&gt;&lt;br /&gt;&#xA;(...)&lt;br /&gt;&#xA;A State Party to the Treaty which has reason to believe that an activity or experiment planned by another State Party in outer space, including the moon and other celestial bodies, would cause potentially harmful interference with activities in the peaceful exploration and use of outer space, including the moon and other celestial bodies, may request consultation concerning the activity or experiment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Article IX roughly means &amp;quot;please don't do that&amp;quot;, while article IV means that it's up to whichever party to the treaty feels obligated to take responsibility.&lt;/p&gt;&#xA;&lt;h3&gt;NASA&lt;/h3&gt;&#xA;&lt;p&gt;NASA has an &lt;em&gt;&lt;a href=&quot;https://planetaryprotection.arc.nasa.gov/about&quot; rel=&quot;nofollow noreferrer&quot;&gt;Office of Planetary Protection&lt;/a&gt;&lt;/em&gt;. Its mission is as follows&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The mission of the Office of Planetary Protection is to promote the responsible exploration of the solar system by implementing and developing efforts that protect the science, explored environments, and Earth. The objectives of planetary protection are several-fold and include:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Preserving our ability to study other worlds as they exist in their natural states;&lt;/li&gt;&#xA;&lt;li&gt;Avoiding the biological contamination of explored environments that may obscure our ability to find life elsewhere – if it exists; and&lt;/li&gt;&#xA;&lt;li&gt;To ensure that we take prudent precautions to protect Earth’s biosphere in case life does exist elsewhere.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;But again, this seems to rely on treaties and cooperation, and seems to be lacking a way to enforce anything.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;But realistically, you need to launch from somewhere on earth. Should the government that controls your launch site become aware of your plans, they would probably shut you down.&lt;br /&gt;&#xA;Even if the country itself might not take responsibility, it's very likely other countries might pressure it to do so.&lt;/p&gt;&#xA;" OwnerUserId="2778" LastEditorUserId="2778" LastEditDate="2021-08-18T08:45:18.647" LastActivityDate="2021-08-18T08:45:18.647" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="17038" PostTypeId="2" ParentId="17003" CreationDate="2016-07-06T12:34:02.630" Score="1" Body="&lt;p&gt;Putting a telescope on any object with an atmosphere is a bad idea.  Any atmosphere will screw up your pictures with clouds and dust.  That's the reason all serious telescopes on Earth are on the highest points possible - to reduce that.  You've still got problems though, just with the air being in the way.  The best images of anything outside our planet all come from Hubble, which is up there precisely because atmospheric refraction is the limiting factor in getting decent pictures.  Even if you're wanting to take pics of something in our solar system, Hubble in space is infinitely better than a telescope on Mars, even if Mars happens to be a bit closer to it (at certain points in its orbit anyway).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's optical.  If you're doing radio astronomy then refraction in air isn't an issue, but if you're looking for serious distance then you're using multiple dishes to give a larger baseline.  Planets impose a natural limit to the baseline of your telescope, based on the size of the planet.  Satellites OTOH can accurately fly in formation at huge distances, providing ludicrously large baselines which would never be possible any other way.&lt;/p&gt;&#xA;" OwnerUserId="15976" LastActivityDate="2016-07-06T12:34:02.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17039" PostTypeId="2" ParentId="17003" CreationDate="2016-07-06T12:49:39.500" Score="2" Body="&lt;p&gt;We have sent telescopes into deep space, often to the Earth-Sun L2 Lagrangian point. This is especially common for infra-red telescopes because water vapor in the atmosphere blocks IR. One example was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Herschel_Space_Observatory&quot; rel=&quot;nofollow&quot;&gt;Herschel Space Observatory&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is little reason to go through the trouble of landing on an other planet when the telescope can just as well stay in space.&lt;/p&gt;&#xA;" OwnerUserId="11510" LastActivityDate="2016-07-06T12:49:39.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17040" PostTypeId="2" ParentId="17035" CreationDate="2016-07-06T13:19:28.260" Score="6" Body="&lt;p&gt;Juno did something similar to this, it went on an initial 2 year orbit, with a flyby of Earth 2 years after launch, after which time it went on to Jupiter, to orbit it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been playing a bit with &lt;a href=&quot;http://trajbrowser.arc.nasa.gov/traj_browser.php?maxMag=25&amp;amp;maxOCC=4&amp;amp;chk_target_list=on&amp;amp;target_list=Jupiter&amp;amp;mission_class=oneway&amp;amp;mission_type=rendezvous&amp;amp;LD1=2016&amp;amp;LD2=2040&amp;amp;maxDT=20&amp;amp;DTunit=yrs&amp;amp;maxDV=7.0&amp;amp;min=DV&amp;amp;wdw_width=365&amp;amp;submit=Search#a_load_results&quot; rel=&quot;noreferrer&quot;&gt;NASA's Trajectory Browser&lt;/a&gt;, and found this possibility:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Launch March 2016- 4.33 km/s beyond Earth Escape&lt;/li&gt;&#xA;&lt;li&gt;Deep space maneuver 1 year later&lt;/li&gt;&#xA;&lt;li&gt;Slight powered flyby of Earth 10 months later&lt;/li&gt;&#xA;&lt;li&gt;Pick the correct course to flyby Jupiter, such that the flight will lead to interstellar space&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Total ~ 5 km/s delta v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b5wqH.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b5wqH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to do a purely Earth based escape, it is also possible, the trick is to periodically return to Earth. So the first return time would be in 2 years, the next might be in 4, and so on until you have left the Solar System. It might take a while, but it could be done.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-06T19:59:15.240" LastActivityDate="2016-07-06T19:59:15.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17041" PostTypeId="1" AcceptedAnswerId="17071" CreationDate="2016-07-06T13:37:45.710" Score="12" ViewCount="2568" Body="&lt;p&gt;Pluto seems to be composed of Nitrogen and other ices that would be fluid or gaseous at higher temperatures. So what would happen if an astronaut using an Apollo era space-suit (crucially with that insulation) would land and step out on the surface of Pluto?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would conduction of heat cause the surface to melt or sublimate, and if so, how fast would this happen? This would be a illustrative example of how different a landing on Pluto would be compared to a landing on the Moon.&lt;/p&gt;&#xA;" OwnerUserId="13732" LastEditorUserId="58" LastEditDate="2016-07-06T15:18:54.260" LastActivityDate="2020-04-10T20:34:35.220" Title="What would happen if an Apollo astronaut landed on Pluto?" Tags="|landing|astronauts|pluto|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17042" PostTypeId="2" ParentId="16989" CreationDate="2016-07-06T14:17:36.210" Score="-1" Body="&lt;p&gt;It's all about gyroscope. If you don't have your physics sharp, you might have to put your imagination to work a little :-)&lt;br&gt;&#xA;It is quite a simple process, and it is all done autonomously (has to be: it takes 48 minutes for a signal to reach Juno from Earth, and, not surprisingly, the same amount of time from Earth to Juno. So 48+48 = DISASTER!!!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 - You have Jupiter's gravity component. It is pulling Juno towards it. Let's call this the &lt;code&gt;down-force&lt;/code&gt;;&lt;br&gt;&#xA;2 - To escape that &lt;code&gt;down-force&lt;/code&gt;, Juno has to accelerate forward so fast that it will eventually do escape, if he keeps on accelerating;&lt;br&gt;&#xA;3 - The gyroscope actually acuses (reads) that acceleration downwards and sets the propulsion rockets  to the &lt;code&gt;forward&lt;/code&gt; direction until the downward acceleration is equal to zero. This is called free fall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These forward rockets have to be set on from time to time (gyroscope sets that - autonomously) because, despite what you might have heard, there is friction (little, but there is) that will slow down the forward speed of Juno when in orbit, because of Jupiter's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Don't confuse acceleration with speed.&lt;br&gt;&#xA;Imagine you in your car. When you step on the gas, you accelerate. You feel the pressure of your back to the car's seat. When you get to the desired speed, you stop accelerating (no more back pressure), and just maintain the gas pedal pressed enough to keep that speed (to overcome friction and other - but no more acceleration).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your imagination is good enough, think that, for you to walk you have to lean your body forward to start falling. To prevent smashing your face on the floor, you stretch your leg forward and put forward speed, until your acceleration is zero and you are enjoying that walk in the park.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nice site about Juno, with lots of videos and nerd facts: &lt;a href=&quot;http://spaceflight101.com/juno/juno-mission-trajectory-design/&quot; rel=&quot;nofollow&quot;&gt;http://spaceflight101.com/juno/juno-mission-trajectory-design/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16028" LastActivityDate="2016-07-06T14:17:36.210" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17044" PostTypeId="2" ParentId="17018" CreationDate="2016-07-06T15:24:08.897" Score="1" Body="&lt;p&gt;A direct flight would require a bigger rocket, bigger than the available ones. A direct flight with an available rocket would require a much smaller probe with less mass for instruments and more fuel. The mission designers tested a lot of different flight plans and looked for a good compromise between flight time and instruments mass. They did not have the option of using a very powerful and fast rocket which existed only in theory but not in practice.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-07-06T15:24:08.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17045" PostTypeId="1" CreationDate="2016-07-06T15:31:14.447" Score="5" ViewCount="102" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/17018/12742&quot;&gt;This question&lt;/a&gt; got me thinking - what types of activities are routinely performed when a probe is in a long transit (like Juno, for example)? What does the staffing typically look like? Does one (or a few) teams manage multiple spacecraft, or does each craft have it's own team?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of man-hours required during the transit is probably very low. Is there even a need for 24-hour monitoring?&lt;/p&gt;&#xA;" OwnerUserId="12742" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-08-05T20:10:19.580" Title="What types of activities are performed when managing a probe in transit?" Tags="|probe|ground-station|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17046" PostTypeId="1" AcceptedAnswerId="20889" CreationDate="2016-07-06T16:44:14.030" Score="9" ViewCount="647" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/q/17045/12102&quot;&gt;question&lt;/a&gt; and especially this &lt;a href=&quot;https://space.stackexchange.com/questions/17045/#comment43533_17045&quot;&gt;comment&lt;/a&gt; address activity associated with the Juno mission, both circa arrival, and the relatively quiet transit period to Jupiter. The idea is that there may have been infrequent contact with Juno, or contact of short duration during transit, but there has probably been quite a burst of activity not that it has arrived.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I wish I could see that, or even make a plot of frequency or duration or just some kind of qualitative metric, for general interest, and for future answers in stack exchange. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a way to get statistics or logging information about what has been displayed on the Deep Space Network NOW page? (&lt;a href=&quot;https://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://eyes.nasa.gov/dsn/dsn.html&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Certainly some kind of log of what information was sent to the server that generates the page exists. I'm not looking for actual communications logs, just a way to summarize or histogram activity as displayed on the web page.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any idea how to do this? Is there a way or a place to suggest this? I know the chances are low that anyone would have to the time to do this, but there may be a &quot;feature request&quot; feature somewhere. Also, it's possible someone is already doing this and doesn't know that anyone else is interested.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-03-30T22:51:05.667" Title="Is there any publicly available logging, or &quot;Wayback Machine&quot; for DSN Now activity?" Tags="|juno|deep-space-network|historical-data|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17047" PostTypeId="1" AcceptedAnswerId="17048" CreationDate="2016-07-06T16:56:22.370" Score="53" ViewCount="6882" Body="&lt;p&gt;When I first heard about the Juno spacecraft, I was surprised that it would be using solar panels to generate power throughout the mission. Why wasn't a &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;radioisotope thermoelectric generator&lt;/a&gt; (RTG) used instead of solar panels? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it because RTGs are more expensive than solar panels are now? All past missions to Jupiter used RTGs for power.&lt;/p&gt;&#xA;" OwnerUserId="14456" LastEditorUserId="286" LastEditDate="2016-07-08T17:20:47.173" LastActivityDate="2018-09-08T20:40:02.810" Title="Why wasn't an RTG used on the Juno spacecraft?" Tags="|design|power|solar-power|juno|rtg|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17048" PostTypeId="2" ParentId="17047" CreationDate="2016-07-06T17:01:04.660" Score="69" Body="&lt;p&gt;That is precisely it. &lt;a href=&quot;https://en.wikipedia.org/wiki/Plutonium-238&quot; rel=&quot;noreferrer&quot;&gt;Plutonium-238&lt;/a&gt;, which is used in the creation of radioisotope thermoelectric generators (RTGs) is very difficult to come by. There are plenty of news articles on this, from &lt;a href=&quot;http://www.popsci.com/nasa-can-make-3-more-nuclear-batteries-and-thats-it&quot; rel=&quot;noreferrer&quot;&gt;Popular Science&lt;/a&gt; to &lt;a href=&quot;http://spacenews.com/u-s-plutonium-stockpile-good-for-two-more-nuclear-batteries-after-mars-2020/&quot; rel=&quot;noreferrer&quot;&gt;Space News&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, it comes down to the fact that plutonium-238 is in short supply now, and it is difficult to make more because of nuclear non-proliferation. Of the stock that NASA has left, they can make three RTGs. One of them is definitely going to be used for &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020&quot; rel=&quot;noreferrer&quot;&gt;Mars 2020&lt;/a&gt;, because it's using the same design as &lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_%28rover%29&quot; rel=&quot;noreferrer&quot;&gt;Curiosity&lt;/a&gt;. The other two have not been allocated yet.  The &lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_Department_of_Energy&quot; rel=&quot;noreferrer&quot;&gt;United States Department of Energy&lt;/a&gt; may start making more plutonium-238, but how much and when is yet to be seen.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2018-09-08T20:40:02.810" LastActivityDate="2018-09-08T20:40:02.810" CommentCount="14" ContentLicense="CC BY-SA 4.0" />
  <row Id="17049" PostTypeId="2" ParentId="17045" CreationDate="2016-07-06T19:35:20.337" Score="2" Body="&lt;p&gt;Juno was placed in hibernation (&lt;a href=&quot;https://www.nasaspaceflight.com/2016/07/nasa-junos-arrival-jupiter/&quot; rel=&quot;nofollow&quot;&gt;https://www.nasaspaceflight.com/2016/07/nasa-junos-arrival-jupiter/&lt;/a&gt;) during its trip so contact would have been sporadic.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight101.com/newhorizons/project-history-mission-profile/&quot; rel=&quot;nofollow&quot;&gt;New Horizons used a similar approach&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Heading into its first long hibernation cycle, New Horizons sent beacons once a month, requiring virtually no support for most of the time, decreasing time needed on the Deep Space Network and personnel at Mission Control. For two months each year, New Horizons was woken up to undergo a complete check of its systems and instruments. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The beacon was a very simple message: all ok/small problems/big problems. So NH would have needed one block of DSN time per month. IIRC a block of 8 hours is standard, maybe a smaller slice suffices for these beacons.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-06T19:35:20.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17050" PostTypeId="2" ParentId="16989" CreationDate="2016-07-06T21:21:19.893" Score="2" Body="&lt;p&gt;The Juno spacecraft uses a combination of techniques to determine its location and trajectory. Onboard systems, known as &quot;star trackers&quot; help the spacecraft to determine its orientation in space. Combined with positioning data determined from radio signal analysis conducted on Earth, the vehicle can &quot;triangulate&quot; its position along its trajectory, and calculate deviations. Once the spacecraft knows its actual current trajectory, it can perform calculations to determine what thrust vectors to apply (using its various engines) to achieve the correct course. In this case, that would be orbital insertion.  Please see &lt;a href=&quot;http://www.npr.org/2016/07/03/484259562&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; for more information.  &lt;/p&gt;&#xA;" OwnerUserId="10641" LastEditorUserId="4660" LastEditDate="2016-07-07T14:43:55.870" LastActivityDate="2016-07-07T14:43:55.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17051" PostTypeId="1" AcceptedAnswerId="17052" CreationDate="2016-07-06T22:26:21.440" Score="7" ViewCount="425" Body="&lt;p&gt;I have not been able to find a definitive answer to this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Mercury to Gemini to Apollo, what was the mechanism for keeping time onboard the spacecraft? Did the Engineers spend energy (and maybe computer cycles) on electric clocks? Or did the astronauts rely on whatever watch was strapped to their wrists?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am assuming (but again have not been able to confirm) that some of the operations required fairly accurate timing.&lt;/p&gt;&#xA;" OwnerUserId="16048" LastActivityDate="2016-07-06T22:42:32.193" Title="How was time kept on board early manned spaceflights?" Tags="|apollo-program|project-mercury|project-gemini|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17052" PostTypeId="2" ParentId="17051" CreationDate="2016-07-06T22:42:32.193" Score="6" Body="&lt;p&gt;Apollo had an &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#Timing&quot; rel=&quot;noreferrer&quot;&gt;onboard guidance computer&lt;/a&gt;, with its own clock.  From Wikipedia:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The AGC timing reference came from a 2.048 MHz crystal clock. The clock was divided by two to produce a four-phase 1.024 MHz clock which the AGC used to perform internal operations. The 1.024 MHz clock was also divided by two to produce a 512 kHz signal called the master frequency; this signal was used to synchronize external Apollo spacecraft systems.&lt;/p&gt;&#xA;&lt;p&gt;The master frequency was further divided through a scaler, first by five using a ring counter to produce a 102.4 kHz signal. This was then divided by two through 17 successive stages called F1 (51.2 kHz) through F17 (0.78125 Hz). The F10 stage (100 Hz) was fed back into the AGC to increment the real-time clock and other involuntary counters using Pinc (discussed below). The F17 stage was used to intermittently run the AGC when it was operating in the standby mode.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Gemini likewise had an onboard &lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_Guidance_Computer&quot; rel=&quot;noreferrer&quot;&gt;guidance computer&lt;/a&gt; with a clock.  According to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Gemini#Reentry_module&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia page on Gemini&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Gemini was the first astronaut-carrying spacecraft to include an onboard computer, the Gemini Guidance Computer, to facilitate management and control of mission maneuvers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Which means that Mercury did not have a computer, and thus they probably had some sort of watch.  The Mercury missions did not have any kind of maneuvering capability, except for reentry, so the need to keep time would be less pressing.&lt;/p&gt;&#xA;&lt;p&gt;Edit: Reading through some of the specific Mercury missions, they make references to astronauts synchronizing clocks or timing things, so I must assume that they did have some clock, probably digital.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-06T22:42:32.193" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17053" PostTypeId="2" ParentId="17047" CreationDate="2016-07-07T00:47:58.143" Score="37" Body="&lt;p&gt;Another interesting note is that this mission more than any other mission to the outer solar system can use solar power. Why? Juno is in a polar orbit, and will continually be in the sun. Solar panels are also becoming more powerful than they have previously. Between the two of these, solar was a more attractive option than it has been in the past. If it was absolutely required, it could use an RTG, but in fact solar panels were possible, and thus they were used. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-07T01:19:19.570" LastActivityDate="2016-07-07T01:19:19.570" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17054" PostTypeId="1" CreationDate="2016-07-07T00:58:38.873" Score="20" ViewCount="2701" Body="&lt;p&gt;I've seen statements to the effect of: &quot;the gravity on the surface of Jupiter is about 2.5 times that of Earth&quot;. The problem with such a statement is that as essentially a ball of gas, Jupiter is not believed to have a solid surface. Behind the &quot;2.5g&quot; claim there must be some criteria applied to select a radius, which when combined with a figure for Jupiter's mass, will yield an acceleration value according to the classic gravitation formula.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The meat of my question pertains to the criteria which produce radius values for the gas planets which presumably are the basis of statements such as I mentioned. As we know, Earth's atmosphere extends quite far into space, just getting thinner and thinner. So as one approaches a gas giant, one would presumably first encounter an extremely thin atmosphere, getting progressively denser, eventually as dense as a liquid. Somewhere along the way, we crossed a point which represents what we deem to be the planet's radius. What is going on at that point? Is it the same for all gas planets, or do the conditions of each planet necessitate different choices? Are the same criteria applied regardless of endeavor (astronomy vs spacecraft engineering vs exometeorology, etc.)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, for what its worth, how does the planet's deemed radius relate to its visible horizon i.e. the visible but indistinct boundary between planet and space?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-07-13T17:52:30.727" Title="What defines the radius of a ball of gas like Jupiter?" Tags="|planetary-science|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17055" PostTypeId="1" AcceptedAnswerId="17057" CreationDate="2016-07-07T01:32:33.370" Score="1" ViewCount="467" Body="&lt;p&gt;I don't understand! Rotating frame? I see Orion pass by around 00:30 to 00:36, that's a clue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why the first two passes are on the &lt;em&gt;same side&lt;/em&gt; (in this view) but the rest alternate? &lt;strong&gt;edit:&lt;/strong&gt; This is because the first orbit has a very different period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of orbits are not considered &quot;Gravity orbits&quot;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is an MWR orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of orbits are in white (not labeled in the legend)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the dark side of Jupiter is mostly facing us - the sunlit face is mostly facing away from us. You can see a crescent of light on the right side at the beginning, then on the left side later on. I thought Juno will &lt;a href=&quot;https://space.stackexchange.com/a/17053/12102&quot;&gt;never experience an eclipse during its time studying Jupiter&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think I see an &quot;Extra science orbit&quot; in yellow at 00:55, but no De-orbit maneuver in red. I was looking forward to the Juno-shattering Kaboom! See &lt;a href=&quot;https://space.stackexchange.com/q/16977/12102&quot;&gt;question 1&lt;/a&gt;, and &lt;a href=&quot;https://space.stackexchange.com/q/17030/12102&quot;&gt;question2&lt;/a&gt;, and &lt;a href=&quot;https://www.youtube.com/watch?v=Z40AsPaktzw#t=68.5&quot; rel=&quot;nofollow noreferrer&quot;&gt;Marvin the Martian circa 1950&lt;/a&gt; for background.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4J7St9Z9GBY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/66Vhi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/66Vhi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-07-07T14:38:10.680" Title="How to understand this NASA animation of Juno's passes near Jupiter?" Tags="|orbital-mechanics|orbit|trajectory|jupiter|juno|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17056" PostTypeId="2" ParentId="17054" CreationDate="2016-07-07T01:37:18.083" Score="24" Body="&lt;p&gt;The radius of Jupiter and the other gas giants is defined, somewhat arbitrarily, to be the radius at which the atmosphere has a pressure of 1 bar. As your question points out, they had to pick something. So that's what they picked. This convention is used for all of the gas giants in our solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a visible boundary definition, you would need to define the opacity threshold and the wavelength, since the opacity depends on wavelength. Even then you would have some ambiguity due to features with different opacity. Would Jupiter have a different radius at the Great Red Spot if your wavelength was red?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though I suppose the pressure definition has a similar problem, as the Great Red Spot is probably a low pressure system. However the uncertainty would be small as the pressure changes very rapidly with altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radius of the Sun used to be defined by opacity (at an optical depth of 2/3). That photosphere radius has some measurement interpretation issues, and remains a subject of investigation. Apparently the IAU got tired of it, so in 2015 the &quot;nominal&quot; radius of the Sun was defined to be &lt;em&gt;exactly&lt;/em&gt; 695,700 km. This allows the use of &quot;solar radii&quot; as a unit without everyone wondering what radius to use to convert that to distance units.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-07-07T04:11:14.247" LastActivityDate="2016-07-07T04:11:14.247" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17057" PostTypeId="2" ParentId="17055" CreationDate="2016-07-07T03:11:01.147" Score="4" Body="&lt;p&gt;Jupiter rotates quite rapidly -- it's rotational period is only ten hours. The camera on that animation is almost keeping up with Jupiter's rotation. The rotation of Jupiter beneath Juno's orbital track is where the apparent twist is coming from -- the path is being rendered Jupiter-surface relative. Note that the very eccentric orbit means that Juno's speed is changing dramatically over the course of each pass -- accelerating toward perijove (near the equator) and decelerating on the way out; it's close to the planet for only a few hours out of each two-week-long orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;MWR is Juno's Microwave Radiometer. I believe the &quot;MWR orbit&quot; versus &quot;gravity orbit&quot; distinction simply tells which instruments are going to be active on each pass -- the MWR versus the gravity experiment --  which has secondary impact on other aspects like the spacecraft's communication protocols. There's more information on those orbits near the end of &lt;a href=&quot;http://spaceflight101.com/juno/juno-mission-trajectory-design/&quot; rel=&quot;nofollow&quot;&gt;this very informative page&lt;/a&gt;. Here's information from Wikipedia about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_(spacecraft)#Scientific_instruments&quot; rel=&quot;nofollow&quot;&gt;instruments&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-07T03:57:09.363" LastActivityDate="2016-07-07T03:57:09.363" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17058" PostTypeId="1" CreationDate="2016-07-07T05:37:44.080" Score="3" ViewCount="209" Body="&lt;p&gt;I have been trying to find this online without too much avail. For my own reference and cross checking, I wanted to know where I could find the emergency checklists and the &quot;made up on the spot&quot; CM restart checklist for the Apollo program. Does anyone have these resources bookmarked or know where I can find said resources without contacting NASA?&lt;/p&gt;&#xA;" OwnerUserId="16058" LastEditorUserId="415" LastEditDate="2016-07-07T08:45:18.600" LastActivityDate="2016-07-07T08:45:18.600" Title="Where can I find the amended/improvised checklists for Apollo 13 post-accident?" Tags="|apollo-program|reference-request|apollo-13|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17059" PostTypeId="1" AcceptedAnswerId="17061" CreationDate="2016-07-07T07:24:00.240" Score="8" ViewCount="1459" Body="&lt;p&gt;In order to better defend my position that &lt;a href=&quot;https://space.stackexchange.com/questions/16977/#comment43611_16992&quot;&gt;trying to track Juno from Earth once it can no longer transmit would be Quixotic&lt;/a&gt;, I'd like to ask what the current record is for the farthest &lt;strong&gt;detection from earth&lt;/strong&gt; (using radar, optical/IR, etc.) of an earth-launched spacecraft or other &lt;em&gt;earth-launched&lt;/em&gt; object that was no longer able to actively transmit or rebroadcast (e.g. &lt;a href=&quot;https://space.stackexchange.com/a/14683/12102&quot;&gt;Doppler ranging&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The recent so-called &lt;a href=&quot;https://physics.stackexchange.com/q/214700/83380&quot;&gt;'WTF' object&lt;/a&gt; comes to mind, but there may be something farther, or otherwise more demonstrative of (at least non-military) capability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bold font phrase is meant to exclude detection of landers on the Moon or Mars from spacecraft orbiting those bodies. I'd also like to exclude &lt;a href=&quot;https://space.stackexchange.com/q/14943/12102&quot;&gt;lunar retro-reflector arrays from Apollo missions&lt;/a&gt; as well as &lt;a href=&quot;https://space.stackexchange.com/a/15329/12102&quot;&gt;LAGEOS&lt;/a&gt; because these can be monitored &lt;a href=&quot;https://space.stackexchange.com/questions/15320/#comment40277_15329&quot;&gt;only because we already know&lt;/a&gt; exactly where &lt;a href=&quot;https://space.stackexchange.com/questions/15320/#comment40284_15329&quot;&gt;they are supposed to be&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-07-09T22:23:41.127" LastActivityDate="2020-12-09T23:56:12.840" Title="What is the current record for the farthest detection of a &quot;dead&quot; spacecraft?" Tags="|radar|record|doppler-ranging|detection|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17060" PostTypeId="2" ParentId="17054" CreationDate="2016-07-07T08:44:48.750" Score="-2" Body="&lt;p&gt;here is the definition for the earth, the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line.&quot; rel=&quot;nofollow&quot;&gt;Kármán line&lt;/a&gt;&#xA;My two cents, generally I think for other planets it would be related to where the density of the atmosphere reduces such that it becomes a rarefied gas. That is where the separation between the fluid molecules becomes significant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think of a body entering an atmosphere it will first encounter molecules spaced very far apart such that they have negligible resistance (that said, over many orbits even this rarefied fluid will contribute to orbital decay). As the fluid becomes more dense there would be a transition from flow through a rarefied fluid to that through a Newtonian fluid. I think that would be a good basis for a drawing a line.&lt;/p&gt;&#xA;" OwnerUserId="16064" LastActivityDate="2016-07-07T08:44:48.750" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17061" PostTypeId="2" ParentId="17059" CreationDate="2016-07-07T12:08:13.860" Score="12" Body="&lt;p&gt;There are two potential candidates that I'm aware of. The first is  &lt;a href=&quot;https://en.wikipedia.org/wiki/J002E3&quot; rel=&quot;noreferrer&quot;&gt;J002E3&lt;/a&gt;, which is believed to be the Apollo 14 Saturn 5 upper stage. At the time of detection, it was orbiting Earth between 1-2 lunar distances away. Another candidate is &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2007/1227.html&quot; rel=&quot;noreferrer&quot;&gt;2007 VN84&lt;/a&gt;, also known as Rosetta, which was discovered via an optical survey while Rosetta was on a close approach path to Earth. Rosetta was transmitting at the time, but the detection was done without intentionally searching for it. According to &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;Horizons&lt;/a&gt;, it was about 6.3 million km away at the time, about 16 times further than the Moon! And point of reference, this detection was first made on October 27th, according to the &lt;a href=&quot;http://www.minorplanetcenter.net/mpec/K07/K07V69.html&quot; rel=&quot;noreferrer&quot;&gt;Minor Planet Center&lt;/a&gt;. On that date, it was twice as far away!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, we can detect spacecraft sized objects at a distance perhaps equal to Earth's hill sphere, very large objects as far as maybe 16-20 lunar distances, but no further.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-08T09:48:00.723" LastActivityDate="2016-07-08T09:48:00.723" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17062" PostTypeId="1" CreationDate="2016-07-07T13:19:27.440" Score="6" ViewCount="888" Body="&lt;p&gt;Did Juno start off its course on the ecliptic plane from the earth orbit and then left it at insertion to Jupiter's orbit over its north pole or it was always in that plane?&lt;/p&gt;&#xA;" OwnerUserId="16061" LastEditorUserId="51" LastEditDate="2016-07-08T13:34:03.907" LastActivityDate="2016-09-01T00:02:09.153" Title="The plane of the orbit of Juno around Jupiter is not the ecliptic plane. How did it get into this plane?" Tags="|orbital-mechanics|spacecraft|jupiter|juno|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17063" PostTypeId="2" ParentId="17055" CreationDate="2016-07-07T13:41:41.860" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How to understand this NASA animation of Juno's passes near Jupiter?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Like much of the stuff coming out of JPL and SWRI regarding Juno, this leaves me unimpressed. The animation has labels that are unexplained, a slowly rotating Jupiter that is unexplained, big jumps in time whose presence was not divulged, and an obviously nonsense star field that I suspect was added just for the &quot;Wow!&quot; effect.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I see Orion pass by around 00:30 to 00:36, that's a clue.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is a clue that this is not a good video. It would have been better to have left the background black.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Here's my best guess with regard to how what you are seeing was created:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Juno's 36 orbits about Jupiter were simulated and then expressed in terms of a Jupiter-fixed frame. Only the parts in the close vicinity of Jupiter are depicted.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;As a time-lapsed animation, almost all (97.6%) of this will be extremely boring because Juno is not in the video frame. The only interesting parts are the 8 hours of the 14 day orbit where Juno passes very close to Jupiter. These 8 hours are the scientifically interesting bits. The boring bits were skipped over.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A frozen 3D image of Jupiter was added to the animation. Having a frozen image is not quite correct as different parts of Jupiter rotate at slightly different rates. Over Juno's 36 orbits, the equatorial regions will have rotated eleven times more than will have the polar regions. This is not shown in the video.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The entire simulation was set into a slow animated rotation to show that these 36 science passes do a nice job of evenly covering Jupiter.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A star field was added for &quot;Wow!&quot; effect. If done correctly, the star field would move quickly (the 8 hour long science pass is more than 80% of Jupiter's rotation period) and then jump to reflect the 13 days and 16 hour time jump, move quickly and then jump, doing that 36 times. This star field is purely synthetic (i.e., it's bogus).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What kind of orbits are not considered &quot;Gravity orbits&quot;?&lt;br&gt;&#xA;  What is an MWR orbit?&lt;br&gt;&#xA;  What kind of orbits are in white (not labeled in the legend)?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Regarding the coloring and labels on the orbits, Two of the nine experiments being conducted by Juno are mutually incompatible with one another. These two experiments turn out to be two of Juno's primary reasons for being, which are peering into Jupiter's atmosphere and assessing its gravitational field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno doesn't have any sensors to measure gravity (sensing gravity is very, very hard). Instead, variations in Doppler shift in the science data return from Juno as received on Earth give clues to Jupiter's gravity field. This means using the high gain antenna, which in turn means that Juno needs to orient itself so the high gain antenna points at Earth. (The high gain antenna is not pointable except by pointing Juno as a whole.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The microwave radiometer (MWR), which is used to peer into Jupiter's atmosphere, has conflicting requirements on Juno's pointing. The radiometer needs to be pointed toward Jupiter and the spacecraft needs to be oriented so as to minimize multipath interference. (If you have a smartphone with a GPS receiver, you may have noticed that it doesn't work so well in downtown areas with tall skyscrapers. That's because of extreme multipath interference.) Pointing the HGA toward Earth is a non-option when the MWR is is use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two orbits in white represent the very first orbits about Jupiter, where the MWR is not yet active and high precision orbit monitoring via X-band and Ka-band Doppler shift is not yet being performed. The &quot;gravity orbits&quot; are where that Doppler monitoring is being performed, and the &quot;MWR orbits&quot; are where the microwave radiometer is active.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-07-07T14:38:10.680" LastActivityDate="2016-07-07T14:38:10.680" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17064" PostTypeId="2" ParentId="17062" CreationDate="2016-07-07T15:04:10.543" Score="6" Body="&lt;p&gt;Basically, Juno came in towards Jupiter from slightly below it.  This allowed it to slow down into an orbit which takes it around the poles of the planet.  Over the course of its journey to Jupiter, it was at a very slight angle, relative to the plane of Jupiter's orbit.  This angle was determined by the flight engineers at its launch, and refined by two deep space maneuvers and an Earth gravitational assist flyby.  This slight angle was enough to put the craft below Jupiter, because its journey was very long.  More information about Juno's trajectory can be found in Spaceflight101's &lt;a href=&quot;http://spaceflight101.com/juno/juno-mission-trajectory-design/&quot; rel=&quot;nofollow&quot;&gt;article&lt;/a&gt;, which includes a &lt;a href=&quot;https://www.youtube.com/watch?v=sYp5p2oL51g&quot; rel=&quot;nofollow&quot;&gt;Juno trajectory animation&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="1142" LastEditDate="2016-09-01T00:02:09.153" LastActivityDate="2016-09-01T00:02:09.153" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17065" PostTypeId="2" ParentId="17062" CreationDate="2016-07-07T15:07:28.467" Score="4" Body="&lt;p&gt;As noted in comments, by selecting the launch time and initial trajectory, any relative inclination between Jupiter and the spacecraft could be achieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, while plane changes measured in &lt;em&gt;degrees&lt;/em&gt; are indeed expensive, for an orbit that goes out to 5 AU, one degree of relative inclination corresponds to a deflection of something like 12 million km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, after Earth flyby, mission control had realized that Juno was on a direct collision course with Jupiter, they could have done a 1-meter-per-second correction &quot;northward&quot;; Juno would then pass over Jupiter's North Pole at a distance of about 60,000km. In the solar frame of reference it's still a low inclination orbit; in Jupiter's reference it's polar.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-07-07T15:07:28.467" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17066" PostTypeId="2" ParentId="17062" CreationDate="2016-07-07T15:29:33.530" Score="9" Body="&lt;p&gt;The plane of the orbit around the Sun is not directly related to the planet-relative plane of the hyperbola on approach to Jupiter, or correspondingly the orbit around Jupiter after orbit insertion. The plane and shape of the orbit around the Sun relative to Jupiter's equatorial plane determines its approach declination to Jupiter. That is essentially the negative of the latitude on Jupiter of your atmospheric entry point if you tried to target your spacecraft to hit the center of Jupiter. The magnitude of the approach declination limits how equatorial the orbit can be, but not how polar it can be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On approach to a body, your target can be anywhere in a plane perpendicular to your approach trajectory, at essentially zero cost. That plane is called the B-plane:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JU5Fk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JU5Fk.png&quot; alt=&quot;b-plane&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For convenience, you target a point on the B-plane that you would intersect if the planet were not there, or if it had no gravity. You actually cross the B-plane inside of that point due to the planet's gravity. You get to pick how far from the center of the planet your target point is, and the angle of that point anywhere around the planet. The distance from the center determines your closest approach distance, or your atmospheric entry flight path angle. The angle determines the plane of your planet-relative approach trajectory, which will be the plane of your orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your approach declination defines a line through the center of the planet entering at a latitude equal to the negative approach declination on one side and exiting at a latitude equal to the positive approach declination on the other side. That line is labeled &quot;S&quot; in the diagram. Imagine a plane that contains that line, and you are allowed to rotate that plane around that line. Those are your allowed orbit planes. One such plane is the &quot;Trajectory Plane&quot; in the diagram. Since your plane must go over the latitude of the magnitude of the approach declination, your orbit plane inclination to the planet equator cannot be less than that. If it were less than that, then your orbit could not go over that latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno's approach declination was about 8°. Then by choosing how you target the Jupiter closest approach, which can be anywhere in a circle around Jupiter, you can pick any final orbit inclination from 8° (prograde) to 90° (polar) to 172° (retrograde).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After orbit insertion, you can do maneuvers or swingbys of satellites to change the orbit plane, e.g. if you wanted to be more equatorial than the approach geometry permitted. Cassini is the champion of this, having dramatically changed its orbit plane many times over the course of its mission at Saturn, using Titan gravity assists.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-07-08T05:30:15.323" LastActivityDate="2016-07-08T05:30:15.323" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17067" PostTypeId="1" AcceptedAnswerId="17070" CreationDate="2016-07-07T15:32:41.660" Score="6" ViewCount="287" Body="&lt;p&gt;Lockheed Martin was responsible for the construction of Juno.  But I was wondering who built the chips Juno uses for its onboard computers, and whether or not they are radiation hardened.  I would assume that, in addition to being in the vault, they are hardened.  Does anybody know where Lockheed got the chips themselves?&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-07-07T22:28:06.343" Title="Who built Juno's chipsets?" Tags="|spacecraft|juno|flight-computer|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="17068" PostTypeId="2" ParentId="17062" CreationDate="2016-07-07T15:50:31.700" Score="-3" Body="&lt;p&gt;I estimate the launch time and coordinates must have been optimally designed so that the gravity assist would already position Juno in an inclined plane much more than what was needed ultimately but adjusted over the long trajectory by the pull of Jupiter into correct entry point.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA would have two tools to angle the shot: earth's orbit and its axial rotation.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd assume they designed the trajectory purposely not in the plane of solar system at all but in an inclined plane winding up and down the apogee of its orbit as needed riding the great roller coaster. Hats off to Isaac Newton.&lt;br&gt;&#xA;quote from Wikipedia page on Ulysses probe   &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;to change the orbital inclination of a spacecraft a large change in heliocentric velocity is needed. However the necessary amount of velocity change to achieve a high inclination orbit of about 80° far exceeded the capabilities of any launch vehicle.&lt;br&gt;&#xA;   &lt;a href=&quot;https://en.wikipedia.org/wiki/Ulysses_%28spacecraft%29&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Ulysses_%28spacecraft%29&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="16061" LastEditorUserId="16061" LastEditDate="2016-07-07T20:27:01.007" LastActivityDate="2016-07-07T20:27:01.007" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17069" PostTypeId="2" ParentId="17047" CreationDate="2016-07-07T16:15:30.637" Score="14" Body="&lt;p&gt;I had the opportunity to tour JPL a few months ago and asked this exact question to our tour guide.  The solar panels on it are enormous and typically, spacecraft going beyond the asteroid belt are equipped with RTGs, so why doesn't Juno have one?  He told us that the US was on very short supply of Plutonium-238 at the time and that they would have had to ask Russia for it.  So instead of being dependent on another country, NASA decided to tackle a much more challenging design feat.  I'm sure that there was a lot more to it than that.  I think the other answers did a good job addressing those other factors, but I just found this very interesting and wanted to share.&lt;/p&gt;&#xA;" OwnerUserId="16073" LastActivityDate="2016-07-07T16:15:30.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17070" PostTypeId="2" ParentId="17067" CreationDate="2016-07-07T16:32:05.997" Score="9" Body="&lt;p&gt;BAE Systems is one supplier for the command and data handling processor.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/RAD750&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/RAD750&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The RAD750 is a radiation-hardened single board computer manufactured by BAE Systems Electronics, Intelligence &amp;amp; Support.&lt;a href=&quot;http://www.baesystems.com/download/BAES_052281/Space-Products--RAD750-component&quot; rel=&quot;noreferrer&quot;&gt;[1]&lt;/a&gt; The successor of the RAD6000, the RAD750 is for use in high radiation environments experienced on board satellites and spacecraft.&lt;a href=&quot;http://www.ballaerospace.com/page.jsp?page=96&quot; rel=&quot;noreferrer&quot;&gt;[2]&lt;/a&gt; The RAD750 was released in 2001, with the first units launched into space in 2005.&lt;a href=&quot;http://www.baesystems.com/download/BAES_052281/Space-Products--RAD750-component&quot; rel=&quot;noreferrer&quot;&gt;[1]&lt;/a&gt;&lt;a href=&quot;http://www.aero.org/conferences/mrqw/2002-papers/A_Burcin.pdf&quot; rel=&quot;noreferrer&quot;&gt;[3]&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The CPU has 10.4 million transistors, nearly an order of magnitude more than the RAD6000 (which had 1.1 million).&lt;a href=&quot;http://www.aero.org/conferences/mrqw/2002-papers/A_Burcin.pdf&quot; rel=&quot;noreferrer&quot;&gt;[3]&lt;/a&gt; It is manufactured using either 250 or 150 nm photolithography and has a die area of 130 mm2.&lt;a href=&quot;http://www.baesystems.com/download/BAES_052281/Space-Products--RAD750-component&quot; rel=&quot;noreferrer&quot;&gt;[1]&lt;/a&gt; It has a core clock of 110 to 200 MHz and can process at 266 MIPS or more.&lt;a href=&quot;http://www.baesystems.com/download/BAES_052281/Space-Products--RAD750-component&quot; rel=&quot;noreferrer&quot;&gt;[1]&lt;/a&gt; The CPU can include an extended L2 cache to improve performance.&lt;a href=&quot;http://www.aero.org/conferences/mrqw/2002-papers/A_Burcin.pdf&quot; rel=&quot;noreferrer&quot;&gt;[3]&lt;/a&gt; The CPU itself can withstand 200,000 to 1,000,000 rads (2,000 to 10,000 gray), temperature ranges between –55 °C and 125 °C and requires 5 watts of power.&lt;a href=&quot;http://www.baesystems.com/download/BAES_052281/Space-Products--RAD750-component&quot; rel=&quot;noreferrer&quot;&gt;[1]&lt;/a&gt;&lt;a href=&quot;http://www.aero.org/conferences/mrqw/2002-papers/A_Burcin.pdf&quot; rel=&quot;noreferrer&quot;&gt;[3]&lt;/a&gt; The standard RAD750 single-board system (CPU and motherboard) can withstand 100,000 rads (1,000 gray), temperature ranges between –55 °C and 70 °C and requires 10 watts of power.&lt;a href=&quot;http://www.aero.org/conferences/mrqw/2002-papers/A_Burcin.pdf&quot; rel=&quot;noreferrer&quot;&gt;[3]&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The RAD750 system has a price that is comparable to the RAD6000 which is US$200,000 per board (per 2002 reference).&lt;a href=&quot;http://mae.pennnet.com/display_article/143464/32/ARTCL/none/none/1/BAE-Systems-moves-into-third-generation-rad-hard-processors/&quot; rel=&quot;noreferrer&quot;&gt;[4]&lt;/a&gt; However customer program requirements and quantities will greatly affect the final unit costs.&lt;/p&gt;&#xA;&lt;p&gt;The RAD750 is based on the PowerPC 750.&lt;a href=&quot;http://www.baesystems.com/download/BAES_052281/Space-Products--RAD750-component&quot; rel=&quot;noreferrer&quot;&gt;[1]&lt;/a&gt; Its packaging and logic functions are completely compatible with the PowerPC 7xx family.&lt;a href=&quot;http://www.aero.org/conferences/mrqw/2002-papers/A_Burcin.pdf&quot; rel=&quot;noreferrer&quot;&gt;[3]&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/press_kits/JunoLaunch.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.jpl.nasa.gov/news/press_kits/JunoLaunch.pdf&lt;/a&gt;, page 15:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Command and Data Handling&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Command and data handling includes a RAD750 flight&#xA;processor with 256 megabytes of flash memory and&#xA;128 megabytes of DRAM local memory. It provides&#xA;100 Mbps total instrument throughput, more than&#xA;enough for payload requirements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="16075" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-07T22:28:06.343" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17071" PostTypeId="2" ParentId="17041" CreationDate="2016-07-07T18:24:48.563" Score="11" Body="&lt;p&gt;The temperature on the moon varies from about 90 K at night to about 400 K during the day. However, the Apollo missions were all timed so that the astronauts would be on the surface around dawn, when the temperatures were relatively mild. Their boots were insulated, and their space suits were designed to be highly reflective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto has temperatures varying from about 33 K to 55 K. What matters to the astronaut inside the suit is the temperature difference between the inside of the suit and the outside. The rate of heat conduction is proportional to that temperature difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you compare lunar nighttime to a balmy summer afternoon on Pluto, the temperature difference is 205 K in the first case versus 240 K in the second case. Those two numbers are really in the same ballpark, so a spacesuit that was designed to handle lunar night temperatures would probably be fine on Pluto. However, the Apollo suits were &lt;em&gt;not&lt;/em&gt; designed for lunar nighttime. Therefore I suspect that the astronauts' feet would have very rapidly frozen, and after that they would have quickly died of hypothermia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also interesting to speculate about what effect the astronauts' footsteps would have on Pluto's surface. I'm imagining some spectacular evaporation of nitrogen ice. It might be so violent that you wouldn't even be able to walk.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2016-07-07T18:24:48.563" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17072" PostTypeId="2" ParentId="14833" CreationDate="2016-07-07T19:55:29.430" Score="5" Body="&lt;p&gt;For &lt;strong&gt;reporting back&lt;/strong&gt; from Alpha Centauri, the &lt;a href=&quot;http://www.deepspace.ucsb.edu/wp-content/uploads/2015/04/A-Roadmap-to-Interstellar-Flight-15-d.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;roadmap&lt;/a&gt;'s pages 25-26 and figure 20 predict that an onboard 10 watt laser with narrow spread (and many assumptions, such as at most 40 photons per bit) could transmit from Alpha Centauri to a 30 m reflector mirror on Earth at 70 Mbps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the power source for a 10 W transmitter is heavy!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plutonium-238's power density is 0.54 watts per gram, so sustained 10 W transmission needs an &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;RTG&lt;/a&gt; of at least 18 grams.  (Materials with more power density, such as polonium-210, would run out of power well before the end of the 20-year voyage.) Burst transmission could be lighter, accumulating the RTG's electrical energy in a supercapacitor, which stores 15 Wh/kg. So a 1 gram supercap stores 54 watt-seconds, enough for a 10 watt 5-second burst transmission; a 1-gram RTG would result in a duty cycle of 1:18, in other words 4 Mbps sustained from Alpha Centauri.  But even that compromise of a power source weighs much more than the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More likely is an RTG+supercap a hundred times lighter, 0.02 grams.  That gives an upper bound of 40 Kbps from Alpha Centauri.  Because the bottleneck is that single 30 m mirror, if the swarm can stagger its broadcasts in sync, and if the mirror can switch its aim from one spacecraft to another fast enough, that number could become 40 Kbps per spacecraft, however many survive the voyage.  (&lt;strong&gt;Edit:&lt;/strong&gt; seen from Earth, the spacecraft won't have spread out enough to need re-aiming.  They'll appear to be transmitting from the same point.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Less likely is replacing the RTG with a nonrechargeable battery, needed only at the destination.  The longest shelf life we know of is 10 years, not nearly enough.  Likewise, although a stored chemical energy propulsion system aka &lt;a href=&quot;https://space.stackexchange.com/a/15496/1235&quot;&gt;SCEPS&lt;/a&gt; has excellent shelf life, it hasn't yet scaled down even to Mars Rover size, nevermind milligram size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Gram-size SCEPS have been attempted.  Kyle Jiang at U. Birmingham built a millimeter-scale &lt;a href=&quot;https://www.quora.com/What-happened-to-the-micro-engine-made-by-Dr-Kyle-Jiang&quot; rel=&quot;nofollow noreferrer&quot;&gt;gasoline engine&lt;/a&gt; 15 years ago, but abandoned it because of technical difficulties.  Around the same time, U. Berkeley built a gram-size 2.5 W butane-powered &lt;a href=&quot;http://www.berkeley.edu/news/berkeleyan/2001/04/10_cmbus.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wankel engine&lt;/a&gt;, and built parts for a &lt;a href=&quot;https://sem.org/wp-content/uploads/2015/12/sem.org-2003-SEM-Ann-Conf-s52p01-Invited-MEMS-Rotary-Engine-Power-System-Project-Overview-Recent.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;1 mm&lt;/a&gt; version (milligram-size), but nothing else that small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, New Horizons transmitted from Jupiter at 38 Kbps, and from Pluto at 1 Kbps.  Too bad NASA didn't build a giant receiving antenna for that, instead of waiting many years for its stored data to trickle back via radio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the full report back from A.C. could take many years.&lt;/p&gt;&#xA;" OwnerUserId="1235" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-08-10T21:07:31.140" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17074" PostTypeId="1" CreationDate="2016-07-08T02:09:18.147" Score="6" ViewCount="227" Body="&lt;p&gt;I'm no engineer, but if I was making a vessel with extending sails, solar panels, mast, antennae, etc. I would make them widest close to the core. But Juno's solar panels narrow toward the core as shown in the image below and get wider further from core. Why is this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D0MvN.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D0MvN.jpg&quot; alt=&quot;Image from NASA.gov&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="6312" LastEditDate="2016-07-08T10:39:53.047" LastActivityDate="2016-07-20T04:44:14.490" Title="Why do Juno's solar panels narrow toward the core?" Tags="|design|solar-power|juno|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17075" PostTypeId="2" ParentId="15224" CreationDate="2016-07-08T05:42:27.807" Score="3" Body="&lt;p&gt;I have never heard of a patented maneuver before. There are maneuver strategies that operators will perform but there are many common ones like a Hohmann Transfer and a pure inclination change.  I would suspect that maneuver strategy are just a closely guarded company secret, not patented.  I worked with many operators who said directly how they maneuver and their maneuver strategy and another would not provide any details in fear their strategy would be leak somehow. There are also consultants that would help you design your trajectory like &lt;a href=&quot;http://www.see.com/&quot; rel=&quot;nofollow&quot;&gt;Space Exploration Engineering Corporation (SEE)&lt;/a&gt; and if they do not have any maneuver strategies, then no one does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, if you were to prove that someone performed your patented maneuver?  There is only one Space Situational Awareness system capable of this, it is the &lt;a href=&quot;http://comspoc.com/about/&quot; rel=&quot;nofollow&quot;&gt;Commercial Space Operations Center (ComSpOC).&lt;/a&gt;  So far there is minimal adoption of this but it will see more interest when the JSPOC implements the same technology as the ComSpOC in the near future.&lt;/p&gt;&#xA;" OwnerUserId="15856" LastActivityDate="2016-07-08T05:42:27.807" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17076" PostTypeId="1" AcceptedAnswerId="17083" CreationDate="2016-07-08T06:28:30.573" Score="3" ViewCount="180" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hitomi_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hitomi&lt;/a&gt; was an Astrophysics science satellite in LEO with four X-ray telescopes, for imaging and spectroscopy of soft and hard X-rays. A paper based on some of the data collected during it's short operational life was just &lt;a href=&quot;http://www.nature.com/nature/journal/v535/n7610/full/nature18627.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;published in Nature&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://global.jaxa.jp/press/2016/04/20160428_hitomi.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JAXA Press Release&lt;/a&gt; links to this &lt;a href=&quot;http://global.jaxa.jp/press/2016/04/files/20160428_hitomi.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;JAXA PDF&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Here is the &lt;a href=&quot;http://global.jaxa.jp/projects/sat/astro_h/files/topics_20160608.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;June 8, 2016 updated JAXA PDF&lt;/a&gt;, much longer and more thorough. There is also a brief synopsis in Nature, but I'm not sure if it is reliable - the title, &quot;&lt;em&gt;&lt;a href=&quot;http://www.nature.com/news/software-error-doomed-japanese-hitomi-spacecraft-1.19835&quot; rel=&quot;nofollow noreferrer&quot;&gt;Software error doomed Japanese Hitomi spacecraft&lt;/a&gt;&lt;/em&gt;&quot; is misleading to say the least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just as in aviation and transportation, an understanding and reporting of events and issues leading to loss of spacecraft is invaluable to the entire community to help avoid losses in the future. Space is hard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, I give several links to the two near-loss and recoveries of SOHO in 1998 &lt;a href=&quot;https://space.stackexchange.com/q/16644/12102&quot;&gt;here&lt;/a&gt;, including the thorough &lt;a href=&quot;http://www.ieec.cat/hosted/web-libpoint/papers/roberts.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Roberts 2002&lt;/a&gt; report. There were many lessons learned there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone help summarize the &lt;a href=&quot;http://global.jaxa.jp/press/2016/04/files/20160428_hitomi.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;JAXA report&lt;/a&gt; here to an easier to understand, if less precise explanation? It seems to be a combination of things, instructions, maneuvers, design, stresses... Could it be said that a realistic worst-case scenario was simply not properly anticipated?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://gizmodo.com/this-is-the-last-thing-japans-lost-black-hole-satellite-1783200724&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gizmodo&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EvQLy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EvQLy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.space.com/32400-japan-hitomi-xray-astronomy-satellite-infographic.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space.com&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q6g9t.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q6g9t.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-07-08T15:52:57.930" Title="Summary of events and issues leading to the loss of Hitomi?" Tags="|mission-design|failure|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17077" PostTypeId="1" AcceptedAnswerId="17078" CreationDate="2016-07-08T10:06:23.887" Score="6" ViewCount="1543" Body="&lt;p&gt;Juno probe has sent few pictures as it approached Jupiter from afar, but Jupiter looks as large as Moon on our sky on those photos. After that it went into (intentional) silence while it was trying to get to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will Juno send any pictures from it's orbit? When, if the answer is yes?&lt;/p&gt;&#xA;" OwnerUserId="1477" LastEditorUserId="1477" LastEditDate="2016-07-13T09:30:17.930" LastActivityDate="2016-07-13T09:30:17.930" Title="When will Juno send us some pictures from it's tight orbit?" Tags="|communication|jupiter|juno|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17078" PostTypeId="2" ParentId="17077" CreationDate="2016-07-08T13:38:35.243" Score="8" Body="&lt;p&gt;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2016/06090600-what-to-expect-from-junocam.html&quot;&gt;http://www.planetary.org/blogs/emily-lakdawalla/2016/06090600-what-to-expect-from-junocam.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first major day for pictures should be August 27th, but image release shouldn't start occurring until September/October.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other dates of note:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flyby 1: August 27th&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flyby 2: October 20th- No pictures planned&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flyby 3: November 2nd&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flyby 4: November 16th- The public can participate in online voting to decide what the camera will target&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then pictures should be taken every 14 days until the camera or Juno dies.&lt;/p&gt;&#xA;" OwnerUserId="15841" LastActivityDate="2016-07-08T13:38:35.243" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17079" PostTypeId="1" AcceptedAnswerId="17087" CreationDate="2016-07-08T14:11:24.637" Score="89" ViewCount="16956" Body="&lt;p&gt;Apollo 16 famously photographed the far side of the Moon, as seen below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IMLGp.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IMLGp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems odd to me. Apollo 16 orbited the Moon at an orbit of only 100 km. It seems too close to get this kind of a photograph of the Moon. How was this picture captured with a global view of the Far Side, from Apollo 16?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="11861" LastEditDate="2016-07-09T02:23:35.357" LastActivityDate="2021-01-12T23:31:02.023" Title="How did Apollo 16 capture this full photograph of the Moon's far side?" Tags="|the-moon|apollo-program|photography|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17080" PostTypeId="2" ParentId="17079" CreationDate="2016-07-08T14:22:28.610" Score="4" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_16#Launch_and_outbound_trip&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Apollo_16#Launch_and_outbound_trip&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When they performed a burn to enter lunar orbit above the far side, they were at ~300km above the surface.  I can't find when this photo was taken, but I'd say there's a good chance it was then.&lt;/p&gt;&#xA;" OwnerUserId="15841" LastActivityDate="2016-07-08T14:22:28.610" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17081" PostTypeId="2" ParentId="16984" CreationDate="2016-07-08T14:58:08.180" Score="3" Body="&lt;p&gt;Jupiter is about 5.2 A.U. from the sun and earth is 1 A.U. from the sun. So distance between Jupiter and Earth ranges from 4.2 to 6.2 A.U.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 A.U. takes about 500 seconds for light to traverse. So light's travel time from earth to Jupiter takes 2100 to 3100 seconds which is 35 to 52 minutes.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-07-08T14:58:08.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17082" PostTypeId="2" ParentId="17079" CreationDate="2016-07-08T15:11:26.160" Score="24" Body="&lt;p&gt;The initial Lunar orbit was roughly 100x300 km, with the further point being at the far side. From the &lt;a href=&quot;http://history.nasa.gov/ap16fj/11_Day4_Pt2.htm&quot; rel=&quot;noreferrer&quot;&gt;mission transcripts&lt;/a&gt;, we learn of a request to photograph the Moon at the terminator, at the far side (Because there was no communication with Earth) Using &lt;a href=&quot;http://www.neoprogrammics.com/spheres/visible_fraction_of_surface.php&quot; rel=&quot;noreferrer&quot;&gt;this simple calculator&lt;/a&gt;, I find that only 7% of the Moon's surface would have been visible at 300 km. The full map of the Far Side can be seen below. It looks like the far side, and the scale is clearly different, but it doesn't seem to be a factor of 7 that would be required!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WyXPl.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WyXPl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, I've found that 10:30 is supposed to be the direction of Lunar north in the image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few things stick out to me. First of all, I believe the entire image is not the far side. In fact, I believe the left most edge is actually the Near Side, as the far side doesn't have many mares, and the line between the two is near perfect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After doing quite a bit of digging around, I've found the source for this image. It turns out it wasn't taken by an astronaut at all, but rather by an instrument known as the Mapping Metric camera, which was used to map the Moon for Apollo 15-17. The film was retrieved in a short EVA on the way home from the Moon. It seems this was one of the photographs taken on the way home from the Moon, as it is one of the last in the series. You can see the entire series at the &lt;a href=&quot;http://wms.lroc.asu.edu/apollo/browse?camera=M&amp;amp;mission=16&amp;amp;page=101&quot; rel=&quot;noreferrer&quot;&gt;Apollo Digital Archive website&lt;/a&gt;. The time of the photograph isn't recorded that I can find anywhere. The last photograph before it that has a time is 3000, 1972-04-24T23:32:28, which starts to show the curvature of the Moon. I believe what actually happened is the Lunar return trajectory started on the far side, and came along a course that allowed for much of the dark side to still be seen. &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_16i_Timeline.htm&quot; rel=&quot;noreferrer&quot;&gt;Trans-Earth Injection happened at 25 Apr 1972 02:15:33&lt;/a&gt;. Likely the photo was taken slightly after TEI, although I can't find the timeline more accurately than that. The photos can be seen to slowly get further away from the Moon, starting at 3000. Before that, they were taken about every 30 seconds. They stopped taking pictures per the standard plan, but at &lt;a href=&quot;http://history.nasa.gov/ap16fj/25_Day9_Pt2.htm&quot; rel=&quot;noreferrer&quot;&gt;200:33&lt;/a&gt; they turned the camera back on for a while. That was 10 minutes after the TEI, which seems like it would be in a good place to capture the pictures they returned of the &quot;far side&quot; of the Moon. It was on for about 2 hours, until about 202:55. For certain at some point during that time a &quot;global&quot; view of the Moon would be visible, and towards the earlier portion the far side should be visible. I can't find the altitude vs time, but this now all makes sense!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-08T19:09:12.127" LastActivityDate="2016-07-08T19:09:12.127" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="17083" PostTypeId="2" ParentId="17076" CreationDate="2016-07-08T15:29:27.753" Score="7" Body="&lt;p&gt;So summarizing the larger report &lt;a href=&quot;http://global.jaxa.jp/projects/sat/astro_h/files/topics_20160608.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, JAXA breaks the failure into 4 distinct events.  These all occurred after a normal attitude adjustment.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A failure in the attitude control system (ACS) determined that the satellite was still rotating, while in fact the maneuver had already been completed. The ACS is made up of two systems, an onboard angular velocity sensor, and a star tracker.  After the maneuver was complete, there was a process to correct the differences in measurements between the two systems, but this process was cut short, and the onboard computer assumed that there must be a problem with the the star tracker (which was actually showing the correct attitude), and ignored it.&lt;/li&gt;&#xA;&lt;li&gt;Only relying on the now incorrectly calibrated angular velocity sensors, the satellite activated its reaction wheels to counter the velocity that it was sensing, but was not actually occurring.&lt;/li&gt;&#xA;&lt;li&gt;The angular momentum limitation was exceeded, and the computer determined that there must be a problem with the reaction wheels.  A system that controls attitude via thrusters was activated.  Somehow this control system malfunctioned as well, and actually increased the speed of rotation.&lt;/li&gt;&#xA;&lt;li&gt;Structural elements exceeded load limits and broke off.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In the end, there is a much more detailed breakdown of events that could have been better, and they primarily focus on potentially better design that could have prevented failure 1 and 3.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The critical failure seems to be in #3.  While the ACS could have had more redundancies, #3 is supposed to be the backup system that catches any malfunction in the reaction wheels.  There were data entry errors in this system, and incomplete verification and testing both on the part of the contractor that developed it and JAXA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some more details on #3 on page 62 and page 80.  I've pulled out some quotes from both sections.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;JAXA held discussions with the support company and decided to change the thruster control parameters according to the actual properties of the thrusters. The company started the process. Note that this operation (changing the parameters) was not described in the documents prepared prior to launch that regulated the operational plan. In addition, details of this operation (which parameters are changed and how) were not shared between the support company and JAXA. &lt;/li&gt;&#xA;&lt;li&gt;There were errors in data input by the support company when the updated thruster control parameters were calculated. Accordingly, inappropriate parameters were derived. &lt;/li&gt;&#xA;&lt;li&gt;The support company was busy with duties on that day. One reason for this was that the company had to perform a task that was not described in the document governing the operational plan.   This situation led to miscommunication of operational instructions between staff members of the company. Thus, a part of required verification was not implemented. &lt;/li&gt;&#xA;&lt;li&gt;JAXA, which was in charge of operations, did not confirm the preparation process for changing the thruster control parameters. Then, JAXA, without noticing the omission of the verification, ordered the implementation of the operation. &lt;/li&gt;&#xA;&lt;li&gt;Operational procedure plans were updated daily, congesting the workload of operational support companies involved with the ACS&lt;/li&gt;&#xA;&lt;li&gt;All tools for parameter setting were positioned as tools for use by experienced developers during development and testing, so no manuals were prepared and no operational training was conducted. There was also no overall manual of procedures for the parameter setting and simulation process.&lt;/li&gt;&#xA;&lt;li&gt;In the end, JAXA did not verify the operational preparation status of parameter changes for thruster control.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="15841" LastEditorUserId="15841" LastEditDate="2016-07-08T15:52:57.930" LastActivityDate="2016-07-08T15:52:57.930" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17085" PostTypeId="2" ParentId="17079" CreationDate="2016-07-08T18:00:28.893" Score="16" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U4pGk.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U4pGk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I roughly got into the right position to match the photo taken.  The eye altitude is a little too low, so ignore that, but I think the lunar longitude is roughly correct (120° E).  I'm very skeptical that this image was taken while at the stated lunar orbit altitudes.  Those altitudes are just way too low to have these features appear as they do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My best guess for when this picture was taken was during the return to Earth, probably not long after the trans-earth insertion burn.  At this point the CSM would be on an escape trajectory leaving the moon, so it'd very rapidly gain altitude on the way out.  It's also consistent with which hemisphere is visible (eastern), since that would be the departing side of the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a16/A16_MissionReport.pdf&quot; rel=&quot;noreferrer&quot;&gt;mission report&lt;/a&gt; also notes on page 5-8 (pdf pg 48) that the mapping camera was also used &quot;vertically&quot; after trans-earth injection.  I would guess that this image is from the mapping camera during that time.  Unfortunately I can't really find any information about exactly when and where this occurred.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another image from a lower altitude:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BdzWO.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BdzWO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I apologize for the cropping on this, Google Earth has a fixed horizontal FOV and changing the window width leaves roughly the same horizontal portion of the moon visible.  However, I think this image is a better match for the picture in the OP.  In this case, approximately 20% of the lunar surface is visible.&lt;/p&gt;&#xA;" OwnerUserId="10313" LastEditorUserId="10313" LastEditDate="2016-07-08T18:35:17.677" LastActivityDate="2016-07-08T18:35:17.677" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17086" PostTypeId="2" ParentId="17047" CreationDate="2016-07-08T19:06:37.807" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/17047/why-wasnt-an-rtg-used-on-the-juno-spacecraft/17048#17048&quot;&gt;Phiteros' answer&lt;/a&gt; states the fact that plutonium is in scarce supply and &lt;a href=&quot;https://space.stackexchange.com/questions/17047/why-wasnt-an-rtg-used-on-the-juno-spacecraft/17053#17053&quot;&gt;PearsonArtPhoto's answer&lt;/a&gt; points out that Juno's mission profile allows it to use solar panels. These issues are legitimate and (the scarcity one anyway) somewhat overriding of other issues. Nonetheless, I wish to bring up something else: Public relations and politics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the opinion of some people, using radioactive materials in a device that could explode is too risky. Thus, every time NASA uses RTG's on a mission, there are protests about the risk of exposing the public to radiation should there be a launch accident.  This brings about negative news reports about the agency. It also likely brings some pressure on NASA from some politicians to find another way to power its spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I'm not agreeing with the position that RTG's are too dangerous to launch into space, just pointing it out.&lt;/p&gt;&#xA;" OwnerUserId="36" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-07-08T20:31:52.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17087" PostTypeId="2" ParentId="17079" CreationDate="2016-07-08T21:38:23.557" Score="168" Body="&lt;p&gt;The photo (frame 3021) appears to have been taken from an approximate altitude of 1180 KM, on the return journey to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We infer it was taken on the return journey as &lt;a href=&quot;http://www.lpi.usra.edu/lunar/missions/apollo/apollo_16/photography/&quot;&gt;frame 3005 was taken after trans-Earth injection&lt;/a&gt;. And, presumably, by &quot;Revolution: TE&quot; in the image's information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.lpi.usra.edu/resources/apollo/frame/?AS16-M-3021&quot;&gt;image information&lt;/a&gt; tells us the photo was taken by the Metric/Mapping Camera. NASA provides details of the &lt;a href=&quot;http://www.history.nasa.gov/afj/simbaycam/fairchild-lunar-mapping-camera.htm&quot;&gt;The Fairchild Lunar Mapping Camera&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The metric camera has a 3-inch (76-mm) focal length, f/4.5 lens. The&#xA;  format is 4½ x 4½ inches on 5-inch-wide film and the angular coverage&#xA;  is 74° by 74°.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Knowing the camera's angle of view and the radius of the moon, we can calculate the minimum altitude required to see a full disc (a 360° horizon) using the law of sines. The line from our camera to the horizon is a tangent, which gives our second angle, a right-angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q1Idk.gif&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q1Idk.gif&quot; alt=&quot;Minium altitude to see a full disc&quot;&gt;&lt;/a&gt;&lt;a href=&quot;https://i.stack.imgur.com/5n8Zr.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5n8Zr.png&quot; alt=&quot;Calculate the altitude given an observer&amp;#39;s angle of view and sphere&amp;#39;s radius&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;h+R/sin 90 = R/sin θ&lt;br&gt;&#xA;h+R/1 = R/sin θ&lt;br&gt;&#xA;h = R/sin θ - R&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moon's equatorial radius: 1738.1 km&lt;br&gt;&#xA;Angle of View: 73.74° (calculated from the focal length and format)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;h = 1738.1/sin(73.74/2) - 1738.1 = 1158.726 km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Knowing the minimum altitude required, and assuming the image shows the entire photo, we can recreate the camera using 3D software and quickly find its general position in relation to a scale model of the moon by lining up the features.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more precise method would probably use known distances between features.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using 3D software we can also see what the image would have looked like had it been taken at an altitude of 300km. (The red highlight indicates the near-side.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/95Vay.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/95Vay.jpg&quot; alt=&quot;Recreated Lunar Mapping Camera position&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The visible footprint viewed from a more familiar angle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oD5sl.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oD5sl.png&quot; alt=&quot;The visible footprint&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16119" LastEditorUserId="16119" LastEditDate="2016-07-10T20:42:26.600" LastActivityDate="2016-07-10T20:42:26.600" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17088" PostTypeId="2" ParentId="17074" CreationDate="2016-07-08T23:29:56.420" Score="1" Body="&lt;p&gt;At first I thought it had something to do with reinforcing the wings, but they cannot support themselves in gravity anyway. Looking into it further I found this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The layout and size of the panels are oriented in that nice symmetrical hexagon so the instruments will have an unconstrained field of view,&quot; Gehling said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;http://www.nasa.gov/mission_pages/juno/launch/Juno_solarpower.html&quot; rel=&quot;nofollow&quot;&gt;NASA: Juno's Solar Cells Ready to Light Up Jupiter Mission&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="16120" LastEditorUserId="6816" LastEditDate="2016-07-08T23:38:36.783" LastActivityDate="2016-07-08T23:38:36.783" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17089" PostTypeId="1" AcceptedAnswerId="17091" CreationDate="2016-07-09T02:07:43.300" Score="6" ViewCount="13043" Body="&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=001IXnp0ogc&quot; rel=&quot;nofollow noreferrer&quot;&gt;this awesome Youtube video&lt;/a&gt; of HD footage from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Civilian_Space_eXploration_Team#Space_Shot_2014_.22GoFast.22_Rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;GoFast 2014 Rocket&lt;/a&gt;, a de-spin maneuver is performed. Is this a &lt;a href=&quot;https://en.wikipedia.org/wiki/Yo-yo_de-spin&quot; rel=&quot;nofollow noreferrer&quot;&gt;yo-yo de-spin&lt;/a&gt;?&lt;/p&gt;&#xA;&lt;p&gt;You can see the de-spin at &lt;a href=&quot;https://youtu.be/001IXnp0ogc?t=56&quot; rel=&quot;nofollow noreferrer&quot;&gt;0:57&lt;/a&gt; and at &lt;a href=&quot;https://youtu.be/001IXnp0ogc?t=225&quot; rel=&quot;nofollow noreferrer&quot;&gt;3:47&lt;/a&gt;, where it really looks like a bolo is flying away, but I can't be certain. It's only a few frames and it's not possible to slow-mo Youtube videos (or frame advance). YouTube frame advance. Pause video. Press period key to advance and comma key to back up one frame at a time.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qM2oU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qM2oU.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The video is great - even though there is no sound in space, the vibrations of the craft are picked up by the cameras.&lt;/p&gt;&#xA;" OwnerUserId="12742" LastEditorUserId="37680" LastEditDate="2020-09-12T03:31:48.097" LastActivityDate="2020-09-12T03:31:48.097" Title="How did the GoFast 2014 Rocket de-spin?" Tags="|amateur-rocketry|" AnswerCount="3" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17090" PostTypeId="2" ParentId="17089" CreationDate="2016-07-09T02:18:08.037" Score="5" Body="&lt;p&gt;I shortly after found &lt;a href=&quot;https://www.youtube.com/watch?v=L4eY_N9IS3w&quot; rel=&quot;nofollow&quot;&gt;this video&lt;/a&gt;, which confirms it's a yo-yo de-spin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder why this guy called his video &quot;flat Earth dome&quot;. I didn't spend any time looking at his other videos, but it appears he's a flat Earth believer, and if so, can't be trusted...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to have a more reputable source.&lt;/p&gt;&#xA;" OwnerUserId="12742" LastEditorUserId="12742" LastEditDate="2016-07-09T02:26:15.257" LastActivityDate="2016-07-09T02:26:15.257" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17091" PostTypeId="2" ParentId="17089" CreationDate="2016-07-09T02:35:33.653" Score="8" Body="&lt;p&gt;I don't have any direct information but I can't think of anything else it could be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it were roll thrusters (cold gas or monoprop) you'd expect to see much slower response (because you'd want them to be no larger/heavier than necessary), they'd have continuous control, and there would be controller artifacts -- overshoot or undershoot. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think a reaction wheel of any reasonable flywheel/battery mass could give that kind of performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sound on the video seems consistent with an unwinding cable of some kind - rasp-and-clunk, with the deceleration on the clunk.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-09T02:44:21.127" LastActivityDate="2016-07-09T02:44:21.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17092" PostTypeId="2" ParentId="1599" CreationDate="2016-07-09T07:59:10.633" Score="5" Body="&lt;p&gt;Yes, it does work.  The first Australian satellite, Australis OSCAR 5, used a magnet to approximately align one axis with the earth's magnetic field AND rods with a large magnetic hysteresis loop to remove spin energy.  AO5 was built in 1966-7 and launched by NASA on a Delta rocket with a Tiros satellite in 1970.  Crude, passive and effective.&lt;/p&gt;&#xA;" OwnerUserId="16123" LastActivityDate="2016-07-09T07:59:10.633" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17093" PostTypeId="1" AcceptedAnswerId="17103" CreationDate="2016-07-09T08:32:40.220" Score="7" ViewCount="3032" Body="&lt;p&gt;When a rocket is traveling through an atmosphere, the component of the aerodynamic force in the direction of motion is called &lt;em&gt;drag&lt;/em&gt;, and the component perpendicular to that is called &lt;em&gt;lift&lt;/em&gt;. Usually a rocket is pointed in &lt;em&gt;nearly the same&lt;/em&gt; direction as motion, but not necessarily exactly. See the NASA illustration below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For simulation, and for flight guidance and control, consideration of lift is essential. &lt;em&gt;Lift is therefore important.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question though: is lift ever &lt;em&gt;useful&lt;/em&gt; when rockets are traveling in an atmosphere? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here, I mean &quot;rocket-shaped&quot; rockets; not car-shaped, or plane-shaped rockets, but the long, approximately cylindrically symmetric ones that tend to fly from low altitudes up into space. (yes, and more frequently these days, back down again).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image from &lt;a href=&quot;https://spaceflightsystems.grc.nasa.gov/education/rocket/rktstab.html&quot; rel=&quot;noreferrer&quot;&gt;https://spaceflightsystems.grc.nasa.gov/education/rocket/rktstab.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7ACyE.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7ACyE.gif&quot; alt=&quot;NASA Rocket Forces&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2022-08-14T18:00:27.620" Title="Is aerodynamic lift ever useful in rocket flight?" Tags="|rockets|atmospheric-drag|guidance|lift|flight-control|" AnswerCount="4" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17094" PostTypeId="1" CreationDate="2016-07-09T10:47:26.670" Score="8" ViewCount="667" Body="&lt;p&gt;Hubble, besides the initial high-profile COSTAR, was upgraded many times giving essentialy a new instrument with the same heavy/expensive glass. It was set in LEO for that purpose even though it has some significant drawbacks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now JWST is going to be at &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Orbit&quot; rel=&quot;noreferrer&quot;&gt;L2 of the Earth&lt;/a&gt;, nearly a million miles out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How will it be serviced and maintained?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This point is dynamicly unstable, so it will use up fuel to stay put. Will it use the last of its fuel to come back, so it can be cheaply refilled?  I recall that it's &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Transport_Network&quot; rel=&quot;noreferrer&quot;&gt;easy to get between L1 and L2&lt;/a&gt; but that doesn’t help with the necessary ∆v needed to park in LEO.&lt;/p&gt;&#xA;" OwnerUserId="6471" LastActivityDate="2016-07-09T13:21:07.050" Title="How will JWST be serviced?" Tags="|spacecraft|james-webb-telescope|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17095" PostTypeId="2" ParentId="17094" CreationDate="2016-07-09T10:53:12.067" Score="12" Body="&lt;p&gt;&lt;a href=&quot;http://jwst.nasa.gov/faq_scientists.html#astronauts&quot; rel=&quot;noreferrer&quot;&gt;It will not be serviced&lt;/a&gt;. When it runs out of fuel or there's a problem, it's End of Mission. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Because Webb, like virtually every satellite ever constructed, will not be serviceable it employs an extensive seven year integration and test program to exercise the system and uncover any issues prior to launch so they might be remedied. Unlike Hubble, which orbits roughly 350 miles above the surface of Earth and was therefore accessible by the Space Shuttle, Webb will orbit the second Lagrange point (L2), which is roughly 1,000,000 miles from Earth. There is currently no servicing capability that can be used for missions orbiting L2, and therefore the Webb mission design does not rely upon a servicing option.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They tried to increase reliability of the mechanical components: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The gyroscopes on HST and Chandra are mechanical devices dependent on bearings for their function, and they face problems typical of such designs. Webb has adopted a different gyroscope technology. The &quot;Hemispherical Resonator Gyroscope&quot; (HRG) uses a quartz hemisphere vibrating at its resonant frequency to sense the inertial rate. The hemisphere is made to resonate in a vacuum, and the hemisphere's rate of motion is sensed by the interaction between the hemisphere and separate sensing electrodes on the HRG housing. The result is an extremely reliable package with no flexible leads and no bearings. The internal HRG operating environment is a vacuum, thus once the gyroscope is in space any housing leaks would actually improve performance. The HRG eliminates the bearing wear-out failure mode, leaving only random failure and radiation susceptibility of the electronics (which all such devices share, and which can be mitigated by screening and shielding).  Stress analyses of HRGs show this design has a &quot;mean time before failure&quot; of 10 million hours. As of June 2011, this type of device had accumulated more than 18 million hours of continuous operation in space on more than 125 spacecraft without a single failure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-07-09T13:21:07.050" LastActivityDate="2016-07-09T13:21:07.050" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17096" PostTypeId="1" AcceptedAnswerId="17098" CreationDate="2016-07-09T11:35:30.160" Score="4" ViewCount="166" Body="&lt;p&gt;Juno has large solar panels on it.   I was curious about their effectiveness when Juno is closest to Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does Jupiter have a significant impact on the solar panels?   Can the solar panels convert Jupiter's radiation into energy?     Does Jupiter's large magnetosphere impede the solar panels ability to convert the sun's energy into power?&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2016-07-10T21:14:11.207" Title="Jupiter's impact on Juno's solar panels" Tags="|jupiter|solar-power|juno|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17097" PostTypeId="2" ParentId="17093" CreationDate="2016-07-09T11:43:49.820" Score="6" Body="&lt;p&gt;You said it yourself, Lift is important for guidance and control. As a matter of fact a rocket is designed in such a way, that the center of pressure is aft of the center of gravity. The distance between CG and CP is also called the caliber stability margin measured in rocket caliber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can use the fins of a rocket to control the direction and magnitude of the lift force and therefore change the positon of the center of pressure and therefore the stability of your rocket in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence, yes, lift is useful. &lt;/p&gt;&#xA;" OwnerUserId="16129" LastEditorUserId="12102" LastEditDate="2016-07-11T08:24:06.107" LastActivityDate="2016-07-11T08:24:06.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17098" PostTypeId="2" ParentId="17096" CreationDate="2016-07-09T12:06:37.090" Score="3" Body="&lt;p&gt;The solar panels cannot convert radiation found in the magnetic fields to energy. The radiation will degrade the solar panels over time, to the point where they eventually will not produce as much power as they do now. The panels were purposely built in such a way to accept this degradation in power over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno will no go into eclipse during it's mission lifetime, except for during the Earth flyby. There will be minimal effect overall from being close to Jupiter, aside from the long term radiation degradation.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-10T21:14:11.207" LastActivityDate="2016-07-10T21:14:11.207" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17099" PostTypeId="1" AcceptedAnswerId="17100" CreationDate="2016-07-09T13:01:58.817" Score="2" ViewCount="812" Body="&lt;p&gt;Drag is aerodynamic force component &lt;em&gt;parallel&lt;/em&gt; to the direction of motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lift is aerodynamic force component &lt;em&gt;perpendicular&lt;/em&gt; to the direction of motion.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Direction of motion &lt;em&gt;with respect to what?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;1) &lt;strong&gt;The instantaneous local atmosphere (the air)?&lt;/strong&gt; If the wind suddenly changes, does the &lt;em&gt;displacement angle&lt;/em&gt; (below) change because the rocket keeps going in one direction, but the aerodynamic force changes because the air is now moving differently?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) &lt;strong&gt;Earth-centered, Earth-rotating frame?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) &lt;strong&gt;Earth-centered, inertial frame?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to write a few lines of code, but I was stopped cold when I realized that a (very strong) gust of wind can induce lift - depending on how the vectors are defined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image from &lt;a href=&quot;https://spaceflightsystems.grc.nasa.gov/education/rocket/rktstab.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://spaceflightsystems.grc.nasa.gov/education/rocket/rktstab.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7ACyE.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7ACyE.gif&quot; alt=&quot;NASA Rocket Forces&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="25" LastEditDate="2016-07-09T13:22:49.687" LastActivityDate="2016-07-09T17:53:06.020" Title="Rocket drag and lift based on flight direction - in which frame of reference?" Tags="|rockets|atmospheric-drag|guidance|lift|flight-control|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17100" PostTypeId="2" ParentId="17099" CreationDate="2016-07-09T13:25:55.800" Score="3" Body="&lt;p&gt;As I believe you suspected, it is based off of the local wind direction. Changing winds will affect the way that lift affects the rocket. As specified at &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_(physics)&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Drag ... is a force acting opposite to the relative motion of any object moving with respect to a surrounding fluid.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-09T13:25:55.800" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17101" PostTypeId="2" ParentId="17093" CreationDate="2016-07-09T15:18:36.840" Score="7" Body="&lt;p&gt;As far as I know, no. In order to make a cylindrical rocket as light as possible, they are flown to minimize the side loads to the structure -- as close to a zero angle of attack as possible. If they wanted to use lift, it would increase the mass of the structure to be able to take substantial drag forces from the side. There would not be sufficient benefit from the lift of a cylindrical structure to offset that mass increase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wings or other lifting surfaces on the other hand (which you are ruling out), could potentially provide some benefit in the early parts of ascent. That was the case for Pegasus.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-07-09T17:08:41.617" LastActivityDate="2016-07-09T17:08:41.617" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17102" PostTypeId="1" CreationDate="2016-07-09T15:58:39.573" Score="10" ViewCount="987" Body="&lt;p&gt;If we are ever able to built a spacecraft that can travel near the speed of light, how can we make it to our destination in one piece? I mean at such speed a tiny tiny rock (even space dust?) would completely destroy the spacecraft. At such speed, you can't really dodge them either, so how would it be possible?&lt;/p&gt;&#xA;" OwnerUserId="16132" LastEditorUserId="25" LastEditDate="2016-07-09T21:11:49.543" LastActivityDate="2016-07-10T21:57:17.540" Title="Debris in interstellar flight" Tags="|interstellar-travel|debris|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17103" PostTypeId="2" ParentId="17093" CreationDate="2016-07-09T17:44:32.107" Score="8" Body="&lt;p&gt;Angling to get lift is going to increase the atmospheric cross-section of the rocket and so increase drag. For any reasonable angle of attack, the drag force is going to be much larger than the lift force, so I believe that for powered ascent it makes the most sense to minimize drag, which means zero AoA and zero lift. This also, as Mark Adler notes, minimizes lateral stresses on the vehicle, allowing structural weight to be minimized. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the total drag losses on a large rocket in ascent to LEO are small -- for Saturn V, about 0.5% of the total ∆v expenditure. Since low-AoA lift is a small fraction of drag, any gain from body lift would be extremely small. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, there have to be minor excursions from zero AoA during a powered ascent (though hopefully only small ones); these should be positive AoA in order to take advantage of what little lift there is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, lift is useful for steering during unpowered flight; the Falcon 9 first stage uses body lift to control its downrange flight to an ASDS. It's also very important for control of reentry vehicles, but those aren't &quot;rocket-shaped rockets&quot; so they're outside of the scope of your question. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-09T19:55:10.693" LastActivityDate="2016-07-09T19:55:10.693" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17104" PostTypeId="2" ParentId="17099" CreationDate="2016-07-09T17:53:06.020" Score="1" Body="&lt;p&gt;Offered up as a supplement to the accepted answer, since I like pictures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rdt3u.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rdt3u.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5hkTS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5hkTS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740026178.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Coordinate Systems for the Space Shuttle Program&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-07-09T17:53:06.020" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17106" PostTypeId="2" ParentId="17102" CreationDate="2016-07-09T23:11:34.430" Score="2" Body="&lt;h2&gt;Wear protection&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it's a joke, just build proper interstellar ship.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Most times, when people talk about destroying ship by particles at 0.9c speed, people forget about what happens with that little(or big) particle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens with that particle when it meets a cloud of particles on 0.9c?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that cloud is gas let say Hydrogen, just take enough of it and it will evaporate most particles you might encounter. I mean it might not be able to evaporate 1km asteroid (depends on how big is shield) but it might work as early detection system too, in that case, so it probably good idea to launch it(or light detection version of it) some light hours ahead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Make ship which is able to generate such cloud from gas or dust upfront. One of possibilities, if you relay on reactive propulsion for slowdown at arrival point - just let work braking engines for enough time to form such cloud. With conventional reactive engines, one day after making of that bubble, bubble will be upfront at distance 1 light second, and each day it will be 1 more light second farther. With ION propulsion systems 1 light second per each 2 hours(70 km/s exhaust velocity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There will be some effective pressure of interstellar media, it will slow down that cloud, and after some time &quot;return&quot; it back in form of plasma at almost same relative velocity, as velocity you have launched that cloud - catch it, launch back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I took it as simple way to demonstrate, but it's not only way to implement such protection. Also you will collect some heavy elements during your journey, nice, mining &quot;gold&quot; with interstellar space ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other ways to retain that shield and implement whole system, as I said, but over all in general it's simple idea.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Clarification and reference and cites&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Similar concept proposed in (link is just abstract of it) &lt;a href=&quot;http://adsabs.harvard.edu/abs/1978JBIS...31S.123B&quot; rel=&quot;nofollow&quot;&gt;A. Bond, Project Daedalus: target system encounter protection, in: A. Bond et al.,&#xA;Project Daedalus: Final Report, JBIS Suppl. (1978) S123-S125&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A concept for protecting the Daedalus vehicle against destructive impacts with large particulate debris during the stellar encounter phase of the mission is presented. In the concept a cloud of fine particles would fly ahead of the vehicle so that any large masses passing through the cloud would be heated and vapourised before the arrival of the main vehicle. Such a protective shield is made possible by virtue of the high specific kinetic energy of the cloud relative to the target star.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Although I do not have access to that report, and being unable to determine similarity and difference in details, but proposed principle is the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But overall it's maybe simpler understand in case of thin foil, or cloud of pieces of such foils, let say same 50 mkM thickness - which may evaporate bodies of significant size and mass on impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I said above, this principle may be implemented in different ways, and efficient in therms of mass needed for that.&lt;/p&gt;&#xA;" OwnerUserId="3039" LastEditorUserId="3039" LastEditDate="2016-07-10T21:57:17.540" LastActivityDate="2016-07-10T21:57:17.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17107" PostTypeId="2" ParentId="14183" CreationDate="2016-07-10T00:17:45.783" Score="8" Body="&lt;p&gt;The Apollo 10 press kit itself says the APS is rated at 3500 lbf (1589kg) thrust, and the Mission Report says the preflight predictions and in-flight inferences were in the 3434-3499 lbf range. Actual thrust isn't measured in flight, but inferred from tank pressure regulators in the system. The number 1650 which appears in the Apollo By The Numbers table happens to be the loaded ascent stage oxidizer weight for that mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, the Apollo 9 press kit gives the 3500 lbf rating, and predicted and inferred thrust values of 3338-3508 lbf. Again, the loaded ascent stage oxidizer value, 2524 lbs, is erroneously entered as the ascent thrust value in ABTN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, my hypothesis 1 was correct: the rated values in ABTN are simply transcription errors from the respective mission reports for Apollo 9 and 10, and the ascent engine was rated 3500 lbf on all flights.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;I feel a little bad about taking someone else's bounty for answering my own question -- but not all that bad.&lt;/em&gt; ;) &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-07-10T00:17:45.783" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17109" PostTypeId="1" CreationDate="2016-07-10T02:20:31.930" Score="3" ViewCount="656" Body="&lt;p&gt;Abstract: &lt;em&gt;&quot;Four, 100kg fusion warheads, launched from a Mars orbiter, can throw into the&#xA; air, enough dust to cover Mars' South Polar Cap, darken it, and cause it to&#xA; sublime through increased solar heating. The added atmospheric pressure will&#xA; set off a runaway advection effect and partially terraform the planet. We have&#xA; the warheads and the orbiters. We can start whenever we like.&quot;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this possible? If it is, current launch vehicles such as f9,delta IV,atlas V could accomplish it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;one issue is the use of an imagined 30kg fusion warhead, using a real warhead, &lt;a href=&quot;https://en.wikipedia.org/wiki/B61_nuclear_bomb&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/B61_nuclear_bomb&lt;/a&gt; weighing it at 300kg, would be more accurate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Paper link: &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.663.7945&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow&quot;&gt;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.663.7945&amp;amp;rep=rep1&amp;amp;type=pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16143" LastEditorUserId="16143" LastEditDate="2016-07-10T02:42:43.957" LastActivityDate="2016-07-10T03:15:59.790" Title="Is terraforming mars with only 4 nuclear bombs/penetrators in 10 years possible as this paper says?" Tags="|mars|nuclear|terraforming|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17110" PostTypeId="2" ParentId="17109" CreationDate="2016-07-10T03:15:59.790" Score="7" Body="&lt;p&gt;I'm not an expert in any of the topics of the paper, but on a quick read I see it's full of naive assumptions about ratios -- to cover 1/20 the area requires 1/20 the energy release, 6% of cap coverage sublimes the cap in 75 years therefore 60% coverage sublimes it in 7 years, and so forth, culminating in the hilarious warhead weight calculation -- nuclear warhead yields aren't anything remotely like linear with mass. (I love back of the envelope math and scaling assumptions, but I wouldn't go anywhere near these.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scheme hinges on a perfectly even distribution of a particular consistent &quot;fluffy&quot; dust structure as a result of bomb detonations, which, absent some experimental results, seems extremely optimistic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The author describes swapping meters for feet as a &quot;minor metrication error&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not saying the basic conclusion of &quot;a handful of fusion bombs can give Mars a dense atmosphere&quot; is wrong, but the paper as it stands is &lt;em&gt;extremely&lt;/em&gt; weak. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-07-10T03:15:59.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17111" PostTypeId="1" AcceptedAnswerId="17112" CreationDate="2016-07-10T04:25:55.167" Score="1" ViewCount="874" Body="&lt;p&gt;In this article about  max Q &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Max_Q&quot; rel=&quot;nofollow&quot;&gt;https://en.m.wikipedia.org/wiki/Max_Q&lt;/a&gt; the shuttle launch is discussed. Since there are four distinct large objects - two boosters, one shuttle and one giant tank - there could be several ways to define an effective max-Q. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are three different aerodynamic pressures on leading surfaces, and two distinct average shear forces on struts connecting the components. Since both the boosters and the shuttle produce thrust, and since each had a different mechanism for reducing thrust around the one-minute mark, this must have been quite an interesting problem compared to a single body cylindrical rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, which was more critical - pressures on the objects and their internal structure, or drag-induced shear forces &lt;em&gt;between&lt;/em&gt; objects due to imbalances of sums of drags and thrusts?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2016-07-10T17:21:08.047" LastActivityDate="2016-07-10T17:21:08.047" Title="How is max Q for the shuttle actually defined?" Tags="|space-shuttle|atmospheric-drag|max-q|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17112" PostTypeId="2" ParentId="17111" CreationDate="2016-07-10T04:42:17.960" Score="4" Body="&lt;p&gt;Max Q is simply the maximum of the dynamic pressure of the external flow, ${1\over 2}\rho v^2$. It has nothing to do with the vehicle, except for the vehicle's speed relative to the undisturbed fluid.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-07-10T05:17:19.743" LastActivityDate="2016-07-10T05:17:19.743" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17113" PostTypeId="1" AcceptedAnswerId="17116" CreationDate="2016-07-10T06:43:53.170" Score="7" ViewCount="1259" Body="&lt;p&gt;This is the question that I should have &lt;a href=&quot;https://space.stackexchange.com/q/17111/12102&quot;&gt;asked here&lt;/a&gt;. The space shuttle and the two solid rocket boosters (SRBs) are mechanically attached to the giant tank. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SRB's, shuttle, and tank all experience forces due to drag, and due to gravity. SRBs and shuttle also experience forces due to thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also mechanical forces between the SRBs and tank, and between shuttle and tank. My question here is about these forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While under flight, the SRBs are lifting the tank, so the forces of the SRBs on the tank are positive. Near the end of their propulsive flight, they separate from the tank - are they still lifting at this moment?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle's three main engines burn LOX &amp;amp; LH2 from the tank. The shuttle sometimes carries a very heavy payload, but sometimes it &lt;em&gt;does not&lt;/em&gt; - meaning the gravitational force on the shuttle can vary substantially from one flight to the next. The shuttle's main engines support the weight of the shuttle. But &lt;em&gt;do they always share in the lifting of the tank?&lt;/em&gt; Or when the shuttle has a very heavy payload, is the tank (+SRBs) actually pulling the shuttle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally, I'd like to see if there is a plot of SRB-tank, and shuttle-tank force (magnitude and sign) as a function of time, for small payload mass and maximum payload mass (Hubble?). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically &lt;em&gt;what-pushes-what vs time?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jIve0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jIve0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;above:&lt;/em&gt; STS-79 (NASA) from &lt;a href=&quot;https://en.wikipedia.org/wiki/Mobile_Launcher_Platform#/media/File:STS-79_rollout.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BlJiv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BlJiv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;above:&lt;/em&gt; STS-51f - Spacelab (NASA) from &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/flyout/multimedia/challenger/1985-07-29.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pw64c.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pw64c.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;above:&lt;/em&gt; STS-132 (NASA) from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Space_Shuttle_Atlantis_launches_on_STS-132.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/27koe.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/27koe.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;above:&lt;/em&gt; STS-117 (NASA) from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:In-flight_close_up_of_Space_Shuttle_Atlantis_during_launch_(STS-117).jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2021-05-12T00:55:40.783" Title="Shear forces between Shuttle, tank, and boosters - what pushes what?" Tags="|space-shuttle|gravity|atmospheric-drag|thrust|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17114" PostTypeId="2" ParentId="16926" CreationDate="2016-07-10T06:53:06.377" Score="6" Body="&lt;p&gt;&lt;strong&gt;tl;dr/update:&lt;/strong&gt; The corner that makes the shock wave appear to have aspects of a rectangular top is real and it is reproducible! But &lt;a href=&quot;https://space.stackexchange.com/a/37710/12102&quot;&gt;@OrganicMarble's &lt;strong&gt;new answer&lt;/strong&gt;&lt;/a&gt; now puts the issue to rest.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Together with the images from 1981 and 2015 here is a video of another test from 2017.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here it is as a GIF, and then as the video. The GIF is 30 fps from about 00:55 to 01:02 (frames 1640 to 1860). I still see a &quot;bump&quot;, and from this angle it is about a quarter of the way from the left edge of the shock wave, almost in line with the left vertical railing post.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7C9Bi.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7C9Bi.gif&quot; alt=&quot;GIF of &amp;quot;rectangular&amp;quot; shape in shock wave&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Feb 22, 2017 NASA video &lt;a href=&quot;https://youtu.be/u4VRD5cTNE4?t=50&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Rainbow View of NASA's RS-25 Engine Test&lt;/a&gt; cued at &lt;code&gt;00:50&lt;/code&gt; just before the inside view:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/u4VRD5cTNE4?start=50&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;However, the shock wave appears to be &quot;circular&quot; in cross-section during flight. Here are some still photos taken from below, during launch, from Wikipedia. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3qPYn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3qPYn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;above: STS-79 (NASA) from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:STS-79_Launch.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; - &lt;strong&gt;CLICK to ENLARGE&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zbtrq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zbtrq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;above: STS-51f, Spacelab, (NASA) from &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/flyout/multimedia/challenger/1985-07-29.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; - &lt;strong&gt;CLICK to ENLARGE&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-07-26T21:44:12.350" LastActivityDate="2019-07-26T21:44:12.350" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17115" PostTypeId="2" ParentId="16821" CreationDate="2016-07-10T07:36:45.670" Score="9" Body="&lt;p&gt;As &lt;a href=&quot;https://space.stackexchange.com/questions/2017/#comment42836_6417&quot;&gt;pointed out&lt;/a&gt; by @Thomas, this nice thesis &lt;a href=&quot;http://mediatum.ub.tum.de/node?id=1243479&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radiation from High Pressure Hydrogen-Oxygen Flames and its Use in Assessing Rocket Combustion Instability - Ph. D. Thesis, Fiala, T., 2015&lt;/a&gt; discusses this phenomenon. The term $\color{blue}{\text{blue radiation}}$ is suggested as the best available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While there are several narrow spectral lines int the near UV (about 310nm) due to excimeric hydroxyl radical $\text{OH}^*$, the blue light in the visible part of the spectrum appears to be a broad, unresolved continuum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Figure 5.2 from &lt;a href=&quot;http://mediatum.ub.tum.de/node?id=1243479&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fiala 2015&lt;/a&gt; shows low resolution spectra from $\text{H}_2 /\text{O}_2$ flames at different pressures. The nonlinearity of the intensity vs pressure can be seen in the substantial jump in intensity around 450nm as the pressure is increased from 21 to 30 bars ('atmospheres'). This may be the same onset that produces the sudden appearance of the blue radiation below the engine in the test-firing images in the question, where a first shock-wave produces a volume of sudden high pressure.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HRzxO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HRzxO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In Section 5.3. &lt;em&gt;Investigation of the Origin of the Blue Radiation&lt;/em&gt; the two most like sources of blue &lt;a href=&quot;https://en.wikipedia.org/wiki/Chemiluminescence&quot; rel=&quot;nofollow noreferrer&quot;&gt;chemiluminescence&lt;/a&gt; are presented and discussed:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hB3M2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hB3M2.png&quot; alt=&quot;Blue Radiation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Through spatial imaging of the luminescence at different pressures, combined with numerical modeling of the kinetics and evolution of the processes within the flame, &lt;a href=&quot;http://mediatum.ub.tum.de/node?id=1243479&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fiala 2015&lt;/a&gt; concludes that only the &lt;em&gt;second reaction&lt;/em&gt; (5.2) leading to the production of chemiluminescence from excimeric $\text{H}_2\text{O}_2^*$ is consistent with all of the current experimental results.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://i.stack.imgur.com/82JOE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle&lt;/a&gt; provides a nice contrasting view of both the intense yellow light presumably from some combination of blackbody radiation soot and the aluminum reaction products in the SRB's exhaust, and the blue radiation from the water-like exhaust from the shuttle's main LH2/LOX engines. The pronounced first static shock wave followed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Shock_diamond&quot; rel=&quot;nofollow noreferrer&quot;&gt;shock-diamond-like&lt;/a&gt; repeats below are clearly evident.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/82JOE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/82JOE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;above: STS-123 (NASA) from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:STS-123_closeup_launch.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; - cropped. The vertical white bar below the shuttle is possibly an anti-lightning mast, and not an exhaust plume. Guy wires can be seen as well.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-03-25T01:02:14.627" LastActivityDate="2018-03-25T01:02:14.627" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17116" PostTypeId="2" ParentId="17113" CreationDate="2016-07-10T08:16:30.800" Score="8" Body="&lt;p&gt;The three SSMEs provide &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;noreferrer&quot;&gt;568 tons&lt;/a&gt; of thrust, the Orbiter has a weight in the region of 100 tons incl payload. The Orbiter always exerts a positive force on the tank from liftoff to MECO, no matter what the payload (0-16 tons in practice).   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRBs are jettisoned when their thrust drops to &lt;a href=&quot;https://space.stackexchange.com/questions/10829/for-a-typical-shuttle-mission-how-much-solid-fuel-is-leftover-at-srb-separation/12134#12134&quot;&gt;100,000 lbf/45 tons&lt;/a&gt;, so by then their thrust/weight ratio is below 1 and they are being pulled along by the Orbiter. Thrust dropoff is very rapid, so this occurs for just a few seconds. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-07-10T14:02:39.463" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17117" PostTypeId="1" CreationDate="2016-07-10T12:37:27.417" Score="6" ViewCount="856" Body="&lt;p&gt;&lt;a href=&quot;https://www.ted.com/talks/stephen_petranek_your_kids_might_live_on_mars_here_s_how_they_ll_survive?language=en&quot; rel=&quot;noreferrer&quot;&gt;Here is the talk in question&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He talks about getting to Mars by 2027.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He talks about a device called a WAVAR which will extract water from humidity.  He also talks about a device called MOXIE which will convert the CO2 from the air into breathable oxygen.  He talks about eating dried foods to eat.  He talks about living in caves because of solar radiation.  Taking these things in isolation, they seem reasonable enough devices.  However, the energy required to run these devices must be huge.  He, no surprise, doesn't touch on the energy component.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He then goes on to talk about terraforming Mars.  He talks about heating up dry ice (CO2) in order to create an atmosphere with a solar sail.  He predicts that doing this will take 20 years before we get a planet with weather and a climate similar to British Columbia.  He does concede that it'll take 1000 years before the atmosphere is breathable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt;&#xA;To elaborate a bit on what the question is:  How much power do these systems need?  How many solar panels (or alternative form of power (ie. nuclear) will that require?  How many orders of magnitude bigger than previous rockets will one destined to Mars with this payload need to be?  Or, how many trips/dropoffs will be necessary?  That sort of thing.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It &lt;em&gt;seems&lt;/em&gt; as though it would take hundreds of dropoffs of materials before humans would be ready to stay there for an extended period of time.  Given that Earth and Mars are only close enough once every two years that means we'd either need to be launching 10s if not 100s of rockets every two years to dropoff materials or some other assumption I've already made is horribly wrong.  For starters/guidance, how many/how badly are my assumptions in the preceding paragraph?&lt;/p&gt;&#xA;" OwnerUserId="12310" LastEditorUserId="25" LastEditDate="2016-07-10T21:34:45.333" LastActivityDate="2017-03-09T00:32:10.593" Title="How plausible are the predictions made by Stephen Petranek at TED?" Tags="|mars|colonization|water|energy|terraforming|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17118" PostTypeId="1" CreationDate="2016-07-10T14:41:50.373" Score="4" ViewCount="2208" Body="&lt;p&gt;As I understand it, the average atmospheric pressure at the surface of Mars is close to the triple point of water. Is this a coincidence or does water have some sort of controlling effect on Mars' atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="16152" LastActivityDate="2016-07-11T02:04:11.117" Title="Mars' atmosphere and the triple point of water" Tags="|mars|atmosphere|water|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17119" PostTypeId="2" ParentId="3772" CreationDate="2016-07-10T14:50:31.817" Score="5" Body="&lt;p&gt;This might be of interest: CERN, in collaboration with the European Space Radiation Super Conducting Shield project are using advances in super conductor technology to develop a super conducting magnetic field to protect spacecraft and their occupants. &lt;a href=&quot;http://www.sciencealert.com/scientists-are-developing-a-magnetic-shield-to-protect-astronauts-from-cosmic-radiation&quot; rel=&quot;nofollow&quot;&gt;The aim is to create a magnetic field 3,000 times stronger than the Earth's to protect astronauts in a region of diameter ten meters enclosing the spacecraft or habitat&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find details of the power requirements. But the mass is 53.8 tons for the &quot;Pumpkin configuration&quot; and it is passively cooled so the main power would just be to generate and maintain the magnetic field in the super conducting magnets. I've put &lt;a href=&quot;http://robertinventor.com/booklets/Online-Case-for-Moon.htm#solar_storms&quot; rel=&quot;nofollow&quot;&gt;a section about it here in my Case for Moon First&lt;/a&gt;, with the cites I was able to find, but I didn't find that much so far. &lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="3038" LastEditDate="2016-07-10T16:58:04.870" LastActivityDate="2016-07-10T16:58:04.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17120" PostTypeId="2" ParentId="17102" CreationDate="2016-07-10T17:08:35.627" Score="7" Body="&lt;p&gt;&lt;a href=&quot;https://arxiv.org/abs/1010.4823&quot;&gt;Here&lt;/a&gt; is a fairly recent paper by Crawford on this topic. He considers the case of a probe traveling for 6 years at 0.1c. It appears that the most problematic material is relatively large dust grains in the 100 μm range. Most interstellar dust particles are orders of magnitude smaller than this, but the distribution has a tail of large particles. Although the tail is poorly characterized, it appears that such a probe would experience something like 2-200 impacts per square meter with particles of this size. Each of these grains would have a kinetic energy of millions of joules, which is equivalent to about a kilogram of TNT. Material shielding works for smaller particles, but can't protect against impacts this energetic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some discussion in the paper of how to solve the problem. It seems extremely speculative, but then, boosting a space probe to relativistic speeds is itself extremely speculative, and is not likely to be something humans can achieve for centuries into the future.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2016-07-10T17:08:35.627" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17121" PostTypeId="2" ParentId="8446" CreationDate="2016-07-10T20:00:30.633" Score="5" Body="&lt;p&gt;Largely automated. Flying the rendezvous by hand on the LM's own resources would have been incredibly hard, although flying it semi-manually with assistance from ground tracking and computers would, in theory, have been possible and the astronauts were provided PADs (lists of times, orientations, etc. for pre-calculated maneuvers) for this eventuality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LM ran a series of programs, using data from its own inertial guidance, and a rendezvous radar which measured the distance, radial velocity, and direction to the CM. The programs first got the LM off the ground and into a safe lunar orbit, then performed a series of phasing and plane-change maneuvers to match orbits with the CM over the course of about two orbits. The initial ascent to orbit was performed using the APS engine, and allowed a fairly wide margin for error; all of the later rendezvous maneuvers were performed using RCS, and involved delta Vs on the order of a few meters per second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CM had its own matching set of programs that computed the maneuvers from the CM's point of view, using its own instruments. While for the most part, the output of these programs was mostly used as a double-check on the LM's navigation, it was possible for the CM to &quot;swoop in&quot; and rendezvous with an LM that was unable to maneuver, once the LM had reached orbit. This never actually occurred.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the programs I've mentioned ran on the primary computer (aka LM Guidance Computer or Primary Navigation and Guidance System), but the LM also carried a second computer, simpler but in theory more reliable, called the Abort Guidance System, that was capable of carrying out these maneuvers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The computer programs would bring the LM to within about a mile of the CM at a small closing velocity, at which point semi-manual control begins, with the computer maintaining an orientation pointing towards the CM, and the astronaut using data from the radar as well as eyeballs to bring the LM to rest a few feet away from the CM. The CM then maneuvers by hand and eye to dock with the LM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/afj/loressay.htm&quot; rel=&quot;noreferrer&quot;&gt;This essay on Lunar Orbit Rendezvous&lt;/a&gt; on the NASA Apollo Flight Journal provides a lot of useful information.&lt;/p&gt;&#xA;" OwnerUserId="3055" LastActivityDate="2016-07-10T20:00:30.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17122" PostTypeId="1" AcceptedAnswerId="17123" CreationDate="2016-07-10T20:40:56.043" Score="8" ViewCount="2362" Body="&lt;p&gt;With ISp defined as ${v_e}\over{g_0}$, the naive approach would tell me, the maximum possible specific impulse, with propellant moving at speed of light relative to the craft, is $c\over{g_0}$ or 30,570,322s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, with relativistic physics naive approach is often wrong and I feel like I'm making some oversimplification here... so, if I am, could someone straighten it out? What happens with Specific Impulse when extreme energies are put into the propulsion, accelerating the propellant to relativistic speeds?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2021-04-02T05:31:31.643" Title="Is there a maximum ISp?" Tags="|specific-impulse|relativistic-rocket|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17123" PostTypeId="2" ParentId="17122" CreationDate="2016-07-10T21:30:09.940" Score="7" Body="&lt;p&gt;A photon rocket should have a &lt;em&gt;V&lt;sub&gt;e&lt;/sub&gt;&lt;/em&gt; of &lt;em&gt;c&lt;/em&gt;, hence ~30 megasecond I&lt;sub&gt;&lt;em&gt;sp&lt;/em&gt;&lt;/sub&gt;. The rocket equation would be tricky to apply, of course -- if you have a magic matter-energy reaction that perfectly converts &quot;fuel&quot; into linear-directed photon exhaust, it works fine, but if energy is going elsewhere, the effective exhaust velocity will be different. You'd &lt;a href=&quot;https://en.wikipedia.org/wiki/Relativistic_rocket#Relativistic_rocket_equation&quot; rel=&quot;noreferrer&quot;&gt;account for relativistic effects in applying the ∆v&lt;/a&gt; using everyone's favorite sigmoid function, the hyperbolic tangent. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;what, you don't have a favorite sigmoid function?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-07-10T21:30:09.940" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17124" PostTypeId="2" ParentId="17117" CreationDate="2016-07-10T21:34:12.283" Score="3" Body="&lt;p&gt;Converting CO2 into oxygen is feasible, and is discussed in a number of sources. Water from humidity is possible, but there isn't enough on Mars. It turns out heating rocks, however, will extract some water, and that could be done, although it might be a bit difficult. Both of these are discussed in &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/The_Case_for_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Case for Mars&lt;/a&gt;&quot;, which I don't have my copy available at the moment. Living in caves is an idea that is discussed fairly heavily, and is a generally agreed good idea for early settlements. The power required would be on the order of kilowatts to manage the water/ oxygen production. Simply put, it's easier to bring the materials needed to produce oxygen and water than it is to bring the water/ oxygen with you from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The terraforming ideas is where things really break down. It would take far longer than 20 years to do any kind of real system. In 1000 years, well, that's a pretty acceptable answer that is commonly used.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="198" LastEditDate="2017-03-09T00:32:10.593" LastActivityDate="2017-03-09T00:32:10.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17125" PostTypeId="1" AcceptedAnswerId="17126" CreationDate="2016-07-10T23:05:22.303" Score="0" ViewCount="2085" Body="&lt;p&gt;What is the typical weight of the propellant required to send a tiny armature rocket into orbit? What is the cost of such a small rocket in today's dollars?&lt;/p&gt;&#xA;" OwnerUserId="16163" LastEditorUserId="6816" LastEditDate="2016-07-10T23:54:37.957" LastActivityDate="2016-07-11T01:00:44.197" Title="What is the cost of sending a small payload to space?" Tags="|rockets|model-rocketry|" AnswerCount="1" CommentCount="3" ClosedDate="2016-07-11T12:09:46.287" ContentLicense="CC BY-SA 3.0" />
  <row Id="17126" PostTypeId="2" ParentId="17125" CreationDate="2016-07-11T00:40:14.473" Score="2" Body="&lt;p&gt;According to &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/nexgen/Nexgen_Downloads/AIAA_JPC_Paper_Review_Measure_Space_Trans_Afford_r8.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA-Kennedy paper&lt;/a&gt;, this plot shows actual launch costs in $/kg for various payload sizes for payloads sent to low-earth orbit (LEO): &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VNnmt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VNnmt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This plot is consistent with the NASA-Marshall's &lt;a href=&quot;http://www.nasa.gov/centers/marshall/news/background/facts/astp.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;website&lt;/a&gt; which states: &quot;Today, it costs $10,000 to put a pound of payload in Earth orbit.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typically, getting a payload to geosynchronous orbit costs three times as much as getting it to LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recently, start-up space companies have been promising much lower costs.  For example, SpaceX is talking about &lt;a href=&quot;http://www.buildtheenterprise.org/spacex-breaking-the-1000-per-pound-launch-cost-barrier&quot; rel=&quot;nofollow noreferrer&quot;&gt;breaking the \$1000 per pound ($2,200/kg) Launch Cost Barrier&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for how much a &quot;small&quot; payload costs, as you can see in the above plot, rockets have economies of scale: the larger payloads typically have lower per kg costs.  For low cost, your best bet may be to 'hitchhike' on a larger payload.&lt;/p&gt;&#xA;" OwnerUserId="11598" LastEditorUserId="11598" LastEditDate="2016-07-11T01:00:44.197" LastActivityDate="2016-07-11T01:00:44.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17127" PostTypeId="2" ParentId="17118" CreationDate="2016-07-11T01:05:17.853" Score="7" Body="&lt;p&gt;Mars' surface atmosphere ranges widely in both temperature and pressure. This range occasionally includes the triple point of water. I think it's fairly safe to say that currently the triple point of water is not having much of a direct, controlling effect on the atmosphere, except an influence on what small amount of water might be found in the atmosphere. However, it certainly may have a large influence on the atmosphere's behavior over billions of years, as it would have the surface water and ice.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;A triple point is a point in a 2D plot phase plot with temperature and pressure as axes. Below is a &lt;a href=&quot;http://www1.lsbu.ac.uk/water/water_phase_diagram.html&quot; rel=&quot;noreferrer&quot;&gt;phase plot for water&lt;/a&gt; which is from &lt;a href=&quot;http://www1.lsbu.ac.uk/water/&quot; rel=&quot;noreferrer&quot;&gt;this amazing site about water&lt;/a&gt;. Note - this site is &lt;code&gt;extensively hyperlinked&lt;/code&gt;. Click on a feature inside the phase plot (on the web page) for example, and the reference PDF will appear!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To the left of the triple point - where temperature is below &lt;a href=&quot;http://www1.lsbu.ac.uk/water/water_properties.html&quot; rel=&quot;noreferrer&quot;&gt;exactly +0.01C&lt;/a&gt;, liquid can not exist very long. Water will tend to change directly between solid and gas. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To the right of the triple point, up to some high temperature - water can exist as a stable liquid over a range of pressures that depends on temperature. At higher temperatures water can become supercritical and outside the scope of this answer (whew, dodged that one). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've added a rounded rectangle to indicate &lt;em&gt;roughly&lt;/em&gt; the range of temperatures and pressures that may occur on the Martian surface. I used ballpark values of 150K to 293K as guidelines for atmospheric temperature extremes (looking at numbers &lt;a href=&quot;https://en.wikipedia.org/wiki/Climate_of_Mars&quot; rel=&quot;noreferrer&quot;&gt;from here&lt;/a&gt;, and I found a table with approximate &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Mars#Structure&quot; rel=&quot;noreferrer&quot;&gt;pressures for extremes of elevations on Mars&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; &quot;Armstrong Limit&quot; &lt;em&gt;does not&lt;/em&gt; refer to a location on mars in that table, although I think it should, and some locations are actually on Earth, not Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The triple point of water falls in the upper right corner of this area. So at very low altitudes, like at the bottom of Hellas Planitia, on a very hot day, liquid water could rest comfortably exposed to atmosphere until it starts cooling down in the afternoon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/opbWR.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/opbWR.png&quot; alt=&quot;Phase diagram of water with MARS info&quot;&gt;&lt;/a&gt;&#xA;&lt;strong&gt;above&lt;/strong&gt; phase diagram of water &lt;a href=&quot;http://www1.lsbu.ac.uk/water/water_phase_diagram.html&quot; rel=&quot;noreferrer&quot;&gt;from here&lt;/a&gt;, with annotations about Mars added.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LP81v.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LP81v.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;strong&gt;above&lt;/strong&gt; surface elevation of Mars &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Mars_topography_(MOLA_dataset)_with_poles_HiRes.jpg&quot; rel=&quot;noreferrer&quot;&gt;from here&lt;/a&gt; (larger size available). The giant depression in deep blue centered around 40S, 70W is &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Mars_topography_(MOLA_dataset)_with_poles_HiRes.jpg&quot; rel=&quot;noreferrer&quot;&gt;Hellas Planitia&lt;/a&gt; - the lowest area on the surface. The highest point is the summit of &lt;a href=&quot;https://en.wikipedia.org/wiki/Olympus_Mons&quot; rel=&quot;noreferrer&quot;&gt;Olympus Mons&lt;/a&gt;, the farthest mountain on the left at 19N, 134W.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-07-11T02:04:11.117" LastActivityDate="2016-07-11T02:04:11.117" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17128" PostTypeId="1" AcceptedAnswerId="17253" CreationDate="2016-07-11T01:12:15.157" Score="2" ViewCount="895" Body="&lt;p&gt;Fixed nozzles designed for vacuum are larger/broader than those designed for atmospheric or combined air/space use, which have less flanged nozzles due to the drag a larger flange would incur in the atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;How much efficiency is conceded by reducing the flange of the nozzle for use at ground launch, compared with a nozzle designed for use purely in a (near) vacuum?&lt;/p&gt;&#xA;&lt;h3&gt;Edit:&lt;/h3&gt;&#xA;&lt;p&gt;Rephrasing the question:&lt;/p&gt;&#xA;&lt;p&gt;How much less efficient is a fixed nozzle designed for optimal sea-level launch efficiency when used in a vacuum when compared with a fixed nozzle designed for optimal in-vacuum efficiency?&lt;/p&gt;&#xA;" OwnerUserId="10584" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-10-11T07:01:21.430" Title="How much efficiency is lost from a fixed De Laval nozzle when modified for atmospheric use?" Tags="|atmosphere|vacuum|nozzle|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17129" PostTypeId="1" AcceptedAnswerId="17130" CreationDate="2016-07-11T01:34:38.920" Score="9" ViewCount="5782" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/17122/12102&quot;&gt;Is there a maximum &lt;span class=&quot;math-container&quot;&gt;$\text{I}_{sp}$&lt;/span&gt;?&lt;/a&gt; reminded me that I once read somewhere that the maximum possible &lt;span class=&quot;math-container&quot;&gt;$\text{I}_{sp}$&lt;/span&gt; for a rocket engine &lt;em&gt;based on expansion driven by exothermic, chemically reacting propellant(s)&lt;/em&gt; is about 450 seconds.&lt;/p&gt;&#xA;&lt;p&gt;(Actually I read 4500 m/s and I'm just dropping a zero instead of dividing by 9.8.)&lt;/p&gt;&#xA;&lt;p&gt;Is this about right? if so, how was that actually determined? Was there just limits on kCal/mole and kg/mole and some thermodynamic wisdom, or something more?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; if there is a more accepted, concise term than &amp;quot;exothermic chemical reaction rockets&amp;quot; or &amp;quot;rocket engine based on expansion driven by exothermic chemically reacting propellant(s)&amp;quot; &lt;em&gt;please tell me&lt;/em&gt;! I want to say 'ya know - normal rockets'.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; the search &lt;code&gt;site:space.stackexchange.com 4500 m/s&lt;/code&gt; turns up a number of locations here. For example &lt;a href=&quot;https://space.stackexchange.com/q/5601/12102&quot;&gt;this question&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; from surface to LEO is 9000 m/s,&lt;/p&gt;&#xA;&lt;p&gt;typical exhaust velocity &lt;span class=&quot;math-container&quot;&gt;$v_e$&lt;/span&gt; = 4500 m/s&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;But is this a practical limit? I don't mean what's the highest &lt;span class=&quot;math-container&quot;&gt;$\text{I}_{sp}$&lt;/span&gt; chemical engine demonstrated, I really would like to learn about a thermodynamic limit based on chemistry and thermodynamics.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2021-09-15T12:17:35.907" Title="Is there a maximum Isp for &quot;exothermic chemical reaction rockets&quot;?" Tags="|rockets|engines|chemistry|specific-impulse|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17130" PostTypeId="2" ParentId="17129" CreationDate="2016-07-11T03:38:41.333" Score="12" Body="&lt;p&gt;450-455s &lt;em&gt;I&lt;sub&gt;sp&lt;/sub&gt;&lt;/em&gt; is typical of H&lt;sub&gt;2&lt;/sub&gt;/O&lt;sub&gt;2&lt;/sub&gt;; according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_rocket_propellant#Propellant_table&quot; rel=&quot;noreferrer&quot;&gt;the Huzel and Huang data&lt;/a&gt;, a hydrogen-beryllium mix combusted with oxygen can hit ~540s. The numbers in that table are for moderate chamber pressure and expansion ratio; higher values are possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The highest specific impulse for a chemical propellant ever test-fired in a rocket engine was 542 seconds (5,320 m/s) with a tripropellant of lithium, fluorine, and hydrogen. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't know what the theoretical limits are. I know that with complex molecules in the exhaust, substantial kinetic energy is held in the form of vibration within the molecular bonds, not contributing to thrust -- this is one of the reasons H&lt;sub&gt;2&lt;/sub&gt;/O&lt;sub&gt;2&lt;/sub&gt; engines are run hydrogen-rich; H&lt;sub&gt;2&lt;/sub&gt; doesn't flex the way  H&lt;sub&gt;2&lt;/sub&gt;O does -- so looking for high-energy reactions in complex compounds comes with diminishing returns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://yarchive.net/space/rocket/fuels/fluorine.html&quot; rel=&quot;noreferrer&quot;&gt;Bruce Dunn post on yarchive&lt;/a&gt; claims without citation:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Isps in the mid 700s are not even theoretically possible, let alone a&#xA;  practical proposition.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-11T15:11:56.590" LastActivityDate="2016-10-11T15:11:56.590" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17131" PostTypeId="1" CreationDate="2016-07-11T07:28:50.613" Score="5" ViewCount="191" Body="&lt;p&gt;This &lt;a href=&quot;https://twitter.com/nasaspaceflight/status/585092870652239872&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Spaceflight Tweet&lt;/a&gt; shows historical photos of (presumably) Antonio Elias wishing Pegasus well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Does anyone know roughly when were these Pegasus photos taken? Could they be circa the first flight in 1990? Before or after?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly it worked in any case. After 25 years and 37/42 successful launches, Pegasus II is coming. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tweet links to this article: &lt;a href=&quot;https://www.nasaspaceflight.com/2015/04/pegasus-celebrates-quarter-century-milestone/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pegasus Celebrates a Quarter of a Century Milestone&lt;/a&gt;. Some more info and photos from that timeframe can be found in the video below.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ku4Tp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ku4Tp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Some more history from &lt;a href=&quot;https://www.youtube.com/watch?v=oY3GclS5VUQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube&lt;/a&gt;, including Kubrick's &lt;em&gt;other&lt;/em&gt; propulsion-related film.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/oY3GclS5VUQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-05-23T09:11:50.000" LastActivityDate="2019-05-23T09:11:50.000" Title="Roughly when were these Pegasus photos taken?" Tags="|history|launch-vehicle|first-flight|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="17132" PostTypeId="2" ParentId="17129" CreationDate="2016-07-11T08:25:23.250" Score="12" Body="&lt;p&gt;The theoretical limit is set by the specific energy of the reaction of combustion of the propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Knowing specific energy $e$ of given substance, we can put a cap on obtainable specific impulse $I_{sp}$ by assuming 100% of conversion of chemical energy to kinetic energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ I_{sp} = {v_e \over g_0} $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ E_{chem} = e m \geqslant  E_k = {1 \over 2 }{ m v_e^2} $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ v_e \leqslant \sqrt{2e } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ I_{sp} \leqslant {\sqrt{2e } \over g_0} $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How close we can approach to this theoretical limit is the matter of engineering and construction of the engine. For example, for the common LH2+LO2 cryofuel, the specific energy is &lt;a href=&quot;https://en.wikipedia.org/wiki/Energy_density_Extended_Reference_Table&quot;&gt;13.43MJ/kg&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$I_{sp}  \leqslant {\sqrt{2 \cdot 13430000 {J\over kg}} \over 9.8 {m \over s^2}} = 528.8s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The practically obtainable 455 seconds of Specific Impulse for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot;&gt;Space Shuttle&lt;/a&gt; mean the SSME achieved 86% of that theoretical maximum (the rest obviously dissipated as heat in the exhaust gasses). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most energetic reaction seems to be (although the claim is unsourced) &lt;a href=&quot;https://en.wikipedia.org/wiki/Beryllium_oxide&quot;&gt;oxidation of beryllium&lt;/a&gt; At &lt;a href=&quot;https://en.wikipedia.org/wiki/Energy_density_Extended_Reference_Table&quot;&gt;23.9MJ/kg&lt;/a&gt; it would purely theoretically allow &lt;strong&gt;705 seconds&lt;/strong&gt; of specific impulse. Purely theoretically, because beryllium oxide is a powder, so there's no adiabatic expansion of gas which creates propulsion. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-11T08:25:23.250" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17133" PostTypeId="1" AcceptedAnswerId="17134" CreationDate="2016-07-11T09:36:52.713" Score="12" ViewCount="2538" Body="&lt;p&gt;What is (or was, if it's no longer in use) the system with lowest launch mass, capable of putting (any) payload in (or reaching) LEO? &quot;How small&quot; did we get with orbital flights?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(and yes, I realize economically that's a rather poor idea; still I'd like to know what was attempted.)&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="58" LastEditDate="2016-07-11T16:17:18.020" LastActivityDate="2019-01-24T02:51:21.013" Title="What is/was the smallest orbital launch system, by total mass?" Tags="|launch-vehicle|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17134" PostTypeId="2" ParentId="17133" CreationDate="2016-07-11T09:53:43.650" Score="15" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_I&quot; rel=&quot;noreferrer&quot;&gt;Juno 1&lt;/a&gt; had a payload of 11 kg to LEO from a start mass of 29 t. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Diamant&quot; rel=&quot;noreferrer&quot;&gt;Diamant-A&lt;/a&gt; had a start weight of 18.4 t, payload 100 kg to LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Black_Arrow&quot; rel=&quot;noreferrer&quot;&gt;Black Arrow&lt;/a&gt; had a start weight of 18.1 t, payload 100 kg to LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;Vanguard&lt;/a&gt;, start weight of 10 t, payload 9 kg to LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lambda_4S&quot; rel=&quot;noreferrer&quot;&gt;Lambda 4S&lt;/a&gt; had a start weight of 9.4 t, payload 26 kg to LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NOTS-EV-2_Caleb&quot; rel=&quot;noreferrer&quot;&gt;Caleb&lt;/a&gt; was an air-launched missile theoretically capable of attaining orbit. Start weight of 1350 kg, payload 7 kg to LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NOTS-EV-1_Pilot&quot; rel=&quot;noreferrer&quot;&gt;Project Pilot&lt;/a&gt; was an air-launched missile theoretically capable of attaining orbit. Start weight of 900 kg, payload 1 kg to LEO. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-07-11T18:48:13.390" LastActivityDate="2016-07-11T18:48:13.390" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="17135" PostTypeId="1" AcceptedAnswerId="17139" CreationDate="2016-07-11T12:55:54.320" Score="3" ViewCount="2189" Body="&lt;p&gt;How many joules of electric energy can you draw out of an RTG battery over its lifetime - and what's the mass of that battery? (whole; shell, thermocouples, fuel and all.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(realizing, that the power drops off asymptotically towards zero over time, pick any reasonable cut-off point you like - say, 10% of initial output or any other you prefer.)&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-12T02:52:53.903" Title="What's the total specific energy of RTG?" Tags="|energy|rtg|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17136" PostTypeId="2" ParentId="16720" CreationDate="2016-07-11T14:04:54.797" Score="2" Body="&lt;p&gt;The iron oxides Fe3O4 and Fe2O3 are magnetic, only FeO is not magnetic. Read here &lt;a href=&quot;https://en.wikipedia.org/wiki/Ferromagnetism&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Ferromagnetism&lt;/a&gt; about magnetic properties of iron and its oxides.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-07-11T14:04:54.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17137" PostTypeId="1" AcceptedAnswerId="17143" CreationDate="2016-07-09T16:39:29.740" Score="17" ViewCount="15163" Body="&lt;p&gt;The following screenshots from the website &lt;a href=&quot;http://stuffin.space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stuff in Space&lt;/a&gt;, which tracks satellites and man-made debris orbiting the earth show us a few things:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;It appears to me that: very few satellites (or pieces of debris) pass over the poles.  Am I correct, and if so, why?&lt;/li&gt;&#xA;&lt;li&gt;GPS satellites seem to avoid the poles almost entirely&lt;/li&gt;&#xA;&lt;li&gt;It appears to me that: there is a high concentration of satellites around the equator. Am I correct, and if so, why?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xjtTkm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xjtTkm.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/oWjEhm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oWjEhm.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/m9fGVm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/m9fGVm.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/EBmd9m.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EBmd9m.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/kY9KZm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kY9KZm.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/pgsMum.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pgsMum.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/HifpYm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HifpYm.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/gAwPbm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gAwPbm.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h1&gt;Is it true that many of our satellites orbit near the equator, and few pass over the poles?  If so, why?&lt;/h1&gt;&#xA;" OwnerUserId="11184" OwnerDisplayName="Wad Cheber" LastEditorUserId="3289" LastEditDate="2020-12-21T15:29:48.110" LastActivityDate="2020-12-21T16:28:11.297" Title="Why do so few satellites pass over the poles, and why do so many focus on the equator?" Tags="|orbital-mechanics|artificial-satellite|orbit|polar-orbit|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="17138" PostTypeId="2" ParentId="17137" CreationDate="2016-07-10T13:16:23.760" Score="1" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;It appears to me that: very few satellites (or pieces of debris) pass over the poles. Am I correct ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;You're completely, totally wrong.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;GPS satellites seem to avoid the poles almost entirely&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This is completely, totally wrong.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It appears to me that: there is a high concentration of satellites around the equator. Am I correct ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Note that geosynchronous satellites can  &lt;em&gt;only be&lt;/em&gt; on the equator.&lt;/p&gt;&#xA;&lt;p&gt;Other than that you're completely wrong. There is no concentration whatsoever of satellites anywhere.&lt;/p&gt;&#xA;&lt;p&gt;(All I see in the images is that there's completely random and even coverage, plus, the few geosynchronous satellites, which matches the known simple facts.)&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;By the way, there are around &lt;strong&gt;400&lt;/strong&gt; geosynchronous satellites (thought to be &amp;quot;402&amp;quot; at time of writing).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There are around &lt;strong&gt;6000&lt;/strong&gt; satellites in total. (About 1/2 are working, about 1/2 are old ones.)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The number is increasing quickly, estimated 10,000 working sats in about 10 years from now.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="3289" OwnerDisplayName="Joe Blow" LastEditorUserId="3289" LastEditDate="2020-12-21T16:28:11.297" LastActivityDate="2020-12-21T16:28:11.297" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="17139" PostTypeId="2" ParentId="17135" CreationDate="2016-07-11T15:24:58.387" Score="4" Body="&lt;p&gt;The lifespan of a Radioisotope Thermoelectric Generator (RTG) is entirely dependent on how big you want it to be.  A larger RTG will live longer, but will, obviously, be more massive.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-Mission_Radioisotope_Thermoelectric_Generator&quot; rel=&quot;nofollow&quot;&gt;Curiosity's RTG&lt;/a&gt; weighs 45 kg, including all the exterior cladding.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the RTGs in use by NASA use Plutonium-238 as a fuel source.  According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Life_span&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, Pu-238 has a half-life of 87.7 years, meaning the power output will decay by about 0.787% per year.  Curiosity's RTG was designed to provide 125 watts of power when it was first built.  Assuming this constant decay rate, it will take 114.35 years for the power delivered by Curiosity's RTG to decay to 10%  Of course, Curiosity requires a great deal more than 12.5 watts of power to run, so it will have failed long before this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doing some rough calculations, over the course of 114.35 years, Curiosity's RTG will supply a total of 248,000,000,000 Joules.  To calculate this, I simply integrated:&#xA;$$&#xA;E = 125watts - (0.00787) \frac{1 year}{3.154*10^7 seconds} (125 watts)*t&#xA;$$&#xA;(I don't have experience with MathJax, so this is the best I could do with the equation formatting.  I am pretty sure this math is correct, though someone might want to check it for me.).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information about the RTGs used on specific missions can be found &lt;a href=&quot;https://solarsystem.nasa.gov/rps/rtg.cfm&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;, as given by Andy in the comment to the original question..&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-07-12T02:40:13.727" LastActivityDate="2016-07-12T02:40:13.727" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17140" PostTypeId="2" ParentId="1619" CreationDate="2016-07-11T17:37:37.667" Score="3" Body="&lt;p&gt;238Pu decays by alpha emission. A charged particle like an alpha is trivial to shield against; it gets blocked by a few centimeters of air, or by a piece of paper. RTGs are enclosed in extremely sturdy cases to reduce the probability of dispersing plutonium into the atmosphere if the rocket blows up. The case will completely block all alpha particles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when 238Pu emits an alpha, 29% of the time you also get a 43 keV x-ray. These would represent about 0.3% of the energy emitted by the 238Pu, the remainder being in the alphas. Although 0.3% is relatively small, a lump of 238Pu this size is going to be a heck of a hot x-ray source. However, 43 keV is a pretty low energy x-ray, so it's not especially penetrating. Some of these x-rays will be blocked by the surrounding plutonium itself. From googling, it looks like people handling 238Pu do it inside a glove box, with lead-lined gloves, plus precautions to get rid of the intense heat. In the photo in AlanSE's answer, I would guess that the woman is using a Geiger counter to survey for any x-rays that escaped the shielding. Or she may be doing some kind of routine periodic check to make sure that no plutonium has escaped into the environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After 238Pu decays, it becomes 234U, which has an extremely long half-life, and therefore does not emit radiation with any significant intensity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aboard the spacecraft, during its mission, any escaping x-rays could conceivably do mischief such as messing up computer chips. However, these probes are designed for the high-radiation environment of deep space.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2016-07-11T17:43:56.353" LastActivityDate="2016-07-11T17:43:56.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17142" PostTypeId="2" ParentId="17122" CreationDate="2016-07-11T21:49:33.480" Score="3" Body="&lt;p&gt;No.  The number you gave is for a photon drive.  However, if you throw particles out the back at relativistic velocity they weigh more than the fuel you drew from the tank.  Since there's no limit on how much relativistic mass you can pile on the particle there's no limit on the ISP.  Note, however, that there's no way you can boost particles like that using onboard energy.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-07-11T21:49:33.480" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17143" PostTypeId="2" ParentId="17137" CreationDate="2016-07-11T21:54:34.267" Score="26" Body="&lt;p&gt;You really asked a number of related questions, so here's the answers to some of the questions.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;All satellites will pass over the equator at some point in time. Only a satellite passing directly over the poles will pass over the poles. &lt;/li&gt;&#xA;&lt;li&gt;GPS satellites pass in regular orbits that can be seen from the whole Earth. They don't need to pass over the poles to do so. &lt;/li&gt;&#xA;&lt;li&gt;Launching into a polar orbit is more difficult than launching into an equatorial orbit, due to the motion of the Earth.&lt;/li&gt;&#xA;&lt;li&gt;Only a LEO satellite has any real reason to be over the poles. In fact, the diagram you showed of debris shows a near ring around the poles, indicating that a fair amount of debris comes close to the poles, but doesn't run right over it.&lt;/li&gt;&#xA;&lt;li&gt;LEO satellites are frequently put into a 98 degree inclination orbit known as Sun Synchronous. This orbit will pass over Earth at the same (solar) time of day every day. That is responsible for the ring you see over the poles.&lt;/li&gt;&#xA;&lt;li&gt;Google Earth's coverage of the poles isn't because they can't be seen (They can be seen from satellites), but because they simply use other data that people have used, and no one is paying for high resolution Polar pictures. Look at other sparse areas away from the poles, like the Amazon Rain Forest, remote islands, Sahara desert, etc, for comparison.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-11T21:54:34.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17144" PostTypeId="1" AcceptedAnswerId="17163" CreationDate="2016-07-12T01:43:37.383" Score="9" ViewCount="4801" Body="&lt;p&gt;I have read many times that the Delta IV is more expensive to launch than the Atlas V. Outside of the Delta IV Heavy configuration, which is in a class by itself for certain payloads and missions, are there other reasons a customer might choose to launch on a Delta instead of an Atlas? There must be, since there have been a number of Delta IV launches in the 15 or so years since the Delta IV family was introduced. But I can't figure it out.&lt;/p&gt;&#xA;" OwnerUserId="15933" LastActivityDate="2016-07-12T16:53:46.460" Title="What are the advantages of Delta IV over Atlas V" Tags="|rockets|cost-to-orbit|atlas-v|delta-iv|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17145" PostTypeId="2" ParentId="17135" CreationDate="2016-07-12T02:18:27.040" Score="3" Body="&lt;p&gt;If you allow the time to go to infinity, on the assumption that power continues to be produced at arbitrarily low heat output (which it doesn't), and that the conversion system maintains the same efficiency (it doesn't), then you get simply:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_\infty=P_0{t_{1/2}\over\log 2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $P_0$ is the initial power and $t_{1/2}$ is the half-life. This assumes that only one decay is contributing to the energy output. It gets more complicated for a chain of daughter products, but that won't matter in this case if you only go to a few hundred years. To cut it off at time $T$, the energy up to that point is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_T=P_0{t_{1/2}\over\log 2}\left(1-e^{-{T\log 2\over t_{1/2}}}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you asked about going down to some fraction of the original power, let's call that $f$, this simplifies to:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_f=P_0{t_{1/2}\over\log 2}\left(1-f\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for $f=0.1$, $t_{1/2}=87.7\,\mathrm{y}$, and $P_0=125\,\mathrm{W}$, we get $E_f=449\,\mathrm{GJ}$. That's a while though, since $f=0.1$ at $T=291\,\mathrm{y}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above significantly underestimates the reduction in power of a real RTG. The degradation of the thermoelectric converters can equal or exceed the decay rate of the Plutonium. See &lt;a href=&quot;http://anstd.ans.org/wp-content/uploads/2015/07/5107_Hammel-et-al.pdf&quot; rel=&quot;nofollow&quot;&gt;this presentation&lt;/a&gt; for more details. The half-life can however be used as shown above to estimate the total &lt;em&gt;heat&lt;/em&gt; output of the RTG, if $P_0$ is the initial heat flux.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-07-12T02:52:53.903" LastActivityDate="2016-07-12T02:52:53.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17146" PostTypeId="2" ParentId="17137" CreationDate="2016-07-12T07:13:03.093" Score="7" Body="&lt;p&gt;Regarding &quot;a high concentration of satellites around the equator&quot;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Geostationary orbit (~36,000 km above the equator) is the only orbit where a satellite appears to be stationary in the sky, so you can communicate with it using a small, fixed dish. In all other orbits you need a dish that can rotate to track the satellite.&lt;br&gt;&#xA;This makes geostationary orbit highly desirable for applications with a large number of users who don't want to pay much for the service, i.e. satellite TV.&lt;br&gt;&#xA;Other applications also benefit from being stationary: communications, weather and other long-term observation, etc. &#xA;And just outside geostationary orbit is another large concentration: this is a graveyard for old geostationary satellites. When a satellite is decommissioned, it is moved to a slightly higher orbit to free up its slot in geostationary orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Geostationary satellites are &lt;em&gt;concentrated&lt;/em&gt; on the equator (because that's the only place where an orbit can be geostationary) but that does not mean those satellites &lt;em&gt;focus&lt;/em&gt; (i.e. aim their antennas and sensors) on the equatorial regions. GEO is so high, those satellites can be seen/see even high latitudes, just about everywhere except the polar regions. I'm at ~50º N, and my country benefits from geostationary communications and meteorology satellites, for example.   &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2019-02-03T19:32:45.300" LastActivityDate="2019-02-03T19:32:45.300" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17147" PostTypeId="1" AcceptedAnswerId="24779" CreationDate="2016-07-12T08:01:34.490" Score="8" ViewCount="3566" Body="&lt;p&gt;If I understand correctly, the term 'gravity turn' refers to a specific way to turn from the initial near-vertical launch direction to a final tangential orbit direction in an efficient manner. It's described as a relatively small tangential nudge, return to essentially zero angle of attack (or displacement angle - &lt;a href=&quot;https://space.stackexchange.com/q/17149/12102&quot;&gt;I have asked separately&lt;/a&gt; about that) and the trajectory will naturally slowly turn over &quot;sideways&quot; into the tangential direction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the &lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Gravity_turn&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kerbal Wiki&lt;/a&gt; wisely teaches us:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Then once a certain altitude is reached, a slight turn is made, called the &lt;strong&gt;pitchover maneuver&lt;/strong&gt;. By turning away from vertical slightly, gravity will pull the velocity vector of the craft down towards that direction and the craft has to tilt to follow it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In modern, large launch vehicles with sophisticated aerodynamic models, and vectored and throttled thrusts, is the classic gravity turn maneuver still approximated - either by design or coincidence - or is the actual turn the result of such a complex optimization problem that a classical gravity turn &lt;em&gt;per se&lt;/em&gt; is no longer recognizable?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-03-12T10:50:51.250" LastActivityDate="2021-03-12T10:50:51.250" Title="Are 'classical' gravity turns still used and recognizable as such?" Tags="|launch|launch-trajectories|flight-profile|gravity-turn|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17148" PostTypeId="1" CreationDate="2016-07-12T08:19:12.027" Score="1" ViewCount="112" Body="&lt;p&gt;How much of the time, if it happens at all, is Phobos the nearest interplanetary object of its size (r=11.3 km) to the Earth-Moon system? Are there maybe larger NEAs always lurking closer by? I can't quite find how many asteroids larger than Phobos are crossing the orbits of Earth, Mars or Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During Earth-Mars conjunctions, half of the time when Phobos is on the sunny side of Mars, and when Venus is not too close, Phobos is the nearest of the planets and their moons. (Same of course goes for Deimos when Phobos is out of the way).&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-07-12T10:24:17.343" LastActivityDate="2016-07-12T11:53:10.600" Title="What is the average distance to any asteroid nearer and larger than Phobos?" Tags="|phobos|near-earth-asteroid|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17149" PostTypeId="1" AcceptedAnswerId="17184" CreationDate="2016-07-12T08:20:05.957" Score="1" ViewCount="4491" Body="&lt;p&gt;When I hear &quot;angle of attack&quot; or AoA, I think of an airplane. Usually, both the airfoil and the plane itself have a clearly defined 'top' and 'bottom'. In this case, the concept of angle of attack is applied in cartesian coordinates, and has a well defined sign convention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Angle_of_attack&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; thinks of an airplane also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/C7c0C.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C7c0C.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a nominally cylindrically symmetric rocket, this graphic found at &lt;a href=&quot;https://spaceflightsystems.grc.nasa.gov/education/rocket/rktstab.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://spaceflightsystems.grc.nasa.gov/education/rocket/rktstab.html&lt;/a&gt; uses &quot;displacement angle&quot;, which if I understand correctly is applied in spherical coordinates and is therefore generally meant to be positive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also &lt;a href=&quot;https://books.google.com/books?id=dQHMBQAAQBAJ&amp;amp;lpg=PA115&amp;amp;ots=pqUy5rK37X&amp;amp;dq=rocket%20displacement%20angle&amp;amp;pg=PA115#v=onepage&amp;amp;q=rocket%20displacement%20angle&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Figure 3.15&lt;/a&gt; in &lt;em&gt;Introduction to Rocket Science and Engineering&lt;/em&gt;, Travis S. Taylor&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rocket which is flying horizontally for example could have a displacement angle up, down, left, or right, and it would be pretty much identical aerodynamically. Not so for an airplane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these two terms therefore not really interchangeable? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; my question applies to 'rocket-shaped' rockets only, not car-shaped or &lt;a href=&quot;https://space.stackexchange.com/a/17104/12102&quot;&gt;plane-shaped&lt;/a&gt; rockets or spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7ACyE.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7ACyE.gif&quot; alt=&quot;NASA Rocket Forces&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ArWeg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ArWeg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-07-13T14:53:19.333" Title="Angle of attack, or displacement angle?" Tags="|rockets|aerodynamics|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17150" PostTypeId="2" ParentId="17148" CreationDate="2016-07-12T08:55:10.133" Score="3" Body="&lt;p&gt;I haven't done an exhaustive search, but &lt;a href=&quot;https://en.wikipedia.org/wiki/433_Eros&quot; rel=&quot;nofollow&quot;&gt;433 Eros&lt;/a&gt; gets a lot closer and is larger (34.4×11.2×11.2 km).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a long list of &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Mars-crossing_minor_planets&quot; rel=&quot;nofollow&quot;&gt;Mars-crossers&lt;/a&gt; with 18 candidates that may be larger than Phobos. It doesn't identify which 18 though. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this &lt;a href=&quot;https://en.wikipedia.org/wiki/Near-Earth_object#Notable_objects&quot; rel=&quot;nofollow&quot;&gt;list of NEOs&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/1036_Ganymed&quot; rel=&quot;nofollow&quot;&gt;1036 Ganymed&lt;/a&gt; is the largest with a diameter of 32-34 km. &#xA;Good luck tracing all their orbits and deciding how much of the time Phobos is nearer than all of them. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-12T08:55:10.133" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17151" PostTypeId="1" AcceptedAnswerId="17162" CreationDate="2016-07-12T09:02:55.213" Score="3" ViewCount="108" Body="&lt;p&gt;Since the tides on the moon have an antieffect on the other side of earth,&#xA;(still don't quite understand how that works), shouldn't the gas giants deform when their moons move around in their orbits? At least for Saturn this should hold true since most bigger moons are close together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the ice layer on Uranus too thick to crack under the tides of its moon?&lt;/p&gt;&#xA;" OwnerUserId="14245" LastEditorDisplayName="user687" LastEditDate="2016-07-12T14:45:10.560" LastActivityDate="2016-07-12T17:11:27.060" Title="Tides on other bodies" Tags="|moons|tides|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17152" PostTypeId="1" AcceptedAnswerId="17160" CreationDate="2016-07-12T09:08:42.630" Score="11" ViewCount="659" Body="&lt;p&gt;As recent research has been started by a team around Stephen Hawking under the name &lt;a href=&quot;https://breakthroughinitiatives.org/Initiative/3&quot;&gt;Breakthrough Starshot&lt;/a&gt; they are going for accelerating little probes up to 0.2~0.25$c$. There isn't that much information about it available yet, but they say there will be different obstacles before they really can start launching the probes. While I read it as its questionable the accelaration method will even work as expected, it seems no one else is having any doubt about it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even critics just see problems in challenges like: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How they want to make probes the size of 1cm&lt;sup&gt;3&lt;/sup&gt; be traveling for 30 years through space, while not hitting any particles at said 0.25$c$ or other stuff from that region. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But no one (or at least I was not able to find) was stating:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;They will never ever be able to accelerate the probes to that speed because of [$reason]&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So is it really &lt;em&gt;that&lt;/em&gt; certain, that we have knowledge available how to accelerate microcontroller like electronics of that size to a quarter of speed of light?&lt;/p&gt;&#xA;" OwnerUserId="11099" LastEditorUserId="9060" LastEditDate="2016-07-13T18:33:01.847" LastActivityDate="2016-07-13T18:33:01.847" Title="Are we actually that close to techniques of accelerating probes to speeds like a quarter $c$?" Tags="|interstellar-travel|laser|acceleration|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17153" PostTypeId="1" AcceptedAnswerId="17161" CreationDate="2016-07-12T09:56:34.713" Score="4" ViewCount="2620" Body="&lt;p&gt;I searched for night pictures of the ISS, but most of them seem to be from before the station passes into Earth's shadow, even though it crossed the terminator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any pictures of the ISS when it is fully shadowed by the Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any floodlights that can be turned on/off?&#xA;What about a picture from within/without?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="13710" LastEditDate="2016-07-12T15:18:48.607" LastActivityDate="2016-07-13T12:39:41.337" Title="What does the ISS look like by night?" Tags="|iss|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17154" PostTypeId="2" ParentId="17128" CreationDate="2016-07-12T10:11:20.447" Score="1" Body="&lt;p&gt;This might be a duplicate as Organic Marble suggests.&#xA;But I think there is a misconception about the Laval Nozzle.&#xA;The Purpose of the Nozzle is to achieve pressure equilibrium between Exhaust gases and the surrounding pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they are not in equelibrium, thrust is lost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Think about the surrounding pressure as a ring directly connected to the nozzle. If the ring is smaller then your nozzle, energy is lost to move around the ring, if it is bigger, energy is lost filling in the gap between nozzle and ring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since bigger nozzles are needed for lower pressure surroundings, the higher drag of a nozzle greater then the rocket body would never occur if engineered for efficiency of exhaust gasses.&lt;/p&gt;&#xA;" OwnerUserId="14245" LastActivityDate="2016-07-12T10:11:20.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17155" PostTypeId="2" ParentId="14964" CreationDate="2016-07-12T10:16:41.703" Score="5" Body="&lt;p&gt;Dawn's potential target was asteroid 145 Adeona:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On April 20, 2016, New Scientist announced that the Dawn team had sent&#xA;  NASA a proposal for an extended mission. According to the report, the&#xA;  spacecraft retains enough xenon fuel to break Ceres orbit and reach a&#xA;  third asteroid.[123] In July 2016, NASA rejected the proposal because&#xA;  remaining in orbit around Ceres has the potential to provide more&#xA;  significant science discoveries than a flyby of 145 Adeona in May&#xA;  2019.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_%28spacecraft%29#Mission_conclusion&quot;&gt;https://en.wikipedia.org/wiki/Dawn_%28spacecraft%29#Mission_conclusion&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13643" LastActivityDate="2016-07-12T10:16:41.703" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17156" PostTypeId="2" ParentId="17148" CreationDate="2016-07-12T11:53:10.600" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://neo.jpl.nasa.gov/cgi-bin/neo_ca?type=NEO&amp;amp;hmax=22&amp;amp;sort=dist_min&amp;amp;sdir=ASC&amp;amp;tlim=all&amp;amp;dmax=0.1AU&amp;amp;max_rows=20&amp;amp;action=Display%20Table&amp;amp;show=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL has a nice data base&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a screen shot:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/LwxH9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LwxH9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice some of the options. You can sort by minimum possible distance. For time range you can choose All available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They measure their distances in LD (Lunar Distance) or AU (Astronomical Unit). To get kilometers, multiply their LD number by 384,400 or their AU number by 150,000,000.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They also give Vinf. This rock's relative velocity to earth at close approach is of interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In many cases asteroid albedo is unknown. So the H (absolute magnitude) doesn't tell us the asteroid's size. &lt;a href=&quot;http://neo.jpl.nasa.gov/glossary/h.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here&lt;/a&gt; is a JPL chart telling us the size range an H number might indicate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is one of the reasons I yearn for a &lt;a href=&quot;https://en.wikipedia.org/wiki/Wide-field_Infrared_Survey_Explorer&quot; rel=&quot;nofollow noreferrer&quot;&gt;WISE&lt;/a&gt; 2.0 parked at the SEL2 (Sun Earth L2). From SEL2's point of view, the sun, earth and moon all occupy the same small region of the sky. So a small shade suffices to block heat from 3 major heat sources. That gives most of the 3 K sky the infra red scope can radiate heat into.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we do a visual and infra red observation at the same time, that tells how much light is being absorbed and how much reflected. When we have a handle on the rock's albedo, then H gives us a better idea of the size.  &lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-07-12T11:53:10.600" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17157" PostTypeId="2" ParentId="17152" CreationDate="2016-07-12T12:05:03.683" Score="12" Body="&lt;p&gt;Given that the interstellar medium (ISM) has a density of about 1 atom per cubic centimeter and given that &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/6.2007-6156&quot; rel=&quot;noreferrer&quot;&gt;laser propulsion&lt;/a&gt; could, in theory, accelerate a spacecraft to 30% of the speed of light in ten minutes, I'm going to say this one is plausible. They are talking about using several probes, so even if a few happened to get destroyed by particles, some should remain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be clear, we are talking about technology that will be available within the next couple of decades. They are not talking about launching today. &lt;a href=&quot;https://breakthroughinitiatives.org/Challenges/3&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; is a list of the challenges that the team has identified that they have to overcome. For example, &lt;a href=&quot;https://breakthroughinitiatives.org/index.php?controller=Forum&amp;amp;action=viewforum&amp;amp;id=20&amp;amp;page=2&quot; rel=&quot;noreferrer&quot;&gt;one&lt;/a&gt; is in fact that protection is needed for the spacecraft from collisions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I don't want to understate the challenge of collision protection. One commenter (David Theil) on the Breakthrough Initiatives page says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At a typical interstellar medium density of 1 atom per cubic cm these can be ignored from a momentum consideration. Dust particles of order 10^-14 g in mass have density of roughly 10^-12 per cubic cm in the local bubble. A cm sized spacecraft could expect to encounter a few million of these beasties on the way to Alpha Cen (roughly 1 parsec.) At a speed of 6x10^9 cm/s (0.2c) each one will deposit about 10^5 ergs into our little spacecraft...not enough to raise the temperature all that much if averaged over the whole 1 gram mass (assumed heat capacity of silicon), BUT presumably enough to sputter away some protective coating. I would want to do some lab experiments to see how different coatings respond to such collisions. Even the LHC can't produce 10^16 eV particles. This is going to be a tough thing to test and will have to rely on modeling.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-07-13T18:15:40.620" LastActivityDate="2016-07-13T18:15:40.620" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="17158" PostTypeId="1" AcceptedAnswerId="17159" CreationDate="2016-07-12T12:52:44.443" Score="6" ViewCount="503" Body="&lt;p&gt;Assume that we need just the following Martian resources for our colony:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;O&lt;sub&gt;2&lt;/sub&gt;&lt;/li&gt;&#xA;&lt;li&gt;N&lt;sub&gt;2&lt;/sub&gt;&lt;/li&gt;&#xA;&lt;li&gt;H&lt;sub&gt;2&lt;/sub&gt;O&lt;/li&gt;&#xA;&lt;li&gt;Regolith for radiation shielding&lt;/li&gt;&#xA;&lt;li&gt;Regolith for construction&lt;/li&gt;&#xA;&lt;li&gt;Regolith for printing tools&lt;/li&gt;&#xA;&lt;li&gt;Martian compounds for rocket fuel&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.ncbi.nlm.nih.gov/pubmed/11539074&quot; rel=&quot;noreferrer&quot;&gt;The resources of Mars for human settlement - Meyer TR and McKay CP&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much surface area do we need on Mars to establish collection of all these resources for a colony of 7 people (max capacity of the Dragon)?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Other things to consider:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Habitat&lt;/li&gt;&#xA;&lt;li&gt;Food production&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Variations in approach&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SF pointed out a variety of assumptions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Assumptions: water recovered from regolith or air? Oxygen by distillation of CO2 or cyanobacteria farm? Power solar or nuclear? Surface/subsurface shelter or cave?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Out of the plausible options that have been presented in scholarly/technical discourse, you should assume that which requires the &lt;em&gt;most&lt;/em&gt; surface area. If you wish, you can also specify the lower surface area requirements of the alternatives.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Landing site for deliveries from Earth (no need to estimate for launch facilities)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-07-12T14:07:02.723" LastActivityDate="2022-05-21T22:03:56.993" Title="How much land does a colony on Mars need to start off?" Tags="|mars|colonization|isru|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17159" PostTypeId="2" ParentId="17158" CreationDate="2016-07-12T14:14:44.753" Score="7" Body="&lt;p&gt;Of these, the only one that isn't well known is nitrogen. The nitrogen levels at Mars are somewhat unknown, but expected to be pretty low overall. Of course, nitrogen isn't required for a colony of what you have.&lt;/p&gt;&#xA;&lt;p&gt;The one thing that actually takes area is to grow food, which you did not include. On Earth, growing crops requires about 1 acre of land per person per year. I believe Mars habitats would be closely monitored in temperature, and have less meat, which would increase the yield by a factor of two. More intense methods could make that to a single acre, as is discussed at &lt;a href=&quot;https://worldbuilding.stackexchange.com/q/9582/244&quot;&gt;World Building&lt;/a&gt;. In fact, it could be much less than this, but...&lt;/p&gt;&#xA;&lt;p&gt;In &lt;em&gt;The Martian&lt;/em&gt;, assuming a 2500 calorie diet of pure potatoes requires about 370 m&lt;sup&gt;2&lt;/sup&gt; of land. Let's increase that to 500 m&lt;sup&gt;2&lt;/sup&gt; to allow for other types of crops to be grown to give at least some variety. That also gives about 1 acre of land (4046 m&lt;sup&gt;2&lt;/sup&gt;) feeding 7 people So far so good, 4000 m&lt;sup&gt;2&lt;/sup&gt; of farmland would be the minimum required.&lt;/p&gt;&#xA;&lt;p&gt;Beyond this, the shelter could in theory be below the farmland and be just fine. Give, say, 500 m&lt;sup&gt;2&lt;/sup&gt; for stuff on the surface, and one would be more than fine. Let's even round up, and say that about 700 m&lt;sup&gt;2&lt;/sup&gt; of land is required per astronaut.&lt;/p&gt;&#xA;&lt;p&gt;Fuel for the return would come from the atmosphere. The amount of regolith required to make the water would vary depending on where you land, but might require some significant investment. If you are planning on purely solar powered operations, an area of 6600 m&lt;sup&gt;2&lt;/sup&gt; of panels would be required to give 100 kWe of power. Nuclear power is highly recommended, not only because it is more dense power wise, but also produces waste heat, which can be used for heating the habitats.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="47149" LastEditDate="2022-05-21T22:03:56.993" LastActivityDate="2022-05-21T22:03:56.993" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="17160" PostTypeId="2" ParentId="17152" CreationDate="2016-07-12T14:36:29.080" Score="6" Body="&lt;p&gt;The solar sail idea just works, tested and true.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very powerful lasers are a reality. Very accurate, narrow-beam lasers are a reality too. Bringing these two together is absolutely doable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Powering that up requires just lots and lots of solar cells. We have these.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very reflective mirrors (so that the probe is accelerated and not obliterated by the laser) are a reality too, used with laboratory lasers extensively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are the essentials to perform the acceleration. It's completely doable from technological and scientific point of view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two important reasons why we might never see it done:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;money&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;politics.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;First, while expensive, development and creation of these probes will not be &lt;em&gt;outrageously&lt;/em&gt; expensive. But the laser propulsion device, while much simpler to build, will need a lot of power, and simply will weigh a lot. Putting that in orbit is going to cost a small fortune. Less than the ISS or the LHC, but still a lot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then - a lot of people may be very unhappy about someone putting an extremely powerful laser of exquisite accuracy in Earth orbit. That thing CAN be misused. And as result, politics kicks in and the whole project may be blocked due to its military potential.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-12T14:36:29.080" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17161" PostTypeId="2" ParentId="17153" CreationDate="2016-07-12T14:49:11.217" Score="5" Body="&lt;p&gt;The best image I can find is &lt;a href=&quot;http://www.nasa.gov/multimedia/imagegallery/image_feature_2184.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xxo6t.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xxo6t.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a few external lights on the spacecraft, mostly to assist docking spacecraft at night. See &lt;a href=&quot;https://space.stackexchange.com/q/5592/25&quot;&gt;Why does the International Space Station have a downward facing light?&lt;/a&gt; for those pictures and where they are. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-07-13T12:39:41.337" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17162" PostTypeId="2" ParentId="17151" CreationDate="2016-07-12T15:53:59.620" Score="4" Body="&lt;p&gt;Short answer: yes, the gas giant will be deformed by its orbiting moon, but the tide raised will be vanishingly small..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason tides happen is because the force of gravity varies with distance.  Newton's Law states that $F_g=\frac{G*M_1*M_2}{R^2}$, and has an inverse square dependence on distance.  When the moon is directly overhead, you are being pulled more by it than when it is on the other side of the Earth.  This means that the material on the Earth is pulled outwards, creating a bulge.  The material on the opposite side of the Earth, however, does not feel the pull as strongly, so it relaxes, creating another bulge.  Here is an image showing how the force of gravity varies across a planet, creating two tidal bulges.  For more information on how that works, check &lt;a href=&quot;https://www.lhup.edu/~dsimanek/scenario/tides.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this website&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hlsXL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hlsXL.jpg&quot; alt=&quot;A vector plot showing the differential gravitational forces on a body due to a satellite.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can estimate the tidal acceleration on the surface of Neptune by Neptune's largest moon, Triton, using the formula given &lt;a href=&quot;http://mb-soft.com/public/tides.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&#xA;$$&#xA;a = G*M_{moon}*\frac{2*D*R_{planet}-{R_{planet}}^2}{D^2*(D-R_{planet})^2}&#xA;$$&#xA;Plugging in numbers for each of those values gives us an approximate acceleration of $6.56*10^{-29}$ m/s^2.  &lt;a href=&quot;http://mb-soft.com/public/tides.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Moon's tidal acceleration on the Earth&lt;/a&gt; is $1.129*10^{-6}$ m/s^2.  Thus, Triton's effect on Neptune is $10^{13}$ times smaller than the Moon's effect on Earth.  I have chosen the Triton/Neptune system because Triton is &lt;a href=&quot;https://en.wikipedia.org/wiki/Uranus#Moons&quot; rel=&quot;nofollow noreferrer&quot;&gt;over twice the mass of all the Uranian satellites combined.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given all this, Triton does not have a significant effect on Neptune.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Neptune has a much bigger effect on Triton.  Reversing the situation, we find that Neptune's tidal acceleration on Triton is $3.36*10^{-4}$m/s^2.  Whether or not that is strong enough to break the frozen nitrogen on Triton's surface depends on the tensile strength.  If you want to see what kinds of stresses result from tidal forces on icy satellites, check out this &lt;a href=&quot;https://github.com/SatStressGUI/SatStressGUI&quot; rel=&quot;nofollow noreferrer&quot;&gt;program&lt;/a&gt; we're working on (right now it only works on Mac).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though, actually, it turns out that one key factor which determines the strength of stressing factors on a moon is its &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc97/pdf/1855.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital eccentricity&lt;/a&gt;.  Triton (despite being in a retrograde orbit), only has an orbital eccentricity of 0.000016.  This results in stresses on the order of 0.05 kPa, not nearly enough to break the ice (stresses due to its orbital obliquity, however, are several orders of magnitude larger, but that is beyond the scope of this question).&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-07-12T17:11:27.060" LastActivityDate="2016-07-12T17:11:27.060" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17163" PostTypeId="2" ParentId="17144" CreationDate="2016-07-12T15:58:15.467" Score="9" Body="&lt;p&gt;Since 2004, it's been policy of the US government to assure there are at least two launcher families available for launching &quot;national security payloads&quot; (e.g. military surveillance and communication satellites). Prior to the entry of Antares and Falcon 9 in the market, this pretty much meant Atlas and Delta, therefore USAF was required to make the necessary orders to keep both rockets in production.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.law.cornell.edu/uscode/text/10/2273&quot;&gt;https://www.law.cornell.edu/uscode/text/10/2273&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-12T16:53:46.460" LastActivityDate="2016-07-12T16:53:46.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17164" PostTypeId="1" AcceptedAnswerId="17165" CreationDate="2016-07-12T16:43:01.693" Score="8" ViewCount="739" Body="&lt;p&gt;Today I've read in the Facebook group of Roscosmos (&lt;a href=&quot;https://i.stack.imgur.com/uTmfK.png&quot; rel=&quot;noreferrer&quot;&gt;original text in Russian&lt;/a&gt;) that 16 years ago on this day the &lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_(ISS_module)&quot; rel=&quot;noreferrer&quot;&gt;Zvezda&lt;/a&gt; module was launched into space. According to the article 2 weeks later it docked to the ISS, which back then consisted of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Unity_(ISS_module)&quot; rel=&quot;noreferrer&quot;&gt;Unity&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Zarya&quot; rel=&quot;noreferrer&quot;&gt;Zarya&lt;/a&gt; modules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIR the Soyuz spacecraft can reach the ISS in 6 to 48 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did it take Zvezda 2 weeks to dock to the ISS? Or is this a typo?&lt;/p&gt;&#xA;" OwnerDisplayName="user3049" LastEditorUserId="25" LastEditDate="2016-07-12T17:07:24.583" LastActivityDate="2016-07-12T17:07:24.583" Title="Did the &quot;Zvezda&quot; module really dock to the ISS 2 weeks after launch?" Tags="|launch|orbital-mechanics|iss|docking|zvezda|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17165" PostTypeId="2" ParentId="17164" CreationDate="2016-07-12T17:06:24.240" Score="11" Body="&lt;p&gt;I've looked through a large number of sources, Zvezda really did take 2 weeks to dock to the ISS, launching July 12 and docking July 26, 2000. &lt;a href=&quot;https://spaceflightnow.com/station/zvezda/000712launch/&quot;&gt;SpaceFlightNow&lt;/a&gt; did a pretty good job of explaining what was happening during those two weeks. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It will take flight controllers two full weeks to activate and check out Zvezda's systems and to maneuver the spacecraft into the proper orbit and orientation for docking with the international space station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The 6 hour orbital option wasn't available then. The first Soyuz took about 48 hours, but they were significantly less complex than Zvezda, which had to deploy large solar panels, wasn't really meant for major movement in space, etc, while Soyuz is designed for more maneuverability in space.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-12T17:06:24.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17166" PostTypeId="1" AcceptedAnswerId="17168" CreationDate="2016-07-12T17:27:16.737" Score="2" ViewCount="909" Body="&lt;p&gt;Venus has an average surface temperature of 735K.  Would it be possible to harness this extreme temperature and use it to generate electricity?  After all, that's the principle a radioisotope thermoelectric generator (RTG) runs on.  And a large portion of RTG electric power goes to heating the spacecraft to keep its electronics and instruments functional.  Solar panels could not be used to power a Venusian lander, because they would get destroyed by the atmosphere, and RTGs are expensive.  But if there's a way to convert the atmospheric heat into electricity, we could have a longer Venus mission without having to worry about power.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-07-13T09:10:59.750" Title="Would it be possible to power a Venus lander using a temperature differential?" Tags="|venus|power|temperature|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17168" PostTypeId="2" ParentId="17166" CreationDate="2016-07-12T18:26:44.083" Score="11" Body="&lt;p&gt;High temperature in itself is useless. You need a temperature differential, just like the RTG exploits the difference between the temperature of the Plutonium and that of outer space. Kinetic energy (heat) flows from the high temperature region to the low temperature region and can do work on the way.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Venus, there's no gradient. Everything has the same temperature. You want to keep your spacecraft a lot cooler than the ambient temperature, but that means expending lots of energy to pump the heat out. Running a Peltier element off the difference between the spacecraft and ambient temperature would only decrease the effectiveness of your cooling system, as you'd be letting the heat back in through the Peltier element. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-12T18:26:44.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17169" PostTypeId="2" ParentId="16976" CreationDate="2016-07-12T18:37:18.720" Score="4" Body="&lt;p&gt;Jonathon Mcdowell of Jonathon's Space Report fame &lt;a href=&quot;http://planet4589.org/space/jsr/back/728.html&quot; rel=&quot;nofollow&quot;&gt;reports&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Juno&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA's Juno probe entered Jupiter orbit on Jul 5. The probe's UK-built&#xA;  Leros-1b engine fired for 35min 2s starting at 0230 UTC, slowing the&#xA;  vehicle by 0.542 km/s to reach a 3900 x 8029000 km x 89.8 deg orbit&#xA;  around the giant planet.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The engine-induced velocity change was, of course small compared to the&#xA;  velocity change caused by Jupiter's gravity as Juno passed through&#xA;  perijove, but enough that as it arced upwards again, it no longer had&#xA;  enough speed to escape the planet's pull - although it won't start&#xA;  falling back down again until Apojove 0 on around Jul 31.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;At 0000 UTC Juno had a velocity of 28.1 km/s relative to Jupiter at a&#xA;  height of 261000 km above the cloud tops. As it fell inward, by the&#xA;  start of the burn at 0230 UTC this had increased to 53.9 km/s at a mere&#xA;  19000 km above the planet at 47 deg N latitude. Maximum jovicentric&#xA;  velocity, 57.95 km/s, was reached at 0248 UTC, with Juno only about 4400&#xA;  km over the Jovian equator. By the end of the burn Juno's speed relative&#xA;  to Jupiter had dropped to 54.2 km/s. On Jul 12, a week after orbit&#xA;  insertion, Juno was travelling only 4.5 km/s relative to and away from&#xA;  the planet, at a distance of almost 5 million km from it.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The almost 58 km/s perijove velocity appears to be the record speed at&#xA;  periapsis relative to the central body during an orbit insertion. As it&#xA;  happens, the probe's heliocentric velocity was almost the same, 59.3&#xA;  km/s. Relative to Earth, the probe was travelling at 61.7 km/s. On its&#xA;  third perijove later this year (Aug 27) the velocity vectors of probe&#xA;  and Earth will be better aligned and although it will again be&#xA;  travelling at 58 km/s relative to Jupiter, relative to Earth it will be&#xA;  going 73.7 km/s according to NASA (the predicted trajectory on Horizons&#xA;  atually reaches 76 km/s), and this is the maximum expected geocentric&#xA;  velocity during the mission.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Fastest ever? - Not so fast!&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So, Juno is orbiting its host planet faster than any planetary orbiter&#xA;  ever. However, this is not, as some media outlets have reported, the&#xA;  fastest ever spacecraft relative to the Earth.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The largest geocentric velocity reached by a spacecraft&#xA;  was 98.9 km/s, by Helios 2. The Helios 2 mission was a joint German-US&#xA;  probe to study the solar wind, placed in an elliptical solar orbit&#xA;  of about 0.28 x 1.0 AU.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;My media contacts tell me that JPL claims (I haven't heard from&#xA;  JPL directly) that the record was only 164000 mph - i.e. about 73 km/s -&#xA;  and was set in Apr 1976.  It is true that the HELIOCENTRIC velocity&#xA;  record was set on 1976 Apr 16 by Helios 2, reaching a velocity of 68.6&#xA;  km/s, beating the 66.1 km/s record of its sibling Helios 1.  And it is&#xA;  true that on that day the geocentric velocity of Helios 2 was 73.4 km/s,&#xA;  the record quoted by NASA.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;But: you don't get the maximum geocentric velocity by taking the date of&#xA;  the maximum heliocentric velocity and converting that one to geocentric&#xA;  (which is what JPL seem to have done) - the +/- 30 km/s modulation&#xA;  caused by the Earth's motion around the Sun means that the heliocentric&#xA;  and geocentric velocities don't peak at the same time. For a fixed&#xA;  elliptical Keplerian orbit around the Sun, the maximum heliocentric&#xA;  velocity always occurs at perihelion and always has the same magnitude.&#xA;  The maximum geocentric velocity will happen when perihelion happens on&#xA;  the opposite side of the Sun from the Earth, and since the orbital&#xA;  periods of probe and Earth are different, that will only happen once&#xA;  every many orbits.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In Apr 1976 the Earth was moving almost at right angles to Helios 2, so&#xA;  the geocentric velocity was not much bigger than its heliocentric one.&#xA;  In contrast, on 1989 Jan 12 I calculate Helios 2 was close to perihelion&#xA;  and moving in the opposite direction to the Earth, so a similar&#xA;  heliocentric velocity translated to a much larger geocentric velocity of&#xA;  98.9 km/s. Caveat: I have extrapolated the 1980 orbital solution&#xA;  without including any perturbations, so the date is almost certainly&#xA;  wrong, but the magnitude of the maximum velocity probably isn't far off.&#xA;  I hope that GSOC and JPL can do a better job. My results, which use&#xA;  orbital elements obtained from NASA/NSSDC in 1993, are in good agreement&#xA;  with the SPK kernels of L. Wennmacher (2011) available at naif.jpl.nasa.gov&#xA;  for the period when they overlap.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;But perhaps you don't want to count the 1989 Helios 2 record, because&#xA;  Helios 2 died in 1980. What is the maximum geocentric velocity of a&#xA;  working space probe? Helios 1 was still transmitting in 1985, and on&#xA;  1980 Dec 5, it reached an impressive geocentric velocity of 96.2 km/s&#xA;  (215000 mph).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Plots of the Helios 1 and 2 geocentric velocity versus time can be seen&#xA;  at &lt;a href=&quot;http://planet4589.org/space/jsr/Helios1Vel.jpg&quot; rel=&quot;nofollow&quot;&gt;http://planet4589.org/space/jsr/Helios1Vel.jpg&lt;/a&gt;&#xA;  and&#xA;  &lt;a href=&quot;http://planet4589.org/space/jsr/Helios2Vel.jpg&quot; rel=&quot;nofollow&quot;&gt;http://planet4589.org/space/jsr/Helios2Vel.jpg&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So to summarize:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Fastest geocentric velocity of human artifact:    Helios 2, 1989 Jan 12?, 98.9 km/s&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Fastest geocentric velocity of active probe:      Helios 1, 1980 Dec  5,  96.2 km/s&lt;/li&gt;&#xA;  &lt;li&gt;Fastest planetocentric velocity of artifact&#xA;  in orbit around that planet:                     Juno, 2016 Aug (expected), 73.7 km/s&lt;/li&gt;&#xA;  &lt;li&gt;Fastest heliocentric velocity of (active or not)&#xA;  human artifact:                                  Helios 2, 1976 Apr 16,    68.6 km/s&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;A new record is expected to be set in Dec 2024 when NASA's Solar Probe Plus mission, scheduled for launch in 2018, will reach the perihelion of an 0.04 x 0.73 AU solar orbit traveling at a searing heliocentric velocity of 205.0 km/s and an even more remarkable geocentric velocity of 234.8 km/s (525000 mph for the metrically impaired).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Supplement: sample Ecliptic1950 orbital elements for Helios 1 and 2&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Helios 1 - Epoch 1980 Feb 24.00  0.310 x 0.985 AU i=0.006 Node=143.33 e=0.522 AOP=114.17 M=180.33&#xA;  Helios 2 - Epoch 1980 May 12.73  0.291 x 0.986 AU i=0.029 Node=138.16 e=0.544 AOP=155.75 M=  0.00&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-12T18:37:18.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17170" PostTypeId="1" AcceptedAnswerId="17171" CreationDate="2016-07-06T05:55:05.770" Score="8" ViewCount="262" Body="&lt;p&gt;As many of us know, Juno spacecraft achieved orbit of Jupiter yesterday (July 4, 2016). Does anyone have an educated guess on approximately when we might see the first optical images from Juno in orbit from NASA? I imagine that they have some testing/calibration of equipment and fine-tuning of the final orbit before seeing the good stuff released to the public.&lt;/p&gt;&#xA;" OwnerUserId="15868" OwnerDisplayName="iMerchant" LastEditorUserId="12102" LastEditDate="2021-09-13T00:03:16.163" LastActivityDate="2021-09-13T00:03:16.163" Title="When can we (public) expect to see the first optical images of Jupiter from the Juno spacecraft?" Tags="|imaging|jupiter|spaceflight-firsts|juno|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17171" PostTypeId="2" ParentId="17170" CreationDate="2016-07-06T11:42:34.180" Score="11" Body="&lt;p&gt;There's an interesting Planetary Society article about this: &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2016/06090600-what-to-expect-from-junocam.html&quot;&gt;What to expect from Junocam&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We won't be able to see spectacular views of Jupiter's belts and zones from Jupiter orbit until the very end of August, and it'll be November before we'll see automated release of high-resolution raw images.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;August 27 is expected to be a day when photography takes place, however there are no plans to release all of the pictures then.  Perhaps selected images will be released to the Press.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;October 20 will see another close approach, but no photography is expected as an orbit change burn is the priority.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems we will only be getting regular, high quality photographs in November or thereabouts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do the pictures take so long to start being released? Apparently, it's partly due to the spacecraft's limited memory and transmission rate, and a lot of testing work that will be needed before the images can be processed to a high quality. So images taken in August may not be released until November.&lt;/p&gt;&#xA;" OwnerUserId="8756" OwnerDisplayName="Andy" LastActivityDate="2016-07-06T11:42:34.180" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17172" PostTypeId="2" ParentId="17170" CreationDate="2016-07-06T15:57:39.433" Score="2" Body="&lt;p&gt;They posted a video of the approach a couple days ago. Here it is, where I linked to the time that they show the approach: &lt;a href=&quot;https://www.youtube.com/watch?v=XpsQimYhNkA&amp;amp;feature=youtu.be&amp;amp;t=32&quot; rel=&quot;nofollow&quot;&gt;https://www.youtube.com/watch?v=XpsQimYhNkA&amp;amp;feature=youtu.be&amp;amp;t=32&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="27089" OwnerDisplayName="Quantic" LastActivityDate="2016-07-06T15:57:39.433" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17173" PostTypeId="2" ParentId="17170" CreationDate="2016-07-07T16:08:58.437" Score="5" Body="&lt;p&gt;It's important to remember that JunoCam is not a primary instrument on this mission. JunoCam's primary reason for being is public outreach. Other instruments take precedence. That its primary reason for being is public outreach and that it isn't a primary instrument is perhaps a blessing in disguise for the public. This means the imagery won't be shrouded in a veil of semi-secrecy until the principal investigators have had a chance to thoroughly investigate the images and publish papers on their findings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also important to remember that Juno is in a highly elliptical orbit, and that JunoCam is designed to work during close approaches. JunoCam is very unlike a modern digital camera. It will not work very well except during close approach. One close approach has already occurred, the July 4/5 orbit insertion. However, the camera, along with all of the other scientific instrumentation, were powered off for that event to avoid problems with power and computer usage. The next close approach (perijove) will not occur until late August. So you'll have to be patient.&lt;/p&gt;&#xA;" OwnerUserId="2752" OwnerDisplayName="David Hammen" LastActivityDate="2016-07-07T16:08:58.437" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17174" PostTypeId="2" ParentId="17166" CreationDate="2016-07-12T20:49:09.803" Score="3" Body="&lt;p&gt;You might be able to do something like OTEC: &lt;a href=&quot;https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion&lt;/a&gt; . Say you attach an anchor at the surface, and connect a balloon to it. Let the balloon float at an altitude of 5 km on a tether, which gives you a temperature difference of about 38 C (roughly twice what OTEC could get). The atmospheric pressure is about 70 times that on earth, so you don't need a very big helium balloon to support quite a large weight. On a logarithmic scale, buoyancy in the venerian atmosphere is more similar to buoyancy in water than buoyancy in earth's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hard part then is the same thing that makes it hard to get OTEC to work. You have to operate a heat engine between heat reservoirs that are separated by a large distance, and the thermodynamic efficiency is going to be fairly low because $\Delta T/T$ is only about 5%. This is probably a far-future, high-tech scenario rather than something that could be exploited for probes in the foreseeable future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wind power from surface winds is probably a lot more practical, although just like with wind power on earth it would go away when there was no wind.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2016-07-12T21:40:56.657" LastActivityDate="2016-07-12T21:40:56.657" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17175" PostTypeId="2" ParentId="17153" CreationDate="2016-07-13T03:32:06.753" Score="8" Body="&lt;p&gt;The reason there aren't a lot of pictures...is that it's really dark.  We spent a lot of time in robotics training emphasizing this.  The ISS arm has spotlights on the tip and on its steerable elbow cameras, and the steerable truss-mounted cameras have lights on them too (some of these lights are burned out, at least in the final STS program era).  The crew had to use all these lights to illuminate the areas of interest if robotic ops were required at night.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: As of Jan 16 it seems that many of the lights are &lt;a href=&quot;https://www.quora.com/Why-isnt-the-International-space-station-lit-up-at-night&quot;&gt;still burned out&lt;/a&gt; and haven't been replaced.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But most of these lights have been in space for 10+ years and now the&#xA;  majority of them are totally dead, while others are dimming.....If they continue to burn out, we'll eventually get priority because it will get to a point where we just can't do close-contact robotics at night without new lights, which could essentially double the time it would take us to do things. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-07-13T12:20:35.027" LastActivityDate="2016-07-13T12:20:35.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17176" PostTypeId="1" AcceptedAnswerId="17177" CreationDate="2016-07-13T05:50:38.980" Score="27" ViewCount="11104" Body="&lt;p&gt;Helicopter drones are awesome fun down here on Earth, but what if... we took one to Mars? This sounds like a good question for &lt;a href=&quot;https://what-if.xkcd.com/54/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Randall Munroe&lt;/a&gt;. But I read about a proposal to send a helicopter drone (MHS) to Mars with &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars 2020&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TXMRu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TXMRu.png&quot; alt=&quot;Mars Heli-drone&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Martian atmosphere's density is a fraction of a percent of Earth's (average &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars#Atmosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.6%&lt;/a&gt; ASL). My off the cuff calculations say that such a drone would have to be 200 times more powerful than its Earth-bound cousins.&lt;/p&gt;&#xA;&lt;p&gt;Is this a realistic proposal?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="46507" LastEditDate="2022-05-23T21:05:43.463" LastActivityDate="2022-05-23T21:05:43.463" Title="Could a helicopter fly on Mars?" Tags="|mars|aerodynamics|technology|helicopter|" AnswerCount="5" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="17177" PostTypeId="2" ParentId="17176" CreationDate="2016-07-13T08:32:39.120" Score="34" Body="&lt;p&gt;Gravity is about a third of Earth's and competitive aerobation helicopter models have a truly excessive power surplus. Just look at &lt;a href=&quot;https://www.youtube.com/watch?v=PptMrBFAO-A&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;There won't be any manned helicopter flight. The helicopter power scales poorly with size - there's a reason we have no VTOL Jumbo Jets. But the same up-scaling issue is our friend when down-scaling. A &lt;a href=&quot;https://www.youtube.com/watch?v=A-PigXQWChs&quot; rel=&quot;nofollow noreferrer&quot;&gt;6kg helicopter can lift 30kg&lt;/a&gt; of payload, so we have equivalent of TWR=6 on Earth.&lt;/p&gt;&#xA;&lt;p&gt;Drag and lift are both identically (quadratically) proportional to airspeed, so the same loss of lift causes loss of the primary limiting force of the helicopter - air drag on the blades. What's left is mechanical losses (friction of bearings), and mechanical durability of the construction, but these can be built with much surplus - plus larger rotor blades will offset need for extreme RPM.&lt;/p&gt;&#xA;&lt;p&gt;Of course other things must be taken into account. These flights will be more like &amp;quot;hops&amp;quot; - a short segment of flight followed by a looong recharging period. And the drone will need to be small - and that means not much scientific equipment. Unfortunately, that also means it won't be fully autonomous as there simply won't be enough room to fit a good radio and antenna to reach Earth, or even the satellites. But it could definitely serve as a &amp;quot;recon vehicle&amp;quot; for a large rover, fetching small samples, testing firmness of ground surface for driving over, and planning the best route.&lt;/p&gt;&#xA;&lt;p&gt;2021.03.16 edit:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://mars.nasa.gov/resources/25828/first-video-of-nasas-ingenuity-mars-helicopter-in-flight/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ingenuity has performed its first flight.&lt;/a&gt;. Most of data in my post predicted Ingenuity's parameters correctly. 2,500 rpm at 60cm rotor radius gives 157m/s top linear speed of the rotor tips, vs 267m/s speed of sound in carbon dioxide (and 90m/s in aerobation RC helicopters), so they remained subsonic. It's lightweight - 1.8kg, comparable to heavier aerobatic models. It uses the rover as a retransmitter to contact the satellites and Earth. The rotor blades are both wider and longer than in aerobation RC helicopters to compensate for the thin atmosphere, but the motor is comparable to mid-shelf models regarding power output. The helicopter is capable of a 30s flight and requires a day to recharge the batteries.&lt;/p&gt;&#xA;&lt;p&gt;2021-09-05 edit:&lt;/p&gt;&#xA;&lt;p&gt;As the first phase - technology demonstration of feasibility of powered flight on Mars has ended, &lt;a href=&quot;https://twitter.com/NASA/status/1423012693439651840?s=20&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ingenuity performs reconnaissance work&lt;/a&gt; helping Curiosity pick interesting targets.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/HLczt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HLczt.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2021-08-05T18:29:33.927" LastActivityDate="2021-08-05T18:29:33.927" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="17178" PostTypeId="2" ParentId="17077" CreationDate="2016-07-13T09:04:50.303" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/feature/jpl/nasa-s-juno-spacecraft-sends-first-in-orbit-view&quot; rel=&quot;nofollow noreferrer&quot;&gt;The first JunoCam picture from orbit was taken on July 10, 2016&lt;/a&gt; and published on July 12.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D0RQD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D0RQD.jpg&quot; alt=&quot;Jupiter as seen from JunoCam&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-07-13T09:10:03.927" LastActivityDate="2016-07-13T09:10:03.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17179" PostTypeId="2" ParentId="17166" CreationDate="2016-07-13T09:10:59.750" Score="2" Body="&lt;p&gt;I'm recently on &quot;specific energy&quot; checking spree, so let's test Ice Water Energy Storage for Venus application.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's see how much energy can we store as temperature gradient on Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surprisingly, Water is a material with about the highest specific heat across a range of temperatures. You might bump it by half a percent or so by additions, but let's take pure water for easier calculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ice water: 2.108 kJ per kilogram per Kelvin&#xA;Latent heat of melting: 334 KJ/kg @ 273.15K&#xA;Liquid water: 4.184 KJ per kilogram per Kelvin&#xA;Latent heat of vaporization: 2264.76 KJ/kg @ 373.15&#xA;Water steam: 1.996 kJ per kilogram per Kelvin&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We cool the block of ice down to 77.2 K while using cold gas thrusters - Liquid Nitrogen, performing the reentry burn. That's the initial temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus surface temperature: 735 K&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, heating the ice up to 0.&#xA;273.15-77.2 = 195.95K. Times 2.108 = 413KJ.&#xA;Then melting it: 334 KJ.&#xA;Then heating water: 4.184 times 100K = 418.4KJ&#xA;Then evaporating: 2264.7KJ&#xA;Then heating the steam: 735-373.15 = 361.85 times 1.996 = 722.2KJ.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4.152MJ per kilogram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's not bad. About the same energy density as thermite, and about 8 times more than Li-Ion batteries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, that places it below Li-Ion batteries as electric energy source. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thermocouples, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%. &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Efficiency&quot; rel=&quot;nofollow&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, with generous 10%, we're at 0.4MJ - specific energy of consumer level alkaline batteries. Poor!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BUT! The rest of energy isn't wasted. It's used to cool the craft, like a sublimator. So the idea isn't entirely bad - it could buy the probe a good couple hours of life! It's just not good as power source alone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...and of course my calculations are waaay off due to Venus pressure. The actual performance will be worse. But I can't really find the data for heats of water at 90bar, and what we have should give you a clue what we're dealing with.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-13T09:10:59.750" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17180" PostTypeId="1" AcceptedAnswerId="17183" CreationDate="2016-07-13T09:45:00.580" Score="14" ViewCount="1143" Body="&lt;p&gt;JunoCam has &lt;a href=&quot;https://twitter.com/NASAJuno/status/753001106498850816&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweeted&lt;/a&gt; us! More background &lt;a href=&quot;http://www.bbc.com/news/science-environment-36780756&quot; rel=&quot;nofollow noreferrer&quot;&gt;from the BBC&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tqlJR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tqlJR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is supposed to have several features that help allow it to work in the high radiation and magnetic fields it will experience during each flyby. Although it has a &lt;a href=&quot;http://www.onsemi.com/pub_link/Collateral/KAI-2020-D.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;4:3 sensor&lt;/a&gt;, according to &lt;a href=&quot;https://en.wikipedia.org/wiki/JunoCam#Specifications&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; it has a very long, narrow 18 x 3.4 degree FOV, and that there are three color filters, not the normal &lt;a href=&quot;https://en.wikipedia.org/wiki/Bayer_filter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bayer&lt;/a&gt; arrangement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does this thing work? Why such a strange aspect ratio? Inquiring minds want to know!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-07-13T12:06:37.990" Title="How is JunoCam different from a normal CCD camera?" Tags="|jupiter|flyby|juno|camera|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17181" PostTypeId="1" AcceptedAnswerId="17182" CreationDate="2016-07-13T10:11:04.993" Score="22" ViewCount="3121" Body="&lt;p&gt;From the question &lt;a href=&quot;https://space.stackexchange.com/questions/12607&quot;&gt;What are the regulations governing (against) beards in space?&lt;/a&gt; we know that hair and beard length are not regulated on the ISS.  For the most part pictures indicate both men and women keep their hair maintained.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know when using electrical trimmers at home there are small pieces of hair, that gravity bring to the ground, and they get washed or vacuumed away.  In the ISS they would just float around until someone inhaled them, or they got caught someplace. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is hair maintenance managed on the ISS?  Is trimming limited to only the head (i.e. is body hair below the neck line trimmed in space)?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-07-13T12:55:29.337" Title="How are hair and beard lengths maintained in space?" Tags="|iss|microgravity|hygiene|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17182" PostTypeId="2" ParentId="17181" CreationDate="2016-07-13T10:41:51.327" Score="26" Body="&lt;p&gt;&lt;a href=&quot;http://www.asc-csa.gc.ca/eng/astronauts/living-in-space/personal-hygiene-in-space.asp&quot;&gt;A haircut&lt;/a&gt; is done with an electric trimmer which has a vacuum hose attached.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wet shaving avoids this by trapping the cuttings in shaving foam, but some astronauts use electric razors, again with a vacuum hose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a video of someone using an electric trimmer: &lt;a href=&quot;https://www.youtube.com/watch?v=or2tzAkkbkM&quot;&gt;Shaving in space&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-07-13T12:55:29.337" LastActivityDate="2016-07-13T12:55:29.337" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17183" PostTypeId="2" ParentId="17180" CreationDate="2016-07-13T12:06:37.990" Score="18" Body="&lt;p&gt;JunoCam used different technologies than does the typical framing camera one buys at a store. A typical digital color camera uses a Bayer filter pattern, a row of alternating tiny blue and green filters, followed by a row of alternating tiny green and red filters, each filter covering a pixel, followed by a row of alternating tiny blue and green filters, and so on. JunoCam instead has four filter strips, each of which is 1600 pixels wide. Three are in the visible range, the standard red, green, and blue filters based on how the human eye works. The fourth is in the near infrared and is designed to see methane. Each of the three visible filter strips are about 150 pixels high; the methane filter strip is a somewhat larger than one of the visible filter strips.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;JunoCam uses two other concepts that you don't see in the typical digital camera. One is time delay and integration. The lighting level at Jupiter is about 1/27th of the lighting level at Earth's orbit (Jupiter orbits at about 5.2 AU). Juno rotates at 2 RPM. A short duration exposure would result in too much noise due to the low lighting. A low duration exposure would be too blurry thanks to that rotation. Time delay and integration means taking a series of short duration exposures and integrating them together by shifting the subsequent images to account for rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other key technique is that JunoCam is a pushframe imager as opposed to a framing imager. A frame results when the time delay and integration is complete (the number of TDI steps is commandable). This frame is pushed onto local memory. JunoCam alternates between visible and methane channel, so that means that another visible frame will be produced a minute later thanks to JunoCam's 2 RPM rotation rate. The result is a jumble of partially overlapping framelets that need extensive ground processing to make sense of the imagery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;JunoCam was designed to yield optimal performance at one hour before and one hour after closest approach. This should give good performance at closest approach, but more importantly should also give good performance over Jupiter's polar regions. JunoCam won't perform so well Juno is far from Jupiter (which is most of its orbit).&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-07-13T12:06:37.990" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17184" PostTypeId="2" ParentId="17149" CreationDate="2016-07-13T14:42:31.067" Score="3" Body="&lt;p&gt;Summarizing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.grc.nasa.gov/www/k-12/airplane/angdva.html&quot; rel=&quot;nofollow&quot;&gt;NASA's introductory material defines &quot;angular displacement&quot;&lt;/a&gt; as &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a vector quantity, which means that angular displacement has a size and a direction associated with it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It goes on to say that the direction is specified by an axis with right-hand rule, so 1 degree of pitch-up would be defined as 1 degree angular displacement around the horizontal axis left-to-right (conventionally this is the +y axis). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For one degree of pitch-down, these three representations would be equivalent:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;-1 degree around +y axis &lt;em&gt;(minimum magnitude, always positive axis)&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;+1 degree around -y axis &lt;em&gt;(positive displacement)&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;+359 degree around +y axis &lt;em&gt;(positive displacement and axis)&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So presumably the &lt;em&gt;displacement angle&lt;/em&gt; is the magnitude part of an angular displacement value, and whether it can be negative is a matter of convention. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Angle_of_attack&quot; rel=&quot;nofollow&quot;&gt;Angle of attack&lt;/a&gt; is in particular the angle between local airflow and the axis of discussion, generally in a particular plane. For a rocket that will generally be the longitudinal axis (often labeled +x) and the pitch plane. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(For a craft with substantial aerodynamic surfaces the angle of attack is usually given with the chord line of the wing as the reference rather than the longitudinal axis of the craft -- they aren't always the same!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rotation around the vertical axis is yaw, and the angle between the longitudinal axis and local airflow in that direction is the &lt;em&gt;sideslip angle&lt;/em&gt;. Angle of attack and sideslip angle are sometimes abbreviated α and β respectively.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-13T14:53:19.333" LastActivityDate="2016-07-13T14:53:19.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17185" PostTypeId="2" ParentId="17176" CreationDate="2016-07-13T15:36:22.940" Score="36" Body="&lt;p&gt;There's nothing like seeing it flying in a Mars-density chamber to answer your question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/vpBsFzjyRO8?t=1m43s&quot; rel=&quot;noreferrer&quot;&gt;Crazy Engineering: Mars Helicopter&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a really nice video of it in controlled flight in the chamber, but I can't find that one on the interwebs yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Update two years later:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks SF for &lt;a href=&quot;https://youtu.be/oOMQOqKRWjU?t=56s&quot; rel=&quot;noreferrer&quot;&gt;this link to nice video&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2018-05-17T16:39:31.907" LastActivityDate="2018-05-17T16:39:31.907" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17186" PostTypeId="2" ParentId="16984" CreationDate="2016-07-13T15:56:41.477" Score="0" Body="&lt;p&gt;Minimum distance of Jupiter from Earth (space.com) $\approx588*10^6 \text{km}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maximum distance of Jupiter from Earth (space.com) $\approx968*10^6 \text{km}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$c \approx 3.0*10^8 m/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Minimum time for signal to reach Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$588*10^6 \text{km} * (1000m / 1\text{km}) * (1/c) * (1\text{min} / 60\text{sec}) \approx 32.67 \text{min}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maximum time for signal to reach Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$968*10^6 \text{km}  * (1000\text{m} / 1\text{km}) * (1/c) * (1\text{min} / 60\text{sec}) \approx 53.78 \text{min}$$&lt;/p&gt;&#xA;" OwnerUserId="16208" LastEditorUserId="16208" LastEditDate="2016-10-14T23:35:23.830" LastActivityDate="2016-10-14T23:35:23.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17188" PostTypeId="2" ParentId="17054" CreationDate="2016-07-13T17:39:59.840" Score="0" Body="&lt;p&gt;As we don't seem to have come up with an official standard definition, I would have a tendency to simply consider the average density of gas per volume as a reference point. That would simply be the &lt;a href=&quot;http://spacemath.gsfc.nasa.gov/earth/RBSP9.pdf&quot; rel=&quot;nofollow&quot;&gt;average number of gas molecules measured per cubic centimeter&lt;/a&gt; at any given time. After we have measured this, all we need to do is define a reference value stating at which measurement we do consider this statistically significant for defining our gas planet radius.&lt;/p&gt;&#xA;" OwnerUserId="16210" LastEditorUserId="6816" LastEditDate="2016-07-13T17:52:30.727" LastActivityDate="2016-07-13T17:52:30.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17190" PostTypeId="2" ParentId="4441" CreationDate="2016-07-13T19:59:11.113" Score="-3" Body="&lt;p&gt;A projectile fired horizontally will fall to earth eventually under the influence of gravity and air friction. It will keep the same face towards the earth throughout its flight if it has not been given an initial spin. If the same projectile is fired in space with enough force that it falls to earth at the same rate as the earth is falling away, it will never fall to earth and will be &quot;in orbit&quot;. Providing it has been given no initial spin then it will always keep the same face towards the earth throughout its orbit. Tidal and other force may disturb the orientation of the satellite and thrusters are needed to maintain the general orientation but the principle condition is to keep the same face towards the earth throughout the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same applies to the moon. It is a projectile that is trying to go in a straight line. The earth's gravity pulls it into orbit and it must therefore keep the same face towards the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is nothing special about the moon's rotation about the earth and it has no magical spin that keeps the same face towards the earth. &lt;/p&gt;&#xA;" OwnerUserId="16212" LastActivityDate="2016-07-13T19:59:11.113" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17191" PostTypeId="1" AcceptedAnswerId="17192" CreationDate="2016-07-13T21:26:29.170" Score="17" ViewCount="1305" Body="&lt;p&gt;When two objects are in different orbits of the same body, no matter how close they are, the inner object will always be faster - it will move farther away from the outer one (until it gets closer again, that is). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when you connect the two objects mechanically, this obviously won't happen: Both now share a common center of mass and both sides of the connection are constantly under some stress (corresponding to the de- and acceleration). If the difference between velocities is too high, the link will break.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this a problem when constructing large orbital structures, like the ISS? Do we need to take into account these forces? Are they even measurable?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="25" LastEditDate="2016-07-14T00:37:32.397" LastActivityDate="2016-07-14T00:37:32.397" Title="Is the difference in orbital velocity of different parts measurable on board the ISS?" Tags="|orbital-mechanics|iss|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17192" PostTypeId="2" ParentId="17191" CreationDate="2016-07-13T23:08:40.160" Score="20" Body="&lt;p&gt;An orbiting spacecraft of finite size experiences '&lt;a href=&quot;http://www.dept.aoe.vt.edu/~cdhall/courses/aoe4140/SatDy.pdf&quot;&gt;gravity gradient torques&lt;/a&gt;'.  These torques, if left uncorrected, tend to align the long axis of the spacecraft so that it points toward the center of the earth.  The ISS certainly experiences such torques due to its great size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the ISS usually flies in a &lt;a href=&quot;http://adsabs.harvard.edu/abs/1993asee.nasa.....T&quot;&gt;Torque Equilibrium Attitude&lt;/a&gt; which is chosen so the torques due to external effects such as gravity gradient, aerodynamics, etc, tend to balance out.  This reduces the amount of effort that the attitude control system must exert.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the gravity gradient torques being measurable, an instrument on the ISS can supposedly detect them, the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/914.html&quot;&gt;MAMS&lt;/a&gt;.  However, the linked web page states that it is only operated during dynamic events such as docking.  &lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-07-13T23:08:40.160" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="17193" PostTypeId="1" AcceptedAnswerId="17194" CreationDate="2016-07-14T01:00:17.567" Score="10" ViewCount="603" Body="&lt;p&gt;I understand that Europa is tidally locked* to Jupiter, so there's a Jovianmost longitude on one side and an anti-Jovian longitude on the other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I can't figure out is which side is which.  Looking at &lt;a href=&quot;http://planetarynames.wr.usgs.gov/Page/EUROPA/target&quot; rel=&quot;noreferrer&quot;&gt;a map of Europa&lt;/a&gt;, is Dyfed Regio (for example) right near the Jovian longitude or the anti-Jovian longitude?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FDaMK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FDaMK.jpg&quot; alt=&quot;map of europa&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;*I've read some articles that say Europa might precess more than we previously expected.  For this question, that doesn't matter.  I'm just asking about its current orientation.&lt;/p&gt;&#xA;" OwnerUserId="552" LastEditorUserId="25" LastEditDate="2016-07-14T01:23:05.257" LastActivityDate="2019-04-09T20:36:38.500" Title="Which side of Europa is which?" Tags="|jupiter|europa|mapping|coordinates|tidal-locking|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17194" PostTypeId="2" ParentId="17193" CreationDate="2016-07-14T01:18:12.363" Score="8" Body="&lt;p&gt;The standard for any tidally locked body, of which Europa is a member, is to have the 0 longitude be the point at the center of the planet-facing side. That being the case, the middle of the map should be the portion facing Jupiter, the edges the part that never faces Jupiter. See &lt;a href=&quot;https://en.wikipedia.org/wiki/Longitude#Length_of_a_degree_of_longitude&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; for the referenced quote below:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Tidally-locked bodies have a natural reference longitude passing through the point nearest to their parent body: 0° the center of the primary-facing hemisphere, 90° the center of the leading hemisphere, 180° the center of the anti-primary hemisphere, and 270° the center of the trailing hemisphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That being said, the source image for your map is found at the &lt;a href=&quot;http://pubs.usgs.gov/imap/i2757/&quot; rel=&quot;noreferrer&quot;&gt;USGS website&lt;/a&gt;. The far right side is 0 degrees longitude, the far left 360, of the image shown below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GcJir.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GcJir.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The darker region, which is the central piece for your image, is centered around 250 degrees longitude. In fact, looking at the &lt;a href=&quot;http://planetarynames.wr.usgs.gov/SearchResults?target=EUROPA&quot; rel=&quot;noreferrer&quot;&gt;database of features on Europa&lt;/a&gt;, we find out that &lt;a href=&quot;http://planetarynames.wr.usgs.gov/Feature/14293;jsessionid=0EDE9FA3CBD068B6DBD5D1D07EA85DA1&quot; rel=&quot;noreferrer&quot;&gt;Dyfed Regio&lt;/a&gt; is 10 lat, 250 longitude. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-14T01:34:57.607" LastActivityDate="2016-07-14T01:34:57.607" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17195" PostTypeId="1" CreationDate="2016-07-14T09:22:10.633" Score="2" ViewCount="207" Body="&lt;p&gt;I'm trying to understand what would shortest paths could looks like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume a few things&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;We have a constant thrust to weight ratio&lt;/li&gt;&#xA;&lt;li&gt;We do not need to decelerate &lt;/li&gt;&#xA;&lt;li&gt;We need to take drag into account&lt;/li&gt;&#xA;&lt;li&gt;No wind/surface terrain&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;How to calculate the quickest path between two points on earth ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the third point might be a bit complicated so some simplification can be made&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-12-18T02:52:04.890" Title="Given a constant thrust to weight ratio, how to calculate the quickest path between two points on earth?" Tags="|rockets|trajectory|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17196" PostTypeId="1" AcceptedAnswerId="17198" CreationDate="2016-07-14T12:44:46.783" Score="31" ViewCount="8344" Body="&lt;p&gt;I'm writing a computer game involving space exploration, where the player will come across space ships/stations that are hundreds/thousands/tens of thousands of years old. I'm also trying to make it relatively realistic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I love the idea of the player coming across an ancient spaceship, it being all spooky, rusted, battered, stained and generally knackered and creepy looking, but that doesn't seem realistic...I can't imagine rust happening, any stainy liquids that leaked out would just boil away etc&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if everyone abandoned a giant space station, turned the power off and just let it drift through space for evermore - would it look any different in 100/1000/100,000+ years? What about if it was in (stable) orbit above a planet, or near a star?&lt;/p&gt;&#xA;" OwnerUserId="16225" OwnerDisplayName="GoatInTheMachine" LastEditorUserId="58" LastEditDate="2016-09-16T15:33:11.600" LastActivityDate="2019-05-15T06:12:49.317" Title="Do things get dirty in space?" Tags="|interstellar-travel|debris|chemistry|" AnswerCount="7" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17198" PostTypeId="2" ParentId="17196" CreationDate="2016-07-14T13:02:25.053" Score="37" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Spooky&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This one is subjective. To some, just finding an abandoned spaceship would be spooky. I'll say there's probably not a lot that has to happen to evoke this feeling.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rusted&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Actually, unless your spaceship never had a breathable atmosphere or the atmosphere was vented before the spaceship was abandoned, rust is totally possible on the inside and even the outside. As the oxygen escaped, rust would be produced on any exterior iron in contact with the venting oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/4154/would-unpainted-iron-or-steel-rust-in-space&quot;&gt;If you're in orbit of a planet with an oxygen atmosphere, the trace amounts could also cause some amount of rust over time.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an example of [apparent] oxidation on the Quest Joint Airlock thermal cover:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i0eWz.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i0eWz.jpg&quot; alt=&quot;Oxidation on the Quest Joint Airlock thermal cover&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Tristan pointed out in the comments, the coloration is not actually oxidation:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the discoloration on the airlock hatch cover is most likely not oxidation but rather contamination due to outgassing from materials that become volatile in the space environment (plastic, rubber, trace organic compounds in the interior atmosphere that get released when the hatch is opened, etc.). The brown color is from the contaminants being altered by UV radiation. To the extent that oxidation is occurring, it would act to bleach the discoloration, rather than contribute to it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Despite that, you can still see both that oxidation occurs, and that even if it wouldn't look like you imagine, there are other effects that can cause a similar appearance to rust on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from actual rust, when the coating and lubrication breaks down on metal parts, they will &lt;a href=&quot;https://en.wikipedia.org/wiki/Cold_welding&quot; rel=&quot;noreferrer&quot;&gt;cold weld&lt;/a&gt; together. &lt;a href=&quot;https://physics.stackexchange.com/questions/10679/how-would-steel-degrade-in-space&quot;&gt;Steel will develop crystallographic defects and a positive charge.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Battered&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the short term, this depends somewhat on where in space you are, but we're talking about an ancient spaceship. By that point, it's almost certain that its going to be battered by cosmic dust. Here's an example of what debris damage might look like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zkh3Im.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zkh3Im.jpg&quot; alt=&quot;Debris damage&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Stained&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A lot of parts will become discolored overtime by radiation exposure. Does that count?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might be hard to visualize this on other parts, but here's an example of UV effects on rope (the rope on the right is new, the left is affected by UV):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CaF8h.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CaF8h.jpg&quot; alt=&quot;UV degradation on rope&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your spaceship is in orbit of a planet, you'll probably have even more junk to run into. I just hope you don't expect the thing to be operable. For that matter, if your spaceship is in too low of an orbit, then its orbit could decay and it could break apart rather quickly due to interaction with the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;You didn't ask, but the spaceship might also get moldy, see this question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/20334/melanized-fungi-on-the-iss-are-they-enjoying-the-elevated-radiation-levels-t&quot;&gt;Melanized fungi on the ISS - are they &amp;#39;enjoying&amp;#39; the elevated radiation levels there?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the fungi would require at least minimal levels of oxygen, so I wouldn't expect the mold to last long on an abandoned ship.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-03-02T17:23:58.160" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17199" PostTypeId="2" ParentId="17196" CreationDate="2016-07-14T13:02:43.293" Score="5" Body="&lt;p&gt;You're right, there's no rust or staining. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the spacecraft is in orbit of a planet, you will get meteorite craters. After 25 years in Low Earth Orbit, the Hubble space telescope has collected thousands of tiny impact craters (from sub-mm particles). Leave something long enough and you'll get bigger impacts too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In interstellar space, you get fewer impact craters. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-14T13:02:43.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17200" PostTypeId="1" CreationDate="2016-07-14T13:04:16.453" Score="1" ViewCount="722" Body="&lt;p&gt;I've learnt that solar flares have so much energy in them, but is it possible to tap the energy from them?&lt;/p&gt;&#xA;" OwnerUserId="16221" LastActivityDate="2020-10-26T12:40:33.203" Title="Is it possible to absorb energy from solar flares" Tags="|energy|solar-flares|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17201" PostTypeId="1" AcceptedAnswerId="17211" CreationDate="2016-07-14T13:39:17.850" Score="3" ViewCount="276" Body="&lt;p&gt;Metals items used in certain types of nuclear power station can become brittle over time due to exposure to radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the amount of cosmic radiation, is embrittlement of metals on space craft an issue that needs to be considered?&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2016-07-14T18:44:07.440" Title="Radioactive Embrittlement of Metals in Space" Tags="|spacecraft|radiation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17202" PostTypeId="1" CreationDate="2016-07-14T14:39:37.810" Score="-2" ViewCount="317" Body="&lt;p&gt;Will US Policy be that there should be two Boeing CST-100 Starliner or SpaceX Dragon V2 craft attached to ISS at all times for the American segment of the station? Will Soyuz continue to be used?&lt;/p&gt;&#xA;" OwnerUserId="16220" LastEditorUserId="51" LastEditDate="2016-07-14T15:30:08.187" LastActivityDate="2016-07-14T16:01:50.287" Title="What will the new rules, guideliness, and safety measure be for the ISS after NASA kicks out Soyuz?" Tags="|iss|crewed-spaceflight|nasa|soyuz-spacecraft|policy|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17203" PostTypeId="2" ParentId="16788" CreationDate="2016-07-14T14:48:51.407" Score="6" Body="&lt;p&gt;As geoffc has explained in his answer SpaceX is working for new modifications or solutions by finding the limits of what could be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another possible option for GTO mission landings in theory could be, including the solid rocket booster and not modifying Falcon 9 FT at all. Using two small solid rocket booster only in GTO missions would improve the performance of Falcon 9 FT and would make possible to save more fuel at the first stage, rising a lot the chances for successful landings. It will rise the cost but providing successful landings and reusability, would drop the price alot and customers will still be satisfied. For example two small solid booster at the same category with &lt;a href=&quot;https://en.wikipedia.org/wiki/Graphite-Epoxy_Motor&quot; rel=&quot;nofollow noreferrer&quot;&gt;GEM-40,GEM-46&lt;/a&gt; (&lt;a href=&quot;https://space.stackexchange.com/questions/13388/why-are-there-no-more-delta-ii-rockets/14843#14843&quot;&gt;$2.5 million one&lt;/a&gt;) of Delta II could work for missions similar to those it has made so far. For greater payloads would help solid booster as powerful as &lt;a href=&quot;https://en.wikipedia.org/wiki/Graphite-Epoxy_Motor&quot; rel=&quot;nofollow noreferrer&quot;&gt;GEM-60&lt;/a&gt; of Delta IV, or &lt;a href=&quot;https://en.wikipedia.org/wiki/AJ-60A&quot; rel=&quot;nofollow noreferrer&quot;&gt;AJ-60A&lt;/a&gt; of Atlas V. So in the future for some type of missions that Falcon 9 FT couldn't make, not necessarily should be used Falcon Heavy (their heaviest version) but a smaller version than it and less costly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their Merlin 1D FT is a great engine and they have done a great job. Projecting and producing a solid booster would not be difficult for SpaceX since they are far less complicated and cheaper. Also they could make them even reusable, since they are trying with payload fairings why not they can do this even with their solid boosters, dropping the cost of using them. Even if they won't reuse them still the price will be ok. Until they produce their solid booster they need to buy from Aerojet Rocketdyne or Orbital ATK. It looks difficult to happen since these companies are rivals with SpaceX. And SpaceX has shown ambitions that wants to make everything by themselves in a original way. But could be as option by thinking that they would make reality the reusability of all their first stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course maybe SpaceX will not need solid boosters, they just will make modifications, improvements or changing procedures that they follow for Falcon 9 FT first stage. They have made a great success with their landings and probably will have 100% success rate in perfection in a couple of months. But remembering the SES-9 GTO mission which was with the heaviest payload until now, SpaceX could have missions with payloads a little heavier or much more heavier than that. It would need a Falcon Heavy, but why using it if a smaller one than it and with more capabilities than Falcon 9 FT, a middle class, could make that mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I consider these as option because is a similar way that Delta IV family rockets operate, or as the project of Atlas V family where was planned even a heavy HLV version with three RD-180 rocket engine boosters. In these way they will resolve the difficulties in GTO missions without forcing, pushing to the limits everything in their rocket, and also their costumers don't need to pay more moneys for a heavier rocket (Falcon Heavy) if another version smaller and cheaper could make it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most important reason it is not to use solid booster configuration as a replacement for Falcon Heavy in some kind of missions. The most important one is to bring back safely 1st stages from a GTO mission. They are attepmting this with the current Falcon 9 FT and in the next year they expect 100% success. But if they continue to face problems and will not make it 100% success, than buying small solid boosters (or producing depends which option is better for them), could save their Falcon 9 FT 1st stage. Total launch price of a reused Falcon 9 FT is around &lt;a href=&quot;https://spaceflightnow.com/2016/03/31/spacex-hopes-to-sell-used-falcon-9-boosters-for-40-million/&quot; rel=&quot;nofollow noreferrer&quot;&gt;43 million&lt;/a&gt;. Including the solid boosters, the cost for a new GTO mission still will be lower than the current price &lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;nofollow noreferrer&quot;&gt;62 million&lt;/a&gt;, so no problems for customers they will be still satisfied. SpaceX will have some costs to adjust solid booster with Falcon 9 FT and to make changes in the launchpad, but still they have to many GTO missions upcoming. In this way this costs will not be a problem for a company like SpaceX, it will win a lot more moneys in the upcoming GTO missions. What is worthy for them it is that they could learn from tests and soft landings from GTO missions where we have high velocities. These difficult landings would help them with useful data for Mars landings which are also difficult and which is their goal also. So they could have the chances to study GTO missions (which are many missions) 1st stages by making them reusable. With 100% success the cost will drop more quickly for reusability, and also most important taking useful data for difficult missions in Mars. SpaceX could do further upgrades in Falcon 9 FT and could make possible 100% success in the next year or sooner. If not maybe this could be as option.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-08-24T17:41:19.777" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17204" PostTypeId="2" ParentId="17202" CreationDate="2016-07-14T14:57:00.760" Score="5" Body="&lt;p&gt;The current plan is that the Russians will maintain a Soyuz vehicle at the station at all times there is crew aboard, allowing for 3 crew members.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;US Commercial Crew (Boeing CST-100 Starliner, or SpaceX Dragon V2/Crew) will be maintained on the station at all times there is a crew aboard, allowing for 4 crew members.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently with 2 Soyuz the maximum (Though of course, there are 4 Russian docking ports, so potentially 4 Soyuz, but usually 2 Soyuz, a Progress or two, and possibly an ESA ATV is the compliment of vehicles) there is a maximum crew size of 6.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By moving to 7, (3 on Soyuz, 4 on Commercial Crew) they gain one more member, and since after maintenance, exercise, and other administrivia on station, the entire crew of six, only manage to accomplish about 40 hours of science experiments a week total. Thus adding one more person, should add 40 more hours a week of available work time, which could double the science throughput. (Assuming that their time is not consumed in incremental tasks based on 6 vs 7 in the crew).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus no one is getting kicked out until the Russians decide to leave the station.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-07-14T16:01:50.287" LastActivityDate="2016-07-14T16:01:50.287" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17205" PostTypeId="1" CreationDate="2016-07-14T16:09:14.277" Score="1" ViewCount="3023" Body="&lt;p&gt;The ISS currently handles a crew of 6 launched by two Soyuz vehicles. With the advent of Commercial Crew bring 4 a mission, the hope is to run a crew of 7.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where in the ISS do the crew sleep?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US Segment has quarters in the Harmony module, but how many?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rcFU7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rcFU7.jpg&quot; alt=&quot;US Module Crew quarters&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian segment has room to sleep, but for how many?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-14T16:21:58.963" Title="Where do the crew sleep on the ISS? Where are the sleeping quarters?" Tags="|iss|crewed-spaceflight|sleep|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17206" PostTypeId="1" CreationDate="2016-07-14T16:16:17.080" Score="3" ViewCount="135" Body="&lt;p&gt;I have heard that some astronaut (maybe Scott Kelly) complained about itchy, sensitive skin after returning from space. Is it true? After growing 2 inches, now he can't sleep on a bed and wear Earth clothes and walk.&lt;/p&gt;&#xA;" OwnerDisplayName="user16223" LastEditorDisplayName="user16223" LastEditDate="2016-07-14T22:57:12.623" LastActivityDate="2016-07-14T22:57:12.623" Title="Astronauts with oversensitive skin after returning from space" Tags="|astronauts|astronaut-lifestyle|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17207" PostTypeId="2" ParentId="17205" CreationDate="2016-07-14T16:21:58.963" Score="2" Body="&lt;p&gt;The Russian segment has two sleep stations in the Zvezda module, which is at the far aft end of the station.  In this &lt;a href=&quot;https://www.youtube.com/watch?v=ICCuD4M9nIs&quot; rel=&quot;nofollow&quot;&gt;video&lt;/a&gt;, Garret Reisman shows the two off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US Harmony node has 4 sleep stations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That accounts for six spots. With the advent of commercial crew, it makes you wonder what the plan for the 7th crew member will be.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-14T16:21:58.963" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17211" PostTypeId="2" ParentId="17201" CreationDate="2016-07-14T18:44:07.440" Score="5" Body="&lt;p&gt;No. You can expect materials embrittlement from space radiation to be less than one-billionth of that taking place in a nuclear reactor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neutron bombardment is the primary (but not only) mechanism for metal embrittlement in nuclear reactors. Neutrons are heavy and fast enough to knock atoms out of place in their crystalline structure, which over time manifests as brittleness. The neutron flux in nuclear reactors is much, much, much more intense than it is in space. From Wikipedia's &quot;Neutron Flux&quot; article: A reactor vessel of a typical nuclear power plant (PWR) endures in 40 years (32 full reactor years) of operation approximately 3.5×10^19 neutrons per square centimeter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, especially outside of the Earth's magnetic field, solar protons dominate the radiation spectrum. They have nearly the same mass as neutrons, although their charge and energy levels cause them to interact differently. Ignoring those differences and looking exclusively at the intensity, the GOES satellite shows an average proton flux well below 10 neutrons/second per square centimeter at earth's distance from the sun. That adds up to fewer than 1x10^10 protons in the same 40-year period.&lt;a href=&quot;http://www.swpc.noaa.gov/products/goes-proton-flux&quot; rel=&quot;noreferrer&quot;&gt;http://www.swpc.noaa.gov/products/goes-proton-flux&lt;/a&gt;  That's roughly one-billionth the flux of a reactor. So from a gross materials-embrittlement perspective, space radiation will not cause the same damage as nuclear reactor radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over time, solar protons can damage sensitive spacecraft systems like solar panels, electronics, and crew. Humans are more fragile than steel reactor vessels. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A paywalled article &lt;a href=&quot;http://link.springer.com/article/10.1007/BF00166645&quot; rel=&quot;noreferrer&quot;&gt;http://link.springer.com/article/10.1007/BF00166645&lt;/a&gt; indicates that the largest source of neutrons in the near-earth region is the Earth itself. Cosmic rays striking the upper atmosphere create particle cascades which includes the release of neutrons. It's interesting that some forms of radiation become less intense when you leave Earth. Overall, though space is a dangerously radioactive place for long duration missions.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-07-14T18:44:07.440" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17212" PostTypeId="1" CreationDate="2016-07-14T19:20:14.700" Score="3" ViewCount="431" Body="&lt;p&gt;In the future(2019-2020), if the American portion of the ISS had an unstable hull breach or worse a sick astronaut and 4 people took to earth. Are Russians qualified to handle the American portion of the ISS? Since they depend on the American power supply. &lt;/p&gt;&#xA;" OwnerDisplayName="user16223" LastEditorUserId="-1" LastEditDate="2016-07-15T12:08:13.357" LastActivityDate="2016-07-25T15:20:54.407" Title="Can Russians manage the ISS without American intervention, given it depends on American solar panels?" Tags="|iss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17213" PostTypeId="1" CreationDate="2016-07-14T20:04:44.880" Score="2" ViewCount="84" Body="&lt;p&gt;What is the highest altitude the ISS can raise itself to? What about with help from Progress-MS? How much would it cost to raise the ISS by 1km with or without progress?&lt;/p&gt;&#xA;" OwnerDisplayName="user16223" LastEditorUserId="13710" LastEditDate="2016-07-14T20:12:03.007" LastActivityDate="2016-07-15T11:43:28.533" Title="Altitude control of ISS via Progress-MS" Tags="|iss|orbit|altitude|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17214" PostTypeId="1" CreationDate="2016-07-14T20:57:25.297" Score="3" ViewCount="136" Body="&lt;p&gt;I read somewhere that the last &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_MS-01&quot; rel=&quot;nofollow noreferrer&quot;&gt;Progress-MS01&lt;/a&gt; strongly misfired completely to side just after docking to ISS which created problems for future Soyuz missions. Why did this happen? What was the effect  on the ISS and future structural airlock safety? &lt;/p&gt;&#xA;" OwnerDisplayName="user16223" LastEditorUserId="6944" LastEditDate="2016-12-04T16:06:29.060" LastActivityDate="2016-12-04T16:06:29.060" Title="Progress-MS01 docking incident" Tags="|iss|progress|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17215" PostTypeId="2" ParentId="17196" CreationDate="2016-07-14T21:20:35.907" Score="5" Body="&lt;p&gt;From the outside, you have many external factors that will cause &quot;rust like&quot; and &quot;stain like&quot; appearances. Things like radiation, lubricants, cold, space dust, micro-meteorites, etc. No one will pick on you if your old ship looks a bit old. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the inside, if there's an atmosphere, or ever was one, then just pretend it's on earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, don't give up your artistic vision of a spooky ship to suite science. Some of the best space battles have awesome pew pew sounds and big kabooms, and engine noise. None of those things are real.&lt;/p&gt;&#xA;" OwnerUserId="11013" LastActivityDate="2016-07-14T21:20:35.907" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17216" PostTypeId="2" ParentId="17212" CreationDate="2016-07-14T22:56:04.723" Score="4" Body="&lt;p&gt;The international partners of the ISS look after the parts of the station they made. This is done from the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station_program#Mission_control_centers&quot; rel=&quot;nofollow noreferrer&quot;&gt;mission control centers&lt;/a&gt; on the ground. All ISS systems are monitored on the ground and are &lt;a href=&quot;https://space.stackexchange.com/q/14901/4660&quot;&gt;adjusted from there&lt;/a&gt;, usually the crew is only involved when maintenance and repairs are needed, or new components are being installed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if an extremely unlikely incident incapacitated everyone except the Russians and they all left, Houston would continue to run the US orbital segment just as they do now. &lt;a href=&quot;http://isslive.com/displays/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;This service&lt;/a&gt; shows some of the readouts from the consoles at the Johnson Space Center. If they needed the remaining crew to help them, they would talk them through it. Cosmonauts tend to have an &lt;a href=&quot;https://www.google.com.mx/?ion=1&amp;amp;espv=2#q=cosmonaut%20engineer&quot; rel=&quot;nofollow noreferrer&quot;&gt;engineering background&lt;/a&gt; and speak English, and would already be familiar with the US side. It wouldn't be a problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is hard to imagine how the ISS could lose contact with the ground without all crew being evacuated. Houston would always be in touch with a crew in such a situation, and would continue to coordinate with Mission Control at Korolev, ESA, and JAXA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we imagine a fictional scenario like a natural disaster or a war, in which Houston can't communicate with the ISS, then the &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100020234.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Backup Control Center takes over&lt;/a&gt;. That center used to be Korolev in Moscow, but has now been switched to Marshall Space Flight Center in Huntsville, Alabama. During Hurricane Rita in 2005, Korolev was used to control the ISS for a brief time, but they no longer have that capability. If the BCC also lost contact with the ISS it is unclear what would happen. Continuous contact with the station would be lost if it was not possible to use NASA's Tracking and Data Relay Satellite System, and would be restricted to when the station is in view of a ground station. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-07-25T15:20:54.407" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17217" PostTypeId="1" AcceptedAnswerId="17249" CreationDate="2016-07-15T00:45:23.743" Score="4" ViewCount="1215" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cold_welding&quot; rel=&quot;nofollow&quot;&gt;Cold welding&lt;/a&gt; is a problem in vacuum of space - where two metal parts come into contact, they may weld accidentally to each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I understand, even a little air or other substance prevents it. My question is - how little? At what altitude do we need to start worrying about it, or what atmospheric pressure is the threshold between cold-welding vacuum, and common underpressure?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="4660" LastEditDate="2016-07-15T01:11:21.373" LastActivityDate="2016-07-16T02:28:25.403" Title="What is the air pressure threshold for cold welding?" Tags="|materials|vacuum|cold-welding|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17218" PostTypeId="1" CreationDate="2016-07-15T07:00:47.610" Score="2" ViewCount="438" Body="&lt;p&gt;Using my cell phone in a city to download lots of data very fast is common, thanks due to the dense net of base stations close to me (~1 km). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But with the space station about 400 km above ground and moving fast, how is that downlink of the live stream established exactly? I am especially interested in the HD live stream from the space station. If the communication &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Communications_and_computers&quot; rel=&quot;nofollow&quot;&gt;as explained here&lt;/a&gt; is almost continuous, why is the live stream sometimes broken for a longer time? Does a broken live stream also mean, that a voice communication is interrupted between the astronauts and the ground station? And what bandwidth is used for the live HD stream?&lt;/p&gt;&#xA;" OwnerUserId="11919" LastEditorUserId="11919" LastEditDate="2016-07-15T13:24:52.460" LastActivityDate="2016-07-15T13:24:52.460" Title="How exactly does the live feed from ISS work?" Tags="|iss|communication|communication-satellite|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17220" PostTypeId="1" AcceptedAnswerId="17222" CreationDate="2016-07-15T07:37:12.877" Score="26" ViewCount="20733" Body="&lt;p&gt;The answer to &lt;a href=&quot;//physics.stackexchange.com/questions/3863/what-happens-to-an-astronaut-whos-floating-in-a-spaceship-in-space-when-it-be&quot;&gt;What happens to an astronaut who's floating in a spaceship (in space) when it begins to move?&lt;/a&gt; is that we can create g-force just by accelerating. So then why go all the way to create rotating ring?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ring makes sense for ISS, but why use it for spaceships like the one I see on &lt;em&gt;The Martian&lt;/em&gt;. Is there any reason why they can't just accelerate to create gravity?&lt;/p&gt;&#xA;" OwnerUserId="16231" LastEditorUserId="6816" LastEditDate="2016-07-15T07:43:54.717" LastActivityDate="2018-11-09T17:30:13.350" Title="Why can't spaceships just accelerate to create gravity?" Tags="|artificial-gravity|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17221" PostTypeId="2" ParentId="17220" CreationDate="2016-07-15T07:56:37.640" Score="10" Body="&lt;p&gt;Basically, we just don't have engines that can accelerate at 1G, or anywhere near that, for more than a few minutes. Not only do we not &lt;em&gt;currently&lt;/em&gt; have such engines, we aren't even sure when we will. Nuclear-thermal designs can get at least twice the &lt;span class=&quot;math-container&quot;&gt;$I_{SP}$&lt;/span&gt; of chemical rockets, meaning the same amount of thrust can be maintained for twice as long for the same starting mass. (This doesn't mean 1G for twice as long, exactly, but it's fairly close.) Of course, twice as long as &quot;a few minutes&quot; still isn't very good. Project Orion and similar nuclear pulse propulsion designs can achieve much higher &lt;span class=&quot;math-container&quot;&gt;$I_{SP}$&lt;/span&gt;, but require setting off 1kt nuclear charges repeatedly. Both of these rely on fairly well-proven principles, but we don't have mature designs for them yet, and the current climate toward nuclear reactors and bombs in space is unfavorable, so there's not much serious research.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, the Medusa variant of Orion (using a sail to intercept the nuclear blast along with long tethers reeled in and out to spread its acceleration bursts over time) can likely achieve &lt;span class=&quot;math-container&quot;&gt;$I_{SP}$&lt;/span&gt; of close to 100000 s. Consider a mission to Mars at a fairly close approach (&amp;lt;75 million km, with the closest approach being about 40 million km) and a constant 0.3g acceleration/deceleration (to match Mars gravity ahead of time). Per &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/torchships.php#brachistochrone&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atomic Rockets&lt;/a&gt;, the equation for total time is &lt;span class=&quot;math-container&quot;&gt;$t = 2 \sqrt{D/a}$&lt;/span&gt; (which comes out to be about 3.7 days), and the equation for required &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt; is &lt;span class=&quot;math-container&quot;&gt;$\Delta V = 2 \sqrt{D \cdot a}$&lt;/span&gt;. Finally, the Tsiolkovsky rocket equation is of course &lt;span class=&quot;math-container&quot;&gt;$$\Delta V = v_e \ln {m_0 \over m_1}$$&lt;/span&gt;&#xA;Assuming a 100 ton dry mass (probably rather low) we can substitute: &lt;span class=&quot;math-container&quot;&gt;$$2 \sqrt{D \cdot a} = v_e \ln {m_0 \over m_1}$$&lt;/span&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$m_0 = m_1 \cdot e^{2 / v_e \cdot \sqrt{D \cdot a}}$$&lt;/span&gt;&#xA;The result is a starting mass of around 270 tons, which means 170 tons of fuel pellets in the form of 1kt nuclear shaped charges, so probably about half highly-enriched plutonium or U-238 and half bomb casing. Needing to acquire close to 100 tons of weapons-grade plutonium for a short trip to Mars is difficult, but not outright impossible. Accepting a lower gravity of, say, 0.1g the whole time (which may or may not be enough for comfort, but over a short duration, in this case about 6.3 days, is not going to affect bone density much) would cut the required propellant mass in half. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other designs either don't have enough thrust to create decent gravity, or don't have enough &lt;span class=&quot;math-container&quot;&gt;$I_{SP}$&lt;/span&gt; for continuous running during a voyage, or are very speculative. (Antimatter rockets, say, are a very long ways from being practical.)&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2018-11-09T17:30:13.350" LastActivityDate="2018-11-09T17:30:13.350" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="17222" PostTypeId="2" ParentId="17220" CreationDate="2016-07-15T07:57:21.683" Score="54" Body="&lt;p&gt;Constant acceleration requires energy. Our current rocket engines need to use propellant to provide that energy. And there just cannot be enough propellant to generate artificial gravity for any meaningful duration. We would need a new type of space drive to be able to use acceleration that way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concept is well known from (science-)fiction (sometimes named &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Torchship&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Torchship&quot;&lt;/a&gt;&lt;/em&gt;) and the artifical gravity provided is actually sort of a &lt;em&gt;side effect&lt;/em&gt;. The main benefit of a ship able to accelerate at $1G$ fo a long time would be the speed with which it can travel across the Solar System - &lt;a href=&quot;https://space.stackexchange.com/a/841&quot;&gt;Mars in two days, Jupiter under one week&lt;/a&gt;. But we are not sure if such propulsion system is even possible in reality. Often cited possibilities which might allow it in theory are fusion and antimatter drives.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-07-15T14:57:17.527" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17223" PostTypeId="2" ParentId="17196" CreationDate="2016-07-15T09:00:28.340" Score="2" Body="&lt;p&gt;it really depends on where in space. In orbit around an inhabited/earlier inhabited planet it can become dirty from space waste and dust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in deep space, the main problem are meteors. They would hit the ship creating dents and over 100 000 years that is bound to happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, radiation may eventually harm the ships systems(depending on where it is in space). If it is near somewhere the sun just died(no black hole) there may be space dust that cover up the windows&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to sum up:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Hull damage from meteors and other objects in space&lt;/li&gt;&#xA;&lt;li&gt;Radiation damage&lt;/li&gt;&#xA;&lt;li&gt;Space dust on windows&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerDisplayName="user16235" LastActivityDate="2016-07-15T09:00:28.340" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17224" PostTypeId="2" ParentId="17196" CreationDate="2016-07-15T11:07:30.637" Score="2" Body="&lt;p&gt;First ask yourself: Do you develop a game or a simulation? Because at a game fun is the ONLY goal. If realism contributes to fun, that's great, but if it doesn't, keep it OUT. So if 'spooky, rusted, battered, stained and generally knackered and creepy looking' improves the player's experience, put it in, no matter how realistic it is. Games don't have to be realistic, they only have to be consistent. So the next ancient station should be spooky etc. also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are doing a simulation, well, forget what I said and try to make it physically correct. But don't expect it to be much fun, even not for the very precise users.&lt;/p&gt;&#xA;" OwnerUserId="16237" LastActivityDate="2016-07-15T11:07:30.637" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17225" PostTypeId="2" ParentId="17213" CreationDate="2016-07-15T11:43:28.533" Score="1" Body="&lt;p&gt;The ISS is designed to operate between 330-435 km. It could be raised to that level if required to by the on-board fuel, or perhaps even higher, but that isn't typically done. The altitude is set to optimize the fuel cost. It takes more fuel to reach the station if it's higher, but it overall requires less fuel. The exact amount of fuel for the various propulsion modules can be found on the &lt;a href=&quot;https://en.wikipedia.org/wiki/ISS_Propulsion_Module&quot; rel=&quot;nofollow&quot;&gt;ISS Propulsion Module Wiki page&lt;/a&gt;. The space station needs about 7000 kg of fuel per year, and &lt;a href=&quot;http://spacemath.gsfc.nasa.gov/weekly/5Page35.pdf&quot; rel=&quot;nofollow&quot;&gt;loses about 90 m/ day&lt;/a&gt;. Given all of that, the ISS needs to use about 210 kg of fuel to raise the altitude 1 km. At any one time, it can raise the altitude about 30 km, although that would hit into reserves that are preferred not to be used.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-15T11:43:28.533" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17226" PostTypeId="2" ParentId="17220" CreationDate="2016-07-15T11:54:58.020" Score="46" Body="&lt;p&gt;jkavalik gives the right answer. But to put this in perspective, let me add some numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say, we use a battery of state-of-the-art ion drives to retain a semi-comfortable, Martian level of gravity, of 3.73 m/s&lt;sup&gt;2&lt;/sup&gt; for a period of 24 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;24 h is 86400 seconds. At 3.73 m/s&lt;sup&gt;2&lt;/sup&gt; this gives 322,272 m/s of delta-V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's use a large number of &lt;a href=&quot;https://en.wikipedia.org/wiki/VASIMR&quot;&gt;VASIMIR&lt;/a&gt; thrusters, of 12,000 s of specific impulse. Substituting to the Rocket Equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \begin{align}&#xA;\Delta v &amp;amp;= I_\mathrm{sp} g_0 \ln { m_0 \over m_\mathrm f }\\&#xA;\ln { m_0 \over m_\mathrm f } &amp;amp;= { \Delta v_0 \over { I_\mathrm{sp} g_0 } }\\&#xA;{ m_0 \over m_\mathrm f } &amp;amp;= \mathrm e^{ \Delta v_0 \over { I_\mathrm{sp} g_0 } }\\&#xA;&amp;amp;= \mathrm e^{ 322272 \over { 12000 \cdot 9.8 } }\\&#xA;&amp;amp;= \mathrm e^{2.74}\\&#xA;&amp;amp;= 15.48&#xA;\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means, that to maintain gravity of Mars, about third of Earth's, for a period of 24 hours, using ones of the best ion drives we have currently, the craft would need to use 14.48 times its own weight in fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To maintain it over another (earlier) 24 h, it would need 14.48 times the weight of craft plus fuel needed for the second day – about 240 times its own weight. Third day? 3700 times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for 3 days of travel at mild Martian gravity, in a capsule of 10 tons, we'd need to deliver 37 thousand tons of propellant to the orbit.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorDisplayName="user10840" LastEditDate="2016-07-15T15:58:57.410" LastActivityDate="2016-07-15T15:58:57.410" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17227" PostTypeId="1" CreationDate="2016-07-15T12:23:13.907" Score="1" ViewCount="135" Body="&lt;p&gt;How much would it cost for a business to place some self-maintaining electronic equipment aboard the ISS? Would it be a monthly fee, or a one-time transportation and installation fee? Would that business be responsible for supplying electricity for the equipment, or will the ISS be able to support the electric power needs for such equipment?&#xA;If this question cannot be answered here, who can I contact that can answer these questions for me? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sorry that I cannot disclose the type of electronic equipment that I'm referring to, this is a business venture that I have to keep under the radar of other businesses, until the equipment is in place, or other businesses will be trying to do this first.&lt;/p&gt;&#xA;" OwnerUserId="16239" LastEditorUserId="25" LastEditDate="2016-07-15T14:57:26.267" LastActivityDate="2016-07-16T23:16:02.843" Title="ISS business venture cost?" Tags="|iss|cost|commercial|equipment|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17229" PostTypeId="2" ParentId="12725" CreationDate="2016-07-15T12:48:15.180" Score="0" Body="&lt;p&gt;it could screw around with all the electronics, and would require the ship to have an outer layer that is ferromagnetic,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but beyond that, it would need a constant source of energy (assuming they're electromagnets) which could be a big issue if that energy runs out&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but if you can get around that then I would think they would be rather useful&lt;/p&gt;&#xA;" OwnerUserId="16241" LastActivityDate="2016-07-15T12:48:15.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17235" PostTypeId="2" ParentId="17220" CreationDate="2016-07-15T13:54:50.667" Score="5" Body="&lt;p&gt;Acceleration costs the energy. Let's ignore everything else - suppose we are in free space, there is no gravity, no drag, no other motions. then energy $E$ needed to gain velocity $v$ is $E=1/2 mv^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to maintain given acceleration $g$ you will ned a power source that can provide output of $P(t)=mgv(t)$ (I've just derived the energy by time) all the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we use rotation to get feeling of gravity we just need to rotate the ring. The &quot;gravity&quot; will be $g=\omega^2 r$ and the energy needed to accomplish that is $E=1/2J\omega^2=\frac{J\cdot g}{2r}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No matter how the engines would work, how effective they would be; the gravity by rotation will be more effective that gravity by propulsion.&lt;/p&gt;&#xA;" OwnerUserId="16245" LastActivityDate="2016-07-15T13:54:50.667" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17236" PostTypeId="2" ParentId="15052" CreationDate="2016-07-15T14:18:54.187" Score="1" Body="&lt;p&gt;I tried this &lt;a href=&quot;http://www.engineersedge.com/calculators/shell_internal_pres_pop.htm&quot; rel=&quot;nofollow&quot;&gt;wall thickness calculator&lt;/a&gt;. For a 500 bar pressure vessel with an internal radius of 2&quot; and perfect joints it gives a wall thickness of 4&quot;.&lt;br&gt;&#xA;The next step would be to calculate thrust and duration from that volume, then a thrust/weight ratio. My prediction is that you won't get a T/W of over 1.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-15T14:18:54.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17237" PostTypeId="2" ParentId="17227" CreationDate="2016-07-15T14:56:57.937" Score="3" Body="&lt;p&gt;NASA tends to do such things as joint partnerships. An excellent example was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Bigelow_Expandable_Activity_Module&quot; rel=&quot;nofollow&quot;&gt;Bigelow BEAM module&lt;/a&gt;. NASA actually paid Bigelow to test their hardware on orbit. However, while Bigelow holds many patents on such technology, they are required to provide data to NASA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, NASA has many such partnerships set up with companies such as SpaceX and Blue Origins, where the commercial partner pays for the cost of things, NASA provides expertise, and the commercial partner provides data in return. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In your case, you probably want to actually launch a dedicated satellite mission to test out the hardware. There are virtually no items that can't be tested on the ground or on a stand alone satellite first. Depending on the size of your payload, it could cost $60 million to launch, or more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you really want to have a commercial venture with a space station, you're best bet is to wait for the &lt;a href=&quot;https://bigelowaerospace.com/b330/&quot; rel=&quot;nofollow&quot;&gt;Bigelow 330&lt;/a&gt;, which is set to launch in 4-5 years. They don't have the rates for what you are asking for available online, but I'm sure if you talk with them you can get a quote for what it would take. Of course, NDAs can be signed both ways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, NASA is &lt;a href=&quot;http://spacenews.com/nasa-seeking-ideas-for-use-of-space-station-docking-port/&quot; rel=&quot;nofollow&quot;&gt;looking to put something&lt;/a&gt; where the Bigelow BEAM module is. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-16T23:16:02.843" LastActivityDate="2016-07-16T23:16:02.843" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17238" PostTypeId="5" CreationDate="2016-07-15T16:34:26.777" Score="0" Body="&lt;p&gt;This is not to be confused with using radio waves for astronomical research.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-07-15T16:45:52.877" LastActivityDate="2016-07-15T16:45:52.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17239" PostTypeId="4" CreationDate="2016-07-15T16:34:26.777" Score="0" Body="Using radio wavelengths to communicate between multiple spacecraft or between a spacecraft and the Earth." OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-07-15T16:45:48.597" LastActivityDate="2016-07-15T16:45:48.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17240" PostTypeId="5" CreationDate="2016-07-15T16:36:24.677" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-07-15T16:36:24.677" LastActivityDate="2016-07-15T16:36:24.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17241" PostTypeId="4" CreationDate="2016-07-15T16:36:24.677" Score="0" Body="Questions which relate to how spacecraft and rockets navigate and stay on course." OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-07-15T16:45:57.237" LastActivityDate="2016-07-15T16:45:57.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17245" PostTypeId="2" ParentId="17196" CreationDate="2016-07-15T19:10:49.373" Score="3" Body="&lt;p&gt;Let's consider some space things that have been around for some time: Mir, the ISS, Hubble, and the Moon.&lt;/p&gt;&#xA;&lt;h1&gt;Micrometerites leave everything pock-marked&lt;/h1&gt;&#xA;&lt;p&gt;There is more debris in orbit around planets, or on other unswept bits of space, where you can expect meteorites, micrometeorites and so on. In space there can be some very large speed differences, making it a bit like an extremely high energy but sparse dust storm, combined with a hail of small sharp bullets.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://pics-about-space.com/dmage-mir-space-station?p=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mir got holes in its solar panels&lt;/a&gt; and other flat surfaces, either as dents or just as holes over the course of a few decades.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://blogs.voanews.com/science-world/2012/03/30/addressing-the-growing-problem-of-space-junk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle window damage was caused by a paint fleck&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;h1&gt;The Moon has had a range of visitors&lt;/h1&gt;&#xA;&lt;p&gt;The lunar surface has lots of circular impact craters across its surface. There are a few factors that have caused that; notably the incredible amount of time it has been there, and also the combination of the Earth and Moon's gravity wells pulling passing objects in.&lt;/p&gt;&#xA;&lt;p&gt;However, like most large, old bodies hanging around the solar system, it is covered in a layer of regolith, a kind of aggregate dust left by many many impacts throwing up a bit of matter which then settles. So over time that has acted to smooth out and cover up the smaller craters like so much grey concealer. So we could expect things that sit near planets or stars to eventually accumulate craters that look like the Moon, just at a smaller scale.&lt;/p&gt;&#xA;&lt;h1&gt;Europa's surface is icy, but young&lt;/h1&gt;&#xA;&lt;p&gt;Compared to other surfaces in the solar system, Europa's is very fresh. Being made of ice but in orbit around a vast planet causing tidal forces that squeeze and pull the moon like a cold stress ball leaves you with an ever-refreshed surface.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/jpl/europas-stunning-surface/&quot; rel=&quot;nofollow noreferrer&quot;&gt;That surface is streaked with lines and holes&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;It is quite conceivable that a worthwhile part of a large space station would be either a dwelling built out of something solid like ice, or even that it would have a large ice ball stuck to it as a long-term store of water. It could even be a repurposed asteroid that is already in a regular or eccentric orbit. So we might expected a station made of something other than metal to look a bit more like that.&lt;/p&gt;&#xA;&lt;h1&gt;So yes, there is an abrasive, impactful decay&lt;/h1&gt;&#xA;&lt;p&gt;of anything that is static.&lt;/p&gt;&#xA;&lt;h1&gt;Unless it heals itself&lt;/h1&gt;&#xA;&lt;p&gt;Spaceships of the future really should have some automated systems going on; self-healing ones to patch up the bodywork, fix glass, fetch water from nearby sources and store it in a giant ice ball.&lt;/p&gt;&#xA;&lt;p&gt;I can imagine some of those systems having unintended effects when left to their own devices for a long time; ships that are more patchwork than original paintwork, ice balls that have grown enormously.&lt;/p&gt;&#xA;" OwnerUserId="519" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-15T19:10:49.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17249" PostTypeId="2" ParentId="17217" CreationDate="2016-07-16T00:18:33.813" Score="4" Body="&lt;p&gt;This is interesting Q-A to read: &lt;a href=&quot;https://space.stackexchange.com/q/4169/3039&quot;&gt;How is the unwanted cold welding prevented in space?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interesting to read paper they refer:&lt;br&gt;&#xA;&lt;a href=&quot;http://esmat.esa.int/Publications/Published_papers/STM-279.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Assessment of Cold Welding between  Separable Contact Surfaces due to Impact and Fretting under Vacuum&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Cold welding test method&lt;/strong&gt;&lt;br&gt;&#xA;  The contact is closed for 10 s and then and opened for another&#xA;  10 s. At impact, the base pressure of the vacuum is less than 5 × 10&lt;sup&gt;–8&lt;/sup&gt; mbar, i.e.&#xA;  the surfaces are not recovered during opening. During a fretting test, a base&#xA;  pressure of 55 × 10&lt;sup&gt;–7&lt;/sup&gt; mbar is sufficient, since the change from oxidative to&#xA;  adhesive wear occurs in the range 0.1–10&lt;sup&gt;–3&lt;/sup&gt; mbar.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In appendix they recommend pressure for testing:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Ug5ui.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ug5ui.png&quot; alt=&quot;Assessment of Cold Welding between  Separable Contact Surfaces due to Impact and Fretting under Vacuum, page 40&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it have to be understood, that process depends from set of factors, including type of interaction between parts, and not induced by vacuum, but induced by lack of gas which will form or restore adhesive and chemical layers over surface of metal fast enough, to prevent cold welding to happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://www.youtube.com/watch?v=Yx6bk0qgl3s&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; as example is also cold welding, there protective layers are destroyed faster then they can form with that metal-metal sealing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So answering the question, starting from medium vacuum(1 – 10&lt;sup&gt;−3&lt;/sup&gt; mbar) we have to consider and expect it to happen, but it highly depend on parts(their material) and their working conditions.&lt;/p&gt;&#xA;" OwnerUserId="3039" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-07-16T02:28:25.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17250" PostTypeId="2" ParentId="12725" CreationDate="2016-07-16T03:32:30.777" Score="8" Body="&lt;p&gt;This turns out to be a pre-space-age thing.  It's not really needed, most of the time it's perfectly fine to float around.  On Skylab they provided an elaborate system of triangular grid floors and special shoes with triangular cleats to lock into them.  The crew hated them and hardly ever used them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OrESb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OrESb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v6iup.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v6iup.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When body positioning is important (like for flying the robot arm), various types of foot restraints can be used, mostly simple straps.  If you are riding the robot arm, there were also foot restraints.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NmDKz.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NmDKz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EVA folks also use tethers to secure themselves to work points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But for just moving around, in or out of a space vehicle? Use your hands and/or push off with your feet (not recommended on EVA).  Walking is highly inefficient in free fall.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-02-04T15:34:05.213" LastActivityDate="2019-02-04T15:34:05.213" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="17251" PostTypeId="1" CreationDate="2016-07-16T06:50:31.870" Score="10" ViewCount="590" Body="&lt;p&gt;With the huge mass of Jupiter and its fast, approximately 10 hours daily rotation, there may be substantial twisting of space-time around it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A distortion of such magnitude that may call for considering the effects of time dilation or twisting and smearing of the Time-space especially because of Juno's elongated orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this still below the tolerance level of the mission or they have had to deal with it?   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today I found this article on Lense-Thirring effect on this subject which is really interesting! &lt;a href=&quot;https://en.wikipedia.org/wiki/Lense%E2%80%93Thirring_precession&quot; rel=&quot;nofollow noreferrer&quot;&gt;space-time frame dragging&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is a bit involved mathematically but basically it says objects in orbit very near a large mass would be under a pseudo torsion opposite the orbit's direction and also the mass of orbiting object will increase if it rotates opposite the induced torsion and decreases if it rotates along the torsion. One way of describing the frame dragging is looking at it like a planetary gear box: the sun gear at the center rotating clockwise induces a torsion anti clock to planet gear and clockwise the ring gear&lt;img src=&quot;https://i.stack.imgur.com/gsGYI.jpg&quot; alt=&quot;![enter image description here&quot;&gt;]&lt;a href=&quot;https://i.stack.imgur.com/gsGYI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16061" LastEditorUserId="16061" LastEditDate="2016-07-19T18:39:13.837" LastActivityDate="2016-07-19T18:39:13.837" Title="Does NASA use Newtonian model of gravity for plotting the course of Juno or general relativity?" Tags="|gravity|interplanetary|jupiter|navigation|juno|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17252" PostTypeId="2" ParentId="17251" CreationDate="2016-07-16T10:57:41.170" Score="9" Body="&lt;p&gt;Not only does it use Einstein's Theory of Relativity, it's actually going to use &lt;a href=&quot;http://spaceflight101.com/juno/juno-one-week-out/&quot;&gt;Jupiter's huge mass&lt;/a&gt; to do a test that hasn't been done yet, on how it affects rotating objects. As it's expected to affect the gyroscopes, that is most certainly something that will have to be taken into account.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-16T10:57:41.170" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17253" PostTypeId="2" ParentId="17128" CreationDate="2016-07-16T11:49:44.923" Score="2" Body="&lt;p&gt;We can use the equation &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F = q V_e + (P_e-P_a) A_e$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $P_e$ is the exit plane pressure, $P_a$ is ambient pressure, and $A_e$ is the exit plane area.  $qV_e$ without the correction term gives the thrust when the exit plane pressure matches ambient.  Here $q$ is mass flow and $V_e$ is exit velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a nozzle optimized for sea level, $P_e$ = $P_a$ so there is no exit plane pressure effect thrust loss at liftoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the vehicle reaches vacuum, the term in parentheses would be equal to $P_e$.  Per the equation you can see that this gives additional thrust equal to the product of $P_e$ $A_e$.  However, this exhaust would be severely underexpanded and would experience losses due to a pattern of expansion waves at the nozzle exit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A picture from &lt;a href=&quot;http://www.cems.uvm.edu/~jmmeyers/ME239/Slides/04%20-%20Over%20and%20Under%20Expansions%20and%20Nozzle%20Configurations%20v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vxgE6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vxgE6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Underexpansion is inevitable since an infinitely long De Laval nozzle would be required to get the exit plane pressure to zero.  (This means that your &quot;fixed nozzle designed for optimal in-vacuum efficiency&quot; is always a compromise - it gets cut off due to weight, packaging considerations, etc.) Underexpansion is also more desirable than overexpansion which can result in flow separation and severe shock losses, hence the use of dual-bell nozzles and other schemes to try to match the exit plane and ambient pressures better.  If these fancy schemes are not practical, some compromise design exit plane pressure is used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The linked presentation does a great job of explaining all this with real world examples and mitigation strategies. &lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-07-16T14:34:56.073" LastActivityDate="2016-07-16T14:34:56.073" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17256" PostTypeId="1" CreationDate="2016-07-16T18:59:29.547" Score="17" ViewCount="2488" Body="&lt;p&gt;Falcon Heavy has a standard Falcon 9 Full Thrust (FT) rocket core with two strap-on boosters derived from the Falcon 9 FT first stage. What about a heavier Falcon Heavy with four strap-on boosters (or maybe three), what kind of challenges will face SpaceX? I am wondering for some possible challenges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With 45 Merlin 1D engines ignited at liftoff it will have a higher probability that one engine failure could cause destruction of entire rocket. But would it have problems in vibrations such as Pogo oscillation even in this case where engines are grouped in five boosters, 9 per each and not being in a single core booster connected with each other all of them (such as N1 rocket with even less engines 30 but still not successful)? The N1 had more powerful engines (NK-15) and this maybe caused higher vibrations. Probably a reason that Falcon Heavy hasn't fly yet and has many delays, could be vibrations caused in launch simulation tests. The other thing, what about the center core? Would it face to much pressure from the side boosters and by the weight of a larger second stage with more engines, more fuel and a new larger interstage? Something else mabye the aerodynamic pressure, could it be an issue? Other challenges could be in other things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of challenges would face SpaceX for such a rocket?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-08-27T20:57:52.290" LastActivityDate="2020-06-07T20:31:23.503" Title="What would the challenges be in developing a Falcon Heavy with three or four strap-on boosters?" Tags="|spacex|design|falcon-heavy|" AnswerCount="2" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17260" PostTypeId="1" CreationDate="2016-07-17T00:36:16.940" Score="1" ViewCount="318" Body="&lt;p&gt;China future plan for spacestation after the lost testbed are more and more turning into reality by time. Now with Long March 5/7 success when will China launch its spacestation and how will it differ from Iss?&lt;/p&gt;&#xA;" OwnerDisplayName="user16238" LastEditorUserId="12102" LastEditorDisplayName="user16238" LastEditDate="2021-08-28T23:19:32.160" LastActivityDate="2021-08-28T23:19:32.160" Title="China. Its very own modular spacestation" Tags="|iss|space-station|cnsa|tiangong-2|tiangong-space-station|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17261" PostTypeId="1" AcceptedAnswerId="17266" CreationDate="2016-07-17T01:13:37.097" Score="6" ViewCount="853" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/17256/12102&quot;&gt;This engineering question&lt;/a&gt; got me wondering. If cross feed technology were sufficiently developed, and stresses could be managed (these are big if's, and there are more I'm sure), how would these various configurations roughly compare?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there numbers out there, or could someone ballpark them, or tell me a simple way to ballpark the missing values?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first two are copied from Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_Heavy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon Heavy&lt;/a&gt;. They are tons to LEO. They also agree with the numbers in this &lt;a href=&quot;https://m.reddit.com/r/spacex/comments/4ibtcb/estimated_payload_of_falcon_heavy_with_crossfeed/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reddit&lt;/a&gt; which is probably not independent, but that table also gives the third number as well.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;F9 FT:      22.8&#xA;FH:         54.4&#xA;FH w/cf:    64.5 &#xA;FQH:         ?&#xA;FQH w/cf:    ?&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;In my very unofficial acronyms, FQH = 'Falcon Quad Heavy' - four F9-like boosters, and w/cf = 'with cross feed'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cross feed for FQH might have boosters 1 and 3 feeding into 2 and 4, and dropping off first, while boosters 2 and 4 are also simultaneously feeding the core. It's a mess of engineering - I'm not proposing it as a real solution - it's more of a best possible (or &lt;em&gt;best imaginable&lt;/em&gt;) upper limit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I'm starting to rethink this question; The potential benefit of the &quot;FQH&quot; might be better realized by optimizing throttling and even staggering engine starts. In other words, maybe this is more difficult to even &quot;ballpark&quot; than I first realized.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2018-05-27T05:42:42.390" Title="Ballpark comparison of a hypothetical Falcon 'Quad' Heavy with cross feeds" Tags="|spacex|design|falcon-heavy|cross-feed|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17266" PostTypeId="2" ParentId="17261" CreationDate="2016-07-17T06:56:43.023" Score="7" Body="&lt;p&gt;Do you know KSP? It simulates unrealistic perfect crossfeed capability which allows for &lt;em&gt;&lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Asparagus_staging&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asparagus staging&lt;/a&gt;&lt;/em&gt; - a very effective technique in the simulation (look at some videos, I remember one showing a rocked with stage separation ocurring each 3 seconds or such all the way to orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with it is that fueling of 1 F9 booster takes roughly 30 minutes and using that fuel through engines only a bit more than 150s (based on MECO times of GTO launches where the remaining fuel was minimal). For the crossfeed to work optimaly in quad or more side boosters configuration, you need to &lt;em&gt;&quot;refuel&quot;&lt;/em&gt; the &quot;inner&quot; boosters with a flow roughly 4 times bigger than during fueling (for quad, n-fold more for each additional &quot;layer&quot; of boosters). You soon find out that the pipes get quite wide and the pumps very heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to the question - I don't have exact numbers, but my understanding is that without &lt;em&gt;crossfeeed&lt;/em&gt; you are looking at a problem with quickly diminishing returns. You either have too much thrust and lose to drag (or lose the vehicle), or you throttle down (or turn off) the &quot;inner&quot; engines and lose to gravity:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;if you just lower the thrust of all or some engines, you will get almost drained fuel tanks after separation.&lt;/li&gt;&#xA;&lt;li&gt;if you disable some engines and start them later to preserve fuel in &quot;inner&quot; boosters, you turned the thing into normal 3-(or-more)-stages rocket, just with small stages on the side instead of bigger one at the bottom.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;With super-crossfeed you can model the entire situation a bit simpler:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;FH w/cf turns to a fully fuelled F9 after side booster separation but with all the height and velocity already imparted by the boosters. You are effectively air launching F9 (but with the advantage of not needing to change velocity from horizontal to vertical). To get the numbers you need to compute the delta-v of two cores of propelant with thrust of 3 cores (but it will have to throttle down for Max-Q etc.) and payload of full F9. You can then add the delta-v of &quot;normal&quot; F9 to get the ability of entire stack.&lt;/li&gt;&#xA;&lt;li&gt;FQH w/cf - analogically, you are air-launching FH, you just now have fuel of 2 boosters to feed 5 sets of engines (= 45 Merlins) so the staging will happen sooner.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;My &quot;launch-math&quot; is not good enough to provide any reasonable estimates for actual numbers unfortunatelly. I just know from my KSP tests that the difference between no crossfeed and full crossfeed is really substantial - where without crossfeed the rocket did not even get out off the atmosphere, the crossfeed variant was able to get straight up to escape velocity.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2018-05-27T05:42:42.390" LastActivityDate="2018-05-27T05:42:42.390" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="17268" PostTypeId="1" AcceptedAnswerId="17270" CreationDate="2016-07-17T11:19:24.873" Score="7" ViewCount="780" Body="&lt;p&gt;Reading &lt;em&gt;&quot;&lt;a href=&quot;http://spacenews.com/britain-selects-u-s-french-british-teams-to-study-spaceport-feasibility/&quot; rel=&quot;noreferrer&quot;&gt;Britain selects U.S., French, British teams to study spaceport feasibility&lt;/a&gt;&quot;&lt;/em&gt; &amp;mdash; I can imagine places in Europe for retrograde or polar launches, but are there places where standard LEO/ISS/GTO launch is possible? For example at Florida KSC there is only a limited range of inclinations to launch to when taking safety considerations into account (no going west, no overfly of the Bahamas etc.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the map I am not sure if any prograde orbital launch is possible from Europe, but maybe I overestimate the needs for a safe zone? Are Norway (high inclination), Denmark, Netherlands or Germany far enough away for launches from the east coast of the UK? Spain/Italy and Africa? What about a possibility to launching over the Black Sea? Or are there any inland zones where launching an orbital rocket would be safe enough?&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="6816" LastEditDate="2017-03-25T06:41:09.757" LastActivityDate="2022-01-31T00:31:35.697" Title="Possible places for a launch complex in UK and continental Europe" Tags="|launch-site|esa|range-safety|uk-space|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17269" PostTypeId="2" ParentId="17268" CreationDate="2016-07-17T11:44:57.510" Score="5" Body="&lt;p&gt;Really a launch so far north you don't want to launch straight east. In fact, if you read the list of rockets they include in the linked article, all of them are sub-orbital except for the proposed Reaction Engine SSTO, which range safety concerns are somewhat less. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that a launch site would be in the northern portion of the UK, with a polar launch inclination being the desired location. I also suspect that they might try for something in the range of the ISS, although that might be more difficult due to range safety. But for sub-orbital, which is the key proposal, the only safety concern is to have a reasonably large area around it free of people, which again, the northern portion of the UK seems to be best.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other reasons why a straight eastward trajectory is a bad thing from so high as Europe. The minimum inclination that you can achieve from a launch point is related to the latitude. The UK being so far north limits things to mostly polar or at the very least a higher inclination orbit. A site to launch at the ISS would be possible, but not much below that inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For continental Europe to orbital ranges, I would look around Germany/Denmark, launching in to the Baltic Sea. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-17T12:37:52.463" LastActivityDate="2016-07-17T12:37:52.463" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17270" PostTypeId="2" ParentId="17268" CreationDate="2016-07-17T12:26:28.677" Score="10" Body="&lt;p&gt;The Baltic Sea is far too crowded to make it the range area. At any point of time you can see at least several ships within view distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the same reason, while it might seem like the isle of Ibiza would provide 2700km of downrange over the sea, the Mediterrean Sea traffic is so high, it's unlikely to be allowed, plus the range of inclinations is pretty narrow. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/izDlb.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/izDlb.png&quot; alt=&quot;Map with the path: Ibiza - south of Corsica - south of Siccily - path ending near Cairo&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming ESA stays on good terms with Russia - which isn't too far-fetched, the terms currently aren't bad - Lapland (north-easern Finland), and northern Norway. The population &quot;downrange&quot; is scarce. The serious disadvantage is massive loss of delta-V due to polar latitudes, harsh climate and limited inclinations available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another alternative with 1000km of safe downrange is near Romanian-Bulgarian border. I believe in case Russia was opposed to it, deals with Azerbaijan and Georgia (whose relations with Russia are outright hostile) could be arranged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H6Cxo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H6Cxo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in the end, why? ESA's spaceport in French Guiana is at a better location than any other!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yYnh7.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yYnh7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2018-06-15T09:22:06.487" LastActivityDate="2018-06-15T09:22:06.487" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="17272" PostTypeId="1" CreationDate="2016-07-17T15:09:48.753" Score="7" ViewCount="1098" Body="&lt;p&gt;I read somewhere that fuel was delivered to the Apollo launch site by tanker truck (for cryogenic propellants) and rail cars (for the kerosene). Were the fuels transferred directly from delivery vehicles to the spacecraft, or was there on-site storage? More importantly, for either Apollo or Shuttle, if the spacecraft had to be de-fueled, where did all that propellant go? Was it transferred to tanker vehicles or piped into storage tanks for later re-use? If there was on-site storage, how close was it to the pad, and what was the capacity in terms of number of launches which could be accommodated? &lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-07-17T16:15:10.987" Title="Apollo/Shuttle launch site fuel handling" Tags="|launch|apollo-program|space-shuttle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17273" PostTypeId="2" ParentId="17272" CreationDate="2016-07-17T15:44:12.293" Score="16" Body="&lt;p&gt;For shuttle, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Liquid oxygen used as an oxidizer by the orbiter main engines is&#xA;  stored in a 900,000-gallon tank on the pad's northwest corner, while&#xA;  the liquid hydrogen used as a fuel is kept in an 850,000-gallon tank&#xA;  on the northeast corner.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can see the storage spheres in this picture.  The tall tank nearer to the pad is water for the deluge system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3OXNM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3OXNM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/launch/LOX-LH2-storage.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as detanking, the propellants went back to the storage spheres, albeit with some inevitable losses.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We drain back from the external tank into the large spheres that you&#xA;  see out at the launch pad. So, we do recover the fuel. Some of it is&#xA;  lost in boiling off during the attempt, but never-the-less we do reuse&#xA;  about 90% of the fuel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts115/launch/qa-leinbach.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as limiting the number of launches, I do not believe that availability of the propellants was ever the limiting factor. Rules about crew and launch team fatigue would kick in before that ever happened. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The space agency usually limits the number of consecutive launch&#xA;  attempts to two in order to give the astronauts and launch team a&#xA;  rest.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Note this is from an &lt;a href=&quot;https://news.google.com/newspapers?nid=1298&amp;amp;dat=20000426&amp;amp;id=eGQzAAAAIBAJ&amp;amp;sjid=cwgGAAAAIBAJ&amp;amp;pg=6609,6730039&amp;amp;hl=en&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; about STS-101 where they planned to attempt an unusual 3rd-try-in-a-row)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-07-17T16:15:10.987" LastActivityDate="2016-07-17T16:15:10.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17276" PostTypeId="1" AcceptedAnswerId="17284" CreationDate="2016-07-18T04:23:06.027" Score="2" ViewCount="1046" Body="&lt;p&gt;The timeline in the &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/spacex_crs9_press_kit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;press kit&lt;/a&gt; for the upcoming CRS-9 SpaceX launch says at about 11 minutes after launch, &quot;Dragon's solar arrays deploy&quot;, and at 2 hours 19 minutes, &quot;Dragon's Guidance, Navigation and Control bay door opens&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can imagine that cameras for guidance would be protected behind a door, but it sounds like there is a lot more there! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What all is there in the Guidance, Navigation and Control bay &lt;em&gt;that would need to be exposed to space&lt;/em&gt; after the door opens?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I found some pics on line - is this the door in question? The gold structure on the inside surface of the door is different in the two pics - but they both look vaguely antenna-like. What are those?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J9eOT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J9eOT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Above:&lt;/strong&gt; from &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/15332256421_a9105be08b_o.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt; - click for full size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/E9xAf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/E9xAf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Above:&lt;/strong&gt; from &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/pdf/DragonLabFactSheet.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX DragonLab Fact Sheet&lt;/a&gt; - click for full size, or just go see the PDF.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-07-18T05:21:24.843" LastActivityDate="2016-07-18T15:22:09.697" Title="What is behind Dragon's Guidance, Navigation and Control bay door?" Tags="|spacex|falcon-9|dragon|launch-sequence|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17277" PostTypeId="2" ParentId="17276" CreationDate="2016-07-18T04:32:50.990" Score="3" Body="&lt;p&gt;Star sensor, DragonEye, and the CanadArm grapple fixture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the first picture in the question, the plate with a triangle/pyramid like structure if the grapple fixture for the CanadArm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second image is just art and I do not think entirely accurate.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-07-18T13:04:43.013" LastActivityDate="2016-07-18T13:04:43.013" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17278" PostTypeId="1" AcceptedAnswerId="17283" CreationDate="2016-07-18T05:10:01.417" Score="1" ViewCount="123" Body="&lt;p&gt;I was taking long-exposure shots of tonight's SpaceX launch and noticed these swirling like maneuvers when I zoomed-in:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are both stages making a roll or something?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Separation&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V4DXs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V4DXs.jpg&quot; alt=&quot;Separation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Stage-1&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/t6qMH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t6qMH.jpg&quot; alt=&quot;stage1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Stage-2&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dE3dp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dE3dp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="688" LastEditorUserId="25" LastEditDate="2016-07-18T12:08:54.787" LastActivityDate="2016-07-18T12:08:54.787" Title="What kind of maneuver Falcon9 &amp; Dragon follows after separation" Tags="|launch|spacex|rockets|falcon-9|dragon|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17279" PostTypeId="1" AcceptedAnswerId="17280" CreationDate="2016-07-18T05:47:47.127" Score="9" ViewCount="1358" Body="&lt;p&gt;A simplified version of the landing choices might be - if there's going to be sufficient propellant, the plan is for the F9 first stage to u-turn and go all the way back to the launch area to land. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, a drone ship is conveniently placed downrange so much less propellant is needed to land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In either situation, are all the tanks topped-off to absolutely 100% of capacity &quot;just in case&quot;, or are they filled only with the anticipated amount needed plus a safety margin?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I'm asking &lt;em&gt;what is actually done&lt;/em&gt; is that I seem to recall that &quot;leftover&quot; propellant might actually be helpful, since the thrust of the (three?) engines on landing is way more than the weight of an empty 1st stage, even when throttled.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="25" LastEditDate="2016-07-18T10:30:03.227" LastActivityDate="2016-08-06T16:33:22.567" Title="Recent Falcon 9 flights - 'topped off' with propellants, or just a margin over what's needed to land?" Tags="|launch|spacex|rockets|falcon-9|landing|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17280" PostTypeId="2" ParentId="17279" CreationDate="2016-07-18T06:13:53.313" Score="6" Body="&lt;p&gt;All tanks of a launch vehicle are always topped to the 100%. It simplifies the things a lot because the rocket weights the same and behaves the same. With tanks &lt;em&gt;&quot;half-empty&quot;&lt;/em&gt; for example the TWR on launch would be different. The weight difference between payloads is something like ~1% of the liftoff mass iirc so that does not change the things much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;just in case&lt;/em&gt; reserve is important too. Recent case of &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_CRS_OA-6#Launch&quot; rel=&quot;nofollow&quot;&gt;Cygnus OA-6&lt;/a&gt; would have been a failure if the &lt;em&gt;Atlas V&lt;/em&gt; and &lt;em&gt;Centaur&lt;/em&gt; were not topped to the max. The same for &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-1#Falcon_9_engine_anomaly&quot; rel=&quot;nofollow&quot;&gt;SpaceX CRS-1&lt;/a&gt;. The &lt;em&gt;engine-out&lt;/em&gt; capability is exactly &lt;em&gt;&quot;if anything bad happens with some engine(s), use the fuel reserves and burn longer, possibly forbidding a landing attempt&quot;&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2016-07-18T10:36:02.963" LastActivityDate="2016-07-18T10:36:02.963" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17281" PostTypeId="2" ParentId="17279" CreationDate="2016-07-18T10:29:50.417" Score="7" Body="&lt;p&gt;There's really no point to not filling the tanks all the way, and every reason to do so. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reasons to fill all the way:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Simplifies procedures&lt;/li&gt;&#xA;&lt;li&gt;Initial liftoff the same for all launches&lt;/li&gt;&#xA;&lt;li&gt;Keeps the maximum acceleration more constant&lt;/li&gt;&#xA;&lt;li&gt;Empty tanks can cause sloshing, which can be bad.&lt;/li&gt;&#xA;&lt;li&gt;Fuel reserve in case of an emergency.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Reasons not to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Saves a very small amount of money (Less than 1% of the launch cost is in fuel)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-18T10:29:50.417" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="17282" PostTypeId="1" CreationDate="2016-07-18T11:50:54.537" Score="6" ViewCount="465" Body="&lt;p&gt;How does the trajectory vary between the two types of missions? Specifically, how does the speed and velocity vary for the booster for a Falcon 9 mission?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6537" LastEditDate="2017-12-12T05:57:13.137" LastActivityDate="2018-04-02T14:22:51.630" Title="How does the flight trajectory of a GEO vs LEO mission vary for a Falcon 9 first stage?" Tags="|launch|spacex|rockets|falcon-9|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17283" PostTypeId="2" ParentId="17278" CreationDate="2016-07-18T12:08:39.660" Score="5" Body="&lt;p&gt;Here's another photo, taken from &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/4tdi30/heres_my_long_exposure_photograph_of_the_falcon_9/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reddit&lt;/a&gt;.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/fFBF1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fFBF1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's virtually certain that what you saw was camera shake. There's no way that the rocket would have such motion, it would seriously degrade the performance of the rocket. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-18T12:08:39.660" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17284" PostTypeId="2" ParentId="17276" CreationDate="2016-07-18T13:23:24.437" Score="3" Body="&lt;p&gt;From the &lt;a href=&quot;http://www.spacex.com/news/2013/05/16/dragon-guidance-navigation-control-gnc-bay&quot; rel=&quot;nofollow&quot;&gt;SpaceX site&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The sealed equipment bay on the forward side of Dragon (shown with the bay door open in the photo below) contains state-of-the-art sensors essential to Dragon Guidance, Navigation, and Control (GNC). Optical sensors, laser-based range sensors, and inertial sensors are used during Dragon free-flight and approach to the space station.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Redundant sensors are also connected to the fault-tolerant avionics suite, increasing the system’s safety and reliability. The sensors are concealed by the GNC bay door, which opens on ascent to allow sensors to take measurements and closes to protect them during reentry.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The gray circle is the Canadarm grapple fixture.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-07-18T15:22:09.697" LastActivityDate="2016-07-18T15:22:09.697" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17285" PostTypeId="1" CreationDate="2016-07-18T13:41:22.593" Score="5" ViewCount="383" Body="&lt;p&gt;Are any parts of the Soyuz capsule re-used or re-manufactured? From a previous post I know it is much too expensive to design the spacecraft for re-use or re-inspection, but can &lt;em&gt;any&lt;/em&gt;  components be remanufactured and reused?&lt;/p&gt;&#xA;" OwnerUserId="16304" LastActivityDate="2016-07-18T14:22:54.523" Title="Are any parts of the Soyuz capsule re-used or re-manufactured?" Tags="|crewed-spaceflight|soyuz-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17286" PostTypeId="1" AcceptedAnswerId="17314" CreationDate="2016-07-18T14:08:15.340" Score="3" ViewCount="880" Body="&lt;p&gt;In the discussion in the comments of &lt;a href=&quot;https://space.stackexchange.com/a/17204/58&quot;&gt;this answer&lt;/a&gt;, I questioned whether the (reported) full 40 hours of designated work time (per week) for a new International Space Station (ISS) crew member could be spent on science (aside from exercise). I asked whether exercise counted as work time, and Organic Marble suggested I ask this as a new question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know we've had a question on how ISS crew splits their time between different activities before:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/11935/how-is-time-split-on-different-activities-on-iss&quot;&gt;How is time split on different activities on ISS?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, that doesn't seem to address whether the allotted exercise time comes out of their 40 hour work week.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is exercise part of their work hours or not?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update from comments:&lt;/strong&gt; A thorough answer to this question needs to address not only whether the exercise is counted in the work week, but how many hours are in the work week and what portion of it is exercise.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-07-20T08:44:43.390" Title="Does ISS crew exercise time count toward their X hour work week?" Tags="|iss|astronaut-lifestyle|physical-activity|policy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17288" PostTypeId="2" ParentId="17285" CreationDate="2016-07-18T14:18:15.717" Score="5" Body="&lt;p&gt;Yes, some parts of descent capsule are reused - seats, computer, altimeter &#xA;(see &lt;a href=&quot;http://www.aex.ru/news/2014/11/20/126949/&quot; rel=&quot;noreferrer&quot;&gt;Article in russian&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="11046" LastEditorUserId="11046" LastEditDate="2016-07-18T14:22:54.523" LastActivityDate="2016-07-18T14:22:54.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17289" PostTypeId="1" AcceptedAnswerId="17290" CreationDate="2016-07-18T14:48:39.477" Score="10" ViewCount="698" Body="&lt;p&gt;I thought only launches to rendezvous (resupply, repair, space station expansion etc) and direct launches into interplanetary trajectory require precisely timed launch window. Then I &lt;a href=&quot;https://www.reddit.com/r/space/comments/4tdha3/spacex_falcon_9_first_stage_lands_on_land/d5gtxlj&quot; rel=&quot;noreferrer&quot;&gt;read&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Geosync launches tend to have a fairly small fixed time window due to the need to get the satellite into sunlight before too much time passes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is that so? The GEO (and GTO too) are so high that the satellite remains in sunlight a most of the time. Is really sunlight a factor that dictates GEO launches? And if so, what hours does the launch window for these fall into?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2016-07-18T15:27:09.187" LastActivityDate="2016-07-18T15:27:09.187" Title="Do geosynchronous satellite launches require a tight launch window?" Tags="|launch|power|geostationary|launch-window|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17290" PostTypeId="2" ParentId="17289" CreationDate="2016-07-18T15:26:53.670" Score="8" Body="&lt;p&gt;The maximum eclipse any GSO satellite will be in is 72 minutes, per &lt;a href=&quot;http://www.intelsat.com/tools-resources/library/satellite-101/eclipse-seasons/&quot; rel=&quot;noreferrer&quot;&gt;Intelsat&lt;/a&gt;. Thus, a launch can't happen that puts the satellite in an orbit with an eclipse period of longer than 72 minutes, or else the satellite might fail. This will be more sensitive for an all electric satellite, which needs to take into account not only the present time but also the future eclipse time. The highly eccentric orbit can lead to longer eclipses than might otherwise be had. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a satellite is launched, it is in a low power state. There is still battery drain, however, and it might take a while to get to a state where it is power positive. The time is longer for the low power state than the 72 minutes, but it still must be reached. A Geostationary launch will drift for a period of time in LEO to get to the optimal second thrust location. For example, &lt;a href=&quot;http://spaceflightnow.com/2016/05/25/timeline-for-falcon-9s-launch-of-thaicom-8/&quot; rel=&quot;noreferrer&quot;&gt;Thaicom was deployed almost 32 minutes after launch&lt;/a&gt;. SpaceX also requires that all customers stop charging their batteries 13 minutes before launch.  By the time the Solar Arrays are fully deployed, it could be almost an hour without power. If it isn't deployed straight from the rocket (There's a certain amount of rotational velocity that can be imparted), then it could take time to orient correctly. All of these factors add up, making launch one of the key requirements for batteries for a mission!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, the optimal eclipse time for these satellites will be close to Earth, where it will pass quicker. For all-electric, this is even better, as the thrusts tend to be required when at higher altitudes. I suspect this is a limiting factor in most GSO launches. The window won't be particularly tight, but I suspect it is on the order of hours. Depending on the exact alignment, there might be two orbits of a launch window where the requirements are met, IE, launch at 00:00-00:15 or 01:30-01:45 (Numbers purely made up), although that kind of launch window usually isn't published.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-18T15:26:53.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17291" PostTypeId="1" AcceptedAnswerId="17607" CreationDate="2016-07-18T16:24:19.357" Score="13" ViewCount="501" Body="&lt;p&gt;Juno has arrived at Jupiter, and will begin taking measurements to determine if the gas giant has a solid core.  But what I cannot seem to find is the implications such a discovery would have. &lt;a href=&quot;https://arxiv.org/abs/1006.5486&quot;&gt;This paper&lt;/a&gt; says that there are essentially two models - the Core Accretion model and the Disk Instability model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Core Accretion follows the idea that gas giants start off as rocky bodies which quickly accrete mass and gas, eventually leading to runaway growth and forming gas giants.  This model may take from one to a few Myr, and requires a fair amount of rocky material in the protoplanetary disk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Disk Instability model says that gas giants form in a similar manner to stars; collapsing in from clouds of gas.  In this instance, no central core is needed to begin accretion.  This happens at a much faster rate, and these gas giants can still produce cores via sedimentation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions are thus:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If Juno discovers a solid core on Jupiter, will that allow us to pick one of the two models, since both could evolve a solid core?&lt;/li&gt;&#xA;&lt;li&gt;If we discover that Jupiter does or does not have a solid core, what does that mean for the development of Earth and life?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I've looked around, but haven't been able to find a source that links the two very neatly.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="25" LastEditDate="2016-07-18T20:23:41.930" LastActivityDate="2016-08-01T19:53:13.253" Title="What are the implications if Juno finds that Jupiter has a solid core?" Tags="|planetary-science|jupiter|juno|solar-system|planet-formation|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17292" PostTypeId="1" AcceptedAnswerId="17294" CreationDate="2016-07-18T16:58:25.967" Score="6" ViewCount="1627" Body="&lt;p&gt;What was the ultimate fate of the Viking 2 Orbiter?  &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1975-083A&quot; rel=&quot;nofollow&quot;&gt;According to NASA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The orbiter developed a leak in its propulsion system that vented its attitude control gas. It was placed in a 302 x 33176 km orbit and turned off on 25 July 1978 after returning almost 16,000 images in 706 orbits around Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But this doesn't say what happened to the orbiter.  Did it crash into Mars's surface?  If so, do we know where it crashed?  Or is it still up there, orbiting?  And what does that orbital notation (302 x 33176 km) mean?  &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/masterCatalog.do?sc=1975-075A&quot; rel=&quot;nofollow&quot;&gt;NASA's catalog&lt;/a&gt; says of the Viking 1 Orbiter:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On 7 August 1980 Viking 1 Orbiter was running low on attitude control gas and its orbit was raised from 357 x 33943 km to 320 x 56000 km to prevent impact with Mars and possible contamination until the year 2019. Operations were terminated on 17 August 1980 after 1485 orbits.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So Viking 1 is still orbiting until 2019 (which is coming up, I wonder if they have any plans on that).  But Viking 2's orbit was lower, so could it have already crashed?  &lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_program#Viking_landers&quot; rel=&quot;nofollow&quot;&gt;The landers were sterilized&lt;/a&gt;, but the orbiters were not, so they could contaminate the surface, if there are any microbes alive on them (not that &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiNjZ_Nvf3NAhUY-GMKHcG3BxsQFggcMAA&amp;amp;url=http%3A%2F%2Fwww.space.com%2F10930-mars-landings-red-planet-exploration.html&amp;amp;usg=AFQjCNEY_c-7QwNcmh_XlyLkCjKEvJD_lw&amp;amp;sig2=bystaqU2WF3MYi_1xPjGRA&quot; rel=&quot;nofollow&quot;&gt;other spacecraft&lt;/a&gt; haven't crashed on Mars).&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="25" LastEditDate="2016-07-18T17:18:42.940" LastActivityDate="2016-07-19T17:20:17.500" Title="What happened to the Viking 2 Orbiter?" Tags="|mars|mars-orbiters|viking|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17293" PostTypeId="2" ParentId="17279" CreationDate="2016-07-18T17:04:53.230" Score="1" Body="&lt;p&gt;For landing, one downside to completely full tanks, on low performance missions is the possible damage a falling stage would cause. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We saw on the several failed landing attempts of the ASDS JRTI and OCISLY the damage that a landing stage (JRTI for CRS-5 (Jan 2015), CRS-6 (Apr 2015), and Jason-3 (Jan 2016) then on OCISLY with SES-9 (Mar 2016)) can do. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When Jason-3 landed, but a leg did not lock and it slowly toppled the explosion of the fuel and oxidizer as the tanks were punctured was impressive.  The more fuel remaining, the bigger the potential bang.  However since the goal is successful landing, not crashing that may be less of a concern.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-18T17:04:53.230" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17294" PostTypeId="2" ParentId="17292" CreationDate="2016-07-18T17:33:23.627" Score="6" Body="&lt;p&gt;It seems pretty likely that it is still in orbit. &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter#Launch_and_orbital_insertion&quot; rel=&quot;nofollow&quot;&gt;MRO&lt;/a&gt; is in an orbit of about 370-400 km, which is quite low, but still doesn't use significant fuel to keep it up. 250 km is considered too high for &lt;a href=&quot;http://mars.jpl.nasa.gov/mgs/sci/aerobrake/SFMech.html&quot; rel=&quot;nofollow&quot;&gt;aerobraking&lt;/a&gt;. Almost certainly Viking 2 is still orbiting Mars, probably in a similar orbit to what was last known. The only real question is if sunlight pushed it into an orbit where it started to dip in to Mars's atmosphere, at about 150 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has, however, been some debate recently as to if the &lt;a href=&quot;http://enu.kz/repository/2009/AIAA-2009-6002.pdf&quot; rel=&quot;nofollow&quot;&gt;Viking Orbiters are still around&lt;/a&gt;. Most of the models indicate that both orbiters are still orbiting to this date, however some of them indicate that they might have crashed in to Mars already.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, the likelihood of any kind of contamination is very small. An uncontrolled re-entry would expose the spacecraft to tremendous heat, the UV radiation will kill anything living on it prior to the time, and the high velocity of impact will also sterilize it. The future (Or present) crash sites of the Viking orbiters are likely to be more sterile than Curiosity, Spirit, Opportunity, and the other landers that have arrived on Mars since Viking.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-19T17:20:17.500" LastActivityDate="2016-07-19T17:20:17.500" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="17296" PostTypeId="1" AcceptedAnswerId="17298" CreationDate="2016-07-19T02:07:33.473" Score="5" ViewCount="369" Body="&lt;p&gt;I saw the following sentence in &lt;a href=&quot;http://thespacereview.com/article/3007/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Space Review&lt;/a&gt;, and became intrigued:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Japan’s Hagoromo lunar orbiter, deployed in 1990, reached orbit but ceased transmitting enroute.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If it ceasted transmitting en-route, how did they know it reached orbit? Usually we know where things are by monitoring the doppler profile of their broadcasts (or re-broadcasts), or they simply tell us where they think they are themselves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radar maybe? But I &lt;a href=&quot;https://space.stackexchange.com/a/17061/12102&quot;&gt;learned here&lt;/a&gt; that optical detection tends to win at larger distances, since sunlight gives you a kilowatt per square meter to start with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After googling and wikipedia-ing, I read in Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Hiten&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hiten&lt;/a&gt; article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On the first lunar swing-by, Hiten released a small orbiter, Hagoromo (はごろも, named after the feather mantle of Hiten), into lunar orbit. The transmitter on Hagoromo failed, but its orbit was visually confirmed from Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is there any documentation of how this &quot;visual confirmation&quot; was actually done? Was there someone who was actually &lt;em&gt;visually looking&lt;/em&gt; through a giant telescope and saw this &lt;strong&gt;36 cm diameter precocious nano satellite&lt;/strong&gt; orbiting the moon about &lt;em&gt;36 billion centimeters&lt;/em&gt; away and shouted &quot;Yep! I see it!&quot; in Japanese, or English, or whichever language they use when they spot a tiny satellite in orbit around the another celestial body?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rQgXp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rQgXp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/muses-a_subsat.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hi65x.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hi65x.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://www.isas.jaxa.jp/j/japan_s_history/picture/06.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jaxa hitorical photos&lt;/a&gt; (found by googling &quot;&lt;em&gt;はごろも JAXA&lt;/em&gt;&quot;). Hagoromo is the little one on the left, deployed for capture into lunar orbit from Hiten on the right as it passed near the moon in its highly elongated orbit around the earth - quite a cool robotic maneuver for a nano satellite in 1990 (and that's only the beginning of Hiten's story.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ujfSb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ujfSb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://usi.kir.jp/CIA/ISAS/USI_cia_DS.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://usi.kir.jp/CIA/ISAS/USI_cia_DS.html&lt;/a&gt; - models (presumably) of Hagoromo sitting on top of Hiten.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-07-19T16:14:30.563" Title="How could tiny Hagoromo have been seen visually from earth confirming its lunar orbit?" Tags="|artificial-satellite|doppler-ranging|hagoromo|hiten|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17297" PostTypeId="1" AcceptedAnswerId="17299" CreationDate="2016-07-19T02:57:33.853" Score="1" ViewCount="155" Body="&lt;p&gt;Hagoromo was a ~36cm nano satellite that was deployed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Hiten&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hiten&lt;/a&gt; while Hiten was in a highly elliptical orbit around the earth, in 1990.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From there, Hagoromo was able to &lt;a href=&quot;http://thespacereview.com/article/3007/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;enter&lt;/a&gt; into an &lt;a href=&quot;https://space.stackexchange.com/a/17061/12102&quot;&gt;orbit&lt;/a&gt; around the &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/muses-a_subsat.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;moon&lt;/a&gt;. Also see &lt;a href=&quot;https://space.stackexchange.com/q/17296/12102&quot;&gt;this question&lt;/a&gt; and any answers that may be there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How was this done exactly? Was it purely ballistic capture (which still requires a tiny amount of thrust I think?) or more of a propulsive maneuver to enter into a lunar orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of propulsion and attitude control (if any) did Hagoromo actually have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rQgXp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rQgXp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/muses-a_subsat.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hi65x.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hi65x.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://www.isas.jaxa.jp/j/japan_s_history/picture/06.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jaxa hitorical photos&lt;/a&gt; (found by googling &quot;&lt;em&gt;はごろも JAXA&lt;/em&gt;&quot;). Hagoromo is the little one on the left, deployed for capture into lunar orbit from Hiten on the right as it passed near the moon in its highly elongated orbit around the earth - quite a cool robotic maneuver for a nano satellite in 1990 (and that's only the beginning of Hiten's story.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ujfSb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ujfSb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://usi.kir.jp/CIA/ISAS/USI_cia_DS.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://usi.kir.jp/CIA/ISAS/USI_cia_DS.html&lt;/a&gt; - models (presumably) of Hagoromo sitting on top of Hiten.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-07-19T05:17:49.667" Title="Was Hagoromo's capture into lunar orbit ballistic capture or more propulsive?" Tags="|artificial-satellite|orbital-maneuver|hagoromo|hiten|ballistic-capture|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17298" PostTypeId="2" ParentId="17296" CreationDate="2016-07-19T05:15:45.750" Score="4" Body="&lt;p&gt;From the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1990-007B&quot; rel=&quot;nofollow&quot;&gt;NASA Space Science Data Coordinated Archive entry on Hagoromo&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ignition of the Hagoromo deceleration rocket was confirmed by ground observation at 20:04:03 UT.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From a few other sites, this was done at the Kiso Observatory in Japan using a 105 cm Schmidt camera. The satellite would have been ~22 magnitude and near the Moon, so hard to see -- except when it was firing its engine. Then it would have been considerably brighter than 22 magnitude.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-07-19T16:14:30.563" LastActivityDate="2016-07-19T16:14:30.563" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17299" PostTypeId="2" ParentId="17297" CreationDate="2016-07-19T05:17:49.667" Score="3" Body="&lt;p&gt;From the &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1990-007B&quot; rel=&quot;nofollow&quot;&gt;NASA Space Science Data Coordinated Archive entry on Hagoromo&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A solid propellant (KM-L) retrorocket with a mass of 4 kg was mounted inside the spacecraft for orbit insertion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-07-19T05:17:49.667" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17301" PostTypeId="1" AcceptedAnswerId="17307" CreationDate="2016-07-19T10:09:24.070" Score="23" ViewCount="47706" Body="&lt;p&gt;There's plenty of people who said it would have been impossible for the astronauts in the Apollo 11 to film or take pictures in the moon due to the high radiation there's in the moon because of the lack of an earth atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They compare it with some recordings of the Chernobyl accident which show some fast lights on the films. They say those are the radioactive particles hitting the film of the camera which doesn't seem to happen in the moon films.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Others say it would have been impossible to take any film outside the magnetic field of the earth as these would have been destroyed by the amount of particles in the outside. &lt;/p&gt;&#xA;" OwnerUserId="16315" LastEditorUserId="6642" LastEditDate="2016-07-21T06:18:21.250" LastActivityDate="2019-09-19T21:49:16.953" Title="How was it possible for the Apollo 11 to film and take pictures with such radiation?" Tags="|the-moon|apollo-program|radiation|camera|myth-busting|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17302" PostTypeId="2" ParentId="17301" CreationDate="2016-07-19T10:30:38.960" Score="21" Body="&lt;p&gt;Radiation can affect film - but bear in mind the radiation around Chernobyl was, truly, extremely high. The radiation in our region of space is not as extreme.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also bear in mind that the earlier &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Orbiter_program&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbiter probes&lt;/a&gt; used film cameras, the pictures were developed and scanned automatically (by machinery on board) and the results transmitted to Earth. The results were pretty good!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/88UUK.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/88UUK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nasa.gov/290262main_smallversion.png&quot; rel=&quot;noreferrer&quot;&gt;Very high resolution version at this link.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-07-20T15:11:50.990" LastActivityDate="2016-07-20T15:11:50.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17303" PostTypeId="2" ParentId="17301" CreationDate="2016-07-19T11:01:06.100" Score="9" Body="&lt;p&gt;NASA used special, temperature-resistant, radiation-proof film for their photos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That kind of film was not readily available in the Soviet Union - &quot;not readily available&quot; meaning the only rolls of the film they had were ones &lt;a href=&quot;https://www.damninteresting.com/faxes-from-the-far-side/&quot; rel=&quot;nofollow noreferrer&quot;&gt;recovered from American spy balloons&lt;/a&gt;. These were used to take photos of the far side of the Moon by the Luna 3 probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more primary account of the Soviets' use of American film is this translation of a St. Petersburg &lt;a href=&quot;http://www.svengrahn.pp.se/trackind/luna3/SpyBalloon.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;newspaper article&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="1235" LastEditDate="2019-09-19T21:49:16.953" LastActivityDate="2019-09-19T21:49:16.953" CommentCount="18" ContentLicense="CC BY-SA 4.0" />
  <row Id="17304" PostTypeId="1" AcceptedAnswerId="17347" CreationDate="2016-07-19T11:04:02.810" Score="4" ViewCount="462" Body="&lt;p&gt;&lt;a href=&quot;http://www.nspo.org.tw/2008e/projects/project5/intro.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Formosat-5&lt;/a&gt; (see also &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/formosat-5.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunters&lt;/a&gt;) is &lt;a href=&quot;https://spacexstats.com/missions/sherpa-flight-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;expected&lt;/a&gt; to be &lt;a href=&quot;https://space.stackexchange.com/a/16666/12102&quot;&gt;deployed&lt;/a&gt; along with &lt;a href=&quot;http://spacenews.com/darpa-satlets-to-fly-on-sherpas-debut-mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sherpa&lt;/a&gt; later this year. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I noticed that before 5 comes 3, and after 5 comes 7. In that part of the world the number 4 is like a black cat with 13 painted on one side and 666 on the other walking under a ladder at midnight. Depending on location, you can find floors enumerated as 4 missing from hotels and room numbers containing 4 missing in hospitals -  you just don't mess around with 4 if you can help it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But 6? Gunter's space page shows little more than a &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/formosat-6.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;placeholder&lt;/a&gt;, and other web sites (see screen shots below) have no mention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was it just cancelled, or was there trepidation also because of a quirk of language. I'm told that the number 6 can also sound like a word for &quot;falls down&quot; and so would be &lt;em&gt;particularly unlucky for a satellite&lt;/em&gt;, but I am not sure if this is a coincidence, an amusing story, or turns out to be at least partly the reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this answer, I need someone who actually knows the answer or can find a credible source, not just a quick copy/paste/forget. Thanks! &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/34J8b.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/34J8b.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://www.nspo.narl.org.tw/en/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nspo.narl.org.tw/en/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hK0m8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hK0m8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://www.nspo.narl.org.tw/tw2015/projects/FORMOSAT-5/program-description.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nspo.narl.org.tw/tw2015/projects/FORMOSAT-5/program-description.html&lt;/a&gt; using google translate in Chrome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uS7md.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uS7md.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; suggestions for a recent internet search&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-07-21T19:46:30.593" Title="What happened to Formosat-4 and Formosat-6?" Tags="|spacex|falcon-9|sherpa|formosat|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17305" PostTypeId="2" ParentId="17301" CreationDate="2016-07-19T11:06:54.980" Score="18" Body="&lt;p&gt;NASA studied &lt;a href=&quot;https://ston.jsc.nasa.gov/collections/trs/_techrep/CR188427.pdf&quot;&gt;the effects of radiation on film&lt;/a&gt;. Bright spots are just one of the possible results. Other effects include an increase in the amount of noise, and a decrease in contrast and color response. These effects are not easily detectable to the untrained eye and without access to the original material. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this study, NASA also experimented with shielding. The shield in use provided little to no benefit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiation levels during the Moon missions were far lower than in Chernobyl. Total mission dose was on the order of &lt;a href=&quot;http://www.braeunig.us/space/hoax.htm&quot;&gt;1 rem&lt;/a&gt; (0.01 Sv).  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="33" LastEditDate="2016-07-19T16:41:13.660" LastActivityDate="2016-07-19T16:41:13.660" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17307" PostTypeId="2" ParentId="17301" CreationDate="2016-07-19T11:46:15.493" Score="38" Body="&lt;p&gt;The radiation dosage for a year on the moon is between &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0032063312002085&quot; rel=&quot;noreferrer&quot;&gt;110 mSv and 380 mSv&lt;/a&gt;. On Earth, that dosage is 2.4 mSv, or higher, depending on where you are exactly. Bottom line, the few days in Lunar orbit would have aged the film due to radiation between 50-150 days/ day in orbit maximum, thus it would be the equivalent of film that was aged a few years at most. The environment at the Moon is more likely to have high energy effects, which I'll get to later. &lt;a href=&quot;http://chernobylgallery.com/chernobyl-disaster/radiation-levels/&quot; rel=&quot;noreferrer&quot;&gt;Chernobyl&lt;/a&gt; produced about 80 mSv/ second after the incident, considerably more than on the surface of the Moon! To this day, the dosage at the center of Chernobyl is around 10 mSv/ second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo missions were launched near the Solar Minimum, which would tend to have more higher cosmic ray strikes, and higher overall radiation, but fewer solar storms. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, there actually ARE signs of radiation in some of the images, if you look carefully. At the very least, it's dust in on the film, the two can be difficult to tell apart. For instance, look in high resolution at the dark portion of this image. The lines that run through it are quite possibly signs of radiation strikes, or even (gasp!) stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j3Z5f.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j3Z5f.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-21T11:54:32.560" LastActivityDate="2016-07-21T11:54:32.560" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17308" PostTypeId="2" ParentId="17147" CreationDate="2016-07-19T11:51:56.513" Score="2" Body="&lt;p&gt;Sometimes you just can't get @RussellBorogove to admit that those really helpful comments can become a really a good answer. So until that time comes... I'll temporarily accept this copy/paste answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/questions/17147/#comment44249_17147&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Any stretch of time during atmospheric ascent where the rocket isn't vertical and its AoA is near zero is gravity turn by definition. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and from &lt;a href=&quot;https://space.stackexchange.com/questions/17147/#comment44250_17147&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(Alternately, if it's supersonic and not falling apart, it's probably pretty close to gravity turn. ;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So a recognizable feature of the general idea of a gravity turn may be the fact that the turn happens without tilting the rocket away from the direction it's moving through the air (very much) with thrust. I think. The added structural mass required to handle side stresses at supersonic speeds are a huge penalty - and if you don't add it, you'd loose structural integrity if you tried to make a substantial turn by vectoring thrust.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-02-03T06:05:47.607" LastActivityDate="2018-02-03T06:05:47.607" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17312" PostTypeId="1" AcceptedAnswerId="17329" CreationDate="2016-07-19T12:46:21.970" Score="4" ViewCount="309" Body="&lt;p&gt;A piece of space debris WT1190F is in the news in part because it has been far from earth (farther than lunar distance) in the past but &lt;s&gt;will&lt;/s&gt; should have already re-enter earth's atmosphere &lt;s&gt;in a few weeks&lt;/s&gt; seven months ago. See this open access news item in Nature: &lt;a href=&quot;http://www.nature.com/news/incoming-space-junk-a-scientific-opportunity-1.18642&quot; rel=&quot;nofollow noreferrer&quot;&gt;Incoming space junk a scientific opportunity&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've read here and other places that the object &lt;s&gt;is&lt;/s&gt; was thought to be relatively low density, based on observations over the past few years. I'm wondering how this can be determined. Perturbation due to solar radiation is one possibility, but how good would the observations need to be to see such a small effect? (I estimate of the order of ~1 meter/second per year for 100kg and 1 square meter).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; based on recommendations &lt;a href=&quot;https://physics.meta.stackexchange.com/a/8961/83380&quot;&gt;here&lt;/a&gt; I am posting here a slightly edited version of &lt;a href=&quot;https://physics.stackexchange.com/q/214700/83380&quot;&gt;this 8 month old question&lt;/a&gt; in physics stackexchange. It's now way over the 60 day limit for migration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit 2:&lt;/strong&gt; I'm asking specifically about the &lt;strong&gt;WT1190F&lt;/strong&gt; object estimated density, not just how it can be done in general. And please include a citation or link so I can read more about how it was done in this case. Thanks!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2016-07-20T14:06:57.580" Title="Why do (did) they think WT1190F is low density?" Tags="|orbital-mechanics|artificial-satellite|planetary-science|debris|near-earth-object|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17314" PostTypeId="2" ParentId="17286" CreationDate="2016-07-19T14:58:33.390" Score="4" Body="&lt;p&gt;No, exercise time is not part of the &quot;40 hour&quot; work week. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the question you linked to:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;06.00 wake up, breakfast&lt;br&gt;&#xA;07.30 planning conference&lt;br&gt;&#xA;08.15 start of the work day: science and maintenance activities plus workouts.&lt;br&gt;&#xA;13.00 lunch&lt;br&gt;&#xA;14.00 resume work&lt;br&gt;&#xA;18.15 work preparation for the next day, another planning conference&lt;br&gt;&#xA;19.30 dinner, then free time&lt;br&gt;&#xA;21.30 sleep &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So work from 7.30 to 13.00 is 5.5 h, 14.00 to 19.30 is another 5.5 h, for a total of 11 h including 2.5 h of exercise and 1.5 h for planning conferences and preparation.&#xA;This leaves 7 h for science and maintenance activities. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That schedule is adhered to for 5 days/wk. On Saturday, astronauts do housekeeping and station maintenance. I'd count that as work as well. Sunday is a day off, unless there's an event (spacecraft arrives or leaves). Some science experiments have daily requirements so these continue though the weekend.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/NACRS_ISSResourceOverview_022014T.pdf&quot; rel=&quot;nofollow&quot;&gt;ISS research resource planning&lt;/a&gt; shows that recent ISS expeditions had an average output of 40 hours/week of science when 6 crew were on board. &#xA;That's 40 hours for 6 people combined. So most of their work day is spent on station operations and maintenance rather than science. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 7th crewmember would be able to work mostly on science (as the maintenance workload doesn't increase much with 7 crew vs. 6). If he can spend all of his working hours Mon-Fri on science, that would add 35 h of science to the total output. Maybe a few hours on Saturday can be spent on science as well.   &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="415" LastEditDate="2016-07-20T08:44:43.390" LastActivityDate="2016-07-20T08:44:43.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17316" PostTypeId="2" ParentId="14242" CreationDate="2016-07-19T16:42:51.453" Score="1" Body="&lt;p&gt;Originally it was thought that the Falcon Heavy landing would use some mechanism to separate in time the landing of the two core boosters.  Perhaps, slow one down, allow one to reenter a bit faster to buy enough time for one to land, perhaps let the dust settle down, before the second one landed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon &lt;a href=&quot;https://twitter.com/elonmusk/status/755166100967550979&quot; rel=&quot;nofollow&quot;&gt;tweeted&lt;/a&gt; that he expects them to land simultaneously. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The contingency pads in the EIS apparently have yet to be built, as late as the CRS-9 landing at LZ-1. So it is unclear yet what the purpose would be, although as a divert location it does make sense, as is stated in Pearson's answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An &lt;a href=&quot;http://www.orlandosentinel.com/news/space/os-spacex-landing-pads-space-coast-20160718-story.html&quot; rel=&quot;nofollow&quot;&gt;article in the Orlando Sentinel&lt;/a&gt; indicates they are looking for permission to build two more pads. It is unclear from the article if they mean the contingency pads, or more pads. The article references within the LZ-1 complex, but never calls them by the contingency pad names.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-19T16:42:51.453" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17317" PostTypeId="2" ParentId="9346" CreationDate="2016-07-19T16:44:35.140" Score="1" Body="&lt;p&gt;After the question was asked, it became clear that they stopped using the Milkstool test stand, and instead started testing all Falcon 9 core stages on the new test pad. It is also clearly intended to test the Falcon Heavy. But it is also capable of testing the regular Falcon 9. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By lowering the exhaust, expending some of its energy against the trench they are also likely reducing the noise footprint for local residents. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-19T16:44:35.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17318" PostTypeId="2" ParentId="10391" CreationDate="2016-07-19T16:46:48.540" Score="1" Body="&lt;p&gt;After they got launching seriously, SpaceX decided that second stage recovery would be too difficult, and still maintain a useful payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Musk recently mused in a &lt;a href=&quot;https://twitter.com/elonmusk/status/755167487017291776&quot; rel=&quot;nofollow&quot;&gt;tweet&lt;/a&gt;, that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Really tempting to redesign upper stage for return too (Falcon Heavy has enough power), but prob best to stay focused on the Mars rocket&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This indicates that the better ISP of the Raptor engine under development for Mars missions might make it possible. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-19T16:46:48.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17319" PostTypeId="2" ParentId="14921" CreationDate="2016-07-19T18:28:34.243" Score="4" Body="&lt;p&gt;Here are a couple of papers on this topic:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Cooperstock, &lt;a href=&quot;http://arxiv.org/abs/astro-ph/9803097v1&quot; rel=&quot;nofollow&quot;&gt;http://arxiv.org/abs/astro-ph/9803097v1&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Carrera, &lt;a href=&quot;http://arxiv.org/abs/0810.2712&quot; rel=&quot;nofollow&quot;&gt;http://arxiv.org/abs/0810.2712&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I also have a brief treatment that may be more accessible (but still requires knowledge of general relativity) in &lt;a href=&quot;http://lightandmatter.com/genrel/&quot; rel=&quot;nofollow&quot;&gt;my GR book&lt;/a&gt;, section 8.2.10. The basic result for a system orbiting with angular frequency $\omega_0$ is that cosmological expansion causes the orbit to expand according to the equation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\frac{\dot{r}}{r_0} = \omega_0^{-2}\frac{d}{dt}\left(\frac{\ddot{a}}{a}\right)$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $a$ is the cosmological scale factor, and dots indicate time derivatives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the effect depends on the &lt;em&gt;acceleration&lt;/em&gt; of cosmological expansion, so it's not something you can just interpret naively as if space expands and therefore the system gets bigger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For purposes of order-of-magnitude estimation, we can take the second factor on the right-hand side to be the cube of the Hubble constant. For a small system like the earth-moon system, $\omega_0$ is high, and therefore the cumulative effect is incredibly small. Plugging in numbers, I find that over the period of time since the Apollo missions, the contribution of cosmological expansion to the recession of the moon is about $10^{-22}$ cm. Cooperstock has a picturesque way of describing the smallness of such effects, which is that the expansion of the earth's orbit since the age of the dinosaurs is less than the diameter of an atomic nucleus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if lunar laser ranging (LLR) had the kind of precision needed in order to measure this effect, it would never see it, because the distance between the earth and the moon is growing for reasons of tidal dynamics. This is the effect that really is seen in LLR, and it's many, many orders of magnitude greater than the cosmological effect.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2016-07-19T18:36:55.353" LastActivityDate="2016-07-19T18:36:55.353" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17320" PostTypeId="2" ParentId="13110" CreationDate="2016-07-19T19:05:58.533" Score="4" Body="&lt;p&gt;I'll start by mentioning the times and acceleration given in the books for this series.&#xA;The claim is that this ice hauler is capable of 1/3 G continuous acceleration, and in fact much higher acceleration is possible (mentioned in the attempt to escape incoming missiles, but no numbers are given).&#xA;A claim is also made that the ship makes 2 round trips a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that a relatively high constant acceleration is used an elliptical orbit is not a good model for this travel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not know how to model orbits with continuous acceleration, so i will fall back on straight line travel just to demonstrate what 1/3G makes possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ceres and Saturn have orbital radii 445,410,000km and 1,509Gm (as per Wikipedia), which gives 1,954Gm apart at the farthest distance.&#xA;Assuming the ship accelerates for the first half of the distance at 1/3G and then flips to apply the same accel in reverse we get a travel time over a straight line of 25.4 days by the following method:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;t=sqrt(2x/a) where x is 977Gm and a is 9.8/3&lt;/p&gt;&#xA;&#xA;&lt;p&gt;t=1093767 seconds, which is 12.7 days for half of a one way trip&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We must double this time again to get the round trip of 51 days, small enough to achieve the authors claim of 2 round trips a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trip times calculated allow us to compare this to existing sea freight times such as 2 weeks to 1 month for crossing the Pacific as given at the following site:&#xA;&lt;a href=&quot;https://www.reference.com/business-finance/long-cargo-ship-china-united-states-b7c6214e137c2e0a&quot; rel=&quot;nofollow&quot;&gt;https://www.reference.com/business-finance/long-cargo-ship-china-united-states-b7c6214e137c2e0a&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the travel times can be compared to the existing cargo ship travel times, then the career issues associated can also be compared. At only 2 times longer round trip, I think they can be compared.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who pays for medical care at sea while working a cargo ship?&#xA;What happens if I quit my job part way through a voyage?&#xA;Is my employer required to drop me off at the same port they picked me up?&#xA;Issues of mutiny and discipline?&#xA;Any other considerations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am aware of a number of problems with my answer, like the travel path going through the sun, the failure to match the origin or destination orbital velocities and the general ignoring of orbital mechanics in an orbital problem. But these numbered were only meant to give an order of magnitude to discuss the affects of the voyage on career choices.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastActivityDate="2016-07-19T19:05:58.533" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17321" PostTypeId="1" AcceptedAnswerId="17322" CreationDate="2016-07-19T21:26:06.257" Score="30" ViewCount="5472" Body="&lt;p&gt;All that I know about the Challenger tragedy was that some o-rings failed in the reusable solid rocket boosters (SRBs) due to the low temperatures that day. I'm curious if such an accident would have happened if the SRBs didn't have to be reusable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, maybe whatever coupling that used the o-rings could have been solid or sealed in another way if the SRBs weren't intended to be retrieved and reused. Or, maybe some other specific implementation detail of the SRBs or the SRBs inherently weren't ideal but the goal of re-usability trumped that.&lt;/p&gt;&#xA;" OwnerUserId="16324" LastEditorUserId="6816" LastEditDate="2016-07-19T21:32:38.393" LastActivityDate="2017-06-14T09:10:14.433" Title="Did the Challenger SRBs fail due to design for reuse?" Tags="|rockets|space-shuttle|reuse|srb|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17322" PostTypeId="2" ParentId="17321" CreationDate="2016-07-19T22:46:13.053" Score="64" Body="&lt;p&gt;The O-rings and the temperature were only the last in the long chain of blunders, and that had little to nothing in common with reusability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The construction of the SRBs wouldn't be much different with no reusability in mind. Indeed, SRBs of very similar design are to be used in SLS, and they are not intended to be reusable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About the most important factor responsible for the disaster was misunderstanding of how the O-rings were meant to function.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The segments of SRBs were a consequence of the technological process; the boosters were manufacturer that way, regardless of the reusability needs.  The segments would be connected with seals consisting of the O-rings, and ablative putty. The putty would act as a sealant keeping the pressure in, keeping the flames from escaping, and providing thermal isolation to the O-rings from the flames. The O-rings were to keep the putty from getting squeezed out by the extreme pressures of combustion. They were never meant to come into contact with flames and keep them from escaping, to any degree.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The putty appeared to be not efficient enough - all of it would burn away during the SRB burn. Then - the O-rings - not meant to withstand any flames - would still withstand them to such a degree, that less than half of them would burn through. And that fact - misunderstanding the fact that the putty burning through entirely was already a critical failure, and treating the degree of ablation of the O-rings as indicative of performance of the seal - allowed continued use of the faulty design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The O-rings, not ever being meant to operate as flame blocker, not tested or prepared for such a role, would operate within specs for their designated task, and entirely out of specs for the task they were not meant to. It was pure dumb luck they would perform as flame blocker at all, and with changing weather conditions of course their behavior as flame blocker changed - nobody ever engineered them not to! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact - reusability - and in particular, recovery of the boosters - was a factor that could have prevented the disaster: the damage to the O-rings might pass unnoticed otherwise. Thanks to the boosters being recoverable, the damage was noticed, analyzed, and had the right conclusion been drawn, it would lead to redesign and fixing the error, e.g. replacing the putty with one that ablates slower. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately - despite that - the problem was ignored, the damage accepted as &quot;within norm&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in the end, the conclusion is pretty much opposite to the one you implied: Reusability is more likely to prevent disasters, than promote them; it allows to notice and analyze any problems that occur and would be missed in non-recoverable rockets until changing external conditions lead to a disaster. It just requires a thorough insight into the intended workings of the rocket, to be able to draw the correct conclusions from the observations. Without reusability we may lack the observations to draw conclusions from.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-07-20T01:13:09.310" LastActivityDate="2016-07-20T01:13:09.310" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="17323" PostTypeId="2" ParentId="17321" CreationDate="2016-07-20T00:14:54.417" Score="26" Body="&lt;p&gt;Not reusability per se, but design and politics.  The competitor to build the SRBs for the Shuttle program was Aerojet, and their design was monolithic.  In other words, no segments.  Aerojet was actually selected as the winning contractor but the NASA administrator Fletcher overrode this and gave it to Morton Thiokol.  Aerojet had already built an &lt;a href=&quot;http://www.abandonedfl.com/aerojet-dade/&quot;&gt;awe-inspiring test facility&lt;/a&gt; in Florida.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, Fletcher was from Utah.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will never be known whether Aerojet's booster design would have had fatal flaws as well, but it would not have had the actual flaw that doomed Challenger and the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.tsgc.utexas.edu/archive/general/ethics/boosters.html&quot;&gt;Texas Space Grant Consortium&lt;/a&gt; (Describes Fletcher's overturn of contract award and subsequent controversy)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://articles.philly.com/1986-03-03/news/26082692_1_o-ring-nasa-rocket-design&quot;&gt;philly.com&lt;/a&gt; (Describes how the Aerojet design was monolithic and presumably safer)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-07-20T00:45:34.130" LastActivityDate="2016-07-20T00:45:34.130" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17324" PostTypeId="2" ParentId="17321" CreationDate="2016-07-20T01:21:57.850" Score="15" Body="&lt;p&gt;Technically, it was the O-rings, but that was not the root cause. The root cause was a poor culture at NASA. The technical solution was easy: Don't launch on that cold of a day. In Florida, even in late January, all you have to do is wait a day or two and temperatures will once again be balmy. Technically savvy people knew well of the O-ring problem in cold weather. They tried to escalate these concerns but were completely rebuffed. &lt;a href=&quot;http://www.spacesafetymagazine.com/space-disasters/challenger-disaster/challenger-management-failure/&quot;&gt;NASA had a schedule to meet&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Poor culture at NASA was also responsible for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_1#Spacecraft_preparation&quot;&gt;Apollo 1 fire&lt;/a&gt;, the &lt;a href=&quot;http://history.nasa.gov/SP-2006-4702/chapters/chapter11.pdf&quot;&gt;Columbia&lt;/a&gt;  &lt;a href=&quot;https://josephhall.org/papers/nasa.pdf&quot;&gt;disaster&lt;/a&gt; (two references), &lt;a href=&quot;http://www.cio.com.au/article/420036/what_went_wrong_hubble_space_telescope_what_managers_can_learn_from_it_/&quot;&gt;Hubble's messed-up optics&lt;/a&gt;, and lots of lesser problems. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-07-20T01:28:02.070" LastActivityDate="2016-07-20T01:28:02.070" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17326" PostTypeId="2" ParentId="17074" CreationDate="2016-07-20T04:44:14.490" Score="-1" Body="&lt;p&gt;Not really familiar with the vehicle or the specifics of how it was mated to its launch vehicle but my first reaction would be that it probably is driven by either a packing constraint or possibly is related to the specifics of the deployment mechanism or sequence.&lt;/p&gt;&#xA;" OwnerUserId="16329" LastActivityDate="2016-07-20T04:44:14.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17327" PostTypeId="2" ParentId="13110" CreationDate="2016-07-20T08:52:28.507" Score="7" Body="&lt;p&gt;Short answer: Maybe, if you allow for an engine like used in the series.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spaceships in &lt;em&gt;The Expanse&lt;/em&gt; do not use Hohmann transfers. Instead they just start to accelerate continuously to the point of intercept with their target, flipping midway and deccelerating again. &#xA;This reduces both traveled distances (because your path is far less arced) and increases average travel speed, both reducing travel time by large amounts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This type of travel is available because the ships do not use &quot;conventional chemical rocket engines&quot; but something called a epstein-drive. This is, according to the books, a high efficiency fusion engine.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How efficient would we have to get this thing to do maneuvers like in the series? Let's do some numbers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A typical travel in the books is in the order of a two week travel, accelerating with 1/3 g, flipping over midway. (this maneuver would bring us from Earth to Jupiter, if Jupiter/Earth is roughly at their median distance.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1/3 g ~ 3m/s²&lt;br&gt;&#xA;2 weeks ~ 1,206,600 seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for this maneuver to work, we need enough fuel for&lt;br&gt;&#xA;1.206.600s *  3m/s² = 3.628.800 m/s delta V.&lt;br&gt;&#xA;(For comparison: our current rockets are like 90% fuel and manage to do ~10.000 m/s delta V).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, our rockets can't do this at all. But remember the epstein drive!&#xA;Fusion drives are in the very early development state here on real Earth...but Wikipedia gives us some numbers for a prototype here: &#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Fusion_rocket&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Fusion_rocket&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;700 km/s exhaust velocity is stated there, let's see what this gives us:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exhaust velocity/Earth gravity gives us the Specific Impulse of an engine, which basically tells us how much fuel we have to spend to change our speed by an set amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using this site:&lt;br&gt;&#xA;&lt;a href=&quot;http://www.quantumg.net/rocketeq.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.quantumg.net/rocketeq.html&lt;/a&gt;&lt;br&gt;&#xA;we can find out how much delta V we get for a rocket of weight X, with fuel y available and a specific Impulse of z.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the Wikipedia-fusion-engine we get a specific impulse of 70,000. But what about ship mass and fuel mass? For ease of imagination, let's use a Boeing 747 as example:&lt;br&gt;&#xA;An empty 747 weights 178,756 kg and has a maximum weight of 396,890 kg. Assuming that all the extra weight is fuel, we get a delta V of 547,179 m/s.&#xA;That's only 15% of the delta V we need.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no direct continuous thrust travel to jupiter...but we can always add a coasting phase into the middle. and if we want &quot;only&quot; to go from Mars to the asteroid belt...and assume that our Wikipedia-fusion-drive can be upgraded a bit...    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer is: &#xA;Maybe not exactly like in &lt;em&gt;The Expanse&lt;/em&gt;. &#xA;But if we get any type of fusion drive working we will use maneuvers much closer to the expanse-like travel than to our current &quot;fuel-save&quot;-mode Hohmann-transfers.&lt;/p&gt;&#xA;" OwnerUserId="11787" LastEditorUserId="1142" LastEditDate="2016-07-20T12:19:38.413" LastActivityDate="2016-07-20T12:19:38.413" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17328" PostTypeId="2" ParentId="14317" CreationDate="2016-07-20T12:53:55.373" Score="1" Body="&lt;p&gt;Since you &quot;modified&quot; your question:&#xA;The purpose of an antenna is to receive a signal not to filter noise.&#xA;The Power voyager receives, could be sun + earth, earth - sun, sun - earth or only sun. The Modulation used is still unknown, so i cant calculate the needed SNR for it to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the normal radiation level shouldn't be a problem for the voyager probe.&#xA;Think of the sun as a constant emitter. Only flares create spikes. The spectrum of these radio waves will have a Gaussian bell curve (Watts/ Hz). The Engineers of Nasa will probably have picked a Frequency spectrum that is on one of the lower ends of this bell curve. The radio power of the sun decreases faster per Meter away from its origin then that of earth. Since earth uses directional communication and the sun is just a big ball of energy.&#xA;So the true problem will be, to always point at voyager correctly. &lt;/p&gt;&#xA;" OwnerUserId="14245" LastEditorUserId="14245" LastEditDate="2016-07-21T13:05:34.790" LastActivityDate="2016-07-21T13:05:34.790" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17329" PostTypeId="2" ParentId="17312" CreationDate="2016-07-20T13:51:26.840" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/WT1190F&quot; rel=&quot;nofollow&quot;&gt;WT1190F&lt;/a&gt; orbited the Earth during the time it was observed. There were a number of varying observations that were made over it's lifetime. Wikipedia shows the lifetime at 3 different years, each with a wildly different orbit. Due to frequent passes by the Moon, as well as the Yarkovsky effect and pertubations of solar radiation, even a small difference could be determined. These are used to calculate the &lt;a href=&quot;http://projectpluto.com/temp/wt1190f.htm&quot; rel=&quot;nofollow&quot;&gt;Area to Mass&lt;/a&gt; ratio. They were able to use 5 months worth of tracking from 2009, some in 2011, and 2013 to determine the orbit of the object. Slight perturbations can hint as to what the mass of the object might be.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-20T13:51:26.840" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17330" PostTypeId="1" AcceptedAnswerId="48024" CreationDate="2016-07-20T14:07:57.607" Score="6" ViewCount="228" Body="&lt;p&gt;This would be a handy reference, to say the least.  There is a paper copy in at least one &lt;a href=&quot;http://www.worldcat.org/title/shuttle-systems-design-criteria-vol-1-shuttle-performance-assessment-databook/oclc/671633839&quot;&gt;library&lt;/a&gt; in Houston, so it's not inconceivable that it's online somewhere, but my searches to date have failed.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-08-30T14:11:16.663" LastActivityDate="2020-10-16T15:10:20.813" Title="Is NSTS 08209 &quot;Shuttle Systems Design Criteria&quot; available online?" Tags="|space-shuttle|history|reference-request|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17331" PostTypeId="1" CreationDate="2016-07-20T15:08:48.477" Score="17" ViewCount="3934" Body="&lt;p&gt;I read that it would be more difficult to land (not crash land) on Mercury or even send a spacecraft in orbit around it because of the gravitational pull of the Sun and the lack of planetary atmosphere. Does current technology allow landing a spacecraft on Mercury to study the surface in detail? &lt;/p&gt;&#xA;" OwnerUserId="11347" LastEditorUserId="25" LastEditDate="2016-07-20T17:12:07.207" LastActivityDate="2016-07-20T17:22:25.133" Title="What are the challenges to landing a spacecraft on Mercury?" Tags="|landing|spacecraft-development|lander|mercury|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17332" PostTypeId="2" ParentId="17330" CreationDate="2016-07-20T15:12:51.360" Score="2" Body="&lt;p&gt;It is possible that a digital copy is stored at the &lt;a href=&quot;https://standards.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;NASA Technical Standards System&lt;/a&gt; which requires a NASA account to access.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Otherwise, there does not seem to be a browser-accessible copy at all.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-07-20T15:12:51.360" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17333" PostTypeId="2" ParentId="17256" CreationDate="2016-07-20T15:16:11.657" Score="8" Body="&lt;p&gt;Most engine failures result in shutdown of the engine without harm to its neighbors, and Merlin appears to be a reliable and safe engine. A 45-engine cluster would barely notice the loss of one or two. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure of the vibration implications but it seems like pogo mitigation is better understood now than it was in 1970. I'm not sure if other vibration resonance issues would be heightened or not - more mass to dampen things and a less concentrated noise spectrum versus more total vibrational energy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Center core loading would indeed be higher at launch; I don't know if the structural strength margin is sufficient or if they would have to revise it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerodynamic pressure shouldn't be an issue; assuming a similar trajectory, the force per cross sectional area will be unchanged. Aerodynamic stability shouldn't change much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without crossfeed, they'd want to shut down some of the booster engines as they go, but that's easily done. They could even shut some down through max-Q and then relight them, if that's more efficient than throttling down. &quot;Asparagus&quot; crossfeed would be best, of course, emptying the top-bottom boosters into the left-right ones while the left-right pair empty into the center, dropping one pair at a time. You'd want to flow from top and bottom to both neighbors at once, otherwise the moving fuel would exert a substantial torque, rolling the ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What might be the biggest problem is that Falcon is horizontally integrated - assembled while lying on its side. It might be possible to assemble the center, payload, and L-R boosters, elevate it at the pad, then attach the T-B boosters, but I think vertical assembly would be more likely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, ULA has released white papers showing 4-booster and even 6-booster Delta IV &quot;superheavy&quot; configurations, with something like 100 ton to LEO payload capacity. On one hand, this suggests that ULA thinks it's just &quot;a small matter of engineering&quot; to do it; on the other, ULA's business and PR practices suggest that they haven't done more than a cursory feasibility study.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, even if it is doable, I would not expect SpaceX to pursue a Falcon Superheavy. F9/FH will suffice for almost all commercial payloads, and they'll look to BFR for the really big missions.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-20T15:31:27.370" LastActivityDate="2016-07-20T15:31:27.370" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17334" PostTypeId="1" CreationDate="2016-07-20T15:42:07.860" Score="8" ViewCount="275" Body="&lt;p&gt;It seems strange to me that given all of the concerns about living space and color palette aboard ISS, the data/power/cooling ports for different mid-deck locker equivalent (MLE) express rack payloads are on the fronts of the lockers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not have some kind of snap-in docking connector which includes all the necessary power, data, air, water, gas, and vacuum connections? This could go on the back of the locker instead of the front, and would therefore be out of the way of the astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it would mean the lockers would lose a tiny bit of interior space, since the locker design would now have to accommodate a pre-ordained set of connectors in a certain arrangement. But MLEs are already almost totally arbitrary anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yes — I realize that not all payloads use all the connectors — but that doesn't necessarily matter. One could simply have dead ends in the MLE for these, or even not connect those particular pipes to the dock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it mostly historical, or is there a good reason locker connections are done the way they are?&lt;/p&gt;&#xA;" OwnerUserId="869" LastEditorUserId="869" LastEditDate="2016-07-21T13:04:59.690" LastActivityDate="2024-01-30T19:02:30.090" Title="Why is there no common &quot;docking&quot; interface for express rack payloads?" Tags="|iss|crewed-spaceflight|payload|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17336" PostTypeId="2" ParentId="17331" CreationDate="2016-07-20T17:11:37.000" Score="8" Body="&lt;p&gt;The closest we have done thus far is to orbit a spacecraft around Mercury. This was accomplished by a number of flybys first, of Earth, Venus, and Mercury. The mass of &lt;a href=&quot;https://en.wikipedia.org/wiki/MESSENGER&quot; rel=&quot;noreferrer&quot;&gt;Messenger&lt;/a&gt; was about 500 kg, minus fuel. The orbit was still highly elliptical. Per &lt;a href=&quot;https://i.stack.imgur.com/piNqI.png&quot; rel=&quot;noreferrer&quot;&gt;this chart&lt;/a&gt;, to land on Mercury from that orbit requires about 4200 m/s, roughly. That is an enormous amount, more than any other spacecraft to date has had! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has been some work towards a lander, however. The closest to succeeding is the &lt;a href=&quot;https://en.wikipedia.org/wiki/BepiColombo&quot; rel=&quot;noreferrer&quot;&gt;BepiColombo mission&lt;/a&gt;, which proposed to send a 7 kg lander to Mercury, known as the Mercury Surface Element. The main mission was approved, but the lander was rejected. Approximately 80% of its mass would be fuel. It would  land near a pole, to avoid the temperature extremes that come due to the long days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it might be possible, but it would be very difficult. Messenger was a low cost mission, and was able to orbit it. A lander was proposed, but nixed, due to it's expense and expected relatively little scientific gain.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-20T17:15:36.597" LastActivityDate="2016-07-20T17:15:36.597" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17338" PostTypeId="2" ParentId="12706" CreationDate="2016-07-20T18:19:21.577" Score="1" Body="&lt;p&gt;Though there are other types of satellites which fit your question, I wanted to point our that there are publicly usable amateur radio satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below quoted from wikipedia (&lt;a href=&quot;https://en.wikipedia.org/wiki/Amateur_radio_satellite&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Amateur_radio_satellite&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These satellites can be used for free by licensed amateur radio operators for voice (FM, SSB) and data communications (AX.25, packet radio, APRS). Currently, over 5 fully operational amateur-satellites in orbit act as repeaters, linear transponders or store and forward digital relays.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It would be possible to listen in on others communications with these satellites, while they were in range, and the type of data could be voice, Morse code, or any of a number of kinds of digital packet data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A site which tracks these satellites:&#xA;&lt;a href=&quot;http://www.n2yo.com/satellites/?c=18&quot; rel=&quot;nofollow&quot;&gt;http://www.n2yo.com/satellites/?c=18&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16322" LastActivityDate="2016-07-20T18:19:21.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17339" PostTypeId="1" CreationDate="2016-07-20T19:06:42.780" Score="4" ViewCount="17363" Body="&lt;p&gt;We've all seen tons of images taken by astronauts and spacecraft, but many times they are artificially colored, include invisible light spectrums, or have a lot of contrast. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WisP2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WisP2.jpg&quot; alt=&quot;Apollo 11 return&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would imagine, without the atmosphere and artificial light from civilization, that the stars and the sun would be even more brilliant from space. But in picture from the ISS or spacecraft the background often looks completely black. Of course, cameras can't capture an image the way a human eye would.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is are there any accounts from astronauts or experts on what space actually looks like from space? &lt;/p&gt;&#xA;" OwnerUserId="15668" LastEditorUserId="4660" LastEditDate="2016-07-20T23:33:04.887" LastActivityDate="2016-07-21T17:42:40.173" Title="What does space look like from space?" Tags="|crewed-spaceflight|spacecraft|observation|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17340" PostTypeId="2" ParentId="17339" CreationDate="2016-07-20T19:22:59.503" Score="1" Body="&lt;p&gt;Like this: &lt;a href=&quot;https://youtu.be/RiGeqsGjhoY?t=60&quot; rel=&quot;nofollow&quot;&gt;Stars from spacestation&lt;/a&gt;&#xA;Sometimes pictures from the Moon show dark sky with no lights because the Earth is very bright and drowns out the much fainter stars. A picture taken with short exposure just does not collect enough light to show those faint objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long exposure, time-lapse kind of shots are much closer to what a human would see. &lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-07-21T17:42:40.173" LastActivityDate="2016-07-21T17:42:40.173" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17341" PostTypeId="2" ParentId="12706" CreationDate="2016-07-21T00:06:17.993" Score="0" Body="&lt;p&gt;I agree with one of the comments above that this is more of a list question, so it's hard to get a definitive answer.  That being said, the source that I personally am most familiar with is &lt;a href=&quot;http://earthexplorer.usgs.gov/&quot; rel=&quot;nofollow&quot;&gt;USGS Earth Explorer&lt;/a&gt;.  Mainly geographic data.&lt;/p&gt;&#xA;" OwnerUserId="15841" LastActivityDate="2016-07-21T00:06:17.993" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17342" PostTypeId="1" AcceptedAnswerId="17445" CreationDate="2016-07-21T04:25:48.900" Score="5" ViewCount="1144" Body="&lt;p&gt;Given the orbital elements of two satellites and the delta-V capabilities of one of the satellites, how can you calculate the maneuvers required to have the earliest possible interception?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference of what I'm trying to do: You have two spacecraft, and one of them is attempting to fire a missile at the other. The orbits are known, as are the properties of the missile; the angle and timing are what's needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: The satellites are in the same orbital plane (I know it would be very different if they weren't).&lt;/p&gt;&#xA;" OwnerUserId="410" LastEditorUserId="25" LastEditDate="2016-07-22T13:11:13.247" LastActivityDate="2016-07-23T14:12:08.223" Title="Intercepting another satellite" Tags="|orbital-mechanics|artificial-satellite|orbital-maneuver|military|" AnswerCount="2" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17343" PostTypeId="1" AcceptedAnswerId="17348" CreationDate="2016-07-21T05:49:18.623" Score="1" ViewCount="177" Body="&lt;p&gt;On earth, the photovoltaic cell junction for mass production tends to be just silicon, although there is a variety of material types - single crystal vs multicrystal or polycrystal, or even amorphous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just read that the Voyagers used indium phosphide (InP), Wikipedia says that &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_panels_on_spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galium Arsenide&lt;/a&gt; (GaAs) are common, and I have read some satellites use &lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-junction_solar_cell&quot; rel=&quot;nofollow noreferrer&quot;&gt;triple-junction solar cells&lt;/a&gt; multi-layered devices used when the absolute highest efficiency is achieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any reason - &lt;em&gt;besides purchase price&lt;/em&gt; - that would keep all new satellites from using the highest efficiency cells? And actually, do different cells have best efficiency under different conditions encountered in spaceflight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pics of Juno's photovoltaics from NASA:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3XaZ0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3XaZ0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vc2lo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vc2lo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ks7ko.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ks7ko.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-07-22T01:12:17.690" Title="Has the choice of photovoltaic junction types for newer spacecraft converged on one, optimal solution?" Tags="|artificial-satellite|design|power|solar-power|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17344" PostTypeId="1" AcceptedAnswerId="17354" CreationDate="2016-07-21T06:11:53.797" Score="4" ViewCount="483" Body="&lt;p&gt;The efficiency of photovoltaic cells is surprisingly strong function of junction temperature. Hotter usually means lower efficiency. If one searches for example for images and sites tagged with &quot;floatovoltaics&quot; it's clear that some effort is being made to use water to cool the junctions to maintain efficiency under maxiumum solar incident flux. I think it's because the higher temperature allows more random recombination within the junction, before the charge can be collected. Any incident solar radiation that is not converted to electrical power (i.e. most of it) will heat the cells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are photovoltaics in spacecraft just 'hanging there' at whatever equilibrium temperature they happen to attain, or is the temperature sometimes actively controlled or at least limited in some way? Can they ever get &lt;em&gt;too cold?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pBEU4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pBEU4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Image from &lt;a href=&quot;http://www.renewableenergyworld.com/articles/2013/11/running-out-of-precious-land-floating-solar-pv-systems-may-be-a-solution.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Renewable Energy World&lt;/a&gt; which it credits to Far Niente, a Napa Valley winery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bBokH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bBokH.jpg&quot; alt=&quot;Floatovoltaics from NGEO from Kyocera&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Image from &lt;a href=&quot;http://news.nationalgeographic.com/news/energy/2015/01/150116-floating-solar-power-japan-yamakura/&quot; rel=&quot;nofollow noreferrer&quot;&gt;National Geographic&lt;/a&gt; which it credits to Kyocera, Inc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HyrUE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HyrUE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Image from an article &lt;em&gt;with many flotovoltaic photos&lt;/em&gt; in &lt;a href=&quot;http://news.nationalgeographic.com/news/energy/2015/01/150116-floating-solar-power-japan-yamakura/&quot; rel=&quot;nofollow noreferrer&quot;&gt;pv-magazine&lt;/a&gt; which it credits to New Jersey American Water&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9vhaK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9vhaK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Image from an article *with many flotovoltaic photos in &lt;a href=&quot;http://news.nationalgeographic.com/news/energy/2015/01/150116-floating-solar-power-japan-yamakura/&quot; rel=&quot;nofollow noreferrer&quot;&gt;pv-magazine&lt;/a&gt; which it credits SGP Solar.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-07-21T06:25:50.297" LastActivityDate="2016-07-26T13:58:43.870" Title="Temperature of photovoltaic cells vs efficiency - is it ever actively controlled?" Tags="|artificial-satellite|power|solar-power|thermal-control|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17345" PostTypeId="2" ParentId="17339" CreationDate="2016-07-21T12:54:50.297" Score="4" Body="&lt;p&gt;What space looks like in space depends on what is around you that might scatter light. The human eye is very adaptable to varying light conditions, in low light conditions the iris dilates to let in as much light as possible, in high light conditions it contracts to restrict light to give the retina a consistent amount of light and to protect it from damage from too much light. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Earth you cannot see the stars in the day because the atmosphere scatters light from the sun. The stars are still there of course but they are drowned out by the other light. Also, your pupil is too small to give the sensitivity to low light emitters. On the earth at night how many stars you see depends on the condition of the atmosphere and the amount of scattered light there is, which is why modern observatories are built in high altitudes far from any light pollution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space what you see is determined by what is around you and what is scattering light in your direction. The surface of the moon is very bright in sunlight, so if you are standing on the surface looking at the moon your pupil will contract and you will see fewer stars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are floating in space no-where near any light scattering source, with the sun out of your field of view then your eyes would dilate out and be at their maximum sensitivity (after 20 minutes - it takes time for your night vision to kick in fully) what you would see is the deepest, deepest black punctuated by the unwavering pinpricks of stars (no twinkling as there's no atmosphere), and a bright glow from the milky way. It wouldn't be that different from looking up at the sky from the palomar observatory (or any other high point) on a clear and moonless night - at palomar you wouldn't be looking through a layer of glass or plastic designed to keep you from asphyxiating, but the view in space would be even more spectacular.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The moon has no atmosphere to speak of to scatter light, so if you lie on the surface of the moon facing up, and the sun and earth are not in your field of view, it would be the same thing as if you were in open space. As soon as the earth, moon or sun came into your view the light will cause your eye to contract and you'd see less stars. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2016-07-21T13:49:47.073" LastActivityDate="2016-07-21T13:49:47.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17346" PostTypeId="2" ParentId="17339" CreationDate="2016-07-21T13:23:15.890" Score="6" Body="&lt;p&gt;To answer your specific question &quot;are there any accounts from astronauts or experts on what space actually looks like from space?&quot;, yes, there are many.  Every astronaut memoir I have read has always addressed this.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an example from Mike Collins' book &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0802131883&quot; rel=&quot;nofollow&quot;&gt;Liftoff&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;My God, the stars are everywhere, even below me. They are somewhat&#xA;  brighter than on earth, but the main difference is that they don't&#xA;  twinkle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From Jerry Ross's book &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1557536317&quot; rel=&quot;nofollow&quot;&gt;Spacewalker&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To see stars at night we had to turn down all the orbiter's interior&#xA;  lights to make it dark and eliminate the reflections in the windows.&#xA;  Then we could view so many more stars than one could ever gaze at from&#xA;  Earth, and during a single orbit we could see both Northern and&#xA;  Southern Hemisphere stars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And this&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/LaUCMzgidvs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-07-21T14:10:34.883" LastActivityDate="2016-07-21T14:10:34.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17347" PostTypeId="2" ParentId="17304" CreationDate="2016-07-21T15:10:35.217" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;http://link.springer.com/article/10.1186/s40562-016-0049-0&quot; rel=&quot;nofollow&quot;&gt;&quot;The fast development of solar terrestrial sciences in Taiwan&quot; by Liu, J., Chang, L.C., Chao, C. et al. &lt;/a&gt; there is no Formosat-4:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The pronunciation of “4” in Chinese sounds like “death” or “bad luck”, which is somewhat similar to “13” in West. Therefore, there is no FORMOSAT-4 mission. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and Formosat-6 was canceled:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;FORMOSAT-6 was designed to have a remote sensing mission and, however, terminated after a careful evaluation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Some of the authors of this paper are among the scientists involved in these projects, so I would consider it an official statement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what is available on &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/formosat-6.htm&quot; rel=&quot;nofollow&quot;&gt;Gunter's&lt;/a&gt;, it is possible that Formosat-6 was cancelled because of the suspension of development of the Taiwan Small Launch Vehicle (TSLV):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;FORMOSAT-6 was targeted to be launched by TSLV (Taiwan Small Launch Vehicle) as the first choice. Due to the fact that there will be no propellant system to perform orbit adjustment, and therefore the satellite will be injected to its final orbit directly. The satellite will also be designed to be compatible with other international piggyback launch opportunities, as the development of TSLV has been suspended.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-07-21T19:46:30.593" LastActivityDate="2016-07-21T19:46:30.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17348" PostTypeId="2" ParentId="17343" CreationDate="2016-07-21T19:05:13.347" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4922696&amp;amp;newsearch=true&amp;amp;queryText=Spacecraft%20solar%20array%20technology%20trends&quot; rel=&quot;nofollow noreferrer&quot;&gt;This article from 2008&lt;/a&gt; was the best survey of trends in PV technology for space that I could find (requires IEEE subscription). It generalizes and essentially states that satellites in GEO have had a constant demand for more power, and that satellites in MEO or LEO have had a constant demand for higher radiation tolerance (higher end of life power). There has been continuous improvements in PV technology on both fronts, and reduced costs. Satellite manufacturers have taken advantage of these improvements, although cautiously to be sure the technology is space qualified. The paper mentions the role of multi junction cells as a big part of this, but doesn't note a particular brand or design as what the industry is converging on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass, power margin, and space available on the rocket are the largest trade offs considered when choosing a solar cell technology. Flight heritage of the solar panel design is also a consideration. Lastly, for some missions it may be cheapest to just use more panels of a standard design offered by a manufacturer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think temperature would have the largest effect on cell operation in different missions. The solar spectrum would be unchanged (although dimmer at further differences). Radiation degradation could also be a consideration, especially for a mission like Juno, but I don't have any source comparing different materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/9714/why-does-the-iss-not-use-the-most-efficient-solar-panels-available?rq=1&quot;&gt;Similar question answered here.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12966" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-07-22T01:12:17.690" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17349" PostTypeId="1" AcceptedAnswerId="17416" CreationDate="2016-07-21T19:26:26.633" Score="10" ViewCount="398" Body="&lt;p&gt;Is there a theoretical/experimental precedent for stellar photosphere aerocapture (if it is in fact, a thing)? Specifically, would using this method of deceleration be in some way more attractive than an aerobraking or magnetoshell aerocapture around a planet if one were traveling at relativistic speeds, say .1c or a bit less? As I understand it, wrongly or not, temperatures in the sun's outer layers are about 6000K as opposed to the upwards of 100,000K ablative temperature of a heat shield during aerobraking. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plot synopsis: a ship traveling at sub-light speed is too low on fuel to decelerate in time to keep from blowing through and past its target solar system. Radical measures are considered.&lt;/p&gt;&#xA;" OwnerUserId="16346" LastEditorUserId="6816" LastEditDate="2016-07-21T19:40:08.913" LastActivityDate="2016-11-08T14:54:36.060" Title="Is stellar photosphere aerocapture possible, and if so, is it a viable option for rapid deceleration from relativistic speeds?" Tags="|interstellar-travel|aerobraking|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17350" PostTypeId="2" ParentId="17349" CreationDate="2016-07-21T20:07:29.333" Score="2" Body="&lt;p&gt;&lt;em&gt;How?&lt;/em&gt;  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Consider the probe that Galileo dropped into Jupiter.  While the objective wasn't aerocapture it serves as a useful comparison.  The probe burned away 1/4 of it's mass in the process.  We simply can't build a heat shield that can survive the maneuver.&lt;/li&gt;&#xA;&lt;li&gt;You're going fast enough that the electromagnetic force no longer controls the interaction of matter--at the interface between probe and star &quot;solid&quot; ceases to be truly solid.  You're going to get a lot of particles embedding themselves in the heat shield rather than being deflected.  Oops--said particles generally dump their kinetic energy as heat--inside the heat shield.  Some will score direct enough hits to cause fusion reactions instead.&lt;/li&gt;&#xA;&lt;li&gt;Wikipedia says the density is about 2E-4 kg/m^3.  Lets take a hypothetical probe that's a cylinder 50m long with an average density equal to water.  Water has a density of about 1000kg/m^3.  We have 1000kg / 2E-4 kg for a stopping distance of 2,000,000 meters or 2000km.  I'm getting a quarter of a million gs if you go for a complete stop.  Simply going for aerocapture will be somewhat less (I'm not sure what orbital velocity you need at that point) but not enough to matter.  (Note: My omission of width is deliberate, it cancels out.)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="276" LastEditorUserId="238" LastEditDate="2016-07-22T08:06:17.710" LastActivityDate="2016-07-22T08:06:17.710" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17351" PostTypeId="1" AcceptedAnswerId="17410" CreationDate="2016-07-21T20:10:51.287" Score="5" ViewCount="1654" Body="&lt;p&gt;Over on SE.Worldbuilding I found myself stuck on figuring what happens to the scale height of an atmosphere if the composition changes (The specific case involved something breathable in place of the Martian atmosphere.)  Google is being quite useless in this regard.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-07-22T10:21:43.790" Title="Atmospheric scale height, how is it calculated?" Tags="|atmosphere|physics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17352" PostTypeId="2" ParentId="17343" CreationDate="2016-07-21T23:16:03.300" Score="0" Body="&lt;p&gt;Of course not. Until the 'perfect' photovoltiac material is determined (when would you expect that to happen? 5 years? 5 decades? 5 centuries?), we'll be forced to use less than perfect materials. Given different criteria - for instance budget, mass, life-time, light intensity (towards the Sun, or away?), the &quot;optimum&quot; will vary.&lt;/p&gt;&#xA;" OwnerUserId="16354" LastActivityDate="2016-07-21T23:16:03.300" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17353" PostTypeId="1" CreationDate="2016-07-21T23:51:31.730" Score="3" ViewCount="1837" Body="&lt;p&gt;NASA's cFS (core Flight System) has been released as open-source.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &lt;a href=&quot;https://cfs.gsfc.nasa.gov&quot; rel=&quot;nofollow&quot;&gt;Core Flight System (cFS)&lt;/a&gt; is a platform and project independent reusable software framework and set of reusable software applications.  There are three key aspects to the cFS architecture: a dynamic run-time environment, layered software, and a component based design.  It is the combination of these key aspects that makes it suitable for reuse on any number of NASA flight projects and/or embedded software systems at a significant cost savings.    &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What are its uses? Where is it really used? What advantage does it bring for the developer community? Can it fly a sat /a rocket/a UAV? Where does NASA use it, other than in &lt;a href=&quot;http://www.engineering.com/Education/EducationArticles/ArticleID/12655/NASA-CubeSats-Incoprate-Raspberry-Pi-as-Learning-Tool.aspx&quot; rel=&quot;nofollow&quot;&gt;cubesats&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="415" LastEditDate="2016-07-22T20:46:53.453" LastActivityDate="2021-07-07T00:10:06.007" Title="What are the uses of the NASA core Flight System (cFS)?" Tags="|nasa|software|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17354" PostTypeId="2" ParentId="17344" CreationDate="2016-07-22T00:41:40.410" Score="2" Body="&lt;p&gt;The only solar cell temperature sensitive mission I could think of was MESSERGER. The mission to Mercury had solar panels that were 2/3 mirror surfaces, and they were programmed to rotate away from the Sun/Mercury to reduce incoming light if the temperature rose too high. This was allowable because the panels produced more power in Mercury orbit than the mission required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://web.archive.org/web/20070824103010/http://www.nasa.gov/pdf/168019main_MESSENGER_71504_PressKit.pdf&quot; rel=&quot;nofollow&quot;&gt;Source&lt;/a&gt;. See page 27.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to look at other missions and see what they did, wikipedia has a great list of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_of_Mercury&quot; rel=&quot;nofollow&quot;&gt;two missions to Mercury&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Observations_and_explorations_of_Venus#Timeline_of_Venus_exploration&quot; rel=&quot;nofollow&quot;&gt;numerous missions to Venus&lt;/a&gt;. These are some of my favorite missions since they are so extreme. I especially like Venera 9, which took the first photos from the surface of another planet (So cool!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Decreasing temperature from room temperature increases efficiency until somewhere below 100 K. Below this temperature the efficiency drops off as the short circuit current decreased. Semiconductor physics is complicated (citation needed), so I glossed over the reasons why this happens.&lt;/p&gt;&#xA;" OwnerUserId="12966" LastEditorUserId="12966" LastEditDate="2016-07-22T21:31:32.163" LastActivityDate="2016-07-22T21:31:32.163" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17355" PostTypeId="1" AcceptedAnswerId="17363" CreationDate="2016-07-22T06:53:40.813" Score="10" ViewCount="4539" Body="&lt;p&gt;See Topic. I know it is true for Sol, but in another solar System, is it possible that the mid or outer planets are completing one full orbit in a shorter amount of time than what a (smaller?) planet, closer to the star, would require ?&lt;/p&gt;&#xA;" OwnerUserId="16357" LastEditorUserId="14443" LastEditDate="2016-07-22T17:56:37.023" LastActivityDate="2016-07-22T17:56:37.023" Title="Do planets further from the sun take longer to complete their orbit?" Tags="|orbital-mechanics|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17356" PostTypeId="2" ParentId="17355" CreationDate="2016-07-22T07:13:52.723" Score="2" Body="&lt;p&gt;No it's impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The size of an orbit is relative to the speed of the orbiting object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a planet moved faster, it will be further, and completing a revolution will take longer because of the increased distance.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-07-22T07:13:52.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17357" PostTypeId="2" ParentId="17355" CreationDate="2016-07-22T07:14:39.600" Score="11" Body="&lt;p&gt;No, the laws of physics are the same wherever you are.  &lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/kepler.html&quot; rel=&quot;noreferrer&quot;&gt;Kepler's Laws&lt;/a&gt; govern what kinds of orbits planets have.  In this case, we are interested in Kepler's Third Law.  It states that the square of the period is proportional to the cube of the semimajor axis.  Mathematically, this is $P^2\varpropto a^3$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, this means that the further away a planet is from its star, the longer it takes to complete an orbit.  This is a universal law, and it does not matter whether the system in question is our own solar system or some other planet going around a faraway star.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-07-22T16:33:48.383" LastActivityDate="2016-07-22T16:33:48.383" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17358" PostTypeId="2" ParentId="17355" CreationDate="2016-07-22T07:22:38.963" Score="0" Body="&lt;p&gt;No, inside a single solar system this is not possible. The larger a circular orbit the longer it will take to complete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two catches:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we have two different solar systems, one with a massive sun (and thus higher speeds needed for the same orbit) and you compare that to a system with a lighter sun then you will see different speeds. But then we are comparing two orbits around two different suns. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, I have no idea how this will work for non-cicular orbits. Especially if you take one circular orbit and one highly elliptical one. Also, how do you declare which is the 'outer planet' if the ellipse takes it both inside and outside the other planets orbit?&lt;/p&gt;&#xA;" OwnerUserId="686" LastEditorUserId="13710" LastEditDate="2016-07-22T07:37:19.473" LastActivityDate="2016-07-22T07:37:19.473" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17359" PostTypeId="1" CreationDate="2016-07-22T07:53:22.920" Score="7" ViewCount="876" Body="&lt;p&gt;In the world of space exploration, the international community is extremely concerned with avoiding contamination of extraterrestrial bodies with life from Earth.  NASA has a &lt;a href=&quot;https://planetaryprotection.arc.nasa.gov/news&quot;&gt;very robust and stringent planetary protection protocol&lt;/a&gt;.  But now that private companies are joining the space exploration scene, how are they going to handle planetary protection?  As private companies, do they fall under the jurisdiction of the United Nations &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot;&gt;Outer Space Treaty&lt;/a&gt;?  Is there any way the international community could prosecute them for contaminating another world?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only source I could find was &lt;a href=&quot;https://www.washingtonpost.com/opinions/spacex-takes-planetary-protection-seriously/2016/06/16/fe1929f0-327c-11e6-ab9d-1da2b0f24f93_story.html&quot;&gt;a statement&lt;/a&gt; from the senior director of communications saying &quot;SpaceX takes planetary protection very seriously&quot;, but this yields almost no useful information.  How is SpaceX going to prevent cross-contamination?  I am not too interested in the technical details of how to sterilize a spacecraft, but I am curious about the broad strokes.  Does SpaceX have a team looking into this?  Do they have a plan put together?&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2020-08-19T15:10:49.530" Title="What plan does SpaceX have for planetary protection?" Tags="|spacex|planetary-protection|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17360" PostTypeId="5" CreationDate="2016-07-22T07:55:38.077" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-07-22T07:55:38.077" LastActivityDate="2016-07-22T07:55:38.077" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17361" PostTypeId="4" CreationDate="2016-07-22T07:55:38.077" Score="0" Body="The careful sterilization of any space probes or missions to prevent contaminating any extraterrestrial worlds with life from Earth.  Not to be confused with [planetary-defense], which protects Earth from asteroids and other physical threats." OwnerUserId="13710" LastEditorUserId="26446" LastEditDate="2021-06-09T20:55:19.880" LastActivityDate="2021-06-09T20:55:19.880" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17362" PostTypeId="2" ParentId="17349" CreationDate="2016-07-22T08:14:04.617" Score="3" Body="&lt;p&gt;At 0.1c any gas colliding with the craft's heat shield would probably undergo nuclear fusion... which would make heating effects far worse than the usual &quot;friction&quot; (compression) considerations. So I'd say any normal material would be unable to survive the process.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-07-22T08:14:04.617" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17363" PostTypeId="2" ParentId="17355" CreationDate="2016-07-22T08:49:51.640" Score="16" Body="&lt;h3&gt;Yes, it is possible.&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;While the other answers show the correct observation that the orbital period is directly derived from the semi-major axis (half the distance of the longest axis of an ellipse), that is not exactly the same as the &lt;em&gt;average&lt;/em&gt; distance. Consider the following example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/egmoIm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/egmoIm.png&quot; alt=&quot;eccentric orbit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, both the circular and the eccentric orbit have the same semi-major axis, and thus the same orbital period. The eccentric orbit is divided roughly in half, with one part closer to the Sun than the circular orbit, and one farther away (red part). However, an object moves faster in its orbit when close to the Sun, so the object is going to spend a lot less time in the inner part than in the outer part. The &lt;strong&gt;average distance&lt;/strong&gt; is then larger than that of the circular orbit. Trading a little of that average distance to get a slightly lower orbital period, your scenario is achieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While what you describe is is technically possible in this way, what you are really after is what the others have explained. This is just a remainder of how orbital mechanics can surprise you once in a while.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:47:13.163" LastActivityDate="2016-07-22T09:47:13.163" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="17364" PostTypeId="5" CreationDate="2016-07-22T09:04:44.580" Score="0" Body="&lt;p&gt;Ballistic capture is a manoeuvre using weak stability boundaries in a three (or $2&amp;lt;n$) body system to brake into orbit with minimal propellant consumption.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:04:44.580" LastActivityDate="2016-07-22T09:04:44.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17365" PostTypeId="4" CreationDate="2016-07-22T09:04:44.580" Score="0" Body="Questions regarding the use of weak stability boundaries to brake into orbit." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:04:44.580" LastActivityDate="2016-07-22T09:04:44.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17366" PostTypeId="5" CreationDate="2016-07-22T09:07:04.483" Score="0" Body="&lt;p&gt;The Hiten spacecraft is notable for using a ballistic capture to achieve a temporally orbit around the Moon.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:07:04.483" LastActivityDate="2016-07-22T09:07:04.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17367" PostTypeId="4" CreationDate="2016-07-22T09:07:04.483" Score="0" Body="Questions regarding the Japanese moon probe." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:07:04.483" LastActivityDate="2016-07-22T09:07:04.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17368" PostTypeId="5" CreationDate="2016-07-22T09:09:00.940" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:09:00.940" LastActivityDate="2016-07-22T09:09:00.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17369" PostTypeId="4" CreationDate="2016-07-22T09:09:00.940" Score="0" Body="Questions regarding the unmanned Viking Mars probes launched by NASA." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:09:00.940" LastActivityDate="2016-07-22T09:09:00.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17370" PostTypeId="5" CreationDate="2016-07-22T09:12:10.143" Score="0" Body="&lt;p&gt;In many cases, the time period where a spacecraft can be launched is limited. The time between the earliest and latest possible launch is the launch window.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:12:10.143" LastActivityDate="2016-07-22T09:12:10.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17371" PostTypeId="4" CreationDate="2016-07-22T09:12:10.143" Score="0" Body="Questions regarding the limited period of time where a specific mission plan can be used." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:12:10.143" LastActivityDate="2016-07-22T09:12:10.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17372" PostTypeId="5" CreationDate="2016-07-22T09:14:14.080" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:14:14.080" LastActivityDate="2016-07-22T09:14:14.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17373" PostTypeId="4" CreationDate="2016-07-22T09:14:14.080" Score="0" Body="Do not use this tag. It sometimes appears during migration between Stack Exchange sites." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:14:14.080" LastActivityDate="2016-07-22T09:14:14.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17374" PostTypeId="5" CreationDate="2016-07-22T09:16:36.230" Score="0" Body="&lt;p&gt;Rockets are launched vertically. The velocity in an orbit is horizontal. Sooner or later, the rocket has to turn over.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:16:36.230" LastActivityDate="2016-07-22T09:16:36.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17375" PostTypeId="4" CreationDate="2016-07-22T09:16:36.230" Score="0" Body="Questions regarding the tilting of a rocket to gain horizontal velocity once the drag is low enough." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:16:36.230" LastActivityDate="2016-07-22T09:16:36.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17376" PostTypeId="5" CreationDate="2016-07-22T09:18:17.823" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:18:17.823" LastActivityDate="2016-07-22T09:18:17.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17377" PostTypeId="4" CreationDate="2016-07-22T09:18:17.823" Score="0" Body="An aircraft using a vertically mounted rotor to stay in the air, giving it the ability to hoover in contrast to a plane." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:18:17.823" LastActivityDate="2016-07-22T09:18:17.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17378" PostTypeId="5" CreationDate="2016-07-22T09:19:09.653" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:19:09.653" LastActivityDate="2016-07-22T09:19:09.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17379" PostTypeId="4" CreationDate="2016-07-22T09:19:09.653" Score="0" Body="Questions regarding pumping of fuel between stages." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:19:09.653" LastActivityDate="2016-07-22T09:19:09.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17380" PostTypeId="5" CreationDate="2016-07-22T09:20:35.623" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:20:35.623" LastActivityDate="2016-07-22T09:20:35.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17381" PostTypeId="4" CreationDate="2016-07-22T09:20:35.623" Score="0" Body="A material used in high heat flux locations (e.g., atmospheric entry, rocket nozzles) that ablates and carries away the heat/energy of the flow protecting the structures behind it." OwnerUserId="8693" LastEditorUserId="40257" LastEditDate="2022-04-27T12:18:23.680" LastActivityDate="2022-04-27T12:18:23.680" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17382" PostTypeId="5" CreationDate="2016-07-22T09:22:02.560" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:22:02.560" LastActivityDate="2016-07-22T09:22:02.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17383" PostTypeId="4" CreationDate="2016-07-22T09:22:02.560" Score="0" Body="Questions regarding co-orbital orbits in a three body system." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:22:02.560" LastActivityDate="2016-07-22T09:22:02.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17384" PostTypeId="5" CreationDate="2016-07-22T09:23:46.090" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:23:46.090" LastActivityDate="2016-07-22T09:23:46.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17385" PostTypeId="4" CreationDate="2016-07-22T09:23:46.090" Score="0" Body="Questions regarding the gigantic sea based rocket concept to achieve economy of scale." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:23:46.090" LastActivityDate="2016-07-22T09:23:46.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17386" PostTypeId="5" CreationDate="2016-07-22T09:27:07.403" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:27:07.403" LastActivityDate="2016-07-22T09:27:07.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17387" PostTypeId="4" CreationDate="2016-07-22T09:27:07.403" Score="0" Body="Questions about changing the orbital plane of a trajectory." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:27:07.403" LastActivityDate="2016-07-22T09:27:07.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17388" PostTypeId="5" CreationDate="2016-07-22T09:33:03.700" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:33:03.700" LastActivityDate="2016-07-22T09:33:03.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17389" PostTypeId="4" CreationDate="2016-07-22T09:33:03.700" Score="0" Body="The point during the launch where the aero-dynamical stress on the launch vehicle is highest." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:33:03.700" LastActivityDate="2016-07-22T09:33:03.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17390" PostTypeId="5" CreationDate="2016-07-22T09:34:30.840" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:34:30.840" LastActivityDate="2016-07-22T09:34:30.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17391" PostTypeId="4" CreationDate="2016-07-22T09:34:30.840" Score="0" Body="Questions regarding acceleration - the rate and/or direction by which an object's speed or velocity vector changes with respect to time." OwnerUserId="8693" LastEditorUserId="12102" LastEditDate="2019-12-19T06:14:40.103" LastActivityDate="2019-12-19T06:14:40.103" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17392" PostTypeId="5" CreationDate="2016-07-22T09:42:07.963" Score="0" Body="&lt;p&gt;Space stations continuously need new supplies. An example of a resupply spacecraft is the Progress spacecraft supplying the ISS.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:42:07.963" LastActivityDate="2016-07-22T09:42:07.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17393" PostTypeId="4" CreationDate="2016-07-22T09:42:07.963" Score="0" Body="Questions about bringing new supplies and components to a location." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:42:07.963" LastActivityDate="2016-07-22T09:42:07.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17394" PostTypeId="5" CreationDate="2016-07-22T09:50:48.773" Score="0" Body="&lt;p&gt;Some distances in the universe are huge, for example the distances between stars. To cross them, a spacecraft must have a very high velocity. Relativistic theory must be applied to properly describe the behaviour of such spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:50:48.773" LastActivityDate="2016-07-22T09:50:48.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17395" PostTypeId="4" CreationDate="2016-07-22T09:50:48.773" Score="0" Body="Questions regarding spacecraft with a velocity some significant fraction of *c*. (The speed of light)" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:50:48.773" LastActivityDate="2016-07-22T09:50:48.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17396" PostTypeId="5" CreationDate="2016-07-22T09:53:00.603" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:53:00.603" LastActivityDate="2016-07-22T09:53:00.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17397" PostTypeId="4" CreationDate="2016-07-22T09:53:00.603" Score="0" Body="Questions about discovering or observing something. Often used about detecting a new object, like an asteroid." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:53:00.603" LastActivityDate="2016-07-22T09:53:00.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17398" PostTypeId="5" CreationDate="2016-07-22T09:54:52.993" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:54:52.993" LastActivityDate="2016-07-22T09:54:52.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17399" PostTypeId="4" CreationDate="2016-07-22T09:54:52.993" Score="0" Body="Questions about finding old accounts, informations and observations if they are still available." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:54:52.993" LastActivityDate="2016-07-22T09:54:52.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17400" PostTypeId="5" CreationDate="2016-07-22T09:56:09.853" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:56:09.853" LastActivityDate="2016-07-22T09:56:09.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17401" PostTypeId="4" CreationDate="2016-07-22T09:56:09.853" Score="0" Body="Questions about the creation process for planets. For questions not directly related to the exploration of space, try Astronomy SE." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:56:09.853" LastActivityDate="2016-07-22T09:56:09.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17402" PostTypeId="5" CreationDate="2016-07-22T09:57:06.227" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:57:06.227" LastActivityDate="2016-07-22T09:57:06.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17403" PostTypeId="4" CreationDate="2016-07-22T09:57:06.227" Score="0" Body="Questions about restoring equipment for reuse after a mission." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:57:06.227" LastActivityDate="2016-07-22T09:57:06.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17404" PostTypeId="5" CreationDate="2016-07-22T09:58:39.963" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:58:39.963" LastActivityDate="2016-07-22T09:58:39.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17405" PostTypeId="4" CreationDate="2016-07-22T09:58:39.963" Score="0" Body="Questions about manoeuvres executed in interplanetary space, far from a planetary gravity well." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T09:58:39.963" LastActivityDate="2016-07-22T09:58:39.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17406" PostTypeId="5" CreationDate="2016-07-22T10:00:20.180" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T10:00:20.180" LastActivityDate="2016-07-22T10:00:20.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17407" PostTypeId="4" CreationDate="2016-07-22T10:00:20.180" Score="0" Body="Questions about the Japanese lunar orbiter Hagoromo." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T10:00:20.180" LastActivityDate="2016-07-22T10:00:20.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17408" PostTypeId="5" CreationDate="2016-07-22T10:03:25.057" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T10:03:25.057" LastActivityDate="2016-07-22T10:03:25.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17409" PostTypeId="4" CreationDate="2016-07-22T10:03:25.057" Score="0" Body="Questions about instruments to collect data from the environment." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T10:03:25.057" LastActivityDate="2016-07-22T10:03:25.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17410" PostTypeId="2" ParentId="17351" CreationDate="2016-07-22T10:21:43.790" Score="3" Body="&lt;p&gt;The scale height of an atmosphere is calculated as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$H=\frac{kT}{Mg}$$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$k$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Boltzmann_constant&quot; rel=&quot;nofollow&quot;&gt;Boltzmann constant&lt;/a&gt;. ($1.38 \cdot 10^{−23} \frac{J}{K}$)&lt;/li&gt;&#xA;&lt;li&gt;$g$ is the acceleration due to gravity.&lt;/li&gt;&#xA;&lt;li&gt;$T$ is the absolute temperature.&lt;/li&gt;&#xA;&lt;li&gt;$M$ is the average molecular mass.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Of the above factors, only the average molecular mass, $M$, changes if you change the composition. For example, oxygen is lighter than carbon dioxide, giving a larger scale height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Be aware that changing the composition of the atmosphere also affects things like the green house effect, possibly also changing the absolute temperature, $T$.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-07-22T10:21:43.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17411" PostTypeId="2" ParentId="17260" CreationDate="2016-07-22T11:00:50.473" Score="1" Body="&lt;p&gt;A bit dated (2011) &lt;a href=&quot;http://europe.chinadaily.com.cn/china/2011-04/26/content_12395890.htm&quot; rel=&quot;nofollow&quot;&gt;article&lt;/a&gt; from China Daily states that the launch of Tiangong-3 is planned around 2020-2022, stating that this is information from The China Manned Space Engineering Office. The guardian had an &lt;a href=&quot;https://www.theguardian.com/world/2011/apr/26/china-space-station-tiangong&quot; rel=&quot;nofollow&quot;&gt;article from around the same time&lt;/a&gt;, giving a similar time range. A &lt;a href=&quot;http://www.spacedaily.com/reports/Chinas_Space_Station_is_Still_On_Track_999.html&quot; rel=&quot;nofollow&quot;&gt;more recent article&lt;/a&gt; from Space Daily states that the plan is not changed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some core differences between Tiangong-3 and the ISS is that while the ISS is an international project using modules from several countries, the Russia and the US being the main partners, China plans to build their station entirely by themselves. Structurally, the station is going to look more like &lt;a href=&quot;https://en.wikipedia.org/wiki/Mir&quot; rel=&quot;nofollow&quot;&gt;Mir&lt;/a&gt;, with modules clustered in a cross around a single central docking node. In contrast, the ISS is built around several nodes and beams.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-07-22T11:00:50.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17412" PostTypeId="2" ParentId="17359" CreationDate="2016-07-22T11:22:45.043" Score="4" Body="&lt;p&gt;NASA has a &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/OPD_docs/NPI_8020_7_.doc&quot; rel=&quot;nofollow noreferrer&quot;&gt;protocol for this&lt;/a&gt;, and SpaceX will follow it.  As 1337Joe notes, the Outer Space Treaty applies:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Article VI of the Outer Space Treaty: &amp;quot;the activities of&#xA;non-governmental entities in outer space, including the Moon and other&#xA;celestial bodies, shall require authorization and continuing&#xA;supervision by the appropriate State Party to the Treaty&amp;quot;. I don't&#xA;know that they have something in place, but by treaty they have the&#xA;responsibility to supervise operations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Things to consider are forward contamination (bring stuff to Mars) and backwards contamination, (bring stuff back to Earth).&lt;/p&gt;&#xA;&lt;p&gt;In terms of forward, that ship is gonna sail pretty early. Sure a probe can be sterilized, kept under a fairing during launch, heated during reentry but once we get to the point of people going to Mars that concern is basically over. SpaceX's goal is to get humans to Mars, so the only relevance of forward contamination will be in their first few unmanned missions.&lt;/p&gt;&#xA;&lt;p&gt;The same is actually true for reverse contamination once someone comes home. Sure a sample return mission could follow protocol and do all the required silly steps.&lt;/p&gt;&#xA;&lt;p&gt;In many ways it is a moot point overall in the world of SpaceX except for maybe the first mission or two.&lt;/p&gt;&#xA;&lt;p&gt;We know they plan a Red Dragon mission in 2018. This will be a Dragon capsule. The contents and interior can easily be sterilized as needed. The exterior is harder, since a Dragon launches with no fairing, so no way to sterilize the outside and then protect it until launch. However, ascent is NOT a fun environment for microbes, and the descent into Mars via retro-propulsion is pretty tough to survive as well. That may not be sufficient for NASA so they may have to take additional steps.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2020-08-19T15:10:49.530" LastActivityDate="2020-08-19T15:10:49.530" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="17413" PostTypeId="5" CreationDate="2016-07-22T12:24:35.210" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T12:24:35.210" LastActivityDate="2016-07-22T12:24:35.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17414" PostTypeId="4" CreationDate="2016-07-22T12:24:35.210" Score="0" Body="Questions regarding how orbits related by simple fractions interact." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-22T12:24:35.210" LastActivityDate="2016-07-22T12:24:35.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17415" PostTypeId="2" ParentId="17342" CreationDate="2016-07-22T13:05:45.730" Score="1" Body="&lt;p&gt;The only answer for this is to build a simulator, and test out a variety of things. Here's a few things to keep in mind:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A lower orbit is a faster orbit, which will allow you to catch up to your target. A higher orbit is slower, so you will slow down to intercept the object.&lt;/li&gt;&#xA;&lt;li&gt;It seems you want a high speed intercept, with some given initial delta v, and nothing else. That seems highly unlikely to work, as you'd have to be spot on, which is very difficult to achieve.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I would take and fire a missile, see how far it is away from the target when the intercept occurs, and then move it in time until it intersects with the target. Then try firing at different angles to see what happens. Iterate until you get the best solution.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-22T13:05:45.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17416" PostTypeId="2" ParentId="17349" CreationDate="2016-07-22T13:18:44.527" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;As I understand it, wrongly or not, temperatures in the sun's outer layers are about 6000K as opposed to the upwards of 100,000K ablative temperature of a heat shield during aerobraking. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The relevant temperature is not the temperature of the star's atmosphere, it's the temperature to which everything is heated by friction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you compare 0.1c with the kinds of velocities we have for interplanetary probes (roughly on the order of magnitude of escape velocity from the solar system), it's about 100 times greater. When you increase the velocity by a factor of 100, you increase the kinetic energy by a factor of $10^4$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second huge factor working against you is that the time is very short. The distance over which your probe has to stop is at most on the order of the diameter of the star, and in reality it's going to be a lot less, since you don't have any way to curve your trajectory to match the curvature of the star. (If you had a way to curve your trajectory like that, it would require a large acceleration, which is what we're trying to accomplish in the first place.) Even taking the stopping distance optimistically to be the diameter of the sun, the time to stop comes out to about 20 seconds, which is extremely short.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you have the combination of these two factors: a huge amount of heat being dissipated, and a short time to get rid of the heat. This is a huge amount of power, and convection simply isn't going to be a fast enough process to get rid of it. If you want to make the maneuver work, you're going to need some exotic and extremely efficient method of getting rid of the heat.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;would using this method of deceleration be in some way more attractive than an aerobraking or magnetoshell aerocapture around a planet&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes. The diameter of the sun is about 100 times greater than the diameter of the earth, or 10 times the diameter of a gas giant. That means that the time considerations get much worse if you try to brake using a planet's atmosphere rather than a star's. By the same logic, a supergiant star would probably be better than a main-sequence star like the sun.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Plot synopsis: a ship traveling at sub-light speed is too low on fuel to decelerate in time to keep from blowing through and past its target solar system. Radical measures are considered.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This makes it sound like you have in mind a crewed ship. To get from 0.1c to rest in a distance equal to the diameter of the sun, you would need a deceleration of 30,000 gees. That would kill the crew.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2016-07-22T14:03:50.060" LastActivityDate="2016-07-22T14:03:50.060" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17417" PostTypeId="1" AcceptedAnswerId="17418" CreationDate="2016-07-22T13:25:01.117" Score="7" ViewCount="1326" Body="&lt;p&gt;I understand that by sending objects to a specific altitude you can get an object to orbit over a specific area of the Earth. What I have a tough time grasping is how that object then follows the shifting tilt of the planet? Does the tilt shift the entire gravitational field and pull everything with it? Are boosters used?&lt;/p&gt;&#xA;" OwnerUserId="12101" LastActivityDate="2016-07-22T13:49:42.727" Title="How do geosynchronous orbits follow the tilt of the earth?" Tags="|orbital-mechanics|orbit|geosynchronous|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17418" PostTypeId="2" ParentId="17417" CreationDate="2016-07-22T13:49:42.727" Score="8" Body="&lt;p&gt;Geostationary satellites don't. Or more actually - the tilt remains the same in relation to &quot;distant stars&quot; and the seasons on Earth - the latitude at which Sun is in zenith - changes due to Earth circling the Sun, while its tilt remains fixed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, the GEO satellites' orbital plane remains just the same in relation to &quot;distant stars&quot; - and as result, to Earth (identical to Earth's equator plane) - and so, it follows the tilt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't follow &lt;a href=&quot;https://en.wikipedia.org/wiki/Axial_precession&quot;&gt;Earth's axial precession&lt;/a&gt; which is the actual change of its tilt, but with one precession period of 26,000 years, the necessary corrections using &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_control_system&quot;&gt;RCS&lt;/a&gt; are minimal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, for &lt;em&gt;geosynchronous&lt;/em&gt; satellites - these remain at about the same altitude, but aren't necessarily in the same plane as Earth's equator. In their case the plane remains fixed as well, but being shifted so, the satellites travel north-south and back, with one travel cycle per day.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-22T13:49:42.727" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17419" PostTypeId="1" CreationDate="2016-07-22T14:23:47.497" Score="11" ViewCount="5464" Body="&lt;p&gt;What is the most distant photo taken of our sun? I am guessing it may be from Voyager 1, but can't find an actual photo anywhere.&lt;/p&gt;&#xA;" OwnerUserId="16364" LastActivityDate="2016-07-23T10:22:24.387" Title="Most distant photo of the sun?" Tags="|voyager|the-sun|photography|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17420" PostTypeId="2" ParentId="17419" CreationDate="2016-07-22T14:39:35.480" Score="9" Body="&lt;p&gt;This was the famous &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA00451&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1 &quot;Family Portrait&quot;&lt;/a&gt; The distance was just over &lt;a href=&quot;https://en.wikipedia.org/wiki/Family_Portrait_(Voyager)&quot; rel=&quot;nofollow noreferrer&quot;&gt;40 AU.&lt;/a&gt; This is actually the furthest image that has ever been taken away from Earth; after this the camera was shut off. Voyager 2 never attempted such a photo. New Horizons isn't that far out yet, although it will take pictures from further away, and I suspect that after all is said and done it might try to do a similar family portrait... but not until all of the data is received from its extended mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bfNeM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bfNeM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="4703" LastEditDate="2016-07-23T01:03:22.923" LastActivityDate="2016-07-23T01:03:22.923" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17421" PostTypeId="2" ParentId="17419" CreationDate="2016-07-22T14:39:54.827" Score="12" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JoOrl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JoOrl.jpg&quot; alt=&quot;Sun from Voyager 1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 1 took an image of the sun on February 14, 1990 at a distance of 6 billion kilometers. The famous &lt;a href=&quot;https://en.wikipedia.org/wiki/Pale_Blue_Dot&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pale Blue Dot&lt;/a&gt; was captured at the same time and is superimposed here as well as a narrow angle view of Venus.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This color image of the sun, Earth and Venus was taken by the Voyager 1 spacecraft Feb. 14, 1990, when it was approximately 32 degrees above the plane of the ecliptic and at a slant-range distance of approximately 4 billion miles. It is the first -- and may be the only -- time that we will ever see our solar system from such a vantage point. The image is a portion of a wide-angle image containing the sun and the region of space where the Earth and Venus were at the time with two narrow-angle pictures centered on each planet. The wide-angle was taken with the camera's darkest filter (a methane absorption band), and the shortest possible exposure (5 thousandths of a second) to avoid saturating the camera's vidicon tube with scattered sunlight. The sun is not large in the sky as seen from Voyager's perspective at the edge of the solar system but is still eight million times brighter than the brightest star in Earth's sky, Sirius. The image of the sun you see is far larger than the actual dimension of the solar disk. The result of the brightness is a bright burned out image with multiple reflections from the optics in the camera. The &quot;rays&quot; around the sun are a diffraction pattern of the calibration lamp which is mounted in front of the wide angle lens. The two narrow-angle frames containing the images of the Earth and Venus have been digitally mosaiced into the wide-angle image at the appropriate scale.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA00450&quot; rel=&quot;nofollow noreferrer&quot;&gt;PIA00450: Solar System Portrait - View of the Sun, Earth and Venus - NASA&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="4558" LastEditDate="2016-07-23T10:22:24.387" LastActivityDate="2016-07-23T10:22:24.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17422" PostTypeId="2" ParentId="17359" CreationDate="2016-07-22T18:09:16.237" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;As private companies, do they fall under the jurisdiction of the United Nations Outer Space Treaty?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All of the leading private space companies are incorporated in the U.S. (e.g., SpaceX, Orbital ATK, Sierra Nevada, Blue Origin, ...). They are subject to state and national law, and also to any international treaties to which the U.S. are party. So yes, these companies fall under the jurisdiction of the Outer Space Treaty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of these companies want to reincorporate outside the U.S.; doing so would risk their contracts with NASA and the DoD, risk their ability to access classified and sensitive but unclassified information, and risk having their intellectual property confiscated on the basis that it is based on U.S. classified / sensitive but unclassified information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right now, there's no risk of this happenning; those companies need access to U.S. launch facilities. The future may be different; there's already a bit of tension in the deliberateness of how the U.S. government operates and the speed at which those private space companies would rather operate. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is there any way the international community could prosecute them for contaminating another world?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's new territory. However, companies that have committed crimes against humanity have been broken up and have had key personnel jailed by international tribunals. Companies that violate international treaties in a lesser way are usually dealt with in the country in which they are incorporated or have their headquarters.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The only source I could find was a statement from the senior director of communications saying &quot;SpaceX takes planetary protection very seriously&quot;, but this yields almost no useful information. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;SpaceX apparently will take planetary protection very seriously for its unmanned missions to Mars (e.g., setting up protocols, ensuring the vehicle is assembled in clean room conditions, performing additional sterilization actions between Earth and Mars). However, SpaceX is very serious about sending humans to Mars. Humans on Mars is game over with respect to planetary protection. How SpaceX and NASA will address this is a problem that is apparently being left to the future.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-07-22T18:09:16.237" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17423" PostTypeId="1" AcceptedAnswerId="17426" CreationDate="2016-07-22T19:27:00.317" Score="5" ViewCount="1432" Body="&lt;p&gt;I was playing with a rocket design simulator, and I noticed that if I set Earth's gravity to be much higher than it is today, it became impossible to design a rocket that could reach escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this a real phenomenon?  Like if Earth was a larger planet, we might not have reached the moon by now?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or am I just horrible at rocket design?  :)&lt;/p&gt;&#xA;" OwnerUserId="16366" LastEditorUserId="6816" LastEditDate="2016-07-22T19:32:50.030" LastActivityDate="2016-07-23T16:30:48.667" Title="Is space travel possible from a large planet?" Tags="|launch|gravity|escape-velocity|" AnswerCount="1" CommentCount="2" ClosedDate="2016-07-23T13:27:09.427" ContentLicense="CC BY-SA 3.0" />
  <row Id="17424" PostTypeId="1" CreationDate="2016-07-22T19:35:14.300" Score="9" ViewCount="726" Body="&lt;p&gt;I finally figured out that the nitrogen gas is used for the MELFI (minus eighty freezers). I can't figure out what the argon would be used for, though I can see how helium might be useful for refrigeration also. But what are these actually used for, if anything?&lt;/p&gt;&#xA;" OwnerUserId="869" LastEditorUserId="6944" LastEditDate="2021-03-28T18:19:33.320" LastActivityDate="2024-02-01T13:08:17.693" Title="What is the purpose of the helium and argon gas supplies on ISS?" Tags="|iss|helium|gasses|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17426" PostTypeId="2" ParentId="17423" CreationDate="2016-07-22T20:06:28.900" Score="5" Body="&lt;p&gt;Yes, if the planet is sufficiently large we would not be able to reach orbit &lt;strong&gt;with our current available materials and engineering techniques / technologies&lt;/strong&gt;, this is partial due to the &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow&quot;&gt;tyranny of the rocket equation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, &lt;strong&gt;at least using rockets for transport&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source:&#xA;&lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the above quote, a greater radius is used, but the greater radius means more surface gravity, so it talks about the same thing.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Obviuosly there are other technologies that could still allow for space travel. The most simple way to increase delta-v on a rocket is staging. However, there are still practical limits for staging. One problem is thrust, The F1 is the biggest rocket engine ever built, and some studies suggest that there is a practical limit of 1.5x the size of an F1 above which engines can't be constructed. Morever you can't stack vertically forever, since the lowest stage could not support the weight of the whole stack. Adding stages radial is also limit, the sheer stress would at some point break the whole rocket apart. So while this might shift the numbers a bit, I don't think it has significant influence on the spirit of the question. It doesn't really matter wether Earth needs to be 1.5x or 2x as big, at some point we can't reach orbit anymore with what we currently have.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other speculative technologies - rail launchers, launch loops etc. that have been research, but those aren't technologies that are currently in use. And they have mainly been researched for application on Earth, so I would be hesistant to deem them appropriate for higher gravity / radius without researching their limits, first (there is a good possibility that the materials can't handle the incresed velocities / stresses).&lt;/p&gt;&#xA;" OwnerUserId="13867" LastEditorUserId="13867" LastEditDate="2016-07-23T08:53:15.050" LastActivityDate="2016-07-23T08:53:15.050" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17427" PostTypeId="2" ParentId="17353" CreationDate="2016-07-22T20:28:42.187" Score="8" Body="&lt;p&gt;You are asking multiple questions at once.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Can it fly a sat /a rocket/a UAV?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, sort of.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Core Flight Executive is a portable, platform independent embedded system framework developed by NASA Goddard Space Flight Center. This framework is used as the &lt;em&gt;basis for the flight software for satellite data systems and instruments&lt;/em&gt;, but can be used on other embedded systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(highlights by me)&lt;br&gt;&#xA;Source: &lt;a href=&quot;https://sourceforge.net/projects/coreflightexec/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://sourceforge.net/projects/coreflightexec/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There exist other packages for it, as it is a pluggable framework, that can do different tasks needed for different spacecraft. Remember, its a &lt;em&gt;framework&lt;/em&gt;, not a completely ready-to-use package that you simply drop into your spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What are its uses? / Where is it really used?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA's cFS has been used in a number of missions, starting with the Lunar Reconnaissance Orbiter which helped fund initial development (&lt;a href=&quot;http://flightsoftware.jhuapl.edu/files/2014/Presentations/Day-1/Session-3/1-FSW2014_Workshop_McComas_CFS.pdf&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;FSW2014 slides (pdf)&quot;&gt;source, pdf&lt;/a&gt;). It has also been used in Moon Express, the prototype robotic lander Mighty Eagle, the recently launched Magnetospheric Multiscale (MMS), and will be used on the upcoming Solar Probe Plus. For more uses, see &lt;a href=&quot;https://cfs.gsfc.nasa.gov/Applications.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://cfs.gsfc.nasa.gov/Applications.html&lt;/a&gt;  and the &quot;Featured Missions&quot; sidebar.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What advantage does it bring for the developer community?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For the normal developer community, not much, unless you have spare money to build a spacecraft and buy a launch for it. It might be possible to study the software to learn about design techniques, but this will be of little interest for the normal software developer.&lt;/p&gt;&#xA;" OwnerUserId="13867" LastEditorUserId="4686" LastEditDate="2017-04-28T12:20:14.883" LastActivityDate="2017-04-28T12:20:14.883" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17428" PostTypeId="1" CreationDate="2016-07-22T21:40:42.113" Score="2" ViewCount="54" Body="&lt;p&gt;If a neutron star or black hole was detected, say, only one parsec from here, would it help space exploration?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could it be used as a gravity assisted flyby portal for interstellar space flight? I suppose that it depends on its speed relative to us and the star we'd like to travel to. But some star remnants have very high velocities because of their violent formation history. And wouldn't a close binary neutron star would be a gold mine for gravity assists.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="10273" LastEditDate="2019-01-06T14:17:24.183" LastActivityDate="2019-01-06T14:17:24.183" Title="How could a local compact star remnant be used for space exploration?" Tags="|interstellar-travel|black-hole|" AnswerCount="0" CommentCount="3" ClosedDate="2016-07-23T00:01:33.840" ContentLicense="CC BY-SA 3.0" />
  <row Id="17429" PostTypeId="1" CreationDate="2016-07-23T02:39:49.757" Score="5" ViewCount="495" Body="&lt;p&gt;&lt;em&gt;I'm working on &lt;a href=&quot;https://space.stackexchange.com/q/14317/12102&quot;&gt;this question&lt;/a&gt;. This is the farthest (known) communications link humans have ever engineered - an one node is running in deep space on 40 year old technology.&lt;/em&gt; Pretty amazing!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;em&gt;DESCANSO Design and Performance Summary Series&lt;/em&gt;, the NASA document &lt;a href=&quot;http://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Article 4 - Voyager Telecommunications&lt;/a&gt; has a nice block diagram as Figure 3.1 on page 13. A screen shot is shown below. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Where can I see the frequency response of the S-band feed horn &lt;em&gt;at the primary focus&lt;/em&gt;, and the S-band receiver &lt;a href=&quot;https://en.wikipedia.org/wiki/RF_front_end&quot; rel=&quot;nofollow noreferrer&quot;&gt;front end&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Intermediate_frequency&quot; rel=&quot;nofollow noreferrer&quot;&gt;IF&lt;/a&gt; engineering specs (pass band(s), out-of-band rejection, etc.) and schematic? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm primarily interested in the parts that have to cope with, and remove most of the powerful, broad-band radio noise from the sun &lt;em&gt;before any of the final demodulation/correlation.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have some representative values that would be great - if you can go ahead and answer the other question linked above - related to rejection of noise from the Sun - that is even greater! But please give me a source, link, or citation or two so I can go reading further. I'm really amazed by this spacecraft and the engineering that went into it - thanks!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QbVXt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QbVXt.png&quot; alt=&quot;Voyager Radio System - NASA&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager Telecommunications&lt;/a&gt; - click for full size.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-07-30T12:07:40.933" Title="Frequency responses of Voyager's S-band high gain antenna's feed-horn and receiver front end &amp; IF?" Tags="|communication|voyager|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17430" PostTypeId="1" AcceptedAnswerId="17433" CreationDate="2016-07-23T03:17:55.113" Score="6" ViewCount="985" Body="&lt;p&gt;I just looked at the Deep Space Network site DSN Now at &lt;a href=&quot;https://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;noreferrer&quot;&gt;https://eyes.nasa.gov/dsn/dsn.html&lt;/a&gt; and I saw that both dish numbers 25 and 26 are receiving a 8.42 GHz downlink signal from Voyager 1. Both dishes &lt;a href=&quot;https://en.wikipedia.org/wiki/Goldstone_Deep_Space_Communications_Complex&quot; rel=&quot;noreferrer&quot;&gt;are 34 meters in diameter&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After watching for 20 more minutes, I can see that while both signal strengths drift by many dB, the CARRIER signal in number 25 always remains about 6dB weaker than the DATA signal in number 26.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does DSN sometimes uses two dishes at the same time to receive Voyager-1?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i8ZLy.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i8ZLy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screen shot montage from NASA's DSN Now page at about 02:52 UTC, July 23, 2016.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2021-05-28T14:11:24.743" Title="Why does DSN sometimes uses two dishes at the same time to receive Voyager-1?" Tags="|voyager|deep-space-network|doppler-ranging|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17431" PostTypeId="2" ParentId="16953" CreationDate="2016-07-23T03:55:25.370" Score="3" Body="&lt;p&gt;Seems to be a mistake based on both common sense and backup from @OrganicMarble's &lt;a href=&quot;https://space.stackexchange.com/questions/16953/#comment44571_16953&quot;&gt;comment above&lt;/a&gt; - not to imply that the later is not also a subset of the former :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently, not everything on the internet is always correct.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-07-23T03:55:25.370" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17432" PostTypeId="1" AcceptedAnswerId="17451" CreationDate="2016-07-23T04:11:06.740" Score="12" ViewCount="867" Body="&lt;p&gt;There is lots of easily accessed information online detailing in what order different ISS modules were added, and who built them.  However, what's more difficult to find, possibly buried under the other stuff, is how exactly the modules are attached.  Specifically, I am interested in how they pressurized each module.  Were the modules pressurized on the ground, and flown up to the station without being opened until they were attached?  Or did they attach them, then fill them with atmosphere from tanks?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did find &lt;a href=&quot;https://www.youtube.com/watch?v=yRqUPjl3tTQ&quot;&gt;this interesting video&lt;/a&gt; showing the order all the compartments were added in.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2024-01-16T15:58:05.057" Title="How were the ISS compartments attached and pressurized?" Tags="|iss|orbital-assembly|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17433" PostTypeId="2" ParentId="17430" CreationDate="2016-07-23T05:06:11.657" Score="8" Body="&lt;p&gt;If you look at the &quot;more detail&quot; section of the DSN status web page, you'll see that 25 and 26 are operating in &quot;array mode&quot;. With two antennas receiving, twice as much incoming signal is being caught, and because the dishes are physically separated, some local noise sources will appear in one receiver and not the other, making it easier to discriminate signal from noise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I note that both dishes are shown as receiving carrier, but only one is shown as receiving data. I suspect that this and the higher SNR figure is a presentation artifact, and both dishes are in reality receiving similar signal strength, but the combined signal strength is being shown for 26. That's just a guess, though, and I would expect a 3db improvement for a pair of antennas, not a 6db, so there may well be something else going on.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-07-23T05:06:11.657" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17434" PostTypeId="1" CreationDate="2016-07-23T08:06:47.697" Score="3" ViewCount="116" Body="&lt;p&gt;Is in general possible to download data from space missions? I mean &quot;raw&quot; data from sensors instead of the few artistic pictures released to public. Or do scientists keep those files in their secret vaults?&lt;/p&gt;&#xA;" OwnerUserId="16372" LastEditorUserId="25" LastEditDate="2016-07-23T11:53:25.287" LastActivityDate="2016-07-23T12:11:02.117" Title="Data from space missions" Tags="|planetary-science|raw-data|scientific-data|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17435" PostTypeId="5" CreationDate="2016-07-23T08:15:02.253" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-07-23T08:15:02.253" LastActivityDate="2016-07-23T08:15:02.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17436" PostTypeId="4" CreationDate="2016-07-23T08:15:02.253" Score="0" Body="Unprocessed information collected from instruments during a space mission." OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2016-07-23T11:51:13.850" LastActivityDate="2016-07-23T11:51:13.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17438" PostTypeId="1" CreationDate="2016-07-23T10:12:31.027" Score="1" ViewCount="133" Body="&lt;p&gt;A new type of &lt;a href=&quot;http://tny.im/5dQ&quot; rel=&quot;nofollow&quot;&gt;radar&lt;/a&gt;. How will &lt;a href=&quot;http://rostec.ru/en/research/tecnology/4517645&quot; rel=&quot;nofollow&quot;&gt;photonic radar&lt;/a&gt; behave in outer space? What will be challenges faced by such &lt;a href=&quot;http://lofi.phys.org/news/2014-03-fully-photonics-based-laser.html&quot; rel=&quot;nofollow&quot;&gt;radar&lt;/a&gt; ? How will it differ from pesa on Soyuz-MS ? Will such radar be termed as space weapon and  what resolution will such radar provide for earth observation ?&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="16355" LastEditDate="2016-07-31T02:31:56.120" LastActivityDate="2016-07-31T02:31:56.120" Title="Fielding Photonic radar in outer space" Tags="|radar|" AnswerCount="1" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="17439" PostTypeId="1" AcceptedAnswerId="17446" CreationDate="2016-07-23T11:25:34.063" Score="26" ViewCount="3507" Body="&lt;p&gt;The cosmonauts from &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_T-13&quot;&gt;Soyuz T-13&lt;/a&gt; mission &quot;wore winter garb, including fur-lined hats, as they entered the station.&quot; &quot;Frost covered the walls and apparatus. &quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, the heat balance of Salyut 7 was definitely negative, requiring active heating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, ISS requires active cooling, and a lot of it, suffering from surplus heat, the radiators and cooling systems being absolutely vital to the station's existence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would ISS freeze over as well, without power? Or do the two designs differ in some significant way that changes their &quot;ambient&quot; temperature?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2016-07-23T11:52:04.360" LastActivityDate="2016-07-25T00:16:04.610" Title="Why did Salyut 7 freeze over, while ISS requires massive cooling system?" Tags="|iss|space-station|thermal-control|salyut|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17440" PostTypeId="2" ParentId="17439" CreationDate="2016-07-23T11:51:20.513" Score="9" Body="&lt;p&gt;A space station must expel the heat from its electronics. This heat can be considerable, which usually requires large radiators. If the station is dead, then the heat is expelled, without having the heat from the electronics to power it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, Salyut 7 was designed to have constant hot water available for the astronauts. The heat from this needed to be removed, and would have been accounted for in the design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind the ISS requires 75-90 kW of power to function. This power will almost entirely end up as heat in the station. The ISS insulation is somewhat better than earlier stations as well, which leads to more trapped heat. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give you an idea, the &lt;a href=&quot;http://www.space.com/3450-nasa-engineers-ponder-temporary-iss-power-loss.html&quot; rel=&quot;noreferrer&quot;&gt;ISS has partially lost power for an entire day&lt;/a&gt;; in fact, several times. During those times, some of the internal heaters are shut off. The &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2001/ast21mar_1/&quot; rel=&quot;noreferrer&quot;&gt;environment is extremely complex&lt;/a&gt;; without proper thermal management there would be a huge temperature swing!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="4703" LastEditDate="2016-07-25T00:16:04.610" LastActivityDate="2016-07-25T00:16:04.610" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17441" PostTypeId="2" ParentId="17434" CreationDate="2016-07-23T11:57:15.480" Score="5" Body="&lt;p&gt;NASA requires all missions to release the raw data on a regular basis, in particular to the &lt;a href=&quot;https://pds.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Planetary Data System&lt;/a&gt;. ESA releases their data to the &lt;a href=&quot;http://www.rssd.esa.int/index.php?project=PSA&quot; rel=&quot;noreferrer&quot;&gt;Planetary Science Archive&lt;/a&gt;. Russia releases it's data to the &lt;a href=&quot;http://www.iki.rssi.ru/ssda.html&quot; rel=&quot;noreferrer&quot;&gt;Solar System Data Archive&lt;/a&gt;. Other organizations are found under the umbrella &quot;&lt;a href=&quot;https://planetarydata.org/&quot; rel=&quot;noreferrer&quot;&gt;International Planetary Data Alliance&lt;/a&gt;&quot;, which has a &lt;a href=&quot;http://meetingorganizer.copernicus.org/EPSC2013/EPSC2013-299.pdf&quot; rel=&quot;noreferrer&quot;&gt;whitepaper here&lt;/a&gt; giving the names of all of the organizations with such data.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-23T11:57:15.480" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17442" PostTypeId="2" ParentId="17434" CreationDate="2016-07-23T11:57:40.803" Score="1" Body="&lt;p&gt;Certainly it is &quot;possible to download data from space missions&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A great example is the FUNcube program which is sponsored by AMSAT-UK.  According to their webpage 'More than 900 stations, including many at schools and colleges around the world, have received and decoded the telemetry.'  I'd suggest taking a look at URL  &lt;a href=&quot;https://funcube.org.uk/&quot; rel=&quot;nofollow&quot;&gt;https://funcube.org.uk/&lt;/a&gt;  which has information about what is involved and links to further details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And FUNcube is by no means the only satellite you can monitor.   &lt;a href=&quot;http://www.pe0sat.vgnet.nl/decoding/satellite-telemetry/&quot; rel=&quot;nofollow&quot;&gt;http://www.pe0sat.vgnet.nl/decoding/satellite-telemetry/&lt;/a&gt;  contains links to a great deal of pertinent &quot;how-to-do-it&quot; information; just choose the &quot;Satellite&quot; dropdown from the menu bar to get an idea of what is possible.&lt;/p&gt;&#xA;" OwnerUserId="14583" LastEditorUserId="14583" LastEditDate="2016-07-23T12:11:02.117" LastActivityDate="2016-07-23T12:11:02.117" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17443" PostTypeId="2" ParentId="17438" CreationDate="2016-07-23T12:10:07.613" Score="1" Body="&lt;p&gt;Edit: I misunderstood the question, here's another attempt. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photonic radar refers to a radar system that uses optical signal processing. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A laser produces a finely-tuned digital signature, which is converted into a radio frequency wave and transmitted from the radar antenna. The returning wave is also converted via laser into a digital signal free from “jitter”. The cables are replaced with fibre optics – it is cheaper, lighter and crucially – it has less interference. It’s very accurate.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In other words, the advantage is you have less noise to deal with. That gives an increase in range and precision. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in space, it has the same issues as ordinary radar: it's useful for distances that are common on Earth, but for distances common in space you need huge amounts of power. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-07-26T10:46:49.527" LastActivityDate="2016-07-26T10:46:49.527" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17444" PostTypeId="1" CreationDate="2016-07-23T12:31:10.977" Score="3" ViewCount="389" Body="&lt;p&gt;For example: the &lt;a href=&quot;http://www.funcubedongle.com/?page_id=1225&quot; rel=&quot;nofollow noreferrer&quot;&gt;FUNcube Dongle Pro+&lt;/a&gt; is a USB &lt;a href=&quot;https://en.wikipedia.org/wiki/Software-defined_radio&quot; rel=&quot;nofollow noreferrer&quot;&gt;Software Defined Radio&lt;/a&gt; receiver, or SDR. It provides a digitized 192kHz wide chunk of IF (intermediate frequency) via high speed USB interface. Generally your computer, Raspberry Pi or whichever platform supports the required USB data rate and power requirements will then run whatever SDR software you like to visualize the spectrum or demodulate the signal any way you like.&lt;/p&gt;&#xA;&lt;p&gt;There are some internal provisions to disable reception of some things, so don't open it up and mess with it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What body of regulations would actually apply to a cubesat with a FUNcube Dongle Pro+ or similar? For this answer, I really need more than speculation, or maybes. Also, I'm not looking for export regulations or voluntary agreements &lt;em&gt;for this particular question.&lt;/em&gt; Of course they are important, but I'm asking about regulations that apply to putting my own dongle into my cubesat, and then having a reputable launch provider put it into orbit.&lt;/p&gt;&#xA;&lt;p&gt;If necessary to identify country of launch, use a country that has a history of managing cubesat launches and following all internationally accepted protocols, rather than one that uses a &lt;a href=&quot;https://space.stackexchange.com/a/27443/12102&quot;&gt;don't ask, don't tell&lt;/a&gt;-like approach.&lt;/p&gt;&#xA;&lt;p&gt;From their document [FCD2ManualV4.pdf][3]&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Specifications&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Frequency range 150kHz-240MHz and 420MHz-1.9GHz&lt;/li&gt;&#xA;&lt;li&gt;Sensitivity Typically 12dB SINAD NBFM for 0.15uV at 145MHz&lt;/li&gt;&#xA;&lt;li&gt;Reference oscillator 1.5ppm 26MHz&lt;/li&gt;&#xA;&lt;li&gt;Sampling rate 192kHz&lt;/li&gt;&#xA;&lt;li&gt;Bit depth 16 bits (32 bits used internally)&lt;/li&gt;&#xA;&lt;li&gt;PC interface USB 1.x Male A Full Speed (12Mbps)&lt;/li&gt;&#xA;&lt;li&gt;RF interface Standard SMA female (not Reverse Polarity [RP])&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Below:&lt;/strong&gt; Further screenshots from their document &lt;a href=&quot;http://www.funcubedongle.com/MyImages/FCD2ManualV4.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;FCD2ManualV4.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/xzRBU.png&quot; alt=&quot;screenshot from the Funcube Dongle Pro manual http://www.funcubedongle.com/MyImages/FCD2ManualV4.pdf&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QNXVR.jpg&quot; alt=&quot;screenshot from the Funcube Dongle Pro manual http://www.funcubedongle.com/MyImages/FCD2ManualV4.pdf&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2023-01-29T21:25:37.223" LastActivityDate="2023-01-29T21:25:37.223" Title="What body of regulations (if any) would apply to a cubesat with an SDR on board?" Tags="|cubesat|law|regulatory|software-defined-radio|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="17445" PostTypeId="2" ParentId="17342" CreationDate="2016-07-23T13:23:54.010" Score="2" Body="&lt;p&gt;Assuming a restricted two-body problem (neglecting the mass of the satellites and no other forces at work) the problem can be solved with search combined with a solver for the Gauss' problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gauss problem is the following: Given two positions and the time between the positions, find the velocites at both positions, which in other words finds the orbit. We will denote this as $(v_1, v_2) = GaussProblem(r_1, r_2, t)$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how can we use the solver to intercept the second satellite? This is done by assuming an intercept time ($t$) (we will iterate to find the best one later). So what we do now is that we calculates the position of the second satellite at the intercept time, which I assume you already can do (using numerical integration or solver for the Kepler problem). I will denote this as $(r', v') = KeplerProblem(r, v, t)$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that we have two positions (the current position of the first satellite, the position of the second satellite at the intercept time), we can use the solver for the Gauss problem to find what the velocity at the first position &lt;em&gt;should&lt;/em&gt; have been, if the first satellite was on the intercept orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So our maneuver is simply then: $\Delta v = v_1 - v_{satellite 1}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The algorithm is the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;For each $t \in [min, max]$ (take some range)&lt;/li&gt;&#xA;&lt;li&gt;$(r', \_) = KeplerProblem(r_{satellite 2}, v_{satellite 2}, t)$&lt;/li&gt;&#xA;&lt;li&gt;$(v_1, v_2) = GaussProblem(r_{satellite 1}, r', t)$&lt;/li&gt;&#xA;&lt;li&gt;$\Delta v = v_1 - v_{satellite 1}$&lt;/li&gt;&#xA;&lt;li&gt;Select the maneuver that best matches your parameters (minimum intercept time given delta velocity requirements).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Some notes:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;For a given problem instance, a solver for the gauss problem may fail.&lt;/li&gt;&#xA;&lt;li&gt;The times are relative, not absolute.&lt;/li&gt;&#xA;&lt;li&gt;The maneuvers are assumed to be executed at the current time. Calculating the intercept at a later time may yield a better intercept in terms of time and deltaV.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Gauss Problem&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not 100% sure which is the correct name for the problem, as in Fundamentals of astrodynamics by Bate, Roger R., Donald D. Mueller, and Jerry E. White denotes this as the Gauss problem, others as Lambert's problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a method for solving the problem, you should either look in the mentioned book, which contains several methods, or look at the following links: &lt;a href=&quot;http://aerospacengineering.net/?p=1614&quot; rel=&quot;nofollow&quot;&gt;http://aerospacengineering.net/?p=1614&lt;/a&gt;, &lt;a href=&quot;http://www.dept.aoe.vt.edu/~cdhall/courses/aoe4134/Apiteration.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.dept.aoe.vt.edu/~cdhall/courses/aoe4134/Apiteration.pdf&lt;/a&gt;, &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Lambert%27s_problem&quot; rel=&quot;nofollow&quot;&gt;https://en.m.wikipedia.org/wiki/Lambert%27s_problem&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="15934" LastActivityDate="2016-07-23T13:23:54.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17446" PostTypeId="2" ParentId="17439" CreationDate="2016-07-23T13:27:53.973" Score="29" Body="&lt;p&gt;Salyut 7 had a active cooling system as well. The heat exchangers are the large white panels, perpendicular to the solar panels, to be seen in photographs of the station. This is necessary on all space stations in active mode as the energy produced by solar panels (apparently 4.5 kW in case of Salyut 7, an order of magnitude more in case of the ISS) needs to be dissipated in some way. The only natural way of cooling a station is by radiation - and this is not a lot: An ideal radiator can emit about 400 W/m² at room temperature while sun delivers almost 1400 W to the same surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for the station to be freezing cold when Soyuz T-13 arrived is that at this point the station was completely without power for almost four months. The only energy heating up the station was sun light. During night, the station lost heat by radiation. The exact equilibrium temperature can not be easily calculated and depends a lot on surface area, volume, attitude and shielding of the station. It also depends on the exact design of the cooling system - if parts of it work without electricity (e.g. by heat conductivity, not requiring pumps for a cooling liquid) it will continue to cool down the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is very likely that the same freezing temperatures would be found on ISS given it was shut down completely for a longer term.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-07-23T13:27:53.973" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17447" PostTypeId="1" CreationDate="2016-07-23T14:13:45.333" Score="4" ViewCount="44" Body="&lt;p&gt;Last night here in Southern California, I was watching the flare of the Iridium 6, I use the Sputnik app to alert me. I was amazed when I saw not one but two bright lights tracking simultaneously.. One in front of the other. I have never seen this before. Can you explain what I saw? Was it a reflection of two parts of the satellite? It was amazing!&lt;/p&gt;&#xA;" OwnerUserId="16374" LastActivityDate="2016-07-23T14:52:49.707" Title="Double satellite flare on iridium 6" Tags="|atmosphere|" AnswerCount="0" CommentCount="1" ClosedDate="2016-07-23T23:36:03.797" ContentLicense="CC BY-SA 3.0" />
  <row Id="17449" PostTypeId="2" ParentId="13817" CreationDate="2016-07-23T18:44:29.997" Score="12" Body="&lt;p&gt;Here are the projected Delta Heavy upgrades from United Launch Alliance (ULA) in different configurations. You can see that the payload value at the name of the rocket version is not the same as the LEO payload value. I guess the name of the rocket version shows the payload for a low earth orbit, 200 km at 28.5 degrees (probably 28.7), that’s why is different. The low earth orbit (LEO) payload value shown here is for 407 km LEO at 28.5 degrees. This data is from &lt;a href=&quot;http://www.astronautix.com/&quot; rel=&quot;noreferrer&quot;&gt;astronautix.com&lt;/a&gt;. In my opinion the low earth orbit, 407 km at 28.5 degrees, should be at 51.6 degrees because I guess it is for the ISS (International Space Station) orbit, but anyway I am referring to the website. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade30t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 30 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 3 CBC (RS-68 engines) with 4 GEM-60 solid rocket boosters (SRB) + 2nd stage (RL-10B). LEO Payload: 27,000 kg (59,000 lb) to a 407 km orbit at 28.50 degrees, Earth Escape (EE) Payload: 10,000 kg (22,000 lb). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade35t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 35 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 3 CBC (RS-68B engines) + 2nd stage AUS-60 upper stage (powered by 2 MB-45 or RL-45). LEO Payload: 32,000 kg (70,000 lb), EE Payload: 12,500 kg (27,500 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade40t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 40 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 3 CBC (RS-68 Regen engines) with 4 GEM-60 SRB + 2nd stage (?). LEO Payload: 36,000 kg (79,000 lb), EE Payload: 14,000 kg (30,000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade42t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 42 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 3 CBC (RS-800 engines) + 2nd stage AUS-60 upper stage (powered by 2 MB-60 or RL-60), aluminum-lithium lightweight alloy in place of the existing aluminum in all stages. LEO Payload: 38,000 kg (83,000 lb), EE Payload: 14,000 kg (30,000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade43t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 43 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 3 CBC (RS-68 Regen engines) with 4 GEM-60 SRB + 2nd stage AUS-60 upper stage (powered by 1 MB-60 or RL-60). LEO Payload: 39,000 kg (85,000 lb), EE Payload: 14,500 kg (31,900 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade45t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 45 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 3 CBC (RS-68B engines) with 6 GEM-60 SRB + 2nd stage AUS-60 upper stage (powered by 1 MB-60 or RL-60). LEO Payload: 41,000 kg (90,000 lb) to a 407 km orbit at 28.50 degrees, EE Payload: 15,000 kg (33,000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade48t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 48 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 3 CBC (RS-68 Regen engines) with 4 (or maybe 6) GEM-60 SRB + 2nd stage ? (maybe 1 MB-60 or RL-60), cross-feed between the strap-ons and core. LEO Payload: 43,000 kg (94,000 lb) to a 407 km orbit at 28.50 degrees, EE Payload: 15,800 kg (34,800 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade53t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 53 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 5 CBC (RS-68 engines) + 2nd stage AUS-60 upper stage powered by 2 MB-60 or RL-60 ), aluminum-lithium lightweight alloy stages. LEO Payload: 48,000 kg (105,000 lb), EE Payload: 18,000 kg (39,000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade67t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 67 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 7 CBC (RS-68 engines) + 2nd stage AUS-60 upper stage (powered by 2 MB-60 or RL-60 ). LEO Payload: 61,000 kg (134,000 lb), EE Payload: 22,500 kg (49,600 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade70t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 70 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 7 CBC (RS-68 engines) + 2nd stage AUS-60 upper stage (powered by 3 MB-60 or RL-60). LEO Payload: 63,000 kg (138,000 lb), EE Payload: 24,000 kg (52,000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade76t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 76 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 7 CBC (RS-68 engines) + 2nd stage AUS-60 upper stage (powered by 3 MB-60 or RL-60), aluminum-lithium lightweight alloy stages. LEO Payload: 69,000 kg (152,000 lb), EE Payload: 26,000 kg (57,000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade87t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 87 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 7 CBC (RS-800K engines) + 2nd stage AUS-60 upper stage (powered by 3 (or maybe 4) MB-60 or RL-60). LEO Payload: 79,000 kg (174,000 lb), EE Payload: 30,000 kg (66,000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/d/deltaivheavyupgrade94t.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Delta IV Heavy Upgrade 94 t:&lt;/strong&gt;&lt;/a&gt; stage 0+1st stage, 7 CBC (RS-800K engines) + 2nd stage AUS-60 upper stage (powered by 4 MB-60 or RL-60), aluminum-lithium lightweight alloy stages. LEO Payload: 85,000 kg (187,000 lb), EE Payload: 32,000 kg (70,000 lb).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of these configurations use different combinations, as we see: a different number of common core boosters(CBC), different number of strap on solid rocket boosters, different engines at the first stages (RS-68, RS-68B, RS-68 Regen, RS-800, RS-800K), and also different upper stages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some of these versions, lightweight aluminum-lithium alloy is used at all rocket stages in place of the existing aluminum (used in the current Delta IV versions and Delta Heavy). In this list are two versions, 70 t and 76 t, both with the same configuration of all stages, but the first uses the existing aluminum at the rocket stages and the second uses aluminum-lithium lightweight alloy. Because of this, the second version of these two can put 6 t more payload at 200 km, 407 km orbits and 2 t more at Earth escape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cross-feed system is planned to be used by Falcon Heavy. But even ULA projects have included this system as a possible option. Here in these versions only one configuration uses the cross-feed system, the 48 t version. I am not sure how much the cross-feed system will affect its configuration to make a comparison. I guess that since these configurations correspond with the PearsonArtPhoto image, it could have 6 solid boosters and a 2nd stage powered by 1 MB-60 or RL-60. If it has 4 solid booster as in the link, then it will have the same configuration as 43 t version. So it means that the cross-feed system gives 5 t more payload at 200 km orbit, 4 t more at 407 km orbit and 1.3 t more at earth escape. As I said before these configurations correspond with the PearsonArtPhoto image, but astronautix.com has more accurate payload values in 200 km, 407 km orbits and Earth escape. Some of these Delta Heavy versions could lift to space much more payload than Falcon Heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; Configurations using RS-68 Regen engine, have this upgraded engines with regeneratively-cooled nozzles to the core vehicles and cryogenic propellant densification.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=32324.0;attach=532976;image&quot; alt=&quot;Delta IV Upgrades&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image shows some other possible configurations. In this image, Delta IV Heavy Upgrade versions are similar or maybe the same with those described above. Delta IV Super Heavy Derived versions are different configurations where one common core booster (should be larger in diameter than the current ones in use) are 2, 3 or 4 engines (RS-68s family, probably RS-68A). In the 1st versions where each CBC has 2 RS-68s the payload in 407 km orbit at 28.5 degrees is around 65 t. In the 2nd version where each CBC has 3 RS-68s the payload looks more than 105 t, and the 3rd one with 4 RS-68s looks to have a payload of more than 145 t. ULA has projected many different configurations for Delta IV Heavy upgrades, but the chances are that none of them will be launched.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-08-24T18:15:01.817" LastActivityDate="2016-08-24T18:15:01.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17450" PostTypeId="1" AcceptedAnswerId="17478" CreationDate="2016-07-23T21:00:35.550" Score="2" ViewCount="675" Body="&lt;p&gt;When designing a rocket engine, what degrees of freedom do you have to get the combustion chamber to a desired pressure?  I assume you have mass flow rate, type and mixture ratio of fuel and oxidizer, and geometry, but what aspects of the geometry matter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a given geometry, is there a way to calculate the minimum pressure to cause choked flow at the nozzle?&lt;/p&gt;&#xA;" OwnerUserId="16376" LastActivityDate="2016-07-25T12:33:57.123" Title="Controlling Combustion Chamber Pressure" Tags="|rockets|thermodynamics|combustion|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17451" PostTypeId="2" ParentId="17432" CreationDate="2016-07-23T22:29:20.407" Score="14" Body="&lt;p&gt;The ISS modules were launched pressurized, if for no other reason than that to pressurize them in orbit would be a drain on ISS resources.&lt;/p&gt;&#xA;&lt;p&gt;The mechanical interface between a new module and the ISS is the Common Berthing Mechanism (CBM), which has been discussed on this site quite a bit.  This answer will focus on the process of opening a pressurized path from the ISS to the new module.&lt;/p&gt;&#xA;&lt;p&gt;The CBM forms a ring around the hatch, shown here.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cc4qr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cc4qr.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Of interest to this answer is the Manual Pressure Equalization Valve towards the bottom of the hatch.&lt;/p&gt;&#xA;&lt;p&gt;Our working example will be the Japanese Experiment Module (JEM), carried up on STS-124.  I couldn't find a photo of the CBM end of the module so here is a graphics rendering of it.  The red A labels the hatch/CBM on the JEM and the red B labels the hatch/CBM on the ISS where the module will be attached.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wAO3u.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wAO3u.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Once the robotics/CBM magic happens and the JEM is mechanically installed, a &amp;quot;vestibule&amp;quot; has been created.  I couldn't find a good schematic of this so I drew up a terrible one.  This tries to show a view looking perpendicular to the long axis of the JEM&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yasLg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yasLg.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This vestibule is formed by the rings of the CBM and the hatches on either side.  Once the module mating is complete, the vestibule is at vacuum and the ISS and new module are at normal pressure.&lt;/p&gt;&#xA;&lt;p&gt;NASA has chosen to not publish any Assembly Ops Checklists on the web but Spaceref.com once did (now gone), for the US Lab.  The procedure is called VESTIBULE PRESSURIZATION AND LEAK CHECK and starts on document page 121.  In brief, it describes how, on the ISS side, a crewperson will open the MPEV and allow air to flow into the vestibule from the ISS.  Once a period of time has elapsed, or the flow stops, the MPEV is then closed, and time is allowed to elapse to ensure that the vestibule does not have a leak.&lt;/p&gt;&#xA;&lt;p&gt;Once everyone is happy that there is not a leak, the MPEV is re-opened, and the ISS hatch is opened.  The MPEV on the new module is then opened, and once equalization is complete, the module hatch is opened.&lt;/p&gt;&#xA;&lt;p&gt;The module can now be accessed and outfitting can begin.&lt;/p&gt;&#xA;&lt;p&gt;Here's an excerpt of the procedure:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/O1aDm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/O1aDm.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2024-01-16T15:58:05.057" LastActivityDate="2024-01-16T15:58:05.057" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="17452" PostTypeId="1" AcceptedAnswerId="17513" CreationDate="2016-07-24T00:02:40.853" Score="2" ViewCount="824" Body="&lt;p&gt;In this NASA Goddard &lt;a href=&quot;https://www.youtube.com/watch?v=CFrP6QfbC2g&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt; titled &quot;One Year on Earth – Seen From 1 Million Miles&quot;, I've gotten stuck on the line &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In this view, EPIC sees the Sun rise in the west, and the Sun set in the east, at least 13 times a day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Listening carefully, it's &quot;...see the sunrise... and the sunset...&quot; &lt;em&gt;and not&lt;/em&gt; &quot;...see the Sun rise... and the Sun set...&quot;, &lt;strong&gt;so my transcription is not correct&lt;/strong&gt;. I'll leave it, along with this note, because the distinction is important. I only caught it after reading the &lt;a href=&quot;https://space.stackexchange.com/a/17453/12102&quot;&gt;nice, concise answer&lt;/a&gt; by @Leorex through carefully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;DSCOVR is in a Lissajous orbit about the Sun-Earth L1 Lagrange point between the Earth and the Sun, and the EPIC camera points towards the earth which remains nearly 100% sunlit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A given point on earth sees about one sunrise and one sunset per day, and the Lissajous orbit gently rocks back and forth slightly with roughly a six month period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't understand what it is that is happening at least thirteen times a day. Can someone help me understand this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OcdLC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OcdLC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen shot during the quoted line. Start playing at &lt;code&gt;00:44&lt;/code&gt; to hear it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/CFrP6QfbC2g?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-06-27T05:49:12.117" Title="Why does DSCOVR's camera EPIC see at least 13 sunrises and sunsets per day?" Tags="|lagrangian-points|dscovr|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17453" PostTypeId="2" ParentId="17452" CreationDate="2016-07-24T01:40:37.970" Score="5" Body="&lt;p&gt;In your embedded YouTube video, at 1m 16s EPIC's Lead Scientist Jay Herman says, &quot;Epic takes at least one set of images about every two hours.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Epic takes a set of images every 1h 50m 46.153s, it will take 13 sets of images in 24 hours, each showing a sunrise and a sunset.&lt;/p&gt;&#xA;" OwnerUserId="16119" LastActivityDate="2016-07-24T01:40:37.970" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17454" PostTypeId="2" ParentId="17452" CreationDate="2016-07-24T02:45:49.863" Score="2" Body="&lt;p&gt;I've accepted the &lt;a href=&quot;https://i.stack.imgur.com/relUV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;great answer&lt;/a&gt; by @Leorex. I went back and found that the video has closed captions (the little [CC] thing) and indeed my original transcription was wrong. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; and then I looked at the whole thing again six months later, after &lt;a href=&quot;https://space.stackexchange.com/a/20063/12102&quot;&gt;this answer&lt;/a&gt; and decided that no, the problem is more than just confusing &quot;sunset&quot; for &quot;sun set&quot;. @DavidHammon's logic is inescapable, as usual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/relUV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/relUV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-02-07T13:08:02.487" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17456" PostTypeId="1" AcceptedAnswerId="17466" CreationDate="2016-07-24T08:26:01.560" Score="2" ViewCount="395" Body="&lt;p&gt;Is it possible to illuminate a satellite (LEO) using a ground based laser or other kind of light sources (visible light LED Array with mirrors) and then take a detailed photo of satellite? also a ballpark estimate of the power needed for illumination and the telescope resolution (+20MP sensor) for a 1-meter object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &quot;Detailed&quot; I mean, it's components be resolvable in the photo, to get an estimate of its surface area, moments of inertia and initial conditions at the time of observation (using subsequent photos) to use for further attitude propagation.&lt;/p&gt;&#xA;" OwnerUserId="16381" LastEditorUserId="16381" LastEditDate="2016-07-31T09:18:54.573" LastActivityDate="2020-07-27T10:50:02.287" Title="Taking detailed photos of satellites using laser illumination" Tags="|artificial-satellite|observation|imaging|photography|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17457" PostTypeId="1" CreationDate="2016-07-24T08:27:59.270" Score="2" ViewCount="998" Body="&lt;p&gt;Planet Earthtoo saw that Earth could put a person in orbit, so they wanted to go to space &lt;em&gt;too&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The planet Earthtoo is twice the diameter of Earth, with the same internal structure - the average density is the same as Earth so the total mass is 8 times that of Earth, and thus the surface gravity is twice that of earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEtooO (Low Earthtoo Orbit) is lower than that on earth because even though the surface pressure is twice that of Earth, the scale height is half. However, Tooian scientists have decided that 200km is their target height for the first tootronaut to safely orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earthtoo also has a 24 hour day. They have no mountains, but a nice coastline on the equator from which to launch and take advantage of the rotation speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tooian rocket scientists have perfected a conventional rocket engine with an Isp of 500 seconds independent of ambient pressure. They can be scaled in size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of their deep admiration of Earth astronaut Don Pettit, they have studied his discussion of &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow&quot;&gt;The Tyranny of gravity&lt;/a&gt; and worked and worked until they could build each stage of their rocket with 96% of its mass as propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass of the orbiter with the tootronaut, life support, and substantial re-entry protection (twice the orbital velocity, twice the gravitational force) was an amazingly low 1000kg. They've been watching earth very carefully and took the best technologies. They sometimes call it the &quot;Flying Tile&quot; because of the substantial use of ceramic tile material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Based on the parameters below and situation described above, what is a rocket design that can put the tootronaut and orbiter into LEtooO? Please choose a widely accepted and recognized spaceflight simulator (Kerbal could be OK for example). Show enough of the answer that other people can independently run your design into orbit and verify that &quot;yep, it works.&quot; How big is this rocket? &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Use the following values:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;radius of planet:   12740 km&#xA;rotation period:    24 hours&#xA;launch location:    equator&#xA;surface gravity:    19.6 m/s^2&#xA;atmosphere density &#xA;    at surface:     2.4 kg/m^3&#xA;scale height:       3.8 km&#xA;&#xA;mass of payload:    1000 kg&#xA;specific impulse*:  500 sec (any size engine, any ambient pressure)&#xA;stage dry mass:     0.04 of total mass when stage is fully fueled.&#xA;&#xA;altitude of orbit:  200 km&#xA;orbital velocity:   15.8 km/s or thereabouts, for a circular orbit&#xA;    (derived)&#xA;orbit inclination:  0 degrees&#xA;&#xA;(*) remember to use 9.8 and NOT 19.6 if you convert Isp to effective thrust velocity!&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-07-25T00:18:09.053" LastActivityDate="2016-07-25T00:18:09.053" Title="Can planet Earthtoo put a Tooian in orbit too?" Tags="|rockets|mission-design|simulation|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17459" PostTypeId="2" ParentId="17457" CreationDate="2016-07-24T12:07:06.267" Score="4" Body="&lt;p&gt;First, we need to find what additional $\Delta v$ the rocket needs to achieve orbit. Earth LEO velocity is for instance 7.8 km/s, but the real $\Delta v$ needed is somewhere around 9-9.5km/s because of drag and gravity losses. That is an additional 1.5km/s. The first thing I notice is that Earthtoo has a denser atmosphere, but that the scale height is lower. Here is a plot comparing the atmosphere to ours:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U1Ce9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U1Ce9.png&quot; alt=&quot;atmosphere of Earthtoo&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After just a few kilometres, the density is in fact lower. Drag is more important when the velocity starts to get high, so I will say that an Earthtoo rocket looses less $\Delta v$ due to it. That should compensate a little for the higher gravity losses. I want more thrust too, so I chose larger engines to compensate for the rest, increasing my dry mass share to 0.05. For a worst case scenario, I then design my rocket for a $\Delta v$ of 18 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Here is my design:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have &lt;strong&gt;3&lt;/strong&gt; stages, each of them 3 times more massive than all the mass they carry.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Payload:    1000kg&#xA;3rd stage:  4000kg ( 200kg dry mass)&#xA;2nd stage: 15000kg ( 750kg dry mass)&#xA;1st stage: 60000kg (3000kg dry mass)&#xA;Total:     80000kg&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The idea is that the mass ratio for each expended stage is then $\frac{1+3}{1+3 \cdot 0.05} = 3.48$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $\Delta v$ added for each of the three stages is then:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v = \ln{3.48} \cdot 500s \cdot 9.81m/s^2 = 6120 m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is 18360m/s in total, slightly more than needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can not see the need to run this in a sophisticated spaceflight simulator, as some of the given parameters are highly unlikely. For instance, a performance of 500s independent of ambient pressure is extremely optimistic, as most launchers $I_{sp}$ at sea level is around 300s, and an atmosphere twice as dense does not help at all. Secondly, complete rocket stages seldom have a better total-to-dry mass ratio than about 15. (0.067), so even the more conservative figure I used (20 instead of 25) is fiction.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-07-24T12:07:06.267" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17460" PostTypeId="2" ParentId="17457" CreationDate="2016-07-24T14:53:04.940" Score="6" Body="&lt;p&gt;Pettit's article doesn't mention the advantage of &lt;a href=&quot;https://en.wikipedia.org/wiki/Multistage_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;staging&lt;/a&gt; at all. All he's actually saying is that a hydrogen fueled single-stage-to-orbit launcher can't work on a planet 50% larger than Earth. Staging doesn't invalidate the Tsiolkovsky equation but it does work around the need to attain extreme propellant mass fractions in a single stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Hohmannfan notes, the Isp and mass ratio assumptions in the question as written are pretty unrealistic, but I think the Tooians can do this with Apollo-era technology and a 4-stage rocket 4 times the size of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the gravity losses are a bigger concern, so I aimed for 20km/s of total ∆v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Payload: Stripped Mercury, 1 t&lt;/li&gt;&#xA;&lt;li&gt;Stage 4: Basically a shortened &lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_(rocket_stage)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centaur&lt;/a&gt;, 2 &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10&quot; rel=&quot;nofollow noreferrer&quot;&gt;RL10-A3&lt;/a&gt;, 13t, 6038 m/s&lt;/li&gt;&#xA;&lt;li&gt;Stage 3: 3x &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_J-2&quot; rel=&quot;nofollow noreferrer&quot;&gt;J-2&lt;/a&gt;, 135t, 5535 m/s&lt;/li&gt;&#xA;&lt;li&gt;Stage 2: 5x &lt;a href=&quot;https://en.wikipedia.org/wiki/M-1_(rocket_engine)&quot; rel=&quot;nofollow noreferrer&quot;&gt;M-1&lt;/a&gt;, 1500t, 5462 m/s&lt;/li&gt;&#xA;&lt;li&gt;Stage 1: 40x &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;F-1&lt;/a&gt;, 9500t, 3261 m/s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Total 11149 tons -- Saturn V is about 3000.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel fractions on the stages are 80%-84%, since I assume the larger structure and higher gravity are concerns. Generally it needs twice as much engine per mass as Terran rockets, which also contributes to dry mass. Here's my worksheet:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UPth2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UPth2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The three upper stages are smaller than the three stages of the Saturn V launcher; there's no question that these could be built. The M-1 engine was never completed and flown but got well into development, and there's no reason to think it wouldn't have been successful. It could be replaced on a 6:1 basis with J-2s if not. The first stage is certainly hefty. Simply finding enough stage-base area to mount so many engines requires that the stages be short and squat (or perhaps conical, like the Russian N-1 design); this means that drag will be of greater relative concern than on Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The max accel values assume that each stage continues burning at full thrust until fuel is expended. Saturn V shut down center engines partway through the burns on the first two stages in order to reduce g-force on the crew, peaking at about 4g. I assume Tooians are a little tougher, but if necessary, this rocket can do something similar. Obviously the first stage needs better than 2g initial acceleration (=1:1 TWR) for liftoff. The initial acceleration of the other stages may seem low, but note that Saturn V second stage lights up at only 0.8g and third stage about 0.55g, while the Tooian rocket maintains better than 1:1 TWR at all points until the last stage. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-25T00:17:28.780" LastActivityDate="2016-07-25T00:17:28.780" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17461" PostTypeId="2" ParentId="6394" CreationDate="2016-07-24T17:31:48.773" Score="2" Body="&lt;p&gt;Usable volume rises quickly with radius (r&lt;sup&gt;2&lt;/sup&gt;) so a cylinder with a diameter of 6.7 m has almost twice the usable volume of a cylinder of 5 m diameter.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-24T17:31:48.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17463" PostTypeId="2" ParentId="17034" CreationDate="2016-07-25T00:31:31.670" Score="2" Body="&lt;p&gt;Here are some desirable properties of an ablator. The first ones are they you mentioned, with some additional detail.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A sublimation point lower than the operation temperature is a must. An ablator that does not evaporate is not an ablator. The shield has to loose some mass in order to get rid of the added energy, in contrast to for example the tiles of the space shuttle. They where designed to &lt;em&gt;not&lt;/em&gt; evaporate, and instead store heat in heat sinks.&lt;/li&gt;&#xA;&lt;li&gt;A low thermal conductivity is similarly important. While the shield can not stand re-entry conditions for ever, the point is to delay the problem for a few minutes during the descent. High heat capacity is related (but not directly dependent), and is together with the conductivity pointing towards low molecular mass, non-metals, and organic materials. Note also that internal voids are also excellent for insulation, giving a spongy or foamy material an edge.&lt;/li&gt;&#xA;&lt;li&gt;In addition, to actually consume energy, a combustion reaction with the air should be endothermic, and any sublimation of material associated with a high enthalpy of fusion.&lt;/li&gt;&#xA;&lt;li&gt;A high vapour pressure is important, as most of the shielding effect is not directly from the shield itself, but rather from the gas it emits, creating a cushion diverting the airflow.&lt;/li&gt;&#xA;&lt;li&gt;Mechanical robustness is in itself important, as a fragile material being shredded to pieces in the extreme conditions is of no use. This does not technically have to be a property of the ablator per se, as an underlying honeycomb lattice or aramid rings in the material can take care of that. (It is still a plus though.)&lt;/li&gt;&#xA;&lt;li&gt;For technical reasons, the materials in question are not practical to cast, so the materials must have good solidifying properties, like epoxy.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-07-25T00:31:31.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17464" PostTypeId="1" CreationDate="2016-07-25T01:32:42.943" Score="1" ViewCount="49" Body="&lt;p&gt;Since the blowing up of third stage of Soyuz 2.1a it has been put at back burner and older version of Soyuz -TM put into survice. My question is since the contract with ESA also expire in 2016 when will we see it on active duty?  When it will cleared safe for human again? &lt;/p&gt;&#xA;" OwnerUserId="16355" LastActivityDate="2016-07-25T01:32:42.943" Title="Supply mission to ISS" Tags="|soyuz-spacecraft|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17466" PostTypeId="2" ParentId="17456" CreationDate="2016-07-25T01:56:51.457" Score="6" Body="&lt;p&gt;A common equation to express the best resolution of an optical system is based on the overlap of two &lt;a href=&quot;https://en.wikipedia.org/wiki/Airy_disk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Airy disks&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ sin(\theta)\approx \theta \approx 1.22 \frac{\lambda}{D} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;You can ask what size aperture on earth would have a diffraction limit of its own size in LEO;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{D}{L} \approx 1.22 \frac{\lambda}{D} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$D^2 \approx 1.22 \lambda L$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$D \approx 1.1 \sqrt{ \lambda L}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Choosing rough, ballpark numbers; &lt;span class=&quot;math-container&quot;&gt;$L_{LEO}=500 \text{km}, \lambda=600 \text{nm}$&lt;/span&gt;, I get 60cm.&lt;/p&gt;&#xA;&lt;p&gt;So the diffraction limit in LEO is about 0.6m for a 0.6m aperture. The relationship is inverse, so it's 0.3m for a 1.2m aperture for example.&lt;/p&gt;&#xA;&lt;p&gt;That applies to an imaging system as well as to a collimator for laser illumination if you wanted it.&lt;/p&gt;&#xA;&lt;p&gt;But that's just diffraction. Tracking smoothly and accurately would be quite a challenge. A normal astronomical telescope mount and drive might not have the right accuracy and smoothness while slewing at almost 1 degree per second.&lt;/p&gt;&#xA;&lt;p&gt;You could build a special satellite tracking drive and I'm sure that's done, or you could use a pulsed laser - basically flash photography. The standard &lt;a href=&quot;https://en.wikipedia.org/wiki/Diode-pumped_solid-state_laser&quot; rel=&quot;nofollow noreferrer&quot;&gt;DPSS&lt;/a&gt; NdYAG laser at 1064nm could be used directly, or frequency doubled to green 532nm. They come in a variety of powers and typically pulse at 1 to 10 kHz and have pulse widths (in time) of the order of 20ns. You can clock your CCD accordingly and overlay the frames later.&lt;/p&gt;&#xA;&lt;p&gt;If you use a pulsed laser, it allows you to use very short exposure times centered at the moment the pulse returns to earth, and an optical filter centered on the laser wavelength to reduce background scattered light (including the daytime sky).&lt;/p&gt;&#xA;&lt;p&gt;Because the distances are so much larger than the coherence length of those - and probably any laser you can buy off the shelf, you can think of the light as incoherent and you won't see any &lt;a href=&quot;https://en.wikipedia.org/wiki/Speckle_pattern&quot; rel=&quot;nofollow noreferrer&quot;&gt;laser speckle&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;It's not going to be as bright as sunlight, which is roughly a kilowatt per square meter, but because of the strobing you can integrate for many seconds (effectively).&lt;/p&gt;&#xA;&lt;p&gt;So after tackling the diffraction limit and motion smearing, you still have atmospheric turbulence. You can't use (at least conventional) &lt;a href=&quot;https://en.wikipedia.org/wiki/Adaptive_optics&quot; rel=&quot;nofollow noreferrer&quot;&gt;adaptive optics&lt;/a&gt;, because it is based on looking through the same column of air for at least milliseconds in order for the mirrors to respond, and slewing at this rate, it's not going to work (easily).&lt;/p&gt;&#xA;&lt;p&gt;However there is the fallback technique of &lt;a href=&quot;https://en.wikipedia.org/wiki/Lucky_imaging&quot; rel=&quot;nofollow noreferrer&quot;&gt;lucky imaging&lt;/a&gt;, and since the satellite is an artificial, engineered, functional object an not an unknown heavenly body, you can process, simulate, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Deconvolution&quot; rel=&quot;nofollow noreferrer&quot;&gt;deconvolve&lt;/a&gt; the heck out of the data to figure out what the satellite would look like without atmospheric effects.&lt;/p&gt;&#xA;&lt;p&gt;But remember - if you use the sun, you have a half-degree-collimated beam at oblique incidence, and if you use a laser, the beam is far narrower. Mirror-like surfaces (flat shiny sections of metal, solar panels) may appear quite dark if illuminated at the wrong angle, but suddenly appear to flare if the right geometry happens. If it's diffuse, or crinkled, it may be much brighter than shiny surfaces away from specular reflection. This can cause the images to be very different than what the satellite would look like if you were standing next to it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Caution:&lt;/strong&gt; Please adhere to all applicable safety measures and legal restrictions. If you are shooing highly collimated lasers outdoors, even into the sky, you can get in a heap of trouble and could potentially do serious damage to people, aircraft, and spacecraft. Don't try this at home!!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-07-27T10:50:02.287" LastActivityDate="2020-07-27T10:50:02.287" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17467" PostTypeId="1" CreationDate="2016-07-25T02:50:39.780" Score="2" ViewCount="139" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/q/17457/12102&quot;&gt;this question&lt;/a&gt; I posed the problem of getting an inhabitant of the planet Earthtoo into low orbit (LEtooO) using conventional rocket. Judging by the great answers there, it is - to my surprise - quite possible. Earthtoo has twice the diameter of the earth and the same average density, so the surface gravity is also twice that of earth. The density of the atmosphere at the surface is also twice that of earth (they need more oxygen to climb stairs), and the scale height is half that of earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the tootronaut is ready to come home. Total spacecraft mass is 1000kg, orbital velocity around Earthtoo is about twice that of Earth, about 15.8 km/s. Yikes! The kinetic energy to be dissipated is about four times from LEtooO than it would be from LEO, and as soon as you start loosing energy, you start falling into an atmosphere which is increasing in density far more rapidly than the Earth's atmosphere would, because of the much smaller scale height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could this be done with similar materials and techniques used for reentry on earth? Or would the higher velocity and shorter scale height be extremely difficult to handle using Earth's current heat shield technology? Would it need to rely heavily on propulsive deceleration? Would it have to be airplane-shaped? The Tooians jokingly call the reentry vehicle the &quot;Flying Tile&quot; for a reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not looking for opinions or maybes here. Please use an appropriate amount of math and physics to back up your answer. Thanks!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-07-25T14:44:28.733" Title="The Tootronaut is ready to enter the Earthtoo atmosphere, will it be twice as hot?" Tags="|reentry|spacecraft-development|simulation|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17469" PostTypeId="1" CreationDate="2016-07-25T04:18:11.307" Score="4" ViewCount="581" Body="&lt;p&gt;I followed the string of notes regarding the disturbances caused by the Voyager tape-recorder. It sounds like the recorder has a net angular momentum when the tape is running and that the hydrazine thrusters are used to compensate this. This was presumably necessary during the planetary encounters when trying to take high-resolution images of planets or moons. The main antenna also has a relatively narrow beam-width. I have several related questions. Why use the hydrazine thrusters - are there no reaction wheels?  Does the antenna pointing get disturbed by the operation of the tape-recorder. Is the axis of the tape-recorders angular momentum along the same axis as the antenna (in which case it doesn't matter much if the spacecraft rotates slightly)? Is Voyager-1 spin-stabilized? &lt;/p&gt;&#xA;" OwnerUserId="14419" LastEditorUserId="12102" LastEditDate="2020-11-03T23:38:09.470" LastActivityDate="2020-11-03T23:38:09.470" Title="Voyager 1's tape recorder and other angular momentum management issues" Tags="|voyager|reaction-wheel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="17470" PostTypeId="1" AcceptedAnswerId="17477" CreationDate="2016-07-25T07:03:28.110" Score="2" ViewCount="1415" Body="&lt;p&gt;Looking at Sol, we can see that earth-like planets are close to the sun, while Gas Giants are at medium distance with Ice Giants being far out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Im wondering if this make-up is actually the norm or if it possible (and not highly unlikely) that terran-like planets could be at medium distance, while gas and ice giants could be Close up or very far out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose it depends on the size and power of the star, so Ice Giants (or planets) are probably always more far out than gas giants or earth-like planets ?&#xA;However, for true gas giants, could they be closer to the star ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;i.e. would it be possible for a configuration that goes like Star - Jupiter alike - Saturn alike - Mercury alike - venus alike - ice giants ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;thanks !&lt;/p&gt;&#xA;" OwnerUserId="16357" LastActivityDate="2016-07-25T12:28:25.693" Title="Could, in another star system, gas giants be closer to the star then earth-like planets?" Tags="|solar-system|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17471" PostTypeId="2" ParentId="17469" CreationDate="2016-07-25T09:33:41.193" Score="5" Body="&lt;p&gt;There are no reaction wheels on Voyager. They were new technology at the time (the first use I can find is the &lt;a href=&quot;http://ethw.org/First-Hand:Lessons_Learned_from_Six_Decades_of_Spacecraft_Guidance_and_Control_Experiences&quot; rel=&quot;noreferrer&quot;&gt;OGO series of satellites&lt;/a&gt;, first launched in 1964), that may have been an argument against their use.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager is not spin-stabilized. It used thrusters for stabilization, to provide a stationary platform for the cameras. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't been able to find information on the orientation of the tape recorder. No diagram of the internals of Voyager's main body, either. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-25T09:33:41.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17472" PostTypeId="2" ParentId="17470" CreationDate="2016-07-25T09:42:41.210" Score="2" Body="&lt;p&gt;Generally, no. When a star ignites, its solar wind starts pushing the matter in the protoplanetary disk outward. Lighter particles are easier to move, so they move farther out. So you get heavy elements near the star, light elements further away.&lt;br&gt;&#xA;Collisions and gravitational disturbances mix them up a little so you can get gas giants with rocky moons.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-25T09:42:41.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17473" PostTypeId="1" AcceptedAnswerId="17485" CreationDate="2016-07-25T10:14:12.717" Score="1" ViewCount="289" Body="&lt;p&gt;What thermal effect does the Sun have on spacecraft that are roughly the same distance from the Sun as the Earth?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, do spacecraft need to constantly rotate so that one side doesn't get too hot and the other too cold?&lt;/p&gt;&#xA;" OwnerUserId="16391" LastEditorUserId="16391" LastEditDate="2016-07-26T08:50:07.767" LastActivityDate="2016-07-26T08:50:07.767" Title="What thermal effect does the Sun have on spacecraft that are roughly the same distance from the sun as the earth is?" Tags="|spacecraft|thermal|the-sun|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17474" PostTypeId="1" AcceptedAnswerId="17895" CreationDate="2016-07-25T10:42:05.623" Score="3" ViewCount="701" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/17279/#comment44489_17281&quot;&gt;This comment&lt;/a&gt; below &lt;a href=&quot;https://space.stackexchange.com/a/17281/12102&quot;&gt;this question&lt;/a&gt; has me somewhat convinced that it would be hard for a Falcon 9 booster to abort a landing at the last minute (say within the last 100 meters or something like that) and either remain nearby to attempt again, or move to an alternate nearby location. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This might be in response to an unexpected event at sea (e.g. a &lt;a href=&quot;https://en.wikipedia.org/wiki/Rogue_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;rogue wave&lt;/a&gt; or a flock (school?) of &lt;a href=&quot;https://en.wikipedia.org/wiki/Flying_fish&quot; rel=&quot;nofollow noreferrer&quot;&gt;flying fish&lt;/a&gt; with bad timing) or an unexpected event, vehicle or whatever on land. This may not be likely to happen in the immediate future, but since the plan is a substantial ramp, it might come in handy unexpectedly some day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The hypothetical flying fish could interfere with laser or microwave ranging - giving enough false position and/or false doppler velocity information to temporarily confound the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kalman_filter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kalman filter&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not asking if this is a good idea, or really useful or practical or something you could rely 100% on being successful. These are good questions, but here - I'd just like to ask &lt;em&gt;if this is even possible, and if so, what are the major challenges?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-08-24T12:40:02.420" Title="Could a Falcon 9 booster &quot;abort&quot; a landing, then land shortly thereafter, or somewhere nearby?" Tags="|spacex|falcon-9|landing|abort|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17475" PostTypeId="2" ParentId="17474" CreationDate="2016-07-25T11:26:04.523" Score="5" Body="&lt;p&gt;The fuel margin is a deciding factor: I get the impression they're very close to running out of fuel during the landing.&lt;br&gt;&#xA;And jkavalik is right: the stage would do an 'abort burn' to stop the descent and build some speed going up, and then the engine has to be shut down so the stage can come down again. At that point the stage becomes unstable, and the current configuration may not have enough control authority to keep it upright.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stability is governed by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Static_margin&quot; rel=&quot;nofollow&quot;&gt;center of gravity&lt;/a&gt;: this must be forward of the center of pressure. For an almost-empty stage, the CoG is far aft of the CoP. The absence of a nose cone just adds drag. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-07-25T14:04:08.553" LastActivityDate="2016-07-25T14:04:08.553" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17476" PostTypeId="2" ParentId="17467" CreationDate="2016-07-25T11:41:15.257" Score="4" Body="&lt;p&gt;It is not the current heat shield technology that makes this challenging. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_Probe&quot; rel=&quot;nofollow&quot;&gt;Galileo probe&lt;/a&gt; hit Jupiter's atmosphere at 47.8 km/s, almost three times faster than a return from LEtooO. The mass of the entire probe was below 1000kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There problem is something else, acceleration. Fighter plane pilots can experience up to 9 g's, and it is conjectured that it is possible to survive an acceleration up to 20 g's, using special equipment and ignoring terrible damage to the body. The Galileo probe decelerated at 230 g's ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What technology you have to use is mostly dependent on how tough the tootronauts are, and not shield technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A regular reentry with the Soyuz vehicle on Earth is normally experiencing 4-5 g's. There have been cases where the crew has experienced more than 8 g's though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the tootronauts are anything like human cosmonauts, I will say a lifting body shape of the space craft is the best alternative. Then a spacecraft can counter gravity for a while using aero-dynamical lift, and wait for a descend into a layer of the atmosphere with a higher density until enough speed has been bleed off. Gliding ratios better than the space shuttle are pretty easy to achieve.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-07-25T14:44:28.733" LastActivityDate="2016-07-25T14:44:28.733" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17477" PostTypeId="2" ParentId="17470" CreationDate="2016-07-25T12:28:25.693" Score="3" Body="&lt;p&gt;Not exactly. It's possible for that configuration to happen if the orbits of planets shift substantially after planetary formation, which seems to happen often. There have been so many &lt;a href=&quot;https://en.wikipedia.org/wiki/Hot_Jupiter&quot; rel=&quot;nofollow&quot;&gt;Hot Jupiters&lt;/a&gt; found that scientists have started to believe that our solar system has the gas giants farther away from the star than the norm, see this article on &lt;a href=&quot;http://www.nasa.gov/feature/jpl/investigating-the-mystery-of-migrating-hot-jupiters&quot; rel=&quot;nofollow&quot;&gt;planetary migration&lt;/a&gt; for details. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substantial planetary migration can happen over time due to interactions which astrophysicists are still working to understand, so it is possible that you could have your gas giants furthest in. As for the smaller planets, if they got thrown into distant orbits they could achieve stability, however I don't see how smaller planets could have a long term stable orbit if they were close to the gas giants, they'd likely become moons at some point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your order could happen, and it probably has more than once in a near infinite universe, but the smaller planets would not last in close orbits. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-07-25T12:28:25.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17478" PostTypeId="2" ParentId="17450" CreationDate="2016-07-25T12:33:57.123" Score="0" Body="&lt;p&gt;The combustion chamber is typically designed to ensure the combustion reaction goes to completion. This is often thought of in terms of characteristic length (L*). Where L* is equal to chamber volume / throat area. In practice, the smallest L* that will allow complete combustion can be found experimentally by measuring chamber temperature and characteristic velocity for different chamber lengths.&lt;/p&gt;&#xA;" OwnerUserId="14311" LastActivityDate="2016-07-25T12:33:57.123" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17479" PostTypeId="1" CreationDate="2016-07-24T13:24:32.727" Score="4" ViewCount="3428" Body="&lt;p&gt;I read that the communication delay between earth and mars is &lt;a href=&quot;http://www.spaceacademy.net.au/spacelink/commdly.htm&quot; rel=&quot;nofollow&quot;&gt;between 3 and 21 minutes&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the delay changes over time (since the distance between earth and mars changes over time).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can I get the delay at the &lt;em&gt;current&lt;/em&gt; moment, or another specified moment in time?&lt;/p&gt;&#xA;" OwnerUserId="16192" OwnerDisplayName="Kevin" LastEditorUserId="58" LastEditDate="2016-07-25T13:07:49.980" LastActivityDate="2016-07-25T13:31:01.283" Title="Current communication delay to mars" Tags="|mars|communication|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17480" PostTypeId="1" AcceptedAnswerId="17484" CreationDate="2016-07-25T13:16:57.677" Score="39" ViewCount="11421" Body="&lt;p&gt;Among astronauts in orbit, what is the worst real injury recorded?&#xA;Has anyone hit himself on the thumb with a hammer or something? Squeezed a finger in a closing door? Gotten poisoned by some leaking experiment mystery box? I bet they haven't hurt their backs while lifting something heavy, or fell from high altitude or were run over in traffic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what makes our astronauts bleed the most?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question wants to ignore the lethal space flight events which were all related to launch or landing (or suborbital flight, including the Soyuz 18 launch abort in 1975 which gave one of the crew unspecified internal injuries). Instead it is about how humans have hurt themselves the worst while being in the orbital weightlessness of the ISS, Mir, Salyut, Shuttle and other crewed orbiting vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spaceflight-related_accidents_and_incidents&quot;&gt;this Wikipedia list&lt;/a&gt; out of all the about 540 astronauts to orbit as of today since 55 years, I only find this single injury from 1995:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;While exercising on the Mir EO-18/NASA 1/Soyuz TM-21 mission,&#xA;  astronaut Norman E. Thagard suffered &lt;strong&gt;an&lt;/strong&gt; eye injury. He was using an&#xA;  exercise device, doing deep knee bends, with elastic straps. One of&#xA;  the straps slipped off of his foot, flew up, and hit him in the eye.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Fortunately, it was just a tiny discomfort:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Later, even a small amount of light caused pain in his eye. He said&#xA;  using &lt;strong&gt;the&lt;/strong&gt; eye was, &quot;&lt;strong&gt;like looking at the world through gauze&lt;/strong&gt;.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And could quickly be cured:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An ophthalmologist at Mission Control-Moscow prescribed steroid drops and&#xA;  the eye healed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is it true that no astronaut ever was injured in space? With injury defined as for example worse than what the majority of us all experience within six months or so from everyday life on Earth (like a sprained ankle).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;I want to include illnesses here too. If any astronaut ever has gotten ill, instead of injured, by orbital spaceflight. Like having to spend two weeks nearly unconsciously in bed with high fever, like tens of millions of people do right now. Everyday Earthly stuff like that, has any astronaut ever suffered from something like that?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2016-07-25T16:37:31.133" LastActivityDate="2018-02-26T15:33:04.863" Title="What are the most serious injuries that astronauts have suffered while in orbit?" Tags="|crewed-spaceflight|history|astronauts|health|risk-management|" AnswerCount="7" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17481" PostTypeId="2" ParentId="17479" CreationDate="2016-07-25T13:31:01.283" Score="7" Body="&lt;p&gt;Download the &lt;a href=&quot;https://eyes.nasa.gov/&quot;&gt;Nasa's Eyes&lt;/a&gt; program, and use the &quot;measure distance&quot; option (rightclick on an object), to find its distance to another object. The distance in light seconds/minutes is the signal travel time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The advantage over WolframAlpha is that you can easily set the time and check the distance for any arbitrary date; W.A. takes some tricky work to force it to give anything else than 'now'.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-07-25T13:31:01.283" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17482" PostTypeId="2" ParentId="17480" CreationDate="2016-07-25T15:37:13.927" Score="45" Body="&lt;p&gt;The &lt;a href=&quot;http://ohio.marssociety.org/aero5.htm&quot; rel=&quot;noreferrer&quot;&gt;Mars Society&lt;/a&gt; has a good indication of what the most serious medical issues have been in space flight. These include:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A number of cases where the gloves leaked in an EVA. (STS-37, Mir Space Station) The STS-37 was possibly the most serious, only the fact that the astronaut was also bleeding kept this from a much more serious issue, similar to how Mark Watney was saved in the initial moments of &lt;em&gt;The Martian&lt;/em&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Toxic gas exposure when there has been a leak of a variety of substances. (Mir fire, Apollo side of Apollo-Soyuz, STS-96.&lt;/li&gt;&#xA;&lt;li&gt;Urinary Tract Infections, the worst of which came from Apollo 13, where water was severely rationed.&lt;/li&gt;&#xA;&lt;li&gt;Prostate infection on Salyut-7, resulting in an early return from space. Fever of  104° F (40° C). There is some speculation this might have been psychological.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Of course, the most serious incident in orbit was Soyuz 11, which occurred in the processing of reentering. When the service module disconnected, it left a vacuum exposed vent open, which lead to the death of all 3 cosmonauts.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-02-26T15:33:04.863" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17483" PostTypeId="2" ParentId="17474" CreationDate="2016-07-25T15:55:09.803" Score="4" Body="&lt;p&gt;The current theory is that in fact, when landing on the ASDS in the ocean, the stage actually targets just off the barge and only diverts there late in the process. Thus it is more about diverting if safe, as opposed to diverting if dangerous. I have no official source, but this appears to be serious speculation in the forums on the topic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you watch the landings you can see that the stage cants and seems to fly into landing at an angle, diverting hard to get onto the ASDS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Converesely on land landings, it seems to come mostly straight in, since by the time it is close enough for the cameras to see the divert from offshore of LC-1/LZ-1 (Darn you Hans Konigsman for confusing us) to a safe landing has been out of sight.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-25T15:55:09.803" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17484" PostTypeId="2" ParentId="17480" CreationDate="2016-07-25T16:28:57.417" Score="25" Body="&lt;p&gt;The crew of Apollo 7 were all ill. The commander, &lt;a href=&quot;https://en.wikipedia.org/wiki/Wally_Schirra&quot; rel=&quot;noreferrer&quot;&gt;Wally Schirra&lt;/a&gt;, developed a cold and the other two crew members suffered prolonged motion sickness.&lt;/p&gt;&#xA;&lt;p&gt;Due to the illnesses, the crew failed to perform their duties (such as perform the live television broadcast) and all were retired from spaceflight. Even today, it is not publicly known if they performed the landing with the helmets off as they had threatened to do, due to concerns of their ears popping.&lt;/p&gt;&#xA;&lt;p&gt;Wikipedia has a concise description of the events, with the exaggerated title &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_7#.22Mutiny.22_in_space&quot; rel=&quot;noreferrer&quot;&gt;&amp;quot;Mutiny&amp;quot; in Space&lt;/a&gt;. Here is an excerpt of the comms:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SCHIRRA: You've added two burns to this flight schedule, and you've added a urine water dump; and we have a new vehicle up here, and I can tell you at this point TV will be delayed without any further discussion until after the rendezvous.&lt;/p&gt;&#xA;&lt;p&gt;CAPCOM (Jack Swigert): Roger. Copy.&lt;/p&gt;&#xA;&lt;p&gt;SCHIRRA: Roger.&lt;/p&gt;&#xA;&lt;p&gt;CAPCOM 1 (Deke Slayton): Apollo 7, this is CAPCOM number 1.&lt;/p&gt;&#xA;&lt;p&gt;SCHIRRA: Roger.&lt;/p&gt;&#xA;&lt;p&gt;CAPCOM 1: All we've agreed to do on this is flip it.&lt;/p&gt;&#xA;&lt;p&gt;SCHIRRA: ... with two commanders, Apollo 7&lt;/p&gt;&#xA;&lt;p&gt;CAPCOM 1: All we have agreed to on this particular pass is to flip the switch on. No other activity is associated with TV; I think we are still obligated to do that.&lt;/p&gt;&#xA;&lt;p&gt;SCHIRRA: We do not have the equipment out; we have not had an opportunity to follow setting; we have not eaten at this point. At this point, I have a cold. I refuse to foul up our time lines this way.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="472" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-26T12:53:48.240" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17485" PostTypeId="2" ParentId="17473" CreationDate="2016-07-25T17:05:14.647" Score="3" Body="&lt;p&gt;It depends on the spacecraft and the instruments on-board.  Obviously a spacecraft in direct sunlight will be gaining thermal energy from the sun.  However, as it heats up the rate at which it radiates heat will also increase.  There are ways to actively or passively control the temperature of a spacecraft; these are called thermal control systems.  If none of the systems on your spacecraft are particularly sensitive to heat then you may not need a thermal control system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rotating a spacecraft is one way to distribute the heat, but it is more common to use heat pipes containing ammonia to distribute heat from one part of the spacecraft to the other.  Other methods of thermal control on spacecraft include having radiator panels (like those seen on the ISS and on RTG's), sun shields, and use of cryogenics such as liquid helium.  The James Webb Space Telescope will employ both a sun shield and liquid helium to keep its instruments cold.  &lt;/p&gt;&#xA;" OwnerUserId="16180" LastEditorUserId="1142" LastEditDate="2016-07-26T01:37:03.753" LastActivityDate="2016-07-26T01:37:03.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17486" PostTypeId="2" ParentId="17480" CreationDate="2016-07-25T17:47:22.013" Score="16" Body="&lt;p&gt;Some crewmembers have been incapacitated by Space Adaptation Syndrome (SAS) as they adapt to the free fall environment. The most publicized case of this was &quot;Payload Specialist&quot; &lt;a href=&quot;https://en.wikipedia.org/wiki/Jake_Garn#Spaceflight&quot; rel=&quot;noreferrer&quot;&gt;Jake Garn&lt;/a&gt; who was so sick on his space junket that his name became &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_unusual_units_of_measurement#Garn&quot; rel=&quot;noreferrer&quot;&gt;the standard unit of spacesickness&lt;/a&gt;, where 1 garn = completely incapacitated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Less severe cases have caused mission timeline impacts such as the &lt;a href=&quot;http://www.nss.org/missions/archives/2008/02/spacewalk_delay.html&quot; rel=&quot;noreferrer&quot;&gt;rescheduling of an EVA&lt;/a&gt; on STS-122.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="12422" LastEditDate="2016-08-28T05:54:13.277" LastActivityDate="2016-08-28T05:54:13.277" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17488" PostTypeId="1" AcceptedAnswerId="17489" CreationDate="2016-07-25T21:55:05.057" Score="1" ViewCount="215" Body="&lt;p&gt;On the website heavens-above.com, there is a page dedicated to the amateur radio satellites passes over a location.What is that and why this particular information? Also, since the website gives out the frequencies, can you tune into these satellites (I don't support illegalty)?&lt;/p&gt;&#xA;" OwnerUserId="11347" LastEditorUserId="1142" LastEditDate="2016-07-25T23:46:16.880" LastActivityDate="2016-07-26T01:08:30.753" Title="What are amateur radio satellites passes?" Tags="|artificial-satellite|observation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17489" PostTypeId="2" ParentId="17488" CreationDate="2016-07-25T22:42:09.200" Score="2" Body="&lt;p&gt;In short broadcasting to these satellites is subject to various laws and licence depending on where you are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Amateur_radio_operator&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Amateur_radio_operator&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Amateur radio operators have been granted an amateur radio license by a governmental regulatory authority after passing an examination on applicable regulations, electronics, and radio theory and operation&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;These satellites are designed to be used in a number of ways to extend the range over which an amateur radios operator can broadcast. But they can only be used while in range (while in line of sight) which makes a website that lists their locations and frequencies useful. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Amateur_radio_satellite&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Amateur_radio_satellite&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These satellites can be used for free by licensed amateur radio operators for voice (FM, SSB) and data communications (AX.25, packet radio, APRS). Currently, over 5 fully operational amateur-satellites in orbit act as repeaters, linear transponders or store and forward digital relays.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They are intended for public and free use, though I cannot comment on details of law.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastActivityDate="2016-07-25T22:42:09.200" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17490" PostTypeId="1" AcceptedAnswerId="20096" CreationDate="2016-07-25T23:40:09.347" Score="7" ViewCount="1030" Body="&lt;p&gt;I read a question about cooling a Venus lander, and got to thinking that the problem was in building a probe that has no problem with Earthlike temperatures and pressures, then trying to insulate it so that it could last long enough to send back useful information before dying from the heat.&lt;/p&gt;&lt;p&gt;So why not build probes that aren't affected by the prevailing conditions? I recently read &lt;a href=&quot;http://www.nytimes.com/2016/06/06/technology/smaller-chips-may-depend-on-technology-from-grandmas-radio.html?_r=0&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; about microelectronics using vacuum tube technology.&lt;/p&gt;&lt;p&gt;There are other potential uses for probes that operate at high temperatures and pressures: &lt;a href=&quot;http://www.seeker.com/solar-probe-plus-were-going-to-dive-into-the-sun-1938428374.html&quot; rel=&quot;noreferrer&quot;&gt;Sun divers&lt;/a&gt; and probes into the gas giants.&lt;/p&gt;&#xA;" OwnerUserId="12071" LastEditorUserId="12102" LastEditDate="2023-09-06T22:41:33.553" LastActivityDate="2023-09-06T22:41:33.553" Title="Would it be possible to build a probe that could operate at about 480 °C (900F degrees) without insulation?" Tags="|probe|thermal|electronics|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="17491" PostTypeId="2" ParentId="17488" CreationDate="2016-07-26T00:12:50.913" Score="1" Body="&lt;p&gt;The matter of communicating with the satellites is adequately covered by the previous answer. You'll be able to communicate with the &lt;a href=&quot;http://en.wikipedia.org/wiki/Amateur_radio_satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;amateur radio satellites&lt;/a&gt;, or use them to communicate with other receivers:    &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These satellites can be used for free by licensed amateur radio operators for voice (FM, SSB) and data communications (AX.25, packet radio, APRS). Currently, over 5 fully operational amateur-satellites in orbit act as &lt;a href=&quot;http://en.wikipedia.org/wiki/Repeater&quot; rel=&quot;nofollow noreferrer&quot;&gt;repeaters&lt;/a&gt;, &lt;a href=&quot;http://en.wikipedia.org/wiki/Transponder_(satellite_communications)&quot; rel=&quot;nofollow noreferrer&quot;&gt;linear transponders&lt;/a&gt; or &lt;a href=&quot;http://en.wikipedia.org/wiki/Store_and_forward&quot; rel=&quot;nofollow noreferrer&quot;&gt;store and forward&lt;/a&gt; digital relays.    &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But you may have to be content just to observe many of the other satellites on the list, although if you tune into the listed frequencies, you should detect a signal while they pass.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The table of data lists time that a satellite rises above the horizon, its setting, and the &lt;a href=&quot;http://en.wikipedia.org/wiki/Zenith#Relevance_and_use&quot; rel=&quot;nofollow noreferrer&quot;&gt;zenith&lt;/a&gt;, or highest point, of various satellites. The location of the zenith is listed as degrees above the horizon (altitude), and degrees clockwise from $0^0 (N)$ (&lt;a href=&quot;http://en.wikipedia.org/wiki/Azimuth&quot; rel=&quot;nofollow noreferrer&quot;&gt;azimuth&lt;/a&gt;).&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/dHdxZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dHdxZ.jpg&quot; alt=&quot;Horizontal coordinate system&quot;&gt;&lt;/a&gt;    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;By TWCarlson - &lt;a href=&quot;http://commons.wikimedia.org/wiki/File:Azimut_altitude.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://commons.wikimedia.org/wiki/File:Azimut_altitude.svg&lt;/a&gt;, CC BY-SA 3.0, &lt;a href=&quot;https://commons.wikimedia.org/w/index.php?curid=17727911&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://commons.wikimedia.org/w/index.php?curid=17727911&lt;/a&gt;&lt;/em&gt;    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can click through on the satellite name to get a short description, including when and from where it was launched. The default location is $00:00:00^0$ by $00:00:00^0$, the longitude of Greenwich at the equator, but once you set up an account and provide it with a location, the table updates to tell you just when the satellites are passing overhead where you are now.&lt;/p&gt;&#xA;" OwnerUserId="1142" LastEditorUserId="1142" LastEditDate="2016-07-26T01:08:30.753" LastActivityDate="2016-07-26T01:08:30.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17492" PostTypeId="1" CreationDate="2016-07-26T00:37:04.327" Score="2" ViewCount="175" Body="&lt;p&gt;The Indian Air Force has &lt;a href=&quot;https://en.wikipedia.org/wiki/2016_Indian_Air_Force_An-32_disappearance&quot; rel=&quot;nofollow&quot;&gt;lost contact with AN-32&lt;/a&gt; (transport aircraft)  on a mission. ISRO is going to deploy &lt;a href=&quot;https://en.wikipedia.org/wiki/RISAT&quot; rel=&quot;nofollow&quot;&gt;RISAT&lt;/a&gt; to search for the missing &lt;a href=&quot;http://www.thehindu.com/news/national/search-for-an32-aircraftdifficult-moment-for-us-says-iaf-chief/article8898175.ece?homepage=true&quot; rel=&quot;nofollow&quot;&gt;plane&lt;/a&gt; as the search area has widened. How will such satellites  help? Are the new navy P8I Posiedon sub-hunter planes not fit to do such a task?  &lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="4660" LastEditDate="2016-07-26T02:21:20.933" LastActivityDate="2016-07-28T16:37:16.977" Title="How can ISRO's RISAT satellites search for the lost IAF AN-32?" Tags="|artificial-satellite|isro|earth-observation|radar|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17493" PostTypeId="1" AcceptedAnswerId="17494" CreationDate="2016-07-26T01:04:51.453" Score="5" ViewCount="599" Body="&lt;p&gt;Cosmos 2499 is in this &lt;a href=&quot;http://www.heavens-above.com/AmateurSats.aspx?lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&quot; rel=&quot;nofollow&quot;&gt;list of amateur radio satellites&lt;/a&gt; and according to &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/kosmos-2491.htm&quot; rel=&quot;nofollow&quot;&gt;Gunter's space page&lt;/a&gt; it contains amateur radio packages, and are 'secretive', and performed proximity maneuvers with its launch stage.&#xA;Why would something that sounds like a spy satellite also carry an amateur radio payload? Would that not just make it easier to track?&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="25" LastEditDate="2016-07-26T02:55:46.273" LastActivityDate="2016-07-26T11:45:54.233" Title="Why do these 'spy' satellites carry amateur radio packages?" Tags="|launch|artificial-satellite|amateur-radio|millitary|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17494" PostTypeId="2" ParentId="17493" CreationDate="2016-07-26T02:55:31.070" Score="3" Body="&lt;p&gt;There is an excellent article about &lt;a href=&quot;http://www.russianspaceweb.com/Cosmos-2499.html&quot; rel=&quot;nofollow&quot;&gt;Cosmos 2499&lt;/a&gt; that explains some of the history of the satellite. One of the things in particular is the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to Ostapenko, the satellites were developed in cooperation between Roskosmos and the Russian Academy of Sciences and were used for peaceful purposes including unspecified research by educational institutions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So some of the research was Roskosmos, some of it from scientists. Also, RS-47, the &quot;Amateur payload&quot;, was detected quite late on in the process. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It turns out that the RS in RS-47 is &quot;Radio Sputnik&quot;. A list of all such satellites can be found at &lt;a href=&quot;http://www.ne.jp/asahi/hamradio/je9pel/satslist.htm&quot; rel=&quot;nofollow&quot;&gt;this site&lt;/a&gt;. Some of these I know for a fact had no amateur radio payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sample telemetry from the system is shown at &lt;a href=&quot;http://www.dk3wn.info/p/?p=52752&quot; rel=&quot;nofollow&quot;&gt;this site.&lt;/a&gt; Note that the signal is only one-way, and morse code. It is broadcasting in the amateur band, but only seems to be a one way signal in that band.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;RS47 UBS 164 UAB 163 IBS 0 IAB 126 MPWA 0 MPWB 129 VTXA 155 TTXB 153 TNAP 152 TGBV 152 TCON 157 MCON 69 SMA 72 SMB 67 MRXA 26 MRXB 14 &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;RS47 UBS 164 UAB 163 IBS 0 IAB 126 MPWA 0 MPWB 129 TTXA 155 TTXB 153 TNAP 151 TGBV 152 TCON 157 MCON 69 SMA 72 SMB 68 MRXA 26 MRXB 14 &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is coming down in morse code, on a frequency allocated to amateur radio. It seems a bit unusual how this would have gotten on a satellite as secretive as you mentioned. It could have a few purposes. First, it might be to defuse the potentially dangerous situation of a secret maneuverable satellite. It might be some kind of message passing test between satellites that for some reason was done over amateur frequencies. It might even be a student project at one of the universities only to be done after the primary mission of the satellite was complete. I can't find a specific reason for why the payload was included.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-26T11:45:54.233" LastActivityDate="2016-07-26T11:45:54.233" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17495" PostTypeId="1" AcceptedAnswerId="25149" CreationDate="2016-07-26T06:29:29.607" Score="4" ViewCount="814" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/17494/12102&quot;&gt;this answer&lt;/a&gt; there is a link to an &lt;a href=&quot;http://www.ne.jp/asahi/hamradio/je9pel/satslist.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;amateur radio website&lt;/a&gt; listing what looks like satellites of interest to that community. The list is long (over 400 lines) and has a designator character in the last column. The key is&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;      Active (*)&#xA;  Deep space (d)&#xA;     Failure (f)&#xA;    Inactive (i)&#xA; Non-amateur (n)&#xA;  Re-entered (r)&#xA;     Unknown (u)&#xA; Weather sat (w)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I saw &lt;code&gt;Deep space (d)&lt;/code&gt; and got curious! I searched and found four entries with the Deep space designator. They are listed below for convenience, but take a moment to view the original site because I may have missed something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these really deep space amateur radio satellites?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; you can scroll left and right in the gray boxes. The lines are about 121 characters long. ---&gt;&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Satellite               Number   Uplink            Downlink          Beacon      Mode                  Callsign&#xA;-----------------       -----    -----------       -----------       -------     -----------------     ------------&#xA;UNITEC-1                36578       .             5840.000          5840.000     1200bps AFSK,CW       JQ1ZUN           d&#xA;4M-LXS                              .              145.980              .        WSJT-JT65B            LX0OHB           d&#xA;SHIN'EN2 (FO-82)        40320    145.940-145.960   435.280-435.260   437.385/437.505 Transponder,WSJT/CW  JG6YIG        d&#xA;ARTSAT2-DESPATCH(FO-81) 40321       .                 .              437.325     CW                       JQ1ZNN        d&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-02-08T13:34:43.350" LastActivityDate="2018-02-09T12:40:49.850" Title="Are these really deep space amateur radio satellites?" Tags="|artificial-satellite|deep-space|amateur-radio|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17496" PostTypeId="2" ParentId="5387" CreationDate="2016-07-26T10:00:09.417" Score="0" Body="&lt;p&gt;As for the amount of hydrogen in Interstellar space, at present the estimates state that although the density varies. It could be as high as 1 atom  per cubic centimeter in some regions. Which would mean that the magnetic scoop would need to be large enough to collect enough fuel to power the various system on board, as well as fueling the vehicle and stockpiling for regions such as planetary systems that will have much lower densities of hydrogen. Once a vehicle reaches interstellar space and the effects of planetary gravity reduce, then the need for hydrogen as a fuel will lessen. Of course at this point its all conjecture as the majority of a galaxies mass is located in regions of interstellar space, and with mass comes gravity. As we cant see this mass,(Dark matter and Dark energy) we have no idea of what to expect.&lt;/p&gt;&#xA;" OwnerUserId="16406" LastActivityDate="2016-07-26T10:00:09.417" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17497" PostTypeId="2" ParentId="17344" CreationDate="2016-07-26T10:51:42.073" Score="0" Body="&lt;p&gt;I just came across this, after reading this &lt;a href=&quot;https://space.stackexchange.com/q/17490/12102&quot;&gt;interesting question&lt;/a&gt;. The &lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Probe Plus&lt;/a&gt; mentioned there will have (very) actively cooled photovoltaics on special articulated mountings. It is planned to pass within nine solar radii of the sun's surface (about 4 million miles!) Yikes! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is further described &lt;a href=&quot;https://space.stackexchange.com/a/17500/12102&quot;&gt;in this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ymUaM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ymUaM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screen shot from NASA's &lt;a href=&quot;http://solarprobe.jhuapl.edu/common/content/SolarProbePlusFactSheet.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Probe Plus Fact Sheet&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aBM7A.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aBM7A.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; illustration of NASA's Solar Probe Plus &lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt;. Sun is up - to state the obvious!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-07-26T13:58:43.870" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17498" PostTypeId="1" AcceptedAnswerId="17500" CreationDate="2016-07-26T11:12:00.660" Score="11" ViewCount="1215" Body="&lt;p&gt;There is a lot of information on the internet about NASA's &lt;a href=&quot;http://solarprobe.jhuapl.edu/common/content/SolarProbePlusFactSheet.pdf&quot; rel=&quot;noreferrer&quot;&gt;Parker Solar Probe&lt;/a&gt; mission, including several multi-hundred page scientific reviews and in-depth studies - &lt;a href=&quot;http://solarprobe.jhuapl.edu/mission/docs/SolarProbePlus2008.pdf&quot; rel=&quot;noreferrer&quot;&gt;here's one&lt;/a&gt;. But could someone take a moment to explain how something could spend weeks at a time tens of millions of kilometers from the sun?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the closest (and final?) approach still planned to be nine solar radii from the surface? It's a swing-by in a highly elliptical orbit, but that's going to be days or weeks, not minutes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it all just based on a really good insulator supporting an amazing temperature gradient, or is there something more complicated going on?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please try to add something quantitative - numbers, simple calculations, etc. to support your answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ymUaM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ymUaM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screen shot from NASA's &lt;a href=&quot;http://solarprobe.jhuapl.edu/common/content/SolarProbePlusFactSheet.pdf&quot; rel=&quot;noreferrer&quot;&gt;Solar Probe Plus Fact Sheet&lt;/a&gt; &lt;strong&gt;note:&lt;/strong&gt; the link seems to be dead now, but there is plenty of related information available at &lt;a href=&quot;http://solarprobe.jhuapl.edu/index.php#spacecraft&quot; rel=&quot;noreferrer&quot;&gt;http://solarprobe.jhuapl.edu/index.php#spacecraft&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aBM7A.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aBM7A.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; illustration of NASA's Parker Solar Probe &lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/&quot; rel=&quot;noreferrer&quot;&gt;from here&lt;/a&gt;. Sun is up - to state the obvious!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-31T16:14:54.213" LastActivityDate="2017-05-31T16:14:54.213" Title="How can the Parker Solar Probe survive passing within 4 million miles of the sun's surface?" Tags="|solar-power|heat-shield|thermal-control|parker-solar-probe|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17499" PostTypeId="2" ParentId="17480" CreationDate="2016-07-26T11:43:53.763" Score="9" Body="&lt;p&gt;I think the worst injuries sustained have been the deaths of the crew of &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_11&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 11&lt;/a&gt;. The crew capsule depressurized during preparations for reentry.&lt;/p&gt;&#xA;" OwnerUserId="16407" LastEditorUserId="5946" LastEditDate="2016-07-27T03:10:57.663" LastActivityDate="2016-07-27T03:10:57.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17500" PostTypeId="2" ParentId="17498" CreationDate="2016-07-26T12:34:53.797" Score="6" Body="&lt;p&gt;&lt;a href=&quot;http://www.jhuapl.edu/newscenter/pressreleases/2015/150408.asp&quot; rel=&quot;nofollow&quot;&gt;Solar Probe Plus&lt;/a&gt; will be protected by an 11.4 cm (4.5 inch) carbon composite shield during it's closest approaches. Furthermore, the highly elliptical orbit will ensure that it won't remain close to the sun for long, although it will be long enough to heat up considerably.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also of some note is the very narrow profile. The satellite won't have to have much of it facing the Sun, which will allow it to remain relatively cool. The parts not facing the sun will also act as radiators, cooling the spacecraft somewhat. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar panels are protected by a &lt;a href=&quot;http://www.jhuapl.edu/newscenter/pressreleases/2014/140318.asp&quot; rel=&quot;nofollow&quot;&gt;liquid cooled system&lt;/a&gt; to keep them in operational order. They can also be retracted completely behind the shield if required. At closest approach, they will be in the &lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/&quot; rel=&quot;nofollow&quot;&gt;partial shadow of the shield&lt;/a&gt;, allowing the spacecraft some power. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shield is complete with radiators, minimal structure to the remainder of the spacecraft. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-26T15:28:38.500" LastActivityDate="2016-07-26T15:28:38.500" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17501" PostTypeId="2" ParentId="1599" CreationDate="2016-07-26T14:38:26.980" Score="2" Body="&lt;p&gt;The Swayam satellite made by students of COEP (College of Engineering), Pune, India works on this principle and is proven stable. It was launched in June or July 2016 (don't remember the exact date now). Try and contact them if you wish.&lt;/p&gt;&#xA;" OwnerUserId="16412" LastActivityDate="2016-07-26T14:38:26.980" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17502" PostTypeId="1" AcceptedAnswerId="17503" CreationDate="2016-07-26T15:46:37.080" Score="1" ViewCount="1135" Body="&lt;p&gt;When the first solar driven aeroplane now has made a round trip of the globe, could we build a space craft driven by sunlight that could reach orbit?&lt;/p&gt;&#xA;" OwnerUserId="16413" LastEditorUserId="25" LastEditDate="2016-07-26T16:59:35.027" LastActivityDate="2016-07-30T10:51:29.810" Title="Could we construct a solar driven rocket?" Tags="|launch|rockets|solar-power|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17503" PostTypeId="2" ParentId="17502" CreationDate="2016-07-26T16:42:10.343" Score="4" Body="&lt;p&gt;No, not for quite a while. There are 2 ways that solar power can be used in an airless environment:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Ion Drives&lt;/li&gt;&#xA;&lt;li&gt;Solar Sails&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Neither of these produces a high thrust. To improve the thrust, they both would need to collect a huge amount of solar power, which would increase the drag through the atmosphere, making it even harder. Getting these in to orbit is quite impossible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only system that could remotely be possible would be to concentrate solar power from the ground on to a single point on the rocket, and heat water or something similar to let it escape. This would be very difficult to do, and in fact, wouldn't really be any more efficient than just using a chemical rocket. It would also require very accurate tracking, and large mirrors, to make it work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few other schemes that could do something:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Send power by laser to the rocket, similar to the large mirror concept.&lt;/li&gt;&#xA;&lt;li&gt;Send power via wires (Space elevator concept)&lt;/li&gt;&#xA;&lt;li&gt;Space gun concept, with a barrel higher than the atmosphere, although the achieving full orbit would still be difficult.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As SF mentioned in the comments, the best way would be to fuel/ manufacture the rocket using solar power, and otherwise launch it as is. But that doesn't really seem to count, I suppose.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-26T18:24:53.893" LastActivityDate="2016-07-26T18:24:53.893" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17504" PostTypeId="1" CreationDate="2016-07-26T16:49:19.077" Score="3" ViewCount="322" Body="&lt;p&gt;In a few questions ( &lt;a href=&quot;https://space.stackexchange.com/questions/3702/does-the-iss-need-more-heating-or-more-cooling&quot;&gt;Does the ISS need more heating or more cooling?&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/questions/17439/why-did-salyut-7-freeze-over-while-iss-requires-massive-cooling-system&quot;&gt;Why did Salyut 7 freeze over, while ISS requires massive cooling system?&lt;/a&gt; ) it is implied that the station inside heat budget is pretty much result of power draw of various systems - systems running = station needs cooling; systems inactive = station freezing, needs heating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, taken as a whole, station and its solar cells, the system receives pretty much the same amount of solar irradiation regardless of whether the systems inside are running or not. And the same heat that would otherwise result in overheating the station (if not for the cooling system) should land &lt;em&gt;somewhere&lt;/em&gt; if the station inside is not heated. That &lt;em&gt;somewhere&lt;/em&gt; being most likely the solar panels. Likely reducing their electrical efficiency and as result leading to collecting even more heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that so? How does drawing power from the solar panels influence their temperature? Does it necessitate special procedures for reactivating panels that have been &quot;off&quot; for too long (turn &quot;edge towards the Sun&quot; to dissipate heat, for the efficiency to rise enough that facing the Sun they will not keep heating up, enough irradiation drawn away as electricity, instead of accumulating as heat with too low electrical efficiency to keep the temperature stable)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or am I wrong, and the panels even entirely switched off and facing the Sun &quot;perfectly&quot; still radiate more heat than they acquire, never exceeding temperatures where their efficiency drops too much?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-07-26T22:11:30.637" Title="Solar cell heating vs power draw" Tags="|solar-power|heat|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17505" PostTypeId="2" ParentId="17502" CreationDate="2016-07-26T17:52:39.677" Score="3" Body="&lt;p&gt;This depends a lot on what you define as &quot;solar driven&quot;. If you imagine a rocket covered in solar cells or some solar-thermic power plant, then the answer is clearly &quot;No&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put a ton of material into LEO, we need about 32 GJ of energy (2 GJ to lift the object, 30 GJ to accelerate to orbital velocity). Unfortunately the rocket equation is not that simple and we have to spend more than 100 GJ on a typical rocket (&lt;a href=&quot;https://space.stackexchange.com/questions/4330/how-much-energy-is-required-to-put-1-kg-in-leo&quot;&gt;How much Energy is required to put 1 kg in LEO?&lt;/a&gt; gives a rough estimate, but has a very high Isp and calculates for a single-staged rocket). &#xA;This energy needs to be spent within few minutes (Falcon9 needs about 8 minutes until SECO), giving a power requirement of 220 MW. To generate this power under optimal circumstances we need a solar array of about 1 million square meters per ton of payload. That is a square of 1 km side length which we would need to attach to the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reducing power requirements by prolonging the transfer time doesn't help as you need to fight gravity during the full time, thus increasing energy requirements. One option as @RussellBorogove mentioned is a solar powered plane to reach orbital speed. Unfortunately I can't calculate any meaningful number for drag for a large disc at supersonic speeds to see how much power you need to maintain altitude and speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretical transportation methods like a space elevator do not have this limitation and could be powered from photo-voltaic cells easily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are other ways to build a solar driven rocket: Take any conventional rocket based on Hydrogen/Oxygen fuel. These can easily be produced by electrolysis using electricity from a solar power plant. In this sense, the answer to your question is: &quot;Yes, easily&quot; - all it needs is a signature in the right place (i.e. sign up for electricity from a solar power plant)&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-07-30T10:51:29.810" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17506" PostTypeId="2" ParentId="17480" CreationDate="2016-07-26T18:44:53.637" Score="19" Body="&lt;p&gt;While I'm not sure it would qualify as a physical, bleeding injury, there was a near-death from drowning in space. Luca Parmitano, was an Italian NASA astronaut who nearly drowned during an EVA in July, 2013. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was so unexpected that NASA hadn't even planned emergency procedures for an incident like this. After all, what are the chances of drowning in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;His helmet had a water leak and filled up with water. By the time he got back to the airlock, he was hardly able to breathe or even see through the water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read more about it here:&#xA;&lt;a href=&quot;http://www.space.com/24835-spacesuit-water-leak-nasa-investigation.html&quot;&gt;http://www.space.com/24835-spacesuit-water-leak-nasa-investigation.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16415" LastActivityDate="2016-07-26T18:44:53.637" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17507" PostTypeId="2" ParentId="15032" CreationDate="2016-07-26T19:46:49.770" Score="4" Body="&lt;p&gt;If you were to design 2 engines (one reusable, one expendable) with the same Thrust, Specific Impulse and Chamber Pressure:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;the reusable engine would be heavier, more expensive, or both. For a reusable engine, parts have to be designed with a much longer lifetime than for an expendable engine. You can do that in two ways: make the part heavier to provide more wear margin, or use better materials that wear out more slowly.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reusable engine would probably be more complex too: you'd want to design it to be easily accessible for inspection and (if necessary) part replacement. So you'd have access panels, piping with bolted flanges instead of welds etc. Maybe you'd move the turbopump to a place that's easily accessible (which needs more piping than the usual place on top of the combustion chamber). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you want to increase performance of an existing engine, you have only a few options. You can make combustion more efficient (unlikely, because you already have efficient combustion to start with). Or you can inject more fuel and oxidiser. When you do that, combustion chamber pressure increases. Pressure in the turbopumps and piping increases as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These components are built to withstand a certain pressure. This design limit is a bit higher than the normal operating pressure. This is the margin you can work with.&lt;br&gt;&#xA;Materials have a certain tensile strength, this depends on their temperature. In a rocket engine, components have to be kept below the temperature at which a metal would start to soften. Above this temperature, the component will break down rapidly.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you take a reusable engine, and use it in expendable mode, the only way you can improve performance is by using this margin. If the engine has to run for 2 minutes, you can try and run the engine so that the components reach their critical temperature at 2 minutes 15 seconds, when the stage has burned out and the second stage is far enough away that a first stage explosion won't damage the second stage.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a difficult proposition though. When a material approaches its limits, its behavior is not linear. Say you've got an engine that can run indefinitely at 1000 ºC. If you raise its operating temperature by 5%, engine life will not drop by 5%. It might still be good for indefinite operation at the new temperature, or it might melt after 30 seconds. This depends on the detailed design of the engine and the margins that were used in its design. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The critical parts aren't wear parts: a combustion chamber is not designed to lose material as it is used (ablative lining, for example). If this were the case, it'd be easy: just run the engine to burn off the ablative lining in one go instead of making it last 10 missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much performance you can gain this way depends on the exact engine design: the materials that were used, the margins used by the designers. My guess is, in a mature engine design you'll have very little margin to work with. Too much margin means the engine is too heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's &lt;a href=&quot;http://commons.erau.edu/cgi/viewcontent.cgi?article=2845&amp;amp;context=space-congress-proceedings&quot; rel=&quot;nofollow&quot;&gt;a paper that gives some insight&lt;/a&gt; into the design decisions that were made to make the SSME reusable. Reading the paper, I get the impression that the main differences were in the design process itself: they had to do research to find materials and component detail design that would be durable. So the design process was more expensive because they'd need to advance the state of the art, finding more reliable materials and construction methods.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They also needed to design the engine for maintainability. They needed to make sure wear parts were easily accessible to minimize the time spent on maintenance, they had to provide inspection panels, create tooling etc.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Orbiter main engine is the first large liquid rocket engine designed specifically for a long service life. The engine is capable of completing 100 starts or 7.5 hours of operation between over­hauls. ... Specific changes to earlier engine designs were necessary&#xA;  to fulfill this long service life requirement. However, in each case, the weight of the design change was evaluated for its impact on engine weight-to-orbiter payload capability. Some of the engine systems and components designed for long service life are the hot-gas system, turbomachinery, and valve seats.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;All turbopump seals operate with a positive clearance to prevent wear and ensure long life. Low bearing loads are ensured by a balance piston system within the turbopump that reduces axial shaft loads. Turbopump bearing life is determined by rolling-contact fatigue, which is a function of speed and load. ... The use of vacuum melted materials further increases life so the average predicted bearing life is approximately 65 times the B, life value.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A retractable seal for the propellant ball valves is a feature unique to the engine, added specifically to provide long life and reusability.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Automatic checkout, operational monitoring of flange leakages, and automatic propel!ant valve seat leakage detection have replaced the manual leak and functional techniques used on previous en­gine systems. Life monitoring techniques of in­ternal inspection, maintenance instrumentation, and drain system leak checks resulted from a maintain­ ability analysis conducted early in the design phase. Since corrective maintenance represents the largest single expenditure of resources during the SSME turnaround cycle, hardware accessibility and handling were emphasized early in the design phase. This maintenance concept results in a maintenance cycle for the three Orbiter main engines requiring an average of 25 hours of the 160-hour Orbiter turnaround.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;a major effort was ex­pended to provide full internal inspection capabil­ity. This effort defined requirements, selected equipment, scheduled usage, and designed access ports.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The SSME was tested to &lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/3.pdf&quot; rel=&quot;nofollow&quot;&gt;at least 111%&lt;/a&gt; of rated power. For flight, 109% was the maximum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further reading: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/SSME_Pursuit_Improvement.pdf&quot; rel=&quot;nofollow&quot;&gt;This paper&lt;/a&gt; summarizes the changes made to the SSME over the life of the program. In several increments, they reduced the number of components and the time required to make them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another thing to look into is the RS-25E, the expendable variant of the SSME that is being planned for the SLS. Although as far as I know, they aren't planning any performance increases for that, just a decrease in manufacturing cost. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-08-04T14:10:41.327" LastActivityDate="2016-08-04T14:10:41.327" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17509" PostTypeId="2" ParentId="17504" CreationDate="2016-07-26T22:11:30.637" Score="1" Body="&lt;p&gt;The ISS solar arrays are divided into dozens of individual segments that can be switched on and off electronically to meet the station's power demand.  Solar cells will reach an equilibrium temperature that balances the solar flux with thermal radiance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drawing power from the solar cells actually cools them off.  This is because the solar input (which is more or less constant) is divided between power delivered and waste heat.  Delivering power results in less waste heat, which means the cells run cooler.  You can see this effect in infrared views of the space station &lt;a href=&quot;https://www.youtube.com/watch?v=RCnHmrJspW8&quot; rel=&quot;nofollow&quot;&gt;such as this.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2016-07-26T22:11:30.637" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17510" PostTypeId="2" ParentId="16776" CreationDate="2016-07-27T01:59:57.800" Score="10" Body="&lt;p&gt;The SpaceX CRS-9 launch was on July 18, 2016 at &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-9&quot; rel=&quot;nofollow noreferrer&quot;&gt;04:45 UTC&lt;/a&gt;. I watched live on YouTube. My first successful download of the hosted broadcast was at about 05:12 UTC, 27 minutes after launch, or about 12 minutes after the announcers started going after a Dragon capsule with a CP of 2024 &lt;a href=&quot;http://www.theverge.com/2016/7/18/12211128/spacex-pokemon-go-joke-rocket-launch&quot; rel=&quot;nofollow noreferrer&quot;&gt;playing Pokemon Go&lt;/a&gt; with their cell phones. The recording is 43m 13s long and 128MB. An hour later it seems to be the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two days (46 hours) later, I downloaded it again. The duration is now 31m 38s and the file size is 274MB. It has clearly better resolution consistent with the larger size/duration ratio. The duration is 11m 42s shorter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several spot checks show that apart from about 10m 38s removed from the beginning and about 1m 4s removed from the end, the two seem to be the same. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may be the same/similar case for the technical web cast, I haven't checked. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Conclusion:&lt;/strong&gt; apart from non-informative content removed from the beginning and end, the broadcast appears to be unchanged based on spot checks, except that a higher resolution format seems to have been posted. The YouTube links that contain video time codes within the url's parameters in the two examples in the question pointed to times &lt;em&gt;beyond the end&lt;/em&gt; of the edited and re-uploaded video.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Advice:&lt;/strong&gt; Always try to work from the launch time clock, not the time in a YouTube video, as that can change. If you can wait a few days until the video seems to be in final format, the video times &lt;em&gt;might be&lt;/em&gt; more stable. Don't use urls with start time parameters - e.g. &lt;code&gt;&amp;amp;t=3083s&lt;/code&gt; - if you have any reason to think the video might be re-uploaded later edited or modified for length.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit (Feb. 2018):&lt;/strong&gt; as discussed in &lt;a href=&quot;https://space.stackexchange.com/a/24882/12102&quot;&gt;this answer&lt;/a&gt; as well as others to the question &lt;a href=&quot;https://space.stackexchange.com/q/24873/12102&quot;&gt;Was the side booster footage from the same camera during the Falcon Heavy test flight stream?&lt;/a&gt; edits to a live broadcast were made before posting the permanent version to correct the use of an incorrect video feed in one of the channels displayed in an inset. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-02-20T02:55:08.547" LastActivityDate="2018-02-20T02:55:08.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17512" PostTypeId="2" ParentId="17495" CreationDate="2016-07-27T04:54:23.547" Score="2" Body="&lt;p&gt;The answer is no they are not Ham Radio Satellites, but they are amateur built satellites.&#xA;These satellites are indeed in deep space, however I would like to point out that there is a difference between 'amateur satellites' and 'amateur radio satellites.' I call them 'amsats' and 'hamsats' respectively. Amsats are built by amateurs, ie non-commercial, non-government, non-military. They are usually built by university teams. They often use amateur radio frequencies for their control and telemetry radios but they are not intended for amateur radio operators. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hamsats, may have scientific experiments and payload, but they also have amateur radio beacons or transponders intended for amateur radio operators to carry out conversations or to transfer signals such as TV or data. All currently active amsats are in Low Earth orbits (LEO).  Hamsats are often built by amateur radio operators in conjunction with university teams.&lt;/p&gt;&#xA;" OwnerUserId="16422" LastEditorUserId="16422" LastEditDate="2016-07-27T04:59:58.177" LastActivityDate="2016-07-27T04:59:58.177" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17513" PostTypeId="2" ParentId="17452" CreationDate="2016-07-27T10:43:50.633" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;In this view, EPIC sees the Sun rise in the west, and the Sun set in the east, at least 13 times a day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is incorrect. This statement is based on the planned activity of sending home at least 12 images a day. Communication link restrictions and the higher priority of the scientific instruments on DSCOVR over EPIC means that this does not always happen. Saying that EPIC takes 12 images per day does not sound very exciting. Saying that EPIC &quot;sees the Sun rise in the west, and the Sun set in the east, at least 13 times a day&quot; somehow does seem exciting. It is however deceiving. Because DSCOVR is in a Lissajous orbit about the Sun-Earth L1 point, almost all of the portion of the Earth visible to DSCOVR (and hence to EPIC) is fully sunlit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Technically, this answer is quite incorrect. A single EPIC image occasionally contains a tiny, tiny part of the Earth that is in sunrise or a tiny, tiny part of the Earth that is in sunset, but only very rarely both. EPIC does not see &quot;the Sun rise in the west and the Sun set in the east, at least 13 times a day.&quot; Because of atmospheric effects, sunrise occurs before any part of the Sun is geometrically above the horizon, and sunset occurs after every part of the Sun is geometrically below the horizon. To simultaneously see sunrise and sunset would require seeing more than half the globe. Simultaneous sunrise and sunset cannot be observed simultaneously from DSCOVR's perspective because it sees slightly less than half the globe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;Editorial comment&lt;/strong&gt;: It would have been much better to simply say that &quot;In this view, EPIC sees almost half the globe, almost all of which is fully sunlit. Because of the Earth's rotation, and because EPIC takes at least 12 images per day, it sees almost every part of the sunlit Earth, every day.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A different satellite (which does not exist) could provide a lot more than 12 fully-sunit images per day, but that satellite does not exist. Twelve pictures a day might not be that much, but it is a lot better than none.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-07-27T16:40:34.110" LastActivityDate="2016-07-27T16:40:34.110" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17514" PostTypeId="2" ParentId="17480" CreationDate="2016-07-27T11:21:59.593" Score="13" Body="&lt;p&gt;It is well known that finger injuries sustained during EVA plagued a lot of astronauts. The &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/a15.clsout1.html&quot; rel=&quot;noreferrer&quot;&gt;Lunar Surface Journal&lt;/a&gt; for Apollo 15 has some good references to this at &lt;strong&gt;126:08:40&lt;/strong&gt;, with links to post-mission photographs of Dave Scott &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/ap15-S71-42195.jpg&quot; rel=&quot;noreferrer&quot;&gt;clearly showing the damage to this fingernails&lt;/a&gt; and giving a good indication of the extent of the blood blisters that formed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, post-mission reports for 15 indicate that it wasn't just fingernails. Gene Cernan even complained of blisters between his thumb and forefingers and reported that his knuckles were rubbed raw and bloody against the inside of his suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The astronauts powered through and kept working, but in long-duration missions this type of injury would surely have to be dealt with. It was largely unforeseen in earlier mission but had the Apollo missions been of a longer duration this would surely have impacted the duration and amount of EVA's performed.&lt;/p&gt;&#xA;" OwnerUserId="8132" LastEditorUserId="8132" LastEditDate="2016-07-27T17:21:39.150" LastActivityDate="2016-07-27T17:21:39.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17515" PostTypeId="2" ParentId="17480" CreationDate="2016-07-27T11:40:00.963" Score="12" Body="&lt;p&gt;A relatively large number of US astronauts who performed EVAs have injured their shoulders badly enough to require surgery.  At the time this study was performed (2012) there was no definite conclusion as to whether the injuries resulted from orbital EVAs or training.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tu6Be.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tu6Be.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;full presentation &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120009404.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-07-27T11:40:00.963" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17516" PostTypeId="1" AcceptedAnswerId="17517" CreationDate="2016-07-27T14:49:05.377" Score="7" ViewCount="1083" Body="&lt;p&gt;I saw the word &quot;shunts&quot; in &lt;a href=&quot;https://space.stackexchange.com/a/8069/12102&quot;&gt;this answer&lt;/a&gt; about DSCOVR's upgraded photovoltaic arrays when it was unmothballed and readied for space. Reading it, I get the idea that shunts are used to dissipate the difference in power between what the satellite needs and what the array is producing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/17509/12102&quot;&gt;this answer&lt;/a&gt; which is actually about silicon PVs on the ISS, it is mentioned that when entire arrays are offline, they get hotter, and I'm assuming this is because they are left open-circuit and so all the electron-hole carriers remain in the junction and recombine, returning their energy as heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I'm also wondering that if you need less than the full amount of energy, if you had a modern switching power supply to condition the voltage, wouldn't you just draw less current, instead of full current and shunting some into a big hot resistor? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to narrow the question down I should ask about modern designs and recently built and launched satellites. I'm thinking communications satellites, but since DSCOVR received a modernized system, that's OK too. Of course if there is something more compelling, that's fine, but the ISS can be a separate question because there are issues of legacy and compatibility. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-07-28T11:07:21.727" Title="When there is more power than needed, are satellite photovoltaics shunted or open circuit?" Tags="|artificial-satellite|solar-power|communication-satellite|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17517" PostTypeId="2" ParentId="17516" CreationDate="2016-07-27T14:54:13.003" Score="9" Body="&lt;p&gt;I wasn't sure if your last sentence meant that you didn't want to hear about the ISS, but in case you do, in the case of the ISS, they are shunted.  The device that handles this is the Sequential Shunt Unit (SSU). The SSU is the primary power regulation device that controls Solar Array Wing (SAW) output. There is one SSU per power channel/SAW. The function&#xA;of the SSU is to manage the output of the solar cells to produce 160 V dc. The SSU&#xA;maintains this voltage by balancing the system demand with the number of connected&#xA;array strings in the SAW. Each array string can be individually connected or&#xA;disconnected from the primary power bus, thus the output from the SSU is the sum of&#xA;all connected strings at any given time. Array strings that are disconnected from the&#xA;power system are shunted (shorted back to the array itself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A high level graphic from &lt;a href=&quot;https://www.nasa.gov/externalflash/ISSRG/pdfs/electrical.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7UQSW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7UQSW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what it's worth, the SSU on power channel 3A shorted out in May 2014 leading to the loss of its associated power channel (1 of 8).  It was replaced &lt;a href=&quot;https://www.nasaspaceflight.com/2014/10/american-eva-prepare-iss-for-commercial-crew/&quot; rel=&quot;noreferrer&quot;&gt;during an EVA&lt;/a&gt; in October 2014.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-07-27T15:16:27.227" LastActivityDate="2016-07-27T15:16:27.227" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17518" PostTypeId="1" CreationDate="2016-07-27T15:48:08.747" Score="17" ViewCount="2321" Body="&lt;p&gt;&lt;a href=&quot;http://spaceflightnow.com/2016/07/25/nasa-books-nuclear-certified-atlas-5-rocket-for-mars-2020-rover-launch/&quot;&gt;This article&lt;/a&gt; says NASA has booked a nuclear-certified Atlas 5 for the launch of the Mars 2020 rover, and says this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;currently, Atlas 5 is the only launch vehicle that holds a NASA&#xA;  certification for launching the nuclear batteries made of plutonium&#xA;  that will power the 2,000-pound rover&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What does it take for a launcher to be nuclear-certified?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="25" LastEditDate="2016-07-27T16:30:30.803" LastActivityDate="2016-07-27T21:38:59.113" Title="What does it mean for a launcher to be 'nuclear-certified'?" Tags="|launch|nuclear|safety|launch-vehicle|regulatory|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17519" PostTypeId="2" ParentId="17518" CreationDate="2016-07-27T16:30:17.037" Score="20" Body="&lt;p&gt;Really what it means is &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/NPD_attachments/AttachmentA_7C.pdf&quot;&gt;&quot;Category 3&quot; certified&lt;/a&gt;, with an additional review of a self-destruct situation to prevent breaking the nuclear payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Category 3 is also what is required to launch humans, and in fact the final milestone in the Commercial Crew is to rate the system Category 3. It is required for any kind of sensitive launch. Note that man rated requires a category 3 rocket, but also extra levels of scrutiny as well. A Category 3 launcher can carry cargo that is rated &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8705_0004_&amp;amp;page_name=main&quot;&gt;NPR 8705.4 categories A-D&lt;/a&gt;. Basically it means that NASA has done an extensive analysis of the rocket system, and/or had flight tests to minimize the risk. The requirements are quite extensive, and are documented at the link above. At the very least 3 consecutive flights are required to get the certification, and depending on the level of NASA review, that could be 14 consecutive flights. Note that one must also seek the certification, it's not automatically given upon the 14th consecutive launch of the rocket!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today there are actually two US rockets that are certified Category 3 by NASA, the Atlas 5, and the Delta II. And of course, the Delta II is being phased out of service, in fact, &lt;a href=&quot;https://spaceflightnow.com/2015/04/08/what-to-do-with-the-final-delta-2-rocket/&quot;&gt;there is only 1 left for sale&lt;/a&gt;. I've also seen a reference to the Delta IV being rated Category 3, which I'm assuming is only the Medium version, not the Heavy version.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to being human rated, there are some additional requirements to being certified to launch a nuclear payload. The best reference I can find is &lt;a href=&quot;https://books.google.com/books?id=py_PCwAAQBAJ&amp;amp;pg=SA6-PA69&amp;amp;lpg=SA6-PA69&amp;amp;dq=NASA%20nuclear%20payload%20certification&amp;amp;source=bl&amp;amp;ots=BTQ9bC2VFE&amp;amp;sig=v1AkAy_K21h4QZLGWXPBPwFEoss&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwi095mAoJTOAhUC7CYKHbXiCiEQ6AEIQzAG#v=onepage&amp;amp;q=NASA%20nuclear%20payload%20certification&amp;amp;f=false&quot;&gt;this book&lt;/a&gt;, which I only have the Google edition with missing pages. The main difference seems to be an analysis of what would happen in the event of a Self-Destruct, specifically relating to high speed shrapnel heading through the payload, potentially damaging the casing protecting the nuclear material.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-27T21:38:59.113" LastActivityDate="2016-07-27T21:38:59.113" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17520" PostTypeId="1" AcceptedAnswerId="17521" CreationDate="2016-07-27T17:11:26.330" Score="5" ViewCount="385" Body="&lt;p&gt;I'm having some trouble understanding the requirements for launch vehicle certification.  According to &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/displayDir.cfm?t=NPD&amp;amp;c=8610&amp;amp;s=7D&quot; rel=&quot;nofollow&quot;&gt;NPD 8610.7D&lt;/a&gt; the requirements for a category three LV are &quot;a demonstrated flight record of a series of 14 consecutive successful flights of a common launch vehicle configuration...&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The document hints that there are other reviews that go into granting a launch vehicle certification but it seems like 14 consecutive successful flights is the key item.  The Delta IV has been fully successful on all flights except for the first flight of the Delta IV heavy which did not reach as high of an orbit as was intended.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the other requirements not made clear in this document and why aren't all the requirements spelled out specifically. &lt;/p&gt;&#xA;" OwnerUserId="16180" LastEditorUserId="25" LastEditDate="2016-07-27T17:18:16.210" LastActivityDate="2016-07-27T18:33:45.840" Title="With 31/32 successful launches why isn't the Delta IV a category 3 certified launch vehicle" Tags="|launch|nasa|launch-vehicle|delta-iv|policy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17521" PostTypeId="2" ParentId="17520" CreationDate="2016-07-27T17:30:59.837" Score="6" Body="&lt;p&gt;The requirement is for the same configuration. The Delta IV is a highly configurable rocket, and has been flown in the following modes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Delta IV-M 3 times, no failures&lt;/li&gt;&#xA;&lt;li&gt;Delta IV-M+ 19 times, no failures&lt;/li&gt;&#xA;&lt;li&gt;Delta IV Heavy- 9 times, 1 failure. 6 in a row success.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, the Delta IV-M+ configuration has the required 14 consecutive flights. Assuming the proper instrumentation was on board as required per the &lt;a href=&quot;http://nodis3.gsfc.nasa.gov/NPD_attachments/AttachmentA_7C.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA flight requirement matrix&lt;/a&gt;, the only thing that would be required is for ULA to present the data to NASA to get the certification. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And in fact, it turns out that the Delta IV &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Commercial_Space/ISSCargoRFIFinal09062007.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;is Category 3 certification&lt;/a&gt; for the Delta IV in 2007. I assume that is only for the M and/or M+ configuration, and not for the Heavy configuration. It, however, does not have Nuclear certification, which is explained in more detail in &lt;a href=&quot;https://space.stackexchange.com/a/17519/25&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-07-27T18:33:45.840" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17522" PostTypeId="1" CreationDate="2016-07-27T21:17:41.430" Score="5" ViewCount="498" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MxPQa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MxPQa.jpg&quot; alt=&quot;Spare NASA rovers&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a picture on Wikipedia that seems suggest that NASA is keeping rovers from previous missions for testing and flight spares.&#xA;Do they have full set of science instruments or are they stripped down versions?&#xA;What happens to them after the mission is over? Are they being used for anything, either as a whole or in part (for example the science instruments)?&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="13710" LastEditDate="2016-07-27T21:54:45.733" LastActivityDate="2016-07-28T00:24:37.163" Title="What happens to spare rovers?" Tags="|mars|nasa|rovers|curiosity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17523" PostTypeId="2" ParentId="17522" CreationDate="2016-07-27T21:52:59.580" Score="6" Body="&lt;p&gt;These are not actually &quot;spare&quot; rovers.  They are either fully functional duplicates or media mockups.  In the case of the former, they are used by NASA's Jet Propulsion Laboratory for testing.  Sometimes when they run into troubles with a rover, or they just want to try making it do something new, they test it on one of these first.  For example, when &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2014/08190630-curiosity-wheel-damage.html&quot;&gt;Curiosity was having wheel troubles&lt;/a&gt;, they brought out their copy and drove it over sharp rocks so that they could better understand what was causing the damage.  JPL has a special area called the &quot;&lt;a href=&quot;https://www-robotics.jpl.nasa.gov/facilities/facility.cfm?Facility=1&quot;&gt;Mars Yard&lt;/a&gt;&quot; to do this testing in.  They also have stripped-down models to be used for more specific tests.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They do have spare parts for some of the instruments and other parts, and sometimes these are used for testing or just kept as mementos.  However, in some cases they are repurposed for use on other missions.  When they built Curiosity, they made some duplicate parts.  The spare heat shield, which was never used, will be used for the Mars 2020 rover.  &lt;a href=&quot;http://www.planetary.org/multimedia/planetary-radio/show/2016/0725-rover-tds.html&quot;&gt;Some of the Entry, Descent, and Landing (EDL) radar will also be reused&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the copies are nonfunctional, or are just minimally functional.  These are used essentially as props, to take to schools and show kids, or to demonstrate to the media different parts of the machine.  They look as close as possible to the real thing, and can sometimes be driven around, but lack the science instruments.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, most notably, any of these copies will lack the RTG power source that Curiosity contains, and are powered via more traditional batteries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the mission is over, they are usually kept, just in case they might be useful for future missions.  NASA reuses a lot of designs for future missions, since they know these designs work.  Otherwise, they're just used as teaching and media aids.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-07-27T21:52:59.580" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17524" PostTypeId="2" ParentId="17516" CreationDate="2016-07-27T23:22:28.880" Score="2" Body="&lt;p&gt;This is just a short answer addressing the headline question. I apologise for the short answer, I don't have time at the moment to dig out references or a full explanation as the answer comes from experience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both shunt regulators and series regulators (open circuit) exist in unmanned satellite power system design. I think that series regulators are more common for smaller satellites though I don't recall the design rationale.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Asides:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Rotation&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Whilst I haven't heard of a real design example, the &quot;rotate to catch less sunlight&quot; would improve cooling but would presumably have to be used in conjunction with one of the regulators in order to allow the voltage to remain stable to changing loads.&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;Benefits&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;One might go a step further and look into whether we care much about reducing the array temperature. A cooler array is usually more efficient, however this question came up in attempting to understand the oversupply situation and so efficiency is irrelevant. A significantly higher prolonged temperature could in principle lead to increased defects and failures though I haven't seen this cited as a degradation factor for typical array temperatures.&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;Thermal IR&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The video of the shuttle leaving the ISS that is linked to from one of the links in the question is curious. Its really interesting to see that the arrays appear to have a striped pattern. This might indicate sections working at different temperatures on account of regulator action. This could occur in a series regulator but this is inconsistent with the assertion of a shunt regulator from Organic Marble's answer. Of course we don't know where the shunt loads are.  Its possible that if the shunt is a direct short then the cells themselves are the shunt load and unused sections could still run hotter than connected sections.   On some satellites, perhaps where the dimensions are lesser, shunt loads can provide useful thermal maintenance for other parts of the satellite.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-07-28T11:07:21.727" LastActivityDate="2016-07-28T11:07:21.727" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17525" PostTypeId="1" CreationDate="2016-07-27T23:57:24.727" Score="8" ViewCount="3294" Body="&lt;p&gt;I just watched &lt;a href=&quot;https://www.youtube.com/watch?v=-E8bnhIUCdY&quot;&gt;a VSauce2 video&lt;/a&gt;, at the end of which certain NASA animations were shown about some of their recent innovations. One of them included use of printers in the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they actually mean a noticeable decrease in cost? I mean, I'm aware that bringing equipment up to these places has huge costs, and I assume 3D printers may decrease the amount of said equipment, as they can create it up there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are they &lt;em&gt;precisely&lt;/em&gt; used for up there?&lt;/p&gt;&#xA;" OwnerUserId="306" LastEditorUserId="4660" LastEditDate="2016-07-28T00:35:33.580" LastActivityDate="2016-07-30T11:27:38.933" Title="Do 3D printers in space stations mean a significant cost reduction?" Tags="|iss|space-station|technology|materials|cost|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17526" PostTypeId="2" ParentId="17525" CreationDate="2016-07-28T00:15:11.033" Score="19" Body="&lt;p&gt;The 3-D printer on the ISS is more for testing purposes than anything. The idea is that for really long duration missions, a 3-D printer will allow them to make something in case something breaks, or they realize they need something that didn't come initially with the ship. So far I can only find a few things that have been made, including a &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/news/3Dratchet_wrench&quot;&gt;ratchet&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/watch?v=T_vpsqVn2A8&quot;&gt;several test items&lt;/a&gt;, and a &lt;a href=&quot;http://www.digitaltrends.com/cool-tech/3d-printed-tools-in-space-iss/&quot;&gt;wrench&lt;/a&gt;. It seems the later was actually used as needed, the rest are just test artifacts. There is a lot of promise, a 3-d printer can potentially &lt;a href=&quot;http://www.wired.com/2016/03/little-3-d-printer-iss-huge-step-space-exploration/&quot;&gt;save a significant amount of launch weight&lt;/a&gt;, not to mention reducing the spares required on a ship, but overall, they are still trying to figure out exactly how to best use them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As has been mentioned, the real benefit will be for those things that break, aren't life critical, but are needed sooner than a few months that can be taken to get the part from the ground. To my knowledge the 3-D printer has been used once for such, to print a tool.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-28T17:12:55.757" LastActivityDate="2016-07-28T17:12:55.757" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17527" PostTypeId="2" ParentId="17522" CreationDate="2016-07-28T00:24:37.163" Score="1" Body="&lt;p&gt;These are known as Engineering Test Units. They can differ from flight units in a number of ways:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;They have to function in Earth's gravity, so they might be more sturdy than the launched items.&lt;/li&gt;&#xA;&lt;li&gt;They might have lower cost parts. Many processors have a space grade vs commercial grade quality, which the difference in cost is quite large. &lt;/li&gt;&#xA;&lt;li&gt;The quality control for these units is typically not as high. They likely wouldn't be built in a clean room, for instance, nor undergo as much testing. Conversely, they might have undergone extreme testing which would ensure the spacecraft could not be used for launch, as the launch artifacts are only qualified to undergo the event once.&lt;/li&gt;&#xA;&lt;li&gt;They are quite useful on the ground. They are used to test Earth based samples to see how they compare to their Martian counterparts, to get out of &lt;a href=&quot;http://mars.nasa.gov/mer/mission/status_spiritAll_2009.html#sol1907&quot; rel=&quot;nofollow&quot;&gt;difficult situations&lt;/a&gt;, and to test unusual command stacks. They are also used to test software on the ground. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Bottom line, they might be able to send them, but the ones on the ground are of lower quality. After a mission is over, they will likely end up as some kind of a museum piece, either internal to NASA, or even somewhere like the Smithsonian. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-28T00:24:37.163" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17528" PostTypeId="2" ParentId="17525" CreationDate="2016-07-28T02:08:25.890" Score="22" Body="&lt;p&gt;A 3D printer on station isn't likely to lead to direct cost savings; the range of items it can replace are limited, and it has to be supplied with feedstock mass in any case; it won't allow for significantly fewer supply launches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary benefit is that it can allow for the repair of a system the astronauts would otherwise have to do without until the next scheduled supply - whether a nonessential but convenient item, or a scientific experiment, or, conceivably, even a mission-critical or safety-critical part. This would have a definite impact in terms of return-on-investment, but that's not the same as cost savings.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-28T15:13:48.830" LastActivityDate="2016-07-28T15:13:48.830" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17529" PostTypeId="2" ParentId="4003" CreationDate="2016-07-28T03:53:51.027" Score="2" Body="&lt;p&gt;Perhaps it is the courier 1b? Launched in 1960, the last information on the satellite published c 2009 lists the nav beacon as still transmitting. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, it might actually be theoretically possible to contact.   With the batteries long dead, the solar cells might do a reboot allowing the satellite to be recontacted...?&lt;/p&gt;&#xA;" OwnerUserId="16433" LastActivityDate="2016-07-28T03:53:51.027" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17530" PostTypeId="1" CreationDate="2016-07-28T06:30:43.683" Score="9" ViewCount="510" Body="&lt;p&gt;What is the reason that Red Dragon is testing supersonic retropropulsion on a Mars mission. Is the Earth's upper atmosphere too different to that of Mars to validate this method as a viable way of landing on Mars?&lt;/p&gt;&#xA;" OwnerUserId="14276" LastEditorUserId="51" LastEditDate="2016-07-28T15:45:23.160" LastActivityDate="2016-07-28T15:45:23.160" Title="Why cannot supersonic retropropulsion, for use in the Red Dragon lander be tested on Earth?" Tags="|mars|spacex|landing|reentry|red-dragon|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17531" PostTypeId="2" ParentId="17525" CreationDate="2016-07-28T08:58:42.987" Score="2" Body="&lt;p&gt;A 3d printer does not give direct cost savings, it's cheaper to launch a pre-made object than the feedstock it takes to print it (there's always some material waste when 3d printing), plus the weight of the printer itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What a 3d printer gives is the ability to prototype. 3d printed plastics are weak compared to cast plastics and nowhere near the strength of metal, you couldn't use them for anything load bearing. You can, however print a part, see if it works, and then tweak the design. Once the design is right the design can be given to someone on the ground to manufacture and send up on a later re-supply. For non-load bearing pieces you could use the 3d printed materials directly, meaning you aren't paying for something to be manufactured. It also gives the astronauts the option to design and implement their own made solutions, meaning less ground support. These may well translate into cost savings but it's impossible to quantify how much. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2016-07-29T08:52:32.517" LastActivityDate="2016-07-29T08:52:32.517" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17532" PostTypeId="1" AcceptedAnswerId="17543" CreationDate="2016-07-28T09:15:44.293" Score="2" ViewCount="501" Body="&lt;p&gt;In this &lt;a href=&quot;https://www.youtube.com/watch?v=txk-VO1hzBY&quot; rel=&quot;nofollow noreferrer&quot;&gt;cool video of a presentation&lt;/a&gt; about SpaceX's GPU-based computational fluid dynamics, there is a slide early on about making rocket fuel on Mars - specifically using water from the ground and carbon dioxide from the atmosphere to make oxygen and methane:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2H2O + CO2 -&amp;gt; CH4 + 2O2&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Going in that direction requires energy, and I'm guessing solar power in some form or other would certainly be one way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be multiple ways to use solar power to drive endothermic chemical reactions. There is photocatalysis, concentrated solar thermal catalysis, photovoltaic for thermal catalaysis and/or electrolysis just for example. &quot;What is the best method...&quot; would be a great question, but it's not this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question: Ballpark figures: if seven people wanted to get in some capsule (could be the same Dragon that got them there or not) now sitting on a methane/LOX burning booster, blast off and get back to Mars orbit to rendezvous with a waiting vehicle to go home, and for some reason they had to make the propellants from water and carbon dioxide, roughly how many square kilometer years of solar power on Mars would be necessary to synthesize those propellants?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume food and water and vehicle for the return is waiting for them in orbit, but they have to make the propellants to get to orbit on Mars as shown in the screen capture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0S0HL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0S0HL.jpg&quot; alt=&quot;SpaceX Methane Fuel on Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/txk-VO1hzBY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="25" LastEditDate="2016-07-28T15:03:19.577" LastActivityDate="2016-07-28T15:03:19.577" Title="Back to oribt from Mars - how many km^2 yr of solar power to make enough propellant?" Tags="|mars|fuel|solar-power|isru|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17533" PostTypeId="2" ParentId="17530" CreationDate="2016-07-28T09:16:10.183" Score="11" Body="&lt;p&gt;They have been &lt;a href=&quot;http://qz.com/656025/how-spacex-is-really-bringing-us-closer-to-mars-no-really/&quot;&gt;testing supersonic retropropulsion&lt;/a&gt; on Earth: the Falcon 9 landings do their reentry burn in circumstances not too different from Mars' atmosphere.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen no announcement of a Dragon supersonic retropropulsion test on Earth, but that doesn't mean there won't be such a test. SpaceX does a lot without publicly announcing it. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-07-28T09:16:10.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17534" PostTypeId="1" AcceptedAnswerId="17535" CreationDate="2016-07-28T09:36:52.497" Score="13" ViewCount="1694" Body="&lt;p&gt;In this &lt;a href=&quot;https://www.youtube.com/watch?v=txk-VO1hzBY&quot; rel=&quot;noreferrer&quot;&gt;cool video of a presentation&lt;/a&gt; about SpaceX's GPU-based computational fluid dynamics, there is a slide early on about making rocket fuel on Mars - specifically using water from the ground and carbon dioxide from the atmosphere to make oxygen and methane:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2H2O + CO2 -&amp;gt; CH4 + 2O2&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Going in that direction requires energy, and I'm guessing solar power in some form or other would certainly be one way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But why not keep it simple (&quot;because it isn't&quot; is a possible answer) and just use solar photovoltaics and electrolysis to make LOX and LH2 from the same water used above:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2H2O  -&amp;gt; 2H2 + O2&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;What are the salient tradeoffs here? If the slide shows methane, and the acompanying discussion talks about the problems of trying to make a methane engine, why is LOX + LH2 not discussed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0S0HL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0S0HL.jpg&quot; alt=&quot;SpaceX Methane Fuel on Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/txk-VO1hzBY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="25" LastEditDate="2016-07-28T15:03:50.517" LastActivityDate="2021-10-25T17:44:44.033" Title="Making propellants on Mars: Why not just LH2/LOX instead of methane?" Tags="|mars|fuel|solar-power|isru|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17535" PostTypeId="2" ParentId="17534" CreationDate="2016-07-28T09:57:34.937" Score="13" Body="&lt;p&gt;Liquid Hydrogen is difficult to deal with. The temperature must be 33 K or lower. Liquid Oxygen requires 90K, and Liquid Methane is similar. The temperature requirements are far less as such. The surface of Mars varies between 140K to 300K. The values for storing Methane/ Oxygen are much closer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methane also requires less hydrogen than the LH2/LOX rocket. It has been assumed that hydrogen is relatively difficult to find on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, Liquid Hydrogen is very difficult to deal with, as is mentioned in this &lt;a href=&quot;http://www.nasa.gov/topics/technology/hydrogen/hydrogen_fuel_of_choice.html&quot;&gt;NASA article&lt;/a&gt;. The long term storage of the substance is something that hasn't yet been achieved.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-28T09:57:34.937" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17536" PostTypeId="2" ParentId="4000" CreationDate="2016-07-28T10:05:14.630" Score="5" Body="&lt;p&gt;I think the best way to get to Sedna is to look to the spaceships that are now in Solar escape velocity.&lt;/p&gt;&#xA;&lt;h3&gt;Escape Velocity&lt;/h3&gt;&#xA;&lt;p&gt;&lt;em&gt;New Horizons&lt;/em&gt; was the first spaceship launched directly into solar escape trajectory. However, the ship used a Jupiter gravity assist. It will reach 100 AU from Sun in 32 years, but most probably it will not be functioning after 2026, when it will run out of power.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Voyager 1 &amp;amp; 2&lt;/em&gt; used multiple gravity assists, to reach their current speed. Voyager 1 reached 100 AU from Sun around 2006, after 29 years of flying. Voyager 2 reached that distance in 2012, after 35 years from launch. Both spacecrafts are still functional, but they can hardly power-up their scientific payload.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;em&gt;Pioneer&lt;/em&gt; spacecraft also used Jupiter and Saturn gravity assists. Both ships are supposed to be over 100 AU from Sun since 2014. However, contact has been lost before they reached this distance.&lt;/p&gt;&#xA;&lt;h3&gt;Trajectory details&lt;/h3&gt;&#xA;&lt;p&gt;As seen above, it is possible to reach Sedna orbit in less than 30 years. Sedna will be closer in 60 years, but the difference will not be much (less then 10% from current distance). So, we could launch a probe now instead of waiting.&lt;/p&gt;&#xA;&lt;p&gt;As New Horizons showed us, it is possible to send a spacecraft directly into solar escape velocity. However, we can use gravity assists, to reduce flight time.&lt;/p&gt;&#xA;&lt;p&gt;Every 12 years, Jupiter is in place for a &lt;em&gt;Jupiter gravity assist&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Every 29 years, Saturn is in place for a &lt;em&gt;Saturn gravity assist&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Every 59 years, both Jupiter and Saturn are in place for a &lt;em&gt;Double gravity assist&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;A flyby mission would reach Sedna in 20 to 30 years. However, sending an orbiter or a landing probe will be very hard, if not impossible.&lt;/p&gt;&#xA;&lt;h3&gt;Task - Flyby&lt;/h3&gt;&#xA;&lt;p&gt;To send a flyby to Sedna is almost like the flyby New Horizons conducted to Pluto. We can use the same spacecraft design, only that we will need a far more efficient RTG (radioactive thermoelectric generator), so that it will keep functioning after 30 years. The probe will conduct a fast flyby, then it will spend a few years sending all data back to Earth. This kind of mission is feasible and affordable with current NASA budget.&lt;/p&gt;&#xA;&lt;h3&gt;Task - orbiter&lt;/h3&gt;&#xA;&lt;p&gt;If we send an orbiter to Sedna, we have two ways: First, we need to slow it down, to enter orbit. Second, we can send it with a very low speed, enough to enter orbit. The first method will require giant amounts of propellant and will dramatically increase ship weight at launch. One alternative is to use an ion engine, powered-up by the ship's RTG, to reduce speed in the last 10 years before encounter. However, the amount of energy still available at that time is limited. Sending a probe with little speed might be a solution, but it will reach Sedna in 100 years. At that time, its RTG would have declined energy production to a very low level and anyway there will be nobody from the building team to see the results.&lt;/p&gt;&#xA;&lt;h3&gt;Task - landing&lt;/h3&gt;&#xA;&lt;p&gt;Sending a lander to Sedna is even more complicated then an orbiter. However, if we conduct a fast flyby, we can use an &lt;em&gt;impactor&lt;/em&gt;, similar to the one used by Deep Impact mission. This will at least allow us get a close picture from the surface and see at impact site sub-surface composition.&lt;/p&gt;&#xA;&lt;h3&gt;Challenges&lt;/h3&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The main problem with Sedna is the amount of time needed for a flyby. RTG energy decay is the main concern.&lt;/li&gt;&#xA;&lt;li&gt;Luminosity is lower than on Pluto, so cameras will need a longer exposure time.&lt;/li&gt;&#xA;&lt;li&gt;Mass and gravity of Sedna are unknown. We only approximate its diameter.&lt;/li&gt;&#xA;&lt;li&gt;It is not known if Sedna has moons, rings or anything in orbit around it that could harm a spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;Communication delay is a problem. Sending a message to the ship, we will need an Earth day to get a response.&lt;/li&gt;&#xA;&lt;li&gt;Signal power is decreasing by the square distance. Signal power at Sedna will be 6.5 times weaker then at Pluto, meaning that a spaceship will need 6.5 times more time to download all its findings. If we needed over an year to download data from New Horizons, we will need probably 10 years for a probe that explored Sedna.&lt;/li&gt;&#xA;&lt;li&gt;Since we don't know major data about Sedna (mass, gravity, orbital axis, possible satellites and space debris), there is a high chance of risk. The ship will need to conduct some preliminary research before flyby, then it will need some strong trajectory correction maneuvers. At first, it will scan for possible moons or rings, to analyze the risk of a collision, then it will determine rotation axis, to see what is the best flyby path. As it gets closer, the probe will get a low-resolution image of the far side of Sedna, then it will get high-detail resolution of its encounter side. Then, as it gets behind Sedna, the probe will capture a solar eclipse, to see if there is any atmosphere. Overall, these were the steps performed also by New Horizons on Pluto.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;h3&gt;Conclusion&lt;/h3&gt;&#xA;&lt;p&gt;A similar mission to New Horizons, but with a more powerful RTG, eventually with the help of Jupiter and Saturn flybys, could get to Sedna in 20 to 30 years.&lt;/p&gt;&#xA;" OwnerUserId="14629" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-07-28T16:03:39.427" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17537" PostTypeId="1" AcceptedAnswerId="27588" CreationDate="2016-07-28T10:25:24.250" Score="13" ViewCount="777" Body="&lt;p&gt;If you take a simple &lt;a href=&quot;https://en.wikipedia.org/wiki/Infrared_thermometer&quot; rel=&quot;nofollow noreferrer&quot;&gt;infrared &amp;quot;thermometer&amp;quot;&lt;/a&gt; which is actually a lot like a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bolometer&quot; rel=&quot;nofollow noreferrer&quot;&gt;bolometer&lt;/a&gt; and point it at a clear sky, it says &amp;quot;cold!&amp;quot; because in the wavelength range that it's using (roughly 5 to 15 microns) the Earth's atmosphere is partially transparent and not really an effective blackbody radiator, so the &amp;quot;thermometer&amp;quot; is really somewhat exposed to the cold of space. Point it at a cloud, and it will register &amp;quot;not as cold&amp;quot;. It's (roughly) why clear nights are colder than cloudy nights.&lt;/p&gt;&#xA;&lt;p&gt;Since the Martial atmosphere is fairly thin (though probably has very roughly similar amount of CO2 as Earth's), if I put a flat chunk of 'black body' on top of an insulator and expose it to the Martian sky at night, is there some simple expression for the effective temperature of the sky - what is the radiative heating/cooling going to be? Is it looking at 100K?&lt;/p&gt;&#xA;&lt;p&gt;There will be of course thermal contact (convective heat transfer) with the atmosphere, and that can be calculated separately. This question is just about radiative heating/cooling from exposure to to the &lt;span class=&quot;math-container&quot;&gt;$2\pi$&lt;/span&gt; SR half-sphere above horizontal.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-04-29T00:26:03.927" LastActivityDate="2021-04-29T00:26:03.927" Title="How cold is the Martian sky at night? Or the day for that matter?" Tags="|mars|thermal|thermal-control|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="17538" PostTypeId="1" CreationDate="2016-07-28T10:56:27.803" Score="8" ViewCount="1578" Body="&lt;p&gt;In a spacecraft, even if a reaction wheel is placed at some distance from the center of mass, the satellite always rotates through an axis which passes through its CoM? What is the reason behind this? Why is the torque vector treated as free vector?&lt;/p&gt;&#xA;" OwnerUserId="16437" LastEditorUserId="4660" LastEditDate="2016-07-28T14:01:41.613" LastActivityDate="2016-07-29T03:23:10.660" Title="Why does the torque due to a reaction wheel shift to the center of mass of the spacecraft?" Tags="|attitude|reaction-wheel|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17539" PostTypeId="1" AcceptedAnswerId="17542" CreationDate="2016-07-28T10:57:36.187" Score="12" ViewCount="1650" Body="&lt;p&gt;I remember reading somewhere that a catalyst was used to change the ratio of ortho- vs para-hydrogen before it is loaded as LH2 propellant on a rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is this exactly, and why is it necessary? I've read this paragraph from &lt;a href=&quot;http://www.hysafe.org/download/997/brhs_ch1_fundamentals-version%201_0_1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hydrogen Fundamentals&lt;/a&gt; on this hydrogen safety website, but I still don't completely understand the implications for rocket fueling.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Liquid hydrogen (LH2) has the advantage of extreme cleanliness and the more economic type of storage, however, on the expense of a significant energy consumption of about one third of its heat of combustion. Another drawback is the unavoidable loss by boil off which is typical to maintain the cold temperature in the tank. The evaporation rate is even enhanced when ortho hydrogen is stored. The heat liberated during the ortho-para conversion at 20 K is huge with 670 kJ/kg compared to a figure of 446 kJ/kg for the latent heat of vaporization at the same temperature. This represents a safety issue requiring a design of the hydrogen loop which is able to remove the heat of conversion in a safe manner. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-04-27T01:49:11.003" LastActivityDate="2019-04-27T01:49:11.003" Title="What is the ortho/para issue with LH2 as a fuel?" Tags="|propulsion|fuel|safety|liquid-fuel|hydrogen|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17540" PostTypeId="2" ParentId="17538" CreationDate="2016-07-28T11:32:14.407" Score="3" Body="&lt;p&gt;&quot;Because Newton's first law says...&quot; is an answer but it's not really a &quot;because&quot; type answer. Newton's laws of motion are observations. He noticed these things about how the world seems to work and wrote them down. They are not explanations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Asking &lt;em&gt;why does an object's center of mass tend not to move (or change velocity) when there is no external force&lt;/em&gt;, and answering &lt;em&gt;because objects tend not to move (or change velocity) when there is no external force - and we call that Newton's Law&lt;/em&gt; is not really an answer to &lt;em&gt;why?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Richard Feynman sometimes got a little prickly when asked &quot;the why question&quot; in physics himself. He is usually very careful to remind us over and over &quot;we don't know why&quot; something happens, but it just does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in my alternate answer, I've expressed Newton's first law in more familiar rather than formal terms. But really, no matter how you look at it, it's simply an observation of how the world appears to work. Every time we check, no matter how we check, it keeps happening this way. Most people call it a law, but this is only because we haven't found a case &lt;strong&gt;yet&lt;/strong&gt; when it doesn't happen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We should really call these &lt;strong&gt;Newton's observations&lt;/strong&gt;. He noticed it and noted his observations, and he couldn't find exceptions, and it helped him predict more complicated things and those thinks also always seem to happen the way he predicted. For humans, a law is something we should or must do. But, in physics, a law is no more than an observation that something seems to keep happening and no exceptions have been seen &lt;strong&gt;yet.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Very good question! I had to think about it. The center of mass can't suddenly move on its own, without an external force - like thrust from it's rocket or somebody pushing on it. Floating there in space, if you don't see it interacting with anything, not thrust or magnetic fields or beams of light or solar wind, then it's gotta just stay right there. That's what Newton sez in this &lt;a href=&quot;https://space.stackexchange.com/a/17541/12102&quot;&gt;excellent answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If something (symmetric like a wheel around its axis) is turning inside the spacecraft - even a &lt;a href=&quot;https://space.stackexchange.com/q/10223/12102&quot;&gt;tape recorder&lt;/a&gt; - the whole thing can rotate in place, but it can't move from one place to another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If something moves (translates) inside, then the whole spacecraft can appear to move from the outside, but its center of mass &lt;em&gt;will still be&lt;/em&gt; in the same place.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-07-29T03:23:10.660" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17541" PostTypeId="2" ParentId="17538" CreationDate="2016-07-28T11:32:25.380" Score="11" Body="&lt;p&gt;The reason for this is Newton's first law of motion: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When viewed in an inertial reference frame, an object either remains&#xA;  at rest or continues to move at a constant velocity, unless acted upon&#xA;  by a net force&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(from &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot;&gt;here&lt;/a&gt;). This is the version for point-like objects. For extended objects, the law applies to the center of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it activates its reaction wheel, the satellite is not acted upon by external forces (ignoring gravity): the wheels only create an internal force, which cannot impact the trajectory of the center of mass. &lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="3141" LastEditDate="2016-07-28T14:05:53.130" LastActivityDate="2016-07-28T14:05:53.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17542" PostTypeId="2" ParentId="17539" CreationDate="2016-07-28T12:06:45.063" Score="19" Body="&lt;p&gt;The problem is that the transition produces enough energy to boil the LH2.  As explained on the old &lt;a href=&quot;http://yarchive.net/space/rocket/fuels/liquid_hydrogen.html&quot;&gt;sci.space.history&lt;/a&gt; group:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Skipping the gory quantum-mechanical details... there are two energy&#xA;  states of the hydrogen molecule, ortho and para.  At room temperature,&#xA;  hydrogen is about 3/4 ortho.  At liquid-hydrogen temperatures, the&#xA;  stable state is almost all para.  But the ortho-para conversion is&#xA;  slow, so if you just liquefy hydrogen, what you get is still 3/4&#xA;  ortho, and it slowly converts itself to para.  This is trouble because&#xA;  the ortho-para transition releases a modest amount of energy, and it&#xA;  doesn't take much energy input to boil off liquid hydrogen.  In fact,&#xA;  the transition will boil off &lt;em&gt;all&lt;/em&gt; the liquid.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The fix is to find a catalyst which will speed up the transition, and&#xA;  put some of that in your hydrogen liquefier, so the transition will&#xA;  happen while the liquefier is sucking heat out of the hydrogen anyway.&#xA;  This is now quite routine.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The difference in properties between the two are quite small; some&#xA;  thermal properties are slightly different, as I recall.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;I suppose ortho-hydrogen ought to have a very slightly higher Isp than&#xA;  para-hydrogen, but the difference is slight and the difficulties of&#xA;  handling and storage loom large.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This particular problem is pretty much unique to hydrogen,&#xA;  fortunately. Hydrogen and helium, and to some extent neon, show&#xA;  oddities in their behavior which are visible effects of quantum&#xA;  mechanics -- they are sometimes spoken of as the &quot;quantum gases&quot;. &#xA;  (This contributes to some of the peculiarities of liquid hydrogen,&#xA;  like its very low boiling point and its very low density.)  Hydrogen&#xA;  is the only one of the three which has molecules containing more than&#xA;  one atom, and hence the only one which shows an ortho-para&#xA;  distinction.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In practical terms this means&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As  a  result  of  this  slow  change,  a  thermally  isolated  tank &#xA;  of  liquid  hydrogen  prepared  without  conversion  to  the &#xA;  para  form  will  lose  about  1  percent  of  its  volume  during&#xA;  the  first  day  of  storage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(this is from the reference linked below)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A number of catalysts have been used: chromic oxide, paramagnetic ferric oxide, hydrous manganese oxide. More info &lt;a href=&quot;http://nvlpubs.nist.gov/nistpubs/jres/60/jresv60n3p221_A1b.pdf&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-07-28T12:12:06.503" LastActivityDate="2016-07-28T12:12:06.503" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17543" PostTypeId="2" ParentId="17532" CreationDate="2016-07-28T14:11:02.343" Score="3" Body="&lt;p&gt;Don't have exact figures but let's estimate it a little...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars orbital velocity is roughly 3.3 km/s. Your dragon capsule has a mass of about 8 tons. So your entire rocket would be something like 50 tons to reach Mars orbit. The methane/oxygen needed in that is roughly 36 tons. Of that, about 12 tons is methane. To generate 1 kg of methane we need 50 MJ of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars distance to the sun is roughly 1.5 AU on average. Thus the solar power generated per m² is about 44% of that of earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Near the equator you have roughly 600 W/m² of solar radiation at midday. Due to planetary rotation you can use half the day to generate power and it will increase from sunrise until midday and then decrease until sunset - you can increase that amount by spacing and rotating the panels though, so let's assume we end up with effective 150 W/m².&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we use really efficient solar panels, we can probably generate up to 50 W/m². You can probably convert 60% of that energy to fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so 12,000 * 50 MJ / 60% ~ 1TJ. With 50 W/m² that would mean about 2 * 10^10 m² * s or 634 m² * year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This does of course not account for losses storing or compressing the fuel yet... with all the assumptions, you might end up with 10-50 times that number though.&lt;/p&gt;&#xA;" OwnerUserId="12555" LastActivityDate="2016-07-28T14:11:02.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17544" PostTypeId="2" ParentId="17532" CreationDate="2016-07-28T15:00:58.737" Score="2" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Sabatier_reaction#Manufacturing_propellant_on_Mars&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;, I learned that 1 kg of methane was produced using 700 W of power in a day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://amzn.to/2a76ZAF&quot; rel=&quot;nofollow&quot;&gt;The Case for Mars&lt;/a&gt; estimates 82 tonnes of Methane/ LOX required for the ERV, which assumes launching directly to Earth. Of this, about 22 tons (20 metric tons) of it is methane. This is for a return directly to Earth vehicle, not just to Mars orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The overall power requirement can then be computed. 20000/365=54.8 kg of fuel required per day. That means 38.3 kW of power on average per day. Given the same efficiency that Adwaenyth listed, that's about 767 m^2 of solar panels to produce the fuel in a year. Of course, a real mission would have 2 years to produce the fuel, at least, more likely 3 years. It would also require a certain amount of cooling. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars Direct plan requires sending a small nuclear reactor, on the order of 80 kW, to supply this power. It also suggest some solar power for backups and remote sites. At the estimated 1 ton/ 5 kW of solar panels, that does make sense.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-07-28T15:00:58.737" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17545" PostTypeId="2" ParentId="17530" CreationDate="2016-07-28T15:45:15.033" Score="4" Body="&lt;p&gt;The Martian atmosphere is very much thinner than Earths.  So SRL on Mars needs to happen in the equivalent of the density of Mars's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus when the Falcon 9 first stage is coming back for a landing on the ASDS ship (No longer a barge! Don't be a barge-ist on this site!) or back at LZ-1, they are collecting reentry data to share with NASA as it passes through similar densities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a great presentation by &lt;a href=&quot;https://www.youtube.com/watch?v=ZoSKHzziLKw&quot; rel=&quot;nofollow&quot;&gt;Larry Lemke, at SETI talks&lt;/a&gt; where he talks about EDL (Entry, Descent, Landing) on Mars, and that there is a 'sporty' way to do it with a lifting body capsule, where you would actually fly down closer to the surface, into the densest part of the atmosphere, and then fly up, to stretch out how long you have to decelerate via aero breaking.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-28T15:45:15.033" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17546" PostTypeId="2" ParentId="17534" CreationDate="2016-07-28T16:26:50.730" Score="9" Body="&lt;p&gt;As briefly mentioned in the previous answer, H&lt;sub&gt;2&lt;/sub&gt; is very tricky to deal with. The temperature is one thing, but what he didn't mention was its extremely low density. If I recall correctly the LH&lt;sub&gt;2&lt;/sub&gt; tanks on the shuttle require around 4-5 times the space of its LOX tanks, if not more. The volume and mass of the tank itself make them very difficult to deal with, and adding cooling system to prevent the fuel from boiling off compounds that problem even further.&lt;/p&gt;&#xA;&lt;p&gt;Methane, on the other hand, is very dense. What it lacks in performance it more than makes up for by requiring a comparably tiny tank volume. What your rocket lacks in ISP it will more than make up for in having more propellant and less dead weight from the tanks.&lt;/p&gt;&#xA;&lt;p&gt;Edit: I forgot to mention that electrolyzing water into H&lt;sub&gt;2&lt;/sub&gt;/O&lt;sub&gt;2&lt;/sub&gt; requires very large amounts of electrical power. The Sabatier reaction (2H&lt;sub&gt;2&lt;/sub&gt;O + CO&lt;sub&gt;2&lt;/sub&gt; =&amp;gt; CH&lt;sub&gt;4&lt;/sub&gt; 2O&lt;sub&gt;2&lt;/sub&gt;) is vastly more energy efficient.&lt;/p&gt;&#xA;&lt;p&gt;So, in addition to my points about the low density and high tank/support system mass involved with LH&lt;sub&gt;2&lt;/sub&gt;, you'd need significant additional powerplant mass, too.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastEditorUserId="7062" LastEditDate="2021-10-25T10:46:30.903" LastActivityDate="2021-10-25T10:46:30.903" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="17547" PostTypeId="2" ParentId="17492" CreationDate="2016-07-28T16:37:16.977" Score="2" Body="&lt;p&gt;RISAT-1 and RISAT-2 both carry &lt;a href=&quot;https://en.wikipedia.org/wiki/Synthetic_aperture_radar&quot; rel=&quot;nofollow&quot;&gt;synthetic aperture radar&lt;/a&gt; systems which can create detailed maps of the surface below.  There has been talk of using these systems to identify wreckage on the ground for quite a while now, but in the late 1990s, when a lot of these ideas were being proposed, the resolution was simply too low to make it a reliable option.  &lt;a href=&quot;http://searchandrescue.gsfc.nasa.gov/techdevelopment/sar2.html&quot; rel=&quot;nofollow&quot;&gt;NASA actually did some research in this area&lt;/a&gt; too, but that's been indefinitely on hold for quite some time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a sample paper from the late 90s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://wmsmir.cits.rncan.gc.ca/index.html/pub/geott/ess_pubs/219/219846/13148.pdf&quot; rel=&quot;nofollow&quot;&gt;http://wmsmir.cits.rncan.gc.ca/index.html/pub/geott/ess_pubs/219/219846/13148.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Abstract: (emphasis mine)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This  paper  summarizes  some  results  of  studies  at  the  Canada  Centre  for  Remote  Sensing  (CCRS)  using  Synthetic  Aperture  Radar  (SAR)  imagery  from  spaceborne  systems  for  the  detection  of  crashed  aircraft.      Studies  have  been  carried  out  using  detected  products  (intensity  values  only)  and  interferometric  methods   (using   complex   imagery).      &lt;strong&gt;Due   to   the   low   resolution   of   single   polarization single frequency spaceborne SAR imagery  (approximately 8 metres ground range being the best currently available from operational remote sensing satellites),  it  is  seen  that  such  imagery  cannot  currently  be  used  with  much  optimism  although  the  techniques  themselves  show  promise.&lt;/strong&gt;  Further  study  is  needed  to  examine  if  the  better  resolutions  that  will  be  available  from  future  systems  such  as  RADARSAT-2  make  possible  the  reliable  detection  of  crashed aircraft.   Other research, not described in this manuscript, is &#xA;  underway at CCRS through  the  support  of  the  National  Search  and  Rescue  Secretariat  examining the  contributions  to  Search  and  Rescue  that  can  be  made  using spaceborne polarimetric SAR systems including RADARSAT-2. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.thehindu.com/sci-tech/technology/article3350204.ece&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt;, RISAT-1 has a resolution that can be varied between 50m and 3m, and also has a &quot;spotlight mode&quot; where this resolution can be as good as 1m.  This is definitely in the range where it could be useful if we have a general idea of where to look.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Realistically, I'm not sure how much good the satellites will do, especially compared with the aircraft and submarines already deployed, but it is an idea that works in theory, so you can't really fault them for going for it.&lt;/p&gt;&#xA;" OwnerUserId="15841" LastActivityDate="2016-07-28T16:37:16.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17548" PostTypeId="2" ParentId="15032" CreationDate="2016-07-28T17:13:52.460" Score="0" Body="&lt;p&gt;I think you're asking the wrong question. Reusable engines will certainly have inferior performance to expendable ones, at least for a given cost/mass/size/TWR/ect. Just think about how cheap and simple model rocket motors are. They're nothing but explosives in a cardboard tube, and I can't imagine how you'd come up with a similar reusable system without making big sacrifices.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real question of re usability is &quot;How is a re-usable engine/vehicle recovered?&quot; Consider how the SRBs from the space shuttle are recovered - by splashing down and being towed back. This makes landing simple - as all you need are parachutes to slow the fall - but introduces the problem of salt-water. Turns out that bathing your SRBs in a corrosive ocean is bad for them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now think of a SpaceX or Blue Origin style landing on land. It's much more complicated because of the need for landing gear, guidance, and the extra fuel to soft land, but I set my rocket down in relatively pristine condition. The engines aren't being eaten by salt water and don't need the same kind of intensive cleaning and refurbishment. That translates into less cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, it's not so much about the engine as it is how you get it back.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastEditorUserId="16440" LastEditDate="2016-07-28T17:50:57.240" LastActivityDate="2016-07-28T17:50:57.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17549" PostTypeId="1" CreationDate="2016-07-28T17:21:19.947" Score="8" ViewCount="221" Body="&lt;p&gt;Visiting friends in the Northern Tablelands of NSW.  I went outside and observed a clear diffuse blob to the north of Scorpio.  At first I thought, comet, however binoculars revealed a characteristic rocket plume shape. The diffuse blob dispersed over the following hour. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I pulled out the camera and tripod, took two sets of shots, one wide area, one more zoomed in on the plume. About 10 images of 15 seconds each, F4.5. I then stacked each set, and average/fused the individual images (using Hugin, on OSX) to produce two images with more subtle detail. This was necessary to capture the plume location with respect to background stars, with just a mid range semi DSLR Camera. There had already been some drift from first sighting of more than a degree. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have then combined the two photos, the close up within the wide view, so as to provide a location image of the plume as seen from this location. The plume moved and dispersed with respect to the background stars over the time it took to take the two series of photographs as indicated in the final composite photo.  (Note purple oval and arrow in photo.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Total sequential image taking took about half an hour. Processing about 3 hours. Another half hour to put this post together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The observation location is in the Australian Eastern Standard Time (AEST) +10 hours to UTC time zone, and I began taking photos close to midnight (within an hour) on 28 July, moving into the early hours of Fri 29 July.  The observational ground location was 151˚ 39' East, and 30˚ 01 Min South. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyone able to identify what rocket launch or other event this might be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/McY6p.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/McY6p.jpg&quot; alt=&quot;Combined photo showing plume location near Scorpio.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15989" LastEditorUserId="15989" LastEditDate="2016-07-28T18:53:18.853" LastActivityDate="2016-07-29T16:06:21.427" Title="Identity of source of Rocket Plume visible Eastern Australia, north of Scorpio, midnight 28 July 2016?" Tags="|rockets|observation|plume|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17551" PostTypeId="1" AcceptedAnswerId="17601" CreationDate="2016-07-28T18:12:29.490" Score="4" ViewCount="669" Body="&lt;p&gt;What parts and tools are carried aboard the ISS for general repairs and maintenance?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mechanical: nuts and bolts, screws, drivers? Wrenches, pliers, saws?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plumbing: rigid or flexible tubing? Heat-shrink fittings? Silicone caulk? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electrical: wires, wire strippers, soldering equipment (I imagine the fumes and fire hazard would be a non-starter!)&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-08-01T15:39:33.707" Title="What spare parts and tools are aboard ISS?" Tags="|iss|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17552" PostTypeId="1" AcceptedAnswerId="17558" CreationDate="2016-07-28T18:16:10.090" Score="3" ViewCount="1856" Body="&lt;p&gt;Suppose you have a manned orbital shipyard. It obviously has no manufacturing capability, but instead it serves as an anchor and assembly point for modules launched from earth. It might have remote manipulator arms for mating the modules together, and the crew would preform EVAs for more delicate assembly tasks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What, if any, advantages would this confer over the rendevous-and-docking style assembly planned for missions like Mars Direct or Constellation?&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2019-03-03T06:34:26.480" Title="Would an orbital scaffold or shipyard be useful?" Tags="|orbit|construction|rendezvous|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17553" PostTypeId="2" ParentId="17549" CreationDate="2016-07-28T18:19:02.227" Score="2" Body="&lt;p&gt;Not sure but it was probably the NROL-61 launch.  A spy satellite for the US National Reconnaissance Office was launched at 1237 GMT July 28 from Cape Canaveral.  I don't know its exact orbit so I can't say for certain whether or not it was what you saw but the spy satellite would have been performing a periphrasis raise maneuver in the vacinity of Australia sometime 45+ minutes after its launch at 2237 AEST which is consistent with your observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://spaceflightnow.com/2016/07/28/spy-satellite-infrastructure-supported-by-atlas-5-rocket-launch/&quot; rel=&quot;nofollow&quot;&gt;https://spaceflightnow.com/2016/07/28/spy-satellite-infrastructure-supported-by-atlas-5-rocket-launch/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16180" LastActivityDate="2016-07-28T18:19:02.227" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="17554" PostTypeId="2" ParentId="17525" CreationDate="2016-07-28T18:52:54.467" Score="2" Body="&lt;p&gt;Yes and no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's pretend that the 3D printer can print anything if it has equal mass feedstock. Let's also pretend there is a static 10% waste in feedstock for the printer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, with those rules. It is more expensive to use the 3D printer in many situations. The 10% waste would totally ruin it. However there are two areas that would really make it worth while. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;It would be cheaper to carry 100 lbs of Feed stock and the plans for 1,000 1 lbs tools then to carry 1,000 1 lbs tools. Replace tools with anything. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;It would be cheaper to carry 100 lbs of feed stock and print a 990 foot sheet, then it would be to try to fly a 990 foot sheet into space. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Now with those two examples you can begin to extrapolate examples where it may be better to print than to carry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real problem is that a 3D printer can't just print anything. It's very limited. Limited by size, shape, material, and function (moving parts). For example you can't print a motherboard with it. Even things like a tennis ball are not gonna happen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why take it into space? Well the biggest reasons I can see are to test its usefulness. There is potential there. And to use it to replace or repair some equipment. Basically, if you just need a washer, print the washer. Don't launch another pod, or evac the station because you need a tiny washer.  &lt;/p&gt;&#xA;" OwnerUserId="11013" LastEditorUserId="415" LastEditDate="2016-07-30T11:27:38.933" LastActivityDate="2016-07-30T11:27:38.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17555" PostTypeId="1" CreationDate="2016-07-28T20:20:43.697" Score="3" ViewCount="358" Body="&lt;p&gt;Red Dragon is &lt;a href=&quot;http://spacenews.com/spacex-spending-about-300-million-on-red-dragon-mission/&quot; rel=&quot;nofollow&quot;&gt;currently in the news again&lt;/a&gt;. Except for its enormous amount of ambition and its relatively large risk of failure (e.g. Falcon Heavy RUD, a SuperDraco failure on Mars, mixing imperial and metrical units etc.), it seems to be relatively straightforward: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Stuff Dragon II with enough fuel for re-entry and landing&lt;/li&gt;&#xA;&lt;li&gt;Put onto Falcon Heavy&lt;/li&gt;&#xA;&lt;li&gt;Aim and shoot&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The &quot;aim&quot; part is even supported by NASA free of cost. However, the whole Mission still will cost around 320Million \$. A single Dragon II launch is proposed for 140Million\$ (still quite expensive) - including SpaceX margin.  That still leaves 180Million$ for the &quot;Mars part&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When SpaceX is going to launch that mission in 2018 and support it until 2020 (next launch), that makes 40Million $ annual budget, or roughly 200 exceptionally qualified engineers. Since SpaceX probably does not pay that many people that well for a single mission: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where does all that money go? &lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="25" LastEditDate="2016-07-28T20:35:20.027" LastActivityDate="2016-07-29T17:26:55.253" Title="What exactly makes Red Dragon costly" Tags="|launch|spacex|mars|cost|red-dragon|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17556" PostTypeId="2" ParentId="17555" CreationDate="2016-07-28T20:41:15.027" Score="3" Body="&lt;p&gt;The first difference is the Red Dragon will use a Falcon Heavy, which runs between \$30-\$80 million more than the Falcon 9. Note that the higher end of this is more likely reality, the Falcon Heavy is expected to be fully expended to get the Red Dragon to Mars, giving it the higher price tag. That alone accounts for almost half of the $180 million difference you mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, they are likely going to have some kind of experiment they run on Mars, which will no doubt cost something. It'd be a waste to get the system there without anything to do with it other than land it. I suspect this will be a decent part of the cost, and in fact might absorb any money left over from other areas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next they have to figure out the communication problem, which is much more complex than in Earth Orbit, although still quite doable. I suspect this will be a \$10-\$20 million effort. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then they have to meet Planetary Protection concerns, which no doubt will add cost. This probably isn't a huge cost, but a few million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There will no doubt be much modeling, simulation development, and other such exercises to ensure that the Dragon will land, software re-writes, etc. No doubt this will be at least $20 million, quite possibly more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, they have to pay to operate the spacecraft for at least 8 months, possibly as long as a year or longer, depending on how long it lasts on Mars. Again, this won't be a huge cost, I suspect NASA is allowing the use of the DSN to make this happen, but it is still a cost, at least to keep the operators required to make sure the spacecraft is working in order, maybe a few million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of that adds up quite a bit. It wouldn't surprise me at all to see that it ends up costing quite a bit more than a standard Dragon launch to the ISS.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-29T13:51:43.433" LastActivityDate="2016-07-29T13:51:43.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17557" PostTypeId="1" AcceptedAnswerId="17561" CreationDate="2016-07-28T20:44:36.840" Score="8" ViewCount="1144" Body="&lt;p&gt;While trying to find the priceof a Falcon Heavy launch, &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-heavy/&quot; rel=&quot;noreferrer&quot;&gt;I noticed the following&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;\$90M (&amp;lt;8.0mT), [\$130M (&gt;8.0mT)]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why is there a $40 million difference in price for the heavier payload? Also, that value seems suspect. Is this even a real price difference?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-07-29T22:27:55.770" LastActivityDate="2018-02-22T16:34:09.010" Title="Why does a Falcon Heavy cost more with a heavier payload?" Tags="|launch|spacex|rockets|cost|falcon-heavy|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="17558" PostTypeId="2" ParentId="17552" CreationDate="2016-07-28T21:00:08.823" Score="6" Body="&lt;p&gt;It depends a lot on what capabilities this shipyard brings. One of the primary advantages could be in tremendous volume savings - by having a dedicated spot to do the majority of the spacecraft assembly, one could send up raw (read: dense) materials to feed into the assemblers on this station/array. The array could then process these into the required components through refining and sub-assembly. This doesn't result in any weight savings, however. This could also be useful for processing any asteroid materials, but that presents another suite of problems and issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another method, and one that is close to being tested, is sending up the raw material required for the largest structures on a spacecraft while sending up the assembled spacecraft bus. An assembly drone could then use the raw material to build the extra structure on top of the pre-built satellite bus (this being what contains the most complicated elements). This would be highly advantageous for building large orbital telescopes, reflectors, or antennae as there is no great way to pack large structures like this into launch fairings (look at the complexity involved in the JWST deployment for example). A few companies have received &lt;a href=&quot;https://www.sbir.gov/&quot; rel=&quot;nofollow&quot;&gt;SBIR&lt;/a&gt; grants for concepts related to this (the company I work for being one of them - I'm too leery of saying what our tech is though).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in summary - the capabilities of the array matter. Assembly alone at the array would provide for some advantages. Assembly and some degree of fabrication extends these capabilities and the worth of such an array, and inclusion of refining allows for usage of materials acquired in space. Further capabilities in manufacturing electronics, silicates, organics, plastics, and so on would advance its worth to be tremendous (and I may have misidentified things here, my apologies).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some interesting links:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/spacetech/niac/2013phaseII_hoyt.html#.V5pxeLgrKUk&quot; rel=&quot;nofollow&quot;&gt;https://www.nasa.gov/spacetech/niac/2013phaseII_hoyt.html#.V5pxeLgrKUk&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ti.arc.nasa.gov/news/IRG-Dragonfly-robot-UI/&quot; rel=&quot;nofollow&quot;&gt;https://ti.arc.nasa.gov/news/IRG-Dragonfly-robot-UI/&lt;/a&gt; - currently underway, attempt to build a large antenna in orbit using robotic methods&lt;/p&gt;&#xA;" OwnerUserId="16197" LastEditorUserId="1142" LastEditDate="2016-07-29T12:53:05.677" LastActivityDate="2016-07-29T12:53:05.677" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17559" PostTypeId="2" ParentId="17552" CreationDate="2016-07-28T21:31:42.380" Score="1" Body="&lt;p&gt;There are several proposed spaceship designs (or rather elements of spaceship design) that require quite large distances:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/radiation.php#id--Radiation_Shielding--Shielding--Shadow_Shield&quot; rel=&quot;nofollow&quot;&gt;Shadow shield&lt;/a&gt; works best far, far away from the crew module&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.projectrho.com/public_html/rocket/artificialgrav.php&quot; rel=&quot;nofollow&quot;&gt;Spin gravity&lt;/a&gt; works better when the RPMs are lower (due to the Coriolis effect)&lt;/li&gt;&#xA;&lt;li&gt;Photovoltaic power tends to require rather large structures&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All these aspects are practically not feasible when you want to launch your spaceship with one single rocket. So a space-yard is necessary for ships with nuclear-thermal propulsion, photovoltaic power support or artificial gravity. &lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-07-28T21:31:42.380" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17560" PostTypeId="2" ParentId="17490" CreationDate="2016-07-28T23:18:20.720" Score="4" Body="&lt;p&gt;In addition to vacuum tube technology, NASA is looking into &lt;a href=&quot;http://spectrum.ieee.org/tech-talk/aerospace/robotic-exploration/nasa-commissions-ultra-high-temp-chips-for-venus-landsailing-rover&quot; rel=&quot;nofollow&quot;&gt;high temperature semiconductors for use with Venus probes&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="16445" LastActivityDate="2016-07-28T23:18:20.720" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17561" PostTypeId="2" ParentId="17557" CreationDate="2016-07-29T00:07:54.927" Score="8" Body="&lt;p&gt;The difference is in the re-usability factor. The number on the &lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9 cost&lt;/a&gt; page is as follows:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon 9 to GTO- 5.5 t to GTO \$62m&lt;/li&gt;&#xA;&lt;li&gt;Falcon Heavy to GTO- 8 t to GTO \$90m&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, it lists the capacities as follows:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Falcon 9- 8.3 t&lt;/li&gt;&#xA;&lt;li&gt;Falcon Heavy- 22 t&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One can only assume that the reduction in price is for a mission that allows for landing the booster again, to try for re-usability. The full capacity, that doesn't allow for landing, costs more. There might even be a likelihood of return computed, a booster that needs to carry more weight has a higher chance of being damaged.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-02-22T16:34:09.010" LastActivityDate="2018-02-22T16:34:09.010" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17562" PostTypeId="1" AcceptedAnswerId="17635" CreationDate="2016-07-29T08:59:25.060" Score="8" ViewCount="2151" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/17500/12102&quot;&gt;this answer&lt;/a&gt; the survival of the &lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/index.php&quot; rel=&quot;noreferrer&quot;&gt;Parker Solar Probe&lt;/a&gt; spacecraft passing as close as  9 solar radii from the Sun's surface is discussed. Using guestimates for apoapsis and periapsis of 1.1 $a_{venus}$, 10 $r_{sun}$. I get velocities of about 11,000 and 190,000 m/s using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;noreferrer&quot;&gt;vis-viva equation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QAreb.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QAreb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screenshot from page ES9 of the &lt;a href=&quot;http://solarprobe.jhuapl.edu/common/content/SolarProbePlusFactSheet.pdf&quot; rel=&quot;noreferrer&quot;&gt;Solar Probe Plus Fact Sheet&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heat shield is &lt;a href=&quot;http://solarprobe.jhuapl.edu/spacecraft/index.php&quot; rel=&quot;noreferrer&quot;&gt;described here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The main feature of the TPS is an 8-foot-diameter, 4.5-inch-thick, carbon-carbon, carbon foam shield that will sit atop the Solar Probe Plus spacecraft body. The system will protect the Parker Solar Probe from temperatures exceeding 2,500 degrees Fahrenheit and impacts from hypervelocity dust particles as it flies through the Sun’s outer atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the GIF below, the shield and a stand-off between the shield and the body of the &lt;a href=&quot;https://space.stackexchange.com/questions/17490/#comment45058_17490&quot;&gt;espresso maker-shaped&lt;/a&gt; spacecraft can be seen. As described in the &lt;a href=&quot;https://space.stackexchange.com/a/17500/12102&quot;&gt;linked answer&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/17498/#comment45311_17500&quot;&gt;here&lt;/a&gt;, the &quot;side panels&quot; are radiators to help cool the back of the shield, and the long legs connecting the shield to the satellite designed to have a very high thermal impedance to minimize heat flow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've asked about the origin of the dust near the Sun &lt;a href=&quot;https://astronomy.stackexchange.com/q/16707/7982&quot;&gt;in this question&lt;/a&gt; If I understand correctly, the dust is slowly falling in to the Sun because particles of a certain size range experience a tangential deceleration force via the &lt;a href=&quot;https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect&quot; rel=&quot;noreferrer&quot;&gt;Poynting Robertson effect&lt;/a&gt;. It's related to astronomical aberration - moving transversely through a photon flux causes a slightly enhanced radiation pressure on the forward facing side of the particle. Since the force is velocity dependent, it is greater at periapsis, and thus tends to circularize the orbits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question is:&lt;/strong&gt; If the heat shield is facing directly towards the Sun so that this long narrow spacecraft can remain in the shadow so that it is &quot;moving sideways&quot; through this dust field, how does the this shield &quot;&lt;em&gt;protect the spacecraft from impacts from hypervelocity dust particles as it flies through the Sun's outer atmosphere?&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there &quot;hypervelocity dust particles&quot; coming straight out of the Sun? Sun soot?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is further highlighted by this paragraph from Section 4.5.3 of this &lt;a href=&quot;http://solarprobe.jhuapl.edu/mission/docs/SolarProbePlus2008.pdf&quot; rel=&quot;noreferrer&quot;&gt;2008 report&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Coronal Dust instrument is mounted on the –Y panel looking in the ram direction to collect the maximum number of dust impacts. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; The constant, slow infall of particles/dust results in a higher concentration near the sun - a collapsing spherical shell has smaller and smaller area. This cloud has been historically observed as the source of &lt;a href=&quot;https://en.wikipedia.org/wiki/Zodiacal_light&quot; rel=&quot;noreferrer&quot;&gt;Zodiactal Light&lt;/a&gt;. Most of the particles are tens or hundreds of microns in size, and one theory for the origin is the collisions of asteroids or from comets. When the cloud gets too close to the Sun, the particles disintegrate, and this &quot;hole&quot; in the cloud is called the &lt;a href=&quot;http://arxiv.org/pdf/0909.1722.pdf&quot; rel=&quot;noreferrer&quot;&gt;Solar Dust Corona&lt;/a&gt;. There is much more to read in the paper nicely titled &lt;a href=&quot;http://solarprobe.gsfc.nasa.gov/dust.pdf&quot; rel=&quot;noreferrer&quot;&gt;Dust Near The Sun&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sYf2r.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sYf2r.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; a long exposure from Paranal showing &lt;a href=&quot;https://en.wikipedia.org/wiki/Zodiacal_light&quot; rel=&quot;noreferrer&quot;&gt;zodiactal light&lt;/a&gt; - this and larger versions &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Zodiacal_Light_Seen_from_Paranal.jpg&quot; rel=&quot;noreferrer&quot;&gt;available in Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/32uRq.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/32uRq.gif&quot; alt=&quot;Solar Probe Plus&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF made from six screenshots, cropped from the &lt;a href=&quot;https://www.youtube.com/watch?v=nDAlf_UmlTk&quot; rel=&quot;noreferrer&quot;&gt;YouTube video&lt;/a&gt; shown separately below, from the Johns Hopkins University Applied Physics Laboratory. The &lt;a href=&quot;http://www.nasa.gov/content/goddard/nasa-gives-green-light-for-johns-hopkins-apl-to-begin-building-solar-probe-plus-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;JHU APL has designed and will build and operate the spacecraft for NASA&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also see details of the articulation of the solar panels - note how they become increasingly shadowed behind the shield as they are withdrawn, until only a tiny section remains highly obliquely illuminated in the incredibly intense flux of light (and everything else) from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/nDAlf_UmlTk?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ymUaM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ymUaM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screen shot from NASA's &lt;a href=&quot;http://solarprobe.jhuapl.edu/common/content/SolarProbePlusFactSheet.pdf&quot; rel=&quot;noreferrer&quot;&gt;Solar Probe Plus Fact Sheet&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-31T16:14:00.130" LastActivityDate="2018-01-07T07:42:06.067" Title="How does the Parker Solar Probe's heat shield protect it from dust near the sun?" Tags="|heat-shield|solar-wind|parker-solar-probe|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17563" PostTypeId="1" AcceptedAnswerId="17566" CreationDate="2016-07-29T14:41:02.633" Score="9" ViewCount="8566" Body="&lt;p&gt;As I stated in the answer to another question, LH2 suffers some serious drawbacks compared to other fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Off the top of my head, they are&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Extremely low density, resulting in:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lower mass-fraction because of high tank mass&lt;/li&gt;&#xA;&lt;li&gt;High aerodynamic drag due to tank volume&lt;/li&gt;&#xA;&lt;li&gt;Larger and heavier vehicle structure due to tank volume&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Boiloff and tank seepage, resulting in:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Even higher tank mass because of the need for insulation&lt;/li&gt;&#xA;&lt;li&gt;More complicated tank arrangements because of the inability to butt LH2 tanks against relatively warm cryopropellants like LOX.&lt;/li&gt;&#xA;&lt;li&gt;Long-term storage issues from propellant boiloff&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The only possible advantages I can think of are&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Marginally higher specific impulse&lt;/li&gt;&#xA;&lt;li&gt;Producing only water as a byproduct, keeping environmentalists happy.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In light of all this, what rationale is there for a LH2/LOX Rocket?&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2022-06-04T08:19:39.413" Title="Pros and Cons of LH2/LOX vs Other Fuels" Tags="|launch|fuel|engines|liquid-fuel|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17564" PostTypeId="1" AcceptedAnswerId="17567" CreationDate="2016-07-29T14:41:32.337" Score="7" ViewCount="1724" Body="&lt;p&gt;I know that rocket launches always have water towers spray water all around when the rocket is launching to suppress the extreme sounds and shockwaves generated by the launch and ensure the rocket doesn't tear itself apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in &lt;a href=&quot;https://www.youtube.com/watch?v=SZQY902xQcw&quot;&gt;this&lt;/a&gt; video of a test fire no water towers are visible. How does SpaceX prevent damage from vibrations in this case?&lt;/p&gt;&#xA;" OwnerUserId="3141" LastActivityDate="2016-07-29T18:48:03.160" Title="Why doesn't this SpaceX rocket test fire use water sprays?" Tags="|spacex|water|testing|sound|vibration|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17565" PostTypeId="2" ParentId="17001" CreationDate="2016-07-29T15:03:18.970" Score="6" Body="&lt;p&gt;Our current best estimate for the Star Shot ground-based approach is it will cost on the order of \$10 B.  We estimate the space-based approach to cost on the order of \$100 B.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We intend to send probes to planets in the habitable zone around nearby stars. The Alpha Centauri system is the closest sun-like star, which is why it is our first target.  About 80% of the nearby stars are dwarf stars; there is a debate if life is possible in these systems.&lt;/p&gt;&#xA;" OwnerUserId="16453" LastEditorUserId="6816" LastEditDate="2016-07-29T16:49:05.420" LastActivityDate="2016-07-29T16:49:05.420" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17566" PostTypeId="2" ParentId="17563" CreationDate="2016-07-29T15:04:34.213" Score="18" Body="&lt;p&gt;It's not a &quot;marginally higher&quot; specific impulse. High performance hydrogen engines typically have a vacuum Isp of around 420-450 seconds, compared to 310-350 for hypergolics or kerosene. That's about 30% delta v advantage, ton for ton, which more than offsets the structural volume penalty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drag penalty is mostly irrelevant for upper stages as long as their diameter is no larger than lower stages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another drawback to hydrogen is that low propellant density yields lower thrust from a similar chamber size; again not a big problem for upper stages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The clean exhaust is nice, but not a huge advantage over kerosene. It is much more attractive than toxic hypergolics, of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider two 22-ton upper stages each pushing a 5 ton payload. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Stage H is 20 tons hydrogen-LOX, 2 tons dry mass, 450s Isp.&lt;/li&gt;&#xA;&lt;li&gt;Stage K is 21 tons kerosene-LOX, 1 ton dry mass, 350s Isp.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;H delivers 5956 m/s of delta-v versus K's 5162 m/s -- a 15% improvement despite hauling twice the dry structural mass.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-01-18T16:18:30.853" LastActivityDate="2017-01-18T16:18:30.853" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="17567" PostTypeId="2" ParentId="17564" CreationDate="2016-07-29T15:18:14.220" Score="8" Body="&lt;p&gt;The amount of white vapor present at the beginning and end of the test suggests that there is indeed a water spray down within the flame trench.  There is no need for above-grade spray because the rocket is tied down and cannot lift off.  The structure of the test stand itself will provide substantial acoustic shielding for the rest of the vehicle.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2016-07-29T15:18:14.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17568" PostTypeId="2" ParentId="17549" CreationDate="2016-07-29T16:06:21.427" Score="3" Body="&lt;p&gt;The most probable answer to date offered for the observed sky feature is the NROL-61 rocket launch. Specifically, a fuel dump from the Centaur stage of the multistage launch, as part of the Centaur's decent procedure back to Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As this Satellite is intended for high geostationary orbit, multiple rocket stages are required. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This view comes from the fact this appears the only significant launch in the time period, within an hour or so, to the phenomena observed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other possible astronomical explanations such as comet, nebula, etc are eliminated by the shape, dynamic change, and drift relative to background stars over relatively short time periods of the phenomena observed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket launches from other nation states unpublished and unknown remain an alternate possibility. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The below quoted note on the NROL-61 launch from the &lt;a href=&quot;http://satobs.org/seesat/Jul-2016/0119.html&quot; rel=&quot;nofollow&quot;&gt;satobs.org site&lt;/a&gt; refers to one part of this multi stage launch process to deliver the satellite to its destination orbit that seems of particular relevance as explanation. (My bold highlighting.) &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Centaur de-orbit MECO 3 is the de-orbit burn of the Centaur. The orbit has been estimated based on an instantaneous change of elements. The MET of 01:05:00 is approximate. &lt;strong&gt;Blowdown of remaining propellants will occur shortly after MECO 3, which likely will take less than 15 minutes to complete.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More on the Centaur on Wikipedia: &lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_(rocket_stage)&quot; rel=&quot;nofollow&quot;&gt;Centaur_(rocket_stage)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What a pleasure to observe casually a new and interesting night sky phenomena, and with the help of several be able to come to a plausible identification and explanation. &lt;/p&gt;&#xA;" OwnerUserId="15989" LastActivityDate="2016-07-29T16:06:21.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17569" PostTypeId="2" ParentId="17555" CreationDate="2016-07-29T17:01:17.360" Score="1" Body="&lt;p&gt;Launch vehicle and lander are &lt;a href=&quot;http://science.nasa.gov/media/medialibrary/2012/01/23/NAC_Science_Meeting_ReportOctober_31-November_1_2011-finalTAGGED.pdf&quot; rel=&quot;nofollow&quot;&gt;estimated to be between 150 and 190 million.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost of operation will be minimal because once the spacecraft is on it's way to Mars very little intervention is required during the trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for payload, SpaceX has been tight lipped but I expect a number of people will be excited to hitch a free ride to Mars, so I don't see that being a major cost driver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the $300 million number includes some of the developmental costs for the Dragon II, such as being able to do a power landing instead of simply splashing down.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-07-29T17:26:55.253" LastActivityDate="2016-07-29T17:26:55.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17570" PostTypeId="2" ParentId="17525" CreationDate="2016-07-29T17:53:56.450" Score="3" Body="&lt;p&gt;I disagree vastly with the majority of the answers here - there are a large number of recent NASA SBIR/other grants that show the agency firmly believes otherwise!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, the 3D printer on the space station is naught but a research one and is a poor one at that. The company that made it is run by a... not so nice fellow that didn't do a good job of building it. It has served limited use due to this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary cost savings could arrive through recycling of the materials onboard. This would allow for reusable utensils, reusable tools (tool broke? if you still have material, it can be reprocessed), purpose-made parts for performing experiments (recycling makes this not at all wasteful), and so on. Utensils especially are useful - if they can be recycled, they will be sanitized when the material is melted at its usual temps! Currently, astronauts must wash all of their utensils with wet wipes since running water isn't the best idea on the ISS. This generates a (hilariously silly) amount of waste in the form of nothing but wet wipes. Which also must be sent up as supplies. And the astronauts say that the utensils can get fairly gnarly after a while... so being able to print single-use recyclable utensils lowers waste generation, lowers supply requirements, and increases quality-of-life (in some small measure) for the crew. All large cost savings! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issue isn't with the technical readiness of 3D printers for space station employment. Even the most basic of printers is capable of printing in zero-g in its stock form. The issue that requires some engineering is bed adhesion, and removal of the byproducts generated by melting ABS/PLA/Ultem-1010. These fumes can be hazardous and must be removed from the build volume with air cycling (or must be contained while the machine is running, then purged).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ERASMUS is the utensil recycling idea I mentioned - can see that it was funded here: &lt;a href=&quot;http://www.parabolicarc.com/2016/05/06/nasa-selects-tethers-unlimited-sbir-awards/&quot; rel=&quot;nofollow&quot;&gt;http://www.parabolicarc.com/2016/05/06/nasa-selects-tethers-unlimited-sbir-awards/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the same company/division is currently building this as well - probably not a coincidence, once you figure one of these out the rest are natural evolutions of the starter concept. Neat stuff! -&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.geekwire.com/2016/nasa-firmamentum-3-d-printer-recycler-space/&quot; rel=&quot;nofollow&quot;&gt;http://www.geekwire.com/2016/nasa-firmamentum-3-d-printer-recycler-space/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16197" LastActivityDate="2016-07-29T17:53:56.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17571" PostTypeId="2" ParentId="17564" CreationDate="2016-07-29T18:27:36.550" Score="0" Body="&lt;p&gt;The two test stands that SpaceX has in McGregor, TX are pretty open at the base. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is the BFT (Big F*****/Falcon) Test stand that they inherited and upgraded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BPwls.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BPwls.jpg&quot; alt=&quot;Big Test Stand&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has since been retired (at least for now... My personal theory is that as the launch tempo increases they will possibly require reactivating it) since the change to super cooled LOX and RP1 would require a lot of ground support equipment (GSE) changes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were in the midst of building a new test stand, we all thought was for the Falcon Heavy, so needed to be big enough to handle three cores at once, though since then it seems they only plan on testing one core at a time, however as the Falcon Heavy launch campaign is getting closer and closer, reality will trump theory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that test stand in this photo:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lKnGt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lKnGt.jpg&quot; alt=&quot;McGregor test stands&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you will notice in both is that they are REALLY open to the world. There is a lot of space for the exhaust to go, and nowhere really to pour water that can absorb the energy (thermal and audible). Apparently they do use a water deluge system, and it is likely at the bottom of the new test stands trench.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-07-29T18:48:03.160" LastActivityDate="2016-07-29T18:48:03.160" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17572" PostTypeId="2" ParentId="17444" CreationDate="2016-07-29T22:51:27.807" Score="3" Body="&lt;p&gt;I don't have a great answer to this, because each mission is rather unique, and the response always seems to be &quot;it depends&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'll likely need to get a frequency allocation from the FCC for your radio transmissions. Now I know there may be ways to get around this if you're low enough power or have a relatively common bandwidth/frequency, regardless it's worth looking into.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other I know of is licensing to image through the NOAA. Depending on your resolution you may be asked to not capture (any) images without a license. The licensing process becomes more stringent with different wavelengths and higher resolutions. At some points (for some of digitalglobe's spacecraft they need to send their imagery directly to a 3 letter government agency first, before they themselves can see them).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apart from that you'll need to comply with launch regulations, which are very unique to the launch vehicle, even within the cubesat community. These take the form much more of design requirements than a licensing process. Things that launch vehicles worry about are typically battery size and storage requirements, deployment sequences (including the sequence to power on your transmitter), any sort of pressure vessels, and of course propulsion and any sort of explosives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many more, and you're on the right track reaching out to the community. I would further recommend reaching out to folks who've built and flown a spacecraft similar to yours - that's usually the best avenue for advice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck.&lt;/p&gt;&#xA;" OwnerUserId="15822" LastActivityDate="2016-07-29T22:51:27.807" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17573" PostTypeId="1" CreationDate="2016-07-30T17:06:52.220" Score="8" ViewCount="635" Body="&lt;p&gt;Much has been made of how the reentry burns of Falcon 9 first stages occur at an altitude where the atmosphere is similar in density to the atmosphere  of Mars, and so they demonstrate that supersonic retropropulsion will work for Mars entry by the Red Dragon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the Falcon 9 first stage slows from around 1.5 km/s to some velocity during that burn, but not to zero. If the stage is &lt;a href=&quot;https://youtu.be/FCCyVCvN2bo?t=1151&quot; rel=&quot;nofollow noreferrer&quot;&gt;returning to Cape Canaveral&lt;/a&gt;, it slows to zero and then speeds up in the opposite direction, but that isn't really the same thing. It isn't spending time at low velocities in that atmosphere as a lander would coming in on Mars. &#xA;As there is no GPS or any beacons on the ground, a lander will spend some time at low speeds making sure it knows exactly how far away the ground is. Plus it would enter the atmosphere at about 3.5 km/s. It would let drag slow it as much as possible but then would it be firing its engines at a speed similar to the F9 1st stage?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Red Dragon is a pretty different shape and mass than an F9 1st stage, and according to &lt;a href=&quot;https://youtu.be/ZoSKHzziLKw?list=WL&amp;amp;t=1646&quot; rel=&quot;nofollow noreferrer&quot;&gt;this presentation&lt;/a&gt; it will incorporate lift to come in pretty level, so it will have a different angle of attack. So, how is information from F9 retropropulsion helping?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My conclusion from this information in &lt;a href=&quot;https://space.stackexchange.com/a/5711/4660&quot;&gt;this answer&lt;/a&gt; by Mark Adler is that control of such a vehicle is challenging and not the same when parameters are changed, which is why I ask:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol start=&quot;4&quot;&gt;&#xA;  &lt;li&gt;&lt;p&gt;Once in the supersonic flow, the aerodynamic effects of the messy business end of the rocket can be complicated, making control a bit of&#xA;  a challenge. You can have counterintuitive effects that redirect flow&#xA;  in unexpected directions at different angles of attack. &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Predicting the effect of the running engines on the drag is a &#xA;  challenge. The thrust plumes tend to reduce the drag, countering in&#xA;  part the intent of firing the engines to increase deceleration. With&#xA;  enough of a thrust to drag ratio, this is not a show stopper, but you&#xA;  need to be able to predict how large the effect is to know if you have&#xA;  enough fuel. This impact on drag also complicates what happens when&#xA;  you gimbal the engine, which is part of the challenge in #4. Again,&#xA;  high thrust to drag ratio can reduce the surprises here.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-08-02T22:37:38.620" Title="How is retropropulsion by the Falcon 9 first stage being used to design retropropulsion entry on Mars?" Tags="|mars|falcon-9|reentry|red-dragon|flight-control|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17574" PostTypeId="5" CreationDate="2016-07-30T17:07:51.907" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-07-30T17:07:51.907" LastActivityDate="2016-07-30T17:07:51.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17575" PostTypeId="4" CreationDate="2016-07-30T17:07:51.907" Score="0" Body="Systems and software used to control a space craft's trajectory." OwnerUserId="4660" LastEditorUserId="12102" LastEditDate="2019-08-04T21:15:25.060" LastActivityDate="2019-08-04T21:15:25.060" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17576" PostTypeId="2" ParentId="14383" CreationDate="2016-07-30T20:53:41.137" Score="156" Body="&lt;p&gt;Because linear increases in delta-v require exponential increases in mass, small changes to the assumptions you make about fuel tank structural mass and engine thrust-to-weight ratio start to make very large changes in the final size of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if you're getting off a 3.6g planet with a 7-stage rocket, the difference between 88% fuel fraction and 92% fuel fraction yields about a 10:1 difference in the total mass of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I don't think it's really reasonable to talk about ultimate theoretical limits; too many engineering factors are involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Locking down a lot of variables, I can tell you what kind of rocket you'd need for a given surface g, though. Let's make these assumptions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;We are placing 1 ton of payload into low planetary orbit.&lt;/li&gt;&#xA;&lt;li&gt;Required delta-v to reach orbit, including atmospheric and gravity losses, is 10,000m/s per surface g. Seems to hold for Earth, Mars, and the &quot;Earthtoo&quot; which was discussed &lt;a href=&quot;https://space.stackexchange.com/questions/17457/can-planet-earthtoo-put-a-tooian-in-orbit-too/17459#17459&quot;&gt;in another Q/A&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;We can build rocket stages of arbitrary size, with a tankage propellant fraction of 90%; the rocket stage mass is the tank mass plus the engine mass -- ullage rockets, interstage, etc. is all handwaved out.&lt;/li&gt;&#xA;&lt;li&gt;We have an infinite supply of Apollo-era rocket engines: RL-10, J-2, M-1, H-1, and F-1.&lt;/li&gt;&#xA;&lt;li&gt;First-stage TWR at ignition must be at least 1.2 (relative to local gravity)&lt;/li&gt;&#xA;&lt;li&gt;Middle-stage TWR at ignition must be at least 0.8&lt;/li&gt;&#xA;&lt;li&gt;Final-stage TWR at ignition must be at least 0.5&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Given those assumptions, here is a table of surface gravity, stage count, first-stage engines, and total rocket mass.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Surface                         First        Total       Saturn V &#xA;Gravity   Stages                Stage      Mass, t     Equivalent&#xA; 0.5           2             1x RL-10          4.5&#xA; 1.0           3             1x   H-1         49.4          0.02&#xA; 1.5           3             1x   F-1        249.2           0.1&#xA; 2.0           4             5x   F-1       1329.0           0.5&#xA; 2.5           5            40x   F-1       8500.9             3&#xA; 3.0           6           274x   F-1      50722.2            17&#xA; 3.5           7          2069x   F-1     331430.9           100&#xA; 4.0           8         20422x   F-1    2836598.4           950&#xA; 4.5           8        392098x   F-1   47 million         15000&#xA; 5.0           9    3.5 million   F-1  391 million        130000&#xA; 6.0          11    400 million   F-1   38 billion      millions&#xA;10.0          18        2.88e19   F-1      1.65e21  quadrillions&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Up above 10g, something really interesting happens that is kind of a theoretical limit. The mass of the rocket reaches &lt;em&gt;a measurable fraction of the mass of the entire planet&lt;/em&gt; it's launching from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 10.3g, rocket mass is 0.035 of the mass of the planet.&#xA;10.4g, rocket mass is one fifth of the mass of the planet. This doesn't actually alter the ∆v requirement -- we're going into orbit around the rocket/planet barycenter!&#xA;At 10.47g, the rocket &lt;em&gt;is&lt;/em&gt; the planet, and we're... just... chewing it up entirely, pulverizing it in a dust cloud expanding at 4km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These extreme conclusions appear to be corroborated by &lt;a href=&quot;https://arxiv.org/pdf/1803.11384.pdf&quot; rel=&quot;noreferrer&quot;&gt;this independently derived paper&lt;/a&gt;, which explores some other related aspects of super-Earth-based chemical rockets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another consideration recently brought up by user @uhoh is that as the linear scale of a given rocket stage increases, its mass, and thus the required thrust force to lift it, goes up by the cube of the scale, but the area available at the base of the rocket to mount engines goes up only by the square of the scale; this problem is made even worse here by the increasing surface gravity. The Saturn V was just about at the point where this relation starts to become problematic; the outboard engines on its first stage are mounted at the very edge of the stage in order to make room for their nozzles to gimbal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid rockets don't have the same dimensional constraints, and have very good thrust-to-weight and thrust-to-cost ratios, so they're probably more likely to be used in lower stages for these very large rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stages much larger than the Saturn V first stage would need to address this with some combination of being shorter and squatter, or compromising engine gimbal range, or mounting engines in pods surrounding the tankage, and there might be fairly hard engineering limits at some point for those reasons. At the 3g mark, for example, the 274 first-stage engines would require a stage about 90 meters in diameter and 9 meters tall, at which point the engineering inefficiencies associated with the fuel tank proportions will be becoming serious.   &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="21600" LastEditDate="2020-06-08T16:20:57.313" LastActivityDate="2020-06-08T16:20:57.313" CommentCount="18" ContentLicense="CC BY-SA 4.0" />
  <row Id="17577" PostTypeId="1" CreationDate="2016-07-30T22:01:07.163" Score="0" ViewCount="1307" Body="&lt;p&gt;I'd like a momento to travel in space, something small and personal. Are there companies that will arrange for transport to space (not near-space) and then return back to earth?  Or, maybe just a one-way trip?&lt;/p&gt;&#xA;" OwnerUserId="16469" LastEditorUserId="16469" LastEditDate="2016-07-30T22:26:33.163" LastActivityDate="2016-08-02T03:32:33.493" Title="How can I send a small object to space?" Tags="|transport|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17578" PostTypeId="2" ParentId="17577" CreationDate="2016-07-30T22:10:52.950" Score="6" Body="&lt;p&gt;The &quot;easiest&quot; way I can think of to accomplish this is to make friends with an astronaut scheduled to fly aboard the ISS. I believe they aren't allowed to fly souvenirs for profit, but I imagine you could get away with asking them to carry a small item for you in their personal gear in exchange for some other favor. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-07-30T23:17:15.833" LastActivityDate="2016-07-30T23:17:15.833" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17579" PostTypeId="1" AcceptedAnswerId="17580" CreationDate="2016-07-30T23:11:02.450" Score="21" ViewCount="6429" Body="&lt;p&gt;Today I have seen &lt;a href=&quot;http://www.theverge.com/2016/7/29/12327380/spacex-falcon-9-reusable-rocket-test-watch&quot; rel=&quot;noreferrer&quot;&gt;this report&lt;/a&gt; of a SpaceX Falcon 9 rocket test firing. The force of a burning rocket is tremendous, and in the video we see an apparently simple construction keeping the rocket (or rocket stage in this case) tied to the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does this construction manage to keep the rocket on the ground? What are its materials and design? Has this same (or similar) design always been used for such tests?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RjPcl.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RjPcl.jpg&quot; alt=&quot;Falcon 9 rocket firing test&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6750" LastEditorUserId="51" LastEditDate="2016-07-31T03:21:15.527" LastActivityDate="2020-04-21T20:49:15.473" Title="How does a simple construction keep a firing rocket on the ground?" Tags="|spacex|rockets|falcon-9|testing|first-stage|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17580" PostTypeId="2" ParentId="17579" CreationDate="2016-07-30T23:17:56.967" Score="21" Body="&lt;p&gt;I can answer this part &quot;Has this same (or similar) design always been used for such tests?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early days of the program Shuttle would occasionally do a pad test where the main engines were fired briefly.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Q_AUgo-RDuU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During this test (and at all other times) the Shuttle/External Tank/Solid Rocket Booster stack was held down to the Mobile Launcher by eight bolts. Four of these bolts connected the aft skirt of each Solid Rocket Booster to the Mobile Launcher.  &lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/reference/shutref/srb/posts.html&quot; rel=&quot;noreferrer&quot;&gt;The bolts were 28 inches long and 3.5 inches in diameter.&lt;/a&gt; You can see the hold-down post which was the interface between the bolts and the Launcher in the picture below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s17J0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s17J0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A schematic of the holddown post and bolt is shown here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zYnnl.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zYnnl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a real launch, the nuts were blown in half by pyrotechnic devices (2 per nut, for redundancy).  Here is a picture of the nuts after launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oqSoj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oqSoj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not sure you can accurately characterize this as &quot;simple&quot; but it was generally reliable.  I am aware of a couple of problems that occurred throughout the years, the amusingly named &quot;&lt;a href=&quot;http://www.nasa.gov/offices/nesc/home/Feature_1.html&quot; rel=&quot;noreferrer&quot;&gt;stud hang ups&lt;/a&gt;&quot; and an incident on STS-112 where &lt;a href=&quot;http://klabs.org/DEI/References/pyros/sts112_pyros.pdf&quot; rel=&quot;noreferrer&quot;&gt;one set of redundant pyros didn't fire&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-07-31T01:08:43.493" LastActivityDate="2016-07-31T01:08:43.493" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17581" PostTypeId="2" ParentId="17579" CreationDate="2016-07-31T03:20:53.417" Score="6" Body="&lt;p&gt;The launch mount used for actual SpaceX launches is capable of holding the full power of the stack for at least 1 second before every launch, and for the hot fire tests that are often several seconds long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SpaceX ones are in some ways simpler (no explosives) but in some ways more complex (they release on demand) than the Space Shuttle ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The stack in the image above is the new test pad at McGregor.  They built it, we think to test a Falcon Heavy at full thrust, which is almost 5 million lbs of thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They used a LOT of concrete, dug down deep into the ground, with lots and lots of dirt/weight on top of it, so that the rocket does not launch with the launch mount attached. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-07-31T03:20:53.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17583" PostTypeId="2" ParentId="17577" CreationDate="2016-07-31T09:10:41.103" Score="0" Body="&lt;p&gt;I personally prefer methods other than sending it with an astronaut friend or a satellite launcher. It feels so great by designing and launching an amateur rocket which is not so hard these days. you can buy commercial off the shelf solid rocket boosters and design an optimal staging sequence and achieve a brief suborbital flight with miniature payloads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd also propose using balloon assisted launches, but it needs more complex telemetry and recovery systems. But still possible as a personal project. This is also one of my ambitions for near future.&lt;/p&gt;&#xA;" OwnerUserId="16381" LastActivityDate="2016-07-31T09:10:41.103" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17584" PostTypeId="1" CreationDate="2016-07-31T09:23:15.003" Score="1" ViewCount="1863" Body="&lt;p&gt;Say a smart person downloads the rocket plans and sets up a center of excellence. Said person gets a cheap commercial laser cutter, welder, tanks, alloy sheets,  wood and steel rods, a used centrifugal pump, cheap fuel, pipes, etc. Why have I not heard of some smart person building a 21st century V2? What makes it so hard?&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="9060" LastEditDate="2016-08-01T17:08:23.993" LastActivityDate="2016-08-01T17:08:23.993" Title="What would it cost to build a V2 rocket today?" Tags="|rockets|" AnswerCount="1" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="17585" PostTypeId="2" ParentId="17579" CreationDate="2016-07-31T11:10:11.283" Score="10" Body="&lt;p&gt;Lets assume 5*10^6 lbs of thrust:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then 4 steel wires of approximately 60mm diameter should be enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The special thing about a rocket is not its maximum force at  zero speed but the fact that the force is generated in a way which is independent from the velocity difference to a medium in which the vehicle flies, and stays constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source for the strength of the steel wire:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://drahtseiltechnik.cgahrens.de/edelstahlseile/bruchlastrechner&quot; rel=&quot;nofollow noreferrer&quot;&gt;DIN3069 wire&lt;/a&gt; (DIN: Deutsche Industrie Norm, usually equivalent to some ISO norm)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.wirerope.com/docs/ctg-industrial.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Breaking load table&lt;/a&gt; (starting from Page 21/22)&lt;/p&gt;&#xA;" OwnerUserId="16471" LastEditorUserId="-1" LastEditDate="2020-04-21T20:49:15.473" LastActivityDate="2020-04-21T20:49:15.473" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="17588" PostTypeId="2" ParentId="13705" CreationDate="2016-07-31T15:00:39.160" Score="7" Body="&lt;p&gt;A 60 mph wind on Mars will produce about the same force as a 6 mph wind on Earth, since the density is ~1/100th, and the force goes as velocity squared. So it's still something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We certainly have taken into account wind for Mars landings, where it is especially significant while the vehicle is descending on the parachute. On Mars Exploration Rover, landing failures could be induced by increased horizontal velocity on impact due to steady winds and swinging of the vehicle due to wind shears, the latter resulting in angle on the terminal descent rockets, themselves increasing the horizontal velocity on impact. Both could contribute to exceeding the horizontal velocity capability of the airbags.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a result on MER, considerable effort went into &lt;a href=&quot;http://www.boulder.swri.edu/~rafkin/kass_et_al_2003_JGR.pdf&quot;&gt;modeling the wind at the candidate and final landing sites&lt;/a&gt;, affecting the landing site selection, and two systems were developed to mitigate the effect of wind, a transverse rocket system to control the angle of the retro rockets when fired&lt;sup&gt;1&lt;/sup&gt;, and a &lt;a href=&quot;https://www-robotics.jpl.nasa.gov/publications/Andrew_Johnson/DIMES-ai-space.pdf&quot;&gt;descent image motion estimation system&lt;/a&gt; to measure the horizontal velocity of the vehicle relative to the ground. Both systems turned out to be important for Spirit's successful landing. (The wind was calm, as predicted, for the Opportunity site.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mesoscale wind modeling was done for Mars Science Laboratory (Curiosity) as well, where there the requirement to land much more precisely than MER was impacted by wind while on the parachute, and the parachute deployment loads could also be adversely impacted by wind.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;San Martin, A. Miguel, and Erik S. Bailey. &quot;The Mars Exploration&#xA;  Rover Transverse Impulse Rocket System: An Active System to Compensate&#xA;  for Martian High Winds During Landing.&quot; Advances in the Astronautical&#xA;  Sciences 121 (2005): 519-539.&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-07-31T15:57:14.920" LastActivityDate="2016-07-31T15:57:14.920" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17590" PostTypeId="1" AcceptedAnswerId="17596" CreationDate="2016-07-31T17:46:12.323" Score="4" ViewCount="426" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Salyut_3&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; indicates that the Salyut 3 was launched with a gun, which could have been used in self-defense or against other satellites, and which was actually tested after the last crew had departed in the Soyuz 15.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this the only example of a gun being deployed onboard a spacecraft? Elsewhere, &lt;a href=&quot;https://en.wikipedia.org/wiki/OPS-4&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; indicates that the unflown OPS-4 station would have been equipped with a different space-to-space system, which would have fired missiles - but I have not come across any other such examples.&lt;/p&gt;&#xA;" OwnerUserId="16160" LastActivityDate="2016-08-01T11:35:48.847" Title="Was the Salyut 3 (OPS-/Almaz-2) the only spacecraft to feature a space-to-space gun?" Tags="|space-station|salyut|almaz|weapon|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17591" PostTypeId="1" CreationDate="2016-07-31T17:54:23.350" Score="2" ViewCount="158" Body="&lt;p&gt;I'm trying to compile a list of current space contracts held by NASA, the CSA, and any other space-related agency so that I can get a feel for what the most heavily researched areas are. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are some resources that might list contracts like these?&lt;/p&gt;&#xA;" OwnerUserId="16472" LastEditorUserId="4660" LastEditDate="2016-07-31T20:40:19.753" LastActivityDate="2016-07-31T22:52:44.603" Title="Where can I find data on current space agency contracts?" Tags="|nasa|reference-request|research|esa|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17592" PostTypeId="2" ParentId="17591" CreationDate="2016-07-31T21:49:04.283" Score="2" Body="&lt;p&gt;As a starting point, for the part of the question aimed at &quot;any other space-related agency&quot; this link takes you to the European Space Agencies tender announcement site:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://emits.sso.esa.int/emits/owa/emits.main&quot; rel=&quot;nofollow&quot;&gt;http://emits.sso.esa.int/emits/owa/emits.main&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It lists up an coming contracts and those that are currently open for competition. Its quite hard to find out what contracts have been placed with whom until some time after the event. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sticking with this site for the moment, you can see by picking any individual item in the Open Invitation To Tender list that it identifies amounts of money and which ESA department. Whilst this responds to the request for lists of contracts you can see that you are right down in the weeds and there is too much to digest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be more interested in a year-ahead or multi-year workplan. I'll add one when I next come accross one. &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-07-31T21:49:04.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17593" PostTypeId="2" ParentId="17591" CreationDate="2016-07-31T22:52:44.603" Score="4" Body="&lt;p&gt;This &lt;a href=&quot;https://prod.nais.nasa.gov/cgibin/npdv/npdv.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;website&lt;/a&gt; has access to &quot;all current NASA contracts.&quot;  The organization seems kinda goofy (it seems to be a way for politicians to find out how much NASA is spending in their district) but if you keep drilling down you can find the info.  for example, there is the 28 billion dollars going to Boeing.  But fear not, that is the total, this year is only 1.8 billion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S3at3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S3at3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-07-31T22:52:44.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17594" PostTypeId="1" AcceptedAnswerId="17696" CreationDate="2016-08-01T07:28:10.797" Score="4" ViewCount="404" Body="&lt;p&gt;The 34 day launch window for the &lt;a href=&quot;http://www.asteroidmission.org/objectives/mission-operations/&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-REx&lt;/a&gt; probe will open in about five weeks (early September 2016), and it's approach to the near earth asteroid &lt;a href=&quot;https://en.wikipedia.org/wiki/101955_Bennu&quot; rel=&quot;nofollow noreferrer&quot;&gt;101955 Bennu (provisional designation 1999 RQ36)&lt;/a&gt; is planned to start about two years later around August 2018. Currently the plan is to stay with the asteroid until the next return to earth opportunity. The plan is to start back in March 2021, and return to earth in September 2023.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the year 2135 Bennu may pass through a &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_keyhole&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational keyhole&lt;/a&gt;&quot; - a small point in position (and velocity) in this case between the Earth and the Moon. At that point, the future trajectory is very sensitive to the exact orbit through the gravitational fields of the Earth and Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One distinguishing feature of this mission is the planned collection of 60 to 2000 grams of material and return it to earth for study. One reason for this might be that Bennu currently has some possibility of eventually striking earth in another roughly 150 years, and a wide range of information about the object will be valuable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photographs and various other measurements will collect information about the surface. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/101955_Bennu&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, detailed study of the surface will be required to understand its interaction with sunlight. Details of the shape, reflectivity and rotational dynamics are necessary to predict the impact of solar photon pressure and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Yarkovsky_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yarkovsky effect&lt;/a&gt;, both of which need to be calculated using a realistic model of the asteroid rather than using that of a sphere with uniform surface properties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a nice &lt;a href=&quot;http://postdocs.jpl.nasa.gov/files/ura/Trajectory_analysis_of_the_potentially_hazardous_asteroid_Bennu.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;presentation&lt;/a&gt; and &lt;a href=&quot;http://echo.jpl.nasa.gov/asteroids/chesley_etal_2014_Bennu.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;paper&lt;/a&gt; on Bennu's orbit and study.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the types of measurements will be done? With which technologies will OSIRIS-REx scan the surface of Bennu? What exactly is hoped to be learned about the shape and properties of the surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2WZSd.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2WZSd.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; subset of frames from the very nice 4MB GIF in &lt;a href=&quot;http://www.asteroidmission.org/objectives/mission-operations/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA webpage&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;side note:&lt;/strong&gt; While the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pioneer anomaly&lt;/a&gt; (discussed also in &lt;a href=&quot;https://space.stackexchange.com/q/15075/12102&quot;&gt;this question&lt;/a&gt; and its answer) was proposed to be related to anisotropy in the radiation of thermal energy due to (mostly) internal heat sources, the excellent &lt;a href=&quot;https://arxiv.org/pdf/1204.2507v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;analysis here&lt;/a&gt; illustrates the important of detailed models of heating, thermal transport, and radiation from a complex body in order to more precisely predict radiative (and also reflective) photon recoil.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2021-02-02T21:39:07.143" Title="How will OSIRIS-REx scan and characterise the near-earth asteroid Bennu?" Tags="|measurement|scientific-data|near-earth-object|near-earth-asteroid|osiris-rex|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17596" PostTypeId="2" ParentId="17590" CreationDate="2016-08-01T10:50:45.970" Score="5" Body="&lt;p&gt;Salyut 3 is the only publicly acknowledged space to space weapon that has been orbited. It's possible that there are others which have been deployed in secret. The outer space treaty does not prohibit the deployment of conventional weapons in space, there doesn't seem to be any hurry to do it though, which is a good thing. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-08-01T10:50:45.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17597" PostTypeId="2" ParentId="17590" CreationDate="2016-08-01T11:35:48.847" Score="3" Body="&lt;p&gt;Space to Space weapons seemed like a good idea, but really isn't that great for orbiting spacecraft. First of all, what is the use case? It would only really work for spacecraft in a similar orbit. Let's imagine that a spacecraft is coming towards you. Firing weapons to destroy the spacecraft might lead to an impact in the firing spacecraft, destroying it. It sounds cool, and as much of the early space program lead to the desire to have such weapons, it has since been discarded as a useful idea. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space.com has a list of the &lt;a href=&quot;http://www.space.com/19-top-10-space-weapons.html&quot; rel=&quot;nofollow&quot;&gt;top 10 unclassified space weapons&lt;/a&gt;. Of these, the only non-theoretical weapon was that possessed by Salyut, or anti-satellite technology launched by ground based facilities. So for now, weapons in space have not been launched, beyond the Salyut mission.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-08-01T11:35:48.847" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17598" PostTypeId="1" AcceptedAnswerId="17599" CreationDate="2016-08-01T12:48:06.610" Score="11" ViewCount="6876" Body="&lt;p&gt;During the course of a day, the geosynchronous satellite's position in the sky forms a 8-like figure.&#xA;I'm having trouble visualizing the movement of geosynchronous satellite when viewed from the surface of the earth. How does the inclination angle vary with respect to time. Why does the oscillation gets smaller as the inclination angle decrease?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P9nN7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P9nN7.jpg&quot; alt=&quot;Seen in the image below is an afternoon analemma photo taken in 1998–99 by Jack Fishburn in Murray Hill, New Jersey, USA.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16479" LastEditorUserId="16479" LastEditDate="2016-08-01T13:31:36.413" LastActivityDate="2016-08-03T15:17:57.323" Title="Why do geosynchronous satellites appear to oscillate in North-Southward direction? How to I trace out their path on the surface of the earth?" Tags="|orbital-mechanics|communication-satellite|geosynchronous|inclination-change|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17599" PostTypeId="2" ParentId="17598" CreationDate="2016-08-01T13:31:37.460" Score="8" Body="&lt;p&gt;Let's first look at a typical orbit first for comparison. Note that the object being orbited could be anywhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1yrZ6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1yrZ6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that it spends a time both above and below the equator. A few things about a Geosynchronous orbit:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The inclination varies with time, although this isn't that important for day to day operations.&lt;/li&gt;&#xA;&lt;li&gt;The period is 24 hours, but a geostationary satellite either isn't perfectly circular, or 0 inclined.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The orbit you have shown is an elliptical inclined &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;geosynchronous orbit&lt;/a&gt;. The shape is actually called an &lt;a href=&quot;https://en.wikipedia.org/wiki/Analemma&quot; rel=&quot;nofollow noreferrer&quot;&gt;Analemma&lt;/a&gt;. In fact, the image you provided is of the Sun doing something similar to what these spacecraft will do on a daily basis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The inclination is responsible for the up and down motion. The sideways part is due to the eccentricity primarily, although an inclined orbit will show a slight variation due to the changing relative velocity compared to the Earth's surface the higher it goes. The spacecraft moves faster when closer to Earth, and thus will be somewhat ahead of the Earth. When it is further, it moves slower, moving back in the other direction.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-08-02T13:07:07.733" LastActivityDate="2016-08-02T13:07:07.733" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="17601" PostTypeId="2" ParentId="17551" CreationDate="2016-08-01T15:39:33.707" Score="5" Body="&lt;p&gt;This answer assumes that the question does not refer to &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_replacement_unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Replacement Units&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The definitive reference needed to answer this question for the US side of the ISS is the International Space Station In-Flight Maintenance Book aka the IFM Checklist.  Unfortunately the &lt;a href=&quot;http://www.spaceref.com/iss/ops/ifm.mrg.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;only one I can find online&lt;/a&gt; is quite old, December 2000.  Johnson Space Center reorganizations have even changed the organization which owns this document.  It is now the Operations Support Officer group, not the Maintenance Repair Group.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Reference section of this document contains tool and hardware lists, too lengthy to reproduce here (especially for an outdated document).  Here's the relevant part of the table of contents, and some representative pages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Nhjlp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Nhjlp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;tool kit list&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ovUgh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ovUgh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spare/diagnostic cabling&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TR4ki.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TR4ki.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bulk of the document contains procedures for performing maintenance and repair.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more up-to-date tool list can be found in the links used in the answer to this &lt;a href=&quot;https://space.stackexchange.com/questions/12391/iss-nuts-and-bolts-metric-sae-or-both/12394&quot;&gt;question&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A link to an updated version of the IFM Checklist would be most welcome.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-08-01T15:39:33.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17602" PostTypeId="2" ParentId="17584" CreationDate="2016-08-01T16:41:00.777" Score="3" Body="&lt;p&gt;Since we're not prepared to build V-2s today, it is probably worthwhile to treat this as if we were making V-2s from scratch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The American &lt;a href=&quot;http://www.astronautix.com/j/jupiterc.html&quot; rel=&quot;nofollow&quot;&gt;Jupiter-C&lt;/a&gt;, which was derived from the V-2, cost &lt;a href=&quot;http://www.astronautix.com/j/jupiterc.html&quot; rel=&quot;nofollow&quot;&gt;$92.500 million in 1959 dollars to develop&lt;/a&gt;. &lt;a href=&quot;https://www.measuringworth.com/uscompare/relativevalue.php&quot; rel=&quot;nofollow&quot;&gt;In today's dollars&lt;/a&gt; that is about \$752.00 million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could improvements be made today to bring down this cost? Possibly, but I think a point-by-point analysis of cost savings for the entire development of the rocket is too broad for an answer here.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;It's worthwhile to note that &lt;a href=&quot;https://en.wikipedia.org/wiki/Copenhagen_Suborbitals&quot; rel=&quot;nofollow&quot;&gt;Copenhagen Suborbitals&lt;/a&gt; (whose members work for free) estimated in 2013 that they had raised under half a million dollars. They have yet to achieve suborbital spaceflight, and they have had only one launch since 2013.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-08-01T16:55:25.273" LastActivityDate="2016-08-01T16:55:25.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17603" PostTypeId="1" CreationDate="2016-08-01T16:47:07.513" Score="2" ViewCount="481" Body="&lt;p&gt;Be it weather, crops, global warming, atmosphere, ocean, roads, birds, forest, intelligence, safety, connectivity, land/space transmission, why does the UN not have its own satellites? It could provide immense benefit to 200+ poor UN member nations and spare the rest of humanity of great expense or American and Russian propaganda.&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="8162" LastEditDate="2016-08-15T19:34:21.533" LastActivityDate="2016-09-30T09:46:06.793" Title="Why does the UN not have its own satellites?" Tags="|communication|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17605" PostTypeId="2" ParentId="17598" CreationDate="2016-08-01T17:19:52.570" Score="5" Body="&lt;p&gt;I suggest building a 3D model to understand it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Start with a nice disk, such as a paper plate or the cardboard packaging of a frozen pizza.  Mark the center and draw regularly spaced radial lines (“slices”) across the face of the plate.  Number the lines at the rim and duplicate the labels farther inward too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cut a ring off the plate, about 1 inch wide.  If you can, leave a tiny bit uncut at opposide sides so the ring is still attached at 2 points and can pivot at that location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tip the ring 60 degrees or so from the plate.  The &lt;em&gt;difference&lt;/em&gt; in drift is larger if the inclination is high, but lining up and sighting might be easier if not so high.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cut tiny notches in the ring and remaining plate, and then a fine string can hold the contraption in place so it doesn’t flop around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, for each marked line on the plate, sight where it lines up visually on the tilted ring now.  You need a right angle: get a small scrap of wood or carboard box and tape a large cardboard triangle to one face.  Now you can line up the bottom of the triangle with a line on the main plate and know it’s standing normal to the plate’s surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now see where the triangle touches the detached ring.  Each mark on the flat plate represents your sightline as the Earth turns. The corresponding mark on the arch is where the satellite is at that time.  So look at the &lt;em&gt;height&lt;/em&gt; to see that the sattelite appears to rise in the sky, and look at how its position will be to the left or right of your sightline: this is the side to side motion you see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where the arch meets the flat plate, you can approximate it in 2 dimensions as a horizontal ruler and a diagonal ruler.  The same length along the diagonal ruler will not not reach as far horizontally (East) so the rising sattelite falls behind the turning Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the top of the arch, the highest point in the sky, the sattelite is directly overhead again. The setting part is a mirror image. So, you see it rise and drift left at first, then back to the right until it reaches the highest point, keeps going right as it sets, then back left again to set at the original position. Below the horizon it does the same thing upside down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If viewing at other than the equator (lift the plate up within the arch model) then the two lobes will be asymmetrical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://en.wikipedia.org/wiki/Analemma#Analemmas_of_geosynchronous_satellites&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt; for &lt;em&gt;Analemma of Geosynchronous Satellites&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;&lt;a href=&quot;https://i.stack.imgur.com/jqxJu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jqxJu.jpg&quot; alt=&quot;image&quot;&gt;&lt;/a&gt;&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The figure 8 you show (taken from &lt;a href=&quot;https://en.wikipedia.org/wiki/Analemma&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and explained on that page) is related but different.  The position of the sun at the same civil time each day will move because the axis is tilted and the eccentric orbit moves faster when closer to the sun and slower when farthest away, though the Earth turns at a constant rate. So you get arcs that move north-south depending on the time of year &lt;em&gt;and&lt;/em&gt; the sun is earlier or later along the arc depending on the time of year.&lt;/p&gt;&#xA;" OwnerUserId="6471" LastEditorUserId="6471" LastEditDate="2016-08-03T15:17:57.323" LastActivityDate="2016-08-03T15:17:57.323" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17606" PostTypeId="1" CreationDate="2016-08-01T18:06:32.213" Score="11" ViewCount="1649" Body="&lt;p&gt;Building off of &lt;a href=&quot;https://space.stackexchange.com/questions/5943/who-what-is-the-international-authority-to-assign-refuse-orbit-to-a-satellite-sp/&quot;&gt;this previous question&lt;/a&gt;, is there a more detailed summary anywhere of the process that a commercial satellite would have to go through to obtain a GEO slot?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The documentation I have found has only clarified that the International Telecommunications Union allocates orbital slots to countries.  That still leaves a lot of room for interpretation on the national level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could work a few different ways:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;National government requests from ITU on an &quot;as needed basis&quot;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Company requests GEO slot from national government.&lt;/li&gt;&#xA;&lt;li&gt;National government requests slot from ITU&lt;/li&gt;&#xA;&lt;li&gt;ITU grants slot to national government&lt;/li&gt;&#xA;&lt;li&gt;National government grants slot to company&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;National government maintains a bank of GEO slots to draw from&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;National government requests multiple GEO slots from ITU for general use over some period of time&lt;/li&gt;&#xA;&lt;li&gt;Company requests GEO slot from national government&lt;/li&gt;&#xA;&lt;li&gt;National government grants slot to company&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Or even:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Company requests GEO slot with permission of national government&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Company gets permission or approval from national government to request a slot&lt;/li&gt;&#xA;&lt;li&gt;Company requests GEO slot directly from ITU&lt;/li&gt;&#xA;&lt;li&gt;ITU grants slot to national government with the understanding that it is for company&lt;/li&gt;&#xA;&lt;li&gt;National government grants slot to company&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Are any of these right?  Or is there another process that commercial satellites need to go through to get a GEO slot from the ITU?&lt;/p&gt;&#xA;" OwnerUserId="15841" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2021-03-02T02:32:37.280" Title="What is the process for allocating GEO slots to commercial satellites?" Tags="|artificial-satellite|geostationary|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17607" PostTypeId="2" ParentId="17291" CreationDate="2016-08-01T19:53:13.253" Score="6" Body="&lt;p&gt;&lt;strong&gt;If Juno discovers a solid core on Jupiter, will that allow us to pick one of the two models, since both could evolve a solid core?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A solid core itself won't be a smoking gun because as you said, both models predict that a solid core is possible.  If Jupiter does &lt;em&gt;not&lt;/em&gt; have a solid core however, the disk instability model begins to look a bit better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something else interesting here is the size of the core.  Most core accretion models for gas giants predict very large cores (on the order of 10 times the mass of Earth).  They also tend to be very dense because they form as a solid body that eventually becomes the core of a much larger planet.  Disk instability models do allow for a solid core, but the cores tend to be smaller and less dense because they don't develop around an already solid body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the data from Juno won't tell us for certain which model is more accurate, but the lack of a solid core, or a relatively small and less dense core will definitely make the disk instability model look better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If we discover that Jupiter does or does not have a solid core, what does that mean for the development of Earth and life?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have to say that this is more of a big picture question that I'm not really qualified to answer, especially when it comes to the connection to life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to keep it very short, determining which of these models is more accurate will allow us to get a better idea of how long it took Jupiter to form.  If core accretion is more accurate, Jupiter likely took millions of years to form, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Protoplanetary_disk&quot;&gt;protoplanetary disk&lt;/a&gt; was around for a long time compared to our estimates of how long they usually last.  Earth was also developing within this disk, so anything we can learn about the solar system while Jupiter was forming can likely be applied to the environment of Earth as it was forming.&lt;/p&gt;&#xA;" OwnerUserId="15841" LastActivityDate="2016-08-01T19:53:13.253" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17608" PostTypeId="2" ParentId="17579" CreationDate="2016-08-01T21:04:58.230" Score="8" Body="&lt;p&gt;The orange top and strong cables seem to be a bit atypical. There is a &lt;a href=&quot;https://i.imgur.com/KO9KEhz.gifv&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt; of F9-019 (Jason-3) on the same stand. &lt;a href=&quot;https://i.stack.imgur.com/JzWUJ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JzWUJ.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&#xA;which shows thinner cables and no special cap. But it is a v1.1 of F9 so some differences may be because of that too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most important part of the hold-down mechanism are the &lt;a href=&quot;https://i.stack.imgur.com/Wms7B.jpg&quot; rel=&quot;noreferrer&quot;&gt;clamps&lt;/a&gt; which keep the rocket fixed to the transporter/erector or a test stand. &lt;a href=&quot;https://i.stack.imgur.com/Vlmmh.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vlmmh.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&#xA;These attach directly to the &lt;em&gt;octoweb&lt;/em&gt; structure just above the engine bells. There is a shot of the release in SpaceX webcast intro (you can see it for example &lt;a href=&quot;https://www.youtube.com/watch?v=wPYOtCFSLKw&amp;amp;t=0m7s&quot; rel=&quot;noreferrer&quot;&gt;at 7s&lt;/a&gt; into &lt;a href=&quot;https://www.youtube.com/watch?v=wPYOtCFSLKw&quot; rel=&quot;noreferrer&quot;&gt;Thaicom 8 webcast&lt;/a&gt; from which I took this image)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/d1gjg.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d1gjg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems to be the old F9 v1.0 if I am not mistaken. Later in the intro there is another detail of the release &lt;a href=&quot;https://www.youtube.com/watch?v=wPYOtCFSLKw&amp;amp;t=0m14s&quot; rel=&quot;noreferrer&quot;&gt;at 14s&lt;/a&gt;. Then in the same webcast there is a closeup on the current version at &lt;a href=&quot;https://www.youtube.com/watch?v=wPYOtCFSLKw&amp;amp;t=21m20s&quot; rel=&quot;noreferrer&quot;&gt;21:20&lt;/a&gt; but unfortunatelly without the actual release.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5fMLE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5fMLE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About the orange cap and thick wires - the speculations are that it is used to simulate the weight and some stressed (from vibrations etc.) of the second stage to check the structural integrity of the returned stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(Used images belong to SpaceX)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2016-08-01T21:04:58.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17609" PostTypeId="1" CreationDate="2016-08-01T21:22:31.020" Score="5" ViewCount="1597" Body="&lt;p&gt;A recent Government Accountability Office (GAO) &lt;a href=&quot;http://www.gao.gov/assets/680/678704.pdf&quot;&gt;report&lt;/a&gt; has cast doubts on the ability of the Orion MPCV to meet its requirements on its already excessive budget. Orion has been in development since 2006 and will likely continue until 2023 with costs in excess of \$20 B before crew fly on it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I understand the commercial crew contracts SpaceX is to receive up to \$2.6 B  and Boeing is to receive up to \$4.2 B for design development and testing of the Dragon 2 and Starliner.  So what I am wondering is what modifications would be needed to be made to these spacecraft to let them fly beyond LEO say to cis-lunar space.  Necessary modifications I can think of include: life support systems to allow longer missions, radiation protection for crew and radiation hardened electronics, larger heat shield to allow for much faster reentry, and more robust structures for greater g-loads upon entry.  Would it be possible to modify the current designs to include these requirements or would these changes require the the development of an entirely new spacecraft which would be about as expensive as the Orion?&lt;/p&gt;&#xA;" OwnerUserId="16180" LastActivityDate="2017-12-19T19:21:08.247" Title="Could SpaceX's Dragon V2 or the Boeing Starliner be upgraded to travel beyond LEO" Tags="|spacex|dragon-v2|orion-spacecraft|cst-100|boeing|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17610" PostTypeId="1" CreationDate="2016-08-01T23:10:06.447" Score="12" ViewCount="1045" Body="&lt;p&gt;Watching Boston Dynamics progress with their walking robots makes it seem like maybe a version adapted to function off-world could compete with wheeled rovers now:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/tf7IEVTDjng?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This looks like a machine that could handle a lot of terrain a wheeled rover can't, and could be more effective with tools as it is capable of bracing itself, and navigating fluidly around something it is investigating or working on. It doesn't seem like such a big step to make legs that can function also as arms at least in a limited way, either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, maybe the mechanisms involved are too delicate. It had occurred to me that maybe joints that aren't rotating like wheels are easier to shield from dust, and have an advantage because the motors are above the dust, and the feet don't kick up dust like a wheel can. But I could be off-base on that. Maybe their power requirements are a big disadvantage, though I haven't found data on how much power they draw.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has it now become feasible to adapt this technology for a walking robot on another world? (I am aware it hasn't been developed and tested for that application, but I'm referring to taking what there is and adapting it.) Does the format have the advantages it seems to have, or does the complexity and fragility of the technology still outweigh possible advantages? I am particularly interested in how they might perform on the Moon, with its nasty dust, and the traction issues that come from low gravity. As my focus is looking at what extensive permanent infrastructure might be like, power usage and supply in that light is my interest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this video, another version handles some more varied and challenging terrain:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/xqMVg5ixhd0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-08-05T17:20:38.490" LastActivityDate="2021-05-24T20:03:49.793" Title="Does new technology make it advantageous to have walking rovers?" Tags="|design|rovers|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17611" PostTypeId="1" AcceptedAnswerId="17612" CreationDate="2016-08-01T23:32:44.073" Score="9" ViewCount="414" Body="&lt;p&gt;In regards to Europa, what are the limitations regarding current data transmission technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say we were to have an orbiter circling Jupiter, a lander on the surface of Europa and a &quot;swimmer&quot; [submarine] that somehow got under the ice, and liquid water existed under the ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using current technology, how could the &quot;swimmer&quot; talk to earth? (Aka &quot;phone home&quot;)&#xA;Would a wire be needed to penetrate the 2-8km of ice?  (Can radar penetrate that far?)&#xA;How far could the &quot;swimmer&quot; communication penetrate?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;From the bottom of the Europa ocean, to the surface?&lt;/li&gt;&#xA;&lt;li&gt;Through the ice, and to an orbiter?  &lt;/li&gt;&#xA;&lt;li&gt;All the way to the deep space network?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I am specifically curious about the limits of current data transmission capabilities of an object buried under 2-8km of ice.  (Is the thickness of the ice relevant?). Data transmission rate isn't necessarily relevant, just distance through various mediums such as ice, water, and open space.&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2016-08-02T18:02:54.693" Title="Do you need a &quot;wire&quot; on a Europa submarine to phone home?" Tags="|data-transmission|radio-communication|europa|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17612" PostTypeId="2" ParentId="17611" CreationDate="2016-08-02T01:48:06.360" Score="9" Body="&lt;p&gt;A wire would be needed to get any real amount of data out. Wikipedia has the &lt;a href=&quot;https://en.wikipedia.org/wiki/Communication_with_submarines&quot; rel=&quot;nofollow noreferrer&quot;&gt;state of the art communication with submarines&lt;/a&gt;. There are basically 2 ways to communicate with them. Using sound waves could work for short period distance, and that might be possible to use from a surface component to the underwater submarine. Very low frequency (3-30 kHz) can communicate a few meters into water, while ELF can communicate using 3-300 Hz waves. Neither can transmit significant data, nor can one be built underwater. They also can't be focused very well, so they would at best communicate to the surface, from there to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://electronics.stackexchange.com/a/106418/2533&quot;&gt;Ice is somewhat better at transmitting RF&lt;/a&gt; than water, but it too, has difficulties. Radar is successful at penetrating large depths of ice, unlike water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only practical way to get data from under the water is to have a lander relay communications between Earth and the submarine, possibly via an orbiter, using either sound waves, or a wire.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:33:08.970" LastActivityDate="2016-08-02T18:02:54.693" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17614" PostTypeId="2" ParentId="17577" CreationDate="2016-08-02T03:32:33.493" Score="0" Body="&lt;p&gt;There are companies that sell the excess capacity on boosters.  It's limited to small satellites and you have to choose from whatever sizes &amp;amp; orbits they have available and you accept that your payload only flies if the real one does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's still very expensive, though.  Obviously these are one-way trips unless you include a deorbit and reentry system--and that would be expensive indeed.  Assuming the object to be orbited contains nothing that could possibly threaten a spacecraft I would expect you could find one that would take it.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-08-02T03:32:33.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17615" PostTypeId="1" CreationDate="2016-08-02T07:00:20.400" Score="18" ViewCount="1954" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HO3hL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HO3hL.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;For me it looks like water droplets, but I doubt that's the case in space.&#xA;Also, I don't understand why this should be a craft's surface feature. Wouldn't one aim to design a spacecraft to be as smooth as possible?&#xA;And even if this is intentionally designed part of the surface, what is this irregularly placed rough stuff intended for?&lt;/p&gt;&#xA;&lt;p&gt;If it is some environmental phenomenon, what is it and how did it get there?&lt;/p&gt;&#xA;&lt;p&gt;Here is a contrast-enhanced detail of the Service Module (click on the photo to see the full image):&lt;br /&gt;&#xA;&lt;a href=&quot;https://www.hq.nasa.gov/alsj/a17/AS17-145-22252HR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CS5VB.png&quot; alt=&quot;Apparent bubbling on parts of the Service Module's aluminium skin&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11099" LastEditorUserId="47149" LastEditDate="2022-06-01T00:16:14.270" LastActivityDate="2022-06-01T00:16:14.270" Title="What is this on the craft from the Apollo 17 mission?" Tags="|apollo-program|spacecraft|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="17619" PostTypeId="2" ParentId="17609" CreationDate="2016-08-02T11:53:59.190" Score="1" Body="&lt;p&gt;The ISS crew cargo craft do not have real facilities for long term stays.  There is no washroom for example. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course any serious beyond-LEO mission in a capsule will likely include a hab module of some kind which is not something CST-100 or Dragon is excluded from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gwynne Shotwell in discussing Red Dragon mentioned that deep space communication systems was one of their big issues. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX claims their heat shield is over designed for LEO reentry with sufficient margin for higher energy return missions. How high, I am sure they will experiment to find out on some mission. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-02T11:53:59.190" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17620" PostTypeId="2" ParentId="17611" CreationDate="2016-08-02T13:20:35.977" Score="6" Body="&lt;p&gt;You would almost certainly need a wire. You have 2 things to contend with on Europa (besides the crippling radiation, extreme cold, vacuum, etc) and that is Ice and Water. Water absorbs radio waves, only extremely low frequency, very powerful radio waves can penetrate much water and these signals are terrible at carrying information as the rate at which data is carried is very low, like one character ever few seconds. ELF is used to communicate with submarines, but it's one way as the transmitter is huge, and the receiving antenna is very long - over a kilometer if I remember. Not practical for a space probe to carry, and there's no way we could send a transmitter to Europa as it's enormous. The probe would only be able to receive, and not send data using an ELF system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sonar is also a very bad carrier for information, it goes pretty far but there's too much attenuation and distortion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The surface ice is a real problem as well, as it also absorbs radio and it will prevent a probe underneath from being able to expose an antenna. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To understand how we would do this on Europa you don't have to look any farther than how we do it on earth. While there are autonomous probes the vast majority of unmanned undersea exploration is done using tethered probes as it's the only way to supply the probe with power, and the only way to get data from the probe in quantity while it is under the surface. Even the autonomous probes have to surface to send information. Navies across the world have spent a great deal of money to find ways to transmit data through water so that submarines can communicate while submerged, but they still have to send data through an antenna exposed to the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So any probe we send will require a wire of some sort to send data back. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="58" LastEditDate="2016-08-02T13:22:47.647" LastActivityDate="2016-08-02T13:22:47.647" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17623" PostTypeId="1" AcceptedAnswerId="17734" CreationDate="2016-08-02T16:59:51.783" Score="7" ViewCount="972" Body="&lt;p&gt;You are on a ship in the cold vacuum of space. You find a body of an astronaut exposed to vacuum (for this instance just assume human, but you never know). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The body was essentially freeze dried and all microbes and moisture 'removed', as &lt;a href=&quot;https://space.stackexchange.com/questions/4864/could-the-void-of-space-act-as-a-good-refrigerator-for-food-in-long-journeys?rq=1&quot;&gt;this question&lt;/a&gt; pointed out. The decay of a body in space has been answered in &lt;a href=&quot;https://space.stackexchange.com/questions/308/decay-of-a-corpse-in-space?rq=1&quot;&gt;this question&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this scenario, let us assume that it hasn't been exposed to the sun's radiation (or at least hasn't been exposed to multiple freezing/thawing cycles) and is still fairly intact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This body is brought on board your spacecraft. What happens to it next?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will the body start to decay once it is 'thawed' out, or will it remain 'perfectly' preserved. I'm curious if all the micro-organisms necessary for decomposition would remain completely dead; no hidden &lt;a href=&quot;http://www.iflscience.com/plants-and-animals/tardigrades-turn-glass-when-dried-out/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tardigrade&lt;/a&gt;-type critters? Would contamination from live humans/animal test subjects provide a source for decomposition to start in the preserved body?&lt;/p&gt;&#xA;" OwnerUserId="16480" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-08-13T11:48:13.427" Title="What happens to a body, that has been exposed to vacuum, once brought back onboard?" Tags="|astronauts|life|medical|vacuum|death|" AnswerCount="2" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17625" PostTypeId="1" CreationDate="2016-08-02T19:41:23.273" Score="9" ViewCount="697" Body="&lt;p&gt;Project Longshot &lt;a href=&quot;http://large.stanford.edu/courses/2012/ph241/klein2/docs/19890007533_1989007533.pdf&quot;&gt;report&lt;/a&gt; (1988) says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A laser with an input power of 250 kilowatts would allow for a&#xA;  data rate of 1000 bits per second at maximum range.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yet Breakthrough Starshot website (2016) &lt;a href=&quot;https://breakthroughinitiatives.org/index.php?controller=Forum&amp;amp;action=viewforum&amp;amp;id=13&amp;amp;page=3&quot;&gt;says&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Images of the target planet could be transmitted by a 1Watt laser onboard the nanocraft&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why do these estimates differ by 5 orders of magnitude? Is one (or two) of them incorrect, or did the technology change so much in 30 years? What are the physical principles behind this?&lt;/p&gt;&#xA;" OwnerUserId="15943" LastActivityDate="2016-08-03T21:50:24.570" Title="How much power would an Alpha Centauri probe require for communication?" Tags="|interstellar-travel|power|radio-communication|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17626" PostTypeId="2" ParentId="17610" CreationDate="2016-08-02T21:09:22.343" Score="4" Body="&lt;p&gt;One possible reason may be the &lt;a href=&quot;https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Technology Readiness Level&lt;/a&gt; of these walking machines, considering both the mechanic and the supporting electronic: wheels solution can be considered &quot;good enough&quot; for the current mission's objectives, and have the best reliability for operating in that environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[EDIT]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks to the useful comments, I realized I gave a very poor supported answer, so I will draw the economical model behind my one-line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qATpc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qATpc.png&quot; alt=&quot;status&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the graph you can read on the y-axis a variable, &quot;Level Of Technology required for the mission scope (LOT)&quot;, that can be thought as the sum of, as example, reliability for the duration of the mission, sturdiness necessary for the explorations planned, energy consumption within mission constraints, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the y-axis we place the year.&#xA;Different lines are different technologies: I plotted wheel-based rovers, leg-movement based robots and, to project further the analysis, an hoverboard-like drone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For every planet exploration mission, the LOT is almost constant: there can be minimal variation in case we want the rover travel on flat terrain or climb a mountain (Mars) or navigate over rather than under ice (Europa), but the main parameters are terrain composition at the rover-size level (density of the soil, operational temperature, and so on). For my model, I consider them constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Model is not in scale, since variable are just a raw approximation of reality, but I consider Mars more challenging that the moon for some known characteristics already explored by rovers and satellites, and Mercury and Europa even more challenging just because of the mission constraint to actually go there (distance) and geology (ice surfaces, extreme temperature ranges).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interception between the wheel-based exploration and Moon LOT represent Apollo missions, while interception with Mars LOT represent martian rovers. If we project this line in the future, we may have advances in technology that will allow wheels also on Mercury and on Europa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other line, leg-based robot are on an unknown trajectory: we can consider them adequate for Earth exploration but not yet for other planets mission. Same for hoverboards: we have even more unknowns. But depending of the speed of advances of these technologies, we may have one or the other reach an intersection point with the wheel-based approach, and in that case there won't be any rational cause to not consider them the &quot;best&quot; option available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As extreme, the hoverboard line is vertical, as we may think that hoverboard will travel on every condition and terrain, therefore they will be the best choice as soon as they're introduced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We may want to populate the model with as many parameters as we can get (e.g. more funding expected for the leg-based technology can tilt up the leg-based line, while new materials to increase energy efficiency may tilt all the lines), but I think we should face that the situation now is this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uABDW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uABDW.png&quot; alt=&quot;known-status&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of info required to produced a precise model is simply too much, and I think a clear answer is premature to give without falling in speculation: we simply do not yet have enough data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(model derived from Clayton Christensen, &quot;&lt;a href=&quot;http://www.claytonchristensen.com/books/the-innovators-dilemma/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Innovator's Dilemma&lt;/a&gt;&quot;)&lt;/p&gt;&#xA;" OwnerUserId="15760" LastEditorUserId="15760" LastEditDate="2016-08-10T12:43:42.950" LastActivityDate="2016-08-10T12:43:42.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17627" PostTypeId="1" CreationDate="2016-08-03T01:17:53.397" Score="3" ViewCount="339" Body="&lt;p&gt;Is the Buddy Secondary Life Support System (BSLSS) available in today's American-made spacesuits used on the International Space Station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The BSLSS enables one astronaut to provide life support (oxygen and coolant) from his space suit via a specialized hose to another astronaut in case of an emergency during a spacewalk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think I saw something like that in the Russian Orlan space suits.&lt;/p&gt;&#xA;" OwnerUserId="16495" LastEditorUserId="6816" LastEditDate="2016-08-03T01:54:01.217" LastActivityDate="2016-08-03T22:47:14.497" Title="Buddy Secondary Life Support System (BSLSS) on American suits" Tags="|iss|spacesuits|life-support|united-states|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17628" PostTypeId="2" ParentId="17627" CreationDate="2016-08-03T02:40:16.880" Score="2" Body="&lt;p&gt;No, there is no such capability in the Shuttle / ISS era US EVA suits (Extravehicular Mobility Units - EMUs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The suit contains a separate Secondary Oxygen Pack that is only used in the case of emergency. It provides approximately 30 minutes of O2 - enough to return to the vehicle. But there is no cross connection capability to another suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/pdf/188963main_Extravehicular_Mobility_Unit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt; (old, and for kids, but still the best description on the internet I know of)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: It's hard to prove a negative, but I can also offer up the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/492872main_EVA_G_H_20.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;EVA Checklist&lt;/a&gt;.  If you look at the Cuff Checklist (which is the checklist attached to the crewmember's wrist and which contains emergency response procedures) there is no reference to any buddy-breathing system.  Here's the table of contents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jXBZa.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jXBZa.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SUIT P EMERG just sends you to ABORT EVA which sends you back inside. So does SOP O2 ON which indicates the Secondary Oxygen Pack has been activated.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-08-03T13:15:48.573" LastActivityDate="2016-08-03T13:15:48.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17629" PostTypeId="1" AcceptedAnswerId="17633" CreationDate="2016-08-03T12:20:55.903" Score="13" ViewCount="2244" Body="&lt;p&gt;Since both Kerosene (RP-1) and liquid hydrogen (LH2) continue to be important liquid fuels used with liquid oxygen (LOX), something can be learned by comparing how they are used. This is covered well within questions and answers here this stackexchange. &lt;a href=&quot;https://space.stackexchange.com/q/17563/12102&quot;&gt;One recent question&lt;/a&gt; piqued my interest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V&lt;/a&gt;, RP-1 was used in the massive first stage, but the 2nd and third stages used LH2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/17566/12102&quot;&gt;this answer&lt;/a&gt; to the question, it's pointed out that the low density of the Hydrogen will have a penalty on the structure (the giant LH2 tank?) somewhat increased drag (again the giant LH2 tank?) and would require either a larger chamber size, or possibly more engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the Saturn V did have to use LH2/LOX in the first stage, &lt;em&gt;roughly&lt;/em&gt; how much different would it look? Just a little bit taller and wider, or really totally different?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much larger would the total LH2/LOX storage on the ground be? 5x? 25x? &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-09-24T23:22:37.657" Title="How would the Saturn V have differed if the first stage was also LH2/LOX?" Tags="|rockets|fuel|saturn-v|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17630" PostTypeId="2" ParentId="17615" CreationDate="2016-08-03T12:25:57.737" Score="9" Body="&lt;p&gt;The Apollo Service module was painted with an &lt;a href=&quot;https://billabbott.wordpress.com/2011/12/30/colors-materials-for-apollo-11-cm-sm-lm-what-the-hardware-looked-like-for-the-dragon-kit/&quot; rel=&quot;noreferrer&quot;&gt;aluminum paint&lt;/a&gt;. Aluminum paint will bubble severely if exposed to temperatures above &lt;a href=&quot;https://www.wolframalpha.com/input/?i=747+K&quot; rel=&quot;noreferrer&quot;&gt;747 K&lt;/a&gt;, if properly cured. One can only assume the paint was exposed to something close to that temperature at some point in it's lifecycle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tracing back, let's see where we can find them. Here's a picture of the SM from the LM after release around the Moon. It's hard to see, but I think I see some evidence of the bubbling near the CM, where there's high glare. This matches the same spot as seen at your image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ck6Tn.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ck6Tn.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The breakdown of the spacecraft is as follows. Note that the image was almost certainly taken from the side hatch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CpYKi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CpYKi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I had to venture a guess, I would say it probably happend when the tower was jettisoned, some small flame reached the paint that caused to to boil somewhat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next image is at the Rendezvous, post-Lunar mission. There is certainly bubbling seen there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wy4cA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wy4cA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="198" LastEditDate="2016-08-08T20:13:48.380" LastActivityDate="2016-08-08T20:13:48.380" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17631" PostTypeId="1" CreationDate="2016-08-03T15:16:08.563" Score="17" ViewCount="2713" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fLSi4.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fLSi4.jpg&quot; alt=&quot;This is a photo of the connector&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have done continuity tests on the 3 pins.&lt;br&gt;&#xA;1 is obviously ( from the continuity tests ) a ground pin.&#xA;1 is switched by the power switch.&#xA;1 has resistance and seems to change impedance when the power is switched on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anyone has any information on this connector, voltages and pinouts that'd be super useful.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some extra pictures:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Z8iTh.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z8iTh.jpg&quot; alt=&quot;This is a custom ( I think ) board&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Custom board?  I think so based on the quality of the fabrication.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VYQg6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VYQg6.jpg&quot; alt=&quot;They pre-modded this to have an exposed swappable CMOS battery... but it&amp;#39;s 6v?&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They pre-modded this to have an exposed swappable CMOS battery... but it's 6v?  The original I think was 3v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CbKSX.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CbKSX.jpg&quot; alt=&quot;Shot of full back w/ high density ports&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shot of full back w/ high density ports&lt;/p&gt;&#xA;" OwnerUserId="806" LastEditorUserId="806" LastEditDate="2016-08-03T18:32:05.753" LastActivityDate="2019-12-02T09:08:29.557" Title="I am looking for information on the power connector used on the Space Shuttle to power a GridCase 1530 laptop" Tags="|space-shuttle|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17632" PostTypeId="2" ParentId="17631" CreationDate="2016-08-03T15:50:47.883" Score="15" Body="&lt;p&gt;The IBM laptops were flown unmodified and a special &quot;brick&quot; was used to convert the Orbiter 28 VDC to the regular power supply voltage and current used by the laptops.  The &quot;brick&quot; is shown as DC PWR SPLY in this excerpt from the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/492873main_ORB_OPS_G_M_10.pdf&quot; rel=&quot;noreferrer&quot;&gt;Orbit Ops Checklist&lt;/a&gt;.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/TWg0F.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TWg0F.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Grid dates from an earlier time but I doubt they modified anything but the connector.  Do you have info on what a regular Grid used? If no, it looks like the company still exists, they might know. &lt;a href=&quot;http://www.griduk.com&quot; rel=&quot;noreferrer&quot;&gt;http://www.griduk.com&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My roommate worked on these in 1983-1984 but I didn't know details. &lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-08-03T15:55:16.633" LastActivityDate="2016-08-03T15:55:16.633" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17633" PostTypeId="2" ParentId="17629" CreationDate="2016-08-03T16:06:10.070" Score="25" Body="&lt;p&gt;The requirements of the first stage are that it deliver about 3340 m/s of delta v to a 690 ton payload (the upper stages and spacecraft), with an initial thrust-to-weight ratio of at least 1.16:1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best candidate for a first-stage hydrogen engine in the Saturn era would be the never-completed &lt;a href=&quot;https://en.wikipedia.org/wiki/M-1_(rocket_engine)&quot; rel=&quot;noreferrer&quot;&gt;M-1&lt;/a&gt;. At sea level, it would be much less powerful than the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot; rel=&quot;noreferrer&quot;&gt;F-1&lt;/a&gt;, but with better specific impulse - 310 seconds vs 263. Without going into too much detail, the result is that the first stage carries 1730 tons of LH2/LOX rather than 2160 tons of kerosene/LOX. However, depending on the exact mixture ratios in use, kerosene/LOX is about 3.5 times denser than LH2/LOX, so even with less fuel mass, the stage needs to get a lot bigger. If the 10 meter diameter is retained, the stage gets stretched from 42m to 92m -- a very long and skinny stage. More reasonable would be a 12-meter diameter, 63 meter long first stage. The overall rocket, all-up for an Apollo J mission, masses 2593 tons instead of 2970 tons. It might look something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7Hkuv.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7Hkuv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drag penalty wouldn't be terribly severe. About 50 m/s of the Saturn V's total potential is lost to drag; the larger diameter stage would probably incur another ~25m/s hit, but I don't want to go back and recalculate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To achieve the initial TWR requirement, 8 M-1s are needed, granting 1.21:1 TWR. As fuel mass was consumed and specific impulse (and thus thrust) increased, 2 or 4 of them would likely be shut off over the course of the burn to limit acceleration for crew comfort, with the remainder left running til fuel-out. Acceleration-by-time curves are shallower for higher Isp engines, so we may have a less efficient ascent than the Saturn V. Engine shutdown might therefore be delayed to make up some speed; this could mean a higher peak acceleration than Saturn V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Total hydrogen carried by the rocket would be about 4x that of the Saturn V; presumably that would quadruple the ground storage requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bear in mind that even though this rocket masses less in total, it probably costs much more because of first stage construction and transport logistics, engine complexity, and hydrogen handling, and on that basis it's somewhat inferior to the Saturn V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another possibility would be using 16x &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;noreferrer&quot;&gt;SSME&lt;/a&gt; instead of 8x M-1. This probably couldn't have flown until the 1980s, but the sea level specific impulse is far better, 366s instead of 310s. This would further reduce the first-stage fuel mass but probably add a lot of cost -- the entire shuttle program only built 42 of those engines.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-09-24T23:22:37.657" LastActivityDate="2016-09-24T23:22:37.657" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17634" PostTypeId="2" ParentId="17631" CreationDate="2016-08-03T17:18:36.430" Score="11" Body="&lt;p&gt;I have an unmodified commercial GRiDCase 1520. The external DC connector is labelled 16V, and the output of the AC/DC convertor brick states that it is 16.25V. The GRiD Compass was the unit designed specifically for NASA/DoD use, the Case was a commercial product, and I doubt that they produced a significantly separate design for Shuttle use (though they have clearly added a more secure DC connector than the commercial barrel connector).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OciiQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OciiQ.jpg&quot; alt=&quot;GRiDCase 1520 rear view, power brick removed&quot;&gt;&lt;/a&gt; &#xA;&lt;a href=&quot;https://i.stack.imgur.com/yXTai.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yXTai.jpg&quot; alt=&quot;GRiDCase 1520 power brick, inner connector&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16499" LastActivityDate="2016-08-03T17:18:36.430" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="17635" PostTypeId="2" ParentId="17562" CreationDate="2016-08-03T18:49:50.417" Score="3" Body="&lt;p&gt;@uhoh - Mistake, no.  Poor phrasing, perhaps yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the &lt;a href=&quot;http://solarprobe.jhuapl.edu/common/content/SolarProbePlusFactSheet.pdf&quot; rel=&quot;nofollow&quot;&gt;APL fact sheet&lt;/a&gt; is implying that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_Probe_Plus&quot; rel=&quot;nofollow&quot;&gt;Solar Probe Plus&lt;/a&gt; heat shield must survive extreme temperatures, radiation, and dust impacts otherwise the the rest of the spacecraft will &lt;a href=&quot;https://en.wikipedia.org/wiki/Ablation&quot; rel=&quot;nofollow&quot;&gt;ablate&lt;/a&gt; near perihelion at closest approach.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If the heat shield is facing directly towards the Sun so that this long narrow spacecraft can remain in the shadow so that it is &quot;moving sideways&quot; through this dust field, how does the this shield &quot;&lt;em&gt;protect the spacecraft from impacts from hypervelocity dust particles as it flies through the Sun's outer atmosphere&lt;/em&gt;&quot;?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's not that the heat shield is meant to protect the spacecraft from dust, since at perihelion it's velocity transverse to the heat shield normal (i.e., several hundred km/s with respect to the sun) will vastly exceed the typical dust speeds expected (i.e., several tens of km/s with respect to the sun).  In other words, in the spacecraft frame at perihelion the trajectory of impacting dust will basically be into the side of the spacecraft, not normal to the heat shield surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, micron-sized dust hitting something at several hundred km/s, even at glancing angles, is not something to ignore.  The teams have done considerable modeling to try and deal with this issue, but the dust distribution inside ~0.3 &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_unit&quot; rel=&quot;nofollow&quot;&gt;AU&lt;/a&gt; is not well known.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Fun Side Note:&lt;/strong&gt;  The FIELDS instrument is planning to look for dust by detecting the electric field pulses resulting from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Plasma_(physics)&quot; rel=&quot;nofollow&quot;&gt;plasma&lt;/a&gt; clouds produced by the impacting particle.  This has been done before with missions like &lt;a href=&quot;https://en.wikipedia.org/wiki/WIND_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;&lt;em&gt;Wind&lt;/em&gt;&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow&quot;&gt;STEREO&lt;/a&gt; (and a few others) and was kind of discovered by accident.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2016-08-03T18:49:50.417" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17636" PostTypeId="2" ParentId="17625" CreationDate="2016-08-03T21:34:53.803" Score="5" Body="&lt;p&gt;Super cool question! It combines a lot of my interests / career areas of focus: concept space mission analysis, communications theory, optics, and deep space exploration.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How much power would an Alpha Centauri probe require for communication?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To fully answer this I'll need to work through the math myself and provide some example solutions based on various assumptions. Stand by for the math... in the meantime here is a quick response.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is one (or two) of them incorrect?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Longshot provides detailed calculations on page 48. I haven't double checked the math but the logic used makes sense. I can do the math and post it here when I get home if requested. Starshot doesn't provide any math which makes me doubt their claim. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest assumption made by Starshot is that they can create a Fresnel lens with enough accuracy to improve the focus beyond the $3.25*10^{-7}rad=0.067''$ Longshot arrives at. Back of the envelope calculation: Note that they mention using 6 250 kW lasers on page 49, giving 1.5 MW of lazing power. To increase the power received by 1.5 million times the area subtended by the Earth over the total beam area would need to increase by $\sqrt{1,500,000}=1225$ (area of a circle). A focal length of infinity is theoretically possible, but would be dependent on the technology of the sail. Further, a thin lens would require a high number of grooves, reducing the efficiency of the lens. Lastly, they don't mention a data rate, so data may be able to be transmitted if sent VERY slowly.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;did the technology change so much in 30 years?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes and no. Physics didn't change. The biggest change I can think of is the invention of Turbo Codes, which revolutionized the communications world in 1993. Low-light imaging technology has improved (not sure by how much, but a lot). The size of large ground based telescopes has increased a bit, and now use adaptive optics to counter atmospheric distortion. &lt;/p&gt;&#xA;" OwnerUserId="12966" LastEditorUserId="12966" LastEditDate="2016-08-03T21:50:24.570" LastActivityDate="2016-08-03T21:50:24.570" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17638" PostTypeId="1" CreationDate="2016-08-03T23:20:26.397" Score="2" ViewCount="95" Body="&lt;p&gt;So I've seen ULA post about their intern rocket launch a couple of weekends ago and was curious:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the performance specs on the motors used for the first/second stage? (Class, quantity, etc.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, here's a video of the rocket I'm asking about (~1 minute):&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/YuoMwCaB2So?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="9030" LastEditorUserId="25" LastEditDate="2016-08-12T19:36:18.760" LastActivityDate="2016-08-12T19:36:18.760" Title="ULA/Ball Aerospace Intern Sport Rocket" Tags="|rockets|ula|performance|amateur-rocketry|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17639" PostTypeId="1" CreationDate="2016-08-04T01:52:41.877" Score="5" ViewCount="322" Body="&lt;p&gt;There is a claim that habitable-zone orbits are stable around both components of the Alpha Centauri binary, and at least one planet has been detected (though not in the habitable zone). Both stars appear to be about as old as the Sun or older. There is now some serious effort to look for an &quot;Earth Proxima&quot; (&lt;a href=&quot;http://missioncentaur.org/project/&quot; rel=&quot;nofollow&quot;&gt;Mission Centaur&lt;/a&gt;) in the Alpha Centauri system. Seems like a lot more to go on than &lt;a href=&quot;http://www.seti.org/&quot; rel=&quot;nofollow&quot;&gt;SETI&lt;/a&gt; has had with other places they've been listening to. With that in mind, has there ever been any effort to listen for artificial signals from Alpha Centauri or to send something like an &quot;Arecibo message&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Alpha Centauri is only about 4 light years away, it would be reasonable to expect that &lt;em&gt;if&lt;/em&gt; there is an intelligence present with the means and desire to listen and reply (admittedly a whole lot of &quot;ifs&quot;), we shouldn't have to wait more than 10 years to get a result if one is at all forthcoming, and it would be a relatively inexpensive exercise to undertake, even if nothing comes of it.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="4660" LastEditDate="2016-08-04T16:55:15.243" LastActivityDate="2016-08-13T15:11:51.433" Title="Has Alpha Centauri been a subject for SETI activities?" Tags="|seti|alpha-centauri|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17640" PostTypeId="1" AcceptedAnswerId="17644" CreationDate="2016-08-04T03:22:53.727" Score="6" ViewCount="264" Body="&lt;p&gt;If I was to knock on a wall or door between a pressurised room and a vacuum, would that sound different from knocking on a wall or door of the same material between two pressurised rooms? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, would there be any easy way to check if a room is depressurised, using only human senses and without opening the door?&lt;/p&gt;&#xA;" OwnerUserId="16504" LastEditorUserId="47149" LastEditorDisplayName="user10509" LastEditDate="2022-06-03T21:54:47.410" LastActivityDate="2022-06-03T21:54:47.410" Title="Does knocking on walls produce a different sound when adjacent to a vacuum?" Tags="|spacecraft|physics|sound|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="17641" PostTypeId="1" AcceptedAnswerId="17642" CreationDate="2016-08-04T04:00:17.890" Score="8" ViewCount="724" Body="&lt;p&gt;Conventional magnetotorquers for cubesats are electromagnets that produce torque in the Earth's magnetic field, and nearly all of the power they use just heats the copper through &lt;span class=&quot;math-container&quot;&gt;$\text{I}^2 \text{R}$&lt;/span&gt; losses. A permanent magnet that was articulated with some kind of &lt;em&gt;gimbal-like mechanism&lt;/em&gt; that could point it in a wide variety of orientations within the cube could in principle do this as well, using only a tiny fraction of the power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a lot of design issues related to a magnetic design that would maximize the effect while keeping weight down. There might need to be a fail-safe parked position so that the satellite would not be &quot;magnetically active&quot; externally until it was fully deployed and operational - a little bit like those &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_base&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetic bases&lt;/a&gt; used in machine shops. A realistic design might require some finite torque in the articulation simply due to forces between the magnet and its &quot;safe position&quot; keeper or other ferro-magnetic materials that could be substantially larger than the torque related to the Earth's magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;This sounds great, but is it functionally realistic for a low-U cubesat?&lt;/li&gt;&#xA;&lt;li&gt;Are there rules about permanent magnets inside cubesats - either regulatory or from individual launch providers - that could be a 'deal-breaker'?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; an example of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_base&quot; rel=&quot;nofollow noreferrer&quot;&gt;Magnetic Base&lt;/a&gt; (rotated) from &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/5/50/MagneticBase.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. The permanent magnet (PM) magnetotorquer would not look like this nor be this heavy. It's just an example of the concept of a PM object with an &quot;off&quot;-like position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Uzvjp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Uzvjpm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-08-11T02:00:26.427" LastActivityDate="2022-05-03T12:44:25.203" Title="Could an articulated permanent magnet work as a low-power cubesat magnetotorquer? Problems?" Tags="|attitude|cubesat|magnetic-field|magnetorquer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17642" PostTypeId="2" ParentId="17641" CreationDate="2016-08-04T06:28:42.073" Score="11" Body="&lt;p&gt;RAX and RAX-2 and possibly other &lt;a href=&quot;http://exploration.engin.umich.edu/blog/&quot; rel=&quot;nofollow noreferrer&quot;&gt;cubesats launched by the University of Michigan&lt;/a&gt; (I can't remember at the moment) used fixed neodymium based magnets on their Z+ axis. Hysteresis was added to dump any residual momentum after P-Pod ejection. On the RAX missions, the magnets then were used to orient the satellite vertically over the poles where the science missions were conducted.&lt;/p&gt;&#xA;&lt;p&gt;I haven't kept close tabs on the smallsat industry lately, but I would estimate that it is totally within the realm of doable, but with its risks. Cubesats are often built out of aluminum. While there may be other ferromagnetic materials used, and current loops in the electronics, I think a well chosen servomotor could over come most issues. The biggest risk in my mind would be dealing with vibration on launch. Moving parts tend to have higher risk.&lt;/p&gt;&#xA;&lt;p&gt;Yes, there are rules about magnets (and just about everything else) on secondary payloads. The primary payload usually has many times more risk invested so they control almost everything about secondary payloads. The launch provider will have a say as well. It's almost always a case by case basis. The largest risk takers are probably any other secondary payloads on board. For example, M-Cubed (another Michigan launched sat) &lt;a href=&quot;http://exploration.engin.umich.edu/blog/wp-content/uploads/2012/05/M-Cubed_CubeSat_Workshop2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;was believed&lt;/a&gt; to have &lt;a href=&quot;https://ieeexplore.ieee.org/document/6497127&quot; rel=&quot;nofollow noreferrer&quot;&gt;stuck to another cubesat&lt;/a&gt; using passive magnetic control, rendering both missions a loss.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Disclaimer: I worked on the missions mentioned here, as well as a few others.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12966" LastEditorUserId="10273" LastEditDate="2022-05-03T12:44:25.203" LastActivityDate="2022-05-03T12:44:25.203" CommentCount="14" ContentLicense="CC BY-SA 4.0" />
  <row Id="17643" PostTypeId="2" ParentId="17640" CreationDate="2016-08-04T09:46:35.543" Score="1" Body="&lt;p&gt;I suspect there would be a difference: a door between a pressurized area and a vacuum is under pressure from one side. Compare to a guitar string: they sound different when you vary the pressure on them.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-04T09:46:35.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17644" PostTypeId="2" ParentId="17640" CreationDate="2016-08-04T10:03:15.670" Score="3" Body="&lt;p&gt;If there is a wall between a pressurised room and a vacuum, the resulting forces to the wall are pretty strong. Strings of a guitar sound different if tension on the string varies. I suspect the wall would sound different. The force on a door between vacuum and pressure is so strong that opening of the door would be impossible or the door is severly damaged. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have seen a normal room door bending by a pressure difference between both adjacent rooms. The pressure difference was very small, only some centimeters of water or a few mbar. It was possible to open the door, but you had to use all your muscles.&#xA;The force to a door 1 m wide and 2 m high from a pressure difference of 1 bar is the same as of a load of 20,000 kg to the ground below. It is better to build not a rectangular door but a circular one.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="15906" LastEditDate="2020-03-08T00:07:48.580" LastActivityDate="2020-03-08T00:07:48.580" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17645" PostTypeId="1" AcceptedAnswerId="17646" CreationDate="2016-08-04T10:15:45.830" Score="1" ViewCount="153" Body="&lt;p&gt;I've read in a side note included in this &lt;a href=&quot;https://space.stackexchange.com/a/17635/12102&quot;&gt;nice answer&lt;/a&gt; that :&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Fun Side Note:&lt;/strong&gt;  The FIELDS instrument is planning to look for dust by detecting the electric field pulses resulting from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Plasma_(physics)&quot; rel=&quot;nofollow noreferrer&quot;&gt;plasma&lt;/a&gt; clouds produced by the impacting particle.  This has been done before with missions like &lt;a href=&quot;https://en.wikipedia.org/wiki/WIND_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Wind&lt;/em&gt;&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow noreferrer&quot;&gt;STEREO&lt;/a&gt; (and a few others) and was kind of discovered by accident.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;in Wikipedia I see &lt;a href=&quot;https://en.wikipedia.org/wiki/WIND_(spacecraft)#Some_discoveries_and.2For_contributions_to_science_by_the_Wind_spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;eighteen discoveries by WIND&lt;/a&gt; but not that particular one, and while the STEREO spacecraft had a cluster of instruments called &quot;&lt;a href=&quot;http://sprg.ssl.berkeley.edu/impact/instruments.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;IMPACT&lt;/a&gt;&quot; the Wikipedia article doesn't mention the detection of dust impacts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How where these plasma clouds first noticed, and eventually attributed to dust impacts?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; There are several good articles about the detection phenomenon cited in &lt;a href=&quot;https://astronomy.stackexchange.com/a/16803/7982&quot;&gt;this great answer&lt;/a&gt;, but I'm wondering if there was a &quot;hey, what the heck are these strange pulses - oh, &lt;em&gt;that's&lt;/em&gt; what they are!&quot; story I can read, or otherwise learn about. It may not be quite as exciting as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pulsar#Discovery&quot; rel=&quot;nofollow noreferrer&quot;&gt;discovery of the first pulsar&lt;/a&gt; but I'd still like to learn more. I have found &lt;a href=&quot;http://swaves.gsfc.nasa.gov/nugget_docs/STEREO%20dust%20detections-StCyr.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this discussion&lt;/a&gt; so far, but it's still a bit thin on the &quot;discovery&quot; aspect.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-10-11T14:50:55.763" LastActivityDate="2020-10-11T14:50:55.763" Title="Where to read about early work in the detection of dust impacts on solar observing spacecraft?" Tags="|impact|meteoroid|space-dust|stereo-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17646" PostTypeId="2" ParentId="17645" CreationDate="2016-08-04T13:26:20.283" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;in Wikipedia I see eighteen discoveries by WIND but not that particular one...&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, I have not had a chance to update the list of &lt;a href=&quot;https://en.wikipedia.org/wiki/WIND_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Wind&lt;/em&gt;&lt;/a&gt; discoveries for quite a while but plan to do so soon.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;...while the STEREO spacecraft had a cluster of instruments called &quot;IMPACT&quot; the Wikipedia article doesn't mention the detection of dust impacts...&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The IMPACT instrument is part of a &lt;a href=&quot;http://stereo.gsfc.nasa.gov/instruments/instruments.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;suite&lt;/a&gt; of instruments on STEREO but has nothing to do with dust particles.  The discovery of dust detections by &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow noreferrer&quot;&gt;STEREO&lt;/a&gt; came from the &lt;a href=&quot;http://swaves.gsfc.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;WAVES&lt;/a&gt; instrument, specifically the &lt;a href=&quot;http://adsabs.harvard.edu/abs/2008SSRv..136..487B&quot; rel=&quot;nofollow noreferrer&quot;&gt;time domain sampler (TDS)&lt;/a&gt; reciever.  The TDS on STEREO produces 16,384 three component vector electric field measurements as a time series snapshot centered on the the peak signal amplitude.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;How where these plasma clouds first noticed, and eventually attributed to dust impacts?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The following is an excerpt from &lt;a href=&quot;http://adsabs.harvard.edu/abs/2016JGRA..121..966K&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Kellogg et al.&lt;/em&gt; [2016]&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At first attributed to interference, Michael Kaiser and Keith Goetz (unpublished STEREO Team meetings) began to explore the idea that the STEREO signals were due to dust impacts, similar to those investigated at Saturn [&lt;a href=&quot;http://adsabs.harvard.edu/abs/1983Icar...53..236G&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Gurnett et al.&lt;/em&gt;, 1983&lt;/a&gt;; &lt;a href=&quot;http://adsabs.harvard.edu/abs/1983GeoRL..10....5A&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Aubier et al.&lt;/em&gt;, 1983&lt;/a&gt;; &lt;em&gt;Meyer-Vernet et al.&lt;/em&gt;, 2009a].  This idea was pursued, especially by &lt;em&gt;Meyer-Vernet et al.&lt;/em&gt; in several papers [2009a, 2009b].  The STEREO data were then extensively analyzed by &lt;em&gt;Zaslavsky et al.&lt;/em&gt; [2012] and by &lt;em&gt;Belheouane et al.&lt;/em&gt; [2012]. Debris simultaneous with signals on antennas has been observed with the optical imagers on STEREO [&lt;em&gt;St. Cyr et al.&lt;/em&gt;, 2009].  Interpretation of these waveforms seems now to be established.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To the best of my knowledge, the first electric field observations were in 1981 but were not properly interpreted until the 1983 papers by &lt;em&gt;Gurnett et al.&lt;/em&gt; and &lt;em&gt;Aubier et al.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dust impacts have been observed by electric field probes on (in no particular order) &lt;a href=&quot;http://en.wikipedia.org/wiki/Cassini-Huygens&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Cassini&lt;/em&gt;&lt;/a&gt; [e.g., &lt;em&gt;Kurth et al.&lt;/em&gt;, 2006], &lt;a href=&quot;http://en.wikipedia.org/wiki/WIND_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Wind&lt;/em&gt;&lt;/a&gt; [e.g., &lt;em&gt;Malaspina et al.&lt;/em&gt;, 2014; &lt;em&gt;Kellogg et al.&lt;/em&gt;, 2016], &lt;a href=&quot;http://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow noreferrer&quot;&gt;STEREO&lt;/a&gt; [e.g., &lt;em&gt;Meyer-Vernet et al.&lt;/em&gt;, 2009], &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager&lt;/a&gt; [e.g., &lt;em&gt;Gurnett et al.&lt;/em&gt;, 1983; &lt;em&gt;Aubier et al.&lt;/em&gt;, 1983], &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vega&lt;/a&gt; [e.g., &lt;em&gt;Laakso et al.&lt;/em&gt;, 1989], &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space 1&lt;/a&gt; [e.g., &lt;em&gt;Tsurutani et al.&lt;/em&gt;, 2004], and &lt;a href=&quot;https://en.wikipedia.org/wiki/MAVEN&quot; rel=&quot;nofollow noreferrer&quot;&gt;MAVEN&lt;/a&gt; [e.g., &lt;em&gt;Andersson et al.&lt;/em&gt;, 2015] spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Ulysses_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Ulysses&lt;/em&gt;&lt;/a&gt; spacecraft has a dedicated dust detection instrument.  It was this instrument that discovered &lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_medium&quot; rel=&quot;nofollow noreferrer&quot;&gt;interstellar&lt;/a&gt; dust (ISD) [&lt;em&gt;Grun et al.&lt;/em&gt;, 1993; &lt;a href=&quot;http://adsabs.harvard.edu/abs/2010ARA%26A..48..173M&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Mann&lt;/em&gt;, 2010&lt;/a&gt;; &lt;a href=&quot;http://adsabs.harvard.edu/abs/2010PPCF...52l4012M&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Mann et al.&lt;/em&gt;, 2010&lt;/a&gt;].&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;...but I'm wondering if there was a &quot;hey, what the heck are these strange pulses - oh, that's what they are!&quot; story I can read, or otherwise learn about...&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The introduction by &lt;em&gt;Kellogg et al.&lt;/em&gt; [2016] and the two &lt;em&gt;Voyager&lt;/em&gt; papers by &lt;em&gt;Gurnett et al.&lt;/em&gt; [1983] and &lt;em&gt;Aubier et al.&lt;/em&gt; [1983] have some interesting comments on the original thoughts regarding these signals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were good reasons to think that the electric field signals were artificial.  Below I show four characteristic examples of dust impacts (i.e., Types A-D) observed by the &lt;em&gt;Wind&lt;/em&gt; spacecraft's WAVES TDS receiver:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7NXrY.png&quot; width=&quot;500&quot; height=&quot;500&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and two characteristic examples instrument interference signals (i.e., Types E and F):  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y7J9C.png&quot; width=&quot;250&quot; height=&quot;500&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that there are several similarities between the dust impact signals and the artificial instrument responses, which is why some of the original observations (e.g., those made in 1981) were interpreted as instrument interference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below I have listed several relevant references but note that this list is certanly not comprehensive and that there are dozens more.  A good place to start would be to dig through the reference lists in &lt;em&gt;Kellogg et al.&lt;/em&gt; [2016], &lt;em&gt;Malaspina et al.&lt;/em&gt; [2014], and the two 2010 papers by I. Mann.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Andersson, L., et al. &quot;Dust observations at orbital altitudes surrounding Mars,&quot; &lt;em&gt;Science&lt;/em&gt; &lt;strong&gt;350&lt;/strong&gt;, pp. 0398, doi:10.1126/science.aad0398, 2015.&lt;/li&gt;&#xA;&lt;li&gt;Aubier, M.G., N. Meyer-Vernet, and B.M. Pedersen &quot;Shot noise from grain and particle impacts in Saturn's ring plane,&quot; &lt;em&gt;Geophys. Res. Lett.&lt;/em&gt; &lt;strong&gt;10&lt;/strong&gt;, pp. 5-8, doi:10.1029/GL010i001p00005, 1983.&lt;/li&gt;&#xA;&lt;li&gt;Gr$\ddot{u}$n, E., et al. &quot;Discovery of Jovian dust streams and interstellar grains by the ULYSSES spacecraft,&quot; &lt;em&gt;Nature&lt;/em&gt; &lt;strong&gt;362&lt;/strong&gt;, pp. 428-430, doi:10.1038/362428a0, 1993.&lt;/li&gt;&#xA;&lt;li&gt;Gurnett, D.A., E. Grun, D. Gallagher, W.S. Kurth, and F.L. Scarf &quot;Micron-sized particles detected near Saturn by the Voyager plasma wave instrument,&quot; &lt;em&gt;Icarus&lt;/em&gt; &lt;strong&gt;53&lt;/strong&gt;, pp. 236-254, doi:10.1016/0019-1035(83)90145-8, 1983.&lt;/li&gt;&#xA;&lt;li&gt;Kellogg, P.J., K. Goetz, and S.J. Monson &quot;Dust impact signals on the Wind spacecraft,&quot; &lt;em&gt;J. Geophys. Res.&lt;/em&gt; &lt;strong&gt;121&lt;/strong&gt;(2), pp. 966-991, doi:10.1002/2015JA021124, 2016.&lt;/li&gt;&#xA;&lt;li&gt;Kurth, W.S., T.F. Averkamp, D.A. Gurnett, and Z. Wang &quot;Cassini RPWS observations of dust in Saturn’s E Ring,&quot; &lt;em&gt;Planet. Space Sci.&lt;/em&gt; &lt;strong&gt;54&lt;/strong&gt;, pp. 988-998, doi:10.1016/j.pss.2006.05.011, 2006.&lt;/li&gt;&#xA;&lt;li&gt;Laakso, H., R. Grard, A. Pedersen, and G. Schwehm &quot;Impacts of large dust particles on the VEGA spacecraft,&quot; &lt;em&gt;Adv. Space Res.&lt;/em&gt; &lt;strong&gt;9&lt;/strong&gt;, pp. 269-272, doi:10.1016/0273-1177(89)90273-1, 1989.&lt;/li&gt;&#xA;&lt;li&gt;Malaspina, D.M., et al., &quot;Interplanetary and interstellar dust observed by the Wind/WAVES electric field instrument,&quot; &lt;em&gt;Geophys. Res. Lett.&lt;/em&gt; &lt;strong&gt;41&lt;/strong&gt;, pp. 266-272, doi:10.1002/2013GL058786, 2014.&lt;/li&gt;&#xA;&lt;li&gt;Mann, I., et al. &quot;Dust in the interplanetary medium,&quot; &lt;em&gt;Plasma Phys. Control. Fusion&lt;/em&gt; &lt;strong&gt;52&lt;/strong&gt;, pp. 124012, doi:10.1088/0741-3335/52/12/124012, 2010.&lt;/li&gt;&#xA;&lt;li&gt;Mann, I. &quot;Interstellar Dust in the Solar System,&quot; &lt;em&gt;Annual Review of Astronomy and Astrophysics&lt;/em&gt; &lt;strong&gt;48&lt;/strong&gt;, pp. 173-203, doi:10.1146/annurev-astro-081309-130846, 2010.&lt;/li&gt;&#xA;&lt;li&gt;Meyer-Vernet, N., et al. &quot;Dust detection by the Wave instrument on STEREO: Nanoparticles picked up by the solar wind?,&quot; &lt;em&gt;Sol. Phys.&lt;/em&gt; &lt;strong&gt;256&lt;/strong&gt;, pp. 463-474, doi:10.1007/s11207-009-9349-2, 2009.&lt;/li&gt;&#xA;&lt;li&gt;Scarf, F.L., D.A. Gurnett, W.S. Kurth, and R.L. Poynter &quot;Voyager plasma wave measurements at Saturn,&quot; &lt;em&gt;J. Geophys. Res.&lt;/em&gt; &lt;strong&gt;88&lt;/strong&gt;, pp. 8971-8984, doi:10.1029/JA088iA11p08971, 1983.&lt;/li&gt;&#xA;&lt;li&gt;Tsurutani, B.T., et al. &quot;Plasma clouds associated with Comet P/Borrelly dust impacts,&quot; &lt;em&gt;Icarus&lt;/em&gt; &lt;strong&gt;167&lt;/strong&gt;, pp. 89-99, doi:10.1016/j.icarus.2003.08.021, 2004.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2016-08-04T13:26:20.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17647" PostTypeId="2" ParentId="6546" CreationDate="2016-08-04T19:43:00.463" Score="1" Body="&lt;p&gt;Explanation in non technical jargon:&#xA;Think of yourself (Earth) sitting with your arms tucked close to your chest on a rotating piano stool. If you were to extend your arms, the rotation speed of the stool seat will slow down.&#xA;Now, because the location on Earth directly under the moon is pulled toward the moon, the radius of the Earth is extended (stretched)towards the direction of the moon, for the same reason as you, on the rotaing piano stool, slow down as you extend your arms away from your body, thereby increasing the radius of your body or the piano stool.&#xA;Technically, you and the Earth are obeying the law of &quot;conservation of angular momentum&quot;:  Mass x speed x radius of a rotating body is constant.&#xA;Should the radius of the body increase, its rotational speed must decrease&#xA;in order to hold constant the value of its original angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another factor slowing Earth rotation is the part of ocean movement caused by the moon's gravity, i.e. tides. The sliding of the ocean water over the Earth's rotating surface acts like a brake, furthering the decrease in Earth rotation rate. Even if the Earth were without large expanses of water--oceans--its rotation rate would still decrease, but not as much.&lt;/p&gt;&#xA;" OwnerUserId="16507" LastEditorUserId="16507" LastEditDate="2016-08-04T20:06:24.533" LastActivityDate="2016-08-04T20:06:24.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17648" PostTypeId="2" ParentId="9267" CreationDate="2016-08-04T22:54:08.773" Score="2" Body="&lt;p&gt;I believe the following may be helpful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager&lt;/a&gt; probes weighed about 825 kg and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_IIIE&quot; rel=&quot;nofollow noreferrer&quot;&gt;Titan IIIE&lt;/a&gt; booster could boost 15,400 kg to &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;low Earth orbit&lt;/a&gt; (LEO).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Juno probe, in contrast, weighs 3,625 kg, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V#Versions&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas 551&lt;/a&gt; booster used to launch it is only capable of 18,814 kg to low Earth orbit, a substantially less boost to payload mass ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure this is over-simplifying, but it seems the boost to weight ratio is a decent indicator of how far past LEO the booster is capable of. The higher the ratio, the more the booster has left upon reaching LEO and heading out to Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno weighed around four times as much as Voyager, but its booster was only about one third more powerful. Somewhere in between would lie the threshold for a faster transit to Jupiter.&lt;/p&gt;&#xA;" OwnerUserId="16508" LastEditorUserId="286" LastEditDate="2019-01-13T22:33:32.850" LastActivityDate="2019-01-13T22:33:32.850" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17649" PostTypeId="1" CreationDate="2016-08-05T15:45:07.463" Score="11" ViewCount="333" Body="&lt;p&gt;When a cargo mission is flown toward the ISS, there are at least four different kinds of liquids that may need to be moved into/out of the station:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Propellant for Zvezda's thrusters&lt;/li&gt;&#xA;&lt;li&gt;Water&lt;/li&gt;&#xA;&lt;li&gt;Wastewater&lt;/li&gt;&#xA;&lt;li&gt;Oxygen&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;How is this managed? Are the docking ports specially equipped for each sort of liquid? How is the liquid moved in the station (i.e. is there plumbing for wastewater)? What drives the liquids? Are there pumps on the station, the vehicle or both?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally: How is this system maintained in microgravity? While you can pump liquids, how do you drain the pipes in case you need to work on them?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="58" LastEditDate="2016-08-05T15:50:25.667" LastActivityDate="2016-08-06T18:17:24.793" Title="Liquids Plumbing on board" Tags="|iss|water|liquid-fuel|movement|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17650" PostTypeId="1" CreationDate="2016-08-05T19:53:06.123" Score="5" ViewCount="471" Body="&lt;p&gt;The harsh radiation environment in space requires that electronics on spacecraft be &quot;radiation hardened.&quot;  However, the radiation environment in LEO is not as intense as say a spacecraft traveling through the Van Allen belts.  I'm wondering what level of radiation hardening is needed for spacecraft in LEO or payloads going to the ISS.  A link to a NASA or other agency requirements document would be appreciated.  &lt;/p&gt;&#xA;" OwnerUserId="16180" LastActivityDate="2016-08-05T19:53:06.123" Title="What level of radiation protection do electronics on the ISS or in LEO need?" Tags="|iss|low-earth-orbit|radiation|electronics|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17651" PostTypeId="1" AcceptedAnswerId="17652" CreationDate="2016-08-05T20:25:43.523" Score="2" ViewCount="146" Body="&lt;p&gt;It's possible/likely that one day we'll be able to post/mail/deliver stuff to a non-terrestrial vehicle, such as an orbiting vehicle or an extraterrestrialLy landed vehicle, &lt;em&gt;without&lt;/em&gt; involving the vehicle's governing organisation (eg NASA).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be its address?&lt;br&gt;&#xA;Who would have jurisdiction over deciding?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some ideas:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;&amp;lt;Recipient&amp;gt;&#xA;&amp;lt;Name of craft&amp;gt;&#xA;&amp;lt;Location&amp;gt; (options of LEO, GSO, HEO, Luna, Mars, LMO)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="10584" LastEditorUserId="10584" LastEditDate="2016-08-07T21:24:24.687" LastActivityDate="2016-08-07T21:24:24.687" Title="What would be the postal/delivery address for an orbiting vehicle?" Tags="|low-earth-orbit|geostationary|" AnswerCount="2" CommentCount="4" ClosedDate="2016-08-05T22:10:34.947" ContentLicense="CC BY-SA 3.0" />
  <row Id="17652" PostTypeId="2" ParentId="17651" CreationDate="2016-08-05T20:42:18.247" Score="5" Body="&lt;p&gt;It would probably just be designated by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_Catalog_Number&quot;&gt;satellite catalogue number&lt;/a&gt; contained in that spacecraft’s &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot;&gt;two line element&lt;/a&gt;.  With that number the orbital postal service could look up your spacecraft’s two line element which would tell them exactly what orbit they would need to be in to rendezvous.   For example, the satellite catalogue number of the ISS is 25544 so if I was sending you a letter the address might look like this&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bohemian&lt;/p&gt;&#xA;&#xA;&lt;p&gt;International Space Station, Habitation Module &lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEO, 25544&lt;/p&gt;&#xA;" OwnerUserId="16180" LastActivityDate="2016-08-05T20:42:18.247" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17653" PostTypeId="2" ParentId="17651" CreationDate="2016-08-05T22:03:38.057" Score="3" Body="&lt;p&gt;Currently, you just use the mailing address of the controlling entity of the satellite/module. For example, mail to astronauts on ISS should go to one of&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&#xA;[Astronaut's name]&#xA;NASA Headquarters&#xA;300 E. Street SW, Suite 5R30&#xA;Washington, DC 20546&#xA;U.S.A&#xA;&#xA;&#xA;[Cosmonaut's name],&#xA;Russian Federal Space Agency&#xA;42 Schepkina st.&#xA;Moscow 107996 &#xA;Russia&#xA;&#xA;&#xA;[Astronaut's name]&#xA;ESA Communication Department&#xA;ESRIN&#xA;Via Galileo Galilei&#xA;Casella Postale 64&#xA;I-00044 Frascati&#xA;Italy &#xA;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;That way the mail will reach the right recipient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until actual postal service is established to the satellites, any direct mailing addresses of these, bypassing the ground HQ, are a pure speculation.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-08-05T22:03:38.057" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17654" PostTypeId="1" CreationDate="2016-08-05T22:48:14.573" Score="5" ViewCount="869" Body="&lt;p&gt;How much would Dragon 2 or Red Dragon need to be changed to act as a lunar lander? What I mean is not just landing but also lifting off from the Moon's surface to a specified orbit to dock with another spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current designs would need some modification. For a Moon mission the heat shield and the parachute system are unnecessary, so in place of their mass there could be fuel available for the mission. But would it have enough fuel to land and bring the astronauts back to orbit? If not, how much more fuel is needed? What would the total mass be for this? Also what kind of other modifications would it need, for example life support systems, internal power, and spacecraft components (computers, electronics, batteries), protection from radiation, and structural material changes with lighter materials (since it will not serve to make a re-entry into Earth's atmosphere just as a lunar lander)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will be costly, since the design could change, but would it be worth it to modify the current Dragon spacecrafts, or would it be better to create an entirely new design with better capabilities for such a mission and that doesn't resemble the current designs?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="13710" LastEditDate="2016-08-05T23:14:02.370" LastActivityDate="2016-08-06T13:16:32.173" Title="How much modification would Dragon 2 or Red Dragon require to serve as a lunar lander?" Tags="|the-moon|design|lunar-landing|dragon-v2|red-dragon|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17655" PostTypeId="2" ParentId="17654" CreationDate="2016-08-05T23:39:00.113" Score="2" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/q/16813/25&quot;&gt;this question&lt;/a&gt;, we know the delta v is about 1700 m/s with the stock configuration of the Dragon 2. The heat shield will be replaced with some extra propellant for a Lunar lander. It's really hard to say how much mass that would be, but let's give it a 10% decrease in mass and the same increase in fuel. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moon requires about 2.3 km/s&lt;/a&gt; It seems reasonable that a light load could land on the Moon. But there would not be enough to return to orbit, let alone back to Earth. Still, I'm sure if they wanted to they could re-design it to make such a trip, mostly by bringing a larger payload to the Moon. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-08-05T23:39:00.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17656" PostTypeId="1" CreationDate="2016-08-06T13:01:44.450" Score="2" ViewCount="48" Body="&lt;p&gt;What kind of systems or technology is developed for ISS (International Space Station)? There are so many debris at higher orbits than ISS, but many of them has started deorbiting so we have a potential risk that some debris could collide with ISS. Illustrating even with examples is ok, such as different damages suffered before by the ISS and which technology served as protection or solution for these sittuations. Another element that has to do with safety is the evacuating process how fast could be done that?&lt;/p&gt;&#xA;" OwnerUserId="16522" LastEditorUserId="16522" LastEditDate="2016-08-06T17:16:39.980" LastActivityDate="2016-08-06T17:21:10.683" Title="What systems or technologies are developed to protect ISS from space debris?" Tags="|iss|debris|safety|" AnswerCount="0" CommentCount="10" ClosedDate="2016-08-06T17:21:27.823" ContentLicense="CC BY-SA 3.0" />
  <row Id="17657" PostTypeId="2" ParentId="17654" CreationDate="2016-08-06T13:16:32.173" Score="4" Body="&lt;p&gt;tl;dr: If you use (nearly) all your cargo capacity for additional propellant and get rid of heatshield etc. you might barely be able to land the Dragon II from Lunar orbit (back-of-the-envelope result). In all other cases you need to strap on additional propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/a/16814/11256&quot;&gt;the answer&lt;/a&gt; to &lt;a href=&quot;https://space.stackexchange.com/questions/16813/dragon-2-propellant-capacity&quot;&gt;this question&lt;/a&gt;, we know that the delta v is more like 450m/s. &#xA;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Launch.2Flanding&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;, we'd need around 3.8km/s for LLO -&gt; Moon -&gt; LLO. According to my calculations that means around 27t of propellant for an unmodified Dragon. That amount can be reduced to 16t of propellant when we reduce the dry mass by 2.5t (the intended cargo return capacity). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we do not want to return from the Moon (e.g. refuel there), the standard Dragon II needs around 7.5t propellant for landing while the reduced version needs around 5t.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-08-06T13:16:32.173" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17659" PostTypeId="2" ParentId="17279" CreationDate="2016-08-06T16:33:22.567" Score="3" Body="&lt;p&gt;No, tanks are not always filled to 100%. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=19228.msg605258#msg605258&quot; rel=&quot;nofollow&quot;&gt;Falcon 9&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Tank fill fraction, especially on second stage, is mission-dependent (optimizing the rocket equation - adding performance margin makes what you learn in class very idealized). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=29457.msg1473370#msg1473370&quot; rel=&quot;nofollow&quot;&gt;Cryogenic tanks&lt;/a&gt; are usually filled to capacity, though.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;a.  it limits the amount of analysis that has to be done&lt;br&gt;&#xA;  b.  There are only empty and full sensors on the tanks&lt;br&gt;&#xA;  c.  hard to determine load by head pressure accurately due to prop boil off.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Just easier to launch full and deal the excess on orbit&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On the Ariane 1-4 it was routine to fill the second stage only to the level required for the mission. This had to do with the guidance system, they decided to set second-stage engine cutoff at a specified speed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Ariane 40 and 42P the first stage was filled to a lower level, because a full tank would result in the thrust/weight ratio being too low. &#xA;(source: Europäische Trägerraketen 1: Von der Diamant zur Ariane 4 – Europas steiniger Weg in den Orbit, B. Leitenberger)&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-06T16:33:22.567" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17660" PostTypeId="2" ParentId="17649" CreationDate="2016-08-06T18:17:24.793" Score="7" Body="&lt;p&gt;With the exception of the propellant for Zvezda's thrusters, which is fairly simple and fairly short plumbing, you are asking about the Space Station's Environmental Control and Life Support Systems (ECLSS). The plumbing for propulsion is quite simple. This plumbing has existed since the dawn of spaceflight. The plumbing needed for life support is anything but simple. A simplified diagram follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OFznf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OFznf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;sub&gt;Source: &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2000/ast13nov_1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; (so no copyright), via &lt;a href=&quot;https://en.wikipedia.org/wiki/File:SpaceStationCycle.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;.&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Providing fuel and oxidizer to the rockets on the Space Station is extremely simple compared to the problem of providing usable water and breathable atmosphere to people on the Space Station. The Space Station has leaks, requiring regular resupplies of breathing atmosphere. People consume water and oxygen and produce carbon dioxide. That CO&lt;sub&gt;2&lt;/sub&gt; must be continuously removed (watch &lt;em&gt;Apollo 13&lt;/em&gt;) from the breathing atmosphere lest the crew be poisoned. The crew can also be poisoned by trace contaminants in the breathing atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water from human waste in the Space Station toilets (both #1 and #2) is carefully scrubbed using specialized equipment. This waste water is then electrolyzed to form hydrogen and oxygen. The hydrogen is vented overboard as waste while the oxygen goes back into the breathing atmosphere. The desiccated products of human #1 and #2 waste are either sent overboard or placed in modules that will burn on reentry into the Earth's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Carbon dioxide is removed from the breathing atmosphere via a complex and somewhat failure-prone set of machinery in both the Russian and US segments of the Space Station. Both use a two segment chamber in which one chamber selectively collects carbon dioxide from the breathing atmosphere while the other vents accumulated carbon dioxide into space. Eventually, the collecting chamber becomes saturated with carbon dioxide and the venting chamber has little left to vent. Valves are repositioned at this point to make the old collecting chamber become the new venting chamber and the old venting chamber become the new collecting chamber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This process of course leaks. Some of the good stuff (nitrogen and oxygen) is vented along with the carbon dioxide. Moreover, the vented carbon dioxide is mostly oxygen. Experiments have been tried that used equipment applicable to Mars that converts that carbon dioxide into methane (which was vented), but those experiments were just that, experiments. (Aside: That these experiments have not become operational and have instead been cancelled says something about the prospects sending humans to Mars.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Station is a leaky vessel. In addition to those expected leaks from hydrogen and carbon dioxide venting, the station leaks fluids with every docking and undocking, and it also leaks at every joint. This latter problem is is becoming more severe as the station ages. Because of this, the Space Station is regularly resupplied with water, nitrogen, and oxygen.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-08-06T18:17:24.793" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17661" PostTypeId="1" AcceptedAnswerId="17676" CreationDate="2016-08-06T18:52:41.607" Score="10" ViewCount="1877" Body="&lt;p&gt;I am studying low-thrust maneuvers, in particular a change of inclination only. According to (Ruggiero et al., 2011)[1], if one writes the perturbation force in the Gauss planetary equations in terms of yaw and pitch angles $\alpha$ and $\beta$, the expression for the change in inclination is as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\frac{\operatorname{d}\!i}{\operatorname{d}\!t}=\left|\vec{f}\right| \frac{r}{h} \cos{(\omega + \nu)} \sin{\beta}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(hence depending only on the out-of-plane angle)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one further derives this equation with respect to $\beta$, arrives to this expression for the optimal out-of-plane angle for the maximum instantaneous change of the inclination:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\beta = \frac{\pi}{2} \operatorname{sgn}\left(\cos{(\omega + \nu)}\right)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, we should change the direction of the thrust vector every half orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My problem with this result is that, intuitively, I would expect a change after every crossing of the line of nodes, and therefore depending on $\sin{(\omega + \nu)}$. In this way I would think a net torque around the node would be exerted and hence the inclination should change. Instead, the change is shifted 90 degrees and I don't understand why.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Can somebody provide some physical explanation, simulation, or alternative derivation that helps me clarify why changing the direction in this way produces a net change of inclination?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1]: Ruggiero, A., P. Pergola, S. Marcuccio, and M. Andrenucci. &quot;Low-thrust maneuvers for the efficient correction of orbital elements.&quot; In &lt;em&gt;32nd International Electric Propulsion Conference&lt;/em&gt;, pp. 11-15. 2011.&lt;/p&gt;&#xA;" OwnerUserId="10716" LastEditorUserId="10716" LastEditDate="2017-01-29T23:32:51.000" LastActivityDate="2023-03-07T06:00:20.117" Title="Change of inclination using low thrust" Tags="|orbital-mechanics|low-energy-transfer|inclination-change|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17662" PostTypeId="1" CreationDate="2016-08-06T22:46:55.030" Score="7" ViewCount="153" Body="&lt;p&gt;SpaceX and ULA are the only two companies currently authorized to launch US national security payloads. These two companies will compete with each other to win the contracts for national military satellites. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What criteria are needed and required to be completed for such contracts? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much different are the contracts for these launches comparing with other commercial launch contracts at general? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we speak for capacity of the rockets than the company which can't support launching such a payload because is heavy would not compete, but if they compete it means that both have capabilities to launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we speak for the launching price than SpaceX would have won all those contracts, it won't be needed a competition. How is the winner chosen?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="51" LastEditDate="2016-08-07T02:08:26.327" LastActivityDate="2016-08-07T02:20:05.853" Title="How is decided the winner for US military satellites launch contracts and what criteria are necessary to be completed for these contracts?" Tags="|spacex|commercial|military|government|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17663" PostTypeId="1" AcceptedAnswerId="17664" CreationDate="2016-08-07T01:39:26.860" Score="4" ViewCount="2442" Body="&lt;p&gt;This question is about the final burn in the landing sequence of the Falcon 9 first stage return to Earth tests.&lt;/p&gt;&#xA;&lt;p&gt;If I understand correctly, there is usually far more thrust than there is weight at this point. If the maneuver took place a little higher up, the first stage would simply reach zero vertical velocity and immediately start accelerating back up again until it became unstable (see &lt;a href=&quot;https://space.stackexchange.com/a/17475/12102&quot;&gt;this answer&lt;/a&gt; and all the associated comments) or ran out of fuel.&lt;/p&gt;&#xA;&lt;p&gt;I'm trying to understand the issues behind a landing &lt;a href=&quot;https://space.stackexchange.com/q/17474/12102&quot;&gt;abort and re-try&lt;/a&gt; and the associated balance of thrust and weight for which one of the many considerations is the &lt;a href=&quot;https://space.stackexchange.com/q/17279/12102&quot;&gt;initial fuel loading&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;An abort and re-try could be made more feasible if two or only one engine were used once this phase began. Three engines might send the stage too high too fast. Shutting down two engines and doing this with a single engine would improve the possibility that this emergency procedure might work.&lt;/p&gt;&#xA;&lt;p&gt;However any further discussion of that belongs with &lt;a href=&quot;https://space.stackexchange.com/q/17474/12102&quot;&gt;this question&lt;/a&gt;. Here, I'm asking specifically: &lt;em&gt;Can Falcon 9 first stage landing final burn using one, two, and three engines&lt;/em&gt; and what are the &lt;em&gt;Tradeoffs?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The following two quotes are from a list of &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9_first-stage_landing_tests#Landing_attempts&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9 first stage landing attempts&lt;/a&gt; - these two contain some information about the number of engines used, but it's not enough to make a definitive list of the number of engines used in the final burns of each landing.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Flight 24:&lt;/strong&gt; Pursuing their experiments to test the limits of the flight envelope, SpaceX opted for a shorter landing burn with three engines instead of the single-engine burns seen in earlier attempts; this approach consumes less fuel by leaving the stage in free fall as long as possible and decelerating more sharply, thereby minimizing the amount of energy expended to counter gravity.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Flight 26:&lt;/strong&gt; The landing failed in its final moments due to low thrust on one of the first stage engines, caused by the exhaustion of its liquid oxygen fuel supply. That caused the engines to shut down early while the first stage was just above the drone's deck, causing a landing failure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-08-07T05:30:09.227" Title="Can Falcon 9 first stage landing final burn using one, two, and three engines? Tradeoffs?" Tags="|spacex|falcon-9|landing|abort|merlin-1d|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17664" PostTypeId="2" ParentId="17663" CreationDate="2016-08-07T02:06:42.710" Score="3" Body="&lt;p&gt;As you note in the question, for higher performance missions, they have tried more aggressive landing burns. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issue is, also as noted, they have thrust at lowest throttle, greater than the mass of the stage at landing. So hovering is out. Coming down gently is out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus they perform their 'hover slam' (They should trademark that!). That means they come down at a terminal velocity they chose (through aerodynamic effects) and then fire the engines as short a time as possible in order to reach 0 altitude at 0 velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even with one engine having more thrust than their mass at landing time, they still need to kill that terminal velocity. The faster they do it, the less fuel they spend doing it. The longer they take to brake the more energy is wasted fighting gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that the first burn, to slow down the forward motion and head back to base (LZ-1 or ASDS) is done with three engines, since they just spent 2 and half minutes getting up to speed with 9 engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next burn is designed to guide them through the atmosphere as things get hot, and coming in too fast will melt the stage. Finally the third burn is to actually land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On that second burn, they have tried three engine burns for shorter in order to save fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the landing burn they have started with three engine burns, then reduced down to one, since three engines is just way to much thrust at landing to meet their goal. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-07T02:06:42.710" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17665" PostTypeId="2" ParentId="17662" CreationDate="2016-08-07T02:20:05.853" Score="1" Body="&lt;p&gt;I am a bit cynical on the topic of ULA. They have been running what I consider a scam for a long time, and because there was no competition, we saw what happens in a monopoly situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ULA has used the claim of reliability and payload size in the past. They have not had a failure of a payload in over a decade and over a hundred launches. Thus for unique and expensive payloads, the suggest that the extra cost is worth it for the record of success in the past.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For very large payloads, the Delta 4 Heavy and Atlas 5 551 are the heaviest launchers, and more powerful than a Falcon 9 1.2. This will remain true until the Falcon Heavy becomes operational and has a few flights under its belt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper stage of a Falcon 9 is its weakest feature. It has a high thrust engine (almost 200,000 lbs of thrust), but not the greatest Isp (311s). Whereas ULA uses the Centaur upper stage on both Delta and Atlas launch vehicles. That uses one or two RL-10 engines, which while much lower thrust (25,000 lbs) has MUCH better Isp (450-465s) which makes a huge difference in an upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondarily the Falcon 9 upper stage cannot coast long periods and relight but a Centaur can. Thus GTO missions (Geo Sync Transfer orbit) where the payload does the extra work to get to GEO are possible, but Falcon cannot directly insert into GEO since it cannot coast and do the final burn the many hours later that is required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;People in the forums have run the numbers for a Falcon 9 with a more Centaur like upper stage and it gets much larger payload numbers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For interplanetary missions, the Isp difference matters a great deal in terms of what can be lofted to various destinations. (Not getting into a discussion of C3 here). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ULA has played these two cards heavily in the past.  For GPS III launches they did not even compete, since GPS III was basically an assembly line payload, and if one was lost, just build another one out of the insurance money.  Based on price alone they had no chance so they did not bid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that SpaceX has a history of 26 or more launches (Depending on when you read this) with only one well understood failure the reliability argument is going away. Some would argue that the re-usability and recover of the first stage will make them more reliable. That is, they have now seen what a stage looks like AFTER it launches and returns. This can only help refine the vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For special one off NRO missions, or maybe the James Webb Telescope, the reliability argument can be a powerful one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon Musk has tweeted that he would love to redesign the upper stage of the Falcon 9 but needs to focus on Mars first. It is good enough for almost every payload they wish to win, so not important to fix right now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When they have a working Raptor LOX/CH4 engine, then they may reconsider a smaller version for a Falcon 9 upper stage, since its improved Isp makes a big difference in performance.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-07T02:20:05.853" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17666" PostTypeId="2" ParentId="17663" CreationDate="2016-08-07T03:34:52.117" Score="4" Body="&lt;p&gt;As noted, the advantage to doing a higher thrust burn is that it takes less time and therefore uses less fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main advantage to a slower final approach is that it gives more time for the guidance system to correct the rocket's trajectory. Any guided landing procedure involves a cycle of estimating the accuracy of the current trajectory and steering to correct the error, and the more time allowed to make corrections, the better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX seems to have settled on using one engine for the very last part of the landing even on three-engine approaches; for symmetry reasons this more or less eliminates the possibility of a two-engine approach, because you'd have to change over from two outboard engines to the center engine in a fraction of a second, and engine start times are slightly variable. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-08-07T05:30:09.227" LastActivityDate="2016-08-07T05:30:09.227" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17667" PostTypeId="1" CreationDate="2016-08-07T06:34:37.573" Score="1" ViewCount="88" Body="&lt;p&gt;I have gone through the slingshot for the Juno with the earth.&#xA;I have also seen the train and ball example as given by NASA.  [&lt;a href=&quot;http://solarsystem.nasa.gov/basics/grav/primer.php]&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://solarsystem.nasa.gov/basics/grav/primer.php]&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, I am still unable to visualize and think on how it is working. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My assumption and thinking:-&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, earth gravity will be more when the Juno is close to it and the vehicle have to maintain its constant speed [not more and not less, since it will go out of the track or it will fall in the earth]. So it is doing a elliptical rotational around the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Original Juno Path: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/xX0MS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xX0MS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, we are starting to slow the speed at the point &quot;A&quot; and juno will travel along with the earth, due to gravity, upto point &quot;B&quot;.  Lets consider it is going to acquire the speed of some &quot;5 kms&quot;. And while it reaches the point &quot;B&quot;, we are going to apply extra speed by turning on the main ignition, to escape from the earths' gravitational orbit and to follow back the juno's orbit and we are going to turn off the main engine back and allow the juno, to travel with its original speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;A&quot; and &quot;B&quot; points, just marked approximately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since, juno crossed the earths' gravitational pull, [and no more earth's gravity is applicable] the juno will travel with the extra gained speed in some straight line/or its own elliptical orbit, and since in space there is no resistance to reduce its speed so it will not lose the speed unless some thing is happening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what I am assuming. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is :- &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;In the original image, the earth is not in the center of the elliptical path of the juno.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;My Image Path:- &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dlwiW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dlwiW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Regret for the bad drawing skill. Lets assume the earth be in the center of the juno's slingshot travel.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;If the earth's flyby is providing extra speed to the vehicle, why the juno is following its own trajactory path. It can loop the earth continuously as much as possible, so it will travel with more speed like more than &quot;5kms&quot; and after some time, we can turn on the main ignition to escape from the earth gravitational pull. Right.. Why they have decided the juno's some thing to be different?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is there any reason why, we have to maintain some distance between the surface of the earth and the juno. I mean, the elliptical path seems to be away from the earths atmosphere atleast by some 100kms. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is there any reason/advantage why the juno have to follow some thing like the perigee and apogee position as the earth is doing with the sun or the moon is doing with the earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Could somebody one, kindly correct me/my assumption and the thought, if wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks.&lt;/p&gt;&#xA;" OwnerUserId="15807" LastActivityDate="2016-08-07T06:34:37.573" Title="How slingshot is used to increase the speed of Juno" Tags="|jupiter|gravity-assist|juno|earth|" AnswerCount="0" CommentCount="4" ClosedDate="2016-08-08T15:09:58.420" ContentLicense="CC BY-SA 3.0" />
  <row Id="17668" PostTypeId="2" ParentId="17610" CreationDate="2016-08-07T08:28:21.110" Score="8" Body="&lt;p&gt;From a first principles point of view, to move around on a rough surface with a reliable robotic vehicle for extended periods of time, it's probably unavoidable that you need to expend energy. Walking 10km on gravel is more work than a flat surface because the gravel moves and dissipates energy. Walking 10km over rocks that don't move still uses energy because you have to go up and down doing work against gravity and we don't recover that energy.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zlBS5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zlBS5.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Plot of some randomly selected potentially interesting solar system bodies. Horizontal axis: surface gravity (somewhat related to energy needed to move around) as a ratio to that on Mars. Vertical axis: Approximate sunlight intensity as a ratio to that on Mars - as estimated by ratios of semi-major axis to the -2 power. Venus is listed twice - at the top of the atmosphere where aircraft such as robot balloons and robot planes can collect substantial amounts of light, and the surface where only a few percent of the redder parts of sunlight reach.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DGlXc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DGlXc.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Page 6 from &lt;a href=&quot;https://spaceflightsystems.grc.nasa.gov/SSPO/SP/VenusUpper/Presentations/landis_Venus_aircraft-2013.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus Aircraft - design evolution 2000-2008, Geoffrey A. Landis, NASA John Glenn Research Center&lt;/a&gt;. Above 50km, there is more sunlight available than there is on earth - closer to the sun, and possibility to collect reflected light from below (as does the ISS around Earth) to make up for some cloud cover.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GVuOT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GVuOT.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Page 32 from &lt;a href=&quot;https://spaceflightsystems.grc.nasa.gov/SSPO/SP/VenusUpper/Presentations/landis_Venus_aircraft-2013.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus Aircraft - design evolution 2000-2008, Geoffrey A. Landis, NASA John Glenn Research Center&lt;/a&gt;. The very dense atmosphere makes powered flight very attractive (ballooning as well). However, flying as fast as the wind would be energetically challenging at most altitudes.&lt;/p&gt;&#xA;&lt;p&gt;Robotic Areal Vehicles may be possible future missions for Venus' atmosphere. It's a long reach, but things taking inspiration from the &lt;a href=&quot;https://aviation.stackexchange.com/a/29427/14213&quot;&gt;Festo Air Penguin discussed at length in this answer&lt;/a&gt; and shown below, and the Festo Air Ray (not shown) might be possible.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/jPGgl5VH5go?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://www.youtube.com/watch?v=jPGgl5VH5go&quot; rel=&quot;nofollow noreferrer&quot;&gt;Festo Air Penguin&lt;/a&gt; discussed more &lt;a href=&quot;https://www.youtube.com/watch?v=jPGgl5VH5go&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/dv6zQr1_C9Q?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://www.youtube.com/watch?v=dv6zQr1_C9Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;Festo Robot Balloon&lt;/a&gt; delivering a bottle of water on demand. This would be much more difficult on Venus for a number of reasons, but the higher atmospheric density means balloons could carry a significantly heavier payload, and it wouldn't be necessary to use Helium for buoyancy.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/nj1yhz5io20?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://www.youtube.com/watch?v=nj1yhz5io20&quot; rel=&quot;nofollow noreferrer&quot;&gt;Festo Bionicopter&lt;/a&gt; could take advantage of the denser atmosphere on Venus. It could also make use of some legs as well!&lt;/p&gt;&#xA;&lt;p&gt;More about the Vega program using robot balloons on Venus in &lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, in &lt;a href=&quot;http://www.wired.com/2013/04/vega-venus-rain/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wired&lt;/a&gt;, and in &lt;a href=&quot;https://www.newscientist.com/article/dn16797-nasa-may-send-fleet-of-spacecraft-to-venus/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The New Scientist&lt;/a&gt;, and future possibilities with &lt;a href=&quot;http://www.lpi.usra.edu/vexag/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Venus Exploration Group (VEXAG)&lt;/a&gt; and &lt;a href=&quot;http://www.univie.ac.at/EVE/index.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA's European Venus Explorer (EVE)&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Wheels have served humans well over thousands of years. Through zillions of km of trial and error as well as amazing engineering, they’ve solved mobility problems for humans here on earth and on several other solar system bodies.&lt;/p&gt;&#xA;&lt;p&gt;By far the largest body of detailed &lt;em&gt;experience, imagery and metrology of wheel performance&lt;/em&gt; on off-world robotic vehicles comes from the three rovers on Mars.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DPnED.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DPnED.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Comparison of &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_rover#/media/File:H_rover-comp_wheels_02.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Rover Wheels&lt;/a&gt;. Left: &lt;a href=&quot;https://en.wikipedia.org/wiki/Sojourner_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sojourner&lt;/a&gt; of &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Pathfinder&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Pathfinder mission&lt;/a&gt;. Center: &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Exploration_Rover&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Exploration Rovers (MER)&lt;/a&gt; (&lt;a href=&quot;https://en.wikipedia.org/wiki/Spirit_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spirit&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Opportunity&lt;/a&gt;). Right: &lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity&lt;/a&gt; of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Science_Laboratory&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Science Laboratory (MSL)&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/abvYR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/abvYR.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Curiosity_Self-Portrait_at_%27Big_Sky%27_Drilling_Site.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity Self Portrait at Big Sky Drilling Site&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LdixC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LdixC.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Detail cropped from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Curiosity_Self-Portrait_at_%27Big_Sky%27_Drilling_Site.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity Self Portrait at Big Sky Drilling Site&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kw3Zn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kw3Zn.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/9/9d/MarsCuriosityRover-LocationMap-Sol-1243-20160204.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Location map - Curiosity rover at the base of Mount Sharp - as viewed from Space (MRO; HiRISE; February 4, 2016/Sol 1243)&lt;/a&gt;.&amp;quot; You have to open this into a separate window and zoom in to see the trail details, starting in the right side of the upper edge. Note that the path is chosen as a compromise between science and where the wheels are judged safe enough to go without getting stuck or damaging the vehicle.&lt;/p&gt;&#xA;&lt;p&gt;One of the jobs of Curiosity's Mobility System is to cary a large package of &lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_(rover)#Instruments&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity's Scientific Instruments&lt;/a&gt; over large distances so that information can be collected from a wide variety of locations.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZaU7P.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZaU7P.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_(rover)#/media/File:MarsCuriosityRover-Drilling-01.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity Robotic Arm&lt;/a&gt; applying a drill to Martian rock. Samples are then collected and transported to locations inside Curiosity for further analysis using a variety of analytical equipment.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vyxhz.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vyxhz.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Curiosity still &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Msl-arm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;...inside the Spacecraft Assembly Facility at NASA's Jet Propulsion Laboratory, Pasadena, Calif.&lt;/a&gt;. Even in Mars's surface gravity only &lt;span class=&quot;math-container&quot;&gt;$\frac{3}{8}$&lt;/span&gt; of Earth's, all of this scientific equipment together has gotta be pretty heavy! &lt;strong&gt;The robot arm is often forgotten because it doesn't show up in many of Curiosity's selfies&lt;/strong&gt; in the same way your hand or the business-end of your selfie stick don't show up. But if you look close in the Big Sky Drilling Site &amp;quot;selfie&amp;quot; a few images above - you can see it's shadow on the surface!!&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;While Boston Dynamics' &lt;a href=&quot;https://www.youtube.com/watch?v=cNZPRsrwumQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;Big Dog&lt;/a&gt; ran on fossil fuel for various reasons (see &lt;a href=&quot;http://www.bostondynamics.com/img/BigDog_IFAC_Apr-8-2008.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Boston Dynamics conference proceeding PDF&lt;/a&gt;) including power density and demonstrations for particular non-science &amp;quot;missions&amp;quot;,  &lt;a href=&quot;https://www.youtube.com/watch?v=M8YjvHYbZ9w&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spot&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/watch?v=tf7IEVTDjng&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpotMini&lt;/a&gt; (shown in the question), and &lt;a href=&quot;http://www.bostondynamics.com/robot_littledog.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;LittleDog&lt;/a&gt; are electrically powered, and LittleDog seems to be built with off-world use in mind, or at least in the back of the mind.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;LittleDog has four legs, each powered by three electric motors. The legs have a large range of motion. The robot is strong enough for climbing and dynamic locomotion gaits. The onboard PC-level computer does sensing, actuator control and communications. LittleDog's sensors measure joint angles, motor currents, body orientation and foot/ground contact. Control programs access the robot through the Boston Dynamics Robot API. Onboard lithium polymer batteries allow for 30 minutes of continuous operation without recharging. Wireless communications and data logging support remote operation and data analysis. LittleDog development is funded by the DARPA Information Processing Technology Office.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s5zfe.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s5zfe.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Little Dog cut-away illustration from &lt;a href=&quot;http://www.bostondynamics.com/robot_littledog.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Boston Dynamics&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/UIipbi0cAVE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Video of &lt;a href=&quot;http://www.bostondynamics.com/robot_littledog.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Little Dog&lt;/a&gt; climbing over terrain from &lt;a href=&quot;https://www.youtube.com/watch?v=UIipbi0cAVE&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/M8YjvHYbZ9w?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Video of Boston Dynamics' battery-powered &lt;a href=&quot;https://www.youtube.com/watch?v=M8YjvHYbZ9w&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spot&lt;/a&gt; climbing over terrain and getting along well with &amp;quot;Hockey Stick Guy&amp;quot; (&lt;a href=&quot;https://www.youtube.com/watch?v=lSCll4ePB_M&quot; rel=&quot;nofollow noreferrer&quot;&gt;here with commentary on YouTube&lt;/a&gt; and in &lt;a href=&quot;http://www.wired.com/2016/02/boston-dynamics-new-robot-wicked-good-getting-bullied/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wired&lt;/a&gt;) despite getting kicked by him.&lt;/p&gt;&#xA;&lt;p&gt;Presumably, a main robot rover may also cary one or several more highly mobile rovers for sample collection. Much in the same way that Curiosity's robotic arm can collect samples and transport them to the &amp;quot;laboratory&amp;quot; inside Curiosity, mini-rovers may be able climb up on to, &lt;strong&gt;as well as down into&lt;/strong&gt; hard-to-reach places for measurements, imagery, mapping (via telemetry or data transfer upon return) as well as some kinds of sample collection. While drilling requires force and Curiosity uses mass and leverage, a clever robot could find leverage between rocks or walls, possibly even moving rocks around to improve the situation.&lt;/p&gt;&#xA;&lt;p&gt;These guys look like they are ready to go anywhere in the solar system!&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/6b4ZZQkcNEo?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Boston Dynamic's &lt;a href=&quot;http://www.bostondynamics.com/robot_sandflea.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sand Flea&lt;/a&gt; launching itself all over the place! Now imagine this happening on a low surface gravity body. Suborbital (except in extreme cases like comets or small asteroids) but it is point-A to point-B transportation. Needs robust electronics and sensors to avoid getting a headache, but possibly fine for sample collection and scouting.&lt;/p&gt;&#xA;&lt;p&gt;Currently it uses stored compressed gas for multiple jumps (see below). Some interesting ideas could be imagined to make the gas rechargeable from an atmosphere, or replaced with an electromagnetic linear motor (tiny captive rail-gun-like thing).&lt;/p&gt;&#xA;&lt;p&gt;The following is from the &lt;a href=&quot;http://www.bostondynamics.com/img/SandFlea%20Datasheet%20v1_0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sand Flea Datasheet&lt;/a&gt; (remember, the specs are for Earth surface gravity!):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SandFlea is a small robot with remarkable mobility. The 11 lb robot drives like a traditional wheeled vehicle on mild terrain, but jumps up to 8m high over difficult terrain. It can jump 25 times using the piston actuator and onboard fuel supply. Jumps of 1-8 m heights are user selectable. Specially designed wheels cushion the shock of landing. Flight and landing attitude of the robot are automatically controlled by an onboard stability system.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Controllable hop height, 1-8 m&lt;/li&gt;&#xA;&lt;li&gt;Controllable launch angle&lt;/li&gt;&#xA;&lt;li&gt;Precision hops through windows or doors, on to tables, up staircases, on to or off of roofs or balconies&lt;/li&gt;&#xA;&lt;li&gt;Piston actuator&lt;/li&gt;&#xA;&lt;li&gt;Laser-based ranging to guide launch&lt;/li&gt;&#xA;&lt;li&gt;Operator control unit (OCU) with live video feed for remote operation&lt;/li&gt;&#xA;&lt;li&gt;Robot and OCU both fit into a small backpack&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TWbNs.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TWbNs.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Boston Dynamics &lt;a href=&quot;http://www.bostondynamics.com/robot_sandflea.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;says&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The robot uses gyro stabilization to stay level during flight, to provide a clear view from the onboard camera, and to ensure a smooth landing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ISznqY3kESI?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Boston Dynamic's &lt;a href=&quot;http://www.bostondynamics.com/robot_rhex.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;RHex &lt;/a&gt; going all over the place - looking for water perhaps?&lt;/p&gt;&#xA;&lt;p&gt;However, they would either need tiny RTGs of their own, solar panels of their own, or have to be charged and then recharged by the main robot. This can be done by contact, or through &lt;a href=&quot;https://arxiv.org/pdf/1509.00940.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;highly resonant inductive charging&lt;/a&gt; - which can actually cover a significant gap of a few meters in a pinch - or just optical charging - laser to special photovoltaics like this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/8hhv9Cu98us?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; quadcopter illustrative example of a small vehicle receiving power from a beam of light.  &lt;strong&gt;Note:&lt;/strong&gt; an aperture of 5 centimeters can &amp;quot;beam&amp;quot; power over many kilometers if atmospheric effects are minimal and motion is minimal.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-08-07T16:54:09.927" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="17669" PostTypeId="1" AcceptedAnswerId="33546" CreationDate="2016-08-07T10:30:31.627" Score="8" ViewCount="1413" Body="&lt;p&gt;&quot;In February 2012, the Chinese government released a complete lunar map constructed from Chang'e 2's data, claiming that it was the highest-resolution map of the entire Moon yet recorded.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_2#Lunar_mission&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Chang%27e_2#Lunar_mission&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where/how do I view this map?&lt;/p&gt;&#xA;" OwnerUserId="15719" LastEditorUserId="15719" LastEditDate="2016-08-07T11:58:38.277" LastActivityDate="2021-04-12T02:42:46.430" Title="Where/how do I view the Chang'e 2 lunar map?" Tags="|the-moon|mapping|chang-e|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17670" PostTypeId="1" AcceptedAnswerId="17671" CreationDate="2016-08-07T11:01:57.730" Score="46" ViewCount="5767" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/17660/12742&quot;&gt;this answer&lt;/a&gt;, it's mentioned that waste $CO_2$ and $H_2$ from the Environmental Control and Life Support Systems (ECLSS) are vented into space from ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there no other uses for this material now or possibly in the future?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, wouldn't it be in the best interest to keep any and all mass local to ISS (or another spacecraft) &quot;just in case&quot; it could be useful? Getting stuff there is expensive, and $H_2$ could be used for fuel I would think.&lt;/p&gt;&#xA;" OwnerUserId="12742" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-01-01T13:45:12.567" Title="Why vent $CO_2$ and $H_2$ waste products to space on ISS?" Tags="|iss|life-support|mass|waste|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17671" PostTypeId="2" ParentId="17670" CreationDate="2016-08-07T13:53:53.950" Score="68" Body="&lt;p&gt;Added complexity is never welcome in a situation where lives are on the line and help isn't a phone call away. There have been a number of ammonia-related events and those are in systems that should theoretically be a lot more reliable and well characterized than a new CO&lt;sub&gt;2&lt;/sub&gt; or H&lt;sub&gt;2&lt;/sub&gt; storage system. If you polled ISS astronauts past and present - basically anyone who's had to think long and hard about the immediacy of technologies effect on life, I'd say the would be mostly thumbs down for keeping an &quot;extra&quot; poisonous gas and explosive gas around &quot;just in case it might come in handy some day.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;HYDROGEN&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To keep a usefully large amount of hydrogen, you'd have to liquefy it. A hydrogen refrigerator may end up being somewhat complex and have moving parts and make vibrations. A cryogenic refrigerator is not unheard of in a spacecraft (infrared detectors often need to be cooled). LH&lt;sub&gt;2&lt;/sub&gt; has to be really really cold to store it without a high pressure (= dangerous and heavy) tank, around 20&amp;nbsp;K.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You do have 35-45 minutes of relatively good exposure to the cold of space on every orbit. All you'd have to do is invent a new kind of passive refrigerator technology that could take advantage of it using some kind of door that opens and closes, then insulate it from the rest of the station and for the almost 1.5 kilowatts of sunlight per square meter the other 45 minutes of each orbit, then fix it every time it breaks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping hydrogen around for a long time has issues with making various metals brittle and can lead to failures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrogen_safety&quot; rel=&quot;noreferrer&quot;&gt;Hydrogen Safety&lt;/a&gt;, hydrogen can be &lt;strong&gt;explosive&lt;/strong&gt; and concentrations of as little as 4% to as much as 75% in air are flammable. A tiny, low energy spark from a static discharge can ignite it - it's about ten times less energy required than for an explosive mixture of gasoline and air. Hydrogen flames are nearly invisible to humans (there is UV so there are hydrogen flame detectors based on that).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are arguments against the storage of hydrogen for use as a propellant in &lt;a href=&quot;https://space.stackexchange.com/q/17534/12102&quot;&gt;answers to this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrogen can diffuse through many materials and attack some of them as well. Elon Musk says it is &quot;&lt;a href=&quot;https://www.youtube.com/watch?v=Y_e7rA4fBAo&quot; rel=&quot;noreferrer&quot;&gt;pernicious&lt;/a&gt;&quot; (10:20 to 12:40 for the entire discussion on hydrogen powered cars).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using LH&lt;sub&gt;2&lt;/sub&gt; for regular burns to maintain ISS altitude would require somehow adding a new set of engines to the ISS , or transferring the hydrogen (and LOX from somewhere) to a docked spacecraft which could be refueled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speaking of LOX - there isn't any on the ISS. In &lt;a href=&quot;https://space.stackexchange.com/a/17677/12102&quot;&gt;this answer&lt;/a&gt; it is explained that oxygen is brought to the ISS in very high pressure gaseous form, and stored as a high pressure gas. To develop significant thrust, it is usually necessary to deliver oxygen and hydrogen in liquid form to get enough mass per second into a small enough reaction volume. So a system to produce and store LOX would also be necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;CARBON DIOXIDE&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CO&lt;sub&gt;2&lt;/sub&gt; is toxic, and even if it doesn't kill you right away, the coughing reflex it triggers can make it more difficult to handle a CO&lt;sub&gt;2&lt;/sub&gt; leak emergency (until you get a breathing apparatus). It can be stored at relatively low pressure and not so cold temperature if reduced to a solid (dry ice), as can be seen in the phase diagram below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's hard to imagine how it would come in handy, unless you are growing so many plants that they are depleting all of the CO&lt;sub&gt;2&lt;/sub&gt; produced by humans. However, if everyone leaves and there are &lt;a href=&quot;https://en.wikipedia.org/wiki/Silent_Running&quot; rel=&quot;noreferrer&quot;&gt;robots taking care of the plants&lt;/a&gt;, a little extra CO&lt;sub&gt;2&lt;/sub&gt; might be helpful - depending on what the robots are doing with the plants after harvesting them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is an interesting - if academic - question; could waste CO&lt;sub&gt;2&lt;/sub&gt; be cooled by exposure to space to form dry ice, then be stored as potential propellant for a &lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket&quot; rel=&quot;noreferrer&quot;&gt;VASIMR&lt;/a&gt; or other ion engine for maintaining altitude. While it is a heavy molecule as a gas, it may be a nightmare in an ion engine. I think carbon would be the last thing you want inside a maintenance-free system with lots of high voltages, and it may be hard to ionize, and maintain ionization since you'd probably end up with both positive and negative ions desperately struggling to recombine with each other, and half of them would end up moving in wrong directions and cause further trouble.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;CONCLUSION:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These gases may not have any practical use, especially without new kinds of technologies and more mass. However, they pose several immediate and continuous risks to the safety of the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nXVH5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nXVH5.png&quot; alt=&quot;Enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Above:&lt;/strong&gt; Phase diagram of CO&lt;sub&gt;2&lt;/sub&gt; from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Carbon_dioxide_pressure-temperature_phase_diagram.svg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-08-08T07:51:19.923" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17673" PostTypeId="2" ParentId="17661" CreationDate="2016-08-07T14:13:43.720" Score="3" Body="&lt;p&gt;This is a well-known phenomenon in helicopter dynamics and in control systems. In a second order control system (which roughly describes your low-thrust scheme), the phase angle change is 90° when the input control frequency is the same as the system's natural frequency. (See any elementary control text and look at the second order system frequency response.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are proving the force at orbital frequency. Very similar to a helicopter where pitch changes to the rotor blades are input 90° ahead. You can think of the pitch change as an inclination change of the rotor plane.&lt;/p&gt;&#xA;" OwnerUserId="16530" LastEditorUserId="6816" LastEditDate="2016-08-07T14:24:12.933" LastActivityDate="2016-08-07T14:24:12.933" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17674" PostTypeId="1" AcceptedAnswerId="17677" CreationDate="2016-08-07T14:54:19.437" Score="6" ViewCount="1854" Body="&lt;p&gt;One can transport and store oxygen more efficiently (volume and weight of tank) if it is liquid under the order of 1 atmosphere. &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_oxygen&quot; rel=&quot;nofollow noreferrer&quot;&gt;Liquid oxygen&lt;/a&gt; (LOX) is kept cryogenic in insulated tanks for short periods of time as a propellant in rockets, and for weeks just about anywhere in well insulated but heavier tanks (see below). I believe oxygen is transported from earth to the ISS as LOX, as suggested in &lt;a href=&quot;https://space.stackexchange.com/q/17649/12102&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the ISS is there a (at least some) supply of oxygen that is stored as LOX? Is the &quot;cold of space&quot; when the sun isn't shining ever used to keep it cold, or just good insulation and a refrigerator? Writing &lt;a href=&quot;https://space.stackexchange.com/a/17671/12102&quot;&gt;this answer&lt;/a&gt; got me thinking about this. LOX boils at about 54K at 1 atmosphere. If heat leaks into the tank through insulation or the liquid access connection or boil-off vent, it is removed by the boil-off process. However if you need to make it last a long time, supplemental cooling might be necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Portable_container_for_liquid_oxygen_DSC06216.JPG&quot; rel=&quot;nofollow noreferrer&quot;&gt;Portable container for transport of liquid oxygen&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zJkOq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zJkOq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-05-18T00:15:17.040" LastActivityDate="2019-05-18T00:15:17.040" Title="Supply of liquid oxygen (LOX) maintained on the ISS? Kept cold using &quot;space&quot;, or refrigerator?" Tags="|iss|oxygen|lox|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="17675" PostTypeId="1" AcceptedAnswerId="18755" CreationDate="2016-08-07T16:04:43.020" Score="7" ViewCount="484" Body="&lt;p&gt;Rover missions are hard - you need to put them down somewhere where they can safely drive around and measure (hopefully a variety of) interesting things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Future rovers may have legs as discussed in &lt;a href=&quot;https://space.stackexchange.com/a/17668/12102&quot;&gt;this answer&lt;/a&gt; and the ability of various legged motilities may open access to much more territory. I'm curious roughly how much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the various Mars rover missions, I am guessing that maps were made of &quot;interesting&quot; areas and &quot;drivable&quot; areas, and then they were overlaid for simultaneous compatibility, though I am sure it was a far more complex process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any kind of rough estimates of what areas of Mars would be relatively safe to land a rover so that it could do a substantial amount of driving, and which of those areas, either due to boulders or sand or inclines or other reasons not so safe? Is there anything like a &quot;rover-friendly region map&quot; estimated for Mars available somewhere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another similar question has been posed for the moon: &lt;a href=&quot;https://space.stackexchange.com/q/19932/26446&quot;&gt;Are there &amp;quot;rover-friendly region maps&amp;quot; for the Moon?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="26446" LastEditDate="2019-08-10T03:27:43.903" LastActivityDate="2022-05-20T18:21:16.343" Title="Are there &quot;rover-friendly region maps&quot; for Mars?" Tags="|mars|rovers|curiosity|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="17676" PostTypeId="2" ParentId="17661" CreationDate="2016-08-07T19:14:08.707" Score="10" Body="&lt;p&gt;I found this kind of counterintuitive as well at first. This is the way I rationalized it to myself: consider a high-thrust maneuver to change inclination. Obviously if it's impulsive, you perform it at the node. If it lasts for, say, one minute, you would burn +/- 30 seconds around the node. Now, take that to the limit, where the burn time is the entire orbit. The burn gets &quot;smeared&quot; around half of the orbit, but it's still centered on the node.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Put another way, your goal is to change the cross-track component of the velocity vector. That component changes sign 90 degrees from the nodes, so your thrust should flip at those points as well.&lt;/p&gt;&#xA;" OwnerUserId="9060" LastActivityDate="2016-08-07T19:14:08.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17677" PostTypeId="2" ParentId="17674" CreationDate="2016-08-07T19:34:37.193" Score="7" Body="&lt;p&gt;Hard to prove a negative, but no, there is no provision for storing LOX on the ISS.  The coldest thing up there is the &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/MELFI.html&quot; rel=&quot;noreferrer&quot;&gt;MELFI&lt;/a&gt;, the Minus Eighty Degree Laboratory Freezer (for) ISS.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oxygen (and nitrogen) is stored as a high pressure gas in the tanks fitted to the outside of the US airlock. (The dog-house shaped objects around the airlock in the picture below).  These tanks were brought up full by the shuttle, for example in this writeup from the &lt;a href=&quot;https://www.nasa.gov/pdf/397216main_sts129_presskit.pdf&quot; rel=&quot;noreferrer&quot;&gt;STS-129 press kit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...a   High Pressure Gas Tank (HPGT) full of oxygen  from  ELC-2  to  a &#xA;  spot  on  the  outside  of  the   Quest  airlock,  placing  it  amidst&#xA;  the  other  dog   house-shaped  tanks  that  are  used  to  replenish &#xA;  atmosphere  lost  when  spacewalkers  enter  and   exit  the  station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(ELC-2 was an External Logistics Carrier)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IwxAZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IwxAZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that the shuttle logistics path isn't available they have to use &lt;a href=&quot;https://www.nasa.gov/content/air-supply-high-pressure-tanks-ready-for-space-station&quot; rel=&quot;noreferrer&quot;&gt;NORS&lt;/a&gt; instead.  The NORS tanks are internal to the station.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-08-07T19:52:19.873" LastActivityDate="2016-08-07T19:52:19.873" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17678" PostTypeId="1" AcceptedAnswerId="17680" CreationDate="2016-08-08T02:39:10.363" Score="0" ViewCount="590" Body="&lt;p&gt;I just wrote the following paragraph as an edit to &lt;a href=&quot;https://space.stackexchange.com/a/17671/12102&quot;&gt;this answer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Using LH&lt;sub&gt;2&lt;/sub&gt; for regular burns to maintain ISS altitude would require a whole new set of engines that may not yet exist anywhere.  While LH&lt;sub&gt;2&lt;/sub&gt; is one of the main workhorses getting big things into space from a launchpad (e.g. Saturn V except 1st stage, Space Shuttle) it's not usually kept stored for long periods, and engines designed for long term usage and multiple starts over time without maintenance would be a technical challenge. Engines that use cryogenic fuels are substantially more complex to build, operate, and maintain, as they have components that require pre-ignition cool downs. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So you can see that &lt;em&gt;I have a hunch&lt;/em&gt; that the answers for &quot;&lt;em&gt;Have reliable, restartable LH2/LOX ever existed? - ever used?&lt;/em&gt;&quot; are probably &quot;No&quot; and &quot;No&quot;, that LH2/LOX is only fueled at launch time and ignited once,&#xA;&lt;em&gt;but I really don't know&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there ever been an effort to build or test a restartable LH2/LOX engine? Even one restart? Here restart means in space, not in a test rig or launch pad on Earth.  &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-08-08T16:38:20.383" Title="Have reliable, restartable LH2/LOX engines ever existed? - ever used?" Tags="|engines|hydrogen|restartable-engine|lox|" AnswerCount="2" CommentCount="8" ClosedDate="2016-08-08T16:38:36.727" ContentLicense="CC BY-SA 3.0" />
  <row Id="17679" PostTypeId="1" CreationDate="2016-08-08T03:17:54.620" Score="6" ViewCount="14633" Body="&lt;p&gt;A new spaceport is proposed for Camden County, Georgia (USA). The latitude is 30.56N and the only available launch azimuth is 123 degrees.  What is the resulting LEO without mid-course maneuvers? Can show me how you calculate this?&lt;/p&gt;&#xA;" OwnerUserId="12814" LastEditorUserId="8693" LastEditDate="2016-08-08T21:29:30.527" LastActivityDate="2021-06-17T10:00:04.870" Title="How to calculate Orbital Inclination knowing the launch azimuth and launch latitude?" Tags="|orbital-mechanics|orbit|inclination-change|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17680" PostTypeId="2" ParentId="17678" CreationDate="2016-08-08T03:20:00.503" Score="2" Body="&lt;p&gt;Both the J-2 engine used on the Saturn V and the RL-10 used on the Centaur and DCSS upper stages are hydrogen-fueled and restartable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V third stage burned once to finish insertion into low Earth orbit, once to send the Apollo&#xA;spacecraft on its way to the moon, and once for its final disposition, either to hit the moon or to go into solar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, the Centaur upper stage has been used frequently to insert satellites into geosynchronous orbits and many interplanetary missions. On multi-satellite missions, Centaur RL-10s have been started more than 7 times, and Rocketdyne is investigating a further development of the RL-10 called CECE which is intended to have 50-start capability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As discussed in this &lt;a href=&quot;https://space.stackexchange.com/questions/15403/why-is-tea-teb-chemical-ignition-used-instead-of-spark-ignition&quot;&gt;Q&amp;amp;A&lt;/a&gt;, hydrogen engines are generally easier to restart than kerosene; they can be started with an electrical spark igniter instead of a more-involved liquid-hypergolic or solid ignition charge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Japan's LE-5, Russia's RD-0146, and China's YF-75D are all upper stage, restartable hydrogen engines in the same general class as the RL-10.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-08-08T15:27:54.210" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17681" PostTypeId="2" ParentId="17678" CreationDate="2016-08-08T03:20:27.237" Score="1" Body="&lt;p&gt;There may be others, but at least the Saturn 5's J2 engines have a operational history of inflight restarts (from &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_J-2&quot; rel=&quot;nofollow&quot;&gt;wikipedia&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Unlike most liquid-fuelled rocket engines in service at the time, the&#xA;  J-2 was designed to be restarted once after shutdown when flown on the&#xA;  Saturn V S-IVB third stage. The first burn, lasting about two minutes,&#xA;  placed the Apollo spacecraft into a low Earth parking orbit. After the&#xA;  crew verified that the spacecraft was operating nominally, the J-2 was&#xA;  re-ignited for translunar injection, a 6.5 minute burn which&#xA;  accelerated the vehicle to a course for the Moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;as did the RL10 engines on various Centaur upper stages&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The  RL10  is  capable  of  multiple restarts  in  space.  In  fact,&#xA;  the  engine  was  started  seven  times  in  one  mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;https://www.asme.org/getmedia/e04882e6-5b54-404f-b634-f7e4d4494067/36-RL-10-Rocket-Engine.aspx&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-08-08T03:20:27.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17682" PostTypeId="2" ParentId="17679" CreationDate="2016-08-08T05:50:28.490" Score="-4" Body="&lt;p&gt;Orbital inclination will be equal to the latitude. The reason is simple - you just need that orbit plane will pass through the center of the Earth and your launch point&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See explanation here: &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/jason-davis/3450.html&quot; rel=&quot;nofollow&quot;&gt;Of inclinations and azimuths&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11046" LastEditorUserId="11046" LastEditDate="2016-08-08T05:58:28.570" LastActivityDate="2016-08-08T05:58:28.570" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17684" PostTypeId="1" CreationDate="2016-08-08T08:36:56.257" Score="4" ViewCount="576" Body="&lt;p&gt;We have seen here the excellent answer to the question: &lt;a href=&quot;https://space.stackexchange.com/questions/17629/how-would-the-saturn-v-have-differed-if-the-first-stage-was-also-lh2-lox&quot;&gt;How would the Saturn V have differed if the first stage was also LH2/LOX?&lt;/a&gt;&#xA;What if liquid methane would be used instead of kerosene?&#xA;Would the tank for methane be smaller than a tank for LH2, but larger than a tank for kerosene?&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-10-11T13:03:38.470" Title="How would the Saturn V have differed if the first stage was using liquid methane and LOX?" Tags="|saturn-v|kerosene|methane|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17685" PostTypeId="2" ParentId="17679" CreationDate="2016-08-08T12:24:43.120" Score="13" Body="&lt;p&gt;&lt;a href=&quot;http://www.orbiterwiki.org/wiki/Launch_Azimuth&quot; rel=&quot;noreferrer&quot;&gt;It's just a matter of some spherical trig.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\cos(inclination) = \cos(lat)*\sin(azimuth)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in your example, the inclination would be equal to:&#xA;$$&#xA;\arccos(\cos(30.56)*\sin(123)) = 43.77\deg&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To simplify the calculation, I assumed that your given azimuth was an inertial one.  The calculations that take into account the rotation of the Earth and what the compass onboard would measure as the azimuth are in the link above.&lt;/p&gt;&#xA;" OwnerUserId="15841" LastEditorUserId="15841" LastEditDate="2016-08-08T13:13:52.323" LastActivityDate="2016-08-08T13:13:52.323" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17686" PostTypeId="2" ParentId="17684" CreationDate="2016-08-08T14:08:27.837" Score="4" Body="&lt;p&gt;Methane/LOX specific impulse is only slightly better than kerosene/LOX and it's about 25% less dense. In this case I think stretching the first stage would be more likely than fattening it; furthermore it might be possible to use a common-bulkhead tank instead of the separate tanks of the S-IC, (the temperature differential between liquid methane and liquid oxygen being relatively modest) so the external stage dimensions might not change much at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Methane wasn't popular as a rocket fuel at the time, of course, so there wouldn't be any viable candidates for engines to power the stage.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-08-08T14:08:27.837" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17687" PostTypeId="1" AcceptedAnswerId="17688" CreationDate="2016-08-08T16:32:34.467" Score="7" ViewCount="758" Body="&lt;p&gt;The recent questions about restarting rocket engines in space made me wonder about this.  Every mission used the OMS a couple of times at least but rendezvous missions increased the usage. The record holder is probably some multiple rendezvous mission but I don't know a good source for this information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KEzTb.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KEzTb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The OMS engines were built by Aerojet and were a member of the &lt;a href=&quot;https://en.wikipedia.org/wiki/AJ10&quot; rel=&quot;noreferrer&quot;&gt;AJ10 engine family&lt;/a&gt;.  They will be used on the Orion service modules (not new ones, the original ones built in the 70s (!)) and will be discarded when the service module is jettisoned.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-08-29T23:45:58.723" LastActivityDate="2020-08-29T23:45:58.723" Title="What is the maximum number of OMS burns executed during a single Shuttle mission?" Tags="|space-shuttle|history|engines|orbital-maneuver|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17688" PostTypeId="2" ParentId="17687" CreationDate="2016-08-08T17:40:25.927" Score="4" Body="&lt;p&gt;After searching google for 'sts shuttle &quot;OMS-10&quot; burn', iterating with an increasing ID number, I think STS-39 is the record holder with some 16 OMS burns. (The burn count isn't precisely defined by the ID number, because OMS-1 is skipped on direct-insertion launches, and ascent-assist and deorbit burns aren't numbered.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, most of them are done with only one engine at a time; I don't know if you would count single versus double burns differently. STS-93 and STS-82 were also well above average and may have more individual OMS engine starts than -39.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://id.scribd.com/mobile/doc/52642908/STS-39-Space-Shuttle-Mission-Report&quot; rel=&quot;nofollow&quot;&gt;https://id.scribd.com/mobile/doc/52642908/STS-39-Space-Shuttle-Mission-Report&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-08-08T17:40:25.927" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17689" PostTypeId="2" ParentId="17679" CreationDate="2016-08-08T19:03:53.800" Score="5" Body="&lt;p&gt;To amplify on the correct answer of @M.A.H.:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To apply this &lt;em&gt;general&lt;/em&gt; spherical triangle (&lt;a href=&quot;https://en.wikipedia.org/wiki/Solution_of_triangles&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Solution_of_triangles&lt;/a&gt;) to the particular problem of orbital inclination:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OyXj0.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OyXj0.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Point $C$ is the geographical North Pole;  Point $B$ is the launch point, so the arc $C-B$ is the &lt;em&gt;co-latitude&lt;/em&gt; of the launch point;  $A$ is the point where the orbit track crosses the equator, so the arc $C-A$ is $90^o$ and the orbital inclination is $\alpha$.  Once the launch azimuth is related to the angle $\beta$ (depending on the exact geometry) the triangle can be solved for $\alpha$, and also $\gamma$, the longitude of the ascending node of the orbit.&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2016-08-08T19:03:53.800" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17691" PostTypeId="2" ParentId="16788" CreationDate="2016-08-08T20:32:50.963" Score="4" Body="&lt;p&gt;One thing SpaceX is working on that I haven't seen mentioned here yet is the landing struts. After their last failed attempt at a barge landing there were some comments about already having new landing struts in the works that would be able to survive the impact of the harder landing and would also be deploy-able earlier to provide more drag for passive speed reduction.&lt;/p&gt;&#xA;" OwnerUserId="16458" LastActivityDate="2016-08-08T20:32:50.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17692" PostTypeId="1" AcceptedAnswerId="17695" CreationDate="2016-08-08T21:05:36.840" Score="8" ViewCount="824" Body="&lt;p&gt;There's a few related questions about rocket self-destruct mechanisms here already, e.g.:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5775/how-are-self-destructs-implemented-in-rocketry&quot;&gt;How are self-destructs implemented in rocketry?&lt;/a&gt; (covers &lt;em&gt;how&lt;/em&gt; they work)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/923/do-all-launches-include-self-destruct-mechanisms&quot;&gt;Do all launches include self-destruct mechanisms?&lt;/a&gt; (covers &lt;em&gt;who&lt;/em&gt; uses them)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/14059/how-can-a-rockets-self-destruct-make-for-a-safer-situation-than-being-out-of-co&quot;&gt;How can a rocket&amp;#39;s self-destruct make for a safer situation than being out-of-control?&lt;/a&gt; (covers &lt;em&gt;why&lt;/em&gt; they exist)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In the control room, I imagine tension might be kind of high. I also imagine that folks are hoping for the best. So it seems like, in a high tension situation, if there were no guidelines, there is a possibility that the person with the self-destruct button might either press it too quickly in a knee-jerk reaction (e.g. in response to a minor but correctable problem), or the possibility that a person may wait too long (perhaps being unsure of their decision, or hoping that it is correctable, or perhaps it is a manned flight).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: What's the typical criteria for initiating a self-destruct on a rocket launch? What guidelines are usually in place that a trained operator can fall back on in a high pressure environment?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if the rocket starts to veer off course; is there a rule that says &quot;wait X seconds after noticing a problem&quot;? Things like that.&lt;/p&gt;&#xA;" OwnerUserId="4634" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-08-08T22:32:35.097" Title="What rocket launch protocols (if any) are in place to prevent premature or late intentional self-destruct?" Tags="|launch|rockets|failure|range-safety|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17695" PostTypeId="2" ParentId="17692" CreationDate="2016-08-08T22:15:16.310" Score="8" Body="&lt;p&gt;tl;dr The Range Safety Officer watches the vehicle trajectory to ensure that it stays within pre-determined trajectory limit lines.  If the vehicle strays outside of those lines, the Officer sends the destruct signal (or signals the crew to tell them to shut off the engines, in the case of the Shuttle second stage).  The inner lines are chosen so that if the vehicle is destroyed when they are crossed, the debris won't fall outside of the impact limit lines.  Obviously these lines are different for every vehicle and mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the shuttle, they could be directed to destroy the vehicle by code word transmission from Mission Control (see Wayne Hale's blog post below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the criteria for destruct at the Eastern Test Range (Florida)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;1.4.1.3  Control of Errant Vehicle Flight&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;a . Range Safety shall verify that all launch vehicles  launched  from  or  onto  the &#xA;  Ranges  have  a positive,  range-approved  method  of  controlling &#xA;  errant  vehicle  flight  to  meet  the  objective  of  minimizing&#xA;  risks to the general public and foreign countries.  NOTE:  Normally,&#xA;  control  systems  on  launch vehicles  using  the  Ranges  shall &#xA;  consist  of  an  airborne  Range  Safety  System  that  shall  meet &#xA;  all  the requirements  of  Chapters  2,  4,  and  7 of  this  document.  A  thrust  termination  system  may  be  considered  as  an &#xA;  alternative  to  a  Range  Safety  System; however,   quantification &#xA;  of   risks   must   be   determined,  and  the  requirements  in &#xA;  Chapter  2  shall  be met.  The  alternative  thrust  termination &#xA;  concept  and design shall be approved by the Range Commander. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;b. Range  Safety  shall  establish  flight  termination criteria  and &#xA;  Range  Safety  mission  flight  rules  to ensure  that  operations  do&#xA;  not  exceed  acceptable public safety limits. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;c .  Range  Safety &#xA;  shall  establish  and  control  Hazardous  Launch  Areas  and &#xA;  procedures  to  protect  the public  on  land,  on  the  sea,  and  in&#xA;  the  air  for  each launch  and  launch  vehicle   using  the  Ranges &#xA;  and  to ensure the following criteria are met:&lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;No intact launch vehicle, scheduled debris, or payload,  or  launch  vehicle  and  payload  subsystems shall be allowed to&#xA;  intentionally impact on land except in the launch area inside the&#xA;  impact limit lines. &lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;  &#xA;  &lt;p&gt;2 . Flight paths and trajectories shall be&#xA;  designed so  that  normal  impact  dispersion  areas  do  not  encompass land. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3 . Safety margins shall be used to avoid overly&#xA;  restrictive flight termination (destruct) limits. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;d .  Range  Safety &#xA;  may  allow  errant  launch  vehicles  to  fly  to  obtain  maximum &#xA;  data  until  they would present an unacceptable risk to the public or&#xA;  until   Range   Safety   can   no   longer   control   the launch&#xA;  vehicle&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Impact Limit Lines are defined as&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Impact limit line&#xA;   - a Hazardous Launch Area; the&#xA;  boundary within which trajectory constraints and&#xA;  FTSs are used to contain an errant launch vehicle&#xA;  and vehicle debris. Mission-essential and Wing-&#xA;  essential personnel are permitted within the ILLs;&#xA;  with Wing Commander approval, non-essential&#xA;  personnel may be permitted within this area. How-&#xA;  ever, the collective risk will not exceed acceptable&#xA;  standards for non-essential personnel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://snebulos.mit.edu/projects/reference/NASA-Generic/EWR/99ewr-c1.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A little more on the lines from &lt;a href=&quot;https://books.google.com/books?id=ko0PuecTtPQC&amp;amp;pg=PA52&amp;amp;lpg=PA52&amp;amp;dq=eastern%20test%20range%20destruct%20lines&amp;amp;source=bl&amp;amp;ots=TPaBsSwVfo&amp;amp;sig=HCm5bzjJFPJP4gKSxlVLOUg5f8w&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjd-6vV6rLOAhVB_WMKHRlcDZwQ6AEIVDAJ#v=onepage&amp;amp;q=eastern%20test%20range%20destruct%20lines&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oVVEH.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oVVEH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A really good writeup on Shuttle range safety and the interaction with Mission Control from former Flight Director and Shuttle Program Manager Wayne Hale's &lt;a href=&quot;https://blogs.nasa.gov/waynehalesblog/tag/range-safety/&quot; rel=&quot;noreferrer&quot;&gt;blog&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;By long standing jointly signed Flight Rules, if the shuttle were to&#xA;  veer off course, spin out of control, or break up, my responsibility&#xA;  as Shuttle Ascent Flight Director was to transmit those Code Words on&#xA;  my loop.  On hearing those words, the FCO would depress the two&#xA;  buttons in front of him to – as we say – ‘terminate the flight’.  That&#xA;  means exactly what you think it means.  I don’t have to spell it out.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This blog post also explains the role of the shuttle pilot and commander in a 2nd stage range safety scenario, as I mentioned above.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You see, the shuttle Commander and Pilot are designated Agents or&#xA;  Deputies of the Range.  The destruct package is built into the Solid&#xA;  Rocket Boosters and those are jettisoned two minutes into an eight and&#xA;  a half minute powered flight.  After that, should the shuttle go off&#xA;  course toward a populated area, the FCO can do nothing about it.  The&#xA;  responsibility which the President of the United States has given to&#xA;  the FCO cannot be accomplished – except to call the crew and tell them&#xA;  to do what is necessary.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So we practice these scenarios – far fetched as they may be – to&#xA;  ensure that the crew knows what to do.  Steer out to sea; shut down&#xA;  the main engines, protect the population along the eastern seaboard. &#xA;  One small problem – that procedure puts the shuttle crew into what is&#xA;  delicately labeled a “black zone”.  If the shuttle is high enough – as&#xA;  it is for much of the boost phase – but with forward velocity&#xA;  significantly below orbital speed – then an unpowered entry will&#xA;  result in the g-loads and heating which builds up too fast, faster&#xA;  than the wings can generate lift.  And the result?  Well.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So the Commander and the Pilot are designated Deputies of the Range. &#xA;  If the really bad thing happens, they are sworn to protect the&#xA;  population of the east coast, even at the expense of their crews’&#xA;  lives.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;It takes courage to fly in space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-08-08T22:32:35.097" LastActivityDate="2016-08-08T22:32:35.097" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17696" PostTypeId="2" ParentId="17594" CreationDate="2016-08-08T22:52:44.870" Score="6" Body="&lt;p&gt;OSIRIS-REx is packed all full of good stuff.  I'll throw together a quick list of the scanning ones you're interested in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also of note is that the entire spacecraft will be making that scanning motion shown in the gif, so as the asteroid rotates, all of these instruments will be able to have full coverage of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;OSIRIS-REx Visible and Infrared Spectrometer (OVIRS)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OVIRS is a point spectrometer, and will be used to create an organic and mineral map of the entire asteroid.  It has a resolution of 20m from 5km away, which will allow for larger scale mapping, and also can shift down to 0.08-2m resolution to create a detailed up-close map of the eventual chosen sample return site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;OSIRIS-REx Thermal Emission Spectrometer (OTES)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OTES is a thermal spectrometer.  It will have a similar function to OVIRS, and will also play a role in measuring the total thermal energy released by Bennu.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Regolith X-ray Imaging Spectrometer (REXIS)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How REXIS works is well above my pay grade, but I think I'll chalk it up to &quot;nuclear magic&quot;.  Seriously though, on a broad level, solar wind and radiation interacts with the surface on the asteroid, and releases radiation that can be detected by REXIS.  This released radiation contains signatures based on the material in the asteroid that released them.  The signatures can be measured, and after processing on the ground, you have an elemental abundance map that can help supplement the spectrometers described above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;PolyCam&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An 8in telescope that will be the first onboard object to see the asteroid.  Once it is closer, it will be used to create high resolution images of the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;MapCam&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A middle/close range camera with 1m resolution, designed to search for asteroid plumes and satelites in the &quot;Preliminary Survey&quot; phase of the mission.  It will also be used for high resolution imagery of the chosen sample site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;SamCam&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Close range camera designed to document the sample acquisition event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;OSIRIS-REx Laser Altimeter (OLA)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lidar&quot; rel=&quot;nofollow&quot;&gt;LIDAR&lt;/a&gt; scanning system to create a detailed topographic map of the surface.  &lt;a href=&quot;http://www.asc-csa.gc.ca/eng/satellites/osiris-rex/ola.asp&quot; rel=&quot;nofollow&quot;&gt;Funded by the Canadian Space Agency in exchange for a fraction of the returned sample&lt;/a&gt;, it is an improved version of a Canadian LIDAR system that has been used on the Phoenix Mars Lander and an Air Force Satellite.  It will be used to create an overall topographic map early on in the mission, and then more detailed maps once potential sites are chosen.  This system will give us the best topographic map of an asteroid we have ever had.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And just to give a better idea of where all of these instruments fit into the mission as a whole:&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;There are 4 major phases of surveying Bennu:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Preliminary Survey: Searching for asteroid plumes (&lt;a href=&quot;http://www.scientificamerican.com/article/massive-asteroid-spurts-plumes-of-water-vapor/&quot; rel=&quot;nofollow&quot;&gt;like here&lt;/a&gt;), any natural satellites, and measuring the &lt;a href=&quot;https://en.wikipedia.org/wiki/Yarkovsky_effect&quot; rel=&quot;nofollow&quot;&gt;Yarkovsky effect&lt;/a&gt;, as you mentioned above.&lt;/li&gt;&#xA;&lt;li&gt;Orbital A: Transition from star based navigation (using the position of the stars as captured by an onboard camera and the orientation of the satellite to determine position) to landmark-based navigation using the preliminary imagery captured by the cameras.&lt;/li&gt;&#xA;&lt;li&gt;Detailed Survey: Making detailed maps of the surface, using not only topographic data from the OLA, but the spectrometers as well.  Scientists will identify potential sample sites during this phase.&lt;/li&gt;&#xA;&lt;li&gt;Orbital B: A continuation of Detailed Survey with more of a focus on the potential sample sites selected in that phase.  This is where we will likely see the much higher resolution modes for the sensors mentioned above.  By the end of this phase, a sample site should be chosen.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.asteroidmission.org/mission/#explore-instruments&quot; rel=&quot;nofollow&quot;&gt;A treasure trove of information about the instruments of OSIRIS-REx can be found here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: &lt;a href=&quot;http://www.nasa.gov/feature/goddard/2016/nasa-to-map-asteroid-bennu-from-the-ground-up&quot; rel=&quot;nofollow&quot;&gt;Today NASA released a profile of the OLA&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15841" LastEditorUserId="15841" LastEditDate="2016-08-12T20:13:00.417" LastActivityDate="2016-08-12T20:13:00.417" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17698" PostTypeId="1" AcceptedAnswerId="17699" CreationDate="2016-08-09T05:54:45.800" Score="6" ViewCount="1113" Body="&lt;p&gt;Could the cosmic radiation of heavy ions which hit us be the exhaust of advanced electric propulsion used by alien space travelers? While supernovae seem to cause part of them, much of it is far from explained AFAIK. Our SEP probes spew out noble gas ions. Fission-fragment propulsion would spew out heavier ions at even higher speed. Almost &lt;a href=&quot;http://www.neumannspace.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;any ion&lt;/a&gt; could be used for propulsion. Is there any physical argument against alien civilization origin of some of the cosmic radiation (ions from outside of the Solar System)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, what kind of SETI could be done today by studying cosmic rays, heavy ions rather than photons?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plots like the one below really make cosmic rays look like a natural astrophysical phenomena. But there are error bars on the very exponential graphs like the one below. And the neat distribution function maybe just reflects what cosmic magnetic fields do to ions after a billion years regardless of origin. Does the spectrum of high energy ions represent cosmic alien market shares for different propulsion brands?&#xA;&lt;a href=&quot;https://i.stack.imgur.com/2KBLg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2KBLg.png&quot; alt=&quot;(Hillas 2006, preprint arXiv:astro-ph/0607109 v2)&quot;&gt;&lt;/a&gt;&#xA;(Hillas 2006)&#xA;&lt;em&gt;Aside, why is there an E² factor on the vertical axis here?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="13941" LastEditDate="2016-08-09T14:00:15.873" LastActivityDate="2016-08-09T23:57:54.083" Title="Could cosmic rays be produced by alien propulsion systems?" Tags="|advanced-propulsion|ion-thruster|cosmic-radiation|seti|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17699" PostTypeId="2" ParentId="17698" CreationDate="2016-08-09T09:16:07.260" Score="13" Body="&lt;p&gt;I can't give a precise answer to your primary question besides &quot;Extremely unlikely&quot;, but here are some facts on cosmic rays that might help coming to a conclusive answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current models are able to describe the distribution of energies and ion masses rather well. What we do not know precisely is the source of this radiation. There are plenty possible sources, but none of them is able to describe the full spectrum of cosmic rays. &#xA;I would like to point you to a summary of our knowledge about cosmic rays at &lt;a href=&quot;http://pdg.lbl.gov/2015/reviews/rpp2015-rev-cosmic-rays.pdf&quot;&gt;http://pdg.lbl.gov/2015/reviews/rpp2015-rev-cosmic-rays.pdf&lt;/a&gt; .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, the total energy of cosmic rays in our neighborhood is huge. Don't forget that Earth is just a tiny &quot;pale blue dot&quot; in space and the fraction of cosmic rays actually hitting us (and our detectors) is vanishingly small. Secondly, the spatial distribution of cosmic rays is very homogeneous, with  fluctuations on the level of 0.001 only. These two facts make it rather unlikely that cosmic rays are produced by technology of aliens. Required power sources must be extremely huge and we would expect more localized sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, we do observe localized sources for extremely high energy rays only (because they need to come from sources near-by and can not travel too far). However, these high energies are rather unlikely to be used in an engine because they are hard to generate. For comparison: Todays ion thrusters can accelerate Xenon up to 1 keV, 6 orders of magnitude to the left of the diagram above. Using the best known particle acceleration technologies known today (about 100 MeV/m) these extremely high energy rays needed an accelerator with a length comparable to the radius of the solar system (70 AU). Lower energy emissions are much more likely to come from an extraterrestrial space ship, but are also much harder to detect because of the huge background.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Further notes on cosmic rays&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;You also commented about the large error bars near the right end of the plot. This is purely due to statistics, not due to some not understood factors. In this region we expect about 1 particle of this energy per square kilometer and year which makes them very hard to measure. For us it is rather fortunate that these events are rather seldom - they deliver the energy needed to lift an apple by ten meters concentrated in a single atomic nucleus!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The factor of E^2 is used in this plot is to give us convenient units. In many cases this plot is made with a factor of E^2.6, which makes the first part of the plot flat and more easy to see the various kinks and bends in the spectrum. Compare your plot to figure 28.8 in the PDF linked above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These various changes in slope of the curve seem to be caused by solar and galactic magnetic fields which can trap particles to some extend within our galaxy while very high energy particles are reaching us directly from extragalactic sources. The steep drop-off at the end of the spectrum is not unexpected as well: Faster particles start to interact with the cosmic microwave background and are slowed down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The PDF also shows relative numbers for various types of nuclei (figure and table 28.1). E.g. iron is about 1/4000 of the number of protons (hydrogen nuclei). As you can see, the shape of the curve is rather similar for all measured ions. The plot shows this distribution only for relatively low energies because it is hard to distinguish masses at very high energies - the mass of the heaviest nuclei is just 250 GeV, compared to a million GeV kinetic energy. There are also no numbers for really heavy ions beyond iron, because count rates are getting lower and lower and error bars are getting bigger.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2016-08-09T18:16:40.447" LastActivityDate="2016-08-09T18:16:40.447" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17701" PostTypeId="1" CreationDate="2016-08-09T10:55:39.483" Score="0" ViewCount="220" Body="&lt;p&gt;I read about the manned mission &lt;a href=&quot;https://www.theguardian.com/news/video/2015/feb/09/volunteers-first-manned-mission-mars-video&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and I have a few questions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What prevents a return trip from Mars. Why can't a spaceship with efficient fuel, food and oxygen supplies be built for a return trip? &lt;/li&gt;&#xA;&lt;li&gt;How much supplies can we send to sustain human life?&lt;/li&gt;&#xA;&lt;li&gt;When is a manned mission necessary rather then sending robots like Curiosity to do research on the planet?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="16563" LastEditorUserId="18879" LastEditDate="2018-12-08T18:51:26.520" LastActivityDate="2018-12-08T18:51:26.520" Title="Is a one-way manned mission to Mars currently possible?" Tags="|mars|colonization|future-missions|return|" AnswerCount="1" CommentCount="6" ClosedDate="2016-08-09T16:30:53.183" ContentLicense="CC BY-SA 4.0" />
  <row Id="17702" PostTypeId="2" ParentId="17701" CreationDate="2016-08-09T13:42:54.230" Score="8" Body="&lt;p&gt;Getting back is not impossible, just very expensive.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mbfks.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mbfks.png&quot; alt=&quot;one Falcon Heavy to get there, ten Saturn V to get there and back again&quot;&gt;&lt;/a&gt;   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://what-if.xkcd.com/38/&quot; rel=&quot;noreferrer&quot;&gt;This site&lt;/a&gt; has a good explanation of the phenomenon. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-09T13:42:54.230" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17703" PostTypeId="2" ParentId="8678" CreationDate="2016-08-09T15:56:56.607" Score="0" Body="&lt;p&gt;It's a function of the fact that at periapsis the ship has the highest velocity and the Oberth effect is about adding velocity when the velocity is already high in some frame of concern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe it also applies to multistage rockets where the upper stage increase in kinetic energy may be substantially greater than the total chemical energy of the propellant of that stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should then also apply to any continuous thrust engine no matter how &#xA;it works where the mechanical power grows linear in V while the liberated power of the engines power source is constant. Energy is conserved as the integrated mechanical power always equates to the ships kinetic energy in any frame.&lt;/p&gt;&#xA;" OwnerUserId="16567" LastActivityDate="2016-08-09T15:56:56.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17704" PostTypeId="2" ParentId="17698" CreationDate="2016-08-09T16:29:59.083" Score="1" Body="&lt;p&gt;Asdfex did an absolutely superlative job explaining why it's probably just background stellar emissions we're seeing, but let me ask you this: Who said a sun can't be an engine?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're willing to posit the existence of Megastructures (that we can't directly detect) then I offer you the &lt;strong&gt;&lt;a href=&quot;http://www.popularmechanics.com/space/deep-space/a10885/the-shkadov-thruster-or-how-to-move-an-entire-solar-system-17000392/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shkadov Thruster:&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kYgYc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kYgYc.jpg&quot; alt=&quot;Well ain&amp;#39;t that a monster&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's basically an immense photon-sail pushed by nearly half the output of a star. Its size allows it to gravity-tractor its powering star (and solar system) along with it, turning it into a literal starship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I don't think this is even remotely possible, I'm just giving you an edge-case where ion emissions can be the result of alien propulsion.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastEditorUserId="1142" LastEditDate="2016-08-09T23:57:54.083" LastActivityDate="2016-08-09T23:57:54.083" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17705" PostTypeId="1" CreationDate="2016-08-09T18:25:26.480" Score="5" ViewCount="287" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Momentum_exchange_tether&quot; rel=&quot;nofollow&quot;&gt;Rotovators&lt;/a&gt; have the big advantage that they don't have as much force on them as a full space elevator and thus can be constructed without such incredibly strong materials.  The big downside is that since they are freestanding objects they only act as momentum storage. As they boost things their orbit decays; if you don't want it destroyed, you have to use it to slow things down also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first glance this means they're not of much use until we have extraterrestrial mining to provide mass to be deorbited.  However, I got to thinking about this and found myself out of my depth on the math:    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The normal scenario presented is that a lesser rocket lifts a payload to the rotovator and the rotovator then hauls that payload to orbit.  However, lets try a different approach:    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Falcon 9 upper stage approaches the rotovator but couples late, not getting the maximum benefit.  It does &lt;strong&gt;not&lt;/strong&gt; detach its payload, the whole rocket attaches.  As the rotovator turns it reaches orbit and the payload is detached from the rocket but the rocket remains attached and continues on up, only detaching at the top when it's going as slow as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rotovator lifted the rocket + payload for less than its full run and then slowed the rocket without payload as much as possible.  Choose the attachment point correctly and the energy balances but I'm not sure about the vectors.  Am I missing something or would this provide a basically free boost to any rocket that was in the right place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(And as a bonus this might make recovery of that upper stage viable.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="1142" LastEditDate="2016-08-09T23:34:29.197" LastActivityDate="2016-08-09T23:34:29.197" Title="Rotovators and booster recovery: is there a free lunch here?" Tags="|launch|space-elevator|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17706" PostTypeId="2" ParentId="17705" CreationDate="2016-08-09T18:47:50.403" Score="1" Body="&lt;p&gt;There is no free lunch but there is a discounted lunch if you are willing to take the risks.  If you are using a rotovator to boost to a lower orbit then the rotovator will be boosted into a higher one and vice versa.  The rotovator then needs to have some sort of propulsion system to move back down to the a lower orbit to be used again.  If the rotovator had an electric propulsion system then it could make the transfer back more efficiently than a spacecraft using a chemical propulsion system.  This is where the potential cost savings come in. A rotovator could deorbit multiple spacecraft for a lower fuel expense if you are willing to wait for it to travel from orbit to orbit which would take a greater amount of time and operator expenses.  In addition to this detaching the tether/spacecraft at the wrong time/position would send the spacecraft and rotovator off into the wrong orbits.  In the case of orbital reentry a seconds delay in detaching could be the difference between a safe reentry and burning up in the atmosphere.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A detailed systems analysis would need to be done to see if it would actually reduce costs.  My gut says by using a rotovator you are adding a lot of complexity and risk for a small mass savings and that it probably is not worth the cost.  &lt;/p&gt;&#xA;" OwnerUserId="16180" LastActivityDate="2016-08-09T18:47:50.403" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17707" PostTypeId="1" AcceptedAnswerId="18571" CreationDate="2016-08-10T07:07:24.990" Score="4" ViewCount="527" Body="&lt;p&gt;I am trying to simulate orbit raising like India's MOM mission did in order to get escape velocity. But I can't determine the best/most-suitable point/time on the current orbit where the probe should start burning the propellant in order to raise to a new orbit, whether the point the burn starts should be at perigee of the current orbit, or it should start somewhere before the perigee. Please keep in mind that all orbits of MOM probe were elliptical.&lt;/p&gt;&#xA;" OwnerUserId="16572" LastEditorUserId="1142" LastEditDate="2016-08-10T23:24:07.480" LastActivityDate="2016-10-10T19:59:11.987" Title="Burn time to raise to an elliptical orbit from an elliptical orbit" Tags="|orbital-mechanics|orbital-maneuver|burn|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17708" PostTypeId="1" CreationDate="2016-08-10T07:08:24.603" Score="5" ViewCount="874" Body="&lt;p&gt;SpaceX just released &lt;a href=&quot;https://www.youtube.com/watch?v=DKqY8sy3nkM&quot; rel=&quot;nofollow noreferrer&quot;&gt;this high-speed footage&lt;/a&gt;, writing in the comments:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Missions in order of appearance: May JCSAT-14; July CRS-9 launch, stage separation, engine plume interaction, and re-entry burn; December 2015 ORBCOMM landing burn; July CRS-9 landing burn.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Matching the video fragments with that description, the fragment from 0:37-0:50 seems to be the &lt;em&gt;engine plume interaction&lt;/em&gt; (the next fragment 0:50-0:55 would then be the re-entry burn):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dxMC9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dxMC9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is that, actually? What interacts? The two separating dots are obviously the upper and first stages, but what are the filaments/fumes?&lt;/p&gt;&#xA;" OwnerDisplayName="user10509" LastEditorUserId="25" LastEditDate="2017-12-19T13:24:49.417" LastActivityDate="2021-06-07T08:14:53.930" Title="What is this 'engine plume interaction'?" Tags="|launch|spacex|falcon-9|engines|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17710" PostTypeId="2" ParentId="17708" CreationDate="2016-08-10T11:59:59.030" Score="1" Body="&lt;p&gt;The two dots are the upper stage boosting to orbit, and the first stage doing its first burn to slow down. (Boostback burn).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is much clearer to see in this &lt;a href=&quot;https://www.instagram.com/p/BI5wLIFj7f7/&quot; rel=&quot;nofollow&quot;&gt;Instagram video.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interaction is the first stage boosting through the plume of the second stage. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="25" LastEditDate="2016-08-10T15:09:42.737" LastActivityDate="2016-08-10T15:09:42.737" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17712" PostTypeId="1" CreationDate="2016-08-10T18:35:29.277" Score="12" ViewCount="2325" Body="&lt;p&gt;Let's say someone pays for the shipping through an as yet unnamed commercial launch company.  And they place something in orbit that is effectively a kinetic weapons platform.  Or at least has the capacity to operate as one while having an alternative primary function.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring the illegality of shipping a weapon to space, I know that's illegal.  But let's say this third party hands over control to another party.  Is the ownership of that platform, once it's in orbit, in and of itself illegal?&lt;/p&gt;&#xA;" OwnerUserId="806" LastActivityDate="2016-08-10T19:27:52.927" Title="Is it illegal for a private citizen of the US to own and operate an orbital kinetic weapons platform?" Tags="|law|orbital|weapon|" AnswerCount="1" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="17713" PostTypeId="2" ParentId="17712" CreationDate="2016-08-10T18:53:42.667" Score="9" Body="&lt;p&gt;The legal definition of &quot;Weapon of Mass Destruction&quot; in US law is quite broad (see &lt;a href=&quot;https://www.schneier.com/blog/archives/2009/04/definition_of_w.html&quot;&gt;https://www.schneier.com/blog/archives/2009/04/definition_of_w.html&lt;/a&gt; ), and an orbital kinetic weapon definitely qualifies if the projectile is big enough to re-enter in one piece. The FBI and/or ATF will want to have a talk with the owner.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-08-10T19:27:52.927" LastActivityDate="2016-08-10T19:27:52.927" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17715" PostTypeId="1" CreationDate="2016-08-10T20:32:47.227" Score="5" ViewCount="63" Body="&lt;p&gt;How much is the radiation dose from cosmic rays lowered if a view only sees the ground, and not the sky? Leave aside shielding, and just taking as a starting point that the sky is completely blocked, but none of the surrounding landscape is, and that landscape is perfectly flat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0032063312002085&quot; rel=&quot;nofollow noreferrer&quot;&gt;This model&lt;/a&gt; seems to have been calculated based on standing unprotected on a flat landscape. In this situation,cosmic ray particles hit the regolith around you, and at some depth strike the nucleus of an atom. That nucleus shatters and its bits fly in all directions. Those particles likely also have enough energy to smash another nucleus should they encounter one. Some portion of the resulting particles head in your direction and pass through you. Some of the particles strike another nucleus inside you with sufficient force to smash it. The charged particles passing through you (including from any atomic nuclei smashed inside your body) break chemical bonds of nearby molecules as they pass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qGbXn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qGbXn.jpg&quot; alt=&quot;particle traverse distance&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know what the mean free path through regolith is taken as being, but my idea was that since the collisions happen under the surface, the average amount of regolith the particle cascade needs to pass through to reach you increases quickly with distance from you, and so drops off quickly. (In fact the quick sketch seems to indicate you could reduce your dose considerably by crawling along the ground.) On the other hand, the collision and particle cascade process means that many times as many particles are created in the ground as come from the sky. So I have no idea how it sums up, or even with how much confidence we can even say how it compares without directly measuring over time.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2017-12-27T14:08:38.137" LastActivityDate="2017-12-27T14:08:38.137" Title="How much does back-scattered ionizing radiation contribute to radiation totals on the Moon?" Tags="|radiation|health|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17716" PostTypeId="2" ParentId="7948" CreationDate="2016-08-10T22:39:53.050" Score="1" Body="&lt;p&gt;I would do it as a numerical simulation and would plan on having a time step of a millisecond or less. Any modern processor should be able to do all the equations for each step in a few milliseconds at worst, so it should be able to run in real time (or my guess, much faster).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First you need to do the thrust and drag to get the forces, then use the mass at that time step to get the acceleration, use that to update the velocity vector and then use that to update the position vector. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that spherical earth washes out since you are outside of it and you can treat it as a point mass at the center. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to get the thrust, you need to figure the baseline engine thrust and then what percentage of it you are running at in each time step. The atmospheric drag will be in the opposite direction of the velocity vector and will be basically some drag constant times projected area times the velocity relative to the air squared. (Yup, drag is a V squared phenominon)  The gravity drag will be a constant. From that and the mass at the timeslice, you should be able to get the acceleration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Start at takeoff with the location vector and velocity vector being zero and then do each time step and check that the results look plausible.&lt;/p&gt;&#xA;" OwnerUserId="13841" LastActivityDate="2016-08-10T22:39:53.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17717" PostTypeId="2" ParentId="13359" CreationDate="2016-08-11T00:09:57.653" Score="1" Body="&lt;p&gt;At the very least, as the middle of 2016 passes, it was noted that they have at least two complete launch teams. It was mentioned in the context of Vandenberg vs Florida, as the end of 2016 has several SLC-4E launches scheduled (Iridium Next F1, Sherpa/FormosaSat, and Iridium Next F2).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-11T00:09:57.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17718" PostTypeId="1" CreationDate="2016-08-11T03:01:51.013" Score="2" ViewCount="425" Body="&lt;p&gt;How can I know the remaining life of a satellite? I have been using satbeam, and try to calculate it based on launch date + expected life, but it seems to have some problems. For example, for this satellite:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.satbeams.com/satellites?norad=25630&quot; rel=&quot;nofollow&quot;&gt;https://www.satbeams.com/satellites?norad=25630&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch date is 1999 and the expected lifetime is 12 years, so the expected end year is 2011, but it is still active now. How should I know when this satellite would retire? Thank you.&lt;/p&gt;&#xA;" OwnerUserId="16587" LastActivityDate="2016-08-11T03:46:35.167" Title="Satellite Remaining Life" Tags="|artificial-satellite|geostationary|communication-satellite|lifetime|" AnswerCount="1" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17719" PostTypeId="1" CreationDate="2016-08-11T03:33:05.740" Score="0" ViewCount="92" Body="&lt;p&gt;Is it possible for a small object (such as a manned capsule) to realize a maneuver using the orbital resonance influence of Saturn's moons Pan and Prometheus?&lt;/p&gt;&#xA;" OwnerUserId="16204" LastEditorUserId="6816" LastEditDate="2016-08-11T04:16:50.250" LastActivityDate="2016-08-11T04:16:50.250" Title="Saturn's Pan and Prometheus orbital resonance" Tags="|orbital-mechanics|orbital-maneuver|moons|saturn|orbital-resonance|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17720" PostTypeId="2" ParentId="17718" CreationDate="2016-08-11T03:46:35.167" Score="3" Body="&lt;p&gt;A satellite should have a planned lifetime, &quot;expected lifetime&quot;, or design lifetime, but it's a balance between a number of factors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gunter's Space Pages is a great place to read about satellites and get a better idea what they are for and any special information about them. Here is the page for &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/jcsat-5.htm&quot; rel=&quot;nofollow&quot;&gt; JCSAT-5 and JCSAT-6&lt;/a&gt;. Take a look there first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This satellite is not in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;nofollow&quot;&gt;low earth orbit&lt;/a&gt; (LEO) so there is no risk of the orbit decaying due to atmospheric friction. It will stay &quot;up there&quot; for a very long time, although not necessarily in the exact location it is supposed to be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites - including telecommunications satellites like this one - have usually been quite expensive to build and put into orbit, so in this example, if you can't reasonably expect it to keep working for 12 years, then it may not be economically worth paying for it to be 1) designed, 2) built, 3) application for and receiving a &quot;spot&quot; in the geostationary space obtained, 4) launched, and 5) maintained by ground control. Business plans to use this telecommunications bandwidth also rely on it's stability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a huge amount of experience with telecom satellite &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_station-keeping&quot; rel=&quot;nofollow&quot;&gt;orbital station keeping&lt;/a&gt; to maintain them in their assigned geostationary orbit position and pointing in the correct direction - &lt;a href=&quot;https://en.wikipedia.org/wiki/Attitude_control&quot; rel=&quot;nofollow&quot;&gt;attitude control&lt;/a&gt;. The satellite usually has more propellant than it is expected to need during its expected lifetime, because if it lasts longer, you can usually or at least often make more money maintaining it and selling the bandwidth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is an expense to maintaining a satellite, so if you are not making money, sometimes you change to an inactive mode so that you don't have to pay nearly as much for regular ground communication bandwidth and control.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-08-11T03:46:35.167" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17721" PostTypeId="1" AcceptedAnswerId="17733" CreationDate="2016-08-11T04:22:56.287" Score="1" ViewCount="650" Body="&lt;p&gt;I learnt that satellite can be equipped with transponder of different bands (L, C, Ku, Ka), and in term of bandwidth, L &amp;lt; C &amp;lt; Ku &amp;lt; Ka. My question is, am I correct to assume that, with a higher bandwidth, the equipment cost should be higher? i.e. Ka band transponder should be the most expensive and L band transponder is the cheapest. And a satellite equipped with only L band transponders should be cheaper to develop that that equipped with Ka band transponders. Thank you.&lt;/p&gt;&#xA;" OwnerUserId="16587" LastActivityDate="2016-08-11T15:43:55.147" Title="Cost of different kind of band in satellite" Tags="|artificial-satellite|orbit|communication-satellite|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17723" PostTypeId="1" AcceptedAnswerId="17724" CreationDate="2016-08-11T08:30:05.160" Score="5" ViewCount="553" Body="&lt;p&gt;Hypothetically, why could the current Dragon not be used for carrying astronauts &lt;strong&gt;from&lt;/strong&gt; space? Since there is always some research material in Dragon, I believe this vehicle has e.g. heat shields. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it just a matter of equipment (seats, belts, etc.), or in material used and structure as well?&lt;/p&gt;&#xA;" OwnerUserId="16590" LastEditorUserId="4660" LastEditDate="2016-08-11T14:30:50.033" LastActivityDate="2016-08-11T14:30:50.033" Title="What prevents the current Dragon capsule being used for crew?" Tags="|spacex|iss|crewed-spaceflight|dragon|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17724" PostTypeId="2" ParentId="17723" CreationDate="2016-08-11T08:59:34.700" Score="3" Body="&lt;p&gt;AFAIK there is no life support system on board. The air in the capsule is not enough to support a human for the ~4 hours it takes from leaving the station to landing.&lt;br&gt;&#xA;You'd also have to install seats designed to handle the G loads (i.e. contoured to properly support the body, and on springs to soften the touchdown). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-08-11T11:18:30.373" LastActivityDate="2016-08-11T11:18:30.373" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17726" PostTypeId="2" ParentId="17723" CreationDate="2016-08-11T10:05:43.310" Score="-4" Body="&lt;p&gt;It is only equipped to go into space but not out of space, you would need much more than just a room and parachutes to touch down safely.&lt;/p&gt;&#xA;" OwnerUserId="16593" LastActivityDate="2016-08-11T10:05:43.310" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17727" PostTypeId="2" ParentId="17723" CreationDate="2016-08-11T10:21:03.577" Score="6" Body="&lt;p&gt;There is a minimal life support system, in the sense that it is pressurized and they carry animal experiments that do not have their own life support systems. But it is not designed to handle the load a full crew were generate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Musk has said that had a person been aboard they would have been fine, on earlier Dragon flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To meet NASA's requirements they need an abort system, which is being added in Dragon V2. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-11T10:21:03.577" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17733" PostTypeId="2" ParentId="17721" CreationDate="2016-08-11T15:43:55.147" Score="4" Body="&lt;p&gt;Hobbes is correct. Higher frequency transponders are more expensive to design and test than low frequency transponders. This is simply because as the wavelength decreases, smaller components within the transponder will have an effect (often unintended). You can build a VHF or UHF radio using a handful of cheap parts. The flip side of this is that higher frequency bands contain more bandwidth, so it is relatively easier to obtain the rights to a large chunk of Ka band than it would be in C or S band.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The transponder is often just a fraction of the entire mission, so it's cost is usually insignificant or minor in comparison to other costs. Also, since higher bandwidth satellites can deliver more data to more users, the cost of the transponder is quickly repaid. Think about the number of users dish satellite has and the revenue they generate.&lt;/p&gt;&#xA;" OwnerUserId="12966" LastActivityDate="2016-08-11T15:43:55.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17734" PostTypeId="2" ParentId="17623" CreationDate="2016-08-11T17:15:40.860" Score="4" Body="&lt;p&gt;Just because there are answers to those questions, doesn't mean they are right. The quote from a magazine in &lt;a href=&quot;https://space.stackexchange.com/a/354/12102&quot;&gt;this answer&lt;/a&gt; but it seems to be a dead link now: &lt;a href=&quot;http://scienceinfocus.co.uk/qa/would-corpse-decay-space&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://scienceinfocus.co.uk/qa/would-corpse-decay-space&lt;/a&gt;. But that quote does point out that it won't necessarily be completely dessicated like &quot;freeze dried instant coffee&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It points out that there are competing processes. While water near the surface will be under osmotic-type pressure to diffuse &lt;em&gt;to&lt;/em&gt; the surface where it can evaporate while it's still warm, the &quot;big chill&quot; will set in. Once ice, the mobility of the water molecules will be greatly, greatly reduced. It's not solid ice - there are pockets of ice in each cell and the cell walls will be busted up, but there is still a &lt;em&gt;whole lot of cell walls&lt;/em&gt; to traverse, and only water molecules that are on surfaces and aren't &lt;em&gt;inside&lt;/em&gt; ice will have mobility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How cold will it be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's try a little physics. Assume we're at 1.5 AU (Mars neighborhood). At earth, the total power is about 1.5 kw/$\text{m}^2$, at Mars $I_{sun}$  would be that times 1.5 $\text{}^{-2}$ or about 670 W. Let's say the cross-sectional area of a space suit intercepting sunlight $A$ is 1.0 m $\text{}^2$ and the total surface area for radiation is 2.5 m $\text{}^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The power input from the sun will then be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P_{in} =  I_{sun} A \alpha$$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\alpha$ is the absorptivity of the white space suit in the visible and near-infrared wavelengths. I'm going to ballpark the diffuse reflectivity at 0.8, and call the absorptivity 1-0.8 = 0.2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting in numbers, I get 134 W. Since it's probably going to be spinning, I'll make a simplification and treat this as an equilibrated object. The whole surface area will be at a uniform temperature, and that temperature will be the value that allows it to re-radiate those 134 Watts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stefan-Boltzmann Law&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P_{out} = A \epsilon \sigma T^4 $$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\sigma$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stephan-Boltzmann constant&lt;/a&gt; and is about 5.67E-08 W m${}^{-2}$ ºK ${}^{-4}$, and $\epsilon$ is the dimensionless &lt;a href=&quot;https://en.wikipedia.org/wiki/Emissivity&quot; rel=&quot;nofollow noreferrer&quot;&gt;emissivity&lt;/a&gt; (between 0 and 1). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you might think that a white space suit with a 0.2 absorptivity should also have an emissivity of 0.2, but these things are not constants. They can depend strongly on wavelength. Most things that look white still have an emissivity at longer wavelength infrared (where &quot;cool stuff&quot; radiates) above 0.9. If you go to the linked Wikipedia article for &lt;a href=&quot;https://en.wikipedia.org/wiki/Emissivity&quot; rel=&quot;nofollow noreferrer&quot;&gt;emissivity&lt;/a&gt; again, it says:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Paint (including white)   0.9&#xA;Paper, roofing or white   0.88 to 0.86&#xA;Snow                      0.8 to 0.9&#xA;Water, pure               0.96&#xA;Concrete, rough           0.91&#xA;Glass, smooth (uncoated)  0.95&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So if you had infrared eyes (say 15 or 20 micron wavelength) all those things would be pretty much black. None of them would be transparent. Snow is black sand. Water is ink, &quot;transparent glass&quot; is obsidian or black marble, and white paint is black paint. Actually all paint is black paint. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clean bare metal is brilliant, but let it oxidize and it will darken as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think about it, those infrared thermometers - while they do or should have an emissivity setting for accuracy - tend to work without it, because &quot;most stuff&quot; is roughly 0.9 at room temperature, and they often have a default of 0.90 or 0.92 or something like that in the firmware, if you don't specify one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nice unoxidized metal surfaces however are good reflectors at visible wavelengths, through infrared all the way down to radio. Those can be down around 0.1 and below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's just chose 0.9&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P_{out} = P_{in} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ A \epsilon \sigma T^4 = P_{in}  $$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$  T^4 = \frac{P_{in}}{A \epsilon \sigma} $$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging the numbers in I get 180 K. That's c-c-c-cold, about -93C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid ice on the moon, exposed to space (but not sunlight) for millions of years is expected to be stable at 100K (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_water&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar water&lt;/a&gt;). While we're not that cold, this is not exposed - it's deeply embedded in a complex biological and crystalline ice matrix, and we may be talking about years or tens of years - faster if David Bowman is in his pod.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The body will probably have a substantial amount of ice. If you bring it in to room temperature it will have some water in it. It will maintain integrity to some degree, which means any bacterial spores (some bacteria have spores) and fungal spores (in other places) may activate, even if only one in every billion bacteria actually survives being frozen in ice, it's going to be a LOT of them. New surface contamination will be present too. It's a race against time between all those competing biological sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not, it won't be a dried out mummy. It will become a problem inside a spacecraft. Keep it frozen, or outside, but I personally prefer to leave it (or me) as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Burial_at_sea&quot; rel=&quot;nofollow noreferrer&quot;&gt;burial at sea&lt;/a&gt; so to speak.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-08-11T17:37:22.313" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17735" PostTypeId="2" ParentId="17623" CreationDate="2016-08-11T18:40:00.300" Score="4" Body="&lt;p&gt;This has happened once, kind of. &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_11#Death_of_crew&quot; rel=&quot;nofollow&quot;&gt;Soyuz 11&lt;/a&gt;, which was re-entering Earth, had a loose valve that subjected it to the vacuum of space. The astronauts returned to Earth eventually, with the capsule in perfect condition. A few things of note:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The autopsies took place at Burdenko Military Hospital and found that the cause of death proper for the cosmonauts was hemorrhaging of the blood vessels in the brain, with lesser amounts of bleeding under their skin, in the inner ear, and in the nasal cavity, all of which occurred as exposure to a vacuum environment caused the oxygen and nitrogen in their bloodstreams to bubble and rupture vessels. Their blood was also found to contain heavy concentrations of lactic acid, a sign of extreme physiologic stress. Although they could have remained conscious for almost a minute after decompression began, less than 20 seconds would have passed before the effects of oxygen starvation made it impossible for them to function.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also of note is this video, showing support crews attempting CPR on the cosmonauts. If you want to see that, check it out at &lt;a href=&quot;https://www.youtube.com/watch?v=DdhwbvKMe3I&quot; rel=&quot;nofollow&quot;&gt;YouTube&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would they decompose after being brought back in to a human environment? Yes, eventually. It would probably be somewhat slower than otherwise. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some evidence that the decay process would be different. Of some note is the &lt;a href=&quot;http://www.smithsonianmag.com/science-nature/forests-around-chernobyl-arent-decaying-properly-180950075/?no-ist&quot; rel=&quot;nofollow&quot;&gt;forest around Chernobyl&lt;/a&gt;. Of some note, they did an experiment where they took leaves that hadn't decomposed for an extended period of time in a high radiation area, and brought them to a low radiation area. The leaves decomposed as expected. So one can expect that the decomposition process would return to normal, given the correct enviroment.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-08-13T11:48:13.427" LastActivityDate="2016-08-13T11:48:13.427" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17736" PostTypeId="1" CreationDate="2016-08-11T20:51:55.757" Score="2" ViewCount="293" Body="&lt;p&gt;The ISS has three (four?) types of docking ports. The Russian side has one standard with two types (active/passive?) and the US side has either a PMA (soon to be updated with the IDA to the NDS standard) or a CBM port.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The PMA (Pressurized Mating Adapter) with the IDA (International Docking Adapter) allow for vehicles to dock and undock without intervention and thus are designed/designated for crew vehicles.  The IDA brings the PMA to the NDS (Nasa Docking Standard) that CST-100, Dragon V2, and possibly Dream Chaser cargo will match.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CBM (Common Berthing Module) is meant for attaching larger modules like the various ISS Modules (Unity, Harmony, Tranquility, Columbus, Kibo, PLM, Quest Airlock, Cupula, BEAM, etc).  Thus it is not really meant for unattended operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus a critique of using a Dragon (cargo/v1 whatever you wish to call it) for crew, even if all other concerns were met, is that it cannot unberth on its own in case of an emergency. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what exactly keeps it from unberthing on it's own?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-11T21:04:56.193" Title="Can an ISS berthed vehicle remotely unberth?" Tags="|spacex|iss|dragon|docking|cbm|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17737" PostTypeId="2" ParentId="17736" CreationDate="2016-08-11T20:55:39.520" Score="4" Body="&lt;p&gt;There are a list of things that have to happen during an unberth operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two CBM ports (one on the vehicle, one on the station module) have hatches that have to be opened and closed. There is a small narrow vestibule in between the two hatches once the two are berthed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, the docking ring of each side mates, and once that is airtight, there is a gap (about a foot wide) between the outside of the ISS and vehicle hatches.  Both those hatches need to be opened to allow passage through. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are bolts that are driven (16, driven in sets of 8) to hold the structures together in the rings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the vehicle is detached, then the CanadArm2 is used to move the visiting vehicle far enough away that its use of thrusters won't impinge on the station in a harmful fashion.  (Big solar panels and big radiators they would like you to avoid getting dirty with hypergolics or their combustion products). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CanadArm2 can be operated from the ground, so that is likely the least of the set of problems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest problem is likely that once the crew departing are inside their vehicle, they cannot close the ISS side inner hatch and seal it air tight.  Thus leaving on their own would likely vent the station to space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case of a truly bad, station is going down situation, a CBM berthed vehicle could likely leave in a way that left the ISS unihabitable if the berthing ring bolts could be driven remotely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sealing off just that ISS Module would not really help, since the open CBM ports used for visiting vehicles are either on Node 1 Unity which is the transit point between the US and Russian segments, or else on Harmony (Node 2) which node connecting Columbus and Kibo to the station.  Closing off either of those modules on boths its docking ports, would also leave the station mostly unusable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A really good reference to the CBM ports is in the answer to this &lt;a href=&quot;https://space.stackexchange.com/questions/13799/how-are-hatches-set-up-in-the-common-berthing-mechanism-cbm&quot;&gt;question.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-08-11T21:04:56.193" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17738" PostTypeId="1" CreationDate="2016-08-12T01:21:38.893" Score="11" ViewCount="613" Body="&lt;p&gt;There was a time, where people touring the SLC-40 facility were not allowed to take pictures of the engine end of the first stage for &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Traffic_in_Arms_Regulations&quot; rel=&quot;nofollow&quot;&gt;ITAR&lt;/a&gt; reasons. It &lt;strong&gt;appears&lt;/strong&gt; that has relaxed since I have seen lots of images of that end of the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But mounting a first stage, with real engines, outside their factory, where any Tom, Dick, or Danny Ocean could stage a heist and steal and engine seems like it might be a real ITAR issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there ITAR restrictions on their static display of the Orbcomm mission F9 first stage?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="25" LastEditDate="2018-12-03T12:50:53.823" LastActivityDate="2018-12-03T12:50:53.823" Title="How does ITAR affect SpaceX's display of the Orbcomm first stage outside their headquarters?" Tags="|spacex|falcon-9|regulatory|itar|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17739" PostTypeId="1" CreationDate="2016-08-12T02:29:43.397" Score="12" ViewCount="1657" Body="&lt;p&gt;Cubesats are (ir-)regularly deployed at very low velocity from the ISS, and they don't usually have propulsion. This tells me that the delta-v is enough to guarantee to those in charge that their orbits won't intersect the ISS again.&lt;/p&gt;&#xA;&lt;p&gt;A tool bag &amp;quot;got away&amp;quot; - I saw this in a text book.&lt;/p&gt;&#xA;&lt;p&gt;I remember an ISS astronaut deploying something intentionally by hand - I think it was a nano-satellite of some kind, maybe a self-deorbit experiment?&lt;/p&gt;&#xA;&lt;p&gt;But these are just from memory.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;What kinds&lt;/em&gt; of things have been tossed out of - or otherwise intentionally deployed from - the ISS?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I don't want a list of everything - well yes I do, but I'm not &lt;em&gt;asking here&lt;/em&gt; for a list of everything. I'm guessing there may be 3 to 6 different classes of things. Cubesats would be one classs. Space suits would be one class, waste would be one (hypothetical) class, for example.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I found this YouTube linked in this discussion of some examples: &lt;a href=&quot;http://www.science20.com/robert_inventor/why_a_ball_thrown_to_earth_from_orbit_boomerangs_can_astronauts_hit_earth_with_a_ball_arrow_or_bullet-158237&quot; rel=&quot;noreferrer&quot;&gt;Why A Ball Thrown To Earth From Orbit &amp;quot;Boomerangs&amp;quot;. Can Astronauts Hit Earth With A Ball, Arrow Or Bullet?&lt;/a&gt; (By inventor &amp;amp; programmer (and debunker) Robert Walker, October 24th 2015)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;uhoh!&lt;/strong&gt; My video link is broken, at least temporarily. It's also (of course) broken in the link above. I will ask a question shortly about this because the caption there is intriguing;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Here is a steel ball thrown out of the ISS to help test how well ground stations can track orbital debris&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Video link &lt;code&gt;https://www.youtube.com/watch?time_continue=24&amp;amp;v=z5HcXnjtICE&lt;/code&gt; no longer works, so instead I've asked &lt;a href=&quot;https://space.stackexchange.com/q/48738/12102&quot;&gt;What radar-trackable steel ball was thrown from the ISS before October 2015?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-11-24T03:34:10.573" LastActivityDate="2021-06-02T11:46:08.427" Title="What kinds of things have been tossed out of the ISS?" Tags="|iss|low-earth-orbit|payload-deployment|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17740" PostTypeId="1" AcceptedAnswerId="17752" CreationDate="2016-08-12T03:21:37.250" Score="6" ViewCount="364" Body="&lt;p&gt;I'm not looking for exact numbers, and I know that it will vary on a case by case basis, but I was thinking about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kodiak_Launch_Complex&quot;&gt;Kodiak Launch Complex&lt;/a&gt; lately:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Last Launch: August 25th, 2014&lt;/li&gt;&#xA;&lt;li&gt;2 Active Pads&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://peninsulaclarion.com/news/2014-12-13/lockheed-martin-wins-alaska-spaceport-bid&quot;&gt;There has also been some talk about its financial struggles, with Alaska threatening to cut funding if it did not bring in more business.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the spaceport's last launch coming two years ago, the lack of revenue is intuitive, but what are the major expenses here?  Would a small spaceport like this have a permanent, full time staff?  What magnitude of upkeep costs are necessary for the launch pads?  Are these smaller spaceports even expected to break even, or are they simply job magnets?&lt;/p&gt;&#xA;" OwnerUserId="15841" LastActivityDate="2016-08-12T21:10:24.047" Title="What are the major expenses for a small spaceport?" Tags="|launch-site|commercial|spaceport|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17741" PostTypeId="2" ParentId="17739" CreationDate="2016-08-12T03:25:44.223" Score="9" Body="&lt;p&gt;An Orlan spacesuit!  aka &lt;a href=&quot;https://en.wikipedia.org/wiki/SuitSat&quot; rel=&quot;noreferrer&quot;&gt;SuitSat 1&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6uZ8m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6uZ8m.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tool bag was lost by &lt;a href=&quot;https://en.wikipedia.org/wiki/Heidemarie_Stefanyshyn-Piper&quot; rel=&quot;noreferrer&quot;&gt;Heidemarie Stefanyshyn-Piper&lt;/a&gt; during the STS-126 mission, who was a fine astronaut and deserves to be remembered for more than this incident. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.universetoday.com/wp-content/uploads/2008/11/lost_bag_iss.jpg&quot; alt=&quot;https://www.universetoday.com/21411/satellite-tracker-captures-lost-toolbag-on-video/&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I am sure the embroiderers of mission patches do not miss her:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/c5Rt1m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/c5Rt1m.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An USB flashdrive&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bbc.com/news/science-environment-35487441&quot; rel=&quot;noreferrer&quot;&gt;http://www.bbc.com/news/science-environment-35487441&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/F2KIv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/F2KIv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;(credits NASA TV)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS was very young then, but Jerry Ross &lt;a href=&quot;http://www.debunker.com/texts/sts88%20and%20black-knight.pdf&quot; rel=&quot;noreferrer&quot;&gt;lost a trunnion thermal cover&lt;/a&gt; during STS-88.  Photos of this caused some UFO nonsense but the linked article debunks it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Tlk0Z.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Tlk0Z.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.833" LastActivityDate="2016-08-12T12:36:03.837" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17742" PostTypeId="1" CreationDate="2016-08-12T05:51:43.623" Score="2" ViewCount="140" Body="&lt;p&gt;&lt;a href=&quot;https://www.newscientist.com/article/2100700-mystery-object-in-weird-orbit-beyond-neptune-cannot-be-explained/&quot; rel=&quot;nofollow&quot;&gt;https://www.newscientist.com/article/2100700-mystery-object-in-weird-orbit-beyond-neptune-cannot-be-explained/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What could be potential reasons for &#xA;a) the object only now being discovered (200km in size expected)&#xA;b) it spinning opposite ?&lt;/p&gt;&#xA;" OwnerUserId="16357" LastActivityDate="2016-08-12T09:04:17.323" Title="Niku - potential object beyond Neptune spinning counterclock-wise" Tags="|orbital-mechanics|solar-system|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17743" PostTypeId="2" ParentId="17739" CreationDate="2016-08-12T08:14:30.217" Score="7" Body="&lt;p&gt;To add to the list, don't forget the large Ammonia Tank that Clay Anderson jettisoned in his EVA (23 Jul 2007):&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nnsTk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nnsTk.jpg&quot; alt=&quot;The Living Moon Image&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sub&gt;&lt;sup&gt;Source: &lt;a href=&quot;https://www.thelivingmoon.com/41pegasus/02files/Space_Debris_03.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Living Moon&lt;/a&gt;&lt;/sup&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;Further jettison 27th January; Glover has just tossed the Columbus Ka Band Antenna Cover over the side:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XuprJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XuprJ.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;On 2nd June a Russian Spacewalk jettisoned the &amp;quot;Zarya Fluid Flow Regulator in an airtight container&amp;quot;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0hidc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0hidc.jpg&quot; alt=&quot;Capture from NASA TV&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="8162" LastEditDate="2021-06-02T11:46:08.427" LastActivityDate="2021-06-02T11:46:08.427" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="17744" PostTypeId="2" ParentId="17742" CreationDate="2016-08-12T09:04:17.323" Score="3" Body="&lt;p&gt;The combination of size and distance is the reason it wasn't discovered earlier.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://xkcd.com/1633/&quot; rel=&quot;nofollow noreferrer&quot;&gt;XKCD graphic&lt;/a&gt; shows this pretty well:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CueSa.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CueSa.png&quot; alt=&quot;Graphic of planet size vs. distance&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anything below and to the right of the areas &quot;stuff we can see through telescopes&quot; and &quot;planets ruled out by the WISE survey&quot; is too small to see from Earth. Improvements in telescopes and data processing are gradually moving those boundaries to the right.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To find a planet, you have to take photos of the sky at intervals and compare them. The stars will be in the same position in both photos, the planets will have moved. Doing such a survey of the whole sky is difficult. Doing it manually is a monumental task (it took 4 years to discover Pluto), these days computers can help with that. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-12T09:04:17.323" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17745" PostTypeId="1" CreationDate="2016-08-12T15:03:35.167" Score="4" ViewCount="102" Body="&lt;p&gt;Gwynne Shotwell at SmallSat 2016 in Utah mentioned that SpaceX would be using the JCSAT-14 recovered first stage as a fleet leader in wear and tear. That they would be mounting it on the McGregor, TX test stand in between operational stage testing campaigns and running as many full duration burn tests as they can before some unspecified end condition occurs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, run the thing to destruction minus one. (A full up RUD on the test stand could potentially damage the stand, which would affect all operational stages, so would be bad, thus the minus one.  Stop just before it destroys the stand or even the stage).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the end of July 2016 reports suggested that three full duration burns (2+ minutes) had already been run on the test stand, and it had to be removed to allow testing of the AMOS-6 first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This raises an interesting (to me, anyway) question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How long does it take, if the stand is empty and ready to go, to mount a first stage, run the test, clean up the stand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then potentially run a second test, or remove the stage and ready the stand for another stage to be tested.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some ways this would implicitly answer another question of the maximum bandwidth of McGregor to test first stages. (Number of work days in a year/number of days to do a test). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This also would provide insight into how much of a dent in operational missions the testing of JCSAT-14 will incur, giving a hint of at some of the costs of reusability development.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-12T15:03:35.167" Title="How long does it take SpaceX to mount a stage on the McGrgeor test stand, run the test, and dismount it?" Tags="|spacex|falcon-9|testing|first-stage|mcgregor-tx|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17746" PostTypeId="1" CreationDate="2016-08-12T16:49:38.813" Score="3" ViewCount="320" Body="&lt;p&gt;Could we place a satellite in the wake of the Earth's orbit so that when the Earth completes its orbit of the sun it would effectively meet back up with said satellite?&lt;/p&gt;&#xA;" OwnerUserId="16598" LastEditorUserId="12978" LastEditDate="2016-08-12T19:38:49.650" LastActivityDate="2019-07-19T01:08:30.770" Title="&quot;Stationary&quot; Space Probe" Tags="|orbit|probe|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17747" PostTypeId="2" ParentId="17746" CreationDate="2016-08-12T17:17:57.253" Score="2" Body="&lt;p&gt;No. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you place a space probe in the wake of earth, outside earth sphere of influence, it will just follow the earth, orbiting at a similar speed, never reaching the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You cannot orbit the earth and remain in it's wake (you wouldn't be orbiting).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To reach the sun from earth, you would need to cancel the speed of your orbit around the sun, not &quot;orbiting in it's wake&quot;&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-08-12T17:17:57.253" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17748" PostTypeId="2" ParentId="17746" CreationDate="2016-08-12T17:19:22.410" Score="5" Body="&lt;p&gt;No.  At least not the way you're thinking of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason that the Earth orbits around the Sun is that the Sun's gravity is always pulling it inwards.  This is not unlike an object being swung around in a circle on a rope:  if you do this, you always have to pull the object inwards to maintain the circular path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A satellite that was somehow made &quot;stationary&quot; in the solar system would still feel the Sun's gravity, but wouldn't be moving around in a circle.  So it would just fall straight into the Sun.  You could, if you wanted to, put a satellite on an elliptical orbit that had a period of one year.  This satellite would leave Earth, travel on its own orbit through the solar system, and then would happen to meet up with Earth a year later.  But this wouldn't be a &quot;stationary&quot; satellite.&lt;/p&gt;&#xA;" OwnerUserId="10884" LastActivityDate="2016-08-12T17:19:22.410" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17751" PostTypeId="2" ParentId="17746" CreationDate="2016-08-12T20:48:42.857" Score="2" Body="&lt;p&gt;Yes, but it wouldn't be a traditional satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth is constantly being pulled towards the Sun by the Sun's gravity.  It orbits because it's also moving sideways fast enough that gravity only acts to bend its motion into a circle.  If you launched a satellite in a way to cancel the speed it had from being on Earth, it would fall into the Sun in short order.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since your probe isn't able to stay out of the Sun by moving sideways, you need some other way to deal with the Sun's gravity.  Fortunately, there is such a way: the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind&quot; rel=&quot;nofollow&quot;&gt;solar wind&lt;/a&gt; is a constant flow of high-energy particles moving away from the Sun, while &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pressure&quot; rel=&quot;nofollow&quot;&gt;light itself can exert a force on objects&lt;/a&gt;.  You can harvest these forces with a &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_sail&quot; rel=&quot;nofollow&quot;&gt;magnetic sail&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;nofollow&quot;&gt;solar sail&lt;/a&gt; to let your probe stay in place while waiting for the Earth to meet up with it again.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2016-08-12T20:48:42.857" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17752" PostTypeId="2" ParentId="17740" CreationDate="2016-08-12T20:52:24.977" Score="5" Body="&lt;p&gt;The first expense is of course rent and taxes for the land the spaceport is on, after that there are a number of infrastructure maintenance expenses involved.  A basic spaceport is going to have a lot to maintain.  For example the Kodiak complex has a clean room for final satellite preparations, a vehicle assembly building, as well as a mission control center with communications and computer systems.  Large buildings like these, especially ones with complicated HVAC systems, like clean rooms, and IT infrastructure, like mission control, will require constant maintenance.  Kodiak seems to only launch solid fuel rockets, but other launch sites use cryogenic liquid fuel or oxidizers, requiring extensive tank farms with added maintenance requirements.  On top of that the pads and roadways will require upkeep as well as other general groundskeeper like tasks (cutting the grass or shoveling the snow in Alaska).&#xA;For a small site this could likely be done with a smallish staff likely &amp;lt;50 people onsite full time, this number would go up with a launch bringing in a lot of mission specialists and other personnel.  There are also going to be the normal materials and equipment costs associated with maintaining the infrastructure.  A big cost is transportation, these launch sites are usually in isolated areas making moving people and materials there expensive. A large part of the expenses are also offsite, from the corporation running it, which has business offices in Anchorage and Huntsville with staff in each office to pay for, normally these would be minor expenses, but it all adds up when you only average 1 launch a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their operating expenses are only around 10-15 million dollars a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kodiak has a lot of information on their website, including annual reports with detailed breakdowns of costs and revenue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://akaerospace.com/about-us/reports&quot; rel=&quot;nofollow&quot;&gt;http://akaerospace.com/about-us/reports&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15824" LastEditorUserId="15824" LastEditDate="2016-08-12T21:10:24.047" LastActivityDate="2016-08-12T21:10:24.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17753" PostTypeId="2" ParentId="5423" CreationDate="2016-08-12T22:15:09.180" Score="10" Body="&lt;h3&gt;The problem&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;The GPS receiver's time has to be synchronized with atomic clocks located in GPS satellites. It is kinda &lt;em&gt;chicken or the egg&lt;/em&gt; problem. The receiver needs precise time to calculate precise distance and precise distance to synchronize time (to calculate the time difference).&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;The answer&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;It is the fourth satellite that gives your receiver the precise time.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;The explanation&lt;/h3&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If you only have distance to satellite A you can be located anywhere on a &lt;strong&gt;sphere&lt;/strong&gt; around A.&lt;/li&gt;&#xA;&lt;li&gt;If you add distance to satellite B, you can be located anywhere in the intersection of spheres around A and B, which is gonna be a &lt;strong&gt;circle&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;li&gt;If you add distance to satellite C, you can be located in &lt;strong&gt;two points&lt;/strong&gt;, which are the intersection of the circle and sphere around C.&lt;/li&gt;&#xA;&lt;li&gt;If you add distance to satellite D, one of the two points becomes &lt;strong&gt;the point&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But this is the perfect scenario - you've got 4 precise distances and they perfectly fit into single point. But precise measurement of distances requires synchronized time on your receiver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what if your receiver's time is not synchronized with satellites? In that case the fourth measurement - the distance to satellite D - will be way off. It won't align with neither of the &lt;strong&gt;two points&lt;/strong&gt;, but it will be notably closer to one of them. Let's call the distance between &lt;strong&gt;the point&lt;/strong&gt; and the sphere around satellite D &lt;strong&gt;the error&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point the receiver solves the problem backwards. We know that if the receiver has precise time and 4 distances it should yield precise position. So by adjusting the unsynchronized time of your receiver and recalculating &lt;strong&gt;the error&lt;/strong&gt; with the adjusted time, &lt;strong&gt;the error&lt;/strong&gt; increases or decreases. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Bottom line&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;The process of synchronizing time is reduced to the problem of minimizing &lt;strong&gt;the error&lt;/strong&gt; by adjusting local time. Once &lt;strong&gt;the error&lt;/strong&gt; is minimized, the precision of receiver's time is maximized.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h3&gt;What's so special about the fourth measurement?&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Imagine you have 4 distances. Pick &lt;strong&gt;any&lt;/strong&gt; 3 distances of them and if their spheres overlap, they will create two points where all of them intersect. No matter how imprecise the 3 distances are, as long as their spheres overlap they will form two perfectly defined points. But that does not mean the points relate to the real world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fourth measurement is the one that cannot be loose, in order to the 4 spheres overlap in single point.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;What's the purpose of the fourth measurement?&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;In theory, 3 satellites can give you enough data to reduce your possible location to two points. And one of the points usually will be in absurd distance or moving absurd speed so it could be ruled out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the fourth satellite is not required to rule out one of the two points but really to synchronize the time of your receiver. Because without that the two points produced by overlapping only three spheres are not reliable &lt;strong&gt;at all.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Does the receiver just assume its time is precise enough for the initial measurements?&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Yes.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h3&gt;Notes&lt;/h3&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;when I say that the distances perfectly fit into single point, I actually mean that the time error is minimized and other sources of error are not considered (atmospheric etc.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;this answers is wrong as @AnthonyX points out &lt;a href=&quot;https://space.stackexchange.com/questions/5423/how-does-gps-receiver-synchronize-time-with-gps-satellites/17753#comment46854_17753&quot;&gt;below&lt;/a&gt;, but still there is little bit of &lt;a href=&quot;https://space.stackexchange.com/questions/5423/how-does-gps-receiver-synchronize-time-with-gps-satellites/17753#comment46858_17753&quot;&gt;truth&lt;/a&gt; (I think) in there&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="16603" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-09-03T10:05:19.260" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17754" PostTypeId="1" CreationDate="2016-08-13T01:09:14.747" Score="10" ViewCount="929" Body="&lt;p&gt;A lot of satellites use hydrazine propellant but the Prospector 1 is using water.  What are the benefits and drawbacks of that decision? &lt;/p&gt;&#xA;" OwnerUserId="16606" LastEditorUserId="6816" LastEditDate="2016-08-13T01:30:58.680" LastActivityDate="2017-09-17T03:37:41.913" Title="Why is the Deep Space Industries Prospector 1 using water propellant instead of hydrazine?" Tags="|propulsion|fuel|asteroid|deep-space|liquid-fuel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17755" PostTypeId="2" ParentId="17754" CreationDate="2016-08-13T01:29:52.100" Score="12" Body="&lt;p&gt;Unlike chemical rockets such as hydrazine (a monopropellant, which doesn't require separate oxidizer and fuel to burn), water is just the reaction mass &amp;mdash; not the energy source. The Comet-1 thruster that the DSi Prospector series is using is electrothermal, so it requires a substantial electrical power supply in order to energize the water (presumably by turning it to steam). According to their &lt;a href=&quot;http://deepspaceindustries.com/wp-content/uploads/2016/08/DSI_Comet1_Thruster_Specs_4.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;data sheet&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Comet-1 thrusters are the ideal balance of cost and performance, occupying a place in the market between low-cost, low-performance cold gas and resistojets, and high-cost, high-performance monopropellant and electric systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In other words, the tradeoff is that in exchange for lower cost (and In-Situ Resource Utilization, &lt;a href=&quot;/questions/tagged/isru&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;isru&amp;#39;&quot; rel=&quot;tag&quot;&gt;isru&lt;/a&gt;), $I_{SP}$ and probably specific power are lower than hydrazine, and $I_{SP}$ is much lower than ion engines. Since the Prospectors are intended to refuel as they go, and therefore they don't need to carry anywhere near all their fuel at launch, low $I_{SP}$ (150&amp;ndash;175 s) isn't quite such a problem, but it's still a limiting factor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose the mission is to harvest two asteroids close enough that only 250 m/s $\Delta V$ is necessary to reach them from a suitable parking orbit. Each trip, the miner retrieves 10% of its dry mass in payload and leaves it in the same parking orbit. (These are made-up numbers, but they shouldn't be too far off, and things get more dramatic with higher $\Delta V$ or more payload.) With a 450 s $I_{SP}$ from a $LH_2 / LO_2$ chemical rocket (the best you're going to get), that requires a fuel fraction of about 21%, which is not too bad. But with the Comet-1, you can launch with only enough reaction mass to get to one of them and a tank large enough to contain a full round trip, then refuel at every asteroid harvested. The result is launching with only about a 14% remass fraction, which (even with the larger tank) is likely a cost improvement. And that stays the same no matter how many missions you go on, whereas with ten asteroids, $H_2 / LO_2$ requires a starting fuel fraction of about 69%, which is 2½ times the wet mass (starting mass) of the two-asteroid mission. (Doubling the asteroids to twenty increases the wet mass by almost another factor of 3, to 8 times the mass for two.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the eventual plan at Planetary Resources is to electrolyze the water brought back and store $LH_2$ and $LO_2$ in orbital fuel depots for further use. This has all the advantage of easy refueling between trips &lt;em&gt;and&lt;/em&gt; the much better $I_{SP}$ of hydrolox without the larger solar panels on the craft, but requires more complicated engines, routine fully automated docking, and, crucially, long-term storage of cryogenic fuel in space, which has not yet been done and probably relies on &lt;a href=&quot;https://space.stackexchange.com/questions/9244/what-cryogenic-selective-surface-materials-have-high-technology-readiness-level&quot;&gt;cryogenic selective surfaces&lt;/a&gt; to be practical.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I used &lt;a href=&quot;http://quantumg.net/rocketeq.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this rocket equation calculator&lt;/a&gt; for the initial figures, then automated calculating $\Delta V$ for ten- and twenty-asteroid missions with this PowerShell script:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;$dry = 1000&#xA;$pay = 100&#xA;$dv = 250&#xA;$v_e = 450*9.81&#xA;$asteroids = 10        # or 20&#xA;$mass = $dry&#xA;1..(2*$asteroids) | ForEach-Object {&#xA;    if ($_ % 2 -eq 0) {&#xA;        # Outgoing trip, no payload&#xA;        $mass -= $pay&#xA;    }&#xA;    else {&#xA;        # Incoming trip, payload&#xA;        $mass += $pay&#xA;    }&#xA;    $mass *= [Math]::Exp($dv / $v_e)&#xA;}&#xA;$mass&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2017-09-17T03:37:41.913" LastActivityDate="2017-09-17T03:37:41.913" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17756" PostTypeId="1" AcceptedAnswerId="17759" CreationDate="2016-08-13T03:20:52.083" Score="3" ViewCount="1666" Body="&lt;p&gt;What are the limitations for the 1st stage liquid fueled rocket engines that are currently in widespread use, what are the factors that limit their total thrust? Why can't you just inject more and more fuel in the same sized engine to produce more thrust for example??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By limitations, I mean the largest factors that stop these engines from producing more thrust. I would assume that it's something like the heating of the engine, or the rate at which the fuel can be burnt / passed through into the combustion chamber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could you also explain how these factors limit the thrust produced?&lt;/p&gt;&#xA;" OwnerUserId="16607" LastEditorUserId="16607" LastEditDate="2016-08-13T13:18:52.127" LastActivityDate="2016-08-19T01:06:16.643" Title="Limiting factors of liquid rocket engine thrust" Tags="|rockets|engine-design|engines|liquid-fuel|thrust|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17757" PostTypeId="2" ParentId="17756" CreationDate="2016-08-13T08:06:25.570" Score="5" Body="&lt;p&gt;Several issues come to mind: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Temperature and pressure of the combustion chamber (increase them enough and the walls will deform or melt). Can be mitigated by cooling the chamber walls.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Combustion instability: the larger the engine, the more chance you have to get instability. This was a &lt;a href=&quot;http://heroicrelics.org/info/f-1/f-1-injector.html&quot; rel=&quot;noreferrer&quot;&gt;big issue in development of the F-1 engine&lt;/a&gt; (Saturn V first stage). The issue was solved through lots of testing of different injector configurations. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Injection density. Higher pressures require more propellant so you need more injectors until most of the chamber wall consists of injector holes.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The F-1 injector had to have what was described as an &quot;extraordinarily high injection density,&quot; approximately 5 pounds of propellant per square inch per second.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-13T08:06:25.570" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17758" PostTypeId="2" ParentId="14317" CreationDate="2016-08-13T13:35:49.503" Score="3" Body="&lt;p&gt;I believe that the Voyager receiver uses a PLL, phase locked loop to acquire the signal and filter out the solar noise. Also, the solar radiation is different to the transmitted signal which Voyager can use to differentiate between the noise with a band pass filter and some other electronics. &lt;/p&gt;&#xA;" OwnerUserId="16607" LastActivityDate="2016-08-13T13:35:49.503" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17759" PostTypeId="2" ParentId="17756" CreationDate="2016-08-13T14:13:58.930" Score="8" Body="&lt;p&gt;To move more propellant faster into the combustion chamber increases the chamber pressure; this requires a larger, more powerful turbopump. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;High pressure, high volume turbopumps are hard to design - most of the problems encountered in developing the space shuttle main engines were pump failures, some catastrophic. There are some terrifying numbers associated with the SSMEs -- each engine's pumps, turning at about 30,000 RPM, produce something like 100,000 HP of mechanical power just to move propellants.   (&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#Turbopumps&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine#Turbopumps&lt;/a&gt; ) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SpaceX Merlin series, while using a much more conservative pump design, is still likely pump-limited; they want to be able to reuse those engines for a long time without the total overhaul required of the SSME, so they can't risk damaging the pumps by driving them too hard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some point it's easier to gain performance by making a larger, lower pressure engine. Taken to the limit, this approach gives you Sea Dragon -- an extremely large, powerful, low-pressure engine. &lt;a href=&quot;https://space.stackexchange.com/questions/16816/how-realistic-would-the-sea-dragon-engine-be-to-produce-given-todays-technology&quot;&gt;How realistic would the Sea Dragon engine be to produce given today&amp;#39;s technology?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-08-13T15:44:32.827" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17760" PostTypeId="2" ParentId="17639" CreationDate="2016-08-13T15:11:51.433" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://physicsdatabase.com/2015/11/21/seti-searching-for-planets-in-alpha-centauri/&quot; rel=&quot;noreferrer&quot;&gt;Yes&lt;/a&gt;, SETI &lt;a href=&quot;http://io9.gizmodo.com/5952357/scientists-discover-a-planet-in-alpha-centauri-the-star-system-nearest-earth&quot; rel=&quot;noreferrer&quot;&gt;has listened&lt;/a&gt; to Alpha Centauri. &lt;a href=&quot;http://iopscience.iop.org/article/10.1086/379320/fulltext/&quot; rel=&quot;noreferrer&quot;&gt;Here's a 2003 list of targets&lt;/a&gt; for SETI, which includes all of the 100 closest stars.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-13T15:11:51.433" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17761" PostTypeId="1" CreationDate="2016-08-13T18:52:27.080" Score="0" ViewCount="3579" Body="&lt;p&gt;The second stage of Apollo 11 generated a greater than 20 deg shock-wave half angle which corresponds to less than $1000 \frac{m}{s}$ forward speed not 6000 mph ($2682.24 \frac{m}{s}$)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I measured the half angle of the shock-wave generated by the second stage of Apollo 11 after separation (see the picture). This angle is greater than 20 deg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using &lt;a href=&quot;http://wordpress.mrreid.org/2012/12/15/sonic-booms-and-mach-cones/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the relation that gives the velocity of an object as a function of the speed of sound and the half angle of the shock-wave&lt;/a&gt;, I got:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${{V}_{Stage\ 2}}&amp;lt;\frac{1225\frac{km}{h}}{\sin \left( 20{}^\circ  \right)}=994.90\frac{m}{s}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.msfc.nasa.gov/saturn_apollo/documents/First_Stage.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;According to NASA&lt;/a&gt;, at burnout, the first stage had 6000 mph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In appears that, after separation, stage II advanced only at 994.9 m/s / 6000 mph = 37.09% of the stated speed (6000 mph). &lt;em&gt;How is it possible?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MIlGt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MIlGt.jpg&quot; alt=&quot;First Stage Separation, Apollo 11&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://www.youtube.com/watch?v=cbavpuNKGxk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Video, First Stage Separation, Apollo 11&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more clear picture of the same event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SJzW7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SJzW7.jpg&quot; alt=&quot;First Stage Separation, Apollo 11&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another picture, from the same video, shows the shock-wave created by the leading edge of Saturn V. It is visible due to the cloud of gas formed around the entire rocket when the first stage retrorockets fired during separation. The same half angle, greater than 20 degree, appears.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aD3yF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aD3yF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://www.youtube.com/watch?v=cbavpuNKGxk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Video, First Stage Separation, Apollo 11&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user16613" LastEditorUserId="1054" LastEditDate="2017-12-18T18:31:55.637" LastActivityDate="2017-12-18T18:31:55.637" Title="The speed of Apollo 11 at first stage separation was apparently only about 40% of what NASA claims? What does this indicate?" Tags="|apollo-11|shockwave|" AnswerCount="3" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17762" PostTypeId="2" ParentId="17761" CreationDate="2016-08-13T19:41:05.213" Score="23" Body="&lt;p&gt;That is not a shockwave. It looks like the edge of the second stage engine exhaust plume.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it were a shockwave, more shockwaves should be visible: one at each point where the diameter of the rocket changes, so escape tower, CM, both interstages.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shockwaves are rarely visible. In the lower atmosphere you can sometimes see condensation in the low-pressure area aft of the shockwave (a vapor cone). You normally need a Schlieren camera to see the shockwave itself.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've reviewed a number of launch videos. No shockwaves visible anywhere. The vapor cone is visible &lt;a href=&quot;https://youtu.be/CXIW1eEzrFM?t=189&quot; rel=&quot;noreferrer&quot;&gt;around Mach 1&lt;/a&gt;, not at higher speeds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stage separation occurred at 67 km, atmospheric pressure at that altitude is very low. I don't think there's enough air (or water in the air) to create a visible shockwave. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the &lt;a href=&quot;http://web.mit.edu/digitalapollo/Documents/Chapter5/saturnas501.pdf&quot; rel=&quot;noreferrer&quot;&gt;separation sequence&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;S1C engine cutoff at 152 seconds after liftoff. &lt;a href=&quot;http://www.cosmoquest.org/forum/showthread.php?45921-Saturn-V-1st-stage-separation&quot; rel=&quot;noreferrer&quot;&gt;After cutoff, it takes about 4 seconds&lt;/a&gt; for thrust to decay to zero.  &lt;/li&gt;&#xA;&lt;li&gt;S1C separates and S2 ullage fires at 153 s&lt;/li&gt;&#xA;&lt;li&gt;S2 engines start at 154 s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update&lt;/strong&gt;:&#xA;Here are some images of actual shockwaves. This is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Shadowgraph&quot; rel=&quot;noreferrer&quot;&gt;shadowgraph&lt;/a&gt; &lt;a href=&quot;http://www.aerospaceweb.org/question/aerodynamics/q0074.shtml&quot; rel=&quot;noreferrer&quot;&gt;of a bullet&lt;/a&gt;. A shadowgraph is a way to photograph objects that makes shockwaves more visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J3kju.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J3kju.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we see a prominent shockwave at the tip of the bullet, plus smaller ones aft which are at a different angle. This means you can't reliably measure speed from the angle of the shockwave alone. &#xA;You can also see the turbulent wake region aft of the bullet. This region has the lowest pressure, and will be the first to be filled with exhaust products.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's &lt;a href=&quot;http://www.billzilla.org/dunnomotor.htm&quot; rel=&quot;noreferrer&quot;&gt;Thrust SSC&lt;/a&gt; going at Mach 1:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/8TsAn.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8TsAn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the shockwave going almost perpendicular to the vehicle, and the exhaust products (mixed with dust) that stay in a much narrower cone. More precisely you don't see the shockwave itself, but the dust that gets picked up off the desert floor by the shockwave. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a final sanity check: if the Saturn V were moving at 1/3 its design speed at first stage separation, the mission would have failed because the CSM wouldn't have reached orbit. We know that didn't happen. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-12-03T09:58:44.457" LastActivityDate="2016-12-03T09:58:44.457" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17763" PostTypeId="2" ParentId="17761" CreationDate="2016-08-13T19:46:13.820" Score="14" Body="&lt;p&gt;The main issue is that the image of the rocket is not taken from the side, but at an angle. We can determine the angle by looking at the apparent length to width ratio of the two stages - note that this screenshot has been taken directly after separation. The first stage seems to be twice as long as wide, the second stage is about three times longer than wide. Comparison with the actual size show a diameter of 10 m and a length of 40 m and 60 m respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means, all lengths seem to be shortened by a factor of 2. In particular, this makes the angle of the Mach cone seem to be twice as big as it actually is. After restoring the correct aspect ratio, I measure 18 degrees, corresponding to a Mach speed of 6.5, or 2200 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the poor quality of the image the number is reasonably similar to the expected 2700 m/s. As Hobbes pointed out, we can not be sure that it actually is a Mach cone, but is rather close to the expected one. I'm also in favor for the explanation as the outer edge of the second stage engine plume.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2016-08-13T20:46:53.697" LastActivityDate="2016-08-13T20:46:53.697" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17764" PostTypeId="1" CreationDate="2016-08-13T21:51:06.023" Score="2" ViewCount="138" Body="&lt;p&gt;According to Wikipedia (&lt;a href=&quot;https://en.wikipedia.org/wiki/Nova_%28rocket%29&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Nova_%28rocket%29&lt;/a&gt;) the &lt;em&gt;late&lt;/em&gt; Nova rocket was a &quot;larger version&quot; of the Saturn 5, meant to send manned missions to Mars. Some requests for proposals were obtained, but the whole project was sadly scrapped once it became apparent NASA would not receive a decent budget any longer once Apollo was cancelled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question: What was the best design among the received proposals? By best here it is meant what NASA regarded as the best at the time. Were they even properly evaluated by NASA before the project was scrapped? If not, then the answer is simply they were not evaluated. Perhaps they have been evaluated later just for academic interest?&lt;/p&gt;&#xA;" OwnerUserId="8707" LastEditorUserId="8707" LastEditDate="2016-08-14T05:16:33.950" LastActivityDate="2016-08-14T05:16:33.950" Title="What was the best late Nova rocket design among the request for proposals NASA received?" Tags="|mars|apollo-program|rocket-equation|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17765" PostTypeId="1" CreationDate="2016-08-14T03:05:03.863" Score="3" ViewCount="1266" Body="&lt;p&gt;If we imagine an extra-terrestrial civilization which is on an exact technological par with Earth, and has been leaking radio signals into space exactly the way we have been doing, how far away can it be from Earth and still be detectable as a technological world? The focus of my question is on the nature of incidental radio emissions into space - both how detectable they will be, and how recognizably artificial they would be to us here and now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider an extra-terrestrial civilization on an Earth-like world develops technologically just as we have. At their equivalent of our 20th century, they start broadcasting radio signals around their world in the same bands and at the same power levels as has been used here. And, just as has happened here, some of those radio waves leak out into space. SETI just happens to have a dish (say one of the VLA dishes) pointed right at their world. How far away could they be before all those signals become indistinguishable from noise? Let's assume a similar ebb and flow of signal strength and clutter just as we have produced over the decades. How would degree of clutter from numerous transmission sources affect the overall signature as a technological world? Would a technological world like Earth have to be relatively nearby for cluttered artificial emissions not to be missed or dismissed as natural? Can we detect (and recognize as such) the incidental emissions leaked from a late-20th-century-equivalent world, if such existed, hundreds or thousands of light years away?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could express the same question in the reverse case: A hypothetical technological world equipped with radio telescopes exactly equivalent to what exist on Earth today, with one pointed at us. At 20 light-years distance, would our circa 1996 emissions be both detectable to such equipment and recognizably artificial? What about our c.1986 emissions arriving on such a world 30 ly away? What happens as we go further out (and further back into Earth's radio history)? Is a less cluttered spectrum more easily recognizable as artificial? As we have gotten noisier, have we become more easily detectable? Have we become more or less easily recognizable as a technological world through our incidental emissions? Have we passed through a peak, and if so, how many Sun-like stars are now inside that radio &quot;shell&quot;?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-08-14T03:05:03.863" Title="From how far away could our SETI searches detect and recognize our technological signals?" Tags="|seti|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17766" PostTypeId="1" AcceptedAnswerId="17769" CreationDate="2016-08-14T04:05:40.930" Score="1" ViewCount="410" Body="&lt;p&gt;In the NYTimes article &lt;a href=&quot;http://www.nytimes.com/2016/08/04/arts/music/third-man-records-space.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Third Man Records Sends a Vinyl Record Into Space&lt;/a&gt; an LP record player was designed and built to be flown in a balloon. It seems to have achieved only about 28.5 km which is definitely not &quot;Space&quot; proper. The &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kármán line&lt;/a&gt; is generally considered to be 100km, and I can't find any example of a balloon even suggested to reach past 50 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What would be the technical challenges that would have to be overcome to do it right - to play a vinyl record (of Carl Sagan &quot;singing&quot;) in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first two &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_Golden_Record&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sounds of Earth&lt;/a&gt; &quot;gold records in space&quot; were carried by Voyager 1 and Voyager 2 and Carl Sagan was a major proponent and influence there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This current effort intends to put a &lt;em&gt;record of Carl Sagan&lt;/em&gt; into space as a tribute to his tremendous positive influence on both the science of &lt;strong&gt;space exploration&lt;/strong&gt;, and the public's understanding and appreciation of &lt;strong&gt;space exploration&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uwECo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uwECo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/z1751oqIKYc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In the words of Professors Carl Sagan and Stephen Hawking:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/zSgiXGELjbc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-08-14T04:29:12.327" LastActivityDate="2019-01-22T09:01:31.540" Title="What technical challenges would be playing this LP in (actual) space?" Tags="|balloons|space-art|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17768" PostTypeId="1" CreationDate="2016-08-14T06:14:29.850" Score="1" ViewCount="508" Body="&lt;p&gt;I think it may be related to the sun, but I don't understand the &quot;tooth&quot; shape in the GIF - is this a shadow of something? Is the spacecraft already spinning for the satellite deployment? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the sources of light. Possibly some combination of Sun, Earthshine, infrared/red thermal radiation from the nozzle, but is there also artificial illumination from the spacecraft for the camera?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't understand exactly what I'm looking at here (the mechanism making this pattern of light on the nozzle).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Full video &lt;a href=&quot;https://www.youtube.com/watch?v=QZTCEO0gvLo&quot; rel=&quot;nofollow noreferrer&quot;&gt;shown here&lt;/a&gt; - use the launch time code &lt;strong&gt;T+ 00:26:30&lt;/strong&gt; to &lt;strong&gt;00:28:30&lt;/strong&gt; for reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ow5WE.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ow5WE.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wt6tR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wt6tR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Early illustration of a 2nd stage, shown &lt;em&gt;without&lt;/em&gt; a large communications satellite payload from the Spaceflight 101 article &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9 v1.1 &amp;amp; F9R Launch Vehicle Overview&lt;/a&gt;. The dimension listed there as about 3.7m x 13.8m &lt;em&gt;without payload adapter&lt;/em&gt;. This is for reference only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second stage looks much shorter in the images &lt;a href=&quot;https://space.stackexchange.com/q/9166/12102&quot;&gt;in this question&lt;/a&gt; but it is described as an &quot;incomplete stage&quot;. It seems it might be &lt;a href=&quot;http://www.spacelaunchreport.com/falcon9.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;from 2010&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mFenC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mFenC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rypGi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rypGi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XjLda.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XjLda.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4DMEj.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4DMEj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9yrOB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9yrOB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oi0oB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oi0oB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-04-21T12:29:32.300" LastActivityDate="2019-04-21T12:29:32.300" Title="What makes this shadow pattern on the 2nd stage nozzle SpaceX JCSat-16?" Tags="|spacex|falcon-9|identify-this-object|" AnswerCount="0" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="17769" PostTypeId="2" ParentId="17766" CreationDate="2016-08-14T06:22:39.763" Score="4" Body="&lt;p&gt;If you want to do this in orbit, the main challenge is lack of gravity. A record player needs to apply a very specific amount of pressure onto the needle (about 1 gram), and just about every design uses an adjustable counterweight on the arm to achieve that. You'd have to add a spring that pulls or pushes the arm down. &#xA;There were a few turntables designed to operate vertically, I don't know what mechanism they used to get the correct needle pressure, though. Might still be gravity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Minor issues: you also need to push the record against the turntable to prevent it floating. And the torque from the turntable has to be handled by the spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://thirdmanrecords.com/news/success-history-is-made-as-third-man-records-plays-the-first-phonographic-record-in-space/&quot; rel=&quot;nofollow&quot;&gt;this article&lt;/a&gt; by the group that ran this mission, there was another issue: vinyl melts at the temperatures reached in space under full illumination by the sun: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As you rise higher and higher into the thinning atmosphere, temperature and increasing vacuum (lack of air) can cause issues. Vinyl has a rather low melting point (160°F), and without air to keep things cool, you could wind-up with a lump of melted plastic on your hands if a record is exposed to the sun for too long. Without air, things in direct sunlight can get very hot while things in shade can get very cold. This constant expansion and contraction can physically distort a vinyl record rendering it unplayable. so our turntable platter also served as a heat-sink in order to keep the vinyl cool in direct sunlight.” The gold plating on the record was another measure to keep the grooves from losing their shape.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The balloon didn't have a pressure capsule, so the record was played in the thin upper atmosphere. When you get high enough, the atmosphere becomes too thin to carry sound. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You also need a mechanism to keep the turntable and record safe during ascent and descent, you don't want the changing G-loading to damage the record or needle and pickup mechanism. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-08-14T11:09:10.887" LastActivityDate="2016-08-14T11:09:10.887" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17770" PostTypeId="2" ParentId="17766" CreationDate="2016-08-14T08:39:13.177" Score="3" Body="&lt;p&gt;As hinted by @Hobbes there were turntables that operated vertically; in fact some of them were designed to work in any orientation and work under variable gravity conditions as they were portables designed to play EPs while moving. A veritable disc based Walkman pre-cassettes. I saw one in my younger days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a similar one on show from the 1970s:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/br9kRKCOVw0?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They used a clutch to grip the disc (as is done with CDs and DVDs) and a spring loaded stylus to track the groove and apply the correct force. (It does, however, still have a tone arm but there is some built in compensation for lateral forces to enable it to resit the jolts and bumps of being used whilst carried and swung about. The only issue is the gyroscopic forces of the spinning disc to any space craft. Details of the operational mechanism is shown here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/cMeyrFgCzkY?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This technology developed through the '80s with the Sharp RP-114:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/u_YAmDbEuPg?start=402&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or the Sony PS-F5 or PS-F9:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/xPtaYbI3mFs?start=12&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other commercially available technology is the &lt;a href=&quot;http://elpj.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;laser phonograph&lt;/a&gt; which also has a clutch grip and uses a non-contact laser. This therefore has no tone arm and is much more resistant to lateral forces:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://elpj.com/wp-content/uploads/2015/04/LT-master-White_noshadow-300x221.jpeg&quot; alt=&quot;ELP laser Turntable&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I personally cannot see why Icarus went to trouble to re-invent what they could have bought on eBay to play the disc!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something like these can then just be sent up as cargo in a Progress or SpaceX; perhaps it would amuse one of the sponsors to do so! :-)&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="10273" LastEditDate="2019-01-22T09:01:31.540" LastActivityDate="2019-01-22T09:01:31.540" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="17771" PostTypeId="2" ParentId="3612" CreationDate="2016-08-14T09:09:52.867" Score="2" Body="&lt;p&gt;A different way to get to the sun is 9.5 km/s for getting to LEO, 3.5 km/s (less if you slingshot around moon) to escape to solar orbit, then 8.8 km/s for solar escape minus Jupiter/Saturn slingshot then before you escape burn retrograde till perisol is below solar surface. Total should be less than 21 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pluto's orbit is on average 10km/s so I'd guess 6-8 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources: Wikipedia, other answers, and I play lots of Kerbal with RSS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: launch into a prograde orbit near equator, otherwise it takes 10 km/s to get to orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note 2: use a fuel that does not evaporate over time.&lt;/p&gt;&#xA;" OwnerUserId="16619" LastEditorUserId="16619" LastEditDate="2016-08-14T09:25:11.240" LastActivityDate="2016-08-14T09:25:11.240" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17773" PostTypeId="1" AcceptedAnswerId="17774" CreationDate="2016-08-14T13:56:06.257" Score="17" ViewCount="3244" Body="&lt;p&gt;With the Saturn V, there were two critical moments during launch that I am curious about:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The vehicle COMMITs to liftoff&lt;/li&gt;&#xA;&lt;li&gt;The vehicle LIFTS OFF&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;LIFTOFF is fairly obvious: the booster has risen some small distance above the launch pad, and is basically on its way (but hasn't yet cleared the launch tower).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;COMMIT was the point, a moment before LIFTOFF, at which the vehicle was &lt;em&gt;committed&lt;/em&gt; to lift-off. In other words, no matter what else happened, it either rose from the launch pad, or things crashed and burned (and the LES likely saw use). Turning off the engines no longer caused it to settle back nicely on the pad with just a few tons less fuel than it had a few seconds earlier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So much for pretext for my actual questions, which I believe to be highly related:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;What specific event or condition defined the COMMIT state&lt;/strong&gt; of the vehicle? In other words, what caused the vehicle to &lt;em&gt;become committed&lt;/em&gt; to lift-off, as opposed to not being committed to lift-off a moment earlier?&lt;/li&gt;&#xA;&lt;li&gt;What specific events, possibly limited to those visible to an ideal external observer (in the physics sense), &lt;strong&gt;occured &lt;em&gt;in between&lt;/em&gt; COMMIT and LIFTOFF?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="415" LastActivityDate="2019-02-21T08:11:22.613" Title="In the Saturn V launch sequence, what defined COMMIT, and what happened between COMMIT and LIFTOFF?" Tags="|launch|launch-sequence|saturn-v|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17774" PostTypeId="2" ParentId="17773" CreationDate="2016-08-14T15:28:18.333" Score="20" Body="&lt;p&gt;&lt;a href=&quot;http://www.americaspace.com/?p=28278&quot;&gt;This article&lt;/a&gt; might offer some clarity. The Saturn V was held down to the pad until T=0. The engines started up several seconds before. The engines took a few seconds to build up to and stabilize at lift-off thrust. During that time, if an anomaly is detected, the engines &lt;em&gt;could&lt;/em&gt; be shut down, leaving the vehicle still safely anchored to the pad. Once the hold-downs are released, the option to shut down on the pad is gone because the vehicle &lt;em&gt;will&lt;/em&gt; lift off, hence &quot;launch commit&quot;. The lift-off call is a confirmation that lift-off has occurred.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-08-14T15:28:18.333" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17777" PostTypeId="1" CreationDate="2016-08-14T18:55:43.883" Score="3" ViewCount="9264" Body="&lt;p&gt;What is the breakdown of a typical rocket- say, a Delta IV? From what I've heard 99% of the cost is engineering labor but I haven't seen any source for that. I just need the manufacturing cost of the rocket, not launching, operating, etc.&lt;/p&gt;&#xA;&lt;p&gt;What is the typical cost percentage going to:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Engineering labor&lt;/p&gt;&#xA;&lt;p&gt;General/Administrative/profit&lt;/p&gt;&#xA;&lt;p&gt;Indirect labor&lt;/p&gt;&#xA;&lt;p&gt;Direct labor&lt;/p&gt;&#xA;&lt;p&gt;Capital&lt;/p&gt;&#xA;&lt;p&gt;Materials&lt;/p&gt;&#xA;&lt;p&gt;Balance of system&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;If that's too much to ask, a simple guess of the percent cost going to materials/labor/capital would be helpful.&lt;/p&gt;&#xA;" OwnerUserId="12649" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-23T19:31:26.313" Title="What is the rough breakdown of rocket costs?" Tags="|launch|rockets|cost|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17778" PostTypeId="1" CreationDate="2016-08-14T19:53:14.480" Score="1" ViewCount="308" Body="&lt;p&gt;A quick calculation for 1g spin gravity at a leisurely 1 round per 20min gives a radius of 360km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume, we find a strong enough material for lightweight cables, what are the challenges for building a Bola-style habitat (i.e. two stations rotating around a common center of mass)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is the large radius a problem on &quot;habitable&quot; earth orbits (tidal forces)?&lt;/li&gt;&#xA;&lt;li&gt;Is it difficult to maintain the common center of mass of such a large (but comparatively lightweight) structure?&lt;/li&gt;&#xA;&lt;li&gt;Is it difficult to start the desired rotation?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Which radius is practically possible?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-08-19T00:55:44.833" Title="What are the challenges of a large Bola style Habitat" Tags="|gravity|space-station|construction|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17780" PostTypeId="1" CreationDate="2016-08-14T22:54:59.410" Score="9" ViewCount="1843" Body="&lt;p&gt;Whenever communications to/from a space vehicle are described or discussed, there is inevitably a reference to the vehicle's &quot;high gain&quot; antenna. What is it about such an antenna (specific technical features) that makes it &quot;high gain&quot;, and what alternative equipment is fitted to a spacecraft that would require use of the qualifier &quot;high gain&quot;? Is there usually some sort of low or not-so-high gain antenna fitted?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2016-08-14T23:44:38.750" LastActivityDate="2016-09-29T23:26:30.170" Title="What makes an antenna &quot;high gain&quot;, and high gain as opposed to what alternative(s)?" Tags="|communication|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17781" PostTypeId="2" ParentId="17780" CreationDate="2016-08-14T23:29:30.127" Score="15" Body="&lt;p&gt;A higher gain antenna is more directional than a low-gain one.  In any given situation use of a high-gain antenna permits higher data rates but requires more accurate pointing of the antenna toward the other station.  In turn this requires that the spacecraft attitude determination and control be operating correctly.   If the HGA is fixed to the body, this also constrains the spacecraft attitude while using the HGA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In most cases a spacecraft will have both high and low gain antennas - high will normally be used for high-speed communications, and low in emergencies or during commissioning before attitude control is established.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;HGA&quot; and &quot;LGA&quot; are relative terms, and the HGA could be anything from Cassini's 4 meter diameter parabolic reflector to the quad array of ~4cm helical X-band antennas on Planet Labs' earth observation cubesats.  The LGA is usually somewhat omnidirectional, though because of the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Hairy_ball_theorem&quot;&gt;hairy ball theorem&lt;/a&gt; no antenna can be perfectly omnidirectional.  A monopole or dipole construction is typical (though the radiation pattern will differ from the classic mono/dipole patterns due to the spacecraft structure) or it could be a low-gain patch antenna which might still require some degree of spacecraft pointing.  Multiple low-gain antennas can be used, either multiplexed or connected to separate transceivers, to get better diversity of coverage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A high gain antenna might be deployable in order to achieve a large aperture while still fitting inside the rocket fairing, though after the Galileo failure designers may be nervous of that.  It could also be steerable either mechanically or electronically.  The LGA is usually fixed, both for reliability and because it doesn't need to be physically or electrically large, and doesn't need to be pointed.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2016-09-29T23:26:30.170" LastActivityDate="2016-09-29T23:26:30.170" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17782" PostTypeId="2" ParentId="4000" CreationDate="2016-08-14T23:38:36.687" Score="3" Body="&lt;p&gt;&lt;strong&gt;Sedna Sample Return&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if this is not feasible with current technology, it is a very interesting subject. Is it possible to send a Sedna Sample Return mission? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get an answer, we must look to space missions that have already been launched. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Vehicle design:&lt;/strong&gt; The spaceship must contain an orbiter and a lander. The lander will reach the surface of Sedna, will take a sample and will return. It must have enough propellant to land and to return. The orbiter must be used as a communication relay to Earth and must have enough propellant to return to Earth. The orbiter must also have an RTG, strong enough to function up to 100 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this, we should look at ESA's Rosetta and Philae spacecrafts. Sedna has a low gravity, but probably no atmosphere. So, we need propellant both at landing and at launch, but not in too large an amount. We don't know Sedna's escape velocity, but Pluto's is 1.2, 10 times smaller then Earth's. Philae had 100 kg. Our lander will have probably 250 kg, with all its fuel tanks full of hydrazine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbiter will have to survey and map Sedna, then it will deploy the lander, then it will wait, then capture the lander and return home. Rosetta had 1500 to 2000 kg, but New Horizons only 500 kg. We will need the lightest spaceship, to preserve hydrazine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lander can be battery powered. The orbiter will need an RTG. Plutonium used in an RTG has a half lifetime of over 80 years, but the thermocouples don't live that long. Basically, the energy produced by an RTG decreases by half in about 25 years. The mission will take probably 100 years, so energy production will decrease to less then 10%. This means that, without at least 100 kg of plutonium, a sample return mission is impossible. It could be possible to use a backup empty RTG, to reinsert plutonium in it. The probe, with lander and without its fuel, must not weight more then 1000 kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vehicle will include a lander, an orbiter, but also a huge amount of fuel needed to decrease ship velocity, and also fuel needed to send the probe back to Earth. We might have to send into space a total of 5000 kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launch.&lt;/strong&gt; The only way to send into space such a large spaceship is with the use of some of the largest existing rockets. An Atlas V would hardly be enough. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Trajectory.&lt;/strong&gt; Sending the probe directly into solar escape velocity is out of question. We have to use the advantages of Jupiter and possible Saturn flybys. If possible, a Mars or Earth flyby are also useful. This will also result in a longer time to reach Sedna. It will take at best 40 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sedna Approach Phase.&lt;/strong&gt; The probe will approach Sedna with high speed, probably 15 km/s. At that speed, it is impossible to be captured into orbit. We have to decrease speed somehow. Using conventional chemical engines is out of the question, because of the huge amount of fuel needed. The only solution will be a ion engine. As Dawn have shown us, with 400 kg of xenon, it managed to produce a total thrust close to 10 km/s. So, it is possible, if the RTG produces enough energy, to decrease speed from 15 km/s to 1 km/s in 12 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bonus Ship.&lt;/strong&gt; One extra mission that can benefit from a Sedna encounter is a fast flyby. We can add a 3rd spacecraft, with its own RTG and suite of science instruments, that will detach from the main ship before slowing down. The bonus ship will continue to move with the same speed and will soon reach the interstellar environment. Its mass can be of 300 kg, not much compared to the 5000 kg already needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sedna Orbit Phase.&lt;/strong&gt; Once in orbit, the probe will first enter a high-altitude orbit, to search for moons and characterize Sedna system. Just as Dawn did at Vesta and later at Ceres, our probe will enter closer orbits, mapping the dwarf planet and in the final phase, looking for a place for the lander. Finally, it will deploy the lander who will have a few days to touchdown, to explore the surface, to drill, to take samples and to return. To return to the orbiter, the lander will need to perform high-accuracy maneuvers. The orbit phase will probably take 5 years to complete. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the end of this phase, the spacecraft will need to detach all unnecessary weight. This might include much of its scientific payload, empty propellant tanks, the lander (except for the tank containing samples) and used RTG. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is also possible to make the whole spaceship land on Sedna, without the need of a special lander. However, this will result in more fuel needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Return phase.&lt;/strong&gt; Sedna has a low gravity. As shown above, the escape velocity for Pluto is a bit above 1 km/s. For Sedna, it is probably 0.7. The orbital speed is only 1.07 km/s. so, a spaceship will only need a 2 km/s thrust to detach Sedna's surface and to get to a 0 km/s speed relative to the Sun. At that point, solar gravity will do all the work and will take the ship on a trajectory towards the Sun. The resulting orbit will not be a straight line, but something looking more like the path of a long-term comet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the probe approaches, its speed will increase gradually. It will move very slowly up to the orbit of Neptune. To speed-up the process, almost all the remaining xenon should be used by the ion engines to increase speed. If not, it could take 400 years to reach Earth. Nobody wishes to wait that long. So, we must accelerate the ship to at least 10 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the probe passes the orbit of Neptune, its speed increases fast. At Earth's orbit, the spaceship can move with 30 km/s faster then Earth. It can become impossible to land on Earth with such a huge speed. To slow down, the best option is to use a Jupiter flyby, and possibly other flybys. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth Landing.&lt;/strong&gt; The final phase of the mission is an Earth landing. At that time, after almost 100 years, the RTG will produce only a very small amount of power. Many devices will not be working. Maybe, it will be a good idea to have onboard a small solar panel, to provide the power needed for the final phase of the mission. The spacecraft will conduct its final trajectory correction maneuvers, then it will detach the tank containing samples from Sedna. Only the samples will return, softly landing with the help of a parachute. The spacecraft will enter a solar orbit or will burn in the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Mission Conclusion:&lt;/strong&gt; After 100 years of travel and with great costs, the mission will return to Earth a sample from Sedna. Probably there will be samples from the surface, materials drilled from a few meters deep and an atmospheric sample (if Sedna has any). Also, the probe can bring dust samples acquired along the journey. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one major problem about this mission. On Sedna, many solid rocks can be in fact gasses on Earth. We know from Pluto, that its crust is made almost entirely of substances that on Earth should be gasses or liquids. Rocks brought from Sedna should melt or even evaporate. In the lab, scientists will have big problems analyzing the samples. The long needed time and degradation of the samples, together with high mission costs, are the reasons why a sample return mission to Sedna is unfeasible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S.: No spacecraft has flown for that long and nobody has ever designed complex electronic devices able to operate for 100 years. A mission to Sedna requires that all devices will still be functioning after many years. Also, for the return phase, we must make sure that navigation computers, engines, the RTG and reaction wheels are working well until the end of the mission. A sample return mission will be at high risk because we have not enough time to test all systems and sub-systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, there are many unknown things. Nobody knows what the mass and gravity of Sedna is. We must design the ship to the highest estimated mass, to make sure we have enough propellant. And, as we don't know the exact mass, there is no way to know how much fuel will remain for the return phase. If we have more fuel available for the return phase, we can get our samples faster, but we will have to re-calculate the return route and Jupiter flyby to decrease speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, since we don't know the mass, we don't know how to enter orbit. All these not known parameters means that many parts of the mission will have to be calculated fast and only when the ship will get to a certain point. There will not be much time for estimations and backup plans. Powerful computers on Earth should decide what to do within hours. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Communication delay is big, because of distance. There will not be much time for analyzing problems. It can be a very good thing to program the ship to automatically find the best trajectory. &lt;/p&gt;&#xA;" OwnerUserId="14629" LastEditorUserId="1142" LastEditDate="2016-08-15T00:35:46.660" LastActivityDate="2016-08-15T00:35:46.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17783" PostTypeId="2" ParentId="6550" CreationDate="2016-08-14T23:57:06.283" Score="1" Body="&lt;p&gt;The Orion emergency escape system is of the old Apollo pull rocket and shroud system.  The CST-100 has a more modern pusher RS-88 rocket system for emergency escape.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CST-100 is designed to be reusable while the Orion is a one use space craft.&lt;/p&gt;&#xA;" OwnerUserId="16623" LastActivityDate="2016-08-14T23:57:06.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17784" PostTypeId="1" AcceptedAnswerId="17795" CreationDate="2016-08-15T02:08:46.900" Score="5" ViewCount="828" Body="&lt;p&gt;This video about Spectrolab's spacecraft power systems mentions Juno several times and it looks like the Juno panels are shown at several points in the video. Multi-junction cells are discussed, they have higher efficiency than silicon and so were the obvious choice for Juno. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I was confused when I saw this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Spectrolab's solar cells are approximately 30% efficient in space and exceed 40% efficiency in terrestrial applications. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why is there such a large difference in these values? Is photovoltaic efficiency defined differently for space and terrestrial applications? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I normally advise against taking a percent of a percent because it can be confusing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ 1- \frac {\text{approximately 30%}} {\text{exceeding 40%}} \approx \text{25% less} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4IBBE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4IBBE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen shot from the video &lt;a href=&quot;https://www.youtube.com/watch?v=9Dtmwbd77UM&quot; rel=&quot;nofollow noreferrer&quot;&gt;found here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/9Dtmwbd77UM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-08-15T03:56:46.430" LastActivityDate="2016-08-15T17:50:14.360" Title="Why are these multi-junction solar cells 40% efficient on earth but 30% in space?" Tags="|artificial-satellite|solar-power|juno|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17785" PostTypeId="1" CreationDate="2016-08-15T03:56:14.677" Score="3" ViewCount="178" Body="&lt;p&gt;Since &lt;em&gt;the shape&lt;/em&gt; (as well as the integral) of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_irradiance&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar irradiance&lt;/a&gt; spectrum above the atmosphere is different than that below it, is the efficiency of photovoltaic solar panels defined and measured differently than those for terrestrial applications? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the related question &lt;a href=&quot;https://space.stackexchange.com/q/17784/12102&quot;&gt;Why are these multi-junction solar cells 40% efficient on earth but 30% in space?&lt;/a&gt; some of the issues mentioned there are related to ratings - which are likely to be conservative lower limits. Here I'm asking about the &lt;em&gt;definition of efficiency and its testing procedure&lt;/em&gt; and how it may differ when applied to panels destined for space and terrestrial applications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VgZPr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VgZPr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Solar spectrum &lt;a href=&quot;https://en.wikipedia.org/wiki/Air_mass_(solar_energy)#/media/File:Solar_Spectrum.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-03-29T14:58:11.700" LastActivityDate="2019-03-29T14:58:11.700" Title="Is solar panel efficiency defined and measured differently for space vs terrestrial applications?" Tags="|artificial-satellite|solar-power|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17786" PostTypeId="1" AcceptedAnswerId="17794" CreationDate="2016-08-15T06:54:32.243" Score="3" ViewCount="611" Body="&lt;p&gt;The GIFs are made from frames of the live broadcast of SpaceX JCSat-16 2nd stage around the time of the second engine cut-off (SECO-2). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the engine moves outward axially as the thrust ends, although I can't be absolutely sure. This is a funny angle - that nozzle is quite foreshortened in this view, and the lens may be a fisheye and quite close, contributing some distortion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Watching the video on-line the speed telemetry shows the acceleration drop to zero &lt;em&gt;just before&lt;/em&gt; the apparent motion. Is this evidence of some kind of spring-loading or deflection of the engine mount?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The camera could be on a mount which is flexing under acceleration, but because of the sort-of binary step nature of the motion, I don't think that's actually what is happening in this case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also the engine is gimbaled, and viewing from this angle with a short focal length lens, it is possible that this is actually rotation about a pivot point further to the right inside the spacecraft rather than axial translation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm curious if this motion can be explained as a relaxation of a hypothetical &quot;shock absorber&quot; as discussed &lt;a href=&quot;https://space.stackexchange.com/q/15463/12102&quot;&gt;in this question&lt;/a&gt; after thrust has stopped.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5iSXD.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5iSXD.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Frames from the broadcast changed to monochrome (R+G+B)/3 and stitched manually into a GIF. There are jumps in time due to the editing of the video on line. The launch time code can be used to estimate time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EaVix.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EaVix.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Two frames from the broadcast changed to monochrome (here just the Blue (B) channel only, in order to improve contrast and ignore thermal radiation) selected 50 frames (50/30 ~ 1.67 seconds) apart. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Full video:&lt;/strong&gt;  &lt;a href=&quot;https://www.youtube.com/watch?v=QZTCEO0gvLo&quot; rel=&quot;nofollow noreferrer&quot;&gt;shown here&lt;/a&gt; use the launch time code &lt;strong&gt;T+ 00:27:32&lt;/strong&gt; to &lt;strong&gt;00:27:38&lt;/strong&gt; for reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/QZTCEO0gvLo?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-10-01T02:36:51.307" Title="When this F9 2nd stage engine suddenly moves upon shutdown, is it moving along the rocket's axis?" Tags="|spacex|rockets|falcon-9|engine-design|restartable-engine|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17787" PostTypeId="1" AcceptedAnswerId="17791" CreationDate="2016-08-15T09:24:35.930" Score="4" ViewCount="755" Body="&lt;p&gt;I was watching SpaceX's launch of the Japanese satellite JCSAT-16 yesterday, and the second stage at a point seemed to decrease in altitude (from 171km to 165km) &lt;a href=&quot;https://youtu.be/OERDIFnFvHs?t=1441&quot; rel=&quot;nofollow&quot;&gt;at about T+00:07:00 in this video&lt;/a&gt;. Is that normal or was that a glitch?&lt;/p&gt;&#xA;" OwnerUserId="16627" LastEditorUserId="6537" LastEditDate="2017-12-12T22:36:18.163" LastActivityDate="2017-12-12T22:36:18.163" Title="SpaceX second stage altitude decrease?" Tags="|launch|spacex|falcon-9|upper-stage|telemetry|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17788" PostTypeId="1" AcceptedAnswerId="17792" CreationDate="2016-08-15T11:09:19.080" Score="1" ViewCount="279" Body="&lt;p&gt;I was looking up oxidizers in two aspects, economy and power. Since much of math goes over my head, I got almost nothing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be most economical  and most  powerful potent oxidizer choice available today? Is there a catalyst which can be helpful here?&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="6816" LastEditDate="2016-08-20T02:02:41.750" LastActivityDate="2016-08-20T02:02:41.750" Title="Different LNG (methane) combinations with oxidizer" Tags="|rockets|fuel|oxidizer|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17789" PostTypeId="2" ParentId="17787" CreationDate="2016-08-15T11:18:36.497" Score="1" Body="&lt;p&gt;There doesn't seem to be any glitch as far as I can tell.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I watched the video. The altitude is expressed in integer kilometers. We don't know for sure if it's rounded or truncated, but a good guess would be truncated, so that 171000 and 171999.99 would both show up as 171 km, '&lt;a href=&quot;https://en.wikipedia.org/wiki/Odometer&quot; rel=&quot;nofollow noreferrer&quot;&gt;odometer&lt;/a&gt; style'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every time I see the altitude change, I hit the spacebar (1/5 second reflexes after coffee, on a good day) and type the T+ time, the altitude, and the speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why speed? for two reasons1&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;em&gt;Always&lt;/em&gt; take more data than you think you need! For a reason, you can choose between &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Just_Do_It&quot; rel=&quot;nofollow noreferrer&quot;&gt;Just do it&lt;/a&gt;&quot; or &quot;&lt;a href=&quot;http://nylottery.ny.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hey, you never know&lt;/a&gt;&quot;. I'm a &lt;em&gt;Hey, you never know&lt;/em&gt; person myself. Data can come in handy for many reasons later on. One example is the next item&lt;/li&gt;&#xA;&lt;li&gt;See if other data has a glitch - is the frame rate smooth, is the telemetry smooth, is the trajectory smooth... helps differentiate between different plausible explanations if you verify a potential glitch.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Here's the result. I don't see a glitch. There is gap in the altitude plot because the 171 never &quot;rolls over&quot; to 172, so we have no information about the altitude except that it's somewhere between 171 and 172. Remember, the data points represent when the odometer rolls over - &lt;em&gt;the only time when we can even guess&lt;/em&gt; at the fractional part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0CFSn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0CFSn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Plot of the data I took from the video, using the Python script below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Discussion:&lt;/strong&gt; For a circular low earth orbit (LEO), or any orbit actually, you can get the orbit velocity $v_{orbit}$ at a distance $r$, as long as you know the semi-major axis $a$, the gravitational constant $G$ times the mass of the body you are orbiting, in this case $M_e$ for the Earth by using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis-viva&lt;/a&gt; equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{orbit}^2=GM_e (\frac{2}{r}-\frac{1}{a})  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $GM_e$ is about 3.9860E+14  m${}^3$ / s${}^2$. If choose the radius of the earth $r_e$ to be 6371 km, and calculate a circular orbit ($r = a$) at an altitude $h$ of 171 km so that $a = r_e+h$, I get an orbital speed of about 7805 m/s. If I project that on to a non-rotating Earth 171km below, that's 7601 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 28 degrees latitude, the earth is spinning at about 410 m/s, so assuming we're in a 28 degree inclination plane and still nearby the launch site, the ground speed will be about 7192 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see in the plot that throughout this part of the video, the 2nd stage is well below orbital speed, and thus is sub-orbital. However it's picking up substantial speed and getting much closer. Still you see a rising and falling arc that you might expect of something suborbital, and the decrease in the rate of fall is consistent with that - thus the asymmetric shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CO4Ov.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CO4Ov.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Odometer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Odometer&lt;/a&gt; rollover from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Odometer_rollover.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;screendata = &quot;&quot;&quot;05:39   165  3646&#xA;05:45   166  3715&#xA;05:50   167  3790&#xA;05:57   168  3879&#xA;06:05   169  3988&#xA;06:14   170  4124&#xA;06:28   171  4333&#xA;06:45  -171  4623&#xA;06:55  -171  4804&#xA;07:05  -171  4990&#xA;07:12   170  5132&#xA;07:28   169  5466&#xA;07:40   168  5739&#xA;07:50   167  6006 &#xA;08:01   166  6299&#xA;08:13   165  6651&quot;&quot;&quot;&#xA;&#xA;import matplotlib.pyplot as plt&#xA;import numpy as np&#xA;&#xA;lines = screendata.split('\n') &#xA;&#xA;time, alt, speed = zip(*[line.split() for line in lines])&#xA;&#xA;time = [ [int(thing) for thing in t.split(':')]&#xA;          for t in time]&#xA;&#xA;time = np.array([ 60*m + s for m, s in time], dtype=float)&#xA;&#xA;alt, speed  = [np.array([int(item) for item in thing], dtype=float)&#xA;               for thing in [alt, speed] ]&#xA;&#xA;alt[alt&amp;lt;0] = np.nan       # those marked with a minus didn't actually change&#xA;&#xA;plt.figure()&#xA;&#xA;plt.subplot(2,1,1)&#xA;&#xA;plt.plot(time, alt)&#xA;plt.plot(time, alt, 'ok')&#xA;&#xA;plt.ylim(min(alt)-1, max(alt)+1)&#xA;plt.title(&quot;altitude (final upon each integer km change in video) vs T+ (sec)&quot;)&#xA;&#xA;plt.subplot(2,1,2)&#xA;&#xA;plt.plot(time, speed)&#xA;plt.plot(time, speed, 'ok')&#xA;&#xA;plt.ylim(min(speed)-1, max(speed)+1)&#xA;plt.title(&quot;speed vs T+ (sec)&quot;)&#xA;&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-08-15T11:58:12.150" LastActivityDate="2016-08-15T11:58:12.150" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17790" PostTypeId="2" ParentId="6031" CreationDate="2016-08-15T11:32:40.840" Score="2" Body="&lt;p&gt;The pre-1.0 version of Kerbal Space Program had a very bad aerodynamic model, that the correct rule of thumb was to keep things near the terminal velocity, otherwise you ran the risk of being significantly decreased. The later versions increased the fidelity to something more akin to what is seen. Keeping your speed under control while in the atmosphere is always a good idea, however, it's now possible to be going at a very high speed inside the atmosphere, as in fact it is possible on Earth. See Hobbes answer for the more specifics in real life how drag works.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-08-15T11:32:40.840" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17791" PostTypeId="2" ParentId="17787" CreationDate="2016-08-15T11:36:38.573" Score="5" Body="&lt;p&gt;This is actually pretty common in rocket launch profile design. For the Ariane rocket, we covered it in &lt;a href=&quot;https://space.stackexchange.com/q/13749/25&quot;&gt;What is the reason for the Ariane 5 launcher with Intelsat 29e losing altitude?&lt;/a&gt;, but the same principal applies in general. When you have a low thrust high ISP upper stage, it is common for it to thrust more horizontally at a certain point than vertically, which might cause the altitude to go down somewhat. The speed eventually will be such that the altitude begins to rise, as the curvature of the Earth drops off more.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-08-15T12:07:41.647" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17792" PostTypeId="2" ParentId="17788" CreationDate="2016-08-15T11:50:52.397" Score="2" Body="&lt;p&gt;Oxygen is the best oxidiser that's also simple to handle and inexpensive. Better oxidisers exist, but they are expensive and can be toxic and/or corrosive, making them difficult to handle. In particular, fluorine can give very high specific impulse, but is nearly impossible to work with safely so it's never been used in production rockets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_rocket_propellant#Lithium_and_fluorine&quot; rel=&quot;nofollow&quot;&gt;good list&lt;/a&gt; of propellants and oxidizers. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-15T11:50:52.397" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17793" PostTypeId="2" ParentId="1599" CreationDate="2016-08-15T12:01:38.613" Score="2" Body="&lt;p&gt;I've managed to obtain a scan of ARRL QST July issue of 1969, where operation of Australis-Oscar 5 satellite is described. This includes spin reduction through permalloy bars and stabilization by a bar magnet:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When Australis is placed into orbit, it will be spinning at some 4 revolutions per minute. This spinning will cause fading in the signal. In order to remove the spin energy, a set of permalloy rods with a very large hysteresis loop at the low flux density has been included. Hysteresis loss along with eddy current loss in the case, will tend to remove the spin, allowing orientation along the local magnetic field vector by a small magnet (see fig. 3) which is also included. Since the v.h.f. antenna lies along the same axis as the bar magnet (the X axis) fading of the v.h.f. signals should be reduced. All antennas are made of flexible steel tape.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cOqOU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cOqOU.png&quot; alt=&quot;Australis-Oscar electronics, with a thin bar magnet going along the whole length of X axis&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-08-15T12:01:38.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17794" PostTypeId="2" ParentId="17786" CreationDate="2016-08-15T13:42:03.080" Score="4" Body="&lt;p&gt;This occurs in the very last moments before the engine is shut down. In fact, as you mentioned, the acceleration drops to 0 as this happens. There's a number of possible explanations, including:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The engine had a slight gimbal that was reverted upon completion of the thrust.&lt;/li&gt;&#xA;&lt;li&gt;There isn't any real movement, but rather the difference in the sun angle to re-position the satellite causes it to look like there is an issue.&lt;/li&gt;&#xA;&lt;li&gt;Stopping the engine has a slight jerk. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/QZTCEO0gvLo?start=2672&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are a number of recent SECO cutoffs posted. All shown are of the second SECO. All show similar behavior. Note that the first SECO is rarely shown because it happens about the same time as the 1st stage landing. I went back as far as CRS-8, and all of them are missing the first SECO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ckjP8stlzxI?start=2694&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/wPYOtCFSLKw?start=2990&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given all of these, I'm pretty sure the truth is the first case, that the engine was slightly gimballed and returned to it's centered state. This most likely makes the stabilization easier required to properly deploy it's payload.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-08-15T13:42:03.080" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17795" PostTypeId="2" ParentId="17784" CreationDate="2016-08-15T17:50:14.360" Score="6" Body="&lt;p&gt;The 40+ percent efficiencies on earth are using solar concentrators, often with concentration ratios as high as 1000x. Most spacecraft including Juno use planar solar arrays without concentration. Record solar cell efficiencies as a function of time are published periodically by NREL and are available as a nice plot here: &lt;a href=&quot;http://www.nrel.gov/ncpv/images/efficiency_chart.jpg&quot; rel=&quot;noreferrer&quot;&gt;http://www.nrel.gov/ncpv/images/efficiency_chart.jpg&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="16632" LastActivityDate="2016-08-15T17:50:14.360" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17796" PostTypeId="1" AcceptedAnswerId="17797" CreationDate="2016-08-15T18:21:13.807" Score="4" ViewCount="681" Body="&lt;p&gt;Have there ever been significant changes in the altitude of the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By significant, I mean changes greater than about 40 km, thus by far exceeding the periodic reboosts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there haven't been, then why not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the ISS technically be able to change its altitude and be able to surive at other altitudes?&lt;/p&gt;&#xA;" OwnerUserId="11099" LastEditorUserId="415" LastEditDate="2016-08-15T19:46:49.030" LastActivityDate="2016-08-15T22:45:17.230" Title="Have there ever been significant changes in the altitude of the ISS?" Tags="|iss|altitude|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17797" PostTypeId="2" ParentId="17796" CreationDate="2016-08-15T18:39:40.910" Score="6" Body="&lt;p&gt;Yes, there have been changes of &gt; 40 km, as shown in this graph.  The X axis scale is not showing, but it is from Nov 1998 to July 2008.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wyzp7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wyzp7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacemath.gsfc.nasa.gov/weekly/6Page30.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reference&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: I checked to make sure the graph is not just a theoretical math problem.  It's not, the same graph appears &lt;a href=&quot;http://www.nbcnews.com/id/17630218/ns/technology_and_science-space/t/space-station-sinks-new-low-its-ok/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-08-15T18:51:23.963" LastActivityDate="2016-08-15T18:51:23.963" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="17798" PostTypeId="1" AcceptedAnswerId="17800" CreationDate="2016-08-15T18:40:16.213" Score="7" ViewCount="710" Body="&lt;p&gt;There might be no reason but I doubt the date had simply no reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe there may have been personal significance to Neil Armstrong.   Alternatively picking a summer date may have been safer.&lt;/p&gt;&#xA;" OwnerUserId="16549" LastEditorUserId="415" LastEditDate="2016-08-16T12:36:31.193" LastActivityDate="2016-08-16T12:36:31.193" Title="What is the scientific reason for selecting July 20 1969 for the moon landing?" Tags="|apollo-11|historical-data|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17799" PostTypeId="1" CreationDate="2016-08-15T18:55:28.613" Score="4" ViewCount="458" Body="&lt;p&gt;I'm using the SGP4 algorithm which outputs Earth-centered, inertial (ECI) satellite coordinates. &#xA;I found some software to visualize orbits, but they all request input as Keplerian elements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can I have 3D visual simulations of earth and the satellite orbit, animated or not, from the ECI-time data I have?&lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="9060" LastEditDate="2016-08-15T19:11:26.847" LastActivityDate="2016-08-15T19:11:26.847" Title="How can I visualize orbits given Earth-centered, inertial (ECI) coordinates?" Tags="|orbit|simulation|visualization|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17800" PostTypeId="2" ParentId="17798" CreationDate="2016-08-15T19:04:22.407" Score="14" Body="&lt;p&gt;The reasons &lt;a href=&quot;http://history.nasa.gov/SP-4029/Apollo_11a_Summary.htm&quot;&gt;for that date&lt;/a&gt; were:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There were a number of considerations which determined the launch windows for a lunar landing mission. These considerations included illumination conditions at launch, launch pad azimuth, translunar injection geometry, sun elevation angle at the lunar landing site, illumination conditions at Earth splashdown, and the number and location of the lunar landing sites.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The time of a lunar landing was determined by the location of the lunar landing site and by the acceptable range of sun elevation angles. The range of these angles was from 5° to 14° and in a direction from east to west. Under these conditions, visible shadows of craters would aid the crew in recognizing topographical features. When the sun angle approached the descent angle, the mean value of which was 16°, visual resolution would be degraded by a “washout” phenomenon where backward reflectance was high enough to eliminate contrast. Sun angles above the flight path were not as desirable because shadows would not be readily visible unless the sun was significantly outside the descent plane. In addition, higher sun angles (greater than 18°) could be eliminated from consideration by planning the landing one day earlier where the lighting is at least 5°. Because lunar sunlight incidence changed about 0.5° per hour, the sun elevation angle restriction established a 16-hour period, which occurred every 29.5 days, when landing at a given site could be attempted. The number of Earth-launch opportunities for a given lunar month was equal to the number of candidate landing sites.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The time of launch was primarily determined by the allowable variation in launch pad azimuth and by the location of the Moon at spacecraft arrival. The spacecraft had to be launched into an orbital plane that contained the position of the Moon and its antipode at spacecraft arrival. A 34° launch pad azimuth variation afforded a launch period of 4 hours 30 minutes. This period was called the “daily launch window,” the time when the direction of launch was within the required range to intercept the Moon.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Two launch windows occurred each day. One was available for a translunar injection out of Earth orbit in the vicinity of the Pacific Ocean, and the other was in the vicinity of the Atlantic Ocean. The injection opportunity over the Pacific Ocean was preferred because it usually permitted a daytime launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So they basically had one launch opportunity per month. I suspect they chose the earliest possible opportunity that could be achieved safely.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-15T19:04:22.407" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17803" PostTypeId="2" ParentId="15498" CreationDate="2016-08-15T23:25:58.480" Score="4" Body="&lt;p&gt;My brother just pointed out that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In October 1998, the Epson Stylus Color 800 became the first ever color printer to be used in space when it was installed in the Space Shuttle Discovery for the 92nd space shuttle mission (STS-95). The printer performed so well that Epson was subsequently selected as the printer supplier for the International Space Station. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.epson.co.in/epson_india/explore_epson/mediacentre/company_information/milestone_products/printer.page?pno=2&amp;amp;tabname=&amp;amp;subcat=&quot; rel=&quot;nofollow noreferrer&quot;&gt;(Epson) MILESTONE EPSON IMAGING TECHNOLOGIES AND PRODUCTS&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3jvWs.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3jvWs.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-08-15T23:25:58.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17805" PostTypeId="2" ParentId="17785" CreationDate="2016-08-16T00:33:39.553" Score="-1" Body="&lt;p&gt;It's based on an approximation of the photon flux (number of photons impinging on the surface per time). I do not know of any reason to differentiate the ground and orbit environments otherwise. I do not know of any efficiency difference. This does not come from a builder, but a user. We need a canonical answer from a builder. ;)&lt;/p&gt;&#xA;" OwnerUserId="16634" LastActivityDate="2016-08-16T00:33:39.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17806" PostTypeId="2" ParentId="17603" CreationDate="2016-08-16T07:24:33.277" Score="6" Body="&lt;p&gt;The UN does not have a space agency comparable to ESA or NASA. Recently though (Sept 2016), UNOOSA announced a partnership that will allow it to organize space flights, with a commercial partner supplying the infrastructure and other technical aspects. See below.&#xA;Apart from UNOOSA, several sub-organizations of the UN run space programs:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The UN meteorology organization WMO set up the &lt;a href=&quot;http://public.wmo.int/en/programmes/global-observing-system&quot; rel=&quot;nofollow noreferrer&quot;&gt;Global Observing System&lt;/a&gt;. This distributes meteo information contributed by various members that operate satellites.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Global Observing System (GOS) provides observations on the state of the atmosphere and ocean surface from the land-based and space-based instruments. This data is used for the preparation of weather analyses, forecasts, advisories and warnings, and for the climate monitoring and environmental activities carried out through other programmes as well as by other international organizations. Global Observing System is operated by National Meteorological and Hydrological Services (NMHSs), national or international satellite agencies, and involves several consortia dealing with specific observing systems or specific geographic regions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The UN organization for Training and Research &lt;a href=&quot;http://www.unitar.org/unosat/what-we-do&quot; rel=&quot;nofollow noreferrer&quot;&gt;UNITAR&lt;/a&gt; has a similar role in GIS and earth observation:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Through the use of Geographic Information Systems (GIS) and satellite imagery, UNOSAT provides timely and high-quality geo-spatial information to UN decision makers, member states, international organizations and non-governmental organizations. UNOSAT develops solutions on integrating field collected data with remote sensing imagery and GIS data through web-mapping and information sharing mechanisms, including remote monitoring of development projects and sharing of geographic data using web-services.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Communications services are not provided by the UN, but by a different international organization: &lt;a href=&quot;https://www.britannica.com/topic/Intelsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;Intelsat&lt;/a&gt; was set up in 1964 (known then as the International Telecommunications Satellite Consortium).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Intelsat was founded as a public-private consortium in 1964 by the telecommunication agencies of 18 nations, including the United States, which proposed the organization. The transmitting and receiving apparatus in each country was owned by the Intelsat member, the telecommunication agency from that country. Within 10 years the membership of Intelsat had grown to include agencies from 86 countries, and by 2001 about 150 countries were members.&lt;/p&gt;&#xA;&lt;p&gt;The consortium contracted with the U.S. National Aeronautics and Space Administration (NASA) to launch its satellites. The first of these was Early Bird, later renamed Intelsat I...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As the communications industry matured, Intelsat's role and position changed.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Intelsat became a private company in 2001, with its public services under the oversight of the International Telecommunications Satellite Organization (ITSO).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;UNOOSA&lt;/h2&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.unoosa.org/oosa/en/aboutus/history/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;United Nations Office for Outer Space Affairs&lt;/a&gt; used to be an organization that dealt with the legal aspects of space.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The United Nations Office for Outer Space Affairs was initially created as a small expert unit within the United Nations Secretariat to service the ad hoc Committee on the Peaceful Uses of Outer Space, established by the General Assembly in its resolution 1348 (XIII) of 13 December 1958.&lt;/p&gt;&#xA;&lt;p&gt;The unit was moved to work under the Department of Political and Security Council Affairs in 1962 and was transformed into the Outer Space Affairs Division of that Department in 1968. In 1992, the Division was transformed into the Office for Outer Space Affairs within the Department for Political Affairs. In 1993, the Office was relocated to the United Nations Office at Vienna. At that time, the Office also assumed responsibility for substantive secretariat services to the Legal Subcommittee, which had previously been provided by the Office of Legal Affairs in New York.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In September 2016, UNOOSA &lt;a href=&quot;http://www.unoosa.org/oosa/en/informationfor/media/2016-unis-os-475.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;announced its first foray into actual spaceflight&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-30T08:20:34.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17807" PostTypeId="1" AcceptedAnswerId="21981" CreationDate="2016-08-16T14:03:16.767" Score="11" ViewCount="1551" Body="&lt;p&gt;China launched the Quantum Experiments at Space Scale (QUESS) on the satellite called MOZI or Micius today, which will be used for quantum communications between China and Europe. &lt;a href=&quot;http://www.popsci.com/chinas-quantum-satellite-could-change-cryptography-forever&quot; rel=&quot;nofollow noreferrer&quot;&gt;Popular Science&lt;/a&gt; describes the science package as such:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;QUESS's function is to test the phenomena of quantum entanglement. Operated by the China Academy of Sciences, this 500kg satellite contains a quantum key communicator, quantum entanglement emitter, entanglement source, processing unit, and a laser communicator.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is not clear to me what this means. Maybe I don't understand the lingo, or maybe they are dumbing it down for the public. How is the entangled signal relayed by this satellite from China to Europe? Are the &quot;quantum key communicator&quot;, &quot;entanglement emitter&quot;, and &quot;laser communicator&quot; really separate devices? &lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="12102" LastEditDate="2017-06-16T15:30:52.940" LastActivityDate="2017-08-15T19:12:44.520" Title="How does QUESS, China's quantum communications satellite experiment, relay entangled signals?" Tags="|communication-satellite|laser|china|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17808" PostTypeId="1" AcceptedAnswerId="17809" CreationDate="2016-08-16T15:19:56.530" Score="31" ViewCount="42749" Body="&lt;p&gt;During the launch of Apollo 12 a major malfunction was caused by 2 lightning strikes. See &lt;a href=&quot;http://www.realclearscience.com/blog/2013/02/apollo-12-struck-by-lightning.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.realclearscience.com/blog/2013/02/apollo-12-struck-by-lightning.html&lt;/a&gt; for more info.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solution to the issue was to 'Switch SCE to AUX', which is pretty clearly to switch 'something' to Auxiliary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was switched to auxiliary? And why was this solution so unexpected?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would also appreciate a link to a more complete story of this event if possible.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="415" LastEditDate="2016-08-17T09:46:04.813" LastActivityDate="2016-08-17T09:46:04.813" Title="What does 'Switch SCE to AUX' mean?" Tags="|launch|apollo-program|failure|apollo-12|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17809" PostTypeId="2" ParentId="17808" CreationDate="2016-08-16T15:33:34.110" Score="41" Body="&lt;p&gt;It stands for Signal Conditioning Equipment. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_12#Launch_and_transfer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The loss of all three fuel cells put the CSM entirely on batteries,&#xA;  which were unable to maintain normal 75-ampere launch loads on the&#xA;  28-volt DC bus. One of the AC inverters dropped offline. These power&#xA;  supply problems lit nearly every warning light on the control panel&#xA;  and caused much of the instrumentation to malfunction.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Electrical, Environmental and Consumables Manager (EECOM) John Aaron&#xA;  remembered the telemetry failure pattern from an earlier test when a&#xA;  power supply malfunctioned in the CSM Signal Conditioning Equipment&#xA;  (SCE), which converted raw signals from instrumentation to standard&#xA;  voltages for the spacecraft instrument displays and telemetry&#xA;  encoders.[3]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Aaron made a call, &quot;Try SCE to aux,&quot; which switched the SCE to a&#xA;  backup power supply. The switch was fairly obscure, and neither Flight&#xA;  Director Gerald Griffin, CAPCOM Gerald Carr, nor Mission Commander&#xA;  Pete Conrad immediately recognized it. Lunar Module Pilot Alan Bean,&#xA;  flying in the right seat as the spacecraft systems engineer,&#xA;  remembered the SCE switch from a training incident a year earlier when&#xA;  the same failure had been simulated. Aaron's quick thinking and Bean's&#xA;  memory saved what could have been an aborted mission, and earned Aaron&#xA;  the reputation of a &quot;steely-eyed missile man&quot;.[4] Bean put the fuel&#xA;  cells back on line, and with telemetry restored, the launch continued&#xA;  successfully. Once in Earth parking orbit, the crew carefully checked&#xA;  out their spacecraft before re-igniting the S-IVB third stage for&#xA;  trans-lunar injection. The lightning strikes had caused no serious&#xA;  permanent damage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And here is &lt;a href=&quot;http://motherboard.vice.com/read/john-aaron-apollo-12-curiosity-luck-and-sce-to-aux&quot;&gt;another good version&lt;/a&gt; of the story.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-08-16T15:39:10.220" LastActivityDate="2016-08-16T15:39:10.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17812" PostTypeId="1" AcceptedAnswerId="17813" CreationDate="2016-08-17T09:35:05.207" Score="12" ViewCount="2535" Body="&lt;p&gt;I understand Curiosity is powered by plutonium decay.  I was wondering if that kind of battery leaks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If humans ever go to Mars, and they want to meet with Curiosity, would they need to take extra precautions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I understand that present-day settlers would need protection from cosmic and solar radiation anyway, due to the thin atmosphere on Mars.  So for the purposes of this question please assume that we have spent 2,000 years or so terraforming Mars to give it an Earth-like atmosphere.  Alternatively, imagine Curiosity finds a way to return to Earth.  I'm just wondering if Curiosity's plutonium battery leaks a significant amount of radiation or not.)&lt;/p&gt;&#xA;" OwnerUserId="1501" LastEditorUserId="6816" LastEditDate="2016-08-17T09:51:11.627" LastActivityDate="2016-08-17T17:21:54.350" Title="Does Curiosity pose a radiation hazard for would be colonists?" Tags="|radiation|curiosity|rtg|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17813" PostTypeId="2" ParentId="17812" CreationDate="2016-08-17T09:42:12.097" Score="24" Body="&lt;p&gt;At first glance, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot;&gt;RTG&lt;/a&gt; does not pose a risk. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;It is powered by Pu-238, which is primarily an alpha emitter throughout its decay chain. Alpha particles can be stopped by a sheet of paper. An astronaut is perfectly safe in his suit, even if the RTG were disassembled and the Pu lying around unprotected. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The RTG is built to survive a launch failure, i.e. it will remain intact if the launcher explodes underneath it. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;When the RTG is intact, no radiation leaks out. The Pu is sealed in several layers of metal.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If they wanted to move the RTG into a pressurized building and work on it, they'd need to check for leaks with a simple Geiger counter. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The story is a bit more complicated than that, though.&#xA;In addition to alpha, an RTG produces a small amount of gamma radiation. This &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690025340.pdf&quot;&gt;NASA report&lt;/a&gt; gives a detailed analysis of the gamma radiation produced by an RTG. I'm having trouble converting this data into an answer to the question 'would standing next to an RTG have damaging consequences for a human?'&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://books.googleusercontent.com/books/content?req=AKW5QaclzBonljS9e7dXiJVYbT-kklhOi6NYZ1eaPp6OZvDn7BJ6RpzjiQF1PSHHvIEP7-P44hOCrFJ3B5A0Z2n6ROybzL6r7BnrssaM_5Sps7LudDZn-d3gyk95KU5-EI3G_N1uFpTwzLN0dOoxqDe9eE7jy8RlW-1pWiob12CMC6zBmv6ghttXo4IGYPMZyRSkEpJ4zZgMbidb_Iv5oeiSGvKixxxLtBRnTfKdmGH35iwsfnvS8qzMyYYUbewYWXNF0resDhkJ&quot;&gt;New Horizons environmental impact statement&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;PuO&lt;sub&gt;2&lt;/sub&gt; emits much less gamma and neutron radiation than Sr-90&#xA;  and Cm-244. Because gamma and neutron radiation are more penetrating than the alpha particles emitted by Pu-238, extensive shielding (not required with PuO&lt;sub&gt;2&lt;/sub&gt;) would be required during production and handling, as well as onboard the spacecraft to protect sensitive components.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-08-17T17:21:54.350" LastActivityDate="2016-08-17T17:21:54.350" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="17814" PostTypeId="5" CreationDate="2016-08-17T09:49:48.833" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-08-17T09:49:48.833" LastActivityDate="2016-08-17T09:49:48.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17815" PostTypeId="4" CreationDate="2016-08-17T09:49:48.833" Score="0" Body="Apollo 12 was the second Moon-landing mission in NASA's Apollo program, and the program's sixth manned mission. It launched on November 14, 1969, had two lunar EVAs on November 19 and 20, and landed on Earth on November 24." OwnerUserId="415" LastEditorUserId="415" LastEditDate="2016-08-17T11:40:36.907" LastActivityDate="2016-08-17T11:40:36.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17816" PostTypeId="1" CreationDate="2016-08-17T13:03:24.973" Score="2" ViewCount="226" Body="&lt;p&gt;Let's imagine we have delivered a very long/tall tower into orbit. It's a rigid construction, a couple hundred kilometers long, at altitude where air friction is entirely negligible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we place it in circular orbit - its center of mass moving at just the right speed for its altitude - but we rotate it to some random angle to the surface, and leave it like that, giving it spin of 1 revolution per day so that - neglecting tidal forces - it would naturally remain in the same orientation to the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It won't stay like that. The linear velocities of all its points relative to Earth are about the same (the bottom end's minimally lower, upper's minimally higher due to that 1RPD spin, but these should be negligible differences).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the end that happens to be at lower altitude, moves slower than orbital velocity proper for that altitude and will tend to be pulled downwards. The other end, exceeding circular orbit's velocity for its altitude, will be pulled upwards. Our tower starts spinning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's the stable equilibrium position at which it would remain still - always facing Earth at the same angle?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2018-04-01T18:49:00.447" Title="Equilibrium position of a very long object in orbit" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17817" PostTypeId="1" AcceptedAnswerId="17818" CreationDate="2016-08-17T13:51:53.623" Score="16" ViewCount="5560" Body="&lt;p&gt;A common picture of someone floating on the Dead Sea or Great Salt Lake, shows how high you float.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;l&lt;a href=&quot;https://i.stack.imgur.com/D3Alp.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D3Alp.jpg&quot; alt=&quot;Man Floating&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you had that water in a bath tub, I don't think you would sink to the bottom, you would just awkwardly float in the tub.  Bathing in small tub where your body does not rest on the bottom would be impractical/impossible. Imagine bobbing around in the tub trying to wash without tipping over and banging your head against the side of the tub. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the gravity on Mars being much less than Earth, would you float in the bath water? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2016-08-18T23:24:36.283" Title="Can you take a bath on Mars?" Tags="|mars|gravity|water|hygiene|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17818" PostTypeId="2" ParentId="17817" CreationDate="2016-08-17T14:10:38.607" Score="38" Body="&lt;p&gt;Short answer, No different from Earth in floating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;A HREF=&quot;https://en.wikipedia.org/wiki/Buoyancy&quot;&gt;Buoyancy&lt;/A&gt; in water or any fluid is based on the weight of water displaced.  Floating is based on the weight of the item displacing water.  This is ultimately ends up in comparing densities.  If the density of the displacing object is greater than the density of the fluid it will weigh more and sink, if it's density is less it will weigh less than the water displaced and float.  On Earth people float higher on very salty water because the water is denser than fresh water, so less fluid must be displaced to support the weight of the person.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a lower gravity field like on mars, your body and the water would both weigh less by the same amount (local gravity), but your densities would compare the same way, so floating or sinking would be unchanged.&lt;/p&gt;&#xA;" OwnerUserId="15824" LastActivityDate="2016-08-17T14:10:38.607" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17819" PostTypeId="2" ParentId="17812" CreationDate="2016-08-17T15:52:35.787" Score="-3" Body="&lt;p&gt;Any Martian explorer stupid enough to not take some obvious necessary precaution (which would be unnecessary in this case) shouldn't have been on Mars in the first place. The initial colonists better be damned smart people, they'll need every bit of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As well, they will all be radiation experts. The journey to Mars is fraught with radiation perils both from cosmic rays and solar events; and the surface of Mars won't be a picnic either for unshielded humans. Habitations will likely be underground for that reason.&lt;/p&gt;&#xA;" OwnerUserId="16646" LastActivityDate="2016-08-17T15:52:35.787" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17820" PostTypeId="1" AcceptedAnswerId="27684" CreationDate="2016-08-17T15:53:27.617" Score="7" ViewCount="425" Body="&lt;p&gt;Propulsion technology for cubesats and nanosatellites is in active development, but there are at least a few based on sound principles that are likely to be tested in the next few years. There are also a few time-tested thruster technologies used on hundreds of satellites now that could in principle be engineered to put in a cubesat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What would be the best way to design a (say) 3U cubesat, deployed from the ISS or similar altitude LEO vehicle, so that at least a 1U section of it could attain a heliocentric orbit with the highest possible aphelion (no longer bound to the Earth/Moon system) using current or near-future cubesat-comaptable propulsion technology?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;If it's helpful for example&lt;/em&gt;, it could be assembled already in the microgravity environment of the ISS, and gently deployed from there, in order to save on structural mass that would have been needed to survive launch from earth. Parts could be 3D printed there as well. Budget for &lt;a href=&quot;https://en.wikipedia.org/wiki/Commercial_off-the-shelf&quot; rel=&quot;nofollow noreferrer&quot;&gt;COTS&lt;/a&gt;-type items is large, and &quot;the shelf&quot; is a few years in the future to allow for things that are not quite ready yet but probably will be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A clever use of the moon's gravity is of course both allowed and encouraged. I just ran across &lt;a href=&quot;https://space.stackexchange.com/a/2488/12102&quot;&gt;this answer&lt;/a&gt;, which links to this NASA page: &lt;a href=&quot;http://stereo.gsfc.nasa.gov/orbit.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://stereo.gsfc.nasa.gov/orbit.shtml&lt;/a&gt;. The first video is a nice illustration of how handy the moon can be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a GIF made from the frames of that &lt;code&gt;.mov&lt;/code&gt; file:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tH0UC.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tH0UC.gif&quot; alt=&quot;STEREO PHASING ANIMATION&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 'at least 1U' part should have some minimal power system so it can measure something to verify its aphelion distance, a starcam snapshot of a few planet positions, or just a measurement of the 1/r^2 drop in solar intensity, or something different. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If spin-stabilization helps, assume that the method of deployment can set the attitude (aim it) and spin it if needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you assemble it on the ISS, &lt;em&gt;it should be considered safe&lt;/em&gt; to assemble on the ISS. Otherwise you'll have to launch from Earth the old fashioned way.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-06-07T04:57:42.113" LastActivityDate="2018-06-07T04:57:42.113" Title="With current or near-future Cubesat propulsion technology, largest aphelion achievable?" Tags="|propulsion|engines|cubesat|ion-thruster|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="17821" PostTypeId="1" CreationDate="2016-08-17T19:03:21.357" Score="4" ViewCount="151" Body="&lt;p&gt;I read about the ESA OPS-SAT satellite, which should &lt;strong&gt;evaluate new hard- and software&lt;/strong&gt; &lt;strong&gt;in orbit&lt;/strong&gt;. &lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/OPS-SAT&quot; rel=&quot;nofollow&quot;&gt;More information here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.mouser.jp/new/criticallink/critical-link-MityARM-5CSX/&quot; rel=&quot;nofollow&quot;&gt;Information on the card used&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there already such satellites or systems similar to this project, or is this some totally new research?&lt;/p&gt;&#xA;" OwnerUserId="13695" LastEditorUserId="58" LastEditDate="2016-08-17T19:18:37.630" LastActivityDate="2021-12-07T18:13:47.303" Title="In orbit test platforms for new hard- and software" Tags="|artificial-satellite|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17822" PostTypeId="1" AcceptedAnswerId="17823" CreationDate="2016-08-17T23:57:44.093" Score="19" ViewCount="5746" Body="&lt;p&gt;I'm pretty sure I recall some images of a lunar module sitting on a slope, although perhaps it just looked that way due to a tilt of the camera.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, my question is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;whether any of the lunar modules had landed sufficiently off level to raise concerns about the return lift off?&lt;/li&gt;&#xA;&lt;li&gt;what was the inclination limit for safe lift off?&lt;/li&gt;&#xA;&lt;li&gt;how close did any of them come to that limit (obviously not exceeded since all returned safely)?&lt;/li&gt;&#xA;&lt;li&gt;was there any contingency plan if an LM had come to rest at an angle too inclined for safe launch... would it have involved EVA activties to jack up or lower one or more legs to attain an acceptable attitude for lift off?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="429" LastActivityDate="2019-11-25T02:13:03.817" Title="Did any of the Apollo lunar modules land significantly off level?" Tags="|apollo-program|lunar-landing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17823" PostTypeId="2" ParentId="17822" CreationDate="2016-08-18T00:33:46.277" Score="31" Body="&lt;p&gt;All the LMs landed a few degrees off-level. Apollo 11 was closest to level at about 4 degrees. The design limit for LM ascent stage lift-off is variously stated as 12 or 15 degrees. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo 15 was tilted somewhere around 11 degrees (6.9 degree pitch-up and 8.6 degrees left roll).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OvL3S.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OvL3S.png&quot; alt=&quot;Apollo 15&amp;#39;s LM landing site showing substantial tilt&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unclear me to what the rationale for the design limit is; the ascent stage has a very good thrust-to-weight ratio and attitude control, and could launch from even a very steep tilt; in fact, if the LM started to fall over after touchdown, it was possible to do an ascent-stage abort liftoff with the LM tilted as much as 60 degrees, or even further with an emergency manual control mode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Aesww.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Aesww.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Upon landing, with the descent stage fuel tanks empty and the ascent stage full, the LM's center of gravity is fairly high. The stationary tip-over limit for the LM is a little over 40 degrees (the point on the diagram where the neutral stability line crosses the vertical axis).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that if one of the landings had come down with an unacceptably steep tilt, the astronauts would have been instructed to dig surface material out from under the high legs until it was more level. Mission control would probably also press Grumman for a detailed risk assessment. If they ran out of consumables time before leveling the LM, they'd obviously have attempted launch anyway; they'd have nothing to lose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've skimmed the Apollo 15 &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/a15.landing.html&quot; rel=&quot;noreferrer&quot;&gt;annotated transcripts&lt;/a&gt; for references to the tilt. The flight director was notified of the tilt (seen via telemetry) a couple minutes after landing, around 104:45. Around 104:50 is this note:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In Houston, the Flight Director asks if the spacecraft tilt is acceptable and wants to review the relevant pre-flight assessments of the margins of safety.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The tilt had some &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/a15.postland.html&quot; rel=&quot;noreferrer&quot;&gt;housekeeping implications&lt;/a&gt;; they had to repoint their S-band antenna and sight on different stars than originally planned to realign their inertial platform.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 116:33:39 there's &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/a15.eva1wake.html&quot; rel=&quot;noreferrer&quot;&gt;discussion about the impact of the LM tilt on rover deployment&lt;/a&gt; -- previously tested and no problem at 15 degrees. Around 120:26 they have some &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/a15.lrvdep.html&quot; rel=&quot;noreferrer&quot;&gt;minor tilt-related difficulty&lt;/a&gt; deploying the rover. Later, there's a water leak in the cabin that goes unnoticed for a while because the tilt is causing it to puddle in one corner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apart from that, throughout the surface stay and departure, there are no other mentions of it, so I have to assume the FD was reassured that the tilt was within spec. In particular, stage separation and ascent engine ignition are two separate manual operations, and there's no discussion of the timing between them, i.e. they didn't seem worried about the ascent stage sliding in between.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-11-25T02:13:03.817" LastActivityDate="2019-11-25T02:13:03.817" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="17824" PostTypeId="1" CreationDate="2016-08-18T03:06:12.247" Score="20" ViewCount="8475" Body="&lt;p&gt;I'm in love with SpaceX and Falcon 9, I watch every live stream and the most special, most amazing, most thrilling part of each launch is the 1st stage landing. Seeing the smoke (or vapor) clear and seeing that 33m tall rocket landed safely within 2 meters of target makes me cheer in my office on the far side of the world. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But for some reason every Falcon 9 landing's live feed from &quot;Of Course I Still Love You&quot; cuts out just as it's coming into view. Why? Oh, why, does does it torture me so?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XZHvf.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XZHvf.png&quot; alt=&quot;SpaceX Falcon 9 half a minute after landing&quot;&gt;&lt;/a&gt;&#xA;I can see it! I can see it right there. But why has the &lt;a href=&quot;https://youtu.be/OERDIFnFvHs?t=25m26s&quot; rel=&quot;noreferrer&quot;&gt;feed dropped out&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="51" LastEditDate="2016-08-18T03:09:28.253" LastActivityDate="2021-06-18T05:53:15.710" Title="Why does the video feed always fail during Falcon 9 landings?" Tags="|spacex|falcon-9|landing|asds|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17825" PostTypeId="2" ParentId="17824" CreationDate="2016-08-18T03:12:16.990" Score="13" Body="&lt;p&gt;Consider what is happening. A rocket firing a 175,000 lb thrust engine, (throttled down as low as it can go, so let's call it 100,000 lbs thrust) is pointed at, and getting closer and closer to a flat surface 170x300 feet in size floating on the ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engine thrust is vibrating the platform so much they lose satellite lock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put that in scale: A Boeing 777 engine has 105,000 lbs of thrust; a 747 (depending on the model and engine choice) had around 60,000 lbs of thrust (in each of four engines) and on close approach to the exhaust stream of a 747, it is possible to roll a school bus from the air flow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ASDS is shaking very badly due to the rapidly approaching rocket. As it gets closer the effect gets worse. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorDisplayName="user10509" LastEditDate="2016-08-18T07:34:07.523" LastActivityDate="2016-08-18T07:34:07.523" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17826" PostTypeId="2" ParentId="17821" CreationDate="2016-08-18T08:43:51.103" Score="3" Body="&lt;p&gt;Do you mean generally? There are quite a lot of (european) test missions (e.g. from ESA the Proba Missions: Proba-1, Proba-2, Proba-V and Proba-3 which is currently in preparation, from DLR e.g. the TET-1 mission). There are probably more which I am not aware of, but most of the ones I listed I had a bit to do with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of them are for testing new technology (hard- and software, insturments, or e.g. Proba-3 for testing precise formation flying in a high elliptical orbit etc).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as far as I understand, OPS-SAT is special in regard of it's architecture as it is an &quot;upgrade&quot; in comparison to more traditional test missions listed above, as it tests more new technology of the platform and communication itself and not so much new payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also don't know much about non-european missions in that direction (NASA, India, China etc.).&lt;/p&gt;&#xA;" OwnerUserId="15975" LastActivityDate="2016-08-18T08:43:51.103" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17827" PostTypeId="1" AcceptedAnswerId="17839" CreationDate="2016-08-18T09:16:57.187" Score="3" ViewCount="433" Body="&lt;p&gt;I'm thinking of how a satellite can be fully passively stabilized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have two passive &lt;em&gt;attractors&lt;/em&gt;, forces that draw the satellite towards a certain orientation. A &lt;a href=&quot;https://space.stackexchange.com/a/17793/103&quot;&gt;bar magnet&lt;/a&gt; will orient along the magnetic field axis, and a &lt;a href=&quot;https://space.stackexchange.com/a/17192/103&quot;&gt;mass on extended arm&lt;/a&gt; will orient the craft along the gravity gradient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These two are sufficient to establish a fixed (or at least known) &quot;preferred orientation&quot; relative to Earth, but they are insufficient to stabilize the craft due to nearly nonexistent friction to extinguish oscillations around the preferred orientation. Besides the &lt;em&gt;attractors&lt;/em&gt;, we need &lt;em&gt;dampers&lt;/em&gt; to create friction-like forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For magnetic field, there's a &lt;a href=&quot;https://space.stackexchange.com/a/17793/103&quot;&gt;permalloy bar&lt;/a&gt; with a hysteresis of magnetic field orientation. In changing magnetic field, it will act like friction, opposing the rotation and dissipating the kinetic energy through eddy currents. But again, it acts along the magnetic field axis only. We still have an oscillation around the axis - in the plane passing through gravity gradient axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can we dampen this motion?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-09-05T18:02:21.440" Title="Passive spin damping system" Tags="|artificial-satellite|attitude|passive-systems|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17829" PostTypeId="1" CreationDate="2016-08-18T15:18:58.813" Score="5" ViewCount="140" Body="&lt;p&gt;I've noticed a lot of variations on the MER image data: we have RDR, EDR, and a whole lot of images that haven't been corrected yet (which appear mostly black in colour). On top of this, in the PDS databases, we have a collection of panoramic images and cropped images. I've read through a few papers on computer vision on the MER, but it hasn't specified which data they use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://an.rsl.wustl.edu/mer/&quot;&gt;Analyst's Notebook&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://geo.pds.nasa.gov/missions/mer/geo_mer_datasets.htm&quot;&gt;PDS Geosciences Node&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm designing an algorithm for terrain estimation using MER data, and in order to get as realistic results as possible, I want to use the data that the rover actually uses to navigate. Which data collections should I be looking at?&lt;/p&gt;&#xA;" OwnerUserId="16665" LastActivityDate="2016-08-19T17:34:20.087" Title="Which images were used primarily for navigation algorithms on the Spirit rover?" Tags="|mars|rovers|spirit|opportunity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17830" PostTypeId="1" AcceptedAnswerId="19744" CreationDate="2016-08-18T15:47:50.893" Score="11" ViewCount="2360" Body="&lt;p&gt;In the related questions &lt;a href=&quot;https://space.stackexchange.com/questions/17817&quot;&gt;Can you take a bath on Mars?&lt;/a&gt; &amp;amp; &lt;a href=&quot;https://space.stackexchange.com/questions/6607&quot;&gt;How would swimming on Mars feel, given the lower gravity?&lt;/a&gt; we learn that bathing and swimming on Mars is much like it is on Earth.  Some &lt;a href=&quot;https://space.stackexchange.com/questions/17817/can-you-take-a-bath-on-mars/17818#comment46163_17818&quot;&gt;comments&lt;/a&gt; on those questions suggest that if the pool is exposed to Martian atmosphere it would boil away quickly. What would really happen if your swimming pool or fish pond was suddenly exposed to  Martian atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some comments suggest the pool magically appears and boils away, but here is a realistic scenario.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A swimming pool &lt;a href=&quot;https://en.wikipedia.org/wiki/Olympic-size_swimming_pool&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;(Olympic size)&lt;/em&gt;&lt;/a&gt; or fish pond is full of water {25–28 °C (77–82 °F)} and covered by an inflatable dome that keeps Earth atmosphere over the pool.  A event occurs: which cuts off power to heat and circulate the water, and the dome is torn badly or complete removed. What happens to the pool now? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2020-06-21T21:34:30.060" Title="Boiling ponds and pools on Mars?" Tags="|mars|atmosphere|water|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17831" PostTypeId="2" ParentId="17830" CreationDate="2016-08-18T16:38:12.793" Score="6" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi; 6.0 mbar), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi; 1.013 bar).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.engineersedge.com/h2o_boil_pressure.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;This chart&lt;/a&gt;&#xA;seems to indicate that the 77°F (25°C) water would be above its boiling point by more than 75°F (the granularity of the chart does not give more info).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would result in a (perhaps explosive) boiling of the pool until it had released enough energy to be below its boiling point in the 6°F to -24°F (-14°C to -31°C) range. At this point the remaining water may freeze and sublimate in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But&lt;/strong&gt;, according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, the surface of Mars is between -226°F (-143°C) and 95F (35°C), so depending on conditions the pool may never cool below its boiling point, and all of it simply evaporates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;@Dean MacGregor correctly points out that at 600Pa water will never be a stable liquid, so the final result (after 'explosive' boiling) will eventually be that all the water evaporates, or a portion boils off, allowing the remainder to cool to ice, which may or may not sublimate. All depending largely on the temperature of this particular part of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a demonstration, this is a &lt;a href=&quot;https://www.youtube.com/watch?v=y4BGV7-1lhs&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; of water being subjected to a vacuum at room temperature. On those conditions the water boils, until it cools to freezing.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2017-01-10T15:53:16.157" LastActivityDate="2017-01-10T15:53:16.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17832" PostTypeId="2" ParentId="17830" CreationDate="2016-08-18T17:36:03.640" Score="11" Body="&lt;p&gt;Here's a phase diagram of water from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Phase_diagram_of_water.svg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PxpJM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PxpJM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you find Mars's atmosphere of 600Pa on the Y-axis and track right you'll see that H20 can't be water on Mars.  It can only be ice or vapor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In your example where you have a dome of Earth like atmosphere protecting your Martian pool that blows away somehow then you'll either have a skating rink or an empty hole but no water.  Going by the average Martian temperature you'd have a skating rink but once the temperature exceeds the sublimation point all your H2O would turn to vapor and that'd be the end of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other answer asserts that it would first boil and perhaps there'd be some H20 left as ice after some of it boiled off.  I suppose this depends on the exact circumstances of losing the dome.  The way OP asks the question, it seems that first heat is lost and then the dome goes away.  If the water has already cooled to below the sublimation point before the pressure drop occurs then there's no need for it to boil.  If everything is working fine and then the dome flies away while the ambient temperature is above boiling temperature then clearly it would boil off.  However, if the ambient temperature is below sublimation when the dome is lost I don't think it is clear that the water would boil rather than freeze.  The very act of decompressing would cool the water so we can't simply say that the pressure can drop faster than the water cools because the pressure dropping speeds the cooling.&lt;/p&gt;&#xA;" OwnerUserId="12310" LastEditorUserId="12102" LastEditDate="2016-09-29T01:32:41.717" LastActivityDate="2016-09-29T01:32:41.717" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17835" PostTypeId="2" ParentId="17778" CreationDate="2016-08-19T00:55:44.833" Score="3" Body="&lt;p&gt;If you are talking about one in earth orbit you have to think about it's orientation. If the axis is north / south then the habitats would be rotating into higher and lower orbits. So tidal forces would probably try to impose vibration. Not to say this couldn't be overcome. The entire system would have a center of mass and I would think if you had enough stiffening structure, it might be manageable. &#xA;You might think of pointing the axis at the earth so the habitats would be at the same altitude all the time, they would effectively be changing inclination. But I believe a rotating mass would want to stabilize it's axis and you couldn't point it at earth unless the axis rotated once every orbit. &#xA;The further you are out, the less the tidal forces would matter. Wernher von Braun wanted to put a spinning torus station a thousand miles out. Maybe that is why. &#xA;Another interesting point in this is that a rotating object wants to spin around it's long axis. So if you built a long spinning weenie as a space colony, after a while it would wind up spinning end over end. That is why Gerard O'Neill proposed space colonies in counter rotating pairs with a truss connecting at the axis. That way the whole system acts as a non spinning object.&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-08-19T00:55:44.833" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17836" PostTypeId="2" ParentId="17756" CreationDate="2016-08-19T01:06:16.643" Score="-2" Body="&lt;p&gt;Ultimately there is the limit of total available energy. There are just so many calories in a pound of hydrogen and oxygen (or whatever fuels) Rockets are actually extremely efficient engines, turning more of the latent energy into movement than most engines. (I wish I had the numbers.)&#xA;That's why some people argue that we need nuclear rockets, the energy density is higher by order of magnitudes. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-08-19T01:06:16.643" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17837" PostTypeId="1" AcceptedAnswerId="17838" CreationDate="2016-08-19T10:50:54.910" Score="5" ViewCount="348" Body="&lt;p&gt;Which are space station projects for a Mars mission using the current propulsion technology capabilities? Many space station concepts are designed by artists before for missions beyond Earth. But these concepts have capabilities and use propulsion systems which aren't designed or tested yet. In the next decades new technology will be developed, but until these days which are the projects or designs using the current propulsion systems?&lt;/p&gt;&#xA;" OwnerUserId="16521" LastEditorUserId="16521" LastEditDate="2016-08-25T11:30:03.187" LastActivityDate="2016-08-25T11:30:03.187" Title="Which are the space station projects for a Mars mission using the current propulsion technology capabilities?" Tags="|mars|propulsion|space-station|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17838" PostTypeId="2" ParentId="17837" CreationDate="2016-08-19T13:44:37.130" Score="9" Body="&lt;p&gt;JPL wrote up &lt;a href=&quot;http://online.liebertpub.com/doi/abs/10.1089/space.2015.0018&quot; rel=&quot;noreferrer&quot;&gt;&quot;A Minimal Architecture for Human Journeys to Mars&quot;&lt;/a&gt; last year, which plots out a likely path given the current technology and budget situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The architecture is a Phobos-first roadmap that has a 2.5 year round-trip to Phobos launching in 2033, with a 300-day stay at Phobos. After the Phobos trip, there would be a short-surface-stay mission to Mars, described as similar to Apollo 17, in 2039. This still doesn't sound quite like what you're asking about, since you're asking about a station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For that, you have to look at the final mission in the architecture, the long-surface-stay Mars mission. The mission would be for a crew of four, and would start around 2040. Like the Phobos mission, there would be about 300 days on the surface starting in 2043. The mission would require 10 SLS launches. Here is an image depicting the stages:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BxpI8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BxpI8.png&quot; alt=&quot;JPL Long Duration Mars mission plan&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each lander would be delivered in a manner almost identical&#xA;  to the crewed lander, with the exception that they could&#xA;  use direct entry and avoid aerocapture as an intermediate step.&#xA;  The crewed segments of the mission would be identical to the&#xA;  previous short-surface-stay mission, except that the full crew&#xA;  would go to the surface. This launch campaign would be implemented&#xA;  with a steady cadence of one SLS launch every 6&#xA;  months. The exception is that once every 2 years, two SLS&#xA;  launches would need to occur within one month of each other.&#xA;  Additional ground infrastructure at the Kennedy Space Center&#xA;  would be required to support those biennial extra launches, and&#xA;  that capability would be needed by about 2040.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Aside from the SLS and the Orion, which are still in development, the mission would also need at least four additional vehicle types that are either planned or still under study: the planned Solar Electric Propulsion (SEP) tug; and the Deep Space Habitat (DSH), In-Space Chemical Propulsion stage, and Mars Lander/Mars Ascent Vehicle (MAV) which are still under study. These are listed for the short duration mission, but are also present on the long duration mission. They do not go into the specifics of the needs for the long duration mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper also details plans to progress toward a permanent outpost. It says &quot;using this example architecture, a new crew of four could be sent to&#xA;Mars every 4 years along with two cargo landers.&quot; These missions would continue to extend the base on Mars.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In addition to consumables, the cargo landers could&#xA;  bring exploration equipment such as pressurized rovers, advanced&#xA;  surface power systems, science equipment, drilling&#xA;  equipment, in situ resource utilization (ISRU) packages, and&#xA;  additional habitation volume. As the Mars base expands, some&#xA;  crew would stay for the minimum cycle time of about 350&#xA;  days, but others could possibly stay for a much longer time&#xA;  and wait for the next Earth return opportunity. In this way, the&#xA;  base could eventually be permanently occupied and evolve&#xA;  toward increasing self-sufficiency.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Taking into account the readiness level of the described technologies, they estimate that this concept should be affordable given the sunset of the ISS in 2028.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They conclude:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Annual budget constraints need to be considered as a design&#xA;  requirement for human journey to Mars architectures&#xA;  since it is likely that the NASA budget will not see a dramatic&#xA;  increase beyond adjustments for inflation. This in turn requires&#xA;  a phased approach toward establishing a permanent&#xA;  outpost on Mars to allow the technical risk and the required&#xA;  funding to be spread out and still deliver significant and publicly engaging milestones along the way. One such approach&#xA;  is presented here as an example. It is a minimal architecture&#xA;  that relies on assets already under development or&#xA;  planned by NASA. A series of missions in cislunar space&#xA;  would lead to a Phobos lander in 2033 to be followed in short&#xA;  order by a dress rehearsal landing on the Moon, and then by a&#xA;  crew to the surface of the red planet by 2039.&#xA;  We hope that the ideas and principles introduced here in&#xA;  whole or in part can be a useful input to the process of structuring&#xA;  an implementable human journey to Mars in our lifetime.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is important to emphasize that at this stage, this is just one example architecture, though the paper notes that other studies have used portions of this as a basis.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-08-19T13:44:37.130" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17839" PostTypeId="2" ParentId="17827" CreationDate="2016-08-19T14:14:15.250" Score="5" Body="&lt;p&gt;A rotational viscous damper is what you're looking for, commonly called a rotational dashpot.  Normally on Earth these would dampen between a fixed ground system and the moving system, in space it would be designed to dampen between the moving satellite and the fluid itself in the damper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most basically it is a cylinder of fluid whose inertia is going to resist rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Attitude_control#Pure_passive_attitude_control&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Attitude_control#Pure_passive_attitude_control&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For mass saving you could design your fuel tanks for an active thruster system to act as the passive damper.&lt;/p&gt;&#xA;" OwnerUserId="15824" LastEditorUserId="15824" LastEditDate="2017-09-05T18:02:21.440" LastActivityDate="2017-09-05T18:02:21.440" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17840" PostTypeId="1" AcceptedAnswerId="17865" CreationDate="2016-08-19T16:14:04.457" Score="4" ViewCount="2333" Body="&lt;p&gt;I've read a bunch on space elevators and from what everyone is saying, it would be required that the elevator is in geosynchronous orbit. From my understanding, that is so that it would stay fixed in one position over the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, from my understanding a space elevator would be some sort of cable that runs up into space and so people would essentially ride that cable up. Now that would also explain why it would need to be in geosynchronous orbit, since the cable would then otherwise wrap itself around the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That got me thinking though, what if you were to build a super-structure from the ground up. Something similar to the image below. If you were able to create a structure strong enough to withstand the all of the elemental forces in the upper atmosphere:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Would it be possible to have the structure sticking out into space, but not in geosynchronous orbit?&lt;/li&gt;&#xA;&lt;li&gt;What would be the challenges in creating a structure like this?&lt;/li&gt;&#xA;&lt;li&gt;Could something like this change the earths rotational period?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ETzYx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ETzYx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16678" LastActivityDate="2021-06-04T01:57:39.320" Title="Why would a space elevator / structure need to be in Geosynchronous orbit?" Tags="|space-elevator|geosynchronous|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17842" PostTypeId="2" ParentId="17829" CreationDate="2016-08-19T17:34:20.087" Score="4" Body="&lt;p&gt;What you're looking for is the images from the &quot;Navigation Camera&quot;. As explained &lt;a href=&quot;http://mars.nasa.gov/mer/home/posters/OpportunityPosterBack.pdf&quot;&gt;here&lt;/a&gt;, the mast-mounted stereo imagers collectively referred to as the &quot;Navigation Camera&quot; were used for navigation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can browse the collection of images from the Navigation Camera here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mars.nasa.gov/mer/gallery/all/spirit.html&quot;&gt;http://mars.nasa.gov/mer/gallery/all/spirit.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You didn't mention Opportunity, but it is available as well:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mars.nasa.gov/mer/gallery/all/opportunity.html&quot;&gt;http://mars.nasa.gov/mer/gallery/all/opportunity.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just select one of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Timekeeping_on_Mars#Sols&quot;&gt;sols&lt;/a&gt; available under the &quot;Navigation Camera&quot; heading and click the &quot;View Selected Sol&quot; button. The images for that sol will appear in the new page.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The body-mounted Hazard Avoidance cameras assisted in navigation as well, and you should also see the headings for them listed at the above links as &quot;Hazcam&quot;.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-08-19T17:34:20.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17843" PostTypeId="2" ParentId="17830" CreationDate="2016-08-19T18:09:33.513" Score="4" Body="&lt;p&gt;Depending on how deep the pool is, some of the water will still be under pressure from the weight of the water on top of it. I don't think the entire volume could boil simultaneously, only the surface would be able to. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A very crude way to calculate the water pressure is to take the depth of the water column, divide it by 10m (on earth, 10m of water equals 1atm), then multiply by 0.38: the strength of martian gravity relative to earth gravity. For example at 1m the pressure would be 0.038atm, and at 2m it would be 0.076atm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The temperature required for water to boil at these pressure would be be 26C at 1m and 38C at 2m (I found these numbers on &lt;a href=&quot;http://www.engineeringtoolbox.com/boiling-point-water-d_926.html&quot; rel=&quot;nofollow&quot;&gt;this site&lt;/a&gt;). If the pool temperature is ~25 degrees then the top 1m of water would immediately start boiling. An Olympic pool is 2-3m deep which would leave 1-2m of water which is under enough pressure to prevent immediate boiling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In accordance with this analysis it would not boil explosively because there just wouldn't be enough vapor pressure at that temperature to &quot;blow apart&quot; the pool against the force of gravity. The top 1m of water would merely boil vigorously, rapidly cooling down in the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As discussed in other answers, the real question is whether the water would boil away to nothing or freeze, I suspect it would freeze and it might be that a liquid layer remains trapped under the ice, which then freezes solid. Over time the ice pool will probably sublimate away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Disclaimer: I am not a physicist. My analysis may be flawed.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2016-08-19T18:09:33.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17844" PostTypeId="1" CreationDate="2016-08-19T21:13:12.487" Score="3" ViewCount="161" Body="&lt;p&gt;Given that the Russian GLONASS system has an orbital altitude of 19,140 Km and a period of revolution of 11 hours and 15 minutes, the US system has an orbital altitude of 20,180 Km and a period of revolution of 11 hours and 58 minutes and finally the Galileo system an orbital altitude of 23,222 Km and a period of revolution of 14 hours and 22 minutes, how does one calculate the period of revolution for the Chinese Compass system with an orbital altitude of 21,150 Km?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I used Kepler's third law in my original calculation, but assumed that the orbital altitudes given are referenced to the centre of the earth.  I made this assumption based on how I interpreted the math, and did not know that the orbital altitudes are referenced to mean sea level on Earth.  Once I added the radius of the earth which is approximately 6,371 Km to the orbital altitudes I was able to calculate the period of revolution.  So the actual orbital distance from the centre of the earth to the Chinese Compass system is 21,150 Km + 6,371 Km.&lt;/p&gt;&#xA;" OwnerUserId="16681" LastEditorUserId="16681" LastEditDate="2016-08-20T00:51:03.690" LastActivityDate="2016-08-20T00:51:03.690" Title="Period of revolution of Chinese Compass system" Tags="|artificial-satellite|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17845" PostTypeId="2" ParentId="17844" CreationDate="2016-08-19T21:51:54.937" Score="3" Body="&lt;p&gt;You can use an online calculator such as &lt;a href=&quot;http://keisan.casio.com/exec/system/1224665242&quot; rel=&quot;nofollow&quot; title=&quot;orbital period calculator&quot;&gt;this&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The math is explained &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_period#Small_body_orbiting_a_central_body&quot; rel=&quot;nofollow&quot; title=&quot;Wiki explaining the math&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case the result is a period of 12 hours 37 minutes at a velocity of 3.8 Km/s.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastActivityDate="2016-08-19T21:51:54.937" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17846" PostTypeId="1" CreationDate="2016-08-20T16:09:42.940" Score="4" ViewCount="275" Body="&lt;p&gt;I am wondering why it takes  two to three years to build this tiny little&#xA;international docking adapter IDA 3? Every year 4-6 Soyouz spaceships are built. &lt;/p&gt;&#xA;" OwnerUserId="13909" LastActivityDate="2016-08-20T18:52:00.967" Title="Why does it take 2-3 years to build the third IDA?" Tags="|docking-port|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17847" PostTypeId="2" ParentId="17846" CreationDate="2016-08-20T16:58:40.407" Score="6" Body="&lt;p&gt;The IDA was never meant for mass production: they needed just 2 for the ISS, and they knew long beforehand when they needed them. So they knew they could spend 2 years building them without impacting the schedule.  They built the things by hand, took their time testing them etc. This is labor-intensive, and labor costs money. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To reduce the build time, you have to spend money too: build tools and jigs, design factory processes (e.g. an assembly line), design the IDA for manufacturability, etc. &#xA;So you have to decide: spend money on labor, or spend money on R&amp;amp;D so you can make the module cheaply? This decision depends on how many modules you need. In this case, only 2. That's an easy decision to make. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, then the launch of IDA 1 went wrong, and the 2-year production time bit them in the ass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, the fact the Russians can produce 6 Soyuz per year doesn't say anything about how long it takes to build one Soyuz: it could still take 2 years to build one spaceship, if they build 12 of them in parallel to get the production rate they need. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-08-20T18:52:00.967" LastActivityDate="2016-08-20T18:52:00.967" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17848" PostTypeId="1" CreationDate="2016-08-20T17:46:22.963" Score="6" ViewCount="299" Body="&lt;p&gt;Although the &lt;a href=&quot;https://en.wikipedia.org/wiki/CST-100_Starliner&quot; rel=&quot;nofollow&quot;&gt;Boeing CST-100 Starliner&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Orion MPCV&lt;/a&gt; are designed for different missions (LEO vs beyond LEO), one might imagine a mission which could be served by either vehicle. Presumably, both will be equipped with the same type of docking adapter, but can either be mated to and launched on the same booster type? Do they have compatible physical/electrical mountings/interconnections? Or do the different mission types and therefore booster requirements preclude the need for any such standardization?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="16606" LastEditDate="2016-08-22T23:06:29.160" LastActivityDate="2016-08-23T18:11:49.823" Title="Are the Boeing CST-100 Starliner and Orion MPCV interchangeable at any level?" Tags="|crewed-spaceflight|orion-spacecraft|cst-100|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17849" PostTypeId="1" CreationDate="2016-08-20T17:50:08.200" Score="3" ViewCount="411" Body="&lt;p&gt;Is there a preferred orientation to approach (or leave) a space station for docking? Is it from below? above? by change in the inclination (port or starboard) from ahead or aft?  I know recently the Soyuz has been using a bi-elliptic transfer orbit to get there. That makes me thing from below. I have also read that they have on occasion changed the station attitude for a docking. The pictures show the docking as always being on the earth side. But I don't know if that's because it's the best angle to approach, or because the docking section is the most massive and therefore oriented to earth by tidal forces. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2018-11-16T07:26:16.767" Title="Best orientation for docking" Tags="|orbital-mechanics|docking|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17850" PostTypeId="1" CreationDate="2016-08-20T18:09:23.773" Score="1" ViewCount="985" Body="&lt;p&gt;Not to sound rhetorical but if a Ion engine was used it would start at 1 mile an hr and then the speed would  continue exponentially.  So theoretically wouldn't that make it possible to break free from earth's gravity?   &lt;/p&gt;&#xA;" OwnerUserId="16685" LastEditorUserId="4660" LastEditDate="2016-08-20T18:43:15.780" LastActivityDate="2016-08-20T22:09:55.947" Title="Why can't ion engines be used for launch?" Tags="|gravity|escape-velocity|" AnswerCount="2" CommentCount="3" ClosedDate="2016-08-21T14:00:40.130" ContentLicense="CC BY-SA 3.0" />
  <row Id="17851" PostTypeId="2" ParentId="17850" CreationDate="2016-08-20T18:33:19.800" Score="2" Body="&lt;p&gt;Ion engines produce far less thrust than their weight on Earth. This means that starting at Earth's surface, the downward acceleration of gravity is much larger than the upward acceleration produced by the ion engine, so it wouldn't break free. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If some other method (e.g. a chemical rocket engine booster) is used to get your ion engine into stable Earth orbit, then yes, you can escape Earth's gravity, but not at 1 mph.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-08-20T18:33:19.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17852" PostTypeId="2" ParentId="17850" CreationDate="2016-08-20T18:37:28.647" Score="2" Body="&lt;p&gt;You need to think about how hard something is being pushed, not how much speed is being added. Unless an engine is pushing a rocket hard enough, which is called its thrust, it can't overcome the force that is pulling on the rocket, gravity. Also, the bigger the whole rocket is compared to the  the engine, the more thrust is needed to move it. So, engineers talk about the thrust-to-weight ratio of a rocket, and normally write that T/W. To be enough to move the rocket upward, the T/W needs to be larger than 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NEXT ion engine has a thrust of 236 milliNewtons, and a Newton is the force it takes to accelerate 1 kg by 1 meter per second in 1 second.  Since an ion engine weighs much more than 236 grams, it can't lift even itself into space. The Space Shuttle solid rocket boosters have a thrust of 12 million Newtons each.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason why ion engines work so well in space, is that when they are turned on, the spacecraft is already moving in orbit, so it can't fall to the ground any more, and it is out of the atmosphere, so there is no friction slowing it down. In that situation, even though the engine is only thrusting a tiny amount, nothing is overpowering it any more, so it slowly adds up and the space craft accelerates. Because ion engines are extremely efficient with fuel, and it takes years for a deep space probe to go places so there is plenty of time for the acceleration to add up, they are very good for ships that have to go very far.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="195" LastEditDate="2016-08-20T22:09:55.947" LastActivityDate="2016-08-20T22:09:55.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17853" PostTypeId="1" CreationDate="2016-08-20T22:47:20.820" Score="10" ViewCount="449" Body="&lt;p&gt;Which are the benefits and the challenges for a new Space Launch System (SLS) version with four Solid Rocket Boosters (SRB)? Benefits i mean what would gain in payload capabilities, how much more payload would put in low Earth orbit and beyond Earth's orbit, or what kind of payload could handle and support (for example an entire space station or &quot;spaceship&quot; for missions beyond Earth's orbit). Of course calculations or simulations would be needed to have very accurate values, but I would like to have an idea and interested for some approximate possible values not neccesarily very accurate. With this configuration and new payload mass capability in orbit or Earth escape, one mission that requires two SLS rocket launches would require only one.  With challenges I mean technical and engineering difficulties not opinions about the cost or if the government would support and fund this SLS version or not. I am speaking for that case where everything has a &quot;go&quot; and supported. Which are the profits and challenges in theory for this design?&lt;/p&gt;&#xA;" OwnerUserId="16522" LastEditorUserId="16522" LastEditDate="2016-12-20T01:03:53.910" LastActivityDate="2016-12-21T18:37:57.177" Title="Which are the benefits and challenges for a new SLS rocket version with four SRB?" Tags="|rockets|nasa|design|engineering|sls|" AnswerCount="2" CommentCount="5" ClosedDate="2016-12-22T10:46:12.920" ContentLicense="CC BY-SA 3.0" />
  <row Id="17854" PostTypeId="1" AcceptedAnswerId="17857" CreationDate="2016-08-21T10:20:46.793" Score="3" ViewCount="324" Body="&lt;p&gt;What is the meaning of the acronym &lt;strong&gt;BODA&lt;/strong&gt;  as used in an &lt;em&gt;Atlas V&lt;/em&gt; launch, specifically the Juno launch in 2011?&lt;/p&gt;&#xA;" OwnerUserId="16692" LastActivityDate="2016-08-21T14:29:43.660" Title="What is the meaning of &quot;BODA&quot;?" Tags="|atlas-v|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17855" PostTypeId="1" CreationDate="2016-08-21T11:22:51.010" Score="2" ViewCount="1153" Body="&lt;p&gt;I keep reading that scientists are looking for life on other planets. However almost always their premise to determine the same is based on life on Earth, i.e. need for water, air, etc. Why do we always think of life on Earth? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't it conceivable that life on other planets may take a different course? Instead of water they may need something else. Instead of breathing air, they might need some other means of life support.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have these questions not been analysed? Or already been discounted?&lt;/p&gt;&#xA;" OwnerUserId="16693" LastEditorDisplayName="user687" LastEditDate="2016-08-21T14:00:24.587" LastActivityDate="2016-08-22T13:21:40.847" Title="Life on other planets not based on water" Tags="|astrobiology|" AnswerCount="3" CommentCount="4" ClosedDate="2016-08-22T17:41:07.330" ContentLicense="CC BY-SA 3.0" />
  <row Id="17856" PostTypeId="2" ParentId="17855" CreationDate="2016-08-21T13:01:08.817" Score="2" Body="&lt;p&gt;Yes, these questions have been analysed extensively. Life based on compounds &lt;a href=&quot;https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry&quot; rel=&quot;nofollow noreferrer&quot;&gt;other than carbon and water&lt;/a&gt; is theoretically conceivable, but &lt;a href=&quot;https://en.wikipedia.org/wiki/Extraterrestrial_life#Biochemistry&quot; rel=&quot;nofollow noreferrer&quot;&gt;unlikely&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It is often hypothesized that life forms elsewhere in the universe would, like life on Earth, be based on carbon chemistry and rely on liquid water. Life forms based on ammonia (rather than water) have been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.[24]&lt;/p&gt;&#xA;&lt;p&gt;About 29 chemical elements are thought to play an active positive role in living organisms on Earth.[25] About 95% of this living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts.[26] The unique characteristics of carbon made it unlikely that any other element could replace carbon, even on another planet, to generate the biochemistry necessary for life. The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins. Carbon forms more compounds than all other elements combined. &lt;strong&gt;The great versatility of the carbon atom makes it the element most likely to provide the bases—even exotic ones—to the chemical composition of life on other planets.&lt;/strong&gt;[27]&lt;/p&gt;&#xA;&lt;p&gt;Life on Earth requires water as its solvent in which biochemical reactions take place. Sufficient quantities of carbon and the other elements along with water, may enable the formation of living organisms on other planets with a chemical make-up and temperature range similar to that of Earth.[28] Terrestrial planets such as Earth are formed in a process that allows for the possibility of having compositions similar to Earth's.[29] The combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can provide structural elements for life. Plants derive energy through the conversion of light energy into chemical energy via photosynthesis. Life, as currently recognized, requires carbon in both reduced (methane derivatives) and partially oxidized (carbon oxides) states. Nitrogen is needed as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(emphasis mine)&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-08-21T16:12:06.330" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17857" PostTypeId="2" ParentId="17854" CreationDate="2016-08-21T14:21:42.400" Score="10" Body="&lt;p&gt;BODA appears to stand for &quot;burn-out detection algorithm&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the launch of an Atlas V equipped with solid rocket boosters, the main engine throttles down after about 75 seconds to limit g-force. Once the &lt;em&gt;burn-out&lt;/em&gt; of the solid rockets is &lt;em&gt;detected&lt;/em&gt;, the main engine throttles back up to 100%. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure if the algorithm is based on determination of pressure inside the booster casings, or on accelerometer readings, or tracking of the trajectory from the ground, or some combination thereof.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(This was a tough one to work out. The acronym expansion was given in &lt;a href=&quot;http://descanso.jpl.nasa.gov/monograph/series13/DeepCommo_Acro_Abbrev--141029.pdf&quot;&gt;this list&lt;/a&gt; but with no further explanation. Launch timelines like &lt;a href=&quot;http://www.hwupgrade.it/forum/archive/index.php/.../t-1112682.html&quot;&gt;this one for New Horizons&lt;/a&gt; mention burnout detection without use of the acronym.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-08-21T14:29:43.660" LastActivityDate="2016-08-21T14:29:43.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17858" PostTypeId="2" ParentId="17855" CreationDate="2016-08-21T14:34:29.230" Score="5" Body="&lt;p&gt;We simply would not have a good idea &lt;em&gt;what&lt;/em&gt; to look for if looking for non-water-based life. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know that life using aqueous chemistry can occupy a wide range of temperatures, pH values, light and dark, using photosynthesis, chemosynthesis - even &lt;a href=&quot;http://www.bbc.com/earth/story/20160613-there-are-microbes-that-eat-and-poo-nothing-but-electricity&quot; rel=&quot;nofollow&quot;&gt;microbes that &quot;eat and poo electricity&quot;&lt;/a&gt; have been discovered in a wide range of environments. (Despite the cute title, the article is a good review of the subject - bacteria and other microbes that derive their metabolic energy directly from external chemical potentials.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This large variety gives us a sound basis for choosing signs or markers of life processes. Things we should look for that would suggest life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we have no examples of life processes without water, and not even any good theories of what it might be like, it is much harder to propose what signs or markers are worth spending $10,000,000 or 100,000,000 or more for a space probe to look for it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SETI - the &lt;a href=&quot;https://en.wikipedia.org/wiki/Search_for_extraterrestrial_intelligence&quot; rel=&quot;nofollow&quot;&gt;Search for ExtraTerrestrial Intelligence&lt;/a&gt; - looks for signs of intelligence, with no bias on chemistry. They certainly don't have a bias towards water, and actually they don't even have a bias towards life. Just intelligence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking for water-based life is a little bit of a case of looking for your keys under the streetlight because that's where the light is, but at least here you know you have lost &lt;em&gt;at least some&lt;/em&gt; of your keys under the light.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-08-22T13:21:40.847" LastActivityDate="2016-08-22T13:21:40.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17859" PostTypeId="1" AcceptedAnswerId="18276" CreationDate="2016-08-21T18:38:17.013" Score="7" ViewCount="184" Body="&lt;p&gt;Is it more likely for a space station like the ISS to suffer micro meteor and space debris strikes on the forward side as oppose to others? (it certainly seems less likely to get them on the underside.)&#xA;The implication of this is in the layout of a space station design. Would it make sense to locate less important components (like storage) on the forward side to protect more critical components behind. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-09-19T16:49:07.897" Title="Angle of meteoroid and debris strikes on ISS" Tags="|iss|debris|meteoroid|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17860" PostTypeId="2" ParentId="797" CreationDate="2016-08-21T19:38:56.397" Score="0" Body="&lt;p&gt;Cosmic radiation can be shielded by mass. This may require hulls several feet thick, of something like ice. &#xA;A two-foot thick hull encompassing a habitat only 15 feet in diameter would comprise a very large portion of its total mass. However, the walls don’t have to be thicker when a habitat is larger. For a habitat that is 100 feet in diameter, 2 foot thick walls is a much smaller portion of its total mass. So using mass to shield looks terrible with small habitats but much more doable with large ones. These walls would also protect against meteors. So there is bound to be a push for larger habitats as we become able to build them.&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="1142" LastEditDate="2016-08-23T02:34:32.870" LastActivityDate="2016-08-23T02:34:32.870" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17863" PostTypeId="2" ParentId="17855" CreationDate="2016-08-22T02:04:00.167" Score="3" Body="&lt;p&gt;This has been thought about a great deal in the field of astrobiology, and is the basis of the idea of &lt;a href=&quot;https://en.wikipedia.org/wiki/Biosignature#In_astrobiology&quot; rel=&quot;nofollow&quot;&gt;biosignatures&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The usefulness of a biosignature is determined, not only by the&#xA;  probability of life creating it, but also by the improbability of&#xA;  nonbiological (abiotic) processes producing it...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Perhaps the best example of biosignatures that don't depend on the presence of anything associated with life as we know it is atmospheres in chemical disequilibrium:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Over billions of years, the processes of life on a planet would result&#xA;  in a mixture of chemicals unlike anything that could form in an&#xA;  ordinary chemical equilibrium. For example, large amounts of oxygen&#xA;  and small amounts of methane are generated by life on Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The example is of chemicals associated with life as we know it, but any chemical that should be short-lived and yet is present in large quantities would count as worth investigating as a possible sign of life. We still can't detect much about exoplanet atmospheres, but scientists have been able to &lt;a href=&quot;http://www.universetoday.com/92962/how-well-can-astronomers-study-exoplanet-atmospheres/&quot; rel=&quot;nofollow&quot;&gt;say a few things&lt;/a&gt; about them. The next generation of &lt;a href=&quot;http://sci.esa.int/sci-fma/47037-exoplanet-spectroscopy-mission-esm/&quot; rel=&quot;nofollow&quot;&gt;infra-red space telescopes&lt;/a&gt; will improve matters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theory for detection of this kind has been worked on for over 20 years now, and a framework has been established for how to determine whether an observed phenomenon could be life. A system out of equilibrium is an initial sign, the work of Christoph Adami goes from there to examine how to take such data and find out if there is a pattern associated with that which looks like life: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/F17_KiAZOxg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using such analysis does require a lot of data, though. It could be applied to places like Europa, Titan, or Enceladus, but it will be a while before we could apply it to any exoplanets.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2016-08-22T02:04:00.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17864" PostTypeId="2" ParentId="8323" CreationDate="2016-08-22T02:47:48.800" Score="0" Body="&lt;p&gt;The one thing you would not want to do is blast something apart and create more space debris. So I would think you would need to have to have a way to determine if the target was small enough to be completely vaporized, and restrict it's use to those objects. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-08-22T02:47:48.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17865" PostTypeId="2" ParentId="17840" CreationDate="2016-08-22T03:05:55.073" Score="13" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I've read a bunch on space elevators and from what everyone is saying, it would be required that the elevator is in geosynchronous orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The center of gravity of a space elevator would need to be at geosynchronous altitude, which means that a space elevator would need to be extend well beyond geosynchronous altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;Would it be possible to have the structure sticking out into space, but not in geosynchronous orbit?&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;What would be the challenges in creating a structure like this?&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The key challenge is strength of materials. The tallest structure built to date is &quot;only&quot; 829.8 meters tall (the Burj Khalifa), and the very top of that structure is exceedingly narrow compared to the base. A building under construction will reach about a kilometer into the sky. It &lt;em&gt;might&lt;/em&gt; be possible to build a structure ten times this tall, but that means the top is still well within the troposphere. Building a structure that is a few hundred kilometers high is not possible given our current understanding of strengths of materials, not even a mountain made of diamond.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, even if it was possible to build a structure that was over a hundred kilometers high, it wouldn't be particularly useful for space exploration. There are two key obstacles regarding space exploration. The lesser of the two is climbing out of the bulk of the Earth's atmosphere. The far more significant obstacle is achieving sufficient horizontal velocity so as to be in orbit, and a one hundred kilometer tall structure will not help solve that problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;A space elevator is a bit beyond the reach of current science and technology, but it's within an order of magnitude. A structure that reaches only partly into the sky is several orders of magnitude beyond the reach of current science and technology.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-08-22T03:05:55.073" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17866" PostTypeId="1" CreationDate="2016-08-22T03:09:37.210" Score="3" ViewCount="246" Body="&lt;p&gt;I am interested in electric propulsion for manned interplanetary craft. There must be a lot of models for a craft able to travel to Mars, for instance. I am wondering what the power requirements seem to be. One Mars mission craft has been described as 300 tons (ISS is 450 tons).  I am imagining a plasma rocket like the VASIMR powered by solar arrays or a reactor. If anyone is familiar with these conversations, I wonder if they are talking about 200Kw, 600Kw, a megawatt or what would have to be supplied to the engines.    &lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="1142" LastEditDate="2016-08-22T23:37:17.530" LastActivityDate="2016-08-23T21:25:18.683" Title="Power requirements for solar electric interplanetary vehicle" Tags="|interplanetary|power|nuclear|magnetoplasmadynamic|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17867" PostTypeId="1" AcceptedAnswerId="17868" CreationDate="2016-08-22T14:49:43.027" Score="6" ViewCount="5502" Body="&lt;p&gt;In our several decades of spaceflight have we ever exposed any living organism to space? I'm pretty sure this hasn't happened to Humans, but have we jettisoned some lab rats into space (&quot;for science&quot;) on purpose or by accident? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I'm not including extremophile bacteria that can live in space.&lt;/p&gt;&#xA;" OwnerUserId="3141" LastActivityDate="2016-08-23T10:00:16.467" Title="Has any living thing been directly exposed to the vacuum of space" Tags="|eva|vacuum|experiment|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17868" PostTypeId="2" ParentId="17867" CreationDate="2016-08-22T15:02:32.847" Score="6" Body="&lt;p&gt;Not quite sure what you mean by &quot;directly&quot;, but yes, animals have been exposed to the vacuum of space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://eea.spaceflight.esa.int/portal/exp/?id=6952&quot; rel=&quot;noreferrer&quot;&gt;European Space Agency (ESA)&lt;/a&gt; conducted an experiment called Biopan on the Russian satellite Foton 11 in 1997/1998 where &lt;em&gt;Artemia&lt;/em&gt; (brine shrimp) were exposed to the vacuum and radiation of space. This was not the first time either. CNES (the French space agency) conducted an earlier experiment with &lt;em&gt;Artemia&lt;/em&gt; on the Soviet satellites Bion 6 and Bion 8 in the 1980s.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-08-22T15:02:32.847" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17869" PostTypeId="5" CreationDate="2016-08-22T15:12:09.977" Score="0" Body="" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-08-22T15:12:09.977" LastActivityDate="2016-08-22T15:12:09.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17870" PostTypeId="4" CreationDate="2016-08-22T15:12:09.977" Score="0" Body="A research program on detonating small nuclear bombs to propel a space craft that took place in the United States between 1958 and 1965. Specific impulse of the engine was expected to be around 6000 seconds, while also offering high thrust. The project was cancelled due to the Partial Test Ban Treaty." OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-08-22T15:12:09.977" LastActivityDate="2016-08-22T15:12:09.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17871" PostTypeId="1" AcceptedAnswerId="17980" CreationDate="2016-08-22T16:03:10.653" Score="5" ViewCount="458" Body="&lt;p&gt;Having a material strong enough to support a space elevator built from the ground up seems like it may never be able to happen. I had also seen another suggestion for building a space elevator starting on the moon and stretching into the earths atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That got me thinking - would it be possible to suspend an object in the stratosphere with a counterweight all the way in space? If so, how could something like that work?&lt;/p&gt;&#xA;" OwnerUserId="16678" LastActivityDate="2016-08-29T19:44:17.780" Title="Would it be possible to suspend a space elevator in the stratosphere?" Tags="|orbital-mechanics|orbit|space-elevator|geosynchronous|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17872" PostTypeId="2" ParentId="17867" CreationDate="2016-08-22T16:51:14.150" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I'm pretty sure this hasn't happened to Humans...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Depends on your definition of 'exposed'...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz 11 suffered an accidental depressuriation when preparing for reentry, when a valve accidentaly opened; the crew were not wearing spacesuits and had no protection against depressuriation. The cabin was in vacuum for &lt;a href=&quot;http://www.spacesafetymagazine.com/space-disasters/soyuz-11/crew-home-misfortunes-soyuz-11/&quot;&gt;around ten minutes&lt;/a&gt; before it reentered the atmosphere, long enough to kill the crew.&lt;/p&gt;&#xA;" OwnerUserId="11259" LastActivityDate="2016-08-22T16:51:14.150" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17873" PostTypeId="2" ParentId="17866" CreationDate="2016-08-22T17:03:59.290" Score="3" Body="&lt;p&gt;On the order of 300 kW (for a 70 t vehicle). In preparation for that, &lt;a href=&quot;https://www.nasa.gov/content/what-is-nasa-s-asteroid-redirect-mission/&quot; rel=&quot;nofollow&quot;&gt;Asteroid Redirect Robotic Mission (ARRM)&lt;/a&gt; will step up our game to around 40 kW.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-08-22T22:19:55.130" LastActivityDate="2016-08-22T22:19:55.130" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17874" PostTypeId="2" ParentId="17866" CreationDate="2016-08-22T20:59:28.937" Score="2" Body="&lt;p&gt;I found a nice paper detailing different mission profiles from Earth to Mars: &lt;a href=&quot;http://www.adastrarocket.com/VASIMR_for_flexible_space_exploration-2012.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.adastrarocket.com/VASIMR_for_flexible_space_exploration-2012.pdf&lt;/a&gt; &#xA;One of their examples is a 2 MW VASIMR delivering 30 t of payload to Mars orbit within 230 days. Please note that this trajectory does not start in LEO, but at ESOI, an orbital height of about 1 million kilometers. Not starting from LEO seems a good idea for a manned low thrust mission - it takes long to escape Earth's gravity well for the main space ship, but passengers could board later using a light conventional rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, here is a back-of-the-envelope calculation on how much power is needed, given the assumptions made are correct:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much power does an ion thruster need to get a 300 t spaceship to Mars? Naturally, the electric power can be arbitrarily low, given a very long transfer time. For a manned mission we should aim for a transit time of less than 2 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A transfer from LEO to LMO (Low Mars Orbit) seems to take about 6-7 km/s on a conventional trajectory. For this question we are going to assume that the same Δv is needed for a low-thrust mission. (Actually, low-thrust missions can not make use of the Oberth effect so that the needed Δv to escape the Earth gravity well is higher than with conventional rockets)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first question is, how much force do we need to accelerate our spaceship by 6 km/s within 2 years? Using the well-known formula $F = m \cdot a $ and $v=a\cdot t$ we get $$F = m \cdot \frac{v}{t}$$&#xA;With $v = 6~\mbox{km/s}$ and $t = 2 \cdot 365 \cdot 86400~\mbox{s}$ we get $F = 28~\mbox{N} $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An ion thruster that could be built today can reach about 50mN/kW force. So we need a thruster with about 600 kW power (scaling linearly, e.g. 1.2 MW for a transfer time of one year).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Earth orbit, sun provides about 1.3 kW per square meter power and a  high-efficiency solar panel can produce about 300 W electrical power from that. This means, the required power could be supplied by a solar array of about 3000 m². But as Mars is further out, intensity of the sun light reaching the panels drops down so that the array size needs to be larger by a factor of 2.3, i.e. 6900 m². Compare this to the ISS solar array size of about 2500 m² (albeit with lower efficiency).&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2016-08-23T21:25:18.683" LastActivityDate="2016-08-23T21:25:18.683" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17875" PostTypeId="2" ParentId="17867" CreationDate="2016-08-23T04:06:33.953" Score="7" Body="&lt;p&gt;In 1996 in a vacuum chamber space suit test Jim le Blanc had his air hose detach and he was exposed to a virtual vacuum at 0.1 psi. It was 87 seconds before the chamber was returned to normal air pressure. The supervising engineer Cliff Hess said “Essentially, he had no pressure on the outside of his body and that’s a very unusual case to get,”&#xA;He passed out but suffered no effects other than an ear ace. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-08-23T04:06:33.953" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17876" PostTypeId="2" ParentId="17867" CreationDate="2016-08-23T04:25:27.990" Score="7" Body="&lt;p&gt;In 1964 and 1965 at Holloman Air Force base in New Mexico a number of chimpanzees were exposed to a vacuum for 30 seconds. They were at 2 mm of mercury, which is 0.038 psi. They were revived with pure oxygen. Their ability to perform tasks were tested after they suffered rapid and total decompression for that time. They all did OK. When Stanley Kubrick first released the movie 2001 a Space Odyssey in 1968 he was so concerned that he would be criticized for the scene where Bowman survived a vacuum that he distributed a pamphlet at the theaters citing the air force experiments. I remember that pamphlet. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-08-23T04:25:27.990" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17877" PostTypeId="2" ParentId="17867" CreationDate="2016-08-23T10:00:16.467" Score="6" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tardigrade&quot; rel=&quot;noreferrer&quot;&gt;Tardigrades&lt;/a&gt; ('water bears') have also been exposed to the vacuum of space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They were on the Biopan-6 research platform provided by European Space Agency (ESA), and were sent into space with the russian FOTON-M3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the &lt;a href=&quot;http://tardigradesinspace.blogspot.nl/&quot; rel=&quot;noreferrer&quot;&gt;TARDIS&lt;/a&gt; blog for an overview.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Paper: &lt;a href=&quot;http://www.cell.com/current-biology/abstract/S0960-9822(08)00805-1?_returnURL=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982208008051%3Fshowall%3Dtrue&quot; rel=&quot;noreferrer&quot;&gt;Tardigrades survive exposure to space in low Earth orbit&lt;/a&gt;&lt;br /&gt;&#xA;K. Ingemar Jönsson, Elke Rabbow, Ralph O. Schill, Mats Harms-Ringdahl, Petra Rettberg&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Vacuum (imposing extreme dehydration) and solar/galactic cosmic radiation prevent survival of most organisms in space. Only anhydrobiotic organisms, which have evolved adaptations to survive more or less complete desiccation, have a potential to survive space vacuum, and few organisms can stand the unfiltered solar radiation in space.&lt;br /&gt;Tardigrades, commonly known as water-bears, are among the most desiccation and radiation-tolerant animals and have been shown to survive extreme levels of ionizing radiation.&lt;br /&gt;Here, we show that tardigrades are also able to survive space vacuum without loss in survival, and that some specimens even recovered after combined exposure to space vacuum and solar radiation. These results add the first animal to the exclusive and short list of organisms that have survived such exposure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerDisplayName="user10509" LastActivityDate="2016-08-23T10:00:16.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17878" PostTypeId="1" AcceptedAnswerId="17881" CreationDate="2016-08-23T12:05:54.053" Score="31" ViewCount="15280" Body="&lt;p&gt;I am very new to the rockets and this can be a very dumb question,&#xA;just that I am not sure if my understanding here is right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the rockets engines are at bottom  which help it take off and after stage separation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does it have to flip 180 degrees, this aligns the engines in direction of travel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does this help slowing down the rocket, it should only increase the speed of reentry when its fired?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the videos the slowing down happens with the engines firing at bottom before landing. Does it do an additional 180 flip some where in between?&lt;/p&gt;&#xA;" OwnerUserId="16708" LastEditorUserId="51" LastEditDate="2016-08-23T13:40:46.960" LastActivityDate="2016-11-25T18:55:38.380" Title="Why does the SpaceX Falcon 9 rocket do a 180 flip for reentry?" Tags="|launch|spacex|falcon-9|reentry|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17879" PostTypeId="2" ParentId="17878" CreationDate="2016-08-23T12:54:33.047" Score="45" Body="&lt;p&gt;Trajectory of the Falcon 9 &lt;a href=&quot;//space.stackexchange.com/questions/7753/why-did-spacex-attempt-to-land-the-booster-on-a-barge-instead-of-somewhere-on-la/13611#13611&quot;&gt;first stage&lt;/a&gt;:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/3zJ0c.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3zJ0c.png&quot; alt=&quot;F9 trajectory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&lt;a href=&quot;https://zlsa.github.io&quot; rel=&quot;nofollow noreferrer&quot;&gt;Graphic courtesy ZLSA Design (zlsa.github.io)&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, before the boostback burn, the stage flips so the engines point in the direction of travel. When the engines fire, this slows down the stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This trajectory is used when the stage returns to the launch site (and for some early experiments where the ASDS landing ship was placed close to the launch site). For landings downrange to the ASDS, the boostback burn is skipped. The booster still has to flip over, so the engines are pointed in the direction of flight for the reentry burn. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="415" LastEditDate="2016-11-25T18:55:38.380" LastActivityDate="2016-11-25T18:55:38.380" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17880" PostTypeId="2" ParentId="17878" CreationDate="2016-08-23T13:46:37.737" Score="9" Body="&lt;p&gt;A rocket launches, initially straight up to get out of the dense part of the atmosphere, but very soon turns to accelerate horizontally. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbiting is all about going fast enough around the planet (or gravity source) that the balance of centripetal force and gravity are the same. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital speed is very high, on the order of 18,000 MPH. So the payload, in order to orbit needs to attain that velocity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the first stage separates, it is about 60 kilometers above the ground, going about 3000 KPH. (Different missions profiles differ a bit with SpaceX since a GTO mission requires more performance from the first stage than a light LEO mission). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to land, the stage had to reenter, which is down, but also slow down, and lose that 3000 KPH velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being empty, (most of the fuel and oxidizer is used) the mass of the stage drops from the 1.5 million lb range down to the 150-200 KLb range. Thus the engine thrust is more effective at tranferring momentum, working on a smaller mass. Thus they only use three engines to slow it down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When there is sufficient margin, (payload was light enough, or orbit was easy enough) they not only cancel their forward velocity (Thus the 180 degree flip) but also to fly back the distance they covered already, to land back on land in Florida (Or potentially Vandenberg, or even Boca Chica in the future).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-23T13:46:37.737" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17881" PostTypeId="2" ParentId="17878" CreationDate="2016-08-23T13:57:41.020" Score="48" Body="&lt;p&gt;Hobbes has already showed you a diagram of the Falcon 9 launch profile, so I won't repeat that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt; &lt;em&gt;This answer is not intended to be a complete, scientific treatment of the subject.&lt;/em&gt; I knowingly and deliberately simplify, gloss over and ignore things in several places, in order to explain this in a way that hopefully makes sense to the OP while still being reasonably correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a common misconception that space is simply &quot;high up&quot;. It is not. You can get to orbital altitude quite easily, and if you do nothing else, once you turn off the engine, you will simply fall back down to the ground. This is because, to take one example, the gravitational attraction of Earth at the altitude of the International Space Station is approximately 90% of that on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rather, space (or more accurately in this case, orbit) is when you are &lt;em&gt;going forward very fast&lt;/em&gt;. In fact, you have to go forward so fast that your forward momentum matches the downward momentum imparted by gravity. In order to establish a stable, low (a few hundred kilometers up) orbit around the Earth, you need to move sideways at somewhere in the range 7,000 to 8,000 meters &lt;em&gt;per second&lt;/em&gt;. This is called &lt;em&gt;orbital velocity&lt;/em&gt;, and the value of it becomes lower as you move farther away from the central body (because the gravitational influence of the central body falls off with the square of the distance). If you are having trouble visualizing how fast this is, consider that the ISS, which is in low Earth orbit (about 400 km altitude) circles the Earth once every 90 minutes. The Earth's orbit around the Sun works in the same way, only that the Earth is moving at about 30,000 m/s to make a whole revolution around the Sun in about 365 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a rocket spends most of its energy to gain that sideways velocity. It obviously spends some energy getting to the correct altitude, because altitude and velocity are intricately linked in the field of study we call &lt;em&gt;orbital mechanics&lt;/em&gt;, but the vast majority is spent gaining forward speed, not altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An orbit has a value known as its &lt;em&gt;eccentricity&lt;/em&gt;. The eccentricity of an orbit is, very basically, a value indicating how circular it is. A perfectly circular orbit has an eccentricity = 0, and an elliptic orbit has an eccentricity in the range $0 &amp;lt; e &amp;lt; 1$. It is also valid to have $e \ge 1$, but we don't need to consider those cases here (those are &lt;em&gt;escape trajectories&lt;/em&gt; because they are leaving the gravitational field of the central body). Perfectly circular orbits are a largely theoretical construct which mostly doesn't exist in real life; all practical orbits have at least a small, non-zero degree of eccentricity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For orbits with very low eccentricity, and where the mass of the object in orbit is far smaller than the mass of the body it is orbiting (such as a spacecraft in circular orbit around the Earth), we can estimate the orbital period $T$ based on $$ v_o \approx \frac{2 \pi a}{T} \Rightarrow v_o T \approx 2 \pi a \Rightarrow T \approx \frac{2 \pi a}{v_o} $$ where $v_o$ is the &lt;em&gt;orbital velocity&lt;/em&gt; and $a$ is what is known as the &lt;em&gt;semi-major axis&lt;/em&gt; (half of the linear distance between the two points farthest apart in the orbit, which for circular orbits becomes the orbit radius). As you can see, there is a close relationship between $T$, $a$ and $v_o$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, if you want to slow down, you can simply stop applying forward thrust. Friction will then eventually (generally quite soon) cause your vehicle to lose forward velocity, and it will eventually come to a stop in one manner or another. This is independent on the construction of your vehicle; it could be a car, a bicycle, an airplane, a helicopter or a race horse, and that doesn't matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, there is no (or rather, for all intents and purposes and over short periods of time, negligible) friction. Hence, if a spacecraft turns off its engine, the spacecraft will simply continue in the orbit caused by its current velocity and current position within the local gravitational field. Unlike an airplane, the spacecraft can be pointing in literally any direction, and orbital mechanics doesn't care; unlike an aircraft, a spacecraft is not operating within any fluid medium. To turn around, change its direction of travel, or stop, in space, a spacecraft needs to apply &lt;em&gt;additional&lt;/em&gt; thrust in some direction &lt;em&gt;other than the direction of travel&lt;/em&gt;. (There are other ways, such as gravitational slingshots, but those are basically a special case of this, and those don't apply to Earth-orbit launches anyway.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such thrust is commonly, but far from always, applied &lt;em&gt;retrograde&lt;/em&gt;. Retrograde is one of those funny words in rocketry and orbital mechanics; here, it means &quot;opposite to the direction of travel&quot;. In a car, it would be called &quot;in reverse and with wheels pointing straight ahead&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Applying thrust partially or fully retrograde has the effect of slowing down the spacecraft's forward speed. Because this means that it can no longer keep up its current orbit, it will start falling in the direction of the prevalent gravitational field, which in the case of a low Earth orbit and thrusting for reentry eventually causes the spacecraft to reenter the Earth's atmosphere as it falls toward the Earth. With less thrust, or by thrusting in a direction other than fully retrograde, it will cause the spacecraft to settle into a different orbit, which may or may not intersect Earth ground or atmosphere. The change of velocity resulting from this thrust is what we call &lt;em&gt;delta-v&lt;/em&gt;, from the Greek letter $\Delta$ which in mathematics is used to indicate difference, and $v$ which by convention is the symbol associated with velocity. The change of velocity needed to settle into a different orbit, even an Earth-intersecting one, is commonly far lower than what was required to establish the original orbit; for example, the Space Shuttle was commonly in an orbit similar to that of the ISS, but only needed &lt;a href=&quot;https://space.stackexchange.com/q/12011/415&quot;&gt;approximately 90 m/s of delta-v under power to commit to landing&lt;/a&gt;. Once committed to landing, the space shuttle did not have enough engine power to get back into a stable orbit around the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the engines are fixed in place at one end of the spacecraft and the spacecraft is generally pointing &lt;em&gt;prograde&lt;/em&gt; (in the direction of travel) after the engines have been shut down, the spacecraft has to turn around before it can apply any significant thrust retrograde. &lt;strong&gt;Thus, it has to flip over.&lt;/strong&gt; Because there is so extremely little friction in space, it can do so leisurely, without really affecting its direction of travel much at all; in an atmosphere, the vehicle would probably start tumbling and possibly break up because of aerodynamic forces, and aircraft need special considerations to be able to perform anything resembling similar maneuvers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest is merely an elaborate dance to get the spacecraft to settle down slowly and safely in an appropriate spot on the Earth's surface. Phew. Putting it that way makes it sound downright &lt;em&gt;simple&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2016-08-24T06:44:10.660" LastActivityDate="2016-08-24T06:44:10.660" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17883" PostTypeId="1" CreationDate="2016-08-23T15:50:47.440" Score="0" ViewCount="170" Body="&lt;p&gt;I am envisioning a magnetic cannon to launch materials directly from Earth's surface to geosynchronous equatorial orbit.  The parameters I have in mind are a near vacuum launch tube, about 10 km in length, running up and through the side of a mountain near the equator (there is a nice, 5700+ M volcano in Ecuador that is only 0.03 degrees north latitude, which seems promising if the earthquakes aren't too bad), launching at ~500 G's of acceleration for 2 seconds, before entering the atmosphere at relatively high altitude.  A tube design that allows the projectile to essentially pack the air into the end of the tube as it fires could smooth the transition from tube to atmosphere, and at the trajectory you need (roughly 70 degrees from horizontal at tube exit), interaction with meaningful atmosphere will only occur for about 7 seconds.  If the gun is stationary, then the projectile will consistently &quot;land&quot; in roughly the same place in orbit, with slight variation based on air conditions; so only slight course correction would be needed on-board to use the system to put mass on orbit to build and feed a space station complex.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first question is: Can a projectile be designed with meaningful cargo space, self contained launch electronics, an engine system and a tiny bit of fuel for course correction once in orbit, which can withstand 2 seconds of 500 G acceleration, and then 7 seconds of atmospheric flight at orbital speed (starting with air pressure at 5700 M altitude, and getting less stressful from there)? What would you make such a projectile out of?  How would you design it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second question is: Assuming a fairly large mass of cargo per launch (10-50 tons), can a magnetic launch system be built to achieve that 500 G acceleration along a 10 km track, without melting itself from its own power requirements?  If so, how would you do it, and what would the power requirement actually be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, how would you deal with the necessity of making the track curved?  Is it better to use a shorter track with greater acceleration force, and fire straight?  How important is the trade off between lower G-force at launch and longer, curved track?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note: This launch system is intended to be freight only.  Humans would die from the G-forces of launch, and will still need to ride rockets, if this thing ever gets built.&lt;/p&gt;&#xA;" OwnerUserId="16710" LastActivityDate="2016-08-23T15:50:47.440" Title="What factors prohibit building a space cannon to launch raw materials?" Tags="|magnetic-field|launch-vehicle|geosynchronous|g-force|launch-system|" AnswerCount="0" CommentCount="8" ClosedDate="2016-08-23T17:06:29.260" ContentLicense="CC BY-SA 3.0" />
  <row Id="17884" PostTypeId="2" ParentId="17848" CreationDate="2016-08-23T18:11:49.823" Score="2" Body="&lt;p&gt;Short principle-based answer: &lt;strong&gt;Yes&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They both use a &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Docking_System&quot; rel=&quot;nofollow&quot;&gt;Low Impact Docking System&lt;/a&gt;, so what one could dock with the other could too. For that matter, they could dock with eachother.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the principle based part; Ever tried to hook up an old monitor to  a new computer? Maybe you have DVI cable for your TV but an HDMI cable on your cable box? Adapters are cheap and easy to come by for these consumer grade items. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same goes for your rocket. Granted, you probably can't buy an Atlas-to-Orion adapter online, but space agencies employ lots of bright and talented people. Engineers exist to solve these problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, vehicle assembly isn't as simple as screwing your payload into a launcher. You're not so much loading it onto a launch vehicle as you are building it into the launch vehicle. Again, your engineers are going to earn their keep - it's part of their job to make sure all the electronics and power hookups are linked together properly.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2016-08-23T18:11:49.823" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17885" PostTypeId="1" AcceptedAnswerId="21010" CreationDate="2016-08-23T18:56:33.593" Score="1" ViewCount="132" Body="&lt;p&gt;We've all seen pictures of micrometeorite and debris damage, and we've all heard the wisdom that at orbital speeds a paint chip is more lethal than a rifle round. We also know that having holes punched in your crew compartment is a bad thing. So, how do you defend against it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm familiar with Whipple shields, but I have a few questions about them.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How do they deal with repeat impacts in the same area?&lt;/li&gt;&#xA;&lt;li&gt;If you were building a long-endurance or reusable orbital vehicle, could sections of shield be easily replaceable?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There's also a problem I haven't seen addressed: Radiators &amp;amp; solar cells&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How would a nuclear powered spacecraft deal with having its TMS (Thermal Management System) radiators punctured?&lt;/li&gt;&#xA;&lt;li&gt;Can a TMS be designed to be self-sealing or contain isolatable loops?&lt;/li&gt;&#xA;&lt;li&gt;If a TMS radiator is holed, how quickly would it leak its fluid, and how much would be needed to refill an emptied coolant loop?&lt;/li&gt;&#xA;&lt;li&gt;If a Solar Array is damaged by debris, how much energy generation is lost?&lt;/li&gt;&#xA;&lt;li&gt;Can Sollar Arrays be field-repaired?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Final bonus question: How does the above impact the overall design of a reusable interplanetary spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2017-04-06T15:04:49.837" Title="Impact shielding and damage control of a manned Interplanetary spacecraft" Tags="|reuse|design|debris|impact|" AnswerCount="1" CommentCount="2" ClosedDate="2017-04-07T10:08:30.630" ContentLicense="CC BY-SA 3.0" />
  <row Id="17887" PostTypeId="2" ParentId="8323" CreationDate="2016-08-23T19:54:30.430" Score="3" Body="&lt;p&gt;Several things, actually&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1: Power&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;As others have said, you need to power this monster. Lasers are very power hungry and inefficient weapons. For every watt of heat you put towards vaporizing your target, you're going to need twenty to both power the thing AND run the cooling system to keep it from melting.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;2: Heat&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Not only are they power inefficient, but lasers also generate huge amounts of waste-heat. On earth this makes them impractical, but since space is a near-perfect insulator they're downright dangerous. Again, expect to use most of your mass and power for a cooling system instead of the actual laser.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;3: Targeting&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Believe it or not, tracking a tiny object at vast distances and incredible speeds with erratic doppler readings by radar is quite a tall order. In addition to cooling and weapon power, we're now doing to need a large, complicated, and delicate sensor array to track debris. It's going to be extremely difficult getting targeting data that's accurate enough for your pinpoint-accurate laser to strike a pinpoint-sized target.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;4: Targeting II: Turret Actuation Boogaloo&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Once you've solved all the other problems, you still have to point your laser in the right direction. Even with perfect targeting data, you can still miss because your turret/control moment gyros have errors in either their manufacturing or programming. Tolerances can only be made so tight.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So yeah, you're trying to hit a gnat with a scalpel, at night, from a thousand miles distance, while said gnat is traveling faster than an SR-71. It's not impossible but the engineering is going to be deeply impressive.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2016-08-23T19:54:30.430" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17888" PostTypeId="1" CreationDate="2016-08-23T21:40:55.447" Score="1" ViewCount="390" Body="&lt;p&gt;Proton started with Proton-K (Proton-Zond) to Proton-M and has now grown to major phases 1-4 and enhanced Block-Dm and Briz-M.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the latest geostationary orbit (GSO) capacity? I could not find it anywhere. Will there be some more work done (before they are being gradually replaced by Angara A5), or is it that the old railroad has it limits? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Found &lt;a href=&quot;http://spacelaunchreport.com/proton.html&quot; rel=&quot;nofollow&quot;&gt;Space Launch Report&lt;/a&gt; but it does not provide easy detail. &lt;a href=&quot;http://russianspaceweb.com/proton-m.html&quot; rel=&quot;nofollow&quot;&gt;RussianSpaceWeb.com&lt;/a&gt; is pretty close. And &lt;a href=&quot;https://en.wikipedia.org/wiki/proton_(rocket_family)&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorDisplayName="user10509" LastEditDate="2016-09-15T16:36:49.553" LastActivityDate="2016-09-15T16:36:49.553" Title="What is the latest GSO capacity for the Russian heavy launch-vehicle Proton-M?" Tags="|rockets|payload|proton-m|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17889" PostTypeId="1" CreationDate="2016-08-24T05:41:06.010" Score="0" ViewCount="96" Body="&lt;p&gt;When I got it right, the big problem to overcome for SpaceX was, to get the right technique to land the first stage properly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This requires the first stage also to have enough fuel, so it can burn the the delta V away before it touches down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But wouldn't it be possible to simply land the first stage by using parachutes? (obvisiously not, as people would do it allready)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why this wouldn't work?&lt;/p&gt;&#xA;" OwnerUserId="11099" LastActivityDate="2016-08-24T05:41:06.010" Title="Why is it such a big thing to recover the first stage in a reusable way?" Tags="|first-stage|recovery|" AnswerCount="0" CommentCount="8" ClosedDate="2016-08-24T08:30:30.400" ContentLicense="CC BY-SA 3.0" />
  <row Id="17890" PostTypeId="1" AcceptedAnswerId="17901" CreationDate="2016-08-24T05:47:20.763" Score="8" ViewCount="1082" Body="&lt;p&gt;I read the following (&lt;a href=&quot;http://www.esquire.com/news-politics/a16681/elon-musk-interview-1212/&quot; rel=&quot;nofollow&quot;&gt;link&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Another time, Musk had an issue with a vendor [...] . The issue was&#xA;  that they were going Russian on him. &quot;We got a big increase from the&#xA;  vendor after the first units were delivered,&quot; says Mark Juncosa,&#xA;  SpaceX's lead structural engineer. &quot;It was like a painter who paints&#xA;  half your house for one price, then wants three times that for the&#xA;  rest. [...]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...] Juncosa walks in to tell him that Alcoa is going Russian on him. The&#xA;  problem is that the domes are made of aluminum, and Alcoa has a&#xA;  special machine for making the aluminum SpaceX needs. They're the only&#xA;  ones who have it, they spent a lot of money on it, and now they want&#xA;  to make SpaceX pay for it.... &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...] he listens to Juncosa tell him that he's found a way to go around  Alcoa, &#xA;  involving smaller pieces of aluminum. &quot;Maybe,&quot; Musk says.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So how did this end? Is SpaceX going arround Alcoa, involving smaller pieces of aluminum? How exactly? Or do they simply pay Alcoa what Alcoa demands?&lt;/p&gt;&#xA;" OwnerUserId="16719" LastEditorUserId="4660" LastEditDate="2016-08-24T15:45:52.500" LastActivityDate="2016-08-24T15:45:52.500" Title="How did SpaceX resolve their Alcoa issue?" Tags="|spacex|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17891" PostTypeId="1" CreationDate="2016-08-24T09:20:59.920" Score="-2" ViewCount="257" Body="&lt;p&gt;SpaceX was founded by Elon Musk 14 years ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tesla, which was also founded by Elon Musk, has made fabrication mistakes in its PREMIUM cars due to its very young age -- especially when it comes to assembling parts.&lt;/p&gt;&#xA;" OwnerUserId="16721" LastEditorUserId="897" LastEditDate="2016-08-24T17:23:25.317" LastActivityDate="2016-08-24T17:23:25.317" Title="Why does NASA trust SpaceX for such important tasks being a very young company?" Tags="|spacex|nasa|" AnswerCount="3" CommentCount="3" ClosedDate="2016-08-24T14:16:25.883" ContentLicense="CC BY-SA 3.0" />
  <row Id="17892" PostTypeId="2" ParentId="17891" CreationDate="2016-08-24T09:44:45.460" Score="5" Body="&lt;p&gt;Because they convinced NASA they could deliver. And they did. For cheaper than their competitors.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-08-24T09:44:45.460" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17893" PostTypeId="2" ParentId="15507" CreationDate="2016-08-24T11:28:35.533" Score="13" Body="&lt;p&gt;The grid fins are a critical control surface to the returning Falcon 9 first stage, but are only useful during certain parts of the flight after the second-stage has separated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The grid fins provide virtually none of the control authority to the descending stage in the final seconds of the landing process, once the &quot;landing burn&quot; has started.  With the landing burn in operation, the rocket has considerable control authority (directional pointing, as well as descent velocity reduction) from the gimbaling mechanism on the rocket engine(s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the grid fins were &lt;em&gt;the&lt;/em&gt; critical control surface during some phases of the earlier part of the reentry.  Let me break it down by phases:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;out-of-the-atmosphere&lt;/strong&gt;, during the initial minute or so of post-separation flight: the grid fins are &lt;strong&gt;useless&lt;/strong&gt;, as there is insufficient atmospheric mass &#xA;for them to interact with.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;atmospheric entry&lt;/strong&gt;, as the descent continues and the very beginning of the &quot;thicker&quot; atmosphere is encountered.  Grid fins are &lt;em&gt;possibly&lt;/em&gt; of some benefit, but &lt;strong&gt;limited&lt;/strong&gt; by the still remarkably thin upper atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;encountering the denser atmosphere&lt;/strong&gt;, this is where the stage surfaces would begin to heat excessively, sufficient to cause damage if unaddressed.  SpaceX novel approach is to fire three of the nine first-stage engines to &quot;retropropulsively&quot; reenter the atmosphere.  Much control authority is available to the descending stage from the gimbaling rocket engines.  The grid fins are &lt;strong&gt;unneeded&lt;/strong&gt; in this phase of the descent.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;high-altitude, high-velocity supersonic descent&lt;/strong&gt; Grid fins operate well in the supersonic flight regime, and are &lt;strong&gt;very useful&lt;/strong&gt; here, as well as the only method the rocket has for control authority at this point.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;transonic&lt;/strong&gt; flight.  This is where the rocket transitions from supersonic to subsonic velocity (simply due to the atmospheric drag that is continually slowing the descending stage), and encounters a great deal of &quot;transonic buffet&quot;, where part of the rocket's surfaces are in supersonic flow while other parts are encountering subsonic flow.  Grid fins are generally quite &lt;strong&gt;ineffective&lt;/strong&gt; in the transonic regime.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;subsonic descent&lt;/strong&gt;.  Once the rocket stage is below about Mach 0.9 to Mach 0.8 velocity, the grid fins are once again &lt;strong&gt;very useful&lt;/strong&gt; and the only control surfaces available to help &quot;point&quot; the rocket until the landing burn is initiated.  The rocket continues to decelerate initially due to atmospheric drag, up until the time that the force of gravity on the descending stage is equal to the force of drag on the stage; at this point, the stage reaches what is called &quot;terminal velocity.&quot;  The grid fins are useful during all of this.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;landing burn firing&lt;/strong&gt;: grid fin value quickly falls as the velocity rapidly decreases from the several hundred kilometers per hour of terminal velocity to less than a few meters per second.  Grid fin effectiveness is a function of velocity of the airstream, so &lt;strong&gt;grid fins really do little or nothing in the final few seconds of the descent and landing&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="16723" LastActivityDate="2016-08-24T11:28:35.533" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17894" PostTypeId="2" ParentId="17891" CreationDate="2016-08-24T11:50:19.653" Score="3" Body="&lt;p&gt;A couple of reasons come to mind, one of public policy and one of effectiveness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a matter of &lt;strong&gt;public policy&lt;/strong&gt;, the US Congress decided to provide (limited) funding in Congressional law for NASA to (begin) to contract with commercial US companies to develop private orbital rocket launch capability.  SpaceX submitted a response to a NASA request for proposals, and SpaceX won the initial competition in 2006. (SpaceX was the top competitor in that competition, and was the first of two companies NASA executed small contracts with; the second company fell out after failing to raise sufficient capital to meet its project plans, and was later replace by Orbital Sciences, now &quot;Orbital ATK&quot;)  SpaceX has continued to achieve a large number of project milestones since, including the first orbital launch of a Falcon 9 in 2010.  SpaceX has delivered on its contracted agreements in such a way that NASA has continued to issue more, and more complex, and higher value contracts to SpaceX for the delivery of space transportation services.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a matter of &lt;strong&gt;effectiveness&lt;/strong&gt;, it is often the case that private economic actors will achieve quite different results than state actors.  Leaving political factors aside, and whether this might be a good or bad thing, we can explain that phenomenon economically merely by pointing out that the various players in each world (the SpaceX company vs. various government workers from members of Congress to NASA employees) face a very different set of incentives.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each entity (state organizations vs. private company organizations) are good at different things.  To take just one example, think about innovation.  How might the design and improvement year-by-year of the personal computer, or the smartphone, be handled differently by each structure given the incentives the various people face in each environment?  Which might do innovation more rapidly and repeatedly over time.  The empirical data show, hands down, if you want innovation, especially rapid and continuous innovation, during the late-20th and early-21st century, private economic actors will deliver much more than state entities.  &lt;/p&gt;&#xA;" OwnerUserId="16723" LastActivityDate="2016-08-24T11:50:19.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17895" PostTypeId="2" ParentId="17474" CreationDate="2016-08-24T12:40:02.420" Score="3" Body="&lt;p&gt;The landing abort burn itself should not be the problem. But getting back from an ascending rocket to a descending rocket fast enough seems impossible to me, for two reasons:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A) Remember, that even with a single engine on at lowest thrust level, the rocket would accelerate away from earth. So after the landing abort, all engines have to be turned off. But with a positive velocity (away from earth), the rocket will experience &lt;em&gt;negative g-forces&lt;/em&gt;, accelerating it to earth &lt;em&gt;faster&lt;/em&gt; than gravity alone would! This is because the rocket in free fall has zero g-force, but with air drag, its movement away from earth is decellerated, meaning, that the rocket is accelerated towards earth faster than by gravity alone!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Negative g-forces have drastic consequences. Whatever little fuel is left at the bottom of each of the two tanks, now suddenly feels, that &quot;up&quot; and &quot;down&quot; have just been reversed, and starts moving towards the new &quot;down&quot;, which is, where the &quot;up&quot; used to be! You can simulate that at home in your kitchen easily: Fill a glass with water (your &quot;fuel&quot;), take the glass in a hand, then push the glas up suddenly (the abort burn), then down again (the phase, where gravity wins). Even though you never turned over the glass, the water spills all over, when you do that fast enough!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the fuel tanks of the rocket are closed, so the fuel will not spill out during the phase of negative G-forces. And the fuel will start flowing back to the bottom, once the rocket has reached its maximum height and starts falling back to earth, because air drag now causes positive G. But will enough &quot;air&quot;-free (more precisely: helium-free) fuel collect at the bottom during the few seconds of free fall?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;B) As already written, the rocket might become instable. While going up with engines off, it gets slower and slower, so that it is harder and harder to control by the grid fins. The fuel sloshing in the tanks add to this instability. The thrusters might be able to keep the rocket upright, but I am not sure, if they can.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;C) During the non-powered phase of flight, the rocket will hardly be able to offset wind drag. Of course, the rocket could already stear in the right direction to counter wind during the abort burn. But in the case of unsteady wind (which on a nominally performing rocket is the most likely problem to require a landing abort in the first place!) the exact required direction might be hard to predict - and the final correction of the wind shift will be hard.&lt;/p&gt;&#xA;" OwnerUserId="16725" LastActivityDate="2016-08-24T12:40:02.420" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17896" PostTypeId="2" ParentId="17891" CreationDate="2016-08-24T12:50:09.807" Score="7" Body="&lt;p&gt;NASA does not just 'trust' its suppliers. It audits them to &lt;em&gt;make sure&lt;/em&gt; the supplier can deliver what it claims. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Commercial_Orbital_Transportation_Services&quot;&gt;contracts for commercial cargo and crew&lt;/a&gt; flights were built around milestones where the supplier had to show (either in audits or via flight tests) that it can deliver.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-24T12:50:09.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17897" PostTypeId="2" ParentId="17849" CreationDate="2016-08-24T14:00:42.047" Score="3" Body="&lt;p&gt;I don't know about ISS procedures, specifically, but in general it'd be preferred to approach from the orbit normal or anti-normal (from above or below), assuming the station is not tumbling. This means the docking port would appear to rotate as you approach it, so you just need to set up a roll rate to match. If you approach from any other directions the docking port appears to rotate away from you, which means you need to translate as well as rotate to keep up. &#xA;In practice, this rate is low (one revolution per orbit) and not a difficult problem to solve, but lacking any other factors (such as lines of sight/clearance, not sprayrng the solar phnels with your thruster exhausts, etc) then an approach along the orbit normal would be easiest.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-08-24T14:00:42.047" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="17898" PostTypeId="1" AcceptedAnswerId="24307" CreationDate="2016-08-24T14:14:06.617" Score="5" ViewCount="870" Body="&lt;p&gt;I am looking for technical details about the soyuz tma, simvol displays and cockpit controls in general. Something like the NASA familiarization manuals, but for Soyuz. And in english :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone point me to some resources? I've already seen that student simulator of docking (excellent) and own the Haines workshop manual, but more would be appreciated. I've even taken a lot of screenshots from Gravity movie.. :)&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="58" LastEditDate="2020-08-05T14:34:04.247" LastActivityDate="2020-08-05T14:34:04.247" Title="What technical details exist about the Soyuz TMA spacecraft and its Simvol Displays &amp; Cockpit Controls?" Tags="|soyuz-spacecraft|instrument|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17899" PostTypeId="2" ParentId="7978" CreationDate="2016-08-24T14:45:11.213" Score="-2" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;ISS orbit  : it is difficult to say how accurate it is. The reason is that its center of mass is not stable due to the astronauts activities inside the ISS. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;GPS satellites orbit: can be determined with accuracy of 2-4 centimeters&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;Geostationary orbit: can reach accuracy of decimeter level.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="16729" LastActivityDate="2016-08-24T14:45:11.213" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17900" PostTypeId="1" CreationDate="2016-08-24T15:28:40.710" Score="3" ViewCount="128" Body="&lt;p&gt;I am utterly confused as to how the ecliptic co ordinate system works.&#xA;My understanding is that the vernal equinox is comparable to greenwich for longitude here on earth. But how do we know where the vernal equinox is normal to on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if I launch a hypothetical satellite into polar orbit, how do I find the right ascension of the ascending node? Presumably it depends on the time it goes through the ascending node?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Complete beginner here utterly lost.&lt;/p&gt;&#xA;" OwnerUserId="16730" LastActivityDate="2016-08-24T15:28:40.710" Title="Regarding the vernal equinox and calculate a TLE code" Tags="|coordinates|two-line-elements|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17901" PostTypeId="2" ParentId="17890" CreationDate="2016-08-24T15:30:26.903" Score="5" Body="&lt;p&gt;In 2014, 2 years after that interview took place, SpaceX &lt;a href=&quot;http://www.cnbc.com/2014/10/02/inside-the-worlds-largest-aluminum-lithium-plant.html&quot;&gt;still got&lt;/a&gt; aluminium-lithium alloys &lt;a href=&quot;http://forgingmagazine.com/picture/alcoa-goes-light-alloys-big-way&quot;&gt;from Alcoa&lt;/a&gt;.&lt;br&gt;&#xA;But &lt;a href=&quot;http://www.constellium.com/markets/aerospace/space/space-success-stories&quot;&gt;they also&lt;/a&gt; have &lt;a href=&quot;http://www.americaspace.com/?p=56744&quot;&gt;another supplier&lt;/a&gt;, &lt;a href=&quot;http://www.aluminiumtoday.com/contentimages/subscriber-pdfs/AIT_sept_oct_2012_editorial_web_res.pdf&quot;&gt;Constellium&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX has since developed a heavy-payload launcher called Falcon 9, which uses AIRWARE® technology extensively for structural components. Its first flight took place in 2010.&#xA;  Almost the entire first and second stages of the launcher are made from our (Constellium's, ed.) materials, with longitudinal seams on both the launcher and Dragon space vehicle joined by Friction Stir Welding. The same welding process is used to weld domes made from our AIRWARE® 2195 and 2219 alloys to the barrel sections. The domes are manufactured by spin forming.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it appears SpaceX found another supplier for the material for the tank domes. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-24T15:30:26.903" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17902" PostTypeId="2" ParentId="6655" CreationDate="2016-08-24T18:38:54.347" Score="0" Body="&lt;p&gt;Good books on this topic include &quot;Interplanetary Mission Analysis and Design&quot; by Stephen Kemble (Springer, 2006) and &quot;Orbit &amp;amp; Constellation Design &amp;amp; Management&quot; by James Wertz et al. (Microcosm, 2001).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The approximate delta-v for a slowly spiralling transfer over many orbits is, as pointed out already by HopDavid, the difference in orbital speeds for a transfer between circular coplanar orbits. This approximation does not apply to a Mars transfer as your 0.00032 m/s^2 acceleration can reach the required delta-v in much less than one orbit. You may however use it as an upper bound and the Hohmann transfer delta-v as a lower bound.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to get to Mars with a low-thrust electrical propulsion system, you will need to (1) escape Earth orbit (2) transfer to Mars and (3) capture into Mars orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft will start in whatever orbit the launch vehicle initially injected it, then use either continuous thrust or thrust-coast arcs (coasting near apogee to increase efficiency) to get into a translunar orbit from where escape is possible by gravity assist (cutting about 800m/s off the delta-v requirement). It is possible that the launch vehicle may put you directly in a trans-lunar orbit but something similar to geosynchronous transfer orbit is more likely. Delta-v to trans-lunar orbit in that case should be around 3km/s for continous thrust (3.2 km/s in the slightly lower acceleration example given by Kemble; much more than the 750m/s for high thrust). To solve the problem with your own numbers you would use Lagrange's orbital equations. (Starting on p. 226 of Kemble or p. 63 of Wertz. This post is already getting too long...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After lunar flyby, you will still not have enough hyperbolic excess to reach Mars. That would be about 3km/s for a typical launch window with high-thrust propulsion minus the small hyperbolic excess you have after the lunar flyby. Since the Oberth effect is no longer working in your favour, you will need roughly this amount of delta-v (with high thrust). That amounts to &gt;3 months of thrust in your example but still much less than the semi-period of the transfer orbit. Expect somewhere in the lower half of 3km/s &amp;lt; delta-v &amp;lt; 6km/s to get you from Earth escape to Mars-crossing orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hyperbolic velocities at Mars insertion will be similar to Earth escape but there is no major moon for gravity assist. (It is possible to get a free injection by making use of the Lagrange points. I will neglect this as taking too long.) How much delta-v this requires will depend on your requirements and I don't think it is part of the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can further reduce the delta-v requirement with only slight increase in transfer time if you assume that the spacecraft performs an Earth flyby a short while after Earth escape (with a deep-space manoeuvre in-between). Mars flyby before insertion is also useful but less so.&lt;/p&gt;&#xA;" OwnerUserId="16733" LastActivityDate="2016-08-24T18:38:54.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17903" PostTypeId="1" CreationDate="2016-08-24T19:17:08.730" Score="3" ViewCount="97" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Control_moment_gyroscope&quot; rel=&quot;nofollow&quot;&gt;Gydrodynes&lt;/a&gt;, aka control moment gyroscopes, are spinning wheels that are torqued at their axis (a motor pushes the axis to make it a different direction) for the purposes of creating counter-torque that rotates the whole spacecraft. They're a way of attitude adjustment without expending any fuel. Instead, they spend electric energy. They're also good for fine attitude control, (though reaction wheels are even finer.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they need to be at the center of mass or can they be located anywhere without losing any efficiency? (rotation per electric energy expended.)&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2016-08-25T20:31:08.833" Title="For the purposes of attitude control, do gyrodynes need to be placed at the center of mass?" Tags="|spacecraft|attitude|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17904" PostTypeId="1" AcceptedAnswerId="17925" CreationDate="2016-08-24T19:40:21.850" Score="3" ViewCount="1652" Body="&lt;p&gt;So, a satellite launch takes a huge amount of fuel to launch a relatively small (compared to total rocket weight at takeoff) satellite into orbit.  Obviously there is a lot of energy expended during the launch, but what percentage of the expended energy is actually needed to get the satellite itself up into orbit?  I suppose it would just be a ratio of the amount of kinetic energy the satellite has at release in orbit compared to the potential energy of the fuel the rocket burns to get it there.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a specific common scenario, consider a SpaceX Falcon9 launch of a max-weight satellite to LEO.  Research on the SpaceX website indicates that its GTO capacity is 22,800kg.  Other research indicates that the Falcon9 carries 136,900kg of RP-1 between its first two stages.  However, I'm struggling to find out how to calculate the kinetic energy of such a satellite in LEO and the potential energy in the fuel.&lt;/p&gt;&#xA;" OwnerUserId="16735" LastActivityDate="2016-08-25T19:36:51.907" Title="Efficiency of rocket launches?" Tags="|launch|spacex|energy|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17906" PostTypeId="1" CreationDate="2016-08-24T22:04:00.980" Score="2" ViewCount="14629" Body="&lt;p&gt;I recenty came a across a doubt...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can a smart phone be used in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously you aren't going to get any data signal, but will the functions like touch screen, GPS, accelerometer, vibrator, etc, work? Will anything get affected by gravity or no air?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sid&lt;/p&gt;&#xA;" OwnerUserId="16740" LastActivityDate="2021-02-04T14:41:10.550" Title="Can a smart phone function in space?" Tags="|gravity|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="17907" PostTypeId="1" AcceptedAnswerId="17914" CreationDate="2016-08-24T22:57:26.303" Score="0" ViewCount="92" Body="&lt;p&gt;During the today (24/08/16) launch of Ariane 5, Stéphane Israël (chairman &amp;amp; CEO) tells that all conditions are good, including a &lt;em&gt;green green weather&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it a shortcut for &lt;em&gt;no wind, no rain&lt;/em&gt;, or something like that ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=74_8VIurLm8#t=8m5s&quot; rel=&quot;nofollow&quot;&gt;Link to the youtube video&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14528" LastEditorUserId="14528" LastEditDate="2016-08-25T08:11:03.200" LastActivityDate="2016-08-25T08:22:45.173" Title="What is 'green green' weather?" Tags="|esa|ariane|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17908" PostTypeId="1" CreationDate="2016-08-24T23:49:32.023" Score="6" ViewCount="221" Body="&lt;p&gt;Which parts (sections or modules) of International Space Station (ISS) belongs to which space agencies or countries that are part of ISS program? I am interested to know statistical information, about the habitable volume in value for all sections of each space agency. A comparison by habitable volume between sections controlled or developed by each space agency or country.&lt;/p&gt;&#xA;" OwnerUserId="16734" LastEditorUserId="16734" LastEditDate="2016-10-10T19:22:21.947" LastActivityDate="2016-10-12T14:39:15.493" Title="Which sections of ISS belongs to each space agency part of ISS program and what is the habitable volume for each section?" Tags="|iss|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17910" PostTypeId="2" ParentId="17903" CreationDate="2016-08-25T01:22:23.980" Score="2" Body="&lt;p&gt;The closer they are to the center the better. However that is often compromised. On ISS the flywheels don't get moved every time a ship docks. I don't think they have ever been moved, even as modules have been added. (I know they got used to death during the construction phase and had to be replaced.) I would think on a ship it is something the pilot gets use to.  &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-08-25T01:22:23.980" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17911" PostTypeId="2" ParentId="7718" CreationDate="2016-08-25T01:37:15.833" Score="3" Body="&lt;p&gt;No one has mentioned the violence of a hot rocket hitting water at an odd angle. Rockets are delicate. Water is heavy. The force of it jetting trough crevices would probably rip wires and tubes off. There is the danger of it shorting circuits. It would blow steam off the nozzles. It's salty, it's dirty. It would penetrate every part of the rocket that wasn't sealed. It would get into the combustion chamber. &#xA;What if you dropped your car into the sea from about 15 feet? I think it would need more repair than just drying it off.&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-08-25T01:37:15.833" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17912" PostTypeId="1" CreationDate="2016-08-25T04:52:14.937" Score="2" ViewCount="82" Body="&lt;p&gt;Besides an epic millennium-long interstellar voyage by a probe, what can we possibly do to learn more about this rocky planet discovered around Proxima Centauri?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that we have some estimate of its size, mass, orbital period, and distance from its star. But what else could we learn, with existing technology (you can either consider cost or not)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What sort of telescope would be required in order to resolve some visible details of a planet that size, over such an immense distance? (And what other interesting things could we do with such a telescope?)&lt;/li&gt;&#xA;&lt;li&gt;Is there any information we can glean based on possible transits of the planet across Proxima Centauri? (ie composition, atmosphere, etc)&lt;/li&gt;&#xA;&lt;li&gt;Is there something else we could do that I have not thought of yet?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="15933" LastActivityDate="2016-08-25T04:52:14.937" Title="Rocky Planet around Proxima Centauri - how can we learn more about it?" Tags="|planet|space-telescope|extra-solar-object|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17913" PostTypeId="1" AcceptedAnswerId="21020" CreationDate="2016-08-25T06:18:11.007" Score="10" ViewCount="556" Body="&lt;p&gt;The August 24, 2016 NASA news item: &lt;a href=&quot;http://www.nasa.gov/image-feature/new-nasa-record-holder-for-cumulative-days-in-space&quot; rel=&quot;nofollow noreferrer&quot;&gt;New NASA Record Holder For Cumulative Days in Space&lt;/a&gt; congratulates ISS station commander Jeff Williams for passing 520 total days in space - a new NASA astronaut record.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I was contemplating the challenges of spending a few percent of one's entire life in a (very large) tin can, I noticed the two microgravity robot things with him. Where can I read more about them, or at least what are they called?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then the one on Cmdr. Williams' left caught my eye - that looks like art! Where can I read more about the pattern on the outside of that unit?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Who is the space artist??&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; from &lt;a href=&quot;http://ssl.mit.edu/spheres/projects/vertigo.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ssl.mit.edu/spheres/projects/vertigo.html&lt;/a&gt; which also shows another photo of the red sphere &lt;em&gt;sans décoration&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IYhBI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IYhBI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below x3:&lt;/strong&gt; Images courtesy of &lt;a href=&quot;http://www.nasa.gov/image-feature/new-nasa-record-holder-for-cumulative-days-in-space&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ntDzk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ntDzk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NabLD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NabLD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iBnFk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iBnFk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-11-09T14:53:10.630" LastActivityDate="2018-11-09T14:53:10.630" Title="Who is the space artist? Patterns on the Spheres robots on the ISS" Tags="|iss|astronauts|robotics|space-art|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="17914" PostTypeId="2" ParentId="17907" CreationDate="2016-08-25T08:22:45.173" Score="1" Body="&lt;p&gt;Yes. During Arianespace launch broadcasts, they sometimes show a list of launch conditions. This is a list of about 15 items shown in red or green. If all are green, the launch is proceeded with. An item shown in red means a delay or scrub.&lt;br&gt;&#xA;This is similar to the voice poll NASA uses (where each department head says 'Go' or 'No go' a few minutes before launch). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example from a Vega launch using a similar system:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Pq52O.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Pq52O.jpg&quot; alt=&quot;Vega launch poll&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-25T08:22:45.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17915" PostTypeId="2" ParentId="17913" CreationDate="2016-08-25T09:36:53.713" Score="12" Body="&lt;p&gt;These are some of the &lt;a href=&quot;http://www.nasa.gov/spheres/spheres-satellites&quot;&gt;SPHERES&lt;/a&gt; satellites. They're little flying robots, driven by pressurised gas, intended to fly around inside the station.  They were created for various indoor experiments, such as docking and formation flying, without needing some extremely complicated (and easily lost) system rated for independent flight outside the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They've been on the station &lt;a href=&quot;http://www.nasa.gov/spheres/the-history-of-spheres&quot;&gt;since April 2006&lt;/a&gt;. They've been modified a lot over the years, your picture shows various add-ons I can't identify. They have been used in schools outreach projects so perhaps that's the source of the arty designs...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yes; various NASA sources admit the idea came from the first &quot;Star Wars&quot; movie; the little flying droid used for Luke's sword training.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-08-25T09:36:53.713" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17916" PostTypeId="1" CreationDate="2016-08-25T11:31:06.233" Score="3" ViewCount="5164" Body="&lt;p&gt;The script I'm wanting to develop uses the cartesian coordinates (XYZ) from a satellite, and in conjunction with the range, elevation and azimuth from a location, I then take a satellite’s orbital information and get the ground longitude/latitude under that satellite at a given time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One step further from this: imagine the signal from a satellite piercing the atmosphere at exactly 300km above sea level. At this particular point when altitude is 300km, I need to calculate the ground longitude/latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the pyephem module there appears to be already a method (ephem.readtle) that can achieve this, but for TLE (two line element) data only. I'd like to use a satellite's cartesian coordinates to develop this. Is there such a method already out there? Or perhaps somebody with experience in this domain can point me in the right direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A similar question already exists &lt;a href=&quot;//stackoverflow.com/questions/15954978/ecef-from-azimuth-elevation-range-and-observer-lat-lon-alt&quot;&gt;referring to ECEF from Azimuth, Elevation, Range and Observer Lat,Lon,Alt&lt;/a&gt;, but it's not the same problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's what I have developed already:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;satellite cartesian coordinates, XYZ&lt;/li&gt;&#xA;&lt;li&gt;azimuth, elevation and range of satellite from ground station&lt;/li&gt;&#xA;&lt;li&gt;ground station coordinates in lat, long, height above sea level&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Here's what I need: ground longitude/latitude under a satellite at a specific epoch, and in particular where the piercing point in the atmosphere (the point which the signal from the satellite pierces the atmosphere) is 300km altitude.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Ok, so I didn't resolve yet the issue with how to resolve for ground tracks for altitudes of 300km, but I believe the method I've wrote converting XYZ to ellipsoidal is complete:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; def cartesian_to_ellipsoidal(self):&#xA;    x = 4433469.9438&#xA;    y = 362672.7267&#xA;    z = 4556211.6409&#xA;    r = np.sqrt(x**2 + y**2 + z**2)&#xA;&#xA;    # WGS-84 PARAMETERS, semimajor and semiminor axis&#xA;    a = 6378137.0&#xA;    b = 6356752.314 &#xA;&#xA;    # Eccentricity&#xA;    e_squared = (a**2 - b**2) / a**2&#xA;&#xA;    # Auxiliary quantities&#xA;    p = np.sqrt(x**2 + y**2)&#xA;&#xA;    # Latitude (phi) &amp;amp; Longitude (lam)&#xA;    phi = np.rad2deg(np.arctan(z / ((1- e_squared) * p)))&#xA;    lam = np.rad2deg(np.arctan(y/x))&#xA;&#xA;    # Radius of curvature in prime vertical               &#xA;    N = a / np.sqrt(1 - e_squared * (np.sin(np.deg2rad(phi)))**2)&#xA;&#xA;    # Altitude&#xA;    h = (p / np.cos(np.deg2rad(phi))) - N&#xA;&#xA;    return lam, phi, h&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The XYZ coordinates are taken as a sample from this Matlab worked example &lt;a href=&quot;http://web.ics.purdue.edu/~ecalais/teaching/gps_geodesy/lab_3.pdf&quot; rel=&quot;nofollow&quot;&gt;Matlab worked example&lt;/a&gt;. The results are to an accuracy I'm satisfied with. What I don't understand and perhaps a dumb question, but when a ground track is calculated along with the altitude, why is the altitude not zero? One would expect since a ground track is mapped to the surface of the earth, the altitude should be zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One last point: the desired ground track was for where the line betweent the satellite and ground station pierces the amosphere 300km above the earth. Considering distances of 20000km (26000 km radius), then would an adjustment to my code - just to compensate for the desired 300km altitude scenario - make much a difference to outcome? If not, ground track data may just suffice.&lt;/p&gt;&#xA;" OwnerUserId="16749" LastEditorUserId="16749" LastEditDate="2016-08-26T14:22:19.823" LastActivityDate="2018-01-06T16:26:45.483" Title="Ground longitude/latitude under a satellite (cartesian coordinates) at a specfic epoch" Tags="|artificial-satellite|gps|ephemeris|ground-track|python|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="17917" PostTypeId="1" AcceptedAnswerId="17926" CreationDate="2016-08-25T12:59:09.480" Score="6" ViewCount="4488" Body="&lt;p&gt;The new discovery of a small planet keeping Proxima Centauri company prompted my imagination into overtime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are four parts to this:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;If we were to deploy a small probe on a light sail how long would it take to get the distance to Proxima Centauri? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What speed would the sail reach before it had to slow down and how long would it take to reach top speed.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How long would it take to come to a stop.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Assuming it made it in one piece would the probe be able to beam data back to us for us to analyse? I know this would be a 4+ year transmission each way.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="16752" LastActivityDate="2023-04-29T09:34:12.420" Title="How long would it take for a light sail to make it to Proxima Centauri and what speed would it get to?" Tags="|light-sail|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17918" PostTypeId="2" ParentId="17917" CreationDate="2016-08-25T14:37:47.227" Score="0" Body="&lt;p&gt;Edward Guinan, an astronomer at Villanova University &lt;a href=&quot;http://wypr.org/post/planet-just-outside-our-solar-system-potentially-habitable#stream/0&quot; rel=&quot;nofollow&quot;&gt;claims&lt;/a&gt; that 0.1C is possible with present technology and thus the trip would take less than 50 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what present technology is he referring to, laser propulsion and solar sail are still largely unproven.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, if you get up to 0.1C, it makes more sense to do a flyby rather than to try to slow down. Having onboard propulsion system means extra weight, and on top of that having to slow down means you have to carry twice the propellent. If you use Earth based laser, you won't have any means of slowing down.&#xA;That's basically why New Horizons did a flyby - they wanted to get there fast and had no means of slowing down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once  you are there you can use radio or laser (still experiment) communications to return the collected science data. There will be a number of challenges - the solar system will be very small at that distance so the antenna has to be pointed very accurately, but I think it's doable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There will be very little back and forth due to the large lag time, I would imagine that the spacecraft will have to do the final course corrections on it's own. Traveling at 0.1C, the spacecraft will cross the Proxima Centauri system in a few weeks, so obviously there is no time for any back and forth.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-08-25T16:22:52.120" LastActivityDate="2016-08-25T16:22:52.120" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17919" PostTypeId="1" CreationDate="2016-08-25T14:53:48.573" Score="0" ViewCount="185" Body="&lt;p&gt;What I'am trying to understand if in space does the asteriod belt &quot;sit&quot; on a linear orbit or does it encompass a 360 degree orbit? (in a plane Cartesian coordinate system)&lt;/p&gt;&#xA;" OwnerUserId="16755" LastActivityDate="2016-08-25T18:02:58.417" Title="is the asteriod belt linear or 360 degrees" Tags="|asteroid|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17920" PostTypeId="2" ParentId="17919" CreationDate="2016-08-25T15:04:09.860" Score="1" Body="&lt;p&gt;The vast majority of the solar system is in the same plane thus the belt is also mostly flat and circular in shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This video explain the reasons:&#xA;&lt;a href=&quot;http://www.universetoday.com/108355/why-is-the-solar-system-flat/&quot; rel=&quot;nofollow&quot;&gt;http://www.universetoday.com/108355/why-is-the-solar-system-flat/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-08-25T18:02:58.417" LastActivityDate="2016-08-25T18:02:58.417" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17921" PostTypeId="2" ParentId="741" CreationDate="2016-08-25T17:17:30.020" Score="5" Body="&lt;p&gt;&lt;em&gt;Right now&lt;/em&gt;, the primary limit is that we're stuck using reaction drives, meaning you have to expend propellant mass to accelerate the spacecraft.  So, your total &amp;Delta;V (change in velocity) is limited by the amount of propellant you can carry and the efficiency of your engines as specified in the Tsiolkovsky rocket equation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V = 9.8 * I_{SP} * ln ( MR ) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where MR is the mass ratio&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$MR = {M_{spacecraft} + M_{propellant} \over M_{spacecraft}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Dawn unmanned spacecraft is using an ion engine with a specific impulse (I&lt;sub&gt;sp&lt;/sub&gt;) of 3100 seconds, which is currently the most efficient engine in use that I know of.  If the spacecraft carries its own mass in propellant ($M_{spacecraft} = M_{propellant}$, for a MR of 2) that means we can get a total &amp;Delta;V of ~21057 m/s.  Fast, but not interstellar travel fast.  If the spacecraft carries 9 times its mass in propellant (MR = 10), we can get up to ~69953 m/s.  Better, but still not good enough for interstellar flight.  For a Dawn-like spacecraft to reach 0.01 &lt;em&gt;c&lt;/em&gt; (~ 3,000,000 m/s) we'd need a mass ratio on the order of $5.0 * 10^{41}$.&lt;sup&gt;1&lt;/sup&gt;    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a practical upper limit to how much mass we can launch from the surface of the Earth, which constrains the amount of propellant we can send with the spacecraft.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two ways around the problem &amp;mdash; one is to accelerate the spacecraft using EM radiation against a sail.  There's a project called &lt;a href=&quot;https://breakthroughinitiatives.org/Initiative/3&quot; rel=&quot;nofollow&quot;&gt;Breakthrough Starshot&lt;/a&gt; that's looking to use a ground-based bank of terawatt lasers to accelerate a gram-scale spacecraft to 0.2 &lt;em&gt;c&lt;/em&gt; (59958491 m/s) in the space of about 10 minutes.  There have also been numerous ideas of using a sail with the solar wind from the Sun.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other is to create a true reactionless drive (such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Alcubierre_drive&quot; rel=&quot;nofollow&quot;&gt;Alcubierre drive&lt;/a&gt; or the &lt;a href=&quot;https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster&quot; rel=&quot;nofollow&quot;&gt;EmDrive&lt;/a&gt;), which has a number of issues (not least of which is how you get around conservation of momentum and things like that).&lt;br&gt;&#xA;&lt;hr&gt;&#xA;&lt;sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The fact that &amp;Delta;V goes up with the &lt;em&gt;logarithm&lt;/em&gt; of the mass ratio is what makes the rocket equation so tyrannical.  Every kg of propellant you add has to be accelerated along with the spacecraft, leading to diminishing returns.  That's why we didn't use rockets to slow down the Apollo spacecraft or Shuttle orbiters before re-entry; it just translated into too much mass to launch.   &#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12922" LastEditorUserId="12922" LastEditDate="2016-08-26T20:53:07.787" LastActivityDate="2016-08-26T20:53:07.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17922" PostTypeId="2" ParentId="17906" CreationDate="2016-08-25T18:15:13.590" Score="3" Body="&lt;p&gt;Multiple android based phone satellites have been launched into space. (At least six &lt;a href=&quot;https://en.wikipedia.org/wiki/PhoneSat&quot; rel=&quot;nofollow&quot;&gt;according to Wikipedia&lt;/a&gt;). I haven't looked into which components they tried using in orbit, but the camera obviously worked.&lt;/p&gt;&#xA;" OwnerUserId="12966" LastActivityDate="2016-08-25T18:15:13.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17923" PostTypeId="1" AcceptedAnswerId="17937" CreationDate="2016-08-25T18:27:21.807" Score="5" ViewCount="816" Body="&lt;p&gt;I've been reading a bit about the concept of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Dyson_sphere#Dyson_swarm&quot; rel=&quot;nofollow&quot;&gt;Dyson Swarm&lt;/a&gt; and how one of the more plausible routes would be to disassemble Mercury to construct ~1km hematite mirrors as the swarm reflectors. The basic concept is that the mirrors would be un-powered and simply reflect sunlight to a central processor that would extract the energy and beam it wherever we need. But that got me wondering; in such a scenario, wouldn't that simply turn the mirror elements into solar sails? Wouldn't they need an equivalent amount of energy in the opposite direction simply to keep them in place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Additional links about the swarm idea &lt;a href=&quot;http://www.popularmechanics.com/space/deep-space/a11098/could-we-build-a-dyson-sphere-17110415/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://www.youtube.com/watch?v=jW55cViXu6s&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10557" LastEditorUserId="10557" LastEditDate="2016-08-29T14:46:33.540" LastActivityDate="2019-06-10T22:57:41.327" Title="What would keep Dyson Swarm reflectors in place?" Tags="|artificial-satellite|solar-power|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17925" PostTypeId="2" ParentId="17904" CreationDate="2016-08-25T19:36:51.907" Score="5" Body="&lt;p&gt;Let's take your example of a Falcon 9. The actual payload to GTO is given as 8300 kg. In this case GTO is likely to be defined as a 185 x 35890 km orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket has to accelerate the payload to orbital speed and also to lift it to the orbit. That is, the payload is lifted by 185km: $$E_h = m \cdot g \cdot h = 1kg \cdot 9.81 m/s \cdot 185km = 1.8 MJ$$&#xA;The orbital speed at this point is calculated as &#xA;$$\sqrt{GM\left(\frac{2}{a}-\frac{1}{r}\right)}$$ &#xA;with the semi major axis $a = \frac{1}{2}(185 + 25890 + 2\cdot 6380)km = 24,250 km$ and $r = 185 km$&#xA;$$v = 10.25 km/s$$&#xA;This is a kinetic energy of 52 MJ/kg. That is, the rocket has to deliver 53.8 MJ per kilogram payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch mass of a Falcon 9 is 550 t, including 155 t RP-1 and 361 t LOX. RP-1 has an enthalpy of 42.9 MJ/kg, the whole rocket releases 6,640 GJ by burning the propellant.  This is used to lift the payload of up to 8300 kg, that is 800 MJ/kg. This makes the overall efficiency 6%.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-08-25T19:36:51.907" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17926" PostTypeId="2" ParentId="17917" CreationDate="2016-08-25T19:56:40.230" Score="3" Body="&lt;p&gt;TL/DR: Over 2000 years for a ridiculously advanced craft travelling at the surface escape velocity of the sun, 620 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The effectiveness of a solar sail is determined by its &quot;lightness factor&quot;, the thrust-to-weight ratio under solar gravity. (Since both gravity and radiation pressure obey inverse square law, this is independent of distance from the sun.) It is sometimes given in terms of the characteristic acceleration or characteristic thrust, which is the value at Earth's distance from the sun. The characteristic acceleration due to solar gravity is 0.005 m/s^2. (Edit: I should have written 0.0059 as 0.006. The basic conclusion will remain the same.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speed that can be achieved is limited by a number of factors. The maximum temperature that the sail can withstand (which limits how closely it can fly by the sun to gain speed) is one limit. Material strength is also an important factor, such a very thin material could easily be torn apart by radiation pressure when pulling against the inertia of a spacecraft near the surface of the sun. Furthermore, dust so near the sun is not that well studied and may pose a risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example, Wolfgang Seboldt &amp;amp; Bernd Dachwald discuss a number of solar sails in &quot;Solar Sails: Propellantless Propulsion for Near- and Medium-Term Deep-Space Missions&quot; (*1) One currently build-able of 400 m^2 at 87 g/m^2, one of 2500 m^2 at 38 g/m^2 likely build-able if existing technology is refined and one of 4900 m^2 at 23 g/m^2 that would require significant research but is considered realistically achievable. (Payload mass not included.) The third could achieve a characteristic thrust of about 40mN, giving a lightness factor of &amp;beta; = a/(0.005 m/s^2 ) = 0.07.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is not enough to reach Proxima in any reasonable amount of time unless the sail is accelerated with large lasers. Instead of working it all out, I will show what happens with a lightness factor of 1 (*2), much higher than the craft above even without payload. If the spacecraft is on an orbit around the sun (ideally highly eccentric) and deploys its solar sail approximately at perihelion, we may assume that the sail exactly cancels solar gravity for the entire trip. This means that the spacecraft will escape the solar system with a constant speed equal to whatever its speed at perihelion was. Unless we also use another propulsion system, we must assume that we start from a bound orbit and so this can at most be the surface escape velocity of the sun, 617 542 m/s (*3).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(3.991 * 10^16 m)/(6.18 * 10^5 m/s) = 6.46 * 10^10 s, or about 2000 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The acceleration period for such a &amp;beta; = 1 craft will consist of the period where it falls in towards the sun, after which it will as already mentioned escape at constant speed. Falling in from Earth would give much less than the mentioned speed; if we assume a Jupiter flyby was used for perihelion lowering then the acceleration time (starting at Jupiter) would be just about a sixth of a Jupiter year, or two years. (Edit: Egg on my face. I wrote half a Jupiter year, when I should have said half an orbit at half of Jupiter's semi-major axis. That makes for 0.5 * 0.5^(3/2) of Jupiter's orbital period.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(*1) Published in &quot;Advanced Propulsion Systems and Technologies Today to 2020&quot; edited by Claudio Bruno &amp;amp; Antonio Accettura, 2008. (American Institute of Aeronautics and Astronautics) p. 448-449&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(*2) See &quot;The Startflight Handbook: A Pioneer's Guide to Interstellar Travel&quot; by Eugene Mallove &amp;amp; Gregory Matloff, 1989. (Wiley) p. 96-97.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(*3) From &quot;Space Mission Engineering: The New SMAD&quot; edited by James Wertz et al., 2011. (Microcosm) p. 955&lt;/p&gt;&#xA;" OwnerUserId="16733" LastEditorUserId="16733" LastEditDate="2016-08-25T20:40:55.610" LastActivityDate="2016-08-25T20:40:55.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17927" PostTypeId="2" ParentId="17903" CreationDate="2016-08-25T20:31:08.833" Score="1" Body="&lt;p&gt;Reaction wheels and the like provide a pure couple or moment. As such it does not matter where they act -- the craft will still rotate about its CG.&lt;/p&gt;&#xA;" OwnerUserId="16759" LastActivityDate="2016-08-25T20:31:08.833" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17928" PostTypeId="2" ParentId="17923" CreationDate="2016-08-25T20:55:56.290" Score="3" Body="&lt;p&gt;In the Wikipedia article you referenced, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dyson_sphere#Dyson_swarm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dyson Swarm&lt;/a&gt; constructs &lt;em&gt;orbit around the star&lt;/em&gt; - standard orbital mechanics would apply, but traffic management might be interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further down in the same article, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dyson_sphere#Dyson_bubble&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dyson Bubble&lt;/a&gt; has non-orbiting (&amp;amp; extremely lightweight) constructs that balance the solar sail pressure with gravity.&lt;/p&gt;&#xA;" OwnerUserId="12978" LastEditorUserId="10273" LastEditDate="2019-06-10T22:57:41.327" LastActivityDate="2019-06-10T22:57:41.327" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="17929" PostTypeId="1" AcceptedAnswerId="17936" CreationDate="2016-08-25T20:59:21.670" Score="16" ViewCount="2736" Body="&lt;p&gt;In reading an answer here: &lt;a href=&quot;https://space.stackexchange.com/questions/6587/how-close-can-i-get-to-a-rocket-launch-and-not-get-killed?rq=1&quot;&gt;How close can I get to a rocket launch and not get killed?&lt;/a&gt;, the comment is made that noise from the launch can be tremendous.  This isn't a big surprise to anyone who's watched a launch on video, or especially has had the privilege of watching one live.  So, fundamentally, why are rocket launches so loud?  What components of a rocket motor firing contribute to the tremendous noise?  I understand that a tremendous amount of energy is being expended in a very short timeframe and concentrated area (rocket nozzle).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_engine_nozzle&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; indicates that a typical nozzle velocity is 2802 m/s, which is around Mach 8.  I'm sure the shock wave of that nozzle velocity (and the 2.8 km/sec escape exhaust velocity) is a significant contributor to the noise, but are there other significant contributors to launch noise?&lt;/p&gt;&#xA;" OwnerUserId="16735" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2021-03-09T17:36:47.940" Title="Why are rocket launches so loud?" Tags="|launch|rockets|exhaust|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17933" PostTypeId="1" CreationDate="2016-08-25T21:42:21.823" Score="-4" ViewCount="222" Body="&lt;p&gt;Boeing and Spacex is practically in race to human flight launch first. It's rocket-ULA Atlas-V have excellent record already in industry. But the Starliner aerodynamic problems aerodynamic/mass due to changes in design/size and water landing(Is orion immune to this?) . How is Starliner doing and human rating of Atlas-V doing so far are they still planing for 2018-2019 launch ferry window? &lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="16355" LastEditDate="2016-09-03T00:05:01.073" LastActivityDate="2016-09-03T00:05:01.073" Title="Boeing starliner launch vehicle" Tags="|nasa|ula|orion-spacecraft|" AnswerCount="1" CommentCount="7" ClosedDate="2016-08-26T17:13:32.097" ContentLicense="CC BY-SA 3.0" />
  <row Id="17934" PostTypeId="2" ParentId="17906" CreationDate="2016-08-25T22:48:40.550" Score="3" Body="&lt;p&gt;No.  Take an ordinary smartphone and put it in outer space and you've got a big problem:  &lt;strong&gt;Inadequate cooling.&lt;/strong&gt;  The phone is going to destroy itself with its own heat.  Normal conduction and convection cooling will not work, all you have is radiation and at those temperatures it's a small part of the cooling.&lt;/p&gt;&#xA;&lt;p&gt;Also see &lt;a href=&quot;https://space.stackexchange.com/a/38614/12102&quot;&gt;this answer&lt;/a&gt; to &lt;em&gt;Do Phonesats and GoPros work with their internal batteries in space vacuum?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="12102" LastEditDate="2020-07-04T03:42:25.073" LastActivityDate="2020-07-04T03:42:25.073" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="17935" PostTypeId="2" ParentId="17933" CreationDate="2016-08-26T00:39:39.280" Score="4" Body="&lt;p&gt;Presumably the aerodynamic problems with the Starliner are specific to its use with the Atlas V launcher. The 4.6m diameter of the capsule has to &quot;neck down&quot; onto the 3m upper stage of the Atlas, and the shape of that transition has significant aerodynamic effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Orion CEV is much too heavy for Atlas V, it will have a different set of problems. Orion's 5 meter diameter matches that of the DCSS-5 used with Delta IV Heavy, so it won't have that kind of &quot;neck down&quot; adapter, so it should be aerodynamically relatively trouble-free on that launcher. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-08-26T00:39:39.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17936" PostTypeId="2" ParentId="17929" CreationDate="2016-08-26T00:56:06.867" Score="19" Body="&lt;p&gt;From &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110012036.pdf&quot; rel=&quot;noreferrer&quot;&gt;Prediction of Acoustic Loads Generated by Propulsion Systems&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The  primary  noise  source  during  rocket  engine  testing  is  the &#xA;  jet  noise  generated  by  the  rocket  exhaust  plume .  The  jet &#xA;  noise  is  produced  directly  from  the  formation,  propagation  and&#xA;  dissipation  of  vortices  or  eddies  that  are  formed in the shear&#xA;  layer of the plume. These eddies are very small in size near the&#xA;  nozzle exit where they originally  form  and  as  they  propagate &#xA;  along  the  plume  they  become  larger  until  they  eventually &#xA;  dissipate.  Once  the  sound  waves  that  are  created  from  the &#xA;  eddies  begin  to  propagate  through  the  air  and  spread  out &#xA;  over  an  area  they  create  pressure  changes  that  will  then  be &#xA;  picked  up  by  the  microphones.  Since  these  sound  waves  vary &#xA;  in  frequency,  a  spectrum  of  varying  sound  pressure  levels &#xA;  will  be  generated  along  the  length  of  the  plume.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gV9cP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gV9cP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="415" LastEditDate="2016-08-26T07:33:42.030" LastActivityDate="2016-08-26T07:33:42.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17937" PostTypeId="2" ParentId="17923" CreationDate="2016-08-26T01:54:26.690" Score="4" Body="&lt;p&gt;If each 'mirror satellite' in the swarm could at least keep the same face towards the sun, then it wouldn't need to expend energy to stay in its orbit. This is because of a handy cancellation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The force of gravity scales as $-m_{sat}/r^2$ and the force of the photon pressure scales as $+A_{sat}/r^2$ where $m_{sat}$ is the mass of the satellite (which might change a little if it has propellant for attitude control) and $A_{sat}$ is the effective area of the satellite. Since they both vary as $1/r^2$ the photon pressure just feels like the gravity of the sun is a little weaker. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the full equations. For &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pressure&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radiation Pressure&lt;/a&gt; of a flat reflector with 100% reflectivity (no absorption/re-radiation):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P_{rad} = \frac{2E_f}{c} cos^2\alpha $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $E_f$ is the incident energy flux density of the sun's photons on the satellite (and $E_f/c$ is the incident momentum density thereof) and $\alpha$ is the tilt angle away from 'straight back towards the sun'. The factor of 2 in momentum comes from the reflection - you reverse the sign of the momentum $p$ of the photon so the mirror gets $2p$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The force is then just pressure times area:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F_{rad} = A_{sat} P_{rad} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy flux density at the satellite's distance $r$ is just the total radiation power of the sun $\mathscr {P}_{sun}$  divided by the area of a sphere at that distance:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_f = \mathscr {P}_{sun} \frac{1}{4 \pi r^2}  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All together, assuming normal incidence (setting $cos^2 \alpha = 1)$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F_{rad} = \frac { \mathscr {P}_{sun}}{2 \pi c} \frac{A_{sat}}{r^2} $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravitational force comes from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation#Modern_form&quot; rel=&quot;nofollow noreferrer&quot;&gt;'modern form' of Newton's law of gravitation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F_{grav} = -Gm_{sun}\frac{m_{sat}}{r^2} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the same $1/r^2$ dependencies are in both forces, and since the signs are opposite, it simply &quot;feels&quot; like slightly weaker gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;side note:&lt;/strong&gt; the cancellation &lt;em&gt;sort-of works&lt;/em&gt; for elliptical orbits too, except for the libration. As the mirror orbits (revolves) around the Sun once, it also rotates around its axis once so that it points towards the sun all the time. But if it's in an elliptical orbit, it is sometimes moving faster and sometimes slower, but the rotation of the mirror around it's axis is constant. For very small eccentricities this 'periodic pointing error' will be 'very very' small (since $cos^2 \alpha$ is quartic about $\alpha = 0$ ), but it's not zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However if you can somehow keep the mirror pointed directly back at the sun, then the cancellation works just fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A graphic example of libration - the moon is rotating about its axis pretty much uniformly, but because of its elliptical orbit around the earth, it &lt;em&gt;appears to be&lt;/em&gt; tilting back and forth. When it's farther away, it appears to be turning to the right faster, when it's close, it can't keep up and appears to move to the left.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Libration&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Libration&lt;/a&gt;&lt;/strong&gt; image from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase2.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lLTA5.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lLTA5.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;At a given circular orbital radius, a shiny flat mirror pointing back towards the sun has a slightly lower orbital velocity that a diffuse white thing that scatters in many direction, and that has a slightly lower velocity than a dark thing that soaks up the power then re-radiates it isotropically. But they all just continue in their stable orbits (unless of course they rotate then it's a total mess!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the problem is that the satellites may change reflectivity due to damage, or effective area and reflection direction due to attitude changes, and the mass may change, and of course they all have gravitational effects on each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ring can accommodate some of those, if they all behave identically. But the more complicated multi-ring orbits will perturb each other both gravitationally and by blocking each other's photon pressure, and that may require some much fancier orbits.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-08-26T07:54:50.817" LastActivityDate="2016-08-26T07:54:50.817" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17939" PostTypeId="1" AcceptedAnswerId="17940" CreationDate="2016-08-26T06:05:45.903" Score="6" ViewCount="662" Body="&lt;p&gt;I was reading &lt;a href=&quot;https://space.stackexchange.com/questions/1393/#comment20620_1395&quot;&gt;this comment&lt;/a&gt; about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pendulum rocket fallacy&lt;/a&gt; under &lt;a href=&quot;https://space.stackexchange.com/a/1395/12102&quot;&gt;this answer&lt;/a&gt; and looked at the picture (below) as I've seen it (or similar) in science books all my life, and I realized that I don't understand how that thing works. I do understand that the engine is near the top, and at least one of the propellant tanks is at the bottom, but does that large &quot;cage&quot; that Goddard is holding on to stay on the ground? The equivalent of the &quot;rocket body&quot; is only the two vertical rods - held at their midpoints? When launched, did it just slide freely through those those U-shaped holders? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And what is the stuff at the bottom - ground level? Is it just holding the rocket from tipping? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And finally, did Goddard really not have the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Use of basic Newtonian mechanics&quot;&lt;/a&gt;? Or is it really that the point of this configuration was to first demonstrate and study &lt;em&gt;successful launch and engine function under acceleration in flight&lt;/em&gt;, and once that was off the ground, the study of in-flight stability* could be done with next generation designs? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IjrXH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IjrXH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tLTvS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tLTvS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped details of a photo of &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Goddard_and_Rocket.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Goddard and Rocket&lt;/a&gt; originally from NASA  and described as &quot;Dr. Robert H. Goddard and a liquid oxygen-gasoline rocket at Auburn, Massachusetts.  8 March 1926 (according to National Air and Space Museum)&quot;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-08-26T14:12:44.670" Title="Which part(s) of the Goddard Rocket is(are) the rocket, and why (actually) no fins?" Tags="|rockets|history|first-flight|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17940" PostTypeId="2" ParentId="17939" CreationDate="2016-08-26T06:31:33.653" Score="7" Body="&lt;p&gt;The A-frame is the &quot;launch pad&quot; and stays on the ground. The bit at the top is the engine; the two vertical rods (pipes) deliver gasoline and LOX from the fuel tanks at the bottom. The tank assembly has a conical shield on the top to deflect exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Robert_H._Goddard#First_liquid-fueled_flight&quot; rel=&quot;nofollow&quot;&gt;Wikipedia article on Goddard&lt;/a&gt; describes the rocket rather well. It also strongly implies that the tractor layout was expected to produce stability:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This layout is no longer used, since the experiment showed that this was no more stable than placing the combustion chamber and nozzle at the base. By May, after a series of modifications to simplify the plumbing, the combustion chamber and nozzle were placed in the now classic position, at the lower end of the rocket.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is generally more difficult to build a fuel-at-the-bottom tractor rocket (because fuel needs to move up) and less efficient (because exhaust needs to be diverted sideways, reducing thrust as the cosine of the deflection angle), so you wouldn't do it if you didn't expect an advantage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trajectory taken by this first rocket probably owed more to a burn-through of the engine nozzle than to the pendulum fallacy, however.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-08-26T14:12:44.670" LastActivityDate="2016-08-26T14:12:44.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17941" PostTypeId="1" AcceptedAnswerId="30475" CreationDate="2016-08-26T08:06:55.433" Score="8" ViewCount="385" Body="&lt;p&gt;JAXA is planning to launch SLATS (Super Low Altitude Test Satellites).&#xA;Theses will orbit for &gt; 2 years using ion propulsion to counteract the substantial drag at the 180-260km high altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They will use Xenon as a propellant, which will probably be the limiting factor for the lifetime of the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the residual atmosphere instead sounds like a very appealing solution: No need to include propellant (Infinite ISP!!), and potentially vastly improved lifetime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the challenges to use the atmosphere as a propellant instead of on board supply ?  &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="415" LastEditDate="2016-08-26T08:20:11.253" LastActivityDate="2018-09-05T19:47:44.423" Title="What are the challenge to use atmosphere as a propellant for super low altitude (180-260 km) satellites?" Tags="|propulsion|ion-thruster|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="17942" PostTypeId="2" ParentId="17939" CreationDate="2016-08-26T14:01:21.687" Score="8" Body="&lt;p&gt;This image shows what's at the bottom: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dPAC7.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dPAC7.png&quot; alt=&quot;Goddard rocket diagram&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's the line for filling the LOX tank. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-26T14:01:21.687" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17943" PostTypeId="1" CreationDate="2016-08-26T17:58:53.667" Score="5" ViewCount="603" Body="&lt;p&gt;What is the &lt;a href=&quot;http://m.sputniknews.com/science/20160826/1044666478/russia-rocket-engine-clean.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;detonation liquid rocket engine (LRE)&lt;/a&gt;? How does this work? How is it better than the average rocket engine?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Russian scientists have successfully tested the first detonation liquid rocket engine (LRE) which uses next-generation clean fuel, according to the Foundation for Advanced Studies (FPI).&lt;br /&gt;&#xA;  “The given results are impressive and unique. We can now firmly admit that ecologically clean rocket fuel is possible, and we made it happen,” said Igor Denisov, the FPI’s deputy.&lt;br /&gt;&#xA;  ...&lt;br /&gt;&#xA;  &quot;This liquid is a mix between oxygen and kerosene,” announced the FPI. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="16355" LastEditorDisplayName="user10509" LastEditDate="2018-09-27T22:09:46.430" LastActivityDate="2018-09-27T22:09:46.430" Title="Detonation Liquid rocket engine" Tags="|rockets|engine-design|combustion|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="17945" PostTypeId="1" CreationDate="2016-08-26T20:29:52.710" Score="0" ViewCount="2243" Body="&lt;p&gt;How much fuel does a Falcon 9 rocket used by SpaceX use, for a trip to the International Space Station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any possibility of Teslas batteries being used in the future?&lt;/p&gt;&#xA;" OwnerUserId="16768" LastEditorUserId="51" LastEditDate="2016-08-26T20:35:09.890" LastActivityDate="2016-08-28T05:22:50.350" Title="Falcon 9 fuel use" Tags="|spacex|falcon-9|fuel|" AnswerCount="2" CommentCount="2" ClosedDate="2016-08-28T16:59:48.727" ContentLicense="CC BY-SA 3.0" />
  <row Id="17946" PostTypeId="2" ParentId="17945" CreationDate="2016-08-26T20:55:26.903" Score="4" Body="&lt;p&gt;Elon Musk has famously said that all transportation will eventually transition to electric EXCEPT rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He included airplanes (and with the release of his Master Plan V2, trucks, buses, become more included) in that, claiming to have an idea for an electric airplane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass of propellant on first stage, fuel + oxidizer is on the order of 395,700kg and it generates 1.5 million lbs of thrust so replacing that with batteries is unlikely to ever happen.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-08-26T20:55:26.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17949" PostTypeId="1" AcceptedAnswerId="17950" CreationDate="2016-08-27T02:26:04.940" Score="5" ViewCount="1123" Body="&lt;p&gt;I was listening to the &lt;a href=&quot;http://www.bbc.co.uk/programmes/p025ynjy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Plutonium episode of &quot;the Elements&quot;&lt;/a&gt; on the BBC, and between 07:00 and 08:30 they talked about uses of plutonium such as spacecraft power and heat. While the returning astronauts in Apollo 13 were not actually &quot;keeping warm&quot; using the RTG, I didn't realize that the spacecraft actually had RTGs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/75/12102&quot;&gt;this answer&lt;/a&gt; I found that indeed the Apollo 13 &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;LEM&lt;/a&gt; contained an RTG.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was it just the one RTG per LEM that was used in Apollo spacecraft, or were there more? Were they just used as a passive, steady source of heat to insure things didn't get too cold, or for a source of power also?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-08-27T03:06:27.793" Title="What were the uses of the RTGs in the Apollo spacecrafts?" Tags="|apollo-program|thermal|thermal-control|rtg|apollo-13|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17950" PostTypeId="2" ParentId="17949" CreationDate="2016-08-27T03:06:27.793" Score="9" Body="&lt;p&gt;The RTGs powered the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package&quot; rel=&quot;noreferrer&quot;&gt;Apollo Lunar Surface Experiments Package&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is also the first thing that comes up when you google &quot;Apollo RTG&quot;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-08-27T03:06:27.793" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17951" PostTypeId="1" AcceptedAnswerId="17957" CreationDate="2016-08-27T04:56:43.447" Score="2" ViewCount="1516" Body="&lt;p&gt;After reading &lt;a href=&quot;https://space.stackexchange.com/a/17950/12102&quot;&gt;this answer&lt;/a&gt; about the RTGs used on the Lunar surface, I added some of the images from the Wikipdia article &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo Lunar Surface Experiments Package&lt;/a&gt; into this &lt;a href=&quot;https://astronomy.stackexchange.com/a/16705/7982&quot;&gt;answer&lt;/a&gt;, where I noted that the Lunar soil at that location was pretty dark by comparing it to the darkness of the radiator fins of the RTG.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then I looked up and noticed that the whole side of the LEM appears to have the texture of crinkled metallized film, but it's extremely dark! Is this a result of the landing, or was it supposed to be like that? I thought the purpose of the metallization was to reflect the sunlight to avoid the more than 1 kilowatt of light per square meter from overheating the spacecraft and crew. Did this darkening cause trouble?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't recall seeing LEMs so dirty - assuming this is lunar soil, was this darkening site-specific?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bwK1d.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bwK1d.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Tpb0i.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Tpb0i.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped and annotated details from: &quot;Astronaut Alan L. Bean from Apollo 12, put the Plutonium 238Pu Fuel from the Lunar Module into the SNAP 27 RTG&quot; from &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package#/media/File:Putting_the_Plutonium_238_fuel_into_the_SNAP_27.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. &lt;strong&gt;note:&lt;/strong&gt; red rectangle highlights possible missing &quot;black stuff&quot;, blue rectangle highlights a shadow from a sunlit part of the &quot;black stuff.&quot; other blue arrows show shadows cast from sunlit rods. Small blue arrows show a dark surface with bright gold circular areas suggesting a pattern of screws or fasteners perhaps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EJnfV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EJnfV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Astronaut Alan L. Bean from Apollo 12, put the Plutonium 238Pu Fuel from the Lunar Module into the SNAP 27 RTG&quot; from &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Surface_Experiments_Package#/media/File:Putting_the_Plutonium_238_fuel_into_the_SNAP_27.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gF90B.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gF90B.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Apollo 11 NASA image &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1969-059C&quot; rel=&quot;nofollow noreferrer&quot;&gt;1969-059C&lt;/a&gt; appears to be the same side of the LEM, although a different mission. The &quot;dark stuff&quot; is there but surrounded by bright film on either side.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2016-08-27T22:16:05.630" Title="Why does the side of the LEM look almost black in this photo? Did that cause problems?" Tags="|apollo-program|materials|lunar-landing|thermal|thermal-control|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17954" PostTypeId="1" AcceptedAnswerId="17956" CreationDate="2016-08-27T07:20:42.593" Score="1" ViewCount="264" Body="&lt;p&gt;There &lt;a href=&quot;http://www.mram-info.com/cobhams-mram-chips-reach-mars-soon&quot; rel=&quot;nofollow noreferrer&quot;&gt;seems to be&lt;/a&gt; (perhaps &lt;a href=&quot;http://www.mram-info.com/aeroflex-launches-16mbit-and-64mbit-mram-chips-based-everspins-technology&quot; rel=&quot;nofollow noreferrer&quot;&gt;these?&lt;/a&gt;) a probe headed to Mars that has incorporated &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetoresistive_random-access_memory&quot; rel=&quot;nofollow noreferrer&quot;&gt;Magnetic Memory&lt;/a&gt; (MRAM) as an integral part of the mission.&lt;/p&gt;&#xA;&lt;p&gt;Is this a PR gimmick, or is MRAM &lt;em&gt;actually the best technology&lt;/em&gt; for this mission to Mars? Are there situations where it would be the best choice for cubesats in LEO also?&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.mram-info.com/cobhams-mram-chips-reach-mars-soon&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;ExoMars is a joint-endeavor between the European Space Agency (ESA) and the Russian Federal Space Agency (Roscosmos). ExoMars 2016 is the first mission, that will consist of a Trace Gas Orbiter (TGO) and an entry, descent and landing demonstrator module (EDM) known as Schiaparelli. The main objectives of this mission are to search for evidence of methane and other trace atmospheric gases that could be signatures of active biological or geological processes on Mars.&lt;/p&gt;&#xA;&lt;p&gt;Cobham's MRAM are used by the Nadir and Occultation for MArs Discovery (NOMAD) instrument on the TGO. NOMAD is a spectrometer suite that will measure the spectrum of sunlight across a wide range of wavelengths, enabling the detection of components of the Martian atmosphere. Cobham says that its MRAM chips were chosen due to the low access time, data retention and footprint, as well as radiation performance. Five MRAMs have been integrated on an interface board on the instrument.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-07-27T05:00:51.030" LastActivityDate="2020-07-27T05:00:51.030" Title="Magnetic Memory to Mars?" Tags="|mars|artificial-satellite|cubesat|electronics|trace-gas-orbiter|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17955" PostTypeId="2" ParentId="17945" CreationDate="2016-08-27T08:25:26.967" Score="3" Body="&lt;p&gt;The only electric rocket engines we use today are ion engines. Those still need a propellant (usually Xenon) and their thrust/weight ratio is far below 1 so we can't use those to launch a rocket from Earth. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6816" LastEditDate="2016-08-28T05:22:50.350" LastActivityDate="2016-08-28T05:22:50.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17956" PostTypeId="2" ParentId="17954" CreationDate="2016-08-27T12:39:08.813" Score="4" Body="&lt;p&gt;ExoMars is not the first use of MRAM. In 2007, &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/spritesat&quot; rel=&quot;nofollow&quot;&gt;SpriteSat&lt;/a&gt; was launched with &lt;a href=&quot;http://www.scientificamerican.com/article/japanese-satellite-mram-freescale/&quot; rel=&quot;nofollow&quot;&gt;MRAM in its magnetometer&lt;/a&gt; subsystem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2012, &lt;a href=&quot;http://nepp.nasa.gov/workshops/etw2012/talks/tuesday/t05_heidecker_mram_technology.pdf&quot; rel=&quot;nofollow&quot;&gt;JPL tested an MRAM unit&lt;/a&gt; for radiation resistance. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;No latchup observed during any testing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;JPL also launched CubeSat On-board Processing Validation Experiment, “&lt;a href=&quot;http://www.jpl.nasa.gov/cubesat/missions/mcubed.php&quot; rel=&quot;nofollow&quot;&gt;COVE&lt;/a&gt;” with an MRAM on board in 2013. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, MRAM's suitability is not a PR gimmick. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-27T12:39:08.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17957" PostTypeId="2" ParentId="17951" CreationDate="2016-08-27T14:33:24.850" Score="6" Body="&lt;p&gt;The black areas are matte black &lt;a href=&quot;http://www.tempil.com/specialty-coatings/pyromark-high-temperature-paint/&quot; rel=&quot;noreferrer&quot;&gt;Pyromark&lt;/a&gt;-painted &lt;a href=&quot;https://en.wikipedia.org/wiki/Inconel&quot; rel=&quot;noreferrer&quot;&gt;Inconel&lt;/a&gt; insulation blankets. The parts of the lunar modules that would experience a lot of heating due to the exhaust of the reaction control thrusters had this material as the outer layer of their insulation blankets. There were four sets of thrusters placed 90&lt;sup&gt;o&lt;/sup&gt; apart, each with four thrusters, and they looked like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XezJU.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XezJU.png&quot; alt=&quot;reaction control thrusters on the Apollo lunar module&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/LM04_Lunar_Module_ppLV1-17.pdf&quot; rel=&quot;noreferrer&quot;&gt;Apollo News Reference Lunar Module Quick Reference Data&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Because the top deck and side panels of the descent stage are&#xA;  subjected to engine exhaust, these areas are extensively protected&#xA;  with a nickel inconel mesh sandwich outboard of the mylar and H-film&#xA;  blankets.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Flat black material both absorbs and emits heat better than any other color material. From &lt;a href=&quot;http://www.thenakedscientists.com/HTML/questions/question/3136/&quot; rel=&quot;noreferrer&quot;&gt;The Naked Scientists&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Essentially, when light is absorbed, a photon of light comes&#xA;  in and hits an electron, giving it some energy. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Now whether it’s absorbed or not will depend on the structure of the&#xA;  electrons [in the atom].  If there's somewhere for the electron to go,&#xA;  and if it can increase it's energy level, then it will absorb the&#xA;  light. If not, then the light can't be absorbed and carries straight&#xA;  on. If it is absorbed, the substance looks black. If it isn't, it’s&#xA;  white [because the light is reflected back].&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This means that, if you heat something up, if there's [a higher energy&#xA;  level] somewhere for the electrons to go up to, and it’s black, then&#xA;  there's also somewhere for the electron to drop down to again, meaning&#xA;  that the substance will also emit energy. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;So if something is good at absorbing light then it’s also good at&#xA;  emitting it.  So a black thing should be much better emitting light than a white thing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://home.earthlink.net/~pfjeld/lmdata/&quot; rel=&quot;noreferrer&quot;&gt;Lunar Module Coatings Page&lt;/a&gt; has diagrams showing where all the different kinds of insulation and coatings were used on the lunar landers. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2016-08-27T15:50:56.120" LastActivityDate="2016-08-27T15:50:56.120" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17958" PostTypeId="1" AcceptedAnswerId="17959" CreationDate="2016-08-27T16:01:20.683" Score="1" ViewCount="414" Body="&lt;p&gt;I read &lt;a href=&quot;http://qz.com/766697/spacexs-biggest-rival-is-developing-space-trucks-to-ferry-cargo-in-an-orbital-economy/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX’s biggest rival is developing “space trucks” to ferry cargo in an orbital economy&lt;/a&gt; in Quartz which linked to this April 2016 outline of ULA's view of &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Commercial_Space/2016_Cislunar.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transportation Enabling a Robust Cislunar Space Economy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is quite an exciting outline of what the logistics of a a cis-lunar space economy might look like, and there is a lot of ideas and new ways of thinking to take in. One of them seems here to be to not have to cary your 2nd stage full of fuel into space between your first stage and your payload - am I understanding this correctly? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does bringing propellants up from the surface of the Earth (the other sources are pretty far out there) allow one to avoid Earth's deep gravitational well? Are they going to collect water from clouds instead? I'm missing something here!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5yzOd.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5yzOd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Slide number 8 from ULA's &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Commercial_Space/2016_Cislunar.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transportation Enabling a Robust Cislunar Space Economy&lt;/a&gt; annotated later to indicate the connection between transport of terrestrial water to orbit while avoiding Earth's deep gravitational well. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-15T16:21:06.463" LastActivityDate="2017-08-15T16:21:06.463" Title="ULA: bringing propellants from Earth &quot;Avoids Earth's Deep Gravity Well&quot;?" Tags="|rockets|fuel|upper-stage|ula|refueling|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17959" PostTypeId="2" ParentId="17958" CreationDate="2016-08-27T16:47:25.767" Score="3" Body="&lt;p&gt;For missions that require more fuel than the standard ACES stage contains, &lt;a href=&quot;https://www.youtube.com/watch?v=BbZKfno1KNc&quot; rel=&quot;nofollow&quot;&gt;they propose to launch&lt;/a&gt; 2 rockets, one with the ACES stage and payload, one with extra LOX en LH.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They also propose using the ACES stage and XEUS lander as space tugs, keeping them in space for a long time instead of having to launch a new one for every mission. That's what the 'avoid Earth's gravitational well' is about. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-27T16:47:25.767" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17960" PostTypeId="2" ParentId="17958" CreationDate="2016-08-27T16:58:55.637" Score="1" Body="&lt;p&gt;I believe the point is that &lt;em&gt;the spacecraft itself&lt;/em&gt; only has to get out of Earth's gravity well once. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until ISRU propellant sources on the moon are up and running, propellant still needs to be brought up from Earth's surface, but regular bulk propellant-only launches to a fuel depot in LEO have different engineering requirements that may allow for cost reduction. This is sometimes referred to as the &quot;big dumb booster&quot; concept. The payload is cheap and easily replaceable, so the launcher doesn't need to be as reliable; the payload doesn't care about g-force, so higher-g trajectories (which lose less energy to gravity because of their shorter ascent time) can be considered, making everything more efficient. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because propellant is generally a large majority of a spacecraft's fueled mass, keeping the spacecraft out of the gravity well while still having to truck propellant isn't a huge win, but it can be a win.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-08-27T16:58:55.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17961" PostTypeId="1" AcceptedAnswerId="17962" CreationDate="2016-08-27T17:38:02.190" Score="5" ViewCount="699" Body="&lt;p&gt;I've seen the term &quot;burp&quot; used, for example, in a description of the &lt;a href=&quot;https://www.awesomestories.com/asset/view/F-1-Engine-Propulsion-of-the-Saturn-V-Moon-Rocket&quot; rel=&quot;noreferrer&quot;&gt;Saturn V F-1 engine&lt;/a&gt; start sequence (or what is perhaps the &lt;a href=&quot;http://history.nasa.gov/ap08fj/pdf/sa503-flightmanual.pdf&quot; rel=&quot;noreferrer&quot;&gt;source document&lt;/a&gt; - see pdf p 62, source page 4-9). What does it mean? What causes it to happen?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2016-08-27T22:02:07.260" LastActivityDate="2016-08-27T22:02:07.260" Title="What is a rocket engine &quot;burp&quot;, and why does it happen?" Tags="|rockets|propulsion|saturn-v|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17962" PostTypeId="2" ParentId="17961" CreationDate="2016-08-27T21:45:25.840" Score="5" Body="&lt;p&gt;&quot;Burping&quot; usually refers to a start-up failure mode in which the combustion chamber pressure rises too quickly relative to the pressure of the inflowing propellants (as the pumps spin up, for example). This can cause the feed system to backflow with undesirable effects upstream.  Page 77 of &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=4&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjyuJq6x-LOAhVG5WMKHdzDBngQFgg1MAM&amp;amp;url=https%3A%2F%2Fdspace.mit.edu%2Fbitstream%2Fhandle%2F1721.1%2F82764%2F54069429-MIT.pdf%3Fsequence%3D2&amp;amp;usg=AFQjCNHzYmW9GI7iiDVHxMhSNUhP6C_cZg&amp;amp;sig2=BhgMDWt2-2BUNbs7SJwo-g&amp;amp;bvm=bv.131286987,d.cGc&quot; rel=&quot;nofollow&quot;&gt;this document&lt;/a&gt; contains an example of someone using the term in that fashion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, it's not clear to me how ignition of the gas generator exhaust in the engine bell of the F1, as quoted in your link, would help to prevent that.  The link seems to have quoted from and slightly changed an Apollo-era diagram found in &lt;a href=&quot;https://www.ibiblio.org/apollo/Documents/StagesToSaturn-Bilstein.pdf&quot; rel=&quot;nofollow&quot;&gt;Stages to Saturn&lt;/a&gt;, page 111.  In the original it reads&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Fuel-rich turbine combustion gas is ignited by flame from igniters.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;a)  Ignition of this gas prevents backfiring and burping&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In this case I believe the writer is referring to the combustion chamber of the gas generator (which could, of course, be affected by &quot;burping&quot;).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-08-27T22:01:09.997" LastActivityDate="2016-08-27T22:01:09.997" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="17963" PostTypeId="1" CreationDate="2016-08-28T00:20:23.903" Score="7" ViewCount="407" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/17960/12102&quot;&gt;answer&lt;/a&gt; suggests that if a payload were simply bulk propellant, such as LH2 and LOX, and not delicate technology such as an interplanetary probe or communications satellite, then there may be some re-optimization of launch trajectory and perhaps the launch vehicle, to incorporate such things as a higher g-force trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that early in a launch, the thrust (and therefore g-force) is sometimes &lt;em&gt;even reduced&lt;/em&gt; so that the total compression forces on the rocket body (rocket thrust pushing up at one end, aerodynamic drag pushing down at the other) is capped at &lt;a href=&quot;https://space.stackexchange.com/a/17112/12102&quot;&gt;&quot;Max-Q&quot;&lt;/a&gt; - see also &lt;a href=&quot;https://en.wikipedia.org/wiki/Max_Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; - so how much could it actually be increased? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How would a bulk payload like fuel (instead of delicate technology) change boost to LEO optimization? &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-03-29T14:56:58.153" LastActivityDate="2019-03-29T14:56:58.153" Title="How would a bulk payload like fuel (instead of delicate technology) change boost to LEO optimization?" Tags="|rockets|low-earth-orbit|trajectory|launch-vehicle|g-force|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="17964" PostTypeId="1" AcceptedAnswerId="18541" CreationDate="2016-08-28T02:44:36.277" Score="1" ViewCount="157" Body="&lt;p&gt;I &quot;watched&quot; Juno's flyby of Jupiter using the simulation in the &lt;a href=&quot;https://eyes.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Eyes app on my laptop&lt;/a&gt;, and later saw the good news &lt;a href=&quot;https://twitter.com/NASAJuno/status/769649749662109696&quot; rel=&quot;nofollow noreferrer&quot;&gt;in this tweet&lt;/a&gt;. Now I want to do a &quot;flyby in Python&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the setup (shown below) in the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;Horizons web interface&lt;/a&gt;, I can easily download the state vectors of the Juno Spacecraft and Jupiter itself and subtract. This is predicted data of course, not real-time telemetry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The header in the data file for the Juno spacecraft seems to suggest that the flyby is covered by the most recent calculation, the last entry in the list entitled &lt;code&gt;ref_160226_180221_160226_V0.1&lt;/code&gt;. Everything after April 18, 2016 is predicted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I would like to see an updated trajectory - which includes measurements taken after this flyby, should I just keep checking back here, or is there somewhere else I can look to see at least notification that a new calculation has been done that is based on more recent data?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q3BAh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q3BAh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; SPACECRAFT TRAJECTORY:  &#xA;   From JPL navigation team. Predicts after 2016-Apr-18.0.&#xA;&#xA;   Trajectory name                    Start (CT)          Stop (CT)&#xA;   ------------------------------   -----------------   -----------------&#xA;   rec_110805_111026_120302         2011-Aug-05 17:19   2011 OCT 26 00:00&#xA;   rec_111026_120308_120726         2011 OCT 26 00:00   2012 MAR 08 12:00   &#xA;   rec_120308_120825_121109         2012 MAR 08 12:00   2012 AUG 25 00:00&#xA;   rec_120825_130515_130708         2012 AUG 25 00:00   2013 MAY 15 00:00&#xA;   rec_130515_131005_131031         2013 MAY 15 00:00   2013 OCT 05 12:00&#xA;   rec_131005_131014_131101_reco    2013 OCT 05 12:00   2013 OCT 14 00:01&#xA;   rec_131014_131114_140222         2013 OCT 14 00:01   2013 NOV 14 06:00&#xA;   ref_131114_171017_140311_V0.2    2013 NOV 14 06:00   2015 MAR 26 00:00&#xA;   ref_150326_180221_150326_V0.1    2015 MAR 26 00:00   2016 FEB 26 00:01&#xA;   ref_160226_180221_160226_V0.1    2016 FEB 26 00:00   2018 FEB 21 11:40&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZQHdj.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZQHdj.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-08-28T11:11:20.760" LastActivityDate="2016-10-07T12:13:57.463" Title="How to know when JPL Horizons is updated with new Juno spacecraft trajectory?" Tags="|orbital-mechanics|spacecraft|orbital-maneuver|juno|jpl-horizons|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17965" PostTypeId="1" CreationDate="2016-08-28T03:53:47.807" Score="6" ViewCount="460" Body="&lt;p&gt;I'm not referring to the obvious development testing, but the life cycle of individual engines. When a rocket is launched, have those specific engine &quot;serial numbers&quot; been previously test-run? Like the way the engine in your brand new car has already been run before it leaves the factory. Is it a necessary Q/A step for all rocket engines, or is launch the first (and presumably only) time some engines have ever run? If they are test fired, how does it compare to actual flight run in terms of duration and thrust level? If some engines are tested pre-flight and others not, what determines which engines do or do not get a pre-flight test firing?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-08-29T09:49:59.110" Title="Does every liquid-fueled rocket engine get a test run before its actual flight operation?" Tags="|rockets|testing|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17966" PostTypeId="2" ParentId="17888" CreationDate="2016-08-28T15:02:35.283" Score="5" Body="&lt;p&gt;Spaceflight101 gives the direct geostationary orbit payload capacity as 3500kg using the Briz-M stage. &lt;a href=&quot;http://spaceflight101.com/spacerockets/proton-m-briz-m/&quot;&gt;http://spaceflight101.com/spacerockets/proton-m-briz-m/&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia lists it as 3250kg, referring to a 2009 mission planner's guide.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-08-28T15:02:35.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17967" PostTypeId="2" ParentId="17965" CreationDate="2016-08-28T17:13:56.043" Score="4" Body="&lt;p&gt;Can't post a complete answer because I only know about Shuttle, but all the SSMEs were &quot;green run&quot; before being flown and after major repairs.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For  the  majority  of  the  Space  Shuttle  Program,  SSMEs  were &#xA;  assembled  at  Pratt  &amp;amp;  Whitney  Rocketdyne’s   Canoga  Park, &#xA;  California  facility.    Engines  were  then  shipped  to  Stennis &#xA;  Space  Center  (SSC)  for  testing.     Development engines remained at SSC.  Flight engines were tested and, pending successful post-test inspections and  a completed engine acceptance review, delivered to Kennedy Space Center  where they were  readied for flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/SSME_Pursuit_Improvement.pdf&quot; rel=&quot;nofollow&quot;&gt;Space Shuttle Main Engine - The Relentless Pursuit of Improvement&lt;/a&gt; page 10.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-08-28T17:13:56.043" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17968" PostTypeId="2" ParentId="17965" CreationDate="2016-08-28T17:58:28.283" Score="9" Body="&lt;p&gt;No. At least one engine wasn't: the NK-15 as used on the N-1 first stage. These engines were rated only for a single start. They would build a batch of engines, &lt;a href=&quot;http://www.bis-space.com/belgium/wp-content/uploads/2015/10/endofa40yearcountdown.pdf&quot;&gt;test some of them&lt;/a&gt;, and any failure was a reason to scrap the whole batch. Clearly, this was not optimal, so its replacement, the NK-33 (which was to be used from the fifth launch onwards) was designed so it could be run more than once. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-08-28T17:58:28.283" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17969" PostTypeId="2" ParentId="17965" CreationDate="2016-08-28T21:22:59.707" Score="5" Body="&lt;p&gt;For the European engines, I can say that they test run every engine for new designs until they are very confident in the manufacturing quality. Then they only test run if they make changes (even pretty minor ones) and also periodically to check that the engines are still identical to the ones that were qualified earlier. It's possible that tiny unintentional changes in manufacturing accumulate to change how the engine runs. Such changes may not be detectable in the sparse data they get from the flight engines, hence the periodic tests.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-08-29T09:49:59.110" LastActivityDate="2016-08-29T09:49:59.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17970" PostTypeId="1" AcceptedAnswerId="17971" CreationDate="2016-08-28T22:19:22.610" Score="-1" ViewCount="146" Body="&lt;p&gt;Don't know if this is already been thought of, if it has it would be cool to know the name?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fundamental problem in how the public sees space transport, the problem is we see it as firing a rocking from object a to go object b however a more elegant solution would be to have a constant motion which gradually speeds up overtime. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason we are not building long distant space craft is because we believe that if we build it now, tomorrow the new space craft will be faster and will overtake it......&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm defining a mission to mars as being long distant I also defining that by tomorrow I mean before the mission has been completed....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes around 150-300 days to get to mars depending on the launch, 50 days give or take thats not bad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay so lets get to the point.... we fire a rocket this rocket takes 150 days to get to mars its unmanned we leave it to continuously slingshot from mars to earth and back again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This provides a continuous supply between Earth and Mars. It would be cool to know the name of this if it exists?&lt;/p&gt;&#xA;" OwnerUserId="16782" LastEditorUserId="16782" LastEditDate="2016-08-29T11:00:03.087" LastActivityDate="2016-08-29T13:34:44.477" Title="A Space Gravitational Bus Route - is there such a thing?" Tags="|mars|spacecraft|gravity|" AnswerCount="1" CommentCount="9" ClosedDate="2016-08-29T17:33:55.120" ContentLicense="CC BY-SA 3.0" />
  <row Id="17971" PostTypeId="2" ParentId="17970" CreationDate="2016-08-29T03:08:13.667" Score="2" Body="&lt;p&gt;For a Mars cycler to provide continuous supply between Earth and Mars, we have to load it at each flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To load it with cargo, we have to perform a rendezvous, that is: match its velocity/timing. So to dock with the cycler, you have to spend the same energy as what would be required to go to/from Mars in the first place! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the cycler can help your logistics. It can provide deep space communication, power facility, life support, ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a very interesting concept for manned flight, but as long as we just send robots/satellites to Mars, the cycler is of very limited interest.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="1142" LastEditDate="2016-08-29T13:34:44.477" LastActivityDate="2016-08-29T13:34:44.477" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17972" PostTypeId="1" AcceptedAnswerId="22011" CreationDate="2016-08-29T03:37:26.773" Score="13" ViewCount="1008" Body="&lt;p&gt;I noticed a small hemisphere protruding from the dorsal side of the Enterprise space shuttle's nose cone, forward of the front landing gear. I have no clue what its purpose is but am far from an expert in aerospace engineering. Does anyone here know what it's for?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BCOML.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BCOML.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;&lt;a href=&quot;https://spaceraceleadership.wordpress.com/2010/08/12/space-history-can-you-name-the-1st-space-shuttle/&quot; rel=&quot;noreferrer&quot;&gt;Image Source: Space Race Leadership&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="16785" LastEditorUserId="6944" LastEditDate="2020-09-01T18:01:25.063" LastActivityDate="2020-09-01T18:01:25.063" Title="Purpose of hemisphere under shuttle nose cone (Enterprise)" Tags="|space-shuttle|engineering|aerodynamics|shuttle-enterprise|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="17973" PostTypeId="1" AcceptedAnswerId="17974" CreationDate="2016-08-29T05:26:03.827" Score="35" ViewCount="7535" Body="&lt;p&gt;NASA recently published this photo taken by Juno, at a distance of about 700,000kms from Jupiter:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TZCPl.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TZCPl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The overall quality (very grainy around the darker regions, a bit washed-out in the brighter areas, and not the greatest dynamic range even in areas that are properly exposed) and resolution of this photo seems to compare poorly against images taken from ground-based telescopes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wIfpv.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wIfpv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...and also from the images that we can get using Hubble:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NDOoY.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NDOoY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...both of which would have been shot at distances of closer to 600 &lt;em&gt;million&lt;/em&gt; kms from Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand Juno will get even closer to Jupiter than it currently is, but at the moment it's nearly 1000x closer to Jupiter than Hubble/Earth-based observatories and returning images that, at best, appear to be on par with what we can accomplish using terrestrial telescopes.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a reason for this (like maybe the images released were shot in a low-resolution/low-bandwidth mode that doesn't actually use the camera's full capabilities), and what sort of improvements in the image quality can be expected as Juno reaches its closest orbital altitude?  If the image quality can't be expected to drastically improve, was there a reason for not equipping the mission with better imaging technology that can at least clearly eclipse what we can attain by using Earth-local resources?&lt;/p&gt;&#xA;" OwnerUserId="8367" LastActivityDate="2019-01-03T11:39:23.247" Title="Why are photos from Juno such low resolution?" Tags="|juno|imaging|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17974" PostTypeId="2" ParentId="17973" CreationDate="2016-08-29T05:59:32.910" Score="55" Body="&lt;p&gt;Short answer: JunoCam is not a scientific instrument; It was put onboard solely to get some neat pictures.  It is not necessary for the scientific mission, and is mostly there just for public interest.  You can &lt;a href=&quot;https://www.missionjuno.swri.edu/junocam&quot;&gt;interact with JunoCam&lt;/a&gt; by voting on what it takes pictures of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer:  There are several reasons which combine to result in Juno only being able to take lower-quality pictures. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Juno itself is spin-stabilized.  It is constantly rotating, at about 2 rpm, so it cannot focus on any spot for long.&lt;/li&gt;&#xA;&lt;li&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/JunoCam&quot;&gt;Wikipedia&lt;/a&gt;, Junocam is &quot;capable of color imaging at 1600 x 1200 pixels.&quot;  This results in a resolution of 15 km/pixel.  Compared to the Mars Reconnaisance Orbiter's HiRISE camera, which is able to take pictures at 0.3 m/pixel, this is not a very high resolution.&lt;/li&gt;&#xA;&lt;li&gt;The environment around Jupiter is filled with radiation.  JunoCam is not inside the radiation vault, because it would be unable to take pictures.  This means that JunoCam is only expected to survive about 8 orbits.  The longer it orbits, the more its pictures will likely deteriorate.  I recall reading an article somewhere about what this deterioration will look like, but I cannot find it anymore.&lt;/li&gt;&#xA;&lt;li&gt;At Jupiter's orbit, data rates are limited.  JunoCam will only be able to return about 40 megabytes of data per orbit.  This means that some quality may have to be sacrificed in order to get more pictures back.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-08-29T05:59:32.910" CommentCount="23" ContentLicense="CC BY-SA 3.0" />
  <row Id="17975" PostTypeId="1" AcceptedAnswerId="17976" CreationDate="2016-08-29T06:25:24.623" Score="9" ViewCount="2609" Body="&lt;p&gt;Going to &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid-fuel_rocket&quot; rel=&quot;noreferrer&quot;&gt;wikipedia's article on solid-fuel rockets&lt;/a&gt;, I come across some graphs of thrust curves for certain bore-hole geometry. Right now I'm interested in the simplest bore-hole: a circular hole right in the center of the fuel grain:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PeERF.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PeERF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This thrust-curve &lt;strong&gt;seems totally wrong&lt;/strong&gt;. For a circular hole with a linear burn rate, the radius should expand linearly. Since C = pi*d, the circumference should also increase linearly. (Burn rate is proportional to the circumference of the exposed fuel grain.) Therefore, the mass flow should also increase linearly. Therefore, the thrust should also increase linearly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(That all assumes that all else is equal. I don't know much about solid-rocket calculations. I've tried working with them twice over the years, and I came to the conclusion that solid rocket calculations are even more complex than liquid rocket calculations, which really bugged me cuz solid rockets appealed to my sense of simplicity due to having no moving parts. I'm not sure if combustion pressure goes up linearly, but it seems it would have to...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, if all this is true, then the thrust curve should not even be a curve at all. It should just be a straight line going up at some angle...maybe steeply, maybe shallowly, but &lt;strong&gt;linearly&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graph shown is radically different than that. Am I missing something?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the thrust curve for a solid rocket with a simple circular hole, and why?&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2016-08-29T18:49:13.870" Title="What is the correct thrust curve for a solid rocket with a simple circular hole, and why?" Tags="|solid-fuel|thrust|calculation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17976" PostTypeId="2" ParentId="17975" CreationDate="2016-08-29T12:31:21.320" Score="11" Body="&lt;p&gt;Hill and Peterson &quot;Mechanics and Thermodynamics of Propulsion&quot;, third printing, November 1970, page 385, has a diagram that agrees with your intuition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tElNE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tElNE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(sorry for poor scan quality)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are correct - as the surface area increases, so does the mass flow rate.  There must be other factors in play in the graph from Wikipedia.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-08-29T12:31:21.320" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17977" PostTypeId="2" ParentId="17975" CreationDate="2016-08-29T12:44:53.683" Score="10" Body="&lt;p&gt;Underneath the diagrams in the Wikipedia article you link, there's a mention of the BATES grain geometry:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Circular bore: if in BATES configuration, produces progressive-regressive thrust curve&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/BATES&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/BATES&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In which there is combustion occurring on both ends of a cylindrical segment as well as the central bore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In such a case, there are two factors which decrease the burn surface area over time, counteracting the linear increase expected from a circular bore. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the length of the bore decreases as the ends burn away. The inner burn area is the product of the increasing circumference and the decreasing length.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, the burning end surfaces are annular, with area decreasing over the burn as the inner diameter of the ring increases. The end-burn area loss is proportional to the square of the burn time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the proportions of the initial bore diameter, outer diameter, and length, a variety of thrust curve shapes can be achieved. A quick experiment in a spreadsheet shows that a grain segment with outer diameter of 5 units, initial inner diameter of 2 units, length 14 units, with double-end burning yields a pretty good match to the curve we're trying to understand. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XjMZn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XjMZn.png&quot; alt=&quot;solid rocket fuel thrust-time curve&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="4660" LastEditDate="2016-08-29T18:49:13.870" LastActivityDate="2016-08-29T18:49:13.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17978" PostTypeId="2" ParentId="9426" CreationDate="2016-08-29T18:28:53.450" Score="3" Body="&lt;p&gt;The rate of &quot;normalization&quot; would vary person to person. Hence why ISS personnel have mandatory exercise in space so they won't be essentially crippled when they return to earth. The exact statistics are unknown but it is reasonable to assume without countermeasures eventually moon gravity would feel similar to earth gravity eventually. IMHO &lt;/p&gt;&#xA;&#xA;&lt;p&gt;IMHO is related to no one being in space for an extremely extended period of time and very few people for any extended period. This leaves too small a pool of data to draw anything other than cursory data. The longest time by anyone was Valeri Polyakov at 14.5 months (almost &lt;a href=&quot;http://www.theverge.com/2015/10/29/9638354/nasa-astronaut-scott-kelly-consecutive-space-flight-216-days-iss&quot; rel=&quot;nofollow noreferrer&quot;&gt;438 days&lt;/a&gt;). I am a retired Registered Nurse and do have extensive knowledge of Anatomy &amp;amp; Physiology. This bone loss begins within the first few days in space. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The most severe loss (of bone) occurs between the second and fifth&#xA;  months in space, although the process continues throughout the entire&#xA;  time spent in microgravity. Extended stays on Mir have resulted in&#xA;  losses of bone mass of as much as 20% (indicating there is a range).&#xA;  Astronauts regain most of their bone mass in the months following&#xA;  their return from space,but not all...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/audience/foreducators/postsecondary/features/F_Bones_in_Space.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ref&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Calf muscles biopsies before flight and after a six months mission on&#xA;  the ISS show that even when crew members did aerobic exercise five&#xA;  hours a week and resistance exercise three to six days per week,&#xA;  muscle volume and peak power both still decrease significantly.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Actually posts study on webpage as well.) In the study they measure muscle strength pre- and post flight in conjunction with biopsies of the soleus and gastrocnemius muscles to measure cell size and the structural changes of fast and slow muscle fibers. Chemical analysis measures muscle fiber structural changes involving myosin, (drives muscle contractions and cell divisions, enzymes, and substrates.) Electron microscopy &quot;determines the relationship between thick and thin filament, the amount of myofilament loss, and changes in membrane-associated protein complexes found in skeletal muscle fibers and connective tissue that help the muscle resist stretch-induced damage.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;MRI studies are done as well. These tests are run at multiple intervals both before and after space flight. It goes into further detail but shows significant variance in muscular atrophy, especially with a small sample set. A short &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/245.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;synopsis&lt;/a&gt; reports &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;overall calf muscle volume significantly decreased by 13¬+2%, peak&#xA;  power decreased significantly by 32%, force-velocity characteristics&#xA;  were reduced significantly to -20 to -29% across the velocity&#xA;  spectrum, and there was a 12-17% shift in myosin&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(As the atrophy progresses, strength will most likely continuously decline to the point where moon gravity would &quot;feel&quot; like earth gravity. &lt;em&gt;(This would be the reason for the IMHO part of my conjecture. Almost all the data collected, and all data in this study, is over a period of 6 months. There is no data for a long enough period of time nor is there any data with the relative gravity of the moon. This data is for micro-gravity and we can only theorize as to how the data would change with the low gravity of the moon)&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should also be noted these are all individuals not only with the exemplary physical conditioning required to be accepted into a space program, but that this also indicates there is most likely relatively minimal variance among them in their level of physical conditioning. This is a small data set of individuals that are all in good health, meeting a high standard for their physical fitness and with relatively low age variance and yet has a statistically significant variance in muscle breakdown over the same time period in the same environment. If in only 6 months peak power decreases by 32%, muscle volume decreases by approximately 13% and force velocity decreases by 20-29%, I don't find it unreasonable to predict the eventual loss of &quot;superpowers&quot;.&lt;/p&gt;&#xA;" OwnerUserId="16496" LastEditorDisplayName="user20636" LastEditDate="2020-03-10T20:09:58.430" LastActivityDate="2020-03-10T20:09:58.430" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="17979" PostTypeId="1" CreationDate="2016-08-29T18:38:31.343" Score="2" ViewCount="570" Body="&lt;p&gt;I have recently read Seveneves and loved the use of certain technologies in it. Has anyone put serious study into the bolo hanglider hanger and possible viable usage of a similar technology?&lt;/p&gt;&#xA;" OwnerUserId="16496" LastActivityDate="2020-02-07T23:14:45.753" Title="Orbiting bolo technology viability?" Tags="|technology|transport|orbital|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="17980" PostTypeId="2" ParentId="17871" CreationDate="2016-08-29T18:57:48.957" Score="4" Body="&lt;p&gt;Low Earth Orbit is a bad place for an elevator. The satellites and dense debris make an impact likely. The tether would probably get cut too often to confer a return exceeding investment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;A TALE OF TWO MOONS&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A series of orbital elevators might be worthwhile. Here is a diagram of possible elevators from the Martian moons Phobos and Deimos:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bE533.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bE533.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tossing a payload from the top of the 937 km Phobos tether will toss it to an ellipse whose apoaerion coincides with the foot of the 2942 km Deimos tether. Speed of the payload at apoaerion matches the speed of the Deimos elevator foot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going the other direction, dropping a payload from the Deimos elevator foot will send a payload to the top of the Phobos elevator. Again, the ellipse's periaerion speed matches the speed of the top of Phobos' elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the two moons could exchange passengers and cargo using virtually no propellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EARTH ORBITAL ELEVATORS&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could we make a similar relay in earth's neighborhood?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a few problems. With Phobos' 1.07e16 kilogram mass and Deimos' 1.5e15 kilogram mass, the moons are huge momentum banks. They could catch and throw payloads for millennia with very little effect on their orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth elevators wouldn't enjoy such a large anchor mass. If a 20 tonne elevator imparts 1 km/s delta V to a 10 tonne payload, the elevator will suffer a velocity change of .5 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To have tethers whose orbits aren't destroyed by a catch or a throw, it's desirable to have an anchor mass that greatly exceeds the payload mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've envisioned &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2016/08/tran-cislunar-railroad.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;3 vertical tethers in earth's neighborhood&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8wUYM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8wUYM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Super Geosynchronous Tether there is a possible source of anchor mass: The dead sats in a graveyard orbit just above geosynchronous orbit. For the other tethers it would be very difficult to get a substantial anchor mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the elevator is massive enough that a few catches or throws don't destroy its orbit, it can be a momentum exchange broker. Catching payloads from above or dropping payloads to lower orbits will boost momentum. Catching payloads from below or throwing payloads to higher orbits will sap momentum. By balancing momentum boosting actions with momentum sapping maneuvers, the elevator orbit can be preserved without use of propellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or if momentum does need to be changed with the use of propellent, ion engines such as Hall thrusters are an option. A xenon exhaust velocity of 30 km/s is nearly 10 times as good as chemical propellents. And if elevator catches and throws are separated by weeks or months, there is time for the low thrust ion engines to build up momentum.  Elevators are a way to have you Ion engine ISP and eat Oberth cake too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;SUMMARY&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe orbital elevators are doable and worthwhile. They would take substantial initial investment, though.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-08-29T19:44:17.780" LastActivityDate="2016-08-29T19:44:17.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17981" PostTypeId="1" CreationDate="2016-08-29T20:29:22.687" Score="5" ViewCount="988" Body="&lt;p&gt;Why do we launch space satellites and vehicles from sea level and not from a higher locale like Denver? Is saving a mile not more fuel efficient? Gravity is less the further away from center or is this negligible in a mile? Atmosphere thinner so less friction and less heat over less time? Must be something simple I am missing.&lt;/p&gt;&#xA;" OwnerUserId="16797" LastEditorDisplayName="user687" LastEditDate="2016-08-30T00:12:00.353" LastActivityDate="2016-08-30T00:18:10.610" Title="Why launch from sea level and not Denver?" Tags="|launch|artificial-satellite|propulsion|" AnswerCount="2" CommentCount="2" ClosedDate="2016-08-30T00:15:19.530" ContentLicense="CC BY-SA 3.0" />
  <row Id="17982" PostTypeId="2" ParentId="17981" CreationDate="2016-08-29T20:31:47.063" Score="3" Body="&lt;p&gt;Space launch is speed, not altitude.  (If you climbed a 200 mile high tower based in Denver, and stepped off, you would NOT be in orbit and you'd fall to the ground).  The closer your launch site is to the equator, the more free rotational speed you get from the Earth (as long as you launch eastwards).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, it is nice to have less populated areas to the east (the ocean, for example) for your boosters to fall in.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-08-29T20:31:47.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17983" PostTypeId="2" ParentId="17981" CreationDate="2016-08-29T20:46:21.680" Score="6" Body="&lt;p&gt;At the surface, this seems like a decent idea.  Why not launch from higher up, so you don't have to go as far?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer is that when choosing a launch location, we are not so much concerned with height as we are with speed. To get into a Low Earth Orbit, you need to be traveling at about 7.8 km/s. At the equator, the Earth is spinning at 1670 km/hr, or ~0.5 km/s. This means that we need 7.3 km/s of delta-V&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://image.gsfc.nasa.gov/poetry/ask/a10840.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;, we can calculate the Earth's rotational velocity at any latitude using the formula $1670*\cos(\theta)$. Denver is at 39 degrees North, so the rotational velocity there is ~1298 km/hr, or ~0.36 km/s. So to launch from Denver we need to increase our speed by 7.44 km/s. This is a difference of 0.14 km/s, which is a significant saving.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, we don't actually launch from the Equator, but we try to get as close to it as we can.  All the different countries around the world try to launch as close to the equator as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other thing to consider is that the Earth is not perfectly spherical.  It has an equatorial bulge formed by its rotation. This means that the radius at the equator is about 26 miles larger than at the poles. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Equatorial_bulge&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, this results in the equator actually being taller than Mount Everest. Of course, the atmosphere and ocean also bulges out like this, so you are still under the same amount of atmosphere, but it does make it easier to get to space. Since you're further from the center of the Earth, you experience more centrifugal force. This helps counter the force of gravity by a very tiny amount, but every little bit helps when trying to get to space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @OrganicMarble points out, it also allows us to drop our boosters in the ocean, to be retrieved later.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-08-30T00:18:10.610" LastActivityDate="2016-08-30T00:18:10.610" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="17984" PostTypeId="1" CreationDate="2016-08-29T21:23:24.363" Score="1" ViewCount="104" Body="&lt;p&gt;I've read that if the Earth was 50% larger, escape velocity would be so great that we could not build a powerful enough rocket to escape. Is this true and how do I do the maths to prove it?&lt;/p&gt;&#xA;" OwnerUserId="16798" LastActivityDate="2016-08-29T21:23:24.363" Title="Escape Velocity" Tags="|orbital-mechanics|rockets|" AnswerCount="0" CommentCount="4" ClosedDate="2016-08-29T21:55:08.940" ContentLicense="CC BY-SA 3.0" />
  <row Id="17985" PostTypeId="1" CreationDate="2016-08-29T22:22:37.267" Score="3" ViewCount="71" Body="&lt;p&gt;There are many Centaur upper stages and a few other objects in the junk yard orbit just beyond the geosynchronous orbit. Is there any reasons why this stuff could not be reused? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are cryogenic tanks that could eventually serve the oxygen production activities on the moon. There are rocket engines that could be used for landing on and launching from the moon's surface or for space tugs. There are some satellites with solar panels and some interesting electronics. Why isn't anyone talking about all this stuff as an In-Situ Resource?&lt;/p&gt;&#xA;" OwnerUserId="16800" LastEditorUserId="16800" LastEditDate="2016-08-31T19:26:03.563" LastActivityDate="2016-08-31T19:26:03.563" Title="Reusing Tanks and Rocket Engines from just beyond Geosynchronus Orbit" Tags="|reuse|commercial|" AnswerCount="0" CommentCount="2" ClosedDate="2016-08-30T11:53:44.030" ContentLicense="CC BY-SA 3.0" />
  <row Id="17986" PostTypeId="2" ParentId="1723" CreationDate="2016-08-29T23:15:19.180" Score="5" Body="&lt;p&gt;By far the most abundant resource on the moon is Oxygen. It comprises a little less than 50% of all the material on the surface of the Moon, and some of it can be extracted reasonably easily. If large production facilities were built using local resources to produce Oxygen, the Oxygen could be used as 89% (by weight) of the fuel needed by rockets in orbit. Hydrogen, the other 11%, could easily be brought up from Earth in large volumes.&lt;/p&gt;&#xA;" OwnerUserId="16800" LastActivityDate="2016-08-29T23:15:19.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="17988" PostTypeId="1" CreationDate="2016-08-30T02:45:20.913" Score="1" ViewCount="136" Body="&lt;p&gt;If we were going to launch a ship meant to take a one way trip and colonize another planet outside of our solar system, what location on earth would serve as the best launch point? Could we use one that already exists or would we have to build a new one? I'm talking about a ship like in the one in the movie 'Pandorum' or the video game 'Civilization, Beyond Earth', with people, technology, food, plants etc. on it.&lt;/p&gt;&#xA;" OwnerUserId="16804" LastActivityDate="2016-08-30T05:15:24.293" Title="When launching a ship meant to colonize a planet beyond our solar system, what would be the best location for the launch?" Tags="|launch|rockets|colonization|interstellar-travel|" AnswerCount="1" CommentCount="2" ClosedDate="2016-08-30T07:27:01.060" ContentLicense="CC BY-SA 3.0" />
  <row Id="17989" PostTypeId="2" ParentId="17988" CreationDate="2016-08-30T05:15:24.293" Score="3" Body="&lt;p&gt;Here is my take on this. As people look at the economics of space, big things in space only start to make sense when the materials to build them are in space. That would mean the refineries and factories are also in space, and the people who work there. This is orbital space, not on the surface of any large planet or moon. So we are talking about orbital settlements harvesting asteroids. Looking at orbital space the most likely spot for asteroid exploitation would be lagrange points. Possibly Earth / Moon lagrange points or the trojan asteroids, the Jupiter lagrange points. If we are already building huge spinning colonies in space, building a multi generational craft to travel to another star is not as big a leap. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-08-30T05:15:24.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17993" PostTypeId="1" AcceptedAnswerId="18002" CreationDate="2016-08-30T19:37:47.877" Score="3" ViewCount="2395" Body="&lt;p&gt;Suppose I have a spaceship and fire up its engines, accelerating to 10000 km / h. Now I turn off my engine and see I have consumed 10 % of my fuel (90 % left).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much fuel does it take to accelerate the starship to 20000 km / h, assuming I don't hit a rock and can keep my speed at 10000 km / h without expending any fuel whatsoever?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will I need another 10 % of my starting fuel, or do I require more fuel, i.e. energy use linear or non linear?&#xA;I am under the impression that the closer I get to lightspeed, the more energy I need...?&lt;/p&gt;&#xA;" OwnerUserId="16357" LastEditorDisplayName="user8858" LastEditDate="2017-06-04T03:24:12.607" LastActivityDate="2021-02-08T00:26:59.977" Title="Amount of fuel for accelerating a spaceship" Tags="|fuel|acceleration|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="17994" PostTypeId="2" ParentId="17993" CreationDate="2016-08-30T22:51:03.297" Score="3" Body="&lt;p&gt;This is governed by the Tsiolkovsky rocket equation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V = ln\left( \frac{M_{i}}{M_{f}} \right)v_e$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\Delta V$ is your total change in velocity, $v_e$ is the effective exhaust velocity of your engine, $M_{i}$ is the mass before the burn, and $M_{f}$ is the mass at the end of the burn.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the issue - it takes a minimum mass ratio to reach a particular $\Delta V$ for a given effective exhaust velocity, as given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{M_i}{M_f} = e^\left(\frac{\Delta V}{v_e}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of the exponential term, the mass ratios get ugly as you aim for higher $\Delta V$.  For anything we're flying today, they get &lt;em&gt;stupid&lt;/em&gt; huge for reaching speeds beyond tiny fractions of $c$.  For that reason, I'm starting with speeds on the order of 0.0001 $c$.  This would all break down at relativistic speeds, but we're not going to get &lt;em&gt;anywhere&lt;/em&gt; near relativistic speeds with conventional reaction drives.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're using an ion engine like the Dawn spacecraft ($v_e$ ~ 30380 m/s), then the mass ratio required to reach 0.0001 $c$ (~30000 m/s) is 2.685; for every kilogram of mass you want to accelerate, you must expend ~1.7 kg of propellant&lt;sup&gt;1&lt;/sup&gt;.  This is &lt;em&gt;the&lt;/em&gt; most efficient engine in use today; the SSME $v_e$ tops out at ~4410 m/s in a vaccuum, giving us a mass ratio of a little over 900 (899 kg of propellant for each kg of final mass).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means we have to work backwards; if we want to do two burns, we have to work out how much propellant we need to reserve for the second burn first, and then factor that into the calculation for the first burn.  We're assuming magical propellant tanks with no mass.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the second burn, we're accelerating the just the dry mass of the spacecraft by 30000 m/s.  If our spacecraft masses 1000 kg and our mass ratio is ~2.7, that means we need to reserve 1700 kg of propellant for that burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the first burn, we're accelerating the spacecraft plus the propellant needed for the second burn.  Our $\Delta V$ is the same, so our mass ratio is the same, so we need 1.7 * (1000 + 1700) = 4590 additional kilograms of propellant, for a total starting mass of 8290 kg.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mentioned that the mass ratios get ugly as your $\Delta V$ goes up.  If you want to accelerate by 0.001 $c$ (300000 m/s), the mass ratio for the Dawn engine jumps up to ~19437; for every kg of mass you want to accelerate, you have to expend over nineteen &lt;em&gt;thousand&lt;/em&gt; kg of propellant.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We're not going to reach anything near light speed using any reaction drive that has to carry its own propellant.  You either need to gather propellant mass as you fly (Bussard ramjets), use solar or laser sails, or use a true reactionless drive like the Star Trek warp drive.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;hr&gt;&#xA;&lt;sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The mass ratio is propellant mass plus final mass divided by final mass.  A mass ratio of 2 means we have 1 kg of propellant for each kg of final mass, a mass ratio of 3 means we have 2 kg of propellant for each kg of final mass, etc.  So we subtract 1 from the mass ratio to figure out how much propellant we use.&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12922" LastEditorUserId="12922" LastEditDate="2016-08-31T13:19:41.293" LastActivityDate="2016-08-31T13:19:41.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17995" PostTypeId="2" ParentId="5463" CreationDate="2016-08-30T23:45:02.717" Score="2" Body="&lt;p&gt;Consider:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As rockets consume their fuel, they get lighter, so the thrust they produce is applied to a progressively reducing mass. The result is increasing acceleration and therefore g-force on the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two examples: Apollo Saturn and the Space Shuttle both limited their acceleration by shutting down or throttling back engines so as not to exceed about 3g. This is both for the benefit of the crew and because the structures couldn't take any more than that. The reason is weight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everything on a rocket has to be made as light as possible to minimize the fuel required to get it into space. That limits how strong things can be. Rocket structures are made only as strong as they need to be, hence they are as light as they can possibly be. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if you want to catapult the thing off a launch pad, you need to make things stronger to take all those extra g-forces (only needed for the duration of the catapult) and therefore everything gets heavier and therefore you need to carry more fuel to get the job done once you're past the end of the catapult stroke.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2016-09-01T01:57:40.363" LastActivityDate="2016-09-01T01:57:40.363" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="17996" PostTypeId="2" ParentId="3693" CreationDate="2016-08-31T00:11:56.793" Score="9" Body="&lt;p&gt;There is another important variable besides chamber pressure and nozzle design to consider--how the engine pumps the propellants into the chamber. The J-2 engine, like the lower stage (and much more powerful but lower ISP kerosene burning F-1 engine) used a gas generator (basically burning oxygen and fuel, here hydrogen) to drive a turbine that pumped the oxygen and hydrogen into itself and the main chamber. The F-1 (using kerosene instead of hydrogen) was designed with high enough gas generator pressure to drive the pumps at launch, but the J-2, designed for vacuum operations, used a lower gas generator pressure to drive its pumps. The very low sea level ISP reflects not just the usual &quot;back up&quot; of rockets by sea level air pressure but also a turbo pump rendered anemic by the same back pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just today I downloaded software called RPA that simulates rocket chambers and nozzle performance for a wide variety of propellant types and chamber pressures and other conditions. My attempt to simulate the J-2A engine (an upgrade of the J-2) suggests, if I did it correctly, that a reasonable sea level ISP for the J series engines with the same type of chamber and nozzle would be a bit under 300 seconds, a reduction of 31 percent. In terms of thrust in Klbs, that would suggest a bit under 160, so at just under 110 poor pump performance at sea level apparently dings it another 22 percent. &lt;/p&gt;&#xA;" OwnerUserId="16823" LastActivityDate="2016-08-31T00:11:56.793" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17997" PostTypeId="1" AcceptedAnswerId="17998" CreationDate="2016-08-31T02:48:58.903" Score="6" ViewCount="539" Body="&lt;p&gt;SETI is listening for messages of extra-terrestrial origin. &lt;a href=&quot;http://www.theverge.com/2016/8/29/12695352/seti-russia-radio-observatory-signal-detected-alien-unlikely&quot;&gt;This article&lt;/a&gt; talks about a candidate signal Russian astronomers received over a year ago but that only recently came to light. There was no independent confirmation of the signal at the time, and current observations focused on &lt;a href=&quot;https://en.wikipedia.org/wiki/HD_164595&quot;&gt;HD 164595&lt;/a&gt;, the apparent source of the signal in question, are apparently not yielding any results. This prompts my question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is protocol related to the detection/confirmation of an extra-terrestrial signal? Let's imagine that a signal is detected and subsequently confirmed as of extra-terrestrial and artificial origin - source location is beyond our solar system, signal composition cannot be explained by any natural phenomenon. For lack of a better example, let's say its primary feature is just like that depicted in Carl Sagan's &quot;Contact&quot; - streams of pulses enumerating prime numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens next? Is there full public disclosure? Is such disclosure relied upon to decode the signal (assume the signal has secondary features which suggest the presence of an actual message)? Is disclosure avoided? Is public disclosure made upon detection, prior to confirmation? Is there even a protocol in place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The story with the Russian detection seems to suggest that an opportunity for confirmation was lost because of a lack of even limited disclosure. What would happen upon a detection event by SETI or some other scientific agency such as a university?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2016-08-31T03:33:43.940" LastActivityDate="2016-09-02T18:54:10.703" Title="What is the protocol surrounding public disclosure of a message of extra-terrestrial origin?" Tags="|seti|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="17998" PostTypeId="2" ParentId="17997" CreationDate="2016-08-31T06:02:09.887" Score="5" Body="&lt;p&gt;SETI has published &quot;a&quot; &lt;a href=&quot;http://www.seti.org/post-detection.html&quot;&gt;protocol&lt;/a&gt; for efforts under their banner, but isn't something that can be enforced on other organizations.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even that protocol is vague in many many areas (doesn't seem to suggest timeframes for example), but it does give a framework for how the followup could proceed.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2016-08-31T06:02:09.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="17999" PostTypeId="2" ParentId="2310" CreationDate="2016-08-31T07:24:45.060" Score="4" Body="&lt;p&gt;Legally, this would be outlawed by the Outer Space Treaty of 1967, as well as the &quot;daughter&quot; treaties which dictate ownership of space objects. There is currently no law of salvage in space, such as there is on the earth's oceans. Whatever is launched belongs to the launching country or launch provider unless specifically given to a third party. So, even if this is a theoretically possible construction, which it would be given a huge amount of fuel, there would still be the ownership of the space junk to overcome. If a piece of space junk can be point sourced, then ownership can always be determined. I have a feeling that this current legal structure will stand until someone is brazen enough to actually get up there and start collecting.&lt;/p&gt;&#xA;" OwnerUserId="13563" LastActivityDate="2016-08-31T07:24:45.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18000" PostTypeId="1" AcceptedAnswerId="18001" CreationDate="2016-08-31T10:28:00.807" Score="7" ViewCount="984" Body="&lt;p&gt;SpaceX company has many months and now becoming years pushing the launch date for the first Falcon Heavy. Yeah right space rocketry is tricky and very difficult, it's important that everything should be 100% safe. But all these delays looks to be a little bit exaggerated. It could be possible that they just don't need it for the moment, they can launch planned payloads for the customers even with the current Falcon 9 FT. Who knows, could be this or other reasons, engineering reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why so many delays?&lt;/p&gt;&#xA;" OwnerUserId="16521" LastActivityDate="2016-09-01T03:14:23.753" Title="Which are the reasons that Falcon Heavy first flight has so many delays?" Tags="|launch|spacex|falcon-heavy|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18001" PostTypeId="2" ParentId="18000" CreationDate="2016-08-31T11:25:16.513" Score="10" Body="&lt;p&gt;There are several likely reasons: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Elon Musk likes to &lt;a href=&quot;http://www.bloomberg.com/news/articles/2016-05-09/elon-musk-s-tesla-strategy-win-big-by-falling-short&quot;&gt;set absurdly optimistic deadlines&lt;/a&gt; to put pressure on everyone to achieve the impossible.&lt;/li&gt;&#xA;&lt;li&gt;With improvements to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9&quot;&gt;Falcon 9&lt;/a&gt; (100% increase in payload to LEO from v1.0 to v1.2/FT) the FH became less necessary.&lt;/li&gt;&#xA;&lt;li&gt;SpaceX needs its engineering resources elsewhere (Return to flight after the CRS-7 failure, First stage recovery). &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The degree to which these 3 are responsible for the delays is guesswork, SpaceX hasn't said anything about it. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="10335" LastEditDate="2016-08-31T17:32:39.967" LastActivityDate="2016-08-31T17:32:39.967" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18002" PostTypeId="2" ParentId="17993" CreationDate="2016-08-31T13:15:22.733" Score="4" Body="&lt;p&gt;Will I need another 1 % of my starting fuel, or do I require more fuel, i.e. energy use linear or non linear?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring relativistic effects, you require less fuel to go from 10000 km/hour to 20000 km/hour as opposed to going from 0 km/hour to 10000 km/hour. While 20000 km/hour sounds fast, it's actually quite slow. That's not even enough to get you from the ground into orbit about the Earth, which requires a Δv somewhere between 34000 km/hour to 40000 km/hour. That 20000 km/hour is also very slow compared to the speed of light, which is a bit over 1000000000 km/hr. Relativistic effects start appearing at about 1% of the speed of light, and aren't significant until about 10% of the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll generously assuming your rocket initially was 90% fuel by mass. (Making a rocket whose mass is initially 85% fuel is hard.) I'll also assume that your rocket is in empty space, well removed from any gravitating bodies. In that case, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;ideal rocket equation&lt;/a&gt; applies:&#xA;$$\Delta v = v_e \ln\left(\frac{m_i}{m_f}\right)$$&#xA;Your rocket attained a Δv of 10000 km/hour after burning 10% of the initial fuel. This means your rocket has an exhaust velocity of 29.45 km/s (106033 km/hr), which puts it in the class of a typical ion propulsion engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the rocket burns all of the fuel while accelerating in a straight line, the final speed would be 244148.9 km/hour. This is more than twice the value naively obtained by multiplying your 10000 km/hour by ten.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-08-31T13:15:22.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18003" PostTypeId="2" ParentId="17993" CreationDate="2016-08-31T15:49:58.070" Score="1" Body="&lt;p&gt;The answers using the Tsiolkovsky rocket equation are fine, but they may not clarify the underlying physics. It's simpler if we take the case of $\Delta V\ll v_e$. In that approximation, the mass of the ship is constant. The question would then be whether the fuel used is proportional to the rocket's final momentum (which goes linearly with $\Delta V$), or its final kinetic energy (which goes like $\Delta V^2$). It seems like a paradox, because in a rocket, the fuel is providing &lt;em&gt;both&lt;/em&gt; the momentum (reaction mass) &lt;em&gt;and&lt;/em&gt; the kinetic energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The resolution of the paradox is that in the final state, there is not just kinetic energy in the rocket. There is also kinetic energy in the fuel. So it's erroneous to assume that we have to use up fuel-energy in proportion to $\Delta V^2$. The fuel used is proportional to $\Delta V$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way to see this is that we can pick a frame of reference that's momentarily at rest relative to the ship at some point in time after it's already been accelerating. In that frame, the efficiency of the motor has to be the same as the efficiency when the motor starts from rest, because in that frame, the rocket &lt;em&gt;is&lt;/em&gt; momentarily at rest.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2016-08-31T15:49:58.070" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18004" PostTypeId="1" AcceptedAnswerId="18101" CreationDate="2016-08-31T17:54:01.820" Score="16" ViewCount="1011" Body="&lt;p&gt;I do &lt;strong&gt;not&lt;/strong&gt; mean multiple combusters as in the linear aerospike. I mean place multiple &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerospike_engine&quot;&gt;aerospikes&lt;/a&gt; next to each other. Imagine a Falcon 9 or Saturn V with aerospikes instead of conventional bell nozzles. Is it possible? Are there some serious downsides?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought that aerospikes kind off expand against the ambient air. But that ambient air would be disturbed if there were multiple other aerospikes, no?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firefly Beta, if it ever arrives, would cluster three cores each with it's own aerospike. Any disturbance between the three aerospikes to be expected?&lt;/p&gt;&#xA;" OwnerUserId="16719" LastEditorUserId="12102" LastEditDate="2021-11-23T00:07:41.760" LastActivityDate="2021-11-23T00:07:41.760" Title="Is it possible to cluster aerospike rocket engines?" Tags="|rockets|firefly-aerospace|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18005" PostTypeId="1" AcceptedAnswerId="18013" CreationDate="2016-08-31T19:52:33.237" Score="4" ViewCount="2127" Body="&lt;p&gt;&lt;a href=&quot;http://durofy.com/leo-meo-geo-satellite-systems/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This article&lt;/a&gt;, &lt;a href=&quot;http://www.harriscaprock.com/blog/high-throughput-satellite-communications-systems-meo-vs-leo-vs-geo/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, and &lt;a href=&quot;https://space.stackexchange.com/questions/14872/is-it-possible-to-deliver-enough-leo-satellites-with-one-launch-to-substitute-fo&quot;&gt;this earlier question&lt;/a&gt; indicate that, given enough satellites in LEO, it's possible to substitute between GEO and LEO. The first article and the question have some indication of the substitution ratios (from the article, 40-80 LEO sats to 3 GEO sats; from the question, 100 LEO sats to 1 GEO sat) for what seem to be telecom applications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question:&lt;/strong&gt; For which applications are LEO and GEO substitutable?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm assuming telecom and imaging are among them; is that correct, and what else might be? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any thoughts or suggestions would be much appreciated. Thank you!&lt;/p&gt;&#xA;" OwnerUserId="15707" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-09-02T16:18:44.547" Title="For which applications are LEO and GEO substitutable?" Tags="|low-earth-orbit|geostationary|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18006" PostTypeId="2" ParentId="18000" CreationDate="2016-09-01T03:14:23.753" Score="5" Body="&lt;p&gt;As noted by others, the payload of the Falcon 9 base has been increased, specifically its capability to GTO to handle all but the largest satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper stage is not that great (Low Isp, where it matters in an upper stage) and cannot direct inject to GSO because of lack of ability to restart many hours later.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the need for the Heavy keeps getting reduced. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as much as anything else, each time they get closer, they end up revising the core vehicle (Falcon 9 1.0 to 1.1 to 1.2/Full Thrust, with a Fuller thrust coming sometime in 2016-2017) and this affects the Heavy design each time, causing at least some re-iteration. They may simply be waiting for the design to finally settle down on the F9 Fuller Thrust before actually building the cores for it. Gwynne Shotwell indicated at least one of the boosters was under construction in May 2016, so should be getting closer.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-09-01T03:14:23.753" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18007" PostTypeId="2" ParentId="18005" CreationDate="2016-09-01T11:26:28.913" Score="3" Body="&lt;p&gt;Moving a system from LEO to GEO has impacts far beyond the number of satellites required to provide constant line of sight to the ground:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;GEO systems are further away, which increases the latency by 60x and the power required by 3600x for telecom applications, compared to a 600km LEO orbit. It also degrades the resolution by a factor 60x for imaging systems, which makes high-resolution imaging satellites in GEO infeasible with current technology. For active systems such as RADAR and LIDAR, the power required in GEO is 13 million times higher.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;GEO systems are fixed in the sky: the ground terminals do not need a tracking antenna to follow them, which decreases a lot the cost and makes it possible to reach a wide market (this is important for satellite TV for instance). &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So depending on your exact application (satellite TV, communications on the move with lightweight terminals like Iridium, hourly low-resolution high-coverage imaging of the Earth, or high-resolution imaging), the tradeoffs will tell you that a LEO or GEO system is optimal. In that sense, they are not subtitutable. I can't think of any application where a GEO and a LEO system deliver the same service at the same price.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="4685" LastEditDate="2016-09-02T16:18:44.547" LastActivityDate="2016-09-02T16:18:44.547" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18008" PostTypeId="2" ParentId="18005" CreationDate="2016-09-01T12:13:08.037" Score="1" Body="&lt;p&gt;TV transmission using LEO satellites would be difficult and expensive for a consumer. A tracking antenna would necessary and for uninterrupted TV even two tracking antennas with a delicate handover from one satellite to the next one. The tracking antennas would need a clear view to the sky for the complete path of the satellite. Compared with a GEO TV satellite a lot of TV consumers could not mount a tracking antenna at the same place useful for a fixed antenna.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-01T12:13:08.037" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18009" PostTypeId="1" CreationDate="2016-09-01T12:54:36.600" Score="0" ViewCount="94" Body="&lt;p&gt;I note that the NASA page on Wallops trajectories has a disclaimer saying that the page has not been updated and is for historical purposes only. Are the northern-most trajectories still restricted to 90 degrees azimuth avoiding launches over Assateague National Seashore?&lt;/p&gt;&#xA;" OwnerUserId="12814" LastActivityDate="2016-09-01T12:54:36.600" Title="What are possible trajectory azimuths at Wallops?" Tags="|launch|trajectory|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18010" PostTypeId="1" AcceptedAnswerId="18012" CreationDate="2016-09-01T14:33:24.177" Score="8" ViewCount="566" Body="&lt;p&gt;I am watching the live stream of today(9/1/16)'s &lt;a href=&quot;http://gizmodo.com/watch-live-as-iss-astronauts-spacewalk-to-install-a-new-1785497722&quot;&gt;spacewalk&lt;/a&gt;. The ISS entered a position in space where the video feed was lost, but they are still able to broadcast the audio conversations between the astronauts. How is it possible to only be able to transmit/receive one, and not the other?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Apologies if this has been asked before, but nothing showed up in the search.)&lt;/p&gt;&#xA;" OwnerUserId="12646" LastEditorUserId="4660" LastEditDate="2016-09-01T14:57:38.437" LastActivityDate="2016-09-01T16:03:07.183" Title="Why are there times when the ISS can transmit audio, but not video?" Tags="|iss|data-transmission|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18011" PostTypeId="1" AcceptedAnswerId="18016" CreationDate="2016-09-01T14:35:58.170" Score="25" ViewCount="3005" Body="&lt;p&gt;The Falcon 9 rocket, complete with the Amos 6 payload, just blew up (Or RUDed) on &lt;a href=&quot;http://arstechnica.com/science/2016/09/spacexs-falcon-9-rocket-apparently-blew-up-during-a-test-firing-thursday/&quot;&gt;the launch pad&lt;/a&gt;. I believe the standard language for most launch insurances is that it is only covered after &quot;Intentional launch&quot; of the vehicle, specifically limiting incidences like that which occurred this morning. Was Amos 6 insured, and how are such issues typically managed?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-09-01T17:54:44.800" LastActivityDate="2016-09-03T11:31:35.877" Title="Was Amos 6 insured for pre-launch operations?" Tags="|launch|spacex|falcon-9|testing|insurance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18012" PostTypeId="2" ParentId="18010" CreationDate="2016-09-01T15:14:30.757" Score="9" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt; after being pointed in the right direction by @Organic Marble:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Video goes out over the Ku-band link (which uses a very directional dish antenna), audio goes over a separate S-band link (which looks to be less directional, so less likely to go down). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-09-01T16:03:07.183" LastActivityDate="2016-09-01T16:03:07.183" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18013" PostTypeId="2" ParentId="18005" CreationDate="2016-09-01T15:15:36.917" Score="2" Body="&lt;p&gt;The choice of LEO or GEO is often a trade off between different systems and frequently comes down to cost. There's not a lot that you can do in GEO that you can't manage in LEO if cost isn't a problem (or visa versa).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few examples of where this is the case;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Earth observation missions - typically your first impulse might be that LEO is a clear winner here, it's closer to the Earth, so your resolution is higher, you can get the data back to Earth quicker and using less power - win win right? What if you want to observe a specific country frequently, your own country for monitoring deforestation, soil moisture levels or air quality. You can do that with a single Geo satellite in position over your country, or many LEO sats. If you want to collect data every few minutes, but don't care too much about the resolution of the data, then GEO would almost certainly be a cheaper option than LEO.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Communications satellites - The further away from the Earth a satellite is, the greater proportion of the surface of the Earth that satellite has line of sight to (up to a point of course). So Geo seems like an easy win, sure it'll cost more power to transmit coms over the greater distance, but in space you've got solar panels and power's easy! But what about making sure you're using that line of sight? If your satellite is facing the wrong way then you can put in as much power as you like, your directional antenna won't make a blip on Earth. Pointing accuracy requires either momentum wheels or attitude thrusters, and usually both (thrusters to de-load the wheels after periods of use). Propellant isn't anywhere near as cheap or easy in space, and the pointing accuracy resolution for GEO is much higher than LEO.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Access to space - so you're a 2nd world country looking at dragging yourself into the intellectual elite by launching your very own space station. You can examine crystal growth, ant colonies and spring oscillations in 0g - who wouldn't want to! Do you go LEO or GEO. Certainly in LEO you're more greatly affected by the mass concentrations on Earth, but GEO is so much farther away. It's not cheap to get the bulk material needed for a space station to GEO, let alone the tons of food, water, air and springs that astronauts consume on a daily basis. LEO is closer and therefore cheaper.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I can only come up with one type of mission that requires you to operate in either LEO or GEO (there may be more);&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Removal of GEO Debris/Servicing of GEO satellites - replace GEO with LEO, it can work either way. The key thing here is that you are targeting a specific area of space, it's one of your mission requirements. You want to clean up GEO, you can't send a satellite to remove debris from LEO, it would have a negligible effect.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1567" LastActivityDate="2016-09-01T15:15:36.917" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18016" PostTypeId="2" ParentId="18011" CreationDate="2016-09-01T18:21:35.953" Score="30" Body="&lt;p&gt;Rewriting with clarification, I apparently misinterpreted the second tweet. Source still is &lt;a href=&quot;https://twitter.com/pbdes&quot; rel=&quot;noreferrer&quot;&gt;this twitter feed&lt;/a&gt; (from a &lt;a href=&quot;http://www.spacenews.com&quot; rel=&quot;noreferrer&quot;&gt;spacenews.com&lt;/a&gt; reporter).&lt;/p&gt;&#xA;&lt;p&gt;The launch insurance doesn't count:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SpaceX explosion didnt involve intentional ignition - E Musk said occurred during 2d stage fueling - &amp;amp; isn't covered by launch insurance. &lt;a href=&quot;https://twitter.com/pbdes/status/771409425475174400&quot; rel=&quot;noreferrer&quot;&gt;(tweet)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;However, the satellite was insured under a different policy until launch:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Spacecom insured Amos-6 for $285M in marine cargo market, not space insurance market. Launch +1 yr policy would kick in at rocket ignition. &lt;a href=&quot;https://twitter.com/pbdes/status/771409983074426881&quot; rel=&quot;noreferrer&quot;&gt;(tweet)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;@cbs_spacenews Sat was insured as marine cargo for pre-launch phase. Launch policy didn't kick in because no ignition-w/-intent-to-launch. &lt;a href=&quot;https://twitter.com/pbdes/status/771410879770456064&quot; rel=&quot;noreferrer&quot;&gt;(tweet)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Verdict appears to be that the satellite was insured (and that reporting with clarity via twitter is hard, the repeated focus on launch insurance is misleading).&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-01T19:23:04.243" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18017" PostTypeId="1" CreationDate="2016-09-01T21:16:08.740" Score="1" ViewCount="135" Body="&lt;p&gt;Does Max Q occur before or after the &lt;a href=&quot;https://en.wikipedia.org/wiki/Prandtl%E2%80%93Meyer_expansion_fan&quot; rel=&quot;nofollow noreferrer&quot;&gt;Prandtl–Meyer expansion fan&lt;/a&gt;? The shape, nature's way of avoiding a single shock wave (per Wikipedia) is visible on the Apollo launches for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vpE5d.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vpE5d.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; Apollo 11, &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Apollo_11_Launch_-_GPN-2000-000628.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16848" LastEditorUserId="12102" LastEditDate="2018-12-03T04:49:46.160" LastActivityDate="2018-12-03T04:49:46.160" Title="Max Q vs Prandtl-Meyer Fan" Tags="|aerodynamics|exhaust|max-q|shockwave|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18018" PostTypeId="1" CreationDate="2016-09-02T00:18:12.220" Score="-3" ViewCount="122" Body="&lt;p&gt;If it took Voyager’s radio signal 4 hours to cross the solar system and reach Earth, roughly how far away in AU was Voyager from Earth when it sent that signal?&lt;/p&gt;&#xA;" OwnerUserId="16852" LastEditorUserId="6816" LastEditDate="2016-09-02T00:38:16.557" LastActivityDate="2016-09-02T01:07:32.600" Title="Voyager's distance from Earth" Tags="|communication|telemetry|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18019" PostTypeId="2" ParentId="18018" CreationDate="2016-09-02T00:30:49.267" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://www.wolframalpha.com/input/?i=speed%20of%20light%20*%204%20hours&quot; rel=&quot;nofollow&quot;&gt;About 29 au away.&lt;/a&gt;  Speed * time = distance (since speed has units of distance/time). For this problem, the units are as follows:&#xA;$$C \text{ meters}/\text{second} * 4 \text{ hours} * (60*60) \text{ seconds}/\text{hour} \approx 4*10^{12} \text{ meters} \approx 29 \text{ au}$$&#xA;I didn't know the conversion off the top of my head so I used trusty Wolfram Alpha.&lt;/p&gt;&#xA;" OwnerUserId="12966" LastEditorUserId="6816" LastEditDate="2016-09-02T01:07:32.600" LastActivityDate="2016-09-02T01:07:32.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18020" PostTypeId="1" AcceptedAnswerId="18021" CreationDate="2016-09-02T00:36:35.787" Score="35" ViewCount="3806" Body="&lt;p&gt;I understand that the static test is a general test of pre-launch procedures, but risking a multi-million dollar payload for a test seems remarkable. Couldn't they use a mass simulator or something? Do they fuel the payload as well for the test, and couldn't that be done separately?&lt;/p&gt;&#xA;" OwnerUserId="16853" LastActivityDate="2016-09-03T21:32:58.927" Title="Why was AMOS-6 mounted on the rocket for a test fire?" Tags="|launch|spacex|falcon-9|failure|testing|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18021" PostTypeId="2" ParentId="18020" CreationDate="2016-09-02T01:00:34.240" Score="38" Body="&lt;p&gt;The static test wants to be done relatively close to launch day, to minimize the likelihood of anything happening to the engines between the test and the launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The more conservative procedure is to attach the payload after the static test, but they'd have to lower the rocket, move it back indoors, stack the payload, take it back out and reelevate it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Static firing after payload integration saves a day or two, but puts the payload at risk if something like this happens. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-09-02T04:05:12.960" LastActivityDate="2016-09-02T04:05:12.960" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18022" PostTypeId="2" ParentId="18005" CreationDate="2016-09-02T01:03:02.283" Score="1" Body="&lt;p&gt;Geostationary orbits only come in one flavor: zero inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means that there is no access to the near-polar areas from GEO (geostationary). No imaging, no reliable data links, &lt;em&gt;nada&lt;/em&gt;, &lt;em&gt;ничего&lt;/em&gt;, &lt;em&gt;zilch&lt;/em&gt;. Ignoring refraction, the &lt;strong&gt;geo&lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;stationary&lt;/strong&gt;&lt;/a&gt; ring is below the horizon when you are beyond roughly 81 degrees North or South latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the first, and very useful &lt;strong&gt;geo&lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;synchronous&lt;/strong&gt;&lt;/a&gt; orbit - the &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Molniya orbit&lt;/a&gt; - can have excellent coverage of one pole by sacrificing good (good = slow moving, long time) coverage of the opposite hemisphere. Three satellites in that orbit will provide continuous coverage, but may require some attention to the pointing direction for antennas which have gain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b3Tys.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b3Tys.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; A Molniya orbit illustration:&quot;&lt;em&gt;The Molniya orbit. Usually the period from perigee +2 hours to perigee +10 hours is used to transmit to the northern hemisphere.&lt;/em&gt;&quot; &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit#/media/File:NASA_molniya_oblique.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5fyLc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5fyLc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; A sphere inside a cone, from &lt;a href=&quot;http://mathcentral.uregina.ca/QQ/database/QQ.09.07/s/juan1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mathcentral.uregina.ca&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-09-02T02:45:36.527" LastActivityDate="2016-09-02T02:45:36.527" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18023" PostTypeId="1" AcceptedAnswerId="18028" CreationDate="2016-09-02T04:30:00.033" Score="11" ViewCount="2372" Body="&lt;p&gt;As the saying goes - space is hard. Rockets are a collection of extremes in temperature, pressure, and energy packed as closely as possible and driven to their limits. Even in 2016 they still occasionally fail catastrophically. For example in todays &lt;a href=&quot;http://www.latimes.com/business/la-fi-spacex-rocket-reliability-1472754973-htmlstory.html&quot; rel=&quot;noreferrer&quot;&gt;Los Angeles Times&lt;/a&gt; business section:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Bill Ostrove, an aerospace and defense analyst at Forecast International, said SpaceX's reliability with the Falcon 9 is 93%, which is &amp;quot;right in the ballpark&amp;quot; of the industry average of 95%.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;While most transportation has a per-trip reliability of four, five, or even six nines (i.ie 99.9999%) putting things into orbit doesn't quite have even two nines yet.&lt;/p&gt;&#xA;&lt;p&gt;But I couldn't help wanting to at least consider if this event could have some non-rocket-related explanation. So I thought I would hypothetically ask out of morbid curiosity, could this event have been caused by some malicious human intent. How vulnerable are space launch vehicles to wackos?&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.discovery.com/tv-shows/mythbusters/mythbusters-database/shooting-gas-tank/&quot; rel=&quot;noreferrer&quot;&gt;Mythbusters showed&lt;/a&gt; that you can't just shoot a car's gasoline tank and make a Hollywood-friendly event:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Explanation: In Hollywood, it seems like all you need to blow a car to smithereens is a gun and a gas tank. A garage-full of flicks depict people destroying automobiles by shooting straight into to the tank, triggering a massive explosion.&lt;/p&gt;&#xA;&lt;p&gt;But when MythBuster Jamie Hyneman bored into a Cadillac's full gas tank with a barrage of bullets, no such detonation went down. His six slugs simply ripped right through the tank and out the other side of the otherwise intact sedan.&lt;/p&gt;&#xA;&lt;p&gt;With no flames to be found, Jamie's gas tank target practice busted the cinematic motor myth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Citing an entertainment TV show like Mythbusters for a source of factual engineering information is a little painful in itself, however in this case it looks like fairly reliable results.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit 1:&lt;/strong&gt; &lt;em&gt;A rocket filled with fuel and oxidizer and various other pressurized gasses is not identical to a car (hat tip to @PeterCordes for pointing that out for us) and cars can not be extrapolated to rockets. So I have asked:&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;Could a few high-velocity rounds have released enough RP-1 and LOX to have initiated the chain of events seen in today's &lt;a href=&quot;https://twitter.com/elonmusk/status/771479910778966016?ref_src=twsrc%5Etfw&quot; rel=&quot;noreferrer&quot;&gt;fast fire (not actually an explosion)&lt;/a&gt;? &lt;em&gt;I am not looking for speculation, or opinions of the likelihood.&lt;/em&gt; From an engineering perspective, could a few shots from a high power rifle have punctured the spacecraft and tanks and lead to the &lt;a href=&quot;http://www.nytimes.com/2016/09/02/science/spacex-rocket-explosion.html?_r=0&quot; rel=&quot;noreferrer&quot;&gt;series of events seen today&lt;/a&gt;?&lt;/p&gt;&#xA;&lt;p&gt;I think the issue of &lt;em&gt;the likelihood that this could actually happen&lt;/em&gt; would make a good parallel question, as I am sure there are several precautions in place.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit 2:&lt;/strong&gt; &lt;a href=&quot;https://twitter.com/elonmusk/status/774153847371501569&quot; rel=&quot;noreferrer&quot;&gt;Tweet from ElonMusk&lt;/a&gt; - there are continuing developments &lt;a href=&quot;http://www.space.com/34029-elon-musk-seeks-help-solving-rocket-explosion.html&quot; rel=&quot;noreferrer&quot;&gt;and discussions&lt;/a&gt;, don't take this tweet out of context:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Particularly trying to understand the quieter bang sound a few seconds before the fireball goes off. May come from rocket or something else.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit 3:&lt;/strong&gt; and there's &lt;a href=&quot;http://www.wired.co.uk/article/quadcopter-drone-fires-gun&quot; rel=&quot;noreferrer&quot;&gt;this stuff&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit 4:&lt;/strong&gt; Snippet from the &lt;a href=&quot;https://www.washingtonpost.com/business/economy/implication-of-sabotage-adds-intrigue-to-spacex-investigation/2016/09/30/5bb60514-874c-11e6-a3ef-f35afb41797f_story.html&quot; rel=&quot;noreferrer&quot;&gt;Washington Post&lt;/a&gt; - please read the full article there.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;...As part of the investigation, SpaceX officials had come across something suspicious they wanted to check out, according to three industry officials with knowledge of the episode. SpaceX had still images from video that appeared to show an odd shadow, then a white spot on the roof of a nearby building leased by ULA, a joint venture between Lockheed Martin and Boeing.&lt;/p&gt;&#xA;&lt;p&gt;The SpaceX representative explained to the ULA officials on site that it was trying to run down all possible leads in what was a cordial, not accusatory, encounter, according to the industry sources, who spoke on the condition of anonymity because of the ongoing investigation.&lt;/p&gt;&#xA;&lt;p&gt;The building, which had been used to refurbish rocket motors known as the SMARF, is just more than a mile away from the launchpad and has a clear line of sight to it...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;They quote Musk as saying:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;“We’ve eliminated all of the obvious possibilities for what occurred there,” he said. “So what remains are the less probable answers.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Which almost sounds like the engineer's version of &amp;quot;&lt;a href=&quot;http://www.bestofsherlock.com/top-10-sherlock-quotes.htm#impossible&quot; rel=&quot;noreferrer&quot;&gt;...when you have eliminated the impossible, whatever remains, however improbable, must be the truth...&lt;/a&gt;&amp;quot;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-04-16T06:01:01.543" Title="How vulnerable could space launch vehicles be to a &quot;lone gunman&quot;?" Tags="|spacex|failure|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18024" PostTypeId="1" CreationDate="2016-09-02T04:31:49.080" Score="11" ViewCount="811" Body="&lt;p&gt;Following the &lt;a href=&quot;https://www.youtube.com/watch?v=_BgJEXQkjNQ&amp;amp;feature=youtu.be&amp;amp;t=1m8s&quot;&gt;SpaceX disaster&lt;/a&gt; there is some obvious damage to the pad/tower. How long will it take to repair damage like that, and what kind of process/inspection is required before launches are allowed from that pad?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2016-09-02T18:56:25.720" Title="How long to repair the pad/tower after the SpaceX disaster?" Tags="|spacex|launch-site|launchpad|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18025" PostTypeId="2" ParentId="18020" CreationDate="2016-09-02T05:14:55.003" Score="11" Body="&lt;p&gt;The Transporter/Erector can only lift a complete first and second stage. That is not to say it requires the payload, but to say it requires the second stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking your question to next logical step, why do a static fire of the first stage engine with a $20 million second stage on board?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, they need it so the T/E can grab it and hold the complete stack. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A better question might be, for a Static Fire, why were they fueling the second stage at all?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having said all that, there is an additional reason to test fire with the second stage, fueled, and with the payload attached is that the vehicle behaves different fully configured than in pieces, and this is a static test of the entire vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/50rr9v/falcon_9_amos6_static_fire_anomaly_faq_summary/&quot; rel=&quot;nofollow&quot;&gt;Reddit&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;According to Peter B. de Selding on Twitter, SpaceX implemented an optional policy of attaching the payload to the vehicle earlier this year with some “insurer upset”, as it saves about a day during of launch preparations, and allows SpaceX to monitor the payload’s interaction with the vehicle. The customer can decline this option if they wish.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6816" LastEditDate="2016-09-03T21:10:09.417" LastActivityDate="2016-09-03T21:10:09.417" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18026" PostTypeId="2" ParentId="18024" CreationDate="2016-09-02T05:18:12.533" Score="5" Body="&lt;p&gt;As always it takes time to discover what exactly was damaged, and then figure out how to repair it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A report from a firefighter who was on scene, but not in any way a representative of anyone other than himself noted on &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/50rr9v/falcon_9_amos6_static_fire_anomaly_faq_summary/d76rt4x&quot; rel=&quot;nofollow&quot;&gt;Reddit&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Quite. Granted, I'm not in construction or an engineer but it seems like it will be months before the pad is usable. Dozens of pressurized vessels and tanks were destroyed including 5-6 pressurized rail cars. The gantry itself, while still standing appears to be a total loss, as may be a lightening arrest tower at the corner of the pad. Several buildings located on or near the pad are either destroyed or severely damaged. There's no power at the moment and I can tell you from first-hand experience that the water mains and hydrants are compromised. Debris made it as far as pad 39A, which is quite a distance. They'll be finding pieces of it in the surrounding woods and beach line for years, just as they've found pieces of 1997's Delta II mishap as recently as a year or two ago.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, we can make some general comments.  SpaceX is much more pad frugal than NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider the RSS (Rotating Service Structure) still left at LC-39A that SpaceX plans on removing. Consider the MLP (Mobile Launch Platform) that Saturn V used. Consider the crawlers used by Saturn and Shuttle launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is some impressive amounts of infrastructure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX's TEL is a set of steel girders on rail bogies that they bought used, with hydraulic cylinders to push it upright.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of stuff to be damaged is much more minimal at a SpaceX pad than at a typical NASA pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So maybe a better comparison would be the MARS (Mid-Atlantic Regional SpacePort) and how long it took to recover from an Antares explosion there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That took from October 29 2014 (date of the crash) to September 30 2015, so about a year. Although repairs were slowed down because they weren't entirely funded from the beginning.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-09-02T18:56:25.720" LastActivityDate="2016-09-02T18:56:25.720" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18027" PostTypeId="2" ParentId="2810" CreationDate="2016-09-02T05:44:05.800" Score="8" Body="&lt;p&gt;First off, here's a picture (source: NASA) of the inside of the MIR node with something connected in every direction:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/File:Mir_node_interior_STS-84,_2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rD38R.jpg&quot; alt=&quot;MIR Docking Module&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also a video &lt;a href=&quot;https://youtu.be/vBONpY3TxN8?t=55s&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; where the node is traversed, but I can't find any pictures from before all the ports were occupied.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not particularly spacious, but in the picture only one cone-shaped drogue hatch cover is clearly visible, as well as one cover near the camera (can't tell its geometry) and what looks like a flat cover through the far hatch. The specs of the base module (from &lt;a href=&quot;https://ston.jsc.nasa.gov/collections/TRS/_techrep/RP1357.pdf&quot; rel=&quot;noreferrer&quot;&gt;Mir Heritage Hardware&lt;/a&gt;, page 104) that specifies 2 docking and 4 berthing ports (docking were on the ends of the module, berthing were on the sides of the node, difference being that berthing ports didn't have an automatic docking system). All ports appear to use the &lt;a href=&quot;https://en.wikipedia.org/wiki/SSVP_docking_system&quot; rel=&quot;noreferrer&quot;&gt;SSVP docking system&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Procedure for connecting a module to a berthing port (also from Mir Heritage Hardware) involved docking the module to the axial port of the node, then using a manipulator arm to move it to a berthing port. I don't see any indication that a different docking mechanism was used, meaning that each berthed node would have needed a drogue port to connect to. I can't currently find a source, but I believe that once connected the probe/drogue could be removed to free up space around the transfer tunnel (and the pictures/video are consistent with this).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, comparing the size of the drogue port cover pictured and the size of the node the drogue cover would take up interior volume, but it would also be on a face of the node that isn't in use. I don't think it would intrude far enough into the node to be much of an obstruction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To directly answer the question: No, it doesn't appear that the docking ports consume much interior space. I think the real reason they didn't include more docking/berthing ports in Zarya/Zvezda is a combination of the close proximity of solar panels (&lt;a href=&quot;https://en.wikipedia.org/wiki/File:Zarya_from_STS-88.jpg&quot; rel=&quot;noreferrer&quot;&gt;zarya&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/File:View_of_the_bottom_of_Zvezda.jpg&quot; rel=&quot;noreferrer&quot;&gt;zvezda&lt;/a&gt; pictures) on the sides and a desire not to add complexity or holes in the space station that weren't expected to be used.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2016-09-02T05:44:05.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18028" PostTypeId="2" ParentId="18023" CreationDate="2016-09-02T07:30:02.573" Score="10" Body="&lt;p&gt;You'd have to get very lucky. Bullets would punch small holes in the RP-1 and LOX tanks and you'd have small streams of both flowing out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the bullet were to hit one of the high-pressure tanks, however (like the helium tank inside one of the first stage propellant tanks), the tank could rupture and release its entire contents at once. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there was a rifle shot, I expect it would have been clearly audible in the video of the explosion. Apparently there was a sound just before the explosion (which I didn't hear on a casual review of the video), but it's not been definitively identified. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-10-03T19:04:29.037" LastActivityDate="2016-10-03T19:04:29.037" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18030" PostTypeId="1" AcceptedAnswerId="18031" CreationDate="2016-09-02T13:44:49.187" Score="5" ViewCount="881" Body="&lt;p&gt;I'm writing an essay titled the Physics of a Shuttle Mission, which is broken down into the journey into orbit, orbital maneuvering and re-entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest problem at the moment is with example calculations. For example the delta v required for OMS-1 and OMS-2. This is because it seems impossible to find info on the Shuttle's velocities and altitudes beyond about T plus 8min. And I believe that OMS-1 occurs at around T plus 10min 30 sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone have any specific numerical data on OMS-1 and OMS-2? Or could point me to a place that has this information.&lt;/p&gt;&#xA;" OwnerUserId="16860" LastActivityDate="2021-03-15T14:45:07.413" Title="Space Shuttle OMS maneuvers" Tags="|space-shuttle|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18031" PostTypeId="2" ParentId="18030" CreationDate="2016-09-02T14:26:00.743" Score="6" Body="&lt;p&gt;The Delta-V for most OMS 1 &amp;amp; 2 burns performed during the Space Shuttle program is available in the ever-useful &lt;a href=&quot;https://ntrs.nasa.gov/api/citations/20110001406/downloads/20110001406.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Missions Summary&lt;/a&gt; document in the LAUNCH SITE LIFTOFF TIME column.&lt;/p&gt;&#xA;&lt;p&gt;For example, here is the data for STS-135. Planned and actual.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rQxui.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rQxui.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Edit: just noticed you asked about altitude as well.  The altitude at OMS-2 burn isn't explicitly called out in that document, but it does give the orbital altitudes for post OMS-2.  Since OMS-2 was a perigee raising burn, and was performed at apogee, you can use the apogee number for &amp;quot;post OMS 2&amp;quot; as a close approximation.  For STS-135 this was 124.3 nm.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-03-15T14:45:07.413" LastActivityDate="2021-03-15T14:45:07.413" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18032" PostTypeId="1" AcceptedAnswerId="18033" CreationDate="2016-09-02T14:53:23.727" Score="4" ViewCount="1796" Body="&lt;p&gt;Continuing the series of questions (&lt;a href=&quot;https://space.stackexchange.com/questions/18011/was-amos-6-insured-for-pre-launch-operations&quot;&gt;part 1&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/questions/18020/why-was-amos-6-mounted-on-the-rocket-for-a-test-fire&quot;&gt;part 2&lt;/a&gt;) inspired by the recent &lt;a href=&quot;http://spacenews.com/developing-explosion-rocks-spacex-falcon-9-pad-at-cape-canaveral/&quot; rel=&quot;nofollow noreferrer&quot;&gt;loss of the AMOS-6 satellite during a static test fire&lt;/a&gt;, here's one thing that puzzles me a bit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What kinds of problems is a static test fire (with the payload in place) supposed to avoid?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would assume, perhaps naïvely, that in order for a static test fire before launch to make sense, there would have to be a reasonably large class of failure types that are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;detectable in a static test fire,&lt;/li&gt;&#xA;&lt;li&gt;not reliably detectable &lt;em&gt;without&lt;/em&gt; the test fire,&lt;/li&gt;&#xA;&lt;li&gt;serious enough to threaten mission success if only detected after launch, and&lt;/li&gt;&#xA;&lt;li&gt;unlikely to result in &quot;rapid unscheduled disassembly&quot; and total loss of the vehicle and payload if they occur during the test fire.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Given that e.g. SpaceX does run a static test fire before launch, and that they're not stupid, I assume that there must be enough of these kinds of problems for the test fire to make sense.  But as a layperson with only a casual interest in rocketry, I'm having a hard time figuring out what they are.  Or perhaps there are some other reasons for the test fire?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, if the test fire was conducted &lt;em&gt;without&lt;/em&gt; the payload, I could see it making sense; in the event of catastrophic failure during the test fire, you'd only lose the really expensive launcher, but not the also really expensive payload.  But with the payload in place, RUD during the test fire is hardly a better outcome than RUD during launch.  I would also expect there to be at least some &quot;bad luck&quot; failure modes that occur only rarely, and are difficult to detect before they do occur (meaning that a test fire before launch effectively just doubles the risk of those failure modes).  Given that a test fire isn't free, either, surely there must be some major advantages to it that outweigh these costs and risks.&lt;/p&gt;&#xA;" OwnerUserId="189" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-09-02T15:02:47.693" Title="What is the advantage of doing a static test fire before launch?" Tags="|spacex|failure|testing|launch-sequence|risk-management|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18033" PostTypeId="2" ParentId="18032" CreationDate="2016-09-02T15:02:47.693" Score="3" Body="&lt;p&gt;Test fires have many purposes. The engines themselves were individually fired at the McGregor, TX facility, then shipped back to Hawthorne, CA to be mated to a stage. That entire stage was shipped to McGregor, TX and test fired as a stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then it was shipped to CCAFS, Florida or Vandenburg to be launched. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Product may settle in shipping?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things can happen. One more test on the pad, ensures everything is go for launch. I read somewhere that Jonathan Mcdowell of Jonathons Space Report fame looked it up and the last RUD during a test fire was in the 1960's. So this is not a common outcome. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the static fire does test that the engines are all good to go after travelling across the country the slow way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it also tests the Ground Support Equipment (GSE) which seems like the likely culprit at the moment (that could change) and we saw during the first several F9 Full Thrust launch campaigns that the GSE was a problem.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-09-02T15:02:47.693" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18034" PostTypeId="1" CreationDate="2016-09-02T15:04:21.203" Score="13" ViewCount="447" Body="&lt;p&gt;Or more specifically, had a Dragon V2 (uncrewed) been atop the AMOS-6 mission stage that blew up during propellant loading before a test fire would the Dragon abort system been armed and thus been able to abort away, unmanned as it would have been? &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2020-01-21T08:56:00.797" Title="When is the Dragon V2 abort system armed?" Tags="|spacex|falcon-9|dragon-v2|abort|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18035" PostTypeId="2" ParentId="18023" CreationDate="2016-09-02T15:07:15.073" Score="8" Body="&lt;p&gt;Max effective range of a .50 caliber rifle is on the order of 2000 yards (~1.8 km).  I'm trying to find a reliable number for the minimum safe distance from the F9 in the event of a kaboom, but I'm pretty sure that's well short of it (I want to say it's on the order of 3 km, but again, trying to find a source).  The shooter would risk severe injury or death.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2016-09-02T15:07:15.073" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="18036" PostTypeId="2" ParentId="18034" CreationDate="2016-09-02T15:11:36.870" Score="15" Body="&lt;p&gt;Now we've had the Crew Dragon In Flight Abort test, this can be answered more definitively.  I've not found a stream where it's absolutely crystal clear, but in the &lt;a href=&quot;https://www.youtube.com/watch?v=JmTVn_pKeTQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASASpaceFlight&lt;/a&gt; coverage, you can hear the countdown net in the background - sometimes being spoken over by Chris G.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A timeline of critical events:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;~&lt;a href=&quot;https://www.youtube.com/watch?v=JmTVn_pKeTQ&amp;amp;t=59m45s&quot; rel=&quot;nofollow noreferrer&quot;&gt;T-45 minutes&lt;/a&gt;, the Crew Access Arm is retracted&lt;/li&gt;&#xA;&lt;li&gt;~&lt;a href=&quot;https://www.youtube.com/watch?v=JmTVn_pKeTQ&amp;amp;t=1h4m33s&quot; rel=&quot;nofollow noreferrer&quot;&gt;T-40 minutes&lt;/a&gt; you can just about hear over the audio net in the background:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On countdown(?) Launch Escape System is armed&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;~&lt;a href=&quot;https://www.youtube.com/watch?v=JmTVn_pKeTQ&amp;amp;t=1h9m35s&quot; rel=&quot;nofollow noreferrer&quot;&gt;T-35 minutes&lt;/a&gt;, prop loading begins&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So yes, the abort system is armed before prop loading, and after the access arm is out of the way for crew missions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Original answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since SpaceX are as private as they are, there's probably not going to be any specific public information on when the system is armed until it's included on a launch timeline, or announced during a launch sequence etc.  That being said, we do know three things:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Static Fires are designed to test all the details in the run up to, but not including, the launch itself&lt;/li&gt;&#xA;&lt;li&gt;Musk was asked this question on Twitter, &lt;a href=&quot;https://twitter.com/elonmusk/status/771479910778966016&quot; rel=&quot;nofollow noreferrer&quot;&gt;and replied&lt;/a&gt;:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This seems instant from a human perspective, but it really a fast fire, not an explosion. Dragon would have been fine.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;After the CRS-7 failure, they looked at the fact the Dragon survived the initial breakup, and &lt;a href=&quot;http://shitelonsays.com/transcript/elon-musk-talks-failed-crs-7-dragon-mission-2015-07-20&quot; rel=&quot;nofollow noreferrer&quot;&gt;would make software modifications&lt;/a&gt;:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In fact, if the software had initiated the parachute deployment, then the Dragon spacecraft, we believe, would have survived. And for future missions, even for the cargo version of the Dragon spacecraft, we're now including contingency software that, if something were to go wrong with the vehicle, Dragon will always attempt to save itself.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So without explicit SpaceX confirmation, it can only be speculation, but all in all, I think it's safe to assume that any and all contingency systems (including Dragon abort) will be in place through all stages of both the static fire and launch, and the Dragon V2 would have a good chance of surviving an incident like this in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not particularly scientific, but there are also numerous videos of last year's pad abort test overlaid on top of the Sept 1st failure, &lt;a href=&quot;https://gfycat.com/ThankfulGoodBadger&quot; rel=&quot;nofollow noreferrer&quot;&gt;such as this one&lt;/a&gt;, indicating that if the trigger for the abort is fast enough, the Dragon should easily outrun the explosion.&lt;/p&gt;&#xA;" OwnerUserId="10686" LastEditorUserId="10686" LastEditDate="2020-01-21T08:56:00.797" LastActivityDate="2020-01-21T08:56:00.797" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18037" PostTypeId="1" AcceptedAnswerId="18039" CreationDate="2016-09-02T17:15:46.303" Score="10" ViewCount="775" Body="&lt;p&gt;Deep Space Network sites in California, Madrid and Canberra perform an excellent job, for example receiving from Voyager 2 (16+ billion km) at -150 dBm power.  However, would it be more advantageous to operate sites in earth orbit ?&lt;/p&gt;&#xA;" OwnerUserId="16863" LastActivityDate="2016-09-02T20:10:05.027" Title="Would an orbital Deep Space Network be more efficient?" Tags="|atmosphere|radio-communication|deep-space-network|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18038" PostTypeId="2" ParentId="18037" CreationDate="2016-09-02T17:30:43.350" Score="6" Body="&lt;p&gt;I would say that no, an orbital DSN would not be more advantageous.  The main problem with receiving transmissions is due to the distance they are being sent from.  The DSN antennas have to be very sensitive because the transmission power falls off with the square of the distance.  Putting the antennas in space would not make them more sensitive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we wanted to place these receivers in space, they would still have to be incredibly large.  It would be very difficult to build and launch such large satellites.  Once they were in orbit, it would also be very difficult to service or upgrade them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, the whole reason we have the DSN is to maintain communication with space probes no matter which direction the Earth turns.  If we used satellites to receive these messages, they would then have to be transferred back down to the Earth.  So we would just have another link in the system.  It's easier just to have them transferred down to Earth directly.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-09-02T17:30:43.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18039" PostTypeId="2" ParentId="18037" CreationDate="2016-09-02T17:50:06.403" Score="13" Body="&lt;p&gt;Let's think about what it would take to do what you're suggesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The DSN uses some pretty large antennas, so you're talking about putting a 70 meter dish like this (and a power supply to run it) in space:&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network#/media/File:Goldstone_DSN_antenna.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ab2dO.jpg&quot; alt=&quot;70m antenna at Goldstone&quot;&gt;&lt;/a&gt;&#xA;(Source: NASA)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could put it in LEO (because that's much closer and cheaper), but then you have to deal with the Earth occluding it frequently. You could put it at the Earth-Moon L2 point (much more expensive), where the moon blocks signals coming from the Earth, as mentioned later, but then you have have to relay all communication to it because the moon is in the way, adding a lot of complexity to the system, especially since you'll want more than one antenna for the purpose of supporting multiple missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you've got it there, you also have to deal with a more noisy environment (radio spectrum-wise) than on the ground. From &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;noreferrer&quot;&gt;wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each facility is situated in semi-mountainous, bowl-shaped terrain to help shield against radio frequency interference.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Where in space you're getting bombarded by a broad spectrum of energy from the sun, as well as having line of sight to an awful lot of the surface of the Earth, some of which will have transmitters pointed up, particularly your own communication system, because what good is a communication system in space that can't talk to the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, when you look into it there are frequencies at which the atmosphere is transparent, so being outside the atmosphere doesn't make a big difference given the right frequencies used:&#xA;&lt;a href=&quot;http://www.spaceacademy.net.au/spacelink/radiospace.htm&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/G7Cil.gif&quot; alt=&quot;Atmospheric windows in the electromagnetic spectrum&quot;&gt;&lt;/a&gt;&#xA;(Source: Australian Space Academy)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long story short, no: you'd spend a lot of money building and launching huge radio antennas into orbit, and once there (where they're much harder to maintain and particularly vulnerable to collisions with space debris due to their size) the environment isn't actually more conducive to communicating with distant satellites anyway.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-09-02T17:56:49.447" LastActivityDate="2016-09-02T17:56:49.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18040" PostTypeId="1" CreationDate="2016-09-02T18:44:30.430" Score="3" ViewCount="458" Body="&lt;p&gt;The AMOS-6 mission Falcon 9 had a RUD (Rapid Unscheduled Disassembly) event on the pad, and it is initially postulated that it was related to fueling the second stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know that SpaceX revamped the GSE (Ground Support Equipment) for the Falcon 9 1.1 (since the size changed and connectors had to move) and for the Falcon 9 Full Thrust (Which requires super cooled LOX and RP1). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know they are working to open LC-39A to this standard, and Vandenburg should be ready to go on this standard, since the first launch after the the West Coast range was reopened was scheduled for two weeks after the AMOS launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally the McGregor, TX test facility requires similar GSE to allow for testing of the various F9 versions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus SpaceX has now built the GSE for:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;McGregor, TX - Falcon 9 1.0 (Old test stand)&lt;/li&gt;&#xA;&lt;li&gt;McGregor, TX - Falcon 9 1.1 (New test stand)&lt;/li&gt;&#xA;&lt;li&gt;McGregor, TX - Falcon 9 Full Thrust (Updated to new test stand)&lt;/li&gt;&#xA;&lt;li&gt;LC-40, CCAFS, FL - Falcon 9 1.0&lt;/li&gt;&#xA;&lt;li&gt;LC-40, CCAFS, FL - Falcon 9 1.1&lt;/li&gt;&#xA;&lt;li&gt;LC-40, CCAFS, FL - Falcon 9 Full Thrust&lt;/li&gt;&#xA;&lt;li&gt;LC-39A, CCAFS, FL - Falcon 9 Full Thrust&lt;/li&gt;&#xA;&lt;li&gt;SLC-4W, VAFB, CA - Falcon 9 1.0&lt;/li&gt;&#xA;&lt;li&gt;SLC-4W, VAFB, CA - Falcon 9 1.1&lt;/li&gt;&#xA;&lt;li&gt;SLC-4W, VAFB, CA - Falcon 9 Full Thrust&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;That is a lot of iterations on GSE to build, and still have a major fault hidden within it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, each of these systems, except maybe the SLC-4W pads, have had LOTS of vehicles run through.  McGregor has tested all stages for Falcon 9 that have ever flown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LC-40 has had all but 2 of the 29 launched stages run through it. (Two launched from VAFB).  The initial Falcon 9 Full Thrust missions had a number of launch attempts where they failed to launch so they fueled, defueled, and fueled again before finally launching.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each Falcon 9 Full Thrust also did a successful hotfire test, which required fueling and defueling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means the GSE has done a very large number of such cycles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus it seems very interesting as what the root cause might be here, if it was GSE related.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now it may be that we have to wait for the official SpaceX answer to this, before we can get a real answer, but I thought it would be worth phrasing the question to get it out there.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="9060" LastEditDate="2016-09-02T19:45:59.440" LastActivityDate="2016-09-02T19:45:59.440" Title="What could be wrong in the ground support equipment to cause the Falcon 9 AMOS on pad explosion?" Tags="|spacex|falcon-9|fuel|" AnswerCount="0" CommentCount="1" ClosedDate="2016-09-02T21:11:22.263" ContentLicense="CC BY-SA 3.0" />
  <row Id="18041" PostTypeId="2" ParentId="17997" CreationDate="2016-09-02T18:46:26.897" Score="0" Body="&lt;p&gt;Since enforcement of any organization's particular protocol on anyone else is next to impossible, there is a good chance that an organization involved in this area of research has its own public as well as a private version of  protocol for disclosure. But anyone's path to disclosure probably would depend heavily on the content of the message, it's intent, the requests the message  made, its perceived impact on society, its perceived impact on religion, etc., and no doubt depends heavily on whether the organization is a military, governmental, scientific or profit-oriented one. &lt;/p&gt;&#xA;" OwnerUserId="1493" LastEditorUserId="1493" LastEditDate="2016-09-02T18:54:10.703" LastActivityDate="2016-09-02T18:54:10.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18045" PostTypeId="1" CreationDate="2016-09-02T21:28:21.660" Score="5" ViewCount="837" Body="&lt;p&gt;Since SpaceX has isolated the AMOS-6 Falcon 9 pad explosion to the upper stage LOX tank during fueling, it seems useful to consider in general terms what was happening at the time and what materials and equipment were around. Rocket explosions usually involve firing engines and/or structural failures in flight, and I have little sense how this explosion was possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current SpaceX statement is thus:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At approximately 9:07 am ET, during a standard pre-launch static fire&#xA;  test for the AMOS-6 mission, there was an anomaly at SpaceX's Cape&#xA;  Canaveral Space Launch Complex 40 resulting in loss of the vehicle.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The anomaly originated around the upper stage oxygen tank and occurred&#xA;  during propellant loading of the vehicle. Per standard operating&#xA;  procedure, all personnel were clear of the pad there there were no&#xA;  injuries.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have read that LOX makes most anything extremely explosive and it takes very little to trigger that. How little does it take? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a spark or some other trigger have been necessary, and if so, what sources of such a thing were nearby? Would it have been easy for some source of ignition to have ended up there from the environment?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a leak of the equipment have been necessary for this to occur?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="23891" LastEditDate="2019-03-31T05:48:05.733" LastActivityDate="2023-03-21T14:37:40.270" Title="How can the elements involved in loading a LOX tank be involved in an explosion?" Tags="|spacex|failure|launch-sequence|lox|rocket-explosion|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18047" PostTypeId="1" CreationDate="2016-09-02T22:44:33.563" Score="8" ViewCount="460" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_(rocket_family)&quot;&gt;Soyuz&lt;/a&gt; family has many variants. Do any of them require different launchpad infrastructure? If multiple active variants require different infrastructure, how are the logistics of that handled? Do they use different pads?&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="58" LastEditDate="2017-02-27T20:29:51.147" LastActivityDate="2017-02-27T22:22:57.530" Title="Do different generations of Soyuz use different launchpad infrastructure?" Tags="|launch|rockets|launch-site|launchpad|soyuz-launcher|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18049" PostTypeId="1" CreationDate="2016-09-03T01:12:15.153" Score="3" ViewCount="441" Body="&lt;p&gt;I can't connect to the Tass news item directly &lt;a href=&quot;http://tass.com/science/896683&quot; rel=&quot;nofollow&quot;&gt;http://tass.com/science/896683&lt;/a&gt; but on a &lt;a href=&quot;http://www.seti.org/seti-institute/a-seti-signal&quot; rel=&quot;nofollow&quot;&gt;SETI page&lt;/a&gt; an item dated August 31, 2016 says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An article that quotes the Russian news agency TASS is suggesting that the signal found using the Russian RATAN-600 radio telescope is, indeed, terrestrial interference. The article, which can be found here – &lt;a href=&quot;http://tass.com/science/896683&quot; rel=&quot;nofollow&quot;&gt;http://tass.com/science/896683&lt;/a&gt; -- says in part “Director of the Institute of Applied Astronomy at the Russian Academy of Sciences Alexander Ipatov told TASS that back at the Soviet period he had been part of a group of young astronomers at the special astrophysical observatory searching for signals from extraterrestrial civilizations. ‘We, indeed, discovered an unusual signal. However, an additional check showed that it was emanating from a Soviet military satellite, which had not been entered into any of the catalogs of celestial bodies,’ Ipatov said.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Are there really &quot;Soviet&quot; (as opposed to Russian) military satellites that are not in &quot;any of the catalogs of celestial bodies&quot;? For example, does the NORAD generated TLE database at space-track.org leave out satellites belonging to other countries' military as a courtesy for some reason?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-04-01T09:10:48.297" LastActivityDate="2022-12-05T12:13:02.390" Title="Was it a &quot;Soviet military satellite not in any of the catalogs?&quot;" Tags="|artificial-satellite|radio-communication|military|seti|radio-astronomy|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18050" PostTypeId="2" ParentId="5423" CreationDate="2016-09-03T03:49:00.453" Score="-5" Body="&lt;p&gt;I'm pretty sure many of the answers above are at least partially off.  You don't need 4 GPS satellites for a solution, you need 3.  As stated above, 1 gets you a sphere, 2 a circle, and 3 one or most likely two points on the circle.  One of those points is far in space or inside the Earth.  The GPS receiver uses the mean surface distance of the Earth, the geode, to determine the correct (closer) one.  Whichever is the closest to the geode wins.  More than 3 satellites just allow better reduction of errors for timing and atmospheric issues, system faults, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of time, I'm not 100% sure, but I would imagine that the actual time of day is not of great importance, but just listening to ephemeris data should get sub-second accuracy for that.  To get the receiver to have near-atomic accuracy I believe the system just makes an initial guess based on the data it has (perhaps an average of all the times it has heard from the satellites + a few MS for the mean distance of receivers to sats?  Just speculation there) and then computers the error in the position.  If the clock is fast, the error is big in one direction; slow produces a large error in the opposite direction.  The receiver's clock is continually sped up or slowed down via a type of Phase-Locked Loop (PLL) until the error is as close to zero as possible; this continues for the entire time the device is running.  If the clock starts to drift again, the PLL will catch it and push it back on track.  By doing this, the clock is kept to nearly the same accuracy as those on the satellite, and thus accurate enough for navigation purposes or other precision timing purposes (NTP, radio transmission synchronization, etc).  I suspect that's also (at least partly) why you see a larger error when you first turn on a GPS receiver that gets progressively smaller as the clock gets progressively more in sync.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PLL's have been around for a long time and are used everywhere.  A VCXO is commonly used with a PLL to provide a cheap, variable, high frequency source based on the output of a lower speed, high quality fixed oscillator in radios and other communications gear, along with computers and other electronics.  Hence where clock multiplier comes from with computing, a slow speed, fixed XO drives a faster bus via PLL to get your FSB speed, your CPU then using another multiplier and PLL for its own speed.   PLL's also have been used for clock synchronization and recovery for communications for a very long time in communications networks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has a decent article on it.  It provides an analogy of cars racing around a track, each one under their own control trying to obtain the fastest safe/practical speed.  When a pace car comes out, they all must get in line and follow the car in front of them, without passing.  The pace car drives a fixed speed based on their speedometer.  Each driver behind it tries to keep a similar distance to it or the car in front of them; they speed up to close gaps or slow down to lengthen them.  Thus they all end up nearly evenly spaced and traveling at almost exactly the same speed.  Remove the reference (pace car), and they're all off at random again.&lt;/p&gt;&#xA;" OwnerUserId="16867" LastActivityDate="2016-09-03T03:49:00.453" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18051" PostTypeId="1" AcceptedAnswerId="18052" CreationDate="2016-09-03T06:22:11.920" Score="1" ViewCount="418" Body="&lt;p&gt;I was reading about &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow&quot;&gt;gravity assist&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This maneuver is sometimes called a gravitational slingshot. But to me it seems more like a sling (like the one the Biblical David used against Goliath) The maneuver makes use of the movement of the planet (like a sling makes use of the movement of the hand) and the orbit is curved, like that of a sling. I don't immediately think of a slingshot when I read about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A slingshot (Y-bow) is stationary. A gravitational &quot;slingshot&quot; would never work if the planet was stationary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, it is a detail, but technical terms are often very precise. Why is this term used? (Or did I miss something and it is not misleading?) (I am interested in the exchange of energy between bodies so my question is not only linguistic.)&lt;/p&gt;&#xA;" OwnerUserId="16871" LastEditorUserId="415" LastEditDate="2016-09-03T09:54:48.447" LastActivityDate="2016-09-03T09:54:48.447" Title="Why use the term &quot;gravitational slingshot&quot;?" Tags="|terminology|gravity-assist|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18052" PostTypeId="2" ParentId="18051" CreationDate="2016-09-03T07:48:39.723" Score="3" Body="&lt;p&gt;The term is ambiguous. A slingshot is also a shot from a sling. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-09-03T07:48:39.723" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18053" PostTypeId="2" ParentId="18051" CreationDate="2016-09-03T07:57:10.577" Score="6" Body="&lt;p&gt;It's a poetic term which everyone understands immediately. You're trying to treat this as a &quot;technical term&quot; when it clearly isn't. It's a descriptive term - as evidenced by the existence of less-poetic terms like &quot;gravity assist&quot;. Einstein said, &quot;Everything should be made as simple as possible, but no simpler.&quot; - the term is immediately understood, it conveys what it's meant to, it causes no confusion - so why complicate the matter? It serves it's purpose. Nothing else matters.&lt;/p&gt;&#xA;" OwnerUserId="16853" LastActivityDate="2016-09-03T07:57:10.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18054" PostTypeId="2" ParentId="12350" CreationDate="2016-09-03T10:39:53.513" Score="-3" Body="&lt;p&gt;It is due to orbital period time of earth and ISS.&#xA;Earth orbits it self each 24 hour however ISS orbits around the earth each 1.5 hour approximately. &#xA;It would be possible to see ISS once each 12 hour  if earth and ISS were at same orbital period.&#xA;However now coinsident of two phenomena is needed for visibility. This coinsident is  is not in a second. It is in several hours. &lt;/p&gt;&#xA;" OwnerUserId="16873" LastActivityDate="2016-09-03T10:39:53.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18056" PostTypeId="1" AcceptedAnswerId="18071" CreationDate="2016-09-03T13:57:39.867" Score="14" ViewCount="1535" Body="&lt;p&gt;How stable are &lt;a href=&quot;https://en.wikipedia.org/wiki/Graveyard_orbit&quot;&gt;graveyard orbits&lt;/a&gt;? How big of an issue is orbital decay?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the wikipedia article states, the graveyard orbit is higher than the synchronos orbit from which the satellites are disposed of. A lower graveyard orbit would make little sense, since you need to cross it when bringing new satellites into position. However, orbital decay might lead to dead satellites entering the &quot;good&quot; synchronous orbit again.&#xA;I guess this is accounted for, has somebody an estimate?&lt;/p&gt;&#xA;" OwnerUserId="16645" LastEditorUserId="16645" LastEditDate="2016-09-03T15:52:44.907" LastActivityDate="2016-09-05T09:02:45.833" Title="How stable are satellites in a graveyard orbit" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18057" PostTypeId="2" ParentId="3987" CreationDate="2016-09-03T17:13:02.540" Score="21" Body="&lt;p&gt;The From the annotated &lt;a href=&quot;http://history.nasa.gov/ap08fj/01launch_ascent.htm&quot; rel=&quot;noreferrer&quot;&gt;Apollo 8 Flight Journal&lt;/a&gt;, in the notes after the 2:36 mark, we see that the interstage separation happens well after the J-2 engines on the second stage are running:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If, at staging, the interstage were to stay with the S-IC, there is a danger that any slight rotation of the massive first stage would cause contact between it and the engine bells on the S-II. Therefore a cut is made directly above the S-IC using a shaped explosive charge. This leaves the interstage attached to the S-II. However, the ring imposes a significant mass penalty on the second stage. So much so that it is a mandatory abort if the interstage doesn't separate. It too is dropped 30 seconds after the first separation. This gives time for the second stage engines to establish smooth acceleration with minimal rotation. [The J-2 engines start less than a second after separation of the first stage.]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So at the time the interstage falls away, the J-2 engines are going full-thrust. Hydrogen-oxygen exhaust in vacuum is essentially clear -- it's mostly water vapor, so it's not obvious that they're running, but they are. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As the [interstage] ring enters the superhot exhaust gases, it glows furiously, presumably as its paint coating is vapourised. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Hydrogen rocket engines run fuel-rich, so there wouldn't be much free oxygen in the exhaust stream to burn things with. However, any paint, cabling, or other material on the inside of the interstage would heat up and outgas quickly, and presumably that gas mixture burns with a yellow flame.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-09-07T20:40:28.500" LastActivityDate="2016-09-07T20:40:28.500" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18058" PostTypeId="1" CreationDate="2016-09-03T20:14:33.717" Score="1" ViewCount="47" Body="&lt;p&gt;The payload for the Falcon 9, the Amos 6 satellite, was placed on top of the rocket during the static fire test. Why was this done?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-03T20:26:47.207" Title="Why was the payload placed on the rocket for a test?" Tags="|launch|spacex|falcon-9|testing|payload|" AnswerCount="0" CommentCount="2" ClosedDate="2016-09-03T21:28:46.863" ContentLicense="CC BY-SA 3.0" />
  <row Id="18061" PostTypeId="2" ParentId="18020" CreationDate="2016-09-03T21:32:58.927" Score="3" Body="&lt;p&gt;SpaceX performs a test fire of all their rockets at the pad, lasting for a few seconds. There is an option to place the satellite on top of the rocket, or to delay the launch by a single day. As the static fire test has been considered a low risk to the spacecraft, this has often been done, as a day can cost the commercial operators lots of money, and there has been low risk. Furthermore, it provides a realistic test of what the spacecraft will do on launch day. They can determine what the thermal environment will be like on the fairing, which might need to be adjusted for flight day. It also is a good chance for the satellite team to have a dress rehearsal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I understand it, there is a cap that is placed on the rocket if it doesn't have the payload. That cap needs to be removed, and the fairing attached, which takes a day.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-03T21:32:58.927" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18063" PostTypeId="1" CreationDate="2016-09-04T05:53:05.587" Score="8" ViewCount="270" Body="&lt;p&gt;Apollo, Soyuz and Orion have used, are using or will use launch escape towers. If it is not needed, it must separate during flight. What if it does not separate? I suppose that the spacecraft would drop its upper stage and land. But these spacecrafts use parachutes to land. Isn't the launch escape tower in their way? And is it safe to land with the added mass of the escape launch tower?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-09-04T16:55:31.087" Title="Can a crewed capsule land safely if its launch escape tower does not separate?" Tags="|soyuz-spacecraft|saturn-v|launch-escape-system|orion-spacecraft|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18064" PostTypeId="1" CreationDate="2016-09-04T07:05:54.110" Score="6" ViewCount="328" Body="&lt;p&gt;France launched its Diamant rocket to orbit between 1965 and 1975, and the UK its Black Arrow 1969-1972. Did they take advantage of NASA's rocket development experiences?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were any Apollo system components developed in Europe, like the service module for Orion today? I know that the still camera used on the Moon was a Hasselblad made in Sweden.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2019-07-11T01:19:19.993" Title="How did the USA cooperate with western allies during the Apollo program?" Tags="|apollo-program|spacecraft-development|international-politics|uk-space|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18065" PostTypeId="1" CreationDate="2016-09-04T08:41:04.300" Score="6" ViewCount="497" Body="&lt;p&gt;In the Wikipedia article for &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Molniya orbit&lt;/a&gt; the introduction mentions that the period should be 0.5 &lt;a href=&quot;https://en.wikipedia.org/wiki/Sidereal_time&quot; rel=&quot;nofollow noreferrer&quot;&gt;sidereal day&lt;/a&gt;, so that its path across the sky every ~12 hours as observed from a given point on Earth remains constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But later in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit#Properties&quot; rel=&quot;nofollow noreferrer&quot;&gt;properties&lt;/a&gt; section it refines that statement, saying that it should actually be 0.5 &lt;a href=&quot;https://en.wikipedia.org/wiki/Synodic_day&quot; rel=&quot;nofollow noreferrer&quot;&gt;synodic day&lt;/a&gt; instead, and goes on to discuss the effect of the earth's oblateness upon the precession of the right ascension of the ascending node &lt;span class=&quot;math-container&quot;&gt;$\Omega$&lt;/span&gt; and the argument of perigee &lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;geosynchronous orbit&lt;/a&gt; needs a period of 1.0 &lt;a href=&quot;https://en.wikipedia.org/wiki/Sidereal_time&quot; rel=&quot;nofollow noreferrer&quot;&gt;sidereal day&lt;/a&gt; to remain fixed in the sky, why is the Molniya referenced to a synodic day instead of a sidereal day? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there a simple way to understand this?&lt;/strong&gt; Some kind of &quot;Aha!&quot; one-liner?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-11-08T16:23:23.027" LastActivityDate="2018-11-08T16:23:23.027" Title="Why should Molniya orbit period be a half-synodic, not a half-sidereal day?" Tags="|orbital-mechanics|molniya|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18066" PostTypeId="1" AcceptedAnswerId="18079" CreationDate="2016-09-04T09:06:25.823" Score="5" ViewCount="832" Body="&lt;p&gt;An Earth orbit with a period of 1 sidereal day (and zero inclination) is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;geosynchronous orbit&lt;/a&gt;, orbits slightly above and below that are &lt;a href=&quot;https://en.wikipedia.org/wiki/Supersynchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;supersynchronous&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Subsynchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;subsynchronous&lt;/a&gt; orbits, and a &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;noreferrer&quot;&gt;Molniya orbit&lt;/a&gt; (or the &lt;a href=&quot;https://en.wikipedia.org/wiki/Medium_Earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;MEO&lt;/a&gt; of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System&quot; rel=&quot;noreferrer&quot;&gt;GPS&lt;/a&gt; satellites) designed with a half-day period is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-synchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;semi-synchronous orbit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am guessing that an orbit with a period of 2 sidereal days might be called a bisynchronous orbit if it turned out to be useful. Right now I don't see that term outside of KSP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would an Earth orbit designed to have a period of a rational fraction of a sidereal day be called. For example a ~16 hour orbit designed to pass over the same location on earth every ~48 hours having a period of 2/3 (2:3) of a sidereal day? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought that would fall under a general category of synchronous orbits, but it seems synchronous refers specifically to 1:1. Is there a different term for the group of orbits synchronized to rational numbers of sidereal days?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-09-05T12:09:33.097" Title="Are there terms for Earth orbits with rational number multiples of 1 sidereal day?" Tags="|orbital-mechanics|artificial-satellite|terminology|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18068" PostTypeId="2" ParentId="18066" CreationDate="2016-09-04T11:19:42.463" Score="1" Body="&lt;p&gt;Molniya is called semisynchronous because it can work as a sort-of geosynchronous satellite for part of its orbit, not because its period is 12h. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've never seen specific names for orbits other than geosynchronous and sun-synchronous. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-09-04T11:19:42.463" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18069" PostTypeId="2" ParentId="18064" CreationDate="2016-09-04T11:35:56.587" Score="7" Body="&lt;p&gt;The UK did take some advantage of American components, the engine programme for &lt;a href=&quot;https://en.wikipedia.org/wiki/Rolls-Royce_RZ2&quot; rel=&quot;nofollow&quot;&gt;Blue Streak&lt;/a&gt; started with licensed American designs that were progressively replaced with indigenous designs. Black Arrow was a purely British development. British companies had some rocket experience because in the 1950s there were several plans (Saunders-Roe SR.53 and SR.177, Avro 720) to use rocket engines to boost military aircraft. This development was superseded by usable afterburners, but not before several rocket engines were developed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The giant radio telescope at Jodrell Bank played a role in the early days of the space race, tracking Sputnik and others via radar, and assisting with (IIRC) some of the early US Moon probes. By the time Apollo started, NASA had enough large antennas of its own and Jodrell was no longer used officially (they still pointed the antenna at the Moon for the Apollo 11 landing and listened in on its communications, IIRC). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;France did not use American technology, Diamant was a purely French development. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both nations' rocket programmes started as military programmes, with the desire to have a nuclear deterrent independent of the Americans. That meant occasional reinventions of the wheel were inevitable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was at least one European contribution to Apollo in the shape of &lt;a href=&quot;http://www.esa.int/About_Us/Welcome_to_ESA/ESA_history/First_flag_on_the_Moon/%28print%29&quot; rel=&quot;nofollow&quot;&gt;scientific experiments&lt;/a&gt;. I don't believe any Apollo spacecraft hardware was built in Europe. NASA did buy some loose items of equipment in Europe though: NASA used Hasselblad cameras (standard cameras with some modifications) and Zeiss lenses (some standard, some specially designed for NASA) in the Apollo program. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-09-12T16:11:44.027" LastActivityDate="2016-09-12T16:11:44.027" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18070" PostTypeId="2" ParentId="18056" CreationDate="2016-09-04T11:37:47.640" Score="3" Body="&lt;p&gt;Orbital decay due to atmospheric drag is not an issue at geosynchronous altitude. According to &lt;a href=&quot;http://www.swpc.noaa.gov/impacts/satellite-drag&quot; rel=&quot;nofollow noreferrer&quot;&gt;NOAA&lt;/a&gt;, drag is a factor worth considering for altitudes below 2000 km. Drag is not constant:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The drag force on satellites increases during times when the Sun is active. When the Sun adds extra energy the atmosphere the low density layers of air at LEO altitudes rise and are replaced by higher density layers that were previously at lower altitudes. As a result, the spacecraft now flies through the higher density layer and experiences a stronger drag force. When the Sun is quiet, satellites in LEO have to boost their orbits about four times per year to make up for atmospheric drag. When solar activity is at its greatest over the 11-year solar cycle, satellites may have to be maneuvered every 2-3 weeks to maintain their orbit &lt;a href=&quot;http://www.swpc.noaa.gov/impacts/satellite-drag&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;In addition to these long-term changes in upper atmospheric temperature and density caused by the solar cycle, interactions between the solar wind and the Earth’s magnetic field during geomagnetic storms can produce large short-term increases in upper atmosphere temperature and density, increasing drag on satellites and changing their orbits. The North American Aerospace Defense Command (NORAD) has to re-identify hundreds of objects and record their new orbits after a large solar storm event (Figure 2). During the March 1989 storm event, for example, the NASA's Solar Maximum Mission (SMM) spacecraft was reported to have &amp;quot;dropped as if it hit a brick wall&amp;quot; due to the increased atmospheric drag.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/NRLMSISE-00&quot; rel=&quot;nofollow noreferrer&quot;&gt;NRLMSISE-00 atmosphere model&lt;/a&gt; uses actual drag data from satellites.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-05T09:02:45.833" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18071" PostTypeId="2" ParentId="18056" CreationDate="2016-09-04T12:40:25.437" Score="18" Body="&lt;p&gt;The answer to your question depends on which part of &quot;stable&quot; you are interested in. The height of the orbit does not change much, atmospheric drag can be neglected almost completely. The orbital lifetime is in the order of many many thousand, possibly millions of years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, the precise orbit is not stable but varies over time. Gravitational influence of the Sun and Moon cause a change of inclination by almost one degree per year - see &lt;a href=&quot;https://space.stackexchange.com/questions/12528/why-is-the-ribbon-of-decommissioned-geosynchronous-satellites-skewed/12532#12532&quot;&gt;Why is the ribbon of decommissioned geosynchronous satellites skewed?&lt;/a&gt;. If you want to dive into the dynamics of these orbits, &lt;a href=&quot;http://hanspeterschaub.info/Papers/Anderson2015c.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; is a nice PDF with details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another major influence on the orbit is the radiation pressure from the Sun. As the orientation of the decommissioned satellites is not fixed, the pressure can be estimated to be equal in all directions (averaged over time). Hence, it does not lead to orbital decay, but orbital perturbations only. For this reason, the height of the graveyard orbit is adapted to the surface-to-mass ratio of the satellite: The higher the expected influence of radiation pressure, the higher the desired orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, a typical graveyard orbit is about 300 km above GEO, consisting of &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;200 km space around GEO to be kept free&lt;/li&gt;&#xA;&lt;li&gt;35 km additional to cope with gravitational disturbances&lt;/li&gt;&#xA;&lt;li&gt;plus another 50 - 100 km for perturbations due to radiation pressure, depending on satellite structure&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-09-04T12:40:25.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18072" PostTypeId="1" CreationDate="2016-09-04T15:50:09.550" Score="4" ViewCount="10353" Body="&lt;p&gt;The above question may sound naive. Still I would be thankful to someone who helps me in understanding  the concept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the following link it is mentioned that earth's rotation speed with respect to  latitude varies as cosine of the latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://image.gsfc.nasa.gov/poetry/ask/a10840.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://image.gsfc.nasa.gov/poetry/ask/a10840.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is how the rotation speed varies with cosine of latitude?&lt;/p&gt;&#xA;" OwnerUserId="14402" LastEditorUserId="12102" LastEditDate="2018-03-07T22:58:44.097" LastActivityDate="2020-06-18T19:46:15.543" Title="Earth's rotation speed vs latitude" Tags="|earth|rotation|" AnswerCount="0" CommentCount="0" FavoriteCount="0" ClosedDate="2020-06-19T09:44:05.597" ContentLicense="CC BY-SA 3.0" />
  <row Id="18073" PostTypeId="2" ParentId="27191" CreationDate="2016-09-04T16:03:44.467" Score="15" Body="&lt;p&gt;&lt;em&gt;NOTE: This answer was provided for a different, very basic question which didn't specify the WGS84 ellipsoid; it's illustrative of the basic principle as applied to a spherical Earth. Still, not bad for an answer written a year and a half before the question was asked...&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;At any latitude, the Earth completes one rotation per day.&lt;/p&gt;&#xA;&lt;p&gt;At the equator, the circumference of the Earth is about 40000 km, so the speed of rotation is 40000 km/day or 463 m/s.&lt;/p&gt;&#xA;&lt;p&gt;If you pick a line of higher latitude and look at it on a globe, you will see that the line of latitude is smaller than the equator.&lt;/p&gt;&#xA;&lt;p&gt;One rotation completed in a day is therefore a shorter distance; at 60 degrees latitude the line is about (cos 60 x 40000) = 20000 km long, so the speed of rotation is 20000 km/day or 230 m/s.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-05-25T16:44:37.000" LastActivityDate="2021-05-25T16:44:37.000" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18074" PostTypeId="2" ParentId="18066" CreationDate="2016-09-04T16:10:17.990" Score="4" Body="&lt;p&gt;I don't know if there's a term that applies just to the (rational-fraction-of-day-period) satellite, but such a satellite could be said to be in a &lt;em&gt;resonant&lt;/em&gt; or &lt;em&gt;harmonic&lt;/em&gt; orbit with a second, synchronous satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could say your 16-hour satellite was in 2:3 resonance with Earth's day, for example.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-09-04T16:10:17.990" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18075" PostTypeId="2" ParentId="18063" CreationDate="2016-09-04T16:41:54.507" Score="1" Body="&lt;p&gt;That'd no doubt interfere with the reentry, and the deployments of the parachutes. I suspect they'd call an abort slightly after the failure and maybe actually use the tower to escape, but I'm guessing there. Perhaps it'd be risky to activate it if didn't get jettisoned correctly, who knows how it is remaining attached..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Details on redundancy jettison operations from apollo sim: &lt;a href=&quot;http://nassp.sourceforge.net/wiki/Launch_Escape_System&quot; rel=&quot;nofollow&quot;&gt;http://nassp.sourceforge.net/wiki/Launch_Escape_System&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And links at the bottom of that page to official docs.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2016-09-04T16:55:31.087" LastActivityDate="2016-09-04T16:55:31.087" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18077" PostTypeId="1" AcceptedAnswerId="18078" CreationDate="2016-09-05T08:11:01.267" Score="4" ViewCount="514" Body="&lt;p&gt;I understand that the Space Technology Mission Directorate's Game Changing Development program &lt;a href=&quot;https://gameon.nasa.gov/&quot; rel=&quot;nofollow&quot;&gt;https://gameon.nasa.gov/&lt;/a&gt; has a broad scope and is not necessarily focused on immediate payoff. Exploration of ideas and concepts to the experimental stage can be beneficial in many ways - both direct and indirect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone help me understand at least the &lt;em&gt;potential benefits&lt;/em&gt; of the towed glider air launch concept with a nice, concise summary? In what situation would it be attractive and/or competitive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've looked at several discussions, but better &quot;carry efficiency&quot; doesn't speak to me. Can this put one or a group of cubesats in LEO at a lower &lt;em&gt;cost per cube&lt;/em&gt; (or &lt;em&gt;per kilogram&lt;/em&gt;) than other current or likely near-future launch services? Or perhaps is easier to get to the desired latitude for a better choice of inclination or altitude?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/drc-012-011_cheaper_safer_more_efficient_air_launch_via_towed_glider_online_1-7-2016.pdf&quot; rel=&quot;nofollow&quot;&gt;Cheaper, Safer, More Efficient Air Launch via Towed Glider&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/FS_TGLAS_150129-1.pdf&quot; rel=&quot;nofollow&quot;&gt;Space Technology, Game Changing Development, Towed Twin-Fuselage Glider Launch System&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/0hEnYyykaL8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-09-05T21:54:20.987" Title="What is the game that the Towed Glider Air-Launch System (actually) changes?" Tags="|artificial-satellite|launch-vehicle|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18078" PostTypeId="2" ParentId="18077" CreationDate="2016-09-05T11:47:24.943" Score="11" Body="&lt;p&gt;Air launching has been around for awhile, see the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pegasus_(rocket)&quot;&gt;Pegasus rocket system&lt;/a&gt; as an example. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Scaled_Composites_White_Knight&quot;&gt;White Knight&lt;/a&gt; is another system. The main goal of much rocket development is cost reduction, and air launching is a way to reduce the size and complexity of a rocket by using an airplane to get it part of the way. So far all the aircraft used for air launch have been manned (B52s and L1011s). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Manned aircraft have to have a certain size and complexity in order to provide a pressure vessel and support systems for the squishy organics inside. This extra complexity and size adds significant design and build cost. Manned aircraft need much more certification because of the safety aspects, adding to the design and build cost. Bigger aircraft need more fuel, bigger engines, etc adding to the build and running costs. This proposal uses advances in pilotless aircraft to reduce the size and complexity of the launch platform by making it unmanned and unpowered (except for a small rocket engine). It essentially outsources the design, certification, and build of the manned, powered aspect to a 3rd party. You don't need an astronaut or test pilot to fly the towing airplane because it's commercial off the shelf - some extra training and similation would certainly be required but not to the level that someone flying the White Knight would need. Because there's no people in the lifter-glider if something goes wrong the towing aircraft can just cut it loose over the sea. The rocket could explode and the tow would simply fly away. A small rocket on the lifter-glider would boost the glider into a better drop angle for the rocket and get it clear after the drop, which would be harder to accomplish on a manned airplane. All these aspects mean you could possibly reduce the price of high-altitude launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The term &quot;game changer&quot; is often over-used, often to get more budget. Whether it is appropriate to this proposal is opinion-based. I see potential to this proposal, at the same time there's a lot that would have to go right with it. Towing a big glider with a rocket on it using a big jet is a bit different from towing a sailplane behind a super-cub! The towing airplane might require significant modifications to handle the stresses, and that could make the proposal much less financially attractive. I personally think this is worth investigation but I'm not dancing a jig just yet. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2016-09-05T12:32:13.167" LastActivityDate="2016-09-05T12:32:13.167" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18079" PostTypeId="2" ParentId="18066" CreationDate="2016-09-05T12:09:33.097" Score="3" Body="&lt;p&gt;This is a total coincidence, but I just ran across this introduction to the paper &lt;a href=&quot;http://www.spacecraftresearch.com/files/StreetmanPeck_JGCD2007.pdf&quot; rel=&quot;nofollow&quot;&gt;New Synchronous Orbits Using the Geomagnetic Lorentz Force&lt;/a&gt; (Brett Streetman and Mason A. Peck: Journal of Guidance, Control, and dynamics, Vol. 30, No. 6, November–December 2007):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a repeat-groundtrack orbit, the subsatellite point traces out a recurring pattern in some integer number of orbital periods. Traditionally, these orbits are achieved by adjusting the period of a satellite such that it completes an integer number of revolutions in exactly an integer number of sidereal Earth days. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it seems that the term &quot;&lt;strong&gt;repeat-groundtrack orbit&lt;/strong&gt;&quot; might be a suitably all-encompassing term for &lt;em&gt;Earth orbits with rational number multiples of 1 sidereal day.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a bit more of the first paragraph in case someone is interested...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Geostationary and geosynchronous Earth orbits (GEOs) are perhaps the most familiar and useful examples. These orbits have a mean motion equal to the spin rate of the Earth. We shall refer to orbits that repeat their groundtrack every orbital period as GT-1 orbits. Thus, all trajectories in GEO are in the GT-1 class. A more general class, the GT-x orbit, repeats its groundtrack every x revolutions. For example, satellites in the GPS constellation are in 12 sidereal hour orbits and can thus be considered GT-2 satellites. Many low-Earth-orbit (LEO) imaging satellites designed for full-Earth coverage also use repeat-track orbits. Every 16 days, over the course of 233 orbits, Landsat 7 covers the full Earth, making it a GT-233 satellite...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-09-05T12:09:33.097" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18080" PostTypeId="2" ParentId="18077" CreationDate="2016-09-05T21:54:20.987" Score="1" Body="&lt;p&gt;Rockets burn a huge percent of their fuel at the first few kilometers of lift off before they get to 10-12 kilometer altitude.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Airplanes can reach the same altitudes much more efficiently and deploy the rocket.&lt;br&gt;&#xA; The difference and advantage of  towed glider as opposed to direct deployment of rocket from under the wings as has been the conventional way, is safety and capacity of lifting heavier rockets.&lt;br&gt;&#xA; Because the rocked weight is not carried by the towing plane more weight and larger mass can be carried, and safety because of introduction of an unmanned vehicle which would play a safer role in case of catastrophic abortion of mission and providing more recovery options.&lt;/p&gt;&#xA;" OwnerUserId="16061" LastActivityDate="2016-09-05T21:54:20.987" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18081" PostTypeId="1" AcceptedAnswerId="18085" CreationDate="2016-09-06T03:48:07.143" Score="2" ViewCount="453" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/18078/12102&quot;&gt;an answer&lt;/a&gt; to a previous question, I learned about the game-changing nature of Towed Glider Air-Launch System  technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the glider in the Towed Glider Air-Launch System (&lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/drc-012-011_cheaper_safer_more_efficient_air_launch_via_towed_glider_online_1-7-2016.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/FS_TGLAS_150129-1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;http://www.nasa.gov/sites/default/files/atoms/files/why-towed-glider-over-direct-carry.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;) started off as a true glider, in &lt;a href=&quot;https://www.youtube.com/watch?v=0hEnYyykaL8&quot; rel=&quot;nofollow noreferrer&quot;&gt;this YouTube video&lt;/a&gt; it now has what looks to me like rocket propulsion. The animation calls it a &quot;sustainer motor&quot;, which I assume means that it can sustain all of the the altitude and airspeed gained during the tow while also reorienting the rocket to near-vertical before ignition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this just an alternative, or is this actually what Towed Glider Air-Launch System has actually needed all along to be as competitive as advertised? It suddenly seems like a much more complex, expensive, and risky technology, but that's just my impression. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZPvFQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZPvFQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Z3kQV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z3kQV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2R7mL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2R7mL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/0hEnYyykaL8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-03-27T21:47:19.607" LastActivityDate="2019-03-27T21:47:19.607" Title="Is the Towed Glider Air-Launch System's glider now also a rocket?" Tags="|artificial-satellite|launch-vehicle|" AnswerCount="1" CommentCount="20" ContentLicense="CC BY-SA 3.0" />
  <row Id="18082" PostTypeId="1" CreationDate="2016-09-06T04:10:32.810" Score="0" ViewCount="236" Body="&lt;p&gt;I just watched the movie 'Interstellar' for the first time. Assume we have reached the point (in real life) where we are able to launch interstellar spaceships meant to colonize other planets. To do this could we launch 3 or more ships from Earth and have them somehow link up or come together in space to form a larger ship, or would the ship need to be assembled in space because of how large it would have to be? I'm imagining the space station near the end of the movie could not be built on earth due to the size of it. The movie was very interesting and it got me thinking and I'm just curious. Thanks in advance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Warning:&lt;/strong&gt; minor spoilers in comments.&lt;/p&gt;&#xA;" OwnerUserId="16892" LastEditorUserId="6642" LastEditDate="2016-09-06T14:00:46.317" LastActivityDate="2016-09-06T14:05:04.150" Title="Could an interstellar spaceship be made up of several ships launched from Earth, or would it have to be built in space?" Tags="|launch|spacecraft|colonization|interstellar-travel|" AnswerCount="1" CommentCount="8" ClosedDate="2016-09-06T10:17:42.487" ContentLicense="CC BY-SA 3.0" />
  <row Id="18083" PostTypeId="2" ParentId="18082" CreationDate="2016-09-06T06:43:08.757" Score="2" Body="&lt;p&gt;With our current understanding of physics, YES we would have to build it in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no way we can scale rockets so high as to build large spacecraft and launch from Earth.&#xA;Of course, as comments pointed it out, it might be beneficial to build the rocket from off-Earth resources, avoiding the launch from Earth issue in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Realistically, engineering and R&amp;amp;D could only scale payload to LEO in the order of thousands of tonnes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not directly related but to give you and idea of how badly rocket scales:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/14383/how-much-bigger-could-earth-be-before-rockets-wouldt-work&quot;&gt;How much bigger could Earth be, before rockets would&amp;#39;t work?&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-09-06T14:05:04.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18084" PostTypeId="1" AcceptedAnswerId="18086" CreationDate="2016-09-06T12:17:32.863" Score="4" ViewCount="432" Body="&lt;p&gt;Let's imagine there are already interplanetary spacecrafts and travel. Would it be possible to create electromagnetic accelerator somewhere in the outer space? It would work this way:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be a long platform, which would emit electromagnetic waves in specific plates in following scenario:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;First plate, the nearest to the spacecraft, would emit electromagnetic wave, in order to pull (or maybe push) the spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;First plate would turn off.&lt;/li&gt;&#xA;&lt;li&gt;Second plate, the nearest to the spacecraft, would turn on and pull the spacecraft etc.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;As the frequency of turning on/off the plates would be increasing, the speed of the spacecraft would be increasing as well (we are in vacuum). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9ZnFJ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9ZnFJ.png&quot; alt=&quot;Electromagnetic accelerator&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could be this approach applicable in reality, could it pull the spacecraft to the desirable speed? I know it would be pretty expensive, but we could avoid burning fuel that can be used e.g. during landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for any constructive advises and explanations.&lt;/p&gt;&#xA;" OwnerUserId="16590" LastEditorUserId="6944" LastEditDate="2016-09-06T13:58:02.277" LastActivityDate="2018-06-18T01:16:18.200" Title="Electromagnetic accelerator in space" Tags="|spacecraft|interplanetary|low-energy-transfer|rail-gun|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18085" PostTypeId="2" ParentId="18081" CreationDate="2016-09-06T12:37:29.317" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is this just an alternative, or is this actually what Towed Glider Air-Launch System has actually needed all along to be as competitive as advertised? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.nasa.gov/sites/default/files/files/FS_TGLAS_150129-1.pdf&quot; rel=&quot;nofollow&quot;&gt;this update&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The project’s next step will be to demonstrate the horizontal launch of a small launch vehicle from the onethird-scale glider. That will be followed by the build up and demonstration of a one-third-scale “sustainer motor” top-mounted to the glider. This variable thrust motor will provide the energy to change the glider and launch vehicle orientation from horizontal to a highly inclined flight path angle to maximize launch efficiency.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it seems that the &quot;sustainer motor&quot; is needed to change the orientation from horizontal to vertical. Vertical air launch of the rocket is only one of the key benefits of this technique, but it's an important one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At a high altitude (necessary to make this an advantages technique) the pitch up maneuver requires a significant amount of time - most of the horizontal momentum must be changed to vertical momentum. As an un-propelled glider, all of that momentum must be transferred to the rarified atmosphere, and that takes time. The rate of descent will accelerate as soon as the pitch results in loss of lift, and more altitude will be lost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without a simulation, I can only speculate what might happen, but without significant lift, but plenty of kinetic energy, the whole thing may become unstable and begin to tumble. In short, at high attack angles, a glider with no propulsion will just stall and then loose it's ability to control its orientation. In order to maintain orientation in near-vertical flight, thrust is required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is likely that rocket propulsion of the glider was always needed, but not included until recently in discussion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming ballpark figures of a 10,000 lbm &quot;glider&quot; carrying a 20,000 lbm launch vehicle* &lt;a href=&quot;http://www.nasa.gov/sites/default/files/atoms/files/why-towed-glider-over-direct-carry.pdf&quot; rel=&quot;nofollow&quot;&gt;from here&lt;/a&gt;, the &quot;sustainer&quot; motor would need more than 30,000 lbf of thrust to maintain a vertical velocity, ignoring drag. That is something like a pair of SuperDraco engines, but with &lt;em&gt;really really deep&lt;/em&gt; throttling since the thrust must be much lower while the flight is horizontal and the drag is only about 1/15 of the weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*although a glider that can cary an 80,000 lbm launch vehicle is &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/drc-012-011_cheaper_safer_more_efficient_air_launch_via_towed_glider_online_1-7-2016.pdf&quot; rel=&quot;nofollow&quot;&gt;mentioned here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-09-06T13:25:26.890" LastActivityDate="2016-09-06T13:25:26.890" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18086" PostTypeId="2" ParentId="18084" CreationDate="2016-09-06T14:09:18.053" Score="3" Body="&lt;p&gt;According to Newton, for every action there is an equal and opposite reaction. As the magnets were applied to the craft, it would apply as much force on the accelerator.  The accelerator would have to be very massive and/or have rocket engines to generally maintain position and maximize the force imparted on the spacecraft. Of course assembling such an accelerator would require many launches plus maintenance flights etc. The economic advantage in creating such a platform could be quite challenging.&lt;/p&gt;&#xA;" OwnerUserId="15744" LastActivityDate="2016-09-06T14:09:18.053" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18087" PostTypeId="2" ParentId="6336" CreationDate="2016-09-06T14:11:20.513" Score="13" Body="&lt;p&gt;To answer the creeping of the exhaust plume up the side of the first stage, it's called Plume Induced Flow Separation (PIFS). This paper (PDF) explains and simulates the computational fluid dynamics (CFD) of the PIFS phenomenon on the Saturn V. It's pretty heady but it explains the physics behind it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110004015.pdf&quot; rel=&quot;nofollow&quot;&gt;USM3D Simulations of Saturn V Plume Induced Flow Separation &lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16898" LastEditorDisplayName="user10509" LastEditDate="2016-09-07T04:01:07.363" LastActivityDate="2016-09-07T04:01:07.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18088" PostTypeId="2" ParentId="14687" CreationDate="2016-09-06T19:35:30.273" Score="0" Body="&lt;p&gt;The COCOM limit is a ITU requirement, it is Global and it has to do with import/export. If your device comes without COCOM limits, it is considered &quot;Dual Use&quot; by the Dept. of State and must be licensed by them for Export (Same with just about every country in the world.) With COCOM limits, you can export with permission of the Dept. of Commerce. If you make it and don't export it you're OK.  If you buy it in the USA and don't export it, you are OK. If you sell one without COCOM limits, you must follow the ITAR rules and regulations or you go to jail.  Most small cheap units seem to have COCOM limits &quot;Hard Wired&quot; and can't be &quot;removed&quot;.  &lt;/p&gt;&#xA;" OwnerUserId="15793" LastActivityDate="2016-09-06T19:35:30.273" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18089" PostTypeId="2" ParentId="14542" CreationDate="2016-09-06T21:42:27.447" Score="3" Body="&lt;p&gt;&quot;Could&quot; is such a tricky phrase. Do you mean near-term and economically viably, or eventually, after a 50-year technology maturation?&#xA;Assuming you mean within 20 years, my answer is &quot;Probably yes, but the system would be less attractive due to financial and other considerations than evolved chemical rockets.&quot;&#xA;The big challenge from a performance perspective is thrust-to-weight, or T/W. Yes, a NERVA-class NTR with its ISP of 700-1000 uses each kg of fuel twice as efficiently as a chemical rocket, but when launching from the ground that doesn't come close to making up for its lower T/W (7 vs 70 for chemical rockets, per Wikipedia). Project Timberwind designed a pebble-bed (more granular-bed) NTR in the 80's and got to a claimed (design) T/W of 23 at sea level and 30 in vacuum. However the Wikipedia article doesn't mention the flow instabilities (read: Localized hot spots leading to failure) that were confirmed when they built a partial test article and flow tested it with inductive heating to simulate nuclear heating of the simulated fuel elements. So a near-term achievable result lies between NERVA's proven T/W of 70 and Timberwind's pushed-too-far goal of 30. &#xA;Meanwhile, Aerojet said &quot;What if we inject extra mass to improve T/W at the cost of ISP during the first 1-2 minutes, called LANTR? T/W climbs impressively, but still not where you want it to compete with chemical rockets for a first stage. &lt;a href=&quot;http://www.nss.org/settlement/moon/LANTR.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nss.org/settlement/moon/LANTR.html&lt;/a&gt; ; &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a454590.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.dtic.mil/dtic/tr/fulltext/u2/a454590.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA looked over NTRs at the behest of Bush Sr for his mars project and said a fresh program to develop a mars-transfer NTR (still not for ground launch but to improve our throw weight to mars) with modern environmental controls would cost a cool \$1B. Applying that number to your problem, I think we can say that a near-term \$1B investment and adding LANTR would give us a functional but underperforming first stage. And considering what's going on with SpaceX and Blue Origins, that $1B could buy a LOT of kerosene and methane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BUT crunching the numbers might yield a kick-ass second stage/mars transfer injection vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, be careful reading claims of Dumbo and Timberwind fans. When you make your fuel passages smaller (small channels for Dumbo, or a granular bed for Timberwind) your heat transfer coefficients go up dramatically. That's great for T/W (less reactor size and weight needed to transfer the same amount of energy to the fuel) but moves you into the territory of Laminar Flow Instability. That's a pesky problem where the hydrogen flows more slowly the hotter it gets. And since the engine is generating energy at a constant rate, hot spots rapidly become hotter spots and then you get sort of a whistle-boom effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, you can get your fuel hotter (thus improving ISP and thrust) if you use metallic fuel elements. Hooray! Hotter exhaust! Oops, the elements weigh twice as much as the lower-performing carbon elements; you improved ISP but reduced T/W. How about an NTR that does the initial 2/3 of its heating with relatively light carbon elements then runs the fuel through a second stage with metallic elements to reach peak exhaust temp? Do you have any idea how much of a nightmare the neutron modeling would be for that? Now you're probably looking at a $2B project, easy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if we decide to colonize an outer planet on a massive scale - say a $500B+ investment over 20 years - AND it's enough of a social imperative to disregard the sprinkling of radioactive exhaust - then second-stage NTR + LANTR would be worth looking at, especially as a second stage/MOI engine. But even then NTRs probably won't make sense as a first stage or SSTO until we achieve a technological leap to gas-phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sorry for the lack of math. &lt;/p&gt;&#xA;" OwnerUserId="8247" LastEditorUserId="58" LastEditDate="2016-09-06T21:52:58.270" LastActivityDate="2016-09-06T21:52:58.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18091" PostTypeId="2" ParentId="955" CreationDate="2016-09-07T01:52:03.253" Score="15" Body="&lt;p&gt;The issue with Sea Dragon and pressure instability is that the likelihood of pressure instability increases exponentially as the size of the combustion chamber and nozzle diameter increases linearly. Sea Dragon's bell was supposed to be over 75 feet in diameter and put out 350 meganewtons of force (about 5000% more than an F-1 engine).  The F-1s had huge issues with combustion stability, which were eventually solved.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Truax wasn't concerned with combustion stability because the engine was going to be a pintle injector type. He believed the natural combustion stability of the pintle injector would allow enormous engines to be highly stable at a variety of pressures. (Variable pressure was a key part of his design, because it allowed a much simpler and sloppier system in which pressure started high and slowly dwindled down as the tanks emptied.)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;TRW (who built the rockets for the Apollo lander) later &lt;a href=&quot;http://www.rocket-propulsion.info/resources/articles/TRW_PINTLE_ENGINE.pdf&quot; rel=&quot;noreferrer&quot;&gt;validated his belief&lt;/a&gt;.  In that paper they point out that pintle injectors have demonstrated stable combustion with motors varying in scale by 50,000:1.  So, Truax was probably right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On an interesting side note, the Soviets had the same problem with combustion stability that we did.  This is why early Soviet engines used four smaller combustion chambers instead of one 1 big one. The Soviet's inability to develop large engines was what lead to the N1 (their moon rocket) to have so many engines (33 in the first stage, I believe).  Which then led to enormous plumbing problems, which led to the failure of the N1, which led to the failure of the Soviet Moon program. &lt;/p&gt;&#xA;" OwnerUserId="16902" LastEditorUserId="195" LastEditDate="2016-09-07T04:02:10.303" LastActivityDate="2016-09-07T04:02:10.303" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18092" PostTypeId="1" AcceptedAnswerId="18095" CreationDate="2016-09-07T04:11:30.773" Score="26" ViewCount="9739" Body="&lt;p&gt;What happens to Soyuz reentry capsules after landing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess some must go to museums, but quite a lot of them have returned now.&#xA;Is there any indication of where and how they are stored? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="26446" LastEditDate="2020-04-18T17:58:31.397" LastActivityDate="2020-04-18T17:58:31.397" Title="What happens to Soyuz reentry capsules after landing?" Tags="|soyuz-spacecraft|museum-exhibits|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18094" PostTypeId="2" ParentId="18092" CreationDate="2016-09-07T06:36:01.753" Score="12" Body="&lt;p&gt;Many are put on display, some just end in a junkyard&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/r/russialocatnedspacecraft.html&quot; rel=&quot;noreferrer&quot;&gt;Location of Russian Manned Spacecraft&lt;/a&gt; lists objects on display.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edited quote from the &lt;a href=&quot;http://www.collectspace.com/ubb/Forum41/HTML/000169.html&quot; rel=&quot;noreferrer&quot;&gt;Soyuz spacecraft: post-landing and recovery&lt;/a&gt; post on Collectspace.com:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Some of the spacecraft flown during Intercosmos missions are now on exhibit in the countries from where their international crew members hailed.&lt;br /&gt;&lt;br /&gt;&#xA;  - Soyuz 29, onboard which Sigmund Jähn flew, is at the Deutsches Museum für Meisterwerke der Naturwissenschaft and Technik (Deutsches Museum, for short) at Munich.&#xA;  - Miroslaw Hermaszewski's Soyuz 30 is in Warsaw, Poland.&lt;br /&gt;&#xA;  - Soyuz T-6, which launched Jean-Loup Chrétien, is on display at the Musee de l'Air near Paris (where the Paris Air Show is staged).&lt;br /&gt;&#xA;  - Soyuz TM-10 is at the National Air and Space Museum in Washington, DC. It was sold at Sotheby's in 1993 for $1.65 million to the Ross Perot Foundation, which in turn loaned it to the museum.&lt;br /&gt;&#xA;  - In 2000, Spacehab offered the Soyuz TM-26 descent module for direct sale, and then auctioned it through Superior Galleries. To my knowledge, it failed to meet its reserve.&lt;br /&gt;&lt;br /&gt;&#xA;  Unlike in the U.S., where the flown spacecraft are property of the government, the Soyuz belong to its manufacturer, RSC Energia. As such, the company's museum outside Moscow is probably the best place to see multiple Soyuz on display.&lt;br /&gt;&#xA;  The majority of the flown capsules are said to be kept in a Soyuz &quot;junkyard&quot; owned by Energia.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Another post at CollectSpace: &lt;a href=&quot;http://www.collectspace.com/ubb/Forum31/HTML/000312.html&quot; rel=&quot;noreferrer&quot;&gt;Russian Soyuz spacecraft on public display&lt;/a&gt; also contains some snippets of information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was unable to find information about that 'Energia junkyard', but look at this abandoned &lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-3119861/Russia-s-forgotten-space-agency-Haunting-images-reveal-two-abandoned-Soviet-shuttles-rotting-giant-derelict-hangar.html&quot; rel=&quot;noreferrer&quot;&gt;Buran in ist construction hangar&lt;/a&gt; (photos by Ralph Mirebs)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/As8hc.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/As8hc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user10509" LastActivityDate="2016-09-07T06:36:01.753" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18095" PostTypeId="2" ParentId="18092" CreationDate="2016-09-07T10:11:07.063" Score="27" Body="&lt;p&gt;There were more than 500 Zenit (including Vostoks and Voskhods) landing capsules; an order of 160 (as of today) manned and unmanned Soyuz variants. &#xA;Plus prototypes used for modelling and testing on the ground.&#xA;That's a pretty large number of used capsules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are &lt;a href=&quot;http://militarynews.ru/story.asp?rid=2&amp;amp;nid=358592&quot; rel=&quot;noreferrer&quot;&gt;65 pieces of equipment&lt;/a&gt; certified for reuse in Soyuz TMA landing module. Such as cosmonaut seats or flight recorders. &#xA;There are radioactive pieces (gamma ray altimeter) and other dangerous or valuable pieces that are removed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After that the shell is utilized one way or another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many go to museums. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In general, &lt;a href=&quot;https://www.google.co.uk/search?q=%D1%81%D0%BF%D1%83%D1%81%D0%BA%D0%B0%D0%B5%D0%BC%D1%8B%D0%B9+%D0%B0%D0%BF%D0%BF%D0%B0%D1%80%D0%B0%D1%82+%D1%81%D0%BE%D1%8E%D0%B7+%D0%BC%D1%83%D0%B7%D0%B5%D0%B9&amp;amp;tbm=isch&quot; rel=&quot;noreferrer&quot;&gt;this image search&lt;/a&gt; should give you an idea.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example some are displayed in &lt;a href=&quot;http://nnm.me/blogs/krapich/muzey_rkk_energiya_im_s_p_koroleva/&quot; rel=&quot;noreferrer&quot;&gt;RSC Energia in-house museum&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/I8KVj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/I8KVj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;photo from &lt;a href=&quot;http://nnm.me/blogs/krapich/muzey_rkk_energiya_im_s_p_koroleva/&quot; rel=&quot;noreferrer&quot;&gt;krapich blog&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside German Titov's museum:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ZOec6.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZOec6.png&quot; alt=&quot;German Titov museum&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;photo from &lt;a href=&quot;http://muzeytitova.ru/fond/veshchestvennye-istochniki/spuskaemyy-apparat-kosmicheskogo-korablya-soyuz-tma-21/&quot; rel=&quot;noreferrer&quot;&gt;German Titov memorial museum&lt;/a&gt;&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A piece of Prokopyevsk regional museum exhibition:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/T3LNK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/T3LNK.jpg&quot; alt=&quot;Prokopyevsk museum&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;photo from &lt;a href=&quot;http://kemoblast.ru/news/miners-day/2012/08/24/v-prokopevskom-kraevedcheskom-muzee-otkryt-novyj.html&quot; rel=&quot;noreferrer&quot;&gt;Kemerovskaya oblats &lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A piece of Tsiolkovsky museum exhibition:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/MDmUM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MDmUM.jpg&quot; alt=&quot;Tsiolkovsky musem&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;photo from &lt;a href=&quot;http://scorokhod.livejournal.com/70220.html&quot; rel=&quot;noreferrer&quot;&gt;Skorokhod blog&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some amount stored on the &lt;a href=&quot;http://rsc-etestpilot.blogspot.ru/2012_07_01_archive.html&quot; rel=&quot;noreferrer&quot;&gt;temporary storage pad&lt;/a&gt; awaiting utilization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3YsDp.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3YsDp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;photo from &lt;a href=&quot;http://rsc-etestpilot.blogspot.ru/2012_07_01_archive.html&quot; rel=&quot;noreferrer&quot;&gt;Mark Serov's blog&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another location we have photos of is probably the white brick building behind the temporary storage pad, but the author of the photos doesn't disclose the location. Not all of those have been into space. The the black-and-white, for example, seems to be a piece of testing equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oPafw.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oPafw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;photo from &lt;a href=&quot;http://saoirse-2010.livejournal.com/19823.html&quot; rel=&quot;noreferrer&quot;&gt;saoirse-2010 blog&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2017-06-04T10:18:17.837" LastActivityDate="2017-06-04T10:18:17.837" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18096" PostTypeId="2" ParentId="12353" CreationDate="2016-09-07T12:23:56.780" Score="5" Body="&lt;p&gt;There are so many reasons why an L2 orbit was chosen for the JWST. Though the specific advantages over LEO include;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;It can use Infrared instruments since the heat from the Earth and Sun can be radiated away from its field of view.&lt;/li&gt;&#xA;&lt;li&gt;It will have a constant, uninterrupted view of deep space.&lt;/li&gt;&#xA;&lt;li&gt;It will require less orbital corrections throughout its lifetime.&lt;/li&gt;&#xA;&lt;li&gt;Obviously, the risk of space debris in LEO is a growing problem.&lt;/li&gt;&#xA;&lt;li&gt;L2 has been a favoured orbit for space observations by astronomers for a long time.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Due to the delicate nature of the telescope's deployment, yes, it would be great if we could get people there in case something goes wrong. However, this is also the reason why it cannot be deployed in LEO and then sent to L2 as it simply isn't structurally designed to withstand that sort of journey and would have to pass through altitudes of even greater levels of debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This website talks about the advantages of the L2 orbit for the JWST:&#xA;&lt;a href=&quot;http://www.nasa.gov/topics/universe/features/webb-l2.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/topics/universe/features/webb-l2.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14065" LastActivityDate="2016-09-07T12:23:56.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18097" PostTypeId="1" AcceptedAnswerId="18107" CreationDate="2016-09-07T14:53:52.180" Score="5" ViewCount="1386" Body="&lt;p&gt;This Teslarati article &lt;a href=&quot;http://www.teslarati.com/tesla-app-reveals-spacex-solar-panel-easter-egg/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.teslarati.com/tesla-app-reveals-spacex-solar-panel-easter-egg/&lt;/a&gt; mentions a SpaceX themed Easter Egg, but says that the solar panels on the SpaceX roof can not be confirmed using Google maps - probably only because they are not updated yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So of course I looked, and the X-shaped pattern made from solar panels can be seen in the Icon (lower left) but not in the large view. Why are these different? Is it real?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HjOt1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HjOt1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/fuTWK.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fuTWK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-09-08T14:39:16.947" LastActivityDate="2016-09-08T14:39:16.947" Title="Why does the SpaceX Easter Egg only show up in the icon in Google Maps?" Tags="|spacex|" AnswerCount="2" CommentCount="13" ClosedDate="2016-09-08T14:45:23.213" ContentLicense="CC BY-SA 3.0" />
  <row Id="18098" PostTypeId="2" ParentId="18097" CreationDate="2016-09-07T14:57:40.047" Score="8" Body="&lt;p&gt;Lower resolution images are updated more often than higher resolution images. Google Maps works with a large variety of images, the large scale ones taken by satellites, and most of the close ones by airplanes. The image on the lower left is probably from a frequently updating satellite, the right from an airplane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While checking this out, I noticed that the SpaceX roof is now updated, and has the X. &lt;a href=&quot;https://www.google.com/maps/place/Hawthorne,+CA/@33.9206825,-118.3292094,391m/data=!3m1!1e3!4m5!3m4!1s0x80c2b42b5acc2ef1:0x62c1711dcc38b301!8m2!3d33.9164032!4d-118.3525748&quot;&gt;https://www.google.com/maps/place/Hawthorne,+CA/@33.9206825,-118.3292094,391m/data=!3m1!1e3!4m5!3m4!1s0x80c2b42b5acc2ef1:0x62c1711dcc38b301!8m2!3d33.9164032!4d-118.3525748&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-07T14:57:40.047" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18099" PostTypeId="1" CreationDate="2016-09-07T15:00:59.250" Score="0" ViewCount="888" Body="&lt;p&gt;The above question may sound naive. I am trying to understand how satellite ground track can be computed when satellite position and velocity vectors are available from on-board GPS receiver of a low earth orbiting satellite.&lt;/p&gt;&#xA;" OwnerUserId="14402" LastActivityDate="2016-09-07T15:00:59.250" Title="Satellite Ground Track From ECEF Frame Position And Velocity Vector" Tags="|ground-track|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18100" PostTypeId="1" AcceptedAnswerId="36952" CreationDate="2016-09-07T15:03:41.733" Score="28" ViewCount="2992" Body="&lt;p&gt;This is an image of the radioisotope thermoelectric generator (RTG) for Cassini:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pu3VQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pu3VQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This one was for one left on the Moon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HatYW.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HatYW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this one is for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-Mission_Radioisotope_Thermoelectric_Generator&quot; rel=&quot;noreferrer&quot;&gt;Multi-Mission RTG&lt;/a&gt;, used by Curiosity on Mars:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5ZrxI.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5ZrxI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One is black, one grey, and one white. Why such a difference in the color of these RTGs? &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="12519" LastEditDate="2016-09-09T18:22:18.110" LastActivityDate="2020-07-25T03:41:39.033" Title="Why are RTGs different colors?" Tags="|design|power|nuclear|thermal|rtg|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18101" PostTypeId="2" ParentId="18004" CreationDate="2016-09-07T15:14:59.923" Score="1" Body="&lt;p&gt;I think you're right, the separate aerospikes would interfere with each other and, depending on the placement, might not benefit from the aerodynamics of the vehicle while in atmosphere. Note that the linear aerospike you mention, intended for the X-33, fills out nearly the entire rear surface of that vehicle. Philip Bono's aerospike designs from the early Sixties (e.g. the &lt;a href=&quot;https://www.flickr.com/photos/sdasmarchives/27719476681/in/album-72157669057850210/&quot; rel=&quot;nofollow&quot;&gt;Hyperion&lt;/a&gt;) also feature large plug nozzles completely covering the underside. Clustering the engines might require some fancy aerodynamics to get the right effect. And besides, it doesn't give you any redundancy benefit that isn't already present in the plug nozzle concept, which already uses multiple pumps and so on.&lt;/p&gt;&#xA;" OwnerUserId="16905" LastActivityDate="2016-09-07T15:14:59.923" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18103" PostTypeId="2" ParentId="18100" CreationDate="2016-09-07T17:58:01.710" Score="28" Body="&lt;p&gt;The big difference between the two darker RTG fins (Black and Grey) and the white RTG fins, is that the white fins were destined for use in an atmosphere (Mars).  The presence of an atmosphere, even as diffuse as Martian air, would allow increased heat transfer from the RTG fins via convection and conduction, vs. the space based versions which would entirely rely on radiation to transfer heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiation heat transfer is affected by color and is likely the reason for the color differences.&lt;/p&gt;&#xA;" OwnerUserId="15824" LastActivityDate="2016-09-07T17:58:01.710" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18104" PostTypeId="1" CreationDate="2016-09-07T18:47:47.857" Score="13" ViewCount="716" Body="&lt;p&gt;I was looking at some pictures of launches (&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/8/85/SDOs_Atlas_V_lifted_off.jpg&quot;&gt;like this one&lt;/a&gt;) and started looking at the stairwells, doors, and other places people are intended to go. It got me wondering what kind of dangers exist on a launch pad after a successful launch. Obviously I would expect heat to be an issue, but I was wondering if there would be other dangers in the area and what kind of precautions are taken before people are allowed to enter the area.&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2020-02-23T16:59:46.453" Title="What dangers are on a launch pad after a successful launch?" Tags="|launch|launch-site|launchpad|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18105" PostTypeId="1" AcceptedAnswerId="18106" CreationDate="2016-09-07T19:32:54.203" Score="4" ViewCount="369" Body="&lt;p&gt;In the answer to this &lt;a href=&quot;https://space.stackexchange.com/questions/6490/how-close-can-i-get-to-a-rocket-launch&quot;&gt;question&lt;/a&gt; it is claimed that 'Submarines often launch orbital vehicles'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this correct? What would they be launching?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was under the impression that only military submarines launched anything to space, and that ICBMs were strictly sub-orbital and that their test launches were infrequent. I suppose this leads to the question of 'what is frequent', but I'm not going to define that.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-09-08T14:19:21.287" Title="Do submarines often launch orbital vehicles?" Tags="|launch|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18106" PostTypeId="2" ParentId="18105" CreationDate="2016-09-07T19:43:05.597" Score="6" Body="&lt;p&gt;The Russians, mostly were reselling ICBM launchers as space payloads. The US never really did much of this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Volna#Performance&quot; rel=&quot;nofollow&quot;&gt;Volna&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Shtil%27#Performance&quot; rel=&quot;nofollow&quot;&gt;Shtil&lt;/a&gt; boosters are converted ICBM's launched from subs.  They are pretty cheap to buy, but are mostly phased out and not in use these days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To the point about ICBM's usually being suborbital and yet they can be used for orbital missions, it is worth noting that the Soyuz booster is the direct descendant of the R-7 ICBM. The Russians (Soviets at the time I guess) had trouble miniaturizing the nuclear warheads, so their boosters were much bigger than the US equivalents to compensate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus when it came time to convert an ICBM to a orbital booster by adding an upper stage, they had the performance to meet the need.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-09-08T14:19:21.287" LastActivityDate="2016-09-08T14:19:21.287" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18107" PostTypeId="2" ParentId="18097" CreationDate="2016-09-07T22:30:30.493" Score="2" Body="&lt;p&gt;The solar panels can be seen both in the icon and in the main window.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WwoRR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WwoRR.jpg&quot; alt=&quot;Google Maps&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't see them in the main window, because you have 3D models enabled in the settings drawer, you can drag the maps while holding Ctrl to see that:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MGOSM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MGOSM.jpg&quot; alt=&quot;Google Maps&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The model is not updated that often, so you can't see the panels on it. You can turn off 3D models to see the new image. You could see it in the small preview window, because the preview does not render 3D models.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You commented that you can indeed see it in a different browser, that is because the other browser does not support 3D models in Google Maps (or the other way around).&lt;/p&gt;&#xA;" OwnerUserId="16908" LastEditorUserId="16908" LastEditDate="2016-09-07T22:38:29.920" LastActivityDate="2016-09-07T22:38:29.920" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18108" PostTypeId="1" AcceptedAnswerId="18110" CreationDate="2016-09-08T03:27:57.463" Score="9" ViewCount="350" Body="&lt;p&gt;Watching the video in this NYTimes article &lt;a href=&quot;http://www.nytimes.com/2016/09/06/science/nasa-osiris-rex-asteroid-bennu-sample.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Aims at an Asteroid Holding Clues to the Solar System’s Roots&lt;/a&gt; I realized for the first time how the samples actually make it back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The chemical composition of the organic and volatile compounds is of great interest for this mission for many reasons. So I would think that it is important to avoid thermally induced chemical reactions, migration, and redistribution of the compounds within the sample matrix. If it gets hotter than it does while on Bennu, the science may be compromised.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; According to the text in this excellent &lt;a href=&quot;http://www.asteroidmission.org/mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asteroid Missions animation&lt;/a&gt; near the end, the re-entry velocity will start at about 12.4 km/sec. See also &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/osiris-rex_press_kit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-REx Press Kit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx#Science_objectives&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia's article on OSIRIS-REx&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Return and analyze a sample of pristine carbonaceous asteroid regolith in an amount sufficient to study the nature, history, and distribution of its constituent minerals and organic material.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is the re-entry kept shallow to avoid peak external temperatures, or is it deep and rapid to &quot;get it over with&quot; before heat can diffuse through (presumably) a lot of insulation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screen shots from the &lt;a href=&quot;http://www.nytimes.com/2016/09/06/science/nasa-osiris-rex-asteroid-bennu-sample.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NYTimes video&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NFg4Y.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NFg4Y.png&quot; alt=&quot;frame from NYTimes video at 1:45&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5ljLO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5ljLO.png&quot; alt=&quot;frame from NYTimes video at 2:03&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fRJYx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fRJYx.png&quot; alt=&quot;frame from NYTimes video at 2:12&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tcy0A.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tcy0A.png&quot; alt=&quot;frame from NYTimes video at 2:25&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/slg1C.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/slg1C.png&quot; alt=&quot;frame from NYTimes video at 2:38&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YYjDX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YYjDX.png&quot; alt=&quot;frame from NYTimes video at 2:39&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6816" LastEditDate="2016-09-10T17:56:13.073" LastActivityDate="2016-09-10T18:02:08.450" Title="How will Bennu's volatile compounds stay cool and unmodified by the heat of re-entry (OSIRIS-REx)?" Tags="|reentry|thermal|sample-return|osiris-rex|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18109" PostTypeId="2" ParentId="18108" CreationDate="2016-09-08T04:20:28.807" Score="1" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.asteroidmission.org/mission/#sample&quot; rel=&quot;nofollow noreferrer&quot;&gt;the OSIRIS-REx website run by NASA&lt;/a&gt; the heat shield on the sample return capsule removes over 99% of the initial kinetic energy:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ULFmm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ULFmm.png&quot; alt=&quot;pic from asteroidmission.org &quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;That should be enough to prevent any chemical/biological changes and preserve the regolith for further scientific study.&lt;/p&gt;&#xA;" OwnerUserId="16606" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-10T18:02:08.450" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18110" PostTypeId="2" ParentId="18108" CreationDate="2016-09-08T11:30:16.270" Score="10" Body="&lt;p&gt;The sample return capsule &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20150000809&quot; rel=&quot;noreferrer&quot;&gt;is designed&lt;/a&gt; to keep its contents below 75 °C. It is a flight-proven design, reusing technology developed for the Stardust mission.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight101.com/osiris-rex/osiris-rex-sample-return-capsule/&quot; rel=&quot;noreferrer&quot;&gt;The return capsule’s structure&lt;/a&gt; consists of a graphite-epoxy material covered with a Thermal Protection System making use of NASA’s PICA heat shield technology – Phenolic-Impregnated Carbon Ablator.&lt;/p&gt;&#xA;&lt;p&gt;PICA is a lightweight material designed to withstand high temperatures and mechanical stress. The technology was developed at NASA’s Ames Research Center and currently is in its third generation after undergoing gradual improvements. SpaceX modified the PICA technology for the company’s Dragon and Dragon 2 spacecraft, enabling re-use of heat shields.&lt;/p&gt;&#xA;&lt;p&gt;The principle behind ablative heat shield technology is to create a boundary layer between the shield’s outer wall and the extremely hot shock layer gas by allowing the heat shield material to slowly burn away and, in the process, generate gaseous reaction products that flow out of the heat shield and keep the shock layer at a separation distance, reducing the overall heat flux experienced by the outer shell of the spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;The processes occurring at the heat shield material include a charring, melting and sublimation on the one hand and pyrolysis on the other.&lt;/p&gt;&#xA;&lt;p&gt;Pyrolysis creates the product gases that are blowing outward and create the desired blockage of convective and catalytic heat flux. Radiative heat flux is reduced by introducing carbon compounds into the boundary layer gas which make it optically opaque.&lt;/p&gt;&#xA;&lt;p&gt;The backshell of the SRC is also covered with thermal protection material, but since it resides in the wake of the hot gas flow, it will not need as much protection as the forward facing side. Its Thermal Protection System is comprised of a cork-based material known as SLA 561V, originally developed for the Viking missions to Mars in the 1970s and in use on a number of missions including the Mars Pathfinder, Genesis and MER rover missions. Also facilitated within the backshell is the parachute mechanism with secure attach points for the parachutes to the capsule.&lt;/p&gt;&#xA;&lt;p&gt;The Sample Canister itself is an aluminum enclosure mounted on a composite equipment deck and residing between the backshell and heat shield. The overall performance of the SRC will maintain the sample at a temperature below 75°C in order to avoid pyrolysis (the thermochemical decomposition) of organic material at higher temperatures.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-08T11:30:16.270" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18111" PostTypeId="2" ParentId="18108" CreationDate="2016-09-08T12:33:25.723" Score="2" Body="&lt;p&gt;This is to supplement the accepted answer by @Hobbes, and this information is from &lt;a href=&quot;http://spaceflight101.com/osiris-rex/osiris-rex-sample-return-capsule/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this link&lt;/a&gt; from that answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The return capsule is a lot larger than I imagined, 81cm diameter, 50cm tall, so there is plenty of room for insulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vAAfw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vAAfw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LHKSq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LHKSq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SgIsa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SgIsa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-09-08T12:33:25.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18112" PostTypeId="1" AcceptedAnswerId="18146" CreationDate="2016-09-08T19:36:47.423" Score="2" ViewCount="298" Body="&lt;p&gt;I know that a momentum wheel has a momentum offset and a &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_wheel&quot; rel=&quot;nofollow&quot;&gt;reaction wheel&lt;/a&gt; does not. So, I'm not expecting just this answer. But my question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any difference between these two in component and internal operation level? Or, let's put it this way: Can a reaction wheel be spun up using an external torque (i.e. thrusters or magnetorquers) and be used as a momentum wheel or become (in a sense) a momentum wheel?&lt;/p&gt;&#xA;" OwnerUserId="16381" LastEditorUserId="16381" LastEditDate="2016-09-10T11:31:02.097" LastActivityDate="2016-09-11T10:03:01.720" Title="Can a reaction wheel be spun up using an external torque and be used as a momentum wheel?" Tags="|artificial-satellite|attitude|reaction-wheel|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18113" PostTypeId="1" CreationDate="2016-09-08T23:12:17.417" Score="-4" ViewCount="783" Body="&lt;p&gt;Using weather balloons pressurized to not to pop in a chain from the ground what is  the highest altitude possible in a chain?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could up scale to use blimps but it is a balloon that can achieve the highest altitude of 53km without a tether. Wind is not a factor in the model. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V60pP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V60pP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would use this to run a power line vertically or just make a ladder to sky dive from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WGqEA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WGqEA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YiUhr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YiUhr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2019-01-06T19:06:22.583" LastActivityDate="2019-01-06T19:06:22.583" Title="Balloon Space Ladder" Tags="|reuse|space-elevator|" AnswerCount="3" CommentCount="12" FavoriteCount="0" ClosedDate="2019-01-10T09:10:43.303" ContentLicense="CC BY-SA 4.0" />
  <row Id="18114" PostTypeId="1" CreationDate="2016-09-08T23:32:25.183" Score="5" ViewCount="79" Body="&lt;p&gt;Does NASA broadcast live video of their launches (specifically from Kennedy) that does &lt;em&gt;not&lt;/em&gt; feature commentary? I appreciate Mike Curie's comments but... sometimes I just want to hear the controllers do their work.&lt;/p&gt;&#xA;" OwnerUserId="9041" LastActivityDate="2016-09-08T23:32:25.183" Title="Commentary-free launch video?" Tags="|launch|nasa|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18116" PostTypeId="1" AcceptedAnswerId="18120" CreationDate="2016-09-09T07:11:30.290" Score="11" ViewCount="2520" Body="&lt;p&gt;Yesterday ISRO operated the GSLV MK2 launch vehicle, which is a medium lift launch vehicle with a capability to lift 5 tonnes to LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just compared Falcon 9 with GSLV MK2. Falcon 9 capability to lift 23 tonnes to LEO is almost 5 times as much as the GSLV. So I expect a similar difference in thrust for each rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GSLV First Stage : 760 kN * 4 (L40 Straps)  + 4,700 kN (Solid core stage) = &lt;strong&gt;7740kN&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 FT First Stage : 825 kN * 9 (Merlin 1D+) = &lt;strong&gt;7425kN&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both LVs first stages burn duration is around 160 seconds. Specific impulse also does not have much difference for both Vikas and Merlin engine. Also GSLV third stage uses a cryogenic engine with very high Specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still what are the main factors which allow Falcon 9 to perform &lt;strong&gt;5 times better&lt;/strong&gt; than GSLV Mk2?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the second stage of GSLV the main reason?&lt;/p&gt;&#xA;" OwnerUserId="10204" LastEditorUserId="-1" LastEditDate="2016-09-14T20:01:54.570" LastActivityDate="2023-08-30T19:39:36.553" Title="GSLV MK2 vs Falcon 9 - Reasons for Huge performance difference" Tags="|launch|spacex|falcon-9|isro|merlin-1d|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18117" PostTypeId="2" ParentId="18113" CreationDate="2016-09-09T10:29:45.473" Score="9" Body="&lt;p&gt;&lt;strong&gt;Summary&lt;/strong&gt;: No, this won't work in any way, shape or form. And if you did get it to work, it'd be more expensive than using rockets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;10 reasons it won't work: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aircraft can get to an altitude of ~30 km. Above that, the atmosphere is so thin you need unfeasibly large wings to provide enough lift. An aircraft that flies next to a rail is no different in this regard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's the method for transferring power from the rail to the aircraft. This has been studied extensively: trains use the same method to transfer power from their overhead catenary wires. At a speed of 500 km/h this is already problematic, as the SNCF found when preparing for the world &lt;a href=&quot;https://en.wikipedia.org/wiki/TGV_world_speed_record&quot; rel=&quot;nofollow noreferrer&quot;&gt;speed record with their TGV&lt;/a&gt;. The pantograph presses against the power wire/rail. This moves the power wire/rail. At low speed, this movement is small enough that the pantograph remains in constant contact with the wire. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At high speed, the pantograph can't follow the wire reliably anymore so you get intermittent contact. Applying more (mechanical) tension to the wire reduces how much the wire moves around, but this is limited by the mechanical strength of the wire. The highest speed ever achieved with such a power transfer method is ~580 km/h or about 200 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then there's the distance between the aircraft and the rail. This has to be small enough to allow a structure like a pantograph to bridge the gap. But it has to be large enough to prevent the aircraft from hitting the rail when it encounters turbulence (which can easily displace the aircraft tens of meters). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you can overcome the turbulence problem, you've carried your rocket to 20 km altitude and 200 m/s. You need 200 km and 8 km/s, so all that effort has given a very small reduction of the rocket size.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Airplane-launched rockets exist (Pegasus, Virgin LauncherOne) but they aren't common. Most rocket companies don't think it's worth bothering with an aircraft as the first stage. An electric-powered aircraft would add several more hurdles to an already difficult problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems you're trying to make the launch cheaper by saving fuel. But the fuel cost is a negligible part of the cost of a rocket. A SpaceX Falcon 9 costs \$60M to launch. Only \$200k of that is the fuel cost.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if you could accelerate to 1000 m/s on your launch system, that leaves 7000 m/s that must be provided by the booster. 7/8 of a Falcon 9 is still a \$60M rocket, it'll just be a bit shorter than the current F9 because it needs less fuel. The only cost advantage is in a barrel section for the fuel tanks you can omit, and those are cheap parts.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in order to save a few hundred thousand per launch, you're proposing a rail system and dozens of blimps. Launch rates would have to be huge to make that economically feasible. Until then, using an aircraft as the first stage is a better option, and even that has its limitations, as no aircraft exist that are large enough to launch e.g. a 4-ton communications satellite into GEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;New: cost, wight, volume calculation&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Catenary wire as used by railroads weighs &lt;a href=&quot;http://www.elandcables.com/cables/railway-catenary-wire&quot; rel=&quot;nofollow noreferrer&quot;&gt;on the order of 600 kg/km&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.isas.jaxa.jp/e/special/2003/yamagami/03.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bu60-1&lt;/a&gt; was 60,000 m&lt;sup&gt;3&lt;/sup&gt;, that's 50 m in diameter and had a payload of 4.6 kg. So a balloon capable of supporting 500 kg would need a volume of ~60 million m&lt;sup&gt;3&lt;/sup&gt;, that's a diameter of 500 m. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://ravenind.com/news/raven-aerostar-s-largest-super-pressure-balloon-proves-persistent-altitude-for-long-duration-capabil&quot; rel=&quot;nofollow noreferrer&quot;&gt;largest high-altitude balloon&lt;/a&gt; is 0.5 million m&lt;sup&gt;3&lt;/sup&gt; and has a lifetime of 30 days. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your balloons would need to be 10 times larger than the largest high-altitude balloon ever made, and you'd need to replace them every 30 days. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The string needs to be at a shallow angle to prevent the aircraft from stalling. Say a 1/10 grade. To reach 50 km altitude, you need a string 500 km long. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A catenary 500 km long doesn't work: railroad catenaries are supplied with power at regular intervals, at 500 km the voltage drop would be severe. &lt;/li&gt;&#xA;&lt;li&gt;to fill the balloons, you need 3 billion m&lt;sup&gt;3&lt;/sup&gt; of helium, that's &lt;a href=&quot;https://en.wikipedia.org/wiki/Helium_production_in_the_United_States&quot; rel=&quot;nofollow noreferrer&quot;&gt;30 times&lt;/a&gt; the world annual production. Cost is on the order of billions of dollars. &lt;/li&gt;&#xA;&lt;li&gt;I've ignored wind effects: high altitude wind exerts lots of force on the balloons and the wire. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-31T21:13:59.980" LastActivityDate="2018-01-31T21:13:59.980" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18118" PostTypeId="2" ParentId="14577" CreationDate="2016-09-09T11:43:50.203" Score="4" Body="&lt;p&gt;The part of propellant burned in the gas generator has a lower temperature than that in the combustion chamber. When the gas from the generator passed the turbine the temperature decreases further. In the combustion chamber this gas with a lower temperature is mixed with the other hot gas, the resulting temperature is a little lower. Lower temperature means a little less pressure and little less exhaust velocity and therefore a little less efficiency. No propellant mass flow is lost for the turbo pumps, but a little heat loss means a loss of energy.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-09T11:43:50.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18119" PostTypeId="2" ParentId="18116" CreationDate="2016-09-09T12:01:17.053" Score="6" Body="&lt;p&gt;There are lots of factors that impact the mass a Launch Vehicle can bring to orbit. In this case, what we need to look at is not only the first stage's thrust, but also its efficiency, measured in rocketry as the Specific Impulse (I&lt;sub&gt;sp&lt;/sub&gt;), which indicates the energy an engine can extract from a given mass of propellants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to wikipedia, the GSLV MkII's first stage has a very low I&lt;sub&gt;sp&lt;/sub&gt; (237s) due to its use of a solid motor, while the boosters have a slightly higher I&lt;sub&gt;sp&lt;/sub&gt; (262), still fairly low for liquid engines. For comparison, the Falcon 9 Full Thrust's Merlin-1D engines rate at 282s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, it shows on the GSLV MkII's page that the first stage burns for 100 seconds, while the strap-on boosters burn for 160 seconds. This means that for the last 60 seconds of their burn, the four liquid boosters are carrying the first stage's casing which has become pure dead weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, looking at the launch mass of both vehicles gives us another reason: Falcon 9 Full thrust takes off at 549 tonnes, while GSLV MkII weighs 414 tonnes. We don't know the exact propellant mass fraction for either, but most rockets are around 90% fuel by mass at takeoff — GSLV just carries much less fuel than Falcon 9 FT&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_Satellite_Launch_Vehicle&quot; rel=&quot;nofollow&quot;&gt;GSLV MkII&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9_Full_Thrust&quot; rel=&quot;nofollow&quot;&gt;Falcon 9 FT&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8841" LastEditorUserId="4660" LastEditDate="2016-09-15T14:18:56.973" LastActivityDate="2016-09-15T14:18:56.973" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="18120" PostTypeId="2" ParentId="18116" CreationDate="2016-09-09T15:08:27.077" Score="12" Body="&lt;p&gt;The biggest factor is probably the &amp;quot;inside-out&amp;quot; staging design, where the solid core burns out but is not jettisoned, while the liquid boosters continue to run.&lt;/p&gt;&#xA;&lt;p&gt;Spaceflight101 gives the empty mass of the (steel-cased) solid stage as 28 tons (with an unremarkable 83% mass fraction); this means the otherwise decent specific impulse of the strap-on engines is horribly compromised by carrying 28 tons of dead weight for a full minute.&lt;/p&gt;&#xA;&lt;p&gt;That unusual design is derived from the earlier PSLV launcher, which uses the same solid rocket first stage, but with smaller and shorter-burning solid strap-on boosters which drop away halfway through the first-stage burn. It was easier and cheaper  to change those out for larger liquid-fueled boosters than to design an all-new high-thrust liquid first stage for GSLV.&lt;/p&gt;&#xA;&lt;p&gt;GSLV MkIII will be a more efficient solids-on-the-outside design, launching 10 tons to LEO from a 630 ton launch weight.&lt;/p&gt;&#xA;&lt;p&gt;In general, you can't expect to learn much about payload by comparing launch thrust. For orbital launchers, liftoff thrust is generally correlated with liftoff weight - typically you start with a 1.15-1.5 ratio -- but liftoff weight is not closely correlated with payload, as discussed in another Q/A: &lt;a href=&quot;https://space.stackexchange.com/questions/10670/why-are-rocket-mass-on-the-launch-pad-and-payload-mass-to-leo-not-strongly-corre&quot;&gt;Why are rocket mass on the launch pad and payload mass to LEO not strongly correlated?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2023-08-30T19:39:36.553" LastActivityDate="2023-08-30T19:39:36.553" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18122" PostTypeId="1" CreationDate="2016-09-09T16:28:56.117" Score="3" ViewCount="291" Body="&lt;p&gt;I saw these boarding passes the other day in JAL museum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Funny enough, since it's printed by JAL, which does not fly to space yet, I guess this should be considered as a correspondence flight :) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/d3PQR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d3PQR.jpg&quot; alt=&quot;Boarding tickets&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;Picture source: Own work.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it a common practice to print this kind of tickets for astronauts ? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="10273" LastEditDate="2018-02-03T04:26:33.623" LastActivityDate="2018-02-03T04:26:33.623" Title="Is printing boarding pass to space a common practice?" Tags="|astronauts|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18123" PostTypeId="2" ParentId="18113" CreationDate="2016-09-09T17:14:59.710" Score="2" Body="&lt;p&gt;It looks like the space shuttle is doing 1500 m/s at 30km altitude, which is about 3350 mph. The fastest jet engine ever built is the mighty J-58 which in the SR-71 produces a top speed of about 2200 mph. I doubt that an e-fan can outdo that and it's already 50% short. You need to be going at least 15,000 mph to make orbit, so I don't see the e-fans adding any value.&lt;/p&gt;&#xA;" OwnerUserId="13841" LastActivityDate="2016-09-09T17:14:59.710" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18124" PostTypeId="2" ParentId="18113" CreationDate="2016-09-09T17:18:04.840" Score="5" Body="&lt;p&gt;The velocity you need to maintain LEO (~200 km) is on the order of 7800 m/s (~28,000 mph).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SR-71, which AFAIK is the fastest-flying jet aircraft, reached a top speed of a little over Mach 3, or around 1029 m/s (~2300 mph).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The X-15 rocket plane topped out at ~2020 m/s (~4520 mph).  Both of these fell well short of what is required to reach orbit (which is what I interpret most people to mean when they say &quot;space&quot;).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, of course, you run out of usable atmosphere to generate lift well short of LEO.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, yes, you &lt;em&gt;definitely&lt;/em&gt; need rockets to actually reach orbit, meaning you need to carry the mass of the rocket engine and propellant with you as you climb the cable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we have to consider the following logistical issues:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Stringing a series of blimps together from ground level to into the stratosphere (20-some-odd kilometers, give or take), such that they stay put and don't pose a hazard to other aircraft;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Suspending a continuous conductor from those blimps (that, at 20-some-odd kilometers long, is going to weigh several tens of tonnes, meaning that it will have to have a high tensile strength);&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Keeping the string of blimps and conductor stable as an aircraft is screaming alongside it, and building it to withstand the shockwaves from the aircraft as it goes supersonic;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Transferring energy from the conductor to the aircraft;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Building an electric turbofan capable of propelling an aircraft to speeds in excess of 2000 m/s, while it's carrying the payload and rocket mass;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Safety protocols if a blimp envelope fails, or if the conductor breaks loose;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Then there are some engineering questions I'm in no way qualified to answer, like how thick the conductor needs to be to transfer sufficient power without melting, how to generate that power, how much is lost to resistance by the time you get to the top of the conductor, etc.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, this is not a practical approach.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Anything&lt;/em&gt; that saves you gravity losses is a good idea, which is why some people have developed airborne launch systems - the rocket isn't fighting gravity for as long as it would when launched from the ground, which does save quite a bit of propellant mass.  However, airborne launch simply isn't practical for anything but small payloads.  It's a lot easier (at least from an engineering perspective) to build bigger rockets than to build bigger airplanes.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastEditorUserId="12922" LastEditDate="2016-09-09T17:37:59.120" LastActivityDate="2016-09-09T17:37:59.120" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18125" PostTypeId="1" CreationDate="2016-09-09T20:09:46.120" Score="8" ViewCount="1604" Body="&lt;p&gt;I've seen many designs for creating artificial gravity within a habitat in space by rotating the habitat about a central axis. With today's science, can artificial gravity be generated in a vehicle, or habitat, that would mimic the gravity of earth (1-g)? &lt;/p&gt;&#xA;" OwnerUserId="16920" LastActivityDate="2016-09-11T05:51:19.620" Title="Is it possible to create artificial gravity in a habitat in space, or on another planet or moon without spinning the hull of the habitat" Tags="|artificial-gravity|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18126" PostTypeId="2" ParentId="18125" CreationDate="2016-09-09T20:21:59.210" Score="-4" Body="&lt;p&gt;The answer is yes. Create a vehicle inside a hollow, stationary, radiation shield - about one-mile in diameter. Have the vehicle run on a magnetically-levitated track around the inside perimeter of the hollow tube. At the proper velocity artificial gravity, (even 1-g) can be produced inside the moving vehicle. Another similar vehicle could run on a separate track to match velocities and on-load, or off-load people or materials to the first vehicle. The vehicles could be pressurized to create conditions similar to an environment at sea-level on earth. This could be done on a moon, another planet, or in space.&lt;/p&gt;&#xA;" OwnerUserId="16920" LastActivityDate="2016-09-09T20:21:59.210" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18127" PostTypeId="2" ParentId="18125" CreationDate="2016-09-09T22:26:58.430" Score="8" Body="&lt;p&gt;Short answer:  No, we cannot create a stationary habitat that has a gravity-like force that does not depend on motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we &quot;feel&quot; the force of gravity, what we actually feel is the ground pushing up on us.  This is called the &quot;Normal Force&quot;, and is a result of &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton.27s_3rd_Law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's Third Law&lt;/a&gt;.  The normal force you feel is the result of the Earth beneath you pushing you up and supporting your weight.  However, if you were to go skydiving, when you jump out of a plane, there is nothing to support you, so you do not feel any force.  You are in &lt;a href=&quot;http://www.physicsclassroom.com/class/1Dkin/u1l5a&quot; rel=&quot;nofollow noreferrer&quot;&gt;free fall&lt;/a&gt;.  This is what the astronauts feel as they orbit the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9jh6J.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9jh6J.png&quot; alt=&quot;A diagram showing the Normal Force&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you're in a spinning space station, your inertia causes your body to want to move in a straight line.  This is &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton.27s_1st_Law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's First Law&lt;/a&gt;.  However, as your body tries to move in a straight line, it meets the wall of the space station, which is curving beneath you.  The space station's wall resists you, and creates a normal force, which you feel like gravity.  Imagine you are in a car that suddenly turns to the right.  When it turns, you feel a &quot;force&quot; which pulls you to the left.  This is your inertia, resisting the acceleration of the car.  You press against the wall of the car, which presses back on you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to recreate this without having a rotating system is by using a constantly, linearly accelerating vehicle, like a rocket.  By firing its thrusters, the rocket would essentially push all of the occupants forward, creating a normal force, which you could interpret as a gravitational force.  It's just like when you're in a car at a stoplight; when the car accelerates, you feel a &quot;force&quot; pushing you back in your seat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, having a rocket that is constantly accelerating would (a) require lots of fuel and (b) soon leave the solar system.  Neither is good for a long-term space station.  A rotating space station, however, is pretty easy, because once we spin the station up, there is nothing to slow it back down.  Having a rotating system on another planet or moon, however, is a lot more difficult, for a host of reasons.  The primary reason, however, is that the speed at which the station (or parts of it) would have to be rotating to create a 1g force is extremely high.  (&lt;a href=&quot;https://space.stackexchange.com/questions/281/what-would-the-size-and-rotation-of-a-station-need-to-be-to-produce-1g-gravity-f&quot;&gt;A similar question was asked here&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because gravity is, in a sense, the result of an object's inertia, there is no way we can create a gravity-like force without having the object undergo some form of acceleration, whether that be rotational or linear.  SciFi movies, like Star Wars and Star Trek, where they have &quot;artificial gravity generators&quot; are entirely fictional, and we have no way of creating one with our understanding of physics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A different explanation can be found &lt;a href=&quot;http://curious.astro.cornell.edu/about-us/150-people-in-astronomy/space-exploration-and-astronauts/general-questions/927-can-artificial-gravity-be-created-in-space-intermediate&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-09-09T22:26:58.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18128" PostTypeId="2" ParentId="18122" CreationDate="2016-09-10T01:50:47.323" Score="2" Body="&lt;p&gt;This appears to be a promotional item.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Koichi_Wakata&quot; rel=&quot;nofollow&quot;&gt;Koichi Wakata&lt;/a&gt; launched to the ISS on a Soyuz (correctly listed as TM-11M) in November 2013 which is close to the dates on the &quot;ticket&quot;.  The return date, however, is not close to the actual return date of May 2014.  BKN is probably supposed to be Baikonur.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would be surprised if Japan Air Lines is actually authorized to issue tickets for Soyuz flights.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-09-10T01:50:47.323" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18129" PostTypeId="1" AcceptedAnswerId="18132" CreationDate="2016-09-10T07:47:29.100" Score="6" ViewCount="1703" Body="&lt;p&gt;To my understanding the main problem with space elevators is that the strength to weight ratio is not high enough to support the cable's weight. Naturally the first thought that would come to mind is to use a cord with varying thickness. Ie. the cord gets thinner as it gets closer to earth. Why is the strength to weight ratio still a problem? &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_elevator&quot; rel=&quot;noreferrer&quot;&gt;Quoted from wikipedia&lt;/a&gt;: Artsutanov's idea was introduced to the Russian-speaking public in an interview published in the Sunday supplement of Komsomolskaya Pravda in 1960,[10] but was not available in English until much later. He also proposed tapering the cable thickness so that the stress in the cable was constant. This gave a thinner cable at ground level that became thickest at the level of geostationary orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="16921" LastActivityDate="2018-04-11T14:12:25.040" Title="Space elevator - cord thickness - why is the strength to weight ratio a problem?" Tags="|space-elevator|" AnswerCount="1" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18130" PostTypeId="1" CreationDate="2016-09-10T09:40:42.050" Score="2" ViewCount="230" Body="&lt;p&gt;Most systems, that I am aware of, use vertical-launch but horizontal-launch has been envisioned for many years. Has horizontal-launch ever been successful?&lt;/p&gt;&#xA;" OwnerUserId="16920" LastActivityDate="2016-09-10T12:57:04.270" Title="Has any system been placed in orbit using horizontal-launch?" Tags="|launch|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18132" PostTypeId="2" ParentId="18129" CreationDate="2016-09-10T12:25:59.310" Score="19" Body="&lt;p&gt;It's the exponential nature of the equation determining the thickness. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's discretize the problem to make it simpler to picture: you have a thread of very strong material, that has a self-breaking length of two kilometers. That means, if you hang more than two kilometers of this rope on iself, it will break under own weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's cut it in kilometer long pieces, so each piece can hold another time its weight. Then let's try to extend it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First two segments are single strings, each a kilometer long, first one with 50% redundancy, the other at breaking limit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third kilometer must hold the weight of two lengths  - so it must be two such pieces in parallel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fourth kilometer holds weight of four lengths (the initial two and the segment with two), so four ropes in parallel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fifth - eight times the thickness. Sixth - sixteen. Seventh - thirty-two. To add the twelth kilometer, the thickness must be 1024 times the initial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not actually AS bad, as with altitude gravity starts dropping both with distance of the central body and with fraction of orbital velocity. But it's a long time and a huge stress before we get there and the thickness starts stabilizing, then dropping as it extends past GEO. Still, with current material engineering, the amount of material needed is huge - and in case of materials less durable than nanotubes the exponential explosion means there's not enough material on Earth to build it, as it quickly gets into continent-sized thicknesses..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of earlier estimates for using nanotubes gave the thickness of 1km around GEO for 1 ton payload. There are newer ones that give more promising estimates, but that should give you a clue why this is a problem.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-09-12T07:51:05.603" LastActivityDate="2016-09-12T07:51:05.603" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18133" PostTypeId="2" ParentId="18130" CreationDate="2016-09-10T12:57:04.270" Score="2" Body="&lt;p&gt;Yes, in the event of plane based launches. Pegasus is the most well known such system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/m7_yyvGxwrE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-10T12:57:04.270" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18134" PostTypeId="1" CreationDate="2016-09-10T13:18:37.023" Score="4" ViewCount="1295" Body="&lt;p&gt;I'm looking for a video showing what it looks like to travel at 8k/s at a low altitude (ignoring air friction)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the orbital videos I've seen are so far from the ground that there's no real sense of speed. But i imagine a video shot from a plane flying quite low and then sped up to the right speed would be awesome. Even a computer rendering would be cool. Anyone ever seen or able to make such a thing?&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-11T01:31:21.627" Title="What does orbital velocity look like near the ground" Tags="|orbit|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18135" PostTypeId="1" CreationDate="2016-09-10T14:33:00.980" Score="7" ViewCount="4906" Body="&lt;p&gt;As part of a school project, I have to build my own reaction wheel to be used for an engineering model of a nanosat orbiting in the LEO. I am kind of lost as how to progress on building my own reaction wheel, and how to predict the actuation accuracy. This is what I know:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reaction wheel would need: a motor driver (basically a current amplifier, or can a microcontroller be used as well?), gyro rate sensor, a DC brushless motor, and a wheel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My questions are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Would I be able to use a microcontroller as my motor driver? I've heard about speed control algorithms as well, but I'm not sure what they are used for.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What should I use as the wheel?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I am required to state the pointing accuracy that I would be able to achieve, I honestly don't know how to gauge this, what I'm thinking of doing is just going with the bare minimum, maybe take the accuracy of a space grade reaction wheel and reduce it by 20-30%.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="16924" LastEditorUserId="716" LastEditDate="2020-04-01T07:25:38.663" LastActivityDate="2020-04-01T07:25:38.663" Title="DIY Reaction wheel and actuation accuracy" Tags="|reaction-wheel|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18137" PostTypeId="1" CreationDate="2016-09-10T15:40:19.323" Score="3" ViewCount="184" Body="&lt;p&gt;Are brown dwarfs useful for gaining velocity via gravitational slingshot (or Oberth Maneuver, I'm still fuzzy on the difference to be honest)  by a theoretical starship with fusion propulsion looking to gain velocity while conserving fuel? I have been informed, correctly or not, that a brown dwarf wouldn't have enough velocity for a ship to gain a significant boost in the turn, (would it make a difference in velocity if it were part of a binary pair?) but I wanted a second opinion. &lt;/p&gt;&#xA;" OwnerUserId="16346" LastEditorUserId="16346" LastEditDate="2016-09-10T16:28:33.053" LastActivityDate="2016-09-12T18:18:30.783" Title="Brown dwarfs: could they be used to gain velocity in either a gravity-assisted slingshot or an Oberth maneuver? Why or why not?" Tags="|crewed-spaceflight|oberth-maneuver|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18139" PostTypeId="2" ParentId="18135" CreationDate="2016-09-10T17:19:55.087" Score="3" Body="&lt;p&gt;Maybe an old hard drive platter would make a good wheel? They are certainly machined to very precise measurements. You'd probably be able to recycle some of the mounting and axle from one too, but as to the motor I'm not sure. HD's probably use something much more complex than a simple dc motor tho.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to accuracy, just guessing isn't going to get you much points, even if you are correct with 20-30%, your method for determining that value is rubbish. At least do some maths involving the mass of the wheel, its rotation speed, make an estimte of its moment of inertia and thus an estimate of its angular momentum. It's your homework after all, we shouldn't do it for you.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-10T17:19:55.087" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18140" PostTypeId="2" ParentId="18108" CreationDate="2016-09-10T17:24:07.523" Score="-2" Body="&lt;p&gt;In the same way re-entering astronauts remain cool and unmodified, by using a heat shield.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-10T17:24:07.523" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18141" PostTypeId="1" AcceptedAnswerId="18250" CreationDate="2016-09-10T18:02:25.383" Score="4" ViewCount="308" Body="&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/_BgJEXQkjNQ?start=219&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is well after the first and second stage were gone, so I suspect it was something about the pad itself that resulted in the second explosion event. Any ideas?&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="23891" LastEditDate="2019-03-31T05:48:13.423" LastActivityDate="2019-03-31T05:48:13.423" Title="What was the cause of the second explosion in SpaceX's pad &quot;Fast Fire&quot;?" Tags="|spacex|falcon-9|failure|launchpad|rocket-explosion|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18142" PostTypeId="1" AcceptedAnswerId="20844" CreationDate="2016-09-10T20:49:28.170" Score="9" ViewCount="1637" Body="&lt;p&gt;Been reading about early space exploration again, especially Sputnik 2 (1957) and Venera 1 (1961). Both of these suffered from overheating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It strikes me that a metal object will easily &quot;absorb&quot; the Sun's rays, which are not mitigated by any atmosphere. A deep space probe will not have the protection of Earth's magnetic field either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it seems to me that a metal object, if warmed by the Sun, will cook anything inside of it. So how is a spacecraft engineered to not overheat? Especially for something like a solar sensor or solar panel, which cannot be covered by a reflective sheet.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="12102" LastEditDate="2021-04-01T22:09:37.750" LastActivityDate="2021-04-02T09:15:46.307" Title="How are spacecraft engineered to prevent overheating?" Tags="|spacecraft|engineering|thermal|thermal-control|heat|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18143" PostTypeId="2" ParentId="18142" CreationDate="2016-09-10T23:08:16.970" Score="8" Body="&lt;p&gt;There are many ways to control the temperature of a satellite and the distribution of heat within a satellite. Often they are wrapped in &lt;a href=&quot;https://space.stackexchange.com/questions/5246/why-is-gold-used-in-space-technology-to-protection-from-heat-radiation&quot;&gt;multi layer insulation&lt;/a&gt;, which gives them their typical golden shine, or other surface treatments like white paint are applied. Other measures include retractable blinds, orienting the satellite in a specific way to the sun, heat pipes/heat bridges and radiators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are just some tools that professional thermal analysts use in the design of satellites to keep the critical components within their respective operating temperature envelopes. Usually most importantly, the batteries must be kept within a few tens of degrees (C) at all times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You also touched briefly on magnetic particles, which are deflected by the earth's magnetic field, but that is another matter altogether, because particles do not cause a significant amount of extra heat flux on spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-09-13T15:45:54.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18144" PostTypeId="2" ParentId="18134" CreationDate="2016-09-11T01:31:21.627" Score="8" Body="&lt;p&gt;Nothing has ever travelled 8km/s in the Earth's lower atmosphere; and to be frank I'm not sure whether that's ever going to be possible&lt;sup&gt;citation needed&lt;/sup&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As uhuh points out in the comments on your post, the velocity you need to orbit Earth at ground level is approximately 7910m/s, or a little over Mach 23.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can however provide you video footage of something traveling at a little under half that, at a more leisurely pace of Mach 10: the &lt;strong&gt;Martin Marietta Sprint missile&lt;/strong&gt;, which was introduced in 1972 as an anti-ballistic missile as part of the Sentinel Program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6tzNG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6tzNG.jpg&quot; alt=&quot;The Sprint missile, developed by Martin Marietta&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the time it reached its cruising speed in under 5 seconds, it had accelerated to a velocity of &lt;strong&gt;Mach 10&lt;/strong&gt; (or 3,430m/s), traveling somewhere between FL50 and FL1000. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;There is video of it in action &lt;a href=&quot;https://www.youtube.com/watch?v=kvZGaMt7UgQ&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sheer amount of air friction generated required the use of an ablative heatshield which glowed white hot at over 3400°C&lt;sup&gt;&lt;a href=&quot;http://www.nuclearabms.info/Sprint.html&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;&lt;/sup&gt;, which you can see in the above video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To travel 2.3x faster than this, you would need to have 5.3x the amount of kinetic energy; which would likely be highly unsurvivable even with the state of materials science today. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2016-09-11T01:31:21.627" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18146" PostTypeId="2" ParentId="18112" CreationDate="2016-09-11T10:03:01.720" Score="2" Body="&lt;p&gt;Yes, many models of reaction wheel/momentum wheel are useable in both modes of operation, it really just comes down to the control algorithm. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a clarification in terminology, for a wheel to rotate with respect to the body of the satellite it is mounted in all that is required is for the wheels drive motor to be turn on. In isolation, an assuming that the satellite is in orbit rather than in a ground test, the reaction through the motor drive will be that both the wheel and the satellite body will adopt a net rotational rate with respect to inertial space. The rates will be of opposite sign and of magnitudes inversely proportional to their inertias. If the satellite body is required to be stationary &lt;strong&gt;then&lt;/strong&gt; some external torque needs to be applied to keep it so.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-09-11T10:03:01.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18148" PostTypeId="1" CreationDate="2016-09-11T15:48:02.087" Score="2" ViewCount="1054" Body="&lt;p&gt;So I was working on my sci-fi novel, and I stumbled across a problem: How much toilet paper do astronauts use in space? As of right now, the furthest human beings from Earth are orbiting just above our heads. So, it would be very easy to deliver new supplies and to remove waste via small cargo craft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, as we venture further out into space (such as to Mars, Jupiter, Saturn, or maybe even to another star), simply restocking the ship becomes pretty much impossible mid flight. So, astronauts will have to either bring everything they need with them, or generate it on board (i.e. growing food mid-flight in an garden). As the distance gets further, however, you will need to bring more supplies and materials with you; which ultimately raises the mass of the ship and requires more fuel to get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because toilet paper is an expendable resource, the crew of a deep space mission would need &lt;strong&gt;a lot&lt;/strong&gt; of it for one mission. Just a flight to Mars could require 6 months worth of TP. A flight to our nearest star, even at 1/5th the speed of light) would require almost &lt;strong&gt;20 years&lt;/strong&gt; worth of it. In order to keep fuel costs down, ship mass light, and butts clean, we need a solution that is either renewable or (and I hate to say it) reusable for a mission beyond the Earth's orbit. So, besides just holding it in, does anyone have any ideas on how to solve this crisis?&lt;/p&gt;&#xA;" OwnerUserId="35454" OwnerDisplayName="Mattias" LastEditorUserId="4660" LastEditDate="2016-09-12T01:04:52.747" LastActivityDate="2016-09-16T08:47:56.543" Title="What is a good alternative to toilet paper for space travel?" Tags="|astronaut-lifestyle|" AnswerCount="2" CommentCount="6" ClosedDate="2016-09-12T13:31:06.293" ContentLicense="CC BY-SA 3.0" />
  <row Id="18149" PostTypeId="1" AcceptedAnswerId="18151" CreationDate="2016-09-11T18:06:18.473" Score="6" ViewCount="264" Body="&lt;p&gt;Reading Twitter comments about the recent Falcon 9 pad explosion, some postulated the problem was that a static discharge ignited the fuel as it was being loaded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since TEA-TEB is required to ignite the Falcon's fuel (something you'd assume would be easy, but it's not!), I am skeptical that static discharge is even a plausible cause of the explosion. However, since I am not a rocket scientist, I thought I'd get some experts' opinions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it even possible to ignite the Falcon's fuel with a static discharge?&lt;/p&gt;&#xA;" OwnerUserId="4603" LastEditorUserId="51" LastEditDate="2016-09-16T20:48:01.717" LastActivityDate="2016-09-16T20:48:01.717" Title="Can Falcon 9's fuel be ignited by static discharge?" Tags="|spacex|falcon-9|fuel|rud|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18150" PostTypeId="2" ParentId="18137" CreationDate="2016-09-11T19:09:22.863" Score="6" Body="&lt;p&gt;It's kind of tricky to explain and it takes some visualization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any massive object can be used for a gravity assist and several planets have been used.  The Moon (edit, Mars has been used, see first comment) has never been used cause it's too small.   Venus and Earth, were both used for the Mercury mission and all 4 of the larger planets have been used.   Gravity assists are a good way to leave the solar-system.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's both the size of the object and the relative motion that make the assist possible.   Jupiter, for example, would provide better gravity assists if it was closer to the sun and orbited faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is, a planet or star can only provide a &quot;push&quot; in the direction it's going and stars like brown dwarfs tend to move in one direction.   Think of throwing a ball into the path of a moving train, the train can only accelerate the ball in the direction it's going.  Same with gravity assists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://solarsystem.nasa.gov/basics/grav/cartoon.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://solarsystem.nasa.gov/basics/grav/cartoon.jpg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you can direct the ball at a different angle, so the spacecraft's bend is anywhere from just above a 0 degree directional change to a full 180 degree turnaround, but it still only accelerates in the direction of the object giving the assist.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 180 degree turn around provides the greatest acceleration (see picture)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mJFt2.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mJFt2.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the obvious problem here with stellar travel is that you'd have to move in the opposite of your destination to gain that maximum velocity and given how far you'd have to travel to get to a star, that would be hugely counter productive.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More probable gravitational assists are usually less than 90 degrees in adjusted direction, where the increase in velocity is measurably less, but that's the tradeoff, you want to accelerate in the direction you want to get to.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xh8Qy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xh8Qy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stars are so far apart and the &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Stellar_kinematics#Space_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;space velocity&lt;/a&gt;&quot; (yes, that's the correct term), between them tends to be quite small compared to distance between them that it's not very convenient or likely to be of much help for interstellar travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planets are far more convenient for gravitational assists because the solar system is all mostly on the same plane and because planets change their directional velocity a full 360 degrees every orbit, and even with planets, there's a window of opportunity, especially for the 4 outer planets, that NASA has to use to take advantage of gravity assists.   The window resets about every orbit or two, which, if you consider Saturn's orbit of 29 years, there's a long wait between windows when Saturn can be used for a gravity assist towards Uranus or Neptune.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In theory, if there was a very fast moving brown dwarf star and it was moving in the right direction and relatively close, that could be used for a gravity assist, but currently there's no such stars in our vicinity and there might not be for hundreds of thousands, maybe millions of years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A star with a large planet in a very low very fast orbit could be used but gravity assist velocity is still quite tiny compared to stellar distances.  Gravity assists aren't likely to ever be all that beneficial to interstellar travel.   Maybe if you were near the center of the galaxy with hyper-velocity stars, or, maybe when Andromeda and the Milky-way merge and you have stars moving in very different directions at significantly higher relative speeds, but generally speaking, yes it could be done, but it wouldn't be useful.   It's probably far more practical to build faster spacecraft.  Traveling interstellar distances with stellar gravity assists would be glacially slow.   For near by stars, you'd probably lose time trying to use stellar gravity assists.   For more distant travel you might find one that could give you a push, but at those speeds and those distances, your travel would still take tens if not hundreds of thousands of years.   It's, in no way, practical.&lt;/p&gt;&#xA;" OwnerUserId="8711" LastEditorUserId="8711" LastEditDate="2016-09-12T18:18:30.783" LastActivityDate="2016-09-12T18:18:30.783" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18151" PostTypeId="2" ParentId="18149" CreationDate="2016-09-11T19:09:55.620" Score="9" Body="&lt;p&gt;The Falcon 9 uses RP-1, which is refined kerosene. Kerosene is routinely ignited via sparks in e.g. gas turbine engines, but that's with finely misted kerosene in hot air. The 25 kV of an ignition system is readily attainable by static discharges. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gasoline fires due to static discharge happen occasionally at petrol stations. Diesel and kerosene are a bit harder to light, but the &lt;a href=&quot;https://tsocorpsite.files.wordpress.com/2014/08/kerosene.pdf&quot; rel=&quot;nofollow&quot;&gt;MSDS for kerosene&lt;/a&gt; warns to take precautions to prevent static discharge when handling kerosene. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-09-12T12:49:30.130" LastActivityDate="2016-09-12T12:49:30.130" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18152" PostTypeId="1" AcceptedAnswerId="18153" CreationDate="2016-09-11T20:06:15.953" Score="7" ViewCount="1925" Body="&lt;p&gt;I was wondering whether there is a limit to the amount in which the perceived g-forces during atmospheric reentry can be reduced by trading fuel consumption for smoothed out deceleration peaks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To define the concept I had in mind; a certain launcher coming from a certain orbit, say the one of the ISS, has a specific reentry corridor (neglecting skipping reentry and straight drops). And the atmosphere has a specific density distribution along it's altitude, so once a certain window of angles, altitudes and velocities for reentry have been selected, I could imagine it is impossible to slow down any slower than that density/atmospheric drag implies, no matter how much fuel you have left to slow the launcher down before the atmospheric drag becomes too dominant (e.g. &gt;2 G's).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Or as XY-problem, since I could not find a lot of data:)&#xA;What generally is the minimal perceived (retrograde) deceleration during for the manned spaceflights to the ISS during atmospheric reentry?&lt;/p&gt;&#xA;" OwnerUserId="16929" LastActivityDate="2021-03-08T01:36:24.930" Title="Limit to reduction of G-forces during reentry?" Tags="|reentry|fuel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18153" PostTypeId="2" ParentId="18152" CreationDate="2016-09-11T21:21:59.127" Score="12" Body="&lt;p&gt;In theory, with sufficient fuel you can come down in a powered reentry as gently as you like to a limit of just above 1g -- this is symmetrical with a theoretical ascent from surface to orbit at just above 1g. The amount of fuel required is prohibitive, however.&lt;/p&gt;&#xA;&lt;p&gt;With a high-lift reentry vehicle, reentry force can be moderated somewhat. If you had sufficient control of both lift and drag, it would be possible to again achieve any g-force profile desired. In practice, heat is the limiting factor; slow deceleration from orbital speed in atmosphere sufficient to provide lift means a sustained period of high temperatures, and the total heat load needs to be dealt with somehow. The US Space Shuttle had the gentlest g-force profile on reentry of any manned spacecraft, &lt;a href=&quot;https://space.stackexchange.com/a/50619/195&quot;&gt;around 1.6g peak&lt;/a&gt;, and this required a very advanced thermal system.&lt;/p&gt;&#xA;&lt;p&gt;Soyuz reentry from the ISS, using body lift to control the trajectory, typically incurs no more than 4.5g force on reentry.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-03-08T01:36:24.930" LastActivityDate="2021-03-08T01:36:24.930" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18154" PostTypeId="1" CreationDate="2016-09-11T21:30:40.057" Score="3" ViewCount="188" Body="&lt;p&gt;Just read this wiki article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Tyuratam&quot; rel=&quot;nofollow&quot;&gt;Tyura-tam&lt;/a&gt;, the actual location of the Cosmodrome. It says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the mid 1950s, the Soviet Union announced that space activities were being conducted from the Baykonur Cosmodrome, which was assumed to be near the city of Baykonur, in the Kazakh SSR. In reality, the launch facilities were located 400 kilometres (250 mi) to the southwest at Tyuratam to obfuscate the west as to its true location.[2]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I checked the cite and it says the same thing about mid-50's announcment as &quot;Baykonur&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I've read many times that after Yuri Gagarin flew on Vostok 1 (1961), Soviet officials filled out a FAI report to claim an official record, and listed his launch site as Baikonur. So I assumed that's when the launch site was officially named.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the wiki article and source say it was actually announced in the mid-50's. Can anyone confirm this? I've been hunting/googling for a TASS archive for a while now, but cannot find one.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2016-09-13T12:03:00.740" Title="Did TASS officially name the Baikonur Cosmodrome in 1950's instead of 1961?" Tags="|history|soviet-union|baikonur-cosmodrome|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18155" PostTypeId="1" AcceptedAnswerId="18167" CreationDate="2016-09-11T21:39:47.687" Score="11" ViewCount="1348" Body="&lt;p&gt;What do astronauts do while traveling to the ISS so they don't get bored?  I know they must check equipment and do science experiments, but what they do in their free time?&lt;/p&gt;&#xA;" OwnerUserId="13559" LastEditorUserId="13710" LastEditDate="2016-09-11T21:42:41.397" LastActivityDate="2016-09-12T21:59:51.567" Title="When astronauts travel to the ISS, how do they not get bored?" Tags="|iss|orbit|spacecraft|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18156" PostTypeId="2" ParentId="5459" CreationDate="2016-09-12T00:33:40.063" Score="3" Body="&lt;p&gt;To be technical, your title and inline questions are different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;Do Russians have means of launching a manned mission to the Moon? Do they have an engine with sufficient thrust and stability needed (RD-170 has bigger thrust than the F-1 engine, if I read the data properly), and a suitable launch vehicle to pull off such a feat?&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sort of yes, but not immediately. Presumably the Energiya booster designs are still lying around somewhere and could be thrown back into production. Energiya was designed to be flexible so it could lift different payloads to orbit, not just Buran (Soviet Space Shuttle). Its first launch put a huge 80-ton Polyus sputnik in orbit. Its second launch put the even huger craft (Buran shuttle) into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, Energiya can put about 100 tons in low earth orbit. Saturn V could do 120. In the 1960's, Soviets had a Moonshot program that needed 95 tons in LEO (it was a small lander only landing 1 cosmonaut on the Moon), so the Energiya is technically big enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;Russia manned Moon landing capability today&lt;/code&gt; ... &lt;code&gt;and do they have a spacecraft capable of landing a manned mission on the Moon's surface and later lifting it off for a likely LOR approach?&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Right. The mission as a whole needs a lander, and other stuff along the way too. Now, there was a lander (&lt;a href=&quot;https://en.wikipedia.org/wiki/LK_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;LK&lt;/a&gt;) which they tested 4 times unmanned in Earth orbit, but that was back in the late 60's and early 70's. It would presumably take much longer to dust off those designs and test them again, because the lander is in many ways a much more complicated piece of work than a launch rocket. It needs 3-axis control, guidance radars, endurance, life support, restartable and throttlable engines, and a rendezvous and dock capability with its &quot;mothership&quot;, the L-3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recommend checking out the &lt;a href=&quot;https://en.wikipedia.org/wiki/Soviet_manned_lunar_programs&quot; rel=&quot;nofollow&quot;&gt;Soviet Moonshot programs&lt;/a&gt; for lots more info.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BTW, &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit_rendezvous&quot; rel=&quot;nofollow&quot;&gt;LOR&lt;/a&gt; as America did it is not the only possibility. Another way is assembling a lunar mission in Earth orbit, much like assembling a space station, and then sending it off to the Moon. The completed stack could still do LOR at the Moon. This could conceivably be done with the Proton rocket today, and the final manned launch is done with Soyuz which rendezvous with the lunar assembly. All options have their advantages and disadvantages, however. This idea doesn't need a huge booster, but requires a lot more launches into precise orbits. If one launch goes wrong, the whole mission would be ruined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To sum up, launch capability is a maybe---depends on how fast they could resurrect the Energiya---but landing capability is a longer way off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you wanna talk other lunar missions, like a manned circumlunar, that could be done relatively quickly by rendezvousing a Soyuz and a 20-ton booster from a Proton.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastActivityDate="2016-09-12T00:33:40.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18157" PostTypeId="1" CreationDate="2016-09-12T01:18:32.323" Score="4" ViewCount="1065" Body="&lt;p&gt;Atmospheric re-entry from orbit seems such a violent affair with tons of speed and energy having to be shed, which necessitates heat shields and what not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do understand that the velocity of the craft has to slow down to de-orbit but does it have to be done in such a rapid fashion?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible for a slower deorbit that doesn't require a heat shield? What would this take? Larger wings on the orbital craft? Big engines? Large parachutes?orbit&lt;/p&gt;&#xA;" OwnerUserId="4676" LastActivityDate="2016-09-12T10:55:49.713" Title="Why can't we deorbit to earth more gently?" Tags="|orbital-mechanics|orbit|landing|reentry|" AnswerCount="3" CommentCount="3" ClosedDate="2016-09-12T10:43:42.277" ContentLicense="CC BY-SA 3.0" />
  <row Id="18158" PostTypeId="2" ParentId="18157" CreationDate="2016-09-12T02:21:47.477" Score="4" Body="&lt;p&gt;When de-orbiting, all the horizontal velocity needs to be transferred into another form of energy. In a normal reentry it is converted to heat. &#xA;If a much larger aerodynamic surface is used, the heat is dispersed over a larger area. There have been suggestions to use balloons (called a Ballute) to increase surface area for reentry. In a very shallow angle or skip reentry, the heat is dispersed over longer time. A space plane can do this.&#xA;Retro rocket landings on earth could avoid reentry heat, but it would probably require atomic rockets to have enough energy. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="16669" LastEditDate="2016-09-12T02:37:24.253" LastActivityDate="2016-09-12T02:37:24.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18159" PostTypeId="1" AcceptedAnswerId="18165" CreationDate="2016-09-12T03:06:44.277" Score="13" ViewCount="853" Body="&lt;p&gt;The Atlas V 411 configuration is interesting because there is a single SRB on one side of the first stage, requiring the main engines to vector substantially to keep the thrust mostly axial.&lt;/p&gt;&#xA;&lt;p&gt;The series of beautiful images in Space Flight Insider &lt;a href=&quot;http://www.spaceflightinsider.com/photo-galleries/photo-gallery-launch-nasas-osiris-rex/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Photo Gallery: Launch of  NASA’S OSIRIS-REx&lt;/a&gt; show the night launch, and details of the single sided booster of the Atlas V 411 configuration.&lt;/p&gt;&#xA;&lt;p&gt;In one image I noticed that the exhaust plumes are expanding differently - the SRB exhaust is expanding nicely, but the exhaust from the main engines appears to contract after it exits. Does this mean that the pressure in the exhaust is actually sub-atmospheric (if I put a rugged pressure meter in there, moving at the same speed as the exhaust would it read below 15 psi?) , or is it moving so fast with respect to the atmosphere that the &amp;quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Bernoulli%27s_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bernoulli effect&lt;/a&gt;&amp;quot; causes a pressure drop across the interface, causing it to contract?&lt;/p&gt;&#xA;&lt;p&gt;I'm looking for more than the simple answer about compromises in expansion ratio - I'd like to know if the pressure in the first stage exhaust is &lt;em&gt;really sub-atmospheric&lt;/em&gt; in a frame moving with the exhaust itself.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ohtjt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ohtjt.png&quot; alt=&quot;cropped detail of a photo of Atlas V 411 launch of OSIRIS-REx from Space Flight Insider&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; cropped detail of photo below&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y3uLt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y3uLtm.png&quot; alt=&quot;a photo of Atlas V 411 launch of OSIRIS-REx from Space Flight Insider&quot; /&gt;&lt;/a&gt; Photo of &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlas V 411&lt;/a&gt; launch of &lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-REx&lt;/a&gt; from Space Flight Insider &lt;a href=&quot;https://en.wikipedia.org/wiki/Bernoulli%27s_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-01-27T20:28:14.260" LastActivityDate="2022-10-02T20:54:41.857" Title="Why do the shapes of these two exhaust plumes from an Atlas V 411 look different?" Tags="|ula|srb|atlas-v|exhaust|osiris-rex|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18161" PostTypeId="2" ParentId="18148" CreationDate="2016-09-12T04:53:01.430" Score="0" Body="&lt;p&gt;Maybe very careful engineering could produce food which can be digested totally, with little to no waste product.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Parenteral_nutrition&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Parenteral_nutrition&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2016-09-16T08:47:56.543" LastActivityDate="2016-09-16T08:47:56.543" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18163" PostTypeId="2" ParentId="18157" CreationDate="2016-09-12T09:38:20.953" Score="3" Body="&lt;p&gt;If you are not going to use atmospheric re-entry the only other option is to use engines to slow down, and that would be incredibly expensive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine if you wanted to land on a body with 1G but no atmosphere, from LEO. Only a rocket will get you down safely in this case. In order to land from LEO you would need a rocket as big as the one that put you up there in the first place. To put a rocket that size up there would require a significant number of launches. It would be fabulously expensive to do it that way when the atmosphere lets us do it practically for free. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, it's not actually a problem in the first place - atmospheric re-entry is not particularly violent compared to launch, both are between 3-5G for most human rated systems. It's not a problem that needs solving. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-09-12T09:38:20.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18164" PostTypeId="2" ParentId="18148" CreationDate="2016-09-12T09:48:51.333" Score="2" Body="&lt;p&gt;Duplicate of &lt;a href=&quot;https://space.stackexchange.com/questions/845/without-renewable-resources-how-do-you-wipe-in-space/852#852&quot;&gt;this question&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would have to adapt a &lt;a href=&quot;http://www.wikihow.com/Use-a-Bidet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bidet&lt;/a&gt; to space.&#xA;See also: &lt;a href=&quot;https://space.stackexchange.com/questions/8337/could-a-bidet-be-used-effectively-in-free-fall&quot;&gt;This question&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-09-12T09:48:51.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18165" PostTypeId="2" ParentId="18159" CreationDate="2016-09-12T10:01:42.763" Score="15" Body="&lt;p&gt;Yes, the pressure of the first stage exhaust is always at least slightly subatmospheric, because that gives the maximum average ISP over the whole burn time. Rockets with boosters attached (parallel staging) often operate at the lowest possible exhaust pressure that prevents the flow from detaching from the nozzle walls. Historically, the &lt;a href=&quot;https://space.stackexchange.com/questions/18555/what-is-the-summerfield-criterion-regarding-rocket-exhaust-expansion&quot;&gt;Summerfield criterion&lt;/a&gt; was used, where the minimum pressure in a nozzle must be greater than 0.35...0.4*atmospheric pressure, but more recent designs use even lower pressures. For example, if I remember correctly, the Vulcain 2 was designed using the &lt;a href=&quot;https://space.stackexchange.com/questions/18555/what-is-the-summerfield-criterion-regarding-rocket-exhaust-expansion&quot;&gt;Schmucker criterion&lt;/a&gt;, which factors in the Mach number as well.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://elib.dlr.de/49253/1/AIAA2005-3940.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;More info&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="37597" LastEditDate="2022-10-02T20:54:41.857" LastActivityDate="2022-10-02T20:54:41.857" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18166" PostTypeId="2" ParentId="18157" CreationDate="2016-09-12T10:55:49.713" Score="1" Body="&lt;p&gt;Larger parachutes would not help. At the speed from the orbit, large (and also small!) parachutes would be destroyed at the upper edge of the atmosphere. Parachutes may be used only if the craft is slowed down by the heat shield to subsonic speed, about 0.2 km/s. But the speed in the orbit is much higher, about 7.8 km/s, therefore more than 97 % of the initial speed must be decelerated without parachutes.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-12T10:55:49.713" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18167" PostTypeId="2" ParentId="18155" CreationDate="2016-09-12T11:46:47.983" Score="8" Body="&lt;p&gt;It depends somewhat. There are basically 2 profiles to getting to the ISS. One of them requires only 6 hours to dock with the station, in which case there is no down time at all. As for the longer, 2 day docking profiles, they most likely don't hit that point. There is a lot to do in those two days, and much like driving across country, there's a lot to see to prevent that boredom from sinking in completely. Early flights were in fact very operator intensive, so there probably wasn't much time to be bored. They do have access to television, but I can't find much more in the ways of entertainment prior to docking with the space station.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-12T11:46:47.983" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18169" PostTypeId="1" CreationDate="2016-09-10T13:26:20.770" Score="-2" ViewCount="279" Body="&lt;p&gt;Can someone out there help explain (guide, give examples etc.), or show a solution of, the following textbook problem that I can't manage to solve?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You’ve just completed an analysis of where the Space Shuttle must be when it performs a critical maneuver. You know the shuttle is in a circular prograde orbit and has a position vector of&#xA;  $r0= 6275.396I + 2007.268J +1089.857K$    (vektor)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In 55 minutes, you predict the orbital parameters are&#xA;  $a = 1.0470357$   ,  $e = 0.000096$,   $i= 28.5°$,   $M= 278.94688°.$&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Is your analysis correct? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Comments: The initial orbit is circular, but the final orbit has eccentricity different from 0, but it is small, perhaps caused by disturbances. Cape Canaveral, from which the Space Shuttle is launched has a latitude of 28.5 degrees.&lt;/p&gt;&#xA;" OwnerDisplayName="Newbie_01" LastEditorUserId="6944" LastEditDate="2018-11-30T02:06:41.293" LastActivityDate="2019-04-29T03:03:45.873" Title="Is the analysis of this maneuver correct?" Tags="|orbital-maneuver|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18170" PostTypeId="2" ParentId="18169" CreationDate="2016-09-10T14:57:54.420" Score="-1" Body="&lt;p&gt;Okay, we have the orbital parameters given. My guess is that we need to validate one of them (or all) . The orbit was circular but put into a little elliptical orbit (since e e is not equal to 0) , which means that we have a Hohmann transfer. &#xA;The phasing time, tau, is 55 minutes (how much it takes for the transfer).&#xA;For elliptical orbit, tau= pi*sqrt((a_trans^3)/my)   (sorry, don't know how to add greek symbols) . my= gravitational parameter and a_trans is radius of periapsis.&#xA;We can calculate a_trans from this equation, and I did that and got 1.3.. Earth Radii, which does not match the given one. Therefore, the answer should be b). am I right? &lt;/p&gt;&#xA;" OwnerDisplayName="Newbie_01" LastActivityDate="2016-09-10T14:57:54.420" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18171" PostTypeId="2" ParentId="18154" CreationDate="2016-09-12T21:34:49.547" Score="4" Body="&lt;p&gt;The references to the name of &lt;em&gt;Baikonur&lt;/em&gt; were used to deliberately delude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a plywood mock launch pad built in the real town of &lt;em&gt;Baikonur&lt;/em&gt; which is hundreds of km away from the actual launch pad.&#xA;So that's what they were referring to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The village and then the city at actual launch pad had different names at different times, most notable of which is &lt;em&gt;Leninsk&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And only after Yuri Gagarin's flight the actual launch pad was named &lt;em&gt;Baikonur Cosmodrome&lt;/em&gt;. At that moment there appeared two different &lt;em&gt;Baikonurs&lt;/em&gt; on the map, some 350km away from each other. The original town and the cosmodrome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The city of &lt;em&gt;Leninsk&lt;/em&gt; hosting the cosmodrome personnel was renamed to &lt;em&gt;Baikonur&lt;/em&gt; only in 1995.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wooden mock cosmodrome in the real &lt;em&gt;Baikonur&lt;/em&gt; town &lt;a href=&quot;http://www.vestnik.com/issues/97/0930/win/snitkov.htm&quot; rel=&quot;nofollow&quot;&gt;is said to have existed up until seventies.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't confirm there were any publilc press releases with the name &lt;em&gt;Baikonur&lt;/em&gt; before Gagarin's flight though. May be it was only used in internal documentation.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2016-09-13T12:03:00.740" LastActivityDate="2016-09-13T12:03:00.740" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18172" PostTypeId="2" ParentId="18155" CreationDate="2016-09-12T21:59:51.567" Score="7" Body="&lt;p&gt;I've asked a couple of astronauts, and apparently they actually do get surprisingly bored!  (This is in the context of long-duration missions on the ISS, not transfer from/to Earth on Soyuz).  They have limited, slow internet access, a library of DVDs and iPads, as well as whatever books they can bring in their Personal Preference Kits (1kg max).  I haven't seen any references to whether Kindles have been approved for flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some musical instruments on the station including a &lt;a href=&quot;http://qz.com/596446/its-surprisingly-difficult-to-play-guitar-in-space/&quot; rel=&quot;noreferrer&quot;&gt;guitar&lt;/a&gt; and a keyboard.  Photography is also a popular pastime.  Suitably licensed astronauts can chat to hams on Earth via amateur radio.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2016-09-12T21:59:51.567" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18173" PostTypeId="2" ParentId="18045" CreationDate="2016-09-12T22:37:13.977" Score="6" Body="&lt;p&gt;Just to mention one facet of this multifaceted problem,&#xA;any electrical field potential that exists (i.e. items that aren't on a common bias electrically) can and usually will at some point create an arc (spark) when it discharges. Coupling and de-coupling of interfaces, feedlines, etc especially would be a danger if their electrical fields are uneven. Also, aside from dynamic mechanical actions just stated, static arc production is always possible with higher potential field imbalances, and would be theoretically more prominent. And as Organic Marble brought forth, with high oxygen concentrations, it doesn't take much more for dust or greasy fingerprints to become a type of fuel in itself, conveying the reactive production of (fire) (flame).&lt;/p&gt;&#xA;" OwnerUserId="16922" LastEditorUserId="6816" LastEditDate="2016-09-12T22:52:00.390" LastActivityDate="2016-09-12T22:52:00.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18174" PostTypeId="1" CreationDate="2016-09-13T00:29:40.747" Score="1" ViewCount="71" Body="&lt;p&gt;Gravity on other planets or moons do not equal that of the Earth. If an Earth-equivalent gravity is required then can this level of gravity, or any level of gravity, be created artificially? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Artificial gravity could be generated in a vehicle by moving the vehicle in a circle. If a banked, circular path were provided, occupants within the vehicle could feel as if they were under an increase in the force of gravity. If several parallel circular paths were provided &amp;mdash; such as maglev tracks &amp;mdash; trains could be placed on the tracks that could act as living quarters for people or materials that might require a gravity similar to Earth. If the diameter of the tracks are approximately one mile, then the occupants would probably not feel the Coriolis effect caused by moving in a circle. People and supplies could be interchanged between vehicles moving along the parallel tracks with the same radial velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there were requirements for communities to be constructed, such that the occupants (families with small children) would feel conditions similar to Earth, or Mars, could these variable-gravity facilities be constructed?&lt;/p&gt;&#xA;" OwnerUserId="16920" LastEditorUserId="16920" LastEditDate="2016-09-15T16:46:06.640" LastActivityDate="2016-09-15T16:46:06.640" Title="How can veriable artificial gravity be created in a full colony on another planet, or moon?" Tags="|colonization|artificial-gravity|" AnswerCount="0" CommentCount="0" ClosedDate="2016-09-15T11:35:01.340" ContentLicense="CC BY-SA 3.0" />
  <row Id="18175" PostTypeId="1" AcceptedAnswerId="21999" CreationDate="2016-09-13T01:03:50.177" Score="0" ViewCount="709" Body="&lt;p&gt;In &lt;a href=&quot;https://www.blueorigin.com/be4&quot; rel=&quot;nofollow&quot;&gt;this explanation&lt;/a&gt; of the Blue Origin BE-4 engine, the text says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;LOWEST COST&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Blue Origin’s BE-4 requires NO taxpayer dollars.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Fully funded&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The BE-4 is fully paid for by the private sector requiring no government funding for development. A recent NASA “Justification for Other Than Full and Open Competition” for buying additional Space Shuttle Main Engines states that a traditional engine development program will cost more than $2.2 billion.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Save taxpayer money&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The BE-4 saves taxpayers an additional $3 billion in national security launch costs over 20 years by providing higher thrust – 1.1 million pounds versus 860,000 pounds for the RD-180 – which enables a greater payload capability and allows for the removal of a solid rocket motor at more than  10 million per flight for comparable missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Do I understand this correctly - the BE-4 engine will save US taxpayers $3 billion over 20 years &lt;em&gt;because it has 28% more thrust than RD-180 engine?&lt;/em&gt; The math is 300 launches times 10 million per launch = 3 billion? Or 150 launches times 20 million (two boosters) per launch? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It sort-of sounds like private investment is considered &quot;free money&quot; and they'll never want to recover their investment by charging more. That's often a criticism of companies using &lt;em&gt;government money&lt;/em&gt; for development -  that it's &quot;not fair&quot; because it's &quot;free money&quot;. But I don't usually follow these arguments very closely so maybe I am missing some subtleties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However there does seem to be a &lt;em&gt;much more extensive&lt;/em&gt; discussion of the issues related to the RD-180 and BE-4 engines and US government satellite launches in the 2014 The Space Review article &lt;a href=&quot;http://www.thespacereview.com/article/2608/1&quot; rel=&quot;nofollow&quot;&gt;ULA, Blue Origin and the BE-4 engine&lt;/a&gt;. Here is a snippet, but there is much more to read in the full article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The companies issued statements to try to dispel the perceived purpose of the Blue Origin BE-4 engine: “The BE-4 is not a direct replacement for the RD-180 that powers ULA’s Atlas V rocket, however two BE-4s are expected to provide the engine thrust for the next generation ULA vehicles. The details related to ULA’s next generation vehicles—which will maintain the key heritage components of ULA’s Atlas and Delta rockets that provide world class mission assurance and reliability—will be announced at a later date.”&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;However, ULA posted this on its Twitter account: “We intend to use a pair of BE-4s on the base Atlas with even better performance.” In addition, the handout for the BE-4 states the engine will be built for “…meeting both commercial requirements and those of the U.S. Air Forces’s Evolved Expendable Launch Vehicle (EELV) program.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-17T08:09:11.650" LastActivityDate="2017-06-17T09:43:38.793" Title="How will the BE-4 engine &quot;Save taxpayer money&quot;?" Tags="|cost|cost-to-orbit|blue-origin|be4-engine|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18176" PostTypeId="1" AcceptedAnswerId="19284" CreationDate="2016-09-13T03:48:12.497" Score="6" ViewCount="766" Body="&lt;p&gt;Clarification: I'm talking about the INNER airlock door.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping the airlock closed in-between spacewalks sounds safer:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;In case of accidental puncture of the outside of the module, this has no effect on the rest of the station.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;This removes a (little bit) of livable space for the astronauts&lt;/li&gt;&#xA;&lt;li&gt;It will need to be actually opened up again once you'll want to depart.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So what do they actually do ?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="415" LastEditDate="2016-12-05T19:35:21.043" LastActivityDate="2016-12-05T19:35:21.043" Title="Is the inner airlock hatch kept open in-between spacewalks on the ISS?" Tags="|iss|procedure|airlock|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18178" PostTypeId="1" AcceptedAnswerId="18180" CreationDate="2016-09-13T09:47:00.683" Score="27" ViewCount="4656" Body="&lt;p&gt;As I understand it, a rocket should produce its maximum thrust on liftoff. As time progresses, the mass of the rocket will decrease, but since the engines produce the same thrust, the rocket will accelerate faster. So, all rocket engines on the first stage (which aren't blocked by other stages) should ignite on lift off, which will reduce the mass of the vehicle quicker, giving more acceleration. This seems true for most rockets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_Satellite_Launch_Vehicle_Mark_III&quot; rel=&quot;noreferrer&quot;&gt;GSLV MK3&lt;/a&gt; by &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Space_Research_Organisation&quot; rel=&quot;noreferrer&quot;&gt;Indian Space Research Organisation (ISRO)&lt;/a&gt;, it seems different:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9Rkyj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9Rkyj.jpg&quot; alt=&quot;GSLV MK3 Staging Timing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GSLV MK3 ignites its core liquid engines (L110) approximately 120 seconds after liftoff. As described on the launch &lt;a href=&quot;https://www.youtube.com/watch?v=aTTd9P690yU&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt;, the exact timing depends on RTC (Real Time Decision). This requires the S200 Solid strapons to lift the full L110 mass for up to 120 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What factors dictate this kind of staging sequence? Why are the L110s not ignited on liftoff? Is max Q the reason?&lt;/p&gt;&#xA;" OwnerUserId="10204" LastEditorUserId="5946" LastEditDate="2016-09-14T03:55:07.500" LastActivityDate="2020-07-29T13:20:12.447" Title="Why do some rockets not ignite all their engines during liftoff? (GSLV MK3 LV)" Tags="|launch|rockets|isro|first-stage|rocket-equation|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18179" PostTypeId="2" ParentId="18178" CreationDate="2016-09-13T10:05:49.123" Score="19" Body="&lt;p&gt;Your assumption that we need max thrust at takeoff is partially wrong.&#xA;Although right at takeoff you &lt;strong&gt;do&lt;/strong&gt; want max thrust, it might be counterproductive short after:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Your rocket and payload are Max G rated. You can't have an arbitrary high acceleration&lt;/li&gt;&#xA;&lt;li&gt;Atmospheric drag is higher at lower altitude and increase with the square of speed.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If you go too fast too quickly you'll lose all your speed to drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As quoted from wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In flight, as the thrust from the S200 boosters begins to tail off,&#xA;  the &lt;strong&gt;decline in acceleration&lt;/strong&gt; is sensed by the rocket’s onboard sensors&#xA;  and the twin Vikas engines on the ‘L110’ liquid propellant core stage&#xA;  are then ignited. Before the S200s separate and fall away from the&#xA;  rocket, the solid boosters as well as the Vikas engines operate&#xA;  together for a short period of time&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As it might be technically challenging to stop then restart the engine in flight, or throttle them too much, the liquid engine is used only as a relay for the solid boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I couldn't find the acceleration curve to support my claim; sorry for that.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="238" LastEditDate="2016-09-13T10:29:40.443" LastActivityDate="2016-09-13T10:29:40.443" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18180" PostTypeId="2" ParentId="18178" CreationDate="2016-09-13T10:20:37.077" Score="32" Body="&lt;p&gt;Besides limiting aerodynamic stress and drag losses as you and Antzi mention, using the core engine only at high altitude means the engine can be optimized for low-pressure use by putting a larger nozzle on it. This optimizes expansion of the exhaust, and in the case of GSLV MkIII contributes to a ~6% increase in specific impulse over the sea-level version of the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that GSLV III isn't unique in doing this; the Titan III and IV launchers also launched using only SRMs, igniting the liquid-fueled core stage as the solids burned out.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-09-13T12:01:09.903" LastActivityDate="2016-09-13T12:01:09.903" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18181" PostTypeId="2" ParentId="18065" CreationDate="2016-09-13T10:33:22.497" Score="4" Body="&lt;p&gt;The wording is quite confusing. First off, the article frivolously uses the term 'orbital period' without ever qualifying which they mean: sidereal, synodic, draconic or anomalistic? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That changes the result, depending on how you understand the orbital period: Is it the time between the satellite reaching periapsis on two consecutive passes, or is it the time its true anomaly repeats?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, a bit about the orbit. The Molinya orbit is to be used with constellations of three satellites; its purpose is to provide observation of two &quot;interesting&quot; regions of Earth in high latitudes; areas on two sides of Earth. This all from altitudes much lower than GEO (both lower distance for contact and observation, and lower launch costs), and thanks to the constellation configuration, 24/7 observation period is assured, at good angle to the surface of the &quot;interesting&quot; regions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, Soviets used it to observe Siberia and northern US. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Molinya orbit has a very high eccentricity. The reason is to keep the satellites of the constellation for above the interesting area of Earth for the longest time (when they move slowly near the apoapsis) and let them dip into periapsis briefly when the interesting area moves out of focus (to be taken over by two next satellites of the constellation), only to return &quot;into position&quot; shortly later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, let's neglect orbital precession of the satellite. The optimal orbital period for this configuration is one sidereal day - the three orbits of the three satellites remaining in the same orientation relative to distant stars, and while the terminator line of Earth moves as Earth circles the Sun (and synodic day shifts relative to sidereal), the whole configuration remains constant relative to &quot;distant stars&quot;, each satellite reaching apoapsis 1/6 sidereal day from the previous, and landing above the same location every sidereal day (and one on the opposite side of Earth, but same hemisphere, half a sidereal day later). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, in this configuration - taking the orbit without precession - it's accurate to say its period is half a sidereal day. That's also the number you get When you write down the orbital elements assuming Earth is a perfect sphere (as is commonly done) - so within this simplification, 0.5 &quot;sidereal day&quot; is accurate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, Earth not being a perfect sphere, perturbs the orbit, causing its precession. The orbit was chosen such, that the perturbation per day is nearly exactly the angular shift between sidereal and synodic day, per day. As result, the orbit's argument of periapsis travels around Earth at one rotation per year, remaining in constant orientation relative to the Sun, and resulting in the orbit being synchronous with Earth surface at given time of synodic day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has little practical consequence, as both all three satellites retain their synchronicity and observation of the designated locations just the same, and the ground stations would have little trouble adapting to synodic period. Except the J2 term of the gravitational field of the Earth does inevitably cause &lt;em&gt;some&lt;/em&gt; precession of any inclined, eccentric orbit - removing it would cost a lot of fuel, so instead the inherent error is transformed into a feature, the orbit chosen such, that it's synchronous to convenient, common sidereal time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angular precession per day is just a bit under 1 degree (360 degrees/365 days), and while the satellite completes the orbit the &quot;sidereal&quot; number of times - reaches the apoapsis 364 times per year - the apoapsis completes one full circle, each day both argument of periapsis and true anomaly of the satellite shifting by that 1 degree, adding one more orbital rotation to the number and resulting in the &quot;synodic day period&quot; result.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-09-14T08:12:17.733" LastActivityDate="2016-09-14T08:12:17.733" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18182" PostTypeId="1" AcceptedAnswerId="18183" CreationDate="2016-09-13T06:33:32.827" Score="3" ViewCount="302" Body="&lt;p&gt;If sending data on earth from mars takes e.g 10 min then how do NASA scientists manage to synchronize stuff with Mars rovers ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose if something goes wrong and it is required to control or guide a rover (from Earth) to perform some immediate tasks to protect itself. So from Earth instructions are sent but it reaches the rover 10 min. later and in those 10 minutes the rover has performed some other actions (which might not be enough to protect itself). So while these things need to be in sync. there should be some technologies or ways to achieve this.&lt;/p&gt;&#xA;" OwnerUserId="16949" OwnerDisplayName="Rahul Rabhadiya" LastEditorUserId="4660" LastEditDate="2016-09-13T13:54:25.367" LastActivityDate="2016-09-13T13:54:25.367" Title="How do Mars rovers synchronize with NASA scientists?" Tags="|mars|data-transmission|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18183" PostTypeId="2" ParentId="18182" CreationDate="2016-09-13T10:39:19.747" Score="8" Body="&lt;p&gt;There is no concept of &quot;immediate&quot; - which leads to situations such as Spirit getting &lt;a href=&quot;http://www.space.com/18766-spirit-rover.html&quot;&gt;stuck&lt;/a&gt; while travelling, and by the time Earth knew about it, the wheels had dug the rover in rather badly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So &quot;synchronisation&quot; itself really doesn't happen. Instead, routes are risk assessed, and instructions given to the rover to move a distance that should be safe. Some of these have been relatively long journeys, and the computer on board can handle simple instructions, but in general any challenge or any risky route requires the rover to stop, send back position data, image data and so on for analysis, and await new instructions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comms is via &lt;a href=&quot;http://mars.nasa.gov/msl/mission/rover/communications/&quot;&gt;radio&lt;/a&gt; (thanks @PeterErwin) and depending on relative positions of Earth and Mars can be anywhere between 3 and 21 minutes. So there is a lot of sitting and waiting. Some of this time is spent usefully by taking photographs, or &quot;doing science&quot; but sometimes it's just waiting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars exploration is Sloooooow!&lt;/p&gt;&#xA;" OwnerUserId="38" OwnerDisplayName="Rory Alsop" LastActivityDate="2016-09-13T12:36:37.137" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18184" PostTypeId="1" AcceptedAnswerId="18186" CreationDate="2016-09-13T14:21:14.780" Score="18" ViewCount="1932" Body="&lt;p&gt;Wikipedia has a &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-OK_No.1&quot; rel=&quot;nofollow noreferrer&quot;&gt;stub article&lt;/a&gt; on the loss of the Soyuz 7K-OK No.1 test flight in 1966-12-14, but this part doesn't make sense to me:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, once the Soyuz rocket's engines ignited, they did not operate&#xA;  at full power and didn't produce enough force to lift the rocket up,&#xA;  causing it to stay on the launch pad.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;30 minutes later:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There were only thought to be three ways to trigger the LES [&lt;em&gt;Launch Escape System&lt;/em&gt;], but there&#xA;  was in fact a fourth, which was triggered by disconnecting the rocket&#xA;  from external power and receiving a launch signal (which the system&#xA;  interpreted as the start of a vertical launch without any other&#xA;  signals by the inertial system), but &lt;strong&gt;the rotation of the earth&#xA;  causing a deviance of more than eight degrees from the initial&#xA;  starting orientation&lt;/strong&gt;, which was an abort criterion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How was the orientation measured, if not relative to the Earth's gravity, which does not change with Earth's rotation? I find no more information about this accident, which is said to have killed &quot;all&quot; ground staff since the unexpected LES activation put the whole rocket on fire (the Wiki list below says only one fatality).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've only found &lt;a href=&quot;https://history.nasa.gov/SP-4408pt1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper source&lt;/a&gt;&#xA;&quot;Siddiq (2000), p. 874.&quot;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="8162" LastEditDate="2019-02-03T11:46:39.007" LastActivityDate="2019-02-03T11:46:39.007" Title="How did the attitude system of the uncrewed Soyuz 7K-OK No.1 fail on the launch pad in 1966, killing ground staff as LES was activated?" Tags="|attitude|soyuz-spacecraft|launch-escape-system|failure|soyuz-launcher|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18185" PostTypeId="1" AcceptedAnswerId="18303" CreationDate="2016-09-13T14:32:11.207" Score="16" ViewCount="857" Body="&lt;p&gt;Up to now two spacecraft have been launched for an asteroid sample return mission. The first one &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;noreferrer&quot;&gt;Hayabusa&lt;/a&gt; is a completed mission and now &lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx&quot; rel=&quot;noreferrer&quot;&gt;OSIRIS-REx&lt;/a&gt; is an ongoing mission. Hayabusa was launched in 2003, many years before OSIRIS-REx. Technology now is much more better. What are the improvements in capabilities for the current asteroid sample return mission? What are the technological differences?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/RCWfY.jpg&quot; alt=&quot;Hayabusa spacecraft, JAXA&quot;&gt;&#xA;&lt;sup&gt;Hayabusa spacecraft, JAXA.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://pbs.twimg.com/media/Cr13DGQWIAAgOjx.jpg&quot; alt=&quot;OSIRIS-REx spacecraft, NASA&quot;&gt;&#xA;&lt;sup&gt;OSIRIS-REx spacecraft, NASA.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-09-16T16:00:25.157" LastActivityDate="2020-06-06T16:10:39.437" Title="What are the differences in technology between OSIRIS-REx and Hayabusa missions?" Tags="|asteroid|technology|sample-return|osiris-rex|hayabusa|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18186" PostTypeId="2" ParentId="18184" CreationDate="2016-09-13T15:29:08.293" Score="25" Body="&lt;p&gt;Rocket guidance systems generally use a fixed inertial platform based on gyroscopes to determine their orientation in space; an accelerometer solution would be useless to determine orientation (though helpful for position determination) as soon as the rocket was in motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_platform&quot;&gt;https://en.wikipedia.org/wiki/Inertial_platform&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once operating, the platform's orientation would be fixed in space, while the rocket, clamped to a rotating planet, turned beneath it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;guidance is internal&quot; call heard shortly before liftoff in American rocket launches marks the time at which the platform calibration is stopped and it becomes free to rotate. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-09-13T15:52:50.073" LastActivityDate="2016-09-13T15:52:50.073" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18188" PostTypeId="2" ParentId="441" CreationDate="2016-09-13T17:06:16.017" Score="1" Body="&lt;p&gt;Creating artificial gravity on another planet or moon could be accomplished the same way it would be done in space, by moving a vehicle in a circle. It would be difficult to move a very large vehicle in a circle such as spinning it, but a smaller vehicle could travel in a circle on a banked track. The system might be similar to cars moving on a banked-track at a motor speedway. Maglev systems would probably work better because of less wear on wheels. The track would need to be about one mile in diameter so that occupants would not feel the Coriolis-effect of moving in a circle as in a small diameter systems. The slope of the track could be varied depending on the velocity required to produced a specific required gravity.&lt;/p&gt;&#xA;" OwnerUserId="16920" LastEditorUserId="16920" LastEditDate="2016-09-13T17:12:56.997" LastActivityDate="2016-09-13T17:12:56.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18189" PostTypeId="1" CreationDate="2016-09-13T19:23:04.787" Score="4" ViewCount="931" Body="&lt;p&gt;Sedna is far away from the Sun and receives only a small amount of light, even if it is close to perihelion. What would a human eye see on the surface of Sedna? Would there be enough light to see where you're going? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if Sedna were at aphelion? Would it still be enough light to see something? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if Sedna has any moons, how would a human eye see them from the surface? &lt;/p&gt;&#xA;" OwnerUserId="14629" LastEditorUserId="1142" LastEditDate="2016-09-15T12:28:51.363" LastActivityDate="2016-09-15T12:28:51.363" Title="How does it look to be on the surface of Sedna?" Tags="|landing|planetary-science|colonization|observation|tno|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18190" PostTypeId="2" ParentId="176" CreationDate="2016-09-13T20:14:04.470" Score="1" Body="&lt;p&gt;There is a site, &lt;a href=&quot;http://terraforming.wikia.com/wiki/Main_Page&quot; rel=&quot;nofollow&quot;&gt;Terraforming Wiki&lt;/a&gt;. Even if it is not written by scientists or supervised by an institution, it still contains some useful information about terraforming and very interesting points of view. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many things to take into consideration when it comes to terraforming. Here, I want to point out only 4 of them. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Gravity&lt;/strong&gt; is very important. A too low gravity will make a celestial body lose its atmosphere. If one day we create an atmosphere around the Moon, it will at some point be lost in space, even if that will take more then a human lifetime. This takes out of consideration smaller moons and dwarf planets. &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Water&lt;/strong&gt; is the second most important. All inner planets and moons (except Earth) have a lack of water. If we don't bring water to them, they will become large deserts. By contrast, moons of the outer planets (except Io), if heated, will become ocean worlds. Also, other volatiles are missing where water is absent. &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Luminosity&lt;/strong&gt; is the third constrain. Plants need a certain amount of light and also they need both red and blue light waves. I made simple experiments with plants &lt;a href=&quot;http://terraforming.wikia.com/wiki/Luminosity&quot; rel=&quot;nofollow&quot;&gt;(see here)&lt;/a&gt;. Beyond the asteroid belt, plants still can grow, but I don't know if they can feed a human colony. Beyond Neptune, plant life is probably impossible. &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Chemistry&lt;/strong&gt; of the target planet or satellite is also important. Some chemical compounds might be dangerous for life. Some substances can be transformed, others cannot. Venus has an atmosphere rich in carbon dioxide and sulfuric acid, but both compounds can be transformed. By contrast, Titan might have an internal ocean &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?release=2014-217&quot; rel=&quot;nofollow&quot;&gt;as salty as the Dead Sea&lt;/a&gt; based on Cassini findings. We still don't know how to desalinize a planetary-sized object. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If we look at these four constrictions, we can say that no celestial body is easy to terraform. Also, there are greenhouse gasses that can rise temperatures even for the distant Pluto to values suitable for life. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, there are many things we don't know. For example, we still don't know what is the amount of water available on Mars. Will it be enough to create an ocean? On the other hand, many of the moons of the gas giants seem to have salty planetary oceans, which might not be suitable for Earth life. We know that Enceladus has an &lt;a href=&quot;https://en.wikipedia.org/wiki/Enceladus#Subsurface_water_ocean&quot; rel=&quot;nofollow&quot;&gt;alkaline ocean with a pH of 11 to 12&lt;/a&gt; and that also Europa has &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=6514&quot; rel=&quot;nofollow&quot;&gt;alkaline salts&lt;/a&gt; dissolved in its ocean. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In conclusion, I don't want to point a specified planet or moon as the best candidate for terraforming. Only two things are to be said: that terraforming is very hard and expensive (much beyond our current technology) and that we need much more research data before pointing to a celestial body or another. &lt;/p&gt;&#xA;" OwnerUserId="14629" LastActivityDate="2016-09-13T20:14:04.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18191" PostTypeId="2" ParentId="18178" CreationDate="2016-09-13T20:28:40.067" Score="8" Body="&lt;p&gt;Any excess energy, potential and kinetic, the boosters carry after separation is wasted. Therefore, you want to separate them at the lowest possible altitude and velocity, saving the core rocket's fuel for when it must no longer drag the boosters with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;This holds true for any stage separation and it's the reason we use staged rockets at all.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It contradicts the objective to produce maximum thrust at takeoff, so any choice of staging is an engineering trade-off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to achieve both is &lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Asparagus_staging&quot;&gt;asparagus staging&lt;/a&gt;, where all engines ignite immediatly, but the core engine is fueled by the boosters' tanks, also saving on it's own fuel. This, however, comes at the price of additional complexity and weight and has consequently never been done in real life. &lt;s&gt;The falcon heavy, scheduled for 2017, is the first rocket to use it.&lt;/s&gt; As @DylanSp pointed out, &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_Heavy#Cancelled_propellant_crossfeed&quot;&gt;crossfeed has been cancelled&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="16943" LastEditorUserId="16943" LastEditDate="2016-09-14T13:16:52.873" LastActivityDate="2016-09-14T13:16:52.873" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18192" PostTypeId="1" AcceptedAnswerId="18608" CreationDate="2016-09-14T02:22:31.953" Score="4" ViewCount="809" Body="&lt;p&gt;Control algorithms are used on spacecraft to operate reaction wheels, RCS thrusters and other actuators. Since of the most common controllers I've come across are the PID and quaternion error command. What are the parameters that are accounted for when selecting a spacecraft attitude controller? How is the trade off carried out between different controllers and how are they optimized?&lt;/p&gt;&#xA;" OwnerUserId="12386" LastActivityDate="2016-10-13T03:58:37.867" Title="How to select/design a control algorithm for spacecraft attitude control?" Tags="|attitude|flight-control|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18193" PostTypeId="2" ParentId="18189" CreationDate="2016-09-14T03:02:55.190" Score="10" Body="&lt;p&gt;Sedna at perihelion is 76 AU from the Sun, meaning it receives about 1/76^2 = 1/5776 as much light from the Sun as Earth does. This distance reduces the sun from a -26.7 apparent magnitude to a -17.3 apparent magnitude. The Moon has an apparent magnitude of -12.6, so a quick answer would be: the Sun illuminates Sedna at perihelion about 100 times as much as a full moon illuminates the Earth. The human eye isn't linear, so 100 times as light doesn't mean &quot;looks 100 times as bright&quot;, but the directional answer seems clear: people can see enough to get around by moonlight, so they should be able to see better than that — more like twilight than moonlight, and enough for some color vision. Compared to either the Sun or moon as seen from Earth, the Sun as seen from Sedna will be much smaller (less than 1 minute of arc in diameter), so it will cast much more sharply-defined shadows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At Sedna's projected aphelion of 936 AU, the Sun drops to an apparent magnitude of -11.9 (a little more than one millionth of its brightness as seen from Earth, and one-half the brightness of a full Moon). Visibility is still possible under these conditions, but limited. The sun would have no apparent size; it would just be an extremely bright star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for any hypothetical moons of Sedna, it's hard to guess about their appearance without knowing anything about their size, orbit, or composition. I'll leave that one alone.&lt;/p&gt;&#xA;" OwnerUserId="3055" LastEditorUserId="3055" LastEditDate="2016-09-14T15:05:15.380" LastActivityDate="2016-09-14T15:05:15.380" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18194" PostTypeId="1" AcceptedAnswerId="18195" CreationDate="2016-09-14T03:23:28.937" Score="3" ViewCount="382" Body="&lt;p&gt;This &lt;a href=&quot;http://arstechnica.com/science/2016/09/did-the-fourth-richest-human-just-tease-plans-to-colonize-the-moon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ars Technica article&lt;/a&gt; has two titles that seem to be a bit gratuitous:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Why Bezos’ rocket is unprecedented — and worth taking seriously&lt;/p&gt;&#xA;&lt;p&gt;Why Blue Origin’s crazy big rocket might fly, and what it means for spaceflight&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and the link itself contains:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;did-the-fourth-richest-human-just-tease-plans-to-colonize-the-moon&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The article then links to a &lt;a href=&quot;http://arstechnica.com/science/2016/04/astronaut-prime-heres-how-jeff-bezos-plans-to-remake-spaceflight/&quot; rel=&quot;nofollow noreferrer&quot;&gt;previous article&lt;/a&gt;, with the Bezos quote:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;The reason I like vertical landing is because it scales so well,&amp;quot; he explained. &amp;quot;New Shepard is about 80 feet tall. It’s the shortest vehicle we will ever make. It gets easier to land the vehicles the bigger they get. It’s the inverted pendulum problem. It's easier to balance bigger things. I like those architectures. Parachutes have the opposite problem; as things get bigger, it's very difficult. You can’t build a parachute 1,000 feet in diameter. Even wings, they scale pretty well to a certain size, but they end up being a lot of dead weight to carry.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;An example of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Inverted_pendulum&quot; rel=&quot;nofollow noreferrer&quot;&gt;inverted pendulum&lt;/a&gt; would be balancing a broom stick on your index finger.&lt;/p&gt;&#xA;&lt;p&gt;Is there a simple way to understand why a substantially larger rocket would be easier to land vertically? All I can think of is that the dry weight would increase faster than size cubed &lt;span class=&quot;math-container&quot;&gt;$(L^{&amp;gt;3})$&lt;/span&gt; to maintain structural integrity (the same reason there can't be &lt;a href=&quot;https://en.wikipedia.org/wiki/Godzilla&quot; rel=&quot;nofollow noreferrer&quot;&gt;Godzilla&lt;/a&gt; sized exoskeleton creatures) but the drag force would scale only as &lt;span class=&quot;math-container&quot;&gt;$(L^{2})$&lt;/span&gt; (keeping aspect ratio constant), but I'm just grasping at straws here. The New Glen first stage will not have the same aspect ratio as the Falcon 9 first stage, as &lt;a href=&quot;https://www.wired.com/2016/09/blue-orgins-new-glenn-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;mentioned in Wired&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But even though, at 270 feet, the New Glenn is much taller than a Falcon 9, the Blue Origin rocket is also much fatter. In terms of aspect ratio, this means it will be easier to control as it comes down.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;…but I don't think Bezos' argument is based solely on aspect ratio, and I don't understand why a smaller aspect ratio would make vertical landing easier - is it the larger moment of inertia?&lt;/p&gt;&#xA;&lt;p&gt;Is this explained or worked out somewhere? If not, is there any physics/engineering rationale that could be expressed here?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VmQ71.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VmQ71.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; size comparison of various rockets &lt;a href=&quot;https://www.wired.com/2016/09/blue-orgins-new-glenn-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;from Wired&lt;/a&gt; with credit to Blue Origin. Right click to view full size image.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LH4Gd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LH4Gd.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; illustration of an &lt;a href=&quot;https://en.wikipedia.org/wiki/Inverted_pendulum&quot; rel=&quot;nofollow noreferrer&quot;&gt;inverted-pendulum-type&lt;/a&gt; problem &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Balancer_with_wine_3.JPG&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-14T19:35:41.270" Title="How would vertical landing scale with size - does bigger = &quot;easier&quot; (actually)?" Tags="|landing|blue-origin|vertical-landing|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="18195" PostTypeId="2" ParentId="18194" CreationDate="2016-09-14T04:09:31.193" Score="5" Body="&lt;p&gt;The mass of a pendulum doesn't affect its period, but the length does; the longer your rocket, the slower it will tip (in degrees-per-second) while off-balance, and the more time you have to compensate for minor shifts before you have a catastrophe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gimbaled rockets have excellent control authority, and the engine thrust will scale with the rocket mass, so in general you can compensate for any small gust of wind or data error if you have sufficient time. A long pendulum gives you that time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the &quot;fatter&quot; stage, perhaps the advantage there is that the engines, being mounted further out from the centerline, can produce more torque at smaller deflection angles? This would reduce the coupling between vertical acceleration and rotation, affording more tolerance in throttle response time.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-09-14T04:46:22.090" LastActivityDate="2016-09-14T04:46:22.090" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18196" PostTypeId="1" AcceptedAnswerId="27902" CreationDate="2016-09-14T04:46:56.037" Score="14" ViewCount="1411" Body="&lt;p&gt;If the Mars rover &lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_(rover)&quot; rel=&quot;noreferrer&quot;&gt;Curiosity&lt;/a&gt; had been equipped with lights, would it be able to drive or do other work in the evening? This could be some combination of LED headlights, wheel-lights, and/or a spotlight on the movable robotic arm,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the chances of a deer crossing in front of the rover are pretty remote, and in general the rocks stay put (although there are &lt;a href=&quot;http://phys.org/news/2014-08-tall-boulder-martian-hill-upright.html&quot; rel=&quot;noreferrer&quot;&gt;notable exceptions&lt;/a&gt;! see below)  they'd only have to flash maybe once per second or even less most of the time. Most efficient LED lights are pulse width modulated anyway, and LED flash units for cameras exist, so this would not necessarily represent a large power drain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two sources of heat already available on board Curiosity to potentially keep things within operational temperatures - the circulating fluids warmed indirectly by thermal infrared radiation from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-Mission_Radioisotope_Thermoelectric_Generator&quot; rel=&quot;noreferrer&quot;&gt;MMRTG&lt;/a&gt;, and electrical heaters powered by the MMRTG electrical output directly. And of course the presence of the MMRTG means that Curiosity (unlike previous Mars rover designs) does not rely on daylight for electrical power. Although I don't know if these are sufficient to enable evening operations. I'm thinking there is residual warmth in the spacecraft and Martian surface in the evening (as opposed to the morning)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not suggesting that this is a good idea for the Curiosity mission, this would add complexity, weight, and risk with marginal benefits. However it seems to me this could be done, though there may be factors I haven't considered. There will be future rover missions on Mars and other bodies, so I am asking about a Curiosity-with-lights as a reference point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Things do occasionally move on Mars. From &lt;a href=&quot;http://phys.org/news/2014-08-tall-boulder-martian-hill-upright.html&quot; rel=&quot;noreferrer&quot;&gt;Tall boulder rolls down martian hill, lands upright&lt;/a&gt;, &quot;The High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter recorded this view on July 3, 2014.&quot; Click for larger view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3CF3X.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3CF3X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-29T15:43:36.180" LastActivityDate="2020-07-24T17:34:35.687" Title="If Curiosity had lights, could it drive or work in the evening?" Tags="|curiosity|rtg|autonomy|" AnswerCount="3" CommentCount="24" ContentLicense="CC BY-SA 3.0" />
  <row Id="18197" PostTypeId="1" CreationDate="2016-09-14T05:10:53.937" Score="1" ViewCount="275" Body="&lt;p&gt;Assuming the magnetorquer and the field generating electronics are both rigidly fixed to the spacecraft cube. Reference to any supporting papers/articles would be great. Thanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the residual field affect the performance of the magnetorquer due to generation of back emf. To cancel which, more current must be passed to get the same torque (in case polarity of emf is opposing) and less current in the other direction where emf polarity is favorable. Constant external field assumed.&lt;/p&gt;&#xA;" OwnerUserId="16976" LastEditorUserId="16976" LastEditDate="2016-09-16T14:12:52.160" LastActivityDate="2019-12-30T03:14:53.657" Title="Will there be torques on spacecraft due to interaction of magnetic torquer with the residual magnetic field of the spacecraft?" Tags="|spacecraft|attitude|magnetorquer|flight-control|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18199" PostTypeId="1" AcceptedAnswerId="18213" CreationDate="2016-09-14T06:20:07.813" Score="2" ViewCount="1233" Body="&lt;p&gt;I have heard many times about interstellar travels. Are centrifuge space craft (artificial gravity) and warp drives technologies being built or still it is just concepts ? Nearest start system is alpha centauri so is it possible to reach there by mentioned technologies ? and if yes then how.&lt;/p&gt;&#xA;" OwnerUserId="16949" LastActivityDate="2016-09-14T21:05:03.763" Title="How close humans are for interstellar travel?" Tags="|spacecraft|interstellar-travel|interplanetary|alpha-centauri|" AnswerCount="3" CommentCount="1" ClosedDate="2016-09-15T13:51:39.813" ContentLicense="CC BY-SA 3.0" />
  <row Id="18200" PostTypeId="2" ParentId="18199" CreationDate="2016-09-14T07:39:50.680" Score="3" Body="&lt;p&gt;A sucessful flight of humans to mars and back might be possible in some years or some decades. But we don't have the technology for a human flight to the outer planets and their moons. Before we did not manage to send an orbiter to pluto and land a rover on pluto we are not able to leave our solar system with human passengers. We should send some spacecrafts to the oort cloud before. There is so much work to be done before. We did not even try a space station in an earth orbit with artifical gravity in a centrifuge yet and we would need many years of experience with humans living there.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-14T07:39:50.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18201" PostTypeId="1" CreationDate="2016-09-14T07:42:29.880" Score="-6" ViewCount="982" Body="&lt;p&gt;&lt;strong&gt;comment:&lt;/strong&gt; I've used a somewhat innocuous abstraction of 'pranks in space' below, but there's no end to the trouble that could be caused and the increasing ease with which a state (nation or otherwise) could cause it. The scope of &lt;a href=&quot;https://en.wikisource.org/wiki/Outer_Space_Treaty_of_1967#Article_IX&quot; rel=&quot;nofollow noreferrer&quot;&gt;Article IX of The Outer Space Treaty &lt;strong&gt;of 1967&lt;/a&gt;&lt;/strong&gt; is pretty narrow - what exactly happens if party A says that party B may be in violation of article IX? Is there a board of arbitration? Are there any mechanisms of 'punishment' established? What about a non-signatory state? Misdeeds on earth are getting easier to detect and report by independent news media, but how many news orgs have NORAD's capability?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The discussion in the recent article in The Conversation: &lt;a href=&quot;https://theconversation.com/spacex-explosion-shows-why-we-must-slow-down-private-space-exploration-until-we-rewrite-law-65019&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX explosion shows why we must slow down private space exploration until we rewrite law&lt;/a&gt; focuses mostly on accident and liability issues, and points out that the 50 year old &lt;a href=&quot;http://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/introouterspacetreaty.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Outer Space Treaty&lt;/a&gt; ratified &lt;a href=&quot;http://www.state.gov/t/isn/5181.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;in 1967&lt;/a&gt; may need to be dusted off and looked at more closely. I'd recommend reading the full article - here is a paragraph near the end:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With the increase in private participation in space experimentation and perhaps even mineral mining, the provisions governing civil liability over mishaps arising from the operations of a space station are likely to become one of the most contested areas of space law. What if a module or component part fails to function on a space station? In the absence of multilateral rules on this point, a patchwork of legal rules is gradually maintained through MOUs (Memorandum of Understanding) and other national laws such as the US Commercial Space Launchings Act (CSLA) of 1978. How will private companies fit into these as they possibly become partners?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Further, the &quot;PrankSat&quot; situations may multiply when it becomes more and more common to add significant propulsion capability to cubesats, and control may not be under the same number of levels of security that is currently used for &quot;serious&quot; satellites. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other terms that may apply in some cases include &quot;tit for tat-sat&quot; and &quot;a proportional response-sat.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uKGgb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uKGgb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Outer Space Treaty parties, signatories, and non-signatories as of March 2015 from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Outer_Space_Treaty_parties.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, modified by pasting legend (sorry Antarctica).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If a hypothetical President Wisenheimer felt secure that his/her country's large supply of neodymium, palladium and &lt;a href=&quot;https://en.wikipedia.org/wiki/Unobtainium&quot; rel=&quot;nofollow noreferrer&quot;&gt;unobtainium&lt;/a&gt; gives them some economic protection, what international regulations, agreements, or other forces could help the rest of the world stop, or at least discourage the president from allowing his/her brother in law's aerospace company &lt;em&gt;OrbitalPranksters&lt;/em&gt; from launching PrankSat and putting it in orbit in front of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2zaxu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2zaxu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kol1O.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kol1O.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pranksters know what a collimator is, and understand that the basic point-to-parallel-to-point method will add to the image on the CCD even if it significantly under-fills the telescope's aperture. It will be in focus at all distances since both have infinity conjugate focci. (image &lt;a href=&quot;http://www.trioptics.com/downloads/&quot; rel=&quot;nofollow noreferrer&quot;&gt;from a randomly chosen internet optics catalog&lt;/a&gt;), and here are some &lt;a href=&quot;http://www.pyser-sgi.com/images/added/pdf/SAC.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;other collimators&lt;/a&gt; that project images into telescopes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NbCMu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NbCMu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PrankSat could navigate near the HST, or - since the divergence of the FOV is small - intercept the scheduled HST view from kilometers away and &quot;Flash&quot;. It could also attach itself to the HST in some way, occasionally move into view, and really become a pest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pFb6M.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pFb6M.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; I found this on the internet. Used with warm regards to &lt;a href=&quot;https://en.wikipedia.org/wiki/Neil_deGrasse_Tyson&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dr. Tyson&lt;/a&gt; and all he does to further the public's understanding of science.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-04-16T05:52:22.400" LastActivityDate="2019-04-16T05:52:22.400" Title="What regulations, agreements, or other forces can help mitigate &quot;PrankSats&quot;?" Tags="|launch|cubesat|regulatory|" AnswerCount="2" CommentCount="11" ClosedDate="2018-03-01T10:26:55.903" ContentLicense="CC BY-SA 4.0" />
  <row Id="18202" PostTypeId="2" ParentId="18045" CreationDate="2016-09-14T08:24:26.180" Score="4" Body="&lt;p&gt;Liquid oxygen contains contains 4000 times more oxygen than normal air (by volume). A lot of materials that are almost not flameable in normal air may burn in liquid oxygen. Even pure oxygen at a pressure of 200 bar is a dangerous thing. Even stainless steel tubes did burn due to a steep pressure rise of the oxygen inside. Even an asphalt floor may be dangerous if liquid oxygen is spilt.&lt;/p&gt;&#xA;&lt;p&gt;A paper about Resonance Tube Hazards in Oxygen Systems &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19820013541.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19820013541.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;NASA SAFETY STANDARD FOR OXYGEN AND OXYGEN SYSTEMS &lt;a href=&quot;https://www.hq.nasa.gov/office/codeq/doctree/canceled/1740151.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.hq.nasa.gov/office/codeq/doctree/canceled/1740151.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;A NASA technical memorandum &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740002978.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Test of LOX Compatibility for Asphalt and Concrete Runway Materials&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="15906" LastEditDate="2023-03-19T17:59:25.980" LastActivityDate="2023-03-19T17:59:25.980" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18203" PostTypeId="2" ParentId="18201" CreationDate="2016-09-14T10:06:01.827" Score="9" Body="&lt;p&gt;For your specific example, the answer is &lt;strong&gt;cost&lt;/strong&gt;.  Nobody is going to spend hundreds of millions (or even just 1% of that) just to prank a community of astronomers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For other examples?  What stops China or Russia from launching a set of rockets to destroy GPS satellites, or military communication satellites?  Neither country wants to initiate World War III.  What stops a private company from doing the same?  Cost, technological capability, laws, negative consequences, lack of positive consequences, etc.  Even though private space industry is coming up, it's still heavily government regulated.  If a private company in Russia were to launch a set of rockets from Russia that just happen to destroy the GPS constellation (why the heck would they do that?), the Kremlin probably should expect a phone call from the White House, and quite likely somewhat more than that.  If Inmarsat were to launch a set of rockets destroying the Iridium constellation, they won't stay in business terribly long.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2018-02-28T23:51:00.573" LastActivityDate="2018-02-28T23:51:00.573" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18204" PostTypeId="2" ParentId="17777" CreationDate="2016-09-14T10:10:31.363" Score="2" Body="&lt;h2&gt;Cost of a single launch&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.gwu.edu/~spi/assets/docs/NASA%20L.Vehicle%20Study%20V-5.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Launch vehicles: an economic perspective&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The 1994 study of DOD launch vehicles (mainly Titan and Atlas) showed&#xA;  the breakdown of the various components of a launch. The vehicle was&#xA;  only about half of the total cost. The remainder was divided among&#xA;  operations (30%), payload integration (5%) and “other” government&#xA;  costs (15%).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;h2&gt;Production cost&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=30749.40&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Production cost: 114m (not launch cost, based on Arianespace's purchase of 35 rockets for 4bn) (in €, 2009)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boosters: ~25m&lt;br&gt;&#xA;Vulcain: ~15m&lt;br&gt;&#xA;Upper stage: ~20m&lt;br&gt;&#xA;EPC (first stage incl. Vulcain), VEB (control system on top of second stage), fairing: 69m  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Production cost is subject to a &lt;a href=&quot;http://www.astronautix.com/c/costpriceanholedarnthing.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;cost curve&lt;/a&gt;: if the first rocket to be built costs 100%, the second will be (roughly) 90% of that, the third will be 90% of the second. &#xA;The curve will look like this:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/WYh0E.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WYh0E.gif&quot; alt=&quot;Cost curve&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the 200th rocket costs 50% of the first one to build. In rocket terms, that's a huge production run.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To produce a rocket, you need expensive tooling:   &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A building large enough to maneuver the stage in.  &lt;/li&gt;&#xA;&lt;li&gt;E.g. to weld the first stage, you need a welding fixture large enough to hold the entire stage, and accurate to within millimeters, if not less.   &lt;/li&gt;&#xA;&lt;li&gt;if you use carbon composites, you need an autoclave (vacuum oven) large enough for the largest piece you make. Some rockets have the outer skin of entire stages built as a single piece of carbon composite. &lt;/li&gt;&#xA;&lt;li&gt;milling and welding equipment for exotic materials, like the high-temperature alloys used in rocket engines. &lt;/li&gt;&#xA;&lt;li&gt;inspection equipment (again, huge systems to inspect an entire first stage tank).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All this tooling has to be paid off over a small (in terms of mass production) number of rockets built. The tooling can cost on the order of billions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of the small production run, you can't automate everything. So there's lots of highly-qualified labor involved. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost of the materials is only a small part of the production cost. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Development budget&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Again, Ariane 5, from 'Europäische Tragerraketen, band 2', Bernd Leitenberger:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Studies and tests 125&lt;br&gt;&#xA;solid boosters 355&lt;br&gt;&#xA;H120 first stage 270&lt;br&gt;&#xA;HM60 (Vulcain) engine and test stands 738&lt;br&gt;&#xA;other elements of the first stage and boosters 95&lt;br&gt;&#xA;upper stage and VEB 200&lt;br&gt;&#xA;ground support in Europe 80&lt;br&gt;&#xA;Buildings and other structures in Kourou (launch pad) 450&lt;br&gt;&#xA;Test flights 185&lt;br&gt;&#xA;Total 2498&lt;br&gt;&#xA;ESA and CNES management 102&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was the budget in million €, as set in 1985. Final cost was on the order of 120% of this budget. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-09-14T14:42:59.330" LastActivityDate="2016-09-14T14:42:59.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18206" PostTypeId="2" ParentId="17777" CreationDate="2016-09-14T13:11:09.820" Score="3" Body="&lt;p&gt;SpaceX does little but launch things in to space, plus R&amp;amp;D for future missions. The current employee count at &lt;a href=&quot;http://www.spacex.com/about&quot; rel=&quot;nofollow&quot;&gt;SpaceX is over 4000&lt;/a&gt;. &lt;a href=&quot;http://www.payscale.com/research/US/Employer=Spacex/Salary&quot; rel=&quot;nofollow&quot;&gt;Payscale&lt;/a&gt; estimates the salaries, to which we can effectively double to estimate the actual cost per employee, including benefits, places to work, etc. From that, I'm going to guess the average salary is about \$90K/employee. That means the rough cost of an employee is more like \$180K. Thus, the costs associated with labor are about \$720 million per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The raw goods are primarily aerospace grade aluminum. The cost is roughly \$1/pound. Much of it is also carbon composite, and more expensive items, so let's double that. The &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/&quot; rel=&quot;nofollow&quot;&gt;inert mass&lt;/a&gt; is roughly 30,000 kg. That means the raw cost is about \$120,000. There will be more when you consider there are some specially ordered components. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fuel costs per launch are estimated at about \$60,000 per launch. There are usually 3 fillings of the fuel per mission, so let's triple that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, let's assume there are 18 launches per year, averaging \$70 million per launch (Including government launches which are more expensive). That gives an income of about \$1.26 billion per year. The raw goods cost are \$300,000 per launch, or \$5.4 million. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In SpaceX's defense, most of the difference, which I suspect is real, is either put in to capital costs, paying for more machines and buildings, future R&amp;amp;D work, Dragon (Which wasn't accounted for), or making money from the years that SpaceX didn't have as many launches in a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, like most expensive product, most of the cost is in labor. The numbers here are guesses, and the number of launches is higher than expected. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-09-14T13:17:08.767" LastActivityDate="2016-09-14T13:17:08.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18207" PostTypeId="1" CreationDate="2016-09-14T13:29:37.463" Score="17" ViewCount="2690" Body="&lt;p&gt;Due to nature of space-rocket launches using &quot;conventional&quot; rocket fuel (fuel+oxidizer, e.g. hydrogen+oxygen), every kilo of fuel that has to be carried impacts the rocket's range in a very nonlinear way (in short, more fuel on board needs more fuel to lift it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, any fuel/weight saving at early stages of rocket flight correspond to the significant increase in rocket's ability (max height or max payload). That's why for example, Virgin Galactic uses WhiteKnight jet powered plane to lift the SpaceShip to relatively meager height of 20 km, even if it is only 1/5 of the final height of 100 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is obvious that turbofan engines have pretty low ceiling with respect to spaceflight, but why not continue the flight (e.g. start of the SpaceShip's ascent) with the ramjet engines, which have the following properties:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Very simple design, much less (none?) moving parts than the turbojet/turbofan&lt;/li&gt;&#xA;&lt;li&gt;Still makes the use of the environmental air, not needing oxidizer, therefore much less weight to carry&lt;/li&gt;&#xA;&lt;li&gt;With increasing speed the ramjet needs regulation of amount of air at intake, which would be naturally compensated by increasingly thinner air during ascent, perhaps not needing regulation at all?&lt;/li&gt;&#xA;&lt;li&gt;Unlike solid fuel rockets it can be turned off if necessary&lt;/li&gt;&#xA;&lt;li&gt;It is far less efficient than turbofan, but due to use of environmental air, still much more efficient than fuel/oxidizer combination?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, why no one is using ramjets for the second stage of the launch? Perhaps even with &quot;classic&quot; design, solid first stage (to get to the minimum velocity for ramjet operation), ramjet second stage and then liquid fuel third stage?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I checked the question (and answers) which was reason for this question was marked a duplicate. I strongly disagree - the previous question asks about jet engines in general, and the answer is quite turbofan-specific. I know the difference between various types of jet engines, therefore I was asking &lt;strong&gt;specifically&lt;/strong&gt; about the &lt;strong&gt;ramjet&lt;/strong&gt; engine, and I also outlined its advantages (turbofan engines, commonly referred to as &quot;jet engines&quot; used in commercial aviation are quite different beast compared to the ramjet engine!) For one, the size, the main disqualification for turbofans in the answers of the previous questions is due to &lt;strong&gt;fan&lt;/strong&gt; part of the &lt;strong&gt;turboFAN&lt;/strong&gt; engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, no, this question is not a duplicate. I asked specifically because there is very little discussion on ramjets used for rocket propulsion. I would kindly ask that the &quot;duplicate&quot; tag is removed, since the question the tag refers to is only distantly related. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SECOND EDIT: I was asking specifically about ramjets for the &lt;strong&gt;second stage&lt;/strong&gt; of the launch, please read the &lt;strong&gt;title&lt;/strong&gt; of my post before directing me to the answer why ramjets cannot be used for the &lt;strong&gt;first stage&lt;/strong&gt;!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, thanks to @Hobbes, we know that &lt;a href=&quot;https://www.nasa.gov/offices/ipp/centers/dfrc/technology/DRC-010-039-Ram-Booster.html&quot; rel=&quot;noreferrer&quot; title=&quot;that exact sequence of propulsion techniques&quot;&gt;NASA has design that incorporates that exact sequence of propulsion techniques&lt;/a&gt;, so my question again: &lt;strong&gt;Why no commercial operators are using it&lt;/strong&gt; - since differently from NASA they are very cost-aware. Or shold be.&lt;/p&gt;&#xA;" OwnerUserId="16952" LastEditorUserId="16952" LastEditDate="2016-09-16T05:48:28.107" LastActivityDate="2017-01-05T06:03:40.620" Title="Why is (conventional) ramjet not used for 2nd stage of rocket propulsion?" Tags="|rockets|propulsion|ramjet|virgin-galactic|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18208" PostTypeId="1" CreationDate="2016-09-14T13:37:56.620" Score="8" ViewCount="1463" Body="&lt;p&gt;Can anyone link me to some resources on the background noises heard inside gemini and apollo capsules? Even the LEM is of interest. I'd like to know what the thruster firings sounded like, and if there were any hissing, humming, banging, thermal pops and pings, things like that. Engine firing too, of course, but mostly the ambient sound and the RCS. Was it possible for the astronauts to monitor or diagnose ship activity by ear? Sounds of valves and relays opening and closing, etc&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2016-09-14T13:59:19.990" LastActivityDate="2018-03-10T05:06:44.940" Title="Spacecraft onboard sounds" Tags="|spacecraft|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18209" PostTypeId="2" ParentId="18207" CreationDate="2016-09-14T14:29:47.857" Score="19" Body="&lt;p&gt;What is the use of a second stage using ramjet in a height where the remaining air is very, very thin? The Falcon 1 first stage is used up to a height of 90 km, the second stage reaches a height of 200 km where the satellite is put into the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would need three stages, the first with a rocket engine to get a speed where a ramjet may be used, the second with a ramjet engine up to the height where the remaining air is to thin for ramjet operation and an additional third stage with a rocket engine to get into a low orbit at 200 km height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A hypothetical example, the ramjet is used from 13 km height and 19 % of the ground air pressure up to 36 km height with 1 % pressure. The first stage is restricted to only 13 km, the second stage adds only 23 km and the rest of 164 km is left to the third stage. Lets look for the speed, if the ramjet operates from mach 2 to mach 5 or from 0.66 km/s to 1.7 km/s. We have to reach a speed of 7.8 km/s for a low orbit. The third stage has to deliver the remaining 6.1 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I looked into Wikipedia for some speed values for a ramjet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Needing three stages instead of two adds complexity and cost and the reliability is smaller, that means the risk to loose the payload due to a malfunction of the whole rocket is higher. The third stage has to contribute more speed and height than the second stage of a conventional rockert solution, therefore needs more fuel and a eventually a stronger rocket engine. The first stage may be a little cheaper, but the third stage gets more expensive and we have the additional cost of the second ramjet stage. I doubt that the total cost is lower.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="13126" LastEditDate="2016-09-20T09:04:11.350" LastActivityDate="2016-09-20T09:04:11.350" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="18211" PostTypeId="1" CreationDate="2016-09-14T17:01:18.333" Score="1" ViewCount="103" Body="&lt;p&gt;My goal is to establish a career in space exploration. What opportunities are there in Europe and how can I best prepare?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am willing to do all it takes to maximise my chances of working in this area, even if it means going to University again to get another degree (I have a Humanities degree). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I really want to help accelerate our multiplanetary future and would be grateful for any advice.&lt;/p&gt;&#xA;" OwnerUserId="16955" LastEditorUserId="16955" LastEditDate="2016-09-14T18:17:31.620" LastActivityDate="2016-09-14T18:17:31.620" Title="Space exploration career opportunities for Europeans?" Tags="|esa|career|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18213" PostTypeId="2" ParentId="18199" CreationDate="2016-09-14T20:30:40.813" Score="7" Body="&lt;p&gt;Manned interstellar space flight is not going to happen anytime soon.  Definitely not within the next 50 years, most likely not within the next 100.  There are too many basic issues that are &lt;em&gt;hard&lt;/em&gt; to work around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest issue (apart from politics and funding) is propulsion.  So far, nobody's figured out how to build an engine that doesn't require some form of reaction mass (propellant) to accelerate, meaning your total &amp;Delta;&lt;em&gt;V&lt;/em&gt; (change in velocity) is limited by how much reaction mass you can carry along, and the amount of reaction mass you need grows &lt;em&gt;exponentially&lt;/em&gt; as &amp;Delta;&lt;em&gt;V&lt;/em&gt; increases (i.e., for &lt;em&gt;N&lt;/em&gt; times more &amp;Delta;&lt;em&gt;V&lt;/em&gt;, you need &lt;em&gt;e&lt;/em&gt;&lt;sup&gt;&lt;em&gt;N&lt;/em&gt;&lt;/sup&gt; times more reaction mass).  Conventional chemical and ion engines are simply not up to the task of interstellar travel; either the amount of propellant necessary is many orders of magnitude larger than the spacecraft itself&lt;sup&gt;1&lt;/sup&gt;, or you accept travel times of thousands to tens of thousands of years (which brings up issues with power, life support, spacecraft maintenance, etc.).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are ideas for engines that use nuclear fission or fusion that provide better performance, but the best of them would still require on the order of a century or so to reach the nearest star.  And again, you're limited by the amount of reaction mass you can carry.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The logical step up from that is to not carry your reaction mass with you; instead, you either gather reaction mass from the interstellar medium as you travel (like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;noreferrer&quot;&gt;Bussard ramjet&lt;/a&gt;), or you use a sail to exchange momentum with the solar wind or a powerful EM source, like a laser array.  There's a project called &lt;a href=&quot;https://breakthroughinitiatives.org/Initiative/3&quot; rel=&quot;noreferrer&quot;&gt;Breakthrough Starshot&lt;/a&gt; that intends to use a massive bank of ground-based lasers to accelerate a gram-scale spacecraft with a large sail to something like 20% &lt;em&gt;c&lt;/em&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with ramjets is that there just isn't that much material to gather, and the drag from the scoops would probably cancel out whatever thrust you could generate.  And the problem with sails in general (and Starshot in particular) is that you have no way to slow down once you reach your destination.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, interstellar travel with reaction drives is not going to happen.  We need to develop some kind of reactionless drive, one that &lt;em&gt;doesn't&lt;/em&gt; require you to exhange momentum with some kind of reaction mass in order to accelerate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's the &lt;a href=&quot;https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster&quot; rel=&quot;noreferrer&quot;&gt;EmDrive&lt;/a&gt;, which &lt;em&gt;allegedly&lt;/em&gt; provides thrust without reaction mass.  There's a lot of skepticism associated with this, with good reason.  At first blush, it seems to violate conservation of momentum.  Also, people who've tried to replicate the results using sane test setups have not been able to reproduce the effect.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, that pretty much leaves us with &lt;a href=&quot;https://en.wikipedia.org/wiki/Alcubierre_drive&quot; rel=&quot;noreferrer&quot;&gt;warp drives&lt;/a&gt;, which...aren't going to happen anytime soon, if ever.  They're &lt;em&gt;mathematically&lt;/em&gt; possible, but that doesn't mean that they will ever be &lt;em&gt;physically&lt;/em&gt; possible, and even if they're physically possible, the energy requirements are such that we could probably never build one.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;hr&gt;&#xA;&lt;sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;To reach 1% &lt;em&gt;c&lt;/em&gt; (3000000 m/s) using the ion engine on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Dawn&lt;/a&gt; spacecraft, you'd need on the order of 10&lt;sup&gt;88&lt;/sup&gt; kg of propellant for every kg of spacecraft mass.  To reach that same velocity using an RL-10 chemical (hydrogen/oxygen) engine, you'd need on the order of 10&lt;sup&gt;295&lt;/sup&gt; kg of propellant for every kg of spacecraft mass.  &#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2016-09-14T20:30:40.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18214" PostTypeId="2" ParentId="368" CreationDate="2016-09-14T20:35:39.620" Score="2" Body="&lt;p&gt;Yeah, it would be great if you could build a module that has small robots in it that grow some kind of nice, sustainable, diet for many people.  I think it's more likely we'll grow into some kind of Iain Banks type planet cloud rather than really live well on other bodies in the Solar System.  That being said, you'd then grow food in an artificial environment that would have to be heavily maintained.  It would probably need to keep being fed somehow with solar/fusion and raw material floating around.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take a look at a TED talk about vertical farming in shipping containers and vertical farming in Europe using greenhouses.  I think that's a good starting place, but you'd need to get numbers for how many plants, which plants, and how much mass of CHNOPS elements you'd need to keep the cycle going.  I imagine that, plus asteroid and comet elements, robots doing the work, and some kind of set mass for your crew.  Plus, maybe some of that new lab-grown meat onboard.&lt;/p&gt;&#xA;" OwnerUserId="14659" LastActivityDate="2016-09-14T20:35:39.620" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18215" PostTypeId="2" ParentId="18199" CreationDate="2016-09-14T21:05:03.763" Score="-1" Body="&lt;p&gt;Humans will NEVER travel to another star!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It requires either so much time, or so radically new technology, that humans cannot exist under such conditions. We are not created to be star travelers, we're deeply rooted in Earth. We're just the guys in the post office. We'll send Earth life out there, but not ourselves.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-09-14T21:05:03.763" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18216" PostTypeId="2" ParentId="17888" CreationDate="2016-09-14T21:16:03.860" Score="2" Body="&lt;p&gt;These are the fresh ideas as published by ILS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ilslaunch.com/sites/default/files/pdf/Proton_Variant_Brochure_Mech.pdf&quot; rel=&quot;nofollow&quot;&gt;Proton Medium / Proton Light&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ilslaunch.com/sites/default/files/2-Press%20Conference%20-%20ENG.PDF&quot; rel=&quot;nofollow&quot;&gt;Expanded Proton Line: Proton Medium &amp;amp; Proton Light&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main development is in the removal of the second stage and the reduction of the first stage.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2016-09-15T10:13:33.850" LastActivityDate="2016-09-15T10:13:33.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18217" PostTypeId="2" ParentId="368" CreationDate="2016-09-14T21:23:19.167" Score="6" Body="&lt;p&gt;China conducted a 105-day controlled environment project with a crew of three scientists in 2014. In the facility (called &lt;a href=&quot;https://en.wikipedia.org/wiki/Yuegong-1&quot; rel=&quot;noreferrer&quot;&gt;Yuegong-1&lt;/a&gt;) five kinds of cereals, fifteen kinds of vegetables, and one kind of fruit were grown. Yellow mealworms were grown as well for protein. The mission completed successfully. China hopes to utilize this facility as a testbed for the controlled environment that will be in place in their completed space station (Tiangong).&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-09-14T21:23:19.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18218" PostTypeId="1" CreationDate="2016-09-15T00:40:26.677" Score="14" ViewCount="800" Body="&lt;p&gt;Jupiter has a very intense radiation field, and thus spacecraft going to study it need heavy radiation shielding, such as &lt;a href=&quot;http://www.nasa.gov/image-feature/goddard/2016/infographic-juno-built-to-withstand-intense-radiation-environments/&quot; rel=&quot;noreferrer&quot;&gt;Juno's radiation vault&lt;/a&gt;.  This protects the delicate instruments and electronics.  However, Juno needs the shielding because it passes by Jupiter very closely, and is exposed to the full brunt of the radiation.  Would a lander on Europa need similar protection, or is Europa sufficiently far away and/or have a thick enough atmosphere that it would not need as much shielding?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: This question has drawn some conflicting answers.  The fact that a lander would have shielding from the moon and that it is always in the intense radiation environment made me feel unsure of which was more important.  If someone could make a more detailed answer including both facts, perhaps mathematically, I would appreciate it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, wouldn't Europa's magnetic field help deflect some of the radiation?  After all, most of the radiation present in the belts is just charged particles.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-09-16T00:49:56.213" LastActivityDate="2016-09-16T15:36:27.107" Title="Would a Europa lander need radiation shielding similar to Juno?" Tags="|radiation|jupiter|shielding|europa|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18221" PostTypeId="2" ParentId="18218" CreationDate="2016-09-15T08:03:35.630" Score="12" Body="&lt;p&gt;A Europa lander would need much more shielding, and/or more radiation tolerant components. Juno's orbit avoids the main radiation belt, but Europa is right in the middle of it.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-09-15T08:03:35.630" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18222" PostTypeId="1" AcceptedAnswerId="18261" CreationDate="2016-09-15T08:24:27.070" Score="18" ViewCount="2024" Body="&lt;p&gt;I was under the impression (when asking &lt;a href=&quot;https://space.stackexchange.com/q/18196/12102&quot;&gt;this question&lt;/a&gt;) that over the past four years that experience by ground operators and programmers had lead to a configuration where the Mars rover Curiosity could do a significant amount of &quot;work&quot; or movement autonomously - meaning not necessarily requiring somewhat-near-real-time comms. link with Earth, where &quot;somewhat-near-real-time&quot; means as fast as the current Earth-Mars separation will allow at the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if a plan for a movement by a few meters, or tens of meters is decided, is Curiosity able to do it? Must the path be pre-planned and validated by people on Earth, or is Curiosity able to plan a path - including obstacle avoidance, and execute it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about drilling and sample collection, or laser ablation and chemical analysis - once a plan is in place, can these run autonomously without comms. for some period of time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, I'm also interested in how commonly these are actually done autonomously, but the my main question is if it has been shown to have this capability.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-01-01T02:43:01.730" LastActivityDate="2021-01-01T02:43:01.730" Title="How much can the Mars rover Curiosity do autonomously, after four years of operation?" Tags="|mars|rovers|curiosity|robotic-missions|autonomy|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18223" PostTypeId="1" AcceptedAnswerId="19820" CreationDate="2016-09-15T11:09:48.187" Score="16" ViewCount="8386" Body="&lt;p&gt;I'm looking for information on atmospheric density in Earth orbit. All the atmospheric density tables and graphs I've found go no higher than 100 km. Definitions like the &lt;a href=&quot;http://www.engineeringtoolbox.com/standard-atmosphere-d_604.html&quot; rel=&quot;noreferrer&quot;&gt;US Standard Atmosphere&lt;/a&gt; don't go higher than 250k ft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in the rest of the graph, all the way to density =0 (or as low as it gets in interplanetary space). This sits occasionally gets questions about e.g. drag on a satellite, and I've never found a good source, just rules of thumb (drag is significant in LEO, but not an issue in GEO).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize atmospheric density fluctuates, I'd be happy with an average value or even better, a bandwidth indication (max. and min. values).  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2019-08-15T07:46:51.653" LastActivityDate="2024-01-26T00:03:57.780" Title="Where can I find data for Atmospheric density vs. altitude?" Tags="|atmosphere|atmospheric-drag|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18224" PostTypeId="1" CreationDate="2016-09-15T11:21:29.763" Score="2" ViewCount="553" Body="&lt;p&gt;Wikipedia gives 0.64 or 0.65 km/s for going from LLO to EML-1 or back. I would like to know what the delta v of the specific burns involved are, and if my initial calculations are close, I don't see how their number can be correct.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here is Wikipedia article about delta-v budget&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;It's complicated because it's a full 3-body problem, so I'm left trying to approximate it with a simplified model in the Moon's sphere of influence. LLO I have taken to be at 110 km, at 1.629 km/s. I have 61,000 km for the distance from the Moon to the EML-1 (from the approximation &lt;span class=&quot;math-container&quot;&gt;$R \cdot \sqrt{\frac{m_1}{3 \cdot m_2}}$&lt;/span&gt;).&lt;/p&gt;&#xA;&lt;p&gt;First burn, I consider getting into an orbit with apogee at 61,000 km, which is at the radius of EML-1. Perigee for the elliptical orbit would be 2.27 km/s, with the difference from LLO being 0.641 km/s, which is our first burn.&lt;/p&gt;&#xA;&lt;p&gt;NOTE: the numbers here are wrong staring with 0.714 km/s; see my answer for details.&lt;/p&gt;&#xA;&lt;p&gt;Second burn, I calculate the speed at apogee of that elliptical orbit to be 0.068. However, EML-1 is circling around the Moon, which I would calculate by:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\frac{2 \cdot \pi \cdot R}{30 \cdot 24 \cdot 60^{2}}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;if R is the 61,000 km distance from the Moon. This gives 0.714 km/s. If the orbit is going in the &amp;quot;right&amp;quot; direction, then I subtract the apogee speed from this to get 0.641 km/s for the second burn.&lt;/p&gt;&#xA;&lt;p&gt;If you added these, it's 1.355 km/s total, and both burns are of about equal magnitude. I don't think that's right. These calculations are probably correct for the equations I'm using, but I strongly suspect that the underlying assumptions are highly flawed. Perhaps Earth's gravity has a major effect in the mechanics? In other cases, the EML-1 and EML-2 points are touted as extremely low delta v points to reach, but these numbers would seem to somewhat undermine that, if the 0.641 km/s burn is going &amp;quot;out of the way&amp;quot;. But I'm not sure.&lt;/p&gt;&#xA;&lt;p&gt;Does anyone have any serious numbers for the transfer from LLO to EML-1?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="47149" LastEditDate="2022-05-27T16:17:28.300" LastActivityDate="2022-05-27T16:17:28.300" Title="Burns for EML-1 to LLO" Tags="|orbital-mechanics|the-moon|lagrangian-points|hohmann-transfer|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="18225" PostTypeId="2" ParentId="18224" CreationDate="2016-09-15T12:43:05.967" Score="1" Body="&lt;p&gt;This is not a complete answer, but it is a start. I think you may not be able to arrive at correct numbers for a full 3BP maneuver doing piecemeal transfer orbit calculation. Your arithmetic may not ever add up to the real values that way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The value of 0.65km/s from a halo orbit around Earth-Moon L1 to LLO can be found on this slide from the ULA presentation &quot;&lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Commercial_Space/2016_Cislunar.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transportation Enabling a Robust Cislunar Space Economy&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also &lt;em&gt;a lot&lt;/em&gt; to read in Zazzera, Topputo, and Massari's &quot;&lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/ARI/ARI%20Study%20Report/ACT-RPT-MAD-ARI-03-4103b-InterplanetaryHighways-Milano.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Assessment of Mission Design Including Utilization of Libration Points and Weak Stability Boundaries&lt;/a&gt;&quot; and Koon, Lo, Marsden and Ross’ (KoLoMaRo’s) ”&lt;a href=&quot;http://www.cds.caltech.edu/~koon/book/KoLoMaRo_DMissionBk.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dynamical Systems, the Three Body Problem, and Space Mission Design&lt;/a&gt;&quot; as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is just one example of getting from LEO to the EML1 halo: Zanzottera, Mingotti, Castelli and Dellnitz's &quot;&lt;a href=&quot;http://www.few.vu.nl/~rci270/publications/PaperZMCD.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth-to-Halo Transfers in the Sun–Earth–Moon Scenario&lt;/a&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bear in mind:&lt;/strong&gt; the ellipses (and for that matter the distances) in this slide are schematic only. Those are not what the orbits actually look like.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZLpuN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZLpuN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I haven't found an independent source for L2, but but at least the article in the &lt;a href=&quot;https://web.archive.org/web/20120426010349/http://www.futureinspaceoperations.com/papers/HumanOps_Beyond_LEO_11_2010.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Web Archive&lt;/a&gt; cited by Wikipedia seems to treat L1 and L2 as roughly equivalently difficult (or easy) to access. &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space.E2.80.94high_thrust&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia's  tabular values&lt;/a&gt; for high thrust:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3dNfw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3dNfw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...and low thrust:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lqCTZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lqCTZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...are all in the 0.6 to 0.8 km/sec range.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-09-15T14:47:21.950" LastActivityDate="2016-09-15T14:47:21.950" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18226" PostTypeId="2" ParentId="17963" CreationDate="2016-09-15T13:04:25.840" Score="1" Body="&lt;p&gt;The bulk payload like fuel may tolerate high g-forces and strong vibrations, but the tanks and the structure of the rocket body must be built for high g-forces also. Not easy if the tanks and the structure should be as light as possible to maximize the amount of bulk payload. A payload of LOX and LH2 would require thermal insulation to limit the loss by boil-out.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-15T13:04:25.840" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18227" PostTypeId="2" ParentId="18223" CreationDate="2016-09-15T14:50:37.077" Score="3" Body="&lt;p&gt;As a rough estimate, you can use&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\frac{P}{P_0}=e^{-y/h},$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $P$ is the pressure, $P_0$ is the pressure at some reference height such as sea level, $y$ is the height above that reference, and $h$, called the scale height, is about 8000 meters. This expression basically follows from thermodynamics (equipartition). It's exactly correct if the gravitational field is constant and  the atmosphere is in thermal equilibrium. In reality, the atmosphere is not in thermal equilibrium, and it gets colder as you go higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this is an exponential, it falls off very rapidly. You don't have to go very high before it becomes totally negligible, and the dominant component will not be the earth's atmosphere but the interplanetary medium.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2016-09-15T14:50:37.077" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18228" PostTypeId="1" CreationDate="2016-09-15T15:02:13.887" Score="4" ViewCount="308" Body="&lt;p&gt;Most horizontal-launch systems envisioned in the past, that used electromagnetics as a launch-assist, were subsonic. In order to conserve fuel during launch and to keep engine-weight to a minimal a combination of electromagnetic-propulsion, i.e. &lt;a href=&quot;https://en.wikipedia.org/wiki/Electromagnetic_propulsion&quot; rel=&quot;nofollow&quot;&gt;maglev or rail-guns&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Scramjet&quot; rel=&quot;nofollow&quot;&gt;scramjet engines&lt;/a&gt; may offer a viable alternative for manned systems to LEO. A lot of electrical-energy storage systems would be required to create the required velocity in the launch infrastructure such that the orbital vehicle scramjet engines could ignite. An example of a lower speed system is &lt;a href=&quot;http://www.popsci.com/technology/article/2010-11/nasa-engineers-propose-combining-rail-gun-and-scramjet-fire-spacecraft-orbit&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;. If the launch velocity could be increased to between Mach-2 and Mach-4 a manned system, using only scramjet engines, could be constructed. Final orbit insertion would need some additional rocket engines. The launch-track would need to be at least 10 miles in length to keep acceleration under 3 g's, and offer launch-abort opportunities.&lt;/p&gt;&#xA;" OwnerUserId="16920" LastEditorUserId="4660" LastEditDate="2016-09-20T15:00:29.527" LastActivityDate="2016-09-20T15:00:29.527" Title="Does the combination of electromagnetics and scramjets offer a viable potential for horizontal-launch?" Tags="|launch|ramjet|mass-driver|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18229" PostTypeId="2" ParentId="18218" CreationDate="2016-09-15T17:01:49.640" Score="7" Body="&lt;p&gt;That depends on where the spacecraft lands. The radiation belts rotate faster than the moon so the trailing side of Europa gets a lot of radiation while the leading side gets relatively little. &#xA;It also depends if the lander will be a separate spacecraft or a part of the orbiter. If it has to go multiple times through the radiation belt with the orbiter, then it will have to be shielded. On the other hand, if the lander stays in a safe orbit until the planet is surveyed and then is commanded to land it might not need shielding.&#xA;The final variable is the duration of the mission. I've seen 10 days being mentioned which will probably not need shielding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is very little info on the lander, so it's mostly a speculation but from what I've read it appears it will probably not be shielded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/van-kane/20160105-nasa-europa-lander.html&quot; rel=&quot;nofollow&quot;&gt;http://www.planetary.org/blogs/guest-blogs/van-kane/20160105-nasa-europa-lander.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astrobio.net/news-exclusive/hiding-from-jupiters-radiation/&quot; rel=&quot;nofollow&quot;&gt;http://www.astrobio.net/news-exclusive/hiding-from-jupiters-radiation/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="4660" LastEditDate="2016-09-16T15:36:27.107" LastActivityDate="2016-09-16T15:36:27.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18231" PostTypeId="1" AcceptedAnswerId="18234" CreationDate="2016-09-15T13:54:42.383" Score="5" ViewCount="1174" Body="&lt;p&gt;From what I have read gravitational waves cannot travel faster than the speed of light. And since gravitational waves are part of the fabric of space, wouldn't that disprove the idea behind an Alcubierre Drive?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An Alcubierre Drive tries to create negative mass in front of it and positive mass behind it. Since the negative mass is dependent upon some form of anti/dark matter this idea is already out of reach. However, what if you can, instead, create gravitational waves in front and behind an object (IE have the crest behind the object and trough in front of it) wouldn't that be the same concept as the below Alcubierre Drive? Thinking about it further, the two are different, because the area with negative mass will want to be filled in with some mass, and if you can keep creating negative mass in front of the object it will stay in motion. Now, change the idea to a wave - would you be able to 'ride' that wave?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RlBtz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RlBtz.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it prove that you can warp the fabric of space to eventually accelerate an object beyond the speed of light? Or, since gravitational waves propagate at the speed of light does that mean we can at least travel at light-speed? If so, how can you prove/disprove that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or does the detection of gravitational waves have no bearing on the possibility for humans to travel faster than the speed of light?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another question would be - is it possible to artificially create gravitational waves to drive said object? What could be a possible method for doing so?&lt;/p&gt;&#xA;" OwnerUserId="16678" OwnerDisplayName="Adjit" LastEditorUserId="25" LastEditDate="2016-09-15T21:48:21.917" LastActivityDate="2016-09-16T16:32:04.413" Title="Does the detection of gravitational waves prove we can travel faster than the speed of light?" Tags="|gravity|advanced-propulsion|alcubierre-drive|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18232" PostTypeId="2" ParentId="18201" CreationDate="2016-09-15T20:41:51.963" Score="-2" Body="&lt;p&gt;The answer, as gerrit has eluded to, is that going to space is &lt;strong&gt;hard&lt;/strong&gt; and &lt;strong&gt;expensive,&lt;/strong&gt; which places it outside the realm of even the most dedicated pranksters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, assuming you can get your pranksat a launch, there is a bevy of launch-vehicle levied requirements on how your pranksat must be designed. Each of those needs to be validated through (most likely) testing, to ensure that your payload will not harm the launch vehicle (for example, by shaking apart in its container.) The facilities and knowledge to execute such testing and design is expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also a ton of national and international bureaucracy involved in going to space. How would you talk to your prospective pranksat? You'd likely use radio, which in the United States places you under the thumb of the FCC. It would be difficult to convince the safety-oriented bureaucrats at the FCC that a pranksat is worth allocating the radio bandwidth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, the act of conducting rendezvous is non-trivial; determining a spacecraft's orientation and position requires  specialized hardware and software that requires considerable expertise to construct. Additionally, flying around near other spacecraft is a great way to collide with them, causing at the very least an international incident and at the worst a major debris-causing incident.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While cubesats are becoming more common, the factors outlined above place hard constraints on the cost and inherent risk of building the type of pranksat you outline. &lt;/p&gt;&#xA;" OwnerUserId="16968" LastActivityDate="2016-09-15T20:41:51.963" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18233" PostTypeId="1" AcceptedAnswerId="18237" CreationDate="2016-09-15T21:22:07.427" Score="13" ViewCount="1187" Body="&lt;p&gt;The Gaia space telescope has two main mirrors that look in different directions, 106⁰ apart. The light of both is focused on a single CCD. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does it have two main mirrors? Is it only to have more opportunities to observe stars as it rotates or is there something else going on, perhaps to do with measuring the distances to stars?&lt;/p&gt;&#xA;" OwnerUserId="16969" LastActivityDate="2021-04-03T02:02:53.400" Title="Why does the Gaia space telescope have two main mirrors" Tags="|space-telescope|gaia|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18234" PostTypeId="2" ParentId="18231" CreationDate="2016-09-15T21:47:56.250" Score="4" Body="&lt;p&gt;No, it does not. Gravity waves can't travel faster than the speed of light in the &quot;direct&quot; path, they will always appear to us as if that is the speed they are traveling. However, if space was stretched away from the source, it would take longer for them to get there, but they still would appear to be traveling at the same speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, nothing has changed as regards to FTL travel as a result of gravity wave detection.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-15T21:47:56.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18236" PostTypeId="2" ParentId="18218" CreationDate="2016-09-15T22:47:09.080" Score="2" Body="&lt;p&gt;No, a Europa (or Io) lander would need much less radiation shielding than Juno.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lander halves its radiation exposure by simply sitting on a body which shields half of its horizon. And by melting its sensitive electronics a few meters into the surface, the radiation problem is pretty much eliminated. And getting down under the surface is pretty much the science goal of a Europa lander anyway. It's not like anyone imagines a Curiosity roving around on Europa's surface. &lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-09-15T22:47:09.080" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18237" PostTypeId="2" ParentId="18233" CreationDate="2016-09-16T03:33:04.360" Score="16" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MFmOF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MFmOF.jpg&quot; alt=&quot;GAIA optical bench and dual primary mirrors&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GAIA's Silicon Carbide Optical Bench, with the two objective mirrors of it's twin telescopes pointing 106.5° apart. From &lt;a href=&quot;http://spaceflight101.com/gaia/gaia-instrument-information/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;, image credit: ESA/Astrium.&lt;/p&gt;&#xA;&lt;p&gt;Cool GIF that is too big to put here: &lt;a href=&quot;http://i.giphy.com/l2Sq1WUTZeWNXaoEM.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://i.giphy.com/l2Sq1WUTZeWNXaoEM.gif&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In order to make extremely precise measurements of positions throughout the sky, GAIA slowly spins and reads out the CCD images in a synchronized fashion - the image on the CCD moves at the same rate as the charge is transferred and read out (clocked) from pixel to pixel. This allows field distortion effects within one telescope to be constantly measured and compensated, since most stars will be measured up to 70 different times passing through a different height (perpendicular to scan direction) across the CCDs multiple times.&lt;/p&gt;&#xA;&lt;p&gt;The multiple measurements as GAIA follows Earth's motion around the sun allows for high precision &lt;a href=&quot;https://en.wikipedia.org/wiki/Parallax&quot; rel=&quot;nofollow noreferrer&quot;&gt;parallax&lt;/a&gt; determination, which can be used to calculate a distance for a very large number of stars in a systematic way.&lt;/p&gt;&#xA;&lt;p&gt;However, the necessary kind of precision over long separations in the sky would be extremely difficult with a single telescope. The &amp;quot;trick&amp;quot; that GAIA uses is to have &lt;strong&gt;both telescopes project on to same CCD array at the same time!&lt;/strong&gt; A star will pass across the CCD array twice, first accompanied by stars that are 106.5° &lt;em&gt;ahead of it&lt;/em&gt; and the second time accompanied by stars that are 106.5° &lt;em&gt;behind it&lt;/em&gt;. This allows the analysis to build up precise relationships at simultaneously very wide and very narrow separations.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VFyB0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VFyB0.png&quot; alt=&quot;GAIA's CCD array, from Spaceflight 101 &amp;amp; ESA&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L2SaT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L2SaT.jpg&quot; alt=&quot;GAIA's CCD array, from Spaceflight 101 &amp;amp; Astrium&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;both above:&lt;/strong&gt; GAIA's CCD array, from &lt;a href=&quot;http://spaceflight101.com/gaia/gaia-instrument-information/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;, image credit: ESA and Astrium respectively.&lt;/p&gt;&#xA;&lt;p&gt;According to the GAIA FAQs which does an excellent job: &lt;a href=&quot;http://www.cosmos.esa.int/web/gaia/faqs&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.cosmos.esa.int/web/gaia/faqs&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Why does GAIA have two telescopes?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The goal of the Gaia mission is to perform global astrometry over the entire celestial sphere. For this purpose, the satellite is equipped with two telescopes whose viewing directions are separated by a large, so-called basic angle (106.5 deg). This allows to make simultaneous measurements of star positions at small and large angular scales, i.e., inside each telescope field of view and between the two fields of view. Since the parallax factor differs between the two fields of view, global astrometry can be performed (as compared to relative astrometry offered by a single field of view). Wide-angle measurements also guarantee a distortion-free and rigid system of coordinates and proper motions over the whole sky.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Why is there an angle of 106.5 degrees between Gaia's 2 telescopes?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The choice of the so-called basic angle of GAIA was a non-trivial one. On the one hand, it should be of order 90 degrees to allow simultaneous measurements of stars separated by large angles on the sky. On the other hand, it should not be a harmonic ratio of a 360-degree circle (e.g., 60 deg, 90 deg, or 120 deg). Taking these considerations into account, acceptable ranges for the basic angle are 96.8 +/- 0.1 deg, 99.4 +/- 0.1 deg, 100.5 +/- 0.1 deg, 105.3 +/- 0.1 deg, 106.5 +/- 0.1 deg, 109.3 +/- 0.1 deg, 109.9 +/- 0.1 deg, etc. Accommodation aspects identified during industrial studies subsequently favoured 106.5 deg as the value finally adopted for Gaia.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;How do you ensure the stability of the basic angle?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The stability of the basic angle is achieved through the design of the spacecraft and mission (profile), including the choice of material for most payload structural parts (silicon-carbide), the absence of moving parts in the spacecraft (except for the thruster valves), the thermo-mechanical decoupling between the service and payload modules, a constant power dissipation of the payload, and the spacecraft orbit and attitude (ensuring a constant solar illumination without eclipses by the Earth). In addition, the basic-angle monitor provides continuous feedback on short-term basic-angle variations (P &amp;lt; 12 h), with long-term variations (P &amp;gt; 12 h) being calibrated within the data processing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-04-03T02:02:53.400" LastActivityDate="2021-04-03T02:02:53.400" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18238" PostTypeId="1" CreationDate="2016-09-16T03:34:10.960" Score="13" ViewCount="956" Body="&lt;p&gt;At first glance, solid rocket launching makes more sense than liquid:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Simple to scale. Once designed, it can be made perfectly the same, all the time.&lt;/li&gt;&#xA;&lt;li&gt;Storage - Keep for as long as you want, launch on demand.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;And modern designs can be made to be precise - if they can be made to hit a house halfway across the world, they should be able to hit a precise orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why so few commercial solid launch vehicles?&lt;/p&gt;&#xA;" OwnerUserId="16972" LastEditorUserId="238" LastEditDate="2016-09-16T04:20:19.663" LastActivityDate="2019-05-12T00:26:33.810" Title="Why are there so few solid rocket satellite launchers?" Tags="|launch|solid-fuel|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18239" PostTypeId="1" CreationDate="2016-09-16T05:49:11.873" Score="5" ViewCount="165" Body="&lt;p&gt;I saw a video of Sunny Williams &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/SGP6Y0Pnhe4?start=94&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;she moved the laptop around. Go to time position 1:34. Is this articulated arm available to purchase for consumers on Earth?&lt;/p&gt;&#xA;" OwnerUserId="16973" LastEditorUserId="4660" LastEditDate="2016-09-16T15:49:35.473" LastActivityDate="2020-11-15T01:30:31.753" Title="what brand/make is the laptop articulated arm in sleeping pod?" Tags="|iss|computing|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18240" PostTypeId="2" ParentId="18238" CreationDate="2016-09-16T06:35:23.410" Score="22" Body="&lt;p&gt;Solid rockets have advantages, but also drawbacks:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;they have low I&lt;sub&gt;sp&lt;/sub&gt;. Especially on upper stages, this means the stage has to be large.  &lt;/li&gt;&#xA;&lt;li&gt;they generate &lt;a href=&quot;https://space.stackexchange.com/questions/1485/how-were-vibrations-supposed-to-be-handled-in-an-ares-i&quot;&gt;more vibration&lt;/a&gt; than a liquid stage. &lt;/li&gt;&#xA;&lt;li&gt;they're not as simple to scale as you think. Casting the solid fuel &lt;a href=&quot;http://www.ndt.net/article/wcndt00/papers/idn202/idn202.htm&quot; rel=&quot;noreferrer&quot;&gt;is a difficult process&lt;/a&gt; that has to be tightly controlled to get repeatable results. &lt;/li&gt;&#xA;&lt;li&gt;because the entire stage is the combustion chamber, the entire stage has to be built to withstand combustion pressure. This means their empty weight is high and fuel fraction is low (85% for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;noreferrer&quot;&gt;Shuttle SRB&lt;/a&gt;, liquid rockets get over 90%). &lt;/li&gt;&#xA;&lt;li&gt;rockets have to be able to reach a variety of orbits. This is much easier with a liquid engine which can be throttled in flight, and shut off before fuel depletion. A solid motor can only be throttled in the design phase (by changing the casting pattern for the propellant), and can only be shut down by blowing holes in the casing (which is not advisable when the motor is still attached to the payload). &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The precision of an ICBM has nothing to do with motor design. The solid motors just propel the missile in the general direction of the target. After the boost phase, accurate targeting is done with liquid-propellant maneuvering thrusters. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-09-16T07:10:18.980" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18241" PostTypeId="1" AcceptedAnswerId="18243" CreationDate="2016-09-16T08:50:23.087" Score="8" ViewCount="1075" Body="&lt;p&gt;In the news item on the website of ABC's affiliate station WFTV in Florida &lt;a href=&quot;http://www.wftv.com/news/local/spacex-explosion-nearly-destroyed-asteroid-seeking-osiris-rex-spacecraft-official-says/444996227&quot; rel=&quot;noreferrer&quot;&gt;Official: SpaceX explosion nearly destroyed asteroid-seeking OSIRIS-REx spacecraft&lt;/a&gt;, there is discussion of a loss of pressure and cooling following the SpaceX 'fast fire' (explosion) during a test on September 1st, 2016. I don't understand what kind of pressure or cooling was lost, and how that could have destroyed OSIRIS-REx.&lt;/p&gt;&#xA;&lt;p&gt;I am not even sure how far apart these two rockets were, or why their systems were coupled in such a way that a failure at one location could so easily lead to the failure of critical systems and potential loss of an expensive payload at a different location. It's not like rockets never explode.&lt;/p&gt;&#xA;&lt;p&gt;Could someone either show a map, or give GPS coordinates that can be googled that illustrate exactly where these two rockets were at the time? What are the &amp;quot;adjoining facilities&amp;quot; mentioned below? It almost sounds like the technicians had to run in to a burning building to fix a refrigerator!&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;OSIRIS-REx was sitting atop an Atlas V rocket at an adjoining facility, set to launch a week later, when the SpaceX rocket exploded during a static test, NASA said.&lt;/p&gt;&#xA;&lt;p&gt;In a release, 45th Space Wing mission support group commander Lt. Col. Greg Lindsey, detailed the emergency response to the explosion at Cape Canaveral Air Force Station.&lt;/p&gt;&#xA;&lt;p&gt;Included in his description was information on how technicians had to fight to save OSIRIS-REx as emergency personnel put out fires from the SpaceX explosion.&lt;/p&gt;&#xA;&lt;p&gt;As his teams responded to the explosion, Lindsey was told that the cooling system at Launch Complex 41 was losing pressure.&lt;/p&gt;&#xA;&lt;p&gt;“Without those chillers, the spacecraft for the next launch (OSIRIS-REx) would be lost,” he said. “Needless to say, at this point I had to reestablish our priorities and get a team working on a way … to allow access for technicians to enter in order to make the necessary repairs.”&lt;/p&gt;&#xA;&lt;p&gt;As Lindsey and his personnel were developing a plan, there was a critical loss of pressure at Launch Complex 41 “and technicians had to get to (OSIRIS-REx) immediately.”&lt;/p&gt;&#xA;&lt;p&gt;With assistance from Kennedy Space Center, a response team from the 45th Space Wing was able to circumvent the site of the explosion, get to Launch Complex 41 and save OSIRIS-REx, Lindsey said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-16T19:49:44.753" Title="Why was OSIRIS-REx almost destroyed because of a loss of pressure following the SpaceX &quot;fast fire&quot;?" Tags="|spacex|failure|atlas-v|osiris-rex|cape-canaveral|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18242" PostTypeId="1" AcceptedAnswerId="18249" CreationDate="2016-09-16T09:04:32.740" Score="2" ViewCount="500" Body="&lt;p&gt;From 2006-2008 DARPA ran a program called &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_Express&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Express&lt;/a&gt;(wikipedia link) which was &quot;a safe and cost-effective approach to autonomously service satellites in orbit.&quot; The program had several goals:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Transfer of hydrazine monopropellant between ASTRO and NextSat.&lt;/li&gt;&#xA;&lt;li&gt;Use of ASTRO’s robotic arm to detach and reattach a flight computer from a bay onboard ASTRO.&lt;/li&gt;&#xA;&lt;li&gt;Use of the arm to detach a battery from its bay on ASTRO and transfer it to a similar bay onboard NextSat.&lt;/li&gt;&#xA;&lt;li&gt;Use of ASTRO’s arm to couple to a NextSat grappling fixture and “berth” the spacecraft, allowing a separate capture mechanism to perform final docking.&lt;/li&gt;&#xA;&lt;li&gt;Multiple rendezvous and capture scenarios with a number of different sensors, at ranges up to seven kilometers (4.4 miles).&lt;/li&gt;&#xA;&lt;li&gt;“Soft” (low momentum transfer) docking with a special, three-fingered capture mechanism.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This whole program looks like it was quite successful and other elements have appeared in DARPAs other satellite servicing programs.  Why wasn't the project expanded and why aren't other organizations working on satellite refueling? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cmsCP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cmsCP.png&quot; alt=&quot;Astro Express parts&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ps would someone who has the reputation please add the DARPA tag to this post? thx :) &lt;/p&gt;&#xA;" OwnerUserId="16606" LastEditorUserId="58" LastEditDate="2016-09-19T15:23:06.103" LastActivityDate="2016-09-19T15:23:06.103" Title="Why Was DARPA's Orbital Express Refueling Program Discontinued?" Tags="|artificial-satellite|military|hydrazine|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18243" PostTypeId="2" ParentId="18241" CreationDate="2016-09-16T11:16:14.800" Score="7" Body="&lt;p&gt;SLC-40 and 41 are neighbors, no other pads in between. Some debris landed further away than SLC-41. They share some systems, e.g. the deluge system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only good source I've found is &lt;a href=&quot;http://www.patrick.af.mil/News/Commentaries/Display/Article/938481/emergency-management-a-behind-the-scenes-look-on-the-eastern-range&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; post by a member of the emergency response team. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Sept. 1, OSIRIS-REx was still in the vertical integration building &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/2016/09/06/atlas-v-asteroid-mission-go-thursday-launch/89927312/&quot; rel=&quot;nofollow&quot;&gt;next to the launch pad&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The explosion triggered the deluge system. The ERT worried the system might empty its storage, running the pumps dry. This could damage the pumps and make a launch from SLC-41 impossible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The post also mentions the chillers, but offers no detail. I've got confirmation this was about the &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=26459.msg1585037#msg1585037&quot; rel=&quot;nofollow&quot;&gt;climate control system for the payload&lt;/a&gt;. The spacecraft was undergoing tests and was powered up. As it was already inside the fairing, that power/heat went into a small enclosed volume, so they run an AC to blow cold air into the fairing.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cause of the chiller failure may have been something simple like a power failure (either due to blast effects, or a precautionary shutdown after the explosion). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-09-16T19:49:44.753" LastActivityDate="2016-09-16T19:49:44.753" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18244" PostTypeId="1" AcceptedAnswerId="18245" CreationDate="2016-09-16T13:56:34.987" Score="19" ViewCount="1300" Body="&lt;p&gt;In the &lt;a href=&quot;https://en.wikipedia.org/wiki/Amos-6&quot; rel=&quot;nofollow&quot;&gt;F9-029 Amos 6&lt;/a&gt; mission failure at the launch pad, a &quot;fast fire&quot; happened near the 2nd stage of Falcon 9 FT. In &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-7&quot; rel=&quot;nofollow&quot;&gt;F9-020 CRS-7&lt;/a&gt; mission, an explosion happened in flight (again in the 2nd stage) a few moments before 1st stage cutoff. Failure of a strut was the cause of the 2nd stage explosion in CRS-7 mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Elon Musk, the Amos 6 mission case is considered the most difficult and complex failure they have ever had in 14 years. SpaceX has also requested support from the public with audio, photo or video materials if they have. This means that the investigation process could last longer and be more difficult than was thought previously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the reasons that it is more difficult to find what caused the disintegration of the Falcon 9 FT rocket in the Amos 6 mission?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-10-19T19:19:52.027" LastActivityDate="2016-10-19T19:19:52.027" Title="Why is the F9-029 Amos 6 case more complex than the F9-020 CRS-7 explosion?" Tags="|launch|spacex|falcon-9|failure|testing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18245" PostTypeId="2" ParentId="18244" CreationDate="2016-09-16T14:06:37.203" Score="20" Body="&lt;p&gt;Several issues have been reported informally, mostly in Elon Musk tweets.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Not as many cameras turned on as usual&lt;/li&gt;&#xA;&lt;li&gt;Storage on site was damaged, and not all data was uploaded during test&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This fueling event, for a static fire, was a really routine thing by this point. They thought they had fueling down pat. The last fueling accident like this in the US was back in the 60's. Thus they were not paying quite as much attention with cameras as they might during a launch, or even 13 minutes later, during the test fire. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem may be in the fueling system, the Ground Support Equipment, the strongback, or any of the other surrounding equipment, which is obviously not as instrumented as the vehicle itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally the data link off site is not as fast as they would like, so they buffer data at the site itself, and upload it later. But the storage devices were damaged to some unknown (at this time) degree.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus they have lots of data from cameras and telemetry, but it is not as much as they might have if they had been paying full attention as they would during a launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be fair, tracking down the CRS-7 root cause to me, was quite impressive as that was a somewhat obscure thing to fail and detect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think when they report the real cause of this one, it should be very interesting.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-09-16T14:13:55.117" LastActivityDate="2016-09-16T14:13:55.117" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18246" PostTypeId="2" ParentId="18241" CreationDate="2016-09-16T15:18:10.640" Score="5" Body="&lt;p&gt;Satellites are finely tuned to work in a specific environment. Space is a tough environment, but usually pretty consistent. One of the things required is to keep the spacecraft at a certain operating temperature at all times, including on the ground and on the rocket. There are a few components that are particularly sensitive, most notably the batteries. Depending on the state of integration, the satellite might have been set such that it was continually running, which is something required prior to launch in remove before flight operations. If that had happened, then the satellite was being used in the tightened environment pre-launch, which could cause some heating issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The incident may or may not have been directly related to the issue, but access to the integration facility was clearly limited being so close to the scene of the accident, and that took what might have been fairly minor problem and compounded it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I doubt that OSIRIS-REX would have been a complete waste if the temperature had gone in to the red lines, but it's performance might have been in question, particularly for any eclipse periods that the spacecraft will enter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that when the incident happened, the entire area was evacuated. The kinds of measurements that are required to determine if there is a problem are routinely monitored, particularly when the launch is so close. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-16T15:18:10.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18247" PostTypeId="2" ParentId="18238" CreationDate="2016-09-16T16:25:57.843" Score="7" Body="&lt;p&gt;Making a solid rocket, that is large enough to be useful for orbital launch is quite hard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You cannot fuel it on the pad, so the fuel has to be cast into the booster where it is manufactured. Thus it is very hard to transport after that, since a large SRB is VERY heavy. And often very long. So you cannot ship it by truck or even train. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the Shuttle SRB's are made in segments so they can be transported from Utah to Florida via train. Length is an issue since the curves of the train tracks limit it before it might impinge on a train on the parellel set of tracks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a great story, how the size of the SRB's is directly related to the size of a horses arse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRB segment in length and width cannot exceed the ability to transport by rail. This is dependent on the gauge of the rail (distance between the two rails. Narrow gauge (Russia) and wide guage (US/Europe in some places) are common.  Toronto subways as a fun example use a third size just to be different) Rail gauge is based on the Roman roads that still exist in some places in Europe. The width of the road was determined by a wagon with pairs of horses pulling it.  Thus the direct connection. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solids can have awe inspiring amounts of thrust (SRB at 2.8 million lbs of thrust, compared to F-1 or RD-170 engines at 1.5 Million lbs thrust. They are sort of the largest liquid engines seriously built) but their ISP is usually lousy.  This is why they are often used as lower stages where thrust matters but ISP is less relevant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is why it is always interesting when you see a design for a booster with a solid upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yet, conversely, kick stages, sometimes a baby 4th stage, or even 5th stage can be solids.  The Star line of motors is pretty common in commercial use. There, solids scale better for the size, do not need huge performance, just need that last little bit of impulse. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2016-09-16T18:03:58.187" LastActivityDate="2016-09-16T18:03:58.187" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18248" PostTypeId="2" ParentId="18231" CreationDate="2016-09-16T16:32:04.413" Score="2" Body="&lt;p&gt;Both gravitational waves and the existence of the Alcubierre metric are predictions of Einstein's theory of general relativity (GR).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GR is strongly supported by a great deal of experimental evidence. In particular, gravitational waves were observed indirectly in 1974 through observations of the Hulse-Taylor binary pulsar, and GR's predictions about gravitational waves were verified in quantitative detail then. What is new within the last year or so is the direct detection of gravitational waves, but there hadn't been any doubt about their existence for decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Observations can never prove a scientific theory. They can disprove it, or they can fail to disprove it. When we accumulate enough observations that fail to disprove a theory, we start to believe that the theory is right, within its realm of applicability. This is what has happened with GR. Direct detection of gravitational waves has little effect on whether we believe GR is valid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Alcubierre metric is a solution to the equations of GR (the Einstein field equations), so additional confirmation of GR is additional confirmation that the Alcubierre metric is mathematically valid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, it's wrong/naive to think of the Alcubierre metric as a recipe for building a faster-than-light (FTL) spaceship, even in theory. You can't build one without exotic matter, which probably doesn't exist. It's probably &lt;a href=&quot;http://arxiv.org/abs/0904.0141&quot; rel=&quot;nofollow&quot;&gt;unstable&lt;/a&gt; due to quantum effects. You can't get in or out of the drive region, which can't be turned on or off.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2016-09-16T16:32:04.413" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18249" PostTypeId="2" ParentId="18242" CreationDate="2016-09-16T20:03:30.067" Score="6" Body="&lt;p&gt;It's mostly due to nature of how DARPA operates in general. They have few 'programs', they are project and result focused. DARPA tends to form a temporary project team, formulate desired outcomes, see it either fail or come through and move on. The results, if any, will end up in literature and in forms of IP in hands of contracted partners etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their projects are time- and resource-bounded, not open ended. Teams and members come and go. There is a good overview article of of how DARPA &lt;a href=&quot;https://hbr.org/2013/10/special-forces-innovation-how-darpa-attacks-problems&quot;&gt;operates here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There also &lt;strong&gt;are&lt;/strong&gt; other organizations working on satellite fuelling, including DLR in Germany with DEOS mission, NASA on ISS with Robotic Refueling Missions and upcoming RESTORE-L project. MDA/Space Systems Loral have kept around a low-key commercial project for this for a decade or so as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;DARPA itself has another upcoming project called RSGS - Robotic Servicing of Geosynchronous Satellites&lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2016-09-16T20:03:30.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18250" PostTypeId="2" ParentId="18141" CreationDate="2016-09-16T20:47:30.450" Score="4" Body="&lt;p&gt;A firefighter on the scene reported that a number of train rail cars, and ground support tanks were burned and blew up in the process.  Could be one of any of those.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-09-16T20:47:30.450" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18251" PostTypeId="1" AcceptedAnswerId="18253" CreationDate="2016-09-16T21:49:07.890" Score="11" ViewCount="1085" Body="&lt;p&gt;Saturn V had RP-1/LOX 1st stage with 5 F-1 rocket engines. The 2nd and 3rd stage used LH2/LOX J-2 rocket engines, 5 and 1 respectively. At that time NASA had an advantage to launch heavy payloads because it was successful in the production of LH2/LOX rocket engines. What situation would it be if even the 2nd and 3rd stages of Saturn V would have been with RP-1/LOX engines? I am speaking for that situation where the 1st stage remains the same. What configuration would the Saturn V have had for the 2nd and 3rd stages (with what rocket engines, how many of them, what amount of fuel in their tanks, or even a possible 4th stage) and what performance? Probably it would have lower payload values and would be a shorter rocket since RP-1 is denser than LH2, but what would be the values and how would it look like?&lt;/p&gt;&#xA;" OwnerUserId="16979" LastEditorUserId="16979" LastEditDate="2016-09-17T03:21:39.213" LastActivityDate="2019-06-15T16:57:28.950" Title="What would be the configuration and performance for Saturn V with all stages RP-1/LOX?" Tags="|rockets|nasa|saturn-v|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18252" PostTypeId="2" ParentId="18251" CreationDate="2016-09-17T01:21:40.340" Score="11" Body="&lt;p&gt;It would probably not have been practical to do a three-stage, single launch LOR &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot;&gt;Saturn&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_(spacecraft)&quot;&gt;Apollo&lt;/a&gt; all on kerosene; the specific impulse advantage of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_J-2&quot;&gt;J-2&lt;/a&gt; engines is just too great. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to my spreadsheet estimates, a four-stage kerosene rocket 3 times the size of Saturn V could do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The translunar stage, S-IV-K, is 138 tons propellant, 12 tons dry, 47 tons payload (Apollo CSM and LM). 1x &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_H-1&quot;&gt;H-1&lt;/a&gt; engine (as used on the Saturn 1B).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third stage, S-III-K, 465 propellant, 35t dry, 1x &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot;&gt;F-1&lt;/a&gt; engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second stage, S-II-K, 1674t propellant, 126t dry, 3x F-1 engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First stage, S-I-K, 6392t propellant, 408t dry, 16x F-1 engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All-up mission launch mass of this &quot;Saturn XVI&quot; would be 9297 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First 3 stages produce ~9600 m/s of delta-v, taking the beast to a 185km circular orbit. Fourth stage produces ~3400 m/s to send the spacecraft to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass could be brought down substantially with more optimized engines; both the F-1 and H-1 were designed as first-stage engines. Larger nozzle extensions, particularly on the 3rd and 4th stages, would improve the specific impulse, without requiring all-new engine designs. With an &quot;H-1V&quot; producing 320s ISP and an &quot;F-1V&quot; producing 337s, total launch mass could be brought down to 6820 tons or so, with &quot;only&quot; 12 engines on the first stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both my answers are, obviously, merely rough Kerbal-style feasibility estimates. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Amended tonnages to reflect more conservative fuel tank mass fractions.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-09-17T23:46:32.470" LastActivityDate="2016-09-17T23:46:32.470" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18253" PostTypeId="2" ParentId="18251" CreationDate="2016-09-17T03:44:42.437" Score="16" Body="&lt;p&gt;If we hold the loaded masses of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V&lt;/a&gt;'s three stages constant, but switch to kerosene tankage, engines, and propellants for the upper stages, we get a rocket that can send perhaps 55 tons to LEO and 9 tons translunar -- not enough for a crewed landing mission, but certainly enough for a flyby and possibly a crewed lunar orbit mission. Two launches and an Earth orbit rendezvous could possibly get a bare bones, Soyuz/LK lander mission, and three launches and EOR could probably pull off an Apollo CSM/LM mission.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First stage remains a 5-engine &lt;a href=&quot;https://en.wikipedia.org/wiki/S-IC&quot; rel=&quot;nofollow noreferrer&quot;&gt;S-IC&lt;/a&gt;, 2148 tons propellant, 132 tons dry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second stage would be 447t propellant, 34t dry, 1x &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;F-1&lt;/a&gt; engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third stage would be 106t propellant, 9t dry, 1x &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_H-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;H-1&lt;/a&gt; engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For translunar flight, the third-stage H-1 would have to be modified to provide restart capability; it would burn once to get into LEO and then again for TLI, like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_J-2&quot; rel=&quot;nofollow noreferrer&quot;&gt;J-2&lt;/a&gt; on the Saturn V third stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All-up launchpad mass would be 2885 tons for the translunar mission, 2931 tons for 60t to LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper stages would be much more compact than on the Saturn V, as you note. The second stage would be about the size of the Saturn V third stage, in fact, so the stack could look rather like the proposed &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_INT-20&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn INT-20&lt;/a&gt; configuration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Amended tonnages to reflect more conservative fuel tank mass fractions.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-06-15T16:57:28.950" LastActivityDate="2019-06-15T16:57:28.950" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18254" PostTypeId="1" CreationDate="2016-09-17T04:17:08.270" Score="0" ViewCount="68" Body="&lt;p&gt;I'm really interested in micro satellites, but I'm a beginner. So, for the year-end project of a course called &quot;Neural networks&quot; at University I want to do something related with this. I searched a lot but I couldn't find anything that blows my mind and that is not too difficult (for example, all things about deep learning and forecasting are awesome, but I don't have so much time to do something like this.). So, I thought that maybe I could make an atmospheric density predictor, and make a nice drag force predictor (as you know, the atmospheric density is the non-linear term in the drag force), but I read that micro satellites don't care about the drag force because they last so less time in space (like 3 years) that it's not so important. As you know, micro satellites work in LEO, where the drag force is significant. What do you think?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone have any other idea? I really want to enter in the aerospace &quot;world&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS: does micro satellites use any propulsion system? How do they move between orbits? If they can't move by themselves, maybe my idea of predicting the drag force is useless&lt;/p&gt;&#xA;" OwnerUserId="16998" LastActivityDate="2016-09-17T04:17:08.270" Title="Neural networks applied to micro satellites missions" Tags="|artificial-satellite|orbit|spacecraft|atmosphere|" AnswerCount="0" CommentCount="0" FavoriteCount="0" ClosedDate="2016-09-17T12:59:34.670" ContentLicense="CC BY-SA 3.0" />
  <row Id="18256" PostTypeId="1" AcceptedAnswerId="18277" CreationDate="2016-09-17T16:57:45.117" Score="1" ViewCount="2026" Body="&lt;p&gt;In many sci-fi novels I have read, interstellar travelers had their ship  accelerate half the way, then decelerate half the way. I wonder why this is the chosen approach. Would not it be better to accelerate all the way, then leave the ship for small landing module(s) and decelerate them while allowing the interstellar ship to go away from the star system? Maybe, even eject the landing modules from the mother ship in opposite direction.&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="4660" LastEditDate="2016-09-17T17:16:18.663" LastActivityDate="2016-11-22T19:51:18.283" Title="Why would interstellar space travelers need to decelerate their whole ship?" Tags="|interstellar-travel|" AnswerCount="9" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18257" PostTypeId="2" ParentId="18256" CreationDate="2016-09-17T17:16:28.543" Score="2" Body="&lt;p&gt;Basically, the answer comes down to the amount of fuel you need to accelerate and decelerate.  When planning an interstellar mission, your payload space is limited by how much fuel you can have onboard, so every available space is filled with essentials.  You don't really have room for luxuries.  If you want to use this escape pod method, those escape pods take up weight and space, contributing to the amount of fuel you need to accelerate the ship in the first place.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The escape pods themselves will also need a very large amount of fuel in order to perform the deceleration necessary.  As we know from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;Tsiolkovsky Rocket Equation&lt;/a&gt;, $\Delta v = v_e ln \frac{m_0}{m_f}$, meaning that as delta-v grows, the fuel needed grows at an exponential rate.  Even if you fire them from the ship in the opposite direction, the ship itself will have to be carrying the fuel necessary to do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Add to this the fact that, since you've decided to let your ship accelerate the ENTIRE way, it's going to be traveling insanely fast, so those little escape pods will need to decelerate a huge amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, while it is possible to do as you suggest, the fuel requirements would be tremendous.  It would almost always be easier just to decelerate the whole ship.  That way you can use all of the space onboard for necessary equipment and personnel.  Plus once it arrives at the destination, you can reuse it, or park it in orbit to act like a space station.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-09-17T17:16:28.543" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18258" PostTypeId="1" CreationDate="2016-09-17T17:48:22.353" Score="2" ViewCount="808" Body="&lt;p&gt;What about Mt. Chimborazo in Ecuador as a starting point, saving some miles, thinner atmosphere etc? It's the farthest point from Earth's center, even compared to Mt. Everest because of the bulge. Has anyone proposed starting here yet? Or using a space bola system starting around this point? But I am unsure if the 1 degree 28 minute 9 second difference from equator would cause too many difficulties with geosynchronous orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could combining space bolas with the elevator make up for the distance?&lt;/p&gt;&#xA;" OwnerUserId="16496" LastEditorUserId="6816" LastEditDate="2016-09-18T04:53:27.663" LastActivityDate="2016-12-28T06:23:20.773" Title="Would 100 miles off the equator be too much for a space elevator to have the right orbital geometry?" Tags="|orbital-mechanics|space-elevator|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18260" PostTypeId="2" ParentId="18222" CreationDate="2016-09-18T01:34:18.667" Score="10" Body="&lt;p&gt;Not only has it autonomous driving capability, just very recently in 2016 it acquired new features for 'autonomously' targeting ChemCam and autonomosly re-schedule some of the science observations by itself. All based on pre-defined priority rules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The auto-targeting software component is called &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=6575&quot; rel=&quot;nofollow noreferrer&quot;&gt;AEGIS&lt;/a&gt;. Previously tested and enabled autonomous driving component is called &lt;a href=&quot;http://www.nasa.gov/mission_pages/msl/multimedia/curiosity20130919.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;AutoNav&lt;/a&gt;, which is being used since 2013. EDIT: as per comment below, AutoNav goes back to MER's, Spirit team first used the software piece in February 2004.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is important to understand there are different levels of autonomy in these processes. For example, important component of AutoNav is simply estimating how far the rover actually moved during driving. Previously this was done back on earth by processing navcam images and wheel odometry data, and now AutoNav is pretty much able to keep an accurate estimate during the drive. This enables longer, bolder drive plans to be uplinked as the rover can be trusted not to be too far off its planned path, by 'closing the control loop' locally. This level of autonomy does not mean a lot of high level planning done by robot.&lt;/p&gt;&#xA;" OwnerUserId="3140" LastEditorUserId="25067" LastEditDate="2018-11-15T10:18:15.097" LastActivityDate="2018-11-15T10:18:15.097" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18261" PostTypeId="2" ParentId="18222" CreationDate="2016-09-18T05:00:18.067" Score="21" Body="&lt;p&gt;AutoNav is simply given a set of waypoints, which the software then has to, on its own, plan and execute a route. You can imagine the waypoints as lawn darts on the 3D terrain map built from the previous sol's images. In fact, in the rover planning tool, the waypoints are displayed as actual lawn darts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan is executed about a meter at a time, heading in the direction of the waypoint as permitted by the terrain. At each step the rover will build a safety map of the terrain ahead and go around obstacles as needed. The drive can terminate before reaching the final waypoint if either no progress can be made safely, or the allotted time has been used up. Regardless of how the drive ended, images are always taken of the final state for use by the rover planners on Earth for the next sol's plan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, arm, drilling, sample movement, instrument operations, etc. can all be executed autonomously over a sol or longer. Those are generally well-specified, deterministic actions (unlike waypoint driving), and the sequence terminates if any unforseen problems are encountered. Drilling and sample movement activities are done over several sols so that the operators on Earth can be in the loop and verify proper progress along the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rovers are designed to be autonomous enough to keep themselves busy for a full sol. They normally get only one set of commands direct from Earth when they wake up in the morning, and then send back the results the sol's activities in the afternoon through a relay orbiter, before going to sleep.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2018-11-13T21:06:30.010" LastActivityDate="2018-11-13T21:06:30.010" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18262" PostTypeId="1" AcceptedAnswerId="18318" CreationDate="2016-09-18T06:16:10.060" Score="21" ViewCount="3957" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current events makes this question even more poignant&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://mars.nasa.gov/mer/mission/status_opportunityAll.html#5183&quot; rel=&quot;noreferrer&quot;&gt;sols 5183 to 5189, Aug. 23, 2018 - Aug. 29, 2018: Team Continues to Listen for Opportunity&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Curiosity selfie has been released just a few days ago, showing better the collection of sand on top of the rover: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA22545&quot; rel=&quot;noreferrer&quot;&gt;Panorama of Vera Rubin Ridge&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After snagging a new rock sample on August 9, 2018 (Sol 2137), NASA's Curiosity rover surveyed its surroundings on Mars, producing a 360-degree panorama of its current location on Vera Rubin Ridge. The scene is presented with a color adjustment that approximates white balancing, to resemble how the rocks and sand would appear under daytime lighting conditions on Earth. Two versions are included here: one with scale bars, and one without.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The panorama includes umber skies, darkened by a fading global dust storm. It also includes a rare view by the Mast Camera of the rover itself, revealing a thin layer of dust on Curiosity's deck. In the foreground is the rover's most recent drill target, named &quot;Stoer&quot; after a town in Scotland near where important discoveries about early life on Earth were made in lakebed sediments.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KH3s0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KH3s0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bQVww.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bQVww.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Surfaces still dust-covered five months later!&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1jXBe.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1jXBe.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; even the inclined surface of the high gain antenna seems covered in dust!&#xA;Cropped from Phys.org's &lt;a href=&quot;https://phys.org/news/2019-01-curiosity-farewell-mars-vera-rubin.html&quot; rel=&quot;noreferrer&quot;&gt;Curiosity says farewell to Mars' Vera Rubin Ridge&#xA;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A selfie taken by NASA's Curiosity Mars rover on Sol 2291 (January 15) at the &quot;Rock Hall&quot; drill site, located on Vera Rubin Ridge. Credit: NASA/JPL-Caltech&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;See also Space.com's &lt;a href=&quot;https://www.space.com/43150-mars-curiosity-selfie-leaving-vera-rubin-ridge.html&quot; rel=&quot;noreferrer&quot;&gt;Curiosity Snaps Epic 'Selfie' on Mars as It Moves On to New Martian Sights&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; In light (or lack of light in this case) of the current dust storm on Mars discussed in the recent NASA news article &lt;a href=&quot;https://www.nasa.gov/feature/jpl/opportunity-hunkers-down-during-dust-storm&quot; rel=&quot;noreferrer&quot;&gt;Opportunity Hunkers Down During Dust Storm&lt;/a&gt; and since almost two years have passed, I've un-accepted @Phiteros' answer to see if there is an update on ExoMARS' dust-on-solar-panels strategy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since it does not have an RTG, ExoMARS relies on efficient collection of solar radiation for its mission to succeed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Xx3DG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xx3DG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;This global map of Mars shows a growing dust storm as of June 6, 2018. The map was produced by the Mars Color Imager (MARCI) camera on NASA's Mars Reconnaissance Orbiter spacecraft. The blue dot indicates the approximate location of Opportunity.&quot; From &lt;a href=&quot;https://www.nasa.gov/feature/jpl/opportunity-hunkers-down-during-dust-storm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. Credits: NASA/JPL-Caltech/MSSS&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I remember reading stories about the Spirit and Opportunity Mars rovers related to solar power and dust - about problems with dust storms, and parking on an incline for the winter in order to collect enough power to 'stay alive'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Curiosity rover carries a large, powerful RTG for both thermal heat (circulating fluids) and of course electrical power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now it seems the &lt;a href=&quot;https://en.wikipedia.org/wiki/ExoMars_(rover)&quot; rel=&quot;noreferrer&quot;&gt;ExoMARS rover&lt;/a&gt; will use solar panels again. The do seem a bit larger compared to the size (or volume) of the craft compared to Spirit and Opportunity, but I am wondering, what management techniques are begin considered for the ExoMARS rover to handle covering of the horizontal panels by dust, or low solar elevation in 'winter'?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2Z0CU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Z0CU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;A 1:4 model of the ExoMars rover at the ESA pavilion, 2007 International Paris Air Show at the Le Bourget airport.&quot; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Paris_Air_Show_2007-06-24_n18.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vVh7y.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vVh7y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;The deck of NASA's Mars Exploration Rover Spirit is so dusty that the rover almost blends into the dusty background in this image assembled from frames taken by the panoramic camera (Pancam) during the period from Spirit's Sol 1,355 through Sol 1,358 (Oct. 26-29, 2007)...&#xA;Dust on the solar panels reduces the amount of electrical power the rover can generate from sunlight each sol.&quot; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Mars_Spirit_rover%27s_solar_panels_covered_with_Dust_-_October_2007.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="79" LastEditDate="2019-01-29T03:34:25.537" LastActivityDate="2020-03-04T04:33:41.903" Title="Curiosity is still dirty! How will the ExoMARS Rover keep its solar panels dust-free and collecting sufficient power?" Tags="|mars|rovers|solar-power|dust|exomars|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="18263" PostTypeId="1" CreationDate="2016-09-18T15:59:05.830" Score="0" ViewCount="41" Body="&lt;p&gt;Is this concept covered somewhere I'm curious about resistance and tension and velocity in space versus tons of chemical fuels? If I've missed that can you point me to it?&lt;/p&gt;&#xA;" OwnerUserId="16997" LastActivityDate="2016-09-18T15:59:05.830" Title="Can we build a sling in space to shoot a ship to Mars faster?" Tags="|orbital-mechanics|mars|fuel|" AnswerCount="0" CommentCount="3" ClosedDate="2016-09-18T21:29:27.620" ContentLicense="CC BY-SA 3.0" />
  <row Id="18266" PostTypeId="1" CreationDate="2016-09-18T20:17:21.537" Score="2" ViewCount="167" Body="&lt;p&gt;Science fiction has given us countless examples of international space campaigns (exp: Star Trek's &quot;Federation&quot;). But what's the largest and most active space campaign from Earth and how much (in tonnage) has been launched into or constructed in space under its united flag?&lt;/p&gt;&#xA;" OwnerDisplayName="user16987" LastActivityDate="2016-09-19T16:19:09.313" Title="What is the largest single project (in tonnage) in which an international space cooperative has launched and or constructed in space?" Tags="|launch|construction|international-politics|radio-telescope|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18267" PostTypeId="2" ParentId="18266" CreationDate="2016-09-18T20:25:03.890" Score="5" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow&quot;&gt;International Space Station&lt;/a&gt;, by far. The mass of the station varies with time, but it's around 420 tons It was constructed over the course of about 13 years, although it is still being tweaked today.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-09-19T16:19:09.313" LastActivityDate="2016-09-19T16:19:09.313" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18268" PostTypeId="1" AcceptedAnswerId="18319" CreationDate="2016-09-18T21:20:47.713" Score="4" ViewCount="313" Body="&lt;p&gt;How can I determine if a rotating thin shell cylinder space station is stable? Its dimensions are:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;radius = 895m&#xA;length = 1150m&#xA;thickness of the shell = 15m&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The interior is filled with lunar regolith, weighing 10+ tons/m&lt;sup&gt;3&lt;/sup&gt;. RPM=1 and rotational speed is 95.2952m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shell is to be composed of kevlar and carbon fiber. The structure is intended to be used as a human settlement. Can you help me to figure out if it's stable or not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What i mean by stable is- As the station rotates generating gravity on the inside, there are forces acting in different directions and if it is not right it might wobble. Here moment of inertia can be taken into account and according to thumbs rule in order for a station to rotate stably it's major axis moment of inertia has to at least 20% greater than the minor ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;em&gt;I am doing this for a competition. The idea is to design a settlement, and I have designed one. I am still in grade 9, but I'm really interested in physics and if one of you could help it would be highly appreciated.&lt;/em&gt;)&lt;/p&gt;&#xA;" OwnerUserId="17000" LastEditorUserId="17000" LastEditDate="2016-09-22T12:34:20.890" LastActivityDate="2016-09-22T14:37:19.517" Title="How can I determine if a rotating space station is stable" Tags="|space-station|geosynchronous|" AnswerCount="1" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="18269" PostTypeId="2" ParentId="18224" CreationDate="2016-09-19T00:34:33.027" Score="2" Body="&lt;p&gt;I realized an error in the math in my question that is significant enough to leave as a formal answer. The error occurred here, and I'll point it out in bold.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, EML-1 is circling around the moon, which I would calculate by 2*pi()*R/(30*24*60^2), &lt;strong&gt;if R is the 61,000 km distance from the moon. This gives 0.714 km/s&lt;/strong&gt;. If the orbit is going in the &quot;right&quot; direction, then I subtract the apogee speed from this to get 0.641 km/s for the second burn.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The distance from the center of the moon to EML-1 is about 61,000 km, but the calculation erroneously plugged in 322,000 km, which is the distance from EML-1 to the center of Earth. If you plug in the intended (and more logical) number, you come to the conclusion that EML-1 is moving at about 150 m/s relative to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a very low number, but reasonable considering the circumstances. EML-1 is much closer to the moon, so if we're considering moon-based SOI calculations, it'll be notably lower than Earth-based SOI calculations. It is, at 150 m/s versus 700 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the orbit direction is the same as the rotation of EML-1 about the moon, then we subtract 150 m/s - 68 m/s = about &lt;strong&gt;81 m/s&lt;/strong&gt; (rounding after the fact here) in order to get the burn necessary to match the velocity vector of EML-1 for a projectile in transfer from LLO. As km/s, that is 0.081, so it would add fairly marginally to a 0.6 km/s burn at a perigee at LLO altitude, for 0.681 km/s total. The altitude of LLO is also a fungible number which can account for the range given in the slide referenced in the other answer, in additional to a possibility of some marginal gravity drag at LLO departure as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me embellish on another detail that I blew through in the question. Should it be 150 - 68 or 150 + 68? What direction do LLO orbits typically go in (for planned space missions)? Probably in the direction of the moon's rotation. So let me go point-by-point:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Sun rises in the East, so Earth rotates counter-clockwise if viewed from N. Hemisphere&lt;/li&gt;&#xA;&lt;li&gt;Moon orbits in same direction as the Earth rotates, so counter-clockwise&lt;/li&gt;&#xA;&lt;li&gt;Moon is tidally locked, so it rotates counter-clockwise as well&lt;/li&gt;&#xA;&lt;li&gt;EML-1 rotates counter-clockwise about the moon by similar statements&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Thus, it should be subtracted, instead of added. Possibility still remains for a 0.13 km/s modification (increase) if the mission is retrograde in LLO, which is admittedly unlikely. That could be why the range goes from 0.6 km/s to 0.8 km/ in the referenced slide, although I find little reason to ever assume the higher number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The validity of the approximation is still highly questionable. Perhaps this the number here is still a pretty good ballpark. On Hop's blog, several mentions are given to a approx 0.5 km/s burn to drop from EML-1 into an Earth grazing orbit (not so different from 0.7). That &lt;em&gt;should&lt;/em&gt; be a 3-body problem, but you can find that the Earth SOI approximation is very acceptable as a ballpark estimation. If the moon-side follows the same pattern, then we would conclude that 80 m/s is relatively close to the true answer.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="26" LastEditDate="2016-09-19T01:40:10.890" LastActivityDate="2016-09-19T01:40:10.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18270" PostTypeId="1" CreationDate="2016-09-19T04:19:04.247" Score="30" ViewCount="6650" Body="&lt;p&gt;Just wondering, why are unmanned rovers almost always using six wheels (adding more weight) as opposed to four wheels? Is it for added traction, redundancy, balance, or a combination of these factors?&lt;/p&gt;&#xA;" OwnerUserId="17002" LastEditorUserId="4703" LastEditDate="2016-09-19T14:08:07.607" LastActivityDate="2017-02-06T23:51:35.740" Title="Planetary rovers - six wheels vs four?" Tags="|rovers|curiosity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18271" PostTypeId="2" ParentId="18270" CreationDate="2016-09-19T04:44:24.297" Score="48" Body="&lt;p&gt;Short answer:  More wheels results in greater stability, allowing the rover to traverse more varied types of terrain.  It also provides redundancy, in case one of the wheels breaks, the rover is not completely crippled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars rovers all use the same design, called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocker-bogie&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocker-Bogie&lt;/a&gt;.  This is a special design which allows the rover to traverse many different terrains while keeping the body of the rover stable.  Since the wheels are not connected to each other, it also allows the two sides of the rover to go on different slopes simultaneously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Rocker_bogie.gif#/media/File:Rocker_bogie.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/5/55/Rocker_bogie.gif&quot; alt=&quot;Rocker bogie.gif&quot;&gt;&lt;/a&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(From &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocker-bogie&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This design allows Curiosity to tilt 45 degrees in any direction without flipping over (though automatic sensors prevent it from going over 30).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, each wheel can be equipped with its own motor.  This allows the rover to rotate in place and to continue functioning even if one of the wheels is damaged.  For example, when the Mars Exploration Rover, &lt;a href=&quot;https://en.wikipedia.org/wiki/Spirit_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spirit&lt;/a&gt;, broke one of its front wheels, the other wheels were able to provide enough power to keep it going.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other designs which differ from Rocker-Bogie suspension, but most of these also use six wheels. (Such as the proposed &lt;a href=&quot;http://vislab-ccom.unh.edu/~schwehr/papers/marsokhod-fsr97.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Russian Marsokhod rover&lt;/a&gt; which never actually launched.).  Having more wheels increases traction and stability, resulting in a safer rover overall.  You could make a design with four wheels, but this would be less stable.  Alternatively, you could put 8 or 10 wheels on, but this would drastically increase the cost and weight of the rover.  Therefore, 6 wheels is a good middle ground that provides lots of traction and stability without being too heavy.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2017-02-06T23:51:35.740" LastActivityDate="2017-02-06T23:51:35.740" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="18272" PostTypeId="1" CreationDate="2016-09-19T13:49:52.973" Score="0" ViewCount="141" Body="&lt;p&gt;If neutrinos travel faster than the speed of light does the calculation for the speed of light become obsolete. IE is there now a new formula for calculating speed/velocity through space/vacuum.&lt;/p&gt;&#xA;" OwnerUserId="16755" LastActivityDate="2016-09-19T13:59:41.457" Title="Neutrinos travel faster than the speed of light" Tags="|escape-velocity|" AnswerCount="1" CommentCount="3" ClosedDate="2016-09-19T14:00:28.057" ContentLicense="CC BY-SA 3.0" />
  <row Id="18273" PostTypeId="2" ParentId="18272" CreationDate="2016-09-19T13:59:41.457" Score="3" Body="&lt;p&gt;The neutrino speeds were found to not be faster than light after all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly#End_results&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-09-19T13:59:41.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18274" PostTypeId="2" ParentId="17859" CreationDate="2016-09-19T15:09:47.440" Score="2" Body="&lt;p&gt;Meteroides are on hyperbolic trajectories relative to earth. It is unlikely to encounter such an object coming from the deck-side, because earth masks most of the possible inbound trajectories and even if if misses the ground, the athmosphere would slow it and probably break it into pieces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space debris is on elliptical orbits. Since most missions start in eastward direction in order to make use of the earth rotation, the debris is also more likely to travel eastward at low to medium inclinations (read: angle relative to equator).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Debris hitting the Forward side would be slower than the ISS. That means, the perigee needs to be below the ISS orbit. Since ISS's perigee is only 401km and athmospheric drag starts at 300km, there is not much room for such objects and they will tend to have less relative velocity to ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Debris hitting the Aft side is faster than ISS. There is no limit for the apogee because there is nothing above to brake or catch such debris. Higher relative speeds to ISS are less likely though (the higher the eccentricity, the more time the debris spends farther away from earth, so less likely to encounter such an object).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most dangerous and the most likely encounter is with an object on nearly circular orbit, but with a different (worst is opposite) ascending node. That means, ISS and debris move in different planes, such debris is approaching from star or portside and will likely have a high relative velocity.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastActivityDate="2016-09-19T15:09:47.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18276" PostTypeId="2" ParentId="17859" CreationDate="2016-09-19T16:49:07.897" Score="2" Body="&lt;p&gt;The ISS orbital debris threat is worst about thirty degrees to the left and right of the velocity vector in the local horizontal plane (see page 9 of &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20150003495&quot; rel=&quot;nofollow&quot;&gt;the pdf linked here&lt;/a&gt;).  At the ISS orbit, pretty much all collisional trajectories are due to differences in orbital plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS meteoroid environment is almost completely shadowed in the nadir direction, with a slight bias toward the velocity vector (see page 4 of &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20140010668&quot; rel=&quot;nofollow&quot;&gt;this pdf&lt;/a&gt;).  It's important to note that the near-homogeneity of the meteoroid environment is due mostly to the environment being expressed in an earth-centered LVLH coordinate system.  In actuality, the meteoroid environment is highly directional in a sun-centric coordinate system, but the orbit around earth, coupled with the steady precession of the orbital plane, tends to &quot;peanut butter&quot; out the variations.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2016-09-19T16:49:07.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18277" PostTypeId="2" ParentId="18256" CreationDate="2016-09-19T17:47:02.940" Score="3" Body="&lt;p&gt;In general, you want travel as light as possible, as this will enhance your delta-v budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With current technology, the mass of your spacecraft will vastly decrease towards end of mission. There will be some consumables that can not be recycled. Most notably, propellant will be vented (as exhaust gas). Thats even true for ion-thrusters. The less consumables/propellant you carry, the less structure your craft needs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ratio of mass reduction vs remaining mission time with current technology is relatively high, so it makes sense to get rid of structure that is not needed any more. Think of ascent stages that get jettisoned as soon as their fuel is depleted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sci-Fi is about fictitious technology. If you manage to travel and sustain life on board with less consumables, that may be a completely different story: what you need in the end may not be much less than what you started with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding &quot;eject in opposite direction&quot;: This is what propulsion is about, eject something heavy with the highest possible velocity. The main ship has much more mass than your thrusters exhaust gases, thats a good thing. But it may be difficult to achieve a decent delta-v on ejection without disintegrating the whole thing. Maybe a good idea would be to have a rail-gun and shoot little pieces of your mother ship in a prograde direction one by a time, thereby spreading the delta-v over a longer time.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastActivityDate="2016-09-19T17:47:02.940" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18278" PostTypeId="2" ParentId="18256" CreationDate="2016-09-19T18:14:26.940" Score="2" Body="&lt;p&gt;This would only make sense if there was a substantial amount of the ship that could be disposed of that was only needed for launch or transit, and not for the final stretch. First of all, let's take a look at some assumptions.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The nearest star- 4.3 light years.&lt;/li&gt;&#xA;&lt;li&gt;Assuming 10% of the speed of light, that's about 43 years, give or take, to get there. &lt;/li&gt;&#xA;&lt;li&gt;Assuming an acceleration of 0.1g, that's about a year to accelerate and decelerate. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, so any mass that you would have to get rid of would have to be during the decelerate portion. That is still a sizable portion of the trip. You could safely dump at that time any empty containers for food, but for that long of a trip, you are probably having to grow food somehow. You could dump anything that was only needed for the launch phase. But I'd be pretty willing to bet that you need most of the ship to sustain life for the year required to decelerate. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-19T18:14:26.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18280" PostTypeId="2" ParentId="18256" CreationDate="2016-09-19T19:23:35.643" Score="1" Body="&lt;p&gt;Short answer: Presumably &quot;the whole ship&quot; and all of it's weight is the very mechanism which provides the ability to dramatically change momentum (whether accelerating or decelerating), so decelerating without the whole ship begs the question: How?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, as Michael Kjörling has pointed out, the primary limiting factor of the acceleration/deceleration rate is probably going to be the force it exerts on the travelers, unless the story invokes some sort of &quot;&lt;a href=&quot;http://stargate.wikia.com/wiki/Inertial_dampener&quot; rel=&quot;nofollow&quot;&gt;inertial dampener&lt;/a&gt;&quot;, which according to modern physics is &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertia_negation&quot; rel=&quot;nofollow&quot;&gt;impossible&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Andreas also makes a good point, that the total useful mass of the ship should be somewhat less by the midpoint of the trip, but if you have already maximized the force you are willing to subject your travelers to for the first half of the trip, then to achieve deceleration without exceeding that same maximum force will still require the deceleration to begin right around that midpoint. What changes is the expenditure of fuel required to achieve that deceleration rate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think you have a good idea at the heart of your question though. At the time of arrival, assuming you have no intention of reusing the vehicle, you have this whole big hunk of interstellar spaceship that is no use to anyone. It strikes me as a fine innovation to find ways to use parts of that hulking machinery as propellant mass. For example, let's say the fuel used for much of the first part of your journey required some sort of containment tanks, most of which are empty near the end of the journey. If these tanks could be ground into fine particles which are somehow injected into the propellant to add mass to the exhaust without completely wrecking the fine balance in the fuel, perhaps it could add to the efficiency of the deceleration and reduce the overall fuel payload. The engine that could make use of this dirty fuel without exploding and the grinder that is itself of some relatively negligible mass probably remain still in the realm of science fiction, but if such a thing were possible, decreasing the fuel payload would be a major gain. Fuel mass is a critical factor in any long range space flight, because adding fuel also adds weight, which requires even more fuel... you see where this is going. &lt;/p&gt;&#xA;" OwnerUserId="945" LastActivityDate="2016-09-19T19:23:35.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18282" PostTypeId="1" CreationDate="2016-09-17T17:03:53.523" Score="0" ViewCount="67" Body="&lt;p&gt;In many sci-fi novels I have read, the interstellar travelers had their ship half the way accelerate, then half the way decelerate. I wonder why this is the chosen approach? Would not it be better to accelerate all the way, then leave the ship for small landing module(s) and decelerate them while allowing the interstellar ship to go away from the star system? Maybe, even eject the landing modules from the mother ship in opposite direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Clarification:&lt;/strong&gt; I meant unidirectional travel aimed at colonization of new worlds.&lt;/p&gt;&#xA;" OwnerUserId="812" OwnerDisplayName="Anixx" LastEditorUserId="58" LastEditDate="2019-04-23T16:19:52.667" LastActivityDate="2019-04-23T16:19:52.667" Title="Why interstellar space travellers would need to decelerate their ship?" Tags="|interstellar-travel|" AnswerCount="0" CommentCount="0" FavoriteCount="0" ClosedDate="2016-09-19T20:48:08.027" ContentLicense="CC BY-SA 3.0" />
  <row Id="18283" PostTypeId="2" ParentId="18256" CreationDate="2016-09-17T17:20:50.007" Score="3" Body="&lt;h1&gt;Reason 1: Leaving the system again&lt;/h1&gt;&#xA;&lt;p&gt;The problem with your proposal is that there's likely no way for the small landing craft to leave the planetary system. In most sci-fi stories/movies, in my experience, the landing craft are small, sleek, and designed to go down to the planet and then return. They have engines powerful enough only to get back to the mothership, and not a lot of fuel. The designs are minimalist.&lt;/p&gt;&#xA;&lt;p&gt;Here's an example. Take &lt;a href=&quot;http://starwars.wikia.com/wiki/Eta-2_Actis-class_light_interceptor&quot; rel=&quot;nofollow noreferrer&quot;&gt;Obi-Wan's Starfighter&lt;/a&gt; from &lt;em&gt;Attack of the Clones&lt;/em&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DRLPu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DRLPu.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;It has &lt;a href=&quot;http://starwars.wikia.com/wiki/Hyperspace_transport_ring&quot; rel=&quot;nofollow noreferrer&quot;&gt;a separate ring&lt;/a&gt; to travel through hyperspace; the engines on the ship itself just aren't powerful enough. At the same time, the ring is not, I would assume, very helpful during re-entry, or when landing, for that matter. You need some other way of catching up with the mothership, or a way to get off the system.&lt;/p&gt;&#xA;&lt;p&gt;If you're writing hard science fiction, then a hyperspace ring is probably not going to feature in your story. Odds are good, then, that your landing craft will need a way to get to the star system and leave again. The mothership needs to stay there so the craft can leave again, unless the crew are on a one-way journey.&lt;/p&gt;&#xA;&lt;h1&gt;Reason 2: Entering the system&lt;/h1&gt;&#xA;&lt;p&gt;At any rate, though, the craft will need to decelerate. If you're traveling at any speed that makes interstellar travel plausible, you're going pretty damn fast. Chances are good that you're going at a speed faster than the escape velocity for the planet, if not faster than the escape velocity for the system. If you intend to have any control whatsoever and land on the planet without crashing, then you should probably decelerate. A lot. Again, the landing craft likely don't have powerful enough engines for this.&lt;/p&gt;&#xA;" OwnerUserId="5963" OwnerDisplayName="HDE 226868" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-09-17T17:20:50.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18284" PostTypeId="2" ParentId="18256" CreationDate="2016-09-17T19:05:49.150" Score="1" Body="&lt;p&gt;There are basic laws in physics, one of them the conservation of momentum and energy, also known to be one of newtons laws. It says: &quot;As long as no force changes the momentum of an object, its momentum and kinetic energy will not change.&quot;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Case A: constant acceleration&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Now, let's take a spaceship, that accelerates all the way from A to B with constant acceleration $\vec a$. As long as it doesn't go relativistic speeds, it will achieve the velocity $\vec v = t \times \vec a$, wherin t is the travel time from A to B. As t is really large, this will be a very very huge speed.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Case B: deceleration at halfway point.&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Now, we have our rocket accelerate with $\vec a$ for half the flight time, and then decelerate with a for the other half. Decelerating is accelerating with $-\vec a$ (we use vectors here!). The velocity at the end will be: $\vec v = \frac 1 2 t \times \vec a + \frac 1 2 t \times \vec -a = \frac 1 2 t \times (\vec a - \vec a) = 0$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But why should we want B We'll need much more time?!&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Discussion&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Our Energy is dependent on the speed: $E_{kin}=\frac 1 2 m v^2$. This kinetic energy is stored in the whole vessel and will continue to keep the ship on the path, as it is directly proportional to its momentum and inertia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, we want to drop our colonist capsule. Let's say the capsule is 1/10th of the weight of the carrier ship. Then the capsule will carry 1/10th of the kinetic energy while still travelling almost at its speed before being launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's look at our cases: In case A, $\vec v$ is still that very huge number, in case B it is 0, or close to, if the deceleration was not for the full second half of the travel but some less. To get into a nice orbit around the destination's sun, the ship should do a bit less deceleration than acceleration.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Solution&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Now, our colonist capsule needs to achieve several steps to actually land, and each one requires it to be slower (in relative terms) than in the step before:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;get an orbit with roughly the same speed and radius around the star as the planet it wants to land on&lt;/li&gt;&#xA;&lt;li&gt;then decelerate into an orbit around that planet&lt;/li&gt;&#xA;&lt;li&gt;then decelerate into a decaying orbit to land on the planet&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So we need to already pack a huge amount of fuel to get through these maneuvers, and if we want to decelerate just the capsule in the solar system, that will not only demand packing even more fuel, it will demand extremely powerful engines lest the ship exits the system before it is out of it again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can however reduce the fuel cost for the deceleration: Whenever a fuel tank is leeched empty, we abandon it to continue on the trajectory to our destination, reducing our mass by a bit and thus the reducing time needed for the next phase of acceleration or deceleration.&lt;/p&gt;&#xA;" OwnerUserId="17008" OwnerDisplayName="Trish" LastActivityDate="2016-09-18T02:51:52.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18285" PostTypeId="2" ParentId="18256" CreationDate="2016-09-18T04:40:08.293" Score="10" Body="&lt;p&gt;&lt;strong&gt;You need to decelerate because you only brought what you needed.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space travel is expensive.  Interstellar space travel, doubly so.  Lots of effort is going to be focused on efficiency.  How little can you send to another planet and still have a viable mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think about it, you can take inventory of what you wont want to bring with you for your new colony.  It should be a very minimal list.  Everything you brought with you should be useful.  Heck, even the hull of your spacecraft is surely highly valuable.  Waste not want not.  When you really get down to it, there's not much mass that you would want to let keep flying into the darkness of space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, consider the strength of the acceleration.  If we're talking about most sci-fi interstellar travel, you spend a lot of time accelerating at a very slow rate.  Ideally, you do so in a fuel efficient manner.  If you jettison close to the planet, you're going to need very powerful retro-rockets to quickly slow your smaller vessels.  That means lots of gee forces.  Too close, and your humans may not survive the deceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, if you have to decelerate your small craft for a long time, then the distinction between your small craft and your larger craft becomes blurry.  Your small craft suddenly needs all of the structure and life support needed to keep your passengers alive during this deceleration period.  If you're not careful, suddenly you find yourself needing so much stuff that you might as well have just decelerated the whole ship!&lt;/p&gt;&#xA;" OwnerUserId="3532" OwnerDisplayName="Cort Ammon" LastActivityDate="2016-09-18T04:40:08.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18286" PostTypeId="2" ParentId="18256" CreationDate="2016-09-18T05:39:05.550" Score="1" Body="&lt;p&gt;The distinction that must be made here is between comfort and speed. A spacecraft accelerates half-way to gain the fastest velocity to reach its destination in the shortest possible time, but at a rate of acceleration that normal humans can go about their business and lives on board that is sufficiently comfortable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the spacecraft has to decelerate it will do so at the same rate of deceleration as its acceleration phase. This would involve decelerating the whole spacecraft. However, if instead of decelerating and stopping the spacecraft, once it had accelerated to the halfway mark, dropped off landing craft those landing craft would have to decelerate all the way until they reached the target system for colonization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason that the landing craft have to decelerate from the halfway point is they have to lose the same amount of velocity as the main spacecraft and at a rate of deceleration that is comfortable and convenient for the human colonists it is carrying.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the landing craft were dropped off closer to the target system the forces of deceleration will increase proportionally. The shorter the distance and the shorter the time to decelerate the higher the g forces will be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless you want your colonists to arrive resembling raspberry jam, a longer and slower deceleration will be the way to go. Almost invariably this will mean if your spacecraft accelerated to halfway mark, it will have to decelerate the rest of the way to arrive safely and in one piece.&lt;/p&gt;&#xA;" OwnerUserId="17013" OwnerDisplayName="a4android" LastActivityDate="2016-09-18T05:39:05.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18287" PostTypeId="2" ParentId="18256" CreationDate="2016-09-19T23:19:27.927" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Would not it be better to accelerate all the way, then leave the ship for small landing module(s) and decelerate them while allowing the interstellar ship to go away from the star system?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This implies that the landing is a one-way trip - if the small modules slow down to land while the mothership keeps accelerating, then they'll never catch up to the mothership again.  If that's the intent, fine.  If not, you've just stranded people on a strange planet forever.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first issue that rears its ugly head is acceleration.  Suppose your mothership has been accelerating at 1 g all the way from the source to the destination.  If you want the load on your landing module to also be 1 g, then you will &lt;em&gt;have&lt;/em&gt; to dispatch it at the halfway point.  Dispatch it any later, and the g load will be higher.  For equipment designed to withstand high g loads, that's not so much of a problem.  For squishy payloads (a.k.a. meat sacks, a.k.a. people), you're going to have to keep it around 1 g or so.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, if your mothership is accelerating a slower rate, you can dispatch the landing module later in the trip to keep the acceleration around 1 g.  But no matter what, you are going to have to account for that acceleration.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This brings us to a second issue - the amount of time spent slowing down.  How long will it take your landing module to slow down at 1 g or so?  A few hours?  A few days?  A few weeks?  A few years?  How much life support does it carry (if any)?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say your mothership is booking along at 10% c (~30000000 m/s).  At 1 g, it would take your lander on the order of a month to slow down (not accounting for any relativistic effects).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, okay, we can deal with acceleration, we can deal with time to slow down.  This brings us to the last issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're using any kind of a reaction drive (chemical rocket, ion engine, nuclear-pulse drive, etc.), you need reaction mass (propellant), and the amount of reaction mass you need rises &lt;em&gt;exponentially&lt;/em&gt; with the change in velocity (&amp;Delta;V).  This comes from the Tsiolkovsky rocket equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V = ln\left(\frac{m_i}{m_f}\right)v_e$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\Delta V$ is the total change in velocity;&lt;br&gt;&lt;/li&gt;&#xA;&lt;li&gt;$m_i$ is the mass before the burn (payload plus propellant);&lt;br&gt;&lt;/li&gt;&#xA;&lt;li&gt;$m_f$ is the mass after the burn (payload only); and&lt;br&gt;&lt;/li&gt;&#xA;&lt;li&gt;$v_e$ is the effective exhaust velocity of your drive (which can also be expressed as $g * I_{sp}$, where $g$ is one standard Earth gravity (9.8 m/s&lt;sup&gt;2&lt;/sup&gt;) and $I_{sp}$ is the specific impulse of your drive).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You may have heard the phrase &quot;the tyranny of the rocket equation&quot;.  That logarithmic term is what makes it so tyrannical.  If you flip it around like so:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{m_i}{m_f} = e^{\left(\frac{\Delta V}{v_e}\right)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;you see that your &lt;em&gt;mass ratio&lt;/em&gt; $\frac{m_i}{m_f}$ grows exponentially with &amp;Delta;V.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The higher the mass ratio, the more kg of propellant you need to accelerate each kg of spacecraft mass.  A MR of 2 means you need 1 kg of propellant for each kg of spacecraft mass.  A MR of 3 means you need 2 kg of propellant for each kg of spacecraft mass.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's assume you're using an engine with a specific impulse ($I_{sp}$) of 1500 seconds (sort of like the ion engines on the Dawn spacecraft).  A &amp;Delta;V of 30000 m/s (~0.01% c) gives us a mass ratio of e^(30000/(9.8*1500)) =~ 7.7.  For every kg of spacecraft you want to accelerate, you have to expend 6.7 kg of propellant. A &amp;Delta;V of 60000 m/s (~0.02% c) gives us a mass ratio of 59.24.  A &amp;Delta;V of 120000 m/s (~0.04% c) gives us a mass ratio of &lt;strong&gt;3510&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this type of engine, to reach 0.28% of light speed (a little over one quarter of one percent), you would need on the order of &lt;em&gt;almost one Earth's mass of propellant&lt;/em&gt; (2.36e24 kg) for &lt;em&gt;each kilogram of spacecraft mass&lt;/em&gt; you want to accelerate.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, we're not flying to another star system using ion engines, or frankly any other reaction drive we have the technology to build.  You'd need drives with specific impulse values on the order of 100000 seconds or better, which we are nowhere near producing, or a true reactionless drive (one that doesn't require exchanging momentum), which are are also nowhere near producing. &lt;/p&gt;&#xA;" OwnerUserId="12922" LastEditorUserId="12922" LastEditDate="2016-11-22T19:51:18.283" LastActivityDate="2016-11-22T19:51:18.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18288" PostTypeId="1" AcceptedAnswerId="18302" CreationDate="2016-09-20T01:02:22.683" Score="5" ViewCount="698" Body="&lt;p&gt;Dark, narrow, straight lines move around within images from &lt;a href=&quot;https://en.wikipedia.org/wiki/Interface_Region_Imaging_Spectrograph&quot; rel=&quot;nofollow noreferrer&quot;&gt;Interface Region Imaging Spectrograph&lt;/a&gt; (IRIS) in such a way that they appear intentional and systematic, and not just some imaging artifact as discussed in &lt;a href=&quot;https://space.stackexchange.com/q/15370/12102&quot;&gt;this SDO question&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/15427/12102&quot;&gt;this SOHO question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What causes this line? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y4E0l.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y4E0l.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Vl96W.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vl96W.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screen shots from YouTube videos of IRIS imagery. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; The full YouTube videos of IRIS imagery. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/VtqadtIkAaQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/SuAjao9e51U?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RKbJi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RKbJi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Photograph of IRIS from the detailed article about the IRIS' optical instrumentation at &lt;a href=&quot;http://spaceflight101.com/iris/spacecraft-instruments/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;. Photo credit LMSAL.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pudte.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pudte.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Details of IRIS' Spectrograph and Slit-Jaw Imager from the article about the IRIS' optical instrumentation at &lt;a href=&quot;http://spaceflight101.com/iris/spacecraft-instruments/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;. Image credit NASA/LMSAL.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-09-20T19:38:56.070" Title="What are these dark lines in IRIS images of the sun?" Tags="|the-sun|space-telescope|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18289" PostTypeId="1" AcceptedAnswerId="18291" CreationDate="2016-09-20T04:01:34.123" Score="14" ViewCount="14379" Body="&lt;p&gt;This should be fairly straightforward, but, how do I get the semi-major axis (a) from at TLE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example if I have the TLE: &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;1 25544U 98067A 01260.91843750 .00059354 00000-0 74277-3 0 4795&#xA;2 25544 51.6396 342.1053 0008148 106.9025 231.8021 15.5918272116154&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I know that 15.5918272 is the mean motion of the body ($n$). I also know that $n = \sqrt{\frac{\mu}{a^3}}$. If I use the given $n$ value I get a semi-major axis $a=11.79$ which is obviously incorrect. What am I missing here?&lt;/p&gt;&#xA;" OwnerUserId="8431" LastActivityDate="2020-06-23T18:40:31.003" Title="How to get semi-major axis from TLE?" Tags="|orbital-mechanics|orbit|low-earth-orbit|orbital-elements|two-line-elements|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18290" PostTypeId="2" ParentId="15010" CreationDate="2016-09-20T04:39:15.370" Score="3" Body="&lt;p&gt;The fraction of launch payload of a ECLSS (environmental control and life support system) depends on a number of factors. The two most important factors to consider are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;What type of systems make up the ECLSS?&lt;/strong&gt; Are your systems completely regenerative or do they require consumables? Most likely the system is a combination of the two. A fully regenerative system wouldn't require extra consumables and thus would result in a lower overall subsystem mass.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;What is the mission profile and how does this influence the safety margin of the ECLS system?&lt;/strong&gt; All human spaceflight missions are designed within a certain safety margin and thus require redundant systems, extra margins with regard to the consumables, and spare parts to fix the systems if/when they break. This is all to say that a mission to LEO which has the ability for a quick abort might allow for a lower safety margin. Such a mission could quickly abort if the ECLSS fails irreparably using an escape vehicle (i.e. Soyuz on the ISS). A mission to Mars does not have the ability for a quick abort and thus the safety margin and the ECLSS mass must increase.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Larsen and Pranke's text &lt;em&gt;Human Spaceflight&lt;/em&gt; gives a rule of thumb of &lt;strong&gt;8% dry mass&lt;/strong&gt; for the ECLSS. This includes pressurizing, oxygen supply, controlling and revitalizing the atmosphere, and active thermal control. This was found via extrapolation from NASA mission data.&lt;/p&gt;&#xA;" OwnerUserId="8431" LastEditorUserId="6816" LastEditDate="2016-09-20T05:01:30.620" LastActivityDate="2016-09-20T05:01:30.620" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18291" PostTypeId="2" ParentId="18289" CreationDate="2016-09-20T05:23:55.860" Score="17" Body="&lt;p&gt;The TLE gives mean motion (&lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt;) in &lt;span class=&quot;math-container&quot;&gt;$\frac{rev}{day}$&lt;/span&gt;. This needs to be converted to &lt;span class=&quot;math-container&quot;&gt;$\frac{rad}{s}$&lt;/span&gt; which can be accomplished by multiplying the &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; TLE value by &lt;span class=&quot;math-container&quot;&gt;$\frac{2\pi}{86400}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Therefore, to go directly from &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; in TLE to the semi-major axis &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt;. We can use the following formula: &lt;span class=&quot;math-container&quot;&gt;$a=\frac{\mu^{1/3}}{\frac{2n\pi}{86400}^{2/3}}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;For &lt;span class=&quot;math-container&quot;&gt;$n=15.5918272 \,\, \frac{rev}{day}$&lt;/span&gt;, we get &lt;span class=&quot;math-container&quot;&gt;$a=6768.16 \,\, km$&lt;/span&gt;.&lt;/p&gt;&#xA;" OwnerUserId="8431" LastEditorUserId="195" LastEditDate="2020-06-23T18:40:31.003" LastActivityDate="2020-06-23T18:40:31.003" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18293" PostTypeId="2" ParentId="18228" CreationDate="2016-09-20T10:33:57.550" Score="2" Body="&lt;p&gt;The potential of a horizontal launch within the earth's atmosphere is very much limited by the atmosphere itself. At a low height the maximum speed is limited by the high atmospheric pressure and at medium height there is not enough oxygen left for the scramjet. But to get into a low orbit, much more speed and height is necessary, about 8 km/s speed instead of some 1.3 km/s for Mach-4 and 200 km height instead of only about 25 km. The difference in kinetic energy is impressive, about 38 times more for 8 km/s instead of 1.3.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-20T10:33:57.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18294" PostTypeId="2" ParentId="2570" CreationDate="2016-09-20T11:00:58.540" Score="-1" Body="&lt;p&gt;You do realise people navigated before compasses existed, right? So obviously they'd do it the same way they did back then; using the sun and stars. &lt;/p&gt;&#xA;" OwnerUserId="17016" LastActivityDate="2016-09-20T11:00:58.540" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18295" PostTypeId="1" AcceptedAnswerId="18296" CreationDate="2016-09-20T12:32:20.100" Score="13" ViewCount="3178" Body="&lt;p&gt;I assume the retrograde orbit ($\theta_{inc}=142°$) is a side-effect of geography - ocean overfly access only lies West of the secured launch site - &lt;a href=&quot;http://spaceflight101.com/israel-launches-advanced-optical-reconnaissance-satellite/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; in Spaceflight 101 makes several points that I don't quite understand. I think if one can actually understand this particular orbit, all of these questions can be answered at the same time, so I'm grouping them together here as a 'what the heck is going on here' type question.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Ofeq satellites are operated by the Israeli Ministry of Defence and built by Israel Aerospace Industries and Elbit Systems. Their primary purpose is to collect data from territories relevant to Israel.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Therefore, the satellites typically operate from equatorial orbits inclined 142 degrees covering Israel and surrounding territories.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;While that orbit prevents the satellites from collecting intelligence on a global scale, it does provide six daylight passes over the Middle East for the first one-and-a-half years of their missions, delivering rapid response imagery of targets of interest to the Israeli intelligence community.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The number of favorable passes per day decreases with mission duration due to orbital mechanics, but the Ofeq satellites can still achieve more relevant passes over the area of interest than a polar orbiting spacecraft that would only have one or two daily passes with favorable look angles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;1) &quot;&lt;em&gt;While that orbit prevents the satellites from collecting intelligence on a global scale...&lt;/em&gt;&quot; Is it the inclination, or the sun-synchrony, or both? Is this a &lt;a href=&quot;https://space.stackexchange.com/a/18079/12102&quot;&gt;repeat ground track orbit&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) &quot;&lt;em&gt;...six daylight passes over the Middle East...&lt;/em&gt;&quot; &lt;strong&gt;per day??&lt;/strong&gt; How does that work??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) &quot;&lt;em&gt;...Ofeq satellites can still achieve more relevant passes over the area of interest than a polar orbiting spacecraft that would only have one or two daily passes with favorable look angles.&lt;/em&gt;&quot; I think I understand the part about polar orbit, the earth's surface at 32N latitude moves about 2100 km in 90 minutes, so at say 700 km altitude you could possibly make two passes looking at $+/- 56°$ degrees, or one pass more vertical and one more oblique?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) &quot;&lt;em&gt;The number of favorable passes per day decreases with mission duration due to orbital mechanics...&lt;/em&gt;&quot; some kind of precession?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-07-06T22:44:53.920" LastActivityDate="2020-07-06T22:44:53.920" Title="The strange orbit of Ofeq 11 - how does it (actually) do this?" Tags="|orbital-mechanics|low-earth-orbit|military|ofeq-spacecraft|israel|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18296" PostTypeId="2" ParentId="18295" CreationDate="2016-09-20T13:02:17.310" Score="13" Body="&lt;p&gt;Okay, first of all, let's collect all of the information we need to do anything with this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The orbit is 600 km.&lt;/li&gt;&#xA;&lt;li&gt;The inclination is 142 degrees. (-38)&lt;/li&gt;&#xA;&lt;li&gt;The Middle East's latitude is about 38 N at the highest point.&lt;/li&gt;&#xA;&lt;li&gt;The orbital period is about 96 minutes.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Okay, so how could you put all of that together to get 6 daylight passes per day? The orbit is somewhat sinusoidal, moving from across the sky per day. It will tend to be at the northern portion of that orbit for most of the time. An object at, say, 30 N would likely be visible for much of that portion of the day, as the satellite passes over roughly 38 degrees at it's highest point. Given the orbital period, and a day of 12 hours, there are 7.5 possible orbits to cover the daylight portion of the day, which is greater than the 6 stated. However, there will be several marginal orbits in that period of time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is further compounded by the fact that the orbit is retrograde. The Earth is actually moving the opposite direction, effectively giving over that 12 hour period of time an extra half-orbit, giving the possible orbits up to 8, which means that the 6 number is more likely to work well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All orbits precess unless you try very hard to make it not so, by using a specific inclination. This is not one of those orbits. The orbital path will eventually shift such that the daylight portion of the orbit is in the southern hemisphere, and then return to the northern hemisphere. As the satellite depends at least in part on daylight, it's effectiveness will diminish with time, although it will eventually return to it's optimal usage.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-09-20T13:02:17.310" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18297" PostTypeId="1" AcceptedAnswerId="18317" CreationDate="2016-09-20T13:33:13.000" Score="17" ViewCount="933" Body="&lt;p&gt;Suppose there is a fire inside a large spacecraft, and there are crewmembers trapped inside.  Is it possible to extinguish the fire by venting and then repressurizing the cabin, without killing the people?&lt;/p&gt;&#xA;" OwnerUserId="10982" LastEditorUserId="7062" LastEditDate="2021-09-13T11:56:43.957" LastActivityDate="2021-09-13T11:56:43.957" Title="Could a person survive a vent of a compartment large enough to extinguish a fire?" Tags="|crewed-spaceflight|fire|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18298" PostTypeId="1" AcceptedAnswerId="18313" CreationDate="2016-09-20T14:03:38.523" Score="7" ViewCount="216" Body="&lt;p&gt;In 2011 the NASA Langley Research Center completed the &lt;a href=&quot;http://www.nasa.gov/centers/langley/exploration/hib.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hydro Impact Basin&lt;/a&gt; for conducting drop tests of the Orion spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much water is the basin filled with for these tests? Does the Orion displace a lot of water in these tests (that is, is much expelled from the basin)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is the basin refilled, if needed? Does it have built-in plumbing, or does NASA run a big hose out to fill the pool?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bonus&lt;/strong&gt;: Does Orion get to invite any friends to the pool party? (Are any non-Orion tests conducted in this pool, or planned to be conducted?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bRxUV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bRxUV.jpg&quot; alt=&quot;Hydro Impact Basin overview&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/vgAFq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vgAFq.jpg&quot; alt=&quot;Orion landing in Hydro Impact Basin&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-09-20T17:10:49.920" LastActivityDate="2016-09-21T17:50:06.080" Title="How does Orion take its bath (Hydro Impact Basin)?" Tags="|nasa|water|testing|orion-spacecraft|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18299" PostTypeId="2" ParentId="18297" CreationDate="2016-09-20T14:52:47.227" Score="10" Body="&lt;p&gt;Scientific American author Anna Goslin reported 2008 on NASA experiences with depressurization in animal tests and also in accidents involving humans. According to &lt;a href=&quot;http://www.scientificamerican.com/article/survival-in-space-unprotected-possible/&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; , surviving a near-vacuum for a short time is possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I doubt however that this would be a viable approach to extinguish an onboard fire. You would need to vent quickly, then repressurize with nitrogen or any other non-oxidizing gas that may be available. The crew would need to use oxygen masks like in aircraft. Venting and repressurizing a compartment of reasonable size within 60 seconds might be too challanging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT after additional thinking:&#xA;Oxygen masks would probably not prevent the crew from going unconscious, as the gas-exchange in their lungs will not work at all at low ambient pressure. The emergency venting valves would need to be designed to not create too much thrust. You must prevent that they suck in any objects and get stuck. The chilling effect of adiabatic expansion of the inert gas would be enormous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2nd EDIT: Read this &lt;a href=&quot;http://aviationweek.com/bca/it-s-not-about-breathing&quot; rel=&quot;noreferrer&quot;&gt;Aviation Week article&lt;/a&gt; to get an impression of gas exchange problems under low pressure conditions.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17004" LastEditDate="2016-10-16T22:05:54.447" LastActivityDate="2016-10-16T22:05:54.447" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18300" PostTypeId="1" AcceptedAnswerId="18361" CreationDate="2016-09-20T16:41:28.247" Score="12" ViewCount="2807" Body="&lt;p&gt;As I first noted in &lt;a href=&quot;https://space.stackexchange.com/questions/18295/#comment47634_18295&quot;&gt;this comment&lt;/a&gt; I couldn't get TLEs for the recently launched OFEQ-11 spy satellites from space-track.com. When I looked at &lt;a href=&quot;http://www.n2yo.com/satellite/?s=41759&quot; rel=&quot;nofollow noreferrer&quot;&gt;Search Resuls: OFEQ 11 Satellite details 2016-056A NORAD 41759 - N2YO.com&lt;/a&gt; I got a map that showed it over it's &amp;quot;target area.&amp;quot; At the bottom of the page it lists what appear to be Two Line Elements, and if I plug them into SkyField using Tel Aviv coordinates, indeed those TLEs put OFEQ-11 above the horizon at the same time.&lt;/p&gt;&#xA;&lt;h2&gt;n2yo:&lt;/h2&gt;&#xA;&lt;p&gt;But the link where n2yo says it's getting the data is now empty &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=2016-056A&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=2016-056A&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h2&gt;calsky:&lt;/h2&gt;&#xA;&lt;p&gt;Onn the appropriate page in calsky.com &lt;a href=&quot;http://www.calsky.com/cs.cgi/Satellites/1?obs=76732742607510&amp;amp;satid=2016-056&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.calsky.com/cs.cgi/Satellites/1?obs=76732742607510&amp;amp;satid=2016-056&lt;/a&gt;  when I click the TLE link gives a long line of predicted TLEs: &lt;a href=&quot;http://www.calsky.com/observer//tle.cgi?satid=16056A&amp;amp;lang=en&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.calsky.com/observer//tle.cgi?satid=16056A&amp;amp;lang=en&lt;/a&gt; which show it burning up in the atmosphere about 100 days from now.&lt;/p&gt;&#xA;&lt;h2&gt;heavens-above:&lt;/h2&gt;&#xA;&lt;p&gt;There is a map and a TLE show here also! &lt;a href=&quot;http://www.heavens-above.com/orbit.aspx?satid=41759&amp;amp;lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.heavens-above.com/orbit.aspx?satid=41759&amp;amp;lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=41158.0&quot; rel=&quot;nofollow noreferrer&quot;&gt;This thread&lt;/a&gt; may be helpful:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Don't expect TLEs from Space-Track - Ofeq TLEs have been classified since 2011.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;So where are these second-hand TLEs coming from, considering the orbit is classified?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eDf4t.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eDf4t.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wYrpB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wYrpB.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SXrOE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SXrOE.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vI2L7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vI2L7.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-05-09T17:33:22.537" LastActivityDate="2022-08-06T19:56:00.887" Title="How (the heck) are military satellites with (apparently) classified TLEs still showing up on sat map websites?" Tags="|orbital-elements|two-line-elements|military|ofeq-spacecraft|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="18301" PostTypeId="2" ParentId="18300" CreationDate="2016-09-20T16:51:19.770" Score="8" Body="&lt;p&gt;There are a few ways.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Visual tracking&lt;/li&gt;&#xA;&lt;li&gt;Tracking of it's RF emissions&lt;/li&gt;&#xA;&lt;li&gt;Radar tracking.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I suspect it's the first two. If you know about where something is, you can see it visually, and track it's RF emissions to boot. People make a hobby of such things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of web sites that track this information. &lt;a href=&quot;https://www.prismnet.com/~mmccants/tles/&quot; rel=&quot;noreferrer&quot;&gt;https://www.prismnet.com/~mmccants/tles/&lt;/a&gt; is one of the first ones that I found, there are other online communities that collect this information. I can only assume that the popular websites collect info from these sources.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-09-20T17:36:34.120" LastActivityDate="2016-09-20T17:36:34.120" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18302" PostTypeId="2" ParentId="18288" CreationDate="2016-09-20T19:38:56.070" Score="6" Body="&lt;p&gt;@uhoh found the answer in one of the publications on the optical system of IRIS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The IRIS telescope has two major measuring devices: First, an imaging device that can take normal photos of the surface of the sun in several different spectral bands. This is the source of the videos shown in the two videos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second device is the source of the black line in these pictures: A slit spectrometer. It consists of a slit jaw (two metal plates forming a narrow aperture) to cut out one line of the image of the sun and a prism that spreads the light depending on its wavelength. A usual CCD sensor is then used to measure the spectral distribution of the incoming light with high resolution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The front of the slit jaw consists of mirrors to reflect the remaining light onto the imaging sensor. Naturally, this reflection misses the light that is used for the spectrometer and shows a black line as can be seen in the pictures. In order to gather spectral information across the whole surface of the Sun, the telescope turns slowly, selecting a different part for each successive image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rather extensive description of the optical system can be found in this &lt;a href=&quot;https://iris.lmsal.com/documents/iris_csr_instrument.pdf&quot;&gt;publication&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The scanning across the sun surface can be seen in this &lt;a href=&quot;http://www.lmsal.com/solarsoft/irisa/data/level2_nrt/2016/09/20/20160920_072933_3610091469/www/l2_20160920_072933_3610091469_SJI_1400_t000_fits_20160920_072933_j.html&quot;&gt;animation&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-09-20T19:38:56.070" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18303" PostTypeId="2" ParentId="18185" CreationDate="2016-09-20T19:46:30.100" Score="8" Body="&lt;p&gt;Here's a quick comparison of the two missions. Differences to follow. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;OSIRIS-REx&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Name: Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer&lt;/li&gt;&#xA;&lt;li&gt;Lead Institution(s): NASA (USA), University of Arizona&lt;/li&gt;&#xA;&lt;li&gt;Principle Investigator (PI): Dante Lauretta (University of Arizona)&lt;/li&gt;&#xA;&lt;li&gt;Target: &lt;a href=&quot;https://en.wikipedia.org/wiki/101955_Bennu&quot; rel=&quot;noreferrer&quot;&gt;101955 Bennu&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Launch date: 8 September 2016&lt;/li&gt;&#xA;&lt;li&gt;Return date: 24 September 2024 (anticipated)&lt;/li&gt;&#xA;&lt;li&gt;Sample mass: 60 grams - 2 kg (anticipated)&lt;/li&gt;&#xA;&lt;li&gt;Science objectives:&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Return and analyze a sample of pristine carbonaceous asteroid regolith in an amount sufficient to study the nature, history, and distribution of its constituent minerals and organic material. &lt;/li&gt;&#xA;&lt;li&gt;Map the global properties, chemistry, and mineralogy of a primitive carbonaceous asteroid to characterize its geologic and dynamic history and provide context for the returned samples. &lt;/li&gt;&#xA;&lt;li&gt;Document the texture, morphology, geochemistry, and spectral properties of the regolith at the sampling site in situ at scales down to millimeters.&lt;/li&gt;&#xA;&lt;li&gt;Measure the Yarkovsky effect (a thermal force on the object) on a potentially hazardous asteroid and constrain the asteroid properties that contribute to this effect. &lt;/li&gt;&#xA;&lt;li&gt;Characterize the integrated global properties of a primitive carbonaceous asteroid to allow for direct comparison with ground-based telescopic data of the entire asteroid&#xA;population.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Hayabusa&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Name: はやぶさ (literally, &quot;Peregrine Falcon&quot;) (&lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Lead Institution(s): JAXA (Japan)&lt;/li&gt;&#xA;&lt;li&gt;Principle Investigator (PI): Junichiro Kawaguchi (Institute of Space and Astronautical Science)&lt;/li&gt;&#xA;&lt;li&gt;Target: &lt;a href=&quot;https://en.wikipedia.org/wiki/25143_Itokawa&quot; rel=&quot;noreferrer&quot;&gt;25143 Itokawa&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Launch date: 9 May 2003&lt;/li&gt;&#xA;&lt;li&gt;Sampling date: November 2005&lt;/li&gt;&#xA;&lt;li&gt;Return date: 13 June 2010 (recovered June 14, 2010)&lt;/li&gt;&#xA;&lt;li&gt;Sample mass: &amp;lt; 1 gram&lt;/li&gt;&#xA;&lt;li&gt;Science objectives: (my brief summary of the info from &lt;a href=&quot;http://neo.jpl.nasa.gov/missions/hayabusa.html&quot; rel=&quot;noreferrer&quot;&gt;this page&lt;/a&gt;)&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Determine and map surface morphology, composition, etc. -- &lt;em&gt;in many ways&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;Search for possible asteroid satellites and dust rings&lt;/li&gt;&#xA;&lt;li&gt;Reveal history of impacts from other asteroid and comet fragments&lt;/li&gt;&#xA;&lt;li&gt;Provide accurate shape and mass determinations&lt;/li&gt;&#xA;&lt;li&gt;Establish relationship between &lt;a href=&quot;https://en.wikipedia.org/wiki/25143_Itokawa&quot; rel=&quot;noreferrer&quot;&gt;25143 Itokawa&lt;/a&gt; and meteorite types (composition, type, history, etc.)&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Differences&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Besides the administration of each project (USA vs. Japan), and the spacecraft themselves, the two missions are fairly similar. There are some notable differences, however, and for the sake of the OP, I'll focus on these. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to some failures, most of the spacecraft burned up upon re-entry. The &lt;em&gt;Hayabusa&lt;/em&gt; mission returned less than a gram of material. According to Dante Lauretta (PI of &lt;em&gt;OSIRIS-REx&lt;/em&gt;) in &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/dante-lauretta/20141020-collaboration-between.html&quot; rel=&quot;noreferrer&quot;&gt;this publication&lt;/a&gt;, the &lt;em&gt;OSIRIS-REx&lt;/em&gt; mission is in part an attempt to continue the work of &lt;em&gt;Hayabusa&lt;/em&gt;, as &lt;em&gt;OSIRIS-REx&lt;/em&gt; was hugely influenced by &lt;em&gt;Hayabusa&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/dante-lauretta/20141020-collaboration-between.html&quot; rel=&quot;noreferrer&quot;&gt;the same publication&lt;/a&gt;, Lauretta points out that Hayabusa's difficulties were due mainly to limited knowledge of Itokawa and insufficient simulation of the sampling process ahead of time. &lt;em&gt;OSIRIS-REx&lt;/em&gt; will spend almost a year characterizing Bennu while orbiting it, however, so scientists on Earth will have a better understanding of the asteroid and (importantly) its surface before actually sampling it. Whereas &lt;em&gt;Hayabusa&lt;/em&gt; actually landed on Itokawa for about 30 minutes, &lt;em&gt;OSIRIS-REx&lt;/em&gt; will touch it only briefly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although both Bennu and Itokawa are of similar size (~400-500 m in diameter), a key difference is that Bennu is a near Earth asteroid, meaning that it could collide with Earth, and Itokawa is not. Also, the shape of Itokawa is more like a slightly bent potato, whereas Bennu is much more rounded. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Itokawa&#xA;&lt;a href=&quot;https://i.stack.imgur.com/wUZgx.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wUZgx.gif&quot; alt=&quot;Image of Itokawa&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bennu&#xA;&lt;a href=&quot;https://i.stack.imgur.com/oJwHX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oJwHX.png&quot; alt=&quot;Artist&amp;#39;s rendering of Bennu&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for technology, &lt;em&gt;Hayabusa&lt;/em&gt; used an explosive harpoon to kick up particles into its collection module. &lt;em&gt;OSIRIS-REx&lt;/em&gt; will essentially cover the sample site with an inverted pan-like contraption and release several bursts of nitrogen gas at the regolith to kick up particles into its collection module. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since &lt;em&gt;OSIRIS-REx&lt;/em&gt; will spend a year orbiting Bennu, it is equipped with a suite of telescopes/imagers, radar, spectroscopes and gravitometers to keep it busy during this time. Many of these instruments allow collaborative, independent analyses of the inferences made by the other instruments on board. While &lt;em&gt;Hayabusa&lt;/em&gt; also had several similar instruments, it did not have as many as &lt;em&gt;OSIRIS-REx&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read about the individual instruments on the official pages for &lt;a href=&quot;http://www.isas.jaxa.jp/e/enterp/missions/hayabusa/index.shtml&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Hayabusa&lt;/em&gt;&lt;/a&gt; (also &lt;a href=&quot;http://neo.jpl.nasa.gov/missions/hayabusa.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;) and &lt;a href=&quot;http://www.nasa.gov/mission_pages/osiris-rex/&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;OSIRIS-REx&lt;/em&gt;&lt;/a&gt; (also &lt;a href=&quot;http://www.asteroidmission.org/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;). These pages, and links therein, can provide you with a wealth of instrumentation and engineering details. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Sources used&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Except where noted, information was taken from Wikipedia's pages on &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;noreferrer&quot;&gt;Hayabusa&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx&quot; rel=&quot;noreferrer&quot;&gt;OSIRIS-REx&lt;/a&gt;. Some information was also based on discussions with colleagues involved with &lt;em&gt;OSIRIS-REx&lt;/em&gt;. &lt;/p&gt;&#xA;" OwnerUserId="11264" LastActivityDate="2016-09-20T19:46:30.100" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18304" PostTypeId="1" CreationDate="2016-09-20T19:54:55.010" Score="9" ViewCount="2068" Body="&lt;p&gt;Is there a well-accepted standard protocol for communicating between a cubesat and ground station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On what basis would I choose the protocol?&lt;/p&gt;&#xA;" OwnerUserId="17021" LastEditorUserId="6816" LastEditDate="2016-09-20T20:24:08.577" LastActivityDate="2016-09-27T04:53:12.097" Title="Standard communications protocol for cubesats" Tags="|communication|cubesat|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18305" PostTypeId="2" ParentId="15218" CreationDate="2016-09-20T23:31:08.717" Score="5" Body="&lt;h2&gt;Tiangong-1 will deorbit sometime in late 2017&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As recently reported in &lt;a href=&quot;http://www.popularmechanics.com/space/satellites/a22936/tiangong-falling-to-earth/&quot; rel=&quot;nofollow&quot;&gt;Popular Mechanics&lt;/a&gt; The China National Space Agency recently announced that Tiangong-1 will deorbit sometime late in 2017, although they can't say exactly when or where the former space station will fall. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://news.xinhuanet.com/english/2016-09/14/c_135687885.htm&quot; rel=&quot;nofollow&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14380" LastEditorUserId="14380" LastEditDate="2016-09-21T20:00:06.823" LastActivityDate="2016-09-21T20:00:06.823" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18306" PostTypeId="2" ParentId="13918" CreationDate="2016-09-21T08:26:55.020" Score="3" Body="&lt;p&gt;I think this project is extremely difficult. To get detailed high quality pictures of Apollo landing sites, you need a very low orbit around the moon. The distance between the spacecraft and the landing site has to be small enough for a detailed picture. But those very low moon orbits are not stable due to mascons of the moon. To avoid a crash, the small spacecraft would need a sophisticated propulsion system and a lot of fuel for orbit control. The navigation must be extremely precise to get an orbit over a landing site. There is no moon GPS available for navigation. Finding the landing sites requires complex and very fast image processing to dectect the remains of a landing within a series of pictures. I doubt that an arduino has enough processing power to do this image processing in real time.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-21T08:26:55.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18307" PostTypeId="2" ParentId="18304" CreationDate="2016-09-21T10:11:46.243" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.itu.int/en/ITU-R/space/workshops/2015-prague-small-sat/Presentations/ESA-Small_Sat.pdf&quot; rel=&quot;nofollow&quot;&gt;This ESA presentation&lt;/a&gt; has some considerations regarding frequency allocation. Getting an allocation from ITU can turn out to be the hardest part of your project, so --- to my knowledge --- all cubesats use amateur radio bands. In the &lt;a href=&quot;http://www.iaru-r1.org/index.php/spectrum-and-band-plans/vhf/2-meter&quot; rel=&quot;nofollow&quot;&gt;2m VHF band&lt;/a&gt;, some frequencies are reserved for satellite telemetry by IARU.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IARU states &quot;all modes&quot;, but according to ITU Radio Regulation 25.2A, your transmissions in amateur radio bands need to be accessible to other radio amateurs. You are free to create a new modulation scheme and/or protocol, but you would need to publish it. To keep things easy, cubesats usually go for &lt;a href=&quot;https://en.m.wikipedia.org/wiki/AX.25&quot; rel=&quot;nofollow&quot;&gt;AX.25&lt;/a&gt;, for which tested hard- and software is available.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastActivityDate="2016-09-21T10:11:46.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18308" PostTypeId="1" AcceptedAnswerId="18310" CreationDate="2016-09-21T10:27:50.210" Score="25" ViewCount="4922" Body="&lt;p&gt;As I was reading &lt;a href=&quot;https://www.cubesat.org/s/cds_rev13_final2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Cubesat standard&lt;/a&gt;, one of requirements is low outgassing &amp;quot;to prevent contamination of other spacecraft&amp;quot;. It also gives a link to a NASA-approved list of low outgassing materials.&lt;/p&gt;&#xA;&lt;p&gt;Is this an actual problem that can be a hurdle and has caused problems in the past, or is it a mere formality like &amp;quot;don't send stuff that will boil off in vacuum, spraying everything around with droplets&amp;quot;?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="46278" LastEditDate="2022-01-18T17:20:15.230" LastActivityDate="2022-01-18T20:55:15.490" Title="How serious a problem can outgassing of satellites be?" Tags="|cubesat|outgassing|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18309" PostTypeId="1" CreationDate="2016-09-21T11:30:15.117" Score="7" ViewCount="1487" Body="&lt;p&gt;I'm trying to run a Matlab simulation for desaturation of a reaction wheel using magnetorquers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is still something that I'm not entirely sure of  :    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that when the reaction wheel reaches its max speed ( in terms of rpm), it becomes saturated and will have 0 acceleration. Assuming that you stop the reaction wheel motor after saturation, will the angular velocity generated by the reaction wheel still cause it to turn? I think this is true as it would then justify using magnetorquers to externally counteract this angular velocity, but I am not entirely sure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, this is my current plan to desaturate the reaction wheel:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the reaction wheel is saturated, a control signal will be sent to active a P controller with the control law : m=-Kp(href-hw) where m is the magnetic moment of the magnetorquer, hw is the momentum of the reaction wheel and href is a reference momentum. This P controller would then be connected to a magnetorquer ( Let's call the this torque tmag). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;tmag would then be added into the dynamic equations for the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would this work?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks! &lt;/p&gt;&#xA;" OwnerUserId="16924" LastEditorUserId="58" LastEditDate="2016-09-21T17:19:34.217" LastActivityDate="2016-09-27T04:46:11.393" Title="Will this magnetorquer configuration effectively desaturate a reaction wheel?" Tags="|attitude|reaction-wheel|magnetorquer|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18310" PostTypeId="2" ParentId="18308" CreationDate="2016-09-21T11:31:22.563" Score="24" Body="&lt;p&gt;Its not a mere formality it is unfortunately one of those things that makes satellites expensive. However I wouldn't be surprised if all kinds of nominally incompatible materials have already been exposed in cubesat missions.  &lt;strong&gt;EDIT&lt;/strong&gt; This report might be useful background: &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960044619.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA contamination control report 1996&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could in principle affect other parts of the cubesat or in the shorter term other cubesats in the dispenser or other systems outside a cubesat dispenser. A consequence of the dispenser being an enclosure is that there will still be  some leak paths and these become the main out-gassing vent routes. The time for this to occur might limit the worst of the hazard to the host cubesat. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mechanisms of adhesion to other surfaces are varied and include condensation on cool surfaces and also UV ionisation and electro-static return to a surface not on a direct flight path. &lt;strong&gt;EDIT&lt;/strong&gt; The link I added above has a short section on this, section 2.3.1.2.2.2, figure reproduced below:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/qXovk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qXovk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note however the reference goes on to say that this particular mechanism is believed to be much more of a problem at GEO altitudes because the plasma screening distance is much greater than at LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outgassing products can be water vapour, most solid objects have a dose of trapped moisture, or something more destructive. See this article &lt;a href=&quot;http://space.skyrocket.de/doc_sat/hs-702.htm&quot; rel=&quot;noreferrer&quot;&gt;http://space.skyrocket.de/doc_sat/hs-702.htm&lt;/a&gt; regarding ouggassing from solar arrays. It says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first version of the 702 used solar arrays with concentrators. These concentrators tended to early fogging, as due to an inherent design flaw the outgassing of the solar cells was higher than expected. This fogging lead to much reduced lifetime. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The latter satellites all weighed several tonnes and I suspect cost over a billion $ between them.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-09-22T08:57:26.977" LastActivityDate="2016-09-22T08:57:26.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18311" PostTypeId="1" CreationDate="2016-09-21T12:06:31.037" Score="3" ViewCount="202" Body="&lt;p&gt;China's first space station Tiangong-1 is expected to deorbit in late 2017. Are NASA or others tracking it? What is its position and condition at present time?    &lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="14380" LastEditDate="2016-09-21T20:19:44.250" LastActivityDate="2016-09-21T20:19:44.250" Title="When and how will the Chinese spacestation reenter?" Tags="|low-earth-orbit|reentry|debris|space-station|" AnswerCount="3" CommentCount="1" ClosedDate="2016-09-21T20:20:37.227" ContentLicense="CC BY-SA 3.0" />
  <row Id="18312" PostTypeId="2" ParentId="18311" CreationDate="2016-09-21T12:21:16.673" Score="2" Body="&lt;p&gt;For your viewing pleasure, the current position of Tiangong 1:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.n2yo.com/satellite/?s=37820&quot; rel=&quot;nofollow&quot;&gt;http://www.n2yo.com/satellite/?s=37820&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2016-09-21T12:21:16.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18313" PostTypeId="2" ParentId="18298" CreationDate="2016-09-21T13:58:53.900" Score="5" Body="&lt;p&gt;The Hydro Impact Basin (HIB) holds about 3.4 million liters of water. This is based on the stated dimensions of 20 feet deep by 100 feet long by 60 feet wide, measured at the water surface. To compare, the &lt;a href=&quot;https://www.reference.com/home-garden/many-gallons-water-average-21-foot-swimming-pool-hold-ee99d3ef00a83005#&quot; rel=&quot;nofollow&quot;&gt;average 21-foot round swimming pool with a depth of 48 inches&lt;/a&gt; holds 51,100 liters. This means that the HIB is about 66 times larger! An Olympic swimming pool holds 2.5 million liters of water, which means the HIB is still about 1.4 times the size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The HIB is, in fact, filled with a hose:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/LZQvg1FTFtE?start=40&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven’t found any information about water consumption, but I did find that operational costs are constrained:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Operational costs (water consumption, grounds maintenance, labor to&#xA;  operate/maintain basin) shall be considered and controlled to a range of&#xA;  \$2,000 to \$2,500 per year, not including actual testing activities.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I also haven’t found whether any other space vehicles will use the HIB (my guess is not), but Orion does get to share the pool with divers who can access the pool from ladders and stairs around the perimeter. The divers standby to assist with recovery in the event of a sinking vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://crgis.ndc.nasa.gov/crgis/images/4/41/HydroBasin.pdf&quot; rel=&quot;nofollow&quot;&gt;Operational Readiness Review for LandIR Hydro Impact Basin - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-09-21T17:50:06.080" LastActivityDate="2016-09-21T17:50:06.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18314" PostTypeId="2" ParentId="18311" CreationDate="2016-09-21T15:18:21.457" Score="4" Body="&lt;p&gt;It is absolutely being tracked, as everything in space large enough to do so, by &lt;a href=&quot;https://www.space-track.org/&quot; rel=&quot;nofollow&quot;&gt;JSPOC&lt;/a&gt;. They release all non-classified spacecraft locations as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it will reenter, and even better, where, are much more difficult questions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact time of it's reentry &lt;a href=&quot;http://www.spacedaily.com/reports/Has_Tiangong_1_gone_rogue_999.html&quot; rel=&quot;nofollow&quot;&gt;can't be accurately predicted in advance&lt;/a&gt;. Right now it is just under 400 km, which, while it will have a short lifespan, likely be less than a year, it won't reenter in the next month. So I predict between 6 months and a year to reenter, at a location that will only be known a few days in advance, if we are lucky. The exact orbit in which a satellite will reenter is very difficult to determine ahead of time, and even just 1 orbit difference could mean thousands of km difference in it's landing position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this time, it is believed that there is no control authority at all with the space station.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-09-21T15:49:12.480" LastActivityDate="2016-09-21T15:49:12.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18315" PostTypeId="2" ParentId="18311" CreationDate="2016-09-21T17:21:59.440" Score="0" Body="&lt;p&gt;If you live more than 43 degrees north or south of the equator you won't get hit by it, it'll be constrained to somewhere within that band.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-21T17:21:59.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18316" PostTypeId="2" ParentId="18308" CreationDate="2016-09-21T17:27:09.490" Score="15" Body="&lt;p&gt;There are a few recorded incidents of spacecraft contamination caused by outgassing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Cassini, some haze was noted in test photographs after launch; this was thought to be due to some outgassing material near the camera itself. It was removed after several attempts using the camera's heaters:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ciclops.org/sci/docs/CassiniImagingScience.pdf&quot;&gt;Cassini Imaging Science report&lt;/a&gt;, 2004, p.475-476.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Stardust, the navigation camera became contaminated after launch. Again spacecraft heaters were used to eventually clear the problem:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://stardust.jpl.nasa.gov/news/vision.html&quot;&gt;NASA page on Stardust's cameras&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also the &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140001406.pdf&quot;&gt;Stardust &quot;Lessons Learned&quot;&lt;/a&gt; report mentions this and an additional problem with sample collection:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...However, despite these precautions the&#xA;  Stardust spacecraft outgassing was sufficient to degrade&#xA;  camera operations, and the aerogel capture media&#xA;  was significantly contaminated during manufacture.&#xA;  We also never completely solved the problem of&#xA;  defining useful limits for organic contaminants of&#xA;  spacecraft hardware, which haunts us as we rather unexpectedly&#xA;  captured primitive cometary organics.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-09-21T17:27:09.490" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18317" PostTypeId="2" ParentId="18297" CreationDate="2016-09-22T02:21:49.970" Score="21" Body="&lt;p&gt;Depressurization causes little if any harm so long as it's neither too fast (if the air in your lungs exits too fast it will do severe damage), nor too long (due to a lack of oxygen.)  So long as the space involved isn't too big these requirements can be met.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if it's a solid material that is burning (and how many flammable liquids or gases would be on a space station anyway?) you would be facing the definite possibility that the fire would simply reignite when you put the air back in the room.  In vacuum there would be little cooling of the hot materials, they very well might remain above ignition temperature.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-09-22T02:21:49.970" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18318" PostTypeId="2" ParentId="18262" CreationDate="2016-09-22T04:54:04.160" Score="16" Body="&lt;p&gt;They will likely be relying on two things:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1029/1999JE001029/epdf&quot; rel=&quot;noreferrer&quot;&gt;Since the rate at which dust will accumulate on the solar panels is pretty well known&lt;/a&gt;, they can estimate a lifespan of the rover.  All space probes have some form of estimated lifespan, and the primary science mission is generally planned to fit within that low timeframe.  However, as we've seen from Spirit and (especially) Opportunity, the actual lifespan of the rover can be much longer because of:&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cleaning_event&quot; rel=&quot;noreferrer&quot;&gt;Cleaning events&lt;/a&gt;.  Basically, Mars has just enough of an atmosphere to have wind that blows the dust around.  This cleans off the solar panels every once in a while, allowing the rovers to keep operating.  Here's a picture of Opportunity before and after one such cleaning event:&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HZi9U.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HZi9U.jpg&quot; alt=&quot;Opportunity Cleaning Event&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spirit and Opportunity were only expected to last 90 days, but when it was discovered that the dust occasionally gets cleared off, the missions became much longer.  Spirit only stopped when it became stuck in soft sand and could no longer angle its solar panels correctly.  Opportunity, on the other hand, is still going, 12 years later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/25577-mars-rover-opportunity-solar-panels-clean.html&quot; rel=&quot;noreferrer&quot;&gt;Here is a news article about one of the more recent cleaning events for Opportunity.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as hibernation during the winter, they will likely follow the same techniques used by Spirit and Opportunity. Unfortunately, I do not know this for sure, but since it worked for MER, they will likely use the same legacy technique.  Some of the instruments will also have their own independent batteries, such as the &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=8&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjmv_fzm6LPAhVI6WMKHc_4B7wQFghYMAc&amp;amp;url=http%3A%2F%2Fwww.edn.com%2FPdf%2FViewPdf%3FcontentItemId%3D4440002&amp;amp;usg=AFQjCNHJ02lzEH_bV6owCsVqMVy9amrHcg&amp;amp;sig2=Tk4F1I-YQNQOb-N7-SMMEw&quot; rel=&quot;noreferrer&quot;&gt;DREAMS experiment&lt;/a&gt;, which will operate during the dust storm season.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2014/05191401-rover-dust-before-and-after.html?referrer=https://www.google.com/&quot; rel=&quot;noreferrer&quot;&gt;Here you can see the difference in dust before and after a cleaning event&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In all likelihood, ExoMars will not have any method to remove the dust.  There are &lt;a href=&quot;https://www.newscientist.com/blogs/shortsharpscience/2008/11/why-dont-the-mars-rovers-have.html&quot; rel=&quot;noreferrer&quot;&gt;several reasons&lt;/a&gt; for this.  A mechanical device would probably have trouble because of the dust's static charge.  Tilting and shaking the rover could work, but it would be complicated and possibly damaging to sensitive instruments.  Other methods involve washing them (which requires liquid), using an electrostatic charge (still requires tilting the panels, and could also damage sensitive electronics), and blowing them off (requires moving a high-powered fan across the panels).  &lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-09-22T05:20:50.217" LastActivityDate="2016-09-22T05:20:50.217" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18319" PostTypeId="2" ParentId="18268" CreationDate="2016-09-22T14:37:19.517" Score="4" Body="&lt;p&gt;The moments of inertia of a hollow cylinder of uniform density without end caps is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$I_z={m\over 2}\left(r_1^2+r_2^2\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$I_x=I_y={m\over 4}\left(r_1^2+r_2^2+{h^2\over 3}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $m$ is the total mass, $r_1$ is the inner diameter, $r_2$ is the outer diameter, and $h$ is the height.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-09-22T14:37:19.517" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18320" PostTypeId="1" CreationDate="2016-09-22T17:27:53.927" Score="3" ViewCount="1129" Body="&lt;p&gt;Are there any examples of pressure fed engines used on launch vehicles?  I already know of the ones on the wikipedia page which include&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Firefly Space Systems&lt;/li&gt;&#xA;&lt;li&gt;OTRAG (rocket)&lt;/li&gt;&#xA;&lt;li&gt;Quad (rocket) of Armadillo Aerospace&lt;/li&gt;&#xA;&lt;li&gt;XCOR EZ-Rocket of XCOR Aerospace&lt;/li&gt;&#xA;&lt;li&gt;Masten Space Systems&lt;/li&gt;&#xA;&lt;li&gt;Aquarius Launch Vehicle&lt;/li&gt;&#xA;&lt;li&gt;NASA Project Morpheus prototype lander&lt;/li&gt;&#xA;&lt;li&gt;NASA Mighty Eagle mini lunar lander&lt;/li&gt;&#xA;&lt;li&gt;CONAE Tronador II upper stage&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="16180" LastEditorUserId="9092" LastEditDate="2021-03-15T10:17:10.740" LastActivityDate="2021-03-15T10:17:10.740" Title="Some examples of pressure fed engines used on launch vehicles" Tags="|engine-design|launch-vehicle|pressure-fed-engine|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18321" PostTypeId="1" AcceptedAnswerId="18333" CreationDate="2016-09-23T06:08:52.203" Score="5" ViewCount="529" Body="&lt;p&gt;Can anyone explain to me the reasoning behind the differences in the placement and orientation of the thruster quad packages on the CSM and LM?  On the CSM a quad is a + shape and are located in a plane parallel to the plane of the quad on the opposite side, whereas on the LM a quads thrusters are not in the same plane, and the quad is located on 'the corner' of the vehicle, so they are not parallel with the opposite quad. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the advantages and disadvantages, and other factors, resulting in these configurations?&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2019-05-25T00:20:53.793" Title="Apollo CSM thruster quad orientations vs LM" Tags="|apollo-program|flight-control|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18322" PostTypeId="2" ParentId="17777" CreationDate="2016-09-23T08:47:54.813" Score="1" Body="&lt;p&gt;There are a lot of good answers here. Let's look also at &lt;em&gt;why&lt;/em&gt; rockets cost so much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google says the average home is about \$250k to buy a home, about a third of which is labor, \$80k. You can see a typical house cost breakdown &lt;a href=&quot;http://porch.com/advice/new-home-construction-costs-numbers/&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.   It also &lt;a href=&quot;https://www.google.com/search?q=average+home+weight&amp;amp;oq=average+home+wei&amp;amp;aqs=chrome.1.69i57j69i59j0l2.4593j0j4&amp;amp;client=ms-android-hms-tmobile-us&amp;amp;sourceid=chrome-mobile&amp;amp;ie=UTF-8&quot; rel=&quot;nofollow&quot;&gt;weighs&lt;/a&gt; about 120k pounds. So labor costs are roughly \$1/lb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, construction workers are paid close to minimum wage, while as Pearson cited rocket manufacturing employees make about ten times more. So that's \$10/lb. Lets double that to \$20/lb assuming a rocket is more complex than a house. As Hobbes cited, about half of the cost is R&amp;amp;D. So let's double to \$40/lb. Legal and other business costs might get us to \$60/lb, assuming again, from the housing example, that overhead is a third of the cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see &lt;a href=&quot;http://www.fool.com/investing/general/2016/04/17/how-profitable-is-spacex-how-much-more-profitable.aspx&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt; that SpaceX will potentially have a 40% profit margin. So in all the cost of a rocket is about \$100/lb. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, a rocket is typically around \$1,000/lb of dry mass, with the fuel making up another 90% of the weight. So there must be waste, bureacracy, testing, or &lt;em&gt;something&lt;/em&gt; making up the rest. Based on &lt;a href=&quot;http://firearmshistory.blogspot.com/2012/06/cost-of-manufacturing.html?m=1&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; example from firearms, products can be marked up sevenfold after passing through layer after layer of middlemen. So something similar to this may be happening with rocketry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, then there must be significant testing expense. If we assume, per the comments and &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; link, that a Falcon 9 costs \$200k to fuel, that means 300 full launch tests can be done for each rocket to bring the cost close to what it sells for, \$60MM. If labor is the main testing cost, then with \$200k cost per employee it requires 300 person-years of testing. Assuming 1 million parts, &lt;a href=&quot;https://www.quora.com/How-many-parts-are-in-an-aircraft&quot; rel=&quot;nofollow&quot;&gt;typical of large aircraft&lt;/a&gt;, each employee-year involves testing one part per work hour.&lt;/p&gt;&#xA;" OwnerUserId="12649" LastEditorUserId="12649" LastEditDate="2016-09-23T19:31:26.313" LastActivityDate="2016-09-23T19:31:26.313" CommentCount="4" CommunityOwnedDate="2016-09-23T19:18:09.247" ContentLicense="CC BY-SA 3.0" />
  <row Id="18323" PostTypeId="1" CreationDate="2016-09-23T10:11:22.980" Score="3" ViewCount="1305" Body="&lt;p&gt;How effective is the thermal control system and the liquid cooling garment of a space suit? Does it prevent sweating only for a small to medium workload? If an astronaut is sweating behind his ears there might be problems with the communication system inside the helmet and the vision might be impaired. Wearing a suit filled with some sweat for many hours during an EVA might be uncomfortable.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="13126" LastEditDate="2016-09-26T06:54:22.057" LastActivityDate="2016-09-26T15:42:25.533" Title="Does an astronaut sweat in a space suit?" Tags="|spacesuits|thermal-control|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18324" PostTypeId="2" ParentId="1302" CreationDate="2016-09-23T12:29:21.353" Score="10" Body="&lt;p&gt;In addition to the above answers, the third crewman on the CSM performed visual checks of the LM after separation before descent, photography duties, regular purging of fuel cells, reconfiguring and relaying communications from the LM to mission control and platform alignments. Some of these may be possible from the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess it was also a big comfort to have a guy on board just in case of some failure which would result in the loss of attitude or communication (and control) of the CSM. It's not just for computer backup. For example, a mechanical failure like some gas leak could push the CSM off attitude and through gimbal lock. That might be easily correctable by a guy on board, yet pose major problems to fix from the ground if communication was lost, and could prevent the LM from docking and kill everyone.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-23T12:29:21.353" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18328" PostTypeId="1" CreationDate="2016-09-24T00:29:26.990" Score="3" ViewCount="616" Body="&lt;p&gt;Is it possible to colonize planetoids using &lt;a href=&quot;http://en.wikipedia.org/wiki/Liquid_breathing&quot; rel=&quot;nofollow noreferrer&quot;&gt;breathable liquid&lt;/a&gt; such as &lt;a href=&quot;http://en.wikipedia.org/wiki/Fluorocarbon&quot; rel=&quot;nofollow noreferrer&quot;&gt;perfluorocarbons&lt;/a&gt; (PFCs) with dissolved oxygen and filling craters and depressions with it instead of building insulated habitats?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this would require much less oxygen than creating a breathable atmosphere and at the same time would not limit the colonists to the inside of their dwellings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides liquid breathing, fluorocarbons have other wonderful properties:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;They are very potent dielectrics unlike water, so no short-circuit.&lt;/li&gt;&#xA;&lt;li&gt;They have high weight so they can form an atmosphere where there is not enough gravity to keep an atmosphere of lighter gases.&lt;/li&gt;&#xA;&lt;li&gt;For terraforming a planet: PFCs are not photodissociated, having highest lifetime in the atmosphere, they are the most potent greenhouse gases (tetraflourocarbon is 6500 times more potent than CO2), which allows to warm up planets such as Mars (this has been already suggested).&lt;/li&gt;&#xA;&lt;li&gt;PFCs can be used as blood substitutes and have anestetic properties. Oxygen-rich PFCs help to heal wounds. They are also used to store living organs for transplantation.&lt;/li&gt;&#xA;&lt;li&gt;Some PFCs are extremely hydrophobic &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As uhoh suggests in a comment, one scenario for breathable liquid environment is &quot;walking on the ceiling&quot;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;if the local gravity were $g′$ and the perfluorocarbon had a density of 1.6 $ g/cm^3 $ then they'd be walking on the ceiling feeling a &quot;gravity&quot; of 0.6 $g′$.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Here is an example of such a buoyant, inverted &quot;ice walker&quot; - starts at 03:27:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/HK5qnpDClRI?start=207&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="812" LastEditorUserId="12102" LastEditDate="2017-02-11T19:42:53.923" LastActivityDate="2021-09-06T17:01:41.923" Title="Is it possible to colonize planetoids by using perfluorocarbon lakes?" Tags="|colonization|terraforming|" AnswerCount="3" CommentCount="26" ContentLicense="CC BY-SA 3.0" />
  <row Id="18329" PostTypeId="1" AcceptedAnswerId="18330" CreationDate="2016-09-24T03:34:03.267" Score="5" ViewCount="2717" Body="&lt;p&gt;A recent &amp;quot;&lt;a href=&quot;http://www.spacex.com/news/2016/09/01/anomaly-updates&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anomaly Update&lt;/a&gt;&amp;quot; dated &lt;code&gt;September 23, 1:00pm EDT&lt;/code&gt; by SpaceX,  includes the sentence:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;At this stage of the investigation, preliminary review of the data and debris suggests that a large breach in the cryogenic helium system of the second stage liquid oxygen tank took place.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This has been repeated in several news outlets, but &lt;a href=&quot;http://spaceflightnow.com/2016/09/23/falcon-9-rocket-explosion-traced-to-upper-stage-helium-system/&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflightnow&lt;/a&gt; adds:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Falcon 9 rocket’s upper stage liquid oxygen tank contains several composite helium vessels, each pressurized to about 5,500 pounds per square inch in flight. The helium is routed through the second stage’s Merlin engine, where the helium warms up and is injected into the rocket’s propellant tanks to pressurize the stage as the launcher burns fuel, keeping the tanks structurally sound.&lt;/p&gt;&#xA;&lt;p&gt;While cryogenic helium was aboard the Falcon 9 at the time of the explosion, the mishap occurred around eight minutes before the rocket’s main engines were scheduled to ignite for the on-the-pad “static fire” test Sept. 1.&lt;/p&gt;&#xA;&lt;p&gt;At that point in the countdown, the propellant tanks are normally not pressurized for launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I'm trying to understand the implications behind: &amp;quot;The Falcon 9 rocket’s upper stage liquid oxygen tank contains several composite helium vessels...&amp;quot;. According to &lt;a href=&quot;https://space.stackexchange.com/a/9848/12102&quot;&gt;this answer&lt;/a&gt; &amp;quot;In the move to Falcon 9 v1.1 and the Merlin 1D engine they moved them into the LOX tanks on both stages. This de-cluttered the engine area a great deal.&amp;quot; - mentioned in &lt;a href=&quot;https://space.stackexchange.com/a/9909/12102&quot;&gt;this answer&lt;/a&gt; as well - I can see that the helium tanks have been relocated &lt;em&gt;inside the LOX tanks&lt;/em&gt; for several practical reasons including layout/arrangement and thermal management - inside the LOX tanks the temperature is much colder than ambient.&lt;/p&gt;&#xA;&lt;p&gt;At a given maximum pressure, a lower temperature allows for more helium to be stored in the tanks, or a smaller/lighter tanks can be used for a fixed amount of helium, although I am not sure the actual reason. Boiling LOX is about 90K (only about one third of ambient absolute temperature), and SpaceX is (often/usually?) subcooling below that to pack even more LOX into the tanks.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Is the helium now substantially colder by moving the tanks inside the LOX tank, or is it simply easier to keep them cold - avoiding coolers and insulation?&lt;/li&gt;&#xA;&lt;li&gt;If the helium tanks are pressurized only in the last minutes before launch, doesn't compressing this gas to roughly 400 atmospheres generate a huge amount of heat? If the tanks are fixed size, the pressure will increase steadily as it is filled, and that makes a heat source inside the LOX tank just before launch. I can understand adding the helium &lt;em&gt;beforehand&lt;/em&gt;, letting it cool a bit after compression, and then filling with LOX, but wouldn't compressing the helium in the tank at the last minute be incompatible with sub-cooled LOX?&lt;/li&gt;&#xA;&lt;li&gt;Or, can &amp;quot;...each pressurized to about 5,500 pounds per square inch in flight.&amp;quot; be taken literally - the tank becomes pressurized in flight? The pressurization &lt;em&gt;occurs in flight&lt;/em&gt;?&lt;/li&gt;&#xA;&lt;li&gt;And does &amp;quot;At that point in the countdown, the propellant tanks are normally not pressurized for launch.&amp;quot; refer to LOX and RP-1 &lt;em&gt;but not&lt;/em&gt; helium? Is the helium pressurized but not the propellants, or does this also refer to the helium being not pressurized?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Consider for a moment the &lt;a href=&quot;https://www.nasa.gov/content/air-supply-high-pressure-tanks-ready-for-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;NORS 6000 psi oxygen tanks&lt;/a&gt; for delivery to ISS as described in &lt;a href=&quot;https://space.stackexchange.com/a/17677/12102&quot;&gt;this answer&lt;/a&gt;. According to that article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Working inside the high bay of the Space Station Processing Facility at Kennedy, about a dozen people perform the operations to fill a single tank following two to three days of inspections and work stand preparations. &lt;strong&gt;They work about 40 feet from the stand with the tank and leave it alone for a day so it can cool down after the pressurization.&lt;/strong&gt; (my emphasis)&lt;/p&gt;&#xA;&lt;p&gt;The team performed several fills on other tanks to make sure the process could be handled safely and performed as planned.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;There's really no other place in the world that operates up to 10,000 pounds per square inch like this in an operational environment,&amp;quot; Bigos said. &amp;quot;We've had to be exceptionally cautious. It's not yet normal day-to-day operations but we're getting there.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It doesn't sound like you'd want to pressurize a 6000 psi tank submersed inside a sub-cooled LOX tank just minutes before liftoff - due to the heat generation if for no other reason. The helium tanks may or may not be smaller, but this is not a location where you'd want to be generating much heat.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bZo2b.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bZo2b.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NORS oxygen tank from &lt;a href=&quot;https://www.nasa.gov/content/air-supply-high-pressure-tanks-ready-for-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Credit: NASA, cropped/rotated.&lt;/p&gt;&#xA;&lt;p&gt;I know it's just the color and general shape, but just looking at that makes me nervous - I think it's reminding me of &lt;a href=&quot;https://i.stack.imgur.com/d1OQ4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;these&lt;/a&gt; for some reason.&lt;/p&gt;&#xA;&lt;p&gt;You can always top-off a boiling LOX tank at the last minute, but I don't think you can really re-sub-cool a LOX tank inside of a rocket. Heat introduced into sub-cooled LOX will raise its temperature and the LOX will expand and its density decrease, unless there are additional &amp;quot;chillers&amp;quot; in there as well.&lt;/p&gt;&#xA;&lt;p&gt;However in any case I now understand that the pressure of the helium is still used &lt;em&gt;outside of the LOX tank&lt;/em&gt; and that they were moved inside for other reasons.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-16T00:32:09.737" Title="Problems compressing the helium tanks inside the F9 2nd stage LOX tank at the last minute?" Tags="|falcon-9|failure|lox|sub-cooled|high-pressure-environment|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18330" PostTypeId="2" ParentId="18329" CreationDate="2016-09-24T08:26:12.123" Score="2" Body="&lt;ol&gt;&#xA;&lt;li&gt;It's much easier to cool the He by placing the tank inside the LOX tank. If you had a He tank outside, you'd have to insulate it, and provide active cooling. Both add weight and complexity.  &lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Propellant tanks can be pressurized at the last moment: you fill them at atmospheric pressure and vent the boiloff (if any). For boiling-point LOX: when the tank is full, you close the vent valve until the boiloff pressurizes the tank to the value you want. For subcooled LOX you close the vent valve and use He to pressurize the tank. Filling at atmospheric pressure works because LOX is a liquid. You can't do this with a tank that contains gaseous He.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they filled the He tanks during propellant loading, they'd introduce a large heat source into the subcooled LOX tank, which would lead to huge amounts of boiloff. Still, that's what they do, according to the &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;user guide&lt;/a&gt;. &quot;Early in the countdown&quot; can be as little as 1 hour before launch.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Early in the countdown, the vehicle performs LOX, RP-1 and pressurant loading,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;No. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;He is not a propellant. It only refers to RP-1 and LOX. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://spaceflight101.com/falcon-9-ft-countdown-timeline/&quot; rel=&quot;nofollow noreferrer&quot;&gt;countdown sequence&lt;/a&gt; confirms: He and LOX are loaded simultaneously at T -30 minutes. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6816" LastEditDate="2017-06-15T23:43:58.920" LastActivityDate="2017-06-15T23:43:58.920" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="18331" PostTypeId="2" ParentId="2253" CreationDate="2016-09-24T09:24:43.320" Score="6" Body="&lt;p&gt;NASA probably learned a lot of lessons following the development of Gemini. Astronaut Gus Grissom (170cm) was instrumental in the cockpit layout and design, and they even nicknamed the spacecraft the Gusmobile, since it's rumoured he was the only one able to fit into it (I read somewhere that it wasn't just Gus who fit, but that a majority of the original astronaut team had problems). Their helmets were touching the hatch when it was closed, and the lack of room made getting in and out of space suits very difficult. NASA revisited the design of the seating to make a bit more room for the remaining astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, go here and search the page for &quot;Gusmobile&quot;: &lt;a href=&quot;http://www.astronautix.com/g/gemini.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.astronautix.com/g/gemini.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo, being larger (NASA learned some lessons?) introduced it's own problems, since with the increased room came increated mobility, and with all the moving around astronauts sometimes became very space-sick.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To also show that maybe some lessons have been learned, the modern American space suit used on the ISS is able to fit the 5th-95th percentile of Americans which allows NASA great flexibility in choosing crew members. Compare to the Russian Sokol suit which only allows the 40-60th percentile, and cosmonauts must fit this profile. (ref: &lt;a href=&quot;http://history.nasa.gov/SP-4225/nasa4/nasa4.htm&quot; rel=&quot;noreferrer&quot;&gt;http://history.nasa.gov/SP-4225/nasa4/nasa4.htm&lt;/a&gt; search page for &quot;EMU&quot; - date of info unknown).  The ISS is also extremely large in comparison, with a dedicated roof and floor to help orientation and reduce space sickness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And finally a bit of speculation, although I don't have any information on the scale of the set for Apollo 13 film, the control panel is really accurately produced, and when compared to photos of inside Apollo I thought it gave a decent representation. Having seen one with my own eyes, the movies (even the real movies) somehow all make it look bigger and roomier. Maybe it's the weightlessness.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-24T09:24:43.320" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18332" PostTypeId="1" AcceptedAnswerId="18349" CreationDate="2016-09-24T16:09:29.657" Score="9" ViewCount="4465" Body="&lt;p&gt;Propellant tank pressurization is a critical aspect of liquid propellant rocket design. Many designs use high pressure helium, heated and recirculated, however the propellant gases themselves can be used - at least in the case of LH2 as explained in &lt;a href=&quot;https://space.stackexchange.com/a/6616/12102&quot;&gt;this answer&lt;/a&gt;. In the case of Falcon 9 this is mentioned &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; as I've learned in the discussion and links associated with &lt;a href=&quot;https://space.stackexchange.com/q/18329/12102&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Another question &lt;a href=&quot;https://space.stackexchange.com/q/9311/12102&quot;&gt;Why do pressure-fed systems have to be pressurized with helium or nitrogen?&lt;/a&gt; addresses the question of &lt;em&gt;why the choice of pressurization gas must be helium or nitrogen&lt;/em&gt;, and self-pressurized by their own boil-off gas. This is a different question. I am asking about the function or purpose of the pressurization, and the relative importance of two following possibilities.&lt;/p&gt;&#xA;&lt;p&gt;I had thought the pressure was necessary only to feed the propellants into the pumps and other plumbing of the engine fast enough, but then I saw this line in the CSMonitor article: &lt;a href=&quot;http://www.csmonitor.com/Science/2016/0924/SpaceX-launch-explosion-traced-to-helium-system.-Now-what&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX launch explosion traced to helium system. Now what?&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Helium is injected into fuel tanks to keep them structurally sound as the launcher burns fuel during flight. This system apparently leaked during the static test.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Thinking about it, overpressure would certainly help maintain rigidity of the tank. Anyone who's seen the &amp;quot;&lt;a href=&quot;https://lecdemo.johnshopkins.edu/oldlecdemo/reese1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;crush the can&lt;/a&gt;&amp;quot; experiment can't forget it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is the pressurization of propellant tanks actually necessary for structural integrity? And while maintaining pressure above ambient may be necessary to prevent buckling, is further positive pressure used in the mechanical design of a rocket to substantially stiffen the structure?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wKcof.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wKcof.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Image of the &amp;quot;&lt;a href=&quot;https://lecdemo.johnshopkins.edu/oldlecdemo/reese1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Crush the Can&lt;/a&gt;&amp;quot; experiment, Ronald Lane Reese, Johns Hopkins University (1999).&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-01-24T23:57:52.037" LastActivityDate="2021-01-24T23:57:52.037" Title="Is the pressurization of propellant tanks necessary for structural integrity?" Tags="|fuel|liquid-fuel|high-pressure-environment|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="18333" PostTypeId="2" ParentId="18321" CreationDate="2016-09-24T16:13:40.327" Score="6" Body="&lt;p&gt;According to Thomas Kelly's &lt;a href=&quot;https://books.google.com/books?id=BZ5fBgAAQBAJ&amp;amp;pg=PT89&amp;amp;lpg=PT89&amp;amp;dq=lunar%20module%20rcs%20placement&amp;amp;source=bl&amp;amp;ots=hwP3qbZRtp&amp;amp;sig=9dAcVWta1bZjYuSm-j_Mr1jZbdE&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwiSqL-8rqjPAhVB1GMKHbUDASoQ6AEIXjAN#v=onepage&amp;amp;q=lunar%20module%20rcs%20placement&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;&amp;quot;Moon Lander: How We Developed the Apollo Lunar Module&amp;quot;&lt;/a&gt;, the primary driver for the positioning of the LM RCS quads was avoiding contamination of the windows with rocket exhaust:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;We argued engineering logic and practical issues, such as jet contamination effects on adjacent areas of the ascent stage. This latter consideration convinced us to abandon the originally proposed RCS configuration, which had four jets close to the front windows, likely to obscure the windows with rocket-exhaust products. ...&lt;/p&gt;&#xA;&lt;p&gt;Bob Grossman jumped up and took the chalk from me, his face glowing. He vigorously erased the blackboard and began to sketch. &amp;quot;Here, how about this?&amp;quot; we said as he continued to sketch. &amp;quot;We'll mount the thrusters at forty-five degrees, keeping them away from the windows. And we'll arrange them as an axial pair [X-axis aligned] and an orthogonal pair (Y/Z axis). ...&lt;/p&gt;&#xA;&lt;p&gt;It looked better than any other the others, but we continued discussing its pros and cons for another hour before choosing it as the preferred design arrangement.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In vacuum, a rocket exhaust plume rapidly expands into a cloud, so each use of the thrusters would leave a thin deposit of material on the windows, clouding them slightly.&lt;/p&gt;&#xA;&lt;p&gt;This leaves unclear why the CSM RCS layout remained with (nearly*) axial placement, particularly since it &lt;a href=&quot;https://www.hq.nasa.gov/alsj/tnD6850LMLandingGearSubsytem.pdf&quot; rel=&quot;noreferrer&quot;&gt;imposed a need for some thermal protection&lt;/a&gt; on the LM's legs due to exhaust impingement during docked flight.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hoXww.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hoXww.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The configurations are, essentially, functionally equivalent, and there's no obvious large advantage to that configuration. It may have been as simple as the CSM design having been more complete before the LM's RCS layout was decided.&lt;/p&gt;&#xA;&lt;p&gt;* note that the service module RCS quads are mounted about 7º off the reference axes of the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-05-25T00:20:53.793" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18335" PostTypeId="1" AcceptedAnswerId="18345" CreationDate="2016-09-24T22:26:20.503" Score="4" ViewCount="281" Body="&lt;p&gt;We know that oxygen is the &quot;king&quot; of oxidizers:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;non-toxic (unlike fluorine)&lt;/li&gt;&#xA;&lt;li&gt;cheap&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;only&lt;/em&gt; an oxidizer (potassium nitrate, for example, comes with K and N which aren't what we want)&lt;/li&gt;&#xA;&lt;li&gt;no need for catalysis ($H_2O_2$)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But with all that being said, we still have to contain it inside a very cold tank (90K) or a very pressurized one (so very heavy&amp;hellip;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With all that in mind, would it be a good idea to use nitrogen dioxide ($NO_2$) and/or dinitrogen tetroxide($N_2O_4$) to stabilize the $O_2$? (I mean &lt;em&gt;raising its boiling point.&lt;/em&gt;)&#xA;It would reduce the need for cooling machines (not cheap) while letting the mix be a good oxidizer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT 1 : As Organic Marble pointed out it seems that $N_2O_4$ could replace $NO_2$ in the question. But because $NO_2$ and $N_2O_4$ are always in chemical equilibrium, filling a tank with one of them is, I guess, filling it with both of them ?&#xA;&lt;br&gt;&lt;br&gt;&#xA;And if any of you think of another good oxidizer that would do the trick, feel free to share the idea! &lt;/p&gt;&#xA;" OwnerUserId="17048" LastEditorUserId="4660" LastEditDate="2016-09-25T20:36:18.303" LastActivityDate="2016-09-26T07:28:16.690" Title="Would LOX+$NO_2$ or LOX+$N_2O_4$ make a good oxidizer mix?" Tags="|oxidizer|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18337" PostTypeId="1" AcceptedAnswerId="18343" CreationDate="2016-09-25T13:43:03.293" Score="6" ViewCount="816" Body="&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is this explanation for the SpaceX 1-Sept-2016 anomaly plausible? Or most of it at least (sans LHe)?&lt;/p&gt;&#xA;&lt;p&gt;In the recent New Atlas article &lt;a href=&quot;http://newatlas.com/spacex-falcon-9-explosion-helium/45594/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX fingers helium as cause of Falcon 9 rocket explosion&lt;/a&gt; a detailed scenario for the September 1st 2016 anomaly (fast fire, explosion) is presented. They say &amp;quot;SpaceX hasn't said exactly how the helium system failed, but the catastrophic nature of the explosion points to a likely scenario.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;I recommend you read it there. I will show a few snippets:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Along with the propellants is the cryogenic helium system, which are insulated gas bottles filled with liquid helium that sit submerged in the LOX to keep them at a frosty −452.2° F (−269° C, 4 K, )...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;NOTE!!:&lt;/strong&gt; The scenario here starts with liquid helium, not high pressure helium gas as is generally discussed here in SXSE. After the helium tanks were moved inside the LOX tanks where the ambient temperature is so much lower - &lt;strong&gt;in fact below 70K for sub-cooled LOX&lt;/strong&gt; - , is there a chance they were then changed to liquid helium tanks instead of gaseous helium tanks?&lt;/p&gt;&#xA;&lt;p&gt;Briefly, they propose a cascade of events where each fluid violently boils when it comes in contact with the next, warmer fluid.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;...the most likely scenario is that one of the helium bottles in the LOX tank suffered a sudden, massive failure, such as bursting open or a valve failing. The helium flashed into gas in the warmer liquid oxygen and the tank suddenly overpressurized.&lt;/p&gt;&#xA;&lt;p&gt;This caused another failure between the LOX tank and the RP-1 section. Because the failure was so fast, the bulkhead most likely ruptured or shifted aside. At this point, the LOX and the RP-1 mixed. The RP-1 that the Falcon 9 uses is super-chilled to 20° F (−7° C, 266° K), but the LOX is cooled to −340° F (−207 °C, 66° K). When they mix, the LOX immediately boiled and the pressure in the tank skyrocketed...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;They continue:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It's likely that either the tank blew from the pressure and the mixture ignited, or it ignited in the tank due to an electrical spark, friction, or some other cause. The rest was a matter of combustion and gravity as the first stage, then the propellants in the satellite payload exploded, and the remaining LOX flooded into underground passages to feed more fires.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2021-03-28T18:21:40.007" LastActivityDate="2021-03-28T18:21:40.007" Title="Is this explanation for the SpaceX 1-Sept-2016 anomaly plausible?" Tags="|spacex|falcon-9|failure|lox|helium|" AnswerCount="1" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="18339" PostTypeId="1" CreationDate="2016-09-25T17:59:03.343" Score="8" ViewCount="4722" Body="&lt;p&gt;I want to build a solid fuelled rocket (I was thinking 3 stage with 2 boosters) that'd launch a  micro-satellite (cubesat size) into space but &lt;strong&gt;first I want to try reach space (100km) with no payload&lt;/strong&gt;. NASA shows the % composition for the fuel they use in their solid fuelled boosters.&lt;a href=&quot;http://www.nasa.gov/returntoflight/system/system_SRB.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/returntoflight/system/system_SRB.html&lt;/a&gt; I think I could use this but use a different binder I've done some looking around and I've found 2 'small' rockets that have reached space &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/bDoh8zQDT38?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; and the GoFast rocket. Judging from this I think my rocket could be around 10-X meters in length and internal radius around 0,3-0,X meters. On Alibaba Ammonium perchlorate was $1-5 per Kg and that is 70% of the fuel mixture. Also from what I understand I just need to reach 11km/s and then even if the rocket Cuts out it'll still enter space as escape velocity has been achieved.&lt;/p&gt;&#xA;" OwnerUserId="17053" LastEditorUserId="17053" LastEditDate="2016-09-28T18:50:45.217" LastActivityDate="2016-09-28T18:50:45.217" Title="Would a 'small' rocket like this reach space?" Tags="|rockets|propulsion|amateur-rocketry|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18340" PostTypeId="2" ParentId="18339" CreationDate="2016-09-25T19:29:48.237" Score="19" Body="&lt;p&gt;First, I suggest watching this tv show: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/BJppeRWSD94?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They set out to put a backyard rocket into space (note: space, not orbit.)  Unfortunately, that turned out not to be feasible, they ended up settling for sending a hybrid rocket up about 10 miles and with a lot of assistance.  The problems that come to mind:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;So far high power solid hobby rocketry is a field that has self-regulated.  If you want anything to do with the current practitioners you have to work within their format--and that requires working your way up in size.&lt;/li&gt;&#xA;&lt;li&gt;FAA permission.  They're not going to be one bit happy with you if you launch something like that without clearance.  Since your bird isn't meaningfully controlled they have to clear the whole airspace your bird could reach.  For high-flying birds you need a remote location far from any airways so clearing it isn't too disruptive.&lt;/li&gt;&#xA;&lt;li&gt;Safety zone.  The usual standard here is 1/4 the max height.  You want to put a rocket over the Karman line, that means a safety zone of more than 13 miles in every direction from the point of liftoff.  Got access to that much land??&lt;/li&gt;&#xA;&lt;li&gt;Note that the safety zone is impossible for the big birds so it's handled differently:  There's somebody watching the flight path of the rocket, if it goes too far off course they push the big red button and the rocket turns into a bunch of little bits.  (For a recent example, the SpaceX CRS-7 mission.  In that case the flight control computer suicided the rocket when it started coming apart.)  Those flight termination systems are made of explosives--and that means an explosives license.&lt;/li&gt;&#xA;&lt;li&gt;Note that even the rocket fuel is considered hazardous material and in those quantities subject to some fairly onerous storage requirements.  You'll have to do all your work on a pretty darn big piece of land to make the regulators happy.  Handle it wrong: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/_1kTAX9uWcw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="276" LastEditorUserId="238" LastEditDate="2016-09-26T04:16:05.367" LastActivityDate="2016-09-26T04:16:05.367" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18341" PostTypeId="1" AcceptedAnswerId="18358" CreationDate="2016-09-26T00:00:09.003" Score="3" ViewCount="621" Body="&lt;p&gt;Is it possible for a launch of a space object originating from Earth to go so badly that it would cause permanent damage to the Solar System or even destroy it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example: a spaceship going off trajectory, ramming into a small planet or satellite, and destroying it or permanently changing its orbit or gravity, therefore changing the flow of light and radiation throughout the entire Solar System.&lt;/p&gt;&#xA;" OwnerUserId="17055" LastEditorUserId="12102" LastEditDate="2017-03-25T01:07:59.520" LastActivityDate="2019-11-11T00:48:22.877" Title="Can a launch of a rocket or ship from Earth go so badly that it damages the Solar System?" Tags="|life|solar-system|astrobiology|travel|" AnswerCount="6" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18343" PostTypeId="2" ParentId="18337" CreationDate="2016-09-26T02:10:52.310" Score="14" Body="&lt;p&gt;Edit Jan 2, 2017: Well, it was a COPV bottle after all. There were buckles in some liners where super-cooled liquid oxygen pooled. From &lt;a href=&quot;https://www.nasaspaceflight.com/2017/01/spacex-return-rtf-falcon-9-iridium-spacecraft/&quot; rel=&quot;noreferrer&quot;&gt;NASAspaceflight&lt;/a&gt; article, quoting the results of the investigation:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each stage of Falcon 9 uses COPVs to store cold helium which is used&#xA;  to maintain tank pressure, and each COPV consists of an aluminum inner&#xA;  liner with a carbon overwrap. The recovered COPVs showed buckles in&#xA;  their liners. Although buckles were not shown to burst a COPV on their&#xA;  own, investigators concluded that super chilled LOX can pool in these&#xA;  buckles under the overwrap.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;When pressurized, oxygen pooled in this buckle can become trapped; in&#xA;  turn, breaking fibers or friction can ignite the oxygen in the&#xA;  overwrap, causing the COPV to fail. In addition, investigators&#xA;  determined that the loading temperature of the helium was cold enough&#xA;  to create solid oxygen (SOX), which exacerbates the possibility of&#xA;  oxygen becoming trapped as well as the likelihood of friction&#xA;  ignition.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, what follows has useful points, but leaned too much on a statement from another NSF article that sources were extremely doubtful the COPVs were the cause. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If by 'plausible' you mean that an exploding helium bottle could have caused an explosion like the one we saw, yes. However, the article's assertion that 'it is the &lt;em&gt;most likely&lt;/em&gt; scenario' is not plausible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is always risky trying to get an accurate analysis of a very complex subject from a news source that isn't dedicated to that area. Gigantic explosions always get a lot of coverage, and a lot of news outlets have covered this that hardly ever mention space industry events. New Atlas reports on these kinds of things regularly, but there are several inaccuracies in the article. In response to the same update from SpaceX on the investigation, two of the top magazines dedicated to the space industry published long articles: &lt;a href=&quot;https://www.nasaspaceflight.com/2016/09/spacex-press-amos-6-failure-root-cause/&quot; rel=&quot;noreferrer&quot;&gt;NASA Space Flight&lt;/a&gt; and &lt;a href=&quot;http://spaceflight101.com/spacex-falcon-9-explosion-narrowed-to-stage-2-helium/&quot; rel=&quot;noreferrer&quot;&gt;Space Flight 101&lt;/a&gt;. These articles use a bit of lingo so are somewhat harder for the general public to follow. Some googling might be needed, but not so much as to be onerous. Most of what follows is taken from them, with a few additional sources which are listed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is what &lt;a href=&quot;http://www.spacex.com/news/2016/09/01/anomaly-updates&quot; rel=&quot;noreferrer&quot;&gt;SpaceX's statement&lt;/a&gt; says: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At this stage of the investigation, preliminary review of the data and&#xA;  debris suggests that a large breach in the cryogenic helium system of&#xA;  the second stage liquid oxygen tank took place. [Updated 09/24: At&#xA;  this time, the cause of the potential breach remains unknown.].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are lines and connectors leading to the helium bottles, so a breach doesn't mean it was in the bottle. Also, because the bottles were being filled at the time, a breach doesn't mean the a piece &lt;em&gt;failed&lt;/em&gt;, per se, in the sense that, if the problem was (for instance) that a pump in the ground equipment failed, it could have caused pressure waves to travel down the lines and into the bottles, pushing them beyond their design limits. In that case it isn't the 'fault' of the lines or the bottles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Certainly because the helium bottles hold so much pressure, if one failed suddenly it could well look like the explosion that happened. The problem is, why would it do that. This is an important point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pressure vessels that hold the helium, the &lt;a href=&quot;https://ston.jsc.nasa.gov/collections/trs/_techrep/SP-2011-573.pdf&quot; rel=&quot;noreferrer&quot;&gt;COPVs&lt;/a&gt;, have had problems in the past. They are spheres made of Kevlar embedded in a polymer, with a plastic or metal liner, that are filled with very cold helium gas up to a pressure of about 5500 psi. &lt;a href=&quot;http://www.spacex.com/news/2015/07/20/crs-7-investigation-update&quot; rel=&quot;noreferrer&quot;&gt;Failure of a strut&lt;/a&gt; holding one caused the loss of the &lt;a href=&quot;https://youtu.be/OAX7UFd70M8?t=165&quot; rel=&quot;noreferrer&quot;&gt;CRS-7 vehicle&lt;/a&gt; when the COPV was thus breached. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That failure was during flight, when the structure is subjected to a lot of vibration and high acceleration. The helium vessel did not explode as a result, though it was at full pressure.  In the AMOS-6 case, the COPVs had only been partially filled, and were not under any of the stresses that occur during launch. The NASA Space Flight article, says 'sources note they are extremely skeptical a COPV could be at fault, due to the amount of focus placed on them after the CRS-7 failure' - which is to say, because they'd lost a vehicle due to a breach in a COPV, they worked to make sure that wouldn't happen again. So, such a helium bottle exploding when it isn't even full, is under no extra g-force, and there is no vibration, would be very hard to explain. Something coming from elsewhere might have subjected such a bottle to excessive forces that led to a rupture in it (or in a line leading to it) but then the issue is that something, not the bottle itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the  CRS-7 flight, there was 0.9 seconds between the first indication of a problem and the loss of telemetry. In the AMOS-6 case, there was only 0.09 seconds between signs of a problem and the loss of telemetry. COPVs hold so much pressure that a massive rupture is like a small bomb going off, so it is awfully tempting to focus on them (and I have speculated a lot about it myself). However, there is a lot at stake in this investigation, and the FAA, NASA, and the Air Force are taking a much more active hand in it than they did last time. If the sources used by NASA Space Flight - and they get the best sources as the magazine is highly respected - say they are extremely skeptical a COPV caused this, then I am too. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2017-01-02T15:52:31.597" LastActivityDate="2017-01-02T15:52:31.597" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18344" PostTypeId="2" ParentId="18341" CreationDate="2016-09-26T05:19:19.713" Score="10" Body="&lt;p&gt;Not in terms of physical damage, no. Rockets and spaceships and all the energy they contain is just so very, very, very, very small compared to everything.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one possibility for large scale damage though, even if it is very unlikely. There is a very small chance that some kind of virus or bacteria carried on a vehicle to some destination might just happen to survive and spread when it gets there, and if so, that could infect, pollute or otherwise damage where it landed. If you were really unlucky, over a very long time such an infection could spread to cover the entire body it landed on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although it is very unlikely, NASA do take this quite seriously, to the point of sterilizing all spacecraft components and assembling them in an ultra clean room, as well as calculating the probability that organic life survives that process, and survives the flight time to the destination, to make sure that the chance of contaminating the destination is sufficiently low. The &lt;a href=&quot;https://planetaryprotection.arc.nasa.gov/about&quot; rel=&quot;noreferrer&quot;&gt;Office of Planetary Protection&lt;/a&gt; is responsible for this.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="6816" LastEditDate="2016-09-26T16:59:53.893" LastActivityDate="2016-09-26T16:59:53.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18345" PostTypeId="2" ParentId="18335" CreationDate="2016-09-26T07:28:16.690" Score="3" Body="&lt;p&gt;If you want to mix LOX with other gases, those gases should be liquid at the temperature of LOX. But NO2 is solid at only -11.2 °C and N2O4 at -11 °C. The turbo pumps might be destroyed by a mixture of a liquid with solids and the valves and injectors for the oxidizer might be blocked. I doubt that a liquid mixture of O2, NO2 and N2O4 would contain more than very little O2.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-26T07:28:16.690" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18346" PostTypeId="2" ParentId="18341" CreationDate="2016-09-26T07:32:28.063" Score="22" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at the numbers:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                          10000 kg. =&amp;gt; Your rocket &#xA;        73420000000000000000000 kg. =&amp;gt; The moon &#xA;      5972200000000000000000000 kg. =&amp;gt; The earth &#xA;1989000000000000000000000000000 kg. =&amp;gt; The sun&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;To put this to scale, the rocket has a similar mass towards the moon as that of one human cell towards a human.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can throw it as fast as you want, it won't have any effect.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="4660" LastEditDate="2016-09-27T14:34:20.177" LastActivityDate="2016-09-27T14:34:20.177" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18347" PostTypeId="1" AcceptedAnswerId="18350" CreationDate="2016-09-26T11:07:50.253" Score="12" ViewCount="1460" Body="&lt;p&gt;I've got an email, that because of the increased solar activity, more cosmic ray-s are hitting the earth. And for this reason, humans experience more stress and headache.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know, it's not true. But I can not disprove it. Is it possible to check on a website, if there is an increased solar activity, or more cosmic rays are hitting the Earth, as usual?&lt;/p&gt;&#xA;" OwnerUserId="8873" LastActivityDate="2016-09-27T01:07:46.973" Title="How to check, if there is currently an increased solar activity?" Tags="|cosmic-radiation|space-weather|solar-wind|the-sun|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18348" PostTypeId="2" ParentId="18332" CreationDate="2016-09-26T11:13:37.503" Score="-1" Body="&lt;p&gt;Lets take a rubber boat as an example. The tubes of the boat must be filled with air to let the boat float and carry some load. But to give the boat stability in the waves, a certain amount of pressure is needed. To much pressure inside the tubes of the boat will destroy them. The pressure has to be kept between certain limits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel tank of a rocket has to be as light as possible, a certain pressure inside will make the tank structure stiff against bending forces. Again the pressure has to be kept within the limits, too low and the tanks may be destroyed when bended (because they are made from metall, not from rubber), too high and the pressure itself destroys the tank. The pressure limits must be observed when the rocket stands at the launch with empty tanks, during the fuel loading, while the rocket is waiting for ignition and LOX and LH2 boils off, when the fuel is pumped into the combustion chamber and also when the air pressure outside decreases from sea level to vacuum. It is not the absolute pressure inside the tank that has to be maintained, it is the relative pressure to the outside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A tank which is stable when the pressure inside is equal to the pressure outside may be build too, but it will be heavier and thus the velocity at engine shutdown will be lower. The tubes of the rubber boat demonstrate what additional stiffness is possible when inside pressure is above the lower limit.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-09-26T11:13:37.503" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18349" PostTypeId="2" ParentId="18332" CreationDate="2016-09-26T12:31:34.540" Score="16" Body="&lt;p&gt;Pressure stabilization is used in some rockets, and to varying degrees.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=16213.0&quot;&gt;Atlas&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_%28rocket_stage%29&quot;&gt;Centaur&lt;/a&gt; use 'full-scale' pressure stabilization. The tank walls were so thin, &lt;a href=&quot;http://ntrs.nasa.gov/search.jsp?R=20040084080&quot;&gt;an unpressurized stage would collapse under its own weight&lt;/a&gt; (huge PDF). The stage had to be pressurized (or kept in a support jig) at all times.  &lt;/li&gt;&#xA;&lt;li&gt;the Falcon 9 uses &lt;a href=&quot;https://aerospaceblog.files.wordpress.com/2010/03/spacex-brochure.pdf&quot;&gt;flight pressure stabilization&lt;/a&gt;. The tank walls are thick enough that a stage can bear its own weight, and does not need pressurization during manufacturing or transport. It does need pressurization in flight, to withstand the flight loads.  &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://books.google.nl/books?id=0iV3KTLaAKYC&amp;amp;pg=PT169&amp;amp;lpg=PT169&amp;amp;dq=%22saturn+v%22+%22self+supporting%22+tank+structure&amp;amp;source=bl&amp;amp;ots=KEEGny08W7&amp;amp;sig=3SCUywHaOkJXYA_tWvR1k_QeGcg&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwipxrfBhK3PAhWKCMAKHdUuAxIQ6AEIHjAA#v=onepage&amp;amp;q=%22saturn%20v%22%20%22self%20supporting%22%20tank%20structure&amp;amp;f=false&quot;&gt;Saturn V&lt;/a&gt; used no pressure stabilization. The stage structure is strong enough to withstand flight loads on its own.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-09-26T12:45:14.303" LastActivityDate="2016-09-26T12:45:14.303" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18350" PostTypeId="2" ParentId="18347" CreationDate="2016-09-26T13:08:49.293" Score="31" Body="&lt;p&gt;Looks pretty darned quiet to me right now:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fvyI1.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fvyI1.gif&quot; alt=&quot;low level of proton events&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find that &lt;a href=&quot;http://www.swpc.noaa.gov/products/goes-proton-flux&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, along with other measures of space weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, cosmic rays and solar activity are two entirely different things. Cosmic rays originate from outside of our solar system. The flux of cosmic rays is relatively constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for your question about stress, indeed, I just experienced some added stress because people propagate stupid emails like that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be fair, I did in fact experience stress and headaches due to solar activity back in 2003. Right around Halloween it looked like this (note that these are log plots):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dgiGy.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dgiGy.gif&quot; alt=&quot;off-the-chart level of proton events&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At that time I was the mission manager responsible for MER-A, also known as Spirit, in cruise on its way to Mars. This was the largest solar event that had ever been seen at that time, and so it wouldn't be MER if we hadn't been hit by it. As you can see, the event was, quite literally, off the chart. Both MER-A and MER-B were disabled by the event, losing their ability to track stars, and therefore unable to do turns or conduct maneuvers. Fortunately they were spinning spacecraft and were able to maintain attitude while they weathered the storm. In the meantime, we on the ground had no idea how long the storm would last, whether the star scanners would ever work again, and whether we would be able to get those spacecraft to their precise entry points at Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, solar activity can cause stress and headaches, if you happen to be a spacecraft operator, or you live in space. Or possibly, if you happen to operate electronic communication systems on the ground that are susceptible to space weather.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-09-27T01:07:46.973" LastActivityDate="2016-09-27T01:07:46.973" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="18352" PostTypeId="2" ParentId="18323" CreationDate="2016-09-26T15:42:25.533" Score="6" Body="&lt;p&gt;While I don't have a source document I can link, from talking with folks in the JSC suit lab, I have learned that in general, the EMU (ISS/shuttle space suit) cooling system is overpowered to a small degree, to the point that crew can become uncomfortably cold while working.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The suit does have a valve that allows crew to adjust the cooling rate.  It's labeled on the right side of the picture below.  Interestingly, most astronauts tend not to use that valve for the fine-grained control for which it was designed -- they just tend to flip it all the way to &quot;C&quot; when they get too hot and all the way to &quot;H&quot; when they get too cold. (again, no documented source -- just from conversations I've had)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/K4v4c.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/K4v4c.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2016-09-26T15:42:25.533" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18353" PostTypeId="2" ParentId="18341" CreationDate="2016-09-26T16:47:56.047" Score="1" Body="&lt;p&gt;One idea that came to mind is near-earth asteroids.  There have been discussions on how to nudge one away from earth, but if you wanted to switch to evil genius mode, you could nudge it towards earth - Wouldn't destroy the solar system, but would make for a bad day/year/millennium/... for earth.&lt;/p&gt;&#xA;" OwnerUserId="17059" LastActivityDate="2016-09-26T16:47:56.047" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18354" PostTypeId="1" AcceptedAnswerId="26705" CreationDate="2016-09-26T17:00:29.373" Score="0" ViewCount="741" Body="&lt;p&gt;I am wondering if the &quot;cryogenic helium&quot; system discussed in articles about the 1-Sept-2016 SpaceX Falcon 9 anomaly (see &lt;a href=&quot;https://space.stackexchange.com/a/18343/12102&quot;&gt;this answer&lt;/a&gt; for a good summary to date) is not actually related to the high pressure gaseous helium in the COPVs &lt;a href=&quot;https://space.stackexchange.com/q/9847/12102&quot;&gt;which were moved inside of the LOX tanks&lt;/a&gt;, but instead this is actually for liquid helium used in more standard cryogenic applications, to either maintain the sub-cooled LOX well below its boiling point, or for fast pumping speed &lt;a href=&quot;https://en.wikipedia.org/wiki/Cold_trap&quot; rel=&quot;nofollow noreferrer&quot;&gt;cold trapping&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Cryopump&quot; rel=&quot;nofollow noreferrer&quot;&gt;cryopumping&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at a typical Falcon 9 countdown timeline &lt;a href=&quot;http://spaceflight101.com/falcon-9-ft-countdown-timeline/&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt; as linked in &lt;a href=&quot;https://space.stackexchange.com/a/18330/12102&quot;&gt;this answer&lt;/a&gt;, I've pasted a selected, discontinuous subset of items below. You should go to the original link for the complete thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This filling near the end seems like a bad idea for an extremely high pressure, heat generating pressurization of the COPVs which will end up swimming in sub-cooled LOX. Instead, this looks like filling and later topping off of cold traps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this Liquid helium for cold trapping and cryopumping? Are there mechanical compressors or is this just dewar-supplied LHe?&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;T-0:33:30   Stage 1 Liquid Oxygen Loading&#xA;&#xA;T-0:29:30   Stage 1 Helium Load&#xA;&#xA;T-0:25:00   All three Cryo Helium Pumps active&#xA;&#xA;T-0:22:00   Stage 2 Fuel Loading Complete&#xA;&#xA;T-0:19:30   Stage 2 Liquid Oxygen Loading&#xA;&#xA;T-0:17:20   Stage 1 LOX Flowrate Adjustment for Stage 2 Fast Fill&#xA;&#xA;T-0:13:15   Stage 2 Helium Loading&#xA;&#xA;T-0:13:00   Stage 2 LOX Flow Adjustment for Helium Cryo Load&#xA;&#xA;T-0:09:15   Stage 1 Helium Topping&#xA;&#xA;T-0:06:45   Stage 2 Helium Transition to Pipeline&#xA;&#xA;T-0:01:25   Helium Loading Termination &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2021-03-28T18:14:37.263" LastActivityDate="2021-03-28T18:14:37.263" Title="Is the &quot;cryogenic helium&quot; system in recent discussions actually LHe for cryopumping?" Tags="|falcon-9|cryogenics|lox|sub-cooled|helium|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18355" PostTypeId="1" AcceptedAnswerId="18357" CreationDate="2016-09-26T17:46:45.327" Score="4" ViewCount="290" Body="&lt;p&gt;During India's PSLV-C35 launch on 26 September 2016, mission control's launch sequence events (stage separations, stage ignitions, etc.) included 'CLG INIT'. What does it mean?&lt;/p&gt;&#xA;" OwnerUserId="3082" LastActivityDate="2016-09-26T18:57:53.877" Title="What does CLG INIT mean?" Tags="|isro|launch-sequence|pslv|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18356" PostTypeId="2" ParentId="18341" CreationDate="2016-09-26T17:56:33.570" Score="2" Body="&lt;p&gt;Let's assume you have a rocket made of anti-matter. Let's give it a 100 metric ton rocket of the stuff. $e=m  c^2$, and with anti-matter, we are converting a mass twice the size of the rocket to pure energy. Let's just say that they have answered that question on &lt;a href=&quot;https://worldbuilding.stackexchange.com/a/13660/244&quot;&gt;World Building&lt;/a&gt;, you are quite a bit short of the limit of $10^{15}kg$ required to completely destroy the Earth alone, let alone the entire Solar System...&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:23.630" LastActivityDate="2016-09-26T17:56:33.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18357" PostTypeId="2" ParentId="18355" CreationDate="2016-09-26T18:12:23.220" Score="8" Body="&lt;p&gt;&quot;CLG INIT&quot; stands for &quot;closed loop guidance initiation&quot;, and means that the launcher is switching into a mode where it actively steers based on its current trajectory rather than a fixed, unconditional program (referred to as &quot;open loop guidance&quot;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the early part of flight, it is safer to use a &quot;pitch-versus-time&quot; or similar simple logic to guide the rocket. Weather events or errors in the measured trajectory might otherwise cause undesirable over-reactions in steering. Once out of the dense atmosphere, the launcher can correct for any difference between the desired trajectory and the actual one; the repeated process of measurement, error determination, and steering to correct the error is the &quot;loop&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For PSLV, the CLG INIT occurs a few seconds after second stage ignition. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-09-26T18:57:53.877" LastActivityDate="2016-09-26T18:57:53.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18358" PostTypeId="2" ParentId="18341" CreationDate="2016-09-26T18:30:56.773" Score="16" Body="&lt;p&gt;First, some perspective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The impact of &lt;em&gt;a single fragment&lt;/em&gt; of the  &lt;a href=&quot;https://en.wikipedia.org/wiki/Comet_Shoemaker%E2%80%93Levy_9&quot; rel=&quot;noreferrer&quot;&gt;Shoemaker-Levy comet&lt;/a&gt; on Jupiter released an estimated energy equivalent of &lt;strong&gt;six million &lt;em&gt;megatons&lt;/em&gt;&lt;/strong&gt; of TNT (approximately 600 times the size of the world's nuclear arsenal), leaving an impact scar that was visible for several months afterward.  That was obviously a bad day for Jupiter, but that impact had little to no effect on anything else, including Jupiter's moons.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We simply cannot build anything that could deliver the kind of energy necessary to cause any kind of damage to anything bigger than a small asteroid (where &quot;small&quot; means on the order of a dozen meters across).  We &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Impact_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;smacked a hammer into a comet&lt;/a&gt; which excavated a lot of material, but didn't significantly alter the comet's path.  We could possibly knock a larger asteroid on a collision course with another planet, but that wouldn't be an &lt;em&gt;accident&lt;/em&gt; - it would require planning and a non-trivial amount of energy.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Objects in the solar system are so massive and so far apart that nothing we could launch would have much of an effect.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2016-09-26T18:30:56.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18359" PostTypeId="2" ParentId="18309" CreationDate="2016-09-27T04:46:11.393" Score="2" Body="&lt;p&gt;If a reaction wheel is spinning at maximum speed and you turn off the motor it will slowly decelerate because of friction. Potentially, depending on how it's designed, it might regeneratively brake as well, which will convert the stored momentum back into electricity. Because of the conservation of angular momentum, as the wheel slows down, the satellite will experience angular acceleration in the opposite direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The purpose of torque rods is not really to de-spin wheels that have spun up as the result of a maneuver. Torque rods are used to desaturate the momentum that builds up in a spacecraft over time due to external disturbance torques caused by effects such as atmospheric drag, gravity gradients, and solar pressure. These torques are small, but integrate up and if left unchecked will eventually require wheels to be spun fully up, which makes the wheels useless for maneuvering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are lots of good papers on the canonical control laws for torque rod control. A common algorithm goes by the name B-dot, because the commanded dipole is proportional to the time-derivative change in the local Earth-generated magnetic field. Here's &lt;a href=&quot;http://paint-chemicals.com/images/prd_1305-78-8_1924750492/763916775A%20B-Dot%20Acquisition%20Controller%20for%20the%20RADARSAT%20Spacecraft.pdf&quot; rel=&quot;nofollow&quot;&gt;one example&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2016-09-27T04:46:11.393" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18360" PostTypeId="2" ParentId="18304" CreationDate="2016-09-27T04:53:12.097" Score="0" Body="&lt;p&gt;I'm not aware of any studies on different comms protocols in use and which ones are most common. I did find this page on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cubesat_Space_Protocol&quot; rel=&quot;nofollow&quot;&gt;Cubesat Space Protocol&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally about space comm protocols, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Consultative_Committee_for_Space_Data_Systems&quot; rel=&quot;nofollow&quot;&gt;CCSDS&lt;/a&gt; publishes a wide set of recommendations and reference implementations at various layers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I was starting from scratch I'd probably first look into what radios were available and if their designs presume anything about comm protocols. Many include packet formats, forward error correction codes, and other details that it will be a pain to work around. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're going to use an existing ground station you should also ask them for an ICD defining what protocols they already use. &lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2016-09-27T04:53:12.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18361" PostTypeId="2" ParentId="18300" CreationDate="2016-09-27T15:39:20.337" Score="17" Body="&lt;p&gt;There is an international network of observers of classified satellites, organized around the Seesat-L mailing list:&#xA;&lt;a href=&quot;http://www.satobs.org/seesat/seesatindex.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.satobs.org/seesat/seesatindex.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They typically look for satellite passes using binoculars and a stopwatch, or using a camera. Then they fit TLEs to those observation, to be able to find the satellite on a later pass. You can get those TLEs on Mike McCants' site:&#xA;&lt;a href=&quot;https://www.prismnet.com/~mmccants/tles/&quot; rel=&quot;noreferrer&quot;&gt;https://www.prismnet.com/~mmccants/tles/&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a little bit on background on them, here is an article on one of the members, Ted Molczan: &lt;a href=&quot;https://www.wired.com/2006/02/spy-3/&quot; rel=&quot;noreferrer&quot;&gt;https://www.wired.com/2006/02/spy-3/&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the website of Marco Langbroek, who tracks geostationary satellites using telescopes: &lt;a href=&quot;http://sattrackcam.blogspot.com/?_sm_au_=i2VrV5PNT5tRFWRP&quot; rel=&quot;noreferrer&quot;&gt;http://sattrackcam.blogspot.com/?_sm_au_=i2VrV5PNT5tRFWRP&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="3065" LastEditDate="2016-09-27T16:13:41.977" LastActivityDate="2016-09-27T16:13:41.977" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18363" PostTypeId="1" AcceptedAnswerId="18365" CreationDate="2016-09-27T18:01:19.590" Score="3" ViewCount="2514" Body="&lt;p&gt;In &lt;a href=&quot;https://astronomy.stackexchange.com/a/18498/7982&quot;&gt;this Astronomy SE answer to &lt;em&gt;How can &amp;quot;Geysers&amp;quot; on Europa reach heights of 100km?&lt;/em&gt;&lt;/a&gt; there is discussion of the possible role of pressure at the top of Europa's ocean in the ability of the recently observed vapor plumes to reach 100 to 200 kilometers above the surface. Europa has a surface gravity of about 1.3 &lt;span class=&quot;math-container&quot;&gt;$m/s^2$&lt;/span&gt; and it would take a velocity of 500 &lt;span class=&quot;math-container&quot;&gt;$m/s$&lt;/span&gt; to rise ballistically to 100 kilometers for example.&lt;/p&gt;&#xA;&lt;p&gt;In this question I am just asking if there have been any determinations, either inferred from observations or from simulations, of the pressure of the water at the top of the ocean where it meets the ice. Something quantitative? A number, even roughly?&lt;/p&gt;&#xA;&lt;p&gt;If it's highly variable with time due to tidal effects, then it's the highest pressure that's more interesting.&lt;/p&gt;&#xA;&lt;p&gt;A first guess might be that the pressure is what you'd get by calculating the weight per unit area of the ice above. In that case, if there is a crack, the water would rise approximately to the surface of the ice and the pressure would drop to approximately zero. What I am after is if there is ever any substantial pressure beyond this - does the pressure ever deviate so much higher than that, that it would become important in the discussion of cryovolcanos, geysers, and jets?&lt;/p&gt;&#xA;&lt;p&gt;But here I'm just asking about determinations of pressure.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pi2p6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pi2p6.png&quot; alt=&quot;Diagram of Europa's Ice surface and subsurface ocean, from the JPL News feature &amp;quot;Scientists Find Evidence of 'Diving' Tectonic Plates on Europa&amp;quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Diagram of Europa's Ice surface and subsurface ocean, from the JPL News feature &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=4285&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scientists Find Evidence of 'Diving' Tectonic Plates on Europa&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2022-02-01T06:00:51.350" LastActivityDate="2022-02-01T06:00:51.350" Title="Have there been any determinations of the water pressure in Europa's ocean just below the ice?" Tags="|water|europa|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="18365" PostTypeId="2" ParentId="18363" CreationDate="2016-09-27T20:34:41.090" Score="7" Body="&lt;p&gt;It is easy to calculate. The ice is reported to be 15 to 25 km deep. Simply take the weight of the ice in Europa's gravity over one square meter of the top of the ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.wolframalpha.com/input/?i=europa%20surface%20gravity%20*%20density%20of%20ice%20*%2020%20km%20in%20pascals&quot; rel=&quot;noreferrer&quot;&gt;Calculation in Wolfram Alpha&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You get 24 MPa, or about 240 atmospheres for 20 km of ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However that ice sitting on the ocean is in equilibrium, just as ice is floating in a glass of water, and so that pressure cannot be a source of energy for cryo-volcanism. You need some other energy input, which in this case would be Jupiter tidal forces.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-09-27T20:34:41.090" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18366" PostTypeId="2" ParentId="18339" CreationDate="2016-09-27T21:18:07.447" Score="6" Body="&lt;p&gt;Since you added that drawing I am adding this answer to specifically address your drawing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No.  It will not reach space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that there is nowhere near enough information in your drawing to reach this conclusion, I am saying that anyone displaying the level of ignorance of the subject that you have has zero chance of reaching space no matter what the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You seem to be showing two strap-ons of approx 300mm x 10m and a central booster of 300mm x 13m.  The nose cone length isn't really relevant here.  However, there is nothing to indicate the thickness of the wall.  You also need to know the weight of the whole thing as well as the specific impulse of whatever you are using as fuel.  Likewise, there's nothing to indicate the thrust of the rocket.  (The higher the thrust the less gravity loss there will be but the more drag loss.  Also, the higher the thrust the thicker the walls must be and thus the heavier the rocket.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I note no means of steering this rocket.  When you light this it's not going to straight up.  A simple test to demonstrate what your rocket is going to do:  Take a normal hobby rocket motor, make a nose cone for it out of paper and glue it to the engine.  Make a little cardboard stand to hold it upright.  Take this out into empty desert terrain (what I am describing is a major fire hazard, you need to ensure there's nothing around that can possibly burn) and bring along a few hundred feet of two-strand electric wire that has alligator clips on the ends.  Insert the igniter, clip the leads to it and back off the length of the wire.  A normal automotive battery should be able to provide the firing current through a wire of that length.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-09-27T21:18:07.447" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18367" PostTypeId="1" CreationDate="2016-09-28T01:17:18.707" Score="2" ViewCount="187" Body="&lt;p&gt;NASA has issued reports to the effect that small bits of Mars have come to Earth due to asteroid impacts - asteroid hits Mars, chunks are ejected into space far enough to eventually fall into Earth's gravity well. The reverse is also plausible - material from Earth could have been transported to Mars the same way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, my question is: how far could such transport reach through the solar system? Could material ejected from an early Earth have made it as far out as Jupiter? What about material from Mars?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-09-28T10:31:06.107" Title="How far could material from Earth or Mars be transported due to asteroid impact?" Tags="|asteroid|earth|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18368" PostTypeId="1" AcceptedAnswerId="23963" CreationDate="2016-09-28T02:15:08.493" Score="2" ViewCount="329" Body="&lt;p&gt;I know some older space stations had fax machine (since Sayult 7 at least).&#xA;Do they still have it on the ISS ? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does is support in/out or only one way communications ? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-12-10T00:00:48.273" Title="Is there a fax machine on the ISS ?" Tags="|iss|communication|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18369" PostTypeId="1" AcceptedAnswerId="18385" CreationDate="2016-09-28T02:45:54.367" Score="16" ViewCount="633" Body="&lt;p&gt;If there are cities on Mars in the second half of this century with say a million people, they will be living in engineered structures and sustained by agriculture in protected, controlled environments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there was an asteroid impact on Mars with the mass and velocity that would classify it as an &lt;a href=&quot;https://en.wikipedia.org/wiki/Extinction_Level_Event&quot;&gt;Extinction Level Event&lt;/a&gt; if it had hit Earth, would they also become extinct?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While Mars is slightly smaller than Earth, it's a lot closer to the asteroid belt, so these kinds of events are just as relevant to &quot;them&quot; as they are to &quot;us&quot;. Even more so perhaps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume the impact is not necessarily a direct hit on the city itself, or that they are widely spaced enough that at least some are far from the impact.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-09-28T11:45:52.143" LastActivityDate="2016-10-07T11:33:49.097" Title="Would the one million people on Mars be killed by an impact equivalent to an Extinction Level Event on Earth" Tags="|mars|colonization|asteroid|impact|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18371" PostTypeId="1" AcceptedAnswerId="27250" CreationDate="2016-09-28T04:20:03.997" Score="7" ViewCount="414" Body="&lt;p&gt;One of my favorite answers in SXSE is &lt;a href=&quot;https://space.stackexchange.com/a/6632/12102&quot;&gt;this one&lt;/a&gt; which addresses subtler effects of low gravity on sports that one might not anticipate. There are other answers there, and other questions and answers about swimming here as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/q/897/12102&quot;&gt;question about jumping&lt;/a&gt; was recently re-opened and I think it's a good question, and the &lt;a href=&quot;https://space.stackexchange.com/a/901/12102&quot;&gt;answer&lt;/a&gt; there almost qualifies as a mini scholarly work in itself. The human &lt;a href=&quot;https://en.wikipedia.org/wiki/Human_musculoskeletal_system&quot; rel=&quot;noreferrer&quot;&gt;musculoskeletal system&lt;/a&gt; is (roughly) optimized for a certain combination of inertial and gravity, and playing sports in a very different gravity might result in some unexpected differences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simple case in point - ISS astronauts and lots of other people as well know that it takes time to learn to carefully and safely move heavy objects in microgravity without injury. &#xA;Weight lifters would have to learn this as well - what new kinds of injuries will result from an overzealous jerk (&lt;a href=&quot;https://en.wikipedia.org/wiki/Clean_and_jerk#Jerk&quot; rel=&quot;noreferrer&quot;&gt;motion&lt;/a&gt;) in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Clean_and_jerk&quot; rel=&quot;noreferrer&quot;&gt;clean and jerk&lt;/a&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, maybe the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shot_put&quot; rel=&quot;noreferrer&quot;&gt;shot put&lt;/a&gt; would not need quite as much interplanetary &lt;a href=&quot;https://en.wikipedia.org/wiki/Cross-training&quot; rel=&quot;noreferrer&quot;&gt;cross training&lt;/a&gt; as it is more about inertia and &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;noreferrer&quot;&gt;Newton's Laws of Motion&lt;/a&gt; than it is gravity - from the point of view of the mechanics of the athlete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And of course there are questions of bone loss in low gravity, but there is not much known about long term effects in reduced gravity yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So I'd like to know:&lt;/strong&gt; has there been any scholarly or serious work in &lt;a href=&quot;https://en.wikipedia.org/wiki/Sports_science&quot; rel=&quot;noreferrer&quot;&gt;Sports Science&lt;/a&gt; for the low surface gravity of Mars or the Moon?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I almost changed this question to &quot;how would the &lt;a href=&quot;https://en.wikipedia.org/wiki/Clean_and_jerk&quot; rel=&quot;noreferrer&quot;&gt;clean and jerk&lt;/a&gt; have to be done differently on Mars&quot;. It's a really good question as well and not so simple. If you have a good quantitative answer, you could ask and answer yourself, or someone could just ask it! But it deserves a thorough treatment.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-05-10T07:19:29.163" LastActivityDate="2018-05-14T07:47:12.760" Title="Any scholarly or serious work in Sports Science for the low surface gravity of Mars or the Moon?" Tags="|mars|colonization|physical-activity|reduced-gravity-sports|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18372" PostTypeId="2" ParentId="6607" CreationDate="2016-09-28T05:55:45.507" Score="2" Body="&lt;p&gt;Wave height will be bigger, right? And of course, droplets and splashes will take longer to fall. As your head leaves the water, surface tension will try to hold water blobs onto your face, which could also interfere with breathing. You might need to adapt your swimming style somewhat to avoid these effects, like shaking your head in a side to side motion and timing your air intake differently.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-28T05:55:45.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18374" PostTypeId="1" AcceptedAnswerId="18391" CreationDate="2016-09-28T06:30:09.457" Score="8" ViewCount="797" Body="&lt;p&gt;The cool plot in &lt;a href=&quot;https://space.stackexchange.com/a/17832/12102&quot;&gt;this answer&lt;/a&gt; (from &lt;a href=&quot;https://en.wikipedia.org/wiki/Ice#/media/File:Phase_diagram_of_water.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;) got me thinking about the more than sixteen distinct forms of ice, and how many of them have been actually seen to occur naturally in the solar system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are wide ranges of temperature and pressure present in the solar system and there seems to be a lot of agreement that water should be present in many of these, most frequently as ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you include amorphous ice, there may be two or three forms of ice observed to occur naturally right here on (or directly above) earth - if you include the atmosphere and even though &lt;a href=&quot;https://en.wikipedia.org/wiki/Noctilucent_cloud&quot; rel=&quot;nofollow noreferrer&quot;&gt;noctilucent clouds&lt;/a&gt; seem to suddenly be a lot more frequent now in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Anthropocene&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anthropocene&lt;/a&gt; you still consider them 'natural' in as much as any effect on the Earth due to life processes is natural (e.g. the appearance of atmospheric oxygen).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So just for example, are there measurements that can look at comets and say &quot;yep, that's probably Ice $n$!&quot; (where $n$ is the kind of ice)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do different ices have any electromagnetic signatures - visible, infrared, microwave - that allows one to deduce the type of ice that might be there? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or other physical differences that could be detected in some way?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Future prospects:&lt;/strong&gt; (or prospecting)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-REx&lt;/a&gt; will return a bit of a comet to Earth, although by the time it collects its sample the surface of Benu will probably be warm enough that any water ice that might be present on or just below the surface somehow will be hexagonal Ice I, and of course the sample's toasty return to Earth via re-entry has a maximum sample temperature &lt;a href=&quot;https://space.stackexchange.com/a/18110/12102&quot;&gt;specced at +75 °C&lt;/a&gt;, this discussion of &lt;a href=&quot;https://en.wikipedia.org/wiki/Ice#Phases&quot; rel=&quot;nofollow noreferrer&quot;&gt;water ice phases&lt;/a&gt; says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Subjected to higher pressures and varying temperatures, ice can form in 16 separate known phases. With care, all these phases except ice X can be recovered at ambient pressure and low temperature in metastable form.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;so maybe some day, we'll see different forms of ice returned to Earth in specially designed capsules, or even see them first hand on an exploration mission.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2022-06-05T00:35:33.243" Title="What forms of water ice have been observed and verified in the solar system?" Tags="|water|comet|measurement|sample-return|ice|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18375" PostTypeId="1" CreationDate="2016-09-28T06:56:39.550" Score="-6" ViewCount="368" Body="&lt;p&gt;In this sketch there is a gun system as a propellant chamber and we are using &lt;em&gt;iron balls&lt;/em&gt; as a propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An electromagnet is attached on the other side of this engine .When we fire these   Iron balls  with the help of this gun system then these balls will move towards forward side and a back force will be create to propel  this engine .  These iron balls will be piled up on the surface of this electromagnet and we can again reuse these iron balls and send back these iron balls into the propellant chamber or refilling the Gun system again and again with the help of a mechanical system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this way this engine will work  and  the propellant (iron balls) will never be reduced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a question that suppose if the iron balls could reduce its velocity  with itself and hit with electromagnet with low velocity then this engine will move or not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose the iron ball is expelling with 5 meter/second speed from Gun system and is hitting the electromagnet or a wall with 2 meter /second velocity then this device should work or not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore I would like to inform you that we will use ferrofluid (magnetic fluid) to reduce the velocity of these Iron balls. These iron balls will be filled up with this magnetic fluid  and at the time of  expelling this fluid will be in solid state but once expelling from the gun system it will convert into liquid form to reduce the velocity of iron balls. In this way this iron ball will reduce its velocity with itself due to ferro liquid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;We can use plastic balls also filled up with this magnetic fluid and wall of anything than electromagnet that's not an issue.&lt;/li&gt;&#xA;&lt;li&gt;The ball  will be expelled from the gun system having spin velocity and fluid has  very much friction.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Please explain your answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Yvbfb.jpg&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="17081" LastEditorUserId="238" LastEditDate="2016-09-28T07:01:02.777" LastActivityDate="2016-09-29T08:00:46.793" Title="Never reducing propellant for space propulsion" Tags="|propulsion|energy|" AnswerCount="3" CommentCount="6" ClosedDate="2016-09-30T15:08:09.823" ContentLicense="CC BY-SA 3.0" />
  <row Id="18376" PostTypeId="2" ParentId="18375" CreationDate="2016-09-28T07:32:28.603" Score="13" Body="&lt;p&gt;&lt;strong&gt;NO&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a rule of thumb: if you think you invented either a way to generate infinite energy or infinite propellant: you are wrong.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;But [...]&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;No. Always.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rxynL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rxynL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your craft will gain a velocity vector of e.&#xA;To retrieve the bullets you need to apply to them a velocity vector of -e.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do this using a liquid, the liquid will accelerate by e.&#xA;If you do this using magnets, the magnets will accelerate by e.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the magnet or liquid are attached to the main craft you achieved a final velocity change of 0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is not attached to you craft, the propellant is not the bullets, it's the liquid/magnet. Which is now moving away from your craft.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-09-28T07:32:28.603" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18377" PostTypeId="2" ParentId="12626" CreationDate="2016-09-28T08:00:31.503" Score="4" Body="&lt;p&gt;I was just researching this now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A working design basically doesn't exist yet, so it's still a question of &quot;send what up?&quot;&#xA;All the major initial results are far more likely the result of thermal currents, which is obviously the case. Since then, the measurements have mostly been within the range of error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There still appears to be promise, but they're still working on test sensitivity and drive design until they get some results that aren't probably the result of electronic noise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also apparently a bunch of electronics that work on earth don't work on space. Who knew?!&lt;/p&gt;&#xA;" OwnerUserId="17084" LastActivityDate="2016-09-28T08:00:31.503" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18378" PostTypeId="2" ParentId="18328" CreationDate="2016-09-28T08:02:49.987" Score="3" Body="&lt;p&gt;Intriguing idea. I looked briefly at whether Mars' lowest lowlands could contain CFC atmospheres with acceptable pressure at the bottom, plus some oxygen, CO2 and nitrogen for open-air ecosystems. (I was inspired in this partly by &lt;a href=&quot;http://en.wikipedia.org/wiki/The_Space_Trilogy&quot; rel=&quot;nofollow&quot;&gt;C.S. Lewis' Mars&lt;/a&gt;, which posited very deep gorges hosting reasonable atmospheric pressure at their lowest elevations; but also by the geological results from Mars rovers, which suggest lots of chlorine and fluorine minerals in the soil.) It never occurred to me that you could have lakes of a breathable liquid. This might require some modifications on people (perhaps prosthetic) to keep the liquid out of their GI tracts; maybe also some neurosurgery to suppress gag reflexes. It might make more sense for agricultural purposes, with people visiting the submerged fields in SCUBA gear and the like.&lt;/p&gt;&#xA;" OwnerUserId="10144" LastEditorUserId="1142" LastEditDate="2016-09-29T23:11:06.143" LastActivityDate="2016-09-29T23:11:06.143" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18379" PostTypeId="2" ParentId="18367" CreationDate="2016-09-28T09:18:00.303" Score="0" Body="&lt;p&gt;There is no limit in either direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kim has shown that matter can leave the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the opposite direction, towards inner planets, it's a matter of angular momentum with respect to the Sun. Since objects move slower the farther out they are, it's easier for them to &lt;em&gt;lose all their momentum&lt;/em&gt; in a collision. Then the Sun's gravity pulls them inwards nearly &lt;em&gt;radially&lt;/em&gt;. It's slow, but given enough time, such an object crosses all inner planet's orbits and hits the Sun. If it didn't collide with another body. (This assumes the original body was close to the ecliptic plane and the collision didn't change that too drastically.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, there are no conservation laws that prohibit rocks from leaving or entering the Solar System as far as they like. Those laws only restrict the time frame and the velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This opens up the (slim) possibility that we might find some &lt;a href=&quot;https://en.wikipedia.org/wiki/Pluto&quot; rel=&quot;nofollow&quot;&gt;Pluto&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/90377_Sedna&quot; rel=&quot;nofollow&quot;&gt;Sedna&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/50000_Quaoar&quot; rel=&quot;nofollow&quot;&gt;Quaoar&lt;/a&gt; debris on Antarctica :-)&lt;/p&gt;&#xA;" OwnerUserId="716" LastEditorUserId="716" LastEditDate="2016-09-28T09:26:16.160" LastActivityDate="2016-09-28T09:26:16.160" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18380" PostTypeId="2" ParentId="18367" CreationDate="2016-09-28T10:31:06.107" Score="3" Body="&lt;p&gt;It is possible for dust and debris to travel basically anywhere in the solar system, given lots and lots of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first major hurdle to this travel is that the impact needs to accelerate the debris to a speed higher than the planets escape velocity. In the case of Mars, that's a speed of about 5km/s. Any debris traveling under this speed will eventually fall back down and land on Mars. Anything higher will drift off into interplanetary space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in interplanetary space, there is a set of pathways known as The Interplanetary Transport Network, which are very low-energy routes between bodies in the solar system. With a small bit more speed than the escape velocity it is possible for the debris to enter a path in this network, and eventually arrive at some other planet in the solar system. The only cost is time; it can take millions or billions of years on some routes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In an even bigger collision, and in some gravitational slingshot scenarios, the debris can reach the sun escape velocity, and head off into interstellar space.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-28T10:31:06.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18381" PostTypeId="2" ParentId="18375" CreationDate="2016-09-28T11:17:57.190" Score="5" Body="&lt;p&gt;What you are proposing violates the law of conservation of momentum. The force required to pull the iron balls back to re-use them is equal to and opposite of the force required to push them in the first place. If it was this easy we'd have a colony in alpha centauri by now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote TARS in Interstellar: &quot;Newton's third law: the only way humans have ever figured out of getting somewhere is to leave something behind.&quot;&lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-09-28T11:17:57.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18382" PostTypeId="1" CreationDate="2016-09-28T11:18:27.517" Score="1" ViewCount="1121" Body="&lt;p&gt;Is it possible to build an atmosphere on Mars without an ozone layer? What would trap the gas? Also what is an ozone layer made of? &lt;/p&gt;&#xA;" OwnerUserId="17086" LastEditorDisplayName="user10509" LastEditDate="2016-09-28T13:27:16.613" LastActivityDate="2020-03-28T15:46:06.453" Title="Does a planet need an ozone layer to maintain an atmosphere?" Tags="|mars|atmosphere|terraforming|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18383" PostTypeId="2" ParentId="18382" CreationDate="2016-09-28T11:35:46.130" Score="7" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Ozone_layer&quot; rel=&quot;noreferrer&quot;&gt;ozone layer&lt;/a&gt; contains more ozone (O&lt;sub&gt;3&lt;/sub&gt;) than most of the remainder of the atmosphere. It absorbs a lot of UV radiation.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An ozone layer is not required to maintain an atmosphere. All gases in the atmosphere are trapped by gravity only.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you build a human-livable atmosphere on Mars, the oxygen in that atmosphere will automatically start creating an ozone layer.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-09-28T11:35:46.130" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18384" PostTypeId="2" ParentId="18339" CreationDate="2016-09-28T13:02:30.257" Score="3" Body="&lt;p&gt;There were some tests out of China Lake in the late 1950s to see if small multi-stage solid fuel rockets could reach orbit. Results were inconclusive because they didn't have the telemetry and radar systems to detect success or failure. These rockets were launched from high altitude jet aircraft. There have been a few hobbyist attempts at repeating this effort the same air-launched style, none reaching the point of significant testing as far as I know. Most recently, there's CubeCab, which proposes multi-stage solids launched from a fighter jet that can reach high altitudes. www.cubecab.com. By launching from F-104s, which can fly both high and fast, you get a kind of reusable first stage that could make it possible to launch something as small as they hope to make all the way to orbit.&lt;/p&gt;&#xA;" OwnerUserId="10144" LastActivityDate="2016-09-28T13:02:30.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18385" PostTypeId="2" ParentId="18369" CreationDate="2016-09-28T13:13:07.543" Score="12" Body="&lt;p&gt;I would guess no. What makes the extinction level events so dangerous for us isn't the impact or the shockwave. A shockwave on Mars won't do much damage since the atmosphere is very thin anyway, and it's likely that colonists are mostly living underground (using the ground as radiation shielding).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What makes the big impact strikes so dangerous on Earth is the dust thrown into the atmosphere which drops the global temperatures long afterwards. On Mars, later this century, open-air agriculture is probably only in the experimental phases, with most of the population still living on food produced in protected, indoor farms. The colonists will likely still live in heated, pressurized habitats rather than on a terraformed surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A catastrophic environmental collapse is unlikely just due to dust, since there's not going to be much environment outdoors. Since Mars already experiences periodic dust storms which would interfere with solar power generation, the colonists will have backup power sources. I suspect more dust in the atmosphere would be an inconvenience for the colonists, rather than life threatening&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-09-28T13:13:07.543" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18386" PostTypeId="1" CreationDate="2016-09-28T13:48:33.773" Score="14" ViewCount="3333" Body="&lt;p&gt;The questions here in SXSE &lt;a href=&quot;//space.stackexchange.com/q/8073/12102&quot;&gt;Can you swim in space?&lt;/a&gt; and in Physics SE &lt;a href=&quot;//physics.stackexchange.com/q/275733/83380&quot;&gt;How to escape the center of a room without gravity? &lt;strong&gt;[closed]&lt;/strong&gt;&lt;/a&gt; both address aspects of how to move if stuck floating in the center of a volume in microgravity with nothing to grab. For the purposes of this question, let's say the nearest thing is now 100 meters away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because I worry about this all the time, I want to build a very small device I can keep on my person on a lanyard or ankle band that will help propel me &lt;em&gt;safely&lt;/em&gt; to a wall. Although according to the answers it should be possible to escape naturally, I want to use a device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How should my tiny device work? I want it to be as small and light-weight as possible. Should it be a laser, or a fan, or tiny tank of compressed gas, or a magnet, or an atmospheric ion thruster, or a rolled up $100 bill that I can bribe someone to help me?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, what would be the smallest, lightest technology device that could propel an average person 100 meters in say 10 minutes in a normal atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the future I will sell them to space tourists, so it needs to be propulsive on its own. Space vacationers will expect getting around to be easier in low gravity; vigorously waving a fan won't be appealing to them.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6816" LastEditDate="2016-10-05T08:39:19.037" LastActivityDate="2020-12-30T19:27:51.283" Title="Tiny emergency propulsive device if stuck floating in a large volume in microgravity" Tags="|propulsion|microgravity|safety|" AnswerCount="9" CommentCount="12" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18387" PostTypeId="2" ParentId="18386" CreationDate="2016-09-28T14:20:56.870" Score="5" Body="&lt;p&gt;Here are a couple of (bad) ideas:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Make a &lt;a href=&quot;https://en.wikipedia.org/wiki/Macram%C3%A9&quot; rel=&quot;noreferrer&quot;&gt;macramé&lt;/a&gt; belt out of 100m of string and make sure the buckle is magnetic.  Take off the belt, untie the knots, then lasso something.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Use a couple of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hand_fan&quot; rel=&quot;noreferrer&quot;&gt;hand fans&lt;/a&gt;.  They'd be reasonably easy to hide and should move more air than your lungs.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Carry a small radio and call for help.  This probably won't work if you're the victim of a prank, but you never know.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I wouldn't bother with a tank of compressed gas - too easy to run out of fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, a giant electromagnet big enough to draw you to your target might be a bit impractical and hard to hide in your hair.&lt;/p&gt;&#xA;" OwnerUserId="12978" LastActivityDate="2016-09-28T14:20:56.870" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18388" PostTypeId="2" ParentId="18386" CreationDate="2016-09-28T14:29:33.303" Score="10" Body="&lt;p&gt;For comparison's sake, &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_Aid_For_EVA_Rescue&quot; rel=&quot;nofollow noreferrer&quot;&gt;SAFER&lt;/a&gt; is what 3m/s-dV of compressed gas looks like. I doubt you'd be able to hide that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hand fans are your best bet, because unlike all your other options they don't carry their own reaction mass (well, if you can call the battery in a laser pointer 'reaction mass'.) They won't move the air particularly fast, but they'll be moving relatively large quantities of it for a long time compared to compressed-gas/mini-ion options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, fans are such a good option &lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_Constellation&quot; rel=&quot;nofollow noreferrer&quot;&gt;many aircraft use&lt;/a&gt; scaled up &lt;a href=&quot;https://en.wikipedia.org/wiki/Bell_UH-1_Iroquois&quot; rel=&quot;nofollow noreferrer&quot;&gt;versions&lt;/a&gt; for highly &lt;a href=&quot;https://en.wikipedia.org/wiki/Rutan_Voyager&quot; rel=&quot;nofollow noreferrer&quot;&gt;efficient propulsion.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the spirit of the question, let's find the simplest possible option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We need to cover 100 meters in 600 seconds (10 minutes). This means we need a velocity of&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$100m/600s \approx 0.16 m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;What if you're a professional baseball player?&lt;/em&gt; The world-record pitching speed is around 40 m/s, so this seems like a good place to start. Let's just assume you have a ball on you at all times, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say you're a little skinny, at around 60 Kg, and the ball is a little heavy, around 150 g,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$150 g / 60 Kg = 0.0025$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ball is going to get 99.75% of the velocity, and you only .25%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$0.0025 * 40 m/s = 0.1 m/s$$&#xA;$$100 m /  0.1 m/s = 1000 s$$&#xA;$$1000 s \approx 16.6\ minutes$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we're going nowhere fast, but we'll get there in a quarter hour. If you want to get there in the 10 minute mark, throw something heavier, or try to break the world record for pitching speed. You could also diet before you're trapped, but that would require some amazing foresight.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastEditorUserId="6816" LastEditDate="2017-02-05T04:42:56.463" LastActivityDate="2017-02-05T04:42:56.463" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18389" PostTypeId="2" ParentId="18375" CreationDate="2016-09-28T15:10:14.157" Score="5" Body="&lt;p&gt;Everyone has this idea at some point, and every time it doesn't work. However, you came to the right place if you want to learn. Antzi explained it very well, but I think there's something deeper to be said: &lt;strong&gt;You can never cheat physics.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you tied a long spool of string to your bullets, firing them quickly and reeling them in slowly might seem like it would work - But it doesn't. &#xA;&lt;strong&gt;Physics doesn't make rounding errors, so the momentum I gain from firing at a high speed is precisely counteracted by slowly stopping them&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;But I'm using magnets, not string.&quot;&lt;/em&gt; True, but have you ever held a magnet near metal? You'll notice that the magnet is being pulled towards something ferrous just as much as said ferrous thing is being pulled towards it. On their own, the magnet and object gain velocity towards eachother, but the combined magnet-object system (Your gun) has a net velocity change of zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&quot;Alright, What if I use gravity to pull the projectiles back in.&quot;&lt;/em&gt; Again, no. Every object has gravity proportional to its mass, your projectiles included. As your ship pulls them back in, the projectiles are pulling the ship aswell. Long and short, &lt;strong&gt;You can't sidestep the issue by changing forces.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, your idea of using a gun for propulsion isn't entirely without merit. The magnet/self-replenishing-fuel-source idea is, but not the gun. If you ditch the magnet and replace your 5m/s pea shooter with a 40km/s Railgun, you have a truly amazing exhaust velocity, and thus a very efficient propulsion system.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2016-09-28T15:10:14.157" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18390" PostTypeId="2" ParentId="18386" CreationDate="2016-09-28T15:32:34.043" Score="2" Body="&lt;p&gt;Your question requires a very low speed. 100 meters in 10 minutes is 0.166m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By my calculations using $Ek=\frac 1 2 mv^2$, assuming you weigh 80kg then you need 1.10224J of energy. If you have something on your person that you can throw away from you at 5m/s, then rearranging the equation gives the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m=\frac 2 {v^2} Ek$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m=\frac 2 {5^2} 1.10224J = 0.088kg= 88g$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe my edited values and units are correct this time, so I think 88g is still low, although would be uncontrolled once moving.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any extra equipment you might carry to help you escape could also be thrown. As UIDAlexD said, a fan could work, but also just throwing the fan away from you would be enough to get you moving.&lt;/p&gt;&#xA;" OwnerUserId="17090" LastEditorUserId="17090" LastEditDate="2016-09-28T17:19:12.673" LastActivityDate="2016-09-28T17:19:12.673" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18391" PostTypeId="2" ParentId="18374" CreationDate="2016-09-28T15:49:16.023" Score="7" Body="&lt;p&gt;The only form of ice that we see naturally in bulk on Earth is Ice I, all within the sub-h variety. There's no place on Earth that gets cold enough for any other form--but that's not necessarily true for the rest of the solar system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately for this question, there is nowhere in the solar system that we could ever find any type of ice that depends on high pressure. The best option for that would be deep under the surface of Europa, obscured by the surface ice and the (possible) liquid ocean under the surface, but the maximum pressure under Europa's ice is around 7 mega-pascals (Many thanks to @Innovine for calculating this), so other forms of ice would not form there.  High-pressure situations simply do not occur on the surface of any planet. And high-pressure situations tend to have higher temperatures, so that restricts other forms of ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, there may be pockets of other forms of ice in our solar system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PxpJM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PxpJM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Phase diagram of water from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Phase_diagram_of_water.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most likely types of ice that will be found in the solar system are Ice I sub-c, or ice XI. These forms of ice occur at lower pressure, and at temperatures that are relatively easy to achieve in our solar system. And indeed, &lt;a href=&quot;http://link.springer.com/chapter/10.1007/978-1-4614-3076-6_11#page-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;ice XI is present on the surface of the Moon, and on Hydra&lt;/a&gt; (one of the moons of Pluto). Ice XI is detected because it absorbs infrared light differently from the absorption profile of Ice I--the differences are discussed in &lt;a href=&quot;http://link.springer.com/chapter/10.1007/978-1-4614-3076-6_11#page-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Amorphous and Crystalline H2O-Ice&lt;/a&gt;, by Rachel M. E. Mastrapa (the article is behind a paywall).&lt;/p&gt;&#xA;" OwnerUserId="17050" LastEditorUserId="28871" LastEditDate="2020-02-09T23:29:49.897" LastActivityDate="2020-02-09T23:29:49.897" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="18392" PostTypeId="1" AcceptedAnswerId="19293" CreationDate="2016-09-28T16:51:23.240" Score="12" ViewCount="957" Body="&lt;p&gt;&lt;a href=&quot;http://satwcomic.com/space-attack&quot; rel=&quot;noreferrer&quot;&gt;This comic&lt;/a&gt; claims that it was to prevent people panicking and attacking the crew. However, Vostok pilots were supposed to land separately from the capsule. Are there any photos supporting this claim? Which ships used such label?&lt;/p&gt;&#xA;" OwnerUserId="17091" LastEditorUserId="-1" LastEditDate="2016-09-30T04:02:26.267" LastActivityDate="2017-11-10T22:41:27.957" Title="Did early Russian capsules really have &quot;Human Inside&quot; labels?" Tags="|crewed-spaceflight|history|russia|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18393" PostTypeId="2" ParentId="18386" CreationDate="2016-09-28T23:42:49.243" Score="-2" Body="&lt;p&gt;&lt;strong&gt;Breath in with mouth, and generate thrust from nostril, continue until you have escaped danger&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Provide external stimulants pass through the nasal hairs to reach the nasal mucosa, allowing one to trigger controlled directional upper body muscle movements (for quickest results).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Honestly can not understand why the down vote, but here are the detail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3613375/&quot; rel=&quot;nofollow&quot;&gt;Airflow Dynamics of Human Jets: Sneezing and Breathing&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;Emergency Forced Nostril Thrust Generator&quot;, has an aim to provide &#xA;controlled directional thrust using nothing more than human body movements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see from the image provided in this journal , the classical theory of utilizing breathing to accumulate velocity in order to escape is inefficient at best for two reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1.The classic theory does not appear to hold in microgravity condition&lt;/strong&gt; &#xA;The traditional notion was, breath in slow from everywhere, breath out fast focused, and off you go.&#xA;However looking at air/fluid movements in the ISS indicate otherwise.&#xA;Here are the maths. Let Air density be &lt;em&gt;p&lt;/em&gt; , surface area of mouth &lt;em&gt;A&lt;/em&gt;, and velocity of airflow from mouth be &lt;em&gt;u&lt;/em&gt;, the following holds when breathing out.&#xA;$-pAu2$ of work is preformed over time. therefore, inverse thrust of $pAu2$ is generated to move forward.&#xA; While the airflow from the lung to the mouth follows a single route, as soon as it reaches outside the mouth, the &lt;strong&gt;airflow scatters to every possible direction&lt;/strong&gt; regardless of shape of nozzle. The net volume of in and out still remains the same unless you change the  inlet and outlet vector to a different direction. If there was even a sight change in airflow direction, this will be due to the density of air itself. (as there is no such thing as natural flow of air under microgravity condition. Remember just WHY the ISS forced air conditioning to be turned on ? ). In a nutshell, when you breath out air in microgravity, the air just sits there, you gain no net thrust if you keep facing the same direction. You can not rely for external shape of airflow to propel your self. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The field of fluid dynamics appear to represent a closer model of airflow under microgravity condition than the classical earth model that we are used to...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2. Motion control problem&lt;/strong&gt;&#xA;When we attempt to use breathing as the self propellent method, the difficulty lies in changing the motion vector. You want to limit the body movement as much as possible here, yet generate a controlled thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nostril, is located to the far end of the body mass, yet faces downwards, towards the center of gravity, with the mouth opening located approximately 90 degrees angle, allowing the inlet and outlet to be positioned on a different plane at any given time, a very desirable property. And the thrust generated is always controlled by how much air one breathes in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why I asked to stick up a alien object up the nose&lt;/strong&gt;&#xA;The question we have is a very real and likely scenario, that can happen in emergency situations.  Under the assumption, it would not be a good idea to just expect life support and other air flow systems to be working (you would not be stuck if they were functioning correctly the first place anyway)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Provide external stimulants pass through the nasal hairs to reach the nasal mucosa, allowing one to trigger controlled directional upper body muscle movements&quot; really is the quickest and most efficient way to instantaneously generate thrust to evacuate danger with minimal resource. (but I guess people just thought I was fooling around.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Reasoning in Design&lt;/strong&gt;&#xA;Given the condition, you really only have Air as possible propellant.&#xA;There is a known engineering problem between size/efficiency for providing&#xA;air thrust by a mechanical turbine engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This leaves us in a well known problem of limited mass, limited power, &#xA;a area that has been worked upon in Earth, under the field of Human powered Aircraft design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key in such constraints is to replace as much engineering components as feasible with the human body, using the &lt;strong&gt;human body as part of the design&lt;/strong&gt; instead of being just a payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When designing for a theoretical thrust generator, you have two options (1) place it at the center of gravity, or (2) at the tip of the structure, directing the nozzle towards the center of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the nostril and facing downwards, I honestly believe that this is a  very elegant solution for providing controlled thrust in a controlled manner.&#xA;It allows one to replace the nozzle and air thrust generator with the nostril and lungs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final piece of puzzle is &quot;controlled upper body muscle movements&quot;, and here I intentionally &lt;strong&gt;did not write sneeze&lt;/strong&gt; as that was not what I meant.&#xA;While it is true, that the MacGyver style experiment (pepper up the nose) proposed is unpleasant, it was to show just how much potential thrust the human body can generate when appropriate muscles are worked upon in the desired order.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may appear silly, but I am always dead serious about my answers.&lt;/p&gt;&#xA;" OwnerUserId="17095" LastEditorUserId="17095" LastEditDate="2016-09-30T07:25:51.597" LastActivityDate="2016-09-30T07:25:51.597" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18394" PostTypeId="2" ParentId="18386" CreationDate="2016-09-29T00:47:32.393" Score="7" Body="&lt;p&gt;Fling out a parachute that pops open when it reaches the end of a tether. Pull it back rapidly.This will give you momentum (more than it took to fling the parachute). It could double as a cape, turban, loincloth or mumu.&lt;/p&gt;&#xA;" OwnerUserId="17097" LastEditorUserId="17097" LastEditDate="2016-09-29T02:38:17.240" LastActivityDate="2016-09-29T02:38:17.240" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18395" PostTypeId="1" CreationDate="2016-09-29T00:48:18.617" Score="9" ViewCount="567" Body="&lt;p&gt;Given that SpaceX have successfully landed several Falcon 9 boosters and that these vehicles use helium pressure-fed engines, when these vehicles land, their propellant tanks will presumably be mostly filled with a fairly large quantity of relatively lower pressure helium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is SpaceX able to recover and reuse this helium?  Do they?  Is it economically worth them doing so?&lt;/p&gt;&#xA;" OwnerUserId="758" LastEditorUserId="6944" LastEditDate="2021-03-28T18:13:41.240" LastActivityDate="2021-03-28T18:13:41.240" Title="Does SpaceX recover and reuse tank pressurization helium?" Tags="|spacex|falcon-9|reuse|fuel-system|helium|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18396" PostTypeId="1" AcceptedAnswerId="18398" CreationDate="2016-09-29T01:57:39.517" Score="11" ViewCount="1208" Body="&lt;p&gt;Is the data presented on this slide realistic? Here's the slide:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wa7Oh.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wa7Oh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the talk at the time the slide is presented, so you can hear what he's saying with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/H7Uyfqi_TE8?start=2384&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can not wrap my head around these numbers. Is there a known published source that could confirm these specific launch windows with extraordinarily low trip times? SpaceX is claiming trip times as low as 80 days, and this is with 6 km/s, with some of that reserved for landing (looks about 1 km/s, for 5 km/s for TMI). They are planning to launch from LEO, so we would have to credit some Oberth effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I ask is because of the immense gulf between these numbers and what known sources say. To quote one, &lt;a href=&quot;http://www.universetoday.com/14841/how-long-does-it-take-to-get-to-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;150-300 days&lt;/a&gt;. The standard transfer calculations, which I can reproduce, &lt;a href=&quot;http://www.phy6.org/stargaze/Smars1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;give 8.5 months&lt;/a&gt;. That number is certainly overly conservative due to the elliptical orbits, but I've seen some give 6 months as the &lt;em&gt;best case&lt;/em&gt; scenario. Even on this site, one answer seems to say that getting 2.5 months &lt;a href=&quot;https://space.stackexchange.com/questions/3223/why-not-travel-to-mars-in-2-months&quot;&gt;would fundamentally destroy the economics&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should this be looked as something discrediting of SpaceX's ambitions? Or is there a realistic set of numbers where this could actually work? If I take the claims (and the background) at face value, it looks like it's a completely and totally impossible. Or is there something this evaluation is missing?&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-09-29T04:29:28.523" Title="How is 80 to 150 days to Mars possible at 6 km/s TMI Delta V?" Tags="|spacex|mars|hohmann-transfer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18397" PostTypeId="2" ParentId="18386" CreationDate="2016-09-29T02:10:23.240" Score="19" Body="&lt;p&gt;What if you just carried a couple of uninflated balloons with you?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you ever get stuck, just inflate the balloon, and then hold it near your center of mass, aim it away from you, and let the air out. Repeat until you make it to the wall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nice thing is that an uninflated balloon is light enough that you could even have a couple of spares.&lt;/p&gt;&#xA;" OwnerUserId="17098" LastActivityDate="2016-09-29T02:10:23.240" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18398" PostTypeId="2" ParentId="18396" CreationDate="2016-09-29T02:55:07.210" Score="9" Body="&lt;p&gt;My &lt;a href=&quot;http://clowder.net/hop/railroad/Hohmann.xls&quot; rel=&quot;noreferrer&quot;&gt;Hohmann spreadsheet&lt;/a&gt; shows TMI from LEO is around 3.6 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://clowder.net/hop/railroad/NonHohmannEarthToMars.xlsx&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; is a Non Hohmann transfer sheet from earth to Mars. Into the pink cells I type .9 A.U. for the transfer orbit's perihelion and 1.6 km/s into the transfer orbit's aphelion. Here's a pic of this transfer orbit:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LKICA.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LKICA.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This trip is about 140 days from earth to Mars.&lt;br&gt;&#xA;TMI from LEO is about 5.2 km/s&lt;br&gt;&#xA;Exiting Hohmann to enter low Mars orbit takes about 3.5 km/s. I am guessing Musk wants to most of the delta V at the Mars end of the trip via aerobraking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All my spreadsheets assume circular, coplanar orbits. Of course Mars gets closer than the 1.52 A.U. my models assume -- Mars perihelion is 1.38 A.U. So trips can be shorter than what my spreadsheet indicates.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2016-09-29T03:00:15.787" LastActivityDate="2016-09-29T03:00:15.787" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18401" PostTypeId="1" CreationDate="2016-09-29T07:32:43.400" Score="2" ViewCount="214" Body="&lt;p&gt;OSIRIS-REx has launched and is on its way to 101955 Bennu to... &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;&lt;a href=&quot;http://www.asteroidmission.org/objectives/&quot; rel=&quot;nofollow&quot;&gt;determine Bennu's physical and chemical properties, which will be critical to know in the event of an impact mitigation mission&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is from &lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx#Mission&quot; rel=&quot;nofollow&quot;&gt;Osiris-Rex's Wikipedia page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...asteroid Bennu is a threatening Earth impactor capable of causing vast destruction and death&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;What is the chance that Bennu will impact Earth?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What is the potential for damage if it does hit?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How will information from Osiris-Rex affect an impact mitigation mission?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Bonus question: When will Bennu (potentially) impact with Earth?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="723" LastEditDate="2016-09-29T08:55:47.287" LastActivityDate="2016-10-21T03:30:06.030" Title="Does Bennu pose a risk to Earth? And how can OSIRIS-REx's mission save us?" Tags="|asteroid|osiris-rex|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18403" PostTypeId="2" ParentId="18401" CreationDate="2016-09-29T08:36:29.467" Score="4" Body="&lt;p&gt;Can OSIRIS-REx save us? No. I &lt;a href=&quot;http://www.asteroidmission.org/objectives/mission-operations/&quot; rel=&quot;nofollow&quot;&gt;found&lt;/a&gt; no &lt;a href=&quot;http://spaceflight101.com/osiris-rex/osiris-rex-mission-profile/&quot; rel=&quot;nofollow&quot;&gt;evidence&lt;/a&gt; that diverting the asteroid is a mission objective. The spacecraft is not equipped to influence the asteroid in any meaningful way.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the Wikipedia article you quoted:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The science objectives of the mission are:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Return and analyze a sample of pristine carbonaceous asteroid regolith in an amount sufficient to study the nature, history, and&#xA;  distribution of its constituent minerals and organic material.  &lt;/li&gt;&#xA;  &lt;li&gt;Map the global properties, chemistry, and mineralogy of a primitive carbonaceous asteroid to characterize its geologic and&#xA;  dynamic history and provide context for the returned samples.  &lt;/li&gt;&#xA;  &lt;li&gt;Document the texture, morphology, geochemistry, and spectral properties of the regolith at the sampling site in situ at scales down&#xA;  to millimeters.  &lt;/li&gt;&#xA;  &lt;li&gt;Measure the Yarkovsky effect (a thermal force on the object) on a potentially hazardous asteroid and constrain the asteroid properties&#xA;  that contribute to this effect.  &lt;/li&gt;&#xA;  &lt;li&gt;Characterize the integrated global properties of a primitive carbonaceous asteroid to allow for direct comparison with ground-based&#xA;  telescopic data of the entire asteroid population.  &lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx#Science_objectives&quot; rel=&quot;nofollow&quot;&gt;Does Bennu pose a risk to Earth?&lt;/a&gt;: recent calculations produced a cumulative probability of 1 in 1410 (or 0.071%) of impact with Earth in the period 2169 to 2199.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first mission that will try to move an asteroid is &lt;a href=&quot;https://www.nasa.gov/mission_pages/asteroids/initiative/index.html&quot; rel=&quot;nofollow&quot;&gt;the Asteroid Redirect Mission&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-09-30T19:19:04.677" LastActivityDate="2016-09-30T19:19:04.677" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18404" PostTypeId="1" CreationDate="2016-09-29T10:14:05.267" Score="2" ViewCount="346" Body="&lt;p&gt;Presumably liquid rocket combustion is never 100% efficient, so what proportion of fuel does typically remain unburnt, venting into the atmosphere, and does this vary for different rocket designs and fuel types - hydrogen, methane etc?&lt;/p&gt;&#xA;" OwnerUserId="17104" LastActivityDate="2016-11-28T13:58:38.690" Title="What proportion of a rockets fuel remains unburnt?" Tags="|rockets|fuel|chemistry|combustion|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18405" PostTypeId="1" AcceptedAnswerId="18406" CreationDate="2016-09-29T10:18:07.290" Score="2" ViewCount="924" Body="&lt;p&gt;Which is the longest distance traveled by a space craft? Manned or unmanned? I believe it's Voyager, but I am not sure about the distance it traveled or is still traveling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the mission would be a one way mission obviously, but I may be wrong.&lt;/p&gt;&#xA;" OwnerUserId="17103" LastEditorUserId="6816" LastEditDate="2016-09-29T15:56:59.523" LastActivityDate="2016-09-29T16:37:04.597" Title="Which is the longest distance traveled by a space craft?" Tags="|spacecraft|travel|long-duration|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18406" PostTypeId="2" ParentId="18405" CreationDate="2016-09-29T12:32:23.267" Score="6" Body="&lt;p&gt;The spacecraft &lt;a href=&quot;https://space.stackexchange.com/questions/3010/what-is-the-farthest-a-spacecraft-has-traveled-away-from-earth&quot;&gt;farthest away from Earth&lt;/a&gt; is Voyager 1. But that's not necessarily the longest distance traveled. If you accept the distance traveled while in orbit, a spacecraft can rack up a lot of miles while staying close to Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's try this for Helios I:&lt;br&gt;&#xA;launched in 1974, so 42 years ago. 367.920 hours, 1.324 *10&lt;sup&gt;9&lt;/sup&gt; seconds, at 45 km/s this is 59 *10&lt;sup&gt;9&lt;/sup&gt; km traveled, much further than Voyager's 19 *10&lt;sup&gt;9&lt;/sup&gt; km.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Helios probes are in an eccentric probe. Assuming a speed of 70 km/s at perihelion and 30 km/s at aphelion (close to Earth's speed), a straight average is 45 km/s.&lt;br&gt;&#xA;A better value can be calculated by finding its orbital period and the length of one orbit, I'm having some trouble figuring out how to find that length. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-09-29T16:37:04.597" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18407" PostTypeId="2" ParentId="18404" CreationDate="2016-09-29T13:06:55.330" Score="1" Body="&lt;p&gt;I managed to track down one reference online, in the SSME (Shuttle) engine, which uses liquid hydrogen/oxygen, &quot;two-stage combustion approximately 99.6% efficient&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;nofollow&quot;&gt;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also see, Modern Engineering for Design of Liquid-Propellant Rocket Engines by Dieter K. Huzel&lt;/p&gt;&#xA;" OwnerUserId="17104" LastActivityDate="2016-09-29T13:06:55.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18408" PostTypeId="1" AcceptedAnswerId="18410" CreationDate="2016-09-29T14:48:51.403" Score="3" ViewCount="750" Body="&lt;p&gt;I'm currently diving into this topic and as I understand from the brief search there are two main types of inter-satellite link: radio and optical. As I can see, one of the main distinction of optical link is that we need to configure the direction (as opposed to radio, where we emit the signal broadly). &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a166761.pdf&quot; rel=&quot;nofollow&quot;&gt;First inter-satellite radio link&lt;/a&gt; was established in 1975, &lt;a href=&quot;https://www.newscientist.com/article/dn1603-first-inter-satellite-laser-link-made/&quot; rel=&quot;nofollow&quot;&gt;first inter-satellite laser link&lt;/a&gt; — in 2001. What I don't yet understand is that whether &lt;em&gt;optical inter-satellite link and laser inter-satellite link are the same thing&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it really is the same thing, then my questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;What are the basic characteristics of such communication channel? Bit rate? angles (which allow to establish connection)? distance? &lt;/li&gt;&#xA;&lt;li&gt;Is the atmosphere an obstacle for optical communication, for satellite-Earth link or for two satellites with a big distance so the beam must go through atmosphere?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="17106" LastEditorUserId="58" LastEditDate="2020-02-26T14:02:14.247" LastActivityDate="2020-02-26T14:02:14.247" Title="Characteristics of the inter-satellite optical link" Tags="|artificial-satellite|communication|laser|optical-communication|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18409" PostTypeId="2" ParentId="18386" CreationDate="2016-09-29T14:58:17.493" Score="5" Body="&lt;p&gt;If you need to be able to cover 100 meters in 10 minutes, and you have a mass of 70 kg, then the minimum momentum you have to be able to give yourself is $70\cdot \frac{100}{600} \approx 12 \rm{~kg~m/s}$. Assuming you have the arm of a professional baseball pitcher, who would be capable of throwing a 145 gram ball at speeds up to 160 km/h (45 m/s), we see you would have to carry two baseballs with you (or objects of equivalent mass). This is why carrying a very small collapsible fan quickly becomes more attractive as an option - you don't need to carry all the mass if you can move the air around you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simple handheld fan might give an air speed of 2 m/s over a disk with a diameter of 20 cm. Such a fan would move 75 g of air per second, for a net force of 0.15 N; applying that force to your 75 kg body, you would accelerate with a=0.002 m/s$^2$. This means that in 10 minutes, you can cover a distance of $\frac12 a t^2 = 380 m, which is plenty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference is in the fact that your fan leverages the mass of the air around you. This is why rockets are so big - they have to carry not only their fuel, but in essence their propulsion is entirely the result of spitting mass out of the tail pipe. When you can move the medium around you, you can be much more effective. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that a can of &quot;emergency air&quot; will again not give you the result you desire. It's that pesky product of mass and velocity that matters...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the question of power: if we assume 25% efficiency of the motor, the electrical power has to be four times thekinetic energy of the air being moved per second. So the motor power has to be&#xA;$$P = 4\cdot \frac12 m~v^2 = 2\cdot 0.075 * 4 = 0.6 \rm{~W}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping this up for 10 minutes would require 0.1 Wh of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A small portable fan with dual AA size batteries would have about 1600 mAh * 3.0 V = 4.8 Wh capacity. Which is 48 times more than you need. Note that as the area of the fan gets smaller, the power required for the same momentum goes up (you need higher velocity of less air, basically). This approach has some scope for further miniaturization - I will leave it up to you to figure out the details of that. Incidentally, this is why &lt;a href=&quot;http://www.aerovelo.com/atlas-helicopter/&quot; rel=&quot;nofollow&quot;&gt;the human-powered &quot;helicopter&quot; that won the Sikorsky challenge had rotors that were 10's of meters in diameter...&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10738" LastEditorUserId="10738" LastEditDate="2016-09-29T15:27:08.997" LastActivityDate="2016-09-29T15:27:08.997" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18410" PostTypeId="2" ParentId="18408" CreationDate="2016-09-29T16:04:21.180" Score="3" Body="&lt;p&gt;Yes, the optical link being discussed is a laser--here's an article (behind a paywall, but the abstract mentions that the technology is laser technology) that discusses the exact satellite link you're talking about: &lt;a href=&quot;http://ieeexplore.ieee.org/document/5475229/&quot; rel=&quot;nofollow&quot;&gt;http://ieeexplore.ieee.org/document/5475229/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lasers do not have to be optical light. They can be anything from infra-red to X-rays, though X-ray lasers take a very large power source. But this particular link is an optical-light laser.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Laser-light does not remain perfectly coherent from transmission to reception, but it remains coherent enough that what starts out as a small beam at transmission (at low-Earth orbit) only expands to a few meters in width at reception (at geosynchronous orbit). A regular light would radiate in all directions, dissipating the beam far too rapidly, and would need to be far more powerful than a laser transmitter.&lt;/p&gt;&#xA;" OwnerUserId="17050" LastActivityDate="2016-09-29T16:04:21.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18411" PostTypeId="1" CreationDate="2016-09-29T17:11:16.187" Score="4" ViewCount="977" Body="&lt;p&gt;I was wondering how a rocket adjusts its position to ensure it takes the desired path. What sensors, or generally technology, are used to gain data on the rocket's position, where it is heading and the rocket's orientation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, is processing of the data from the sensors required before feeding the data to the output devices or actuators of the rocket? For example the engine or the fins in order to adjust the rocket's position. If so what processing occurs?&lt;/p&gt;&#xA;" OwnerUserId="17107" LastEditorUserId="6816" LastEditDate="2016-09-29T17:39:41.650" LastActivityDate="2016-10-02T04:15:14.547" Title="Generally, what types of sensors are used in rocket guidance systems?" Tags="|rockets|software|guidance|sensors|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18412" PostTypeId="2" ParentId="18411" CreationDate="2016-09-29T17:35:57.277" Score="6" Body="&lt;p&gt;Traditionally, just an inertial measurement unit. More recently GPS has come into play as an additional sensor to better nail down the state.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, a great deal of processing takes place to convert the data from the sensors into a filtered state, comparing that state to the desired state at that moment in time, and coming up with various commands, kept within their limits, to guide the vehicle back to the desired trajectory, as well as deciding when to cut off burns and stage.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-09-29T17:35:57.277" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18413" PostTypeId="1" CreationDate="2016-09-29T18:47:01.657" Score="1" ViewCount="195" Body="&lt;p&gt;&lt;a href=&quot;http://www.celestialmechanics.co.uk/vortex.html&quot; rel=&quot;nofollow&quot;&gt;Orbitec's vortex rocket engine&lt;/a&gt; injects oxidizer from near the nozzle up the wall of the engine chamber in a vortex. The oxidizer injector being in this position seems unusual. Are there any complications of placing the injector in this position? How does Orbitec plan to work around these issues?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.orbitec.com/documents/VHRE_2006.pdf&quot; rel=&quot;nofollow&quot;&gt;Some more (brief) information on the vortex hybrid rocket engine&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-10-04T13:55:45.297" LastActivityDate="2016-10-04T13:55:45.297" Title="Are there any complications of the position of the oxidizer injector on Orbitec's vortex rocket engine?" Tags="|rockets|fuel|oxidizer|hybrid-rocket|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18415" PostTypeId="2" ParentId="17603" CreationDate="2016-09-29T21:53:07.993" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.theverge.com/2016/9/28/13098630/united-nations-space-mission-developing-countries-climate-change-food-security&quot; rel=&quot;nofollow&quot;&gt;It will soon&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The United Nations Office of Outer Space Affairs (UNOOSA) is accepting proposals &quot;on anything from developing materials that resist corrosion in space to studying climate change and food security,&quot; &lt;a href=&quot;http://motherboard.vice.com/read/the-un-announced-its-first-ever-space-mission%20&quot; rel=&quot;nofollow&quot;&gt;Motherboard reports&lt;/a&gt;. Developing countries get first dibs, but the mission is open to all UN member states. The payloads will be selected by UNOOSA in 2018 and will be launched into low Earth orbit — up to about 1,200 miles [1,900 km] above Earth — in 2021.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is research, not infrastructure; presumably the UN does not feel it is worthwhile to invest in parallel infrastructure just for the sake of it.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2016-09-30T09:46:06.793" LastActivityDate="2016-09-30T09:46:06.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18416" PostTypeId="1" CreationDate="2016-09-29T22:01:25.487" Score="3" ViewCount="139" Body="&lt;p&gt;Currently Rosetta is on a free fall collision course with 67P/Churyumov–Gerasimenko. If I understand correctly the probe will send back data and pictures during the descent, and once it has crashed onto the comet no communication will be possible anymore. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This raises the question, why not use some of the (presumably still available) fuel to have the space probe hover close to the comet so that there is enough time for measurements taken very close to the surface to be transmitted back? Or alternatively, if hovering is too complicated a maneuver while taking measurements, why not have multiple descents that get iteratively closer to the surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason why I am asking is that the impact speed that I found mentioned was 1m/s, and the current data transmission rate is of the order of dial-up speed [45kbps], which doesn't allow for much data to be sent back in the very last stages closest to the surface, which presumably is a region of great interest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the camera is concerned I understand that it can't focus on anything closer than 1km, but presumably it could be pointed sideways at surface features during the decent? And even if not, the other instruments might generate interesting data very close to the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as far as I can see there might be two answers to this: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The above maneuvers cannot be performed or &lt;/li&gt;&#xA;&lt;li&gt;The data close to the comet wouldn't actually be all that interesting [e.g. due to the instruments not being built/optimized for that region].&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="11397" LastEditorUserId="238" LastEditDate="2016-09-29T23:16:03.947" LastActivityDate="2016-09-30T11:19:00.623" Title="Why is Rosetta not going to hover or descend multiple times before crashing into 67P/Churyumov–Gerasimenko?" Tags="|probe|rosetta|comet|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18417" PostTypeId="1" CreationDate="2016-09-29T22:50:05.407" Score="6" ViewCount="2207" Body="&lt;p&gt;Elon Musk just announced his concept for traveling to Mars.  During the Q&amp;amp;A he commented that the risk of radiation danger to the passengers was low.  I seem to remember concerns for the astronauts during the Apollo era if flares occurred during the passage to and from the moon. Would there not be risks of multiple incidents during the longer flight to Mars?  What levels of exposure might be considered reasonable especially considering these Martians will also have greater lifetime exposures because of the limited protection Mars affords them throughout their lifetime on the planet? How great is the risk traveling in space for months during the trip with the risk of exposure from cosmic sources and multiple solar flare events?&lt;/p&gt;&#xA;" OwnerUserId="15744" LastEditorUserId="17004" LastEditDate="2016-09-30T09:19:13.600" LastActivityDate="2017-09-12T19:05:07.043" Title="How big is the risk/potential damage from radiation on a Mars trip?" Tags="|spacex|mars|radiation|cosmic-radiation|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18418" PostTypeId="1" CreationDate="2016-09-29T23:32:32.210" Score="8" ViewCount="615" Body="&lt;p&gt;If Spacex succeeds, the first ship of colonists will arrive on Mars sometime during the 2020's.  Developing the spacecraft is a monumental undertaking and I am amazed that a private company may be able to put it all together to get us there with potentially hundreds of people and a lot of cargo.  But what do they do when they get there?  Spacex may develop the transportation but there is  a lot more new technology needed once we get there to support these people such as habitats, food farms, power plants, water and atmosphere generators, recycling systems, and a whole litany of other technologies to support the residents.  The transporter design that Elon Musk presented was merely a shell. I suspect Spacex will spend all their development time and resources filling in the interior with all the vital systems needed to get there.  Other innovators will need to step up to develop the infrastructure needed for the Mars village.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that NASA is working on various technologies, but I think none intended to support several astronauts for a short stay. Some companies are working on some aspects of a permanent settlement, such as Bigelow inflatable habitats, but I suspect there are many missing teeth in developing the necessary systems.  So I have several related questions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What organizations,companies, or research groups are working on major subsystems needed for a Mars village that readers on this site are aware of?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any emerging organizations/leaders who can potentially coordinate the efforts of the many focused technology efforts to build a community consistent with scale or Spacex's transport plan?&lt;/p&gt;&#xA;" OwnerUserId="15744" LastActivityDate="2016-10-27T09:42:29.447" Title="What work is being done on Mars infrastructure once Elon drops us off on the planet?" Tags="|mars|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18419" PostTypeId="2" ParentId="18417" CreationDate="2016-09-29T23:42:03.413" Score="5" Body="&lt;p&gt;There was a sensor called &lt;a href=&quot;http://mars.nasa.gov/msl/mission/instruments/radiationdetectors/rad/&quot; rel=&quot;noreferrer&quot;&gt;RAD (Radiation Assessment Detector)&lt;/a&gt; on the Curiosity rover, for measuring the inflight radiation does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the one hand, the results showed that there was a significant radiation dose in flight. On the other hand as Robert Zubrin notes, there was a similar sensor on an earlier flight that showed the same basic levels, and then it was evidence it was safe, and now it is evidence it is not safe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zubrin also notes that the radiation dose is almost identical to that the ISS astronauts take if they were to stay in orbit for similar amounts of time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus there is some debate on this topic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar Flares which are heavy protons and slower than light, (so when detected at Earth or a satellite closer to the sun, there is time to react even when messaging at light speed) can be stopped with several inches of water. In that case, a storm shelter aboard, where the water/food are layered around it, can suffice. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;High energy photons/gamma rays/cosmic rays are harder to shield from, and just have to be accepted as risk.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-09-29T23:42:03.413" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18420" PostTypeId="1" CreationDate="2016-09-30T01:33:46.650" Score="-6" ViewCount="209" Body="&lt;p&gt;in this sketch suppose the velocity of ball is 0 and it is out from the system and stayed in 0 velocity due to two electromagnets and these two magnets are attached with two springs and both springs are in stretched position towards up and down side . The ball is expelled from the gun system and got 0 velocity after expelling from the system by using these electromagnets and a force is also applying on these magnets and springs are in stretched position. you will say that engine will not move as net force is ZERO but if we release these spring then what will be happened with engine as after releasing these springs both will work to propel the engine ? should it move or not or can we modify the direction of this engine ? please keep aside the issue of ball retrieving. Thanks&lt;a href=&quot;https://i.stack.imgur.com/vdfaa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vdfaa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17082" LastActivityDate="2016-09-30T02:20:55.810" Title="Never reducing propellant for deep space propulsion" Tags="|energy|" AnswerCount="1" CommentCount="6" ClosedDate="2016-09-30T08:25:52.850" ContentLicense="CC BY-SA 3.0" />
  <row Id="18422" PostTypeId="2" ParentId="18420" CreationDate="2016-09-30T02:20:55.810" Score="2" Body="&lt;p&gt;When you fire the ball, you use the gun to put a force on it, pushing it backwards.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton.27s_third_law&quot; rel=&quot;nofollow&quot;&gt;Newton's Third Law&lt;/a&gt; tells us that &quot;for every action, there is an equal and opposite reaction&quot;.  This means that the gun will experience a force pushing it forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to stop the ball, you must apply a force to it.  However, in doing so, you will also apply a force to whatever is stopping the ball.  In this case, it would be the electromagnets attached to the springs.  This will cause the springs to stretch out.  However, this will put a force on the gun, since the springs are attached to the gun.  This will pull the gun back to its original position and stop it, resulting in no net motion and no net velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You cannot create an engine that does not expend fuel or energy to move.  The &lt;a href=&quot;https://en.wikipedia.org/wiki/Laws_of_thermodynamics&quot; rel=&quot;nofollow&quot;&gt;Laws of Thermodynamics&lt;/a&gt; state that in order to do any kind of work, such as moving something, we will lose energy.  If you think you have found a way to propel a spacecraft without ever losing propellant, you haven't.  No matter what you think, &lt;strong&gt;&lt;em&gt;it cannot be done&lt;/em&gt;&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-09-30T02:20:55.810" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="18423" PostTypeId="2" ParentId="10845" CreationDate="2016-09-30T02:54:28.030" Score="1" Body="&lt;p&gt;I've heard several terms for various planetary systems. The one account that each of them agrees on is that Jupiter's planetary system is called the &quot;Jovian System&quot;. I have heard the Saturn planetary system referred to as the &quot;Cronian System&quot;, the Uranus planetary system referred to as the &quot;Urianian System&quot;, and Neptune is &quot;Neptunian&quot;. I have also heard, on some occasions, Mars and its moons referred to as the &quot;Martian System&quot;. For planets like Venus and Mercury, I don't think there are any, but usually objects from Venus are called &quot;Venusian&quot; or more rarely &quot;Venerian&quot; after the Russian word for Venus; Venera.&lt;/p&gt;&#xA;" OwnerUserId="17113" LastActivityDate="2016-09-30T02:54:28.030" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18424" PostTypeId="2" ParentId="18411" CreationDate="2016-09-30T04:02:09.267" Score="2" Body="&lt;p&gt;Astronauts need to know 2 things; the attitude the ship is in, and where the ship is. Internal guidance can store both of these types of information with gyros and accelerometers, but it has to be updated. &#xA;So whereas the Apollo had an internal gyroscope that tracked the attitude, it was updated / corrected with star sightings by an astronaut. &#xA;Where the ship is, is tricker because it is not in a place, it is in a vector, and that changes constantly from one moment to the next. &#xA;A computer model of movement predicts it's trajectory and that needs to be updated as it drifts. In earth orbit, the ship can be tracked from the ground where there are more people and many antennas to triangulate between. &#xA;Beyond earth (and GPS) like a trip to Mars, astronomical observation of the planets would have to be done to update the trajectory information. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="16669" LastEditDate="2016-10-02T04:15:14.547" LastActivityDate="2016-10-02T04:15:14.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18425" PostTypeId="2" ParentId="18417" CreationDate="2016-09-30T06:48:27.587" Score="16" Body="&lt;p&gt;If you just stay here on Earth, you have an 18% chance of dying of cancer. Let's first consider a one-way trip, ignoring solar flares and radiation after you land. So just consider the cosmic rays, whose flux is pretty constant and which cannot be reasonably shielded against. If you go to Mars in six months, you'll get 0.3 sieverts, increasing your chance of getting cancer to about 20% or 21%. That's actually below the OSHA career limit of a 3% increase. So that would be considered &quot;safe&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the surface of Mars, the atmosphere provides a very good start at shielding, and you can pack a bunch of dirt on your habitat to significantly improve on that. If you're on Mars for only a year and half waiting for the return trip, you can keep the excess cancer pretty small. Though if you're going to Mars to stay for the rest of your life, it would be difficult to put up enough shielding to avoid shortening somewhat the rest of your life. To replicate the shielding we have from our atmosphere here, you would need to be under something like 20 feet of dirt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you return, giving you another six months in space, you have now exceeded the OSHA career limit, increasing your chance of getting cancer 20 years later by 5% to about 23%. Oh no! Clearly going from an 18% chance to a 23% chance of getting cancer is much too high a risk to accept in order to be the among the &lt;em&gt;first humans to walk on Mars!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other effects, but excess cancer fatalities are the largest given our current understanding. Here is &lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3797711/&quot;&gt;a good article on the subject&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for big solar flares, those things can definitely kill you if you're not prepared. Forget cancer. This could result in near-immediate death from nervous system damage. You are pretty unlikely to be hit by one. However, a spacecraft that I am very familiar with that was on its way to Mars in 2003 got hit by a giant solar flare that would definitely kill a human with only typical spacecraft walls to shield them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The good news is that a) you can get hours of warning if you put some spacecraft around the Sun to watch for the events, since the protons move much slower than light and x-rays, and b) you can reasonably shield humans in a spacecraft from the protons. Generally water, food, and human waste will make for good shielding. You can have a small storm shelter that the crew can crowd into for the duration of the event, which could be on the order of a day or two. The shelter needs shielding in all directions. Since the protons in the plasma are moving much faster than the cloud is as a whole, they are coming at you from all directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, yes, there is some small increase in risk from the radiation. Small, assuming that you are prepared for flares. However that risk pales in comparison to the many other risks of journey, like surviving launch, surviving landing on Mars, or surviving every day, day after day, either in space or on Mars, always with a fraction of an inch of material between you and instant death. Or the risk of losing your source of heat and freezing to death in your otherwise safe habitat on Mars, where it is way, way colder than Antarctica. Or losing your means of producing oxygen. Or drinkable water. Or food. I could go on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wouldn't sweat the radiation risk at all until those other risks get down to the percent level, which could be quite a long time.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-09-30T06:48:27.587" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18426" PostTypeId="1" CreationDate="2016-09-30T10:24:22.090" Score="3" ViewCount="353" Body="&lt;p&gt;We tend to assume that water and human waste are an ideal shield for interplanetary trips. Mostly because we need it in the first place. &#xA;However the only serious proposal does has one other element in much larger proportions: rocket fuel. (Methane and O2)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it be better suited than water for shielding and how does it compare ?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="51" LastEditDate="2016-09-30T12:45:31.580" LastActivityDate="2016-09-30T12:45:31.580" Title="How does methane and LOX compare to water for radiation shielding" Tags="|spacex|radiation|cosmic-radiation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18427" PostTypeId="2" ParentId="18416" CreationDate="2016-09-30T10:24:49.297" Score="2" Body="&lt;p&gt;This question has been asked several times in different places and I would think, that it was also carefully considered by ESA. The pros of such a strategy seems obvious. The probe will be abandoned anyway, so there seems to be not much of a risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hovering is not an option, the probe would not be able to control and maintain this state. If there is position/attitude uncertainty (after a thruster burn or after passing an inhomogeneous gravity field), it takes a relatively long time to assert the state vector again. This is by no means possible in near real time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A descent with an abort maneuver was done before by Rosetta, when it set Philae on its impact trajectory. Technically speaking, the descent is an orbit that intersects the comets surface. This would enable the probe to reach perspectives that are not available with flybys. The abort is time critical and probably needs to be done without prior state determination, while a flyby gives you much more time to reassert the state vector. ESAs flight dynamics team has some first hand experience with close flybys and the pointing errors that result from going so close.  The famous lander image f.e. was planned to have the suspected landing spot &quot;Abydos&quot; centered. In the end, they were lucky to have it covered at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is a descent without abort different from that and why should it be better? The scientific phase could last a little bit longer, but at the same time, you are less confident to capture anything of interest because of pointing error. The time to transfer the data will be hard limited, in fact data from the nearest approach will never reach us for sure. When you look at this with exploration on your mind, it is certainly worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ESA also wants to make history with a success story. An ambitious maneuver may leave a strange taste if it does not work out as expected. And finally, there are also expenses that need to be planned. I think, the scientific return from the finale will not add too much to the overall achievements. It is just an emotional thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Paolo Ferri on ESA-TV: &quot;We wanted a clean end.&quot;&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17004" LastEditDate="2016-09-30T11:19:00.623" LastActivityDate="2016-09-30T11:19:00.623" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18428" PostTypeId="1" CreationDate="2016-09-30T12:38:46.497" Score="12" ViewCount="305" Body="&lt;p&gt;Now, since Rosetta touched down on &quot;Tschurri&quot;, as the comet is called in the german speaking world, I am asking myself how firmly does Rosetta sit on its comet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since on the comet very little gravity is acting due to the comets low mass, it would probably not take too much effort (force) to lift Rosetta from the comet's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So could during one of the comet's next active periods Rosetta be blown from the comet's surface by e.g. outgassing?&lt;/p&gt;&#xA;" OwnerUserId="16645" LastActivityDate="2016-10-06T14:43:59.547" Title="How firmly does Rosetta rest on 67P/Churyumov-Gerasimenko?" Tags="|microgravity|rosetta|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18429" PostTypeId="2" ParentId="18426" CreationDate="2016-09-30T12:45:15.623" Score="1" Body="&lt;p&gt;In general, most approaches to flare shelters start with finding the 'thickest' part of the vehicle on one side of the shelter.  This is usually the base where the engines might be, and often where the fuel tanks might be. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless of fuel type (LOX/LH2, hypergolics, LOX/RP1, LOX/CH4) a few feet of them, as any fuel tank would have is better than nothing.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lockheed Martin is spending a lot of time and effort in the Orion design on placement of equipment, storage, tankage, etc so as to maximize their value in radiation mitigation. I listened to a podcast with an engineer from there, but do not recall the source, where they discussed some custom software they had spent a long time developing that helps optimize for this aspect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the goal is first, place as much mass/density between humans and the radiation.  Use what you have in hand, then add what can be added/moved as needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any mission will have fuel and water and food. At least some amount. Each has differing amount of value in terms radiation shielding but with planning all can and should be used as best they can.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-09-30T12:45:15.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18430" PostTypeId="2" ParentId="18428" CreationDate="2016-09-30T17:17:58.617" Score="4" Body="&lt;p&gt;Sadly, we just don't know what happened to it, and we probably never will.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rosetta may have come gently to rest in the snow-drift texture of the&#xA;  comet’s surface or bounced back into space, as the Philae lander did&#xA;  when it was dispatched in 2014. Rosetta’s precise fate will never be&#xA;  known, because it was pre-programmed to shut down all communication on&#xA;  contact and there are no telescopes on Earth powerful enough to see&#xA;  it.&#xA;  (&lt;a href=&quot;https://www.theguardian.com/science/2016/sep/30/rosetta-probe-lands-on-comet-67p-ending-12-year-mission&quot; rel=&quot;nofollow&quot;&gt;Source&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Philae hit the surface around 1m/s and bounced quite high (twice) (&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2014/11131025-philae-status-a-day-later.html&quot; rel=&quot;nofollow&quot;&gt;at 38 cm/s for the first bounce&lt;/a&gt;).  Rosetta hit the surface at 90 cm/s (same source as the quote above).  Its possible Rosetta did the same and took damage when it landed.  But this is just speculation.  We won't know unless we send another probe to 67p to take pictures of the remains.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no clue if Rosetta could be blown off the surface by outgassing.&lt;/p&gt;&#xA;" OwnerUserId="16599" LastEditorUserId="16599" LastEditDate="2016-10-05T20:24:55.957" LastActivityDate="2016-10-05T20:24:55.957" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18432" PostTypeId="1" CreationDate="2016-09-30T19:55:58.087" Score="2" ViewCount="770" Body="&lt;p&gt;I keep reading about SpaceX advancements toward the long-term goal, Mars colonization. Primarily, I read the excellent Wait But Why articles about the topic, which gave a not-so-technical version of Musk's plan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it seems to me that there a lot of financial and technological leaps (mainly in the colonization part, not the travel itself) in their plan. Will they get enough money to build the gigantic rockets they're talking about now? Will they have enough technology to safely colonize Mars?&lt;/p&gt;&#xA;" OwnerUserId="17117" LastActivityDate="2016-09-30T21:26:00.743" Title="Criticism of SpaceX Mars colonization plan" Tags="|spacex|mars|colonization|technology|" AnswerCount="1" CommentCount="6" ClosedDate="2016-10-01T09:44:58.403" ContentLicense="CC BY-SA 3.0" />
  <row Id="18433" PostTypeId="2" ParentId="15463" CreationDate="2016-09-30T21:17:26.377" Score="9" Body="&lt;p&gt;I have managed to confirm that at least the SSME mounting to the Space Shuttle Orbiter does not have any shock absorber, as shown in this diagram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dsch2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dsch2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The top of the device bolts directly to the Orbiter thrust structure.  The bottom bolts directly to the SSME powerhead.  The interface is a &quot;simple&quot; spherical bearing.  There is no provision for any kind of compliance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drawing is from the &lt;a href=&quot;https://archive.org/download/PocketDataRIRD87142SpaceShuttleMainEngine/Pocket_Data_RI-RD87-142_Space_Shuttle_Main_Engine.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocketdyne Space Shuttle Main Engine Pocket Data Book RI/RD87-142&lt;/a&gt;, page 2-78.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwiVhemTvbjPAhWh6YMKHSVnB74QFggeMAA&amp;amp;url=http%3A%2F%2Flarge.stanford.edu%2Fcourses%2F2011%2Fph240%2Fnguyen1%2Fdocs%2FSSME_PRESENTATION.pdf&amp;amp;usg=AFQjCNE8N8kdYHGKVYKlxhgJFjyLwxi9UQ&amp;amp;sig2=28lnPwRN26PLOJo71Yx8uQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;This document&lt;/a&gt; has more on the gimbal bearing.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The gimbal bearing provides a means of attaching the engine to the&#xA;  vehicle while allowing the engine to be pivoted (gimballed) around its&#xA;  two axes. This is necessary in order to point the engine thrust vector&#xA;  for vehicle steering, in the manner of a ship’s rudder. The gimbal&#xA;  bearing is bolted to the vehicle by its upper flange and to the engine&#xA;  by its lower flange. It supports 7,480 pounds of engine weight and&#xA;  withstands over 500,000 pounds of thrust. It is a ball-and- socket&#xA;  universal joint in which concave and convex spherical surfaces on the&#xA;  seat, body, and block intermesh. Sliding contact occurs between these&#xA;  surfaces as the bearing is angulated. Fabroid inserts located at the&#xA;  sliding contact surfaces reduce friction that occurs during gimbal&#xA;  bearing angulation. The bearing, which is installed during engine&#xA;  assembly, measures approximately 11 by 14 inches, weighs about 105&#xA;  pounds, and is made of a titanium alloy.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I cropped &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/e/e5/STS-133_Discovery_outfitted_with_one_SSME.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;this picture&lt;/a&gt; to show the engine interface area.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ECb7p.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ECb7p.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;You can see the vehicle side of the spherical bearing in the center of the opening.  The bolt pattern matches the drawing!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The green pushrod devices are the thrust vector control actuators, the red circles are covers for the holes where the low pressure turbopumps mount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: info on the thrust structure &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_coord.html#aft_fuselage&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The internal thrust structure supports the three SSMEs. The upper&#xA;  section of the thrust structure supports the upper SSME, and the lower&#xA;  section of the thrust structure supports the two lower SSMEs. The&#xA;  internal thrust structure includes the SSMEs, load reaction truss&#xA;  structures, engine interface fittings and the actuator support&#xA;  structure. It supports the SSMEs, the SSME low-pressure turbopumps and&#xA;  propellant lines. The two orbiter/external tank aft attach points&#xA;  interface at the longeron fittings.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The internal thrust structure is composed mainly of 28 machined,&#xA;  diffusion-bonded truss members. In diffusion bonding, titanium strips&#xA;  are bonded together under heat, pressure and time. This fuses the&#xA;  titanium strips into a single hollow, homogeneous mass that is lighter&#xA;  and stronger than a forged part. In looking at the cross section of a&#xA;  diffusion bond, one sees no weld line. It is a homogeneous parent&#xA;  metal, yet composed of pieces joined by diffusion bonding. (In OV-105,&#xA;  the internal thrust structure is a forging.) In selected areas, the&#xA;  titanium construction is reinforced with boron/epoxy tubular struts to&#xA;  minimize weight and add stiffness. This reduced the weight by 21&#xA;  percent, approximately 900 pounds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Finally found a decent picture of the thrust structure. It's from the Dennis Jenkins book Space Shuttle, the 1992 edition, page 140.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LGaOa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LGaOa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-11-04T19:53:37.937" LastActivityDate="2018-11-04T19:53:37.937" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="18434" PostTypeId="2" ParentId="18432" CreationDate="2016-09-30T21:26:00.743" Score="7" Body="&lt;p&gt;SpaceX is focused on the transportation angle, because frankly that's the &lt;em&gt;easy&lt;/em&gt; part (funding aside).  Elon has effectively said that other people will have to figure out the harder parts (power, air, water, food, waste recycling, psychological effects of living in a tin can for the rest of your life, etc.).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thinking about it, though, I see a method in the madness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As long as &lt;em&gt;getting to&lt;/em&gt; Mars (or any other body) is too expensive or impractical for more than a couple of token manned missions, nobody's going to put much effort into researching long-term survival on the Martian surface.  There's no point because we'd never go, not in any kind of numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By presenting what looks like a workable, practical, and above all &lt;em&gt;affordable&lt;/em&gt; (for suitably loose definitions of &lt;em&gt;affordable&lt;/em&gt;) architecture for getting &lt;em&gt;large numbers&lt;/em&gt; of people to Mars, SpaceX is providing an impetus for scaling up that kind of research.  If you have someone who's serious about putting people on Mars in the next few decades and has the means to do it, then suddenly that research isn't quite such a career backwater.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ITS is the kick in the pants necessary to shake loose some money for more research into the &lt;em&gt;hard&lt;/em&gt; problems of living on another planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may or may not work.  The ITS may never get built.  But it's an interesting approach.&lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2016-09-30T21:26:00.743" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18435" PostTypeId="2" ParentId="14363" CreationDate="2016-09-30T21:55:22.887" Score="3" Body="&lt;p&gt;In the book, he tows the second rover (IIRC, the second rover isn't in the film) behind the first. There's not enough space inside the second rover for the Hab life support equipment pieces (can't recall the technical names) that reclaimed oxygen from carbon dioxide, regulated oxygen levels, and reclaimed water that he had to bring with him. All of these needed to be inside the pressurized space. At least one of them was too tall to fit inside, so he had to make a &quot;balloon&quot; with &quot;canvas&quot; (not the flimsy Hefty garbage bags you see in the film, but flexible multilayered carbon-fiber composite). The two rovers share power and atmosphere through an umbilical, so he can breathe fine even through the life support is in the trailer.&lt;/p&gt;&#xA;" OwnerUserId="17118" LastActivityDate="2016-09-30T21:55:22.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18436" PostTypeId="2" ParentId="9412" CreationDate="2016-09-30T22:30:50.220" Score="0" Body="&lt;p&gt;One additional risk from a convoy:  If you do suffer a nasty strike a convoy means debris would be more likely to hit another ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the flip side, a convoy means you have a better chance of rescuing someone from a damaged spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-09-30T22:30:50.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18437" PostTypeId="1" CreationDate="2016-10-01T00:27:14.590" Score="5" ViewCount="254" Body="&lt;p&gt;Obviously, it's now an essentially inert artifact not going anywhere under its own thrust. The perihelion of 67P is about 20% larger than Earth's aphelion. Assuming its orbit is stable, neither the comet nor any debris or human artifact it may shed could be expected to come anywhere near Earth. But, is its orbit stable? If not, in what way is it changing? Would it get closer to Earth in the future - perhaps centuries or millenia from now?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-10-05T02:56:00.827" Title="Could Rosetta find its way back to Earth?" Tags="|rosetta|comet|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18438" PostTypeId="2" ParentId="18437" CreationDate="2016-10-01T05:32:53.080" Score="6" Body="&lt;p&gt;I used the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; database and downloaded the predicted positions (state vectors) of Earth and Comet 67P at 10 day intervals from 1600-Jan-01 to 2500-Dec-01. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; As @pericynthion pointed out in comments, since comets are subject to various non-gravitational forces, including &quot;propulsion&quot; by gasses vented unevenly and unpredictably by the rotating (and potentially wobbly) comet, long term predictions are not necessarily as reliable as they would be for far more massive planets. I'll see what I can to do find out more about this particular ephemeris for this particularly well-characterized comet that never gets closer than 1.2 AU from the sun (within this prediction).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the ephemeris I've used here, the three Marsden non-gravitational force parameters $A_1, A_2, A_3$ are shown and presumably are active. This is an empirical way to try to parameterize and approximate all of the forces besides gravity that affect a comet's orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MGKOr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MGKOr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the results. I'm not sure this answers your question yet. It's a good question - why there are a series of closer approaches between the years 1950 and 2200, and the answer is what it usually is - Jupiter! A close approach around 1959 bumps it into an orbit that occasionally gets ear earth, and later another close approach around 2209 bumps it out again. That darn Jupiter, always keeping things interesting!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lRZb0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lRZb0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wvu1t.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wvu1t.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T02:56:00.827" LastActivityDate="2016-10-05T02:56:00.827" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18440" PostTypeId="1" CreationDate="2016-10-01T22:15:35.607" Score="0" ViewCount="208" Body="&lt;p&gt;I need to know if there's any relationship between the drag force of any point in LEO with the quantity of particles or something like that that are flying in the air that makes quality of the satellite imagery worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can the drag force or the atmospheric density modify the quality of satellite imagery?&lt;/p&gt;&#xA;" OwnerUserId="16998" LastEditorUserId="6816" LastEditDate="2016-10-02T20:12:47.650" LastActivityDate="2016-10-03T10:10:30.067" Title="Atmospheric influences on satellite imagery" Tags="|low-earth-orbit|atmospheric-drag|remote-sensing|" AnswerCount="1" CommentCount="5" ClosedDate="2016-10-03T11:12:28.570" ContentLicense="CC BY-SA 3.0" />
  <row Id="18441" PostTypeId="1" CreationDate="2016-10-01T22:19:32.417" Score="8" ViewCount="6588" Body="&lt;p&gt;In his IAC 2016 talk, Elon Musk said (&lt;a href=&quot;https://www.youtube.com/watch?v=H7Uyfqi_TE8&amp;amp;feature=youtu.be&amp;amp;t=28m4s&quot; rel=&quot;nofollow noreferrer&quot;&gt;at 28m 4s in the video&lt;/a&gt;) that the ITS booster tanks would use autogenous pressurization. This means there is gaseous oxygen resp. methane in the tank. To stay gaseous, its temperature must be &lt;strong&gt;above&lt;/strong&gt; the boiling point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, the liquids are sub-cooled, i.e. their temperatures are &lt;strong&gt;far below&lt;/strong&gt; the boiling point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we have &quot;warm&quot; gas and &quot;cold&quot; liquid in the same tank. How does this work out? Wouldn't the gas quickly condense, causing the pressure to drop? And respectively, wouldn't the liquid heat up and eventually boil?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could imagine that it works for the booster, as the tanks are emptied so quickly that there won't be time to reach equilibrium anyway. But what about the spaceship? Is it also using autogenous pressurization? The diagram shows smaller tanks embedded in the spaceship tanks, could this be a helium pressurization system like in the Falcon 9?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H4RtH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H4RtH.png&quot; alt=&quot;schema of ITS cut open to show the inside of the tanks&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: &lt;a href=&quot;http://webbook.nist.gov/chemistry/fluid/&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Thermophysical Properties of Fluid Systems&quot;&gt;This web-based tool by NIST&lt;/a&gt; may be useful to determine and visualize thermophysical properties of fluids used in rockets.&lt;/p&gt;&#xA;" OwnerUserId="548" LastEditorUserId="12102" LastEditDate="2016-10-05T01:31:34.043" LastActivityDate="2021-08-21T00:29:17.967" Title="Autogenous pressurization with sub-cooled propellant" Tags="|spacex|liquid-fuel|cryogenics|fuel-system|sub-cooled|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18442" PostTypeId="1" CreationDate="2016-10-02T02:04:13.203" Score="12" ViewCount="1161" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b1cuS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b1cuS.jpg&quot; alt=&quot;Interplanetary Transport System engine&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know the reason why, in SpaceX's newly-announced &lt;a href=&quot;https://en.wikipedia.org/wiki/ITS_launch_vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;ITS launch vehicle&lt;/a&gt;, only the inner cluster gimbals, and not the whole engine system?&lt;/p&gt;&#xA;" OwnerUserId="17124" LastEditorUserId="6852" LastEditDate="2017-05-08T05:57:32.697" LastActivityDate="2017-10-03T02:38:59.487" Title="SpaceX's ITS - why gimbal only the inner cluster?" Tags="|launch|spacex|attitude|engine-design|its|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18443" PostTypeId="2" ParentId="18442" CreationDate="2016-10-02T03:43:57.523" Score="17" Body="&lt;p&gt;The main issue is one of size. Each engine bell requires space and with 42 that is a lot of space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to gimbal there needs to be room for the bell to actually move, which means you need to space the engines further apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But just allowing the inner ring to gimbal they are the only ones who need the extra space, which you can see in the image cited in the question. Since this suffices for control, why do more? Also the hardware to gimbal is more mass, more cost, so if possible to minimize, why wouldn't you?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-10-02T03:43:57.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18444" PostTypeId="2" ParentId="18440" CreationDate="2016-10-02T06:32:34.323" Score="2" Body="&lt;p&gt;Flying lower means a smaller portion of the atmosphere is below your satellite. If anything, image quality would be better. But as @pericynthion commented, the amount of atmosphere at these altitudes is too little to matter for image quality.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only potential problem is turbulence causing your satellite to vibrate, but by the time that's a problem you're hours away from reentry (i.e. way too low to be in a usable orbit).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-10-03T10:10:30.067" LastActivityDate="2016-10-03T10:10:30.067" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18445" PostTypeId="1" CreationDate="2016-10-02T08:02:20.413" Score="2" ViewCount="598" Body="&lt;p&gt;All the current proposals for a space elevator seem to be for one to the GSO.However is there some proposal for a space elevator to a low earth orbit? I know that it would require continuous power to just maintain it in a synchronized position, but it still seems to be worth it.&lt;/p&gt;&#xA;" OwnerUserId="16627" LastEditorUserId="16627" LastEditDate="2016-10-02T08:20:44.907" LastActivityDate="2016-10-02T19:13:01.317" Title="Space elevator: Is there some way you could make one to a low earth orbit?" Tags="|space-elevator|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18447" PostTypeId="2" ParentId="18445" CreationDate="2016-10-02T08:36:18.310" Score="3" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;The reason it's hard to get to orbit isn't that space is high up.&lt;/p&gt;&#xA;&lt;p&gt;It's&#xA;hard to get to orbit because you have to go so &lt;em&gt;fast&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Space isn't like this:&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S5q5M.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S5q5M.png&quot; alt=&quot;Not actual size&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Space is like &lt;em&gt;this&lt;/em&gt;:&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rHNTI.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rHNTI.png&quot; alt=&quot;You know what, sure, actual size.&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Any explanation I would produce cannot be possibly better, so I shamelessly copied it from the canonical source &lt;a href=&quot;https://what-if.xkcd.com/58/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://what-if.xkcd.com/58/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The answers to the existing question &lt;a href=&quot;https://space.stackexchange.com/questions/2699/what-is-a-space-elevator&quot;&gt;&amp;quot;What is a space elevator?&amp;quot;&lt;/a&gt; also answer your question; they explain that elevator needs to go all the way up to the GEO and beyond.&lt;/p&gt;&#xA;" OwnerUserId="12519" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-10-02T08:36:18.310" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18448" PostTypeId="2" ParentId="18445" CreationDate="2016-10-02T09:49:58.233" Score="1" Body="&lt;p&gt;From the linked question in kubanczyk's answer you can understand the principle of operation of a space elevator. As to what to do about getting to another orbit such as LEO you have a choice. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example, if you had a &quot;conventional&quot; supersynch elevator and released something in a high MEO it would not enter a circular MEO orbit but an ellipse with its apogee in MEO and perigee at some lower point in LEO. You'd have to work out the target MEO release point in order to minimise the delta-V to attain your desired LEO orbit. This might be large, e.g. &gt; 1km/s, depending very much upon your definition of &quot;LEO&quot;, but you have at least sidestepped the ~9.5km/s to go straight to LEO from the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Clarification&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By &quot;conventional supersynch elevator&quot; I mean that, i.e. one that terminates well beyond GSO and is balanced appropriately, see geoffc's answer to the other question and the comments after it. Its entirely up to the user where on the structure to release an object. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thought experiment: If it is released at GSO altitude then it will adopt a circular orbit. If it is released slightly sub-GSO then it will have a GSO velocity at an altitude for which a circular orbit would have to have a higher velocity. As a result it will adopt an elliptical path with a lower perigee. Keep on lowering the release point and explore what happens to the perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Related questions&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some helpful explanations and diagrams in CoAstroGeek's answer to this question &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/5277/needed-height-from-the-release-of-a-space-elevator-to-get-a-stable-elliptical-or?rq=1&quot;&gt;Needed height from the release of a space elevator to get a stable elliptical orbit?&lt;/a&gt;&#xA;and HopDavid's answer to this one &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/5163/if-we-build-a-space-elevator-from-earth-surface-to-geo-could-i-step-off-it-at-g?rq=1&quot;&gt;If we build a space elevator from Earth surface to GEO, could I step off it at GEO and remain in GEO?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-10-02T16:08:59.787" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18449" PostTypeId="1" CreationDate="2016-10-02T11:26:09.697" Score="9" ViewCount="2228" Body="&lt;p&gt;If we consider nuclear-rocket technology not feasible (whether due to technological or political limitations), the second best options for future transportation in solar system are perhaps solar-electric ships... there are many concepts to use ion-thrusters in combination with photovoltaics in large scale - both &lt;a href=&quot;http://www.astronautix.com/m/mars1989.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;historical&lt;/a&gt; and &lt;a href=&quot;https://www.newscientist.com/article/dn17476-ion-engine-could-one-day-power-39-day-trips-to-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;more recent&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thin-film solar cells can ultimately provide power source with density something like &lt;a href=&quot;http://news.mit.edu/2016/ultrathin-flexible-solar-cells-0226&quot; rel=&quot;nofollow noreferrer&quot;&gt;6W/g&lt;/a&gt;. In future it can be in principle developed may be up to 100-400 W/g (10-40% conversion efficiency in 1 micron foil at Earth orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore it makes sense to ask what are the inherent limits (&lt;strong&gt;maximum thrust-to-weight ratio&lt;/strong&gt;) of the ion thruster, assuming the problem of sufficiently light power source is solved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now what about Ion thrusters? &lt;strong&gt;What makes them heavy?&lt;/strong&gt; I guess current experimental realizations do not try to push thrust-to-weight up hard enough, simply because they don't have enough power to feed them. But in combination with these thin-film solar cell, the bottleneck starts to be the weight of thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Intuitively I assume &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/f/f2/Electrostatic_ion_thruster-en.svg/2000px-Electrostatic_ion_thruster-en.svg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;classical ion thrusters with grid electrodes&lt;/a&gt; can be made lighter than &lt;a href=&quot;https://en.wikipedia.org/wiki/Hall-effect_thruster#/media/File:Wfm_hall_thruster.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hall thrusters&lt;/a&gt;. While the grids can be made of thin metallic foils or wires, the Hall thrusters need quite heavy electromagnets.  But I don't really know the engineering challenges which have to be solved in order to make them lighter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I really like with this respect is &lt;a href=&quot;http://www.nextbigfuture.com/2016/09/nasa-niac-brane-craft-that-is-solar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this concept&lt;/a&gt; where the thrusters are distributed over the solar array  &lt;/p&gt;&#xA;" OwnerUserId="8393" LastEditorUserId="8393" LastEditDate="2020-01-05T14:44:45.997" LastActivityDate="2022-01-02T06:39:52.053" Title="What limits thrust-to-weight ratio of ion thusters? (beside power density of energy source)" Tags="|propulsion|solar-power|ion-thruster|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18450" PostTypeId="2" ParentId="18442" CreationDate="2016-10-02T14:40:51.407" Score="3" Body="&lt;p&gt;Optimally packing the engines, the outer rings would contain 12 and 18 respectively, for a total count of 37 engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Requiring space around the inner 7 to gimbal them, means adding more engines in the outer rings, allowing the designers to bring the total count up to 42.&lt;/p&gt;&#xA;" OwnerUserId="6864" LastActivityDate="2016-10-02T14:40:51.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18451" PostTypeId="2" ParentId="18445" CreationDate="2016-10-02T19:13:01.317" Score="2" Body="&lt;p&gt;While in a technical sense a space elevator to LEO can't exist because it would just fall down there is another option called a rotovator--and it's much easier to build, besides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You take a long cable and put it in orbit, the whole cable is spinning on the &lt;strong&gt;short&lt;/strong&gt; axis.  You orient this so the end facing the planet is on the backwards part of it's spin.  Note that the tip of this cable will be moving far below orbital speed, for the biggest rotovator it can even come to a complete stop.  (But picking something up off the ground isn't practical for safety reasons.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the rotovator has two weaknesses:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1)  You have to dock with it on the fly, you can't just board it like you can an elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2)  An elevator is in continual outward tension, so long as the weight in transit is less than the outward tension it's fine.  It's like a printing press--continually turning out mass in orbit.  A rotovator isn't anchored, it has zero outward tension.  Everything you lift lowers the rotovator.  Thus it functions like a bank--you can only withdraw what you put in.  The only reason this is of any use is that what you put in and what you take out need not be the same thing.  You can boost spacecraft and deorbit trash.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-10-02T19:13:01.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18452" PostTypeId="1" CreationDate="2016-10-02T19:41:10.370" Score="4" ViewCount="669" Body="&lt;p&gt;The primary motivation for the development of large capacity launch vehicles seems to be that of efficiency - a bigger vehicle will be more efficient at delivering a given payload to LEO than multiple smaller vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there seems to be a number of difficulties in scaling up. Factors like chamber pressure and fuel chemistry seem to place a hard limit on the amount of thrust and ISP possible for a given size of engine. Engines only seem to get so big; one factor is combustion instability. Bigger launchers then seem to require larger numbers of engines to achieve a given total thrust. For example, the proposed SpaceX ITS booster would have 42 Raptor engines. That raises questions of reliability; more things that can go wrong increases the prospect that something may go wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: do issues associated with scaling up ultimately place a practical limit on the maximum payload to LEO for a single vehicle launch? Is it a hard limit or one which could increase over time with the further evolution of space technology? What is the current practical limit? What could be the limit in 10 years, 20 years, or 50 years?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2016-10-02T22:39:23.493" LastActivityDate="2016-10-03T22:46:11.480" Title="Optimal size for heavy-lift launch vehicle" Tags="|rockets|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18453" PostTypeId="2" ParentId="18452" CreationDate="2016-10-02T22:29:28.603" Score="3" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Sizes_of_Orion_vehicles&quot; rel=&quot;nofollow&quot;&gt;Project Orion&lt;/a&gt; claimed to be able to put 6100 tons in LEO in a single launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on what you are willing to allow for &quot;the future evolution of space technology&quot;, the limit could be pretty large.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-10-02T22:29:28.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18454" PostTypeId="2" ParentId="18449" CreationDate="2016-10-03T00:14:42.153" Score="0" Body="&lt;p&gt;There are 2 things that limit basically all electric rockets. One is the power supply. An electric rocket is only as good as it’s power supply. Whether it is solar or nuclear, in many designs, the weight of the power generation is so great that it usurps the advantage of needing little propellant. As you pointed out, the increasing performance and light weight of solar cells is encouraging. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other big  limitation is the density of the exhaust stream. Because these are extremely hot, they are naturally of low mass and large volume. So the problem becomes how to compress the exhaust stream so you can have more thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don’t know that the weight of the engine itself is as much of a problem. As you pointed out a design with magnetic coils (like the VASMIR) is heavier, but the coils enable it to squeeze the exhaust stream to make it more dense, so maybe it cancels out the weight burden. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-10-03T00:14:42.153" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18455" PostTypeId="2" ParentId="18452" CreationDate="2016-10-03T00:22:14.727" Score="4" Body="&lt;p&gt;Let me present a very simple mechanism that would limit the size of rockets. Consider, at some &lt;em&gt;height&lt;/em&gt; the added weight from structural materials will outweigh any benefit gained from further reduction of dynamic pressure in the climb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Computing this exactly would be quite difficult, since dynamic pressure varies greatly over the trajectory. However, I'm not interested in complex calculations here. I'll do the simplified version. I'll consider a rocket moving straight up, at its max Q.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fraction of mass contained in structural materials, approx:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \alpha = \frac{ 3 x 3 \rho_{rocket} g h }{ \sigma_{yield} } \frac{\rho_{rocket} }{ \rho_{steel} } $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The retarding pressure from the weight of the steel would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P_{steel} = \alpha \rho_{steel} 3 g $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tradeoff between more steel vs. higher aerodynamic efficiency will start to become acute when P_steel is roughly in the same ballpark as the dynamic pressure. This is totally a cop-out to obtain a general ballpark figure. Using the above expressions, I can do:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;9*(1 g/cm^3)^2*(9.8 m/s^2)^2*(80 m)^2/((500 MPa))*3&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;(you can plug this into Google)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I've done here, is that I manipulated the value of &lt;em&gt;h&lt;/em&gt; until the pressure came out to around 33 kPa, which I found &lt;a href=&quot;http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml&quot; rel=&quot;nofollow&quot;&gt;from this sound&lt;/a&gt; was about the Max Q of the shuttle. The values from steel and other stuff are all just educated guesses. Anyway, my answer is &lt;strong&gt;80 meters&lt;/strong&gt;. A rocket becomes less efficient if it is taller than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V was actually 110 meters tall, but I think that was counting a relatively slender cone on the top. So historically large rockets might already be close to the &quot;limit&quot;. However, this isn't truly a mass limit, just height. Saturn V was very slender. You could imagine its width growing until it was basically a blocky shape... which probably isn't far from what Orion was proposed for. Beyond that, you'd be building some kind of pancake shape rocket, and intuitively something just sounds wrong with that.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2016-10-03T00:22:14.727" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18456" PostTypeId="1" AcceptedAnswerId="18459" CreationDate="2016-10-03T07:20:30.823" Score="3" ViewCount="316" Body="&lt;p&gt;I just noticed when reading about comet 67P that there were &lt;a href=&quot;https://planetgate.mps.mpg.de/Image_of_the_Day/public/IofD_archive.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS images&lt;/a&gt; taken by the OSIRIS camera on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta&lt;/a&gt; spacecraft. Since those six letters also dominate the name of the spacecraft &lt;a href=&quot;http://www.asteroidmission.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-REx&lt;/a&gt;, I was wondering if there is a backstory here, or if it is only a coincidence or acronym collision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit 1:&lt;/strong&gt; After I initially posted I noticed that there is a new space related video game also &lt;a href=&quot;https://en.wikipedia.org/wiki/Osiris:_New_Dawn&quot; rel=&quot;nofollow noreferrer&quot;&gt;called OSIRIS&lt;/a&gt; which just won Best Survival game at PAX West 2016. Looks a &lt;a href=&quot;https://www.destructoid.com/ul/389864-/Osiris2-noscale.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;tiny bit realistic&lt;/a&gt; perhaps but since I haven't played any space games since &lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroids_(video_game)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asteroids&lt;/a&gt; (the real one, in an arcade, using quarters) I have no idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit 2:&lt;/strong&gt; Aboard the upcoming CRS-12 launch is the Penn State University cubest &lt;a href=&quot;https://sites.psu.edu/sspl/osiris-3u/&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-3U&lt;/a&gt; (also &lt;a href=&quot;https://space.skyrocket.de/doc_sdat/osiris-3u.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://spaceflight101.com/dragon-spx12/osiris-3u/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit 3:&lt;/strong&gt; Phys.org's article &lt;a href=&quot;https://phys.org/news/2018-02-astronomers-s0-star-ready-big.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astronomers discover S0-2 star is single and ready for big Einstein test&lt;/a&gt; describes W. M. Keck Observatory's OH-Suppressing Infrared Imaging Spectrograph (OSIRIS) and Laser Guide Star Adaptive Optics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit 4:&lt;/strong&gt; OSIRIS (Optical Space Infrared Downlink System) highly compact optical communication payloads for small spacecrafts in a LEO &lt;a href=&quot;https://gomspace.com/dlr.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;, &lt;a href=&quot;https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10524/105240R/The-OSIRIS-program-at-DLR/10.1117/12.2290726.short?SSO=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;2&lt;/a&gt;, &lt;a href=&quot;https://core.ac.uk/download/pdf/31011065.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;3&lt;/a&gt;, &lt;a href=&quot;http://www.parabolicarc.com/2018/04/16/worlds-smallest-satellite-laser-communication-terminal-passes-cdr/&quot; rel=&quot;nofollow noreferrer&quot;&gt;4&lt;/a&gt;, &lt;a href=&quot;http://www.tesat.de/en/media-center/press/699-pi1288-world-s-smallest-laser-communication-terminal-from-tesat-on-track-cdr-for-cubel-successfully-held&quot; rel=&quot;nofollow noreferrer&quot;&gt;5&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-12-03T22:47:01.673" LastActivityDate="2018-12-03T22:47:01.673" Title="Any relationship between Rosetta's OSIRIS camera, OSIRIS-REx, OSIRIS-3U, OSIRIS optical comms, OSIRIS spectrograph, &amp; OSIRIS game?" Tags="|rosetta|osiris-rex|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18457" PostTypeId="1" AcceptedAnswerId="18460" CreationDate="2016-10-03T08:44:30.497" Score="4" ViewCount="1012" Body="&lt;p&gt;In certain chemical reactions, catalysts can be used to decrease the activation energy of those reactions. For example, &lt;a href=&quot;https://www.youtube.com/watch?v=bBBy4-0uUC4&quot; rel=&quot;nofollow&quot;&gt;platinum is a catalyst&lt;/a&gt; for &lt;a href=&quot;http://www.technology.matthey.com/pdf/pmr-v10-i2-060-064.pdf&quot; rel=&quot;nofollow&quot;&gt;hydrogen/oxygen&lt;/a&gt; reactions, so, therefore, the resulting gases have more energy leading to a higher ISP. Right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, then I understand why there aren't platinum catalysts in the reaction chamber of the engine as it would melt the platinum. However, what would be the issue with suspending the platinum within the hydrogen fuel to form a colloid, therefore removing that aspect of the problem?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The video linked is an interesting watch too&lt;/p&gt;&#xA;" OwnerUserId="16607" LastEditorUserId="16607" LastEditDate="2016-10-03T11:13:18.460" LastActivityDate="2016-10-03T21:59:39.893" Title="Platinum Colloid in Liquid Hydrogen fuel to increase fuel efficiency" Tags="|liquid-fuel|hydrogen|combustion|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18458" PostTypeId="2" ParentId="18408" CreationDate="2016-10-03T08:50:10.420" Score="2" Body="&lt;p&gt;A bit of sparse data are available for the EDRS system from ESA:&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/European_Data_Relay_System&quot; rel=&quot;nofollow&quot;&gt;EDRS&lt;/a&gt;&#xA;And a bit from Airbus &lt;a href=&quot;http://www.edrs-spacedatahighway.com/cutting-edge-technology/key-system-features&quot; rel=&quot;nofollow&quot;&gt;EDRS&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least some basic data rates and distances.&lt;/p&gt;&#xA;" OwnerUserId="15975" LastActivityDate="2016-10-03T08:50:10.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18459" PostTypeId="2" ParentId="18456" CreationDate="2016-10-03T11:09:13.603" Score="4" Body="&lt;ul&gt;&#xA;&lt;li&gt;Rosetta: Optical, Spectroscopic, and Infrared Remote Imaging System  &lt;/li&gt;&#xA;&lt;li&gt;OSIRIS-REx: Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I haven't found an obvious connection, I'd say it's a coincidence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The origin of the name &lt;a href=&quot;https://dslauretta.com/2015/01/22/osiris-rex-whats-in-a-name/&quot; rel=&quot;nofollow&quot;&gt;OSIRIS-REx&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Michael Drake, the original principal investigator on OSIRIS-REx, first asked me to help him develop the mission concept in 2004. My job was to define and implement the science investigation for the mission. I started jotting down the big ideas that an asteroid sample return mission would go after. My first list included the words Origins, Spectroscopy, Resources, and Security – these were the themes that I would build the mission science plan around. I realized right away that I had almost spelled out Osiris – all that were needed were a couple of vowels&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-10-03T15:07:00.777" LastActivityDate="2016-10-03T15:07:00.777" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18460" PostTypeId="2" ParentId="18457" CreationDate="2016-10-03T12:00:39.280" Score="20" Body="&lt;p&gt;Short answer: no, it won't increase the performance of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Platinum (or other catalysts) can be used in fuel cells, where the purpose is to get a chemical reaction at (relatively) lower temperatures. That means without having to burn the gases, in other words. (It achieves this by reducing the &quot;activation energy&quot; of the reaction as the question correctly states.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combustion however does not need a catalyst. Once ignited the conditions within fire provide enough energy to get the reaction to proceed, no catalyst is needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So adding platinum to the fuel isn't going to have any overall effect on the combustion. (If you were adding something that actually combusted, that would be a different matter - but then it would just be a different sort of fuel. Similarly if you can show the platinum itself is combusting - which I doubt - then that might add some heat.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's fairly well-known chemistry that catalysts reduce the activation energy of the reaction, but not the overall heat (enthalpy, if you like) of reaction.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastActivityDate="2016-10-03T12:00:39.280" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18461" PostTypeId="2" ParentId="18452" CreationDate="2016-10-03T12:11:31.513" Score="5" Body="&lt;p&gt;Note; larger rockets can definitely be cheaper per unit mass to LEO than smaller rockets, through production methods. The Sea Dragon design was intended to be produced in a dry dock, using 8mm steel, and with simple, large engine designs. The intention was to reduce the component count in the engine by two orders of magnitude, compared to the Saturn V. &#xA;&lt;a href=&quot;https://en.m.wikipedia.org/wiki/Sea_Dragon_(rocket)&quot; rel=&quot;nofollow&quot;&gt;https://en.m.wikipedia.org/wiki/Sea_Dragon_(rocket)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the SpaceX ITS, 42 Raptors allows a moderate degree of redundancy in the event of several failing, and further, Elon has stated that the optimisation favoured smaller engines, rather than larger ones. Since there are so many engines, the forces can be balanced by throttling or deactivating engines opposite to the defect. &#xA;In the event of several engines failing; This wouldn't be as much of an issue, as the payloads then go into a parking orbit. So if several engines failed such that the ascent was severely slowed, the second stage would likely still be able to reach orbit, although at a lower level. As it either has to be refueled, or to deliver fuel, this likely wouldn't be a major issue, as burning more fuel would allow the satellite to orbit as previously planned. This could result in further refuelings to compensate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure if the ITS first stage will be able to land when mostly fueled. I'd hope so, although perhaps it is more difficult landing a taller rocket, with fuel mass too. Although the crew compartment would be able to eject from the first stage and land on Earth, as SpaceX implemented in their Dragon design. (Dragon2?). &lt;/p&gt;&#xA;" OwnerUserId="17132" LastEditorUserId="17132" LastEditDate="2016-10-03T22:46:11.480" LastActivityDate="2016-10-03T22:46:11.480" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18462" PostTypeId="1" AcceptedAnswerId="18472" CreationDate="2016-10-03T13:29:07.283" Score="2" ViewCount="325" Body="&lt;p&gt;As in a title, I'm searching for a way of reserving a place on a rocket to send small satellites into space. Can it be only done by some kind of launch broker company? Is there any information on how those companies work with SpaceX or any other companies providing satellite launch?&lt;/p&gt;&#xA;" OwnerUserId="17134" LastEditorUserId="8162" LastEditDate="2019-07-05T07:08:04.873" LastActivityDate="2019-07-05T07:08:04.873" Title="How to reserve places on a SpaceX, Chinese or Indian rocket to send small satellites?" Tags="|launch|rockets|cubesat|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18463" PostTypeId="2" ParentId="18456" CreationDate="2016-10-03T14:54:58.433" Score="8" Body="&lt;p&gt;Just speculating here, but Osiris in an Egyptian myth &lt;a href=&quot;https://books.google.com/books?id=C1zI3P2jq8QC&amp;amp;pg=PA167&amp;amp;lpg=PA167&amp;amp;dq=osiris+ladder+to+heaven&amp;amp;source=bl&amp;amp;ots=3RW86kypOZ&amp;amp;sig=YayaO9PhhMcIIX9EkApOiHCGhiU&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwi86o_mtZbZAhUDL6wKHef0BnEQ6AEISTAJ#v=onepage&amp;amp;q=osiris%20ladder%20to%20heaven&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;ascended a ladder into the heavens&lt;/a&gt;. So it makes sense to name a probe (or a component thereof) after an &quot;astronaut&quot; from Egyptian mythology. Why Osiris as opposed to a host of other potential namesakes? That probably comes down to what letters they had available to make an acronym with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are project-specific reasons for going with Egyptian tradition, however:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_(spacecraft)#Naming&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Rosetta&lt;/em&gt;&lt;/a&gt; and its lander &lt;em&gt;Philae&lt;/em&gt; are named after Egyptian artifacts.&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;OSIRIS-REx&lt;/em&gt; was sent to explore &lt;a href=&quot;https://en.wikipedia.org/wiki/101955_Bennu#Name&quot; rel=&quot;nofollow noreferrer&quot;&gt;101955 Bennu&lt;/a&gt;, which is named for an Egyptian mythological bird (a firebird).&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://steamcommunity.com/app/402710/discussions/0/141136086924628678/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Osiris: New Dawn&lt;/em&gt;&lt;/a&gt; was a mission sent to create a new beginning for civilization in another planetary system, so the name Osiris was selected because of the god's association with rebirth.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://sites.psu.edu/sspl/osiris-3u/&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS-3U&lt;/a&gt; was being launched from the ISS, which some pronounce &quot;Isis&quot; (the name of the god Osiris's wife).&lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://www2.keck.hawaii.edu/inst/osiris/OSIRIS_Manual_v2.3.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS spectrograph&lt;/a&gt; is designed to work with the Keck Adaptive Optics System (KAOS). The god Osiris's rival was the god of chaos, Set.&lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10524/105240R/The-OSIRIS-program-at-DLR/10.1117/12.2290726.short&quot; rel=&quot;nofollow noreferrer&quot;&gt;OSIRIS optical system&lt;/a&gt; is on the BIROS satellite which is part of the FireBird constellation (see Bennu above).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2018-12-03T20:41:31.377" LastActivityDate="2018-12-03T20:41:31.377" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18464" PostTypeId="1" CreationDate="2016-10-03T15:32:30.873" Score="15" ViewCount="841" Body="&lt;p&gt;There are a couple of quick ways to check the number of people in space, right now &lt;a href=&quot;http://www.howmanypeopleareinspacerightnow.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;there are 3&lt;/a&gt;. Usually the ISS has a crew of 6. The launch planned for the other 3 crew &lt;a href=&quot;https://spaceflightnow.com/2016/09/17/russians-delay-next-crew-launch-to-space-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;was delayed indefinitely&lt;/a&gt; due to a technical issue with the Soyuz craft:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;in a one-sentence announcement on its web page early Saturday,&#xA;  Roscosmos, the Russian federal space agency, said the launch had been&#xA;  delayed “for technical reasons after tests at the Baikonur Space&#xA;  Center.”&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;It is not yet know how long the Soyuz might be grounded or what impact&#xA;  the delay might have on downstream flights.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sometimes when crews are rotated, briefly there are &lt;a href=&quot;https://space.stackexchange.com/a/11917/4660&quot;&gt;9 people on the ISS&lt;/a&gt; until the crew being relieved go home. I had thought that would be the preferred method, actually, to avoid being short-staffed, in case something just like this happened. Is it common for there briefly to be only 3 people on the station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks possible that this delay could stretch out to weeks. Will the current crew be able to perform all the needed duties alright if they are in this situation for that long? Could some duties be foregone to keep their work load manageable?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-10-03T16:48:10.020" Title="Does having only 3 people on the ISS create workload problems?" Tags="|iss|astronauts|risk-management|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18465" PostTypeId="1" AcceptedAnswerId="18467" CreationDate="2016-10-03T15:38:58.590" Score="8" ViewCount="1273" Body="&lt;p&gt;A quick search the meaning of the abbreviations for the previous versions of the Soyuz spacecraft easily tells me their meaning:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;code&gt;Soyuz T&lt;/code&gt;. '&lt;em&gt;T&lt;/em&gt;' is for транспортный, transport.&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;Souyz TM&lt;/code&gt;. The '&lt;em&gt;M&lt;/em&gt;' is for модифицированный, modified.&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;Soyuz TMA&lt;/code&gt;. The '&lt;em&gt;A&lt;/em&gt;' is for антропометрический, anthropometric.&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;Soyuz TMA-M&lt;/code&gt;. What is the '&lt;em&gt;M&lt;/em&gt;'?&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;Soyuz TMA-MS&lt;/code&gt;. And what is the '&lt;em&gt;S&lt;/em&gt;'?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I know there are more than one designation system, for instance, reading through my notes, the TMA-M had the internal designation 11F732A47. But the TMA-MS name (or just MS) is in common use, so I believe the M and the S must mean something. I am not able to figure it out, so what is it?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-10-03T17:52:06.877" Title="What does the abbreviation &quot;MS&quot; for the current Soyuz version name mean?" Tags="|spacecraft|soyuz-spacecraft|terminology|russia|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18466" PostTypeId="2" ParentId="18464" CreationDate="2016-10-03T16:48:10.020" Score="9" Body="&lt;p&gt;Yes, this could cause issues. Normally about 5 people are required full-time for maintenance. They need 6 people to get any science done.&lt;br&gt;&#xA;So they're going to be looking at contingency plans right now.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The handover planning may be caused by a shortage of docking ports (you'd need 3 Soyuz docked simultaneously to have 9 people on board). But I don't know the current station configuration. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-03T16:48:10.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18467" PostTypeId="2" ParentId="18465" CreationDate="2016-10-03T17:21:45.637" Score="9" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.vesti.ru/doc.html?id=2660696&quot;&gt;this news website&lt;/a&gt;, &quot;MS&quot; stands for &quot;modernized systems&quot; (модернизированные системы).&lt;/p&gt;&#xA;" OwnerUserId="812" LastActivityDate="2016-10-03T17:21:45.637" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18468" PostTypeId="2" ParentId="18465" CreationDate="2016-10-03T17:36:42.170" Score="4" Body="&lt;p&gt;This doesn't answer the question as well as the existing answer, but I ended up with some good supplementary information even if I couldn't find references for the 'M' and 'S' letters so I'm posting as an answer anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://ru.wikipedia.org/wiki/%D0%A1%D0%BE%D1%8E%D0%B7_%28%D0%BA%D0%BE%D1%81%D0%BC%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%B8%D0%B9_%D0%BA%D0%BE%D1%80%D0%B0%D0%B1%D0%BB%D1%8C%29&quot; rel=&quot;nofollow&quot;&gt;Russian wikipedia page for Союз&lt;/a&gt; actually doesn't fully agree with the listing you have (with much thanks to the &lt;a href=&quot;https://translate.google.com/translate?hl=en&amp;amp;sl=ru&amp;amp;u=https://ru.wikipedia.org/wiki/%25D0%25A1%25D0%25BE%25D1%258E%25D0%25B7_%28%25D0%25BA%25D0%25BE%25D1%2581%25D0%25BC%25D0%25B8%25D1%2587%25D0%25B5%25D1%2581%25D0%25BA%25D0%25B8%25D0%25B9_%25D0%25BA%25D0%25BE%25D1%2580%25D0%25B0%25D0%25B1%25D0%25BB%25D1%258C%29&amp;amp;prev=search&quot; rel=&quot;nofollow&quot;&gt;google translate of the page&lt;/a&gt; because my Russian isn't really that good):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;code&gt;«Союз Т» (Т — транспортный)&lt;/code&gt; = &quot;Soyuz T&quot; (T - transport)&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;«Союз ТМ» (ТМ — транспортный модернизированный)&lt;/code&gt; = &quot;Soyuz TM&quot; (TM - modernized transport)&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;«Союз ТМА» (А — антропометрический)&lt;/code&gt; =  &quot;Soyuz TMA&quot; (A - anthropometric)&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;«Союз TMA-M»&lt;/code&gt; = Soyuz TMA-M&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;I couldn't actually find anything (that page, it's references, other searches in English and Russian) that spelled it out explicitly, but several of the pages referred to the changes between this and the previous version being модернизированных, or &quot;modernized&quot;, systems.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;code&gt;«Союз MC»&lt;/code&gt; = Soyuz MS&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Note that most, if not all, of the references I found called it Soyuz MS, not Soyuz TMA-MS. This is expected to be the last iteration on the Soyuz design, to be eventually replaced by «Федерация», or &quot;Federation&quot;.&lt;/li&gt;&#xA;&lt;li&gt;For completeness, as per the accepted answer this is модернизированные системы, or Modernized Systems&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2016-10-03T17:52:06.877" LastActivityDate="2016-10-03T17:52:06.877" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18469" PostTypeId="2" ParentId="2881" CreationDate="2016-10-03T18:19:46.663" Score="3" Body="&lt;p&gt;At first glance, this looks like a horribly complicated problem. Here is an example of a tadpole orbit around around $L_5$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xCNbO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xCNbO.png&quot; alt=&quot;tadpole orbit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Geometrically, it is very irregular. That, however, is a projection in a co-rotating frame of reference. The same orbit in the inertial frame is actually an almost perfect ellipse:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wzkNL.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wzkNL.png&quot; alt=&quot;almost an ellipse&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In for instance the Sun-Earth $L_4$ and $L_5$ points, the gravitational influence of the Earth on the object is almost negligible. (And the minuscule influence that exists is self-dampening by the very nature of the Lagrange points).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are of course cases where this is not true. Firstly, if the mass ratio of the two massive objects is small, and secondly if the amplitude is very large (bordering a horseshoe orbit). However, in &lt;em&gt;both&lt;/em&gt; of these cases, stability is lost. The triangular $L_4$ and $L_5$ points are not stable if the mass ratio is smaller than 24.96, and a too close minimal distance to the second most massive body will cause the amplitude to change rapidly and chaotically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, the oscillation of an orbit somewhat offset from one of the $L$ points is not primarily caused by gravitational interactions with the second most massive object, it is simply a matter of orbital velocity being higher in the periapse of an elliptical orbit than in the apoapse. The property you are looking for to describe the magnitude of the oscillation is then best described by the &lt;strong&gt;eccentricity&lt;/strong&gt; of the elliptical orbit.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-10-03T18:19:46.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18470" PostTypeId="2" ParentId="18441" CreationDate="2016-10-03T19:03:58.637" Score="4" Body="&lt;p&gt;There is nothing wrong with a substance existing in both a liquid form and a gas form at the same time, like in this case. In fact, that is where the equilibrium is!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, there is not really such a thing as a liquid in a vacuum, it will always attempt to fill it*, as the boiling point gets lower with the decreased pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Le_Chatelier%27s_principle&quot; rel=&quot;nofollow&quot;&gt;Le Chatelier's principle&lt;/a&gt; is the law preventing the process from going towards any extreme, either all gas or all liquid:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If the liquid starts to &lt;strong&gt;boil&lt;/strong&gt;, the pressure increases, rising the boiling point, and the phase transitions consumes heat, lowering the temperature&lt;/li&gt;&#xA;&lt;li&gt;If the gas starts to &lt;strong&gt;condense&lt;/strong&gt;, the pressure decreases, lowering the boiling point, and the process is exothermic, releasing heat so the temperature rises.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Any attempt by the system to go towards either extreme is resisted.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;sub-cooled&quot; as the term is used in this setting, means that the temperature of the liquid is below the equilibrium point (the boiling point), but a transition is not going to go fast. The only way to get the liquid to leave the sub-cooled state more quickly would be to supply heat externally, as the process is endothermic. Also, the system is self-balancing: If you remove fuel from the tank and lower the pressure, some of the liquid boils until an equilibrium is reached again. Then it stops boiling. lastly, fuel tanks are well insulated, and once surrounded by vacuum, the heat-flow is &lt;em&gt;extremely&lt;/em&gt; slow. Like any other cryogenic system, it requires a little refrigeration to be kept liquid in the long term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;*There are factors that can keep the liquid from boiling to fill the vacuum, like surface tension and some intermolecular forces, but those factors can generally be ignored.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-10-03T19:03:58.637" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18471" PostTypeId="2" ParentId="18457" CreationDate="2016-10-03T21:59:39.893" Score="4" Body="&lt;p&gt;It is possible that the platinum would improve the performance, but not in the way you're thinking. Metal particles in rocket fuels have been used experimentally as a way of increasing the density impulse. This is discussed in chapter 12 of &lt;em&gt;Ignition: an Informal History of Liquid Rocket Propellants&lt;/em&gt; by John D. Clark (available in PDF on the Internet if you look in the right places, but I'm not sure how officially so I won't link to them myself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mercury does work well in certain applications: its toxicity is a big problem, but its low melting and boiling points, plus its monoatomicity, are very favourable. Solid metal in suspension, however, has a tendency to settle; it was tried as far back as 1929 in Germany, and revived in the late 1950s in the US as part of a push to use thixotropic gels as propellant in order for the US Navy to mitigate the risk of propellant leakage, but when Clark wrote his book in 1972 he noted that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And there have been other aluminized fuels, but none of them are ready for operational use. In my own opinion, it will be a long time before they are operational, if they ever are.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6259" LastActivityDate="2016-10-03T21:59:39.893" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18472" PostTypeId="2" ParentId="18462" CreationDate="2016-10-03T22:45:41.750" Score="5" Body="&lt;p&gt;Generally, if you are only intending to fly e.g. one cubesat, the major launch providers won't be interested in dealing with you directly.  It's not worth their time, and besides generally their primary customer &quot;owns&quot; a particular launch, so the space-available isn't theirs to sell.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead you would go through a broker such as &lt;a href=&quot;http://www.spaceflight.com/launch/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Services&lt;/a&gt; or &lt;a href=&quot;http://www.isispace.nl/&quot; rel=&quot;noreferrer&quot;&gt;ISISpace&lt;/a&gt;.  If you're willing to be deployed from the ISS, you could try &lt;a href=&quot;http://nanoracks.com/products/smallsat-deployment/&quot; rel=&quot;noreferrer&quot;&gt;Nanoracks&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As called2voyage mentioned, there is the &lt;a href=&quot;https://www.fbo.gov/index?s=opportunity&amp;amp;mode=form&amp;amp;tab=core&amp;amp;id=2b5567e469589112120af29570514b23&quot; rel=&quot;noreferrer&quot;&gt;NASA Cubesat launch initiative&lt;/a&gt; for nonprofits and universities that don't mind waiting a while.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some up-and-coming small launch providers such as &lt;a href=&quot;http://book.rocketlabusa.com/&quot; rel=&quot;noreferrer&quot;&gt;Rocket Lab&lt;/a&gt; may be an exception and willing to work with you directly.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2016-10-03T22:45:41.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18473" PostTypeId="2" ParentId="18441" CreationDate="2016-10-04T00:50:07.403" Score="0" Body="&lt;p&gt;Here is my current understanding of the situation.&lt;/p&gt;&#xA;&lt;p&gt;Reading &lt;a href=&quot;https://space.stackexchange.com/q/6323/12102&quot;&gt;Disposition of the Oxidizer Tank in Rockets with Autogenous Pressurization&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/6605/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/6616/12102&quot;&gt;this&lt;/a&gt; answer, it seems that autogenous pressurization of propellants is not used on sub-cooled propellants but instead happens when the temperature of the propellants (and tank internal surface) is at the boiling point. Adding additional heat will build pressure, or maintain pressure while propellants are leaving the tank.&lt;/p&gt;&#xA;&lt;p&gt;The main advantage of sub-cooling is to increase the density so that a tank which is completely full can hold a larger mass of propellant. Once a propellant begins to be depleted (used) and say 10% or 20% has been used, there is no further major advantage for it to be at a temperature below boing point, except perhaps a bit of thermal inertial if heating in space is not sufficiently insulated. However, until that point, an inert gas should be used for pressurization.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; as @oefe pointed out in a comment below, in the recent video &lt;a href=&quot;https://youtu.be/H7Uyfqi_TE8&quot; rel=&quot;nofollow noreferrer&quot;&gt;Making Humans a Multiplanetary Species&lt;/a&gt; starting at about 32:38, Elon Musk discusses advantages of maintaining sub-cooled temperatures for liquid propellants including reduction in cavitation within the turbo pumps. This is discussed further in &lt;a href=&quot;https://space.stackexchange.com/q/18486/12102&quot;&gt;How (actually) do sub-cooled propellants reduce cavitation within turbo pumps and make feed easier?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I don't think a propellant can easily be both sub-cooled and self-pressurized &amp;quot;autogenously&amp;quot; simultaneously, which I think is the key point of asking your question.&lt;/p&gt;&#xA;&lt;p&gt;Heat could be added to the closed system within the tank or by passing a small amount of propellant through a heater and re-introducing it. I think in general &amp;quot;autogenous pressurization&amp;quot; refers to such a system, which may or may not meet the thermodynamic definition of a &amp;quot;closed system&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;(some plots of published data on LOX and RP-1 density vs temperature can be found in &lt;a href=&quot;https://space.stackexchange.com/q/14456/12102&quot;&gt;Does the NK-33 engine require subcooled kerosene so cold that it turns to wax?&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-21T00:29:17.967" LastActivityDate="2021-08-21T00:29:17.967" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18474" PostTypeId="1" CreationDate="2016-10-04T03:48:44.510" Score="6" ViewCount="317" Body="&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Since the presence of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Rosetta&lt;/a&gt; spacecraft near comet 67P allowed for a detailed mass measurement, extremely precise position and velocity determinations, and physcal measurement and imaging of ejecta from the comet as it passed through perihelion, &lt;em&gt;did Rosetta improve on models of non-gravitational effects on comet 67P's orbit&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Background:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The calculation of the orbits of comets can be more difficult than those of most asteroids for a number of reasons. Some comets have such highly eccentric orbits that aphelion is too far for the comet to be observed continuously, or the period is so long that only one pass has been observed and a period can not be calculated, or it passes so close to the sun that it's orbit is highly modified. However comet &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;noreferrer&quot;&gt;67P/Churyumov–Gerasimenko&lt;/a&gt; currently has a period of only about 6.4 years a perihelion/aphelion of 1.2 AU and 5.7 AU respectively. While it's orbit is within a so called &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_System#Inner_Solar_System&quot; rel=&quot;noreferrer&quot;&gt;frost line&lt;/a&gt;&quot; it remains further from the sun than the Earth's orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/18438/12102&quot;&gt;this answer&lt;/a&gt; I plot some data from a recent &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;NASA JPL Horizons&lt;/a&gt; ephemeris for comet 67P. The current default solution &lt;code&gt;soln ref.= JPL#K084/25, data arc: 1995-07-03 to 2016-05-30&lt;/code&gt; appears to use Marsden coefficients to model non-gravitational forces on the comet. &lt;a href=&quot;https://en.wikipedia.org/wiki/Brian_G._Marsden&quot; rel=&quot;noreferrer&quot;&gt;Brian . Marsden&lt;/a&gt; was a British astronomer who contributed greatly to the field of cometary orbits. (See also &lt;a href=&quot;http://www.nytimes.com/1998/03/13/us/man-in-the-news-a-cheery-herald-of-fear-brian-geoffrey-marsden.html?_r=0&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://www.columbia.edu/cu/21stC/issue-3.2/reichhardt.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.) While exact modeling of non-gravitational forces on comets would be extremely complex, he introduced a simple empirical parameterization that provides a framework to discuss the magnitude and potential effects of these forces on the orbits of comets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the following convention: $\hat{\mathbf{e}}_R, \ \hat{\mathbf{e}}_T, \ \hat{\mathbf{e}}_N$ are unit vectors at the location of the comet in the radial, transverse, and normal directions where $\hat{\mathbf{e}}_R$ points away from the sun, $\hat{\mathbf{e}}_N$ is the direction of the angular momentum vector (perpendicular to the orbit plane) and $\hat{\mathbf{e}}_T$ is perpendicular to the first two and approximately in the direction of motion, non-gravitational accelerations &lt;a href=&quot;http://adsabs.harvard.edu/full/1983A%26A...126..307F&quot; rel=&quot;noreferrer&quot;&gt;can be parameterized&lt;/a&gt; using the empirical equations:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{a}_{NG} = (&#xA;A_1\hat{\mathbf{e}}_R \ +  \&#xA;A_2\hat{\mathbf{e}}_T \ +  \ &#xA;A_3\hat{\mathbf{e}}_N) \ g(r),    $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$g(r)= 0.111262\left(\frac{r}{2.808}\right)^{-2.15} \left(1+\left(\frac{r}{2.808}\right)^{5.093}\right)^{-4.6142}, $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the acceleration coeficients $A_1,A_2,A_3$ commonly have units of $AU / day^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've reproduced these here to illustrate the basic idea. There are further considerations including a delay term and effects of rotation. However with this parameterization it is possible to discuss and at least get a handle on non-gravitational effects without a detailed physical model. These effects might the &lt;a href=&quot;https://en.wikipedia.org/wiki/Yarkovsky_effect&quot; rel=&quot;noreferrer&quot;&gt;Yarkovsky&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effect&quot; rel=&quot;noreferrer&quot;&gt;Poynting-Robertson&lt;/a&gt; effects, and of course recoil from material energetically ejected from the comet, especially as it approaches the sun and is heated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The parameters $A_1,A_2,A_3$ in model can be used to express effects from physical models of comets, but they can also be used as fitting parameters to improve orbital solutions for comets based on observational data.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wtjMr.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wtjMr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; linear and semi-log plots of $g(r)$ between 1.2 and 5.7 AU.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P1HAZ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P1HAZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Example of the non-gravitational parameters used in the most recent &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; ephemeris for comet 67P. The coefficients have units of $AU/day^2$. For a comparison, the gravitational acceleration at a distance of 1.2 $AU$ is about 0.0041 $m/s^2$ or about 2.1E-04 $AU/day^2$. This suggests that the non-gravitational forces used here have a parts-per-million effect per orbit which will become substantial over a large number of orbits.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-09T00:58:10.710" LastActivityDate="2021-09-09T00:58:10.710" Title="Did Rosetta improve on models of non-gravitational effects on comet 67P's orbit?" Tags="|orbital-mechanics|planetary-science|solar-system|comet|rosetta|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18475" PostTypeId="1" AcceptedAnswerId="23707" CreationDate="2016-10-04T04:05:11.230" Score="13" ViewCount="5598" Body="&lt;p&gt;What is the current maximum bandwidth between Mars and Earth (at their closest), accounting for any satellite/prober/orbiter that is either at Mars or already launched and en route?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far I've found that the Mars Reconnaissance Orbiter can achieve up to &lt;a href=&quot;https://web.archive.org/web/20060317102639/http://mars.jpl.nasa.gov/mro/mission/sc_telecomm.html&quot; rel=&quot;noreferrer&quot;&gt;6 Megabits/second&lt;/a&gt;, but I haven't been able to find capabilities of newer missions such as the Mars Atmosphere and Volatile EvolutioN Mission or the ExoMars Trace Gas Orbiter.&lt;/p&gt;&#xA;" OwnerUserId="17139" LastEditorUserId="12102" LastEditDate="2020-07-27T04:59:35.513" LastActivityDate="2022-08-09T09:28:20.570" Title="Current maximum bandwidth between Mars and Earth?" Tags="|mars|data-transmission|exomars|mars-reconnaissance-orbiter|trace-gas-orbiter|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18476" PostTypeId="1" CreationDate="2016-10-04T06:19:37.577" Score="1" ViewCount="187" Body="&lt;p&gt;Would impacting the planet with comets at the right angle (tangent) give it enough spin to turn its iron core, kickstarting a magnetic field while bringing enough water to it to reduce the amount of carbon in its atmosphere? &lt;/p&gt;&#xA;" OwnerUserId="17140" LastEditorUserId="6816" LastEditDate="2016-10-04T06:27:22.883" LastActivityDate="2016-10-04T12:23:27.690" Title="Can engineered comet impacts kickstart Venus's magnetic field?" Tags="|venus|terraforming|magnetic-field|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18477" PostTypeId="2" ParentId="18476" CreationDate="2016-10-04T12:10:21.450" Score="3" Body="&lt;p&gt;Magnetic fields don't appear on planets just from spinning. It doesn't seem to be well known how planets get magnetic fields. Juno will explore this presently. At Jupiter only Ganymede, the Jovian (=large) moon with the next to the longest period of orbit = rotation as they are tidally locked, has a significant magnetic field. Mercury takes 2,800 hours to spin, but it has a magnetic field. Mars spins at almost exactly the same rate as Earth, but it's magnetic field strength at the surface is about 20-30 times weaker than that of Earth's and insufficient for its mass, composition and distance to the Sun to keep a substantial atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And why would you need a magnetic field? There's no correlation between atmosphere and magnetic field. See Mercury and Ganymede (magnetic fields but no atmospheres), Titan and Venus (atmospheres but no magnetic fields) for the black sheep in that assumption. For radiation shielding, either helps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe a Pluto sized comet hitting Venus could disintegrate all of it and reassemble it in a way so that the new planet gets a magnetic field and a thinner atmosphere, and why not a moon. But that's well beyond engineering. Maybe a giant comet impact made Venus the way it is today, resurfacing it and bringing volatiles to create its thick CO2 atmosphere. Comets carry a fair share of carbon, so I doubt that a tangential impact would remove any of it.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-10-04T12:23:27.690" LastActivityDate="2016-10-04T12:23:27.690" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18478" PostTypeId="2" ParentId="15163" CreationDate="2016-10-04T14:54:25.573" Score="3" Body="&lt;p&gt;Sexual psychology in space could in fact be an issue, as you have surmised.&lt;/p&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;https://books.google.com/books?id=1U72hLZzZEQC&amp;amp;lpg=PR14&amp;amp;ots=vRFJTzAHe0&amp;amp;dq=Nick%20Kanas%20Human%20Interactions%20in%20Space&amp;amp;pg=PA91#v=onepage&amp;amp;q=gender&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Psychology and Psychiatry&lt;/a&gt; by Nick Kanas and Dietrich Manzey, there is a section on gender issues which states:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Rosnet and colleagues [2004] found the presence of seduction behavior, rivalry, and sexual harassment in their polar station when the women were about the same age as the men.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It continues:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The possibility of pairing and sexual contact also needs to be considered during long-duration space missions. Will such activities lead to jealousies and problems in crew cohesion? In a recent space simulation project conducted in Moscow that involved several multinational teams of isolated and confined individuals (called SFINCSS, or Simulation of a Flight of International Crew on Space Station), a female participant reported unwanted sexual advances (including kissing) from a male participant. This resulted in a breakdown of cohesion and group rancor that affected not only the isolated teams but also the participating agencies [Inoue et al., 2004; Kass and Kass, 2001; Sandal, 2004]. Stuster [1996] has pointed out that similar unwanted sexual attention has occurred during Antarctic missions, and that disruptions in cohesion have taken place as a result of male-female pairings. He also stated that if a woman chooses to have a relationship during her stay in the Antarctic, it often is with one man, with a preference for senior over junior personnel. Although the other men usually accept the situation, disruptions may occur if the relationship involves the station leader, who is seen as having an unfair advantage. Along these lines, it is interesting that in the days of the polar explorers, the commanding officer of the ship or the expedition leader was permitted the luxury of taking his wife or mistress with him on the long voyage [Stuster, 1996]. Buckey [2006] has reviewed a number of sexual and non-sexual tensions that might occur in a mixed-gender crew going to Mars. He suggests that the crew-members could be observed under isolated and confined conditions during training to see how they come together as a team in reference to possible problems with harassment, flirtatiousness, or jealousy. Should such problems occur, further training or even replacement of offending crewmembers might be necessary for the actual mission.&lt;/p&gt;&#xA;&lt;p&gt;One might argue that future crews should consist of married couples or stable male-female pairs in order to minimize competition and conflict. However, there is no reason to expect that such a crew composition would prevent secret liaisons and jealousies, since infidelity and extra-marital relations occur on Earth in less stressful interpersonal environments. Enforced platonic relationships and sexual abstinence also is a possibility, but it is difficult to imagine this as a realistic scenario for healthy energetic people who are confined together for long periods of time. Perhaps novel social systems and customs will evolve in space that are similar to those found in communes, where pairing and unpairings will be tolerated with a minimum of conflict and animosity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From this it would seem that there is currently no existing policy (beyond whatever sexual harassment policy we can assume to be in place), but that this issue is one under heavy consideration, especially as the time of human interplanetary travel approaches.&lt;/p&gt;&#xA;&lt;p&gt;What we can be sure of is that this issue is under study, and that the astronauts are given counseling on top of their extensive training. The screening and training process probably minimizes the issues that are experienced, but &lt;a href=&quot;https://science.nasa.gov/science-news/science-at-nasa/2002/21oct_getalong/&quot; rel=&quot;nofollow noreferrer&quot;&gt;it has been noted&lt;/a&gt; that even in these conditions suppressing such issues only postpones an inevitable reaction of some form. Instead, it would seem that healthy outlets need to be provided.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-10-04T14:54:25.573" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18480" PostTypeId="1" AcceptedAnswerId="33713" CreationDate="2016-08-14T18:37:36.210" Score="4" ViewCount="4133" Body="&lt;p&gt;What are the mechanisms used to avoid destruction of space probes and satellites during meteor showers? Are they affected by them?&lt;/p&gt;&#xA;" OwnerUserId="17150" OwnerDisplayName="DRz" LastEditorUserId="58" LastEditDate="2016-10-04T17:04:29.657" LastActivityDate="2019-01-19T23:14:34.453" Title="Why are space probes and satellites not destroyed by meteor showers such as the Perseid?" Tags="|artificial-satellite|probe|collision-avoidance|meteors|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18481" PostTypeId="2" ParentId="18441" CreationDate="2016-10-04T19:43:20.740" Score="5" Body="&lt;p&gt;Once the tanks are no longer full, there's no point in keeping the propellants subcooled. When you start the engines, you just have to control the rate at which the tank and propellant warm up, to keep the pressure below its limit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The warm gas you inject into the headspace to pressurize the tank will start warming up the surface of the liquid. You'll have to stay ahead of the cooling down and condensation of this gas.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there's a balancing act between two limits. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-04T19:43:20.740" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18482" PostTypeId="1" AcceptedAnswerId="18503" CreationDate="2016-10-04T20:06:07.633" Score="31" ViewCount="5661" Body="&lt;p&gt;It is written &lt;a href=&quot;http://www.bbc.com/news/science-environment-36607608&quot;&gt;here&lt;/a&gt; that before the final descent Rosetta was programmed for &quot;auto shutoff&quot; after reaching the surface of the comet, to prevent the possibility of the further communications in the case of unlikely survival.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the reason for wasting time in programming such a behavior? If the probe is no longer required, why cannot it just be abandoned? And maybe it could take a couple of extra pictures from the surface of the comet before that, or at least finish the transmission of data collected during the descent? And if not, then not, since the chances are low. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the team of tele-operators costs money, the probe has been flying for many years. It would be strange to hear that it is not possible to afford this team for the extra month or two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems so strange that I even suspect that the &quot;auto shutoff&quot; may be a kind of urban legend. Could this be true?&lt;/p&gt;&#xA;" OwnerUserId="3042" LastEditorUserId="-1" LastEditDate="2016-10-05T17:13:27.743" LastActivityDate="2016-10-05T17:13:41.950" Title="Why was the Rosetta probe programmed to &quot;auto shutoff&quot; at the moment of hitting the surface?" Tags="|rosetta|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18483" PostTypeId="2" ParentId="18482" CreationDate="2016-10-04T21:59:43.980" Score="34" Body="&lt;p&gt;It is true.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As soon as Rosetta hits the surface, its main systems will be turned&#xA;  off, including the attitude and control systems, as well as the main&#xA;  transmitter, &lt;strong&gt;the latter in order to meet regulations aimed at avoiding&#xA;  interference on deep space network communications channels.&lt;/strong&gt; &#xA;  The software that will enable this ‘passivation’ will be uploaded to the&#xA;  spacecraft a week prior to the planned end of mission, and it will be&#xA;  activated around the time of the collision course manoeuvre,&#xA;  approximately ten hours prior to impact. No automated re-activation&#xA;  will be possible after the systems have shutdown on impact. In any&#xA;  case, as soon as Rosetta hits the surface, its high-gain antenna will&#xA;  very likely no longer be pointing towards Earth, making any potential&#xA;  communications impossible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_s_grand_finale_frequently_asked_questions&quot;&gt;this Rosetta FAQ page&lt;/a&gt;. (Emphasis added.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-10-05T17:13:41.950" LastActivityDate="2016-10-05T17:13:41.950" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18484" PostTypeId="1" AcceptedAnswerId="18500" CreationDate="2016-10-04T22:00:28.787" Score="4" ViewCount="604" Body="&lt;p&gt;I've read about &lt;a href=&quot;http://www.nasa.gov/centers/glenn/about/fs21grc.html&quot; rel=&quot;nofollow&quot;&gt;NASA Glenn's Next&lt;/a&gt;'s and &lt;a href=&quot;http://dawn.jpl.nasa.gov/technology/ion_prop.asp&quot; rel=&quot;nofollow&quot;&gt;JPL's DAWN&lt;/a&gt; ion engines using xenon (or cheaper argon and krypton) to go on trips to asteroids, bring a small one back to lunar orbit, bring back mined minerals, and Ad Astra's one to &lt;a href=&quot;http://www.adastrarocket.com/aarc/asteroid-deflection&quot; rel=&quot;nofollow&quot;&gt;deflect asteroids&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These ion engines all use neutral gases, bringing several tons of fuel with them. If the craft could use gases extracted from the asteroid itself, there would be plenty for deflecting the asteroid or bringing it nearer to earth for mining, and it wouldn't have to launch with as much fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While an ion thruster that uses in-situ resources sounds at first glance like a good idea, there's probably a reason I haven't seen articles about them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would such an ISRU ion thruster be infeasible or otherwise inferior to other ways for moving an asteroid?&lt;/p&gt;&#xA;" OwnerUserId="17149" LastEditorUserId="17149" LastEditDate="2016-10-04T22:23:20.697" LastActivityDate="2016-10-05T08:01:43.777" Title="Could an ion engine use an asteroid as an in-situ resource for fuel?" Tags="|propulsion|asteroid-redirect-mission|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18485" PostTypeId="2" ParentId="18386" CreationDate="2016-10-04T22:35:35.750" Score="27" Body="&lt;p&gt;I found a real world test of this.  Dan Barry tried it when STS-96 was docked to the ISS.  I've scanned his account from the book &amp;quot;&lt;a href=&quot;http://www.collectspace.com/ubb/Forum9/HTML/000127.html&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle: the first 20 years.&lt;/a&gt;&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;tl;dr - he escaped by &lt;em&gt;throwing his clothes&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MQUsX.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MQUsX.jpg&quot; alt=&quot;Image of Text: Dan Barry; Stranded in the middle of the room&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Transcription:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;DAN BARRY | &lt;strong&gt;Stranded in the middle of the room&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;STS-96&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;Entering the space station from the orbiter for the first time in 1999&#xA;was an exhilarating experience.  Even with just two modules-Unity and&#xA;Zarya-in place, the station's volume was 10 times that of&#xA;&lt;em&gt;Discovery's&lt;/em&gt; flight deck and mid-deck combined.  I floated exactly to the center of Unity, where I could not reach the walls, and got&#xA;stranded in the middle of the room.  It's not easy to get stranded-I&#xA;had to have my friends help me get perfectly still.  Once I was&#xA;stationary, my brain remained convinced that I could somehow maneuver&#xA;or kick myself over to the wall.  But when I reached out an arm, my&#xA;body moved back and my center remained in the middle of the room. I&#xA;instinctively tried moving fast, then slow, then bicycled my legs.&#xA;None of it helped. I just had to wait for the air currents to drift me&#xA;to the wall. Sneezing and spitting didn't do much good either.  On the&#xA;other hand, throwing clothing as fast as I could produced enough&#xA;reaction to send me to the opposite wall.&lt;/p&gt;&#xA;&lt;p&gt;AN OUTPOST IN THE SKY    &lt;strong&gt;117&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="18663" LastEditDate="2020-12-30T19:27:51.283" LastActivityDate="2020-12-30T19:27:51.283" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="18486" PostTypeId="1" AcceptedAnswerId="18573" CreationDate="2016-10-05T00:19:13.567" Score="8" ViewCount="2567" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZmaEP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZmaEP.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/H7Uyfqi_TE8?start=1958&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;Starting at about 32:38:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;We sub-cool the oxygen and methane to densify it, so compared to... propellants normally used close to their boiling point in most rockets, in our case we actually load the propellants close to their freezing point, and that can result in a density improvement of up to around 10 to 12 percent, which makes an enormous difference in the actual results of the rocket.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;It also makes the... it gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high pressure turbo pump if you have very cold propellant.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In the video of the recent presentation &lt;em&gt;&lt;a href=&quot;https://youtu.be/H7Uyfqi_TE8&quot; rel=&quot;noreferrer&quot;&gt;Making Humans a Multiplanetary Species&lt;/a&gt;&lt;/em&gt; Elon Musk mentions that in addition to obtaining higher loading densities of the LOX and LCH4 propellants, sub-cooling them decreases the chances of cavitation in the turbo pumps and improves other aspects of propellant feed to the engines.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How (actually) do sub-cooled propellants reduce cavitation within turbo pumps and make feed easier?&lt;/p&gt;&#xA;&lt;p&gt;In the Wikipedia article sub-section &lt;a href=&quot;https://en.wikipedia.org/wiki/Cavitation#Cavitation_solutions&quot; rel=&quot;noreferrer&quot;&gt;Cavitation#Cavitation_solutions&lt;/a&gt; it says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;&lt;strong&gt;To overcome cavitation:&lt;/strong&gt; Increase suction pressure if possible. &lt;strong&gt;Decrease liquid temperature if possible&lt;/strong&gt;. Throttle back on the discharge valve to decrease flow-rate. Vent gases off the pump casing.&amp;quot; (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;but there is no citation for that particular sentence. Further along in the same article in &lt;a href=&quot;https://en.wikipedia.org/wiki/Cavitation#Vascular_plants&quot; rel=&quot;noreferrer&quot;&gt;Cavitation#Vascular_plants&lt;/a&gt; it says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Deciduous trees shed leaves in the autumn partly because &lt;strong&gt;cavitation increases as temperatures decrease&lt;/strong&gt;.&lt;span class=&quot;math-container&quot;&gt;${}^{32}$&lt;/span&gt;&amp;quot; (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; As pointed out by @Andy in the comments, there appears to be some problem in the original Wikipedia article here. Since it's a complex biological system with complex chemistry it probably isn't so meaningful here.&lt;/p&gt;&#xA;&lt;p&gt;which seems to point in the opposite direction.&lt;/p&gt;&#xA;&lt;p&gt;The article also links to the on-line book &lt;a href=&quot;http://authors.library.caltech.edu/25017/1/cavbubdynam.pdf&quot; rel=&quot;noreferrer&quot;&gt;CAVITATION AND&#xA;BUBBLE DYNAMICS&lt;/a&gt; by Christopher Earls Brennen which may be helpful here.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; While I can imagine that a liquid at its boiling point might boil with even a small additional amount of heat, this isn't necessarily the same as true cavitation. What I'm looking for here is a solid technical explanation of &lt;em&gt;How (actually) do sub-cooled propellants reduce cavitation within turbo pumps and make feed easier?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-10-27T10:12:17.803" Title="How (actually) do sub-cooled propellants reduce cavitation within turbo pumps and make feed easier?" Tags="|spacex|lox|cavitation|lch4|sub-cooled|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18487" PostTypeId="5" CreationDate="2016-10-05T01:00:26.063" Score="0" Body="" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T01:00:26.063" LastActivityDate="2016-10-05T01:00:26.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18488" PostTypeId="4" CreationDate="2016-10-05T01:00:26.063" Score="0" Body="Liquid methane rocket propellant LCH4, sometimes (slightly incorrectly) called liquid natural gas or LNG as that has several heavier hydrocarbons you might not want." OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T01:00:26.063" LastActivityDate="2016-10-05T01:00:26.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18489" PostTypeId="5" CreationDate="2016-10-05T01:03:10.243" Score="0" Body="" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T01:03:10.243" LastActivityDate="2016-10-05T01:03:10.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18490" PostTypeId="4" CreationDate="2016-10-05T01:03:10.243" Score="0" Body="Cooling liquid propellants well below their boiling point - increases density and other benefits." OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T01:03:10.243" LastActivityDate="2016-10-05T01:03:10.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18491" PostTypeId="5" CreationDate="2016-10-05T01:04:30.477" Score="0" Body="" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T01:04:30.477" LastActivityDate="2016-10-05T01:04:30.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18492" PostTypeId="4" CreationDate="2016-10-05T01:04:30.477" Score="0" Body="Sudden occurrence of &quot;bubbles&quot; or voids in liquids under certain conditions - can damage materials on contact." OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T01:04:30.477" LastActivityDate="2016-10-05T01:04:30.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18493" PostTypeId="5" CreationDate="2016-10-05T01:29:42.423" Score="0" Body="" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T01:29:42.423" LastActivityDate="2016-10-05T01:29:42.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18494" PostTypeId="4" CreationDate="2016-10-05T01:29:42.423" Score="0" Body="Cooling liquid propellants well below their boiling point - increases density and other benefits.&#xD;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-05T01:29:42.423" LastActivityDate="2016-10-05T01:29:42.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18495" PostTypeId="1" AcceptedAnswerId="18511" CreationDate="2016-10-05T02:27:35.593" Score="4" ViewCount="467" Body="&lt;p&gt;Are there any known and verified examples of sabotage of a launch vehicle destined for Earth orbit or beyond?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...or at least widely regarded as 'very likely'? Attempted but unsuccessful examples could count if they came really close. Rock throwing wouldn't count for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sub-orbital launches are excluded, as they could be weapons delivery (actual or tests) and sabotage could be considered self-defense. &quot;Pilot error&quot; type events are also excluded. I'm asking about things like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YA5bM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YA5bM.jpg&quot; alt=&quot;Gary Seven&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen shot or file photo from the Star Trek original series episode &lt;em&gt;&lt;a href=&quot;http://memory-alpha.wikia.com/wiki/Assignment:_Earth_(episode)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Assignment: Earth&lt;/a&gt;&lt;/em&gt;, (#20, Season 2) where &lt;a href=&quot;http://memory-alpha.wikia.com/wiki/Gary_Seven&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gary Seven&lt;/a&gt; is sabotaging a rocket launch. This is the only image of rocket sabotage I could find, but this kind of thing wouldn't count for this question as it was a sub-orbital launch. Photo &lt;a href=&quot;http://memory-alpha.wikia.com/wiki/Assignment:_Earth_(episode)&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-10-06T12:32:51.347" Title="Any known, verified examples of sabotage of a launch vehicle bound for orbit?" Tags="|launch|rockets|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18496" PostTypeId="2" ParentId="18486" CreationDate="2016-10-05T03:38:36.943" Score="12" Body="&lt;p&gt;Cavitation is boiling, in this case it is caused by reduced pressure in the wake of blades. A boiling point is a combination of 2 factors; heat and pressure. If you can't do anything about the reduced pressure you can manipulate the other factor, which is the temperature. A liquid can stay in it's liquid state at a lower pressure if it is colder. I think the cause &amp;amp; effect is clear. You can see the charts on Wikipedia or even calculate it here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.trimen.pl/witek/calculators/wrzenie.html&quot;&gt;http://www.trimen.pl/witek/calculators/wrzenie.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-10-05T03:38:36.943" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18497" PostTypeId="2" ParentId="18442" CreationDate="2016-10-05T07:26:31.737" Score="2" Body="&lt;p&gt;Here's an alternate angle to contemplate (no pun intended).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vectoring engines carries some risk factors the engineers must consider, as with most any sub-system comprising the overall vehicle.&lt;br&gt;&#xA;In a failure scenario, a hard over gimbal actuator presents a flight emergency that of course they would want to be able to compensate for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps the central cluster provides ample response and adequate control gain without needing to have the added complexity of all engines vectoring. It's not hard to imagine the cost increases associated with implementing the added gimbal mechanics, and the proportional increases in flight safety risks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another aspect which I'm admittedly and obviously not versed in regarding the actual flight control and stabilization techniques employed on this vehicle, would be the possibility (if it isn't like this, then I've maybe spawned some unique approach) that having adequate vector influence on the flight profile with the center vectoring only, the outer (ring) of non moving engines can be throttled differentially for some degree of angular deflection of vehicle's flight path&amp;mdash; since those engines are ringing the outer portion of rocket cross-section, and would inherently have some appreciable effect if they were throttled in coordinated manner to induce deflection of vehicle trajectory.&#xA;In that spirit, flight control might be achievable with no gimbals at all, had there not been the intended capability of a powered descent and landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, there are multiple ways to approach solutions like this, and unless you are experienced in developing vehicles like this and working through the process that results in deciding the methods that solidify the final design &#xA;(i.e. spending a lot of money, winning some gambles and scraping by risk factors without accident) we just have no real idea why things end up configured as they do&amp;hellip; although it's safe to say that we are not required to know squat about it for it to work in the capacity intended.&lt;/p&gt;&#xA;" OwnerUserId="16922" LastEditorUserId="6816" LastEditDate="2017-10-02T04:54:00.653" LastActivityDate="2017-10-02T04:54:00.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18498" PostTypeId="1" AcceptedAnswerId="18502" CreationDate="2016-10-05T07:55:00.837" Score="14" ViewCount="2062" Body="&lt;p&gt;All the variants of the payload fairing for Ariane 5 are &lt;a href=&quot;http://www.spacelaunchreport.com/ariane5.html&quot; rel=&quot;noreferrer&quot;&gt;5.4 meters&lt;/a&gt; in diameter, the same as the core stage of the rocket. At the same time, the mirror of the James Webb Space Telescope is &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Optics&quot; rel=&quot;noreferrer&quot;&gt;6.5 meters&lt;/a&gt;. That looks like a very basic geometric problem to me. Although the mirror is modular, it is supposed to be launched in one piece, and not assembled in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is even a diagram of the telescope nicely fitting in the fairing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BfyWJm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BfyWJm.png&quot; alt=&quot;JWST in payload fairing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;&lt;sub&gt;By NASA (James Webb Space Telescope in Ariane 5 fairing) [Public domain], via Wikimedia Commons&lt;/sub&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only possibility I can see is that the payload fairing is custom made significantly wider than any earlier fairing.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2016-10-05T09:00:40.833" LastActivityDate="2016-10-05T10:35:36.527" Title="How can the 6.5 m primary mirror of the JWST fit inside the 5.4 m fairing of Ariane 5?" Tags="|payload|james-webb-telescope|ariane|fairings|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18499" PostTypeId="2" ParentId="18484" CreationDate="2016-10-05T07:58:09.803" Score="0" Body="&lt;p&gt;Thats a good way to look at it, and a good way to create solutions in unexpected ways.  Indeed the ION engine uses inert or otherwise incombustible material to gain what I tend to view as &quot;traction&quot; rather than calling the material a &quot;fuel&quot;... As the &quot;fuel&quot; I would say was the source of the electrical  field employed to eject the inert material for &quot;thrust&quot;&#xA;The material being carried along in similar fashion as the fuel burned in a combustion reaction process on conventional vehicles, does need to be replaced at some point, if you want to keep going... Why not dust or dirt from intercepted objects (micro meteorites) harvested parasitically for use as the inert material being thrown out the rear of craft for &quot;traction&quot; ???&#xA;It doesn't take a whole lot of the inert stuff to do what it does in the ION engine application... Sounds like something I would want to investigate.&lt;/p&gt;&#xA;" OwnerUserId="16922" LastActivityDate="2016-10-05T07:58:09.803" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18500" PostTypeId="2" ParentId="18484" CreationDate="2016-10-05T08:01:43.777" Score="3" Body="&lt;p&gt;Using an asteroid's mass for propulsion is definitely possible. You can chop up pieces of one and use it in a mass accelerator, or ablate it with a nuclear weapon, or use material as fuel in some sort of rocket - provided the body in question has water or gas which can be used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with ion engines is time. Ion engines are efficient but achingly low thrust. It would take decades or centuries to move an asteroid in this way. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-10-05T08:01:43.777" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18501" PostTypeId="2" ParentId="18386" CreationDate="2016-10-05T08:24:14.037" Score="1" Body="&lt;p&gt;A small relatively cheap device (as specified) that would employ not one but several forms of motive force, so that it could be used in a variety of scenarios. I would have a reliable and capable power source 1st of all... like a decent lithium polymer storage battery for starters. That would be your foundation for this device. Then employ a form of electromagnet that can be extended out on a short extension like a selfie stick... allowing you to locate a magnetic compliment in your immediate environment to draw toward.&#xA;If there is no complimentary magnetic (ferrous) objects to leverage for motile influence, then option (B) would be a small canister of expendable inert gas (Like CO2 or helium) to be used in intractable situations where you are truly marooned without anything else to assist your immobility.&#xA;Between those two, and the little spring loaded dart(on a retracting string) with a suction cup, adhesive sticky pad or hook device on its business end ... I think you could tackle most microgravity maroonings &#xA;(especially if your little rescue pack also had a small pneumatic piston pump that you could use to compress whatever environmental gasses around you into the emptied gas container, for an adequate replacement thrust if youve already spent your compressed gas cylinder)  &lt;/p&gt;&#xA;" OwnerUserId="16922" LastActivityDate="2016-10-05T08:24:14.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18502" PostTypeId="2" ParentId="18498" CreationDate="2016-10-05T08:50:25.480" Score="21" Body="&lt;p&gt;The JWST is made to fold up, to fit inside the standard fairing. You can sort of see this in your image, 3 mirror segments are visible (the hexagons in the middle), other segments are viewed side-on and aren't visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=30Lv8JjCqhU&quot;&gt;Folding animation&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=KGaagak_ACE&quot;&gt;Time lapse showing the folding during assembly&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorDisplayName="user10509" LastEditDate="2016-10-05T10:35:36.527" LastActivityDate="2016-10-05T10:35:36.527" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18503" PostTypeId="2" ParentId="18482" CreationDate="2016-10-05T09:58:30.307" Score="31" Body="&lt;p&gt;At first it might seem a bit crazy to crash a still working satellite into the comet. You could say that if you wait long enough the comet will come around again providing enough energy for more research.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This does bring some risks however, to name 2 obvious ones, the satellite could slowly drift out of orbit or it could never wake up again due to the harsh conditions it is in.&#xA;In both cases, you lost your valuable satellite and now have debris that can endanger other missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because space debris already is a growing problem an international treaty was signed to reduce the amount of debris in space. This states that all missions should have a end of life plan. In this case a crash landing on the comet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what if the satellite crashes into the comet, yet still survives the crash? In that case, each time the satellite receives enough energy, it will start to transmit desperately trying to contact earth.&#xA;Now instead of the useless satellite debris, there is a comet that sends a useless signal, which can in fact disrupt the signals from other missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to make sure it is completely harmless, it is not only crashed, but also turned off, just in case it survives.&lt;/p&gt;&#xA;" OwnerUserId="17153" LastActivityDate="2016-10-05T09:58:30.307" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="18504" PostTypeId="1" AcceptedAnswerId="18505" CreationDate="2016-10-05T11:13:52.737" Score="14" ViewCount="2134" Body="&lt;p&gt;In this &lt;a href=&quot;https://space.stackexchange.com/a/18261/12102&quot;&gt;great answer&lt;/a&gt; I learned that the Mars rover Curiosity can be given some tasks and it will go ahead and manage the work and navigation by itself, to at least a certain limit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The James Webb Space Telescope needs to maintain its attitude so that the giant mirror and other optics and experiments stay dark and cold - about 50 Kelvin, while its solar panels get enough sunlight to produce about 2 kilowatts of electricity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will also be in a somewhat unstable halo orbit around Sun-Earth L2. Typically spacecraft are monitored from Earth stations, and doppler measurements are precise enough to decide when orbit correcting thrust maneuvers are necessary to keep it from wandering away very much. Without careful, regular small corrections, much larger fuel-gobbling corrections would be needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These corrections are usual surprisingly small, of the order of a kilometer, and deep sub-meter-per-second in velocity. Precise doppler measurements are combined with detailed orbit modeling including perturbations are used to calculate each maneuver on earth, then checked and double-checked before the instructions are sent to a spacecraft in a halo orbit. Afterwards a second set of measurements are made to confirm that the maneuver had the intended correcting effect on the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the JWST manage its own regular orbit correction maneuvers without help from Earth if absolutely necessary? Suppose there's a failure or a meteorite hit that caused a communications problem - can it at least maintain an orbit until a repair mission (crewed or automated) comes to fix it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, how can it measure its own position precisely enough so that the regular corrections are small? While star cams can be used to ascertain attitude, how will position be measured precisely? Are occasional GPS measurements possible at that distance? Does it have good enough visible light cameras to make super-precision terminator measurements or even pattern recognition of landscape features or occultation timings of/by the Earth and the Moon?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2019-07-24T18:19:04.723" Title="Can the James Webb Space Telescope basically manage its own orbit if necessary?" Tags="|orbit|lagrangian-points|james-webb-telescope|halo-orbit|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18505" PostTypeId="2" ParentId="18504" CreationDate="2016-10-05T13:50:51.527" Score="10" Body="&lt;p&gt;I don't have too much time to research as I'm about to head off to work, but a quick investigation of the JWST doesn't show anything that could be used for Astrogation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity can do its own pathfinding in part because of the programming genius of its creators, and also because of how easily detectable the obstacles are. A camera can easily detect a hazardous boulder a few meters in front of you because it's a large, obvious object. After that detection, everything else is an exercise in pathfinding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astrogation is a lot more difficult, because you're pathfinding through empty space and not a boulder field. Unlike rocks and valleys, Lagrange points aren't directly detectable. You have to infer the location of a Lagrange point from other objects, and without any reference points there's nothing you can do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, from what I've gleaned the JWST doesn't have any Earth-facing telescopes or rangefinders it could use to ascertain its own position. Without position data from Earth, there'd be no way for it to figure out where the Lagrange point is relative to its own location, and stationkeeping would be impossible. In a sense, Earth is more of a Beacon or Reference-Frame than it is command-and-control.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2016-10-05T13:50:51.527" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18506" PostTypeId="1" AcceptedAnswerId="18520" CreationDate="2016-10-05T16:13:35.857" Score="2" ViewCount="1403" Body="&lt;p&gt;Electronics engineer here trying to dive into aviation and aerospace engineering. I was wondering which (advanced) image processing techniques are being used in satellites nowadays. And which purposes do they have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example in some industrial processes computer vision is used to monitor the growth of vegetables.&#xA;A more complex case related to automobile security is where you develop a system for dead angles. So this involves specific hardware with software which uses person detection to detect whether a biker (not an object), jogger, pedestrian or whatsoever is in a range which is considered as too close to the vehicle.&#xA;Another security related case can use person detection and facial recognition.&#xA;A last example: using computervision to detect whether an older person fell and is unconscious. (in other words not sleeping nor falling and standing up again). If the systems detects that the person is unconscious it will automatically call a doctor or whatsoever.&#xA;Those were some easy and very difficult examples. They combine many image processing algorithms (e.g. edge detection, smoothing et cetera). Now I would like to know in which cases image processing can be used in an aerospace (satellites or aviation) context. I will probably look at it and try to implement it myself as a side project for autodidactical purposes :)&lt;/p&gt;&#xA;" OwnerUserId="13091" LastActivityDate="2018-12-01T06:43:14.183" Title="how and why is image processing used on spacecrafts?" Tags="|artificial-satellite|software|imaging|electronics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18507" PostTypeId="2" ParentId="15334" CreationDate="2016-10-05T16:21:37.220" Score="0" Body="&lt;p&gt;&lt;a href=&quot;http://www.u3p.net/prop/simu/sjava_a.htm&quot; rel=&quot;nofollow&quot;&gt;The Union pour la Promotion de la Propulsion Photonique has a Java solar sail simulator&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By default it starts with a circular orbit at 1 AU, and remains in that plane.  It is not quite what you are after, but does allow one to change orientation and lightness number.&lt;/p&gt;&#xA;" OwnerUserId="12999" LastEditorUserId="6816" LastEditDate="2016-10-18T00:31:47.847" LastActivityDate="2016-10-18T00:31:47.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18508" PostTypeId="1" CreationDate="2016-10-05T16:41:12.247" Score="6" ViewCount="11742" Body="&lt;p&gt;In &lt;a href=&quot;https://en.wikipedia.org/wiki/Kerbal_Space_Program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kerbal Space Program&lt;/a&gt; (KSP), optimum &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust-to-weight_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;thrust-to-weight ratio&lt;/a&gt; (TWR) is around 1.5-3. When looking at NASA, SpaceX, Blue Origin spacecraft, the launch TWR is around 1.25-1.3. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it because the atmosphere is much larger, and an optimal &lt;a href=&quot;https://en.wikipedia.org/wiki/Goddard_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Goddard solution&lt;/a&gt; is achieved higher in the atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="17160" LastEditorUserId="12102" LastEditDate="2016-12-23T00:16:30.660" LastActivityDate="2020-02-05T22:49:08.317" Title="Why are optimal launch TWRs so different in KSP than on earth?" Tags="|thrust|kerbal-space-program|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18509" PostTypeId="1" AcceptedAnswerId="18522" CreationDate="2016-10-05T16:52:50.407" Score="3" ViewCount="541" Body="&lt;p&gt;One of the biggest criticisms of the Mars colony is that it won’t be financially feasible and that SpaceX will have to receive massive amounts of funding from NASA and other external organizations to achieve colonization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm inclined to disagree. The way I've always thought of it (though I don't have the exact numbers) is like this: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;SpaceX, the company, is primarily a business which makes tons of money by&#xA;launching super-affordable rockets for its varied repertoire of&#xA;clients. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;It then allocates a portion of those profits to the various aspects&#xA;of the Mars colonization plan, such as the building of the ITS, the&#xA;new spacesuit, and ultimately the launch of the ITS to Mars and the&#xA;establishment of the colony. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;However, at present, SpaceX makes zero money on its Mars&#xA;colonization efforts.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Am I correct in my assumptions? Can SpaceX truly accomplish these objectives exclusively using the money that it makes as a business, or will it need external funding?&lt;/p&gt;&#xA;" OwnerUserId="11786" LastActivityDate="2016-10-06T14:13:51.570" Title="Will SpaceX require funding from NASA to establish a colony on Mars?" Tags="|spacex|mars|nasa|colonization|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18510" PostTypeId="2" ParentId="18509" CreationDate="2016-10-05T17:22:09.563" Score="4" Body="&lt;p&gt;The big question is, how much will it cost? The estimates that I've seen indicate it would cost about &lt;a href=&quot;http://www.theverge.com/2016/9/28/13087110/spacex-elon-musk-mars-plan-habitat-radiation-funding-questions&quot; rel=&quot;nofollow&quot;&gt;\$560 million per launch&lt;/a&gt;, assuming no re-usability. If that is all it takes to get to Mars, then Elon Musk can get the money fairly easily, from a number of sources. Prior to launching humans to Mars, they will have launched one cargo mission unmanned, several LEO ranged tests, done R&amp;amp;D, and at least 3, maybe more, Red Dragon Missions (Estimated around \$150 million, on the low side). Put all of that together, and there's easily a \$2 billion cost in non-R&amp;amp;D costs. R&amp;amp;D is likely on the order of \$3 billion, so that's around \$5 billion minimum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of some question is how much they can sell of the initial runs to space. I can imagine someone paying for cargo launched to Mars at very low cost, for instance. Get the price low enough, and many people would have payloads they are interested to sending to Mars on a Red Dragon. NASA might even pay for loads taken there, for instance, and they might even completely make up the cost of the mission for the subsequent missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon has stated that his potential funding sources include &quot;Stealing underpants, Launch Satellites, Send Cargo to the ISS, Kickstarter, and Profit&quot;. More seriously, he committed to using his life's fortune to make this happen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will NASA fund it? Maybe. I can't imagine that NASA would turn down an opportunity to be on that first spacecraft to Mars that SpaceX has, and I can't imagine SpaceX refusing to give them that opportunity. The first missions will be far more than the \$200,000 end goal, but let's say that they assume no reusability for the first launches, and the \$560 million price tag holds. That would lead to a cost of only \$5.6 million! NASA is already going to pay SpaceX around \$25 million per seat to the ISS, possibly more. I can't imagine that if they meet the required safety margin, NASA wouldn't book seats on the first SpaceX manned mission to Mars. They might even pay a premium to be the first out of the hatch;-) But I don't think they will pay for SpaceX to do the R&amp;amp;D, at least, not directly. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-05T17:22:09.563" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18511" PostTypeId="2" ParentId="18495" CreationDate="2016-10-05T18:00:18.287" Score="7" Body="&lt;p&gt;A speculative example for the &lt;a href=&quot;https://www.rt.com/news/165024-proton-booster-sabotage-investigation/&quot;&gt;Proton&lt;/a&gt; can be found. &#xA;This incident is apparently not a closed case, but it is the best I could find.&#xA;According to &lt;a href=&quot;http://tass.com/russia/697388&quot;&gt;TASS&lt;/a&gt; 3 of the 6 angular rate sensors were installed upside-down, and according to sites which do not seem to list their sources (so I have not linked them) the sensors are very hard to install upside-down, but that is conjecture as far as I'm concerned because I cannot find any good sources for that data.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2016-10-05T18:05:27.180" LastActivityDate="2016-10-05T18:05:27.180" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18512" PostTypeId="1" CreationDate="2016-10-05T20:29:00.897" Score="3" ViewCount="845" Body="&lt;p&gt;Some applications on Earth replace wires with radio communication, standard stuff like Bluetooth. It saves mass and maybe complexity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this done for intra-satellite communication and deep space probes too, i.e. within a spacecraft as replacement for wires? And on the space station or other human space flight missions? Are there special problems making it less desirable up there?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorDisplayName="user10509" LastEditDate="2016-10-07T16:11:08.513" LastActivityDate="2020-05-29T03:29:22.823" Title="Are and should satellites use wireless communication internally, rather than cables?" Tags="|engineering|radio-communication|reliability|" AnswerCount="5" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="18513" PostTypeId="1" AcceptedAnswerId="18516" CreationDate="2016-10-05T21:03:50.563" Score="5" ViewCount="7244" Body="&lt;p&gt;As far as I know, time dilation has little effect on Mars compared to Earth, and I'm not asking with that in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm rather asking if the low surface gravity (38% of Earth's) could allow humans to live longer because the lower gravity would strain the body less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I read a study (I'll try to search for the ref.) that if you over-train frequently you could potentially decrease your lifespan, so wouldn't it make sense that if your body had much less stress applied to it (gravity) you could survive longer? Wouldn't the lower gravity also mean that you would have to train more in order to not degrade your muscles too much?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;PS: Could it be that native Martians (if humans are ever born there) would be unable to ever come to earth since their bodies would be adjusted to the martian gravity? Maybe if they exercised a lot?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="17164" LastEditorUserId="7062" LastEditDate="2022-06-09T01:22:47.147" LastActivityDate="2022-06-09T01:22:47.147" Title="Would you age slower on Mars?" Tags="|mars|life|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="18514" PostTypeId="2" ParentId="18428" CreationDate="2016-10-06T02:19:51.763" Score="3" Body="&lt;p&gt;It's very unlikely that the force of outgassing would be sufficient to shift Rosetta or Philae's position on the comet. When Rosetta was initially approaching 67P, the comet was estimated to be losing about a liter of water every second; averaged over a sun-side surface area of roughly 50 square kilometers, this amounts to something like .02 &lt;em&gt;milligrams&lt;/em&gt; of water per square meter per second evaporating. The exact force exerted would depend on the velocity of the departing water molecules, but it's obviously not going to amount to much. Further, note that evaporation will be much slower directly beneath the spacecraft, because the surface will be shadowed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that Philae has been at rest on the surface of 67P for about two years, a period which included 67P's closest approach to the sun, and therefore its most furious outgassing. It is not known for certain that it hasn't shifted position significantly, but its position in pictures taken from Rosetta in September of 2016 is consistent with its estimated position in mid-2015.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-06T14:43:59.547" LastActivityDate="2016-10-06T14:43:59.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18515" PostTypeId="1" CreationDate="2016-10-06T02:32:37.517" Score="3" ViewCount="149" Body="&lt;p&gt;As a follow up of the comments here: &lt;a href=&quot;https://space.stackexchange.com/questions/18482/why-was-the-rosetta-probe-programmed-to-auto-shutoff-at-the-moment-of-hitting/18503?noredirect=1#comment48373_18503&quot;&gt;Why was the Rosetta probe programmed to &amp;quot;auto shutoff&amp;quot; at the moment of hitting the surface?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm fairly convince that a satellite cannot maintain an orbit above a comet, being quickly ousted or crashing by &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The comet pressure&lt;/li&gt;&#xA;&lt;li&gt;The effect of other bodies perturbing the orbit&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Not helping is the very irregular gravity field of the comet.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-10-06T09:29:25.360" Title="How long could a satellite orbit a comet without any control?" Tags="|orbit|rosetta|comet|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18516" PostTypeId="2" ParentId="18513" CreationDate="2016-10-06T02:46:22.210" Score="7" Body="&lt;p&gt;&lt;strong&gt;Short answer:&lt;/strong&gt; Most likely not, but we don't really know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are theories about how gravity affects the physiology of our body, and we know what aspects get affected by lack of gravity. The overwhelming majority of effects noted due to low gravity are negative.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Effects of low gravity&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;As a data point, NASA has determined that the extremely low gravity of space induces symptoms akin to the aging processes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Studies of age-related health problems have shown that the process of physiological adaptation to the low gravity of space induces symptoms also seen in aging (some effects of aging appear to be due to inactivity rather than the aging process itself).&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight.nasa.gov/shuttle/archives/sts-95/factsheets/aging_research.html&quot;&gt;Link&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Other environmental impacts&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;So, gravity by the evidence appears to be a net negative, so what about other factors?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiation comes to mind as a primary consideration&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The surface of Mars has higher radiation levels compared to earth. For example, in the US total natural and artificial radiation exposure is ~1.7mrad/day, whereas the surface of Mars is ~20mrad/day. SPEs, which Mars is more exposed to due lacking magnetosphere and no ozone layer, drastically increase exposure to radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Increased radiation exposure posed a &gt;= 0 increased risk in cancer, so this most likely decreases the average life span of a human on Mars with a best case of not increasing life spans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://spaceradiation.jsc.nasa.gov/irModels/TP-2013-217375.pdf&quot;&gt;Link&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17139" LastActivityDate="2016-10-06T02:46:22.210" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18517" PostTypeId="2" ParentId="18512" CreationDate="2016-10-06T06:46:37.527" Score="15" Body="&lt;p&gt;Radio may reduce some complexity, but it will introduce problems that wiring doesn't have.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Limited bandwidth. &lt;/li&gt;&#xA;&lt;li&gt;Interference. You can separate wiring so it doesn't influence each other. Can't do that with radio: each receiver can hear all transmitters. &lt;/li&gt;&#xA;&lt;li&gt;You're introducing new points of failure. A radio transmitter or receiver contains loads of active components, giving many more failure points than a wire. A wire that's been properly installed is extremely unlikely to break. In my experience, radio links are always less reliable than wires. &lt;/li&gt;&#xA;&lt;li&gt;A radio transmitter uses more power than sending information through a wire. &lt;/li&gt;&#xA;&lt;li&gt;Timing problems. You've added electronics to the link, these take time so sending a signal takes longer. You may have to retransmit after a collision with another transmission, adding more delay. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Radio is not used in cars beyond the Bluetooth link for your phone. Cars use a system like CANbus, which reduces the amount of wiring you need: instead of a switched wire for every application, you just have a 12V lead and a network wire, where all components attached to the network can talk to each other.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radio links are used for convenience, not to reduce complexity.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-10-07T10:16:22.663" LastActivityDate="2016-10-07T10:16:22.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18519" PostTypeId="2" ParentId="18515" CreationDate="2016-10-06T09:29:25.360" Score="4" Body="&lt;p&gt;Comet 67P/Churyumov–Gerasimenko has a mass of about 9.982 gigatonnes, a semimajor axis of 3.4630 AU, and an eccentricity of 0.64102. If it was a point mass, it's Hill sphere would be about &lt;a href=&quot;http://www.wolframalpha.com/input/?i=(3.4630%20AU)*(1-0.64102)*((9.982e12%20kg)%2F(3*(solar%20mass)))%5E(1%2F3)&quot; rel=&quot;nofollow&quot;&gt;221 km&lt;/a&gt;. The Hill sphere is an approximation regarding the volume within which orbits are stable. A more reasonable limit is 1/3 to 1/2 the Hill sphere radius, or 74 to 110 km. I'll use 100 km as a nice round number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;67P is anything but a point mass. The highly non-spherical nature of its gravitational field will definitely be felt at ~100 km from the center of mass. This would make the issues with &lt;a href=&quot;https://science.nasa.gov/science-news/science-at-nasa/2006/06nov_loworbit&quot; rel=&quot;nofollow&quot;&gt;lunar mascons&lt;/a&gt; tiny in comparison.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plus it's a comet. It emits gases and sometimes boulders when it is close to perihelion. The odds of not being plumed or hit while the comet is near perihelion are astronomically low. Even if a body orbiting the comet isn't plumed or hit by a boulder, the non-gravitational forces on the comet itself will almost certainly be enough to make any orbit about the comet unstable.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-10-06T09:29:25.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18520" PostTypeId="2" ParentId="18506" CreationDate="2016-10-06T11:47:15.700" Score="3" Body="&lt;p&gt;The Earth Obervation satellites do use some image processing, especially image compression to reduce the size of the downlinked data. A typical compression pipeline looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;detector -&gt; offset correction -&gt; gain correction -&gt; bad pixel correction -&gt; compression&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of the volume of data, this is typically done on dedicated hardware, shuch as this:&lt;a href=&quot;http://www.space-airbusds.com/media/document/ens_4_coreci_2014_bd.pdf&quot; rel=&quot;nofollow&quot;&gt;http://www.space-airbusds.com/media/document/ens_4_coreci_2014_bd.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Computer vision can also be used for docking guidance. Generally, high-level algorithms that do not need to run in real-time are executed on the ground, where the hardware constraints are not a problem. Onboard spacecrafts, mass and power are limited, and if you use radiation-hardened hardware it is much less capable than off-the-shelf processors.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastActivityDate="2016-10-06T11:47:15.700" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18522" PostTypeId="2" ParentId="18509" CreationDate="2016-10-06T14:13:51.570" Score="7" Body="&lt;p&gt;One thing people noted heavily that was missing in the IAC presentation by Elon Musk was any mention of what it looks like ON Mars, once people arrive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I.e. He focused on getting there, admittedly the hard part right now. He sort of dodged on the radiation question (Sort of fairy since it is a very unclear issue) which is an In-Transit issue.  But on the topic of living on Mars, he did not even mention it in passing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So would NASA fund the missions there?  Possibly they would buy passage from SpaceX since if a mission is going, is offering room/mass, of course they would take it. Especially since it seems likely it will be cheaper than any mission NASA has launched. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would NASA fund a base?  Unclear but if nothing else, they, and people like Zubrin's Mars Society have been working on issues a base would face for a long time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX's approach was literally stated as being the &quot;railroad&quot; to Mars. What is built there, is left open to others.  The railroad built stations at the end.  Likely built some housing for the people building it at the end point.  Likely built some housing for the staff manning the station, but they did not build the city at the end of it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They may have built beautiful monstrous terminals at the end of the line (Union Stations in most US cities) which contribute to the city at the end. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But following that metaphor, the idea is once a railroad to Mars is available, large scale, with people going, and mass available, then companies and individuals will be interested in developing the endpoint.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-10-06T14:13:51.570" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18525" PostTypeId="1" AcceptedAnswerId="18537" CreationDate="2016-10-06T22:12:34.330" Score="3" ViewCount="80" Body="&lt;p&gt;What do most of the jobs done on spacewalks consist of? Changing small pieces on the ISS? Unbolting/unplugging a piece and putting the new one in? &lt;/p&gt;&#xA;" OwnerUserId="17176" LastEditorUserId="17176" LastEditDate="2016-10-07T03:07:50.270" LastActivityDate="2016-10-07T03:18:49.017" Title="Work done on spacewalks" Tags="|iss|eva|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18526" PostTypeId="1" CreationDate="2016-10-06T22:33:23.973" Score="4" ViewCount="2244" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5423/how-does-gps-receiver-synchronize-time-with-gps-satellites&quot;&gt;This&lt;/a&gt; question and its answers explain the basic theory behind GPS. I particularly like the second answer by @jah138. Using jah138's notation, GPS satellites provide extremely accurate $x_i$, $y_i$, and $z_i$. Meanwhile, $c$ and $\rho_i$ are both known with extreme accuracy. $\Delta t$ is known from the cell phone. With multiple satellites, you end up with a system of equations, and can solve for $x_u$, $y_u$, and $z_u$, make relativistic corrections, blah blah blah, you have your answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where I get held up on is that $\Delta t$ is not known with extreme accuracy since (as I understand, and this is where I think I'm wrong) it comes from my relatively cheap and inaccurate cell phone. GPS obviously works which I believe means it requires a very accurate knowledge of $\Delta t$. &lt;strong&gt;How does a cell phone get extremely accurate $\Delta t$ without use of an on board atomic clock?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="12320" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-10-07T11:15:48.007" Title="How does a cell phone GPS receiver work without an onboard atomic clock" Tags="|gps|accuracy|" AnswerCount="2" CommentCount="8" ClosedDate="2016-10-07T12:05:06.933" ContentLicense="CC BY-SA 3.0" />
  <row Id="18527" PostTypeId="1" AcceptedAnswerId="18530" CreationDate="2016-10-06T22:53:38.207" Score="3" ViewCount="236" Body="&lt;p&gt;How do astronauts move around when on space walks? Do they slowly make their way to their destination while keeping a harness connected to the ISS and changing the connections point as they progress?&lt;/p&gt;&#xA;" OwnerUserId="17176" LastActivityDate="2016-10-07T03:25:39.967" Title="How does astronauts move from point a to b on spacewalks" Tags="|iss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18528" PostTypeId="1" AcceptedAnswerId="18536" CreationDate="2016-10-06T23:36:41.023" Score="2" ViewCount="254" Body="&lt;p&gt;Would it be possible to use electromagnets on the outside surface of the ISS to stay anchored to it or is it made with non-ferromagnetic materials?&lt;/p&gt;&#xA;" OwnerUserId="17176" LastActivityDate="2022-11-20T23:53:54.283" Title="ISS Electromagnetic anchoring" Tags="|iss|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18529" PostTypeId="1" CreationDate="2016-10-06T23:42:00.523" Score="38" ViewCount="6153" Body="&lt;p&gt;I was watching a feed on the latest footage of Hurricane Matthew as seen from the ISS just now, relayed from NASA TV. Suddenly, I spotted something poking out of the ISS hull in the footage that appeared to be a shockmounted 180-degree-dual-microphone setup. I just can't understand what's it for even though I've worked in the A/V production field for more than 15 years of my life as both an audio and video technician/engineer.&lt;/p&gt;&#xA;&lt;p&gt;Now, as far as I know, space is supposed to be a vacuum, and what I have been taught all my life and professional career is that sound requires a &lt;em&gt;&lt;strong&gt;medium&lt;/strong&gt;&lt;/em&gt; (i.e. air, water) to travel through. Would someone tell me why on earth there's an array consisting of at least two pieces of 180-degree-separated, shockmounted and foam-windscreened (!??!!) microphones sticking out of the ISS that is supposed to be in orbit outside our atmosphere?&lt;/p&gt;&#xA;&lt;p&gt;It seems to be a permanent installation as well. Only a few M/S microphone setups would make sense in a 180-degree-L-R-setup like that down here, and even in a case of M/S micing, there should be a third condenser mic somewhere.&lt;/p&gt;&#xA;&lt;p&gt;And that would make remote sense only IF this was taking place inside the atmosphere or somewhere underwater. The mics seem to be shockmounted and have a windscreen foam around their capsules. This makes absolutely no sense either, since as far as I know, no sound is transmitted through the vacuum of space, apart from what's being &amp;quot;amplified&amp;quot; through the so-called artificial atmospheres such as the astronaut suits or the oxygen-pressurized hull of a spaceship or a space station.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VNeJy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VNeJy.png&quot; alt=&quot;180 degree stereo pair microphone setup outside the ISS?&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Please see the pic above for reference. Original URL of the live stream relay is/was at &lt;s&gt;https://www.youtube.com/watch?v=9x3HpNaaOVM&lt;/s&gt; (broken video link with no title given)&lt;/p&gt;&#xA;" OwnerUserId="17178" LastEditorUserId="12102" LastEditDate="2021-08-03T23:50:06.640" LastActivityDate="2021-08-03T23:50:06.640" Title="Why does there appear to be a 180-degree stereo microphone array outside the ISS?" Tags="|iss|space-station|data-transmission|vacuum|identify-this-object|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="18530" PostTypeId="2" ParentId="18527" CreationDate="2016-10-07T00:06:53.897" Score="5" Body="&lt;p&gt;They carry the SAFER emergency pack for case of falling off the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They clip themselves around the station with tethers and connection points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian segment has a couple of 'cranes' called &lt;a href=&quot;https://en.wikipedia.org/wiki/Strela_(crane)&quot; rel=&quot;nofollow&quot;&gt;Strela&lt;/a&gt; that start at the airlocks, and are telescoping poles, to make a shortcut to other modules on the Russian segment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For larger moves around the rest of the station, there is the CanadArm2 and on the truss itself, there is a cart, that runs on a track along the truss, specifically for this purpose. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6944" LastEditDate="2016-10-07T03:25:39.967" LastActivityDate="2016-10-07T03:25:39.967" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18532" PostTypeId="1" AcceptedAnswerId="18533" CreationDate="2016-10-07T00:49:30.547" Score="4" ViewCount="266" Body="&lt;p&gt;How is the cargo organized inside a payload ship like spacex's dragon?&#xA;Is there different size containers for different items inside the ship? How are they secured inside?&#xA;Finally, how is it unloaded into the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(not asking about the dragon specifically) &lt;/p&gt;&#xA;" OwnerUserId="17176" LastActivityDate="2016-10-07T08:13:15.260" Title="Cargo ship inner organisation" Tags="|spacex|iss|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18533" PostTypeId="2" ParentId="18532" CreationDate="2016-10-07T01:30:07.037" Score="6" Body="&lt;p&gt;For the MPLMs that were carried up by Shuttle, everything was packed in soft-ish fabric bags strapped to the walls.  A lot of custom cut foam was used to secure the items. So when you opened one of the bags, you'd see foam, and you had to pull all that out to get the items.  Typically this foam would go back down as garbage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bags can be custom sizes for large items.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An aisle is left down the middle for access, as shown in this &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/4/4c/STS-135_Sandy_Magnus_floats_freely_in_the_Raffaello_MPLM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;picture&lt;/a&gt; of Sandy Magnus inside the MPLM that STS-135 took up. (this MPLM is probably partially unpacked)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/B8HIT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/B8HIT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the Cygnus is basically an MPLM, I imagine it is the same.  No personal knowledge of Dragon, but it sure looks &lt;a href=&quot;https://nssphoenix.files.wordpress.com/2012/04/dragon-interior-cargo-manifest-01.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;similar&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ym8KZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ym8KZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, how is it unloaded - by hand.  On shuttle flights one of the mission specialists was designated the Loadmaster who kept up with and directed the unloading, unpacking, and reloading.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a funny (if somewhat sexist) shot in one of the NASA IMAX movies (I think Space Station 3D) where the male crewmembers &quot;unload&quot; Marsha Ivins as though she were cargo and pass her up through the hatches just like the other items of cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Loading the cargo into the MPLM was no picnic either.  In this picture I took in the SSPF, you can see the MPLM is in a cradle that lets it rotate on its long axis.  The white cabinet thing that blocks our view into the MPLM controls a movable access arm that allows workers to get in and reach any part of the MPLM to attach the cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0Av7l.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0Av7l.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-10-07T01:46:01.387" LastActivityDate="2016-10-07T01:46:01.387" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18535" PostTypeId="2" ParentId="18529" CreationDate="2016-10-07T02:09:06.240" Score="55" Body="&lt;p&gt;That is a UHF antenna.  It was well placed on the Lab to get in the way of robotics ops during space station assembly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a picture of a different UHF antenna unit (this one is on the P1 truss segment) but it's clearly the same device.  Fortunately this is from a credible source, NASA's &lt;a href=&quot;https://www.nasa.gov/pdf/167129main_Systems.pdf&quot; rel=&quot;noreferrer&quot;&gt;ISS Flight Systems brochure&lt;/a&gt; (warning, pdf). I can't quite figure out the page numbering in this pdf but I think it's page 60.  Anyway, if you search the pdf for &quot;UHF&quot; you will find it quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1Xy3y.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1Xy3y.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, all the UHF antennas I know of on the shuttle / station look like that &quot;mike&quot; shape.  You can see a single one on the shuttle airlock in this picture (the ISS antenna have two of the &quot;mikes&quot; each).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/C8swa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C8swa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, here is an &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/209964main_useva12.pdf&quot; rel=&quot;noreferrer&quot;&gt;EVA checklist&lt;/a&gt; listing hazards for the crew to avoid.  You will note that UHF antennae are listed on the Lab and P1, the exact locations of these antennae shown in my answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CKoSY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CKoSY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-10-07T14:36:51.833" LastActivityDate="2016-10-07T14:36:51.833" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="18536" PostTypeId="2" ParentId="18528" CreationDate="2016-10-07T02:55:27.130" Score="4" Body="&lt;p&gt;Sadly, they are made mostly of aluminum, as explained in this &lt;a href=&quot;https://www.quora.com/What-types-of-materials-is-the-International-Space-Stations-exterior-and-interior-made-of&quot; rel=&quot;nofollow&quot;&gt;quora answer&lt;/a&gt; and also on this &lt;a href=&quot;http://science1.nasa.gov/science-news/science-at-nasa/2001/ast14mar_1/&quot; rel=&quot;nofollow&quot;&gt;NASA page&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Because each of the aluminum-can shaped components of the Station has&#xA;  to be lifted into orbit, minimizing weight is crucial. Lightweight&#xA;  aluminum, rather than steel, comprises most of the outer shell for the&#xA;  modules.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-10-07T02:55:27.130" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18537" PostTypeId="2" ParentId="18525" CreationDate="2016-10-07T03:18:49.017" Score="3" Body="&lt;p&gt;You can read a brief summary of every ISS EVA ever (there have been almost 200 of them) at &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_International_Space_Station_spacewalks&quot; rel=&quot;nofollow&quot;&gt;this wikipedia page&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I were to attempt to summarize, there would be a few common categories.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Connecting cables and wires&lt;/li&gt;&#xA;&lt;li&gt;Mounting devices such as antennae, Orbital Replacement Units (pumps, relay boxes, etc), cameras, et al&lt;/li&gt;&#xA;&lt;li&gt;Working with the robotic arms to attach large elements to the station with bolts and remove their launch restraints&lt;/li&gt;&#xA;&lt;li&gt;Various remove-and-replace repair jobs (including greasing bearings)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-10-07T03:18:49.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18538" PostTypeId="1" AcceptedAnswerId="18539" CreationDate="2016-10-07T03:30:20.813" Score="12" ViewCount="10621" Body="&lt;p&gt;I just found this incredible video &lt;a href=&quot;https://www.youtube.com/watch?v=4QXZ2RzN_Oo&quot; rel=&quot;noreferrer&quot;&gt;KSP Doesn't Teach: Rocket Engine Plumbing&lt;/a&gt; and ran through it a few times. It's very helpful in the way it outlines the various issues first, and then designs the designs that address them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a &lt;em&gt;Full FLow Staged Combustion Cycle&lt;/em&gt; engine there are two combustion chambers driving two turbines; one to drive the fuel pump, and the other to drive the oxidizer pump. This &quot;two shaft&quot; solution solves problems that can be caused by leaky seals as explained in the video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I understand correctly, these combustion chambers are run either fuel-rich or oxidizer-rich so that they can deliver enough power without running so hot that the turbine could not handle it. By running on either side of the optimal mixture, the unused gas still expands and produces power by absorbing some of the heat generated by the reacted gases. And since the system is closed, it gets used later anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I am still having trouble understanding this diagram. If oxidizer and fuel are both cryogenic, why is it only the fuel that gets pre-heated? And are there additional propellant feeds from the pumps to the main combustion chamber, or does ALL of the propellant enter via the turbines in the form of incompletely combusted exhaust from the two smaller combustion chambers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0l9pF.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0l9pF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4QXZ2RzN_Oo?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-07T08:15:16.013" LastActivityDate="2017-09-08T18:32:27.553" Title="Rocket Engine Plumbing: Still don't understand the Full Flow Staged Combustion Cycle" Tags="|spacex|propulsion|engine-design|liquid-fuel|raptor|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18539" PostTypeId="2" ParentId="18538" CreationDate="2016-10-07T03:44:36.673" Score="13" Body="&lt;p&gt;1) If oxidizer and fuel are both cryogenic, why is it only the fuel that gets pre-heated? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume you are talking about the H2 that flows through the nozzle? You have it a bit backwards.  This is to cool the nozzle, not heat the hydrogen (although both happen of course) and H2 is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Coolant&quot;&gt;great heat transfer agent&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) And are there additional propellant feeds from the pumps to the main combustion chamber, or does ALL of the propellant enter via the turbines in the form of incompletely combusted exhaust from the two smaller combustion chambers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole point of the staged combustion cycle is that all propellant leaves via the nozzle so that you get efficient thrusting from all of it.  In the schematic you show, a &quot;Full Flow&quot; staged combustion engine, all the propellants flow through the turbines.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle Main Engine, although staged combustion, was not full flow, because much of the O2 entered the main combustion chamber without passing through the turbines.  They had enough problems without dealing with the O2 rich turbine gas.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-10-07T03:47:48.573" LastActivityDate="2016-10-07T03:47:48.573" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18540" PostTypeId="2" ParentId="18532" CreationDate="2016-10-07T08:13:15.260" Score="2" Body="&lt;p&gt;In the ESA ATV, &lt;a href=&quot;http://www.dlr.de/iss/en/DesktopDefault.aspx/tabid-4616/7592_read-11376/gallery-1/gallery_read-Image.1.4723/&quot; rel=&quot;nofollow noreferrer&quot;&gt;racks were installed&lt;/a&gt;, like this:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dfVaF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dfVaF.jpg&quot; alt=&quot;Empty ATV&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bags were &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Crew_in_ATV_with_Jules_Verne_manuscript.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;secured to the racks&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RdUCd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RdUCd.jpg&quot; alt=&quot;ATV loaded&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-07T08:13:15.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18541" PostTypeId="2" ParentId="17964" CreationDate="2016-10-07T08:29:06.923" Score="1" Body="&lt;p&gt;Well one way is to just keep checking manually, until you find that something has changed. If there is a better way please post an answer!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On August 31st, a few days after the flyby, a new solution was generated. If you look for the &lt;code&gt;&amp;lt;- HERE!&lt;/code&gt; markers you can see where older and newer calculations are spliced together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flyby in the new solution is the same distance from Jupiter as the old one, and only about 30 seconds earlier:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j9KSV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j9KSV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Plot of the distance from the Juno spacecraft to the center of Jupiter minus an equatorial radius of 71492 km. Solid line is the newer solution, dashed line is the older solution.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;OLD:&lt;/h2&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; SPACECRAFT TRAJECTORY:  &#xA;   From JPL navigation team. Predicts after 2016-Apr-18.0.   &amp;lt;- HERE!&#xA;&#xA;   Trajectory name                    Start (CT)          Stop (CT)&#xA;   ------------------------------   -----------------   -----------------&#xA;   rec_110805_111026_120302         2011-Aug-05 17:19   2011 OCT 26 00:00&#xA;   rec_111026_120308_120726         2011 OCT 26 00:00   2012 MAR 08 12:00   &#xA;   rec_120308_120825_121109         2012 MAR 08 12:00   2012 AUG 25 00:00&#xA;   rec_120825_130515_130708         2012 AUG 25 00:00   2013 MAY 15 00:00&#xA;   rec_130515_131005_131031         2013 MAY 15 00:00   2013 OCT 05 12:00&#xA;   rec_131005_131014_131101_reco    2013 OCT 05 12:00   2013 OCT 14 00:01&#xA;   rec_131014_131114_140222         2013 OCT 14 00:01   2013 NOV 14 06:00&#xA;   ref_131114_171017_140311_V0.2    2013 NOV 14 06:00   2015 MAR 26 00:00&#xA;   ref_150326_180221_150326_V0.1    2015 MAR 26 00:00   2016 FEB 26 00:01&#xA;&#xA;   ref_160226_180221_160226_V0.1    2016 FEB 26 00:00   2018 FEB 21 11:40   &amp;lt;- HERE!&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;h2&gt;NEWER:&lt;/h2&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;SPACECRAFT TRAJECTORY:  &#xA;   From JPL navigation team. Predicts after 2016-Aug-22.   &amp;lt;- HERE!&#xA;&#xA;   Trajectory name                       Start (TDB)         Stop (TDB)&#xA;   ------------------------------     -----------------   -----------------&#xA;   rec_110805_111026_120302           2011-Aug-05 17:19   2011 OCT 26 00:00&#xA;   rec_111026_120308_120726           2011 OCT 26 00:00   2012 MAR 08 12:00   &#xA;   rec_120308_120825_121109           2012 MAR 08 12:00   2012 AUG 25 00:00&#xA;   rec_120825_130515_130708           2012 AUG 25 00:00   2013 MAY 15 00:00&#xA;   rec_130515_131005_131031           2013 MAY 15 00:00   2013 OCT 05 12:00&#xA;   rec_131005_131014_131101_reco      2013 OCT 05 12:00   2013 OCT 14 00:01&#xA;   rec_131014_131114_140222           2013 OCT 14 00:01   2013 NOV 14 06:00&#xA;   ref_131114_171017_140311_V0.2      2013 NOV 14 06:00   2015 MAR 26 00:00&#xA;   ref_150326_180221_150326_V0.1      2015 MAR 26 00:00   2016 FEB 26 00:01&#xA;&#xA;   ref_160226_180221_160226_V0.1      2016 FEB 26 00:00   2016 APR 13 18:00   &amp;lt;- HERE!&#xA;   pre_160413_161016_160818_pj01_V0.1 2016 APR 13 18:00   2016 OCT 16 23:00&#xA;   ref_160226_180221_160226_V0.5      2016 OCT 16 23:00   2018 FEB 21 11:40&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-07T12:13:57.463" LastActivityDate="2016-10-07T12:13:57.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18544" PostTypeId="2" ParentId="18512" CreationDate="2016-10-07T10:34:28.723" Score="2" Body="&lt;p&gt;Satellites are unmanned, nobody and nothing onboard will appreciate the comfort of a wireless connection. Every electronic subsystem is fixed to its location inside the spacecraft. Therefore any needed interconnection is possible and done using wires, cables, serial busses but not wireless. Using wireless connections would only add unnecessary complexitiy, additional power consumption, additional heat, more failure points and more possiblities for design errors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any interconnection problem has to be solved as simple and reliable as possible, thus any wireless connection has to be avoided.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-10-07T10:34:28.723" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18545" PostTypeId="2" ParentId="18526" CreationDate="2016-10-07T10:38:51.977" Score="1" Body="&lt;p&gt;As you said yourself, you get a system of equations from which the location can be calculated.&#xA;Here the Δt consists of the difference between the time of the signal being sent at the satellite and the time of arrival. Now it is important that if we have 2 satellites next to each other in space, the Δt should be the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This requires that the 2 clocks in the satellites should be equal, hence the atomic clocks which are synchronized from earth. But the clock in the receiver is in both cases the same, so even if your phone is of by 2 hours, the Δt will still be the same in both cases. &lt;/p&gt;&#xA;" OwnerUserId="17153" LastActivityDate="2016-10-07T10:38:51.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18546" PostTypeId="2" ParentId="18526" CreationDate="2016-10-07T11:08:25.523" Score="4" Body="&lt;p&gt;Actually, the accuracy of a quartz clock is sufficient, since only very short periods are measured by the GPS receiver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following is a simple sketch on how GPS works: Three satellites A, B, C send out a signal at the same time, containing their position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the receiver knows the position of the satellites, but since it does not have an atomic clock, it does not know the time of flight, i.e. the distance to each satellite. Instead, it measures the delay between the signals, and so the difference in distance to the satellites.&lt;br&gt;&#xA;Mathematically, the receiver must be on a hyperbola, the blue one from the delay between A and B and green from the delay between A and C. (Usually, a hyperbola consist of two curves, but since the signal from A came later than that of B, the receiver must be on the curve closer to B. I didn't draw the other curve.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HSoCz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HSoCz.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, a quartz clock isn't as precise as an atomic clock. It is said that it can drift up to 30s per month, i.e. the clock is too slow/fast by a factor of 0.00001. If the delay between the signals of two satellites is about 1 second, this gives an error of 300000km/s*0.00001s = 3km.&lt;br&gt;&#xA;But GPS satellites don't send just a  single &quot;ping&quot; each second, they continuously send a data stream including time stamp. From the time stamp, a coarse delay between the signals is calculated. Looking at the waveform of the signals, the single bits have a duration of about 1ms, so it is possible to determine the delay between two signals with a precision of better than 1ms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If the quartz clock has to measure time delays of not more than 1ms, the drift error corresponds to not more than 3 meters!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there are many other uncertainties e.g. due to weather, the receiver uses more satellites than necessary to calculate the most probable position. In my sketch, only timing A&amp;lt;-&gt;B and A&amp;lt;-&gt;C was used, if we use B&amp;lt;-&gt;C, too, we get three possible positions, and the truth is somewhere in the middle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xqLlE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xqLlE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The three curves would cross on one point if there were no uncertainties)&lt;/p&gt;&#xA;" OwnerUserId="11749" LastEditorUserId="11749" LastEditDate="2016-10-07T11:15:48.007" LastActivityDate="2016-10-07T11:15:48.007" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18549" PostTypeId="2" ParentId="18512" CreationDate="2016-10-07T12:08:34.720" Score="5" Body="&lt;p&gt;There's another reason that makes this a bad idea. Many satellites are communications satellites of some kind, receiving communication data from a long distance and rebroadcasting it. Those depend on having a very quiet band. Transmitters tend to leak RF energy outside of their band, and it would make it difficult to hear those far away signals if there are additional RF transmitters on the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-07T12:08:34.720" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18550" PostTypeId="2" ParentId="18512" CreationDate="2016-10-07T12:59:16.660" Score="6" Body="&lt;p&gt;Alright, let's put on our engineering caps. To have a wireless connection, we need:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Wires connecting the subsystem to the wireless transmitter&lt;/li&gt;&#xA;&lt;li&gt;Wires connecting the transmitter to the power supply&lt;/li&gt;&#xA;&lt;li&gt;An internal environment that's fairly well shielded against RF, but allows wireless signals in its volume&lt;/li&gt;&#xA;&lt;li&gt;Wires connecting the receiver to the element the subsystem is trying to communicate with&lt;/li&gt;&#xA;&lt;li&gt;Wires connecting the receiver to the power supply&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And this is just for one-way communication between a single set of systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, for wired communication, we need:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A wire&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In the grand scheme of things, wired connections (Over the short distances inside a vehicle, anyway) are simpler, cheaper, more reliable, and lighter. Going wireless would add huge amounts of cost, complexity, mass, and potential failure points - all of which space programs like to avoid if possible.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2016-10-07T12:59:16.660" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18551" PostTypeId="1" AcceptedAnswerId="18552" CreationDate="2016-10-07T16:56:49.147" Score="5" ViewCount="383" Body="&lt;p&gt;Are the tools like the driver and the bolts on the ISS magnetized to prevent losing them when doing repair/maintenance? &#xA;Is there a special way to prevent it other than magnets?&lt;/p&gt;&#xA;" OwnerUserId="17176" LastEditorUserId="6944" LastEditDate="2016-10-07T17:19:34.600" LastActivityDate="2020-11-10T16:11:50.553" Title="ISS Repair/Maintenance Tools" Tags="|iss|eva|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18552" PostTypeId="2" ParentId="18551" CreationDate="2016-10-07T17:06:12.340" Score="7" Body="&lt;p&gt;Not magnetized, just good old fashioned tethers. All the tools have a place to connect a tether.  You can look at a catalog of all the EVA tools (it's kind of old, and a huge 750 page pdf) at this &lt;a href=&quot;https://archive.org/details/eva-tools-and-equipment-reference-book-nov-83-2&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;For example, here's everyone's favorite tool, the hammer. Notice the tether point.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3bJNE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3bJNE.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-11-10T16:11:50.553" LastActivityDate="2020-11-10T16:11:50.553" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18553" PostTypeId="1" AcceptedAnswerId="18554" CreationDate="2016-10-07T21:07:29.483" Score="26" ViewCount="15782" Body="&lt;p&gt;Okay, time for a stupid question, but it keeps me up at night trying to figure it out.&lt;/p&gt;&#xA;&lt;p&gt;The ISS 'route' appears on paper as such:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YI9Xi.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YI9Xi.png&quot; alt=&quot;ISS path&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I am imagining it has to do with a combination of its orbit and the tilt of the Earth, but if this is the case, on spring/fall equinox it should be a flat line, no?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Can someone explain why it's in a sine wave?&lt;/strong&gt;  I've been playing with my globe trying to see how the tilt of the Earth might affect the path.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-03-07T15:04:56.120" Title="Why does the ISS track appear to be sinusoidal?" Tags="|orbital-mechanics|iss|" AnswerCount="6" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18554" PostTypeId="2" ParentId="18553" CreationDate="2016-10-07T21:29:23.357" Score="40" Body="&lt;p&gt;The trajectory of the ISS only appears to be sinusoidal when it is mapped onto the flat plane. In reality the orbit around Earth looks like that:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FY7Z1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FY7Z1.jpg&quot; alt=&quot;Orbit of the ISS&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer to why the ISS' orbit appears to be looking like a sine wave is &lt;a href=&quot;https://en.wikipedia.org/wiki/Simple_harmonic_motion&quot; rel=&quot;noreferrer&quot;&gt;harmonic motion&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Circular_motion&quot; rel=&quot;noreferrer&quot;&gt;circular motion&lt;/a&gt;. The ISS is moving along the circumference of a circle (the orbit) around the earth:&lt;a href=&quot;https://i.stack.imgur.com/qfuBe.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qfuBe.png&quot; alt=&quot;circular motion&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\omega$ is the &lt;em&gt;angular speed&lt;/em&gt;, $\Phi$ is the &lt;em&gt;initial angle&lt;/em&gt; (phase), $r$ is the &lt;em&gt;radius&lt;/em&gt; of the orbit and $P$ is the position of the ISS. The angle at time &lt;em&gt;t&lt;/em&gt; is given by $ \omega t + \Phi$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you observe the position $P$ at time $t$ you can express the $x$- and $y$-coordinate as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\ P_{x}(t)=rcos(\omega t+\Phi)$ and similarly $\ P_{y}(t)=rsin(\omega t+\Phi)$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In your case the ISS moves in circular motion around Earth and it's position is simply $P$ at time $t$. If you observe this point, standing at the origion $O$ of the above circle and project that onto the $OY$ axis you get&#xA;$\ y(t)=rsin(\omega t+\Phi)$.&#xA;Then plotting the function you get something like this:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/xAull.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xAull.png&quot; alt=&quot;ISS orbit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the border between day and night also appears to be curved due to the same reason. The line isn't closed because while the ISS revolves around Earth once, Earth itself has rotated by a bit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When mapping the surface of a sphere onto the plane you can only keep one line straight. The equator of the earth appears as a straight line in both pictures.&#xA;Also keep in mind that this is a simplification of different projection methods used and only explains why the orbit seems to look like a sine-wave. In reality more complex projection methods are used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This answer is a rewrite using my original answer and the technical details from &lt;a href=&quot;/users/17200/where-is-tftp&quot;&gt;where_is_tftp&lt;/a&gt;'s &lt;a href=&quot;/a/18561/415&quot;&gt;answer&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="17194" LastEditorUserId="415" LastEditDate="2016-10-08T21:56:00.790" LastActivityDate="2016-10-08T21:56:00.790" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="18555" PostTypeId="1" AcceptedAnswerId="18556" CreationDate="2016-10-07T22:41:27.927" Score="4" ViewCount="3346" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/18165/12102&quot;&gt;this answer&lt;/a&gt; to &lt;em&gt;Why do the shapes of these two exhaust plumes from an Atlas V 411 look different?&lt;/em&gt; the 'Summerfield criterion' is mentioned, saying:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Historically, the Summerfield criterion was used, where the minimum pressure in a nozzle must be greater than 0.35...0.4*atmospheric pressure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;What is the 'Summerfield criterion'? And can you explain how the exhaust pressure of 0.35 - 0.4 atm was arrived at?&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="12102" LastEditDate="2021-01-27T20:19:55.290" LastActivityDate="2021-01-27T20:19:55.290" Title="What is the 'Summerfield criterion' regarding rocket exhaust expansion" Tags="|engine-design|exhaust|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18556" PostTypeId="2" ParentId="18555" CreationDate="2016-10-07T23:41:08.080" Score="6" Body="&lt;p&gt;The &lt;a href=&quot;http://elib.dlr.de/49253/1/AIAA2005-3940.pdf&quot; rel=&quot;nofollow&quot;&gt;Ralf Stark paper linked&lt;/a&gt; from the answer which mentions the Summerfield and Schmucker criteria ends with a list of references; among them are a 1973 Schmucker paper (translated in 1984, &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19840011402.pdf&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;) and a widely cited 1954 Summerfield paper, “Flow Separation in Overexpanded Supersonic Exhaust Nozzles”, Jet propulsion, Vol. 24, No. 9, page 319-321, 1954. I cannot find the latter paper online with a brief search.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Summerfield criterion in question is a rule of thumb, stating that flow separation in an overexpanded nozzle occurs at about 0.4 times ambient pressure. From the Schmucker paper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Initial investigations on flow separation in nozzles were conducted by Buchner, Prandtl, Meyer, Flugel and Stanton and published by Stodola. After World War II the increased research work in the area of rocket engines led to numerous investigations of this problem. Forster and Cowles at the California Institute of Technology undertook the first publicized hot-gas tests with a small nitric acid/aniline motor. The result of this work was the separation condition that in an over-expansion to 40 percent of the surrounding pressure, the flow separates from the wall. This number is often called the &quot;Summerfield Criterion&quot; and is today considered to be a conservative design value.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to the Schmucker paper, Summerfield's results are for a relatively low-pressure engine, where chamber pressure is 15-20 times ambient. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact value varies with the operating characteristics of the engine; there seems to be a half-century tradition of experimental papers giving different flow separation criteria for different sets of conditions and citing everyone else who's done the same. The appendix of the Stark paper contains a table listing a number of these.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that Martin Summerfield has two criteria named after him! His &lt;a href=&quot;http://congress.cimne.com/eucass2013/admin/files/fileabstract/a574.pdf&quot; rel=&quot;nofollow&quot;&gt;earlier one&lt;/a&gt;, jointly formulated with his colleague Frank Malina (and so, often referred to as the Summerfield-Malina criterion) in 1946, states that (given certain assumptions) an optimal multistage rocket has equal payload mass ratios between stages.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-09T15:46:16.840" LastActivityDate="2016-10-09T15:46:16.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18557" PostTypeId="1" CreationDate="2016-10-08T00:39:42.803" Score="1" ViewCount="134" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/18528/12102&quot;&gt;ISS Electromagnetic anchoring&lt;/a&gt; asks if the hull of the ISS were ferromagnetic, which would allow astronauts on EVAs to use magnetic grapples to assist in moving around. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer &lt;a href=&quot;https://space.stackexchange.com/a/18536/12102&quot;&gt;sadly, no&lt;/a&gt; made me wonder if perhaps there might might be &lt;em&gt;an up side&lt;/em&gt; to the hull being non-magnetic as well (besides of course the reasons given in the answer).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the hull of the ISS were substantially ferromagnetic, what kinds of problems would that cause?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-10-08T00:39:42.803" Title="If the ISS hull were ferromagnetic what kinds of problems would that cause?" Tags="|iss|spacecraft|magnetic-field|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18558" PostTypeId="1" AcceptedAnswerId="19586" CreationDate="2016-10-08T00:56:13.837" Score="4" ViewCount="1632" Body="&lt;p&gt;I just read in &lt;a href=&quot;http://www.satnews.com/story.php?number=31922882&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satnews Daily&lt;/a&gt; that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A 3U CubeSat funded by NASA and led by the University of Colorado’s Laboratory for Atmospheric and Space Physics, MinXSS requires precision pointing to carry out its scientific objectives.  Integrating an array of high-performance miniaturized space sensors and actuators, including BCT’s Nano Star Tracker and Micro Reaction Wheels, the XACT unit provides state-of-the-art attitude control for small spacecraft like MinXSS.  On-orbit telemetry from the XACT unit and independent spacecraft instrumentation indicates &lt;strong&gt;1-sigma cross-axis pointing error better than 8 arcseconds&lt;/strong&gt;, which is less than the width of the Empire State Building as viewed from Los Angeles.  (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;8 arcseconds is only about 0.002 degrees! Of course a &quot;big sat&quot; like a major telescope with high magnification (or high precision) can do much better, I thought that in general 1U to 3U cubesats were in the 0.1 to 10 degree range - mostly determined by a balance between cost and requirements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this some kind of record for a 3U cubesat?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; the &lt;a href=&quot;http://www.nasa.gov/feature/goddard/2016/minxss-cubesat-deployed-from-iss-to-study-sun-s-soft-x-rays&quot; rel=&quot;nofollow noreferrer&quot;&gt;MinXSS&lt;/a&gt; (and &lt;a href=&quot;http://www.nasa.gov/content/goddard/nasa-funded-mission-studies-the-sun-in-soft-x-rays&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://arxiv.org/abs/1508.05354&quot; rel=&quot;nofollow noreferrer&quot;&gt;ArXiv&lt;/a&gt;) generally &lt;strong&gt;points at the sun&lt;/strong&gt; to measure low energy X-ray spectra. So when it is making measurements it is not rotating with respect to the stars, as an earth-pointing spacecraft would have to do. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I figured out where the need for high pointing accuracy might come from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9j83o.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9j83o.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF made from frames of the video &lt;a href=&quot;https://www.youtube.com/watch?v=hmosNUuCC_I&quot; rel=&quot;nofollow noreferrer&quot;&gt;AR1226 M2.5 Flare and CME, June 7, 2011 05:00-13:00 AIA 211/193/171 Composite&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The X-ray detector inside the &lt;a href=&quot;http://amptek.com/products/x-123-complete-x-ray-spectrometer-with-si-pin-detector/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Amptek X123&lt;/a&gt; (and &lt;a href=&quot;http://amptek.com/products/x-123sdd-complete-x-ray-spectrometer-with-silicon-drift-detector-sdd/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;) is a single silicon PIN diode, about 500 microns thick, with areas between 6 and 25 square millimeters shown as standard on the website. An optional &lt;a href=&quot;http://amptek.com/products/exvc-x-ray-collimator-kit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;pinhole collimator&lt;/a&gt; is also available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The collimator is actually two pinholes spaced about 37 millimeters apart, and they come standard down to 25 microns in diameter. So it's possible that the FWHM could be of the order of 0.04 degrees configured off-the-shelf, and even narrower if a larger distance between pinholes was chosen. They may be able to point at the corona or a CME specifically and record a soft X-ray spectrum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/hmosNUuCC_I?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-08T01:47:47.960" LastActivityDate="2016-12-27T18:23:10.880" Title="Cubesat with 8 arcsecond pointing accuracy - is that typical, or exceptional?" Tags="|attitude|cubesat|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18559" PostTypeId="2" ParentId="18558" CreationDate="2016-10-08T01:06:13.417" Score="7" Body="&lt;p&gt;I'm not sure whether it's a record, but that's definitely an unusually fine pointing accuracy for a cubesat.  Your range of 0.1 to 10 degrees is indeed fairly typical.  It's mostly about requirements - because cubesats are physically small, they can't have very high gain antennas that need to be pointed (5 degrees is typically sufficient even for a high-end cubesat downlink such as Planet Labs' 200MBps X-band transmitter).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise the limited aperture means they don't tend to have telescopes or other instruments with very narrow fields of view &amp;lt;&amp;lt; 1 degree.  So mostly there's no necessity to point better than a degree or so, which is readily achievable with low-cost magnetometers, sun sensors, MEMS gyroscopes, magnetorquers and simple reaction wheels.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2016-10-08T01:06:13.417" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18560" PostTypeId="2" ParentId="18528" CreationDate="2016-10-08T07:54:54.563" Score="1" Body="&lt;p&gt;Let's see if there are any advantages to using magnetic grapples instead of the current system.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those magnetic grapples would have to be electromagnets. Permanent magnets strong enough to prevent an astronaut from flying away would be very hard to remove.&lt;br&gt;&#xA;So instead of passive elements (two short cables and carabiners), you'd need a power source, electric cables and switches, and two magnets, plus a backup (two short cables and carabiners) in case you have a power failure. So you'd add several kg worth of equipment, plus two more cables to get tangled in.&lt;br&gt;&#xA;On the plus side, you'd be able to place the magnet anywhere on the hull (if you made the entire outer hull/Whipple shield out of steel), and you'd save a few seconds because you don't have to manipulate a carabiner through those really thick gloves.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-08T07:54:54.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18561" PostTypeId="2" ParentId="18553" CreationDate="2016-10-08T12:21:19.657" Score="11" Body="&lt;p&gt;The answer is &lt;a href=&quot;https://en.wikipedia.org/wiki/Simple_harmonic_motion&quot; rel=&quot;noreferrer&quot;&gt;harmonic motion&lt;/a&gt; and circular motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you observe a point moving in circular motion over the circumference&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/t57DR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t57DR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and you map this move into the diameter of length $x_m$ in $OX$ axis - you get a harmonic oscillation equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$x(t) = x_m cos(\omega t + \phi)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you map this into $OY$ axis you get the same motion, just seen differently as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$y(t) = x_m sin(\omega t + \phi)$$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In your case the ISS moves in circular motion and it's coordinate point is $P'$ in a time $t$. But the projection of this point (seen from observer located on the Earth in the middle $O$ of our circle) onto $OY$ axis is $$y(t) = x_m sin(\omega t + \phi)$$ and when observed continuously the point seem to be moving in harmonic motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have just rediscovered what Galileo Galilei found in 1610, so you may well be on a good way to other discoveries - good luck!&lt;/p&gt;&#xA;" OwnerUserId="17200" LastActivityDate="2016-10-08T12:21:19.657" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18562" PostTypeId="1" CreationDate="2016-10-08T12:58:42.087" Score="2" ViewCount="1107" Body="&lt;p&gt;As the title states, I am trying to find a way to measure the dipole moment(m) for my magnetorquer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am planning to use magnetorquers for desaturating my reaction wheels for a project. I understand that there is a relation m=nIA where m is the dipole moment, I is the current, n is the no. of turns and A is the cross sectional area of the torque rod. I intend to use that formula to calculate how much voltage to apply to my magnetorquer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I would also like to check that I am actually getting the correct dipole moment physically. Are there any ways to experimentally measure the dipole moment strength of my magnetorquer? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have done some research and came across this formula on this wikipedia page:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kr8aY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kr8aY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic_moment&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Magnetic_moment&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My plan is to use a magnetometer to measure the magnetic field strength at a distance r ( This would then be |r| ) away from the magnetorquer, and assume that the position vector and magnetic moment vector are parallel to simply the dot product of m and r.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would this work?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks&lt;/p&gt;&#xA;" OwnerUserId="16924" LastActivityDate="2023-04-15T03:44:49.943" Title="Ways to measure dipole moment of magnetorquer" Tags="|magnetorquer|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18563" PostTypeId="1" CreationDate="2016-10-08T13:38:16.723" Score="5" ViewCount="490" Body="&lt;p&gt;I'm quite interested in small rockets-balloons that can reach more than 50km, &#xA;thus I want to build one myself (With my school financial support). I just want it to carry like a 350g-450g equipment. My aim is to make some really high quality footage up in there. I (almost) get that a meteorological balloon would be quite ideal, but still, it just wouldn't be original to vindicate the investment.&#xA;That said is it even possible to make it for under 600 dollars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm also not sure about the availability of the things since I live in the central Europe&lt;/p&gt;&#xA;" OwnerUserId="17201" LastEditorUserId="8693" LastEditDate="2016-10-08T17:17:55.847" LastActivityDate="2017-03-23T11:53:00.280" Title="Building a ultra-small almost space vehicle" Tags="|low-earth-orbit|cost|camera|balloons|amateur-rocketry|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18565" PostTypeId="2" ParentId="18553" CreationDate="2016-10-08T14:18:08.727" Score="5" Body="&lt;p&gt;Bear in mind that the orientation of an axis of rotation or of an orbit is essentially fixed in space. This means that there is a fixed relationship between the ISS orbit and the Earth's polar axis. Its orbital plane must be centered on Earth's center of mass; as it happens, the ISS orbit is inclined with respect to the Earth's equator (the ISS orbital parameters are by design), so the ISS ground track will appear to move up and down across the equator. Since the map is laid out such that the Earth's surface is unfolded into a flat sheet with North always up, the ground track will appear sinusoidal (it's a math/geometry thing).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what it's worth: the track shifts from orbit to orbit because the Earth has moved (rotated) under it. The ISS orbital period is about 92 minutes, so when the ISS has completed one orbit, the Earth has rotated about 23 degrees under it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The track doesn't undergo seasonal change because neither the Earth's axis nor the ISS orbit change with the seasons. What changes through the seasons is the Sun's aspect relative to the Earth due to Earth's axial tilt relative to its orbital path around the Sun. The key point is how I introduced my answer - the ISS orbit is fixed in its relationship to Earth's axis, which is all that matters for this question.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2016-10-08T16:48:44.900" LastActivityDate="2016-10-08T16:48:44.900" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18566" PostTypeId="2" ParentId="18563" CreationDate="2016-10-08T19:22:21.127" Score="5" Body="&lt;p&gt;You will have quite a challenge to get to 50 km with a balloon or reasonable sized rocket.  Even weather balloons don't hit those altitudes.  Probably one of the best places to start for this kind of information is the &lt;a href=&quot;http://www.arhab.org&quot; rel=&quot;noreferrer&quot;&gt;Amateur Radio High Altitude Ballooning&lt;/a&gt; page.  In particular, check out their &quot;How To PowerPoints&quot; section.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what you want to do, there is a good deal of info out there to start with.  If you really want to go for altitude, people have &lt;a href=&quot;https://www.youtube.com/watch?v=H-AcSucRBbw&quot; rel=&quot;noreferrer&quot;&gt;launched rockets from balloons&lt;/a&gt;, which are often called &quot;Rockoons&quot;.  But even so, for a rocket that has enough thrust to get to thousands of feet, those end up being pretty big, so they are harder to lift with a balloon.&lt;/p&gt;&#xA;" OwnerUserId="12389" LastActivityDate="2016-10-08T19:22:21.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18567" PostTypeId="1" CreationDate="2016-10-08T21:12:00.480" Score="1" ViewCount="176" Body="&lt;p&gt;In about 2014, i was told the ISS was about the size of one American football field.  In American football (not soccer), the field is 100-120 yards long by 53 yards wide.  In 2016, some cub scouts recently said it was over two football fields in size and the ball of light appears bigger.  Is there a short history of its growth in terms of football field size?  Soccer Field size?&lt;/p&gt;&#xA;" OwnerUserId="11832" LastEditorUserId="11832" LastEditDate="2016-10-10T12:21:38.477" LastActivityDate="2016-10-10T12:21:38.477" Title="What are the rough dimensions of the ISS?" Tags="|iss|history|dimensions|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18568" PostTypeId="1" CreationDate="2016-10-08T21:47:21.513" Score="7" ViewCount="336" Body="&lt;p&gt;Remember the pressurized repair / assembly pods from 2001 A Space Odyssey and so many other space visionary works? Has there been any programs to design &amp;amp; build tiny vehicles like that with manipulator arms, for space stations or construction in space?    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WhgVx.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WhgVx.jpg&quot; alt=&quot;EVA pod&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="1142" LastEditDate="2017-01-04T02:37:38.550" LastActivityDate="2017-01-04T02:37:38.550" Title="Has there been any program to develop tiny pod-like vehicles with manipulator arms for extra-vehicular activities?" Tags="|eva|orbital-assembly|maintenance|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18569" PostTypeId="2" ParentId="18562" CreationDate="2016-10-09T00:12:40.710" Score="3" Body="&lt;p&gt;Understanding how to test and qualify systems of a satellite prototype is certainly on-topic in Space Exploration Stackexchange. Cubesat development is always conceptual at first, and you definitely need to use some physics to develop your understanding. That doesn't mean the question should be moved to Physics Stackexchange.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Measure in the plane perpendicular to the $\mathbb{m}$ direction and centered on the center of the device so that the $\mathbf{m \cdot r} $ term is zero. Then point the sensing direction of the magnetometer parallel to the $ \mathbb{m}$ direction. Now you have a scalar equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$B = \mu H = \frac{4\pi \times 10^{-7}}{4\pi} \frac{m}{r^3}  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$B = \mu H = 10^{-7} \frac{m}{r^3}  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$m = 10^{7} B r^3 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So move the magnetometer close enough so that you are getting a good signal, then plug in the distance as $r$ and the measured $B$ and you have an approximate measurement of $m$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A second way to do this is to drive your magnetotorquer with a low frequency sine wave and use a second small &quot;sense coil&quot; to measure the EMF at a distance. This can often be more accurate than a magnetometer, but it is tricky to do correctly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also remember that the magnetotorquer has a permeable material - the core that the coil is wrapped around that is long and skinny) and that's not necessarily going to be linear, so you should make a series of measurements at different currents. You should also make sure that $r$ is somewhat larger than the length of the rod, or measure at a few different distances. The magnetotorquer is not at all a perfect point dipole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also make sure there are no other ferromagnetic materials nearby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to deal with the Earth's magnetic field, you can just turn the current on and off and look at the &lt;em&gt;change in B&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is at low fields, &lt;em&gt;low cost&lt;/em&gt; magnetometers are not very accurate, so if you want to use an &quot;Arduino class chip&quot; (hobby type) or one in your phone, you may get results that are a bit off. But it could be a good sanity check or starting point at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is actually not an easy measurement to do correctly. But you can get experiment with it even using an &quot;unofficial magnetotorquer&quot; to get a feeling for the process.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-09T01:27:29.017" LastActivityDate="2016-10-09T01:27:29.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18570" PostTypeId="2" ParentId="18567" CreationDate="2016-10-09T14:35:56.263" Score="4" Body="&lt;p&gt;Dimensions of the ISS (in real units) at each step of its construction can be found in these documents:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://athena.ecs.csus.edu/~grandajj/ME296M/space.pdf&quot; rel=&quot;nofollow&quot;&gt;On-Orbit Assembly, Modeling, and Mass Properties Data Book volume 1&lt;/a&gt;, 2002  &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://athena.ecs.csus.edu/~grandajj/ME296M/RevAB_Volume%20I%20Signed_updated.pdf&quot; rel=&quot;nofollow&quot;&gt;On-Orbit Assembly, Modeling, and Mass Properties Data Book volume 1&lt;/a&gt;, 2008 &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://athena.ecs.csus.edu/~grandajj/ME296M/RevAB_Volume%20II%20Signed_updated.pdf&quot; rel=&quot;nofollow&quot;&gt;On-Orbit Assembly, Modeling, and Mass Properties Data Book volume 2&lt;/a&gt;, 2008  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I haven't found a more recent edition than 2008. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Converting to football fields is left as an exercise to the reader. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-09T14:35:56.263" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18571" PostTypeId="2" ParentId="17707" CreationDate="2016-10-10T01:51:43.120" Score="6" Body="&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt; — &lt;em&gt;Center&lt;/em&gt; the burn around periapsis.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Your objective is to increase the specific energy of the orbit by the desired amount, while minimizing the burn time. The specific energy is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathcal{E}={v^2\over 2}-{\mu\over r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathcal{E}$ is a constant of motion of an unperturbed orbit, and can be calculated from the velocity $v$ and distance from the center of the body $r$ at any time in the orbit (with those two measured at the same time of course). $\mu$ is the $GM$ of the body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A change to the energy at any instant in the orbit is then:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$d\mathcal{E}=v\,dv+{\mu\over r^2}dr$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get the most change, we want $v$ to be large and $r$ to be small. Those are both achieved at periapsis. Hence why we do these burns around periapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For small excursions from periapsis, $dr$, the change in $r$ due to either the orbit itself or the burn, is very small. So we will drop that, leaving:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$d\mathcal{E}=v\,dv$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a given instantaneous $\Delta V$ of $dv$, we get the most $d\mathcal{E}$ when $v$ is maximized. That is right at periapsis. However we have finite thrust, so we can't do the whole burn there. $v$ is lower on each side of periapsis, by approximately the same amount for the same time before or after periapsis. To maximize the average $v$ during the burn, we would &lt;em&gt;center&lt;/em&gt; the burn on periapsis, so it starts and stops at the same $v$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I said &quot;approximately&quot; since the burn itself is changing $v$. So while centering the burn is a good starting point, the optimal start and end time is little different. What's more, the $dv$ as a function of time, $dv/dt$ or the acceleration, is increasing over the burn assuming constant thrust, due to the fact that the mass of the vehicle is decreasing as it expends propellant. You can simulate all of that and just change one variable, the start time, and integrate each start time until the desired change in $\mathcal{E}$ is achieved. Then the minimum of that curve is the optimal start time for the burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We haven't talked about the optimal direction for the burn to maximize $d\mathcal{E}$. That would be in the current instantaneous velocity direction, with the derivation left to the reader. Turning during the burn adds complexity and perhaps some risk to the maneuver, so the burn is sometimes done at a fixed inertial attitude, accepting some loss of efficiency for simplicity. The optimal attitude is very close to the velocity direction at periapsis, adjusting it slightly for the realities outlined above.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-10-10T19:59:11.987" LastActivityDate="2016-10-10T19:59:11.987" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18572" PostTypeId="2" ParentId="18506" CreationDate="2016-10-10T02:24:17.123" Score="2" Body="&lt;p&gt;The &lt;a href=&quot;http://mars.nasa.gov/mars2020/mission/overview/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars2020 Rover&lt;/a&gt; mission will use image processing for navigation on the surface &lt;a href=&quot;http://mars.nasa.gov/mars2020/news/whatsnew/index.cfm?FuseAction=ShowNews&amp;amp;NewsID=1930&quot; rel=&quot;nofollow noreferrer&quot;&gt;in a similar way&lt;/a&gt; to the current &lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity Rover&lt;/a&gt;, as its design is rooted on Curiosity's highly successful architecture.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Yjv2w.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Yjv2w.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; illustration of Mars2020 Rover design from &lt;a href=&quot;http://mars.nasa.gov/mars2020/news/whatsnew/index.cfm?FuseAction=ShowNews&amp;amp;NewsID=1930&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;However, image processing will also be used in &lt;em&gt;several new ways&lt;/em&gt; during the descent and landing phase.&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;http://mars.nasa.gov/mars2020/mission/technology/entry-descent-landing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Entry, Descent, and Landing Technologies&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;RANGE TRIGGER - IT'S ALL ABOUT TIMING&lt;/p&gt;&#xA;&lt;p&gt;The key to the new precision landing technique is choosing the right moment to pull the &amp;quot;trigger&amp;quot; that releases the spacecraft's parachute. &amp;quot;Range Trigger&amp;quot; is the name of the technique that Mars 2020 uses to time the parachute's deployment. Earlier missions deployed their parachutes as early as possible after the spacecraft reached a desired velocity. Instead of deploying as early as possible, Mars 2020's Range Trigger deploys the parachute based on the spacecraft's position relative to the desired landing target.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dUs2t.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dUs2t.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Illustration of the &lt;a href=&quot;http://mars.nasa.gov/mars2020/mission/technology/entry-descent-landing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Range Trigger&lt;/a&gt; concept - using the historical &lt;a href=&quot;https://www.nasa.gov/mission_pages/msl/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;/Mars Science Laboratory(MSL)/Curiosity&lt;/a&gt; landing site as an example.&lt;/p&gt;&#xA;&lt;p&gt;Also, during the descent stage, images will be compared in real time to stored terrain maps to improve the accuracy of the landing location:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;HOW TERRAIN-RELATIVE NAVIGATION IMPROVES ENTRY, DESCENT, &amp;amp; LANDING&lt;/p&gt;&#xA;&lt;p&gt;Terrain-Relative Navigation significantly improves estimates of the rover's position relative to the ground. Improvements in accuracy have a lot to do with when the estimates are made.&lt;/p&gt;&#xA;&lt;p&gt;In prior missions, the spacecraft carrying the rover estimated its location relative to the ground before entering the Martian atmosphere, as well as during entry, based on an initial guess from radiometric data provided through the Deep Space Network. That technique had an estimation error prior to EDL of about 0.6 - 1.2 miles (about 1-2 kilometers), which grows to about (2 - 3 kilometers) during entry.&lt;/p&gt;&#xA;&lt;p&gt;Using Terrain-Relative Navigation, the Mars 2020 rover will estimates its location while descending through the Martian atmosphere on its parachute. That allows the rover to determine its position relative to the ground with an accuracy of about 200 feet (60 meters) or less.&lt;/p&gt;&#xA;&lt;p&gt;It takes two things to reduce the risks of entry, descent, and landing: accurately knowing where the rover is headed and an ability to divert to a safer place when headed toward tricky terrain.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UjF7g.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UjF7g.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Illustration of Terrain-Relative Navigation. &amp;quot;&lt;em&gt;Terrain-Relative Navigation helps us land safely on Mars - especially when the land below is full of hazards like steep slopes and large rocks!&lt;/em&gt; From &lt;a href=&quot;http://mars.nasa.gov/mars2020/mission/technology/entry-descent-landing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;In December 2014 the vision system &lt;a href=&quot;http://phys.org/news/2016-10-nasa-flight-mars-lander-vision.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;was tested in the Mojave Desert&lt;/a&gt;. :&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oAcyO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oAcyO.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;&lt;em&gt;A prototype of the Lander Vision System for NASA's Mars 2020 mission was tested in this Dec. 9, 2014, flight of a Masten Space Systems &amp;quot;Xombie&amp;quot; vehicle at Mojave Air and Space Port in California. Credit: NASA Photo/Tom Tschida&lt;/em&gt;&amp;quot; From &lt;a href=&quot;http://phys.org/news/2016-10-nasa-flight-mars-lander-vision.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/A5b9LnzjGgU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NASA tested new &amp;quot;eyes&amp;quot; for its next Mars rover mission on a rocket built by Masten Space Systems in Mojave, California, thanks in part to NASA's &lt;a href=&quot;https://flightopportunities.nasa.gov/technologies/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flight Opportunities Program&lt;/a&gt;, or FOP.&lt;/p&gt;&#xA;&lt;p&gt;The agency's Jet Propulsion Laboratory in Pasadena, California, is leading development of the Mars 2020 rover and its Lander Vision System, or LVS. In 2014, the prototype vision system launched 1,066 feet (325 meters) into the air aboard Masten's rocket-powered &amp;quot;Xombie&amp;quot; test platform and helped guide the rocket to a precise landing at a predesignated target. LVS flew as part of a larger system of experimental landing technologies called the Autonomous Descent and Ascent Powered-flight Testbed, or ADAPT.&lt;/p&gt;&#xA;&lt;p&gt;LVS, a camera-based navigation system, photographs the terrain beneath a descending spacecraft and matches it with onboard maps allowing the craft to detect its location relative to landing hazards, such as boulders and outcroppings.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/17XqW.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/17XqW.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; cropped image of a prototype of the &lt;a href=&quot;https://www.astrobotic.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astrobotics&lt;/a&gt; vision system under testing, from: &lt;a href=&quot;http://www.nasa.gov/centers/Armstrong/Features/XombieTestsAstroboticAutolandingSystem.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nasa.gov/centers/Armstrong/Features/XombieTestsAstroboticAutolandingSystem.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-12-01T06:43:14.183" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18573" PostTypeId="2" ParentId="18486" CreationDate="2016-10-10T08:06:30.480" Score="1" Body="&lt;p&gt;The quote:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;We sub-cool the oxygen and methane to densify it, so compared to... propellants normally used close to their boiling point in most rockets, in our case we actually load the propellants close to their freezing point, and that can result in a density improvement of up to around 10 to 12 percent, which makes an enormous difference in the actual results of the rocket.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;It also makes the... it gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high pressure turbo pump if you have very cold propellant.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; &lt;a href=&quot;https://engineering.stackexchange.com/a/12038/6264&quot;&gt;This answer&lt;/a&gt; to the question &amp;quot;&lt;em&gt;What fundamentally distinguishes cavitation and boiling as different phenomena?&lt;/em&gt;&amp;quot; by a self-described mechanical engineer and former US Navy nuke is particularly helpful!&lt;/p&gt;&#xA;&lt;h2&gt;Background:&lt;/h2&gt;&#xA;&lt;p&gt;Some examples of process which include some material undergoing a phase changing from liquid to gas are &lt;code&gt;evaporation&lt;/code&gt;, &lt;code&gt;boiling&lt;/code&gt;, and &lt;code&gt;cavitation&lt;/code&gt;.&lt;/p&gt;&#xA;&lt;p&gt;As we usually see it, evaporation takes place at a pre-existing and often stable boundary between a liquid and a gas. Even if the temperatures are low and pressure is high, evaporation will continue slowly until the &lt;em&gt;partial pressure&lt;/em&gt; of the material that is the liquid is reached in the gas. So if you have room-temperature water sitting in air and they are both at atmospheric pressure, the water will continue to evaporate until the partial pressure of water in the air reaches the vapor pressure of water at those conditions.&lt;/p&gt;&#xA;&lt;p&gt;But boiling and cavitation generally happen inside a liquid, not outside or above it, and so they depend on both temperature and absolute pressure on the liquid, not just partial pressure as in the case of evaporation.&lt;/p&gt;&#xA;&lt;p&gt;In both cases, a bubble of vapor is produced within the liquid. When the vapor pressure has been increased above the local hydrostatic pressure in the liquid  by the application of heat, we usually call this boiling, and when the hydrostatic pressure has been lowered below the vapor pressure in a transient or highly localized manner, we usually call it cavitation.&lt;/p&gt;&#xA;&lt;p&gt;But let's just consider the effects of reduced pressure, and not temperature.&lt;/p&gt;&#xA;&lt;p&gt;If the hydrostatic pressure of a volume of a liquid is lowered below its vapor pressure in bulk, as in the case of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bubble_chamber&quot; rel=&quot;nofollow noreferrer&quot;&gt;bubble chamber&lt;/a&gt; or even a vacuum flask of water attached to a vacuum pump pulling almost an atmosphere of pressure, &lt;em&gt;the liquid will boil&lt;/em&gt;. The bubbles may reach the surface and release the vapor, or over time they may re-condense within the liquid, especially if the pressure is restored.&lt;/p&gt;&#xA;&lt;p&gt;The actual nucleation and growth of such bubbles a complicated process - liquids have a tensile strength just as solids do, and usually nucleation sites will dominate in the actually formation of the bubble - tearing a hole in the liquid so to speak.&lt;/p&gt;&#xA;&lt;p&gt;In engineering, the use of the word cavitation is generally restricted to situations where the bulk pressure of the liquid is not by itself responsible for the bubble formation. Instead, the term cavitation generally refers to a situation where there is a highly localized and transient drop in pressure below the vapor pressure. Almost immediately after the bubble is formed, the conditions change and the pressure is above the vapor pressure again. While the process is complex, an important feature is the rapidity with which the bubble collapses under the influence of the higher pressure of the liquid.&lt;/p&gt;&#xA;&lt;p&gt;The terms &lt;code&gt;boiling&lt;/code&gt; and &lt;code&gt;cavitation&lt;/code&gt; refer to complex phenomena which are related but different. The distinction is between the evolution of the pressure immediately after the bubble is formed. Conceptually there could be a conitnuum of situations between these two extremes, &lt;em&gt;but that does not mean they are the same thing&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Lwk9Bi3j58o?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;This video shows an example of water being boiled using an immersion heater. The temperature of the heating element is much higher than the water. Between about 01:00 and 02:00 the sounds gets louder and louder, and while a few bubbles can be seen, the process making the sounds is the production and collapse of mostly invisible vapor bubbles. While the temperature at certain points on the heater is higher than the boiling point, as soon as a bubble begins to form, part of the bubble expands into cooler areas of the liquid causing the vapor to quickly condense.&lt;/p&gt;&#xA;&lt;p&gt;This results in a phenomenon similar to cavitation, but on a somewhat longer timescale so that it will not damage the heating element. Boiling water will not cause cavitation damage.&lt;/p&gt;&#xA;&lt;p&gt;Cavitation is bad because the rapid and violent collapse of the bubble is a result of the very rapid change in pressure over time and or distance. It is this rapidity and localization of bubble collapse that generally determines if a phenomenon is called boiling or cavitation.&lt;/p&gt;&#xA;&lt;h2&gt;Subcooling:&lt;/h2&gt;&#xA;&lt;p&gt;Subcooling of liquid propellant as introduced into a turbo pump on a rocket engine can reduce the possibility of cavitation. At lower temperatures the vapor pressure is lower, and thus it is less likely that a given turpbo pump speed can produce a pressure below this point. However, subcooling also raises the density significantly - one of the main reasons that it is being adopted. This means that the turbo pump can rotate more slowly to deliver the same mass per unit time. Slower rotation means less cavitation even at a fixed temperature.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-10-27T10:12:17.803" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18574" PostTypeId="1" AcceptedAnswerId="18575" CreationDate="2016-10-10T13:23:17.660" Score="9" ViewCount="1903" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler&lt;/a&gt; telescope was launched from Earth on March 7 2009 and placed in an independent orbit around the Sun, with a semimajor axis of 1.0132 AU, period of 372.53 days, and an eccentricity of 0.036 according to Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kepler needs to be far enough from Earth so that Earthshine (sunlight reflected by Earth) can not contaminate it's measurements. The camera is considered a photometer - Kepler's main mission is to detect extrasolar planets by measuring tiny shifts in the brightness of stars caused by distant planets transiting their stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe that there are other reasons to keep Kepler away from Earth as well. But I am wondering why it was decided to put Kepler in an orbit that continues to move farther and farther from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kepler's design lifetime 4 years but it is continuing to produce scientific data even now, over seven years since it was originally commissioned. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is about its orbit. Currently it is about 137 million kilometers from Earth, almost 1 AU, and it continues to move farther from Earth all the time (see plots below). I understand that Kepler performs a substantial amount of preprocessing of the roughly 93 million pixel images before sending them to Earth, and downloads are done periodically, perhaps once per month. So the long distance does not seem to be causing too much of a communications problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I'm wondering why it was decided to put Kepler in an orbit which constantly drifts farther from Earth. Is there some benefit to this, or is it the only way to keep it from returning to the vicinity of Earth too quickly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0jmUl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0jmUl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; plot of the motion of the inner solar system bodies projected on to the plane of the Ecliptic in a rotating (synodic) frame based on Earth's orbit. Data from the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; ephemeris database. The Earth is the blue &quot;blob&quot; to the right, and the Kepler telescope moves slowly behind the Earth every year. The wiggles are due to the eccentricity of both the Earth and Kepler.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J1JtZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J1JtZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; plot of the distance between Kepler and Earth as a function of time. Data from the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; ephemeris database. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NGoTc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NGoTc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; plot of the solar angle between Kepler and Earth as a function of time. Data from the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; ephemeris database. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pOAwQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pOAwQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; CCD array of &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_photometer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler's Photometer&lt;/a&gt;, credit: NASA.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-10T14:49:51.340" LastActivityDate="2016-10-10T14:49:51.340" Title="Why was Kepler's orbit chosen to continue to drift away from Earth?" Tags="|orbital-mechanics|orbit|spacecraft|data-transmission|kepler-telescope|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18575" PostTypeId="2" ParentId="18574" CreationDate="2016-10-10T14:28:43.227" Score="12" Body="&lt;p&gt;Initially, Kepler was slated &lt;a href=&quot;http://www.airspacemag.com/daily-planet/keplers-unusual-orbit-54411507/?no-ist&quot;&gt;to go to L2&lt;/a&gt;. However, when preparing Spitzer, NASA found that a heliocentric, Earth-trailing orbit &lt;a href=&quot;http://appel.nasa.gov/2006/10/01/finding-a-way-the-spitzer-space-telescope-story/&quot;&gt;takes less propellant to reach than L2&lt;/a&gt;. For Spitzer, this enabled the switch to a smaller launcher. This finding also applies to Kepler.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-10T14:28:43.227" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18576" PostTypeId="1" AcceptedAnswerId="18577" CreationDate="2016-10-10T22:37:40.203" Score="7" ViewCount="2893" Body="&lt;p&gt;For the Apollo missions in the 1960s, NASA used a combination of kerosene and liquid oxygen to get the rockets out of Earth's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did they do that? If you look at the &lt;a href=&quot;http://www.space.com/18422-apollo-saturn-v-moon-rocket-nasa-infographic.html&quot; rel=&quot;noreferrer&quot;&gt;statistics&lt;/a&gt;, the second stage and beyond for the Apollo rocket had a combination liquid oxygen and liquid hydrogen while &lt;em&gt;only&lt;/em&gt; the first stage had kerosene.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the mixture of oxygen and hydrogen provide a sort of advantage in space than Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm asking this question mainly because I'm working on a sort of Cities In Space project and am wondering if there is a better or more efficient way of escaping Earth's atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="17203" LastEditorUserId="12102" LastEditDate="2016-10-10T23:16:55.117" LastActivityDate="2016-10-11T15:26:34.447" Title="Combination of liquid hydrogen and liquid oxygen" Tags="|apollo-program|fuel|saturn-v|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18577" PostTypeId="2" ParentId="18576" CreationDate="2016-10-10T23:35:54.467" Score="15" Body="&lt;p&gt;Hydrogen-oxygen rocket engines are the most efficient chemical rockets that are reasonably safe and practical to use, by propellant  mass. The metric normally used for this comparison is called specific impulse or Isp, (typically given in units of seconds for historical reasons) and for the Saturn V's upper stages it's about 421 seconds. For comparison, kerosene-oxygen specific impulse usually runs around 300 seconds (for first-stage engines of that era).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For upper stages, mass efficiency is incredibly important, because the lower stages have to lift that mass. Thus hydrogen-oxygen is an extremely common choice for that role.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the first stage, however, mass efficiency is much less important than cost. Kerosene-oxygen engines produce much more thrust per dollar for a number of reasons: kerosene is far denser than hydrogen, so you build a physically much smaller stage, reducing assembly and transport costs; hydrogen plumbing is much trickier; kerosene's energy density means the engine is physically smaller and thus easier to build, transport, install, etc.; liquid hydrogen must be kept much colder than liquid oxygen, and so forth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The Q/As linked by @uhoh in comments illustrate the implications of these tradeoffs for Saturn V.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same pressures that made kerosene attractive for the Saturn V first stage apply even more to solid rocket engines -- lower Isp still, but much more compact, simple, and cost-effective, which is why you see them used as boosters in many modern launchers.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-11T15:26:34.447" LastActivityDate="2016-10-11T15:26:34.447" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18578" PostTypeId="1" AcceptedAnswerId="18579" CreationDate="2016-10-10T23:41:03.640" Score="13" ViewCount="3575" Body="&lt;p&gt;While watching a launch of an Atlas V, I couldn't help but notice that the boosters were positioned asymmetrically. There was one booster on one side of the rocket, and &lt;em&gt;two&lt;/em&gt; boosters on the other side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further research revealed that the positioning for the booster was intentional. Which is odd, because off-setting the boosters will give more thrust to one side of the rocket, and in turn, end in a disaster!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that didn't happen during the launch. The rocket managed to fly perfectly upwards! How did they manage to do that? Why did they even position the boosters that way! Does it hold some sort of benefit? And if so, why don't people use that more and more often?&lt;/p&gt;&#xA;" OwnerUserId="17203" LastEditorUserId="6816" LastEditDate="2018-01-24T17:16:37.070" LastActivityDate="2023-02-23T15:34:18.700" Title="How does Atlas V maintain stable flight with asymmetric solid rocket boosters?" Tags="|design|booster|atlas-v|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18579" PostTypeId="2" ParentId="18578" CreationDate="2016-10-11T00:21:48.520" Score="13" Body="&lt;p&gt;The nozzles of the SRBs are canted outward, so the line of thrust from each SRB passes close to the center of mass of the rocket, so that the net torque from asymmetrical booster arrangements is minimized.&#xA;The main engine on the Atlas, as with most orbital launchers, is gimballed such that it can direct thrust to offset any remaining imbalanced thrust from the boosters (or any other disturbance).&lt;/p&gt;&#xA;&lt;p&gt;Atlas uses only as many boosters as it needs for a given payload -- from zero to 5 -- and some of those configurations are asymmetrical. It looks especially odd using only a single booster, as in this video:&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/dME5amgp0Ug?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;The fundamental reason for the asymmetry seems to be the &lt;a href=&quot;https://m.reddit.com/r/spaceflight/comments/3d3a7u/how_does_atlas_v_launch_with_1_solid_rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;orientation of the paired nozzles of the core engine, combined with the positioning of the LOX feed line running along the outside side of the first-stage tank&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2023-02-23T15:34:18.700" LastActivityDate="2023-02-23T15:34:18.700" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18580" PostTypeId="2" ParentId="13477" CreationDate="2016-10-11T01:47:26.157" Score="9" Body="&lt;p&gt;In case anyone else is curious, I did find some information on this, in a document dated Jan 29, 2003, three days before the Columbia accident.  It shows projected flights going out to STS-147 in late 2010.  After assembly complete, most of the flights just are listed as &quot;ISS flight&quot; but some have the &quot;crew rotation&quot; nomenclature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last ISS assembly flight listed was STS-136 in Jan 2008. It was to carry up the cupola.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only flights shown for Columbia after STS-107 are STS-118, an ISS flight in November 2003, and a Hubble servicing flight in November 2004.  Then a huge gap until November 2009, with STS-144, which is listed as 'Hubble Retrieval' (!) Now that is interesting.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rIBiH.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rIBiH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/0jCEn.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0jCEn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-04-21T16:46:40.867" LastActivityDate="2018-04-21T16:46:40.867" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18581" PostTypeId="2" ParentId="18576" CreationDate="2016-10-11T04:37:19.927" Score="8" Body="&lt;p&gt;My personal favorite source of Saturn V information, SP-4206 &quot;Stages to Saturn,&quot; weighs in on the RP-1 choices.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://history.nasa.gov/SP-4206/ch7.htm&quot;&gt;Chapter 7&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When the contract to build the biggest stage of the Saturn V, the S-IC first stage, was awarded to Boeing on 15 December 1961, general outlines of the first-stage booster were already fairly well delineated. The main configuration of the S-IC had already been established by MSFC, including the decision to use RP-1, as opposed to the LH2 fuel used in the upper stages. Although LH2 promised greater power, some quick figuring indicated that it would not work for the first stage booster.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Liquid hydrogen was only one half as dense as kerosene. This density ratio indicated that, for the necessary propellant, an LH2 tank design would require a far larger tank volume than required for RP-1. The size would create unacceptable penalties in tank weight and aerodynamic design. So, RP-1 became the fuel. In addition, because both the fuel and oxidant were relatively dense, engineers chose a separate, rather than integral, container configuration with a common bulkhead. The leading issue prior to the contract awards related to the number of engines the first stage would mount.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That chapter goes into a lot of detail about the design of the first stage tanks, which are enormous as-is.  I think it's fair to say that even larger LH2 tanks would've compounded some of the construction problems that were had (though they could've been overcome).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://history.nasa.gov/SP-4206/ch4.htm&quot;&gt;Chapter 4&lt;/a&gt; is more about the engines, and implies that technology readiness is a factor:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA's contract award to Rocketdyne in 1959, calling for an engine with a thrust of 6.7 million newtons (1.5 million pounds), was a significant jump beyond anything else in operation at the time. Executives within the space program looked on the big engine as a calculated gamble to overtake the Russians and realize American hopes for manned lunar missions. It seemed within the realm of possibility too, by using engine design concepts already proven in lower thrusts and by relying on conventional liquid oxygen and RP-1 propellants.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's common conservatism in aerospace engineering to take incremental steps forward, so for the newly developed high-thrust F-1 engines for the first stage, they stuck with otherwise-proven hydrocarbon fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not immediately finding a clean source for this last claim, so it might deserve to be edited out, but I believe there's also a specific &lt;em&gt;thrust&lt;/em&gt; advantage owing to the density of the fuel, and so for a first stage with a relatively short burn time it can be more efficient for the overall system to pay the specific &lt;em&gt;impulse&lt;/em&gt; penalty to get the bird off the ground and out of dense atmosphere with the additional thrust, then drop the stage and switch propellants to something higher-impulse.  Similar arguments apply for the strap-on solid boosters commonly used with several launch systems today.&lt;/p&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2016-10-11T04:37:19.927" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18582" PostTypeId="2" ParentId="18553" CreationDate="2016-10-11T05:54:22.240" Score="17" Body="&lt;p&gt;An orbit takes place on a single 2d plane, through the center of the planet. A sphere and a plane through its center intersect in a circle. See diagram:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/DWjvg.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no actual sine wave movement going on, the ISS moves around the planet above the red line, in a circle. The apparent sine motion of the ground track is entirely due to the Mercator projection being used when the map is 'unfolded' from globe to flat surface. Hopefully this image allows you to more easily see the projection, note how the red line &quot;curves downwards&quot; at the front of the image, crosses the equator, and goes up around the back of the sphere. When the checkerboard surface is stretched out into a 2d grid, the red line gets stretched out to be a sine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the sinusoidal motion is also due to the orbit being almost circular. If the orbit was a highly eccentric ellipse, like this:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/JL8q6.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JL8q6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;you get a ground track like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/b/bf/Molniya.jpg&quot; alt=&quot;alternative ground track&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is very useful for observing the ground in the northern hemisphere, since the satellite spends a lot of time in that hemisphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going to even further extremes, the ground track of a geosynchronous satellite is a point. For something nearly in geosynchronous orbit, you can get figure-8 ground tracks. &lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2016-10-11T13:50:05.063" LastActivityDate="2016-10-11T13:50:05.063" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18583" PostTypeId="1" AcceptedAnswerId="18636" CreationDate="2016-10-11T06:20:10.063" Score="8" ViewCount="831" Body="&lt;p&gt;This is something I've wondered about quite frequently. Is there really any common naming procedure used to name spacecraft? It seems that some spacecraft are named after greek and roman gods, such as:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Orion(Spacecraft): Named after the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_(mythology)&quot; rel=&quot;nofollow noreferrer&quot;&gt;mythological giant huntsman Orion&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Apollo(Program): Named after the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo&quot; rel=&quot;nofollow noreferrer&quot;&gt;roman and greek god Apollo&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Gemini(Spacecraft): A name given to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Castor_and_Pollux&quot; rel=&quot;nofollow noreferrer&quot;&gt;mythological twins, Castor and Pollux&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;just to name a few. However, most of the names NASA gives to their spacecraft seem all over the place. Some are named for their mission, others are named in honor of certain people, and others &lt;sup&gt;1&lt;/sup&gt;&lt;a href=&quot;https://web.archive.org/web/20200223234717/http://spacecovers.com/misc/ap1-17_mission_info.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;are just plain weird&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;This lead me to wonder, if there is any set naming convention for naming spacecraft, or is each one just named at random and &amp;quot;on the fly&amp;quot;?&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;&lt;sub&gt;Just start reading each paragraph near the ends. Each one list the names of the command module/service module, and lunar module....&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="16704" LastEditorUserId="17478" LastEditDate="2020-11-18T12:47:00.340" LastActivityDate="2020-11-18T12:47:00.340" Title="What is the procedure for naming spacecraft?" Tags="|spacecraft|history|" AnswerCount="2" CommentCount="11" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18584" PostTypeId="2" ParentId="18578" CreationDate="2016-10-11T09:19:36.723" Score="9" Body="&lt;p&gt;There are tradeoffs at work here. &#xA;The current configuration allows them to use one first stage design for 1-5 boosters. A first stage that has symmetric attachment points, needs 2+4+4 = 10 attachment points for the 2, 3, 4, and 5 booster cases. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another approach would be to discard the odd numbered configurations altogether and go to 2, 4, 6, 8 boosters. That means a higher manufacturing cost and more dead weight (more attachment points, less efficient boosters).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In both cases there's a tradeoff between manufacturing cost and complexity, and rocket efficiency (performance loss due to non-optimal configurations). Apparently the tradeoff was favorable for the current 1-5 booster configuration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Atlas V first stage is built to in a single configuration. &lt;a href=&quot;https://dslauretta.com/2016/08/23/the-atlas-v-411-av-067-our-ride-to-space/&quot; rel=&quot;noreferrer&quot;&gt;Here's the first stage for OSIRIS-REx&lt;/a&gt;, an Atlas V 441 launch with one booster attached. I've indicated attachment points for 2 boosters, indicating they will fly with unused attachment points. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U9ptj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U9ptj.jpg&quot; alt=&quot;Atlas V 441&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This photo also shows there's lots of places around the circumference of the stage where you can't attach boosters due to LOX lines or electronics attached to the outside. This is why e.g. the 4-booster variant is asymmetrical. &lt;a href=&quot;http://forum.kerbalspaceprogram.com/index.php?/topic/122764-atlas-v-411-just-why/&amp;amp;page=2&quot; rel=&quot;noreferrer&quot;&gt;Apparently,&lt;/a&gt; the Common Core Booster was not initially designed to be used with solid boosters, and when they had to be added later they didn't want to rearrange the rest of the stage.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being designed for government contracts in an era with very little competition, it's feasible they didn't want to spend money wringing out the last bit of performance from the design. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-10-12T11:54:14.683" LastActivityDate="2016-10-12T11:54:14.683" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18585" PostTypeId="2" ParentId="18576" CreationDate="2016-10-11T11:22:57.387" Score="-4" Body="&lt;p&gt;Its probably got to do with burn rate, the second fuel burns faster to give a quicker push. Once you near the burning velocity of the first fuel it is less effective at accelerating you. A rocket is accelerated by the explosion hitting the roof of the chamber. Summary: its like first and second gear in your car.&lt;/p&gt;&#xA;" OwnerUserId="17225" LastActivityDate="2016-10-11T11:22:57.387" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18587" PostTypeId="2" ParentId="17908" CreationDate="2016-10-11T16:39:01.903" Score="2" Body="&lt;p&gt;US segment (NASA, ESA, CSA, JAXA) includes:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Unity&quot; rel=&quot;nofollow&quot;&gt;Unity&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Harmony&quot; rel=&quot;nofollow&quot;&gt;Harmony&lt;/a&gt; 75.5 m^3&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Tranquility&quot; rel=&quot;nofollow&quot;&gt;Tranquility&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Cupola&quot; rel=&quot;nofollow&quot;&gt;Cupola&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Destiny&quot; rel=&quot;nofollow&quot;&gt;Destiny&lt;/a&gt; 106 m^3 &lt;strong&gt;(1)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Columbus&quot; rel=&quot;nofollow&quot;&gt;Columbus&lt;/a&gt; Lab (ESA)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Kib.C5.8D&quot; rel=&quot;nofollow&quot;&gt;Kibo&lt;/a&gt; Lab (JAXA)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Quest&quot; rel=&quot;nofollow&quot;&gt;Quest&lt;/a&gt; Airlock&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Leonardo&quot; rel=&quot;nofollow&quot;&gt;Leonardo&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Pressurized Mating Adapter&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Russian Orbital Segment&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Zvezda&quot; rel=&quot;nofollow&quot;&gt;Zvezda&lt;/a&gt; 46,7 m^3&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Zarya&quot; rel=&quot;nofollow&quot;&gt;Zarya&lt;/a&gt; 71.5 m^3 &lt;strong&gt;(1)&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Rassvet&quot; rel=&quot;nofollow&quot;&gt;Rassvett&lt;/a&gt; 5.85 m^3&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Pirs_and_Poisk&quot; rel=&quot;nofollow&quot;&gt;Pirs&lt;/a&gt; 13 m^3 &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Pirs_and_Poisk&quot; rel=&quot;nofollow&quot;&gt;Poisk&lt;/a&gt; 10.7 m^3&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I'll try adding the volumes later, but most of them can be found on Wikipedia.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;(1) Pressured area, not living area&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="415" LastEditDate="2016-10-12T14:39:15.493" LastActivityDate="2016-10-12T14:39:15.493" CommentCount="4" CommunityOwnedDate="2016-10-12T17:35:49.617" ContentLicense="CC BY-SA 3.0" />
  <row Id="18588" PostTypeId="1" AcceptedAnswerId="18589" CreationDate="2016-10-11T19:25:32.000" Score="8" ViewCount="591" Body="&lt;p&gt;Suppose Mars Odyssey (ODY) and the Mars Reconnaissance Orbiter (MRO) are being tracked by antenna &lt;strong&gt;A&lt;/strong&gt; (just for example) at the Goldstone &lt;a href=&quot;https://deepspace.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; complex in &lt;a href=&quot;https://mars.nasa.gov/mer/mission/comm_busy.html&quot; rel=&quot;noreferrer&quot; title=&quot;Multi-Spacecraft Per Aperture&quot;&gt;MSPA mode&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible for antennas &lt;strong&gt;A&lt;/strong&gt; and &lt;strong&gt;B&lt;/strong&gt; to track ODY together (creating an array), while at the same time antennas &lt;strong&gt;A&lt;/strong&gt; and &lt;strong&gt;C&lt;/strong&gt; track MRO together (creating another array)?&lt;/p&gt;&#xA;" OwnerUserId="17234" LastEditorUserId="6816" LastEditDate="2016-10-11T20:24:07.797" LastActivityDate="2016-10-11T22:02:54.940" Title="Is it possible for the Deep Space Network to use MSPA and antenna arraying simultaneously?" Tags="|communication|tracking|deep-space-network|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18589" PostTypeId="2" ParentId="18588" CreationDate="2016-10-11T22:02:54.940" Score="9" Body="&lt;p&gt;If you have three apertures pointed at Mars, then you would array all three for both orbiters. There would be no point in throwing away the signal of one of the orbiters on any of the antennas. You can array four antennas if you like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The DSN catalog does mention that combining MSPA and arraying is good bit more work for them, so it has to be negotiated with them when planning the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way &quot;track&quot; isn't the right word here, since that implies two-way communication for Doppler or range tracking. MSPA supports multiple spacecraft on the receive side, but only one spacecraft on the transmit side, per antenna. You are receiving data from multiple spacecraft simultaneously, not tracking them simultaneously. Again, per antenna.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-10-11T22:02:54.940" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18590" PostTypeId="1" AcceptedAnswerId="18592" CreationDate="2016-10-11T22:32:50.810" Score="4" ViewCount="268" Body="&lt;p&gt;Having read &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Wiki page&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/a/6034/13678&quot;&gt;this Space SE answer&lt;/a&gt;, and &lt;a href=&quot;http://hps.org/publicinformation/ate/faqs/spaceradiation.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this link&lt;/a&gt;, I'm curious:  Is a &quot;coronal mass ejection&quot; the same thing as (synonymous with) a &quot;solar particle event&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, then what is/are the difference(s)?&lt;/p&gt;&#xA;" OwnerUserId="13678" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-10-12T00:52:14.970" Title="Is a coronal mass ejection (CME) synonymous with a solar particle event (SPE)?" Tags="|terminology|cme|solar-flares|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18592" PostTypeId="2" ParentId="18590" CreationDate="2016-10-12T00:52:14.970" Score="2" Body="&lt;p&gt;The two are not the same, though they can be caused by similar phenomena.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the Wikipedia article you cite mentions, a coronal mass ejection is an unusual release of plasma. That plasma is released into the solar wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What makes a solar particle event different is that the plasma, once released, is then accelerated to greater speed/energy. The particles in an SPE can penetrate the atmosphere and cause ionization: particles in a CME are less energetic, but still sufficiently powerful to distort the magnetosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe you will find the Wikipedia article on solar particle events to be interesting: &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_particle_event&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Solar_particle_event&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17050" LastActivityDate="2016-10-12T00:52:14.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18593" PostTypeId="1" AcceptedAnswerId="18595" CreationDate="2016-10-12T05:15:41.093" Score="11" ViewCount="2247" Body="&lt;p&gt;I understand the Mars ISRU reasoning for SpaceX's Raptor which is also going to be methalox based.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But why would ULA, who is the prime customer for BE-4 besides Blue Origin itself, chose to support a riskier, untried, cryogenic fuel for their next generation launcher? The performance bonus vs RP-1, if any even exists, doesn't seem worth it.&lt;/p&gt;&#xA;" OwnerUserId="17241" LastActivityDate="2016-10-12T16:28:58.333" Title="Why does the BE-4 use methane as fuel?" Tags="|rockets|liquid-fuel|ula|blue-origin|be4-engine|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18595" PostTypeId="2" ParentId="18593" CreationDate="2016-10-12T13:56:55.750" Score="18" Body="&lt;p&gt;Reusability is the big factor. Kerosene engines have issues with &quot;coking&quot;, where solid carbon is deposited throughout the engine's pipes. (See &lt;a href=&quot;http://hanson.stanford.edu/dissertations/MacDonald_2012.pdf&quot;&gt;this dissertation&lt;/a&gt; and &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a533333.pdf&quot;&gt;its supporting research&lt;/a&gt; for more details.) This isn't a major issue with expendable engines, but it drives up costs when trying to reuse the engines. The BO New Glenn's first stage is &lt;a href=&quot;https://www.nasaspaceflight.com/2016/09/blue-origin-new-glenn-orbital-lv/&quot;&gt;planned to be fully reusable&lt;/a&gt;, similar to the Falcon 9; the ULA Vulcan &lt;a href=&quot;https://spaceflightnow.com/2015/04/14/ula-chief-explains-reusability-and-innovation-of-new-rocket/&quot;&gt;plans&lt;/a&gt; to reuse its engines by detaching them in a pod, then catching that pod with a helicopter. Methane burns more cleanly due to its simple chemical structure, so it doesn't have this issue, making it a good choice for a reusable lower-stage engine.&lt;/p&gt;&#xA;" OwnerUserId="13208" LastEditorUserId="13208" LastEditDate="2016-10-12T14:56:54.650" LastActivityDate="2016-10-12T14:56:54.650" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18596" PostTypeId="2" ParentId="18593" CreationDate="2016-10-12T14:33:46.057" Score="7" Body="&lt;p&gt;Let's look at &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_rocket_propellant#Bipropellants&quot;&gt;Wikipedia&lt;/a&gt; for possible liquid rocket fuels. The key item is the Exhaust Velocity. Let's remove any that could be toxic, so no Beryllium, Florine, or Boron, despite the fact that each could make a very efficient rocket. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$H_2{\space}LOX$- 3816&lt;/li&gt;&#xA;&lt;li&gt;$CH_4{\space}LOX$- 3034&lt;/li&gt;&#xA;&lt;li&gt;$C_2H_6{\space}LOX$- 3006&lt;/li&gt;&#xA;&lt;li&gt;$C_2H_4{\space}LOX$- 3053&lt;/li&gt;&#xA;&lt;li&gt;$\textrm{RP1}{\space}LOX$- 2941&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So Methane, Ethane, and Ethylene all perform similar to RP1, and in fact slightly better. None compares, however, to Hydrogen/Oxygen. Still, liquid hydrogen has issues that make it difficult to work with. So, what are some of the side effects of these various fuels?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$H_2{\space}LOX$- Difficult to work with, requires larger tanks than traditional fuel. Requires extreme cold.&lt;/li&gt;&#xA;&lt;li&gt;$CH_4{\space}LOX$- Burns clean. Is less energy dense than other fuels, requiring a larger tank.&lt;/li&gt;&#xA;&lt;li&gt;$C_2H_6{\space}LOX$ / $C_2H_4{\space}LOX$ / $\textrm{RP1}{\space}LOX$ Basically these gradually become more energy dense, burn less clean, etc.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Burning clean is important for re-usability, no need to clean the engine periodically. That is probably the major reason why it would be invested for Earth based technologies. They also could be looking for it as a path to Mars, as SpaceX has been.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, Elon Musk broke out SpaceX's decision to use Methane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/H7Uyfqi_TE8?start=906&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of these, only the last doesn't apply. Methane is much cheaper than RP1, hence, if you have a vehicle where most of the cost is the fuel, then Methane makes more sense. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-10-12T16:28:58.333" LastActivityDate="2016-10-12T16:28:58.333" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18597" PostTypeId="1" CreationDate="2016-10-12T14:41:51.707" Score="2" ViewCount="2106" Body="&lt;p&gt;Does Earth's magnetic field block (or divert) only lower energy radiation while the hardest, high speed heavy ions, run through to LEO? Are the hardest rays dampened? Do the van Allen Belts, not far from LEO, actually generate electron rays that hit the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to know what (the potentially harmful part of) the radiation spectrum in the ISS looks like.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="3185" LastEditDate="2016-10-12T16:44:19.917" LastActivityDate="2016-10-12T21:47:55.497" Title="What kinds of radiation hit the ISS?" Tags="|iss|low-earth-orbit|radiation|cosmic-radiation|van-allen-belt|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18599" PostTypeId="2" ParentId="14123" CreationDate="2016-10-12T18:14:34.913" Score="2" Body="&lt;p&gt;It's not that much of a coincidence -- about 1 in 52 for 10 or more odd numbers out of 12. You would have been just as surprised by ten or more &lt;em&gt;even&lt;/em&gt; numbers, so you can bring that down to 1 in 26. And you would have been just as surprised to see such a discrepancy in the Soviet space program, instead of the US program, so you can bring it down to 1 in 13.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To sum up: your claim that there &quot;must have been a reason for this policy&quot; is unfounded.&lt;/p&gt;&#xA;" OwnerUserId="15997" LastEditorUserId="15997" LastEditDate="2016-10-13T11:18:43.397" LastActivityDate="2016-10-13T11:18:43.397" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18600" PostTypeId="2" ParentId="18597" CreationDate="2016-10-12T21:06:17.093" Score="2" Body="&lt;p&gt;As far as radiation, the biggest thing that NASA is worried about seems to be as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Gamma rays&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While fusion in the core does produce gamma rays, these high-energy photons are absorbed and re-emitted many, many times at lower frequencies in the solar plasma. However, gamma rays can enter the solar system from outside the system: these cosmic gamma rays do pose a hazard to the astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;X-rays&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sun produces this form of radiation during solar flares. They can pose hazard to the astronauts if they get too intense, but so far there's been no indication of a solar storm severe enough to warrant emergency measures (which would probably mean evacuation of the station and a return to Earth for all astronauts).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Protons and Neutrons&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These forms of radiation can be problematic, and are monitored on the ISS. (Electron radiation--called beta radiation--cannot penetrate the walls of the ISS.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much radiation is there? Enough that the ISS monitors those radiation levels constantly, but not enough for radiation poisoning or anything of the like. Astronauts are at a higher risk for cancer, cataracts, and cellular damage, but usually they don't experience enough for acute radiation-related health issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All information from &lt;a href=&quot;http://www.nasa.gov/mission_pages/station/research/experiments/1043.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/mission_pages/station/research/experiments/1043.html&lt;/a&gt; and &lt;a href=&quot;http://www.universetoday.com/60065/radiation-from-the-sun/&quot; rel=&quot;nofollow&quot;&gt;http://www.universetoday.com/60065/radiation-from-the-sun/&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="17050" LastEditorUserId="17050" LastEditDate="2016-10-12T21:42:20.617" LastActivityDate="2016-10-12T21:42:20.617" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18601" PostTypeId="1" AcceptedAnswerId="18642" CreationDate="2016-10-12T21:47:40.330" Score="4" ViewCount="1624" Body="&lt;p&gt;This answer on Worldbuilding (to &lt;a href=&quot;https://worldbuilding.stackexchange.com/a/58073/634&quot;&gt;&quot;How to solve the old 'gun on a spaceship' problem?&quot;&lt;/a&gt;) posits that ISS can easily withstand a bullet fired from a handgun&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Our current micrometeorite shielding can protect against the energies of bullets at point blank range, by several times. An aluminium slug of 7mm at 7kms can be stopped by whipple shields. That's a kinetic energy of ~36kJ. A handgun bullet at point blank range is more like ~8kJ. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, this is theoretical guess on an SE answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Space Shuttle (as well as most other space efforts through history) has been - at the very least - &lt;a href=&quot;https://space.stackexchange.com/questions/1398/did-the-sso-have-military-uses&quot;&gt;of interest - to the military&lt;/a&gt;, &lt;strong&gt;&lt;em&gt;have any space vehicles or stations that were designed for actual launch been actually tested on how well their walls can withstand firearm-discharged bullets&lt;/em&gt;&lt;/strong&gt;?&lt;/p&gt;&#xA;" OwnerUserId="570" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-10-15T22:46:45.397" Title="Have spaceships been tested to withstand bullets?" Tags="|spacecraft|testing|military|weapon|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18602" PostTypeId="2" ParentId="18597" CreationDate="2016-10-12T21:47:55.497" Score="3" Body="&lt;p&gt;The ISS gets its radiation from galactic cosmic rays, solar cosmic rays, and the Van Allen belts.  You are correct that the magnetic field does a pretty good job protecting the ISS from solar radiation, but the high energy and/or high mass galactic cosmic rays plow on through; fortunately, the flux of galactic cosmic rays that make it through is much less than that from the other sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Van Allen belts do cause problems for the ISS.  If the magnetic field was symmetric around the Earth, it wouldn't be too much of a problem because the ISS would fly below the belts, but the field is offset such that the belts dip down closer to the Earth over the South Atlantic causing the South Atlantic Anomaly, which the station does have to fly through.  It is the inner belt protons and the outer belt electrons that are problematic.  I read a National Academies report years ago that mentioned that on the Russian station Mir, about half of the radiation dose the cosmonauts received came from the couple percent of the time they spent in the SAA at the station flew through it.&lt;/p&gt;&#xA;" OwnerUserId="12389" LastActivityDate="2016-10-12T21:47:55.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18603" PostTypeId="2" ParentId="14123" CreationDate="2016-10-13T00:03:02.113" Score="2" Body="&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikiquote.org/wiki/Richard_Feynman&quot; rel=&quot;nofollow noreferrer&quot;&gt;Richard Feynman&lt;/a&gt;:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;You know, the most amazing thing happened to me tonight. I was coming here, on the way to the lecture, and I came in through the parking lot. And you won’t believe what happened. I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight?&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Amazing!&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Based on the excellent answer by @TokyK let's look at the numbers with python:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;even_odd = [[(i/2**n)%2 for n in range(12)] for i in range(2**12)]&#xA;&#xA;sums = [sum(x) for x in even_odd]&#xA;&#xA;A, B = np.histogram(sums, bins = range(14))&#xA;&#xA;list(A) &#xA;&#xA;[1, 12, 66, 220, 495, 792, 924, 792, 495, 220, 66, 12, 1]&#xA;&#xA;4096. / (1 + 12 + 66 + 66 + 12 + 1)&#xA;&#xA;25.924050632911392&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;So chances are 1 in 26 that there are two or less groups that are even or odd.&lt;/p&gt;&#xA;&lt;p&gt;What are the chances that - considering all of the things that happened in the Apollo space program - one of them had a chance of 1 in 26 or less of happening?&lt;/p&gt;&#xA;&lt;p&gt;Essentially 100%.&lt;/p&gt;&#xA;&lt;p&gt;Per day?&lt;/p&gt;&#xA;&lt;p&gt;Essentially 100%.&lt;/p&gt;&#xA;&lt;p&gt;Per person?&lt;/p&gt;&#xA;&lt;p&gt;Essentially 100%.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Stuff happens.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-03-07T02:35:17.590" LastActivityDate="2021-03-07T02:35:17.590" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18604" PostTypeId="1" CreationDate="2016-10-13T00:31:30.113" Score="3" ViewCount="229" Body="&lt;p&gt;If you want to colonize the Moon, and have a self-sustaining economy, then what are the procedures that you would have to take?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, what are the priorities you want to set straight and do first, what are some problems you'll have to address and solve etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just want a general idea on how to tackle this. I'm working on a project and would like a  &lt;em&gt;boost&lt;/em&gt; to generate possible ideas.&lt;/p&gt;&#xA;" OwnerUserId="17203" LastEditorUserId="195" LastEditDate="2016-10-13T02:03:14.257" LastActivityDate="2016-10-13T16:34:49.797" Title="What would be the procedure to colonize the Moon?" Tags="|the-moon|colonization|" AnswerCount="1" CommentCount="2" ClosedDate="2016-10-14T06:50:53.990" ContentLicense="CC BY-SA 3.0" />
  <row Id="18605" PostTypeId="1" CreationDate="2016-10-13T02:00:07.040" Score="1" ViewCount="70" Body="&lt;p&gt;I want to create an antenna array of DSS-25 of Goldstone Deep space Communication Complex (DSCC) and DSS-35 of Canberra complex to communicate with Mars Reconnoissance Orbiter (MRO). Suppose that DSS-25 of Goldstone DSCC is currently communicating with the MRO and DSS-35 of Canberra DSCC has next communication pass scheduled with the MRO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On average, what is the duration of the overlap in the incoming (Canberra) and outgoing (Goldstone) station's communication windows during which they could be used as antenna array? &lt;/p&gt;&#xA;" OwnerUserId="17234" LastActivityDate="2016-10-13T02:00:07.040" Title="Overlap in the incoming and outgoing Deep Space Station’s (DSS's) uplink/downlink windows" Tags="|deep-space-network|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18607" PostTypeId="2" ParentId="18486" CreationDate="2016-10-13T03:23:42.550" Score="5" Body="&lt;p&gt;This answer is based mostly on the NPSH link Organic Marble shared (&lt;a href=&quot;http://www.pumpschool.com/applications/NPSH.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.pumpschool.com/applications/NPSH.pdf&lt;/a&gt;) and the various properties you can infer about subcooled LOX.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Subcooling lowers vapor pressure, which helps prevent bubble formation.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Subcooling densifies the propellant, which&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;allows lower fluid velocities at the same mass-flow rates, which&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;allows lower pump RPMs, reducing the pressure drop in the wake of the impeller, consequently reducing the incidence of vapor bubble formation (and the severity of re-collapse if they do)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;helps to reduce friction losses and associated NPSH_A&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;On this part of the quote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;It also makes the... it gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high pressure turbo pump if you have very cold propellant.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm just going to disagree with your interpretation that &quot;despite the casual appearance, this guy usually chooses his words very carefully and makes his distinctions clear.&quot;  This reads as one thought, which he decided to rephrase on the fly--the cavitation risk is directly connected to the ease of feeding the pump.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example-by-contrast, this usenet archive post about high cavitation risk making things hard, from &lt;a href=&quot;http://yarchive.net/ac/cavitation.html&quot; rel=&quot;noreferrer&quot;&gt;http://yarchive.net/ac/cavitation.html&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Liquid pumps for propane (and refrigerants) are very easy to cavitate, since&#xA;  the vapor/liquid are at or close to equalibrium, and the slightest pressure&#xA;  drop (a pump trying to &quot;suck&quot; liquid) will create huge amounts of flash&#xA;  gas and and pump cavitation.  Any thing like undersized pump input lines,&#xA;  or trying to suck liquid uphill or thru fine screens, etc, leads to&#xA;  extreme cavitation and quickly destroys a pump.  I once had to build a&#xA;  3 ton refrigeration unit to subcool liquid 40-50 degrees due to someone's&#xA;  dumbass attack of running liquid refrigerant lines uphill into the pump&#xA;  inlet and it was easier to build the subcooler than to get the piping&#xA;  changed.  Now they listen.  THe subcooling reduced the pump cavitation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2016-10-13T03:23:42.550" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18608" PostTypeId="2" ParentId="18192" CreationDate="2016-10-13T03:46:12.737" Score="6" Body="&lt;p&gt;I'm an aerospace engineer, and took courses in college on spacecraft navigation and control theory.  However, I have not programmed a spacecraft attitude controller in my work, and I can't say I've heard of a &quot;Quaternion Error Command&quot; controller before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, these are the considerations I'd have off the top of my head when selecting a controller:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;is the controller already known?  There's a lot of heritage around, and if we're in a decent-sized firm we may already have experts in some controller type.  The time and money savings resulting are not to be overlooked.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;is the controller stable?  Is it provably stable, given expected disturbances and the control and environmental models you have for your spacecraft?  Spacecraft attitude control almost always requires fuel, and where it doesn't it requires power and tends to saturate.  Stability was one of the primary concerns of the Controls class we took.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;what kind of feedback does the controller need?  (it's presumably closed-loop)  Can your spacecraft hardware provide sufficient fidelity of feedback to retain the stability?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;does the controller run in the hardware constraints you have in place?  Does it still run with the rest of the software you need to run for the spacecraft to do anything?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For further reading on the trade studies I'd immediately turn to &quot;Spacecraft Mission Analaysis and Design,&quot; but my copy is at work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;How are they optimized&quot; is really a whole other question, which depends on the controller you've selected.  In practice it's application of theory followed by repeated tweaking of the controller parameters in response to simulation and (if you're lucky) flight data.&lt;/p&gt;&#xA;" OwnerUserId="11262" LastEditorUserId="11262" LastEditDate="2016-10-13T03:58:37.867" LastActivityDate="2016-10-13T03:58:37.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18609" PostTypeId="2" ParentId="18604" CreationDate="2016-10-13T04:20:32.013" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.thespaceshow.com/search-page?keys=spudis&quot; rel=&quot;nofollow&quot;&gt;Dr. Paul Spudis&lt;/a&gt; is a serious Lunar veteran who has lead several robotic Lunar missions and is a most prominent Lunar geologist. &lt;a href=&quot;http://www.spudislunarresources.com/Papers/Affordable_Lunar_Base.pdf&quot; rel=&quot;nofollow&quot;&gt;Here's&lt;/a&gt; a plan of his to start out a Lunar colony with affordable small rovers, using the likely frozen volatiles at the poles as a resource for rocket fuel and human life support. &lt;a href=&quot;http://www.spudislunarresources.com/Papers/&quot; rel=&quot;nofollow&quot;&gt;More&lt;/a&gt; of his ideas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The costs of settling the Moon consist to a large degree of the cost of transporting mass there from the Earth. By extracting basic resources, like rocket fuel and water/air for life support on the Moon, it can become much more affordable to build and support a settlement on that shore of the sky's sea, than the Apollo program. The Moon's axis is not tilted as much as that of the Earth, its poles have almost no seasons. The bottom of some craters are always in the shadow, as in since millions of years. The coldest places in the Solar System, it has been said. Some mountain ridges are almost always in sunlight. In the shadow there is water ice and other volatiles which are useful for producing rocket fuel and for life support. In the nearly constantly lit up places Solar panels could power a settlement. And they are located next to each other. A settlement there could enjoy both Solar and mined chemical power. And avoid the challenge of the half month long Lunar night that plagues all of the non-polar Moon and requires large power storages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the Moon is nearby and geocentric, it is quickly reachable within a few days any time of the year. And robotic assets there can be remotely controlled from Earth with only a few seconds delay. So Dr. Spudis proposed concept is to begin with launching a series of small robotic missions to the Moon that provide a satellite communications network, stationary Solar power plants and, rather before that, rovers to examine the volatiles believed to exist in those nearly eternally shadowed craters and how they might be extracted since it is yet unknown in what mineralogical form those volatiles exist there. The &lt;a href=&quot;http://nesf2015.arc.nasa.gov/resource-prospector&quot; rel=&quot;nofollow&quot;&gt;Lunar Resource Prospector&lt;/a&gt; is a mission in this spirit, a rover that in 2020 will roll into those eternal and deeply cold shadows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If fuel factories are established on the Lunar poles, visions like &lt;a href=&quot;http://www.moonwards.com/&quot; rel=&quot;nofollow&quot;&gt;moonwards&lt;/a&gt; near the Lunar equator could be supplied by &lt;a href=&quot;http://hopsblog-hop.blogspot.se/2014/06/travel-on-airless-worlds.html&quot; rel=&quot;nofollow&quot;&gt;Lunar suborbital transports&lt;/a&gt; to complement the non-polar resources with extracted polar volatile resources. Very much cheaper than bringing it from Earth. Power and fuel made indigenous, only the engineered equipment (and the workers) needs to be imported from Earth as it wears out or becomes outdated.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-10-13T16:34:49.797" LastActivityDate="2016-10-13T16:34:49.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18610" PostTypeId="1" CreationDate="2016-10-13T04:49:59.747" Score="4" ViewCount="388" Body="&lt;p&gt;I just saw the news item in Lightwave's online website: &lt;a href=&quot;http://www.lightwaveonline.com/articles/2016/10/laser-light-to-launch-space-based-laser-network-for-optical-satellite-as-a-service.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laser Light to leverage free-space optics in space for Optical Satellite as a Service&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;All Optical Hybrid Global Network (or HALO, as laser Light calls it)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That's great - space and optics are made for each other - like coffee and coffee cups.&lt;/p&gt;&#xA;&lt;p&gt;But after looking closely, I believe they are claiming to plan to use optical links from MEO to surface as well. Can this be possible?&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;what kind of laser power and transmitting aperture would a satellite in MEO need to maintain a low BER 100-Gbps link to a receiver on earth that didn't look like the Hale telescope on Mt. Palomar?&lt;/li&gt;&#xA;&lt;li&gt;what about clouds?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Laser Light Communications (US) and its affiliate firm, Laser Light Global, Ltd. (UK) has selected Equinix, Inc. (NASDAQ: EQIX) as its strategic interconnection provider for a network that will combine spaced-based optics and terrestrial fiber-optic network infrastructure to create an Optical Satellite as a Service (OSaaS) offering it calls SpaceCable. The resulting All Optical Hybrid Global Network (or HALO, as laser Light calls it) will offer 100-Gbps connectivity to carriers, enterprises, and government customers via Equinix facilities.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KPRFf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KPRFf.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Graphic from &lt;a href=&quot;http://www.satellitetoday.com/technology/2015/05/04/the-speed-of-light-laser-light-and-optus-explain-optical-communications-partnership/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Speed of Light: Laser Light and Optus Explain Optical Communications Partnership to Via Satellite Magazine&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;It continues...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;According to the company's website, Laser Light will leverage 8 to 12 satellites in medium Earth orbit to create a network that will offer an initial service capacity of 7.2 Tbps. &lt;strong&gt;The satellites will pass signals among themselves and to the ground via free-space optics.&lt;/strong&gt; The space interconnections will include 48 links of 200 Gbps apiece, as well as 72 steerable up/down links to Earth at 100 Gbps. (my emphasis)&lt;/p&gt;&#xA;&lt;p&gt;The satellites and terrestrial network should be ready sometime in 2018, according to information on the company's website.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Additional discussion can be found &lt;a href=&quot;http://www.satellitetoday.com/technology/2015/05/04/the-speed-of-light-laser-light-and-optus-explain-optical-communications-partnership/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;related question: &lt;a href=&quot;https://space.stackexchange.com/q/18408/12102&quot;&gt;Characteristics of the inter-satellite optical link&lt;/a&gt;, also see &lt;a href=&quot;https://space.stackexchange.com/a/15544/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/15542/12102&quot;&gt;this&lt;/a&gt; answer to my previous question.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2020-02-26T14:01:53.073" Title="All Optical Global Network - how does the space-to-ground link avoid clouds?" Tags="|communication|atmosphere|communication-satellite|optical-communication|" AnswerCount="0" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18611" PostTypeId="1" AcceptedAnswerId="18612" CreationDate="2016-10-13T12:09:49.523" Score="3" ViewCount="381" Body="&lt;p&gt;I meant to include the more recent amateur satellites such as cubesats, and also amateur radio satellites - where the heritage of amateur satellite building (mostly) has its roots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have amateur satellites ever experimented with optical communications - basic studies or more?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="58" LastEditDate="2020-02-26T14:01:26.680" LastActivityDate="2023-04-20T11:07:02.893" Title="Have amateur satellites ever experimented with optical communications?" Tags="|cubesat|amateur-radio|optical-communication|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18612" PostTypeId="2" ParentId="18611" CreationDate="2016-10-13T12:38:08.947" Score="3" Body="&lt;p&gt;There was one that I can find, a beacon that used infrared light to communicate at a rate or 400 bits/second, on AO-40, which was a technology demonstration mission by AMSAT.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Z7eIMU9OYWE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Laser_communication_in_space&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt; doesn't include any amateur missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://amsat-uk.org/2014/12/13/cubesat-amateur-laser-communications/&quot; rel=&quot;nofollow&quot;&gt;It is proposed&lt;/a&gt; to use laser based communication for satellites beyond Earth's orbit, such as the Moon or Mars. I can't find any reference of them being used currently, however.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-13T12:38:08.947" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18613" PostTypeId="1" CreationDate="2016-10-13T17:29:20.493" Score="13" ViewCount="3895" Body="&lt;p&gt;If the space elevator platform (The very top) is orbiting a planet in a geosynchronous orbit, wouldn't the forces on the elevator &quot;rope&quot; be minimal?&lt;/p&gt;&#xA;" OwnerUserId="17259" LastEditorUserId="25" LastEditDate="2016-10-13T18:11:27.867" LastActivityDate="2016-10-15T01:15:53.183" Title="Why does the material for space elevators have to be really strong?" Tags="|space-station|materials|space-elevator|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18614" PostTypeId="2" ParentId="18613" CreationDate="2016-10-13T17:52:29.343" Score="6" Body="&lt;p&gt;The force on the rope is due to the weight of the rope. You can imagine a rope 36,000 km long weighs a lot. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-13T17:52:29.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18615" PostTypeId="2" ParentId="18613" CreationDate="2016-10-13T18:09:38.790" Score="3" Body="&lt;p&gt;Imagine building a stack of dirt. For a short period of time, it might stick straight up, but eventually it will fall down, forming something of a pyramid. Every material has an amount at which you can build with it and not have it collapse. Going to a pyramidal shape helps somewhat, but it doesn't solve everything. Specifically what is important is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_strength&quot; rel=&quot;nofollow noreferrer&quot;&gt;Breaking Length&lt;/a&gt;, which can be defined as the maximum weight of an equal area vertical structure that can be supported for the given material. If a material can sustain a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_elevator#Cable&quot; rel=&quot;nofollow noreferrer&quot;&gt;5000 km&lt;/a&gt; vertical tower (As measured at sea level), it is strong enough to use for building a space elevator.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-13T18:09:38.790" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18616" PostTypeId="2" ParentId="18613" CreationDate="2016-10-13T19:31:41.870" Score="23" Body="&lt;p&gt;Supplemental to the other answers; you are correct that the &lt;em&gt;net&lt;/em&gt; force on the tether would be minimal, since the rotation of the counterweight would counteract the force of gravity.  But, the individual components of this net force aren't being distributed evenly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider, say, the first kilometer of tether from the ground.  This is being pulled down by gravity and &quot;held up&quot; by the counterweight, but the counterweight is many many kilometers farther away whereas gravity happens &lt;em&gt;right there.&lt;/em&gt;  So, the rest of the tether is being &quot;pulled apart&quot; by these two forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Incidentally, the counterweight at the top of the tether is going to be well above what &quot;geosynchronous orbit&quot; would be for an ordinary satellite.  Instead the center of gravity for the entire counterweight-tether system would be at geosynchronous orbit altitude, or a bit beyond.)&lt;/p&gt;&#xA;" OwnerUserId="17263" LastEditorUserId="17263" LastEditDate="2016-10-14T22:40:02.663" LastActivityDate="2016-10-14T22:40:02.663" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18617" PostTypeId="1" CreationDate="2016-10-13T20:39:50.333" Score="0" ViewCount="46" Body="&lt;p&gt;How much does it costs NASA for an astronaut to perform a single spacewalk? Not including salary just the cost of actually sending someone into space. &lt;/p&gt;&#xA;" OwnerUserId="17176" LastEditorUserId="25" LastEditDate="2016-10-13T22:32:43.130" LastActivityDate="2016-10-13T23:01:40.023" Title="Average cost of SpaceWalk" Tags="|iss|eva|cost|" AnswerCount="0" CommentCount="2" FavoriteCount="0" ClosedDate="2016-10-13T23:03:12.140" ContentLicense="CC BY-SA 3.0" />
  <row Id="18618" PostTypeId="1" CreationDate="2016-10-13T22:19:19.113" Score="1" ViewCount="89" Body="&lt;p&gt;This never really happened in the Apollo missions, or any other rocket launch... but what would theoretically happen if something exploded (let's say, the oxygen tanks) and caused a hole in the side of the spacecraft?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the crew be sucked out  and killed instantly, similar to an airplane, or would the spacecraft automatically try to correct the error and seal the hole?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, in the unlikely chance that a hole appears on the side of a spacecraft, what is the normal NASA procedure?&lt;/p&gt;&#xA;" OwnerUserId="17203" LastActivityDate="2016-10-13T22:19:19.113" Title="What would astronauts do in the unlikely event an hole appeared on the side of their spacecraft?" Tags="|crewed-spaceflight|spacecraft|" AnswerCount="0" CommentCount="8" ClosedDate="2016-10-14T12:29:38.243" ContentLicense="CC BY-SA 3.0" />
  <row Id="18619" PostTypeId="2" ParentId="14119" CreationDate="2016-10-13T23:01:40.023" Score="5" Body="&lt;p&gt;Assuming that a person is already in space, the main costs are consumables lost (Small), and space suits (Larger). A &lt;a href=&quot;http://history.nasa.gov/spacesuits.pdf&quot; rel=&quot;noreferrer&quot;&gt;space suit&lt;/a&gt; reportedly costs about $2 million. It's hard to tell how often they are used before being retired. One thing that can be tracked is the amount of supplies dedicated to EVAs launched by the &lt;a href=&quot;http://www.nasa.gov/content/commercial-resupply-media-resources&quot; rel=&quot;noreferrer&quot;&gt;Commercial Resupply missions&lt;/a&gt; to the ISS. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SpaceX CRS-2- 3 kg&lt;/li&gt;&#xA;&lt;li&gt;SpaceX CRS-3- 173 kg&lt;/li&gt;&#xA;&lt;li&gt;Orbital CRS-2- 39.4 kg&lt;/li&gt;&#xA;&lt;li&gt;SpaceX CRS-4- 25 kg&lt;/li&gt;&#xA;&lt;li&gt;Orbital CRS-3- 33 kg (Failed mission)&lt;/li&gt;&#xA;&lt;li&gt;SpaceX CRS-5 23 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These missions cover 2 years of time. I'm guessing that the large cargo on CRS-3 was a replacement space suit, and the rest are supplies required, maybe legs or arms, unique to each astronaut. There are 2 American space suits on the station at any one time, so let's say one every 4 years is good.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, all that being said, let's add up some costs. I'm going to use the total amount of the ISS related to EVA costs by the US as a starting point, over the 2 years.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Space Suit: \$2 million&lt;/li&gt;&#xA;&lt;li&gt;Carrying Cargo to ISS- 296 kg lifted, about &lt;a href=&quot;http://spacenews.com/spacex-wins-5-new-space-station-cargo-missions-in-nasa-contract-estimated-at-700-million/&quot; rel=&quot;noreferrer&quot;&gt;\$80,000/kg&lt;/a&gt; gives \$23.68 million.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.businessinsider.com/space-travel-per-seat-cost-soyuz-2016-9&quot; rel=&quot;noreferrer&quot;&gt;Cost for Astronaut to reach ISS&lt;/a&gt;: \$81 million&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are about 9 EVAs/year, always done in pairs. Assume half use the American suits. The cost per EVA per person is then about \$1.5 million. The far larger cost is in getting the astronaut to the ISS to take said spacewalk!&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-13T23:01:40.023" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18621" PostTypeId="1" CreationDate="2016-10-13T23:57:13.823" Score="5" ViewCount="308" Body="&lt;p&gt;The gas generator power cycle is an open cycle, with pre-burner exhaust vented after driving the turbine instead of contributing to thrust as in more advanced power cycles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What percentage of the propellant is usually used to feed the pre burner? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, what percentage of the potential energy is &quot;wasted&quot;? That is, the energy that could have been used for thrust minus the energy that, while not used for thrust, is used for driving the turbines and therefore is not wasted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparing Isp of staged combustion engines with gas generator engines, of course limiting to the same propellants, leads me to a result of around 5-15%, but there really are too many independent variables for a meaningful comparison.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also found a mention of 2-7% on the Braeunig site, but unfortunately with no support or theory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few examples to get a ballpark figure will also suffice.&lt;/p&gt;&#xA;" OwnerUserId="17241" LastEditorUserId="17241" LastEditDate="2016-10-14T13:49:23.973" LastActivityDate="2016-10-14T13:49:23.973" Title="How much propellant is &quot;wasted&quot; in the gas generator power cycle?" Tags="|engine-design|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18622" PostTypeId="1" AcceptedAnswerId="18623" CreationDate="2016-10-14T04:24:01.507" Score="17" ViewCount="3212" Body="&lt;p&gt;Sputnik was only launched a few hundred kilometers above Earth, and transmitted only a simple beeping signal. What was the purpose of having four antennas? Wouldn't one be powerful enough?&lt;/p&gt;&#xA;" OwnerUserId="17272" LastActivityDate="2016-10-15T05:55:02.727" Title="Why did Sputnik 1 have four antennas?" Tags="|probe|russia|soviet-union|sputnik|antenna|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18623" PostTypeId="2" ParentId="18622" CreationDate="2016-10-14T11:42:23.007" Score="15" Body="&lt;p&gt;Sputnik was the first satellite. It was set up before we had an understanding of how difficult it would be to maintain a satellite's position, and in fact was a very simple system overall. The 4 antennas gives an omnidirectional broadcast pattern. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VWtch.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VWtch.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simplest form of an antenna, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Dipole_antenna&quot; rel=&quot;noreferrer&quot;&gt;dipole&lt;/a&gt;, has two beams going in opposite directions. It's beam pattern looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wpdqa.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wpdqa.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the direction wasn't pointed at Earth, it could end up in a null, meaning that the signal couldn't be received. The pattern that they gave allowed for Sputnik to be in any orientation and be able to transmit signal to anywhere visible on the ground below.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-14T11:42:23.007" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18624" PostTypeId="1" AcceptedAnswerId="18638" CreationDate="2016-10-14T12:03:57.087" Score="4" ViewCount="4590" Body="&lt;p&gt;I am in the process of designing a reaction wheel to be used in a cubesat for my final year project. I have already chosen my motor ( Faulhaber 2610T006B ) , and am currently in the process of sizing my flywheel. But I have gotten stuck halfway through. The following is my working:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Desired \space slew \space rate: 3° per \space sec \approx 0.0523599 \space rad \space per \space sec$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\theta = \frac12 \frac\tau J t^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$For \space 90° rotation \space at \space 3° per \space sec: \space Time \space taken = \frac{90}{3} = 30s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\therefore \tau_{min} = \frac{2\theta J}{t^2} = \frac{2\times0.5 \times \pi \times 0.03}{30^2} = 1.047 \times 10^{-4}Nm,$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$where \space \tau_{min} = Minimum \space torque \space required \space to \space achieve \space 90° rotation \space within \space 30s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$For \space reaction \space wheel, \space \tau = I\alpha,$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$where \space I = moment \space of \space inertia \space of \space flywheel,$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\alpha_{max} = max. \space angular \space acceleration \space of \space the \space motor(\frac{rad}{s^2})$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Set \space \tau_{min} = \tau , \therefore I = \frac{\tau_{min}}{\alpha_{max}}\rvert$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Finding \space \alpha_{max},$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F = ma, \space a = radius \times \alpha_{max},$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$where \space m = weight \space of \space flywheel = 50g \approx 0.05Kg, \space a = linear \space acceleration \space in \space \frac{m}{s^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\require{extpfeil}\Newextarrow{\xRightarrow}{5,5}{0x21D2}\tau = Fdsin(\theta) \xRightarrow[\theta = 90°]{} Fd, where \space d = length \space of \space the \space rotor = 0.006meters$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\therefore \tau \space should \space be \space the \space torque \space required \space to \space move \space the \space load, what \space should \space \tau \space be?$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As can be seen from the working above, I am trying to achieve a slew rate of 3 degrees per second. But I am having trouble finding alpha max, that is, the maximum angular acceleration when my flywheel is attached to my motor. I also forgot to mention that J is the principal moment of inertia about one axis and is 0.03kgm^2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My flywheel will be a cylindrical shape, once I am able to find alpha max, I can find my required I(moment of inertia of flywheel) and then begin sizing it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I approaching this the right way? And how should I find alpha max?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would also like to ask if I were to factor in all three reaction wheels operating at once, how much would my requirements for momentum storage change by?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="16924" LastEditorUserId="58" LastEditDate="2016-10-19T16:13:18.560" LastActivityDate="2017-10-28T10:19:43.763" Title="Help with reaction wheel flywheel sizing" Tags="|cubesat|reaction-wheel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18625" PostTypeId="1" AcceptedAnswerId="18626" CreationDate="2016-10-14T13:22:20.483" Score="6" ViewCount="359" Body="&lt;p&gt;At this orbit wouldn't all space junk be traveling at the same speed as the spaceship/satellite transversing it? To be dangerous it should be traveling at a much faster (or slower) rate than the object it is putting in danger. But at this altitude everything should be at the same rate?&lt;/p&gt;&#xA;" OwnerUserId="6000" LastActivityDate="2016-10-16T17:40:35.790" Title="How can space junk be dangerous at geosynchronous orbits?" Tags="|debris|geosynchronous|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18626" PostTypeId="2" ParentId="18625" CreationDate="2016-10-14T13:36:31.990" Score="8" Body="&lt;p&gt;If everything is going in the same orbit, that's true, anything impacting will be a low speed. However, there's a few things to consider.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Two large low-speed objects impacting can cause damage. The best known example of this was the &lt;a href=&quot;http://history.nasa.gov/SP-4225/multimedia/progress-collision.htm&quot; rel=&quot;nofollow&quot;&gt;Progress-MIR collision&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Objects in Geostationary orbit don't stay there naturally, it requires fuel to keep them there. They will naturally keep the 24 hour cycle, but will tend to increase their inclination with time. This inclination can be up to &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Geostationary_orbit#Orbital_stability&quot; rel=&quot;nofollow&quot;&gt;15 degrees&lt;/a&gt;. A 15 degree impact at 3 km/s still is a difference of 770 m/s, or faster than a speeding bullet. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-10-16T16:54:41.040" LastActivityDate="2016-10-16T16:54:41.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18627" PostTypeId="1" AcceptedAnswerId="18629" CreationDate="2016-10-14T14:05:14.323" Score="22" ViewCount="3291" Body="&lt;p&gt;The (currently only) answer to &lt;a href=&quot;https://space.stackexchange.com/q/18622/415&quot;&gt;Why did Sputnik 1 have four antennas?&lt;/a&gt; discusses how Sputnik 1 had four antennas in order to ensure that the Earth would never be in the null of any of its antennas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PearsonArtPhoto's answer specifically states that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Sputnik] was set up before we had an understanding of how difficult it would be to maintain a satellite's position, and in fact was a very simple system overall.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This would seem to beg the question, &lt;strong&gt;did Sputnik 1 have any attitude control at all?&lt;/strong&gt; (By means of thrusters or otherwise.) Or was it simply tumbling along in its orbit in whatever attitude and roll rates it happened to end up in after booster separation?&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="1221" LastEditDate="2016-10-14T22:44:24.803" LastActivityDate="2018-02-21T20:51:14.223" Title="Did Sputnik 1 have attitude control?" Tags="|artificial-satellite|history|attitude|sputnik|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18628" PostTypeId="2" ParentId="18627" CreationDate="2016-10-14T14:50:02.307" Score="17" Body="&lt;p&gt;No, &lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_1#Design&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sputnik&lt;/a&gt; did not have any such system. It would have added complexity and power requirements. The Soviets were trying to beat the Americans in to space, and succeeded by the launch of Sputnik, but they had little time to design it well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were basically 3 systems in the spacecraft, radio, power, and thermal management. The power was mostly 3 batteries, 2 connected to the radio and one to the thermal control system. There was no power left over for other systems to interface with it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the upper stage of &lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sputnik 2&lt;/a&gt; did have an attitude control system, basically a &quot;cold gas&quot; system to maintain stability, although Sputnik did not. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other early systems basically were spin stabilized if stabilized at all. I can't find the first reference to a fully stable satellite, but I'm sure it happened quite early.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-02-21T20:51:14.223" LastActivityDate="2018-02-21T20:51:14.223" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18629" PostTypeId="2" ParentId="18627" CreationDate="2016-10-14T14:53:18.530" Score="22" Body="&lt;p&gt;Sputnik-1, and Sputnik-2, had no attitude control whatsoever, and would have tumbled freely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;[Like] its predecessor, Sputnik-2 would have no attitude control system.&quot; &lt;a href=&quot;http://www.russianspaceweb.com/sputnik2_decision.html&quot;&gt;http://www.russianspaceweb.com/sputnik2_decision.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sputnik technical specifications and diagram: &lt;a href=&quot;http://www.russianspaceweb.com/sputnik_design.html&quot;&gt;http://www.russianspaceweb.com/sputnik_design.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17050" LastEditorUserId="4703" LastEditDate="2016-10-14T19:01:49.077" LastActivityDate="2016-10-14T19:01:49.077" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18630" PostTypeId="2" ParentId="18613" CreationDate="2016-10-14T15:55:48.060" Score="2" Body="&lt;p&gt;A space elavtor works by balancing gravity with centrifugal force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you get further from the center of the earth (which is both the center fo gravity and the center of rotation) gravity decreases and centrifugal force increases. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each segment of the &quot;rope&quot; can be assigned an &quot;effective weight&quot; consisting of weight minus centrifugal force (both weight and centrifugal force are proportional to mass). For segments below geostatoinary orbit the effective weight will be positive, for segments above geostationary orbit the effective weight will be negative.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tension in the rope is highest at geostationary orbit where all the rope with negative effective weight is pulling on all the rope with postive effective weight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the effective weight of the rope below geostationary orbit will be lower than it's actual weight it will still be a substantial fraction of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can to some extent work arround this by making the rope non-uniform but just as with a rocket you need high ISP to avoid ludicrous mass ratios with a space elevator cable you need high strength to weight ratio to avoid ludicrous thickness ratios.&lt;/p&gt;&#xA;" OwnerUserId="12039" LastEditorUserId="12039" LastEditDate="2016-10-14T16:11:59.780" LastActivityDate="2016-10-14T16:11:59.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18631" PostTypeId="2" ParentId="18622" CreationDate="2016-10-14T22:54:47.280" Score="17" Body="&lt;p&gt;I think @PearsonArtPhoto 's answer misses several major points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.cs.mcgill.ca/~rwest/link-suggestion/wpcd_2008-09_augmented/wp/s/Sputnik_1.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.cs.mcgill.ca/~rwest/link-suggestion/wpcd_2008-09_augmented/wp/s/Sputnik_1.htm&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The satellite carried two antennas designed by the Antenna Laboratory of OKB-1 led by M.V.Krayushkin. Each antenna was made up of two whip-like parts: 2.4 and 2.9 meters (7.9 and 9.5 ft) in length, and had an almost spherical radiation pattern, so that the satellite beeps were transmitted with equal power in all directions; making reception of the transmitted signal independent of the satellite's rotation. The whip-like pairs of antennas resembled four long &quot;whiskers&quot; pointing to one side, at equal 35 degrees angles with the longitudinal axis of the satellite.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There were two transmitters, &lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_1&quot; rel=&quot;noreferrer&quot;&gt;at 20 MHz and 40 MHz&lt;/a&gt;. Each transmitter &lt;strong&gt;was connected to only one pair of antennas&lt;/strong&gt; and this is important. &lt;strong&gt;The opening angle between the two radiators in a given pair is 70°, close to right angles!&lt;/strong&gt; This means that in the far field the polarizations of the two are nearly perpendicular. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The significance here is that if two waves have orthogonal polarizations, they would not strongly interfere with each other. You can (roughly!) add their radiation patterns algebraically - one antenna's null would be &quot;filled in&quot; to some extent by the power from the other antenna. If they were not close perpendicular, they'd have to be added strictly as vectors and that would just produce new and different null patterns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One pair was connected to the 20 MHz transmitter, the other pair to the 40 MHz transmitter. These have wavelengths of 15 and 7.5 meters, respectively, so the radiation patterns would have been substantially different for each individual element of a pair, at each frequency. While the pattern wouldn't actually be spherical, the almost-perpendicular orientation of the two radiators in a given pair ensures that the null of one would tend to be filled in by the other, and interference was minimized thereby avoiding the creation of new nulls via interference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A similar design was used by several NASA Explorer satellites as well:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H0aEJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H0aEJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Image of Explorer XVII from &lt;a href=&quot;http://www.nasa.gov/image-feature/explorer-xvii-satellite&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ujsjc.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ujsjc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_SLV-2&quot; rel=&quot;noreferrer&quot;&gt;Vanguard SLV-2&lt;/a&gt; design, NASA image from &lt;a href=&quot;http://www.nasa.gov/content/vanguard-satellite-1958&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; A pair of true dipoles crossed at 90 degrees with perpendicular polarizations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MLpgC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MLpgC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Image from &lt;a href=&quot;http://www.fantastic-plastic.com/ProjectVanguardSatellitePage.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; - original artwork on the circa-1958 plastic model of a Vanguard series satellite - antenna pattern is more easily visualize in this representation. More about Vanguard models can be found in &lt;a href=&quot;http://www.scalemodelnews.com/2015/09/space-technology-from-six-decades-ago.html&quot; rel=&quot;noreferrer&quot;&gt;Space Technology from Six Decades Ago: 1:5 Scale Hawk Project Vanguard Satellite&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fjfAB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fjfAB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Vanguard 1 NASA image from &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1958-002B&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. Three true dipoles appear to be mutually perpendicular.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-15T05:55:02.727" LastActivityDate="2016-10-15T05:55:02.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18632" PostTypeId="1" CreationDate="2016-10-14T23:26:41.513" Score="1" ViewCount="240" Body="&lt;p&gt;Reading about NASA testing the SLS (Space Launch System) booster and they are chilling it to 40 degrees Fahrenheit. This confused me because they are going to fill it with liquid oxygen and liquid hydrogen which are extremely cold (LOX is 90K, hydrogen is 20K). So I'm thinking maybe the booster has insulated walls. And since one of the tanks is above the other, there must be a pipe to carry the the fluid to engine. So where is this pipe located? And how big is it? And is it insulated as well?&lt;/p&gt;&#xA;" OwnerUserId="17284" LastEditorUserId="415" LastEditDate="2016-10-15T17:36:11.503" LastActivityDate="2016-10-15T17:36:11.503" Title="Are rocket boosters insulated?" Tags="|rockets|sls|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18633" PostTypeId="2" ParentId="18632" CreationDate="2016-10-15T00:09:25.080" Score="4" Body="&lt;p&gt;This number and description probably comes from the Wikipedia Space Launch System article and it refers to the two solid fuel boosters attached to the sides of the main core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid fuel boosters do not use cryogenic (very cold) fuel, using solid fuel instead. And so, the temperature testing required is only that which covers the ambient temperature of the launch site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For SLS, cryogenic fuel is used for the core and upper stage.&lt;/p&gt;&#xA;" OwnerUserId="17241" LastActivityDate="2016-10-15T00:09:25.080" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18634" PostTypeId="2" ParentId="18613" CreationDate="2016-10-15T01:15:53.183" Score="0" Body="&lt;p&gt;The cable is under tension - maximum in the center - because every foot you go up the cable, that's one more foot of cable hanging from it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can be addressed by tapering the cable:  the cable cross-section increases as you approach the center in order to support the increasing amount of cable hanging from it.  There is a 'taper ratio' equation (I read an article about all this, IIRC published in either &quot;New Destinies&quot; or &quot;Analog&quot; at least ten years ago).  Given carbon fiber, the taper ratios start getting down to (again, IIRC) about 4.5.&lt;/p&gt;&#xA;" OwnerUserId="12601" LastActivityDate="2016-10-15T01:15:53.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18636" PostTypeId="2" ParentId="18583" CreationDate="2016-10-15T06:57:15.263" Score="3" Body="&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt;Special thanks to @David, for the great link he provided.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Seeing that this question has garnered some interest, And nobody has answered yet, I guess I'll take a crack at it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After reading over the paper mentioned above, and researching other &lt;sup&gt;2&lt;/sup&gt;sources I decided I'd just compile a selected list of rocket and satellite name origins. This list is by no means complete, and feel free to add on if you know a certain spacecraft's name origin. I should mention that the paper above provides more than just rocket and satellite name origins, and I highly recommend reading it for yourself. With that in mind:&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Rockets&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/RM-81_Agena&quot; rel=&quot;nofollow&quot;&gt;Agena&lt;/a&gt;: The Agena rocket is an upper stage that has its origins with the &lt;a href=&quot;http://www.lockheedmartin.com/us/what-we-do/space.html&quot; rel=&quot;nofollow&quot;&gt;Lockheed Missiles &amp;amp; Space Company&lt;/a&gt;. The name &quot;Agena&quot; was proposed for the star Agena in the constellation Centaurus, because of the fact the Agena was an upper stage rocket, igniting in the sky.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_(rocket_family)&quot; rel=&quot;nofollow&quot;&gt;Atlas&lt;/a&gt;: The Atlas rocket was a powerful first stage, used in various applications. In the early '50's, Karel J. Bossart, head of the design team at Convair (Consolidated Vultee Aircraft Corporation), Decied to ask his staff for name ideas for the then new Atlas rocket. In the end &quot;Atlas&quot; was chosen because the missile they were designing at Covair was going to be the biggest of any missile to date. Bossart named it for the Atlas, who, in Greek mythology, is a powerful God that holds up the earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Little_Joe_(rocket)&quot; rel=&quot;nofollow&quot;&gt;Little Joe&lt;/a&gt;: The Little Joe rocket was a inexpensive rocket for testing the escape sequence of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Mercury&quot; rel=&quot;nofollow&quot;&gt;Mercuy&lt;/a&gt; spacecraft. The rocket earned its name, because of its engine being drawn with four holes up, which resembled a &quot;double deuce&quot; throw, in &lt;a href=&quot;https://en.wikipedia.org/wiki/Craps&quot; rel=&quot;nofollow&quot;&gt;Craps&lt;/a&gt;. The that type of throw is sometimes referred to as a &quot;Little Joe&quot;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/s/shotput.html&quot; rel=&quot;nofollow&quot;&gt;Shotput&lt;/a&gt;: The Shotput rocket was&#xA;a rocket specifically made for testing balloon-satellite ejection&#xA;and inflation in space. It carried the &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Echo&quot; rel=&quot;nofollow&quot;&gt;Echo satellite&lt;/a&gt; payload. It gained its name from the fact that it &quot;tossed&quot; the Echo sphere above earth's atmosphere, in a vertical trajectory.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow&quot;&gt;Saturn V&lt;/a&gt;: The Saturn V(Saturn Five) rocket, was the rocket used to place the Apollo vehicles in Lunar Orbit, as well as launch the Skylab projects into Low Earth Orbit. Nominations were submitted to NASA for the name final name for the Saturn V. The front runner for some time was &quot;Krons&quot;, until the Committee suggested-Through the Assistant Administrator for Public Affairs George L. Simpson, Jr- The name &quot;Saturn V&quot;. And Saturn V was chosen.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/PGM-11_Redstone&quot; rel=&quot;nofollow&quot;&gt;Redstone&lt;/a&gt;: The Redstone rocket has its origins as a battlefield missile developed by the U.S. Army. NASA went on to modify it and use it for launching sub-orbital &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Mercury&quot; rel=&quot;nofollow&quot;&gt;Mercury&lt;/a&gt; flights. After having several Nicknames, the booster was officially named &quot;Redstone&quot; due to its development taking place in the Redstone Arsenal in Huntsville, Alabama.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Able_(rocket_stage)&quot; rel=&quot;nofollow&quot;&gt;Able&lt;/a&gt;: Since this name explanation was brief, I thought it might be best to directly cite the &lt;sup&gt;1&lt;/sup&gt;paper:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Able upper stage was one of several derived in 1958 by the Department&#xA;  of Defense's Advanced Research Projects Agency, Douglas Aircraft&#xA;  Company, and Space Technology Laboratories from Vanguard launch vehicle&#xA;  components. It was used with Thor or Atlas first stages. The name&#xA;  signified &quot;A&quot; or &quot;first&quot; (from military phonetic communications practice&#xA;  of stipulating key words beginning with each letter of the alphabet)...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Satellites&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sphinx_(satellite)&quot; rel=&quot;nofollow&quot;&gt;Sphinx&lt;/a&gt;: The Sphinx Satellite was planned to be one of the smallest NASA satellites-Weighing in at just 113 Kilograms(249.122). It gained its name from a experiment acronym-&quot;Space Plasma High Voltage Interaction Experiment.&quot; &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Pegasus_(satellite)&quot; rel=&quot;nofollow&quot;&gt;Pegasus&lt;/a&gt;: The Pegasus spacecraft's most striking feature, is its impressive 96 meters wing span. In terms of satellites, is was the heaviest satellite to date. Thus being consider a &quot;horse&quot; sized payload. due to its massive wing span, and unprecedented size, the name &quot;Pegasus&quot; was chosen for the mythological winged horse.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Nimbus_program&quot; rel=&quot;nofollow&quot;&gt;Nimbus&lt;/a&gt;: The Nimbus satellite was named for the term &quot;Nimbus&quot; in meteorology meaning &quot;precipitating cloud. Its name was Suggested by Edgar M. Courtright, Chief of NASA's Advance Technology Program.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Explorer_1&quot; rel=&quot;nofollow&quot;&gt;Explorer 1&lt;/a&gt;: The &quot;Explorer&quot; name  has its origins even before the birth NASA. before NASA was established, the name was used for US military weather balloons. Wilbur M. Brucker-secretary of the Army-announced the name. Its name reflected its mission, and the mark of the first US satellite.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Echo&quot; rel=&quot;nofollow&quot;&gt;Echo&lt;/a&gt;: Echo-an inflatable satellite, conceived by William J. O'Sullivan-gained its name from its mission- to be used as a reflector for radio signals. In radio terms, echo&quot; is often used to describe the deflection of radio signals from earth to to an orbiting balloon. The satellite was thus named &quot;Echo&quot;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h2&gt;Sources&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Main_Page&quot; rel=&quot;nofollow&quot;&gt;Wikipedia&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770010038.pdf&quot; rel=&quot;nofollow&quot;&gt;Origins of NASA Names paper&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;a href=&quot;http://www.astronautix.com/&quot; rel=&quot;nofollow&quot;&gt;astronautix.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Note to contributors:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;If anyone does deiced to add on to my answer, please make sure it's posted under the correct category. if a category does not exist, feel free to create a new one.&lt;/p&gt;&#xA;" OwnerUserId="16704" LastEditorUserId="8162" LastEditDate="2016-10-15T15:13:50.547" LastActivityDate="2016-10-15T15:13:50.547" CommentCount="0" CommunityOwnedDate="2016-10-15T07:03:49.607" ContentLicense="CC BY-SA 3.0" />
  <row Id="18638" PostTypeId="2" ParentId="18624" CreationDate="2016-10-15T13:47:12.687" Score="3" Body="&lt;p&gt;Think of reaction wheel torque and momentum as separate parameters that are separately sized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Max wheel momentum is based on the moment of inertia (MOI) of the wheel about its rotation axis multiplied by the maximum rotation rate. The max rotation rate is dictated by the electrical properties of the motor, friction, and balance. The last two depend on the design and the manufacturing. That said, &lt;em&gt;you can start with a rule-of-thumb number for sizing like 6000 rpm&lt;/em&gt;, which is roughly what these &lt;a href=&quot;http://www.sinclairinterplanetary.com/reactionwheels&quot; rel=&quot;nofollow&quot;&gt;commonly used small RWs have for max top speed&lt;/a&gt;. (Disclosure: I have used this companies wheels.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You want your satellite to have a max slew of: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$0.0523599 \space rad \space per \space sec$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you can back out require wheel MOI directly from that:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ MOI_{wheel} = MOI_{spacecraft}\frac{\omega_{max_{spacecraft}}}{\omega_{max_{wheel}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, the larger your spacecraft MOI, the larger your wheel's MOI needs to be; the faster you want the spacecraft to rotate the larger the wheel; the faster the wheel can spin the smaller the wheel. Holding the other variables constant of course. Plug in 6000 rpm, 3°/sec and your spacecraft MOI and you're done (use the same units of course).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;You were on the right track for solving for required torque. Think of the maneuver as a trapezoid. Wheel speed will increase linearly to max speed, then sustain there, then decrease linearly back to zero. Maneuver duration in angle and time are set and you know max spacecraft rotation rate, so you can solve for required slope of curve and use that to back out required torque.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's lots of freedom to increase the torque of a wheel by increasing the size of the electric motor that drives it and the power consumption during acceleration. Sanity check your results against existing technology.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2016-10-15T13:47:12.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18639" PostTypeId="1" CreationDate="2016-10-15T13:50:12.043" Score="5" ViewCount="635" Body="&lt;p&gt;I am going to be using air coil magnetorquers for a satellite development project. These air coil magnetorquers come embedded in solar panels. I am currently using &lt;a href=&quot;http://gomspace.com/index.php?p=products-p110&quot; rel=&quot;nofollow noreferrer&quot;&gt;this magnetorquer , it's a solar panel with a magnetorquer embedded into it.&lt;/a&gt; The magnetorquers are going to be used to produce a magnetic moment ($m$) to desaturate the reaction wheels in space. I have a controller that will provide me with the $m$ that I require, but I need a way to experimentally verify that I am getting the correct $m$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am pretty familiar with the math behind the ferromagnetic magnetorquers, alot of information is provided by &lt;a href=&quot;https://space.stackexchange.com/questions/2239/what-is-the-math-behind-magnetorquers?noredirect=1&amp;amp;lq=1&quot;&gt;this link.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mainly have two questions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly,I am working with the magnetorquers integrated with the solar panels, would the mathematics remain the same? I also know that for normal ferremagnetic magnetorquers , $m=nIA$ , where $m$ is the magnetic dipole moment, $I$ is the current through the magnetorquer and $A$ is the area of the magnetorquer. Are these equations still valid for the air coil magnetorquers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, how would I be able to experimentally measure the magnetic dipole moment generated by the air coil? For the ferromagnetic coil magnetoquers, I found &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=27CB6ED1C42A4F31A3B8EC3597973002?doi=10.1.1.565.8909&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this research paper&lt;/a&gt; to measure the magnetic dipole moment for a normal ferromagnetic magnetorquer, but I don't know if it would work for an air coil magnetorquer. I am also worried about how the solar panel itself would interfere with the measurements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks.&lt;/p&gt;&#xA;" OwnerUserId="16924" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-02-13T19:46:50.187" Title="Relation between air-core and ferromagnetic solenoid -core magnetorquers and measurement of dipole moment" Tags="|magnetorquer|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="18640" PostTypeId="1" AcceptedAnswerId="18971" CreationDate="2016-10-15T18:12:29.977" Score="4" ViewCount="808" Body="&lt;p&gt;All the existent satellite constellations and the proposed ones talk about &lt;strong&gt;inter-plane&lt;/strong&gt; communications as a very challenging issue due to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;High relative velocities between satellites&lt;/li&gt;&#xA;&lt;li&gt;Tracking control problems as antennas must slew around&lt;/li&gt;&#xA;&lt;li&gt;Doppler shift&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I wonder why a different approach is not considered. Is it possible for a hybrid topology: combining a ring topology with a star topology?&#xA;In other words, satellites from one plane can communicate among themselves (ring), and if they want to &quot;talk&quot; to a satellite in another plane, they could use a higher orbit satellite (something as a &lt;strong&gt;hub&lt;/strong&gt;) and do the whole routing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the pros and cons fo this approach against typical inter-planar communication?&lt;/p&gt;&#xA;" OwnerUserId="17292" LastEditorUserId="35037" LastEditDate="2020-05-07T16:20:37.930" LastActivityDate="2020-05-07T16:33:59.703" Title="Inter-satellite linking for satellite constellations?" Tags="|artificial-satellite|communication-satellite|radio-communication|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18642" PostTypeId="2" ParentId="18601" CreationDate="2016-10-15T22:35:16.333" Score="2" Body="&lt;p&gt;Not an actual bullet, but researchers fire plenty of things at spacecraft shields to test them. Here's one I found in full &lt;a href=&quot;http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1079&amp;amp;context=nasapub&quot; rel=&quot;nofollow noreferrer&quot;&gt;Performance of Whipple shields at impact velocities above 9 km/s&lt;/a&gt;. They fired aluminum balls in the range of .004g to .3g at 7 to 9.9km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shield failed against .3g at 7km/s. That's 7,350J or about three times what a typical battle rifle fires (see below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It held up against .03g at 9km/s = 800J. That's a bit more than a typical pistol round (again, see below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before you go concluding a spacecraft is &quot;bulletproof&quot; against a handgun, there's a few caveats.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;That's just for the shields they tested&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;This particular test was comparing some new designs against a baseline shield. Different spacecraft will have different shielding. The same spacecraft will have different shielding over different areas. This just gives a ballpark estimate.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Every hit damages the shield&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jz45l.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jz45l.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Bulletproof&quot; armor isn't. Every piece of armor has a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bulletproof_vest#Ballistic_testing_V50_and_V0&quot; rel=&quot;nofollow noreferrer&quot;&gt;V&lt;sub&gt;0&lt;/sub&gt; and a V&lt;sub&gt;50&lt;/sub&gt;&lt;/a&gt; rating. These are it's design limits to stop a certain amount of energy 100% and 50% of the time over a certain amount of rounds. Eventually it will fail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, Whipple shields are designed to take damage to reduce the energy of the impact. A thin layer of metal vaporizes on impact dispersing some of the energy of the impact. This is known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Ablative_armor&quot; rel=&quot;nofollow noreferrer&quot;&gt;ablative armor&lt;/a&gt;. Hit it again in the same spot and the armor will be weakened.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the amount of damage these shields took from single impacts, I'll bet unloading a clip of 9mm will give you at least &lt;a href=&quot;https://en.wikipedia.org/wiki/Spalling&quot; rel=&quot;nofollow noreferrer&quot;&gt;spalling&lt;/a&gt; if not full penetration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more detail on ballistic protection, see &lt;a href=&quot;https://worldbuilding.stackexchange.com/a/55130/760&quot;&gt;my attempt to make Captain America's Shield feasible&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some of the energy numbers for bullets in the question and comments are WAY out of scale. It's worthwhile to review the energies of bullets. (Note: all numbers are approximate for the typical load. The same round has all sorts of variations in bullet weight and powder load. Barrel length also maters, a shorter barrel means lower velocity means less energy.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, typical pistol rounds.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/9%C3%9719mm_Parabellum&quot; rel=&quot;nofollow noreferrer&quot;&gt;9x19mm Parabellum&lt;/a&gt;, the typical &quot;nine&quot; pistol round. 7.5 grams at .4km/s = 600J.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/.45_ACP&quot; rel=&quot;nofollow noreferrer&quot;&gt;.45 ACP&lt;/a&gt;, the famous &quot;Colt 45&quot;. 12g at .3km/s = 600J.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now the most powerful pistol rounds.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/.44_Magnum&quot; rel=&quot;nofollow noreferrer&quot;&gt;.44 Magnum&lt;/a&gt;, &quot;the most powerful handgun int he world&quot; according to Dirty Harry. 16g at .4km/s = 1400J.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/.50_Action_Express&quot; rel=&quot;nofollow noreferrer&quot;&gt;.50 Action Express&lt;/a&gt; made famous by the Desert Eagle. 19g at .45km/s = 2000J.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As you can see, that's no where near the 8000J claimed in the question. These are a full order of magnitude below the velocity of even the low end of an expected micrometeorite strike. That translates into 100 times less energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, modern battle rifles.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO&quot; rel=&quot;nofollow noreferrer&quot;&gt;5.56 NATO&lt;/a&gt; the standard western rifle round like from an M16, 4g at .9km/s = 1800J.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/7.62%C3%9739mm&quot; rel=&quot;nofollow noreferrer&quot;&gt;7.62x39mm&lt;/a&gt; used by modern Russian rifles like the AKM, 8g at .7km/s = 1600J.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/7.62%C3%9754mmR&quot; rel=&quot;nofollow noreferrer&quot;&gt;7.62x54mmR&lt;/a&gt; used by the AK-47 (but not modern Russian rifles). 12g at .8km/s = 3600J.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That the 4g 5.56 NATO and 8g 7.62x39mm have nearly the same energy says a lot about how important a small boost in velocity is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now WWII era stuff you're not likely to see, but this is the most energetic a bullet gets without getting into specialty ammo and guns.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/.30-06_Springfield&quot; rel=&quot;nofollow noreferrer&quot;&gt;.30-06&lt;/a&gt;, the standard US round in WWII. 12g at .8km/s = 4000J.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/.50_BMG&quot; rel=&quot;nofollow noreferrer&quot;&gt;.50 BMG&lt;/a&gt;, used by US WWII aircraft and still used today in vehicle mounted heavy machine guns. 45g at .9km/s = 19000J.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;4,000J is the biggest bullet you're likely to see fired at a spacecraft... on the ground I guess, but you can go up to 20,000J. These are all moving at less than 1km/s.&lt;/p&gt;&#xA;" OwnerUserId="4704" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:23.630" LastActivityDate="2016-10-15T22:46:45.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18643" PostTypeId="1" AcceptedAnswerId="18644" CreationDate="2016-10-16T01:58:28.217" Score="0" ViewCount="323" Body="&lt;p&gt;In the ESA video &lt;a href=&quot;http://www.esa.int/spaceinvideos/Videos/2016/10/ExoMars_at_the_Red_Planet&quot; rel=&quot;nofollow noreferrer&quot;&gt;ExoMars at the Red Planet&lt;/a&gt; after about 02:10, it says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With ExoMars, ESA is going to use for the first time, a method called aerobraking for a spacecraft in orbit around Mars...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Will this be:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;the first time ESA has used aerobraking&lt;/li&gt;&#xA;&lt;li&gt;the first time ESA has used aerobraking around Mars&lt;/li&gt;&#xA;&lt;li&gt;the first time aerobraking has been used around MARS&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;or something else?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This video can be seen in YouTube:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/_qTiRYkGp9k?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-05-31T03:05:34.303" LastActivityDate="2020-05-31T03:05:34.303" Title="Is this a first Mars aerobraking for ESA, or for anyone?" Tags="|mars|record|esa|aerobraking|exomars|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="18644" PostTypeId="2" ParentId="18643" CreationDate="2016-10-16T02:03:04.270" Score="6" Body="&lt;p&gt;ESA has done aerobraking with Venus Express. NASA has done aerobraking at Mars several times, as well as at Venus.  This will be the first time that &lt;em&gt;ESA&lt;/em&gt; has done aerobraking at &lt;em&gt;Mars&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-10-16T05:09:57.477" LastActivityDate="2016-10-16T05:09:57.477" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18645" PostTypeId="1" AcceptedAnswerId="18647" CreationDate="2016-10-16T11:14:11.660" Score="5" ViewCount="1718" Body="&lt;p&gt;I'm using &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow&quot;&gt;NASA Horizons system&lt;/a&gt; to get Earth orbital elements with following settings:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Ephemeris Type : ELEMENTS&#xA;Target Body :    Earth-Moon Barycenter [EMB] [3]&#xA;Center :         Solar System Barycenter (SSB) [500@0]&#xA;Time Span :      Start=2000-01-01, Stop=2001-01-01, Step=12 h&#xA;Table Settings : output units=KM-S&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;This is what it says in the output:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Output units    : KM-S, deg, Julian Day Number (Tp)&#xA;Output type     : GEOMETRIC osculating elements&#xA;Output format   : 10&#xA;Reference frame : ICRF/J2000.0&#xA;Coordinate systm: Ecliptic and Mean Equinox of Reference Epoch&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;And here are the orbital elements for the J2000 epoch:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2451545.000000000 = A.D. 2000-Jan-01 12:00:00.0000 TDB &#xA; EC= 1.487370652262564E-02 QR= 1.466664481240442E+08 IN= 1.196622093669550E-02&#xA; OM= 9.348586355904454E+00 W = 6.534678781572920E+01 Tp=  2451519.451250376645&#xA; N = 1.149765722413008E-05 MA= 2.538006615432194E+01 TA= 2.612305949633834E+01&#xA; A = 1.488808583174952E+08 AD= 1.510952685109462E+08 PR= 3.131072643603165E+07&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So as you can see the inclination is about 0.012&amp;deg; which is not 0 (by the way, same goes both for Earth and Earth-Moon Barycenter). I thought inclination of the Earth orbit in J2000 reference frame must be 0 at J2000 epoch by definition. Could someone clarify this please?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. Now that I think about it and look at &lt;a href=&quot;http://images.slideplayer.com/18/5687394/slides/slide_10.jpg&quot; rel=&quot;nofollow&quot;&gt;this picture&lt;/a&gt; my question becomes more like &quot;Why is the inclination not 23.4&amp;deg;?&quot;&lt;/p&gt;&#xA;" OwnerUserId="17295" LastEditorUserId="6816" LastEditDate="2016-10-16T13:08:26.893" LastActivityDate="2019-07-14T15:53:44.330" Title="Why does NASA Horizons say that Earth inclination is not 0 at J2000 epoch?" Tags="|orbital-mechanics|orbital-elements|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18646" PostTypeId="2" ParentId="18645" CreationDate="2016-10-16T12:50:29.257" Score="2" Body="&lt;p&gt;I'll venture an answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, if you look at the actual data output from the ephemeris, you can see that around the year 2000 the Earth+Moon center of mass (barycenter) is moving &lt;em&gt;in a plane parallel to the ecliptic within a few hundred kilometers&lt;/em&gt;. Both the Earth and the Sun slowly drift up and down because the origin of the coordinate system is actually the center of mass of the solar system, and the other planets, Jupiter and Uranus in particular, have to be taken into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I never really use those generated orbital elements. They are an approximation - a fit - to the motion of the body at that particular time. You can propagate the motion of the body for a short time using them, but they are not meant to be used to describe long-term motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar system is a bunch of objects all interacting with each other. The sun is not (really) in the center, and the planets do not (exactly) even orbit around the Sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for example if you look at the same orbital elements &lt;em&gt;around the Sun&lt;/em&gt; instead of around the entire solar system barycenter, you'll see that the inclination drops to 0.0001 degrees. If at all possible, stick to the cartesian (x, y, z) data to avoid having to think about these issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8COQf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8COQf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ager2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ager2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; data from JPL Horizons database. These are the z-coordiantes only - perpendicular to the ecliptic. Units are kilometers. Solid smooth line is the Earth-Moon barycenter. Solid wiggly line is the Earth Geocenter, and dashed line is the sun. TOP: raw z-coordinates, BOTTOM: z-coordinates minus z-coordinates of the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Away from the year 2000,  the Earth is moving up and down once per year relative to the sun. It flattens out around 2000, when the Earth's orbit coincides with the ecliptic by definition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the python script I used to read the data and make the plot. It's not pretty but it does the job.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;fnames = ['J2000 Earth Barycenter horizons_results.txt',&#xA;          'J2000 Earth Geocenter horizons_results.txt',&#xA;          'J2000 Sun horizons_results.txt']&#xA;&#xA;names  = ['Earth_Barycenter', 'Earth_Geocenter', 'Sun']&#xA;&#xA;JDs, positions = [], []&#xA;&#xA;for fname in fnames:&#xA;&#xA;    with open(fname, 'r') as infile:&#xA;&#xA;        lines = infile.read().splitlines()&#xA;&#xA;        iSOE = [i for i, line in enumerate(lines) if &quot;$$SOE&quot; in line][0]&#xA;        iEOE = [i for i, line in enumerate(lines) if &quot;$$EOE&quot; in line][0]&#xA;&#xA;        print iSOE, iEOE, lines[iSOE], lines[iEOE]&#xA;&#xA;        lines = [line.split(',') for line in lines[iSOE+1:iEOE]]&#xA;        JD  = np.array([float(line[0]) for line in lines])&#xA;        pos = np.array([[float(item) for item in line[2:5]] for line in lines])&#xA;&#xA;        JDs.append(JD)&#xA;        positions.append(pos)&#xA;&#xA;JD = JDs[0]  # assume they are identical &#xA;&#xA;class Obj(object):&#xA;    def __init__(self, name):&#xA;        self.name = name&#xA;&#xA;things = []&#xA;&#xA;for name, pos, JD in zip(names, positions, JDs):&#xA;&#xA;    thing     = Obj(name)&#xA;    thing.pos = pos&#xA;    thing.JD  = JD&#xA;&#xA;    things.append(thing)&#xA;&#xA;earth_bary, earth_geo, sun = things&#xA;&#xA;years = 2000. + (JD - JD[1826])/365.2564&#xA;&#xA;if 1 == 1:&#xA;    fig = plt.figure()&#xA;&#xA;    ax1 = plt.subplot(2,1,1)&#xA;    ax1.plot(years, earth_bary.pos.T[2])&#xA;    ax1.plot(years, earth_geo.pos.T[2])&#xA;    ax1.plot(years, sun.pos.T[2], '--')&#xA;    ax1.set_xlim(1995, 2005)&#xA;    ax1.get_xaxis().get_major_formatter().set_useOffset(False)&#xA;&#xA;    ax2 = plt.subplot(2,1,2)&#xA;    ax2.plot(years, earth_bary.pos.T[2]-sun.pos.T[2])&#xA;    ax2.plot(years, earth_geo.pos.T[2]-sun.pos.T[2])&#xA;    ax2.plot(years, sun.pos.T[2]-sun.pos.T[2], '--')&#xA;    ax2.set_xlim(1995, 2005)&#xA;    ax2.get_xaxis().get_major_formatter().set_useOffset(False)&#xA;&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-16T13:19:04.157" LastActivityDate="2016-10-16T13:19:04.157" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18647" PostTypeId="2" ParentId="18645" CreationDate="2016-10-16T16:34:04.550" Score="11" Body="&lt;p&gt;Here's the heart of your problem:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;&lt;pre&gt;&lt;code&gt;Center :         Solar System Barycenter (SSB) [500@0]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The ecliptic is defined in terms of the Earth's mean (or average) orbit about the Sun at the epoch rather than about the solar system barycenter. Had you instead chosen the Sun as the center you would have found an inclination of 0.000266 degrees (about one arc second) at noon TDB on 1 Jan 20000. Aside: The Earth-Moon barycenter had an inclination of about 0.0001 degrees at that time per the Horizons web site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of reasons for that small one arc second discrepancy. One is that JPL's HORIZONS system currently uses the DE431 JPL ephemerides to compute the positions and velocities of solar system bodies. This ephemeris was released in 2014, decades after the J2000.0 frames were defined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason is the way JPL generates its ephemerides. JPL uses a batch least squares technique to iteratively refine the initial states for an integrated model of the solar system in an attempt to minimize the weighted sum squared error between observations and what this integrated model predicts would be observed. JPL does not release those epoch states. Instead, it releases Chebyshev polynomial coefficients that describe the positions of solar system bodies in something close to a minimum absolute error sense. This is one of the beauties of Chebyshev polynomials; they come very close to the ideal of a minimum absolute error predictor. However, the generator only looks at position; it does not look at velocity. Both position and velocity are needed to compute the osculating orbital elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet another reason is that those Chebyshev polynomial coefficients are expressed in terms of the ICRF frame rather than an ecliptic frame. A transformation is needed to convert those ICRF positions and velocities to an ecliptic frame. Small numerical errors (e.g., see &lt;a href=&quot;https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html&lt;/a&gt;) will creep in while performing those transformations. Other numerical errors will creep in while calculating the inclination. These numerical errors alone explain a good portion of that one arc second discrepancy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, the transformation from the J2000 Earth mean equator / Earth mean equinox frame (which is very close to the ICRF frame) to the J2000 ecliptic / Earth mean equinox frame was computed using a much older version of JPL's solar system model, and it was done using the integrated model rather than the Chebyshev polynomial coefficients.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2019-07-14T15:53:44.330" LastActivityDate="2019-07-14T15:53:44.330" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18648" PostTypeId="2" ParentId="18639" CreationDate="2016-10-16T17:01:03.237" Score="1" Body="&lt;p&gt;See &lt;a href=&quot;https://www.researchgate.net/publication/242270492_On_Determining_Dipole_Moments_of_a_Magnetic_Torquer_Rod_-_Experiments_and_Discussions&quot; rel=&quot;nofollow&quot;&gt;this paper&lt;/a&gt; for a good description of how to measure magnetic dipoles. To summarize, the approach is to use a calibrated magnetometer at a fixed distance from the torque rod and back out the dipole from the measured magnetic field. The test should be conducted with as little ferrous material in the test environment as possible. Obviously, the background field must also be subtracted out from the measurement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the equation, the difference between an air core and a magnetic core is reflected in a missing term known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Permeability_(electromagnetism)&quot; rel=&quot;nofollow&quot;&gt;magnetic permeability&lt;/a&gt;, $\mu$, which is a measure of how much the magnetic field is boosted by the presence of the core material. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your equation is a good approximation for your air-coil, but for an iron-core torque rod you need to account for the core material and geometry as described in the first link.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2016-10-16T17:01:03.237" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18649" PostTypeId="2" ParentId="18625" CreationDate="2016-10-16T17:40:35.790" Score="1" Body="&lt;p&gt;There are orbits which oscillate around a geosynchronous point, which could overlap and cause a collision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More importantly, however, the amount of geosynchronous positions is limited, and junk and dead satellites hanging around there occupying valuable positions is frowned upon. In accordance with an informal (as best I understand) agreement, most satellites, on reaching the end of their fuel, use the last few drops to depart from the geosynchronous spot.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-10-16T17:40:35.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18650" PostTypeId="1" CreationDate="2016-10-16T18:04:08.540" Score="3" ViewCount="152" Body="&lt;p&gt;How deep have spacecrafts gone into an atmosphere for doing aerobraking for orbital insertion (or as tech demos)? I suppose it is the braking force of the atmosphere that counts, is that closely correlated with atmospheric density or pressure and does elemental composition matter much?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-10-16T20:44:03.180" Title="In what atmospheric densities or pressures have aerobrakings been performed?" Tags="|atmospheric-drag|aerobraking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18651" PostTypeId="2" ParentId="18650" CreationDate="2016-10-16T19:57:34.533" Score="5" Body="&lt;p&gt;No spacecraft has yet used aerobraking for orbital &lt;em&gt;insertion&lt;/em&gt;. That's called aerocapture. Aerobraking has been used by orbiters only &lt;em&gt;after&lt;/em&gt; a propulsive orbital insertion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What matters for the drag of aerobraking passes is the atmospheric density profile during the pass, the velocity relative to the atmosphere during the pass, and the presented cross-sectional area of the vehicle during the pass. Corrosion of the vehicle could possibly depend on the composition, e.g. the atomic oxygen found in low-Earth orbit. But it doesn't matter for the drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Typical densities are around ${10}^{-8}\,\mathrm{kg/m^3}$. A desired dynamic pressure, $q={1\over 2}\rho v^2$ is targeted, with the periapsis raised and lowered as needed to get to that $q$, where too low of a $q$ makes the process take too long, and too high of a $q$ can damage the spacecraft due to forces from the drag.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-10-16T20:44:03.180" LastActivityDate="2016-10-16T20:44:03.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18653" PostTypeId="1" AcceptedAnswerId="19588" CreationDate="2016-10-16T23:16:15.920" Score="4" ViewCount="836" Body="&lt;p&gt;I know it will likely be a yellow haze at the surface level, but I've seen little info about what a Venus sky looks like above the cloud tops or in the midsts of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will the sky be blue like on earth or yellowish at daytime? Will sunset be reddish as on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How about night, will it be really dark without a moon, or will e.g. Mercury be similar in light strength as the moon?&lt;/p&gt;&#xA;" OwnerUserId="11308" LastEditorUserId="18879" LastEditDate="2016-12-24T20:21:58.007" LastActivityDate="2017-03-27T20:57:00.327" Title="What would a Venus sky look like above the cloud tops night and day?" Tags="|atmosphere|venus|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18654" PostTypeId="1" AcceptedAnswerId="19089" CreationDate="2016-10-16T23:32:17.870" Score="4" ViewCount="601" Body="&lt;p&gt;Since there is no magnetic field compass is out. No GPS satellites either. I guess some old school methods could be used but I see some potential problems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume we are over the cloud tops, can we actually see the night sky as clearly as on earth, e.g. if we look at stars for navigation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no moon to use as a reference point. Will that cause problems navigating using say a sextant?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How about the length of the Venus day at 243 earth days, would that cause any kind of problems? You can't exactly wait for noon and check the hight of the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I speculated whether you can use fixed points on the surface, but with the thick clouds and atmosphere I guess you can't really see the surface anyway. Say you are 50 km above surface are there simply ways of &quot;looking&quot; at the surface whether by radar, infrared, sound or whatever?&lt;/p&gt;&#xA;" OwnerUserId="11308" LastActivityDate="2016-11-21T14:07:37.127" Title="How would you navigate an airship on Venus?" Tags="|venus|navigation|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18655" PostTypeId="1" CreationDate="2016-10-16T23:42:02.293" Score="4" ViewCount="2778" Body="&lt;p&gt;On earth we can divide our day into 24 hours, and 365 days in the year. I am trying to imagine how people living on airships above the cloud-tops would think about time on Venus given that a single day lasts longer than a Venus year 243 earth day, vs 224 earth days for a year. While at the same time the wind blows you around the planet in about 50 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That makes it hard to  decide what  would be the most practical way of thinking about time and dates on Venus.&lt;/p&gt;&#xA;" OwnerUserId="11308" LastActivityDate="2017-12-19T00:22:04.380" Title="How would people measure time on Venus?" Tags="|venus|time|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18656" PostTypeId="2" ParentId="6170" CreationDate="2016-10-16T23:54:19.973" Score="0" Body="&lt;p&gt;Solid fuel rockets of the 1960s used an oxidizer rich in chlorine, the fumes of which were widely dangerous with a 260-inch engines.  The military had been using something cheaper in ww2, but switched to per chloride as it gave greater range to rockets fired from planes, tanks, landing craft.  The current discussion is to use ammonium nitrate as the oxydizer and wax and 15% aluminum powderfor newer solid fuel boosters.  The exhausts would not be as toxic&lt;/p&gt;&#xA;" OwnerUserId="17303" LastActivityDate="2016-10-16T23:54:19.973" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18657" PostTypeId="1" AcceptedAnswerId="20190" CreationDate="2016-10-16T23:59:38.120" Score="19" ViewCount="3321" Body="&lt;p&gt;Watching the launch of the Shenzhou-11 mission on a Long March 2F, I noticed a number of items falling off the exterior of the rocket. I'm used to seeing chunks of ice sheet coming off of cryogenic launchers, but Long March uses non-cryogenic hypergolic fuels, and the pieces were squared-off, clearly manufactured items. There were quite a few of them - at least 10 coming off the launcher in the first 20 seconds of flight. Here are a few screenshots showing the debris:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/m2w2B.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/m2w2B.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/W5ueX.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/W5ueX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/nPsEx.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nPsEx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q3sAQ.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q3sAQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't see any similar items coming off in &lt;a href=&quot;https://www.youtube.com/watch?v=7RTSGTIe-Rk&quot;&gt;this video of Shenzhou-9&lt;/a&gt; or &lt;a href=&quot;https://www.youtube.com/watch?v=f45YvWGtrDs&quot;&gt;this one of Shenzhou-10&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="26446" LastEditDate="2020-11-02T17:24:34.463" LastActivityDate="2020-11-02T17:24:34.463" Title="What's falling off the Shenzhou/Long March stack?" Tags="|launch|long-march|shenzhou|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18658" PostTypeId="1" AcceptedAnswerId="18671" CreationDate="2016-10-17T00:44:56.197" Score="9" ViewCount="894" Body="&lt;p&gt;Near the top of the Long March 2F rocket (middle of the fairing?) that launched today with the Shenzhou-11 crew, there are four square wafffle-like structures that remind me of grid fins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are they?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XKlBP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XKlBP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7zW0X.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7zW0X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; from &lt;a href=&quot;http://www.cctv-america.com/2016/10/14/heavenly-vessel-chinas-shenzhou-11-ready-for-liftoff&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.cctv-america.com/2016/10/14/heavenly-vessel-chinas-shenzhou-11-ready-for-liftoff&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8J7yg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8J7yg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8Wu5n.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8Wu5n.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; screen captures from the YouTube video &lt;a href=&quot;https://www.youtube.com/watch?v=7Jhx4J2j_kw&quot; rel=&quot;nofollow noreferrer&quot;&gt;Launch of Manned China Mission with Shenzhou 11 to Tiangong-2&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2020-09-12T18:04:12.597" LastActivityDate="2020-09-12T18:04:12.597" Title="What are these four &quot;waffle-like&quot; structures on top of the Long March 2F just launched?" Tags="|grid-fins|long-march|shenzhou|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="18659" PostTypeId="1" AcceptedAnswerId="18660" CreationDate="2016-10-17T00:57:02.817" Score="0" ViewCount="211" Body="&lt;p&gt;It appears that the crew capsule in today's launch of the Shenzhou-11 crew has no windows. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Windows present a lot of design tradeoffs, but can come in handy in some situations. Is this capsule windowless?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XKlBP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XKlBP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7zW0X.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7zW0X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; from &lt;a href=&quot;http://www.cctv-america.com/2016/10/14/heavenly-vessel-chinas-shenzhou-11-ready-for-liftoff&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.cctv-america.com/2016/10/14/heavenly-vessel-chinas-shenzhou-11-ready-for-liftoff&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/A5YpI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/A5YpI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screen capture from the YouTube video &lt;a href=&quot;https://www.youtube.com/watch?v=7Jhx4J2j_kw&quot; rel=&quot;nofollow noreferrer&quot;&gt;Launch of Manned China Mission with Shenzhou 11 to Tiangong-2&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/7Jhx4J2j_kw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-10-17T01:52:06.137" Title="Is the Shenzhou-11 crew's capsule just launched windowless?" Tags="|capsule|shenzhou|spacecraft-window|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18660" PostTypeId="2" ParentId="18659" CreationDate="2016-10-17T01:52:06.137" Score="4" Body="&lt;p&gt;Clearly there is a window. Upper left, watch lighting change within the capsule and on astronaut's face after fairing separation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kCJpw.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kCJpw.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF made from screen captures from the following YouTube video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/7Jhx4J2j_kw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-10-17T01:52:06.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18661" PostTypeId="1" CreationDate="2016-10-17T02:07:35.430" Score="1" ViewCount="147" Body="&lt;p&gt;I can't exactly figure out where the first stage ends and the second stage begins and where all three components of the Shenzhou-11 space craft are located inside the fairing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia image of the Shenzhou spacecraft doesn't seem to match the shape of (what I believe is) the capsule in the launchpad photos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a clear diagram of the whole thing somewhere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've looked at &lt;a href=&quot;http://spaceflight101.com/spacecraft/shenzhou-spacecraft-overview/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt; which is very informative, but doesn't show a complete diagram. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the following sketch about right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eNmfI.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eNmfI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XKlBP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XKlBP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7zW0X.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7zW0X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; from &lt;a href=&quot;http://www.cctv-america.com/2016/10/14/heavenly-vessel-chinas-shenzhou-11-ready-for-liftoff&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.cctv-america.com/2016/10/14/heavenly-vessel-chinas-shenzhou-11-ready-for-liftoff&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6OCaK.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6OCaK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; image of a Shenzhou spacecraft from &lt;a href=&quot;http://news.xinhuanet.com/english/photo/2012-06/16/131657418_51n.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8AhwB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8AhwB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Shenzhou spacecraft from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Post_S-7_Shenzhou_spacecraft.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-17T07:20:28.577" LastActivityDate="2016-10-17T07:20:28.577" Title="How are the major components arranged in the Shenzhou-11 at launch?" Tags="|shenzhou|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18662" PostTypeId="1" CreationDate="2016-10-17T03:11:40.950" Score="0" ViewCount="115" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Long_March_2F&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; the single engine on the 2nd stage is a &lt;a href=&quot;https://en.wikipedia.org/wiki/YF-20&quot; rel=&quot;nofollow noreferrer&quot;&gt;YF-20 series&lt;/a&gt; engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is causing the faint yellow light on the side of the nozzle where there is a tube wrapped around it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pQmQt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pQmQt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen captures from this &lt;a href=&quot;https://www.youtube.com/watch?v=7Jhx4J2j_kw&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube vide&lt;/a&gt; Arrow is added  indicate the &quot;blob&quot; in question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gZZTt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gZZTt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen captures from &lt;em&gt;this other&lt;/em&gt; &lt;a href=&quot;https://www.youtube.com/watch?v=0vMUO3T0nEg&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube vide&lt;/a&gt; Similar type of shot. This is the previous launch of Tiangong-2. Seems to be heavily colorized?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-17T03:19:10.383" LastActivityDate="2016-10-17T03:19:10.383" Title="What is this yellow light on the side of the 2nd stage nozzle - Shenzhou-11 launch" Tags="|engines|exhaust|nozzle|shenzhou|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18664" PostTypeId="1" AcceptedAnswerId="18888" CreationDate="2016-10-17T06:31:12.917" Score="7" ViewCount="104" Body="&lt;p&gt;I know that Exo Mars TGO is intended to be a relay for Exo Mars Schiaparelli and Exo Mars Rover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the orbiter could also serve as a relay for other missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there currently any plan for doing so ? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-11-01T01:24:43.420" Title="Will Exo Mars serve as a relay for non ESA/Roscosmos missions?" Tags="|deep-space-network|exomars|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18665" PostTypeId="1" AcceptedAnswerId="18667" CreationDate="2016-10-17T09:12:59.620" Score="21" ViewCount="7212" Body="&lt;p&gt;Whenever a fan of Kerbal Space Program mentions the super-efficient 'asparagus staging' in any professional context, the automatic response is 'That won't work. KSP fuel pipes are pure magic.'&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of '&lt;a href=&quot;http://wiki.kerbalspaceprogram.com/wiki/Asparagus_staging&quot;&gt;asparagus staging&lt;/a&gt;' is that external boosters of the rocket feed their own fuel into the main engine, or into boosters that would separate later, so that at the moment of separation the craft's remaining tanks are still full, meanwhile, during the launch all engines are engaged - engines of later stages are not carried as dead weight, but contribute to thrust from moment one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yet, we had a system, where an external, detachable fuel tank would feed massive amounts of LOX bipropellant to engines in a separate stage - the shuttle's fuel tank feeding the SSME. At a glance, it looks like it's just a notch away from what would be needed for the 'asparagus'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone provide an overview of the crossfeed system - in particular the detachable joint that fed LOX to to the shuttle from the tank? Would it be efficient if reused for 'asparagus-staged boosters'? Or was it plagued with enough problems - or even just too expensive or inefficient for such use?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="415" LastEditDate="2016-10-17T09:40:14.303" LastActivityDate="2021-05-11T18:16:25.233" Title="How was fuel crossfeed achieved, between the main tank and the Shuttle?" Tags="|liquid-fuel|ssme|fuel-system|cross-feed|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18666" PostTypeId="2" ParentId="18665" CreationDate="2016-10-17T10:04:08.083" Score="18" Body="&lt;p&gt;The crossfeed seen in KSP works by pumping fuel from one fuel tank to another, i.e. against tank pressure. To do this, you need pumps (independent of the engine turbopumps). Pumping large amounts of fuel quickly is not easy.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need to stop pumping cleanly when the tank is empty (or risk a pump explosion when the pump runs dry), this is difficult: you have to shut down a massive flow without causing a &lt;a href=&quot;https://en.wikipedia.org/wiki/Water_hammer&quot; rel=&quot;nofollow&quot;&gt;water hammer&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle had a much easier setup: there was only one tank feeding propellants to the engines, no flow from one tank to another. The only pumps involved were the engine turbopumps. Shutdown was done by shutting down the engines, no need to close valves against a huge flow. The only thing the Shuttle's ET has in common with asparagus staging is the in-flight disconnection. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="415" LastEditDate="2016-10-18T13:08:47.527" LastActivityDate="2016-10-18T13:08:47.527" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18667" PostTypeId="2" ParentId="18665" CreationDate="2016-10-17T10:06:13.017" Score="25" Body="&lt;p&gt;&lt;strong&gt;1) Can someone provide an overview of the crossfeed system - in particular the detachable joint that fed LOX to to the shuttle from the tank?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The buzzwords to use for googling this topic are &amp;quot;ET Umbilical&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;The system as implemented on Shuttle was complicated but caused major problems only once in the program (see below).&lt;/p&gt;&#xA;&lt;p&gt;There were two umbilical areas towards the rear of the shuttle belly.  Five fluid pathways connected through these two umbilicals: 17&amp;quot; liquid oxygen and liquid hydrogen lines, 3&amp;quot; gaseous oxygen and hydrogen repressurization lines, and a smaller hydrogen recirculation line.  There were also numerous electrical and data lines. Large electrically operated doors (latched open at launch by electrically operated latches) covered the umbilical openings and also the aft structural connections between the ET and orbiter.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hsssm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hsssm.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The outside of the doors was covered with shuttle TPS tiles.&lt;/p&gt;&#xA;&lt;p&gt;At launch, obviously the doors were open and all umbilicals mated (the actual mating surface was called the ET umbilical plate).  Each fluid connection had a shutoff valve on both sides of the interface.&lt;/p&gt;&#xA;&lt;p&gt;After the SSMEs shut down, the ET separation sequence began.  First the valves on each side of the fluid connections closed. Three explosive bolts on each umbilical plate fired to free the interface. Then triple redundant hydraulic actuators pulled the orbiter side umbilical plates into the Orbiter below the outer mold line. The system then checked to ensure that all valves were closed.  If not, a waiting period began to let the systems blow down through the open valves and a warning was issued to the crew.  Finally pyrotechnics blew the structural interface apart and the Orbiter fired down-firing jets to fly away from the ET.  Then the pilot flipped switches to release the centerline latches on the doors, close the doors, and latch them closed.&lt;/p&gt;&#xA;&lt;p&gt;The concerns with the system as designed were pretty obvious from the description above.  If the fluid disconnect valves on the ET side did not close, the residual prop from the ET would vent out.  If the ET was mechanically separated while this happened, it would tumble, and could hit the orbiter.  Hence the waiting period.  Failure of the umbilicals to retract into the orbiter could prevent the doors from closing, which would result in an imperfect heat shield for entry.  This same concern existed for electrical or mechanical failures in the door closing mechanism.  There was a documented EVA procedure for the crew to go out and crank the doors closed manually.  This was straightforward except for accessing the area which was far aft and underneath the orbiter belly.  There was also a concern that the disconnect valves could slam closed during operation of the SSMEs resulting in destruction of the vehicle.  During the safety upgrades carried out after the Challenger accident, pneumatically operated latches were added to hold the valves open.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;2) Would it be efficient if reused for 'asparagus-staged boosters'&lt;/strong&gt;?&lt;/p&gt;&#xA;&lt;p&gt;I cannot answer this, but my opinion is that the shuttle system design was unique to its application.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;3) Or was it plagued with enough problems - or even just too expensive or inefficient for such use?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Despite the risks involved, the only actual major issue I am aware of with the system occurred during the STS-35 launch preparation, when the umbilicals experienced massive hydrogen leaks that delayed the flight significantly and resulted in the stack being rolled back off the pad into the assembly building.&lt;/p&gt;&#xA;&lt;p&gt;Sources&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://archive.org/download/PressInformationSpaceShuttleTransportationSystemMarch1982/Press_Information_Space_Shuttle_Transportation_System_March_1982.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;1988 Shuttle News Reference Manual, p. 142 fwd and p. 92 fwd&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-35&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/STS-35&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_asm.html#et_mods&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_asm.html#et_mods&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Personal experiences&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-05-11T18:16:25.233" LastActivityDate="2021-05-11T18:16:25.233" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18669" PostTypeId="1" CreationDate="2016-10-17T13:46:16.307" Score="0" ViewCount="45" Body="&lt;p&gt;According to &lt;a href=&quot;http://economictimes.indiatimes.com/slideshows/science-technology/isro-might-put-a-telescope-on-the-moon/detailed-understanding-of-the-universe/slideshow/54895963.cms&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; report:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As a follow-on mission to India's first space X-ray observatory, ASTROSAT, ISRO is planning to set up a telescope on moon.ISRO chairman A S Kiran Kumar said that the telescope can have lot of advantages as there were no atmosphere or atmospheric effects associated with it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What advantages would this telescope have over the present space based telescopes in orbit around the Earth or the Sun?&lt;/p&gt;&#xA;" OwnerUserId="17309" LastActivityDate="2016-10-17T13:46:16.307" Title="Why put a telescope on moon?" Tags="|the-moon|telescope|" AnswerCount="0" CommentCount="2" ClosedDate="2016-10-17T14:02:42.650" ContentLicense="CC BY-SA 3.0" />
  <row Id="18670" PostTypeId="1" CreationDate="2016-10-17T13:51:43.093" Score="4" ViewCount="96" Body="&lt;p&gt;&lt;a href=&quot;http://www.asri.org.au/satellites/wresat&quot; rel=&quot;nofollow noreferrer&quot;&gt;WRESAT&lt;/a&gt; had a hydraulic loop energy dissipation system to change its axis of spin.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An interesting point to note about the WRESAT design is that, even though WRESAT was spinning around its long axis at orbital insertion, this was not the preferred orientation for the scientific payload. Close inspection of the WRESAT schematic to the right shows an energy dissipator which was a closed hydraulic loop of silicone oil which dissipated the rotational energy as heat. The net result is WRESAT eventually tumbling around it's short axis which places the axis of rotation pointing out into space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Was this silicone oil energy dissipation system passive or active? In other words, was it a rotational viscous dampener or was there a pump involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This related &lt;a href=&quot;https://space.stackexchange.com/questions/17827/passive-spin-damping-system&quot;&gt;question&lt;/a&gt; asks about passive spin/oscillation dampening systems. &lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-11-17T18:31:18.990" Title="WRESAT spin dissipation system, was it passive or active?" Tags="|artificial-satellite|attitude|passive-systems|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18671" PostTypeId="2" ParentId="18658" CreationDate="2016-10-17T14:34:08.043" Score="14" Body="&lt;p&gt;From Spaceflight101:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For Shenzhou Missions, a stretched version of the typical Long March fairing is used ... Four aerodynamic stabilizers are attached to the upper part of the fairing that is part of the Launch Escape System in a escape scenario, those would deploy to stabilize the vehicle while flying under the Launch Abort Engines. The Fairing’s ‘wings’ are in a stowed position for flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight101.com/spacerockets/long-march-2f/&quot;&gt;http://spaceflight101.com/spacerockets/long-march-2f/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As with Soyuz, these are grid fin stabilizers; here's a clear picture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://s-media-cache-ak0.pinimg.com/236x/c0/1a/67/c01a67b4f8140abcc9aa2601fb57f5ef.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't look like the stabilizers are articulated beyond hinging out to a deployed position, so unlike the grid fins found on e.g. Falcon 9, they don't give any active steering capability, they just slow the capsule and keep it oriented into the airstream, like the feathers of a badminton shuttlecock. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-17T17:35:27.930" LastActivityDate="2016-10-17T17:35:27.930" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18675" PostTypeId="2" ParentId="18611" CreationDate="2016-10-17T19:48:18.293" Score="4" Body="&lt;p&gt;Not really amateur, but CubeSats for sure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/aerocube-7-ocsd.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;AeroCube 7 / OCSD&lt;/a&gt; would have already tested it, but had attitude control problems. Should relaunch in a few months. One can find plenty of detailed publications too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/argomoon.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ArgoMoon&lt;/a&gt; will be one of the next. Testing it from beyond Moon orbit, but not much is known yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit 2018-12-08:&lt;/strong&gt; &#xA;AeroCube-7B successfully demonstrated laser communications from a CubeSat in 2018. More details from Small Sat 2018 &lt;a href=&quot;https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=4298&amp;amp;context=smallsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;presentation&lt;/a&gt; and official &lt;a href=&quot;https://aerospace.org/press-release/laser-communications-demonstrated-cubesats-first-time&quot; rel=&quot;nofollow noreferrer&quot;&gt;press release&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://shoutout.wix.com/so/9MIB1bpv#/main&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radix&lt;/a&gt; from Analytical Space was launched in the summer of 2018 and also includes optical communications module. Radix-2 is planned for January 2019 based on this Constellations &lt;a href=&quot;http://www.kratoscomms.com/constellations-podcast/new-space-earth-observation&quot; rel=&quot;nofollow noreferrer&quot;&gt;podcast&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are over 10 CubeSats with laser transmitter payloads in development now. Several of them will launch in 2019 (Golbriak Space module on FSSCat A/B, Tesat-Spacecom module on OSIRIS etc).&lt;/p&gt;&#xA;" OwnerUserId="17302" LastEditorUserId="17302" LastEditDate="2018-12-08T21:17:28.560" LastActivityDate="2018-12-08T21:17:28.560" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18676" PostTypeId="1" CreationDate="2016-10-18T02:18:31.207" Score="3" ViewCount="392" Body="&lt;p&gt;My wife and I watched ISS over NM tonight (10/17/2016).  We both thought that it looked larger than point-like. Can the ISS look like an extended object? Are there docked, or nearby, rockets that could make it appear extended?&lt;/p&gt;&#xA;" OwnerUserId="17317" LastActivityDate="2016-10-19T01:14:48.787" Title="Naked Eye View of ISS: Resolution?" Tags="|iss|naked-eye|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18677" PostTypeId="1" AcceptedAnswerId="18683" CreationDate="2016-10-18T02:22:32.130" Score="2" ViewCount="658" Body="&lt;p&gt;&lt;a href=&quot;http://spacenews.com/spacexs-musk-says-sabotage-unlikely-cause-of-sept-1-explosion-but-still-a-worry/&quot; rel=&quot;nofollow&quot;&gt;This&lt;/a&gt; article about the 2016 Sept 1 Falcon 9 explosion mention 'cryogenic geyser cycling' in reference to the filling of the LOX tanks as though it were a particular method of filling tanks.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In cryogenic geyser cycling environments, such as SpaceX’s rapid filling of a LOX tank at 66 Kelvin, you have violent variations of temperature between the interior and exterior.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is 'cryogenic geyser cycling'?&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2016-10-18T14:07:44.367" LastActivityDate="2017-10-10T09:32:51.473" Title="Filling of LOX tanks - 'cryogenic geyser cycling'" Tags="|terminology|lox|fuel-system|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18678" PostTypeId="2" ParentId="18676" CreationDate="2016-10-18T03:46:46.287" Score="0" Body="&lt;p&gt;&lt;strong&gt;Not Really...&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Docked or nearby ships would probably not elongate the ship to the naked eye. The main reason that the ISS is so bright is because of its acre of solar panels that tend to reflect a lot of sunlight. However, on-board lights sneaking through the windows are just not bright enough to be visible with the naked eye on the ground. This is why NASA only posts sightings that are happening at dusk and dawn; it is not bright enough in the day, and it is not bright enough at night because the solar panels are not reflecting enough light. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A nearby craft is not bright enough to make the ISS appear elongated because the solar panels are too small to reflect and interior lighting does not generate enough light. However, you can make out some pretty good detail with a simple pair of binoculars.&lt;/p&gt;&#xA;" OwnerUserId="17272" LastActivityDate="2016-10-18T03:46:46.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18679" PostTypeId="1" CreationDate="2016-10-18T03:56:36.237" Score="3" ViewCount="195" Body="&lt;p&gt;I was reading over the recent Antares &lt;a href=&quot;http://www.collectspace.com/review/orbital_a-one_overview01-lg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;launch profile&lt;/a&gt; and was surprised to see that the main engine cutoff is at T+230s, but the second stage does not ignite until almost 100 seconds later.  Why is this time so long?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iKi2r.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iKi2r.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would have assumed that 100 seconds of deceleration would cost a substantial amount of fuel to recover, making the system heavier, more expensive, lower payload, and all sorts of other bad things.  I'm certain there's a good reason (no one burns thousands of extra kilos of propellant 'just because, right?)... but I can't figure out what that reason is.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All my (admittedly basic) physics leads me to believe that you'd want to do as much of the initial burn as you can early in the profile to minimize the amount of total energy you have to generate (or maximize the payload you can carry, which are really two sides of the same coin)... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:  thanks for the helpful answers - the key component I was missing is that by the time the first stage cuts off, the vehicle is high enough that there is virtually zero atmospheric drag left, and as such the coast phase costs very little in terms of lost energy (velocity).  I found the &lt;a href=&quot;https://www.orbitalatk.com/flight-systems/space-launch-vehicles/antares/docs/Antares_UsersGuide.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Antares User Guide&lt;/a&gt; extremely instructive, specifically figure 3.2-1, which shows that the velocity reduction between MECO and the second stage ignition to be only ~160m/s.  The gravity turn is also a concept that I hadn't heard of, but makes perfect sense now that I read about it.   ;-)&lt;/p&gt;&#xA;" OwnerUserId="17318" LastEditorUserId="17318" LastEditDate="2016-10-18T18:16:36.027" LastActivityDate="2016-10-18T18:16:36.027" Title="Why is the delay between main engine cutoff and second stage ignition so long?" Tags="|flight-profile|" AnswerCount="0" CommentCount="4" ClosedDate="2016-10-18T18:06:39.700" ContentLicense="CC BY-SA 3.0" />
  <row Id="18680" PostTypeId="2" ParentId="18676" CreationDate="2016-10-18T04:31:04.417" Score="4" Body="&lt;p&gt;The angular width of the ISS seen from Earth's surface is about 0.8 minutes of arc, which is quite close to the angular resolution of the human eye. This is big enough to make it distinctly less &quot;twinkly&quot; than a star, and possibly to perceive some elongation.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-10-18T04:31:04.417" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18681" PostTypeId="1" CreationDate="2016-10-18T05:46:57.557" Score="3" ViewCount="382" Body="&lt;p&gt;While reading about the effects of microgravity on reproduction I was thinking about all the other aspects of space flight and how they may decrease fertility in both men or women (such as changes in cardiovascular health, radiation, the effect of microgravity on menstruation and possibly ovulation, etc&amp;hellip;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize that most astronauts are on the older side and usually already have families which will make having children less common and so a negative result doesn't mean much.  The duration in space would of course be one of the most important factors, so I realize mentioning Joe Starman who spent 8 days on STS-XXX isn't really that useful. I'm just interested in the longest-term examples of astronauts (male or female) who successfully had a child after being returning from space, as that would give a very general example of the possibility.&lt;/p&gt;&#xA;" OwnerUserId="17319" LastEditorUserId="6816" LastEditDate="2016-10-18T05:56:14.847" LastActivityDate="2016-10-18T06:55:29.710" Title="How long can an astronaut stay in space and later have children?" Tags="|astronauts|microgravity|radiation|life|astronaut-lifestyle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18682" PostTypeId="2" ParentId="18681" CreationDate="2016-10-18T06:55:29.710" Score="1" Body="&lt;p&gt;I'm pretty sure this is not the answer you wanted, but... given the right preparation, indefinitely. Semen and ova can be extracted, and stored cryogenically for a long, long time; the storage is small enough to shield it efficiently against cosmic radiation too, even if it were to be sent to Mars, and the in-vitro procedure has been studied and advanced sufficiently that it could be performed 'in space colony conditions'. So, while plain old sex might not work, there is still an efficient workaround.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A separate problem is theorized (please excuse me for not providing the source, it's been a while since I read this and don't have it readily available): humans are not easily rendered sterile by cosmic radiation, but it is expected that the radiation may damage the ova in such a way that the offspring is born infertile. The period of exposure and likelihood of such damage haven't been determined yet.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-10-18T06:55:29.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18683" PostTypeId="2" ParentId="18677" CreationDate="2016-10-18T08:00:05.827" Score="5" Body="&lt;p&gt;A &lt;a href=&quot;https://books.google.nl/books?id=K3gtSIuOkXgC&amp;amp;pg=PA59&amp;amp;lpg=PA59&amp;amp;dq=cryogenic%20geyser&amp;amp;source=bl&amp;amp;ots=NTvEjpOFzM&amp;amp;sig=bHJh0tqDPAZtFr8FPg7xakkDx7Y&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjE8PDg7-PPAhVLthoKHTA4BsoQ6AEIITAA#v=onepage&amp;amp;q=cryogenic%20geyser&amp;amp;f=false&quot;&gt;cryogenic geyser&lt;/a&gt; occurs when a volume of cryogenic liquid inside e.g. a pipe suddenly boils, propelling the liquid/gas mixture through the pipe and into its destination at high speed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pipe can then refill (potentially causing a water hammer), and the process may repeat. If it does, you have cryogenic geyser cycling. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it's not so much a filling process as a phenomenon you want to avoid. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-18T08:00:05.827" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18684" PostTypeId="1" AcceptedAnswerId="18685" CreationDate="2016-10-18T08:21:35.850" Score="1" ViewCount="2936" Body="&lt;p&gt;While looking from Earth's surface it gives the impression that all planets, stars, sun etc are above our heads. When a rocket travels from earth to outer space, it takes a trajectory and gives the feeling that in order to explore space one always have to travel horizontally regardless of interplanetary journey or out of solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it true? I mean if one has to get out of solar system he will still have to travel horizontally instead of going up, up and up? Kind of gives impression that our universe is a long rectangle.&lt;/p&gt;&#xA;" OwnerUserId="17320" LastEditorUserId="6816" LastEditDate="2016-10-18T08:30:36.037" LastActivityDate="2016-11-25T05:38:07.480" Title="Do we travel horizontally in same plane or vertically while exploring space?" Tags="|interstellar-travel|trajectory|travel|" AnswerCount="2" CommentCount="7" ClosedDate="2016-10-19T08:17:05.237" ContentLicense="CC BY-SA 3.0" />
  <row Id="18685" PostTypeId="2" ParentId="18684" CreationDate="2016-10-18T09:02:58.640" Score="9" Body="&lt;p&gt;This is the result of two physical facts: that gravity, pointing toward the center of the Earth, is the only way to define &quot;down&quot;, and that most of the Solar System is quite close to a single &lt;a href=&quot;https://en.wikipedia.org/wiki/Plane_%28geometry%29&quot;&gt;geometric plane&lt;/a&gt;, the &lt;em&gt;ecliptic&lt;/em&gt;, which sort of is like the rectangle you describe. It's also partly because most of our communications are essentially two-dimensional, so depicting the actual three-dimensional nature of space is rather difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first of those physical facts is pretty straightforward. Because &quot;down&quot; is always toward the Earth's center, to go anywhere away from the Earth we have to go &quot;up&quot;. That's really all there is to it. (Although, in practice, most spacecraft have to achieve orbit before leaving Earth's vicinity, and orbit is moving sideways at very high speeds while falling, such that the inertia of that high speed counters the acceleration of falling. So while spacecraft do need to go up, they also need to go sideways comparatively much faster until they're quite far out from Earth: tens if not hundreds of thousands of kilometers.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second of these is a bit more involved. The orbit of a body always lies in a single plane (discounting the various perturbing effects that can shift it slightly), so if the Sun had a single planet and nothing else it would, by definition, be a system with the &quot;long rectangle&quot; effect you describe. But in fact our actual Solar System is very close to being that same &quot;long rectangle&quot;, despite the numerous bodies in it: the planets orbit within just a few degrees of the same plane, called the ecliptic (which is defined as the plane that the Earth orbits in), and many of their moons aren't much farther off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since it's so close to the truth most of the time to depict a trajectory from &quot;above&quot; the ecliptic without worrying about it going a little up or down, and since it's so much more difficult to print or display a full three-dimensional orbital track, most publications simplify it the way you've seen. But the actual inclination of orbits does need to be tracked for interplanetary probes, of course, any of which would miss essentially any target by millions of kilometers if not more if they simply assumed everything was in the ecliptic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of interstellar missions, things get more complicated. Stars aren't distributed very tidily in relation to the ecliptic. However, practical interstellar exploration, if we ever have the technology to do it, will still follow &lt;em&gt;a&lt;/em&gt; single plane for most of the mission, or as close as makes no matter, and since the distance between stars is vastly larger than the entire Solar System, diagrams can just represent it as a dot on one side of the page and show a nice tidy long slightly curved line to a dot on the other side. The actual trajectory from Earth might look like a series of turns around the Earth and Sun before a slowly straightening path off toward the destination, but you can understand why the groundside view is not very practical to put in a single picture &amp;mdash; you can't see the spacecraft most of the time, and when you can, you can't really see what it's doing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we ever get to the point of scheduling multiple star system stops in different Sun-relative inclinations in the same trip, &lt;em&gt;then&lt;/em&gt; things will finally need to be represented more accurately as a routine. But that's so far in the unforeseeable future there's no real point worrying about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's fair to mention, though, that the Voyager spacecraft in particular have rather high inclinations at present; they used their last gravity assists (from Saturn and Neptune respectively) to &lt;a href=&quot;http://voyager.jpl.nasa.gov/science/hyperbolic.html&quot;&gt;shoot off at a fairly sharp angle&lt;/a&gt; of 36&amp;deg; and 79&amp;deg; respectively relative to the ecliptic. Prior to that their inclinations varied within the normal few degrees of the ecliptic. Accurately showing the Voyager 1/2 trajectory from 9.5 AU (Saturn) or 30 AU (Neptune) to their current position (which, at least in Voyager 1's case, is well over 100 AU from the Sun) would still &lt;em&gt;mostly&lt;/em&gt; focus on the post-ecliptic phase, but there's enough beforehand that a good diagram might really need to work on showing the transition three-dimensionally.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2016-10-18T10:16:26.160" LastActivityDate="2016-10-18T10:16:26.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18686" PostTypeId="1" AcceptedAnswerId="20253" CreationDate="2016-10-18T11:50:14.163" Score="2" ViewCount="358" Body="&lt;p&gt;I'm trying to understand where &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno&lt;/a&gt; will pick up most of the slow, cumulative radiation damage within it's orbit.&lt;/p&gt;&#xA;&lt;p&gt;Currently it is in the higher, 53 day orbit. The original plan was that tomorrow (2016-10-19) it would execute a roughly 395 m/s delta-v burn in order to drop down to a 14 day orbit.&lt;/p&gt;&#xA;&lt;p&gt;If I understand correctly, most of the &lt;em&gt;radiation that will damage Juno's instruments&lt;/em&gt; is associated with Jupiter's magnetic field and lies within a few Jovian diameters of the planet. The 54 day orbit spends slightly less time near jupiter than a 14 day orbit of the same perijove would, but it's a small difference.&lt;/p&gt;&#xA;&lt;p&gt;If Juno is accumulating most of its radiation damage near the planet, then it should be able to last for 30 or more of these longer period orbits. However, I had just read that Juno would not last for the same number of flybys (over 30) if it remained in this higher orbit, due to accumulated radiation damage.&lt;/p&gt;&#xA;&lt;p&gt;Here is one example. The lower, 14 day orbit is called the &amp;quot;Science Orbit&amp;quot; in several different articles I've read.&lt;/p&gt;&#xA;&lt;p&gt;From the Spaceflight 101 article &lt;a href=&quot;http://spaceflight101.com/juno/juno-prm-postponed/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Burn postponed for NASA’s Juno, to remain in elongated Jupiter Orbit until December&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Passing Jupiter on Wednesday, Juno will be outbound again with its next close pass of the planet on December 11 which will be the next opportunity for the Period Reduction Maneuver – pending evaluations of the suspect check valve signature.&lt;/p&gt;&#xA;&lt;p&gt;The option of a third Capture Orbit has be prepared as part of numerous contingency scenarios worked out for this mission, &lt;strong&gt;however, there is some urgency in getting Juno down into the science orbit due to the limited lifetime of the spacecraft in the extreme radiation environment of Jupiter.&lt;/strong&gt; (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Are my assumptions about the location of the highest damaging parts of the orbit incorrect, and a substantial amount of radiation damage happens far from Jupiter, and thus in each 54 day orbit the accumulated dose is much larger than in each 14 day lower, &amp;quot;Science&amp;quot; orbit?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZzqDW.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZzqDW.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Capture Orbit Design – Image: NASA/JPL/LASP&amp;quot;. From &lt;a href=&quot;http://spaceflight101.com/juno/juno-completes-closest-pass-of-jupiter-ahead-of-orbital-trim-maneuver/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0tDMfm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0tDMfm.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; sketch of a few of Juno's lower 14-day orbits and illustration of the strongest radiation bands near Jupiter. From the &lt;a href=&quot;http://www.latimes.com/local/california/la-sci-sn-juno-jupiter-arrival-updates-why-the-long-1467415666-htmlstory.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lost Angeles Times&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ixhc3.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ixhc3.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Animation of Jupiter's Synchrotron Radiation Torus. Credit: NASA/JPL - Caltech&amp;quot;. From the &lt;a href=&quot;http://www.vofoundation.org/blog/nasas-juno-spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vatican Observatory Blog&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-04-11T03:34:22.037" Title="Why would Juno's originally planned orbit lowering partially mitigate radiation damage?" Tags="|orbit|orbital-maneuver|radiation|jupiter|juno|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="18687" PostTypeId="1" AcceptedAnswerId="18689" CreationDate="2016-10-18T12:02:19.900" Score="5" ViewCount="1815" Body="&lt;p&gt;In the recent Spaceflight 101 article &lt;a href=&quot;http://spaceflight101.com/juno/juno-prm-postponed/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Burn postponed for NASA’s Juno, to remain in elongated Jupiter Orbit until December&lt;/a&gt; it says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Juno uses a bi-propellant main propulsion system with Hydrazine fuel and Mixed Oxides of Nitrogen as oxidizer while the craft’s Reaction Control System uses Hydrazine monopropellant.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I looked &lt;a href=&quot;http://spaceflight101.com/juno/spacecraft-information/&quot; rel=&quot;nofollow noreferrer&quot;&gt;elsewhere there&lt;/a&gt; but couldn't find anything further. According to this article in &lt;a href=&quot;http://spaceflightnow.com/news/n0006/27arcengine/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt; the Leros 1C Liquid Apogee Engine uses Hydrazine and Nitrogen Tetroxide (N2O4), so I wonder, &lt;em&gt;what are the &quot;mixed oxides of nitrogen?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have just read &lt;a href=&quot;http://www.prnewswire.com/news-releases/leros-engine-propels-the-juno-spacecraft-on-its-historic-voyage-to-jupiter-133466283.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; that the Leros 1B uses Hydrazine and MON-3. Does that stand for &quot;Mixed Oxides of Nitrogen-3&quot;? &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="195" LastEditDate="2016-11-29T05:45:45.657" LastActivityDate="2019-04-21T00:12:47.793" Title="Why does Juno use &quot;mixed oxides of nitrogen&quot; oxidizer for propulsion? Which ones?" Tags="|fuel|juno|oxidizer|hydrazine|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18688" PostTypeId="2" ParentId="18665" CreationDate="2016-10-18T12:52:43.793" Score="4" Body="&lt;p&gt;There are lots of factors in KSP's simulation model which make &quot;asparagus&quot; staging viable and beneficial. The big one is the weak drag model (at least in early versions): the penalty for making the rocket wider isn't too bad. Conversely, the weak coupling between rigid components in KSP causes very tall rockets to &quot;wobble&quot;, and the overpowered and under-damped reaction wheel control systems can exacerbate that. So KSP rockets tend to have a much lower &quot;aspect ratio&quot; than real rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel crossfeed model is unrealistic in that it allows you to connect tanks A and B in any orientation and will always drain A first, regardless of having to pump against G forces. It's also possible to detach the stages in KSP while the engines are running - the Space Shuttle only detached the tank when the engines had shut down. There's no consideration of the risks of leaking drips of fuel and oxidiser from the crossfeed. Detaching a real side tank while under acceleration involves complex forces and may be hard to do cleanly; it quite often goes wrong in KSP!&lt;/p&gt;&#xA;" OwnerUserId="1521" LastActivityDate="2016-10-18T12:52:43.793" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18689" PostTypeId="2" ParentId="18687" CreationDate="2016-10-18T13:03:52.363" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mixed_oxides_of_nitrogen&quot; rel=&quot;nofollow noreferrer&quot;&gt;MON&lt;/a&gt; according to Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mixed oxides of nitrogen (MON) are solutions of nitric oxide (NO) in dinitrogen tetroxide/nitrogen dioxide (N2O4 and NO2).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The addition of a small amount of nitric oxide (1%-10%) makes the oxidizer less corrosive, but slightly less powerful as well, and changes the freezing point of the liquid. MON3 means 3% nitric oxide by mass, which is a common formulation for American engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Clark's &quot;Ignition!&quot;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NO is an extremely effective freezing point depressant for N2O4. ... G. R. Makepeace and his associates, at NOTS, were able to show, in 1948, that  25 percent of NO would bring the freezing point of nitrogen tetroxide down below the required -65 F, and that 30 percent would depress it well below the magic -100 F. However, the vapor pressure of the latter mixture at 160 F was unacceptably high, about 300 psi.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(The freezing point of plain N2O4 is 16 F; Clark says -9 F, but this likely varies with contaminants. Military services wanted reliable storable fuels for all climates.)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Several agencies tried the mixed oxides of nitrogen (MON-25 or MON-30 or whatever, with the number designating the percentage of NO in the mix) with various fuels, and discovered that it was more difficult to get a good performance with MON than with neat nitrogen tetroxide. ... For this reason, and because of its high vapor pressure, investigators turned away from MON for some years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In full, the usual modern hypergolic oxidizer is &quot;an &lt;a href=&quot;https://en.wikipedia.org/wiki/Chemical_equilibrium&quot; rel=&quot;nofollow noreferrer&quot;&gt;equilibrium mixture&lt;/a&gt; of dinitrogen tetroxide and nitrogen dioxide with an admixture of nitric oxide&quot;; it's no wonder this gets abbreviated to &quot;mixed oxides of nitrogen&quot; and then to MON. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-01-21T04:32:56.310" LastActivityDate="2017-01-21T04:32:56.310" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18690" PostTypeId="5" CreationDate="2016-10-18T13:18:07.017" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2016-10-18T13:18:07.017" LastActivityDate="2016-10-18T13:18:07.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18691" PostTypeId="4" CreationDate="2016-10-18T13:18:07.017" Score="0" Body="LOX (liquid oxygen) is pure oxygen kept at a temperature at which oxygen is a liquid, between approximately 54.4 K and 90.2 K (corresponding to -219 °C to -183 °C, or -362 °F to -297 °F). LOX can be used as an oxidizer in liquid-fuel rocket engines." OwnerUserId="415" LastEditorUserId="415" LastEditDate="2016-10-18T13:22:19.740" LastActivityDate="2016-10-18T13:22:19.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18693" PostTypeId="1" AcceptedAnswerId="19214" CreationDate="2016-10-18T15:02:54.343" Score="5" ViewCount="361" Body="&lt;p&gt;In the section about &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_(spacecraft)#Orbit_and_environment&quot; rel=&quot;noreferrer&quot;&gt;Orbit and Environment&lt;/a&gt; the Wikipedia article about the Juno spacecraft says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In comparison, Juno will receive much lower levels of radiation than the Galileo orbiter at its equatorial orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The reason is partly because Galileo had a more equatorial orbit, in order for it to execute a large number of close flyby's of the Jovian moons for data collection. Juno carefully avoids that by remaining in an essentially polar orbit that mostly avoids the intense radiation belts, at least in the beginning of the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Galileo operated for roughly seven years. It was inserted into Jupiter orbit December 1995, the camera operated until January 2002, and the spacecraft was de-orbited - still under control - in September 2003. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Juno will receive less radiation than Galileo and yet it's systems will start failing within a few months and it seems mostly within one year, even with the electronics in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_(spacecraft)#Orbit_and_environment&quot; rel=&quot;noreferrer&quot;&gt;titanium radiation vault&lt;/a&gt;, why would it fail so much faster?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that it's a side effect of Moore's law. Pixels are much smaller, and transistors are much much smaller 20 years later, perhaps 100 to 1000 times smaller volume per element, so while it's much faster and more powerful, it's much, much more sensitive to radiation. But that's only a guess. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;If Juno will get much less radiation than Galileo did, why is it expected to fail so much faster?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-12-02T14:03:27.157" Title="If Juno will experience lower levels of radiation than Galileo did why it will fail so much faster?" Tags="|radiation|jupiter|juno|electronics|galileo|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18694" PostTypeId="1" CreationDate="2016-10-18T15:56:25.633" Score="9" ViewCount="3432" Body="&lt;p&gt;Almost all satellites in LEO have been launched to the east, to take advantage of the Earth's surface rotational speed of about 5% of required orbital speed (Israel being a small exception for geopolitical reasons). Vandenberg and Satish Dhawan in India launch polar satellites to the south out of geographical reasons. But from Kodak and Plesetsk they are launched to the North instead. I don't know about inland Baikonur and Chinese space ports.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it matter substantially for the space debris risk that polar orbits are opposite? Since it results in an impact speed of about 18 km/s, which should be much higher than two roughly equatorial prograde objects colliding.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="58" LastEditDate="2016-10-19T16:37:31.253" LastActivityDate="2016-10-20T16:03:15.290" Title="Are most Earth polar satellites launched to the South or to the North? And why?" Tags="|launch|artificial-satellite|debris|collision-avoidance|polar-orbit|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="18695" PostTypeId="2" ParentId="18694" CreationDate="2016-10-18T16:26:48.090" Score="24" Body="&lt;p&gt;There's no real distinction once you're in orbit; a northbound LEO orbit at a given longitude at a given time is a southbound orbit at nearly the opposite longitude 45 minutes later. The rotation of the Earth brings any chosen launch site under the ground track of every polar-orbit satellite, in both directions, daily. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, the choice of northerly or southerly launch usually comes down to geopolitical considerations, as noted. For the early part of ascent, over-ocean trajectories are better than over-land; flying over unpopulated areas is better than over populated ones; flying over ally nations is better than flying over rivals. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe Baikonur polar launches are to the north for these reasons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know whether more current polar satellites were originally launched to north or south, but as noted, once in orbit that distinction is not important.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-19T16:13:10.803" LastActivityDate="2016-10-19T16:13:10.803" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18696" PostTypeId="1" CreationDate="2016-10-18T16:44:06.453" Score="2" ViewCount="220" Body="&lt;p&gt;Batteries contain metals with some structural rigidity themselves, and need encapsulation anyway. Could the batteries a spacecraft needs be used to build part of the physical structure, such as the arms holding up a solar panel on a com sat or a section of the hull protecting a crew in a spaceship? The point being to save some of the aluminium mass needed, rather than having the batteries being a box of payload that needs extra external physical support itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do batteries need radiation protection?&#xA;Do they cause bad secondary radiation? Li-ion batteries seem to contain iron massed metals like manganese and cobalt. Are there other risks like overheating, explosion, poisonous contamination.&#xA;Has it even been done?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2020-05-25T02:47:31.560" Title="Can Li-ion batteries replace parts of the physical structure of a spacecraft, such as the hull of a crewed spaceship?" Tags="|spacecraft|design|construction|shielding|battery|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18698" PostTypeId="2" ParentId="18670" CreationDate="2016-10-18T17:47:24.893" Score="1" Body="&lt;p&gt;Everything I'm finding on the energy dissipation system indicates that it was passive. This makes sense--introduce a pump, and you add complexity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The research on this type of passive system was first published in 1971: &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710025114.pdf&quot; rel=&quot;nofollow&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710025114.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a passive system like this, you're looking at the friction of the viscous fluid on the walls of the energy dampening system being used to slow down the spin. That friction is then dissipated s heat.&lt;/p&gt;&#xA;" OwnerUserId="17050" LastActivityDate="2016-10-18T17:47:24.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18699" PostTypeId="2" ParentId="18684" CreationDate="2016-10-18T18:08:46.823" Score="1" Body="&lt;p&gt;A free program to visualize the solar system may help you; try &lt;a href=&quot;http://celestiaproject.net&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestia&lt;/a&gt;, it's quite good. Turn on the visualizations of the orbits and you will see how they are nearly all in the same plane. Since there's no up or down in space, it's meaningless to talk about horizontal or vertical. Since the solar system is very close to a plane we could call that flat, horizontal, aka the ecliptic, whatever, but it's an arbitrary thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One sort of up and down in space is in terms of gravity, and we can climb up and down the &quot;gravity well&quot; by traveling faster or slower. Imagine space as a stretched rubber sheet, with each planet a weight on the sheet, causing it to bend downwards. To leave a planet's surface, it doesn't matter what direction you'd go, it's uphill in every direction. Space is sort of the same. With a lot of energy you can get up out of the planet's gravity well, and move across the sheet and eventually fall into a different depression.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you wanted to leave the solar system, and have a lot of energy (like a massive rocket) you could do it in any direction you want, aiming at any star. You just need enough energy to get up out of the gravity well. But in practice we don't have that kind of energy, so we need to have space probes swing by a planet or two in a slingshot maneuver, which gives them a boost of speed. This means they often travel in the ecliptic &quot;horizontal&quot; plane to start with, since that's where the planets are.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="6816" LastEditDate="2016-11-25T05:38:07.480" LastActivityDate="2016-11-25T05:38:07.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18701" PostTypeId="1" AcceptedAnswerId="18705" CreationDate="2016-10-18T22:31:22.660" Score="3" ViewCount="356" Body="&lt;p&gt;The objective is to launch a crewed spacecraft to lower Earth orbit. Except the perigee must be higher than 100km altitude, there is no other constrain on the orbit parameters. The emphasis is that the launch itself is not planned in advance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If an organization that is capable of sending people into orbit (NASA, Roscosmos, CNSA, ULS, SpaceX, etc.) starts working today, how long will it take to have a launch?&lt;/p&gt;&#xA;" OwnerUserId="340" LastEditorUserId="340" LastEditDate="2016-10-19T04:48:15.707" LastActivityDate="2016-10-19T10:59:32.653" Title="how long does it take to have an emergency crewed space launch?" Tags="|launch|crewed-spaceflight|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18702" PostTypeId="1" CreationDate="2016-10-19T00:14:38.817" Score="1" ViewCount="113" Body="&lt;p&gt;What is the requirement of a material to deflect or block radioactive particles? Is it the number of electrons surrounding the atom? The density of the nucleus? The strength of the bond of the atoms in the molecule? What gives a material its radiation blocking properties?&lt;/p&gt;&#xA;" OwnerUserId="17327" LastActivityDate="2016-10-19T00:14:38.817" Title="What properties of a material make for good radiation shielding?" Tags="|radiation|cosmic-radiation|" AnswerCount="0" CommentCount="3" ClosedDate="2016-10-19T13:07:14.493" ContentLicense="CC BY-SA 3.0" />
  <row Id="18704" PostTypeId="1" AcceptedAnswerId="18870" CreationDate="2016-10-19T06:55:10.460" Score="1" ViewCount="609" Body="&lt;p&gt;&lt;strong&gt;update 2:&lt;/strong&gt; It appears Schiaparelli has already been located by satellite. This still represents the &lt;em&gt;first laser ranging retroreflector array deployed beyond the Earth-Moon system&lt;/em&gt;! So assuming it is now oriented such that the array can be seen from space - is anything else needed, or now that the position is know is it ready for science?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; At the moment signal reception from Schiaparelli (ExoMars EDM) has not been re-established after the parachutes should have opened. Assuming it has landed right-side up (or at least close) can the retroreflector array still be used for science? Would imagery from orbit be able to eventually pinpoint the location so that it can be targeted by lasers from future satellites and meaningful measurements be performed?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Retroreflector arrays have many uses in space exploration and physics. See &lt;a href=&quot;https://space.stackexchange.com/a/15329/12102&quot;&gt;this answer&lt;/a&gt;, and &lt;a href=&quot;https://space.stackexchange.com/a/15448/12102&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/14943/12102&quot;&gt;here&lt;/a&gt; for examples.&lt;/p&gt;&#xA;&lt;p&gt;The Schiaparelli Lander carries one that seems to be the first optical retroreflector to be deployed beyond Earth orbit.&lt;/p&gt;&#xA;&lt;p&gt;This description lists &lt;em&gt;examples of what it could be used for&lt;/em&gt;, but are there any specific plans to bounce beams off of it any time soon?&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;How can Schiaparelli's retroreflector array be used if communications is not established?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;From: &lt;a href=&quot;http://spaceflight101.com/exomars/schiaparelli-instrument-overview/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spaceflight101.com/exomars/schiaparelli-instrument-overview/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The INRRI Corner-Cube Retroreflector weighs under 25 grams, is 5.5 by 2.0 centimeters in size and is envisioned to be tracked by Mars orbiters capable of laser ranging or laser altimetry, or even laser communications. Its presence on Mars may also benefit future Martian geodesy or general relativity studies. INRRI has an aluminum body with eight fused silica Corner-Cube Reflectors within it, attached by a silicone rubber.&lt;/p&gt;&#xA;&lt;p&gt;Installing a retroreflector on a rover or lander will enable precise georeferencing during the surface exploration activity. Having precise georeferenced data is useful in tracking sites that may be of interest for future exploration or sample return. Establishing a catalog of georeferenced sites of particular interest is a priority for NASA and ESA in the agencies’ long-term goals for Mars exploration.&lt;/p&gt;&#xA;&lt;p&gt;The INRRI reflector is wavelength-independent, providing an initial platform to explore ranging on Mars, and prove the principles of laser communications between a surface vehicle and an orbiter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FjHYS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FjHYS.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-30T05:47:55.183" Title="Could Schiaparelli's retroreflector array (still) be used?" Tags="|mars|lander|exomars|retroreflector|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18705" PostTypeId="2" ParentId="18701" CreationDate="2016-10-19T07:07:22.037" Score="7" Body="&lt;p&gt;It depends.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know anything about the chinese agency so I'll focus on Roscosmos &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes about 2 years to build a new Soyuz (Mir era data). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However what would happen is that we'd repurpose a scheduled vehicle for the emergency mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we launch about 4 Soyuz a year, this mean that &lt;strong&gt;on average&lt;/strong&gt;, a Soyuz should be ready within 45 days, at most 90 depending on the date of the next scheduled launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, there is probably room for corner cutting and extra hard work. Especially if you are ready to send a rescue vehicle for the rescue vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you needed to send one today (October 19 2016)... You'd be lucky since Soyuz MS-02 is already on the launchpad. Few hours to go ! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 3 months figure is really an average: next launch will be just one month from now:&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6642" LastEditDate="2016-10-19T10:59:32.653" LastActivityDate="2016-10-19T10:59:32.653" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18706" PostTypeId="2" ParentId="566" CreationDate="2016-10-19T07:11:20.253" Score="3" Body="&lt;p&gt;The new Soyuz MS does too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_MS&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Soyuz_MS&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Instead of relying on ground stations for orbital determination and&#xA;  correction, the now included Satellite Navigation System ASN-K&#xA;  (Russian: (АСН-К, Аппаратура Спутниковой Навигации) relying on GLONASS&#xA;  and GPS signals for navigation.[2][11] It uses four fixed antennas to&#xA;  achieve a positioning accuracy of 5 m (16 ft), with the objective to&#xA;  reduce that number to as little as 3 cm (1.2 in) and an attitude&#xA;  accuracy of 0.5°.[12]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-10-19T07:11:20.253" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18707" PostTypeId="1" AcceptedAnswerId="18878" CreationDate="2016-10-19T07:43:19.070" Score="5" ViewCount="323" Body="&lt;p&gt;The ESA blog post &lt;a href=&quot;http://blogs.esa.int/rocketscience/2016/10/18/listening-to-an-alien-landing/&quot; rel=&quot;noreferrer&quot;&gt;Listening to an Alien Landing&lt;/a&gt; describes the plan to monitor a UHF beacon transmitted by Schiaparelli during it's upcoming descent and landing on Mars (2016-10-19 about 14:45 UT). The signal will be weak, and so 28 dishes of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Giant_Metrewave_Radio_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Giant Metrewave Radio Telescope&lt;/a&gt; (GMRT) will be used to listen (read more &lt;a href=&quot;http://www.mso.anu.edu.au/~plah/Home_Page_Stuff/GMRT_2006/GMRT_images_2006.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;). According to the article it was specially fitted with JPL equipment in order to optimize detection of the spacecraft beacon, and relay it live to the ESA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the GMRT the only instrument that is suited for this or just the most convenient? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Was the spacecraft landing time chosen so that the GMRT would be optimally located to receive the beacon?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Nblbv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Nblbv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fjkUU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fjkUU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; Photo of a few of the central dishes in the GMRT array, from &lt;a href=&quot;http://www.mso.anu.edu.au/~plah/Home_Page_Stuff/GMRT_2006/GMRT_images_2006.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zttoq.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zttoq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; altiude and azimuth of Mars as seen from the GMRT in Pune, India calculated using &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;noreferrer&quot;&gt;Skyfield&lt;/a&gt;  &lt;strong&gt;note:&lt;/strong&gt; the light time delay is about 10 minutes.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from skyfield.api import load&#xA;&#xA;data = load('de421.bsp')&#xA;&#xA;mars  = data['mars']&#xA;earth = data['earth']&#xA;GMRT  = earth.topos(19.0965, 74.0497)&#xA;&#xA;ts    = load.timescale()&#xA;hours = np.arange(6, 18.1, 0.5)&#xA;time  = ts.utc(2016, 10, 19, hours)&#xA;&#xA;alt, az, d = GMRT.at(time).observe(mars).apparent().altaz()&#xA;&#xA;plt.figure()&#xA;plt.subplot(2,1,1)&#xA;plt.plot(hours, alt.degrees)&#xA;plt.ylim(0, None)&#xA;plt.title(&quot;Mars altitude from GMRT vs hours, 2016-10-19 UT&quot;)&#xA;plt.subplot(2,1,2)&#xA;plt.plot(hours, az.degrees)&#xA;plt.title(&quot;Mars azimuth from GMRT vs hours, 2016-10-19 UT&quot;)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="265" LastEditDate="2016-10-31T16:24:11.857" LastActivityDate="2016-10-31T16:24:11.857" Title="Was the time of Schiaparelli's landing chosen specifically so the Giant Meter Wave Radiotelescope could listen?" Tags="|data-transmission|radio-telescope|skyfield|exomars|schiaparelli-lander|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18708" PostTypeId="1" AcceptedAnswerId="18709" CreationDate="2016-10-19T09:20:44.743" Score="2" ViewCount="435" Body="&lt;p&gt;The caption in this &lt;a href=&quot;http://www.space.com/34434-europe-landing-exomars-capsule-on-mars-today.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space dot com video&lt;/a&gt; says that Schiaparelli will drop the last two meters, and the shock will be absorbed by a crushable structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is described in the text accompanying this video &lt;a href=&quot;http://www.esa.int/spaceinvideos/Videos/2016/10/Schiaparelli_s_descent_to_Mars_in_real_time&quot; rel=&quot;nofollow noreferrer&quot;&gt;Schiaparelli's Descent to Mars in Real Time&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In six minutes it will use a heatshield, parachute and thrusters to brake from 21 000 km/h to a near standstill 2 m above the surface, where a crushable structure on its underside will absorb the final shock.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, the &quot;Two Meters of Terror&quot; look a little too fast in the video in this &lt;a href=&quot;http://www.bbc.com/news/science-environment-37700042&quot; rel=&quot;nofollow noreferrer&quot;&gt;BBC website article&lt;/a&gt; considering Mars' low gravity. It looks a little more realistic here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bTmcZ.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bTmcZ.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF animation of Schiaparelli landing from &lt;a href=&quot;https://www.inverse.com/article/22286-esa-schiaparelli-lander-mars-six-minutes&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there are plenty of rocks on Mars of all sizes, how will Schiaparelli manage to land so that its crushable structure hits first? Am I understanding the image correctly - &lt;em&gt;is the entire underside of Schiaparelli one big crushable-structure?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the images below the RDA assembly seems to be delicate instrumentation that will actually hit the surface first. I don't know why the crushed/damaged areas on the edge/periphery are representative, I'd expect the center to hit first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jqWPN.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jqWPN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Crushable Landing Structure after test drop – Photo: ESA&quot; from &lt;a href=&quot;http://spaceflight101.com/exomars/schiaparelli-edm/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is it only crushed on two opposite edges simultaneously, but not in the middle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/awWWx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/awWWx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Crushable Landing Structure Design – Image: ESA&quot; from &lt;a href=&quot;http://spaceflight101.com/exomars/schiaparelli-edm/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3ToPl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3ToPl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Schiaparelli - View from Beneath&quot; from &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2016/07/Schiaparelli_view_from_beneath&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-19T12:52:46.630" LastActivityDate="2016-10-19T13:28:40.573" Title="Is the entire underside of Schiaparelli one big crushable-structure?" Tags="|mars|lander|schiaparelli-lander|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18709" PostTypeId="2" ParentId="18708" CreationDate="2016-10-19T11:13:12.420" Score="3" Body="&lt;p&gt;The &lt;a href=&quot;http://spaceflight101.com/exomars/schiaparelli-edm/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight101 article you linked&lt;/a&gt; to has this nice graphic:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/thx60.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/thx60.jpg&quot; alt=&quot;Schiaparelli diagram showing that the entire underside is a crushable structure&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes, the entire underside, except for a section in the middle where the radar sits, is a crushable structure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cross-section you added to the question shows the radar is attached to a flange that sits on a ridge of the crushable structure, and there's a void above the radar. So this happens when the radar hits the ground first: the forces are transmitted to the ridge, and the entire radar is pushed up into the void. The radar may also absorb some of the force by deforming.   &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-10-19T13:28:40.573" LastActivityDate="2016-10-19T13:28:40.573" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18710" PostTypeId="1" CreationDate="2016-10-19T12:25:49.360" Score="1" ViewCount="464" Body="&lt;p&gt;Is it possible to have a synchronized orbit between the Moon and the Earth, using the gravity of both bodies to hold its position  (with some assistance, since the Moon's orbit is not circular). This could lead to so much: tethered lift transport, communications, deep space launches, etc.&lt;/p&gt;&#xA;" OwnerUserId="17333" LastEditorUserId="17174" LastEditDate="2016-10-20T17:34:05.297" LastActivityDate="2016-10-20T17:34:05.297" Title="synchronized orbit between Earth and Moon" Tags="|the-moon|orbit|earth|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18712" PostTypeId="2" ParentId="18710" CreationDate="2016-10-19T12:46:35.550" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is it possible to have a synchronise orbit between the moon and the Earth,using gravity of both bodies to hold its position?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes and no. The &quot;Yes&quot; part: This is the Earth-Moon L1 point. The &quot;No&quot; part: The amount of fuel needed to stay at EML1 is huge. It's much better to be in a pseudo-orbit about those unstable Lagrange points. But now the vehicle is not staying at a single point. Many scientific probes have already been placed at the Earth-Sun L1 and L2 points. The Earth-Moon Lagrange points have not yet been utilized extensively.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This could lead to so much; tethered lift transport, communications, deep space launches ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Not for the tethered lift transport. That doesn't make any sense. A space elevator would need to have it's center of gravity at geosynchronous altitude, making the entire mechanism orbit the Earth once per sidereal day. On the other hand, deep space launches are certainly a possibility with an EML1 or EML2 space station. An EML1 or EML2 space station / fuel depot has been seriously investigated as a waypoint to interplanetary space.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-10-19T12:51:42.727" LastActivityDate="2016-10-19T12:51:42.727" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18713" PostTypeId="2" ParentId="18710" CreationDate="2016-10-19T13:34:23.643" Score="1" Body="&lt;p&gt;As mentioned, the L1 point is between Earth and the Moon. An elevator straight from Earth to the Moon isn't feasible, as they have two different rotation speeds, however, the L1 point is interesting for space elevators. It is one of two key point (The other being the L2 point) in making a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_space_elevator&quot; rel=&quot;nofollow&quot;&gt;lunar space elevator&lt;/a&gt; possible. From that point, a payload could be lifted from Earth to orbit using a Space Elevator, then move to the Moon, and finally go from the Moon to it's surface using such a space elevator.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-19T13:34:23.643" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18714" PostTypeId="2" ParentId="18694" CreationDate="2016-10-19T16:29:30.680" Score="2" Body="&lt;p&gt;Once in orbit, they are the same effectively. Once a satellite is in orbit, it essentially stays fixed while the Earth rotates. When a satellite is heading north on one hemisphere, on the other hemisphere it will be going south. Wait about 12 hours, and the same satellite would then be going south over your location, while a point on the opposite side of the world it will be going north. (The exact timing depends on the inclination of the orbit)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, the difference between launching north and launching south for a polar orbit is only the time of the launch, which will differ by about 12 hours.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-10-20T16:03:15.290" LastActivityDate="2016-10-20T16:03:15.290" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18715" PostTypeId="1" CreationDate="2016-10-19T17:57:50.933" Score="1" ViewCount="43" Body="&lt;p&gt;Does NASA currently use weather forecasts of an upcoming passes to assign appropriate channel data rate with Mars Reconnaissance Orbiter (MRO) and Mars Odyssey Orbiter (ODY)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was looking at DSN NOW &lt;a href=&quot;https://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;nofollow&quot;&gt;https://eyes.nasa.gov/dsn/dsn.html&lt;/a&gt;&#xA;and noticed that the ODY used one telemetry data rate during first half of the pass and used different data rate during second half. Was that achieved by using weather forecasts? &lt;/p&gt;&#xA;" OwnerUserId="17234" LastEditorUserId="17234" LastEditDate="2016-10-20T17:27:36.897" LastActivityDate="2016-10-20T17:27:36.897" Title="Weather forecasts of upcoming passes" Tags="|deep-space-network|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18716" PostTypeId="1" AcceptedAnswerId="18719" CreationDate="2016-10-19T18:01:34.643" Score="3" ViewCount="402" Body="&lt;p&gt;Gravity assists are pretty important for interplanetary travel. NASA uses them mainly because they can save time, energy, propellant, and expenses. But so far, all of the gravity slingshots have used the gravity of a large planet (such as Jupiter) to propel the probe farther into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to use something smaller, such as a comet, to perform a gravity assist for spacecraft such as Orion?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is this possible, but only with smaller spacecraft/probes?&lt;/p&gt;&#xA;" OwnerUserId="17203" LastEditorUserId="6816" LastEditDate="2016-10-19T19:28:28.213" LastActivityDate="2019-04-16T07:34:52.463" Title="Gravitational assists from bodies other than planets" Tags="|orbital-mechanics|gravity|gravity-assist|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18717" PostTypeId="2" ParentId="18716" CreationDate="2016-10-19T18:20:35.397" Score="4" Body="&lt;p&gt;It is possible, but it likely won't do much. In order to perform a gravity slingshot, the object's trajectory has to be bent. It's a bit of a simplification, but imagine the escape velocity at the altitude you are passing over. The vector pulling the spacecraft down will be added to the incoming velocity of the object to bend the object's motion. If the velocity of the spacecraft is high, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow&quot;&gt;escape velocity&lt;/a&gt; low, then no practical gravity assist can be obtained. If there was a super dense small object, such as a black hole, passing very close to it could yield a usable gravity assist, but otherwise isn't possible. And there are issues with getting that close to a very dense object, namely that you have a window of only a few feet, as compared to 10s of miles for something like Jupiter.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-19T18:20:35.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18718" PostTypeId="1" CreationDate="2016-10-19T20:43:39.440" Score="2" ViewCount="4162" Body="&lt;p&gt;I am trying to find the position of planets and moon at any specific date which I was able to find using the JPL ephemeris ASCII files.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am however stuck now at finding the positions of the moon's ascending nodes and descending nodes (intersection points of moon's orbit and the ecliptic) from that data. How can I perform the calculations to get those positions at any given point in time?&lt;/p&gt;&#xA;" OwnerUserId="17337" LastEditorUserId="6816" LastEditDate="2016-10-19T20:49:08.400" LastActivityDate="2022-11-08T05:05:13.627" Title="Calculating positions of Moon's ascending and descending nodes from ephemeris files" Tags="|the-moon|orbit|ephemeris|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18719" PostTypeId="2" ParentId="18716" CreationDate="2016-10-19T23:41:53.447" Score="5" Body="&lt;p&gt;The Moon has been used as a gravitational assist in different ways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/a/2488/12102&quot;&gt;this answer&lt;/a&gt;, which links to &lt;a href=&quot;http://stereo.gsfc.nasa.gov/orbit.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA page&lt;/a&gt;. The first video shows the two &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow noreferrer&quot;&gt;STEREO&lt;/a&gt; spacecraft use two and three passes by the Moon to leave Earth orbit and enter into an orbit around the sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a GIF made from the frames of that &lt;code&gt;.mov&lt;/code&gt; file:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tH0UC.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tH0UC.gif&quot; alt=&quot;STEREO PHASING ANIMATION&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read more about it in &lt;a href=&quot;https://space.stackexchange.com/a/4028/12102&quot;&gt;this answer&lt;/a&gt;, which also includes a discussion of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nozomi_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nozomi spacecraft&lt;/a&gt;. I'm not sure if this counts towards your answer, since technically it is not interplanetary - the final orbit is around the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As long as the spacecraft is much less massive than the body used in the assist, it doesn't matter if the spacecraft is large or small. While the force depends on the mass of the spacecraft, the resulting acceleration is F/m. The simplest way to remember that to remember that a feather and a heavy weight fall to earth (or any body) at the same speed, in a vacuum. There are videos of a feather and hammer being dropped by an astronaut on the moon and of course in vacuum on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth-Moon system:&lt;/strong&gt; The Moon's gravity is used in many different cases and scenarios to maintain or adjust orbits of spacecraft that will remain in the Earth-Moon system gravity, and are not therefore interplanetary. Some of those are also described in &lt;a href=&quot;https://space.stackexchange.com/a/4028/12102&quot;&gt;this answer&lt;/a&gt;, and you can read more about it in the pdf &lt;a href=&quot;http://cbboff.org/UCBoulderCourse/documents/ASPapercosIOK.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Art and Science of Lunar Gravity Assist&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-04-16T07:34:52.463" LastActivityDate="2019-04-16T07:34:52.463" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18720" PostTypeId="1" AcceptedAnswerId="18742" CreationDate="2016-10-20T09:37:01.420" Score="9" ViewCount="373" Body="&lt;p&gt;I was not able to find any technical documentation on the closed loop guidance system that was supposed to accomplish the powered descent of ESA's Schiaparelli Entry Demonstrator Module. Several publications mention that Doppler Radar altimeters and inertial measurement units are used as sensors, but it is not obvious how they wanted to null horizontal movement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any hints about the employed control algorithms, performance estimations, region of convergence and stability, and fallback mechanisms would be welcome.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17174" LastEditDate="2016-10-20T17:34:09.817" LastActivityDate="2016-11-15T19:13:16.273" Title="How does Schiaparelli's guidance system work?" Tags="|landing|guidance|schiaparelli-lander|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18721" PostTypeId="1" AcceptedAnswerId="18733" CreationDate="2016-10-20T10:11:33.420" Score="2" ViewCount="219" Body="&lt;p&gt;The &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/ExoMars/How_Schiaparelli_will_land_on_Mars&quot; rel=&quot;nofollow&quot;&gt;ExoMars Entry, Descent and Landing Demonstrator Module&lt;/a&gt; (EDM) &lt;a href=&quot;https://en.wikipedia.org/wiki/Schiaparelli_EDM_lander&quot; rel=&quot;nofollow&quot;&gt;Schiaparelli&lt;/a&gt; was configured to broadcast a UHF beacon throughout the entry, descent and landing phases using multiple transmitters and antennae. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Giant_Metrewave_Radio_Telescope&quot; rel=&quot;nofollow&quot;&gt;Giant Meter Wave Radiotelescope&lt;/a&gt; (GMWR) - an array of roughly 30 dishes of 45 meters diameter each - was able to receive the signal during at least the first part of the descent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were the previous Mars landers configured to transmit beacon signals, or even data during entry, descent, and/or landing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't have to be intended to be heard on Earth - configuration to signal an orbiting spacecraft counts too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I had originally read (somewhere) that Schiaparelli was only transmitting a beacon signal. But after reading this &lt;a href=&quot;http://www.bbc.com/news/science-environment-37715202&quot; rel=&quot;nofollow&quot;&gt;BBC article&lt;/a&gt; I found this ESA update &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/ExoMars/Schiaparelli_descent_data_decoding_underway&quot; rel=&quot;nofollow&quot;&gt;Schiaparelli Descent Data: Decoding Underway&lt;/a&gt; that explains that the transmissions were spacecraft telemetry, and it was received both by the GMWR on Earth, and by the orbiter ExoMars TGO satellite - even while it was busy putting itself into Mars orbit.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-20T15:48:49.547" LastActivityDate="2017-11-02T15:30:30.590" Title="Were the previous Mars landers configured to transmit beacons or data during descent?" Tags="|mars|data-transmission|telemetry|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18723" PostTypeId="1" AcceptedAnswerId="18724" CreationDate="2016-10-20T12:07:15.613" Score="4" ViewCount="9718" Body="&lt;p&gt;Could Mars's magnetic field be restarted by its own moon? I've done some research (and I am definitely no scientist) but I am looking for answers. I know that Mars has two moons, one of which is on a orbit that could end in it hitting the red planet. Could this lead to the restarting of Mars's magnetic field, blocking deadly solar radiation, allowing liquid water to form and stay, and creating weather--and ultimately, a habitable planet apart from Earth on which we could start our first ever colony?&lt;/p&gt;&#xA;" OwnerUserId="17322" LastEditorUserId="17174" LastEditDate="2016-10-20T13:04:33.243" LastActivityDate="2016-10-21T14:04:47.653" Title="Can we restart Mars's magnetic field?" Tags="|mars|colonization|" AnswerCount="1" CommentCount="3" ClosedDate="2016-10-21T20:31:34.363" ContentLicense="CC BY-SA 3.0" />
  <row Id="18724" PostTypeId="2" ParentId="18723" CreationDate="2016-10-20T12:18:52.830" Score="1" Body="&lt;p&gt;Short answer: No&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply hitting a planet with a big rock does not create a magnetic field. Earth has a magnetic field because it has a molten core which spins. Mars may or may not have a partially molten core, but even if it's partially molten it's not molten enough. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to have a magnetic field on mars you'd have to melt the entire core somehow, and to do that from the outside you'd have to turn the whole planet molten again. That's a lot of small rocks or a few big ones. The trouble with that method is you'd then have to wait the thousands or million years for the outside to solidify enough to be habitable again. We are barely able to move a single small space rock at the moment, and that's only theoretical as it hasn't been tested in practice. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, is it theoretically possible to give Mars a molten enough core to have a magnetic field? Yes, if you have massive resources, incredible technology, and are willing to wait thousands of generations. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2016-10-21T14:04:47.653" LastActivityDate="2016-10-21T14:04:47.653" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18725" PostTypeId="2" ParentId="18721" CreationDate="2016-10-20T15:39:08.557" Score="2" Body="&lt;p&gt;Yes, in fact, there have been several. It's not easy to figure out exactly which ones had such a system, but I'm sure Curiosity did. Usually what they have is a very low bandwidth signal or beacon. The signal changes to indicate key events have occurred, like parachute deployment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's very hard to have meaningful telemetry during the decent, because the higher link antennas can't be pointed at the host satellite correctly, and the entry can cause other interference. But this kind of low bandwidth telemetry can be enough to at least point in the right direction of what failed, and has assisted with such failure before.Phoenix, for instance, landed on the edge of uncertainty with respect to it's landing, which was traced to the parachute opening at the wrong time. This was discovered in part by these beacon signals.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-20T15:39:08.557" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18727" PostTypeId="2" ParentId="14346" CreationDate="2016-10-20T20:18:22.360" Score="20" Body="&lt;p&gt;As of now, it does not and that's not likely to change.  I asked this question of leaders of the JWST project at Goddard Space Flight Center.  It isn't that it wouldn't be useful, the problem is funds and time.  Considering the current cost of the project, any new addition would increase the cost and could impact the scheduled launch date of October 2018.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The camera would need to work in a vacuum. Is MLI (multi-layer insulation) sufficient to keep it's temperature in its operating range?  How and where would you mount it?  Would you need to supply artificial lighting?  Where would you find the bandwidth to send the images back?  Resolving these issues would require resources that are in extremely short supply now.&lt;/p&gt;&#xA;" OwnerUserId="17341" LastEditorUserId="17341" LastEditDate="2016-10-20T21:31:50.370" LastActivityDate="2016-10-20T21:31:50.370" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18728" PostTypeId="2" ParentId="12906" CreationDate="2016-10-20T21:28:31.633" Score="3" Body="&lt;p&gt;Relative to your 3rd question, a common suite of instruments for every probe doesn't work for several reasons.  First, the targets of the missions or their environments may vary considerably.  For example, although both New Horizons and Dawn were designed to study &quot;dwarf planets&quot;, the illumination differs greatly. The asteroid belt is about 2.7 ± 0.5 AU (astronomical units = Earth-Sun distance) from the Sun, while Pluto is currently just under 40 AU away.  So the solar illumination at Pluto is more than 200 times less than that at the asteroid belt.  Second, the purpose of missions vary, so the instruments necessary to serve that purpose vary.  The suite of instruments on a given mission are chosen to meet the requirements of the scientists that will use the data and the particular characteristics of the target(s).  &lt;/p&gt;&#xA;" OwnerUserId="17341" LastActivityDate="2016-10-20T21:28:31.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18729" PostTypeId="2" ParentId="4187" CreationDate="2016-10-21T03:11:45.443" Score="4" Body="&lt;p&gt;One reason to stay attached and coast is to let the lower stage finish it's burnout. One of the early SpaceX launch tests failed because they separated too soon. There was still residual fuel exiting the nozzle after separation and the first stage bumped into the second stage. So on the next launch, they lengthened the attached coasting time, to be sure the first stage was finished before separation. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="25" LastEditDate="2017-12-19T13:23:12.427" LastActivityDate="2017-12-19T13:23:12.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18730" PostTypeId="2" ParentId="18401" CreationDate="2016-10-21T03:20:02.193" Score="3" Body="&lt;p&gt;Relative to the third part of your question, OSIRIS-Rex will provide new data that may be helpful in more accurately estimating the future path of potentially hazardous asteroids by characterizing the Yarkovsky effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea of the Yarkovsky effect is a body in space absorbs light from the sun.  This absorbed energy is re-emitted as infrared radiation.  If the body is spinning or tumbling, this re-emitted radiation is in a different direction.  Although this photon pressure (think light sails) is extremely small, it doesn't go away.  So over several decades, an asteroid that we calculate (without taking the effect into account) will miss Earth by, say, 30,000 miles, might not miss at all!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bennu (aka RQ36) was chosen partly because it is so black.  It has an extremely low albedo, reflecting only about 4%, meaning it will absorb about 96% of the Sun's light.  This provides a worst case that OSIRIS-Rex can observe, providing useful experimental data.&lt;/p&gt;&#xA;" OwnerUserId="17341" LastEditorUserId="17341" LastEditDate="2016-10-21T03:30:06.030" LastActivityDate="2016-10-21T03:30:06.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18731" PostTypeId="2" ParentId="208" CreationDate="2016-10-21T06:02:10.293" Score="8" Body="&lt;p&gt;To answer your original question regarding the state of affairs of space telescopes, from the US (NASA), besides JWST, there are two large space telescope projects.  Work has started on the WFIRST (wide field infrared survey telescope).  The LUVOIR (large ultraviolet/optical/infrared survey) telescope is in the study phase. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;WFIRST was originally a dark energy project that morphed into a facility that will enable research in many fields, including dark energy.  WFIRST will have a 2.4 meter diameter primary mirror with a field of view of 0.79 by 0.43 degrees.  The primary is one of the donated ones mentioned in the answer above.  Although the primary is the same diameter as the Hubble primary, the optical systems are different.  The Hubble is a two mirror Ritchey-Chretien, while the WFIRST will be a three mirror anastigmat to provide a much wider useful field of view. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;LUVOIR is intended to combine an enhanced version of the capabilities of Hubble plus a planetary coronagraph.  Its primary mirror will be between 8 and 16 meters in diameter.  It will include instruments working from the far UV to the near infrared (about 100 nm to 1700 nm) and be diffraction limited at 500 nm.  However, to be chosen by the decadal survey, LUVOIR will have to compete with other NASA missions.  Likely competitors include a dedicated planetary coronagraph, a major X-Ray telescope, and a space-based gravitational wave sensor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've provided links to the official sites for &lt;a href=&quot;https://wfirst.gsfc.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;WFIRST&lt;/a&gt; and &lt;a href=&quot;http://asd.gsfc.nasa.gov/luvoir/&quot; rel=&quot;nofollow noreferrer&quot;&gt;LUVOIR&lt;/a&gt;.  I've also provided a link to the AURA Report which you may not find so easily on your own.  The AURA (the Association of Universities for Research in Astronomy) Report provides a &lt;a href=&quot;http://www.hdstvision.org/report/&quot; rel=&quot;nofollow noreferrer&quot;&gt;detailed list of the areas of astronomical research that astronomers hope to explore in the next decades&lt;/a&gt; and the requirements for telescope facilities, like WFIRST and LUVOIR, that are necessary to perform this research.  Lastly, I've provided a link to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Advanced_Technology_Large-Aperture_Space_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;ATLAST entry on Wikipedia&lt;/a&gt;.  &quot;ATLAST&quot; (Advanced Technology Large-Aperture Space Telescope) was the previous name for what is now mostly called LUVOIR.&lt;/p&gt;&#xA;" OwnerUserId="17341" LastEditorUserId="6816" LastEditDate="2016-11-11T07:59:22.703" LastActivityDate="2016-11-11T07:59:22.703" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18733" PostTypeId="2" ParentId="18721" CreationDate="2016-10-22T00:54:57.673" Score="3" Body="&lt;p&gt;Only Mars Polar Lander and Beagle 2 didn't. (Both when we would have wanted it the most, of course.) Mars Pathfinder's was limited to carrier, Doppler, and one subcarrier semaphore every 10 seconds on a direct-to-Earth link, since there were no relay orbiters there yet.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-11-02T15:30:30.590" LastActivityDate="2017-11-02T15:30:30.590" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18734" PostTypeId="1" AcceptedAnswerId="18735" CreationDate="2016-10-22T02:02:58.927" Score="5" ViewCount="1074" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Why doesn't NASA or SpaceX use the command module as their lander, instead of having another spacecraft as the lander?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I was playing Kerbal Space Program the other day, and noticed how many people used the &lt;em&gt;command module&lt;/em&gt; as their lander. Which is something NASA doesn't do at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has another spacecraft that serves as the lander, and another spacecraft as the command module. The lander lands on the Moon, and the command module orbits around. After the astronauts are done collecting samples on the Moon, they rendezvous with the command module and return back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what if NASA had instead of adding an extra spacecraft, they used the command module as both a lander and the command module. So instead of having two spacecraft, you can have the command module land on the Moon, and then blast off back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Is there something wrong with my thinking, because I'm sure NASA would've thought of this idea a long time ago.&lt;/p&gt;&#xA;" OwnerUserId="17203" LastEditorUserId="195" LastEditDate="2016-10-22T21:22:46.787" LastActivityDate="2016-10-27T11:07:04.293" Title="Using command module as a lander" Tags="|the-moon|apollo-program|lunar-landing|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18735" PostTypeId="2" ParentId="18734" CreationDate="2016-10-22T02:34:15.380" Score="21" Body="&lt;p&gt;What you are describing was termed the &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Direct_ascent&quot;&gt;Direct Ascent&lt;/a&gt;&quot; mission mode. As the Wikipedia article says, it was considered but rejected because of the large launch mass requirement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you think about it, there are a few good reasons in favor of the architecture they went with. Some that come immediately to mind:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;It included a service module which represented a lot of mass that did not have to be round-tripped to the lunar surface. Although the SM engine was designed around the direct ascent mode, it was done before the final choice of Lunar orbit rendezvous was made.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The command module (the only portion of the vehicle to go all the way to lunar orbit AND soft-land back on Earth) was designed for re-entry (cone with heat shield on the bottom). This would have been problematic for lunar landing (no downward view as afforded by the lunar lander).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;As demonstrated by Apollo 13, the separate lander offered a degree of redundancy (albeit imperfectly so). If an Apollo 13-like disaster befell a &quot;direct ascent&quot; type vehicle, it might not have been survivable.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Launch from Earth was (and remains) a complex operation requiring considerable ground support. Lunar liftoff had to be accomplished without any external support, favoring a small, simple ascent vehicle.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2016-10-22T13:53:49.740" LastActivityDate="2016-10-22T13:53:49.740" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18736" PostTypeId="1" AcceptedAnswerId="18738" CreationDate="2016-10-22T08:14:09.423" Score="11" ViewCount="671" Body="&lt;p&gt;I am NOT trying to be lewd, but I had a student ask this.  Yes, she may have been tongue-in-cheek, but I promised to answer her.  And yes, I am trying to keep this as practical as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If and when someone passes gas while in a space station (contained environment in microgravity), would there be enough acceleration to push the person into another position?  I don't need direct math, or even biology, but I'd like to return an answer to my student.  My suspicion is, physically in a vacuum, yes of course, but on, say, the ISS, would it really direct you?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;I am not trying to be lewd&lt;/strong&gt;, but genuinely want to know the &lt;strong&gt;magnitude of order&lt;/strong&gt; of a fart within the confines of a habitable space station in microgravity to answer her question.  If there is literature on it, that's great.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have great respect for the SE community: so please don't think I'm being silly; I'm happy to have this removed if you find it inappropriate; but I promised a space answer.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2018-06-25T11:30:55.770" Title="Can (human) gas propel someone in a contained space station?" Tags="|propulsion|microgravity|physics|life|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18738" PostTypeId="2" ParentId="18736" CreationDate="2016-10-22T10:38:54.557" Score="13" Body="&lt;p&gt;I'll try. Please don't judge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say, 5 liters of carbon dioxide (a guesstimate of about how much of bowel volume could be occupied by a single pocket of gas; may not be average or maximum, but is within realm of 'possible'), at 30km/h (when I happen to release one while wiping myself, it feels roughly as if I put my hand out of a car window at such speed; again, a 'possible' speed.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wet mass to dry mass ratio (...sorry) is small enough that we won't need the rocket equation. CO2 density of 1.98 kg/m3, rounding to 2 (pressurized!) gives 10 grams to 5 liters. With conservation of momentum, the gas gets 8.3m/s*0.01kg = 0.083 kg*m/s. If the astronaut is 50kg,  it will yield 1.6mm/second worth of delta-V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With constant air circulation enforced by ventilation of a space station  the acceleration effect would be entirely unnoticeable&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-10-22T10:38:54.557" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18739" PostTypeId="1" AcceptedAnswerId="18785" CreationDate="2016-10-22T15:43:57.217" Score="11" ViewCount="1113" Body="&lt;p&gt;Assuming that Elon Musk's plan to send colonists to Mars is successful and I'm one of the chosen few, and assuming there's the mass budget for it, can I take my bicycle with me?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What difficulties might I have when attempting to ride a standard Earth-format bicycle on Mars? How might we design and engineer a Mars-format bicycle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's, for the moment, assume the following:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;There exists a spacesuit flexible enough to pedal in.&lt;/li&gt;&#xA;&lt;li&gt;There are no &quot;roads&quot;; this is off-road. We're talking BMX or Mountain Biking.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What effect would the lower gravity and less dense atmosphere have? Is there anything peculiar about the terrain that would require (e.g.) beefed up suspension? Is the soil particularly hard on tyres, etc.?&lt;/p&gt;&#xA;" OwnerUserId="10450" LastEditorUserId="10450" LastEditDate="2016-10-23T14:19:47.813" LastActivityDate="2016-10-25T23:22:16.710" Title="Designing a bicycle for Mars?" Tags="|mars|reduced-gravity-sports|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="18740" PostTypeId="2" ParentId="18734" CreationDate="2016-10-22T18:06:07.747" Score="7" Body="&lt;p&gt;The &quot;direct ascent&quot; version of Apollo would have been about twice as heavy as the actual Apollo spacecraft, and would have required a launcher more than 50% larger than the Saturn V. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once en route to the moon, a lunar landing mission needs to make four significant &quot;burns&quot;: one to enter lunar orbit (&quot;lunar orbit insertion&quot; or LOI), one to kill orbital velocity and find a safe place to land (descent), one to lift off and return to orbit (ascent), one to leave lunar orbit and return to Earth (&quot;trans-Earth injection&quot; or TEI). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The velocity change for the LOI and TEI burns are about 700 m/s each. The velocity change for the ascent burn is about 1750 m/s; the descent burn is theoretically similar but in practice extra fuel needs to be carried to allow the pilot time to find a safe landing site, so the Apollo LM's descent stage carried about 2500 m/s worth of fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the descent and ascent burns are much larger than the LOI and TEI, a great deal of propellant mass can be saved by doing the descent and ascent with the smallest practical spacecraft. In particular, the Apollo LM doesn't have to carry the fuel for the return to Earth all the way to the moon's surface and back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo CSM, if used in direct ascent mode, has just enough fuel to leave the moon and return to Earth, so it would need a larger landing stage to execute the LOI and descent burns. The combined CSM and Descent stage would mass about 89 tons, as compared to 47 tons for the CSM+LM. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This spacecraft mass increase would in turn necessitate a much larger launcher than Saturn V, such as the proposed &lt;a href=&quot;http://www.astronautix.com/s/saturnc-8.html&quot;&gt;Saturn C-8&lt;/a&gt; configuration. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-10-22T18:06:07.747" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18741" PostTypeId="2" ParentId="18104" CreationDate="2016-10-23T04:40:29.187" Score="1" Body="&lt;p&gt;One of the issues is grass fires.  There were grass fires after the successful launch of EO-1 from Vandenberg.  So no one, except the fire fighters, were allowed to go closer for quite some time.  Not surprisingly, according to the link below, this is quite common.  Also, even with a water sound abatement system, the launches are really, really loud--not good for nearby instrumentation or wildlife.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See: &lt;a href=&quot;https://www.facebook.com/spaceheadnews/photos/pcb.493904570803006/493900457470084/?type=3&quot; rel=&quot;nofollow&quot;&gt;https://www.facebook.com/spaceheadnews/photos/pcb.493904570803006/493900457470084/?type=3&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17341" LastEditorUserId="17341" LastEditDate="2016-10-23T08:00:17.153" LastActivityDate="2016-10-23T08:00:17.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18742" PostTypeId="2" ParentId="18720" CreationDate="2016-10-23T10:45:45.777" Score="10" Body="&lt;p&gt;The 2011 Thales Alenia Space presentation on &lt;a href=&quot;https://solarsystem.nasa.gov/docs/3_Portigliotti_ExoMars_GNC.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;EXOMARS-2016 GNC Approach for Entry Descent and Landing Demonstrator&lt;/a&gt; might have something that may help.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From that:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The parachute is deployed based on an acceleration trigger, which is chosen to keep the deployment within the required Mach-dynamic pressure box over the dispersed entry conditions. The parachute is deployed at about 10 km above ground level, at Mach 1.95 and a q of about 700 Pascals.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The heat-shield is deployed a fixed time from parachute deploy, shortly after which the RADAR Doppler altimeter is turned on.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The RADAR-determined altitude and velocity is used to determine when to deploy the backshell with parachute. This occurs (was supposed to occur) at 1.6 km above the ground at about 70 m/s vertical velocity.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The descent engines fire up one second later, and execute a short backshell-avoidance maneuver sideways, to make sure that the surface module does not just accelerate right back up into the backshell and large parachute that was just separated above.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Doppler altimeter combined with inertial measurement data feeds the state determination, which then controls the throttling of the engines to perform a gravity turn to the surface, nulling the vertical and horizontal velocities both to less than 1 m/s. A constant deceleration is targeted. The thruster command rate is 10 Hz. This phase is 22 to 30 seconds, and goes from 1.3 km above the ground to 2 meters, and 70 m/s to 0.5 m/s. Starting at 10 meters above the ground, the state determination ignores the altimeter, using only inertial guidance from that point on.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;At 2 meters above the ground, the thrusters are shut off and the surface module free-falls to the ground. It impacts at 4 m/s vertical and less than 1 m/s horizontal velocity. The crushable material on the bottom of the surface module absorbs the impact.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="17367" LastEditorUserId="6816" LastEditDate="2016-11-15T19:13:16.273" LastActivityDate="2016-11-15T19:13:16.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18744" PostTypeId="2" ParentId="16661" CreationDate="2016-10-23T15:20:04.797" Score="12" Body="&lt;p&gt;At that surface gravity, I don't see how it would be possible to ride a bicycle. The friction between the tires and the road is how the motion of the wheels is converted to motion of the bicycle and the rider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0.3 milligee is practically no gravity at all. It would take 80 seconds to fall to the surface from a height of one meter - not so much a fall as a lazy drift. I don't think a person could even walk effectively on the surface of Deimos. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-10-23T15:20:04.797" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18746" PostTypeId="1" AcceptedAnswerId="18747" CreationDate="2016-10-23T17:37:00.820" Score="16" ViewCount="7784" Body="&lt;p&gt;If a nuclear weapon is detonated on the moon it would launch debris into space. The debris would then fall to earth like asteroids. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would this impact wipe out life on Earth? In other words, are our present nuclear weapons capable of breaking off enough big chunks of the moon to kill life on Earth?&lt;/p&gt;&#xA;" OwnerUserId="14280" LastEditorDisplayName="user10509" LastEditDate="2016-10-24T13:17:14.160" LastActivityDate="2016-10-24T15:38:59.183" Title="Can a nuclear detonation on Moon destroy life on Earth?" Tags="|the-moon|nuclear|impact|" AnswerCount="6" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18747" PostTypeId="2" ParentId="18746" CreationDate="2016-10-23T18:37:36.850" Score="21" Body="&lt;p&gt;Probably not. Just to give you an idea, &lt;a href=&quot;http://meteorites.wustl.edu/lunar/moon_meteorites.htm&quot;&gt;lunar rocks&lt;/a&gt; hit the Earth on a somewhat regular basis. The power required to have a rock hit Earth is equivalent to that of making a 450 m crater. This comes from a 30m asteroid, roughly. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Tunguska_event&quot;&gt;Tunguska&lt;/a&gt; event was caused by a 60m rock, and had an explosive power equivalent to that of a large nuclear weapon, around 15 MT. And that was far too little to damage Earth, so it's really unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume you have a diabolical setup. Let's say you have a perfect cannon, such that the entire force of the nuke will be lifted to a perfect Earth intercept trajectory. The velocity &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot;&gt;leaving the Moon&lt;/a&gt; would be about 2.74 km/s. The velocity at Earth would be about 11.2 km/s, which is Earth Escape Velocity (Probably slightly less). Let's say you have the highest theoretical yield weapon that has been planned, 100 MT. Energy effectively scales as the square of the velocity, thus there would be about a 16 times multiplier on the force of your nuke, in absolutely perfect conditions. That would become a 1.6 GT yield weapon equivalent. That's roughly the equivalent of a 300m &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot;&gt;asteroid impact&lt;/a&gt;, which occurs on average every 73,000 years. It would cause some local damage, maybe even regional, but certainly wouldn't end life on Earth as we know it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the rock broke in to several chunks, it could again cause substantial damage, even more than indicated above, but would most likely be limited to a region, causing significant damage over the area of something like Australia, and might cause a global cooling event, but would most likely not be a major event.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-10-24T09:46:50.820" LastActivityDate="2016-10-24T09:46:50.820" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18748" PostTypeId="2" ParentId="18746" CreationDate="2016-10-23T18:38:37.797" Score="5" Body="&lt;p&gt;As a first approximation, ignoring many details: for any nuke detonated on Earth, our atmosphere has proved to be a impenetrable barrier: no debris made it into outer space. Detonating a nuke on the Moon will have the same effect: Earth's atmosphere stops the debris from arriving on Earth's surface. The debris will burn up as it enters the atmosphere. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-10-24T15:38:59.183" LastActivityDate="2016-10-24T15:38:59.183" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="18749" PostTypeId="2" ParentId="18746" CreationDate="2016-10-23T18:45:55.923" Score="6" Body="&lt;p&gt;No, even a 100 megaton bomb would not endanger life on earth.  A nuclear weapon on the surface of the moon might knock some small rocks loose, but mostly it would vaporize material. Under-surface weapons wouldn't do that much either, you only have to look at the history of &lt;a href=&quot;http://nuclearweaponarchive.org/Library/Effects/UndergroundEffects.html&quot;&gt;underground nuclear explosions on earth&lt;/a&gt; to see that nuclear explosions underground pulverize the material above it rather than blowing solid chunks free. A large nuke optimally below the surface would throw up lot of small pieces, some of which would make it to the earth and then harmlessly burn up. The rest would fall back to the surface of the moon. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-10-23T18:45:55.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18750" PostTypeId="2" ParentId="18395" CreationDate="2016-10-23T19:39:31.433" Score="2" Body="&lt;p&gt;The helium would no longer be pure.  Especially in the liquid oxygen tank, the helium would have significant quantities of other gases mixed in.  Fractional distillation could be used to separate them out again, but that might require major machinery and significant time.&lt;/p&gt;&#xA;" OwnerUserId="3361" LastActivityDate="2016-10-23T19:39:31.433" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18751" PostTypeId="2" ParentId="16661" CreationDate="2016-10-23T21:18:57.003" Score="9" Body="&lt;p&gt;Yes, with suitable modifications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The loop must be ferrous.  Your bike needs strong magnets in the wheels.  You will start out with low friction and have to pedal very gently but as you build up your speed your friction will increase.  Arrange the loop so you can go round and round as many times as you want before leaving it.  You could build up substantial speed that way, plenty sufficient for an escape orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your guidance was good enough you could land the same way--enter the loop and &lt;strong&gt;gently&lt;/strong&gt; apply your brakes.  You wouldn't have much margin, though, I can't imagine such a landing system ever being actually used.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-10-23T21:18:57.003" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18752" PostTypeId="1" CreationDate="2016-10-23T22:20:51.537" Score="1" ViewCount="96" Body="&lt;p&gt;Today I was reading a children's book (&lt;em&gt;Ich hab einen Freund, der ist Astronaut&lt;/em&gt; ... my translation: &quot;I have a friend who's an astronaut&quot;) to my son.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On one page it claims that (translation by me):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sometimes astronauts use the gripper arm [of the space shuttle] to capture defective satellites. On earth these [satellites] are repaired and then put back to space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From all I know this is just wrong. But to be sure:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Was a satellite ever brought back to earth, repaired and then put back into space?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that Space Shuttles and other &quot;transporters&quot; don't count.&lt;/p&gt;&#xA;" OwnerUserId="90" LastEditorUserId="90" LastEditDate="2016-10-23T22:27:05.847" LastActivityDate="2016-10-23T22:27:05.847" Title="Was a satellite ever brought back to earth and then launched again?" Tags="|artificial-satellite|reuse|" AnswerCount="0" CommentCount="3" ClosedDate="2016-10-24T01:24:41.387" ContentLicense="CC BY-SA 3.0" />
  <row Id="18753" PostTypeId="2" ParentId="18746" CreationDate="2016-10-23T23:45:39.033" Score="2" Body="&lt;p&gt;With no atmosphere on the moon, &lt;a href=&quot;http://history.nasa.gov/conghand/nuclear.htm&quot; rel=&quot;nofollow&quot;&gt;a nuclear detonation would not have a blast effect like on Earth&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above answers about how any moon rocks hitting Earth from a nuclear explosion there all said the effect wouldn't be very much, but it would be especially little without atmosphere to result in a shock wave to cause rocks to go anywhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, extreme no.&lt;/p&gt;&#xA;" OwnerUserId="13449" LastActivityDate="2016-10-23T23:45:39.033" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18754" PostTypeId="1" AcceptedAnswerId="19152" CreationDate="2016-10-23T23:50:59.913" Score="3" ViewCount="309" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Considering that the recent (23-Nov-2016) &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/ExoMars/Schiaparelli_landing_investigation_makes_progress&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA update&lt;/a&gt; suggests that a &lt;em&gt;saturation of the rotational sensors in the IMU&lt;/em&gt; may have set off a series of miscalculations leading to mission loss, the quote below continues to be relevant. I'm still looking for a posted answer to &lt;em&gt;What are transonic oscillations (wrist mode) and would Schiaparelli have been transonic at 2 to 4 km altitude?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;In the &lt;em&gt;jan woerner's blog&lt;/em&gt; (director-general of ESA) post titled &lt;a href=&quot;http://blogs.esa.int/janwoerner/2016/10/21/spacecraft-are-tricky-and-engineering-is-an-art-form/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacecraft are Tricky ... and Engineering is an Art Form&lt;/a&gt; there is a comment that I don't understand. Could someone explain what is being discussed here more clearly to a non-rocket-scientist? And add a bit about how it applies to Schiaparelli's entry, descent, and landing?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;What about transonic oscillation ? .&lt;/p&gt;&#xA;&lt;p&gt;MSL descent scenario planned that wrist mode oscillations be mitigated by using little thrusters of its reaction control system to counteract these oscillations.&lt;/p&gt;&#xA;&lt;p&gt;Contact was lost at 2 to 4 km altitude in the crtical area of transonic speed inducing resonant wave with possible parachute twist,.disturbing attitude control system and leading to a crtically reduced rocket burst.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I've used the NASA JPL site for the &lt;a href=&quot;http://mars.nasa.gov/msl/mission/technology/insituexploration/edl/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Science Laboratory - Entry Descent and Landing&lt;/a&gt; and found that at 4km to 2km altitude the velocity of the MSL drops from about 95 to 80 meters per second. I know that the atmosphere is cold, but that still sounds safely sub-sonic.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Was Schiaparelli in a transonic state in that altitude range?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; some items incorporated from comments...&lt;/p&gt;&#xA;&lt;p&gt;Wrist mode:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://solarsystem.nasa.gov/docs/p491.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lessons Learned and Flight Experience from Planetary Parachute Development&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2012/06290700-how-curiosity-land-part-2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;How Curiosity Will Land on Mars, Part 2: Descent by Emily Lakdawalla&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Update:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.nature.com/news/computing-glitch-may-have-doomed-mars-lander-1.20861&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nature: Computing glitch may have doomed Mars lander&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-11-24T10:50:13.933" Title="What are transonic oscillations (wrist mode) and would Schiaparelli have been transonic at 2 to 4 km altitude?" Tags="|mars|exomars|schiaparelli-lander|entry-descent-landing|mars-science-laboratory|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18755" PostTypeId="2" ParentId="17675" CreationDate="2016-10-24T00:48:21.317" Score="4" Body="&lt;p&gt;Mars is mapped to a degree, you can explore Mars on &lt;a href=&quot;http://mars.nasa.gov/maps/explore-mars-map/fullscreen/&quot; rel=&quot;nofollow&quot;&gt;&quot;Mars Trek&quot;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MRO (Mars Reconnaissance Orbiter) is used to look for potential obstacles for the various rovers.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mars Reconnaissance Orbiter is looking at small-scale features&#xA;  In its survey of the red planet, the Mars Reconnaissance Orbiter is increasing tenfold the number of spots surveyed close-up. One of the orbiter's cameras is the largest ever flown on a planetary mission. Though previous cameras on other Mars orbiters could identify objects no smaller than a school bus, this camera can spot something as small as a dinner table. That capability has allowed the orbiter to identify obstacles such as large rocks that could jeopardize the safety of landers and rovers, including the Phoenix mission and Mars Science Laboratory mission. Its imaging spectrometer looks at small-scale areas about five times smaller than a football field, a scale perfect for identifying any hot springs or other small water features.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Still, the MRO's resolution is limited so the rovers take images and send them back to the planners for analysis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Lunar equivalent of the MRO is unimaginatively called the LRO. Its mission is to map the lunar surface and identify potential landing sites, find resources that could be used in a future, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think there are other spacecraft dedicated to mapping the Moon, the Indian Chandrayaan-1 comes to mind, but I don't know what else is currently there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mars.nasa.gov/mars2020/mission/timeline/prelaunch/landing-site-selection/&quot; rel=&quot;nofollow&quot;&gt;Here is a good article&lt;/a&gt; that describes what sites NASA is considering for the Mars 2020 rover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, the two main considerations are what's the scientific value of the site and given the engineering constraints, how much of the objectives can be achieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You probably want to consider elevation data - it seems the rovers we do send to Mars are meant to work on fairly flat surfaces. You can kind of see flatness on &quot;Mars Trek&quot; but there are also &lt;a href=&quot;http://%20jmars.mars.asu.edu/maps/?layer=MOLA_Color&quot; rel=&quot;nofollow&quot;&gt;elevation maps&lt;/a&gt;.&#xA;Dust cover, verticle roughness and other datasets can be found &lt;a href=&quot;http://mars.asu.edu/data&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.&#xA;If the rover has solar panels you probably care about illumination data as well as some of the craters might be in permanent darkness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rover-friendliness of the potential landing site is just one factor, the scientific value of the site is usually given higher importance and the design of the rover is a compromise between science value and development cost.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-10-25T16:30:28.973" LastActivityDate="2016-10-25T16:30:28.973" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18756" PostTypeId="2" ParentId="18746" CreationDate="2016-10-24T03:51:37.747" Score="0" Body="&lt;p&gt;Well, theoretically at least, if you were to destroy a sizable portion of the moon, (which might take quite a few nuclear weapons) you could change its centre of mass, destabilising its orbit and eventually causing it to careen into the earth as an extinction event on steroids. If you're not too worried about crunching numbers it could be plausible enough (although you may need the entire nuclear arsenal of earth to pull it off)&lt;/p&gt;&#xA;" OwnerUserId="17372" LastActivityDate="2016-10-24T03:51:37.747" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18757" PostTypeId="1" CreationDate="2016-10-24T05:59:53.513" Score="3" ViewCount="808" Body="&lt;p&gt;Reading about orbital manoeuvres, I found a few reference to the &quot;Broken Plane Manoeuvre&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One illustration  describes it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dyf93.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dyf93.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Source: &lt;a href=&quot;http://hopsblog-hop.blogspot.jp/2013/01/deboning-porkchop-plot.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://hopsblog-hop.blogspot.jp/2013/01/deboning-porkchop-plot.html&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I understand, it is called life this because the burn which appear to instantaneous when scaled towards a sun orbit, appear to be broken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that sense, any manoeuvre that include a plane change is a &quot;broken plane&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any manoeuvre (with sufficient thrust) should appear broken the same way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do I understand correctly ?&#xA;Why do we use the adjective &quot;broken&quot; only for plane (inclination) changes ?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-10-24T15:22:07.477" Title="Broken plane manoeuvre" Tags="|orbital-maneuver|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18759" PostTypeId="2" ParentId="12207" CreationDate="2016-10-24T07:55:48.843" Score="0" Body="&lt;p&gt;A nuclear reactor could be located near the base; buried, as in Kim Stanley Robinson's &quot;Red Mars&quot;.  Then, a route could be selected that passed close to or right by selected investigatory sites.  A means could be selected to pass current from the reactor to the vehicle as it went along the main route, providing electricity for quite rapid, but safe propulsion, while at the same time giving the rover's batteries a full charge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the rover reached an exit point along the main route, the crew would simply go on internal power and complete their mission, returning to the main route to return to base.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me the size and/or number of batteries would be much less a limiting factor than relying on solar power.  A vehicle could be created that would easily transport a crew of 3 or 4 plus a generous equipment and supply list for extended missions, and do it at a fairly high speed, at least on the portion travelled on the main prepared, electrified road.&#xA;A series of these electrified roads could be created to satisfy any mission requirement.  A series of auxiliary nuclear power plants could also be placed along these roads to insure safety through redundancy, and also to power outposts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The selection of buried nuclear power plants is a natural.  With millions of watts available and reliable, everything becomes much easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, wouldn't it be a great idea to turn Kim Stanley Robinson's trilogy into 3 major motion pictures at the time we are getting ready to really launch into the program.  KSR would love the idea, I think, and the pictures would help drum up worldwide support for and interest in the program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In closing, while I loved &quot;The Martian&quot; and other films about going to Mars, they all involve a catastrophe of sorts and the struggle to repair the damage and get back to earth.  The actual program would likely involve landing several payloads ahead of humans and extensive use of robotics before our arrival to do site prep, drilling for water, excavation and tunneling and some construction.  The actual arrival on Mars would be almost routine were it not for the extraordinary hopes and emotions tied to the event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, probably there will be one, maybe two, &quot;single event&quot; landings, successful or not, preceding the above event, but the slow, complete approach is the best, I think.&lt;/p&gt;&#xA;" OwnerUserId="17376" LastEditorUserId="58" LastEditDate="2016-10-25T16:31:35.280" LastActivityDate="2016-10-25T16:31:35.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18760" PostTypeId="1" CreationDate="2016-10-24T08:54:21.613" Score="2" ViewCount="193" Body="&lt;p&gt;Comparing photos of combustion chamber designs from the the 1940's and 1950's to those of today, it looks to the amateur observer such as myself that the size of the chamber proper,  where fuel and oxidizer are mixed and burn, as compared to the divergent sections of nozzles,  has gotten very much smaller. An example is comparing the early V2 engines to those of the NASA Saturn rocket and shuttle engines. Is this true and if so, how does it reflect advances in design efficiency ?&lt;/p&gt;&#xA;" OwnerUserId="17377" LastActivityDate="2016-10-24T15:34:09.837" Title="combustion chamber evolution" Tags="|combustion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18762" PostTypeId="2" ParentId="18746" CreationDate="2016-10-24T15:15:13.860" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;If a nuclear weapon is donated on the moon it would launch debris into space. The debris would then fall to earth like asteroids.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is incorrect, and reflects a common misconception about space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not trying to ridicule you - this is a very common point of confusion shown by &lt;a href=&quot;https://what-if.xkcd.com/58/&quot; rel=&quot;nofollow&quot;&gt;these XKCD submitters&lt;/a&gt; and the producers of &lt;a href=&quot;https://youtu.be/jOKf5r_JMAo?t=30s&quot; rel=&quot;nofollow&quot;&gt;GI Joe: Retaliation&lt;/a&gt;. But let's oversimplify it:  Lunar astronauts are not dangling upside down like Australians.  They do not hang from the underside of the moon by the soles of their shoes, where they could bend up towards their feet, grab a rock and stretch down towards the earth, and then drop the rock on the earth with catastrophic results. They are well planted on the moon, and chucking a rock, hitting a golf ball, blowing up a small bomb, or even firing a high-speed rifle (1000 m/s muzzle velocity) at the Earth high overhead would simply cause it to land on the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even when holding onto the ISS, throwing the rock down at the earth would (counterintuitively) not cause it to hit the Earth! It would simply change the shape of the orbit, rotating it about the Earth. Eventually, the rock would intersect your path again. Throwing it radially inward, you would need to either throw it incredibly fast so that the shape of the new orbit intersects the Earth before it gets 1/4 of the way around - which might take as little as 22 minutes.  That projectile in GI Joe was not moving nearly fast enough to reach the Earth again in minutes - it appeared to be traveling prograde, meaning it would end up further from the Earth, but even going retrograde that launch speed would have required days or weeks to deorbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get off the surface of the moon way, way up into low-Earth orbit requires about 5 km/s of Delta-V.  From there it's downward onto the surface, requiring about 8 km/s. If you could direct the energy of your nuclear weapon into motion of a projectile - producing a big gun with a nuke instead of gunpowder - you could launch $m$ kg from the moon to low-Earth orbit with $K$ terajoules of energy according to: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;K = \frac{1}{2}mv^2&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $m$ is in kg, $v$ is in meters (not kilometers) per second, and $K$ is in joules.  1 kg requires 12.5 MJ of energy.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And on reentry, you'd loose a lot of that energy.  It would slow down and ablate in the atmosphere. You're going up by 5 km/s to LEO, then down from LEO by 5 km/s to get back to your original energy level, and then you've got an additional 3 km/s of energy which needs to make up for the losses of hoisting the nuke to the moon and all that atmospheric drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're probably better off redirecting an asteroid from further out.  Rather than being deep in the Moon's gravity well, you just start way above that of Earth.  Speaking of gravity wells, that reminds me of another &lt;a href=&quot;http://xkcd.com/681_large/&quot; rel=&quot;nofollow&quot;&gt;xkcd source&lt;/a&gt; which appears to show the Moon's gravity well as being much shallower than I interpreted it as based on &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Delta-vs_between_Earth.2C_Moon_and_Mars&quot; rel=&quot;nofollow&quot;&gt;these Delta-V numbers&lt;/a&gt;, which cite some inaccuracy.  Check my work, please!&lt;/p&gt;&#xA;" OwnerUserId="11500" LastActivityDate="2016-10-24T15:15:13.860" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18763" PostTypeId="2" ParentId="18757" CreationDate="2016-10-24T15:22:07.477" Score="6" Body="&lt;p&gt;The &quot;pure&quot; Hohmann transfer only includes entry and exit burns, and therefore the transfer itself occurs entirely in one plane. As an earlier diagram in your linked article shows, this requires nearly 90 degree plane changes at entry and exit, which is absurdly inefficient. For small inclination changes it is much more efficient to do a midcourse plane change, which &lt;em&gt;breaks the plane&lt;/em&gt; of the Hohmann transfer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, nearly any interplanetary transfer in our solar system will likely include a plane change like this, unless the position of the bodies aligns with the ascending and descending nodes between the planes.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-10-24T15:22:07.477" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18764" PostTypeId="2" ParentId="18760" CreationDate="2016-10-24T15:34:09.837" Score="4" Body="&lt;p&gt;It's more likely that you're seeing a trend toward relatively larger nozzles, in particular on upper-stage engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The size of the combustion chamber is more or less fixed by the chemistry of the fuel and the thrust of the engine. None of the advances made since the 50s have significantly altered this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The size of the nozzle relative to the combustion chamber depends on pressure; the expanding cross section of the nozzle lowers the pressure of the exhaust as it flows, and ideally you want the pressure at nozzle exit to match that of the atmosphere outside. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a sea-level engine running at low chamber pressure, this means a relatively small nozzle. For an engine running in vacuum, you want a much larger nozzle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Early rockets were single stage, so early engines were optimized for low altitude. Later engines also tend to run at higher pressures, which require longer nozzles to take full advantage of. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-10-24T15:34:09.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18766" PostTypeId="2" ParentId="14866" CreationDate="2016-10-24T20:16:26.707" Score="1" Body="&lt;p&gt;This discussion is missing a lot of the main advantages of a lunar telescope. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Radio telescope - yes radio silence for the far side of the Moon - and easy to construct telescopes to investigate the longer wavelengths just using a rover to trail long cables over the surface. It is &lt;a href=&quot;http://www.lpi.usra.edu/lunar/strategies/objectives/ast_observations_moon.pdf&quot; rel=&quot;nofollow&quot;&gt;a particularly good site for frequencies below 100 MHz, as these are blocked by the ionosphere on Earth&lt;/a&gt;. This would open up the last unexplored frequency regime in our radio observations of the universe. &lt;/li&gt;&#xA;&lt;li&gt;Can construct ginormous radio telescopes in the craters. Frank Drake once calculated that it should be possible to create &lt;a href=&quot;http://www.lpi.usra.edu/lunar/strategies/objectives/ast_observations_moon.pdf&quot; rel=&quot;nofollow&quot;&gt;Arecibo type telescopes with a diameter of 30 km or more on the Moon (see abstract on page 91)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Construct passively cooled infrared telescopes in the lunar craters at the poles using liquid mirrors. Because the Moon librates so much - rocks up and down and back and forth relative to Earth as it orbits us - a telescope like this would see much more of the sky than a comparable telescope on Earth. See &lt;a href=&quot;http://science.nasa.gov/science-news/science-at-nasa/2008/09oct_liquidmirror/&quot; rel=&quot;nofollow&quot;&gt;Liquid Mirror Telescopes on the Moon (NASA news, 2008)&lt;/a&gt;. This infrared telescope could detect objects &lt;a href=&quot;http://arxiv.org/abs/0806.2241&quot; rel=&quot;nofollow&quot;&gt;a hundred times fainter than the ones visible to the planned James Web Space Telescope&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The stable lunar surface is ideal for optical interferometry - so you could use large networks of telescopes See &lt;a href=&quot;http://mit.sustech.edu/NR/rdonlyres/Aeronautics-and-Astronautics/16-89JSpring-2007/3BAB4C5D-E5AE-46C4-A669-3442D9F831EF/0/report.pdf&quot; rel=&quot;nofollow&quot;&gt;Lunar Telescope Facility Final Design Report (MIT, 2007)&lt;/a&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is mainly from the &lt;a href=&quot;http://robertinventor.com/booklets/Online-Case-for-Moon.htm#telescope_moon&quot; rel=&quot;nofollow&quot;&gt;telescopes on the Moon section of my Case for Moon First&lt;/a&gt; (except I forgot to mention interferometry there - will do with next update).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dust can be dealt with, for instance by making a region around the telescope into glass, which is easy to do on the Moon as use of a microwave turns the regolith into glass as easily as you boil a kettle. Unlike Mars, there are no dust storms to deal with. Just electrostatic elevation of the dust. See &lt;a href=&quot;http://robertinventor.com/booklets/Online-Case-for-Moon.htm#zzee_link_13_1460157186&quot; rel=&quot;nofollow&quot;&gt;Lunar Glass&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="3038" LastEditDate="2016-10-24T22:40:05.060" LastActivityDate="2016-10-24T22:40:05.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18767" PostTypeId="1" AcceptedAnswerId="18780" CreationDate="2016-10-24T23:24:53.193" Score="2" ViewCount="181" Body="&lt;p&gt;The OP of the question &lt;a href=&quot;https://space.stackexchange.com/q/14866/12102&quot;&gt;Would a lunar telescope provide a significant improvement over terrestrial based equipment?&lt;/a&gt; got slightly trounced for asking &quot;the wrong question&quot;, but &lt;a href=&quot;https://space.stackexchange.com/a/14877/12102&quot;&gt;this answer&lt;/a&gt; does address both the question and &quot;(t)he real question (which) is what advantages, as well as disadvantages, does a telescope on the Moon have over a telescope &lt;em&gt;somewhere else in space&lt;/em&gt;, e.g. in Earth orbit or at a Lagrange point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I saw the bit about interferometry being done in space via:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;precision formation flying&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I became really interested! For example, the individual telescopes of the &lt;a href=&quot;https://ca.wikipedia.org/wiki/Atacama_Large_Millimeter_Array&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atacama Large Millimeter Array&lt;/a&gt; (ALMA) are moved around to different mounting sites (by trucks) to reconfigure the array, but the location of those mounts is known to essentially millimeter accuracy. This is important because the 3D configuration of the elements of the array must be known to do the correlation properly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that &quot;known&quot; does not also mean fixed. Once characterized, orbits in space can be incredibly well known or at least predictable, especially if &quot;double checked&quot; at regular intervals. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I'm a firm believer in &quot;Space and Optics were made for each other.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I am wondering, &lt;em&gt;how will &quot;precision formation flying&quot; likely be first demonstrated?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k8XKL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k8XKL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; an ALMA dish moving in precision, albeit deeply sub-orbital motion. &quot;&lt;em&gt;The first successful movement of an ALMA antenna took place at the Operations Support Facility (OSF) on 8 July 2008...&lt;/em&gt;&quot; From &lt;a href=&quot;http://www.thelivingmoon.com/45jack_files/03files/ALMA_Atacama_Large_Millimeter_Array.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-05T23:09:19.570" LastActivityDate="2021-09-05T23:09:19.570" Title="How will &quot;precision formation flying&quot; likely be first demonstrated?" Tags="|spaceflight-firsts|radio-telescope|interferometry|formation-flying|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18768" PostTypeId="1" AcceptedAnswerId="18769" CreationDate="2016-10-25T00:01:58.680" Score="4" ViewCount="3916" Body="&lt;p&gt;The big news in &lt;a href=&quot;http://www.businessinsider.com/spacex-carbon-fiber-fuel-tank-ocean-ship-test-2016-10&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; is the extremely large carbon-fiber fuel tank. However, the drawing in the article does not show it well. Instead what stands out like a sore thumb in the drawing is the smaller sphere, in a location which suggests it would be suspended inside the &quot;fuel tank&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It reminds me of a similarly shaped (i.e. spherical) object in science fiction, but I don't think it is related. (See below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the sphere a helium tank, coated in carbon and suspended in oxygen? Or have I got it all wrong?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/d8bQl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d8bQl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; SpaceX large rocket conceptual design, found &lt;a href=&quot;http://www.businessinsider.com/spacex-carbon-fiber-fuel-tank-ocean-ship-test-2016-10&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RFdzg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RFdzg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Similarly shaped sphere, conceptual, from &lt;a href=&quot;http://futuredude.com/star-trek-2009-is-not-star-trek/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6537" LastEditDate="2017-12-12T05:51:20.010" LastActivityDate="2019-10-10T03:30:59.757" Title="What is the spherical tank in this drawing of the BFS?" Tags="|spacex|rockets|fuel|liquid-fuel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18769" PostTypeId="2" ParentId="18768" CreationDate="2016-10-25T00:26:09.573" Score="12" Body="&lt;p&gt;Firstly, the image you provided is not of the BFR, but the BFS (Big F***ing Spaceship) or ITS (Interplanetary Transport System) spaceship (&lt;a href=&quot;https://www.reddit.com/r/spacex/comments/590wi9/i_am_elon_musk_ask_me_anything_about_becoming_a/d94trp9/?context=3&quot; rel=&quot;noreferrer&quot;&gt;Musk is purportedly not happy with either name&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;Elon Musk answered a question in his Reddit AMA on r/SpaceX recently that describes the exact purpose of the spheres. Long story short, yes, they are propellant tanks. They are not so much suspended but held in place inside the tanks. Enough of me talking though.&lt;/p&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;https://www.reddit.com/r/spacex/comments/590wi9/i_am_elon_musk_ask_me_anything_about_becoming_a/d94r7ef/&quot; rel=&quot;noreferrer&quot;&gt;Question&lt;/a&gt;&lt;/h1&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;ITS Spaceship design question II.:&#xA;The ITS Spaceship has two mystical spherical tanks, marked green in this slightly edited image. The whole tank design looks very exciting, and there's rampant speculation on this sub about the purpose of those spherical tanks:&lt;/p&gt;&#xA;&lt;p&gt;are they for landing fuel?&lt;br /&gt;&#xA;... or are they storing 'hot' gaseous propellants as part of the autogenous propellant pressurization system?&lt;br /&gt;&#xA;... or are they used for on-orbit propellant densification to store vapor before it's liquefied again?&lt;/p&gt;&#xA;&lt;p&gt;All of the above perhaps?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h1&gt;&lt;a href=&quot;https://www.reddit.com/r/spacex/comments/590wi9/i_am_elon_musk_ask_me_anything_about_becoming_a/d94vcmw/&quot; rel=&quot;noreferrer&quot;&gt;Answer by Elon Musk&lt;/a&gt;&lt;/h1&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Those are the header tanks that contain the landing propellant. They are separate in order to have greater insulation and minimize boil-off, avoid sloshing on entry and not have to press up the whole main tank.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As an addendum, SpaceX's Interplanetary Transport System does not use Helium, instead opting for autogenous pressurization whereby you use the propellant to pressurize itself (gaseous Methane and gaseous Oxygen, respectively).&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-10-10T03:30:59.757" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="18770" PostTypeId="1" CreationDate="2016-10-25T02:23:20.810" Score="1" ViewCount="723" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How would you make a breathable atmosphere in an artificial closure on the Moon?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I originally thought that simply obtaining water from the Moon, sending an electrical current through it to get Hydrogen and Oxygen was sufficient enough to have humans breathe on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But upon further research, I learned that &lt;em&gt;too much&lt;/em&gt; oxygen is actually toxic to your body, and it &lt;em&gt;will&lt;/em&gt; kill you. Meaning you need the right balance of other gases in the atmosphere to make it breathable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is simple: What are the required gases needed in the atmosphere, such that humans can breathe and do normal activities? The atmosphere is composed of $78\%$ Nitrogen, $21\%$ Oxygen, $1\%$ Argon and then the rest is traces of other gas. Such as Helium, Carbon Dioxide, Carbon Monoxide, Methane, Krypton, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which of these gasses should you keep, and which should you drop when creating a breathable atmosphere on the Moon? I think you'll have to keep Nitrogen, Oxygen and Argon, but I'm not sure how you can obtain Nitrogen and Argon.&lt;/p&gt;&#xA;" OwnerUserId="17203" LastEditorUserId="17203" LastEditDate="2016-10-25T17:16:11.730" LastActivityDate="2016-10-26T00:59:14.970" Title="Atmosphere on the Moon" Tags="|the-moon|atmosphere|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18771" PostTypeId="1" CreationDate="2016-10-25T03:56:34.600" Score="6" ViewCount="2761" Body="&lt;p&gt;What kind of propulsion method are known to man that can be used for drone racing on moon land and sky(near surface) ? What role moon atmosphere might play?  &lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="12102" LastEditDate="2016-10-25T05:48:18.727" LastActivityDate="2016-10-25T20:52:14.793" Title="Drone Racing on moon" Tags="|the-moon|reduced-gravity-sports|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18772" PostTypeId="2" ParentId="18771" CreationDate="2016-10-25T04:10:34.573" Score="1" Body="&lt;p&gt;The moon has no atmosphere, so there is not much point in talking about drone racing. There are only 2 ways to race drones on the moon:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Hold the races inside (in a large sealed cave or dome that is filled with air). In that case a normal drone will do the job, but it will be able to fly longer because, due to the fact that lunar gravity is only one sixth of the earth's, the engines will use less power than on earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;To hold the races outside, you need to use rocket engines, just as you would for anything else that needs to lift off from the lunar surface. There are no rocket-propelled &quot;drones&quot; on earth. The military use some jet-powered ones, but these rely on air to burn the fuel, so they will not work on the moon either. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;About the only thing we currently have that could be called a &quot;rocket powered drone&quot; are the re-usable rockets of SpaceX and Orbital. They can take off under rocket power and land again. I'm guessing that's not what you had in mind.&lt;/p&gt;&#xA;" OwnerUserId="17390" LastEditorUserId="17390" LastEditDate="2016-10-25T04:16:06.873" LastActivityDate="2016-10-25T04:16:06.873" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="18774" PostTypeId="2" ParentId="8806" CreationDate="2016-10-25T07:23:28.113" Score="1" Body="&lt;p&gt;The main object of any rocket engine is to give the highest impulse to the&#xA;rocket. mv(rocket)=mv(fuel) so for a given mass of fuel you want the maximum&#xA;velocity possible. The larger nozzle divergence bell makes the better use&#xA;of the vacuum to reach the highest possible velocity.&lt;/p&gt;&#xA;" OwnerUserId="17392" LastEditorUserId="6816" LastEditDate="2016-10-25T07:34:36.383" LastActivityDate="2016-10-25T07:34:36.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18775" PostTypeId="2" ParentId="18771" CreationDate="2016-10-25T08:16:48.017" Score="12" Body="&lt;p&gt;Basicly you have 2 Options (and a third hybrid one):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fastest: &#xA;Do a sub-orbital hop with a reaction engine. It doesn't matter if this is a cold-gas thruster, a rocket engine, a anti-matter engine... you need a thrust/weight ratio above moon gravity, than the amount of deltaV defines how far you can travel. Keep in mind that you need half of your deltaV to slow down again before impact!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slowest: &#xA;move along the ground, using a mechanical device. Wheels, tracks, legs. Your speed won't be too high since you have only the very low moon gravity to stick your movementsystem to the ground. Go to fast and you loose contact and control. But as long as you have electricity(solarcells anyone) you can keep on going.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hybrid solution: &#xA;Use a mechanical pusher-plate-system to jump in a sequence of small sub-orbital hops. Because you don't have to stick to the ground you can go faster than a wheeled system, but you don't run out of fuel like a rocket. But you can't slow down before impact so your hops need to stay small. You need some way to control the landing, and you need to make your drone robust enough to survive multiple impacts...&lt;/p&gt;&#xA;" OwnerUserId="11787" LastEditorUserId="11787" LastEditDate="2016-10-25T08:21:57.557" LastActivityDate="2016-10-25T08:21:57.557" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18776" PostTypeId="2" ParentId="18771" CreationDate="2016-10-25T11:28:32.747" Score="7" Body="&lt;p&gt;Small automated flying probes have been considered for the exploration of lunar lava tubes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://ssed.gsfc.nasa.gov/IPM/PDF/1087.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flying, Hopping Pit-Bots for Cave and Lava Tube Exploration on the Moon and Mars.&lt;/a&gt; by  J. Thangavelautham et al:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QjMJw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QjMJw.png&quot; alt=&quot;Pit-bot diagram of the lander&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fuel is non-cryogenic - RP1 and Hydrogen Peroxide. This feeds a single rocket used mainly for lift and hovering.  For attitude control there are several attitude control jets powered by Hydrogen Peroxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper also gives an overall mass budget of 3 Kg for this small probe, overall size of perhaps 30cm. Naturally for proposed racing applications, something larger would be preferred for more than a few minutes of endurance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the interested there is a &lt;a href=&quot;http://www.hou.usra.edu/meetings/leag2014/pdf/3025.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;second paper&lt;/a&gt; which gives less information about the robots, but some information on the overall mission profile for exploring a lava tunnel.&lt;/p&gt;&#xA;" OwnerUserId="8756" LastEditorUserId="8756" LastEditDate="2016-10-25T14:17:50.153" LastActivityDate="2016-10-25T14:17:50.153" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18777" PostTypeId="2" ParentId="18770" CreationDate="2016-10-25T12:22:31.640" Score="2" Body="&lt;p&gt;As stated before, there is no need for anything besides oxygen to sustain human life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pure oxygen has been used (and still is used) when there is need for a low pressure inside a spacecraft. As moon has a much lower gravity this might be same when we would consider a hypothetial atmosphere. Unfortunatelly its gravity is so low it can not hold any substancial atmosphere...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It still does have a little of an atmosphere, but is is so thin one can not really call it anything.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When considering the inside of a moon-station there are different methods how to create an atmosphere inside - from 100% oxygen on low pressures ( = thin hulls ) to a &quot;normal&quot; atmosphere (though anything beside oxygen and nitrogen (and a little humidity) is not used by humans...) on normal pressures.&lt;/p&gt;&#xA;" OwnerUserId="17398" LastActivityDate="2016-10-25T12:22:31.640" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18778" PostTypeId="1" CreationDate="2016-10-25T12:55:38.913" Score="16" ViewCount="462" Body="&lt;p&gt;As of 2016 there are multiple operational missions in orbit around Mars. There are possibly more non-operational missions in orbit around Mars, simply because we do not know the final fate of all missions to Mars. Each mission carries at least one radio link designed to communicate data back to Earth, otherwise the mission would not be very helpful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, satellites are essentially moving in the sky or geosynchronous (which really means not moving in the sky from the position of an observer on the ground). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a geosynchronous satellite, RF spectrum is generally  coordinated as a position in the sky and a range of frequencies in use. Even if multiple satellites occupy the same position in sky, each one can use a unique range of frequencies to avoid interference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For satellites that move across the  sky the situation is a bit more complex, but there is still some coordination amongst operators. For example, no one would put a satellite into orbit with a transponder on 121.5 MHz because it is the aircraft emergency frequency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But a satellite in orbit on Mars is a significantly different situation. The satellite is generally oriented so that it's high gain antenna is pointing at the Earth at all possible times. From the perspective of an observer on Earth, Mars is just a point in the sky. It doesn't matter how accurate the positioning data you have is, there is no way to point a receiver at a specific object in orbit around Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So basically every single receive site on Earth is pointing at the same spot in the sky. If two satellites in orbit around Mars were attempting to use the same frequency, the receiver on Earth would get interference between them. How is RF spectrum usage coordinated for transmitting from Mars orbit back to Earth?&lt;/p&gt;&#xA;" OwnerUserId="14488" LastActivityDate="2020-07-27T01:42:24.870" Title="How is RF spectrum usage coordinated in Mars orbit?" Tags="|mars|radio-communication|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18779" PostTypeId="2" ParentId="18771" CreationDate="2016-10-25T13:35:44.137" Score="1" Body="&lt;p&gt;You can improve on a reaction engine by (essentially) throwing a rubber ball at the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rubber ball is just reaction mass, of course, but over rock, if accurately thrown, it will bounce back, conveying further momentum when you catch it, as well as preserving reaction mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over dust, of course, it dissipates its energy instead of bouncing, and if inaccurately thrown, it will rebound off target. So some secondary propulsion (or a supply of rubber balls) is essential.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If tethered, recovery is possible but inconvenient, but your height is limited to the tether. This becomes equivalent to the &quot;pusher plate&quot; in another answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall, it's unlikely to be a practical means of transport, despite the reaction mass saving, but it may be capable of providing stable flight at a focus point above an ideally shaped crater, for example.&lt;/p&gt;&#xA;" OwnerUserId="6864" LastActivityDate="2016-10-25T13:35:44.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18780" PostTypeId="2" ParentId="18767" CreationDate="2016-10-25T13:43:26.603" Score="1" Body="&lt;p&gt;The first precision formation flying mission is ESA's &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Engineering_Technology/Proba_Missions/About_Proba-3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Proba-3&lt;/a&gt;, slated for launch in 2018.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Proba-3 is ESA’s – and the world’s – first precision formation flying mission. A pair of satellites will fly together maintaining a fixed configuration as a ‘large rigid structure’ in space to prove formation flying technologies.&lt;/p&gt;&#xA;&lt;p&gt;The mission will demonstrate formation flying in the context of a large-scale science experiment. The paired satellites will together form a 150-m long solar coronagraph to study the Sun’s faint corona closer to the solar rim than has ever before been achieved. Beside its scientific interest, the experiment will be a perfect instrument to measure the achievement of the precise positioning of the two spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-10-25T13:43:26.603" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18781" PostTypeId="2" ParentId="18771" CreationDate="2016-10-25T20:52:14.793" Score="1" Body="&lt;p&gt;There is no atmosphere on the moon, unless you use rockets you're looking at ground based racing. There are two interesting variations on ground based racing to consider, taking advantage of the lower gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MAGNETIC LEVITATION&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The low gravity will make magnetic levitation a lot easier than on Earth. Vehicles like the maglev train will be a lot more practical. One of the issues with magnetic levitation is the need for a special track, which on Earth is expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Night time temperature on the moon may be low enough to keep some superconductors below their tranistion temperature (on earth superconducting magnets have to be cooled as they lose their power at temperature well below ambient.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With alternating magnetic fields it is possible to hover over a simple metal surface such as aluminium using only eddy currents (an alternating magnetic field generates currents in metals that repel the magnet.) This is impractical on earth but would be more practical on the moon. A large aluminium arena could be built.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HOVERCRAFT&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could also be referred to as &quot;ground effect rocket.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A hovercraft could float on a cushion of air (or better, hydrogen) expelled directly into the vacuum. Such a device would be less efficient than a combustion system, but would possibly be viable on the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFTERTHOUGHT: BANKING&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tracks for wheeled racers on the moon would have to be much more steeply banked than on Earth. A 45 degree angle is needed when centripetal acceleration matches gravitational acceleration. This would occur at a lower speed on the moon than on Earth. This acceleration is given by &lt;code&gt;velocity^2/radius&lt;/code&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6117" LastActivityDate="2016-10-25T20:52:14.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18782" PostTypeId="2" ParentId="18395" CreationDate="2016-10-25T20:57:53.417" Score="3" Body="&lt;p&gt;As far as I know they vent any remaining propellants after landing for safety reasons so I would assume most of the helium would end up being vented along with that. So my assumption is no they don't recapture it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be worth it? I doubt it. 20k might sound like a lot but when you are talking about a 60M a mission it ends up being 0.03% of the cost of the mission. Add in the cost of actually capturing and separating back out the helium and your savings are even smaller.&lt;/p&gt;&#xA;" OwnerUserId="16458" LastActivityDate="2016-10-25T20:57:53.417" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18783" PostTypeId="1" AcceptedAnswerId="18784" CreationDate="2016-10-25T21:59:07.840" Score="25" ViewCount="22995" Body="&lt;p&gt;What allows the Raptor engine to have a high chamber pressure of 300 Bar? Compared to other rocket engines which have been or that are now operational, it has the highest chamber pressure. What does it have different from other engines, and what kinds of new developments were made in this engine?&lt;/p&gt;&#xA;" OwnerUserId="17400" LastEditorUserId="198" LastEditDate="2016-10-31T20:53:58.517" LastActivityDate="2017-05-06T23:20:30.080" Title="What's so special about SpaceX's Raptor rocket engine with 300 Bar chamber pressure?" Tags="|spacex|engine-design|raptor|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18784" PostTypeId="2" ParentId="18783" CreationDate="2016-10-25T22:38:29.963" Score="20" Body="&lt;p&gt;The Raptor is a full-flow staged combustion methalox engine. There is a lot of new technology in that sentence.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;em&gt;Staged Combustion&lt;/em&gt;&lt;br&gt;&#xA;Fuel and/or oxidizer is ignited to run a turbine that spins a turbopump, increasing the feed pressure into the combustion chamber. The gases in the turbine are then fed into the combustion chamber instead of being discarded. Usually the fuel-rich side goes through this process but there are also oxidizer rich staged combustion engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Staged_combustion_cycle&quot; rel=&quot;noreferrer&quot;&gt;More on Staged Combustion on Wikipedia&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;em&gt;Full-Flow&lt;/em&gt; Staged Combustion&lt;br&gt;&#xA;All of the mass of propellants in the engines passes through the turbopumps and/or turbines before entering the combustion chamber. That means there is a fuel-rich turbopump and a oxidizer rich turbopump.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;em&gt;Methalox&lt;/em&gt;&lt;br&gt;&#xA;The engine uses liquid methane and liquid oxygen as propellant. No methane-powered engines have been used for orbital rockets.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now the big surprise from the IAC talk was the high chamber pressure of the rocket. This allows for many advantages.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Size: The Raptor engine is roughly the same size as the Merlin engine but has 3x the chamber pressure and 3x the thrust.&lt;/li&gt;&#xA;&lt;li&gt;Pressure: The de Leval nozzles that rocket engines use take highly pressurized, low velocity gas and transforms it to lower pressure, high velocity gas. The better the ratio between chamber pressure and exit pressure, the more efficient the engine.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Raptor has a very high chamber pressure compared to other high-performance engines.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;noreferrer&quot;&gt;The SSME has a chamber pressure of ~20MPa&lt;/a&gt; for instance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for technological developments unique to Raptor, check out &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/590wi9/i_am_elon_musk_ask_me_anything_about_becoming_a/d94tbej/&quot; rel=&quot;noreferrer&quot;&gt;this quote by Elon Musk in his AMA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It used to be developing a new metal alloy that is extremely resistant to oxidation for the hot oxygen-rich turbopump, which is operating at insane pressure to feed a 300 bar main chamber. Anything that can burn, will burn. We seem to have that under control, as the Raptor turbopump didn't show erosion in the test firings, but there is still room for optimization.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="10398" LastEditorUserId="6816" LastEditDate="2017-01-24T22:15:05.667" LastActivityDate="2017-01-24T22:15:05.667" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18785" PostTypeId="2" ParentId="18739" CreationDate="2016-10-25T23:22:16.710" Score="5" Body="&lt;p&gt;Regarding safety, a bicycle designed for Martian conditions would quite likely use &lt;strong&gt;more than two tires&lt;/strong&gt; for stability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why? A regular bicycle with two tires is &lt;em&gt;mostly&lt;/em&gt; stable, even in rough terrain, and there are no obvious reasons that is also the case on Mars (I personally use 1 or 2 tires).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, accidents happen. Most bicycle accidents are minor, and result in a negligible amount of harm. Perhaps you get a bruise, or scratch your clothes a little. No big deal.&lt;br&gt;&#xA;&lt;strong&gt;But what does a small scratch do to your pressure suit?&lt;/strong&gt; Potentially a whole lot more damage, with death being a possibility. In essence, small accidents suddenly carry more risk. Some factors also increments the risk as well:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The cosmonauts would have no prior experience with how forces balance in the environment. Remember that even though the gravitational acceleration is lower, the inertia stays the same. That takes some time to adjust to.&lt;/li&gt;&#xA;&lt;li&gt;Even the most flexible of pressure suits is not as flexible as no suit at all. Limited movement and slow reactions are bad for balance.&lt;/li&gt;&#xA;&lt;li&gt;Natural terrain. There are no roads or paths to follow. Even if the terrain is carefully inspected before hand, there may still be invisible bumps covered in dust.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Elevated risks are unacceptable so far away from the closest hospital and with an entire mission at stake, so therefore a plausible bicycle design uses a wide, stable base, and multiple tires touching the ground.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-10-25T23:22:16.710" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18786" PostTypeId="1" AcceptedAnswerId="18796" CreationDate="2016-10-26T00:17:10.670" Score="2" ViewCount="811" Body="&lt;p&gt;In Elon Musk's recent Reddit AMA (ask me anything), he talked about &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Initially, glass panes with carbon fiber frames&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For colonization of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question:&#xA;Does a glass roof provide sufficient protection for plants/humans [against radiation] to create edible food on Mars?   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Extra credit:&#xA;Is ISRU glass production possible with minimal manufacturing capabilities?    Or is spaceX forced to transport the glass?   (Isn't glass heavy, and non-maleable making it difficult to transport?)&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2016-10-26T14:45:02.210" Title="Is a glass habitat on Mars viable?" Tags="|spacex|mars|radiation|habitat|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18787" PostTypeId="2" ParentId="18770" CreationDate="2016-10-26T00:59:14.970" Score="4" Body="&lt;p&gt;The only gases relevant to life will be oxygen, nitrogen, carbon dioxide, water vapor, and methane (not counting all the volatile compounds of CHNOPS present in trace amounts). Of those, the only one necessary for human life support is oxygen; the rest are either helpful/necessary for plant life or a by-product of life you would likely need to manage or remove.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For human habitation, you will need 3psi partial pressure of oxygen to make available the same amount as in sea level air. But a 3psi atmosphere might not be very human-friendly - if warm enough for comfort at rest, you could overheat in it when active. Add some nitrogen; could probably get away with much less than sea-level equivalent. Some humidity would be a good thing too. &lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-10-26T00:59:14.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18788" PostTypeId="2" ParentId="12207" CreationDate="2016-10-26T01:51:11.010" Score="2" Body="&lt;p&gt;You have to also differentiate between peak and average power. Just giving your mobile unit large enough batteries will give it substantial peak power capacity to move fast or do any other power hungry thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curiosity has to drag it's radioisotope power source along with its batteries due to its mission, but for many of the things you mention, this will not be required. It will be entirely feasible to have a large stationary power source ( either solar or nuclear, there really aren't any other near term options ) charging multiple mobility platforms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Batteries, and also regenerative fuel cells work just fine for decoupling the power source from the usage site.&lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2016-10-26T01:51:11.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18789" PostTypeId="1" CreationDate="2016-10-26T05:11:54.570" Score="31" ViewCount="10005" Body="&lt;p&gt;To what extent might a booster designed to place an extremely light weight &quot;micro&quot; satellite with radio transmitter,  say 100 grams for the sake of argument,  be scaled down before factors such as increased aerodynamic drag on smaller objects,  mass ratios etc.  make such a venture impossible ?  Has anyone done speculative calculations for such as project ?&lt;/p&gt;&#xA;" OwnerUserId="17377" LastEditorUserId="17308" LastEditDate="2016-10-28T12:54:38.217" LastActivityDate="2021-10-07T08:16:58.857" Title="How small could an orbital rocket be?" Tags="|rockets|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18790" PostTypeId="1" AcceptedAnswerId="18794" CreationDate="2016-10-26T09:07:32.947" Score="14" ViewCount="2605" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/18771/#comment49276_18776&quot;&gt;This comment&lt;/a&gt; below &lt;a href=&quot;https://space.stackexchange.com/a/18776/12102&quot;&gt;this answer&lt;/a&gt; about 'spacecraft' or at least 'craft' that can maneuver near the surface of low gravity bodies links to &lt;a href=&quot;https://www.youtube.com/watch?v=KBMU6l6GsdM&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; about Lockeed Martin's &lt;a href=&quot;https://en.wikipedia.org/wiki/Multiple_Kill_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Multiple Kill Vehicle&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I understand correctly it is designed to operate for a short time (destructively) in space, but this test happens on Earth, presumably in normal atmosphere. I don't know the size but &lt;a href=&quot;http://gizmodo.com/5104917/video-of-multiple-kill-vehicle-test-scares-me-silly&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gizmodo says&lt;/a&gt; it was at an altitude of 23 feet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see a &quot;main&quot; thruster to maintain altitude (in the test, against Earth gravity), and a &quot;left&quot; and &quot;right&quot; thruster for translation and stopping, and four diagonal thrusters for &quot;fine positioning&quot;, perhaps? Although the axes of the top pair meet above the spacecraft axis, and that of the bottom pair meet below, so perhaps they add translation and torque simultaneously?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No matter though, it looks like all thrusters lie within a single plane of symmetry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The attitude seems quite stable throughout, even after falling and landing. By the way it moves, does it look like it has rotating intertia wheels of some kind, to maintain attitude? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.youtube.com/watch?v=KBMU6l6GsdM&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt; shows the same 30 second test &quot;flight&quot; from three different angles:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P1Iep.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P1Iep.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF made from frame numbers 300 to 326 extracted from this &lt;a href=&quot;https://www.youtube.com/watch?v=KBMU6l6GsdM&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TirXy.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TirXy.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF made from frame numbers 459 and 460 extracted from this &lt;a href=&quot;https://www.youtube.com/watch?v=KBMU6l6GsdM&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/KBMU6l6GsdM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/M3PUt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M3PUt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Lockeed's Martian Multiple Kill Vehicle - cropped from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Mkv-L_hover_test.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="195" LastEditDate="2023-12-21T23:58:48.570" LastActivityDate="2023-12-21T23:58:48.570" Title="Does this kill vehicle contain momentum wheels? Watch how it moves!" Tags="|attitude|reaction-wheel|flight-control|attitude-determination-and-control|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="18791" PostTypeId="2" ParentId="18790" CreationDate="2016-10-26T09:41:07.393" Score="2" Body="&lt;p&gt;As you can see in &lt;a href=&quot;https://www.youtube.com/watch?v=LC97wdQOmfI&quot; rel=&quot;nofollow&quot;&gt;this&lt;/a&gt; older video, the main thrusters at center of mass (probably 4, but the upper one never fires; no need for it due to gravity pull) - are used for translation (dorsal/ventral; port/star) . Four smaller diagonal thrusters are used to provide pitch and yaw stabilization. There doesn't seem to be any fore/aft control because this being an interceptor/impactor it will be traveling in the 'fore' direction with as high a speed as the delivery vehicle can provide, no need to regulate it. The only remaining axis is roll, and it seems like indeed it's either maintained by a (single) reaction wheel - or not maintained at all - the device could roll and simply another thruster would take over given direction, but the test takes too short for that to matter. Alternatively the thrusters for it are minimal - comparing the thrust necessary to maintain roll comparing to other axis, they may be too minimal to notice their activity.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-10-26T09:41:07.393" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18792" PostTypeId="1" AcceptedAnswerId="18795" CreationDate="2016-10-26T10:20:01.610" Score="1" ViewCount="484" Body="&lt;p&gt;In the &lt;a href=&quot;https://www.youtube.com/watch?v=LC97wdQOmfI&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; linked in &lt;a href=&quot;https://space.stackexchange.com/a/18791/12102&quot;&gt;this answer&lt;/a&gt; the spacecraft is said to be &quot;gyroscopically balanced&quot; (at about 00:20). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm going to ask about the terminology associated with the phrase &quot;gyroscopic stabilization&quot; - attitude control of a spacecraft which involves the use of gyroscopes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is what I believe is true: A gyroscope is traditionally a rotating wheel in a mount that contains additional degrees of freedom. The term &quot;scope&quot; suggests some kind of measurement, and while a gyroscope can be a rotating wheel, it can also be a &lt;a href=&quot;https://en.wikipedia.org/wiki/Fibre_optic_gyroscope&quot; rel=&quot;nofollow noreferrer&quot;&gt;ring of optical fiber&lt;/a&gt; (or other waveguide) or a micro-fabricated &lt;a href=&quot;https://en.wikipedia.org/wiki/Gyroscope#MEMS&quot; rel=&quot;nofollow noreferrer&quot;&gt;Micro-electromechanical systems (MEMS) chip&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if a spacecraft uses gyroscopes that do not contain rotating wheels, and it uses thrusters or magnetotorquers or solar reflectors for torque, can we still correctly say that it is gyroscopically stablized without the use of any rotating parts?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-10-26T14:00:20.057" Title="Can gyroscopic stabilization be done without any rotating parts?" Tags="|attitude|attitude-determination-and-control|gyroscope|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18793" PostTypeId="2" ParentId="18790" CreationDate="2016-10-26T10:25:55.563" Score="2" Body="&lt;h2&gt;No, I don't see the need for Inertia wheels&lt;/h2&gt;&#xA;&lt;p&gt;I can only see 2 sets of engines used: Strong main engines (one on each surface) for up and side movement and 4 RCS ports for stability.&lt;/p&gt;&#xA;&lt;p&gt;As I viewed the video, I saw it was firing 2 RCS ports in almost each spot to maintain the position. This does indicate to me, that the whole stability is achieved by those RCS ports, as the sum of 2 orthogonal ports firing at the same time gives us a force along the mid of that angle between.&lt;/p&gt;&#xA;&lt;p&gt;To fight or induce a roll, just one of the ports would need to open or stays open a split moment longer.&lt;/p&gt;&#xA;&lt;p&gt;As this device seems to aim to be used in space, I would deem inertia wheels harmful even: the use of many moving parts in a device operating in a 3K environment (space) needs a very reliable method to reduce friction on permanently used parts like the axis of the inertia wheels. Such a method that also works equally good in a 0G environment I don't know of, indeed, &lt;a href=&quot;http://llis.nasa.gov/lesson/11501&quot; rel=&quot;nofollow noreferrer&quot;&gt;a dry grease resulted in a catastrophic failure of a rover prototype&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The different engines this device will need for mobility already demand the use at least the same ammount of motors or electric valves for the main engines and RCS ports to work and several more.&lt;/p&gt;&#xA;&lt;p&gt;However, I see some problems as that machine on the video does not yet seem to have a front/back engine set and lack RCS to control Pitch/Yaw too.&lt;/p&gt;&#xA;" OwnerUserId="17008" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-10-26T10:39:34.940" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18794" PostTypeId="2" ParentId="18790" CreationDate="2016-10-26T11:09:25.150" Score="12" Body="&lt;p&gt;The thruster configuration can be seen better in this image:&lt;a href=&quot;https://i.stack.imgur.com/gb4HA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gb4HA.jpg&quot; alt=&quot;MKV-L image&quot;&gt;&lt;/a&gt;(cropped from &lt;a href=&quot;http://www.dtic.mil/get-tr-doc/pdf?AD=ADA499423&quot; rel=&quot;noreferrer&quot;&gt;this document&lt;/a&gt; ).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The attitude control thrusters are not in plane with the direction control thrusters. &lt;a href=&quot;http://patentimages.storage.googleapis.com/pdfs/US7494089.pdf&quot; rel=&quot;noreferrer&quot;&gt;This Raytheon patent&lt;/a&gt; on MKVs states that &quot;attitude control system includes multiple thrusters offset from the center of gravity that provide yaw, pitch and roll control.&quot; All four of them are offset towards the aft, and the top and bottom pair are offset from the roll-axis. Firing one thruster provides torque on all three body axes, firing a second one compensates two of these axes and enforces the third.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This does not rule out that reaction control wheels are used in addition. &lt;a href=&quot;https://youtu.be/LC97wdQOmfI&quot; rel=&quot;noreferrer&quot;&gt;This older video&lt;/a&gt; mentions that the platform is &quot;gyroscopicly balanced&quot;, which probably means that an IMU is used as guidance input.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the design objective of this device is to have many vehicles with independent steering at low cost and effort. This makes it feasible to choose such a simple thruster arrangement and put the burden on the guidance system (hard- vs. software). It is unlikely that reaction control wheels are employed from this perspective.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17004" LastEditDate="2016-10-26T12:56:33.823" LastActivityDate="2016-10-26T12:56:33.823" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18795" PostTypeId="2" ParentId="18792" CreationDate="2016-10-26T12:21:32.193" Score="3" Body="&lt;p&gt;Gyroscopes used as sensors can be mechanical, ie with spinning parts, or electrical in which case lasers are often used. Movement is measured and the output is some sort of data which goes to a guidance system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gyroscopes used to provide physical force to stabilize or orient a craft have to be mechanical as the electric signals or lasers used in electronic gyroscopes have essentially no mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are right that both can be used for stabilization, however for something to be called gyrostabilized it has to have the mechanical element providing the force changing orientation to be a gyroscope. At least that's the dictionary definition. Common usage seems to be a bit different calling anything gyroscopically stabilized if a gyroscope is involved at all. So it depends, I'd generally go with common usage. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2016-10-26T14:00:20.057" LastActivityDate="2016-10-26T14:00:20.057" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18796" PostTypeId="2" ParentId="18786" CreationDate="2016-10-26T14:45:02.210" Score="4" Body="&lt;p&gt;You don't want to filter out all radiation, of course - that's the point of using a transparent material. You want to filter out the high-energy part (UV) while leaving lower frequencies (visible light). Glass can do that, even better than air. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This doesn't filter out very high energy radiation which on earth are shielded by the magnetic field, but for crops that's probably not a major issue. They don't grow old anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Glass is chiefly made from silicon and oxygen. As it happens, those are &lt;em&gt;incredibly&lt;/em&gt; common on Mars, even more than iron. However, if we'd only use those, we'd get a problem with UV. With no additives, we'd make fused quartz, and that is very transparent to UV. Normal glass contains also a sizeable fraction of sodium oxide (soda) and calcium oxide (lime). Those two are fairly common on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Making glass is not too hard: melt the ingredients, mix, pour on a hot tin bath, let it cool.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2016-10-26T14:45:02.210" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18799" PostTypeId="1" AcceptedAnswerId="18800" CreationDate="2016-10-26T22:28:34.130" Score="2" ViewCount="624" Body="&lt;p&gt;What is the mass capacity for the Delta IV Heavy for a trans-lunar injection? Or is there a way to take its capacity to GEO or LEO and substitute it into a formula which would bring the approximate capacity for a trans-lunar injection?&lt;/p&gt;&#xA;" OwnerUserId="17166" LastEditorUserId="195" LastEditDate="2016-10-27T13:31:35.853" LastActivityDate="2016-10-27T13:31:35.853" Title="Delta IV Heavy trans-lunar injection mass" Tags="|the-moon|payload|delta-iv-heavy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18800" PostTypeId="2" ParentId="18799" CreationDate="2016-10-26T23:43:13.133" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.silverbirdastronautics.com/LVperform.html&quot; rel=&quot;nofollow&quot;&gt;John Schilling's calculator&lt;/a&gt;, Delta IV Heavy should be able do about 9.8 tons to TLI (defined as a 185km perigee, 400000km apogee Earth orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow&quot;&gt;nonlinearity of the rocket equation&lt;/a&gt; and wide range of launcher performance characteristics, there's not a general rule for estimating TLI capacity from LEO or GEO capacity, unfortunately. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-27T13:29:54.950" LastActivityDate="2016-10-27T13:29:54.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18801" PostTypeId="1" CreationDate="2016-10-27T00:01:33.703" Score="1" ViewCount="65" Body="&lt;p&gt;I would like to calculate the length of the semi-major axis of Juno's initial heliocentric ellipse from launch to aphelion based on the time from launch to aphelion. Do I use the full 394 days from launch to aphelion or do I subtract the time from launch to leaving Earth's sphere of influence from that 394?&lt;/p&gt;&#xA;" OwnerUserId="16692" LastActivityDate="2016-10-27T01:53:06.220" Title="Juno launch to aphelion" Tags="|orbital-mechanics|juno|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18802" PostTypeId="1" AcceptedAnswerId="18825" CreationDate="2016-10-27T01:03:58.967" Score="7" ViewCount="2346" Body="&lt;p&gt;These two good answers (&lt;a href=&quot;https://space.stackexchange.com/a/17132/12102&quot;&gt;one&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/17130/12102&quot;&gt;two&lt;/a&gt;) to the question &amp;quot;&lt;em&gt;Is there a maximum Isp for 'exothermic chemical reaction rockets?&lt;/em&gt;'&amp;quot; put the limits around 500-550 seconds for the limits of practical chemical reaction engines and in the 700's for &lt;em&gt;beyond practical&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;So when I saw the last line in the table below, I become very interested. This is not an &amp;quot;Electric Solid Propellant Rocket&amp;quot; as described &lt;a href=&quot;https://space.stackexchange.com/a/16951/12102&quot;&gt;in this excellent answer to &lt;em&gt;How does an Electric Solid Propellant rocket work?&lt;/em&gt;&lt;/a&gt;, &lt;s&gt;and it flies in the face of Elon Musk's oft-quoted &lt;em&gt;all modes of transport will become fully electric with ironic exception of rockets&lt;/em&gt;.&lt;/s&gt; (see comments)&lt;/p&gt;&#xA;&lt;p&gt;So to which kind of alternative engine design might the last line of this table refer? Is it something that has ever been tried, or just hypothetical? Does it really refer to an electrical arc passing through monatomic hydrogen? That's 13.6 eV to ionize each proton, which is quite a lot of work in the physics sense of the word, therefore it's not really likely to be a conventional ion propulsion engine (is it)?&lt;/p&gt;&#xA;&lt;p&gt;Can it be miniaturized? For example, a 3U cube sat with plenty of solar panels, some space-rated batteries, and a small tank of hydrogen gas (diatomic) - could this kind of propulsion be implemented in that setting?&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;        Fuel          Isp(seconds)&#xA;gun powder               350 &#xA;kerosene + LOX**         360&#xA;LH2 + LOX**              462&#xA;H2 + Nuclear             800 &#xA;H + electrical arc      1600 &#xA; &#xA;  *Measured on the surface of the Earth&#xA;  **LOX: Liquid Oxygen&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RVPtv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RVPtv.png&quot; alt=&quot;table of some unusual Isp values from California State University Long Beach's course ENGR 370I, Astronautics and Space&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Table from &lt;a href=&quot;http://web.csulb.edu/colleges/coe/ae/engr370i/ch05/ch4.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this page&lt;/a&gt; from California State University Long Beach's course &lt;a href=&quot;http://web.csulb.edu/colleges/coe/ae/engr370i/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ENGR 370I, Astronautics and Space&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-09T01:25:16.953" LastActivityDate="2021-08-09T01:25:16.953" Title="What kind of engine does this Isp = 1600 refer to? Is it cubesat-friendly?" Tags="|propulsion|engine-design|advanced-propulsion|ammonia|" AnswerCount="4" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="18803" PostTypeId="1" CreationDate="2016-10-27T01:16:19.710" Score="0" ViewCount="91" Body="&lt;p&gt;Osiris rode on an Atlas V 411. Why not use a larger rocket and skip the gravity assist flyby?&lt;/p&gt;&#xA;" OwnerUserId="16692" LastActivityDate="2016-10-27T01:42:04.677" Title="Why is Osiris-REx using an Earth flyby gravity assist?" Tags="|gravity-assist|osiris-rex|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18804" PostTypeId="2" ParentId="18803" CreationDate="2016-10-27T01:42:04.677" Score="5" Body="&lt;p&gt;Money. Why spend more if you don't have to? Especially if you're in a competition with other similar proposals. Which it was.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-10-27T01:42:04.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18805" PostTypeId="2" ParentId="18801" CreationDate="2016-10-27T01:53:06.220" Score="2" Body="&lt;p&gt;If the perihelion is at Earth, which is what it appears you are assuming since you think half the orbit starts at or around launch, then you don't need the time. You can just use the aphelion distance from the Sun and Earth's distance from the Sun to compute the semi-major axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the perihelion of the orbit may not have been at Earth, if it was injected at some inward or outward angle to Earth's orbit velocity vector, in which case your time is not half of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, you should use the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow&quot;&gt;JPL Horizons system&lt;/a&gt;, in which you can select the Juno spacecraft (id -61) around the Solar System barycenter, and get the orbital elements directly. The initial semi-major axis was about 1.62 AU. That gives a 753-day orbit, half of which is 376.5 days. So 394 days is not half of the orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-10-27T01:53:06.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18806" PostTypeId="2" ParentId="18802" CreationDate="2016-10-27T04:36:33.967" Score="3" Body="&lt;p&gt;Antzi's correct, that's an arcjet. Like ion engines, specific impulse is really good, but thrust is extremely low. &lt;a href=&quot;http://www.iepc2013.org/get?id=238&quot; rel=&quot;nofollow&quot;&gt;This article&lt;/a&gt; covers some of the state of the art as of 2013 (spoiler: 100 milliNewton at 800 watts.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking around, I see references to hydrogen arcjets achieving between 1200 and 2000s specific impulse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aerojet has some &lt;a href=&quot;http://www.rocket.com/files/aerojet/documents/Capabilities/PDFs/Electric%20Propulsion%20Data%20Sheets.pdf&quot; rel=&quot;nofollow&quot;&gt;small arcjet thrusters&lt;/a&gt; that would fit in a cubesat (linked datasheet has a couple of arcjets in addition to Hall effect and other units) -- but their power conditioning units (scaled for ~2kW operation) wouldn't fit. Those are lower-efficiency units using hydrazine at ~600s specific impulse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anode/nozzle erosion seems to be the biggest drawback, with run times limited to the order of 1000 hours -- that seems like a long time, but you need it at those thrust levels, and ion engines can run for at least 10 times as long.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-28T14:56:32.853" LastActivityDate="2016-10-28T14:56:32.853" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18807" PostTypeId="2" ParentId="18418" CreationDate="2016-10-27T08:57:19.490" Score="7" Body="&lt;p&gt;As a Mars enthusiast, aware of the current affairs in space,  I asked that very same question to myself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The short answer is there are a few groups involved, and show potential,  but still lack in expertise, willingness, or capability to step up and lead this on a large scale as you have asked.  As a civilization, we have done little in terms of solving the challenges of colonizing Mars so far, and people are just starting to understand the situation.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are a few close candidates:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;SpaceX: A great company that would be able to come up with designs if they had the resources.  Right now they have no room in the budget for including a martian base within the scope of their design.  Currently the ITS is only 5% of their resources, if they even wanted to pursue it.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;NASA: Solid at the logistics of space programs.  Potentially in the future, however it isn't currently part of their plans for Mars. Also, there are restrictions on funding due to being a government body.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mars One: A small and passionate group who hopes to colonize Mars. While they have the desire to make it happen,  currently they seem to be more of an astronaut selection company dealing with finding suitable colonists.  They do not have detailed plans of their colony design, prototypes, or strong financial backing. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mars Society: An organization with a simulated desert and Arctic base. It isn't a commercial enterprise, but a leader thus far in colony simulations. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are very few organizations working on colonizing Mars. This is why it is so important to raise awareness of the fact we're going there in a few years. The challenges need to be sorted out as soon as possible for us to have better chances of survival . Companies need to start developing the technology to sustain a colony, because there are many challenges we are going to face living there.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The good news in all this is that there are now more student initiatives than ever pertaining to a Mars colony.  For universities there are design team challenges such as &lt;a href=&quot;http://rascal.nianet.org&quot; rel=&quot;noreferrer&quot;&gt;R.A.S.C.A.L&lt;/a&gt; - which the upcoming project is on designing an airlock.  As well,  there is a new &lt;a href=&quot;https://marscitydesign.com&quot; rel=&quot;noreferrer&quot;&gt;Mars City Design&lt;/a&gt; competition.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realized that there are not many companies out there focused on bringing a Mars Colony into existence. This is why I created a new engineering team at my university called UBC Mars Colony.  We need to show the world Mars is happening,  and that we must solve the challenges of tomorrow - today.  Even though we're only a group of 40 students, if we can show the world it's not that difficult to make an airlock, biosphere, or recycling system,  then people will be motivated to make it happen for real.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Tl;DR&lt;/strong&gt; &#xA;There are no perfect leaders currently involved in establishing a Martian Colony. SpaceX or NASA would be ideal, however they are busy in other projects with tight budgets. Mars One is idealistic, but so far hasn't performed to their own expectations. The Mars Society is a good mix of both research and development, but the challenges will be making this on a larger scale. People need to start making a difference themselves, with companies developing the technology if they want to see a future on the Red Planet.&lt;/p&gt;&#xA;" OwnerUserId="17412" LastEditorUserId="17412" LastEditDate="2016-10-27T09:42:29.447" LastActivityDate="2016-10-27T09:42:29.447" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18808" PostTypeId="1" CreationDate="2016-10-27T10:06:45.423" Score="1" ViewCount="158" Body="&lt;p&gt;What are the most desired features of microcontrollers used in space in on-board computers? Are there some special implementations, special functionality? Are features of some microcontrollers not accepted at all in space and thus some architectures are not taken into account?&lt;/p&gt;&#xA;" OwnerUserId="17414" LastActivityDate="2016-10-27T10:06:45.423" Title="What are the features of microcontrollers used in space?" Tags="|technology|flight-computer|" AnswerCount="0" CommentCount="3" ClosedDate="2016-10-27T14:27:44.867" ContentLicense="CC BY-SA 3.0" />
  <row Id="18809" PostTypeId="2" ParentId="18734" CreationDate="2016-10-27T11:07:04.293" Score="0" Body="&lt;p&gt;The first plan was the direct ascent. It avoided any rendezvous maneuvers in earth or moon orbit. But one very clever engineer found out that this would require a very heavy launcher, much larger than Saturn V. But his plan with the special moon lander and the moon orbit rendezvous maneuver made it possible with Saturn V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This man was John Houbolt: &lt;a href=&quot;https://en.wikipedia.org/wiki/John_Houbolt&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/John_Houbolt&lt;/a&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/content/john-c-houbolt-unsung-hero-of-the-apollo-program-dies-at-age-95/#.WBHa0_SGPFQ&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/content/john-c-houbolt-unsung-hero-of-the-apollo-program-dies-at-age-95/#.WBHa0_SGPFQ&lt;/a&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/centers/langley/news/factsheets/Rendezvous.html&quot; rel=&quot;nofollow&quot;&gt;http://www.nasa.gov/centers/langley/news/factsheets/Rendezvous.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without his plan a successful moon landing and return to earth before the end of the decade would not have been possible.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-10-27T11:07:04.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18810" PostTypeId="1" CreationDate="2016-10-27T13:13:57.773" Score="6" ViewCount="1212" Body="&lt;p&gt;How did JFK's promise of landing one man on the Moon grow into a whole series of landings of 12 men on 7 missions, with even more planned? Given the motivation of the program, shouldn't Apollo have been discontinued after the successful Apollo 11?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did this idea of continuous exploration of the Moon emerge as a de facto space policy for a decade or so? Especially by using the Apollo architecture which was optimized for going there once. Being uneconomical for a lasting human presence on the Moon. It seems to make neither engineering nor political sense.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2016-10-27T13:30:29.220" LastActivityDate="2016-10-27T20:20:11.623" Title="Why did the Apollo program become a MULTIPLE human landing program?" Tags="|the-moon|nasa|apollo-program|history|policy|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18811" PostTypeId="2" ParentId="18810" CreationDate="2016-10-27T13:27:49.957" Score="6" Body="&lt;p&gt;Short answer: it was always intended as a multiple landing program. &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program#Spacecraft_feasibility_studies&quot; rel=&quot;nofollow&quot;&gt;The Apollo program existed before Kennedy's moon speech&lt;/a&gt;. NASA was committed to space exploration, and moon landings -- and more than two and a half hours of science on the moon -- were an essential and obvious part of that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kennedy's challenge set a goal date, but that didn't have any particular formal or legal weight -- and even less so after Kennedy's death. There was no more reason to stop space exploration on July 25, 1969 than there was on November 23, 1963.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-10-27T20:20:11.623" LastActivityDate="2016-10-27T20:20:11.623" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18812" PostTypeId="2" ParentId="18810" CreationDate="2016-10-27T13:29:48.787" Score="17" Body="&lt;p&gt;Most of the cost in such a program is in the capital expenditures, the R&amp;amp;D, infrastructure, etc. Launching an additional Apollo mission is much less costly than a single mission would be. See &lt;a href=&quot;http://www.thespacereview.com/article/1579/1&quot; rel=&quot;noreferrer&quot;&gt;the inflation adjusted chart below&lt;/a&gt; for how the cost of Apollo varied with time. Note that the cost for Apollo peaked in 1968, the cost went down dramatically once the only cost was building additional missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rsY5I.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rsY5I.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, multiple landings were required for a number of missions:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;If there were any significant problems discovered early on, additional test flights might have been required.&lt;/li&gt;&#xA;&lt;li&gt;If Apollo 11 had met with the same fate as Apollo 13, no landing would have occurred.&lt;/li&gt;&#xA;&lt;li&gt;Doing it once could have been a fluke, 2 or more missions was really required to really demonstrate we had that capability. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In addition, NASA wished to accomplish more science than they could do for a single mission. They wanted to learn as much as they could, to take advantage of it as you said.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that there had to be backup missions planned, it only made sense to have those missions in progress. The lead time for building a new rocket was on the order of years, including the training of the crew. Without having a backup mission or two in line, it could have been as long as a year before another rocket could have been built to accomplish the mission. And once you have spent the money, you might as well launch the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After Apollo 11's mission was successful, many people questioned the continuing use of the Apollo program, and over time the public stopped watching the missions, stopped following the progress, and they almost became routine. Despite the fact that there was more hardware available, the Apollo program was cancelled, in large part due to what you mentioned (The goal was to send a single mission there)&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-10-27T13:37:47.247" LastActivityDate="2016-10-27T13:37:47.247" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18814" PostTypeId="1" AcceptedAnswerId="18820" CreationDate="2016-10-27T23:26:18.623" Score="6" ViewCount="2366" Body="&lt;p&gt;We all have seen basic water rockets, where pressurized air pushes out water creating downward force. Is it theoretically possible that a water rocket could create enough force, for a long enough duration, to reach orbit? If not is it possible that it could assist a solid or liquid booster engine?&lt;/p&gt;&#xA;" OwnerUserId="17166" LastActivityDate="2021-11-21T10:48:53.290" Title="Water Powered Rocket to Orbit?" Tags="|rockets|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18815" PostTypeId="2" ParentId="18789" CreationDate="2016-10-28T00:07:37.670" Score="27" Body="&lt;p&gt;In general there's a minimum size of rocket that can place nothing other than itself in orbit (because of the energy requirements to accelerate the rocket's own parts to orbital velocity while lifting it to orbital altitude--at  mostly the same time or problems occur).  There are diminishing returns as the payload sizes gets smaller and you asymptotically approach the smallest possible rocket given the technologies you apply to it, which leads to a couple preliminary conclusions:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Existing small launchers will end up being pretty good models of what the limits are&lt;/li&gt;&#xA;&lt;li&gt;Being a secondary payload is generally much cheaper than flying primary on a small rocket, so small rocket development hasn't taken off, so we haven't seen a lot of real exploration of the limits of small rocketry; you're usually stuck with thought experiments&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;That said, there have been a few papers on this, so I'll share what I've found, minus frankly distressing amounts of link rot.  I want to say I've seen a study at least once, too, but am having trouble finding one in my notes.  The motivations are typically putting up payloads the size of cubesats or smaller, e.g. &lt;a href=&quot;http://www.news.cornell.edu/stories/2014/04/cracker-sized-satellites-launch-orbit&quot; rel=&quot;noreferrer&quot;&gt;http://www.news.cornell.edu/stories/2014/04/cracker-sized-satellites-launch-orbit&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I came across a lot of this a few years ago while toying around with the idea of an orbital rocket that could be transported inside a typical 42-foot semi trailer.  That never became a design or paper, but maybe some day.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Dimotakis et. al concluded in 2000 that a 100lbm payload, which would be in the nanosatellite weight class, could be put into LEO for $300,000 per launch with an aircraft-launched two- or three- stage chemical rocket.  &lt;a href=&quot;https://fas.org/irp/agency/dod/jason/leo.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://fas.org/irp/agency/dod/jason/leo.pdf&lt;/a&gt;  (I think that conclusion's nonsense, but it sure looks like they got paid for that paper and there certainly were companies thinking of doing exactly that for a while, but I think the price of being a secondary payload has dropped since then...)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A Smithsonian Air&amp;amp;Space article primarily about Mars Sample Return concurs with the drag limitations stated in the comments:&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A terrestrial rocket has to push through a plug of air equivalent to a 30-foot column of water, and physics dictates that the smallest vehicle capable of moving all that atmospheric mass without paying a penalty in momentum is about 30 feet long.&lt;/p&gt;&#xA;&lt;p&gt;Read more: &lt;a href=&quot;http://www.airspacemag.com/space/the-one-pound-problem-718812/#zZldMZJXdaAdsFOL.99&quot; rel=&quot;noreferrer&quot;&gt;http://www.airspacemag.com/space/the-one-pound-problem-718812/#zZldMZJXdaAdsFOL.99&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;There are several places I've heard that the Japanese Lambda 4S is the &amp;quot;smallest ground based launch vehicle to put a satellite into orbit&amp;quot; (quoted from the link soon to follow).  &lt;a href=&quot;http://orbitalaspirations.blogspot.com/2011/10/japanese-lambda-4s-launcher.html&quot; rel=&quot;noreferrer&quot;&gt;http://orbitalaspirations.blogspot.com/2011/10/japanese-lambda-4s-launcher.html&lt;/a&gt; does a good job breaking down the rocket equation math for the launcher.  I'd imagine its numbers could be beaten by actually having guidance (which it didn't!) and swapping out the solid rockets for more efficient motors after the first stage, but there probably isn't a ton of margin to win.  She's about 10 tons and 55 feet long.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Vector Space Systems is working on Vector-R, a 50kg to LEO launcher, at 12m tall x 1.2m diameter, 5000 kg design dimensions, though those figures change from page to page in the site...&#xA;&lt;a href=&quot;http://vectorspacesystems.com/technology-4&quot; rel=&quot;noreferrer&quot;&gt;http://vectorspacesystems.com/technology-4&lt;/a&gt;&#xA;&lt;a href=&quot;http://vectorspacesystems.com/technology-5&quot; rel=&quot;noreferrer&quot;&gt;http://vectorspacesystems.com/technology-5&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Rocket Labs' Electron is 16x1.2m for 150kg to a 500km sun-synchronous orbit.  Somewhat more payload and to a higher orbit,  similar-ish size  &lt;a href=&quot;https://www.rocketlabusa.com/&quot; rel=&quot;noreferrer&quot;&gt;https://www.rocketlabusa.com/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;JAXA is planning to launch the SS-520-4, 9.54m x 0.52m, 2600kg at launch, lofting 3kg of 3U cubesat to (for TRICOM-1) an orbit of 180kmx1500km.  All per &lt;a href=&quot;http://spaceflight101.com/ss-520-4-smallest-orbital-rocket-set-for-launch/&quot; rel=&quot;noreferrer&quot;&gt;http://spaceflight101.com/ss-520-4-smallest-orbital-rocket-set-for-launch/&lt;/a&gt;  This is based on a sounding rocket, and I'd expect any orbital rockets at this size to be similar--sounding rockets with very small upper stages added to them.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Astra's &lt;a href=&quot;https://en.wikipedia.org/wiki/Rockets_by_Astra#Rocket_3&quot; rel=&quot;noreferrer&quot;&gt;Rocket 3&lt;/a&gt; family is 11.6m long, placing it solidly in the same class as the other rockets listed here.  The SS-520-4 beat it to making a successful orbital launch (and is also smaller), and none of the Rocket 3 launches have been entirely successful as of early 2021, but I think it deserves to be listed.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11262" LastEditorUserId="11262" LastEditDate="2021-03-03T11:01:53.060" LastActivityDate="2021-03-03T11:01:53.060" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18816" PostTypeId="1" AcceptedAnswerId="18817" CreationDate="2016-10-28T01:01:17.493" Score="10" ViewCount="5620" Body="&lt;p&gt;When you ask about rotating artificial gravity (yes, yes, technically radial acceleration), most answers boil down to &quot;you'd need a 200m diameter spacecraft, and we can't build that yet&quot;&lt;br&gt;&#xA;&lt;img src=&quot;https://www.nasa.gov/sites/default/files/images/226363main_2001_station_t_full.jpg&quot; alt=&quot;2001 torus&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, of course, why would you need the entire circle to be a continuous space station? Just tie two modules together with a cable and extend, right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are the engineering problems just too difficult and numerous to even consider that? I can see a few already:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Moving people between the modules, or to a stationary part of the station, would require an EVA. Not only that, but an EVA from a rapidly moving module.&lt;/li&gt;&#xA;&lt;li&gt;Making orbital or attitude adjustments would be much more complicated.&lt;/li&gt;&#xA;&lt;li&gt;Docking to visiting spacecraft would require a stationary module or stopping the rotation entirely.&lt;/li&gt;&#xA;&lt;li&gt;Moving resources between the modules and/or a stationary section would require long cables and pipes, and possibly seals between moving pieces.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Of course, one can think of solutions to all of these. Most can be addressed by temporarily halting the rotation and maybe winching the modules back together. But none of the solutions seem simple. Is it all just too much to even contemplate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;:&lt;br&gt;&#xA;I didn't think I needed to, but I guess I'll clarify the motivation. I've read plenty of people questioning whether Mars gravity (0.38g) is sufficient to ameliorate the many negative effects of long-term weightlessness. It seems reckless to wait until we're there, in the middle of an 18-month mission, to find out.&lt;/p&gt;&#xA;" OwnerUserId="11410" LastEditorUserId="-1" LastEditDate="2017-03-10T09:42:49.863" LastActivityDate="2023-02-08T15:36:36.550" Title="Why is tethered artificial gravity hardly ever considered?" Tags="|space-station|artificial-gravity|" AnswerCount="5" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="18817" PostTypeId="2" ParentId="18816" CreationDate="2016-10-28T01:44:56.090" Score="10" Body="&lt;p&gt;Tethering two habitable modules together splits your already cramped living space into two smaller spaces that can't be easily traversed; this means a lot of equipment like life support and bathroom facilities need to be duplicated (presumably power and comms could be on an umbilical alongside the tether.) That's an unacceptable efficiency/mass hit, so let's reject it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could put an inert mass on a long tether; this would involve launching a lot of dead weight, which is also unattractive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gemini XI did some tether experiments with an Agena spacecraft, with some odd &quot;jump-rope&quot; oscillations and jerkiness; I don't know if that's an insurmountable problem. The Gemini experiment achieved only milli-gee levels of acceleration. &lt;a href=&quot;http://www.spacesafetymagazine.com/space-exploration/gemini/m-equals-1-all-up-mission-gemini-xi-part-2/&quot; rel=&quot;noreferrer&quot;&gt;http://www.spacesafetymagazine.com/space-exploration/gemini/m-equals-1-all-up-mission-gemini-xi-part-2/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd have to reel in the weight and de-spin every time you wanted to dock a visiting spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, one of the most significant purposes of having a space station is to do experiments in zero g. Spinning the station defeats that purpose.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-10-28T01:44:56.090" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18819" PostTypeId="2" ParentId="18816" CreationDate="2016-10-28T07:56:47.530" Score="8" Body="&lt;p&gt;There's no need for simulated gravity in human spaceflight the next several decades. It might be of interest to basic biological research, but it's up to them to buy a biological research station in LEO, it is of no concern for agencies and companies planning human space flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It will be some time before humans go further than Mars, and it only takes a 6-9 months economical Hohmann transfer to get to Mars. Hundreds of astronauts have spent that much time in microgravity and enjoyed it. No one has ever been hurt by microgravity, and reduced gravity isn't exactly epidemic on Earth, so simulated gravity has very low priority in both the space and the medical communities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Microgravity has the benefit of increasing human utilization of spaceship space. One can squeeze more people into a weightless room. It helps reducing the size and mass of a spaceship. Structures such as solar arrays and antennas can be made lighter and more delicate in microgravity. A spinning spacecraft suffers a mass penalty throughout the design. Also, gravity hurts. It breaks backs, it kills people falling, you drop things on your toes. It is a blessing for human health to get rid of the great cause of accidents, wear and toil which gravity is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reduced gravity, such as 16% or 38% as on the Moon and Mars, should take care of several of the problems experienced in microgravity. It should be enough to somewhat normalize the fluid pressure in the upper body, to give load on muscles and skeleton, to greatly increase the effect of exercise, to make dust fall down instead of floating in everybody's faces (reducing the need and noise of ventilation), to deactivate microbes which in microgravity seem to react as if they were buoyant in water which activates them, to sit or sleep without strapping down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The resources for a  simulated gravity station are better needed on a real mission to Mars to lower real risks of launch, landing and any kind of hardware or software failure. The best place to find out about the effects of reduced gravity is on the Moon and on Mars! We don't need to recreate that which we already have got handy by nature. Gravitational effects interact with radiational, chemical and psychological effects. No examination could be more complete than actually spending a long time on the Moon or Mars. If people can stay on the Moon for a year, they certainly can stay on Mars with more than double the gravity for a year too. And actually, people have spent a year in microgravity, so there's the question about what purpose simulated gravity would have.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-10-28T08:13:18.330" LastActivityDate="2016-10-28T08:13:18.330" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18820" PostTypeId="2" ParentId="18814" CreationDate="2016-10-28T08:01:29.313" Score="11" Body="&lt;p&gt;A water bottle rocket is similar to &lt;a href=&quot;https://space.stackexchange.com/questions/8809/pumped-propellant-propulsion-is-it-viable/8814#8814&quot;&gt;this question&lt;/a&gt; about using a pump to expel a liquid at high speed. We concluded that the highest possible exhaust velocity with this strategy is very low (1.3 km/s, about 1/3 of the Shuttle main engines).&lt;/p&gt;&#xA;&lt;p&gt;A water bottle that stores water plus air at high pressure is even less efficient, because to contain the pressure you need a thick, heavy tank (my estimate: 60 cm thick steel walls for 6000 bar).&lt;/p&gt;&#xA;&lt;p&gt;So how about using ordinary Coke bottles (chosen because they're common and among the strongest bottles available in the supermarket)? We'd have to use a lot of them in parallel.&#xA;Randall Munroe &lt;a href=&quot;https://what-if.xkcd.com/24/&quot; rel=&quot;noreferrer&quot;&gt;did the physics on a similar concept&lt;/a&gt;: using model rocket engines to launch into space.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2EZB9.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2EZB9.png&quot; alt=&quot;Rocket pyramid&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;If you try to produce an orbital rocket using the same design math we used for the suborbital rocket, it spits out a description of a pancake-shaped mountain of model rocket engines over a mile wide. It would taper to a 10-meter-high spire in the center and would weigh about as much as the Great Pyramid.&lt;/p&gt;&#xA;&lt;p&gt;Not only would this vehicle never get out of the atmosphere, it would probably not stand up under its own weight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So, with enough bottles to provide the thrust we need, we have an unstable structure. This is not a feasible way to get to orbit.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-10-28T08:01:29.313" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18821" PostTypeId="2" ParentId="18814" CreationDate="2016-10-28T08:34:31.517" Score="7" Body="&lt;p&gt;There have been hot water rockets used for crash tests of cars to accelerate on a short distance. The water in the tank was heated electrically to a very high temperature and pressure. When the valve to the nozzle was opened, the water turned into steam and left the nozzle with high speed. This principle is far more effective than just water and air under high pressure in a tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These type of rocket is very simple, only one tank, no turbo pumps, only one valve, no combustion chamber and no need to cool the nozzle walls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the tank must be build for very high pressure at high temperature and will be very heavy. Some tank isolation might be necessary to limit the heat loss on the start pad and during the flight. The temperature in the nozzle will be much lower compared with a LH2/LOX rocket and therefore also the specific impulse too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have a rocket with a very bad ratio of structural mass to fuel and a low exhaust velocity. To calculate the necessary number of stages, we need a good guess of structural mass and exhaust velocity, I would think about 30 % and some 300 m/s, but we need an educated guess I can't give. Here &lt;a href=&quot;https://en.wikipedia.org/wiki/Steam_rocket&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Steam_rocket&lt;/a&gt; is a value for the Isp, 195 s, not much compared with 448 s of a modern hydrogen rocket engine. Here &lt;a href=&quot;http://www.tecaeromex.com/ingles/vapori.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.tecaeromex.com/ingles/vapori.html&lt;/a&gt; I found an exhaust velocity of 457 m/s. If somebody may confirm my guess of 30 % structural mass, we may calculate the number of stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the meantime I calculated the number of stages for 8 km/s, 10 % payload, 30 % structural mass and 60 % fuel (hot water), but the result is absurd: 19 to 20 stages. Even with 20 % structural mass and 70 % water we need 15 stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The start weight of a Saturn V was 2,935 t, the fourth stage of the hot water rocket for a payload of 1 t only (counted from top, not from bottom as usual) would have a weight of 10,000 t with fuel.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="13126" LastEditDate="2016-11-14T15:38:01.433" LastActivityDate="2016-11-14T15:38:01.433" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18822" PostTypeId="1" AcceptedAnswerId="18828" CreationDate="2016-10-28T09:57:33.780" Score="3" ViewCount="246" Body="&lt;p&gt;I've developed an n-body orbital mechanics simulator that allows you to explore the solar system with a propulsion system of your choice (Saturn V, Falcon 9 and so on). The orbital state vectors that I use for simulating the orbits of the planets and their moons are obtained for JPL, and it all works pretty well provided you employ a time step that is appropriate for the system being simulated. You can perform a trans-lunar injection, hohmann transfer to get to Mars and so on. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I've heard that a planet has been discovered around Proxima Centauri and not knowing any better, I would of course like to simulate a mission to this planet with a propulsion system of immense power that I have dreamt up with little concern for what can actually be achieved (toying with the idea of using the sun for a gravitational sling-shot, although I'm way too ignorant when it comes to orbital mechanics to know if this would actually work out, disregarding the fact that no known materials would survive the heat and tidal forces a spacecraft would experience in such close proximity to the sun). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem, though, is that I don't have any state vectors for the centauri system... You know; x, y, z, vx, vy, vz. Nor do I have state vectors for the path the bary center of our solar system takes through interstellar space. Any chance somebody could point me to a source containing these vectors or alternatively a way to calculate them from the stellar catalogues that are out there (haven't been able to find such catalogues with vector data, but then again I might have looked in the wrong place, in which case I apologise for wasting your time)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cheerio!&lt;/p&gt;&#xA;" OwnerUserId="17423" LastActivityDate="2016-10-28T14:04:54.083" Title="Orbital State Vectors for the Centauri System" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18823" PostTypeId="1" AcceptedAnswerId="18824" CreationDate="2016-10-28T09:58:11.173" Score="11" ViewCount="2591" Body="&lt;p&gt;In this article about how the &lt;a href=&quot;http://www.universityherald.com/articles/45558/20161022/new-thesis-shows-spacex-falcon-9-explosion-affects-iridium.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX anomaly will affect Iridium&lt;/a&gt; it says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In its &lt;a href=&quot;http://www.universityherald.com/articles/45558/20161022/new-thesis-shows-spacex-falcon-9-explosion-affects-iridium.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;official website&lt;/a&gt;, Iridium describes itself as &quot;the world's only truly global mobile satellite communications company.&quot; It offers global voice and data communications coverage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is Iridium the only mobile satellite communications network that has polar orbits, and therefore has line-of-sight service to satellite telephones at the poles?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quote and screen shots from: &lt;a href=&quot;https://www.iridium.com/company/companyprofile&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.iridium.com/company/companyprofile&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Reaching across land, sea, and air, including the polar regions&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rk9eP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rk9eP.png&quot; alt=&quot;Company Profile: The world&amp;#39;s only truly global mobile satellite communications company&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jErcn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jErcn.png&quot; alt=&quot;Image of a satellite&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="268" LastEditDate="2016-10-28T13:56:38.053" LastActivityDate="2016-10-29T18:44:59.523" Title="What (actually) makes Iridium &quot;the world's only truly global mobile satellite communications company&quot;?" Tags="|communication|communication-satellite|iridium|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18824" PostTypeId="2" ParentId="18823" CreationDate="2016-10-28T10:17:46.790" Score="17" Body="&lt;p&gt;As of today (October 2016), Iridium is the only public satcomm provider using a constellation of low earth orbit satellites with high inclination orbits to provide a service with global coverage. You can compare coverage easily on &lt;a href=&quot;https://www.globaltelesat.co.uk/coverage/#tabscoverage1&quot; rel=&quot;nofollow noreferrer&quot;&gt;this website&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dk3nF.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dk3nF.gif&quot; alt=&quot;Iridium constellation&quot;&gt;&lt;/a&gt;&#xA;(this image is from &lt;a href=&quot;https://commons.m.wikimedia.org/wiki/File:Iridium_Coverage_Animation.gif#mw-jump-to-license&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inmarsat and Thuraya in contrast use geostationary satellites, Thuraya uses spot beams to cover specific regions, while Inmarsat has wide beams. Globalstar uses low earth orbits, but the inclination of 52° does not allow to reach polar regions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Military may have additional non-public networks. A non-public LEO/MEO constellation would not go unnoticed, as more than 30  spacecraft are needed depending on orbital height (Iridium uses 66 plus spares), so all LEO and MEO constellations tend to be known. Iridium is the only such constellation with continuous coverage of polar regions. This makes it unlikely, that military has a non-public network with coverage and latency comparable to Iridium.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17004" LastEditDate="2016-10-29T18:44:59.523" LastActivityDate="2016-10-29T18:44:59.523" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18825" PostTypeId="2" ParentId="18802" CreationDate="2016-10-28T11:55:57.977" Score="14" Body="&lt;p&gt;The specific entry of the table appears to be the &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/1993index/IEPC1993-221.pdf&quot;&gt;HIPARC-R hydrogen arcjet thruster&lt;/a&gt; developed by &lt;a href=&quot;http://www.irs.uni-stuttgart.de/index.html&quot;&gt;Space Travel Institute&lt;/a&gt; of University of Stuttgart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3fyz3.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3fyz3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concept of Arcjets is to use the propellant as conductor between two electrodes, creating intense electric arc, and exciting the propellant into superheated plasma. This allows to infuse it with more energy than a chemical reaction would provide - turning electricity into heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there is no inherent limit on how much electricity one can push through the plasma, there is no theoretical limit on the energy of the gas and as result, the exhaust velocity and specific impulse. The practical limit is of course of engineering nature - the electricity won't heat only the plasma, and superheated plasma is very aggressive against any structures it passes through. This both necessitates cooling of the whole thruster and limits the applicable energies to levels at which the thruster won't sustain critical damage from operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless, the energies used are massive - HIPARC-R operates around 100 kilowatt, so even disregarding the scale of the thruster itself comparing to cubesats (about 3U)  or the propellant forbidden on cubesats there is no way to keep it powered with a cubesat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Travel Institute developed also &lt;a href=&quot;http://www.irs.uni-stuttgart.de/forschung/elektrische_raumfahrtantriebe/triebwerke/elektrotherm-tw/therm-lichtbogen-tw/index.html&quot;&gt;other, smaller arcjets&lt;/a&gt;. For example, &lt;a href=&quot;http://www.irs.uni-stuttgart.de/forschung/elektrische_raumfahrtantriebe/triebwerke/elektrotherm-tw/therm-lichtbogen-tw/tlt-atos.html&quot;&gt;ATOS&lt;/a&gt; operating on 750W, with 480s of specific impulse, 480 gram of own weight, 24 mg/s mass flow, 115 mN of thrust, and running on ammonia, might be applicable on 6U cubesats with extendable solar panels, &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2016-10-28T13:58:56.590" LastActivityDate="2016-10-28T13:58:56.590" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18826" PostTypeId="2" ParentId="18823" CreationDate="2016-10-28T11:59:34.617" Score="3" Body="&lt;p&gt;As explained on the Wikipedia page (&lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&quot; rel=&quot;nofollow&quot;&gt;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&lt;/a&gt;), Iridium constellation uses polar orbits. Those orbits are intended for a full &quot;north-south&quot; coverage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The downside of the Polar orbit is that each orbit has a shift in longitude. This issue is corrected by the use of 6 different polar orbits (equidistant) with each individual coverage overlapping the next one (see the GIF on the Wikipedia page).&lt;/p&gt;&#xA;" OwnerUserId="17308" LastActivityDate="2016-10-28T11:59:34.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18827" PostTypeId="2" ParentId="4753" CreationDate="2016-10-28T13:51:11.420" Score="6" Body="&lt;p&gt;There should be. It needs a huge impact on Earth to send material all the way to Mars but we get them occasionally. You need something of the order of the Chicxulub impact 66 million years ago. The debris would take about a hundred years, for the first material to get there and the material would continue to arrive on Mars for up to 20 million years from that date. So probably the youngest meteorite from Earth would be over 40 million years old.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though you didn't ask it, but to follow up on the other answers, it is very hard to get meteorites from Venus to Earth. The problem is the thick atmosphere. An impact like the Chicxulub impact would send meteorites to Earth if its atmosphere was similar to Earth's. But with its thick atmosphere, it would need to be much larger. And from the impact history on the Moon, Mercury and Mars then the larger 100 km scale impacts all date back to well over 3 billion years ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However way back then, then Venus might also have had a thin atmosphere. So there could be Venusian material on Earth, a bit of it, but it would be impossible to recognize after all that time and weathering. But if this is right, then it's probably there on the Moon, and much easier to spot as it wouldn't be weathered, and there might be Venusian meteorites on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There could also be Mercurian meteorites on Earth - and Mars also. &lt;a href=&quot;https://arxiv.org/ftp/arxiv/papers/0801/0801.4038.pdf&quot; rel=&quot;noreferrer&quot;&gt;Mercurian Impact Ejecta: Meteorites and Mantle&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the best place to look for Earth meteorites is on the Moon. If they landed in frozen areas of the Moon, they may preserve organics from millions or even billions of years. They could also preserve fossils as fossil diatoms are still recognizable, and the smallest ones intact after a simulated impact on the Moon. There may be as much as 200 kilograms of material from Earth per square kilometer of the lunar surface. Details see my &lt;a href=&quot;http://robertinventor.com/booklets/Online-Case-for-Moon.htm#astrobiology_moon&quot; rel=&quot;noreferrer&quot;&gt;Record keeper of inner solar system (including astrobiology and early life on Earth)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="3038" LastEditDate="2016-10-28T20:38:18.703" LastActivityDate="2016-10-28T20:38:18.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18828" PostTypeId="2" ParentId="18822" CreationDate="2016-10-28T14:04:54.083" Score="4" Body="&lt;p&gt;You need a &lt;em&gt;designator&lt;/em&gt; first for the celestial body. There are multiple alamacs with different designators, so you may want to note all designators for Proxima Centauri that you can find. &quot;alpha Centauri C&quot; (the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Flamsteed_designation&quot; rel=&quot;nofollow&quot;&gt;Flamsteedt Bayer&lt;/a&gt; designator) is a good start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With these designators, search in public almanacs. You will find your star in &lt;a href=&quot;http://asa.usno.navy.mil/static/files/2016/brightstar_2016.txt&quot; rel=&quot;nofollow&quot;&gt;bright stars&lt;/a&gt; in the navies &lt;a href=&quot;http://asa.usno.navy.mil/SecH/Section_H.html&quot; rel=&quot;nofollow&quot;&gt;astronomical almanac&lt;/a&gt; or in &lt;a href=&quot;http://simbad.u-strasbg.fr/simbad/&quot; rel=&quot;nofollow&quot;&gt;SIMBAD&lt;/a&gt;. Wikipedia also has links to these databases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will typically find &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Right_ascension&quot; rel=&quot;nofollow&quot;&gt;right ascension&lt;/a&gt; and &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Declination&quot; rel=&quot;nofollow&quot;&gt;declination&lt;/a&gt; in the &lt;a href=&quot;http://aa.usno.navy.mil/faq/docs/ICRS_doc.php&quot; rel=&quot;nofollow&quot;&gt;ICRS reference frame&lt;/a&gt; there, together with values for &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Proper_motion&quot; rel=&quot;nofollow&quot;&gt;proper motion&lt;/a&gt; and radial velocity. These are polar coordinates and rates, you will need to convert them to euclidian coordinates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All these values are not known with a precision that would allow for a travel without (many) midcourse observations and corrections, a problem that already exists for missions to smaller bodies in our solar system (confer &quot;approach phase&quot; of &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Rosetta&quot; rel=&quot;nofollow&quot;&gt;Rosetta mission&lt;/a&gt;). The &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Gaia_(spacecraft)&quot; rel=&quot;nofollow&quot;&gt;Gaia mission&lt;/a&gt; will improve precision a little bit, but your state vector will include a good amount of guessing.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastActivityDate="2016-10-28T14:04:54.083" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18830" PostTypeId="2" ParentId="13399" CreationDate="2016-10-28T17:00:57.390" Score="2" Body="&lt;p&gt;If such a complex system like a multi stage rocket should have a very low accident rate of 2 to 4 per million, it must be built from very reliable parts. But if there are some thousands of parts, each one should have a reliability of less than 1 failure per billion. But it is impossible to proof such low failure rates by testing. If you do one test per minute  during the full year, you would need more than 1900 years for a billion of tests.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2016-10-28T17:00:57.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18831" PostTypeId="1" CreationDate="2016-10-28T17:47:27.650" Score="2" ViewCount="442" Body="&lt;p&gt;What might happen to the lunar atmosphere after a time of major human presence or activity on Earth's natural satellite? After hundreds of touchdowns and flyoffs and disturbances to environment?&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="12102" LastEditDate="2016-10-29T00:31:52.397" LastActivityDate="2017-04-02T01:01:40.830" Title="Moon atmosphere after long human presence" Tags="|the-moon|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18832" PostTypeId="1" AcceptedAnswerId="18871" CreationDate="2016-10-28T18:19:15.473" Score="6" ViewCount="1477" Body="&lt;p&gt;We've covered &lt;a href=&quot;https://space.stackexchange.com/questions/18593/why-does-the-be-4-use-methane-as-fuel&quot;&gt;why the BE-4 uses methane as fuel&lt;/a&gt;; what I want to know is, why did Blue Origin go with hydrogen for the BE-3? Liquid hydrogen is notoriously hard to work with; it seems like an odd choice for a new company's engine. It also doesn't seem like BO is taking advantage of hydrogen's biggest advantage, its great performance as an upper-stage fuel. So why'd they go with hydrogen instead of kerosene for the New Shepard's engine?&lt;/p&gt;&#xA;" OwnerUserId="13208" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-03-08T02:43:53.893" Title="Why does the BE-3 use hydrogen as fuel?" Tags="|rockets|fuel|liquid-fuel|hydrogen|blue-origin|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18833" PostTypeId="2" ParentId="9244" CreationDate="2016-10-28T19:04:47.690" Score="1" Body="&lt;p&gt;A common low A/high E material is silver teflon.  I don't know it's official TRL, but it's been around for quite a while.  The first link below is from 1973.  There are some issues with EOL (End of Life) performance.  Unfortunately, I don't know the details of the degradation.  You can find a good bit of information by searching the web for &quot;silver Teflon emissivity&quot;.  The links below are a start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Silver-Teflon:&#xA;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740012426.pdf&quot; rel=&quot;nofollow&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740012426.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermal control surfaces, including Silver Teflon:&#xA;&lt;a href=&quot;http://www.qrg.northwestern.edu/projects/vss/docs/thermal/3-what-materials-are-used-for-thermal-control.html&quot; rel=&quot;nofollow&quot;&gt;http://www.qrg.northwestern.edu/projects/vss/docs/thermal/3-what-materials-are-used-for-thermal-control.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermo-Optical Properties:&#xA;&lt;a href=&quot;http://webserver.dmt.upm.es/~isidoro/dat1/Thermooptical.pdf&quot; rel=&quot;nofollow&quot;&gt;http://webserver.dmt.upm.es/~isidoro/dat1/Thermooptical.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17341" LastActivityDate="2016-10-28T19:04:47.690" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18834" PostTypeId="2" ParentId="14739" CreationDate="2016-10-29T00:44:58.567" Score="3" Body="&lt;p&gt;Space service volume (i.e. Altitudes above GPS) is a region of interest from all constellation providers. But the GNSS antenna main lobe coverage is limitedly very less. Efforts are being made in order to regulate the power levels available over the limb. As for as GPS is concerned the power levels will be moderated from the next generation of satellites GPS III &lt;a href=&quot;http://www.gps.gov/governance/advisory/meetings/2016-05/parker.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;update on GPS modernisation&lt;/a&gt;and it will be included Signal specification (targeting high altitude space users). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These &lt;a href=&quot;https://space.stackexchange.com/a/12863/8501&quot;&gt;plots&lt;/a&gt;  are for GPS, but for other systems the antenna patterns are not available in detail. Only the main lobe antenna beam width angle is mentioned in some documents for other GNSS constellations.&lt;a href=&quot;http://www.unoosa.org/oosa/en/ourwork/icg/meetings/icg-10/presentations.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;UNOOSA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When considering different frequency band to enable multi-GNSS navigation, L5 (frequency band from all constellation) has much wider range of antenna coverage than the primary FREQUENCY bands (such as L1) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Definitely CIS lunar space is region of interest but the power levels beyond GEO orbit is significantly low (-185dB @GEO) and this calls for the receiver capable to efficiently acquire and track weaker signals. NASA has already developed one receiver and flown it on the MMS mission @76,000 km. The results from the mission suggest that side lobes are more important above GPS constellation &lt;a href=&quot;http://hdl.handle.net/2060/20160001693&quot; rel=&quot;nofollow noreferrer&quot;&gt;GPS Navigation Above 76,000 km for the MMS Mission&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also few feasibility studies on using GNSS navigation for Lunar transfer orbit Recent ones include &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576515002520&quot; rel=&quot;nofollow noreferrer&quot;&gt;Feasibility study of GNSS as navigation system to reach the Moon&lt;/a&gt;, &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S027311771630045X&quot; rel=&quot;nofollow noreferrer&quot;&gt;Design of a High Sensitivity GNSS receiver for Lunar missions&lt;/a&gt;. There are several other studies which will give you idea on the requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope it answers your question.&lt;/p&gt;&#xA;" OwnerUserId="8501" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-10-29T00:44:58.567" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18837" PostTypeId="2" ParentId="18802" CreationDate="2016-10-29T03:18:13.533" Score="6" Body="&lt;p&gt;This answer is supplementary information I've &lt;a href=&quot;https://space.stackexchange.com/questions/18802/#comment49369_18806&quot;&gt;collected&lt;/a&gt; in the past few days. I've accepted the other answer because it's excellent and identifies the actual source of the &amp;quot;1600&amp;quot; mystery value.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; Cubesat-Friendly Developments are discussed at the end.&lt;/p&gt;&#xA;&lt;h2&gt;The Principle&lt;/h2&gt;&#xA;&lt;p&gt;The idea is to heat a propellant using electrical current instead of chemical reaction producing even higher temperatures and therefore higher specific impulse.&lt;/p&gt;&#xA;&lt;p&gt;For a fixed temperature, higher thrust velocity comes from lower mass species, so hydrogen-rich propellants are desireable. Hydrazine is populare both because of its hydrogen content and good space experience and often pre-existing availability within spacecraft. Ammonia is another option.&lt;/p&gt;&#xA;&lt;p&gt;The random kinetic energy of the hot species is converted to cold, directed kinetic energy upon expansion. For a given temperature, lower mass results in higher veolocity.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ E = \frac{1}{2}m v^2$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$v = \sqrt{2E/m}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Propellant at a rate of the order of 100 mg/sec passes through a restriction of the order of 1 millimeter in diameter where an electrical current of order 10-100 Amperes passes through the dense, almost completely ionized gas. The ohmic heating of the plasma through electron-ion (and electron-atom) collisions transfers roughly a third of the power into direct heating of the ions to temperatures of 10,000 to 20,000 K. The hot plasma then expands, cools, and passes out the nozzle as directed thrust.&lt;/p&gt;&#xA;&lt;p&gt;Because of the complex interaction of the plasma, current, and heat flow, a &amp;quot;sweet spot&amp;quot; exits in the 1kw to 30 kW range, and this is where most of the work has been done. The engine requires significant (and heavy) power conditioning and thermal isolation from the spacecraft, since more than half of the electrical power results in heat of the engine components.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8wBuM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8wBuM.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;image above and text below:&lt;/strong&gt; from &lt;em&gt;&lt;a href=&quot;http://link.springer.com/article/10.1007/s11090-015-9610-4&quot; rel=&quot;nofollow noreferrer&quot;&gt;Status and Prospects on Nonequilibrium Modeling of High Velocity Plasma Flow in an Arcjet Thruster&lt;/a&gt;&lt;/em&gt;, Hai-Xing Wang, Su-Rong Sun, Wei-Ping Sun, Plasma Chem Plasma Process (2015) 35:543–564 DOI 10.1007/s11090-015-9610-4&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;The key physics of the constricted arc discharge in arcjet are depicted in Fig. 1. An arc is stuck between the central, conical-tipped cathode and the coaxial, nozzle shaped anode. Working gas, injected with high swirl velocity near the cathode tip, passes through the constrictor region and is ohmically heated by the arc. The energy transferred to the gas is thought to result predominantly from electron–ion or electron–neutral collisions as elec- trons are the dominant current carriers. Extremely small constrictor size, extremely high gas velocity at the nozzle exit and operation at relatively low arc current are a few of the primary features of these kinds of thrusters.&lt;/p&gt;&#xA;&lt;p&gt;Typical low power arcjets have conical converging–diverging nozzle with constrictor diameter on the order of 0.5 mm, expansion half angle of 20°, and exit diameter of 3.5 mm [8–10]. The physical characteristics of the arcjet flow field vary from a nearly fully ionized plasma with temperature in excess of 20,000 K near the cathode tip to a relatively cold plasma (1,000–2,000 K) at the anode wall. Moreover, velocities vary from approximately 10 km/s on centerline to zero at the wall.*&amp;quot; from &lt;a href=&quot;http://link.springer.com/article/10.1007/s11090-015-9610-4&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (paywalled)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There are many examples of arcjet thrusters in the 500 to 2000 Watt range so I'll talk about those first.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oYwAc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oYwAc.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;http://arc.aiaa.org/doi/pdf/10.2514/2.5403&quot; rel=&quot;nofollow noreferrer&quot;&gt;Performance Computation of a Low-Power Hydrogen Arcjet&lt;/a&gt; Kazuhisa Fujita &amp;amp; Yoshihiro Arakawa, J. Propulsion. and Power v15, n 1, Jan-Feb 1999 (paywalled)&lt;/p&gt;&#xA;&lt;p&gt;NASA started the Arcjet Thruster Research and Technology (ATRT) program in 1984. &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870015092.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;For example&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/h0jah.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h0jah.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; illustration of the arcjet principle from &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870015092.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Vxsxs.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vxsxs.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Illustration of an arcjet start-up process, including spiral-fed propellant. Figure 3-6 &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900004962.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The Wikipedia article &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spacecraft_with_electric_propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;List of spacecraft with electric propulsion&lt;/a&gt; currently lists a few spacecraft that carried arcjet thrusters. The Telstar 401 is the first commercial use, launched in 1993. Four arcjet thrusters were used for North-South and East-West station keeping in Geosynchronous orbit.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Telstar 401,                MR-508, Hydrazine, Comms (AS-7000)&#xA;Telstar 402R (Telstar 4),   MR-508, Hydrazine, Comms (AS-7000)&#xA;A2100,                      MR-510, Hydrazine, Comms&#xA;ARGOS (P91-1),              ESEX,   Ammonia,   Experimental Military &#xA;AMSAT-Phase 3-D (OSCAR-40), ATOS,   Ammonia,   OSCAR Sat cold gas mode&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S1270963810001562&quot; rel=&quot;nofollow noreferrer&quot;&gt;Performance and preliminary life test of a low power hydrazine engineering design model arcjet&lt;/a&gt; (Tang et al. 2015, Aerospace Science and Technology, v 15, n 7, Oct–Nov 2011, 577–588)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Telstar 401 (a geostationary communication satellite of Lockheed Martin Corporation) equipped with PRIMEX Aerospace Company’s (originally Rocket Research Company, RRC) MR-508 hy- drazine arcjet propulsion system was successfully launched in 1993, which is the first application of thermal arcjet propulsion systems (11). &lt;strong&gt;From then on, hydrazine arcjet propulsion systems have operated successfully on more than 29 spacecrafts&lt;/strong&gt;, showing substantial performance and reliability (12). (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;MR-508, -509, -510 series Hydrazine Arcjets&lt;/h2&gt;&#xA;&lt;p&gt;The MR-508, -509, -510 series of Hydrazine arcjet thrusters can be seen at &lt;a href=&quot;http://www.rocket.com/propulsion-systems/electric-propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aerojet Rocketdyne&lt;/a&gt; - here is a set of four MR-510 and the &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/1993index/IEPC1993-048.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Power Conditioning Unit&lt;/a&gt; (PCU). They provided an Isp above 500 seconds using hydrazine, and therefore provided more efficient use of the hydrazine mass.&lt;/p&gt;&#xA;&lt;p&gt;However, in this power range, the power supply and control electronics can be much heavier than the engine itself!&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0CcbX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0CcbX.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/1997index/7082.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flight Qualification of the 2.2 kW MR-510 Hydrazine Arcjet System&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/ge-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARC-1, ARC-2, ARC-3 (use Lockheed Martin A2100 bus)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h2&gt;ESEX Ammonia Arcjet&lt;/h2&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/argos.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARGOS&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/e/esexarcjet.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESEX Arcjet (ARGOS)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/a388006.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESEX Arcjet report (ARGOS)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA406095&quot; rel=&quot;nofollow noreferrer&quot;&gt;AIAA 99-2706, An Overview of the On-Orbit Results from the ESEX Flight Experiment&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EINxZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EINxZ.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; ESEX Arcjet from &lt;a href=&quot;http://www.astronautix.com/e/esexarcjet.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H1rd3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H1rd3.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Propellant supply system for ESEX (Electric Propulsion Space Experiment) from &lt;a href=&quot;http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA406095&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; on &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/argos#4rHHF1Herb&quot; rel=&quot;nofollow noreferrer&quot;&gt;ARGOS&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;h2&gt;Cubesat-Friendly Developments&lt;/h2&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2003index/0078-0303iepc-full.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Discharge Characteristics of a Very Low-Power Arcjet&lt;/a&gt;, Hideyuki Horisawa, Hotaka Ashiya and Itsuro Kimura, presents results of an experiment to understand the issues of scaling an arjet thruster down to the 1 to 10 Watt range and about 4 centimeters in length. A quartz window was included so that the plasma in the restriction region could be observed as experimental parameters were varied. In &lt;em&gt;these early studies&lt;/em&gt;, &amp;quot;typical thrust was 1.5 ~ 2.0 mN, Isp: ~ 100 sec for input power of 1 ~ 5 watts and propellant (N2) mass flow rate of 0.6 ~ 2 mg/sec&amp;quot;, and based on these results the possibility of higher Isp in this power range can be explored.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vQWOi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vQWOi.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8xTWX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8xTWX.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Direct Drive&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150016486.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Testing of an Arcjet Thruster with Capability of Direct-Drive Operation&lt;/a&gt; (A. K. Martin et al., NASA-Marshall Space Flight Center, American Institute of Aeronautics and Astronautics) an arject engine capable of being driven directly from a solar array (without substnatial power conditioning) has been demonstrated.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The testing indicated that an operating point exists within the I-V characteristics that is compatible with direct-drive solar-electric operation; for a flow rate of 20 mg/s (argon) the arc could be sustained at a voltage of about 20 V and a current of 25 A (500W).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4xBhS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4xBhS.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wR2wR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wR2wR.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Cubesats have several restrictions on chemical propellants and storage of chemical energy. One step towards addressing this problem has been discussed in &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2013index/968x932g.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Performance Characteristics of Low-Power Arcjet Thrusters Using Low Toxicity Propellant HAN Decomposed Gas&lt;/a&gt; Matsumoto et al.   IEPC-2013-095, (33st International Electric Propulsion Conference, The George Washington University, USA October 6 – 10, 2013).&lt;/p&gt;&#xA;&lt;p&gt;HAN or Hydroxylammonium nitrate is a potential new propellant. However early results showed significant corrosion with conventional materials.&lt;/p&gt;&#xA;&lt;p&gt;&lt;s&gt;There has been some work on production of hydrogen-containing vapor from solid Teflon [citation needed].&lt;/s&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-12-13T22:31:56.640" LastActivityDate="2020-12-13T22:31:56.640" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18838" PostTypeId="1" AcceptedAnswerId="18854" CreationDate="2016-10-29T03:39:39.787" Score="4" ViewCount="1043" Body="&lt;p&gt;I just read about the &quot;green&quot; propellant &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydroxylammonium_nitrate&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hydroxylammonium nitrate&lt;/a&gt; writing &lt;a href=&quot;https://space.stackexchange.com/a/18837/12102&quot;&gt;this answer&lt;/a&gt;. It will be tested in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Green_Propellant_Infusion_Mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Green Propellant Infusion Mission&lt;/a&gt; (also see &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140012587.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;GPIM AF-M315E Propulsion System&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ipcbee.com/vol10/39-V10034.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Catalytic Decomposition of Hydroxylammonium Nitrate Monopropellant&lt;/a&gt; discusses preparation and testing of HAN-water blends and the use of Iridium as catalyst.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Is Hydroxylammonium nitrate propellant &quot;green&quot; enough to ever be used in a cubesat?&lt;/em&gt; In this context, &quot;green&quot; refers to its lower toxicity when compared with hydrazine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is not so much compatibility with the cubesat itself as it is with the safety requirements of the most common space deployment options and cubesat guidelines related to stored chemical energy.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydroxylammonium_nitrate&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; &lt;em&gt;NH3OHNO3 is a fuel/oxidizer blend&lt;/em&gt;, also known as AF-M315E. It would likely be used in a spacecraft as an aqueous solution liquid propellant (the solid salt has stability issues), and may be more efficient (per volume) than Hydrazine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/g/gpim&quot; rel=&quot;nofollow noreferrer&quot;&gt;GPIM (Green Propellant Infusion Mission) / STP-2&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Delivering approximately 50% higher ρΙsp than hydrazine (5% higher Isp combined with a 46% higher density, AF-M315E offers comparable performance to traditional storable bipropellants for low V missions while employing roughly half the number of components, thereby retaining the well-established increased reliability and reduced cost of traditional monopropellants. Many design issues and failure modes associated with long-duration interplanetary missions (e.g. control of mixture ratio, of propellant vapor diffusion and reaction, oxidizer flow decay) do not apply to an equally capable AF-M315E system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U29WO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U29WO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Bim4f.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Bim4f.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-09T01:24:40.390" LastActivityDate="2021-08-09T01:24:40.390" Title="Is HAN-water propellant &quot;green&quot; enough to ever be used in a cubesat?" Tags="|propulsion|fuel|liquid-fuel|green-propellant|ammonia|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18840" PostTypeId="1" CreationDate="2016-10-29T10:25:42.497" Score="3" ViewCount="318" Body="&lt;p&gt;If the ISS orbits say 16 times a day, averaging 90 minutes, it only crosses the Greenwich Meridian (0&amp;deg; longitude) 15 times in a day, or averaging 96 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A kid asked me how often it comes over a day. Which is the right answer? He was presumably meaning coming over our country.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Sometimes we can see consecutive passes, but of course you can't take timings from them, it depends when it is in sunlight.)&lt;/p&gt;&#xA;" OwnerUserId="17418" LastEditorUserId="6816" LastEditDate="2016-10-29T10:34:30.937" LastActivityDate="2016-10-29T15:39:28.617" Title="ISS number of orbits with reference to what?" Tags="|iss|orbit|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18841" PostTypeId="2" ParentId="18840" CreationDate="2016-10-29T12:23:01.477" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;With reference to what?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;With respect to &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth-centered_inertial&quot; rel=&quot;nofollow&quot;&gt;Earth-centered inertial&lt;/a&gt; rather than &lt;a href=&quot;https://en.wikipedia.org/wiki/ECEF&quot; rel=&quot;nofollow&quot;&gt;Earth-centered, Earth-fixed&lt;/a&gt;. Consider the extreme case of a satellite in a geostationary orbit. From the perspective of an Earth-centered inertial frame, such a satellite orbits the Earth once per sidereal day. From the perspective of an Earth-centered, Earth-fixed frame, the orbital period of a geostationary satellite is infinite.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-10-29T12:23:01.477" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18842" PostTypeId="2" ParentId="18840" CreationDate="2016-10-29T15:39:28.617" Score="1" Body="&lt;p&gt;The official ISS orbit count increments when the station crosses the equator (presumably in the southern direction, although I haven't turned up a specific reference to this detail).  On May 16, 2016 &lt;a href=&quot;http://www.space.com/32893-international-space-station-100000-orbits.html#ooid=NnajlqMzE6jZhWDh6qCsCxiFwTJvI_Ye&quot; rel=&quot;nofollow&quot;&gt;this count reached 100,000&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-10-29T15:39:28.617" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18843" PostTypeId="1" CreationDate="2016-10-29T17:10:31.347" Score="15" ViewCount="951" Body="&lt;p&gt;Has an SRB been considered for use in orbit to launch to escape velocity to deep space? Consider SRB disposable and deep space vehicle continues on liquid propulsion. SRB conserves liquid propellants for later orbit maneuvers for deep space vehicle.&lt;/p&gt;&#xA;" OwnerUserId="16880" LastEditorUserId="16880" LastEditDate="2016-10-29T17:20:40.200" LastActivityDate="2016-10-30T02:24:48.250" Title="Has an SRB been considered for use in orbit to launch to escape velocity?" Tags="|deep-space|srb|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18844" PostTypeId="2" ParentId="18843" CreationDate="2016-10-29T17:50:54.680" Score="8" Body="&lt;p&gt;There's at least one example, the Inertial Upper Stage (nee Interim Upper Stage) &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_Upper_Stage&quot;&gt;https://en.wikipedia.org/wiki/Inertial_Upper_Stage&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia's summary is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Inertial Upper Stage (IUS), originally designated the Interim Upper Stage, was a two-stage solid-fueled rocket upper stage developed by Boeing for the United States Air Force beginning in 1976 for raising payloads from low Earth orbit to higher orbits or interplanetary trajectories following launch aboard a Titan 34D or Titan IV rocket, or from the payload bay of the Space Shuttle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A pertinent search phrase would be &quot;solid upper stages.&quot;&lt;/p&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2016-10-29T17:50:54.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18845" PostTypeId="2" ParentId="18843" CreationDate="2016-10-29T18:15:21.503" Score="7" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Payload_Assist_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;PAM-D&lt;/a&gt; was another solid fuel upper stage used on the Space Shuttle and Delta II launchers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SUrMN.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SUrMN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Shuttle application, one or more PAMs were carried in cradles in the payload bay.  They were spring launched out of the bay, as in this picture, and the SRM ignited at a safe distance.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-10-30T02:24:48.250" LastActivityDate="2016-10-30T02:24:48.250" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18846" PostTypeId="2" ParentId="18843" CreationDate="2016-10-29T18:37:13.420" Score="16" Body="&lt;p&gt;This is referred to as 'kick stage' motor and is commonly used. One example being &lt;a href=&quot;https://en.wikipedia.org/wiki/Star_37&quot;&gt;Star 37&lt;/a&gt;. A larger &lt;a href=&quot;https://en.wikipedia.org/wiki/Star_48&quot;&gt;Star 48&lt;/a&gt; is what sent New Horizons on the way to Pluto&lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2016-10-29T18:37:13.420" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18848" PostTypeId="1" AcceptedAnswerId="18852" CreationDate="2016-10-30T01:01:58.110" Score="12" ViewCount="668" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Atmosphere_and_Dust_Environment_Explorer&quot;&gt;Lunar Atmosphere and Dust Environment Explorer&lt;/a&gt; (LADEE) entered lunar orbit 2013-10-06, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_3&quot;&gt;Chang'e 3&lt;/a&gt; landed 69 days later producing dust and gas on 2013-12-14.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These were both quite interesting missions, the Chang'e-3 is the first lunar lander of the 21st century, operated an ultraviolet telescope on the moon, and deployed it's companion Yutu - the first lunar rover of the 21st century as well. LADEE made super-sensitive measurements of trace gas and tiny dust particles above the moon and tested an &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Atmosphere_and_Dust_Environment_Explorer#Lunar_Laser_Communication_Demonstration&quot;&gt;optical comms link&lt;/a&gt; between lunar orbit and Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember reading about the timing of the two events - that Chang'e-3 was announced to be landing during LADEE's initial baseline calibration measurements, some of which are summarized in &lt;a href=&quot;http://www.space.com/23675-china-moon-lander-trouble-nasa-ladee.html&quot;&gt;China's 1st Moon Lander May Cause Trouble for NASA Lunar Dust Mission&lt;/a&gt;, but I never heard what actually happened.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia mentions it &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Atmosphere_and_Dust_Environment_Explorer#Chinese_lander&quot;&gt;here&lt;/a&gt; but is inconclusive. So was the baseline calibration disrupted or &lt;em&gt;did the landing provide additional science value&lt;/em&gt;? If the latter, please cite some example, thanks!&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;China's Chang'e 3 spacecraft, which was launched on December 1, 2013, and entered lunar orbit on December 6, (25) was expected to contaminate the tenuous lunar exosphere with both propellant from engine firings and lunar dust from the vehicle's landing. (26) While concern was expressed that this could disrupt LADEE's mission, (26) such as its baseline readings of the Moon's exosphere, it may instead provide additional science value since both the quantity and composition of the spacecraft's propulsion system exhaust are known. (27) Data from LADEE may be used to track the distribution and eventual dissipation of the exhaust and dust in the Moon's exosphere. (27)(28) It may also be possible to observe the migration of water, one component of the exhaust, giving insight on how it is transported and becomes trapped around the lunar poles. (29)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-10-30T01:40:42.470" LastActivityDate="2016-10-30T16:37:53.983" Title="When Chang'e-3 landed on the moon, did LADEE notice?" Tags="|atmosphere|lunar-landing|dust|chang-e|ladee|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18849" PostTypeId="1" AcceptedAnswerId="18855" CreationDate="2016-10-30T01:29:59.650" Score="2" ViewCount="531" Body="&lt;p&gt;This &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_artificial_objects_on_the_Moon&quot; rel=&quot;nofollow&quot;&gt;List of Artificial Objects on the Moon&lt;/a&gt; lists 21 objects as &quot;intentionally crashed&quot;. Because of the moon's lumpy gravity due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)&quot; rel=&quot;nofollow&quot;&gt;sub-surface mass concentrations&lt;/a&gt; most low lunar &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects&quot; rel=&quot;nofollow&quot;&gt;orbits will be fairly unstable&lt;/a&gt; and sooner or later intersect the surface (i.e. crash).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the detailed gravity distribuion, and identification of &quot;safe&quot; orbits wasn't &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects&quot; rel=&quot;nofollow&quot;&gt;worked out until 2001&lt;/a&gt;. Further, before the Apollo landings, I think there were no seismometers on the moon. Crashes are potential sources of seismic signals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So were all 21 of these objects suddenly maneuvered &lt;em&gt;intentionally&lt;/em&gt; out of orbit in a way that targets the surface, or were some &lt;em&gt;intentionally&lt;/em&gt; put into orbits that basically didn't stand a chance of lasting and the idea was to let them hit sooner or later? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Group A (below) why were those &lt;em&gt;intentionally&lt;/em&gt; crashed? What was the point or motivation behind the intent?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Group A: Before lunar seismometers&lt;/li&gt;&#xA;&lt;li&gt;Broup B: After seismometers, before full mascon-awareness&lt;/li&gt;&#xA;&lt;li&gt;Broup C: After full mascon-awareness&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-10-30T18:07:21.630" Title="What does it (actually) mean when spacecraft are &quot;intentionally crashed&quot; on the moon?" Tags="|the-moon|lunar-landing|lunar-orbit|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18850" PostTypeId="2" ParentId="18849" CreationDate="2016-10-30T01:43:01.083" Score="6" Body="&lt;p&gt;Most of the early missions, such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ranger_program&quot; rel=&quot;noreferrer&quot;&gt;Ranger missions&lt;/a&gt;, mostly just aimed to get to the moon.  Before our understanding of orbital mechanics was as good as it is today, and before computers were powerful and compact, just getting something to the moon was a huge challenge.  Many of these early programs were testing to see if we could even get to the Moon.  Notice how two of the Ranger missions (3 and 5) missed the moon entirely.  These missions were meant to test whether or not we could accurately reach the moon, and they contained no devices for slowing down and getting into orbit, as that was beyond our ability at the time.  Instead, they simply crashed into the surface, taking as many pictures as they could on the way down and relaying them back to base.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the later missions, such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Prospector&quot; rel=&quot;noreferrer&quot;&gt;Lunar Prospector&lt;/a&gt; were intentionally crashed in order to find out more about the lunar surface.  In the case of the Lunar Prospector, its collision with the moon was aimed specifically at a shadowed crater rim, so that spectrometers could analyze the rock and dust it kicked up, looking for water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other spacecraft that are listed as having intentionally crashed into the moon are objects that were merely used as boosters, such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/S-IVB&quot; rel=&quot;noreferrer&quot;&gt;Apollo S-IVB&lt;/a&gt;.  These were used to test the seismic monitors.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During Apollo 13, Apollo 14, Apollo 15, Apollo 16 and Apollo 17, the S-IVB stages were crashed into the Moon to perform seismic measurements used for characterizing the lunar interior.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The rest are satellites that were deorbited to prevent them from crashing uncontrolled into the lunar surface, possibly damaging important sites.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-10-30T01:50:40.773" LastActivityDate="2016-10-30T01:50:40.773" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18851" PostTypeId="1" CreationDate="2016-10-30T09:32:03.603" Score="3" ViewCount="390" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; A simple web search in Chinese may turn up something, but I don't know anyone personally who is both both fluent and interested in space exploration. This is a pretty cool orbit - there must be something somewhere!&lt;/p&gt;&#xA;&lt;p&gt;I tried to find &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chang'e-2&lt;/a&gt; in the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; database but couldn't find it. Since I had previously gotten data for &lt;a href=&quot;https://en.wikipedia.org/wiki/Rosetta_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa&lt;/a&gt; just by typing their names, I didn't realize that those were unusual cases and it's pretty much only NASA missions there.&lt;/p&gt;&#xA;&lt;p&gt;Chang'e-2 has had quite an interesting orbit. From Earth to Lunar orbit, then the &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_2#L2_mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;first ever transfer directly from lunar orbit to Earth-Sun L2&lt;/a&gt;, then a &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_2#4179_Toutatis_mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;flyby (within 3.2 km) of 4179&lt;/a&gt;, and now... where is it?&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Is there any kind of numerical data available so I can reconstruct this in 3D?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;I'd like to follow the orbit. Here are some updates from &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/c-missions/chang-e-2&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Mission status:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;As of 2015, the Chang'e-2 has reached a distance of over 100 million km from Earth.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;In June 2014, the Chang'e-2 Probe is more than 90 million km away from the Earth, &lt;a href=&quot;http://www.unoosa.org/pdf/pres/copuos2014/tech-06.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;and is expected to fly back to a position 7 million km from the Earth in 2029&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;On July 14, 2013, the Chang'e-2 Probe had reached a distance of 50 million km from Earth, marking a new height in the nation's deep space exploration...&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xLUT1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xLUT1.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From Hao et al. 2012 &lt;a href=&quot;http://images.spaceref.com/news/2012/chang%27e-moon-iac2012.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chang'e-2 Satellite Lagrange L2 Point Mission&lt;/a&gt; 63rd International Astronautical Congress, Naples, Italy.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BkVrp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BkVrp.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Artist's conception of Chang'e-2's orbit in space, from &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/c-missions/chang-e-2&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I found 102 spacecraft by asking Horizons for a list of spacecraft:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JkYV0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JkYV0.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;-136134  &#xA;-399050  2015 PDC&#xA;-937001  2017 PDC&#xA;-651     ARTEMIS-P1 (spacecraft)&#xA;-652     ARTEMIS-P2 (spacecraft)&#xA;-399101  Apollo 10 LM (spacecraft) (Snoopy)&#xA;-399100  Apollo 10 S-IVB (spacecraft)&#xA;-399110  Apollo 11 S-IVB (spacecraft)&#xA;-399120  Apollo 12 S-IVB (spacecraft)&#xA;-399080  Apollo 8 S-IVB (spacecraft)&#xA;-399090  Apollo 9 S-IVB (spacecraft)&#xA;-86      CH1&#xA;-204     CONTOUR-A (SPACECRAFT FRAGMENT)&#xA;-205     CONTOUR-B (SPACECRAFT FRAGMENT)&#xA;-206     CONTOUR-C (SPACECRAFT FRAGMENT)&#xA;-82      Cassini (spacecraft)&#xA;-150     Cassini Huygens (spacecraft)&#xA;-151     Chandra Observatory (spacecraft)&#xA;-40      Clementine (spacecraft) [DSPSE]&#xA;-78      DSCOVR (spacecraft) [Triana]&#xA;-203     Dawn (spacecraft)&#xA;-140     Deep Impact Flyby - EPOXI (spacecraft) [EPOXI]&#xA;-70      Deep Impact IMPACTOR (spacecraft)&#xA;-30      Deep Space 1 (spacecraft) [DS1 DS-1]&#xA;-129494  DirecTV-9S (spacecraft)&#xA;-143     ExoMars TGO (spacecraft)&#xA;-127783  GALEX (spacecraft)&#xA;-108366  GOES-1 (spacecraft)&#xA;-112472  GOES-5 (spacecraft)&#xA;-114050  GOES-6 (spacecraft)&#xA;-123051  GOES-8 (spacecraft)&#xA;-177     GRAIL-A (spacecraft) [Ebb]&#xA;-181     GRAIL-B (spacecraft) [Flow]&#xA;-176     GRAIL-SS Second Stage (spacecraft)&#xA;-139479  Gaia (spacecraft)&#xA;-77      Galileo (spacecraft)&#xA;-344     Galileo Probe (spacecraft)&#xA;-47900   Genesis (SRC) (spacecraft)&#xA;-47      Genesis (bus) (spacecraft)&#xA;-130     Hayabusa (spacecraft) [Muses-C]&#xA;-37      Hayabusa 2 (spacecraft)&#xA;-486     Herschel Space Observatory (spacecraft)&#xA;-48      Hubble Space Telescope (HST (spacecraft))&#xA;-111     ICE (spacecraft) [ISEE-3]&#xA;-198     INTEGRAL (spacecraft)&#xA;-640     Infrared Space Observatory (spacecraft) [ISO]&#xA;-125544  International Space Station (spacecraft) [ISS]&#xA;-99999   James Webb Space Telescope (spacecraft) [JWST]&#xA;-61      Juno (spacecraft)&#xA;-610     Juno Centaur Stage (spacecraft)&#xA;-227     Kepler (spacecraft)&#xA;-134381  Kepler Booster (Third Stage) (spacecraft)&#xA;-12      LADEE (spacecraft)&#xA;-18      LCROSS (spacecraft)&#xA;-141043  LISA Pathfinder (spacecraft) [LPF]&#xA;-999742  LISA Pathfinder Propulsion Module (spacecraft) [PRM]&#xA;-85      LRO (spacecraft)&#xA;-25      Lunar Prospector (LP) (spacecraft)&#xA;-202     MAVEN (spacecraft)&#xA;-236     MESSENGER (spacecraft)&#xA;-74900   MRO Centaur Stage (spacecraft)&#xA;-760     MSL Centaur Stage spacecraft&#xA;-2       Mariner 2 (spacecraft)&#xA;-41      Mars Express (spacecraft) [MEX]&#xA;-53      Mars Odyssey (spacecraft)&#xA;-3       Mars Orbiter Mission (MOM) (spacecraft) [Mangalyaan ISRO]&#xA;-74      Mars Reconnaissance Orbiter (spacecraft) [MRO]&#xA;-76      Mars Science Laboratory (spacecraft) (MSL)&#xA;-93      NEAR (spacecraft)&#xA;-997     NEOCP 6Q0B44E [B44E]&#xA;-998     NEOCP J002E3&#xA;-98      New Horizons (spacecraft) [NH New_Horizons]&#xA;-178     Nozomi (spacecraft) [Planet-B]&#xA;-64      OSIRIS-REx (spacecraft) [ORX]&#xA;-253     Opportunity (spacecraft) [MER]&#xA;-39      Ouna (spacecraft)&#xA;-84      Phoenix (spacecraft)&#xA;-23      Pioneer 10 (spacecraft)&#xA;-24      Pioneer 11 (spacecraft)&#xA;-6       Pioneer 6 (spacecraft)&#xA;-20      Pioneer 8 (spacecraft)&#xA;-489     Planck Space Observatory (spacecraft)&#xA;-5       Planet-C (spacecraft) [VCO Akatsuki]&#xA;-226     Rosetta (spacecraft)&#xA;-21      SOHO (spacecraft)&#xA;-234900  STEREO Third Stage (spacecraft)&#xA;-234     STEREO-A (spacecraft) [AHEAD]&#xA;-235     STEREO-B (spacecraft) [BEHIND]&#xA;-136395  Solar Dynamics Observatory (spacecraft) [SDO]&#xA;-557     Spektr-R Observatory (spacecraft)&#xA;-254     Spirit (spacecraft) [MER]&#xA;-79      Spitzer Space Telescope [SST SIRTF (spacecraft)]&#xA;-29900   Stardust (SRC) (spacecraft)&#xA;-29      Stardust (bus) (spacecraft) (NExT)&#xA;-128485  Swift Observatory (spacecraft)&#xA;-55      Ulysses (spacecraft)&#xA;-248     Venus Express (spacecraft) [VEX]&#xA;-31      Voyager 1 (spacecraft)&#xA;-32      Voyager 2 (spacecraft)&#xA;-163     WISE (spacecraft)&#xA;-165     WMAP (spacecraft)&#xA;-125989  XMM-Newton (spacecraft)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2023-08-01T00:48:20.117" Title="What was Chang'e-2's 3D orbit in space? (since it's not in Horizons)" Tags="|orbital-mechanics|jpl-horizons|china|chang-e|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18852" PostTypeId="2" ParentId="18848" CreationDate="2016-10-30T16:37:53.983" Score="11" Body="&lt;p&gt;It did perform the observations, but nothing was detected. &lt;a href=&quot;https://www.nasa.gov/content/ladee-project-scientist-update-intial-observations-of-chang-e-3-landing/&quot;&gt;Here is an article that covers this&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Surprisingly, the LADEE science teams' preliminary evaluation of the&#xA;  data has not revealed any effects that can be attributed to Chang'e 3.&#xA;  No increase in dust was observed by LDEX, no change was seen by UVS,&#xA;  no propulsion products were measured by NMS. Evidently, the normal&#xA;  native lunar atmospheric species seen by UVS and NMS were unaffected&#xA;  as well. It is actually an important and useful result for LADEE not&#xA;  to have detected the descent and landing. It indicates that exhaust&#xA;  products from a large robotic lander do not overwhelm the native lunar&#xA;  exosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2016-10-30T16:37:53.983" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18853" PostTypeId="2" ParentId="18849" CreationDate="2016-10-30T17:04:20.513" Score="2" Body="&lt;p&gt;As well as those crashed for scientific reasons (ie seismic measurements) there are two very good reasons to deliberately crash all end-of-life space probes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;you don't want junk floating round in space, as it can damage satellites or other missions, so you want it down somewhere&lt;/li&gt;&#xA;&lt;li&gt;you don't want it to hit something on the surface accidentally, so a controlled impact in an unimportant area is preferred&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="38" LastActivityDate="2016-10-30T17:04:20.513" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18854" PostTypeId="2" ParentId="18838" CreationDate="2016-10-30T17:52:09.587" Score="9" Body="&lt;p&gt;It probably couldn't be used on a cubesat, but not primarily for environmental reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The restrictions on cubesats generally relate to the risk of it damaging the launch vehicle, primary payload or people involved in the wider mission (including astronauts for cubesats deployed from the ISS). The system being developed for GPIM would break several of the restrictions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;While it's a lot less unpleasant than hydrazine, the safety data still says &quot;Explosive, Carcinogenic, Toxic, Harmful, Irritant, Dangerous for the environment&quot;. I would guess that this would completely rule it out from being deployed from the ISS, and is likely to be challenging for other launcher systems. I haven't looked into it enough to say how big the explosion risk is - doubtless less than hydrazaine, which basically doesn't want to exist at all - but expect a lot of &quot;you're not putting that amateur explosive device anywhere near my $100M satellite&quot;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Feed pressure of 6.8 to 27.6 bar; cubesats need a waiver for anything above 1.2 bar (for good reason: a pressurised tank going bang can make a mess of anything nearby).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12831" LastActivityDate="2016-10-30T17:52:09.587" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18855" PostTypeId="2" ParentId="18849" CreationDate="2016-10-30T18:07:21.630" Score="1" Body="&lt;p&gt;Group A are:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the Rangers, which were built to crash, and to send photos/telemetry during the descent. &lt;/li&gt;&#xA;&lt;li&gt;Luna 2: built to crash and spread Soviet hammer/sickle symbols on the Moon. &lt;/li&gt;&#xA;&lt;li&gt;Luna 2 transfer stage: Luna 2 had no thrusters, so the transfer stage had to aim it at the Moon. I haven't been able to find why they crashed the stage, but it may be as simple as not having enough propellant to adjust its trajectory to miss the Moon. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The only one that could possibly be unintentional was the Luna 2 transfer stage. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-30T18:07:21.630" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18856" PostTypeId="1" AcceptedAnswerId="18862" CreationDate="2016-10-30T18:11:36.280" Score="27" ViewCount="5121" Body="&lt;p&gt;Not a trick question, but a real &lt;em&gt;puzzler - which spacecraft or spacecrafts incorporated real wood structural elements?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-09-12T02:37:09.320" LastActivityDate="2021-06-11T13:06:37.027" Title="Puzzler - which spacecraft(s) incorporated real wood structural elements?" Tags="|spacecraft|construction|" AnswerCount="10" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="18857" PostTypeId="2" ParentId="18856" CreationDate="2016-10-30T18:34:05.783" Score="14" Body="&lt;p&gt;Many Mars landers have used heatshields made of cork powder in phenolic resin, including Viking and Schiaparelli. (ESA uses a material called Norcoat Liège, &quot;liège&quot; being French for cork.)&lt;/p&gt;&#xA;" OwnerUserId="12831" LastActivityDate="2016-10-30T18:34:05.783" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18858" PostTypeId="1" CreationDate="2016-10-30T19:39:54.523" Score="7" ViewCount="3739" Body="&lt;p&gt;I'm curious about the electronics and other instruments on Voyager and similar spacecraft.  Are the electronic components kept in open vacuum or in a sealed atmosphere? I imagine cooling gets tricky if there's no air to help. I can't imagine the Voyager or Galileo tape recorder working in a vacuum without horrible stiction problems. HDDs wouldn't work at all. There must be issues with many instruments which work completely reliably in Earth's atmosphere and suddenly find themselves in a vacuum. Do mechanical bearings (grease or metal-on-metal) continue to work ok?   &lt;/p&gt;&#xA;" OwnerUserId="14419" LastEditorUserId="38" LastEditDate="2016-10-31T09:02:15.503" LastActivityDate="2016-12-15T21:20:25.613" Title="Do electronics and mechanical components work in a vacuum or require a sealed controlled atmosphere?" Tags="|design|voyager|electronics|vacuum|galileo|" AnswerCount="6" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18859" PostTypeId="2" ParentId="18856" CreationDate="2016-10-30T19:41:53.347" Score="14" Body="&lt;p&gt;I think that Soyuz launcher uses wooden parts in engine ignition process &#xA;(proof link in Russian: &lt;a href=&quot;https://geektimes.ru/post/273782/&quot;&gt;https://geektimes.ru/post/273782/&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="11046" LastActivityDate="2016-10-30T19:41:53.347" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18861" PostTypeId="2" ParentId="18856" CreationDate="2016-10-30T22:23:55.140" Score="5" Body="&lt;p&gt;The Apollo SLA had thin cork panels for insulating the LM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The roof of the LM looks to me to be perforated chipboard, like the back of an old tv, but i've no references to back that up :)&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2016-10-31T08:00:26.980" LastActivityDate="2016-10-31T08:00:26.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18862" PostTypeId="2" ParentId="18856" CreationDate="2016-10-30T23:39:32.870" Score="40" Body="&lt;p&gt;Rangers 3, 4, and 5 each had a seismometer encased in balsa wood to limit the impact loads.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YKl49.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YKl49.jpg&quot; alt=&quot;balsa wood sphere&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/cl4wd.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cl4wd.jpg&quot; alt=&quot;diagram of Ranger Block II&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-10-30T23:39:32.870" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18864" PostTypeId="2" ParentId="18858" CreationDate="2016-10-31T01:18:37.750" Score="3" Body="&lt;p&gt;Most modern spacecraft have their electronics work in a vacuum state, at least most of them. Particularly things with moving parts, like Reaction Wheels, might require some atmosphere. These small components are the only components that are sealed, all others are opened to the vacuum of space. Very early spacecraft were not designed as such.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't actually find what components on Voyager are vacuum sealed, but if there are parts, there aren't many.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They perform vacuum testing on Earth to ensure there are no problems. This is often referred to as the T-VAC test, or Thermal Vacuum, where they vacuum out a chamber of all air, and test the spacecraft with different thermal cycles.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-31T01:18:37.750" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18869" PostTypeId="1" CreationDate="2016-10-31T03:02:50.460" Score="6" ViewCount="1645" Body="&lt;p&gt;I am writing a piece of software which propagates the orbit of a spacecraft from an initial Earth orbit to an interplanetary orbit. My simple two-body propagation works fine in either a geocentric or heliocentric orbit. However, I cannot seem to handle the transition correctly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the steps of that transition (I can share relevant code if useful):&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Get ecliptic J200 position of Earth (via &lt;a href=&quot;https://github.com/soniakeys/meeus/blob/master/planetposition/planetposition.go#L195&quot; rel=&quot;noreferrer&quot;&gt;https://github.com/soniakeys/meeus/blob/master/planetposition/planetposition.go#L195&lt;/a&gt; ) and compute the velocity of Earth (via equations in the first pages of Vallado - Fundamentals in Astrodynamics). Note that the &quot;meeus/planetposition&quot; library returns the ecliptic positions in L, B, R, which I convert to cartesian coordinates (via &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic_coordinate_system#Rectangular_coordinates&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Ecliptic_coordinate_system#Rectangular_coordinates&lt;/a&gt; )&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Rotate the radius and velocity vectors of my geocentric orbit about the first axis and by the axial tilt of Earth (as per &lt;a href=&quot;https://en.wikipedia.org/wiki/Ecliptic_coordinate_system#Converting_Cartesian_vectors&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Ecliptic_coordinate_system#Converting_Cartesian_vectors&lt;/a&gt; )&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Add the spacecraft R and V vectors computed in step 2 to the radius and velocity vectors of the planet computed in step 1 (since $r_{\text{sc}_{\text{helio}}} = r_{\text{sc}_{\text{earth}}} + r_{\text{earth}_{\text{helio}}}$)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;When visualizing both trajectories in Cosmographia, I can definitely tell that the computation is wrong (cf. the two screenshots below). I have been stuck on this issue for a few hours now (about twelve I'd say), so any help would be greatly appreciated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bpMBd.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bpMBd.png&quot; alt=&quot;Earth seen from the expected position (geocentric vectors)&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/LiPXr.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LiPXr.png&quot; alt=&quot;Earth seen from the incorrectly computed heliocentric position&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2019-07-25T21:14:09.217" Title="Help converting from ECEF frame to ICRF frame for interplanetary mission design" Tags="|orbital-mechanics|interplanetary|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18870" PostTypeId="2" ParentId="18704" CreationDate="2016-10-31T03:19:39.450" Score="1" Body="&lt;p&gt;Probably not. As mentioned in &lt;a href=&quot;https://space.stackexchange.com/questions/18704/could-schiaparellis-retroreflector-array-still-be-used/18870#comment49089_18704&quot;&gt;the comment&lt;/a&gt; by @OrganicMarble,  Schiaparelli fell without propulsion or parachutes for several kilometers and left a &quot;crater&quot; or at least a significant depression in the Martial soil, possibly accompanied by an explosion of the unused propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even individual corner reflectors - if separated - would probably be covered by debris or soil.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is &quot;&lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/ExoMars/Detailed_images_of_Schiaparelli_and_its_descent_hardware_on_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;(t)he latest image... taken on 25 October by the high-resolution camera on NASA’s Mars Reconnaissance Orbiter.&quot;&lt;/a&gt; from &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2016/10/Zooming_in_on_Schiaparelli_components_on_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; - cropped full size image, and the full reduced-file-size version.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HqvO6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HqvO6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MqQzE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MqQzE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-10-31T03:19:39.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18871" PostTypeId="2" ParentId="18832" CreationDate="2016-10-31T03:34:43.910" Score="9" Body="&lt;p&gt;Blue Origin (BO) chosed LH2 for BE-3 rocket engine because of the performance.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Performance drove the decision to use hydrogen fuel in the BE-3,&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://m.aviationweek.com/awin/blue-origin-tests-new-engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;said Rob Meyerson in an interview in 2013&lt;/a&gt;, president and program manager in BO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking in to perspective, BO had ambitions for space exploration and they would not stop the development only with a suborbital rocket. They had in plan also orbital rockets. We should combine step by step some elements to come in a conclusion. &lt;a href=&quot;https://www.blueorigin.com/technology#engine_stories_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;BE-3 engine&lt;/a&gt; was planned that it would serve for a suborbital and orbital rocket, for a booster stage and upper stage not only for BO future rockets but also for gonverment and commercial launch systems. Since LH2 has a lot of advantages in an upper stage then they would need an LH2 rocket engine. They also had plans for reusability of their suborbital booster, doing tests how to land a booster returning from space. And also had plans to send turists in to space. New Shepard would serve to test landing a booster returned from space and also make a trip for turists in to space. BE-3 was announced to have a thrust of 440 kN. In this way it would be ok as an upper stage for BO orbital rocket and also would give enough power for a suborbital rocket like New Shepard. The better performance LH2 engine would give more chances making more easy for New Shepard booster to land safely and sending turists in space with a reused booster making their trip a low cost trip. To land a booster stage is needed  a considerable amount of fuel, so a high performance engine gives more room for the success. They could use one type of engine in two projects. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the same interview Meyerson says that in their orbital rocket (didn't have a name at that time) they were planing to use in all stages the BE-3 LH2 engine. The same philosophy as in Falcon 9 (using Merlin 1D) which reduces the cost and not having two programs for two differnt types of rocket engines (LH2 and RP-1). Again in their orbital rocket they would have better performance and better chances for reusability with a LH2 BE-3 engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BO a company with ambitions also had considered Methane engine (BE-4) for an orbital rocket and they started project in 2011. They hadn't decided yet at that time what type of engine would use.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“We have ideas. Some things are in development for other engines that we're developing, but we're not ready to discuss those today. Those would provide other options and other architectures,” said Meyerson. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Let say that depending on the success of BE-4 they would decide to chose a LH2 or Methane engine for the 1st stage of orbital rocket. Now they have announced New Glenn using BE-4 Methane engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If they would build an RP-1 engine for New Shepard, an LH2 engine for their orbital rocket upper stage since were focused in performance (high performance of LH2 and advantages at the &lt;a href=&quot;https://space.stackexchange.com/questions/17563/pros-and-cons-of-lh2-lox-vs-other-fuels&quot;&gt;link&lt;/a&gt; of called2voyage comment), including the ambitions to make also a Methane engine with its advantages would make a more complicated situation and costly for BO with three different rocket engine projects.&#xA;Something else for BE-3 engine was that to make things easier for a LH2 engine, they decided for their BE-3 to have a design based on the combustion “tap-off” engine cycle. It has a high Isp, it is simpler then pre burning staged combustion and is well suited to human spaceflight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;BO won experience step by step learning with some small engines, and yes they had made before an engine using RP-1 (Kerosene) as fuel. BE-1 using peroxide, BE-2 using kerosene+peroxide, BE-3 using LH2/LOX, BE-4 using LCH4/LOX. We can't judge why they chosed to build their third generation engine using LH2 or for their fourth using Methane (LCH4). They had right knoweleges and technology. If they feel that they can do then they should try. BO has made gradually their developments and projects, by learning step by step to be successful.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-10-31T13:46:40.083" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18872" PostTypeId="2" ParentId="18838" CreationDate="2016-10-31T08:00:05.377" Score="-1" Body="&lt;p&gt;The problem with HAN is that it wasn't anywhere near as stable and &quot;detonation resistant&quot; as it's proponents suggested it was. That's what doomed the Air Force's ALASA program.&lt;/p&gt;&#xA;" OwnerUserId="17443" LastActivityDate="2016-10-31T08:00:05.377" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18873" PostTypeId="2" ParentId="18858" CreationDate="2016-10-31T08:53:46.717" Score="12" Body="&lt;p&gt;Generally speaking, all &quot;air-tight&quot; containers leak to some small degree. On Earth this manifests itself as an inability to maintain a vacuum indefinitely; in space, it manifests itself as an inability to maintain an atmosphere indefinitely without a source of replenishment gases (which themselves are going to leak over time, so you are really only delaying the inevitable). If you were to place a perfect pressure sensor in either, its value would gradually approach that of the ambient environment outside of the pressure vessel. The leak rate can be made small enough to be considered negligible or at least inconsequential during the intended primary mission of the spacecraft, but it's still going to be non-zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a consequence, for spacecraft that are designed to operate in space for many years or decades, it makes sense to minimize the number of parts that need to operate in an atmosphere. By minimizing the pressure vessel volume (and pressure), at identical mass, the leak rate can be reduced. The ultimate reduction of pressure vessel volume is removing the pressure vessel altogether, leaving everything exposed to the vacuum of space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, anything that does not actually &lt;em&gt;have&lt;/em&gt; to operate within an atmosphere would be approved for operation in a vacuum, and likely exposed to it simply because it makes things easier. A consequence of this is the difficulties in getting rid of waste heat; in a vacuum, you can't exactly just run a cooling fan and be done with it!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for your specific examples:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the Voyager or Galileo tape recorder working in a vacuum without horrible stiction problems&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't really see how that follows, even if common equipment as used on the ground would have those problems (which I'm not sure would be a given). Materials can be chosen to have the desired properties in a vacuum; it's not like a spacecraft is thrown together willy-nilly using off-the-shelf parts with no thought put into how it's going to behave in the spacecraft's native environment.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;HDDs wouldn't work at all&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;HDDs aren't really suitable for spacecraft in the first place because of vibrations, particularly during launch, and their reliance on intricate moving parts. It is possible to make a HDD that doesn't rely on air turbulence to keep the read/write head at the proper height above the platter, but that doesn't solve any of the other problems that HDDs would have in spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;many instruments which work completely reliably in Earth's atmosphere and suddenly find themselves in a vacuum&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is nothing sudden about a spacecraft at all. Designing and launching a reasonably large and complex spacecraft easily costs in the hundreds of millions to billions of dollars. Given such base costs, a several million premium for vacuum-rated parts and solid testing is easily justifiable if it means the difference between the spacecraft &lt;em&gt;perhaps&lt;/em&gt; completing its primary mission successfully or it &lt;em&gt;very likely&lt;/em&gt; completing its primary mission successfully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anything, I suspect it's harder for designers of spacecraft that need to work &lt;em&gt;both&lt;/em&gt; in an atmosphere &lt;em&gt;as well as&lt;/em&gt; in the vacuum of space.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2016-11-03T08:35:15.227" LastActivityDate="2016-11-03T08:35:15.227" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="18874" PostTypeId="1" AcceptedAnswerId="18877" CreationDate="2016-10-31T10:03:03.233" Score="3" ViewCount="454" Body="&lt;p&gt;Is there some general trend for the operating pressure of reaction wheels? I can imagine low to no pressure has obvious advantages of low to no drag (more drag requiring more power, beyond that necessary to compensate for bearing friction), but sometimes lubricants, cooling, and electrical issues benefit from some atmosphere. It can help reduce migration of oil, help cool things, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A poor analogy is a spinning hard disk drive. They usually require at least a half an atmosphere to maintaining proper head gap (the &lt;a href=&quot;http://www.almaobservatory.org/en/press-room/press-releases/520-supercomputer-ready-to-make-alma-a-powerful-telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;ALMA correlator&lt;/a&gt; (also &lt;a href=&quot;https://www.eso.org/public/news/eso1253/&quot; rel=&quot;nofollow noreferrer&quot;&gt;All Systems Go for Highest Altitude Supercomputer&lt;/a&gt;) has no magnetic hard drives because of the altitude) so those would required sealed pressurization to work in low pressure or vacuum. Obviously reaction wheels have different constraints.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I am wondering, &lt;em&gt;do reaction wheels generally run in vacuum, or are they pressurized?&lt;/em&gt; Are they generally sealed or vented? I'm asking based on &lt;a href=&quot;https://space.stackexchange.com/questions/18858/#comment49508_18864&quot;&gt;this comment&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-11-01T09:21:22.023" Title="Do reaction wheels generally run in vacuum, or are they pressurized? Sealed or vented?" Tags="|reaction-wheel|attitude-determination-and-control|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18875" PostTypeId="2" ParentId="18858" CreationDate="2016-10-31T10:20:19.790" Score="1" Body="&lt;p&gt;Using a pressure vessel for electronics is rare. I know of only a few spacecraft that did it: &lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_1&quot; rel=&quot;nofollow&quot;&gt;Sputnik 1&lt;/a&gt;, the &lt;a href=&quot;https://books.google.nl/books?id=sLhPCwAAQBAJ&amp;amp;pg=PA60&amp;amp;lpg=PA60&amp;amp;dq=Lunokhod%20pressure%20vessel&amp;amp;source=bl&amp;amp;ots=ZYKY3YD1kW&amp;amp;sig=z85_anjujYC1tGd5tGOqBEIvH-A&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwiMoMu06ITQAhXB1xoKHRdbDvcQ6AEIHjAA#v=onepage&amp;amp;q=Lunokhod%20pressure%20vessel&amp;amp;f=false&quot; rel=&quot;nofollow&quot;&gt;Lunokhod rovers&lt;/a&gt; and the Venera probes. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-10-31T10:20:19.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18876" PostTypeId="1" CreationDate="2016-10-31T11:33:36.240" Score="5" ViewCount="176" Body="&lt;p&gt;Is there any charging / ESD threat for a docking program at 700 km orbit?&lt;/p&gt;&#xA;" OwnerUserId="17445" LastEditorUserId="17004" LastEditDate="2016-10-31T22:49:28.193" LastActivityDate="2016-10-31T22:49:28.193" Title="Does electrostatic discharge pose a threat for docking or capture in a 700km polar orbit?" Tags="|spacecraft|low-earth-orbit|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18877" PostTypeId="2" ParentId="18874" CreationDate="2016-10-31T12:15:59.750" Score="2" Body="&lt;p&gt;I've found several examples of reaction wheels that are designed to run in a vacuum.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://bradford-space.com/userfiles/be_paper_AAS%2012-077%20RCF%20Wagner%20Airey.pdf&quot; rel=&quot;nofollow&quot;&gt;ESA reaction wheel test facility overview&lt;/a&gt;: &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As reaction wheels are generally enclosed in a vacuum enclosure, they are under stress at ambient pressure and this stress will affect the  microvibration behaviour. It was therefore decided to make the new facility also compatible with testing equipments under light vacuum conditions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.sinclairinterplanetary.com/reactionwheels&quot; rel=&quot;nofollow&quot;&gt;Some examples of reaction wheels designed to be run in a vacuum&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Other reaction wheels run &lt;a href=&quot;http://cdn.intechopen.com/pdfs-wm/13470.pdf&quot; rel=&quot;nofollow&quot;&gt;inside a pressurized enclosure&lt;/a&gt;. Gas pressure is usually in the region of 0.01 bar. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-11-01T09:21:22.023" LastActivityDate="2016-11-01T09:21:22.023" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18878" PostTypeId="2" ParentId="18707" CreationDate="2016-10-31T13:52:01.880" Score="8" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing time is determined by the interplanetary trajectory, largely fixed by launch energy constraints, and then by the landing site location, mainly the longitude. &lt;a href=&quot;https://solarsystem.nasa.gov/docs/p507.pdf&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; identifies the landing site selection criteria, which were almost entirely about the safety of the landing. And appropriately so for an EDL demonstration mission. The landing site selected, Meridiani Planum, is the safest landing site on Mars that we have ever examined, and is where Opportunity landed and continues to operate today. Some consideration was given to the post-landing science potential, for which Meridiani is also a very good choice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given those choices, which antennas on Earth can see Mars during the few minutes of EDL is then a matter of chance. GMRT is an incredible resource for acquiring UHF data from Mars. Its impressive total aperture provided a much higher high signal-to-noise ratio carrier, as well as the Doppler signature, than we have ever seen before on UHF using the Parkes or Green Bank antennas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the primary means to monitor the EDM entry, descent, and landing was the relay radio on TGO itself, and it is that telemetry that is providing the most insight into what happened to the EDM. Having GMRT in view of the landing was not a requirement or a factor in the landing site selection. The use of GMRT was a first-time experiment conducted by JPL (we didn't even know if it would work), which ESA called a &lt;a href=&quot;http://blogs.esa.int/rocketscience/2016/10/18/listening-to-an-alien-landing/&quot; rel=&quot;noreferrer&quot;&gt;&quot;a 'nice-to-have' to allow us to watch the descent in real time&quot;&lt;/a&gt;. As it turned out, it was very nice to have, providing quite clear, real-time information that could not be obtained from TGO. The data from TGO was returned a few hours after landing and orbit insertion.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-10-31T16:20:34.397" LastActivityDate="2016-10-31T16:20:34.397" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18880" PostTypeId="1" CreationDate="2016-10-31T14:53:09.670" Score="1" ViewCount="228" Body="&lt;p&gt;&lt;strong&gt;NOTE:&lt;/strong&gt; Looking at this 2+ years later, I realize now that a careful observer can likely answer this one fairly quickly. I'll do it myself eventually, but someone else is welcome to first.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In this video recognize what looks like a Project Echo reflective balloon, and what looks like the famous &lt;a href=&quot;https://en.wikipedia.org/wiki/Holmdel_Horn_Antenna&quot; rel=&quot;nofollow noreferrer&quot;&gt;Holmdel N.J. Microwave Horn antenna&lt;/a&gt;, but then there are one or more rockets shown, and several other tracking dishes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Discounting the Project Echo stuff, are those two different rockets and just random dishes, or is there some coherence here?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(ignore the transoceanic cable and other underwater stuff).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jChs2.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jChs2.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screenshots from &lt;a href=&quot;https://www.youtube.com/watch?v=F7bKe_Zgk4o&quot; rel=&quot;nofollow noreferrer&quot;&gt;This YouTube video&lt;/a&gt; of Boards of Canada's &quot;Music is Math.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/F7bKe_Zgk4o?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://www.youtube.com/watch?v=F7bKe_Zgk4o&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt; of Boards of Canada's &quot;Music is Math&quot;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-01-20T12:37:59.410" LastActivityDate="2019-01-20T12:37:59.410" Title="Is this video made from one coherent set of images or a mixture of several missions or even agencies?" Tags="|rockets|history|space-art|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="18881" PostTypeId="2" ParentId="18874" CreationDate="2016-10-31T15:03:05.140" Score="4" Body="&lt;p&gt;Most reaction wheel assemblies use ball bearings between the rotor and housing, with some kind of lubricant, typically liquid, coating the ball bearings. There's a problem with liquid lubricants: They evaporate in vacuum. For this reason, most reaction wheel assemblies are hermitically sealed with a low pressure inert gas inside the container.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatives to the use of liquid lubricants are using solid lubricants or forgoing the use of ball bearings entirely, using magnetic suspension instead. Both can operate in vacuum, but both have disadvantages. Friction is greater with solid lubricants compared to liquid lubricants, and the lack of a housing creates contamination issues. System complexity is considerably greater for magnetically suspended reaction wheels than those that use ball bearings.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-10-31T15:03:05.140" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18882" PostTypeId="1" CreationDate="2016-10-31T15:23:15.757" Score="11" ViewCount="511" Body="&lt;p&gt;Supposing that in a rocket failure engineers don't find what the root cause was, what will happen next?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general if the root cause will never be fully understood is there a law, rule, or standard procedure in spaceflight that tells what to do next for moving forward and starting to operate again? If not and these are situations where the procedure depends by the case, then how will it be managed?&lt;/p&gt;&#xA;" OwnerUserId="16522" LastEditorUserId="6816" LastEditDate="2016-12-22T18:53:05.543" LastActivityDate="2016-12-29T14:18:34.517" Title="What happens next if the root cause of one rocket failure won't be identified?" Tags="|launch|rockets|failure|engineering|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18884" PostTypeId="2" ParentId="18876" CreationDate="2016-10-31T17:43:47.057" Score="2" Body="&lt;p&gt;It depends on the orbit. Unlikely a risk at equatorial orbit, but at polar orbits this would be a significant issue. &lt;a href=&quot;http://press.princeton.edu/chapters/s9500.pdf&quot; rel=&quot;nofollow&quot;&gt;Here is a good introduction&lt;/a&gt; that talks about charging risks in different environments, and &lt;a href=&quot;https://arxiv.org/ftp/arxiv/papers/0906/0906.3884.pdf&quot; rel=&quot;nofollow&quot;&gt;another&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Summary: it's not to be ignored, but unlikely at risk at low latitudes in LEO region.  You might also look into &lt;a href=&quot;https://standards.nasa.gov/file/642/download?token=TTB1k_Ly&quot; rel=&quot;nofollow&quot;&gt;NASA-STD-4005 Low earth orbit spacecraft charging design standard&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2016-10-31T17:43:47.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18885" PostTypeId="1" AcceptedAnswerId="18886" CreationDate="2016-10-31T19:13:30.210" Score="15" ViewCount="1538" Body="&lt;p&gt;New Shepard of Blue Origin separates the crew capsule from the booster. The crew capsule is in free fall for a while, then the parachutes open and finally the landing rockets give a brief impulse to land it softly. Meanwhile the booster free falls without parachutes and fires to almost hover to also touch down softly. Actually, it looks to me that the crew capsule dingles more than the booster does (during the last test linked to below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Wouldn't it be simpler and safer to skip the separation and&lt;/strong&gt; the parachutes and the double touch down impulses, to &lt;strong&gt;instead keep the crew capsule on top of the booster to land both at once still integrated?&lt;/strong&gt; The crew capsule could rely on its launch abort system to function also as a landing abort system. To separate only when it increases safety to do so, instead of every time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=t6p_J6n0fgo&quot;&gt;this video&lt;/a&gt; of the test launch 19 June 2016, the booster landing looks softer than the crewed capsule landing. The time in free fall for the booster seems to last 5 minutes (from about 2:00 to 7:00 in the video). For the crew capsule, free fall lasts only 40 seconds longer (until the drogue parachutes unfold) or maybe 100 seconds longer until the main parachutes are unfolded. Is that difference of free fall time, which I suppose is what their customers are in demand of, worth the extra complexity and risk? Couldn't it be made up with a simpler integrated design? Is it maybe designed to give experiences more relevant to planned orbital launches, and thus by intent in itself not optimal for suborbital crew launches?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="12102" LastEditDate="2019-12-19T00:45:05.190" LastActivityDate="2019-12-19T00:45:05.190" Title="Why does Blue Origin land the crew capsule separate from the booster?" Tags="|landing|sub-orbital|blue-origin|stage-separation|new-shepard|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18886" PostTypeId="2" ParentId="18885" CreationDate="2016-10-31T19:51:23.033" Score="16" Body="&lt;p&gt;It would be extremely unstable. There are 2 things that weight a lot on an empty rocket, the capsule and the engine. With the two of them on opposite sides, the rocket would become extremely unstable. Separating the two allows for the entire system to be more stable. This is particularly important when it is on the ground, where it could easily tip over if the capsule was still attached.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, having the system designed to separate allows for an easier integration of the launch abort system. The two have to separate already, minimizing most of the gain that would otherwise be found.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-10-31T19:51:23.033" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18887" PostTypeId="2" ParentId="18696" CreationDate="2016-10-31T21:40:29.443" Score="6" Body="&lt;p&gt;The biggest argument against this is thermal control. Batteries are one of the most thermally sensitive components of spacecraft, and normally require careful thermal design to make sure they stay in operating ranges. Even for terrestrial applications, like electric cars, this is a significant pack engineering constraint, second being safety.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Less importantly, spreading out battery cells will require more wiring mass to connect the cells with balancing taps and all, even further complicating connections if this is goes beyond hinged parts like solar arrays.&lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2016-10-31T21:40:29.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18888" PostTypeId="2" ParentId="18664" CreationDate="2016-11-01T00:45:50.670" Score="5" Body="&lt;p&gt;Yes, Electra radio package is provided by NASA/JPL, and NASA fully expects to &lt;a href=&quot;http://marsmobile.jpl.nasa.gov/news/whatsnew/index.cfm?FuseAction=ShowNews&amp;amp;NewsID=1664&quot;&gt;utilize it's capabilities&lt;/a&gt; for it's own future missions.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Plans call for the TGO to use its Electra radios for communication&#xA;  with ESA's 2018 ExoMars Rover and Russia's Lander as well as with&#xA;  NASA's 2016 Mars lander and 2020 Mars rover.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;2016 Mars lander is the Phoenix-derived InSight ( now delayed to 2018 ), and of course 2020 MSL/Curiosity heritage rover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe it's important to mention that not all Mars orbiters have equal contribution opportunities as relays, due to their chosen science orbits. So &lt;a href=&quot;http://www.nasa.gov/feature/jpl/europes-new-mars-mission-bringing-nasa-radios-along&quot;&gt;TGO will add valuable&lt;/a&gt; capacity thanks to it's favorable orbit.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;TGO's Electra radios use a design from JPL with special features for&#xA;  relaying data from a rover or stationary lander to an orbiter passing&#xA;  overhead. Relay of information from Mars-surface craft to Mars&#xA;  orbiters, then from Mars orbit to Earth, enables receiving much more&#xA;  data from the surface missions than would otherwise be possible.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;As an example of Electra capabilities, during a relay session between&#xA;  an Electra on the surface and one on an orbiter, the radios can&#xA;  maximize data volume by actively adjusting the data rate to be slower&#xA;  when the orbiter is near the horizon from the surface robot's&#xA;  perspective, faster when it is overhead.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA's Curiosity Mars rover and Mars Reconnaissance Orbiter (MRO)&#xA;  already use Electra technology for relay of data. NASA's Mars&#xA;  Atmosphere and Volatile Evolution (MAVEN) spacecraft, in orbit since&#xA;  2014, also carries an Electra radio.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Due to improvements in the newest Electra radios and reduced&#xA;  interference levels compared with MRO, TGO's relay radios are expected&#xA;  to have a relay signal about twice as strong as MRO's. Compared to&#xA;  MAVEN's highly elongated orbit, TGO has a planned orbit similar to&#xA;  MRO's relay-favorable orbit at about 250 miles (400 kilometers) in&#xA;  altitude and nearly circular in shape.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="3140" LastEditorUserId="3140" LastEditDate="2016-11-01T01:24:43.420" LastActivityDate="2016-11-01T01:24:43.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18889" PostTypeId="1" CreationDate="2016-11-01T06:04:22.577" Score="22" ViewCount="6683" Body="&lt;p&gt;As we know, cars need a front window because drivers need to see the road to control the car, but how about the windows in space shuttles? As far as I know, the orbits of ISS or other space devices are mostly controlled by a computer, not by humans directly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it just to allow people in space look back to Earth so that they don't feel bored? Or do the windows have other uses?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="6944" LastEditDate="2020-09-09T12:11:20.193" LastActivityDate="2020-09-09T12:11:20.193" Title="Do windows in space stations, the space shuttle, other spacecraft have practical usage?" Tags="|iss|space-shuttle|spacecraft|spacecraft-window|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="18890" PostTypeId="2" ParentId="18885" CreationDate="2016-11-01T10:30:29.370" Score="6" Body="&lt;p&gt;There's 2 major reasons: safety and practicality&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Safety:&lt;/strong&gt; If you look at the history of booster landings it is not a proven technology, the reliability is not good enough for safety. Don't get me wrong, it's amazing that they can land a booster for re-use, and it works a good portion of the time, however when it goes wrong it goes very, very wrong very fast. If a leg doesn't lock properly, a thruster cuts out at the wrong time, or the booster lands just a bit too fast then you will have a fireball and then you are dependent on the launch abort system to save the lives of the crew. Imagine you have a situation where a leg fails on landing: the booster starts to lean, then the launch abort system activates but sends the capsule off at an angle which doesn't leave enough height for chute deployment the crew in that scenario is dead. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Practicality&lt;/strong&gt;: the capsule is heavy, therefore to land with it the booster would need a significant amount of additional engineering to handle the loads involved. That adds weight. The booster would need significantly more fuel in order to land that additional weight, which requires more fuel to lift that extra fuel. All that additional fuel means a significantly bigger rocket, therefore more expensive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's cheaper and safer to use a parachute system. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-11-01T10:30:29.370" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18891" PostTypeId="2" ParentId="18889" CreationDate="2016-11-01T10:43:08.437" Score="23" Body="&lt;p&gt;For shuttle, the crew flew the last few minutes of the landing manually. Manual flying was a contingency mode for most parts of the entry.  Some kind of outside view is required for that. Given video technology in the 70s when shuttle was designed, they went with windows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/8W4cfIyNvts?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The aft and overhead windows were important for robotic arm and docking operations. Camera views were used as well but window views were often crucial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xO4ST.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xO4ST.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VecqS.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VecqS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-11-01T13:04:02.517" LastActivityDate="2016-11-01T13:04:02.517" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18892" PostTypeId="2" ParentId="18889" CreationDate="2016-11-01T14:03:34.390" Score="16" Body="&lt;p&gt;Video technology of the MIR space station was obviously still not good enough during &lt;a href=&quot;http://history.nasa.gov/SP-4225/multimedia/progress-collision.htm&quot; rel=&quot;noreferrer&quot;&gt;the 1997 MIR-Progress collision&lt;/a&gt;. The radar had been shut down because of technical problems, but the docking attempt was ordered to go ahead anyway, by a crew member remotely operating the approaching Progress supply spacecraft via a video monitor&lt;/p&gt;&#xA;&lt;p&gt;Just afterwards, in the power outage caused by the collision (they had to cut the main power cable in order to close the door to the punctured module to save the breathable indoor atmosphere and survive):&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;&amp;quot;So, in the dark and in the silence, [astronaut Mike] Foale went to the &lt;strong&gt;windows&lt;/strong&gt; in the airlock and held his thumb up to the field of stars. Combining a sailor’s technique with a scientist’s knowledge of physics, Foale estimated the spin rate of the space station.&amp;quot;&lt;/em&gt;&#xA;&lt;a href=&quot;https://www.youtube.com/watch?v=tM7fTLLmgbk&quot; rel=&quot;noreferrer&quot;&gt;Dramatization&lt;/a&gt; of the event.&lt;/p&gt;&#xA;&lt;p&gt;So at least as late as 1997 windows played a useful role in helping determine the rotation, and I suppose thereby determining the Earth communication and solar power periods of a crewed space station.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-11-01T14:09:41.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18893" PostTypeId="2" ParentId="18889" CreationDate="2016-11-01T15:17:35.513" Score="5" Body="&lt;p&gt;On the ISS, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cupola_%28ISS_module%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cupola&lt;/a&gt; is used to observe operations: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The International Space Station Cupola was first conceived in 1987 by Space Station Man-Systems Architectural Control Manager Gary Kitmacher as a workstation for operating the station’s Canadarm2 robotic arm, maneuvering vehicles outside the station, and observing and supporting spacewalks. He likened the use as similar to that of the Shuttle Orbiter Aft Flight Deck.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wouldn't dismiss giving the astronauts a window to look out of as unimportant. Several astronauts I've heard speak, said the most profound change they went through was in the way they look at Earth: they came to understand how fragile Earth's society and ecosystem is, and how rapidly the ecosystem is changing for the worse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source for the last paragraph: a 3-hour talk by astronaut André Kuipers, almost half of it was spent sharing his observations of Earth and the changes he's seen between 2 missions ~10 years apart. Some changes are obvious to the naked eye, others can be easily seen if you have 2 photos of the same region taken at different times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More examples: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/IAmA/comments/3pxrfr/i_am_chris_hadfield_ama/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chris Hadfield:&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Orbiting Earth 2593 times, what I really came to appreciate was the commonality of the human experience. From orbit you see the repeated patterns of human settlement and civilisation, and inevitably start to sense that each of us inherently wants the same things out of life - joy, grace, time and stability to think, better opportunities for our children, laughter, someone to love. The precept of 'Us' and 'Them' is one that is taught; it's not the fundamental reality. Seeing the whole world as 1 place every 92 minutes drove that home within me, forever. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Edgar Mitchell: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty. You want to grab a politician by the scruff of the neck and drag him a quarter of a million miles out and say, &quot;Look at that, you son of a bitch.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.lonelyplanet.com/news/2016/06/20/returned-astronaut-tim-peake-iew-of-earth-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tim Peake&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The view of Earth is the biggest thing, and it is probably the thing that has the biggest impact on you, so it is quite natural that is the thing you will miss the most&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://blogs.nature.com/news/2012/05/earth-overburdened-by-soaring-consumption-says-wwf-report.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;André Kuipers&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;From up here I can see humanity’s footprint, including forest fires, air pollution and erosion&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://old.mfb-geo.com/lev0/News_old_e/news_old_e6.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Example photo&lt;/a&gt; taken from the ISS of the Amazon rainforest. Dark green = rainforest. Lighter colors = forest chopped down to be replaced with agriculture.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s9naJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s9naJ.jpg&quot; alt=&quot;Deforestation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-11-02T16:49:51.137" LastActivityDate="2016-11-02T16:49:51.137" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="18894" PostTypeId="2" ParentId="12777" CreationDate="2016-11-01T15:33:57.007" Score="4" Body="&lt;p&gt;I have a (probably exhaustive) list of every smelting experiments that where done on the space stations (Sayult 5-7; mir) I could post it but it would take time to compile (There was many experiments from different countries; including USA, Cuba, and Europeans).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many of them where simple alloys. The idea was to study the whole process. Some of the experiments where filmed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eventually to obtain greater homogeny that could be obtained on earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many crystals and semiconductor crystals where also processed; both bigger and purer than that we could do on earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The astronauts moving inside the station significantly worsened the results. Some experiments where done at night and during vacancy times to avoid this issue.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-11-01T15:33:57.007" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18895" PostTypeId="1" AcceptedAnswerId="18901" CreationDate="2016-11-01T20:36:14.463" Score="8" ViewCount="532" Body="&lt;p&gt;When the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble space telescope&lt;/a&gt; was originally launched, there was a significant error in the way the main lens was ground, causing major spherical aberration. A repair mission was scheduled and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Corrective_Optics_Space_Telescope_Axial_Replacement&quot; rel=&quot;noreferrer&quot;&gt;COSTAR&lt;/a&gt; was fitted to the telescope and, evidently, fixed the issue. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it have been possible at that time to &quot;just&quot; perform software correction of the spherical aberration? If a similar problem were to happen to a modern telescope, do we now have techniques or hardware that wasn't available in 1990 that would enable a software/firmware-only solution to the problem?&lt;/p&gt;&#xA;" OwnerUserId="11653" LastActivityDate="2016-11-02T01:11:50.747" Title="Could the spherical aberration of Hubble have been solved using only software techniques?" Tags="|space-telescope|hubble|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18896" PostTypeId="2" ParentId="4765" CreationDate="2016-11-01T22:23:30.307" Score="2" Body="&lt;p&gt;I think the where of the question is pretty important. Earth has a fairly strong gravity well and a poor efficiency rocket may not even be able to launch itself to orbit let alone an actual payload. Someplace like Mars with it's lower gravity or already in space is a different story however. For example for SpaceX's mars plan the launch from earth is a ship on top of a rocket which then gets refueled in orbit to get to mars but the return trip is just the ship launching straight from mars and not refueling at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not a rocket scientist either but from what I've ready you basically design your rocket around your engine and choice of fuel. Every little aspect ends up being impacted by those choices and each domino impacts other pieces. As was mentioned by multiple other posts the choice of fuel affects the ratios of the fuels and therefore the sizes of the tanks. Even something as simple as SpaceX's change to use sub-cooled fuels resulted in a change in the sizes of the two tanks. They were able to cool and densify the LOx more than the JP1 so they had to increase the relative size of the JP1 tank compared to the LOx tank to compensate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume similar adjustments had to be made to the pumps because you ultimately want the same ratio of JP1 to LOx but need a different flow rate to accomplish that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm pretty sure you could build a rocket that could run on different fuels but it would undoubtedly be more complex and/or less efficient than a single fuel rocket and for launching from earth that might be a complete no go. Just look at how difficult SSTO (single stage to orbit) designs are when trying to get something with usable capacity.&lt;/p&gt;&#xA;" OwnerUserId="16458" LastActivityDate="2016-11-01T22:23:30.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18897" PostTypeId="1" CreationDate="2016-11-01T22:30:06.307" Score="4" ViewCount="237" Body="&lt;p&gt;Laser fading seems to be a known issue with ring laser gyros. With their STEREO-B probe, NASA experienced a different, &quot;unexpected&quot; failure mode, giving false readings. The Honeywell MIMU's were well beyond the expected lifetime but still, there may be some lessons to be learned from this. Having a second IMU in hot standby increases redundancy in critical mission phases. During extended operations however, any spare unit that ages may better be sent into cold standby.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Do ring laser gyros age (in deep space environment) with laser and dithering disabled?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Were laser and dither disabled on the spare unit for the STEREO mission?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Can the failure mode of STEREO-B's IMU's be attributed to the dithering mechanism?&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Some context to the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA's &lt;a href=&quot;https://stereo.gsfc.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;STEREO&lt;/a&gt;-B is in uncontrolled spin (as of Oct 2016). Efforts to regain control &lt;a href=&quot;http://swrc.gsfc.nasa.gov/main//sites/swrc/presentations/2015_NASA_SWx_Workshop/Wilson_STEREO-LivingPastYourWarranty_20150923.pdf&quot; rel=&quot;noreferrer&quot;&gt;are ongoing&lt;/a&gt; following a &lt;a href=&quot;http://www.theverge.com/2016/8/22/12589406/nasa-stereo-b-spacecraft-contact-deep-space-network&quot; rel=&quot;noreferrer&quot;&gt;telemetry downlink in August 2016&lt;/a&gt; after two years of communication break. Control of the spacecraft &lt;a href=&quot;http://spaceref.com/solar-physics/loss-of-contact-with-stereo-behind.html&quot; rel=&quot;noreferrer&quot;&gt;was lost&lt;/a&gt; on Oct. 1 2014 due to an unlucky double failure during a planned reset sequence &lt;a href=&quot;https://svs.gsfc.nasa.gov/11602&quot; rel=&quot;noreferrer&quot;&gt;in preparation&lt;/a&gt; to solar conjunction (star tracker timeout + IMU fail).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sequence of events is analyzed in this &lt;a href=&quot;http://swrc.gsfc.nasa.gov/main//sites/swrc/presentations/2015_NASA_SWx_Workshop/Wilson_STEREO-LivingPastYourWarranty_20150923.pdf&quot; rel=&quot;noreferrer&quot;&gt;presentation&lt;/a&gt;, prepared by mission specialists at John Hopkins Lab.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastActivityDate="2016-11-01T22:30:06.307" Title="Do ring laser gyros age with laser and dithering disabled?" Tags="|nasa|attitude|deep-space|gyroscope|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18898" PostTypeId="2" ParentId="18718" CreationDate="2016-11-01T23:02:12.690" Score="4" Body="&lt;p&gt;Here's a way to do it entirely online if you don't want to download the CSPICE libraries. Visit &lt;a href=&quot;https://naif.jpl.nasa.gov/naif/webgeocalc.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://naif.jpl.nasa.gov/naif/webgeocalc.html&lt;/a&gt; -&amp;gt; click &lt;a href=&quot;https://wgc.jpl.nasa.gov:8443/webgeocalc/#NewCalculation&quot; rel=&quot;nofollow noreferrer&quot;&gt;WebGeocalc&lt;/a&gt; and click position finder:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1zTOP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1zTOP.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Fill out the data like this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Vuyzq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vuyzq.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EpuSi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EpuSi.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Note that a planetocentric latitude of 0 means the ecliptic latitude is 0, since we're using the ECLIPDATE (ecliptic of the date) reference frame.&lt;/p&gt;&#xA;&lt;p&gt;Hit calculate:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7lRsg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7lRsg.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The results are all the times the moon crosses the ecliptic between 1970 and 2040. This includes both ascending nodes and descending nodes.&lt;/p&gt;&#xA;&lt;p&gt;Of course, you don't want the times, you want the actual positions, so click &amp;quot;Save All Intervals&amp;quot;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Aked9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Aked9.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Go back to the calculation menu and hit &amp;quot;state vector&amp;quot;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4N1DU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4N1DU.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Fill it out as follows:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Nmhr9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Nmhr9.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L7zLF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L7zLF.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Note that you may need to drag your saved intervals list to the &amp;quot;List of Intervals&amp;quot; box, or it may automatically be filled in.&lt;/p&gt;&#xA;&lt;p&gt;After filling out, hit calculate:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H0C1S.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H0C1S.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;And there you have it: the list of lunar positions for when the moon is at its descending and ascending nodes.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wyQL5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wyQL5.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Notice the right ascension of the node moves somewhat linearly:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ybhKC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ybhKC.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;but not exactly.&lt;/p&gt;&#xA;&lt;p&gt;If you were to plot the node's ecliptic longitude, you would see a more linear fit because of the consistent cycle between the moon's draconic and sidereal months.&lt;/p&gt;&#xA;" OwnerDisplayName="user7073" LastEditorUserId="5980" LastEditDate="2021-06-23T02:59:34.937" LastActivityDate="2021-06-23T02:59:34.937" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="18899" PostTypeId="1" CreationDate="2016-11-02T00:59:11.400" Score="2" ViewCount="353" Body="&lt;p&gt;Would it be  technically possible, to build any 'small' space craft that could reach Mars, launching from earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The requirement would only be for the craft to make impact with the planet, crashing into it is totally fine. There is no requirement to carry cargo or passengers, or to land/orbit. There are no budget constraints. The challenge is simply to start on the surface of Earth, and end up on Mars. It would be necessary however to verify that the object did indeed reach its target. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, what are the limits of 'small'? The size of a car? The size of a cell phone? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the main limiting factors be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As this is a very broad question, I am not looking for specific technical detail, but rather what the general issues/constraints might be.&lt;/p&gt;&#xA;" OwnerUserId="17455" LastEditorUserId="17455" LastEditDate="2016-11-03T11:59:45.133" LastActivityDate="2016-11-03T11:59:45.133" Title="What is the smallest starting object that could reach Mars?" Tags="|mars|" AnswerCount="2" CommentCount="11" ClosedDate="2016-11-02T07:38:35.947" ContentLicense="CC BY-SA 3.0" />
  <row Id="18900" PostTypeId="2" ParentId="18889" CreationDate="2016-11-02T00:59:51.047" Score="7" Body="&lt;p&gt;I would like to add one additional effect. Keeping the astronauts sane. Really, ever sit in a small tight room for a week with no windows and no confirmation of anything beyond. In prison they use that as a punishment. Just the ability to look out and confirm that you &quot;can&quot; go there, and you don't &quot;have  to&quot; live the rest of your life in this little tin can, is an amazing effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Try taking a cruise, and getting a small cabin with no window, then nit being allowed to leave that small cabin on penalty of death. You's do pretty much anything to have a tiny window to look out of.&lt;/p&gt;&#xA;" OwnerUserId="11013" LastActivityDate="2016-11-02T00:59:51.047" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18901" PostTypeId="2" ParentId="18895" CreationDate="2016-11-02T01:11:50.747" Score="8" Body="&lt;p&gt;Some effects of the mirror imperfection could have been reverted, others not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Citing from &lt;a href=&quot;http://www.stsci.edu/hst/HST_overview/documents/RestorationofHSTImagesandSpectra.pdf&quot; rel=&quot;noreferrer&quot;&gt;a workshop on HST image restauration&lt;/a&gt;: &quot;The fundamental loss of HST imaging science as a result from spherical aberration is not a loss of resolution; rather, it is a loss in the ability to detect faint objects, especially in crowded fields.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On this workshop, different approaches to enhance HST images were presented. A deblurring by deconvolution was possible in this specials case because of a peculiarity of the error: The mirror mapped stars on the WFPC sensor as &quot;tight core containing about 15% of the light, surrounding plateau containing most of the energy and tendrils extending in apparently random directions&quot;. The &quot;core&quot; provided  enough information to restore resolution. The processing was done on the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YsLlY.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YsLlY.jpg&quot; alt=&quot;an actual HST star image before correction&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HST was designed with replaceable instruments in mind. Leaving the error uncorrected for new instruments was never an option, but fixing it in different ways was, as can be seen from a long list of options in the appendices of the &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910009598.pdf&quot; rel=&quot;noreferrer&quot;&gt;Report of the HST strategy panel&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;No new technologies were developed since 1990 that would allow for the reversal of a lossy signal transformation.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastActivityDate="2016-11-02T01:11:50.747" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18902" PostTypeId="1" CreationDate="2016-11-02T02:01:45.920" Score="-1" ViewCount="1734" Body="&lt;p&gt;They state there are about 500 or so satellites orbiting around the moon. However according to NASA back when they said we landed on the moon, when the astronauts orbited around the dark side of the moon they stated&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;radio transmission got cut off and was a bit of a tense moment until the radio transmission resumed when they came around again.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why would you have 500 so-called satellites orbiting the moon if they only work half the time since according to NASA there is &lt;em&gt;no&lt;/em&gt; communication on the dark side?&lt;/p&gt;&#xA;" OwnerUserId="17456" LastEditorUserId="6816" LastEditDate="2016-11-02T02:48:19.197" LastActivityDate="2021-12-18T20:21:12.780" Title="Why are there so many satellites around the moon if they are blocked out of radio communication half the time?" Tags="|artificial-satellite|the-moon|communication|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18903" PostTypeId="1" AcceptedAnswerId="18922" CreationDate="2016-11-02T03:13:29.543" Score="1" ViewCount="290" Body="&lt;p&gt;I watched a presentation on a manned mission to Mars and learned it would take two years until the Earth and Mars were properly aligned for the return mission from Mars.  Are there any major technological hurdles to surviving on the planet and producing the fuel necessary for the return?&lt;/p&gt;&#xA;" OwnerUserId="17457" LastEditorUserId="37047" LastEditDate="2020-11-25T10:46:55.087" LastActivityDate="2020-11-25T10:46:55.087" Title="What are the main hurdles to surviving on Mars until a return trip becomes possible?" Tags="|mars|habitat|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="18904" PostTypeId="1" AcceptedAnswerId="18916" CreationDate="2016-11-02T03:33:03.207" Score="36" ViewCount="13780" Body="&lt;p&gt;Why do rocket nozzles open wider at the end than, let's say, get narrower?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Let me explain:&lt;/p&gt;&#xA;&lt;p&gt;A jet engine works by having this amazing thing called a combustion chamber. The combustion chamber ignites the air to expand it, thus, causing  more thrust. And most jet engines (such as turbofan engines) &lt;em&gt;increase&lt;/em&gt; the amount of thrust by simply having the engine get narrower as you get behind. (i.e hole getting smaller).&lt;/p&gt;&#xA;&lt;p&gt;But for rockets, the nozzle gets larger and larger. Such as below.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/neTqB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/neTqB.jpg&quot; alt=&quot;typical rocket nozzle&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Wouldn't it be theoretically more efficient to have the nozzle end get slightly narrower similar to a jet engine, to gain more thrust? I'm assuming people have already thought of this idea, because it seems like such a basic concept. Yet, I can't find/think of an answer why...&lt;/p&gt;&#xA;" OwnerUserId="17203" LastEditorUserId="1235" LastEditDate="2020-09-15T20:46:27.970" LastActivityDate="2020-09-15T20:46:27.970" Title="Why do rocket nozzles flare?" Tags="|rockets|engines|engine-design|nozzle|combustion|" AnswerCount="9" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="18905" PostTypeId="2" ParentId="18902" CreationDate="2016-11-02T04:46:53.580" Score="8" Body="&lt;p&gt;I find the 500 figure very, very questionable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are times that you simply accept the blackouts.  Any satellite whose objective is a substantial body other than the Earth or Sun will experience blackouts caused by that body.  It's the price you pay for sending the mission there.  The satellites simply have to be built to talk during the time they have a line of sight.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-11-02T04:46:53.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18906" PostTypeId="2" ParentId="18902" CreationDate="2016-11-02T05:46:38.993" Score="18" Body="&lt;p&gt;Your premise is incorrect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only 37 or so spacecraft have ever orbited the moon. Of these, only 3-4 are active in lunar orbit today. &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_extraterrestrial_orbiters#Moon&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/List_of_extraterrestrial_orbiters#Moon&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the time of the Apollo moon landing missions, there was only one American satellite orbiting the moon, Explorer 35. It was a science mission, not a communication satellite.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-11-02T21:15:02.170" LastActivityDate="2016-11-02T21:15:02.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18907" PostTypeId="2" ParentId="18899" CreationDate="2016-11-02T06:13:14.560" Score="2" Body="&lt;p&gt;For verification, let's say that we're launching 100kg of cyan powder which will be explosively deployed at low altitude shortly before impact. We will need a small computer, altimeter, pyrotechnics, heat shield, parachute decelerator, and so forth - probably at least another 100kg there. Radiothermal generator or solar panels, communication antenna, 50kg. So our total entry-descent-and-splatter package comes to ~250kg. You could probably make it smaller at additional cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now you need a launcher that can send that package to Mars. From Earth orbit you could do the Mars transfer with a 2-3 ton stage. So any launcher that can put a small satellite into orbit could do it. The Vega launcher could do it for relatively cheap. &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Vega_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.m.wikipedia.org/wiki/Vega_(rocket)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A smaller launcher like the Japanese Lambda 4S couldn't do it -- it puts only 26kg in Earth orbit, and there's no practical way to build an interplanetary stage that small. &lt;a href=&quot;https://en.wikipedia.org/wiki/Lambda_4S&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Lambda_4S&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-11-02T06:13:14.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18908" PostTypeId="2" ParentId="18899" CreationDate="2016-11-02T06:41:33.473" Score="3" Body="&lt;p&gt;The main challenge is interplanetary navigation, to actually be able to hit Mars. &lt;a href=&quot;https://solarsystem.nasa.gov/basics/bsf13-1.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacecraft do not navigate by themselves - navigation is done on the ground&lt;/a&gt;, with help of Deep Space Network(s), precisely timing the radio signals and calculating the trajectories correspondingly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally, a launch vehicle upper stage sends a spacecraft en route to Mars, ground tracking determines the trajectory and calculates the necessary corrections. Hence, your spacecraft needs a radio to be trackable. Radio needs power as well, normally solar power, so you need to add some solar panels. In theory, it would be possible to carefully calculate power required for every communication session and just use primary batteries, which might further simplify things, but it would be quite risky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, you need to be able to perform a few trajectory correction maneuvers. For that you need a propulsion system with enough delta-V budget to perform the corrections, which regularly is 3-4 maneuvers in total. With precise insertion, good tracking, the trajectory corrections should be limited to tens of meters/second of delta-V total, so the propulsion system does not need to be big or powerful. &lt;a href=&quot;http://mars.nasa.gov/mro/mission/timeline/mtcruise/cruisetcms/&quot; rel=&quot;nofollow noreferrer&quot;&gt;To correctly perform TCMs&lt;/a&gt;, you do need attitude control with fairly accurate pointing capability, which you likely need for radio communications anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This should be sufficient to make something impact Mars, and if you add it all up, you are going to find that a regular &lt;a href=&quot;https://en.wikipedia.org/wiki/CubeSat&quot; rel=&quot;nofollow noreferrer&quot;&gt;1U cubesat&lt;/a&gt;, weighing 1Kg is not enough to fit everything in. You'll need something bigger, on the order of a hundred or few hundred kilograms, or with really advanced design maybe a 12U cubesat. There are &lt;a href=&quot;http://www.nasa.gov/press-release/nasa-prepares-for-first-interplanetary-cubesats-on-agency-s-next-mission-to-mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;multiple teams&lt;/a&gt; &lt;a href=&quot;https://icubesat.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;around the world&lt;/a&gt; trying to figure out how to make something as small work for interplanetary flight, and it's going to be a few years at least before this can happen. And even then, these will first use a &quot;mothership&quot; approach, where a bigger spacecraft takes the smaller ones to the target orbit first. Note that there are even &lt;a href=&quot;https://icubesat.files.wordpress.com/2015/05/icubesat-2015_org_b-1-2_tyvak_coelho.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;some rare commercial opportunities&lt;/a&gt; available to launch interplanetary cubesats in up to 12U sizes, but not on a Mars-bound trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To detect if the impact took place, you'll know exactly how Shiaparelli team learned about it - radio transmission will terminate, abruptly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For some examples of how such mission might be accomplished, you can look at early history of interplanetary flights. &lt;a href=&quot;https://en.wikipedia.org/wiki/Mariner_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mariner 2&lt;/a&gt; weighed about 200 kilograms and was the first probe to make it to another planet. Some of the more modern small spacecraft in the same weight class have been &lt;a href=&quot;https://en.wikipedia.org/wiki/SMART-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;SMART-1&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Prospector&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar prospector&lt;/a&gt;. Also Japanese &lt;a href=&quot;https://en.wikipedia.org/wiki/Suisei_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sagigake&lt;/a&gt; and it's sister Suisei, weighing in at only 140kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly enough, there is an extra barrier for hitting Mars: planetary protection. Whatever you intend to impact on Mars, needs to adhere to some agreed upon cleanliness requirements. For more info, you can refer to this paper titled &lt;a href=&quot;http://issfd.org/2015/files/downloads/papers/008_Martin-Mur.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Interplanetary Cubesat Navigational Challenges&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3140" LastEditorUserId="3140" LastEditDate="2016-11-02T14:06:06.487" LastActivityDate="2016-11-02T14:06:06.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18909" PostTypeId="2" ParentId="18858" CreationDate="2016-11-02T08:43:44.350" Score="2" Body="&lt;p&gt;Mechanical bearings are a problem in the vacuum of space. Grease will vaporize. If thin oxide layers on the surface of metal parts are removed by friction, cold welding of those parts is possible.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-11-02T08:43:44.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18910" PostTypeId="2" ParentId="18856" CreationDate="2016-11-02T08:50:14.617" Score="3" Body="&lt;p&gt;Balsa wood was discussed for the thermal insulation of cryonic LH2 tanks of the Saturn rocket. But there was not enough balsa wood available, therefore glas fiber reinforced polyurethane foam was used.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="13126" LastEditDate="2016-11-02T12:52:02.867" LastActivityDate="2016-11-02T12:52:02.867" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18911" PostTypeId="2" ParentId="18904" CreationDate="2016-11-02T11:22:39.187" Score="8" Body="&lt;p&gt;The combustion of propellants is an exothermic process, it mainly provides heat. &lt;em&gt;Initial&lt;/em&gt; velocity (think of the turbo pumps) and the changed specific gas constant of the combustion product are negligible. Heat also translates into pressure via the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Ideal_gas_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;gas law&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat and pressure are somewhat useless once the exhaust gas does not interact with our rocket any more. They are lost when the gas mixes with the atmosphere or simply expands in empty space. The fact that these processes are irreversible is expressed as an increase in &lt;a href=&quot;https://en.m.wikipedia.org/wiki/History_of_entropy&quot; rel=&quot;nofollow noreferrer&quot;&gt;entropy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Only momentum of the gas provides thrust and that is proportional to velocity. The task of combustion chamber, choke and nozzle is to convert pressure into velocity efficiently in an adiabatic expansion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For good thermodynamic efficiency, the gas flow &lt;em&gt;should&lt;/em&gt; be &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Isentropic_nozzle_flow&quot; rel=&quot;nofollow noreferrer&quot;&gt;isentropic&lt;/a&gt; (but the stagnation point shifts depending on ambient pressure and speed, so it's always a compromise for ascent stages). This design goal dictates the form of the exhaust duct. For a subsonic flow, a narrowing duct will generate backpressure, effectively reducing the pressure at the choke and increasing velocity (&lt;a href=&quot;https://en.m.wikipedia.org/wiki/Bernoulli%27s_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Benoulli's principle&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Backpressure does however not work for supersonic flows. The pressure will simply not propagate backwards, instead shockwaves would build up. &lt;strong&gt;The optimum profile of a duct does therefore have its narrowest point where the flow reaches Mach 1 and does widen after that  choke.&lt;/strong&gt; A formal derivation of this fact from the state equation for a compressible fluid flow can be found &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Isentropic_nozzle_flow#Supersonic_flow&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Physics.SE also has &lt;a href=&quot;https://physics.stackexchange.com/q/57497&quot;&gt;an answer on the theoretical background&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aircraft capable of supersonic cruise also have convergent divergent nozzles, because they need to generate a supersonic exhaust flow to maintain their speed. Note that supersonic nozzles can be used for subsonic movement (as is the case for a rocket at liftoff) but not the other way around.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2016-11-02T14:25:53.337" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18912" PostTypeId="2" ParentId="18904" CreationDate="2016-11-02T11:32:17.230" Score="2" Body="&lt;p&gt;This page explains it really well (better than me): &lt;a href=&quot;https://spaceflightsystems.grc.nasa.gov/education/rocket/nozzle.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://spaceflightsystems.grc.nasa.gov/education/rocket/nozzle.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As explained there, the exhausted mass is constant but its parameters aren't. At the entry of the nozzle, the gas is hot and has a very high pressure. At the output of the nozzle, the pressure is a lot smaller (as it's the one from the atmosphere or space). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nozzle is used to expand the exhaust gas from that high pressure to the very small external pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pressure and temperature are related to velocity. Hence, reducing the pressure allows to gain in exhaust velocity and thus in thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will also notice that the nozzle is bigger on second stages than on first stages because of the difference of pressure of the environment (atmosphere versus space).&lt;/p&gt;&#xA;" OwnerUserId="17308" LastEditorUserId="17308" LastEditDate="2016-11-02T13:52:44.037" LastActivityDate="2016-11-02T13:52:44.037" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18913" PostTypeId="2" ParentId="18904" CreationDate="2016-11-02T12:16:34.717" Score="6" Body="&lt;p&gt;You're missing one thing: the combustion chamber (hidden behind the piping at the top of the photo). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a cross-section of a rocket engine:&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/CTLFz.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CTLFz.gif&quot; alt=&quot;Nozzle section&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.braeunig.us/space/propuls.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thrust is defined&lt;/a&gt; as: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;F = qVe +(Pe-Pa)Ae&lt;/p&gt;&#xA;&#xA;&lt;p&gt;F = thrust force&lt;br&gt;&#xA;q = the amount of mass going out&lt;br&gt;&#xA;Ve = exhaust velocity&lt;br&gt;&#xA;Pe = pressure at the nozzle end&lt;br&gt;&#xA;Pa = ambient pressure&lt;br&gt;&#xA;Ae = area of the nozzle end  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt; to remove an incorrect statement: &#xA;Borrowing from OrganicMarble's answer: the flow is accelerated by a diverging passage, so you want a large nozzle diameter. &#xA;The nozzle diameter is limited by ambient pressure, though, roughly because you need the pressure in the nozzle to be higher than ambient. It's &lt;a href=&quot;https://space.stackexchange.com/questions/18555/what-is-the-summerfield-criterion-regarding-rocket-exhaust-expansion/18556#18556&quot;&gt;a bit more complicated&lt;/a&gt; than that, apparently Pe can be lower than ambient&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pc is large, so if you want to make Pe equal to Pa, you need a nozzle opening much larger than the throat (the spot where the combustion chamber meets the nozzle). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2019-07-21T10:55:24.517" LastActivityDate="2019-07-21T10:55:24.517" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="18914" PostTypeId="1" CreationDate="2016-11-02T13:55:11.397" Score="1" ViewCount="452" Body="&lt;p&gt;I'm currently modeling the lunar landing in 2D. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I set up the momentum balance: $\ddot{\theta} = \frac{dF_l}{J}$ where $d$ is the distance between the sidethruster and the center of mass. $F_l$ is the force from the side thruster and $J$ is the moment of inertia. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't really have a background in physics, and was wondering what a decent value for $J$ could be. At the moment it's at 100 000 and the distance is 4. But I'd like to be able to explain why I chose these values. &#xA;Any input would be enormously helpful&lt;/p&gt;&#xA;" OwnerUserId="17462" LastActivityDate="2016-11-02T13:55:11.397" Title="Moment of inertia of apollo 11 lunar module" Tags="|apollo-program|lunar-landing|physics|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18915" PostTypeId="2" ParentId="18889" CreationDate="2016-11-02T14:12:02.297" Score="2" Body="&lt;p&gt;Other people have already pointed out practical reasons for being able to see outside; for controlling robot arms, landing shuttles manually or general failover. Cameras are not only more fragile, but much more complex and expensive than a simple window (camera + monitor + camera controls + cable management through the outer wall of the vehicle + power usage, and so on). There is nothing particularly difficult about replacing a section of wall with a window, on the other hand. We do it in industrial settings all over the place (glasses for high pressure, high temperature apparatusses etc.).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is it just to allow people in space look back to Earth so that they don't feel bored?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But let me stress that you can easily leave out the word &quot;just&quot; in this sentence, and will hit at least part of the truth right on the head.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;people in space&quot; are not just circumstantial, but they are the most important part of any manned (sic) space program. I don't have numbers, but I'd wager that the cost of selection, training, life support systems etc., make a great, if not huge part of the overall cost (both money and time) of manned spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence, even if there were no other reasons, the relatively minor cost of adding a few windows might offset huge costs for otherwise necessary measures to keep the humans on the ISS sane. Keep in mind that other things are also done &quot;just&quot; to keep the astronauts happy. Like a small weight allowance to bring personal stuff like music instruments and the like. If that were not a psychological necessity, you can be sure that it would not happen.&lt;/p&gt;&#xA;" OwnerUserId="16024" LastEditorUserId="16024" LastEditDate="2016-11-02T22:29:39.493" LastActivityDate="2016-11-02T22:29:39.493" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18916" PostTypeId="2" ParentId="18904" CreationDate="2016-11-02T14:14:04.053" Score="50" Body="&lt;p&gt;The purpose of this nozzle is to achieve maximum acceleration of the flow to obtain the highest possible exit velocity.&lt;/p&gt;&#xA;&lt;p&gt;The shape of convergent / divergent (de Laval) nozzles is dictated by the thermodynamic properties of gases.&lt;/p&gt;&#xA;&lt;p&gt;For a subsonic gas flow, a converging passage accelerates the flow.  The physics are opposite for supersonic flows: they are accelerated by a diverging passage.  So the relatively short converging portion of the nozzle you see is where the flow accelerates to supersonic speed in the throat, then the long diverging portion accelerates the supersonic flow.&lt;/p&gt;&#xA;&lt;p&gt;Where you cut the nozzle off depends on what you want the nozzle exit plane pressure to be.&lt;/p&gt;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://books.google.com/books/about/The_Dynamics_and_Thermodynamics_of_Compr.html?id=OvxQAAAAMAAJ&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Dynamics and Thermodynamics of Compressible Fluid Flow&lt;/a&gt;, Ascher Shapiro, Volume I (1953). See &lt;strong&gt;Converging-Diverging Nozzles&lt;/strong&gt; p.93 and especially the discussion of figure 4-12 &lt;em&gt;Operation of converging-diverging nozzle at various back pressures&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-09-15T01:56:51.977" LastActivityDate="2020-09-15T01:56:51.977" CommentCount="14" ContentLicense="CC BY-SA 4.0" />
  <row Id="18917" PostTypeId="2" ParentId="18904" CreationDate="2016-11-02T14:42:38.393" Score="25" Body="&lt;p&gt;The gas at the narrowest part (the throat) of a convergent-divergent nozzle used in a rocket engine is ideally moving at the Mach 1, the speed of sound. This creates a choked flow condition. After the throat, the gas expands, the temperature drops, and because of the Venturi effect, it speeds to beyond Mach 1. A convergent-divergent nozzle thusly converts some of the thermal energy in the exhaust into kinetic energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are limits to the extent to which a rocket can convert that thermal energy into kinetic energy. A rocket operating in vacuum could, in theory, convert almost all of that thermal energy into kinetic energy. The exhaust would leave the nozzle at close to zero Kelvin, and with the exhaust being almost perfectly columnated. This would however require an infinitely long nozzle. At some point, adding to the nozzle becomes a net detriment rather than a benefit. A rocket operating in the atmosphere ideally has the exit pressure equal to ambient. Increasing the nozzle beyond this ideal results in back pressure by the atmosphere against the rocket, while decreasing the nozzle below this ideal results in decreased exit velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about aircraft? Subsonic aircraft don't use a convergent-divergent nozzle because the flow isn't choked. There's a performance penalty to be paid for flows that exceed the speed of sound. That is a price that must be paid to escape the Earth's atmosphere and go into orbit. Some hypersonic aircraft do use a convergent-divergent nozzle, but it's often hidden inside the engine. The nozzle is a conical shape that is widest at the throat and tapers toward the rear of the engine. Instead of flowing through the nozzle, the exhaust flows around the nozzle in a jet engine.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-11-02T14:47:57.817" LastActivityDate="2016-11-02T14:47:57.817" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18918" PostTypeId="1" CreationDate="2016-11-02T17:36:47.783" Score="4" ViewCount="187" Body="&lt;p&gt;I was also intrigued by the same problem posed on &lt;a href=&quot;https://space.stackexchange.com/questions/14089/which-computer-monitors-will-work-on-mars&quot;&gt;Which computer monitors will work on Mars?&lt;/a&gt; I would like to take it further to work out which specs (and technologies) should a tablet computer have in order to work either on Mars or outer space? What would be a workable concept?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will define, for our purposes a &lt;strong&gt;tablet&lt;/strong&gt; as a handheld computer device with a sensitive screen that would allow interactions with a human being without a hardware keyboard. Aside of resisting to harsh conditions (low pressure, cold, radiations, etc.) it would have to be &lt;em&gt;usable&lt;/em&gt; by a human being who will presumably be wearing a space suit with handgloves. Since a naked finger is not available, what kind of medium could be used? (stylus, adapting the glove, etc.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess we should accept some &quot;degradation&quot; compared to a regular tablet, i.e. that it could be tethered for energy, and that instead of weighing a pound, it could be  heavier. But at least it should be &lt;em&gt;portable&lt;/em&gt; (i.e. not require exagerate effort, complications or machinery to move around). It would have to be usable outside of a habitat on Mars, or outside of a spacecraft, during a space walk. I assume, also, that these two conditions would define two different &quot;grades&quot; of &quot;space fitness&quot;, like those used for &quot;waterproof&quot;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, if the technology is not available yet, it would be OK to count on it if it could be &lt;em&gt;developed&lt;/em&gt; (if it is reasonable within 10 years and does not involve scientific speculations).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(If you feel that the framing of the question should be slightly revised, please say so.)&lt;/p&gt;&#xA;" OwnerUserId="17465" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-12-02T20:01:41.117" Title="Which specifications should a tablet computer have in order to work in outer space?" Tags="|mars|equipment|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18919" PostTypeId="2" ParentId="18918" CreationDate="2016-11-02T17:57:09.320" Score="2" Body="&lt;p&gt;They'd have to use resistive touch, or have a glove that conducts the users capacitance (too many complications in an already high-risk environment).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside of a spacecraft would require a cooling system to keep the tablet in the operating temperature range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacewalks usually last 5-8 hours.  So battery life would be an issue. But, being weightless would only mean bulkier, not heavier batteries.&lt;/p&gt;&#xA;" OwnerUserId="17351" LastEditorUserId="58" LastEditDate="2016-11-02T18:32:29.550" LastActivityDate="2016-11-02T18:32:29.550" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18920" PostTypeId="2" ParentId="18889" CreationDate="2016-11-02T19:11:17.600" Score="5" Body="&lt;p&gt;Windows have been primarily required for&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Docking operations (Gemini, Apollo CM and LM, STS, ISS)&lt;/li&gt;&#xA;&lt;li&gt;Landings (Apollo LM, STS)&lt;/li&gt;&#xA;&lt;li&gt;Remote manipulation, e.g. Canadarm (STS, ISS)&lt;/li&gt;&#xA;&lt;li&gt;Visual inspection (Apollo, STS, ISS)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;We're still not at a point where every last thing is automated or instrumented, and some manual intervention and inspection is still necessary.  Not to mention &#xA;that instruments occasionally fail, and having a Mark I Eyeball as a backup&#xA;is not a bad thing.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And do not discount the psychological value of having a window.  I've worked in offices with and without windows, and just having a bit of natural light filtering in can make a &lt;em&gt;huge&lt;/em&gt; difference in mood and productivity.  Yes, &#xA;astronauts &lt;em&gt;should&lt;/em&gt; have the psychological stamina to be able to work in closed spaces for extended periods of time, but if having a window makes it easier for them to do their jobs, then why not have one?&lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2016-11-02T19:11:17.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18922" PostTypeId="2" ParentId="18903" CreationDate="2016-11-02T21:38:00.323" Score="2" Body="&lt;p&gt;One of the big hurdles to surviving on Mars long term is radiation. Mars hasn't got a very strong magnetosphere (about 1500 nanotesla compared to Earth's 65000). This is what deflects radiation and solar winds and stops it from hitting us. This is also thought to be one of the reasons Mars has a thin atmosphere, about 0.6% of what Earth has.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means in the long term we would have to protect the astronauts and equipment from the radiation and solar winds.&lt;/p&gt;&#xA;" OwnerUserId="17469" LastEditorUserId="17469" LastEditDate="2016-11-03T07:10:16.230" LastActivityDate="2016-11-03T07:10:16.230" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18923" PostTypeId="2" ParentId="18831" CreationDate="2016-11-02T21:49:01.753" Score="-2" Body="&lt;p&gt;The Moon does have an atmosphere. It is about 1,000,000 particles per m2, this is not very much compared to Earths 10,000,000,000,000,000,000. It is about comparable to the atmosphere that exists in the same region that the ISS orbits - 330 - 435km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you launched a rocket whatever gases were produced would likely not hang around long, of course there would be trace amounts and exactly what gases would depend on the fuel/oxidizer used in the rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The moon's atmosphere currently consists of helium, argon and possibly neon, ammonia, methane and carbon dioxide. This discovery was made by the Lunar Atmospheric Composition Experiment (LACE) that was placed on the lunar surface on the Apollo 17 mission. On Earth we have used telescopes to detect sodium and potassium in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source &lt;a href=&quot;https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphere.html#.WBpdT_mLSUk&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA: Is there an atmosphere on the moon?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17469" LastActivityDate="2016-11-02T21:49:01.753" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18924" PostTypeId="1" CreationDate="2016-11-03T03:24:28.960" Score="3" ViewCount="812" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/18877/12102&quot;&gt;This answer&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/18881/12102&quot;&gt;this answer&lt;/a&gt; address the question &lt;em&gt;Do reaction wheels generally run in vacuum, or are they pressurized? Sealed or vented?&lt;/em&gt; and they both mention that sometimes momentum wheels are sealed with a low pressure (inert) gas rather than vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I found the following in &lt;a href=&quot;http://utias-sfl.net/wp-content/uploads/SSC07-X-3.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sinclair et al. SSC07-X-3, 21st Annual AIAA/USU Conference on Small Satellites&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Many small satellite wheels use a hermetic housing filled with a low-pressure gas. The internal atmosphere allows the use of bearing lubricants with modest vapour pressures as well as protecting parts from contamination or damage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Gas would create additional friction for the system in addition to bearing friction (and possibly eddy currents), which means additional low power. It also means if power to the motor is not available for a period of time, angular moment would couple back to the spacecraft more quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there must be some benefit to the presence of gas in some applications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/18881/12102&quot;&gt;This answer&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There's a problem with liquid lubricants: They evaporate in vacuum. For this reason, most reaction wheel assemblies are hermitically sealed with a low pressure inert gas inside the container.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But my understanding is that &lt;a href=&quot;https://en.wikipedia.org/wiki/Evaporation&quot; rel=&quot;nofollow noreferrer&quot;&gt;evaporation&lt;/a&gt; is really more a function of the &lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Partial_pressure&quot; rel=&quot;nofollow noreferrer&quot;&gt;partial pressure&lt;/a&gt;&lt;/strong&gt; of the material in question within the gas, &lt;strong&gt;not the total pressure&lt;/strong&gt; of the ambient inert gas. I don't understand how the gas could inhibit evaporation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I don't understand why some reaction wheels would be sealed with a low pressure gas, and others with vacuum. What are the trade-offs and why is vacuum chosen in some cases and low pressure gas chosen in others?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The table below (open in new window for full size) is from &lt;a href=&quot;http://cdn.intechopen.com/pdfs-wm/13470.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lubrication of Attitude Control Systems&lt;/a&gt; and was originally linked &lt;a href=&quot;https://space.stackexchange.com/a/18877/12102&quot;&gt;here&lt;/a&gt;. It's a helpful review of the subject, and the vapor pressures of various lubricants (some more vacuum-friendly than others) are listed near the bottom row. It's worth reading the PDF, it's a nice presentation of the issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The right hand side of the table is cut off in the original PDF.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DjI0S.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DjI0S.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-09-17T22:50:23.337" LastActivityDate="2019-09-17T22:50:23.337" Title="Why are some reaction wheels sealed with low pressure gas, others with vacuum?" Tags="|design|physics|attitude-determination-and-control|reaction-wheel|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="18925" PostTypeId="1" AcceptedAnswerId="18931" CreationDate="2016-11-03T03:59:01.157" Score="8" ViewCount="2332" Body="&lt;p&gt;The cargo bay doors of the Space Shuttle were used on-orbit as radiators for thermal control. The cargo bay doors were closed before the reentry burn and descent to the surface, but could there have been a way to utilize the radiators for further active thermal control?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reasons I believe they were not used this way:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.nas.nasa.gov/Software/VWT/streamlines.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;The streamlines&lt;/a&gt; (streaklines?) going over the leading edge extensions look like they are going straight into where the doors would be, in their open position. This would likely either break them off or melt them.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;a href=&quot;https://i.vimeocdn.com/video/476158023.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;stagnation temperature behind the radiators&lt;/a&gt; is still higher than the temperature of the shuttle, so this would potentially have the opposite effect, at least according to this visualization.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Some unknown destabilizing effect on the elevons / vertical tail&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="11653" LastEditorUserId="11653" LastEditDate="2018-09-18T19:52:48.323" LastActivityDate="2019-02-02T23:32:09.510" Title="What effect (if any) would leaving the cargo bay doors open have had during a shuttle reentry?" Tags="|reentry|space-shuttle|radiators|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="18927" PostTypeId="2" ParentId="18903" CreationDate="2016-11-03T08:40:50.403" Score="2" Body="&lt;p&gt;Keeping the life support system running for the habitat and the space suits is the first problem. If fuel is produced there all systems necessary for that must work during the stay. The space ship(s) needed for return will need some maintenance and a lot of checks before the countdown sequence for return to earth. If a space ship for return is left alone in an orbit, it must be prepared for return after two years of hibernation in some weeks or months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But all repairs, checks and services must be done by the astronauts themselves. They can ask the experts on the earth for advice, but nobody is there to help them. All necessary parts and tools for repairs must be there, nothing can be ordered. If someone gets sick and needs medical help, they will get advice from earth, but a surgery must be done by the other astronauts. Every medication that might be needed must be there and all necessary medical equipment. But only small and lightweight medical equipment will be there in case of an emergency.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-11-03T08:40:50.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18929" PostTypeId="2" ParentId="18904" CreationDate="2016-11-03T11:55:10.870" Score="4" Body="&lt;p&gt;The role of the combustion chamber is to burn as much of the fuel+oxidizer as possible; never discard any unburnt fuel or oxidizer because it had to be carried there at huge expense of fuel and oxidizer earlier - every gram counts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jets carry only fuel, they have air available in abundance, so as long as all fuel is burnt, surplus of air not having reacted with the fuel doesn't hurt - and actually helps; heated it expands and provides thrust, without need for huge exhaust velocity which would be hard on the turbines; more gas expanding by less, instead of a small amount of gas expanding by a huge factor - in rockets a tiny amount of mass provides a lot of thrust. In jets, the amount of mass carried by the plane is even smaller, but the mass providing thrust - intake air - is much larger, the airplane over a single flight pushing many times its own weight in air through the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then there's aerodynamics. Refer to &lt;a href=&quot;https://space.stackexchange.com/questions/8806/what-are-the-differences-between-a-standard-merlin-engine-and-the-merlin-vacuum&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two engines on the left have nozzles for atmospheric use. By the time the exhaust gas reaches the opening, its pressure isn't much higher than atmospheric, and it can't provide much more thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0A9SH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0A9SH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is the bell nozzle attached to the third of the above engines - meant for vacuum.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/YC9rw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YC9rw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every last bit of momentum is squeezed from the exhaust gas, which would otherwise escape uselessly sideways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Installing such a thing on an airplane would be completely counter-productive because the huge nozzle itself would introduce so much air drag (through its outside in the air stream) it would completely nullify all the benefits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although, your question does have a significant merit. Nothing beats the bell nozzle in void; it's the most efficient way to harvest momentum out of gas expanding into void. But bell nozzles for atmospheric engines are a crutch, an unoptimal reduction of the void bell nozzle problem to border conditions of the atmospheric pressure. They work, they work well, but they don't work optimally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The counterpart of the nozzle of an airplane jet engine in rocketry is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerospike_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aerospike engine&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OcGQk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OcGQk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=DacGl9drefg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Video of aerospike in action&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The aerospike is definitely superior to bell nozzle in atmospheric conditions. The problem though, is that large-scale implementation of aerospike engines would require a lot of new research, while bell nozzles are 'tried and true', tested, well known and readily available. And so, because nobody wants to pay for &quot;being the first&quot;, we're still stuck with bell nozzles for atmospheric rocketry.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-11-03T11:55:10.870" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18930" PostTypeId="1" CreationDate="2016-11-03T13:16:02.887" Score="5" ViewCount="145" Body="&lt;p&gt;The Sun's magnetic field now and then accelerates a gigantic amount of high velocity protons in different directions. Regardless of the types and causes, let's call it &quot;proton storm&quot; when it is dangerous enough to cause radiation acute sickness to astronauts outside of the useful natural protection of Earth's magnetic field and atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the probability that a deep space mission gets hit by such a Solar proton storm? The Apollo missions all were lucky in this respect. But how lucky were they? &lt;em&gt;As a benchmark, let's assume here the same radiation protection that the Apollo missions had, which I understand wasn't much at all.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's the probability for a half-month Apollo type crew on the sunny side of the Moon being hit by an acutely unhealthy Solar proton storm?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's the probability of a 26 months Hohmann conjunction mission to, and on, Mars having to experience such an event? If they did it the Apollo way in terms of radiation protection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The further out from the Sun, the lower the impact, I suppose. Would it for example be a non-issue for humans at Saturn? Is there a neat probability distribution as a function of distance and time?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2016-11-03T13:24:49.610" LastActivityDate="2021-01-20T00:00:26.110" Title="What is the probability that a deep space crew is hit by a Solar proton storm?" Tags="|radiation|health|humans|risk-management|solar-flares|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18931" PostTypeId="2" ParentId="18925" CreationDate="2016-11-03T17:04:56.220" Score="19" Body="&lt;p&gt;The shuttle was not certified to enter with the payload bay doors open.  In fact, it was not even certified to enter with more than one of the latches that held the doors closed failed open.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space Shuttle Flight rule A10-23 states that with even a single latch gang failed open steps will be taken to minimize entry structural loads.  For more than one failed open, an EVA would be performed to install latch tools to hold the doors closed. (sorry about all caps, that's the way they wrote the rules)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;FOR FAILURE OF ANY TWO PLBD LATCH GANGS TO FULLY ENGAGE&#xA;(CENTERLINE OR BULKHEAD), AN EVA WILL BE PERFORMED TO INSTALL&#xA;MANUAL LATCH TOOLS ON AT LEAST ONE OF THE FAILED LATCH GANGS.&#xA;&#xA;B. FOR FAILURE OF ANY SINGLE OR MULTIPLE PLBD LATCH GANGS TO&#xA;FULLY ENGAGE (CENTERLINE OR BULKHEAD), ENTRY LOADS WILL BE&#xA;MINIMIZED PER THE FOLLOWING:&#xA;&#xA;1. ENTRY TEST MANEUVERS WILL NOT BE PERFORMED, AND ANY&#xA;PROGRAMMED TEST INPUTS (PTI’S) WILL BE INHIBITED.&#xA;&#xA;2. CONSIDERATION WILL BE GIVEN TO TARGETING A RUNWAY/HAC&#xA;APPROACH AT THE PLS OR SLS TO MINIMIZE TAILWIND AT HAC&#xA;INITIATION AND TURBULENCE/SURFACE WINDS.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The rationale section is as follows: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is no analysis which indicates that entry with multiple latch&#xA;  gang failures is an acceptable condition. Although the latch tools&#xA;  have not been proven to provide the same structural integrity as a&#xA;  nominal latch gang, performing an EVA to install these latch tools&#xA;  provides a workaround to recover at least some load-carrying&#xA;  capability. Two latch tools are required to secure a single failed&#xA;  latch gang; however, only two centerline and two bulkhead latch tools&#xA;  are flown. When two of the same type gangs have failed, only one gang&#xA;  can be secured, as both available tools are required to secure one of&#xA;  these latch gangs.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A 1991 Rockwell assessment (which refined analyses performed in 1981&#xA;  and 1987/1988) for a PLBD&#xA;      single latch-out (centerline or bulkhead) certified a nominal entry trajectory in AUTO flight control by&#xA;      exhibiting structural, thermal, and aerodynamic margins above the required 1.4 safety factor. This&#xA;      analysis shows that steps and gaps may occur as a result of the payload bay-to-ambient delta pressure&#xA;      and latch disengagement. Temperature exceedances may occur at these steps and gaps, but the resulting&#xA;      thermal damage poses no flight safety concerns. The extent of the postflight repairs can be limited by&#xA;      minimizing side slip and steady pitch-up maneuvers, according to the 1991 analysis. (Reference&#xA;      Structural Assessment of Shuttle Payload Bay Doors for Entry with a Latch Gang Out, SSD91D0276,&#xA;      May 1991, and A/E FTP #96, 12/18/92.)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Although entry test maneuvers and PTI’s should not expose the vehicle&#xA;  to conditions outside of its&#xA;      certification limits, flying a nominal entry trajectory minimizes adverse mechanical, thermal, and&#xA;      aerodynamic loads. Vehicle accelerations, heating, and compartment-to-ambient pressure differentials&#xA;      can be limited by avoiding these maneuvers. Selecting a runway/HAC that minimizes HAC winds (most&#xA;      importantly, tailwind at HAC initiation) will help to minimize normal loads, and selecting a landing site&#xA;      with benign turbulence and surface wind conditions reduces the need for any abrupt directional control&#xA;      maneuvers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://archive.org/details/flight_rules_generic&quot; rel=&quot;nofollow noreferrer&quot;&gt;Generic Flight Rules&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To sum up, analysis showed that with even one latch gang out, a safe landing could be performed, but there would be thermal damage to the doors.  Since the shuttle was by nature reusable, damage to its structure was highly undesireable, so there was no need to perform further analysis about the effects of multiple latches open (or the doors open).  One latch out was bad enough.  So steps were taken to generate procedures that tried to ensure that a maximum of one latch would be out for entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, no exact analysis was ever done of the case you describe, but even the first steps of getting into that case were highly undesirable and to be avoided.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-02-02T23:32:09.510" LastActivityDate="2019-02-02T23:32:09.510" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="18932" PostTypeId="1" AcceptedAnswerId="19176" CreationDate="2016-11-03T18:12:10.517" Score="15" ViewCount="2506" Body="&lt;p&gt;(Inspired by &lt;a href=&quot;https://space.stackexchange.com/questions/18925/what-effect-if-any-would-leaving-the-cargo-bay-doors-open-have-had-during-a-sh&quot;&gt;this question&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the distinguishing features of Shuttle Columbia was its Shuttle Infrared Leeside Temperature Sensing (SILTS) pod. This was an experiment that took infrared imagery of the left wing and parts of the fuselage during entry.  (It was deactivated long before the STS-107 accident so there was no data from it contributing to the accident investigation.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H3TQM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H3TQM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any imagery from this experiment available online?  I found &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=8&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjuoI3Gl43QAhWLr1QKHVP-BzkQFghJMAc&amp;amp;url=http%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fdownload%3Fdoi%3D10.1.1.37.8683%26rep%3Drep1%26type%3Dpdf&amp;amp;usg=AFQjCNHCmZzbgOIAkg0sRwXRXMK9yuhU9w&amp;amp;sig2=_671qiGsp2OJdiA5pkZEGw&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt; (warning, pdf) that uses some results, but no actual imagery.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="29286" LastEditDate="2020-08-31T18:07:29.247" LastActivityDate="2020-08-31T18:07:29.247" Title="Any imagery from Shuttle Columbia's SILTS pod on the internet?" Tags="|space-shuttle|reentry|imaging|shuttle-columbia|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18933" PostTypeId="1" CreationDate="2016-11-03T19:56:49.607" Score="17" ViewCount="711" Body="&lt;p&gt;Recently we had the 15 year anniversary of Expedition 1 of the ISS. That particular crew launched in a Soyuz (TM-31) and returned on a Space Shuttle (Discovery, STS-102), which I found a remarkable aspect for a space mission!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which led me to wonder who else has returned to Earth in a different type of vehicle than the one in which they launched? After a little searching around I found that ISS Expedition 6 launched on a Space Shuttle (Endeavour, STS-113) and returned in a Soyuz (Soyuz TMA-1), but my quick superficial search didn't uncover anyone else. I thus tallied a total of six people from trips to the ISS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only other missions I found were Shuttle trips to Mir. For example it looks like STS-71 (Atlantis) picked up three Russian cosmonauts who had launched on Soyuz TM-21, and dropped off two at Mir to return on Soyuz TM-21. There are probably more, but I had no easy way of searching. So if anyone has an easy way to extract that information, maybe we can compile the full list? I've collected the ones I have found so far in a comment below.&lt;/p&gt;&#xA;" OwnerUserId="11397" LastEditorUserId="11397" LastEditDate="2016-11-03T21:32:05.470" LastActivityDate="2019-07-21T05:01:46.090" Title="How many people have returned to Earth in a different type of vehicle than the one that launched them into space?" Tags="|crewed-spaceflight|spacecraft|space-shuttle|soyuz-spacecraft|mir|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18934" PostTypeId="1" AcceptedAnswerId="18935" CreationDate="2016-11-03T20:28:57.220" Score="5" ViewCount="1182" Body="&lt;p&gt;I just watched a &lt;a href=&quot;http://www.natgeotv.com/uk/shows/natgeo/exomars-the-hunt-for-life&quot; rel=&quot;noreferrer&quot;&gt;National Geographic documentary on ExoMars&lt;/a&gt;. In it, they mentioned there were &lt;a href=&quot;http://exploration.esa.int/mars/56505-exomars-2016-targets-march-launch-window/&quot; rel=&quot;noreferrer&quot;&gt;2 launch windows&lt;/a&gt; in early 2016 for the TGO/Schiaparelli launch, one in January and one in March.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quote from ESA site showing this is not just a gaffe by NatGeo:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The later window is open 14-25 March and, thanks to the relative orbital positions of Earth and Mars, the mission will still arrive at Mars in October, just as if launched in January.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How does that work? &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_of_Mars#Launch_windows&quot; rel=&quot;noreferrer&quot;&gt;As I understand it,&lt;/a&gt; there is one optimal date every 780 days, with the required launch energy going up slowly either side of the optimal date. So where do two launch windows, 2 months apart, come from? &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-03T22:33:48.400" Title="Launch windows to Mars" Tags="|orbital-mechanics|mars|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18935" PostTypeId="2" ParentId="18934" CreationDate="2016-11-03T21:57:54.483" Score="7" Body="&lt;p&gt;Depending on how you define &quot;optimal&quot;, there are usually two local minima in the impulsive trajectory characteristics for a given opportunity, called &quot;Type 1&quot; and &quot;Type 2&quot;. The Type 1 trajectories go a bit less than 180° around the Sun, and the Type 2 trajectories go a bit more than 180° around the Sun. This bifurcation occurs due to the relative inclination of the orbit of Earth and the orbit of Mars. If the orbits of Earth and Mars were coplanar, there would be one local minima going 180° around the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This can be seen in the traditional &quot;pork chop&quot; plots, which show various trajectory characteristics, such as departure $C_3$ or arrival $V_\infty$, as a function of departure and arrival dates. Here is an example from the 2005 opportunity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hFwDF.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hFwDF.gif&quot; alt=&quot;2005 Earth-Mars pork chop plot&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Type 1 is the smaller region on the bottom, and the Type 2 is the larger region on the top.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TGO changed from a Type 2 to a Type 1, when they moved their launch out two months, but kept the same arrival date. TGO had a large deep space maneuver, so the pork chop plot doesn't have enough dimensions to represent the trade space, and the trajectories weren't the classic Type 1 and Type 2 impulsive trajectories (i.e. a single burn at Earth). But the basic principles still apply.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-11-03T22:33:48.400" LastActivityDate="2016-11-03T22:33:48.400" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18937" PostTypeId="1" AcceptedAnswerId="18947" CreationDate="2016-11-04T01:24:03.777" Score="5" ViewCount="515" Body="&lt;p&gt;Why do we not add an everyday (with some modifications for protection obviously) digital camera to spacecraft so that we can get a (semi) truecolor image of what they are seeing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was reading about how with the Mars rovers, it is really just scientific estimations of what the true color really is (that humans would see) Why not just implement a standard digital camera to see this?&lt;/p&gt;&#xA;" OwnerUserId="14305" LastActivityDate="2016-11-07T20:46:52.927" Title="Why not a digital camera?" Tags="|spacecraft|camera|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="18938" PostTypeId="1" AcceptedAnswerId="18940" CreationDate="2016-11-04T02:50:10.540" Score="6" ViewCount="2079" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Adaptive_optics&quot; rel=&quot;nofollow noreferrer&quot;&gt;Adaptive Optics&lt;/a&gt; is generally used with ground telescopes to correct for atmospheric distortions or &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_seeing&quot; rel=&quot;nofollow noreferrer&quot;&gt;seeing&lt;/a&gt;. Right now it is used mostly in the infrared but is starting to be used in longer visible wavelengths (see excellent answers &lt;a href=&quot;https://astronomy.stackexchange.com/a/14825/7982&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://astronomy.stackexchange.com/a/15012/7982&quot;&gt;here&lt;/a&gt;), and is applied in software to &lt;a href=&quot;https://astronomy.stackexchange.com/a/13967/7982&quot;&gt;radio telescope arrays&lt;/a&gt; as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://en.wikipedia.org/wiki/John_C._Mather&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Scientist John Mather&lt;/a&gt;'s recent &lt;a href=&quot;http://jwst.nasa.gov/faq_ama1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMA #1&lt;/a&gt; the use of adaptive optics in large aperture ground telescopes to potentially image exoplanets is mentioned as well. The &lt;a href=&quot;http://jwst.nasa.gov/faq.html#whyspace&quot; rel=&quot;nofollow noreferrer&quot;&gt;JWST FAQ #7&lt;/a&gt; touts putting a telescope in space as an alternative to adaptive optics on the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Business Insider article &lt;a href=&quot;http://www.businessinsider.com/nasa-telescope-video-secret-2016-11&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA is trying to keep part of its giant golden telescope a secret&lt;/a&gt; the use of adaptive optics on the JWST secondary mirror is mentioned, and shown in the large image below with a red arrow. Also two screen shots from the &lt;a href=&quot;https://www.youtube.com/watch?v=r1ybe4yr2no&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Goddard YouTube video&lt;/a&gt; are shown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Since the JWST is outside the atmosphere and therefore doesn't need adaptive optics, what would the adaptive optics be needed for?&lt;/em&gt; Or is the BI article wrong and this is simply an active mirror mount &lt;em&gt;sans AO&lt;/em&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xo7q4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xo7q4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;  &lt;a href=&quot;https://i.stack.imgur.com/hNlma.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hNlma.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Two frame captures from the &lt;a href=&quot;https://www.youtube.com/watch?v=r1ybe4yr2no&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Goddard YouTube video&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L1eJO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L1eJO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; JWST secondary mirror mechanism annotated with arrow by &lt;a href=&quot;http://www.businessinsider.com/nasa-telescope-video-secret-2016-11&quot; rel=&quot;nofollow noreferrer&quot;&gt;Business Insider&lt;/a&gt; - credit NASA/Chris Gunn &lt;a href=&quot;http://www.nasa.gov/feature/goddard/2016/nasas-james-webb-space-telescope-secondary-mirror-installed&quot; rel=&quot;nofollow noreferrer&quot;&gt;Original Image&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/r1ybe4yr2no?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2022-07-24T06:04:17.370" Title="Why would the James Webb Space Telescope need adaptive optics outside the atmosphere?" Tags="|imaging|space-telescope|james-webb-telescope|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18939" PostTypeId="1" AcceptedAnswerId="18949" CreationDate="2016-11-04T10:00:00.120" Score="5" ViewCount="478" Body="&lt;ul&gt;&#xA;&lt;li&gt;CZ-5 = &lt;a href=&quot;https://en.wikipedia.org/wiki/Long_March_5&quot; rel=&quot;nofollow noreferrer&quot;&gt;Long March 5&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;YZ-2 = &lt;a href=&quot;https://en.wikipedia.org/wiki/Yuanzheng-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yuanzheng 2&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;SJ-17 = &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/sj-17.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shijian 17&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/sj-17.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page says&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SJ 17 (Shijian 17) is a Chinese experimental satellite flown on the maiden &lt;a href=&quot;http://space.skyrocket.de/doc_lau/cz-5.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;CZ-5/YZ2&lt;/a&gt; launch.&lt;/p&gt;&#xA;&lt;p&gt;It is reportedly carrying out experiments with ion propulsion for station keeping.&lt;/p&gt;&#xA;&lt;p&gt;The satellite will be &lt;strong&gt;directly inserted into geostationary orbit by the YZ-2 upper stage of the launch vehicle.&lt;/strong&gt; (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;CZ-5 &lt;a href=&quot;https://en.wikipedia.org/wiki/Long_March_5&quot; rel=&quot;nofollow noreferrer&quot;&gt;has two stages according to Wikipedia&lt;/a&gt;, and second stage uses a pair of LH2/LOX restartable &lt;a href=&quot;https://en.wikipedia.org/wiki/YF-75D&quot; rel=&quot;nofollow noreferrer&quot;&gt;YF-75D engines&lt;/a&gt;. According to the Spaceflight 101 article &lt;a href=&quot;http://spaceflight101.com/long-march-5-inaugural-launch-success/&quot; rel=&quot;nofollow noreferrer&quot;&gt;China’s Long March 5 Heavy-Lift Rocket achieves full Success in Inaugural Mission&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The two engine nozzles began glowing bright orange again when the second stage re-started on a burn of just over four minutes to boost the stack into a highly elliptical sub-GTO-type orbit. Loud cheers emerged when the Yuanzheng-2 upper stage separated from the booster right at the half-hour mark into the flight, marking Mission Success for the Long March 5 rocket.&lt;/p&gt;&#xA;&lt;p&gt;Assuming control of the flight, Yuanzheng-2 immediately fired its hypergolic main engine for a short supplementary burn to raise the high point of the orbit to Geostationary Altitude at around 36,000 Kilometers. Next was a coast phase of around five and a half hours to allow YZ-2 to climb to the apogee of the orbit so that its second burn can circularize the orbit and reduce its inclination to reach a Geostationary Orbit for spacecraft separation at 18:57 UTC.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The hypergolic-fueled Yuanzheng-2 seems to be considered an &amp;quot;upper stage&amp;quot;, and yet, separation is considered the completion of the Long March 5's mission. Is the YZ-2 considered a 3rd stage?&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;How many stages does the Long March 5 actually have?&lt;/em&gt; Two, or three?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QXCSA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QXCSA.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Long March-5 Rocket from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Long_March_Rocket_at_Launchpad.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-05-23T16:20:01.280" Title="CZ-5 plus YZ-2 put SJ-17 in GEO - third stage or separate spacecraft?" Tags="|orbital-maneuver|geostationary|upper-stage|china|long-march|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18940" PostTypeId="2" ParentId="18938" CreationDate="2016-11-04T10:29:49.827" Score="11" Body="&lt;p&gt;The primary and secondary mirrors on JWST &lt;a href=&quot;http://www.nasa.gov/topics/technology/features/webb-actuator.html&quot; rel=&quot;noreferrer&quot;&gt;can be adjusted&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Launching a mirror this large into space isn’t feasible. Instead, Webb engineers and scientists innovated a unique solution – building 18 mirrors that will act in unison as one large mirror. These mirrors are packaged together into three sections that fold up - much easier to fit inside a rocket. Each mirror is made from beryllium and weighs approximately 20 kilograms (46 pounds). Once in space, getting these mirrors to focus correctly on faraway galaxies is another challenge entirely. Actuators, or tiny mechanical motors, provide the answer to achieving a single perfect focus.&lt;/p&gt;&#xA;&lt;p&gt;The primary and secondary mirror segments are both moved by six actuators that are attached to the back of the mirrors. The primary segment has an additional actuator at the center of the mirror that adjusts its curvature. The third mirror segment remains stationary.&lt;/p&gt;&#xA;&lt;p&gt;Lee Feinberg, Webb Optical Telescope Element Manager at NASA's Goddard Space Flight Center in Greenbelt, Md. explained &amp;quot;Aligning the primary mirror segments as though they are a single large mirror means each mirror is aligned to 1/10,000th the thickness of a human hair. This alignment has to be done at 50 degrees above absolute zero! What's even more amazing is that the engineers and scientists working on the Webb telescope literally had to invent how to do this.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jwst.nasa.gov/mirrors.html&quot; rel=&quot;noreferrer&quot;&gt;Each mirror can be adjusted in only 7 spots&lt;/a&gt;: each corner of the hexagon and the middle. This seems to be too few spots to really do adaptive optics. In adaptive optics, you adjust the shape of the mirror itself in multiple locations to compensate for changes in the light wavefront as it's distorted.&lt;/p&gt;&#xA;&lt;p&gt;Those 7 spots are adjustable for one reason: to align the 18 segments of the primary mirror to each other, and to adjust the primary and secondary mirrors to the rest of the optical path after launch.&lt;/p&gt;&#xA;&lt;p&gt;Given the amount of information available on NASA's own website, the Business Insider article looks like clickbait. There's nothing being kept secret there.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-11-04T10:29:49.827" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="18941" PostTypeId="1" AcceptedAnswerId="18942" CreationDate="2016-11-04T10:30:55.447" Score="1" ViewCount="167" Body="&lt;p&gt;The Long March 5's first launch used a LH2/LOX main engine with four RP-1/LOX Liquid Rocket Boosters (LRBs). Is this the first time that RP-1/LOX LRBs have been used in a successful launch to orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/w8F2g.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/w8F2g.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NiqVp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NiqVp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; Screenshots from &lt;a href=&quot;https://www.youtube.com/watch?v=5jb7qE8pKvU&quot; rel=&quot;nofollow noreferrer&quot;&gt;the YouTube video&lt;/a&gt; shown below. Four RP-1/LOX LRBs emit bright white light while LH2/LOX main engine's exhaust is dimmer due to lack of thermal radiation from soot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ysirM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ysirM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Long March-5 from &lt;a href=&quot;https://chinaspacereport.com/2016/10/29/long-march-5-to-make-debut-flight-in-early-november/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chinaspacereport&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-05T23:16:25.507" LastActivityDate="2021-09-05T23:16:25.507" Title="First launch of Long March-5 using 4 RP-1/LOX boosters. First use of RP-1/LOX LRBs?" Tags="|booster|spaceflight-firsts|long-march|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18942" PostTypeId="2" ParentId="18941" CreationDate="2016-11-04T11:05:32.623" Score="5" Body="&lt;p&gt;No. The 4 &lt;a href=&quot;http://www.astronautix.com/r/rd-107-8d74.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz boosters&lt;/a&gt; are RP1/LOX, this is a design that goes back to the R-7 that launched Sputnik 1. Thousands have been launched.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Liquid boosters are a less common design than solids, but have been produced in greater numbers. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-11-04T20:34:49.333" LastActivityDate="2016-11-04T20:34:49.333" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18944" PostTypeId="1" AcceptedAnswerId="38321" CreationDate="2016-11-04T13:14:34.537" Score="13" ViewCount="1358" Body="&lt;p&gt;Why does the YF-75D nozzle material have such a strong spiral pattern? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it a result/biproduct of a manufacturing process only, or does it have some specific function? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; The last, cropped images that have enhanced contrast and sharpening show a bead-like appearance around the bottom edge of the nozzle. This gives the impression that the nozzles is fabricated from fused rods or &quot;ropes&quot; of metal. This is consistent with them being nearly vertical at the top, and increasingly spiraled as the bell shape widens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could these have been made by some additive process?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kE3GM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kE3GM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; YF-75D engine cropped from &lt;a href=&quot;http://tech.southcn.com/t/images/attachement/jpg/site4/20141111/c03fd5725abe15cc05da28.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CflWg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CflWg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; YF-75D engine cropped from &lt;a href=&quot;https://www.chinaspaceflight.com/engine/liquid-hydrogen-oxygen/YF-75D.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chinaspaceflight&lt;/a&gt;. (In Chinese)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Qq2Ai.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qq2Ai.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; YF-75D engine from &lt;a href=&quot;https://www.chinaspaceflight.com/engine/liquid-hydrogen-oxygen/YF-75D.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chinaspaceflight&lt;/a&gt;. (In Chinese)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qDlwH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qDlwH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/XEw67.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XEw67.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; YF-75D engine nozzle detail, cropped from &lt;a href=&quot;https://www.chinaspaceflight.com/engine/liquid-hydrogen-oxygen/YF-75D.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chinaspaceflight&lt;/a&gt;. (In Chinese) &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-08-09T11:35:06.780" LastActivityDate="2019-08-22T22:29:01.457" Title="Why does the YF-75D nozzle have such a strong spiral pattern?" Tags="|engine-design|design|engineering|nozzle|long-march|" AnswerCount="1" CommentCount="18" ContentLicense="CC BY-SA 4.0" />
  <row Id="18945" PostTypeId="2" ParentId="18941" CreationDate="2016-11-04T13:24:42.167" Score="1" Body="&lt;p&gt;No.  The &lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot; rel=&quot;nofollow noreferrer&quot;&gt;Energia&lt;/a&gt; used kerolox strapons.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-11-04T13:24:42.167" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18946" PostTypeId="2" ParentId="9346" CreationDate="2016-11-04T13:29:52.763" Score="4" Body="&lt;p&gt;Looks like now it has been answered (though not officially), but here's a video from SpaceX with JCSAT-14 booster test. In this video, you can clearly see the test stand and you also can see it looks like also designed with three core Falcon Heavy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/SZQY902xQcw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; &lt;/p&gt;&#xA;" OwnerUserId="17490" LastActivityDate="2016-11-04T13:29:52.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18947" PostTypeId="2" ParentId="18937" CreationDate="2016-11-04T16:35:51.327" Score="15" Body="&lt;p&gt;&quot;With some modifications&quot; for space environment is a time and money-intensive activity, at least if you want it to work. You'd need some serious justification for such a camera on a mission that is already highly cost-constrained.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is always a color camera on a Mars rover anyway, usually with much more than three color filters, so its color vision is better than ours. The filters don't line up exactly with our three, each being much narrower for science reasons. But by combining images from filters you can get a very good approximation to how it would look to us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which gets us to how it would look to us. That requires some analysis and adjustment as well, due to the very different direct and scattered light at Mars, both in wavelength distribution and intensity. Even at Earth these differences make whether a digital image &quot;looks like it would to us&quot; a very subjective matter. Do a little reading on white balance and color temperature to see how really tricky it is on Earth to get a picture to look &quot;realistic&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, we could fly commercial cameras if we wanted to. But even if they work, they don't automatically solve the &quot;truecolor&quot; problem you have posed. In fact it would be much more difficult to solve that problem with the data you can get out of a commercial camera, as compared to the data we get from our science cameras.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-11-04T17:53:04.110" LastActivityDate="2016-11-04T17:53:04.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18948" PostTypeId="1" AcceptedAnswerId="18951" CreationDate="2016-11-04T18:14:34.433" Score="3" ViewCount="495" Body="&lt;p&gt;The recent question &lt;a href=&quot;https://space.stackexchange.com/questions/18934&quot;&gt;Launch windows to Mars&lt;/a&gt; got me thinking.  Why is there only one or two launch windows to Mars ever other year?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you leave Earth heading for the sun, you should get a &lt;a href=&quot;https://space.stackexchange.com/questions/889/gravity-assist-for-manned-travel&quot;&gt;gravity assist&lt;/a&gt; on flyby.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least in my mind, the travel time to Mars would be the same no matter where the two plants are.  I imagine that the fastest anything man made ever went was on a powered trajectory towards Sol. The same amount of fuel needed to get out to Mars, must be enough to get to the Mars via the Sun.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-11-05T02:52:26.080" Title="Is the fastest way to Mars a solar flyby?" Tags="|orbital-mechanics|mars|gravity-assist|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18949" PostTypeId="2" ParentId="18939" CreationDate="2016-11-04T19:04:01.760" Score="4" Body="&lt;p&gt;A Honda Civic is a two axle vehicle. It can tow a trailer. When it's towing a trailer, Civic-plus-trailer is functionally a three axle vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no essential difference between a spacecraft with propulsion and a rocket stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, Long March 5 is a two-stage rocket because it's designed and built separately from the YZ stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long March 5 + YZ is functionally a three stage rocket.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-11-04T19:04:01.760" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18951" PostTypeId="2" ParentId="18948" CreationDate="2016-11-04T19:56:42.743" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;If you leave Earth heading for the sun, you should get a gravity assist on flyby.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;When you travel in space, strictly speaking all masses attract each other (&lt;a href=&quot;https://en.m.wikipedia.org/wiki/N-body_problem&quot; rel=&quot;noreferrer&quot;&gt;n-body-problem&lt;/a&gt;). However most time, there is one central body, whose gravity is &lt;em&gt;dominant&lt;/em&gt; and exceeds other forces by several orders of magnitude. For traveling between planets in our solar system, this is clearly the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;noreferrer&quot;&gt;gravity-assist&lt;/a&gt; or &lt;em&gt;slingshot&lt;/em&gt; means you approach another body, so its gravity exceeds the suns for a short time, the &lt;em&gt;dominant central body&lt;/em&gt; changes. If you look at this in a sun reference frame, this bends your trajectory. The trick of this maneuver is, that in the assisting planets frame, you do not loose or gain energy, but in the suns frame you do, because the frames a moving against each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Can our sun itself assist you? No, it can't&lt;/strong&gt;, because you are already under it's influence, the trick with changing the &lt;em&gt;dominant body&lt;/em&gt; does not work here (unless you come from outside our solar system).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It happens that also other assists (except luna) will not help, because reaching the assisting body needs more energy than reaching mars directly. A &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;noreferrer&quot;&gt;Hohmann-transfer&lt;/a&gt; is the cheapest way to get there.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At least in my mind, the travel time to Mars would be the same no matter where the two plants are. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now Hohmann will arrange for getting to mars' orbit, but theres nothing to be found there unless mars happens to be right at this point when you arrive. &lt;em&gt;Braking&lt;/em&gt; there and waiting until mars arrives does also not work, you need to keep your speed relative to the sun in order to stay there. So you and mars would be traveling at the same speed and never meet. &lt;strong&gt;Starting at the right time is essential.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The same amount of fuel needed to get out to Mars, must be enough to get to the Mars via the Sun.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;No&lt;/strong&gt;, Walter Hohmann showed in his work &lt;a href=&quot;https://archive.org/details/nasa_techdoc_19980230631&quot; rel=&quot;noreferrer&quot;&gt;The attainability of heavenly bodies&lt;/a&gt; that &lt;strong&gt;this is not the case&lt;/strong&gt;. In order to approach sun, you need to slow down in suns reference frame to lower centrifugal forces. To get to mars, you need to accelerate, because it is farther out.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is the fastest way...?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you can afford the thrust all the time, a direct intercept to your target is always fastest. The pointing of your thrusters needs to be tilted somewhat to counteract the Coriolis force. Low consumption thrusters strong enough to escape from earth are not available (as of 2016).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With limited propellants, &lt;em&gt;mission duration&lt;/em&gt; will be shortest if you use them in the beginning and at the end, which again means a Hohmann trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Earliest arrival after a given point in time&lt;/em&gt; is different, because you will not be able to delay your start until a good launch opportunity. For earliest arrival, you may need a completely different trajectory. Gravity-assists &lt;em&gt;may&lt;/em&gt; be beneficial in this case to leave the ecliptic.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17004" LastEditDate="2016-11-05T02:52:26.080" LastActivityDate="2016-11-05T02:52:26.080" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="18952" PostTypeId="1" AcceptedAnswerId="18972" CreationDate="2016-11-04T20:18:14.943" Score="3" ViewCount="396" Body="&lt;p&gt;Solar atmosphere causes difficulties in communication during opposition (when Mars is almost behind the sun w.r.t. Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is: is it possible to define an extended radius for the sun, which the Earth-Mars line of sight should never pass through it? if there is, how much is it, in terms of angular separation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've read in some mainstream and NASA news that there is a two week blackout period during each synodic period of Earth-Mars. But I want to know the exact numbers. I'd appreciate any answer with technical references. &lt;/p&gt;&#xA;" OwnerUserId="16381" LastActivityDate="2016-11-07T06:39:12.190" Title="Earth-Mars Radio Blackout: Is it possible to define an extended solar radius to solve this as a geometric problem?" Tags="|mars|communication|communication-satellite|data-transmission|radio-communication|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18953" PostTypeId="1" AcceptedAnswerId="18954" CreationDate="2016-11-04T20:51:33.897" Score="6" ViewCount="964" Body="&lt;p&gt;The Saturn V had an initial production run of 15 vehicles (plus 3 test articles). Twelve were used for Apollo 4, 6, and 8-17, one was used for Skylab, the components of the other two are on display in various locations. I've heard there was originally going to be a second production run of the Saturn V, probably including the uprated F-1A engines among other improvements. Was this ever officially considered? If it was, when was it cancelled? &lt;/p&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2022-01-11T10:58:24.040" Title="When was the second production run of the Saturn V cancelled?" Tags="|rockets|history|saturn-v|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18954" PostTypeId="2" ParentId="18953" CreationDate="2016-11-04T21:28:54.443" Score="4" Body="&lt;p&gt;No additional production run was ever ordered, so there was nothing to cancel. The various upgrades considered never went beyond the proposal stage into detailed designs, so there was nothing to order. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-11-04T21:28:54.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18955" PostTypeId="2" ParentId="9824" CreationDate="2016-11-04T23:57:12.947" Score="5" Body="&lt;p&gt;This document contains some technical details:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsndocs/810-005/101/101E.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://deepspace.jpl.nasa.gov/dsndocs/810-005/101/101E.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In page 12 you will see that the S-Band transmitter has a 400 kW maximum output power. Others have 20 kW max.&lt;/p&gt;&#xA;" OwnerUserId="17492" LastActivityDate="2016-11-04T23:57:12.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18956" PostTypeId="1" AcceptedAnswerId="19330" CreationDate="2016-11-05T01:38:05.840" Score="19" ViewCount="1764" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/18955/12102&quot;&gt;This answer&lt;/a&gt; notes that while most &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; transmitters are 20 kW of power or less, one dish (&lt;a href=&quot;http://www.nasa.gov/directorates/heo/scan/txt_news_dss43.html&quot; rel=&quot;noreferrer&quot;&gt;DSS-43 in Canberra&lt;/a&gt;) is equipped with a transmitter capable of 400 kW.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As described in &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsndocs/810-005/101/101E.pdf&quot; rel=&quot;noreferrer&quot;&gt;101, 70-m Subnet Telecommunications Interfaces&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An additional 400-kW S-band transmitter is available at DSS-43 (Canberra) for Voyager communication &lt;strong&gt;and general emergency commanding&lt;/strong&gt;. Special airspace coordination is required when using the 400-kW transmitter at a power above 100 kW, but in no case can the 400-kW transmitter be used below 17 degrees elevation, no matter what the power is. (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Has DSS-43 ever been used in high power mode (&gt;&gt;20 kW) for an emergency situation?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D3Qb9.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D3Qb9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;http://deepspace.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;DSN's&lt;/a&gt; 70m antenna &lt;a href=&quot;http://www.cdscc.nasa.gov/Pages/Antennas/dss43.html&quot; rel=&quot;noreferrer&quot;&gt;DSS-43&lt;/a&gt; from &lt;a href=&quot;http://www.planetary.org/multimedia/space-images/spacecraft/dss-43-the-70-meter-antenna-at-canberra.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, one of DSN's &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2011/3277.html&quot; rel=&quot;noreferrer&quot;&gt;three 70m dishes&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-11-27T21:34:39.330" LastActivityDate="2022-01-03T17:43:41.803" Title="Has DSS-43 ever been used in high power mode (&gt;&gt;20 kW) for an emergency situation?" Tags="|communication|radio-communication|spaceflight-firsts|deep-space-network|emergency|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18957" PostTypeId="1" CreationDate="2016-11-05T04:19:10.700" Score="1" ViewCount="92" Body="&lt;p&gt;Has the possibility of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Pioneer Anomaly&quot;&lt;/a&gt; as caused by currently inadequate conceptual models of gravity been put to rest by explaining it as caused by on board thermal radiation ? What is the current status of plans, or a lack thereof, to compare thermal acceleration effects demonstrated by the New Horizons spacecraft with that of Pioneer ? &lt;/p&gt;&#xA;" OwnerUserId="17377" LastEditorUserId="238" LastEditDate="2016-11-05T21:48:21.967" LastActivityDate="2016-11-05T21:48:21.967" Title="Pioneer spacecraft anomaly" Tags="|new-horizons|pioneer|" AnswerCount="0" CommentCount="2" ClosedDate="2016-11-07T15:54:51.127" ContentLicense="CC BY-SA 3.0" />
  <row Id="18958" PostTypeId="1" AcceptedAnswerId="19087" CreationDate="2016-11-05T13:51:47.183" Score="4" ViewCount="328" Body="&lt;p&gt;To get more statistical significance for the suspected &lt;a href=&quot;http://www.space.com/5037-nasa-baffled-unexplained-force-acting-space-probes.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;flyby-anomaly&lt;/a&gt; (or to refute it), it would be desirable to track as many hyperbolic earth flybys to within 1mm/s as we can get. &lt;strong&gt;Can we track natural objects&lt;/strong&gt; approaching/leaving earth &lt;strong&gt;with this accuracy&lt;/strong&gt; by long-term observations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Laser tracking of manmade objects may be easier. Think of a swarm of passive reflectors, spring-emitted way before perigee of an earth gravity-assist, so they pass with different distance and leave in different directions. But I fear that we will never see such a maneuver with a significant plane change, because missions rarely ever leave the ecliptic. With the observations so far, in- and outbound declination &lt;a href=&quot;http://repository.tudelft.nl/assets/uuid:a537388f-c526-45ee-b762-514a8d0ed14f/321195.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;seems significant&lt;/a&gt;, especially going from high declination back to low would possibly give insight. Will the effect be reversed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I doubt we will ever see a mission dedicated to this effect, so studying it with natural objects would be useful.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="415" LastEditDate="2016-11-21T15:22:15.683" LastActivityDate="2016-11-21T15:22:15.683" Title="How accurate can near earth asteroids be tracked?" Tags="|orbital-mechanics|gravity-assist|tracking|laser|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="18959" PostTypeId="1" CreationDate="2016-11-05T14:41:07.007" Score="3" ViewCount="812" Body="&lt;p&gt;I am interested for a comparison of solid rocket fuels by their performance and cost. Comparison by specific impulse (Isp), energy that gives (which one is more energitic), density, cost to manifacture them, cost per kg (or pound), and also other elements if you could find. Comparing them in a table would be better. Even if there are included some of the values for these elements, it is still ok. For any further info that you will find later about them, you can edit the answer.&lt;/p&gt;&#xA;" OwnerUserId="16979" LastActivityDate="2016-11-07T01:18:59.523" Title="Comparison between solid rocket fuels. Which are better by performance and cost?" Tags="|fuel|cost|solid-fuel|specific-impulse|performance|" AnswerCount="1" CommentCount="1" ClosedDate="2016-11-08T16:03:38.443" ContentLicense="CC BY-SA 3.0" />
  <row Id="18960" PostTypeId="1" CreationDate="2016-11-05T19:16:45.823" Score="1" ViewCount="65" Body="&lt;p&gt;Since loss of muscle is such a big problem in extended space travel I wondered (and practicalty is a different issue) if astronauts could maintain muscle mass by donning oxygen masks and playing a game for a couple hours a day in a clear liquid with a high viscosity? It would seem this would work every muscle in the body.&lt;/p&gt;&#xA;" OwnerUserId="17497" LastEditorUserId="548" LastEditDate="2016-11-05T19:47:54.820" LastActivityDate="2016-11-15T14:43:10.697" Title="Muscle loss in space" Tags="|crewed-spaceflight|health|long-duration|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18961" PostTypeId="1" CreationDate="2016-11-05T20:11:02.527" Score="2" ViewCount="674" Body="&lt;p&gt;what's the meaning of Scaled pixel width and scaled pixel height of LROC EDR image obtained by LROC image search tool?&#xA;&lt;a href=&quot;https://i.stack.imgur.com/5qFK4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5qFK4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17498" LastEditorUserId="265" LastEditDate="2016-11-06T14:48:40.233" LastActivityDate="2018-02-25T03:06:56.087" Title="Mapping the moon: how to use the Lunar Reconnaissance Orbiter (LRO) image data?" Tags="|the-moon|imaging|mapping|raw-data|lro|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18963" PostTypeId="1" CreationDate="2016-11-05T21:28:33.040" Score="1" ViewCount="126" Body="&lt;p&gt;I'm curious about timeline and the procedures related to the spacecraft separation during Falcon 9 (and perhaps other rockets if such information is available) launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume a mission with one major satellite to LEO (SSO) and number of piggyback CubeSats. I assume that they are deployed after the main payload separation. But does the upper stage change its orbit before or after the deployment? Are the CubeSats deployed all at once or sequentially?&lt;/p&gt;&#xA;" OwnerUserId="13900" LastActivityDate="2016-11-05T21:28:33.040" Title="What is the sequence of the main spacecraft and CubeSats separation during Falcon 9 launch?" Tags="|launch|rockets|falcon-9|cubesat|launch-sequence|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18964" PostTypeId="1" CreationDate="2016-11-05T22:49:27.713" Score="5" ViewCount="3371" Body="&lt;p&gt;I am working on high school presentation about rocket engine materials and I'd like to have a part about Merlin 1D engine as it is supposed to be reusable, but I can't find anything about materials used for this kind of engine. Found a lot of data about single-use engines and solid rockets, but nothing about materials for reusable one.&#xA;Could you please provide anything about materials used? I have literally nothing to take off from...&#xA;Thanks in advance.&lt;/p&gt;&#xA;" OwnerUserId="17500" LastEditorUserId="17166" LastEditDate="2016-11-06T00:10:31.870" LastActivityDate="2017-06-26T22:01:44.283" Title="What material is the Merlin 1D made from?" Tags="|spacex|materials|merlin-1d|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18965" PostTypeId="1" AcceptedAnswerId="18968" CreationDate="2016-11-06T09:04:38.627" Score="2" ViewCount="603" Body="&lt;p&gt;Mars hasn't got a very strong magnetosphere. I often read that even equipment and technology needs to be protected against the radiation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question: &#xA;How do they protect the equipment from radiation? Take the rover Opportunity for example, who's been there for 12 years now. &lt;/p&gt;&#xA;" OwnerUserId="17502" LastActivityDate="2016-11-07T00:50:49.260" Title="Protecting technology from radiation on mars" Tags="|mars|radiation|technology|cosmic-radiation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18966" PostTypeId="2" ParentId="18961" CreationDate="2016-11-06T11:43:14.337" Score="4" Body="&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt; Those numbers are meters per pixel. But you have to be careful because they are scans along the spacecraft orbit with the moon rotating underneath, &lt;strong&gt;so x and y are skewed.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This answer will get you started. The LRO is an amazing spacecraft with a &lt;a href=&quot;https://www.nasa.gov/mission_pages/LRO/spacecraft/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;wide variety of instruments&lt;/a&gt; to map the moon's surface by a number of characteristics as well as &lt;a href=&quot;https://lola.gsfc.nasa.gov/images/zuber_LR_SSR.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;simultaneous laser ranging information for LRO-Earth and LRO-Moon paths&lt;/a&gt; improving lunar gravity models. There is a very helpful one page overview in &lt;a href=&quot;http://www.hou.usra.edu/meetings/leag2014/presentations/keller.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Lunar Reconnaissance Orbiter and the New Moon: Mission Highlights and Two More Years of Science From Lunar Orbit!&lt;/a&gt; by project scientist &lt;a href=&quot;https://www.nasa.gov/mission_pages/LRO/team/keller.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;John Keller&lt;/a&gt;. You are interested in the LROC/NAC (LRO Camera / Narrow-Angle Cameras)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k2Esj.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k2Esj.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LRO Cameras are described in detail in &lt;a href=&quot;http://lroc.sese.asu.edu/files/DOCS/LROCinstrumentOverview.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Reconnaissance Orbiter Camera (LROC) Instrument Overview&lt;/a&gt; (Robinson et al. 2010, Space Sci Rev (2010) 150: 81–124 DOI 10.1007/s11214-010-9634-2):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U4PoH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U4PoH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0XQI6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0XQI6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lnTkO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lnTkO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two essentially identical narrow angle cameras, named &quot;L&quot; and &quot;R&quot; in the data arcives. Each camera uses a roughly 5000 pixel by 1 pixel CDD, and the LRO's orbits in the &quot;X&quot; direction continiously scans like a FAX machine or flatbed scanner. The FOV of the cameras slightly overlap in the middle, but extend to the left and right of the centerlin thus the &quot;L&quot; and &quot;R&quot; designations in the image data archives as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much of the data is taken in a roughly polar orbit, including the data in January 2012 you've mentioned, so the ~5000 pixels of a given camera in this case are nearly parallel to lines of lunar latitude.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Upper right latitude    42.08&#xA;Upper right longitude  340.64&#xA;Lower right latitude    44.76&#xA;Lower right longitude  340.7&#xA;Lower left  latitude    44.76&#xA;Lower left  longitude  340.33&#xA;Upper left  latitude    42.09&#xA;Upper left  longitude  340.28&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;From that we can plot the approximate border of the image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7sQGr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7sQGr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The skewing is due to some combination of the inclination orbit rotation of the moon under the spacecraft while it scans. &lt;strong&gt;Remember to consider the skewing when trying to use these images!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The width is about $340.7-340.33\approx0.37$ degrees in latitude. The data below show that the camera is looking nearly straight down.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Spacecraft altitude       158.79&#xA;Emission angle              1.79&#xA;Sub spacecraft latitude    43.42&#xA;Sub spacecraft longitude  340.7&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The width of the field is roughly 0.37 degrees of longitude, correcting that for the latitude, the width on the surface assuming small angles is: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ w \approx r_{moon} \ cos(latitude) \  \frac{\pi \ 0°.37}{180°} \approx 8.150 \ (\text{km}) $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;8150 meters divided by about 5000 pixels &lt;strong&gt;gives about 1.63 meters per pixel in y&lt;/strong&gt;.  And what does the data say...&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Scaled pixel width   1.59&#xA;Scaled pixel height  1.55&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So of course the scales in the metadata are going to be more reliable than my approximation, and you should use those numbers because a correct calculation would be much more complex! But my calculations serves as a) a sanity check (did I pass?) and b) an instructive illustration of what's actually going on.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here is a quicker way to get at the scale, but it doesn't clue you in on the skewing.  The table below shows that the LEFT camera has &lt;code&gt;10.00 microradians per pixel&lt;/code&gt;. Multiply that by the distance from LRO to the imaging location on the moon of &lt;code&gt;158.79&lt;/code&gt; kilometers and you get...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1.59 meters/pixel! Same as the pixel scaling given.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q3nAy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q3nAy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For fun, you can go to &lt;a href=&quot;https://lrostk.gsfc.nasa.gov/preview.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://lrostk.gsfc.nasa.gov/preview.cgi&lt;/a&gt; and see a real-time animation of what LRO is scanning right now!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jsg0q.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jsg0q.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-02-25T03:06:56.087" LastActivityDate="2018-02-25T03:06:56.087" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18968" PostTypeId="2" ParentId="18965" CreationDate="2016-11-07T00:50:49.260" Score="4" Body="&lt;p&gt;Mostly by designing this equipment to be radiation tolerant in the first place, and also providing redundancies where needed. The materials and components selection are are a careful process, with previous spaceflight heritage being an important design driver. Electronics, joints, bearings, lubricants - all of these need special attention. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiation is most immediately harmful to electronics and electronics is a mix of very expensive &quot;radiation hardened by design&quot; or RHBD components, and carefully picked COTS components in things like motor driver circuits, that fit the temperature and fault tolerance criteria. But long term, radiation affects other components, too, although the effects on martian surface are somewhat more benign than in deep space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More than radiation, parts of the rovers also need careful thermal management, to keep the components in their required operating temperature range. Hence some of the key electronics parts are placed in whats commonly called a &lt;a href=&quot;http://mars.nasa.gov/mer/mission/spacecraft_rover_body.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;warm electronics box&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In extreme environments like Jupiter's magnetosphere, spacecraft may need extra layer of radiation protection like &lt;a href=&quot;https://www.nasa.gov/mission_pages/juno/multimedia/pia13258-4x3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno's radiation vault&lt;/a&gt;, which is effectively a heavy titanium box that contains most mission-critical electronics elements.&lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2016-11-07T00:50:49.260" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18969" PostTypeId="2" ParentId="18959" CreationDate="2016-11-07T01:18:59.523" Score="0" Body="&lt;p&gt;You will find this information in books and materials labeled mostly with &quot;pyrotechnics&quot;. Here &lt;a href=&quot;http://www.lr.tudelft.nl/en/organisation/departments/space-engineering/space-systems-engineering/expertise-areas/space-propulsion/system-design/generate-candidates/comparison-of-rockets/&quot; rel=&quot;nofollow noreferrer&quot;&gt;is a table&lt;/a&gt; that lists some of the common solid propellant combinations, and here &lt;a href=&quot;http://pyrodata.com/composition/rocket-propellants&quot; rel=&quot;nofollow noreferrer&quot;&gt;is a online database&lt;/a&gt;. And here is a short &lt;a href=&quot;http://www.nasa.gov/centers/dryden/pdf/88635main_H-2330.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;historical overview&lt;/a&gt; of what came from where.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think you'll find good cost data anywhere except shopping around for Estes motors by yourself, but I'd recommend looking around on &lt;a href=&quot;http://www.nakka-rocketry.net/propel.html#Introduction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Richard Nakka's excellent&lt;/a&gt; hobby solid propellant project pages.&lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2016-11-07T01:18:59.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18970" PostTypeId="2" ParentId="18952" CreationDate="2016-11-07T06:08:09.250" Score="1" Body="&lt;p&gt;This is not going to be a fixed number for all applications. The noise from the solar atmosphere will reduce the received signal to noise ratio on each end (Earth/Mars) with the right equipment you could probably squeeze a few extra minutes/hours out of the comm link if you simply up the transmitter power accordingly.&lt;/p&gt;&#xA;" OwnerUserId="17507" LastActivityDate="2016-11-07T06:08:09.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18971" PostTypeId="2" ParentId="18640" CreationDate="2016-11-07T06:36:04.770" Score="3" Body="&lt;p&gt;It's a good solution to a sticky problem, pointing accuracy is a real pain in the butt on Earth (when you're stationary) let alone when in orbit (not quite so stationary).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Cons...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;&lt;strong&gt;&lt;em&gt;MONEY&lt;/em&gt;&lt;/strong&gt;  - this is a big one. It's not cheap to get a satellite to orbit. Depending on your requirements, might be as cheap as a&#xA;  couple million dollars or as expensive as a couple hundred&#xA;  million dollars. Whichever number you think is feasible, double it.&#xA;  Now double it again. 1 satellite means the Earth might be between your&#xA;  constellation and your communication hub; 2 could probably do the job&#xA;  though, but what if something does wrong.. heck let's stick a couple&#xA;  of redundancies in orbit as well... which brings me to...&lt;/li&gt;&#xA;  &lt;li&gt;To solve the problem of a constellation you've made a constellation - seriously.. try selling this to the guys who count the money. All these extra satellites actually increase all sorts of risks (eg. Space&#xA;  debris collision, launch failure, etc.).&lt;/li&gt;&#xA;  &lt;li&gt;Does this really fix the issue? A satellite in a high orbit does move slower, and if it's further from the final inter-plane satellite&#xA;  then the &lt;em&gt;relative angular change&lt;/em&gt; would be lower. However, the&#xA;  question of what's the ideal orbit soon brings up other problems like&#xA;  communication power requirements (further = bad).&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Pros...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;The chosen one - the hub satellite (or satellites) could be fitted&#xA;  with better momentum wheels and communication systems so they can&#xA;  easily target the out of plane satellites. This hardware would likely&#xA;  be heavy and expensive  (my phone autocorrected to expansive and it&#xA;  could be that too). You can't afford to outfit your 57 constellation&#xA;  satellites with this kind of gear, who are you Tony Stark! &lt;/li&gt;&#xA;  &lt;li&gt;Duel&#xA;  purpose - or triple, quadruple etc.. This hub satellite system could&#xA;  be used by everyone (for a small fee I'm sure). That means less stuff&#xA;  in orbit, better standardization of satellite hardware and more cash&#xA;  in your pocket!&lt;/li&gt;&#xA;  &lt;li&gt;Redundant redundancies - having satellites in more&#xA;  than one plane helps mitigate the possibility that you'll lose&#xA;  everything in orbit when I test my anti-satellite weapons. No&#xA;  offensive intended.. I picked your constellations orbit by chance.. no&#xA;  it has nothing to do with my competing communications constellation.&#xA;  When something collides or explodes in orbit it could be truly&#xA;  catastrophic for that orbital region.. and I'm talking for hundreds,&#xA;  thousands or millions of years (increases exponentially with&#xA;  altitude).&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So that's a few Pros and Cons. Certainly not an exhaustive list. If I ever figure out how to include hyperlinks on the mobile version of SE I'll pepper this with appropriate websites.&lt;/p&gt;&#xA;" OwnerUserId="17507" LastEditorUserId="35037" LastEditDate="2020-05-07T16:33:59.703" LastActivityDate="2020-05-07T16:33:59.703" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="18972" PostTypeId="2" ParentId="18952" CreationDate="2016-11-07T06:39:12.190" Score="3" Body="&lt;p&gt;The conjunction blackout is defined to be when the Sun-Earth-probe angle is less than 2°. That's about as tight as it gets, with operations usually suspended for several days before and after 2°.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-11-07T06:39:12.190" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="18973" PostTypeId="1" AcceptedAnswerId="18974" CreationDate="2016-11-07T10:17:57.100" Score="3" ViewCount="331" Body="&lt;p&gt;If i were to simulate a lunar module with: $x_0$, $\dot{x}_0$, $y_0$, $\dot{y}_0$. $x$ is horizontal position, and $y$ is vertical position. What would be appropriate values?&lt;/p&gt;&#xA;" OwnerUserId="17462" LastActivityDate="2016-11-07T13:01:03.630" Title="Lunar lander, apollo 11, start position" Tags="|apollo-program|lunar-landing|simulation|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="18974" PostTypeId="2" ParentId="18973" CreationDate="2016-11-07T13:01:03.630" Score="5" Body="&lt;p&gt;I fetched the data I think you are interested in from JPL's Horizon application. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eagle separates from the command module at 6:12 am on the 20'th of July:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CSM-LM separation        July 20 06:11:53&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the time of separation, the state vectors for the command module (at the time of separation the lander and command module presumably had the same state vectors, although I'm sure this is a very poor assumption, but at least it gives you something to work with) were as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2440422.758333333 = A.D. 1969-Jul-20 06:12:00.0000 TDB &lt;/p&gt;&#xA;&#xA;&lt;p&gt;X:  -2.870360413463610E-04 Y: 4.500679223401464E-05 Z: -1.732432604852359E-05&#xA;VX: -6.504365861559985E-04 VY: -1.604671519715920E-05 VZ: -5.403321203240985E-05&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The origin of the coordinate system is the center of the Moon and the time step 1 minute. Distances are in astronomical units.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lander conducted several burns after separating from the command module, though, so if you want to model the descent of the lander, you have to take those burns into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have a nice day!&lt;/p&gt;&#xA;" OwnerUserId="17423" LastActivityDate="2016-11-07T13:01:03.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18975" PostTypeId="2" ParentId="18789" CreationDate="2016-11-07T13:47:57.610" Score="4" Body="&lt;p&gt;I'm no rocket scientist, but I have it from two real rocket scientists that no launcher under about one metric tonne can get anything to orbit, at least if it has to start from the Earth's surface. Air launch can theoretically help, though, if it can bypass a lot of the atmosphere. There were some experiments back in the late 1950s at China Lake with multi-stage solids launched from fighter jets. They might have put small payloads into orbit, but at the time, they lacked the telemetry and tracking to verify orbit. Back then, it was hard to make anything practical with such small payload mass limits, so the approach was abandoned. Those limits aren't what they used to be, though -- significantly functional satellites fit in the cubesat format and perhaps smaller. CubeCab is trying this fighter-plane air launch approach again, planning to exploit the F-104's flight ceiling (in excess of 50,000 feet, with some versions going much higher) and launching one 3U cubesat at a time. Mach 2 isn't a very big fraction of orbital velocity, but it also helps a little. I'm not sure how big CubeCab's stack is, but it's probably much lighter than 1 metric tonne.&lt;/p&gt;&#xA;" OwnerUserId="10144" LastActivityDate="2016-11-07T13:47:57.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18977" PostTypeId="2" ParentId="18964" CreationDate="2016-11-07T18:35:29.123" Score="2" Body="&lt;p&gt;The only information I have been able to find only gives what one part of the engine is made of. That is the merlin vacuum variant bell extension.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_%28rocket_engine_family%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;wiki&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Its combustion chamber is regeneratively cooled, while the 2.7 metres (9 ft)-long niobium alloy expansion nozzle is radiatively cooled.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Specifically, it uses niobium alloy c103 (89-90 Nb, 9-10 Hf, 1Ti), per &lt;a href=&quot;http://www.alkane.com.au/images/pdf/asx/2017/20170516.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.alkane.com.au/images/pdf/asx/2017/20170516.pdf&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This backs up Hobbes comment that a lot of this information simply is not public.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="12375" LastEditDate="2017-06-26T22:01:44.283" LastActivityDate="2017-06-26T22:01:44.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18978" PostTypeId="2" ParentId="18937" CreationDate="2016-11-07T20:46:52.927" Score="1" Body="&lt;p&gt;Deep space probes have tight mass, power, and bandwidth budgets.  Imaging must serve the needs of the mission, and more often than not those needs have nothing to do with what something looks like to the naked eye.  Think about the bandwith limits on the New Horizons spacecraft (roughly 1 kbit/s at Pluto).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;True-color imagery would be great PR, but it would come at too high a cost.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2016-11-07T20:46:52.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18980" PostTypeId="2" ParentId="18858" CreationDate="2016-11-08T04:07:12.090" Score="2" Body="&lt;p&gt;This is a bit of a tangent, but the Voskhod manned spacecraft had a pressurized electronics chamber. It used vacuum tubes and they were cooled with blowing air. I suppose this air must have also gone over a heat sink or something. Not sure how that worked. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-11-08T04:07:12.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18981" PostTypeId="2" ParentId="3033" CreationDate="2016-11-08T04:29:30.617" Score="0" Body="&lt;p&gt;If we build small (few kilometers) Hyperloop-like booster in orbit, and use 'small' ground piece of Hyperloop to put aircraft into orbit altitude (200-300 kilometers), orbital part of the Hyperloop will catch aircraft and increase its speed to 7-8 kilometers per second. This concept is explained here – &lt;a href=&quot;http://www.launchplatform.space&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.launchplatform.space&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="17518" LastActivityDate="2016-11-08T04:29:30.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18982" PostTypeId="2" ParentId="3033" CreationDate="2016-11-08T08:37:25.577" Score="4" Body="&lt;p&gt;A rotovator &lt;a href=&quot;https://en.wikipedia.org/wiki/Skyhook_(structure)#Rotating&quot; rel=&quot;nofollow noreferrer&quot;&gt;rotating skyhook&lt;/a&gt; is within our technological ability - kevlar tether would be able to sustain it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some important problems with the concept, of course. Besides the point that it would be very expensive to build (good 200-400km of kevlar tether shaped so that it can sustain itself against several g and keep tons of payload), expensive to maintain (subject to significant atmospheric drag, needs constant propulsion - although fuel delivery is no longer prohibitively expensive); the whole idea is nutty - the end of the tether dips into the atmosphere nearly vertically as the rotovator &quot;rolls&quot; over the upper atmosphere, and a perfectly synchronized airplane lands, or hooks a payload to it, right in the nick of time before the tip yanks it up and out of the atmosphere, then releases into an eccentric orbital trajectory. So the logistics of the operation are devilishly tricky - imagine a supersonic cargo aircraft, landing on an aircraft carrier which is a submarine that surfaces only for a minute or so.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-11-08T08:37:25.577" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18983" PostTypeId="1" CreationDate="2016-11-08T09:39:11.720" Score="3" ViewCount="1136" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bussard Ramjet&lt;/a&gt; assumes a huge scoop picking interstellar hydrogen, compressing it into plasma, subjecting to nuclear fusion (harvesting the fusion energy of hydrogen) and ejecting the products of fusion at energy higher than what was needed to pick it up. So, free energy and free reaction mass in one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole concept falls apart at the point of &quot;scooping interstellar hydrogen and compressing it&quot; - the scoop would act as a huge parachute, the energy extracted from hydrogen not able to break even with the drag the scoop creates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we already have very good sources of energy - a nuclear reactor, RTG, beamed power, even decent fuel cells. It's the reaction mass that hurts us in the tyranny of rocket equation. We don't need scooping the energy. Let's do away with the funnel and fusion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's imagine a ion drive, where instead of a propellant tank, a valve and a nozzle releasing minuscule amounts of gas to be ionized and accelerated, we have an open end, a straightforward intake open to the void. Any interstellar medium particle, if not already ionized, is ionized, then propelled by electric field towards the back of the drive, and ejected at a speed higher than the one it entered at. Regardless of by how much we accelerate the particles, and how many of them we accelerate, we're gaining momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drive's throughput area would need to be considerably larger than the craft's profile, so that drag from hitting the medium is outweigh by medium that passes through unobstructed and is accelerated instead; and likely the thrust would be minuscule; the area of intakes would need to be huge comparing to the whole craft - likely huge grids of electrodes extending sideways. But are there any other problems I'm not aware of? Is this concept studied? Is it referred anywhere?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2020-05-24T07:00:27.233" Title="Using interstellar medium as propellant for ion propulsion" Tags="|ion-thruster|interstellar-medium|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18984" PostTypeId="2" ParentId="18924" CreationDate="2016-11-08T12:52:54.827" Score="4" Body="&lt;p&gt;I think you are right, the pressure of the gas does not inhibit the evaporation of lubricants. The same is true for air and water vapour. If we compress air from outdoors to 200 bar, a lot of water condenses, but the partial pressure of water is the same, it depends only on temperature, but not on ambient pressure. If the compressed air is expanded to 1 bar again, it is very dry. Not the rise of pressure lets the water condense, it is the loss of volume.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if the lubricants are not sealed within the compartment of the reaction wheel, they would continue to evaporize into space until nothing is left in the bearings. But if the compartment is sealed with or without a gas inside, the evaporation of the lubricant will stop when the vapour pressure is reached within the compartment. The gas does not stop evaporation, but the closed compartment does.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-11-08T12:52:54.827" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="18985" PostTypeId="2" ParentId="17349" CreationDate="2016-11-08T14:54:36.060" Score="1" Body="&lt;p&gt;As the other answers have pretty well shown that stellar aerobraking from 0.1c is not really an option, I wanted to find you some alternatives.&#xA;The following formula will give you the acceleration (in Earth gravitys) given the final velocity, starting velocity and stopping distance.&#xA;$$\frac{V_f^2 - V_i^2)}{\frac{(2*d)}{9.8}}=a$$&#xA;For starters I used the speed you gave and Plutos greatest distance from the Sun. &#xA;$$\frac{0^2 - (299792458/10)^2)}{\frac{(2*7378000000)}{9.8}}=a$$&#xA;Gives -6215Gs, that is a very dead crew.&#xA;I think this realy shows just how fast a fraction of the speed of light is.&#xA;If we instead do the same maneuver from 1/1000th C, we get -0.62Gs. So, by reducing the initial speed, you may be able to find a way to save your crew from being crushed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this is using some undefined braking mechanism.&#xA;Perhaps consider:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Giant solar sail used the create drag on the stellar wind &lt;/li&gt;&#xA;&lt;li&gt;Multiple aerobraking in the event 2 planets or the star are in line with your path&lt;/li&gt;&#xA;&lt;li&gt;Electric tether propulsion, not sure this actually works in this direction of travel&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These methods are meant to spread the deceleration out so that the crew could survive the experience.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastActivityDate="2016-11-08T14:54:36.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18986" PostTypeId="2" ParentId="18983" CreationDate="2016-11-08T16:10:11.837" Score="3" Body="&lt;p&gt;The problem you have with this concept is that there simply isn't enough mass out there to accelerate to make this worthwhile. The average density of hydrogen in space is &lt;a href=&quot;http://hypertextbook.com/facts/2000/DaWeiCai.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;1 atom per cubic centimeter&lt;/a&gt;, which is essentially nothing. Say you had an engine which could impel the hydrogen atoms in 10 cubic meters of space per second, that would be 10,000,000 hydrogen atoms per second, which equates to 1.67 * 10 to the -17 grams of mass per second or .0000000000000000167g. In order to impel those atoms anywhere near fast enough to move your spacecraft it would have to be a very powerful magnetic field, which would mean that it's likely pretty heavy. It would also need fuel - nuclear fission reactors need refueling, you'd use far more nuclear fuel trying to get moving than you would save with this approach. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-11-08T16:10:11.837" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18987" PostTypeId="2" ParentId="3033" CreationDate="2016-11-08T19:46:11.447" Score="2" Body="&lt;p&gt;The Wikipedia article you cited spells out your answer in the far right column of the comparison table. Technology Readiness Levels range from 2 to 9. Rockets (proven technology) score a 9. Gas guns and HARP canons have demonstrated amazing muzzle velocities and projectile altitudes in real hardware, so they rank 6. Almost everything else has a good theoretical background, small proof-of-concept models, and few obvious deal-breakers, but would require significant additional development, so they mostly score 2s. &#xA;There is a chicken-and-egg nature to your question. If you are going to invest large sums in a launch system, you need to know what you intend to launch. Small packets of acceleration-insensitive materials like fuel? Gas gun or space canon would be great and are already at TRL6. Satellite constellations or space construction project sub-components? Now you need payloads in the 1 ton - 20 ton range and limited accelerations. I'm a big fan of Sky Tram and electromagnetic launch for that type of payload. I'm pretty sure we could adapt a lot of payloads to survive ~30g launch acceleration, which opens the door to an Electromagnetic launch system that's on the order of 130 km long. If you want to transport people in large quantities or payloads which can't be designed for high-gee acceleration, then you need to look toward spaceplanes and spaceplane/skyhook hybrid systems, which (in general) move larger payloads gently and infrequently.IMHO, given a few tens of billions of dollars most of the TRL2 systems in the table could be brought to TRL9. What is less certain is the reliability, durability, and ACTUAL $/kg these systems would achieve, and what specific goal they would support. &lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2016-11-08T19:46:11.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="18989" PostTypeId="1" CreationDate="2016-11-09T01:37:01.493" Score="5" ViewCount="2487" Body="&lt;p&gt;If you were to use a spacecraft built of copper in Earth's magnetic field, could you produce enough energy to propel yourself?&lt;/p&gt;&#xA;" OwnerUserId="17527" LastEditorUserId="1142" LastEditDate="2016-12-31T00:00:00.780" LastActivityDate="2017-10-11T02:22:21.473" Title="Using Earth's magnetic field for an electric propulsion system" Tags="|magnetic-field|advanced-propulsion|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="18991" PostTypeId="1" AcceptedAnswerId="18998" CreationDate="2016-11-09T02:45:04.443" Score="4" ViewCount="870" Body="&lt;p&gt;&lt;em&gt;Eight years ago&lt;/em&gt;, on September 27, 2008, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shenzhou_7#Controversy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shenzhou-7&lt;/a&gt; carying three taikonauts deployed the microsatellite &lt;a href=&quot;https://en.wikipedia.org/wiki/Banxing&quot; rel=&quot;nofollow noreferrer&quot;&gt;Banxing-1&lt;/a&gt; for testing maneuvers and external imaging. A few hours later at 15:07 UTC the pair passed the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS&lt;/a&gt; with only about 45 kilometers between Shenzhou-7 and the ISS. Banxing-1 is &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/b/bx-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;described in detail here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Read more at the WSJ commentary &lt;a href=&quot;http://www.wsj.com/articles/SB122539460905385099&quot; rel=&quot;nofollow noreferrer&quot;&gt;China's Close Call&lt;/a&gt; (also &lt;a href=&quot;https://web.archive.org/web/20160309001054/http://www.wsj.com/articles/SB122539460905385099&quot; rel=&quot;nofollow noreferrer&quot;&gt;archived&lt;/a&gt;):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On September 27 at 3:07 p.m. Greenwich Mean Time, the Shenzhou-7 ship passed within 45 kilometers of the International Space Station, according to the U.S. Strategic Command...&lt;/p&gt;&#xA;&lt;p&gt;No one has offered a full explanation for why Beijing would do this. But China's track record of using all of its Shenzhou missions since 1999 for dual military-civil missions justifies speculation that it might have been part of a test of a new antisatellite missile technology. About four hours before Shenzhou's point of closest approach to the space station, Shenzhou-7 started one of its main missions: the launching of a 40-kilogram maneuvering microsatellite called BanXing-1 or BX-1...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It seems that after it was deployed, Banxing-1 was out of control during this close pass. The intended purpose was to remain and maneuver close to Shenzhou-7 and photograph it. Luckily it was drifting &lt;em&gt;on the opposite side from the ISS&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Chinese state television reported on October 2 that &amp;quot;after the satellite was released by the Shenzhou-7 last weekend, it quickly started drifting away from its intended trajectory.&amp;quot; NASA has not responded to questions about the BX-1 ventured closer to the space station than the Shenzhou incident. Neither has Washington.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Is that unusually close to the ISS to deploy and test a new satellite's propulsion and maneuvering without warning? &lt;em&gt;The relative velocity seems to be about 3 kilometers per second!&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;Below are plots made from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLEs&lt;/a&gt;. I've rotated the data into the frame of the ISS (black dot at the origin) with the thin black line pointing Earthwards (Nadir). Distances in kilometers. Blue/solid = Shenzhou-7, red/dashed = Banxing-1.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RcQH0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RcQH0.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1cdDu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1cdDu.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9BvG6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9BvG6.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ynetu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ynetu.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-31T15:55:37.463" Title="Is 45 km unusually close to pass the ISS and deploy a microsatellite without warning?" Tags="|iss|orbital-maneuver|safety|collision-avoidance|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="18996" PostTypeId="2" ParentId="18989" CreationDate="2016-11-09T19:53:01.400" Score="4" Body="&lt;p&gt;There is a &lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/60239/how-to-harness-jupiters-gravitational-energy&quot;&gt;question on the World Building Stack Exchange&lt;/a&gt; relevant to this.  The consensus there seems to be &lt;strong&gt;no you can't&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://worldbuilding.stackexchange.com/a/60264/25206&quot;&gt;This answer&lt;/a&gt; in particular by user Schwern seems to have the answer to your question.  Note that the original question was about power generation for a living space in Jupiter's orbit, but it answers your question too.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Jupiter has an enormous magnetic field, and your house is moving&#xA;  through it. You could wrap some wire around a coil and extract&#xA;  electrical energy from this field! Brilliant!&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;...except by doing so you're creating an oppositely charged magnet&#xA;  which Jupiter's magnetic field draws in creating a drag on your&#xA;  house's orbit. You're mortgaging your house's orbit for electricity.&#xA;  This is a theme.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;You could kick the electro-magnet out into space. Then it will be in&#xA;  its own orbit. It would send the energy back to you with a laser (also&#xA;  a theme) and eventually crash into Jupiter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="16599" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:23.630" LastActivityDate="2016-11-09T19:53:01.400" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="18998" PostTypeId="2" ParentId="18991" CreationDate="2016-11-09T22:14:25.153" Score="4" Body="&lt;p&gt;I know nothing of this specific incident, but the generic ISS COLA (COLlision Avoidance) &quot;box&quot; is sized such that a body passing at 45 km could be inside the COLA box, but wouldn't trigger the &quot;notification&quot; rule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/n2l4R.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/n2l4R.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; (&lt;a href=&quot;http://iaassconference2013.space-safety.org/wp-content/uploads/sites/26/2013/06/1420_Schultz.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-11-09T23:29:07.323" LastActivityDate="2016-11-09T23:29:07.323" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="18999" PostTypeId="1" AcceptedAnswerId="19001" CreationDate="2016-11-09T22:44:07.620" Score="15" ViewCount="8292" Body="&lt;p&gt;According to SpaceX's website, the Falcon 9 uses kerosene as fuel. According to &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Energy_density&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, diesel fuel beats kerosene in energy density in terms mass by 2.8 MJ/Kg and in volume by 5.6MJ/L, so why would they use kerosene instead? In addition, liquid hydrogen beats kerosene by over three times in terms of energy density by mass, although it is about 1/4 the energy density in terms of volume, but couldn't the rocket just be built taller to accommodate the extra volume, and still retain a lower total mass since the fuel makes up most of the rocket's weight?&lt;/p&gt;&#xA;" OwnerUserId="17532" LastEditorUserId="6816" LastEditDate="2016-11-09T23:09:17.863" LastActivityDate="2016-11-12T18:20:35.490" Title="Why use Kerosene?" Tags="|falcon-9|fuel|specific-impulse|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19000" PostTypeId="2" ParentId="16845" CreationDate="2016-11-10T02:47:45.727" Score="6" Body="&lt;p&gt;A family of spacecraft in the 1980s were designed around the Multi-mission Modular Spacecraft (MMS) bus.  This bus was designed specifically to allow for retrieval and repair of the spacecraft by the Space Shuttle. (&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050071087.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;As far as I know, the only MMS-based satellite that was actually repaired by a Shuttle mission was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_Maximum_Mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Maximum Mission&lt;/a&gt;. This spacecraft was disabled by failures in its electrical system, and was retrieved and repaired (not without some associated drama) by Shuttle mission &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-41-C&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-41C&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DZyBi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DZyBi.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Incidentally, 41-C was a prime example of an ambitious pre-Challenger accident Shuttle mission, featuring an untethered EVA by an astronaut flying the Manned Maneuvering Unit, near-real-time replanning of robotics operations, and deployment of a science satellite, the Long Duration Exposure Facility.  It was also the first mission to fly a &amp;quot;direct insertion&amp;quot; ascent, obviating the need for an OMS-1 burn.&lt;/p&gt;&#xA;&lt;p&gt;Quite a bit of footage from this mission showing the capture and repair is included in the early IMAX film &lt;a href=&quot;https://en.wikipedia.org/wiki/The_Dream_Is_Alive&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Dream is Alive&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2022-09-06T22:28:47.833" LastActivityDate="2022-09-06T22:28:47.833" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="19001" PostTypeId="2" ParentId="18999" CreationDate="2016-11-10T02:57:51.577" Score="21" Body="&lt;p&gt;The hydrogen side of this question has been addressed here: &lt;a href=&quot;https://space.stackexchange.com/questions/18576/combination-of-liquid-hydrogen-and-liquid-oxygen&quot;&gt;Combination of liquid hydrogen and liquid oxygen&lt;/a&gt; -- short answer is that a hydrogen first stage would be lighter, loaded, but much more expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern &quot;kerosene&quot;-burning rockets like the Falcon 9 use &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;noreferrer&quot;&gt;RP-1&lt;/a&gt; (or RG-1 in Russian engines); this is a refined form of kerosene containing a relatively narrow mix of hydrocarbons rather than the wider spectrum found in common kerosene. RP-1 is often used as a coolant as well as a propellant, piped along the walls of the combustion chamber before entering it. The different boiling points and polymerization characteristics of different hydrocarbons causes problems:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Many early rockets had burned kerosene, but as burn times, combustion efficiencies, and combustion-chamber pressures grew, and as engine masses shrank, the engine temperatures became unmanageable. Raw kerosene used as coolant would dissociate and polymerize. Lightweight products in the form of gas bubbles, and heavy ones in the form of engine deposits, then blocked the narrow cooling passages. The coolant starvation raised temperatures further, accelerating breakdown. This cycle would escalate rapidly (i.e., thermal runaway would occur) until an engine wall ruptured.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Common diesel fuel is even more prone to polymerization and thermal breakdown, so the same pressures that drove refinement of kerosene into RP-1 apply. For SpaceX in particular, who intends to reuse their rocket engines, minimizing residue in the plumbing is a significant factor, and this is one of the reasons they are moving to methane as the fuel in their next generation of engines.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-11-10T15:28:05.483" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19002" PostTypeId="2" ParentId="18856" CreationDate="2016-11-10T10:38:59.737" Score="13" Body="&lt;h2&gt;A wooden Space Shuttle?&lt;/h2&gt;&#xA;&lt;p&gt;Before the Space Shuttle ever flew, a lot of testing had to be done. Among the test items were the OV-101 (Enterprise), STA-099 (later OV-099, Challenger) and the Pathfinder (later OV-098, no STA designation known). The latter was partly built of wood.&lt;/p&gt;&#xA;&lt;h2&gt;OV-101 Enterprise&lt;/h2&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0Em0G.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0Em0G.jpg&quot; alt=&quot;OV-101 Enterprise&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Enterprise was built without engines or functional heat shield. Also a lot of other systems were missing. It was intended as a flight test article first, but was planned to be upgraded to full orbiter status after conclusion of testing, becoming the second orbiter after OV-102 Columbia. Design changes during the construction of Columbia forced NASA to reconsider this. After the Challenger disaster, refitting Enterprise was again brought up but ultimately decided against.&lt;/p&gt;&#xA;&lt;h2&gt;STA-099 Challenger&lt;/h2&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zwIfQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zwIfQ.jpg&quot; alt=&quot;STA-099 Challenger&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Like the Enterprise, Challenger was first built for testing. It was a full airframe with a mockup crew module and partial heat shielding. It was put to the test for 11 months. After that it was fitted with a functional crew module, parts were reworked and was launched on orbital missions as OV-099.&lt;/p&gt;&#xA;&lt;h2&gt;OV-098 Pathfinder&lt;/h2&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qJYwc.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qJYwc.jpg&quot; alt=&quot;OV-098 Pathfinder&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/resources/orbiters/pathfinder.html&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Pathfinder&lt;/a&gt; (retroactively/honorarily assigned number OV-098) was another full sized orbiter test article, but built out of steel and wood. It was mainly used for testing all shuttle ground facilities: cranes, roadways and other heavy equipment. It completed 6 missions, but zero of those actually went into space. It was later refurbished into a fully equipped model using a test tank (MPTA-ET) and two prototype Advanced Solid Rocket Booster casings.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s6kz7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s6kz7.jpg&quot; alt=&quot;Refurbished Pathfinder&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13692" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-11-15T09:12:15.080" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19004" PostTypeId="1" CreationDate="2016-11-10T20:02:57.243" Score="1" ViewCount="200" Body="&lt;p&gt;Where can I find more detailed information about &lt;a href=&quot;https://en.wikipedia.org/wiki/Spektr-UV&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spectr-UV&lt;/a&gt;? I'm interested to find an in-depth article and/or images of it's components. It seems that it could have offered a unique capability, why haven't there been more space-based UV observatories?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3UYnn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3UYnn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; drawing of Spektr-UV from &lt;a href=&quot;http://www.russianspaceweb.com/spektr_uf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;russianspaceweb.org&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="16355" LastEditorUserId="12102" LastEditDate="2017-02-12T04:01:30.923" LastActivityDate="2017-02-12T04:05:51.147" Title="Russian space observatory Spektr-UV" Tags="|spacecraft|imaging|space-telescope|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19005" PostTypeId="2" ParentId="19004" CreationDate="2016-11-10T21:45:09.433" Score="3" Body="&lt;p&gt;You can find some information on Spectr-UV by searching the web.  I've included three links to the Spektr-UV/Spectr-UF mission below. The third gives details of the three planned spectrometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been more than a dozen space based dedicated UV telescopes/instruments plus two on Hubble (see the link to the Wikipedia List of Space Telescopes article).  During the most recent Hubble Servicing Mission in 2009, STIS (the Space Telescope Imaging Spectrograph) was repaired and COSTAR was replaced with COS (the Cosmic Origins Spectrograph).  Both of these are UV instruments.  The planned follow on to Hubble, LUVOIR (the Large UltraViolet, Optical, InfraRed survey telescope) will have at least one multi-purpose ultraviolet instrument.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spectr-UV links:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Spektr-UV&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Spektr-UV&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.russianspaceweb.com/spektr_uf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.russianspaceweb.com/spektr_uf.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/spektr-uf.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://space.skyrocket.de/doc_sdat/spektr-uf.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;List of space telescopes (in the Contents list, click on Ultraviolet): &#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_space_telescopes&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/List_of_space_telescopes&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STIS (on the HST) links: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.spacetelescope.org/about/general/instruments/stis/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.spacetelescope.org/about/general/instruments/stis/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Telescope_Imaging_Spectrograph&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Space_Telescope_Imaging_Spectrograph&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;COS (on the HST) links:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_Origins_Spectrograph&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Cosmic_Origins_Spectrograph&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.stsci.edu/hst/cos&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.stsci.edu/hst/cos&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.spacetelescope.org/about/general/instruments/cos/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.spacetelescope.org/about/general/instruments/cos/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17341" LastEditorUserId="12102" LastEditDate="2017-02-12T04:05:51.147" LastActivityDate="2017-02-12T04:05:51.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19006" PostTypeId="1" AcceptedAnswerId="20522" CreationDate="2016-11-11T16:04:29.813" Score="34" ViewCount="12292" Body="&lt;p&gt;Is there any possibility to learn and use the operating system languages used in the space shuttle? &lt;/p&gt;&#xA;" OwnerUserId="17539" LastEditorUserId="58" LastEditDate="2016-11-11T17:59:50.570" LastActivityDate="2022-06-04T02:04:14.367" Title="What operating system(s) were used in the space shuttle?" Tags="|space-shuttle|software|" AnswerCount="6" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19007" PostTypeId="1" AcceptedAnswerId="63119" CreationDate="2016-11-12T17:28:22.917" Score="5" ViewCount="1087" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Cyclone_Global_Navigation_Satellite_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cyclone Global Navigation Satellite System&lt;/a&gt; is a group of 8 smallsats which will orbit near the equator (inc=30°) and work as a constellation to take data on typhoons and hurricanes. While they are meant to map wind speed they don't actually do that. They measure ocean surface roughness, but they don't even to do that. Each of the eight smallsats will carefully monitor up to four GPS satellite signals, and for each signal the direct beam and the reflected beam off of the ocean will both be measured.&lt;/p&gt;&#xA;&lt;p&gt;How (actually) will the &lt;a href=&quot;http://clasp-research.engin.umich.edu/missions/cygnss/science.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;CYGNSS satellites&lt;/a&gt; interpret ocean roughness by comparing direct and reflected signals from GPS satellites? I'm interested in an explanation of how the received signals will be compared to extract a number for ocean roughness.&lt;/p&gt;&#xA;&lt;p&gt;See these items also:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://clasp-research.engin.umich.edu/missions/cygnss/docs/CYGNSS_FactSheet_October2014.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://clasp-research.engin.umich.edu/missions/cygnss/docs/CYGNSS_FactSheet_October2014.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://clasp-research.engin.umich.edu/missions/cygnss/science.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://clasp-research.engin.umich.edu/missions/cygnss/science.php&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/cygnss/overview&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/cygnss/overview&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PUYp3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PUYp3.png&quot; alt=&quot;CYGNSS&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Av4by.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Av4by.png&quot; alt=&quot;CYGNSS&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; screen shots from &lt;a href=&quot;http://clasp-research.engin.umich.edu/missions/cygnss/docs/CYGNSS_FactSheet_October2014.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;CYGNSS_FactSheet_October2014&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-07T00:02:47.530" LastActivityDate="2023-03-19T10:08:36.987" Title="How can the CYGNSS spacecrafts (actually) measure ocean roughness?" Tags="|earth-observation|gps|scientific-data|smallsat|cygnss|" AnswerCount="7" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="19008" PostTypeId="2" ParentId="19006" CreationDate="2016-11-12T17:45:29.857" Score="12" Body="&lt;p&gt;For the flight control computers, aka General Purpose Computers (GPCs) there is a good writeup on this topic in the ever-useful &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiZw_vQ4qPQAhWFwlQKHZAlBiIQFggdMAA&amp;amp;url=https%3A%2F%2Fwww.nasa.gov%2Fcenters%2Fjohnson%2Fpdf%2F390651main_shuttle_crew_operations_manual.pdf&amp;amp;usg=AFQjCNENi1Wfkn12N9uV7BJ3FfCXYBLgag&amp;amp;sig2=MlXtSrl1JHfxCIpurPUVfg&quot; rel=&quot;noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt; page 2.6-20.&lt;/p&gt;&#xA;&lt;p&gt;DPS in this quote stands for Data Processing System&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;DPS software is divided into two major groups, system software and&#xA;applications software. The two groups are combined to form a memory&#xA;configuration for a specific mission phase. &lt;strong&gt;The programs are written in HAL/S (high-order assembly language/shuttle) specifically developed for real-time space flight applications&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;System software is the&#xA;GPC operating system software that controls the interfaces among the&#xA;computers and the rest of the DPS.&lt;/strong&gt; It is loaded into the computer when&#xA;it is first initialized. It always resides in the GPC main memory and&#xA;is common to all memory configurations. The system software controls&#xA;GPC input and output, loads new memory configurations, keeps time,&#xA;monitors discretes into the GPCs, and performs many other DPS&#xA;operational functions. The system software consists of three sets of&#xA;programs. The &lt;em&gt;flight computer operating system&lt;/em&gt; (FCOS) (the executive)&#xA;controls the processors, monitors key system parameters, allocates&#xA;computer resources, provides for orderly program interrupts for higher&#xA;priority activities, and updates computer memory. The &lt;em&gt;user interface&#xA;programs&lt;/em&gt; provide instructions for processing flight crew commands or&#xA;requests. The &lt;em&gt;system control program&lt;/em&gt; initializes each GPC and arranges&#xA;for multi-GPC operation during flight-critical phases.&lt;/p&gt;&#xA;&lt;p&gt;One of the&#xA;system software functions is to manage the GPC input and output&#xA;operations, which includes assigning computers as com- manders and&#xA;listeners on the data buses and exercising the logic involved in&#xA;sending commands to these data buses at specified rates and upon&#xA;request from the applications software.&lt;/p&gt;&#xA;&lt;p&gt;The applications software performs the functions required to fly and&#xA;operate the vehicle. To conserve main memory, the applications software is divided into three major functions: Guidance, navigation,&#xA;and control (GNC), Systems management (SM), and Payload (PL).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(I summarized the last sentence in the last paragraph, it is not a direct quote.  Also, the bolding is mine.)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-11-12T17:50:47.430" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19009" PostTypeId="2" ParentId="18999" CreationDate="2016-11-12T18:20:35.490" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;According to Wikipedia, diesel fuel beats kerosene in energy density in terms mass by 2.8 MJ/Kg and in volume by 5.6MJ/L, so why would they use kerosene instead?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;First off, that's wikipedia for you, using unreferenced numbers that are in fact incorrect. The conflicting wikipedia page on &lt;a href=&quot;https://en.wikipedia.org/wiki/Heat_of_combustion#Heat_of_combustion_tables&quot; rel=&quot;noreferrer&quot;&gt;heat of combustion&lt;/a&gt; states lower heating values of 43.0 and 43.4 MJ/kg for kerosene and diesel, respectively. The conflicting wikipedia pages on &lt;a href=&quot;https://en.wikipedia.org/wiki/Kerosene&quot; rel=&quot;noreferrer&quot;&gt;kerosene&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Diesel_fuel&quot; rel=&quot;noreferrer&quot;&gt;diesel fuel&lt;/a&gt; state lower (or net) heating values of 43.1 MJ/kg for both kerosene and diesel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside: The lower and higher heating values of a substance reflect whether the water produced by the combustion exits the system in gaseous or liquid form. As water exits a rocket in a gaseous state with chemical rocket engines, it's the lower (or net) heating values that are relevant to rocket engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Factors that argue against using diesel fuel:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Those heating values do not account for the mass of the oxygen.&lt;br&gt;&#xA;This is fine for combustion on the surface of the Earth, but is completely incorrect with regard to rockets because rockets have to carry both the fuel and the oxidizer. Long hydrocarbon chains require more oxygen per unit mass than do shorter saturated hydrocarbon chains.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Looking at heating values alone does not account for how rocket engines work.&lt;br&gt;&#xA;The thrust produced by a rocket is proportional to $\sqrt{Q/M}$, where $Q$ is the net heating value and $M$ is the mean molecular mass of the exhaust. Water vapor in rocket is highly preferable compared to carbon dioxide because water vapor has a molecular mass of about 18 while carbon dioxide has a molecular mass of about 44. This is the key reason why LH2-LOX rocket engines look so good on paper. This also argues against long chain hydrocarbons.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Carbon deposits present a danger to rocket engines.&lt;br&gt;&#xA;The longer the hydrocarbon chain, the greater the risk for carbon deposits.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The slight advantage (if any) of diesel fuel over kerosene based on heating value becomes a disadvantage after accounting for those other factors.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-11-12T18:20:35.490" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19012" PostTypeId="2" ParentId="19007" CreationDate="2016-11-13T04:31:12.720" Score="5" Body="&lt;p&gt;The CYGNSS spacecraft will use a technique called &quot;&lt;a href=&quot;http://clasp-research.engin.umich.edu/missions/cygnss/technology-ddmi.php&quot; rel=&quot;noreferrer&quot;&gt;Delay Doppler Mapping&lt;/a&gt;&quot;.  Each satellite will be equipped with a Delay Doppler Mapping Instrument (DDMI), which is capable of receiving four DDM's at once.  &lt;a href=&quot;http://www.altimetry.info/radar-altimetry-tutorial/how-altimetry-works/delay-doppler-or-sar-altimetry/&quot; rel=&quot;noreferrer&quot;&gt;DDM&lt;/a&gt; is only slightly different from standard Radar Altimetry, which measures the distance to an object by tracking how long it takes a signal bounced off the surface to return.  That is the 'Delay' part of DDM.  When using DDM, you 'look' at the surface of the object for much longer, allowing you to integrate over the full time.  This allows the signals to be weaker, because they will be integrated over a longer amount of time.  In addition, DDM differs from radar because it uses the Doppler effect to track how fast (and in what direction) something is moving.  DDM can be used to calculate how fast the wind is moving at the surface (1), which is one of the main goals of the study.  CYGNSS will be using special (and highly complex) algorithms (2) to retrieve the wind surface speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of CYGNSS, the signals will originate from orbiting GPS satellites.  The satellites will also beam the signal directly to CYGNSS, so that they can compare the reflected response to the emitted signal.  This idea was proposed back in 1993 (3).  Since CYGNSS consists of eight smallsats, each capable of measuring 4 areas, the mission will be able to reconstruct the shape of the ocean surface and windspeeds there over a significant area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the shape of the ocean surface, researchers will be able to infer how rough the ocean is at that particular location.  While there have been attempts to create a &lt;a href=&quot;http://journals.ametsoc.org/doi/pdf/10.1175/JPO2704.1&quot; rel=&quot;noreferrer&quot;&gt;standard parameter for roughness&lt;/a&gt;, there currently is not one in use (as far as I could find).  Instead, researchers will rely on factors such as the wave height, spacing, speed, and steepness to determine how rough the surface is.  Higher and more closely spaced waves (thus steeper waves) indicate a rougher sea than shallow, long waves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Emily Lakdawalla does a good job of explaining how DDM is used to get the shapes of asteroids on &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2010/2462.html&quot; rel=&quot;noreferrer&quot;&gt;her blog&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Citations:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Chen Li, Weimin Huang.  &quot;Sea surface wind retrieval from GNSS delay-Doppler map using two-dimension least-squares fitting&quot; (2013).  DOI: &lt;a href=&quot;http://dx.doi.org/10.1109/OCEANS-Bergen.2013.6608019&quot; rel=&quot;noreferrer&quot;&gt;10.1109/OCEANS-Bergen.2013.6608019&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Maria Paola Clarizia, Christopher S. Ruf.  &quot;Wind Speed Retrieval Algorithm for the Cyclone Global Navigation Satellite System (CYGNSS) Mission&quot; (2016).  DOI: &lt;a href=&quot;http://dx.doi.org/10.1109/TGRS.2016.2541343&quot; rel=&quot;noreferrer&quot;&gt;10.1109/TGRS.2016.2541343&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Daniel Pascual, et. al. &quot;Precision Bounds in GNSS-R Ocean Altimetry&quot; (2014). DOI: &lt;a href=&quot;http://dx.doi.org/10.1109/JSTARS.2014.2303251&quot; rel=&quot;noreferrer&quot;&gt;10.1109/JSTARS.2014.2303251&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2016-11-14T15:41:10.820" LastActivityDate="2016-11-14T15:41:10.820" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19015" PostTypeId="1" AcceptedAnswerId="19016" CreationDate="2016-11-13T16:32:00.393" Score="0" ViewCount="656" Body="&lt;p&gt;Modern chemical rockets have so far traveled relatively short distances (i.e. to the moon) and there are many newer, more efficient ideas being theorised and tested for longer journeys.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, without considering factors such as cost or fuel efficiency, is there going to be a limit to where such rockets can go, or will newer fuels and larger craft be capable of travelling to wherever the next furthest destination is?&lt;/p&gt;&#xA;" OwnerUserId="17552" LastEditorUserId="17552" LastEditDate="2016-11-24T11:52:53.470" LastActivityDate="2016-11-24T11:52:53.470" Title="Are chemical rockets limited other than their bad efficiency?" Tags="|propulsion|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19016" PostTypeId="2" ParentId="19015" CreationDate="2016-11-14T00:40:21.250" Score="7" Body="&lt;p&gt;In practice, yes, chemical rockets are very limited in what they can achieve, because they consume fuel to accelerate, and by bringing more fuel they grow in weight, yielding diminishing returns from each additional kilogram of fuel. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt; describes exactly how that works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With chemical propulsion, we can do one way trips to the edge of our solar system in about 30-40 years (the Voyagers are there now; New Horizons will be, before very long). The nearest stars are about 1000 times further away than that, and we don't have any practical way of building a machine that can last for 30,000 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the next phases of exploration of our own solar system, various &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrically_powered_spacecraft_propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;electrical rocket propulsion technologies&lt;/a&gt; are attractive; their exhaust velocities are much higher than those of chemical rockets, so for the same mass of propellant they can achieve much higher speeds. That still requires thousands of years for interstellar travel. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-11-14T00:40:21.250" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19017" PostTypeId="1" CreationDate="2016-11-14T04:48:07.603" Score="9" ViewCount="1965" Body="&lt;p&gt;Solid rocket motors are &lt;em&gt;so&lt;/em&gt; much simpler than liquid fueled. Why isn't e.g. SpaceX making a Falcon 9 equivalent with solid fuel only, and rescuing the segment cases with parachutes like the shuttle SRBs?&lt;/p&gt;&#xA;" OwnerUserId="16937" LastActivityDate="2022-10-26T19:03:25.270" Title="Why isn't someone building a fully reusable solid fuel rocket?" Tags="|reuse|solid-fuel|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19018" PostTypeId="2" ParentId="19017" CreationDate="2016-11-14T06:25:43.623" Score="15" Body="&lt;p&gt;The reason is refurbishment cost and turnaround:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a liquid booster, you can more or less pump more fuel and you are ready to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a SRB, it isn't so easy:&#xA;You need to dismantle everything, check it and build it again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space Shuttle's SRB refurbishment has been quoted as a marginal improvement over building them again from scratch, altho I couldn't find a reference on it.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-11-14T06:25:43.623" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19019" PostTypeId="2" ParentId="19007" CreationDate="2016-11-14T12:20:18.947" Score="8" Body="&lt;p&gt;Addressing multipath has long been a challenge with regard to GPS. Carrying a GPS-enabled smartphone toward the heart of a large city results in GPS-estimated positions and altitudes that bounce around. The problem is signals reflecting off of buildings, sometimes multiple times. Those reflecting surfaces result in multiple paths that a GPS signal can follow to reach a GPS receiver, hence the term &quot;multipath&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rather than being confused by multipath, CYGNSS takes advantage of it. Each CYGNSS satellite receives a direct GPS signal that tells the satellite where it is. The satellites also receive multiple indirect signals due to reflections from the surface of the Earth. The spectrum of how those bounced signals trail off over time provide information regarding the local roughness of an ocean's surface. The spectrum of delays from a very smooth surface versus a very rough surface differ markedly.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-11-14T12:20:18.947" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19020" PostTypeId="2" ParentId="19017" CreationDate="2016-11-14T13:33:07.527" Score="12" Body="&lt;p&gt;There are a few reasons that re-usable solid rocket boosters are not being built. The biggest reason is that people don't want solid rockets in the first place. Solid rockets can be stored loaded long term so they are useful if you need a rocket to be instantly ready to go, for instance military purposes, but they have a much lower specific impulse than liquid fuel rockets, so a lot less power. The military doesn't have a use case for recovering rockets, and the space industry isn't looking to solid boosters for launch, so no use case no development. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid boosters are also harder and more expensive to recover because once they are lit then cannot be throttled or stopped and re-lit. You can't have a solid rocket land itself like some of the liquid fuel rockets are, once they cut off they are going to be downrange and you'll have to go find them, haul them out of the water (assuming they wet land), then bring them back and refurb them. Refurbing them means you have to take them apart, clean all the spent propellant out (a nasty and toxic job), and put them back together again. Once the economics of it are worked out the cost of recovery often doesn't make it worth the trouble - look at the shuttle's SRBs as an example. It's possible that technology could be developed to allow the fine control of solid boosters, making them more easily recovered, and the maybe they will look at the possibility. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-11-14T13:33:07.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19021" PostTypeId="2" ParentId="17963" CreationDate="2016-11-14T14:52:11.617" Score="1" Body="&lt;p&gt;As a rather violent &quot;boost optimization&quot;, I'd like to point out to the existing question about unconventional LEO launching, where &lt;a href=&quot;https://space.stackexchange.com/a/3034/12519&quot;&gt;this answer&lt;/a&gt; provides a nice presentation of a &lt;strong&gt;gas gun&lt;/strong&gt;. A quick summary is: gas gun was tested to provide at least 3 km/s initial delta-v. The bullet is planned to be a small rocket. The technology is currently ready for a high-G rocket - with electronics included! Fuel is explicitly said to be an ideal high-G payload. Cost to LEO per pound of fuel is calculated there, but it's so absurdly low I'd dare not repeat it here :)&lt;/p&gt;&#xA;" OwnerUserId="12519" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-11-14T15:26:33.140" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19022" PostTypeId="2" ParentId="9134" CreationDate="2016-11-14T15:31:17.380" Score="1" Body="&lt;p&gt;The analytical solution for an orbit around the moon would not help you because mascons are ignored, the theory only works for a hypothetical moon of homogenous density. A numerical simulation of an orbit over the known mascons would require very precise data about the mascons and a lot of computing time for the simulation of a few centuries. How much numerical precision is necessary to simulate such a long time? Finding the floor for stable orbits is try and error. A lot of expensive computing time on large parallel processor might be necessary. If a candidate of a stable orbit is found, it must be tested if it is stable even for small variations of orbit height and plane.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-11-14T15:31:17.380" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19023" PostTypeId="1" AcceptedAnswerId="19024" CreationDate="2016-11-15T00:10:49.727" Score="6" ViewCount="538" Body="&lt;p&gt;Can we efficiently reflect or capture photons of that wavelength?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More precisely, $ 2.426×10^{-12} $ meters, or $ 1.236 × 10^{20} Hz$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Preferably reflect, but capture followed by re-emission as photons of other frequencies, like infrared is okay too.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;for a rough orientation if the numbers are meaningless to you - they fall pretty deep into cosmic radiation spectrum, way deep frequency of gamma rays, nowhere near to mild X rays.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Why such an oddly specific request? This is related to the concept of antimatter drive. In most of answers that looked at efficiencies achieved by a spaceship exploiting antimatter, the answer is qualified with &quot;assuming annihilation energy can be converted to kinetic energy at near 100% efficiency&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, it can. And quite easily too. A plain parabolic mirror will suffice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one drive of absolutely maximum possible specific impulse (~$3 × 10^7 s$) is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Photon_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;photon drive&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A photon drive is dead easy to make. Everyone's seen a photon drive, most of us have a couple of them installed in our cars. We call them headlights. A source of photons (lightbulb) and a reflector to make them mostly unidirectional. Frequency doesn't matter much; visible, infrared, ultraviolet, whatever we can reflect. And yet, instead of our spiffy photon drives we use the terrible inefficient internal combustion engines for accelerating the cars. That's due to the unfortunate properties of the photon drive which are a pathetic thrust and absolutely terrible wet:dry mass ratio in case of all conventional energy sources - even including nuclear!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only energy source that assures 100% fuel-&gt;reaction mass conversion for a photon drive is antimatter. Reaction mass being photons, fuel being positrons and electrons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ 2.426×10^{-12} $ meters happens to be the low-energy annihilation photon wavelength. (we don't care about high-energy; whatever means we find to infuse the positrons with extra energy would be less energy-efficient than what we get out of the annihilation.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we can build a mirror that reflects great most of these photons, and shape it as a pretty long parabolic mirror (quite similar to De Laval nozzle in shape actually!) we have it. And even if we can't reflect them, if we can absorb them and let them be re-emitted, say, as infrared=heat radiation, just into the nozzle and not anywhere else (...right placement of radiators) that still works - a photon is a photon, as long as it's emitted &quot;backwards&quot; it still gives us the propulsion at the awesome $1 {c \over g_0}$ seconds of specific impulse. Of course the heat management problems arise, that are mostly absent in case of the mirror, but that's just an engineering issue ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, other than that we still need ways to produce, store and manipulate antimatter of course. But &lt;em&gt;assuming we can&lt;/em&gt;, if this question is positively answerable, the near-100%-efficient antimatter drive is within our reach.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="32284" LastEditDate="2020-08-17T18:52:41.733" LastActivityDate="2020-08-17T18:52:41.733" Title="What is our ability to capture or reflect photons with a wavelength of $2.5×10^{-12}$ meters?" Tags="|advanced-propulsion|antimatter|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19024" PostTypeId="2" ParentId="19023" CreationDate="2016-11-15T02:04:48.880" Score="8" Body="&lt;p&gt;Electromagnetic radiation with frequencies above $10^{19}Hz$ are conventionally called Gamma Rays. Gamma rays will penetrate instead of reflect of most materials. Reflection is only possible at very shallow angles of incidence, and even then it works best for lower energies like X-rays, below $10^{19}Hz$. This method is used in the European X-Ray satellite &lt;a href=&quot;https://en.wikipedia.org/wiki/ROSAT&quot; rel=&quot;noreferrer&quot;&gt;ROSAT&lt;/a&gt;, using a &lt;a href=&quot;https://en.wikipedia.org/wiki/Wolter_telescope&quot; rel=&quot;noreferrer&quot;&gt;Wolter Telescope&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At higher frequencies or higher incidence angles, the radiation will either pass through the mirror or, especially in the case of gamma rays, it will interact with the mirror material. Most of the energy will then be re-radiated, most of it going in the same direction as the incoming gammas, rather than backwards as you require.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These reflectors are also only used at very low energies - they're used to detect radiation from stars and galaxies. In your case you're talking about megawatts or gigawatts. At those energies, the mirror material will likely vaporise. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, there is no material that will do what you require.&lt;/p&gt;&#xA;" OwnerUserId="17390" LastActivityDate="2016-11-15T02:04:48.880" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19025" PostTypeId="2" ParentId="19023" CreationDate="2016-11-15T02:50:24.083" Score="7" Body="&lt;p&gt;The wavelength you mention corresponds to about 510 KeV.  As hdhondt says, conventional optics, such as a parabolic mirror, don't work at this range.  Wolter telescopes use grazing incidence reflectors and can work up to about 100 KeV.  The NuSTAR X-ray telescope is a Wolter with a maximum energy of 79 KeV.  The BAT (Burst Alert Telescope) on Swift uses a coded mask that can image up to 150 KeV.  For higher energies, rather than an optical imaging system, the detectors are in stacks of layers, essentially directional detectors.  The Fermi Large Area Telescope used this approach to image up to 300 GeV!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question you bring up is an interesting one.  How would you, even inefficiently, convert the gamma rays from matter/anti-matter collisions into useful energy?  Since the wavelengths are much smaller that the space between atoms in an ordinary material, just absorbing the gamma rays (to convert to heat) seems a challenging issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NuSTAR links:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/NuSTAR&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/NuSTAR&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/nustar/main/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/mission_pages/nustar/main/index.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Swift/BAT links:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Swift_Gamma-Ray_Burst_Mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Swift_Gamma-Ray_Burst_Mission&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://swift.gsfc.nasa.gov/about_swift/bat_desc.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://swift.gsfc.nasa.gov/about_swift/bat_desc.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fermi Large Area Telescope links:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www-glast.stanford.edu/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www-glast.stanford.edu/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://arxiv.org/abs/0902.1089&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://arxiv.org/abs/0902.1089&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17341" LastActivityDate="2016-11-15T02:50:24.083" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19028" PostTypeId="2" ParentId="13632" CreationDate="2016-11-15T11:36:00.157" Score="-2" Body="&lt;p&gt;I am not sure that the  bitangential transfer between two coplanar orbits is exist, we have to check that the focus of the transfer orbit is the same as center of attraction.&lt;/p&gt;&#xA;" OwnerUserId="17563" LastActivityDate="2016-11-15T11:36:00.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19029" PostTypeId="2" ParentId="13632" CreationDate="2016-11-15T11:40:36.463" Score="-2" Body="&lt;p&gt;there is some direct and indirect method to solve the problem, for direct solution, we need to achieve the first derivative of delta V versus of the independent parameters. For the indirect method, we need a numerical solution such as GA and PSO. There is some paper which have already solved some numerical example. However, I am publishing a paper in which I solved lots of different cases to find an appropriate fitting function.&lt;/p&gt;&#xA;" OwnerUserId="17563" LastActivityDate="2016-11-15T11:40:36.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19030" PostTypeId="2" ParentId="18960" CreationDate="2016-11-15T14:43:10.697" Score="1" Body="&lt;p&gt;Could astronauts maintain muscle mass by playing a game in a viscous fluid - probably but it would depend on the game.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The loss of muscle mass is due to the muscles not being used as frequently and/or not being used for as high a mechanical load as they were on Earth. In space no one can hear you scream, but more importantly there's almost no perceived gravity either - everything is (for the purposes of this answer at least) weightless. Big and massive things are actually harder to get moving than less massive things due to inertia but once they are moving the only thing you need to worry about is air resistance (assuming they are not in contact with another surface). On Earth this isn't the case because whatever you do, a dense object will quickly become in contact with another surface due to gravity - so you constantly have to hold off the acceleration due to gravity &lt;strong&gt;and&lt;/strong&gt; move the object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the viscous fluid would increase the 'air resistance' (no longer air of course) significantly and require more force to move and keep things moving. This would not necessarily work every muscle in the body though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I swim a lot, and you hear people say swimming works every muscle in the body (Apollo Creed even implies it to Rocky in Rocky III). It doesn't, there are many muscles that you don't naturally use when swimming, so there might need to be a stricter training schedule than simply playing a game. That being said some games would certainly be better than others. Which leads me to my final thought on the type of game;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When trying to maintain muscle mass by playing in a viscous fluid in micro gravity it's better to play something like tag than something like GTA V [citation needed].&lt;/p&gt;&#xA;" OwnerUserId="17507" LastActivityDate="2016-11-15T14:43:10.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19031" PostTypeId="1" CreationDate="2016-11-15T15:51:51.143" Score="3" ViewCount="252" Body="&lt;p&gt;It is often suggested that gravity should be simulated by rotating the entire habitat module of astronauts in microgravity. But wouldn't &lt;strong&gt;pole dancing&lt;/strong&gt; be enough? The individual astronaut swinging around a pole stick, like a daring strip dancer/acrobat holding on to it with her hands while throwing herself around it with her body at 90 degree angle to it. An astronaut could maybe swing like this in a sleeping bag, or while doing some exercise. With minimal design requirements for the spacecraft. Wouldn't this be enough for the bodily fluids to learn about in what ends the head and the bottom are?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've got an example of a Skylab pole-artistronaut &lt;a href=&quot;https://youtu.be/S_p7LiyOUx0?t=57&quot; rel=&quot;nofollow noreferrer&quot;&gt;right here&lt;/a&gt;:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/kJEtf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kJEtf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2022-11-29T03:32:02.360" Title="Would pole dancing cure microgravity health issues?" Tags="|microgravity|health|artificial-gravity|skylab|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19033" PostTypeId="1" AcceptedAnswerId="38332" CreationDate="2016-11-16T06:10:44.740" Score="14" ViewCount="2410" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/HAL/S&quot; rel=&quot;noreferrer&quot;&gt;HAL/S programming language&lt;/a&gt; is used in the general purpose computers in the space shuttle. I have written a simple HAL/S program of my own. Is there any compiler available to turn it into executable form?&lt;/p&gt;&#xA;" OwnerUserId="17539" LastEditorUserId="415" LastEditDate="2016-11-16T07:10:59.560" LastActivityDate="2023-07-06T01:35:55.600" Title="Is there any compiler available for HAL/S?" Tags="|space-shuttle|software|" AnswerCount="2" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="19034" PostTypeId="2" ParentId="457" CreationDate="2016-11-16T09:28:44.577" Score="4" Body="&lt;h1&gt;The ideal launch site&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The previous answers point out that there are many factors that go into choosing a suitable launch site. However, assuming a strategic requirement for long term space access, the predominant factors are good clearance devoid of populated areas east of the launch site and as close to the equator as possible.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Why launch eastward&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The earth's rotation can assist a rocket launch by boosting its payload capacity. The surface of the earth has a tangential velocity (parallel to the surface of the earth) which is towards the east. This is the prime reason why companies such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Sea_Launch&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sea Launch&lt;/a&gt; developed a sea platform that they pull out to the equator to get the maximum speed boost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a mathematical demonstration as an answer to a similar question on &lt;a href=&quot;https://space.stackexchange.com/questions/998/how-much-of-an-advantage-do-equator-proximal-launch-sites-provide/15275#15275?newreg=cb8dedbd8fdb43e4827ff734c412b614&quot;&gt;this thread&lt;/a&gt; using Tsiolkovsky Rocket Equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_f=v_eln(\frac{m_i}{m_f})$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which allows you to calculate the initial mass of your rocket $m_i$&#xA;as a function of your final mass $m_f$ (empty rocket + payload), the final orbital velocity $v_f$ and the exhaust velocity of the rocket $v_e$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping in mind that this equation doesn't take into consideration the gravitational force acting on the rocket, we can still get an understanding of how the rotational speed of the earth plays a role in the economics of rocket launches.  $v_e$ and the final mass $m_f$ are constants for a given rocket + payload combination. The final orbital velocity $v_f$ actually changes depending on the position of the launch pad relative to the equator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tangential speed of the Earth at the equator is its circumference ($2\pi r$, $r$ =6000 odd kilometres) divided by the time it takes to go round, a little less than 24 hours, giving a total speed of 465 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So plugging in all the numbers into the equation allows one to calculate the initial mass $m_i$ of our rocket if launched from the equator or from a position with a lower tangential speed. The closer to the equator, the lower $m_i$ to produce the same total $v_f$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 2 alternative ways to calculate this, by keeping $m_i$ constant increasing $m_f$ allowing for a larger payload for equatorial launches, or to keep both $m_f$ and $m_i$ constant and calculate a larger $v_f$ allowing for higher orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original thread actually calculates these for the Soyuz rocket launched at different latitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equatorial launch does make a difference to the payload a rocket can carry because of the exponential nature of the equation.  If we compare LEO orbits (regardless of their orientation be it equatorial or polar or in between), gains are quite small, on the order of hundreds of kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if we compare GTO orbits, where the majority of commercial payloads are sent, then the gains can go up by &lt;a href=&quot;https://en.wikipedia.org/wiki/Sea_Launch#Advantages_of_equatorial_ocean-platform_based_launches&quot; rel=&quot;nofollow noreferrer&quot;&gt;as much as 25%&lt;/a&gt;.  The main reason for this is the fact that an orbital plane change needs to take effect, which is the reason why Sea Launch developed their sea-platform launch pad as the economic savings were worth it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, a &lt;a href=&quot;https://www.hq.nasa.gov/pao/History/SP-4204/ch1-2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA study dating back to 1959 made a detailed calculation of the cost reduction of launching a Saturn rocket from the equator as compared to Cape Canaveral&lt;/a&gt; and concluded that equatorial LEOs are 80% less expensive propellant-wise due to required orbital plane changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more detailed derivation of the rocket equation with gravity force acting on the rocket can be viewed online on the &lt;a href=&quot;http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node103.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;MIT website&lt;/a&gt;.  Note also that staging rockets reduces the actual payload capacity. If we say that a 2 stage rocket is 80% fuel, 10% rocket and 10% payload, the first stage's payload is the 2nd stage, therefore the actual payload is 10% of the 2nd stage, or 10% of 10% of the initial rocket, ie 1%.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Why did Russia launch so many satellites from Plesetsk&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Plesetsk is not the ideal launching site, however, the Russians needed to develop alternate orbital solutions to meet their needs.  Accessing &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;GSO orbits&lt;/a&gt; for communication/weather/spy satellites was too expensive, and they therefore developed an alternative solution which instead leveraged the disadvantages of the Plesetsk site location into an opportunity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;High latitude launch sites are better suited for accessing &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;polar orbits&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Highly_elliptical_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;highly elliptical orbits&lt;/a&gt; (or HEO orbits).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russia's need for communication and weather satellites were met by perfecting the HEO orbit known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Molniya orbit&lt;/a&gt; (Lightning in Russian).  Molniya orbits are variable speed orbits, slower on the apogee (point furthest away from the Earth) and faster at the perigee (closest approach to Earth).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians perfected this orbit to the point that the satellite would orbit twice the earth in a day, with an apogee centred on Russian and another one centred on the USA.  This led to dual function satellites, spying on the USA 9 hours of the day, then zipping round the bottom of the Earth and serving as communication/broadcasting/weather satellites for the Soviet Union for another 9 hours of the day (see this &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit#/media/File:Molniya.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;illustration&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The downside of this orbit is that it requires several satellites to provide a continuous cover for the entire surface of Russia.  The satellites are also exposed to the Van Allan radiation belts 4 times a day, resulting in shorter life-times.  The Russians upgraded the satellites regularly, refining the orbits as well as the satellite technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plesetsk is actually ideally situated to launch satellites into Molniya orbits, and as a result saw many more launches than Baikonour.&lt;/p&gt;&#xA;" OwnerUserId="17534" LastEditorUserId="6816" LastEditDate="2018-07-21T23:45:56.163" LastActivityDate="2018-07-21T23:45:56.163" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19035" PostTypeId="1" AcceptedAnswerId="19037" CreationDate="2016-11-16T11:07:24.147" Score="4" ViewCount="664" Body="&lt;p&gt;I am wondering if somebody could help me figure out how to convert orbital state vectors from one origin to another?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say I am running a simulation of the Earth, Moon and Sun system. When calculating the trajectories of the masses under consideration, the center of the coordinate system is the barycenter of the solar system, but for the purpose of animating this simulation, I would like to be able to convert the position vectors that I calculate every iteration into vectors that have the Earth-Moon barycenter as the origin of the coordinate system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would I go about doing this?&lt;/p&gt;&#xA;" OwnerUserId="17423" LastActivityDate="2016-11-16T12:36:03.877" Title="Converting orbital state vectors from one origin to another" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19036" PostTypeId="1" CreationDate="2016-11-16T11:22:47.990" Score="6" ViewCount="294" Body="&lt;p&gt;ESA, the European Space Agency, is (formally) independent of other international cooperations like the European Union. After the fall of the Soviet Union, were there ever any proposals for ESA to join with the Russian space agency? ESA does accept new smaller member states now and then, including the former Soviet republic of Estonia in 2015. In the 1990s Roscosmos had big financial problems but also world leading space competence and assets. Among them the flight ready Energia/Buran launch system. There seems to have been potentially great synergies with such a merger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, Ukraine and Kazakhstan had important space industries and assets, so I'd like to include them in this question.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2018-08-12T17:04:33.793" Title="Was it ever suggested to merge ESA with Roscosmos?" Tags="|history|esa|roscosmos|international-politics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19037" PostTypeId="2" ParentId="19035" CreationDate="2016-11-16T11:33:37.857" Score="3" Body="&lt;p&gt;If you are using &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL's Horizons&lt;/a&gt;, then choose your origin as &quot;@3&quot; which is the Earth-Moon barycenter. Like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Fi8QR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Fi8QR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;..and you'll get numbers for X and Y that are +/- 4,000 km roughly, which is the Earth's orbit around the Earth-Moon barycenter. Z will move a few hundred km because the Moon's orbit is so far out of the ecliptic that it pulls the Earth up and down a little.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Original answer:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Barycenter&quot; rel=&quot;nofollow noreferrer&quot;&gt;barycenter&lt;/a&gt; of the Earth - Moon system would be the weighted average of the two positions which is just the center of mass. If the masses of the Earth and Moon are $m_E$ and $m_M$, and their position vectors are $\mathbf{x}_E$ and $\mathbf{x}_M$, then the location of the barycenter is just:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{x}_{CM} \ = \ \mathbf{x}_E\frac{m_E}{m_E+m_M} + \mathbf{x}_M\frac{m_M}{m_E+m_M}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So get the Earth's and Moon's position in the E-M barycenter coordinates just subtract, :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{x}_{E,CM} \ = \ \mathbf{x}_E \ - \mathbf{x}_{CM}$$&#xA;$$\mathbf{x}_{M,CM} \ = \ \mathbf{x}_M \ - \mathbf{x}_{CM}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The earth will move around the E-M barycenter which will be inside the Earth, roughly 4,000 km from the center. The algebra can be simplified a bit but I'll leave it like this to make it clear what's going on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just for fun, here's a series of images of Pluto and it's moon Charon. In this case the barycenter is outside of pluto, in between the two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/et0qI.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/et0qI.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Barycentric view of the Pluto–Charon system as seen by New Horizons. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Barycentric_view_of_Pluto_and_Charon_29_May-3_June_by_Ralph_in_near-true_colours.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is how I do it using &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt; in Python (for the year 2016). With Horizons you have to do some housekeeping after downloading text file outputs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OOD6x.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OOD6x.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from skyfield.api import load&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;ephem = load('de421.bsp')&#xA;&#xA;earth = ephem['earth']&#xA;moon = ephem['moon']&#xA;sun = ephem['sun']&#xA;&#xA;ts = load.timescale()&#xA;&#xA;days = np.arange(365.)&#xA;&#xA;time = ts.utc(2016, 1, 1+days)&#xA;&#xA;epos   = earth.at(time).position.km&#xA;mpos   = moon.at(time).position.km&#xA;spos   = sun.at(time).position.km&#xA;&#xA;me, mm = 5.972E+24, 7.342E+22&#xA;&#xA;cmpos = (epos*me + mpos*mm) / (me + mm)&#xA;&#xA;epos_cm = epos - cmpos&#xA;mpos_cm = mpos - cmpos&#xA;&#xA;names = ['Earth', 'Moon', 'Moon-Earth', 'Sun', 'Earth_cm', 'Moon_cm']&#xA;posns = [epos, mpos, mpos-epos, spos, epos_cm, mpos_cm]&#xA;&#xA;plt.figure()&#xA;for i, (name, pos) in enumerate(zip(names, posns)):&#xA;    plt.subplot(len(posns), 1, i+1)&#xA;    for thing in pos:&#xA;        plt.plot(days, thing)&#xA;    plt.title(name)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-11-16T12:36:03.877" LastActivityDate="2016-11-16T12:36:03.877" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19038" PostTypeId="2" ParentId="19035" CreationDate="2016-11-16T12:14:20.250" Score="3" Body="&lt;p&gt;Let's say that your simulation produces the position vectors $\mathbf{r}_{bs}$, $\mathbf{r}_{be}$, and $\mathbf{r}_{bm}$  of the Sun ($s$), the Earth ($e$), and the Moon ($m$) relative to the barycenter of the solar system ($b$).  It also produces the velocity vectors $\mathbf{v}_{bs}=\mathbf{\dot{r}}_{bs}$, $\mathbf{v}_{be}=\mathbf{\dot{r}}_{be}$, and $\mathbf{v}_{bm}=\mathbf{\dot{r}}_{bm}$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I understand your question correctly, you want to determine the position vectors $\mathbf{r}_{cs}$, $\mathbf{r}_{ce}$, and $\mathbf{r}_{cm}$ and velocity vectors $\mathbf{v}_{cs}$, $\mathbf{v}_{ce}$, and $\mathbf{v}_{cm}$ relative to the barycenter of the Earth-Moon system ($c$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note&lt;/strong&gt;: I am using the subscript notation $\mathbf{r}_{be}$ to indicate the displacement vector &lt;em&gt;from&lt;/em&gt; $b$ &lt;em&gt;to&lt;/em&gt; $e$, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, you need to calculate $\mathbf{r}_{bc}$, the position vector from the solar system barycenter to the Earth-Moon barycenter:&#xA;$$&#xA;\mathbf{r}_{bc} = \frac{m_e\mathbf{r}_{be}+m_m\mathbf{r}_{bm}}{m_e+m_m}&#xA;$$&#xA;where $m_e$ is the mass of the Earth, and $m_m$ is the mass of the Moon.&#xA;Then you can calculate the position vectors $\mathbf{r}_{cs}$, $\mathbf{r}_{ce}$, and $\mathbf{r}_{cm}$  of the Sun ($s$), the Earth ($e$), and the Moon ($m$) relative to the barycenter of the Earth-Moon system ($c$) using&#xA;\begin{eqnarray*}&#xA;\mathbf{r}_{cs} &amp;amp;=&amp;amp; \mathbf{r}_{bs} - \mathbf{r}_{bc} \\&#xA;\mathbf{r}_{ce} &amp;amp;=&amp;amp; \mathbf{r}_{be} - \mathbf{r}_{bc} \\&#xA;\mathbf{r}_{cm} &amp;amp;=&amp;amp; \mathbf{r}_{bm} - \mathbf{r}_{bc}&#xA;\end{eqnarray*} &#xA;The velocity vectors are &#xA;$$&#xA;\mathbf{v}_{cs} = \mathbf{\dot{r}}_{cs} &#xA;= \mathbf{\dot{r}}_{bs} - \mathbf{\dot{r}}_{bc}&#xA;= \mathbf{v}_{bs} - \mathbf{v}_{bc}&#xA;$$&#xA;and so on, where&#xA;$$&#xA;\mathbf{v}_{bc} =&#xA;\mathbf{\dot{r}}_{bc} &#xA;= \frac{m_e\mathbf{\dot{r}}_{be}+m_m\mathbf{\dot{r}}_{bm}}{m_e+m_m}&#xA;= \frac{m_e\mathbf{v}_{be}+m_m\mathbf{v}_{bm}}{m_e+m_m}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that I also assumed that the reference frames fixed to the solar system barycenter and the Earth-Moon barycenter are not rotating relative to each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this helps!&lt;/p&gt;&#xA;" OwnerUserId="8321" LastActivityDate="2016-11-16T12:14:20.250" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19040" PostTypeId="1" AcceptedAnswerId="19059" CreationDate="2016-11-16T15:59:23.200" Score="14" ViewCount="3354" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Katyusha_rocket_launcher&quot; rel=&quot;noreferrer&quot;&gt;The Katyushas&lt;/a&gt; were invented by the Soviets and used in WW2. They were trucks with rails firing a bunch of small rockets with a range of &lt;a href=&quot;https://en.wikipedia.org/wiki/Katyusha_rocket_launcher#Rocket_variants&quot; rel=&quot;noreferrer&quot;&gt;3 to 12 km&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qda6J.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qda6J.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These were solid-fuel rockets, but I would like to know &lt;strong&gt;what exact rocket fuel&lt;/strong&gt; they used, because it relates to a few of my other questions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://hsm.stackexchange.com/questions/2654/why-was-the-v-2-not-a-solid-rocket-and-or-what-is-the-scientific-history-of-sol&quot;&gt;Why was the V2 not a solid rocket?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/10842/what-was-the-first-large-rocket-to-use-apcp-solid-fuel&quot;&gt;What was the first rocket to use solid APCP fuel?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Forgot to post my research. So far the closest thing I found was &lt;a href=&quot;http://www.russianspaceweb.com/rt1.html&quot; rel=&quot;noreferrer&quot;&gt;this page&lt;/a&gt; from Russianspaceweb.com on the RT-1, apparently Russia's first large solid rocket. It says &quot;These engines burned essentially same powder fuel which had propelled the famous 1930s-vintage Katyusha rockets. (177)&quot; and down in the chart it says &quot;Propellant Solid --- RSD-4K&quot;. I tried googling that but got nothing.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-11-20T19:50:11.867" Title="What solid fuel was used in the Katyushas rocket artillery of WW2?" Tags="|rockets|history|solid-fuel|weapon|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19042" PostTypeId="1" AcceptedAnswerId="19044" CreationDate="2016-11-17T02:29:04.810" Score="8" ViewCount="1133" Body="&lt;p&gt;The Discover Magazine article &lt;a href=&quot;http://blogs.discovermagazine.com/outthere/2015/07/10/how-new-horizons-survived-the-40-year-glitch-and-made-it-to-pluto&quot; rel=&quot;noreferrer&quot;&gt;How New Horizons Survived the 40-Year-Glitch and Made it to Pluto&lt;/a&gt; is a little confusing. It wraps NASA history together with several different space missions to get the &quot;40-year&quot; value, since New Horizons was launched only about eleven years ago.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The engineers have become so good at fixing problems that most of the time the public has no idea what they are up against—until something goes wrong, as happened to New Horizons last weekend, when a software glitch caused the probe to shut down into “safe” mode. For a moment, this was a news story. Then, once again, the engineers stepped up and &lt;strong&gt;&lt;a href=&quot;http://www.nasa.gov/nh/new-horizons-plans-july-7-return-to-normal-science-operations&quot; rel=&quot;noreferrer&quot;&gt;solved the problem&lt;/a&gt;&lt;/strong&gt; (caused by an obscure timing flaw in a command sequence sent to the probe in preparation for flyby). Within three days, all was back to normal.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the linked NASA News item &lt;a href=&quot;http://www.nasa.gov/nh/new-horizons-plans-july-7-return-to-normal-science-operations&quot; rel=&quot;noreferrer&quot;&gt;NASA’s New Horizons Plans July 7 Return to Normal Science Operations&lt;/a&gt;, the issue is also mentioned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The investigation into the anomaly that caused New Horizons to enter “safe mode” on July 4 has concluded that no hardware or software fault occurred on the spacecraft. The underlying cause of the incident was a hard-to-detect timing flaw in the spacecraft command sequence that occurred during an operation to prepare for the close flyby. No similar operations are planned for the remainder of the Pluto encounter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Q:&lt;/strong&gt; Is the glitch described in any more detail anywhere? Was it purely a software/computing timing issue or did the timing involve comms or something mechanical as well?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-03T04:14:32.480" LastActivityDate="2017-10-03T04:14:32.480" Title="What caused New Horizon's so-called &quot;40-year glitch&quot;?" Tags="|communication|new-horizons|software|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19043" PostTypeId="1" AcceptedAnswerId="26863" CreationDate="2016-11-17T02:44:45.103" Score="1" ViewCount="291" Body="&lt;p&gt;When reading the Discover Magazine article &lt;a href=&quot;http://blogs.discovermagazine.com/outthere/2015/07/10/how-new-horizons-survived-the-40-year-glitch-and-made-it-to-pluto&quot; rel=&quot;nofollow noreferrer&quot;&gt;How New Horizons Survived the 40-Year-Glitch and Made it to Pluto&lt;/a&gt; while writing &lt;a href=&quot;https://space.stackexchange.com/q/19042/12102&quot;&gt;this question&lt;/a&gt; I saw &quot;...the new NASA administrator tried to kill New Horizons in favor of a high-tech fission-powered mission.&quot; (see below for the context):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What was the high-tech fission-powered mission? Does it have a name - and is it described somewhere?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The missions had such hopeful names: Pluto 350, Pluto Fast Flyby, Pluto Express. Finally, Stern came up with the winning concept, New Horizons. His final proposal was due one week after 9/11, while the offices of the Johns Hopkins University Applied Physics Lab (which operates the mission) were still shut. Stern completed the proposal and NASA approved it.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The following year, the new NASA administrator tried to kill New Horizons in favor of a high-tech fission-powered mission. He failed, and New Horizons survived. One benefit of its long, difficult competition is that Stern and the mission engineers came up with a relatively inexpensive, flyweight spacecraft. New Horizons weighs about half as much as Voyager 1, and in real dollars costs roughly one-third as much. Placing a lightweight probe atop a large Atlas V rocket yielded the fastest object ever launched when New Horizons took off in 2006... &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2018-04-24T00:39:16.013" Title="What &quot;high-tech fission-powered mission&quot; almost &quot;killed&quot; New Horizons?" Tags="|new-horizons|nuclear|electric-propulsion|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19044" PostTypeId="2" ParentId="19042" CreationDate="2016-11-17T02:56:38.853" Score="10" Body="&lt;p&gt;I found a bit more detail by googling for &quot;New Horizons Anomaly Review Board Report&quot;.   The best writeup was from &lt;a href=&quot;https://hub.jhu.edu/magazine/2015/fall/new-horizons-pluto/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;By 4 p.m., the mission's Anomaly Review Board had convened to be&#xA;  briefed on what had transpired and to discuss the best way forward.&#xA;  Midway through the spacecraft's recovery, they determined there was no&#xA;  fault in the hardware or software. But there had been a conflict when&#xA;  the spacecraft tried to commit to flash memory the complete command&#xA;  sequence for the nine days of the flyby, which it had just received&#xA;  from Earth, while at the same time compressing science data that had&#xA;  been gathered by its instruments. All that simultaneous activity had&#xA;  triggered an overload of the main computer, prompting the autonomy&#xA;  software to switch to the backup processor, point New Horizons toward&#xA;  Earth, and put the craft into safe mode.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It sounds like the ground sent up a lengthy command sequence while the system already had a heavy work load compressing science data, and the main computer crashed due to an overload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the so-called &quot;timing flaw&quot; sounds like it was, basically, just a bad choice of the time at which to send that command sequence.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-11-17T03:08:49.790" LastActivityDate="2016-11-17T03:08:49.790" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19047" PostTypeId="2" ParentId="18654" CreationDate="2016-11-17T12:54:55.143" Score="2" Body="&lt;p&gt;Pressure and temperature on the surface of Venus is so high that a manned landing is impossible. If this airship is manned, the only way to return is getting back into the orbit of the space ship, but this extremly difficult and requires a huge airship including a rocket for return into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this airship should be unmanned, you need a data transmission link back to earth. For this link some satellites in an orbit around Venus may be used. If there are enough satellites for a continous data link, they may be used for navigation too. Radar was used from satellites, therefore it may be used from an airship too. But if the airship gets down to a height of 50 to 20 km above the surface without a clear optical view to the ground, what new important scientific results we may expect?&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-11-17T12:54:55.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19048" PostTypeId="1" CreationDate="2016-11-17T16:10:43.910" Score="2" ViewCount="177" Body="&lt;p&gt;The only poster imaging all astronauts at that time I am aware is the one&#xA;created by NASA in 1967. Are there any follow ups with meanwhile 550 space travellers?&lt;/p&gt;&#xA;" OwnerUserId="13909" LastActivityDate="2016-11-17T16:31:42.343" Title="are there any posters available with all astronauts/cosmonauts?" Tags="|crewed-spaceflight|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19049" PostTypeId="2" ParentId="19048" CreationDate="2016-11-17T16:31:42.343" Score="2" Body="&lt;p&gt;I know of no posters that show all the world's astronauts.  The NASA Space Flight Awareness &lt;a href=&quot;https://www.nasa.gov/directorates/heo/sfa/sfa-posters&quot; rel=&quot;nofollow noreferrer&quot;&gt;page&lt;/a&gt; has some down-loadable posters going back to the  days of a crowded Shuttle Astronaut Office before the Columbia accident, and as recent as 2015.  As well as the occasionally amusing ISS Expedition crew posters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/x3bqv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/x3bqv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-11-17T16:31:42.343" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19051" PostTypeId="1" AcceptedAnswerId="19061" CreationDate="2016-11-17T21:09:20.990" Score="2" ViewCount="1081" Body="&lt;p&gt;I saw a space jump where someone had exceeded the speed of sound before the air slowed him down enough to deploy a parachute, because the air was not dense enough to tear him apart in the upper atmosphere.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Mars: Can a blimp also enter the atmosphere at an angle with a skip entry and will the gradual increase of air pressure be enough to lessen the need for retrorockets or heat shield?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be inflated outside the atmosphere and inflating during entry with a stiff under body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If an entry is only possible at slower speeds for this, could a blimp skip all the way around the planet like a skipping stone to water or skis to water to slow down if the surface on the bottom is large enough and the blimp light enough? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mWSXJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mWSXJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;other related questions: &lt;a href=&quot;https://physics.stackexchange.com/questions/293671/can-a-blimp-skip-on-the-atmosphere&quot;&gt;https://physics.stackexchange.com/questions/293671/can-a-blimp-skip-on-the-atmosphere&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/19559/could-a-huge-blimp-be-lowered-from-orbit-into-the-atmosphere-of-mars&quot;&gt;Blimp on Earth, but a glider on Mars?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2018-12-14T00:08:15.987" LastActivityDate="2019-08-10T22:18:53.337" Title="Can a blimp dead drop on Mars or Earth this way?" Tags="|mars|heat-shield|payload-deployment|entry-descent-landing|" AnswerCount="5" CommentCount="10" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19052" PostTypeId="2" ParentId="19051" CreationDate="2016-11-17T21:30:01.273" Score="4" Body="&lt;p&gt;Yes, at least in theory, but in practice there is very little atmosphere on Mars, so your blimp will have to be huge, or the payload very small. &lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2016-11-17T21:30:01.273" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19053" PostTypeId="1" AcceptedAnswerId="19054" CreationDate="2016-11-17T21:35:26.583" Score="6" ViewCount="263" Body="&lt;p&gt;According to the Wikipedia page for the song &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Happy_Birthday_to_You&quot; rel=&quot;nofollow noreferrer&quot;&gt;Happy Birthday To You&lt;/a&gt;&quot;, the Mars rover Curiosity sang to itself on its birthday in 2013, using its SAM as a musical instrument.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The page for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sample_Analysis_at_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;SAM&lt;/a&gt; indicated it is a suite of tools for chemical analysis.  So how exactly did it produce notes?  Were there centrifuges spinning at a specific rate, were gasses allowed to &quot;squeak&quot; out?  Did they plan this ahead of time, did they test it's singing functionality before its trip to Mars?&lt;/p&gt;&#xA;" OwnerUserId="10948" LastActivityDate="2016-11-17T23:41:38.017" Title="How did Curiosity's SAM &quot;sing&quot; to itself?" Tags="|nasa|curiosity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19054" PostTypeId="2" ParentId="19053" CreationDate="2016-11-17T23:41:38.017" Score="8" Body="&lt;p&gt;This &lt;a href=&quot;https://youtu.be/uxVVgBAosqg&quot; title=&quot;NASA video&quot;&gt;NASA video&lt;/a&gt; about the birthday song answers most of your questions:  the SAM unit uses vibration to &quot;make the soil samples go down&quot;, and the frequencies used include audible ones. So by commanding the SAM to vibrate at the right frequencies a simple tune can be played. The Goddard Spaceflight Centre has an exact duplicate of the SAM on Mars which used to &quot;test our experiments before it is transmitted to Mars&quot; (which demonstrates the tune in the video), so it seems that the planning was done with the duplicate.&lt;/p&gt;&#xA;" OwnerUserId="14637" LastActivityDate="2016-11-17T23:41:38.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19055" PostTypeId="1" AcceptedAnswerId="19070" CreationDate="2016-11-17T23:50:26.173" Score="27" ViewCount="4670" Body="&lt;p&gt;The BBC article &lt;a href=&quot;http://www.bbc.com/news/technology-38012930&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX aims to launch internet from space&lt;/a&gt; discusses the SpaceX proposal for a dense array of over 4,000 LEO satellites for global internet coverage, and links to &lt;a href=&quot;http://cdn.geekwire.com/wp-content/uploads/2016/11/Technical-Attachment.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The initial configuration will be the first group of 1,600 active satellites (plus spares) at 32 orbital planes of 50 satellites each, all at an inclination of 53 degrees and a LEO altitude of 1,150 km. The final configuration adds to that four more sets of planes at four more inclinations and altitudes, for a grand total of 4,425 active satellites (plus spares).&lt;/p&gt;&#xA;&lt;p&gt;If I understand correctly, this configuration is supposed to provide global coverage such that any place on earth will see at least one satellite above 40 degrees elevation at all times.&lt;/p&gt;&#xA;&lt;p&gt;My question is - how does this constellation's configuration actually work? Do these inclinations and altitudes satisfy some expected behavior for &lt;a href=&quot;https://en.wikipedia.org/wiki/Nodal_precession&quot; rel=&quot;nofollow noreferrer&quot;&gt;nodal precession&lt;/a&gt;? Is there some ellegant orchestration here? There are five slightly different altitudes, which means each group will have a slightly different period, so I don't think the different groups are phased relative to each other, only within a group or at least within each plane of each group.&lt;/p&gt;&#xA;&lt;p&gt;Are there five different altitudes simply for collision avoidance - which is more difficult between planes of mixed inclinations than between planes of identical inclination (but different nodes)?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Q:&lt;/strong&gt; &lt;em&gt;How does this constellation's configuration actually work? - what is the method behind the madness? (idomatically speaking)&lt;/em&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Here are some simple plots. I'm working on better ones - hopefully an animation or a 2D map with groundtracks.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eKuGt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eKuGt.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pAQYG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pAQYG.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3Db8V.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3Db8V.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wo302.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wo302.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; screen captures from &lt;a href=&quot;http://cdn.geekwire.com/wp-content/uploads/2016/11/Technical-Attachment.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="46507" LastEditDate="2022-08-16T21:17:56.873" LastActivityDate="2022-08-16T21:18:45.843" Title="SpaceX's 4,425 satellite constellation - what's the method to the madness?" Tags="|spacex|orbital-mechanics|low-earth-orbit|communication-satellite|satellite-constellation|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="19056" PostTypeId="1" AcceptedAnswerId="19060" CreationDate="2016-11-18T00:00:26.453" Score="5" ViewCount="337" Body="&lt;p&gt;Ran out of fuel it too shallow answer.&#xA;There should be something else like uncontrolled spinning and making waste radio noise. But this could be good experiment how long probe would survive so close to sun.&lt;/p&gt;&#xA;" OwnerUserId="17578" LastActivityDate="2019-03-22T15:49:05.600" Title="Why Mariner 10 was instructed to turn off [transmitter]?" Tags="|probe|mariner|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19058" PostTypeId="2" ParentId="16851" CreationDate="2016-11-18T04:03:54.257" Score="3" Body="&lt;p&gt;A propellant deflagrates, rather than explodes.  The difference between them is that deflagration relies on the thermal energy of the flame front, rather than the energy of the shockwave caused by detonation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key to this process is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Deflagration#Flame_physics&quot; rel=&quot;nofollow noreferrer&quot;&gt;balance&lt;/a&gt; between two factors, the activation energy of the propellant and the temperature at which the propellant burns.  This balance is set against the thermal diffusion of the propellant.  If the heat cannot diffuse fast enough into the propellant, it won't stay lit.  If it diffuses too fast... well... you wondered what keeps them from going boom?&lt;/p&gt;&#xA;" OwnerUserId="3532" LastActivityDate="2016-11-18T04:03:54.257" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19059" PostTypeId="2" ParentId="19040" CreationDate="2016-11-18T05:08:15.133" Score="14" Body="&lt;p&gt;From &lt;a href=&quot;https://ru.wikipedia.org/wiki/%D0%A0%D0%A1-82&quot; rel=&quot;noreferrer&quot;&gt;Russian wiki on RS-82&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RS-82 and RS-132 are unguided air-to-air, air-to-ground rockets developed during 1929-1937. Further developed into М-8, М-13 &quot;Katyusha&quot; rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4VIzE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4VIzE.jpg&quot; alt=&quot;М-8 rocket&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems the final propellant was a &lt;strong&gt;solid fuel N&lt;/strong&gt; developed by a group lead by A. Bakaev in mid 30s:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;nitrocellulose (collodion cotton) - 57%&lt;/li&gt;&#xA;&lt;li&gt;nitroglycerine - 28%&lt;/li&gt;&#xA;&lt;li&gt;dinitrotoluene - 11%&lt;/li&gt;&#xA;&lt;li&gt;centralite - 3%&lt;/li&gt;&#xA;&lt;li&gt;vaseline - 1%&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Production was organized in south of Ukraine.&lt;/p&gt;&#xA;" OwnerUserId="17580" LastEditorUserId="17580" LastEditDate="2016-11-18T05:25:27.387" LastActivityDate="2016-11-18T05:25:27.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19060" PostTypeId="2" ParentId="19056" CreationDate="2016-11-18T09:33:42.517" Score="9" Body="&lt;p&gt;When Mariner 10 ran out of fuel, it no longer had the ability to point its high-gain antenna at Earth. There was no benefit to leaving the radio on, and leaving it on meant there was a chance its transmissions would interfere with other missions, so it was switched off. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-18T09:33:42.517" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19061" PostTypeId="2" ParentId="19051" CreationDate="2016-11-18T13:24:32.687" Score="2" Body="&lt;p&gt;That should prove to be very difficult, but I think it depends how long the blimp would have the form of a blimp. Let me say why: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all:&lt;br&gt;&#xA;The jump you are referring to, was a PR-event so I'm not gonna mention names. However the jump was not a re-entry from space, it started in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stratosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stratosphere&lt;/a&gt;, some 25km above the ground, while low-earth-orbit (LEO) is at about 300-500 km.&lt;br&gt;&#xA;Reaching the speed of sound up there (or Mach 1) is easier, because the speed of sound changes with height, along with atmospheric temperature, as seen in this plot from wikipedia (follow the blue curve):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y3JHc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y3JHc.png&quot; alt=&quot;Various temperature profiles&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the alleged record is actually a cheat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But now to your blimp:&lt;br&gt;&#xA;Your blimp will come in at way higher speeds than Mach 1. The temperature on Mars is roughly a factor of &lt;span class=&quot;math-container&quot;&gt;$\sim 2$&lt;/span&gt; lower than on Earth (measured in Kelvin), so the speed of sound there is also a factor of 2 lower. But then an object coming in with the same velocity as on Earth will have Mach 2 instead of Mach 1. Or 60 instead of 30.&lt;br&gt;&#xA;This is important, because objects coming even from LEO on Earth reach Mach numbers of 30 (like the ex-space shuttle) and then have trouble getting rid of the heat.&lt;br&gt;&#xA;And the higher the Mach number and the bigger the surface area of your object is, the more heat you'll have produced during re-entry. So you will want to make your blimp very compact for Mars atmospheric entry, and inflate it later. Then you need boosters to slow down, as well as tanks for the blimp becoming a blimp. I have no engineering perspective on this, how feasible that would be.&lt;br&gt;&#xA;Additionally you'd have to be a VERY big blimp, because buoyancy forces depend linearly on the atmospheric density surrounding the blimp, which is ~100 times lower than on Earth.&lt;br&gt;&#xA;Thus, your blimp would need to have a &lt;span class=&quot;math-container&quot;&gt;$\sim 4.6$&lt;/span&gt; times bigger on all axes to reach the same stability as on Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to summarize:&lt;br&gt;&#xA;The blimp will not enter the atmosphere of Mars with Mach 1, if coming from space. You need to slow it down beforehands, but for it to survive this, the blimp should be packed. And then you might want to think if a blimp on Mars makes sense at all.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastEditorUserId="18879" LastEditDate="2018-09-22T16:43:26.190" LastActivityDate="2018-09-22T16:43:26.190" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="19063" PostTypeId="2" ParentId="19051" CreationDate="2016-11-18T14:56:24.623" Score="10" Body="&lt;p&gt;NASA is working on an &lt;a href=&quot;https://www.nasa.gov/centers/ames/orgs/exploration-tech/entry-systems-div/hiad.html&quot; rel=&quot;noreferrer&quot;&gt;inflatable heat shield&lt;/a&gt;. Not quite a blimp, but a heat shield that's inflatable to a size much larger than the probe it's attached to. The extra area helps decelerate the spacecraft. So inflatable structures can be made to withstand the heat of reentry.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because Mars' atmospheric density is only 0.6% of Earth's, a blimp's lift will only be 0.6% of its lift on Earth.&lt;br&gt;&#xA;The Hindenburg could carry &lt;a href=&quot;https://en.wikipedia.org/wiki/Hindenburg-class_airship&quot; rel=&quot;noreferrer&quot;&gt;90 passengers&lt;/a&gt; on Earth. It weighed about 200 t. It'd be able to carry about 1 passenger on Mars. A blimp would be lighter (Hindenburg was a rigid airship), but you can see you'd need a ridiculously large structure to ensure a soft landing. &#xA;To compare with curent landing methods: one passenger is well within the load capacity of Curiosity's sky crane.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-18T14:56:24.623" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19064" PostTypeId="1" AcceptedAnswerId="21556" CreationDate="2016-11-18T16:25:13.297" Score="3" ViewCount="785" Body="&lt;p&gt;GOES-R is scheduled to launch soon and put in geostationary orbit over North America for high bandwidth continuous monitoring of Earth, with only 120 minutes per year interruptions caused by stationkeeping. It will simultaneously monitor the sun with several instruments and of course it's solar panel. The &lt;a href=&quot;http://www.goes-r.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;GOES-R Website&lt;/a&gt; describes the satellite in detail throughout many pages and links, and two images are shown below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume that over the course of each day, the shaft that supports the solar pannels and sun-pointing insturments rotates 360 degrees relative to the body of the spacecraft. &lt;em&gt;How do all of the wires for power from the pannels and signal and control connections for the sun-pointing instruments not get twisted up and break after a few days of continuous rotation?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Watch &lt;a href=&quot;https://youtu.be/xmxzehJJBDY&quot; rel=&quot;nofollow noreferrer&quot;&gt;the video&lt;/a&gt; below to get a perspecive of the size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/xmxzehJJBDY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4gc8C.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4gc8C.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screenshot from &lt;a href=&quot;https://youtu.be/xmxzehJJBDY&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0cKXi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0cKXi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kXNSq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kXNSq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; annotated illustrations of GOES-R spacecraft and its nadir-pointing and sun-pointing instruments.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-15T18:53:19.660" LastActivityDate="2017-05-18T10:55:53.827" Title="How will GOES-R simultaneously point some instruments down at Earth and others sunward?" Tags="|artificial-satellite|spacecraft|solar-power|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19065" PostTypeId="1" CreationDate="2016-11-19T16:44:31.773" Score="1" ViewCount="66" Body="&lt;p&gt;With SpaceX recent announcement of a huge satellite network in low earth orbit, I was wondering if such a network would motivate a sustainable large-scale launch capacity, i.e. a increase of scale of production and usage of launchers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, one needs quite a lot of rockets to put 4k satellites into orbit, but for how long?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Is further miniaturization of these ku-band communication satellites possible, or do factors like frequency (antennas) or power supply limit this?&lt;/li&gt;&#xA;&lt;li&gt;Is there any reliable estimate about the number of launches needed to maintain the network?&lt;/li&gt;&#xA;&lt;li&gt;Is the high number of satellites a necessity (i.e. motivated by the geometry of the network) or might it be overcome by a later generation of satellites?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2016-11-19T16:44:31.773" Title="Does a low-orbit communication network drive a sustainable launch market?" Tags="|spacex|communication-satellite|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19070" PostTypeId="2" ParentId="19055" CreationDate="2016-11-20T01:05:38.967" Score="28" Body="&lt;p&gt;Nodal precession doesn't matter for a plane of satellites like this, they will rotate around in unison, so the coverage will remain the same.&lt;/p&gt;&#xA;&lt;p&gt;Okay. So, why the unusual dual inclination constellation? The inclination of a satellite band tends to let you know what latitude it will work best at. A 0 degree satellite works best at the equator, a 90 at the poles. 53 will work best for customers around 40-60 latitude, north or south.&lt;/p&gt;&#xA;&lt;p&gt;To have complete coverage, there are two areas that can slip. First of all is the poles, which won't receive any coverage at all. A smaller number of planes could cover the poles quite effectively.&lt;/p&gt;&#xA;&lt;p&gt;The other is a bit less obvious, but is the equator. The equator will have 32 bands of satellites over it, crossing twice per orbit. In essence then there will be coverage for 64 satellites at a time. Over the higher latitudes, the distance around the globe at that latitude is lower, and thus the density will be higher. A single constellation of equatorial satellites will cover this gap quite effectively.&lt;/p&gt;&#xA;&lt;p&gt;Looking at SpaceX's plans, they figure the 32 plane constellation is sufficient for the equatorial regions. The 3 inclinations are 70, 74, and 81, with 5, 8, and 6 planes respectively. My guess is the initial deployment is deemed to be good enough for most of the world's population, but they will eventually want to have global coverage. To do that with the fewest satellites possible, they decided to add in the additional 19 planes. It seems this is required because the FCC dictates that coverage must include Alaska to use that frequency band, which makes things particularly difficult.&lt;/p&gt;&#xA;&lt;p&gt;Thinking this through a bit more, it seems 9 lateral degrees is the limit of coverage. Given 53 for optimal US coverage, 81 covers the poles, there needs to be two sparse groups of satellites to cover the rest if the area. I believe the two were chosen to optimize the satellite count by a simulation of some kind.&lt;/p&gt;&#xA;&lt;p&gt;Note also that the higher the orbit, the more satellites are in the plane. I suspect this is to make up some of the margin that would otherwise have been lost.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="46507" LastEditDate="2022-08-16T21:18:45.843" LastActivityDate="2022-08-16T21:18:45.843" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="19071" PostTypeId="1" AcceptedAnswerId="19075" CreationDate="2016-11-20T04:11:38.450" Score="5" ViewCount="214" Body="&lt;p&gt;I've been reading a few things about NOAA's GOES-R satellite, and I noticed &lt;a href=&quot;http://www.goes-r.gov/mission/mission.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; on the GOES-R site: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...will be placed in the 89.5° checkout orbit where it will undergo an&#xA;  extended checkout and validation phase of approximately one year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What are they doing in that validation to take a full year? It seems like they could make sure it's working a lot faster than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, it looks like future launches for other GOES satellites won't take as long for that part of the process. I haven't yet found exact timelines for GOES-S, GOES-T, and GOES-U, but their test phase isn't nearly as long as GOES-R. Why not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tLfvG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tLfvG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2016-11-22T20:04:34.137" Title="Why will GOES-R take a whole year for validation?" Tags="|artificial-satellite|testing|geostationary|noaa|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19072" PostTypeId="1" AcceptedAnswerId="19073" CreationDate="2016-11-20T06:10:27.507" Score="4" ViewCount="521" Body="&lt;p&gt;The BBC article &lt;a href=&quot;http://www.bbc.com/news/technology-38012930&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX aims to launch internet from space&lt;/a&gt; discusses the SpaceX proposal for a dense array of 4,425 LEO satellites distributed among 83 orbital planes in &lt;em&gt;five inclination groups&lt;/em&gt; as described in the linked document &lt;a href=&quot;http://cdn.geekwire.com/wp-content/uploads/2016/11/Technical-Attachment.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The altitudes of the five groups are all between 1110 and 1325 kilometers. Within each alt/inc group, collisions can be avoided by adjusting the phasing between planes. While there is nodal precession, it will be essentially identical for the entire group as nicely pointed out in this &lt;a href=&quot;https://space.stackexchange.com/a/19070/12102&quot;&gt;excellent, well thought-out answer&lt;/a&gt;. However, each &lt;em&gt;inclination group&lt;/em&gt; will precess differently, so in order to avoid a nightmare of stationkeeping, the inclination groups are placed at slightly different altitudes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think the particular values of these five altitudes were selected randomly, but I don't see any compelling orbital mechanics arguments. So I'm thinking they were chosen to avoid stuff that's already up there in established orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'm wondering &lt;em&gt;for constellations of circular LEO satellites, are there allocations of available &quot;slots&quot; in altitudes?&lt;/em&gt; Assuming these were chosen for that reason, were they assigned, or allocated, or just dreamed up by SpaceX on-the-fly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3Db8V.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3Db8V.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screen captures from &lt;a href=&quot;http://cdn.geekwire.com/wp-content/uploads/2016/11/Technical-Attachment.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eKuGt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eKuGt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pAQYG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pAQYG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2018-07-19T23:56:00.310" Title="For constellations of circular LEO satellites, are there allocations of available &quot;slots&quot; in altitudes?" Tags="|spacex|orbital-mechanics|low-earth-orbit|communication-satellite|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19073" PostTypeId="2" ParentId="19072" CreationDate="2016-11-20T12:33:17.467" Score="7" Body="&lt;p&gt;There aren't really, because it's not practical. Any satellite in a given orbital altitude could potentially impact another satellite in the same orbital altitude. If there were very very tight constraints, one could in theory do such orbital planning, but it would be rather difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe these orbits were created by SpaceX to optimally allocate their satellites, given all of the constraints. I suspect there is a desire to launch out of Vandenburg, which is constrained to 51-145 inclination, which would match very well with the 53 inclination for most of the satellites. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, for any satellite launched, the launching country approves it. There has to be some justification for the orbits, but in general they are more worried about RF than orbital collisions, at least for now.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-11-20T12:33:17.467" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19075" PostTypeId="2" ParentId="19071" CreationDate="2016-11-20T15:18:58.843" Score="6" Body="&lt;p&gt;There are two questions in this question. I'll answer the second question first because (a) this second question is easier to answer, and (b) the answer to this question leads to the answer to the key question. The second question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I haven't yet found exact timelines for GOES-S, GOES-T, and GOES-U, but their test phase isn't nearly as long as GOES-R. Why not?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;GOES-S, -T, and -U are intended to be exact duplicates of GOES-R. They will have the same sensors and the same operational processes as will GOES-R. Intents are one thing, reality is another. The sensors on the follow-on satellites will inherently differ slightly from those on GOES-R. The validation period for those follow-on satellites will address the slight differences between those satellites and the already validated GOES-R.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The validation period for those follow-on satellites will not address the complex ground processes that transform the raw data transmitted by the satellite to Level 1A, Level 1B, and Level 2 products. Those processes will already have been validated by the time those follow-on satellites are launched. That will take considerably less time than will the validation effort for GOES-R.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the key question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why will GOES-R take a whole year for validation?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is primarily because GOES-R essentially is a brand new instrument. The goal is to reach what's called &quot;provisional status&quot; within six months after launch. This means that all of the processes that transform the raw downlink to Level 1A products to Level 1B products to Level 2 products are functioning correcting, and that the underlying science in those transformations is basically correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several key challenges here:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Are the scientific instruments measuring what the scientists think they are measuring?&lt;/li&gt;&#xA;&lt;li&gt;Is the processing from raw data to Level 1A products correct?&lt;/li&gt;&#xA;&lt;li&gt;Is the processing from Level 1A to Level 1B products correct?&lt;/li&gt;&#xA;&lt;li&gt;Is the processing from Level 1B to Level 2 products correct?&lt;/li&gt;&#xA;&lt;li&gt;Can all of that required processing keep up with the much increased volume of data transmitted by the satellite?&lt;/li&gt;&#xA;&lt;li&gt;Do those Level 2 products agree with alternative observations of the same effect?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are a lot of ways things can go wrong in the above. The instruments themselves might have problems, the underlying science might have problems, and the complex software that performs the transformations from raw data to derived products might have problems. The system has to function properly, end-to-end, and the derived products have to agree with already validated (but perhaps localized) alternative measures of assessing those derived products.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal is to have the system at provisional status within six months after launch. That's not quite good enough. Having the system achieve operational status and have the products compare favorably year-round with validation data will take another six to nine months.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-11-20T15:18:58.843" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19076" PostTypeId="1" CreationDate="2016-11-20T19:45:30.263" Score="5" ViewCount="367" Body="&lt;p&gt;I am trying to get information on the maxon drive motors on the Curiosity Rover. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;know what model of DC brushless motors are used &lt;/li&gt;&#xA;&lt;li&gt;what kind of gear speed reducer is used. Is it a spur? &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Also any other helpful sources on the drive motor system/ power transmission systems would be so helpful. &lt;/p&gt;&#xA;" OwnerUserId="17594" LastEditorUserId="12102" LastEditDate="2020-01-14T19:12:18.837" LastActivityDate="2020-01-14T19:12:18.837" Title="Mars curiosity rover's Maxon drive motors" Tags="|mars|spacecraft|curiosity|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="19078" PostTypeId="1" AcceptedAnswerId="19086" CreationDate="2016-11-20T21:43:07.900" Score="14" ViewCount="5541" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mojf8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mojf8.png&quot; alt=&quot;SpaceX Falcon 9 Launch Profile&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;By boosting &lt;strong&gt;up&lt;/strong&gt; and back after separation aren't you just putting more energy in that you'll have to counter with additional deceleration later?&lt;/p&gt;&#xA;&lt;p&gt;Why not just accelerate back, which will use less energy and conserve fuel?&lt;/p&gt;&#xA;&lt;p&gt;Or am I missing something only a rocket scientist would know?&lt;/p&gt;&#xA;" OwnerUserId="17597" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-09-06T01:29:32.347" Title="Why does the SpaceX first stage booster boost up and back after separation not just back?" Tags="|spacex|landing|falcon-9|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19080" PostTypeId="2" ParentId="16851" CreationDate="2016-11-21T00:54:58.833" Score="4" Body="&lt;p&gt;I've asked &lt;a href=&quot;https://chemistry.stackexchange.com/questions/54282/how-is-combustion-speed-regulated-in-solid-propellants&quot;&gt;the same question&lt;/a&gt; on Chemistry.SE and got some more factors than mentioned in the answers here.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;flexible binding agent (rubber), that prevents forming cracks under pressure, is essential to maintaining stable, low deflagration rate vastly lower than in blasting charges of the same composition.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;modifying ratio of oxidizers (perchlorate, ammonium nitrate) regulates the speed; perchlorate burns faster.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;catalysts like carbon and metals can increase the deflagration rate.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://everyspec.com/MIL-STD/MIL-STD-0100-0299/MIL-STD-286C_8618/&quot; rel=&quot;nofollow noreferrer&quot;&gt;MIL-STD-286C.&lt;/a&gt; defines a method of determining linear burning rate of propellants.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So, essentially, first - applying a rubber-like binder is the critical part that reduces deflagration rate to &quot;propellant speeds&quot;. Then, through &quot;trial and error&quot; the rate can be fine-tuned - adapted to requirements of construction, geometry and purpose of specific SRB/missile - through modifying oxidizer composition and catalysts. The final part is the grain shape inside the rocket, which regulates how area of deflagration changes over time, regulating thrust of the rocket over time - and the nozzle area, limiting the pressure (and deflagration rate) inside the SRB.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:57:06.083" LastActivityDate="2016-11-21T00:54:58.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19083" PostTypeId="1" CreationDate="2016-11-21T05:42:12.540" Score="1" ViewCount="633" Body="&lt;p&gt;Time goes faster the weaker the gravitational field you are in as long as your velocity is constant. The Voyager probes have moved a long way from the sun and its planets so I presume time must be going slowly faster for them. Is this true and if so do we know how much faster time is going for them than Earth?&lt;/p&gt;&#xA;" OwnerUserId="17601" LastEditorUserId="13710" LastEditDate="2016-11-21T06:09:48.230" LastActivityDate="2016-11-21T07:08:07.087" Title="Do the Voyager probes experience time moving faster due to relativity?" Tags="|voyager|time|spacetime|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19084" PostTypeId="2" ParentId="19083" CreationDate="2016-11-21T07:08:07.087" Score="4" Body="&lt;p&gt;You can see &lt;a href=&quot;https://space.stackexchange.com/a/6008/265&quot;&gt;this answer&lt;/a&gt; for the formulae. We would expect that Voyager 1's local time is faster than Earth time by about one part in one hundred million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think that the Voyager oscillator is stable enough to measure that small of a difference. So we can't directly answer &quot;is this true&quot;. However it would be a mind-blowing discovery throwing our most basic understanding of physics into disarray if it &lt;em&gt;weren't&lt;/em&gt; true.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-11-21T07:08:07.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19085" PostTypeId="2" ParentId="19078" CreationDate="2016-11-21T08:25:02.430" Score="9" Body="&lt;p&gt;The graphic doesn't make it very clear, but at the moment of separation, the rocket &lt;strong&gt;still has vertical speed&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it &lt;em&gt;does&lt;/em&gt; only boost back, and not up, to cancel the horizontal speed. But the rocket will nonetheless go up until it cancels its vertical speed before it goes down.&lt;/p&gt;&#xA;" OwnerUserId="10274" LastActivityDate="2016-11-21T08:25:02.430" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19086" PostTypeId="2" ParentId="19078" CreationDate="2016-11-21T10:57:54.607" Score="11" Body="&lt;p&gt;This graphic was made for one of the first landing attempts, and it shows an unusual scenario: a landing on the droneship where the ship is positioned much closer to the coast than on later missions.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a normal droneship landing, the stage does two burns: reentry and landing. Its trajectory is roughly a parabola, and doesn't exhibit this extra peak.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For this attempt, they did 3 burns: boostback, reentry and landing. The boostback burn has to kill all forward velocity, then accelerate the rocket on a parabolic trajectory towards the landing site. They did this to test the boostback burn without having the stage land on Cape Canaveral (which was considered too risky for the early landing attempts). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the launch, the rocket is powered through the entire trajectory. At stage separation, the stage moves at 5000 km/h.&lt;br&gt;&#xA;For the return flight to the landing site, the trajectory is an unpowered ballistic trajectory. The boostback burn has to provide enough energy to 'throw' the stage all the way back to the landing site. There are 2 ways to do this:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;accelerate the stage to 5000 km/h. This takes a lot of energy.  &lt;/li&gt;&#xA;&lt;li&gt;accelerate the stage to a lower speed, and provide some energy by boosting it to a higher altitude. The width of a ballistic trajectory is determined by the speed of the rocket and its altitude. This takes less energy.   &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The graphic is also usable for landings back at Cape Canaveral.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-11-21T12:51:26.367" LastActivityDate="2016-11-21T12:51:26.367" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19087" PostTypeId="2" ParentId="18958" CreationDate="2016-11-21T11:38:57.997" Score="4" Body="&lt;p&gt;Arecibo can measure speeds with an &lt;a href=&quot;http://www.atmos-chem-phys.net/4/621/2004/acp-4-621-2004.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;accuracy on the order of 1%&lt;/a&gt; in scanning mode (i.e. just observing asteroids as they pass through the field of view). &lt;a href=&quot;https://www.nasa.gov/topics/universe/features/yarkosky-asteroid.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Distances can be measured to&lt;/a&gt; 10&lt;sup&gt;-8&lt;/sup&gt; (10 ppb), I suspect this is done in a different, more accurate mode. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Emily Lakdawalla &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2013/05230912-radar-tracking-2013fb3-position-uncertainty.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;discusses this in her blog&lt;/a&gt;, inlcuding this graph that shows the position accuracy of one asteroid using optical observations only, or using Goldstone radar observations, with radar being 50 times more accurate:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QdGQw.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QdGQw.gif&quot; alt=&quot;Radar is much more accurate than optical observations only&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/13533-huge-asteroid-2005-yu55-radar-tracking.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;This article&lt;/a&gt; quotes position accuracy of 10 m, and axial speed accuracy of 1 mm/s.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-21T11:38:57.997" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19088" PostTypeId="1" CreationDate="2016-11-21T13:47:52.167" Score="2" ViewCount="212" Body="&lt;p&gt;Antimatter is theoretically an excellent fuel, providing large thrust/mass ratio and very efficient energy transfer. Therefore, we wouldn't need to use a lot of it to achieve similar thrusts that chemical rockets do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without considering other factors such as being safe distances away from people and other compenents and the initial launch from Earth, if a craft is using antimatter as a fuel, could it shrink down to a size much smaller than modern rockets, given how its fuel has a lot less volume? &lt;/p&gt;&#xA;" OwnerUserId="17552" LastActivityDate="2016-12-21T17:26:09.457" Title="Antimatter fuel compartment" Tags="|advanced-propulsion|antimatter|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19089" PostTypeId="2" ParentId="18654" CreationDate="2016-11-21T14:07:37.127" Score="1" Body="&lt;p&gt;I think simply looking at stars might be good enough. &lt;a href=&quot;https://en.wikipedia.org/wiki/Celestial_navigation#Modern_celestial_navigation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on celestial navigation&lt;/a&gt; says, that already in the 1960s automated celestial navigation was very accurate:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[the systems were] able to lock onto up to 11 stars (even in daytime) and resolve the craft's position to less than 300 feet (91 m)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;With all the improvements in imaging technique and computer processing since the 60s, I believe looking at stars from 50km altitude could easily provide accurate navigation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, we have relatively high-quality radar maps of Venus, so the radar option would also be likely to work well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all, I think navigation would not be a great concern at Venus.&lt;/p&gt;&#xA;" OwnerUserId="6917" LastActivityDate="2016-11-21T14:07:37.127" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19090" PostTypeId="2" ParentId="19088" CreationDate="2016-11-21T14:54:14.547" Score="2" Body="&lt;p&gt;It's bad when matter and anti-matter come in direct contact&lt;sup&gt;[reference needed]&lt;/sup&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, the only way to &lt;a href=&quot;http://newscenter.lbl.gov/2010/11/17/antimatter-atoms/&quot; rel=&quot;nofollow noreferrer&quot;&gt;store antimatter&lt;/a&gt; is to trap it inside some very strong magnetic fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, the size of the containment system would greatly exceed the savings in volume gained by using anti-matter.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those magnetic fields have to be very strong, which implies a lot of very heavy hardware to generate them.&lt;/p&gt;&#xA;" OwnerUserId="12978" LastActivityDate="2016-11-21T14:54:14.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19091" PostTypeId="1" CreationDate="2016-11-19T14:39:08.863" Score="3" ViewCount="260" Body="&lt;p&gt;It's often said (or at least, I have read it somewhere on some occasion), that using nukes to combat a potential incoming meteoroid is not viable because it may split it into multiple incoming bogies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some questions though:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;If the nuke was detonated on one side of it, or several were detonated on one side, wouldn't that be enough to push it slightly off course and it would then sail right on by the earth. It would have to be done when it's still some distance away so that even a small nudge would have an effect.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Even if it was split into a gazillion pieces - surely this would be better than one big ass piece as those gazillion pieces would just burn up in the atmosphere and some lucky gits will see the meteor shower of the century.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="17678" OwnerDisplayName="Richard Heyes" LastActivityDate="2017-10-31T06:29:15.377" Title="Using nukes to push an incoming meteoroid off course" Tags="|meteoroid|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19092" PostTypeId="1" CreationDate="2016-11-21T15:29:40.627" Score="42" ViewCount="9798" Body="&lt;p&gt;Mark Adler wrote in &lt;a href=&quot;https://space.stackexchange.com/a/19084/415&quot;&gt;an answer to a different question&lt;/a&gt; that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We would expect that Voyager 1's local time is faster than Earth time by about one part in one hundred million.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;I don't think that the Voyager oscillator is stable enough to measure that small of a difference.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This effect would be due to gravitational time dilation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that &quot;all&quot; that is needed would appear to be a highly stable and accurate oscillator and counter, it seems a &lt;em&gt;somewhat&lt;/em&gt; simple thing to test that would not add a lot to the mass or power budgets of a spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Have we ever launched any probe that included an experiment to prove or disprove gravitational time dilation?&lt;/strong&gt; If yes, then what were the results thus obtained?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Particularly if not, then &lt;strong&gt;are any such probes currently known to be in the idea or planning stages?&lt;/strong&gt; Also, what are the major obstacles in designing and launching such a probe?&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2016-11-22T20:27:58.273" Title="Have we attempted to experimentally confirm gravitational time dilation?" Tags="|probe|gravity|time|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19093" PostTypeId="2" ParentId="19092" CreationDate="2016-11-21T15:40:59.927" Score="40" Body="&lt;p&gt;Yes, time dilation was experimentally confirmed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_Probe_A&quot; rel=&quot;noreferrer&quot;&gt;Gravity Probe A&lt;/a&gt;, launched by NASA on June 18, 1976.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The clock rates of two masers (one on the probe and one on Earth) were compared, and it was found that the difference matched what was predicted with an accuracy of about 70 parts per million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To address your question of challenges in designing the mission:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To yield an accurate and inexpensive experiment, GP-A required a flight path with a large change in the gravitational potential to provide a large gravitational redshift, and it required a flight path that kept the flight Hydrogen MASER in contact with the ground Hydrogen MASER during data collection. [&lt;a href=&quot;http://einstein.stanford.edu/content/faqs/faqs.html#gp-a&quot; rel=&quot;noreferrer&quot;&gt;Stanford&lt;/a&gt;]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-11-21T15:52:13.727" LastActivityDate="2016-11-21T15:52:13.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19094" PostTypeId="2" ParentId="19092" CreationDate="2016-11-21T15:42:45.333" Score="52" Body="&lt;p&gt;In addition to specific probes like the one mentioned by called2voyage, the effect is significant enough that it affects everyday operations. For example, &lt;a href=&quot;http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html&quot; rel=&quot;noreferrer&quot;&gt;the GPS constellation needs regular clock corrections&lt;/a&gt; because the satellite hardware sits much higher up the gravity well than the ground hardware. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_time_dilation&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia page for gravitational time dilation&lt;/a&gt; mentions other confirmations in equipment flying in airplanes, and even between two points only one meter distant in height under laboratory conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, any instrument taking precise measurements over a distance must take relativistic effects into account as well, especially if it and its target are moving at significantly different speeds or are at significantly different heights. Physics SE may be able to better answer exactly when the effects become impossible to ignore.&lt;/p&gt;&#xA;" OwnerUserId="5851" LastEditorUserId="5851" LastEditDate="2016-11-21T15:48:27.617" LastActivityDate="2016-11-21T15:48:27.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19095" PostTypeId="2" ParentId="19092" CreationDate="2016-11-21T16:59:52.963" Score="70" Body="&lt;p&gt;You don't need a space probe. Or an aircraft. Or even a car. &lt;a href=&quot;https://www.nist.gov/news-events/news/2010/09/nist-pair-aluminum-atomic-clocks-reveal-einsteins-relativity-personal-scale&quot; rel=&quot;noreferrer&quot;&gt;NIST has measured the predicted general relativity time dilation due to a change in altitude on Earth of &lt;em&gt;one foot!&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-11-22T20:24:07.323" LastActivityDate="2016-11-22T20:24:07.323" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19096" PostTypeId="2" ParentId="19091" CreationDate="2016-11-21T17:06:56.470" Score="3" Body="&lt;p&gt;If you have enough warning, there are more gentle and much more predictable approaches than using a nuke. If you don't have enough warning, then you're probably only going to get one shot at it, so you definitely don't want the unpredictability of a nuke imparting unknown velocities to unknown size pieces, only one of which would need to be large enough to be a significant threat to make this a risky venture. If you know that the object is not a rubble pile, e.g. a metallic asteroid, and you have very little time to get it off course, then a nuke might be a good option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You certainly shouldn't throw nukes out of your toolbox, but their application needs to be considered very carefully. That statement probably applies to all potential uses of nukes.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-11-21T17:06:56.470" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19097" PostTypeId="1" AcceptedAnswerId="19098" CreationDate="2016-11-21T18:51:59.863" Score="2" ViewCount="246" Body="&lt;p&gt;Regarding a private organization what would be there options for communication with a probe or rover in deep space such as Mars or farther. Would they be able to use the Deep Space Network? If so what would be the fee? What would their other options be?&lt;/p&gt;&#xA;" OwnerUserId="17166" LastEditorUserId="415" LastEditDate="2016-11-21T19:54:14.553" LastActivityDate="2016-11-22T19:32:34.987" Title="How could a private organization ensure access to communications with a deep space probe or rover?" Tags="|mars|communication|deep-space-network|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19098" PostTypeId="2" ParentId="19097" CreationDate="2016-11-21T19:02:28.777" Score="3" Body="&lt;p&gt;SpaceX is said to be in negotiations with NASA to use the DSN for their first Mars missions. I haven't seen cost figures for that.&lt;br&gt;&#xA;A new dish antenna of the type used by the DSN (34 m diameter) costs on the order of $50M. SpaceX would need 3 for continuous coverage. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-21T19:02:28.777" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19101" PostTypeId="1" CreationDate="2016-11-21T23:31:42.540" Score="0" ViewCount="110" Body="&lt;p&gt;What's the reason why we don't let the spacecraft enter the atmosphere at a low angle, thus decreasing the vertical velocity and minimizing the crash-landing risks? If the speed and atmosphere density is right, some kind of wings could even provide a lift, thus giving a way to control the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is is because the speed loss at the higher levels of atmosphere is too significant and the craft will have too high vertical velocity when the speed and atmosphere density conditions allow using anything that resembles a wing?&lt;/p&gt;&#xA;" OwnerUserId="2759" LastActivityDate="2016-11-21T23:31:42.540" Title="Why can't we use wings for reentry?" Tags="|reentry|" AnswerCount="0" CommentCount="2" ClosedDate="2016-11-22T02:13:36.110" ContentLicense="CC BY-SA 3.0" />
  <row Id="19103" PostTypeId="2" ParentId="19091" CreationDate="2016-11-22T01:16:00.433" Score="3" Body="&lt;p&gt;As for the first part:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nukes have one great advantage over any other scheme that has been proposed.  Every other suggested system requires matching orbits with the target before you can attempt to move it.  So long as your guidance system is good enough a nuke doesn't need this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your target is closing on the Earth at 10 km/sec, you have to fly out there, then cancel your outbound velocity and add 10 km/sec back towards Earth before  you've done anything--and at that point even with the biggest booster your payload is awfully small.  Your nuke, however, doesn't care if it's going 20 km/sec relative to the target.  So long as the fuse explodes it at the right altitude it does it's job.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also the little detail that nukes pack an awful lot of energy per kg and the number of kg we can get to the target is limited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the second part of this question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason breaking it up is a very bad thing is the square-cube law.  The energy expends in all three dimensions, thus it drops off at the third power of distance.  We live on the surface, though, the area goes up only at the square of the distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at what happens if we split a rock into 8 equal parts.  Energy = 1/8, distance = 1/2, area = 1/4.  Each piece does 1/4 the damage of the whole rock but there are 8 pieces for a total of twice the damage.  (More energy is expended on blowing things up, less on making a crater.)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Breaking the rock up is the worst thing you can do unless you can break it small enough the pieces expend their energy in the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2016-11-22T01:21:45.487" LastActivityDate="2016-11-22T01:21:45.487" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="19104" PostTypeId="1" AcceptedAnswerId="19126" CreationDate="2016-11-22T01:42:04.650" Score="3" ViewCount="1038" Body="&lt;p&gt;Scott Manley &lt;a href=&quot;https://www.youtube.com/watch?v=JGcvxg7jJTs&quot; rel=&quot;nofollow noreferrer&quot;&gt;tells us&lt;/a&gt; (see &lt;a href=&quot;https://www.engadget.com/2016/11/21/nasa-demonstrates-em-drive-theory-but-dont-get-too-excited/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Engadget&lt;/a&gt; also) that the reaction-less drive peer reviewed paper is out, and nicely explains the basics. If it works, it would violate the long-held belief that linear momentum is conserved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I looked at &lt;a href=&quot;http://arc.aiaa.org/doi/10.2514/1.B36120&quot; rel=&quot;nofollow noreferrer&quot;&gt;the paper&lt;/a&gt; and it didn't take long to find a strange word. (&lt;a href=&quot;https://i.stack.imgur.com/USYPQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;screen capture&lt;/a&gt;) The first sentence of the abstract reads:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A vacuum test campaign evaluating the impulsive thrust performance of a tapered radio-frequency test article&#xA;  excited in the transverse magnitude 212 mode at 1937 MHz has been completed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is a &quot;transverse magnitude 212 mode&quot;? Could it be a *transverse magnetic&quot; mode actually? That's a real term, and would be written $TM212$, having three indices instead of two because it's a more complex shape than a cylinder. Or is it a new kind of resonant mode? Is this what publication in a peer-reviewed journal means? All reviewers, and the journal's editor approved this paper including the abstract?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, the RF amplifier is on the torsional balance, and DC power is supplied through sliding liquid metal contacts &quot;to eliminate cable forces'. A current flowing through a closed circuit &lt;strong&gt;will experience a Lorenz force tending to push the loop to larger area&lt;/strong&gt;. (See the &quot;&lt;a href=&quot;http://web.mit.edu/8.02t/www/materials/StudyGuide/guide10.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;sliding bar problem, fig. 10.12.1&lt;/a&gt;) The magnitude of the force would scale linearly with current (and therefore with the RF power). I don't see any discussion of this force on the torsional balance, or estimation of it's magnitude, but it looks like when they tested reverse thrust, the sliding contacts were also moved to the other side as well. Was this force addressed in earlier publications, or did the reviewers miss this as well?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aJzx5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aJzx5.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Example of a reactionless electromagnetic drive being tested inside a vacuum chamber, from &lt;a href=&quot;http://news.nationalgeographic.com/2016/11/nasa-impossible-emdrive-physics-peer-review-space-science/&quot; rel=&quot;nofollow noreferrer&quot;&gt;National Geographic&lt;/a&gt;. Note, the drive is supposed to produce of the order of 30 to 100 micro-Newtons, which is about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)&quot; rel=&quot;nofollow noreferrer&quot;&gt;weight of a few mosquito&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9WbnO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9WbnO.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; The &quot;sliding bar&quot; problem from &lt;a href=&quot;http://web.mit.edu/8.02t/www/materials/StudyGuide/guide10.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6816" LastEditDate="2016-11-23T01:09:37.773" LastActivityDate="2016-11-23T01:09:37.773" Title="The &quot;Em Drive&quot; paper is out - need some help understanding it" Tags="|propulsion|emdrive|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19105" PostTypeId="1" AcceptedAnswerId="19115" CreationDate="2016-11-22T03:26:33.413" Score="2" ViewCount="694" Body="&lt;p&gt;While descending from orbit can the air be thin enough for a glider type blimp to slowly shed speed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could a blimp skim the atmosphere around a planet, slowly decreasing air speed, without the need of a traditional heat shield? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like a hydroplaning ship on water, the glider-like blimp would be using the air under it to skip most upper atmosphere during orbital entry. I don't think this has been attempted yet, or does this have a name other than 'skip reentry'?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.newscientist.com/article/dn10288-inflatable-cushions-to-act-as-spacecraft-heat-shields/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.newscientist.com/article/dn10288-inflatable-cushions-to-act-as-spacecraft-heat-shields/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would look like the picture below, but with more skips at smaller angles of attack, and more orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y5oJg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y5oJg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PRsKF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PRsKF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="23891" LastEditDate="2019-07-23T03:22:47.377" LastActivityDate="2019-07-23T03:22:47.377" Title="While descending from orbit, can a blimp glider skip an atmosphere to shed speed?" Tags="|orbital-mechanics|orbit|reentry|reuse|trajectory|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="19106" PostTypeId="1" CreationDate="2016-11-22T03:28:49.440" Score="4" ViewCount="572" Body="&lt;p&gt;Not one probe that has or will leave the Solar System (Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizons) has a Solar Power System. This leads to the question, is there a reason behind this? And secondly if a probe were to be launched out of the Solar System with a solar power system couldn't it theoretically communicate with earth indefinitely? The reasoning behind that is even billions of miles out they could turn off all systems for say 3 months, let the batteries recharge and then after the 3 months turn the power back on,communicate with it for a few days and then repeat the process. &lt;/p&gt;&#xA;" OwnerUserId="17166" LastActivityDate="2016-11-29T18:25:27.923" Title="Probes on a Solar System Escape Trajectory" Tags="|probe|solar-power|deep-space|" AnswerCount="4" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19107" PostTypeId="2" ParentId="19106" CreationDate="2016-11-22T04:02:27.830" Score="5" Body="&lt;p&gt;The primary missions of the Voyagers, Pioneers, and New Horizons was exploration of the planets of our solar system, not beyond it. Continuous power during their approaches to our system's planets was much more important than their ability to tell us about interplanetary space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;RTGs provide more continuous power per unit mass than solar panels at Jupiter distance from the sun or further. Any mass dedicated to solar panels would be mass that couldn't be spent on scientific instruments.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-11-22T23:10:02.567" LastActivityDate="2016-11-22T23:10:02.567" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19108" PostTypeId="2" ParentId="19091" CreationDate="2016-11-22T06:50:03.127" Score="3" Body="&lt;p&gt;Judging by the amount of rock vaporized in underground nuke tests, and the size of an average asteroid, todays nuclear weapons are easily capable of vaporizing all but the most massive of asteroids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The largest nuclear detonation by the US was Castle Bravo at 15Mt. This created a crater 2km in diameter and 75m deep. Thats 3.1x10^8 cubic meters. But thats a cone shape, so a lot of energy radiated in upward directions. Lets say very approximately half, giving 6x10^8 m^3 of rock vaporization and/or dispersal. That volume is equivalent to a sphere of radius 530m, assuming we send Bruce Willis to bore a hole to the center of the asteroid. According to wikipedia, there are about 31 million asteroids of less than that size, and about 1m greater. So given that the Russian Tsar Bomba was several times larger again, I think its fair to say that with the most powerful of todays nukes detonated &lt;em&gt;at the core&lt;/em&gt; we are talking about enough energy to totally vaporize the vast majority of asteroids. Any chunks that might remain would be blown outwards at high speed by the extremes of pressure in the center of the vaporized and rapidly expanding plasma cloud, and so be thrown far enough from the original impact trajectory that they'd miss earth (assuming you hit it some weeks or months away).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;i didnt manage to find much more than speculation regarding the effects of nuclear detonations in open space, beside an asteroid. Lacking any atmosphere for blast waves, the energy is released as radiation, which would likely vaporize the surface of the asteroid, propelling it sideways and off course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also found this, which seems interesting, although i didnt watch all of it.&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/_cZUnsSIvBM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2016-11-22T18:16:48.003" LastActivityDate="2016-11-22T18:16:48.003" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19109" PostTypeId="2" ParentId="19106" CreationDate="2016-11-22T07:07:42.047" Score="7" Body="&lt;p&gt;In addition to a few hundred W of electrical power, an RTG produces a few kW of thermal power which is used to keep the spacecraft warm. Without heaters, the spacecraft's temperature would drop to -200 °C. It needs to be warmed up for the electronics to work well, and the thermal cycling would soon break stuff. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-22T07:07:42.047" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19110" PostTypeId="2" ParentId="19106" CreationDate="2016-11-22T10:42:34.263" Score="10" Body="&lt;p&gt;There isn't enough solar energy to power a spacecraft once you get a certain distance from the sun, even if you had 100% power conversion efficiency. The intensity of sunlight diminishes inversely proportional to the square of the distance from the Sun. The intensity of sunlight at Mars is half what it is at Earth, and at Jupiter it's only 5%. At the Earth's orbit the sun's intensity is about 1400 W/m2 (watts per meter squared), but at Uranus it's only 4 W/m2. Juno's solar array, which is about 70 square meters and 10%-ish efficiency, generates almost 500 Watts of power at Jupiter, but would only generate 28 Watts at Uranus, which is nowhere near what it would need to function. Pluto averages 1 W/m2, which equates to about 7 Watts. Even if you had 100% efficiency a 70m2 array would generate 70 Watts at that distance. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-11-22T10:42:34.263" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19111" PostTypeId="2" ParentId="4561" CreationDate="2016-11-22T12:55:54.970" Score="3" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Mataveri_International_Airport&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mataveri Airport&lt;/a&gt; on Easter Island was a planned abort site for the Space Shuttle. Since the Space Shuttle has no means of propulsion, you might want to have a fairly tight mesh of potential landing sites in event of an emergency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a nice side-effect, the long runway allowed for larger passenger aircraft to service the island, which boosted tourism. A nice win-win situation.&lt;/p&gt;&#xA;" OwnerUserId="16645" LastActivityDate="2016-11-22T12:55:54.970" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19112" PostTypeId="2" ParentId="19104" CreationDate="2016-11-22T13:41:49.733" Score="6" Body="&lt;p&gt;By parts,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What is a &quot;transverse magnitude 212 mode&quot;?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I didn't find anything in literature about &quot;transverse magnitude&quot;. At a first glance you're right, probably they meant &quot;transverse magnetic mode&quot;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is this what publication in a peer-reviewed journal means?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Peer reviewing is a long and painful process with numerous revision cycles that could last for years. It is understandable that they can overlook some typos and simple errors. Perhaps the authors can correct this in future.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Was this force addressed in earlier publications, or did the reviewers miss this as well?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Torsion balances have a long history measuring forces down to the nano-newton level, and many of them use liquid metal contacts as &quot;friction-less&quot; contacts, if the Lorentz force was a real problem, probably any research group would have reported it before. &#xA;The problem here is that you do not have neither an external static magnetic field, nor a larger area to be pushed at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some articles about thrust balances that could be helpful (perhaps not exactly the same case as in the paper): &lt;a href=&quot;http://scitation.aip.org/content/aip/journal/rsi/84/9/10.1063/1.4819252&quot; rel=&quot;nofollow noreferrer&quot;&gt;article 1&lt;/a&gt;, &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2013index/vvs6dqil.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;article 2&lt;/a&gt;, &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2005index/011.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;article 3&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can also suggest you to have a look in the many papers published by the electric propulsion community about thrust balances, you can find more stuff than I did: &lt;a href=&quot;http://erps.spacegrant.org/index.php?page=iepc-download-88-07&quot; rel=&quot;nofollow noreferrer&quot;&gt;ERPS article archive&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerDisplayName="user17622" LastEditorDisplayName="user17622" LastEditDate="2016-11-22T17:29:54.180" LastActivityDate="2016-11-22T17:29:54.180" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19113" PostTypeId="1" CreationDate="2016-11-22T14:08:30.393" Score="1" ViewCount="497" Body="&lt;p&gt;Did the Space Shuttle solid rocket boosters (SRBs) ever collide with each other after separation from the Shuttle stack?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To clarify, I'm not asking if an SRB ever collided with the Shuttle itself or the external tank.  I'm asking if the SRBs ever collided with each other while in free fall after separation.&lt;/p&gt;&#xA;" OwnerUserId="17623" LastEditorUserId="415" LastEditDate="2016-11-22T15:31:59.543" LastActivityDate="2016-11-22T15:31:59.543" Title="Was there ever an incident of collision between SRBs after SRB jettison, during launch of the Space Shuttle?" Tags="|launch|space-shuttle|srb|" AnswerCount="0" CommentCount="5" ClosedDate="2016-11-22T16:08:23.490" ContentLicense="CC BY-SA 3.0" />
  <row Id="19114" PostTypeId="2" ParentId="19106" CreationDate="2016-11-22T15:48:17.210" Score="3" Body="&lt;p&gt;A simple estimate, at a distance of 50 AU (1 AU is the distance earth to sun) the solar power is 1/2500 of that at earths orbit. Collecting energy for three months and use it in three days is only a factor of 30, but we need more than a factor of 2500. 3*2500 is 7500 days or more than 20 years. As others wrote, the self discharge of the batteries would be much faster than this slow charge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Kuiper belt goes from some 40 to 75 AU, I used 50 AU for the estimate.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-11-22T15:48:17.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19115" PostTypeId="2" ParentId="19105" CreationDate="2016-11-22T17:04:24.260" Score="6" Body="&lt;p&gt;Let's begin by the last question: a company named JP Aerospace proposed an &quot;double&quot; airship that could reach LEO using electric propulsion. &lt;a href=&quot;https://space.stackexchange.com/q/6548/17622&quot;&gt;See this previous question.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Concerning reentry, the problem is to do the aerobreaking fast enough so your vehicle can withstand the heat generated and aerodynamic forces of the atmosphere at the given altitude. Perhaps in a planet that have a very thin atmosphere this could be possible, but on Earth you would need retro rockets to decelerate to safe velocity levels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This happens because the atmosphere's density has an exponential behavior so when you are gradually slowing using just the drag, you will fall in a high drag zone very fast (even with the buoyancy, since the density is still very low for considerable force).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbital decay by atmospheric drag usually follows a profile as illustrated in the image: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QeYcu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QeYcu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user17622" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-11-22T17:04:24.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19118" PostTypeId="2" ParentId="19097" CreationDate="2016-11-22T18:14:48.427" Score="0" Body="&lt;p&gt;The &lt;a href=&quot;https://deepspace.jpl.nasa.gov/files/dsn/DSN%20Intro_4_Mar20151.pdf&quot; rel=&quot;nofollow&quot;&gt;DSN Mission Support Definition&lt;/a&gt; lists NASA, other government agencies and international partners as &lt;em&gt;customers&lt;/em&gt; (slide 3). The cost structure is depicted on slide 18ff, aperture fee is one of the variable items. Looking at the &lt;a href=&quot;https://deepspace.jpl.nasa.gov/files/dsn/875-0001-G1.pdf&quot; rel=&quot;nofollow&quot;&gt;Mission Service Policies&lt;/a&gt;, I would also expect considerable project costs before you exchange the first bit of telemetry with your craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other agencies have their own networks. &lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/Estrack/Estrack_ground_stations&quot; rel=&quot;nofollow&quot;&gt;ESTRACK&lt;/a&gt;, ESAs network, also has quite &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2013/01/Network_map&quot; rel=&quot;nofollow&quot;&gt;an impressive list of sites&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the other end of the spectrum, there are numerous sites with swivel mounted (smaller) dishes, that will work perfectly for deep space, depending on link budget. &lt;a href=&quot;http://www.amsat.org&quot; rel=&quot;nofollow&quot;&gt;AMSAT&lt;/a&gt; for example is a primary user of the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Bochum_Observatory&quot; rel=&quot;nofollow&quot;&gt;Bochum Radio Observatory&lt;/a&gt; (a private initiative!). They were able to &lt;a href=&quot;http://www.amsat.org/articles/g3ruh/127.html&quot; rel=&quot;nofollow&quot;&gt;receive STEREO telemetry&lt;/a&gt; at Bochum, in fact they even helped NASA with the &lt;a href=&quot;https://amsat-uk.org/2013/04/07/stereo-spacecraft-now-using-turbo-code-software-written-by-amsat-dl/&quot; rel=&quot;nofollow&quot;&gt;implementation&lt;/a&gt; of turbo codes for this mission. AMSATs mars mission P5A is planning to use this dish. I would bet you get some communication time in Bochum if you help to fund P5A.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastActivityDate="2016-11-22T18:14:48.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19119" PostTypeId="1" AcceptedAnswerId="19122" CreationDate="2016-11-22T18:44:30.770" Score="3" ViewCount="3354" Body="&lt;p&gt;I'm wondering if there a way to determine how thick (from the surface to the edge of space(karman line)) a planet's atmosphere would be. Assume an earth body like Earth or Mars (not a gas giant). I imagine there a lots of variables involved, like atmospheric composition, density, gravity, planet size...The possibilities seem endless, but I wonder if anybody has ever come up with a sort of generic equation that could give a ballpark figure as the thickness of a given planet's atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="17628" LastEditorUserId="17628" LastEditDate="2016-11-22T19:33:24.673" LastActivityDate="2016-11-22T20:45:14.033" Title="Is there a calculation for determining the thickness of a planet's atmosphere?" Tags="|planetary-science|atmosphere|planet|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19120" PostTypeId="2" ParentId="19097" CreationDate="2016-11-22T19:32:34.987" Score="1" Body="&lt;p&gt;One alternative is laser communications. You could have much smaller and more affordable apertures on Earth, e.g. a meter, as well as on the spacecraft, for the same data rates. However you would then need to keep the spacecraft and its laser pointed very accurately to get any data at all. If for some reason there is a failure that results in the spacecraft not being able point its laser to your light bucket on Earth, then it's game over.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-11-22T19:32:34.987" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19122" PostTypeId="2" ParentId="19119" CreationDate="2016-11-22T19:41:14.160" Score="6" Body="&lt;p&gt;What you're possibly asking about is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Scale_height&quot; rel=&quot;nofollow noreferrer&quot;&gt;scale height&lt;/a&gt; of an atmosphere.&lt;/p&gt;&#xA;&lt;p&gt;To clarify, atmospheres don't simply abruptly stop existing at a certain height. They decrease in density and pressure exponentially with height, while how strongly that happens is controlled by temperature &lt;span class=&quot;math-container&quot;&gt;$T$&lt;/span&gt;, mean molecular weight &lt;span class=&quot;math-container&quot;&gt;$\mu$&lt;/span&gt; of the atmospheric constituents and local strength of gravity &lt;span class=&quot;math-container&quot;&gt;$g$&lt;/span&gt;.&lt;br /&gt;&#xA;Taken together the scale height &lt;span class=&quot;math-container&quot;&gt;$H$&lt;/span&gt; is&lt;br /&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$$ H = \frac{k_B T}{\mu g} $$&lt;/span&gt;&#xA;and then the density &lt;span class=&quot;math-container&quot;&gt;$\rho$&lt;/span&gt; decreases relative to some reference density &lt;span class=&quot;math-container&quot;&gt;$\rho_0$&lt;/span&gt; as&#xA;&lt;span class=&quot;math-container&quot;&gt;$$ \rho (z) = \rho_0 \cdot exp(-z/H)$$&lt;/span&gt;&#xA;and &lt;span class=&quot;math-container&quot;&gt;$z$&lt;/span&gt; being the distance to this reference level. Same goes for the pressure with height, but not for the temperature.&lt;br /&gt;&#xA;This very simple approximation to atmospheric structure can be derived from the hydrostatic law of fluid dynamics and is usually too simple for all science purposes due to the many approximations involved.&lt;br /&gt;&#xA;However it is useful for the purpose you're asking about, as ,for example, in Earth's atmosphere in a layer &lt;span class=&quot;math-container&quot;&gt;$1.0 \cdot H$&lt;/span&gt; thick, 63.2% of the total atmospheric mass is contained.&lt;/p&gt;&#xA;&lt;p&gt;However as I said, atmospheres continue up to far into space (which is arbitrarily defined to start at &lt;span class=&quot;math-container&quot;&gt;$z=100km$&lt;/span&gt; for Earth) and connect smoothly to the interplanetary medium. Also because of this, any body's atmosphere is always escaping. So what 'thickness' really means, is dependent on how much physics you want to neglect.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-11-22T20:45:14.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19123" PostTypeId="2" ParentId="19071" CreationDate="2016-11-22T19:56:32.213" Score="1" Body="&lt;p&gt;Let me add a few other points on top of David's already excellent answer.  There are some other fixed timeframes that occur after launch but before the Satellite can become operational:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Orbit-raising&lt;/strong&gt;: It will take GOES-R &lt;a href=&quot;http://www.goes-r.gov/downloads/GOES-R_Launch_Timeline.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;about 2 weeks&lt;/a&gt; to get to geostationary orbit.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Outgassing&lt;/strong&gt;: It can take quite a while for all of the atmospheric gasses that &quot;hitchhiked&quot; in the satellite to be dispersed, to enable all the instruments to operate properly.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The above items will be, of course, common to all the launches (R, S, T, U).  Additionally, the GOES-R satellite series includes new products and instruments, not yet flown on earlier GOES missions.  So, in addition to testing the ABI imager and Ground Station software's ability to process the data from the imager at higher resolutions, as David mentioned, there are three times as many spectral channels to check-out.  Also, there are some brand-new instruments on GOES-R, such as the GLM (Geostationary Lightning Mapper), that the ground team could only test using simulated data (instead of, for example, scaling GOES-NOP data to use as test data) up to this point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a nice publicly accessible document that goes into more of the details of what is planned for testing: &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160004356.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Post Launch Calibration and Testing of the Advanced Baseline Imager&#xA;on the GOES-R Satellite&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="17631" LastEditorUserId="6816" LastEditDate="2016-11-22T20:04:34.137" LastActivityDate="2016-11-22T20:04:34.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19124" PostTypeId="1" CreationDate="2016-11-22T21:09:49.640" Score="4" ViewCount="677" Body="&lt;p&gt;So we are spending a lot of money to get things landed vertically. Its a challenge, and obviously tough too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here is the idea. &#xA;Get to mars. land over there, no need to return to earth (if no human is there)&#xA;Get the bots to build a long air strip. Maybe something that can be easily made and transported there and that can be quickly put up there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you can have more ships sent out there with lesser risks. &#xA;What would some of the challenges in this idea, and what would make this not feasible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE: So assume at some point we do colonize and then does it make sense to have an air strip. &#xA;Or would you rather have small shuttles that can take off vertically?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE: So we have concluded that due to atmosphere thickness, it would not be enough for present aircrafts to fly.&#xA;What if your aircraft was 1/3rd lighter by the use of advanced materials or carbon fibre, nylon, or something meant only for mars takeoff to space only? Is there a possibility then?&lt;/p&gt;&#xA;" OwnerUserId="16673" LastEditorUserId="16673" LastEditDate="2016-11-24T07:04:56.780" LastActivityDate="2018-07-18T06:59:02.840" Title="What would be a few challenges to build a landing strip on mars?" Tags="|mars|spacecraft|landing|" AnswerCount="6" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19125" PostTypeId="2" ParentId="19124" CreationDate="2016-11-22T21:16:42.853" Score="9" Body="&lt;p&gt;On Mars, the atmosphere is thin enough (around 1% of Earth's) that it's impractical to use aerodynamic lift during the terminal phase of landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, a horizontally landing craft would be much less efficient than a vertical lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even for Earth-returning spacecraft, where aerodynamic lift is relatively easy to get, the mass requirements for wings and control surfaces plus their thermal protection systems are such that they are only attractive if precision landing and reusability is required -- and even then, vertical propulsive landing SpaceX style is competitive.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-11-22T21:16:42.853" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19126" PostTypeId="2" ParentId="19104" CreationDate="2016-11-22T21:55:31.587" Score="4" Body="&lt;p&gt;Self-interacting Lorentz forces were addressed in section 8, point 3, along with other similar errors:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The third error is magnetic interaction, which has the potential for a false positive resulting from dc currents in power cables interacting during test article operation with ambient magnetic fields (e.g., local Earth field, magnetic damper) to generate a torque displacement on the pendulum. All dc power cables are a twisted pair or twisted shielded pair to minimize magnetic interaction. The test article is tested in forward, reverse, and null thrust orientations, but dc power cable routing and orientation is the same for all three configurations (power cables come in from the top of the test article), meaning any false positives will be the same magnitude and polarity for all three tests. This is not observed during the test campaign.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;http://arc.aiaa.org/doi/10.2514/1.B36120&quot; rel=&quot;nofollow noreferrer&quot;&gt;Original paper&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the conductors were mounted in the same way for all tests, any Lorentz forces from the liquid metal would always appear in the same direction and with approximately the same amount of power as for any other test at the same DC current. But the test rig's measured thrust was highly sensitive to precise tuning of the RF PLL oscillator, and the thrust measured changed directions or disappeared appropriately with the different setups.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2016-11-22T21:55:31.587" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19127" PostTypeId="2" ParentId="19124" CreationDate="2016-11-22T22:54:00.253" Score="3" Body="&lt;p&gt;Basically all the challenges of a mid-scale construction project, except done by remote control from several light minutes away, and you first need to land all the construction equipment on Mars.&lt;/p&gt;&#xA;" OwnerUserId="12831" LastActivityDate="2016-11-22T22:54:00.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19129" PostTypeId="2" ParentId="19124" CreationDate="2016-11-23T08:19:38.820" Score="7" Body="&lt;p&gt;The problem with a landing strip on Mars would be size. Because Mars' atmosphere is so thin you'd have to be going very, very fast (several hundred mph/kph) in order to get enough lift to arrest your descent enough to land safely. At that speed touchdown would be incredibly hard on the wheels, which would most likely pop instantly - a skid makes more sense I'd guess although that's still a big materials science problem. Say you did manage to get the wheels/skid on the ground, you'd then have to slow down and stop. The air is too thin for aerodynamic braking to matter much and the gravity is low so you wouldn't get the same traction, and would need to brake much more gently - again a skid would probably work better on Mars, but in either case it's going to take much longer to work because there's less gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So because of these factors a landing strip would have to be many, many miles long. The very thin atmosphere also makes controlling an airplane challenging, and because of the high speeds involved an airplane's turning radius would be huge, so you would need a very wide runway as well to compensate. So you'd need a landing strip the size of Manhattan, or even bigger, to be able to land an airplane on Mars. That would be hugely costly compared to other alternatives. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2018-07-18T06:59:02.840" LastActivityDate="2018-07-18T06:59:02.840" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="19131" PostTypeId="1" AcceptedAnswerId="19132" CreationDate="2016-11-23T11:59:45.753" Score="7" ViewCount="440" Body="&lt;p&gt;Let's assume that Elon Musk's plan to go to Mars is a success and there are several spacecraft which regularly go to Mars and back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point, how feasible would it be to start exploring the rest of the solar system? Since the capacity of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Interplanetary_Transport_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Interplanetary Transport System (ITS)&lt;/a&gt; would be about 100 people, it appears to be quite possible, although the crew would have to be reduced due to the increases in flight time. And what would be the potential targets for such missions anyway?&lt;/p&gt;&#xA;" OwnerUserId="16099" LastEditorUserId="1142" LastEditDate="2017-03-05T11:16:54.720" LastActivityDate="2017-03-05T11:38:26.113" Title="SpaceX ITS - going beyond Mars" Tags="|spacex|asteroid|interplanetary|jupiter|its|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19132" PostTypeId="2" ParentId="19131" CreationDate="2016-11-23T12:23:02.733" Score="8" Body="&lt;p&gt;ITS (to be renamed, I heard) as presented seems to bypass the health threats from microgravity and cosmic/Solar radiation by using a very short travel time to Mars. Nothing was mentioned about simulating gravity by rotation or using massive radiation shields. The further out one goes &quot;naked&quot;, the more challenging it gets in these respects simply because of the longer travel time required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew wouldn't necessarily have to be reduced, if water is recycled. Dried food storage is not heavy enough to be a significant factor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Potential targets beyond Mars would be asteroids in the Asteroid Belt, Trojan asteroids of Jupiter, maybe moons of Jupiter outside of its intense radiation belts, moons of Saturn and other outer planets, comets. With the exception of some select asteroids at certain launch windows, all of those targets are very very much harder to reach than Mars. One might consider a space station simply orbiting the Sun somewhere.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-11-23T12:23:02.733" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19133" PostTypeId="1" CreationDate="2016-11-23T13:01:31.720" Score="10" ViewCount="3933" Body="&lt;p&gt;Is it feasible to redesign the Space Launch System to use a pair of reusable Falcon 9 first stages instead of the five-segment solid boosters now being developed? The reason would be to lower launch costs while still retaining a Saturn V class capability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But could those creatures get married rationally? For example, would having to fuel the boosters with kerosene while the main stage needs hydrogen mess up the launch pad infrastructure? And SLS is vertically integrated while Falcon is horizontally integrated. And do the attachment points of the Falcon Heavy common cores and the SLS+boosters match each other? And whatnot. Is there any obvious deal breaker?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some key figures through Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Thrust&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid  16,000 kN each&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon  7,600 kN each&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Specific impulse&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid  269 seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 282 seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Burn time&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid  124 seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 162 seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(Some blogger loosely through out this idea because of the recent 2016 November NASA RFI about making SLS cheaper. But I can't find that source now to be credited or blamed.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="51" LastEditDate="2016-11-23T14:38:56.260" LastActivityDate="2019-12-06T18:21:36.593" Title="Could the solid boosters of SLS be replaced by reusable Falcon 9 first stages?" Tags="|spacex|falcon-9|sls|booster|booster-flyback|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19134" PostTypeId="2" ParentId="12841" CreationDate="2016-11-23T13:30:31.637" Score="10" Body="&lt;p&gt;I know that the question is a bit old, but I find it very interesting. So let's go. First of all, I should say that I can't guarantee that I will present the &lt;em&gt;highest numbers of all&lt;/em&gt;, since research is continuely going on. But let's give it a shot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We should recall the equation of thrust efficiency, $\eta_T$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\eta_T=\frac{g_0}{2}\frac{TI_{sp}}{P_{in}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $T$ is the thrust, $I_{sp}$ is the specific impulse, $P_{in}$ is the input power and $g_0$ is the standard gravity acceleration. Then, yes, you are right: with a fixed $\eta_T$ and $P_{in}$ more thrust means less specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another important metric here (you will see why) is the thrust-to-power ratio, which is defined by the same equation simply as &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{T}{P_{in}} = \frac{2\eta_T}{g_0I_{sp}}$$&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Highest power&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Depends on what you define as power: average or instantaneous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Average power&lt;/em&gt; is usually considered when talking about steady-state thrusters, as &lt;a href=&quot;https://en.wikipedia.org/wiki/Hall-effect_thruster&quot; rel=&quot;noreferrer&quot;&gt;Hall effect thrusters (HET)&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gridded_ion_thruster&quot; rel=&quot;noreferrer&quot;&gt;Gridded ion engines (GIT)&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Field-emission_electric_propulsion&quot; rel=&quot;noreferrer&quot;&gt;Field-emission electric propulsion (FEEP)&lt;/a&gt;, the VASIMR (I will not speak about it here since you already cited it), etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you consider &lt;em&gt;instantaneous power&lt;/em&gt; probably you are talking about thrusters that work on pulsed or quasi-steady regime, as &lt;a href=&quot;https://en.wikipedia.org/wiki/Pulsed_plasma_thruster&quot; rel=&quot;noreferrer&quot;&gt;Pulsed plasma thrusters (PPT)&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster&quot; rel=&quot;noreferrer&quot;&gt;Magnetoplasmadynamic thrusters (MPD)&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Pulsed_inductive_thruster&quot; rel=&quot;noreferrer&quot;&gt;Pulsed inductive thrusters (PIT)&lt;/a&gt;, etc.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Average power&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The thrusters types that achieved highest power in ground testing are HETs and Arcjets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://erps.spacegrant.org/uploads/images/2015Presentations/IEPC-2015-125_ISTS-2015-b-125.pdf&quot; rel=&quot;noreferrer&quot;&gt;The X3 Nested Hall Thruster (NHT)&lt;/a&gt; developed by NASA and the University of Michigan achieved about 30 kW. The plan is to operate it up to 100 kW. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Arcjets are essentially &lt;em&gt;thermoelectric&lt;/em&gt; thrusters, thus presenting usually high thrust and low specific impulse. Some groups at NASA &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120016430.pdf&quot; rel=&quot;noreferrer&quot;&gt;operated an arcjet thruster at 1 MW.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Instantaneous power&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Since in the pulsed regime one can discharge a great amount of energy stored, for example, in a capacitor bank on a short amount of time, the instantaneous power a in pulse can achieve very high power levels without many complex equipment.&#xA;This happens because most of the high power cabling will be only between the thruster and the capacitos. The charging of the capacitors can be done by a &quot;lower power&quot; and less expensive rig.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this spectrum, MPDs usually achieve the highest power levels: at Princeton, &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/3.25596?journalCode=jsr&quot; rel=&quot;noreferrer&quot;&gt;researchers reported&lt;/a&gt; MPDs operating at power levels up to 15 MW in pulsed mode, at &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20010094060.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA up to 10 MW&lt;/a&gt;, and in the &lt;a href=&quot;http://arc.aiaa.org/doi/abs/10.2514/3.23210&quot; rel=&quot;noreferrer&quot;&gt;Osaka University up to 6 MW&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940028464.pdf&quot; rel=&quot;noreferrer&quot;&gt;A group at Los Alamos tested&lt;/a&gt; an experimental thruster called &quot;coaxial thruster&quot;, with a power input up to 40 MW. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of their great scalability, &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120001636.pdf&quot; rel=&quot;noreferrer&quot;&gt;many authors consider&lt;/a&gt; the MPDs very good candidates for interplanetary transportation.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Highest specific impulse&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The theoretical maximum specific impulse can be achieved only by a &lt;a href=&quot;https://en.wikipedia.org/wiki/Photonic_laser_thruster&quot; rel=&quot;noreferrer&quot;&gt;photonic thruster&lt;/a&gt;, that generates thrust by emitting photons at the speed of light, thus $I_{sp} \approx 30559883.50$ seconds. &#xA;But I'm sure this was not the kind of answer you were looking for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering only common thrusters, the types that generated highest specific impulse on ground testing were FEEPs and some experimental GIEs. Note that both of them accelerate charged particles by the direct application of an static electric field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-IEPC%202007-DS4G%20diagnostics%202007-050.pdf&quot; rel=&quot;noreferrer&quot;&gt;ESA claimed&lt;/a&gt; to achieve about 14000 seconds with their experimental Dual Stage GIE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ngpdlab.engin.umich.edu/electric-propulsion/field-emission-electric-propulsion&quot; rel=&quot;noreferrer&quot;&gt;It is claimed in literature&lt;/a&gt; that FEEP thrusters can achieve up to 12000 seconds, but I didn't find any example. But, for example, &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2003index/0161-0303iepc-full.pdf&quot; rel=&quot;noreferrer&quot;&gt;a group from Austria reported&lt;/a&gt; specific impulse values up to 8000 seconds.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Summing up&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;In 2014, the european company OHB published some very interesting plots showing the performances of some the most important propulsion systems that have been ground-tested until that time (It's not clear in their presentation if &lt;em&gt;all&lt;/em&gt; the thrusters shown were actually tested). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first shows a simple plot of Power against $I_{sp}$ and you can see that the behavior is consistent with what I commented before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second figure is a plot of the thrust-to-power ratio against $I_{sp}$. The behavior of the constante efficiency curves are modelled by our first equation. Thus there is a top curve of $\eta_T=1$, and the performance of all thrusters should remain bellow that.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sFo0J.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sFo0J.png&quot; alt=&quot;Figure 1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jXAwy.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jXAwy.png&quot; alt=&quot;Figure 2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both pictures were taken from: Peukert, M. and Wollenhaupt, B. &quot;OHB-System‘s View on Electric Propulsion Needs&quot;, EPIC Workshop, 2014.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OBS: As soon as I find more info about other high-power thrusters I will update the answer.&lt;/p&gt;&#xA;" OwnerDisplayName="user17622" LastActivityDate="2016-11-23T13:30:31.637" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19135" PostTypeId="1" AcceptedAnswerId="19138" CreationDate="2016-11-23T13:46:45.770" Score="3" ViewCount="307" Body="&lt;p&gt;I just read about &lt;a href=&quot;http://astronomynow.com/2016/11/23/saturn-probe-prepares-for-dramatic-ring-grazing-orbits/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini's upcoming orbital maneuvers&lt;/a&gt; where there's a nice &lt;a href=&quot;https://saturn.jpl.nasa.gov/system/news_items/main_images/2966_CSM_Traj_wFR_RevC.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;animation showing Cassini's orbits&lt;/a&gt;. Here's a frame of the animation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/M97yL.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M97yL.png&quot; alt=&quot;Cassini orbits&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see a lot of plane changes, i.e. the probe's orbital plane is rotated very often with respect to its previous revolution. I always thought that these maneuvers are very expensive (need to spend quite a lot of fuel): you need to fire around the point where you want the old and new orbits to &quot;touch&quot;, the plane change axis, and from what I remember from the days when I was playing KSP that these involve quite a lot of delta-v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how are the Cassini people able to pull that off? Either they do have more than enough fuel left to spend it that way, or there must be a way reduce the cost of these maneuvers that I'm not aware of yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a way to reduce the amount of fuel spent to rotate the orbital plane to such extreme degrees?&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2016-11-23T17:11:25.420" Title="Save energy in plane changing orbital maneuver" Tags="|orbital-mechanics|orbital-maneuver|cassini|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19136" PostTypeId="2" ParentId="19133" CreationDate="2016-11-23T14:19:45.990" Score="11" Body="&lt;p&gt;The SRBs for the Shuttle and SLS are used to get the stack moving and off the pad, so that the more efficient LOX/LH2 engines can do the hard work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of the Space Shuttle, an amazing amount of thrust is needed. I like using lbs of thrust because it is more inspiring. The SRBs produced 2.8 million lbs of thrust. That is just awe inspiring huge.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While a Falcon 9 Full Thrust is working its way up to 1.7 million lbs thrust, that is not even close to what an SRB puts out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first stage (half stage, whatever you want to call strap on boosters) needs high thrust, performance is less important, and is needed to literally get the booster off the pad. A F9 first stage just won't provide the thrust needed.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorDisplayName="user8269" LastEditDate="2018-02-11T12:18:42.397" LastActivityDate="2018-02-11T12:18:42.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19137" PostTypeId="2" ParentId="19133" CreationDate="2016-11-23T14:32:03.043" Score="6" Body="&lt;p&gt;Pad infrastructure seems simple enough: just add plumbing for kerosene. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd go from 2 to 5 boosters (with more thrust, so higher loads), attachment points won't match and you'd need to qualify the new combination of stages. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-23T14:32:03.043" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19138" PostTypeId="2" ParentId="19135" CreationDate="2016-11-23T17:11:25.420" Score="7" Body="&lt;p&gt;Titan. This is all done using gravity assists from Saturn's largest moon, Titan. Only tiny amounts of propellant are used to target the Titan flybys.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-11-23T17:11:25.420" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19139" PostTypeId="1" AcceptedAnswerId="19170" CreationDate="2016-11-23T17:59:58.567" Score="4" ViewCount="786" Body="&lt;p&gt;According to the pictures of the mission Soyuz MS-03, the crew is wearing a new &quot;Sokol&quot; space suit. The previous crew was still wearing the 1980 &quot;vintage&quot; KV2 version. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/c/cb/Expedition_50_Crew_Board_Soyuz_%28NHQ201611170001%29.jpg&quot; height=&quot;400&quot; alt=&quot;Soyuz MS-03 crew before launch&quot; /&gt;&#xA;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/0/0c/Expedition_49_Preflight_%28NHQ201610190006%29.jpg&quot; height=&quot;400&quot; alt=&quot;Soyuz MS-02 crew before launch&quot; /&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any information about this new model?&lt;/p&gt;&#xA;" OwnerUserId="17642" LastEditorUserId="3200" LastEditDate="2016-11-25T22:54:50.707" LastActivityDate="2016-11-25T22:54:50.707" Title="New Russian Sokol space suit" Tags="|soyuz-spacecraft|spacesuits|sokol-spacesuit|" AnswerCount="1" CommentCount="2" ClosedDate="2016-11-26T13:14:56.263" ContentLicense="CC BY-SA 3.0" />
  <row Id="19140" PostTypeId="1" AcceptedAnswerId="19142" CreationDate="2016-11-23T19:07:36.217" Score="6" ViewCount="354" Body="&lt;p&gt;Read &lt;a href=&quot;http://spacenews.com/esa-mars-lander-crash-caused-by-1-second-inertial-measurement-error/&quot; rel=&quot;noreferrer&quot;&gt;this SpaceNews article&lt;/a&gt; which states,'1 second of Inertial Measurement error' caused the devastation in the Schiaparelli's landing sequence, causing it to free fall for 3.7 kms. What is exactly meant by 1 second of inertial measurement error? Is it gyroscope's angular measurement error? Not much is specified in the article except repeatedly mentioned 'erroneous data supplied to the on-board computer' which made it appear to the lander as though it had landed. It is intuitive in such case for the ground systems to activate. Although these questions remain&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What exactly had/could have happened with the IMU? How would it affect the lander and cause the 'premature release of parachute' and 'brief firing of thrusters'?&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2017-05-25T18:35:49.813" Title="Insight on EDM Schiaparelli's IMU error and subsequent failed landing" Tags="|failure|lander|esa|exomars|schiaparelli-lander|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19141" PostTypeId="1" AcceptedAnswerId="19143" CreationDate="2016-11-23T19:22:06.747" Score="4" ViewCount="727" Body="&lt;p&gt;From my (basic) understanding the most impressive thing about the EmDrive is it violates Newton's third law. No propellant is needed, nothing is jettisoned away, yet it, allegedly, accelerates in a given direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've heard that a flashlight in space, turned on, will eventually accelerate due to the energy of the light going out away from it. How different do the examples of the EmDrive and flashlight differ; is it just the EmDrive isn't even shooting light/radiation out?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition how could the EmDrive be much more efficient than light propulsion?&lt;/p&gt;&#xA;" OwnerUserId="17109" LastEditorUserId="6816" LastEditDate="2016-11-23T20:23:40.453" LastActivityDate="2016-11-24T02:37:43.157" Title="EmDrive compared to flashlight in space" Tags="|propulsion|advanced-propulsion|emdrive|" AnswerCount="3" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19142" PostTypeId="2" ParentId="19140" CreationDate="2016-11-23T20:00:04.397" Score="6" Body="&lt;p&gt;Apparently the lander was moving around too much. Shaking, swinging under the parachute or something like that. The IMU went out of its measurement range and reported incorrect values for 1 second.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The incorrect values led the guidance system to calculate a negative altitude, which led to the conclusion Schiaparelli had landed when it was in fact still more than 3 km up.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/ExoMars/Schiaparelli_landing_investigation_makes_progress&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA investigation&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As Schiaparelli descended under its parachute, its radar Doppler altimeter functioned correctly and the measurements were included in the guidance, navigation and control system. However, saturation – maximum measurement – of the Inertial Measurement Unit (IMU) had occurred shortly after the parachute deployment. The IMU measures the rotation rates of the vehicle. Its output was generally as predicted except for this event, which persisted for about one second – longer than would be expected.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;When merged into the navigation system, the erroneous information generated an estimated altitude that was negative – that is, below ground level. This in turn successively triggered a premature release of the parachute and the backshell, a brief firing of the braking thrusters and finally activation of the on-ground systems as if Schiaparelli had already landed. In reality, the vehicle was still at an altitude of around 3.7 km. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;h2&gt;EDIT:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/ExoMars/Schiaparelli_landing_investigation_completed&quot; rel=&quot;nofollow noreferrer&quot;&gt;final report has been released&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;THE CIRCUMSTANCES OF THE ANOMALY&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The sequence (timeline) of the events is reported below with respect to UTC time.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;a) Separation from TGO on 16/10/2016 at 14:42:00.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;b) Awakening from hibernation on 19/10/2016 at 13:29:48.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;c) Entry in the Mars atmosphere (EIP) detected at 14:42:22 through accelerometers.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;d) Between EIP and Parachute Deployment triggering, an unexpected evolution in the spin rate of the EDM was noticed.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;e) At 14:45:23 the parachute deployment was triggered (trigger is the g-level). - The dynamic conditions at the moment of parachute deployment derived from telemetry showed a total angle of attack (AOA) estimated of about 6.5 deg and a lateral angular rate &amp;lt; 3 deg/s&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;f) Parachute deployment time (time from mortar firing to peak load factor) was circa 1 sec (in line with the predictions).&lt;br&gt;&#xA;   - The parachute was deployed, and the parachute inflation triggered some oscillations of Schiaparelli at a frequency of approximately 2.5 Hz.&lt;br&gt;&#xA;   - About 0.2 sec after the peak load of the parachute inflation, the IMU measured a pitch angular rate (angular rate around Z-EDM axis) larger than expected.&lt;br&gt;&#xA;   - The IMU raised a saturation flag,.&lt;br&gt;&#xA;  - During the period the IMU saturation flag was set, the GNC Software integrated an angular rate assumed to be equal to the saturation threshold rate. The integration of this constant angular rate, during which the EDM was in reality oscillating, led to an error in the GNC estimated attitude of the EDM of about 165 degrees. This would correspond to an EDM nearly turned downside up with the front shield side pointing to quasi-zenith.&lt;br&gt;&#xA;   - After the parachute inflation, the oscillatory motion of Schiaparelli under its parachute was mostly damped and Schiaparelli was descending at a nominal descent rate, with very small oscillations (&amp;lt; 3 deg) around pitch and yaw axis.&lt;br&gt;&#xA;  - After parachute inflation the angular acceleration around the spin axis changed again&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;g) The Front Shield was jettisoned as planned 40s after parachute deployment (timer based command) at 14:46:03&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;h) The RDA was switched on at 14:46:19 (15s after Front Shield separation acknowledgment) and provided coherent slant ranges, without any indication of anomalies;&lt;br&gt;&#xA;  - Once the RDA is on, RIL mode, “consistency checks” between IMU and RDA measurements are performed. The parameters checked are: delta velocity and delta altitude. The altitude is obtained using the GNC estimated attitude to project the RDA slant ranges on the vertical.&lt;br&gt;&#xA;  - Because of the error in the estimated attitude that occurred at parachute inflation, the GNC Software projected the RDA range measurements with an erroneous off-vertical angle and deduced a negative altitude (cosinus of angles &gt; 90 degrees are negative). There was no check on board of the plausibility of this altitude calculation&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;i) Consequently the “consistency check” failed for more than 5 sec. after which the RDA was forced anyway into the loop based on the logic that landing was impossible without the RDA. The correctness of the other contributor to the altitude estimation, i.e. the attitude estimate, was not put in question. The RDA was put in the loop (event signalled by RIL time-out flag at 14:46:46).&lt;br&gt;&#xA;  - The GNC mode entered was TERMINAL DESCENT where the altitude is scrutinized to release the Back-Shell and parachute if the altitude is below an on board calculated limit. EXOMARS 2016 - Schiaparelli Anomaly Inquiry Reference: DG-I/2017/546/TTN Date 18/05/2017 Issue 1 Rev 0 Page 13 - Because of the incorrect attitude estimation leading to an estimated negative altitude, the GNC Software validated the conditions for separating the back-shell and parachute&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;j) Back-shell separation at 14:46:49.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;k) Switch-on of the Reaction Control System (RCS). - First RCS thruster operation was at 14:46:51 (no backshell avoidance manoeuvre)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;l) Switch-off of the RCS 3 seconds later at 14:46:54.&lt;br&gt;&#xA;  - The criterion for the RCS switch-off was based on the estimation of the EDM energy (as combination of the altitude and vertical velocity) being lower than a pre-set threshold. Since the estimation of the altitude was negative and very big, the negative potential energy was much higher than the positive kinetic energy (square of the velocity) and this criterion was immediately satisfied the RCS was commanded off as soon as allowed by the thruster modulation logic. This occurred just 3 seconds after the RCS switch on command when the capsule was at an altitude of about 3.7 km, leading to a free fall of Schiaparelli and to the impact on Mars surface about 34 seconds later. m) The Touch Down occurred at 14:47:28 corresponding to the crash of the surface platform on the surface of Mars at an estimated velocity of ≈150 m/s. The expected landing time was 14:48:05 (some 37s later).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-25T18:35:49.813" LastActivityDate="2017-05-25T18:35:49.813" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19143" PostTypeId="2" ParentId="19141" CreationDate="2016-11-23T20:21:54.860" Score="10" Body="&lt;p&gt;A photon drive (the flashlight example) doesn't use reaction mass, just the momentum of (massless) photons. So if it's supplied with power, e.g. from the sun, it can keep thrusting indefinitely. The most obvious example is a solar sail. However, photon drives have a thrust/power ratio of about 3.34 nN/W, which means that ~300 MW is required for 1 N of thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The EmDrive is similarly free from any need for reaction mass, but it doesn't emit photons, and has a much higher thrust/power, about 1.2 µN/W with the rather crude NASA Eagleworks test article, so almost three orders of magnitude less power required for the same thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An ion engine (the main competition for immediately useable high-ISP engines) has a thrust/power ratio of around 64 µN/W (&lt;a href=&quot;http://www.projectrho.com/public_html/rocket/enginelist.php#ion&quot; rel=&quot;noreferrer&quot;&gt;Dawn's NSTARs&lt;/a&gt;), but has an $I_{SP}$ in the low thousands of seconds (3100 s, specifically), requiring a substantial reaction mass to go anywhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/enginelist.php#vasimr&quot; rel=&quot;noreferrer&quot;&gt;VASIMR&lt;/a&gt; (an improved electrical propulsion drive that's not yet ready to fly) has a similar thrust/power ratio of about 70 µN/W in low gear and a similar $I_{SP}$ of a bit under 3000 s, or as little as 7 µN/W in high gear with an $I_{SP}$ of almost 30000 s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the EmDrive has roughly similar thrust/power as the high-gear VASIMR (intended for interplanetary travel after leaving gravity wells), but doesn't require any reaction mass, just the same power supply. So it would be cheaper to construct (without the significant fuel tanks) and quite possibly accelerate faster (with much lower initial mass). And, of course, it doesn't need to be refueled, just maintained from time to time, which makes interplanetary shuttle reuse much easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, &lt;em&gt;how&lt;/em&gt; this actually can be justified, physically, is the subject of the controversy. There are some wild hypotheses, which might actually pan out, but the majority of scientists are still pretty skeptical, because it mostly doesn't seem to add up.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2016-11-23T20:21:54.860" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19144" PostTypeId="1" CreationDate="2016-11-23T23:36:26.073" Score="1" ViewCount="154" Body="&lt;p&gt;Notice: I deeply apologize if I'm missing a Logical Issue to this Plan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of ending Juno's mission in 2018 by deorbiting it into Jupiter why not follow these steps.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Allow the amount of time the Scientists for Juno want to spend studying Jupiter be used. (Until Feb 2018) &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Around February 1st Juno departs from Jupiter towards the predicted location of mars at arrival time.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/f3VBX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f3VBX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Juno enters mars orbit.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How Juno is already prepared for such a mission:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The area of Juno's solar arrays will be able to easily power itself, its solar arrays are much larger than any mission before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2: Mars is much closer to the Sun then Jupiter is so Juno will get a &lt;strong&gt;giant&lt;/strong&gt; increase in power, 8x more.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Benefits that will come from such a mission:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Nasa raises interest to the General Public and proves that they are  ready to get moving on the mission to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.Juno could be used as an additional communications relay satellite for Opportunity and Curiosity along with future missions.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="17166" LastEditorUserId="17166" LastEditDate="2016-11-24T01:06:11.027" LastActivityDate="2016-11-24T16:45:38.437" Title="Juno possible retirement adjustment" Tags="|mars|jupiter|juno|future-missions|conceptual|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19145" PostTypeId="2" ParentId="19133" CreationDate="2016-11-24T00:21:29.270" Score="2" Body="&lt;p&gt;In addition to the engineering answers already provided there's an even bigger one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only real mission for the SLS is to be pork.  Using existing Falcon boosters rather than paying somebody to make new ones does not meet this essential mission requirement.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-11-24T00:21:29.270" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19146" PostTypeId="2" ParentId="19141" CreationDate="2016-11-24T00:28:46.393" Score="0" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;In addition how could the EmDrive be much more efficient than light propulsion?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It &lt;em&gt;probably&lt;/em&gt; isn't. The most likely explanation is that there is a measurement error affecting the results. The tip-off to this is that the various experimenters testing the EmDrive keep delivering different values for the thrust. It's not like one study has actually confirmed the results of another by getting &lt;em&gt;the same result&lt;/em&gt;. We keep seeing (admittedly difficult to explain) but &lt;em&gt;differing&lt;/em&gt; values for the efficiency of the EmDrive.&lt;/p&gt;&#xA;" OwnerUserId="17648" LastActivityDate="2016-11-24T00:28:46.393" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19147" PostTypeId="2" ParentId="5494" CreationDate="2016-11-24T00:31:16.593" Score="11" Body="&lt;p&gt;Here is an approximate answer for the Budget year of 2005.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2005 1.8 Billion was spent on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$140 Million&lt;/strong&gt; was classified as Cargo/Crew&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$350 Million&lt;/strong&gt; was classified as Operation Program Integrity&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$25 Million&lt;/strong&gt; was classified as Medical support&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$425 Million&lt;/strong&gt; was classified as Launch and Operations (Does Not include Shuttle Launch Costs)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$70 Million&lt;/strong&gt; was classified as Development&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$650 Million&lt;/strong&gt; was classified as Craft Operations&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;$150 Million&lt;/strong&gt; was classified as Flight and Crew Systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we can safely say that with no crew on-board that the following categories can be excluded: Cargo/Crew, Medical Support and Flight and Crew Systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total cost of those categories adds up to 315 Million.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you are left with two categories that may or may not overlap with crew related activities. &#xA;Launch and Operations, and Operation Program Integrity.&#xA;Which together compose of $775 Million of the total budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you can add together the definite and probable cost sectors for the Human Aspect of the ISS which will land you somewhere between 315 Million and 1 Billion (17.5%-55%). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;End Result : Depending on how you look at it, the Human aspect of the 2005 Budget for the ISS will be within the range of 315 Million and 1 Billion (17.5%-55% of the total budget).&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="17166" LastEditorUserId="17166" LastEditDate="2016-11-26T23:12:55.647" LastActivityDate="2016-11-26T23:12:55.647" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19148" PostTypeId="2" ParentId="19144" CreationDate="2016-11-24T01:34:56.803" Score="6" Body="&lt;p&gt;Considering Juno is at the destination of it's planned journey (i.e. Jupiter orbit), it's reasonable to expect that much to most of its propellant have been used already. There are still more, substantial maneuvers planned before 2018 - at least the &lt;a href=&quot;https://space.stackexchange.com/q/18686/12102&quot;&gt;lowering to a shorter period orbit to partially mitigate radiation damage&lt;/a&gt; may still be performed.&lt;/p&gt;&#xA;&lt;p&gt;Considering that it was assisted by the 2nd burn of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_(spacecraft)#Launch&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centaur 2nd stage&lt;/a&gt; to get much of it's velocity (and also an Earth flyby) it may not have enough propellant (delta-v) - nor (possibly) a suitable engine - to:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Get out of Jupiter's strong gravity&lt;/li&gt;&#xA;&lt;li&gt;Lose enough orbital speed to fall all the way back to Mars' orbit&lt;/li&gt;&#xA;&lt;li&gt;Lose more orbital speed to match Mars and then orbit it.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Because of things like conservation of energy and linear and angular momentum, you &lt;em&gt;have to work just as hard to &amp;quot;undo&amp;quot; all of the things Juno had to do to get to to Jupiter in the first place&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;It reminds me of the famous quote: &lt;a href=&quot;https://en.wikiquote.org/wiki/Ginger_Rogers&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ginger Rogers did everything Fred Astaire did, except backwards and in high heels.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-11-24T16:45:38.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19149" PostTypeId="2" ParentId="19141" CreationDate="2016-11-24T02:37:43.157" Score="-3" Body="&lt;p&gt;Photons do have mass, albeit it is very small. They change trejectory based on the curviture of spacetime whereas neutrinos do not. The Em Drive uses radio waves which are composed of photons. But unlike the flash light where the photons are ejected as the propellant,  the Em Drive the photon are directed against a plate before being allowed to escape.&lt;/p&gt;&#xA;" OwnerUserId="17650" LastActivityDate="2016-11-24T02:37:43.157" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19150" PostTypeId="2" ParentId="19124" CreationDate="2016-11-24T08:44:23.467" Score="6" Body="&lt;p&gt;&lt;em&gt;Besides&lt;/em&gt; the extremely thin atmosphere on Mars and construction obstacles, there are &lt;strong&gt;two other reasons&lt;/strong&gt; to not build a landing strip on Mars that I can see.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Why go to the same place twice?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Mars is big. It's not as big as Earth, but there's still a whole lot of unexplored terrain that would be of interest to study. By building a landing strip, even if you can build one and it's viable in the first place (which, given the other answers so far, appears highly doubtful), you commit significant resources to a single location. Just like you can't get a good idea of what the Earth looks like by studying only Manhattan or the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mariana_Trench&quot; rel=&quot;noreferrer&quot;&gt;Mariana trench&lt;/a&gt;, missions have deliberately picked &lt;a href=&quot;https://space.stackexchange.com/q/8087/415&quot;&gt;highly disparate landing sites&lt;/a&gt; at least in part precisely to study different parts of Mars to get a better idea of the planet's &lt;em&gt;overall&lt;/em&gt; geology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/8089/415&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/112O1.jpg&quot; alt=&quot;NASA Mars landing sites&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Landings aren't precise enough&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Our Mars landings just aren't precise enough for a landing strip to make sense. You are probably thinking in terms of doing this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.mars-one.com/images/tabs/_tab/roadmap2024.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IQnWD.jpg&quot; alt=&quot;Mars base&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;while reality is like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/msl/multimedia/pia16039.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6hHsx.jpg&quot; alt=&quot;Mars landing ellipses&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at that 2012 Curiosity landing. A landing ellipse of 12 x 4 miles, or 19 x 6.5 km, with touchdown &lt;a href=&quot;https://space.stackexchange.com/a/13983/415&quot;&gt;2.4 km from the centre&lt;/a&gt;, is &lt;strong&gt;far&lt;/strong&gt; too imprecise for a landing strip to make any sense. Not only would you have to actually land &lt;em&gt;on&lt;/em&gt; the landing strip in the first place; for a horizontal landing onto a landing strip to make any sense at all, you have to be going in just about exactly the right direction when you hit the ground, and hit very close to the correct spot on the landing strip. We aren't talking a few kilometers here; we are talking more like on the order of tens to a few hundreds of meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We simply aren't there yet, and I wonder if we will ever be there before we have significant human presence on Mars, at which point there's more productive things for those people to do than flatten and cover a large swath of Mars surface near their base where it is &lt;em&gt;most&lt;/em&gt; accessible for study.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle was able to do that kind of landing, but it did so on Earth (with an atmosphere hundreds of times thicker, with corresponding differences in aerobraking and lift capability), and it had significant constraints on landing. There's a reason why the deorbit burn was called a landing &lt;strong&gt;commitment.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2016-11-24T08:44:23.467" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19151" PostTypeId="2" ParentId="19144" CreationDate="2016-11-24T09:47:59.913" Score="1" Body="&lt;p&gt;NASA has spent a lot of money building Juno and getting it to Jupiter in the first place. The spacecraft has been built to survive in the harsh locale, and has sensors and instruments specifically designed and built to study aspects of Jupiter in ways that weren't possible in previous missions. Even if it is possible to get Juno to Mars (doubtful, probably not enough fuel), it would not be able to do much science because it's designed to study a totally different planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sending Juno to Mars would not raise positive awareness, in fact probably the opposite. NASA already has 2 functioning rovers on the surface of Mars and there are numerous probes in orbit, so Mars exploration is very much in the public eye. See &lt;a href=&quot;https://www.nasa.gov/mission_pages/mars/missions/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; for a list of current NASA Mars missions. If NASA were to divert Juno to Mars people would question their Judgement. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-11-24T09:47:59.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19152" PostTypeId="2" ParentId="18754" CreationDate="2016-11-24T10:13:24.813" Score="3" Body="&lt;p&gt;&lt;a href=&quot;https://solarsystem.nasa.gov/docs/po489.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wrist mode&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These large changes in parachute loading can couple with the “wrist mode” of the (MSL) entry body where, as the two bodies descend, the entry body rotates about its center of mass. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At 4 km, &lt;a href=&quot;http://exploration.esa.int/mars/57464-exomars-2016-schiaparelli-descent-sequence/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Schiaparelli was planned to be subsonic&lt;/a&gt;, at ~300 km/h:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UUEHV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UUEHV.jpg&quot; alt=&quot;Schiaparelli descent sequence&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that speed decreases rapidly from 1700 km/h at 11 km altitude to 320 km/h at 7 km altitude, the transonic region is between these altitudes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(at 4 km, &lt;a href=&quot;http://www.aerospaceweb.org/question/atmosphere/q0249.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mach 1 is about 800 km/h&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-11-24T10:50:13.933" LastActivityDate="2016-11-24T10:50:13.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19153" PostTypeId="1" AcceptedAnswerId="19166" CreationDate="2016-11-24T13:19:47.580" Score="6" ViewCount="1324" Body="&lt;p&gt;Okay. This is not directly related to &lt;em&gt;actual&lt;/em&gt; space travel, but it is semi-related, and definitely a practical, actual problem I face. It's about &lt;em&gt;Kerbal Space Program&lt;/em&gt; and SSTO spaceplanes. And I believe this is the place where I'm most likely to get an answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They need good thrust on ascent, requiring a chemical engine operating on Liquid Fuel + Oxidizer mix. Once in orbit, they switch to efficient, high ISp thermal nuclear engines, which run only on Liquid Fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes, though, they arrive into orbit with some oxidizer to spare.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In real life scenario, at this point the crew would likely vent the excess oxidizer into space. The game engine doesn't allow this though. Either I burn the spare oxidizer in stochiometric ratio with liquid fuel, at the low ISp of the high-thrust chemical engine, or haul it along the whole journey with me, and this is the decision I must make, looking at curent tank contents (which will be different every time, so $m_{ship}, m_{lf}, m_{ox}$ are variables in the equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using Tsiolkovski's Rocket Equation, this is as far as I got...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\begin{align}&#xA;&amp;amp;\Delta v = { {I_{sp} }\cdot  {g_0}} ln {m_{full} \over m_{dry}} \\&#xA;&amp;amp;1) &amp;amp;\\&#xA;&amp;amp;m_{full} = m_{ship}+m_{ox}+m_{lf} \\&#xA;&amp;amp;m_{dry} = m_{ship}+m_{ox} \\&#xA;&amp;amp;I_{sp} = A \\&#xA;&amp;amp;2) \\&#xA;&amp;amp;m_{full_1} = m_{ship}+m_{ox}+m_{lf} \\&#xA;&amp;amp;m_{dry_1} = m_{ship}+m_{lf}-m_{ox}\cdot R \\&#xA;&amp;amp;I_{sp_1} = B \\&#xA;&amp;amp;m_{full_2} = m_{dry_1} = m_{ship}+m_{lf}-m_{ox} \cdot R \\&#xA;&amp;amp;m_{dry_2} = m_{ship} \\&#xA;&amp;amp;I_{sp_2} = A \\&#xA;\\&#xA;&amp;amp;\Delta v_1 &amp;gt; \Delta v_2 ?\\&#xA;\\&#xA;&amp;amp;R = 9/11 = 0.818\\&#xA;&amp;amp;A = 800 \\&#xA;&amp;amp;B = 305\\&#xA;&amp;amp;m_{lf} &amp;gt; m_{ox} \cdot R\\&#xA;\\&#xA;&amp;amp;\Delta v_1 = { {A }\cdot  {g_0}} ln {{m_{ship}+m_{ox}+m_{lf}} \over {m_{ship}+m_{ox}}} =  ln \left({{m_{ship}+m_{ox}+m_{lf}} \over {m_{ship}+m_{ox}}}\right)^{ {A }\cdot  {g_0}}\\&#xA;\\&#xA;&amp;amp;\Delta v_2 = { { B }\cdot  {g_0}} ln {{m_{ship}+m_{ox}+m_{lf}} \over { m_{ship}+m_{lf}-m_{ox}\cdot R}} +  { {A }\cdot  {g_0}} ln {{m_{ship}+m_{lf}-m_{ox}\cdot R} \over {m_{ship}}} \\&#xA;&amp;amp; =   ln \left({{m_{ship}+m_{ox}+m_{lf}} \over { m_{ship}+m_{lf}-m_{ox}\cdot R}}\right)^{ { B }\cdot  {g_0}} \cdot  \left({{m_{ship}+m_{lf}-m_{ox}\cdot R} \over {m_{ship}}}\right)^{ {A }\cdot  {g_0}} \\&#xA;&amp;amp; =   ln {({m_{ship}+m_{ox}+m_{lf}})^{{B}\cdot  {g_0}} \over {(m_{ship}+m_{lf}-m_{ox}\cdot R})^{{B}\cdot  {g_0}}} \cdot  {({m_{ship}+m_{lf}-m_{ox}\cdot R})^{{A}\cdot  {g_0}} \over ({m_{ship}})^{{A}\cdot  {g_0}}} \\&#xA;&amp;amp; =   ln {({m_{ship}+m_{ox}+m_{lf}})^{{B}\cdot  {g_0}}  \cdot  ({m_{ship}+m_{lf}-m_{ox}\cdot R})^{{(A-B)}\cdot  {g_0}} \over ({m_{ship}})^{{A}\cdot  {g_0}}} \\&#xA;\\&#xA;&amp;amp; \left({{m_{ship}+m_{ox}+m_{lf}} \over {m_{ship}+m_{ox}}}\right)^{ {A }\cdot  {g_0}}&#xA; &amp;gt; {({m_{ship}+m_{ox}+m_{lf}})^{{B}\cdot  {g_0}}  \cdot  ({m_{ship}+m_{lf}-m_{ox}\cdot R})^{{(A-B)}\cdot  {g_0}} \over ({m_{ship}})^{{A}\cdot  {g_0}}}\\&#xA;&amp;amp; {({{m_{ship}+m_{ox}+m_{lf}} })^{ {(A-B) }\cdot  {g_0}}\over ({m_{ship}+m_{ox}})^{ {A}\cdot  {g_0}}}&#xA; &amp;gt; {({m_{ship}+m_{lf}-m_{ox}\cdot R})^{{(A-B)}\cdot  {g_0}} \over ({m_{ship}})^{{A}\cdot  {g_0}}}\\&#xA;&amp;amp; {({m_{ship}})^{{A}\cdot  {g_0}} \over ({m_{ship}+m_{ox}})^{ {A}\cdot  {g_0}}}&#xA; &amp;gt; {({m_{ship}+m_{lf}-m_{ox}\cdot R})^{{(A-B)}\cdot  {g_0}} \over ({{m_{ship}+m_{ox}+m_{lf}} })^{ {(A-B) }\cdot  {g_0}}}\\&#xA;&amp;amp; \left({m_{ship} \over {m_{ship}+m_{ox}}}\right)^{ {A}\cdot  {g_0}}&#xA; &amp;gt; \left({{m_{ship}+m_{lf}-m_{ox}\cdot R} \over {{m_{ship}+m_{ox}+m_{lf}} }}\right)^{ {(A-B) }\cdot  {g_0}}\\&#xA;&amp;amp; \left({m_{ship} \over {m_{ship}+m_{ox}}}\right)^A&#xA; &amp;gt; \left({{m_{ship}+m_{lf}-m_{ox}\cdot R} \over {{m_{ship}+m_{ox}+m_{lf}} }}\right)^ {(A-B) }\\&#xA;\end{align}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;....and...uh, I'm swamped and probably going nowhere. Help?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorDisplayName="user10509" LastEditDate="2016-11-25T09:37:32.263" LastActivityDate="2016-11-27T09:00:11.457" Title="Burn spare oxidizer or carry it along?" Tags="|ssto|kerbal-space-program|rocket-equation|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19154" PostTypeId="1" CreationDate="2016-11-24T13:21:50.793" Score="3" ViewCount="1462" Body="&lt;p&gt;I've seen varying times for spacecraft/rockets getting into LEO, e.g. according do the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Atlas_7&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mercury-Atlas 7 Wikipedia article&lt;/a&gt;, it took just over 5 minutes to get into a 259x154km orbit, whereas the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V#S-IVB_sequence&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V article&lt;/a&gt; says that it took a &quot;typical&quot; Saturn V launch nearly 12 minutes to get into orbit.&lt;/p&gt;&#xA;" OwnerUserId="17652" LastEditorUserId="3065" LastEditDate="2016-11-24T20:46:44.533" LastActivityDate="2016-11-24T20:46:44.533" Title="How long does it take to get into Low Earth Orbit?" Tags="|launch|orbit|low-earth-orbit|" AnswerCount="0" CommentCount="2" ClosedDate="2016-11-25T11:18:36.970" ContentLicense="CC BY-SA 3.0" />
  <row Id="19155" PostTypeId="1" CreationDate="2016-11-24T14:10:38.120" Score="4" ViewCount="173" Body="&lt;p&gt;From what I know, space scientists always work on two prototypes just in case something goes wrong. Why wasn't this the case this time? and are there plans for a repeat of this mission soon?&lt;/p&gt;&#xA;" OwnerUserId="17653" LastActivityDate="2016-11-25T05:56:25.017" Title="Schiaparelli 2?" Tags="|mars|exomars|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19156" PostTypeId="2" ParentId="19155" CreationDate="2016-11-24T15:16:24.573" Score="6" Body="&lt;p&gt;Schiaparelli was a demonstration mission, not a science mission, and as far as I know there are no plans to repeat it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are however plans for a joint ESA-Russia Mars lander, currently planned to launch in 2020. That one is a science mission, larger than TDM with a different architecture. Interestingly, that 2020 lander plans to inherit the guidance system from TDM, the &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/ExoMars/Schiaparelli_landing_investigation_makes_progress&quot; rel=&quot;nofollow noreferrer&quot;&gt;one thing that was known to fail on TDM&lt;/a&gt;. Fortunately it was also the thing that they received a lot of telemetry on before it crashed, so it is very likely that they can diagnose and remedy what happened.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately the TDM did not complete the later phases that that guidance system was reponsible for, in particular the touchy operation of propulsively bringing the vehicle to near-zero vertical and horizontal velocity at near-zero altitude using the RADAR and IMU data. Since that phase was not validated by TDM, I'd expect that they're going to be planning a lot more testing on Earth of that phase before relying on it for their 2020 lander. (Or maybe it will become their 2022 lander, if this or a combination of things results in a delay.)&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-11-24T15:16:24.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19157" PostTypeId="1" CreationDate="2016-11-24T15:20:53.703" Score="3" ViewCount="272" Body="&lt;p&gt;Scientists believe liquid water to be sporadically located near the equator of Mars. Is the water believed to be above-ground and if so do we know how deep? Or is it just within the soil near the surface? &lt;/p&gt;&#xA;" OwnerUserId="17166" LastEditorUserId="17166" LastEditDate="2016-11-25T02:56:16.533" LastActivityDate="2016-11-25T17:01:12.757" Title="Liquid water on Mars" Tags="|mars|water|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19158" PostTypeId="2" ParentId="19155" CreationDate="2016-11-24T15:54:34.210" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Space scientists always work on two prototypes just in case something&#xA;  goes wrong&quot; &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Well, this assumption is partially wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not really.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, it is already very expensive to create one mission, let alone make a backup for it.&#xA;Second, you often only get one chance to get it right:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;You might not have the funding for another launch, even if you have the hardware.&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The launch windows to other planets are usually very small (in the order of months); so you don't have enough time to prepare a second launch (provided you can figure out the issue and fix it before).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Moreover, by the time you are ready for the second launch, science will have advanced, making your probe already partially obsolete.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are many examples of such failures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The latest ones being:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Schiaparelli 03/14/2016 ✝ 19/10/2016 &lt;/li&gt;&#xA;&lt;li&gt;Hitomi 17/02/2015 ✝ 27/03/2016&lt;/li&gt;&#xA;&lt;li&gt;Fobos Grunt 09/11/2011 ✝ 15/01/2012&lt;/li&gt;&#xA;&lt;li&gt;...&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;While Schiaparelli was mostly a success despite the crash landing, Hitomi failed before it started operational life and no replacement is scheduled (some science will be achieved tho).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pray that nothing will happen to the &gt; $9 000 000 000 James Webb: We don't have a James Webb Junior.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="238" LastEditDate="2016-11-25T05:56:25.017" LastActivityDate="2016-11-25T05:56:25.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19159" PostTypeId="2" ParentId="19153" CreationDate="2016-11-24T16:42:38.160" Score="2" Body="&lt;p&gt;If the mass of extra oxidizer is much smaller than the mass of the craft, you can treat it as a linear problem instead of using the rocket equation -- the exponential curves described by the rocket equation are, at small scales, approximately linear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, you want the greatest delta-v, which is acceleration times time. Isp is &quot;mass-specific impulse&quot;, that is, impulse produced per mass unit of propellant, so compute chemical rocket Isp times (oxidizer mass + fuel mass) versus NTR Isp times (fuel mass), then divide by average ship mass over the burn. (Impulse is force times time; force over mass is acceleration, so this gets you acceleration times time which is delta-v). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the results are not close, you have a clear winner, and if the results are very close then it doesn't matter much which you use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In stock KSP, for small oxidizer quantities, due to the high density of KSP LiquidFuel, you should find the NTR to be the winner. Over larger burns, or using Realism Overhaul, the balance might shift to the chemical rocket.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-11-24T20:18:30.247" LastActivityDate="2016-11-24T20:18:30.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19161" PostTypeId="2" ParentId="19153" CreationDate="2016-11-25T10:50:36.573" Score="-2" Body="&lt;p&gt;You can partially fill the tanks prior to launch in the VAB. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;just subtract the excess amount and you should arrive in orbit with only trace amounts left. how fine you cut this margin is up to you. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, this requires a new launch, but maybe that's ok for you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another idea is to have a small tank which you can jettison. Launch with it empty, transfer the remaining undesired oxidizer into it, and throw it away?&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2016-11-25T11:17:23.970" LastActivityDate="2016-11-25T11:17:23.970" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19162" PostTypeId="1" CreationDate="2016-11-25T10:52:59.823" Score="3" ViewCount="733" Body="&lt;p&gt;How is inclination change performed for satellites inserted into GEO?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With most launches performed from spaceports at various high latitudes, it's impossible to launch directly into equatorial inclination. And I know inclination change maneuvers are awfully expensive in terms of delta-V, plus there are quite a few strange tricks to save up on that expense (including both a Moon flyby and atmospheric entry with wings...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is the maneuver performed typically, in case of GEO insertions though? Where? Which engines/stages are used? What's the typical delta-V?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="25" LastEditDate="2016-11-26T10:33:48.820" LastActivityDate="2016-11-26T10:33:48.820" Title="Inclination change to Geostationary Orbit" Tags="|launch|orbital-mechanics|orbital-maneuver|geostationary|inclination-change|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19163" PostTypeId="2" ParentId="721" CreationDate="2016-11-25T11:50:27.797" Score="2" Body="&lt;p&gt;The person that actually developed the algorithms for searching distant radio signals for signs of intelligence is Kent Cullers (portrayed as Kent Clark in the movie &lt;em&gt;Contact&lt;/em&gt;).  He worked at NASA Ames and the SETI Institute until he retired in 2005.  Although blind since birth, he has a contagious enthusiasm which you pretty much need in his line of work.  Any papers by him are a good place to start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, many papers cost money unless your institute's library subscribes to the journal.  I did find a public source in a proceedings that you can find from the search below.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the link from the search is about 300 characters long, the simplest way to find the work is to do a Google search for: &lt;strong&gt;Proceedings of The Search for Extraterrestrial Life: Recent Developments: Kent Cullers&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For info about Kent, see: &lt;a href=&quot;http://www.seti.org/kent-cullers&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.seti.org/kent-cullers&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17341" LastEditorUserId="17341" LastEditDate="2016-11-25T18:18:51.230" LastActivityDate="2016-11-25T18:18:51.230" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19164" PostTypeId="2" ParentId="19162" CreationDate="2016-11-25T12:08:27.757" Score="2" Body="&lt;p&gt;There's two ways that are typically done. The first is to use a &quot;Super Synchronous&quot; orbit to reduce the delta-v overall. Essentially, the initial orbit is around twice geostationary orbit, which makes the inclination change cheaper. The second is to just do it directly. They are always done as far out in the orbit as they can. There's a number of orbital arrangements that allow for this to happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engines can vary dramatically. Progress is being made to slowly move to an electric system for maintaining and achieving the proper orbit. Usually it is a mono-propellant system on the satellite. Occasionally it is the last stage in a rocket system, but that is considered far more expensive, and is usually reserved for military purposes. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-11-25T13:06:48.747" LastActivityDate="2016-11-25T13:06:48.747" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19165" PostTypeId="2" ParentId="19157" CreationDate="2016-11-25T17:01:12.757" Score="2" Body="&lt;p&gt;Liquid water is under debate, but not likely on the surface of Mars.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Seasonal dark streaks on Mars that have become one of the hottest topics in interplanetary research don't hold much water, according to new findings from a NASA Mars orbiter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mars.jpl.nasa.gov/news/whatsnew/index.cfm?FuseAction=ShowNews&amp;amp;NewsID=1929&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mars.jpl.nasa.gov/news/whatsnew/index.cfm?FuseAction=ShowNews&amp;amp;NewsID=1929&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, recent observations have yielded a large resevoir of ice just beneath the surface, between 3 to 33 feet deep.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Frozen beneath a region of cracked and pitted plains on Mars lies about as much water as what's in Lake Superior, largest of the Great Lakes, researchers using NASA's Mars Reconnaissance Orbiter have determined.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Scientists examined part of Mars' Utopia Planitia region, in the mid-northern latitudes, with the orbiter's ground-penetrating Shallow Radar (SHARAD) instrument. Analyses of data from more than 600 overhead passes with the onboard radar instrument reveal a deposit more extensive in area than the state of New Mexico. The deposit ranges in thickness from about 260 feet (80 meters) to about 560 feet (170 meters), with a composition that's 50 to 85 percent water ice, mixed with dust or larger rocky particles.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;At the latitude of this deposit -- about halfway from the equator to the pole -- water ice cannot persist on the surface of Mars today. It sublimes into water vapor in the planet's thin, dry atmosphere. The Utopia deposit is shielded from the atmosphere by a soil covering estimated to be about 3 to 33 feet (1 to 10 meters) thick.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=6680&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.jpl.nasa.gov/news/news.php?feature=6680&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2016-11-25T17:01:12.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19166" PostTypeId="2" ParentId="19153" CreationDate="2016-11-25T17:11:19.400" Score="3" Body="&lt;p&gt;When you burn that oxidiser, you are losing $\Delta v$ due to using engines with lower $I_{sp}$, but gaining $\Delta v$ because you are getting rid of unnecessary mass. Then the right question to ask is &quot;How much oxidiser should be burnt to maximise the $\Delta v$?&quot;. In some cases the best solution might even be &quot;burn some oxidiser, but not all of it.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To calculate that we take the formula for $\Delta v$ based on the starting mass, because that's our constant:&#xA;$$\Delta v=B\cdot g_0\cdot ln\left({{m_{start}}\over{m_{start}-{20 \over 11}m_{ox}}}\right)+A\cdot g_0\cdot ln\left({{m_{start}-{20 \over 11}m_{ox}}\over m_{start}-m_{lf}-m_{ox}}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$=B\cdot g_0\cdot ln\left({{m_{start}}}\right)+(A-B)\cdot g_0\cdot ln\left({{m_{start}-{20 \over 11}m_{ox}}}\right)-A\cdot g_0\cdot ln\left({{m_{start}-m_{ox}-m_{lf}}}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To maximise the $\Delta v$ in terms of burnt oxidiser we then take the derivative of $\Delta v$ in terms of $m_{ox}$ and equate it to zero (in all honesty I used an online derivative calculator for this step):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${{\partial \Delta v} \over \partial{m_{ox}}} = {{A}\over{m_{start}-m_{ox}-m_{lf}}}-{{20\cdot (A-B)}\over{11\cdot(m_{start}-{20\over 11}\cdot m_{ox})}}=0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After some rearranging we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$(9\cdot A-20\cdot B)\cdot m_{start} - 20 \cdot (A-B)\cdot  m_{lf} + 20 \cdot B \cdot m_{ox}=0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in the numbers we have, we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$m_{ox}={11\over 61}(9\cdot m_{lf} - m_{start}) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, &lt;em&gt;for these specific $I_{sp}$&lt;/em&gt; you want to &lt;strong&gt;burn that oxidiser as long as at least $1\over 9$ of your current mass is liquid fuel&lt;/strong&gt;. If you have less liquid fuel than that, the gains from the decreasing mass become lower than the losses from the lower $I_{sp}$ engines.&lt;/p&gt;&#xA;" OwnerUserId="17667" LastEditorUserId="17667" LastEditDate="2016-11-27T09:00:11.457" LastActivityDate="2016-11-27T09:00:11.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19167" PostTypeId="1" CreationDate="2016-11-25T17:26:09.233" Score="2" ViewCount="113" Body="&lt;p&gt;Per JPL, Recent observations have discovered a resevoir of ice beneath the surface of mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How practical, from an ISRU perspective, is this discovery?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Frozen beneath a region of cracked and pitted plains on Mars lies about as much water as what's in Lake Superior, largest of the Great Lakes, researchers using NASA's Mars Reconnaissance Orbiter have determined.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Scientists examined part of Mars' Utopia Planitia region, in the mid-northern latitudes, with the orbiter's ground-penetrating Shallow Radar (SHARAD) instrument. Analyses of data from more than 600 overhead passes with the onboard radar instrument reveal a deposit more extensive in area than the state of New Mexico. The deposit ranges in thickness from about 260 feet (80 meters) to about 560 feet (170 meters), with a composition that's 50 to 85 percent water ice, mixed with dust or larger rocky particles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=6680&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.jpl.nasa.gov/news/news.php?feature=6680&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3-33 feet is a really large range. Does that mean, some ice is closer to the surface then others?  Or is this due to lack of scientific precision, and most likely the ice is ~18 feet underground?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Utopia deposit is shielded from the atmosphere by a soil covering&#xA;  estimated to be about 3 to 33 feet (1 to 10 meters) thick.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=6680&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.jpl.nasa.gov/news/news.php?feature=6680&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the general composition of the soil in the Utopia deposit?  (Is it practical that future explorers can actually get to this water without heavy mining equipment?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main question is on the overall practicality of utilizing this specific ice resevoir for ISRU by future explorers.   I provided specific probing questions, but other considerations that I am missing are acceptable answers.&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2016-11-25T17:26:09.233" Title="Practical ice deposits on Mars?" Tags="|mars|colonization|water|isru|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19168" PostTypeId="1" CreationDate="2016-11-25T18:00:39.633" Score="5" ViewCount="268" Body="&lt;p&gt;There are large amounts of ice underground on Mars.   Some are closer to the surface then others.  Assuming this ice has been mined and is not safe for human consumption without purification, is it safe for irrigation?&lt;/p&gt;&#xA;&lt;p&gt;Do the intermixed dust particles provide nutritional value for the plants?   Or is the risk of toxins (to both the plant, and the human assuming the plants produces food) too great?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Mars Ice Deposit Holds as Much Water as Lake Superior&lt;/p&gt;&#xA;&lt;p&gt;with a composition that's 50 to 85 percent water ice, mixed with dust or larger rocky particles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=6680&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.jpl.nasa.gov/news/news.php?feature=6680&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8653" LastEditorUserId="7062" LastEditDate="2022-06-09T01:40:38.817" LastActivityDate="2022-06-09T01:40:38.817" Title="Is unpurified Martian ice/water safe for irrigation?" Tags="|mars|water|isru|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19169" PostTypeId="2" ParentId="19168" CreationDate="2016-11-25T18:28:36.320" Score="3" Body="&lt;p&gt;No one quite knows at the moment. This is the sort of thing, where the actual content of the water, needs to be examined and purification methods tested against it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, distilling the water is one way to get back to pure water, but energy intensive. Many of the purification processes on Earth potentially could be used, but often are large, and energy intensive. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyone serious about colonizing Mars would be wise to consider investigating this issue experimentally. The good news is SpaceX who have considered Mars seriously, are planning on a Red Dragon mission (using the 2018 window), that will be the largest payload ever landed on the red planet, and likely will repeat that mission each launch window to do the work they need done.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-11-25T18:28:36.320" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19170" PostTypeId="2" ParentId="19139" CreationDate="2016-11-25T22:22:13.903" Score="7" Body="&lt;p&gt;As far as I understand this is just a new &lt;strong&gt;overcoat&lt;/strong&gt; as an additional insulation for cold environments, underneath wich they still wear their usual Cокол suit. It consists of several pieces which can easily be removed by opening some velcro strips.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/pGkvK.jpg&quot; width=&quot;300&quot; /&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4AAWwnwskS4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;http://www.russianspaceweb.com/soyuz-ms-03.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.russianspaceweb.com/soyuz-ms-03.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3200" LastEditorUserId="3200" LastEditDate="2016-11-25T22:29:20.917" LastActivityDate="2016-11-25T22:29:20.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19172" PostTypeId="1" CreationDate="2016-11-26T10:59:08.027" Score="4" ViewCount="850" Body="&lt;p&gt;I understand the the thrust of a rocket is given by &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F_x = M_e V_e - M_i V_i + (P_e - P_i) A_e$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However when I try to derive it myself using Reynold's transport theorem I get the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is my control volume of the rocket thrust chamber&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q1RB2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q1RB2.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$A_i$ is the injector face area &lt;/li&gt;&#xA;&lt;li&gt;$P_i$ is the fluid inlet pressure, in absolute pressure (assuming pressurized by a turbopump)&lt;/li&gt;&#xA;&lt;li&gt;$A_e$ is the nozzle exit area&lt;/li&gt;&#xA;&lt;li&gt;$P_e$ is the atmospheric pressure&lt;/li&gt;&#xA;&lt;li&gt;$R_x$ is the reaction force required to hold the thrust chamber in place &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Using Reynold's transport theorem, I get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ΣF_x = M_e V_e - M_i V_i$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P_i A_i - P_e A_e + R_x = M_e V_e - M_i V_i$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R_x = M_e V_e - M_i V_i - P_i A_i + P_e A_e$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is different from what is mentioned in the third line above.&lt;/p&gt;&#xA;" OwnerUserId="17676" LastEditorUserId="6816" LastEditDate="2016-11-26T19:08:46.753" LastActivityDate="2017-12-16T18:32:43.143" Title="Deriving the thrust of a rocket" Tags="|rockets|thrust|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19173" PostTypeId="1" AcceptedAnswerId="19175" CreationDate="2016-11-26T18:41:03.477" Score="4" ViewCount="1898" Body="&lt;p&gt;The title more or less sums it up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rocket engine works pretty much by combining fuel and oxidizer, and expelling some propellant resulting in foward motion as described by Newton's third law of motion (colloquially, to each action there is an equal and opposite reaction).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Combining oxygen and hydrogen gives water in a highly energetic reaction: $$ 2\text{H}_{2} + \text{O}_{2} \rightarrow \text{H}_2\text{O} + \text{energy} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The water (vapor) has to go somewhere, and as such is ideal to throw out the back of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consequently, &lt;strong&gt;is it correct to say that a LH2/LOX rocket engine uses water as propellant?&lt;/strong&gt; If not, then where is the flaw in my reasoning?&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2017-09-19T15:26:28.530" Title="Is it correct to say that a LH2/LOX rocket uses water as propellant?" Tags="|rockets|propulsion|fuel|liquid-fuel|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19175" PostTypeId="2" ParentId="19173" CreationDate="2016-11-26T22:29:16.060" Score="18" Body="&lt;p&gt;&lt;em&gt;No, it is not.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exhaust is water, not the propellant. The propellant is a bi-propellant consisting of liquid H&lt;sub&gt;2&lt;/sub&gt; and liquid O&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bi-propellant is injected into the combustion chamber, where it reacts and produces H&lt;sub&gt;2&lt;/sub&gt;O (water) as exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just because the exhaust is used to push the rocket forward that does not make it the propellant. The propellant is what is used in the production of energy or exhaust gas - which is H&lt;sub&gt;2&lt;/sub&gt; and O&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In rockets and aircraft, &lt;strong&gt;propellants are used to produce a gas&lt;/strong&gt; that can be directed through a nozzle, thereby producing thrust. In rockets, &lt;strong&gt;rocket propellant produces an exhaust&lt;/strong&gt;, and the exhausted material is usually expelled under pressure through a nozzle. The pressure may be from a compressed gas, or a gas produced by a chemical reaction. The exhaust material may be a gas, liquid, plasma, or, before the chemical reaction, a solid, liquid, or gel. In aircraft, the propellant is usually a fuel and is combusted with the air.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;https://en.wikipedia.org/wiki/Propellant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;, Highlights mine)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So water is the exhaust, not the propellant.&lt;/p&gt;&#xA;" OwnerUserId="13867" LastEditorUserId="13867" LastEditDate="2017-09-19T15:26:28.530" LastActivityDate="2017-09-19T15:26:28.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19176" PostTypeId="2" ParentId="18932" CreationDate="2016-11-27T03:44:15.167" Score="9" Body="&lt;p&gt;Wikipedia has one image that is attributed to the SILTS pod:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/0/0d/SILTS_Image.jpg&quot; alt=&quot;SILTS IR Image&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/File:SILTS_Image.jpg&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the metadata doesn't give any clue as to where it came from, or where more might be found.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just happened to be reading the &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-28&quot; rel=&quot;noreferrer&quot;&gt;STS-28 page&lt;/a&gt;, noticed the image, and remembered this question.&lt;/p&gt;&#xA;" OwnerUserId="1342" LastActivityDate="2016-11-27T03:44:15.167" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19177" PostTypeId="1" AcceptedAnswerId="19179" CreationDate="2016-11-27T09:50:27.333" Score="16" ViewCount="4276" Body="&lt;p&gt;Currently six great folks are working in the International Space Station (ISS).  I am wondering if it is ever vacant and if there's a rule of thumb (magnitude of order of days, an average or anything) that it might be so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;On a high level estimation, how frequently - if ever - is the ISS not manned &lt;em&gt;on board&lt;/em&gt;?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2020-11-04T13:25:33.123" Title="How often has the ISS been physically vacant (un-manned)?" Tags="|iss|crewed-spaceflight|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19178" PostTypeId="1" AcceptedAnswerId="19263" CreationDate="2016-11-27T10:01:59.440" Score="4" ViewCount="555" Body="&lt;p&gt;Because of the &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=6648&quot; rel=&quot;nofollow noreferrer&quot;&gt;slow helium valve responses&lt;/a&gt;, Juno was not instructed to execute the large delta-v maneuver to drop from the ~53 day orbit to the ~14 day orbit originally scheduled for 2016-Oct-19.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd originally extracted the planned orbit from &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt;  mentioned &lt;a href=&quot;https://space.stackexchange.com/q/17964/12102&quot;&gt;here&lt;/a&gt; months ago. Now it's gone - or at least I can't find it. I'd like to retrieve it again as an exercise to understand it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just for one example, for &lt;a href=&quot;https://space.stackexchange.com/q/19181/12102&quot;&gt;this question&lt;/a&gt; I'd like to use Horizons to quickly give me the approximate orbital elements (a and $\epsilon$ at least) for the 14 day orbits around Jupiter. I can figure out how to extract them from the state vectors but it would be great to use this handy system.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;   Trajectory name                       Start (TDB)         Stop (TDB)&#xA;   ------------------------------     -----------------   -----------------&#xA;   rec_110805_111026_120302           2011-Aug-05 17:19   2011 OCT 26 00:00&#xA;   rec_111026_120308_120726           2011 OCT 26 00:00   2012 MAR 08 12:00   &#xA;   rec_120308_120825_121109           2012 MAR 08 12:00   2012 AUG 25 00:00&#xA;   rec_120825_130515_130708           2012 AUG 25 00:00   2013 MAY 15 00:00&#xA;   rec_130515_131005_131031           2013 MAY 15 00:00   2013 OCT 05 12:00&#xA;   rec_131005_131014_131101_reco      2013 OCT 05 12:00   2013 OCT 14 00:01&#xA;   rec_131014_131114_140222           2013 OCT 14 00:01   2013 NOV 14 06:00&#xA;   ref_131114_171017_140311_V0.2      2013 NOV 14 06:00   2015 MAR 26 00:00&#xA;   ref_150326_180221_150326_V0.1      2015 MAR 26 00:00   2016 FEB 26 00:01&#xA;   ref_160226_180221_160226_V0.1      2016 FEB 26 00:00   2016 APR 13 18:00&#xA;   pre_160413_161016_160818_pj01_V0.1 2016 APR 13 18:00   2016 OCT 16 23:00&#xA;   ref_160226_180221_160226_V0.5      2016 OCT 16 23:00   2018 FEB 21 11:40&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Now the last two lines are:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;   pre_160413_161016_160818_pj01_V0.1 2016 APR 13 18:00   2016 JUL 29 21:00&#xA;   nob_160729_170201_161019.V0.1      2016 JUN 29 21:00   2017 FEB 01 00:00&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Primary question:&lt;/strong&gt; I would like to access the original reference orbit before the update. Is there any way I can get it again using Horizions? I'd like to get state vectors with fine granularity using the original &lt;code&gt;ref_160226_180221_160226_V0.5&lt;/code&gt; throughout all of 2017 at least. Is there any way to do this through the web, or telnet, or by e-mail? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Secondary question:&lt;/strong&gt; If you look at the tables, the trajectories are patched - date/time that one ends matches the date/time the next begins, except sometimes for an overlap of 00:01 (a minute) which could be rounding in the table display.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in the last two lines of the updated table, the second-to-last line ends at &lt;code&gt;2016 JUL 29 21:00&lt;/code&gt; while the last line begins &lt;em&gt;one month earlier&lt;/em&gt; at &lt;code&gt;2016 JUN 29 21:00&lt;/code&gt;. Is this a typographical error, or are they somehow averaged together during that month?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dENnD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dENnD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Calculated distance between Juno Spacecraft and Jupiter Barycenter from JPL Horizons. Thin (blue) line: original reference orbit, Thick (green) line: updated orbit shown until 2016-Dec-10 - roughly when final committment for orbit &quot;lowering&quot; burn must be made. Note - data was downloaded at 2 hour time-point spacing, so uneven minimum distances is due to coarse sampling of the incredibly fast flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is the data of the thin (blue) line I would like to access again.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-14T06:25:36.727" Title="JPL Horizons original reference orbit data for Juno - how to retrieve now that it's been updated?" Tags="|orbital-maneuver|jupiter|juno|jpl-horizons|skyfield|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="19179" PostTypeId="2" ParentId="19177" CreationDate="2016-11-27T10:12:39.643" Score="28" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_human_spaceflights_to_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;wikipedia&lt;/a&gt;, the ISS has been manned ever since the first crew of &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_1&quot; rel=&quot;noreferrer&quot;&gt;Expedition 1&lt;/a&gt; entered on November 2nd 2000.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: As @Tristan pointed out the ISS was technically unmanned for a couple of times so far when there was just one crew on board which had to relocate the soyuz to another docking port, see comments below.&lt;/p&gt;&#xA;" OwnerUserId="3200" LastEditorUserId="3200" LastEditDate="2016-11-30T19:50:36.583" LastActivityDate="2016-11-30T19:50:36.583" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19180" PostTypeId="1" CreationDate="2016-11-27T13:35:49.580" Score="9" ViewCount="3039" Body="&lt;p&gt;Space based internet systems, like the one recently proposed by SpaceX and Google, are said to use satellites to communicate directly with end users, without using cellular ground stations. But how much power would a consumer device like a smartphone need to send data 1,000+ km? Wouldn't a separate outdoor antenna and amplifier be required anyway, reducing the mobility?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6471" LastEditDate="2016-11-28T08:54:10.760" LastActivityDate="2016-11-28T08:54:10.760" Title="Could a smartphone really transmit to satellites in a space based internet system?" Tags="|communication-satellite|radio-communication|internet|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19181" PostTypeId="1" AcceptedAnswerId="19188" CreationDate="2016-11-27T14:44:29.260" Score="5" ViewCount="686" Body="&lt;p&gt;Below is a plot of Juno's originally planned orbit around Jupiter, extracted from &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt;. It's shown in J2000 ecliptic coordinates, centered on the Jupiter barycenter. It turns out the orbit is essentially polar (inclination of about 90 degrees) and almost completely within a &lt;span class=&quot;math-container&quot;&gt;$yz$&lt;/span&gt; plane in those coordinates. (Other plots on the internet look different because they rotate the coordinates to keep the solar direction constant.) The black dots represent approximate Jovian apoapses.&lt;/p&gt;&#xA;&lt;p&gt;The plot shows the same orbit viewed side-on (along x axis) and front-on (along y axis), the big red dot is Jupiter.&lt;/p&gt;&#xA;&lt;p&gt;While the inclination of the orbit stays about 90 degrees, the plot shows what looks like a very pronounced &lt;a href=&quot;https://en.wikipedia.org/wiki/Apsidal_precession&quot; rel=&quot;nofollow noreferrer&quot;&gt;apsidal precession&lt;/a&gt;. It seems that during the close flyby of Jupiter's equatorial bulge the extra attraction beyond the overall &lt;span class=&quot;math-container&quot;&gt;$1/r^2$&lt;/span&gt; causes the orbit to advance substantially. The second plot shows the movement of the apoJove over time, showing an apsidal precession of about 31.2 degrees in 477 days, or about &lt;span class=&quot;math-container&quot;&gt;$1.3 \times 10^{-8}$&lt;/span&gt; rads/sec.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;My question is:&lt;/strong&gt; Is this motion actually apsidal precession due to Jupiters non-spherically-symmetric gravitational potential, or is it something else, or even a spacecraft maneuver? &lt;em&gt;If it is indeed precession, where can I find a mathematical expression for the apsidal precession rate?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Mxepq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mxepq.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FAOJi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FAOJi.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; plots of Juno's orbit around Jupiter as described above, data from &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-10T12:54:44.913" LastActivityDate="2021-08-10T12:54:44.913" Title="Juno's original orbit around Jupiter - is this apsidal precession? If so, need expression" Tags="|orbital-mechanics|jupiter|juno|jpl-horizons|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="19182" PostTypeId="2" ParentId="19180" CreationDate="2016-11-27T15:57:09.013" Score="4" Body="&lt;p&gt;Presumably, yes, having a satellite smart phone is possible without fundamental technological innovation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's mobile satellite phones that aren't bigger then late 90s / early 2000s cell phones. Look for Iridium or Thuraya phones.&lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastActivityDate="2016-11-27T15:57:09.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19183" PostTypeId="2" ParentId="19180" CreationDate="2016-11-27T15:57:54.743" Score="11" Body="&lt;p&gt;While power requirements are higher than for regular GSM service, they are not as high as one might think. &#xA;Current satellite telephones use handsets of the size of 2000-era mobile phones and are able to transmit 15 kBit/s to geosynchronous satellites (the Thuraya system). These satellites are more than 30 times farther from Earth than the planned SpaceX system. Therefore, the received power is about 1000 times weaker. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compared to e.g. Iridium, the planned satellite network has the advantage of a much larger number of satellites (4000 compared to 66), which allows the use of highly directional antennas with way better gain, even in the phone itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For high speed data transfer a larger antenna will be required, but lower bandwidth services are well within reach for smart phones, albeit with a slightly changed design to house a larger antenna. I don't have precise numbers, but a bandwidth of well above 100 kBit/s seems well possible when communicating with a satellite in LEO. &lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-11-27T15:57:54.743" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19184" PostTypeId="2" ParentId="19181" CreationDate="2016-11-27T21:25:00.037" Score="2" Body="&lt;p&gt;This probably isn't helpful, but it has an image, so can't be a comment. Try looking at the elliptical elements directly, instead of plotting the path:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5tgzu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5tgzu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the first &quot;few&quot; days, Juno is traveling to Jupiter, so the results won't make much sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do plot the path, consider using the IAU_JUPITER frame, since Juno is effectively in orbit around Jupiter, and J2000 coordinates really aren't appropriate.&lt;/p&gt;&#xA;" OwnerDisplayName="user7073" LastActivityDate="2016-11-27T21:25:00.037" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19185" PostTypeId="2" ParentId="19180" CreationDate="2016-11-28T03:47:01.220" Score="-2" Body="&lt;p&gt;If it is a geosynchronous satellite it would not be a issue. The satellite itself would transmit with sufficient power to connect with the smart phone. The satellite would a high enough gain to receive the smart phone. Any phone within the footprint of the satellite would connect easily. This is because the path of the signal is straight upwards and as a result the signal would not have and terrestrial obstructions or signal propagation issues blocking the signal. The satellites transponder would down link the to the phone company.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A low earth orbiting (LEO) satellite would not be feasible for a any cell phone service due to elliptical nature of the orbit. It would be prone only being with usable for 6-10 minutes during each pass overhead. The satellite would then take 90 mins give or take to complete an orbit of the earth. The satellites orbit would not change. However since it is orbiting Planet Earth which is rotating east to west-90  mins later the satellite would 15 degrees west of you and would not be overhead for hours if not days. Furthermore because of the elliptical orbit the speed as it passes changes. This causes something referred to as Doppler Shift as a result both smart phones and satellite would constantly have to adjust their frequencies to remain in contact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doppler Shift is the effect one hears when stand next to a railway track as a passing train blows the whistle. As it approaches the pitch is higher, in front of you the actual pitch, and as it passes the frequency drops in pitch.&lt;/p&gt;&#xA;" OwnerUserId="14378" LastActivityDate="2016-11-28T03:47:01.220" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19187" PostTypeId="1" AcceptedAnswerId="19190" CreationDate="2016-11-28T11:00:22.473" Score="5" ViewCount="153" Body="&lt;p&gt;The BBC article &lt;a href=&quot;http://www.bbc.com/news/science-environment-38108858&quot; rel=&quot;noreferrer&quot;&gt;Europe's science ministers to decide on ExoMars rover&lt;/a&gt; describes a substantial amount of discussion and soul-searching about the level of funding and capabilities for the rover part of the much larger ExoMars project which involves multiple spacecrafts/devices/launches. The section titled &lt;em&gt;'Dumbed-down' rover&lt;/em&gt; describes continued debate about inclusion of autonomous navigation capability, which can make a substantial improvement on productivity and science if the duration is limited, but will add significant complexity and development costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently it looks like the launch is 4 years away (2020).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm 200% all-in for rovers and robotic exploration. and it needs time and accumulation of real off-world experience, so since spacecraft and rover autonomy is inevitable, the sooner an agency gets started the better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is narrowly focused on the science capability of the ExoMars rover. In addition to the wealth of experience that opperating it will provide, does it have any unique science capabilities beyond being in a new location? A new class of measurement for example?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For that matter, will the chosen location offer any pre-established new science? (not including new science not pre-established - who knows, maybe it will find strawberries this time instead of &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_spherules&quot; rel=&quot;noreferrer&quot;&gt;Blueberries&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-11-29T12:56:52.930" Title="Will the ExoMars rover offer some unique science capability?" Tags="|planetary-science|rovers|exomars|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19188" PostTypeId="2" ParentId="19181" CreationDate="2016-11-28T15:08:52.757" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;My question is: Is this motion actually apsidal precession due to Jupiters non-spherically-symmetric gravitational potential?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, that apsidal precession results from Jupiter's oblateness. Gravitationally, this oblateness effect is expressed in terms of a planet's second dynamic form, or $J_2$. Jupiter's $J_2$ is over ten times that of the Earth due to Jupiter's high rotation rate. Note that Juno's orbit also suffers a bit of nodal precession, as seen in various diagrams of the orbit from above. The average apsidal and nodal precession rates over the course of an orbit due to a planet's oblateness are&#xA;$$\begin{aligned}&#xA;\dot\omega &amp;amp;= \phantom{-} \frac 3 4 J_2 \left(\frac R p\right)^2 n\,(5 \cos^2 i -1) \\&#xA;\dot\Omega &amp;amp;= - \frac 3 2 J_2 \left(\frac R p\right)^2 n \cos i&#xA;\end{aligned}$$&#xA;where $R$ is the equatorial radius of the planet in question, $J_2$ is the planet's second dynamic form, $p=a(1-e^2)$ is the semi-latus rectum, $a$ is the semi-major axis length of the orbit, $e$ is the eccentricity of the orbit, $n$ is the mean motion, and $i$ is the inclination of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://farside.ph.utexas.edu/teaching/celestial/Celestial/node93.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Effect of terrestrial oblateness on artificial satellite orbits&lt;/a&gt; for a derivation of these expressions. This derivation can be found in many other places. The key search terms needed to find these derivations are &quot;Lagrange's planetary equations&quot; and &quot;oblateness&quot;. The key concepts needed to understand these derivations are perturbation techniques and Lagrange's planetary equations.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-11-28T15:20:38.583" LastActivityDate="2016-11-28T15:20:38.583" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19190" PostTypeId="2" ParentId="19187" CreationDate="2016-11-28T18:41:17.117" Score="7" Body="&lt;p&gt;The rover &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/ExoMars/ExoMars_2020_rover&quot; rel=&quot;nofollow noreferrer&quot;&gt;will be the first&lt;/a&gt; that can drill to a depth of 2 m (compared to a few cm for Curiosity). Its instruments for analyzing the drill samples will be more advanced than those on Curiosity. &lt;a href=&quot;http://exploration.esa.int/mars/45103-rover-instruments/&quot; rel=&quot;nofollow noreferrer&quot;&gt;And it&lt;/a&gt; will carry a ground-penetrating radar, that hasn't been done in a Mars rover before.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drill &lt;a href=&quot;http://www.leonardocompany.com/documents/63265270/65638648/Copy+of+ExoMars_LQ_mm07795_.pdf?download_file&quot; rel=&quot;nofollow noreferrer&quot;&gt;is designed to work&lt;/a&gt; in loose soil, but also in frozen soil and soft rock types like sandstone. &lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2790.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;It'll also work in basalt&lt;/a&gt; (from lava). There's no hard cutoff; in harder rock drilling will take longer (15 cm/h in pumice, down to 1 cm/h in harder basalt). The ground-penetrating radar can help assess subsurface hardness so they won't have to waste time drilling into very hard rock. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-11-29T12:56:52.930" LastActivityDate="2016-11-29T12:56:52.930" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19191" PostTypeId="1" AcceptedAnswerId="19192" CreationDate="2016-11-28T20:03:18.017" Score="42" ViewCount="15747" Body="&lt;p&gt;After watching the &lt;a href=&quot;http://channel.nationalgeographic.com/mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;MARS&lt;/a&gt; series on National Geographic I noticed that they sent one of their crew members down a chasm to check for ice. I wondered, why they did not just take a regular commercial drone with them and explore the chasm with that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Immediately after that, I realised, we have only been sending &lt;em&gt;rovers&lt;/em&gt; to explore Mars till now (which was confirmed &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_artificial_objects_on_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;). I think this is far too inconvenient to explore Mars. I understand a rover is more useful if you want to analyse the compounds in the surface of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drones could cover a large area in a short time (depending on sunlight). Rovers have covered 40km &lt;strong&gt;max&lt;/strong&gt; on their own:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/19c1D.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/19c1D.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we are actually searching for present or past life on Mars, drones would come in way easier; they can fly right over large stones, (or into) chasms and other uneveness on the surface of Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my eyes, only energy supply would be a problem; however, this is solvable by landing the drones in sunny spots... which drones can find easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, &lt;strong&gt;what's the deal?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update (2018-05)&lt;/strong&gt;: &lt;a href=&quot;https://www.nasa.gov/press-release/mars-helicopter-to-fly-on-nasa-s-next-red-planet-rover-mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA is sending a helicopter to Mars&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="11358" LastEditorUserId="11358" LastEditDate="2019-04-08T18:35:53.700" LastActivityDate="2019-08-03T20:30:53.173" Title="Why aren't we sending quadcopters to mars?" Tags="|mars|rovers|" AnswerCount="5" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="19192" PostTypeId="2" ParentId="19191" CreationDate="2016-11-28T20:16:20.797" Score="67" Body="&lt;p&gt;Mars has two disadvantages compared to Earth: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it's farther form the Sun, so you get less sunlight. &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.grc.nasa.gov/WWW/K-12/airplane/atmosmrm.html&quot; rel=&quot;noreferrer&quot;&gt;Atmospheric density&lt;/a&gt; on Mars is 0.011 kg/m&lt;sup&gt;3&lt;/sup&gt; (at the surface, at a temperature of 0 °C). On Earth: 1.2 kg/m&lt;sup&gt;3&lt;/sup&gt;, so Mars' atmosphere has 1% of the density of Earth's. &lt;a href=&quot;https://en.wikipedia.org/wiki/Lift_%28force%29&quot; rel=&quot;noreferrer&quot;&gt;Lift is proportional to air density&lt;/a&gt;, so a wing will have 1% of the lift on Mars.  &lt;/li&gt;&#xA;&lt;li&gt;Gravity is 38% that of Earth.   &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A consumer drone that can lift 0.5 kg (total weight of airframe and payload) on Earth, can lift 7.5 grams on Mars. This means you need a large aircraft to accomplish anything.&lt;br&gt;&#xA;(thanks to a comment by Russell Borogove): A drone massing 0.5 kg carrying 0.5 kg of additional payload on Earth must produce 1.0 kg-f of lift to hover; on Mars the same drone produces about 0.01 kg-f of lift (or less) but needs 38% of 0.5 = 0.19 kg-f to get off the ground with no payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This also explains why it hasn't been done yet: only recently have batteries achieved the power density you need to get a drone to lift off at all on Mars. &lt;a href=&quot;https://www.quora.com/Is-it-true-that-battery-energy-density-improves-5-8-per-year&quot; rel=&quot;noreferrer&quot;&gt;In the last 30 years, batteries have doubled in energy density&lt;/a&gt;, with most of that change happening in the last 10 years due to the increased popularity of mobile phones.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XoqM1.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XoqM1.png&quot; alt=&quot;Energy density over time&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until now, helicopter flight on Mars just wasn't possible because an engine and power source powerful enough to fly the aircraft would be too heavy to be lifted off the ground.&lt;br&gt;&#xA;And electric power is the only feasible technology for aircraft on Mars. An internal combustion engine would need oxygen as well as fuel, and you'd have to transport them to the aircraft (too expensive), or make them in-situ (theoretically possible, but has not been demonstrated yet). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA is looking into adding a drone &lt;a href=&quot;http://www.space.com/28360-nasa-mars-helicopter-drone.html&quot; rel=&quot;noreferrer&quot;&gt;to an upcoming mission&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Individual daily flights would be limited to just 3 minutes or so due to power constraints. The helicopter would likely fly as high as 330 feet (100 m) and cover a ground track of about 2,000 feet (600 m) each day, the researchers said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="195" LastEditDate="2017-05-25T15:48:52.773" LastActivityDate="2017-05-25T15:48:52.773" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="19193" PostTypeId="2" ParentId="19191" CreationDate="2016-11-28T21:52:30.213" Score="24" Body="&lt;p&gt;An additional issue that needs to be addressed: such a drone would need to be completely autonomous, or at least have an autonomous controller (rover?) on the Mars surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drone in your example is piloted remotely, but the signal delay between Earth and Mars varies between 4 and 24 minutes (one way), depending on their relative orbital locations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some level, this implies the military-grade hardware you wanted to avoid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To address some of the comments, don't forget that you don't have any GPS for guidance. Your drone needs to be able to take an entire sequence of commands, execute them with complete autonomy in harsh, uncertain conditions, and report results for further analysis. Yes, it's &quot;just software&quot;, but that's handwaving away some pretty significant hurdles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Military grade doesn't mean it has to be developed by the military, but that it has to be developed to a precision that goes beyond most consumer applications.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So here are some of the harsh conditions that such a device would need to deal with:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dust. Dust everywhere.&lt;/li&gt;&#xA;&lt;li&gt;Global dust storms that can last for months&lt;/li&gt;&#xA;&lt;li&gt;Variable winds (during dust storms these have been measured with gusts up to 94 km/h)&lt;/li&gt;&#xA;&lt;li&gt;Low thermal inertia - rapid heating/cooling cycles, around 100 K per daily cycle.&lt;/li&gt;&#xA;&lt;li&gt;Along with that, extreme cold. Different regions are obviously different, but polar temperatures can go as low as -153 C; equatorial regions can reach a balmy 20 C at summer noon, but the nightly lows can fall to -100 or more.&lt;/li&gt;&#xA;&lt;li&gt;No magnetic field, so high cosmic radiation. And no magnetic guidance either.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="17700" LastEditorUserId="17700" LastEditDate="2016-11-29T14:32:20.023" LastActivityDate="2016-11-29T14:32:20.023" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="19194" PostTypeId="2" ParentId="19191" CreationDate="2016-11-28T22:31:49.860" Score="11" Body="&lt;p&gt;&lt;strong&gt;You're looking on the wrong planet.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As others have said, Mars's atmosphere is too thin for a drone to generate sufficient lift. It's also worth pointing out that satellites can do and have done a very good job of mapping planets and moons from orbit, even being able to detect possible water using spectrographic techniques.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus, on the other hand, has a very thick atmosphere, and surface temperatures that are inhospitably high. Flying probes, both of the heavier than atmosphere type and the balloon / airship type have been proposed for that planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trouble is, there's not that much of interest to be seen at height, so some proposed missions have decided to have a surface rover for exploration, too, with the main computers and interplanetary radio flying around in the relative cool of the upper atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an example proposal. We'll see if it becomes reality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.newscientist.com/article/dn7354-to-conquer-venus-try-a-plane-with-a-brain/&quot; rel=&quot;noreferrer&quot;&gt;https://www.newscientist.com/article/dn7354-to-conquer-venus-try-a-plane-with-a-brain/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="6117" LastEditDate="2016-11-28T22:36:54.317" LastActivityDate="2016-11-28T22:36:54.317" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19195" PostTypeId="1" AcceptedAnswerId="19204" CreationDate="2016-11-28T23:50:22.593" Score="0" ViewCount="686" Body="&lt;p&gt;I just read that when Cassini makes it's first close pass of Saturn's F-ring on December 4, it will be the 183rd time the engine fires. I googled Cassini's engine and found this site which has a cool 3D WebGL(?) model of Cassini's engine(s) that you can rotate with your cursor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://saturn.jpl.nasa.gov/engine/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://saturn.jpl.nasa.gov/engine/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I show a screen shot below. The accompanying text says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The main engine is used for spacecraft velocity and trajectory correction changes. To be on the safe side, there are two identical main engines: One is in use and the other is a backup. There are also 16 monopropellant hydrazine thrusters of which eight are prime and eight are backups. The thrusters are used for attitude control and also for small velocity-change maneuvers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;... and...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Bipropellant system- Nitrogen Tetroxide (NTO)/Monomethylhydrazine (MMH)&#xA;  Main (445 Newton) engine for propulsive maneuvers – Burns can be blow-down or pressurized (with Helium)&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;Engines are protected from micrometeoroid particles by articulating cover&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Redundant main engine (never used in flight)&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The two engine nozzles in the 3D model appear to be slightly angled so that they are both axial with the spacecraft center of mass, but the view is not orthographic and the shapes of the nozzles themselves are highly distorted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality, are the two nozzles really oriented so that their axes both intercept the spacecraft center of mass? THough there are thrusters, it seems like you'd want to at least try to minimize the torque of the laterally offset engine(s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IVEgT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IVEgT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;aboveL&lt;/strong&gt; Screenshot of &lt;a href=&quot;https://saturn.jpl.nasa.gov/engine/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this 3D interactive model&lt;/a&gt; of Cassini's engines.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-11-29T04:51:36.250" Title="Does Cassini have one or two identical main engines - if two, how does that work?" Tags="|spacecraft|cassini|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19196" PostTypeId="1" AcceptedAnswerId="19200" CreationDate="2016-11-28T23:58:53.607" Score="0" ViewCount="744" Body="&lt;p&gt;I just read that when Cassini makes it's first close pass of Saturn's F-ring on December 4, it will be the 183rd time the engine fires. I googled Cassini's engine and found this site which has a cool 3D WebGL(?) model of Cassini's engine(s) that you can rotate with your cursor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://saturn.jpl.nasa.gov/engine/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://saturn.jpl.nasa.gov/engine/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edit: here's another: &lt;a href=&quot;https://saturn.jpl.nasa.gov/the-journey/the-spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://saturn.jpl.nasa.gov/the-journey/the-spacecraft/&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I show a screen shot below. The accompanying text says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The main engine is used for spacecraft velocity and trajectory correction changes. To be on the safe side, there are two identical main engines: One is in use and the other is a backup. There are also 16 monopropellant hydrazine thrusters of which eight are prime and eight are backups. The thrusters are used for attitude control and also for small velocity-change maneuvers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;... and...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Bipropellant system- Nitrogen Tetroxide (NTO)/Monomethylhydrazine (MMH)&#xA;  Main (445 Newton) engine for propulsive maneuvers – Burns can be blow-down or pressurized (with Helium)&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;Engines are protected from micrometeoroid particles by articulating cover&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Redundant main engine (never used in flight)&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What does it actually mean that the main engines' &quot;burns can be blow-down or pressurized (with Helium)?&quot; Are both of these &quot;modes&quot; actually used by Cassini for different purposes? If so, how do they differ in performance?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IVEgT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IVEgT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;aboveL&lt;/strong&gt; Screenshot of &lt;a href=&quot;https://saturn.jpl.nasa.gov/engine/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this 3D interactive model&lt;/a&gt; of Cassini's engines.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2021-03-28T18:19:53.120" LastActivityDate="2021-03-29T23:22:30.550" Title="Cassini's main engines' burns &quot;can be blow-down or pressurized (with Helium)&quot; - why?" Tags="|spacecraft|engines|cassini|helium|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19197" PostTypeId="2" ParentId="19195" CreationDate="2016-11-29T00:01:25.177" Score="3" Body="&lt;p&gt;I haven't found a pure orthographic drawing of Cassini, but the illustration on &lt;a href=&quot;https://solarsystem.nasa.gov/basics/bsf11-2.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;this page&lt;/a&gt; seems pretty clear that the thrusters are canted outward, and there's good reason to believe they would be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also a &lt;a href=&quot;https://saturn.jpl.nasa.gov/the-journey/the-spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;full view interactive 3D model&lt;/a&gt; of the spacecraft on the JPL site, using a &quot;longer lens&quot; (thus less perspective distortion) than the engine model you linked; after playing with different viewing angles there I'm satisfied that the thrusters are canted.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-11-29T04:51:36.250" LastActivityDate="2016-11-29T04:51:36.250" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19198" PostTypeId="1" AcceptedAnswerId="19199" CreationDate="2016-11-29T00:02:45.670" Score="5" ViewCount="481" Body="&lt;p&gt;I've &lt;a href=&quot;http://www.theverge.com/2016/11/28/13762994/nasa-cassini-saturn-ring-grazing-orbits-titan&quot; rel=&quot;nofollow noreferrer&quot;&gt;just read&lt;/a&gt; that when Cassini makes its first close pass of Saturn's F-ring on December 4, it will be the 183rd engine burn which sounds like a lot for orbital maneuvers using a spacecraft's main engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is not only excellent engine reliability, it's great from an orbital maneuver perspective as well. Each maneuver requires careful measurements, calculation, simulation, generation of instruction sets, execution, and follow-up measurements. That's a lot of work for the humans!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;So has Cassini's long series of orbit maneuvers using repeated restarts of the main engine some kind of record for the engine?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the New York Times article &lt;a href=&quot;http://www.nytimes.com/2016/11/25/science/cassini-saturn-rings.html?_r=0&quot; rel=&quot;nofollow noreferrer&quot;&gt;How Cassini Will Begin Its Date With Death on Saturn&lt;/a&gt; there is a cool &lt;a href=&quot;https://static01.nyt.com/science/gifs/cassini_600.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;3D rotating GIF&lt;/a&gt; of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is an unattractive 3D plot of the Horizons data, the &lt;a href=&quot;http://www.nytimes.com/2016/11/25/science/cassini-saturn-rings.html?_r=0&quot; rel=&quot;nofollow noreferrer&quot;&gt;NYT article&lt;/a&gt; and &lt;a href=&quot;https://static01.nyt.com/science/gifs/cassini_600.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;associated GIF&lt;/a&gt; are easier on the eyes, but too big to include here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mMsH6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mMsH6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-14T05:10:15.227" LastActivityDate="2017-09-14T14:48:56.277" Title="Is Cassini's 183rd burn; some kind of engine record?" Tags="|spacecraft|orbital-maneuver|cassini|jpl-horizons|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19199" PostTypeId="2" ParentId="19198" CreationDate="2016-11-29T00:27:18.500" Score="4" Body="&lt;p&gt;Hard to say. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cassini's &quot;main engine&quot; is a modernized version of the attitude thrusters used on the Apollo spacecraft: MMH/NTO bipropellant thrusters delivering about 440N thrust. Every maneuver made by the Apollo CSM or LM would have used several of these thrusters in short bursts. It would not surprise me if that amounted to several hundred fires per chamber. &lt;a href=&quot;http://www.rocket.com/files/aerojet/documents/Capabilities/PDFs/Bipropellant%20Data%20Sheets.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The modern version of these thrusters&lt;/a&gt; are rated for ~20000 pulses or 12000 seconds continuous fire. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-14T14:48:56.277" LastActivityDate="2017-09-14T14:48:56.277" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19200" PostTypeId="2" ParentId="19196" CreationDate="2016-11-29T00:40:15.123" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;http://mae-nas.eng.usu.edu/MAE_5540_Web/propulsion_systems/subpages/Rocket_Propulsion_Elements.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sutton&lt;/a&gt;, both blow-down and conventional pressurization use a pressurant (like helium in this case).&lt;/p&gt;&#xA;&lt;p&gt;Conventional systems have a pressure regulator between a pressurant tank and fuel tank, so they yield constant pressure and constant thrust until out of pressurant. In blow-down, per Sutton:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Here the gas is stored under pressure inside the propellant tank together with the propellants. As the gas expands during operation, the tank pressure, the thrust, and chamber pressure decrease steadily down to perhaps 25 to 45% of the initial values. This system is simpler (no separate gas tank and no regulator or gas valve), but the mixture ratio can change slightly during the operation, the total volume occupied by the feed system is larger, and the average specific impulse is somewhat lower.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The thruster itself can operate in either mode simply by virtue of handling a wide range of inlet pressures.&lt;/p&gt;&#xA;&lt;p&gt;Cassini seems to have been designed to switch over to blowdown pressurization in the event of helium regulator problems; the regulator &lt;a href=&quot;http://www.astronomy.com/news/2004/08/cassini-adjusts-its-orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;began leaking in 1997&lt;/a&gt; but redundant valves allowed the system to remain sealed between engine firings.&#xA;When it could no longer maintain regulated pressure, &lt;a href=&quot;https://books.google.com/books?id=KHx45zPLRuYC&amp;amp;pg=PA261&amp;amp;lpg=PA261&amp;amp;dq=cassini%20blowdown&amp;amp;source=bl&amp;amp;ots=_DTIS7zPFc&amp;amp;sig=iYME3dW96NkIWoUSQ65JCiepAjQ&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwi73PudmM3QAhWoqFQKHVEQAQcQ6AEIMjAD#v=onepage&amp;amp;q=cassini%20blowdown&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini switched to blow-down mode&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As this [periapsis raise maneuver of August 2004] was to be the last fully pressurised burn of the primary mission (subsequent firings would be made in 'blowdown' mode, using the residual pressure in the propellant tanks) the latch on the helium flow was left open for 33 minutes after the burn in order to build up the pressure in the propellant tanks, then closed to isolate the system from the leaking outflow regulator.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-03-29T23:22:30.550" LastActivityDate="2021-03-29T23:22:30.550" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="19203" PostTypeId="1" AcceptedAnswerId="19207" CreationDate="2016-11-29T04:11:27.553" Score="5" ViewCount="342" Body="&lt;p&gt;First, I already understand the basics of radio communication at this point, as that is all I've found searching elsewhere on the web for my question. (For example, &lt;a href=&quot;https://space.stackexchange.com/questions/4751/how-does-nasa-get-information-from-space-probes&quot;&gt;How does NASA get information from space probes?&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I really want to know is what a scientist might see at a workplace for tracking probes. Specifically, if something goes wrong with a major spacecraft, does someone see a literal red light start blinking? How can scientists find out when something goes wrong?&lt;/p&gt;&#xA;" OwnerUserId="17707" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-11-30T11:40:42.883" Title="What do scientists see when they track space probes?" Tags="|probe|communication|procedure|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19204" PostTypeId="2" ParentId="19195" CreationDate="2016-11-29T04:30:24.707" Score="7" Body="&lt;p&gt;Not only are they canted, they are gimbaled in two axes. The gimbals are used to fine tune the pointing through the CG that moves as propellants are expended.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-11-29T04:30:24.707" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19206" PostTypeId="2" ParentId="19191" CreationDate="2016-11-29T10:21:28.743" Score="15" Body="&lt;p&gt;To complement Hobbes' answer with a few numbers, a basic comparison of the power requirements for a lifting rotor on Earth and Mars is presented:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The approximate ratio of surface atmospheric pressures of Mars and Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$R_M/R_E = 0.0123$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The approximate ratio of surface gravity, and therefore required thrust on Mars and Earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$T_M/T_E = 0.379$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From actuator disk theory, the power requirements of a rotor scales via the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$P_M/P_E = (T_M/T_E)^{1.5} (R_E/R_M)^{0.5}$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, the same rotorcraft on Earth would need around 2.1 times the power to lift itself on the surface of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side note, to achieve the same power requirements, you'd need to beef up the rotors to 4.42 times the disk area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actuator disk theory is the most basic and idealized analysis you can put forward for this kind of problem. It neglects a whole host of effects like blade drag, compressibility, etc, which all tend to increase the power requirements even more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that as with all aircraft, the more power you need, the bigger the batteries and motors you require to fly. This leads to greater required thrust to lift those bigger motors and batteries which increases power requirements. Extend this argument and you can see that the power requirements and hence the weight of the aircraft tends to snowball. Pop this on a rocket and it tends to snowball the cost of the launch and delivery system too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Added note: another factor to take into account is Mars low sound velocity due to its low temperature, and the fact that to produce enough lift, a rotor would have to rotate at very high speeds. In such case, a great proportion of the rotor would be in a supersonic flow, and in such cases, the lift is generally entirely lost due to the aerodynamic shock. One side note: if we didn't have this phenomenon, the rotor would naturally rotate at much higher speeds for the same power than on Earth since the air resistance from Marsian athmosphere is so low.&lt;/p&gt;&#xA;" OwnerUserId="17713" LastEditorUserId="19335" LastEditDate="2019-08-03T20:30:53.173" LastActivityDate="2019-08-03T20:30:53.173" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19207" PostTypeId="2" ParentId="19203" CreationDate="2016-11-29T12:10:42.143" Score="6" Body="&lt;p&gt;It's best to read a first-hand and also two second-hand accounts of a real red-light spacecraft emergency. &lt;a href=&quot;https://sohowww.nascom.nasa.gov/about/about.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHO&lt;/a&gt;, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar and Heliospheric Observatory&lt;/a&gt; was almost lost - more than once! It was - and still is, thankfully - in a halo orbit around the Sun-Earth Lagrange or Libration L1 point - about a million miles in front of the Earth towards the sun. This is roughly the same general neighborhood as &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSCOVR&lt;/a&gt;, where they both keep an eye on the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Briefly, SOHO's orbit was not stable, so it required regular station keeping maneuvers - short burns every few weeks or months's. It's design and orbit were configured so that it would slowly drift towards the Earth, and a sunward burn - perpendicular to its orbit, would put it back in place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read a short, medium, or long version of the exciting story below. I strongly recommend it!&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://sohowww.nascom.nasa.gov/operations/Recovery/aerospace.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saving Soho&lt;/a&gt; Leonard David, 1999&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://sohowww.nascom.nasa.gov/operations/Recovery/vandenbu.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHO’s Recovery – An Unprecedented Success Story&lt;/a&gt; F.C. Vandenbussche, 1999&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.ieec.cat/hosted/web-libpoint/papers/roberts.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The SOHO Mission L1 Halo Orbit Recovery From the Attitude Control Anomalies of 1998&lt;/a&gt; Craig E. Roberts, 2002&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Roberts 2002 is the definitive report and review of the entire situation, problem, and recovery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, you get a hold of some radio telescopes and listen carefully, analyze doppler shifts, and run computer simulations to compare and try to figure out what is happening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see some more information about SOHO's orbit &lt;a href=&quot;https://space.stackexchange.com/q/16644/12102&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-11-29T12:10:42.143" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19208" PostTypeId="1" AcceptedAnswerId="19224" CreationDate="2016-11-29T13:09:41.113" Score="3" ViewCount="859" Body="&lt;p&gt;I'm starting to see a pattern. Both &lt;a href=&quot;https://space.stackexchange.com/a/19204/12102&quot;&gt;this answer&lt;/a&gt; about Cassini liquid propellant engine and &lt;a href=&quot;https://space.stackexchange.com/a/16678/12102&quot;&gt;this answer&lt;/a&gt; about ion thrusters discuss a particular necessity for gimbaling engines to keep thrust aligned with the spacecraft center of mass (on average).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While thrusters could be used (in at least some cases) to correct for mis-aligned main engine thrust, it's a waste of propellant compared to simply pointing the engine's thrust in a direction that doesn't produce a torque on the spacecraft to begin with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it pretty much a given that all (or at least practically all) deep-space spacecraft have gimbaled engines? Or have some used vectored thrust, or found other solutions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; to clarify, I'm asking about actual existing deep-space spacecraft, not hypotheticals or future plans or ideas. If it's still on the ground but almost finished and funded for launch, that's close enough to being an existing spacecraft for this question.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-01-19T22:00:42.637" Title="Is it pretty much a given that all deep-space spacecraft have gimbaled engines?" Tags="|spacecraft|thrust-vectoring|gimbaled-engines|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19209" PostTypeId="1" AcceptedAnswerId="19517" CreationDate="2016-11-29T16:11:00.623" Score="5" ViewCount="672" Body="&lt;p&gt;My spacecraft knows it wants to add a certain delta-v in a certain direction to its motion, relative to the stars. It will calculate current mass based on propellant usage history, which hasn't been much so far, and it knows the engine's well-characterized thrust, so taken together, it knows that it needs to burn for a time T in order to produce the correct magnitude of delta-v.&lt;/p&gt;&#xA;&lt;p&gt;Its internal contents tend to shift, so that the precise location of the spacecraft's center of mass is unpredictable. Luckily the engine is gimbaled, and it can adjust the gimbals continuously during the maneuver to make sure it keeps the center of mass directly on the thrust axis. It does this by detecting rotation with the star cameras and/or gyros and gimbaling to null the rotation.&lt;/p&gt;&#xA;&lt;p&gt;How does my spacecraft know if the thrust is actually pointing in the right direction? The stars are very far away and there are no handy planets or asteroids nearby, so while it knows attitude, how can it determine that the direction of the delta-v vector is correct?&lt;/p&gt;&#xA;&lt;p&gt;I suppose internal accelerometers (inertial guidance) that have been cross-calibrated with the star cameras would be one solution, and exchange of radio communication and doppler information with Earth would be another.&lt;/p&gt;&#xA;&lt;p&gt;Is that it? Inertial sensors and doppler, or is there any other currently used deep-space spacecraft technology that can measure the direction of delta-v in real time?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JrTDM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JrTDM.png&quot; alt=&quot;spacecraft with uncalibrated engine pointing and uncertain mass distribution&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-10-27T01:15:07.890" LastActivityDate="2020-10-27T01:15:07.890" Title="How can a deep-space spacecraft determine in real time the direction of delta-v?" Tags="|navigation|guidance|gimbaled-engines|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="19211" PostTypeId="1" AcceptedAnswerId="19213" CreationDate="2016-11-29T15:17:59.553" Score="36" ViewCount="4081" Body="&lt;p&gt;A common point of drama in science fiction is the loss of a tether during a spacewalk threatening to (or actually causing) send an unlucky astronaut hurtling into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality, is such a thing actually an unrecoverable situation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If an astronaut is tethered to a spacecraft and the line is severed, the astronaut becomes a new orbiting body.  The energy released from a snapping tether can't be too considerable given the forces usually used to move spacecraft around.  If some kind of explosive decompression (assuming our hero is properly suited) provides a considerable ejection force, I can see recovery being a little more difficult, but probably still not impossible.  This of course assumes that a severed tether is not his only form of life support.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the astronaut's point of view, the craft may drift slowly away (and vice versa) but both are still cycling around the planet below.  Assuming the astronaut could keep calm and keep his wits about him and isn't too impacted by atmospheric drag, wouldn't it be possible to do some small pulses of thrust on either a small propulsion unit with the astronaut, or a thruster on the craft such that they can rendezvous on the next or a future orbit and let him climb back aboard?&lt;/p&gt;&#xA;" OwnerUserId="17722" OwnerDisplayName="Omaha" LastEditorUserId="26446" LastEditDate="2021-07-05T04:06:46.270" LastActivityDate="2021-07-05T04:59:42.830" Title="The Loose Astronaut" Tags="|orbit|eva|astronaut-propulsion|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19212" PostTypeId="2" ParentId="19211" CreationDate="2016-11-29T15:34:45.707" Score="4" Body="&lt;p&gt;You're right, losing a spacewalk tether by itself isn't an unrecoverable situation. In fact, on a normal spacewalk, anytime the tether isn't taut, it's effectively the same as not having it. With a bit of maneuvering, it's certainly possible to recover an extra-vehicular object, without having to reel it in by tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem, at least in movies, is that when the tether breaks, there's usually something else that's gone very wrong - an explosion, impact by space junk, etc. Now there's a serious situation that demands the attention of the crew on board, and may affect maneuverability or other spacecraft systems. If the person left outside isn't brought on board quickly, they could drift away, have their orbit decay, lose life support, or otherwise become much more difficult to recover.&lt;/p&gt;&#xA;" OwnerUserId="13714" OwnerDisplayName="Matt" LastActivityDate="2016-11-29T15:34:45.707" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19213" PostTypeId="2" ParentId="19211" CreationDate="2016-11-29T16:37:04.860" Score="28" Body="&lt;p&gt;This is a great question. I wanted to provide an answer which cited some specific, real-world situations. Currently the only people in space are those aboard the International Space Station. If anyone could potentially get into a scenario as you describe in your question, it would be them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, on spacewalks, a huge number of safety procedures are used to ensure the safety of the astronaut. One primary component is the use of tethers to keep the astronaut tied to the actual space station. If this tether were to break, there are secondary means of keeping the astronaut from drifting into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the astronaut comes untethered, his primary concern is to remove any velocity he may have which is taking him away from the space station. The cause of this velocity can depend on a variety of factors. Often in movies, its because something exploded and the astronaut is flying away from his station/shuttle quite rapidly. In a scenario like this, the astronaut can use his &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_Aid_For_EVA_Rescue&quot; rel=&quot;noreferrer&quot;&gt;SAFER unit&lt;/a&gt;. This is a pack worn by astronauts during extra-vehicular activities (EVA) which is designed for just such an emergency. It has thrusters on it capable of slowing down the astronaut and sending him back towards the station/shuttle. These thrusters are of course limited and can only handle changing the astronaut's velocity by about 3 m/s. If they're moving faster than that, they need an additional way to slow themselves down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, for whatever reason they can't use the thrusters, or it isn't enough, they're likely going to have to rely on Newton's Third Law &amp;mdash; for every action, there is an equal and opposite reaction. What this means is that they can slow themselves down by throwing any objects they have on hand. If they happened to have a wrench in their hand when the unfortunate event occurred, they could throw the wrench in the direction they were traveling. The act of ejecting the wrench in a particular direction would cause them to slow their motion in that direction. At this point, the SAFER unit might still be useful, even if it is out of propellant. They can always throw that! If they need to, they can try throwing anything they can safely dispose of which can help slow their speed and bring them back to the space station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one final saving grace that I can think of. The ISS in particular has robotic arms on the outside. These arms are meant for doing space work by those inside the ISS and include things like performing repairs and helping ships to dock. It is quite possible that an arm could be used to grab onto a free-floating astronaut, or else provide the astronaut with something to grab onto. This of course relies on the fact that 1) the astronaut is close enough, and 2) the arm has not been destroyed during the accident that sent the astronaut into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So ultimately, a free-floating astronaut who has been untethered is not in an ideal situation, but certainly not unrecoverable. There are several redundant mechanisms which the astronaut can use to save themselves in emergency situations. Redundancy is the key to safe space travel!&lt;/p&gt;&#xA;" OwnerUserId="17765" OwnerDisplayName="zephyr" LastEditorUserId="6816" LastEditDate="2016-11-30T10:06:15.207" LastActivityDate="2016-11-30T10:06:15.207" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19214" PostTypeId="2" ParentId="18693" CreationDate="2016-11-29T18:07:17.960" Score="3" Body="&lt;p&gt;It looks like Juno has a much lower apoapsis (e.g., &lt;a href=&quot;http://spaceflight101.com/juno/juno-mission-trajectory-design/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spaceflight101.com/juno/juno-mission-trajectory-design/&lt;/a&gt;) than did Galileo (e.g., &lt;a href=&quot;http://solarsystem.nasa.gov/galileo/mission/journey-orbital.cfm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://solarsystem.nasa.gov/galileo/mission/journey-orbital.cfm&lt;/a&gt;). However, Juno is only scheduled to undergo 36 orbits while Galileo went through 34, so similar but the higher apoapsis of Galileo resulted in a longer duration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit/Updates&lt;/strong&gt;&lt;br&gt;&#xA;It is not clearly stated on the &lt;a href=&quot;https://www.missionjuno.swri.edu&quot; rel=&quot;nofollow noreferrer&quot;&gt;mission website&lt;/a&gt; or elsewhere (that I could find) why Juno would last for a shorter period of time than &lt;a href=&quot;http://www.jpl.nasa.gov/missions/galileo/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo&lt;/a&gt; but I have a few guesses, in no particular order:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Galileo did not rely upon solar arrays which degrade under such harsh radiation environments.&lt;/li&gt;&#xA;&lt;li&gt;Galileo used older electronics (e.g., much larger transistors) which by their size alone makes them more robust to harsh radiation environments.&lt;/li&gt;&#xA;&lt;li&gt;The larger apoapsis, as stated above, would result in a longer mission duration than Juno for the same number of Jovian orbits.&lt;/li&gt;&#xA;&lt;li&gt;(This one is purely speculation.) Fuel may also be a constraint since one of the primary mission requirements is a deorbit that will cause Juno to burn up in the Jovian atmosphere.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I should also note that many of Galileo's instruments had failed (due to radiation damage, among other things) prior to the deorbit into the Jovian atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastEditorUserId="12508" LastEditDate="2016-12-02T14:03:27.157" LastActivityDate="2016-12-02T14:03:27.157" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19215" PostTypeId="2" ParentId="19211" CreationDate="2016-11-29T18:10:48.397" Score="20" Body="&lt;p&gt;Once the astronaut &amp;amp; space vehicle part ways, they're on two separate orbits.  If the force that separated them is impulsive (instantaneous force in a single direction - as in pushing off the spacecraft and forgetting your tether) those orbits intersect at the point of departure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can wait 1 rev (about 90 minutes in LEO) you will cross the spacecraft orbit at this same point with the same relative velocity that you left it.  In fact, if you departed perpendicular to the orbit plane (cross-track), such that you created a plane change, you'll come back in just half a rev.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as David points out, if your departure maneuver had an in-track component such that your orbital period changed relative to the spacecraft, you will cross its path ahead of or behind the vehicle.  In this case, the spacecraft will need to perform a phasing maneuver to sync up.  The best place to do this is likely at the intersection of the orbit planes, so you might have to wait two revs or more depending on how much delta v your crew is willing to spend to get you back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is assuming unperturbed 2 body orbits - if there's a large drag difference between the space vehicle and the astronaut you could end up with a bigger difference when you come back around.  Similarly, J2 will push things around a bit if the the eccentricities or inclinations of the orbits are significantly different.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="4709" LastEditDate="2016-11-29T21:20:43.493" LastActivityDate="2016-11-29T21:20:43.493" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19217" PostTypeId="2" ParentId="19209" CreationDate="2016-11-29T19:19:35.247" Score="1" Body="&lt;p&gt;Generally a spacecraft will use a gyroscopic &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_navigation_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;inertial&lt;/a&gt; platform &amp;mdash; a set of powered gyroscopes holding a fixed orientation in space, mounted in nested gimbals allowing the spacecraft to rotate around it. The relative rotation of the gimbals is measured several times a second to determine the direction the craft is pointing and how fast the direction is changing, which lets the guidance system adjust the engine gimbal to keep the craft on course in a continuous feedback loop.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="6816" LastEditDate="2016-12-20T20:38:11.333" LastActivityDate="2016-12-20T20:38:11.333" CommentCount="22" ContentLicense="CC BY-SA 3.0" />
  <row Id="19218" PostTypeId="2" ParentId="19208" CreationDate="2016-11-29T20:02:54.467" Score="3" Body="&lt;p&gt;For deep space craft, another alternative to a gimbaled main engine is to use differential thrust with multiple engines; four small thrusters in square or diamond layout gives straightforward two-axis steering by firing them in different duty cycles (or throttle levels). If no large-delta-v maneuvers are needed (e.g. the spacecraft is performing flyby rather than orbital insertion) this can be done with the attitude control thrusters and there might be no &quot;main engines&quot; at all.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-11-29T20:34:45.087" LastActivityDate="2016-11-29T20:34:45.087" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19219" PostTypeId="1" CreationDate="2016-11-29T21:07:49.857" Score="3" ViewCount="201" Body="&lt;p&gt;It seems like you at least need the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A self-drilling wellhead to reach the subsurface water, storage tank&lt;/li&gt;&#xA;&lt;li&gt;Some process for refining the regolith into the optimal composition, storage tank&lt;/li&gt;&#xA;&lt;li&gt;A mechanism for composing the concrete into a structure.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;To support all three, you need #4, a power source. I'm guessing probably an RTG. What I'm asking about here mostly is #2 - I don't know beans about concrete on either planet - what would be the process of collecting and processing that material, and what else have I overlooked here?&lt;/p&gt;&#xA;" OwnerUserId="5965" LastActivityDate="2016-11-29T21:29:59.520" Title="What series of devices would you need to deliver to the Martian surface to manufacture concrete entirely in-situ?" Tags="|mars|colonization|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19220" PostTypeId="2" ParentId="19219" CreationDate="2016-11-29T21:24:21.333" Score="3" Body="&lt;p&gt;Concrete is composed of Portland cement, water, and aggregate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The manufacture of Portland cement is an energy and space intensive process.  To wit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The most common way to manufacture portland cement is through a dry method. The first step is to quarry the principal raw materials,&#xA;  mainly limestone, clay, and other materials. After quarrying the rock&#xA;  is crushed. This involves several stages. The first crushing reduces&#xA;  the rock to a maximum size of about 6 inches. The rock then goes to&#xA;  secondary crushers or hammer mills for reduction to about 3 inches or&#xA;  smaller.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed, and fed to a cement kiln.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The cement kiln heats all the ingredients to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special&#xA;  firebrick. Kilns are frequently as much as 12 feet in diameter—large&#xA;  enough to accommodate an automobile and longer in many instances than&#xA;  the height of a 40-story building. The large kilns are mounted with&#xA;  the axis inclined slightly from the horizontal.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The finely ground raw material or the slurry is fed into the higher&#xA;  end. At the lower end is a roaring blast of flame, produced by&#xA;  precisely controlled burning of powdered coal, oil, alternative fuels,&#xA;  or gas under forced draft.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;As the material moves through the kiln, certain elements are driven&#xA;  off in the form of gases. The remaining elements unite to form a new&#xA;  substance called clinker. Clinker comes out of the kiln as grey balls,&#xA;  about the size of marbles.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Clinker is discharged red-hot from the lower end of the kiln and&#xA;  generally is brought down to handling temperature in various types of&#xA;  coolers. The heated air from the coolers is returned to the kilns, a&#xA;  process that saves fuel and increases burning efficiency.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;After the clinker is cooled, cement plants grind it and mix it with&#xA;  small amounts of gypsum and limestone. Cement is so fine that 1 pound&#xA;  of cement contains 150 billion grains.  The cement is now ready for&#xA;  transport to ready-mix concrete companies to be used in a variety of&#xA;  construction projects.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://www.cement.org/cement-concrete-basics/how-cement-is-made&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems unlikely that limestone and clay are available on Mars, not to mention the huge facilities.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probably best to come up with a different substance to build your colony out of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: experiments have been done on making cement from lunar soil simulants. The process of producing the cement is &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunarcrete&quot; rel=&quot;nofollow noreferrer&quot;&gt;apparently the same though&lt;/a&gt;, although it has only been carried out on a tiny scale.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-11-29T21:29:59.520" LastActivityDate="2016-11-29T21:29:59.520" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19221" PostTypeId="2" ParentId="19211" CreationDate="2016-11-29T22:36:31.150" Score="5" Body="&lt;p&gt;If your loose astronaut has no thruster capability of her own, all may not be lost if she was operating from a capable spacecraft with a crew member aboard. Any of Gemini, Apollo, or the US space shuttle were able to make small translational maneuvers under pilot control. At tethered distances the spacewalker and pilot should be able to see each other, and absent major damage to the craft, recovery should be possible if nothing else goes wrong.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-07-05T04:59:42.830" LastActivityDate="2021-07-05T04:59:42.830" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19222" PostTypeId="1" CreationDate="2016-11-30T02:53:15.677" Score="1" ViewCount="1165" Body="&lt;p&gt;Only earth has north/south poles. This is not found in space. How can electronics work outside our earth?&lt;/p&gt;&#xA;" OwnerUserId="17739" LastEditorUserId="6816" LastEditDate="2016-11-30T05:47:33.817" LastActivityDate="2016-11-30T07:55:32.137" Title="If electronics are magnetic, how do they work in space?" Tags="|electronics|" AnswerCount="2" CommentCount="4" ClosedDate="2016-11-30T16:39:13.547" ContentLicense="CC BY-SA 3.0" />
  <row Id="19223" PostTypeId="2" ParentId="19209" CreationDate="2016-11-30T05:53:30.057" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How does my spacecraft know if the thrust is actually pointing in the right direction? The stars are very far away and there are no handy planets or asteroids nearby.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That the stars are very far away is a huge advantage. Parallax will not be an issue for a star tracker that looks for distant bright stars, at least not within the solar system. The inherent errors in even state-of-the-art star trackers are significantly larger than the errors induced by parallax. Parallax might become an issue in the distant future, but by then our children's children's spacecraft will be using quasar trackers.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;So while it knows attitude, how can it determine that the direction of the delta-v vector is correct?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In many cases, the spacecraft does not know that the direction of the delta-v vector is correct. Their flight software is rather primitive. Deep space probes typically receive delta-V commands from Earth. These spacecraft have rather limited autonomy and intelligence, and simply execute the commands transmitted to them. Suppose the Applied Physics Lab had mistaken commanded the &lt;em&gt;New Horizons&lt;/em&gt; spacecraft to perform a maneuver that would have made that spacecraft smack into Pluto. The spacecraft would have done exactly what it had been commanded to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Making a spacecraft truly autonomous is a task for the next generation of rocket scientists. For now, autonomy is limited to those phases of flight where intervention from Earth is impossible. This includes automated rendezvous; automated entry, descent and landing; and autonomous pointing (e.g., &lt;em&gt;New Horizons&lt;/em&gt; pointing towards Pluto).  In all of these cases, the vehicle will have more than the standard set of navigation sensors, and the flight software will be rather complex.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-11-30T05:53:30.057" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19224" PostTypeId="2" ParentId="19208" CreationDate="2016-11-30T06:20:53.080" Score="5" Body="&lt;p&gt;The Mars Reconnaissance Orbiter and the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft have six non-gimbaled main engines, relying on smaller attitude control thrusters to counter any torques.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-11-30T06:20:53.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19225" PostTypeId="2" ParentId="19222" CreationDate="2016-11-30T06:50:19.250" Score="5" Body="&lt;p&gt;Electronics do not need a magnetic field to work. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-30T06:50:19.250" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19226" PostTypeId="2" ParentId="19222" CreationDate="2016-11-30T07:55:32.137" Score="23" Body="&lt;p&gt;Almost every premise of your question is wrong, unfortunately. Earth is not the only celestial body with a magnetic field &amp;mdash; Jupiter, Saturn, and essentially every star (including the Sun), to name a few, all have powerful magnetospheres and north and south magnetic poles accordingly. (Mars and Venus are examples of planets that do &lt;em&gt;not&lt;/em&gt; have magnetospheres of significant strength.) And Earth's magnetosphere extends far enough out to affect many satellites anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Hobbes already mentioned, electronics do not require an external magnetic field to function. Instead, they use internally generated electromagnetic effects, such as induction, semiconductivity, capacitance, and so forth. The presence of an external magnetic field doesn't influence those to any real extent, unless it's rapidly varying or ridiculously strong (e.g. close to a neutron star or some similarly exotic environment). So in a basic sense, not much really changes, and it's possible, with a bit of work, to use off-the-shelf components to do the same job out in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there's a subtler way in which earth-bound electronics &lt;em&gt;do&lt;/em&gt; benefit from Earth's magnetic field. It traps much of the inbound cosmic radiation and solar wind, which are charged particles moving at high speed that can cause errors in calculation if they strike electronic components. (The atmosphere catches most of the rest.) So satellite components must be designed to tolerate higher levels of radiation than the equivalent on Earth, even if they're in a low earth orbit (LEO) where the magnetosphere still protects them, but especially if they operate in geostationary orbit (GEO) or farther out, outside the majority of geomagnetic radiation protection. This includes all interplanetary and lunar probes.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2016-11-30T07:55:32.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19227" PostTypeId="1" CreationDate="2016-11-30T08:58:07.140" Score="1" ViewCount="692" Body="&lt;p&gt;There are some proposed methods like &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;extracting oxygen from ice in polar caps &lt;/li&gt;&#xA;&lt;li&gt;getting it from the CO2 found in Mars' atmosphere &lt;/li&gt;&#xA;&lt;li&gt;or ...&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But let's say we finally find a feasible method to produce large scale amounts of oxygen on mars and we successfully did that, will the air then become breathable for humans and stay like that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By this question I mean in detail&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is there anything other than Oxygen we need to consider to be able to remove our masks?&lt;/li&gt;&#xA;&lt;li&gt;Will environment hold on to the created oxygen or will it soon be gone into other forms not breathable or out into the space altogether?&lt;/li&gt;&#xA;&lt;li&gt;Might that oxygen have any harmful impact on the planet to harm humans in other ways?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="17742" LastEditorUserId="17742" LastEditDate="2016-11-30T09:04:48.553" LastActivityDate="2016-11-30T12:40:50.917" Title="Will Mars ever become breathable for humans?" Tags="|mars|atmosphere|oxygen|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19228" PostTypeId="2" ParentId="19203" CreationDate="2016-11-30T11:40:42.883" Score="3" Body="&lt;p&gt;This depends on the space probe, or more exactly, the design of the software that's used in the operations center. These days software like LabView is often used to convert the incoming data into a presentation. When data falls outside its normal values, the presentation can be modified, including blinking red lights if desired.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this depends on how well you can define &quot;data falls outside its normal values&quot;. When a tank pressure suddenly drops from $value to 0, that's a pretty good indication of a problem. But when the error is more subtle, you may not be able to use automated means to detect that there is a problem. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-11-30T11:40:42.883" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19229" PostTypeId="2" ParentId="19227" CreationDate="2016-11-30T12:19:46.057" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is there anything other than Oxygen we need to consider to be able to&#xA;  remove our masks?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I then take an approximate 20 kPa oxygen level as a given, and yes, there are other things to consider as well. For instance, the current temperature on Mars is about -55ºC (218 K) but varies a lot. Such temperatures are not healthy for your lungs. In the process of changing the atmosphere the temperature will certainly change, but most likely it would still be considerably lower than on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another important factor is the level of carbon dioxide. Humans can sense the presence of $CO_2$ Already at the 1 kPa partial pressure level, 2-3 kPa causes major discomfort and nausea, and levels above 5 kPa are generally deadly over time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Removing our masks will also expose us to small particles and dust in the Martian environment. Those particles can be considerably sharper and cause more damage due to the fact that the processes rounding and degrading them on Earth are mostly not present on Mars.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Will environment hold on to the created oxygen or will it soon be gone&#xA;  into other forms not breathable or out into the space altogether?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Free oxygen is a very reactive substance, and the amount of it will shrink over time in &lt;em&gt;any&lt;/em&gt; environment if there is no natural process replenishing it. In particular, the ground will absorb some when previously unexposed  substances begin to oxidise. The solar wind is also causing atoms in the Martian atmosphere to escape, including oxygen. However, the timescale here is millions of years.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Might that oxygen have any harmful impact on the planet to harm humans&#xA;  in other ways?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Harmful for who? There is no current human usage of the planet Mars, so it is difficult to predict what harm the oxygen can cause in the future. One important aspect is the still unknown question regarding the existence of life on Mars. The sudden presence of an oxygen atmosphere will most likely kill off any possible life forms, and also effectively erase any trace of previous existence. As an engineering problem, you have now also introduced rust and corrosion of metal structures.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2016-11-30T12:19:46.057" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19230" PostTypeId="2" ParentId="19227" CreationDate="2016-11-30T12:40:50.917" Score="3" Body="&lt;p&gt;Short answer: It's unlikely &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Longer answer: Although humans can survive for short periods at low atmospheric pressure, long term exposure is harmful in a number of ways. Oxygen is a corrosive, and at high concentrations and pressures is harmful to humans. Plus, if you want any ecology at all you'd need water, CO2, and large quantities of nitrogen so in order for humans to survive long term you'd need a similar composition of atmosphere to the earth, which is mostly nitrogen, at a reasonably high pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would require billions, maybe trillions of tons of these gases, and where will they come from? Mars is not likely to have them in the quantities required, and even if it does it would require incredible amounts of energy to be expended using technologies we can't begin to design much less transport in order to free them from their sources. It would also take a very long time to do it. If you have the technology to do all that chances are you don't really need to do it on Mars. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-11-30T12:40:50.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19232" PostTypeId="1" AcceptedAnswerId="19235" CreationDate="2016-11-30T15:40:44.737" Score="40" ViewCount="11018" Body="&lt;p&gt;In most American rocket launches, the &quot;counter&quot; will say: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;3,2,1 [awkward pause], and liftoff....&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So what truly happens at T-0? Is there any standardized event that this time indicates?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the pause only the delay between engine's ignition and moment of liftoff?&lt;/p&gt;&#xA;" OwnerUserId="17599" LastEditorUserId="5851" LastEditDate="2016-11-30T19:23:34.867" LastActivityDate="2018-04-30T08:35:53.357" Title="What &quot;actually&quot; happens at T-minus-0" Tags="|launch|rockets|launch-sequence|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19234" PostTypeId="1" CreationDate="2016-11-30T15:58:06.650" Score="6" ViewCount="2427" Body="&lt;p&gt;When a big volcano erupts like Krakatoa or Mt St Helens does it send steam, smoke, dust, rocks high enough and fast enough to escape Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could any lightweight matter from a volcano actually get enough of a push to escape into outer space?&lt;/p&gt;&#xA;" OwnerUserId="17752" LastEditorUserId="415" LastEditDate="2016-12-01T09:00:26.173" LastActivityDate="2016-12-04T09:03:52.273" Title="Could an Earth volcano impart escape velocity on lightweight matter?" Tags="|gravity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19235" PostTypeId="2" ParentId="19232" CreationDate="2016-11-30T16:10:42.953" Score="59" Body="&lt;p&gt;&lt;strong&gt;T minus zero appears to &lt;em&gt;generally&lt;/em&gt; indicate the moment of booster liftoff, but can also indicate something different. To borrow Cort Ammon's wording, it's an action or event that results in &lt;em&gt;a substantial change in the amount of control you have over the situation.&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking &lt;a href=&quot;https://space.stackexchange.com/a/1310/415&quot;&gt;the countdown for the Falcon 9&lt;/a&gt; as an example, T-00:00:00 is the instant of lift-off; in other words, when the rocket has started rising ever so slightly off the launch pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other alternatives. For example, the Apollo &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V#Lunar_mission_launch_sequence&quot; rel=&quot;noreferrer&quot;&gt;Saturn V launch sequence&lt;/a&gt; had first stage engine ignition beginning at T minus 8.9 seconds, and the actual engine ignition procedure took almost a second. There, &lt;a href=&quot;https://space.stackexchange.com/a/17774/415&quot;&gt;T minus 0 indicated release of the booster hold-down arms&lt;/a&gt;. If I recall correctly, the hold-down arms release was required to be simultaneous to the tune of something like 50 ms, to ensure that the rocket stayed upright. Releasing the hold-down arms allowed the rocket to start climbing, but wasn't &lt;em&gt;by itself&lt;/em&gt; the definition of liftoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle#Launch&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle launch sequence&lt;/a&gt; had the main engine ignition beginning at T-6.6 seconds, launch thrust required at T-3 seconds, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;noreferrer&quot;&gt;solid rocket booster (SRB)&lt;/a&gt; ignition at T-0 seconds. With the Shuttle, the SRBs were required to generate sufficient thrust for liftoff, but also &lt;a href=&quot;https://space.stackexchange.com/a/1379/415&quot;&gt;committed the spacecraft to at least 127 seconds of powered flight&lt;/a&gt; until SRB flameout. &lt;a href=&quot;https://space.stackexchange.com/a/10605/415&quot;&gt;It was not possible to safely jettison&lt;/a&gt; the SRBs while they were providing a significant amount of thrust, and to try would likely have resulted in a loss of the vehicle (and crew).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that there is a huge number of things that happen both before and after liftoff. The mission doesn't begin at T minus zero; one can even argue that &lt;a href=&quot;https://space.stackexchange.com/a/19420/415&quot;&gt;the mission begins years before any launch date has been picked&lt;/a&gt;, and the pre-launch countdown is quite specific long before the launch day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The exact event defining T-0&lt;/strong&gt; &lt;em&gt;is&lt;/em&gt; &lt;a href=&quot;https://space.stackexchange.com/questions/1295/what-is-the-purpose-of-having-a-countdown-during-a-rocket-launch?rq=1#comment2588_1301&quot;&gt;somewhat arbitrary&lt;/a&gt;, and different spacecraft operators can easily pick a different event to define it. Besides liftoff, I can certainly see value in defining T-0 as, for example, disconnection of some specific launch tower &lt;a href=&quot;https://en.wikipedia.org/wiki/Umbilical_cable&quot; rel=&quot;noreferrer&quot;&gt;umbilical&lt;/a&gt; or, in the case of manned spacecraft, sealing of the crew access hatch. In reality, T-0 is a point of synchronization, and it's advantageous to keep it very close to liftoff as well as easy to define to a high degree of precision, but it is not really much more than that.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2016-12-14T15:45:21.317" LastActivityDate="2016-12-14T15:45:21.317" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19236" PostTypeId="2" ParentId="19234" CreationDate="2016-11-30T16:33:06.490" Score="13" Body="&lt;p&gt;No way. Escape velocity from Earth is ~11km/s. Volcanic eruptions can be supersonic in the case of &lt;a href=&quot;https://en.wikipedia.org/wiki/Volcanic_pipe&quot; rel=&quot;noreferrer&quot;&gt;Volcanic Pipes&lt;/a&gt;, but that's only ~1000m/s, not even 1% of the kinetic energy required to escape Earth's gravity.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2016-11-30T16:33:06.490" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19237" PostTypeId="2" ParentId="19232" CreationDate="2016-11-30T17:09:05.543" Score="11" Body="&lt;p&gt;The awkward pause is mostly due to the commentator. He counts along with an on-screen counter until T-0. Then he waits until he sees the rocket move, &lt;em&gt;or he hears ground control confirm liftoff&lt;/em&gt;, before he says 'liftoff'. Generally, at T-0, the hold-down clamps are released and the rocket begins moving, but it may be a few seconds before that's visible to the naked eye.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been rockets where liftoff occurs a few seconds after T-0, though. And others where T-0 is the planned start time, but the rocket's control system is free to delay liftoff until all the engines are running at nominal values. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-12-15T10:28:45.387" LastActivityDate="2016-12-15T10:28:45.387" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19241" PostTypeId="2" ParentId="19234" CreationDate="2016-12-01T14:31:10.207" Score="6" Body="&lt;p&gt;&lt;strong&gt;It's not possible.&lt;/strong&gt; Escape velocity is too high (11.186 km/s), an eruption cannot impart such speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if such velocity could be achieved, the possible object thrown out (e.g. a rock or a shard) would just disintegrate because of the heat generated by the friction of the air.&lt;/p&gt;&#xA;" OwnerUserId="17775" LastActivityDate="2016-12-01T14:31:10.207" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19242" PostTypeId="1" AcceptedAnswerId="19246" CreationDate="2016-12-01T16:11:49.933" Score="2" ViewCount="764" Body="&lt;p&gt;I took this pic on 16 th November 2016 two days after supermoon using a small telescope and accidentally captured some sort of satellite pic along with the moon.i am curious to know howmany such satellites are currently orbiting moon.&lt;img src=&quot;https://i.stack.imgur.com/SRoZ2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="17778" LastEditorUserId="25" LastEditDate="2016-12-02T13:34:28.120" LastActivityDate="2016-12-02T21:13:03.150" Title="curious to know about artificial satellites orbiting moon" Tags="|artificial-satellite|the-moon|orbit|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19243" PostTypeId="2" ParentId="19242" CreationDate="2016-12-01T18:17:35.313" Score="7" Body="&lt;p&gt;Unless you had a VERY powerful telescope, you didn't capture an image of a satellite orbiting the moon. From &lt;a href=&quot;https://space.stackexchange.com/q/221/25&quot;&gt;Which artificial satellites in lunar orbit are currently active?&lt;/a&gt; we have 3 spacecraft that are known to be orbiting the Moon, and the largest of these is LRO. The solar panels are the largest part, at &lt;a href=&quot;http://pds-geosciences.wustl.edu/missions/lro/docs/lro_insthost.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;10.7 m^2 area&lt;/a&gt;. It's too small to even be seen by Hubble, which can only see an object the distance of the moon about 200m in one dimension! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, the orbiting period of a satellite would be about 113 minutes, as LRO's period is. The orbital of the object would be about 1/16th of an orbit, so you would have had to have an exposure of about 7 minutes to capture that!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More likely you are either seeing an aircraft, or possibly a satellite orbiting the Earth. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-12-02T21:13:03.150" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19244" PostTypeId="2" ParentId="19242" CreationDate="2016-12-01T23:41:52.840" Score="3" Body="&lt;p&gt;This is what a satellite in orbit around the moon &lt;em&gt;actually looks like in an actual photograph&lt;/em&gt;. After careful calculations and preparation, LRO was able to photograph LADEE while both of them were orbiting the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft were separated by only 9 kilometers. By comparison, on the evening of your photograph the moon was about 361,000 kilometers from the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uwSPf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uwSPf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Photograph of LADEE taken by LRO, both in orbit around the moon, from &lt;a href=&quot;https://lpod.wikispaces.com/January+30%2C+2014&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, seemingly cropped, from &lt;a href=&quot;http://lroc.sese.asu.edu/posts/736&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-12-01T23:41:52.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19246" PostTypeId="2" ParentId="19242" CreationDate="2016-12-02T07:12:18.303" Score="5" Body="&lt;p&gt;For any camera, the Moon is approximately 0.5 degrees across.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Measuring the photo on my monitor, I find the Moon is 10.5 cm across, while the &quot;satellite&quot; is 3.0 cm from top to bottom.  So by simple ratio, the &quot;satellite&quot; is 0.143 degrees across. This converts to a much more useful 0.0025 radians.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if the &quot;satellite&quot; was at lunar distance, its length would be:$$Length_{lunar}=0.0025*384000=960 \text{ km}$$If the &quot;satellite&quot; is in geosynchronous earth orbit, its length would be:$$L_{GEO}=0.0025*36000=90 \text{ km}$$If the &quot;satellite&quot; is in low earth orbit, its length would be:$$L_{LEO}=0.0025*400=1 \text{ km}$$If the &quot;satellite&quot; is a plane at around $10,000$ feet, its length (wingspan) would be:$$L_{Plane}=0.0025*3000=7.5 \text{ m}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which of these seem even remotely possible?&lt;/p&gt;&#xA;" OwnerUserId="895" LastActivityDate="2016-12-02T07:12:18.303" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19247" PostTypeId="2" ParentId="18918" CreationDate="2016-12-02T20:01:41.117" Score="2" Body="&lt;p&gt;A computer that must be usable when the user is inside a space suit? Build the computer into the suit. That way the computer can take advantage of the existing life support system (power, cooling, communications), you eliminate a loose item by building it in (important: anything loose can and will float away/get its tether tangled up), and you can use controls e.g. inside the gloves instead of bulky controls built to be used while wearing gloves.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;the biggest remaining issue is radiation. The computer must be radiation-hardened or radiation-tolerant. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-12-02T20:01:41.117" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19248" PostTypeId="1" AcceptedAnswerId="19249" CreationDate="2016-12-02T23:25:35.460" Score="1" ViewCount="415" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NLBhD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NLBhD.jpg&quot; alt=&quot;NEOCam&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Profoundly not-to-scale illustration of NEOCam in an orbit around the Sun-Earth libration point L1, about 1.5 million kilometers from Earth. Presumably Sun-shield and Earth-shield block light (both infrared and visible) from the Sun and the Earth in order for the instrument to work at cold temperature necessary to detect the faint infrared light radiated from NEOs.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2zIPt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2zIPt.jpg&quot; alt=&quot;NEOCam&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Infrared astronomer &lt;a href=&quot;https://en.wikipedia.org/wiki/Amy_Mainzer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Amy Mainzer&lt;/a&gt; illustrates how asteroids warmed by the sun will stand out more brightly in the infrared compared to reflected visible light from the sun. One coffee cup is black the other white in the false-color infrared thermal image. From &lt;a href=&quot;http://neocam.ipac.caltech.edu/page/mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question: Why not L2?&lt;/strong&gt; It receives about 4% less radiation from the sun (&lt;span class=&quot;math-container&quot;&gt;$1/r^2$&lt;/span&gt;) and would only need one shield to block heating from both the Earth and the Sun at the same time, and the optical data receivers on Earth would be looking through a dark, night-time sky instead of a bright daytime sky. These are probably mild advantages at best of course, but both L1 and L2 must have been originally considered and L1 was selected. So what were the benefits of L1 over L2 — why was L2 less suitable for the mission?&lt;/p&gt;&#xA;&lt;p&gt;In either case NEOCam would be in an orbit around the libration point, not at it, so its solar panels would still receive sunlight at L2, though also receiving 4% less light.&lt;/p&gt;&#xA;&lt;p&gt;The following is from the &lt;a href=&quot;http://neocam.ipac.caltech.edu/page/mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL NEOCam web page about the orbit&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NEOCam's orbit has been carefully designed to maximize scientific discovery while minimizing cost, complexity, and risk.  Similar to NASA's SOHO and Genesis missions, NEOCam will occupy a region of space fairly close to the Earth (in astronomical terms) called the Earth-Sun L1 Lagrange point.  This vantage point at L1, which is about four times further away than the Moon and interior to the Earth along the Earth-Sun line, allows NEOCam to view a large fraction of the Earth's orbit at any given time, and the sunshade (based on the 1983 IRAS mission) allows it to look close to the Sun.&lt;/p&gt;&#xA;&lt;p&gt;This region of space is ideal for NEOCam.  It will allow the observatory to maintain a nearly constant distance from Earth (about 1 million kilometers): far enough away to provide a stable, cold environment, yet close enough to support the high-speed radio communications needed to send NEOCam's large-format images back to Earth.  The return of these large-format images will allow astronomers to detect even the faintest asteroids and comets with great sensitivity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-07-10T11:47:25.693" LastActivityDate="2021-09-14T14:39:19.903" Title="Why has the Earth-Sun libration point L1 been chosen over L2 for NEOCam (now NEO Surveyor) to detect new NEOs?" Tags="|lagrangian-points|space-telescope|halo-orbit|near-earth-object|neo-surveyor|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19249" PostTypeId="2" ParentId="19248" CreationDate="2016-12-03T00:06:49.273" Score="4" Body="&lt;p&gt;We want to find NEOs that are inside Earth's orbit, like Atens, and telescopes don't like looking close to the Sun. So the more inside the orbit of the Earth you can get, the more new NEOs you will find without having to look at the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally you'd like a NEOCAM near the orbit of Venus. Then you'd be able to catch 'em all. But at E-S L1, you'll find most of the threats to Earth.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-12-03T00:06:49.273" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19250" PostTypeId="1" AcceptedAnswerId="35643" CreationDate="2016-12-03T00:26:30.570" Score="5" ViewCount="340" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Possibly helpful information in &lt;a href=&quot;https://gizmodo.com/nasa-is-sending-a-game-of-thrones-inspired-robot-into-s-1792385591&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gizmodo&lt;/a&gt; and &lt;a href=&quot;https://www.youtube.com/watch?time_continue=3&amp;amp;v=KF7d7K29ZrU&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general spacecraft are supplied with propulsion systems often with substantial redundancy in the plumbing and engines or thrusters so they can dependably and reliably perform orbital maneuvers in space. The emphasis is on the word &lt;strong&gt;perform&lt;/strong&gt;. They wait for commands from Earth before doing anything with it. You could say that spacecraft are carefully micro-managed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For deep space missions, this is done with an abundance of caution to protect the substantial investment of time, people, and resources and the science to be gained. For Earth orbit, in addition to protecting all of those (with commercial gain replacing the scientific gain in most cases) there is the additional responsibility of not interfering with all of the other satellites in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was surprised that even the very expensive James Web Space Telescope does not seem to have the ability stay put by itself as &lt;a href=&quot;https://space.stackexchange.com/a/18505/12102&quot;&gt;hypothesized in this answer&lt;/a&gt;, and without constant station-keeping instructions successfully received from Earth, would just drift away from it's libration point orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To contrast, planetary explorers (as opposed to spacecraft) such as rovers and in the future flyers and/or hoppers execute sequences of tasks in rapid succession that require first-hand knowledge of the local environment which makes it beneficial to operate semi-autonomously. For example, based on experience from a number of past and present rovers on Mars, the Curiosity rover on Mars is capable of keeping itself busy for a day or so both in movement and sample taking and analyzing samples as &lt;a href=&quot;https://space.stackexchange.com/a/18261/12102&quot;&gt;described in this answer&lt;/a&gt;, and the EXOMars rover will &lt;a href=&quot;https://space.stackexchange.com/q/19187/12102&quot;&gt;potentially have a degree of autonomy&lt;/a&gt; as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So my question is&lt;/strong&gt;, What are the future prospects for spacecraft autonomy?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-06-30T05:49:01.730" LastActivityDate="2019-04-20T22:52:08.577" Title="What are the future prospects for spacecraft autonomy" Tags="|spacecraft|robotic-missions|autonomy|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="19252" PostTypeId="1" AcceptedAnswerId="19916" CreationDate="2016-12-03T04:06:08.050" Score="3" ViewCount="611" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/18919/12102&quot;&gt;This answer&lt;/a&gt; to the question &lt;a href=&quot;https://space.stackexchange.com/q/18918/12102&quot;&gt;Which specifications should a tablet computer have in order to work in outer space?&lt;/a&gt; points out that capacitive-sensitive touch pads, mouse pads and touch-sensitive screens could be difficult to use if you are wearing the glove of a pressure suit in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can the laptops seen in various locations on the ISS be used when wearing spacesuits? Can the cursor be moved (if the software needs it)? Actually can one even reliably push individual keys and buttons on the laptops while wearing any of the possible suits one might be wearing in an emergency in various scenarios?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-01-26T16:04:24.250" Title="If an ISS emergency requires pressure suits temporarily, can they still use the laptops? Move the cursor?" Tags="|iss|spacesuits|emergency|" AnswerCount="1" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="19253" PostTypeId="1" CreationDate="2016-12-03T04:38:59.527" Score="0" ViewCount="125" Body="&lt;p&gt;Researching the word &lt;em&gt;occultation&lt;/em&gt;, it is evident that it is a newly incorporated term in respect to space and eclipses. It just recently began to populate space periodicals and online articles, but hardcopy publications are rare. Possible leads may include the US Department of Defense, US Department of Education, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What authority first commissioned the word 'occultation' in respect to space and transits of bodies?&lt;/p&gt;&#xA;" OwnerDisplayName="user16987" LastEditorUserId="1142" LastEditDate="2016-12-07T00:31:03.870" LastActivityDate="2019-08-29T06:51:52.053" Title="What publishing authority (and year of publishing) was the first to use the term 'occultation'?" Tags="|terminology|research|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19254" PostTypeId="1" CreationDate="2016-12-03T09:52:57.337" Score="0" ViewCount="49" Body="&lt;p&gt;Cassini makes frequent flyby's of moons at Saturn. If one similarly put a probe in retrograde heliocentric orbit in the Asteroid Belt, how frequent could one expect to make a near flyby, given the maneuverability of a (solar powered) Cassini class spacecraft? Several times per year?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2016-12-03T09:52:57.337" Title="How many asteroids could a retrograde probe in the Asteroid Belt flyby?" Tags="|probe|flyby|asteroid-belt|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19255" PostTypeId="2" ParentId="19253" CreationDate="2016-12-03T10:34:28.160" Score="6" Body="&lt;p&gt;It was not &quot;commissioned&quot; in any sense; the word has been around for that purpose in scientific discourse for a very long time. See, for example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://rstl.royalsocietypublishing.org/content/11/123-132/721.full.pdf+html&quot; rel=&quot;noreferrer&quot;&gt;The Occultation of the Planet Mars by the Moon, Observed by Mons. Hevelius, Mr. Flamstead and Mr. Hally&lt;/a&gt;, &lt;em&gt;Philosopical Transactions of the Royal Society&lt;/em&gt;, 1676. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Yes, those are the astronomers you think they are...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, it is in Latin (&quot;occultiato&quot;) but the English form was in use in print a few decades later (eg &lt;a href=&quot;http://rstl.royalsocietypublishing.org/content/40/445-451/100.full.pdf+html&quot; rel=&quot;noreferrer&quot;&gt;1736&lt;/a&gt;) and would certainly have been understood by contemporaries.&lt;/p&gt;&#xA;" OwnerUserId="11259" LastActivityDate="2016-12-03T10:34:28.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19256" PostTypeId="2" ParentId="19253" CreationDate="2016-12-03T15:14:48.700" Score="7" Body="&lt;p&gt;A &lt;a href=&quot;https://books.google.com/ngrams/graph?content=occultation&amp;amp;case_insensitive=on&amp;amp;year_start=1500&amp;amp;year_end=2000&amp;amp;corpus=15&amp;amp;smoothing=3&amp;amp;share=&amp;amp;direct_url=t4%3B%2Coccultation%3B%2Cc0%3B%2Cs0%3B%3Boccultation%3B%2Cc0%3B%3BOccultation%3B%2Cc0%3B%3BOCCULTATION%3B%2Cc0&quot; rel=&quot;noreferrer&quot;&gt;search for 'occultation' through the &lt;em&gt;English&lt;/em&gt; books indexed for Google's n-gram viewer&lt;/a&gt; suggest the term has been in use since from around 1710, with an earlier spike in the 1650's:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Oy57H.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Oy57H.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further search suggest that the term&lt;sup&gt;*&lt;/sup&gt; was used in the &lt;em&gt;Latin&lt;/em&gt; &lt;a href=&quot;https://books.google.nl/books?id=FElRAAAAcAAJ&amp;amp;pg=PA661&amp;amp;dq=occultation&amp;amp;hl=en&amp;amp;sa=X&amp;amp;redir_esc=y&quot; rel=&quot;noreferrer&quot;&gt;Almagestum Novum Astronomiam Veterem Novamque Complectens Observationibus Aliorum, Et Propriis Novisque Theorematibus, Problematibus, ac Tabulis promotam&lt;/a&gt; by &lt;a href=&quot;https://en.wikipedia.org/wiki/Giovanni_Battista_Riccioli&quot; rel=&quot;noreferrer&quot;&gt;Giovanni Battista Riccioli&lt;/a&gt;, published in 1651.&lt;br /&gt;&#xA;You can &lt;a href=&quot;https://books.google.nl/books/reader?id=JklRAAAAcAAJ&amp;amp;printsec=frontcover&amp;amp;output=reader&amp;amp;source=ebookstore&amp;amp;pg=GBS.PP9&quot; rel=&quot;noreferrer&quot;&gt;view it here&lt;/a&gt; or &lt;a href=&quot;http://gallica.bnf.fr/ark:/12148/bpt6k51332p/f1.image&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; but those are scans that are not searchable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;* Not necessarily as a noun, maybe as the conjugation of a verb&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user10509" LastEditorDisplayName="user10509" LastEditDate="2016-12-03T15:30:37.170" LastActivityDate="2016-12-03T15:30:37.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19257" PostTypeId="2" ParentId="18208" CreationDate="2016-12-03T16:23:19.620" Score="1" Body="&lt;p&gt;This is an interesting topic, this isn't quite what you want but its a start.  There was an audio tape made from accelerometer recordings on-board the Olympus communications satellite, launched 1989. Of course this doesn't show what it would sound like to be present there in a pressurised cabin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My memory is that one could hear the wheels running: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;sounded like the wall of sound one hears in the depths of a ship&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;and thrusters firing in pulsed mode to off-load momentum built up by the wheels: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;sounded like a machine gun&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I had a quick search just now to see if it had found its way onto the internet but without luck.  It seems it can't be beyond the wit of someone to rig something similar up with a modern satellite so perhaps something else will surface.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2016-12-03T16:23:19.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19259" PostTypeId="1" CreationDate="2016-12-03T21:25:52.040" Score="2" ViewCount="740" Body="&lt;p&gt;I've obviously not seen every orbit but every one I have seen switches from rear view to front view just before it would have been aiming towards Antarctica. I ask this as I am interested to see what is happening in and above Earth's atmosphere in this region.&lt;/p&gt;&#xA;" OwnerUserId="17795" LastEditorUserId="6816" LastEditDate="2016-12-03T21:46:28.823" LastActivityDate="2016-12-09T04:23:03.817" Title="Why does the ISS HDEV camera switch from rear view to front view just before rear view is aiming towards Antarctica?" Tags="|iss|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19260" PostTypeId="1" CreationDate="2016-12-04T01:11:31.310" Score="3" ViewCount="417" Body="&lt;p&gt;I came across this detailed article on NASA's JPL website about how mission teams are able to calculate the trajectories (and by extension the position) of spacecraft at faraway distances. However, I was curious if it was possible for a spacecraft equipped with reasonable instrumentation (such as those found on any previous spacecraft design, or feasible planned design) to determine the distance to a target astronomical body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, suppose a spacecraft in orbit around the sun needed to determine the distance between itself and another body relatively far away (up to 5AU). Could the craft do this with, say, a radar beam aimed at the target body?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: I'm going to make the assumption for simplicity that the spacecraft already knows its position and velocity vector with reference to the sun, but that the orbital elements of other known bodies are not already pre-programmed into the craft.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;References:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://solarsystem.nasa.gov/basics/bsf13-1.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Basics of Space Flight - Section II - Chapter 13 - Spacecraft Navigation&lt;/a&gt; | NASA Jet Propulsion Laboratory @ California Institute of Technology&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/2152/can-radio-waves-be-formed-into-a-pencil-beam#2154&quot;&gt;Physics Stack Exchange - Can Radio Waves be Formed into a Pencil Beam?&lt;/a&gt; | Q: Theodor, A: ptomato&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.aoc.nrao.edu/intro/planets.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radio Images of the Solar System&lt;/a&gt; | National Radio Astronomy Observatory&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="15890" LastEditorUserId="15890" LastEditDate="2016-12-04T14:50:24.200" LastActivityDate="2016-12-05T12:25:51.890" Title="Are there Practical Methods to Measure the Distance between a Spacecraft and an Astronomical Body?" Tags="|orbital-mechanics|communication|design|trajectory|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19261" PostTypeId="1" CreationDate="2016-12-04T07:23:04.593" Score="4" ViewCount="520" Body="&lt;p&gt;I'm writing a book chapter about Kepler, both the man and the spacecraft. I can't find much about the computer system and memory. In particular, I'd like to know if the 16 GB memory is DRAM, since I also want to explain how DRAM works in this chapter.&lt;/p&gt;&#xA;" OwnerUserId="14419" LastEditorUserId="6816" LastEditDate="2016-12-20T20:28:26.717" LastActivityDate="2016-12-20T20:28:26.717" Title="Kepler has 16 GB of memory. Is this DRAM?" Tags="|electronics|kepler-telescope|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19263" PostTypeId="2" ParentId="19178" CreationDate="2016-12-04T09:38:40.253" Score="5" Body="&lt;p&gt;Comments from @oefe pointed me to the solution! I'll post it here. Since I'm not an expert I'll keep the explanations minimal to avoid saying anything misleading.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good reference is the SPK required reading:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was &lt;a href=&quot;https://space.stackexchange.com/questions/19178/#comment50423_19178&quot;&gt;pointed to&lt;/a&gt; the following folder, which has solutions in kernel form as &lt;code&gt;.bsp&lt;/code&gt; files: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/JUNO/kernels/spk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://naif.jpl.nasa.gov/pub/naif/JUNO/kernels/spk/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one I'm looking for is a reference orbit &lt;code&gt;spk_ref_160226_180221_160226.bps&lt;/code&gt; which was calculated in February 2016 before Juno reached Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional helpful &quot;clues&quot; are found in the same file name with extension &lt;code&gt;.bsp.lbl&lt;/code&gt; with information such as dates, center and bodyID codes, which I've used in the Python script below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was also &lt;a href=&quot;https://space.stackexchange.com/questions/19178/#comment50671_19178&quot;&gt;pointed to&lt;/a&gt; this discussion in the python-jplephem Github which points out that &quot;data type 1 (Modified Difference Arrays)&quot; kernels have a different structure and could not be read by jplephem:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://github.com/brandon-rhodes/python-jplephem/issues/14&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://github.com/brandon-rhodes/python-jplephem/issues/14&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and that discussion pointed to a solution - a Python script &lt;code&gt;spktype01&lt;/code&gt; which has been written by Shushi Uetsuki (whiskie14142). It seems to be a faithful transcription/conversion into Python from the original FORTRAN, and is well - and candidly - commented. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://github.com/whiskie14142/spktype01&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://github.com/whiskie14142/spktype01&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since I am running Python in an environment where Skyfield is already installed, it seems that the jplephem modules are already available. So I just run this in a folder containing only script below and the  &lt;code&gt;spktype01.py&lt;/code&gt; file from github. It works! Thank you to everyone involved!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below I show that coarse sampling displays varying minimum distances between Juno and the Jupiter system barycenter. By finding each of those minima and resampling at sub-minute intervals, one can see that the periJoves are all essentially the same distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EtR4t.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EtR4t.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from spktype01 import SPKType01&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;Juno_kernel = SPKType01.open('spk_ref_160226_180221_160226.bsp')&#xA;&#xA;JD_coarse   = np.arange(2457558.5, 2457754.5+1, 0.2)&#xA;# print(Juno_kernel)&#xA;&#xA;center, bodyID = 5, -61  # Jupiter barycenter, Juno Spacecraft&#xA;&#xA;coarse_data = []&#xA;for JD in JD_coarse:&#xA;&#xA;    spkpos, spkvel = Juno_kernel.compute_type01(center, bodyID, JD)&#xA;&#xA;    coarse_data.append(spkpos)&#xA;&#xA;coarse_data = np.array(coarse_data)&#xA;&#xA;rc = np.sqrt((coarse_data**2).sum(axis=1))&#xA;&#xA;inflections = (rc[2:] - rc[1:-1]) * (rc[1:-1] - rc[:-2]) &amp;lt;= 0.0 &#xA;i_inflects  = np.where(inflections)[0] + 1&#xA;i_peris     = [i for i in i_inflects if rc[i] &amp;lt; 1E+06]   # I am lazy&#xA;JD_peris = JD_coarse[i_peris]&#xA;&#xA;dminutes = np.linspace(-180, 180, 1001)&#xA;ddays    = dminutes / (24. * 60.)&#xA;&#xA;periJoves = []&#xA;for JD_peri in JD_peris:&#xA;&#xA;    JD_fine = JD_peri + ddays&#xA;&#xA;    fine_data = []&#xA;    for JD in JD_fine:&#xA;&#xA;        spkpos, spkvel = Juno_kernel.compute_type01(center, bodyID, JD)&#xA;&#xA;        fine_data.append(spkpos)&#xA;&#xA;    fine_data = np.array(fine_data)&#xA;&#xA;    r_fine = np.sqrt((fine_data**2).sum(axis=1))&#xA;&#xA;    periJoves.append(r_fine)&#xA;&#xA;if 1 == 1:&#xA;    plt.figure()&#xA;&#xA;    plt.subplot(1,2,1)&#xA;    plt.plot(JD_coarse - JD_coarse[0], rc)&#xA;    plt.yscale('log')&#xA;    plt.ylim(1E+04, 1E+07)&#xA;    plt.xlabel('days (arb)', fontsize=14)&#xA;&#xA;&#xA;    plt.subplot(1,2,2)&#xA;    for periJove in periJoves:&#xA;        plt.plot(dminutes, periJove)&#xA;    plt.yscale('log')&#xA;    plt.ylim(1E+04, 1E+07)&#xA;    plt.xlabel('minutes (arb)', fontsize=14)&#xA;&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-03-14T06:25:36.727" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19264" PostTypeId="2" ParentId="17214" CreationDate="2016-12-04T10:30:21.603" Score="3" Body="&lt;p&gt;You can read detailed accounts of the event e.g. on &lt;a href=&quot;http://www.russianspaceweb.com/progress-ms.html#toru&quot; rel=&quot;nofollow noreferrer&quot;&gt;RussianSpaceWeb.com&lt;/a&gt; and &lt;a href=&quot;https://www.nasaspaceflight.com/2016/07/progress-ms-01-departs-following-off-nominal-test/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASASpaceFlight.com&lt;/a&gt;. During a manual docking exercise, the spacecraft pitched while already in contact with the docking port, due to apparently unexpected thruster firings. However, Roskosmos nevertheless considered the exercise as success. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSVP docking system &lt;a href=&quot;http://www.russianspaceweb.com/docking.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;is designed to withstand accidental loads during docking&lt;/a&gt;. It doesn't seem to have caused any lasting damage, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_MS-03&quot; rel=&quot;nofollow noreferrer&quot;&gt;Progress MS-03&lt;/a&gt; docked to the same port only two weeks afterwards.&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2016-12-04T10:30:21.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19265" PostTypeId="2" ParentId="19261" CreationDate="2016-12-04T13:11:27.507" Score="9" Body="&lt;p&gt;Kepler has a &lt;a href=&quot;https://www.nasa.gov/pdf/314125main_Kepler_presskit_2-19_smfile.pdf&quot; rel=&quot;noreferrer&quot;&gt;16 GigaByte synchronous dynamic random access memory solid-state recorder&lt;/a&gt;, so that would be a kind of DRAM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, note that this is not the processor's main memory. The &lt;a href=&quot;https://en.wikipedia.org/wiki/RAD750&quot; rel=&quot;noreferrer&quot;&gt;RAD750&lt;/a&gt; used in Kepler is a 32-Bit processor, and wouldn't be able to address 16 GB of main memory. In practice, memory for RAD750-based systems is more in the range of 128 MB or less (e.g. see the &lt;a href=&quot;http://www.baesystems.com/en-us/download-en-us/20151109141738/1434554723043.&quot; rel=&quot;noreferrer&quot;&gt;BAE specification sheet for their current RAD750-based single-board computers&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2016-12-04T13:11:27.507" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19266" PostTypeId="1" CreationDate="2016-12-04T14:51:19.353" Score="2" ViewCount="104" Body="&lt;p&gt;For a student project we are looking into the possibility of detecting micrometeoroid (micrometer-size) impacts on the lunar surface with cubesats equipped with a thermal imaging device. We have found sources suggesting that the temperature on impact would be around 3000K. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are now looking at using a thermal imaging device with a Noise Equivalent Temperature difference (NEdT) of 25mK. This NEdT however is for a heat source very close to the sensor and also with a much lower temperature than what our system would try to detect. This sensitivity apparently increases when the temperature of the heat source increases. What we don't know is how this sensitivity scales and whether an IR camera with this sensitivity could detect these temperature differences.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our cubesat will be delivered into a 500km circular orbit (orientation is yet to be chosen) but we are allowed to carry propulsion systems so the orbit can be lowered. &lt;/p&gt;&#xA;" OwnerUserId="17799" LastActivityDate="2016-12-04T14:51:19.353" Title="Detection of micrometeoroid impacts on the lunar surface using thermal imaging?" Tags="|the-moon|cubesat|remote-sensing|meteoroid|sensors|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19267" PostTypeId="2" ParentId="19260" CreationDate="2016-12-04T15:30:44.133" Score="2" Body="&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt;: won't work with any spacecraft further away than one third of the larger semiaxis of the earth's path, even for fully-armed battlestation sized spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Radar (more exactly, time-of-flight) is limited by two things:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;speed of light: The nearest star  is 4 light years away. So anything that we send into that direction could, earliest, return in 8 years. Now, this is not really the problem at hand, because&lt;/li&gt;&#xA;&lt;li&gt;free space attenuation is the fact that for any wave front, the power density decreases by the same amount the sphere surface increases with radius. I.e., you get $\frac1{4\pi d^2}$ of the original power density at distance $d$.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Now, the result is the so-called &lt;em&gt;Radar Equation&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P_r = {{P_t G_t G_r  \sigma \lambda^2}\over{{(4\pi)}^3 R^4}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;with&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}&#xA;P_r &amp;amp;&amp;amp; \text{received signal power}\\&#xA;P_t &amp;amp;&amp;amp; \text{transmitted signal power}\\&#xA;\lambda &amp;amp;&amp;amp; \text{wavelength}\\&#xA;G_t,\,G_r &amp;amp;&amp;amp; \text{directional gain of the transmit, receive antennas}\\&#xA;\sigma&amp;amp;&amp;amp;\text{radar cross section, &quot;effective reflection area&quot;}\\&#xA;R &amp;amp;&amp;amp; \text{the distance between you and the radar target}&#xA;\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's plug in a few numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, let's assume your radar spacecraft has enough power, and sends 1 MW. It's also got an excellent receiver and lots of signal processing, so that it can even detect a reflected signal at far below thermal noise level at 20°C. Let's say it can work with -180dBm of power – that's $10^{-21}$ W. Pretty much nothing. (in fact, we're getting close to action quantization here)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, we come to the following reasoning for our maximum distance $R$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}&#xA;10^{-21} \text{ W}&amp;amp;= \frac{10^{6}  \text{ W} G_t G_r \lambda^2 \sigma}{{(4\pi)}^3 R^4}\\&#xA;10^{-27} &amp;amp;= \frac{{ G_t G_r\lambda^2 \sigma}}{{(4\pi)}^3 R^4}&#xA;\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's furthermore assume your spacecraft has something slightly smaller than the Arecibo Observatory (72dBi) as antenna – something with a gain of 60 dBi, and let's also assume you use that for both transmitting and receiving, $G_t=G_r=G$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}&#xA;10^{-27} &amp;amp;= \frac{{ G_t G_r \lambda^2\sigma}}{{(4\pi)}^3 R^4}\\&#xA;&amp;amp;= \frac{{ G^2 \sigma \lambda^2}}{{(4\pi)}^3 R^4}\\&#xA;&amp;amp;= \frac{{ 10^{12} \sigma \lambda^2}}{{(4\pi)}^3 R^4}\\&#xA;10^{-39}&amp;amp;= \frac{{ \sigma \lambda^2}}{{(4\pi)}^3 R^4}\\&#xA;\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question remains: What's a good estimate for the radar cross section of your target? So, we need to pick a target. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I arbitrarily chose the Imperial &lt;em&gt;Death Star&lt;/em&gt;. Which is nearly spherical, so we can analytically determine its RCS based on its radius $r$, assuming they have a nice, flat, metal surface freshly polished for the visit of the emperor (first Death Star had a $r=70\text{ km}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}&#xA;\sigma &amp;amp;= \pi r^2\\&#xA;&amp;amp;=\pi {(7\cdot 10^4)}^2\text m^2\\&#xA;&amp;amp;\approx 3\cdot 50 \cdot 10^5 \text m^2\\&#xA;&amp;amp;= 1.5\cdot 10^7 \text m^2\text{ .}&#xA;\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back to our maximum distance:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}&#xA;10^{-39}&amp;amp;= \frac{{ 1.5\cdot 10^7 \text m^2 \lambda^2}}{{(4\pi)}^3 R^4}\\&#xA;6.67\cdot10^{-47}&amp;amp;= \frac{{m^2 \lambda^2}}{{(4\pi)}^3 R^4}\\&#xA;\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume we're doing some 1 GHz as frequency, so we have a wavelength of &#xA;$$\begin{align}&#xA;\lambda &amp;amp;= \frac cf\\&#xA;&amp;amp;=\frac{3\cdot 10^8 \frac{\text m}{\text s}}{10^9\frac1{\text s}}\\&#xA;&amp;amp;=3\cdot 10^{-1}\text{ m .}&#xA;\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not a lower frequency, you ask? Simply because the size of an antenna of 60 dBi gain scales linearly with the wavelength. We need to get that antenna to space, so we can't have it being arbitrarily large (and as you've noticed, I'm overly concerned with realism).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It follows that&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align}&#xA;6.67\cdot10^{-47}&amp;amp;= \frac{{m^2 \lambda^2}}{{(4\pi)}^3 R^4}\\&#xA;&amp;amp;=  \frac{{9\cdot 10^{-2} m^4}}{{(4\pi)}^3 R^4}\\&#xA;0.74\cdot10^{-46}\text{ m}^{-4}&amp;amp;= \frac{1}{{(4\pi)}^3 R^4}\\&#xA;{(4\pi)}^3 \cdot 0.74\cdot10^{-46}\text{ m}^{-4}&amp;amp;= \frac{1}{ R^4}\\&#xA;R^4 &amp;amp;= \frac{1}{{(4\pi)}^3 \cdot 0.74\cdot10^{-46}}\text{ m}^{4}\\&#xA;&amp;amp;\approx \frac{1}{2000 \cdot 0.74\cdot10^{-46}}\text{ m}^{4}\\&#xA;&amp;amp;\approx \frac{1}{2 \cdot 0.74\cdot10^{-43}}\text{ m}^{4}\\&#xA;&amp;amp;\approx \frac{1}{1.5\cdot 10^{-43}}\text{ m}^{4}\\&#xA;&amp;amp;= \frac{1}{1.5}10^{43}\text{ m}^{4}\\&#xA;&amp;amp;= \frac{2}{3}10^{43}\text{ m}^{4}\\&#xA;R&amp;amp;=\sqrt[4]{\frac{2}{3}10^{43}}\text{ m}\\&#xA;&amp;amp;=\sqrt[4]{\frac{2}{3}10^{3}}\cdot\sqrt[4]{10^{40}}\text{ m}\\&#xA;&amp;amp;=\sqrt[4]{\frac{2}{3}10^{3}}\cdot 10^{10}\text{ m}\\&#xA;&amp;amp;\approx 5\cdot 10^{10}\text{ m}\\&#xA;&amp;amp;\approx 0.334 \text{ AU .}&#xA;\end{align}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since from the formula we see that radius of the target only contributes to maximum range with the square root, to get a max distance of 5 AU, we'd need to increase the radius by a factor of $\left(\frac5{0.334}\right)^2\approx 15^2=225$, ie. that body would need to have a diameter of 31,500 km at least – about one fourth of the diameter of Jupiter! &lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastEditorDisplayName="user17550" LastEditDate="2016-12-04T16:34:51.743" LastActivityDate="2016-12-04T16:34:51.743" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="19268" PostTypeId="2" ParentId="19260" CreationDate="2016-12-04T16:28:58.673" Score="1" Body="&lt;p&gt;Well, the classical far-distance unit is the parsec – a unit relative to the distance of something that is, perpendicular to the plane in which your own body is moving, so far away that, if you follow a circle with 1AU radius, the angle under which it's observed changes by 2 arcseconds (1 arcsecond relative to the center of that circle and any point on it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That definition, on the other hand, can be used to estimate the distance of something if you know how large your own ellipse is – and have appropriate angle measurement.&lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastActivityDate="2016-12-04T16:28:58.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19269" PostTypeId="1" CreationDate="2016-12-04T19:11:41.693" Score="-1" ViewCount="276" Body="&lt;p&gt;If the cosmic speed limit of electromagnetic waves is $c$ then is there any reason to use this to communicate as over long (interstellar) distances? Can this be an effective means of communication? Even between Earth and Mars the lag is problematic.&lt;/p&gt;&#xA;" OwnerUserId="6474" LastEditorUserId="195" LastEditDate="2016-12-04T19:31:58.820" LastActivityDate="2016-12-05T09:35:28.343" Title="Is SETI a waste of time?" Tags="|seti|" AnswerCount="1" CommentCount="0" ClosedDate="2016-12-04T21:17:55.577" ContentLicense="CC BY-SA 3.0" />
  <row Id="19270" PostTypeId="2" ParentId="19269" CreationDate="2016-12-04T19:43:41.790" Score="4" Body="&lt;p&gt;Light speed does make it more difficult to have a conversation. It doesn't make it impossible though.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But SETI is about one question: are we alone in the universe? We don't need to be able to communicate to answer that.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A comment asked for 'other options than EM' for communications, assuming there are ways to communicate faster than light speed (FTL). &#xA;The problem with that is, we don't even have a theoretical idea of how that might work, let alone a working FTL communications device.  So we use what we have, which is EM. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-12-05T09:35:28.343" LastActivityDate="2016-12-05T09:35:28.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19272" PostTypeId="1" AcceptedAnswerId="19275" CreationDate="2016-12-04T23:28:32.507" Score="23" ViewCount="7820" Body="&lt;p&gt;When the last batch of astronauts were sent to ISS, I read that the crew and rocket were blessed by a priest.  Was this a one-time thing, or is it standard?&lt;/p&gt;&#xA;" OwnerUserId="17817" LastEditorUserId="-1" LastEditDate="2016-12-05T13:56:19.177" LastActivityDate="2016-12-05T18:22:33.320" Title="Are all rockets sent to ISS blessed by a priest?" Tags="|launch|iss|procedure|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19274" PostTypeId="1" AcceptedAnswerId="19283" CreationDate="2016-12-05T02:43:14.380" Score="-9" ViewCount="700" Body="&lt;p&gt;&lt;em&gt;Originally this question asked about using blimps and electricity in replacement of boosters. As written then it was no good and got down votes. I had a few question marks and each question really needed there own question. I have split up the question into other questions and apologize if your answer does not perfectly align with this question anymore. I hope this is a better question? Below in the links are questions pertaining to this one. Thanks and Enjoy.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the picture below the Stratolaunch plane is towing a rocket in place of using stages.&#xA;Could the Stratolaunch Plane shown below&lt;a href=&quot;https://i.stack.imgur.com/Jnw9I.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Jnw9I.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; be replaced with a wing shaped blimp with an electric turbine make a launch stage that would follow &lt;a href=&quot;https://space.stackexchange.com/questions/30545/solar-powered-inflatable-launch-tower&quot;&gt;a floating electric power line&lt;/a&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/I8N1y.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/I8N1y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://aviation.stackexchange.com/questions/31370/could-a-plane-with-an-electric-turbine-engine-generate-enough-lift-as-an-alterna&quot;&gt;https://aviation.stackexchange.com/questions/31370/could-a-plane-with-an-electric-turbine-engine-generate-enough-lift-as-an-alterna&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/88075/sky-train-blimp-plane&quot;&gt;https://worldbuilding.stackexchange.com/questions/88075/sky-train-blimp-plane&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://robotics.stackexchange.com/questions/11449/can-high-voltage-power-lines-provide-a-super-highway-for-drones&quot;&gt;https://robotics.stackexchange.com/questions/11449/can-high-voltage-power-lines-provide-a-super-highway-for-drones&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/26173/would-a-balloon-pop-if-dropped-from-space&quot;&gt;Would a balloon pop if dropped from space?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2019-01-11T19:54:32.840" LastActivityDate="2019-01-11T19:54:32.840" Title="Can a reusable electric powered blimp stage for launch possible?" Tags="|launch|interstellar-travel|aerodynamics|entry-descent-landing|" AnswerCount="2" CommentCount="5" FavoriteCount="0" ClosedDate="2022-08-07T08:34:33.187" ContentLicense="CC BY-SA 4.0" />
  <row Id="19275" PostTypeId="2" ParentId="19272" CreationDate="2016-12-05T02:49:06.610" Score="32" Body="&lt;p&gt;It's standard procedure in Russia. Here's an &lt;a href=&quot;http://time.com/3759467/rocket-blessing-soyuz/&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; reporting on &lt;a href=&quot;https://en.wikipedia.org/wiki/Scott_Kelly_(astronaut)#Year-long_mission&quot; rel=&quot;noreferrer&quot;&gt;Scott Kelly&lt;/a&gt;'s taxi ride to his year-long mission to the ISS in 2015:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The barricades to faith fell along with the Berlin Wall and religion now thrives in Russia and the cities and nations of the old empire. That includes Baikonur, where cosmonauts Gennady Padalka and Mikhail Kornienko and astronaut Scott Kelly will take off in the early hours of March 28 for a long-duration stay aboard the International Space Station—and where less than 37 hours before launch, three Russian Orthodox priests arrived to perform a blessing of the Soyuz rocket that will carry the men. . . .    &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Terrible things can happen when people dare to fly to space. It’s in the nature of human beings to make such presumptuous journeys anyway. And it’s in our nature too to seek a little safety and comfort before we do.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="1142" LastActivityDate="2016-12-05T02:49:06.610" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19276" PostTypeId="2" ParentId="19274" CreationDate="2016-12-05T05:31:10.467" Score="7" Body="&lt;h1&gt;Context&lt;/h1&gt;&#xA;&lt;p&gt;Mars density is less than 1% of earth's.&#xA;This is what earth looks like from 30 000 meters high (where air is about as dense as Mars)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ElH1i.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ElH1i.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h2&gt;It's going to be too heavy&lt;/h2&gt;&#xA;&lt;p&gt;Using &lt;a href=&quot;https://books.google.co.jp/books?id=_iEHI7Nh6yYC&amp;amp;pg=PA47&amp;amp;lpg=PA47&amp;amp;dq=mass%20to%20payload%20ratio%20of%20stratospheric%20balloon&amp;amp;source=bl&amp;amp;ots=Fu2nJX3bW4&amp;amp;sig=yKHvR5yUymYn9LufUw2Pl6ZUfo4&amp;amp;hl=fr&amp;amp;sa=X&amp;amp;ved=0ahUKEwjS9unon9zQAhWFq5QKHU9xDigQ6AEIcDAJ#v=onepage&amp;amp;q=mass%20to%20payload%20ratio%20of%20stratospheric%20balloon&amp;amp;f=false%20book&quot; rel=&quot;noreferrer&quot;&gt;this book&lt;/a&gt; as a reference, we can see that to lift 1T to 30 000m we need a 10T balloon (accounting for balloon and lifting gazes).&lt;/p&gt;&#xA;&lt;p&gt;Of course you'll need more since you'll want to produce positive lift before touching the ground.&lt;/p&gt;&#xA;&lt;p&gt;An empty (without payload and fuel) Ariane 5 is 50T.&lt;/p&gt;&#xA;&lt;p&gt;To lift it to 30 000m, the blimp + rocket would be already 500T!&lt;/p&gt;&#xA;&lt;p&gt;Now that you are at 30 000m, you need to accelerate the whole 500T by 13 500km/h just to reach mars (And I'm not even counting any deceleration).&lt;/p&gt;&#xA;&lt;p&gt;In order to do that, you would need an incredible amount of fuel, that would in turn increase the mass of your blimp, making the whole thing unsustainable due to tyranny of the rocket equation.&lt;/p&gt;&#xA;&lt;p&gt;To go to space, you don't need to get really high, you only need to get really fast.&lt;/p&gt;&#xA;&lt;h2&gt;It would not survive reentry&lt;/h2&gt;&#xA;&lt;p&gt;Assuming that you succeeded to sent your blimp to mars (no, you did'nt), you are moving at a an incredibly high speed.&lt;/p&gt;&#xA;&lt;p&gt;As a reference, the heat shield for the mars exploration rover was 78kg for a 1T payload. To survive the aerodynamic forces (&amp;gt;15G for curiosity), you would need a really strong structure, which can only translate to more mass, which translate to even more mass when the blimp and rocket equations are factored.&lt;/p&gt;&#xA;&lt;h1&gt;Conclusion:&lt;/h1&gt;&#xA;&lt;p&gt;It is impractical because&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The blimp itself would be much too heavy&lt;/li&gt;&#xA;&lt;li&gt;Mars atmosphere is much too thin&lt;/li&gt;&#xA;&lt;li&gt;It offers no advantage compare to a rocket&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;And that's using very optimistic numbers and discounting many factors who would, by themselves make it fail.&lt;/p&gt;&#xA;&lt;p&gt;When mass is a premium, you don't want to carry around a heavy blimp.&lt;/p&gt;&#xA;&lt;p&gt;In orbit, your mass is still the same as it was on earth, and a the blimp that initially  lifted you up of the atmosphere is actually heavy.&lt;/p&gt;&#xA;&lt;h2&gt;Tl;dnR&lt;/h2&gt;&#xA;&lt;p&gt;No.&lt;/p&gt;&#xA;&lt;h1&gt;How big would be a blimp if you wanted to lift at 1atm ?&lt;/h1&gt;&#xA;&lt;p&gt;Since your title question seems to be slightly different, let's try to answer it.&lt;/p&gt;&#xA;&lt;p&gt;The Hindenburg could lift 10 000kg for a volume of 200 000m3.&#xA;A falcon 9 fully fuelled is 750 000kg.&lt;/p&gt;&#xA;&lt;p&gt;Therefor you need 200000/10000*750000 = 15 000 000 m3 to lift the rocket.&#xA;the cruise altitude was 200m, so don't expect going much higher than that.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-01-09T06:00:55.973" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="19277" PostTypeId="2" ParentId="19272" CreationDate="2016-12-05T05:45:35.020" Score="9" Body="&lt;p&gt;For the first time it was in 1994, when Mir commander Alexander Viktorenko asked a priest to bless his rocket before launch&#xA;See &lt;a href=&quot;https://meduza.io/cards/zachem-osvyaschayut-rakety&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://meduza.io/cards/zachem-osvyaschayut-rakety&lt;/a&gt; (in Russian, use Google translate)&lt;/p&gt;&#xA;" OwnerUserId="11046" LastActivityDate="2016-12-05T05:45:35.020" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19279" PostTypeId="2" ParentId="19260" CreationDate="2016-12-05T12:25:51.890" Score="4" Body="&lt;p&gt;Yes. The spacecraft doesn't need radar or lasers or anything active (which won't work due to the incredible distances).. A decent telescope is all that's required. The telescope will measure the targets movement against the background starfield (known as Parallax). Since the spacecrafts velocity and position is known, this movement can be used to make estimates of the distance, which can be refined over time. This is how we knew the distances to the various moons and planets before sending spacecraft there..&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-12-05T12:25:51.890" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19280" PostTypeId="2" ParentId="18176" CreationDate="2016-12-05T15:39:29.587" Score="0" Body="&lt;p&gt;Since the airlock acts as a buffer between &quot;inside pressure&quot; and &quot;space pressure&quot;, I assume you would always want it to be at one of these pressure levels. By keeping a door open, you are sure that the airlock doesn't get its own third pressure level. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's like avoiding floating potentials when building an electronic circuit.&lt;/p&gt;&#xA;" OwnerUserId="17809" LastActivityDate="2016-12-05T15:39:29.587" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19281" PostTypeId="2" ParentId="19259" CreationDate="2016-12-05T15:45:06.790" Score="3" Body="&lt;p&gt;What you're describing is pure coincidence.  The HDEV cameras don't know where the ISS is over the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://nasa.gov/mission_pages/station/research/experiments/917.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://nasa.gov/mission_pages/station/research/experiments/917.html&lt;/a&gt; states that normal operation is to automatically cycle through the cameras at regular intervals (which can be adjusted by ground command). The cycle rate is discussed in this answer: &lt;a href=&quot;https://space.stackexchange.com/a/9571/512&quot;&gt;https://space.stackexchange.com/a/9571/512&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-12-06T18:16:37.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19283" PostTypeId="2" ParentId="19274" CreationDate="2016-12-05T16:29:23.123" Score="5" Body="&lt;p&gt;Using a blimp for takeoff &lt;a href=&quot;https://space.stackexchange.com/questions/1635/could-it-be-possible-to-launch-a-rocket-from-a-balloon&quot;&gt;is pointless&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hard part of going to space is not the distance you travel to get out of the atmosphere. It is the speed. To get to space and stay there, you need to travel at 28,000 km/h. Once you have a vehicle capable of doing that, making it travel 200 km upwards is trivial.&lt;br&gt;&#xA;Using a balloon doesn't save fuel, it doesn't make the launch easier and it doesn't make the rocket cheaper or easier to build.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You also can't just add rocket engines to a blimp. A rocket engine generates a huge amount of thrust (to get to 28,000 km/h in a reasonable amount of time), and no balloon is built to withstand this acceleration so the balloon would collapse. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="18879" LastEditDate="2019-01-09T06:01:44.700" LastActivityDate="2019-01-09T06:01:44.700" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="19284" PostTypeId="2" ParentId="18176" CreationDate="2016-12-05T16:33:14.890" Score="3" Body="&lt;p&gt;The configuration between EVAs is - Inner hatch open, Outer Hatch closed, Inner hatch equalization valves closed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/83108main_sts-113evachecklist.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS EVA Checklist&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at the POST EVA steps.  The first few actions configure the hatch and valves.  There are no further steps in POST EVA reconfiguring the hatches or equalization valves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a screen shot of the first page of the procedure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Lhbps.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Lhbps.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-12-05T16:33:14.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19287" PostTypeId="2" ParentId="19259" CreationDate="2016-12-06T02:37:40.637" Score="4" Body="&lt;p&gt;Because of the path of the orbit of the ISS, you can never see Antarctica from the space station, even in the most ideal circumstances.&lt;a href=&quot;https://i.stack.imgur.com/tWt0u.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tWt0u.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;This Aerospace Corporation map shows the potential re-entry paths of Russia's doomed Progress 59 spacecraft, with re-entry plots for Friday, May 8 at 1:40 a.m. ET (0540 GMT), plus or minus 5 hours.&#xA;Credit: The Aerospace Corporation.  Source Accessed: &lt;a href=&quot;http://www.space.com/29344-falling-russian-spacecraft-reentry-tonight.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;space.com&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the path of the space station (Yellow+Blue lines) passes a few hundred miles north of the northern most point in Antarctica. You might be able to just barely see the tip of the Antarctic Peninsula, but certainly no more than that.&lt;/p&gt;&#xA;" OwnerUserId="16702" LastEditorUserId="6773" LastEditDate="2016-12-06T14:56:56.067" LastActivityDate="2016-12-06T14:56:56.067" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19288" PostTypeId="1" AcceptedAnswerId="19311" CreationDate="2016-12-06T12:29:03.463" Score="1" ViewCount="1782" Body="&lt;p&gt;How much uplink capacity (in terms of GB per day) is typical for a satellite in LEO?  What aspects determine how much data can be transferred back to the earth?&lt;/p&gt;&#xA;" OwnerUserId="17824" LastActivityDate="2016-12-07T13:57:43.007" Title="What is the limit of bandwidth of a satellite in LEO and what determines that" Tags="|artificial-satellite|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19290" PostTypeId="1" CreationDate="2016-12-06T13:11:40.187" Score="13" ViewCount="1258" Body="&lt;p&gt;How bright (in Watts, preferably, or lumens) is the plume from a rocket launch (e.g. a SpaceX launch)? Obviously in its direction of travel it isn't particularly bright, but what is its effective isotropic radiative power abeam?&lt;/p&gt;&#xA;" OwnerUserId="11740" LastEditorUserId="12102" LastEditDate="2017-08-16T15:00:08.210" LastActivityDate="2018-10-21T18:41:55.630" Title="How bright is a rocket launch?" Tags="|launch|rockets|visibility-of-spacecraft|plume|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19291" PostTypeId="1" CreationDate="2016-12-06T14:19:38.230" Score="1" ViewCount="76" Body="&lt;p&gt;Assume a spacecraft with a less-than-perfect maneuvering thruster configuration such that in a particular attitude while attempting to dock, the thrusters will not allow translation along a particular axis.  As a result, the spacecraft has to roll and translate in another direction to compensate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's like a linear analogue to gimbal lock.  Is there a term for this situation?&lt;/p&gt;&#xA;" OwnerUserId="17722" LastActivityDate="2016-12-06T14:19:38.230" Title="Linear Control Lock?" Tags="|thrust|flight-control|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19293" PostTypeId="2" ParentId="18392" CreationDate="2016-12-06T17:32:35.363" Score="11" Body="&lt;p&gt;Yes, the Soyuz TM-10 capsule had such a label next to the exterior hatch, according to &lt;a href=&quot;http://discovermagazine.com/1994/apr/thebloconthebloc363&quot; rel=&quot;noreferrer&quot;&gt;The Bloc on the Block&lt;/a&gt; by Jeffrey Kluger in the April 1994 issue of Discover Magazine:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Then there were lots 154 and 175, the Kosmos 1443 and Soyuz TM-10 space capsules. Sotheby's hoped to get top dollar for these artifacts, and the catalog included photographs of some of the ships' more interesting details in an effort to spark the bidding. (One photo showed the message stenciled in Russian and English next to the Soyuz's exterior hatch, in case the spacecraft was found by a passerby before recovery teams could reach it: MAN INSIDE! HELP! OPEN THE HATCH! TAKE THE KEY! PUT INTO THE HOLE!)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is a picture &lt;a href=&quot;https://www.flickr.com/photos/vruba/12080725555&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, for those interested. (I won't be including the picture in the post due to copyright restrictions.)&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-12-06T17:52:09.380" LastActivityDate="2016-12-06T17:52:09.380" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19294" PostTypeId="2" ParentId="16898" CreationDate="2016-12-06T18:35:35.157" Score="5" Body="&lt;p&gt;The mass of Mars is 6.419 * 10E23 kg, the mass of the largest object in the asteroid belt, the dwarf planet Ceres is 9.393 * 10E20 kg. Adding Ceres to Mars would result in 6.4284 * 10E23 kg, a very small change, only from 100 to 100.146%. But moving Ceres to Mars using our present propulsion methods would require a lot more mass of propellants than the mass of Ceres itself. The idea of increasing the mass of Mars is not realizable using our present level of science and technology.&lt;/p&gt;&#xA;&lt;p&gt;The change of gravity would be measurable using very sensitive gravimeters, but not noticeable by astronauts.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2021-12-05T21:18:44.643" LastActivityDate="2021-12-05T21:18:44.643" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19295" PostTypeId="1" CreationDate="2016-12-06T21:35:09.800" Score="7" ViewCount="437" Body="&lt;p&gt;Let's suppose I want to create a pressurized dome structure on the surface of Mars, but I want to avoid the time/effort/expense/engineering of sealing the base. Let's further stipulate that I've got a good subsurface geological survey of the area, and I've confirmed that there are no voids or caves beneath the area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do I engineer the seal between the dome and the surface? Does the surface need to extend subsurface? Is surface concrete plus the weight of the dome enough to seal? How do you protect against subsidence around the seal? &lt;/p&gt;&#xA;" OwnerUserId="5965" LastActivityDate="2016-12-07T11:54:12.063" Title="How can a non-floored habitat be sealed on Mars?" Tags="|mars|habitat|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19296" PostTypeId="1" AcceptedAnswerId="19298" CreationDate="2016-12-06T21:48:33.207" Score="12" ViewCount="11690" Body="&lt;p&gt;I am just watching Space Race part 3 and the cosmonauts just got their first view of Wostok. The scientist who designed the retrorockets told the soon-to-be spacemen that the rockets would have to be fired precisely in order to not:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;reenter too steep (hence to fast) and crash or&lt;/li&gt;&#xA;&lt;li&gt;reenter too shallow and bounce off the atmosphere &lt;strong&gt;into a higher orbit&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The first point is, of course, well taken (even though one has to wonder why the rockets should be designed with too much power in the first place), but the second point seems dubious to me: After playing KSP, I would suspect that the Wostok's orbit would decay nonetheless after even a minimal braking. How could you enter a higher orbit by touching the atmosphere? That would seem like a great way to get to the moon, after all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: &lt;a href=&quot;https://youtu.be/8B94ftc9UVA?t=726&quot; rel=&quot;noreferrer&quot;&gt;Here is the part I am referencing&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2022-07-11T22:50:24.570" Title="Bounce off the atmosphere at reentry?" Tags="|reentry|atmosphere|aerobraking|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19297" PostTypeId="1" AcceptedAnswerId="22110" CreationDate="2016-12-06T22:00:24.510" Score="40" ViewCount="12166" Body="&lt;p&gt;I understand that U.S. Congress passed a law in 2011 barring space agency collaboration with &lt;a href=&quot;https://en.wikipedia.org/wiki/China_National_Space_Administration&quot; rel=&quot;noreferrer&quot;&gt;CNSA&lt;/a&gt; because of concerns over &quot;national security&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also get that China &lt;a href=&quot;https://en.wikipedia.org/wiki/2007_Chinese_anti-satellite_missile_test&quot; rel=&quot;noreferrer&quot;&gt;blew-up one of their own satellites&lt;/a&gt; causing a massive distribution of orbital debris, which was understandably unpopular.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But other than that, is there any real, solid, practical reason I'm &lt;strong&gt;missing&lt;/strong&gt;, besides just &lt;em&gt;politik&lt;/em&gt;? (would be perfectly acceptable answer if so)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not inviting China to the ISS means China is now building their own space station, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tiangong-1&quot; rel=&quot;noreferrer&quot;&gt;Tiangong&lt;/a&gt;, so if the worry is &quot;technology over-sharing&quot;, well, they're already replicating most of our abilities already. (China's capsule, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shenzhou_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Shenzhou&lt;/a&gt;, looks like the design borrows mostly from the Soyuz anyway.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are we in a cold war with China? The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Race&quot; rel=&quot;noreferrer&quot;&gt;U.S.-Soviet cold war&lt;/a&gt; did bring a truly exciting pace to space exploration, even if we were duplicating each others' work and spending unprecedented amounts to do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issue is, a ticket to Mars is very expensive-looking on a politician's budget review sheet, so while they don't want to be the ones who say &quot;no,&quot; I'm sure they'd welcome support from countries willing to share a chunk of that price tag. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Side Note:&lt;/strong&gt; I've also just learned that &lt;a href=&quot;https://en.wikipedia.org/wiki/Politics_of_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;India&lt;/a&gt; is not part of the ISS program either? This is even more strange, because it seems to me like they've had more launches per year, more successful missions, and their &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;PSLV&lt;/a&gt; has come along nicely and is looking impressive, even though the ISRO isn't as &quot;big&quot; as China's. Plus, I don't recall US having any major political beefs with India. And I'm sure they'd love to go to Mars with us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, why does the &quot;International&quot; Space Station seem to wave a &quot;Members Only&quot; flag?&lt;/p&gt;&#xA;" OwnerUserId="192" LastEditorUserId="22571" LastEditDate="2018-05-29T05:05:21.667" LastActivityDate="2020-12-18T18:40:32.623" Title="Why is China still not invited to the ISS? (Or, why do certain countries not get invited/rejected to work on the ISS?)" Tags="|iss|isro|china|" AnswerCount="3" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="19298" PostTypeId="2" ParentId="19296" CreationDate="2016-12-06T23:12:15.727" Score="24" Body="&lt;p&gt;Yes, a capsule cannot literally bounce off the atmosphere and its kinetic energy must be reduced by an encounter with the atmosphere, rather it would just pass through the atmosphere and back into space, having failed to lose enough velocity to stay in the atmosphere. After going partially around the planet it will reenter the atmosphere, that is actually where the real problem lies, the capsule will come down at the wrong location at the wrong time. There may also be problems with power / oxygen reserves running out or heat shielding failure, for example the design may require jettisoning the heat shield to get rid of the the accumulated heat, or rely on convection in the atmosphere to cool the vessel. So there are lots of things that would go wrong if the vessel goes back into space instead of deeper into the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With that said, there is something of a lack of precise, every day terminology to describe the problem in a pithy way and ordinary people do not understand the distinction between &quot;in orbit&quot; and &quot;high up&quot; anyway, the capsule would literally go back high up into space, and with the right framing (i.e. a graph of altitude vs time) the trajectory would kind of look like a bounce.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2016-12-06T23:24:48.600" LastActivityDate="2016-12-06T23:24:48.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19299" PostTypeId="1" AcceptedAnswerId="19302" CreationDate="2016-12-06T23:42:58.047" Score="4" ViewCount="1645" Body="&lt;p&gt;Isn't electrolysis of water less efficient for a spacecraft than pressurized oxygen tanks? The ISS uses electrolysis but to do this you have to send water to space which weighs more than oxygen and only contains only 1 of every 3 parts oxygen. So wouldn't it be better just to send oxygen up, and obviously mix it with other gases?&lt;/p&gt;&#xA;" OwnerUserId="17166" LastEditorUserId="6816" LastEditDate="2016-12-07T04:45:31.300" LastActivityDate="2017-06-22T20:53:54.577" Title="Isn't electrolysis of water less efficient for a space craft than pressurized oxygen tanks?" Tags="|iss|oxygen|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19300" PostTypeId="2" ParentId="19299" CreationDate="2016-12-07T01:13:27.520" Score="7" Body="&lt;p&gt;The ISS does receive regular shipments of oxygen as a gas in specially built, very high pressure (about 6000 psi or 400 atmospheres), scary looking tanks, as pointed out in &lt;a href=&quot;https://space.stackexchange.com/a/17677/12102&quot;&gt;this interesting answer&lt;/a&gt;. They are called NORS (Nitrogen/Oxygen Recharge System). You can read more about them &lt;a href=&quot;https://www.nasa.gov/content/air-supply-high-pressure-tanks-ready-for-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. They require careful handling and high pressure feed connections. A tank of water is a lot easier to handle, and you can easily transfer water from one tank to another using low pressure tubes and a simple pump. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides the very high pressure, handling oxygen and even transfer in high pressure tubes is potentially dangerous because a leak can lead to fires. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This pressurized supply of oxygen is &lt;a href=&quot;https://space.stackexchange.com/a/19303/12102&quot;&gt;used to re-pressurize oxygen tanks for space suits&lt;/a&gt;, and in an emergency can be an immediate source of oxygen to (re)pressurize the station. But for a steady, lower rate supply of oxygen, electrolysis of plain old water works quite well, considering there is plenty of electrical power. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One important caveat - it is very important to vent the hydrogen to space, since it is &lt;a href=&quot;https://space.stackexchange.com/a/17671/12102&quot;&gt;dangerously flammable and explosive in some cases&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ClbID.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ClbID.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NASA NORS tank, photo from &lt;a href=&quot;https://www.nasa.gov/content/air-supply-high-pressure-tanks-ready-for-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2016-12-07T06:36:21.980" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19301" PostTypeId="1" AcceptedAnswerId="19303" CreationDate="2016-12-07T01:19:58.300" Score="2" ViewCount="354" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/19300/12102&quot;&gt;this answer&lt;/a&gt; I said:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In an emergency a pressurized tank of oxygen gas can be an immediate source of oxygen to pressurize the station, and (possibly) other thinks such as tanks for suits. But for a steady, lower rate supply of oxygen, electrolysis of plain old water works quite well, considering there is plenty of electrical power.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is this how the various space suit air supply systems are provided with oxygen, now that NORS is the preferred method for &lt;a href=&quot;https://space.stackexchange.com/a/17677/12102&quot;&gt;oxygen transport and storage&lt;/a&gt; on the ISS? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it mixed with other gases first, or do the suits use pure oxygen at reduced pressure?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-12-07T03:51:26.103" Title="Can the ISS's NORS tanks be used to recharge spacesuit or spacecraft oxygen supplies?" Tags="|iss|spacesuits|oxygen|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19302" PostTypeId="2" ParentId="19299" CreationDate="2016-12-07T02:07:06.013" Score="12" Body="&lt;p&gt;As Andrew Thompson's comment hints, it's a matter of tankage mass vs the trifling extra mass of hydrogen. Tankage is no small thing; it's the main reason hydrolox ($H_2 / O_2$) rockets, despite their unparalleled* $I_{SP}$, don't have much better performance than other liquid fuels with considerably lower specific impulse but much better &lt;em&gt;density impulse&lt;/em&gt; &amp;mdash; the hydrogen requires much larger tanks. Carrying oxygen efficiently in liquid form requires a pressure vessel and cryogenic handling (or else a much larger pressure vessel for compressed gas), and has extra safety requirements, since pure oxygen is a severe fire danger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, a &lt;a href=&quot;https://www.alibaba.com/product-detail/Liquid-gas-dewar-cylinder-liquid-Dewar_60297988100.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;large tank I found for sale&lt;/a&gt; contains up to 250 kg of lox at 2.3 MPa, and the tank itself is 151 kg, which is a really unpleasant 60% overhead (although this tank is not, of course, optimized for aerospace work). Compared to water, which can be contained in a much lighter tank and with an overhead of 11.9% hydrogen, you can see that it's a significant penalty, even without the additional handling hassles and safety problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;*&lt;em&gt;Well, unparalleled among practical, vaguely safe chemical fuels, anyway. $H_2/F_2$, $H_2/Li/F_2$, and other weirder combinations have even higher $I_{SP}$, but require enormously complex and failure-prone tanks and engines.&lt;/em&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2017-06-22T20:53:54.577" LastActivityDate="2017-06-22T20:53:54.577" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19303" PostTypeId="2" ParentId="19301" CreationDate="2016-12-07T03:51:26.103" Score="2" Body="&lt;p&gt;Yes, O2 for the EMUs must come either from NORS or from the external O2 tanks.  There is no other supply of high pressure O2 on the ISS.  See &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150010425.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; for an overview of the O2 and N2 systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The EMUs use pure O2, no other gas is mixed in.  Refer to &lt;a href=&quot;https://www.nasa.gov/pdf/188963main_Extravehicular_Mobility_Unit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Suited For Spacewalking&lt;/a&gt; for the best overview of the EMU I know of. It lists the regulated suit pressure as 29.6 kilopascal (4.3 psi).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-12-07T03:51:26.103" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19304" PostTypeId="2" ParentId="1308" CreationDate="2016-12-07T04:41:54.580" Score="1" Body="&lt;p&gt;The use of centripetal force is a very feasible solution. The reason that no current spacecraft is using rotation for artificial gravity is because no space travel to date, or currently planned, is of long enough duration to require it.&lt;/p&gt;&#xA;" OwnerUserId="17815" LastActivityDate="2016-12-07T04:41:54.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19305" PostTypeId="2" ParentId="18512" CreationDate="2016-12-07T06:27:08.637" Score="2" Body="&lt;p&gt;This is actually an open research area and despite everyone here seeming to think it's a bad idea, companies still want to do it. Why? Well the simplest reason is 10-15% reduction in mass due to cable elimination and the volume reduction. Both of these are extremely expensive in space applications and any savings that can be made are sought after.&lt;/p&gt;&#xA;" OwnerUserId="17837" LastActivityDate="2016-12-07T06:27:08.637" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19306" PostTypeId="1" CreationDate="2016-12-07T07:16:28.410" Score="3" ViewCount="316" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/19305/12102&quot;&gt;this answer&lt;/a&gt; the recent question &lt;a href=&quot;https://space.stackexchange.com/q/18512/12102&quot;&gt;Are and should satellites use wireless communication internally, rather than cables?&lt;/a&gt; it is stated that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This is actually an open research area and despite everyone here seeming to think it's a bad idea, companies still want to do it....&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; I'm going to use the term 'radio' for internal point-to-point data links. While some would call it 'wireless', other's would say that wireless includes free-space optical (IRDA would be am historical example but there will be others)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the goal for those companies is to reduce spacecraft mass by replacing data communications cables (one example of that would be Ethernet cables but there are others) and their associated electronics with radio (one example of which would be WiFi but there are others), wouldn't a better choice be the use of Plastic Optical Fiber (POF)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This could potentially provide the same high speed network architecture as a hard wired network, while radio might create a nightmare of hot-spots and dead spots within a spacecraft, requiring major mechanical redesign. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Multi-mode POF is often made from PMMA with a diameter of a few hundred microns to almost 1 millimeter, and I don't know if there are issues with radiation damage - would the attenuation of light in the range 750 to 950 through say 10 meters of the stuff be a problem? For short distances (tens of meters) POF has been demonstrated to Gigabit/sec speeds and is much more forgiving to bends and connection issues compared to the single mode or few mode glass optical fibers used in long haul.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;I'm not in any way advocating this&lt;/em&gt;, I'm asking if this &lt;em&gt;would be a better choice than intra-spacecraft radio&lt;/em&gt; when spacecraft mass reduction is the goal, as discussed in the linked question and answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Read more at ieee802(dot)org (two sources in case one link rots): &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.ieee802.org/3/cfi/0314_1/CFI_01_0314.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ieee802.org/3/cfi/0314_1/CFI_01_0314.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.ieee802.org/3/GEPOFSG/public/CFI/GigPOF%20CFI%20v_1_0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ieee802.org/3/GEPOFSG/public/CFI/GigPOF%20CFI%20v_1_0.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Random examples - no affiliation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.homefibre.at/en/products/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.homefibre.at/en/products/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.kdpof.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.kdpof.com/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.fiberoptics4sale.com/blogs/archive-posts/95052806-ethernet-over-plastic-optical-fiber&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.fiberoptics4sale.com/blogs/archive-posts/95052806-ethernet-over-plastic-optical-fiber&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ww1.microchip.com/downloads/en/AppNotes/EPOF-WP.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ww1.microchip.com/downloads/en/AppNotes/EPOF-WP.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.broadcom.com/products/fiber-optic-modules-components/industrial/industrial-control-general-purpose/650nm/afbr-1521cz&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.broadcom.com/products/fiber-optic-modules-components/industrial/industrial-control-general-purpose/650nm/afbr-1521cz&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="58" LastEditDate="2020-02-26T14:01:02.523" LastActivityDate="2020-02-26T14:01:02.523" Title="Could multimode (polymer) optical fiber Gigabit Ethernet be a better choice than WiFi inside future satellites?" Tags="|radio-communication|engineering|reliability|optical-fiber|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="19307" PostTypeId="1" CreationDate="2016-12-07T10:58:39.750" Score="2" ViewCount="87" Body="&lt;p&gt;In a breakdown of inter-program spending for research projects and federal grants, how much money did the US authorize for Space exploration in 2016?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Answers may include manned/unmanned missions, Maintenance/supply, training, IT/Communication, educational, etc.&lt;/p&gt;&#xA;" OwnerDisplayName="user16987" LastActivityDate="2016-12-07T20:12:36.217" Title="How much in US dollars were designated for space exploration research grants in the US fiscal 2016?" Tags="|research|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19308" PostTypeId="2" ParentId="19288" CreationDate="2016-12-07T11:49:38.213" Score="3" Body="&lt;p&gt;The record is &lt;a href=&quot;http://www.guinnessworldrecords.com/world-records/highest-capacity-communications-satellite/&quot; rel=&quot;nofollow noreferrer&quot;&gt;134 Gbit/s&lt;/a&gt;. But that's a new geostationary communications satellite, i.e. the whole satellite does nothing but receive and transmit data. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-12-07T11:49:38.213" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19309" PostTypeId="2" ParentId="19295" CreationDate="2016-12-07T11:54:12.063" Score="5" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/10473/6599&quot;&gt;an answer to another question&lt;/a&gt;, I suggested that a double or nested dome could be used:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jxLIc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jxLIc.png&quot; alt=&quot;Double Dome&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic idea is that if the outer dome is a bit leaky it's not a big deal because the leakage can be continually replaced with compressed martian atmosphere using big dumb compressors. Leakage from the inner dome to the outer dome would be relatively slow due to roughly equalized pressure. In this way a measure of leakage can be actively accommodated without putting undue stress on the machines responsible for creating a breathable atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A dome with an unsealed floor would want to behave in a manner not unlike a hovercraft, with the air wanting to rush out under the sides and also lift the dome up. So it'd be necessary to both bury the skirt and have some beefy regolith anchors - alternatively the pressure could be contained by burying the dome or making the dome incredibly thick; about 4m of regolith-concrete or 10m of ice for 0.4atm of pressure. Either way, I suspect that using a completely sealed dome and carting the regolith inside would involve much less digging.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-12-07T11:54:12.063" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19310" PostTypeId="1" CreationDate="2016-12-07T11:57:20.357" Score="4" ViewCount="705" Body="&lt;p&gt;For electronic components at the level of say electrolytic capacitors, ethernet connectors connectors, and possibly in the future &lt;a href=&quot;https://space.stackexchange.com/q/19306/12102&quot;&gt;fiber optic connectors/electrical signal interfaces&lt;/a&gt; are there well established, recognized certifications that commercial spacecraft manufactures would like to see (or require) for individual components?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If for example a component fails after deployment, and someone asks &quot;you did use all 'commercial space capacitors' didn't you?&quot; is there a widely recognized industrial certification of some kind that can be pointed to while saying &quot;yep, all 100% commercial space-grade electronic components!&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The background of this question is &lt;a href=&quot;https://space.stackexchange.com/q/19306/12102&quot;&gt;this question&lt;/a&gt; where I'm asking about small plastic multimode fiber connectors which would contain high speed LED or laser chips and/or photodiodes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since these contain a variety of both semiconductor materials and optical materials, would there be a certification process (and roadmap thereof) such that without it, a commercial spacecraft manufacturer would be hesitant  to use such a component, even if it was internally validated through exhaustive testing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be military and NASA grades and certifications, but I'm wondering if there are also commercial space specs, or if they just make due with other existing certification procedures.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-11-12T16:50:57.620" LastActivityDate="2019-10-29T20:40:11.080" Title="Is there a space-certification procedure used by commercial spacecraft manufactures for electronic components?" Tags="|engineering|electronics|optical-fiber|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19311" PostTypeId="2" ParentId="19288" CreationDate="2016-12-07T12:49:12.973" Score="4" Body="&lt;p&gt;Worldview-4, a high-resolution imaging satellite that has just been launched in LEO, downlinks up to 18 Terabytes per day:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.lockheedmartin.com/us/news/press-releases/2016/september/ssc-space-WorldView4-Launch.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.lockheedmartin.com/us/news/press-releases/2016/september/ssc-space-WorldView4-Launch.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is achieved  through a network of ground stations all over the globe, and a high-speed X-band datalink (800 Mbps). Note Worldview-4 is extremely expensive, cutting-edge technology and most (if not all) LEO imaging satellites downlink lower volumes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current LEO communication satellites have lower throughputs. For instance, the current Iridium satellites have only 40Mbps of crosslink throughput (&lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&lt;/a&gt;), so even if they are used at maximum capacity all day it only aggregates to 0.4 TB per day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Typically maximum downlink volume is determined by: link budget (basically your communication hardware), energy budget (how much energy your solar panels give you and how much you allocate to the comms system), and comms passes over ground station (depends on your ground station network, you can use inter-satellite links instead). This has to match the required downlink volume, which is the data produced by your sensor (basically depends on the number of pixels of you sensor for an optical camera, and the data compression methods used)&#xA;@pericynthion can probably give you real-world orders of magnitudes for cubesats.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="4685" LastEditDate="2016-12-07T13:57:43.007" LastActivityDate="2016-12-07T13:57:43.007" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19312" PostTypeId="2" ParentId="19310" CreationDate="2016-12-07T13:15:48.373" Score="1" Body="&lt;p&gt;There is, kind of. The US Military has a number of standards, called mil specs, which are required for certain components in certain kinds of environments. Among these is IEST-STD-CC1246D, which specifies cleanliness requirements for space hardware, MIL-STD-883 for electronic environmental concerns, and especially &lt;a href=&quot;https://snebulos.mit.edu/projects/reference/MIL-STD/MIL-STD-1540D.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;MIL-STD-1540D&lt;/a&gt;, which is entitled &quot;PRODUCT VERIFICATION REQUIREMENTS FOR LAUNCH, UPPER STAGE, AND SPACE VEHICLES&quot;. No doubt there are others that I am missing as well, but the general idea is the US Military puts out specifications that are used in the commercial world.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-12-07T13:15:48.373" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19313" PostTypeId="1" AcceptedAnswerId="19314" CreationDate="2016-12-07T16:01:37.833" Score="8" ViewCount="2654" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;The 'impossible' EM Drive is about to be tested in space&#xA;  Time to get to the bottom of this crazy thing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sciencealert.com/the-impossible-em-drive-is-about-to-be-tested-in-space&quot; rel=&quot;noreferrer&quot;&gt;http://www.sciencealert.com/the-impossible-em-drive-is-about-to-be-tested-in-space&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back in September I saw a bunch of articles about actually testing the EMdrive in space...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it really going to space?   Has it actually booked a trip?   Does NASA/eagleworks have the budget for this?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I didn't notice this experiment mentioned in any of the recent EMdrive peer review discussions and was surprised.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I couldn't find any raw details other then space tabloids claiming it booked a flight, and wanted to know more details.&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2016-12-15T01:13:14.277" Title="Is the 'impossible' EMdrive going to space?" Tags="|launch|emdrive|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19314" PostTypeId="2" ParentId="19313" CreationDate="2016-12-07T16:08:32.980" Score="14" Body="&lt;p&gt;&lt;strong&gt;UPDATE (12/14/2016):&lt;/strong&gt; Apparently, aside from the Cannae launch, the EmDrive is actually already in space, according to &lt;a href=&quot;http://www.ibtimes.co.uk/emdrive-chinese-space-agency-put-controversial-tech-onto-satellites-soon-possible-1596328&quot; rel=&quot;nofollow noreferrer&quot;&gt;a recent announcement by China&lt;/a&gt;, who claims to be testing it in orbit.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;No, the media has confused things. Cannae, whose tech is being launched, issued a &lt;a href=&quot;http://cannae.com/cubesat-mission-clarification/&quot; rel=&quot;nofollow noreferrer&quot;&gt;release&lt;/a&gt; clarifying things:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There has been a lot of erroneous information in media articles regarding Cannae’s upcoming launch of a cubesat mission into LEO. To clarify our previous post and press release: Cannae is not using an EmDrive thruster in our upcoming launch. Cannae is using it’s [sic] own proprietary thruster technology which requires no on-board propellant to generate thrust. In addition, this project is being done as a private venture. Cannae is only working with our private commercial partners on the upcoming mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-12-15T01:13:14.277" LastActivityDate="2016-12-15T01:13:14.277" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19315" PostTypeId="1" AcceptedAnswerId="19318" CreationDate="2016-12-07T16:26:17.493" Score="10" ViewCount="10737" Body="&lt;p&gt;From &lt;a href=&quot;https://space.stackexchange.com/questions/5416/how-many-satellites-are-orbiting-the-earth&quot;&gt;this question&lt;/a&gt; we know that there are quite a few  satellites orbiting Earth. But how do &lt;em&gt;ESA&lt;/em&gt; and &lt;em&gt;NASA&lt;/em&gt; avoid their rockets colliding with these satellites (is there some kind of database of (man-made) satellites)? And is it possible that countries have &lt;strong&gt;secretly&lt;/strong&gt; sent up spy satellites which Space organisations cannot track?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am also interested if there have ever been collisions between rockets and satellites before...&lt;/p&gt;&#xA;" OwnerUserId="11358" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2020-10-03T15:00:51.827" Title="How do rockets avoid satellites?" Tags="|rockets|artificial-satellite|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19316" PostTypeId="1" CreationDate="2016-12-07T16:28:47.147" Score="3" ViewCount="209" Body="&lt;p&gt;Is there any safety advantage/disadvantage to a particular attitude in space - lined up with the sun, sideways, tumbling or spinning?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming a long cylindrical space craft on a long journey within the solar system. It is subject to solar radiation, meteoroids/debris and cosmic radiation, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could see a possible advantage in minimizing shielding materials if lined up with the sun but the other things seem unaffected.&lt;/p&gt;&#xA;" OwnerUserId="17815" LastEditorUserId="17815" LastEditDate="2016-12-07T16:37:56.157" LastActivityDate="2016-12-07T17:35:31.690" Title="Is there any safety advantage/disadvantage to a particular attitude of a vessel in space?" Tags="|spacecraft|cosmic-radiation|solar-wind|meteoroid|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19317" PostTypeId="2" ParentId="19307" CreationDate="2016-12-07T17:09:37.267" Score="3" Body="&lt;p&gt;NASA asked for \$18.5 billion of the US budget for 2016, and NASA was granted \$19.3 billion. Here you can see how NASA planned to break down that budget:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HYG7Y.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HYG7Y.jpg&quot; alt=&quot;NASA FY 2016 Budget Request&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Thomas W pointed out in the comments, a PDF version of the budget estimates is &lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/NASA_FY_2016_Budget_Estimates.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;also available&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/NASA_FY2016_Summary_Brief_corrected.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Fiscal Year 2016 Budget Estimates&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.theatlantic.com/science/archive/2015/12/nasa-budget-2016-mars-rover-europa-mission-planetary-science/420912/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's About to Have Its Biggest Budget in a Decade - The Atlantic&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-12-07T20:12:36.217" LastActivityDate="2016-12-07T20:12:36.217" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19318" PostTypeId="2" ParentId="19315" CreationDate="2016-12-07T17:30:39.817" Score="12" Body="&lt;p&gt;Space agencies actively track not only satellites but debris in orbit using radar, and they do have a database of these objects and powerful computer which can calculate where these objects will be based on their parameters. When a launch is planned these objects are taken into account and the trajectory planned to avoid them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible (although unlikely) that stealthy satellites exist which are not tracked. In this case the controlling agency of that satellite will almost certainly do their own calculations to make sure their asset remains clear, and if not the sheer volume of space would make the probability of any collision very very low.  &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-12-07T17:30:39.817" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19319" PostTypeId="2" ParentId="19316" CreationDate="2016-12-07T17:35:31.690" Score="6" Body="&lt;p&gt;&lt;strong&gt;Radiation&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you have rightly noted, a long cylinder pointing end on to the radiation source (probably the sun, especially during periods of high activity), can be shielded by a smaller, and thus lighter, shield.  On the other hand if you need shielding facing all directions, then a sphere is a better choice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;(Micro-)meteorites&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spaceship is moving fast, in some direction given by where it is, where it wants to be, and orbital mechanics.  Dust, rocks and debris will be moving somewhat at random, but will tend to have a preferred direction.  They mostly go round the sun/planet/moon the same way the rest of the solar system rotates, and there won't be many rocks on orbits which pass through another body.  So in some cases, there will be a direction from which more rocks come than others.  It makes sense to point one end of the long cylinder in that direction to reduce the chance of being hit.  Note that the ISS is constructed/orientated with this at least partly borne in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Solar heating&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This isn't something which drives the orientation as much as something which might make it unsafe to vary from the orientation the designers planned.  You probably have solar panels which want to face the sun, and radiators which don't.  If you don't keep an orientation which allows them to do so, and you don't have alternate energy sources/heat sinks, you're going to have problems. Similarly, if the design was expected to face the sun and ends up spinning, the rapid heating/cooling cycles might do bad things to the skin. If it was intended to spin and is now static, thermal management could be difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Stability&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 3D object spinning in space is stable when spinning in some directions, but not others. &lt;a href=&quot;https://www.youtube.com/watch?v=1n-HMSCDYtM.&quot; rel=&quot;noreferrer&quot;&gt;Don't be this shape&lt;/a&gt;.  It would be wise to pick a rotation which is stable, even if stuff moves around inside the ship.  Otherwise you'll be wasting propellant or electricity (on reaction wheels/gyros) staying stable, and have additional risks if that subsystem fails. This isn't about the external shape as much as it is about the distribution of mass inside that shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Artificial Gravity&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you ask about a large ship, and are concerned about radiation, I wonder if you are thinking of a future manned ship.  If so, you'll want to set the spin up to give reasonable gravity, uniform gravity within the crew compartment, and minimal Coriolis effect. That way your astronauts are less likely to misjudge it and get hurt, or spend too much time throwing up. Overall safety is going to depend on them making good decisions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are probably other considerations I haven't though of...&lt;/p&gt;&#xA;" OwnerUserId="11611" LastActivityDate="2016-12-07T17:35:31.690" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19320" PostTypeId="2" ParentId="19261" CreationDate="2016-12-07T20:47:40.827" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Kepler has 16 GB of memory. Is this DRAM?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes and no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The yes part: It's SDRAM. The no part: It's not the computer's main memory. It's instead the equivalent of a tape recorder, only using solid state technology instead of moving parts. That solid state data recorder is used to record the data gathered by Kepler's sensors in between intermittent downloads via the Deep Space Network.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tape recorders have a number of problems. They're large, massive, power hungry, slow, and prone to mechanical failures. NASA began using random access memory as the basis for solid state recorders in 1996 with the Inflatable Antenna Experiment flown on STS-77.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concerns of accessing each of the 16 gigabytes individually is not an issue precisely because this isn't the computer's main memory. There's no reason to make this recorder addressable at the byte level. Kepler captures big chunks of data at once, which are then processed and written as somewhat smaller (but still large) chunks to the recorder. During download via the Deep Space Network, even bigger chunks are read and transmitted to the ground.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-12-08T17:44:02.070" LastActivityDate="2016-12-08T17:44:02.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19322" PostTypeId="1" CreationDate="2016-12-07T23:54:53.953" Score="15" ViewCount="30404" Body="&lt;p&gt;&lt;strong&gt;Updated 2016-12-12&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;I'm writing an orbital mechanics program in &lt;a href=&quot;https://unity3d.com/&quot; rel=&quot;noreferrer&quot;&gt;Unity&lt;/a&gt;. I convert an orbiting object's position and velocity into orbital elements, then converting the orbital elements back into cartesian position vectors so that I can plot the entire orbit. I followed the equations on these two links:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;//space.stackexchange.com/questions/1904/how-to-programmatically-calculate-orbital-elements-using-position-velocity-vecto&quot;&gt;Cartesian State Vectors to Keplerian Elements&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://downloads.rene-schwarz.com/download/M001-Keplerian_Orbit_Elements_to_Cartesian_State_Vectors.pdf&quot; rel=&quot;noreferrer&quot;&gt;Keplerian Elements to Cartesian State Vectors&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The equations I am following to calculate state vectors assume the reference plane is X-Y. In &lt;a href=&quot;https://unity3d.com/&quot; rel=&quot;noreferrer&quot;&gt;Unity&lt;/a&gt; the reference plane is X-Z, with Y being the &amp;quot;up direction&amp;quot;. So what I did was subtract 90 degrees from the inclination such that an orbit on the X-Z plane would be zero inclination, which ended up being the cause of my issues. So I deleted that modification, and now &lt;em&gt;most&lt;/em&gt; of my orbits display correctly, for orbits that begin at the periapsis. But orbits beginning at the apoapsis are reversed. See the images below:&lt;/p&gt;&#xA;&lt;p&gt;Here is a properly drawn orbit. I have a little spaceship orbiting, and it starts on the -X axis. I've drawn an incomplete orbit so that I can see the starting point of the orbit, which you can see begins on the -X axis.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/lrtqy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lrtqy.jpg&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here I changed the starting state of the ship so that it starts at apoapsis. As you can see the starting point of the orbit is 180 degrees off, lying on the +X axis instead of the -X axis. In fact, the orbit is always drawn starting from the periapsis. &lt;strong&gt;What I'm trying to do is have the first point of the orbit be the ship's position at the point in time when I call the ConvertToCartesian function&lt;/strong&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/qA6pL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qA6pL.jpg&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here's my code:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Vector3 ConvertToCartesian(float e, float a, float i, float O, float w, float t, float t0){&#xA;    float mu = G * M;&#xA;    float Mt; //Mean anomaly at time t&#xA;    float T = 2 * Mathf.PI * Mathf.Sqrt(a * a * a / mu); //Orbital period&#xA;    if (t == t0) {&#xA;        t = t0;&#xA;        Mt = 0; &#xA;    } else {&#xA;        float deltaT = (T/60) * (t - t0); //divide time increments into 1/60th of the orbital period&#xA;        Mt = deltaT * Mathf.Sqrt (mu / Mathf.Pow (a, 3));&#xA;    }&#xA;    //Calculate eccentric anomaly using Newton's method&#xA;    float E = Mt;&#xA;    float F = E - e * Mathf.Sin (E) - Mt;&#xA;    int j = 0, maxIter = 30;&#xA;    float delta = 0.000001f;&#xA;    while (Mathf.Abs(F) &amp;gt; delta &amp;amp;&amp;amp; j &amp;lt; maxIter) {&#xA;        E = E - F / (1 - e * Mathf.Cos (E));&#xA;        F = E - e * Mathf.Sin (E) - Mt;&#xA;        j++;&#xA;    }&#xA;    float nu = 2 * Mathf.Atan2 (Mathf.Sqrt (1 + e) * Mathf.Sin (E / 2), Mathf.Sqrt (1 - e) * Mathf.Cos (E / 2)); //True anomaly&#xA;&#xA;    float rc = a * (1 - e * Mathf.Cos(E)); //Distance to central body&#xA;&#xA;    Vector3 o = new Vector3(rc * Mathf.Cos(nu), rc * Mathf.Sin(nu), 0); &#xA;                &#xA;    Vector3 odot = new Vector3 (Mathf.Sin (E), Mathf.Sqrt (1 - e * e) * Mathf.Cos (E), 0); //Velocity vector in the orbital frame&#xA;    odot = (Mathf.Sqrt (mu * a) / rc) * odot;&#xA;&#xA;    float rx, ry, rz;&#xA;    rx = o.x; ry = o.y; rz = o.z;&#xA;&#xA;    rx = ( o.x * (Mathf.Cos (w) * Mathf.Cos (O) - Mathf.Sin (w) * Mathf.Cos (i) * Mathf.Sin (O)) -&#xA;            o.y * (Mathf.Sin (w) * Mathf.Cos (O) + Mathf.Cos (w) * Mathf.Cos (i) * Mathf.Sin (O)));&#xA;    ry = (o.x * (Mathf.Cos (w) * Mathf.Sin (O) + Mathf.Sin (w) * Mathf.Cos (i) * Mathf.Cos (O)) +&#xA;        o.y * (Mathf.Cos (w) * Mathf.Cos (i) * Mathf.Cos (O) - Mathf.Sin (w) * Mathf.Sin (O)));&#xA;    rz = (o.x * (Mathf.Sin (w) * Mathf.Sin (i)) + o.y * (Mathf.Cos (w) * Mathf.Sin (i)));&#xA;&#xA;    Vector3 r = new Vector3(rx, ry, rz); //Position vector&#xA;&#xA;    return r / objectScale;&#xA;&#xA;}&#xA;&#xA;&#xA;/* Convert a body's cartesian state vectors (position, r, and velocity, v) into Kepler elements&#xA; */&#xA;void ConvertToKeplerElements(Vector3 r, Vector3 v){&#xA;    float mu = G * M;&#xA;    Vector3 h = Vector3.Cross (r, v); //Orbital momentum&#xA;    Vector3 n = Vector3.Cross (Vector3.forward, h); &#xA;    eVector = ((v.magnitude * v.magnitude - mu / r.magnitude) * r - Vector3.Dot(r, v) * v) / mu; //Eccentricity&#xA;    float emag = eVector.magnitude;&#xA;    float E = v.magnitude * v.magnitude / 2 - mu / r.magnitude; //Specific mechanical energy&#xA;    if (eVector.magnitude != 1) {&#xA;        a = -mu / (2 * E); //Semi major axis&#xA;        float p = a * (1 - emag * emag);&#xA;    } else {&#xA;        //a = infinity&#xA;        a = 0;&#xA;        float p = h.magnitude * h.magnitude / mu;&#xA;    }&#xA;&#xA;    inc = Mathf.Acos (h.z / h.magnitude); //Inclination&#xA;&#xA;    //Longitude of ascending node (LAN), angle of body to node vector&#xA;    if (inc == 0 || inc == Mathf.PI) { &#xA;        O = 0; //For an equatorial orbit (i = 0), LAN is undefined. By convention, set to 0. &#xA;    } else {&#xA;        O = Mathf.Acos (n.x / n.magnitude); &#xA;    }&#xA;    if (n.y &amp;lt; 0) {&#xA;        O = 2 * Mathf.PI - O;&#xA;    }&#xA;&#xA;    //Argument of periapsis&#xA;    if (eVector.magnitude == 0) {&#xA;        w = 0; //For a circular orbit, by convention place w at ascending node (w = 0)&#xA;    } else {&#xA;        if (inc == 0 || inc == Mathf.PI) {&#xA;            w = Mathf.Atan2 (eVector.y, eVector.x);&#xA;        } else {&#xA;            w = Mathf.Acos (Vector3.Dot (n, eVector) / (n.magnitude * eVector.magnitude)); &#xA;        }&#xA;    }&#xA;    if (eVector.z &amp;lt; 0) {&#xA;        w = 2 * Mathf.PI - w;&#xA;    }&#xA;&#xA;    //True anomaly&#xA;    if (eVector.magnitude == 0) {&#xA;        if (inc == 0 || inc == Mathf.PI) {&#xA;            nu = Mathf.Acos (r.z/ r.magnitude); //For a circular non-inclined orbit, nu is angle of body from the x axis&#xA;            //(True Longitude)&#xA;        } else {&#xA;            //For a circular inclined orbit, nu is the angle between ascending node position vectors, (Argument of Latitude)&#xA;            nu = Mathf.Acos (Vector3.Dot (n, r) / (n.magnitude * r.magnitude));&#xA;        }&#xA;    } else {&#xA;        nu = Mathf.Acos (Vector3.Dot (eVector, r) / (eVector.magnitude * r.magnitude)); //True anomaly is angle between eccentricity and position vectors&#xA;        if (Vector3.Dot (r, v) &amp;lt; 0) {&#xA;            nu = 2 * Mathf.PI - nu;&#xA;        }&#xA;    }&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;To draw the orbit line I just call the function ConvertToCartesian in a loop with increasing values of t to get an array of position vectors.&lt;/p&gt;&#xA;&lt;p&gt;Values: M = 3.3e16, GM ~= 2.2e6, r = (-50000, 0,0)&#xA;For a circular orbit, v = (0,0, sqrt(GM/50000))&lt;/p&gt;&#xA;&lt;p&gt;Follow up question: since the equations assume X-Y reference plane, how can I change it so that it conforms to the X-Z reference plane I'm using in my program?&lt;/p&gt;&#xA;&lt;h1&gt;Update&lt;/h1&gt;&#xA;&lt;p&gt;I updated the method ConvertToCartesian, such that it takes the True Anomaly (nu) calculated from the ConvertToKeplerElements function as an input. Then it calculates Eccentric Anomaly (E), then calculates Mean Anomaly (Mt) from that. When ConvertToCartesian is called in subsequent loops, Mt is updated with the relevant time step. Then I calculate E again from the updated Mt, then calculate nu from the updated E. This solves most cases. Problem is, sometimes nu is calculated 180 degrees off from where it should be, making the starting point of the displayed orbit 180 degrees off.&lt;/p&gt;&#xA;&lt;p&gt;In my ConvertToKeplerElements function, according to the math, nu = 2pi - nu if r.v &amp;lt; 0. This happens, for example, when I set the initial r, v vectors such that the ship starts neither at periapsis or apoapsis but somewhere between. I know that this is correct, &lt;strong&gt;but when I calculate nu-&amp;gt;E-&amp;gt;nu, that 180 degree flip is lost in the calculation.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here's the relevant parts of the updated method ConvertToCartesian.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;        float mu = G * M;&#xA;    float E = Mathf.Acos((e + Mathf.Cos(nu)) / (1 + e * Mathf.Cos(nu)));&#xA;    float Mt = E - e * Mathf.Sin (E);&#xA;&#xA;    //Mean anomaly at time t&#xA;    float T = 2 * Mathf.PI * Mathf.Sqrt(a * a * a / mu); //Orbital period&#xA;    float deltaT = (T/60) * (t - t0); //divide time increments into 1/60th of the orbital period&#xA;    Mt = Mt + deltaT * Mathf.Sqrt (mu / Mathf.Pow (a, 3));&#xA;&#xA;    //Calculate eccentric anomaly using Newton's method&#xA;    float F = E - e * Mathf.Sin (E) - Mt;&#xA;    int j = 0, maxIter = 30;&#xA;    float delta = 0.000001f;&#xA;    while (Mathf.Abs(F) &amp;gt; delta &amp;amp;&amp;amp; j &amp;lt; maxIter) {&#xA;        E = E - F / (1 - e * Mathf.Cos (E));&#xA;        F = E - e * Mathf.Sin (E) - Mt;&#xA;        j++;&#xA;    }&#xA;        &#xA;    //True anomaly&#xA;    nu = 2 * Mathf.Atan2 (Mathf.Sqrt (1 + e) * Mathf.Sin (E / 2), Mathf.Sqrt (1 - e) * Mathf.Cos (E / 2)); &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Here's the bad result I'm getting: &lt;a href=&quot;https://i.stack.imgur.com/WqE8z.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WqE8z.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The true anomaly at the start of the orbit should be 1.5pi (~4.71), but when calculating it from E it is 0.5pi(~1.57)&lt;/p&gt;&#xA;" OwnerUserId="17821" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2021-01-20T11:38:56.643" Title="Converting Orbital Elements to Cartesian State Vectors" Tags="|orbital-mechanics|orbit|orbital-elements|mathematics|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19323" PostTypeId="1" AcceptedAnswerId="19376" CreationDate="2016-12-08T03:51:42.580" Score="13" ViewCount="1162" Body="&lt;p&gt;I'd always heard the story about how Apollo 12 was struck by lightning and wondered why they let it head up in a storm. They always check the weather before a launch, and if there is any chance of even a little rain they delay the flight. What was the reason for Apollo 12's exception?&lt;/p&gt;&#xA;" OwnerUserId="17851" LastActivityDate="2016-12-18T17:11:29.247" Title="Why was Apollo 12 allowed to go up in such bad weather?" Tags="|launch|apollo-program|apollo-12|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19324" PostTypeId="2" ParentId="19051" CreationDate="2016-12-08T03:53:29.770" Score="4" Body="&lt;p&gt;Regarding your recent edit: No you can't just bounce around the atmosphere to slow down with a blimp.&lt;/p&gt;&#xA;&lt;p&gt;Unlike water, there is no clear limit between space and atmosphere, which leaves you with two options:&lt;/p&gt;&#xA;&lt;h2&gt;Phase 1:&lt;/h2&gt;&#xA;&lt;p&gt;You can decelerate at very high altitude where there is only faint trace of atmosphere&lt;/p&gt;&#xA;&lt;p&gt;=&amp;gt; This will eventually decrease &lt;strong&gt;some&lt;/strong&gt; (i.e. not nearly enough) of your speed, until you will enter phase 2.&lt;/p&gt;&#xA;&lt;p&gt;This has been used multiple times by satellites, and was first used by the Hiten mission&lt;/p&gt;&#xA;&lt;h2&gt;Phase 2:&lt;/h2&gt;&#xA;&lt;p&gt;To generate enough lift to ascent and skip the atmosphere like you describe, you need too have substantial atmosphere. By the time you reach the point where you can do this, this is what you looks like:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xdhIm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xdhIm.jpg&quot; alt=&quot;STS reentry&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;A giant ball of fire.&#xA;A blimp cannot survive this since it cannot afford to carry any significant weight (shield) that would allow it to reenter safely.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;There is no in-between&lt;/strong&gt;: you cannot maintain altitude and slow down gradually until you reach survivable speed.&lt;/p&gt;&#xA;&lt;p&gt;As a side note: Orbital altitude is a function of speed. If you decrease speed you decrease altitude. That's why you can't do that.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-12-08T03:53:29.770" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19326" PostTypeId="2" ParentId="19315" CreationDate="2016-12-08T09:07:03.297" Score="4" Body="&lt;p&gt;Nasa have records of data about debris and satellite orbiting the earth which it share with other agencies out there.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Tracking Debris&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Department of Defense maintains a highly accurate satellite&#xA;  catalog on objects in Earth orbit that are larger than a softball.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;NASA and the DoD cooperate and share responsibilities for&#xA;  characterizing the satellite (including orbital debris) environment.&#xA;  DoD’s Space Surveillance Network tracks discrete objects as small as 2&#xA;  inches (5 centimeters) in diameter in low Earth orbit and about 1 yard&#xA;  (1 meter) in geosynchronous orbit. Currently, about 15,000 officially&#xA;  cataloged objects are still in orbit. The total number of tracked&#xA;  objects exceeds 21,000. Using special ground-based sensors and&#xA;  inspections of returned satellite surfaces, NASA statistically&#xA;  determines the extent of the population for objects less than 4 inches&#xA;  (10 centimeters) in diameter.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Collision risks are divided into three categories depending upon size&#xA;  of threat. For objects 4 inches (10 centimeters) and larger,&#xA;  conjunction assessments and collision avoidance maneuvers are&#xA;  effective in countering objects which can be tracked by the Space&#xA;  Surveillance Network. Objects smaller than this usually are too small&#xA;  to track and too large to shield against. Debris shields can be&#xA;  effective in withstanding impacts of particles smaller than half an&#xA;  inch (1 centimeter)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/news/orbital_debris.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Debris and Human Spacecraft&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isro uses the data complied by Nasa with a Radar specifically design to track multiple objects at same time called MOTR right now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacedaily.com/reports/India_to_test_its_home_made_multi_object_tracking_radar_next_month_999.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;MOTR&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14616" LastActivityDate="2016-12-08T09:07:03.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19327" PostTypeId="2" ParentId="19323" CreationDate="2016-12-08T09:12:59.350" Score="-1" Body="&lt;p&gt;May be they did not delay the flight this time because President Richard Nixon attended the launch, see &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_12&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Apollo_12&lt;/a&gt; and the discussion here &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/001509.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.collectspace.com/ubb/Forum29/HTML/001509.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-12-08T09:12:59.350" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19328" PostTypeId="1" CreationDate="2016-12-08T12:06:30.890" Score="2" ViewCount="276" Body="&lt;p&gt;If a spacecraft is going to use solar power as its source of electricity (either for propulsion or just components or both) and it's also going to travel far out into the solar system, to quite high AU, it's going to need larger and more powerful solar arrays to generate the required power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's the best way of folding or somehow condensing an array of a large surface area into a small volume? I know some can operate like blinds but others work like extendable fans. Is there an outright most efficient way to do this?&lt;/p&gt;&#xA;" OwnerUserId="17552" LastEditorUserId="17552" LastEditDate="2016-12-08T19:26:57.817" LastActivityDate="2017-01-12T17:03:29.083" Title="Solar Panel folding" Tags="|solar-power|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19329" PostTypeId="1" CreationDate="2016-12-08T14:12:37.953" Score="6" ViewCount="2405" Body="&lt;p&gt;Is it possible to use a hypersonic jet that goes at extreme speeds in the atmosphere, then once it's at a high altitude it points down to just above the horizon and speeds up. Then it flies by that point that was the horizon which it is now going up because earth is curved and its traveling in a straight line. Would this work to create enough momentum and speed to escape Earth's atmosphere with a single stage vehicle?&lt;img src=&quot;https://i.stack.imgur.com/bRGyN.jpg&quot; alt=&quot;this is the idea behind it&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="17856" OwnerDisplayName="sirmaxalot305" LastEditorUserId="17856" LastEditDate="2016-12-08T15:45:44.167" LastActivityDate="2021-06-08T20:06:37.083" Title="Could you use a hypersonic space plane to fly into space?" Tags="|spaceplane|supersonic|" AnswerCount="5" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="19330" PostTypeId="2" ParentId="18956" CreationDate="2016-12-08T15:44:01.240" Score="11" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: I modified the answer in 2022, added new data on communications with Voyager.&lt;/p&gt;&#xA;&lt;p&gt;The sort of emergency that would require this power level is rare. You're looking at events where a remote (deep-space) spacecraft loses its orientation and can't use its high-gain antenna.&lt;/p&gt;&#xA;&lt;p&gt;For Voyager 2, communication at more than 20 kW is used routinely.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the regular 'reset the timer, keep doing what you're doing' command is sent at 18 kW.&lt;/li&gt;&#xA;&lt;li&gt;if more commands have to be sent, they &lt;a href=&quot;https://youtu.be/FzRP1qdwPKw?t=562&quot; rel=&quot;nofollow noreferrer&quot;&gt;transmit at 75 kW&lt;/a&gt;(as of 2017).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The receiver on Voyager 2 has a failed capacitor which means the frequency it's tuned to is not stable. They have to find the correct frequency each time (by sending the same command at a bunch of frequencies until they get a response).&#xA;For command sequences, they want to avoid a communications session getting interrupted in the middle by this capacitor drifting, so they send at more power which gives the receiver more margin.&lt;/p&gt;&#xA;&lt;p&gt;Older info remains here:&lt;/p&gt;&#xA;&lt;p&gt;I found one occasion where DSS-43 was used at 95 kW: &lt;a href=&quot;http://sci.esa.int/rosetta/48763-rosetta-status-report-no-147/&quot; rel=&quot;nofollow noreferrer&quot;&gt;during tests for communications with Rosetta&lt;/a&gt; just before it entered hibernation mode in 2010:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;During the hibernation entry window next year, Rosetta will be between 3.7 and 4.4 AU from Earth. At those distances, and once Rosetta is spinning, it will be necessary to be able to command Rosetta using the low-gain antenna (LGA) and to detect the strobing signal coming from the high-gain antenna (HGA).&lt;/p&gt;&#xA;&lt;p&gt;On 19 November, a test similar to the one conducted on 20 August was repeated to validate the RF activities and performance to be expected at the end of the DSHM entry window in July 2011 at a geocentric distance of about 4.4 AU. The tested elements were as follows:&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;viability of LGA-F S-band telecommand link (7.8 bps) from DSN 70-metre dish (DSS-43, Canberra) with 95 kW uplink power&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I haven't found other instances, but that doesn't mean it's never happened. I don't remember ever reading about an outer planetary system mission losing orientation and requiring recovery via MGA or LGA.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2022-01-03T17:43:41.803" LastActivityDate="2022-01-03T17:43:41.803" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="19331" PostTypeId="1" CreationDate="2016-12-08T16:23:40.377" Score="12" ViewCount="744" Body="&lt;p&gt;I've just read in Eshleman et al 1977 &lt;a href=&quot;http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1977SSRv...21..207E&amp;amp;defaultprint=YES&amp;amp;filetype=.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radio Science Investigations with Voyager&lt;/a&gt; that the voyagers have:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...a novel attitude-control thruster configuration that minimizes accelerations along the Earth-spacecraft line-of-sight;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;I am guessing&lt;/strong&gt; that the goal would be to minimize any tiny amount of delta-v that would affect doppler measurements of velocity, since there is so much science available in these precision measurements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this just mean that the &quot;novel attitude-control thruster configuration&quot; is designed to balance the impulse from the thrusters used so that only torque is applied about the spacecraft center of mass? If so, how was this novel minimization accomplished?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it doesn't, then what does it mean?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-08-09T11:36:08.283" LastActivityDate="2021-06-09T16:43:37.183" Title="How are the Voyagers' thrusters configured in a novel way to minimize accelerations along Earth-spacecraft axis?" Tags="|engines|attitude|voyager|attitude-determination-and-control|doppler-ranging|" AnswerCount="2" CommentCount="26" ContentLicense="CC BY-SA 4.0" />
  <row Id="19332" PostTypeId="2" ParentId="19323" CreationDate="2016-12-08T18:07:02.460" Score="4" Body="&lt;p&gt;Apollo 12 actually was not struck by lightning, despite the common misconception. It did fly through rain, and was the only manned spacecraft to fly through rain. In that flight, there was a massive static electricity buildup that lead to the two electrical events. &lt;a href=&quot;https://ams.confex.com/ams/pdfpapers/65020.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a rule, 1-404, that states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“The vehicle will not be launched when its flight path will carry it through a a cumulonimbus (thunderstorm) cloud formation.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That in fact was not violated, as it was not actually struck by lightning. The controllers realized they were close to violating that rule, but opted to go ahead and fly the mission. Still, they learned, and as mentioned earlier, this was the last time they attempted to launch in rain, let alone a potential thunderstorm.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-12-08T18:07:02.460" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19333" PostTypeId="1" AcceptedAnswerId="19334" CreationDate="2016-12-08T18:26:32.307" Score="74" ViewCount="16790" Body="&lt;p&gt;In other words, why didn't NASA make their main space launch area in Texas? It has a point below the 30th parallel, which means that it is close enough to the equator. Why did NASA pick Florida?&lt;/p&gt;&#xA;" OwnerUserId="17599" LastEditorUserId="-1" LastEditDate="2016-12-08T21:53:18.693" LastActivityDate="2016-12-13T10:42:32.597" Title="Why is the American space program launch site based in Florida and not Texas?" Tags="|nasa|launch-site|cape-canaveral|" AnswerCount="7" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19334" PostTypeId="2" ParentId="19333" CreationDate="2016-12-08T18:36:16.567" Score="108" Body="&lt;p&gt;Basically, the most prominent reason is so that &lt;a href=&quot;https://www.scientificamerican.com/article/space-shuttle-weather-florida/&quot;&gt;if something happens during launch, it happens over the Atlantic and not someone else&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anything launching over the Gulf of Mexico will probably cross over land a couple of times before going over the Atlantic. As geoffc pointed out in the comments, the Atlantic is a lot wider than the Gulf. Once the rocket has crossed the Gulf it will still be low enough that anything that falls over land could still cause damage. Once the rocket has crossed the Atlantic, it should already be high enough that this is not really much of an issue (stuff burns up in the atmosphere as it falls).&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2016-12-12T14:36:58.273" LastActivityDate="2016-12-12T14:36:58.273" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="19335" PostTypeId="2" ParentId="19322" CreationDate="2016-12-08T19:11:09.073" Score="12" Body="&lt;p&gt;You can find better instructions here:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://web.archive.org/web/20160418175843/https://ccar.colorado.edu/asen5070/handouts/cart2kep2002.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cartesian to Keplerian (PDF)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://web.archive.org/web/20170810015111/http://ccar.colorado.edu/asen5070/handouts/kep2cart_2002.doc&quot; rel=&quot;nofollow noreferrer&quot;&gt;Keplerian to Cartesian (DOC)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I codded all steps given in these documents to check if I could find your error but it seems to work. I will give my code below (in Python) and perhaps if you compare it to yours you can find your error.&lt;/p&gt;&#xA;&lt;p&gt;Running the code I got the follow output:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;[ -9.31322575e-10  -1.09686156e-20   4.12053763e-05]&#xA;[  2.80478539e-13   9.09494702e-13  -8.42143002e-19]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Considering that &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; is in meters and &lt;span class=&quot;math-container&quot;&gt;$\dot{r}$&lt;/span&gt; is in meters per second these errors are quite acceptable.&lt;/p&gt;&#xA;&lt;p&gt;OBS: If you want to transform &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\dot{r}$&lt;/span&gt; to Earth Centered and Fixed there is an additional transformation described in the document!&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;from astropy.constants import G, M_earth, R_earth&#xA;from astropy import units as u&#xA;import numpy as np&#xA;&#xA;mu = G.value*M_earth.value&#xA;Re = R_earth.value&#xA;&#xA;#Test vectors&#xA;r_test = np.array([Re + 600.0*1000, 0, 50])&#xA;v_test = np.array([0, 6.5 * 1000, 0])&#xA;t = 0&#xA;&#xA;&#xA;def cart_2_kep(r_vec,v_vec):&#xA;    #1&#xA;    h_bar = np.cross(r_vec,v_vec)&#xA;    h = np.linalg.norm(h_bar)&#xA;    #2&#xA;    r = np.linalg.norm(r_vec)&#xA;    v = np.linalg.norm(v_vec)&#xA;    #3&#xA;    E = 0.5*(v**2) - mu/r&#xA;    #4&#xA;    a = -mu/(2*E)&#xA;    #5&#xA;    e = np.sqrt(1 - (h**2)/(a*mu))&#xA;    #6&#xA;    i = np.arccos(h_bar[2]/h)&#xA;    #7&#xA;    omega_LAN = np.arctan2(h_bar[0],-h_bar[1])&#xA;    #8&#xA;    #beware of division by zero here&#xA;    lat = np.arctan2(np.divide(r_vec[2],(np.sin(i))),\&#xA;    (r_vec[0]*np.cos(omega_LAN) + r_vec[1]*np.sin(omega_LAN)))&#xA;    #9&#xA;    p = a*(1-e**2)&#xA;    nu = np.arctan2(np.sqrt(p/mu) * np.dot(r_vec,v_vec), p-r)&#xA;    #10&#xA;    omega_AP = lat - nu&#xA;    #11&#xA;    EA = 2*np.arctan(np.sqrt((1-e)/(1+e)) * np.tan(nu/2))&#xA;    #12&#xA;    n = np.sqrt(mu/(a**3))&#xA;    T = t - (1/n)*(EA - e*np.sin(EA))&#xA;&#xA;    return a,e,i,omega_AP,omega_LAN,T, EA&#xA;&#xA;def kep_2_cart(a,e,i,omega_AP,omega_LAN,T, EA):&#xA;&#xA;    #1&#xA;    n = np.sqrt(mu/(a**3))&#xA;    M = n*(t - T)&#xA;    #2&#xA;    MA = EA - e*np.sin(EA)&#xA;    #3&#xA;    #&#xA;    # ERROR WAS HERE&#xA;   #nu = 2*np.arctan(np.sqrt((1-e)/(1+e)) * np.tan(EA/2))&#xA;    nu = 2*np.arctan(np.sqrt((1+e)/(1-e)) * np.tan(EA/2))&#xA;    #4&#xA;    r = a*(1 - e*np.cos(EA))&#xA;    #5&#xA;    h = np.sqrt(mu*a * (1 - e**2))&#xA;    #6&#xA;    Om = omega_LAN&#xA;    w =  omega_AP&#xA;&#xA;    X = r*(np.cos(Om)*np.cos(w+nu) - np.sin(Om)*np.sin(w+nu)*np.cos(i))&#xA;    Y = r*(np.sin(Om)*np.cos(w+nu) + np.cos(Om)*np.sin(w+nu)*np.cos(i))&#xA;    Z = r*(np.sin(i)*np.sin(w+nu))&#xA;&#xA;    #7&#xA;    p = a*(1-e**2)&#xA;&#xA;    V_X = (X*h*e/(r*p))*np.sin(nu) - (h/r)*(np.cos(Om)*np.sin(w+nu) + \&#xA;    np.sin(Om)*np.cos(w+nu)*np.cos(i))&#xA;    V_Y = (Y*h*e/(r*p))*np.sin(nu) - (h/r)*(np.sin(Om)*np.sin(w+nu) - \&#xA;    np.cos(Om)*np.cos(w+nu)*np.cos(i))&#xA;    V_Z = (Z*h*e/(r*p))*np.sin(nu) + (h/r)*(np.cos(w+nu)*np.sin(i))&#xA;&#xA;    return [X,Y,Z],[V_X,V_Y,V_Z]&#xA;&#xA;a,e,i,omega_AP,omega_LAN,T, EA = cart_2_kep(r_test,v_test)&#xA;r_test2, v_test2 = kep_2_cart(a,e,i,omega_AP,omega_LAN,T, EA)&#xA;&#xA;&#xA;print(r_test2 - r_test)&#xA;print(v_test2 - v_test)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Edit: I was able to plot these against JPL's Horizon data and with the small edit they are indeed accurate.&lt;/p&gt;&#xA;&lt;p&gt;Edit2: There was an error in the implementation of #9 in &lt;code&gt;cart_2_kep&lt;/code&gt;. It would never output all possible angles. The alternate formula using &lt;code&gt;atan2&lt;/code&gt; fixes this.&lt;/p&gt;&#xA;&lt;p&gt;Edit3: Equation of V_z corrected, from a &amp;quot;-&amp;quot; to a &amp;quot;+&amp;quot;&lt;/p&gt;&#xA;" OwnerDisplayName="user17622" LastEditorUserId="38975" LastEditDate="2021-01-20T11:38:56.643" LastActivityDate="2021-01-20T11:38:56.643" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="19336" PostTypeId="2" ParentId="19333" CreationDate="2016-12-08T20:11:00.877" Score="13" Body="&lt;p&gt;The whole operation was supposed to be based in Florida. Houston got mission control because of some pork barrel bill in congress, IIRC.&lt;/p&gt;&#xA;" OwnerUserId="17858" LastActivityDate="2016-12-08T20:11:00.877" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="19337" PostTypeId="2" ParentId="19333" CreationDate="2016-12-08T21:23:19.690" Score="2" Body="&lt;p&gt;Besides these other answers, there was a great post-WW2 effort to develop the American South. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the space program was spread between the southern states, e.g, New Mexico, Texas and Florida.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the command centre is in Texas, the launch site in Florida and the landing strips in Florida and New Mexico, among other sites. &lt;/p&gt;&#xA;" OwnerUserId="17861" LastEditorUserId="6816" LastEditDate="2016-12-08T21:38:34.220" LastActivityDate="2016-12-08T21:38:34.220" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19339" PostTypeId="1" AcceptedAnswerId="40218" CreationDate="2016-12-09T00:42:02.030" Score="3" ViewCount="4834" Body="&lt;p&gt;I wanted to make a quick ground-track plot, so I used the Python package &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt; to propagate a TLE and return Earth-Centered Inertial coordinates. Then I created a &lt;code&gt;Topos()&lt;/code&gt; object fixed to the Earth's surface at lat/lon = 0, 0 and used it to generate the rotation of the earth in order to &quot;unwind&quot; the ISS position on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;side note:&lt;/strong&gt; This is not a good way to do this, but it gives results good enough to make a small map for an illustration. Problems include assuming the earth's axis is still in the z direction, and of course treating the earth's surface as spherical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a better way to do this within Skyfield without using a method that starts with an underscore - in other words using methods intended to be completely public for the user? Also, is there a way to get the Earth centered, Earth fixed (i.e. rotating Earth) coordinates directly in Skyfield without unwinding like this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; I've adjusted the script since it's been over a year and Skyfield v 1.0 has been released.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;ISS_TLE = &quot;&quot;&quot;1 25544U 98067A   16341.96974289  .00003303  00000-0  57769-4 0  9996&#xA;2 25544  51.6456 276.4739 0005937 300.1004 104.8148 15.53811586 31866&quot;&quot;&quot;&#xA;L1, L2 = ISS_TLE.splitlines()&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from skyfield.api import Loader, EarthSatellite, Topos&#xA;&#xA;degs     = 180./np.pi&#xA;&#xA;r_earth  = 6371.  # for ROUGH approx. ground track, just use a spherical Earth&#xA;&#xA;load    = Loader('~/Documents/YourNameHere/SkyData')&#xA;data    = load('de421.bsp')&#xA;earth   = data['earth']&#xA;topoZZ  = Topos(latitude_degrees=0.0, longitude_degrees=0.0)&#xA;&#xA;location = earth + topoZZ&#xA;&#xA;ISS      = earth + EarthSatellite(L1, L2)&#xA;&#xA;ts       = load.timescale()&#xA;&#xA;minutes  = np.arange(0, 140, 0.5)&#xA;time     = ts.utc(2016, 12, 7, 12, minutes)&#xA;&#xA;Epos     = earth.at(time).position.km&#xA;ZZpos    = topoZZ.at(time).position.km    ## Position of (0.0N, 0.0E) to get rotation&#xA;ISSpos   = ISS.at(time).position.km - Epos&#xA;&#xA;theta_ZZ = np.arctan2(ZZpos[1], ZZpos[0])   # calculate Earth's rotaion&#xA;&#xA;sth, cth         = np.sin(-theta_ZZ), np.cos(-theta_ZZ) # unwind&#xA;xISS, yISS, zISS = ISSpos&#xA;xISSnew, yISSnew = xISS*cth - yISS*sth, xISS*sth + yISS*cth # rotate ISS data to match Earth&#xA;ISSnew           = np.vstack((xISSnew, yISSnew, zISS))&#xA;&#xA;x, y, z = ISSnew&#xA;r       = np.sqrt((ISSpos**2).sum(axis=0))&#xA;rxy     = np.sqrt(x**2 + y**2)&#xA;ISSlat, ISSlon   = np.arctan2(z, rxy), np.arctan2(y, x)&#xA;&#xA;plt.figure()&#xA;plt.plot(degs*ISSlon, degs*ISSlat, 'ok')&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ArgEP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ArgEP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-12-17T12:38:36.070" LastActivityDate="2019-12-03T07:04:55.073" Title="Better way to get approximate ground track for a satellite using Skyfield?" Tags="|python|skyfield|coordinates|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19340" PostTypeId="2" ParentId="19333" CreationDate="2016-12-09T02:13:43.727" Score="65" Body="&lt;p&gt;Until 1949, the U.S. launched rockets from Wallops Island in Virginia and the White Sands Missile Range (WSMR) in New Mexico. The Rockets launched from Wallops were of American origin while the rockets launched from White Sands were V-2s, supported by a hundred or so German rocket scientists who had been smuggled out of Germany (along with some V-2s) via Operation Paperclip.&lt;/p&gt;&#xA;&lt;p&gt;Then this happened:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The most spectacular flight in the annals of WSMR occurred May 29, 1947, when an experimental V-2, weighing four and a half tons, headed south after liftoff instead of north and landed, some five minutes later, a mile and a half south of Juarez, Mexico. Though no damage was done, the rocket narrowly missed an ammunition dump where Mexican mining companies stored powder and dynamite.&lt;br /&gt;&#xA;&lt;sub&gt;Source: &lt;a href=&quot;https://history.msfc.nasa.gov/german/v-2_whitesands.html&quot; rel=&quot;noreferrer&quot;&gt;https://history.msfc.nasa.gov/german/v-2_whitesands.html&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Raining rockets down on U.S. civilians is one thing. Raining them down on other countries is quite another. There were a number of near misses prior to that incident. A search for a better launch site was already underway.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In October, 1946 the Joint Research and Development Board under the Joint Chiefs of Staff established the Committee on the Long Range Proving Ground to analyze possible locations for a new missile range to be shared by the various branches of the military.&lt;/p&gt;&#xA;&lt;p&gt;Three potential sites emerged. One was based on the coast of northern Washington, with a range along the Aleutian Islands of Alaska. A second was based at El Centro, California, with a range along the coast of Baja, Mexico. A third was based at the Banana River Naval Air Station, with launches from Cape Canaveral and a range over the Atlantic Ocean.&lt;/p&gt;&#xA;&lt;p&gt;In September, 1947 the Committee on the Long Range Proving Ground announced its decision to recommend the establishment of a missile proving ground at the California site, with Cape Canaveral offered as the second choice. The Washington site had been quickly rejected due to its isolation and poor weather.&lt;/p&gt;&#xA;&lt;p&gt;The project was officially designated the Joint Long Range Proving Ground, with development responsibility granted to the Joint Long Range Proving Ground Group. Although plans continued initially for the establishment of a missile range based in California, political problems arose in 1948.&lt;/p&gt;&#xA;&lt;p&gt;Although it would have been a suitable site very close to existing missile manufacturers, the California site had to be rejected when Mexican President Aleman refused to agree to allow missiles to fly over the Baja region. This was largely a result of bad timing, since a wayward V-2 rocket launched from White Sands, New Mexico had recently crashed near Juarez, Mexico.&lt;/p&gt;&#xA;&lt;p&gt;The British, however, were quick to express their willingness to allow missiles to fly over the Bahamas. They also were willing to lease island land to the U.S. military for the establishment of tracking stations. That, coupled with inherent strengths of Cape Canaveral, sealed its selection as the first U.S. long range missile proving ground.&lt;br /&gt;&#xA;&lt;sub&gt;Source: &lt;a href=&quot;http://www.spaceline.org/capehistory/2a.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.spaceline.org/capehistory/2a.html&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Shortly after this selection, those German rocket scientists were moved from Fort Bliss to the Redstone Arsenal near Huntsville, Alabama. Ten years later, those German rocket scientists, along with thousands of American rocket scientists would be transferred once again, but this time the transfer distance was short. NASA's Marshall Space Center is located within Redstone Arsenal.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-12-09T02:13:43.727" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="19341" PostTypeId="2" ParentId="19333" CreationDate="2016-12-09T03:13:01.833" Score="6" Body="&lt;p&gt;If launching into a low-inclination orbit, you want to launch due east from the lowest latitude possible. This gives you the advantage of starting the flight with the speed of the Earth's rotation at your launch latitude; that is &quot;free&quot; speed that the rocket does not need to impart. Furthermore, you want to avoid overflying land masses so malfunctioning rockets to do not land on people's heads, which can have negative budget implications. When launching into higher inclination orbits you adjust your launch azimuth accordingly. Your launch azimuth determines your ground track and thus which land masses you might hit with rocket debris. Typically the launch azimuth for an orbital flight is going to be between due North and due South, on the right-hand side of the compass (0 to +180 deg). Launching to the east from the Texas coast would result in overlying lots of land mass (US, Canada, Cuba, Mexico, S.America). Launching from Florida mitigates this for easterly launches. Polar orbit launches are commonly done from Vandenberg AFB in CA, flying south over the Pacific. High-inclination and polar launches are conducted from Wallops VA. Johnson Space Center's Houston location is a result of politics (President Johnson was from Texas).&lt;/p&gt;&#xA;" OwnerUserId="17869" LastEditorUserId="17869" LastEditDate="2016-12-09T03:20:10.933" LastActivityDate="2016-12-09T03:20:10.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19342" PostTypeId="1" AcceptedAnswerId="19350" CreationDate="2016-12-09T03:37:09.350" Score="7" ViewCount="1053" Body="&lt;p&gt;The Washington Post &lt;a href=&quot;https://www.washingtonpost.com/news/post-nation/wp/2016/12/08/that-was-a-real-fireball-what-happened-when-john-glenn-orbited-the-earth-in-1962/&quot; rel=&quot;noreferrer&quot;&gt;just republished&lt;/a&gt; it's February 21, 1962 article covering the launch, Earth orbit, and safe re-entry and recovery of (Senator) John H. Glenn, in recognition of his service and recent passing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yxdGM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yxdGM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Parachute Drops Capsule, Flier Into the Atlantic&#xA;  By John G. Norris, Staff Reporter&#xA;  Wednesday, February 21, 1962; Page A1&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is worth taking time to read how spaceflight was described a half century ago. I was surprised to read how many Earth stations were involved, and that NASA astronauts were posted at several of them and if I understand correctly were in voice communication with him.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This must have been quite a network. Since things could change rapidly, it was probably a goal to keep in voice communication with the astronaut as continuously as possible. The netowrk itself deserves a separate question. Here I'm wondering:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;How many of the Earth stations involved in receiving telemetry from the spacecraft while in orbit had astronauts stationed on site? Was this coincidental or was there intention to have a fellow astronaut 'on the other end of the line'? If intentional, what was the rationale?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-12-09T17:09:48.957" LastActivityDate="2016-12-14T08:40:19.780" Title="How many earth stations had astronauts on-site during John Glenn's orbit in Friendship-7?" Tags="|communication|astronauts|project-mercury|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19343" PostTypeId="2" ParentId="19259" CreationDate="2016-12-09T04:23:03.817" Score="1" Body="&lt;p&gt;This is a little math just to add to &lt;a href=&quot;https://space.stackexchange.com/a/19287/12102&quot;&gt;@BrianC's answer&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;I used a current TLE for the ISS and &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt; to make an approximate ground-track plot on top of a &lt;a href=&quot;http://visibleearth.nasa.gov/view.php?id=73909&quot; rel=&quot;nofollow noreferrer&quot;&gt;Blue Marble Next Generation w/ Topography and Bathymetry&lt;/a&gt; image from NASA. Surface locations potentially visible from the ISS are those within the cartesian distance &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; calculated from the simplest distance to horizon formula:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$r = \sqrt{(R_{earth} + altitude)^2 - R_{earth}^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Of course if you are looking at thinks &lt;em&gt;above the surface&lt;/em&gt; then this is an underestimate, it also potentially underestimates by not accounting for atmospheric refraction. But it's good enough to get a rough idea.&lt;/p&gt;&#xA;&lt;p&gt;I've plotted dots at 1 minute intervals for 12 hours as an example. In the first image I've reset the value of all pixels that are never within view of the ISS to black. If I ran for longer, it would just be a band around the Earth extending to about +/- 72 degrees latitude. The second image is without masking, to roughky show how much of Antarctica is not visible. Again, this applies to the surface and not high in the atmosphere or in space above antarctica.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WJymI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WJymI.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Xxfty.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xxfty.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-12-09T04:23:03.817" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19345" PostTypeId="2" ParentId="19329" CreationDate="2016-12-09T08:13:30.127" Score="9" Body="&lt;p&gt;I don't think it's that bad a question that it deserves all the criticism it's gotten.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basic idea, fly really high and as fast as possible, then point towards the Earth and use Earth's gravity to go even faster and escape the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem #1.   Flying really high doesn't look like your diagram.   The highest aircraft, if the Earth is the size of a beach-ball, say, 1 meter in diameter, the highest an aircraft could reach would be less than 1/2 inch (1.3 cm) off the beach-ball, so there's not much of an angle available to point downwards.   There's also an atmosphere problem.  Point downwards too much and the craft runs into wind resistance and slows down.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This diagram gives you an idea of how close objects in Near Earth Orbit are, and the jet you're talking about would be much less high than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/8/82/Orbitalaltitudes.jpg&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit#/media/File:Orbitalaltitudes.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;bigger version&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem #2, any velocity added by using the Earth's gravity also brings the jet closer to the Earth so more velocity is needed to escape.   You can't use a planet's gravity to escape the planet's gravity unless the object is already in orbit, and that kind of gravity/velocity approach is always zero sum game.   You can't add velocity that way unless you're talking relative to a 3rd body, which your example isn't.  &lt;/p&gt;&#xA;" OwnerUserId="8711" LastEditorUserId="8711" LastEditDate="2016-12-13T20:27:59.343" LastActivityDate="2016-12-13T20:27:59.343" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19346" PostTypeId="2" ParentId="6486" CreationDate="2016-12-09T10:12:08.540" Score="1" Body="&lt;p&gt;The protection against lightning using this towers is explained by the Rolling Sphere Method, see &lt;a href=&quot;http://www.lightningman.com.au/direct_strike.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.lightningman.com.au/direct_strike.html&lt;/a&gt; &lt;a href=&quot;http://www.lightningsafety.com/nlsi_lhm/Lightning-Discharge-and-Fundamentals-of-Lightning-Protection-Rakov.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.lightningsafety.com/nlsi_lhm/Lightning-Discharge-and-Fundamentals-of-Lightning-Protection-Rakov.pdf&lt;/a&gt; and &lt;a href=&quot;http://www.ijme.us/cd_06/PDF/IT%20P501-124.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ijme.us/cd_06/PDF/IT%20P501-124.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This model of a sphere rolled over the launch site shows that the lightning towers should be higher than the protected rocket. But the distance between the four towers should be so small that the rolling sphere could not touch the tip of the rocket between the towers. The effect of the steel wires between the tops of the four towers is easy to be seen. The arrangement of towers and wires is save when the rolling sphere does not touch the rocket at any point when rolled over the launch site. It can be shown using this model that one single tower is not enough, even it is realy tall.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="13126" LastEditDate="2016-12-09T10:45:25.027" LastActivityDate="2016-12-09T10:45:25.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19347" PostTypeId="2" ParentId="19299" CreationDate="2016-12-09T12:05:37.187" Score="4" Body="&lt;p&gt;Some numbers about density, liquid oxygen has 1.141 kg/l, water is 1.00 kg/l and 88.8 % of its mass is oxygen and compressed oxygen at 300 bar has a density of only 0.406 kg/l. I had no numbers for oxygen at 400 bar, but the density should be lower than 0.53 kg/l due to Van der Waals forces. But compressed oxygen requires a very heavy container and liquid oxygen a very good isolated container. The mass of a container for water is much lower, the total overhead for container and the mass of the hydrogen in the water is about 15 to 20 %. Thus water is the most effective way to transport oxygen to the ISS.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-12-09T12:05:37.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19348" PostTypeId="2" ParentId="19333" CreationDate="2016-12-09T13:09:32.403" Score="6" Body="&lt;p&gt;I agree with the other answers that point out that there is no land east of Florida, while there is significant populated land east of Texas. But I think it is work pointing out that Texas has no advantage over Florida in terms of latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The southernmost point of Texas is Brownsville (Lat 26 deg N) Which by the way, shares a border with Mexico, with which the USA sometimes has less than perfect relations. On the one hand the USA might feel there are some security issues. On the other, Mexico would be quite justified in complaining if an out of control rocket landed in their territory. Houston / Johnson is at Lat 29.5 deg N.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Canaveral / Kennedy is at Lat 28.5 deg N. The southermost tip of Florida is at 25 deg N, but is very close to the population of Miami and the sovereign nation of the Bahamas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is worth checking the advantage that would be obtained by launching from the southern tip of Florida instead of the existing site:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Equatorial peripheral speed: 40000km / 24h = 1666km/h, 0.462km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Peripheral Speed at Canaveral 28.5 deg: 1666 cos 28.5 = 1464km/h, 0.405km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Peripheral Speed at 25 deg: 1666 cos 25 = 1509km/h, 0.419km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary the lowest latitude point on the US mainland is in Florida, not Texas and the advantage compared with the current location would be just 45km/h (28mph.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hawaii's big island has a latitude ranging from 19 to 20 degrees, but again the advantage is small and probably does not outweigh the logistical issues.&lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="6117" LastEditDate="2016-12-09T15:55:48.897" LastActivityDate="2016-12-09T15:55:48.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19349" PostTypeId="2" ParentId="19329" CreationDate="2016-12-09T13:13:57.023" Score="-2" Body="&lt;p&gt;You need to get at least 160 kilometers up to be in orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that the momentum of the fastest planes cannot take them much higher than 35 kilometers, you would need a plane that is 5-10-20 times faster more powerful than a stealth bomber for the same weight. the maths are difficult but it is mathematically possible if new stronger and more energy dense materials were invented. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would make more sense from an energy point of view to use a cannon built on the slope of mountain or in the sea. Also, you need thrusters to steer that momentum, because a plane cannot steer in low atmospheric pressure, and after 32 kilometers, the oxygen supply is less than one percent of ground density, and at 20 kilometers it's around 5%. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can make a plane 20 times faster than a stealth bomber, you should be able to get to at least 50 or 100km altitude, but the time to inflect the airplane upwards would be going at 5 kilometers per second, so you would have about 5 seconds to turn your forwards momentum upwards, you would need big strong flaps and thrusters!!! At that kind of velocity, you could go around the planet in about 2 hours flat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;low earth orbit at 160km takes 88 minutes, so if you can get around the earth in 88 minutes, there's good chances you can get into orbit. That would be 30 times faster than the current fastest runway airplanes. &lt;/p&gt;&#xA;" OwnerUserId="17876" LastActivityDate="2016-12-09T13:13:57.023" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19350" PostTypeId="2" ParentId="19342" CreationDate="2016-12-09T18:00:14.733" Score="11" Body="&lt;p&gt;According to &lt;a href=&quot;http://science.ksc.nasa.gov/history/mercury/ma-6/docs/ma-6-results.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA's mission report&lt;/a&gt; on Mercury-Atlas 6, the following ground stations in the Mercury network offered voice communication and spacecraft telemetry (in order of eastward travel):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Atlantic Missile Range (i.e. Cape Canaveral)&lt;/li&gt;&#xA;&lt;li&gt;Bermuda&lt;/li&gt;&#xA;&lt;li&gt;Mid-Atlantic Ship&lt;/li&gt;&#xA;&lt;li&gt;Canary Islands&lt;/li&gt;&#xA;&lt;li&gt;Kano, Nigeria&lt;/li&gt;&#xA;&lt;li&gt;Zanzibar&lt;/li&gt;&#xA;&lt;li&gt;Indian Ocean Ship&lt;/li&gt;&#xA;&lt;li&gt;Muchea, Australia&lt;/li&gt;&#xA;&lt;li&gt;Woomera, Australia&lt;/li&gt;&#xA;&lt;li&gt;Canton Island&lt;/li&gt;&#xA;&lt;li&gt;Hawaii&lt;/li&gt;&#xA;&lt;li&gt;Southern California&lt;/li&gt;&#xA;&lt;li&gt;Guaymas, Mexico&lt;/li&gt;&#xA;&lt;li&gt;Corpus Christi, Texas&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are two additional stations with radar tracking capability but not communications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of these stations made contact with Glenn on the first orbit, as you can determine by reading &lt;a href=&quot;http://mercury6.spacelog.org/page/&quot; rel=&quot;noreferrer&quot;&gt;the transcripts&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With fourteen stations there obviously weren't enough astronauts for &quot;CAPCOM is an astronaut&quot; to be a hard rule. In addition, at several of the stations, the initial contact is made by a technician; e.g. the transcript shows call sign &quot;Canton Com Tech&quot; before the call sign &quot;Canton CAPCOM&quot; takes over. In a few cases the surgeon at the site addresses Glenn directly (&quot;Friendship Seven, this is Surgeon Zanzibar&quot;) to discuss blood pressure tests and the like. Apart from Cooper I don't think any of the CAPCOMs are addressed by name.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.thespacerace.com/encyclopedia/people/astronauts/&quot; rel=&quot;noreferrer&quot;&gt;The Space Race Encyclopedia&lt;/a&gt;, Cooper was at Muchea, Grissom at Bermuda, and Schirra at California. Shepard was CAPCOM at Canaveral according to &lt;em&gt;The Right Stuff&lt;/em&gt; and other sources. As Carpenter was Glenn's backup crew, I assume he had to be at Canaveral as well. Slayton's autobiography places him at Canaveral (&quot;sitting next to&quot; Shepard), so I believe the answer is that there were four astronaut CAPCOMs at different sites on this particular flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, after losing contact with Woomera on the first orbit, Glenn calls &quot;broadcasting in the blind to the Mercury network&quot;, and gets replies from Canaveral, Kano, and Zanzibar -- saying they did not read the transmission! I infer from this that another ground station (Woomera or Canton?) did receive, and relayed to the others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller#Capsule_Communicator_.28CAPCOM.29&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, &quot;NASA believes that an astronaut is most able to understand the situation in the spacecraft and pass information in the clearest way.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More than anyone else, the astronauts know each other and the interior of the spacecraft as seen by the pilot; they develop their own jargon. (If you've ever given technical support over the phone, you know how much easier it is to do so when the person you're talking to speaks the same technical jargon dialect that you do.) If communication is fragmented, the astronaut CAPCOM can piece it together. If the astronaut pilot is in the middle of a procedure and says &quot;ah crud it's doing that thing again&quot;, the astronaut CAPCOM knows what he means. CAPCOM can then translate for the rest of mission control at leisure, as necessary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even for the very first American manned flight, it was considered important for Alan Shepard to be in communication with someone who had undergone the same training (Deke Slayton); for the second flight, Shepard was the natural choice for Grissom's CAPCOM because he was the only one who'd been through a flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trust was also a factor. The Mercury 7 astronauts, while competitive, were a very close-knit group, and knowing that one of their own was in the comm loop reassured them that they had support in case of any disagreement between the crewman and mission control. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-12-10T04:45:49.507" LastActivityDate="2016-12-10T04:45:49.507" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19351" PostTypeId="2" ParentId="19342" CreationDate="2016-12-09T18:14:27.780" Score="0" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.encyclopedia.com/people/science-and-technology/space-exploration-biographies/capcom&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, there were 13 capcom stations, of which the most critical 6 had astronauts at them, and the other 7 were manned by &quot;recent college graduates&quot;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-12-09T18:14:27.780" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19353" PostTypeId="1" AcceptedAnswerId="19354" CreationDate="2016-12-09T18:26:22.073" Score="8" ViewCount="1712" Body="&lt;p&gt;While browsing concepts of Space Station Freedom and other '80s NASA concepts (I love concept art), I noticed that there are some interesting differences in how the Shuttle was intended to dock with a station than in real life. The concepts, noticeably &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/Space_Station_Freedom_design_1991.jpg/800px-Space_Station_Freedom_design_1991.jpg&quot; alt=&quot;this concept&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/f/f5/SSF_-_1991.jpg&quot; alt=&quot;this one&quot;&gt; have no real docking port inside the Shuttle. In fact, the docking adapter is on the station instead of inside the shuttle. &lt;img src=&quot;https://s-media-cache-ak0.pinimg.com/originals/ad/66/cb/ad66cb9380f15700d1cf371f71b73402.jpg&quot; alt=&quot;real adapter&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why would NASA not choose to go with their conceptual design when building the ISS, and how would it have worked? Surely keeping the adapter on the station would increase payload mass/volume, which is pretty critical when building a station.&lt;/p&gt;&#xA;" OwnerUserId="17857" LastActivityDate="2022-01-14T09:00:11.890" Title="Why did the design for Space Shuttle docking change?" Tags="|iss|crewed-spaceflight|space-shuttle|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19354" PostTypeId="2" ParentId="19353" CreationDate="2016-12-09T19:42:01.203" Score="10" Body="&lt;p&gt;This diagram from 1973 (shortly after the contract was awarded to Rockwell) shows that the external docking adapter was already planned.  I suspect those concept art photos are playing fast and loose with reality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WuFk2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WuFk2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FfqOB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FfqOB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Jenkins &quot;Space Shuttle&quot;, 1992 edition.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-12-10T00:20:33.967" LastActivityDate="2016-12-10T00:20:33.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19355" PostTypeId="2" ParentId="19353" CreationDate="2016-12-09T19:56:33.573" Score="6" Body="&lt;p&gt;All docking ports require a part both on the passive spacecraft (in this case the space station) and a part on the active spacecraft (in this case the shuttle). If I had to guess, the concept art used a reference photograph of the space shuttles, which at the time did not have docking adapters installed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oz2h3.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oz2h3.jpg&quot; alt=&quot;Here you can see Challenger in orbit in 1983&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image is a little small, but you can see that there is no docking port on the spacecraft. The docking adapters were only installed for the first Shuttle-Mir missions, and later found use on the International Space Station.&lt;/p&gt;&#xA;" OwnerUserId="17851" LastActivityDate="2016-12-09T19:56:33.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19357" PostTypeId="2" ParentId="19329" CreationDate="2016-12-10T08:43:51.320" Score="6" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Skylon_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Skylon&lt;/a&gt;'s design is, in fact, for a hypersonic SSTO that is partially air-breathing, much as you describe. It has some important differences, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first of these is that it does not use a dive to speed up. &lt;a href=&quot;//space.stackexchange.com/questions/19329/could-you-use-a-hypersonic-space-plane-to-fly-into-space#19345&quot;&gt;userLTK's answer explains why using gravity acceleration to get fast enough to get to orbit won't work&lt;/a&gt;; briefly, orbit is when you &lt;em&gt;already are&lt;/em&gt; going fast enough to do precisely this, continuously. Instead, it just runs its engines the whole way, just like a rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second is that its engines are very carefully designed to work both as rockets (taking oxygen along when the air runs out) and as jet engines of a unique type (ramjets with heavily precooled intakes and a way to self-start at low speeds). Neither one has been done before, as far as I know, and especially the precooling is extremely challenging. But it's essential, as otherwise the heating just from ram-compressing the thin air at high altitudes for the jet mode will leave the air too hot to expand further, which means the jet stops thrusting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third is that its wings, while important most of the way up, become vestigial in the upper atmosphere; it simply uses a gravity turn at that point, letting its inertia curve it from a shallow climb closer and closer into an orbit as it reaches orbital altitude and speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your title question can probably be answered in the affirmative: there's a company spending hundreds of millions of pounds on a design that may well be solid enough to do the job, so it's very likely possible in principle. But the specific method isn't quite the way you describe it; it's much more challenging to get to orbit than most people can readily grasp.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2016-12-10T20:25:39.720" LastActivityDate="2016-12-10T20:25:39.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19358" PostTypeId="2" ParentId="19322" CreationDate="2016-12-10T10:11:47.970" Score="3" Body="&lt;p&gt;I'm not sure - maybe I'm missing something, but your image seems to show left-handed cartesian coordinates.  This could cause a complete orbit to look right (especially if there are no arrows to show the direction of motion), but an incomplete orbit would indeed look wrong because it would highlight the direction of motion (a plot of the velocity vector part of the state vector could also show this!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is your image - if I used the right-hand rule, my thumb seems to point in your &lt;em&gt;negative z direction.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3B3CN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3B3CN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what I have in Beldner, and I'll bet AutoCad and other 3D engineering programs look similar:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6nB78.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6nB78.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screenshot from the 3D window of &lt;a href=&quot;https://www.blender.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Blender&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are random images of the right hand Cartesian Coordinates. They look just like the Blender coordinates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nZhIv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nZhIv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:3D_Vector.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/75S56.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/75S56.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Coord_system_CA_0.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And these shows the difference between right-handed and left-handed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GUMwi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GUMwi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cartesian coordinate handedness from &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Cartesian_coordinate_system_handedness.svg/640px-Cartesian_coordinate_system_handedness.svg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SI5eh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SI5eh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cartesian Coordinate handedness from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:3D_Cartesian_Coodinate_Handedness.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2016-12-10T10:11:47.970" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19360" PostTypeId="1" AcceptedAnswerId="19361" CreationDate="2016-12-10T11:52:35.873" Score="13" ViewCount="3087" Body="&lt;p&gt;There have been some questions (&lt;em&gt;and answers&lt;/em&gt;) that talk about using an airplane as part (&lt;em&gt;or all&lt;/em&gt;) of the launch to space.  There are active designs that use wings as part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylon_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;launch&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot; rel=&quot;noreferrer&quot;&gt;landing&lt;/a&gt; configuration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting to &quot;space&quot; while not easy, it is simple enough that &lt;a href=&quot;https://space.stackexchange.com/questions/2076&quot;&gt;college groups have made serious attempts to reach space&lt;/a&gt; But getting to height is not the important part, going fast is.  Being in orbit is about going fast enough so that as gravity pulls you to earth you keep missing it.  Getting the speed is the hard part.  If you have wings providing lift, you don't need to spend energy on lift it can mostly go to gaining speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In theory if you &lt;a href=&quot;https://space.stackexchange.com/questions/19329/could-you-use-a-hypersonic-space-plane-to-fly-into-space/19349#19349&quot;&gt;fly around the Earth in a plane fast enough&lt;/a&gt; you are in orbit.  As you approach orbital speed, your wings start being used to provide downward force so you can stay in the atmosphere and gain more speed with your air breathing engine.  Once you have reached optimal speed, point the plane up and you are in the orbit your speed will support. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle&lt;/a&gt; weighs in around 2,000 tons with fuel, while the &lt;a href=&quot;https://en.wikipedia.org/wiki/Antonov_An-225_Mriya&quot; rel=&quot;noreferrer&quot;&gt;Antonov An-225 Mriya&lt;/a&gt; wieghs in around 640 tons fully loaded. It seems probable that with the lighter fuel requirements of an air breathing engine, a plane that could carry enough fuel to get to the same orbital speeds as the space shuttle is plausible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is keeping us from reaching orbit in a plane with an air breathing engine? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/19329&quot;&gt;Could you use a hypersonic space plane to fly into space?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1184&quot;&gt;Horizontal space launch&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2019-07-31T14:06:08.100" Title="Can you get to orbital speed with an air breathing engine?" Tags="|launch|orbit|spaceplane|" AnswerCount="5" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="19361" PostTypeId="2" ParentId="19360" CreationDate="2016-12-10T15:11:00.417" Score="14" Body="&lt;p&gt;No. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fastest airbreathing engines we have are &lt;a href=&quot;https://en.wikipedia.org/wiki/Scramjet#Progress_in_the_2000s&quot; rel=&quot;noreferrer&quot;&gt;scramjets&lt;/a&gt; (supersonic combustion ramjet), i.e. a duct that compresses the air while allowing it to flow at supersonic speeds. The fastest scramjet flight so far reached Mach 5, or about 1/5 of orbital speed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other airbreathing engines are slower, mainly because they only work when the air going into the engine is slowed to subsonic speeds. The J-85 engines of the SR-71 are just about the limit for that sort of technology, at about Mach 3.5.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difficulty with a scramjet is igniting the air/fuel mixture at supersonic speeds and keeping it lit. The incoming air constantly tries to extinguish the combustion.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most promising development in this area is the &lt;a href=&quot;https://en.wikipedia.org/wiki/SABRE_%28rocket_engine%29&quot; rel=&quot;noreferrer&quot;&gt;Reaction Engines SABRE&lt;/a&gt;, which is designed to run as an airbreathing engine up to Mach 5, and switch to a pure rocket mode above that.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting to orbital speed inside the atmosphere is difficult. At Mach 2, a Concorde is about 1 ft longer than when it's on the ground because the heat from air friction expands its structure. At Mach 3 (1/8 of orbital speed), the airframe of the SR-71 had to be built in titanium because aluminium wouldn't withstand the heat.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a rocket accelerating to orbital speed is in a race: it has to get out of the atmosphere before air friction melts it. That means you have a short window in which you could use airbreathing engines, after which those engines are dead weight (unless you can keep using them, as in the SABRE concept).  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-12-10T15:11:00.417" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19362" PostTypeId="2" ParentId="19360" CreationDate="2016-12-10T15:20:18.780" Score="11" Body="&lt;p&gt;It may be possible, but not with existing technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wgfix.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wgfix.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://launiusr.wordpress.com/2012/03/05/hatever-happened-to-the-national-aero-space-plane/&quot; rel=&quot;noreferrer&quot;&gt;The US spent almost 2 billion dollars developing an airbreathing single-stage-to-orbit vehicle in the 1990s&lt;/a&gt;.  This &quot;National Aerospace Plane&quot; (NASP) aka the &quot;Orient Express&quot; project never produced any flight hardware, although a test version (the Rockwell X-30) was planned but never built.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For such a vehicle to succeed, a working &lt;a href=&quot;https://en.wikipedia.org/wiki/Scramjet&quot; rel=&quot;noreferrer&quot;&gt;scramjet&lt;/a&gt; engine is required.  Turbojet and ramjet engines must decelerate the incoming flow to subsonic speeds in their combustors, resulting in high drag and inefficiencies.  A scramjet (the first two letters of which stands for Supersonic Combustion) does not require this deceleration and therefore skips the drag penalties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scramjets do not look like regular engines.  The engine for the NASP was basically its entire underside - it was a waverider configuration where the underbelly compressed the flow for use in the engine.  While an ingenious design, no one has actually gotten this to work for more than a few seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The entire possibility of airbreathing SSTO hinges on a practical, working scramjet engine.  Until this technology is achieved, it cannot be done. This was the major factor in the failure of the NASP.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-12-10T17:30:06.640" LastActivityDate="2016-12-10T17:30:06.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19363" PostTypeId="1" CreationDate="2016-12-10T20:32:09.350" Score="-1" ViewCount="216" Body="&lt;p&gt;Can you land a ferris wheel-like ship on Mars ….sideways? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Envision a typical sci-fi ferris wheel kind of spacecraft, with three habitat elements separated from the central hub by girder type booms (attached low on the hub). There would also be triangulating girder stays to the top of the hub, The central hub houses a docking collar, low G work spaces, fuel tanks and rocket engines.&#xA;All this is pretty standard sci-fi stuff but now I am proposing that you LAND this thing on Mars as total structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;= The greatly increased moment of inertia due to the large diameter would make for a very stable landing platform (not spinning of course).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;= Increased opportunity for aerodynamic surfaces for reentry and landing (along each boom as well as the possibility of sail like membranes between booms).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;We would have to have landing legs on the habitats as well as the center (engine) unit. (To attempt to put some numbers on this - the landing configuration might be maybe about 180 ft diameter, assuming 50 ft booms, a 30 foot habitat length and a 10 ft radius connection ring at the hub. Of course these numbers are complete speculation and it could be much different. The main point is that it is going to be pretty darn BIG!)&lt;/li&gt;&#xA;&lt;li&gt;The main booms can be telescoping types so that, during the long journey, the habitats can be extended outward for increased pseudo gravity. Assuming 50 foot booms extending three times to make approximately a plus 300’ diameter spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;Habitat to habitat movement could be by small “cars” tethered to the central hub, moving along the perimeter. Airlocks on the cars and habitats make easier transfer possible. Movement to the hub would use the same cars but thrusters would be required to compensate for the speed-up/slow-down while moving radially.&lt;/li&gt;&#xA;&lt;li&gt;The configuration would be robust enough to allow low power course corrections even during spin.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="17815" LastActivityDate="2016-12-11T14:00:50.280" Title="Can you land a ferris wheel-like ship on Mars ….Sideways?" Tags="|mars|landing|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19364" PostTypeId="1" CreationDate="2016-12-10T20:51:18.873" Score="5" ViewCount="136" Body="&lt;p&gt;I am searching for an overview of &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_robot&quot; rel=&quot;nofollow noreferrer&quot;&gt;autonomous&lt;/a&gt; space robots integrating &lt;a href=&quot;https://en.wikipedia.org/wiki/Motion_planning&quot; rel=&quot;nofollow noreferrer&quot;&gt;motion planning capabilities&lt;/a&gt;, such as a list of robots (past and present).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far, I have found these robots:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars exploration rover &lt;strong&gt;Spirit&lt;/strong&gt; &lt;/li&gt;&#xA;&lt;li&gt;Mars exploration rover &lt;strong&gt;Opportunity&lt;/strong&gt; &lt;/li&gt;&#xA;&lt;li&gt;Mars exploration rover &lt;strong&gt;Curiosity&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;I excluded the Huygens probe, the ETS-VII satellite and the Sojourner rover&lt;/strong&gt;, as their capabilities are unclear to me. &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20030054507&quot; rel=&quot;nofollow noreferrer&quot;&gt;This paper&lt;/a&gt; claims the Sojourner rover &quot;exhibited only rudimentary autonomy&quot; and had &quot;unused capability for autonomously avoiding obstacles in its path.&quot; The difficulty is in finding information detailed enough to determine whether these robots were truly autonomous. &lt;/p&gt;&#xA;" OwnerDisplayName="user17898" LastEditorUserId="6944" LastEditorDisplayName="user10509" LastEditDate="2021-01-01T14:50:41.340" LastActivityDate="2021-01-01T14:50:41.340" Title="Which robotic spacecraft have autonomous operation capabilities?" Tags="|artificial-satellite|rovers|robotic-missions|robotics|autonomy|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="19366" PostTypeId="2" ParentId="19329" CreationDate="2016-12-11T00:05:12.623" Score="2" Body="&lt;p&gt;At the risk of repeating an existing answer or two...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't take advantage of the gravity of the body you want to escape as a means to make that escape. Earth's gravity gives every object within its reach some amount of potential energy due to that object's mass and distance. The further an object, the greater its potential energy. We are all familiar with falling objects accelerating; that is an exchange of potential energy for kinetic energy (downward). Newtonian mechanics includes a law stating that energy must be conserved. So, ignoring air resistance for the moment, the falling object gains exactly as much kinetic energy as it lost potential energy. Now your hypersonic spaceplane, as it dives from high altitude does the same thing, except it is losing energy to atmospheric drag. When it climbs back out, the best it can hope for is to get back to the same energy state it had when it began the dive, except it won't because of drag losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravitational &quot;slingshots&quot; work because of a nifty bit of physics. Let's say you're on a spaceship cruising along a highly elliptical solar orbit out from Earth kinda in the general direction of Jupiter. Now Jupiter is moving along its orbital path around the Sun, but you are on more of an outward path, and you don't have much velocity in a direction parallel with Jupiter's motion. But, you will make a very close approach to Jupiter, passing on its trailing side. As you approach, your ship is accelerated toward Jupiter by its gravity. As it passes just ahead of you, your trajectory is bent in the direction of Jupiter's motion. You fall into and then climb out of its gravity well. Since you started off outside of its &quot;sphere of gravitational influence&quot;, you escape from it. But you have picked up speed with respect to the solar system. You actually robbed Jupiter of a little of its orbital energy around the Sun and took it with you to help escape the solar system. This was used by the Voyager spacecraft, among others, and it only works because the spacecraft were never in orbit around Jupiter to begin with, and chose a path which allowed them to be pulled along the plane of the planetary orbits as a way to pick up speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theoretically, a hypersonic craft could fly all the way from the ground into orbit. There are a number of challenges to doing so. Escape velocity or even a low Earth orbital velocity is very fast - about 17,000 mph or more; travel at such speeds at normal aviation altitudes is essentially impossible because of the drag and heat that would be produced. It's only possible in the thinnest air which is practically space, and even then, there are heat management issues - any vehicle returning from space has to deal with them. Nevertheless, &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylon_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skylon&lt;/a&gt; seems to address all the issues, but it's still facing a steep development path. Even then, if it proves to be technically feasible, there remains the question of whether it will be commercially feasible. Consider &lt;a href=&quot;https://en.wikipedia.org/wiki/Concorde&quot; rel=&quot;nofollow noreferrer&quot;&gt;Concorde&lt;/a&gt; - a marvel of 1960s aerospace engineering. At the time, it was thought to be the future of air travel. It turned out to be too expensive for most travelers, and ultimately went out of service because it couldn't be operated at a profit. The Space Shuttle also was retired because it failed to meet its principal design objective - routine, economic, scheduleable access to space. As a concept, an SSTO spaceplace looks very attractive, but it is yet to be proven if the hoped-for benefits can be realized.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-12-11T00:05:12.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19367" PostTypeId="2" ParentId="19322" CreationDate="2016-12-11T01:53:17.710" Score="2" Body="&lt;p&gt;First of all &lt;em&gt;kudos for showing your work&lt;/em&gt; and the results illustrating your question so clearly! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the two suggestions at the bottom shed any light on it? Lighty edited from &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; the Keplerian orbital elements are:&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The main two elements that define the shape and size of the ellipse:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Eccentricity&lt;/strong&gt; (&lt;strong&gt;e&lt;/strong&gt;)—shape of the ellipse...&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Semimajor axis&lt;/strong&gt; (&lt;strong&gt;a&lt;/strong&gt;)—the sum of the periapsis and apoapsis distances divided by two.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Two elements define the orientation of the orbital plane in which the ellipse is embedded:&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;&lt;strong&gt;Inclination&lt;/strong&gt; (&lt;strong&gt;i&lt;/strong&gt;)—vertical tilt of the ellipse with respect to the reference plane&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Longitude of the ascending node&lt;/strong&gt; (&lt;strong&gt;ω&lt;/strong&gt; or &lt;strong&gt;Ω&lt;/strong&gt;)—horizontally orients the ascending node with respect to the reference frame's vernal point&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;And finally:&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;5&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Argument of periapsis&lt;/strong&gt; (&lt;strong&gt;ω&lt;/strong&gt;) the orientation of the ellipse in the orbital plane, measured from the ascending node to periapsis &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;True anomaly&lt;/strong&gt; (&lt;strong&gt;ν&lt;/strong&gt;, &lt;strong&gt;θ&lt;/strong&gt;, or &lt;strong&gt;f&lt;/strong&gt;) at epoch ($\mathbf{M_0}$ defines the position at a specific time (&lt;em&gt;also known as the &quot;epoch&quot;&lt;/em&gt;).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;&lt;strong&gt;check your units:&lt;/strong&gt;&lt;/h2&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;float deltaT = (T/60) * (t - t0); //divide time increments into 1/60th of the orbital period&#xA;Mt = deltaT * Mathf.Sqrt (mu / Mathf.Pow (a, 3));&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;If &lt;code&gt;60&lt;/code&gt; is unitelss, then &lt;code&gt;deltaT&lt;/code&gt; has units of time squared. That gives &lt;code&gt;Mt&lt;/code&gt; units of time. Then that means:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;float E = Mt;&#xA;float F = E - e * Mathf.Sin (E) - Mt;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;gives &lt;code&gt;E&lt;/code&gt; units of time, and things that go inside trig functions like &lt;code&gt;Sin()&lt;/code&gt; should be unitless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note;&lt;/em&gt; the use of &lt;code&gt;60&lt;/code&gt; to convert seconds to minutes for example, which would be called &lt;em&gt;seconds per minute&lt;/em&gt; is unitless.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;&lt;strong&gt;try different true anomalies&lt;/strong&gt; ( &lt;code&gt;t0&lt;/code&gt;'s )&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The true anomaly (#6) is called &lt;code&gt;t0&lt;/code&gt; in your program - have you tried just passing different values? If you pass different values between 0 and the period T, you should get the orbit to start at different places. $t0 = 0.5T$ should start as apoapsis. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-12-11T08:34:34.243" LastActivityDate="2016-12-11T08:34:34.243" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19368" PostTypeId="1" AcceptedAnswerId="21327" CreationDate="2016-12-11T07:38:09.967" Score="5" ViewCount="664" Body="&lt;p&gt;I'm looking at some old satellite documentation which mentions the use of a &quot;Q frame&quot; coordinate system to keep track of the satellite attitude. What does this &quot;Q frame&quot; generally mean in the field of satellite localization?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: The documentation can be found here: &lt;a href=&quot;https://store.cloudeo-ag.com/sites/default/files/product_details/EROS%20METADATA%20v3.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://store.cloudeo-ag.com/sites/default/files/product_details/EROS%20METADATA%20v3.0.pdf&lt;/a&gt;, page 11, items 18 and 19 in the table.&lt;/p&gt;&#xA;" OwnerUserId="17905" LastEditorUserId="17905" LastEditDate="2016-12-11T11:43:57.053" LastActivityDate="2017-05-01T15:06:03.457" Title="What does &quot;Q frame&quot; means in terms of satellite coordinates?" Tags="|attitude|terminology|coordinates|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19369" PostTypeId="1" CreationDate="2016-12-11T12:23:39.910" Score="18" ViewCount="6606" Body="&lt;p&gt;The title says it all. &lt;a href=&quot;https://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation&quot; rel=&quot;noreferrer&quot;&gt;Quaternions&lt;/a&gt; are widely used to represent the orientation of a spacecraft. Why is that, and how do quaternions compare to other alternatives?&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="12102" LastEditDate="2018-02-20T00:03:13.560" LastActivityDate="2018-02-20T00:03:13.560" Title="What are quaternions and how are they used to represent spacecraft dynamics?" Tags="|spacecraft|attitude|quaternion|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19370" PostTypeId="2" ParentId="19363" CreationDate="2016-12-11T13:50:27.583" Score="3" Body="&lt;p&gt;I'd be tempted to say that this question, as it stands, would be more on topic on &lt;a href=&quot;https://worldbuilding.stackexchange.com/&quot;&gt;Worldbuilding&lt;/a&gt;, as it is almost entirely speculative. In reality, no-one would ever want to do this. Let's take it back to basics:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing anything on Earth is a challenge. The only large flying objects are Zeppelin type craft, which are buoyant. The largest of which was around 250m long. Down from that the next largest things are jets such as the Antonov 225, Boeing 747 and Dreamliner. These are aerodynamic craft, with wingspan around the 80m mark.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, we can land very small non-aerodynamic craft on Earth - these are capsules under parachutes or using rockets. And they are a huge challenge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scale that up and imagine what you need to provide consistent drag/thrust to a large structure without it warping or snapping due to turbulence, especially through that initial move from orbit to entering the atmosphere at sub-orbital velocity, and you'll see one reason why you would never do it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now think about doing that millions of miles away on Mars...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason is that it has no benefit on a planet. Rotation is useful in space to supply an artificial gravity equivalent, but not for any other reason, or in any other environment. If you build it in space, you'll want to keep it in space.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2017-04-13T12:52:23.630" LastActivityDate="2016-12-11T14:00:50.280" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19371" PostTypeId="1" CreationDate="2016-12-11T15:54:58.137" Score="4" ViewCount="152" Body="&lt;p&gt;In US dollars, how much revenue did NASA project in 2015 for its fiscal 2016 budget and what percentage was manifested during or before Q4 2016?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer may include departmental, management or contract based revenue, grants, etc.&lt;/p&gt;&#xA;" OwnerDisplayName="user16987" LastEditorDisplayName="user16987" LastEditDate="2016-12-13T21:43:02.647" LastActivityDate="2016-12-13T21:43:02.647" Title="How much in revenue did NASA report for the US fiscal 2016?" Tags="|cost|operations|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19372" PostTypeId="2" ParentId="14983" CreationDate="2016-12-11T16:44:41.713" Score="7" Body="&lt;p&gt;The whole mathematical apparatus describing the rocket flight &amp;mdash; gravity, rocket equation &amp;mdash; it's a set of functions that take some parameters allowing you to model the process. If we set the thrust vector constant, we end with a complete equation of motion where we just plug parameters (Earth standard gravity, vehicle mass, $I_{SP}$ etc.) and we can numerically integrate it into the future. But if we want thrust to change direction, we must describe how it changes: parametrize it. We must have a function that models this change using some constants, so that we can integrate the trajectory using given constants OR retrieve them given the a trajectory. LTG (linear tangent steering law) is such a function &amp;mdash; it only allows you to model control over the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to retrieve the guidance constants you need a guidance algorithm, such as Powered Explicit Guidance. &lt;a href=&quot;http://www.orbiterwiki.org/wiki/Powered_Explicit_Guidance&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Orbiter wiki describes PEG in a tutorial way&lt;/a&gt; so that you can try to implement it yourself. I found it quite hard to understand at first, so I'll also point you towards the &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660006073.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;original paper&lt;/a&gt; &amp;mdash; with those two sources together you should be able to put it together. There are several others, such as Apollo's IGM (Iterative Guidance Mode) or Space Shuttle's UPFG (Unified Powered Flight Guidance), but the first does not have any easily available documentation and the other is just very complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Feel free to check out &lt;a href=&quot;https://github.com/Noiredd/PEGAS&quot; rel=&quot;nofollow noreferrer&quot;&gt;my own implementation on GitHub&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="17912" LastEditorUserId="17912" LastEditDate="2017-05-22T09:04:15.890" LastActivityDate="2017-05-22T09:04:15.890" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19373" PostTypeId="1" CreationDate="2016-12-11T17:19:11.127" Score="0" ViewCount="464" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.cnet.com/news/elon-musk-spacex-iss-mars-jeff-bezos-blue-origin/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this CNET article&lt;/a&gt;, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The last time a human broke free from Earth's orbit was the Apollo 17 mission in 1972. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I thought that the moon orbited the Earth, and that visits to it were still within Earth's orbit, as with anywhere else in Earth's gravity well that's not going fast enough to depart. Is CNET just wrong, or am I missing something?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a search of this site, I only found &lt;a href=&quot;https://space.stackexchange.com/questions/6277/is-orion-space-craft-evident-that-nasa-didnt-send-human-beyond-the-earth-orbit&quot;&gt;this&lt;/a&gt; suggesting that higher than 200 miles might be considered &quot;beyond Earth's orbit,&quot; but that seems to be a pretty unreliable source with nothing to back it up. &lt;/p&gt;&#xA;" OwnerUserId="11181" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-12-12T13:30:43.067" Title="What's considered the boundary for &quot;earth orbit?&quot;" Tags="|orbital-mechanics|orbit|apollo-program|gravity|history|" AnswerCount="2" CommentCount="12" ClosedDate="2016-12-13T09:40:00.133" ContentLicense="CC BY-SA 3.0" />
  <row Id="19374" PostTypeId="1" AcceptedAnswerId="19377" CreationDate="2016-12-11T18:07:39.200" Score="45" ViewCount="6370" Body="&lt;p&gt;When we look at various Russian rockets we often see a metal structure looking like a kind of garden fence separating some stages. To my knowledge, only the R7 family (Vostok, Voshkod, Soyuz) and N1 are (were) using it, but nobody else.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other rockets I know of use a solid fairing to separate stages. So why did the Russian engineers chose (and kept) this solution but nobody else? Apart from being very light-weight, are there any other advantages to these &quot;fences&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some images to show what I'm talking about:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/1/12/Soyuz_rocket_assembly.jpg/640px-Soyuz_rocket_assembly.jpg&quot; alt=&quot;Soyuz during assembly&quot;&gt; (&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Soyuz_rocket_assembly.jpg&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/c/c1/N1_white.svg/135px-N1_white.svg.png&quot; alt=&quot;N1&quot;&gt; (&lt;a href=&quot;https://en.wikipedia.org/wiki/File:N1_white.svg&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="198" LastEditDate="2020-02-03T23:09:04.913" LastActivityDate="2020-02-03T23:09:04.913" Title="Why do the Russians use these fence-like interstage fairings?" Tags="|rockets|design|soyuz-launcher|stage-separation|n-1|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19375" PostTypeId="2" ParentId="19373" CreationDate="2016-12-11T18:59:07.710" Score="6" Body="&lt;p&gt;The CNET article is wrong, I think what it is trying to say is the last time humans left Low Earth Orbit (LEO) was in Apollo 17, which is completely true. Apollo spacecraft did not accelerate enough to leave earth's gravity well, they accelerated enough to get to the point that the moon's gravity well became stronger than the Earth's.  &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2016-12-11T18:59:07.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19376" PostTypeId="2" ParentId="19323" CreationDate="2016-12-11T20:12:26.447" Score="7" Body="&lt;p&gt;From NASA's own website:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;With a half hour to go, Merritt Island was experiencing peak winds of 14 knots, light rain showers, broken clouds at 240 meters, and overcast skies at 3,000 meters. But the ceiling exceeded the minimum requirement of 150 meters, and the ground winds were within limits. The Apollo design permitted launch during rain. The possibility of lightning concerned Launch Operations Director Kapryan, however, and he considered a hold. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(...) A weather report from the Eastern Test Range helped Kapryan make up his mind. An Air Force plane reported only mild turbulence and no indication of lightning within 32 kilometers of LC-39. &lt;em&gt;Moonport, Ch22-2&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you want more information on why they continued to launch during the special weather conditions, see these links:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;Lightning Strikes&quot; (&lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4204/ch22-2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Moonport, Ch22-2&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;&quot;Why and Wherefores of Lightning&quot; (&lt;a href=&quot;http://www.hq.nasa.gov/office/pao/History/SP-4204/ch22-3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Moonport Ch22-3&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As response to &lt;a href=&quot;https://space.stackexchange.com/questions/19323/why-was-apollo-12-allowed-to-go-up-in-such-bad-weather/19327#19327&quot;&gt;Uwe's&lt;/a&gt; anwser: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After the launch some newspapers suggested that President Nixon's presence influenced Kapryan's decision. The launch director denied it. &lt;em&gt;Moonport, Ch22-2&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="11358" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2016-12-18T17:11:29.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19377" PostTypeId="2" ParentId="19374" CreationDate="2016-12-11T23:08:55.423" Score="56" Body="&lt;p&gt;It's all to do with &lt;strong&gt;ullage&lt;/strong&gt; in the fuel tanks. Newton's laws of motion mean that when a rocket is no longer firing and no force is being applied, the rocket receives no acceleration. It continues at its same velocity (if we assume a perfect model). The fuel in the tanks goes into free-fall, just as the astronauts do when they reach orbit. You've seen those soft toys going floating around the cabin when the Soyuz engines cut off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel also acts a bit like a passenger in a car when you brake hard. Instead of being pushed back in their seats when accelerating, they fly the other way, towards the windscreen. That's why we use seatbelts. The same for the fuel in those huge tanks. It goes whoosh; right to the other end of the tank, the farthest distance from the rocket motor. The problem is, you want it at the rocket motor end. If the fuel is not at the &quot;bottom&quot; of the tank the pumps spin on empty, and can burn out with nothing to suck on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two solutions to this problem. Some craft have &lt;strong&gt;ullage motors&lt;/strong&gt; which are little engines, sometimes just thruster jets or a release of compressed gas. You can see them in many videos of other craft that do not have this mesh. (Not to be confused with the pump exhaust nozzle,  but that is another matter entirely!) It often looks like a small exhaust pipe near the engine. They can sometimes just be a small solid fuel engine, much like a firecracker. Their only function is to move (slosh) the fuel down to the pumps. The pumps start, and then the engine fires. Whoosh; rocket starts. Now all this fuel sloshing about can cause stability problems for the craft, so they design the tank with special anti-slosh baffles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here we can see those various features in the Saturn V second stage:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.hq.nasa.gov/pao/History/SP-4204/images/m406b.jpg&quot; alt=&quot;Source NASA&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Source: &lt;a href=&quot;https://www.hq.nasa.gov/pao/History/SP-4204/images/m406b.jpg&quot; rel=&quot;noreferrer&quot;&gt;https://www.hq.nasa.gov/pao/History/SP-4204/images/m406b.jpg&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to avoid the extra complexity, controls, fuel, mechanisms and just plain mass and money in your rocket, you can do it the Russian way. Just turn on the next stage while the previous one is still applying force. No need for ullage equipment. However, you have to make somewhere for all that hot exhaust gas (and its force) to go. The easiest way is to make a slight gap between the stages so you then get less of a baba-boom. It also has the nice advantage of pushing the completed stage away from the craft and back towards the planet, which is usually what you want. It might make the rocket taller, but it keeps the mass and complexity down, which is usually what you want.&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="8162" LastEditDate="2016-12-11T23:23:11.923" LastActivityDate="2016-12-11T23:23:11.923" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19378" PostTypeId="1" AcceptedAnswerId="19380" CreationDate="2016-12-12T00:33:36.003" Score="2" ViewCount="234" Body="&lt;p&gt;During the HTV-6 launch I saw a large, enclosed fire pit next to the launch pad. (On the left of the picture below). What is its purpose?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LINQA.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LINQA.png&quot; alt=&quot;Tanegashima&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Video of the fire pit (@ 24:54):&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/bHmcV6bzeCY?start=1494&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="10477" LastActivityDate="2016-12-12T11:50:47.240" Title="Fire pit at Tanegashima space center" Tags="|launchpad|jaxa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19380" PostTypeId="2" ParentId="19378" CreationDate="2016-12-12T01:14:41.823" Score="6" Body="&lt;p&gt;They're burning off hydrogen in what is called a &quot;flare stack&quot;. Probably the hydrogen boiled off from chilling down the engine and/or the hydrogen boiling off from the tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These dim orange flames are typical hydrogen diffusion flames.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The water is there to prevent anything from catching fire. Hydrogen burns pretty hot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Daylight picture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wWoiV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wWoiV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;sub&gt;Source JAXA (&lt;a href=&quot;http://global.jaxa.jp/activity/pr/brochure/files/centers02_e.pdf#page=2&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://global.jaxa.jp/activity/pr/brochure/files/centers02_e.pdf#page=2&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a video of the flare stack at KSC launch pad 39A:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/D_QHJQmfq0o?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2016-12-12T11:50:47.240" LastActivityDate="2016-12-12T11:50:47.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19382" PostTypeId="1" CreationDate="2016-12-12T06:42:31.290" Score="5" ViewCount="600" Body="&lt;p&gt;Imagine I'm sitting on a geostationary satellite. I can see the Earth in front of me. It's about the size of a soccer ball at arm's length. To the left and right, I can see similar satellites in the immediately adjacent slots. They're 0.1 degrees or 74 km away. You can almost discern their shape since their solar cells span about 25 meters and human visual resolution is about 1 arcsecond. But how bright will they appear?  [ Maybe I can get this by looking up their magnitude seen from Earth and using the inverse square law? ]&lt;/p&gt;&#xA;" OwnerUserId="14419" LastActivityDate="2017-01-12T00:57:22.163" Title="At what magnitude would adjacent large geostationary communications satellites see each other?" Tags="|communication-satellite|geostationary|naked-eye|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19383" PostTypeId="1" AcceptedAnswerId="33334" CreationDate="2016-12-12T07:00:50.027" Score="8" ViewCount="1259" Body="&lt;p&gt;I would like to know what is the closest separation achieved by two satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's restrict it to &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Non experimental satellites (or else it would be 0)&lt;/li&gt;&#xA;&lt;li&gt;Operational satellites (no collision)&lt;/li&gt;&#xA;&lt;li&gt;Post orbital insertion (1 m higher in the fairing doesn't count)&lt;/li&gt;&#xA;&lt;li&gt;Intentional&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/3867/6642&quot;&gt;This answer&lt;/a&gt; state that it would be &quot;Few kilometres&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whereas ESA (non commercial entity) achieved &lt;a href=&quot;http://blogs.esa.int/rocketscience/2013/09/16/esas-cluster-satellites-make-closest-ever-dance-in-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;4km&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="12102" LastEditDate="2019-12-19T00:41:14.230" LastActivityDate="2019-12-19T00:41:14.230" Title="What is the closest intentional separation achieved by two satellites?" Tags="|orbital-maneuver|commercial|conjunction|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19384" PostTypeId="1" CreationDate="2016-12-12T07:48:30.550" Score="4" ViewCount="212" Body="&lt;p&gt;I'm interested in the stated goals of Russia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_Piloted_Assembly_and_Experiment_Complex&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Piloted Assembly and Experiment Complex&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears NASA is &lt;a href=&quot;https://www.engadget.com/2015/03/28/nasa-is-working-with-russia-on-a-new-space-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;not entirely on board&lt;/a&gt;, but still amicable about the concept of using elements of the Russian Orbital Segment to construct a new Russian space station called OPSEK.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering: &lt;strong&gt;What support is there currently for OPSEK, &lt;em&gt;outside&lt;/em&gt; of Russia and their stated intentions?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2016-12-13T07:07:10.497" Title="Is there any non-Russian support for Russia's conceptual OPSEK?" Tags="|iss|space-station|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19385" PostTypeId="2" ParentId="19382" CreationDate="2016-12-12T08:43:07.487" Score="8" Body="&lt;p&gt;Here is a profoundly simplistic way to get a feeling for how bright a sunlit object 74 km away might appear in space. Your milage may vary by one or two orders of magnitude depending on details of satellite shape and materials, and geometry of the sun-satellite-observer angle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this 0th order model the illumination by Earthshine is ignored, but can be added later. There will be simpler expressions for this, certainly from radar texts for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assumptions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The observed satellite is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Spherical_cow&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spherical Cow&lt;/a&gt; with a radius $R_{cow}$ of 2 meters. Hereafter known as CowSat.&lt;/li&gt;&#xA;&lt;li&gt;CowSat is a &lt;a href=&quot;https://physics.stackexchange.com/q/282106/83380&quot;&gt;Holstein&lt;/a&gt; with 70% of it's area acting as a simplified diffuse reflector - 50% of the light striking the white area is reflected hemi-isotropically into $2\pi$ sr.&lt;/li&gt;&#xA;&lt;li&gt;Geometry is optimal for cow brilliance. You are between CowSat and the Sun so that CowSat is fully illuminated.&lt;/li&gt;&#xA;&lt;li&gt;The Sun's visual magnitude is -27&lt;/li&gt;&#xA;&lt;li&gt;You are smart and so never look directly at the sun to keep your night vision. This leaves your pupils with a radius ($R_{pupil}$) of 0.003 meters. This actually factors out in the end anyway.&lt;/li&gt;&#xA;&lt;li&gt;You are 150 million kilometers or 1.5E+11 meters ($R_{SunEarth}$) from the sun.&lt;/li&gt;&#xA;&lt;li&gt;At that distance, your pupil takes in $(\pi R_{pupil}^2)/(4\pi R_{SunEarth}^2)$ or 1.0E-28 of the Sun's output.&lt;/li&gt;&#xA;&lt;li&gt;At the same distance, CowSat receives $(\pi R_{CowSat}^2)/(4\pi R_{SunEarth}^2)$ or 4.4E-23 of the Sun's output, and reflects 0.7*0.5 of that (or 1.6E-23) into $2\pi$ sr. &lt;/li&gt;&#xA;&lt;li&gt;At a distance $R_{satsep}$ of 74,000 meters, your pupil takes in $(\pi R_{pupil}^2)/(2\pi R_{satsep}^2)$ or 8.2E-16 of CowSat's reflected light.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Direct to Pupil: &lt;code&gt;1.0E-28 suns&lt;/code&gt;&#xA;Reflected to Pupil: 1.6E-23 * 8.2E-16 = &lt;code&gt;1.3E-38 suns&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CowSat/Sun = 1.3E-10 or 24.7 magnitudes dimmer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;-27 + 24.7 = -2.3 magnitude.  CowSat will certianly be visible and could be very bright - as in Venus seen from Earth bright. In reality it will vary quite a bit, but in general next-door-neighbor geostationary satellites one or two tenths of a degree apart will be easily visible to each other - even using a cellphone camera, a significant fraction of the time when the basic geometry is favorable (viewed satellite is front or at least side-lit from the sun as seen by viewer satellite).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put things in perspective, even a simple 100 mA lensed green LED 18 kilometers away &lt;a href=&quot;https://physics.stackexchange.com/q/282106/83380&quot;&gt;will still appear to be as bright as a 0 magnitude star&lt;/a&gt;!  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8F18Y.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8F18Y.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; CowSat, without question, outstanding in its field. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Cow_female_black_white.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2016-12-12T10:34:56.797" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19386" PostTypeId="1" AcceptedAnswerId="19424" CreationDate="2016-12-12T12:12:29.420" Score="12" ViewCount="701" Body="&lt;p&gt;In April of 2010 the Geo-not-so-stationary satellite Galaxy-15 stopped responding to ground commands. It operates as a &quot;bent-pipe&quot; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Transponder_(satellite_communications)&quot; rel=&quot;noreferrer&quot;&gt;Transponder&lt;/a&gt; system, which means - roughly speaking - it will amplify and rebroadcast back to earth whatever TV programming it happens to receive from Earth (within prescribed frequencies). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Geostationary satellites need to perform regular station-keeping maneuvers to stay put, and since commercial satellites are typically not able to do this, regular instructions are sent from the ground and must be received in order to keep the satellite wherever it's supposed to be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So when Galaxy-15 stopped responding to commands, it started to drift uncontrolled within the geostationary &quot;belt&quot;. That is a potential collision problem, but the other problem was that all the other systems worked. It maintained it's solar panels pointing towards the sun and kept &quot;Earth lock&quot; meaning that it had its receiving and transmitting antennas pointed down towards Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once it drifted into another &quot;box&quot; which happened to have TV signals beamed up from the ground in the same microwave band, it would dutifully amplify them and broadcast them back to Earth, and so even without a physical collision it could cause serious interruptions to other satellite's duties. This mindless capture and rebroadcast without control is what makes the term ZombieSat fitting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following is from &lt;a href=&quot;http://goes.gsfc.nasa.gov/text/Zombiesats.pdf&quot; rel=&quot;noreferrer&quot;&gt;Dealing with Galaxy 15: Zombiesats and on-orbit servicing&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Whatever malfunction did occur did not affect either the satellite’s ability to re-broadcast signals or its ability to keep its transponders pointed at the Earth and solar panels aligned with the Sun (known as “Earth lock”). This allowed the spacecraft to continue to receive and transmit signals. What it did affect was the ability of Intelsat’s ground controllers to maneuver Galaxy 15 to maintain its orbital position. Intelsat issued between 150,000 and 200,000 commands to the satellite in an attempt to get a response to either turn off its communications payload or maneuver. &lt;strong&gt;When these efforts failed, the company attempted to send an even stronger signal to try and force an overload of the satellite’s power system and cause it to shut down&lt;/strong&gt;. This too failed. As a result, the satellite continued to drift slowly eastward through the GEO belt. What had seemed like a small problem was about to get much bigger.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question:&lt;/strong&gt; Is this &lt;a href=&quot;https://i.stack.imgur.com/kP4Gj.jpg&quot; rel=&quot;noreferrer&quot;&gt;a bug, or a feature&lt;/a&gt;? Isn't there some kind of automatic gain control or &lt;a href=&quot;https://en.wikipedia.org/wiki/Automatic_gain_control&quot; rel=&quot;noreferrer&quot;&gt;AGC&lt;/a&gt;? I am guessing that the idea in this attempt to shut down the transponders is to rely on the (preseumably) fixed gain of the amplifier - that stronger-in means stronger-out - and so one can trip something analagous to a circuit breaker by shouting really loud into the microphone so to speak. Was the inability of the amplifier to limit its gain designed in as a feature (a brute force shut-down when all else fails), or was it necessary due to other constraints, or did someone just forget to add an AGC?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One might argue that an AGC might diminish weak signals too far while attempting to limit a strong signal, but dynamic frequency-dependent gain has been around in fiber optic Dense Wavelength Division Multiplexing (DWDM) with 40 or 80 wavelengths, (although those are a different types of modulation e.g. digital QPSK)) so I'm thinking that that would not be a reason to avoid AGC. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That they attempted this suggests that either there was no AGC or that they knew it may be inadequate under extreme conditions. So its not even clear to me if the hoped-for shut down would have been exptected behavior, or wishful thinking. Could somebody with a really big steerable dish start shutting down lots of communication satellites this way? Could a day come when billions of people would experience cable TV withdrawl syndrom all at the same time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NGbzv.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NGbzv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Basic +20dB, automatic gain control cell - as used in 1970s telephone networks.&quot; A simple example of a circuit which will adjust the amplifiers gain if the signal is too strong, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Vactrol_AGC_(Yushin).PNG&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-07T09:16:44.303" LastActivityDate="2021-08-07T09:16:44.303" Title="How can just making the ground broadcast stronger neutralize a zombie(sat)?" Tags="|artificial-satellite|communication-satellite|radio-communication|failure|station-keeping|" AnswerCount="2" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19387" PostTypeId="2" ParentId="19373" CreationDate="2016-12-12T12:51:32.053" Score="3" Body="&lt;p&gt;I think you are right to be troubled by the phrase &quot;beyond Earth's orbit&quot; because it's a poor choice of words that muddies what's actually a slightly complicated situation when your looking at three bodies - in this case the Sun, the Moon, and the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So lets forget the &quot;beyond&quot; part for a moment because that implies a certain distance or at least a certain clearly delineated boundary. Even if you treat the Sun Earth Moon system as a 2D plane, that boundary would probably have to be drawn in four dimensions - at each point x,y there can be some velocity vectors vx, vy that will keep a spacecraft going around the Earth, while another vx, vy might put it free of the Earth to orbit the sun, and another vx, vy that will put it in at least a temporary orbit around the Moon. In the real 3D space it's a boundary in six dimensions in real space. In a CR3BP - a mathematical idealized system with perfectly circular orbits, it's fewer than six, but that's not the real world in a number of ways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I would propose you just ignore the literal interpretation of &quot;beyond&quot; as simply greater than a certain distance. Think of it as &lt;em&gt;no longer orbiting just the Earth alone&lt;/em&gt;.  Orbiting a body can be though of as simply &quot;goes around and around&quot; the body, and so you are right, a spacecraft orbiting the moon is also orbiting the Earth, and is also orbiting the Sun, and is also orbiting the center of the galaxy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are fairly complicated shaped orbits that look like a hybrid between an Earth orbit and a Lunar orbit, or even an orbit which literally &lt;em&gt;stays between the Earth and the Moon&lt;/em&gt; and those orbits get named after dead mathematicians, or athletic feats like (just for example) &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Aleksandr_Lyapunov&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lyapunov&lt;/a&gt;&quot; and &quot;&lt;a href=&quot;https://space.stackexchange.com/q/3889/12102&quot;&gt;back-flip or Lunar Cycler&lt;/a&gt;&quot; orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But when a spacecraft starts circling the moon, going around and around the moon, it is really common for people to say that it is no longer orbiting the Earth. It certainly is also in orbit around the Earth, but usually people will say it's &quot;primary&quot; is now the Moon since that's the body it is now most closely associated with now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Experienced &quot;rocket scientists&quot; know what each other mean when they say it's not in orbit around the Earth any more, and in the back of their mind they still know it is indeed orbiting the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to answer your question: &lt;strong&gt;There is no simple, absolute, closed boundary in 3D (x, y, z) space where you can point and just say anything on one side is definitely in Earth orbit, and anything on the other side is definitely not in Earth orbit&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me know if this helps!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-12-12T13:30:43.067" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19389" PostTypeId="2" ParentId="19297" CreationDate="2016-12-12T17:35:49.393" Score="5" Body="&lt;p&gt;ISS didn't have many countries as part of it especially China and India IMO have many and varying reason.&lt;/p&gt;&#xA;&lt;p&gt;China is somewhat capable on his own for going on venture like this having space station as you know &lt;a href=&quot;https://en.wikipedia.org/wiki/Tiangong-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tiangong&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;If you compare space program of China with nations participating in ISS excluding USA, China has far more program running concurrently and joining ISS means China has to spend a considerable amount and resources for it's part in ISS. China is still a part of &lt;a href=&quot;https://en.wikipedia.org/wiki/Newly_industrialized_country&quot; rel=&quot;nofollow noreferrer&quot;&gt;newly industrialize nations&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Also china is part of &lt;a href=&quot;https://en.wikipedia.org/wiki/Comparison_of_Asian_national_space_programs&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asian Space Race&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;China has to prove itself that it has been there as early as possible to compete with nations like India and Japan.&lt;/p&gt;&#xA;&lt;p&gt;Regarding India there has been talks of letting participate ISRO in ISS but in my opinion this is somewhat not possible because of lack of budget and technological barriers.&lt;/p&gt;&#xA;&lt;p&gt;Isro didn't have any experience in projecting human in outer space neither have any crew sustainability module, although development is in undergoing for above points.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The annual running costs for the Japanese Experiment Module will be&#xA;totally around $350-400 million (almost half of ISRO’s annual budget).&#xA;Which means that if India has to participate meaningfully and do some&#xA;interesting science, ISRO will need an almost 50% hike (to Rs.3,500&#xA;crore) in its budget. Although this is one-tenth of the cost of having&#xA;our own manned space programmes, this is also the cost of having 50&#xA;Mars Orbiter Missions a year!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://thewire.in/28441/will-isro-participate-in-the-international-space-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://thewire.in/28441/will-isro-participate-in-the-international-space-station/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.thespacereview.com/article/1253/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.thespacereview.com/article/1253/1&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14616" LastEditorUserId="33864" LastEditDate="2020-12-18T18:40:32.623" LastActivityDate="2020-12-18T18:40:32.623" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19390" PostTypeId="2" ParentId="19369" CreationDate="2016-12-12T18:28:54.397" Score="27" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What are quaternions and how are they used in spacecraft dynamics?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;h2&gt;Background&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Newtonian mechanics says we live in a universe with three spatial dimensions, and a universal time that is the independent variable, in which we can describe translation and rotation. Relativity theory says we live in a universe with three space-like dimensions and one time-like dimension that are interweaved. This answer ignores relativistic aspects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this assumption, describing translation is easy, at least compared to rotation. An object's center of mass has a position in some frame of reference (preferably inertial) that changes over time. Gravitation, drag, and other interactions such as firing the translational thrusters change this value as a function of time. Simulate these interactions and you have a three degree of freedom (3DOF) simulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What makes 3DOF modeling so easy is that it is a commutative space. Walk north ten meters and then east five meters and you'll arrive at a certain spot. Alternatively, walking east five meters and the north ten meters will bring you to that same spot. Translation in Euclidean space commutes. Rotation in three dimensional space is not commutative. Pick up a book and rotate it about an axis parallel to the book's spine and then rotate the book about an axis perpendicular to the book's face. Do these actions differently (first rotate about an axis perpendicular to the book's face and then about it's spine) and you'll get a different end result.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Rotations in three-dimensional space&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As rotations in three-dimensional space do not commute, whatever they are, they are not vectors. Commutativity is an essential quality of being a vector. Since rotations aren't vectors, how to represent them? It turns out that there are a many different ways to represent rotations in three dimensional space. Euler was one of the first to investigate this issue,  realizing a number of key concepts. One is that a rigid body rotating in three dimensional space can always be described as instantaneously rotating about a single axis. This concept is unique to 3D space, and is also key as to why quaternions (unit quaternions) are useful for describing rotations. I'll address this a bit later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He also realized that the orientation of a planet, or of a rigid body in general, can be described in terms of three rotations. Euler used a sequence of three rotations: A rotation about the Z axis, then another about the once-rotated X axis, and then yet another about the twice-rotated Z axis. There's nothing special about this canonical Z-X'-Z'' sequence. All that is needed is that the first and third rotations be about the same body-fixed axis and that the middle element be about a body-fixed axis orthogonal to the other axis. A related sequence is to use three orthogonal body-fixed axes. This choice results in (for example) a roll-pitch-yaw sequence. There are six classical Euler sequences (e.g., Z-X'-Z''), plus six Tait-Bryan sequences (e.g., roll-pitch-yaw). Yet another variation on this theme is to rotate about a fixed set of axes. This leads to the extrinsic Euler sequences. All together, there are 24 different things that can be called Euler rotation sequences.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way to represent rotations is via a proper orthogonal matrix. An orthogonal matrix is an NxN matrix in which each row (or each column) is a unit vector and is orthogonal to all of the other rows (or columns). This representation is generic; it applies to any N-dimensional cartesian space. The 3x3 representation is of special interest because we live in a 3D universe. A key problem with an NxN matrix used to represent orientation is that rotations in N dimensional space have $(N^2-N)/2$ degrees of freedom while the matrix has $N^2$ elements. Over half of the elements of the matrix are redundant. (This problem of redundancy also applies to quaternions. Rotations in 3D space have three degrees of freedom. Quaternions have four elements.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet another way to represent orientation / rotation in 3D space is via Euler's single axis theorem. No matter how weirdly a rigid body is oriented, there always exists an axis combined with a rotation by some angle about that axis that can be used to represent the orientation of the body in question with respect to some given frame of reference. This axis/angle representation is very close to a quaternionic representation. Given an unit vector $\hat u$ and an angle $\theta$, the quaternion whose scalar part is $\cos(\theta/2)$ and whose vector part is either $\hat u \sin(\theta/2)$ or $-\hat u \sin(\theta/2)$ captures all of the information needed to represent the orientation, completely avoiding the singularity issues associated with Euler sequences, and almost avoiding the over-specification issue associated with matrices.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;WTF are quaternions?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The above begs the question. I wrote about quaternions before discussing what quaternions are. William Rowan Hamilton struggled to find an extension of the complex numbers, where $i=\sqrt{-1}$. He finally had an insight while taking a walk with his wife and invented the quaternions. To ensure he wouldn't forget this incredible insight, he carved some graffiti on a bridge:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$i^2=j^2=k^2=ijk=-1$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0fi9v.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0fi9v.jpg&quot; alt=&quot;plaque on bridge with equation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seemingly simple graffiti is incredibly complex (pun intended). Complex numbers have two elements, one real and one imaginary part. Negative one has two square roots in the complex numbers. Quaternions have four elements, one real part and three imaginary parts. Negative one has an uncountably many square roots in the quaternions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a natural progression from the quaternions, discovered post-Hamilton. The octonions have eight elements, one of which is real part and the other seven are (somehow) imaginary. The sedenions have sixteen elements, one of which is real and the other fifteen are imaginary, and so on, with powers of two. The problem is that each step up the ladder drops a key mathematical concept. Quaternions don't commute (multiplicatively), octonions don't even associate. Sedenions and higher are generally rather worthless.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Quaternion mathematics&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Quaternions have two key binary operators, addition and multiplication, plus additive and multiplicative identity elements, additive inverses, and except for zero, multiplicative inverses. Addition works analogously to addition of a pair of four dimensional Cartesian vectors. Multiplication is where quaternions get tricky. From Hamilton's graffiti, one can deduce (for example) that $ij=k$ but that $ji=-k$. Quaternion multiplication is noncommutative. If one views quaternions as comprising a real scalar part and a vectorial imaginary part, the product of two quaternions $(a+\vec b)(c+\vec d)$ is ($ac-\vec b \cdot \vec d) + (a\vec d + c\vec b + \vec b \times \vec d)$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives quaternions just the right amount of messiness needed to describe the rotational difference between two frames of reference in three-dimensional space. In particular, given some quaternion $q$ and a vector $v$ viewed as a pure imaginary quaternion, the quaternion operation $q\,v\,q^{-1}$ is another pure imaginary quaternion. This operation rotates the vector $v$ in three dimensional space. So does the operation $q^{-1}\,v\,q$.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Unit quaternions&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Because we're dealing with inverses, it makes sense to scale those quaternions so they are unitary. The inverse of a quaternion is its conjugate divided by the square of the magnitude. For a unit quaternion $q$, its inverse and quaternion conjugate are one and the same: $q^{-1} = q^\ast$ for all unit quaternions $q$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reason to use unit quaternions is that they form the right kind of mathematical structure needed to represent rotations in 3D space. The unit quaternions are a mathematical group, as are rotations in 3D space. Every unit quaternion represents some rotation in 3D space, and every rotation in 3D space can be represented by a unit quaternion. Actually, every rotation in 3D space can be represented by two unit quaternions. Multiply a unit quaternion by -1 and you'll get another unit quaternion that represents the same rotation as the first one. Unit quaternions form a double cover on rotations in 3D space.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;How are quaternions used to represent rotations in 3D space?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;This is the key question. The answer is that there is a very simple relation between the single axis representation of a rotation and a unit quaternion. The scalar part (aka real part) of this quaternion is $\cos(\theta/2)$, where $\theta$ is the single axis rotation angle, and the imaginary part is either $\hat u \sin(\theta/2)$ or $-\hat u \sin(\theta/2)$, where $\hat u$ is the unit vector along the axis of rotation. Whether one uses a plus or minus sign is completely arbitrary. This of course results in a number of arguments over &quot;the right way to do it.&quot; There is no right way. Both approaches are equally valid.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;What are some of the gotchas?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;While quaternions aren't evil as are Euler angles, there are issues associated with their use.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Left versus right quaternions.&lt;br&gt;&#xA;There are two ways to use unit quaternions to represent a rotation or transformation in 3D space: $q v q^{\ast}$ or $q^{\ast} v q$. Both approaches are equally valid mathematically; the only difference is whether the unconjugated quaternion is on the precedes or follows the vector to be transformed or rotated.&lt;/li&gt;&#xA;&lt;li&gt;Transformation versus rotation quaternions. &#xA;Quaternions can be used to represent the physical rotation of a vector in 3D space and to represent the transformation of the same vector from one coordinate system to another. These turn out to be conjugate operations.&lt;/li&gt;&#xA;&lt;li&gt;Order of the elements.&lt;br&gt;&#xA;Quaternions have four elements, a scalar real part and a vectorial imaginary part. Which should go first, the real or imaginary part, when storing or exchanging them with someone else? The answer is that there is no right answer.&lt;/li&gt;&#xA;&lt;li&gt;How to numerically integrate them.&lt;br&gt;&#xA;The above three are representational issues only. This final issue is complex enough to be handled separately.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The first issues mean that when working with someone else (e.g., a joint integrated simulation), one had better get the nomenclature the two teams straightened out. The odds are strong that the two groups will not be using the same representations, and the odds are also strong that neither team will change their internal scheme. Fortunately, the conversion from one to another is easily handled. There will be lots of finger-pointing it's not handled.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;How to numerically integrate unit quaternions?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;There are a number of reasons one may need to integrate a unit quaternion over time. A non-zero angular velocity changes a spacecraft's orientation. I use left transformation unit quaternions in my work. The time derivative of such a quaternion is $\dot q_{I\to B} = -\frac 1 2 \omega \, q_{I\to B}$ where $q_{I\to B}$ is the inertial to spacecraft body left transformation quaternion and $\omega$ is the spacecraft's angular velocity expressed in the body frame coordinates. Numerical integration inevitably involves steps along the lines of $x(t+\Delta t) = x(t) + \dot x(t)\Delta t$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is mathematically invalid for unit quaternions; the unit quaternions do not have an addition operator. It is valid for quaternions in general, but the result is inevitably a non-unit quaternion. One simple expedient is to normalize the result. This works,a bit, for very small steps. There has been a huge amount of work on geometric integrators over the last two decades, in particular, Lie group integrators. The unit quaternions are a fairly simple Lie group. The tangent space is the pure imaginary quaternions. This is the space in which angular velocities live. The pure imaginary quaternions form an algebra, a Lie algebra in particular. All of the work done on Lie group integration techniques applies directly to the unit quaternions.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2016-12-14T15:24:14.857" LastActivityDate="2016-12-14T15:24:14.857" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="19392" PostTypeId="2" ParentId="19384" CreationDate="2016-12-13T07:07:10.497" Score="3" Body="&lt;p&gt;Although OPSEK is currently a proposal and creating a space station requires a huge amount of budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russia may extend it's hand to ESA if there is any possibility, there are discussions happened like what after 2025?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Cooperation with Europe and the US&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In 2008, Russian plans for maintaining presence in the low-Earth orbit&#xA;  in general and the creation of a successor to the ISS in particular,&#xA;  had been met enthusiastically in Europe. As ESA had little hope to&#xA;  match the US effort to return to the Moon at the beginning of the 21st&#xA;  century, preserving a destination in low-Earth orbit seemed critical&#xA;  for the political support of the manned space flight on the continent.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In June 2009, Simonetta Di Pippo, ESA director of human space flight,&#xA;  told the editor of RussianSpaceWeb.com that she shared the Russian&#xA;  vision of the future space station as a platform for deep space&#xA;  missions. &quot;I have continuous consultations with officials in Russia.&#xA;  We meet every month, month and a half, and now we are going to start&#xA;  jointly, the study how to proceed beyond 2025, Di Pippo said, ...&quot;and&#xA;  we have a common idea that we would like to preserve presence in the&#xA;  lower orbit. We are studying different scenarios, whether we need&#xA;  permanent presence or, maybe, a human-tended capability, and we can&#xA;  end up with a totally different solution in the end, but I don’t&#xA;  believe we can leave Earth orbit.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Di Pippo also said that although current NASA plans for return to the&#xA;  Moon reserved no essential role for the station, it could change in&#xA;  the future. &quot;Even on the NASA side, they have too many different&#xA;  developments (associated with the Earth orbit), including commercial&#xA;  involvement, which they can not immediately give up,&quot; Di Pippo said.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;By the end of 2010, all partners in the ISS project were expected to&#xA;  agree on an extension of the ISS lifespan from 2015 to 2020 or even&#xA;  2025. Once end of life for the ISS had been decided, active planning for post-ISS manned space flight could begin in Russia, Europe and&#xA;  possibly the US.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is also an open option for Russia from it's partners from &lt;a href=&quot;https://in.rbth.com/economics/technology/2016/11/02/space-agencies-from-brics-sign-cooperation-protocol_644343&quot; rel=&quot;nofollow noreferrer&quot;&gt;BRICS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;China possess own space station program with ambitious objective lift&#xA;  first module of new fully modular station in years 2020-2022. One of&#xA;  the countries possibly interested in cooperation with Russia could be&#xA;  India. ISRO still have not announced any news about their plans for&#xA;  creating independent space station and their astronauts training&#xA;  center in Bangalore is at the beginning of the development.&#xA;  Cooperation with Russia, experienced with manned missions, could be&#xA;  vital for Indian manned space program and change its position from&#xA;  launch service provider to country with manned space program and share&#xA;  in space station. One from Arab countries, like Saudi Arabia (with&#xA;  signed agreement for further cooperation with Roscosmos on June 2015)&#xA;  could also participate in program; in this way it could easily put its&#xA;  space program  forward and get independent access to space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Although only china and india has remarkable presence in outer space and they have their own space ambitions and projects to complete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.russianspaceweb.com/opsek.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.russianspaceweb.com/opsek.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflights.news/?p=29258&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spaceflights.news/?p=29258&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14616" LastActivityDate="2016-12-13T07:07:10.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19393" PostTypeId="1" AcceptedAnswerId="25889" CreationDate="2016-12-13T09:38:21.567" Score="13" ViewCount="4343" Body="&lt;p&gt;I looked for images of a nice sunlit ISS crew cabin interior and predictably didn't have much luck. Most hits were mentions of the sunlight the astronaut would leave behind on earth, or enjoy again after returning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I found a few images of astronauts (possibly) enjoying some sunshine. I found one interior photo of Terry Virts with sunshine on his face, and a few exterior shots of Andre Kuipers, Paolo Nespoli and a group shot of Alexander Gerst, Reid Wiseman and Max Suraev, looking out through a cupola window with sunlit faces. Paolo Nespoli is shielding his eyes from direct sunlight, and it looks like Andre Kuipers is wearing sunglasses??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ubqVE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ubqVE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WIZNX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WIZNX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; TOP: Andre Kuipers, Exp. 30, BOTTOM: Paolo Nespoli, cropped/enlarged from image below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can imagine there are a number of reasons why the cupola windows would generally be set to block the sun, so letting sunshine into the crew area may be limited or discouraged, but I'm wondering if astronauts sometimes feel like opening the shades and letting natural light in &lt;em&gt;at least once in a while&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Do ISS astronauts &lt;em&gt;occasionally&lt;/em&gt; let the sunshine in for a natural light &quot;fix&quot; - in other words to get a refreshing few minutes in natural lighting for a change? If so, are there limitations or restrictions associated with doing so?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Bmv7M.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Bmv7M.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; ISS042e016586 (11/26/2014) --- NASA astronaut Terry Virts in the International Space Station on Earth sunrise Nov. 26, 2014 looks through the cupola window while checking the &quot;dosimeter&quot;. The cupola allows the crew 360 degree vision around the station for both photos and operating the Canada arm to pull spacecraft up to the station ports. From &lt;a href=&quot;https://www.nasa.gov/content/nasa-astronaut-terry-virts-in-the-international-space-station&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JtxQI.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JtxQI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; European Space Agency astronaut Andre Kuipers, Expedition 30 flight engineer, is pictured in a window of the Cupola of the International Space Station, backdropped by the blackness of space. ISS030-E-270550 (21 April 2012). From &lt;a href=&quot;http://spaceref.com/onorbit/photos-putting-canadarm2-through-its-paces-from-the-space-station-cupola.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vzMFC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vzMFC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Paolo Nespoli in the Cupola of the International Space Station. It has seven windows and provides a pressurized observation and work area for astronauts, according to the European Space Agency. From &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2011/01/Paolo_Nespoli_in_the_Cupola&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TggV6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TggV6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; (From left) Expedition 40/41 crew members Alexander Gerst, Reid Wiseman and Max Suraev peer out of the cupola. From &lt;a href=&quot;https://blogs.nasa.gov/spacestation/2014/11/06/med-studies-as-departing-trio-trains-for-landing/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2016-12-13T10:20:14.763" LastActivityDate="2018-03-07T15:56:00.993" Title="Do ISS astronauts occasionally let the sunshine in for a natural light &quot;fix&quot;?" Tags="|iss|sunlight|spacecraft-window|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19395" PostTypeId="2" ParentId="19333" CreationDate="2016-12-13T10:42:32.597" Score="2" Body="&lt;p&gt;As many others have stated Florida is a great place to launch rockets because if the rocket suffers an issue during launch debris from it will land in the Atlantic ocean rather than crashing down into inhabited land, possibly damaging property or killing civilians. &#xA;Another reason that i don't think has been brought up but bears mentioning is that in 1949 swampland in Florida was considerably cheaper to buy in bulk than the alternative sites they were considering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: Heard this on tour of Kennedy Space Centre.&lt;/p&gt;&#xA;" OwnerUserId="16504" LastActivityDate="2016-12-13T10:42:32.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19396" PostTypeId="1" CreationDate="2016-12-13T12:17:34.093" Score="2" ViewCount="2210" Body="&lt;p&gt;Suppose I am in the center of a hypothetical 4mx4m room and there is no gravity. I am wearing a spacesuit with no means of propulsion (e.g. venting, ejecting, etc..). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would I be able to reach &quot;the door&quot;, i.e. start movement in a certain direction, by body movements of some kind, or would I float in the center of the room forever?&lt;/p&gt;&#xA;" OwnerUserId="17929" LastActivityDate="2022-01-30T07:23:10.857" Title="Can a human body change direction when floating in a space without gravity?" Tags="|microgravity|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19397" PostTypeId="1" CreationDate="2016-12-13T13:15:36.730" Score="8" ViewCount="413" Body="&lt;p&gt;If we want a nuclear power plant in space with a capacity similar to that of a conventional water boiling turbine nuclear power plant on Earth today, what kinds of fundamental design changes would have to be made? The largest reactor in operation delivers about 6-7 GW. How could that be replicated in space, either free floating or on a surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do thermionic emission reactors, like the Russian TOPAZ, scale up to Gigawatts? I suppose that radio thermal generators are out of the question on that scale, if for no other reasons the cost of creating and handling plutonium-238. Is water boiling the big future for nuclear power also in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to emphasize &lt;em&gt;operations&lt;/em&gt; in space, not manufacturing. Assume manufacturing on Earth and launched, at most piecemeal for simple docking assembly. I want to ask about what kind of BIG nuclear reactor actually would work in space, rather than how it would get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To clarify, I am not asking about nuclear thermal propulsion systems, but reactors optimized for the production of electric power.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="18744" LastEditDate="2017-03-02T19:31:45.513" LastActivityDate="2017-03-02T19:48:13.790" Title="What kinds of nuclear reactors are suitable for operations in space? (Beyond thermionic and RTG)" Tags="|power|nuclear|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19398" PostTypeId="1" AcceptedAnswerId="19407" CreationDate="2016-12-13T14:56:42.090" Score="8" ViewCount="6037" Body="&lt;p&gt;The first four STS missions were flown by &lt;em&gt;Columbia&lt;/em&gt; with two pilots and had ejection seats as an option.  These seats were eventually disabled (by STS 5) and removed (by STS-61-C).  They could only be used to about Mach 4, in the first 1/5th of the Shuttle's powered ascent.&lt;/p&gt;&#xA;&lt;p&gt;From researching I found that this was accomplished through ejection panels on the top of the Shuttle's cabin, using the same powered seats at the SR-71.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2Ub9Z.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Ub9Z.jpg&quot; alt=&quot;Diagram of ejection panels for seats&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.collectspace.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.collectspace.com/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.buran-energia.com/bourane-buran/bourane-secu-sieges.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Buran seems to have also had ejection seats installed&lt;/a&gt;, but never flew with them.  Their seats seem to be designed to take the pilots 500 meters from the rocket itself.&lt;/p&gt;&#xA;&lt;p&gt;However, with the gimbled direction of the SSME's firing, and the two extremely powerful solid rockets on the Shuttle stack, would being outside the craft have been even survivable at that close a proximity?  Has any analysis on a best case/worst case scenario for these ejection seats ever been done?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="29286" LastEditDate="2020-08-31T19:54:40.053" LastActivityDate="2020-08-31T19:54:40.053" Title="Were ejection seats on the Space Shuttle a practical safety system?" Tags="|space-shuttle|safety|launch-escape-system|shuttle-columbia|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19399" PostTypeId="2" ParentId="19342" CreationDate="2016-12-13T15:57:08.177" Score="3" Body="&lt;p&gt;I did some statistics with the data from &lt;a href=&quot;http://mercury6.spacelog.org/original/144/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mercury6.spacelog.org/original/144/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first orbit was 1:32:47 with voice transmisson for a total of 1:25:01 and hand over gaps of 0:07:46. All time values in hours:minutes:seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second orbit was 1:34:13, voice transmission for 1:26:32 and gaps for 0:07:41.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third orbit was 1:33:18, communication for 1:25:02 and gaps for 0:08:16.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The longest gap was 0:04:54, a lot of gaps less than 30 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The times of telemetry data look a bit different, a total of 0:23:57 of the first orbit without telemetry, second orbit 0:23:40 and third orbit a total of 0:40:09. The longest gaps without telemetry were 0:14:30 and 0:15:30, both in the third orbit. But voice transmission was possible during this longer gaps without telemetry. There have been short phases with simultaneous reception of telemetry by two ground stations from 10 seconds up to 10 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the low orbit with an inclination of 32.54 and the limited number of ground stations it was not possible to achieve a continous communication. Only during the second orbit all ground stations could communicate.&#xA;A radio link was established even with Mercury 6 very low above or even below the horizon and a distance of up to 1100 nautical miles.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="13126" LastEditDate="2016-12-14T08:40:19.780" LastActivityDate="2016-12-14T08:40:19.780" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19400" PostTypeId="1" CreationDate="2016-12-13T16:05:45.600" Score="5" ViewCount="348" Body="&lt;p&gt;Is there a way to recover any of the energy of reentry of a Mars lander?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Tremendous energy is generated as a craft enters the atmosphere – admittedly over a fairly short time - and only one time.&lt;/li&gt;&#xA;&lt;li&gt;We have seen regenerative braking on various vehicles, starting with diesel-electric locomotives, now electric cars and some F-1 racing cars which are capable of significant energy recover over very short time spans.&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The recover could be mechanical, thermal or electrical but would have to be worth lugging along a system. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;For example, an ice supply could be heated to water, saving the energy to do it later on the surface. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;Water could be converted to superheated steam for some use, either during reentry or later on the surface). It is conceivable that ice freighters (from an asteroid) might be bringing large cargos to the surface. A water or ice system might help cool the reentry shield.&lt;/li&gt;&#xA;&lt;li&gt;An auto rotating propeller might assist in aerodynamic deceleration at a later stage in a reentry, while also storing some energy. Such propellers have been proposed for earth reentry although the thin Martian atmosphere is a problem.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="17815" LastEditorUserId="17815" LastEditDate="2016-12-13T17:32:03.347" LastActivityDate="2016-12-13T20:38:44.467" Title="Is there a way to recover any of the energy of reentry of a Mars lander?" Tags="|mars|recovery|entry-descent-landing|energy|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19401" PostTypeId="2" ParentId="19328" CreationDate="2016-12-13T16:30:18.343" Score="3" Body="&lt;p&gt;There is another way to get more power, not to increase the area by folding but to stack several cells for different wavelengths and improving the conversion efficiency. Dawn uses triple-junction cells, see &lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-junction_solar_cell&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Multi-junction_solar_cell&lt;/a&gt;&#xA;But the best efficiency was achieved with laboratory examples using concentrated sunlight. Concentrating sunlight is possible on earth, but not in a spacecraft. Hopefully multi-junction cells are developed which work well even far away from the sun.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-12-13T16:30:18.343" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19402" PostTypeId="1" AcceptedAnswerId="21105" CreationDate="2016-12-13T18:24:00.443" Score="10" ViewCount="1216" Body="&lt;p&gt;After reading &lt;a href=&quot;https://space.stackexchange.com/a/19399/12102&quot;&gt;this answer to &lt;em&gt;How many earth stations had astronauts on-site during John Glenn's orbit in Friendship-7?&lt;/em&gt;&lt;/a&gt; I decided that for my own use I'd like to plot the orbit of Mercury-Atlas 6, the spacecraft piloted by John Glenn on February 20, 1962. This &lt;a href=&quot;https://space.stackexchange.com/a/19350/12102&quot;&gt;thorough answer there&lt;/a&gt; lists fourteen (14) ground stations plus two additional radar tracking stations for telemetry and/or voice communication, spread out - roughly along the general path that the spacecraft wold follow.&lt;/p&gt;&#xA;&lt;p&gt;Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Atlas_6&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mercury-Atlas 6&lt;/a&gt; lists some parameters for an elliptical orbit in geocentric (inertial) coordinates. From these I should be able to find an ellipse whose ground track &lt;em&gt;comes close&lt;/em&gt; to intersecting the launch and landing coordinates. There's some uncertainty there, but I'll see what happens tomorrow.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Perigee:       149 kilometers (80 nautical miles)&#xA;Apogee:        248 kilometers (134 nautical miles)&#xA;Inclination:   32.5 degrees&#xA;Launch site:   Cape Canaveral LC-14&#xA;Landing site:  21°20′N 68°40′W&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;While the TLE catalog available from &lt;a href=&quot;https://www.space-track.org/#/catalog&quot; rel=&quot;nofollow noreferrer&quot;&gt;space-track.org&lt;/a&gt; is convenient to use and indeed lists &lt;code&gt;SatCat: 1962-003A&lt;/code&gt;, &lt;code&gt;NORAD: 00240&lt;/code&gt;, it doesn't seem to have TLEs available. I requested data from the &lt;a href=&quot;https://celestrak.org/NORAD/archives/request.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak &amp;quot;requestor&amp;quot;&lt;/a&gt; and have now received a second e-mail with the attached &lt;code&gt;.txt&lt;/code&gt; file. No dice.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;=====================================================================&#xA;  00240 1962-003A   MERCURY ATLAS 6                                  &#xA;  Launched: 1962-02-20 (051)           Start Date: 1962-02-19 (050)  &#xA;   Decayed: 1962-02-20 (051)            Stop Date: 1962-02-21 (052)  &#xA;=====================================================================&#xA;No data found for Catalog Number 00240&#xA;&amp;lt;End of file&amp;gt;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Where to I look for historical or reconstructed orbit data for early NASA missions - Mercury-Atlas 6 for example.&lt;/p&gt;&#xA;&lt;p&gt;Of course if there are more missions that would be great! I thought that I'd read years ago that NASA had put a collection of at least approximate, reconstructed trajectories somewhere for the public to access, but I can't find it now. I'm flexible on the form of the data, for example a table of state vectors, TLEs, or even one or more sets of Kepler orbital elements.&lt;/p&gt;&#xA;&lt;p&gt;I suppose when I am done I can generate something like this display board shown below, but now I will also be able to plot approximate altitude, azimuth, and distance to each of the tracking stations as a function of time.&lt;/p&gt;&#xA;&lt;p&gt;I'm not looking for such a plot, the answer I'm looking for will help suggest where I might find reconstructed orbit data.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CfB5z.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CfB5z.jpg&quot; alt=&quot;View of Mercury Control at Cape Canaveral Air Force Station, in Florida (USA) during the Mercury-Atlas 6 mission, in February 1962&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; View of Mercury Control at Cape Canaveral Air Force Station, in Florida (USA) during the Mercury-Atlas 6 mission, in February 1962. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Mercury_Control_Center_during_Mercury-Atlas_6_mission_1962.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZXTLi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZXTLi.jpg&quot; alt=&quot; A ground-track map for Mercury-6 from https://www.hq.nasa.gov/office/pao/History/SP-45/fig8.1.htm&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; A ground-track map for Mercury-6 from &lt;a href=&quot;https://www.hq.nasa.gov/office/pao/History/SP-45/fig8.1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2021-10-15T23:04:14.110" Title="Where to look for historical or reconstructed orbit data for early NASA missions - Mercury-Atlas 6 for example" Tags="|orbital-elements|project-mercury|two-line-elements|historical-data|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="19404" PostTypeId="2" ParentId="19400" CreationDate="2016-12-13T19:21:22.317" Score="3" Body="&lt;p&gt;I'm going to say no, not directly. Just one of the many problems is the energy is merely heat. Relatively speaking heat is easy to get, just make a big concentrating solar panel and you have high grade heat all day. What you want to do is preserve the energy in a valuable form: kinetic or orbital energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is hypothetically possible to recover much of the orbital energy &lt;em&gt;before&lt;/em&gt; reentry and thus reduce the amount of energy dissipated during reentry - although the energy recovered wont be available to the lander itself. This involves transferring the kinetic energy of the lander to another object in orbit. The basic concept is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Skyhook_(structure)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyhook&lt;/a&gt;, while normally conceived as a way of lifting payloads out of Earth's atmosphere, they can also be used in reverse, catching an object which is in a high orbit, swinging it around the anchor and depositing it at lower speeds in the atmosphere. In the process, much of the kinetic energy of the lander is transferred to the skyhook assembly, raising the orbit of the skyhook, that energy can then be later reused to lift another object out of the atmosphere and into orbit, for example a payload destined for Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course there would be immense technical challenges to actually building the skyhook and executing the maneuvers, but it is physically possible.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2016-12-13T19:21:22.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19405" PostTypeId="1" AcceptedAnswerId="19421" CreationDate="2016-12-13T19:47:30.317" Score="54" ViewCount="10974" Body="&lt;p&gt;While researching for &lt;a href=&quot;https://space.stackexchange.com/q/19402/12102&quot;&gt;this question&lt;/a&gt; I ran  across something called &lt;a href=&quot;https://celestrak.org/cgi-bin/searchSOCRATES.pl?IDENT=NAME&amp;amp;NAME_TEXT1=&amp;amp;NAME_TEXT2=&amp;amp;ORDER=MINRANGE&amp;amp;MAX=10&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak Socrates&lt;/a&gt;. I requested the ten closest predicted approaches, and saw distances of closest approach of the order of 100 meters or even less. I recalled &lt;a href=&quot;https://space.stackexchange.com/q/19383/12102&quot;&gt;this question&lt;/a&gt; but this kind of close approach doesn't apply, since I don't think Iridium and OrbComm got together and planned this.&lt;/p&gt;&#xA;&lt;p&gt;I figured there must be some mistake, so I clicked &amp;quot;Analysis&amp;quot; and got the TLEs for the two spacecraft, Iridium 55 and &amp;quot;OrbComm FM18. I plugged them into &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt; and sure enough, at about 17 seconds after &lt;code&gt;2016-12-15 13:14:00 UTC&lt;/code&gt; it shows them passing about 100 meters from each other at 6,674 m/s relative velocity. I had to use 1 millisecond time spacing to resolve the event.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is 75 meters an exceptionally close distance for two satellites to pass at &amp;gt;6,000 m/s? I get the feeling that since this is a &amp;quot;weekly top-10&amp;quot; list, this kind of close approach must be happening frequently. Are these satellite constellations under control to such precision that a ~100 meter miss is routinely acceptable, or did I stumble upon a fluke somehow?&lt;/p&gt;&#xA;&lt;p&gt;I suppose this will happen even more often if SpaceX put's &lt;a href=&quot;https://space.stackexchange.com/q/19055/12102&quot;&gt;their first 1,600 satellites&lt;/a&gt; at a single altitude/inclination in 32 intersecting planes.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1Kvdv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1Kvdv.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;IRIDIUM 55&#xA;1 25272U 98019A   16348.53470940  .00000004  00000-0 -55193-5 0  9992&#xA;2 25272  86.3948   4.6359 0002006  88.0295 272.1131 14.34219879979454&#xA;ORBCOMM FM18&#xA;1 25414U 98046B   16347.49264999 -.00000013  00000-0  46447-4 0  9990&#xA;2 25414  44.9965  56.3156 0006146 236.8164 123.2093 14.32449704958255&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kNsQL.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kNsQL.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/98hVM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/98hVM.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Celestrak Socrates screen shot circa 2016-12-13 18:30 UTC&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Wow!!&lt;/h2&gt;&#xA;&lt;p&gt;I went back to the page (about an hour and a half later, I've added estimated times of page loadings before screen shootings) and the predicted distance of closest approach has changed from 0.075 kilometers to 0.113 kilometers, which is precisely the value I get from Skyfield. The TLEs have not changed, gee, what's going on?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; Figured this out at least. The epoch for the Iridium 55 TLE is &lt;code&gt;16348.53470940&lt;/code&gt; which is about 12:50 UTC. Clicking the Analysis button around 18:00 UTC seems to have retreived the updated TLE, and not the TLE that was used to generate the display. It could very well be that if I had updated the page at that moment, the new minimum distance 0.113 km would have appeared.&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;https://celestrak.org/SOCRATES/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Methodology&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Twice each day, CSSI runs a list of all satellite payloads on orbit against a list of all objects on orbit using the catalog of all unclassified NORAD two-line element sets (TLEs) releasable to the public to look for satellite conjunctions over the next seven days. The reason for limiting the search to conjunctions with payloads is to give satellite operators an opportunity to plan—or obtain planning services—to move their satellites out of harm's way (if their satellite is maneuverable), or to take other appropriate measures. Since there currently is no way of knowing which payloads are still active, all payloads are considered.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NVsrk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NVsrk.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Celestrak Socrates screen shot circa 2016-12-13 20:12 UTC&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2016-12-14T17:24:53.850" Title="Is 75 meters an exceptionally close distance for two satellites to pass at &gt;6,000 m/s?" Tags="|collision-avoidance|skyfield|satellite-constellation|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19406" PostTypeId="2" ParentId="19405" CreationDate="2016-12-13T20:05:39.783" Score="28" Body="&lt;p&gt;Exceptionally rare? No. Uncommon, well, yes. From what I can recall, these happened about once every 10 years per satellite for the constellation of satellites I worked with, at least, prior to doing a collision avoidance maneuver. That of course included debris/ satellite close approaches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, I note that the two you listed are both live satellites, which makes things a bit more interesting. With live satellites, the two have to coordinate with each other to make sure they don't do the same maneuver at the same time, cancelling each other out. These are a bit rarer, but it seems like it was about one per, say, 50 years per satellite that one of these would happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at the same page, there are another pair of satellites, one not active, with a close approach even close, although the predicted approach is further in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I strongly suspect that one or the other of those two satellites will perform a collision avoidance maneuver, and by so doing ensure the distance is at least a couple of km away at that time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-12-13T20:05:39.783" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19407" PostTypeId="2" ParentId="19398" CreationDate="2016-12-13T20:15:04.833" Score="7" Body="&lt;p&gt;The seats were ineffective above 80K feet on ascent due to the expansion of the SRB plumes - if you ejected after that, you would pass through the plumes making a bad day worse.  A call was made at that point &quot;&lt;a href=&quot;https://blogs.nasa.gov/waynehalesblog/tag/ejection-seats/&quot; rel=&quot;noreferrer&quot;&gt;Negative Seats&lt;/a&gt;&quot;. [Note that the reference cites a thermal limit on the faceplate - I remember it was the plumes - who are you going to believe, me or a former Flight Director and Space Shuttle Program Manager?] &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a &quot;speed limit&quot; on ejection during entry above which the crewmember would be going too fast to thermally survive the entry.  Sadly I don't remember what that point was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the ejection seats were usable only for a short time during ascent, and a short time during entry.  Worth it?  At the time it was thought yes, due to the fact that no un-crewed test flights were made.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FWIW, here's a picture from the STS-3 training era in the Shuttle Mission Simulator showing what the cockpit looked like then with the ejection seats with the large &quot;rails&quot; that connected the deck to the overhead.  Jack Lousma in the commander's seat and Gordon Fullerton is in the pilot's seat; I believe the lady is Mrs. Fullerton.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UBuny.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UBuny.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-12-13T20:26:08.010" LastActivityDate="2016-12-13T20:26:08.010" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19408" PostTypeId="2" ParentId="19400" CreationDate="2016-12-13T20:38:44.467" Score="3" Body="&lt;p&gt;While it would be hard to do, in part because of the very short time, and would be of questionable value, here is a concept for &lt;a href=&quot;http://solarsystem.nasa.gov/docs/Ali_In%20Situ%20Magnetohydrodynamic%20Energy%20Generation%20for%20Planetary%20Entry%20Vehicles_Updated_Abstract%208046.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;using MHD to do regenerative braking during Mars entry&lt;/a&gt; from the generated plasma.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-12-13T20:38:44.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19409" PostTypeId="2" ParentId="19007" CreationDate="2016-12-13T21:22:11.860" Score="2" Body="&lt;p&gt;This is explained in the mission &lt;a href=&quot;https://youtu.be/lXDBaQJI7-8?t685&quot; rel=&quot;nofollow noreferrer&quot;&gt;Science Briefing&lt;/a&gt;. Basically, they are measuring the distortion of the GPS signal reflected by the water surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a calm flat surface, the reflection is &lt;em&gt;specular&lt;/em&gt;, I.e undistorted. The more rough the surface is, the more the reflection becomes &lt;em&gt;diffuse&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can observe the same effect if you observe the reflection of e.g. the moon on a calm resp. rough lake surface.&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2016-12-13T21:22:11.860" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19410" PostTypeId="2" ParentId="19396" CreationDate="2016-12-13T21:23:27.267" Score="4" Body="&lt;p&gt;You cannot change the location of your center of mass without some form of propulsion.  You can, however change your attitude &lt;a href=&quot;https://youtu.be/960CsCgI6Jg?t=104&quot; rel=&quot;nofollow noreferrer&quot;&gt;by wiggling around like a falling cat.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2016-12-13T21:23:27.267" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19411" PostTypeId="2" ParentId="19396" CreationDate="2016-12-13T21:43:44.763" Score="5" Body="&lt;p&gt;You haven't specified if you're in vacuum or not (though the spacesuit is suggestive). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're in vacuum, you're stuck, unless you started with some motion relative to the room, or something accelerates the room itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're in a fluid medium, even air, you can &quot;swim&quot;, pushing the medium around you preferentially in one direction to accelerate yourself in the opposite direction. This would be slow going in air -- each stroke produces very little momentum -- but you could escape.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-12-13T21:43:44.763" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19413" PostTypeId="2" ParentId="19382" CreationDate="2016-12-13T22:41:42.303" Score="3" Body="&lt;p&gt;thanks @uhoh. I used to keep Jersey dairy cows (brown rather than black and white, but the albedo is probably similar) so I just love the spherical-cow back-of-the-envelope approach and, with my British roots certainly relate to the Monte Python link. &#xA;I found this information about geostationary satellites on &lt;a href=&quot;http://www.satobs.org/geosats.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.satobs.org/geosats.html&lt;/a&gt;: &#xA;&quot;Typically the satellite will be in the mag. +11 to +14 range&quot;. &#xA;The correction of square-law brightness with distance is &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$2.5log_{10}((R_{GEO}/r_{nn})^2) \approx 14$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where the geosynchronous radius and nearest-neighbor satellite separation $R_{GEO}$ and $r_{nn}$ are 42164 and 74 kilometers respectively. This would bring the brightness range between 0 and -3 magnitude, in very good agreement with CowSat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnitude would fall off as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$2.5log_{10}\left(\left(\frac{2 \ R_{GEO} \ sin(N \ \theta / 2)} {2 \ R_{GEO} \ sin( \theta / 2)}\right)^2\right) \approx 5 log_{10}\left(\frac{sin(N \ \theta / 2)}{sin(\theta / 2)}\right)  \approx 5 log_{10}(N)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for the nearer satellites. So the brightness (magnitude) drops off quite slowly, e.g. 0, 1.5, 2.3, 3, 3.5, 3.9, 4.2, 4.5, 4.8, ... &#xA;so, in the densely populated part of the orbit, quite a few of these will be visible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All these satellites will appear to the observer as being in a straight line, but, most interestingly, they will appear all equispaced, which I wasn't expecting. This arises because the chord of a circle subtends the same angle everwhere on the circumference. Or, conversely, an observer fixed on the &#xA;circumference will see a chord of a given length (74 km) subtending the same visual angle (0.05 degrees) wherever the chord is placed on the circle.&#xA;So someone sitting on one of these satellites and looking around would instantly recognise their neighbors not only because they didn't rotate &#xA;every 24 hours like the rest of the stars, but also because of strange linear equispaced pattern with monotonically diminishing brightness.      &lt;/p&gt;&#xA;" OwnerUserId="14419" LastEditorUserId="12102" LastEditDate="2016-12-14T05:01:48.470" LastActivityDate="2016-12-14T05:01:48.470" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19418" PostTypeId="1" CreationDate="2016-12-14T13:58:16.137" Score="4" ViewCount="1043" Body="&lt;p&gt;If a spacecraft was designed to propel itself using the annihilation of matter and antimatter, what potential dangers would this create and how could they be circumvented or at least prepared for?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could there be more risks than modern rockets have now?&lt;/p&gt;&#xA;" OwnerUserId="17552" LastEditorUserId="415" LastEditDate="2016-12-14T15:57:24.170" LastActivityDate="2024-03-27T19:34:57.507" Title="What would be the dangers of matter/antimatter annihilation propulsion, and what can be done about them?" Tags="|propulsion|antimatter|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19419" PostTypeId="1" AcceptedAnswerId="19420" CreationDate="2016-12-14T14:15:21.447" Score="7" ViewCount="1530" Body="&lt;p&gt;We are going to visit Florida in early-mid February and were wondering if we could watch a launch. The schedule, however, does not seem to include February yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the purposes of planning our trip - at what time would we be able to see a launch date being set if there was going to be one? The NASA homepage does not seem to have much info on this topic.&lt;/p&gt;&#xA;" OwnerUserId="17939" LastActivityDate="2016-12-15T04:24:12.850" Title="How far in advance are launch dates set?" Tags="|launch|nasa|kennedy-space-center|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19420" PostTypeId="2" ParentId="19419" CreationDate="2016-12-14T14:36:51.690" Score="11" Body="&lt;p&gt;One of the best online launch manifests is the one complied by &lt;a href=&quot;http://spaceflightnow.com/launch-schedule/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Now.&lt;/a&gt; It has planned launch dates all the way out to the 3rd quarter of 2017.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the only launches in February with definite dates are out of Kourou and Baikonur. You may get lucky with a SpaceX launch though. The SpaceX manifest is still in flux after the the AMOS-6 failure, but they will hopefully be launching &lt;em&gt;something&lt;/em&gt; in February. You may even get to see the Falcon Heavy demo flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, launch dates are sometimes set many years in advance. For interplanetary missions the window for launch may be known well before the design of the spacecraft even begins.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="6312" LastEditDate="2016-12-14T15:45:04.547" LastActivityDate="2016-12-14T15:45:04.547" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19421" PostTypeId="2" ParentId="19405" CreationDate="2016-12-14T14:39:35.070" Score="33" Body="&lt;p&gt;No, 75 meters is not &lt;em&gt;exceptionally&lt;/em&gt; close — several satellites show predicted passages that close or closer every single week of the year. But it is close enough to potentially appear in the top 10 list of satellite conjunctions for the week:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://celestrak.org/cgi-bin/searchSOCRATES.pl?IDENT=NAME&amp;amp;NAME_TEXT1=&amp;amp;NAME_TEXT2=&amp;amp;ORDER=MINRANGE&amp;amp;MAX=10&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://celestrak.org/cgi-bin/searchSOCRATES.pl?IDENT=NAME&amp;amp;NAME_TEXT1=&amp;amp;NAME_TEXT2=&amp;amp;ORDER=MINRANGE&amp;amp;MAX=10&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Satellites do routinely pass each other safely with such small margins of error, but the passage can occasionally be dangerous. Since TLE files are rated as being accurate to only a few kilometers, the last I checked, a predicted 100 meter passage could wind up being much more — or much less.&lt;/p&gt;&#xA;&lt;p&gt;In an incident in 2009, a pair of satellites whose TLEs predicted that they would pass each other by 584 meters failed to do so successfully. A pair of debris fields appeared and were tracked in the positions that the satellites should be in:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Qg2oJ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qg2oJ.png&quot; alt=&quot;Debris clouds of Iridium 33 and Cosmos 2251 ten minutes after the moment they were predicted to pass 584 meters from each other&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;You can read a full report about the prediction, and about satellite close passages in general, at CelesTrak:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://celestrak.org/events/collision.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://celestrak.org/events/collision.asp&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;That particular satellite pair had not even made that day’s Top Ten list of close approaches, and — perhaps as a result? — no evasive maneuver was planned to increase the margin of error. In fact, Iridium 33 made safe passage on very many occasions when the margin of error was much less! Quoting the report linked above:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Iridium 33 saw between 10 to 15 times each week when something was predicted to come within 5 km of it—out of the 1,007 to 1,095 such predicted conjunctions for the entire Iridium constellation. Determining that the predicted conjunction for Iridium 33 and Cosmos 2251 was more significant than the many dozens of other Iridium conjunctions for that week is simply not possible using the TLE data.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So, to answer your question: no, the conjunction of 100 meters that you have found is not at all unusual. Again from the report:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;close approaches with satellites in Earth orbit are not uncommon events. Over the period examined here, between 11,169 and 11,493 conjunctions were reported each week—over 1,600 times a day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The system they described to predict close approaches is still operating, and you can visit its web page: &lt;a href=&quot;https://celestrak.org/SOCRATES/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOCRATES, the Satellite Orbital Conjunction Reports Assessing Threatening Encounters in Space&lt;/a&gt;. You will find that the encounter of 100 meters you calculated is quite routine. The top encounter at the moment when I click on their “Top 10 Conjunctions by Minimum Range” report shows two satellites predicted to pass only 15 meters from each other!&lt;/p&gt;&#xA;" OwnerUserId="8590" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2016-12-14T14:49:23.370" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19423" PostTypeId="1" AcceptedAnswerId="19428" CreationDate="2016-12-14T15:56:31.127" Score="6" ViewCount="2546" Body="&lt;p&gt;Friends of mine who have taken cruises consistently mention the very careful measures that cruise ships take to prevent illness among passengers. Not infrequently, entire voyages are ruined because of widespread sickness among the passengers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It makes sense that cruise ships would be at risk for this, since there are lots of people in an enclosed space. But then I wondered, how is this not even &lt;em&gt;more&lt;/em&gt; of a problem aboard the ISS? After all, it's a hermetically sealed place, it's difficult to bathe effectively, and based on what I've read about the toilet situation, it would be hard to keep fastidiously clean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/2431/how-is-the-iss-equipped-to-deal-with-the-outbreak-of-a-contagion/2432#2432&quot;&gt;This question&lt;/a&gt; addresses communicable diseases. Basic diarrhea seems like a more run-of-the-mill type of illness; I don't know whether it would be possible to detect whether someone was going to have an incident in the next few weeks. And at that point, why wouldn't it just spread all around the station and remain an ongoing problem?&lt;/p&gt;&#xA;" OwnerUserId="17941" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-12-15T10:23:51.970" Title="Why isn't diarrhea a problem on the ISS like it is on cruise ships?" Tags="|iss|health|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19424" PostTypeId="2" ParentId="19386" CreationDate="2016-12-14T16:01:02.780" Score="4" Body="&lt;p&gt;Without the transcript of the meetings where the design of the signal conditioning electronics was agreed I don't think we can answer this with certainty. However, I can speculate a bit more:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The language used &quot;force an overload... cause it to shut down.&quot; is very &lt;strong&gt;terminal&lt;/strong&gt;. Intelsat seem to have been primarily concerned with ensuring Galaxy-15 didn't start to interfere with any other Geo sats. I would guess that the effect of the overload would have been permanent. That they tried it suggests that they thought it had a reasonable chance of success. They probably got the design schematics out and asked their engineers to work out a way to break it any way they could.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One other interesting (and highly unlikely) reason for their urgency might have been the presences of two L band Geostationary Communications and Control Segment transponders on Galaxy-15. Officially the GCCS is managed by the FAA to improve the accuracy of GPS navigation, but it wouldn't be very surprising if other, classified, payloads where included alongside the primary. The cause of the loss of control hasn't officially been established, and the possibility of a foreign goverment ~cough~ &lt;em&gt;China&lt;/em&gt; ~cough~ taking control couldn't immediately  be ruled out. If the USG thought that their toys were about to walk off on their own they quite possibly ordered Intelsat to nuke their bird with every watt they could aim skyward. Unlikely, but an interesting possibility. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Returning to the AGC part of your question: Only &lt;s&gt;an idiot&lt;/s&gt; someone with a good reason would design an amplifier without any sort of overload protection, especially when the amplifier may be exposed to broad-spectrum solar flares. The fact that the attempt to kill Galaxy-15 failed is probable testament to this. I'd expect modern satellites to use a digitally controlled amplifier system, so they would use logic to control the gain rather than an AGC. Combined with a high input impedance this would make it very hard to break the amplifier. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could you kill a satellite with a large enough signal? Absolutely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the AGC failed? Probably not. EMF excitation somewhere else in the spacecraft would probably be fatal before the input protection broke down in a way that would permanently damage something.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2016-12-14T16:01:02.780" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19425" PostTypeId="2" ParentId="19405" CreationDate="2016-12-14T16:46:43.133" Score="17" Body="&lt;p&gt;75 meters isn't all that rare - for the full public catalog there are 75 meter predictions about 2-3 times per day.  What you have to consider is the uncertainty in that number.  For TLE data, that's 75 meters +- several kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 75 meter miss distance is from the nominal position of each object at closest approach.  But TLE data has a significant amount of error.  For what we're concerned with here - high relative velocity conjunctions - that can be characterized by a position covariance. Think of it as a football shaped ellipsoid surrounding the nominal position which characterizes the position error of the TLE.  This is a 3 dimensional Gaussian distribution of the position error.  For TLE data this is typically maybe a few hundred meters at the elset epoch (depending on orbit regime, priority, size, drag regime, etc.), and grows with time.  Several km of error isn't very unusual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So again, when you say that the objects will pass within 75 meters of each other, what you really mean is that the centroid of the (potentially very large) position covariance ellipses pass within 75 meters of each other.  The actual miss distance could be less, or likely much larger.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, miss distance alone is a notoriously bad indicator of actual collision risk - there are techniques for calculating metrics such as probability of collision, Mahalanobis distance, and others based on the position vectors, uncertainty estimates, conjunction geometry, object sizes, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are a few references on probability of collision for space objects.  Not sure what I can give you on the error size estimates - I'll take a look later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Foster, J. L., and Estes, H. S., &quot;A Parametric Analysis of Orbital Debris Collision Probability and Maneuver Rate for Space&#xA;Vehicles,&quot; NASA/JSC-25898, August 1992.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chan, K., &quot;Collision Probability Analyses for Earth-Orbiting Satellites,&quot; Proceedings of the 7th International Space&#xA;Conference of Pacific Basin Societies, Nagasaki, Japan, July 1997.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Patera, R. P. &quot;General Method for Calculating Satellite Collision Probability,&quot; Journal of Guidance, Control, and Dynamics,&#xA;Vol. 24, No. 4, July-August 2001, pp. 716-722.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="4709" LastEditDate="2016-12-14T17:24:53.850" LastActivityDate="2016-12-14T17:24:53.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19426" PostTypeId="2" ParentId="19402" CreationDate="2016-12-14T16:49:15.763" Score="1" Body="&lt;p&gt;This isn't super helpful, but too long for a comment: you might look the archived PDS SPK files at: &lt;a href=&quot;http://naif.jpl.nasa.gov/naif/data_operational.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://naif.jpl.nasa.gov/naif/data_operational.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More specific links:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://naif.jpl.nasa.gov/naif/data_lunar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://naif.jpl.nasa.gov/naif/data_lunar.html&lt;/a&gt; includes a little bit of Apollo 15 from 1971.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://naif.jpl.nasa.gov/pub/naif/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://naif.jpl.nasa.gov/pub/naif/&lt;/a&gt; is the whole list of missions. Mercury/Atlas isn't in the list, but some other older missions are.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://naif.jpl.nasa.gov/naif/data_pds_archived.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://naif.jpl.nasa.gov/naif/data_pds_archived.html&lt;/a&gt; is a general reference to older data.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Poke around the site a bit more: you probably won't find Mercury Atlas, but might other interesting missions.&lt;/p&gt;&#xA;" OwnerDisplayName="user7073" LastActivityDate="2016-12-14T16:49:15.763" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19427" PostTypeId="2" ParentId="19418" CreationDate="2016-12-14T16:50:27.190" Score="0" Body="&lt;p&gt;The major reason why modern rockets have not implemented a matter-antimatter propulsion system is because of the dangers that antimatter and its annihilation with matter pose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the antimatter problems are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Storage:&#xA;As we know the storage of antimatter is very difficult and cumbersome. Its storage is energy consuming which will make the rocket more heavier. This will make it impractical to have a rocket with antimatter. Even if a speck of antimatter leaks it can cause severe damage to the rocket and its crew (if any).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Matter-antimatter annihilation:&#xA;The annihilation of 1 kg of both matter and antimatter will release energy in the range of 43 megatons of TNT. This is a significant amount of energy and it is also required to be directed in the correct direction for proper propulsion to be achieved.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;To address the above problems we must devise a renewable energy source and an efficient way to store antimatter and learn how to correctly direct the resulting energy burst.&lt;/p&gt;&#xA;" OwnerUserId="13522" LastEditorUserId="13522" LastEditDate="2016-12-15T07:50:18.600" LastActivityDate="2016-12-15T07:50:18.600" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19428" PostTypeId="2" ParentId="19423" CreationDate="2016-12-14T17:51:52.297" Score="10" Body="&lt;p&gt;They undergo a medical examination 10 days before launch and are then quarantined. Then they are examined again 2 days before launch and again shortly before suiting up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/6628-routine-quarantine-helps-astronauts-avoid-illness-launch.html&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Diarrhea in particular is usually caused by a virus or bacterial infection, allergies, and eating foods that upset the digestive system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.webmd.com/digestive-disorders/digestive-diseases-diarrhea#1&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the astronauts are thoroughly examined, the risk is substantially lowered.&#xA;In addition astronauts' diet is strictly watched by NASA and the water and air are recycled in a fashion that reduces chances of contamination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for cruise ships, it appears that recent incidents have been linked to poor sanitation according to news reports.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="12318" LastEditDate="2016-12-14T18:02:31.173" LastActivityDate="2016-12-14T18:02:31.173" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19429" PostTypeId="2" ParentId="19419" CreationDate="2016-12-14T18:51:28.053" Score="1" Body="&lt;p&gt;Launches are scheduled all the time, with varying firmness in those dates. It's really hard to known anything further than a week in advance if a launch attempt will happen, and stuff happens all the time. If you are lucky, Falcon 9 will be back and fully operational, they will probably do a launch every 2 weeks. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The surest launch times are for interplanetary missions, because they can't wait, planetary alignment waits for no man! The most common launches are for Earth orbiting satellites, and those rarely have to be launched on a specific date, but rather as soon as they can launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;February seems to be a particularly poor month for launches. The only launches that are possible are Falcon launches, and those are a big maybe right now. If you could go in March you'd be much more likely to see a launch, but...&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-12-14T18:51:28.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19430" PostTypeId="2" ParentId="19419" CreationDate="2016-12-15T04:24:12.850" Score="2" Body="&lt;p&gt;There are commercial and non commercial launches are happening at any day of month from different parts of world. only few countries are capable of launching independently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So commercial launches are somewhat subjected to the situation if agency has task to launch satellite for their client.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For interplanetary and government related mission launch time and ETA are decided in their definition phase for example ISRO's MOM mission used the hohmann transfer orbit this condition only arrive in approx 2 years interval.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is very good space calender compiled by &lt;a href=&quot;http://www.spaceflightinsider.com/launch-calendar/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space flight insider&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.isro.gov.in/pslv-c25-mars-orbiter-mission/mars-orbiter-mission-profile&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14616" LastActivityDate="2016-12-15T04:24:12.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19431" PostTypeId="2" ParentId="19423" CreationDate="2016-12-15T10:23:51.970" Score="4" Body="&lt;p&gt;&lt;strong&gt;Cruise&lt;/strong&gt;:&lt;br&gt;&#xA;Thousands of passengers from all over the world interacting with each other for the first time and with thousands more at every port they visit. Millions of infection vectors. &#xA;Add to this the difficulty in sourcing preparing thousands of meals without a single slipup in hygiene. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Spaceflight&lt;/strong&gt;:&lt;br&gt;&#xA;Less than 10 people who were carefully screened for medical problems before flight, and who have been living in the same area and interacting with each other for years. Their immune systems are already 'used to' the other crewmembers. Very few visitors that can bring in new pathogens. Meals are prepared on a cost-no-object basis.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than being in a confined space, the two have little in common. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-12-15T10:23:51.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19432" PostTypeId="1" AcceptedAnswerId="19435" CreationDate="2016-12-15T12:10:04.123" Score="11" ViewCount="1493" Body="&lt;p&gt;I was looking at the Thaicom 8 falcon 9 launch and landing, trying to get a bit more info about the trajectory the first stage takes. The camera is however damaged and we miss most of the atmospheric footage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems however that the camera was damaged because a grid fin on the rocket was lit on fire by the monopropellant. You can see the event in &lt;a href=&quot;https://youtu.be/_uztxi4fE94?t=7m18s&quot;&gt;this video&lt;/a&gt; (It'll start 07:18 which is T+00:06:53).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know which RCS fuel they use but I'd imagine it coming out pretty hot and being able to light some materials on fire. It seems like the rocket is a bit sideways to the air flow, which brings the smut from the fire straight to the camera lens. &#xA;Does anybody know what the grid fins are made of? The paint on them might only be the flammable part too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, as a side note the landing seems really badly filmed, I'd imagine the would have had more cameras on the rocket and the barge, not only one which doesn't catch the landing and then freezes a few seconds after completely (they just show the last image I think there. Of course the rocket might have damaged their antennas but seems quite easy to shield them from a nominal landing)&lt;/p&gt;&#xA;" OwnerUserId="17842" LastEditorUserId="12102" LastEditDate="2016-12-15T15:23:00.770" LastActivityDate="2017-02-03T15:59:40.873" Title="Is SpaceX grid fin material flammable?" Tags="|spacex|falcon-9|grid-fins|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19433" PostTypeId="1" CreationDate="2016-12-15T13:54:24.347" Score="3" ViewCount="118" Body="&lt;p&gt;I've just watched the live CYGNSS launch, and I'm wondering why there was so much unpowered coasting between stage 2 burnout and stage 3 ignition? The vehicle was not yet in orbit, so presumably it was losing velocity from stage 2?&lt;/p&gt;&#xA;" OwnerUserId="17952" LastEditorUserId="12102" LastEditDate="2021-08-07T00:06:10.617" LastActivityDate="2021-08-07T00:06:10.617" Title="CYGNSS launch - a lot of unpowered coasting" Tags="|cygnss|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19434" PostTypeId="1" CreationDate="2016-12-15T14:28:00.030" Score="4" ViewCount="715" Body="&lt;p&gt;I'm trying to help a student solve the following problem:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A 15 meter in diameter asteroid with a mass of approximately 10,000 metric tons going 20 km/s. The entry and time for disintegration was 28 seconds. Entry angle to horizontal is 18 degrees and the terminal art of the fireball was at about 25 km. What is the straight line distance the meteor traveled?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How is the angle of entry factored into the calculation of the straight line distance?&lt;/p&gt;&#xA;" OwnerUserId="17954" LastEditorUserId="58" LastEditDate="2016-12-15T16:37:44.717" LastActivityDate="2016-12-15T16:37:44.717" Title="How does the entry angle of a meteor affect the straight line distance traveled?" Tags="|reentry|asteroid|" AnswerCount="0" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19435" PostTypeId="2" ParentId="19432" CreationDate="2016-12-15T14:50:37.937" Score="11" Body="&lt;p&gt;As far as I know, the RCS uses nitrogen cold gas thrusters. The grid fins are covered in an ablative material, they are heated by atmospheric friction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing is badly filmed because this is a recording of the live webcast. The transmissions from the barge &lt;a href=&quot;https://space.stackexchange.com/questions/17824/why-does-the-video-feed-always-fail-during-falcon-9-landings&quot;&gt;get disrupted by the rocket landing&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-12-15T14:50:37.937" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19436" PostTypeId="1" AcceptedAnswerId="19440" CreationDate="2016-12-15T16:21:14.090" Score="26" ViewCount="2410" Body="&lt;p&gt;I just watched the &lt;a href=&quot;https://youtu.be/9yRWhu0UGYw&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA JPL video&lt;/a&gt; of the 2014 test of the &lt;a href=&quot;https://www.nasa.gov/?search=ldsd&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low-Density Supersonic Decelerator (LDSD)&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Tangential thrusters are used to spin up the craft for stability before the main rocket ignites. The first screenshot shows a pair; the second one fires a few seconds after the first.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question(s):&lt;/strong&gt; The spin-up looks so nice. Is this pair balanced by at least one more on the opposite side? After engine cut-off, a pair of tangential thrusters facing the opposite direction fires sequentially and it looks like the craft is almost perfectly de-spun with almost no residual rotation. Are these precision calibrated or is there some kind of control to stop them at the right instant?&lt;/p&gt;&#xA;&lt;p&gt;I have two small side-questions. I can split them off if someone would like to answer them separately.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;In the &amp;quot;previously unreleased &lt;em&gt;high definition, high resolution, high speed video&lt;/em&gt;&amp;quot; (2nd screen shot, starting at about 01:10) what is holding the camera?&lt;/li&gt;&#xA;&lt;li&gt;Each step seems to be accompanied by sound. Is there enough air at that altitude to carry sound to a microphone, or is it &lt;a href=&quot;https://en.wikipedia.org/wiki/Foley_(filmmaking)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Foley&lt;/a&gt;?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sdNyK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sdNyK.jpg&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oMu4g.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oMu4g.png&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;OK here is the actual video:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/9yRWhu0UGYw?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2024-03-22T16:38:49.927" LastActivityDate="2024-03-22T16:38:49.927" Title="How did the tangential thrusters for the 2014 LDSD test spin-up then spin-down so nicely?" Tags="|engines|ldsd|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19437" PostTypeId="2" ParentId="19436" CreationDate="2016-12-15T16:56:48.077" Score="10" Body="&lt;p&gt;I would guess that the total impulse of the tangential rockets is calibrated closely enough that no active control is needed. They aren't imparting a huge amount of spin, and a small residual wouldn't be a problem. (They look like solids, and while it is possible to design solids for early cutoff, it's unusual.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Foley is extremely unlikely on a video like this; the primary goal isn't entertainment and no one wants to give crackpots ammunition for claims of fakery. I'd guess that some of the sound is conducted to the microphone through the body of the vehicle, but also a substantial amount via air. If air pressure is ~1% of sea level, if I've done my math right, you should expect about a 40 dB reduction in sound volume, which is substantial but not extreme - like turning a volume knob about halfway down.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-12-15T17:57:18.407" LastActivityDate="2016-12-15T17:57:18.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19438" PostTypeId="2" ParentId="16654" CreationDate="2016-12-15T17:38:38.550" Score="1" Body="&lt;p&gt;There are also a number of examples from fiction. To give a few interesting examples:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;em&gt;From the Earth to the Moon&lt;/em&gt; by Jules Verne (1865) has a gun-launched spacecraft.&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;The War of the Worlds&lt;/em&gt; by H.G. Wells (1898) also has a gun-launched spacecraft, this time taking Martians to Earth.&lt;/li&gt;&#xA;&lt;li&gt;&lt;em&gt;A Trip to Venus&lt;/em&gt; by John Munro (1897) has a spacecraft the workings of which I don't think are fully explained (I only read part of the book). At one point the engine overheats and the protagonists use rifles as a reaction drive, comparing it to using a rocket.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="16733" LastActivityDate="2016-12-15T17:38:38.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19440" PostTypeId="2" ParentId="19436" CreationDate="2016-12-15T19:07:35.593" Score="47" Body="&lt;p&gt;There was no active control for spin/de-spin. We did a very good job (if we do say so ourselves) controlling the mass properties of the vehicle with spin testing and balance masses. Yes, there were balanced pairs of spin motors on each side. They were all made in a single lot for consistency in performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the screen shot you can see the boom holding the camera in the bottom of the picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of the sound you hear is real, picked up by the GoPro microphone. You might notice the sound go down in intensity through the main motor burn. That is due to the vehicle increasing in altitude, with the lower densities reducing the sound transmission.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2016-12-15T19:07:35.593" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19441" PostTypeId="2" ParentId="18858" CreationDate="2016-12-15T21:20:25.613" Score="2" Body="&lt;p&gt;Appreciate all the discussion on this. To try to summarize: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Today's electronics are designed to work in a vacuum. It's still possible there are some situations where it's desirable to seal the electronics to reduce outgassing that could affect some sensitive sensor. I imagine space telescopes might want to seal their electronics - though still perhaps with a small breather and labyrinth and filter.     &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;On bearings, it seems that there are oils and greases designed for use in open vacuum operation over a reasonable range of temperatures. Generally, sealing a unit such as a reaction wheel seems like a good idea to limit lubricant evaporation. Why it's necessary to introduce any gas into the unit isn't clear. There are some comments at: &lt;a href=&quot;https://www.grc.nasa.gov/WWW/spacemech/vol1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.grc.nasa.gov/WWW/spacemech/vol1.html&lt;/a&gt; saying, for example, &quot;Nye 179 was considered the best choice for this application.  Its performance in vacuum was far better than that of SRG-40, and its performance in helium was the best of all oils tested.  [Kalegoras]&#xA;The use of oxygen as a component of the fill gas of reaction wheels was determined to be unnecessary and generally harmful to the life of the bearing lubricant.  [Kalegoras]&quot; &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;One thing I did discover definitely is that tape recorders are always sealed. There has been a lot of work done on this. Tape requires controlled humidity ideally in the range 30-50%. Below this range, electrostatic discharge and head-wear occur. Above this range, hydrolysis of the plastic binder causes sticky deposits and 'brown-stain' to build up on the head. The Galileo tape recorder is sealed in a nitrogen/helium atmosphere with the appropriate humidity. There is an excellent detailed article: &#xA;M. Johnson, G. Levanas, &quot;The Galileo Tape Recorder Rewind Operation Anomaly&quot;, NASA Conference Publication 3350, pp. 231-248, 31st Aerospace Mechanisms Symposium, Huntsville, Alabama, May 14-16, 1997&#xA;&lt;a href=&quot;http://hdl.handle.net/2060/19970021613&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://hdl.handle.net/2060/19970021613&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="14419" LastActivityDate="2016-12-15T21:20:25.613" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19442" PostTypeId="1" CreationDate="2016-12-15T21:27:18.113" Score="7" ViewCount="184" Body="&lt;p&gt;I feel like we have reached a time where technology is allowing for a great deal of automation, as well as remotely-controlled machines.  Testing and operation of automated motorized vehicles has been underway for &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_car#History&quot; rel=&quot;noreferrer&quot;&gt;several decades&lt;/a&gt;.  Obviously there is some remote-control of spacecraft today as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Are there any studies or initiatives related to a manned spacecraft (as in, passengers) that require no pilot?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note: I'm fine if there's a pilot 'just-in-case,' but that it is largely hands-off.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2016-12-16T02:07:25.290" Title="Are there any initiatives for automated (not remotely-controlled), manned space missions?" Tags="|technology|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19443" PostTypeId="2" ParentId="19442" CreationDate="2016-12-15T22:28:04.153" Score="7" Body="&lt;p&gt;Yes. In fact, many crewed vehicles that have flown require little to no crew intervention, especially during the ascent and during adjustment burns - these maneuvers require such precise timing that they are better left to a computer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even the Space Shuttle, which as a space plane had an uncommonly complex re-entry program, required very little in the way of crew input. From &lt;a href=&quot;http://www.nytimes.com/1981/04/10/us/computers-to-have-the-last-word-on-shuttle-flight.html&quot; rel=&quot;noreferrer&quot;&gt;this New York Times article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The computers will be capable of autonomously handling virtually every&#xA;  step of the mission. The only jobs the astronauts must do are putting&#xA;  down the landing gear and braking on the runway.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Russia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Buran_%28spacecraft%29&quot; rel=&quot;noreferrer&quot;&gt;Buran orbiter&lt;/a&gt; &lt;em&gt;only&lt;/em&gt; flew autonomously on its single test flight, though it was designed to carry a crew much like the American shuttle.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I remember hearing a rumor that even in the earliest days of the Mercury program, the engineers initially wanted to strap in the astronauts and send them up as passengers with no control over the craft. They didn't even want to give them windows. As a group mostly consisting of test pilots, the Mercury 7 overruled this pretty quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember hearing a similar rumor that Shuttle wasn't originally supposed to have flight controls (ditching them would save weight and complexity) but again, the astronauts balked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wasn't able to substantiate either rumor (if anyone has a source, please speak up), but it helps to show how ingrained the idea of robotic flights is in the industry.&lt;/p&gt;&#xA;" OwnerUserId="5851" LastActivityDate="2016-12-15T22:28:04.153" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19444" PostTypeId="2" ParentId="19442" CreationDate="2016-12-16T02:07:25.290" Score="3" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NhaYs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NhaYs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The (cancelled) US lifeboat for the ISS was planned to be completely automatic.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When operational, the CRV will be an emergency vehicle to return up to&#xA;  seven International Space Station (ISS) crewmembers to Earth. It will&#xA;  be carried to the space station in the cargo bay of a space shuttle&#xA;  and attached to a docking port. If an emergency arose that forced the&#xA;  ISS crew to leave the space station, the CRV would be undocked and -&#xA;  after a deorbit engine burn - the vehicle would return to Earth much&#xA;  like a space shuttle. The vehicle's life support system will have a&#xA;  duration of about seven hours. A steerable parafoil parachute would be&#xA;  deployed at an altitude of about 40,000 feet to carry it through the&#xA;  final descent and the landing. &lt;em&gt;The CRV is being designed to fly&#xA;  automatically from orbit to landing using onboard navigation and&#xA;  flight control systems. Backup systems will allow the crew to pick a&#xA;  landing site and steer the parafoil to a landing, if necessary.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note the lack of any windows!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;http://nasa.gov/centers/armstrong/news/FactSheets/FS-038-DFRC.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; / Italics mine&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2016-12-16T02:07:25.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19447" PostTypeId="2" ParentId="19360" CreationDate="2016-12-16T09:46:48.707" Score="3" Body="&lt;p&gt;What happens at a reentry? An object with orbital speed enters the atmosphere and the compressed air just before this object gets very hot. If the object is not protected by a well designed and complete heat shield, it is destroyed by the heat very fast. The object will loose its speed very rapidly. But this happens not only when leaving an orbit. If you try to reach orbital speed within the atmosphere, you will need a very good heat shield and a lot of power to overcome the friction within the atmosphere and to reach orbital speed. You may save some oxygen by air breathing, but you will need a lot more fuel to overcome the friction. But also the fuel must be protected by a heat shield. In the end you will need a lot more weight for the extra heat shield and fuel than the total weight of a conventional rocket solution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have now thousands of objects in an orbit by using a two stage rocket, leaving the atmosphere as fast as possible and accelerating to final orbital speed outside the atmosphere. But nothing has reached an orbit yet by using a single stage airbreathing vehicle.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-12-16T09:46:48.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19448" PostTypeId="1" CreationDate="2016-12-16T10:20:09.933" Score="15" ViewCount="8553" Body="&lt;p&gt;SLS is to a large degree a continuation of the canceled Ares V, basically the STS without the orbiter. But what changes in design were made from Ares V to SLS? What were the reasons for those changes? Were there for example problems discovered with Ares V that motivated some of the design changes? Or changes in the basic specifications for the launcher, for example its compatibilities with launch pads and industrial capabilities? I understand that the change happened for political reasons, but I suppose that it gave engineers an opportunity to improve on the previous design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see on Wikipedia that Ares V was to have had 5 Space Shuttle Main Engines compared to SLS' 4, or 5-6 RS-68 engines which is what Delta IV uses. Is the fewer number of engines compensated for by extending the solid boosters for SLS? Is there any particular reason for the change in number of engines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it correct to say that both Ares V and SLS were designed to be a little more powerful than Saturn V, in order to send astronauts directly from Earth's surface to the Lunar surface without having to leave half the spacecraft in Lunar orbit?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2018-12-09T12:58:38.677" Title="What are the differences between SLS and Ares V?" Tags="|rockets|design|sls|ares-v|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19449" PostTypeId="2" ParentId="19296" CreationDate="2016-12-16T10:28:49.610" Score="3" Body="&lt;p&gt;Energy conservation is valid for orbits too. You need energy to get from a lower orbit to a higher one.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But &quot;bouncing&quot; of the atmosphere does not add energy, some energy is lost to air friction. Finally you will leave the orbit anyway when entering the upper atmosphere even for short time due to an elliptical orbit. If not at the first time, then at the next times at the lowest point of the elliptical orbit. Each short pass of the atmosphere will take some of the orbit energy again. Removing some orbit energy results in a lower orbit. If the orbit is too low finally, reentry is unavoidable. But a delayed reentry may be dangerous for the crew, the reserves for electrical energy and oxygen for the capsule are very limited.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="15906" LastEditDate="2018-02-22T15:24:07.783" LastActivityDate="2018-02-22T15:24:07.783" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19450" PostTypeId="1" AcceptedAnswerId="19452" CreationDate="2016-12-16T10:51:03.670" Score="31" ViewCount="9811" Body="&lt;p&gt;During the Apollo missions the Lunar lander was separated, turned around and docked during flight. And the command module was left in Lunar orbit during the surface mission. The Soviets planned to have the single cosmonaut to land on the surface do a space walk between the Soyuz-like orbital module and the lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it correct to say that both these mission designs were compromises made because the launchers were slightly too small for a direct launch from Earth to the Lunar surface? Or was it perhaps a deliberate choice after all, maybe in order to win time in the competitive space race?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why weren't the launchers designed to be larger so that the Lunar missions could be simplified? How much larger would they have had to be, 10%? 20%? Were they maybe as large as it was feasible to build launchers back in the 1960s, or did they during the development process fail to achieve the initial specifications? Is SLS Block II designed to perform a direct launch of astronauts to the Moon without dockings and EVA's?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kx1FZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kx1FZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;Image from &lt;a href=&quot;http://www.spaceistheplace.ca/page_6_2013.html&quot; rel=&quot;noreferrer&quot;&gt;SpaceIsThePlace&lt;/a&gt;. A model of the two cosmonauts transferring a box with Moon rocks from the Lunar lander to Soyuz.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2018-12-04T11:59:45.297" Title="Why weren't Saturn V and the Soviet N-1 Moon rockets made larger in order to simplify Lunar missions?" Tags="|the-moon|design|mission-design|saturn-v|n-1|" AnswerCount="7" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19451" PostTypeId="1" CreationDate="2016-12-16T11:35:20.887" Score="0" ViewCount="210" Body="&lt;p&gt;I really like the science and space related thing and I was reading somewhere on the internet that the Moon is set in place 4.53 billion years ago. So my question is &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Q:-&lt;/strong&gt; Is &lt;strong&gt;Moon&lt;/strong&gt; all natural? or is it created by an advanced &lt;strong&gt;extra-terrestrial&lt;/strong&gt;&#xA;  ancient civilization? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It may sound stupid but there are some rumors about it that in &lt;strong&gt;1970&lt;/strong&gt; &#xA;&lt;strong&gt;NASA&lt;/strong&gt; had crashed a spacecraft on the surface of Moon and its surface had vibrated for almost an hour.&lt;/p&gt;&#xA;" OwnerUserId="17960" LastActivityDate="2016-12-16T11:55:28.993" Title="Is moon all natural?" Tags="|the-moon|spacecraft|nasa|" AnswerCount="1" CommentCount="3" ClosedDate="2016-12-16T12:51:35.613" ContentLicense="CC BY-SA 3.0" />
  <row Id="19452" PostTypeId="2" ParentId="19450" CreationDate="2016-12-16T11:53:03.063" Score="29" Body="&lt;p&gt;In order to use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Direct_ascent&quot; rel=&quot;noreferrer&quot;&gt;direct ascent&lt;/a&gt; method of landing on the moon, which is where the entire vehicle descends and leaves the moon, you would need a rocket an order of magnitude bigger than the Saturn V, not just a bit bigger. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an early comparison NASA made back before they decided to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit_rendezvous&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbit Rendezvous&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8zwbU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8zwbU.jpg&quot; alt=&quot;Nova rocket comparison&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The C1 became the Saturn I, the C-5 became the Saturn V, the Nova never got off the drawing board because it was going to be very expensive and hard to build considering that with LOR you could do it for much less.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a &lt;a href=&quot;http://www.astronautix.com/n/nova.html&quot; rel=&quot;noreferrer&quot;&gt;good article on the history of the Nova rocket&lt;/a&gt;. Here's an excerpt:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nova was NASA's ultimate launch vehicle, studied intently from 1959 to&#xA;  1962. Originally conceived to allow a direct manned landing on the moon, in its final iteration it was to put a million-pound payload&#xA;  into low earth orbit to support manned Mars expeditions. It was&#xA;  abandoned in NASA advanced mission planning thereafter in favor of&#xA;  growth versions of the Saturn V.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As time went on the rocket grew and grew and grew until it was a true monster! See some of the concepts in the image below, they make the Saturn V puny in comparison. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qpLjH.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qpLjH.gif&quot; alt=&quot;Nova - Martin Marietta Advanced Designs&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, the program died because it wasn't needed, there was no plan for a manned mission to Mars, which was the only use case for it. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="10335" LastEditDate="2016-12-17T12:49:33.613" LastActivityDate="2016-12-17T12:49:33.613" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19453" PostTypeId="2" ParentId="19451" CreationDate="2016-12-16T11:55:28.993" Score="8" Body="&lt;p&gt;The Moon is natural, just like Earth. In all our studies of the Moon, from Earth, through the Apollo program and through unmanned Moon missions we have found nothing that points to other origins than natural evolution. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA has crashed several spacecraft on the Moon, including Apollo rocket stages. With seismometers on the surface, &lt;a href=&quot;https://science.nasa.gov/science-news/science-at-nasa/2006/15mar_moonquakes/&quot; rel=&quot;nofollow noreferrer&quot;&gt;they found that the moonquakes&lt;/a&gt; from those impacts lasted very long (up to an hour, as you say). This is because the composition of the Moon is a bit different from Earth.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-12-16T11:55:28.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19454" PostTypeId="2" ParentId="19450" CreationDate="2016-12-16T15:30:50.567" Score="12" Body="&lt;p&gt;On the Soviet side, Vladimir Chelomey's &lt;a href=&quot;http://www.astronautix.com/u/ur-700.html&quot; rel=&quot;noreferrer&quot;&gt;UR-700/LK-700&lt;/a&gt; project would have used a direct ascent mission profile. The UR-700 was 76 meters tall and 17.5 meters wide at the base; it would have looked something like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2xwSA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2xwSA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k9Dl5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k9Dl5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(left-to-right: N-1, Saturn V, UR-700)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The UR-700 didn't have much design work done on it, but there were problems developing the high-thrust &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-270&quot; rel=&quot;noreferrer&quot;&gt;RD-270&lt;/a&gt; engines powering the whole thing. It ended up overlapping too much with the already-existing N-1 project, and never received further development.&lt;/p&gt;&#xA;" OwnerUserId="13208" LastEditorUserId="13208" LastEditDate="2016-12-16T23:19:13.310" LastActivityDate="2016-12-16T23:19:13.310" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19455" PostTypeId="2" ParentId="19450" CreationDate="2016-12-16T15:32:46.287" Score="22" Body="&lt;p&gt;During the early part of the Apollo program, the &quot;direct ascent&quot; mode was favored and Lunar-orbit rendezvous (LOR) was considered far too complex. In fact, the specifications of the Apollo service module were set by the direct ascent plan: the SPS engine is sized to lift off from the moon, and the fuel tankage is sufficient for lunar ascent and return to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Nova rocket needed for direct ascent would have been about 60% heavier than the Saturn V (~4750 tons versus ~2970 tons). Development of that large rocket was going to take a very long time. Kennedy's &quot;before the decade was out&quot; speech set a deadline for the program, and Nova would take too long. The project took another look at LOR, concluded it was the fastest and cheapest approach, and decided to commit to it. This shaped the Gemini program; if the lunar mission required rendezvous, NASA needed to learn how to do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kennedy's deadline also effectively set a deadline for the Soviet space program, so it's likely that the same logic drove them to a rendezvous strategy. The great advantage of LOR is that the smaller the lander, the less fuel needs to be carried for lunar descent and ascent, and the Soviet LK was indeed tiny and compromised, carrying one man instead of two, requiring the crewman to EVA to board, and so on. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-02-26T02:29:27.273" LastActivityDate="2017-02-26T02:29:27.273" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="19456" PostTypeId="2" ParentId="19450" CreationDate="2016-12-16T16:04:10.053" Score="7" Body="&lt;p&gt;Its not rocket science, well actually, it is rocket science. Rockets are not very efficient at all and the worst bit is that to carry anything useful, they have to carry the fuel to push the useful bit, then they have to carry the fuel to carry the fuel to push the useful bit and so on. Saturn V was about three thousand tons at takeoff, and was able to push 45 tons towards the moon, so only about 1.5% of the take-off weight left the earth towards the moon. To increase the moon lander to 50 tons, Saturn V would have had to increase to almost 4,000 tons. The Apollo 17 space capsule they picked up out of the pacific probable weighed not much more then three tons, giving a total efficiency of 0.1%.  &lt;/p&gt;&#xA;" OwnerUserId="10877" LastActivityDate="2016-12-16T16:04:10.053" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19457" PostTypeId="2" ParentId="19450" CreationDate="2016-12-16T16:08:08.007" Score="4" Body="&lt;p&gt;Compare the extra work to build a bigger rocket (and don't forget that costs have the ugly tendency to be exponentialised, thanks to our good friend, the Tsiolkovski equation) with a few hours for the docking. Rockets aren't cheap.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main thing to consider is that you need to dock anyway - to get the lunar crew back from the Moon. So you're not saving on anything but those few hours; all the required docking equipment is still required with a bigger rocket. Unless you're considering having the landing module powerful enough to land, ascend and fly back to Earth and land there. In which case, refer to paragraph one - absurdly expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soviets didn't use docking. But this wasn't because they thought docking is undesirable - it was because they simply couldn't do it. They didn't have the capability. So instead, their astronaut was expected to make a space walk, and they couldn't exploit the (remainder of the) landing module for the trip there and back again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that even with all the modern technology we have now, we still use the same basic approach. The Space Shuttle was still a staged rocket, though the bit returning back to Earth was considerably larger than any earlier attempt. Modern private rockets experiment a lot with recovering the spent stages, but they still use the same basic staging approaches. This is because our engines are horribly inefficient, which in addition to having to carry fuel to carry fuel means that the costs of additional weight don't scale very well - we tend to use the smallest possible rocket for the task at hand. You use a big rocket to get out of atmospheric drag, a smaller rocket to make the rest of the trip (and velocity) to orbit, a smaller rocket yet to circularise your orbit or start an injection, even smaller rocket for the landing and even smaller for taking off again and going back home (do you think Mars is within our easy reach nowadays? For a single trip, kind of; add a return trip, and we're back to &quot;oh boy&quot;).&lt;/p&gt;&#xA;" OwnerUserId="2861" LastEditorUserId="2861" LastEditDate="2016-12-16T16:14:55.247" LastActivityDate="2016-12-16T16:14:55.247" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19458" PostTypeId="1" AcceptedAnswerId="19464" CreationDate="2016-12-16T16:41:26.810" Score="10" ViewCount="808" Body="&lt;p&gt;Hypothetical question: Are we able launch an orbiter from Earth to Mars in a way that such orbiter ends on &quot;geostationary&quot; position, hovering always one specific place over Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My thought process behind this is that if we are able to do it, such orbiter could ease future communication with ground level missions&lt;/p&gt;&#xA;" OwnerUserId="5754" LastActivityDate="2016-12-19T02:50:03.310" Title="Is it possible to deploy &quot;geostationary&quot; Mars orbiter?" Tags="|mars|mars-orbiters|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19459" PostTypeId="2" ParentId="19448" CreationDate="2016-12-16T16:55:31.190" Score="8" Body="&lt;p&gt;Ares V was proposed to be quite a bit more powerful than SLS, putting 188 tons into LEO as compared to 130 for SLS. Besides the first stage engine differences, the Ares second stage is substantially more powerful than SLS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ares was designed for a manned lunar landing mission with the Altair lander, but this would be a lunar-orbit rendezvous with an Orion spacecraft, just like Apollo, rather than a direct-ascent mission. The larger rocket is needed to send about six people to the moon for a somewhat longer stay than Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS doesn't presently have a manned lunar landing mission. Single launch LOR with Orion plus a lander is probably out of its reach; it's not significantly more powerful than Saturn V.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2016-12-16T16:55:31.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19460" PostTypeId="1" AcceptedAnswerId="19479" CreationDate="2016-12-16T18:18:34.153" Score="8" ViewCount="1682" Body="&lt;p&gt;The design of the Constellation program called for two main launch vehicles, The Ares I and the Ares V. The Ares I was said to be human-rated and was to be used to carry the Orion vehicle and its crew to Low Earth Orbit. On the other hand, the Ares V was said to be purely for lifting cargo. &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_V#Constellation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia provides an explanation for the reason behind this decision&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Unlike the Saturn V and Space Shuttle, where the crew and cargo were launched together on the same rocket, Project Constellation was planned to use two separate launch vehicles, the Ares I and the Ares V, for crew and cargo respectively. &lt;strong&gt;This configuration would have allowed the two launch vehicles to be optimized for their respective missions&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;(emphasis mine)&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the cancellation of the Constellation program, the Space Launch System started development. Its goal is to replace both Ares launch vehicles. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article on the Space Launch System&lt;/a&gt; notes this change:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The NASA Authorization Act of 2010 envisions the transformation of the Constellation program's &lt;strong&gt;Ares I and Ares V vehicle designs into a single launch vehicle&lt;/strong&gt; usable for both crew and cargo, similar to the Ares IV.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;(emphasis mine)&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did NASA choose to create a single launch vehicle instead of making two separate spacecraft? Why did they choose to go back to the conventional method of launching both crew and cargo in one launch? It seems that there would be many benefits to creating two vehicles. One for crew one for cargo. Was it for cost reasons, or was there more to it?&lt;/p&gt;&#xA;" OwnerUserId="16704" LastEditorUserId="1142" LastEditDate="2017-01-27T02:54:14.987" LastActivityDate="2017-01-27T02:54:14.987" Title="Why did NASA design the SLS do the job of both the Ares I and Ares V?" Tags="|rockets|nasa|design|sls|constellation-program|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19461" PostTypeId="2" ParentId="19460" CreationDate="2016-12-16T19:55:37.150" Score="2" Body="&lt;p&gt;Not specific to the overall architecture change (manned smaller booster, unmanned larger booster) but specific to the actual design of the Ares I they found that the SRB based booster had a number of problems that were looking hard to solve.  If the SRB were to fail, and the Orion aborted, it was looking like the explosion would be big enough to shred the parachute of the capsule trying to escape. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus at some level as the new administration decided to cancel Constellation, and resurect SLS, they did not seem to want to bring back a design they knew had a big flaw.  (Now it seems likely to be have been a solvable problem, better abort capacity, and it may just have been an excuse).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2016-12-16T19:55:37.150" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19463" PostTypeId="2" ParentId="19458" CreationDate="2016-12-16T20:49:00.843" Score="7" Body="&lt;p&gt;Such an orbit is called an &lt;a href=&quot;https://en.wikipedia.org/wiki/Areostationary_orbit&quot; rel=&quot;noreferrer&quot;&gt;areostationary orbit&lt;/a&gt;. It's possible; the orbit would be about 17,000 km above Mars's surface. It's never been done, though.&lt;/p&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2016-12-16T20:49:00.843" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19464" PostTypeId="2" ParentId="19458" CreationDate="2016-12-16T21:47:39.500" Score="14" Body="&lt;p&gt;In general, (.*)stationary orbits are simply the altitude where a circular orbit has a period equal to the rotational period of the central body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to find this altitude, we can transform Kepler's third law to solve for R:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kepler's third law:&#xA;$\frac{T^2}{R^3} = \frac{4\pi^2}{G M_{central}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solved for $R$: &#xA;$R = \sqrt[3]{\frac{T^2 G M_{central}}{4\pi^2}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using $M_{earth}$ and $T_{earth}$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$R = \sqrt[3]{\frac{(86164)^2 * (6.67e-11) * (5.97e24)}{4\pi^2}} = \sqrt[3]{\frac{2.97e24}{39.5}} = 4.22e7m = 42,200km$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where 86164 seconds is the approximate length of Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Sidereal_time&quot; rel=&quot;noreferrer&quot;&gt;sidereal day&lt;/a&gt; (~23h, 56m, 4s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Subtracting the radius of Earth which is ~6370km, this is an altitude of ~35,800km. This can be &lt;a href=&quot;https://www.google.com/search?q=geostationary+orbit+altitude&amp;amp;oq=geostationary+orbit+altitu&amp;amp;aqs=chrome.0.0j69i57j0l4.3412j0j7&amp;amp;sourceid=chrome&amp;amp;ie=UTF-8&quot; rel=&quot;noreferrer&quot;&gt;confirmed by google&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using $M_{mars}$ and $T_{mars}$ (which is 40 minutes longer than an Earth day):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$R = \sqrt[3]{\frac{(88643)^2 * (6.67e-11) * (6.39e23)}{4\pi^2}} = \sqrt[3]{\frac{3.36e23}{39.5}} = 2.04e7m = 20,400km$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where 88643 seconds is the approximate length of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Timekeeping_on_Mars#Time_of_day&quot; rel=&quot;noreferrer&quot;&gt;Martian sidereal day&lt;/a&gt; (~24h, 37m, 23s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Subtracting the radius of Mars which is ~3390km, this gives an altitude of about 17,000km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can easily look up and substitute the numbers of any celestial body for this!&lt;/p&gt;&#xA;" OwnerUserId="17968" LastEditorUserId="12102" LastEditDate="2016-12-19T02:50:03.310" LastActivityDate="2016-12-19T02:50:03.310" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19465" PostTypeId="2" ParentId="19448" CreationDate="2016-12-16T21:55:23.570" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;But what changes in design were made from Ares V to SLS? What were the reasons for those changes? Were there for example problems discovered with Ares V that motivated some of the design changes?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article on the Space Launch&#xA;   System&lt;/a&gt; states&#xA;   that the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Authorization_Act_of_2010&quot; rel=&quot;noreferrer&quot;&gt;NASA Authorization Act of&#xA;   2010&lt;/a&gt;&#xA;   conceptualized a space craft that would shape the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I&quot; rel=&quot;noreferrer&quot;&gt;Ares&#xA;   I&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_V&quot; rel=&quot;noreferrer&quot;&gt;Ares&#xA;   V&lt;/a&gt; into a single launch vehicle&#xA;   resembling the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_V#Ares_IV&quot; rel=&quot;noreferrer&quot;&gt;Ares&#xA;   IV&lt;/a&gt;. It would be used&#xA;   for both cargo and crew transportation. This means that the SLS was designed to human-rated, while the Ares V design was to purely for lifting power. The reason for this probably being that NASA considered using one vehicle instead of two to be more efficient, cost wise as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The payload capacity was also very different. The Ares V was projected to be able to lift roughly 188,000kg to Low Earth Orbit, while the Space Launch System is projected to lift between 70,000kg to 130,000kg to Low Earth Orbit.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I see on Wikipedia that Ares V was to have had 5 Space Shuttle Main Engines compared to SLS' 4, or 5-6 RS-68 engines which is what Delta IV uses. Is the fewer number of engines compensated for by extending the solid boosters for SLS?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Actual no. The Space Launch System is suppose to use a &quot;beefed-up&quot; version of the Space shuttle boosters with five segments instead of four. The Wikipedia on the Space Launch System notes this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Blocks 1 and 1B of the SLS will use two &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#Five-segment_booster&quot; rel=&quot;noreferrer&quot;&gt;five-segment Solid Rocket Boosters&lt;/a&gt; (SRBs), which are based on the four-segment &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Solid Rocket Boosters&lt;/a&gt;. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is there any particular reason for the change in number of engines?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The most likely reason is because the Space Launch System - as I've already said above - is designed to be safe enough for human travel, while the Ares V was not. The reduced power would probably help limit the force of oscillations. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is it correct to say that both Ares V and SLS were designed to be a little more powerful than Saturn V, in order to send astronauts directly from Earth's surface to the Lunar surface without having to leave half the spacecraft in Lunar orbit?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Ares V was designed to lift the Altair lander into Low Earth Orbit and, after rendezvousing with the Orion spacecraft, put it on a trajectory towards the moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The total mass of the Altair lander paired with the total mass of the Orion spacecraft, would need a considerable amount of force to put the two space craft in a Trans-luanr Injection. More force than the Saturn V could provide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the Space Launch System, it would probably have a hard time matching up with the Ares V, and would fall short. And as &#xA;@Russell Borogove has already said, it doesn't seem to be that much more powerful than the Saturn V.&lt;/p&gt;&#xA;" OwnerUserId="16704" LastActivityDate="2016-12-16T21:55:23.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19466" PostTypeId="1" AcceptedAnswerId="19480" CreationDate="2016-12-17T00:15:11.330" Score="4" ViewCount="634" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/19463/12102&quot;&gt;This answer&lt;/a&gt; introduced me to the word &lt;a href=&quot;https://en.wikipedia.org/wiki/Areostationary_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Areostationary Orbit&lt;/a&gt; - (hint, comes from Ares), an orbit that would remain stationary above a point on the equator of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article mentions that &lt;a href=&quot;https://en.wikipedia.org/wiki/Areostationary_orbit#Stationkeeping&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational interaction with the two moons of Mars&lt;/a&gt;, Phobos in particular, would be a factor that would require station keeping. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://arxiv.org/abs/1203.1775&quot; rel=&quot;nofollow noreferrer&quot;&gt;2012 paper&lt;/a&gt; discusses locations in the gravity field of mars where a Areostationary orbit would be possible due to small variations in the gravity field of the planet. Certain points seem to be slightly stable, and satellites put in &lt;em&gt;certain particular locations&lt;/em&gt; above the Martian equator would tend to stay there, although they would be orbiting these locations by a few degrees. If the wobble it &lt;em&gt;too large&lt;/em&gt;, it sort-of defeats the &quot;stationary&quot; part. You'd have to do some steering, and at that point it would be better to just call it a synchronous orbit. (Fixed dishes will be superseded by steerable phased arrays well before then).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, once I started seeing terms like &lt;em&gt;monodromy matrix&lt;/em&gt; and &lt;em&gt;heteroclinic orbits&lt;/em&gt; in that paper, I decided to ask here instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What factors would make station-keeping of an Areostationary Mars satellite necessary? Would the pull of the Moons destabilize orbits at Mars' natural gravitational Areostationary points over the period of years, requiring station-keeping?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2016-12-17T21:27:31.743" Title="What factors would make station-keeping of an Areostationary Mars satellite necessary?" Tags="|mars|geostationary|station-keeping|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19467" PostTypeId="1" AcceptedAnswerId="19469" CreationDate="2016-12-17T01:05:14.237" Score="13" ViewCount="2297" Body="&lt;p&gt;Doubling the life support, almost doubling the crew space, carrying lots of extra mass - the &lt;a href=&quot;https://space.stackexchange.com/questions/19450/why-werent-saturn-v-and-the-soviet-n-1-moon-rockets-made-larger-in-order-to-sim&quot;&gt;recent question about direct ascent&lt;/a&gt; made me ponder this: the fundamental role of the Command Module was to provide transfer both ways and reentry capabilities to the Apollo missions. None of these requires a crewed module. Why wasn't the 'command module' just an umnanned tug?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's imagine this: a single crewed cabin, with lunar descent and ascent modules (both detachable once used up) that can undock/redock to a heat shield, which in order is attached to a module with high ISp low thrust propulsion for transfer burns (with radio relay capacity too). There should be huge mass savings on not making that module crewed. They could be redirected to making a larger lander, or just reduce the cost. Additionally, docking with crew transfer capability is way more complex than without it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I missing something here?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="195" LastEditDate="2018-05-17T18:06:26.020" LastActivityDate="2018-10-24T19:21:28.560" Title="What did Apollo need the crewed Command Module for?" Tags="|apollo-program|lunar-landing|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="19469" PostTypeId="2" ParentId="19467" CreationDate="2016-12-17T02:43:05.373" Score="28" Body="&lt;p&gt;The primary function of the command module was to keep three crewmen alive and allow them to survive re-entry. Those requirements weren't negotiable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soyuz spacecraft to some extent divides those functions into two modules, and so its re-entry module is more compact than the Apollo CM (Command Module), but this comes with an overall complexity penalty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point of the LOR (Lunar Orbit Rendezvous) strategy is that it allowed the lunar landing delta-v cost to be paid by a smaller craft. The LM (Lunar Module) cabin supported about 8 man-days of operation in very cramped quarters; the CM supported 36 man-days in relative luxury. Landing the CM would require a much larger descent-ascent system than the LM — greatly exceeding the mass of the &quot;redundant&quot; LM cabin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In essence, you'd be calling for a small optimization of the direct ascent strategy — leaving the TEI (Trans-Earth Injection) fuel and possibly the heat shield in lunar orbit for the landing, at the cost of requiring a rendezvous (like LOR).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fueled mass of the Apollo LM (15 tons) is more than 7.5 times the mass of the cabin (ascent stage, dry, is 2.1 tons, and consists of the cabin, ascent engine, tankage, and various additional equipment).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo CM, less the heat shield, is about 4.7 tons. Even stripped down, say to 4 tons, you'd need 30 tons for a complete lunar descent and ascent at the same mass ratio as the LM. So to save the mass of the 2-ton LM cabin you have to incur a 15-ton hit in the LM.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you define a two-man mission instead, the CM could be scaled down to perhaps 3 tons, which leads to something like a 5 ton penalty. Remember, though, the Apollo CSM (Command/Service Module) was conceived as a general purpose 3-man spacecraft to ferry crews to space stations and so forth, not solely for moon landings. (After the Apollo lunar program, the spacecraft was used for Skylab missions and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot; rel=&quot;noreferrer&quot;&gt;Apollo-Soyuz Test Project&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A single cabin solution also removes the lifeboat capability that saved the lives of the Apollo 13 crew, though that wasn't a primary driver of the LOR architecture.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="2778" LastEditDate="2018-10-24T08:35:07.393" LastActivityDate="2018-10-24T08:35:07.393" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19471" PostTypeId="2" ParentId="19448" CreationDate="2016-12-17T08:09:35.767" Score="16" Body="&lt;p&gt;As it turns out, SLS is actually very similar to the original Ares V configuration, proposed around 2002 (I believe). During this time it was known simply as 'CaLV,' or Cargo Launch Vehicle, as a launcher to loft 125 tonnes to LEO for Constellation lunar missions. This rocket featured STS 8.4m tanks on both the core and second stages. Main sustainer propulsion came from 5 SSMEs, while the upper stage was to use a single J-2X. A pair of 5-segment boosters would augment the stack.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS is nearly identical, as it also features 8.4m core stage tankage to reuse shuttle tooling. In addition, the 5-segment boosters have been carried over. Upper stage options have changed, though, now featuring either 8.4m or 5m diameters with Rl-10 engines. The CaLV throw weights are very close to SLS Block 1B because it is basically just that; the Block 1B should throw around 118 tonnes (though this may include upper stage propellant).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This all seems pretty similar, but what makes this a good question is the constant growth the Ares V design had to absorb to keep up with the demands of its mission as Altair and Orion came of age.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CaLV was an inline shuttle-derived vehicle that could meet the needs of Constellation's EOR architecture alongside Ares I ('The Stick'). It was to have 8.4m core and upper stages with 5 RS-25s and one J-2X, respectively. Two 5-segment SRBs would shove it off the pad. With a payload to LEO of 125 tonnes, it was fine for the early 2000s plans... but these were short-lived.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lunar lander, known as 'Altair,' was its primary payload and was growing in mass. Orion, too, was getting heavier for the TLI burn. As such, a lunar injection mass of over 60 tonnes was needed. Thus, CaLV morphed into Ares V, with a new design packing more power. The engine of choice became the less efficient but more powerful and cheaper RS-68. But being more thirsty, the tanks had to change. The core stage grew to 10m in diameter and 65m in length, and to support this the boosters each received an extra half-segment for greater thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next to grow was the upper stage, or EDS. Altair hit 45 tonnes, meaning a combined TLI mass of over 65 tonnes. Previously 8.4m, it also widened to 10m while retaining the single J-2X. More mass at the top of the rocket meant the bottom had to adjust, and the core added a 6th RS-68 - in fact, the updated RS-68A was planned. Ares V was, if it ever flew, to end up using the RS-68B, with even greater thrust and a newer ablative nozzle. The core also stretched to 71m in length, leading to a rocket capable of lifting 188t to LEO, 71t to TLI, standing 116m tall and weighing 3,705t on the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, problems started to surface. Notably, base heating with the RS-68 cluster and SRB exhaust turned out to be severely problematic to efficiency. The ablative lining could not dissipate heat with nozzles packed together, unlike the RS-25 and its regenerative cooling system. Along with this, pad infrastructure changes and the need for 10m tooling meant money, and lots of it. The Augustine Commission called for a smaller Ares V Lite that would be cheaper to operate, with a 140t LEO payload, 5 RS-68s and the old 8.4m upper stage. This vehicle could also have eliminated the need for Ares I and all of its development costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This advice was not adhered to, and Ares V continued to grow up until it's death. Near the end it was a beast of grand proportions; some studies showed 6.0- or even 6.5-segment boosters (!) on either side of a 7-engined RS-68B core. Just like science fiction, mythical giants may seem incredible until it is revealed they are too heavy for their legs to support them. Ares V was almost the same: it could have worked, but it's 'authors' kept writing too long into the night, and when they stood back the 9-billion dollar tag could not keep it afloat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS is similar to the Ares V Lite in that it also sits in the niche of throw weight that allows it to act as the sole vehicle in any EOR mission architecture, rather than the Ares I/Ares V combo. It is perhaps more sensible for keeping to existing shuttle and EELV tooling for its 8.4m core stage and 5m Block I upper stage. Also, it uses the 5-segment boosters that were already tested for Ares I. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The operation of SLS is similar to the Ares with a few exceptions. Both feature hydrolox cores and upper stages flanked by solid boosters. However, the Ares used its core stage as more of a first stage (closer to Saturn V operation than that of SLS); it had over twice the thrust (with less efficiency) and a shorter burn time, meaning it's upper stage was larger because it would have to generate a larger portion of the ascent delta-V. On the other hand, SLS' core stage uses very efficient but less powerful engines that burn for a longer duration - SLS is a lighter rocket by several hundred tonnes. Therefore, the core is really a sustainer stage that fires for nearly all of the ascent, and the small RL-10 powered upper stages only need to give a little push to orbit. In fact, the SLS core stage dropped the fifth engine both to preserve engine stockpiles and to extend its burn time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To increase payload, therefore, Ares grew its first stage - the core and its RS-68s, and the boosters grew both to lift its greater mass and to line up with the rising intertank distance from the thrust structure. SLS' sustainer remains the same, but Block 2 will increase payload through the use of liquid-fuel boosters because they constitute most of its first stage power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Key differences, then, are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Core stage diameter (Ares = 10m, SLS = 8.4)&lt;/li&gt;&#xA;&lt;li&gt;Ascent operation (First stage vs. sustainer)&lt;/li&gt;&#xA;&lt;li&gt;Mission requirements (Ares EOR means more TLI mass)&lt;/li&gt;&#xA;&lt;li&gt;Booster type (reflects ascent operation)&lt;/li&gt;&#xA;&lt;li&gt;Vehicle scale and mass (Ares is larger)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It has always been interesting to me when I look back at Ares V as a 'child,' in its CaLV days. The rocket was all there, taken straight from shuttle hardware, with just a bit of stretching, no RS-25 uprating needed, and without the J-2X in the trash can as soon as it had proven itself. CaLV fits into the gap SLS fills, and that Ares V created and could never stoop to; it just did it well before any of them. Which begs the question regarding why twenty years of development and thirty billion dollars have brought about a rocket with one less engine than it started with...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Written on my phone, so please forgive me of any spelling errors or horrendous grammar!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Al.&lt;/p&gt;&#xA;" OwnerUserId="12678" LastEditorUserId="12678" LastEditDate="2018-12-09T12:58:38.677" LastActivityDate="2018-12-09T12:58:38.677" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19472" PostTypeId="2" ParentId="19360" CreationDate="2016-12-17T12:31:43.693" Score="2" Body="&lt;p&gt;Well, another approach is to think of the exhaust velocity of the engines. You would have to be able to push the air backwards faster than it comes in in order to get a net thrust forward. You can get more thrust from the additional fuel you burn up but at that point a rocket engine would be more efficient as the intake air is just slowing you down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, in order to get to orbital speeds you'd need about 8km/s exhaust velocity (NOT effective exhaust velocity) from your engine, which is basically impossible. Rocket engine exhaust velocity is easy to calculate from Isp and the highest Isp ever achieved with chemical propulsion is around &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse#Examples&quot; rel=&quot;nofollow noreferrer&quot;&gt;542s&lt;/a&gt; (for a non-practical tripropellant concept engine) which translates to exhaust velocity of 5.3km/s. 8km/s is almost certainly never going to be achieved with chemical reactions. (It would for example need temperatures of &gt;5000C, which is not containable in large enough quantities for an engine to actually produce thrust for anything practical)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anybody can find actual exhaust velocity for scramjets or similar, I'm definitely interested, I couldn't find any. I might be wrong with those, I know more of rocket propulsion.&lt;/p&gt;&#xA;" OwnerUserId="17842" LastActivityDate="2016-12-17T12:31:43.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19474" PostTypeId="1" CreationDate="2016-12-17T14:57:23.110" Score="10" ViewCount="939" Body="&lt;p&gt;Reading through several old reports, I have come across studies conducted by MSFC on improved Saturn V designs with greater payload capacity, and with uprated engines. As a result of increasing launch vehicle propellant masses, the first stage 'MS-IC' engines had to produce more thrust to lift the upper stages efficiently. Rocketdyne's F-1 would have continued on as UF-1 and F-1A variants. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Marshall reports from 1965 make reference to the F-1 qualification engine, which was rate down at a peak sea level thrust of 1,522,000 pounds force. The UF-1 ('Uprated F-1') was intended to serve as an interim model that would generate higher thrust levels but not require intensive redevelopment required for substantial boosting. As such, the UF-1 would settle at a thrust of 1,650,000 pounds force. The F-1A would make further improvements not including completely new component design to achieve 1,800,000 pounds force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The UF-1 is the model that interests me, because there isn't much writing on it elsewhere. MSFC researchers intended to increase its thrust, but also stated that the standard 1,522k F-1 thrust was limited by the horsepower of its '35 inch turbopump.' To update to 1,650k the UF-1 would feature a new 30 inch turbine and improved pump inducers to support it, and also a strengthened gas generator operating at a lower lox/fuel ratio. The 30 inch turbine would then limit the thrust because of its critical turbine speed. However, the F-1A would surpass this with the same 30 inch model by adding new, increased diameter impellers (as well as larger gas generators and exhaust manifolds).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But here's where it gets interesting: I have also been reading 'Stages to Saturn' by Roger E. Bilstein, in which the author writes of a test run of an early completed thrust chamber at Edwards AFB. On April 6th 1961, 'the thrust of the prototype chamber peaked at 7,295,000 newtons (1,640,000 pounds).' If originally developed to be limited to 1,522k pounds force by turbine horsepower constraints, how could this engine generate so much more thrust without the 1965-listed improvements to the turbine and GG?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This reminds me of various other articles listing F-1 thrusts beyond 1,522k pounds throughout Saturn V operation: from 1,530k on Apollo 11 to 1,565k on Apollo 15 and 1,580k on Skylab. But 1640k? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here are the questions:&#xA;- How did the old turbine allow for 1,640klbf thrust?&#xA;- Would chamber pressure or specific impulse be increased?&#xA;- Could Saturn V have been propelled with this thrust level?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to know if 1,640k could be sustained...&lt;/p&gt;&#xA;" OwnerUserId="12678" LastActivityDate="2016-12-20T13:33:19.403" Title="What was the maximum thrust of the Rocketdyne F-1 engine?" Tags="|rockets|apollo-program|engine-design|saturn-v|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19476" PostTypeId="1" CreationDate="2016-12-17T17:34:01.407" Score="4" ViewCount="19661" Body="&lt;p&gt;I'm theoretically trying to launch a rocket for a school project and therefore I chose Saturn V. I took its weight of 140.000kg (308 647 pounds) and I'm trying to calculate how much fuel it'd take to launch it at two different locations, one on the equator and the other one at the poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravity acceleration I've already calculated (Equator: $9,797 m/s^2$; Poles: $9,863m/s^2$) as well as the distance, which is $1,414213*10^7$ meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I'm stuck, for I don't know how to involve the gravitational force declination in my calculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to calculate how many joules (J) are needed to launch a rocket to a height of $1,414213*10^7$ meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first I did that with the formula F_{res}=g$*m$, but that doesn't involve the declination of gravitational force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for air resistance, I'd like to calculate that as well, but I think I'll manage on my own.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What formulas or rules of thumb should I use?&lt;/p&gt;&#xA;" OwnerUserId="17976" LastEditorUserId="17976" LastEditDate="2016-12-17T21:33:45.657" LastActivityDate="2021-05-09T10:49:52.247" Title="What is the formula to calculate how much fuel is needed for a rocket?" Tags="|rockets|fuel|gravity|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19479" PostTypeId="2" ParentId="19460" CreationDate="2016-12-17T21:17:58.430" Score="7" Body="&lt;p&gt;Keep in mind that the development of a launch vehicle takes considerable time and money, so having to build two new rockets is harder on funding. Also, looking at the proposed vehicles themselves, we see that both launchers would require new tooling and infrastructure, with Ares I requiring 5.5m stages and Ares V needing 10m stages. The massive launch weight of the Ares V would also require extensive strengthening of the launch sites, to support the 8.2 million pound vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS instead keeps the 8.4m and 5m tank tooling used for current launchers, and the solid boosters will be slightly shorter at 5 segments (rather than 5.5 or more), meaning less strain on the crawler. It seems the operation of SLS will be closer to that of the shuttle than either Ares rockets, meaning easier transition of labour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is also a question of usefulness in the launch industry. This was an issue with both Ares I and Ares V. For Ares I, with a LEO payload capacity of 25 tonnes, it may have been hard-pressed to compete with other vehicles such as Atlas V and Delta IV, and in its proposed form it would not compete in BEO performance. Ares V would likely be the opposite, sitting in a class of its own and having the ability to lift any mission payload thrown at it; however it was perhaps too large, and would see little to no use as even a deep space launcher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS loses to Ares V in pure performance but it is a more sensibly proportioned vehicle, having around 1,000 tonnes less liftoff mass. LEO and BEO payload masses are still high, but it is more useful as a super-heavy or deep-space lifter between manned Orion missions. This could also have been a factor.&lt;/p&gt;&#xA;" OwnerUserId="12678" LastActivityDate="2016-12-17T21:17:58.430" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19480" PostTypeId="2" ParentId="19466" CreationDate="2016-12-17T21:27:31.743" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What factors would make station-keeping of an Areostationary Mars satellite necessary?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Mars' rather lumpy gravitational field, The Sun's gravity, Mars' two moons, solid body tides, and solar radiation pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That 2012 article addressed Mars' rather lumpy gravitational field (partly), but it didn't address any of those other perturbing factors. Those other factors will exist, even at the supposed stationary points investigated in that article. Those stationary points also exist for geosynchronous satellites. However, for a  geosynchronous satellite, the fuel penalty for not being near one of those stationary points are small. The perturbations from the Moon and Sun dominate, requiring considerable fuel (about 40 m/s per year) for north-south stationkeeping. The total stationkeeping budget for a geostationary satellite is about 50 m/s per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That north-south stationkeeping will still be needed for an aerostationary satellite. Mars axial tilt is a bit more than is the Earth's. This tilt makes the orbits of equatorial orbits become inclined due to third body effects (e.g., the Sun, the Moon in the case of the Earth, and Mars' two moons in the case of Mars). Solar radiation pressure causes circular orbits to become non-circular. Stationkeeping will still be needed, even in the vicinity of those stationary points.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-12-17T21:27:31.743" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19481" PostTypeId="1" CreationDate="2016-12-17T22:24:45.887" Score="6" ViewCount="1016" Body="&lt;p&gt;The Rocketdyne F-1 was to be updated with a new turbopump and gas generator assembly, without other major component redesign. This was to result in a thrust increase from around 1,522,000 pounds force for the F-1 to 1,800,000 pounds for the F-1A. Specific impulse at sea level was to have grown from 265 second to between 269 and 271 seconds, and chamber pressure would be increased from 1015psi to 1,161psi.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the combustion chamber dimensions and nozzle area ratio were not altered; so why does the higher-pressure F-1A feature a lower listed vacuum specific impulse of 303 seconds compared to the F-1 at 304s to 305s?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am aware Astronautix applies a 'six second improvement in specific impulse' for both vacuum and sea level Isp - meaning 270s SL and 310s Vac. However other reports disagree; and during my searching the opposition has grown in number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why would a pressure improvement with no changes in chamber dimensions cause lower efficiency?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could it have been a case of testing at sea level (F-1A was never flown), where vacuum trials were actually conducted at imperfect low air pressure rather than none?&lt;/p&gt;&#xA;" OwnerUserId="12678" LastActivityDate="2018-12-25T13:37:42.213" Title="How does the efficiency of the Rocketdyne F-1A compare to the F-1?" Tags="|rockets|propulsion|apollo-program|saturn-v|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19482" PostTypeId="2" ParentId="19476" CreationDate="2016-12-17T22:25:33.240" Score="3" Body="&lt;p&gt;The problem as asked is impossible to solve.  The problem is the energy needed is dependent on your rocket's acceleration and on drag losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I have no proof there is no equation that will answer your question I certainly have never heard of one or seen any indication thereof.  You almost certainly have to do a brute force simulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, while gravity losses will be constant for a rocket going straight up, if your target altitude is high enough it can be more efficient to burn horizontally instead to reduce your gravity loss.  As you build horizontal velocity the gravity loss will drop.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2016-12-17T22:25:33.240" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19483" PostTypeId="1" CreationDate="2016-12-18T01:39:11.880" Score="2" ViewCount="123" Body="&lt;p&gt;The Apollo program was motivated by politics; no matter that the Apollo program was conceived before Kennedy's speech, the commitment to make it happen was driven by the need to demonstrate an American lead over the then Soviet Union in the space race.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before, during, and after Apollo, there was also talk about sending people to Mars. Apollo 11 achieved Kennedy's goal and asserted a U.S. lead in space. So what would have been the motivation at that time to send people to Mars rather than perform robotic exploration? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was Mars &quot;the next high ground&quot; in case Apollo failed? Were there ideas of commercial development (resource extraction) driving the interest in going there? Is the absence of a political motive (such as drove Apollo) the reason human missions to Mars were not pursued at the time? What was the motive fueling the development of all the Mars mission concepts which were floated at the time?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2016-12-18T01:39:11.880" Title="Rationale for human presence on Mars in the Apollo era" Tags="|mars|crewed-spaceflight|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19484" PostTypeId="1" CreationDate="2016-12-18T01:44:41.080" Score="0" ViewCount="115" Body="&lt;p&gt;Ask around and find out what gases, vapors, and/or fumes, in the atmosphere are the result of advanced technology. Then use a spectroscopy to look for those compounds in the atmospheres of earth like exo-planets. Who is doing this?&lt;/p&gt;&#xA;" OwnerUserId="17906" LastEditorUserId="238" LastEditDate="2016-12-18T01:56:32.757" LastActivityDate="2016-12-18T07:18:20.613" Title="The gases of technology" Tags="|exoplanet|" AnswerCount="1" CommentCount="12" FavoriteCount="0" ClosedDate="2016-12-19T22:05:12.490" ContentLicense="CC BY-SA 3.0" />
  <row Id="19486" PostTypeId="1" CreationDate="2016-12-18T02:58:44.583" Score="1" ViewCount="242" Body="&lt;p&gt;I'm writing about a spaceship that drops past L-5 from Mars to the sun, performs a tight Oberth maneuver (@ 10 million K from sun?), has 1.5g acceleration at perihelion, but loses thrust before finishing its maneuver and swings wide. It needs a Jupiter gravity assist to change orbital planes, and I'm thinking that orbit to Jupiter will be a long one--13 AU? 15? the ship needs to &quot;chase&quot; Jupiter. It needs two or three months to get to Jupiter, time for the crew to repair systems that will be needed there, damaged in the CME that knocked out their propulsion. They have limited, intermittent propulsion (1/2 time) 2-3 days after leaving closest approach; I'm thinking I should limit that to .1 or .2 g acceleration. Several weeks later they get their badly-damaged main engine (a fusion-powered VASIMR) back online; that gives them ~.1 g (?) acceleration until almost Jupiter, when they finally get their inertial-reduction coils rebuilt and can thrust at 1g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can do simple acceleration-time-distance etc. calculations, and when I remember to add in all of the conversions, my answers are close to those produced by online relativistic spaceship calculators--and .1 or .2 g seems plenty.. But nothing I find online tells me how to account for the &quot;gravity tax,&quot; the need to subtract out solar escape velocity from 10 diameters out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anybody here know that math?&lt;/p&gt;&#xA;" OwnerUserId="17982" LastActivityDate="2016-12-18T02:58:44.583" Title="Can someone show me the math behind an Oberth maneuver around the sun, and acceleration to Jupiter?" Tags="|orbital-mechanics|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19487" PostTypeId="2" ParentId="18953" CreationDate="2016-12-18T05:40:35.413" Score="6" Body="&lt;p&gt;Russell is correct; no second run was ever officially ordered, which is why we never saw more Saturn Vs lift off. However, when you ask if it was ever considered, know that it certainly was - in a big way!&#xA;I suggest you look into Saturn MLV and ELV studies...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MLV stood for 'Modified Launch Vehicle' while ELV stood for 'Evolved (or Earth) Launch Vehicle.' These launchers would have been improved Saturns with greater payload capacity, better economy, or both.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Saturns SA-501 through SA-512 were all three-stage Saturn Vs used for Apollo missions, while SA-513 was a two-stage vehicle (not &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_INT-21&quot; rel=&quot;nofollow noreferrer&quot;&gt;INT-21&lt;/a&gt;!) that launched Skylab. SA-514 and -515 were to launch another Apollo mission and a backup Skylab, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 'second run' would begin with standard Saturn Vs marked SA-516 onwards, with perhaps some two-stage launches and the eventual introduction of INT-21. But the MLV variants would begin at SA-522 with the Saturn MLV-V-1A: stretched stages, six F-1s and seven S-II J-2s. SA-526 would introduce the first of the F-1A powered Saturns, the MLV-V-1, which would eventually use J-2S and J-2T.&#xA;SA-536 would see the MLV-V-3 launcher with the high-pressure HG-3 upper stage engine, and later evolve into the V-3B with toroidal upper stage engines. These later variants would approach 170t to LEO and 75t to TLI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Between these milestones, it is likely that solid booster assisted Saturns such as the MLV-V-4(S) would arise. 120in and 156in strap ons could boost payloads to around 160 tonnes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After this, some studies get crazy. Launchers such as the ELV models of the Saturn V-25(S)U would use 260in diameter solid boosters and stretched stages to lift 250 tonne NERVA stages to LEO for mars exploration. The V-4/260 would also use 260in solids, but with massive 23m fairings and auxiliary tanks to lift over 360 tonnes. My favourite design involves a vehicle consisting of four stretched Saturn Vs joined under a single fairing to allow payloads of 520 tonnes and over!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many, is not all, of these improved designs would likely never be intended to fly. A true second run would probably only see more standard Saturn Vs, joined by a few INT-21s, MLV-V-1s and various solid-boosted models. Sometime during the production run the rockets would no longer feature fins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But of course, consideration does not mean real, in-depth planning. It seems the second run was canned around 1968, when Apollos 18 to 20 were also scrapped. Much of this appears to be designer dreaming without a goal to be achieved before the next decade was out...&lt;/p&gt;&#xA;" OwnerUserId="12678" LastEditorUserId="415" LastEditDate="2017-01-03T12:53:43.797" LastActivityDate="2017-01-03T12:53:43.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19488" PostTypeId="2" ParentId="19484" CreationDate="2016-12-18T07:18:20.613" Score="8" Body="&lt;p&gt;The best method of analyzing exoplanet atmospheres is called &lt;a href=&quot;http://spiff.rit.edu/classes/extrasol/lectures/spectra/spectra.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transit Absorption Spectroscopy&lt;/a&gt;.  First, we analyze the spectrum of an exoplanet's star.  Then, we wait for the planet to pass in front of the star.  When it does so, some of the star's light is absorbed by the planet's atmosphere, changing its spectrum.  Analyzing this change will tell us what chemicals are present in the planet's atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to detect life, there are several compounds which researchers look for.  One of these is Oxygen.  Oxygen is a highly reactive element, and will disappear from a planet's atmosphere very rapidly if it is not being produced anywhere.  Life uses oxygen because it is so reactive, so if oxygen is present in a planet's atmosphere, it is a strong indicator of the presence of life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to look for intelligent, technologically advanced life, researchers search for compounds which are not present in nature.  One of the most notable compounds is the family of chlorofluorocarbons (CFC's).  CFC's have &lt;a href=&quot;http://www.nature.com/nature/journal/v399/n6738/full/399749a0.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;no known natural source&lt;/a&gt;, and have a highly noticeable &lt;a href=&quot;http://iopscience.iop.org/article/10.1088/2041-8205/792/1/L7/meta&quot; rel=&quot;nofollow noreferrer&quot;&gt;infrared absorption line&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the absorption lines produced by these gases are in the infrared.  This means that you need a special infrared telescope to observe them.  Unfortunately, since our atmosphere is opaque at some of the frequencies required to find CFC's, you need to use space-based telescopes.  There are hopes that the James Webb Space Telescope and the Transit Exoplanet Survey Satellite will be able to do this work &lt;a href=&quot;https://arxiv.org/pdf/1509.08922.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;when they launch&lt;/a&gt;.  As &lt;a href=&quot;http://iopscience.iop.org/article/10.1088/2041-8205/792/1/L7/meta&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt; shows, the JWST would have to focus on a planet for ~5 hours to determine the atmosphere's composition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This work can be done &lt;a href=&quot;http://rsta.royalsocietypublishing.org/content/372/2014/20130083.short&quot; rel=&quot;nofollow noreferrer&quot;&gt;from the ground&lt;/a&gt;, but it is more difficult and has more uncertainty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More sources:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astrobio.net/news-exclusive/unintelligent-life-cfcs/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.astrobio.net/news-exclusive/unintelligent-life-cfcs/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2016-12-18T07:18:20.613" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19489" PostTypeId="2" ParentId="16948" CreationDate="2016-12-18T18:02:09.493" Score="12" Body="&lt;p&gt;So far (2016) the biggest &amp;quot;room&amp;quot; in orbit was the Orbital Workshop of Skylab.&lt;/p&gt;&#xA;&lt;p&gt;Here's a &lt;a href=&quot;https://www.youtube.com/watch?v=d1sr6aVzW9M&quot; rel=&quot;noreferrer&quot;&gt;video showing astronauts moving in the largest part of Skylab&lt;/a&gt;. You never get to see the &amp;quot;floor&amp;quot; in this one, unfortunately. The room was so big that they were even able to test an astronaut maneuvering unit (jetpack) inside (see pictures at the end of this answer).&lt;/p&gt;&#xA;&lt;p&gt;I found numbers for the Orbital Workshop volume ranging from about &lt;a href=&quot;http://www.encyclopedia.com/science-and-technology/astronomy-and-space-exploration/space-exploration/skylab&quot; rel=&quot;noreferrer&quot;&gt;270 m³ (9,550 cubic feet)&lt;/a&gt; to &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab#Dry_workshop&quot; rel=&quot;noreferrer&quot;&gt;about 302 m³ (10,664 cubic feet)&lt;/a&gt; (total habital volume of the station is usually cited as 351–361 m³). That seems to include the crew quarters section. When looking at the &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Skylab_innen.jpg&quot; rel=&quot;noreferrer&quot;&gt;cutaway diagram&lt;/a&gt; I estimate that the crew quarters sections (divided from the rest of the lab by a wire mesh grid) took up about a fourth of that volume, leaving the main &amp;quot;room&amp;quot; with &lt;strong&gt;about 202–226 m³&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The largest module (on the inside) of the ISS is the Kibō module with a pressurized volume of 155 m³. So Sklyab beats that.&lt;/p&gt;&#xA;&lt;p&gt;Skylabs large volume actually caused problems: apart from the obvious &lt;a href=&quot;https://space.stackexchange.com/questions/5232/is-there-an-upper-limit-for-the-internal-size-of-space-stations?noredirect=1&amp;amp;lq=1&quot;&gt;&amp;quot;getting stuck&amp;quot; issue&lt;/a&gt; (Skylab crews learned that you can indeed &lt;a href=&quot;https://space.stackexchange.com/questions/8073/can-you-swim-in-space&quot;&gt;&amp;quot;swim&amp;quot; in the station&lt;/a&gt;) it seemed that the large volume contributed to &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_adaptation_syndrome&quot; rel=&quot;noreferrer&quot;&gt;Space Adaptation Syndrome&lt;/a&gt; (&lt;em&gt;citation needed; I read about it recently but can't find a good source now&lt;/em&gt;).&lt;/p&gt;&#xA;&lt;p&gt;Images from the actual Skylab:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://history.nasa.gov/SP-400/ch7.htm&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://history.nasa.gov/SP-400/p118.jpg&quot; alt=&quot;An astronaut maneuvering unit, shown here, was flown in the workshop to test it under weightless conditions for possible future application.&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://airandspace.si.edu/exhibitions/space-race/online/sec500/sec514.htm&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://airandspace.si.edu/exhibitions/space-race/online/sec500img/512l1p1f.jpg&quot; alt=&quot;Astronaut Maneuvering Equipment&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image from trainer:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.jsc.nasa.gov/history/jsc40/jsc40_pg13.htm&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LIsIk.jpg&quot; alt=&quot;An interior view of the Skylab Orbital Workshop (OWS) trainer located in Building 5&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://www.jsc.nasa.gov/history/jsc40/gallery/lores/S77-23480.jpg&quot; rel=&quot;noreferrer&quot;&gt;nasa.gov&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-03-15T20:54:10.483" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19490" PostTypeId="2" ParentId="19386" CreationDate="2016-12-18T18:07:43.343" Score="2" Body="&lt;p&gt;I agree with ForgeMonkey that there is little to say based on public domain sources. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, for the necessarily speculative nature of an answer I would instead suggest the following as typical approaches within geostationary communications satellites:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;my starting point is that the Question asks about the transponders, i.e. the payload, rather than the command uplink. I'll come back to commanding later.  Also please note that this question does not seem to be addressing the actual recovery process, it is just asking about the use of a high power ground uplink.&lt;/li&gt;&#xA;&lt;li&gt;some satellites used fixed gain, and some use a form of AGC for&#xA;getting the right level of the signal, it is more likely to be&#xA;applied at the level of each transponder (Galaxy 15 had 24) than in a digital control loop at the whole payload level. &lt;/li&gt;&#xA;&lt;li&gt;payload protection may not have contemplated a high-power uplink,&#xA;it is a civil satellite, but even if it did it doesn't help the satellite as a whole because it is the command link, not the payload, that has a problem.&lt;/li&gt;&#xA;&lt;li&gt;satellite level protection may also have assumed payload shedding in&#xA;the case of a loss of power, it would be usual for the payload to be&#xA;shed before the TT&amp;amp;C system (the part that receives commands), &lt;/li&gt;&#xA;&lt;li&gt;that said it is not impossible that the same satellite protection&#xA;may have instigated a computer reset or swap to redundant units in case of a power short-fall. For this to make sense we have to imagine that over-demand for power from the transponders could cause a bus undervoltage and thus appear to have the same symptoms as a lack of power.  &lt;strong&gt;EDIT&lt;/strong&gt; In this sense AGC at the transponder level could actually hinder the reset by preventing a bus undervoltage, though they could be developed into a form of emergency command path if the central unit was programmed to respond to &quot;high uplink power =&gt; reset TTC&quot;.&lt;/li&gt;&#xA;&lt;li&gt;Lastly, the quoted source does not seem to make the same assumption that the high power r.f. was directed at the payload transponders. If it was instead aimed at the command receiver then that device instead would need its own protection logic that triggered a reset if the power level were too high. Its not impossible but it sounds like it would more likely be implemented as a response to an internal failure that had similar electrical symptoms.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Putting this all together it seems we are left with No. 5 or No. 6, though even then its all rather sketchy.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2016-12-19T22:03:38.237" LastActivityDate="2016-12-19T22:03:38.237" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19491" PostTypeId="1" CreationDate="2016-12-18T22:12:58.420" Score="1" ViewCount="48" Body="&lt;p&gt;Suppose I have an idea for a better way to go from planet A to planet B. What do I need to do to determine the delta-V required and efficiency in terms of reducing necessary shielding mass on the vehicle that actually travels? What software should I use to simulate the orbits and figure out the windows? Are there any guides to this process?&lt;/p&gt;&#xA;" OwnerUserId="17986" LastEditorUserId="13710" LastEditDate="2016-12-23T18:35:18.360" LastActivityDate="2016-12-23T18:35:18.360" Title="How can I refine an orbital plan to get from one planet to another?" Tags="|orbital-mechanics|mission-design|" AnswerCount="0" CommentCount="3" ClosedDate="2016-12-19T10:05:35.660" ContentLicense="CC BY-SA 3.0" />
  <row Id="19492" PostTypeId="1" AcceptedAnswerId="19493" CreationDate="2016-12-19T02:40:41.397" Score="11" ViewCount="670" Body="&lt;p&gt;As far as I researched the Freon compressor to cool air has to stay up right not to leak oil into the system? So how does one work in space? What do they use to cool of the air?&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="6944" LastEditDate="2016-12-19T16:06:26.620" LastActivityDate="2016-12-19T16:06:26.620" Title="How does the freon compressor work at 0 gravity?" Tags="|crewed-spaceflight|space-shuttle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19493" PostTypeId="2" ParentId="19492" CreationDate="2016-12-19T04:03:07.717" Score="12" Body="&lt;p&gt;The space shuttle used Freon in some of its cooling loops but they did not work like a home air conditioning system with expansion valves, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Schematic of &lt;a href=&quot;http://www.carsondunlop.com/news/air-conditioner-maintenance-beat-the-heat-before-it-gets-here/&quot; rel=&quot;noreferrer&quot;&gt;home AC system&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/E0STV.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/E0STV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Schematic of shuttle Freon loop:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2SZLB.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2SZLB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead the Freon was used as the heat transfer medium in two parallel cooling loops.  Redundant electrical pumps pumped the Freon through a series of heat exchangers which picked up heat either directly from components mounted on coldplates, or from heat exchangers with other thermal loops.  These other loops included the fuel cell coolant loops, the hydraulic systems, and the water loops, which cooled coldplate mounted equipment inside the crew cabin and also picked up heat from the cabin air through the cabin heat exchanger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of this heat collected by the Freon loops was rejected to space through the large radiators mounted on the inside of the payload bay doors.  Other heat rejection devices available during various mission phases were a heat exchanger with the ground service equipment, an ammonia boiler system, and a water flash evaporator system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The diagram above is extracted from the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/383445main_eclss_21002.pdf&quot; rel=&quot;noreferrer&quot;&gt;Environmental Control and Life Support Training Manual&lt;/a&gt; which you may wish to peruse for further information about the shuttle's Freon, water, and air coolant loops.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-12-19T04:17:01.670" LastActivityDate="2016-12-19T04:17:01.670" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19494" PostTypeId="1" CreationDate="2016-12-19T06:24:24.073" Score="0" ViewCount="181" Body="&lt;p&gt;Why is there no talk (that I hear) of starting colonies on the moon? The main drawback I see to the moon is the lower gravity (~one sixth of earth vs ~one third on Mars), but that could be addressed with, say, daily centripetal force sessions. &#xA;I don't have a problem with going to Mars eventually, just think we could work out a lot of bugs in this venture by starting significantly closer to home. &lt;/p&gt;&#xA;" OwnerUserId="17988" LastActivityDate="2016-12-19T18:59:34.597" Title="Destination Moon vs Mars" Tags="|the-moon|" AnswerCount="1" CommentCount="4" ClosedDate="2016-12-19T22:04:57.403" ContentLicense="CC BY-SA 3.0" />
  <row Id="19497" PostTypeId="2" ParentId="19494" CreationDate="2016-12-19T12:49:58.827" Score="5" Body="&lt;p&gt;The thin atmosphere of Mars and the 24 hour day both give a substantial benefit to Mars. The atmosphere of Mars allows for ease of getting oxygen and carbon dioxide, both of which are quite essential. The atmosphere allows it to block micro-meteors, and some radiation protection. It also allows for less pressure requirements to be placed on it, allowing for transparent materials to be used. That allows for growing plants much easier than it would be with an opaque dome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has the near 24 hour day, which makes things quite a bit easier to survive, from solar power to plants, it is preferred. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-12-19T18:59:34.597" LastActivityDate="2016-12-19T18:59:34.597" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19498" PostTypeId="1" AcceptedAnswerId="19502" CreationDate="2016-12-19T13:44:47.077" Score="46" ViewCount="5516" Body="&lt;p&gt;The bond energy of $O_2$ is &lt;a href=&quot;https://en.wikipedia.org/wiki/Bond-dissociation_energy#Homolytic_versus_heterolytic_dissociation&quot; rel=&quot;noreferrer&quot;&gt;498kJ/mol&lt;/a&gt;. The bond energy of $O_3$ is &lt;a href=&quot;https://www.quora.com/Calculate-the-average-bond-energy-of-one-O3-bond-And-what-wavelength-of-light-has-just-the-right-amount-of-energy-to-break-this-bond&quot; rel=&quot;noreferrer&quot;&gt;364kJ/mol&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means the energy to break up the particles into 3 moles of monoatomic oxygen (which then binds with fuel to release energy) is 747kJ for $O_2$ and only 364kJ for $O_3$. You're saving 127kJ/mol of $O$ using ozone instead of diatomic oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Oxyhydrogen&quot; rel=&quot;noreferrer&quot;&gt;241.8 kJ&lt;/a&gt; of energy surplus is produced burning a mole of hydrogen using (half a mole of) $O_2$. It looks like using ozone could bump this by over a half! 50% better specific energy of LFO fuel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...why aren't we doing this? Why isn't ozone at least a considerable part of the oxidizer composition? Or did I make a mistake somewhere? Or is ozone so hard to obtain or otherwise ungrateful that it's not worth the headache?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="16322" LastEditDate="2016-12-19T23:06:21.783" LastActivityDate="2016-12-20T17:33:38.417" Title="Using ozone as oxidizer" Tags="|fuel|oxidizer|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19499" PostTypeId="2" ParentId="12824" CreationDate="2016-12-19T14:26:06.963" Score="4" Body="&lt;p&gt;This will vary as we develop new technologies but there are some very lightweight cells. &lt;a href=&quot;http://www.spacefuture.com/archive/early_commercial_demonstration_of_space_solar_power_using_ultra_lightweight_arrays.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Future&lt;/a&gt; mentions 4300 W/kg, which should be the current best, but we will surely see improvements and new concepts.&lt;/p&gt;&#xA;" OwnerUserId="11655" LastEditorUserId="6816" LastEditDate="2016-12-26T03:37:50.107" LastActivityDate="2016-12-26T03:37:50.107" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19500" PostTypeId="2" ParentId="19498" CreationDate="2016-12-19T14:36:55.423" Score="22" Body="&lt;p&gt;Ozone is not stable, storing in liquid form as a solution in LOX requires stabilizers. It will oxidise most metals. A gaseous mixture of ozone and oxygen may decompose explosively. And it is poisonous in low concentrations. Nobody wants such boiloff gases at the launch pad. See &lt;a href=&quot;https://en.wikipedia.org/wiki/Ozone&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Ozone&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="13126" LastEditDate="2016-12-19T14:59:24.957" LastActivityDate="2016-12-19T14:59:24.957" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19501" PostTypeId="2" ParentId="19450" CreationDate="2016-12-19T14:52:44.397" Score="5" Body="&lt;p&gt;There was another argument against the direct ascent mode. A very large and complex rocket should be landed and started on and from the moon surface. But rockets of this size would require on earth a launch pad, a start tower, connections to a control room, a count down with many steps and a lot of personnel. But the rocket motors of the service module and the lunar module used for lunar rendezvous mode could be built very simple and reliable. No turbo pumps, no gas generators necessary, just only compressed nitrogen from tanks for the pressurization of propellants. These motors could be ignited in zero gravity. The hypergolic fuel made unlimited reignition possible.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2016-12-19T14:52:44.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19502" PostTypeId="2" ParentId="19498" CreationDate="2016-12-19T16:06:40.987" Score="55" Body="&lt;p&gt;The book 'Ignition!' tells the history of propellant research and has this to say about ozone from page 112 available &lt;a href=&quot;http://www.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For it has its drawbacks. The least of these is that it's at least as&#xA;  toxic as fluorine. (People who speak of the invigorating odor of ozone&#xA;  have never met a real concentration of it!) Much more important is&#xA;  the fact that it's unstable — murderously so. At the slightest provocation&#xA;  and sometimes for no apparent reason, it may revert explosively&#xA;  to oxygen. And this reversion is catalyzed by water, chlorine, metal&#xA;  oxides, alkalis —and by, apparently, certain substances which have&#xA;  not been identified. Compared to ozone, hydrogen peroxide has the&#xA;  sensitivity of a heavyweight wrestler.&#xA;  Since pure ozone was so lethal, work was concentrated on solutions&#xA;  of ozone in oxygen, which could be expected to be less dangerous.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Further reading of that chapter will explain more the issues with dealing with ozone. One of the major issues was that when mixed with O2 to increase stability and reduce toxicity at 75% O2 and 25% O3, it tended to separate from O2 which has a lower boiling point (at 90K compared to ozone at 161K) in effect distilling into higher and higher concentrations of ozone, so we are back to the more volatile, more toxic product. (see page 113 of Ignition for this info)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In conclusion, from page 114&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For ozone still explodes. Some investigators believe that the explosions&#xA;  are initiated by traces of organic peroxides in the stuff,&#xA;  which come from traces, say, of oil in the oxygen it was made of.&#xA;  Other workers are convinced that it's just the nature of ozone to&#xA;  explode, and still others are sure that original sin has something to do&#xA;  with it. So although ozone research has been continuing in a desultory&#xA;  fashion, there are very few true believers left, who are still convinced&#xA;  that ozone will somehow, someday, come into its own. Im not one&#xA;  of them.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2016-12-20T17:33:38.417" LastActivityDate="2016-12-20T17:33:38.417" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19503" PostTypeId="2" ParentId="19418" CreationDate="2016-12-19T17:25:39.573" Score="1" Body="&lt;p&gt;This is essentially an engineering question. Dangers, or risks as you'd typically call them in engineering, have two aspects: probability and outcome. Something can be a high risk because of a high probability, or because of a serious outcome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of anti-matter, any significant containment failure would result in a catastrophic loss, so it's an important risk regardless of probability. The main reason for this is that the amount of energy on even a small containment failure released is very high, which will likely lead to the loss of the whole containment system, and thus the instant release of all energy on-board. And this release will be in a barrage of high-energy radiation, some of which itself will be anti-matter (fast positrons). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, you probably can't entire eliminate all leakage. There will be the occasional atom escaping the containment, and violently reacting. You'll need a secondary liner which should survive the duration of the trip; this is very much sacrificial. The main goal is to handle the secondary radiation; it should harm neither the confinement nor the ship itself, all the while being lightweight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would be true even if we somehow could manage to store the antimatter &lt;em&gt;outside&lt;/em&gt; the actual spacecraft. This isn't easy, but we might be able to keep it confined just outside the ship, in the near vacuum of space. Still, that would be close to the ship, and probably close to half the anti-matter escaping confinement would still hit the hull. But that's OK, the hull was designed for that anyway.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2016-12-19T17:25:39.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19504" PostTypeId="1" CreationDate="2016-12-19T18:30:28.970" Score="12" ViewCount="2815" Body="&lt;p&gt;The CubeSat standard (&lt;a href=&quot;http://www.cubesat.org/s/cds_rev13_final2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;CubeSat Design Specification Revision 13&lt;/a&gt;) requires that CubeSats have a remove before flight (RBF) pin that &quot;shall cut all power to the satellite once it is inserted into the satellite.&quot; Most space missions require an RBF pin in order to protect the operators who integrate the spacecraft into the rocket's payload bay. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.pumpkininc.com/content/doc/forms/pricelist.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pumpkin's pricing list&lt;/a&gt; (pgs. 10, 22) includes a custom, Pumpkin RBF bracket kit and pin, and &lt;a href=&quot;https://electronics.stackexchange.com/questions/225581/remove-pin-to-close-switch&quot;&gt;this forum&lt;/a&gt; suggests some ideas for using non-switches as the RBF pin. However, I want to know: are there standard, mass-market electronic switches that come in a pin inhibit form that spacecraft developers use or are most RBF pins custom made by the spacecraft developer?&lt;/p&gt;&#xA;" OwnerUserId="6570" LastEditorUserId="-1" LastEditDate="2017-04-13T12:33:08.970" LastActivityDate="2017-06-16T04:56:21.187" Title="How are remove before flight (RBF) pins made?" Tags="|rockets|artificial-satellite|cubesat|spacecraft-development|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19505" PostTypeId="2" ParentId="19474" CreationDate="2016-12-19T23:47:24.900" Score="5" Body="&lt;p&gt;My explanation for this is in theory, since I haven't found yet a source that confirms what the reason is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that book the author writes that was a test run. Also other sources in internet says that it was a static firing testing. It doesn't explain how successful that test was. It doesn't mean that the F-1 engine ran for about 176.9 seconds which is the burning time of F-1 engine in S-IC first stage of Saturn V launch (168 seconds after liftoff + 8.9 seconds before liftoff). Maybe F-1 engine was capable of reaching 1640k pounds but there was no guarantee that it would resist for 177 seconds or even less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the time that F-1 engine was certified to be operational it had a thrust of 1500k pounds. This was the level of thrust where engineers proved that the engine was within acceptable parameters, it was 100% of its nominal value (further info about this you find &lt;a href=&quot;//space.stackexchange.com/questions/5744/why-are-rocket-engines-operating-above-100-often-considered-nominal&quot;&gt;here&lt;/a&gt;). I am suggesting that the F-1 engine with the technology of 1961 was capable of operating at 109%-110%, but it would last for only a few seconds. Static firing test are very short compared to burning time in a mission. Until now info for that test doesn't clarify what happened next, if the engine was damaged, if yes how much it was and if it could be operational or tested again. F-1 engines in those years were prototypes. Many tests and experiments were done at that time. But this was their first test, so doesn't exclude the possibility that the real purpose of static firing short test was just for the level of thrust and not trying to make the engine reliable.&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2016-12-20T13:33:19.403" LastActivityDate="2016-12-20T13:33:19.403" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19506" PostTypeId="1" AcceptedAnswerId="19507" CreationDate="2016-12-20T03:54:32.260" Score="9" ViewCount="209" Body="&lt;ul&gt;&#xA;&lt;li&gt;Liquid hydrogen is $0.071 g/cm^3$&lt;/li&gt;&#xA;&lt;li&gt;Solid  hydrogen is $0.086 g/cm^3$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Since denser fuel is desirable, has solid hydrogen been considered for use in space exploration?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the very low temperature (hydrogen melting point is 14K) to be impossible to manage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe hydrogen ice cubes in the tanks would help reduce boil off?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6816" LastEditDate="2016-12-20T05:05:49.440" LastActivityDate="2016-12-20T05:19:05.827" Title="Has solidifying liquid propellant been considered for space exploration?" Tags="|oxygen|cryogenics|hydrogen|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19507" PostTypeId="2" ParentId="19506" CreationDate="2016-12-20T05:19:05.827" Score="9" Body="&lt;p&gt;The melting point of hydrogen isn't much lower than its boiling point (6K), so the temperature isn't necessarily &lt;em&gt;that&lt;/em&gt; much of an obstacle. However, using solid fuel requires either melting it, or burning it in place. Trying to burn solid hydrogen would likely result in the whole thing flash-boiling from the radiated heat, so that's a bit of a non-starter. And having to melt fuel out of the tank is a serious problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, &lt;a href=&quot;https://en.wikipedia.org/wiki/Slush_hydrogen&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia notes that there &lt;em&gt;are&lt;/em&gt; ideas to use hydrogen slush&lt;/a&gt; for fuel, as that will, indeed, increase density a bit. It should also reduce boil-off as long as at least some of the tank is emptied early on; otherwise, the decreasing density of the melting slush is guaranteed to require more space or a &lt;em&gt;much&lt;/em&gt; higher-pressure tank.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2016-12-20T05:19:05.827" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19508" PostTypeId="2" ParentId="4646" CreationDate="2016-12-20T10:18:08.487" Score="1" Body="&lt;p&gt;How safe a space settlement is depends on how well-established it is. The very first lunar base using inflatable structures would be vulnerable to radiation and small meteors as well as a critical malfunction, such as oxygen storage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A well established base would probably be underground, at the pole, and have it’s own repair and refining capability. In time they would develop the ability to refine oxygen, water, aluminum, silicone and other local resources. The evolution from a temporary base to a self-sustaining colony could happen over time. The goal would be to be able to supply more needs locally and import less from the earth every year. That self-sufficiency can't happen in Earth orbit because there are no local resources. With self-sufficiency comes safety. Instead of fragile, lightweight, high-performance objects lifted from the earth, the structure of the colony and everything in it could be more massive and heavy duty, if it were made from lunar materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A well established lunar colony would need a completely reusable travel infrastructure. This would best be imagined in three legs, with completely reusable spacecraft. The first leg to low Earth orbit. The second leg would be and electric or nuclear rocket going from Earth orbit to lunar orbit. Finally would be a lunar lander going from the lunar surface to lunar orbit. This approach would not require a huge delta Vee for any of the rockets. That way they could be built robustly, to be practically reusable. So the Apollo model just doesn't apply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lunar settlement would be more vulnerable at first, but much safer in the long run.&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-12-20T10:18:08.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19509" PostTypeId="2" ParentId="9776" CreationDate="2016-12-20T11:23:43.890" Score="4" Body="&lt;p&gt;The one big advantage Mars has over the moon is that it has a very wide range of chemicals available there. Probably everything we would eventually need is available on Mars. This is not the case with the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However a Settlement on Mars and a settlement on the Moon would be essentially the same thing: large structures underground. Mars has almost no more radiation protection than the moon. The Martian atmosphere is so close to a vacuum, it is of no help in terms of the pressure differential around the spacesuit. The fact that a Martian day approximates an Earth day would not matter much since everybody would be living underground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the very long run a martian colony could live without imports from elsewhere. A lunar colony would probably always need imports. However a lunar colony is also much more capable of doing trade. With lower gravity, and no atmosphere to create aerodynamic demands on spacecraft, a lunar settlement is more accessible. The moon could participate in commerce involving exploitation of asteroids much more easily than a martian settlement could. If large orbiting structures are to be built, it’s not hard to imagine lunar aluminum and glass being more economical than lifting the same stuff from the earth. That gives the lunar colony a product to sell in exchange for it’s imports like carbonaceous chondrite asteroid material for agriculture. It’s hard to picture anything that martians could economically export.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While a martian settlement could become self sufficient, a lunar settlement would probably be much more a part of a moon / earth / asteroid economic system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recent studies have speculated that a lunar cave, either man made or natural, could be quite large. &#xA;&lt;a href=&quot;http://www.airspacemag.com/daily-planet/cave-living-moon-180961494/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.airspacemag.com/daily-planet/cave-living-moon-180961494/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-12-20T11:23:43.890" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19510" PostTypeId="2" ParentId="19360" CreationDate="2016-12-20T11:37:33.460" Score="5" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keyword: &lt;a href=&quot;https://aviation.stackexchange.com/questions/1640/what-is-the-difference-between-oat-rat-tat-and-sat/1642#1642&quot;&gt;Ram Rise&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we neglect temperature, you can technically have a scramjet that gets enough air by going fast enough, maintaining proper climb rate so that lift, drag and intake air remain roughly constant as speed rises.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This all breaks if we look at how heating works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Ram~Rise=SAT\times0.2\times{M}^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$RAT=SAT+k\times{Ram~Rise}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SAT - Static Air Temperature. As read from immobile thermometer. This will be of order of 200-300K. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;M - airspeed in Mach. Orbital speed is around Mach 23.2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;k - Ram coefficient, a shape adjustment factor. It will not play a very big role here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RAT - the temperature as &quot;experienced&quot; by the craft surface&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you substitute the numbers, our baseline starts somewhere around 25,000 Kelvins. Quite likely more. Ram coefficient may reduce this by an order of magnitude maybe, but we're still exceeding melting temperature of tungsten. In short, our craft will totally overheat and burn up. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:54.153" LastActivityDate="2016-12-20T11:37:33.460" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19511" PostTypeId="1" AcceptedAnswerId="19512" CreationDate="2016-12-20T16:31:04.467" Score="7" ViewCount="594" Body="&lt;p&gt;One of the drawbacks of jet engines is that they suffer from turbo lag, whereby a throttle change will take some amount of time to translate to a change in thrust due to the momentum of the turbine itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've never heard liquid-fuel rocket engines discussed with this drawback, even though they are fed by turbopumps.  Isn't turbo lag a pandemic problem in all turbomachinery, and if so, why does it not affect the ability of a liquid-fueld rocket to respond quickly to throttle changes?&lt;/p&gt;&#xA;" OwnerUserId="17722" LastActivityDate="2017-09-06T21:58:48.423" Title="Rocket Throttle vs. Jet Throttle" Tags="|rockets|engine-design|turbomachinery|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19512" PostTypeId="2" ParentId="19511" CreationDate="2016-12-20T18:33:32.857" Score="9" Body="&lt;p&gt;Most large liquid-fueled engines, if they throttle at all, follow a simple throttle program, either with a single large step-down or gradual reduction to manage acceleration limits as a stage gets lighter. In these cases, a lag time of a second or two is of little consequence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The major exception to this is in engines used for powered descent such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D&quot; rel=&quot;noreferrer&quot;&gt;Merlins&lt;/a&gt; on SpaceX's &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9&quot; rel=&quot;noreferrer&quot;&gt;Falcon 9&lt;/a&gt;. In this case, the descent algorithm must &quot;plan ahead&quot; knowing the effect of throttle lag. Assuming that the system has access to good test data regarding the throttle lag, it's not too difficult to do this under computer control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Turbine lag is only a problem in jet engines because, at the points when throttle management is most time sensitive (takeoffs and, especially, landings), the throttle is being driven by a human. Humans are pretty good at this -- thousands of airliners land safely every day -- but computers are better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket engines that have been landed by humans under manual control, as far as I know, have all been smaller pressure-fed engines with no turbopump lag to deal with.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-06T21:58:48.423" LastActivityDate="2017-09-06T21:58:48.423" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19513" PostTypeId="2" ParentId="17853" CreationDate="2016-12-20T19:31:17.433" Score="7" Body="&lt;p&gt;Let me be clear: Rockets are not LEGO elements.  Real life is nothing like KSP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The capability to selectively include multiple SRBs would have to be designed into a vehicle from the very beginning of its design.  The presence or absence of boosters, especially ones as large as SLS's, means changes at just about every level of the program: structural design, launch trajectory, control systems, sizing of stages, payload environment, abort capability, ground segment design (up to and including building a whole new launch pad to support the massive weight).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A four-booster SLS would not be SLS -- it would be a wholly different rocket.  As such, this question isn't really that meaningful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, adding boosters would mean you have more liftoff thrust, which means that in general, you can launch heavier payloads to orbit.  It's not really that simple though, and you play a game of diminishing returns, wherein more is simply more, not better.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2016-12-20T19:31:17.433" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19514" PostTypeId="1" CreationDate="2016-12-20T23:23:50.417" Score="7" ViewCount="641" Body="&lt;p&gt;I am trying to understand a basic formula in a Celestial Mechanics &lt;a href=&quot;http://farside.ph.utexas.edu/teaching/336k/Newtonhtml/node113.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference&lt;/a&gt;. The formula is for the position of the Moon in a geocentric frame. The reference claims in line (981) that this should be given by &#xA;$$ \vec{r}_M \approx a_M ( \cos \lambda_M, \sin \lambda_M, \iota_M \sin(\lambda_M - \bar{\omega}_M) ),$$&#xA;where $\lambda_M$ is the Moon's ecliptic longitude, $\bar{\omega}_M$ is the lunar ecliptic longitude of the ascending node, $\iota_M$ is the small inclination of the Moon's orbit with respect to the ecliptic, and $a_M$ is the distance from the Moon to the Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that in the frame in question, the Earth's angular velocity is given by $\vec{\omega} = \omega (-\sin(\theta) \sin(\phi), \sin(\theta) \cos(\phi), \cos(\theta))$. $\phi$ is the angle of diurnal rotation, $\theta$ is the angle between the Earth's diurnal rotation axis and the ecliptic. &lt;strong&gt;Can someone show me the derivation of $\vec{r}_M$ in terms of explicit rotation matrices&lt;/strong&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can quickly reproduce the formula for the Earth's angular velocity about its axis $\vec{\omega}$. I assume that the Earth's rotation axis is about $(0,0,1)$ to begin with, and then apply a clockwise rotation of $\theta$ about the $x$-axis, followed by a counterclockwise rotation about the $z$-axis to obtain &#xA;$$ \vec{\omega} = \left( \begin{matrix} \cos \phi &amp;amp; -\sin \phi &amp;amp; 0 \\ \sin \phi &amp;amp; \cos \phi &amp;amp; 0 \\ 0 &amp;amp; 0 &amp;amp; 1 \end{matrix} \right) \left( \begin{matrix} 1 &amp;amp; 0 &amp;amp; 0 \\ 0 &amp;amp; \cos \theta &amp;amp; \sin \theta \\ 0 &amp;amp; -\sin \theta &amp;amp; \cos \theta \end{matrix} \right) \left( \begin{matrix} 0 \\ 0 \\ 1 \end{matrix} \right) = \left( \begin{matrix} - \sin \phi \sin \theta \\ \cos \phi \sin \theta \\ \cos \theta \end{matrix} \right), $$&#xA;precisely as claimed. How can we find the result for the lunar position $\vec{r}_M$? &lt;/p&gt;&#xA;" OwnerUserId="18006" LastEditorUserId="6816" LastEditDate="2016-12-22T22:46:51.963" LastActivityDate="2016-12-22T22:46:51.963" Title="Understanding formula for geocentric position of Moon" Tags="|orbital-mechanics|mathematics|celestial-mechanics|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19515" PostTypeId="1" AcceptedAnswerId="23035" CreationDate="2016-12-21T07:05:12.387" Score="7" ViewCount="633" Body="&lt;p&gt;The cool GIF on the NASA JPL Cassini resources page &lt;a href=&quot;https://saturn.jpl.nasa.gov/resources/7393/&quot; rel=&quot;nofollow noreferrer&quot;&gt;All Orbits (2004-2017): The Ball of Yarn&lt;/a&gt; is quite beautiful to look at, but it also hints at some beautiful engineering and orbital mechanics as well. Take a moment to look at the real thing there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the page &lt;a href=&quot;https://saturn.jpl.nasa.gov/news/59/saturn-tour-dates-2016/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn tour dates 2016&lt;/a&gt;, in the next few days Cassini will execute &quot;short burn number 468&quot; after it starts &quot;rev 254&quot; of its orbit around Saturn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my recent question &lt;a href=&quot;https://space.stackexchange.com/q/19198/12102&quot;&gt;Is Cassini's 183rd burn - or the associated orbital mechanics - some kind of record?&lt;/a&gt; I've noted that a &lt;a href=&quot;https://i.stack.imgur.com/qcnxG.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;popular news article&lt;/a&gt; states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Cassini’s first graze of the F ring is scheduled to occur on December 4th. During that pass, Cassini will also briefly ignite its main engine to help fine-tune its orbit. It will be the 183rd time the spacecraft turns on its engine, as well as its last time, if all goes according to plan.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm wondering how to reconcile the &quot;short burn&quot; count &gt; 460 with the number of times the engine has been turned on &amp;lt; 190.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is there a place where &lt;em&gt;all of&lt;/em&gt; Cassini's propulsive maneuvers post-insertion (big and small, or at least big) are tabulated? Perhaps as a list of time and delta-v?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I looked at the orbital elements available in &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; and I can clearly see when many (most?) of the major changes in orbit happen, but just looking for changes here, or in changes in velocity that can't be explained by Saturn's gravity is a sketchy way to do this considering the complexity of this mission. Also, that would not distinguish propulsive maneuvers from gravitational deflections by Cassini's many close flyby's of Saturn's many satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qcnxG.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qcnxG.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Teaser GIF to get you to enjoy &lt;a href=&quot;https://saturn.jpl.nasa.gov/resources/7393/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the real thing &lt;strong&gt;here&lt;/strong&gt;&lt;/a&gt; (since even the low-res version is larger than the stackexchange imgur's limit of 2 &lt;s&gt;MB&lt;/s&gt;  oops! &lt;a href=&quot;https://space.stackexchange.com/questions/19261/#comment50812_19261&quot;&gt;MiB&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Some of the orbital elements of the Cassini spacecraft around the planet Jupiter from &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt;. Discrete changes (steps) in the orbital elements hint at propulsive maneuvers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Zhuuq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Zhuuq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pVapN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pVapN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;EC: Eccentricity e&#xA;IN: Inclination i (deg)&#xA;OM: Longitude of Ascending Node OMEGA (deg)&#xA;W:  Argument of Perifocus w (deg)&#xA;A:  Semi-major axis a (km)&#xA;&#xA;*******************************************************************************&#xA;Ephemeris / WWW_USER Tue Dec 20 20:03:00 2016 Pasadena, USA      / Horizons&#xA;*******************************************************************************&#xA;Target body name: Cassini (spacecraft) (-82)      {source: cassini}&#xA;Center body name: Saturn (699)                    {source: cassini}&#xA;Center-site name: BODY CENTER&#xA;*******************************************************************************&#xA;Start time      : A.D. 2004-Jun-01 00:00:00.0000 TDB&#xA;Stop  time      : A.D. 2017-Sep-29 00:00:00.0000 TDB&#xA;Step-size       : 720 minutes&#xA;*******************************************************************************&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-08-25T13:58:38.407" LastActivityDate="2018-08-25T13:58:38.407" Title="Unravelling Cassini's &quot;ball of yarn&quot; orbit around Saturn, tabulation of propulsive maneuvers?" Tags="|orbital-maneuver|cassini|jpl-horizons|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="19516" PostTypeId="2" ParentId="18778" CreationDate="2016-12-21T09:48:10.790" Score="3" Body="&lt;p&gt;I went through the &lt;a href=&quot;http://www.itu.int/ITU-R/go/space-plans/en&quot; rel=&quot;nofollow noreferrer&quot;&gt;ITU's Space Plans&lt;/a&gt; for information on this. They are the internationally agreed authoritative body (some exceptions, perhaps, but most spacefaring countries have signed up) The wording is actually very simple (despite there being extensive documentation - 430 pages!) as it is developed from the ITU's more general documentation on radio frequency usage on land, sea and air so I've pulled out some of the most relevant:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Article 4: Assignment and use of frequencies&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;4.3  Any new assignment or any change of frequency or other basic characteristic of an existing assignment (see Appendix 4) shall be made in such a way as to avoid causing harmful interference to services rendered by stations using frequencies assigned in accordance with the Table of Frequency Allocations in this Chapter and the other provisions of these Regulations, the characteristics of which assignments are recorded in the Master International Frequency Register.&lt;/p&gt;&#xA;&lt;p&gt;4.4  Administrations of the Member States shall not assign to a station any frequency in derogation of either the Table of Frequency Allocations in this Chapter or the other provisions of these Regulations, except on the express condition that such a station, when using such a frequency assignment, shall not cause harmful interference to, and shall not claim protection from harmful interference caused by, a station operating in accordance with the provisions of the Constitution, the Convention and these Regulations.&lt;/p&gt;&#xA;&lt;p&gt;4.5  The frequency assigned to a station of a given service shall be separated from the limits of the band allocated to this service in such a way that, taking account of the frequency band assigned to a station, no harmful interference is caused to services to which frequency bands immediately adjoining are allocated.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So allocation of frequencies is the main control here to avoid collisions. Section 1 deals with interference and there are further provisions, for example:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;15.4 a) locations of transmitting stations and, where the nature of the service permits, locations of receiving stations shall be selected with particular care;&lt;/p&gt;&#xA;&lt;p&gt;15.5 b) radiation in and reception from unnecessary directions shall be minimized by taking the maximum practical advantage of the properties of directional antennas whenever the nature of the service permits;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;For specific frequencies it's worth getting a copy and reading The Table of Frequency Allocations, Regional Allocations, and the full range of frequencies detailed in Chapter II.&lt;/p&gt;&#xA;&lt;p&gt;Countries and regions are given allocations, and can decide where in their ranges they want to transmit. They need to tell the ITU their choice, so it can be incorporated in the allocation table.&lt;/p&gt;&#xA;&lt;p&gt;If they need to use a previously allocated frequency, this can be arbitrated through the ITU, but each country is expected to stay within their allocated ranges.&lt;/p&gt;&#xA;&lt;p&gt;Interestingly though, in emergency, 4.9 comes into play:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;4.9  No provision of these Regulations prevents the use by a station in distress, or by a station providing assistance to it, of any means of radiocommunication at its disposal to attract attention, make known the condition and location of the station in distress, and obtain or provide assistance.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-12-21T16:17:49.533" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19517" PostTypeId="2" ParentId="19209" CreationDate="2016-12-21T10:51:31.920" Score="1" Body="&lt;p&gt;I see the caveat of your question... and simultaneosly, I don't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem: randomly, non-deterministically shifting center of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solution: as craft tries to turn, this is detected and compensated by engine gimbal, so that the thrust vector always points through CoM, unless it's compensating for the tilt right now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side effect: discrepancy between heading and bearing; thrust misaligned with ship's geometrical axis (which is misaligned with the misplaced CoM), propelling it in a different direction than originally intended.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Resulting problem: how to determine the value of the error - shift in velocity vs intended?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solution: using exactly the same software that is used to drive the gimbal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You know the value of thrust (force), you have a very good idea about mass (fuel mass flow and ship's total mass), and you know the direction of thrust at any point of time, along with direction of the exhaust-CoM vector (calculated from shift vs stars, necessary to drive the gimbal operation.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Split the thrust vector into component vectors along the exhaust-COM axis and perpendicular. Integrate force $\rightarrow$ acceleration resultant from the parallel component over time to get the velocity change; integrate (2nd degree) displacement from the perpendicular component (perpendicular component of velocity afterwards will be near zero since the rotation is extinguished, and never allowed to grow significantly).&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-12-21T10:51:31.920" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="19518" PostTypeId="1" CreationDate="2016-12-21T12:41:38.590" Score="11" ViewCount="562" Body="&lt;p&gt;I was searching for comparisons between space rockets regarding their payload mass in low Earth orbit and Earth escaping. First one to see capability of the space booster (the first stages) and second one to see results and efficiency of upper stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparisons could be found in &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Comparison_of_orbital_launch_systems&quot; rel=&quot;noreferrer&quot;&gt;wikipedia&lt;/a&gt;, but these aren't a fair comparison since low orbit heights and inclinations for these rockets are different. If we compare something should be used the same standard. Many USA rockets are launched in 28.7° (or 28.5° or 28°) inclination, and their low orbit values are for 185 km or 200 km. Russian rockets are launched from Baikonour in 51.6° and values for low orbit are mostly from 200km or 220km. ESA rockets are launched from Kourou site at 5° inclination. Soyuz rocket is launched from both these sites ( Baikonour 51.6°, Kourou 5°). We could understand how much influences inclination for this Soyuz from &lt;a href=&quot;http://www.russianspaceweb.com/soyuz2_lv.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. But with Soyuz case we are lucky since it is launched in different inclinations. Question is what about all other rockets. Values would be different for low Earth orbits in 185 km - 200 km - 220 km, and in different inclinations. Maybe their influence would not be to much but still the comparison is not real and fair.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For low Earth orbit we need the same orbit and inclination. For Earth escape were orbit's heigh doesn't matter still the inclination should be the same. Is there any program or website that could compare rockets with the same standard and in the same conditions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until now i found this site &lt;a href=&quot;http://www.silverbirdastronautics.com/LVperform.html&quot; rel=&quot;noreferrer&quot;&gt;silverbirdastronautics&lt;/a&gt; were we could change inclination and orbit's height, but this is not a reliable site because payload values are differnt from official results for those rockets and it hasn't a high level of accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could we make a fair comparison of space rockets regarding their low Earth orbit and Earth escape payloads?&lt;/p&gt;&#xA;" OwnerUserId="17400" LastEditorUserId="17400" LastEditDate="2016-12-21T12:50:20.063" LastActivityDate="2020-09-18T19:07:47.217" Title="How could we make a fair comparison between space rockets regarding their payload mass in low Earth orbit and Earth escaping?" Tags="|rockets|low-earth-orbit|launch-site|payload|escape-velocity|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19519" PostTypeId="2" ParentId="19088" CreationDate="2016-12-21T17:26:09.457" Score="1" Body="&lt;p&gt;An anti-matter engine could potentially be very efficient, as you have mentioned it is extremely energy efficient for its mass. The tradeoff, however, is that you need a lot larger engine to contain the antimatter fuel. The only theory for trapping the fuel is to use careful magnetic fuel. That requires a significant size of a field typically, and thus requires very careful manipulation of the charge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, a similar proposal was &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Orion&lt;/a&gt;, which proposed to use nuclear bombs to explode forward a spacecraft. A single launch could launch tens of thousands of tons, for a rocket that was of compatible height to, say, the Saturn V. They have their issues, landing in particular is quite difficult, but they could be overcome. Antimatter would be even more efficient, and easier to control than it would be otherwise.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2016-12-21T17:26:09.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19520" PostTypeId="2" ParentId="17853" CreationDate="2016-12-21T17:46:26.633" Score="5" Body="&lt;p&gt;Some preliminary numbers.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going from &lt;a href=&quot;http://www.braeunig.us/space/specs/sls.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site&lt;/a&gt;, each SRB has around 16000 kN of thrust and a gross liftoff mass of 731885 kg.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Leaving everything else equal (which as Tristan points out isn't realistic, but I'm not qualified to do realistic), adding 2 boosters gives you about a 57% increase in gross liftoff mass, 80% increase in total liftoff thrust, and about a 16% increase in liftoff T/W.  You also gain an additional 27% or so of cross-sectional area, so you get a proportional increase in aerodynamic drag.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So using liftoff T/W as a &lt;em&gt;very&lt;/em&gt; rough proxy, you're looking at 16% or less increase in payload for a given mission (for SLS Block 1, that means going from 70 t to a little over 81 t).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt; Note that Block 2 offers an 85% increase in payload (130 t) by still using 2 boosters (lighter and more powerful than the current set) and a more powerful upper stage.  Improving the upper stage gets you the most bang for your buck.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastEditorUserId="12922" LastEditDate="2016-12-21T18:37:57.177" LastActivityDate="2016-12-21T18:37:57.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19521" PostTypeId="2" ParentId="18508" CreationDate="2016-12-21T20:58:08.027" Score="2" Body="&lt;p&gt;There are also other considerations that real spacecraft need to deal with, like gee loading.  An initially high TWR will mean that your TWR at MECO will be even higher, potentially causing payload damage.  Also, a lower TWR means MaxQ will be at a lower pressure.  In real life, engine burn times are also much longer (usually) than in KSP, and generally that relates to a lower initial TWR.&lt;/p&gt;&#xA;" OwnerUserId="8481" LastActivityDate="2016-12-21T20:58:08.027" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19522" PostTypeId="1" AcceptedAnswerId="20799" CreationDate="2016-12-21T22:03:10.553" Score="8" ViewCount="440" Body="&lt;p&gt;This year Santa has decided to try a new strategy to deliver presents which he hopes will require less physics-defying magic, since magic is in short supply these days. Instead of personally flying gifts to every child in the world in one night, Santa would like to launch one or more rockets from the North Pole that enter into an orbit around the Earth and then drop presents over each country in the world. From there locally stationed elves will distribute the presents to each house.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Don't worry about the propulsion, weight, or distribution. Santa has those aspects covered. You can assume that the payload starts out at a miraculous 195 kg. A kilo is dropped for each country. If you require multiple rockets, the payload must be appropriately divided between them (1 kg per drop). You can drop anywhere over the country. You don't really have any time constraints. Santa wants all the drops done before the first country experiences midnight December 25, but that shouldn't be a problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Optimizing for both number of orbits and number of burns (Santa wants to minimize $orbits \times burns$), what is the minimum number of orbits required to perform all 195 drops, where an orbit is one revolution of one rocket around the Earth?&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2018-02-22T19:06:16.143" LastActivityDate="2018-02-22T19:06:16.143" Title="How many orbits will it take for Santa to deliver all the presents?" Tags="|orbital-mechanics|reentry|orbital-maneuver|santa-claus|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19523" PostTypeId="1" AcceptedAnswerId="19536" CreationDate="2016-12-21T22:10:58.873" Score="3" ViewCount="511" Body="&lt;p&gt;As I know, Space Shuttle used the same amount of fuel for every mission. And used OMS for finishing orbit. Then how it is possible that, for example STS-1 which hadn't payload and was in roughly 266km*271km orbit, used same amount of fuel as STS-31 who carried Hubble Telescope (approximately 11t) into orbit of 585km*615km?&lt;/p&gt;&#xA;" OwnerUserId="13559" LastActivityDate="2016-12-22T15:51:57.047" Title="Space Shuttle fuel" Tags="|fuel|space-shuttle|hubble|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19524" PostTypeId="1" CreationDate="2016-12-21T22:43:27.927" Score="0" ViewCount="282" Body="&lt;p&gt;NASA has the &lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/One_Small_Step_Comparison_720p.mov&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; of Neil Armstrong on the moon. I'm interesting in the technology how the signal was created and transmitted. There are some details about it in Wikipedia but I'd like to know in-depth. Could it be a feasible project for me to recreate the NASA technology using my own hardware and methods?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or at least learn how it was done in detailed theory, electronically, physically and mathematically?&lt;/p&gt;&#xA;" OwnerUserId="18012" LastActivityDate="2016-12-22T03:31:46.150" Title="Details of Apollo 11 video transmission" Tags="|the-moon|communication|radio-communication|apollo-11|" AnswerCount="2" CommentCount="0" ClosedDate="2016-12-22T03:47:22.530" ContentLicense="CC BY-SA 3.0" />
  <row Id="19526" PostTypeId="2" ParentId="19524" CreationDate="2016-12-21T23:39:41.293" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Could it be a feasible project for me to recreate the NASA technology using my own hardware and methods?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sure, but beware the costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1960's cameras where impressive beasts, made from components (camera tubes) that no one uses these days anymore. To top this up, NASA certainly didn't just buy a stock TV camera and put a sticker on it, they certainly put thought into weight reduction and operation in space and with astronaut fingers...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Or at least learn how it was done in detailed theory, electronically, physically and mathematically?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'd recommend you start out by studying electrical engineering, which is the theory of electronics, the physics behind electronic devices and the math behind practical signals. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have the basics of linear electrical networks down, it's probably worth taking a while to study signal theory, after which you turn to understanding tube devices (something that is not commonly taught these days anymore – tubes aren't used for these kind of things anymore, but are typically specialty devices for high-power microwaves etc).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your signal theory will nicely prepare you for both understanding what the electrical signals coming out of the camera tube itself are, as well as understanding the basics of RF and RF communications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A bit of self-study on legacy analog TV standards will probably be easy after that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, you would probably be able to understand the functionaly of each (ok, most) components in such a camera in a detailed way.&lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastActivityDate="2016-12-21T23:39:41.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19528" PostTypeId="2" ParentId="14636" CreationDate="2016-12-22T02:16:06.923" Score="1" Body="&lt;p&gt;The book Rocket Propulsion Elements, Sutton, has a sketch of a missile that use solid grain to generate gas that pressurize a tank of liquid propellant separated from the hot gas with a piston&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/l0nWt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/l0nWt.png&quot; alt=&quot;Propulsion system pressurized by solid grain&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The advantage is that a missile can be stored for much longer time, it is not necessary to worry about the wear suffered by the turbo pumps or possible leaks of a pressurized system to helium. I believe that the drawback is the weight of the pistons, and a eventuali leak betwen the hot gases and the liquid propelant could cause an explosion. &lt;/p&gt;&#xA;" OwnerUserId="18015" LastEditorUserId="18015" LastEditDate="2016-12-22T21:37:29.310" LastActivityDate="2016-12-22T21:37:29.310" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19529" PostTypeId="1" AcceptedAnswerId="19531" CreationDate="2016-12-22T03:12:13.527" Score="3" ViewCount="185" Body="&lt;p&gt;Suppose large settlements are established on the moon with lots of people and industrial activity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on what we know, what chemicals that are needed for human life and industry might be lacking? We know the basic breakdown of rock composition from Apollo: oxygen, silicon, iron, calcium, aluminum, magnesium are there. And we are trying to find out how much water might be at the poles in dark zones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about carbon and nitrogen that are needed for agriculture and plastics? What other chemicals would be needed that have not been found in exploitable concentrations?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="6816" LastEditDate="2016-12-22T04:41:35.040" LastActivityDate="2016-12-22T04:41:35.040" Title="Chemical resources on the moon in relation to human occupation" Tags="|the-moon|colonization|mining|natural-resources|geology|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19530" PostTypeId="2" ParentId="19524" CreationDate="2016-12-22T03:31:46.150" Score="2" Body="&lt;p&gt;Their system was very proprietary. Here is an interesting historical note on that. When the &quot;Great Step for Man&quot; was taking place, the video was being displayed on a special monitor (while being recorded in the background.) The TV News crew could not process the direct signal, so they just pointed their cameras at the monitor. That grainy image shot off the screen is what everyone has seen. The original was much better. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in a terrible bureaucratic faux pas, NASA not only never transcoded the original video, they accidentally ERASED it. That’s why the only surviving video is the grainy TV news video shot off NASA’s screen. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2016-12-22T03:31:46.150" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19531" PostTypeId="2" ParentId="19529" CreationDate="2016-12-22T04:27:12.227" Score="4" Body="&lt;p&gt;The Moon is depleted of carbon, and what little carbon there is is in form of nonorganic compounds. Except for CO&lt;sub&gt;2&lt;/sub&gt; (which is nonorganic), life likes to work with organic compounds as opposed to nonorganic compounds. The Moon is also depleted of nitrogen (not surprising), but also potassium, sodium, and zinc. Potassium and sodium are critical elements for life. Zinc is an essential trace element.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-12-22T04:27:12.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19533" PostTypeId="2" ParentId="19523" CreationDate="2016-12-22T10:24:23.483" Score="2" Body="&lt;p&gt;Three things I can think of:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fuel loaded does not equal fuel used.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The shuttle was loaded with nearly the same amount of fuel for every launch (that being &quot;full tanks&quot;), but that doesn't mean the same amount was consumed.  For a normal launch, main engine cut-off would be determined by the orbiter reaching a target velocity.  Any fuel remaining in the tank at that point is discarded. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For some missions, there would have been a wider margin between necessary and loaded fuel.  For other missions, the margin was very tight.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Different orbital inclination&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Besides altitude, different inclinations require different amounts of fuel to reach.  From Florida, any inclination other than 28.5 degrees requires more fuel.  For Hubble, STS-31 launched into this most efficient orbit.  I don't know what the mission reason was, but the STS-1 orbit was 40 degrees.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Flight profile&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Early shuttle flights flew a &quot;standard insertion&quot; profile, while later flights (including STS-31) flew direct insertion, which was more efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the flight profiles were different, comparing OMS burns is difficult, but STS-1 used 161s of OMS and STS-31 used 305s of OMS to reach orbit.  So at least some of the difference is there.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2016-12-22T10:24:23.483" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19534" PostTypeId="2" ParentId="18778" CreationDate="2016-12-22T12:32:06.297" Score="2" Body="&lt;p&gt;The procedure for coordination is &lt;a href=&quot;http://www.itu.int/en/ITU-R/space/plans/Pages/MIFR.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;specified&lt;/a&gt; in the links provided by Rory Alsop. To recap:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A space organization submits a request to the Radiocommunications Bureau of the ITU.  &lt;/li&gt;&#xA;&lt;li&gt;The RB examines the request, makes sure there are no collisions with current frequency allocations. &lt;/li&gt;&#xA;&lt;li&gt;On approval, the space organization is free to use this frequency. The Master Register (list of allocated frequencies) is updated and published. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When an administration intends to bring into use its assignment(s) in the appropriate Regional Plan or Regions 1 &amp;amp; 3 List, it notifies this to the Bureau. The procedure of Article 5 of Appendices 30 and 30A is then applied.  The Bureau examines the submission to assure that the information received is complete, that the data elements are in conformity with Appendix 4, and that the notified characteristics comply with those of the entries in the Plans or Lists. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At first glance, this applies to Earth-orbiting satellites only. I'm looking for more details. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ITU has reserved several slices of the radio spectrum for deep-space use. In the US, use of these slices is coordinated by &lt;a href=&quot;http://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201B.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Through international agreements, the International Telecommunications Union (ITU) allocates and regulates portions of the frequency spectrum for both commercial and government use. The primary objective of the ITU is to establish regulatory procedures for the coordinated use of frequencies by those agencies permitted to operate in the allocated bands. The&#xA;  ITU has allocated certain bands to deep space (Category B) research. In some cases, the deep space missions may be required to conditionally share a frequency band between multiple users in the same band.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Consultative Committee for Space Data Systems (CCSDS) is an international organization for space agencies interested in mutually developing standard transmission and data handling techniques to support space research, including space science and applications. As a member of the CCSDS, NASA has submitted recommendations for various space systems applications.  &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The National Telecommunications and Information Administration (NTIA), an&#xA;  agency of the U.S. Department of Commerce, is the Executive Branch's principal authority on domestic and international telecommunications and information technology issues. During the planning phase of all missions using the DSN, the proposed operating frequencies and other operating parameters are reviewed by the NTIA for approval through the System Review process. The NTIA evaluations are based upon the technical and regulatory criteria for the efficient and coordinated use of the frequency spectrum by NASA missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-12-22T13:15:17.217" LastActivityDate="2016-12-22T13:15:17.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19535" PostTypeId="2" ParentId="18508" CreationDate="2016-12-22T12:42:37.740" Score="28" Body="&lt;p&gt;It is no longer generally considered optimal to use a launch TWR of 1.5-3 in KSP. A little background: In KSP players who were optimizing for launcher weight would go for a TWR of 2.0 or higher, so players wishing bragging rights of &quot;I launched 100t into orbit for only X tonnes of rocket&quot; would go for a TWR of about 2.0 or even higher and this worked because a high TWR is extremely effective at mitigating gravity drag, a rocket with a TWR of 1.25 loses 80% of its thrust to gravity, while a rocket with a TWR of 2.0 loses only 50% of its thrust to gravity, a much greater fraction of the thrust is thus available for building velocity rather than fighting gravity and that makes the rocket significantly more efficient in terms of weight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as KSP development continued, career mode was released and the cost of rocket parts was balanced. At this time &lt;em&gt;cost per tonne to orbit&lt;/em&gt; became a new and more legitimate metric for judging rocket performance. In KSP when you want to build a rocket which launches payload as cheaply as possible, often a launch TWR of around 1.25-1.3 is exactly what you want to use - at least for rockets which aren't using SRBs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, why is this the optimal TWR for making cost-effective rockets both in KSP and IRL? What it comes down to is &lt;em&gt;rocket engines are expensive&lt;/em&gt; but &lt;em&gt;fuel and tankage is cheap&lt;/em&gt;, relatively speaking. Thus it is desirable to pile almost as much fuel onto the engine as it can lift to get maximum burn time out of the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If in real life, there was some incentive to make launch vehicles which were as light as possible rather than as cheap as possible they would also have a higher TWR.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2016-12-22T19:36:49.120" LastActivityDate="2016-12-22T19:36:49.120" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19536" PostTypeId="2" ParentId="19523" CreationDate="2016-12-22T15:28:14.580" Score="5" Body="&lt;p&gt;If the question refers to the Orbital Maneuvering System (OMS) propellants, as &lt;a href=&quot;https://space.stackexchange.com/users/12422/digger&quot;&gt;Digger&lt;/a&gt; commented, it is based on a false assumption that the same amount of propellant was used for every mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many old Space Shuttle Mission reports are available on the &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp&quot; rel=&quot;nofollow noreferrer&quot;&gt;NTRS&lt;/a&gt; server.&#xA;Here are two examples showing the different usages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19940023717&amp;amp;hterms=sts-57%20space%20shuttle%20mission%20report&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3Dsts-57%2520space%2520shuttle%2520mission%2520report%26Ntx%3Dmode%2520matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-57&lt;/a&gt;: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/txJM0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/txJM0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19970016099&amp;amp;hterms=sts-81%20shuttle%20mission%20report&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3Dsts-81%2520shuttle%2520mission%2520report%26Ntx%3Dmode%2520matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-81&lt;/a&gt;:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/0azvp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0azvp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-12-22T15:51:57.047" LastActivityDate="2016-12-22T15:51:57.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19537" PostTypeId="1" AcceptedAnswerId="19548" CreationDate="2016-12-22T15:59:58.863" Score="33" ViewCount="5238" Body="&lt;p&gt;Will satellite imagery be able to see the backs of big animals such as lions, tigers, elephants, horses or zebras from space when they venture out in the open? I've not been able to find satellite imagery that captures such animals in the open.&lt;/p&gt;&#xA;" OwnerUserId="18021" LastActivityDate="2021-10-15T00:48:57.890" Title="Is it possible to see animals from space?" Tags="|artificial-satellite|earth-observation|image-processing|" AnswerCount="7" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19538" PostTypeId="2" ParentId="19537" CreationDate="2016-12-22T16:16:28.443" Score="35" Body="&lt;p&gt;Yes, but it requires exceptional resolution. The &lt;a href=&quot;https://en.wikipedia.org/wiki/GeoEye-1&quot; rel=&quot;noreferrer&quot;&gt;GeoEye-1&lt;/a&gt; satellite has 0.41 m resolution, and it &lt;a href=&quot;https://www.itc.nl/Pub/News-overview/in2015/in2015-january/Animal-populations-counted-using-satellites-for-the-first-time.html&quot; rel=&quot;noreferrer&quot;&gt;has been used to count animals&lt;/a&gt;. Here you can see a picture it took of wildebeests:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hu0gd.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hu0gd.png&quot; alt=&quot;GeoEye-1 Satellite Image of wildebeest population&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Image Credit: ITC&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The black and white image is the satellite image (though GeoEye-1 can do color imagery as well in 1.65 m resolution). The tiny dots peppered across the park are wildebeests. &lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2016-12-22T16:16:28.443" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19539" PostTypeId="1" AcceptedAnswerId="19542" CreationDate="2016-12-22T17:14:49.777" Score="5" ViewCount="2304" Body="&lt;p&gt;Here is a brief dramatic of the Space Shuttle launch and reentry:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch :&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You get on this plane like thing that is strapped to an orange fuel tank that looks more like a &lt;strong&gt;huge bomb&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;li&gt;The fuel tank is strapped to &lt;strong&gt;2 missile&lt;/strong&gt; looking SRBs that actually have explosives charge in them.&lt;/li&gt;&#xA;&lt;li&gt;Takeoff (&lt;strong&gt;YAY&lt;/strong&gt;), but then, your stuck to this constellation of things for 2 minutes until the 2 SRBs gets jettisonned and you know what, there is &lt;strong&gt;no abort&lt;/strong&gt; / launch escape system like Apollo or Soyuz where you press a button then you go flying in the air then fall back down safely.&lt;/li&gt;&#xA;&lt;li&gt;Once the 2 SRBs are gone, you're flying plane with no interior fuel tank is flying with this orange thing until you wanna get rid of it.&lt;/li&gt;&#xA;&lt;li&gt;Congrats, you are now in orbit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Landing:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You're travelling at a speed of... let's just say way too fast.&lt;/li&gt;&#xA;&lt;li&gt;You only have 2 &lt;em&gt;little&lt;/em&gt; propusion things that can slow you down because, &lt;strong&gt;duh, you threw out the only fuel tank for the main engines like long ago during your way to orbit&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Once you get into the atmosphere, you turn around and use the air to slow you down. &lt;strong&gt;BUT WAIT&lt;/strong&gt;, that air is also trying to burn you toast. So NASA helped you by putting on a heat shield.&lt;/li&gt;&#xA;&lt;li&gt;Now that you can get back without being all burned and crunchy, you have wings.&lt;/li&gt;&#xA;&lt;li&gt;Wings provide lift, you have speed, &lt;strong&gt;BUT YOU DON'T WANT TO GO BACK INTO SPACE&lt;/strong&gt;. So you have to do some weird maneover to go down slowly.&lt;/li&gt;&#xA;&lt;li&gt;Now you see KSC runway 33, great, but guess what, &lt;strong&gt;you are a flying brick&lt;/strong&gt;. You know how NASA trains their pilots, by putting them in a small plane, going up high, putting the engines on reverse and opening the gears to augment drag.&lt;/li&gt;&#xA;&lt;li&gt;Now remember, you have no fuel and only one shot to land. Your descent angle is &lt;strong&gt;way worse then a 747&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;li&gt;You landed.&lt;/li&gt;&#xA;&lt;li&gt;Congrats, you're on earth.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So which one is actually harder??&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="17599" LastEditorUserId="58" LastEditDate="2016-12-22T22:36:31.723" LastActivityDate="2016-12-24T09:04:24.940" Title="Is getting IN or OUT of orbit easier for the Space Shuttle?" Tags="|launch|reentry|space-shuttle|" AnswerCount="2" CommentCount="10" ClosedDate="2016-12-28T10:31:56.597" ContentLicense="CC BY-SA 3.0" />
  <row Id="19540" PostTypeId="1" CreationDate="2016-12-22T17:23:21.860" Score="2" ViewCount="1824" Body="&lt;p&gt;Im building a rocket simulation engine and I would like to try and calculate the components of acceleration required to get the rocket into geostationary orbit. For example, the yellow and green lines in the photo below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BIsy8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BIsy8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that geostationary orbit is around 39000km, how do I calculate the trajectory that the rocket would need to travel through become tangential to the geostationary orbit.&lt;/p&gt;&#xA;" OwnerUserId="18024" LastActivityDate="2016-12-22T19:24:21.147" Title="How to calculate the trajectory needed to enter geostationary orbit?" Tags="|orbital-mechanics|geostationary|mathematics|simulation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19541" PostTypeId="2" ParentId="19540" CreationDate="2016-12-22T17:27:30.977" Score="3" Body="&lt;p&gt;Take a look at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann transfer&lt;/a&gt;.  This technique can be used to construct an orbit which will be tangent to your initial circular orbit, and final circular orbit  - similar elliptical orbits in your diagram assuming they started from a circular orbit at the perigee side.  It should be fine for what it looks like you're doing.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="4709" LastEditDate="2016-12-22T19:22:52.183" LastActivityDate="2016-12-22T19:22:52.183" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19542" PostTypeId="2" ParentId="19539" CreationDate="2016-12-22T17:32:45.880" Score="24" Body="&lt;p&gt;Although it seems opinion-based, this is actually answerable if you interpret &quot;harder&quot; to mean &quot;riskier&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the Columbia accident a &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100005659.pdf&quot; rel=&quot;noreferrer&quot;&gt;Probablistic Risk Assessment&lt;/a&gt; was done of shuttle missions. The results show that ascent was riskier.*&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(LOCV means Loss Of Crew and Vehicle.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/10KVE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/10KVE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*The two columns refer to when the LOCV-causing incident actually occurs (on the left) and when it affects the vehicle (on the right).  For example, the STS-107 incident occurred during ascent, but did not affect the vehicle until entry.  You could sum this up by saying &quot;There is a higher chance that a problem will occur during ascent that will destroy the vehicle.  However, it may not show up until entry.&quot;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-12-22T17:43:43.103" LastActivityDate="2016-12-22T17:43:43.103" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19545" PostTypeId="2" ParentId="19540" CreationDate="2016-12-22T19:24:21.147" Score="3" Body="&lt;p&gt;The Hohmann orbit tangent to both LEO and GEO would have an perigee of 6678 km and an apogee of 42164 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ju9s3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ju9s3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope it's evident that the semi-major axis, a, of this transfer orbit is (6678 km + 42,216)/2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis viva equation&lt;/a&gt; can be used to calculate the speed of a spacecraft on this orbit provided you know the distance from earth's center, r.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v=\sqrt{GM(2/r - 1/a)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;G=&lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravitational Constant&lt;/a&gt;&lt;br&gt;&#xA;M=&lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mass of Earth&lt;/a&gt; (see right column under &lt;strong&gt;Physical characteristics&lt;/strong&gt;)&lt;br&gt;&#xA;a=24,421 km for this transfer orbit.&lt;br&gt;&#xA;r=6,678 km when ship's at perigee and 42,164 km at apogee  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there's the tools to get transfer orbit speed at perigee and apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what's the speed of the LEO and GEO orbits?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, with a circular orbit, r = a.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v=\sqrt{GM(2/r - 1/a)}$&lt;br&gt;&#xA;Substituting a = r,&lt;br&gt;&#xA;$v=\sqrt{GM(2/r - 1/r)}$&lt;br&gt;&#xA;reduces to&lt;br&gt;&#xA;$v=\sqrt{GM/r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can use the above to get speeds of both LEO and GEO orbits. At LEO r=6678, at GEO r=42,164.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The burn from LEO would be transfer orbit's perigee speed minus LEO speed. The circularization burn at apogee would be GEO orbit speed minus the transfer orbit's speed at apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope I've given you enough to work this out on your own.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2016-12-22T19:24:21.147" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19548" PostTypeId="2" ParentId="19537" CreationDate="2016-12-23T10:17:36.263" Score="23" Body="&lt;p&gt;It is technically possible to see big animals, but we are still far from being able to identify them without context information. For commercial satellites, the highest spatial resolution is 31 cm at nadir view (worldview-3), but only in the panchromatic band. For colors, the resolution is still very good (1m24) but only large animals are visible. Only animals with a high contrast are therefore visible (easier to see a horse than a lion) BUT you can also use their shadows for a better detection (helps to identify a giraffe). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One very useful feature to detect animals is the use of thermal infrared. However, this is currently not possible with commercial satellites due to the coarser spatial resolution in this wavelength. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To conclude, even if there are some studies in &lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4281221/&quot; rel=&quot;noreferrer&quot;&gt;open savanna&lt;/a&gt; or &lt;a href=&quot;http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0101513&quot; rel=&quot;noreferrer&quot;&gt;arctic areas&lt;/a&gt; where spatial remote sensing is applied to animal counting, it is still very difficult to differentiate the species and even counting is sometimes difficult (like you must look at the spots that moved, but you cannot tell that any spot is an animal). However, given the trends in spatial resolution of commercial satellites (which is no more legally limited to 50cm), one can expect 10-25 cm in a near future, which would look like the cows below (from an aerial photo at 25 cm in true colors, satellites are not (yet) at this resolution. And of course, I am only mentioning commercial satellites for the currently available resolution).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CdlA5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CdlA5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12909" LastEditorUserId="716" LastEditDate="2016-12-24T11:29:13.387" LastActivityDate="2016-12-24T11:29:13.387" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19549" PostTypeId="1" CreationDate="2016-12-23T11:41:19.920" Score="0" ViewCount="285" Body="&lt;p&gt;I don't know how much greenhouse gases are on our planet but what would happen to Mars if we could contain the gases on Earth somehow and transfer it to the surface of Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring the fact that solar winds would strip away progress made if any, would all the greenhouse gases on Earth be enough to warm Mars considerably? I've look around google but no questions of mine have been asked apparently... &lt;/p&gt;&#xA;" OwnerUserId="18034" LastEditorDisplayName="user10509" LastEditDate="2016-12-23T14:21:33.490" LastActivityDate="2016-12-24T20:19:29.057" Title="What would the effect be if we placed all of our greenhouse gases on Mars?" Tags="|mars|atmosphere|terraforming|conceptual|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19550" PostTypeId="2" ParentId="18207" CreationDate="2016-12-23T12:12:30.177" Score="8" Body="&lt;p&gt;Thanks to many people answering and commenting, I think it could be summed up as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) For suborbital flight, height is important. Ramjets could be used for that. NASA even researched ramjet as a propulsion option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) BUT, if your goal is to reach orbit, the height does not matter that much. You can lift your space vehicle to an altitude, and it will fall down like a rock. What matters in orbital and interplanetary flight, is the velocity - (see &lt;em&gt;escape velocity&lt;/em&gt;), so the large part of the energy invested is used for &lt;em&gt;accelerating&lt;/em&gt; the vehicle, not purely lifting it. In that case, ramjets get significantly less useful due to the narrow range of velocities.&lt;/p&gt;&#xA;" OwnerUserId="16952" LastEditorUserId="16952" LastEditDate="2017-01-05T06:03:40.620" LastActivityDate="2017-01-05T06:03:40.620" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19551" PostTypeId="2" ParentId="19549" CreationDate="2016-12-23T12:40:05.770" Score="7" Body="&lt;p&gt;The total mass of &lt;a href=&quot;https://micpohling.wordpress.com/2007/03/30/math-how-much-co2-by-weight-in-the-atmosphere/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth's atmosphere&lt;/a&gt; is about $5.1480 \times 10^{18}$ kg. The carbon dioxide portion of that is about 0.058% by mass. Of that, about 40% is &quot;excess&quot; carbon dioxide, above the pre-industrial average. So that all comes to be about 1.2 trillion metric tons ($1.2 \times 10^{15}$ kg), which getting that to Mars would represent a significant challenge in and of itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmosphere of Mars is about &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;25 trillion metric tons&lt;/a&gt; ($2.5 \times 10^{16}$ kg), so in fact that would increase the atmospheric pressure of Mars by about 5%, which isn't that signficant. I'm not an expert in the math, but I believe that would be not be enough to trigger a &quot;Runaway greenhouse effect&quot; that would melt a large part of the Martian polar ice caps, which would in turn cause more running away of the temperatures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note there are much cheaper ways both to get rid of carbon dioxide on Earth and to bring greenhouse gases to Mars. See &lt;a href=&quot;http://www.space.com/764-mars-habitable-inject-greenhouse-gas.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, which proposes manufacturing greenhouse gasses on Mars which are much more potent than Carbon Dioxide.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2016-12-24T20:19:29.057" LastActivityDate="2016-12-24T20:19:29.057" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19552" PostTypeId="1" CreationDate="2016-12-23T17:14:26.577" Score="10" ViewCount="1575" Body="&lt;p&gt;I've read &lt;a href=&quot;https://space.stackexchange.com/questions/10898/how-much-wobble-does-a-typical-geostationary-satellite-experience&quot;&gt;this&lt;/a&gt; already and some Wikipedia articles, but found no real estimates on how much does a satellite deviate from its standard location. Say that the standard sub-satellite point (SSP) is &lt;code&gt;(0°N,0°E)&lt;/code&gt; at a satellite height of &lt;code&gt;42164km&lt;/code&gt;. While the satellite is still fully operational, how much is the SSP expected to deviate from this point on these two directions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;perpendicular to the equatorial plane (N-S)&lt;/li&gt;&#xA;&lt;li&gt;tangent to the circular orbit (E-W)?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What about the satellite height? Obviously, it cannot increase by more than 300km (since this is approximately where the graveyard orbit is).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've also read about the 8-like figure described by the satellite motion on a daily basis after corrections are stopped being performed on the N-S direction. What is the amplitude of this 8-like figure in terms of the SSP? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I really do not need very precise figures (I am just guessing they are mission specific anyway), but rather some estimates.&lt;/p&gt;&#xA;" OwnerUserId="11737" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-01-14T18:35:02.453" Title="How much is a geostationary satellite expected to deviate from the geostationary orbit?" Tags="|geostationary|diurnal-cycle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19553" PostTypeId="1" CreationDate="2016-12-23T17:49:30.860" Score="6" ViewCount="636" Body="&lt;p&gt;ISS has an inclination of about 56 degrees but because his orbit is not exactly a whole part of 24 hours, ISS will the next day be above other places. So after a while he has 'visited' most of the planet (exept north and south pole) but how long does this take?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or with other words, when ISS travels above your house how does it take before he will cross it again?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope it is clear, but perhaps I mixed things up?&lt;/p&gt;&#xA;" OwnerUserId="13303" LastActivityDate="2016-12-24T04:34:35.760" Title="How long does it take for ISS to travel over all possible places of the world one time?" Tags="|artificial-satellite|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19556" PostTypeId="2" ParentId="19553" CreationDate="2016-12-24T03:07:51.670" Score="11" Body="&lt;p&gt;Let's consider different interpretations of what &quot;travel over&quot; a place means to get a ballpark idea.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;The ISS is literally &quot;over&quot; a given place:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;The ISS&lt;/a&gt; is about 108.5 meters wide, at an altitude (recently) of about 400 km, and the Earth's equatorial radius (where the coverage is most sparse) is &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth_radius&quot; rel=&quot;nofollow noreferrer&quot;&gt;6378 km&lt;/a&gt;. So on the surface that projects down to a 102 meter wide &quot;footprint&quot; on the surface at the equator. If each equator-crossing is at an angle of 51.6 degrees with respect to the equator, it paints a section of the equator that is 102 meters / cos(90-51.6) ~ 130 meters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the Earth's equator is about 40 million meters in circumference, it would take an &lt;em&gt;absolute minimum&lt;/em&gt; of 308,000 crossings to paint the equator. At a period of 93 minutes, that would be about 19,900 days or &lt;strong&gt;a minimum of 57 years&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That lower limit is unrealistic. Since the ISS isn't tightly coupled to the Earth's surface in a &lt;a href=&quot;https://space.stackexchange.com/a/18079/12102&quot;&gt;repeat ground-track orbit&lt;/a&gt; one could try a statistical analysis assuming that over the decades the passes were distributed randomly. In that case it turns out only about 63% of the equator's circumference would be painted in the first 57 years, and something like 63% of the leftover would be painted in the next 57 years, etc. (see python script at the end)&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;The ISS passes within 1 degree of the zenith:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;That's about 7 kilometers, so the equator would be 63% painted after 5,700 orbits (about one year).&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;The ISS passes within 45 degrees of the zenith:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;That's about 800 kilometers, so the equator would be 63% painted after 50 orbits (three days). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember this is just statistical, so there could be much longer periods where some areas go without seeing the ISS, but only very careful (and pointless) planning could prevent any one given place on the equator from not having the ISS within 45 degrees of the zenith within a given year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; for points above the equator, but within 51.6 degrees North and South latitude, the track's motion is increasingly parallel to the ground, so the probabilities become higher. Equator coverage is the hardest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a quick python calculator to confirm the &quot;about 63%&quot; statistical estimate:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;&#xA;total_meters = int(6378 * 1000 * 2 * np.pi)&#xA;n_orbits     = int(total_meters / 130)&#xA;equator      = np.zeros(total_meters, dtype=bool)&#xA;positions    = (total_meters * np.random.random(n_orbits)).astype(int)&#xA;&#xA;for pos in positions:&#xA;    equator[pos:pos+130] = True  # ignore wrapping - can use np.put()&#xA;&#xA;print equator.sum().astype(float)/len(equator)&#xA;&#xA;print &quot;FYI: 1-exp(-1) = &quot;, 1 - np.exp(-1)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;gives for example:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;0.631416058554&#xA;FYI: 1-exp(-1) =  0.632120558829&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2016-12-24T04:34:35.760" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19557" PostTypeId="2" ParentId="19539" CreationDate="2016-12-24T09:04:24.940" Score="-1" Body="&lt;p&gt;Humans were able to place objects into space long before they were able to recover them. During ICBM development the launcher technology was widely understood at the time, whereas blunt, ablative heat shields were a highly classified and protected military secret. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More crews have been killed on reentry than any other mission phase.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-12-24T09:04:24.940" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19558" PostTypeId="1" CreationDate="2016-12-24T16:32:55.463" Score="7" ViewCount="239" Body="&lt;p&gt;My question relates more to the possibilities of life on Mars than to mining mineral deposits. QUESTION: Is it possible to have oxidized iron in the quantities that we see on Mars with only available free oxygen molecules, and without the existence of huge quantities of O2? I am trying to determine the possibility of the existence of &quot;stromatolite&quot; types of life early on in the life of Mars, whereas a parallel to Earth's life origins may have existed. I simply do knot know if the existence of &quot;free oxygen&quot; in lesser quantities than mass photosynthesized O2 is enough to rust out the entire planet. The existence of iron deposits in early oceanic seabeds may provide the best evidence of this, but I have yet to hear of any discovery of layered iron deposits. Thank you for your time and consideration.&lt;/p&gt;&#xA;" OwnerUserId="18043" LastActivityDate="2017-03-24T16:04:17.380" Title="Iron Oxidation Mechanism on Mars" Tags="|mars|astrobiology|oxygen|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19559" PostTypeId="1" AcceptedAnswerId="19569" CreationDate="2016-12-24T17:31:15.857" Score="3" ViewCount="1112" Body="&lt;p&gt;&lt;strong&gt;Could enough surface area be created with a blimp to shed speed using Mars' atmosphere to have a controlled soft landing to minimize the need for retrorockets?&lt;/strong&gt;  Runway would need to be in place. While floating on Earth it would have to behave as a high speed glider on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The heat shield bellow in the 2nd picture is a high tech balloon made by NASA. Could it be added to the nose of a hybrid Airship built or inflated in space work for descending from orbit without the need heavy heat shielding? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a stratospheric launch could a blimp like the 1st picture achieve orbital speeds as a rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bWr3M.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bWr3M.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4bRNv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4bRNv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.newscientist.com/article/dn10288-inflatable-cushions-to-act-as-spacecraft-heat-shields/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.newscientist.com/article/dn10288-inflatable-cushions-to-act-as-spacecraft-heat-shields/&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/5tRcv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5tRcv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/dqVqa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dqVqa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/centers/ames/orgs/exploration-tech/entry-systems-div/hiad.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/centers/ames/orgs/exploration-tech/entry-systems-div/hiad.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.newscientist.com/article/dn10288-inflatable-cushions-to-act-as-spacecraft-heat-shields/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.newscientist.com/article/dn10288-inflatable-cushions-to-act-as-spacecraft-heat-shields/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2018-02-18T22:08:13.793" LastActivityDate="2018-12-19T20:31:28.493" Title="Blimp on Earth, but a glider on Mars?" Tags="|mars|crewed-spaceflight|experiment|entry-descent-landing|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19564" PostTypeId="1" AcceptedAnswerId="44546" CreationDate="2016-12-25T02:35:18.183" Score="24" ViewCount="3033" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iZNt3.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iZNt3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are three of these on the ISS. One connects the Russian modules to the rest. One was used for docking the shuttles. One... kinda just sits there, with modules that were to be berthed canceled. (Edit: PMA-2 and PMA-3 are getting International Docking Adapters (IDA-1 and IDA-3, IDA-2 was lost in the CRS-7 loss of mission) attached for Dragon 2 and CST-100 docking ports.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any particular reason for the peculiar shape? One would think having the exits line up semi-concentrically, or having one of the edges line up would be the intuitive approach. So why the heavy slant?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="51" LastEditDate="2018-01-10T14:19:06.440" LastActivityDate="2022-01-29T04:04:32.533" Title="Why is the Pressurized Mating Adapter slanted?" Tags="|iss|berthing|pma|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19565" PostTypeId="1" CreationDate="2016-12-25T06:11:43.840" Score="2" ViewCount="68" Body="&lt;p&gt;Astronauts need to regularly go out of ISS for the purpose of maintenance and repair. What happens if they start feeling itching inside the spacesuit? Do they have to go back inside? Do they use any medicine for not having itching? Or is it that in space it doesn't itch at all? &lt;/p&gt;&#xA;" OwnerUserId="18047" LastEditorUserId="6816" LastEditDate="2016-12-25T06:22:21.570" LastActivityDate="2016-12-25T06:22:21.570" Title="What do astronauts do if they feel like itching during space-walk?" Tags="|iss|eva|" AnswerCount="0" CommentCount="1" ClosedDate="2016-12-26T01:09:39.853" ContentLicense="CC BY-SA 3.0" />
  <row Id="19566" PostTypeId="2" ParentId="18258" CreationDate="2016-12-25T07:59:19.670" Score="3" Body="&lt;p&gt;Shouldn't be a problem. I think these are the correct equations, but I haven't tried to solve them explicitly. I think the terminal mass will end up just slightly north of the equator depending on exactly how long the tether is (assumes ideal perfectly-light, perfectly-strong tether cable). If you anchored near the north pole it would be nearly horizontal and you could drive your car right onto it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/u2MNQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/u2MNQ.png&quot; alt=&quot;force diagram for space tether anchored at non-zero latitude &quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a simple example where I've set two angles equal. the terminal mass will be at about 25% greater radius than for geostationary. The greater 'centrifugal force' is exactly compensated by the sum of the weaker gravitational force plus the identical tether force. Nothing about being practical, just interesting solutions &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/It5za.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/It5za.png&quot; alt=&quot;simple symmetric configuration easy to analyse&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14419" LastEditorUserId="14419" LastEditDate="2016-12-28T06:23:20.773" LastActivityDate="2016-12-28T06:23:20.773" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19567" PostTypeId="1" CreationDate="2016-12-25T16:48:14.390" Score="6" ViewCount="1775" Body="&lt;p&gt;How much power did the voice and data radios have on the Apollo missions? Additionally how much broadcast power is used on data transmissions from the surface of Mars?&lt;/p&gt;&#xA;" OwnerUserId="18050" LastEditorUserId="6816" LastEditDate="2016-12-25T19:11:49.590" LastActivityDate="2017-12-10T09:29:31.553" Title="How much power do missions to other planets use for radio transmissions?" Tags="|apollo-program|radio-communication|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19568" PostTypeId="1" AcceptedAnswerId="24544" CreationDate="2016-12-25T18:27:50.763" Score="0" ViewCount="275" Body="&lt;p&gt;What is the easiest orbit or most most fuel efficient orbit to achieve around Mars? How difficult is it to go from one type of orbit to another? I want to see if it is more fuel efficient and less turbulent for a ship to be dropped fro geosynchronous orbit verses other methods.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;related question:&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/19559/could-a-blimp-shed-speed-this-way-entering-the-atmosphere-of-mars&quot;&gt;Could a blimp shed speed this way entering the atmosphere of Mars?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2018-01-18T21:19:32.117" LastActivityDate="2018-01-20T05:19:35.617" Title="Can a ship go straight into geosynchronous orbit of Mars?" Tags="|mars|orbital-mechanics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19569" PostTypeId="2" ParentId="19559" CreationDate="2016-12-25T22:14:18.043" Score="9" Body="&lt;p&gt;Unfortunately my answer won't contain the crucial &quot;yes&quot; or &quot;no&quot; - for the simple reason that one hinges upon fine parameters and would likely require a good NASA study for actual answer. But the answer I can give without that currently is &quot;Quite likely so.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) matter of sustaining the blimp in void. That one's easy &quot;yes&quot;. If the blimp's envelope rated pressure differential is 500Pa, inflate it to 200Pa and it will stay inflated in void. Any risk of bursting can be handled through a dumb passive safety valve. And the pressure can be provided from pressurized gas tank - let me say if your target pressure is 0.006 bar, then a small pressure tank inflated to (pretty standard) 100 bar can fill a huuuge volume. Never mind storing liquid or even solid fuel and diverting cooled combustion products into the envelope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) Lowering it into the atmosphere. This is the one where I can shrug helplessly and say &quot;maybe&quot;. We know that objects light enough (surface area to mass ratio high enough) can reenter from LEO into atmosphere without burning up. How light? What ratio? Where to draw the border? The reentry profile would probably involve insertion into a relatively high orbit, where the blimp would lose most of its orbital velocity due to atmospheric drag, even before falling below low orbit altitude - and maintaining buoyancy, reduce descent rate to such degree, that &lt;a href=&quot;https://space.stackexchange.com/questions/19360/can-you-get-to-orbital-speed-with-an-air-breathing-engine/19510#19510&quot;&gt;ram rise&lt;/a&gt; heating would be low enough due to the medium being sparse enough that the envelope wouldn't burn up. This is the crucial point - having the drag high enough at altitudes high enough that adiabatic compression of the medium can't damage the envelope before it slows down to terminal speed. And this is a calculation for a good paper by professional scientists, not for a post on a Q&amp;amp;A site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) Atmospheric descent. Absolutely doable; heating the contents of the envelope would regulate buoyancy allowing to fine-tune the descent speed and allowing for a soft touchdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would such a blimp be usable on Mars? Possibly so; while not really useful as an airship (with air so thin, the lift would be pathetic and the required size would make it a victim of extremely fast winds), but the envelope could be repurposed as a layer of shell of a colony dome, not a full 1 bar but a partial pressure to allow inner layers to have a lower pressure delta until the innermost one provides the full pressure. Also due to need to keep the whole thing very light (for airbraking) there would be nearly no payload - the envelope itself would be the actual payload, with only a minimal amount of active systems for guiding the descent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But - providing we can make a blimp light enough and durable enough to withstand the aerobraking - this would be a very viable method of delivery of the dome construction material - likely way more robust than powered landers with deflated material.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2016-12-25T22:14:18.043" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="19572" PostTypeId="2" ParentId="5846" CreationDate="2016-12-26T11:32:31.030" Score="3" Body="&lt;p&gt;As you said, for the first stage, gaining thrust for the first few minutes is much important than efficiency, and solid propellants provide much more thrust than the liquid propellants. plus they are comparatively cheaper. Due to presence of high atmospheric pressure, lower stages of rockets are designed to give very high thrust, so that the rocket could climb to high altitude very fast where atmospheric pressure will be low. To do that either solid rocket engines or multiple stages of liquid or cryogenic engines are used.&#xA;LVM3 launch vehicle used two S200 solid propellant booster for lift off. Specific impulse for these were 274.5 sec and the thrust provided was 9316 KN. The core stage of LVM3 consisted L110 stage, which uses two Vikas engines (liquid), with specific impulse of 293 sec and the thrust provided was 1598 KN.&#xA;                         So for the first stage of a rocket, i think solid propellants are more advantageous to liquid ones in terms of thrust and serve as a better option but unlike liquid ones, they do not account for safety and possess a higher risk.&lt;/p&gt;&#xA;" OwnerUserId="18054" LastActivityDate="2016-12-26T11:32:31.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19573" PostTypeId="2" ParentId="18258" CreationDate="2016-12-26T17:50:16.540" Score="0" Body="&lt;p&gt;I tend to agree with Roger Wood: It's not impossible to anchor a space elevator on a point 1.5 degrees off the equator. This point is 'just' off by 170 km, compared to the distance to GEO of 36,000 km.&#xA;Nevertheless, this comes with some downsides:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;You will not reach a geostationary orbit. As Roger shows in his drawing, you will get close to the desired orbit, but won't reach it. Every payload will have to use its own propulsion to get into a stable, geo-synchronous orbit and adjust the inclination to get to geo-stationary orbit.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There will be a constant pull on the tether. This will increase the total stress on it, but this increase should be close to negligible. Finding a material that can stand the stress is the most prominent problem, but let's assume this slight increase in load can be covered once we have the technology to build it.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You have to get all your payload up onto a mountain in the middle of nowhere. People will need quite some time to adapt to the thin air before they are able to work. A level place close to the ocean would be much easier to operate.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The savings are close to nothing - you get to climb 6 km less, compared to a total of 36,000 km distance. Note that a space elevator is not a rocket - atmosphere doesn't do any harm because the speed is low.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-12-26T17:50:16.540" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19574" PostTypeId="1" AcceptedAnswerId="19590" CreationDate="2016-12-26T22:06:09.257" Score="8" ViewCount="4308" Body="&lt;p&gt;Quite a bit has been written about railguns launching stuff to the earth. But has any work been done on using the moon as a pit stop between the earth and mars.&lt;/p&gt;&#xA;" OwnerUserId="16627" LastActivityDate="2023-09-03T08:39:48.280" Title="Can we use a rail gun on the moon to launch stuff directly to mars?" Tags="|mars|the-moon|" AnswerCount="5" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19575" PostTypeId="1" AcceptedAnswerId="19577" CreationDate="2016-12-27T00:20:32.570" Score="8" ViewCount="516" Body="&lt;p&gt;Consider the following video (taken from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:790106-0203_Voyager_58M_to_31M_reduced.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/a/a3/790106-0203_Voyager_58M_to_31M_reduced.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/a/a3/790106-0203_Voyager_58M_to_31M_reduced.gif&quot; alt=&quot;Voyager approach to Jupiter&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To me it looks like Voyager 1 will crash into Jupiter. Obviously Voyager 1 didn't crash into Jupiter, but at a glance it looks like it will.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What exactly is happening in that video/animation? How did it avoid crashing? I'm assuming the spacecraft didn't have enough fuel to perform a burn to insert itself into orbit, but maybe it executed a burn to slingshot around it?&lt;/p&gt;&#xA;" OwnerUserId="8707" LastEditorUserId="775" LastEditDate="2018-08-13T12:14:04.330" LastActivityDate="2018-08-13T12:14:04.330" Title="How did Voyager 1 avoid crashing into Jupiter?" Tags="|orbital-mechanics|jupiter|voyager|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19576" PostTypeId="2" ParentId="18882" CreationDate="2016-12-27T00:22:53.293" Score="6" Body="&lt;p&gt;Its hard to answer this in the &quot;typical case&quot; let alone the general case because it will depend upon the manufacturer and customer relationships. Failure review board terms of reference can quite broad and &quot;simply&quot; say that the board must determine the cause and prognosis without reference cost and schedule concerns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One concept that often arises in commercial, and perhaps wider, circumstances is the &quot;most probable root cause&quot;.  At this point Organic Marble's description is a good start:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Many times the exact root cause of an incident is never known. A fault tree is generated and all the branches are run down to closure if possible, but often it is not possible. Any open branches (possible causes) will be addressed, so more than one fix could be applied. Sometimes this process is executed and all the answers are wrong (like the initial try of fixing the foam shedding problem after the Columbia accident, and then there was a giant chunk of foam shed on the next flight).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is quite common in both satellite and launch failure analysis to see many branches of a fault tree given a &quot;not credible&quot; assessment and then to find several attributed to as either &quot;credible&quot; or &quot;credible but unlikely&quot;.  It may well seem uncomfortable to the outsider but it is not impossible to see lot of money spent on producing evidence against every branch of a fault tree through ground tests and generation of a range of corrective actions and yet the failure review board has only narrowed the range of possibilities for the root cause.  Then the &quot;most probable&quot; root cause simply becomes a coin flip between a number of scenarios, all of which happen to be plausible but unlikely. It can be quite unsettling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anyone is wondering what I'm adding to this answer myself, it would be that I have taken part in plenty of failure review meetings at equipment/subsystem/system level (mainly for satellites) and I have come to the view that this is a far softer-edged and human process than many people are comfortable with.  Satellites and launch vehicles aren't naturally occurring phenomena, every failure comes down eventually a human failing in design or workmanship. Correspondingly every decision to go ahead with a design change also has to be a human made compromise.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="415" LastEditDate="2016-12-27T15:28:06.047" LastActivityDate="2016-12-27T15:28:06.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19577" PostTypeId="2" ParentId="19575" CreationDate="2016-12-27T01:22:25.417" Score="12" Body="&lt;p&gt;Voyager was continually pointing its camera towards Jupiter, while passing alongside it, with its closest approach being 349,000 km. With no visible fixed references behind the planet, it's not obvious that the camera direction is changing during the approach. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-12-27T02:45:12.017" LastActivityDate="2016-12-27T02:45:12.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19578" PostTypeId="2" ParentId="19574" CreationDate="2016-12-27T03:32:23.093" Score="6" Body="&lt;p&gt;Yes, but it would be extremely hard, you would need to accelerate it fast enough to escape the moon's gravity, Earth's gravity, then raise your orbit of the sun to achieve a Mars transfer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that's not all! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the railgun would need to be aimed perfectly: being one centimeter off on the moon will make you thousands of kilometers off at mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, you would need to be in a specific place on the moon. This would be a problem, since the moon's orbit takes 27 days, and since the moon is tidally locked, it may take longer than the transfer window to be in the correct position to fire the cannon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, the amount of energy required. From low earth orbit, to a Mars transfer orbit, you need 4.3 kilometers/second of delta v. Back to the energy, the US navy's railgun requires 25 megawatts of energy. For comparison, the ISS's acre of solar panels produce 120 kilowatts at max. And the gun only accelerates a 2.5 kilogram projectile to 2.4 kilometers a second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In conclusion, it's possible, but not feasible with modern technology. It would require too much energy, have so little times it could be used it would be almost useless, and the tiniest mistake could make it fail.&lt;/p&gt;&#xA;" OwnerUserId="18052" LastEditorUserId="6816" LastEditDate="2016-12-27T20:13:08.293" LastActivityDate="2016-12-27T20:13:08.293" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19579" PostTypeId="2" ParentId="19567" CreationDate="2016-12-27T04:08:05.587" Score="5" Body="&lt;p&gt;Apollo used about 20 watts. The total power available from the fuel cells in the service module was about 4 kW.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Unified_S-band#Emergency_key&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Unified_S-band#Emergency_key&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Electrical_power_system&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Electrical_power_system&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mars.nasa.gov/MPF/rovercom/rovfaq.html#faq6&quot; rel=&quot;noreferrer&quot;&gt;http://mars.nasa.gov/MPF/rovercom/rovfaq.html#faq6&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sojouner rover used 100 mW for radio transmissions back to the Pathfinder lander. They tested the radio link on earth to a range of about half a mile. The maximum power from its solar array was only about 16 W. Pathfinder relayed the signal back to earth with about 12 W of power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://mars.jpl.nasa.gov/MPF/rovercom/itworks.html&quot; rel=&quot;noreferrer&quot;&gt;https://mars.jpl.nasa.gov/MPF/rovercom/itworks.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://mars.jpl.nasa.gov/MPF/roverpwr/power.html&quot; rel=&quot;noreferrer&quot;&gt;https://mars.jpl.nasa.gov/MPF/roverpwr/power.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2016-12-27T04:24:37.327" LastActivityDate="2016-12-27T04:24:37.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19582" PostTypeId="1" AcceptedAnswerId="19601" CreationDate="2016-12-27T13:29:06.437" Score="6" ViewCount="1765" Body="&lt;p&gt;I haven't seen the film &lt;a href=&quot;https://en.wikipedia.org/wiki/Hidden_Figures&quot; rel=&quot;noreferrer&quot;&gt;Hidden Figures&lt;/a&gt; yet, but I have read and listened to some dialogue clips in &lt;a href=&quot;http://www.npr.org/2016/12/26/506486155/fences-and-hidden-figures-explore-race-and-more-in-midcentury-america&quot; rel=&quot;noreferrer&quot;&gt;this NPR Review&lt;/a&gt;. In it they mention a &lt;a href=&quot;https://en.wikipedia.org/wiki/Frenet%E2%80%93Serret_formulas&quot; rel=&quot;noreferrer&quot;&gt;Frenet frame&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gram%E2%80%93Schmidt_process&quot; rel=&quot;noreferrer&quot;&gt;Gram–Schmidt process&lt;/a&gt; for orthonormalization of vectors.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;COSTNER: (As Al Harrison) You think you can find the Frenet frame for this data using the Gram-Schmidt...&lt;/p&gt;&#xA;&lt;p&gt;HENSON: (As Katherine G. Johnson) Orthogonalization algorithm, yes, sir. I prefer it over Euclidean coordinates.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How were Frenet frames and Gram–Schmidt orthonormalization used in spacecraft orbit calculations, and why was this especially important in the Mercury and Apollo eras?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;For a reminder of what electronic computers were actually like in the 1960's, below are some screen shots from the &lt;a href=&quot;https://www.youtube.com/watch?v=RK8xHq6dfAo&quot; rel=&quot;noreferrer&quot;&gt;official trailer&lt;/a&gt;. Floating point ops with Python on a Raspberry Pi 2 is about 10,000 times faster than an &lt;a href=&quot;https://en.wikipedia.org/wiki/IBM_7090&quot; rel=&quot;noreferrer&quot;&gt;IBM 7090&lt;/a&gt; for example.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gppk8.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gppk8.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MVgj2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MVgj2.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uzTpX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uzTpX.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; That looks like 100μs per division.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2016-12-28T19:09:23.713" Title="How were Frenet frames and Gram–Schmidt orthonormalization used in spacecraft orbit calculations?" Tags="|nasa|history|mathematics|calculation|computing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19584" PostTypeId="1" AcceptedAnswerId="19589" CreationDate="2016-12-27T16:49:00.527" Score="2" ViewCount="352" Body="&lt;p&gt;Two factors at odds: &quot;have experienced crew on the station, efficient since day zero&quot; vs &quot;have more astronauts with the station operations experience, not depending on the same small crew&quot; - how often do astronauts &lt;em&gt;return&lt;/em&gt; to ISS after having &quot;served a term&quot; there? What's the average ratio of astronauts &lt;em&gt;returning to ISS&lt;/em&gt; vs newcomers, first time there?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-01-31T04:43:09.170" Title="How frequently do astronauts revisit ISS?" Tags="|iss|crewed-spaceflight|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19585" PostTypeId="2" ParentId="19518" CreationDate="2016-12-27T17:04:25.850" Score="5" Body="&lt;p&gt;Launch vehicles are complex machines and each one behave in a complete different way, i.e. each rocket have its own variation of the gravity turn maneuver having different aerodynamic and gravity loss profiles. To account all effects and precisely determine the mass to &quot;LEO&quot;, as said in the comments, the only way is to &lt;a href=&quot;http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=2578&amp;amp;context=etd&quot; rel=&quot;noreferrer&quot;&gt;integrate the trajectory&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another important aspect is to note that metrics like &quot;payload to LEO&quot;, &quot;payload to LTO&quot;, &quot;payload to GTO&quot;, etc. are, &lt;em&gt;per se&lt;/em&gt;, inaccurate measurements, since, as you pointed out, none of these orbits are strictly defined. These data are usually considered only in the zeroth iteration of the mission design, when one is selecting something like a rocket-class requirement and only a rough estimate of the rocket's payload capability is needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make a truly precise characterization and comparison one would need to standardize the final orbit, launch site, weather conditions, and simulate with a full model, or measure it in the case of an experimental study.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that this characterization would be extremely complex, and that the rough &quot;payload to LEO&quot; estimate is enough for a preliminary design, this detailed study in general makes no sense. In a second phase, the team would jump from this approximate number to a detailed simulation to ensure that the system can accomplish the mission and what is its real capability.&lt;/p&gt;&#xA;" OwnerDisplayName="user17622" LastActivityDate="2016-12-27T17:04:25.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19586" PostTypeId="2" ParentId="18558" CreationDate="2016-12-27T18:23:10.880" Score="8" Body="&lt;p&gt;I'm one of the leads on MinXSS. Yes, that is unusually high pointing accuracy for a CubeSat but it's probably not a record. It's about to become commonplace because we were just the first to fly the Blue Canyon Technologies XACT attitude determination and control system (ADCS). There are already lots of them out there ready to be flown, including two at JPL that will be flying to Mars independently with the InSight mission in about a year. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We needed better pointing on MinXSS than is typical for a CubeSat because the Sun is half a degree across and we needed to keep it in our field of view of ±4 degrees. Obviously, 8 arcsec is way better than that, so it's overkill for us but BCT is just down the street from us and we have a strong partnership with them and we had to focus our development efforts elsewhere to ensure success. ADCS is hard, so we were grateful that they were just coming out with the XACT product right at the time we needed it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are measuring soft x-ray spectra from the sun, so we don't actually image the sun at all. We just need to make sure that it's in our field of view and we're okay. Now that we've demonstrated the XACT works even better than the specification sheet says, we're going to be flying it on lots of future missions that require higher pointing accuracy than was needed on MinXSS. &lt;/p&gt;&#xA;" OwnerUserId="14500" LastActivityDate="2016-12-27T18:23:10.880" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19587" PostTypeId="2" ParentId="19567" CreationDate="2016-12-27T18:25:26.453" Score="3" Body="&lt;p&gt;MER uses a 15 W X-band solid state power amplifier, which actually delivers about 17 W RF out. The UHF relay radio has a 12 W amplifier, which delivers more like 14 W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MSL also has a 15 W X-band SSPA for the surface (it has a 100 W X-band TWTA for cruise), and an 8.5 W UHF RF amplifier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://descanso.jpl.nasa.gov/DPSummary/MER_article_cmp20051028.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Exploration Rover Telecommunications&lt;/a&gt; and &lt;a href=&quot;http://descanso.jpl.nasa.gov/DPSummary/Descanso14_MSL_Telecom.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Science Laboratory Telecommunications System Design&lt;/a&gt; for more information.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2016-12-27T19:14:44.360" LastActivityDate="2016-12-27T19:14:44.360" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19588" PostTypeId="2" ParentId="18653" CreationDate="2016-12-27T19:18:12.453" Score="2" Body="&lt;p&gt;It is probably going to be blue for the exact same reason that Earth's sky is blue, shorter wavelengths of light are scattered more so the scattered light in the atmosphere is biased towards the blue end of the spectrum. Sunsets would also be red for the same reason as on Earth, probably even redder because as the sun sets it would be able to pass through a greater thickness of atmosphere than on Earth (as we are observing from a vantage point above the clouds).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With that said, human color perception is complicated and depends on both the eyes and the brain, the sunlight above the clouds on Venus would be about twice as bright as on Earth, I don't know if that would have an effect on how our eyes perceive the color of the sky, but it could. It would probably still be somewhere between blue and white though.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2016-12-27T19:30:44.220" LastActivityDate="2016-12-27T19:30:44.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19589" PostTypeId="2" ParentId="19584" CreationDate="2016-12-27T19:41:46.050" Score="8" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_International_Space_Station_visitors&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, there have been 226 visitors to the ISS.  Of these, 1 person made five visits, 6 people made four visits, 28 made three, and 84 made two.  This means that 119 of the visitors made more than one visit, which is ~53% of all visitors.  However, this is just people going to the ISS.  Some of these missions were Space Shuttle missions, where the crew of the Space Shuttle was only there for a short amount of time before returning to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For crew members only, I went through &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_International_Space_Station_expeditions&quot; rel=&quot;nofollow noreferrer&quot;&gt;this list&lt;/a&gt; to count the number of times each name showed up.  I found that, of the 106 crew members, 2 went four times, 5 went three times, and 21 went two times.  This means that only 26% of ISS crew members had previously been on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you examine the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_International_Space_Station_expeditions&quot; rel=&quot;nofollow noreferrer&quot;&gt;aformentioned list&lt;/a&gt;, it appears that since 2009, when we stopped using the Space Shuttle to ferry crew members, the common practice is for the ISS to have a crew of 6 members.  Whenever a new Soyuz capsule docks, the three members who had been there longest depart, leaving the ISS with three veterans and three new crew members.  Then, when the next Soyuz arrives, the old veterans leave, once again leaving three veterans and three new members.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2017-01-31T04:43:09.170" LastActivityDate="2017-01-31T04:43:09.170" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19590" PostTypeId="2" ParentId="19574" CreationDate="2016-12-27T22:30:06.407" Score="13" Body="&lt;p&gt;It would be a bad idea but on two technicalities.  Railguns produce huge accelerations in a short distance, what's needed for a mobile weapons platform.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a launcher what you want is a linear motor, not a railgun.  Think of a maglev train, but without a speed limit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be a &lt;strong&gt;major&lt;/strong&gt; engineering project that would certainly have to be built almost entirely from lunar or asteroid resources.  The track would be hundreds of miles long but you have the advantage that the forces on it are much more predictable than here on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The longer the track and the higher acceleration you will tolerate on the track the farther you can go.  If you take this to its logical conclusion you get a doubled track wrapped around the lunar equator.  You build a second track upside-down and overhead and the cars that ride on it are built to ride either track.  For 5G on that track you can get an ejection velocity sufficient to take you anywhere from practically grazing the sun to an escape orbit.  You still need to bring along some way to stop but your main boost is done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clayton brought up transfer windows.  I see this as a non-issue.  Yes, the launcher will only be pointed in the right direction once a month--but there's no deep space target with a launch window that comes that frequently anyway.  It might not be a perfect launch window but such windows aren't a binary state, a sightly mistimed launch just costs you a bit more fuel--but your &quot;fuel&quot; is electricity, dirt cheap compared to rocket fuel you need to haul along.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clayton also brought up the issue of accuracy.  Yes, a perfect ejection isn't going to happen.  You're not going to be able to fire artillery and hit Mars.  Rocket launches suffer the same limit, the reality is midcourse corrections are needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LocalFluff (in a comment to Clayton's post) also addressed the issue of plane changes.  Do them in a flyby, it's still a lot cheaper than the whole burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, the second technicality:  It's cheaper to go to Mars than to go to the Moon, even if you figure that your facility on the moon can pluck a craft out of a very low orbit and land it.  (You have a car riding on a track, it shoots cables up as the spacecraft flies overhead.  Get it right, they connect and the spacecraft is hauled down and decelerated.  Miss and you try again next orbit.)  The only reason to use a lunar facility to go to Mars would be if you wanted to do it when you didn't have a decent launch window.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the &quot;it's cheaper to go to Mars than the moon&quot; bit:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/gyaQe.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gyaQe.png&quot; alt=&quot;Solar System ΔV map&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Start at Earth (bottom left) and work your way up:&#xA;9.4 km/sec to reach low orbit&#xA;Another 2.44 km/sec to reach geosynchronous orbit&#xA;Another .68 km/sec to reach the moon&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, from here there are two paths:&#xA;Going to the moon:&#xA;.14 km/sec for capture&#xA;.68 km/sec for low orbit&#xA;1.73 km/sec for landing.&#xA;Note that I was only counting the first two numbers in talking about capturing a vessel in orbit.  In reality the cost would be higher as you're not going to get a capture in a 100 km orbit.  1 km periapsis would be more like it.  I was simply counting .82 km/sec as the cost of the Moon, but if you have to do it with rockets all the way it's 2.55 km/sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going to Mars:&#xA;.09 km/sec to leave Earth's SOI&#xA;.39 km/sec for a Martian transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the total cost is .48 km/sec, well below the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's that I hear from the peanut gallery about the .67 km/sec for capture at Mars?  Note that red arrow on that line.  You get that .67 km/sec from a dip in the Martian atmosphere, not from your engine.  If your objective is landing you can go all the way down without burning, if you want to orbit Mars you will need a small burn to circularize once you have used the atmosphere all you can.  Even with that circularization burn it's still going to be less than the cost of going to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One point--you would never actually move around like that diagram shows and many of those numbers aren't actually accurate for the shown maneuver.  In practice you always burn as much as you can as close to as massive a body as possible.  Thus if your objective is a low orbit about the moon you don't do the ~10 burns (5 arrows + 5 circularization burns) that chart shows, but rather:&#xA;1)  Launch burn&#xA;2)  Low orbit circularization burn.  (Note: Some rockets are able to do one continuous burn for these two steps.  This saves the need to use a restartable engine for this purpose.)&#xA;3)  Lunar transfer burn.  This will take you to the desired altitude above the moon.&#xA;4)  Circularization burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In theory burns 1, 2 and 3 can be combined into a single burn but that gives you an extremely narrow launch window and doesn't actually gain you much of anything as you need to get above the atmosphere before piling on too much velocity anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, you would not find that .39 km/sec would suffice to get you from Earth orbit to Mars, that number is assuming you do the burn in low Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you really want to learn about orbital mechanics I strongly suggest the game Kerbal Space Program.  While it definitely makes some simplifications the orbital mechanics are pretty accurate (although if you want real numbers you will have to download a mod that gives you our solar system rather than the game one.  The stock game uses a more compact solar system to make things happen faster--transfer time to the most distant planet in the game is under 2 years.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2016-12-28T03:12:26.017" LastActivityDate="2016-12-28T03:12:26.017" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19591" PostTypeId="1" AcceptedAnswerId="19598" CreationDate="2016-12-28T02:14:36.880" Score="9" ViewCount="2131" Body="&lt;p&gt;One of my earliest contacts with the concept of orbital mechanics was learning about the four &lt;a href=&quot;http://dsp.agh.edu.pl/_media/en:dydaktyka:cosmic_velocities.pdf&quot; rel=&quot;noreferrer&quot;&gt;cosmic velocities&lt;/a&gt;. They were in my primary school handbook. They were referenced in amateur astronomer's guidebook. They appeared during the university lectures. I believe they appeared in some sci-fi novels from the eastern bloc. They are the &quot;daily bread&quot; to anyone interested in space exploration around here.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;First cosmic velocity is the minimal velocity to orbit the planet.&lt;/li&gt;&#xA;&lt;li&gt;Second cosmic velocity is the minimal velocity to escape the planet's gravitational influence.&lt;/li&gt;&#xA;&lt;li&gt;Third cosmic velocity is the minimum to escape the planet's solar system.&lt;/li&gt;&#xA;&lt;li&gt;Fourth cosmic velocity is the minimum to escape the galaxy.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(and it was a bit of a pain to find any - even as simple and crude as the linked one - English language reference to these.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://ru.wikipedia.org/wiki/%D0%9A%D0%BE%D1%81%D0%BC%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%B0%D1%8F_%D1%81%D0%BA%D0%BE%D1%80%D0%BE%D1%81%D1%82%D1%8C&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; is only available in Russian, Belarussian, French, Polish, Armenian and Bulgarian. And I've never seen anyone on this site ever mention &quot;first cosmic velocity&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any specific reason why this set of terms, so common and fundamental in the former eastern bloc is absolutely nonexistent in the western space exploration community?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-12-28T23:16:00.293" Title="Why don't I find the concept of cosmic velocities in western space-related materials?" Tags="|terminology|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19592" PostTypeId="1" AcceptedAnswerId="19879" CreationDate="2016-12-28T06:11:58.270" Score="6" ViewCount="1514" Body="&lt;p&gt;I'm thinking about the feasibility of making a CubeSat to test the EM drive thruster. Assuming the microwave electronics and resonant cavity could be scaled down to CubeSat size (5 watts, let's say), and we expect the most recently NASA reported ~1 millinewton per kilowatt, my quick calculations (check my math if they seem off) say that we'd expect to see a delta-V of around 3 m/s over a month's time, and thus a resulting orbit altitude increase/decrease of 5km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, there's also drag acting on the spacecraft, so you'd probably have to fire it for month in one direction, and then a month in the other, to identify if there's delta-V independent to that of drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for this experiment to be useful, we'd need a way to pretty accurately measure the orbit of the satellite over time. How is this done normally? I'm imagining things like ground-based radar, or perhaps just tracking the communications radio, or maybe some sort of onboard computer and sensors, but I'm not sure if any/all of those are feasible or accurate. Is there a way to get enough accuracy that this experiment seems feasible?&lt;/p&gt;&#xA;" OwnerUserId="12096" LastEditorUserId="6816" LastEditDate="2016-12-28T06:26:56.140" LastActivityDate="2017-01-22T14:14:15.037" Title="CubeSat Orbit Determination" Tags="|cubesat|tracking|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19593" PostTypeId="2" ParentId="19591" CreationDate="2016-12-28T07:23:59.133" Score="0" Body="&lt;p&gt;I could only find the Western counterpart of second cosmic velocity: &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Escape_velocity&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18065" LastActivityDate="2016-12-28T07:23:59.133" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19597" PostTypeId="1" CreationDate="2016-12-28T14:07:34.890" Score="6" ViewCount="1232" Body="&lt;p&gt;In a perfect scenario the space elevator is in Geosynchronous Orbit, it could be suspended a papers width above an egg, the egg would never be harmed and there would always be just enough room to slide a piece of paper between the egg and the elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality there are a lot variables. The elevators orbit will be impacted by the moon (think tides). The land under the elevator will move (think continental drift). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do you make or control the interaction between the elevator and the Earth? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it was even possible (which I doubt) to anchor the elevator to the continental plate the forces would be much greater then &lt;a href=&quot;https://en.wikipedia.org/wiki/Post-glacial_rebound&quot; rel=&quot;nofollow noreferrer&quot;&gt;Post-glacial rebound&lt;/a&gt; resulting in... well I don't know, but I think it would be bad. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&#xA;To Clarify: While it is possible that the end of the space elevator that touches the ground is is a thin flexible rope/wire that drifts around ground and absorbs fluctuation in distance between GSO and the surface like a kit string.  In this question I am imagining that the elevator is solid inflexible tower, stretching into the sky.&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2016-12-29T16:27:46.140" LastActivityDate="2016-12-31T17:53:55.020" Title="How would the foundation/anchor for a space elevator work?" Tags="|space-elevator|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19598" PostTypeId="2" ParentId="19591" CreationDate="2016-12-28T16:05:05.390" Score="11" Body="&lt;p&gt;I think this boils down to a difference of scientific pedagogical culture. In general, science is so international that we are used to terminology translating easily from one language to another. But, especially during the Soviet era, there was less scientific communication across the boundary between the Russian sphere and the rest of the world. The idea of the four cosmic velocities sounds like something that was introduced by a particular author at a particular point in time, as a way of emphasizing how much more is required to achieve each new &quot;level of escape&quot; in space exploration. People probably found it vivid, and it became a part of the culture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the books where that idea was spread were all Eastern European books. People didn't read them in the west, or at least not in English, and so the idea didn't make it here. People just talked about orbital velocity and escape velocity in general, as concepts that can be applied to anything with gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently, the concept did make it to Germany, but that makes sense, as part of Germany was once part of the Soviet sphere. Here is an &lt;a href=&quot;http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-15602/8702_page-4/&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; where it is discussed, and Russian scientists are credited with inventing the concept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, basically, you don't find it here because it just didn't (or hasn't yet) spread here from its birthplace in Russia.&lt;/p&gt;&#xA;" OwnerUserId="10259" LastActivityDate="2016-12-28T16:05:05.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19599" PostTypeId="2" ParentId="19518" CreationDate="2016-12-28T16:12:42.960" Score="2" Body="&lt;p&gt;I don't know if I'm right, but it could be interesting to compare the final energy of the payload rather than its mass. It would give you (I think) a better metric and a more fair one.&#xA;&lt;br&gt;&#xA;The formula could be this :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_{total} = \frac{1}{2}mv² + mgz$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With m the mass of the payload, z the altitude of its orbit and v its speed.&#xA;We could also use the fact that orbital velocity is directed by :&#xA;$$ {\sqrt {\frac {GM}{R}}} \leq v \leq{\sqrt {\frac {2GM}{R}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With M the mass of the Earth and R the distence between the center of the Earth and the satelite.&lt;/p&gt;&#xA;" OwnerUserId="17048" LastActivityDate="2016-12-28T16:12:42.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19600" PostTypeId="2" ParentId="14521" CreationDate="2016-12-28T19:09:21.767" Score="0" Body="&lt;p&gt;Assuming you don't get too near any stars, it is pretty easy to travel in temperatures that will support &lt;a href=&quot;https://en.wikipedia.org/wiki/Superconductivity&quot; rel=&quot;nofollow noreferrer&quot;&gt;Super Conductors&lt;/a&gt;.  Also assuming you can shield the heat of your ship from the device...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Build a carousal that will spin on a super conductor.  Put it it outside in the cool vacuum of space. Cover it with something to protect from micrometeorite impacts. Point a camera at it. Start it spinning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other then the initial push there is no outside energy impacting the perpetual motion machine. &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2016-12-28T19:09:21.767" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19601" PostTypeId="2" ParentId="19582" CreationDate="2016-12-28T19:09:23.713" Score="8" Body="&lt;p&gt;From my experience, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Fundamental_theorem_of_curves&quot; rel=&quot;noreferrer&quot;&gt;fundamental theorem of space curves&lt;/a&gt; isn't all that fundamental with regard to spaceflight. This is just a movie, and even the most historically accurate of movies get fundamental things wrong. That said, there are a couple of places where the Frenet-Serret frame (or the Serret-Frenet frame) might well have been useful in the early 1960s, and that would be launch and entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During launch, the launch vehicle is very close to being in a zero angle of attack attitude with respect to the local wind, at least for the portion the rocket is in the atmosphere. The rocket thrusts against opposite the unit tangent vector, accelerating the vehicle parallel to the unit tangent, while drag counteracts this acceleration to some extent. Other forces (some of them fictitious) yield curvature and torsion. And there's the Frenet-Serret frame, with physics. Another name for this is the so-called gravity turn (but this term is not quite correct).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A similar problem occurs with capsule entry, where once again the vehicle's is oriented along the angle of attack with respect to the local wind. The vehicle would burn up with any other orientation.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2016-12-28T19:09:23.713" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19602" PostTypeId="2" ParentId="19518" CreationDate="2016-12-28T19:45:22.727" Score="1" Body="&lt;p&gt;I think Lui Txai Calvoso Habl has the right answer, that is, your question is beyond the scope of this forum to answer well. Your best bet for accuracy without spending a lot of money is to look at the mission planner guides for each launch vehicle, usually available at the vendor websites.&lt;/p&gt;&#xA;&lt;p&gt;If instead you want to &amp;quot;get a feel&amp;quot; for things without the concern of being too precise then read on.&lt;/p&gt;&#xA;&lt;p&gt;I had a quick look at the &lt;a href=&quot;http://www.silverbirdastronautics.com/LVperform.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;silverbird&lt;/a&gt; site you mentioned in the question. As it seems to be something of a &amp;quot;black box&amp;quot; its hard to see what assumptions it is making though, in addition to the point raised by Lui Txai Calvoso Habl for the type of trajectory and gravity turn, it is also plausible that the silverbird site is using time average estimates for thrust and specific impulse. In the grand scheme of things I don't think that's too bad at all if it helps one get a rough understanding of each launcher.&lt;/p&gt;&#xA;&lt;p&gt;As &lt;strong&gt;an alternative&lt;/strong&gt; you might like to explore the &lt;a href=&quot;http://www.b14643.de/Spacerockets/index.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Launch Vehicles&lt;/a&gt; site. Of particular interest are the tables such as these for &lt;a href=&quot;http://www.b14643.de/Spacerockets_1/East_Europe_1/Soyuz-2/Description/Frame.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz&lt;/a&gt; where the author has collected together parameters and turned them into a Total Impulse, in mega Newton Seconds, MNs, for each stage. I think this is quite a good way of understanding more about the hardware. Of course it still doesn't help you with the problems of gravity turns, dogleg manoeuvres, time spent in the atmosphere and the time average values of thrust and Isp, but you can do your own sums and see how far out you are from the tabulated figures.&lt;/p&gt;&#xA;&lt;p&gt;Lastly, if you really want to be able to hypothesise the performance of a given vehicle launched from a different launch latitude then you could  simply rank the launch vehicles by total impulse to get an idea of which would be best. Once again, its not going to solve the detailed sums but it might help understand the big picture.&lt;/p&gt;&#xA;&lt;p&gt;Lastly, this &lt;a href=&quot;https://space.stackexchange.com/questions/10670/why-are-rocket-mass-on-the-launch-pad-and-payload-mass-to-leo-not-strongly-corre?rq=1&quot;&gt;previous question&lt;/a&gt; addresses the idea of comparing launch vehicles in a rough-and-ready way and is quite illuminating.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2020-09-18T19:07:47.217" LastActivityDate="2020-09-18T19:07:47.217" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19603" PostTypeId="2" ParentId="19591" CreationDate="2016-12-28T23:16:00.293" Score="6" Body="&lt;p&gt;There's a similar and unrelated terminology difference in statistics: what in Russian (at least in my university) is usually called &quot;type I error&quot; and &quot;type II error&quot; [1], in English is more often referred to as &quot;false positive&quot; and &quot;false negative&quot;. I find the latter terminology superior: the names are descriptive, there are no arbitrary numbers to remember. I feel the same way about the cosmic velocities: the names &quot;orbital speed&quot; and &quot;escape velocity&quot; just seem more descriptive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there doesn't seem to be much value in numbering these velocities (as long as each of them has some name), it's not surprising that this terminology didn't spread from eastern Europe to the rest of the world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1] &lt;a href=&quot;https://en.wikipedia.org/wiki/Type_I_and_type_II_errors&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Type_I_and_type_II_errors&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15943" LastActivityDate="2016-12-28T23:16:00.293" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19604" PostTypeId="2" ParentId="5846" CreationDate="2016-12-29T02:31:24.447" Score="5" Body="&lt;p&gt;This all depends on the grounds of the competition, whether it is launch vehicle development or operations costs, system safety, or just sheer performance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Often solid boosters are quoted as being simpler to develop and fly, as they do not require plumbing systems and do not separate 'tanks' from engines. Liquid propellant stages will likely feature more complex engines - in many cases more than one chamber - and may also have to account for the issues of cryogenic storage. This is perhaps mitigated with pressure-fed liquid fuel designs (like Sea Dragon). Also, with breakthroughs from commercial companies, reusability may soon allow for liquid stages to simply be refilled and flown. SRBs must have their propellant cast again in a lengthy process involving some disassembly, if they are to fly again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;System safety is anyone's game in flight, though it appears more sway towards liquid fuels for their ability to shut off propulsion in an abort scenario. Solid fuel motors cannot throttle (but thrust profiles can be tailored before launch), and cannot shut down. However, we must remember that the increased complexity of many current liquid fuel launchers gives the possibility of many more potential failure modes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I commonly see the analysis of safety only extend to in-flight dangers. It is crucial to remember that a rocket spends a long time in preparation on the ground while awaiting launch, and here solid fuel motors lose. A motor casing must have its propellant cast well before launch, and this includes during vehicle assembly. As such, workers are placed in the presence of live stages (think Shuttle in the VAB). Liquid fuel staged need only be fuelled on the pad, just before launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Performance is liquid fuel's game for the most part, with its generally much higher specific impulse. However, the battle for Isp is not most important during the first portion of ascent. From reading papers on proposed Shuttle liquid booster programs, it is 'impulse density' that wins here. Rather than simply exhaust velocity, an engine must combine that with reaction mass to achieve optimum figures (or something like that - where is that paper anyway!?) As you may see, this appears to say high thrust and fuel storage are what you're looking for, and SRBs have it all there - dense propellant and massive burn rates. This holds true for many liquid rocket stages as well - the Saturn V first stage's heavy launch mass and low specific impulse were less of an issue, because combined with its massive thrust it could generate delta-V against resistance from gravity and drag losses. It is these losses that ultimately define the design of lower stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About the rockets you listed - the Shuttle, Ariane 5 and Titan IV. Do you notice any similarity between their designs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All three feature two SRBs around a high-performance, and long burning, liquid fuel core stage. The solid boosters provide most of the thrust at liftoff and then separate after a short burn, while the core does most of the work in reaching orbital velocity. This makes the core into what is known as a 'sustainer.' Basically, it is what makes a two-stage rocket out of a 'stage-and-a-half' boosted design (Titan's first stage is perhaps less suited to this, but it's core is not a LH2 stage and is less efficient than the others - thus it has more upper stages). The first stage, according to this, is the two solid boosters AND the core, together. The second stage is that very same core, without boosters. The core features an engine with high Isp but lower thrust and fuel mass, great for an upper stage but not for a first. The boosters feature a low Isp but massive thrust and fuel capacity - so, when combined, the boosters lift the core and fuel through atmospheric resistance, while the core slightly increases the overall Isp of the first part of the ascent. It's like having a normal first stage that is slightly more efficient than the boosters themselves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer short: solids shouldn't have to compete - they're best when they work with liquids, not by themselves. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just no messing around when the thing is on the ground...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Al.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(By the way, that 'impulse density' stuff has escaped me for the moment. I hope it's correct, but if not, please forgive my forgetfulness!)&lt;/p&gt;&#xA;" OwnerUserId="12678" LastEditorDisplayName="user8269" LastEditDate="2018-06-18T04:25:03.873" LastActivityDate="2018-06-18T04:25:03.873" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19606" PostTypeId="2" ParentId="18882" CreationDate="2016-12-29T14:18:34.517" Score="0" Body="&lt;p&gt;If no root cause for the failure is found it should be discussed what to do: another ground test, repeat the launch without any change, add more telemetry and repeat the launch or do more tests during manufacture and assembly of the rocket. The problem is what extra sensors for telemetry might help to identify the cause. Additional high speed cameras inside the rocket may help to understand what is going wrong. But even if the next starts are a full success the team does not know if the problem still exists.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2016-12-29T14:18:34.517" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19607" PostTypeId="2" ParentId="7986" CreationDate="2016-12-29T18:02:43.997" Score="-1" Body="&lt;p&gt;For Florida, surrounded by water and with a reputation for electric storms, weather is a big factor. Early morning and evening is preferred for stable atmospheric conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following is surmise: &#xA;Whatever the launch site, the economic factors mentionned in the other answers would favorize a night-time or early morning launch. Human factors such as the sleep cycle would tend to an early morning launch. Any launch emergency would be better dealt with in daytime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be a compromise among these preferences which would be followed subject to compatiblity with the principal constraint of orbital criteria&lt;/p&gt;&#xA;" OwnerUserId="18075" LastActivityDate="2016-12-29T18:02:43.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19608" PostTypeId="1" AcceptedAnswerId="19609" CreationDate="2016-12-29T20:54:46.343" Score="30" ViewCount="2027" Body="&lt;p&gt;In John D. Clarke's &lt;em&gt;&lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;noreferrer&quot;&gt;Ignition!&lt;/a&gt;&lt;/em&gt; (1972), the author spends the last chapter making predictions about the future of liquid rocket propellants. I thought these were very interesting, but I realize the book is decades old. So what's the status of these predictions now, based on contemporary works?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Chemical rockets will never have more than 600 seconds specific impulse.&lt;/li&gt;&#xA;&lt;li&gt;Storing free radicals in propellant to defeat this limit is impractical.&lt;/li&gt;&#xA;&lt;li&gt;~500km range rockets will use chlorine pentafluoride and a hydrazine derivative.&lt;/li&gt;&#xA;&lt;li&gt;Monopropellants, gels, and slurries will not be used in main propulsion.&lt;/li&gt;&#xA;&lt;li&gt;Long-range missiles will use N2O4 and hydrazine for the foreseeable future.&lt;/li&gt;&#xA;&lt;li&gt;First-stage space boosters will use liquid oxygen and RP-1, even with reusable boosters.&lt;/li&gt;&#xA;&lt;li&gt;Upper-stage boosters will use J-2 hydrogen-oxygen. The final stage may use hydrogen-fluorine or hydrogen-lithium-fluorine.&lt;/li&gt;&#xA;&lt;li&gt;Past this point, nuclear rockets will have to come into play.&lt;/li&gt;&#xA;&lt;li&gt;Lunar landers and other extraterrestrial modules will use N2O4 and hydrazine.&lt;/li&gt;&#xA;&lt;li&gt;Deep space probes will use methane, ethane, and diborane, possibly propane, with OF2, ONF3, or NO2F as oxidizers. Perchloryl fluoride (ClO3F) will be useful out to Jupiter.&lt;/li&gt;&#xA;&lt;li&gt;Hydrogen peroxide will be used as a monopropellant, but not as an oxidizer for main propulsion.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="17138" LastEditorUserId="17138" LastEditDate="2016-12-30T03:56:02.570" LastActivityDate="2020-02-03T20:14:03.337" Title="What was the result of the propellant predictions in the last chapter of &quot;Ignition!&quot;?" Tags="|fuel|nuclear|hydrogen|monopropellant|hydrazine|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19609" PostTypeId="2" ParentId="19608" CreationDate="2016-12-29T21:09:30.433" Score="35" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Chemical rockets will never have more than 600 seconds specific impulse. Storing free radicals in propellant to defeat this limit is impractical.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Validated. Chemical rockets in use top out at 450-460 seconds, with a demonstrated test-stand record of 542 seconds.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;~500km range rockets will use chlorine pentafluoride and a hydrazine derivative.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As far as I know, most rockets in that range are using hydrazine derivatives with NTO (N2O4) or MON oxidizer, or are solid fueled. Chlorine and fluorine compounds are not in wide use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know why Clark thought anyone would use ClF5/hydrazine; its specific impulse is comparable to kerosene/LOX and its bulk density is quite good, but it's finicky and dangerous. He goes into detail about the hazards of the similar ClF3, and notes that chemists and engineers have very different attitudes towards these things.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Monopropellants, gels, and slurries will not be used in main propulsion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Correct. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Long-range missiles will use N2O4 and hydrazine for the foreseeable future.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;US likes solids in ICBMs for their relative safety. Russia used to use UDMH/N2O4 liquid fueled ICBMs but seem to also be preferring solids nowadays. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;First-stage space boosters will use liquid oxygen and RP-1, even with reusable boosters.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Again overlooking solids, which are more cost effective. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Upper-stage boosters will use J-2 hydrogen-oxygen. The final stage may use hydrogen-fluorine or hydrogen-lithium-fluorine.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Fluorine is just too unsafe to be worth its small specific impulse advantage. Upper stages generally use hydrogen. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Past this point, nuclear rockets will have to come into play.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Politically infeasible currently.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Lunar landers and other extraterrestrial modules will use N2O4 and hydrazine.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Deep space probes will use methane, ethane, and diborane, possibly propane, with OF2, ONF3, or NO2F as oxidizers. Perchloryl fluoride (ClO3F) will be useful out to Jupiter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Again, Fluorine has been rejected. Deep space probes use hydrazine/NTO or electric propulsion such as ion engines (gaining an order of magnitude in specific impulse instead of the marginal increase possible from fluorine, at the price of very low thrust, which is acceptable for certain interplanetary missions).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hydrogen peroxide will be used as a monopropellant, but not as an oxidizer for main propulsion.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;True.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall it's a decent prediction record, mostly overestimating the industry's tolerance for very hazardous propellants. Solids partially displaced more efficient liquid hypergolics, and fluorine never supplanted LOX, for a combination of interrelated safety, environmental, and cost concerns.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2016-12-30T16:43:05.353" LastActivityDate="2016-12-30T16:43:05.353" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="19610" PostTypeId="2" ParentId="19208" CreationDate="2016-12-29T21:42:56.373" Score="2" Body="&lt;p&gt;One example of a deep-space spacecraft that uses multiple fixed thrusters pointed in different directions instead of a gimbaled engine is &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Propulsion_and_attitude_control&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="17071" LastEditorUserId="17071" LastEditDate="2017-01-19T22:00:42.637" LastActivityDate="2017-01-19T22:00:42.637" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19611" PostTypeId="1" AcceptedAnswerId="19614" CreationDate="2016-12-29T22:26:33.783" Score="7" ViewCount="445" Body="&lt;p&gt;I remember hearing a story from some two decades ago, about a publicity stunt by one of the two - can't remember which - Pepsi or Coca-Cola.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The action was allegedly deployment of a gigantic balloon bearing the company's logo into Earth orbit, for publicity/advertisement purposes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't give more details, as I only heard about as much. I don't even remember the source but I'm fairly sure it wasn't a very authoritative one - &#xA;likely word on mouth. I can't find any references to the event so it's possible it was merely an urban legend or some other event misshapen by retelling. Thus the question - can anyone confirm or rebuke the story?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="6944" LastEditDate="2017-03-22T17:36:27.327" LastActivityDate="2017-03-22T17:36:27.327" Title="Was a commercial (ad) balloon satellite ever deployed?" Tags="|artificial-satellite|commercial|balloons|mir|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19612" PostTypeId="2" ParentId="19611" CreationDate="2016-12-29T22:36:40.107" Score="1" Body="&lt;p&gt;It would be kind of pointless - at least for a free-flying satellite.  Even the largest objects in low earth orbit require a telescope, a good pass (location, lighting, weather, etc.) and no small amount of skill to even resolve the shape of the object.  So the audience of such an advertisement would be extremely small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a great example of imaging ISS &amp;amp; Discovery with an 8&quot; telescope.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/sQIHs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sQIHs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image Credit: &lt;a href=&quot;http://blogs.discovermagazine.com/badastronomy/2011/02/28/ridiculously-awesome-pic-of-discovery-and-the-iss-taken-from-the-ground/#.WGWO0LPSSeg&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://blogs.discovermagazine.com/badastronomy/2011/02/28/ridiculously-awesome-pic-of-discovery-and-the-iss-taken-from-the-ground/#.WGWO0LPSSeg&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="4709" LastEditDate="2016-12-29T23:18:25.027" LastActivityDate="2016-12-29T23:18:25.027" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19613" PostTypeId="2" ParentId="19597" CreationDate="2016-12-29T22:42:52.670" Score="5" Body="&lt;p&gt;One proposed concept:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Central shaft vertical with branching diagonally downwards tunnels; constructed using the mining techniques. The tunnels filled with reinforced concrete (steel beams immersed in concrete). The tether branching/split into multiple strands, each anchored to a separate beam.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each of the beams can support several hundred tons, and is about impossible to remove - it would require displacing thousands of tons of soil to rip the beam free; it's more likely to rip the beam apart than break it loose. This solution can be easily scaled as the shaft can go kilometers deep, and provide hundreds of such anchors; the total anchor strength can easily exceed durability of the tether.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2016-12-29T22:42:52.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19614" PostTypeId="2" ParentId="19611" CreationDate="2016-12-29T22:59:38.540" Score="11" Body="&lt;p&gt;Yes, this was done during the EO-21 mission via a spacewalk from Mir in May 1996.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XiuAB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XiuAB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The deployment was filmed (or videotaped) for use in a TV commercial.  The balloon was not intended to be visible directly from the surface of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional info &lt;a href=&quot;http://www.whitelabelspace.com/2008/08/greatest-space-ads-part-iii-pepsi-on.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2016-12-29T23:26:11.820" LastActivityDate="2016-12-29T23:26:11.820" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19615" PostTypeId="2" ParentId="14521" CreationDate="2016-12-29T23:02:48.140" Score="1" Body="&lt;p&gt;This sounds a lot like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://100yenshopping.com/wp-content/uploads/2016/06/SolarPoweredSwingingLuckyCat-FrontView.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://100yenshopping.com/wp-content/uploads/2016/06/SolarPoweredSwingingLuckyCat-FrontView.jpg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(CC picture coming later)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It needs a bit of sun, and can sustain a similar semi perpetual motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course a plastic one would'nt last centuries, but this is just an engineering issue.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2016-12-29T23:02:48.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19616" PostTypeId="2" ParentId="10195" CreationDate="2016-12-30T08:48:05.157" Score="4" Body="&lt;p&gt;I posted this image in the question where are the solid motors of Pioneer and voyager spacecraft? &lt;a href=&quot;https://space.stackexchange.com/posts/19616/edit&quot;&gt;Jupiter gravity assit&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/gkLde.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gkLde.jpg&quot; alt=&quot;trajectory plots&quot;&gt;&lt;/a&gt;  It helps explain when a spacecraft speeds up for a gravity assist and when it does not.Use this image along with Neptune/Triton flyby posted above.Objects that flyby in front of a planet orbital motion or polar regions slow down.&#xA;The other option for Voyager 1 or 2 was to use a gravity assist to visit Pluto.Voyager 1 visited Titan instead of a closer Saturn flyby for a Pluto mission&lt;a href=&quot;http://www.astronomy.com/year-of-pluto/2015/06/why-didnt-voyager-visit-pluto&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1 Saturn trajectory&lt;/a&gt;&#xA;Another citation of the Voyager astrodynamics at Saturn to set up the Uranus-Neptune tour &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.558.8977&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyagers at Saturn an astrodynamics study&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3297" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2016-12-30T12:00:26.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19618" PostTypeId="2" ParentId="14044" CreationDate="2016-12-30T13:53:27.883" Score="4" Body="&lt;p&gt;Here is a good way to visualize this  &lt;a href=&quot;https://i.stack.imgur.com/b8uHe.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b8uHe.jpg&quot; alt=&quot;pioneer 11 trajectory error &quot;&gt;&lt;/a&gt; Notice pioneer 11 did not meet its aimpoint plan so the Pioneer 11 solid third stage is most likely is in a Ulyssis solar polar orbit of some kind &lt;/p&gt;&#xA;" OwnerUserId="3297" LastActivityDate="2016-12-30T13:53:27.883" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19619" PostTypeId="2" ParentId="6170" CreationDate="2016-12-30T15:12:34.357" Score="7" Body="&lt;p&gt;MSFC studies did include in-house analysis of Saturn V solid booster modifications. There are several reports from 1965 detailing what was known as a Saturn MLV 'Modified Launch Vehicle' program. Within these documents are design, operation, cost and management plans for enhanced, larger Saturns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first such solid booster-augmented Saturn V design was the Saturn MLV-V-4(S), essentially a standard Saturn V (as it had the same stage lengths) with a strengthened S-IC first stage that also mounted four 120-inch diameter SRBs. It is interesting to note that this design offered very similar payload capacity to the standard, booster-free model. This could be a case of propellant offloading.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless, those guys at Marshall knew there was some potential there, and set about detailing the parameters for the improved Saturn MLV-V-4(S)A and V-4(S)B. The A variant would feature a first stage with a 337-inch stretch, but keep standard F-1 engines because the solids would help them lift the heavier first stage early on (otherwise TWR was below 1). The B variant would improve the second stage to carry up to 970k to one million pounds of propellant. LEO payloads were to be 160.9 tonnes for the A and around 172 tonnes for the B. Solid boosters allowed for the Saturn V core stack to be upgraded less extensively to surpass 160t LEO performance. SRB-free Saturns MLV-V-3 and V-3B needed much longer second and third stages, F-1A engines, and either the advanced HG-3 or 400klbf-class toroidal hydrogen engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving on from MLV, we have it's bigger brother in the form of the ELV concepts. ELV, or 'Evolved Launch Vehicle,' would push the Saturn V to the limits of its design possibilities, perhaps beyond practicality, and even past sanity. Of the three largest models, two would rely on solid fuel boosters to lift their weight off the pad. Smallest (not at all small) was the Saturn ELV V-25(S)U, which is interestingly also called the 'Earth Launch Vehicle' given it was to send people to mars in the 1980s. It would do so by lifting the NERVA stages required for TMI into 300nmi orbits for docking. Each stage would have a total mass of 249 tonnes, so its maximum LEO capacity would be just a bit beyond that. In this case, the solid boosters were 156-inch diameter strapons (four of these) around an F-1A powered first stage stretched 498 inches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The V-4/260 is the largest booster-assisted Saturn V design I can find. A V-25(S) core with a stretched upper stage, surrounded by four 260-inch solids. The boosters and first stage would generate 55 million pounds of thrust together, and the SRBs were so large they carried extra kerosene and LO2 for the core to increase burn time. Payload was to be 363 tonnes to LEO, so high that only the V-4X(U) could beat it (four stretched Saturn Vs joined together under a single 520 tonne payload).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These rockets, perhaps not including the later madness, were actually considered and you can find a lot down to even acoustics and fuel fill data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are two main reasons why they weren't flown: first, there was the great cost of  new launch facilities; second; there was the lack of missions and funding after Apollo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, developing a launch vehicle also means developing the ground systems for it. Adding to one means adding to existing hardware whether it flies or not. The scale of the moon rocket made things just that much bigger and more difficult, meaning more costly. Adding SRBs meant new launch pads, new flame tranches (and so new testing for sound and heat stresses, etc.) and a new crawler to support the even greater weight of the rocket. This is important. Solid boosters must be fuelled prior to transport, meaning the crawler doesn't just carry more stage mass if the boosters were liquids, but it also carries dense fuel. The boosterless MLVs would just use the existing trenches, saving money they would have used to make their more complex stages, but also saving time. In addition, payload gains could be so great that the 410-foot tall crane height of the VAB would have to be raised. If so, you have so shell out money to raise the roof of that structure. If not, you can't use your product to the best of its ability - the only ability that justifies its existence, lifting more than the original.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, following the repeated demonstrations of the standard Saturn V'S capability to launch nearly 50 tonnes to the moon, US manned space flight began to focus on LEO. The moon rocket showed its worth orbiting an entire space station in one hit. But in truth, Apollo lunar sorties were to only mission that really required the booster at the time. Skylab was originally planned as a wet workshop launched with the smaller Saturn 1B. With no need for more Skylabs, mission planners sought new ways to put the Saturn V to work, including slapping a Centaur as a fourth stage for massive interplanetary probes - but none of these were waiting either. Funding for NASA dried up between Apollo and STS, so no Development money could be spared for solid booster augmented Saturns. The issue was, the normal one was already too big. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cheers if you get through all that, and I hope it provides at least something helpful to you!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Al.&lt;/p&gt;&#xA;" OwnerUserId="12678" LastActivityDate="2016-12-30T15:12:34.357" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19620" PostTypeId="2" ParentId="19567" CreationDate="2016-12-30T16:06:38.413" Score="1" Body="&lt;p&gt;The transmitters used in the spaceships were small to save weight and power. The HF output power was small, about 20 W.&#xA;For downlink to earth this was compensated by using very large antenna dishes and high gain low noise preamplifiers in the ground stations. For uplink a lot of power could be used, about 20 kW and even more. The antenna dishes used at the spaceships were much smaller than those on earth, about 2 m diameter in space and 34 to 70 m on earth. But the smaller dishes had an advantage, the beam width was larger and required less precise alignment to the ground station. The large dishes on earth and their small beam width required better alignment, but using heavy equipment this was easier on earth than in space.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2016-12-30T16:06:38.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19622" PostTypeId="1" AcceptedAnswerId="19623" CreationDate="2016-12-30T17:57:21.537" Score="-2" ViewCount="983" Body="&lt;p&gt;There are many satellites revolving around earth, then why any $2$ of them collide to each other.$$OR$$ Why all of  them have different orbits? What the scientists do to separate their orbits so that they could not collide? &lt;/p&gt;&#xA;" OwnerUserId="17909" LastActivityDate="2017-11-10T07:37:49.613" Title="Why the different satellites have different orbits?" Tags="|artificial-satellite|orbit|" AnswerCount="2" CommentCount="1" ClosedDate="2016-12-31T15:30:29.057" ContentLicense="CC BY-SA 3.0" />
  <row Id="19623" PostTypeId="2" ParentId="19622" CreationDate="2016-12-30T18:41:38.870" Score="7" Body="&lt;p&gt;The short answer is simply the vastness of space between the satellites. Here is a recent satellite orbit map showing the somewhat ordered madness of active satellites. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/28sFO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/28sFO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;The first thing to notice is that there are groups of satellites at much larger orbital distances from earth than the cluster of satellites near earth. The common satellite orbit altitudes are:  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Low Earth Orbit (LEO): 160-2,000 km&lt;/li&gt;&#xA;&lt;li&gt;Medium Earth Orbit (MEO): Includes geocentric orbits ranging from 2,000 km to just below geosynchronous orbit at 35,786 km. These are most commonly at 20,200 km or 20,650 km with orbital periods of approximately 12 hours. &lt;/li&gt;&#xA;&lt;li&gt;High Earth Orbit (HEO): Geocentric orbits above the altitude of geosynchronous altitude of 35,786 km. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Obviously, these satellites are small in nature, but are spaced in such a way that the odds of colliding with another is very slim, since they are all mostly moving in the same direction at various altitudes. Once again, this is simply due to the vastness of space for which the satellites orbit within. Additionally, as noticed in the picture, there are satellites at various inclination angles at similar altitudes. This is done primarily for covering more of Earths northern and southern surfaces, however, also aids in avoiding collisions. But honestly, the chances of a collision are extremely small, even in LEO.&lt;/p&gt;&#xA;" OwnerUserId="18022" LastEditorUserId="18022" LastEditDate="2017-11-10T07:37:49.613" LastActivityDate="2017-11-10T07:37:49.613" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19624" PostTypeId="2" ParentId="19622" CreationDate="2016-12-30T19:53:03.247" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://hitchhikersguidequotes.tumblr.com/post/13945214509/space-is-big-really-big-you-just-wont-believe&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space is big. Really big&lt;/a&gt;. As TRF pointed out, satellites are tiny compared to the vastness around them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there is a valid worry. From Wikipedia - &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kessler_syndrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kessler Syndrome&lt;/a&gt; proposed by the NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade where each collision generates space debris that increases the likelihood of further collisions. One implication is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges unfeasible for many generations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But even that worry is really only a concern for LEO - which has the highest density of satellites - and various programmes are looking at solutions.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2016-12-30T19:53:03.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19625" PostTypeId="1" AcceptedAnswerId="19626" CreationDate="2016-12-30T19:56:31.157" Score="2" ViewCount="134" Body="&lt;p&gt;If a ladder attached to the International Space Station were oriented &quot;vertically&quot;, that is, perpendicular to the Earth's surface, how much force would an astronaut with a mass of 100 kg climbing the ladder have to exert to take each step upward?&lt;/p&gt;&#xA;" OwnerUserId="17927" LastActivityDate="2016-12-30T21:54:27.407" Title="Climbing a ladder attached to an extended object while in Earth Orbit" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19626" PostTypeId="2" ParentId="19625" CreationDate="2016-12-30T21:54:27.407" Score="8" Body="&lt;p&gt;If the ladder is mounted to the top of the station: None, he actually needs force to climb down!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The center of gravity of the station is in free fall around the Earth. At this point, centrifugal force and gravity cancel out precisely. Any other point of the station experiences a small residual force (hence the term &quot;microgravity&quot;).&#xA;A point above the center of mass is in an orbit with the same angular velocity, but further away from Earth. An orbit at this height would have a angular velocity that is slightly lower, so he is too fast and feels and outwards force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can assume that gravity is constant on this small scale and centrifugal force scales as $F = m \omega^2 r$. ISS is in an orbit with about $r=6800km$ while the astronaut is in an orbit $r'=6800km + x$. The difference in centrifugal force is now just the relative difference of these radii. If the ladder is e.g. 68m long, the astronaut feels about 1/100000 of his natural weight pushing him outwards, about 1/10 N.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2016-12-30T21:54:27.407" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19629" PostTypeId="1" AcceptedAnswerId="19632" CreationDate="2016-12-31T10:39:14.563" Score="1" ViewCount="270" Body="&lt;p&gt;Military grade lasers shooting down incoming missiles have been in the news since Ronald Reagan was the US president. The problems, as I recall are mostly about diffusion in atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Kessler_syndrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kessler syndrome&lt;/a&gt; suggests at some point the amount of debris in LEO is going to be so great it creates significant issues. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When LEO is so full of debris that you can no longer find a safe window to launch through, could you use laser mounted on your rocket to clear a launch path? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2016-12-31T16:51:11.463" Title="Can a laser be used to clear the launch path of a rocket?" Tags="|launch|artificial-satellite|low-earth-orbit|laser|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19630" PostTypeId="1" CreationDate="2016-12-31T11:50:54.990" Score="2" ViewCount="354" Body="&lt;p&gt;You might have seen large rotating wheel space stations with central de-spun hubs or platforms in sci-fi movies like The Martian.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/MP7HD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MP7HD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;My question is, how could the sealing between the rotational and stationary modules work? It must be able to contain the pressure difference of near 1 atmosphere and meanwhile have the lowest friction possible.&#xA;Any insights on the technologies involved and studies done? published papers, etc.&lt;/p&gt;&#xA;" OwnerUserId="16381" LastActivityDate="2016-12-31T12:46:36.170" Title="How the interface between de-spun platform and rotating parts of a spacecraft with artificial gravity could work?" Tags="|crewed-spaceflight|spacecraft|artificial-gravity|rotation|" AnswerCount="1" CommentCount="0" ClosedDate="2016-12-31T19:48:25.003" ContentLicense="CC BY-SA 3.0" />
  <row Id="19631" PostTypeId="2" ParentId="19630" CreationDate="2016-12-31T12:46:36.170" Score="1" Body="&lt;p&gt;The station could be built as a stationary cylinder with a rotating wheel inside. No problem with rotating seals and their friction. But the volume and mass of such a station would be much bigger. But also a stationary torus containing a rotating wheel inside seems possible. The volume and mass of the torus would be much smaller. Adding spokes to the torus is possible. The rotation of the wheel might be stopped and started as necessary to enable an astronaut to change between zero and artificial gravity. But this would cause the whole station to rotate. But some kind of a rotating lift for transfer between stationary and rotating parts of the station would be possible.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2016-12-31T12:46:36.170" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19632" PostTypeId="2" ParentId="19629" CreationDate="2016-12-31T13:14:37.240" Score="5" Body="&lt;p&gt;Well, for a laser to help in this situation, you'd need to vaporize the debris. That means heating the entire piece to its evaporation temperature. This needs either a very powerful laser, or enough dwell time at a lower energy level.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both are difficult to achieve. For high laser power, you're talking about the sort of power that is generated by the first stage engines, plus tons of laser equipment.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At lower power, you need to account for the fact that the debris you're worried about has a speed on the order of kilometers per second relative to the launch vehicle. This makes it tricky to target the debris accurately. If you need 5 seconds dwell time for an object 10 cm across, you need to hit it when it's 5-50 km away and keep hitting it continuously. Objects that small are very difficult to hit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd be better off putting the laser on a satellite, where it can shoot at targets at leisure, and not care how long it takes. That way you can also do selective ablation on larger pieces, and produce retrograde thrust to deorbit them. Power's still a problem though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2016-12-31T16:51:11.463" LastActivityDate="2016-12-31T16:51:11.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19633" PostTypeId="1" AcceptedAnswerId="19635" CreationDate="2016-12-31T17:39:50.827" Score="4" ViewCount="228" Body="&lt;p&gt;My idea is to send an hypothetical lander/rover to Titan, and recover from this some samples of rocks...&lt;br&gt;&#xA;To create a very efficient engine, I suppose to carry only liquid oxygen, and burn the atmospheric tholins.&lt;br&gt;&#xA;Do you think it's possible?&lt;/p&gt;&#xA;" OwnerUserId="17682" LastActivityDate="2016-12-31T20:04:05.927" Title="Build an engine that burns tholins" Tags="|engine-design|lander|titan|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19634" PostTypeId="2" ParentId="19597" CreationDate="2016-12-31T17:53:55.020" Score="1" Body="&lt;p&gt;The foundations would be more concerned with getting traffic on and off the cable in an efficient way, rather than cable engineering, especially since you are thinking of a rigid static cable. I imagine a curved, conical tower construction similar to a tall volcano shape, where cargo trains would approach the elevator site, start going uphill, and smoothly transfer onto the cable without pause. Descending trains, on the other tracks on the cable, would detach from the cable and slide down the mountainside on rails to flatten out and continue away from the site (probably rolling to a stop in warehouses for unloading and prepping for launch again). &lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2016-12-31T17:53:55.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19635" PostTypeId="2" ParentId="19633" CreationDate="2016-12-31T20:04:05.927" Score="7" Body="&lt;p&gt;Tholins encompass a large range of hydrocarbon compounds, could be anything from heavy oil-like substances to something more akin to plastics. So it'll burn, the question is if you can use it in an engine. Heavy oil needs a large engine, plastics aren't really usable.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two ways around this: either burn the tholins directly and build a steam engine (or e.g. a Stirling engine), or crack the tholins into lighter compounds and burn those. Cracking needs a much larger installation than a steam engine. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2016-12-31T20:04:05.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19639" PostTypeId="2" ParentId="4597" CreationDate="2017-01-02T00:43:52.207" Score="3" Body="&lt;p&gt;2014 MU69 is the primary mission for now, but there are at least a few other things being done.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Images are being taken of other KBOs, which given the much higher solar angle than anything which can be done at Earth can provide interesting observations.&lt;/li&gt;&#xA;&lt;li&gt;Other potential targets are being discussed. The odds of finding one isn't great, but if all goes according to plan, there will be plenty of left over fuel to provide another target.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-01-02T00:43:52.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19640" PostTypeId="1" CreationDate="2017-01-02T05:06:41.310" Score="6" ViewCount="2796" Body="&lt;p&gt;What caused John Glenn to see what he described as &quot;fireflies&quot;?  I've heard three different explanations.  If it's true that it was pee and sweat, could you explain how and why it was collected, and how it was expelled from the ship?&lt;/p&gt;&#xA;" OwnerUserId="12675" LastActivityDate="2017-01-02T17:32:07.620" Title="What caused John Glenn to see what he described as &quot;fireflies&quot;?" Tags="|crewed-spaceflight|project-mercury|waste|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19642" PostTypeId="2" ParentId="19640" CreationDate="2017-01-02T17:25:21.730" Score="3" Body="&lt;p&gt;Googling the subject produces results which all seem to quote each other: the fireflies were produced by condensation on the outer walls of the spacecraft.  When Carpenter knocked on the wall, he could produce fireflies at will.  The question really boils down (sorry) to what produced the condensation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mercury capsules rejected heat to space from their cooling loops through what we called in shuttle a &quot;flash evaporator&quot;.  Water is sprayed over a heat exchanger, picks up heat from the cooling loop(s), and is then exhausted into space.  This diagram shows the Mercury cooling loops including the evaporators (the exhaust is denoted as &quot;overflow&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UEsHS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UEsHS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This diagram is from the wonderfully detailed &quot;&lt;a href=&quot;https://www.scottcarpenter.com/Project%20Mercury%20Familiarization%20Manual.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Mercury Familiarization Manual&lt;/a&gt;&quot; but it contains not one word about liquid waste disposal. Solid waste is mentioned to be managed by astronaut diet!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 1969 NASA &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690023882.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;publication&lt;/a&gt; &quot;A review of Spacecraft Waste-Management Systems&quot; describes the Mercury liquid waste disposal system:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During the first manned Mercury missions, spacecraft waste-management&#xA;  system requirements were nominal, primarily because of the limited&#xA;  duration of the flights. For subsequent Mercury flights of somewhat&#xA;  longer duration, a simplified waste-management system, which consisted&#xA;  of an in-suit urination bag, was the only requirement. However, on the&#xA;  extended flight of Mercury-Atlas 9 (MA-9), a more complex&#xA;  waste-management system was required. The system (fig. 1) consisted of&#xA;  two units: (1) the urination bag affixed to a quick disconnect and (2)&#xA;  a storage bag, a syringe-type pump, a hose assembly, and a quick&#xA;  disconnect.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The referenced diagram shows only a storage tank with no provision for dumps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gAeoG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gAeoG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on all NASA publications I can find, it appears that there was in fact no provision for dumping urine from the Mercury spacecraft.  To address sweat, note that the condensate tank has no provision for being dumped. Water was certainly emitted and it is plausible that it would condense on the spacecraft.  I vote for condensed cooling water as the source of the fireflies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You mention a third explanation - if I have not addressed that one, please edit your question and say what it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also please note - the Gemini system did contain provision for dumping urine overboard. Hence the famous Schirra quote about the Constellation Urion.  But your question asked about Glenn so I have limited my discussion to the Mercury capsule.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-01-02T17:32:07.620" LastActivityDate="2017-01-02T17:32:07.620" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19643" PostTypeId="1" AcceptedAnswerId="19645" CreationDate="2017-01-02T22:10:43.197" Score="3" ViewCount="661" Body="&lt;p&gt;As a tangent while looking for numbers for the L/D ratio impact on entry profile, I came upon &lt;a href=&quot;http://www.orbiterwiki.org/wiki/GPIS_6:_Reentry&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site&lt;/a&gt; which seemed to favor skipping reentries in almost all cases. Reading further, I realized that it's not dictated by fundamental physics and is intentionally designed in the trajectory planning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Skip_reentry&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia page&lt;/a&gt; gave the following reason for using skips:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;to achieve greater entry range or to slow the spacecraft before final entry, which helps to dissipate the huge amount of heat that is usually generated on faster descents&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This doesn't make sense to me, because by dipping further into the atmosphere (to generate more heat, in order to get upward velocity), the vehicle would generate more heat, which means that the time spent outside of the atmosphere would merely be compensating for that heat generation. It's not obviously better on-balance. In the long run, you still have to generate the same time-integrated amount of drag...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone give a complete argument as to how skipping reduces the thermal load on the craft?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2017-01-03T03:48:51.850" Title="Mathematically, what is the argument in favor of skipping reentries?" Tags="|reentry|thermal|aerobraking|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19645" PostTypeId="2" ParentId="19643" CreationDate="2017-01-03T00:01:57.157" Score="10" Body="&lt;p&gt;The two systems that I know of that were qualified for skip entry, Apollo and Shuttle, did so purely for cross-range capability. The skip entry would have been used for an emergency return that could not wait for a more favorable alignment with respect to the landing/splashing site. The purpose of qualifying those vehicles for skip entry was not related to heating. (As it happens, neither vehicle ever made use of the skip entry capability.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A skip entry will reduce peak heating due to the shallower entry, at least on the first entry. The lower entry velocity of the second entry would also permit a shallower entry flight path angle than for a single entry that does not skip out. The double entry may result in increased heat load (the integral of the heat rate). On the other hand, it can also provide a short period of heat dissipation in space to cool the heatshield before the next entry, reducing the consequences of the total heat load across the two entries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an extreme example, aero-braking orbiters at Mars and Venus use &lt;em&gt;many&lt;/em&gt; &quot;skip entries&quot; at very high altitude to reduce apoapsis with very low heat rates and complete dissipation of the heat load between skips.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not understand &quot;by dipping further into the atmosphere (to generate more heat, in order to get upward velocity)&quot;. A skip entry dips less into the atmosphere, not more. You don't need &quot;upward velocity&quot;. The curve of the trajectory still goes down. It just curves less than the curvature of the planet, so it exits the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drag goes with the square of velocity, whereas the heating goes more like the cube of the velocity. (It is more complicated than just the cube, but thinking of it as the cube suffices for this discussion.) As a result the total heat load depends on the profile of the velocity, and is not directly related to the total integrated drag.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-01-03T00:07:38.773" LastActivityDate="2017-01-03T00:07:38.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19646" PostTypeId="1" AcceptedAnswerId="19666" CreationDate="2017-01-03T03:05:23.163" Score="4" ViewCount="609" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/q/18337/12102&quot;&gt;this question&lt;/a&gt; there is discussion of the large differences in temperature between liquid helium-4, subcooled liquid oxygen (sub-LOX) and cold kerosene (RP-1). Approximate temperatures are &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_helium#Data&quot; rel=&quot;nofollow noreferrer&quot;&gt;4.2K&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9_Full_Thrust#Modifications_from_previous_model&quot; rel=&quot;nofollow noreferrer&quot;&gt;~70K&lt;/a&gt; also &lt;a href=&quot;https://twitter.com/elonmusk/status/677666779494248449&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9_Full_Thrust#Modifications_from_previous_model&quot; rel=&quot;nofollow noreferrer&quot;&gt;~286K&lt;/a&gt; respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following question is related to this, but not necessarily based on SpaceX designs or plans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The phase diagram for helium-4 (${}^4$He) is shown below. There is further helpful discussion &lt;a href=&quot;http://www.dartmouth.edu/~pchem/75/pdfs/HePhaseDiagrams.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://ltl.tkk.fi/research/theory/helium.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above its &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_helium#Data&quot; rel=&quot;nofollow noreferrer&quot;&gt;critical point of 5.2K&lt;/a&gt; helium-4 becomes a &lt;a href=&quot;https://en.wikipedia.org/wiki/Supercritical_fluid&quot; rel=&quot;nofollow noreferrer&quot;&gt;supercritical fluid&lt;/a&gt; where the distinction between liquid and gas no longer applies, and so 'boiling until it reaches equilibrium' is not the right way to think above 5.2K. If you close it off at ambient pressure at 4.2K, it will reach &lt;a href=&quot;https://en.wikipedia.org/wiki/Critical_points_of_the_elements_(data_page)&quot; rel=&quot;nofollow noreferrer&quot;&gt;2.24 atmospheres at 5.19K&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; But if you continue to warm this sealed, very strong tank up to 90K (LOX at ambient pressure) or even ~70K (sub-cooled LOX), what would be the helium pressure now?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note 1:&lt;/strong&gt; that's a &lt;em&gt;supercritical fluid&lt;/em&gt; above 5.2K, not to be confused with &lt;em&gt;superfluid&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note 2:&lt;/strong&gt; since putting tanks of helium-4 inside tanks of LOX has become a critical technology in space exploration, this particular question - what actually happens - should be still considered on topic here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p2rQX.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p2rQX.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UHwGp.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UHwGp.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; Phase diagrams of &lt;a href=&quot;http://ltl.tkk.fi/research/theory/helium.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;helium-4 from here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2021-03-28T18:18:39.790" LastActivityDate="2021-03-28T18:18:39.790" Title="Liquid Helium (4.2K) sealed, then raised to sub-LOX temperature (~70K) - what is the new pressure?" Tags="|spacex|liquid-fuel|cryogenics|helium|sub-cooled|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19647" PostTypeId="1" CreationDate="2017-01-03T03:30:38.370" Score="10" ViewCount="4790" Body="&lt;p&gt;When pumping fuel and oxidizer into a rocket engine, I presume the pump must create more pressure than the inside of the combustion chamber, otherwise the liquid would back-flow. Is this right? &#xA;Then a rocket that uses a pressurized fuel tank instead of pumps would have to have more pressure in the whole fuel tank than in the engine! So why have the engine at all? the pumps have already achieved the pressure! &#xA;The full flow Raptor engine really baffles me in this regard. There must be more to it. Can someone explain how this works?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="6816" LastEditDate="2017-01-03T23:17:59.083" LastActivityDate="2017-01-20T07:00:05.647" Title="Overcoming pressure differential to feed fuel into rocket engines" Tags="|fuel|engine-design|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19648" PostTypeId="2" ParentId="19643" CreationDate="2017-01-03T03:48:51.850" Score="0" Body="&lt;p&gt;It seems to work for me. Let’s say you have X number of BTUs you have to generate to get a spacecraft to the surface. Let’s say a direct reentry is 6 minutes to terminal velocity. So that’s X BTUs in 6 minutes. But if you skipped several times you might be creating X BTUs in 20 or 30 total minutes.  Also there is the fact that you would be skimming higher up and creating lower temperatures. Plus there is the radiating off heat between skips. By the time you couldn’t skip anymore and had to go down, your velocity would be a lot less. I see a lot of advantages there. Is there a flaw in that logic somewhere?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2017-01-03T03:48:51.850" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19649" PostTypeId="2" ParentId="646" CreationDate="2017-01-03T03:59:44.153" Score="2" Body="&lt;p&gt;Check out Saber Astronautics' PIGI. &lt;a href=&quot;https://saberastro.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://saberastro.com/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By far the best graphics and great ease of use, awesome for visualizing orbits on all the planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their casual license starts at just $15 a month, so worth looking at. PC and Mac.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://saberastro.com/products/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://saberastro.com/products/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18104" LastActivityDate="2017-01-03T03:59:44.153" CommentCount="0" CommunityOwnedDate="2017-01-03T03:59:44.153" ContentLicense="CC BY-SA 3.0" />
  <row Id="19650" PostTypeId="1" CreationDate="2017-01-03T04:02:01.547" Score="38" ViewCount="2841" Body="&lt;p&gt;When a spacecraft lands on the moon, on unprepared ground, it must fling a lot of rocks and dust all around. If you are building a lunar base, the other components that are already there would be pelted with rocks and dust. So there is a dilemma of it being far enough away to be safe, but close enough for efficient access. It seems to me one of the first things they would have to build is a landing pad.  I wonder what speeds and distances retro rocket debris could reach. Is anyone aware of studies of this hazard in regards to lunar base plans? &lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="6816" LastEditDate="2017-01-03T23:22:10.987" LastActivityDate="2017-11-15T07:03:56.360" Title="Flung debris in lunar landing" Tags="|colonization|debris|lunar-landing|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19651" PostTypeId="2" ParentId="19647" CreationDate="2017-01-03T04:25:27.537" Score="4" Body="&lt;p&gt;You are correct that for a pressure fed engine, the combustion chamber pressure must be lower than the tank pressure.  But as for &quot;why have the engine at all&quot;.... an important thing happens in the &lt;em&gt;combustion&lt;/em&gt; chamber. Namely, combustion! That process uses the chemical energy of the propellants to accelerate the resultant gases through the nozzle. And acceleration of gas to high velocity is what it's all about.  Thrust is exhaust velocity times mass flow (plus a pressure term related to the exit plane pressure...typically a much smaller contributor to the thrust [sometimes a detractor]).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not sure what your question about the Raptor engine is.  It has been discussed in &lt;a href=&quot;https://space.stackexchange.com/questions/18783/whats-so-special-about-spacexs-raptor-rocket-engine-with-300-bar-chamber-press/18784#18784&quot;&gt;this question&lt;/a&gt; and full flow staged combustion has been discussed in &lt;a href=&quot;https://space.stackexchange.com/questions/18538/rocket-engine-plumbing-still-dont-understand-the-full-flow-staged-combustion-c&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-01-03T12:28:39.780" LastActivityDate="2017-01-03T12:28:39.780" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19652" PostTypeId="1" CreationDate="2017-01-03T06:44:24.620" Score="4" ViewCount="262" Body="&lt;p&gt;I was speculating that geostationary satellites could be around possibly for billions of years (i.e. much beyond the point where plate tectonics would have erased all traces of human civilization on Earth) and might make interesting artifacts for future aliens to find. But I'm realizing I'm on very shaky ground for all sorts or reasons.  I hadn't appreciated that these satellites are in an unstable equilibrium radially. With a slight decrease in radius, atmospheric drag and tidal effects cause radius to reduce further, and vice-versa for an increase in radius - albeit very very slowly. I understand there are various gravitational disturbances (Sun and Moon) that cause the orbits to move a few degrees off off the equatorial plane and also that the Earth is slightly flattened which causes orbits to slowly precess. And finally the Earth is gravitionally lumpy and satellites tend to drift to one of two stable longitudes. I guess my first question is: are these local points of longitudinal stability sufficient to lock the orbital period and overcome the unstable radial equilibrium? &lt;/p&gt;&#xA;" OwnerUserId="14419" LastActivityDate="2017-01-03T06:44:24.620" Title="extreme longevity of geostationary orbits" Tags="|orbital-mechanics|gravity|geostationary|station-keeping|" AnswerCount="0" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19653" PostTypeId="2" ParentId="19650" CreationDate="2017-01-03T07:17:41.973" Score="35" Body="&lt;p&gt;Great question. The exhaust velocity from typical landing engines is about 3 km/s. You can imagine good-sized particles being accelerated to a significant fraction of that, say 1 km/s, which is the muzzle velocity of a rifle. There is nothing in the vacuum to slow them down, other than eventually hitting something, or the ground. 1 km/s is already a good fraction of the orbit velocity (about 1.6 km/s), so you basically have bullet-speed, sand-sized ICBM particles being ejected from the landing location, going tremendous distances. I'd think that no matter how far away you have the landing location for your base, you'd need to plan for some protection from particles coming from that direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure that there have been studies of this, but I don't have any handy references. A literature search will likely turn up something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And in fact TonioElGringo &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880017751.pdf&quot; rel=&quot;noreferrer&quot;&gt;turned up something&lt;/a&gt;. The very first paragraph from the relevant section:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The effects of rocket engine blast on both prepared and unprepared&#xA;  pads is probably the single most important and the most complex to&#xA;  analyze of any affecting pad designs. Blast from the lander rocket&#xA;  engine will present a broad range of issues affecting virtually every&#xA;  aspect of lunar base design.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The study got much lower velocities than I was guessing above, but it was also a rough-order-of-magnitude calculation. They didn't have computational fluid dynamics (CFD) back then. Their general conclusion was that your habitat surfaces would sustain relatively little damage if they were 2 km or more away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;https://www.nasa.gov/sites/default/files/617743main_NASA-USG_LUNAR_HISTORIC_SITES_RevA-508.pdf&quot; rel=&quot;noreferrer&quot;&gt;more recent reference&lt;/a&gt; that states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA analysis using gas flow codes has indicated that rocket exhaust&#xA;  plumes from the landing stages can induce high injection velocities of&#xA;  the top layer of the lunar surface;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Analysis further indicates that these particles can achieve ejection&#xA;  velocities between 300 and 2000 meters per second (m/s) with the&#xA;  smaller particles generally traveling faster. Because there is&#xA;  negligible ambient lunar atmosphere outside the plume, the particles&#xA;  continue at that velocity until striking the lunar surface far away.&#xA;  Some particles travel almost all the way around the moon before&#xA;  impact. The smallest, dust-sized particles achieve near-lunar escape&#xA;  velocity, 2.37 km/s, and even exceed it by a significant margin,&#xA;  sending them into solar orbit, according to some plume simulations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That study also concluded that keeping your landing site 2 km away from what you are trying to protect would be adequate. This is due mainly to the curvature of the Moon's surface, where at that distance the highest-speed horizontally accelerated material would fly mostly over the artifacts.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-01-04T14:04:40.650" LastActivityDate="2017-01-04T14:04:40.650" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19654" PostTypeId="2" ParentId="19647" CreationDate="2017-01-03T07:32:43.823" Score="17" Body="&lt;p&gt;You are correct, the injection pressure is larger than the chamber pressure of the engine. According to &quot;&lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1563470136&quot; rel=&quot;noreferrer&quot;&gt;Modern Engineering Design of Liquid Propellant Rocket Engines&lt;/a&gt;&quot;, the Space Shuttle Main Engine featured turbopump characteristics as follows:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1.) High-Pressure Oxidizer Turbopump (HPOTP) boost discharge pressure is 6,952.2 psia, which flows into the oxidizer preburner.&lt;br&gt;&#xA;2.) High-Pressure Fuel Turbopump (HPFTP) discharge pressure is 6,024.8 psia, which flows into the fuel preburner.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the while, the SSME had a nominal chamber pressure of approximately, 3,000 psia. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for your question, why even have the engines, if the turbopumps can supply propellant at such high pressures? Well, the answer to your question is the role of &lt;em&gt;combustion&lt;/em&gt;. First off, we must look at what is our equation for thrust. From simple control volume analysis, we obtain a thrust equation of the form,&#xA;$$F = \dot{m}v_2 + (p_2-p_3)A_2 $$&#xA;where $F$ is the thrust force, $\dot{m}$ is the engine flow rate, $v_2$ is the gas velocity at the nozzle exit, $p_2$ is the gas pressure at the nozzle exit, $A_2$ is the cross sectional area at the nozzle exit, and $p_3$ is the ambient pressure outside the engine. Using quasi one-dimensional gas dynamics and isentropic relations, we can write the nozzle exit velocity in the following form,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ v_2 = \sqrt[]{\frac{2\gamma}{\gamma-1} R T_1 \left[1-\left(\frac{p_2}{p_1}\right)^{(\gamma-1)/\gamma}\right]} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\gamma$ is the ratio of specific heats, $R$ is the specific gas constant, and $T_1$ is the chamber temperature (very important for combustion). As shown here, $v_2 \sim \sqrt[]{T_1}$. Similarly, the mass flow rate through the engine can be obtained from the mass flow parameter, which we will assume choked flow at the throat,&#xA;$$\dot{m} = \frac{A_t p_1}{\sqrt[]{T_1}} \ \sqrt[]{\frac{\gamma}{R}} \left(\frac{\gamma+1}{2}\right)^{-(\gamma+1)/[2(\gamma-1)]}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $p_1$ is the chamber pressure. Once again, we can see that mass flow rate of the engine is influence by the chamber temperature as $\dot{m} \sim 1/\sqrt[]{T_1}$. Combining these two expressions into the equation of thrust yields,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ F = A_t p_1 \sqrt[]{\frac{2\gamma^2}{\gamma-1}\left(\frac{2}{\gamma+1}\right)^{(\gamma+1)/(\gamma-1)}\left[1-\left(\frac{p_2}{p_1}\right)^{(\gamma-1)/\gamma}\right]} + (p_2-p_3)A_2 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we will consider some hypothetical scenarios in regard to your question. First we will look at the conventional rocket engine. Suppose we have a chamber pressure similar to the SSME at approximately, $p_1$ = 3,000 psia. Also, for simplifying analysis, suppose we have a nozzle that perfectly expands the gas to atmospheric pressure at sea level, which would yield a nozzle pressure ratio of $p_2/p_1$ = 14.7/3000 = 0.0049. Additionally, we will assume the throat area is 0.5 ft$^2$. Since we expand into the atmospheric condition, $p_2-p_3 = 0$ and we have no pressure area force in our equation and the thrust is purely momentum flux through the nozzle. The final thing to determine is the ratio of specific heat $\gamma$ for the engine. We will assume we are using LOX/LH2 propellant, which features a burned product ratio of specific heat of approximately $\gamma \approx$ 1.2 at 6,000 F. Substituting all of this into our expression for thrust we obtain for a rocket (very similar to the SSME, &lt;em&gt;as expected&lt;/em&gt;):&#xA;$$ \boxed{F = 372,064 \ \ \text{lbf} \\ \dot{m} = 1,030 \ \text{lbm/s} \\ v_2 = 11,614 \ \text{ft/s} \\ I_{sp} = 361 \ \text{sec} }$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let us consider your case, what if we only used the turbopump injection pressure as our means of thrust generation. For this case, we will assume the injection pressure is the discharge pressure from the SSME and nominally equal to $p_1$ = 6,500 psia. In this case, as we are abandoning the engine concept, hence, we do not require combustion and can focus on simply blasting O$_2$ at 6,500 psia through a nozzle that perfectly expands to atmospheric sea level condtitions. Now visual inspection of the &lt;a href=&quot;https://i.stack.imgur.com/4ZDiY.png&quot; rel=&quot;noreferrer&quot;&gt;SSME plumbing system&lt;/a&gt; indicates that the 6,500 psia is going into a plumbing network that is roughly $\approx$1/5th the diameter of the primary injection lines at a temperature of -258 F (above boiling point for LOX). Moreover, the primary thrust chamber feed lines are operating nominally at roughly 3,100 psia and 728 F, so we can consider 2 cases for your proposed question. The first, is considering an injection pressure and temperature of $p_1$ = 6,500 psia and $T_1$ = -258 F, but with a throat area of 1/5th our rocket example above for choked flow. Now in these cases we do not have combustion and our ratio of specific heats will stay at roughly $\gamma$ = 1.4. The second case we will consider is maintaining the same throat area from the previous rocket example, but with an injection pressure and temperature of 3,100 psia and 728 F. The results for your case go as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your case 1: $p_1$ = 6,500 psi, $T_1$ = -258 F, and $A_t$ = (1/25)(0.5 ft$^2$) = 0.02 ft$^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \boxed{F = 30,796 \ \text{lbf} \\ \dot{m} = 215 \ \text{lbm/s} \\ v_2 = 4,613 \ \text{ft/s} \\ I_{sp} = 143 \ \text{sec} }$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your case 2: $p_1$ = 3,100 psi, $T_1$ = 758 F, and $A_t$ = 0.5 ft$^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \boxed{F = 357,865 \ \text{lbf} \\ \dot{m} = 6,212 \ \text{lbm/s} \\ v_2 = 1,854 \ \text{ft/s} \\ I_{sp} = 58 \ \text{sec} }$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In both of your cases, we come up short as far as the thrust produced and specific impulse without combustion. In fact, the thrust produced in the second scenario is comparable to the thrust produced for a engine similar to the SSME, however, the specific impulse is horrendous. Operating the rocket in this fashion would be light years beyond inefficient. The inefficiency comes from the low exhaust velocity as a consequence of $v_2 \sim \sqrt[]{T_1}$ and the large mass flow rate as a consequence of $\dot{m} \sim 1/\sqrt[]{T_1}$. Hence, a way to obtain high $v_2$ and low $\dot{m}$ for the same throat area, is to obtain a large $T_1$, which requires combustion. Even if we considered using the H$_2$ line instead of the O$_2$, we would only obtain an $I_{sp}$ of 167 for the latter scenario, which is comparable to a solid rocket booster, while using a liquid propellant, i.e. awful performance. &lt;/p&gt;&#xA;" OwnerUserId="18022" LastEditorUserId="18022" LastEditDate="2017-01-20T04:55:18.727" LastActivityDate="2017-01-20T04:55:18.727" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19656" PostTypeId="1" CreationDate="2017-01-03T13:27:14.887" Score="4" ViewCount="446" Body="&lt;p&gt;A couple of days ago I asked &lt;a href=&quot;https://space.stackexchange.com/questions/19629&quot;&gt;Can a laser be used to clear the launch path of a rocket?&lt;/a&gt; turns out this is not a good plan but the &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/19629/can-a-laser-be-used-to-clear-the-launch-path-of-a-rocket/19632#19632&quot;&gt;answer by Hobbes&lt;/a&gt; suggests the idea of clearing debris with a laser is not completely out of the question.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...laser on a satellite, where it can shoot at targets at leisure, and not care how long it takes. That way you can also do selective ablation on larger pieces, and produce retrograde thrust to deorbit them. Power's still a problem though.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If we wanted to put laser(s) in orbit to and use them deorbit junk, at a rate of say 10% quicker then we are currently creating orbital debris, what would it take?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the laser currently exist that can create the retrograde thrust in the target? Could it be powered by solar or would it need another power source?   &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-01-03T17:09:38.583" Title="Deorbit LEO debris with a higher orbit laser?" Tags="|artificial-satellite|low-earth-orbit|debris|laser|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19657" PostTypeId="2" ParentId="19656" CreationDate="2017-01-03T13:38:11.560" Score="3" Body="&lt;p&gt;Truth be told, they don't even need to be in orbit to make it work. There are plans to deorbit small objects using Earth based lasers (Fire as they are approaching you). For an object in LEO, it only take a few m/s of delta-v change to really make a difference, which can be imparted by a fairly powerful laser on a small object pretty easily. According to &lt;a href=&quot;https://www.technologyreview.com/s/423302/nasa-studies-laser-for-removing-space-junk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“Focused onto a piece of junk for an hour or two every day, they calculate that a 5 KW laser could do the trick”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note that for objects that are trackable from Earth, that would be enough to prevent a collision if planned enough in advance, but not enough to deorbit. It will work for small paint flakes and that size of object, which are difficult to track. There are methods in place to track these smaller debris objects, using on-orbit sensors, but they do have their limits. A more powerful laser (~ 1000 times more powerful) would work to deorbit debris that can be seen, but those are considerably more expensive.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-01-03T17:09:38.583" LastActivityDate="2017-01-03T17:09:38.583" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19660" PostTypeId="2" ParentId="19650" CreationDate="2017-01-03T15:36:51.127" Score="13" Body="&lt;p&gt;Damage to existing equipment from exhaust plumes is indeed an issue for lunar outpost planning. The best information currently available about this is from the Apollo 12 mission that landed close (155m) from the Surveyor 3 Lander that had been on the moon for two and a half years. Astronauts Conrad and Bean removed pieces of the probe that where taken back to Earth for analysis. The Lander was found to be sand blasted by the exhaust plume.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For further information see &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_12#Moon_landing&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Apollo_12#Moon_landing&lt;/a&gt;&#xA;and a 2011 paper from NASA &lt;a href=&quot;http://dx.doi.org/10.1016/j.icarus.2010.11.013&quot; rel=&quot;noreferrer&quot;&gt;http://dx.doi.org/10.1016/j.icarus.2010.11.013&lt;/a&gt; &quot;Apollo 12 Lunar Module exhaust plume impingement on Lunar Surveyor III&quot; that uses electron microscopy and other methods to analyze the damage.        &lt;/p&gt;&#xA;" OwnerUserId="18111" LastActivityDate="2017-01-03T15:36:51.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19661" PostTypeId="1" AcceptedAnswerId="34697" CreationDate="2017-01-03T17:13:43.760" Score="10" ViewCount="1306" Body="&lt;p&gt;I've just read the phys.org new item which summarizes the conclusions by SpaceX about the cause of the September 1st 2016 anomaly.&lt;/p&gt;&#xA;&lt;p&gt;I'm citing quotes in the news item which come from a SpaceX announcement which is further quoting investigators, so I am not sure I'm getting the correct picture yet:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Investigators concluded that super chilled LOX can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Does the term &amp;quot;buckles&amp;quot; refer to existing static indentations of some kind in the as-fabricated aluminum liner, or to spaces that are formed by the aluminum liner actually buckling during the loading process?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt; The 2018 Florida Today article &lt;a href=&quot;https://www.floridatoday.com/story/tech/science/space/2018/01/12/after-launch-pad-explosion-spacex-updating-falcon-9-astronauts-nasa-safety-panel-eyes-falcon-9-rocke/1029228001/&quot; rel=&quot;nofollow noreferrer&quot;&gt;After 2016 launch pad explosion, SpaceX updating Falcon 9 for astronauts&lt;/a&gt; suggests that it is the tank that was buckled (lower your volume to avoid advertisements):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Changes in helium loading operations prevented a repeat of the problem believed to have caused the explosion: &lt;strong&gt;buckles in helium tanks that trapped slushy liquid oxygen in gaps between the tanks and their composite wraps&lt;/strong&gt;. Friction &amp;quot;or other mechanisms&amp;quot; ignited oxygen in the upper stage.&lt;/p&gt;&#xA;&lt;p&gt;While Falcon launches resumed fairly quickly, a redesigned helium bottle has not yet completed testing nor won NASA’s approval for crew launches.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6z4qX.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6z4qX.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The next launch date has now been announced as January 8, 2017, just a few days away. Does the &lt;em&gt;plan to avoid this happening in the immediate future&lt;/em&gt; consist primarily of ways to change the helium loading process to keep it from getting so cold that liquid oxygen in these trapped spaces simply can't freeze into solid oxygen (SOX), or are they also going to try to prevent these pocket of LOX from forming between the Aluminum liner and the carbon overwrap in the first place?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Aluminum in contact with pure oxygen, carbon in contact with pure oxygen, huge stresses, huge vibrations, only a bit of friction necessary to start a reaction... no physical barrier?&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;He built gradually to a crying jag, during which he claimed to be deeply touched by the idea of an inhabited planet with an atmosphere that was eager to combine violently with almost everything the inhabitants held dear. He was speaking of Earth and the element oxygen.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;“When you think about it boys”, he said brokenly, “that’s what holds us together more than anything else, except maybe gravity. We few, we happy few, we band of brothers - joined in the serious business of keeping our food, shelter, clothing and loved ones from combining with oxygen.”&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;--Kurt Vonnegut, &lt;em&gt;God Bless You, Mr. Rosewater&lt;/em&gt; (1965)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-03-09T01:33:14.327" Title="Final conclusion/description of the cause of the SpaceX Sept. 1, 2016 anomaly? What's a &quot;buckle&quot;?" Tags="|spacex|falcon-9|failure|lox|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="19663" PostTypeId="1" AcceptedAnswerId="19665" CreationDate="2017-01-03T18:47:49.913" Score="9" ViewCount="1363" Body="&lt;p&gt;If the orbits are co-planar (lie in the same plane), the required minimal $\Delta v$ is calculated by the well-known &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann transfers&lt;/a&gt;. But what is the case if they aren't co-planar?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many orbital corrections are needed in this case, and in which direction?&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="12102" LastEditDate="2017-01-05T05:19:53.390" LastActivityDate="2019-04-14T23:21:06.387" Title="How is the minimal $\Delta v$ transfer between non-coplanar orbits calculated?" Tags="|orbital-mechanics|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19665" PostTypeId="2" ParentId="19663" CreationDate="2017-01-03T20:31:11.180" Score="4" Body="&lt;p&gt;A single plane change maneuver is required to make the orbits coplanar.  Plane change maneuvers are expensive and are most easily applied where the original and desired orbital planes intersect.  The burn is more efficient the closer it is to the apogee of the orbit -- so it is often better to do your in-plane adjustments first.  You can get the direction of the burn by normalizing the cross product of the velocity vector and radial vector at that point.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2017-01-05T16:10:55.480" LastActivityDate="2017-01-05T16:10:55.480" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19666" PostTypeId="2" ParentId="19646" CreationDate="2017-01-03T20:46:46.037" Score="5" Body="&lt;p&gt;We don't need to look into the phase diagram here, because it doesn't matter on which path we reach a certain end point. We can safely assume that we heat up the helium at constant pressure and then shrink its volume again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Density of liquid helium boiling at 1 atm pressure is 0.125 $\rm g/cm^3$.&#xA;Density of gaseous helium at 273 K is 0.18 $\rm mg/cm^3$. Hence, it will have the 700-fold volume, or, if volume is constraint, a pressure of 700 bar. &#xA;At temperature of LOX, i.e. 70 K we can treat Helium as a perfect gas that behaves according to the ideal gas law as a first approximation. That is, $pV/T = const$. So, pressure will be a factor $T_{Lox}/273 \rm K = 0.26$ lower. A volume of liquid helium sealed and heated to 70 K will be at a pressure of 180 bar. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is only true if we can assume that Helium still behaves like an ideal gas even at an pressure of 18 MPa. This deviation can be found in the compressibility factor and has been measured (See e.g. S.W. Van Sciver, Helium Cryogenics, Appendix 1). This factor is about 1.2 to 1.3 at our conditions, hence actual pressure will be about 230 bar. &lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2017-01-04T10:32:38.663" LastActivityDate="2017-01-04T10:32:38.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19667" PostTypeId="2" ParentId="19663" CreationDate="2017-01-03T23:23:42.123" Score="2" Body="&lt;p&gt;I'm going to assume you mean non-coplanar rather then parallel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First - a Hohmann transfer isn't always the most efficient transfer between coplanar orbits.  When the ratio between the initial &amp;amp; final semi major axis is large enough, (&gt; approximately 12) a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bi-Elliptic Transfer&lt;/a&gt; may be more efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt; for &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination_change&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbit inclination change&lt;/a&gt; is given by &lt;span class=&quot;math-container&quot;&gt;$\Delta v_i=2v \sin \frac{\Delta i}{2}$&lt;/span&gt;, where &lt;span class=&quot;math-container&quot;&gt;$v$&lt;/span&gt; is the orbital velocity.  So, for the simple case of a transfer between circular orbits, you'll want to do you the majority of the  inclination change at the apogee of the Hohmann transfer. By combining the Hohmann apogee burn with the plane transfer you gain an additional efficiency from the vector addition of the velocity changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I say the majority, because I believe when you do all the math the optimal solution performs a fraction of the inclination change at perigee as well.  I'll see if I can find the reference for that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's one reference I was thinking of for the general solution: &lt;a href=&quot;https://arxiv.org/pdf/1509.05550v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;On non-coplanar Hohmann Transfer using angles as parameters&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reference from AlanSE looks good as well, from a quick look: &lt;a href=&quot;http://www.kdm.p.lodz.pl/articles/2011/V15_2_07KAM_SOL_2_2011.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;On The Generalized Hohmann Transfer with Plane Change Using Energy Concepts Part I&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="1054" LastEditDate="2019-04-14T23:21:06.387" LastActivityDate="2019-04-14T23:21:06.387" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19668" PostTypeId="2" ParentId="18568" CreationDate="2017-01-04T00:20:18.170" Score="3" Body="&lt;p&gt;There has been at least one &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120016580.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;semi-recent (2012) paper&lt;/a&gt; on the topic.  The design is called the Flexcraft. There is a wikipedia article &lt;a href=&quot;https://en.wikipedia.org/wiki/FlexCraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VNAmO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VNAmO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears to be purely in the proposal stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The concept was more common in the '50s and '60s.  You can see a lot of concept art at the wonderful &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/spacesuits.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atomic Rockets&lt;/a&gt; page (search the page for &quot;A space pod is a small pressurized vehicle&quot; - the page is gigantic).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-01-04T00:20:18.170" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19669" PostTypeId="2" ParentId="19650" CreationDate="2017-01-04T01:44:39.477" Score="4" Body="&lt;p&gt;I do not believe this would be a problem with a long term base with a properly placed landing area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thing is the base needs a radiation shield anyway and the easiest way to do that is simply pile up stuff already laying on the moon.  You need two meters (of actual material.  Since it won't pack perfectly you'll need a thicker pile) of material, that's going to shrug off landing/takeoff debris without appreciable damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus all you need to actually protect is exposed instruments--and they can be sheltered with a berm and designate your area for rockets to be on the other side of the berm.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-01-04T01:44:39.477" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="19670" PostTypeId="2" ParentId="19663" CreationDate="2017-01-04T01:46:21.283" Score="3" Body="&lt;p&gt;Another answer started to hit at the right answer, but I'm not &lt;em&gt;entirely&lt;/em&gt; sure that it's completely correct from start to finish:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;By combining the Hohmann apogee burn with the plane transfer you gain an additional efficiency from the vector addition of the velocity changes.&lt;/p&gt;&#xA;&lt;p&gt;I say the majority, because I believe when you do all the math the optimal solution performs a fraction of the inclination change at perigee as well.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I believe this is right in that a plane change is done both at apogee and at perigee. However, I believe that it throws around some erroneous assumptions about why and how much.&lt;/p&gt;&#xA;&lt;p&gt;For a Hohmann transfer, your first burn is at perigee. Think about 2 separate planes for a second. If the 2nd burn is to be done at apogee, then &lt;em&gt;that alone&lt;/em&gt; determines the angle that the perigee burn must have. This doesn't have anything to do with optimization at this point. If we are doing a perigee + apogee burn to complete the entire transfer, then we have necessarily set the angles that each will require. They are determined by geometry.&lt;/p&gt;&#xA;&lt;p&gt;Here's some academic stuff:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.kdm.p.lodz.pl/articles/2011/V15_2_07KAM_SOL_2_2011.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.kdm.p.lodz.pl/articles/2011/V15_2_07KAM_SOL_2_2011.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;And from that, allow me to restate a proper name for it - &amp;quot;Hohmann orbit transfer with split–plane change&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;You can consider alternatives, but you'll back yourself into a corner regarding optimization if it looks anything like a Hohmann transfer. If you burned within your own plane at perigee and waited to do a plane change, then you'll need to cover more distance in a shorter time, and it doesn't make sense. If you separated the plane change and the Hohmann burns, then you're trading the hypotenuse of a triangle for the sum of its sides.&lt;/p&gt;&#xA;&lt;p&gt;Bi-Elliptic is still a possibility for energy minimization, but I do not know how to categorize what that combined (plane change plus other parameter changes) maneuver would look like, and obviously, it would only be relevant for a subset of all possible transfers.&lt;/p&gt;&#xA;" OwnerUserId="26" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-04T01:46:21.283" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19671" PostTypeId="1" CreationDate="2017-01-04T08:35:26.687" Score="9" ViewCount="310" Body="&lt;p&gt;Choice of launcher seems to be an early decision in the design of spacecrafts. There would be an advantage to having the option to easily change launcher, especially for an interplanetary mission with a rarely recurring launch window, in case the intended launcher is grounded for a long time. A mission prepared to deal with an emergency, such as a space tug to save a failed orbital insertion of a crewed or otherwise valuable spacecraft, or for last-minute deflection of an asteroid, would also benefit from being able to launch from any out of several frequently launched rocket types (replacing the payload of the next one prepared to go).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most frequently launched rockets nowadays are Atlas V, Falcon 9, Ariane 5 and Proton (assuming that Soyuz is too feeble). Incidentally, two of them have been grounded for 4 and 7 months now.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Could one design a spacecraft to go with at least three out of these four?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Would it need a bulky adapter stage?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Could one make adapters that could take almost any satellite or spacecraft on any of the launchers? (To steal the competitors' customers)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Proton is horizontally integrated, the others vertically. Is that outright disqualifying for inter-launcher payload compatibility?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I suppose one &quot;could&quot;, and that it comes down to how much would it would cost in mass and extra engineering. If it would be cheaper to have a dedicated launcher on some level of standby instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delta IV and the Japanese H-IIB are AFAIK both based on Delta, so they might be quite compatible. However, they are the most expensive launchers and do not launch very often.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-01-07T04:15:59.880" Title="What does it take to make a spacecraft able to launch on any out of several different launcher types?" Tags="|payload|launch-vehicle|integration|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19672" PostTypeId="2" ParentId="19671" CreationDate="2017-01-04T10:46:52.070" Score="10" Body="&lt;p&gt;The physical adapter is the easy part. These have standard dimensions. Commercial launch providers were quick to realize it'd be advantageous to offer compatibility. E.g. the Ariane 5 payload fairing dimensions were based on the Shuttle payload bay. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the user manuals for various launchers, you can see the differences: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;vibration and other environmental factors (a satellite has to be tested and qualified for the launcher environment, so you have to budget for more tests)&lt;/li&gt;&#xA;&lt;li&gt;maybe: electrical interfaces, although there are commonalities here too, for example the Ariane 5 and Falcon 9 use the same electrical connectors between launcher and satellite. I haven't had time yet to do a thorough comparison. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So provisionally: yes, you should be able to design a payload that's compatible with multiple rockets, without too much of a mass penalty.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-04T10:46:52.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19673" PostTypeId="1" CreationDate="2017-01-04T10:58:00.543" Score="2" ViewCount="176" Body="&lt;p&gt;If a spaceship were traversing a galaxy, which trajectory would require less energy: cut straight through from one spiral arm to the next, or keep curving within the spiral arms themselves. I guess what I'm wondering about here, is whether there is a denser field of gravity within the arms dragging on the vessel, or not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for your insight!&lt;/p&gt;&#xA;" OwnerUserId="18121" LastEditorUserId="195" LastEditDate="2017-01-04T11:09:09.073" LastActivityDate="2017-01-04T12:09:53.360" Title="travelling the galaxy within its spiral arms, or 'jump' from one to the other?" Tags="|interstellar-travel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19674" PostTypeId="2" ParentId="19673" CreationDate="2017-01-04T12:09:53.360" Score="4" Body="&lt;p&gt;There are about as many stars between the arms as in the arms! The spiral arms are visible because that's where stars are formed. Larger stars are brighter and burn their fuel faster, therefor large bright stars are necessarily young and are only found near where they were formed. Older stars just orbit straight through the arms. The spiral shape is some kind of standing wave phenomena that concentrates the cold gas out of which stars are formed into regions of such a shape. And that's where I leave the answer to astronomers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there's much more normal matter mass in the shape of gas than in the shape of stars, I suppose that spiral arms could have slightly higher mass density than the gaps, because cold gas somehow tends to concentrate there. But since many generations of short lived stars in the bright arms have ended up as star remnants (black holes, neutron stars, white dwarfs) which do not remain in the arms, I would think that the current short lived population of young large bright stars makes up only a small fraction of a galaxy's mass. And dark matter in some kind of spherical halo in and around the galaxy has several times the mass of all gas and stars anyway. &lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-01-04T12:09:53.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19676" PostTypeId="2" ParentId="19671" CreationDate="2017-01-04T14:18:45.507" Score="1" Body="&lt;p&gt;The launcher is chosen early is that once you know what launcher you intend to use, then you can determine how big (in particular how heavy) your satellite can be.  That would have a BIG impact on the design of the satellite.  I'm pretty sure that there have been times that a mission has had to change to a larger (typically more expensive) launcher because the satellite was too heavy, but can't come up with a reference right now.&lt;/p&gt;&#xA;" OwnerUserId="17059" LastActivityDate="2017-01-04T14:18:45.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19677" PostTypeId="2" ParentId="19671" CreationDate="2017-01-04T14:55:52.213" Score="6" Body="&lt;p&gt;Changing launchers late in the development cycle is unusual, but it does happen. For example, the next &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_(spacecraft)&quot;&gt;Cygnus&lt;/a&gt; mission (OA-7) has been &lt;a href=&quot;http://spacenews.com/orbital-to-launch-next-cygnus-mission-on-atlas-5/&quot;&gt;moved on to an Altas 5 from Antares in order to carry more cargo&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, it appears that OA and NASA made the decision to switch simply because the Atlas 5 can carry more mass to orbit. The Cygnus missions OA-4 and OA-6 &lt;a href=&quot;https://www.nasa.gov/image-feature/ula-atlas-v-rocket-with-cygnus-spacecraft-at-the-launch-pad&quot;&gt;also launched on Atlas&lt;/a&gt;, and the more powerful launcher enabled those missions to carry &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_(spacecraft)&quot;&gt;better than 2000 kgs&lt;/a&gt; more payload mass to ISS than the missions launched on Antares. It's desirable for OA-7 to do the same because of timing: both JAXA and SpaceX have experienced delays in their resupply contracts recently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Hobbes hints in his answer, the biggest difficulty in switching launchers is that nearly everything will need to be requalified. NASA requirements are exacting, but even fully commercial launches must convince &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_insurance&quot;&gt;their insurers&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Office_of_Commercial_Space_Transportation&quot;&gt;government regulators&lt;/a&gt; that their payload will not cause a hazard during launch - and in Cygnus's case, it also can't cause a hazard to ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everything from the structure to the software must pass qualification tests, and these tests are expensive in both time and money to perform (months of time and thousands of man hours, speaking from experience). It isn't worth the expense to do for other launchers, which is why you're unlikely to see vehicles &quot;designed for three of the four&quot;, as you put it. That said, if there's a legitimate need to switch, it's certainly possible - engineers are pretty resourceful.&lt;/p&gt;&#xA;" OwnerUserId="5851" LastActivityDate="2017-01-04T14:55:52.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19678" PostTypeId="1" AcceptedAnswerId="19680" CreationDate="2017-01-04T16:06:19.473" Score="3" ViewCount="421" Body="&lt;p&gt;The standard J-2 engine measures 3.38 metres in total length, with a nozzle exit diameter of 2.03 metres, to support its 27.5:1 expansion ratio. However, these dimensions are apparently identical for the simplified, tap-off cycle J-2S.&#xA;But the J-2S features an area ratio of 40:1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is that so?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question might have been answered if I could find any certain information about a change in combustion chamber dimensions. Unfortunately, the only graphic I have seen portrays a 'recontoured' nozzle but does not list any chamber redesign.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could it be down to a change in throat area for the J-2S? I seem to recall a mention of the engine sporting a De Laval type nozzle, with a smaller throat diameter permitted by the higher chamber pressure. It might help support the Isp jump, but does not appear very widely - and there are no dimension changes listed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is the J-2S engine nozzle actually larger than the J-2, and misinformation took its toll over the past half-century?&lt;/p&gt;&#xA;" OwnerUserId="12678" LastActivityDate="2017-01-04T22:48:13.067" Title="How is the Rocketdyne J-2S area ratio larger than that of the J-2?" Tags="|rockets|apollo-program|engine-design|saturn-v|nozzle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19680" PostTypeId="2" ParentId="19678" CreationDate="2017-01-04T17:40:25.140" Score="6" Body="&lt;p&gt;It appears that the J-2S has a smaller throat diameter than the J-2, and hence a greater expansion ratio with the same final nozzle diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/816454.pdf&quot; rel=&quot;noreferrer&quot;&gt;Altitude Testing of the J-2 Rocket Engine in Propulsion Engine Test Cell (J-4)&lt;/a&gt;, the 1966 J-2 had a throat area of 170.4 square inches, implying a throat diameter of 14.73&quot;. A 27.1:1 expansion ratio yields a 76.7&quot; exit diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The similarly-named 1970 paper, &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/874400.pdf&quot; rel=&quot;noreferrer&quot;&gt;Altitude Developmental Testing of the J-2S&lt;/a&gt;, gives a throat diameter of 12.192&quot; and 39.62:1 expansion ratio for the J-2S engine, for a throat area of 116.7 square inches and the same 76.7&quot; nozzle exit diameter.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-01-04T22:48:13.067" LastActivityDate="2017-01-04T22:48:13.067" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19681" PostTypeId="1" AcceptedAnswerId="24351" CreationDate="2017-01-04T20:06:56.450" Score="10" ViewCount="1177" Body="&lt;p&gt;Regarding a manned shelter on the surface of Mars...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With all of this esoteric talk of ice shelters, why not simply create concrete (Marscrete) buildings? The ice probably provides better radiation protection, but surely the difference is not that great.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, how thick do walls have to be?&lt;/p&gt;&#xA;" OwnerUserId="5965" LastEditorUserId="25" LastEditDate="2018-01-08T16:25:25.197" LastActivityDate="2023-02-20T17:53:13.223" Title="How thick would a Marscrete structure need to be to provide adequate protection against radiation?" Tags="|mars|colonization|radiation|isru|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19682" PostTypeId="2" ParentId="19671" CreationDate="2017-01-04T21:44:20.320" Score="6" Body="&lt;p&gt;In response to the part of the question &quot;Could one design a spacecraft to go with at least three out of these four?&quot; the answer is yes.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;This is actually quite common for geostationary communications satellites. It is quite typical for a customer to request that competing satellite manufacturers make their satellites compatible with two or three launch vehicles and the four that you mention are certainly chosen from.&lt;/li&gt;&#xA;&lt;li&gt;In a commercial satellite procurement that could take three years the customer could organise the tender amongst satellite manufacturers first and then either run the launch vehicle tender themselves in collaboration with the satellite manufacturer or simply delegate it to the satellite manufacturer. &lt;/li&gt;&#xA;&lt;li&gt;There can typically be two distinct stages; very early on the compatibility with several launch vehicles is soft, it is just enough to ensure real competition amongst the launchers. Later, once the satellite design is frozen and its build is underway two launch vehicles can have contracts as prime and back-up to allow a late change should there be scheduling problems. &lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Examples of real launch vehicle changes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacenews.com/echostar-books-a-commercial-atlas-5-for-jupiter-2/&quot; rel=&quot;nofollow noreferrer&quot;&gt;EchoStar Explains Why Jupiter-2 is Launching on Atlas instead of Ariane&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacenews.com/inmarsat-worried-about-spacex-falcon-heavy-delays-books-reservation-for-ils-proton-launch/&quot; rel=&quot;nofollow noreferrer&quot;&gt;For Europasat/Hellas-sat 3: &quot;Fearing SpaceX Falcon Heavy delays, Inmarsat reserves ILS Proton&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/viasat-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Viasat 2: &quot;was booked on SpaceX's Falcon-Heavy rocket for a launch in late 2016, but was moved to an Ariane-5ECA rocket in 2017 due to schedule uncertainies&quot;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This &quot;standardised&quot; situation I've described is probably more specific to GEO communications satellites where the main commercial platform designs are already structurally qualified for the main launch vehicle choices. Even in this context there is a lot to do in terms of routine analysis effort and perhaps changes that need to reflect:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The specifics of each type of payload in terms of its mass and thus LV performance impacts&lt;/li&gt;&#xA;&lt;li&gt;The vibration spectrum of each launch vehicle option that is cascaded down to mission specific components&lt;/li&gt;&#xA;&lt;li&gt;The couple loads analysis &amp;mdash; most relevant for any unusual items such as folded antennas&lt;/li&gt;&#xA;&lt;li&gt;The stowed accommodation inside the fairing for each LV type&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="6816" LastEditDate="2017-01-07T04:15:59.880" LastActivityDate="2017-01-07T04:15:59.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19683" PostTypeId="1" AcceptedAnswerId="19684" CreationDate="2017-01-04T23:11:22.273" Score="5" ViewCount="617" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_J-2&quot;&gt;J-2&lt;/a&gt; engine used on the upper stages of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_(rocket_family)&quot;&gt;Saturn launchers&lt;/a&gt; has a specific impulse in vacuum of 421 seconds. This is substantially lower than that of the smaller &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10&quot;&gt;RL10&lt;/a&gt;, 440-460 seconds depending on the model. The RL10 has a greater nozzle expansion ratio -- 47:1 up to 280:1 depending on the model, compared to 27:1 for J-2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the 5 J-2 engines were fairly crowded in the base of the &lt;a href=&quot;https://en.wikipedia.org/wiki/S-II&quot;&gt;Saturn V second stage&lt;/a&gt;, the single engine in the &lt;a href=&quot;https://en.wikipedia.org/wiki/S-IVB&quot;&gt;third stage&lt;/a&gt; is not; a nozzle extension doubling its 2.1 m diameter to 4.2 m would still allow for reasonable clearance versus the 6.6 m stage diameter. This would increase the nozzle ratio to around 100:1, significantly better than the ratio of the 450 s versions of the RL-10, and with a higher chamber pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nozzle extension and the necessary lengthening of the interstage would add mass -- 5 tons or so, at a guess -- but even with that penalty, achieving 450 second specific impulse on the third stage could increase translunar payload by around 2 tons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not particularly surprising that this approach wasn't used for Saturn/Apollo; developing and deploying a variant engine would come at a cost that might be better spent elsewhere. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Was this possibility at least considered during the Apollo program, or during post-Apollo proposals for further Saturn development?&lt;/li&gt;&#xA;&lt;li&gt;Is the difference in ISP between J-2 and RL10 attributable primarily to nozzle expansion ratio, or is propellant mix ratio or another factor contributing significantly? Would a 100:1 expansion ratio J-2 be able to achieve 450 s specific impulse at 5260 kPa chamber pressure?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-01-05T18:03:18.987" Title="Was a large-nozzle variant of the J-2 ever considered?" Tags="|engines|saturn-v|specific-impulse|nozzle|j-2|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19684" PostTypeId="2" ParentId="19683" CreationDate="2017-01-05T02:13:00.440" Score="5" Body="&lt;p&gt;Remember when the J-2X engine was proposed for the Ares I and V rockets? That featured a nozzle area ratio of 55:1. This ratio was the same as that of the original J-2X proposal. However, unlike the new design that would add performance through gas generator and chamber enhancements, the old J-2X was a simplification study.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The modern J-2X and its 55:1 expansion ratio could achieve a vacuum Isp of 448s or 451s, although this would operate at a chamber pressure above 1,200psi instead of the original 763psi.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you mentioned, the clustered S-II arrangement of five engines would not allow for any increase in nozzle dimensions. You are also correct about S-IVB interstage length, which limited the J-2 size. But we must remember, too, that there are RL-10 variants with extending nozzles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.astronautix.com/graphics/r/rl10b2a.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;RL-10B-2&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1Sch4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1Sch4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The J-2X was to be essentially a standard J-2 with an extension kit attached to the original nozzle exit. In the stowed position the J-2X would have the same length as the J-2, and the effective area ratio would still be 27.5:1. Upon actuation of the nozzle, it would extend (in a required time of under two seconds) and produce a 55:1 area ratio nozzle. This increased nozzle diameter could easily pass through the S-II/S-IVB interstage. The studies required that the extension kit weigh less than 204kg (450lb). Using this expansion ratio, the J-2 engine's vacuum specific impulse at a 5.5:1 LOX:fuel ratio would be increased by 10 to 11 seconds. This would be sufficient for a 3,200 to 4,200 pound improvement in Saturn V payload (I believe this was to TLI), minus the weight of the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three different methods of extending the nozzle in flight were proposed. One system involved a one-piece nozzle extension that would surround the engine when stowed and lower down with hydraulic arms when deployed (much like the RL-10B-2). This proposal was discontinued due to its similarity with the XLR-129 engine under study (the shuttle engine that lost out to the SSME). A similar proposal would have broken the extended nozzle into several 'rings' that would telescope down over each other. However, the added complexity of the system caused it to be eliminated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More info here:&#xA;&lt;a href=&quot;http://www.secretprojects.co.uk/forum/index.php?topic=16981.5;wap2&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.secretprojects.co.uk/forum/index.php?topic=16981.5;wap2&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That left the Airmat design. This would involve a nozzle constructed from a strong woven mesh that would inflate into the desired nozzle shape as soon as the engine was started. Exhaust from the gas generator would be routed through the mesh nozzle, and this would also cool the material. This gas could only vent inwards, allowing film cooling. not only was this design simple in operation, but it was also lightweight. Therefore, it would likely promise the greatest payload gain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/J-2X_Airmat_DWG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;graphic&lt;/a&gt; shows the basic principle of the 'Airmat' design, complete with the exhaust venting through the inner wall for film cooling:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/kCWUF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kCWUF.png&quot; alt=&quot;http://www.alternatewars.com/BBOW/Space_Engines/J-2X_Airmat_DWG.png&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The three concepts that made the grade for final analysis:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/NmmK3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NmmK3.png&quot; alt=&quot;http://www.alternatewars.com/BBOW/Space_Engines/J-2X_NozzleConcepts.png&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope this is helpful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Al.&lt;/p&gt;&#xA;" OwnerUserId="12678" LastEditorUserId="6944" LastEditDate="2017-01-05T18:03:18.987" LastActivityDate="2017-01-05T18:03:18.987" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="19686" PostTypeId="2" ParentId="84" CreationDate="2017-01-05T12:37:15.317" Score="1" Body="&lt;p&gt;Yes the Moon has most of the building block materials but without the helping industries such as mining, purification, molding, etc., you cannot have your parts ready for the spacecraft.&#xA;I also think that even if we send everything from Earth to the Moon just to see if we can successfully assemble those parts it would be very very difficult since we don't have facilities over there. &lt;/p&gt;&#xA;" OwnerUserId="18010" LastEditorUserId="6816" LastEditDate="2017-01-05T19:51:22.083" LastActivityDate="2017-01-05T19:51:22.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19687" PostTypeId="1" CreationDate="2017-01-02T14:48:55.450" Score="-5" ViewCount="382" Body="&lt;p&gt;I'm gonna write some things based on high school physics. Suppose we need to send a person to the moon. Then, we need to calculate two things, the speed v and the angle $\theta$ with which the spacecraft should be launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose we have considered all the factors like gravitational forces of Earth and moon and sun and of other planets also and taken into consideration Earth's rotation and all other factors and have calculated accurate values of these two quantities. I don't know what the accurate value of v will be but since escape velocity is 11.6Kmps and moon is much closer than infinity, so let's take v=5Kmps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, for the spacecraft, all we need is a strong box, maybe of iron, having necessary oxygen and food for the astronauts. We just have to launch this box with the astronatus inside at 5Kmps. If the combined mass of astronauts with the box is 1000 Kg. Then, the kinetic energy of the spacecraft will be 1.25*$10^{10}$J. So, we have to do this much work on the box.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But now we have nuclear energy, so we don't really need to burn diesel or natural gas. 1/5g of uranium-235 can produce more than that. And then we just need to supply that heat to a high specific heat capacity to raise its temperature, transfer that heat to a very cool temperature body like very cool ice, so that the efficiency of the engine is maximum and we'll obtain the work required from that heat engine. Maybe such engine will hit the box of astronauts with tremendous force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we can minimize the force by a hydraulic lift. The engine will push a piston of larger area with great force through smaller distance and hence the piston of smaller area will hit the box with less force through a larger distance. So, the box will get the required energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this is not a feasible idea, then what are some corrections that you would apply to this? I think that the apparent weight of astronauts when the box is accelerating is not a problem, because we can use magnetic suits and magnets to balance the pseudo force and keep their weight normal. I'm only minimizing the force to the extent the material of the box can endure and not according to the weight experienced by the astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wikipedia article says that the working stress for AISI 1018 Steel with a 4 factor of safety is 110MPa. So, if our box is a cube made of this material of surface area, say, 5m$^2$, then it can endure a force of 550MPa, which means an acceleraton of 550 Kms$^{-2}$ for our 1000 Kg box. But our magnetic forces are limited, so we'll somewhat minimize this force by some orders of magnitude, so that 11Kmps is reached over a convenient distance on the second piston and such that our magnetic forces can somewhat cancel the pseudo force. Does anyone have an idea for landing?&lt;/p&gt;&#xA;" OwnerDisplayName="Dove" LastActivityDate="2017-01-05T22:48:34.230" Title="Is this apparatus enough to send a person to Moon?" Tags="|rockets|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19688" PostTypeId="2" ParentId="19687" CreationDate="2017-01-02T15:41:58.697" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I don't know what the accurate value of v will be but since escape velocity is 11.6Kmps and moon is much closer than infinity, so let's take v=5Kmps.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Mistake number one. The delta V cost to launch traditionally into low Earth orbit is about 10 km/s. The delta V cost to get near the Moon is almost Earth escape velocity. The delta V needed to land on the Moon is more than Earth escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Now, for the spacecraft, all we need is a strong box, maybe of iron, having necessary oxygen and food for the astronauts. We just have to launch this box with the astronatus inside at 5Kmps. If the combined mass of astronauts with the box is 1000 Kg.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Mistake number two. &quot;Just&quot; is one of the most dangerous words in engineering (and software development, for that matter). Unless that box carries a rocket and propellant, you &quot;just&quot; smacked that box into the Moon at at least 2.4 km/s. Your box needs to be a lot bigger in order to softly land the box and its content on the Moon. It needs to be considerably bigger yet if the people in the box are to come back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Maybe such engine will hit the box of astronauts with tremendous force.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Maybe? That's mistake number three. Suppose we want to accelerate an object to 11 km/s via a rail launcher / big gun / piston / whatever, while keeping the acceleration to 30 m/s&lt;sup&gt;2&lt;/sup&gt; (a bit over 3*g*). The rail launcher / big gun / piston / whatever would have to be over 2000 km long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we have yet another problem, which is atmospheric drag. Objects going at Mach 5 a dozen kilometers or so above the surface of the Earth have a nasty tendency to melt. Orbital velocity (low Earth orbit) is Mach 25 or higher. Your 11 km/s is Mach 32 at standard temperature and pressure. The heating rate due to atmospheric drag is roughly proportional to the cube of velocity. An object going at Mach 32 at STP accumulates heat at 260 times the rate needed to start melting things. &lt;/p&gt;&#xA;" OwnerUserId="2752" OwnerDisplayName="David Hammen" LastActivityDate="2017-01-02T15:41:58.697" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19689" PostTypeId="2" ParentId="19687" CreationDate="2017-01-02T17:11:29.923" Score="3" Body="&lt;p&gt;First, the Moon is actually quite &quot;close to infinity&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The minimum velocity can be calculated from energy conservation. I.e. you start with some velocity $v_0$ at Earth radius $R$ and end up with Moon's orbital velocity $v_m$ (so that you were in rest in Moon frame of reference) at its orbit $r$, then energy conservation reads,&#xA;\begin{equation}&#xA;-G\frac{Mm}{r}+\frac{mv_m^2}{2}=-G\frac{Mm}{R}+\frac{mv_0^2}{2}&#xA;\end{equation}&#xA;However $R/r\sim 60$ and because of that the potential energy at Earth radius $G\frac{Mm}{R}$ happens to be much higher than potential energy at Moon's orbit and corresponding kinetic energy. If you calculate it, you'll get $v_0$ about $50$ m/s less than Earth escape velocity which is about $0.5\%$ difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then when you approach Moon it's gravity starts playing and in practice considering ship approaching Moon from infinity is good approximation too. So no matter what you do at Earth, the ship will approach Moon surface with approximately its escape velocity about $2.4$ km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now what you should remember when you talk about launch and landing is that humans are quite squishy. You can have very hard ship but it's acceleration felt by humans that's important in the end. Let's take $10g$ as maximum &quot;safe&quot; acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now your ship may have bumper. When ship lands bumper's front hits surface first and as it deforms slowers the crew cabin. Let's assume that all this time crew cabin has $10g$ acceleration. The problem is that to slower from $2.4$ km/s with this acceleration one needs about $t=\frac{v}{10g}\simeq24.5$ s. In that time the ship will travel $10g t^2/2\simeq 29$ km. So to slower the ship with bumper it has to be at least $29$ km thick... Even worse with your piston dampering the hit at the launchpad idea because now you have to take $v\simeq 11$ km/s.&lt;/p&gt;&#xA;" OwnerUserId="18138" OwnerDisplayName="OON" LastActivityDate="2017-01-02T17:11:29.923" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19691" PostTypeId="2" ParentId="19687" CreationDate="2017-01-05T22:48:34.230" Score="3" Body="&lt;p&gt;You need to leave atmosphere with 12.52 km/sec to reach the moon.  You need to leave the atmosphere with 12.61 km/sec to reach infinity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition you have to shed 2.55 km/sec of velocity that you will get from the moon as you fall towards it.  You actually need more than this as once you light your rocket you start taking gravity losses.  With a sufficiently developed infrastructure on the moon there could be some capture system but that's for well down the road as it's a huge piece of machinery that must operate on tight tolerances between extremely rapidly moving parts.  You'll still need maneuvering rockets even then because your launch accuracy won't be perfect and drag in space isn't perfectly predictable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trying to anything like a piston arrangement simply isn't feasible.  Instead, look at magnetic levitation trains.  Since there are no wheels involved there is no actual speed limit, boosting a train car to interplanetary velocities is simply major engineering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Barring a revolution in physics I expect to see such a system built--&lt;strong&gt;on the moon&lt;/strong&gt;.  If we can tack a few nines onto the reliability we might even build such a system &lt;strong&gt;above&lt;/strong&gt; Earth.  (This would require a support structure based on tossing projectiles back and forth at hypersonic velocities.  While the numbers work it's dynamic--stop throwing the projectiles and it comes crashing to Earth and the anchor stations do crude approximations of atomic bombs.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We will never build one &lt;strong&gt;on&lt;/strong&gt; Earth, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heating has been mentioned in other posts but that's not even the biggest deal at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First there's the shockwave as your capsule goes down the track.  That shockwave carries a lot of energy and we don't just have a track underneath.  The usual designs involve a string of hoops--but they won't survive the shockwaves.  Nobody has come up with a means of making the track survive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest to an answer to this anyone has proposed so far is that the track be evacuated.  At that point boosting it to the 12.52 km/sec is simply major engineering.  It can't stay in the track forever, though--you need to somehow build an airlock that allows it out while keeping the air away.  (Probably something that snaps open at the last instant) but this simply reduces the shockwave problem--you still have incredibly destructive shockwaves as the capsule exits the tube.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next comes a problem for which I have never seen anything resembling an answer: drag.  Your astronauts die when the capsule hits the air.  I don't have the physics knowledge to compute the best answer here but from what I have been able to find I have gotten some crude answers that are more than an order of magnitude over what the human body can take.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, for a manned launch you have another problem--your capsule has to be gargantuan if it so reach space at all.  The problem is that to make the boost survivable it must be almost perfectly horizontal because there's a limit to how deep underground you can go.  While a horizontal launch is no problem from an airless body it's a very big problem if there's an atmosphere in the way.  A very high speed projectile will expend it's energy once it has displaced a mass equal to it's own mass.  Your capsule needs to be very long and thin and very heavy in order to punch through all the air that it will encounter this way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We just might see something along these lines for some cargo launches.  Bore your launch tube straight down into a mountain and boost at gun type accelerations.  Even then it's extremely tricky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mankind has launched one projectile from the Earth's surface at something above escape velocity.  Perhaps even at solar system escape velocity.  (The photographs can assign a minimum velocity but they can't measure the velocity.)  It didn't head for the stars, though--it was no doubt vaporized by it's passage through the air.  (And don't look to reproduce it, either--the boosting force was a hydrogen bomb.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-01-05T22:48:34.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19692" PostTypeId="1" CreationDate="2017-01-05T23:17:55.117" Score="1" ViewCount="137" Body="&lt;p&gt;If I am an astronaut on a spacewalk in zero-g, and I reach out and hold onto a pole (sticking out from a large space station) on front of me (with the pole parallel to my head-to-feet axis), and then I twist the pole in the direction of my fingers (around the poles long axis) I should be able to rotate myself around the pole, right? Now what happens when I let go of the pole... Do I start to rotate around my CoM, and also gain a translation at a tangent to where I let go?&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="12102" LastEditDate="2017-01-07T10:46:41.410" LastActivityDate="2017-01-07T10:46:41.410" Title="Zero-g astronaut motion" Tags="|astronaut-lifestyle|reduced-gravity-sports|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19693" PostTypeId="2" ParentId="1059" CreationDate="2017-01-05T23:31:25.773" Score="4" Body="&lt;p&gt;ISRO is &lt;a href=&quot;http://economictimes.indiatimes.com/news/science/isro-to-launch-record-103-satellites-in-one-go-in-february/articleshow/56334137.cms&quot; rel=&quot;nofollow noreferrer&quot;&gt;scheduled to launch 103 satellites on a single PSLV next month (Feb 2017)&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="18140" LastEditorUserId="6816" LastEditDate="2017-01-06T01:05:23.223" LastActivityDate="2017-01-06T01:05:23.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19694" PostTypeId="2" ParentId="19692" CreationDate="2017-01-06T01:14:15.807" Score="2" Body="&lt;p&gt;Yes and yes, whatever linear and angular momentum you have immediately before you let go will continue indefinitely after you let go. Basically Newton's first law.&lt;/p&gt;&#xA;" OwnerUserId="14419" LastActivityDate="2017-01-06T01:14:15.807" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19696" PostTypeId="1" AcceptedAnswerId="19703" CreationDate="2017-01-06T12:10:44.090" Score="4" ViewCount="1578" Body="&lt;p&gt;Boeing and the Marshall Space Flight Centre brought to pass many studies of improved Saturn V launch vehicles. The rockets proposed under the MLV and ELV projects would feature longer stages for more propellant capacity, and would be fitted with new, uprated engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://archive.org/stream/NASA_NTRS_Archive_19650020081#page/n13/mode/2up&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://archive.org/stream/NASA_NTRS_Archive_19650020081#page/n13/mode/2up&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/h/hg-3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.astronautix.com/h/hg-3.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/HG-3_(rocket_engine)&quot; rel=&quot;nofollow noreferrer&quot;&gt;HG-3&lt;/a&gt; was one such engine. Some basic information on the engine is as follows.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Total length = 3.38 metres&lt;/li&gt;&#xA;&lt;li&gt;Nozzle exit diameter = 2.03 metres&lt;/li&gt;&#xA;&lt;li&gt;Vacuum Isp = 451 seconds&lt;/li&gt;&#xA;&lt;li&gt;Vacuum thrust = 314,900lbf (to be increased to 375klbf and 400klbf)&lt;/li&gt;&#xA;&lt;li&gt;Sea level Isp = 280 seconds&lt;/li&gt;&#xA;&lt;li&gt;Chamber pressure = 3,000psia&lt;/li&gt;&#xA;&lt;li&gt;LOX:fuel mixture ratio = 5.1:1 to 5.3:1&lt;/li&gt;&#xA;&lt;li&gt;Operating cycle = Staged combustion&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, there is very little other information available on the engine, as it was never built – no test data or proper geometry. However, there are certain traits it shares with other engines that may permit close estimates. The HG-3 ‘formed the basis’ for the design of the SSME, that engine also being a hydrogen/oxygen staged combustion unit with a vacuum Isp of 453 seconds and a very similar chamber pressure. The most powerful 400klbf HG-3s would have been roughly 80,000 pounds short on vacuum thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, the HG-3 designs show a very short nozzle that expands quickly. The engine also features a much larger power head and turbopump assembly than the J-2S.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Av1hR.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Av1hR.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, given this similar performance and close heritage to the RS-25:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Could the nozzle area ratio of the HG-3 have been similar to that of the SSME’s 69:1?&lt;/strong&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;What causes the large disparity of sea level specific impulse (280s vs 366s) even with the nearly identical chamber pressures?&lt;/strong&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A sea level-optimised variant is also featured on Astronautix. Vacuum Isp barely changes while sea level Isp rises dramatically to 360 seconds. The only change listed is a lower expansion ratio nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/h/hg-3-sl.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.astronautix.com/h/hg-3-sl.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Why is the vacuum Isp change so small while the sea level change is so large?&lt;/strong&gt; (Changing area ratio often does this, but here it seems disproportionally large)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Al.&lt;/p&gt;&#xA;" OwnerUserId="12678" LastEditorUserId="198" LastEditDate="2017-01-07T20:13:03.967" LastActivityDate="2017-01-07T20:13:03.967" Title="Is it possible to estimate the nozzle characteristics of the Rocketdyne HG3 engine?" Tags="|apollo-program|space-shuttle|engine-design|saturn-v|ssme|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19701" PostTypeId="2" ParentId="6468" CreationDate="2017-01-06T20:25:59.890" Score="2" Body="&lt;p&gt;The &quot;Suitport&quot; images you have seen are conceptual. They serve to illustrate the concept and capture people's imaginations, and showing any additional structure would be counter to that purpose. If we ever move forward seriously with the concept it's entirely likely that some sort of method to protect the suits when not in use will be added. This may be something as light and simple as a removable &quot;garment bag&quot; or retractable tarp covering, a lightweight non-pressurizable porch or vestibule, or all the way up to a heavy pressurizable &quot;garage.&quot; So much about a manned Mars mission is still conceptual that it's not worth the effort at this time to do detailed studies of debris threats to external suits and the various means of protecting them. When we get really serious about a crewed Mars mission, then we will nail down debris threat standards and MTBF targets for the suits, compare them against the mission duration and mission implications of losing a single suit, and make an informed decision. My crystal ball says when the time comes (30 years?) we WILL go with the Suitport concept, the suits will be protected by a sort of tarp or sleeping bag, and they will throw in a spare suit just in case. A spare suit should be much lighter than an additional hard protective structure, and protects against multiple contingencies, not just ones brought about by external suit storage.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2017-01-06T20:25:59.890" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19702" PostTypeId="2" ParentId="12447" CreationDate="2017-01-06T20:53:01.610" Score="3" Body="&lt;p&gt;The answer to your headline question is that gloves with fingers have been used so far because remote-controlled robot hands with good tactile feedback are still an unsolved problem even now. Force-feedback mechanisms are expensive and tricky to get right, and they can be dangerous if the motors you use to transmit the force are too strong. Creating a way to transmit the feeling of, say, an edge to a person's fingertips is still more challenging. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the manipulator end, there are issues of power, complexity, and reliability to consider. Parallel to the problem of synthesizing the feeling of an edge on the user's fingers, is the problem of developing an appropriate array of sensors on the manipulator end. Those images you showed are, I'm pretty sure, research prototypes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could even make the case that, once you have solved the problem of remote manipulators, it no longer makes sense to put the person in a space suit at all. That is not quite true, since balance and walking are still issues, for instance. But the full telepresence robot requires roughly the same overall level of technical prowess as the hands alone, if you imagine research going on in parallel in different areas.&lt;/p&gt;&#xA;" OwnerUserId="10259" LastActivityDate="2017-01-06T20:53:01.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19703" PostTypeId="2" ParentId="19696" CreationDate="2017-01-06T22:00:57.157" Score="6" Body="&lt;p&gt;Here is a mathematical answer to your question. Using the numbers you provided for the HG-3, we have,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$I_{sp} = 451 \ \text{s} \ (\text{vac})$$&#xA;$$F = 1,400,745 \ \text{N} \ \ (314,900 \ \text{lbf}) \ (\text{vac})$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this alone, we can get an estimate of the steady mass flow rate through the engine, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \dot{m} = \frac{F}{I_{sp} g} = 317 \ \text{kg/s}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we can figure out the size of the throat from the choked flow condition at the nozzle throat. The mass flow parameter can be rearranged to the form,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ A_t = \frac{\dot{m} \sqrt{T_1}} {p_1} \sqrt{\frac{R}{\gamma}}  \left(\frac{\gamma+1}{2}\right)^{\frac{(\gamma+1)}{2(\gamma-1)}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\dot{m}$ is the flow rate through the engine, $T_1$ is the chamber temperature, $p_1$ is the chamber pressure, $R$ is the specific gas constant of the burned products, and $\gamma$ is the ratio of specific heats of the burned products. Using the NASA Chemical Equilibrium with Applications (CEA) code for an average LOX:fuel mixture ratio of 5.2:1 at a chamber pressure of 3,000 psia, we obtain,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ T_1 = 3,414 \ \text{K} $$&#xA;$$ R = 678 \ \text{J/kg$\cdot$K} $$&#xA;$$ \gamma = 1.16 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting these values with the previously calculated mass flow rate yields,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ A_t = A^{*} = 0.0363 \ \text{m$^2$} \ \ (0.3910 \ \text{ft$^2$})$$&#xA;$$ d_t = d^{*} = 0.215 \ \text{m} \ \ (0.705 \ \text{ft})$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now from your provided information, we were given that $d_e$ = 2.03 m, namely, $A_e$ = 3.235 m$^2$. So our nozzle expansion area ratio required to obtain a vacuum $I_{sp} = 451 \ \text{s}$ is given by,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \boxed{ \epsilon = \frac{A_e}{A_t} = \frac{A_e}{A^*} = 89.1 }$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is the answer to your first question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we want to look at the sea-level performance. We will assume the mass flow rate through the engine is similar to the vacuum scenario, since the LOX/fuel mixture ratio only fluctuates by $\pm$ 1 and we already took the average for sizing the nozzle throat above. We need to determine the nozzle exit parameters. For idealized quasi one-dimensional compressible flow, the Area-Mach relation can be used to estimate the nozzle exit Mach number. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{A_e}{A_t} = \frac{A_e}{A^*} = \frac{1}{M_e}\left[\frac{2}{\gamma+1} \frac{\gamma-1}{2}M_e^2\right]^{\frac{\gamma+1}{2(\gamma-1)}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the Newton-Raphson numerical technique to solve the above for $M_e$ given $A_e/A_t$ = 89.1 we obtain,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ M_e = 4.52 $$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is conventional in idealized rocket analysis to assume isentropic expansion of the burned products through the nozzle. Hence, from isentropic relations we have at the nozzle exit plane,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ p_e = p_1 \left(1 + \frac{\gamma-1}{2} M_e^2 \right)^{-\frac{\gamma}{\gamma-1}} $$&#xA;$$ T_e = T_1 \left(1 + \frac{\gamma-1}{2} M_e^2 \right)^{-1} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To which we obtain,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$p_e = 18,735 \ \text{Pa} \ \ (2.72 \ \text{psia}) $$&#xA;$$T_e = 1,285 \ \text{K} \ \ (1,853 \ \text{$^{\circ}$F}) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further, the sound speed on the nozzle exit plane is given by,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a_e = \sqrt{\gamma R T_e} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To which we obtain,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ a_e = 1,006.5 \ \text{m/s} \ \ (3,302 \ \text{ft/s}) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence, our nozzle exit velocity is simply,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ V_e = a_e M_e = 4,551 \ \text{m/s} \ \ (14,931 \ \text{ft/s}) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now our sea-level thrust is given by,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ F = \dot{m} V_e + \left(p_e - p_a\right) A_e $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where at sea-level, $p_a$ = 101,325 Pa (14.7 psia). Substituting all of the calculated parameters yields,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ F = 1,173,595 \ \text{N} \ \ (263,834 \ \text{lbf} ) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, the sea-level specific impulse is given by,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ I_{sp} = 378 \ \text{s} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is obviously based on idealized rocket performance analysis. Additionally, the sea-level specific impulse is very large compared to the 280 you presented in the question. So what is going on here? Well, it turns out we are going to suffer performance losses from over-expansion with a nozzle that has a nozzle expansion area ratio of roughly 89. According to Sutton (Rocket Propulsion Elements), if $p_a$ is slightly higher than $p_e$ then we will have our idealized $I_{sp} \approx$ 378 and our nozzle will continue to flow full. He further states that the nozzle will continue to flow full while developing a slight contraction until the nozzle exit pressure drops to about 40% of the ambient. However, in our case we have very severe over-expansion at the sea-level condition, where our nozzle exit pressure is only approximately 18% of the sea-level ambient back pressure condition. According to the Summerfield criterion (Flow Separation in Over-expanded Supersonic Exhaust Nozzles”, Jet propulsion, Vol. 24, No. 9, page 319-321, 1954), which is a general rule of thumb, the nozzle flow will separate at only $p_e/p_a$ = 0.4. So in our case, we will definitely have flow separation in the divergent portion of the nozzle, which will drastically undercut our anticipated ideal $I_{sp}$. Below is a schematic from Sutton's text that demonstrates the effects of flow separation in the diverging portion of the nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gkA2X.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gkA2X.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The general answer to your questions go as follows:&lt;br&gt;&#xA;1.) Yes the HG-3 engine could achieve a vacuum $I_{sp} = 451 \ \text{s}$ provided the nozzle expansion area ratio was roughly $\epsilon = 89$.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.) Nozzle expansion area ratio plays a large role in the over-expansion effects regarding the nozzle flow losses and separation. Cutting back the expansion area ratio to something similar to the SSME (69) alleviates this issue at the sea-level condition. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;3.) The sensitivity of sea-level performance to over-expansion is much more severe than the performance gains from higher expansion area-ratios in a vacuum back pressure condition. From the NASA CEA code and the conditions previously mentioned, the vacuum specific impulse goes as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \begin{array}{c|c}&#xA;\epsilon = \frac{A_e}{A_t} &amp;amp; I_{sp} - \text{s} \ (\text{vac}) \\&#xA;\hline&#xA;60 &amp;amp; 445 \\&#xA;70 &amp;amp; 447 \\&#xA;80 &amp;amp; 449 \\&#xA;90 &amp;amp; 451 \\&#xA;100 &amp;amp; 452 \end{array} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could also tabulate the sea-level performance, however, it is also the ideal sea-level performance and won't account for the performance losses due to over-expansion effects. Determining the losses due to over-expansion effects requires either full numerical solutions to the Navier-Stokes equations, or experimental testing. &lt;/p&gt;&#xA;" OwnerUserId="18022" LastActivityDate="2017-01-06T22:00:57.157" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19704" PostTypeId="2" ParentId="5848" CreationDate="2017-01-06T22:40:32.673" Score="19" Body="&lt;h1&gt;TL;DR&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Without SRBs, by maintaining the real stack's thrust to weight ratio at ignition, you run out of fuel at about 167 seconds with (if we maintain the flight profile of the real stack) a velocity around 1.4 km/s and an altitude of around 70 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without SRBs, with an initial thrust to weight ratio of 1, you run out of fuel at about 289 seconds with (if we maintain the flight profile of the real stack) a velocity around 2.4 km/s and an altitude of around 100 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To attain a stable, low Earth orbit, you need around 7.4 km/s at approximately 110 km altitude.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;A few data points&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Orbiter (specifically OV-105) had an &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_orbiter#Shuttle_Orbiter_Specifications_.28OV-105.29&quot; rel=&quot;noreferrer&quot;&gt;empty weight of 68,585 kg&lt;/a&gt; (68.6 Mg) or &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_shuttle#Specifications&quot; rel=&quot;noreferrer&quot;&gt;110,000 kg&lt;/a&gt; (110 Mg) at liftoff&lt;/li&gt;&#xA;&lt;li&gt;The Super Lightweight External Tank had an &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_external_tank#Super_Lightweight_Tank&quot; rel=&quot;noreferrer&quot;&gt;empty weight of 26,500 kg&lt;/a&gt; (26.5 Mg) or &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_shuttle#Specifications&quot; rel=&quot;noreferrer&quot;&gt;756,000 kg&lt;/a&gt; (756 Mg) when fully fueled&lt;/li&gt;&#xA;&lt;li&gt;The Orbiter plus the Super Lightweight External Tank thus have a combined empty weight of 95.1 Mg.&lt;/li&gt;&#xA;&lt;li&gt;The Orbiter (again OV-105) three main engines provided 1,752 kN thrust each (when running at &lt;a href=&quot;https://space.stackexchange.com/q/16971/415&quot; title=&quot;Which STS mission raised the normal engine throttle above 100%, and what change to the SSME made that possible?&quot;&gt;104%&lt;/a&gt; at sea level) for a total of 5,255 kN or 536 Mgf on SSMEs only&lt;/li&gt;&#xA;&lt;li&gt;The two &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_shuttle#Specifications&quot; rel=&quot;noreferrer&quot;&gt;solid rocket boosters&lt;/a&gt; (SRBs) massed 571 Mg each at liftoff, and contributed 12,500 kN or 1,274 Mgf of thrust each&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Common assumptions&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Given that the SSMEs burned through 629,340 kg of LOX and 106,261 kg of LH2 in a &lt;strong&gt;480 seconds normal ascent burn,&lt;/strong&gt; the burn rate is about 1.3 Mg/second LOX and 0.22 Mg/second LH2. We also see that the LOX to LH2 mass flow ratio is about 5.92:1 as $\frac{629\,340}{106\,261} \approx \frac{5.92}{1}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For simplicity, I'm not allowing for any payload whatsoever. In this scenario, if you want to carry a payload, doing so will necessarily reduce the amount of fuel you can bring, because you are basically transferring liftoff mass from fuel (which you need to get off the ground) to, as far as propulsion is concerned, dead weight.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Realistic case: thrust to weight ratio at ignition same as a normal Space Shuttle including SRBs&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;For a normal Space Shuttle, including external tank and SRBs, the total thrust provided by all five engines (three main engines plus two SRBs) is $3 \times 1\,752 + 2 \times 12\,500 = 30\,255$ kN or about 3,085 Mgf at liftoff. At the same time, the total mass of the fully equipped spacecraft is $110 + 756 + 2 \times 571 = 2\,008$ Mg. This gives a thrust to weight ratio (TWR) of $\frac{3\,085}{2\,008} \approx 1.536$ at the moment of ignition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because absent the SRBs we have 536 Mgf of thrust, we can allow for $\frac{536}{1.536} \approx 349$ Mg of gross liftoff weight while maintaining the thrust to weight ratio. Subtracting the empty weight of the Orbiter and the SLET leaves about 254 Mg that we can use for fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping the 5.92:1 LOX/LH2 ratio, we can load up about 36.7 Mg of LH2 and 217 Mg of LOX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By burning 1.3 Mg/second LOX and 0.22 Mg/second LH2, this fuel load lasts $\frac{217}{1.3} \approx \frac{36.7}{0.22} \approx 167$ seconds. Consequently, &lt;strong&gt;we run out of gas in about 167 seconds.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Absolute best theoretical case: thrust to weight ratio = 1 at ignition&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Since the orbiter plus external tank with no payload whatsoever weighs 95 Mg, and you have 536 Mgf of thrust, this leaves you with about 440 Mg that you can use for fuel and still get the spacecraft to move at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keeping the LOX/LH2 mass flow ratio the same, in 440 Mg we can fill the ET with some 376 Mg LOX and 63.6 Mg LH2 ($376 \approx 63.6 \times 5.92$). Assuming that the mass flow rate is unchanged with this lower initial mass, &lt;strong&gt;this fuel load will be depleted after 289 seconds,&lt;/strong&gt; give or take a fraction of a second or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this configuration will require very different launch facilities to keep the spacecraft from being destroyed by reflected energy before it has time to climb sufficiently that this is not a major problem. But since the Space Shuttle absent the SRBs is already a quite different spacecraft from what we had, I think it's safe to say that we can also adjust the launch facilities to accomodate the different spacecraft design.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Where does that leave us?&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;I couldn't find any nice official NASA graph for altitude versus time for the Space Shuttle (if anyone knows of one, please comment!), but velocity versus time was easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://forums.xkcd.com/viewtopic.php?t=103771#p3419255&quot; rel=&quot;noreferrer&quot;&gt;This xkcd forums post&lt;/a&gt; by davidstarlingm references &lt;a href=&quot;https://science.ksc.nasa.gov/shuttle/missions/sts-30/sts-30-press-kit.txt&quot; rel=&quot;noreferrer&quot;&gt;the NASA STS-30 press kit&lt;/a&gt;, which among a few other notable points in the trajectory puts &quot;Negative Return&quot; at 238 seconds with a velocity of 6,915 ft/s which is a hair under 2,108 m/s and an altitude of about 319,000 feet or 97 km. While I doubt the real curve is this nice (taking into account for example the engine dethrottling late in the ascent to maintain a maximum 3 G acceleration should result in a more S-shaped curve), looking at the plot of the velocity versus time data and the function describing the data points available:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VAU4q.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VAU4q.png&quot; alt=&quot;y = 0.0002x^3 - 0.0753x^2 + 38.683x - 362.56&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;puts &lt;strong&gt;the Space Shuttle velocity around the 290 second mark at somewhere around 8,000 ft/s or 2,400 m/s.&lt;/strong&gt; That's the theoretical best case, where we started out with a TWR = 1. As a quick approximation, &lt;a href=&quot;https://space.stackexchange.com/a/2077/415&quot; title=&quot;How far do you have to be from Earth to be “in space”?&quot;&gt;we actually got to space today&lt;/a&gt; in terms of altitude; the Kármán line is at 100 km altitude, and we at least passed 97 km altitude some time before our engines shut down due to fuel exhaustion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the more realistic case maintaining the original stack's &lt;strong&gt;TWR = 1.536 at T-0, at 167 seconds the spacecraft's velocity is around 5,000 ft/s or 1,500 m/s.&lt;/strong&gt; At the same time you are about halfway between the real spacecraft's SRB staging at 125 seconds (153,405 ft altitude in the case of STS-30) and negative return at 238 seconds (STS-30: 319,008 ft altitude), so let's split the difference and call it 236,200 ft or 72 km. In this case, we are nowhere near space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare these to the real STS-30 trajectory, which was attained with main engines cutoff (MECO) at 511 seconds (so actually a somewhat longer burn) and 24,286 ft/s (7.4 km/s) velocity at about 362,000 ft or 110 km altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No matter how you slice it, from 1.5 km/s or 2.4 km/s to 7.4 km/s is quite a long way to go, and especially in the case of TWR = 1.536, we also still have a fair distance to climb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a consequence, &lt;strong&gt;at these points the Space Shuttle Orbiter is an unguided, unpowered ballistic projectile on its way back toward the ground.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2018-05-31T20:20:57.693" LastActivityDate="2018-05-31T20:20:57.693" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="19705" PostTypeId="1" CreationDate="2017-01-07T10:43:21.100" Score="3" ViewCount="164" Body="&lt;p&gt;Is it possible for a &quot;home made&quot; rocket to launch an object into a stable orbit assuming a person has calculated the right trajectory?&lt;/p&gt;&#xA;" OwnerUserId="18148" LastActivityDate="2017-01-07T10:43:21.100" Title="Can a DIY rocket put an object into a stable orbit around earth?" Tags="|amateur-rocketry|model-rocketry|hybrid-rocket|" AnswerCount="0" CommentCount="1" ClosedDate="2017-01-07T14:22:52.780" ContentLicense="CC BY-SA 3.0" />
  <row Id="19706" PostTypeId="1" AcceptedAnswerId="23291" CreationDate="2017-01-07T10:44:01.503" Score="5" ViewCount="590" Body="&lt;p&gt;I was enjoying the &lt;a href=&quot;https://www.youtube.com/watch?v=ekWOS818g2w&quot; rel=&quot;noreferrer&quot;&gt;Trailer&lt;/a&gt; to the YouTube site &lt;a href=&quot;https://www.youtube.com/channel/UCf9iavSS2yRObwyHpCcmz0Q&quot; rel=&quot;noreferrer&quot;&gt;Kinematic&lt;/a&gt;, and I noticed that the satellite or model shown in the beginning looks different than how I remember the original Sputnik - specifically the location and direction of the antennas. While the satellite is shown, the narration begins:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Sixty years ago, the Russians beat us into space. We didn't deny Sputnik was up there. We didn't argue about the science...&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is this a representation of a Sputnik? If not, what is it supposed to be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&quot;Bonus points&quot;&lt;/strong&gt; Where is this clip from - is it old &quot;found&quot; footage, perhaps from some 1960's documentary? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lFyIe.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lFyIe.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF reconstructed from screen shots from the &lt;a href=&quot;https://www.youtube.com/watch?v=ekWOS818g2w&quot; rel=&quot;noreferrer&quot;&gt;Kinematic Trailer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ekWOS818g2w?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-05-12T01:45:02.017" LastActivityDate="2021-05-12T01:45:02.017" Title="Puzzler: Is this a Sputnik?" Tags="|artificial-satellite|history|space-art|museum-exhibits|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19707" PostTypeId="1" AcceptedAnswerId="19709" CreationDate="2017-01-07T11:20:21.773" Score="6" ViewCount="1953" Body="&lt;p&gt;There is the popular question “What is the gravity at the center of the Earth?”. And the answer is zero, because the forces cancel out. And then the gravity increases linearly as you move to the surface. Would the same be true for a rotating cylinder?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I mean, if, for example, I have a cylinder with a radius 270 of meters, rotating at 2 rpm, you can calculate, that the gravity will be about 1.21 g on the surface of the cylinder, right? &#xA;And then, if I am inside and I go deeper (towards the center) in the cylinder while it is still spinning, would the gravitational force decrease to zero (for example, when I'm 80 meters in, so in about 190 meters distance from the center, would be in a place with 0.8 g)?&lt;/p&gt;&#xA;" OwnerUserId="18149" LastActivityDate="2017-10-14T15:23:39.607" Title="What is the gravity inside a rotating cylinder?" Tags="|gravity|artificial-gravity|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19708" PostTypeId="1" AcceptedAnswerId="19710" CreationDate="2017-01-07T11:41:19.340" Score="9" ViewCount="575" Body="&lt;p&gt;I saw this message on &lt;a href=&quot;https://www.space-track.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;space-track.org&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The JSpOC has identified a close approach between two non-maneuverable satellites in a sun-synchronous orbit (approximately 800km altitude) with a time of closest approach at 21:53:00 UTC (16:53 EST) on 7 January 2017. The probability of collision has been predicted as high as 44%. Affected operators have been notified.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Which two satellites are involved? I'd like to calculate the ground location below where the two orbits intersect in the unlikely even that there will be an &amp;quot;Earth-shattering Kaboom!&amp;quot; but I can not find an event at January 7, near 21:53 in &lt;a href=&quot;https://celestrak.org/cgi-bin/searchSOCRATES.pl?IDENT=NAME&amp;amp;NAME_TEXT1=&amp;amp;NAME_TEXT2=&amp;amp;ORDER=MAXPROB&amp;amp;MAX=100&quot; rel=&quot;nofollow noreferrer&quot;&gt;Socrates&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The multiple answers to &lt;a href=&quot;https://space.stackexchange.com/q/19405/12102&quot;&gt;this question&lt;/a&gt; are worth reading for additional background.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; &lt;a href=&quot;https://space.stackexchange.com/questions/19708/#comment52295_19708&quot;&gt;@Chris noted&lt;/a&gt; that today's &lt;a href=&quot;http://spaceflight101.com/close-orbital-encounter-january-7-2017/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101 article&lt;/a&gt; discusses this particular conjunction.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eHKfi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eHKfi.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Marvin_the_Martian&quot; rel=&quot;nofollow noreferrer&quot;&gt;Marvin the Martian&lt;/a&gt; has appeared on the &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Nasa_mer_marvin.png#Fair-use_rationale_for_Marvin_the_Martian&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Mission patch&lt;/a&gt; of Mars Exploration Rover (Spirit).&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/QuUJfYcn3V4?start=22&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;Or perhaps this is more appropriate for a potential collision:&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/LkbWF-kDK8Y?start=35&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2017-01-08T11:32:31.660" Title="Which two satellites had a 44% probability of collision at 2017-01-07 21:53 UTC?" Tags="|artificial-satellite|impact|collision-avoidance|two-line-elements|space-track|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19709" PostTypeId="2" ParentId="19707" CreationDate="2017-01-07T12:32:53.720" Score="5" Body="&lt;p&gt;Yes and no. &#xA;As long as you're on a solid surface: yes. 'Gravity' (centripetal force, really) decreases proportionally with distance to the center.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if the cylinder has a vacuum inside (so no air mass that rotates with the cylinder), you'd be weightless as soon as you left the surface. If you were to cancel out your forward speed (=the rotation speed of the surface you were standing on), you'd be able to float just above the surface. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-07T12:32:53.720" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19710" PostTypeId="2" ParentId="19708" CreationDate="2017-01-08T01:47:55.463" Score="7" Body="&lt;p&gt;First of all, it was a miss. Whew!&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The close approach predicted at 21:53:00 UTC on 7 Jan 2017 has passed without incident. The JSpOC has confirmed that both satellites are being tracked as single objects, indicating that no collision has occurred. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A few tidbits that might help narrow it down:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The percentage is very high, which means the objects must be large.&lt;/li&gt;&#xA;&lt;li&gt;They are in a Sun-Synchronous orbit.&lt;/li&gt;&#xA;&lt;li&gt;Neither object can maneuver. This means they are either inactive, or have fuel depleted.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The most popular theory, although it might never be confirmed publicly, is DMSP F15 &amp;amp; Meteor 1-26. DMSP F15 is a military weather satellite, meaning it's exact orbital predictions are likely classified (Or at least not for public release), which would explain why the difficulties in identifying the satellites. Meteor is a defunct USSR weather satellite.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-01-08T01:47:55.463" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19712" PostTypeId="1" AcceptedAnswerId="39923" CreationDate="2017-01-08T08:17:38.970" Score="8" ViewCount="310" Body="&lt;p&gt;The phys.org new item &lt;a href=&quot;http://phys.org/news/2017-01-image-small-satellite-deployed-space.html&quot; rel=&quot;noreferrer&quot;&gt;Image: Small satellite deployed from the Space Station&lt;/a&gt; shows the cubesat craft Space Tethered Autonomous Robotic Satellite (STARS-C) being deployed from the ISS.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A satellite is ejected from the Japan Aerospace Exploration Agency (JAXA) Small Satellite Orbital Deployer on the International Space Station on Dec. 19, 2016. The satellite is actually two small satellites that, once at a safe distance from the station, separated from each other, but were still connected by a 100-meter-long Kevlar tether.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is the third in a series of STARS tethered cubesat missions, and the 100 meter tether is a significant step beyond the 10 meter tether of &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/stars-2&quot; rel=&quot;noreferrer&quot;&gt;STARS-2&lt;/a&gt; for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of scientific and technical issues to be addressed by this series of missions, my question is only about the &lt;em&gt;tether tensions method of attitude control&lt;/em&gt; of the 'daughter satellite'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bMmCi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bMmCi.jpg&quot; alt=&quot;Space Tethered Autonomous Robotic Satellite-2)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Figure 8: Attitude control model of the daughter sub-satellite by the arm link technique (image credit: Kagawa University)&quot; from &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/stars-2&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am wondering about the dynamics of this system during an attitude change. If I understand correctly, the daughter sub-satellite holds the tether line with an articulated &quot;arm&quot;, and changing the orientation of the arm (articulating the arm's  &quot;shoulder&quot;) will change the sub-satellite's attitude with respect to the tether direction, and therefore the nadir.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a significant attitude change (say 30 degrees) made this way tend to set up oscillations requiring damping? Could it be used to point a small telescope or directional antenna to track one point on the Earth (about 1.1 degrees/sec at 400km) for say 30 seconds, then slew to the next point within 5 or 10 seconds, then track that point, or would the whole thing just start oscillating in some way, requiring some kind of damping mechanism?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't mean perfect ground tracking, just something to get rid of most of the approximately 7 km/sec apparent ground motion of the earth seen through a telephoto lens camera or small telescope so that reasonable shutter speeds (or frame capture rates) can be used with modest f/no.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-01-24T00:16:21.233" LastActivityDate="2019-11-15T17:14:15.380" Title="How well could the tether tension method of attitude control actually ultimately work for a cubesat system?" Tags="|cubesat|attitude-determination-and-control|space-tether|formation-flying|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="19713" PostTypeId="2" ParentId="13211" CreationDate="2017-01-08T12:07:26.653" Score="0" Body="&lt;p&gt;The F9 first stage remains hypersonic until only shortly before landing - this is evidenced by the sonic boom emitted shortly before landing as it transitions to subsonic speed probably less than 10 seconds before landing.&lt;/p&gt;&#xA;" OwnerUserId="18155" LastActivityDate="2017-01-08T12:07:26.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19714" PostTypeId="1" CreationDate="2017-01-08T15:00:34.377" Score="3" ViewCount="999" Body="&lt;p&gt;Is there an ultimate limit to the thrust outputs of electric propulsion thrusters?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although they work best providing low amounts of thrust for long periods of time, could improvements in technology allow their thrust to compete with that of modern chemical rockets, or is it just their nature to have a low thrust output (despite a high specific impulse)?&lt;/p&gt;&#xA;" OwnerUserId="17552" LastActivityDate="2017-01-10T19:03:51.480" Title="Electrical Propulsion Thrust" Tags="|electric-propulsion|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19715" PostTypeId="2" ParentId="13211" CreationDate="2017-01-08T16:10:01.067" Score="6" Body="&lt;p&gt;Addressing the &quot;has SpaceX exceeded the record&quot; part of the question: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.spaceacademy.net.au/spacelink/blackout.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;graph&lt;/a&gt; shows that a Space Shuttle reentry was in the hypersonic flight regime for approximately 25 minutes.  (I took 1.7 km/s ~ Mach 5 ~ end of hypersonic regime).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nRgw4.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nRgw4.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm pretty sure that's longer than the entire flight time for a recovered Falcon 9 first stage.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6537" LastEditDate="2017-12-18T19:56:49.887" LastActivityDate="2017-12-18T19:56:49.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19716" PostTypeId="2" ParentId="19714" CreationDate="2017-01-08T16:16:50.920" Score="5" Body="&lt;p&gt;In principle, ion thrusters can be scaled to very large thrusts, but their thrust per watt of input power is inherently low. This means that the power supply for any electrical thruster is going to be very heavy, so the thrust-to-weight ratio is always going to be low. This holds for solar, radiothermal, and even nuclear reactor supplies; it's possible that future power plant technology could shift this balance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft solar panels are very lightweight and efficient -- they deliver on the order of 100 watts per kilogram of panel at Earth's distance from the sun, but the delivered power rapidly decreases as you get farther from the sun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;RTGs produce only 2-5 watts per kilogram, and the few nuclear fission plants that have been used in spacecraft delivered no more than about 15 watts per kilogram.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-01-08T18:17:30.443" LastActivityDate="2017-01-08T18:17:30.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19717" PostTypeId="1" CreationDate="2017-01-08T16:29:39.353" Score="1" ViewCount="286" Body="&lt;p&gt;With advances in chemical engineering, could chemical propulsion have a fuel (or fuels) that would allow it to compete with newer, more advanced form of propulsion (such as electrical or nuclear) or are the chemicals being used now likely to be as good as it's going to get?&lt;/p&gt;&#xA;" OwnerUserId="17552" LastActivityDate="2017-01-08T20:06:08.410" Title="Chemical Fuel Improvements" Tags="|fuel|chemistry|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19718" PostTypeId="1" AcceptedAnswerId="19719" CreationDate="2017-01-08T16:36:44.987" Score="5" ViewCount="3117" Body="&lt;p&gt;We all know ISS is orbiting the earth so it has translational motion, but, does it rotate on its own axis as well?&#xA;Thanks&lt;/p&gt;&#xA;" OwnerUserId="18156" LastActivityDate="2020-01-18T20:00:53.720" Title="Does the ISS have a rotational motion in addition to its translational motion?" Tags="|iss|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19719" PostTypeId="2" ParentId="19718" CreationDate="2017-01-08T16:56:21.820" Score="8" Body="&lt;p&gt;Yep! Since the same side of the ISS is always facing down towards the earth, every time it orbits once around the Earth, it rotates once around its axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually, every time an ocean liner travels around the Earth once, it has also rotated around it's port-&lt;s&gt;stern&lt;/s&gt; starboard axis. A better example would be a plane, it slowly rotates around the axis through it's wings as it travels around the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some nice images in some of these &lt;a href=&quot;https://www.quora.com/In-space-travel-which-way-is-up&quot; rel=&quot;nofollow noreferrer&quot;&gt;Quora answers&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually I will repost one of them here. The ISS is traveling in the direction of it's own $X_{LVLH}$ axis, and so it rotates about it's $Y_{LVLH}$ axis once per orbit. LVLH stands for &quot;Local Vertical, Local Horizontal.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WkrA1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WkrA1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OpZev.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OpZev.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; ISS spacecraft directions, from &lt;a href=&quot;https://www.nasa.gov/images/content/409334main_iss_orientation.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-01-09T00:51:58.557" LastActivityDate="2017-01-09T00:51:58.557" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="19720" PostTypeId="2" ParentId="19714" CreationDate="2017-01-08T17:05:48.537" Score="8" Body="&lt;p&gt;Thrust isn't the bottle neck. Rather thrust to mass. In other words, acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 10 kilowatt Hall thruster exerts about half a newton. Half a newton is about a tenth of a pound or a little more than an ounce.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the power source, Hall Thruster and payload have a total mass of 500 kilograms, an ounce of force doesn't give much acceleration. About $1 millimeter/second^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can you get low mass power sources that provide enough juice? Possibly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very thin solar panels might do it. But deploying solar panels thinner than Saran Wrap® might be difficult. There would also need to be supporting structure as well as gimbals to point the array.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How about a nuclear power source? There is a need to dump waste heat. Here on earth, nuclear power plants often are located near a river and water carries away waste heat. This is a lot harder in space — vacuum is a great insulator. You would likely need many square meters of radiator surface to dump the heat. Again, the need for a lot of area but little mass mandates thin, fragile structures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ratio of mass to power is sometimes called Alpha. I talk about this more in my blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/the-need-for-better-alpha.html&quot; rel=&quot;noreferrer&quot;&gt;The Need For a Better Alpha&lt;/a&gt;. Another related article is &lt;a href=&quot;https://hopsblog-hop.blogspot.com/2016/09/xenon.html&quot; rel=&quot;noreferrer&quot;&gt;Xenon&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-01-08T17:05:48.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19721" PostTypeId="2" ParentId="19717" CreationDate="2017-01-08T18:55:43.477" Score="1" Body="&lt;p&gt;Exceeding the specific impulse (i.e. mass efficiency) of electrical or nuclear propulsion is unlikely with chemical propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Practical chemical rockets top out around &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10#Current_design&quot; rel=&quot;nofollow noreferrer&quot;&gt;460 seconds Isp&lt;/a&gt;, with exotic, but impractical, propellants going up into the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tripropellant_rocket#Simultaneous_burn&quot; rel=&quot;nofollow noreferrer&quot;&gt;500s&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nuclear-thermal rockets (using the heat of a controlled fission reaction to accelerate hydrogen or other propellants) were expected to deliver better than 800 seconds Isp. Such engines were developed (&lt;a href=&quot;https://en.wikipedia.org/wiki/NERVA&quot; rel=&quot;nofollow noreferrer&quot;&gt;NERVA&lt;/a&gt;, &lt;a href=&quot;http://www.astronautix.com/r/rd-0410.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;RD-0410&lt;/a&gt;) but never flown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electrical thrusters (&lt;a href=&quot;https://en.wikipedia.org/wiki/Hall-effect_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hall effect&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;ion&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;VASIMR&lt;/a&gt;, etc) produce Isp in the 2000-5000 second range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Competing on thrust instead of specific impulse, of course, the rankings are reversed; electrical thrusters have extremely low thrust-to-mass ratios, and chemical propellants very high ratios, with NTR in between.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-01-08T19:01:57.350" LastActivityDate="2017-01-08T19:01:57.350" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19722" PostTypeId="1" AcceptedAnswerId="19723" CreationDate="2017-01-08T19:09:07.860" Score="10" ViewCount="2407" Body="&lt;p&gt;I watched the Kimbrough and Whitson's EVA on 01/06.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I'm wondering why astronauts use custom screwdriver and not the usual ones (like Makita or Dewalt, for example) ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/d/d5/STS-131_EVA3_Rick_Mastracchio_2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bUK1C.jpg&quot; &gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://image.prntscr.com/image/6acc25988f79468398e5e42a1d34eba1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://image.prntscr.com/image/6acc25988f79468398e5e42a1d34eba1.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it because of the battery technology ? Rotation speed ? Something else ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there something that makes the usual tools do not work in space ?&lt;/p&gt;&#xA;" OwnerUserId="18158" LastEditorUserId="10273" LastEditDate="2019-01-24T02:29:59.200" LastActivityDate="2019-01-24T02:29:59.200" Title="Spatial screwdriver" Tags="|eva|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="19723" PostTypeId="2" ParentId="19722" CreationDate="2017-01-08T19:37:22.960" Score="24" Body="&lt;p&gt;The &lt;a href=&quot;http://www.techbriefs.com/Briefs/July99/GSC13706.html&quot; rel=&quot;noreferrer&quot;&gt;Pistol Grip Tool&lt;/a&gt; has some features not found in DIY or even professional cordless drills. &#xA;It can control its torque and speed exactly, and torque/speed/turn count values can be programmed. It also measures and records the applied torque.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.popularmechanics.com/space/a24639/space-drill-used-by-nasa-astronauts/&quot; rel=&quot;noreferrer&quot;&gt;In general, space tools&lt;/a&gt; also need to be usable while wearing thick gloves, and they need to work in extreme temperatures and a vacuum, and they need to be tested to make sure they don't interfere with anything. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-01-08T19:48:30.930" LastActivityDate="2017-01-08T19:48:30.930" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19724" PostTypeId="2" ParentId="19717" CreationDate="2017-01-08T20:06:08.410" Score="1" Body="&lt;p&gt;Honestly, the efficiency of chemical rockets we use today are nearly maxed out for practical fuel combinations. Also, chemical rocket engines will most likely never reach the efficiency metrics of electrical and nuclear propulsion systems (the propulsive energy conversion is fundamentally different). However, when you need a substantial amount of thrust, the thrust to weight ratio of chemical propulsion systems dominates the playing field. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The combustion process used today in all rockets/gas-turbines is called deflagration, which is a relatively slow release of heat and a constant pressure combustion process. The only thing that can be done to raise the $I_{sp}$ for chemical propulsion systems (and is currently being studied) is detonating the fuel, which is a pressure gain combustion process. Detonation based propulsion systems come in two varieties (at least at the research and experimental level). The first is called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pulse_detonation_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;pulsed detonation engine&lt;/a&gt; (PDE), which has received a great deal of treatment since the early 90s. The second is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rotating_detonation_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;rotating detonation engine&lt;/a&gt; (RDE), which is heavily active in current aerospace research. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a plot from an analytical study (Morris, C. I., “Numerical Modeling of Single-Pulse Gasdynamics and Performance of Pulse Detonation Rocket Engines,” Journal of Propulsion and Power, Vol. 21, No. 3, 2005, pp. 527-538.) that details the idealized performance of a single-cycle pulse detonation rocket engine vs the conventional steady state rocket engine (SSRE) for oxyhydrogen fuel. Note, the blowdown pressure ratio does not go to infinity (i.e. a vacuum) so comparison with the vacuum level performance is not reported, however, the idealized PDRE is more efficient than the conventional rocket engine over the entire operational blowdown pressure ratio range. Now granted this is all idealized performance, and harnessing the energy of detonation wave for thrust applications is generally very challenging. Here are some &lt;a href=&quot;https://www.youtube.com/watch?v=rYxsilgRxi4&quot; rel=&quot;nofollow noreferrer&quot;&gt;attempts&lt;/a&gt; in the laboratory setting at the academic level. This testing at the academic level is conducted by PhD students, however more sophisticated operations exist that are not open source at facilities like the Air Force Research Laboratory. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NtcKS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NtcKS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RDE is a more sophisticated concept for a detonation based engine. It is more practical in application than the PDE, due to its non-pulse operation. Here is a schematic of the operation of the RDE. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/czsa5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/czsa5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/IzdnT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IzdnT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RDE is actively being researched at many universities and industry companies (Aerojet/Rocketdyne and the Aerospace corporation to name a few). The primary goal as of now (economically convincing for funding) is to integrate the PDE/RDE into next generation gas turbine engines. However, the rocket application remains wide open given the maturing technology. Here are some videos of RDE testing at various universities:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1.) &lt;a href=&quot;https://www.youtube.com/watch?v=QbJgOk2rDbI&quot; rel=&quot;nofollow noreferrer&quot;&gt;University of Texas at Arlington&lt;/a&gt;&lt;br&gt;&#xA;2.) &lt;a href=&quot;https://www.youtube.com/watch?v=pHcxI-8GtZg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Purdue University&lt;/a&gt;&lt;br&gt;&#xA;3.) &lt;a href=&quot;https://www.youtube.com/watch?v=AP6dOa0AcCw&quot; rel=&quot;nofollow noreferrer&quot;&gt;University of Cincinnati&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18022" LastActivityDate="2017-01-08T20:06:08.410" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19726" PostTypeId="1" AcceptedAnswerId="19733" CreationDate="2017-01-09T07:38:11.407" Score="10" ViewCount="705" Body="&lt;p&gt;Suppose for some reason it was particularly advantageous to stop the ISS from &lt;a href=&quot;https://space.stackexchange.com/a/19719/12102&quot;&gt;rotating once per orbit&lt;/a&gt;, so that it maintained a constant attitude with respect to the stars (or the Sun) instead of with respect to the Earth. Maybe some kind of emergency procedure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the ISS de-spin itself in say a half-day - or better yet a few hours - remain that way for a few days, and then spin up again for normal operation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in the attitude control - does propulsion exist to do this, and could the attitude control system either handle this or be put into a safe state and allow the maneuver to be managed and monitored by the crew? I didn't say ground because I am not sure if comms link could be managed in this state without advanced planning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any structural issues caused by all the torques, stresses and tensions involved?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2019-07-30T22:05:14.967" Title="Could the ISS stop rotating wrt the stars for a few days, then start again?" Tags="|iss|attitude|attitude-determination-and-control|emergency|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19727" PostTypeId="1" AcceptedAnswerId="19729" CreationDate="2017-01-09T07:57:31.793" Score="9" ViewCount="4762" Body="&lt;p&gt;Assume the approximation that the Earth is spherically symmetric &amp;mdash; without higher order gravitational multipoles, and other effects that might be related to gravity from other bodies or the Earth's orbit around the Sun are ignored. Given a circular orbit, &quot;down&quot; would be a vector pointed towards the center of the Earth, and I think the &quot;nadir&quot; would be in exactly the same direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in a realistic circular orbit, do &quot;down&quot; and &quot;nadir&quot; have any more nuanced meaning by convention or usage? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, there is a vector that is perpendicular to instantaneous velocity and orbital angular momentum, another that is the local gravity gradient, and yet another that points towards the Earth's center of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of these could be thought of as roughly &quot;down&quot; or in the direction of the &quot;nadir&quot;, but are any of these (or something else) used preferentially or by convention? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; This question is asking a lot more than just &lt;a href=&quot;//space.stackexchange.com/q/3631&quot;&gt;What are these orientations called in orbit?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6816" LastEditDate="2017-01-09T17:07:19.313" LastActivityDate="2017-01-10T13:32:40.327" Title="In &quot;spacecraft talk&quot; is nadir just a fancy word for &quot;down&quot;?" Tags="|artificial-satellite|mathematics|frames|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19729" PostTypeId="2" ParentId="19727" CreationDate="2017-01-09T13:15:25.293" Score="10" Body="&lt;p&gt;There are some subtleties here. The fields where the concept of nadir are most important are nadir-pointing Earth observation satellites, satellites formation flying, and rendezvous and proximity operations. The latter two almost inevitably use some form of a local vertical / local horizontal frame (aka a Hill frame), in which vertical (i.e., nadir and zenith) points to / away from the center of the Earth. This is primarily because of the use of the linearized Clohessy-Wiltshire equations in these fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first almost inevitably uses nadir as pointing toward the subsatellite point, which is usually taken to mean the geodetic subsatellite point rather than the geocentric subsatellite point. The geocentric subsatellite point is where the line connecting the center of the Earth with the satellite intersects the surface of the Earth. The geodetic subsatellite point is that point on the surface of the reference ellipsoid that is closest to the satellite. This is equivalent to the point on the ellipsoid where the satellite is at zenith (&quot;straight up&quot;). Using nadir as pointing to the center of the Earth will miss this geodetic subsatellite point by up to 21 km for a satellite in a sun synchronous orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An even more subtle meaning is that nadir points in the direction of the point on the geoid that is closest to the satellite. This concept is important in satellite altimetry measurements of sea surface height. If this point is within the beam width of the radar altimeter, it is this point that will register as the satellite's altitude. Some other point will give an erroneous reading if this point is outside the altimeter's beam width. The distance between this point and the geodetic subsatellite point can be nearly a kilometer.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-01-10T13:32:40.327" LastActivityDate="2017-01-10T13:32:40.327" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19730" PostTypeId="2" ParentId="8078" CreationDate="2017-01-09T13:19:57.010" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://docs.google.com/viewer?a=v&amp;amp;pid=sites&amp;amp;srcid=ZGVmYXVsdGRvbWFpbnxjYXN0YXdheW1pc3Npb258Z3g6NzRhZDRmYTBmMjczNDg3Yg&quot; rel=&quot;nofollow noreferrer&quot;&gt;CASTAway&lt;/a&gt; (PDF) is a mission proposed for ESA that would flyby 10 asteroids in the Main Belt and observe tens thousands telescopically to determine their composition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4sfVz.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4sfVz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-01-09T13:19:57.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19731" PostTypeId="2" ParentId="19727" CreationDate="2017-01-09T16:29:14.057" Score="4" Body="&lt;p&gt;In the ISS Operations community, &quot;zenith&quot; and &quot;nadir&quot; are commonly used to refer to the Z axis directions relative to the ISS, instead of specifically Earth-pointing directions.  (Although they got these names because the ISS does normally fly in an attitude that points the so-called nadir direction at the earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an example from a &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/123838main_iss_shuttle_joint_ops_book.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;crew procedure&lt;/a&gt; where one of the Node docking ports is referred to as the Nadir port - &quot;ISS coordinates assume MPLM on Node Nadir&quot;.  Pitching the station 180 degrees wouldn't cause this name to change, of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dSWIt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dSWIt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an image from a &quot;ISS Location Coding&quot; lesson showing the same concept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P99jS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P99jS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-01-09T16:36:08.347" LastActivityDate="2017-01-09T16:36:08.347" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19732" PostTypeId="2" ParentId="19726" CreationDate="2017-01-09T16:54:00.350" Score="8" Body="&lt;p&gt;I don't think it's happened in a long time, but in the early stages of assembly, the ISS sometimes flew in a &quot;XPH&quot; attitude when the beta angle* was between 10 and 75 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vjsVd.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vjsVd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only public info I could find on this is from a rather annoying &lt;a href=&quot;http://web.archive.org/web/20160303180117/https://spaceflight.nasa.gov/station/flash/start.swf&quot; rel=&quot;nofollow noreferrer&quot;&gt;flash animation NASA page&lt;/a&gt;; it shows some animations of the orbits, here are two frames from the XPH example showing that it's at least quasi-inertial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qlkm1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qlkm1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/s5vxV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s5vxV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this stage, the proto-ISS would fly in XVV (X axis to velocity vector), the above mentioned XPH, or YVV (Y axis to velocity vector), for various values of the beta angle, maneuvering between them as required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Note: the beta angle is defined as the angle between the Sun vector and the ISS orbital plane.  High values of the beta angle imply the ISS is in sunlight longer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/22Ecf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/22Ecf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="32596" LastEditDate="2019-07-30T22:05:14.967" LastActivityDate="2019-07-30T22:05:14.967" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19733" PostTypeId="2" ParentId="19726" CreationDate="2017-01-09T19:09:37.663" Score="11" Body="&lt;p&gt;This is not a full answer, but some numbers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/11924/what-are-the-iss-moments-of-inertia-around-design-axes&quot;&gt;What are the ISS moments of inertia around design axes?&lt;/a&gt;&#xA;The total moment of inertia of the station is about $M = 55\cdot 10^6 \rm kg m^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/323/how-often-must-the-iss-desaturate-its-control-moment-gyros&quot;&gt;How often must the ISS desaturate its control moment gyros?&lt;/a&gt;&#xA;The reaction wheels are desaturated when they reach $13000 \rm ft lbf sec$ which is $L = 17 \rm kJs = 17 \rm kgm^2s^{-1}$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dividing the two gives us $L/M = 1/3200~\rm rad/s = 1/20,000 Hz$. This means, if we just spin up a reaction wheel to full power, the station will start to rotate once every 5.5 hours. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In its usual orbit, the station rotates once every 90 minutes, about three and a half times as fast. That means, reaction wheels are not sufficient and thrusters needs to be used. As desaturation of reaction wheels takes place every few months, we can conclude that this is well within reach of the capabilities of ISS.&#xA;I can not give an estimate how fast the change in rotation can be done with respect to stability of the station.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-01-09T21:33:47.597" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19734" PostTypeId="1" CreationDate="2017-01-09T20:05:33.903" Score="1" ViewCount="1285" Body="&lt;p&gt;There are multiple simulator-games around (orbiter, lunar flight), but how accurate they are? Is there &quot;more scientific&quot; (not a game)  simulator?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Q:&#xA;What is the most accurate and commercially available Space Flight Simulator? &#xA;In the Space Flight Simulator:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;User can Control/manipulate Spacecraft and It`s flight trajectory&lt;/li&gt;&#xA;&lt;li&gt;Flight trajectory (orbital or any) is realistic&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18169" LastEditorUserId="18169" LastEditDate="2017-01-10T23:12:49.827" LastActivityDate="2017-01-10T23:12:49.827" Title="Space Flight Simulator" Tags="|spacecraft|trajectory|simulation|" AnswerCount="1" CommentCount="11" ClosedDate="2017-01-10T08:20:24.433" ContentLicense="CC BY-SA 3.0" />
  <row Id="19736" PostTypeId="1" CreationDate="2017-01-10T05:46:41.807" Score="4" ViewCount="808" Body="&lt;p&gt;Why did the Moon form differently on the near side and the far side?  The near side has many mare, where the crust was thinner and lava came out after the moon was impacted by meteors.  But why was the crust on the near side thinner?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related: &lt;a href=&quot;https://space.stackexchange.com/q/2631/58&quot;&gt;Why is the Far Side of the Moon so different from the Near Side?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note: The above explains that the crust is thinner, but not why.)&lt;/p&gt;&#xA;" OwnerUserId="18173" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-01-11T00:49:34.283" Title="Why is the crust on the near side of the Moon thinner than on the far side?" Tags="|the-moon|planetary-science|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19737" PostTypeId="2" ParentId="19734" CreationDate="2017-01-10T07:38:18.400" Score="1" Body="&lt;p&gt;&lt;a href=&quot;http://www.space-shuttle-mission.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Mission&lt;/a&gt; is an excellent simulation of what the crew do inside the shuttle, ie, flipping switches. But it is not a realistic orbital flight simulator and has little to no orbital mechanics simulation, it is all on rails with pre-designed flights, whichis realistic).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbiter&lt;/a&gt; is a good general purpose simulator, if you stick to realistic vessels. Not as much cockpit realism as ssm2007 but better flight (you can choose your own orbits and perform unscripted rendevous and docking manouvers for instance)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are much more accurate orbital mechanics simulators and calculators available than the above, but these generally don't include any kind of flying or piloting aspect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have been playing a little with &lt;a href=&quot;http://trajectorysolution.com/ZOOM%20Program.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ZOOM&lt;/a&gt; (a trajectory and flight simulator) but haven't done enough with it yet to say if its good or not.. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, kerbal space program isn't very accurate but its quite good at simulating an engineering mindset and space program development approach.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2017-01-10T15:56:58.320" LastActivityDate="2017-01-10T15:56:58.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19739" PostTypeId="1" CreationDate="2017-01-10T10:56:42.440" Score="7" ViewCount="327" Body="&lt;p&gt;I am curious to know what will happen to an average satellite over a 1 million year time span. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, how high does the perigee need to be in order to not deorbit due to atmospheric friction during that time frame, and is it likely or unlikely that other large bodies eventually perturb the satellite so that it leaves orbit (burns up or escapes is irrelevant).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;what effects will radiation pressure and solar wind (1-6x10^-9N/m^2) have on an average satellite (about 3000kg?) Can the satellite be pushed out of orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much erosion might occur to the satellite due to micro impacts and solar wind impacting on it etc.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2017-01-10T11:15:10.690" LastActivityDate="2017-01-10T23:35:59.023" Title="What happens to satellites over a million years" Tags="|orbital-mechanics|solar-wind|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19740" PostTypeId="2" ParentId="19714" CreationDate="2017-01-10T11:03:21.920" Score="1" Body="&lt;p&gt;TL;DR: If you want more thrust, you can either get more energy or just reduce the propellant efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long version:&#xA;You can create electric propulsion for any range of thrust/Isp (propellant efficiency).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It basically boils down to the impulse formula and the kinetic energy formula.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All types of electric propulsion turn electric energy into kinetic energy. The amount of electric energy depends on your power source. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have an amount of energy X you can turn it into kinetic energy by &#xA;$E= 0.5mv^2$.&#xA;&lt;strong&gt;But&lt;/strong&gt;, the amount of thrust you generate depends on the impulse formula:&#xA;$I = mv$.&#xA;There is your problem: to get more thrust you can increase either mass expelled per second or the speed at which you expel that mass. But increasing speed of propellant requires an exponential increase in energy ($v^2$ remember) while an increase in mass requires just a linear increase in energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This boils down to a simple formula: If you have a fixed amount of energy available you basically can trade between propellant efficiency and amount of thrust generated: halving your exhaust velocity means generating four times the thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, since a rocket has to carry all the propellant with it from the start, you normally want it to be as propellant-efficient as possible (basically in space, mass is a far more valuable thing than time). This is why electric propulsion normally trades thrust for as high an Isp as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if you find a way to generate more &lt;em&gt;energy&lt;/em&gt; for the same mass&amp;hellip;than you can keep your high-Isp electric propulsion engine and increase the thrust for the same amount. (Or increase your Isp a tiny bit more.)&lt;/p&gt;&#xA;" OwnerUserId="11787" LastEditorUserId="6816" LastEditDate="2017-01-10T19:03:51.480" LastActivityDate="2017-01-10T19:03:51.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19741" PostTypeId="1" CreationDate="2017-01-10T13:11:04.347" Score="3" ViewCount="6141" Body="&lt;p&gt;How many countries have sent astronauts or other trained personnel to the International Space Station?&lt;/p&gt;&#xA;" OwnerDisplayName="user16987" LastEditorUserId="6944" LastEditDate="2017-01-11T00:09:09.693" LastActivityDate="2018-07-21T04:14:21.347" Title="How many countries have sent personnel to the ISS?" Tags="|iss|astronauts|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19742" PostTypeId="2" ParentId="19741" CreationDate="2017-01-10T14:03:11.740" Score="16" Body="&lt;p&gt;Wikipedia has a &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_International_Space_Station_crew&quot;&gt;list of International Space Station crew&lt;/a&gt; where you can also find relevant statistics. As of January 2017, 10 nations have sent &lt;em&gt;crew&lt;/em&gt; to the ISS: United States, Russia, Japan, Italy, Canada, Germany, France, Belgium, Netherlands and the United Kingdom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, more people/nations have &lt;em&gt;visited&lt;/em&gt; the ISS: &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_International_Space_Station_visitors&quot;&gt;List of International Space Station visitors&lt;/a&gt;. This adds another 8 nations to the list: Sweden, Brazil, Denmark, Kazakhstan, Malaysia, South Africa, South Korea and Spain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in total, people from 18 nations have been to the ISS as of January 2017.&lt;/p&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="14549" LastEditDate="2017-01-10T15:53:58.650" LastActivityDate="2017-01-10T15:53:58.650" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19744" PostTypeId="2" ParentId="17830" CreationDate="2017-01-10T18:41:09.510" Score="12" Body="&lt;p&gt;The other answers forget latent heat/enthalpy of phase change, and many neglect pressure rise with column of water. :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Enthalpy of evaporation of liquid water is considerably higher than its latent heat of fusion. It takes way more energy to bring a 100C liquid water into a 100C steam at 1 bar, than is released as same mass of 0C water freezes into 0C ice. The values vary with pressure, but not by a very large margin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water pressure in Earth gravity changes by about 1bar per 10 meters. With Mars, that will be a ballpark of 1 bar, or 100KPa per 30 meters - meaning scarce centimeters deep the water is above the triple point pressure. That means the boil-off would be occurring only near the very surface. It would also drain heat very rapidly, freezing water below. It would very quickly form a layer of ice, which would then proceed to evaporate very slowly, draining heat from water below (ice is a lousy heat conductor, so it would take a lot of time to drain the energy needed for sublimation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, your pool's thin top-most layer would boil rapidly for a short moment, and a very short distance below the boiling layer a thin layer of ice would form, which then, subsequently, would thicken over time, as water deeper below freezes, and its level would keep dropping slowly (much slower than ice layer thickness growth) as its surface would evaporate.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-01-10T18:41:09.510" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19745" PostTypeId="2" ParentId="19739" CreationDate="2017-01-10T23:35:59.023" Score="3" Body="&lt;p&gt;This is an interesting question to ask but unfortunately is one with: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;a) a complex answer requiring much study to explore the variables and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;b) many different regimes according to altitude and the various perturbations that apply&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A million years is a long time, perhaps a lot comes down to how much of an orbit disturbance is significant to you over that time. I don't wish to put you off but this could be several weeks study with a good orbit propagator.  My first reaction would have been to ask for more information on the candidate orbit you have in mind but, realistically this is not going to help in the millions of years time frame because, I believe, this problem is intractable through integration errors. Consider that each of the disposal zones for satellites (LEO - de-orbit, MEO and GEO to local graveyard) has been the subject of stability assessments, i.e. funded studies, that are specific to the satellites in question even over the much shorter periods of a few tens of decades. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your three tonne suggestion may have nothing like the very specialised characteristics of the 60cm, highly reflective, low area to mass LAGEOS satellites which are at almost 6000km. Even for these most favourable conditions I'd be surprised if the &lt;a href=&quot;https://ilrs.cddis.eosdis.nasa.gov/missions/satellite_missions/current_missions/lag1_general.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;8 million year&lt;/a&gt; estimate is anything better than an order of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over long periods all sorts of inter-relationships between lunar/solar/Earth precession topics come in to play at different timescales.  My hazy recollection is that what might be considered to be a stable orbit over decades might not stay that way over hundreds of years.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short of that effort, from memory and in the immediate timescale:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;solar radiation pressure begins to dominate over atmospheric drag beyond 1000km - 2000km. &lt;/li&gt;&#xA;&lt;li&gt;a tumbling object will behave differently to an inertially stable or Earth pointing mission.&lt;/li&gt;&#xA;&lt;li&gt;objects in a high Earth orbit with a large Area to Mass ratio, e.g. extended solar arrays, will adopt an increased eccentricity from solar radiation pressure.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are plausibly hand-calculation territory for a few years. After that you are going to have to bite the bullet and do some serious simulations yourself and at some point in that process you will have to recognise where the uncertainties dominate over the result.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the slightly longer term:&#xA; - the Sun's effect on the atmosphere can't be relied upon as a constant, consider the Maunder sunspot minimum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the effects of the environment on the satellite itself it is not unreasonable to expect an increase in thermal absorption from outgassing, atomic oxygen at LEO, UV and micrometeoroids in that order. Whether that is sufficient to cause any structural damage would be down to the details of design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, its an interesting topic, I'd be happy to hear comments and corrections.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-01-10T23:35:59.023" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19746" PostTypeId="2" ParentId="19736" CreationDate="2017-01-10T23:37:46.627" Score="3" Body="&lt;p&gt;It has been &lt;a href=&quot;https://arxiv.org/pdf/1406.2020v1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;proposed&lt;/a&gt; that Earthshine, i.e. light and heat from the molten Earth after the Moon forming impact when the Moon was much closer, heated the near side of the tidally locked Moon and that this caused the differences in crust thickness by allowing different materials accrete and condensate as the Moon cooled. Basically, more of the crusty materials condensated on the colder far side, to put it in the blunt way that I can relate to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I (not being a scientist) would think that tidal forces played an important part too. Causing different volcanic activities on the near and far sides. The only somewhat similar system is the tidally locked Pluto/Charon system and maybe something could be learned from how their crusts are differentiated from facing and opposing hemispheres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars too has a basic &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_dichotomy&quot; rel=&quot;nofollow noreferrer&quot;&gt;dichotomy&lt;/a&gt; in its topography, although not obviously related to any companion present or past.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-01-10T23:46:53.907" LastActivityDate="2017-01-10T23:46:53.907" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19747" PostTypeId="1" CreationDate="2017-01-11T10:38:02.873" Score="7" ViewCount="163" Body="&lt;p&gt;It is sometimes suggested that fuel produced on the Moon or on asteroids could be launched more cheaply to dock with and refuel for example communication satellites in GEO to extend their operating lifetimes (as in orbital station keeping). This has of course to compete with simply launching more fuel from Earth together with the satellite to begin with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But isn't the economic lifetime in GEO limited more by the aging of the electronics than by the fuel consumption? Hubble Space Telescope was upgraded with new instruments several times and practically worked as a whole series of different telescopes in some respects. If com sats are designed to be upgraded robotically, wouldn't it require much less launch mass to do so than to refuel them in mid-life or launch a completely new satellite? An upgrade mission could even be launched as a secondary payload with another new satellite to GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't it in general more economical to launch the latest &quot;smart mass&quot; that in the end is producing the commercial service, than bulk mass like fuel, if one is to go through the hassle of docking anyway?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-01-14T02:02:40.410" Title="What are the economics of upgrading versus refueling satellites?" Tags="|artificial-satellite|economics|hubble|fuel-depots|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="19748" PostTypeId="1" CreationDate="2017-01-11T14:04:37.610" Score="7" ViewCount="1492" Body="&lt;p&gt;Instead of launching a payload to orbit, consider one of the big launchers of today (like Atlas V, Ariane 5, Falcon 9, Proton) being used as a sounding rocket for astronomical observation purposes. Lifting its payload thousands of kilometers into space and then falling down to Earth within an hour or a few.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My presumptions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Compared to an orbital insertion, a sounding rocket mission would allow for much more payload mass since no acceleration to orbital velocity is needed.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;It would be easier for a reusable first stage to land. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The design of the payload could be much simplified: A battery as power supply. No need for on board propulsion. No communication needed, the data collected could be landed as a physical memory chip. No radiation hardening needed for such a short exposure so the cheapest and latest off-the-shelf electronics could be used.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Sounding rockets do contribute alot to science already, so I wonder how it might be expanded to big sounding payloads. The payloads could either be discarded after their one and only use (as in making one observation of e.g. an exoplanet when it is known to transit), or be soft landed and upgraded and reconfigured for another mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why aren't sounding rockets made as large as orbital launchers?&lt;/strong&gt; Are there show stoppers here? For example that orbital launchers are ill suited for sounding missions. Or that there aren't as many simplifications to be made on the payload as I suggest here. Or that the science opportunities don't motivate say $100,000,000 for one or two hours of observation above the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-01-18T22:20:42.373" Title="Using large orbital launchers as sounding rockets. What are the opportunities and challenges?" Tags="|payload|launch-vehicle|science|sounding-rocket|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19749" PostTypeId="2" ParentId="19748" CreationDate="2017-01-11T14:48:17.503" Score="16" Body="&lt;p&gt;Suborbital sounding rockets cost about 1/100 as much as an orbital launcher. &lt;a href=&quot;http://en.wikipedia.org/wiki/Black_Brant_(rocket)#Black_Brant_XII&quot; rel=&quot;nofollow noreferrer&quot;&gt;Black Brant XII&lt;/a&gt;, one of the most advanced sounding rockets in use, can take 100-400kg payload into space for \$600K. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a Falcon-9-based sounding rocket approach to make any sense, you'd have to describe a single payload that delivered as much science as 100 individual 100kg payloads. I'm hard-pressed to imagine such a thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even a single-stage, fully-reusable Falcon 9 isn't likely to drive launch costs low enough to compete with that. I would guess that the single-launch staffing, logistics, fuel, and refurbishment costs would pay for ten Black Brants -- and SpaceX hasn't yet demonstrated any reusability. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="1142" LastEditDate="2017-01-18T22:20:42.373" LastActivityDate="2017-01-18T22:20:42.373" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19750" PostTypeId="1" CreationDate="2017-01-11T16:23:27.950" Score="3" ViewCount="319" Body="&lt;p&gt;After having visited the L4 leading Trojans (&quot;Greeks&quot;) of Jupiter, almost a decade after launch &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2017/nasa-selects-mission-to-study-jupiter-s-trojan-asteroids&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lucy&lt;/a&gt; will spend almost 4½ years moving on to the trailing L5 Trojans, 5 AU away. Is there a special window of opportunity of trajectories that motivates this huge traverse across the inner Solar system in order to fly by another asteroid?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bJMTW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bJMTW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&lt;a href=&quot;http://www.hou.usra.edu/meetings/lpsc2016/pdf/2061.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Image&lt;/a&gt; is from a document if the Lunar and Planetary Science Conference 2016.&lt;/em&gt;&#xA;The Lucy mission is commented by blogger van Kane &lt;a href=&quot;http://futureplanets.blogspot.se/2017/01/lucy-and-psyche-asteroid-missions.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-01-12T08:29:02.733" LastActivityDate="2017-01-12T08:29:02.733" Title="How will the Lucy spacecraft move from Jupiter's L4 to its L5 Trojan asteroids?" Tags="|trajectory|asteroid|mission-design|lagrangian-points|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19751" PostTypeId="2" ParentId="19750" CreationDate="2017-01-11T17:40:20.983" Score="5" Body="&lt;p&gt;To get from Earth orbit to Jovian orbit takes a lot of energy - You need to turn your nearly circular orbit round Earth into a Hohmann transfer orbit (an ellipse with the other end out by Jupiter's orbit) and then once you are there you need to reduce the elliptical extremes by expending more energy to move your orbit to match Jupiter. If you then wanted to come back in towards the sun you need to expend yet more energy to move the ellipse once more. These changes are your delta-v budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However in this case Lucy is not in a Jovian orbit. It is in a highly elliptical orbit, as @Hobbes pointed out, and one end of that ellipse is already within Earth's orbit. So it doesn't require any further delta-v to come back in towards the Sun and out again. All that is required is a much smaller energy cost to aim the outermost end to be at L5 rather than L4.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2017-01-11T17:40:20.983" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19752" PostTypeId="1" AcceptedAnswerId="58981" CreationDate="2017-01-11T22:01:20.663" Score="6" ViewCount="91" Body="&lt;p&gt;Before anyone comments - obviously, we don't have any experimental data. But do perhaps we have ISS or other data that can be extrapolated from?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I'm asking specifically is, what do we currently know about the development of, for example, a fertilized chicken egg in Mars g? For the purposes of this question, assume that we've handled any radiation problems...&lt;/p&gt;&#xA;" OwnerUserId="5965" LastEditorUserId="1235" LastEditDate="2022-04-25T20:36:26.207" LastActivityDate="2022-04-25T20:36:26.207" Title="What is the current state of knowledge regarding embryology (from an animal husbandry perspective) in Mars-g gravity?" Tags="|mars|astrobiology|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19754" PostTypeId="2" ParentId="1059" CreationDate="2017-01-12T02:58:37.507" Score="-2" Body="&lt;p&gt;1) PSLV     February ,2017  ISRO is going to launch 103 satellites in this PSLV. The present record for launching the highest number of satellites at one launch is held by Russia which launched 37 satellites on June 19, 2004 earlier this was planned to be launched in January with 83 satellites&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) PSLV     28 December 2017    This launch is contracted by Team Indus, for Google Lunar X Prize , this would be an exceptional launch since this would be carrying 3 rovers to moon ,while never have a rocket carried even 2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) &#xA;GSLV Mk III     20 January 2017     This will launch GSAT-19E , a successful launch will make ISRO self reliable for launching satellites of 3200 kgs range to geostationary orbits&lt;/p&gt;&#xA;" OwnerUserId="18185" LastActivityDate="2017-01-12T02:58:37.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19755" PostTypeId="1" AcceptedAnswerId="19789" CreationDate="2017-01-12T03:25:50.273" Score="8" ViewCount="1827" Body="&lt;p&gt;On Mars studies have shown that carbon dioxide solidifies to dry ice at the poles. During the winter months can electrolysis be applied to collected and then contained liquid &lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt; to replenish oxygen reserves by creating an engine that uses &lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt; as the oxidizer? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could solar powered electrolysis of liquid &lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$H_2O$&lt;/span&gt; during the summer provide liquid &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt; and Hydrogen to power a generator in place of using batteries for electrical storage for generator during winter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What can be added to liquid CO&lt;sub&gt;2&lt;/sub&gt; to allow better electrolysis to crack &lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt; or to used as an oxidizer for &lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt; combustion engine? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main component of the atmosphere of Mars is carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) at 95.9%. Each pole is in continual darkness during its hemisphere's winter, and the surface gets so cold that as much as 25% of the atmospheric CO&lt;sub&gt;2&lt;/sub&gt; condenses at the polar caps into solid CO&lt;sub&gt;2&lt;/sub&gt; ice (dry ice). When the pole is again exposed to sunlight during summer, the CO&lt;sub&gt;2&lt;/sub&gt; ice sublimes back into the atmosphere. This process leads to a significant annual variation in the atmospheric pressure and atmospheric composition around the Martian poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bl98f.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bl98f.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Atmosphere_of_Mars&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt; sublimes but it can be self collected in vast containers or an underground cave that can be capped open during winter, closed and kept as a super liquid during summer to power generators similar to steam engines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://chemistry.stackexchange.com/questions/66265/can-co2-be-broken-into-its-component-elements-by-electrolysis&quot;&gt;https://chemistry.stackexchange.com/questions/66265/can-co2-be-broken-into-its-component-elements-by-electrolysis&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/31724/martian-carbon-dioxide-turbine-oxygen-generator&quot;&gt;Martian Carbon Dioxide Turbine Oxygen Generator&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://chemistry.stackexchange.com/questions/112136/what-is-the-ideal-temperature-to-crack-water&quot;&gt;https://chemistry.stackexchange.com/questions/112136/what-is-the-ideal-temperature-to-crack-water&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2019-04-04T13:58:02.890" LastActivityDate="2022-05-28T22:56:14.007" Title="Can a machine process and run on $CO_2$ and $H_2O$ for an $O_2$ by product?" Tags="|mars|colonization|oxygen|gasses|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="19756" PostTypeId="2" ParentId="19755" CreationDate="2017-01-12T03:37:44.867" Score="6" Body="&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; the question has been significantly modified (potentially improved) - I'll leave this slightly-outdated answer here as-is for reference.&lt;/p&gt;&#xA;&lt;p&gt;There probably are no studies that have shown that carbon dioxide liquifies naturally on Mars. Below carbon dioxide's triple point of &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbon_dioxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;5.1 atmospheres&lt;/a&gt; it cannot exist in liquid phase at the surface.&lt;/p&gt;&#xA;&lt;p&gt;I should mention that a few papers several years ago discussed the possibility of frozen CO&lt;sub&gt;2&lt;/sub&gt; in the soil creating &amp;quot;pockets&amp;quot; where the pressure &lt;em&gt;might possibly&lt;/em&gt; rise above the triple point, creating small, temporary pockets of CO&lt;sub&gt;2&lt;/sub&gt; as liquid, but I don't think that concept panned out.&lt;/p&gt;&#xA;&lt;p&gt;Electrolysis of CO&lt;sub&gt;2&lt;/sub&gt; is definitely a way to make oxygen, but the method being looked at with the greatest interest is high temperature (800 - 1000 °C) gaseous electrolysis with conductive oxide solid electrodes.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/13869/12102&quot;&gt;This answer&lt;/a&gt; to the question &lt;a href=&quot;https://space.stackexchange.com/q/13865/12102&quot;&gt;How to produce oxygen out of Venusian atmosphere's carbon dioxide?&lt;/a&gt; does a good job of outlining some options for obtaining oxygen from atmospheric CO&lt;sub&gt;2&lt;/sub&gt;. It points to this very nice, long answer about &lt;a href=&quot;https://space.stackexchange.com/a/5096/12102&quot;&gt;MOXIE (Mars OXygen In situ resource utilization Experiment)&lt;/a&gt; which is a solid electrode, high temperature gaseous CO&lt;sub&gt;2&lt;/sub&gt; electrolytic cell which reduces CO&lt;sub&gt;2&lt;/sub&gt; to oxygen and carbon monoxide. It is one of the experiments planned for the Mars 2020 Rover. These answers are very well written and &lt;em&gt;the answer linked for MOXIE contains extensive links for further reading&lt;/em&gt;, so I won't discuss it any more here.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Now your question about liquid CO&lt;sub&gt;2&lt;/sub&gt;. While I could find some discussion of CO&lt;sub&gt;2&lt;/sub&gt; dissolved in other ionic liquids, so far I haven't found anything about trying to do electrolysis on pure liquid CO&lt;sub&gt;2&lt;/sub&gt;. Since CO&lt;sub&gt;2&lt;/sub&gt; is a stable, covalently bonded molecule, I think we can assume that it will have extremely poor electrical conductivity as a liquid. The reason water works so well is that it dissociates readily - pH 7 means (oh my gosh, I'm going to say something about high school chemistry) one in every roughly 10&lt;sup&gt;-7&lt;/sup&gt; of the water molecules is split all of the time, resulting in H&lt;sup&gt;+&lt;/sup&gt; (or H&lt;sub&gt;3&lt;/sub&gt;O&lt;sup&gt;+&lt;/sup&gt;) and OH&lt;sup&gt;-&lt;/sup&gt; Add some salt, or other ionic compounds and the conductivity goes up even farther. Liquid CO&lt;sub&gt;2&lt;/sub&gt;? possibly no useful conductivity.&lt;/p&gt;&#xA;&lt;p&gt;It looks like gaseous high temperature electrolysis has been identified as the best way to go on Mars.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nXVH5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nXVH5.png&quot; alt=&quot;Enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Above:&lt;/strong&gt; Phase diagram of CO&lt;sub&gt;2&lt;/sub&gt; from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Carbon_dioxide_pressure-temperature_phase_diagram.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="47149" LastEditDate="2022-05-28T22:56:14.007" LastActivityDate="2022-05-28T22:56:14.007" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="19757" PostTypeId="1" CreationDate="2017-01-12T05:32:14.953" Score="8" ViewCount="2372" Body="&lt;p&gt;The Apollo Command Module and the LEM were built in clean rooms free of dust on earth and yet (knowing that they didn't have air locks on either module) the astronauts came in directly from the moon with dust on them. They even drove around in the Lunar Rover causing &quot;rooster tails&quot; of dust.  Photos show they had dust all over their suits. There was no way to clean their suits.  Why didn't the moon dust cause problems on their return flights?&lt;/p&gt;&#xA;" OwnerUserId="18188" LastEditorUserId="415" LastEditDate="2017-01-12T08:48:07.370" LastActivityDate="2018-05-22T19:05:07.660" Title="Why did moon dust not cause problems on Apollo during the return flight from the Moon?" Tags="|apollo-program|eva|dust|clean-room|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19758" PostTypeId="1" CreationDate="2017-01-12T06:49:05.393" Score="4" ViewCount="175" Body="&lt;p&gt;As I'm reading &quot;&lt;a href=&quot;http://www.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ignition!&lt;/a&gt;&quot; I'm finding one point baffling: vapor pressure being such a big deal. Outgassing of both fuels and oxidizers would cause dangerous pressure build-up. The book describes the hair-raising stress of handling materials that release oxygen. The utter horror of watching technicians estimate the pressure by measuring barrel bulging. The stringent requirement for periodic manual venting. Abandoning one promising line of research purely due to unacceptable vapor pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;https://en.wikipedia.org/wiki/Check_valve&quot; rel=&quot;nofollow noreferrer&quot;&gt;check valve&lt;/a&gt; is a really simple, cheap, small device that would neatly protect the contents of the container from environment, while allowing whatever accumulates in the container to vent once the pressure inside rises above a preset value. Even equipping individual barrels with these would cost pennies. Did no-one think of that, or is there some other reason making this impossible or impractical to such a degree that vapor pressure would become an insurmountable problem?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-01-12T20:37:02.760" Title="Why were hypergolics tanks vented manually?" Tags="|liquid-fuel|hypergolics|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19759" PostTypeId="2" ParentId="19757" CreationDate="2017-01-12T07:39:51.567" Score="6" Body="&lt;p&gt;If the CM and LEM were built in clean rooms, it wasn't because of dust itself being a problem. They were trying to prevent materials from Earth contaminating the Moon rock samples.&lt;br&gt;&#xA;The Moon dust turned out to be somewhat of a problem (it's nasty stuff). But there was no weight budget for an airlock. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-12T07:39:51.567" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19760" PostTypeId="1" CreationDate="2017-01-12T09:13:12.257" Score="3" ViewCount="2682" Body="&lt;p&gt;It is possible to supply spacecrafts in orbit in cis-Lunar space with oxygen from fuel production facilities on the Moon or on asteroids. What is the size of this potential in-space oxygen market today? I would like to see estimates of how many active satellites there are in orbit and how much oxygen fuel they together consume for station keeping and other maneuvers after orbital insertion. And how much is lost by leakage if that is a relevant quantity. Especially for geosynchronous orbit separately since it is relatively easy to reach several satellites in a single supply run there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose that the space station consumes a significant fraction of all fuel in space because of its great mass and the atmospheric drag. Water and breathing oxygen for the ISS crew would also be part of the market demand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since it is oxygen that would be supplied from in-space, I'd like to see estimates of how much oxygen is consumed in relation to other volatilities (such as hydrogen, kerosene, hypergolic fuels, solid fuels, cold CO2, xenon, helium, whathaveyou).&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-01-12T09:19:57.797" LastActivityDate="2017-01-12T15:23:06.753" Title="How much fuel is being consumed by all spacecrafts in orbit?" Tags="|artificial-satellite|fuel|isru|cislunar|supply-mission|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19761" PostTypeId="2" ParentId="19760" CreationDate="2017-01-12T12:36:54.830" Score="4" Body="&lt;p&gt;BOE: 1000 active satellites, launched with 3 tons of fuel+oxidizer each for an expected lifetime of 15 years is 3000/15=200 tons of fuel+oxidizer needed per year.&lt;br&gt;&#xA;&lt;strong&gt;Note&lt;/strong&gt;: 3 tons is too much, as many satellites will have a total mass that's less than that. I could be an order of magnitude off. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of these will be hypergolics so no oxygen, the remainder is electric propulsion so no oxygen either. Oxygen is avoided because it's difficult to store long-term due to boiloff. In the current market, basically only the ISS would benefit from on-orbit availability of oxygen. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-01-12T15:23:06.753" LastActivityDate="2017-01-12T15:23:06.753" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19762" PostTypeId="2" ParentId="19757" CreationDate="2017-01-12T19:33:23.173" Score="12" Body="&lt;p&gt;Moon dust &lt;em&gt;was&lt;/em&gt; a problem.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Lunar Module dust, dirt and debris contamination presented numerous challenges for several of the Apollo missions. The Apollo 12 crew observed that at 1/6-g the cabin atmosphere was excellent, however, after orbital insertion lunar dust filled the atmosphere and caused eye and nose irritation. [...] The Apollo 16 crew appears to have had themost difficulty with dust, citing the following: Velcro on the floor was caked with dust; crew feet,&#xA;  hands and arms were covered with dust that was transferred into the suit upon donning; the midstep was covered with dust; eye and mouth irritation; and, the cabin fan in zero-g did not appear to clean the atmosphere. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.hq.nasa.gov/alsj/TP-2006-213726.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;The Apollo Experience Lessons Learned for Constellation Lunar Dust Management&quot;&lt;/a&gt;, page 10ff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there were plenty of problems with the dust as you can see from that report.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An airlock would not have prevented the dust from getting into the LM as you can see from those reports, since it was stuck on the surface of the suits, e.g.:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the Apollo 14 debrief, Alan Shepard spoke of dust contamination. “We did find that we had to take the boots off because there’s so much dust in your overshoes that we did take those off before we went to bed.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and a bit later&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I guess the dust brush worked fairly well. It got most of it, but we were still pretty dirty&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="13867" LastEditorUserId="13867" LastEditDate="2018-05-22T18:49:27.023" LastActivityDate="2018-05-22T18:49:27.023" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19763" PostTypeId="2" ParentId="19758" CreationDate="2017-01-12T20:37:02.760" Score="4" Body="&lt;p&gt;It's worth remembering a lot of the research John D. Clarke recalls in Ignition! was not primarily being conducted with the requirements of civilian space flight in mind. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The military were interested in propellants that would allow munitions to be fuelled in the factory, then stored indefinitely, but ready for immediate use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Setting aside any risk of toxicity or flammability resulting from the vented propellants, any losses through check valves could mean that the performance of any rocket could vary over a wide range.&lt;/p&gt;&#xA;" OwnerUserId="13122" LastActivityDate="2017-01-12T20:37:02.760" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19764" PostTypeId="2" ParentId="13241" CreationDate="2017-01-12T20:37:44.847" Score="2" Body="&lt;p&gt;The marine zones &lt;em&gt;should&lt;/em&gt; be in the USCG local notice to mariners&#xA;&lt;a href=&quot;http://www.navcen.uscg.gov/pdf/lnms/lnm11012017.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.navcen.uscg.gov/pdf/lnms/lnm11012017.pdf&lt;/a&gt;&#xA;(that doesn't have an example, but it should...)&lt;/p&gt;&#xA;" OwnerUserId="18193" LastActivityDate="2017-01-12T20:37:44.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19766" PostTypeId="2" ParentId="1325" CreationDate="2017-01-13T06:18:42.990" Score="1" Body="&lt;p&gt;&lt;strong&gt;An effective launch escape system was not incorporated into the STS because of weight considerations that now seem to be both short sighted and even absurd.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Possible Escape Systems.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1)&lt;/strong&gt; An escape ejection capsule system similar to the one used in the &lt;strong&gt;B-58 Hustler&lt;/strong&gt; was a possibility. But the ejection capsule weighed several times as much as the seats on the shuttle and were thus not developed.&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/nJ7yhvhBe9E?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2)&lt;/strong&gt; Then there is the fact that the entire crew compartment could (should) have been designed to separate as was the case with the &lt;strong&gt;F-111 escape system&lt;/strong&gt;. However the system would have once again been in NASA's opinion too heavy. &#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/M8l79lSjVMg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;3)&lt;/strong&gt; verum potest esse penale&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reality is I believe that NASA didn't want to reduce the payload capacity of the STS by including an effective crew escape system. It seems that 50,000lbs of payload to LEO was the 'magic' number NASA wanted and reducing that for the sake of crew safety down to 40,000 to 45,000 pounds of payload to LEO was an undesirable effect of incorporating such an escape system. A system mind you that could have safely protected the crew members from a stationary position on the pad up to a speed of +Mach 2. &lt;/p&gt;&#xA;" OwnerUserId="18198" LastActivityDate="2017-01-13T06:18:42.990" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19768" PostTypeId="1" AcceptedAnswerId="19769" CreationDate="2017-01-13T14:06:43.380" Score="3" ViewCount="982" Body="&lt;p&gt;I know it would take a lengthy amount of time to reach the asteroids and meteors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After hearing of the Europeans landing a probe on a comet, the first thing I could think of was: &quot;Could this be used to increase the speed of a spacecraft?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Think about it; a comet moves between 10-70&amp;nbsp;km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can attach a spacecraft (safely)/probe to a 70&amp;nbsp;km/s object and increase the speed of your ship to 70&amp;nbsp;km/s then detach yourself from the comet, would that not steamtrain the time it would take for us to explore the universe?&lt;/p&gt;&#xA;" OwnerUserId="18202" LastEditorUserId="286" LastEditDate="2017-01-13T17:59:29.433" LastActivityDate="2017-01-13T17:59:29.433" Title="Why are meteors, asteroids and comets not used as propulsion for spacecrafts?" Tags="|crewed-spaceflight|spacecraft|probe|interstellar-travel|" AnswerCount="2" CommentCount="3" ClosedDate="2017-01-13T18:53:28.773" ContentLicense="CC BY-SA 3.0" />
  <row Id="19769" PostTypeId="2" ParentId="19768" CreationDate="2017-01-13T14:14:19.677" Score="9" Body="&lt;p&gt;The problem is, to even reach the comet, you have to achieve that speed already by other means - but once you have reached that speed, the relative motion of the comet is 0 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is actually something like what you might consider, only a little different. If you fly towards a large mass (like a planet) you can use some of the planet's momentum to accelerate your spacecraft if you plot your path carefully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is already being done for a long time and is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity assist&lt;/a&gt; - in fact the Rosetta mission you describe did it multiple times. The problem is, that you need a huge mass to assist you (a comet would likely be too little) and it needs to travel in the right distance at the right speed at the right time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oh and btw. Earth itself moves around the sun at 30 km/s. The speed noted in comets is either orbital speed (wich is similar to Earth's speed itself) or speed towards or from Earth. So the comets usually aren't that much faster or slower than planets anyways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for interstellar travel, 70 km/s might sound very fast, but compared to light speed, it is only a small fraction of 1/4285 c. So the nearest star system (Proxima Centauri) is still 18'425 years away at 70 km/s - and only that if those comet is rushing directly towards Proxima.&lt;/p&gt;&#xA;" OwnerUserId="12555" LastEditorUserId="12555" LastEditDate="2017-01-13T14:31:58.517" LastActivityDate="2017-01-13T14:31:58.517" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19770" PostTypeId="2" ParentId="19768" CreationDate="2017-01-13T14:24:35.437" Score="4" Body="&lt;p&gt;Rosetta achieved, by its own propulsion and gravity assists, the same speed and trajectory of the comet it entered orbit around. And it took it almost 10 years to do so. And it was the easiest to reach comet available at the time. One could send a probe to meet a comet at 70 km/s speed relative to it, but that would then be called an &amp;quot;impactor&amp;quot; for obvious reasons. The probe would be vaporized by the very violent collision. Some of the gas would indeed follow the comet back out to the Oort cloud, but no working structure could survive it.&lt;/p&gt;&#xA;&lt;p&gt;But then there is the &lt;a href=&quot;https://www.nasa.gov/content/niac-overview&quot; rel=&quot;noreferrer&quot;&gt;NIAC&lt;/a&gt; (NASA Innovative Advanced Concepts) that finances very small investigations into very futuristic space missions, almost none of which will actually ever be possible. So there's &lt;a href=&quot;https://www.nasa.gov/content/comet-hitchhiker-harvesting-kinetic-energy-from-small-bodies-to-enable-fast-and-low-cost&quot; rel=&quot;noreferrer&quot;&gt;this guy&lt;/a&gt; who convinced them to pay him for a year or so to look into hitchhiking with a comet by using some kind of tether system, like a spider's web. Gaining up to 10 km/s speed, which would be alot. But it is still just a question being asked.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-13T14:24:35.437" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19771" PostTypeId="1" AcceptedAnswerId="19774" CreationDate="2017-01-13T15:56:21.377" Score="20" ViewCount="2028" Body="&lt;p&gt;A recent comment in a discussion of satellite refueling mentioned the &#xA;possiblity of a spacecraft dipping into the atmosphere from LEO to &#xA;obtain oxygen. Similarly, the science fiction RPG &lt;em&gt;Traveller&lt;/em&gt; proposes&#xA;scooping hydrogen from gas giant atmospheres to refuel fusion-powered&#xA;starships. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these techniques actually feasible? Specifically, I'm asking:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;From stable LEO, is it possible to drop perigee into Earth's atmosphere, &#xA;collect material, return to stable orbit, and filter the collection down &#xA;to oxygen with a net gain in propellant mass, or will fighting drag use more propellant than is collected? Assume that the ship has &#xA;high-thrust H2/O2 engines at around 450s Isp, and starts&#xA;with a surplus of fuel to oxidize. &#xA;Would the filtration need to be magically instantaneous (i.e. only oxygen &#xA;is scooped), or could it &#xA;still work if it scooped mixed atmosphere, rejecting the nitrogen, CO2, etc after regaining safe orbit?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;From stable low orbit around a gas giant or ice giant -- Jupiter, Saturn, Uranus, or Neptune --  is it possible to skim the atmosphere, collect material, and regain orbit, again with a net gain of propellant mass? Again with H2/O2 engines, but this time the ship is assumed to start oxidizer-rich. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-04-11T16:13:03.347" LastActivityDate="2018-04-12T05:59:19.517" Title="Is atmospheric skimming for propellant feasible?" Tags="|fuel|atmosphere|oxygen|hydrogen|fuel-depots|" AnswerCount="5" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19773" PostTypeId="2" ParentId="19552" CreationDate="2017-01-13T17:39:06.923" Score="12" Body="&lt;p&gt;&lt;strong&gt;Regulatory limits&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The deviations depend upon the radio frequency licensing agreement applicable to each satellite. This is a feature of the International Telecommunications Union, ITU.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Limits in terms of degrees can be typically +- 0.05deg in both longitude and latitude or, especially for some older registrations possibly +-0.1 deg longitude and unlimited in latitude.&lt;br&gt;&#xA;In terms of km this becomes&lt;/p&gt;&#xA;&#xA;&lt;p&gt;+-0.05 deg   =   +-37km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;+-0.1 deg    =   +-74km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Inclined orbits&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, operating a satellite with a latitude other than 0.0 deg largely means in an inclined orbit, and this means that the North-South excusions are equal and opposite over a 24 hour cycle. To do so also introduces an East-West excusion naturally (this follows geometrically). From the Earth looking up at a satellite with a slight inclined orbit at geosynchronous altitude the satellites motion appears circular for very small inclinations less than half a degree or so but becomes the &quot;figure of 8&quot; motion for larger inclinations.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Eccentricity&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The eccentricity of the orbit will affect firstly the orbital height and then as a consequence its velocity at apogee and perigee and thus its longitude and also the shape of the figure-of-8. &#xA;An example eccentricity is 0.0003, which would lead to a ~12km excursion in height from the Geostationary radius. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Natural perturbations&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A non manouvering object left exactly at GEO will drift firstly in longitude because the Earth is not exactly spherical. Correcting this is called East-West Station-keeping and is usually done every couple of weeks, depending on the type of propulsion system, and this is roughly sufficient for the +-0.05 deg limit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The effects of gravity of the Sun and the Moon, &quot;luni-solar&quot; perturbations, cause the satellite's inclination to change. It also causes the Right Ascension of the Ascending Node, RAAN, to change though we can come back to that. If left uncorrected this will cause a North South drift of roughly 1 deg in the first year. North South Station Keeping, NSSK, is also often performed once every week or two but is far more costly in energetic terms and has driven the development of more efficient propulsion to keep propellent requirements down.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many satellites are actually operated without NSSK towards the end of their working lives to save propellant though this depends on the ground users still being able to obtain a good signal from the displaced satellite.  Interestingly, objects left to drift indefinitely reach a maximum inclination of ~15deg before their inclination drifts down again over several decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The physical size of the satellite, its Area/Mass ratio and reflective properties, also make the satellite subject to Solar Radiation Pressure. This can cause the eccentricity to grow, which increases the daily East-West drift, and thus reduces the time before an East West manouevre is needed. Hence Eccentricity correction, reducing the eccentricity and making the orbit more circular is usually performed at the same time as the East-West manoeuvres themselves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Collision avoidance&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also international guidelines for debris mitigation which suggest that the operational zone for GEO in height terms is 42164.5 +- 50km and that the rest of the way up to +-200km is a &quot;manoeuvre corridor&quot; for satellites undergoing a change of station.  This isn't mandatory but is, I believe, followed by a large fraction of operators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Footnotes (EDIT responding to comments)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;By &quot;largely means in an inclined orbit&quot; I am only referring to the possibility, not necessarily practical, of operating with some continual thrust or solar radiation pressure so as to achieve a non-keplerian orbit. I'm not aware of any satellite that does this, though proposals for a &quot;Statite&quot; could be in this territory.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;By &quot;not exactly spherical&quot; I am referring to the tri-axial shape of the Earth rather than its oblateness. The former refers to the land mass features we call continents and the latter relates to slight flattening at the poles. Tri-axiality causes accelerations in longitude East or West according to the longitude relative to two stable longitudes. There are two corresponding unstable longitudes also. The acceleration and thus East-West Station Keeping burden is least at these four points.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-01-14T18:35:02.453" LastActivityDate="2017-01-14T18:35:02.453" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19774" PostTypeId="2" ParentId="19771" CreationDate="2017-01-13T17:46:15.173" Score="15" Body="&lt;p&gt;It is easy enough to analyze this in terms of conservation of momentum, we'll assume the velocity through the atmosphere to be 7.8km/s, that is LEO orbital velocity - in actuality it'll be a little different due to rotation of the Earth and eccentricity of the atmosphere-grazing orbit, but not different enough to change the conclusion. Now assume the ship scoops up 1kg of atmosphere; logically it would be necessary to fling that entire 1kg out the back at 7.8km/s to regain the velocity lost when catching it and that would leave the ship with no reaction mass gained, to actually gain reaction mass would require ejecting a portion of the captured gas out the back at faster than 7.8km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the table of &lt;a href=&quot;https://en.wikipedia.org/wiki/Spacecraft_propulsion#Table_of_methods&quot; rel=&quot;noreferrer&quot;&gt;exhaust velocities&lt;/a&gt; a chemical rocket engine exhaust velocity maxes out at around 4.4km/s which is a serious problem because even if somehow all the oxygen and nitrogen can be used as oxidizer (it can't, though maybe some of the nitrogen can) and that the fuel is weightless, the exhaust velocity is still much too low to recover the lost momentum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this analysis would suggest that for chemical rockets an atmospheric scoop is a total non-starter, it's so far from something that would work we don't even need to consider all the other problems such as inefficiency. Nevertheless Nuclear or Electric propulsion could potentially reach the required exhaust velocities, assuming they can use nitrogen for reaction mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the gas giants, with orbital velocity being over 15km/s even for Neptune and Uranus the sheer exhaust velocity required would be a serious challenge even for electric propulsion. Now, if the atmosphere were pure hydrogen it could just about work with onboard oxidizer because the hydrogen is so light compared with the oxidizer - as such catching 1kg of hydrogen at 15km/s could be offset by ejecting 5kg of H₂O at 3km/s, so there is margin for velocity gain there. Unfortunately, the atmosphere is not pure hydrogen and also contains about 26% helium by mass which pretty much eliminates that margin unless you have some kind of magic filter. Also, you would be bleeding oxidizer like crazy making it an exercise of dubious usefulness.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2017-01-13T19:55:01.680" LastActivityDate="2017-01-13T19:55:01.680" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="19775" PostTypeId="1" CreationDate="2017-01-13T18:28:48.253" Score="6" ViewCount="1046" Body="&lt;p&gt;About 2.5 million light years from the Milky Way is the Andromeda Galaxy.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have about &lt;a href=&quot;https://en.wikipedia.org/wiki/Andromeda_Galaxy#Collision_with_the_Milky_Way&quot; rel=&quot;nofollow noreferrer&quot;&gt;4 billion years before the two collide&lt;/a&gt; as they approach each other things are getting to get interesting. But for the next 2 or 3 billion years it should be fairly calm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a Lagrangian point with these two galaxies?  If so how long would it be stable/present for? &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2017-01-14T09:50:29.100" Title="Does the Milky Way have a Lagrangian point?" Tags="|lagrangian-points|galactic|" AnswerCount="1" CommentCount="0" ClosedDate="2017-01-14T12:31:24.930" ContentLicense="CC BY-SA 3.0" />
  <row Id="19776" PostTypeId="1" AcceptedAnswerId="19800" CreationDate="2017-01-14T00:24:20.423" Score="3" ViewCount="1898" Body="&lt;p&gt;With all this talk about lunar bases being at the poles, especially Shackleton Crater, I was thinking about the navigation. I am assuming that the lunar landers would stay there and ferry people &amp;amp; stuff to and from the surface. So the trip from Earth would be to lunar orbit for rendezvous, not to the surface. I am seeing the Earth side being a similar situation. A shuttle would lift off to a space station and a different ship would go orbit to orbit. We would have to do orbital insertion in both directions, and disregard atmospheric breaking or liftoff. &#xA;Here's my thinking. Since the lunar day is once a month, the rotation advantage of being near the equator is nil. Since the moon is on average about 240,000 miles away and the diameter is about 2K, that would mean, from the earth's point of view, there wouldn't be much of a difference between shooting for the lunar equator or the lunar pole. &#xA;So I am thinking the delta V requirements of achieving lunar polar orbit wouldn't be much different than the delta V for lunar equilateral orbit.  Can anybody do the math on that?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="6816" LastEditDate="2017-01-14T20:51:42.533" LastActivityDate="2017-01-15T23:43:53.647" Title="Lunar Polar Orbit" Tags="|orbital-mechanics|the-moon|polar-orbit|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19777" PostTypeId="2" ParentId="19747" CreationDate="2017-01-14T02:02:40.410" Score="2" Body="&lt;p&gt;&lt;em&gt;Warning I've written this answer and there have been no opportunities for citations - that means you're just trusting this random guy on the Internet if you believe me!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the figure of merit for judging which is better (upgrading or refueling) is more a question of practicality than anything else.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are extremely unlikely to develop a new significantly more effect fuel that can work with existing propulsion systems. We might be able to make small improvements on the fuel, or great new fuels that require different a propulsion system, but the key thing here is the old fuel worked well enough and is likely to do so for the foreseeable future (station keeping operations are reasonably secular).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We are extremely likely to develop new technology to make satellites harder/better/faster/stronger (or maybe more efficient/ faster down link/ lower power usage). This is great news if you run a satellite manufacturing/launch business because &lt;em&gt;we can make a satellite that's 5 times better every 10 years&lt;/em&gt; (not correct numbers). But if you're the client/end user it sucks, for the same reason my old PC motherboards can't be used to play Kerbal Space Program:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To upgrade you need all interfaces to be forward compatible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means supplying the correct power (right voltages), having data transfer and storage that can handle increased loads, and even making sure the thermal system can keep your new hardware hot/cold enough. It might be a simple coefficient of thermal expansion problem or it might be that you have a square plug and a round hole. (For reference my old motherboards don't have the appropriate slots for graphics interfaces).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So although upgrading hardware sounds better in an ideal situation it's likely to be almost impossible to implement without a whole lot of tinkering.&lt;/p&gt;&#xA;" OwnerUserId="17507" LastActivityDate="2017-01-14T02:02:40.410" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19778" PostTypeId="2" ParentId="19775" CreationDate="2017-01-14T04:02:56.817" Score="5" Body="&lt;p&gt;Nope!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lagrange points&lt;/a&gt;, also called libration points, are (in practice) extended locations that are well defined for a pair of bodies that are in a roughly circular, and periodic orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Andromeda and Milky way galaxies are moving almost straight at each other, on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision&quot; rel=&quot;nofollow noreferrer&quot;&gt;collision course&lt;/a&gt;. Since their path is nothing like a circular or modestly elliptical orbit, classical Lagrange points are not defined. Once they collide, they will strongly interact, and cease being independent self-contained galaxies, so there is no periodicity to their motion either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further discussion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mathematically, if the two galaxies were treated as point masses, then one might ask if they could be considered bound in a highly elliptical orbit about their center of mass, or not. If so, and if gravity from other nearby galaxies in the local group can be ignored, then one might further ask if there are libration points or orbits defined. The elliptical restricted 3 body problem can have something similar to Lagrange points, but they would be &lt;a href=&quot;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Dissertations/1992_Hiday.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;small orbits themselves, not points&lt;/a&gt;. Since one or more of these caveats are probably not valid in the galactic collision example, it doesn't make sense to explore &quot;mathematical curiosities&quot; further here. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, see this &lt;a href=&quot;https://space.stackexchange.com/a/3654/12102&quot;&gt;helpful answer&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-01-14T09:50:29.100" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19779" PostTypeId="2" ParentId="19771" CreationDate="2017-01-14T04:03:22.883" Score="10" Body="&lt;p&gt;It can be made to work, but it would be quite different from the typical SF depiction.&lt;/p&gt;&#xA;&lt;p&gt;As Blake points out, you can't gain anything from mass that is already moving faster than rocket exhaust. However, you do not have to collect the gas at orbital velocity. In the extreme case you could slow to a complete stop and refuel while landed, as long as your fuel tank is big enough to allow a single stage to orbit.&lt;/p&gt;&#xA;&lt;p&gt;There are serious plans to refuel that way on Mars (~5 km/s) using current technology, so nothing impossible there.&lt;/p&gt;&#xA;&lt;p&gt;Earth (~10 km/s) is close to the practical limit for a single stage - enough that we've never actually built one, but it could be done if there weren't better options available.&lt;/p&gt;&#xA;&lt;p&gt;Assuming that refilling the tank takes less than a year, there should be some point between orbit and the ground where you don't have excessive drag, but also don't have to empty the tank to get back to orbit.&lt;/p&gt;&#xA;&lt;h2&gt;Nuclear rockets&lt;/h2&gt;&#xA;&lt;p&gt;Using chemical rockets is probably not going to work - I prefer to avoid atmospheres composed of perfectly mixed rocket fuel and oxidizer - so I'll start with a nuclear thermal rocket that has similar performance to H2/O2 but will burn anything that can be sprayed onto hot rocks. That doesn't exist, but seems reasonable with current technology.&lt;/p&gt;&#xA;&lt;p&gt;I'm also going to assume separate fuel gathering ships rather than a small add on to a larger ship - burning 90% of the collected fuel to return to orbit only gets you a full tank if you can make ten trips.&lt;/p&gt;&#xA;&lt;p&gt;Mars gets closest to basic atmosphere skimming - orbital velocity is lower than exhaust velocity, so you don't need to slow down to gain a small amount of fuel on each pass through the atmosphere.  Completely filling the tank may be possible.&lt;/p&gt;&#xA;&lt;p&gt;Earth requires slowing to at least 4 km/s below orbital velocity. You'll need a very fast pump, as you have to either produce enough thrust to hover or fill the tanks in the few minutes before you hit the ground or thick atmosphere. Getting back into orbit will use up at least half of the collected fuel. Starting with enough fuel to return to orbit isn't really an option, so the pumps will need to be very reliable as well as fast.&lt;/p&gt;&#xA;&lt;p&gt;Jupiter requires slowing by 10 km/s, so there is really no margin for error. Each trip into the atmosphere uses up 99% of the collected fuel - you need hundreds of flights to fill the tank, and abort to orbit isn't even a theoretical possibility.&lt;/p&gt;&#xA;&lt;h2&gt;Chemical rockets&lt;/h2&gt;&#xA;&lt;p&gt;Fully fueling a chemical rocket this way probably isn't going to happen. If you do find all the necessary elements in one place, it is probably as water. The energy to process that into rocket fuel probably means a nuclear reactor, which you may as well just use more directly.&lt;/p&gt;&#xA;&lt;p&gt;Collecting only the heavier oxidizer is more reasonable, but probably not worth it in most cases - even if Jupiter had a pure oxygen atmosphere you would be using 99 tons of hydrogen to get 8 tons of oxygen.&lt;/p&gt;&#xA;&lt;p&gt;On Earth you might be able to come out ahead -  Starting with 100 tons of hydrogen you could collect 800 tons of oxygen and return to orbit with 50 tons of hydrogen and 400 tons of oxygen.&lt;/p&gt;&#xA;&lt;h2&gt;Engineering challenges&lt;/h2&gt;&#xA;&lt;p&gt;The rocket itself is relatively easy - several suitable designs already exist, although nothing flown.&lt;/p&gt;&#xA;&lt;p&gt;To store the collected fuel, you need something about the size and weight of the space shuttle external tank, which is easy enough, but it also needs to be able to survive reentry, possibly hundreds of times, without adding too much mass to get back to orbit.&lt;/p&gt;&#xA;&lt;p&gt;To fit a useful amount of air in the tank, you probably need to liquify it. A high pressure tank may also work, but is likely heavier. Either option generates a lot of heat. That isn't usually a problem, but you are trying to do this while flying at mach 10 attached to both a nuclear reactor and a rocket engine.&lt;/p&gt;&#xA;&lt;h2&gt;Would it ever be worth doing?&lt;/h2&gt;&#xA;&lt;p&gt;On Earth, it might happen if sufficiently reliable technology was already used for something else, but otherwise there are better options. Using a falcon style ground based first stage gets you most of the benefits with none of the risk.&lt;/p&gt;&#xA;&lt;p&gt;An Earth-like planet without infrastructure could make sense, but you are probably better off landing - use a rocket that can reach orbit empty or act as the first stage for a large load of fuel.&lt;/p&gt;&#xA;&lt;p&gt;Venus actually makes it a look like a decent option - mid air fuel production is just as dangerous as anywhere else, but the surface environment manages to be even more unpleasant.&lt;/p&gt;&#xA;&lt;p&gt;Jupiter needs either a lot of flights or a lot of ships. If you have sufficient local infrastructure to support a hundred ships, you may as well build a space elevator or something. Without that, entering the atmosphere of a gas giant is reserved for the truly insane starship captain who somehow missed all the moons and ring system full of easily accessible water ice.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-14T04:03:22.883" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19780" PostTypeId="1" CreationDate="2017-01-14T06:16:39.767" Score="7" ViewCount="331" Body="&lt;p&gt;Are the propellant mass and transfer time from orbit A to orbit B the same as from orbit B to orbit A, considering low-thrust propulsion? That is, in the case of dealing with Keplerian orbit parameters and having both points in the sphere of influence of Earth with any third body or perturbation effects neglected. In other words, can we say that the return transfer is a &quot;mirror copy&quot; of the forward one? If that is not generally the case, then under what conditions can we witness such symmetrical transfers?&lt;/p&gt;&#xA;" OwnerUserId="18208" LastEditorUserId="1142" LastEditDate="2017-01-17T01:51:51.977" LastActivityDate="2017-01-17T01:51:51.977" Title="Are transfers between two orbits commutative?" Tags="|orbital-mechanics|artificial-satellite|orbit|orbital-maneuver|delta-v|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19783" PostTypeId="1" AcceptedAnswerId="19786" CreationDate="2017-01-14T10:53:49.127" Score="3" ViewCount="156" Body="&lt;p&gt;Why will the RP-1 loading for SpaceX's Iridium-1 mission begin at &lt;code&gt;T -00:70:00&lt;/code&gt; - wouldn't it usually be written as &lt;code&gt;T -01:10:00&lt;/code&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\hspace{6cm}$&lt;img src=&quot;https://i.stack.imgur.com/USkrM.png&quot; alt=&quot;screen shot SpaceX&amp;#39;s Iridium-1 press kit&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screen shots (above and below) from: &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/spacex_iridium1_press_kit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spacex.com/sites/spacex/files/spacex_iridium1_press_kit.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\hspace{6cm}$&lt;img src=&quot;https://i.stack.imgur.com/54maf.png&quot; alt=&quot;screen shot SpaceX&amp;#39;s Iridium-1 press kit&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="415" LastEditDate="2017-01-15T15:35:26.267" LastActivityDate="2017-01-15T15:35:26.267" Title="Why will the RP-1 loading for SpaceX's Iridium-1 mission begin at T-00:70:00 as opposed to T-01:10:00?" Tags="|spacex|launch-sequence|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19784" PostTypeId="1" CreationDate="2017-01-14T11:16:37.350" Score="1" ViewCount="1968" Body="&lt;p&gt;What percent of the total available fuel onboard the space shuttle, external tank, and rocket boosters was required to launch and get the space shuttle into orbit? Or phrased another way, what percent of the initial fuel load was left for maneuvering and reentry?&lt;/p&gt;&#xA;" OwnerUserId="18210" LastEditorUserId="415" LastEditDate="2017-01-15T15:48:30.707" LastActivityDate="2017-01-15T15:48:30.707" Title="How large a percentage of its fuel did the Space Shuttle use to get to orbit?" Tags="|launch|fuel|space-shuttle|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19785" PostTypeId="2" ParentId="19784" CreationDate="2017-01-14T11:52:00.300" Score="2" Body="&lt;p&gt;The Shuttle had three propulsion systems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;nofollow noreferrer&quot;&gt;solid rocket boosters&lt;/a&gt; (SRB), mounted at the side&lt;/li&gt;&#xA;&lt;li&gt;the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;main engines&lt;/a&gt; (SSME), mounted in the orbiter and fed from the external tank&lt;/li&gt;&#xA;&lt;li&gt;the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Orbital_Maneuvering_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital manoeuvring system&lt;/a&gt; (OMS), mounted in the orbiter and fed from tanks inside the orbiter&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;During launch, the SRBs were fired until they burned out, and the SSMEs were run until the external tank was almost dry. The SRBs were still generating a minute amount of thrust when they were dropped, but we can effectively assume 100% fuel usage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The external tank still had a small amount of fuel in it when it was dropped, because it was safer and more controllable to shut down the main engines rather than let them run dry. I don't have a figure to hand, but I suspect this would be &amp;lt;1% of the total fuel mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After this point, the SSMEs played no further role in the launch. The Shuttle was still on a suborbital trajectory (or a highly elliptical orbit), and the OMS was used to perform the final orbital insertion burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm finding it difficult to nail down an exact figure (it would depend heavily on mission profile and payload) but Jenkins' &lt;strong&gt;The Space Shuttle&lt;/strong&gt; (2001 ed, p. 390) suggests an average of about 500fps of fuel was used during ascent, out of a total of 1000fps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in total - 100% (or effectively 100%) of the SRBs and external tank, and ~50% of the OMS fuel carried in the orbiter.&lt;/p&gt;&#xA;" OwnerUserId="11259" LastActivityDate="2017-01-14T11:52:00.300" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19786" PostTypeId="2" ParentId="19783" CreationDate="2017-01-14T12:35:19.827" Score="4" Body="&lt;p&gt;Somebody messed up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It absolutely should be &lt;code&gt;-01:10:00&lt;/code&gt;. My guess is that someone was counting backwards from &lt;code&gt;T = 0&lt;/code&gt;, and didn't think long enough about what they were typing. Press kits are usually put together by PR or media relations people, who at SpaceX are probably on just as tight deadlines as the engineers. Mistakes will be made.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastActivityDate="2017-01-14T12:35:19.827" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19787" PostTypeId="1" AcceptedAnswerId="19790" CreationDate="2017-01-14T12:42:01.800" Score="5" ViewCount="457" Body="&lt;p&gt;The first ten of eleven OrbComm satellites were deployed in opposing pairs at five different times. The 2nd stage performed roll maneuvers to make sure the initial velocity and/or attitude were optimal. Special thanks to whomever for the forethought of putting the moon in the field of view to make this historic sequence both even more visually rich and educational. At the top you can see the opposing satellite deployed symmetrically at the same time as one at the bottom of the view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lDzYi.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lDzYi.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; SpaceX deployment of the OrbComm-2 satellites GIF made from cropped screenshots of the &lt;a href=&quot;https://www.youtube.com/watch?v=O5bTbVbe4e4&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX technical broadcast&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the Iridium satellites are in a pentagonal arrangement. Will they be deployed as vertical pairs (doubtful - too close together) or perhaps offset pairs - not exactly coplanar along with the 2nd stage axis?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Iridium satellites configured for launch by SpaceX Falcon 9, from &lt;a href=&quot;https://twitter.com/IridiumComm/status/813844414846173186&quot; rel=&quot;nofollow noreferrer&quot;&gt;Iridium Tweet&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5CMmN.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5CMmN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-07T00:15:39.410" LastActivityDate="2021-08-07T00:15:39.410" Title="What pattern of satellite release will the pentagonal Iridium arrangement use?" Tags="|spacex|payload-deployment|iridium|orbcomm-2|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19789" PostTypeId="2" ParentId="19755" CreationDate="2017-01-14T20:23:00.190" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Can electrolysis of water provide fuel for the generator where both water and liquid carbon dioxide is utilized in one self sustaining machine?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;NO&lt;/strong&gt;. A 'self sustaining machine' would be a perpetual motion machine, and those are impossible. You have to provide energy from an external source. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electrolysis of water costs energy, about &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrolysis_of_water#Efficiency&quot; rel=&quot;noreferrer&quot;&gt;50 kWh per kg&lt;/a&gt; of hydrogen produced, the process is about 80% efficient. So when you burn that hydrogen, you can recover 80% of the energy you used to electrolyze it. And part of that 80% is lost converting the heat (from burning the hydrogen) into usable forms of energy (motion, electricity). Fuel cells are about 80% efficient. You can also burn the hydrogen in an internal combustion engine, but those are usually less than 50% efficient. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, cracking CO&lt;sub&gt;2&lt;/sub&gt; costs a lot of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current processes for cracking CO&lt;sub&gt;2&lt;/sub&gt; use H&lt;sub&gt;2&lt;/sub&gt; to react with CO&lt;sub&gt;2&lt;/sub&gt; to &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrochemical_reduction_of_carbon_dioxide&quot; rel=&quot;noreferrer&quot;&gt;reduce the CO&lt;sub&gt;2&lt;/sub&gt; to CO plus H&lt;sub&gt;2&lt;/sub&gt;O&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA &lt;a href=&quot;https://www.nasa.gov/content/direct-generation-of-oxygen-via-electrocatalytic-reduction-of-carbon-dioxide-in-an-ionic/&quot; rel=&quot;noreferrer&quot;&gt;is investigating ways&lt;/a&gt; to reduce CO&lt;sub&gt;2&lt;/sub&gt; directly to carbon and O&lt;sub&gt;2&lt;/sub&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Could we not add something to liquid CO&lt;sub&gt;2&lt;/sub&gt; to allow better electrolysis?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Possibly, yes. That's the subject of NASA's research: to find an ionic liquid that can serve as a catalyst for CO&lt;sub&gt;2&lt;/sub&gt; reduction. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="18879" LastEditDate="2019-03-09T03:16:53.507" LastActivityDate="2019-03-09T03:16:53.507" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="19790" PostTypeId="2" ParentId="19787" CreationDate="2017-01-14T21:37:29.397" Score="3" Body="&lt;p&gt;According to the &lt;a href=&quot;https://youtu.be/tTmbSur4fcs?t=4805&quot; rel=&quot;nofollow noreferrer&quot;&gt;webcast&lt;/a&gt; (at T+01:00:25) they were released one at a time, 100 seconds apart. &lt;/p&gt;&#xA;" OwnerUserId="548" LastEditorUserId="548" LastEditDate="2017-01-15T17:08:56.160" LastActivityDate="2017-01-15T17:08:56.160" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19792" PostTypeId="1" CreationDate="2017-01-15T08:25:42.013" Score="0" ViewCount="633" Body="&lt;p&gt;I read that there is a satellite orbiting Mars that the rovers send data to but I still find it amazing how that satellite can send signals tens of millions of miles to earth without any amplifications. How is that possible?&lt;/p&gt;&#xA;" OwnerUserId="18217" LastEditorUserId="25" LastEditDate="2017-01-15T16:33:53.873" LastActivityDate="2017-01-18T19:30:43.057" Title="Why don't signals from rovers on Mars get lost on the long trip to Earth?" Tags="|mars|artificial-satellite|communication|rovers|" AnswerCount="3" CommentCount="4" ClosedDate="2017-01-18T05:14:30.677" ContentLicense="CC BY-SA 3.0" />
  <row Id="19794" PostTypeId="2" ParentId="18789" CreationDate="2017-01-15T09:11:02.650" Score="11" Body="&lt;p&gt;&lt;em&gt;This is just an addendum to&lt;/em&gt; &lt;a href=&quot;https://space.stackexchange.com/a/18815/12102&quot;&gt;@ErinAnne's thorough answer&lt;/a&gt; above, there is a new update in &lt;a href=&quot;http://spaceflight101.com/ss-520-4-rocket-launches-on-experimental-mission/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Experimental Launch of World’s Smallest Orbital Space Rocket ends in Failure&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While they have called it the &quot;World’s Smallest Orbital Space Rocket&quot;, so far it hasn't put it's payload into orbit yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yt04y.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yt04y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; SS-520-4 rocket ready for launch. From &lt;a href=&quot;http://spaceflight101.com/ss-520-4-rocket-launches-on-experimental-mission/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, Photo: JAXA&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dNKD1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dNKD1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; SS-520-4 rocket. From &lt;a href=&quot;http://spaceflight101.com/ss-520-4-rocket-launches-on-experimental-mission/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, Image: JAXA&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SS-520-4 is a three-stage solid-fueled rocket standing 9.54 meters tall, measuring 52 centimeters in diameter and weighing in at 2,600 Kilograms – smaller and lighter than any previous ground-based orbital launch vehicle. It is based on the SS-520 sounding rocket design, modified with a small third stage tasked with injecting a payload into Low Earth Orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NgdHr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NgdHr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; TRICOM-1 in Launch Configuration. From &lt;a href=&quot;http://spaceflight101.com/ss-520-4-rocket-launches-on-experimental-mission/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, Photo: JAXA&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/A7wCC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/A7wCC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Person with TRICOM-1 for scale - in this case Professor Hiroto Habu of the Japan Aerospace Exploration Agency. Image from &lt;a href=&quot;https://english.kyodonews.jp/photos/2016/11/445500.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-01-17T03:10:23.440" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19795" PostTypeId="2" ParentId="14687" CreationDate="2017-01-15T09:59:44.407" Score="2" Body="&lt;p&gt;I have just run across the following note in Section &lt;a href=&quot;https://sst-soa.arc.nasa.gov/05-guidance-navigation-and-control&quot; rel=&quot;nofollow noreferrer&quot;&gt;05. Guidance, Navigation and Control&lt;/a&gt; of the NASA site [State of the Art of Small Spacecraft Technology]:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;GPS units are controlled under the Export Administration Regulations (EAR) and must be licensed to remove COCOM limits&lt;span class=&quot;math-container&quot;&gt;${}^{15}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;${}^{15}$&lt;/span&gt;Office of the Federal Register, “FOREIGN AVAILABILITY DETERMINATION PROCEDURES AND CRITERIA,” vol. Title 15 Part 768.7, 2015.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I then found this at the &lt;a href=&quot;https://www.law.cornell.edu/cfr/text/15/768.2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cornell Legal Access Institute&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;§ 768.2 Foreign availability described.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;(a) Foreign availability. Foreign availability exists when the Secretary determines that an item is comparable in quality to an item subject to U.S. national security export controls, and is available-in-fact to a country, from a non-U.S. source, in sufficient quantities to render the U.S. export control of that item or the denial of a license ineffective. For a controlled country, such control or denial is “ineffective” when maintaining such control or denying a specific license would not restrict the availability of items that would make a significant contribution to the military potential of the controlled country or combination of countries detrimental to the national security of the United States (see sections 5(a) and 3(2)(A) of the EAA.)&lt;/p&gt;&#xA;&lt;p&gt;(b) Types of foreign availability. There are two types of foreign availability:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;List item (1) Foreign availability to a controlled country; and&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;(2) Foreign availability to a non-controlled country.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Note to paragraph (B) of this section:&lt;/p&gt;&#xA;&lt;p&gt;See §768.7 of this part for delineation of the foreign availability assessment procedures, and § 768.6 of this part for the criteria used in determining foreign availability.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;§768.7 is quite long, and details a specific set of procedures for the determination of &lt;em&gt;Foreign Availability.&lt;/em&gt;  From what I understand, this is essentially saying if it is determined that the item or technology is already readily available for purchase outside the US, then one can apply for permission to export it from the US.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;This applies to exporting from the US. So far I haven't found any rules against building a LEO-capable GPS unit in the US for ones own cubesat to be launched within the US or by a US launch provider, nor selling one to someone else within the US. (I've chosen the US as an example of a &amp;quot;CoCom country&amp;quot; here.)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-15T09:59:44.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19796" PostTypeId="2" ParentId="19792" CreationDate="2017-01-15T11:38:59.233" Score="3" Body="&lt;p&gt;How a satellite is able to communicate with Earth is better answered with &lt;a href=&quot;https://space.stackexchange.com/q/958/12102&quot;&gt;How does Voyager 1 send signals to Earth?&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/4751/12102&quot;&gt;How does NASA get information from space probes?&lt;/a&gt;. I'm only going to answer why it is much easier to have an orbiting satellite communicate with Earth than a rover, which I believe is the crux of your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, there are landers that have communicated directly with Earth. Opportunity and Curiosity can both talk directly to Earth. However, they can't have as large of a dish as an orbiting satellite can. In fact, the landers don't have a steerable antenna, as that would be too difficult, and thus can only communicate with Earth when Earth is in about the right part of the sky (Directly overhead), which should happen once a day. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Communication with an orbiting spacecraft is much easier, as the distance is as much as a million times closer. Power is lost as a square of distance, thus that is a very significant factor! This allows for a lower gain antenna, which allows the two to communicate much easier, and allows the orbiting spacecraft with it's much larger steerable antenna to do the heavy lifting!&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-01-15T11:38:59.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19797" PostTypeId="2" ParentId="19792" CreationDate="2017-01-15T13:37:28.413" Score="2" Body="&lt;p&gt;I think the key to your question is in your pre-condition &quot;&lt;em&gt;without any amplification&lt;/em&gt;&quot;, in which case it would indeed be amazing, or at least surprising. However power amplifiers are always used to raise the signal strength at the transmitter so that it can be received correctly.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That point made, PearsonArtPhoto's answer and also the other referenced question/answers should be easier to understand.  For example the level of amplification at both transmit and receive ends, the types of antenna at each end, frequency, bandwidth and the distance all play a role in the rate of information transfer.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-01-18T19:30:43.057" LastActivityDate="2017-01-18T19:30:43.057" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19798" PostTypeId="2" ParentId="19780" CreationDate="2017-01-15T16:42:34.807" Score="3" Body="&lt;p&gt;Given no third body effects, atmospheric drag, etc, and an instantaneous burn, the amount of fuel required to switch to an orbit is the same as is required to return to the previous orbit. This assumes the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The starting mass is the same.&lt;/li&gt;&#xA;&lt;li&gt;No fuel spent on station keeping.&lt;/li&gt;&#xA;&lt;li&gt;The exact reverse trajectory is kept.&lt;/li&gt;&#xA;&lt;li&gt;The profile of the thrust is the same.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;For the low thrust case, however, the profile of the thrust changes. You have more thrust at the end of the maneuver, which will change the optimal trajectory slightly. I suspect moving to the closer orbit would take less fuel then moving to the higher one, but I haven't do a simulation to actually prove that. I suspect that except in extreme cases, where most of the mass of the spacecraft is fuel, then this is essentially the same.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-01-15T17:30:30.650" LastActivityDate="2017-01-15T17:30:30.650" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19799" PostTypeId="2" ParentId="18856" CreationDate="2017-01-15T21:03:14.263" Score="10" Body="&lt;p&gt;In the mid 1970's China had a series of reconnaissance satellites called &lt;a href=&quot;http://en.wikipedia.org/wiki/Fanhui_Shi_Weixing&quot; rel=&quot;noreferrer&quot;&gt;Fanhui Shi Weixing (FSW)&lt;/a&gt;. These were designed pretty early on. They were recoverable and mostly used for mapping and internal geography of China. They were recoverable because at first they had film cameras like early American spy satellites. These had a heat shield make of white oak. It apparently worked well since there were quite a few, (20 something?) missions. &#xA;Later on they tried to evolve them into manned spacecraft. In the west it was believed they had a failure causing the loss of the astronaut, so that program was discontinued. I believe FSW (with improvements) were used again for reconnaissance. Later they got people up in the Shenzhou program.&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="1142" LastEditDate="2017-01-17T01:18:45.097" LastActivityDate="2017-01-17T01:18:45.097" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19800" PostTypeId="2" ParentId="19776" CreationDate="2017-01-15T23:36:45.237" Score="4" Body="&lt;p&gt;&lt;strong&gt;Ordinary Lunar Landing in Lower Latitudes&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A LEO to Moon transfer orbit is a 300 km x 378022 km orbit. These numbers refer to distances above earth's surface, not distance from earth's center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perigee speed of this transfer orbit is 10.9 km/s while LEO speed is about 7.7 km/s. TLI (Trans Lunar Injection) is about 3.2 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At apogee of this transfer orbit the space craft is moving .19 km/s with regard to earth while the moon is moving 1.02 km/s with regard to the earth. If the transfer orbit and moon's orbit are coplanar, the arrival &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vinfinity&lt;/a&gt; would be (1.02-.19) km/s or about .83 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the spacecraft enters the moon's sphere of influence, the path can be modeled as a hyperbolic orbit with regard to the moon. Speed of something on a hyperbolic path is $\sqrt{V_{esc}^2+V_{inf}^2}$. At the moon's surface Vesc is about 2.376 km/s. So speed of a spacecraft near the surface is $\sqrt{.83^2+2.376^2}$ km/s or about 2.516 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may notice I'm using three significant figures here. The moon's orbit varies so it's not possible to give a general answer with this precision. However I do have a motive as you'll see later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding the LEO TLI burn of 3.2 km/s to this 2.516 km/s and you get 5.716 km/s. I round up to 6 km/s since there would likely be gravity loss as you land on the moon as well as course corrections enroute.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Lunar Landing At the Poles&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd still have a LEO TLI burn of about 3.2 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But with my route to the poles, the spacecraft enters the moon's sphere of influence somewhat above the moon and the Vinf velocity vector lies in the same plane as the line from lunar north to south pole. This can be accomplished if the transfer orbit and and lunar orbit aren't coplanar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the orbits aren't coplanar, the launch needs to be timed so the spaceship arrives at the moon as it's crossing the line of nodes, where the two planes intersect. If departing from a equatorial LEO, launch windows open every two weeks or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cvIjR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cvIjR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But now the apogee velocity vector isn't anti-parallel to the moon's velocity vector. So you can't just subtract one from the other to get your Vinf.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you need to use vector subtraction. Assuming the two vectors differ by 29º, you get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hoPLl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hoPLl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the new Vinf is .86 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's plug that into &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\sqrt{V_{esc}^2+V_{inf}^2}$ km/s =&lt;br&gt;&#xA;$\sqrt{.86^2+2.375^2}$ km/s =  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.526 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now you see why I used more significant figures than I'm comfortable with. Departing from an equatorial orbit and landing on a pole only increases surface hyperbola velocity by .01 km/s. I would still round up to 6 km/s for equatorial LEO to moon's pole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in terms of delta V, the hit is insignificant (in my opinion). Take into account that an equatorial LEO is easier to enter from a place like French Guiana's spaceport and it might be even be a little easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The big disadvantage is constraints on launch windows. The moon only crosses a line of nodes each two weeks so you don't have any time return. However launch windows each two weeks is still pretty good (in my opinion).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another disadvantage is the poles lie on the moon's terminator. So if you have sunlight, the shadows are long. This might make landing harder if you're relying on visual input.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbits are time reversible so same goes for the trip back. However with the return trip aerobraking can shed delta V when you arrive at perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Irrelevant Note&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm feeling mortal and have been thinking more about my bucket list. A list item is making a comic book with hard science fiction and some science fact. I'm a big fan of yours and am hoping to get some of your illustrations in my comic book. Especially the polynesians whose population has reached the limits their little island can support.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-01-15T23:43:53.647" LastActivityDate="2017-01-15T23:43:53.647" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19801" PostTypeId="1" CreationDate="2017-01-15T23:36:57.953" Score="7" ViewCount="1122" Body="&lt;p&gt;The 1U cubesat structure is a 10*10*10 cm metal cube with frames. But it costs around 2000-3000€, which is really expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've found it for example on these sites:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.isispace.nl/product/1-unit-cubesat-structure/&quot; rel=&quot;noreferrer&quot;&gt;https://www.isispace.nl/product/1-unit-cubesat-structure/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.cubesatshop.com/product/1-unit-cubesat-structure/&quot; rel=&quot;noreferrer&quot;&gt;https://www.cubesatshop.com/product/1-unit-cubesat-structure/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What is the reason behind it? Is it created from a special material? Or is it expensive because of the low demand, so it can't be mass-produced?&lt;/p&gt;&#xA;" OwnerUserId="8873" LastActivityDate="2021-01-07T18:29:01.003" Title="Why is the cubesat structure so expensive?" Tags="|cost|cubesat|manufacturing|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19802" PostTypeId="2" ParentId="17761" CreationDate="2017-01-16T00:26:16.353" Score="-3" Body="&lt;p&gt;At stage separation, the second-stage engine has begun its burn and an optimally-designed diverging cone of the nozzle will underexpand the jet of gas in the early part of the burn, causing wall separation within the nozzle where the shock wave forms. Consequently, the shock wave will be within the jet plume, and is usually visible as shock diamonds in the exhaust, the number of diamonds corresponding to the Mach number of the jet exhaust speed.&lt;/p&gt;&#xA;" OwnerUserId="18221" LastActivityDate="2017-01-16T00:26:16.353" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19803" PostTypeId="2" ParentId="13005" CreationDate="2017-01-16T03:14:11.073" Score="2" Body="&lt;p&gt;Your explosion would be limited to either above the chromosphere, or approximately on the surface using a leading ballistic plow to get further penetration, like what bunker buster bombs do. Either the leading plow, or the bomb itself would necessarily have to have an ablative sphere, because that region has an extremely low-density gas. It's not so important that there's a low-density, but the product of density times you speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's reasonable to imagine a dive-bomb initiated from the outer solar system, like around Jupiter. Using gravitational assist it's not unreasonable to kill your orbital momentum, so I can imagine a trajectory that is basically outright perpendicular to the sun's surface. This will have you hitting the surface at around its full escape Delta V, which is 617 km/s. Your dynamic pressure goes with $\rho v^2$, so every particle that hits you hits with with about 10,000 as much oomph as particles causing atmospheric drag on the ISS. Because of this, the mass-thickness you could hope to plow through would be very little.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The density of gases around the sun's &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/3/32/Sun_Atmosphere_Temperature_and_Density_SkyLab.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;photosphere increases rapidly&lt;/a&gt;, but considering these numbers, not fast enough for you to get deep enough to do anything interesting, even with an insanely huge continent-killing bomb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In these regions, radiative heat load would be a lot, but I disagree that it will be the dominating factor. Cooling can be done by boiling off some liquid that is stored on-board. This will work (for the purely radiative zone) because you're simply not spending much time within the area. Let's start with the assumption that the heat loads of Solar Probe Plus are survivable. The dive bomb would be going 211 km/s at the point it passes it perihelion. With this, I can develop a lower-bound estimate of 7 hours to reach the surface, more realistically like 3 hours. Yeah, it would be extremely difficult engineering, but reflective coating would buy you a lot, and active circulation of cooling fluid below that surface that is expended by boiling would help, and good insulation would also help.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let's get to the point of this fireworks show. I'll say our goal is to create a tsunami wave on the surface of the sun that can be viewed with telescopic observations. In order to be &quot;visible&quot; this wave needs to be have a meaningful effect within the photosphere. I don't think this is possible. Ballpark a 1,000 km region within the chromosphere. Say it takes a 1 km/s particle to dislodge a single particle in the heat shield. The v^2 term gets us to a factor of like 360,000, and say the ablative shield can be 0.1 m thick. That will allow me to ballpark a tolerable mass-thickness, which combined with the 1,000 km assumption for the region's thickness, gives a density ballpark, of about 1e-10 km/m^3, or 1e-13 g/cm^3. This leaves us in the upper regions of the chromosphere before our craft is torn apart molecule-by-molecule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to get into the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stokes&amp;#39;_law_of_sound_attenuation&quot; rel=&quot;nofollow noreferrer&quot;&gt;blast wave attenuation equation&lt;/a&gt; for this problem. I lack a specific viscosity for these regions, but it looks like it won't matter. It's hard for mechanical power to transfer effectively from the very low-density regions to the high-density regions, and we need this to create a well-observable event. Don't get me wrong, some wave propagation will still be happening through the photosphere, but it'll be relatively weak by that point.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2017-01-16T03:14:11.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19804" PostTypeId="1" AcceptedAnswerId="19808" CreationDate="2017-01-16T03:19:07.513" Score="17" ViewCount="3589" Body="&lt;p&gt;I was surprised to find out from @JohnnyRobinson's &lt;a href=&quot;https://space.stackexchange.com/a/19799/12102&quot;&gt;new answer&lt;/a&gt; that China's series of &lt;a href=&quot;https://en.wikipedia.org/wiki/Fanhui_Shi_Weixing&quot; rel=&quot;noreferrer&quot;&gt;Fanhui Shi Weixing&lt;/a&gt; spacecraft (FSW) used impregnated oak heat shields. I found this in &lt;a href=&quot;http://www.astronautix.com/f/fsw.html&quot; rel=&quot;noreferrer&quot;&gt;astronautix&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The capsule for the FSW, &lt;strong&gt;like that of the US Discoverer/KH-1 spy satellite&lt;/strong&gt;, was mounted heat shield-forward on top of the launch vehicle. The ablative impregnated-oak nose cap covered electrical equipment. The spherical aft dome contained the recovery parachute. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Did these spacecraft re-enter narrow-end or &quot;nose&quot; first? If so, was this aerodynamically stable? Most re-entry spacecraft (manned capsules for example) tend to be larger, blunt-end first. Why would nose-first have been chosen early-on?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qCx8h.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qCx8h.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Labeled &quot;JB-2&quot; in &lt;a href=&quot;http://www.astronautix.com/f/fsw.html&quot; rel=&quot;noreferrer&quot;&gt;astronautix&lt;/a&gt;, presumably a FSW spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kRg4M.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kRg4M.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Recovery of FSW-17 photoreconnaissance satellite.&quot; From &lt;a href=&quot;http://www.astronautix.com/f/fsw.html&quot; rel=&quot;noreferrer&quot;&gt;astronautix&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lypgK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lypgK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Image of FSW-3 or FSW-4 from &lt;a href=&quot;http://www.globalsecurity.org/space/world/china/fsw-3.htm&quot; rel=&quot;noreferrer&quot;&gt;globalsecurity.org&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hWVBa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hWVBa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;FSW reconnaissance satellite in assembly.&quot; From &lt;a href=&quot;http://www.astronautix.com/f/fsw.html&quot; rel=&quot;noreferrer&quot;&gt;astronautix&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZbmUU.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZbmUU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen shot from &lt;a href=&quot;https://books.google.com.tw/books?id=XaqK7LOVsc0C&amp;amp;printsec=frontcover#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;China's Space Program - From Conception to Manned Spaceflight&lt;/a&gt; by Brian Harvey, 2004, Springer-Verlag London, 978-1-85233-566-3, (also &lt;a href=&quot;http://www.springer.com/gp/book/9781852335663&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MOcPM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MOcPM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen shot from &lt;a href=&quot;https://www.cia.gov/library/center-for-the-study-of-intelligence/csi-publications/books-and-monographs/corona.pdf&quot; rel=&quot;noreferrer&quot;&gt;CORONA: America’s First Satellite Program&lt;/a&gt; Kevin C. Ruffner, Ed., Center for the Study of Intelligence, Central Intelligence Agency, Washington, D-C. 1995 &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-01-16T23:48:08.960" Title="Why did early satellites (e.g. China's Fanhui Shi Weixing) re-enter the atmosphere narrow end (nose)-first?" Tags="|artificial-satellite|reentry|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19805" PostTypeId="2" ParentId="19780" CreationDate="2017-01-16T05:45:15.757" Score="4" Body="&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt;: No, continuous thrust maneuvers introduce too many complexities for the &quot;return&quot; trajectory to be a mirror copy.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For continuous thrust maneuvers, we need to consider how many orbital parameters we want to change at once, and whether we plan on having a minimum efficiency during that transfer. Even if only one orbital parameter is changed at once, then we still can't have a mirror trajectory: this is because each orbital element will have an important effect on the optimum thrusting to change that element, as shown in Fig 1 below. Then, if attempting to change multiple orbital elements at once, we have to select which of the six &quot;summation&quot; control laws we wish to use (Petropoulos, Ruggiero, Naasz, and a few more). Each will emphasize the change of one orbital element more than another, and they all will lead to different &quot;return&quot; trajectories.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, changing semi major axis (a) from 42164km to 24396km (around Earth) requires 21kg of fuel (supposing a Snecma PPS1350 engine) and 45 days, whereas going from 24396km to 42164km requires 17kg and only 37 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/axxIO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/axxIO.png&quot; alt=&quot;Fig.1 (Ruggiero and Pergola - IEPC 2011-102)&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Dbhg5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Dbhg5.png&quot; alt=&quot;Fig.2 (same source)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;NOTE:&lt;/em&gt; Both figures are from Ruggiero and Pergola - IEPC 2011-102.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources: those of my MSc. thesis on continuous thrust applied to return interplanetary mission designs. I also hope to open source my propagator in the coming weeks or months, which should allow anyone to try out a variety of missions and control laws for continuous thrust missions.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-01-16T05:45:15.757" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19806" PostTypeId="1" AcceptedAnswerId="19880" CreationDate="2017-01-16T05:51:49.500" Score="3" ViewCount="1294" Body="&lt;p&gt;What is the plan for a thermal protection system on the Dream Chaser and what advantages will it have over the Space Shuttle TPS?&lt;/p&gt;&#xA;" OwnerUserId="17857" LastEditorUserId="1142" LastEditDate="2017-01-22T16:49:19.723" LastActivityDate="2017-01-22T21:10:21.237" Title="What kind of thermal protection system does the Dream Chaser have?" Tags="|iss|crewed-spaceflight|reentry|design|thermal|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19807" PostTypeId="1" CreationDate="2017-01-16T07:29:29.487" Score="5" ViewCount="1106" Body="&lt;p&gt;Can someone explain (in general or specific terms) how SpaceX deployed these 10 satellites? I'm trying to imagine the mechanisms and process of launching these objects with the required precision. Is there a satellite &quot;dispenser&quot; of some sort (a &quot;tube&quot;) with these objects stacked up inside?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: I read the possible duplicate question but it didn't go into the actual mechanism(s) involved or provide any details about how this sort of precision launch is accomplished (e.g. a servo spins up the object to a specified rotation speed and then...). (I didn't watch the linked webcast).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.space.com/35338-spacex-return-to-flight-rocket-launch-landing-success.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX Returns to Flight with 10-Satellite Launch, Rocket Landing&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX's two-stage Falcon 9 rocket lifted off from a launch pad just&#xA;  a few hundred meters from the scenic California coastline here today&#xA;  at 12:54 p.m. EST (1754 GMT; 9:54 a.m. local California time),&#xA;  carrying 10 communications satellites to low-Earth orbit for the&#xA;  Virginia-based company Iridium. After a week of heavy rain and strong&#xA;  winds, only a few clouds obscured the view of the rocket as it climbed&#xA;  skyward.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Deployment of the satellites began 59 minutes after liftoff and took&#xA;  about 15 minutes, SpaceX representatives said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="16501" LastEditorUserId="25" LastEditDate="2017-10-10T01:21:53.760" LastActivityDate="2017-10-10T01:21:53.760" Title="How were the 10 Iridium NEXT satellites deployed by SpaceX?" Tags="|spacex|artificial-satellite|falcon-9|payload-deployment|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19808" PostTypeId="2" ParentId="19804" CreationDate="2017-01-16T08:26:09.437" Score="37" Body="&lt;p&gt;The advantages of the blunt end first design were known well before either vehicle was launched (1958, a few years earlier for spy satellite designers). However, pointy end first is the simplest design that works.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Falling objects generally want to go heavy end first, draggy end last. With a heavy heat shield on the front and a light but bulky parachute in the back, pointy end first is more stable.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Most rockets provide a cone shaped space for the payload. Launching blunt end first would need a large fairing, and putting the heat shield at the back makes connections to the rest of the spacecraft a whole lot more complicated. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That makes it perfect for early designs or when reliability is more important than launch mass. However, there are a couple of issues that make it impractical for large / manned capsules.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;It requires significantly more thermal shielding. Part of this is simple geometry - the blunt end has less area to cover. On top of that the shockwave from a blunt object keeps most of the heat away from the vehicle, so a blunt end first shield can be thinner as well.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Acceleration and deceleration are in opposite directions. Equipment can be secured in all directions easily enough, but an astronaut would experience reentry as 8 g while tied to the ceiling, which seems distinctly unpleasant.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-01-16T08:26:09.437" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19809" PostTypeId="1" AcceptedAnswerId="40417" CreationDate="2017-01-16T10:33:29.340" Score="8" ViewCount="274" Body="&lt;p&gt;I was among many watching SpaceX's exciting OrbComm-2 mission (&lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9_Flight_20&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon-9 Flight 20&lt;/a&gt;, 22-Dec-2015), and besides the first successful landing of a 1st stage after an orbital mission, the deployment of the eleven OrbComm satellites made for great imagery. By watching the (nearly full) Moon, you can even see the 2nd stage perform roll maneuvers between each successive deployment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see this in SpaceX's 'promo trailers' for example. In fact it is &lt;em&gt;this specific visual&lt;/em&gt; that brought me to SXSE (Space Exploration Stackexchange) a year ago - as evidenced by my first salvo of questions here! Since I hadn't heard of TLEs yet, I was &quot;scraping&quot; websites to get the positions of the satellites so I could watch them drift apart. The first image (below) is from my &lt;a href=&quot;https://space.stackexchange.com/q/13217/12102&quot;&gt;very first SXSE question&lt;/a&gt;, and the second is from one of my &lt;a href=&quot;https://space.stackexchange.com/q/19787/12102&quot;&gt;most recent questions&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while I've learned an amazing amount about satellites and space exploration in hundreds of little pieces, I'd like to see how several of the pieces fit together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What goes into the planning and execution &lt;em&gt;of the deployment of groups of LEO satellites&lt;/em&gt;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think a good answer might include at least a cursory outline (e.g. 2-3 sentences maximum) of things like:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2nd stage orbit altitude and circularization&lt;/li&gt;&#xA;&lt;li&gt;attitude adjustment prior to each satellite release (sans &lt;a href=&quot;https://space.stackexchange.com/a/14815/12102&quot;&gt;baseball bat&lt;/a&gt;) &lt;/li&gt;&#xA;&lt;li&gt;the &quot;kick&quot; - mechanical release of the individual satellite(s)&lt;/li&gt;&#xA;&lt;li&gt;subsequent satellite propulsion - engines and/or boosters on each&lt;/li&gt;&#xA;&lt;li&gt;first TLE's available confirming (to us) successful deployment&lt;/li&gt;&#xA;&lt;li&gt;the plan for them to migrate to their final locations (using minimal propellants in some cases)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; By &quot;cursory outline&quot; of an item I mean name it, describe what it is and why it's important in a very basic way. I'm not looking for a technical description of any item at depth. Instead, just looking for a basic view of the breadth of the task. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recent relevant constellations include &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Orbcomm-2&lt;/strong&gt; (Orbcomm/SpaceX) I've mentioned. cf. &lt;a href=&quot;https://space.stackexchange.com/q/13494/12102&quot;&gt;this question&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;CYGNSS&lt;/strong&gt; (NASA/Orbital ATK) Propulsion-less to final configuration. cf. &lt;a href=&quot;https://space.stackexchange.com/q/19007/12102&quot;&gt;this question&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Iridium-1&lt;/strong&gt; (Iridium/SpaceX) cf. &lt;a href=&quot;https://space.stackexchange.com/q/19787/12102&quot;&gt;this question&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Each of those questions include some links to the missions and images of the stacking of the satellites.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ui4zF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ui4zF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; 3D plot of the eleven OrbComm satellites flying in formation interpolated to a single point in time (14:00:00 UTC 24-DEC-2015) from &lt;a href=&quot;https://space.stackexchange.com/q/13217/12102&quot;&gt;my first question here&lt;/a&gt;. Units are kilometers, the axes are approximately Earth Centered Inertial (ECI), the origin is the average position of the group, and the short line segments behind each point represent the previous 5 seconds of movement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lDzYi.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lDzYi.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; SpaceX deployment of the OrbComm-2 satellites GIF made from cropped screenshots of the &lt;a href=&quot;https://www.youtube.com/watch?v=O5bTbVbe4e4&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX technical broadcast&lt;/a&gt;. The &lt;em&gt;nearly full Moon&lt;/em&gt; is the white circular object at the lower extreme left.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-07T00:15:16.223" LastActivityDate="2021-08-07T00:15:16.223" Title="What goes into the planning and execution of the deployment of groups of LEO satellites?" Tags="|low-earth-orbit|satellite-constellation|payload-deployment|cygnss|orbcomm-2|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19810" PostTypeId="2" ParentId="19801" CreationDate="2017-01-16T11:14:03.683" Score="14" Body="&lt;p&gt;It's expensive because of the quality requirements.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Material cost is small, low demand is a factor but a structure that size can be made much cheaper than this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main cost factors are : &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The design has to be qualified for use in space, this requires testing. So the R&amp;amp;D cost is high. Low demand means you have to amortize this cost over a small number of units.   &lt;/li&gt;&#xA;&lt;li&gt;The manufacturing process has lots of verification steps plus a paper trail for auditing. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-16T11:14:03.683" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19811" PostTypeId="1" AcceptedAnswerId="19813" CreationDate="2017-01-16T11:58:49.093" Score="18" ViewCount="1536" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tSuFm.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tSuFm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image was attached to the Wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_1&quot;&gt;Vostok 1&lt;/a&gt; with a perfunctory caption &quot;Part of the Vostok 1 instrument panel&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in the globe in the center. What is the name of this instrument? Is it just a passive map, or did it automatically follow the craft position? If it did, how did it do it (obtain the data to position the globe)? The article mentions considerable problems with determining the orbit; Gagarin didn't learn he was in the right orbit nearly until reentry burn time. Any more information on this device?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-04T20:01:04.350" Title="What was the globe instrument in Vostok 1?" Tags="|navigation|instrument|orbit-determination|vostok-program|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19812" PostTypeId="1" AcceptedAnswerId="19815" CreationDate="2017-01-16T12:05:53.927" Score="4" ViewCount="437" Body="&lt;p&gt;Citing the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The orbit's perigee and apogee had been selected to cause reentry due to orbital decay within 10 days (the limit of the life support system function) in the event of retrorocket malfunction. However, the actual orbit differed from the planned and would not have allowed descent until 20 days.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The obvious case where Gagarin would be stuck in orbit is the retro thruster not firing. But from miscellaneous sources &lt;sup&gt;;-)&lt;/sup&gt; I know one needs very little delta-V to perform a retrograde burn at the apogee that drops perigee enough for aerodynamic forces to grow enough to deorbit the craft. Assuming inability to start the main retro engine, could the nitrogen thrusters of Vostok-1 have provided enough delta-V to reduce orbital decay below the critical 10 days?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="7062" LastEditDate="2017-01-17T07:15:18.027" LastActivityDate="2017-01-19T21:52:28.593" Title="Could Vostok 1 deorbit on its cold gas thrusters?" Tags="|rcs|vostok-program|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19813" PostTypeId="2" ParentId="19811" CreationDate="2017-01-16T14:33:55.620" Score="16" Body="&lt;p&gt;From &lt;a href=&quot;https://ru.wikipedia.org/wiki/%D0%92%D0%BE%D1%81%D1%82%D0%BE%D0%BA_(%D0%BA%D0%BE%D1%81%D0%BC%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%B8%D0%B9_%D0%BA%D0%BE%D1%80%D0%B0%D0%B1%D0%BB%D1%8C)&quot;&gt;russian wikipedia&lt;/a&gt;:&#xA;Another instrument helped to the pilot to decide when to start manual operation to return to Earth - it was a small globe with a clock mechanism, which shows the (calculated, not measured) current location over Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It followed the craft position as calculated with the mechanical computer inside, following parameters initially entered (using the knobs) as given by ground station. The discs above the globe displayed latitude and longitude; the device also used potentiometers coupled to the mechanical computer shafts to feed current craft position information to other instruments. Upon press of a button, the globe would turn from current craft position to a predicted point of landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparing the indication of &quot;Globus&quot; with ground features seen through &quot;Vzor&quot; (a small window with aiming cross lines, pointed at ship's nadir), the astronaut could determine deviations from planned path. &lt;/p&gt;&#xA;" OwnerUserId="11046" LastEditorUserId="103" LastEditDate="2017-01-17T09:23:38.880" LastActivityDate="2017-01-17T09:23:38.880" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19814" PostTypeId="2" ParentId="19780" CreationDate="2017-01-16T15:14:19.577" Score="3" Body="&lt;p&gt;The answer is no except in the mathematically idealized case where the reaction mass used is negligible and the engine is an idealized device capable of making any force up to some maximum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's consider a Hohmann transfer orbit from a lower orbit to a higher orbit. This requires a $\Delta \textbf{v}_1$ out of the lower orbit and a $\Delta \textbf{v}_2$ to insert into the higher orbit. The force exerted on  the spacecraft during one of the burns is $\textbf{F}=m\textbf{a}$, where $m$ is the mass of the spacecraft, which is a function of time because we're expending reaction mass. The mass is large during the low-altitude burn and smaller during the high-altitude burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's time-reverse the motion. Under time reversal, $\textbf{a}$ stays the same, which means that $\textbf{F}$ has the same direction. But $|\textbf{F}|$ doesn't stay the same, because $m(t)$ is now a different function. In this version, the mass is large during the high-altitude burn and smaller during the low-altitude burn. This breaks the symmetry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when the reaction mass is negligible and  the engine is an idealized device capable of making any force up to some maximum, $m(t)$ is a constant function, and time-reversing the orbit keeps $\textbf{F}$  the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This argument does not depend on the assumption of a Hohmann transfer orbit, which was just illustrative. The only assumption was that the gravitational field was static.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of this actually depends in principle on whether it's low-thrust or high-thrust propulsion, continuous acceleration or acceleration in short burns. However, because of the high exhaust velocities used in low-thrust propulsion, it may be a better approximation to say that the reaction mass is negligible. I would assume that this is why, in the numerical example given by ChrisR's answer, there is at least approximate symmetry.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2017-01-16T15:31:32.237" LastActivityDate="2017-01-16T15:31:32.237" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19815" PostTypeId="2" ParentId="19812" CreationDate="2017-01-16T15:47:09.787" Score="6" Body="&lt;p&gt;Not likely. For early manned missions, rendezvous wasn't a requirement, so thrusters provided only for rotation, not for translation. According to &lt;a href=&quot;http://vostoksupersite.weebly.com/thruster-system.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this reference&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In each of the two [redundant] sections of 8 [thrusters] there were 2 thrusters each for pitch and yaw and 4 for roll. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Depending on how the thrusters were offset from the craft's center of gravity, it's possible that some alternating pattern of thruster use would give some translational force, but that would be terribly inefficient. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if the thrusters were arranged for translation, they wouldn't have been able to de-orbit the craft. There was a total of 20kg of gas carried out of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_(spacecraft)#Vostok_3KA&quot; rel=&quot;nofollow noreferrer&quot;&gt;spacecraft mass&lt;/a&gt; of 4.7 tons; nitrogen gas specific impulse is around 73 seconds. According to the rocket equation this yields only about 3 m/s worth of delta-V. At a guess that might cut a few hours off the natural orbital decay time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does seem a little strange to provide 10 days of consumables for what was supposed to be a two hour flight, rather than spending the mass on some sort of backup retrofire solution. The follow-on Voskhod design did have a backup, solid-fuel retrorocket.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-01-19T21:52:28.593" LastActivityDate="2017-01-19T21:52:28.593" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19816" PostTypeId="1" AcceptedAnswerId="19817" CreationDate="2017-01-16T16:14:43.437" Score="6" ViewCount="968" Body="&lt;p&gt;In April 2014 &lt;a href=&quot;https://space.stackexchange.com/questions/4250/#comment8670_4250&quot;&gt;this comment&lt;/a&gt; was made below the question &lt;a href=&quot;https://space.stackexchange.com/q/4250/12102&quot;&gt;What propulsion methods does the ISS use for station-keeping?&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In November they said in three years, so perhaps late 2016. Since such claims from a company seeking investment funds are always optimistic, more likely 2017 or later. – Mark Adler Apr 13 '14 at 13:05&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is there a chance the ISS will make a big step in the &quot;all electric&quot; direction in 2017? Is there sufficient electric power? Will &lt;a href=&quot;https://www.nasaspaceflight.com/2017/01/spacewalkers-upgrading-iss-batteries/&quot; rel=&quot;noreferrer&quot;&gt;the new lithium ion batteries&lt;/a&gt; help in any way? See also &lt;a href=&quot;http://www.sciencealert.com/a-spacewalking-robot-has-replaced-the-18-year-old-batteries-on-board-the-iss&quot; rel=&quot;noreferrer&quot;&gt;The ISS just got its first battery refresh in 18 years&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-10-30T17:22:00.903" Title="Will the ISS have electric propulsion to maintain altitude? Is there enough power for it?" Tags="|iss|station-keeping|electric-propulsion|vasimr|" AnswerCount="1" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19817" PostTypeId="2" ParentId="19816" CreationDate="2017-01-16T16:27:06.663" Score="7" Body="&lt;p&gt;the VASIMR test planned by Ad Astra &lt;a href=&quot;https://integratedspaceanalytics.com/cms/portal/vasimr-testing-iss&quot; rel=&quot;noreferrer&quot;&gt;has been cancelled&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On December 8, 2008, Ad Astra signed an agreement with NASA to arrange&#xA;the placement and testing of a flight version of the VASIMR, the&#xA;VF-200, on the International Space Station (ISS),[24] but the plan was&#xA;scrapped in 2015.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://sen.com/blogs/irene-klotz/nasa-nixes-ad-astra-rocket-test-on-the-space-station&quot; rel=&quot;noreferrer&quot;&gt;reason&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But a series of agreements with NASA, dating back to June 2005, ended in December 2014, with the U.S. space agency determining that the space station “was not an ideal demonstration platform for the desired performance level of the engines,” NASA spokeswoman Rachel Kraft wrote in an email.&lt;/p&gt;&#xA;&lt;p&gt;NASA cited VASIMR’s power consumption and test duration requirements as reasons for ending plans to fly the engine on the station.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-16T17:30:27.697" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19819" PostTypeId="1" AcceptedAnswerId="19909" CreationDate="2017-01-16T16:36:49.190" Score="9" ViewCount="650" Body="&lt;p&gt;Due to the Oberth Effect, it is most efficient to accelerate a spacecraft along its orbit at or as close-as-possible-to periapsis, and decelerate within close proximity to apoapsis.  But all discussions of this seem to be centered around pure tangential orbital acceleration, for example performing transfers within the same plane or reducing speed to allow orbital re-entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Does the same logic apply to non-tangential maneuvers, say, adjusting orbital inclination?&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume we have two spacecraft in stable orbits of a planet that wish to rendezvous, and that these two orbits that are not co-planar.  One must perform a transverse engine burn to adjust its own inclination to match the other. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At two points on each orbit, the orbital planes intersect.  It is at one of these two points that acceleration must be applied to align the orbits.  Regardless of whether this maneuver is performed at the ascending or descending node (defined by comparing their individual inclinations relative to the equator of the planet), at least some of the acceleration will be &lt;strong&gt;against&lt;/strong&gt; the spacecraft's actual velocity at that time, because the goal is to reduce the relative inclination.  Because of this, &lt;strong&gt;does the Oberth Effect make it most efficient to perform this maneuver at the node closer to apoapsis since the vessel's speed will be lower than at the opposing node&lt;/strong&gt;?&lt;/p&gt;&#xA;" OwnerUserId="17722" LastActivityDate="2017-01-26T15:25:30.240" Title="Oberth Effect for non-tangential acceleration" Tags="|orbital-mechanics|orbital-maneuver|inclination-change|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19820" PostTypeId="2" ParentId="18223" CreationDate="2017-01-16T17:38:07.013" Score="15" Body="&lt;p&gt;In the Shuttle Mission Simulator we used the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jacchia_Reference_Atmosphere&quot; rel=&quot;noreferrer&quot;&gt;Jacchia Reference Atmosphere&lt;/a&gt;, it's good to 2500 km.  IIRC it's not good low down so we used a standard atmosphere model for atmospheric flight regimes and Jacchia above 182 km (600K feet).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-01-16T17:49:58.117" LastActivityDate="2017-01-16T17:49:58.117" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19822" PostTypeId="1" AcceptedAnswerId="19943" CreationDate="2017-01-16T20:32:27.583" Score="4" ViewCount="1165" Body="&lt;p&gt;What could the link be between the «white performance bars show» below and the «actual size of the rocket»?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The quote below seems to imply there is one.&#xA;But isn't that white scale purely arbitrary? It’s my current understanding that SpaceX could have increased the scale of the bar across the board so that Saturn V performance would also be above its height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hKtBn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hKtBn.png&quot; alt=&quot;Vehicles by performance&quot;&gt;&lt;/a&gt; source page: 28 &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/mars_presentation.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spacex.com/sites/spacex/files/mars_presentation.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The white bars show the performance of the vehicles. In other words, the payload to orbit of the vehicle. What it represents is what the size efficiency of the vehicle is. And mostly including ours that are currently flying, including those; the performance bar is only a small percentage of the actual size of the rocket. With the interplanetary system initially to be used for Mars, we have been able to, we believe, massively improve the design performance. &lt;strong&gt;It's the first time the rocket performance bar will exceed the physical size of the rocket.&lt;/strong&gt; -Elon&lt;br&gt;&#xA;  Source: &lt;a href=&quot;https://diyhpl.us/wiki/transcripts/spacex/elon-musk-making-humans-a-multiplanetary-species/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://diyhpl.us/wiki/transcripts/spacex/elon-musk-making-humans-a-multiplanetary-species/&lt;/a&gt; or &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/H7Uyfqi_TE8?start=1759&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.tylervigen.com/spurious-correlations&quot; rel=&quot;nofollow noreferrer&quot;&gt;This link illustrates my point&lt;/a&gt; in what is meant about not being correlated (its source is, somewhat ironically, &lt;a href=&quot;https://twitter.com/elonmusk/status/765111090430607360&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="6852" LastEditorUserId="6852" LastEditDate="2017-01-28T19:29:03.310" LastActivityDate="2017-01-30T21:52:54.113" Title="Size of rocket correlation with its performance" Tags="|spacex|interplanetary|its|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19824" PostTypeId="2" ParentId="19822" CreationDate="2017-01-16T21:13:47.153" Score="6" Body="&lt;p&gt;The white bars are payload in kg. They look roughly proportional to me, so in that sense they're accurate. The size of the white bars vs. the dimensions of the rocket is misleading. The performance bar is one-dimensional. The rockets, on the other hand, grow in 3 dimensions. So when you compare a Falcon 9 to a Saturn V, the performance difference is much larger than the height difference suggests. If you were to do a graph of takeoff weight vs payload weight for these rockets, you'd get a less misleading graph. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, there is a correlation between rocket size and performance. But that correlation seems to be fairly linear. Rocket stages of very different sizes have a mass fraction of around 95% (i.e. 95% of the stage weight is fuel).&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-16T21:13:47.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19826" PostTypeId="1" AcceptedAnswerId="19832" CreationDate="2017-01-16T23:21:06.743" Score="11" ViewCount="4603" Body="&lt;p&gt;I know I'm not the only one who has this question, but I still have trouble wrapping my head around how a rocket engine preburner works. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that the preburner is what gets the rocket's turbopumps to run, and that's how the fuel enters the combustion chamber. That's not the difficult part. &#xA;I also understand how the preburner is fed. (It basically feeds itself, as a portion of the fuel and oxidizer is fed into the preburner, which turns a turbine, and that turbine moves the turbopumps.)&#xA;Finally, I do know that the rocket's tanks are pressurized by heating an inert gas, and then feeding that gas into the tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, how does the preburner begin running? As I mentioned above, I understand how the preburner can keep itself running, but how does mission control start the preburner?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anyone can answer my question, it'd be extremely appreciated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also: If I stated something incorrectly, please correct me. I don't want to misunderstand anything about these complex machines.&lt;/p&gt;&#xA;" OwnerUserId="18229" LastEditorUserId="6944" LastEditDate="2017-01-17T21:09:44.963" LastActivityDate="2017-09-12T02:33:30.433" Title="Rocket Engine Plumbing: Still Confused about the Preburner" Tags="|rockets|propulsion|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19827" PostTypeId="2" ParentId="19826" CreationDate="2017-01-17T00:21:16.823" Score="8" Body="&lt;p&gt;The preburner is essentially a standard turbine engine, which is a little easier to find information on; you need compressed air from an external source, or a separate motor to rotate the compressor, and a spark plug to ignite the pressurized fuel/air mix.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The details will vary between engine designs, but a preburner can be simpler than a full turbine engine: if the fuel and oxidizer both come from sufficiently pressurized tanks, you can leave off the compressor stage and the spark plug alone is enough to start the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX Raptor Preburner at Stennis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qwMlj.jpg&quot; alt=&quot;Raptor&amp;#39;s Pre-burner]([![Oxyidizer-rich exhaust from aSpaceX Raptor Pre-burner&quot;&gt;]&lt;a href=&quot;https://i.stack.imgur.com/qwMlj.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18207" LastEditorUserId="20970" LastEditDate="2017-09-12T02:33:30.433" LastActivityDate="2017-09-12T02:33:30.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19829" PostTypeId="1" AcceptedAnswerId="19835" CreationDate="2017-01-17T01:44:27.787" Score="1" ViewCount="168" Body="&lt;p&gt;In the BBC news article &lt;a href=&quot;http://www.bbc.com/news/science-environment-38599645&quot; rel=&quot;nofollow noreferrer&quot;&gt;To see finally the face of Peggy&lt;/a&gt; (Peggy is a very small moonlet interacting with a ring) there is an audio interview with Carl Murray of Queen Mary University of London. After about 08:45 he discusses the masses of the rings, including this (approximately transcribed) sentence:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And that’s what you get with Cassini, in the final orbits, the Grand Finale orbits, we will get the mass of not only the rings, but the mass of the B ring, which is the most massive of the ring system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Will this be enough of a constraint to interpret &quot;get the mass&quot; as an approximate mass measurement? How is this done - by sampling and characterization, or through actual gravitational interaction? If so, actually how?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article also links to Linda Joyce Spilker's talk abstract &lt;a href=&quot;https://agu.confex.com/agu/fm16/meetingapp.cgi/Paper/167247&quot; rel=&quot;nofollow noreferrer&quot;&gt;P33B-2136: Cassini’s Grand Finale&lt;/a&gt; at the American Geophysical Union's 2016 fall meeting.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-01-17T15:47:19.970" Title="How will the Cassini spacecraft help constrain the mass of Saturn's rings?" Tags="|saturn|cassini|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19830" PostTypeId="2" ParentId="19792" CreationDate="2017-01-17T02:23:24.967" Score="0" Body="&lt;p&gt;I don't know if this is an answer, but in general huge radio telescopes are used to receive signals from space probes. The transmitter that is millions of miles away might be 20 or 30 watts, this is comparable to the power of a police squad car which is 15 to 30 watts. A normal desktop radio might start to loose a 15 watt signal in a couple of miles. Radio telescopes have a huge reflective surface that is all angled to favor a precise point. With that and very sensitive receivers, they can focus on very weak signals in a tiny portion of the sky. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2017-01-17T02:23:24.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19832" PostTypeId="2" ParentId="19826" CreationDate="2017-01-17T03:54:45.350" Score="10" Body="&lt;p&gt;&quot;However, how does the preburner begin running?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Refer to this simplified schematic from the crew checklist for the Shuttle system I'm about to describe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WAelE.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WAelE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Head and ullage pressure in the propellant tanks is the initial impetus for propellant to flow into the preburner(s).  At least in the Shuttle, the tanks were pressurized on the pad by helium supplied by the ground, after their respective vent valves closed.  The O2 tank was way above the engines at the tip of the ET so there was significant head pressure too.  Not so much head for the LH2, it does not have the density for it.  Then when the valves in the plumbing between the tanks and the engines opened, and the engine valves did too, prop flowed into the preburner driven by tank outlet pressure.  Then the electrical igniters kicked on, the preburners ignited, the turbines began to spin, etc, as you have described.  (reference the &lt;a href=&quot;http://www.data-core.de/dac/blog/docs/NASA.space.shuttle.manual.1988.pdf&quot;&gt;Space Shuttle News Reference Manual&lt;/a&gt;, page 185 of the pdf) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is info about the start sequence in the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/13095/what-are-the-phases-of-ignition-and-flameout-of-a-liquid-fuel-engine&quot;&gt;this question&lt;/a&gt; but it does not specifically talk about what drives the flow initially.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-01-17T04:04:38.870" LastActivityDate="2017-01-17T04:04:38.870" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19833" PostTypeId="1" AcceptedAnswerId="19841" CreationDate="2017-01-17T05:22:21.330" Score="2" ViewCount="1765" Body="&lt;p&gt;Can anyone help to find data or book on Isp for hybrid rocket engines &lt;strong&gt;using LOX&lt;/strong&gt; as oxidizer?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am interested to know Isp for LOX combinations with carbon, Al+HTPB, parafin, sugar and others.&lt;/p&gt;&#xA;" OwnerUserId="11701" LastActivityDate="2017-01-19T04:08:26.373" Title="Isp of LOX-based hybrid rocket engines" Tags="|hybrid-rocket|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19834" PostTypeId="2" ParentId="19819" CreationDate="2017-01-17T13:01:07.787" Score="2" Body="&lt;p&gt;I think I've answered my own question.  As I mentioned, acceleration to alter inclination would be perpendicular to the orbital velocity, so any effect that is a function of that velocity should not apply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...right?&lt;/p&gt;&#xA;" OwnerUserId="17722" LastEditorUserId="17722" LastEditDate="2017-01-17T13:24:06.520" LastActivityDate="2017-01-17T13:24:06.520" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19835" PostTypeId="2" ParentId="19829" CreationDate="2017-01-17T15:47:19.970" Score="3" Body="&lt;p&gt;From &lt;a href=&quot;https://www.newscientist.com/article/2085745-cassini-gears-up-for-final-fiery-plunge-into-saturns-atmosphere/&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Scientist&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At such close quarters, Cassini’s instruments can gather unprecedented information about the composition of the planet’s ionosphere and atmosphere. The craft will also collect data on radiation belts girding the planet, and will be in a position to use gravity measurements to give the best estimate yet of the mass of the rings.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Cassini has already contributed to a &lt;a href=&quot;https://saturn.jpl.nasa.gov/news/2802/saturns-rings-less-than-meets-the-eye/&quot; rel=&quot;nofollow noreferrer&quot;&gt;better understanding&lt;/a&gt; of the B ring's mass, but with this information there's still a &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0019103516000117&quot; rel=&quot;nofollow noreferrer&quot;&gt;± 50% uncertainty&lt;/a&gt; (link to the original paper): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The scientists found that, while the opacity of the B ring varied by a large amount across its width, the mass – or amount of material – did not vary much from place to place. They &quot;weighed&quot; the nearly opaque center of the B ring for the first time -- technically, they determined its mass density in several places -- by analyzing spiral density waves. These are fine-scale ring features created by gravity tugging on ring particles from Saturn's moons, and the planet's own gravity. The structure of each wave depends directly on the amount of mass in the part of the rings where the wave is located.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-17T15:47:19.970" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19836" PostTypeId="1" CreationDate="2017-01-17T17:24:40.980" Score="3" ViewCount="1384" Body="&lt;p&gt;I apologize if this question is confusing, I will try to clarify a little... so, this is talking about 'space' in the form of space that exists outside of our atmosphere. Outer space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I once read that the temperature of outer space varies, but is almost always above &lt;a href=&quot;https://en.wikipedia.org/wiki/Absolute_zero&quot; rel=&quot;nofollow noreferrer&quot;&gt;absolute zero&lt;/a&gt; with the theoretical exception of the &lt;a href=&quot;https://en.wikipedia.org/wiki/CMB_cold_spot&quot; rel=&quot;nofollow noreferrer&quot;&gt;eridanus supervoid&lt;/a&gt; (and relating voids), where the amount of heat sources present is few to none. But why is outer space above absolute zero in the first place? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If space is defined as the absence of everything, why is the ambient temperature of the absence of everything above absolute zero?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: For example, the region of space immediately surrounding a star (past the atmosphere). Why is the temperature of this space 'warm' even though there are no particles to heat up or excite?&lt;/p&gt;&#xA;" OwnerUserId="18236" LastActivityDate="2017-01-21T04:42:14.460" Title="If 'space' is defined as the absence of everything, then why can it carry heat?" Tags="|temperature|" AnswerCount="3" CommentCount="5" ClosedDate="2017-01-18T08:46:55.887" ContentLicense="CC BY-SA 3.0" />
  <row Id="19837" PostTypeId="2" ParentId="19836" CreationDate="2017-01-17T18:24:14.073" Score="0" Body="&lt;p&gt;Particles are heated up &lt;em&gt;because&lt;/em&gt; of the presence of heat, but they are not the medium through which heat is necessarily transferred.  Heat is radiation, which travels through vacuum just as well as (actually, better than) it does through matter.&lt;/p&gt;&#xA;" OwnerUserId="17722" LastActivityDate="2017-01-17T18:24:14.073" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19838" PostTypeId="2" ParentId="19836" CreationDate="2017-01-17T22:26:55.943" Score="4" Body="&lt;p&gt;The concept of temperature started as a term in classic macroscopic physics. When dealing with steam engines, you transfer heat by steam convection and conduction through the walls of a heat-exchanger. These are the major mechanisms in a high pressure system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outer space is near vacuum. Convection and conduction do not play a significant role. It would be impossible to bring a sensor (which is a thermal reservoir of its own) into equilibrium with the few particles in outer space. The concept of temperature as mean kinetic energy of particles is meaningless in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each body radiates &quot;black body radiation&quot; with a (Planck-) spectrum depending on its temperature.  At the same time, it receives radiation from other bodies. This mechanism also establishes an equilibrium without the bodies being in contact or exchanging particles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, if you are far from any star, not much of your field of view may be filled with matter to exchange radiation with. Most will be void. But even the cosmic background is not at absolute zero Kelvin; it has a radiation spectrum which can be translated into a temperature. This is &lt;em&gt;the temperature of space&lt;/em&gt;. The radiation is not constant in all directions, but these inconsistencies don't relate to particles: a &quot;cold spot&quot; from earth's perspective in the cosmic background does not imply any particles in that direction are less thermally excited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question of background radiation is interesting for physicists and astronomers (including space-based telescopes). It it totally unimportant for space travel, because spacecraft never reach thermal equilibrium with the cosmic background.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="5851" LastEditDate="2017-01-18T15:09:24.950" LastActivityDate="2017-01-18T15:09:24.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19839" PostTypeId="1" AcceptedAnswerId="20870" CreationDate="2017-01-17T23:26:22.007" Score="29" ViewCount="2706" Body="&lt;p&gt;The movie &lt;em&gt;&lt;a href=&quot;//en.wikipedia.org/wiki/Hidden_Figures&quot;&gt;Hidden Figures&lt;/a&gt;&lt;/em&gt; tells the story of three black women who were &quot;computers&quot; for the Mercury space program. They performed calculations related to  orbits, launches, aerodynamics, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are some specific examples of the calculations they did? In what form would they receive a problem, and what would their completed work look like? Are there are any examples of their original work publicly available? The level of detail I'm looking for is such that I could, if I wanted, spend a day replicating a piece of work they did.&lt;/p&gt;&#xA;" OwnerUserId="10473" LastEditorUserId="198" LastEditDate="2017-01-18T14:19:00.993" LastActivityDate="2017-04-15T18:00:31.357" Title="What are some specific examples of the calculations human &quot;computers&quot; did for the Mercury space program?" Tags="|history|mathematics|project-mercury|computing|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19840" PostTypeId="2" ParentId="19836" CreationDate="2017-01-18T02:37:28.360" Score="6" Body="&lt;p&gt;The short answer is that heat is &lt;em&gt;transmitted&lt;/em&gt; through space by radiation, but space &lt;em&gt;has a temperature&lt;/em&gt; because it isn't truly empty. Or, better, it is not &lt;em&gt;space&lt;/em&gt; that has a temperature at all, but the (very sparse) light and matter found in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Transmitting heat is not the same thing as having a temperature. You may remember learning in school about convection, conduction, and radiation as the three ways of transmitting heat. Convection, which is stuff that has heat moving around and carrying the heat to new places, does not work in perfectly empty space. Conduction, which is what you call it when rapidly jiggling atoms knock against their neighbors and make them jiggle faster, spreading the heat through the material, also doesn't work in empty space. But radiation, which basically means photons, can carry heat through perfectly empty space from one object to another. (By &quot;perfectly empty&quot;, I mean &quot;perfectly empty of matter&quot;, which I think is what you mean too.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you also ask a different question about how people can talk about the temperature of the space outside the atmosphere of a star. And the answer is that space is never completely empty, if you look on a large enough scale. Even between galaxies, there is estimated to be about an atom every cubic meter. And those atoms are moving. Since temperature is a measure of the energy of how much random motion there is, you can meaningfully assign a temperature to even the sparsest collection of atoms. You can also assign a temperature to the electromagnetic radiation, since photons have kinetic energy too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even though temperature has meaning in this case, it would do almost nothing to heat you up if you were out there. The interstellar medium can be immensely hot, but there is so little of it that the few atoms hitting you have essentially no effect, and you end up cooling off to near absolute zero by emitting radiation. This is similar to the way air at 450 degrees coming out of an oven is harmless to you, but you would burn yourself touching glass at the same temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a lot of subtleties here. In a normal gas, atoms collide all the time, and so their speeds tend to get averaged out to be roughly similar. Some are slower and some are a good amount faster, but there's a formula for how many particles would be moving at a given speed that's different from the average. In space, they hardly collide at all, so those formulas don't apply. That changes the rules a bit, but people still find it useful to use the concept of temperature.&lt;/p&gt;&#xA;" OwnerUserId="10259" LastEditorUserId="10259" LastEditDate="2017-01-21T04:42:14.460" LastActivityDate="2017-01-21T04:42:14.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19841" PostTypeId="2" ParentId="19833" CreationDate="2017-01-18T04:12:28.110" Score="4" Body="&lt;p&gt;Here's &lt;a href=&quot;http://www.lr.tudelft.nl/nl/organisatie/afdelingen/space-engineering/space-systems-engineering/expertise-areas/space-propulsion/propulsion-options/chemical-rockets/hybrid/&quot; rel=&quot;nofollow noreferrer&quot;&gt;some information from Delft University&lt;/a&gt; with a table showing 275-300s specific impulses for various HTPB and PE hybrids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a &lt;a href=&quot;http://www.eucass-proceedings.eu/articles/eucass/pdf/2012/01/eucass2p353.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;nice hybrid overview paper&lt;/a&gt; with a broader table, giving LOX-carbon Isp 249s, LOX-HTPB/Al 40% Isp 274s, LOX-paraffin 281s. These figures are all for chamber pressure of 3.5MPa (~35 atmospheres), exhausting at sea level pressure, but expansion ratio not specified.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closest thing in the latter table to sugar is cellulose, which gives 247s burned with gaseous oxygen. (Sucrose is C&lt;sub&gt;6&lt;/sub&gt;H&lt;sub&gt;12&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt;, sorbitol is C&lt;sub&gt;6&lt;/sub&gt;H&lt;sub&gt;14&lt;/sub&gt;O&lt;sub&gt;6&lt;/sub&gt;, cellulose C&lt;sub&gt;6&lt;/sub&gt;H&lt;sub&gt;10&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-01-19T04:08:26.373" LastActivityDate="2017-01-19T04:08:26.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19843" PostTypeId="1" AcceptedAnswerId="19845" CreationDate="2017-01-18T13:40:26.447" Score="2" ViewCount="154" Body="&lt;p&gt;On page 94 of &lt;a href=&quot;http://www.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ignition!&lt;/a&gt; an equation for &quot;Frozen equilibrium calculation&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;c = \left[ 2H/M \right]^{1/2} \left[ 1- \left( {P_e \over P_c} \right) ^{R/C_p} \right]^{1/2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All quantities are explained, except for $C_p$ - it only appears in the sentence &lt;em&gt;&quot;$\gamma$ is the ratio of specific heats, $C_p/C_v$ of the chamber gases&quot;&lt;/em&gt; in a prior equation, but without explaining which of these two is specific heat of what. Then it's later referenced frequently, being an essential parameter, but always only by the $C_p$ symbol. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is baffling especially in light of the page 99 quote: &lt;em&gt;&quot;As for the solids, $C$, $Al_2 O_3$,and $B_2 O_3$, their $R/C_p$ is precisely zero.&quot;&lt;/em&gt; Since $R$ is a constant, that would imply infinite $C_p$ - an infinite specific heat? what the heck? What is that quantity?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-01-18T14:59:28.047" Title="What is $C_p$ in the fuel efficiency equation?" Tags="|fuel|chemistry|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19844" PostTypeId="2" ParentId="12170" CreationDate="2017-01-18T13:56:27.890" Score="0" Body="&lt;p&gt;One aspect of this issue that seems to be overlooked in most discussions of it I see is experience and technological advancement.&#xA;When we were trying to get people to orbit and then another planet for the first time, our primary criteria was to do whatever we had to in order to increase our odds of success.  We used H2/LOX because we wanted every bit of ISP we could, no matter what the cost.  50 years ago, we were doing what no one had ever done, and had far less technological prowess and tools to attack the problem with.  We had to &quot;brute force&quot; and &quot;guesstimate&quot; much more than we do now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now getting to LEO is routine rather than a great experiment.  Now we know that thrust/mass ratio is considerably more important in getting to LEO than we did back then.  We know that lower Isp is worth it if we get a large enough improvement in thrust/mass ratio.  We have analytical tools, materials science advances, and manufacturing methods that are far better than those of 50 years ago.  We can now store CH4 and LOX at close to their freezing points much more economically.  We can design and build simpler, more reliable full flow combustion engines.  Raptor's thrust/mass ratio is going to be greater than 2x that of the Saturn V's F1 engine.  It's probably going to be more than 3x more than that of the SSME.  And it's going to be far more reusable.  All those advantages more than make up for the lower ISP of CH4/LOX vs that of H2/LOX. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, we have more experience along with more materials and tools available to us to take maximum advantage of that experience with.  &lt;/p&gt;&#xA;" OwnerUserId="18245" LastActivityDate="2017-01-18T13:56:27.890" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19845" PostTypeId="2" ParentId="19843" CreationDate="2017-01-18T14:26:09.583" Score="4" Body="&lt;p&gt;Cp is the &quot;specific heat at constant pressure&quot;, just as Cv is the &quot;specific heat at constant volume&quot;. You can see the derivation &lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/shegas.html#c2&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.  The values are stated for a given ideal gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since these values are related to ideal gases, they have no meaning for solids, perhaps leading to Clark's comment on page 99. R has no meaning for a solid either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A table of example values of the specific heats, from &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwjx2ff188vRAhWJilQKHWpdAsQQFggcMAA&amp;amp;url=https%3A%2F%2Feclass.upatras.gr%2Fmodules%2Fdocument%2Ffile.php%2FMECH1206%2FCENGEL_%2520PROPERTY%2520Tables%25206%2520NEW-SIappendix1.pdf&amp;amp;usg=AFQjCNG9iWYPf29QmQHdOHddHmpvt4A7sQ&amp;amp;sig2=XT6rEiqr46carkVU5P-eFA&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/alfgP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/alfgP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-01-18T14:59:28.047" LastActivityDate="2017-01-18T14:59:28.047" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19846" PostTypeId="1" AcceptedAnswerId="19848" CreationDate="2017-01-18T18:28:56.490" Score="8" ViewCount="402" Body="&lt;p&gt;After reading up on the matter I noticed that there currently are only two options to remain in space for longer than a few hours:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;International Space Station (US and Russia) assembled in orbit: altitude 409 kilometers (221 nautical miles), 51.65° inclination&lt;/li&gt;&#xA;&lt;li&gt;Tiangong-2 (Chinese): altitude 392 kilometers (212 nautical miles); crews transported by Shenzhou spacecraft&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Human_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on human spaceflight&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And two upcoming commercial alternatives that provide a longer stay in space (24+ hours):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Bigelow_Commercial_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bigelow Commercial Space Station&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://orbitaltechnologies.ru/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Technologies' Commercial Space Station&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Which provide hope as steppingstones for human space exploration. &#xA;Especially since Bigelow Aerospace has already been testing their inflatable module on the ISS!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though the initial press message of Orbital stated it would be available in 2016, which it was not. On top of that the website of Orbital Technologies had its most recent messages in 2011.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Orbital Technologies' prospects don't look too promising anymore, which led me to question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Are there any other commercial competitors which intend to provide long (24h+) space stay?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="18246" LastEditorUserId="17138" LastEditDate="2017-01-19T00:59:30.177" LastActivityDate="2017-01-19T00:59:30.177" Title="Are there other companies than Bigelow Aerospace planning to offer a space hotel?" Tags="|crewed-spaceflight|space-station|commercial|tourism|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19848" PostTypeId="2" ParentId="19846" CreationDate="2017-01-18T20:21:38.457" Score="6" Body="&lt;p&gt;&lt;a href=&quot;https://www.axiomspace.com/&quot;&gt;Axiom Space&lt;/a&gt;, run by people with management and astronaut experience from the ISS, plans to have commercial customers in space by 2020. First with their module docked to the ISS, and when the ISS is retired, as its own multi-modular space station. Here's a long radio interview on &lt;a href=&quot;http://www.thespaceshow.com/show/06-jan-2017/broadcast-2839-mike-suffredini-amir-blachman&quot;&gt;The Spaceshow&lt;/a&gt; this January 6th.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Cu72M.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Cu72M.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Sketch from their website.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-01-18T20:32:48.307" LastActivityDate="2017-01-18T20:32:48.307" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19849" PostTypeId="1" CreationDate="2017-01-18T21:15:58.253" Score="2" ViewCount="356" Body="&lt;p&gt;How different is the communication to ground stations between passive and active ADCS? Also, is there any data or insight into probabilities of ground station contact with passive?&lt;/p&gt;&#xA;" OwnerUserId="18248" LastEditorUserId="1142" LastEditDate="2017-01-20T01:01:19.697" LastActivityDate="2017-01-20T01:43:15.203" Title="How different is communication between passive and active ADCS?" Tags="|communication|attitude|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19850" PostTypeId="1" CreationDate="2017-01-19T11:19:16.180" Score="5" ViewCount="385" Body="&lt;p&gt;In a hybrid LOX rocket, only the flow of LOX may be controlled. The amount of paraffin melted by heat is determined by the very complex heat transfer from gas to solid paraffin. If the mixture of propellants is too fuel rich or too oxygen rich, the efficiency will suffer. The temperature of the paraffin before launch may influence the efficiency. A start in a very hot climate may be impossible.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastEditorUserId="1142" LastEditDate="2017-01-19T14:07:35.280" LastActivityDate="2017-01-19T16:35:34.623" Title="How to prevent melting too much paraffin in a hybrid LOX-paraffin rocket?" Tags="|engine-design|hybrid-rocket|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19851" PostTypeId="2" ParentId="19850" CreationDate="2017-01-19T12:20:22.673" Score="5" Body="&lt;p&gt;You just found the primary shortcoming of hybrid engines: fuel flow cannot be controlled on the fly. The engine can be shut down and restarted unlike common SRB, but throttlability is lousy and lossy. And lots and lots of promising fuels were abandoned due to bad melting or evaporation temperatures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can prepare the optimal fuel a'priori by choosing the parafin composition - longer hydrocarbons will have higher melting point. Other additives may reduce impact of parafin melting; composite the parafin with fibrous material (cellulose/paper, cotton, polyethylene fiber) and despite parafin melting, the shape will be retained until the fiber burns out. Or just tune your combustion parameters (oxidizer supply, chamber pressure) to such that parafin surface ablates before the heat from the surface gets to transfer deeper. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-01-19T16:35:34.623" LastActivityDate="2017-01-19T16:35:34.623" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19852" PostTypeId="1" AcceptedAnswerId="22303" CreationDate="2017-01-19T12:33:50.560" Score="14" ViewCount="5849" Body="&lt;p&gt;One of notes near the end of &quot;Ignition!&quot; included:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Using lithium and fluorine alone (no hydrogen)&#xA;  their maximum specific impulse was 458 seconds. But when they&#xA;  proportioned the lithium and fluorine to burn stoichiometrically to&#xA;  LiF, and injected hydrogen to make up 30 percent of the mass flow,&#xA;  they measured &lt;strong&gt;542 seconds&lt;/strong&gt; —probably the highest measured specific&#xA;  impulse ever attained by anything except a nuclear motor. And the&#xA;  chamber temperature was only 2200 K! Performance like that is&#xA;  worth fighting for. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(emphasis mine).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That was about 60 years ago. And it really leaves LH2/LOX performance in the dirt. Nuclear rockets are a political impossibility, and electric propulsion makes only sense for very light spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this tripropellant combination entirely forgotten, or does it find any use - in actual plans, or just projects - or was there some terrible fault found that made it not viable at all even out of atmosphere? What is its current status?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2019-06-22T05:06:40.467" Title="Where is the Lithium-Fluorine-Hydrogen tripropellant currently?" Tags="|fuel|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="19853" PostTypeId="1" AcceptedAnswerId="21621" CreationDate="2017-01-19T22:13:54.287" Score="11" ViewCount="987" Body="&lt;p&gt;A fair amount of &amp;quot;what-if&amp;quot; discussion surrounds rocket engine failure at critical points in various crewed missions -- lunar ascent and trans-Earth injection on lunar landing missions, retrorocket failure on LEO missions.&lt;/p&gt;&#xA;&lt;p&gt;A popular approach to managing these risks is to use a single engine made reliable through simplicity: hypergolic propellants and pressure feed. This has been quite successful.&lt;/p&gt;&#xA;&lt;p&gt;Including uncrewed missions, &lt;strong&gt;when is the last time a mission-critical, pressure-fed, hypergolic engine failed to ignite on command in space?&lt;/strong&gt; Some technicalities:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;I would not count a stuck circuit that was solved via something like a circuit breaker reset in short order, i.e. in less than an hour with no screwdriver-level intervention&lt;/li&gt;&#xA;&lt;li&gt;I would not count a single failing attitude thruster that didn't prevent a desired attitude change via other available thrusters&lt;/li&gt;&#xA;&lt;li&gt;I would count something like a single STS OMS engine failing a planned burn in a way that required using the other OMS instead&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="195" LastEditorUserId="9092" LastEditDate="2021-03-15T10:14:30.193" LastActivityDate="2021-03-15T10:14:30.193" Title="How reliable are pressure fed hypergolic engines?" Tags="|engines|hypergolics|reliability|pressure-fed-engine|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="19855" PostTypeId="2" ParentId="1234" CreationDate="2017-01-20T00:08:56.827" Score="10" Body="&lt;p&gt;Gills aren't purely for respiration. Fish constantly excrete ammonia and urea from their gills and without sufficient water to wash the waste away this would soon cause death, much like how too small of an aquarium allows these substances to quickly accumulate to toxic levels (which begs the question about how waste and hygiene would be managed in the first place). Maybe a particular species of fish with a unique respiratory system and strong immunity could thrive in a novel microgravity environment though?&lt;/p&gt;&#xA;" OwnerUserId="18259" LastActivityDate="2017-01-20T00:08:56.827" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19856" PostTypeId="2" ParentId="19849" CreationDate="2017-01-20T01:43:15.203" Score="2" Body="&lt;p&gt;This is largely dependant on mission requirements - although that should also define whether you need active or passive ADCS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key piece of hardware that will be defined by the active/passive choice of attitude control is the antenna. Without going into too much detail on either antenna theory or frequency allocations it's safe to say the following;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;If you have no attitude control you almost certainly want an isotropic antenna (gain = 1)&lt;/li&gt;&#xA;&lt;li&gt;If you have perfect ADC and position measurement you want a very small beam width antenna (high gain)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are exceptions to these rules. For example you might have a small data transfer requirement and could conceivably retransmit all data until a ground station sends a 'message received' signal - this would allow no ADC but a non-isotropic antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you satellite is large enough to be tracked or it can self determine it's location (and downlink that) then you can quite reasonably deal with an isotropic antenna on the satellite and a high gain (eg. Yaggi) antenna at the ground station. With approximately 1 downlink a day and a 30% beam width of the receiving antenna you should be able to reestablish the link each day (from the position and velocity of the satellite during current downlink you can reasonably estimate it's future position at 25 hours assuming it's perigee altitude is above ~300 km).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for ground station downlink opportunities it's reasonably easy to predict. If you have an isotropic antenna then the avaliable link time is just a bit of geometry. If you have a directional antenna on the satellite then it becomes a question of geometry, statistics (likelihood of both the satellites attitude and position being correct during a flyover) and, astrodynmamics (how you ADCS effects changes to your attitude).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're interested in a specific satellite  post it in the question/comments and I'll dust off some old code to go into more detail on the specifics of flyovers for your lat/long.&lt;/p&gt;&#xA;" OwnerUserId="17507" LastActivityDate="2017-01-20T01:43:15.203" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19857" PostTypeId="2" ParentId="19592" CreationDate="2017-01-20T02:21:00.373" Score="2" Body="&lt;p&gt;As some of the comments have mentioned space-track provides reasonably upto date TLEs of anything they can track (including cubesats), although I wouldn't think this would be the best option for your use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GPS trackers are now pretty accurate, from experience the error rates in TLEs don't come close to the accuracy you would get from a GPS tracker on your cubesat. The problem with using GPS on a satellite is that most GPS chips are specifically built to make it harder for rogue nations to make course correcting missiles.... no, seriously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GPS is your phone and in your car sat nav won't work if you either &lt;a href=&quot;http://www.eugenewei.com/blog/2015/8/3/cocom-limits&quot; rel=&quot;nofollow noreferrer&quot;&gt;travel faster than 1,900 km/h or higher than 18 km altitude&lt;/a&gt;. LEO satellites travel at around 28,000 km/h and are at least 300 km in altitude. These are the CoCom limits. The rational behind this is that you can't retro fit a missile with an iPhone and then use the GPS system to get the missile position for autonomous course correction (amoung other, less cool things I guess).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The good news is that it's not all the hard to get a GPS chips without these CoCom limits, I'm pretty sure you end up on a list somewhere (NSA, etc.) but hey, if you're on the space exploration stack exchange then you've probably got at least 1 very dangerous thing just sitting in your desk (rocket fuel, mini rail gun etc.) so you're already on a few lists. Actually I'm not sure this was good news after all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;The use of continous thrust would change an orbit. Whether or not this would be negligible is another question. You're talking about 0.001 N/Kw and a power of 5 Watts, so a total thrust of 0.000005 N. Now over time this would make a change in velocity significant enough to get you to leave Earth orbit &lt;em&gt;if there were no other pertubations&lt;/em&gt;. A back of the envelope drag calculation gives 0.000075 N/m2; for a cubesat that is 10x10 cm you'd have a drag force of 1/6th of the force your trying to measure. The bad news is that the drag force calculations are very rough and hugely altitude dependant, you could easily double/half the number by changing your altitude from my test case of 400 km. The good news is the higher your altitude the less of an effect drag will have. The key thing here is that there are many pertubations and your small force could convieveably get lost in the noise. Two cubesats, one with and one without the drag device would likely provide more conclusive evidence! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a side note - be careful when using TLEs for propagation. You &lt;em&gt;must&lt;/em&gt; use the simplified perturbation models to propagate TLE data forward or your results will be incorrect!&lt;/p&gt;&#xA;" OwnerUserId="17507" LastEditorUserId="17507" LastEditDate="2017-01-21T02:44:41.307" LastActivityDate="2017-01-21T02:44:41.307" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="19859" PostTypeId="1" CreationDate="2017-01-20T06:42:03.817" Score="5" ViewCount="139" Body="&lt;p&gt;Could a New Horizons class probe aimed to fly by a Kuiper Belt object 100 AU from the Sun use its camera, built to image the Kuiper Belt object, to also measure distances to stars (and distant Solar system dwarf planets) better than what specialized observatories using the so called Solar parallax of 2 AU can do?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a specialized instrument is needed for parallax measurements, what demands (e.g. mass) would it put on the spacecraft for carrying and operating it? I wonder if it makes sense to combine the two purposes.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-01-20T10:32:50.863" Title="Could a Kuiper Belt object fly by probe measure star distances by parallax better than Gaia?" Tags="|observation|measurement|kuiper-belt|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19860" PostTypeId="2" ParentId="19647" CreationDate="2017-01-20T07:00:05.647" Score="12" Body="&lt;p&gt;Let me try to bite, putting it in easy terms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The purpose of engine is to provide thrust - by ejecting propellant (combustion products) at as high velocity as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuels store a lot of chemical energy - more than any closed chamber could contain. If they were allowed to convert all that energy to pressure in one place, the chamber would explode, period. The pressure would be many orders of magnitude of what the pumps can provide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's why the combustion chamber has a throat and a nozzle. As the fuels burn, they &lt;em&gt;would&lt;/em&gt; increase pressure, but they are simultaneously vented through the nozzle - they expand, pressure drops, their velocity increases. The turbopump doesn't have to overcome the pressure that would result from combustion in enclosed space - it just needs to overcome the equilibrium pressure between what builds up from combustion and what is lost due to venting that through the nozzle. Pressure is kept in balance - volume grows. This growth of volume directly converts into movement - gas ejected at high speed through the nozzle - and propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plus the pumps need to pump liquid fuel instead of combustion gasses - pump's work is proportional to pressure and &lt;em&gt;volume&lt;/em&gt; pumped, not &lt;em&gt;mass&lt;/em&gt;. Pumping 100l of water at 10 bar is the same work as pumping 100l of steam at 10 bar, created through boiling a liter of water. Same here - most of work is done when burning the fuel, creating a lot of fast-moving gas. A pump to move the same &lt;em&gt;mass&lt;/em&gt; of gas at the same pressure would need to provide far, far more work - because the volume of that gas would be vastly bigger.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-01-20T07:00:05.647" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19861" PostTypeId="2" ParentId="19859" CreationDate="2017-01-20T10:32:50.863" Score="5" Body="&lt;p&gt;To use the longer base line of 100 AU instead of 2 AU, we need measurements from both the probe in the Kuiper Belt and another probe near the earth. But the data transfer over 100 AU distance is very slow. But such parallax measurements are done for thousands to millions of stars, see the pages about the probes Hipparcos &lt;a href=&quot;https://en.wikipedia.org/wiki/Hipparcos&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Hipparcos&lt;/a&gt; and Gaia &lt;a href=&quot;https://en.wikipedia.org/wiki/Gaia_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Gaia_(spacecraft)&lt;/a&gt;. The amount of data is impressive: &quot;Gaia sends back data for about eight hours every day at about 5 Mbit/s. ESA's two most sensitive ground stations, the 35 m diameter radio dishes in Cebreros, Spain, and New Norcia, Australia, receive the data&quot; If the data rate from the Kuiper Belt probe is only about 100 to 700 Bit/s, only some hundred stars can be measured. It would be difficult to generate the same electrical power for operation of the probe as the Gaia probe uses 1910 W. Using solar cells to generate so much power so far away from sun is impossible. New Horizons has only about 200 W available now, that is one tenth only. The launch mass of Gaia with 2029 kg and New Horizons of 478 kg are very different too.&lt;/p&gt;&#xA;" OwnerUserId="13126" LastActivityDate="2017-01-20T10:32:50.863" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19862" PostTypeId="2" ParentId="17563" CreationDate="2017-01-20T10:56:20.143" Score="3" Body="&lt;p&gt;Another advantage (relating to efficiency) is that it's easier to build a &lt;a href=&quot;https://en.wikipedia.org/wiki/Staged_combustion_cycle&quot; rel=&quot;nofollow noreferrer&quot;&gt;staged combustion&lt;/a&gt; rocket engine using LOX/LH2. Specifically, you can use LH2 for a &lt;em&gt;fuel-rich&lt;/em&gt; staged combustion engine, like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Main_Engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle main engines&lt;/a&gt; (and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot; rel=&quot;nofollow noreferrer&quot;&gt;Energia&lt;/a&gt; main engines). Most other common rocket fuels don't work for fuel-rich combustion, I believe due to the risk of coking. &lt;em&gt;Oxidizer-rich&lt;/em&gt; cycles don't have that problem, but require very advanced metallurgy to safely handle the (extremely corrosive) hot oxidizer. Russia / the USSR figured out how to do that in the 60s, but the USA never built a working oxygen-rich (or full-flow) staged combustion engine until the 2000s.&lt;/p&gt;&#xA;&lt;p&gt;In the end, efficiency is efficiency. Even if, hypothetically, some new propellants could get 600s ISP with a good TWR but could only be used in a gas generator cycle (or something similarly wasteful), it would still be more efficient than the best staged combustion engines available today. With the currently available propellants, though, the fact that LH2 makes it easier to build higher-efficiency rockets (not just because of the inherent suitability of the propellants for high ISP) is - or at least was - significant. (It still is for rockets that need maximum efficiency, but some focus on other desiderata now, like storage, transportation, cost, in-situ generation off earth, etc.)&lt;/p&gt;&#xA;" OwnerUserId="11532" LastEditorUserId="11532" LastEditDate="2022-06-04T08:19:39.413" LastActivityDate="2022-06-04T08:19:39.413" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="19863" PostTypeId="1" AcceptedAnswerId="19864" CreationDate="2017-01-20T12:50:23.653" Score="5" ViewCount="733" Body="&lt;p&gt;GAIA is an &lt;a href=&quot;https://space.stackexchange.com/q/18233/12102&quot;&gt;amazing &lt;em&gt;pair&lt;/em&gt; of space telescopes&lt;/a&gt; that share a single Gigapixel CCD array, with a complex optical system capable of repeated, extremely precise position, brightness, parallax, proper motion, and other measurement of millions of objects. It operates by scanning continuously via a very smooth, steady rotation of the entire spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;GAIA is located near the Sun-Earth L2 position, about 1.5 million kilometers from Earth. It generates a huge amount of data even after substantial processing by the spacecraft. This results in a data stream of between several and 8 MB/s for 8 hours every day - a period chosen for best uninterrupted reception by a single DSN station I assume.&lt;/p&gt;&#xA;&lt;p&gt;To produce a strong enough signal on Earth, GAIA uses a &amp;quot;medium gain&amp;quot; antenna, which means it has significant directionality (&lt;a href=&quot;https://en.wikipedia.org/wiki/Directivity&quot; rel=&quot;nofollow noreferrer&quot;&gt;directivity&lt;/a&gt;). The antenna must dynamically point towards Earth while the spacecraft continues to rotate, in order to preserve the data collection quality and quantity. However, since the level of precision necessary and vibration and microphonics minimization precludes a mechanically rotating mechanism for the antenna, GAIA uses a complex phased array antenna to electronically &amp;quot;point&amp;quot; the beam in Earth's general direction continuously.&lt;/p&gt;&#xA;&lt;p&gt;So far I haven't been able to find any details of how the phased array is configured mechanically, or electronically. Thus:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; &amp;quot;How is GAIA's phased array configured, mechanically and electrically?&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;In the ESA's GAIA update &lt;a href=&quot;http://sci.esa.int/gaia/50523-gaia-phased-array-antenna-delivered-and-integrated/&quot; rel=&quot;nofollow noreferrer&quot;&gt;#04: Gaia Phased Array Antenna Delivered and Integrated&lt;/a&gt; it says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Radiating Cone is a 14-sided truncated pyramid; its primary structure is machined from solid aluminium alloy. Each facet has two sub-arrays, each comprised of six radiating elements; each sub-array has a series of RF couplers that split the incoming signal to provide the amplitude weighting that determines the radiation pattern of the sub-array. The overall antenna radiation pattern is obtained by combining the radiation patterns from the 14 sub-arrays on the Earth-pointing side of the radiating cone.&lt;/p&gt;&#xA;&lt;p&gt;The 28 antenna sub-arrays are fed by seven 'Quadri-Modules', which provide low-level gain, phase and amplitude control, power amplification and routing of signals to the appropriate sub-arrays. The Quadri-Modules are, in turn, fed by a 2:7-way splitter that allows the spacecraft's nominal or cold-redundant transponder to feed the PAA.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I think that this is referring to the truncated pyramid in the center here, which is smaller than I expected. Also, although it is hard to tell for sure, it looks like the pyramid is quite steep.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GYMfX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GYMfX.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;http://sci.esa.int/gaia/50525-rear-face-of-gaia-antenna-support-panel/&quot; rel=&quot;nofollow noreferrer&quot;&gt;GAIA Antenna Support Panel on Ground Handling Trolley&lt;/a&gt;. &amp;quot;The Gaia Antenna Support Panel on its ground handling trolley, showing the Sun-pointing face; the Radiating Cone has two sub-arrays of six radiating elements on each of its 14 facets.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/34rUx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/34rUx.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;http://sci.esa.int/gaia/50525-rear-face-of-gaia-antenna-support-panel/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rear Face of GAIA Antenna Support Pannel&lt;/a&gt;. &amp;quot;The Gaia Antenna Support Panel on its ground handling trolley, showing the rear face and electronics modules. Of the boxes arranged in a circle around the rear of the Radiating Cone (centre of panel), the Electronic Power and Interface Controller (EPIC) is positioned at about seven-o-clock. The remaining boxes are the seven 'Quadri-Modules', which provide low-level gain, phase and amplitude control, power amplification and routing of signals to the appropriate sub-arrays. The 2:7-way splitter that allows the spacecraft's nominal or cold-redundant transponder to feed the Quadri-Modules is at the upper right&amp;quot;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-21T06:38:48.597" Title="How is GAIA's phased array configured, mechanically and electrically?" Tags="|antenna|gaia|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19864" PostTypeId="2" ParentId="19863" CreationDate="2017-01-20T13:10:41.197" Score="5" Body="&lt;p&gt;There are some papers about antenna design on Gaia, and this one by EADS CASA Espacio has many details on GAIA Antenna:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://ieeexplore.ieee.org/document/5505027/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ieeexplore.ieee.org/document/5505027/&lt;/a&gt;&#xA;Allica, J. C., Alonso, E., Amado, M., Bazán, A., Casares, F., de Witte, E., ... &amp;amp; Serrano, J. L. (2010, April). &lt;em&gt;&amp;quot;Architecture of GAIA satellite phased array antenna&amp;quot;&lt;/em&gt; - 2010 Proceedings of the Fourth European Conference on Antennas and Propagation (EuCAP), 12-16 April 2010, pages 4056-4059&#xA;(&lt;a href=&quot;https://scholar.google.com/scholar?cluster=4948320994472611099&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://scholar.google.com/scholar?cluster=4948320994472611099&lt;/a&gt;)&lt;/p&gt;&#xA;&lt;p&gt;It is behind IEEE &lt;a href=&quot;https://en.wikipedia.org/wiki/Paywall&quot; rel=&quot;nofollow noreferrer&quot;&gt;paywall&lt;/a&gt;, and I have only &lt;a href=&quot;https://i.stack.imgur.com/pqc3k.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;first page preview&lt;/a&gt;. But there is &lt;a href=&quot;http://web.archive.org/web/20131213013018/http://utopia.duth.gr/%7Eiaitidis/ESA%20conference%202010/Papers/posters/FCXNL-10C09-2004739-1-2004739garcia.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;very similar paper&lt;/a&gt; &lt;strong&gt;&amp;quot;GAIA SATELLITE PHASE ARRAY ANTENNA&amp;quot;&lt;/strong&gt; from same authors presented probably at some ESA conference later in 2010.&lt;/p&gt;&#xA;&lt;p&gt;Main requirements for Antenna design: 8465 MHz center frequency, 10 MHz bandwidth, EIRP (&lt;a href=&quot;https://en.wikipedia.org/wiki/Equivalent_isotropically_radiated_power&quot; rel=&quot;nofollow noreferrer&quot;&gt;Equivalent Isotropically Radiated Power&lt;/a&gt;) of +34 dBW, power on antenna input: +33 dBm. Required coverage: 360 degrees of azimuth and 45 +- 15 degrees of elevation with antenna scanning speed of &amp;quot;1deg/min in&#xA;azimuth and 1.5deg/day in elevation&amp;quot;. PAA subsystem has two RF inputs (one per transponder) and 7 outputs (&amp;quot;2:7 Input Splitter&amp;quot; made with microstrips). 7 signals are connected with coaxial cables to 7 amplifiers/beamformers:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ZmgW3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZmgW3.jpg&quot; alt=&quot;GAIA PAA subsystem block scheme by EADS CASA Espacio, 2010&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Each amplifier/beamformers has 4 units of &amp;quot;QUAM SSPA&amp;quot; (The Quadrimodule) with total 4 outputs (28 signals from 7 amplifier/beamformers). Their outputs are connected to subarrays of antenna grouped by two into facets, each facet has two &amp;quot;arrays of six circular disk patch antennas&amp;quot;. With 14 facets the PAA Radiating core is built (4 real facets are visible, other are mass dummies):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/oUsGu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oUsGu.png&quot; alt=&quot;figure 2: EM PAA radiating cone by EADS CASA Espacio, 2010&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;With requirement of 45 +- 15 degrees elevation and this photo of test cap installation&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Ek1zw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://i.stack.imgur.com/Ek1zw.png&lt;/a&gt; from the paper we may assume that facets are fixed at angle of 45 degrees to the honeycomb panel visible in your photos.&lt;/p&gt;&#xA;&lt;p&gt;Sizes of antenna are listed in  &lt;a href=&quot;http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF&lt;/a&gt; &amp;quot;&lt;em&gt;X Band Phased Array Antenna - PAA- Requirements Specification  GAIA.ASU.SP.ESM.00002&lt;/em&gt;&amp;quot;, EADS Astrium 2006&#xA;..for external units of PAA assembly:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;shall be included inside a cylinder of 0.40 m diameter (TBC) and 0.45 m height [TBC], of which the LGA shall be assumed to be 0.20m (TBC), centred on the PAA panel axis. ...  PAA external units shall be compatible with  installation of a low gain antenna (mass = 0.5 kg) and associated RF harness and/or waveguides feeding through the PAA panel, with more than 2*Pi steradian field of view&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There is also the drawings - &amp;quot;Figure 1.4-1: Side View of Gaia Service Module (SVM) with PAA Panel&amp;quot; and &amp;quot;Figure 1.4-2: Possible Configuration of PAA Assembly on Antenna Panel (side view)&amp;quot; at page 8 and 9 of &lt;a href=&quot;http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;You may also check some info about &amp;quot;Semi-active Conformal Arrays&amp;quot; (&amp;quot;Concept is adopted for ESA science satellite GAIA&amp;quot;; ESA patent?) from &lt;a href=&quot;http://data.cedupoint.cz/oppa_Minerva/Roederer_A0M17NKA_prezentace.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://data.cedupoint.cz/oppa_Minerva/Roederer_A0M17NKA_prezentace.pdf&lt;/a&gt; &amp;quot;The Design and Construction of Antennas&amp;quot; by Dr. Antoine Roederer, Deft TU (which has great overview of beam antennas for satellites) page 33 and below; and GAIA antenna on page 38:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;28 linear sub-arrays of 6 radiating elements&lt;/li&gt;&#xA;&lt;li&gt;14 hybrid couplers&lt;/li&gt;&#xA;&lt;li&gt;Phase-shifters at low level steer the power around the array&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Several other papers mention main parameters of antenna array too:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://emits.sso.esa.int/emits-doc/ASTRIUMLIM/GAIA_Phased_Array_Antenna/GAIA.ASU.SP.ESM.00002.01.00.PDF&lt;/a&gt; &amp;quot;X Band Phased Array Antenna - PAA- Requirements Specification&amp;quot; 2006&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The PAA allows for a Beam Pointing capability of 360°in Azimuth and 45°±15°in Elevation&lt;/p&gt;&#xA;&lt;p&gt;The antenna shall provide a coverage of at least&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;• 45°± 15° in Elevation&lt;/li&gt;&#xA;&lt;li&gt;•  360° in Azimuth.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;All angles shall be referenced to the antenna coordinate system (Xa,Ya,Za)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://arxiv.org/pdf/1609.04153.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://arxiv.org/pdf/1609.04153.pdf&lt;/a&gt; &amp;quot;The Gaia mission&amp;quot; 2016&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;these numbers guarantee that the Earth is maximally 15◦ away from the boresight of the phased-array antenna, which is used for science data transmission (Sect. 3.2.2)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Two overviews of GAIA:&#xA;&lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/gaia-satellite-mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/gaia-satellite-mission&lt;/a&gt;&#xA;&lt;a href=&quot;http://spaceflight101.com/gaia/gaia-spacecraft-overview/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spaceflight101.com/gaia/gaia-spacecraft-overview/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2843" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-20T23:34:42.943" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19866" PostTypeId="2" ParentId="19476" CreationDate="2017-01-21T09:13:58.087" Score="4" Body="&lt;p&gt;Not a simple task, it may take awhile to understand if you don't have prior knowledge to this field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming you are talking about Rocketdyne F-1 which is the main engine for Saturn V, only calculating the first stage and neglating drag with launch angle of 80 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specs:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;35100 KN in Atm&lt;/li&gt;&#xA;&lt;li&gt;$I_{sp}=263 s \text{ (atm)}$&lt;/li&gt;&#xA;&lt;li&gt;$I_{sp}=304s \text{ (vac)}$&lt;/li&gt;&#xA;&lt;li&gt;Weight with propellant = 5040000 lbs  &lt;/li&gt;&#xA;&lt;li&gt;Net weight = 287000&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For convenience I am just going to take the average specific impulse which is&#xA;$${(263/304)\over 2}=283.5$$&#xA;Mass Flow Rate: 4753000 lb/165 seconds = 212.72 lb/s&lt;br&gt;&#xA;Burn Time = 165 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now use the formula to determine initial acceleration in y-axis&#xA;$$(a_0)_y=g_0[F sin \Theta/w)−1]$$&#xA;Where&#xA;$g_0=9.81m/s^2$ or $32.17ft/s^2$&lt;br&gt;&#xA;F=force=35100KN&lt;br&gt;&#xA;w=weight with propellant&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we get $$32.17ft/s∗[{35100KN∗0.9848\over 22419.03KN}]−1=17.43ft/s^2$$&#xA;For the x-axis use the formula&#xA;$$(a_0)_x=g_0[Fcos\Theta/w]$$&#xA;$$32.17ft/s∗[{351000KN∗0.1736\over 22419.03KN}]=87.44ft/s^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the terminal velocity (where the burn ends):&#xA;$$(u_p)_y=cIn(m_0/m_f)sin\Theta−t_pg_0$$&#xA;c = exhaust velocity&lt;br&gt;&#xA;In = natural log&lt;br&gt;&#xA;$m_0$ = weight with propellant&lt;br&gt;&#xA;$m_f$ = weight after propellant is consumed&lt;br&gt;&#xA;$t_p$ = burn time&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$32.17∗283.5In(5040000/287000)∗0.984−165∗32.17= 20409.33 ft/s^2$$ or 6220.76m/s or 13915.5mph&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roughly 0.52 times the earth escape velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;(Originally &lt;a href=&quot;https://www.quora.com/How-can-I-find-out-the-speed-of-Saturn-V-rocket/answer/Sihao-Feng?srid=usWFB&amp;amp;share=8e531b6d&quot; rel=&quot;noreferrer&quot;&gt;posted on Quora: Finding Delta-V for the First Stage of Saturn V&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="18270" LastEditorUserId="4660" LastEditDate="2017-01-21T21:05:52.460" LastActivityDate="2017-01-21T21:05:52.460" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="19867" PostTypeId="1" CreationDate="2017-01-21T09:24:10.837" Score="5" ViewCount="2074" Body="&lt;p&gt;From the Wikipedia, E-D nozzle works similarly to aerospike nozzle which compensates the exhaust gas in the atmosphere to increase specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/f2pzv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f2pzv.jpg&quot; alt=&quot;Reference&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/ZUzhy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZUzhy.jpg&quot; alt=&quot;Simplified Diagram &quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18270" LastEditorDisplayName="user20636" LastEditDate="2019-02-14T23:42:44.570" LastActivityDate="2019-02-14T23:45:25.107" Title="What are the pros and cons of expansion-deflection nozzle?" Tags="|rockets|nozzle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19868" PostTypeId="1" AcceptedAnswerId="19870" CreationDate="2017-01-21T10:43:28.563" Score="7" ViewCount="1191" Body="&lt;p&gt;One of the things that makes ion thrusters so bulky and problematic are the magnetic shields required to protect from high temperatures, right? And we have high temperatures because we use plasma, right? So why don't we just use fine metallic dust, charge it and feed it into an ion thruster to get rid of the temperature problem.&lt;/p&gt;&#xA;" OwnerUserId="16627" LastEditorUserId="33864" LastEditDate="2021-01-10T09:11:25.243" LastActivityDate="2021-07-10T04:17:11.323" Title="Why can't we use charged fine metal dust instead of plasma for ion thruster?" Tags="|propulsion|ion-thruster|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="19869" PostTypeId="2" ParentId="19868" CreationDate="2017-01-21T12:21:39.233" Score="9" Body="&lt;p&gt;Ion thrusters work so well because they have a very high exhaust velocity. This is made possible by the fact you're using individual atoms as the propellant. Metal powder consists of much larger, heavier particles, and the speed they can reach would be much lower, making the thruster less efficient. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-21T12:21:39.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19870" PostTypeId="2" ParentId="19868" CreationDate="2017-01-21T13:09:37.017" Score="20" Body="&lt;p&gt;In an ion thruster, particles are accelerated because of their electrical charge. The force acting on them is proportional to the charge (and the external field applied, which we can treat as fixed for a specific engine design). Naturally, the heavier the particle is, the less it is accelerated by this force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An extended particle we can describe as a capacitor and, as such it has a capacity given by &lt;span class=&quot;math-container&quot;&gt;$C = 4\pi\varepsilon_0 R$&lt;/span&gt;. If we put R as 10&lt;span class=&quot;math-container&quot;&gt;$~\rm\mu m$&lt;/span&gt;, the resulting capacity is &lt;span class=&quot;math-container&quot;&gt;$10^{-15}\rm F$&lt;/span&gt;. Now we can use an external voltage to charge this particle up. A reasonable voltage might be 100 kV - resulting in a charge of &lt;span class=&quot;math-container&quot;&gt;$10^{-10}\rm C$&lt;/span&gt;. As the elementary charge is &lt;span class=&quot;math-container&quot;&gt;$1.6\cdot 10^{-19}\rm C$&lt;/span&gt;, this means we are removing about 1.6 billion electrons from the particle. On the other hand, such a particle weighs about 10 ng and contains about &lt;span class=&quot;math-container&quot;&gt;$10^{14}$&lt;/span&gt; atoms or &lt;span class=&quot;math-container&quot;&gt;$2.6\cdot10^{15}$&lt;/span&gt; electrons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means, our engine can just remove about one in a million of all electrons available.&#xA;Compare this to the plasma: Here we can remove a large fraction of electrons, although ion engines often use single-charged ions. That means we can remove &lt;span class=&quot;math-container&quot;&gt;$10^{14}$&lt;/span&gt; electrons instead of &lt;span class=&quot;math-container&quot;&gt;$10^{9}$&lt;/span&gt; when using ions instead of particles. The metal particles have a charge-to-weight ratio which is worse by a factor of 100,000. That gives a lot less thrust per amount of mass ejected, as the momentum scales with the square root of the mass for a constant power. What makes the ratio even worse is the fact that higher voltages and therefore larger engines are needed as well, reducing the thrust-to-weight ratio of the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side remark:&#xA;Removing the same amount of electrons from the metal powder is just impossible: Imagine you were to remove 0.1% of the electrons of a larger object, such as the moon. This would exert such a high Coulomb force on the object, it would blow apart in a violent explosion, despite all its mass and gravity.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2020-04-15T08:23:54.610" LastActivityDate="2020-04-15T08:23:54.610" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="19872" PostTypeId="1" CreationDate="2017-01-21T20:25:12.507" Score="15" ViewCount="2563" Body="&lt;p&gt;Do companies/governments ever sell satellites that are already in space to other companies/governments for use? I'm wondering about the sale of the actual physical satellites. Has it ever happened? Is it at all common?&lt;/p&gt;&#xA;" OwnerUserId="10234" LastActivityDate="2017-01-23T08:35:30.307" Title="Do companies/governments ever sell satellites that are already in space?" Tags="|artificial-satellite|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19873" PostTypeId="2" ParentId="19872" CreationDate="2017-01-21T22:37:18.093" Score="17" Body="&lt;p&gt;Yes, it has happened. This news article shows an example: &lt;a href=&quot;http://spacenews.com/urthecast-buying-deimos-imaging-division-and-its-2-satellites/&quot;&gt;Urthecast Buying Deimos’ Imaging Division and its 2 Satellites&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Deimos Space of Spain and Urthecast Corp. of Canada announced a&#xA;  strategic partnership in which Urthecast will purchase Deimos Space’s&#xA;  fast-growing Deimos Imaging division and its two satellites, Deimos-1&#xA;  and Deimos-2 — both in orbit — for 74.2 million euros ($84.5 million),&#xA;  or about twice the division’s forecasted 2015 revenue.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't think this is particularly common, but neither is it the only time this has happened.&lt;/p&gt;&#xA;" OwnerUserId="12831" LastActivityDate="2017-01-21T22:37:18.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19875" PostTypeId="2" ParentId="19872" CreationDate="2017-01-22T03:57:54.723" Score="10" Body="&lt;p&gt;It seems &lt;a href=&quot;https://space.stackexchange.com/a/19873/12102&quot;&gt;@djr's statement&lt;/a&gt; is correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A similar thing happened in 2008. &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/amc-14.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMC-14&lt;/a&gt; failed to achieve geostationary orbit, and was left in a &lt;a href=&quot;https://en.wikipedia.org/wiki/AMC-14#Satellite.27s_fate&quot; rel=&quot;nofollow noreferrer&quot;&gt;highly elliptical and inclined orbit&lt;/a&gt;. After insurance issues were addressed (with an interesting subtext involving &lt;a href=&quot;https://space.stackexchange.com/q/15224/12102&quot;&gt;orbital maneuver patents&lt;/a&gt; &lt;em&gt;note: needs answer!&lt;/em&gt;) the US government purchased the satellite, and have used its thrusters to achieve an inclined Geosynchronous, but not Geostationary (GEO) orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further reading:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space Travel: &lt;a href=&quot;http://www.space-travel.com/reports/Boeing_Patent_Shuts_Down_AMC_14_Lunar_Flyby_Salvage_Attempt_999.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;Boeing Patent Shuts Down AMC-14 Lunar Flyby Salvage Attempt&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space Daily: &lt;a href=&quot;http://www.spacedaily.com/reports/SES_Negotiating_To_Sell_AMC-14_To_US_Government_Agency_999.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SES Negotiating To Sell AMC-14 To US Government Agency&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space News: &lt;a href=&quot;http://spacenews.com/insurers-sell-amc-14-satellite-pentagon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Insurers Sell AMC-14 Satellite to Pentagon&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia: &lt;a href=&quot;https://en.wikipedia.org/wiki/AMC-14&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMC-14&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gunter's Space Page: &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/amc-14.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;AMC-14&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Defense Aerospace: &lt;a href=&quot;http://www.defense-aerospace.com/articles-view/release/3/93168/ses-americom%E2%80%99s-amc_14-satellite-declared-a-total-loss.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;SES Americom Declares AMC-14 Satellite a Total Loss&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-01-23T08:35:30.307" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19876" PostTypeId="2" ParentId="19867" CreationDate="2017-01-22T04:39:42.140" Score="3" Body="&lt;p&gt;From Hill &amp;amp; Peterson, &quot;Mechanics and Thermodynamics of Propulsion&quot;, 3rd printing, November 1970.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I summarize from the relevant portion of Chapter 13, &lt;em&gt;Chemical Rockets: Expansion in Nozzles&lt;/em&gt; (pp 413 &amp;amp; 414):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pros:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;...the presence of a free jet boundary to the expansion process lends&#xA;a kind of self-adjustment to back-pressure variations.&lt;/li&gt;&#xA;&lt;li&gt;...[they are] substantially shorter than comparable internal-flow&#xA;nozzles.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Con(s):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The main difficulties associated with these configurations arise in&#xA;the process of adequately supporting and cooling the central plug.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-01-22T04:39:42.140" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19877" PostTypeId="2" ParentId="19872" CreationDate="2017-01-22T05:53:48.070" Score="8" Body="&lt;p&gt;Some of the companies owning the satellites either merge or sell out to other companies, like &lt;a href=&quot;https://en.wikipedia.org/wiki/PanAmSat&quot;&gt;PanAmSat&lt;/a&gt;, sold to Hughes Electronics, News Corp, then a private consortium, and finally Intelsat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you have &lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&quot;&gt;Iridium&lt;/a&gt;, where the company went bankrupt and a new company bought them.&lt;/p&gt;&#xA;" OwnerUserId="18275" LastEditorUserId="6816" LastEditDate="2017-01-22T06:07:18.680" LastActivityDate="2017-01-22T06:07:18.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19878" PostTypeId="2" ParentId="19476" CreationDate="2017-01-22T08:17:35.287" Score="3" Body="&lt;p&gt;You haven't provided sufficient information about the specific impulse of the engines you've decided to use, rocket geometry, and flight trajectory. Without these, I cannot help you narrow down on propellant mass to reach final height and velocity (assuming you're simulating an actual Saturn V flight with orbital insertion of payload).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Use rocket equations (I call them momentum equations). $\Delta m$ obtained for imparting the required $\Delta v$ will be the propellant mass required. Realistically, you'd want to add some contingent fuel on top of that to account for drag. If you want a realistic figure, run simulations (using MATLAB?) with changing air density with altitude and flight velocity to compute for instantaneous drag. $(\Delta v + dv)$ will be your new $\Delta v$ for the engine to impart with $dv$ being velocity lost from drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can account for change in gravity based on launch latitude by increasing burn time to achieve same $\Delta v$ because it's not practical to redesign the engine and fuel tanks for greater thrust. The burn time can be computed by writing momentum equations from your inertial frame of reference and then, multiply specific impulse by obtained $\Delta m$ all divided by thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mathematically, the work done in joules by the engines will be the integral of the thrust curve. Assuming a single-staged rocket with constant linear acceleration, work done will be thrust multiplied by burn time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember, there are multiple ways to mathematically approach this problem. What approach you take depends on the variables you have established and have yet to establish.&lt;/p&gt;&#xA;" OwnerUserId="18272" LastEditorUserId="6816" LastEditDate="2017-01-22T08:31:42.257" LastActivityDate="2017-01-22T08:31:42.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19879" PostTypeId="2" ParentId="19592" CreationDate="2017-01-22T12:50:00.337" Score="2" Body="&lt;p&gt;I'll try to answer your immediate question first, then propose some alternate and more sensitive variations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's start with a cubesat (maybe 3U) with a mass of 5kg and a miniaturized EM drive thruster (&lt;a href=&quot;http://arc.aiaa.org/doi/10.2514/1.B36120&quot; rel=&quot;nofollow noreferrer&quot;&gt;first peer-reviewed EM drive paper&lt;/a&gt; and &lt;a href=&quot;http://arc.aiaa.org/doi/pdf/10.2514/1.B36120&quot; rel=&quot;nofollow noreferrer&quot;&gt;pdf&lt;/a&gt; thereof) has been put inside, or maybe once in orbit the 3U &quot;pops open&quot; and a folded resonator made of a springy material with conductive inner coating expands to a nicer resonator size. One way or the other, let's assume the TM212 mode resonates properly with a high Q approaching something like the 7000 indicated in Figure 4 of that paper. That's pretty high, but let's just go with the flow. It does (potentially) violate basic physical laws anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring the issue raised in &lt;a href=&quot;https://space.stackexchange.com/questions/19592/cubesat-orbit-determination/19857#comment51997_19592&quot;&gt;@AndrewThompson 's comment&lt;/a&gt; about the thrust potentially scaling with the size of the cavity, let's use a simply scaled propulsive force for 5 Watts: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;0.005 kW $\times$ 1 mN/kW = 5E-06 Newtons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the force is so tiny, we should at least try to avoid as much atmospheric drag as possible. Let's choose an initial circular orbit with altitude of 800 km. Using an equatorial radius of &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;6378&lt;/a&gt; km. the initial semi-major axis will be (in meters):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a_i = (6378 + 800) \ \times \ 1000$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis-viva&lt;/a&gt; equation, setting r = a for a circular orbit and using a value of 3.986004418E+14 m^3/s^2 for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;standard gravitational parameter&lt;/a&gt; of Earth $GM_e$, the initial orbital speed will be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_i = \sqrt{GM_e \ / \ a}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or about 7451.9 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starting with some basic &lt;a href=&quot;https://en.wikipedia.org/wiki/Classical_mechanics&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newtonian physics&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \Delta p = Force \times time$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \Delta v = Force \times time \ / \ mass$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Five micro-Newtons for 30 days against a mass of 5kg gives a $\Delta v$ of 2.6 m/s, so &lt;strong&gt;yes&lt;/strong&gt; your quick calculation is good.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now a really interesting result. This &lt;a href=&quot;https://space.stackexchange.com/a/8421/12102&quot;&gt;illuminating answer&lt;/a&gt; by @MarkAdler confirms that, for a good rule-of-thumb for a very low tangential-only accelerations resulting in a gradual spiral between two co-planar circular orbits, the change in orbital speed will be negative the total $\Delta v$. So in the limit of zero thrust and infinite time, a total $\Delta v$ in the tangential direction of motion will actually &lt;em&gt;slow the space craft's orbital speed&lt;/em&gt; by $\Delta v$ while simultaneously &lt;em&gt;raising the altitude&lt;/em&gt; of the orbit!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that gives a final orbital velocity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_f = v_i - \Delta v$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or 7451.9 - 2.6 = 7449.3 m/s. Using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis-viva&lt;/a&gt; equation again &quot;in reverse&quot; the final semi-major axis will be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a_f = (GM_e / v_f)^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or about 7183.0 km, an increase of 5 km. The period of a circular orbit is just the circumphrence divided by the speed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ T = 2 \pi a / v$$,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so the period will change by 6 seconds, from about 6052 to 6058 seconds, which means the positions of cube sats in the two orbits &lt;strong&gt;would drift an additional 42 kilometers apart after every single 100 minute orbit!!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So actually, &lt;strong&gt;de-phasing&lt;/strong&gt; will be a far more sensitive method than looking for an altitude change (as &lt;a href=&quot;https://space.stackexchange.com/questions/19592/cubesat-orbit-determination/19857#comment52919_19857&quot;&gt;I've already mentioned here&lt;/a&gt;), and without considering all the other problems, it should be sensitive to $\Delta v$'s much lower than this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you can really turn the EM drive ON for one day, then off for one day, then repeat, and as long as you are tracking it by radar several times a day, some kind of test could be performed. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I am going to briefly suggest modifications to consider.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;deploy a pair of matched cubesats, have them measure their relative positions by time-of-flight (light or radio) and have them alternate one OFF, the other ON every few hours or few days, depending on details.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;give them GPS capability, but use a commercially available, orbit-qualified cubesat GPS unit, and &lt;a href=&quot;https://space.stackexchange.com/a/15110/12102&quot;&gt;not somebody's phone&lt;/a&gt;!&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;connect a pair &lt;em&gt;together&lt;/em&gt; with a tether, or tether one to a dummy ballast, and (try to) get them to spin around their center of mass, or at least to deviate from their equilibrium position in orbit (the basis of an interesting follow-up question??)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-01-22T14:14:15.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19880" PostTypeId="2" ParentId="19806" CreationDate="2017-01-22T17:27:51.357" Score="1" Body="&lt;p&gt;According to a June 2015 SNC press release, the TPS will consist of (at least in part) &quot;a high-temperature material called TUFROC.  TUFROC will be used on the high-temperature nose and wing leading edges of the Dream Chaser spacecraft.&quot;  According to &lt;a href=&quot;https://technology.nasa.gov/patent/TOP2-241&quot; rel=&quot;nofollow noreferrer&quot;&gt;this page&lt;/a&gt; TUFROC stands for &quot;Toughened Uni-Piece Fibrous Reinforced Oxidation-Resistant Composite&quot;.  TUFROC has been &lt;a href=&quot;http://www.parabolicarc.com/2012/04/22/nasa-ames-wins-invention-of-the-year-award-for-x-37b-heat-shield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;proven in flight&lt;/a&gt; by its use on the X-37B. This would seem to be used in place of the reinforced carbon-carbon used in similar high-temperature areas on the Shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The press release mentions &quot;a new silica tile coating&quot; implying that other parts of the TPS will consist of tiles. &quot;SNC’s assessments show that these new coatings offer the same thermal protection as previously flown tile coatings, but at a greatly reduced cost.&quot;  I would interpret this to mean that the tiles will be similar to Shuttle tiles, but with a different coating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it appears that the TPS will be similar to that of Shuttle but with certain improvements/cost saving measures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The press release is &lt;a href=&quot;http://www.sncspace.com/AboutUs/NewsDetails/1411&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dream_Chaser#cite_note-Mark-7&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; on Dream Chaser refers to &quot;an ablative tile created by NASA's Ames Research Center.&quot;  However, the linked &lt;a href=&quot;https://www.youtube.com/watch?v=D7sWtEAddkM&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference&lt;/a&gt; for this statement - a video of a press conference from 2011 - contains only a brief mention of &quot;a latest generation tile with an ablative on top&quot; (about 40 minutes into the video). This ablative may be the coating referred to in the press release, or the design may have changed in the 4 years between the video and the press release.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-01-22T21:10:21.237" LastActivityDate="2017-01-22T21:10:21.237" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19881" PostTypeId="1" CreationDate="2017-01-23T07:20:30.520" Score="6" ViewCount="631" Body="&lt;p&gt;The NASA news article &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2016/nasa-s-navcube-could-support-an-x-ray-communications-demonstration-in-space-a-nasa-first&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA’s NavCube Could Support an X-ray Communications Demonstration in Space — A NASA First&lt;/a&gt; mentions the potential use of X-ray wavelengths for improved range of deep space communications. So far the only advantage I can see over optical communications is the shorter wavelength, which means &lt;em&gt;potentially&lt;/em&gt; less divergence for a given aperture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, using round numbers, a 1 eV photon has a wavelength of roughly 1 micron. Transmitting back to Earth with a 10cm aperture, the divergence angle would then be 1E-05 radians (about 2 arc seconds), but use of that requires:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Diffraction-limited optics&lt;/li&gt;&#xA;&lt;li&gt;Diffraction-limited source size, (e.g. optical fiber semiconductor laser)&lt;/li&gt;&#xA;&lt;li&gt;Nicely filled aperture&lt;/li&gt;&#xA;&lt;li&gt;Mechanical alignment within the system to micron precision&lt;/li&gt;&#xA;&lt;li&gt;Beam pointing accuracy and stability to single-digit arc-seconds&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are all certainly doable. Fine control of beam pointing could be done for example with an actuated MEMS tracking device at the focal plane, but locking on to and tracking an optical beacon would get hard because of the significant optical delays. Let's assumed this is solved somehow &amp;mdash; perhaps by tracking incidental stars through the same optical path (off-axis).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For X-ray energies of for example 100 eV, 1 keV and 10 keV, the wavelengths are of order &lt;strong&gt;100, 10, and 1 ångström!&lt;/strong&gt; To take advantage of these, it seems many if not all of the items above would have to be between 100 and 10,000 times better than the optical system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is there any research, or even speculation of what an X-ray transmitter in deep space would look like? Or is the advantage of X-rays for deep space not actually related to diffraction-limited optics?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt; Even a prototype will do, it doesn't have to be a read-for-prime-time deep-space-capable transmitter.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-07-08T23:54:53.357" LastActivityDate="2020-07-14T05:51:51.200" Title="What will the X-ray communications (XCOM) transmitter look like?" Tags="|communication|deep-space|nicer|x-ray-communication|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="19882" PostTypeId="1" AcceptedAnswerId="19884" CreationDate="2017-01-23T07:34:18.030" Score="2" ViewCount="251" Body="&lt;p&gt;According to the NASA news article &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2016/nasa-s-navcube-could-support-an-x-ray-communications-demonstration-in-space-a-nasa-first&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA’s NavCube Could Support an X-ray Communications Demonstration in Space — A NASA First&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The new technology, called NavCube, combines NASA’s SpaceCube, a reconfigurable and fast flight computing platform, with the Navigator Global Positioning System (GPS) flight receiver. Navigator&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;[...]&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;“A Match Made in Heaven”&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;As part of the potential XCOM demonstration, NavCube will drive the electronics for a device called the Modulated X-ray Source, or MXS, which generates rapid-fire X-ray pulses, turning on and off many times per second.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From this article I understand that NavCube is a unique, fast computer for space applications, combined with a unique, sensitive GPS receiver also optimized for space applications.&lt;/p&gt;&#xA;&lt;p&gt;The X-ray communications investigation/demonstration will put an X-ray transmitter at one end of the ISS and an X-ray receiver at the other. In this case the receiver will be &lt;a href=&quot;https://heasarc.gsfc.nasa.gov/docs/nicer/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NICER&lt;/a&gt;, an array of X-ray telescopes that will primarily be used to study astronomical sources such as neutron stars, and to look into the possible use of timing from pulsars as a deep space positioning system for navigation.&lt;/p&gt;&#xA;&lt;p&gt;Would NavCube then actually just serve as a signal generator for some high-speed pulsed X-ray generator?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7TAMi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7TAMi.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;&lt;em&gt;NavCube, the product of a merger between the Goddard-developed SpaceCube 2.0 and Navigator GPS technologies, could play a vital role helping to demonstrate X-ray communications in space — a potential NASA first&lt;/em&gt;. Credits: NASA/W. Hrybyk&amp;quot; from &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2016/nasa-s-navcube-could-support-an-x-ray-communications-demonstration-in-space-a-nasa-first&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/crpch.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/crpch.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NICER, from &lt;a href=&quot;https://heasarc.gsfc.nasa.gov/docs/nicer/nicer_about.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astrophysics on the International Space Station -&#xA;Understanding ultra-dense matter through soft X-ray timing&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V64w2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V64w2.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NICER X-ray timing telescope/concentrator array, from &lt;a href=&quot;https://www.nasa.gov/content/about-nicer&quot; rel=&quot;nofollow noreferrer&quot;&gt;About NICER&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-23T18:43:29.520" Title="How will NavCube (actually) be important for the XCOM testing and demonstration?" Tags="|iss|communication|nicer|x-ray-communication|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19883" PostTypeId="2" ParentId="5260" CreationDate="2017-01-23T12:27:02.533" Score="4" Body="&lt;p&gt;You can't make the velocity zero and expect to stay there. you will need to constantly fire the engine to hold youself in position (this is possible, until you run out of fuel).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, it is possible to fire the engine and lift off earth into space, and then fire in the opposite direction to earths movement until you are still, relative to the sun. But if you now turn the engine off, the suns gravity will pull you down and directly into the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;its the earths forward motion that prevents it falling into the sun&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2017-01-23T12:27:02.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19884" PostTypeId="2" ParentId="19882" CreationDate="2017-01-23T14:33:54.617" Score="2" Body="&lt;p&gt;Yes, it appears that NavCube will primarily serve as a source of signal for the X-ray generator. In fact, &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2016/nasa-s-navcube-could-support-an-x-ray-communications-demonstration-in-space-a-nasa-first&quot; rel=&quot;nofollow noreferrer&quot;&gt;one article&lt;/a&gt; explicitly confirms this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NavCube’s job is to run MXS’s on-and-off switch, said Jason Mitchell,&#xA;  an engineer at Goddard who helped advance the MXS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I agree that this initially doesn't sound like much of a job. However, the article also says this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It also may help demonstrate — for the first time — X-ray communications in space, a capability that would allow the transmission of gigabits per second throughout the solar system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If we are to take this literally, a multiple gigabit throughput would be an enormous upgrade. The ISS's &lt;a href=&quot;http://www.upi.com/Science_News/2016/12/13/Space-Network-upgrade-to-double-data-rates-on-International-Space-Station/8801481664796/&quot; rel=&quot;nofollow noreferrer&quot;&gt;current data downlink&lt;/a&gt; appears to be on the order of 300 megabits. Except for possible internal optical fiber links, there's simply no reason network hardware capable of supporting gigabit rates would be on the Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given this, I think it's likely that NavCube is flying a clock and FPGAs that will enable proof of concept for the entire link.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the X-ray source could be driven at a lower rate, that would only test portions of the technology. NavCube may enable emulation of a much more flight-like system, hence the comments about a &quot;&quot;match made in heaven&quot;.&lt;/p&gt;&#xA;" OwnerUserId="5851" LastActivityDate="2017-01-23T14:33:54.617" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19886" PostTypeId="1" CreationDate="2017-01-24T06:03:50.410" Score="24" ViewCount="1275" Body="&lt;p&gt;&lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2016/08/Sentinel-1A_fragment_impact_in_space&quot; rel=&quot;noreferrer&quot;&gt;According to ESA&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A sudden small power reduction was observed in a solar array of Sentinel-1A, orbiting at 700 km altitude, at 17:07 GMT on 23 August. Slight changes in the orientation and the orbit of the satellite were also measured at the same time.&lt;/p&gt;&#xA;&lt;p&gt;Following a preliminary investigation, the operations team at ESA’s control centre in Darmstadt, Germany suspected a possible impact by space debris or micrometeoroid on the solar wing.&lt;/p&gt;&#xA;&lt;p&gt;Detailed analyses of the satellite’s status were performed to understand the cause of this power loss. In addition, the engineers decided to activate the board cameras to acquire pictures of the array. These cameras were originally carried to monitor the deployment of the solar wings, which occurred just a few hours after launch in April 2014, and were not intended to be used afterwards.&lt;/p&gt;&#xA;&lt;p&gt;Following their switch-on, one camera provided a picture that clearly shows the strike on the solar panel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Of course all Mars rovers's cameras are incredibly useful for monitoring the rover's condition and were almost certainly essential for their long mission lives, but I'm interested in &lt;em&gt;applications in space&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://space.stackexchange.com/a/18727/12102&quot;&gt;this answer&lt;/a&gt;, despite the substantial and mechanically complex series of events that allow the [JWST] to unfold and deploy both a giant, precision optical system and a giant multi-layered heat shield, there does not appear to be a plan to add self-monitoring cameras to it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;So:&lt;/strong&gt; I am wondering how common it is for (at least recent) spacecraft to have &amp;quot;selfie cams&amp;quot; or status, health, self-inspection or function verification cameras which can provide ground personnel with images.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uQo8Z.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uQo8Z.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Photo credit: ESA, from &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2016/08/Sentinel-1A_fragment_impact_in_space&quot; rel=&quot;noreferrer&quot;&gt;SENTINEL-1A Fragment Impact in Space&lt;/a&gt;. Open this image in separate tab for large size.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-11-17T09:23:49.487" Title="How common is it for spacecraft to have &quot;selfie&quot; capability?" Tags="|james-webb-telescope|" AnswerCount="8" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19887" PostTypeId="1" AcceptedAnswerId="27555" CreationDate="2017-01-24T10:49:53.490" Score="9" ViewCount="990" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/q/19402/12102&quot;&gt;this question&lt;/a&gt; I describe looking for a TLE for Mercury-Atlas 6, the spacecraft piloted by John Glenn on February 20, 1962. I'll post here the relevant information:&lt;/p&gt;&#xA;&lt;p&gt;While the TLE catalog available from &lt;a href=&quot;https://www.space-track.org/#/catalog&quot; rel=&quot;nofollow noreferrer&quot;&gt;space-track.org&lt;/a&gt; is convenient to use and indeed lists &lt;code&gt;SatCat: 1962-003A&lt;/code&gt;, &lt;code&gt;NORAD: 00240&lt;/code&gt;, it doesn't seem to have TLEs available. I requested data from the &lt;a href=&quot;https://celestrak.org/NORAD/archives/request.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak &amp;quot;requestor&amp;quot;&lt;/a&gt; and have now received a second e-mail with the attached &lt;code&gt;.txt&lt;/code&gt; file. No dice.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;=====================================================================&#xA;  00240 1962-003A   MERCURY ATLAS 6                                  &#xA;  Launched: 1962-02-20 (051)           Start Date: 1962-02-19 (050)  &#xA;   Decayed: 1962-02-20 (051)            Stop Date: 1962-02-21 (052)  &#xA;=====================================================================&#xA;&#xA;No data found for Catalog Number 00240&#xA;&amp;lt;End of file&amp;gt;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;I am not sure this is because there was a TLE that was &amp;quot;retired&amp;quot; or if this is just a placeholder for the mission but no TLEs (worth posting at least) were generated. So I am not sure if the earliest TLE I might find available in space-track in 2017 would be the first official TLE ever generated.&lt;/p&gt;&#xA;&lt;p&gt;1957 is of course an important year. TLE epoch values have only two digits for the year, so values below 57 are assumed to be 21st century (e.g. 2056) while those equal to or above 57 are assumed to be 20th century (e.g. 1958). But I doubt there were actual Two Line Elements at that time.&lt;/p&gt;&#xA;&lt;p&gt;One could generate historical TLEs from other forms of orbit data or parameterizations, but I'm not interested in the TLE with the earliest epoch.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Thus:&lt;/strong&gt; What was the first proper Two Line Element to be generated?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2021-08-18T01:47:59.660" Title="What was the first proper Two Line Element to be generated?" Tags="|two-line-elements|spaceflight-firsts|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19888" PostTypeId="1" CreationDate="2017-01-24T11:00:48.937" Score="6" ViewCount="786" Body="&lt;p&gt;I'd like to know how a thermal control system of a spacecraft basically works. As the only possibility to get rid of the thermal heat is through radiation, I'd like to get a basic qualitative understanding (without going to much into the maths) of how this specific process of &quot;getting rid&quot; of the heat through radiation works.&lt;/p&gt;&#xA;" OwnerUserId="8968" LastEditorUserId="9092" LastEditDate="2021-03-22T22:46:02.093" LastActivityDate="2021-03-22T22:46:02.093" Title="How does a thermal control system of a spacecraft basicaly work?" Tags="|spacecraft|engineering|heat|thermal-control|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19889" PostTypeId="2" ParentId="19888" CreationDate="2017-01-24T12:21:06.377" Score="3" Body="&lt;p&gt;Similarly to a common refrigerator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a working liquid of a low evaporation temperature (below room temperature) - e.g. ammonia. Flowing through sealed pipes in radiators of equipment and air circulation machinery, it evaporates, absorbing heat from the environment, evaporating in the process. Then it's pumped into radiator panels - large structures outside the spacecraft, mounted on pivot mounts, turning in such a way that narrow edge is directed at the Sun absorbing least amount of solar heat and large surfaces face the deep space, radiating heat out. The vapor gives out heat and condenses back into liquid, to be returned to the radiators inside the craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This may work on minimal pressure differential and only heat of condensation and evaporation (safer, but less efficient), or may be augmented by additional pressurization: the working fluid has even lower evaporation temperature at ambient pressure, but can be liquified by moderate pressurization. The vapor pumped into the radiators through a stronger pump, condenses due to high pressure; that additionally heats it; the extra heat is radiated out. Then, inside the ship, the heat-absorbing radiators contain nozzles where the pressure is released - the decompression process combined with vaporization draws much more heat from the environment than vaporization alone would.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-01-24T12:21:06.377" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19890" PostTypeId="2" ParentId="1805" CreationDate="2017-01-24T16:00:34.060" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UPPcj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UPPcj.jpg&quot; alt=&quot;Columbia debris field&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just adding another map. I got it &lt;a href=&quot;http://web.archive.org/web/20161014213445/http://caib1.nasa.gov/news/report/pdf/vol2/part10.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="3038" LastActivityDate="2017-01-24T16:00:34.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19891" PostTypeId="1" CreationDate="2017-01-24T16:37:38.737" Score="9" ViewCount="970" Body="&lt;p&gt;Watch any of last year's SpaceX LEO satellite deployment missions's broadcasts in YouTube, and you can see views from at least several different cameras. At least two on the 2nd stage nozzle, at least one on the payload, at least one on a gridded fin, one in the 2nd stage LOX tank, and I suspect there are many more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cameras serve several purposes, possibly including marketing and HR/recruiting, but primarily they are used to collect additional information about the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Roughly how many self-viewing cameras are present in a Falcon 9 LEO mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Details may be confidential or proprietary, but are there closer to a dozen or a hundred? Closer on a log scale or linear?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5iSXD.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5iSXD.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Frames from the broadcast changed to monochrome (R+G+B)/3 and stitched manually into a GIF. From &lt;a href=&quot;https://space.stackexchange.com/q/17786/12102&quot;&gt;When this F9 2nd stage engine suddenly moves upon shutdown, is it moving along the rocket's axis?&lt;/a&gt; (currently no accepted answer).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Es9Z.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Es9Z.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF (from giphy) of SpaceX's &lt;a href=&quot;https://www.youtube.com/watch?v=_uztxi4fE94&amp;amp;feature=youtu.be&amp;amp;t=7m18s&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thaicom-8 Technical Broadcast&lt;/a&gt; at around &lt;code&gt;T+ 00:07:00&lt;/code&gt; post launch, showing a &lt;a href=&quot;https://space.stackexchange.com/a/19435/12102&quot;&gt;flaming grid fin&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lPVpU.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lPVpU.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF (from giphy) of (probably) SpaceX's CRS-5 Broadcast, from &lt;a href=&quot;https://www.youtube.com/watch?v=8HZrrHI34x4&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (giphy's limit is 15 minutes). See &lt;a href=&quot;https://space.stackexchange.com/q/7737/12102&quot;&gt;SpaceX CRS-5 Launch: What is the footage that appears to contain bubbles?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-05-31T15:28:20.080" LastActivityDate="2019-05-31T15:28:20.080" Title="Roughly how many self-viewing cameras are present in a Falcon 9 LEO mission?" Tags="|spacex|falcon-9|camera|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="19892" PostTypeId="1" AcceptedAnswerId="19899" CreationDate="2017-01-24T18:25:01.187" Score="6" ViewCount="239" Body="&lt;p&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/banxing-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Banxing-2&lt;/a&gt; is a 47 kg LEO satellite deployed from China's &lt;a href=&quot;https://space.skyrocket.de/doc_sdat/tg-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tiangong-2&lt;/a&gt; on 22 October 2016. It is planned to say in an orbit matched to Tiangong-2 so that it stays in close proximity.&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/banxing-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Banxing 2 was developed and built at the Shanghai Academy of Spaceflight Technology (SAST).&lt;/p&gt;&#xA;&lt;p&gt;It will photograph Tiangong 2 in orbit and will monitor space debris. It features an Ammonia based propulsion system&lt;/p&gt;&#xA;&lt;p&gt;The satellite features 25 Megapixel high-resolution full-frame visible light camera and fisheye cameras.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;While some spacecraft have been able to image parts of themselves] (e.g. Sentinel-1 &lt;a href=&quot;https://space.stackexchange.com/q/19886/12102&quot;&gt;How common is it for spacecraft to have &amp;quot;selfie&amp;quot; capability?&lt;/a&gt; and Falcon-9), it has been pointed out that spacecraft-mounted cameras, even on articulated &amp;quot;selfie sticks&amp;quot; would be impractical for full spacecraft coverage, but small, maneuverable robotic satellites with cameras and other instruments might be a much better approach for larger spacecraft to increase awareness of their condition once deployed in space.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/19886/12102&quot;&gt;How common is it for spacecraft to have &amp;quot;selfie&amp;quot; capability?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/19891/12102&quot;&gt;Roughly how many self-viewing cameras are present in a Falcon 9 LEO mission?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/19886/#comment53026_19886&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/19886/#comment53028_19886&quot;&gt;this&lt;/a&gt; comment&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Have there been any other, publicly acknowledged examples of camera carrying robotic satellites that have had the ability to carefully navigate around and photograph other spacecraft at close range?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TSNk1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TSNk1.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Photo of Shenzou-11 docked with Tiangong-2 taken by Banxing-2. From &lt;a href=&quot;http://www.globaltimes.cn/content/1015143.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Photo credit: CNSA&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jLIUE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jLIUE.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Photo of Shenzou-11 docked with Tiangong-2 taken by Banxing-2. From &lt;a href=&quot;http://www.globaltimes.cn/content/1015143.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Photo credit: CNSA&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JqnXA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JqnXA.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Artist's conception of Banxing-2. From &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/banxing-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Photo credit: CNSA&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/NfqFwz0rFck?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-12-26T04:06:16.240" LastActivityDate="2021-12-26T04:06:16.240" Title="Have there been any other &quot;camsats&quot; besides Banxing-2?" Tags="|camera|smallsat|shenzhou|tiangong-2|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="19893" PostTypeId="2" ParentId="19888" CreationDate="2017-01-24T18:40:35.313" Score="6" Body="&lt;p&gt;In the general case of &quot;spacecraft&quot; meaning any satellite, manned or otherwise, or upper stage the hierarchy for thermal control looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Passive systems&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;materials chosen for their radiative finish (paint, mirrors etc)&lt;/li&gt;&#xA;&lt;li&gt;materials chosen for solid conductivity or insulating properties (aluminium face-skins on honeycomb panels, interface filler, CFRP spacers as insulators)&lt;/li&gt;&#xA;&lt;li&gt;aluminised mylar or kapton blanket material combining the above properties (ten layers with dacron spacers to reduce radiation)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Active systems&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;electrical heaters (some argue these are so simple they count as passive)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;simple heat pipes (again some argue these are passive)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;loop heat pipes &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;mechanically pumped fluid loops&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;More specialised items (louvres, stirling coolers)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Simple heat pipe and loop heat pipes are two phase fluid systems where the heat causes evaporation and pressurisation of a liquid to gas which in turn causes fluid flow from hot to cold parts. The geometry is arranged so that the fluid condenses, i.e. loses heat, to the external radiator. A mechanically driven pump by contrast does not need to be two phase and gives more control to the operator as to what heat transfer capacity it has at a given time.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt; The driving power for a heat pump comes from the heat that is to be transported from a point of power dissipation in the payload to a comparatively cool radiator surface that can see deep space. The whole point is that it is hotter at one end of the heat pipe than the other. The increased gas pressure at the hot end of the pipe spreads out to the cold end causing mass transfer in the gas phase. The role of surface tension, or capillary action, is to even out the meniscus shapes by transferring liquid in the grooves from the cold end back to the hot end. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simple heat pipes don't work well against acceleration. This means whilst they work in orbit they need to be tested on Earth whilst horizontal. Some loop heat pipes, which are a more complex design, can work against a 1-g acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Synthesis&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually the design of an equipment item or a whole satellite will be to identify the power dissipating elements and then choose a primary heat loss path which is specifically made to have well known characteristics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An example of passive design would be to finish items in interior cavities in paint so as to make a high emmisivity surface and thus create a quasi isothermal cavity through efficient exchange of radiation. The walls of the cavity could be an aluminium box or a honeycomb sheet with a dense interior mesh.  Particularly high dissipating elements would be mounted directly on the outer walls, inside or outside. The outside of the honeycomb could be given white paint, or vacuum deposited aluminium or second surface mirrors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the power dissipated is too great for this the design might then progress to a fluid based system. Heat pipes are used extensively in high power geostationary satellites to spread heat within an outer wall and thus make best use of the radiator area.  Loop heat pipes are used where the demand is highest.  Mechanically driven pumps are used in some unmanned situations but, I believe, are more a feature of manned systems.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-01-25T11:05:11.983" LastActivityDate="2017-01-25T11:05:11.983" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19894" PostTypeId="2" ParentId="19886" CreationDate="2017-01-24T19:39:41.737" Score="5" Body="&lt;p&gt;In my experience it is very uncommon for spacecraft to have cameras for self-inspection. For most missions it is not particularly justified since there are much simpler methods to get the telemetry needed. Sentinel-1 had a particularly complex deployment (see &lt;a href=&quot;http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1/Sentinel-1_performs_opening_dance_routine&quot; rel=&quot;noreferrer&quot;&gt;Sentinel-1 performs opening dance routine&lt;/a&gt;) which no doubt justified something more sophisticated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only other case I've come across is &lt;a href=&quot;https://www.youtube.com/watch?v=cQEw9iTf8d0&quot; rel=&quot;noreferrer&quot;&gt;ISRO deployment cameras on RESOURCESAT-2A&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, I'm aware of operations people who have this sort of facility on their wish list even for simple deployments. So, maybe it is something that will used more in the future. &lt;/p&gt;&#xA;" OwnerUserId="18180" LastActivityDate="2017-01-24T19:39:41.737" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19895" PostTypeId="2" ParentId="18783" CreationDate="2017-01-24T23:39:30.177" Score="-1" Body="&lt;p&gt;One very simple thing, exceptionally high specific impulse due to high chamber pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Merlin 1D has a chamber pressure of 97 ATM and atmospheric specific impulse of 282 seconds. Raptor has an atmospheric specific impulse of overwhelmingly 334 seconds (which is dreamlike). Suppose an imaginary rocket with the Merlin-1D has a $\Delta V$ of 10000m/s. If the Raptor was mounted on the rocket the $\Delta V$ would be increased by 1843 m/s!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is life-changing for SpaceX and I highly doubt anything in two years would beat Raptor's exceptional high specific impulse.&lt;/p&gt;&#xA;" OwnerUserId="18270" LastEditorUserId="6816" LastEditDate="2017-01-25T00:14:00.643" LastActivityDate="2017-01-25T00:14:00.643" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19896" PostTypeId="1" AcceptedAnswerId="19897" CreationDate="2017-01-25T00:09:28.950" Score="9" ViewCount="494" Body="&lt;p&gt;The Sojourner rover was deployed as part of the Mars Pathfinder mission. At the end of the mission, the Sojourner rover was still operable. But the base station that the Mars Pathfinder carried failed. So communication with the rover was lost. Presumably the base station is completely inoperable after this failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find any specifications on the frequency used to communicate from the base station to the rover. But the rover has a prominent dipole antenna sticking out of it. This is presumably it's only radio antenna. Is the rover just sitting there waiting for its next set of commands? If I had the correct antenna and transmitter in orbit around Mars, could I start using the Sojourner rover again?&lt;/p&gt;&#xA;" OwnerUserId="14488" LastActivityDate="2017-01-27T16:03:57.450" Title="With the correct antenna and transmitter, could the Sojourner rover be reactivated?" Tags="|rovers|radio-communication|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19897" PostTypeId="2" ParentId="19896" CreationDate="2017-01-25T01:22:12.597" Score="12" Body="&lt;p&gt;If it were still working and you have a powerful enough transmitter and sensitive enough receiver on your orbiter (likely requiring a high-gain antenna), then I suppose. However there is no way that it's still working.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The modem on the Sojourner rover is designed to communicate with a companion modem on the Mars Pathfinder lander no more than a few meters away. It would take some effort to design something to communicate with Sojourner from orbit, but with sufficient determination, it could probably be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sojourner rover primary batteries are dead. The solar cells are covered in dust. The rover has gone through many deep thermal cycles over several Martian years, with the mechanical thermal mismatches very likely breaking electrical connections, rendering the rover unrecoverable. It is most sincerely dead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When Sojourner continued to operate after the lander died, it did not sit still. It was programmed to drive around in case it somehow got out of range of modem communication, hoping that the driving would bring it back in range. We have evidence from orbital images that in fact Sojourner is not currently where we last saw it with the lander camera. It drove, maybe several times, before it finally succumbed to the Martian environment.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-01-25T01:22:12.597" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19899" PostTypeId="2" ParentId="19892" CreationDate="2017-01-25T03:49:21.573" Score="10" Body="&lt;p&gt;In the early days of ISS, when they were building it with the shuttle, there was a soccer ball sized robot camera called AER Cam Sprint. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DWsQ9.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DWsQ9.jpg&quot; alt=&quot;AER CAM PIC&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;NASA, public domain. Astronaut for scale&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They tested it successfully out of the shuttle bay. It had stereo cameras, a light, and cold nitrogen thrusters. It was not autonomous, but was controlled by an astronaut inside. It was envisioned to assist EVA's access the ISS or spacecraft and so on. They lifted of lot of the technology from suit backpacks. &lt;a href=&quot;https://spaceflight.nasa.gov/station/assembly/sprint/&quot; rel=&quot;noreferrer&quot;&gt;https://spaceflight.nasa.gov/station/assembly/sprint/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="6642" LastEditDate="2017-01-25T05:09:45.323" LastActivityDate="2017-01-25T05:09:45.323" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19900" PostTypeId="1" CreationDate="2017-01-25T14:08:34.053" Score="1" ViewCount="159" Body="&lt;p&gt;What is the best resolution of infrared photo taken from space of earth?&#xA;Is it possible to see on this picture, for example, animals or humans (temperature of body is different than ground)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to download or buy such photos for any areas?&lt;/p&gt;&#xA;" OwnerUserId="18783" OwnerDisplayName="lubom" LastEditorUserId="58" LastEditDate="2017-01-25T15:18:51.570" LastActivityDate="2017-03-03T02:07:00.300" Title="What is the highest resolution of infrared photography of Earth available from satellites?" Tags="|earth|photography|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19902" PostTypeId="1" CreationDate="2017-01-25T18:26:57.557" Score="2" ViewCount="427" Body="&lt;p&gt;I am a layman when it comes to this stuff, but I don't understand why we don't build a large ship like the ISS in orbit.  Then we can launch the propulsion portion separately and attach to the cargo and living areas.  Leave the whole mess in orbit around Mars on arrival and descend in a vehicle solely built to get crew to the surface and back. Is this not doable?  It would solve a lot of the issues with the long voyage.&lt;/p&gt;&#xA;" OwnerUserId="18297" LastEditorUserId="6816" LastEditDate="2017-01-25T19:58:32.083" LastActivityDate="2017-01-25T19:58:32.083" Title="Building ship to Mars in orbit" Tags="|mars|construction|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19903" PostTypeId="2" ParentId="19902" CreationDate="2017-01-25T19:06:30.887" Score="0" Body="&lt;p&gt;As I understand it, building interplanetary ships in orbit is the long term plan.  Building the individual pieces is the hard part.  Building a propulsion system that meets the mission requirements, building the long term, deep space human habitation module, and building the cargo section, for example, are the parts that we're having problems with.  Especially the part where you have to keep the humans alive.  Not to mention that building and launching each individual pieces is really expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might find these questions interesting relating to the individual pieces:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/78/is-it-more-efficient-to-assemble-spacecraft-in-orbit-or-assemble-on-the-ground&quot;&gt;Is it more efficient to assemble spacecraft in orbit, or assemble on the ground and launch?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/8833/could-nasa-send-animals-to-mars-and-back-as-a-rehearsal-before-a-human-mission/8835#8835&quot;&gt;Could NASA send animals to Mars and back as a rehearsal before a human mission?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/8289/could-the-iss-be-propelled-to-mars&quot;&gt;Could the ISS be propelled to Mars?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16599" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-01-25T19:06:30.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19904" PostTypeId="2" ParentId="19902" CreationDate="2017-01-25T19:20:20.433" Score="1" Body="&lt;p&gt;There is considerable disagreement on the topic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Direct&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Direct&lt;/a&gt; initiative is probably the most famous in advocating for sending humans to Mars in smaller, Earth-built craft. Since you ask why not, I'll focus on their reasons (rather than those why we should build craft in space, or why we shouldn't go at all). The argument is naturally long but I'll point out:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Proponents feel that orbital construction will never finish, as political winds and budget cuts will inevitably get the project cancelled.&lt;/li&gt;&#xA;&lt;li&gt;The engineering challenges in constructing a large, complex craft in orbit have yet to be overcome. Mars Direct could be achieved with existing technology.&lt;/li&gt;&#xA;&lt;li&gt;Estimated cost is much lower for a Mars Direct-style mission.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="16733" LastActivityDate="2017-01-25T19:20:20.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19905" PostTypeId="2" ParentId="19887" CreationDate="2017-01-25T23:49:42.367" Score="6" Body="&lt;p&gt;The earliest TLE in the public catalog, according to elset epoch is for catalog number 15, Explorer 6, with an epoch of 1959-01-11 01:49:23&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;1 00015U 59004  A 59011.07596599  .00141866 +00000-0 +00000-0 0 09660&#xA;2 00015 047.0999 025.2139 7504899 076.7382 327.1285 02.03707726003008&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I'm not sure why MERCURY 6 doesn't have a TLE, but suspect it's due to the short duration of the flight (3 revs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mercury 8 &amp;amp; 9 (6 and 22 revs respectively) both have a single TLE:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;1 00433U 62052  A 62276.56409039  .02123437 +00000-0 +00000-0 0 00010&#xA;2 00433 032.5500 058.6365 0098501 074.3803 143.5476 16.19408693000016&#xA;&#xA;1 00576U 63015  A 63135.72080739  .00862827 +00000-0 +00000-0 0 00014&#xA;2 00576 032.5430 289.4700 0077884 072.4399 288.2710 16.21829859000037&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="4709" LastActivityDate="2017-01-25T23:49:42.367" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19906" PostTypeId="1" CreationDate="2017-01-26T00:17:50.677" Score="14" ViewCount="5353" Body="&lt;p&gt;On Earth we have the idea of constant ground temperature. Builders in North America think of it as 55 degrees at 5 feet down (not quite true, but close enough). At the surface it varies with the day. A little deeper and it averages the week. The deeper you go, the more it lags behind until you get to 30 feet where it is constant, roughly equal to the annual average of the area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In considering an underground lunar settlement, the constant ground temperature in the first 60 meters or so, where you might tunnel is a really relevant. Since the lunar day is so long and the temperature varies so much, one would think the depth at which constant temperature occurs would be deeper than earth. But maybe soil in a vacuum transmits heat less? I think it would probably vary a lot by latitude as well. The thought of having an underground settlement where the ground temperature is naturally comfortable to people is fascinating. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we know what the constant lunar underground temperature is? And at what depth it stabilizes? Is there any place on the moon where it might be in the 40˚ to 80˚F range?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="6816" LastEditDate="2017-01-26T18:40:55.570" LastActivityDate="2018-04-07T15:20:30.173" Title="Constant lunar sub-surface temperature" Tags="|the-moon|colonization|geology|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19907" PostTypeId="1" AcceptedAnswerId="31290" CreationDate="2017-01-26T04:45:11.130" Score="16" ViewCount="838" Body="&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt; Newly published paper in Nature Geoscience &lt;a href=&quot;https://www.nature.com/articles/s41561-018-0157-x&quot; rel=&quot;noreferrer&quot;&gt;Atmospheric mountain wave generation on Venus and its influence on the solid planet’s rotation rate&lt;/a&gt; has open-access links in &lt;s&gt;&lt;a href=&quot;https://www.sciencenews.org/article/venus-thick-atmosphere-speeds-planets-spin&quot; rel=&quot;noreferrer&quot;&gt;Science News&lt;/a&gt;&lt;/s&gt;, &lt;a href=&quot;https://motherboard.vice.com/en_us/article/8xe7w3/venus-atmosphere-is-so-thick-it-moves-mountains&quot; rel=&quot;noreferrer&quot;&gt;Motherboard&lt;/a&gt;, and &lt;a href=&quot;http://www.sciencemag.org/news/2018/06/wind-hitting-venus-s-mountains-makes-planet-rotate-faster&quot; rel=&quot;noreferrer&quot;&gt;Science&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;There is a stationary gravity wave in Venus's very dense atmosphere. It is intermittent, but has been detected several times. See the NYTimes article &lt;a href=&quot;https://www.nytimes.com/2017/01/16/science/venus-wave-akatsuki.html&quot; rel=&quot;noreferrer&quot;&gt;Venus Smiled, With a Mysterious Wave Across Its Atmosphere&lt;/a&gt; for a discussion of recent observations by JAXA. See &lt;a href=&quot;http://www.isas.jaxa.jp/en/missions/spacecraft/current/akatsuki.html&quot; rel=&quot;noreferrer&quot;&gt;Akatsuki&lt;/a&gt; and &lt;a href=&quot;http://global.jaxa.jp/projects/sat/planet_c/&quot; rel=&quot;noreferrer&quot;&gt;Happy Birthday, Akatsuki!&lt;/a&gt;, celebrating it's first Venusian year at Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus&quot; rel=&quot;noreferrer&quot;&gt;extremely high density&lt;/a&gt; of the atmosphere near the surface, would it be possible for a suitably shaped spacecraft to &quot;surf&quot; or somehow remain aloft in this wave without propulsion, or even use it to rise high in the atmosphere, taking vertical data (e.g. composition, temperature, radar surface imagery and doppler profiling) and then gently return to the surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mG2jA.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mG2jA.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;A sequence of images showing the stationary nature of the bow-shape wave above Venus when it was observed in December 2015. Planet-C&quot; from &lt;a href=&quot;https://www.nytimes.com/2017/01/16/science/venus-wave-akatsuki.html&quot; rel=&quot;noreferrer&quot;&gt;NYTimes&lt;/a&gt;. credit: Planet-C/JAXA&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RksBA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RksBA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;An illustration of how gravity waves travel up mountains and into Venus’s atmosphere. Credit ESA&quot; From &lt;a href=&quot;https://www.nytimes.com/2016/07/21/science/venus-express-clouds-mountains.html&quot; rel=&quot;noreferrer&quot;&gt;How Mountains Obscured by Venus’s Clouds Reveal Themselves&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-01-04T03:10:10.743" LastActivityDate="2020-04-03T17:13:28.163" Title="Would it be possible to &quot;ride the wave&quot; on Venus?" Tags="|planetary-science|venus|jaxa|akatsuki|" AnswerCount="1" CommentCount="20" ContentLicense="CC BY-SA 4.0" />
  <row Id="19908" PostTypeId="1" CreationDate="2017-01-26T04:55:37.780" Score="3" ViewCount="859" Body="&lt;p&gt;What is the distinction (if any) between spacecraft data and spacecraft telemetry? Do these terms tend to represent &lt;em&gt;different kinds&lt;/em&gt; of information? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While each case is different, in what ways do they tend to impact spacecraft resources (e.g. memory, power, bandwidth, weight) differently?&lt;/p&gt;&#xA;" OwnerDisplayName="user16987" LastEditorUserId="12102" LastEditDate="2017-01-27T02:59:10.310" LastActivityDate="2018-04-18T06:19:13.470" Title="Distinction between spacecraft data and telemetry - difference in spacecraft resource use?" Tags="|orbital-mechanics|calculation|telemetry|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19909" PostTypeId="2" ParentId="19819" CreationDate="2017-01-26T09:07:24.593" Score="5" Body="&lt;p&gt;While delta-V - or change in velocity - remains the same, the effect on the trajectory depends in large part on the absolute velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While a small change of speed (a small modification of the velocity vector) at periapsis can convert to large apoapsis change, maneuvers like plane change (normal/antinormal burn) or moving argument of periapsis (rotating the orbit major axis around the central body) require changing direction of the velocity vector by a significant angle - and the larger the velocity, the more delta-V is needed to change that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An easy way to picture it is: you want to change your orbital inclination by 45 degrees by a burn perpendicular to the current trajectory - so you need to turn the velocity vector by 45 degrees. If your current speed is 20m/s, it's a 20m/s burn. If your current velocity is 2000m/s, you need a 2000m/s burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus inclination changes are best performed at speeds as low as possible - e.g. near a distant apoapsis of a highly eccentric orbit - or through gravity assist of an extra body, e.g. the Moon. Oberth effect is actively detrimental to these maneuvers.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-01-26T15:25:30.240" LastActivityDate="2017-01-26T15:25:30.240" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19910" PostTypeId="1" CreationDate="2017-01-26T09:35:24.593" Score="4" ViewCount="4432" Body="&lt;p&gt;If I want to rotate an eccentric orbit around the central body - retain orbital plane, retain apoapsis and periapsis altitudes, but have the orbit rotated in its orbital plane - change the argument of periapsis - what is the optimal maneuver to that end?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know an easy way to achieve this effect is performing a radial burn (towards the center of the central body) at periapsis, at thrust such that the craft retains altitude, against centripetal acceleration; moving in circular path around the body; &quot;dragging the periapsis along&quot; - the moment the engines are cut off, it enters the new trajectory. I'm also aware this method may be awfully costly, especially for highly eccentric orbits and large changes of argument of periapsis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another method is circularizing the orbit at apoapsis, and then returning to desired eccentricity back upon achieving the desired argument of periapsis. This one has a fixed cost, which will be excessive in case the orbit is very eccentric and the desired shift in angle is small. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also a method involving only tangential burns (pro/retrograde) at various points of the orbit, but I only have a rough hunch of how it works, no good solid recipe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an universal strategy to optimally perform this change?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-01-26T12:20:06.440" Title="Optimal change of argument of periapsis?" Tags="|orbital-mechanics|orbital-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19911" PostTypeId="1" AcceptedAnswerId="19915" CreationDate="2017-01-26T09:59:17.217" Score="14" ViewCount="2755" Body="&lt;p&gt;Moon's synodic day is 2551443 seconds. Its circumference is 10921 km. At sustained 15.4km/hour a rover traveling along its equator would retain full sunlight at all times; at higher latitudes that speed would be even lower. On earth, we have all-terrain vehicles for which this kind of speed is trivial to maintain - and making the speed a little higher, there would be time for stops, detours, sample pickup, data transmission and so on. The 3s radio roundtrip and good maps would allow for fully interactive remote control, so no autonomous drive problems at these speeds. And the rover would be able to analyze many locations and spot &quot;anomalies&quot; which are simply impossible to find with a lander due to its limited area coverage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were there ever plans for such a rover? Specific obstacles, or reasons why it would not be viable?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-01-27T01:39:59.687" Title="a &quot;speedy&quot; lunar rover?" Tags="|rovers|lunar-landing|" AnswerCount="3" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="19912" PostTypeId="2" ParentId="19911" CreationDate="2017-01-26T10:58:04.127" Score="16" Body="&lt;p&gt;The Soviets tried direct remote control in the Lunokhod program. The radio delay was found to be a huge obstacle. They used a driver plus a spotter and a commander. The driver had to continuously integrate looking ahead (predicting the future, with only low-resolution B&amp;amp;W images to guide him) with controlling the rover via a delay. This was exhausting work and could only be done in short stints.&lt;/p&gt;&#xA;&lt;p&gt;This is shown in a Lunokhod documentary (Tank on the moon?), I'll see if I can find it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.airspacemag.com/space/the-other-moon-landings-6457729/?all&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; has more detail:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Only one member of each crew would drive the rover. Behind him would sit the crew commander, who would oversee the driver’s handling of the rover. Joining them in the control room would be a navigator, a radio antenna operator, and the flight engineer, who would monitor the rover’s systems. Each crew would operate the rover for two hours; then the other crew would take control.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Latypov and Dovgan’s [the drivers] only guidance came from a monitor, which displayed images from Lunokhod’s two low-resolution television cameras. To any video game enthusiast it sounds simple—but this was nothing like a video game. The cameras did not send a continuous stream of images, but rather single frames, like a slide show, at intervals that varied from seven to 20 seconds. And because radio signals took three seconds to travel round trip between Earth and the moon, the driver didn’t see the results of his actions for many long moments. For this reason, if crew commanders Nikolai Yeremenko and Igor Fyodorov saw Lunokhod heading toward catastrophe, they could push a button to halt the rover.&lt;/p&gt;&#xA;&lt;p&gt;Dovgan, now 66, was well prepared by intensive training. “Driving on the moon felt even easier than it was in the lunodrome,” he says, but his comment belies the difficulties of navigating the rover. The low resolution of the slide show made it difficult to spot craters and boulders, especially at high sun angles, and there was a “dead zone”—a three-foot-wide area immediately in front of the rover that Lunokhod’s cameras could not see. The only solution, according to Dogvan, was to memorize the features in this area from the previous image, before the rover reached it. “&lt;strong&gt;When we were looking ahead and thinking of the obstacles that we did see, we also had to remember what was just behind&lt;/strong&gt;,” he says.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;(emphasis mine)&lt;/p&gt;&#xA;&lt;p&gt;Also, like the Apollo crews, they found it &lt;a href=&quot;https://www.cia.gov/library/readingroom/document/ws-soviet-lunokhod-control-crew-experiences-moon-orientation-problems&quot; rel=&quot;noreferrer&quot;&gt;difficult to estimate distances&lt;/a&gt; due to the lack of landmarks that have a known size.&lt;/p&gt;&#xA;&lt;p&gt;Now, some of these problems can be alleviated with modern technology. You can get better imaging than 1 frame/10 seconds. You can improve the view around the vehicle. You can add computer analysis, and maybe integration of ground-based images with overhead map data to get a better idea of distances, slopes etc.  But the fundamental problems of delay and having to interpret alien terrain remain.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-26T13:36:23.530" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="19913" PostTypeId="2" ParentId="668" CreationDate="2017-01-26T12:05:32.577" Score="4" Body="&lt;p&gt;Look up &quot;&lt;strong&gt;thrown track&lt;/strong&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tracked vehicles actually quite often lose one of their tracks, especially when turning. This is a huge issue, for both civilian an military vehicles. With enough manpower, time and winches, they can be re-tracked - on Earth. For a rover, such event would end a mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For space use, tracks are ridiculously unreliable.&lt;/p&gt;&#xA;" OwnerUserId="16410" LastActivityDate="2017-01-26T12:05:32.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19914" PostTypeId="2" ParentId="19910" CreationDate="2017-01-26T12:20:06.440" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is there an universal strategy to optimally perform this change?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes. Since the orbital plane (inclination and right ascension of ascending node) and orbital shape (semi-major axis and eccentricity, or periapsis and apoapsis distances), the two orbits must necessarily intersect in two points. A single impulsive burn at either of those two points is all that is needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an expensive operation. Suppose $\Delta \omega$ is the angle by which you wish to change the argument of periapsis. The instantaneous delta V needed to perform that optimal change is $$\Delta v = 2\sqrt{\frac{\mu}{a(1-e^2)}}\,\sin\left(\frac {\Delta \omega} 2\right)$$&#xA;Note that this is very similar in form to the $\Delta v$ needed to change the inclination by an angle $\Delta i$.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-01-26T12:20:06.440" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19915" PostTypeId="2" ParentId="19911" CreationDate="2017-01-26T13:53:06.480" Score="17" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;3s radio roundtrip and good maps would allow for fully interactive remote control, so no autonomous drive problems at these speeds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;15.4km/hour is 4.28m/s, so during that 3s round trip the rover travels nearly 13m. It would make an interesting video game to try driving a rover at that speed with that input latency, but I suspect you'd find people crashing a lot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The good news is that autonomous driving is improving all the time. It's &lt;a href=&quot;http://www.jpl.nasa.gov/news/news.php?feature=3884&quot;&gt;being used on Mars&lt;/a&gt;, and in various driver assist technologies for earth cars at highway speeds. I think any such high-speed rover would have similar driver assist to avoid potholes, boulders, and control stability in dustbowls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that, while CoM stability is the same in static analysis on the moon, it has much less righting ability in dynamic analysis because the gravitational force is lower. If you take the same vehicle on Earth and the Moon to a 45 degree angle then drop it, the Earth one will level out much faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another consideration in continuous driving is power. If you're going for solar rather than nuclear then the amount you get from vehicle-sized panels tends to be disappointing. Especially if you need to divert it to heaters, instruments, LIDAR, computers, etc. This is part of the reason that Spirit and Opportunity are so slow.&lt;/p&gt;&#xA;" OwnerUserId="1521" LastActivityDate="2017-01-26T13:53:06.480" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19916" PostTypeId="2" ParentId="19252" CreationDate="2017-01-26T16:04:24.250" Score="1" Body="&lt;p&gt;I'll &quot;amalgamate&quot; the comments below the question. The scenario is &lt;em&gt;highly unlikely&lt;/em&gt;, but either through loss of pressure or contamination of the atmosphere, either ISS crew or an arriving crew could possibly need to be in the ISS while wearing a pressure or EVA suit. The OP asks if the laptops could be operated by someone wearing said suit, pointing out that the ThinkPads seen on the ISS are equipped with built-in capacitive track pads but no external mouse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was pointed out that the ThinkPads also have the little red Trackpoint - a sort of nano-joystick, but probably those would be fairly hard to use with a gloved hand as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was then pointed out that the laptops are for non-essential tasks, so there may be little need to use them, unless of course &lt;a href=&quot;http://memory-alpha.wikia.com/wiki/The_Doomsday_Machine_(episode)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Commodore Decker&lt;/a&gt; left his final log entries on one of the laptops.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was then pointed out that in the case of a substantial or complete pressure loss, if they worked at all, they would overheat quickly. (Oh yeah, fans). Of course if the issue were a contaminant (e.g. ammonia, or CO2, or smoke) or biohazard in the atmosphere, the laptops might still work fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Enter 21st century touch-screen controls, possibly touch-screen everything. While voice command might possibly help in some cases, like they say, &lt;a href=&quot;http://www.imdb.com/title/tt0078748/taglines&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;in space, Siri can't hear you say OK Google.&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, enter Boeing's new suit, including gloves with &lt;a href=&quot;https://www.wired.com/2010/02/korean-iphone-stylus-its-made-of-meat/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;s&gt;sausages&lt;/s&gt;&lt;/a&gt; er... &lt;a href=&quot;https://www.wired.com/2010/02/korean-iphone-stylus-its-made-of-meat/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;s&gt;cheese sticks&lt;/s&gt;&lt;/a&gt;, ok advanced materials for fingertips that will work on capacitive touch sensitive screens:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/l7tyJGC0n4M?start=98&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-01-26T16:04:24.250" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19917" PostTypeId="2" ParentId="19911" CreationDate="2017-01-26T16:12:28.047" Score="5" Body="&lt;p&gt;A rover like this might be technically possible, however it's not viable because there's no benefit in doing it. The science being performed on the surface of bodies in the solar system requires staying in place for quite some time, drilling, taking samples, taking pictures, lasing things, etc. Having a rover which would have to remain almost constantly on the move would make little scientific sense.   &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2017-01-26T16:12:28.047" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19918" PostTypeId="1" CreationDate="2017-01-26T21:06:17.517" Score="1" ViewCount="211" Body="&lt;p&gt;I am working on a little orbit simulation game. I already got it working previously. But I am changing my system up a bit on how I calculate it. Previously I calculated the true anomaly with time and offset to periapsis. And now i want to change the offset to continuously updated true anomaly. But I cant get the thing to work. Please help me how I can calculate the True Anomaly in delatime seconds in the future, given Semi Major Axis and Eccentricity, and True Anomaly. Also I already tried calculating the mean anomaly from the true anomaly, and add mean motion to it, but i reversed the orbit, and made the apoapsis the fastest point. Please help me!&lt;/p&gt;&#xA;" OwnerUserId="18301" LastActivityDate="2017-01-26T21:06:17.517" Title="Calculate True Anomaly in future" Tags="|orbital-mechanics|orbit|orbital-elements|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19924" PostTypeId="1" AcceptedAnswerId="19925" CreationDate="2017-01-27T04:07:36.007" Score="7" ViewCount="8073" Body="&lt;p&gt;I was wondering if a base on Mars could be powered by solar energy alone? Since Mars is further away from the Sun wouldn't that mean lower efficiency  thus making solar not scalable? Let's assume a base the size of the Amundsen station in Antarctica, maybe a couple of hundred people.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastEditorUserId="265" LastEditDate="2017-01-27T04:56:43.063" LastActivityDate="2018-12-26T19:55:36.107" Title="Solar panels on Mars?" Tags="|mars|solar-power|energy|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19925" PostTypeId="2" ParentId="19924" CreationDate="2017-01-27T06:43:42.413" Score="12" Body="&lt;p&gt;Well, we don't really know how much energy a Mars base would need, but we can make some rough estimates.  Bases like &lt;a href=&quot;https://scienceroadshow.wordpress.com/2013/01/26/power-up-what-keeps-mcmurdo-going/&quot; rel=&quot;noreferrer&quot;&gt;McMurdo&lt;/a&gt; and &lt;a href=&quot;http://www.antarctica.gov.au/living-and-working/station-life-and-activities/power-generation&quot; rel=&quot;noreferrer&quot;&gt;Mawson&lt;/a&gt; have power capacities of several hundred to several thousand kilowatts.  Now, we're talking about a Mars base, presumably we're going to need a little more power than that.  Let's estimate that Mars Base One will require 10,000 kilowatts of power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An average solar panel will have an efficiency of ~&lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_cell_efficiency&quot; rel=&quot;noreferrer&quot;&gt;20%&lt;/a&gt;.  Some can get as high as &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjenYWI1-HRAhWJs1QKHcJjDVYQFgg1MAQ&amp;amp;url=https%3A%2F%2Fwww.greentechmedia.com%2Farticles%2Fread%2FSunPower-Again-Holds-Record-For-Worlds-Most-Efficient-Rooftop-Solar-Panel&amp;amp;usg=AFQjCNF1q7W9WX2dHZHhaaf4Fc20n3yI9Q&amp;amp;sig2=IY4zEkWtTPMU8IKqlZqAiw&quot; rel=&quot;noreferrer&quot;&gt;25%&lt;/a&gt; or &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjenYWI1-HRAhWJs1QKHcJjDVYQFgg8MAU&amp;amp;url=https%3A%2F%2Fwww.sciencedaily.com%2Freleases%2F2016%2F05%2F160517121811.htm&amp;amp;usg=AFQjCNEGIwAxvWFTxaF63Bm0uHK8doq3qw&amp;amp;sig2=YMpxVzmGP0zMXRkWuUpxGQ&quot; rel=&quot;noreferrer&quot;&gt;35%&lt;/a&gt;, but most are in the 20's, so we'll say that our solar panels have 20% efficiency.  (Hopefully by the time we launch to Mars we'll have some even better solar panels).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars receives a &lt;a href=&quot;http://www.atmos.washington.edu/2002Q4/211/notes_greenhouse.html&quot; rel=&quot;noreferrer&quot;&gt;flux&lt;/a&gt; of ~593 W/m$^2$ from the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can do a &lt;a href=&quot;http://photovoltaic-software.com/PV-solar-energy-calculation.php&quot; rel=&quot;noreferrer&quot;&gt;quick calculation&lt;/a&gt; to find how much power our solar panels will output:&#xA;$$&#xA;E = A \epsilon F r_p&#xA;$$&#xA;where $E$ is the energy, $A$ is the size of the solar panel, $\epsilon$ is the efficiency, $F$ is the flux received, and $r_p$ is the &quot;Performance Ratio&quot; of the solar panels (losses due to shading, dust, circuitry, etc).  If we assume that we have a 25% performance ratio, we can solve this equation to see how large our solar panels would need to be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$A=$300,000m$^2$, or 0.3km$^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is incredibly large.  A football field is ~5000 m$^2$, so we would have to have a total collection area of 60 football fields.  If we were somehow able to get our power usage down to ~100 kW, we could get away with having a single football field of solar panels, which is still much too expensive to lift into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can get away with using solar panels for our small rovers because they only need a &lt;a href=&quot;http://mars.nasa.gov/mer/mission/spacecraft_rover_energy.html&quot; rel=&quot;noreferrer&quot;&gt;small amount of power&lt;/a&gt;, which their solar panels can provide.  So we will need to increase the efficiency of our photovoltaic systems, decrease the amount of energy we need, or explore other sources of energy (like &lt;a href=&quot;http://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx&quot; rel=&quot;noreferrer&quot;&gt;nuclear power&lt;/a&gt; - the best form of power generation in space).&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2017-01-27T06:43:42.413" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19926" PostTypeId="2" ParentId="19908" CreationDate="2017-01-27T07:39:25.527" Score="3" Body="&lt;p&gt;Telemetry is a subset of data. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Telemetry: data related to the status of the spacecraft itself. Fuel level, temperature, engine speed, position information etc. This data is needed for the operation of the satellite.&lt;/li&gt;&#xA;&lt;li&gt;In addition to telemetry, the spacecraft's payload generates data. This is data that is useful for people other than the operator of the spacecraft. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-27T07:39:25.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19928" PostTypeId="1" AcceptedAnswerId="19931" CreationDate="2017-01-27T16:02:34.310" Score="21" ViewCount="3038" Body="&lt;p&gt;I've previously asked &lt;a href=&quot;https://space.stackexchange.com/q/19891/12102&quot;&gt;Roughly how many self-viewing cameras are present in a Falcon 9 LEO mission?&lt;/a&gt; asking if it's closer to a dozen or a hundred on a log scale, because I have a hunch there's probably three dozen imaging devices &lt;em&gt;present&lt;/em&gt; altogether, even if they're not all active at the same time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then I saw &lt;a href=&quot;https://space.stackexchange.com/a/19927/12102&quot;&gt;this answer&lt;/a&gt; - the last paragraph says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To sum it up, selfie capabilities offer no scientific value but add costs. A selfie capability is however important for marketing and PR purposes, as it allows the operator to share footage with the public. I am thinking about the glorious footage from the Falcon 9, or the propaganda-heavy Chinese Space Station footage taken from its companion satellite Banxing-2.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I think this is probably wrong as it is written - considering the continuous and rapid rate of technology evolution going on with each new launch.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-30T01:01:50.950" LastActivityDate="2018-01-12T16:49:12.350" Title="Does SpaceX use any of Falcon-9's camera data for engineering or status information, or are they &quot;just for PR&quot;?" Tags="|spacex|rockets|spacecraft-development|telemetry|camera|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19930" PostTypeId="2" ParentId="17675" CreationDate="2017-01-27T16:13:50.260" Score="2" Body="&lt;p&gt;From the Lunar Reconnaissance Orbiter (LRO), there is laser altimeter, temperature and imaging data available:&#xA;&lt;a href=&quot;https://lunar.gsfc.nasa.gov/resources.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://lunar.gsfc.nasa.gov/resources.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The altimeter data is pretty sparse, so it is good to get information about craters, large obstacles and overall slope, but not good for finding rover-sized rocks. However, the temperature data is related to the frequency of rocks so it can be used to help estimate how many smaller rocks there are in a region.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually discussion of the LRO data (for example in a  scientific paper) references Apollo data and images to make some assumptions about surface conditions.&lt;/p&gt;&#xA;" OwnerUserId="13178" LastActivityDate="2017-01-27T16:13:50.260" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19931" PostTypeId="2" ParentId="19928" CreationDate="2017-01-27T16:23:08.340" Score="36" Body="&lt;p&gt;A caveat about this answer: it's not about SpaceX directly, more about the use of self-inspection cameras in general across space and launch vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is used for engineering and status information. &quot;Selfie&quot; footage has been standard (at least on launch vehicles) since Apollo.  Telemetry offers a very limited view of things and is prone to misinterpretation in the event of unanticipated failures.  We only need to look at the last Progress failure to see how the telemetry failed to tell the whole story.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While self-inspection capability may offer no scientific value (to the extent that a satellite offers scientific value), it offers &lt;em&gt;operational&lt;/em&gt; value by providing a means to confirm or rule out potential failure situations, which may inform operational decisions on how or whether to mitigate them.  The decision whether or not to include this self-inspection capability is ultimately driven by a trade on whether the additional mass and complexity is balanced by the operational benefit it provides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the ISS, which is admittedly in a class of its own, it offers enormous benefit as an unparalleled means of continuously monitoring vehicle health.  This is of particular importance for detecting things for which there is no real plausible telemetry alternative, or for which the available telemetry is not sensitive enough to paint a complete picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To say that these cameras only offer PR value is flatly false.  The strongest evidence for this is the fact that the overwhelming majority of imagery produced by launch and space vehicle &quot;selfie&quot; cameras is never released to the public.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="-1" LastEditDate="2017-01-28T18:18:08.393" LastActivityDate="2017-01-28T18:18:08.393" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19932" PostTypeId="1" AcceptedAnswerId="38639" CreationDate="2017-01-27T16:49:36.703" Score="4" ViewCount="205" Body="&lt;p&gt;Rover missions are hard - you need to put them down somewhere where they can safely drive around and measure (hopefully a variety of) interesting things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While there has been quite a lapse in wheeled vehicles on the Moon, with &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chang'e 3&lt;/a&gt; and it's rover &lt;a href=&quot;https://en.wikipedia.org/wiki/Yutu_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yutu&lt;/a&gt;, a new era of lunar roving has begun, and with all the &lt;a href=&quot;https://en.wikipedia.org/wiki/Google_Lunar_X_Prize&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar X Prize&lt;/a&gt; activity, things are going to get interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pinpointing a landing on the moon will be difficult for the smaller craft with limited resources, there's no GPS, and the Lunar X Prize contestants may not have the resources to monitor doppler from orbit or from Earth, so the potential landing area will have to be large.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I've split this off from my earlier question &lt;a href=&quot;https://space.stackexchange.com/q/17675/12102&quot;&gt;Are there &quot;rover-friendly region maps&quot; for Mars? What about the Moon?&lt;/a&gt; which has an accepted answer for Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; &lt;em&gt;Are there &quot;rover-friendly region maps&quot; for the Moon?&lt;/em&gt; If not, at least what kind of data could be used to help make one?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-08-09T14:28:49.093" LastActivityDate="2019-09-07T17:54:12.367" Title="Are there &quot;rover-friendly region maps&quot; for the Moon?" Tags="|the-moon|rovers|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="19933" PostTypeId="2" ParentId="19928" CreationDate="2017-01-27T16:51:11.047" Score="15" Body="&lt;p&gt;There is value added. I was an operator of a satellite that had a video of the satellite being deployed. We were able to see from the video that the deployment was clean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume if nothing goes wrong, the only value is PR, but if something does go wrong, video can help considerably.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: This kind of thing is exactly what might be useful from the ZUMA mission debacle. By having the footage available they can presumably remove several branches from a root cause of failure analysis, and provide another truth to verify the sensor. For instance, if the contact sensor reported it was disconnected, you could see if that was the case. You could spot mechanical failures, which telemetry might indicate there was a problem, you could spot exactly where it happened.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-12T16:49:12.350" LastActivityDate="2018-01-12T16:49:12.350" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19934" PostTypeId="1" CreationDate="2017-01-27T17:04:00.590" Score="1" ViewCount="217" Body="&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; Please note the &lt;a href=&quot;http://lunar.xprize.org/about/guidelines&quot; rel=&quot;nofollow noreferrer&quot;&gt;following items&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Mission Requirements for claiming the &lt;strong&gt;Grand or Second Place Prize&lt;/strong&gt; include the following:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Landing – A team must land its craft on the surface of the Moon after providing advance notice of its launch and intended landing site to XPRIZE.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Mobility – After landing, a team must move its craft a distance of at least five hundred meters below, on, or above the lunar surface along an interesting path in a deliberate manner. The distance can be a straight line or may be a series of waypoints approved by the Judging Panel.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As in some games of pool, you have to indicate your intended landing area ahead of time. Your actual landing site should be close, &lt;em&gt;and&lt;/em&gt; allow for an acceptable course of travel for your rover. If you are driving a &lt;a href=&quot;https://youtu.be/6b4ZZQkcNEo&quot; rel=&quot;nofollow noreferrer&quot;&gt;lunarized Sand Flea&lt;/a&gt; maybe the second part is relatively easy. Otherwise, you don't want to end up at the bottom of a hole or wedged in a crevasse.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;As I've &lt;a href=&quot;https://space.stackexchange.com/q/19932/12102&quot;&gt;said here&lt;/a&gt; there's no GPS on the moon. I am presuming the &lt;a href=&quot;https://en.wikipedia.org/wiki/Google_Lunar_X_Prize&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar X Prize&lt;/a&gt; landers &lt;s&gt;will start&lt;/s&gt; would have started from an established lunar orbit (where they &lt;s&gt;can get&lt;/s&gt; could have gotten their bearing and wait for a final &amp;quot;Go&amp;quot; from Earth). This gives them time to receive any of various potential beacon and telemetry signals from Earth, and data from their cameras (e.g. Earth, Moon, Sun, Stars...)&lt;/p&gt;&#xA;&lt;p&gt;But once descent has started, I am not sure which of these &lt;s&gt;are&lt;/s&gt; would have been still helpful, during attitude, and especially propulsive maneuvers.&lt;/p&gt;&#xA;&lt;p&gt;I don't want to know about any competitive secrets, or &amp;quot;special sauce&amp;quot; that that various teams &lt;s&gt;are&lt;/s&gt; were counting on, but from a general perspective, what navigational information is likely to have been useful for these small, limited budget, limited development, limited previous experience lunar landing craft?&lt;/p&gt;&#xA;&lt;p&gt;If the Moon were a smooth sphere with a painted pattern on it, a nice landing might be done with some nice cameras. But the Moon has substantial topography - would they need to be ready to do a &amp;quot;Neil Armstrong&amp;quot; and look around for what looks like a good place to land? Or could they use a &lt;a href=&quot;https://space.stackexchange.com/q/19932/12102&quot;&gt;&amp;quot;rover-friendly&amp;quot; region map&lt;/a&gt; and have precise enough navigational data to put themselves down within a few meters or tens of meters of an absolute spot?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2022-02-15T12:10:36.557" LastActivityDate="2022-02-15T12:10:36.557" Title="How might the Lunar X Prize contestant spacecraft have navigated their descent from orbit, to landing?" Tags="|the-moon|lunar-landing|navigation|xprize|" AnswerCount="0" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="19935" PostTypeId="1" CreationDate="2017-01-27T18:13:59.910" Score="4" ViewCount="1591" Body="&lt;p&gt;In space, there is a lot of electromagnetic radiation from sun and other space objects. Amidst these, how are signals from Mars rovers and other space probes received?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see images of Mars by rovers, how is it noise-free and sharp? Wouldn't the image signal be distorted in space when transmitted to Earth?&lt;/p&gt;&#xA;" OwnerUserId="18315" LastEditorUserId="6816" LastEditDate="2017-01-27T18:22:27.293" LastActivityDate="2021-02-19T09:21:54.387" Title="How are signals transmitted in space?" Tags="|communication|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19936" PostTypeId="1" AcceptedAnswerId="20564" CreationDate="2017-01-27T19:31:17.607" Score="7" ViewCount="1261" Body="&lt;p&gt;It's lunar new year here and people will be lighting fire crackers and launching bottle rockets off and on until dawn, so I thought I would read up on it. EarthSky.org's Moon Phases update &lt;a href=&quot;http://earthsky.org/moon-phases/new-moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;Your new moon is January 27 or 28&lt;/a&gt; is really interesting, including some photos of actual new moons by &lt;a href=&quot;http://www.astrophoto.fr/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thierry Legault&lt;/a&gt;, the photographer who's taking those amazing photos of the ISS passing in front of the &lt;a href=&quot;http://www.astrophoto.fr/iss_atlantis_transit2_2010.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun&lt;/a&gt;, or the &lt;a href=&quot;http://www.astrophoto.fr/eclipse-ISS-20150928.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moon&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;But the next item down, &lt;a href=&quot;http://earthsky.org/astronomy-essentials/lengths-of-lunar-months-in-2017&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lengths of lunar months in 2017&lt;/a&gt;, is also really interesting both technically and visually, because it has this amazing GIF of the moon's changes in appearance over one orbit, shown below. This version is much better quality than the one from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase2.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia&lt;/a&gt; that I've used in &lt;a href=&quot;https://physics.stackexchange.com/q/251700/83380&quot;&gt;How is a result of no time variation in the gravitational constant G related to a measurement of no local expansion?&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/14943/12102&quot;&gt;What are these structures on the Lunar Ranging Retro Reflector (LRRR) arrays for?&lt;/a&gt; But I am not sure where it's from or how to read about it.&lt;/p&gt;&#xA;&lt;p&gt;The information for the Wikipedia-sourced animation is difficult for me to understand already. The period is a &amp;quot;Draconic Month of nodes&amp;quot;, but is that actually the same thing as the time between successive apogees? The issue here is how to generate a smooth, cyclic GIF, since an exact simulation would be jumpy - no two orbits are the same in the real world.&lt;/p&gt;&#xA;&lt;p&gt;Also, how is the shading calculated, &lt;a href=&quot;https://en.wikipedia.org/wiki/Shader&quot; rel=&quot;nofollow noreferrer&quot;&gt;what kind of shader&lt;/a&gt; is used (diffuse reflectivity model) for the oblique sunlight incidence?&lt;/p&gt;&#xA;&lt;p&gt;While an actual answer would be fantastic, a link to a source of the information would be helpful as well.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Source bitmap for projection from NRL's Clementine Spacecraft:&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;USGS: Global simple cylindrical projection at 10 km/pixel.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Animation&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;50 frames were created, equally spaced in time.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Period used: 27.21222 days (Draconic month of nodes)&lt;/li&gt;&#xA;&lt;li&gt;Animation began and ended at Apogee to minimize &amp;quot;jumping&amp;quot; when cycles back to the start.&lt;/li&gt;&#xA;&lt;li&gt;New moon given an unreal 1% intensity &amp;quot;Ambient light&amp;quot; to show motion in all phases.&lt;/li&gt;&#xA;&lt;li&gt;View assumed from the center of the earth. (Geometrically impossible, but best demonstration of monthly libration caused by moon's motion versus the shorter daily libration that would be included from the surface of the earth.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Animation used in &lt;a href=&quot;http://earthsky.org/moon-phases/new-moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;EarthSky.org&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; As &lt;a href=&quot;https://space.stackexchange.com/a/20564/12102&quot;&gt;pointed out here&lt;/a&gt; by the GIF's creator, the original image is actually in &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Lunar_libration_with_phase_Oct_2007.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wstX2.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wstX2.gif&quot; alt=&quot;libration of the Moon from http://earthsky.org/moon-phases/new-moon&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Animation from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase2.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/y6oHW.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y6oHW.gif&quot; alt=&quot;libration of the Moon from https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase2.gif&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-12-31T18:35:24.893" LastActivityDate="2022-05-23T00:33:13.200" Title="Who does these mesmerizing simulations of the phases of the Moon? And how?" Tags="|the-moon|simulation|space-art|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19937" PostTypeId="1" CreationDate="2017-01-27T20:39:59.557" Score="5" ViewCount="1797" Body="&lt;p&gt;Thinking of the primary propellants used for typical rocket launches (such as Hydrogen, RP-1, solid fuel), what are the odors of the exhaust and burned fuel components?  I'd think that the Hydrogen would produce minimal odor due to essentially just water being the result, but wondering what RP-1 and solid fuel exhaust would smell like.  &lt;/p&gt;&#xA;" OwnerUserId="16735" LastActivityDate="2017-01-28T03:06:33.503" Title="Odor of a rocket launch?" Tags="|launch|fuel|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19938" PostTypeId="2" ParentId="19935" CreationDate="2017-01-28T01:04:37.653" Score="6" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;In space, there is a lot of electromagnetic radiation from sun and other space objects.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, there isn't. In classical physics, electromagnetic radiation is a wave phenomenon. Normally, waves of light pass through each other unperturbed. In quantum physics, there is such a thing as two-photon physics, but this takes very energetic photons. Neither the visible light from the Sun nor the radio waves from a spacecraft qualify as &quot;very energetic&quot;. In short, there is no interference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a problem if (for example) Mars is close to being directly behind the Sun as seen from the perspective of the Earth. In this case, an Earth-bound antenna pointing toward Mars will also be pointing toward the Sun. This would be a very bad day for that Earth-bound antenna. This means that communications with Earth are suspended for about a week or two every other year for vehicles on or near Mars. Communications with Mars is not a problem outside of these solar connections.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I see images of Mars by rovers, how is it noise-free and sharp? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;By&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Designing communication protocols that are (to some extent) loss tolerant. This is, in part, the job of the Consultative Committee for Space Data Systems.&lt;/li&gt;&#xA;&lt;li&gt;Designing the communications capabilities of the spacecraft to be in line with the data to be gathered by the spacecraft. This is a collaborative effort between the designers of the spacecraft's science team, the designers of the spacecraft's communications system, the operators of the Deep Space Network.&lt;/li&gt;&#xA;&lt;li&gt;Making sure the spacecraft's antennae are properly aimed toward the Earth. This is one of the jobs of the spacecraft's guidance, navigation, and control system.&lt;/li&gt;&#xA;&lt;li&gt;Filtering downloaded data to remove effects such as salt and pepper noise. This is one of the jobs of the spacecraft data processing team.&lt;/li&gt;&#xA;&lt;li&gt;Plus doing a whole lot of other stuff that I didn't mention above.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-01-28T01:04:37.653" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19939" PostTypeId="1" AcceptedAnswerId="42122" CreationDate="2017-01-28T02:48:47.877" Score="2" ViewCount="529" Body="&lt;p&gt;I'm going to try to do a modest simulation of the Moon's libration as seen from the Earth (geocenter), similar to the technical one's I've shown in &lt;a href=&quot;https://space.stackexchange.com/q/19936/12102&quot;&gt;this question&lt;/a&gt;. I'll do it in Python and render it in Blender.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm familliar with getting Ephemeris type data from &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; and from &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt;, both of which use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Development Ephemerides&lt;/a&gt; (e.g. DE405,...), (see also Horizon's &lt;a href=&quot;http://ssd.jpl.nasa.gov/?ephemerides&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ephemerides page&lt;/a&gt; and links therein). From those I generate local tables and interpolate them for animations and other non-critical calculations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;ftp://ssd.jpl.nasa.gov/pub/eph/planets/ioms/de405.iom.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;I know that&lt;/a&gt; at least some of the DE's contain lunar body motion (e.g. in DE405, but in DE406 it comes separately? Also &lt;a href=&quot;https://github.com/skyfielders/python-skyfield/issues/79&quot; rel=&quot;nofollow noreferrer&quot;&gt;discussed here&lt;/a&gt;) as &lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html#Loading%20Auxiliary%20Files&quot; rel=&quot;nofollow noreferrer&quot;&gt;Loading Auxiliary Files/Dynamic Frames&lt;/a&gt;, but without first-hand knowledge of reading the Spice Kernel's directly, is there a way I can get something close through Horizons? For example, with the position of a fixed position on the Moon with respect to Earth's Geocenter, or vice-versa in addition to the motion of the two centers I could reconstruct it well enough for a fairly accurate animation. However, this is as far as I've gotten - I don't know how to specify a lat/lon position on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could fudge it by just spinning the moon on its axis at a fixed rotational angular velocity, but if it's possible to get a handle on the actual lunar rigid body motion, that would be much more helpful to me in the long run. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/c8NKq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/c8NKq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="8590" LastEditDate="2020-03-28T01:27:07.757" LastActivityDate="2020-03-28T01:27:07.757" Title="Ephemeris for lunar body-centered body-fixed coordinates?" Tags="|the-moon|celestial-mechanics|skyfield|jpl-horizons|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="19940" PostTypeId="2" ParentId="19937" CreationDate="2017-01-28T03:06:33.503" Score="5" Body="&lt;p&gt;So a caveat to this answer; these are qualitative descriptions based on my experience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;LOx/H2&lt;/strong&gt;: I haven't been close enough to a launch or static fire to say.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;LOx/RP1&lt;/strong&gt;: As Anthony X mentioned, RP1 is highly refined kerosene, so the smell is &lt;em&gt;similar&lt;/em&gt; to that of a barbecue lighter. One thing that I noticed is that the smell of gaseous RP1 is significantly stronger than that of the LOx/RP1 combustion products.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Solids&lt;/strong&gt;: There are dozens of propellant combinations for solids, so I'll just cover the one that I am the most familiar with - APCP. The smell is somewhat sulfuric and foul. After smelling this enough, and getting used to it, it isn't so bad. That being said, this is definitely something you don't want to smell (NO2).&lt;/p&gt;&#xA;" OwnerUserId="18321" LastActivityDate="2017-01-28T03:06:33.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19941" PostTypeId="2" ParentId="19939" CreationDate="2017-01-28T03:13:09.730" Score="1" Body="&lt;p&gt;I'm going to jump in and give an answer before Mark Adler jumps in and tells you to use Spice in lieu of some (quite possibly outdated) Development Ephemeris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My answer: &lt;strong&gt;Use Spice&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spice provides kernels for the orientations of the Earth and of the Moon with respect to the ICRF as functions of time. From those, you can compute the relative orientation of the Moon with respect to the Earth at any moment in time.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-01-28T03:13:09.730" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="19942" PostTypeId="1" CreationDate="2017-01-28T13:07:22.487" Score="2" ViewCount="311" Body="&lt;p&gt;So hypothetically if we could kick-start the core of the planet Mars and &quot;turn on&quot; its magnetic field (by say inducing an asteroid moon-ring for lack of a better description) thus shielding its atmosphere from the Sun and begin &quot;terraforming it. My question is would the magnetic field speed up the rotation of Mars and would that pull Mars in a bit closer inside the habitable zone of its orbit around our Sun thus warming the planet even more and increasing the terraforming process? Would this become a danger to our own world?&lt;/p&gt;&#xA;" OwnerUserId="18323" LastEditorUserId="6944" LastEditDate="2021-11-12T00:29:59.387" LastActivityDate="2021-11-12T00:29:59.387" Title="Mars core and its magnetic field" Tags="|mars|terraforming|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19943" PostTypeId="2" ParentId="19822" CreationDate="2017-01-28T13:21:05.777" Score="9" Body="&lt;p&gt;Completing  on Hobbes answer, user sol3tosol4 seems to have nailed my last misunderstanding.&lt;br&gt;&#xA;A simple edit to Elon's quote, make it a nice gems of information:&lt;br&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;«It's the first time the [ITS] rocket [... payload to LEO] &lt;br&gt;&#xA;will [actually] exceed the &lt;s&gt;physical size&lt;/s&gt; [dry mass] of the  rocket»&lt;/em&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Sometimes Elon tries to &amp;quot;dumb down&amp;quot; his explanations by using&#xA;imprecise language, and when he does people tend to think he's gone&#xA;crazy. Slide 28 &amp;quot;Vehicles by Performance&amp;quot; didn't make any sense to me&#xA;either, so I decided to think about it for a while. Here's what I&#xA;think:&lt;/p&gt;&#xA;&lt;p&gt;When Elon presented that slide, he was trying to convey &lt;em&gt;three&lt;/em&gt;&#xA;pieces of information, and they got garbled together.&lt;/p&gt;&#xA;&lt;p&gt;The first piece&#xA;of information was the appearance and relative size of the different&#xA;launch vehicles, which was copied over from the previous slide.&lt;/p&gt;&#xA;&lt;p&gt;The second piece of information&#xA;was the bar graph, which showed the&#xA;relative amount of payload each launcher could send to low Earth orbit&#xA;(LEO). The bar graph doesn't need a vertical scale, because it's&#xA;relative amount of payload. For example, the proposed ITS (Mars&#xA;Vehicle) could put up to 550,000 kg into LEO, and the Saturn V could&#xA;put 135,000 kg into LEO. 550,000 divided by 135,000 is about 4.07, and&#xA;the bar behind the ITS vehicle is about 4.07 times as high as the bar&#xA;behind the Saturn V vehicle. The payload numbers are already printed&#xA;below each launcher, but the bar graph makes the relative payload to&#xA;LEO of each launcher visually obvious.&lt;/p&gt;&#xA;&lt;p&gt;The third piece of information,&#xA;which was not on the slide but which Elon described, was the &amp;quot;maximum&#xA;payload to LEO&amp;quot; of each launcher, relative to the dry mass (the mass&#xA;with no payload or propellant loaded) of the launcher. For example &lt;strong&gt;the&#xA;ITS&lt;/strong&gt; (&amp;quot;Mars Vehicle&amp;quot;) can lift 550,000 kg to LEO, and the dry mass of&#xA;the rocket (from other slides) is 425,000 kg, &lt;strong&gt;a ratio of 1.29: for the&#xA;first time, a rocket can lift into orbit more than its own mass&lt;/strong&gt;, which&#xA;is a remarkable accomplishment. From what numbers I can find, the&#xA;&lt;strong&gt;Saturn V&lt;/strong&gt; could lift 135,000 kg into LEO and had 241,000 kg dry mass, a&#xA;&lt;strong&gt;ratio of only 0.559&lt;/strong&gt;, the &lt;strong&gt;Falcon Heavy&lt;/strong&gt; a ratio of about 0.67, and the&#xA;Falcon 9 Full Thrust a &lt;strong&gt;ratio of about 0.86&lt;/strong&gt; - very good but still less&#xA;than one. What Elon said about that metric had nothing to do with the&#xA;bar graph, and it was just a coincidence that the bar for the Mars&#xA;Vehicle was taller than the picture of the rocket.&lt;/p&gt;&#xA;&lt;p&gt;What Elon said was&#xA;&amp;quot;It's the first time a rocket's performance bar will actually exceed&#xA;the physical size of the rocket&amp;quot;. Change that to &amp;quot;It's the first time&#xA;a rocket's payload to LEO will actually exceed the dry mass of the&#xA;rocket&amp;quot;, and it makes sense. I'd prefer it if Elon didn't try to dumb&#xA;down the terminology, since it's confusing, but I guess he thinks it's&#xA;necessary when the audience isn't all rocket designers. At another&#xA;point in the talk he started to discuss the TEA-TEB ignition fluid&#xA;used by the Falcon rockets, but changed his mind and skipped that&#xA;part.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Source &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/54j2y4/rspacex_official_mars_architecture/#siteTable_t1_d85hec5&quot; rel=&quot;nofollow noreferrer&quot;&gt;r/SpaceX Official Mars Architecture Announcement/IAC 2016 Live Thread - Updates &amp;amp; Discussion&lt;/a&gt; from user &lt;a href=&quot;https://www.reddit.com/user/sol3tosol4&quot; rel=&quot;nofollow noreferrer&quot;&gt;sol3tosol4&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6852" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-01-30T21:52:54.113" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19944" PostTypeId="1" AcceptedAnswerId="19951" CreationDate="2017-01-28T15:21:35.497" Score="8" ViewCount="1477" Body="&lt;p&gt;I am looking for examples of station-keeping delta-V budgets from lunar orbiters to verify the results of an orbital-perturbations programm that I wrote for my master thesis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any indication on where to look? Or is this information hard to come by? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the moment the only values that I found were in &lt;a href=&quot;http://arc.aiaa.org/doi/pdf/10.2514/6.1998-4288&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper describing the mission of the Lunar Prospector on its frozen orbit (with some station-keeping maneuvers' delta-V)&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="18324" LastEditorUserId="6816" LastEditDate="2017-10-22T12:45:45.270" LastActivityDate="2017-10-22T12:45:45.270" Title="Moon orbit station-keeping delta-V budget" Tags="|orbital-mechanics|the-moon|delta-v|station-keeping|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19946" PostTypeId="2" ParentId="13403" CreationDate="2017-01-28T23:54:04.780" Score="3" Body="&lt;p&gt;You've also got the factor in that the cost of reliable reusable components is &lt;em&gt;much&lt;/em&gt; higher than single use ones, since they have to withstand much more abuse. This is one reason why Russia never seriously considered picking up the Buran programme again. Soyuz production and launches have been honed to a fine art, and it'd be very difficult to make a reusable system that's cost competitive and can be turned around as fast as a new Soyuz can roll off the production line. Wasteful? Yes. But also cost-effective in terms of cold, hard cash and production realities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are good elements about the Dragon though, and I personally suspect they will be kept and that in the long run the basics will be retained, but rationalised and cost reduced into a single-use, serial production unit &amp;mdash; provided the US decides to stay in the human spaceflight game, which is debatable. If not the Dragon module might well be licenced out to other operators who will mount it (as a single use disposable) on their own launchers &amp;mdash; Jaxa seems a likely candidate &amp;mdash; much as China uses a modified Soyuz crew section on a Long March launcher.&lt;/p&gt;&#xA;" OwnerUserId="18327" LastEditorUserId="6816" LastEditDate="2017-01-29T00:01:38.253" LastActivityDate="2017-01-29T00:01:38.253" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19947" PostTypeId="1" AcceptedAnswerId="19949" CreationDate="2017-01-29T00:26:38.517" Score="2" ViewCount="622" Body="&lt;p&gt;I'll just point to the following two questions and their associated answers for background:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/q/308155/83380&quot;&gt;For the recently reported production (January 2017) of metallic hydrogen in the laboratory - what is the evidence exactly?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/q/308290/83380&quot;&gt;Are there predictions that hydrogen could remain metallic at ambient pressure?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;http://www.bbc.com/news/science-environment-38768683&quot; rel=&quot;nofollow noreferrer&quot;&gt;comment&lt;/a&gt; then led me to the BBC news article &lt;a href=&quot;http://www.bbc.com/news/science-environment-38768683&quot; rel=&quot;nofollow noreferrer&quot;&gt;Claim made for hydrogen 'wonder material'&lt;/a&gt;, quoted extensively in the second question. One line in the discussion about the possibility of hydrogen remaining in a metallic state after removing the initial, incredibly high pressure (about 5 million atmospheres or 500 GPa):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The US space agency is also fascinated by the material. Already super-cold liquid hydrogen makes for a very powerful rocket propellant, but the dense metallic form of hydrogen promises to deliver really colossal levels of thrust that would enable huge payloads to be lifted off Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So I have to ask:&lt;/strong&gt; Has (theoretically) metastable metallic hydrogen been considered for use as a propellant?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am guessing that if hydrogen could remain metallic at ambient pressure, the density would be significantly higher than liquid hydrogen, overcoming one of the disadvantages of LH2 - giant, heavy tanks. But is there thinking that the thrust would also be significantly enhanced? &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:40:14.797" LastActivityDate="2017-01-29T12:18:13.537" Title="Has (theoretically) metastable metallic hydrogen been considered for use as a propellant?" Tags="|nasa|fuel|hydrogen|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19948" PostTypeId="2" ParentId="19947" CreationDate="2017-01-29T10:22:46.357" Score="2" Body="&lt;p&gt;We have to wait. The generation of metallic hydrogen should be reproduced by other laboratories and the stability at low pressure should be shown. Is it possible to produce not only very small samples but also large volumes necessary for rockets? How to tank a rocket with solid metallic hydrogen? How to keep it stable for hours, days, weeks at the launch site and in the rocket? How to pump it into the combustion chamber? How to convert it from the solid state in a liquid or gaseous state? How to use it for the cooling of the combustion chamber?&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-01-29T10:22:46.357" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19949" PostTypeId="2" ParentId="19947" CreationDate="2017-01-29T12:18:13.537" Score="5" Body="&lt;p&gt;Considered, yes. See e.g. &lt;a href=&quot;https://dash.harvard.edu/bitstream/handle/1/9569212/Silvera_Metallic.pdf?sequence=2&quot; rel=&quot;noreferrer&quot;&gt;this study&lt;/a&gt; from 2010: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We discuss the applications of metastable metallic hydrogen to rocketry. Metastable metallic hydrogen would be a very light-weight, low volume, powerful rocket propellant.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The main attraction of metallic hydrogen is its I&lt;sub&gt;sp&lt;/sub&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;metallic hydrogen has a theoretical I&lt;sub&gt;sp&lt;/sub&gt; of 1700 s.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And this &lt;a href=&quot;http://aip.scitation.org/doi/abs/10.1063/1.2845066&quot; rel=&quot;noreferrer&quot;&gt;paper from 2008:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If one assumes that metallic hydrogen is stable at usable temperatures and pressures, and that it can be affordably produced, handled, and stored, then it may be a useful rocket propellant. Assuming further that the available specific energy can be determined from the recombination of the atoms into molecules (216 MJ/kg), then conceptual engines and launch vehicle concepts can be developed. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;NASA is &lt;a href=&quot;https://www.nasa.gov/offices/oct/early_stage_innovation/niac/silvera_metallic_hydrogen.html&quot; rel=&quot;noreferrer&quot;&gt;looking into this&lt;/a&gt; in its 'Early stage innovation' program. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-29T12:18:13.537" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19950" PostTypeId="1" AcceptedAnswerId="20176" CreationDate="2017-01-29T15:18:22.443" Score="5" ViewCount="2304" Body="&lt;p&gt;In this &lt;a href=&quot;https://space.stackexchange.com/q/19947/12102&quot;&gt;answer&lt;/a&gt; there is discussion of some research on the potential, future use of a solid metallic hydrogen phase that would be metastable at ambient pressure as a rocket propellant. The work is futuristic of course, but there is interest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is more information in the following questions and their answers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/q/308155/83380&quot;&gt;For the recently reported production (January 2017) of metallic hydrogen in the laboratory - what is the evidence exactly?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://physics.stackexchange.com/q/308290/83380&quot;&gt;Are there predictions that hydrogen could remain metallic at ambient pressure?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...and the BBC news article &lt;a href=&quot;http://www.bbc.com/news/science-environment-38768683&quot; rel=&quot;nofollow noreferrer&quot;&gt;Claim made for hydrogen 'wonder material'&lt;/a&gt;, quoted extensively in the second question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the thinking is that it would be used as a monopropellant? Is it possible that the much heavier LOX needed for a conventional LH2/LOX engine would not even be needed?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-02-15T17:20:25.787" Title="Would (theoretically) metastable solid metallic hydrogen even need an oxidizer?" Tags="|fuel|hydrogen|monopropellant|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19951" PostTypeId="2" ParentId="19944" CreationDate="2017-01-29T18:50:17.937" Score="9" Body="&lt;p&gt;Here's a &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070035736.pdf&quot; rel=&quot;noreferrer&quot;&gt;paper on LRO stationkeeping&lt;/a&gt;. The fundamental problem is articulated on page 3:    &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;LRO will fly in a mean 50 km mission orbit that, for scientific purposes, would ideally be&#xA;  a perfectly circular orbit. Due to the lunar non-spherical gravity, however, long term&#xA;  propagations of low lunar orbits show significant altitude variations and secular drift in&#xA;  periselene and aposelene altitudes. In fact, this drift will cause the orbit's periselene&#xA;  altitude to decrease until the spacecraft collides with the surface of the moon. . . . .&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A spacecraft in an initially circular polar orbit about the moon at an altitude of 50 km will impact the Moon in approximately 41 days, assuming an altitude of 12 km for impact - the height of the tallest mountains on the moon. At 100 km altitude, the impact is delayed until about 150 days. At 200 km altitude, impact is avoided as the altitude variations are bounded. Therefore, a 200 km orbit is valuable since stationkeeping costs are eliminated. For orbits at 100 km and below, stationkeeping maneuvers are required to maintain altitude control.     &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The paper goes on to analyse stationkeeping requirements and maneuvers, and concludes:    &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In its $\Delta$V budget, LRO has 150 m/s allocated to stationkeeping for a one year nominal&#xA;  mission duration. As demonstrated through analysis, this allocation is sufficient to meet&#xA;  the five mission constraints for stationkeeping maneuvers. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="17295" LastEditorUserId="1142" LastEditDate="2017-01-29T23:46:10.013" LastActivityDate="2017-01-29T23:46:10.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19952" PostTypeId="2" ParentId="13403" CreationDate="2017-01-29T19:16:16.447" Score="4" Body="&lt;p&gt;The early Dragons had significant saltwater incursion after splashdown - not to the extent that it endangered the cargo, but enough to impact reusability.  This has improved over time but making everything completely immune to the sea costs mass and development time.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-01-29T19:16:16.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19953" PostTypeId="1" CreationDate="2017-01-29T19:53:42.943" Score="4" ViewCount="252" Body="&lt;p&gt;I'm looking at &lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/generic_kernels/pck/pck00010.tpc&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; SPICE kernel, which contains orientation data for planets and other bodies. Model description and examples in &quot;Orientation models&quot; section look pretty simple. But actual numbers make me confused.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, it says that Earth rotation along its axis (prime meridian angle) is 190.147 degrees at J2000 epoch, and I don't get it. From my understanding this angle must be between the equinox vector and prime meridian which is right in front of the sun at J2000. On 21 of December the north pole is turned directly away from the sun, so at J2000 it is about the same since J2000 is the 1st of January, the difference is about 10 degrees. So if the north pole looks away from the sun, the equinox vector should be about parallel to the Earth orbit, which is about perpendicular to the sun direction. So the prime meridian angle should be not 190 degrees, but more like 280 degrees. Why is it like so?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I even checked it in Cosmographia, a tool downloaded from SPICE website, and this &lt;a href=&quot;http://tossha.ru/p/wfueicsrhv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;visualisation&lt;/a&gt; just confirms my thoughts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, the numbers say that the declination of the Earth rotation axis starts from 90 degrees and decreases by 0.557 degrees every century. Which means that in about 16000 years Earth rotation axis will lay in ecliptic plane and it doesn't make any sense. Is it because this model should be applied on short time scales only?&lt;/p&gt;&#xA;" OwnerUserId="17295" LastEditorUserId="12102" LastEditDate="2017-01-30T02:44:27.900" LastActivityDate="2022-02-23T15:34:22.513" Title="Spice PCK planet orientation model" Tags="|orbital-mechanics|attitude|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19954" PostTypeId="2" ParentId="17675" CreationDate="2017-01-29T20:35:59.667" Score="3" Body="&lt;p&gt;A rover-friendly map needs more than altitude/slope data. It also needs information on the geology of the terrain: composition, grain size, etc. It needs to show features like a hard crust with soft sand underneath (which tripped up Spirit and Opportunity), and things like sharp rocks (which have damaged Curiosity's wheels).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of this can be derived from photos, but you may also need spectroscopic data and/or imaging in other wavelengths.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The experience of Spirit, Opportunity and Curiosity suggests we're not quite yet at the point where imaging from orbit provides enough data to be able to plot a route that doesn't contain surprises. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-29T20:35:59.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19956" PostTypeId="2" ParentId="10043" CreationDate="2017-01-29T21:00:11.580" Score="-5" Body="&lt;p&gt;The mass of the satellite definitely affects where it stays in the orbit (leo, meo or geo). Gravitational force between two objects is directly proportional to the product of their masses and indirectly proportional to the square of the distance between them. If a balloon is launched about 2km away from the Earth, it stays up. With its mass and at that distance, the gravitational force on it could be zero. Same cannot be said of a metal ball of higher mass. Objects with higher mass will require longer distance for the force of gravity to be ineffective. &lt;/p&gt;&#xA;" OwnerUserId="18338" LastActivityDate="2017-01-29T21:00:11.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19958" PostTypeId="1" AcceptedAnswerId="19976" CreationDate="2017-01-29T23:31:18.353" Score="4" ViewCount="220" Body="&lt;p&gt;According to (Burt, 1967)[1], the transverse component of the perturbing force (that is: contained in the plane of the orbit and perpendicular to the radius vector) is the only one that affects secular changes of both the semimajor axis and the eccentricity (eqs. (17) and (15)):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\widetilde{\frac{\operatorname{d}\!a}{\operatorname{d}\!t}}=\frac{2 a^{3/2}}{\mu^{1/2}} \left(1 - e_0^2 \left( \frac{a_0}{a}\right)^{3/2} \right)^{1/2} f_2&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\widetilde{\frac{\operatorname{d}\!e}{\operatorname{d}\!t}}=-\frac{3}{2} \left(\frac{p}{\mu}\right)^{1/2} e f_2&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Later in the same paper it's stated that tangential thrust $f_T$, which is effectively a combination of radial $f_1$ and transverse $f_2$ thrust, &quot;leads to a slightly more rapid enlargement of the orbit&quot;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;It is well known that the quasi-optimal strategy for eccentricity change involves exerting thrust along a inertially fixed direction in space perpendicular to the semimajor axis (Edelbaum 1961, Pollard 1997, Ruggiero 2011). Moreover, (Pollard, 1997)[2] states that the secular variation of the eccentricity with such a program is&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\left|\widetilde{\frac{\operatorname{d}\!e}{\operatorname{d}\!t}}\right|=\frac{3}{2} f \sqrt{\frac{a}{\mu}} \sqrt{1 - e^2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which is &lt;em&gt;very close&lt;/em&gt; to the Burt result &lt;em&gt;except for an $e$ factor&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How do these results not contradict the fact that only transverse $f_2$ thrust produce secular changes on semimajor axis $a$ and eccentricity $e$?&lt;/strong&gt; If $f_2$ is the only component producing secular changes, shouldn't the radial component present in $f_T$ and the inertially fixed thrust program be immaterial to the evolution of $a$ and $e$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1]: Burt, E. G. C. &quot;On space manoeuvres with continuous thrust.&quot; &lt;em&gt;Planetary and Space Science&lt;/em&gt; 15.1 (1967): 103-122.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[2]: Pollard, James E. &quot;Simplified approach for assessment of low-thrust elliptical orbit transfers.&quot; &lt;em&gt;25th International Electric Propulsion Conference, Cleveland, OH&lt;/em&gt;. 1997.&lt;/p&gt;&#xA;" OwnerUserId="10716" LastEditorUserId="10716" LastEditDate="2017-01-30T11:27:10.133" LastActivityDate="2017-01-31T08:51:04.003" Title="Secular variation of orbital elements and influence of force components" Tags="|orbital-mechanics|low-energy-transfer|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="19959" PostTypeId="2" ParentId="8224" CreationDate="2017-01-29T23:34:34.023" Score="7" Body="&lt;p&gt;The composite image cannot be made from our standard jpegs as they are log-scaled to improve the contrast. You put the linearly-scaled data into the color channels of an sRBG image and then log scale the result. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The AIA instrument filters are multi-layer metallic filters deposited on glass mirrors that were ground to the right shape. All is explained in Lemen's article that is freely available at &lt;a href=&quot;http://adsabs.harvard.edu/abs/2012SoPh..275...17L&quot;&gt;http://adsabs.harvard.edu/abs/2012SoPh..275...17L&lt;/a&gt; . Other articles describe the calibration and operations. A front filter keeps out the visible light in most channels. The CCD is not greatly heated by the radiation that gets through the filter but is cooled by a radiator pointing towards space.&lt;/p&gt;&#xA;" OwnerUserId="18341" LastActivityDate="2017-01-29T23:34:34.023" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19961" PostTypeId="1" CreationDate="2017-01-29T23:40:56.807" Score="-3" ViewCount="561" Body="&lt;p&gt;Why don't the ISS cameras ever experience technical problems at night? I never have seen the banner flash on the screen stating they are experiencing technical difficulties or switching cameras. Also, how cheap are these cameras and why would you waste potential expensive time replacing this type camera and not use high quality? It would be the first time I heard of a US government agency trying to be frugal.&lt;/p&gt;&#xA;" OwnerUserId="18342" LastActivityDate="2019-08-25T08:09:37.343" Title="Whats with all the ISS camera troubles?" Tags="|iss|camera|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19963" PostTypeId="1" CreationDate="2017-01-30T04:13:26.493" Score="35" ViewCount="60513" Body="&lt;p&gt;Currently I am 13 years old and the thought of space amazes me. I am 5 feet 7 inches (1.7 m) tall. I researched how tall you &lt;em&gt;had&lt;/em&gt; to be to be an astronaut. Sites agreed and disagreed, throwing out a bunch of different size references. The most common was 62 inches (1.6 m). That would put me at approximately 67 inches (1.7 m). I just wanted to make sure I qualified as of right now. 20/15 (6/4.5) eye sight, both eyes. Seems the space suits are expensive, and they wouldn't make a specialized one, would I qualify to become an astronaut?&lt;/p&gt;&#xA;" OwnerUserId="18343" LastEditorUserId="26446" LastEditDate="2021-09-22T08:23:16.603" LastActivityDate="2021-09-22T08:23:16.603" Title="How tall do you have to be to be an astronaut?" Tags="|astronauts|health|astronaut-recruitment|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="19964" PostTypeId="1" AcceptedAnswerId="19968" CreationDate="2017-01-30T04:16:12.187" Score="0" ViewCount="195" Body="&lt;p&gt;Considering the relativity of military at NASA, the question here is &quot;Does NASA have it's own &lt;a href=&quot;https://en.wikipedia.org/wiki/Army_and_Air_Force_Exchange_Service&quot; rel=&quot;nofollow noreferrer&quot;&gt;Army and Air Force Exchange Service&lt;/a&gt; (AAFES), or does it share with the Department of Defense?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The AAFES provides goods and services to members of the American military, so that they still have access to them even overseas.&lt;/p&gt;&#xA;" OwnerDisplayName="user16987" LastEditorUserId="16322" LastEditDate="2017-01-30T20:29:02.880" LastActivityDate="2017-01-30T20:29:02.880" Title="Does NASA have an Army and Air Force Exchange Service?" Tags="|nasa|food|ground-station|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19965" PostTypeId="2" ParentId="19963" CreationDate="2017-01-30T04:39:16.300" Score="38" Body="&lt;p&gt;Straight from the &lt;a href=&quot;https://www.nasa.gov/audience/forstudents/postsecondary/features/F_Astronaut_Requirements.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA website&lt;/a&gt;, there are actually just a few requirements you need to meet to become an astronaut:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A Bachelor's degree in engineering, biological science, physical science, or mathematics.&lt;/li&gt;&#xA;&lt;li&gt;Three years of related work experience&lt;/li&gt;&#xA;&lt;li&gt;Ability to pass a NASA space physical, which includes: having vision 20/200 (6/60) uncorrected or better, correctable to 20/20 (6/6); blood pressure of 140/90 mmHg (19/12 kPa) when sitting.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Height between 58.5 and 76 inches.&lt;/strong&gt; (from 4 feet, 10.5 inches to 6 feet, 4 inches or 149 cm to 193 cm)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If you want to become a Commander or a Pilot, the requirements are a little more stringent.  You have to have uncorrected vision of 20/100 (6/60) or better  (also correctable to 20/20 (6/6)), and a height between 62 and 75 inches (1.6 to 1.9 m).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More detailed information about how the selection process for astronauts works can be found on the &lt;a href=&quot;http://astronauts.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astronauts home page&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="33" LastEditDate="2019-10-18T08:06:18.007" LastActivityDate="2019-10-18T08:06:18.007" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="19966" PostTypeId="2" ParentId="19963" CreationDate="2017-01-30T05:02:19.357" Score="12" Body="&lt;p&gt;Qualification to become an astronaut isn't determined by ability to fit in a space suit, or at least it is unlikely to be a consideration for you. What matters is absence of any issues that would impair your ability to perform the tasks you are there to perform (and not become a liability to your crewmates), and, very importantly, the skills, knowledge, and discipline to do the job you are there to do under the circumstances it needs to be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're only going to be ready to apply in, say, 10-15 years from now, many things could be very different. The current fleet of space suits will probably be completely replaced by newer designs with a wider range of accommodation. However, some things are less flexible and less likely to become more accommodating. A spacecraft pilot station, like the driver's seat of a car or the pilot's seat of an aircraft is going to be constrained in just how wide of a range of adjustment it can offer its occupant. Nevertheless, it seems like you would have no issue in this regard.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-01-30T05:02:19.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19967" PostTypeId="2" ParentId="17661" CreationDate="2017-01-30T05:24:34.567" Score="0" Body="&lt;p&gt;To minimize thrust, you need to fire the engines when $\operatorname{d}\!v_z$ (change in v along z-plane) produced from engine can cause greatest $\operatorname{d}\!i$. Mathematically, a second-order system will incur greatest change in gain (K) when the second and first derivate of z-position variable are greatest i.e., when changing acceleration and moving rapidly. In this context, the transfer function you provided is modeled periodically and therefore, the greatest inclination change is incurred everytime the satellite crosses the point of intersection of the equatorial and orbital plane along its orbit because increased acceleration along the z-plane which is coaxial with inclination plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Practically speaking, a highly eccentric (elliptical) orbit will ease such a maneuver than say with a circular orbit, etc.&lt;/p&gt;&#xA;" OwnerUserId="18272" LastEditorUserId="12102" LastEditDate="2017-01-30T06:29:06.360" LastActivityDate="2017-01-30T06:29:06.360" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19968" PostTypeId="2" ParentId="19964" CreationDate="2017-01-30T15:50:37.653" Score="3" Body="&lt;p&gt;If I understand your question correctly, no.  The key reason is that NASA doesn't have &quot;bases&quot; the way the various defense services do -- nobody lives on-site at any of the NASA centers, even the fairly remote ones.  Thus, the only services provided are typical of large office campuses: cafeteria, gift shop, etc.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-01-30T15:50:37.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19969" PostTypeId="2" ParentId="19963" CreationDate="2017-01-30T16:37:59.437" Score="9" Body="&lt;p&gt;Remember that the first round of NASA astronauts were all experienced test pilots and aviators who &lt;em&gt;already&lt;/em&gt; had to meet a stringent range of physical &lt;em&gt;and mental&lt;/em&gt; requirements.  It's &lt;em&gt;hard&lt;/em&gt; to build aircraft and spacecraft that can accomodate more than a few inches' difference in height and width (at least not without a mass penalty).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mercury, Gemini, and Apollo spacecraft were &lt;em&gt;tiny&lt;/em&gt; - Mercury astronauts didn't sit in the capsule as much as &lt;em&gt;wear&lt;/em&gt; it, and the Apollo command module wasn't much bigger than a modern SUV in terms of interior volume.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Requirements were relaxed as more civilians joined the astronaut corps, but there will still be limits on size and general health.  The ride uphill puts a &lt;em&gt;lot&lt;/em&gt; of stress on the body&lt;sup&gt;1&lt;/sup&gt;, and because mass budgets will always be tight, seats and controls are going to be fixed at &quot;one size fits most&quot;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Having said all that&lt;/em&gt;...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As long as you take care of yourself, eat properly, move around enough to get your heart working every day, give your eyes a break (meaning, don't spend &lt;em&gt;all&lt;/em&gt; your free time in front of a screen reading tiny, tiny text) and take advantage of every educational opportunity that comes your way, you shouldn't have any problems being accepted into the astronaut corps&lt;sup&gt;2&lt;/sup&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;hr&gt;&#xA;&lt;sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;I can personally attest that riding even a moderate roller coaster like &lt;em&gt;California Screamin'&lt;/em&gt; at Disneyland is &lt;em&gt;not&lt;/em&gt; good for someone with high blood pressure.  I could feel the blood vessels in my brain screaming &quot;I canna hold her together much longer, Captain!&quot;&#xA;&lt;li&gt;Frankly, the bigger issue for prospective astronauts is whether NASA will have a viable manned program in the future.  Right now, things don't look good, but in another 10 to 15 years, who knows? &#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2017-01-30T16:37:59.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19970" PostTypeId="1" AcceptedAnswerId="19974" CreationDate="2017-01-30T17:19:21.490" Score="1" ViewCount="591" Body="&lt;p&gt;Say I have two different orbits: orbit A and orbit B , defined from these Keplerian orbital elements, which I know relative to some refence plane and reference direction (which are the same for these two orbits):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$i$: Inclination&lt;br&gt;&#xA;$\Omega$: longitude of ascending node&lt;br&gt;&#xA;$\omega$: argument of periapsis&lt;br&gt;&#xA;$v$: true anomaly of  at a specific time&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The elements are illustrated here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/21vx0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/21vx0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a way for me to figure out these orbital elements of orbit B but with orbit A as reference plane, and the true anomaly of orbit A as the reference direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would prefer a simple calculation, but since I am going to use this in a space flight simulation a recursive algorithm solution should also be okay. &lt;/p&gt;&#xA;" OwnerUserId="18350" LastEditorUserId="6750" LastEditDate="2017-01-30T18:43:58.573" LastActivityDate="2017-01-30T22:44:58.463" Title="Find keplarian orbital elements between orbits" Tags="|orbital-mechanics|orbital-elements|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19971" PostTypeId="2" ParentId="19963" CreationDate="2017-01-30T19:36:05.523" Score="19" Body="&lt;p&gt;As other answers have already explained, the basic height requirement is 4 feet, 10.5 inches to 6 feet, 4 inches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, just meeting that (and the other &lt;a href=&quot;https://space.stackexchange.com/a/19965/11652&quot;&gt;basic specifications&lt;/a&gt;) isn't going to get you far. I actually applied for &lt;a href=&quot;https://astronauts.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;the NASA candidate program&lt;/a&gt; several years back (2012 or so?), shortly after I'd met all of the listed requirements (they have to pick &lt;em&gt;someone&lt;/em&gt;, right?). I got a brief response from NASA a couple months later (&lt;strong&gt;heavily&lt;/strong&gt; paraphrased): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Thanks for your application, you are one of 2500+ applicants nationwide whose resume didn't go straight in the trash for not even meeting the requirements. Please don't send follow up letters, or &lt;a href=&quot;https://topatoco.com/products/qw-astro&quot; rel=&quot;noreferrer&quot;&gt;expect any further response&lt;/a&gt; unless you are one of the 50-ish candidates we short-list to interview in person for the 15 crew and back-up crew positions we expect to be available in 3-4 years.&quot;&lt;br&gt;&#xA;  &lt;sub&gt;I may still have the letter somewhere if people really want to see the exact phrasing?&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If this is something you really, really are willing to dedicate your life to, look at the typical profiles of &lt;a href=&quot;https://www.nasa.gov/astronauts/biographies/astronauts/former&quot; rel=&quot;noreferrer&quot;&gt;past crew members&lt;/a&gt; for reference and try to acquire some additional skills above and beyond the short list NASA gives. Based on my own research and on some things mentioned in the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/606877main_FS-2011-11-057-JSC-astro_trng.pdf&quot; rel=&quot;noreferrer&quot;&gt;application brochure&lt;/a&gt;, some of your options for standing out include (but certainly aren't limited to):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Become a military pilot (to be able to land the return craft if needed)&lt;/li&gt;&#xA;&lt;li&gt;Get (and stay) in top physical shape (no need to body build, just all-around endurance)&lt;/li&gt;&#xA;&lt;li&gt;Get 1 or more advanced degrees in aerospace-related engineering/technology fields (understanding and practicing all the different fields involved)&lt;/li&gt;&#xA;&lt;li&gt;Teach/work in an engineering/technology field for a few years&lt;/li&gt;&#xA;&lt;li&gt;Learn Russian (lots of interaction with the Russian program for the foreseeable future)&lt;/li&gt;&#xA;&lt;li&gt;Get scuba certified and experienced (micro-gravity activities in awkward gear)&lt;/li&gt;&#xA;&lt;li&gt;Get EMT certified (medical care in emergency situations)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Even with all of the above though, there just aren't that many spots on the NASA program. However commercial programs (SpaceX, Blue Horizon, etc) are reasonably likely to have manned programs by the time you are old enough to participate, and are likely to have similar needs/requirements. But even if you never did get into space (I doubt I will at this point), there are many ways to be involved in space exploration, and any and all preparation you may make to be an astronaut will open some amazing career opportunities to you in any field you decide to pursue.&lt;/p&gt;&#xA;" OwnerUserId="11652" LastEditorUserId="11652" LastEditDate="2018-03-11T03:44:33.083" LastActivityDate="2018-03-11T03:44:33.083" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19972" PostTypeId="5" CreationDate="2017-01-30T19:45:35.753" Score="0" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Transport_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Interplanetary Transport System&lt;/a&gt; (ITS), formerly known as the Mars Colonial Transporter (MCT), is SpaceX's privately funded development project to design and build a system of spaceflight technology and remote human settlements on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initially in crew with cargo form and a tanker/space-fuel-depot form. Further engineering would lead to a &quot;full cargo form&quot; bringing up to 450 tons to Mars. With demand and established Mars infrastructure a &quot;full crew form&quot; could bring up to 200 passengers.&lt;/p&gt;&#xA;" OwnerUserId="6852" LastEditorUserId="6816" LastEditDate="2017-02-01T03:52:55.717" LastActivityDate="2017-02-01T03:52:55.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19973" PostTypeId="4" CreationDate="2017-01-30T19:45:35.753" Score="0" Body="The Interplanetary Transport System (ITS) is a super-heavy-lift launch vehicle in development by SpaceX, composed of a booster (also known as BFR) and a spaceship (also known as BFS)." OwnerUserId="6852" LastEditorUserId="6816" LastEditDate="2017-02-01T03:53:17.047" LastActivityDate="2017-02-01T03:53:17.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19974" PostTypeId="2" ParentId="19970" CreationDate="2017-01-30T22:44:58.463" Score="2" Body="&lt;p&gt;I'm also not sure I totally understand the question, so correct me if I misunderstood you. Here's what I understood: you have the orbital elements of two orbits A and B, and would like to define orbit A as the new reference frame, and express the orbit B with respect to that reference frame A.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, yes, that's possible, and not to difficult. In effect, an orbit is simply a rotation. We can compute rotations in different ways. I prefer Modified Rodriguez Parameters, but these aren't that popular yet, so let's use Euler Parameters (also called Euler Angles).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me explain. Start from an Earth Inertial Frame. First, do a rotation by the Z (or third) axis of your right angle of the ascending node. Then do a rotation by the X axis (or first axis) by your inclination. Finally, do a rotation about the Z axis (again) of your true anomaly. So, in terms of Euler parameters, you've done a 3-1-3 rotation (i.e. multiply the R3, R1 and R3 matrices together computed with the correct angles to get your direct cosine matrix (DCM)).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using DCMs, you just need to multiply both DCMs to change the reference frame, and of course multiply that with the R vector of your spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I highly recommend &quot;Analytical Mechanics of Space Systems (AIAA Education Series) 2nd Edition&quot; by Dr. Schaub and Dr. Junkins for any astrodynamics question by the way. The above computation is thoroughly explained in chapter 3.)&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-01-30T22:44:58.463" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19976" PostTypeId="2" ParentId="19958" CreationDate="2017-01-31T08:51:04.003" Score="4" Body="&lt;p&gt;The formulas (15) and (17) are given under the assumption that both $f_1$ and $f_2$ (the radial and the transverse components of the perturbing force) stay &quot;sensibly constant over one orbital period&quot;. In particular, it means that the angle between the force and the radius vector stays sensibly constant. The two cases you are talking about do not satisfy this assumption: if the force is tangential and the orbit is not circular, then the angle between the force and the radius vector depends on the body's position on the orbit. If the force is applied in an inertially fixed direction, the angle varies even more.&lt;/p&gt;&#xA;" OwnerUserId="12448" LastActivityDate="2017-01-31T08:51:04.003" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19977" PostTypeId="1" AcceptedAnswerId="19978" CreationDate="2017-01-31T12:48:13.863" Score="5" ViewCount="646" Body="&lt;p&gt;I wonder how useful SpaceX' second stage would be as a propulsion stage docking in LEO or so with separately launched payload. It seems to &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/&quot; rel=&quot;noreferrer&quot;&gt;carry 92 tons&lt;/a&gt; of propellant on the launch pad. How much could it retain after reaching LEO or some other specific trajectory of common interest, on a Falcon 9 and a Falcon Heavy respectively? Is there any good reason not to use the second stage itself as an in-space booster?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-02-21T19:57:15.167" Title="How much fuel can Falcon 9's upper stage bring to orbit, without any other payload?" Tags="|spacex|falcon-9|upper-stage|mass-fraction|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19978" PostTypeId="2" ParentId="19977" CreationDate="2017-01-31T13:09:14.660" Score="6" Body="&lt;p&gt;Should be pretty close to the payload to LEO: 22.8 tons. Maybe a bit more if you design a new second stage with larger tanks and a nose cone. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-01-31T13:09:14.660" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="19979" PostTypeId="1" CreationDate="2017-01-31T14:02:18.227" Score="3" ViewCount="701" Body="&lt;p&gt;I am working on a project of simulating orbit maneuvering. I have interfaced SGP4 with my current simulation code, however I realized that the orbit maneuvering cannot be done using SGP4. I am aware of the methods where all the perturbations are integrated and the state propagates. However, I haven't been able to find resources for doing the same. What are some good resources/modules which can do so. In GMAT I cannot add custom orientations as thrust that I am giving is dependent on the orientation of the Satellite. I also want to know the actual process which is followed while simulating orbit maneuvering of Satellites. (From the state vectors to the propagate step to the error convergence) &lt;/p&gt;&#xA;" OwnerUserId="16437" LastActivityDate="2017-11-19T18:30:25.723" Title="Orbit Maneuvering" Tags="|orbital-mechanics|orbit|orbital-maneuver|thrust|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19980" PostTypeId="2" ParentId="19979" CreationDate="2017-01-31T16:24:17.920" Score="4" Body="&lt;p&gt;It's difficult to do with SGP4, since it's a mean element theory - here's a pretty good post on what that means: &lt;a href=&quot;https://space.stackexchange.com/questions/14731/nuances-of-the-terms-mean-osculating-keplerian-orbital-elements&quot;&gt;Nuances of the terms (mean / osculating / Keplerian / orbital) elements&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you take the state vector given by SGP4, apply a delta-V to it, and try to stuff the elements back into a TLE, you're not going to get exactly what you expect.  It really depends on what level of accuracy you need and how far out you're interested in propagating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's an ugly hack, but I have seen people simulate low, continuous thrust maneuvers by manipulating the drag parameter of the TLE.&lt;/p&gt;&#xA;" OwnerUserId="4709" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-01-31T16:24:17.920" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19982" PostTypeId="1" AcceptedAnswerId="20004" CreationDate="2017-01-31T17:27:52.690" Score="3" ViewCount="743" Body="&lt;p&gt;There's this ingenious little hack video: &quot;&lt;a href=&quot;https://www.youtube.com/watch?v=8fOG5IZLECk&quot; rel=&quot;nofollow noreferrer&quot;&gt;How to make tritium nuclear battery at home&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea is very simple: &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tritium_radioluminescence&quot; rel=&quot;nofollow noreferrer&quot;&gt;tritium keychain lights&lt;/a&gt;&quot; - tubes with tritium and luminophor - placed between solar batteries, they provide light, converted by the batteries to electricity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The battery in the video provides very little electricity, but it looks like the solution could be easily scaled up - it only uses two tiny keychain lights, &lt;a href=&quot;https://www.aliexpress.com/item/High-qaulity-1-piece-Automatic-light-15-years-2mm-12mm-betalight-tritium-tube-for-key-chain/32706718460.html?spm=2114.01010208.3.67.8fe1LE&amp;amp;ws_ab_test=searchweb0_0,searchweb201602_4_10065_10068_10000032_119_10000025_10000029_430_10000028_10060_10062_10056_10055_10000062_10054_10059_10099_10000022_10000012_10103_10000015_10102_10096_10000018_10000019_10000056_10000059_10052_10053_10107_10050_10106_10051_10000053_10000007_10000050_10084_10118_10083_10000047_10080_10082_10081_10110_10111_10112_10113_10114_10115_10037_10000041_10000044_10078_10079_10077_10000038_429_10073_10000035_10121,searchweb201603_1,afswitch_4,single_sort_1_default&amp;amp;btsid=07de43df-2a38-4c81-ad46-f9d3cb96ccf2&quot; rel=&quot;nofollow noreferrer&quot;&gt;$33/pc&lt;/a&gt;, and cheapest, dollar-store &quot;garden solar light&quot; solar batteries. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Half-life of Tritium is 12.3 years, so it would not perform nearly as well as 238-Pu with half-life of about 87,7 years on long-term missions like Voyager, but the simplicity, low mass and accessibility are so far superior, it seems like a tritium battery based on thin film solars, of the same mass as a typical Pu-238 based RTG, on timescale of ~25 years - missions like MSL Curiosity rover - could outperform the Plutonium counterpart by strides, and in fraction of the price.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why isn't this kind of power source not more popular?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-02-02T12:58:15.870" Title="Tritium as energy source for probes?" Tags="|power|nuclear|rtg|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19983" PostTypeId="1" CreationDate="2017-01-31T18:52:50.223" Score="1" ViewCount="147" Body="&lt;p&gt;Why solid rocket boosters or hybrid engines are favored towards larger hydrogen content, not carbon (C+LOX, C+NO2, C+N2H4O3)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any known data on Isp and issues of such combinations (both solid and hybrid with significant solid carbon content)?&lt;/p&gt;&#xA;" OwnerUserId="11701" LastActivityDate="2017-01-31T21:09:30.690" Title="Hybrid / solid rocket engines with large carbon content" Tags="|rockets|hybrid-rocket|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19984" PostTypeId="1" CreationDate="2017-01-31T20:03:12.413" Score="4" ViewCount="2697" Body="&lt;p&gt;The Earth's moon is one of the largest natural resources of our planet. My question: how much would it cost to build an internationally accessible commercial facility (moonbase) on the Moon?&lt;/p&gt;&#xA;" OwnerDisplayName="user16987" LastEditorUserId="1142" LastEditDate="2017-02-02T01:40:35.577" LastActivityDate="2017-03-04T19:37:59.757" Title="How much would it cost to build an international commercial moonbase?" Tags="|the-moon|commercial|international-politics|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19985" PostTypeId="2" ParentId="19983" CreationDate="2017-01-31T20:56:16.417" Score="2" Body="&lt;p&gt;Specific impulse of an engine is directly proportional to exhaust velocity - and hydrogen, being the lightest of elements (a H2 particle being just two protons + 2 electrons), using the same amount of energy can be accelerated to much higher velocities than any other elements. A single Carbon is about six times heavier as a H2 particle - and only in compounds, like CO or CO2 it takes gaseous form (the resulting particles even heavier) - raw carbon particles are a solid (soot) that on top of having a total zero of adiabatic expansion ratio (consuming energy of combustion as heat, without producing any thrust), tends to gunk up the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The development of fuels is always trying to find the middle ground between reaction as energetic as possible, and combustion products as light as possible; hydrogen reacting with many other substances can produce good energy and particles that are light - HF, HCl, H2O. Carbon can provide more energy, but with its four bounds it will necessitate much heavier exhaust compounds. While you still do gain ISp by increase of combustion reaction energy, you run into new problems: chamber pressure, chamber temperature, residue build-up, and so on. Lighter exhaust products give higher performance without these trade-offs - at cost of lower fuel energy density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for data, I can recommend the book &lt;a href=&quot;http://www.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ignition!&lt;/a&gt; by John D. Clark. The chapter &quot;Performance&quot; explains the theory behind achieving as high performance as possible, and the whole book is filled with examples of more or less successful struggles to find propellants where the exhaust is as light as possible (and the even less successful ones where the exhaust products were heavier, or contained solids).&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-01-31T21:09:30.690" LastActivityDate="2017-01-31T21:09:30.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19986" PostTypeId="2" ParentId="19982" CreationDate="2017-01-31T21:57:14.093" Score="1" Body="&lt;p&gt;Among other issues, tritium is hard to come by ($$). For a long time the US was not producing it at all. Only about &lt;a href=&quot;https://en.wikipedia.org/wiki/Tritium#Production_history&quot; rel=&quot;nofollow noreferrer&quot;&gt;500 pounds&lt;/a&gt; have ever been produced in the US.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-01-31T22:51:13.010" LastActivityDate="2017-01-31T22:51:13.010" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="19988" PostTypeId="1" AcceptedAnswerId="19996" CreationDate="2017-01-31T22:07:47.910" Score="3" ViewCount="279" Body="&lt;p&gt;In other words, how big should an &lt;a href=&quot;http://en.wikipedia.org/wiki/Entomopter&quot; rel=&quot;nofollow noreferrer&quot;&gt;entomopter&lt;/a&gt; be so that it could fly in Mars' atmosphere and carry a payload of 1-2 kg? I'm trying to find out if an entomopter could be used to carry a spectral sensor camera (special kind of spectral sensor that would be smaller than those already existing). I'm also considering a robotic &lt;a href=&quot;http://en.wikipedia.org/wiki/Ornithopter&quot; rel=&quot;nofollow noreferrer&quot;&gt;ornithopter&lt;/a&gt; for that mission. Do you know any other kinds of &lt;a href=&quot;http://en.wikipedia.org/wiki/Micro_air_vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;MAV&lt;/a&gt;'s or UAV's suitable for carrying spectral sensors?&lt;/p&gt;&#xA;" OwnerUserId="18363" LastEditorUserId="1142" LastEditDate="2017-02-01T14:48:21.503" LastActivityDate="2017-02-01T18:01:05.443" Title="Is it possible for entomopter to fly on Mars carrying payload of 1-2kg?" Tags="|mars|design|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="19989" PostTypeId="2" ParentId="19961" CreationDate="2017-01-31T23:13:16.740" Score="2" Body="&lt;p&gt;The technical difficulties NASA refers to are those regarding communications (LOS) with the space station's feed and/or cameras switching. The ISS tracking stations lose contact with the space station during certain times of the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, don't compare consumer-level electronics with those used in aerospace. Lots of factors such as temperature resistance, radiation hardening, camera interface, power requirements, etc. go into choosing or designing a camera to go on the ISS.&lt;/p&gt;&#xA;" OwnerUserId="18272" LastActivityDate="2017-01-31T23:13:16.740" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19990" PostTypeId="2" ParentId="8715" CreationDate="2017-02-01T00:09:17.590" Score="3" Body="&lt;p&gt;This information can be found in a document produced by the Historic American Engineering Record as part of their effort to document the Space Transportation System.  The pdf document is located &lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/1d.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (too long to post the log).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-02-01T00:09:17.590" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="19991" PostTypeId="1" CreationDate="2017-02-01T03:37:10.920" Score="10" ViewCount="698" Body="&lt;p&gt;I am interested in orbital refueling, not for satellites (though that’s a good idea) but for manned exploration and infrastructure. For instance, to support a lunar base we would want regular flights with reusable craft. The best mission profile would depend on an inter-orbital transfer vehicle that never lands. Instead it shuttles between LEO and lunar rendezvous with landers that stay at the moon and never return to earth. There is a lot of refueling in these schemes. Space-X’s Mars plan for instance, depends heavily on refueling in orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quick search shows there have been some refueling studies as well as tests and at least some space station refueling. They discuss the problem of boil-off of cryogenic fuels, and list various solutions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But one thing I have not seen much about is liquid surging and settling. When under thrust it’s not a problem, but in micro gravity, how can we keep the liquid at the drain? Bladders wouldn’t work with most fuels. Centrifugal propellant settling would certainly work, but how? A rotating baffle could spin the liquid in the tank, but it would create turbulence against the walls. Also the way oxygen tanks like to explode, it would be nice to keep machinery out of the tanks. That’s what happened to Apollo 13. &#xA;One could rotate the whole depot and either have a counter-rotating connection, or spin the ship when it docks, &lt;em&gt;a la&lt;/em&gt; 2001. But that’s an awful lot of work just to feed a drain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone know how it is envisioned to handle weightless liquids in tanks?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="12102" LastEditDate="2017-08-15T16:23:35.410" LastActivityDate="2020-07-18T19:06:43.210" Title="Liquid settling in tanks during orbital refueling" Tags="|fuel-depots|refueling|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19992" PostTypeId="2" ParentId="19991" CreationDate="2017-02-01T06:25:53.927" Score="4" Body="&lt;p&gt;Ullage (i.e. getting the fuel at the end the tank you want it to be at and the pressurant at the other end) for large rocket stages in free-fall is usually handled with small auxiliary thrusters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For small thrusters, it's practical to use bladder pressurization or some other solution; you use the small thrusters to slightly accelerate the large stage, and the fuel settles at the tail end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems reasonable to use the same strategy for a fuel depot. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-02-01T06:25:53.927" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19993" PostTypeId="2" ParentId="19984" CreationDate="2017-02-01T08:08:39.590" Score="5" Body="&lt;p&gt;Since we don't have any moonbases yet, this is going to be difficult to answer. So the following will guesswork with a few bits of data in between. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only large-scale construction we've built in space is the ISS. &#xA;Building the ISS &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station_program#Costs&quot; rel=&quot;nofollow noreferrer&quot;&gt;cost on the order of $100 billion&lt;/a&gt;. The ISS can house about 10 people, which is reasonable for a scientific outpost, but really small for a commercial venture.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we assume mining is the goal of this venture: a large gold mine on Earth employs ~&lt;a href=&quot;https://en.wikipedia.org/wiki/Super_Pit_gold_mine&quot; rel=&quot;nofollow noreferrer&quot;&gt;500 people&lt;/a&gt;, and uses thousands of tons of equipment to move 15 million tons of rock. It produces 28 tons of gold/year, sold for $420 million/year. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Transporting the moonbase and industrial equipment to the Moon isn't going to be easy. A Saturn V could only transport about &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;15 tons&lt;/a&gt; to the Moon's surface (and about 5 tons of that would take off again). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today, going to the Moon is just about feasible for scientific missions. Thousand-ton commercial ventures are way out of reach. Even the most precious materials mined on the Moon would be orders of magnitude more expensive than their counterparts mined on Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Projects like the &lt;a href=&quot;https://en.wikipedia.org/wiki/ITS_launch_vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX ITS&lt;/a&gt; aim to change that, with a projected per-launch cost of $60 million for 300 tons to LEO. That's 2 times the payload of the Saturn V, so it might be able to land a single ISS module (weight around 20 tons for the large modules) in one piece. &#xA;Let's try transporting 1000 tons of equipment to the Moon this way. That's 50 launches, or 3 billion in launch cost. Just transporting the materials to the Moon costs more than you can possibly make mining expensive stuff like gold. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-03-04T19:37:59.757" LastActivityDate="2017-03-04T19:37:59.757" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="19994" PostTypeId="2" ParentId="19991" CreationDate="2017-02-01T14:00:35.423" Score="6" Body="&lt;p&gt;Shuttle orbital manuevering / attitude system tankage used screens to ensure that fluid always remained at the tank exit. Sufficient fluid would cling to the screens to get the engines started.&lt;/p&gt;&#xA;&lt;p&gt;The screens are shown in this schematic of an Orbital Maneuvering System tank.&#xA;In this case the screens are only at the aft because this system always provided thrust in roughly the tank axial direction. (schematic from the Crew Operations Manual, linked below)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zR0zY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zR0zY.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The spherical Reaction Control System tanks needed to feed at any (or no) acceleration vector and had screened galleries that girdled the inside of the tanks.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sc1eq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sc1eq.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image source: &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19840019702.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;DESIGN AND DEVELOPMENT OF SHUTTLE GET-AWAY-SPECIAL EXPERIMENT 6-0074&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt; states:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The forward RCS propellant tanks have propellant acquisition devices&#xA;designed to operate primarily in a low- gravity environment, whereas&#xA;the aft RCS propellant tanks are designed to operate in both high and&#xA;low gravity, ensuring adequate propellant flow during all phases of&#xA;flight. A compartmental tank with individual wire mesh screen devices&#xA;in both the upper and lower compartments supplies propellant&#xA;independent of tank load or orientation. A barrier separates the upper&#xA;and lower compartments in each tank.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It's conceivable that such screen/acquisition gallery technology may be applicable to large-scale free-fall refueling designs.  It does add weight to the tankage, but at least has the advantage of being a completely passive system.  I don't know if it's been tested in a free-fall, high-flow situation (as I assume the refueling ops would be).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-07-18T19:06:43.210" LastActivityDate="2020-07-18T19:06:43.210" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="19995" PostTypeId="2" ParentId="19988" CreationDate="2017-02-01T17:16:45.543" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/522Colozza.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This study&lt;/a&gt; suggests that an entomopter with reasonable wingspan (on the order of 1 m) could achieve a 1 kg payload, if powered by a highly efficient engine running on e.g. hydrazine.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Based on the research conducted during this NIAC Phase II study, a design space has been identified in which Mars Entomopter flight is practical. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-01T17:16:45.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19996" PostTypeId="2" ParentId="19988" CreationDate="2017-02-01T17:55:16.350" Score="7" Body="&lt;p&gt;As Hobbes mentioned, there was study funded by the seed program &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Institute_for_Advanced_Concepts&quot;&gt;NASA Institute for Advanced Concepts&lt;/a&gt; in 2002 to look into the viability of entomopters for Mars exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The results are available &lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/522Colozza.pdf&quot;&gt;online&lt;/a&gt;. The team relied mainly on design analysis, but appears to have built a working version of the vehicle's &quot;reciprocating chemical muscle&quot; (RCM) for a small prototype.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The study's primary design point on page 49 suggests an entomopter with a 1.2m wingspan could have a relative lifting capacity of 1.5kg on Mars. Later, the study clarifies that only some of this mass is available as payload:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The weight of the RCM system plus a 10 minute fuel charge is on the order of 0.42 kg. &lt;strong&gt;Based on Table 3-2, this would leave 1.08 kg for mission payload equipment.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The RCM system in the proposed entomopter uses fuel, not electricity, meaning a user looking for more than 10 minutes of flight would have to sacrifice even more payload mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The study itself suggests using in-situ resource production to help overcome this, but this is itself another unproven technology. Even returning to a refueling station containing stockpiled fuel seems likely to be difficult - the proposed vehicle is not capable of hovering, making a precision landing unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In conclusion: it seems plausible that an entomopter could one day lift a payload on the scale you describe on Mars, but the vehicle does not seem like a great fit for the current model of slow, long term robotic exploration. To my knowledge, NASA has not funded a follow-up study for the concept.&lt;/p&gt;&#xA;" OwnerUserId="5851" LastEditorUserId="5851" LastEditDate="2017-02-01T18:01:05.443" LastActivityDate="2017-02-01T18:01:05.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="19997" PostTypeId="1" AcceptedAnswerId="19998" CreationDate="2017-02-01T20:42:28.473" Score="4" ViewCount="616" Body="&lt;p&gt;The Apollo moon landing missions used a Lunar Orbit Rendezvous (LRO) approach where the Command Service Module would be in a low lunar orbit (LLO) while the Lunar Module would land on the moon. This approach saved a significant amount of delta-v when compared to the direct ascent approach, because the heavy CSM did not need to be landed on the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did NASA consider extending the LOR approach to have the CSM be in a highly elliptic orbit around the moon, for example 50,000 km by 110 km, which should save delta-v compared to the LLO? If it was considered, why was the LLO approach used instead of the highly elliptic lunar orbit approach?&lt;/p&gt;&#xA;" OwnerUserId="11820" LastActivityDate="2017-02-03T06:06:09.360" Title="Was an elliptical orbit considered for the CSM when planning the Apollo moon landing missions?" Tags="|orbit|apollo-program|delta-v|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="19998" PostTypeId="2" ParentId="19997" CreationDate="2017-02-01T20:54:09.057" Score="6" Body="&lt;p&gt;I cannot see into their minds fifty years ago, but I'm sure that executing the rendezvous itself was a significant factor. It is far easier to achieve a rendezvous with a circular or nearly circular orbit. Six elements to match become four, and there are simple approaches to phasing and terminal rendezvous in a circular orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-02-01T20:54:09.057" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20001" PostTypeId="2" ParentId="19991" CreationDate="2017-02-02T06:33:29.857" Score="4" Body="&lt;p&gt;For the Salyut 6 space station, both Progress cargos and the station used bladder tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tanks were pressurised using nitrogen. The nitrogen could be recovered once the operation was done (at least on the station side).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To vent the pipes, they just let them open, exposed to vacuum for a week then purged with nitrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Source:The Story of Space Station Mir by David M Harland&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6642" LastEditDate="2017-02-03T01:59:06.913" LastActivityDate="2017-02-03T01:59:06.913" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20002" PostTypeId="1" AcceptedAnswerId="20003" CreationDate="2017-02-02T10:04:16.493" Score="20" ViewCount="2489" Body="&lt;p&gt;On the recent &lt;a href=&quot;https://youtu.be/7WimRhydggo?t=23m22s&quot; rel=&quot;noreferrer&quot;&gt;Iridium 1 launch&lt;/a&gt; the separation of the first and second stages can be seen at T+3:50. In the following screenshot we see the second stage burning at the lower left of the frame and the first stage dropping away at the upper right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What are the two bright objects perceived between the first and second stages in the image?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cO8Ka.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cO8Ka.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11357" LastEditorUserId="51" LastEditDate="2017-02-02T18:26:44.213" LastActivityDate="2017-02-02T18:26:44.213" Title="What are the two bright objects separating from the Falcon 9?" Tags="|launch|spacex|reuse|falcon-9|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20003" PostTypeId="2" ParentId="20002" CreationDate="2017-02-02T10:25:38.530" Score="24" Body="&lt;p&gt;I think that's the fairing (the shell around the satellites on top of the second stage).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fairing separates into 2 halves, those are the only objects large enough to be visible in this photo. At T+3:18 of that video, you can hear one of the technicians say &quot;Fairing separation confirmed&quot;. Right after that, the first bright object separates from the second stage. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-02T10:25:38.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20004" PostTypeId="2" ParentId="19982" CreationDate="2017-02-02T12:58:15.870" Score="6" Body="&lt;p&gt;In a keychain light, you have maybe 5 cm&lt;sup&gt;3&lt;/sup&gt; of tritium, or 0,00045 grams of &lt;sup&gt;3&lt;/sup&gt;H. This makes a tritium light cheap even though Tritium costs \$30,000/kg to make. If 5 cm&lt;sup&gt;3&lt;/sup&gt; can &lt;a href=&quot;https://www.youtube.com/watch?v=KKdzhPiOqqg&quot;&gt;generate 1 $\mu$W&lt;/a&gt;, you need 224.5 kg to make 500 W, or about as much as has ever been produced in the world. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would cost $6.7 million at current prices, which is unrealistic because you'd need to invest in new production facilities to make this much.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With 224 kg of tritium alone (plus solar panels and a pressure vessel), your device would be much heavier than an RTG (a 500W RTG is in the region of 80 kg). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-02T12:58:15.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20006" PostTypeId="2" ParentId="20002" CreationDate="2017-02-02T15:37:11.853" Score="10" Body="&lt;p&gt;They look like the fairing separating. It turns out that if you reuse the first stage, but throw away the fairing and second stage, the next most valuable thing that might be worth recovering is actually the fairing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are large, somewhat complex structures. Need to be light, strong and large. There has been speculation that they are attaching thrusters of some kind to the fairings to test different approaches to recovering the fairings from launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has been speculation that one of the recovery ships, based on its location during ASDS operations is trying to recover the fairing if it made it back through the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The beauty of the SpaceX model is that with so many launches, they can keep trying different ideas until they get it just right. Their model includes the cost of the fairing in the launch already, so experimenting with it after separation can be a cheap way to test things that are very hard to test on the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see this with their recovery of the first stage attempts, where they kept trying with minor tweaks (grid fins, more hydraulic fluid, better legs, 3 burns, 2 burns, 1 engine burns, 3 engine burns) until they got it right. Now they seem like it is odd that EcoStar 23 will be an expendable flight. Normal has become recovering the stage. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-02-02T15:37:11.853" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20007" PostTypeId="1" AcceptedAnswerId="20008" CreationDate="2017-02-02T17:54:25.273" Score="28" ViewCount="10631" Body="&lt;p&gt;I've read in &lt;a href=&quot;https://space.stackexchange.com/a/20006/12102&quot;&gt;@geoff's answer&lt;/a&gt; that SpaceX is doing some experimentation that may lead to fairing recovery capability. In fact I remember hearing Elon Musk mentioning a few years ago that it was being seriously looked at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Originally I hadn't given them much consideration, and in the back of my head I probably thought of them as big fiberglass shells that kept the wind off of the payload. However within the first minute of each launch there's always the mention of max-Q, and in some un-manned launch cases thrust is reduced during max-Q specifically to decrease stresses in the rocket's frame - thrust is pushing up on the frame while aerodynamic drag on the noise is simultaneously pushing down on it. And the &quot;nose&quot; which experiences this stress is actually the fairing, something that has to separate into pieces and reliably clear the area shortly thereafter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I am wondering, what goes into the manufacture of a 21st century fairing that might make it expensive enough to recover and refurbish, and to then convince a customer your going to protect their payload with a used fairing?  Expensive high strength alloys? Expensive manufacturing techniques? Integrated sensors and actuators? &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The &quot;Skybox&quot; and the &quot;Atlantis&quot; - selected for their &lt;em&gt;SXSE sounding&lt;/em&gt; names. 21st century fairings are much more than passive rooftop boxes to &quot;keep the wind off of the customer's stuff.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CnfBi.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CnfBi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9ccDx.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9ccDx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-04-10T19:23:04.937" Title="What makes 21st century fairings so valuable that they'd potentially be recovered and re-used?" Tags="|spacex|falcon-9|recovery|fairings|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="20008" PostTypeId="2" ParentId="20007" CreationDate="2017-02-02T18:20:29.100" Score="32" Body="&lt;p&gt;It is not so much that in the 21st century it is hard or expensive to make a fairing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rather they are just REALLY REALLY big.  13 meters by 4.6 meters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EDVbG.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EDVbG.jpg&quot; alt=&quot;Falcon 9 Fairing dimensions&quot;&gt;&lt;/a&gt;&#xA;That is about 40 feet long, and 14 feet wide. The common description is that a school bus would fit in it. (Sort of like the Space Shuttle cargo bay size).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These need to be very light, as every gram/pound of mass, is a reduction in chargeable payload, but they need to be strong, since they are the aerodynamic shell around a very fragile payload as it accelerates from a dead stop at sea level to Mach 25 (I do not know what speed they are at, when they ditch the fairing, but orbital is Mach 25 or so) in orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the aero loads on it can be quite high. The size is quite large. The need to manage weight is quite critical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All these items conspire to make it cost enough to matter, that recovering it, if possible to do cheaply enough, is a good idea. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of actual construction it is mostly carbon fibre reinforced where needed. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-02-02T18:20:29.100" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="20009" PostTypeId="1" AcceptedAnswerId="21940" CreationDate="2017-02-02T20:42:15.397" Score="17" ViewCount="27322" Body="&lt;p&gt;What are tentative launch costs for 1U/3U cubesat on popular launchers? I checked up ISRO, SpaceX and others. Only RocketLab provides exact cost for cubesat as a piggy-back launch: \$77k / \$240k for 1U/3U cubesats on its Electron rocket. Any idea about other launch providers?&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="9060" LastEditDate="2017-02-02T23:04:57.500" LastActivityDate="2018-12-07T14:20:32.390" Title="Cubesat Launch Costs" Tags="|launch|cubesat|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20010" PostTypeId="2" ParentId="20007" CreationDate="2017-02-02T20:44:14.447" Score="23" Body="&lt;p&gt;The fairings are large structures, made in a way that's difficult to automate. The structure consists of an aluminium honeycomb core with carbon fibre inner and outer panels laminated onto it. Carbon fibre is a labor-intensive material. Then there's the quality assurance that makes everything rocket-related expensive.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roof boxes, OTOH are injection moulded plastic so the entire shell goes from plastic granules to finished product in 10 seconds. And they are made by the million so it's cost-effective to automate the process. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-02T20:44:14.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20012" PostTypeId="1" AcceptedAnswerId="23969" CreationDate="2017-02-03T01:52:02.640" Score="18" ViewCount="2290" Body="&lt;p&gt;In the question &lt;a href=&quot;https://space.stackexchange.com/q/20007/12102&quot;&gt;What makes 21st century fairings so valuable that they'd potentially be recovered and re-used?&lt;/a&gt; I said:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;21st century fairings are much more than passive rooftop boxes to &amp;quot;keep the wind off of the customer's stuff.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In the discussion in comments below &lt;a href=&quot;https://space.stackexchange.com/a/20008/12102&quot;&gt;this answer&lt;/a&gt;, the criteria for a safe altitude for fairing ejection was discussed. I did a quick check of five SpaceX webcast videos and their corresponding press kits, and found that the five fairing deployments were tightly clustered around an altitude of about 110 km, independent of (ground) speed or time after secondary engine start.&lt;/p&gt;&#xA;&lt;p&gt;I've included an estimate of the target inclination. With that and the latitude at deployment, the actual air speed could be estimated, but I'm really looking only for rough ballpark numbers.&lt;/p&gt;&#xA;&lt;div class=&quot;s-table-container&quot;&gt;&#xA;&lt;table class=&quot;s-table&quot;&gt;&#xA;&lt;thead&gt;&#xA;&lt;tr&gt;&#xA;&lt;th&gt;&lt;sub&gt;MISSION&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Ground speed (m/s)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Gr. s. (km/h)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Altitude (km)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Post 2nd stage ignition (sec)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;th&gt;&lt;sub&gt;Inclination (estim.)&lt;/sub&gt;&lt;/th&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/thead&gt;&#xA;&lt;tbody&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;sub&gt;ORBCOMM-2 &lt;/sub&gt;&lt;/td&gt;&#xA;&lt;td&gt;1571&lt;/td&gt;&#xA;&lt;td&gt;5657&lt;/td&gt;&#xA;&lt;td&gt;111&lt;/td&gt;&#xA;&lt;td&gt;13&lt;/td&gt;&#xA;&lt;td&gt;45°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;sub&gt;THAICOM-8&lt;/sub&gt;&lt;/td&gt;&#xA;&lt;td&gt;2536&lt;/td&gt;&#xA;&lt;td&gt;9131&lt;/td&gt;&#xA;&lt;td&gt;111&lt;/td&gt;&#xA;&lt;td&gt;51&lt;/td&gt;&#xA;&lt;td&gt;~0°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;sub&gt; Eutelsat/ABS &lt;/sub&gt;&lt;/td&gt;&#xA;&lt;td&gt;2517&lt;/td&gt;&#xA;&lt;td&gt;9061&lt;/td&gt;&#xA;&lt;td&gt;111&lt;/td&gt;&#xA;&lt;td&gt;47&lt;/td&gt;&#xA;&lt;td&gt;~0°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;sub&gt; JCSAT-16  &lt;/sub&gt;&lt;/td&gt;&#xA;&lt;td&gt;2459&lt;/td&gt;&#xA;&lt;td&gt;8852&lt;/td&gt;&#xA;&lt;td&gt;110&lt;/td&gt;&#xA;&lt;td&gt;48&lt;/td&gt;&#xA;&lt;td&gt;~0°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;sub&gt;Iridium-1&lt;/sub&gt;&lt;/td&gt;&#xA;&lt;td&gt;2034&lt;/td&gt;&#xA;&lt;td&gt;7322&lt;/td&gt;&#xA;&lt;td&gt;107&lt;/td&gt;&#xA;&lt;td&gt;40&lt;/td&gt;&#xA;&lt;td&gt;86°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;tr&gt;&#xA;&lt;td&gt;&lt;sub&gt; Echostar XXIII&lt;/sub&gt;&lt;/td&gt;&#xA;&lt;td&gt;2847&lt;/td&gt;&#xA;&lt;td&gt;10248&lt;/td&gt;&#xA;&lt;td&gt;114&lt;/td&gt;&#xA;&lt;td&gt;38&lt;/td&gt;&#xA;&lt;td&gt;~0°&lt;/td&gt;&#xA;&lt;/tr&gt;&#xA;&lt;/tbody&gt;&#xA;&lt;/table&gt;&#xA;&lt;/div&gt;&#xA;&lt;p&gt;The atmosphere is quite rarified at 110 km; I estimate about &lt;span class=&quot;math-container&quot;&gt;$2 \cdot 10^{-6}$&lt;/span&gt; of sea level's pressure using &lt;span class=&quot;math-container&quot;&gt;$exp(\frac{\text{-110 km}}{H})$&lt;/span&gt; with a scale height &lt;span class=&quot;math-container&quot;&gt;$H$&lt;/span&gt; of 8.4 km.&lt;/p&gt;&#xA;&lt;p&gt;Satellites are built to endure huge g-forces during launch, but those forces on each component are proportional to the mass of the component. So any given section of all that light weight (Kapton or Mylar?) film wrapping the satellite may not experience a very large g-force in terms of newtons, but once the fairing deploys, some surfaces will be exposed to very low pressure but very high velocity winds.&lt;/p&gt;&#xA;&lt;p&gt;In addition to the force of the wind (momentum transfer) there is also energy transfer, and that will scale faster with velocity than the momentum transfer. So even if the bulk mechanical forces turn out to be low, there can be local drag-induced heating, and things that conduct heat poorly (like thin polymer films for example) could develop hot spots.&lt;/p&gt;&#xA;&lt;p&gt;Further, considering that 110 km is between the E and F layers of the ionosphere, there will be a substantial electron and ion wind, which might also cause trouble.&lt;/p&gt;&#xA;&lt;p&gt;Thus:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How strong &lt;em&gt;and &amp;quot;hot&amp;quot;&lt;/em&gt; is the wind on the payload after the fairing is deployed at ~110km?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bmaUt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bmaUt.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Above:&lt;/strong&gt; Photo from Spaceflight Insider's &lt;a href=&quot;http://www.spaceflightinsider.com/organizations/space-exploration-technologies/sierra-nevada-delivers-the-final-11-ormcomm-generation-2-satellites-for-launch&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sierra Nevada delivers final 11 ORMCOMM OG2 satellites for launch&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="47149" LastEditDate="2022-05-21T01:01:57.413" LastActivityDate="2022-05-21T01:01:57.413" Title="How strong and &quot;hot&quot; is the wind on the payload after the fairing is deployed at ~110km?" Tags="|spacex|atmosphere|fairings|" AnswerCount="1" CommentCount="19" ContentLicense="CC BY-SA 4.0" />
  <row Id="20013" PostTypeId="1" AcceptedAnswerId="20016" CreationDate="2017-02-03T03:11:01.103" Score="7" ViewCount="756" Body="&lt;p&gt;While the WSJ headline sounds ominous: &lt;a href=&quot;https://www.wsj.com/articles/congressional-investigators-warn-of-spacex-rocket-defects-1486067874&quot; rel=&quot;noreferrer&quot;&gt;Congressional Investigators Warn of SpaceX Rocket Defects&lt;/a&gt;, Quartz's &lt;a href=&quot;https://qz.com/901774/elon-musks-spacex-needs-to-redesign-its-engine-to-meet-nasas-human-safety-standards/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX needs to redesign its engine to ensure it is safe for human spaceflight&lt;/a&gt; seems to be a better description. Cracking in the blades seems to be known and not unusual, and has been taken into account in turbopump design. It seems SpaceX is already working on some improvements to address these concerns with regard to manned spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the issue and concern that it increases the likelihood of an engine failure - especially 2nd stage when there is only one engine, or is it like throwing a piston rod - where stories claim it can come shooting out of the hood and damage anything else in its path, possibly a propellant tank or two?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or are there other issues related to pump failure - burst lines, leaking propellants, etc?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What are the issues and concerns related to turbopump blade cracks and manned spaceflight?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-03T10:22:04.820" LastActivityDate="2018-04-05T07:50:43.260" Title="What are the issues and concerns related to turbopump blade cracks and manned spaceflight?" Tags="|engine-design|engines|safety|turbomachinery|turbopump|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20015" PostTypeId="2" ParentId="19997" CreationDate="2017-02-03T06:06:09.360" Score="0" Body="&lt;p&gt;Having the CM in a low, circular orbit means it completes an orbit more often. I guess this gives the LM more frequent launch windows in case they need to abort?&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2017-02-03T06:06:09.360" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20016" PostTypeId="2" ParentId="20013" CreationDate="2017-02-03T09:10:39.423" Score="7" Body="&lt;p&gt;Turbo pumps have parts spinning at thousands of RPM, which means the blades in the pump have a great deal of energy. A turbine blade crack can lead to a fracture which will fling the failed turbine part into the pump. The best case would be for the pump to fail in a contained way, either by the piece jamming something or ripping bits of it apart, which would lead to a loss of power in the rocket which may or may not cause an abort. The worst case scenario (and more likely given the energies involved) is an uncontained failure of the pump which will perforate the rocket in multiple places, including other pumps, fuel lines and tanks, which will lead to a catastrophic explosion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Turbine failures in rocket motors have happened. In the Space Shuttle program there were &lt;a href=&quot;http://www.enginehistory.org/Rockets/SSME/SSME6.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;contained pump failures&lt;/a&gt; where after a failure in testing:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A post-test inspection revealed that a first-stage turbine blade had&#xA;  broken off and inflicted significant damage to both turbine stages.&#xA;  Figure 17 shows the damage to the first-stage wheel. The engine was&#xA;  shut down safely with no other engine damage. Two weeks later, on&#xA;  December 1, 1977, Test 901-147 on Engine 0103 experienced a similar&#xA;  failure at slightly above 80 percent power level. This time the damage&#xA;  was more severe. The turbine blade debris caused the rotor to seize&#xA;  up, resulting in the cessation of fuel flow and very LOX rich&#xA;  operation. Major burning throughout the hot gas system followed; but,&#xA;  although significant damage was sustained, it was contained within the&#xA;  engine with no external burnthrough.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More recently an &lt;a href=&quot;http://spaceflightnow.com/2015/11/05/nasa-quietly-posts-images-of-explosive-antares-failure/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Antares rocket exploded just above the pad&lt;/a&gt; when a turbine failed in flight:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The failure began in one of the Antares rocket’s two AJ26 engines,&#xA;  when a spinning rotor inside a liquid oxygen turbopump contacted&#xA;  another component inside the powerplant, triggering an explosion,&#xA;  according to engineers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This resulted in:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/jStzw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jStzw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once a rocket lifts off any kind of abort will require the activation of the launch escape system for the crew to survive, which is not a scenario to be taken lightly as there's a great deal of risk involved. If a satellite is destroyed on launch it's a shame but you can build an identical one, if a person is killed you don't have that option, which is why there's this scrutiny. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2806" LastEditDate="2018-04-05T07:50:43.260" LastActivityDate="2018-04-05T07:50:43.260" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20017" PostTypeId="1" AcceptedAnswerId="20031" CreationDate="2017-02-03T11:31:46.043" Score="24" ViewCount="7962" Body="&lt;p&gt;Ion thrusters need a lot of energy. But I don't understand why. So, one thing I understand is that it takes a lot of energy to achieve a high exhaust velocity (since kinetic energy is proportional to velocity squared) but we get little momentum. This is all great. But, the energy required is coming from the grids. The charge present on these grids doesn't change (due to ion optics), hence they can accelerate particles forever as long as the ionization chamber has sufficient pressure to let the ions enter the grid space. Now, it takes energy to ionize the gas. But when I calculated it, its not in kilowatts obviously.&lt;/p&gt;&#xA;&lt;p&gt;The chambers' walls are positively charged. The walls act like an anode and take in the electrons from the plasma. So, the thing that I was wondering about was whether the constant charging of these walls could take up that much energy. I then tried to calculate the energy needed to do this through the knowledge of the metals work function and found out that it requires less than 100 watts of energy. &lt;em&gt;I am pretty sure this is wrong&lt;/em&gt;, but I don't understand why ion thrusters are so energy hungry.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/f5t2a.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f5t2a.png&quot; alt=&quot;Schematic of ion thruster&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I am talking about what we are using energy for, and how it translates into the ion beam energy. Like ionization, etc; not how to calculate the energy needed.&lt;/p&gt;&#xA;" OwnerUserId="18356" LastEditorUserId="29286" LastEditDate="2020-07-23T13:00:05.790" LastActivityDate="2020-07-23T13:00:05.790" Title="Why are Ion Thrusters so energy hungry?" Tags="|propulsion|energy|ion-thruster|" AnswerCount="6" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20018" PostTypeId="2" ParentId="20017" CreationDate="2017-02-03T12:31:46.097" Score="15" Body="&lt;p&gt;A conventional thruster with two liquid propellants requires energy too. But it is chemical energy stored in the propellants. Ion thrusters use no chemical energy at all, all the energy of the ion beam is from the electrical energy used by the thruster. In fact, a conventional rocket engine with a lot more thrust than a ion thruster uses a lot more of chemical energy.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-02-03T12:31:46.097" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20019" PostTypeId="2" ParentId="20017" CreationDate="2017-02-03T13:38:37.520" Score="12" Body="&lt;p&gt;First, I recommend you having a look at the section 2.6 of the book &quot;Fundamentals of electric propulsion (...)&quot; from NASA researchers Dan Goebel and Ira Katz. It is free on the internet and they show a pretty nice energy balance for a conventional thruster. Here is the link for the pdf: &lt;a href=&quot;http://descanso.jpl.nasa.gov/SciTechBook/series1/Goebel__cmprsd_opt.pdf&quot;&gt;Fundamentals of Electric Propulsion&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, the equation to calculate the overall power consumption would be: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P = \frac{g_0 T I_{sp}}{2\eta_T} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, for a thruster to produce 150 mN of thrust with a specific impulse of 2000 seconds, and a good efficiency of 0.8, the power consumption would be already 1.875 kW. This equation is demonstrated in the section 2.5 of the same book, and it is derived simply from the relation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \eta_T = \frac{P_{jet}}{P_{in}} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;em&gt;kinetic power of the beam&lt;/em&gt;, called the jet power, is defined as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P_{jet} = \frac{1}{2} \dot{m}_p v_e^2 = \frac{T^2}{2\dot{m}_p}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, as discussed in the book, most of the power is simply used to the sheer acceleration of the beam. All the other power consumption (ionization, wall losses, collisions at the grids, etc) are accounted in the efficiency, and is minor in comparison to the electromagnetic acceleration of the beam.  &lt;/p&gt;&#xA;" OwnerDisplayName="user17622" LastEditorDisplayName="user17622" LastEditDate="2017-02-03T17:54:44.907" LastActivityDate="2017-02-03T17:54:44.907" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20020" PostTypeId="1" CreationDate="2017-01-25T14:19:09.357" Score="3" ViewCount="253" Body="&lt;p&gt;I'm working on some delta-V calcs for both Hohmann and non-Hohmann transfers, but the data I'm getting is conflicting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I'm looking for much broader applicability than transfer to Mars, since that's the most common case cited, let's use that.  When I run &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;the formula on Wikipedia&lt;/a&gt;, I get 3,45 km/s for the transfer.  This of course assumes that neither Earth nor Mars are there.  One can find more complicated formulae for including these, for example, &lt;a href=&quot;https://space.stackexchange.com/questions/1380/how-to-calculate-delta-v-required-for-a-planet-to-planet-hohmann-transfer&quot;&gt;How to calculate delta-v required for a planet-to-planet Hohmann transfer?&lt;/a&gt; Which you can find in spreadsheets like &lt;a href=&quot;http://clowder.net/hop/railroad/Hohmann.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; or &lt;a href=&quot;http://clowder.net/hop/railroad/NonHohmannEarthToMars.xlsx&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that's still not clear how you factor into account an aerocapture transfer.  We can compare to datasets like &lt;a href=&quot;https://web.archive.org/web/20070701211813/http://www.pma.caltech.edu/~chirata/deltav.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chris Hirata's&lt;/a&gt;, which make it look like from Earth escape (for which $\Delta V$ is easy to calculate), it's 0,6 km/s to Mars aerocapture outbound, and 0,9km/s to Earth aerocapture inbound. So where exactly do these numbers come from?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I manually calculate Earth escape from a 250km LEO, I get 3,13 km/s additional $\Delta V$ needed.  The difference between that and the &quot;simple&quot; Hohmann transfer formula is 0,3km/s, which matches no information I'm seeing elsewhere or calculating.  For the spreadsheets, they have a number of periapses and apoapses to enter, but it's not clear what's being used for what. One spreadsheet suggests that for aerocapture &quot;apoapsis should be within the planet's sphere of influence&quot;, so if I put in for Earth 250km for both periapsis and apoapsis, and for Mars a 50km periapsis / 570000km apoapsis (just barely within the sphere of influence), it gives &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Departure Vinf 2,9448 km/s&lt;br&gt;&#xA;Arrival Vinf 2,6490 km/s&lt;br&gt;&#xA;Total DV 5,5937 km/s&lt;br&gt;&#xA;Earth: Insertion burn from periapsis 3,6001 km/s&lt;br&gt;&#xA;Mars: Insertion burn from periapsis 0.6749 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I change the Earth apoapsis to just under its sphere of influence (say, 911900km), I get the same except for &quot;Earth: Insertion burn from periapsis 0,4279 km/s&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soooooooo..... how am I supposed to interpret this?  Where is this 0,6 km/s from Earth escape to Mars aerocapture, 0,9 km/s from Mars escape to Earth aerocapture supposed to come from?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had actually spent some time trying to get real-world optimized transfer scenarios calculated in GMAT, but the built-in GMAT targeter is terrible (it always just gets stuck oscillating), and the better plugin targeters are a nightmare to try to compile....  :Þ&lt;/p&gt;&#xA;" OwnerUserId="13465" OwnerDisplayName="KarenRei" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2022-08-31T01:07:42.620" Title="$\Delta V$ to aerocapture - reconciling conflicting data" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20021" PostTypeId="2" ParentId="12400" CreationDate="2017-02-03T15:45:48.527" Score="1" Body="&lt;p&gt;Hydrogenated Boron Nitride Nanotubes are being developed with the general aim of being used in various areas of radiation protection, including the hulls of spacecrafts. I am not qualified enough to answer this from a perspective of science, but for fiction, here is a reason why it is prefect for your requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are competitions called space settlement design competitions, where high school students collaborate to make somewhat feasible designs for a future settlement in space. Locations vary from Mercury to Low Earth Orbit to the surface of Mars. However, HBNNT is a not-so-commonly used material in these competitions, and often makes a proposal stand out because of its unique qualities. I am often in charge of the materials section in my team while designing such proposals, and it is a material I have found that is perfect for hull requirements. It is enough to pass scrutiny by a panel of judges who are NASA engineers/scientists, even though we are technically dealing with a mix of fact and fantasy. It should be good enough for your novel. Unless you were actually going to present the idea for construction, I cannot see why using it should be a problem.&lt;/p&gt;&#xA;" OwnerUserId="14148" LastActivityDate="2017-02-03T15:45:48.527" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20023" PostTypeId="2" ParentId="19661" CreationDate="2017-02-03T15:58:08.183" Score="1" Body="&lt;p&gt;My reading was that the buckles were in the carbon fibre overwrap, but on the underside, thus the &quot;buckles under the overwrap&quot;. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-02-03T15:58:08.183" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20024" PostTypeId="2" ParentId="19432" CreationDate="2017-02-03T15:59:40.873" Score="1" Body="&lt;p&gt;You can see in the various photos of the grid fins after landing (a unique feature of a SpaceX first stage, you get to see it after flight) that they have ablated quite a bit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The true proof will be when the first reused first stage flies, and the state of refurbishment on the grid fins. This should be the SES-10 mission in early 2017. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fins have an ablative covering.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-02-03T15:59:40.873" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20027" PostTypeId="1" AcceptedAnswerId="20688" CreationDate="2017-02-03T17:24:50.153" Score="8" ViewCount="901" Body="&lt;p&gt;Recently,I've  posted a question on ion thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1Br1C.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1Br1C.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought that it would be enough for the grids to be charged to accelerate ions. If I have a positive plate and a negative plate, separated by a distance 'd', a positively charged particle placed in the space would migrate towards the negatively charged plate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;-But a few of you guys pointed out that just 'Charging the plates' wouldn't be enough to accelerate the ion beam. There needs to be a current passing through them.&#xA;-I don't understand why there has to be a current passing through them and how is the current supposed to pass?? (Please draw the circuit diagram showing the direction of the current if possible)&lt;/p&gt;&#xA;" OwnerUserId="18356" LastActivityDate="2017-03-20T04:08:19.137" Title="Why is there supposed be a current passing through the grids in an Ion Thruster?" Tags="|propulsion|ion-thruster|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20028" PostTypeId="2" ParentId="20017" CreationDate="2017-02-03T18:25:55.207" Score="9" Body="&lt;p&gt;There is a sense in which the wonderfully high &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot;&gt;ISP&lt;/a&gt; of ion thrusters is the cause of their energy hunger, independently of the detail of their operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It works like this: we need a certain amount of delta-v (that's what a thruster is for after all) or more formally a certain amount of impulse $J$, and we are going to get it by throwing mass overboard. &#xA;$$J = \Delta m_\text{fuel} v_\text{exhaust} \;.$$&#xA;(Here we take $\Delta m_\text{fuel}$ to be small enough to neglect high order terms. This can be written in differential terms to formalize that, but it doesn't really change the argument.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we compute the energy cost $W$ of this choice in term of how much mass we throw overboard (writing $K$ for kinetic energy of the exhaust relative the spacecraft):&#xA;\begin{align*}&#xA; W &#xA;&amp;amp;= \Delta K \\&#xA;&amp;amp;= \frac{1}{2} (\Delta m_\text{fuel}) v_\text{exhaust}^2 \\&#xA;&amp;amp;= \frac{J^2}{2\,\Delta m_\text{fuel}} \;. &#xA;\end{align*}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, for a fixed impulse, using more mass of fuel requires less energy (neglecting the efficiency of the drive mechanism).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this point of view, engines with very high exhaust velocities are worse rather than better!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we use them anyway for several reasons. First, the tyranny of the rocket equation (with it's exponential dependence on mass ratio) overshadows other consideration, second launch from the surface requires high thrust at almost whatever cost and third the energy cost for chemical propellants is payed at manufacture rather than when rocket is operated and that means ground-side where energy is plentiful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of satellite and probe engines it is the mass-fraction issue that dominates.&lt;/p&gt;&#xA;" OwnerUserId="702" LastEditorUserId="702" LastEditDate="2017-02-03T20:11:55.163" LastActivityDate="2017-02-03T20:11:55.163" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20029" PostTypeId="2" ParentId="20017" CreationDate="2017-02-03T18:41:58.187" Score="5" Body="&lt;p&gt;The foundation of rocketry is Conservation of Momentum:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$p=mv$ (momentum = mass * velocity)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since momentum is conserved, the change in momentum of the propellant is equal to the change in momentum of the rocket body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also: energy of an object in motion is defined by the Kinetic Energy equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$KE = {1\over2}mv^2$ (Kinetic Energy is equal to one half the mass times the velocity squared)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the velocity term is linear in the first equation, and squared in the second.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you, for example, doubled the velocity of your propellant and while you halved the mass, the change in momentum would remain the same, but the energy required to achieve that change would have doubled!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Ion Engines appear &quot;energy inefficient&quot; because they are accelerating low mass propellant to very high velocities.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some applications this is perfectly fine: it is expensive to put mass into orbit, so it can make sense to use a less &quot;energy efficient&quot; engine to save mass in propellant.  Propellant speed also defines the upper limit in the rocketry equation, so higher end speeds are theoretically possible.  In other cases, this characteristic is not desirable.&lt;/p&gt;&#xA;" OwnerUserId="11115" LastEditorUserId="6816" LastEditDate="2017-02-03T23:23:24.417" LastActivityDate="2017-02-03T23:23:24.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20030" PostTypeId="2" ParentId="20017" CreationDate="2017-02-03T21:20:17.113" Score="3" Body="&lt;p&gt;On top of what is the primary cause described by others: about all exhaust energy coming from the electric supply, as opposed to chemical energy of the propellant, in more advanced ion thrusters like VASIMR there's one more &quot;sink&quot; for all that electricity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your normal chemical engine is made from superior alloys resistant to high temperature and high pressure, with active cooling system to keep them from melting and being torn apart - and all that for propellant energy an order of magnitude lower than what ion engines handle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ionized propellant, accelerated to dozens of kilometers per second, would act as an extremely hot, extremely abrasive plasma that would make a short work of any nozzles, grids, generally all solid structural elements of the drive. Simpler drives deal with it simply - let it happen, shortening the drive's useful life and putting an upper limit on achievable delta-V because before you run out of propellant, the drive dies. The more advanced ones use the only &quot;material&quot; that can handle this kind of conditions, and is fully, immediately &quot;self-healing&quot;: the magnetic field. In VASIMR, for example, at no point past the ionization system does the gas ever touch the structural elements; it's entirely trapped within a whole, rather complex &quot;plumbing system&quot; made entirely out of magnetic fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...and you need a lot of electricity to keep these electromagnets running. After all, they must be strong enough to stop and divert particles moving at several dozen km/s - completely independently from all the power-hungry systems that accelerate the particles to these speeds.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-02-03T21:20:17.113" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20031" PostTypeId="2" ParentId="20017" CreationDate="2017-02-03T22:59:34.363" Score="12" Body="&lt;p&gt;The thing to notice about this type of drive is that the ions encounter the positively charged accelerating grid first. The ions that provide the thrust to the rocket are positively charged as well, so they will be repelled and contained within the plasma chamber. The ions' only means of reaching the interior of the accelerating grid is diffusion, meaning an ion must already have sufficient kinetic energy to approach and traverse the positive accelerating grid. If an ion does not have enough kinetic energy to get past the positive grid, it will be reflected back into the plasma chamber and not contribute to the acceleration of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ions in the plasma chamber behave like a gas in that they have a wide range of kinetic energies. Only the highest energy ions reach the accelerating field between the grids. There are two ways of increasing the rate of ions entering the accelerating field between the grids: increasing the temperature of the plasma and increasing the density. The first way increases the average kinetic energy of the ions, allowing more of them to get past the positive grid and reach the accelerating field. The second creates more ions, leading to more accelerated ions just as a matter of probability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the power expenditure comes from keeping the plasma hot and dense enough to get enough ions into the accelerating field at a fast enough rate for the desired thrust. This would be true even in an ideal rocket with no losses due to heat loss to the walls of the chamber, charge loss on the grid due to ion/electron absorption, ion loss due to recombination, etc. If there were no power input, the plasma would cool off and thin out as it lost its highest energy ions to the exhaust (evaporative cooling). Eventually, the plasma chamber would have nothing but low energy ions contained by the walls and the positively charged grid, unable to escape.&lt;/p&gt;&#xA;" OwnerUserId="18404" LastEditorUserId="18404" LastEditDate="2017-02-16T09:23:56.673" LastActivityDate="2017-02-16T09:23:56.673" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20032" PostTypeId="2" ParentId="20027" CreationDate="2017-02-03T23:07:34.000" Score="-1" Body="&lt;p&gt;The current isn't flowing through &lt;em&gt;each&lt;/em&gt; grid, but is flowing &lt;em&gt;from one grid to the other&lt;/em&gt;.  A current is a flow of charge. The propellant ions are charged particles. They are moving, therefore there is a flow of charge, therefore there is a current.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-02-03T23:07:34.000" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20034" PostTypeId="2" ParentId="2102" CreationDate="2017-02-04T01:50:16.423" Score="4" Body="&lt;p&gt;I will elaborate a bit on Mark's answer starting from the basic formula &#xA;$R\rho = 2m/(A*Cl)$. Above the troposphere, the density falls exponentially as $rho = rho_0 \exp(-h/H_0)$ where $H_0$ is the scale height. This means you have to solve an equation like:&#xA;$$rhs = (R+h)*\exp(-h/H_0), rhs = 2m/(A*Cl*\rho_0)$$&#xA;This can be solved analytically using the Lambert W function, which is the inverse $w(x)$ of $x*\exp(x)$. The answer turns out to be $r = -H_0W(-1,y)-R, y=-\exp(-R/H_0)*rhs/H_0$. The notation $W(-1,x)$ indicate that the -1 branch of &lt;a href=&quot;https://en.wikipedia.org/wiki/Lambert_W_function&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Lambert W function&lt;/a&gt; is used. Using the standard ISA atmosphere model above the troposphere, m~10000 kg, Cl~ 0.5, A=30 (typical small fighter aircraft) the answer is about 93 km. Using the atmospheric composition of other planets it should be relatively easy to calculate this.&lt;/p&gt;&#xA;" OwnerUserId="18409" LastEditorUserId="6816" LastEditDate="2017-02-04T02:09:32.553" LastActivityDate="2017-02-04T02:09:32.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20036" PostTypeId="1" CreationDate="2017-02-04T15:18:38.537" Score="5" ViewCount="760" Body="&lt;p&gt;In the article &lt;a href=&quot;https://www.extremetech.com/extreme/243007-breakthrough-starshot-tricks-vlt-make-lean-mean-planet-hunting-machine&quot; rel=&quot;nofollow noreferrer&quot;&gt;Breakthrough Starshot tricks out massive telescope for planet hunting&lt;/a&gt; there is casual mention of a &quot;phased array of lasers in the 100GW range&quot; that will propel a gram scale spacecraft to 0.2 c in a matter of minutes. That gave me pause, but usually when you read about huge powers of laser beams they are pulsed at nanoseconds or picoseconds, and the actual average power is modest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it works out. Just doing approximate math, the momentum of the spacecraft (non relativistic approximation) will be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$p_{sat} = m_{sat}v_{sat} = 0.2mc = \text{0.2} \times \text{0.001} \times \text{3E+08} = \text{6E+04 kg m/s.}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For photons, the energy is related to the momentum as simply:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E = pc$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so you don't have to specify any particular wavelength. If the sail were a perfect mirror at normal incidence, reflecting light gives you double the momentum kick, since you're actually reversing the momentum of the photon, but let's assume 50% efficiency. Thus&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_{tot} = p_{sat}c = \text{6E+04} \times \text{3E+08 kg m}{}^2 / \text{s}^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or 1.8E+13 Joules, or, yes, 100 Gigawatts for the time it takes to cook a three minute egg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For three full minutes, this laser array will be using power at roughly the same rate that the US uses at peak demand. I think the idea is that you can't do it slowly, or the satellite will get too far away too soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There can't be a single laser with this much power, it would be a very large phased array of smaller lasers. That means they would have to be phase-coherent at optical frequencies to synthesize a beam spot on the moving satellite narrow enough to get most of the power on to it's sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any quantitative discussion how this power might be stored and then delivered, and how atmospheric seeing might be compensated coherently across the entire distributed array simultaneously?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-06-08T17:04:40.097" LastActivityDate="2021-06-08T17:04:40.097" Title="Breakthrough Starshot's 100 Gigawatt laser array - what's the current thinking how this might work?" Tags="|interstellar-travel|laser|breakthrough-starshot|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20037" PostTypeId="1" CreationDate="2017-02-04T17:07:40.770" Score="3" ViewCount="214" Body="&lt;p&gt;I found this site with quite a substantial amount of data, tabulations, and more for many space exploration missions. There are detailed information on the Cassini mission here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dmuller.net/spaceflight/mission.php?mission=cassini&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.dmuller.net/spaceflight/mission.php?mission=cassini&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the start until May 2008, event descriptions that include the word maneuvre (maneuver) is found only as &quot;Trajectory maneuvre&quot;, and they are numbered 1 through 157 in June 2008. However after that, they are labeled &quot;Orbital trim maneuvre&quot; and numbered OTM-164 to OTM-255. After 2010 there are no maneuvers listed at all, although the detailed list of events continues into the future - probably end of mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an official distinction here between Trajectory and Orbital trim maneuvers, or is this just the language used for this particular site. Is there an official list of maneuvers beyond 2010 somewhere else that I could look at?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-09-18T23:23:45.797" Title="Cassini: difference between trajectory and orbital trim maneuvers in this list?" Tags="|orbital-maneuver|cassini|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20039" PostTypeId="1" CreationDate="2017-02-04T20:36:39.887" Score="-4" ViewCount="274" Body="&lt;p&gt;First Neil was taking photos then there was a video camera showing both Buzz and Neil in the picture, and the video is showing Neil is still taking photos of Buzz sticking some weather thing! So prove me wrong please, if that was a live feed, where is the cable from the video recorder to the space craft so they can transmit to all on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;they were the only two there on the moon the third man, mr. M Collins he was still hovering in the space craft he was never on the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did that happen?&lt;/p&gt;&#xA;" OwnerUserId="18415" LastEditorUserId="897" LastEditDate="2017-02-04T21:02:16.557" LastActivityDate="2017-02-04T21:11:21.123" Title="How were the video broadcasts from the Moon made on Apollo 11?" Tags="|nasa|" AnswerCount="1" CommentCount="2" ClosedDate="2017-02-06T08:55:11.703" ContentLicense="CC BY-SA 3.0" />
  <row Id="20040" PostTypeId="2" ParentId="20039" CreationDate="2017-02-04T21:11:21.123" Score="7" Body="&lt;p&gt;Apollo 11 carried a &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_TV_camera#Westinghouse_Apollo_Lunar_Television_Camera&quot;&gt;single B&amp;amp;W TV camera&lt;/a&gt;. It was installed on a tray in a storage compartment on the LM. This tray could be folded out to give a view of the ladder. This position was used to make the 'first step' footage. This is a photo of a mockup of the lander, with the tray in the open position.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Qek0.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Qek0.jpg&quot; alt=&quot;TV camera on tray&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Later on, one of the astronauts removed the camera from this tray and placed it on a &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Apollo_11_TV_Camera.JPG&quot;&gt;tripod&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FsCy8.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FsCy8.jpg&quot; alt=&quot;Apollo 11 TV camera&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the cable running from the tripod to the lander (out of frame on the left). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-04T21:11:21.123" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20041" PostTypeId="1" AcceptedAnswerId="20046" CreationDate="2017-02-05T01:45:55.200" Score="4" ViewCount="1088" Body="&lt;p&gt;I understand that the ISS is somewhat capable of unmanned control unless a catastrophic error occurs with core instruments, however when the crew is onboard, there is usually a crew of around 6 at any given time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, to be able to safely control and maintain the station including spacewalks and interior functionality, &lt;strong&gt;how many people are required? Can it be run with a minimum of 2 people? One to do spacewalks and the other suits them up, etc?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user14355" LastActivityDate="2017-02-09T15:07:45.977" Title="Lowest amount of people on the ISS at one time" Tags="|iss|crewed-spaceflight|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20042" PostTypeId="1" CreationDate="2017-02-05T03:15:00.030" Score="5" ViewCount="3496" Body="&lt;p&gt;High &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust-to-weight_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thrust to Weight Ratio&lt;/a&gt; (TWR) rockets means get into fairly quickly and constantly experiencing G-Force, are there any downsides to high TWR rockets other than engineering? Seems like there are no problems accelerating 50 m / s^2 and get into orbit in 3 minutes. &lt;/p&gt;&#xA;" OwnerUserId="18270" LastEditorUserId="12102" LastEditDate="2017-02-05T17:28:59.983" LastActivityDate="2018-07-22T00:06:16.533" Title="Are there any downsides to high TWR rockets?" Tags="|launch|rockets|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20043" PostTypeId="1" AcceptedAnswerId="20044" CreationDate="2017-02-05T04:23:34.500" Score="7" ViewCount="1761" Body="&lt;p&gt;I was curious about how effective farting/urination would be as a propulsion system, but I couldn't find any data on the specific impulse of human farts/urination. Could you actually get a couple of centermeters per second of delta-v from it?&lt;/p&gt;&#xA;" OwnerUserId="18421" LastEditorUserId="18421" LastEditDate="2017-02-05T12:50:35.337" LastActivityDate="2017-02-06T12:31:34.917" Title="What's the specific impulse of a human farts/urination?" Tags="|fuel|specific-impulse|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20044" PostTypeId="2" ParentId="20043" CreationDate="2017-02-05T05:13:12.457" Score="12" Body="&lt;p&gt;From the ballistics, I estimate the &quot;exhaust velocity&quot; of a urination stream at about 0.5-1.0 m/s&lt;sup&gt;2&lt;/sup&gt; (i.e. 0.05-0.1 sec specific impulse) though a number of factors can influence that. Assuming a liter of fluid in the bladder massing about 1kg, and a person massing 75kg &quot;dry&quot;, we get via &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot;&gt;Tsiolkovsky&lt;/a&gt; a delta-v figure of $$ \sim 0.75 \, ln \, \frac {76} {75} \approx 0.01 $$ i.e. on the order of 1 cm/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assure you that it's a lack of data rather than any kind of dignity that prevents me from making a similar calculation for flatulence.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-02-05T05:34:59.163" LastActivityDate="2017-02-05T05:34:59.163" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20045" PostTypeId="1" CreationDate="2017-02-05T07:03:48.317" Score="9" ViewCount="409" Body="&lt;p&gt;In the earlier, pre-New Horizons Pluto flyby question &lt;a href=&quot;https://space.stackexchange.com/q/9777/12102&quot;&gt;How long have we known Pluto is red?&lt;/a&gt; about the general, slightly reddish tinge of Pluto, the answer is &amp;quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tholin&quot; rel=&quot;noreferrer&quot;&gt;tholins&lt;/a&gt;&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;Now that New Horizons flyby is over and &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Post-Pluto_events&quot; rel=&quot;noreferrer&quot;&gt;all of the data has been transmitted to Earth&lt;/a&gt;, there are color, high resolution images of Pluto with substantial variation of color across the surface.&lt;/p&gt;&#xA;&lt;p&gt;Why does pluto have a variations of colors? In other word, Do we know each substance that produces each color?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA19957&quot; rel=&quot;noreferrer&quot;&gt;PIA19957: 'Snakeskin' Terrain&lt;/a&gt;: &amp;quot;In this extended color image of Pluto taken by NASA's New Horizons spacecraft, rounded and bizarrely textured mountains, informally named the Tartarus Dorsa, rise up along Pluto's day-night terminator and show intricate but puzzling patterns of blue-gray ridges and reddish material in between. This view, roughly 330 miles (530 kilometers) across, combines blue, red and infrared images taken by the Ralph/Multispectral Visual Imaging Camera (MVIC) on July 14, 2015, and resolves details and colors on scales as small as 0.8 miles (1.3 kilometers).&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NyUcD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NyUcD.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA19957&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The new “extended color” view of Pluto – taken by New Horizons’ wide-angle Ralph/Multispectral Visual Imaging Camera (MVIC) on July 14 and downlinked to Earth on Sept. 19 – shows the extraordinarily rich color palette of Pluto.&lt;/p&gt;&#xA;&lt;p&gt;“We used MVIC’s infrared channel to extend our spectral view of Pluto,” said John Spencer, a GGI deputy lead from Southwest Research Institute (SwRI) in Boulder, Colorado. “Pluto’s surface colors were enhanced in this view to reveal subtle details in a rainbow of pale blues, yellows, oranges, and deep reds. Many landforms have their own distinct colors, telling a wonderfully complex geological and climatological story that we have only just begun to decode.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/32bkR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/32bkR.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18422" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-04-14T18:03:43.407" Title="Compounds causing the various colors on Pluto's surface?" Tags="|pluto|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20046" PostTypeId="2" ParentId="20041" CreationDate="2017-02-05T09:18:36.770" Score="6" Body="&lt;p&gt;ISS crew regularly drops to 3 these days. But in the early days, expeditions 7-12 had 2 crew members. Without the Shuttle and before COTS, ATV and HTV, it wasn't practicable to keep a crew of 3 supplied with Progress alone. (thanks @Pericynthion for the addition)  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These days, the ISS needs about 5 people to keep up with its maintenance requirements. With less than 5 aboard, you'll get behind on scheduled maintenance, so I suspect a crew of 3 is not a situation the planners will want to allow for long periods.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there won't be a hard limit. You could leave the station empty for a couple of hours or days, but maintenance starts to pile up soon. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-02-09T07:46:53.637" LastActivityDate="2017-02-09T07:46:53.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20047" PostTypeId="2" ParentId="20042" CreationDate="2017-02-05T09:22:31.413" Score="1" Body="&lt;p&gt;For manned launches, 5G is very uncomfortable, so manned rockets are often designed with a lower TWR and an ascent profile where engines are shut down to keep acceleration within limits.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is less of an issue for unmanned rockets, but at some point G-loads will impose structural demands on the spacecraft that will make it heavier than it needs to be. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-05T09:22:31.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20048" PostTypeId="2" ParentId="20042" CreationDate="2017-02-05T10:06:44.843" Score="0" Body="&lt;p&gt;As a spacecraft uses propellant, it loses weight which means the thrust-to-weight ratio of the spacecraft continues to increase. In many cases half or more of the weight could be propellant, meaning that it can increase substantially.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This make it more difficult to do carefully controlled thrust maneuvers, such as small delta-v corrections. These are usually executed by time, and the smaller the duration, the larger the possible error, depending on the level of control and stability and repeatability of ignition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a potential disadvantage would be increased difficulty with small delta-v maneuvers, or the necessity of developing, testing, and calibrating additional throttling capability, or the addition of additional small thrusters that might not have been necessary with a lower TWR engine.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-02-05T10:06:44.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20049" PostTypeId="1" CreationDate="2017-02-05T12:24:22.920" Score="1" ViewCount="302" Body="&lt;p&gt;Just a silly theory to illustrate my question. If we were to build a globe (or a detached runway, could be any structure really as long as it goes around earth and meets on both ends) around the earth, which itself travels at 17500 miles per hour. From there we launch space shuttles, does this mean they could go into orbit at a reduced speed and cost? As they have a starting speed of 17500 miles per hour. And does the height of the globe (in relation to earth) matter?   &lt;/p&gt;&#xA;" OwnerUserId="18425" LastEditorUserId="265" LastEditDate="2017-02-05T18:08:48.223" LastActivityDate="2017-09-30T19:19:34.960" Title="Is it possible, using known acceleration laws, to drastically lower force required to leave Earth's Gravitational pull?" Tags="|gravity|g-force|acceleration|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20050" PostTypeId="2" ParentId="20049" CreationDate="2017-02-05T12:44:51.933" Score="4" Body="&lt;p&gt;A structure like that would reduce the amount of energy needed to get to orbit. But:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;getting a structure up to 17,500 mph would take vast amounts of energy. &lt;/li&gt;&#xA;&lt;li&gt;transporting the spacecraft from Earth to that structure still means you have to accelerate the spacecraft to 17,500 mph. So you're not saving any energy.&lt;/li&gt;&#xA;&lt;li&gt;a ring around the Earth would consume a massive amount of resources, making it impractical to build.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The most practical implementation of what you're describing is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_elevator&quot; rel=&quot;nofollow noreferrer&quot;&gt;space elevator&lt;/a&gt;: a vertical structure attached to Earth. If you climb this structure to 36,000 km up, your speed will be orbital speed. You can step off the elevator and be in orbit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-05T12:44:51.933" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20051" PostTypeId="2" ParentId="20042" CreationDate="2017-02-05T14:13:04.990" Score="5" Body="&lt;p&gt;The main disadvantage of high TWR is that engines are expensive but fuel and tankage is relatively cheap. This makes it more economical to use smaller engines and lower TWR, even though more fuel will be burned counteracting gravity the fuel is of negligible cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, an engine capable of putting out more thrust generally needs to be larger and heavier - but that engine is not itself useful payload. Accelerating a heavy engine halfway to orbital velocity will reduce available delta-V compared with using a lighter engine which is still able to provide an acceptable TWR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid Fuel, which is naturally conducive to a high TWR, also tends to be more expensive than liquid fuel. While solid fuels have their uses, it is cheaper if the bulk of the required orbital velocity comes from liquid fuels which gets back to using smaller engines at the lowest TWR that is still practical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Higher thrust will also inflict more damage to the launchpad and surrounding infrastructure. I don't know whether that counts as a mere engineering problem, but it would certainly require a more robust launchpad or more repairs after a launch.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastEditorUserId="6599" LastEditDate="2017-02-06T13:00:15.867" LastActivityDate="2017-02-06T13:00:15.867" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20052" PostTypeId="2" ParentId="20042" CreationDate="2017-02-05T15:40:55.953" Score="9" Body="&lt;p&gt;An extremely high TWR at launch makes for a rocket that will be traveling very fast in relatively dense air. This means more energy is spent fighting aerodynamic drag, and leads to greater friction heating and aerodynamic stress on the rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that an initial TWR of 5:1 will increase rapidly as fuel is consumed, and implies a peak acceleration on the order of 20g or more. This is way too much for manned flights and a big issue for unmanned - most space cargos are rated for something around 6g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, there's no real advantage to getting into orbit quickly, except in the possible case of military applications. It takes months or years to prepare for a launch and it takes 92 minutes to go around once in LEO; what are you going to do with the 10 minutes saved in the middle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, the ideal TWR is usually the lowest TWR that gets the launcher safely clear of the tower in a reasonable amount of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thought experiment: Consider a 75-ton launcher with a TWR off the pad of 2:1 (i.e. the engines produce 150 tons of thrust at sea level). If you were to add 25 tons of &quot;drop tanks&quot; - say 1 ton of plumbing and separation equipment, 2 tons of tank structure, and 22 tons of fuel - you'd raise the liftoff mass to 100 tons, and you'd have a liftoff TWR of 1.5:1. If the drop tanks feed propellant into the main tanks at the same rate the engines consume them, then at the point when you empty the tanks and drop them, you effectively have the exact same 75-ton rocket you started with (okay, with a very small penalty in plumbing mass), with full tanks, but you've added some altitude and vertical velocity - something like 7m/s for each second it took to empty the drop tanks. In practice, the mass of empty tankage is so small that real launchers don't even bother with drop tanks - they simply design for a very low initial TWR. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9's liftoff TWR is around 1.2. Saturn V was around 1.16. Shuttle was on the high side at about 1.5. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-07-22T00:06:16.533" LastActivityDate="2018-07-22T00:06:16.533" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="20053" PostTypeId="2" ParentId="20049" CreationDate="2017-02-05T22:31:03.190" Score="3" Body="&lt;p&gt;Your basic question seems to be whether we can build megastructures that use passive momentum and other orbital characteristics to make entering orbit simpler. We can, but the specific example isn't a very good one: it uses far too much material and has an unclear method of use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A better example is the classic space elevator. This uses the principle that required orbital speed gets lower and lower at higher altitudes, along with the increase of speed with a fixed angular velocity, to produce an upward force at certain altitudes that matches the downward pull of gravity at lower ones. This way, a single continuous cable's weight in its lower half is supported by its &quot;negative weight&quot; in its upper half. And, in principle, one can simply attach a load to the cable and crank it up with solar power, beamed power from earth, or something similar, and when it gets high enough it's automatically orbiting. Unfortunately, there are &lt;a href=&quot;//space.stackexchange.com/questions/2/what-technological-barriers-do-we-need-to-overcome-to-build-a-space-elevator&quot;&gt;many very serious problems with the basic science required to make a space elevator&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also possible to make skyhooks, which function somewhat similarly and have most of the same challenges, but can be made to orbit at lower altitudes and higher speeds, trading off the ease of connecting for a much cheaper infrastructure. They can also be made to counter-rotate to allow more time and less energy to connect. Skyhooks all need some way to balance the momentum lost to accelerating their payloads; this can either be decelerated masses reentering in the opposite way, or high-efficiency, low-thrust ion engines or the like, which are too low-powered to be of any use in conventional rocket launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, there's the launch loop, which takes the basic premise that &lt;em&gt;something&lt;/em&gt; needs to be moving faster than orbital speed and uses magnetic levitation to separate it from its container, which is the only part that needs to be attached to the planet and directly support payloads. Specifically, the launch loop idea is a very long loop of vacuum tubing magnetically guiding and supported by a chain of iron cylinders moving considerably faster than normal orbital speed. The two sides of the loop would be parallel for most of the length, which would be on the order of 3000 km, and use tethers from the ground to stay at an altitude of 100 km or so (low enough to avoid most debris, but high enough to ignore ram heating even at orbital speeds). Payloads would use magnetic induction to transfer momentum from the circulating chain to themselves as they moved along the chosen main track, and the chain would be re-accelerated in the end loops to compensate for loss of speed, and radiate the extra heat from induction all along its length. The launch loop idea has several considerable advantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it's practical with current materials and knowledge&lt;/li&gt;&#xA;&lt;li&gt;it can largely be built on earth, then lofted just by accelerating the chain to design speeds, rather than needing rockets&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, it is undeniably still very expensive (tens of billions of dollars, very likely) and somewhat challenging to place and secure (presumably somewhere in the ocean, on the equator).&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2017-09-17T02:49:30.540" LastActivityDate="2017-09-17T02:49:30.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20054" PostTypeId="1" CreationDate="2017-02-06T10:03:12.100" Score="6" ViewCount="5063" Body="&lt;p&gt;Suppose I want to send a rocket to Mars, or to the Moon, from earth.&#xA;How much fuel (in percentage of the full load) is needed for the rocket lift-off as a function of the altitude?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know if it exist an explicit formula, but for the sake of concreteness, let say that I'm interested in the following checkpoints:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Exosphere: 700 to 10,000 km (440 to 6,200 miles)&lt;/li&gt;&#xA;&lt;li&gt;Thermosphere: 80 to 700 km (50 to 440 miles)&lt;/li&gt;&#xA;&lt;li&gt;Mesosphere: 50 to 80 km (31 to 50 miles)&lt;/li&gt;&#xA;&lt;li&gt;Stratosphere: 12 to 50 km (7 to 31 miles)&lt;/li&gt;&#xA;&lt;li&gt;Troposphere: 0 to 12 km (0 to 7 miles)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I guess that the most difficult one is the first one, since the air is thicker, gravity falls off with the square of the distance, and the weight of the rocket is at his maximum.&#xA;An answer would be something like: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;30% &lt;/li&gt;&#xA;&lt;li&gt;40% &lt;/li&gt;&#xA;&lt;li&gt;60% &lt;/li&gt;&#xA;&lt;li&gt;80% &lt;/li&gt;&#xA;&lt;li&gt;95%&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If you know good references on the topic, even books, please insert them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: To clarify, here I'm referring to a launch from the surface and to measure at each checkpoint how much fuel I've used in percentage. Such a putative formula should be parametrized by fuel mass, rocket mass, and kinematics (time spent accelerating, acceleration) and might contain some constant taking into account  atmospheric factors (mainly air density and temperature I guess) and gravity (such that I could use the same formula starting from another planet but changing these constants).&lt;br&gt;&#xA;It's true that I've asked this question in order to roughly figure out how (and if) an air launch can be convenient in some situations.&lt;/p&gt;&#xA;" OwnerUserId="16049" LastEditorUserId="16049" LastEditDate="2017-02-09T00:15:59.890" LastActivityDate="2017-12-31T12:16:53.270" Title="Fuel needed for rocket lift-off as a function of the altitude" Tags="|rockets|mass-fraction|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20055" PostTypeId="2" ParentId="20043" CreationDate="2017-02-06T12:31:34.917" Score="3" Body="&lt;p&gt;A fart, or equally breathing out, would approximate a cold gas thruster, for which Isp is often taken as &lt;strong&gt;about 60s&lt;/strong&gt;, i.e. Ve = 600 m/s, though this depends on a few factors.  I'm presuming that the relationship to space exploration is that this is the familiar hypothetical &quot;stranded in the middle of a large void in a space station&quot; question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sums&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a very rough supporting argument with as little a reference to thermodynamics as possible: Each propellant molecule as three degrees of translational freedom, each with an energy of &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;(1/2) k T   &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Just thinking practically, it wouldn't be valid to say that all the translational energy from three dimensions, (3/2) k T, is available propusively as we don't necessarily have a good nozzle, but lets use 3/2 as an upper limit.&#xA;Each molecule has a mass and a velocity that we can relate to the thermal energy through &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;(1/2) m Ve^2&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;k, Boltzmanns constant, is 1.38 x 10^-23 J/K&lt;/li&gt;&#xA;&lt;li&gt;T, human core temperature is (273 + 37) deg K&lt;/li&gt;&#xA;&lt;li&gt;One atomic mass unit is 1.66E-27 kg&lt;/li&gt;&#xA;&lt;li&gt;Lets assume the composition is methane, CH4, with a molar mass of 16&#xA;AMU&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Put all this together and you get 70s for (3/2) kT, or 40s for just a (1/2) kT which envelopes the 60s rule of thumb we started with.  Note that this does not account for the presence of air in said space station which will affect the degree of nozzle expansion that is possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Practicalities&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whilst the Isp is above Russell Borogove's answer for urination we haven't yet considered the thrust level which will be rather lower so the stranded astronaut would have to do a trade-off to decide what to do, not least to think how they would maintain attitude control during the manoeuvre. &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-02-06T12:31:34.917" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20056" PostTypeId="1" AcceptedAnswerId="20061" CreationDate="2017-02-06T13:40:20.860" Score="2" ViewCount="1478" Body="&lt;p&gt;Loren Petchel &lt;a href=&quot;https://space.stackexchange.com/a/20052/103&quot;&gt;wrote&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;gravity loss is 9.8m/s * (1 - your fraction of orbital speed) so you want to build horizontal speed as soon as it won't cost you too much drag.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;My &quot;feel&quot; in Kerbal Space Program tells me otherwise. A low-TWR upper stage (order of TWR 0.3) is doomed to plunge into the atmosphere if it's missing 1/4 of its orbital velocity, even if burning at an angle &quot;upwards&quot;. Actual reduction of gravitational drag &quot;feels&quot; to be non-linear, only last 200-300m/s out of Kerbin's 2200m/s really changes things. This is just a subjective feel in a quite inaccurate space simulator. How is it for real? Is Loren's equation accurate or is my &quot;feel&quot; supported by actual hard maths?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2022-10-18T13:27:22.593" Title="How are gravitational losses on ascent related to horizontal velocity?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20057" PostTypeId="1" AcceptedAnswerId="20069" CreationDate="2017-02-06T13:43:23.157" Score="5" ViewCount="813" Body="&lt;p&gt;I've looked at &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/5ou7mt/is_iridium_launch_object_m_a_test_satellite/&quot; rel=&quot;noreferrer&quot;&gt;Is Iridium launch object M a test satellite?&lt;/a&gt; and I don't understand what seems to have happened.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Iridium launch produced 11 orbiting &lt;a href=&quot;http://stuffin.space/?intldes=2017-003M&amp;amp;search=2017-003&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;objects&lt;/strong&gt;&lt;/a&gt; when only 10 Iridium satellites were launched. There was speculation that S2 had failed to de-orbit into the South Pacific as planned.&lt;/p&gt;&#xA;&lt;p&gt;However looking at object M it has now changed orbit to an inclination of 89.9 degrees with an apogee of 868 km and perigee of 739 km. This is not an Iridium orbit so is this in fact a test satellite for the SpaceX constellation?&lt;/p&gt;&#xA;&lt;p&gt;If so it would have been launched from inside the payload adapter after the Iridium satellites had been deployed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;notes:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;the letter 'I' is skipped because it can be confused with &amp;quot;1&amp;quot;, thus 2017-003A, B, C, D, E, F, G, H, J, K are the SATCAN numbers of the ten Iridium communications satellites&lt;/li&gt;&#xA;&lt;li&gt;what (if anything) was or is 2017-003&lt;strong&gt;L&lt;/strong&gt;?&lt;/li&gt;&#xA;&lt;li&gt;what (if anything) was or is 2017-003&lt;strong&gt;M&lt;/strong&gt;?&lt;/li&gt;&#xA;&lt;li&gt;if you haven't already, go back and click on the &lt;strong&gt;objects&lt;/strong&gt; link to see stuff in space.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2021-12-23T04:05:31.813" Title="What is the final conclusion about 'object M'? What happened?" Tags="|spacex|artificial-satellite|low-earth-orbit|payload-deployment|iridium|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20058" PostTypeId="2" ParentId="18270" CreationDate="2017-02-06T13:53:47.990" Score="2" Body="&lt;p&gt;The rocker-bogie suspension has its origins in a 1969 study by M. G. Bekker, 'Introduction to Terrain-Vehicle Systems' which I haven't been able to find online.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By 1994, NASA had designed &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19940025127&amp;amp;hterms=mars%20rover%20mechanisms%20Rocky&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3Dmars%2520rover%2520mechanisms%2520Rocky%26Ntx%3Dmode%2520matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;a series of rover concepts named 'Rocky'&lt;/a&gt;. They started with an articulated 6-wheeled rover. By the 4th iteration (yes, Rocky IV), the 6-wheel rocker bogie design was established. The 6th iteration would lead to the Sojourner design which flew in 1997 as part of the Pathfinder mission.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A discussion of rocker-bogie vs other designs can be found in 'Sojourner: An Insider's View of the Mars Pathfinder Mission' by A. Mishkin.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-02-06T14:11:02.940" LastActivityDate="2017-02-06T14:11:02.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20059" PostTypeId="2" ParentId="20007" CreationDate="2017-02-06T19:15:50.307" Score="6" Body="&lt;p&gt;It's also possible that they don't want the negative publicity of these farings floating on the surface of the ocean downrange and being either navigational hazards, or a target for environmental concerns.  I'd guess they're light enough that they wouldn't burn up and probably would just float after hitting the water.&lt;/p&gt;&#xA;" OwnerUserId="16735" LastActivityDate="2017-02-06T19:15:50.307" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20061" PostTypeId="2" ParentId="20056" CreationDate="2017-02-06T21:10:15.450" Score="4" Body="&lt;p&gt;Since &amp;quot;gravitational losses&amp;quot; is a rather loose term with no firm definition, let's introduce a term (already common in the Kerbal Space Program community) which allows quantifying them well: gravitational drag.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_drag&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravitational drag&lt;/a&gt;&lt;/strong&gt; can be understood as weight of a body in motion over planet's surface, adjusted for centripetal acceleration caused by motion relative to the planet's curvature. For immobile craft it will be its weight; for a craft in circular orbit it's zero.&lt;/p&gt;&#xA;&lt;p&gt;Gravitational drag is an especially handy tool for calculations regarding spaceplanes - since it's the force directly counteracted by airplane's lift, it makes a very natural appearance in equations regarding spaceplane aerodynamics, in place where airplane equations use weight.&lt;/p&gt;&#xA;&lt;p&gt;For rockets, it's offset by the vertical component of thrust, and rocket's total delta-V wasted on fighting gravitational drag is directly proportional to value of the gravitational drag and time spent under its influence; that way it's a good measure of the rate at which delta-V is lost.&lt;/p&gt;&#xA;&lt;p&gt;It seems I was right; the relation between gravitational drag and difference between current and orbital speed is quadratic:&lt;/p&gt;&#xA;&lt;p&gt;Centripetal force in circular motion is&#xA;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;F_c ={{m v^2} \over r}&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Gravitational force:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$&#xA;F_g = G {{m M} \over r^2} &#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Gravitational drag will be the difference:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ &#xA;F_d = F_g-F_c  = G {{m M} \over r^2} - {{m r v^2} \over r^2} = \left( {m \over r^2} \right) (G M - rv^2)&#xA;$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;So, with constant altitude and masses, gravitational drag is proportionla to &lt;span class=&quot;math-container&quot;&gt;$(G M - rv^2)$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;In practice, the effect is exacerbated by quite a few more factors:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mass of the craft is not constant - it drops with fuel burnt, making the part &lt;span class=&quot;math-container&quot;&gt;$ \left( {m \over r^2} \right)$&lt;/span&gt; drop along with &lt;span class=&quot;math-container&quot;&gt;$m$&lt;/span&gt; - extra drop of gravitational drag.&lt;/li&gt;&#xA;&lt;li&gt;Mass dropping causes craft's TWR - and acceleration - to increase, so &lt;span class=&quot;math-container&quot;&gt;$v$&lt;/span&gt; growth is greater than linear; thus rate of gravitational drag drop over &lt;em&gt;time&lt;/em&gt; is growing in more than quadratic proportion.&lt;/li&gt;&#xA;&lt;li&gt;The craft is frequently not moving horizontally, but still climbing, so the &lt;span class=&quot;math-container&quot;&gt;$r^2$&lt;/span&gt; in nominator keeps growing &lt;span class=&quot;math-container&quot;&gt;$ \left( {m \over r^2} \right)$&lt;/span&gt; drops; as factor of &lt;span class=&quot;math-container&quot;&gt;$v^2$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; grows too, increasing the argument. This effect is not as pronounced, as &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; is counted from center of Earth, and nearing apoapsis the craft loses vertical velocity.&lt;/li&gt;&#xA;&lt;li&gt;the craft escaping atmosphere loses atmospheric drag (increasing acceleration again)&lt;/li&gt;&#xA;&lt;li&gt;the engine operates in vacuum on a higher pressure differential than in atmosphere, resulting in higher thrust (higher acceleration again!)&lt;/li&gt;&#xA;&lt;li&gt;the craft burns nearly horizontally, about all of its thrust contributing towards the horizontal speed, instead of being split between climb component and horizontal acceleration component.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;While any of these side effects doesn't contribute as much as sheer horizontal velocity, they all add up, the result being gravitational drag vanishing way more rapidly near the end of the orbital insertion burn than during the earlier phase of flight - its drop in relation to burn time (as opposed to just velocity) becoming a good bit steeper than just quadratic.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Since the term 'gravitational drag' causes some controversy, let me present some arguments in its defense:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Gravitational drag is a valuable tool in case of spaceplanes. At low airspeeds it degrades to weight, and so substituting it for weight in classic aviation equations allows extending them to cases where orbital mechanics begin to matter.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;With these equations, the four fundamental forces in aviation become thrust, aerodynamic drag (opposing thrust), lift, and gravitational drag (opposing lift).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Atmospheric drag is linearly proportional to air density and drag coefficient of the craft. Gravitational drag is proportional to gravitational field strength &lt;span class=&quot;math-container&quot;&gt;$GM \over r^2$&lt;/span&gt; (think &amp;quot;density of gravity&amp;quot;) and mass of the craft.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Atmospheric drag is inversely proportional to square of speed relative to air. Gravitational drag is inversely proportional to square of speed relative to orbital speed.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;It's pointless to use above orbital speed, and not needed at low speeds, but it's a very helpful quantity for ascent calculations.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2022-10-18T13:27:22.593" LastActivityDate="2022-10-18T13:27:22.593" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="20062" PostTypeId="1" CreationDate="2017-02-06T21:41:24.687" Score="1" ViewCount="366" Body="&lt;p&gt;I read from &lt;a href=&quot;http://www.nature.com/articles/ncomms1772&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; that there’s this material called &lt;em&gt;Organic photovoltaic (&lt;strong&gt;OPV&lt;/strong&gt;) solar cells&lt;/em&gt; that could be used, for example, as wing material for an ornithopter. How much energy would it be able to provide and would that alone be enough for a &lt;a href=&quot;http://robotics.eecs.berkeley.edu/~ronf/Ornithopter/&quot; rel=&quot;nofollow noreferrer&quot;&gt;robotic ornithopter&lt;/a&gt; to fly on Mars, and possibly carry a small amount of payload (like 1kg)?&lt;/p&gt;&#xA;" OwnerUserId="18363" LastEditorUserId="1142" LastEditDate="2017-03-19T13:45:06.253" LastActivityDate="2017-03-19T13:45:06.253" Title="Could robotic ornithopter have all energy it needs to be able to fly from solar power?" Tags="|mars|design|solar-power|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20063" PostTypeId="2" ParentId="17452" CreationDate="2017-02-06T22:00:47.770" Score="1" Body="&lt;p&gt;This may be less confusing:&#xA;The camera shot includes either the sunrise terminator in the west or the sunset terminator in the east, at least 13 times per Earth solar day.&lt;/p&gt;&#xA;" OwnerUserId="18435" LastEditorUserId="6816" LastEditDate="2017-02-06T22:26:32.627" LastActivityDate="2017-02-06T22:26:32.627" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20064" PostTypeId="2" ParentId="20057" CreationDate="2017-02-06T22:49:17.353" Score="1" Body="&lt;p&gt;JSpOC's orbit determination and object identification is a long way from perfect, and particularly suffers when several objects are released into similar orbits, hence the temporary misidentification of an extra object associated with the launch. There was no such object. The 2nd stage reentry was normal.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="12102" LastEditDate="2017-02-07T09:08:14.897" LastActivityDate="2017-02-07T09:08:14.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20067" PostTypeId="1" AcceptedAnswerId="25629" CreationDate="2017-02-07T03:23:14.513" Score="13" ViewCount="1208" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/20008/12102&quot;&gt;This&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/20010/12102&quot;&gt;this&lt;/a&gt; answer describe the Falcon 9 fairings as being quite large, with the payload volume being roughly &quot;the size of a bus&quot; or simply &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;REALLY REALLY big. 13 meters by 4.6 meters&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and being made of an aluminum honeycomb and carbon fiber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then this &lt;a href=&quot;https://space.stackexchange.com/a/20059/12102&quot;&gt;more recent answer&lt;/a&gt; got me thinking further.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the very large area compared to mass and intrinsic strength (built for max-Q aerodynamic load and durability under launch vibrations) and the relatively low ground speed at which they are deployed, they could potentially survive re-entry and remain on the surface of the ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this what happens, at least sometimes? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so, their buoyancy would result in perhaps of order one meter height, making them large, but low visibility objects and potential navigation risks for smaller surface craft, both visually and perhaps radar as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would F9 fairing recovery be part 'good Earth citizenry' in  addition to good economics?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2023-07-25T04:07:09.200" Title="Do 'size of a bus' sized F9 fairings ever float after impact? Are they navigation hazards?" Tags="|spacex|falcon-9|recovery|fairings|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20069" PostTypeId="2" ParentId="20057" CreationDate="2017-02-07T10:25:32.083" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Joint_Space_Operations_Center&quot; rel=&quot;nofollow noreferrer&quot;&gt;Joint Space Operations Center&lt;/a&gt; or JSpOC:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;...is the organization responsible for performing all of the orbit determination activity necessary to maintain the US space catalogue.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;During announced space launches and satellite deployment (and of course unannounced ones as well!) various space monitoring technologies including ground based radar and optical telescopes make a series of observations of space objects for the purposes of &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_determination&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbit determination&lt;/a&gt;. At some point if an object crosses some threshold of potential to be a new object, it is assigned a NORAD Catalog Number. If an observed object is later determined to be a known object, the identity assigned to the catalog number is updated. Thus one object can easily appear under more than one catalog number.&lt;/p&gt;&#xA;&lt;p&gt;International designators are applied to identifiable components of a space launch or a space mission. The naming is getting more complex as components attach and leave the international space station, or are shuttled there then deployed to space at a later time (e.g. cubesats getting tossed out of an airlock).&lt;/p&gt;&#xA;&lt;p&gt;In this case 2017-003 is the third launch of 2017 to be assigned an international designator.&lt;/p&gt;&#xA;&lt;p&gt;While observing the deployment, eleven objects were observed in addition to the Falcon 9 2nd stage, and assigned NORAD catalog numbers 41917 through 41927, and the letters A, B, C, D, E, F, G, H, J, K and M were assigned to them. It is not clear why the letter 'L' was skipped, but the letter 'I' is always skipped because of a potential ambiguity - it can be confused with the digit '1'. 41917 through 41926 likely matched the 10 Iridium satellites in character, and were later identified as IRIDIUM 102, 103, 104, 105, 106, 108, 109, 111, 112, 114 (not in that order, see below) the satellites known to be in the F9 2nd stage payload.&lt;/p&gt;&#xA;&lt;p&gt;However, the eleventh object, number 41927 was in question. It seems to have been given the international designator 2017-003&lt;strong&gt;M&lt;/strong&gt;, but was also called 2017-003&lt;strong&gt;L&lt;/strong&gt;. From &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/5nsaqm/rspacex_iridium_next_constellation_mission_1/dclyc7c/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;I've been in email contact with Dr T.S. Kelso, who runs Celestrak.com. The first thing he did is got the orbital information from Iridium and matched them up, so the Celestrak database now has the satellites named. He has changed Object M to Object L in his database, as he can see no reason, &lt;em&gt;other than an input error, for the letter L to be missed&lt;/em&gt;. If it was left for the stage, then there would have also been a spare. Catalog ID as well. (&lt;em&gt;emphasis added&lt;/em&gt;)&lt;/p&gt;&#xA;&lt;p&gt;As for the 11th object:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;In all likelihood, OBJECT L is some kind of upper stage, unless there were deployment mechanisms expected to be large enough to track. We don’t have any RCS data yet (and even that can be unreliable), so there is no way to know if this is something large like a rocket body or something else. All we can do is continue to monitor at this point.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Today I've done a search on the celestrak website: &lt;a href=&quot;https://celestrak.org/satcat/search.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://celestrak.org/satcat/search.asp&lt;/a&gt; and by playing around with the search input, including the text in the un-selected data field, I could get the chunk of catalog numbers that included the ROI. You can see that number 41927 has been identified, and there are mysteriously un-named catalog numbers immediately following.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uFfsH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uFfsH.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/chJxX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/chJxX.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kSDWi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kSDWi.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:51.853" LastActivityDate="2017-02-09T11:15:32.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20070" PostTypeId="2" ParentId="20067" CreationDate="2017-02-07T10:28:51.357" Score="12" Body="&lt;p&gt;The fairing consists of an aluminium honeycomb sandwiched between carbon fibre sheets. The amount of air trapped in the honeycomb determines if the structure will float, but as we don't know how thick the honeycomb and the CF cheets are, there's no way to say for sure whether it'll float.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shock of landing may damage the fairing, but we don't know its landing speed so we don't know for sure yet. The only fairing I've seen after landing was washed up on shore after being at sea for a while, so no conclusions can be drawn from that one either.&lt;br&gt;&#xA;If it floats, it won't do so indefinitely. Wave action will deform the fairing until the fibre delaminates and water can get into the honeycomb. At that point, it'll sink.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-02-07T10:47:43.813" LastActivityDate="2017-02-07T10:47:43.813" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20071" PostTypeId="1" CreationDate="2017-02-07T16:01:24.783" Score="3" ViewCount="1192" Body="&lt;p&gt;This photo has been bothering me for quite a while, so I decided to look into it further. It is shown in Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Opposition_surge&quot; rel=&quot;nofollow noreferrer&quot;&gt;Opposition Surge&lt;/a&gt; article, which is a fascinating topic that I'd like to avoid &lt;em&gt;here&lt;/em&gt; if possible. Currently the article includes the first photo shown below, a &lt;em&gt;sepia-toned photo of Neil Armstrong, taken by Neil Armstrong&lt;/em&gt;, with, apparently, opposition surge happening around the helmet area of the shadow on the lunar surface. The shadow is of course Buzz Aldrin, and Neil Armstrong is in the white spacesuit on the right side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hYBDs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hYBDs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few clicks more and one can see that this image has been cropped and zoomed from the Buzz Aldrin's visor in a much larger image, shown below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question is&lt;/strong&gt; about the connection of this photograph to the phenomena of opposition surge, coherent backscatter, or shadow-hiding. Is this just a randomly found example by an author of the Wikipedia article, or was the &quot;discovery&quot; of opposition surge in this &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11-5903history.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;already-famous-for-another-reason&lt;/a&gt;-or-two photo actually a notable thing? Are there other examples of this effect in local, human-scale lunar photos? Was it ever photographed or measured on purpose in subsequent Apollo missions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chang'e 3&lt;/a&gt; look for it by photographing it's own shadow? Or, &lt;a href=&quot;https://en.wikipedia.org/wiki/Yutu_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;YuTu&lt;/a&gt; could have carried a little convex mirror and duplicated the shot!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v0Cgm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v0Cgm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Yutu_(rover)&quot; rel=&quot;nofollow noreferrer&quot;&gt;YuTu&lt;/a&gt; lunar rover photographed by &lt;a href=&quot;https://en.wikipedia.org/wiki/Chang%27e_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chang'e 3&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IdkOh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IdkOh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NASA/Apollo photo AS11-40-5903 from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:AS11-40-5903_-_Buzz_Aldrin_by_Neil_Armstrong_(full_frame).jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (larger size available there and at the &lt;a href=&quot;http://www.lpi.usra.edu/resources/apollo/frame/?AS11-40-5903&quot; rel=&quot;nofollow noreferrer&quot;&gt;bottom of this page&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-12T08:27:50.747" LastActivityDate="2019-12-14T01:59:56.017" Title="What's the story behind this Apollo-era image?" Tags="|the-moon|photography|apollo-11|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20072" PostTypeId="1" CreationDate="2017-02-07T16:46:59.880" Score="4" ViewCount="628" Body="&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; The stories in Phys.org &lt;a href=&quot;https://phys.org/news/2017-05-nasa-epic-view-earth.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's EPIC view spots flashes on Earth&lt;/a&gt; and Live Science &lt;a href=&quot;http://www.livescience.com/59117-earth-satellite-images-shown-flashing-lights.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mysterious Light Flashes &lt;strong&gt;1st Spotted by Carl Sagan&lt;/strong&gt; Explained&lt;/a&gt; seems to have information potentially relevant to this. Apparently &lt;em&gt;these have been seen for decades by several different spacecraft&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Journal Article now viewable&lt;/strong&gt; as epdf: &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/2017GL073248/epdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://onlinelibrary.wiley.com/doi/10.1002/2017GL073248/epdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Watch the NASA Goddard video &lt;a href=&quot;https://youtu.be/YN4aSHc0n0w&quot; rel=&quot;nofollow noreferrer&quot;&gt;EPIC Observations of Ice in Earth's Atmosphere&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/YN4aSHc0n0w?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/10356&quot;&gt;This answer&lt;/a&gt; has been bugging me for a while, so I would like to challenge it by asking for a technical reference, journal article, or something that explains the phenomenon more fully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This purple dot appeared around the sub-solar point in an &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory#EPIC&quot; rel=&quot;nofollow noreferrer&quot;&gt;EPIC&lt;/a&gt; photo of the Earth, from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSCOVR&lt;/a&gt; satellite in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lissajou&lt;/a&gt; orbit around the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun-Earth L1&lt;/a&gt; point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the purple dot really caused by an effect of &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassegrain_reflector&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassegrain optics&lt;/a&gt; and/or &lt;a href=&quot;https://en.wikipedia.org/wiki/Coherent_backscattering&quot; rel=&quot;nofollow noreferrer&quot;&gt;coherent backscattering&lt;/a&gt;? If so, can I read further on the technical explanation of small, highly collimated bright spots at sub-solar points on Earth seen from space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note, this looks to be of the order of 10km or less at a distance of 1,500,000km! That's what I mean by highly collimated!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that it is of the order of 1 pixel of the EPIC's camera as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TLl9X.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TLl9X.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Bright spot at sub-solar point taken from &lt;a href=&quot;https://space.stackexchange.com/a/10356&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tiFa3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tiFa3.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen capture from Google maps, annotated with a box marking the approximate location and size of the cropped image with the purple dot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MdmzW.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MdmzW.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; The full DSCOVR EPIC image taken from &lt;a href=&quot;https://space.stackexchange.com/a/10356&quot;&gt;this question&lt;/a&gt;. If you right click and open in a separate view, and zoom, you can see the purple spot.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Side note:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bright, highly collimated reflections could be produced by highly reflective, fairly flat surfaces, and water is one such example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Watching &lt;a href=&quot;https://www.youtube.com/watch?v=CFrP6QfbC2g&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; one can clearly see occasional bright flaring near the sub-solar point when it is on water, presumably the water is unusually calm and a much larger fraction of sunlight is directed into a cone around normal incidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zcpa0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zcpa0.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Screen shot from DSCOVR &lt;a href=&quot;https://www.youtube.com/watch?v=CFrP6QfbC2g&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; showing an example of a common bright reflection from calm seas near the sub-solar point.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-17T10:15:13.867" LastActivityDate="2017-05-17T10:15:13.867" Title="Now what do we think? Is this purple dot really caused by Cassegrain optics and coherent backscattering?" Tags="|imaging|instrument|dscovr|camera|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20073" PostTypeId="1" AcceptedAnswerId="20075" CreationDate="2017-02-07T17:31:39.553" Score="2" ViewCount="2676" Body="&lt;p&gt;Say, I'm given the data typical to the Rocket equation in its many variants: $\Delta v, I_{sp}, m_f, m_0, ρ, T$ (thrust) and the likes. And someone asks me &quot;What's the horsepower of the space shuttle main engine?&quot; - Assuming I don't want to cuss them off, or ask you for an answer, how would I go about calculating the power of the engine? Or &quot;how many kilotons TNT worth of energy is spent on getting a Soyuz from launchpad to orbit&quot; - how to calculate the total energy expenditure? And how does even Work fit into that whole image?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2022-05-29T17:24:28.280" Title="What is the relation between the rocket equation and work, energy and power?" Tags="|power|energy|rocket-equation|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20074" PostTypeId="1" CreationDate="2017-02-07T18:10:49.477" Score="0" ViewCount="159" Body="&lt;p&gt;In the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/3211/where-could-we-build-a-space-elevator-today-2014&quot;&gt;this question about space elevators&lt;/a&gt;, the poster mentioned that the elevator cable would pass through a debris cloud as it moved through lower orbits.  I presume most of that material is &quot;space junk&quot;, but some of it must be functioning satellites.  If we were to discover technology that enabled us to build a space elevator tomorrow, how would we clear a place for it to go?&lt;/p&gt;&#xA;" OwnerUserId="14104" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-02-07T18:10:49.477" Title="How to clear the way for a space elevator?" Tags="|low-earth-orbit|debris|space-elevator|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20075" PostTypeId="2" ParentId="20073" CreationDate="2017-02-07T20:55:03.630" Score="9" Body="&lt;p&gt;If you're in vacuum, the power represented by the kinetic energy of the thrust is the thrust force times the exhaust velocity (g times Isp) over two:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P={g I_{sp}T\over 2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One SSME in vacuum delivers &lt;a href=&quot;http://www.wolframalpha.com/input/?i=512300+lbf+*+standard+gravity+*+452.3+seconds+%2F+2+in+horsepower&quot; rel=&quot;noreferrer&quot;&gt;almost seven million horsepower&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I think they might have forgotten to divide by two on &lt;a href=&quot;http://www.rocket.com/space-shuttle-main-engine&quot; rel=&quot;noreferrer&quot;&gt;this Aerojet Rocketdyne page&lt;/a&gt;, or maybe they were calculating something else.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual work per unit time done on the rocket is different. That is the force of the thrust times the velocity of the rocket. For example, the power applied to the rocket while the SSMEs are running full throttle is zero, while the whole thing is still bolted to the pad. Right before MECO, the maximum velocity while the engines are still running, I get one SSME could do about 24 million horsepower of work per unit time on the orbiter, if at full throttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure that either horsepower number provides a useful comparison with, say, an automobile engine horsepower, if that's what you're looking for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SSME pump ratings on the other hand are directly comparable to an automobile engine's.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-02-07T21:33:38.637" LastActivityDate="2017-02-07T21:33:38.637" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20076" PostTypeId="2" ParentId="20072" CreationDate="2017-02-07T21:03:04.460" Score="1" Body="&lt;p&gt;I think this was answered perfectly well in both the answer and the comment left by Tildal.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;So it would appear as an overexposed region, that might appear as pretty much anything after post-processing, including the Moiré fringed dot with typical read and blue channel interference pattern around it on the image that you noticed, once the RGB sources, or slightly different angles, or both, are interpolated. Notice that there's also a bit more of color bleeding around the central dot, than elsewhere on the image&lt;/p&gt;&#xA;&lt;p&gt;Moiré pattern removal tool removes color bands nicely without loss of sharpness, leaving an artifact resembling a spherical object roughly 40 km (25 mi) in diameter. It doesn't appear to be a dead pixel on the sensor, but a post-processing artifact of 5x5 pixels in the central region of a 2048x2048 pixel source&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The fact it is purple is almost irrelevant - post processing of images can leave you with results of any colour.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-07T21:03:04.460" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20077" PostTypeId="1" AcceptedAnswerId="20079" CreationDate="2017-02-08T04:40:58.873" Score="0" ViewCount="1183" Body="&lt;p&gt;Ion thrusters require a lot of energy because of their high exhaust velocities. But why can't they just pump a mixture of hydrogen and xenon into the chamber, mix it with oxygen and burn it? Doesn't this ionize the gas and also provide enough thermal heat to push the plasma through the grids?&lt;/p&gt;&#xA;&lt;p&gt;Maybe not hydrogen and oxygen, but some other exothermic reaction. Not to completely avoid electricity input but reduce it. Maybe reducing the size of the solar panels. I am talking about deep space missions.&lt;/p&gt;&#xA;&lt;p&gt;This would also be much more efficient than energy production through turbines, wouldn't it?&lt;/p&gt;&#xA;&lt;p&gt;Is there any drawback of using exothermic reactions?&lt;/p&gt;&#xA;" OwnerUserId="18356" LastEditorUserId="47149" LastEditDate="2022-06-01T14:18:39.770" LastActivityDate="2022-06-01T14:18:39.770" Title="Why don't ion thrusters use exothermic reactions directly to ionize the gas and heat up the plasma?" Tags="|physics|ion-thruster|energy|chemistry|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="20078" PostTypeId="2" ParentId="20077" CreationDate="2017-02-08T04:58:34.593" Score="1" Body="&lt;p&gt;Because what you are just describing is regular liquid fueled rocket.&lt;/p&gt;&#xA;&lt;p&gt;The efficiency of a rocket fuel is dictated by its exhaust velocity.&#xA;This velocity has a maximum value for each kind of fuel.&lt;/p&gt;&#xA;&lt;p&gt;If you are going to burn hydrogen, you can achieve about &lt;span class=&quot;math-container&quot;&gt;$4\ 400 \frac{\text{m}}{\text{s}}$&lt;/span&gt;.&#xA;Ion thrusters reach about &lt;span class=&quot;math-container&quot;&gt;$29\ 000 \frac{\text{m}}{\text{s}}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Moreover, we cannot store liquid hydrogen/oxygen for a long time. For a deep space mission, this is an issue.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="47149" LastEditDate="2022-06-01T06:34:25.217" LastActivityDate="2022-06-01T06:34:25.217" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="20079" PostTypeId="2" ParentId="20077" CreationDate="2017-02-08T05:33:06.373" Score="3" Body="&lt;h2&gt;Ion engines need the ions to stay ionized&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;To be usefully efficient, an ion engine needs to ionize a substantial fraction of the propellant, and keep it ionized until it passes through most of the positive electrostatic potential gradient to whatever potential the local free space is assigned. I won't say &quot;ground&quot; here, but the assignment of zero is arbitrary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's the electrostatic acceleration of the ions that is the &quot;push&quot; of the engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means the plasma needs to be low enough pressure so that the ions and electrons don't recombine too much. In some designs using electron cyclotron resonance, the vacuum must be so low that the electrons can accelerate to high energy before a useful, ionizing collision. That low pressure is way too low to allow a sustained chemical reaction. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Stuff doesn't burn in a near-vacuum&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;An ion engine's biggest job is to keep generating high energy electrons to continue to ionize the neutral gas constantly introduced into the chamber. There are then so few collisions that the reaction of one oxygen/hydrogen atom pair wouldn't trigger any nearby atoms to undergo a similar reaction. It wouldn't be self-sustaining, and in fact almost all the hydrogen and oxygen would pass out of the engine unreacted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/C8TFY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C8TFY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from this YouTube Video &lt;a href=&quot;https://www.youtube.com/watch?v=grU8g9jnS4w&quot; rel=&quot;nofollow noreferrer&quot;&gt;Introduction to Ion Thrusters&lt;/a&gt;. Right-click and open in a new view for full size readability and detail. Image is placed here to avoid link rot if the YouTube video is ever moved or deleted.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here is a link to the actual video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/grU8g9jnS4w?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-08T05:43:09.147" LastActivityDate="2017-02-08T05:43:09.147" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20080" PostTypeId="1" AcceptedAnswerId="20081" CreationDate="2017-02-08T05:36:43.217" Score="4" ViewCount="1042" Body="&lt;p&gt;For example, can a space ship overshoot Mars in a reverse gravitational slingshot mixed with a little air breaking, steering to keep the spacecraft from orbit and then land on Mars with a near 0 ground speed during entry? No sharp turns it would just be going slow enough to stall at the edge of the planet's gravity and start falling back to the planet. The picture is not a perfect model on how this could work. The space after each pass would give the heat shield time to cool.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.newscientist.com/article/dn10288-inflatable-cushions-to-act-as-spacecraft-heat-shields/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.newscientist.com/article/dn10288-inflatable-cushions-to-act-as-spacecraft-heat-shields/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/19559/could-a-dynastat-enter-near-0-mph-ground-speed-into-the-atmosphere-of-mars&quot;&gt;Could a Dynastat enter near 0 MPH ground speed into the atmosphere of Mars?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bX5mG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bX5mG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yl6MZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yl6MZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5tRcv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5tRcv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2017-12-24T21:17:26.247" LastActivityDate="2017-12-24T21:17:26.247" Title="Can we significantly reduce entry velocity with this method by airbraking alone for interplanetary travel?" Tags="|orbital-mechanics|reentry|gravity-assist|entry-descent-landing|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20081" PostTypeId="2" ParentId="20080" CreationDate="2017-02-08T09:11:10.057" Score="8" Body="&lt;p&gt;You can use &lt;a href=&quot;https://en.wikipedia.org/wiki/Skip_reentry&quot; rel=&quot;nofollow noreferrer&quot;&gt;skip reentry&lt;/a&gt; to gradually reduce your speed when arriving from an interplanetary journey. NASA also studies skip reentry for the Shuttle, as a way to reduce the heat load during reentry. So you could use this technique to get below orbital speed before the final reentry starts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lower your speed gets, the more difficult it will be to keep skipping. When you're in orbit, you're falling towards the planet (because its gravity attract you), you just keep missing the planet because your forward speed balances out the speed at which you drop. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you reduce speed, you predominantly reduce the forward component of your speed vector. The downward force remains the same. So as your forward speed reduces, you'll need more and more lift to stay at the high altitude you need for the skip maneuver. There comes a point where the lift you need for the next skip is more than the lift your spacecraft can provide, and then you're committed to going down. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;No sharp turns it would just be going slow enough to stall at the edge of the planet's gravity and start falling back to the planet. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That will not work out the way you think. You can put the spacecraft in a very elliptical orbit and make it 'stall' (sort of, you can get the speed at apogee to a very low value). But when you're past apogee and the spacecraft starts moving towards the planet, the gravitational pull will start to accelerate the spacecraft until you're at perigee with your original speed.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So any deceleration will have to be done by aerobraking (i.e. the skip reentry described above). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect you can use a gravitational slingshot to reduce the spacecraft's speed only when the spacecraft is not in orbit. Once you're in orbit, you can't transfer any more momentum from the spacecraft to the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a gravitational slingshot, the spacecraft accelerates (under the gravitational pull of the planet) until you're at the closest approach point. Then the spacecraft will be decelerated while it moves away from the planet.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-02-18T10:43:41.717" LastActivityDate="2017-02-18T10:43:41.717" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20083" PostTypeId="1" CreationDate="2017-02-08T15:55:11.700" Score="1" ViewCount="865" Body="&lt;p&gt;I saw these two links, and they gave me a question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.space.com/35577-looking-at-alpha-centauri-stars-next-door.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.space.com/35577-looking-at-alpha-centauri-stars-next-door.html&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://io9.gizmodo.com/5786083/what-are-the-fastest-spacecrafts-ever-built&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://io9.gizmodo.com/5786083/what-are-the-fastest-spacecrafts-ever-built&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If the solar probes Helios I and II can be 4.1x faster than Voyager, and if it would take a Voyager-like probe 70,000 years to transit the 40-trillion kilometers to Alpha Centauri, what is the shortest plausible time to send a probe to Alpha Centauri with a solar-slingshot?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible that we could have a probe half-way to Alpha Centauri in 25 years using only today's technology?&lt;/p&gt;&#xA;" OwnerUserId="18449" OwnerDisplayName="EngrStudent" LastEditorUserId="18449" LastEditDate="2017-02-08T18:25:35.810" LastActivityDate="2017-02-09T19:36:23.150" Title="Best time to Alpha Centauri with Solar Slingshot?" Tags="|interstellar-travel|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20084" PostTypeId="2" ParentId="20083" CreationDate="2017-02-08T16:50:16.537" Score="7" Body="&lt;p&gt;The claim about very high Helios probes speed is made for their &lt;em&gt;perihelion&lt;/em&gt; i.e. in the point of their orbit closest to the Sun. It originates from the Sun gravity as the probe &quot;falls&quot; closer and is eaten as it goes farther. Here's the depiction of their orbit at Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YVFSs.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YVFSs.png&quot; alt=&quot;Helios probe trajectory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the probe can't go far from the Sun this high speed is completely useless for interstellar trip. Instead you should look what speed the probe will have when it goes (if it does) to the boundary of the Solar system when its decrease due to the Solar gravity becomes very small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also seems that you heard about gravitational slingshot maneuvers however don't understand it correctly. The gravitational slingshot uses planet relative motion to the sun with some speed $u$. When probe approaches the planet relatively to the latter it goes in the hyperbolic trajectory approaching at speed $\vec{v}_0$ and leaving its sphere of influence at speed $\vec{v}_f$ similar in module $|\vec{v}_0|\simeq |\vec{v}_f|$ but with different direction. However relatively to the Sun you have initial speed $\vec{v}_0+\vec{u}$ and the final speed $\vec{v}_f+\vec{u}$. If the direction changes significantly in the planet reference frame it will lead to significant change in the speed relatively to the Sun. You can't however make any &quot;slingshot&quot; if simply orbit around the sun &amp;mdash; all speed boost in the perihelion is eaten as you rise to the aphelion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If our Solar system had massive planet closer to the Sun we could use this boost to perform nice gravity slingshot at that planet. However both Venus and Mercury are quite light and are not well-suited for such a maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you &lt;em&gt;can&lt;/em&gt; get from this boost may come from using Oberth effect &amp;mdash; the changes of speed when it's high lead to higher changes of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway all these gains are of order of km/s, 10 km/s at max. The distance to Proxima Centauri is 4.26 light-years. To get there in 50 years you need to have about 8% of lightspeed. It's about 25000 km/s not 25 km/s!&lt;/p&gt;&#xA;" OwnerUserId="18138" OwnerDisplayName="OON" LastEditorUserId="6816" LastEditDate="2017-02-09T19:36:23.150" LastActivityDate="2017-02-09T19:36:23.150" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20085" PostTypeId="1" AcceptedAnswerId="20089" CreationDate="2017-02-08T18:50:10.367" Score="7" ViewCount="3079" Body="&lt;p&gt;Given:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Semi Major Axis (negative)&lt;/li&gt;&#xA;&lt;li&gt;Eccentricity (&gt;1)&lt;/li&gt;&#xA;&lt;li&gt;Argument of Periapsis&lt;/li&gt;&#xA;&lt;li&gt;Inclination&lt;/li&gt;&#xA;&lt;li&gt;Longitude of Ascending Node&lt;/li&gt;&#xA;&lt;li&gt;Current True Anomaly&lt;/li&gt;&#xA;&lt;li&gt;Mass of Central Body&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;How can one calculate the true anomaly at a future point in time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Doing it with elliptical orbits is easy:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Calculate Eccentric Anomaly (E) from True Anomaly&lt;/li&gt;&#xA;&lt;li&gt;Calculate Mean Anomaly (M) with &#xA; $$M = E -  e*\sin(E)$$&lt;/li&gt;&#xA;&lt;li&gt;Calculate Mean Motion ($n$)&#xA; $$n = \sqrt{(G*mass)/a^3}$$&lt;/li&gt;&#xA;&lt;li&gt;Add Mean Motion to Mean Anomaly&lt;/li&gt;&#xA;&lt;li&gt;Convert new Mean Anomaly back to True Anomaly&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;However, with hyperbolic orbits:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;I can't convert the True Anomaly to the Eccentric Anomaly:&#xA;$$ \sin E = \sin(f)*\sqrt{1-e^2}/(1+e*\cos(f))$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\cos E = (e+\cos(f)) / (1+e*\cos(f))$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E = \textrm{atan2}(\sin E,\cos E)$$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;since e is &gt;1 I get a negative square root which is impossible.&lt;/li&gt;&#xA;&lt;li&gt;since I cant calculate the Eccentric Anomaly I can't get the Mean Anomaly&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Since a hyperbolic orbit is &quot;endless&quot; my mean motion is just zero:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;because $n = 2\pi/Period$ &amp;rarr; $2\pi/Infinity = 0$&lt;/li&gt;&#xA;&lt;li&gt;also hyperbolic orbits have a semimajor axis &amp;lt; 0: $\sqrt{(G*mass)/a^3} = error$&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So can someone please help me how I can calculate the true anomaly on a hyperbolic orbit in &quot;x&quot; time, given current true anomaly and all other keplerian elements.&lt;/p&gt;&#xA;" OwnerUserId="18450" LastEditorUserId="195" LastEditDate="2019-07-02T01:43:48.283" LastActivityDate="2022-11-16T21:26:56.670" Title="Calculate True Anomaly at future point in time with hyperbolic orbits" Tags="|orbital-mechanics|orbit|orbital-elements|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20086" PostTypeId="2" ParentId="20077" CreationDate="2017-02-08T19:36:16.213" Score="2" Body="&lt;p&gt;The ion thruster should be as simple as possible and it should use the propellant mass effectively. Why use three gases if using xenon only works? The propellant mass should be accelerated totally, not only the xenon part. If you use the full heat of a reaction of hydrogen with oxygen, the combustion chamber must be cooled to prevent melting. An ion thruster is used for long missions due to the low thrust, but liquid hydrogen and oxygen can't be stored that long. High pressure tanks for gaseous hydrogen and oxygen are heavy. Xenon is a very heavy gas, a tank for the same mass of gas could be much smaller and lighter for xenon than for hydrogen or oxygen. The amount of electrical energy used for the heating and ionization of xenon is much smaller than that used for the acceleration of xenon. You don't need additional consumables to heat the xenon, just a part of the solar panels is enough.&lt;/p&gt;&#xA;&lt;p&gt;Chemical energy could only deliver about 2 % of the energy that an electric accelerated ion gets.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2022-06-01T11:54:13.670" LastActivityDate="2022-06-01T11:54:13.670" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20087" PostTypeId="2" ParentId="20083" CreationDate="2017-02-09T00:43:09.483" Score="2" Body="&lt;p&gt;This is a supplemental answer to help show how to calculate the speeds and see that the high speed of spacecraft passing close to the sun &lt;em&gt;is only available close to the sun&lt;/em&gt; - and not to suggest that the 2011 &lt;a href=&quot;http://www.centauri-dreams.org/?p=17282&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centauri Dreams&lt;/a&gt; article &lt;a href=&quot;http://io9.gizmodo.com/5786083/what-are-the-fastest-spacecrafts-ever-built&quot; rel=&quot;nofollow noreferrer&quot;&gt;reprinted in Gizmodo&lt;/a&gt; is just a little confusing if not misleading about this by citing the speed of a spacecraft near the sun and comparing it to a spacecraft that has left the solar system. (There are of course slingshot maneuvers, but then that's beyond the spirit of the original article)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Total energy:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E_{tot}=\frac{1}{2}mv^2 - \frac{GMm}{r}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Divide through by the mass of the satellite, it becomes &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_orbital_energy&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific orbital energy&lt;/a&gt;;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathscr{E}_{tot} = \frac{1}{2}v^2 - \frac{GM}{r}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Make hand-waving gestures, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_orbital_energy#Equation_forms_for_different_orbits&quot; rel=&quot;nofollow noreferrer&quot;&gt;read here&lt;/a&gt; about basic orbital mechanics:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathscr{E}_{tot} = - \frac{GM}{2a}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $a$ is the semi-major axis. The total energy is constant, and can be calculated from the semi-major axis alone (don't need eccentricity). Put them together;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$- \frac{GM}{2a} = \frac{1}{2}v^2 - \frac{GM}{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...and you get the very handy &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis-viva equation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v^2 = GM\left(\frac{2}{r}-\frac{1}{a}\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Helios_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, Helios-B perihelion and aphelion are 0.29 and 0.98 AU, or about 43 and 147 million km, and the semi-major axis $a$ is half of the sum (half of the major axis), or about 95 million km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Standard Gravitational Parameter&lt;/a&gt; of the sun is about 1.33E+11 km^3/s^2 (1.33E+20 km^3/s^2 in standard mks units), so you can calculate the velocities are about 69 km/s at periapsis, but drop to about 20km at apoapsis. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-02-09T00:43:09.483" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20088" PostTypeId="1" AcceptedAnswerId="20090" CreationDate="2017-02-09T02:54:27.810" Score="8" ViewCount="492" Body="&lt;p&gt;This recent meteor over the midwest. Reports described it at blue / green. I have seen blue / white, red and yellow meteors. I presume red means iron, green means copper. Can meteoritics or meteor specialists (or whatever you call them) say a lot about the content of a meteor based or the color it makes during entry?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2017-02-09T10:31:14.437" Title="Meteor color hints at chemistry" Tags="|chemistry|entry-descent-landing|meteors|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20089" PostTypeId="2" ParentId="20085" CreationDate="2017-02-09T04:37:31.620" Score="12" Body="&lt;p&gt;Not surprisingly, one needs to use hyperbolic functions as opposed to trigonometric functions with regard to hyperbolic trajectories. The motivation is  simple. Let's start with Kepler's equation, $M = E - e\sin E$. We're going to run into issues (but not impossibilities) with negative square roots. Kepler's equation works quite fine, as is, with hyperbolic orbits so long as one realizes that the sine function can be extended to the trajectories complex plane. That isn't the best way to proceed. A better approach is to use hyperbolic functions. There's a clear relationship between the trigonometric and hyperbolic functions. For example, $\sin(ix) = i\sinh x$ (which will come in very handy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you multiple both sides of Kepler's equations by $i$ and substitute $E=iH$, you will find the hyperbolic equivalent of Kepler's equation, $iM = i(e\sinh H - H)$. Meanwhile, the mean anomaly $M$ is also imaginary. The best thing to do is to redefine mean anomaly so that it carries the same real concept for elliptical orbits but is also real for hyperbolic orbits. The end result is that $M = e\sinh H - H$ for hyperbolic orbits, with (once again) $M$ being zero at periapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about mean motion? Instead of using $n = \sqrt{\frac \mu {a^3}}$, one uses $n = \sqrt{\frac \mu {|a|^3}}$. This has zero effect on elliptical orbits but it has the desired effect of taking mean anomaly for hyperbolic trajectories into the realm of the real numbers. Whether the semi-major axis is positive or negative, mean motion is a simple linear relationship with respect to time, $M(t) = M_0 + n (t-t_0)$. The relationship between hyperbolic anomaly and true anomaly is also quite simple: $$\tanh \left(\frac H 2\right) = \sqrt{\frac {e-1}{e+1}} \tan\left(\frac f 2\right)$$&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-02-09T04:37:31.620" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20090" PostTypeId="2" ParentId="20088" CreationDate="2017-02-09T10:31:14.437" Score="8" Body="&lt;p&gt;Absolutely, it's the same as heating up metals with a blowtorch. The colour of the flame tells you what is present. Wikipedia actually has a &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Meteoroid&quot;&gt;handy list&lt;/a&gt; giving an indication of the significant elements present:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Orange-yellow (sodium)&lt;/li&gt;&#xA;&lt;li&gt;Yellow (iron)&lt;/li&gt;&#xA;&lt;li&gt;Blue-green (magnesium)&lt;/li&gt;&#xA;&lt;li&gt;Violet (calcium)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Red (atmospheric nitrogen and oxygen)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="38" LastActivityDate="2017-02-09T10:31:14.437" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20091" PostTypeId="1" CreationDate="2017-02-09T10:36:07.147" Score="1" ViewCount="1435" Body="&lt;p&gt;The total length of the J-2 rocket engine is 133 inches, but what is its nozzle length from throat to exit?&lt;/p&gt;&#xA;" OwnerUserId="18454" LastEditorUserId="6816" LastEditDate="2017-02-09T11:03:28.343" LastActivityDate="2017-02-09T23:45:26.923" Title="J-2 rocket nozzle length" Tags="|nozzle|j-2|dimensions|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20092" PostTypeId="2" ParentId="20091" CreationDate="2017-02-09T11:56:13.877" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/SP-8120_Excerpt.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This&lt;/a&gt; (which I &lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/Rocketdyne_Engines.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;found here&lt;/a&gt;) says 90 inches &lt;strong&gt;throat&lt;/strong&gt; to exit. Two other items below show &lt;strong&gt;gimbal center&lt;/strong&gt; to exit of 116 inches. So far I can not find the relationship between the throat and the gimbal center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5ZgXR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5ZgXR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;strong&gt;Throat&lt;/strong&gt; to exit - 90 inches. found &lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/Rocketdyne_Engines.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lButC.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lButC.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;strong&gt;Gimbal center&lt;/strong&gt; to exit - 116 inches. See the &quot;J-2S Side Drawing&quot; &lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/Rocketdyne_Engines.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;link here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PTRRq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PTRRq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;strong&gt;Gimbal center&lt;/strong&gt; to exit - 116 inches. See the &quot;J-2 vs J-2S, McDonnell Phase B Shuttle Study&quot; &lt;a href=&quot;http://www.alternatewars.com/BBOW/Space_Engines/Rocketdyne_Engines.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;link here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-09T23:45:26.923" LastActivityDate="2017-02-09T23:45:26.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20093" PostTypeId="2" ParentId="20041" CreationDate="2017-02-09T15:07:45.977" Score="6" Body="&lt;p&gt;While the already accepted answer is great, I just want to add that I made a plot about a year ago where you can see exactly how many people were in space for every day since Gagarin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the picture, it's about 20k pixels wide (sorry for russian labels):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/a1JxN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/a1JxN.png&quot; alt=&quot;http://tossha.com/space/plot_1961_01_01_2016_04_30.png&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here's my post about the project with some historic highlights (entierly russian as well):&#xA;&lt;a href=&quot;https://geektimes.ru/post/274988/&quot; rel=&quot;noreferrer&quot;&gt;https://geektimes.ru/post/274988/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as you can see the last time there were two people on the ISS was in the end of 2009. And before that there was such period from May 2003 to July 2006.&lt;/p&gt;&#xA;" OwnerUserId="17295" LastActivityDate="2017-02-09T15:07:45.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20095" PostTypeId="1" CreationDate="2017-02-09T15:45:05.410" Score="3" ViewCount="491" Body="&lt;p&gt;Suppose you've got a spacecraft in an orbit similar to that of ISS (let's say 51.6 degrees, but about 19 km lower). Suppose also you want to launch a second spacecraft to rendezvous with the first, and you don't have the margin in your launch vehicle to do a dog-leg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that you're looking to launch the second when the ground track of the first is near the launch site again (i.e. the ground track repeats). But the ground track doesn't repeat exactly &amp;mdash; so how close does it need to be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've written this function (in Ruby) to find the ground track repeats:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;# Return the number of days (not sidereal days) to the &#xA;# next ground track repeat.&#xA;#&#xA;# Parameters:&#xA;# * period:         orbital period (seconds)&#xA;# * within_degrees: how close the ground track must be (degrees)&#xA;# * max_passes:     maximum passes over the launch site to consider&#xA;# * sidereal_day:   length of planetary sidereal day (seconds)&#xA;#&#xA;# Returns:&#xA;# * decimal time in days, if found; otherwise nil&#xA;# * error in degrees, if found; otherwise nil&#xA;#&#xA;def days_to_next_launch_opportunity period, within_degrees: 1.5, max_passes: 100, sidereal_day: 86164.1&#xA;  orbits_per_sidereal_day = sidereal_day / period&#xA;  orbits_per_day          = 3600.0 * 24.0 / period&#xA;  ground_track_precession = 360.0 / orbits_per_sidereal_day&#xA;&#xA;  max_passes.times do |m|&#xA;    approx_orbits = (m+1)*360 / ground_track_precession&#xA;&#xA;    orbits =&#xA;    if approx_orbits - approx_orbits.floor &amp;lt; approx_orbits.ceil - approx_orbits&#xA;      approx_orbits.floor&#xA;    else&#xA;      approx_orbits.ceil&#xA;    end&#xA;&#xA;    degrees_away = orbits * ground_track_precession - (m+1)*360&#xA;    return [orbits / orbits_per_day, degrees_away] if degrees_away.abs &amp;lt; within_degrees&#xA;  end&#xA;&#xA;  return [nil, nil] # not found&#xA;end&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;This function tells me most launch opportunities for such an orbit are about 2 days apart (within 2 degrees of a ground track repeat), even varying the inclination, eccentricity, altitude, and longitude of the ascending node slightly. Very occasionally, I get 9 days or 11 days separation between launch windows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, we don't want to &lt;em&gt;hit&lt;/em&gt; the first spacecraft with the second. Supposing we want to launch to the same altitude, but 1–1.5 degrees behind or ahead of it so we can phase to proximity operations within 6 or so hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean we actually want to offset the ground track slightly? By how much? I'm sure there's some really simple trigonometry for this, but I can't find it in Wertz &amp;amp; Larson or in Vallado. Where should I look?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that there's &lt;a href=&quot;https://space.stackexchange.com/questions/6129/how-is-an-orbital-rendezvous-planned&quot;&gt;a related question here&lt;/a&gt;, but it seems more theoretical, and I'm asking about the math.)&lt;/p&gt;&#xA;" OwnerUserId="869" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-02-09T15:45:05.410" Title="Launch windows for on-orbit rendezvous" Tags="|launch|orbital-mechanics|rendezvous|" AnswerCount="0" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20096" PostTypeId="2" ParentId="17490" CreationDate="2017-02-09T16:28:41.990" Score="11" Body="&lt;p&gt;@DavidVomLehn is &lt;a href=&quot;https://space.stackexchange.com/a/17560/12102&quot;&gt;right&lt;/a&gt;. I've just read news today of a the recent paper &lt;a href=&quot;http://aip.scitation.org/doi/10.1063/1.4973429&quot; rel=&quot;noreferrer&quot;&gt;Prolonged silicon carbide integrated circuit operation in Venus surface atmospheric conditions&lt;/a&gt;. See also Ars Technica's &lt;a href=&quot;https://arstechnica.com/science/2017/02/venus-computer-chip/&quot; rel=&quot;noreferrer&quot;&gt;We finally have a computer that can survive the surface of Venus&lt;/a&gt;. Small circuits were fabricated on semiconductor wafers made from silicon carbide rather than silicon, and there were tested for extended periods of time while they were subjected to a Venus-like temperature of 460C and Venus-like pressure of 9.4 Mbar (about 94 standard atmospheres).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The circuits were &lt;a href=&quot;https://en.wikipedia.org/wiki/Ring_oscillator&quot; rel=&quot;noreferrer&quot;&gt;ring oscillators&lt;/a&gt;, a standard test structure used in IC testing. SiC JFETs were used to build rings of NOT gates (inverters) containing an odd number of gates; read &lt;a href=&quot;ftp://ftp.aip.org/epaps/aip_advances/E-AAIDBI-6-068612/GEER2SupplementaryAIP.pdf&quot; rel=&quot;noreferrer&quot;&gt;more here&lt;/a&gt;. This configuration doesn't have naturally stable state, so it naturally oscillates. Changes in the oscillator's waveform shape, amplitude and frequency provide information on changes in the circuit, which caused by several different effects involving both the silicon carbide crystal itself, and the implanted/diffused dopants, metallizations and insulators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SiC can crystallize into several different structures, and 4H-SiC was probably chosen for it's very high band gap. For comparison, silicon is only 1.1 eV. When we say &quot;room temperature is 1/40th of an eV (or 0.025 eV) that's the product of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Boltzmann_constant&quot; rel=&quot;noreferrer&quot;&gt;Boltzmann constant&lt;/a&gt; &lt;span class=&quot;math-container&quot;&gt;$k_B$&lt;/span&gt; (about 8.62E-05 eV/K) and the temperature (20C or 293K). While the high temperature and pressure and naughty chemistry presents several mechanical and chemical insults on the chip, the temperature has a profound effect on the electronic properties of the semiconductor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the math for doped semiconductor devices is more complex, the simple exponential term from the carrier concentration of an intrinsic semiconductor is enough to show the importance of high band gap;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$n_c \approx e^{-\frac{E_{bg}}{k_B T}}$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For silicon with a band gap of about 1.1 eV, that value starts at about &lt;span class=&quot;math-container&quot;&gt;$10^{-19}$&lt;/span&gt; at 293K but rises to the order of &lt;span class=&quot;math-container&quot;&gt;$10^{-8}$&lt;/span&gt; at 733K! It would be so highly ionized (so many of the silicon atoms would have produced carrier electrons) that it would be far too conductive to function as a semiconductor or near-insulator when needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, in the case of 4H-SiC with a bandgap of 3.2 eV, those numbers go from an amazingly low &lt;span class=&quot;math-container&quot;&gt;$10^{-55}$&lt;/span&gt; at 293K (you'd think of it as a ceramic or crystaline insulator, not a semiconductor) and rises only to the order of &lt;span class=&quot;math-container&quot;&gt;$10^{-22}$&lt;/span&gt; at 733K! SiC at Venus temperatures would be a suitable starting semiconductor material in the same way that silicon would be at room temperature. Of course the carrier concentration of intrinsic SiC could be raised by doping, but there's no way to recover from a too-high value for silicon on Venus. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sO4l6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sO4l6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Figure 1 from &lt;a href=&quot;http://aip.scitation.org/doi/10.1063/1.4973429&quot; rel=&quot;noreferrer&quot;&gt;the paper&lt;/a&gt;. Before and after being tested under Venusian atmosphere parameters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aVKoO.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aVKoO.png&quot; alt=&quot;Silicon Carbide Ring Oscillators&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Figure 2 from &lt;a href=&quot;http://aip.scitation.org/doi/10.1063/1.4973429&quot; rel=&quot;noreferrer&quot;&gt;the paper&lt;/a&gt;. The drop in ring oscillator frequency shows the JFETs switch faster at elevated temperature, which is an effect that can be tolerated with proper circuit design. The loss of signal from the 11 stage oscillator was found to be a connection problem - the device continued to work once better connections could be made after the conclusion of testing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ORQd4.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ORQd4.png&quot; alt=&quot;Band Gaps various forms of Silicon Carbide&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Band gap energy of various crystal forms of silicon carbide, vs temperature. From &lt;a href=&quot;http://www.ioffe.ru/SVA/NSM/Semicond/SiC/bandstr.html#Dependence%2FTemperature&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-10-10T02:10:29.003" LastActivityDate="2018-10-10T02:10:29.003" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20097" PostTypeId="1" CreationDate="2017-02-09T17:33:37.280" Score="2" ViewCount="582" Body="&lt;p&gt;I am trying to model the Merlin 1D+ using inventor. I was wondering what the major differences between the Merlin 1D and 1D+ were. I find when I search trying to find a 1D+ engine all I come across is 1D.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I know the basic parts of a rocket engine and I know the Merlin is a gas generator cycle. However, I am wondering if the heat exchanger on the nozzle is for heating the helium and cooling the nozzle or heating the LOX while still cooling the nozzle?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="6816" LastEditDate="2017-02-09T19:49:43.557" LastActivityDate="2017-02-09T19:49:43.557" Title="What are the differences between the SpaceX Merlin 1D and 1D+ engines?" Tags="|spacex|engines|merlin-1d|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20098" PostTypeId="2" ParentId="20097" CreationDate="2017-02-09T18:27:01.203" Score="2" Body="&lt;p&gt;SpaceX is behaving very unlike traditional aerospace companies by iterating their design on a regular basis. While they carved out major updates as Merlin 1A, 1B, 1C, and 1D, it has been a constant that they tweak within versions quite regularly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is unclear if there is a defined product Merlin 1D+ as opposed to the Full Thrust Merlin 1D.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-02-09T18:27:01.203" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20099" PostTypeId="2" ParentId="20091" CreationDate="2017-02-09T21:23:50.073" Score="2" Body="&lt;p&gt;FWIW, here are J2-S numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The paper &quot;&lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/874400.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Altitude Developmental Testing of the J2-S&lt;/a&gt;&quot; gives the throat diameter as 12.192 inches and the expansion ratio as 39.62. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Paragraph 2.1.1&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Thrust Chamber — The tubular-walled, bell-shaped thrust chamber&#xA;  consists of an 18.6-in.-diam combustion chamber with a throat diameter&#xA;  of 12.192 in., a characteristic length (L*) of 35.4, and a divergent&#xA;  nozzle with an expansion ratio of 39.62. &lt;strong&gt;Thrust chamber length (from&#xA;  the injector flange to the nozzle exit) is 108.6 in.&lt;/strong&gt; Cooling is&#xA;  accomplished by the circulation of engine fuel flow downward from the&#xA;  fuel manifold through 180 tubes and then upward through 360 tubes to&#xA;  the injector and by film cooling inside the combustion chamber.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(emphasis mine)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-02-09T21:23:50.073" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20101" PostTypeId="1" CreationDate="2017-02-10T05:53:08.480" Score="0" ViewCount="387" Body="&lt;p&gt;Scramjet technology allows for propulsion once Mach4 is reached, without needing oxidizer thus reducing the weight up to 80% (as read in &lt;a href=&quot;https://medium.com/@rsn/spacex-falcon-9-vs-isros-reusable-launch-vehicle-c52d4d56f87d#.5sjgv3mv1&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://medium.com/@rsn/spacex-falcon-9-vs-isros-reusable-launch-vehicle-c52d4d56f87d#.5sjgv3mv1&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why doesnt spaceX plan to use scramjets for propulsion ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a combination of maglev mass-driver (to reach mach3) with scramjet (to continue to space) make sense ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much could that reduce the cost of rocket launching ?&lt;/p&gt;&#xA;" OwnerUserId="18461" LastActivityDate="2017-02-10T06:04:34.380" Title="Could maglev + scramjet be used to launch rockets to space?" Tags="|launch|spacex|rockets|propulsion|ramjet|" AnswerCount="1" CommentCount="0" ClosedDate="2017-02-10T08:52:43.540" ContentLicense="CC BY-SA 3.0" />
  <row Id="20102" PostTypeId="2" ParentId="20101" CreationDate="2017-02-10T06:04:34.380" Score="4" Body="&lt;p&gt;Your base assumption is wrong: &lt;strong&gt;scramjets needs oxidiser&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, they are designed to use atmospheric O2 as their oxidiser supply.&#xA;Since the rocket needs to go to space, where there is no significant amount of oxygen, they cannot work up there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Indeed, they could be used for the narrow part between insufficient speed and too high altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is probably too narrow to be useful, and would render the design much more complex, making it impractical.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-02-10T06:04:34.380" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20103" PostTypeId="1" AcceptedAnswerId="20113" CreationDate="2017-02-10T08:24:01.403" Score="3" ViewCount="1525" Body="&lt;p&gt;I was lately reading about making pictures of ISS. The pictures are not really great from &quot;home telescopes&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering what the usual altitude of espionage satellites is, given those pictures of i.e. cars are pretty detailed.&lt;/p&gt;&#xA;" OwnerUserId="18463" LastEditorUserId="12102" LastEditDate="2017-02-11T10:17:18.237" LastActivityDate="2017-02-11T10:17:18.237" Title="Reconnaissance satellite altitudes" Tags="|low-earth-orbit|reconnaissance-satellite|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20104" PostTypeId="2" ParentId="20103" CreationDate="2017-02-10T08:32:34.367" Score="8" Body="&lt;p&gt;Reconnaisance satellites are at similar altitudes to the ISS. The ISS is at 330-400 km, recon sats are in &lt;a href=&quot;https://en.wikipedia.org/wiki/KH-11_Kennen&quot;&gt;elliptical orbits&lt;/a&gt; with a perigee on the order of 250 km.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The big difference is the optics. A photo recon satellite has a telescope with a mirror 2.4 m in diameter (dimensions similar to the Hubble space telescope). Ground-based telescopes that big would have a hard time taking photos of the ISS because they can't be moved quickly enough to keep the ISS in sight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is &lt;a href=&quot;https://apod.nasa.gov/apod/ap070628.html&quot;&gt;the best ground-based photo of the ISS&lt;/a&gt; I've seen yet. It was made with a 0.64 m telescope:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UPsnU.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UPsnU.jpg&quot; alt=&quot;ISS&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This site lists &lt;a href=&quot;http://darwinsastroworld.com/ISS.htm&quot;&gt;exposure times&lt;/a&gt; of around 1/250 s. Bigger telescopes will use shorter exposure times.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the main reason large telescopes haven't been used to take pictures of the ISS is that observation time is at a premium: every observatory has a waiting list of people doing actual science. Taking time out to take pictures just for fun wouldn't sit well with the telescope's users. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-02-10T10:56:16.420" LastActivityDate="2017-02-10T10:56:16.420" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20105" PostTypeId="1" CreationDate="2017-02-10T10:05:50.993" Score="8" ViewCount="1317" Body="&lt;p&gt;Since the transit method of detecting exoplanets requires perfect alignment of orbits, would a Kepler-type telescope yield more exoplanet detections if sent out of our solar system even to relatively realistic interstellar distances (say, 0.1-0.5 light years)? Essentially, what I mean is changing the coordinates at which we'd do the observing.&lt;/p&gt;&#xA;" OwnerUserId="18465" LastActivityDate="2017-02-10T19:11:23.430" Title="Would an interstellar exoplanet telescope make sense?" Tags="|interstellar-travel|space-telescope|exoplanet|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20106" PostTypeId="1" AcceptedAnswerId="20108" CreationDate="2017-02-10T10:27:22.683" Score="10" ViewCount="526" Body="&lt;p&gt;When looking at the day sky, you see a solid blue, with no stars, Moon only rarely, barely visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But photos of Earth show clear colors of the continents, yellow Sahara, green amazon forests. There's the same amount of air for the light to pass through when looking &quot;Up&quot; as &quot;down&quot;, so why aren't all photos of Earth from space just of featureless blue ball with clouds - or at least strongly tinted blue?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2023-01-07T09:08:52.187" Title="Why doesn't Rayleigh Scattering obstruct Earth from space?" Tags="|photography|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20107" PostTypeId="1" AcceptedAnswerId="20111" CreationDate="2017-02-10T11:03:40.620" Score="7" ViewCount="514" Body="&lt;p&gt;Are there qualitative differences between observing satellite from earth surface vs. observing earth from the same satellite. Given the same optics on both places.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it matter which way we look through a foggy window?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_seeing&quot; rel=&quot;nofollow noreferrer&quot;&gt;atmospheric seeing&lt;/a&gt; affect maximum useful aperture for resolution (without using adaptive optics) differently, looking down to the Earth vs up to the sky?&lt;/p&gt;&#xA;" OwnerUserId="18463" LastEditorUserId="12102" LastEditDate="2018-02-18T13:53:41.580" LastActivityDate="2018-02-18T13:53:41.580" Title="Atmospheric influences on earth to satellite visibility and vice versa (e.g. atmospheric seeing)" Tags="|observation|reconnaissance-satellite|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20108" PostTypeId="2" ParentId="20106" CreationDate="2017-02-10T11:07:01.137" Score="6" Body="&lt;p&gt;Well we do see the day lit part of the moon if we actually look. If we knew where to look we'd see some brighter planets and stars as well. See &lt;a href=&quot;http://earthsky.org/space/10-surprising-things-to-see-in-the-daytime-sky&quot; rel=&quot;noreferrer&quot;&gt;10 surprising space objects to see in the daytime sky&lt;/a&gt; and also read through &lt;a href=&quot;http://www.nakedeyeplanets.com/venus-conjunctions.htm&quot; rel=&quot;noreferrer&quot;&gt;Naked-Eye Venus Apparitions, Conjunctions &amp;amp; Elongations 2010 to 2020&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Raleigh scattering of visible light in Earth's atmosphere is wide angle. Photons that get kicked out of their initial direction go all over the place &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are looking at the moon, most of the photons from the moon traveling towards your eye make it there un-scattered, so no definition is lost, and the ones that are scattered go in very different directions, so the moon is lightly dimmer but just as clear and crisp as it would be at night. However the scattered light from the Sun brightens the scene, so the dimly lit part of the moon (lit only by Earthshine) is not easily noticed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you put a large black spot on the Earth and viewed it from LEO, it would appear sky blue to you as well!  However because the reflected light from the Earth is so bright it would be hard for you to see it, your eyes would be overwhelmed by the bright reflected light from the Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However if you used a straw or hole in a piece of paper or whatever you had available on the ISS and blocked the excess light out, you'd see the blue sky above the (now mysterious) large back spot easily. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YLnUw.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YLnUw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Venus and the Moon during the day, from &lt;a href=&quot;http://earthsky.org/space/10-surprising-things-to-see-in-the-daytime-sky&quot; rel=&quot;noreferrer&quot;&gt;Earthsky.org&lt;/a&gt;, photo cretid &lt;a href=&quot;http://www.nakedeyeplanets.com/venus-conjunctions.htm&quot; rel=&quot;noreferrer&quot;&gt;nakedeyeplanets.com&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NU8DG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NU8DG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Venus seen through a telescope during the day, from &lt;a href=&quot;http://www.princeton.edu/~rvdb/images/Questar/venus.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/La7Gz.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/La7Gz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; The Moon seen through a telescope during the day, from &lt;a href=&quot;http://www.princeton.edu/~rvdb/images/Questar/venus.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-02-10T11:07:01.137" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20109" PostTypeId="2" ParentId="20105" CreationDate="2017-02-10T11:18:35.883" Score="9" Body="&lt;p&gt;@SF. is right. It is true that a location far from Earth could have access to different orbital planes and thus potentially see a different set of occultations an transits, you would have to move very very far away from the Earth to make any difference. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same budget and time would be much better spent developing a next generation &lt;strong&gt;optical&lt;/strong&gt; space telescope which uses mirror segments that are no longer even part of the same spacecraft, but instead are separated by tens or hundred of meters from each other but can still combine their signals (optically or otherwise) to synthesize an extremely high resolution optical aperture as big as the distance between the segments. This is the same principle used by radio telescope arrays. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a larger effective aperture you can resolve objects farther away, which opens up a much larger volume of space for direct imaging of solar systems instead of relying on geometry for transit measurements alone. Right now only a handful of systems close to the Earth have any kind of direct imaging data.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-10T19:11:23.430" LastActivityDate="2017-02-10T19:11:23.430" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="20111" PostTypeId="2" ParentId="20107" CreationDate="2017-02-10T15:18:55.247" Score="5" Body="&lt;p&gt;Yes there is. The foggy window is much closer to the ground than to the orbiting satellite. Atmospheric turbulence takes place between 0 and 20km high, and satellites orbit over 200km high.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To give you an order of magnitude, atmospheric turbulence degrades the image for telescopes with diameters larger than 15cm. For space-based telescopes in low orbit, it starts to have an influence around 2m of diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: see &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690003603.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690003603.pdf&lt;/a&gt;  for mathematical derivations.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="4685" LastEditDate="2017-02-10T20:31:38.407" LastActivityDate="2017-02-10T20:31:38.407" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20112" PostTypeId="2" ParentId="20105" CreationDate="2017-02-10T15:46:10.420" Score="6" Body="&lt;p&gt;There are several problems with that idea:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;As mentioned by uhoh and SF, reaching a point far enough away from our solar system to make a different observation than Kepler's takes so much time, that the telescope and the data will in fact be antiquated by the time they reach that point - even IF (and that's a very very big IF) we could even manage to build it that it would still work in that distance (power source, transmission array, controlling the telescope's orientation etc.), it would be of limited use.&lt;/li&gt;&#xA;&lt;li&gt;The angle to watch would still be the same. Yes, you would be far out in space, but you would STILL only be able to detect most planets with a transit method, simply because now that you might be a little closer to the Alpha Centauri System, you would still be light years away from almost any other star and still face the same problem as you face on earth. You might detect a few different planets, due to a slightly different angle, but they wouldn't be that much different unless you consider moving hundreds of light years - which would take millions of years with our current technology.&lt;/li&gt;&#xA;&lt;li&gt;Direct detection by visual imaging with current technology is out of the question unless you get very close to a star system or the planet is sufficiently large and far away from its parent star. The limiting factor in this is, that the parent star is so bright, it outshines the reflected light of its planet by several orders of magnitude. If they are close to the parent star, the star will be too bright for them to be imaged directly (be it in microwave, IR, visual or UV range). If they are far away from their parent star, the reflected light is far too faint to be detected unless that telescope would be close to the star system observed - and then it could only detect the planets of that specific star system.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="12555" LastActivityDate="2017-02-10T15:46:10.420" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20113" PostTypeId="2" ParentId="20103" CreationDate="2017-02-10T16:09:07.640" Score="5" Body="&lt;p&gt;That's a great question! So your question is: why are images taken from observation satellites much better than the typical images of the ISS taken from the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;observation satellites generally have higher diameters and focal length than your typical amateur telescope.&lt;/li&gt;&#xA;&lt;li&gt;observation satellites are very stable, so the image is not degraded by vibration. Plus they take pictures only when they are not rotating. On the contrary, taking ISS photos requires moving the telescope around quickly, which introduces a lot of vibrations in most amateur telescopes.&lt;/li&gt;&#xA;&lt;li&gt;most observation satellites have a special sensor wich compensates for object movement when taking pictures (that's called time-delay integration). Ground sensor do not have that (that's why they have to move around to track the target).&lt;/li&gt;&#xA;&lt;li&gt;the atmosphere degrades the images taken by ground telescopes much more than those taken by satellites&lt;/li&gt;&#xA;&lt;li&gt;however observation satellites tend to orbit higher than the ISS (typically 600 to 700km, with the exception of a few spy satellites that fly lower)&lt;/li&gt;&#xA;&lt;li&gt;the ISS has very high contrasts, so it's hard to have an image in which you can see details in both the brightly lit parts and the parts in darkness&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="4685" LastEditDate="2017-02-10T16:14:23.210" LastActivityDate="2017-02-10T16:14:23.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20114" PostTypeId="2" ParentId="20105" CreationDate="2017-02-10T16:39:07.670" Score="0" Body="&lt;p&gt;Somebody else can tackle the detailed maths, but the longer your baseline, and the more observatories, the more complete the coverage. Two observatories in the same orbit as Pluto but on opposite sides of the sun would still only be 0.0012 of a light year apart. Three would be better and they would still be close enough (just) to allow for the communication necessary to allow coordinated observations necessary for parallax type measurements and comparisons. Going much further doesn't actually help since there would be no way to get the information back to Earth to be useful. &lt;/p&gt;&#xA;" OwnerUserId="10877" LastActivityDate="2017-02-10T16:39:07.670" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20115" PostTypeId="2" ParentId="20106" CreationDate="2017-02-10T17:13:51.703" Score="8" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;Why doesn't Rayleigh Scattering obstruct Earth from space?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It does, to some extent. The Earth isn't called the &amp;quot;Blue Marble&amp;quot; for nothing, and it's not because the oceans are blue. It's called the Blue Marble because the sky is blue. A true color image from space, shows this:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/R24ss.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R24ss.png&quot; alt=&quot;&amp;quot;Blue Marble&amp;quot; image from the HIMAWARI satellite&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://www.nnvl.noaa.gov/images/low_resolution/1841v1_20160209-HIMAWARI.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;noaa.gov&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;p&gt;Note the fuzziness of Asia in the image. That's mostly because of Rayleigh scattering. The blue oceans? The deep oceans are a very dim blue as seen from space, also thanks to Rayleigh scattering. However, the much more intense Rayleigh scattering from the atmosphere makes the oceans appear to be blue as seen from space. If the Earth's atmosphere was predominantly helium instead of nitrogen and oxygen, and if the Earth was still covered with 70% water, the Earth would be better called the Almost Black Marble.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="10273" LastEditDate="2023-01-07T09:08:52.187" LastActivityDate="2023-01-07T09:08:52.187" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="20116" PostTypeId="2" ParentId="20107" CreationDate="2017-02-10T18:03:04.803" Score="3" Body="&lt;p&gt;Atmospheric effects are mostly confined to altitudes where there is appreciable  -er- atmosphere. Say below 100,000 feet (nonlinear, so probably 50,000 feet or less is where most turbulence occurs). Recon satellites orbit at ~150 miles or so (KH-11 figure, but commercial earth observation satellites like to orbit above ~400 miles to avoid residual drag). Imagine a line drawn between a ground telescope and the satellite (represents your sight line). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First case, ground to satellite, anchored on the ground... You're turbulence, so grab the line at the 10 mile up mark and wiggle. The satellite end of the (sight)line will be displaced by some distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second case, satellite to ground, anchored  at satellite... Again, you're turbulence, so grab the line at the 10 mile mark and wiggle. The ground end of the (sight)line will also move, but over a much smaller distance than the first case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my (possibly incorrect) conclusion is that observing the earth from a satellite is less affected by atmospheric conditions than observing a satellite from the earth.&lt;/p&gt;&#xA;" OwnerUserId="14632" LastEditorUserId="14632" LastEditDate="2017-02-10T18:19:48.253" LastActivityDate="2017-02-10T18:19:48.253" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20117" PostTypeId="1" AcceptedAnswerId="20119" CreationDate="2017-02-10T20:25:31.073" Score="7" ViewCount="929" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/20104/12102&quot;&gt;This answer&lt;/a&gt; mentions that (optical) spy satellites are often in elliptical orbits, and when I look &lt;a href=&quot;https://en.wikipedia.org/wiki/KH-11_Kennen&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; sure enough the orbits are elliptical, with periapsis altitudes around 250 to 400 km, and apoapsis altitudes around 400 to 1000 km. I'm guessing it is some kind of compromise involving resolution limit by distance and orbit decay, maybe even atomic oxygen, but why would a satellite with a minimum altitude of 400km still go up to 1000 km every orbit instead of being circular?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-02-11T05:50:40.340" Title="Why are spy satellites in elliptical orbits?" Tags="|artificial-satellite|imaging|reconnaissance-satellite|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20118" PostTypeId="2" ParentId="20080" CreationDate="2017-02-11T01:41:00.317" Score="5" Body="&lt;p&gt;Sometimes, and not as well as you are after.  The problem here is that you don't understand the gravitational slingshot maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the reference frame &lt;strong&gt;of the planet&lt;/strong&gt; there's no net velocity change.  The spacecraft leaves with the same speed as it approaches, just on a different bearing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is only in the reference frame of a larger body that you see a speed gain (or loss--you can do the same thing in reverse in order to slow a spacecraft, say to get it close to the sun.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If this were the whole story it would be completely useless for an approach.  However, the same thing applies to moons and planets as applies to planets and the sun.  You can do gravitational slingshots off a moon to shed velocity--that how planets capture asteroids and turn them into baby moons.  You won't get below orbital velocity this way, though.  It's a capture maneuver (not that it can be done in most places due to the lack of an adequate moon), not a landing maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It can also work the other way, ask the Kerbal (Kerbal Space Program.  Play that and you'll get a better feel for orbital maneuvering than any amount of looking at formulas will give you) I've chased all the way to Eve.  I didn't see whether it was the Mun or Minmus that caused the ejection.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-02-11T01:41:00.317" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20119" PostTypeId="2" ParentId="20117" CreationDate="2017-02-11T05:50:40.340" Score="14" Body="&lt;p&gt;This is just an educated guess; I'm not connected with the National Geospatial-Intelligence Agency, or any related agency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Europe is in the northern hemisphere, as is the Middle East and all of mainland Asia. Those are the primary targets of the United State's satellite spying efforts. What about the Southern Hemisphere? It's mostly water (not a threat), a all of Australia and New Zealand (both allies; neither is a threat), parts of South America (some are allies, none are technical threats), parts of Africa (no technical threats there, either), and a bunch of islands (not threats).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the Southern Hemisphere is &quot;mostly harmless&quot;, it makes sense to have an elliptical orbit with perigee occurring over the Northern Hemisphere and apogee over the Southern Hemisphere. A 250 km circular orbit would either require a lot of fuel for altitude maintenance or would require frequent launches. An elliptical orbit with the perigee small over potential threats but apogee a bit larger over the &quot;mostly harmless&quot; areas reduces overall drag and extends the life of the satellite.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-02-11T05:50:40.340" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20120" PostTypeId="1" AcceptedAnswerId="20123" CreationDate="2017-02-11T06:45:38.200" Score="2" ViewCount="1555" Body="&lt;p&gt;Does Popular Science's &lt;a href=&quot;http://www.popsci.com/gaofen-4-worlds-most-powerful-geo-spy-satellite-continues-chinas-great-leap-forward-into-space&quot; rel=&quot;nofollow noreferrer&quot;&gt;GAOFEN 4, The world's most powerful GEO spy satellite, continues China's great leap forward into space&lt;/a&gt; label as most powerful GEO spy satellite refer to optical resolution, electrical power, or ability to knock out satellites on the other side of the GEO orbit with laser beams? That last one is meant in jest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it is optical, and the resolution at Earth's surface is 50 meters (as mentioned in the article) at 36000 km that's 1.6E-06 rad. With a wavelength of 500 nm $\displaystyle 1.22 \frac{\lambda}{\theta}$ suggests an aperture of 40 cm.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's about the same size as the cameras on deep space or planetary science missions, LEO spy satellites have apertures of 1 or 2 meters (or even more perhaps). Here's what a &lt;a href=&quot;https://space.stackexchange.com/a/14846/12102&quot;&gt;1.5 meter spysat looks like&lt;/a&gt;. Even Hubble in LEO is 2.4 meters and if it were just a &lt;em&gt;21st century technology telescope and imager&lt;/em&gt; with a lighter mirror and none of the other science packages, I think it could have been lofted to GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it really a roughly 40 cm camera (16 inches) that is making Gaofen-4 the most powerful GEO spy satellite? It certainly looks like a 40cm telescope in this photo:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dKvFF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dKvFF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from Spaceflight 101's &lt;a href=&quot;http://spaceflight101.com/gaofen-4-earth-watching-satellite-put-into-on-target-orbit-by-long-march-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gaofen-4 Earth-Watching Satellite put into on-target Orbit by Long March Rocket&lt;/a&gt;. Photo credit: CAST via Xinhua&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasaspaceflight.com/2015/12/long-march-3b-gaofen-4-close-2015/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASASpaceflight.com states&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Gaofen-2&lt;/strong&gt; employs CS-L3000A bus, configured with one 1 meter panchromatic/4m multi-spectral camera, with designed lifespan of over 5 years. (my emphasis)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The payload fairing is 4.2 meters in diameter. &lt;s&gt;If Gaofen-4's multispectral camera is using a 4 meter aperture, that sounds powerful to me, but I'm not sure why it would be limited to 50 meter resolution on Earth. And if 40cm is in fact the most powerful camera in GEO, I'll also be surprised.&lt;/s&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; the answer was clear, I just needed reminding that this part was about Gaofen-2 and not Gaofen-4.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-02-11T10:13:03.233" Title="What makes Gaofen-4 &quot;the most powerful GEO spy satellite&quot;?" Tags="|china|reconnaissance-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20123" PostTypeId="2" ParentId="20120" CreationDate="2017-02-11T09:48:09.917" Score="6" Body="&lt;p&gt;To public knowledge, it is the highest-resolution imager in GEO. The 2nd place is occupied by weather sats with instruments like GOCI (&lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_Ocean_Color_Imager&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Geostationary_Ocean_Color_Imager&lt;/a&gt;), with resolutions around 500m. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thing is there is a compromise to be made between field of view and resolution: at 50m resolution, GF-4 has a 400km field of view (source &lt;a href=&quot;https://i.stack.imgur.com/qoLaD.jpg&quot; rel=&quot;noreferrer&quot;&gt;https://i.stack.imgur.com/qoLaD.jpg&lt;/a&gt;). That makes it useful for spotting ships at sea, over a wide area. If it had a 2.4m mirror, it would have a 8m resolution and a much lower field of view. You'll notice that 8m resolution is still not great, and it is not enough to detect vehicles for instance, so the increased resolution does not make the satellite much more useful. On the contrary, it makes it heavier, slower to orient, and less productive due to the lower FOV.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an overview of the surveillance capabilities from high orbits, you can have a look at &lt;a href=&quot;https://satelliteobservation.wordpress.com/2016/10/17/persistent-surveillance/&quot; rel=&quot;noreferrer&quot;&gt;https://satelliteobservation.wordpress.com/2016/10/17/persistent-surveillance/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that regarding GF-2, &quot;4m multi-spectral&quot; means 4m resolution, not aperture. The aperture is much smaller.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastActivityDate="2017-02-11T09:48:09.917" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20124" PostTypeId="1" CreationDate="2017-02-11T17:33:37.443" Score="-2" ViewCount="362" Body="&lt;p&gt;If you build a space station in the shape of a cylindrical wheel and construct it so that it spins around a central docking point in order to create artificial gravity would it be logical to orient the interior structures so that the crew would have their feet aligned with the centripetal force (i.e., parallel to the radius of the circle)? &lt;/p&gt;&#xA;" OwnerUserId="18473" LastEditorUserId="6816" LastEditDate="2017-02-11T21:41:20.647" LastActivityDate="2017-02-11T23:23:59.470" Title="The best shape and orientation for a space station" Tags="|artificial-gravity|habitat|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20125" PostTypeId="1" CreationDate="2017-02-11T18:24:48.457" Score="8" ViewCount="2557" Body="&lt;p&gt;In the famous &quot;pale blue dot&quot; picture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ef61h.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ef61h.jpg&quot; alt=&quot;Pale Blue Dot&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;we see Earth in a brown band. What is that band? Is it an artifact of the camera? Is it a ring of dust orbiting around the Sun akin to the rings of Saturn and we are in the midst of it? Or is it something else? And what of the other bands?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="12102" LastEditDate="2017-02-12T04:07:08.470" LastActivityDate="2017-02-12T04:07:08.470" Title="What is the brown band in which we find the pale blue dot that is Earth?" Tags="|voyager|imaging|earth|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20126" PostTypeId="1" AcceptedAnswerId="26708" CreationDate="2017-02-11T19:21:15.023" Score="4" ViewCount="620" Body="&lt;p&gt;The &lt;a href=&quot;https://spaceflight.nasa.gov/outreach/SignificantIncidents/assets/rogers_commission_report.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rogers Commission Report&lt;/a&gt; (PDF, page 73) describes the Space Shuttle Solid Rocket Boosters as&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[...] a faulty design unacceptably sensitive to a number of factors.&#xA;  These factors were the effects of temperature,&#xA;  physical dimensions, the character of materials,&#xA;  the effects of reusability, processing, and the&#xA;  reaction of the joint to dynamic loading.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What are the failure modes for segmented solid rocket boosters?&lt;/p&gt;&#xA;" OwnerUserId="716" LastActivityDate="2018-04-16T06:26:39.147" Title="Failure modes of segmented solid rocket boosters" Tags="|space-shuttle|failure|srb|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20127" PostTypeId="1" AcceptedAnswerId="20128" CreationDate="2017-02-11T19:51:39.370" Score="4" ViewCount="506" Body="&lt;p&gt;A short while ago I visited a real life dissected rocket and it came to my attention that its nozzle is ribbed across its surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why&lt;/strong&gt; would engineers build it this way and what are &lt;strong&gt;the advantages&lt;/strong&gt; of such a particular design with more even surfaces? &lt;/p&gt;&#xA;" OwnerUserId="18474" LastEditorUserId="6816" LastEditDate="2017-02-11T21:39:33.230" LastActivityDate="2017-02-11T21:39:33.230" Title="Ribbed rocket nozzles" Tags="|rockets|design|engine-design|nozzle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20128" PostTypeId="2" ParentId="20127" CreationDate="2017-02-11T20:16:23.913" Score="12" Body="&lt;p&gt;Rocket engine nozzles often are cooled by pumping (some of) the propellant through pipes inside the nozzle wall or outside of it before they are burned inside the engine. An example of this is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot;&gt;Saturn V's F-1 rocket engine&lt;/a&gt;. This is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Regenerative_cooling_(rocket)&quot;&gt;regenerative cooling&lt;/a&gt;. It's likely these &quot;ribs&quot; you were seeing.&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2017-02-11T20:16:23.913" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20130" PostTypeId="2" ParentId="20124" CreationDate="2017-02-11T23:23:59.470" Score="1" Body="&lt;p&gt;If we ignore the shape you decide to go with for the moment, yes, of course you want interior structures aligned so that &quot;down&quot; as perceived by the crew is the same as the &quot;down&quot; direction something would fall when dropped. This way things on a desk will stay there, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a cylindrical wheel, the curved floor will be the skin of the cylinder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am unsure what you mean by &quot;aligned parallel to the radius of the circle&quot; though.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2017-02-11T23:23:59.470" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20132" PostTypeId="2" ParentId="20125" CreationDate="2017-02-12T00:32:28.577" Score="7" Body="&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/TL;DR&quot; rel=&quot;nofollow noreferrer&quot;&gt;tl;dr&lt;/a&gt;:&lt;/strong&gt; At Voyager 1's distance of over 6 billion km, the Sun and the Earth were separated by only about 0.8 degrees. The streaks are scattered light inside the telescope (roughly speaking, &amp;quot;glare from the Sun&amp;quot;). At this distance, Earth's light is very weak, so even a small amount of scattered light will show up in the image.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The image is listed as &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA00452&quot; rel=&quot;nofollow noreferrer&quot;&gt;PIA00452: &amp;quot;Solar System Portrait - Earth as 'Pale Blue Dot'&amp;quot;&lt;/a&gt;.  You can read further about it and photos of other planets that Voyager 1 took at about the same time in @Hobbes's &lt;a href=&quot;https://space.stackexchange.com/a/3758/12102&quot;&gt;helpful answer&lt;/a&gt;, and some more background about glare from the sun in Voyager 1's cameras in @called2voyage's &lt;a href=&quot;https://space.stackexchange.com/a/17421/12102&quot;&gt;helpful answer&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Original Caption Released with Image:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This narrow-angle color image of the Earth, dubbed 'Pale Blue Dot', is a part of the first ever 'portrait' of the solar system taken by Voyager 1. The spacecraft acquired a total of 60 frames for a mosaic of the solar system from a distance of more than 4 billion miles from Earth and about 32 degrees above the ecliptic. From Voyager's great distance Earth is a mere point of light, less than the size of a picture element even in the narrow-angle camera. Earth was a crescent only 0.12 pixel in size. &lt;strong&gt;Coincidentally, Earth lies right in the center of one of the scattered light rays resulting from taking the image so close to the sun&lt;/strong&gt;. This blown-up image of the Earth was taken through three color filters -- violet, blue and green -- and recombined to produce the color image. The background features in the image are artifacts resulting from the magnification. (emphasis added)&lt;/p&gt;&#xA;&lt;p&gt;Image Credit: NASA/JPL&lt;/p&gt;&#xA;&lt;p&gt;Image Addition Date: 1996-09-12&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Pale_Blue_Dot&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Pale Blue Dot is a photograph of planet Earth taken on &lt;strong&gt;February 14, 1990, by the Voyager 1 space probe&lt;/strong&gt; from a record distance of about 6 billion kilometers (3.7 billion miles, 40.5 AU), as part of the Family Portrait series of images of the Solar System. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I looked up the position of Voyager 1, the Sun, and the Earth using the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA JPL Horizons&lt;/a&gt; website, in J2000 Ecliptic coordinates. You can see that by this time Voyager 1 has &amp;quot;made it's move&amp;quot; and is already far out of the plane of the ecliptic (large negative Z value).&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;JD: 2447926.500000000, A.D. 1990-Feb-04 00:00:00.0000&#xA;&#xA;Reference frame : ICRF/J2000.0                                                 &#xA;Coordinate systm: Ecliptic and Mean Equinox of Reference Epoch                 &#xA;Center body name: Solar System Barycenter (0)     {source: DE431mx}&#xA;&#xA;           X (km)         Y (km)         Z (km)&#xA;Voy_1:  -2.352905E+09, -4.483837E+09,  3.260641E+09&#xA;Sun:    -7.232418E+04,  3.420464E+04, -4.602815E+03&#xA;Earth:  -1.044322E+08,  1.042282E+08, -2.923541E+03&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;Using vectors pointing from Voyager 1 to the Sun and Earth, we can calculate the angular separation between the two as seen from the spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{r_E}=\mathbf{x_E}-\mathbf{x_{V1}} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{r_S}=\mathbf{x_S}-\mathbf{x_{V1}} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$cos(\theta) = (\mathbf{r_S} \cdot \mathbf{r_E}) \ / \ (r_Sr_E)  $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;I get 0.821 degrees on February 14, 1990. So the streaks (scattered light artifacts) are the result of pointing at the Earth, which was separated by less than a degree from the sun. The active &lt;a href=&quot;http://pds-rings.seti.org/voyager/iss/inst_cat_na1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;field of view (FOV) of the camera is 0.42 degrees square&lt;/a&gt;, but light at angles larger than that can still sometimes enter the telescope and reach the image sensor.&lt;/p&gt;&#xA;&lt;p&gt;Even with various baffles in the optical system, a small amount of light was scattered. At over 6 billion kilometers (4 billion miles) from Earth, it is very faint compared to the sun, so even tiny amounts of scattered light inside the telescope will show up on an exposure long enough to make a bright dot when splitting Earth's light between four pixels.&lt;/p&gt;&#xA;&lt;p&gt;The description of &lt;a href=&quot;http://photojournal.jpl.nasa.gov/catalog/PIA00450&quot; rel=&quot;nofollow noreferrer&quot;&gt;image PIA00450&lt;/a&gt; includes more information about this image(PIA00452):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Although the planetary pictures were taken with the narrow-angle camera (1500 mm focal length) and were not pointed directly at the sun, they show the effects of the glare from the nearby sun, in the form of long linear streaks resulting from the scattering of sunlight off parts of the camera and its sun shade.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y0D62.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y0D62.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; sketch of IRIS, Voyager's f=1500mm, f/8.5 camera, from &lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/instruments_iss_na.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/shEgn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/shEgn.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; magnified and cropped part of the image, showing the bale blue light from our pale blue dot shared by four pixels, from Petapixel's &lt;a href=&quot;http://voyager.jpl.nasa.gov/spacecraft/instruments_iss_na.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Pale Blue Dot: A Portrait of Earth Shot From More Than 4 Billion Miles Away&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r65co.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r65co.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Drawing of a Voyager spacecraft. The narrow angle camera is shown in the cluster of optical instruments at the top of the drawing. From &lt;a href=&quot;http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Brian_Herold/spacecraft.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; but a similar drawing is found on page 3 of &lt;a href=&quot;http://planetary.s3.amazonaws.com/voyager_msb/VoyagerMSB13.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager Mission Status Bulletin, January 5, 1978&lt;/a&gt;. Found also &lt;a href=&quot;http://jugandi.com/ebooks/Systems/11%20Exotic%20Systems.htm#_11.4.1.7_Antennas&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; without credit, but with a caption.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-12T01:34:56.020" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20133" PostTypeId="1" AcceptedAnswerId="20134" CreationDate="2017-02-12T10:35:04.210" Score="9" ViewCount="590" Body="&lt;p&gt;I understand that an ion-thruster will continuously charge its spacecraft with a charge opposite of the one of the ions it thrusts. Over a long journey this charge would be considerable. What would happen when such a spacecraft comes in contact with another celestial body? Do engineers need to come up with a mechanism to dissipate the charge in a controlled manner?&lt;/p&gt;&#xA;" OwnerUserId="18480" LastEditorUserId="12102" LastEditDate="2017-02-14T04:26:45.267" LastActivityDate="2020-07-23T09:32:41.493" Title="How is charge dissipated in ion-propulsed spacecraft?" Tags="|propulsion|spacecraft-charging|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20134" PostTypeId="2" ParentId="20133" CreationDate="2017-02-12T10:49:18.500" Score="13" Body="&lt;p&gt;Equalizing the charge of an ion thruster is a real problem. It is not possible to eject only positive xenon ions over a long time. The space craft would charge negatively and ions would be &amp;quot;pulled back&amp;quot; to the spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;To avoid this, also negative electrons are ejected to neutralize the spacecraft and the ion beam. There is a special electron ejector besides the ion ejector. The sum of the related electrical currents of charged ions and electrons should be zero to avoid a increasing charge of the spacecraft over time.&lt;/p&gt;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/questions/20017/why-are-ion-thrusters-so-energy-hungry&quot;&gt;Why are Ion Thrusters so energy hungry?&lt;/a&gt; for a picture of an ion thruster with the electron injector.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2020-07-23T09:03:56.677" LastActivityDate="2020-07-23T09:03:56.677" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20135" PostTypeId="1" AcceptedAnswerId="20136" CreationDate="2017-02-12T17:29:23.133" Score="35" ViewCount="4750" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/18862/12102&quot;&gt;Wood is good&lt;/a&gt; for certain niche space exploration applications, but the one below is not for spaceflight. According to the documentation for the archived photo:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Several spacecraft were built for the Mariner Mars 1964 mission. The ones that were actually launched were referred to as Mariner C-2 and Mariner C-3 until they were renamed Mariner 3 and Mariner 4, respectively. There was also a Proof Test Model (PTM, or Mariner C-1) and a Structural Test Model (STM). This photo shows Mariner C-2 configured for system tests in May 1964. It appears to be in the Spacecraft Assembly Facility, with the observation area at the top of the photo.&lt;/p&gt;&#xA;&lt;p&gt;[...]&lt;/p&gt;&#xA;&lt;p&gt;Documentation found in the Archives does not identify the purpose of the sphere covering the magnetometer during this test.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;What is it for?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Why is it thick, heavy wood?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;I can speculate - non magnetic materials near magnetometers, but a plexiglass or fiberglass box could have been used, both of those were already around at the time. For that matter, what about aluminum?&lt;/p&gt;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;Is it really hanging off of Mariner 3's magnetometer boom?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;This wooden shell looks so heavy (it's not &lt;a href=&quot;https://space.stackexchange.com/a/18862/12102&quot;&gt;Balsa this time&lt;/a&gt;)!&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lb4Ek.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lb4Ek.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Detail cropped from &lt;a href=&quot;http://www.jpl.nasa.gov/blog/2014/12/mariner-3-systems-test-configuration&quot; rel=&quot;nofollow noreferrer&quot;&gt;Photograph Number 293-6619Ac&lt;/a&gt;. Image credit: NASA/JPL-Caltech.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dihsu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dihsu.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;http://www.jpl.nasa.gov/blog/2014/12/mariner-3-systems-test-configuration&quot; rel=&quot;nofollow noreferrer&quot;&gt;Photograph Number 293-6619Ac&lt;/a&gt;. Image credit: NASA/JPL-Caltech.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-07-26T04:38:36.817" LastActivityDate="2020-08-01T21:19:37.327" Title="Why is there a large wooden ball on Mariner 3's magnetometer?" Tags="|spacecraft|testing|mariner|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20136" PostTypeId="2" ParentId="20135" CreationDate="2017-02-12T18:42:51.207" Score="63" Body="&lt;p&gt;I suspect the wooden sphere is a three-dimensional &lt;a href=&quot;https://en.wikipedia.org/wiki/Helmholtz_coil&quot; rel=&quot;nofollow noreferrer&quot;&gt;Helmholtz coil&lt;/a&gt;. A Helmholtz coil is a pair of circular coils, the radius of the coils should be equal to the distance of them. There is a coil pair for each of the three dimensions. Each coil should have the same number of windings.&lt;/p&gt;&#xA;&lt;p&gt;There are circular groves visible filled with silicone or something similar. The groves contain the Helmholtz coils, one pair for each direction. An electrical current flowing through the coils generates the magnetic field. The Helmholtz pair of coils minimizes the nonuniformity of the field at the center of the coils. It is used as a magnetic field simulator for the magnetometer to do tests on Earth.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MI7Kd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MI7Kd.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;The two horizontal coils (marked with a green freehand line) generate the vertical component of the magnetic field.&#xA;The two coils marked with cyan for one horizontal magnetic field component.&#xA;The other horizontal magnetic field component coils marked with magenta, only one coil is visible. Each coil pair is vertical to the two other coil pairs.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ntEf4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ntEf4.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;*I used Python with Matplotlib for a plot of the coil configuration. All coil radii and all pair distances are equal. The green arrow shows the direction of the magnetic field caused by the green coils, the same is true for the colors magenta and cyan.&lt;/p&gt;&#xA;&lt;p&gt;The sphere is symmetric in all three dimensions. The upper row of plots are the views vertical to each coil pair. Hidden lines are not removed.*&lt;/p&gt;&#xA;&lt;p&gt;This configuration of Helmholtz coils may be used to compensate the local earth magnetic field and to generate a magnetic field of variable direction and intensity for a thorough test of the magnetometer to measure sensitivity and linearity in all three dimensions.&lt;/p&gt;&#xA;&lt;p&gt;The sphere is made from wood to avoid any metal and unwanted magnetic fields. Wood does not conduct electrical currents, eddy currents are not possible. It is used for preflight tests only, the weight is no problem. If they used plexiglass or fiberglass for that, they would need a model made from wood to shape the fiberglass. It was easier to use the wood sphere itself as only one piece was necessary. They did not use aluminium because any metal should be avoided. The manufacture of the sphere was easier using wood than aluminium and the better stability of aluminium was not needed there. Using thick and heavy wood provided the necessary stability of the sphere to avoid any deformation during the tests and also during the manufacture of the sphere.&lt;/p&gt;&#xA;&lt;p&gt;The Python code to draw the three axes Helmholtz coils. Run the code to turn the coils arrangement manually and look to it from all sides.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;# This import registers the 3D projection, but is otherwise unused.&#xA;from mpl_toolkits.mplot3d import Axes3D  # noqa: F401 unused import&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;plt.rcParams['legend.fontsize'] = 8&#xA;&#xA;fig = plt.figure()&#xA;ax = fig.gca(projection='3d', proj_type = 'ortho')&#xA;&#xA;# Prepare arrays u, v, w1, w2&#xA;r = 10.0&#xA;d = 0.5 * r&#xA;theta = np.linspace(-np.pi, np.pi, 100)&#xA;u, v = [r * f(theta) for f in (np.sin, np.cos)]&#xA;w1, w2 = np.full_like(theta, d), np.full_like(theta, -d)&#xA;&#xA;ax.plot(u, v, w1, color = 'green')   # coil in XY plane&#xA;ax.plot(u, v, w2, color = 'green')&#xA;&#xA;ax.plot(w1, u, v, color = 'cyan')    # coil in YZ plane&#xA;ax.plot(w2, u, v, color = 'cyan')&#xA;&#xA;ax.plot(v, w1, u, color = 'magenta') # coil in XZ plane&#xA;ax.plot(v, w2, u, color = 'magenta')&#xA;&#xA;ax.set_xlabel('X'), ax.set_ylabel('Y'), ax.set_zlabel('Z')&#xA;&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2020-08-01T21:19:37.327" LastActivityDate="2020-08-01T21:19:37.327" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="20137" PostTypeId="1" CreationDate="2017-02-12T18:46:33.613" Score="15" ViewCount="7565" Body="&lt;p&gt;Mars gets less than half the light that we get on Earth and there are dust storms, but the atmosphere is much thinner and there are no clouds. After all factors have been considered, how effective are solar cells on Mars (compared with those on Earth)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, in other words (if you prefer a more precisely-defined question), how much harvestable solar &lt;em&gt;power&lt;/em&gt; is available (per unit of area) &lt;em&gt;on average&lt;/em&gt; on the respective equators of Earth and Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm asking in order to understand possible power solutions for hypothetical future colonies. So assume expensive panels with all the bells and whistles like heat regulation, solar tracking (if that's a thing), etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since cloud coverage varies around the Earth, I'd like to get best and worst case scenarios (no clouds and 100% cloudy all the time, respectively) to put the average solar power output potential of Mars into perspective.&lt;/p&gt;&#xA;" OwnerUserId="18482" LastEditorUserId="18482" LastEditDate="2017-02-13T05:57:34.157" LastActivityDate="2018-09-07T02:06:58.610" Title="How does the efficacy of solar cells on Mars compare with Earth?" Tags="|mars|solar-power|earth|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20139" PostTypeId="2" ParentId="18135" CreationDate="2017-02-13T00:08:29.000" Score="2" Body="&lt;p&gt;OK. Let's go in order:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;You could, but you have to take in account that driving a wheel to be used in attitude control isn't that simple. For one thing, you have to compute the radiation dose that your electronics will have to survive, then use the appropriate components for the task. Even if you won't be using rad hard components for your project you have to be sure that the proper components exist so your model will be representative of what the actual wheel will be. Then, you will most likely use a sine wave controller to lower the wheel rotation noise (which will have an impact on the attitude control accuracy). You may live with a simpler controller, but if you want to do it properly, this is the way to go.&lt;/li&gt;&#xA;&lt;li&gt;You should ask someone with a lathe to manufacture a proper wheel (maximizing the MOI while lowering the mass). Professional reaction wheels usually have the rotor integral to the wheel, but you won't be doing that. Be careful with the balance of the wheel! Your attitude control will see any dynamic imbalance that it will have (in fact, you should find a way to properly balance it). By the way, you will want to use a sensored BLDC motor, with three phases... And be careful with the lubricants because most of them won't behave properly in space (vacuum, radiation, temperature cycles).&lt;/li&gt;&#xA;&lt;li&gt;As for the accuracy, it is a very difficult question. It depends on a lot of things. Usually you start with a system level requirement, then you build your control and compute the requirements for all the parts (usually there are several tradeoffs involved, since each subsystem responsible will try to make his/her life easier, usually making the life of the other people harder). Back to the wheel, the parameters which will affect the accuracy are the imbalance, rotation jitter and vibration and torque bit. You will also want to have a look to the maximum momentum storage capacity of the wheel, whether if it will have to be reversible (will the satellite be momentum stabilized?... be aware that the dynamics of the wheel will change for low RPMs...).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="18484" LastEditorUserId="6816" LastEditDate="2017-02-13T00:38:22.713" LastActivityDate="2017-02-13T00:38:22.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20140" PostTypeId="2" ParentId="20135" CreationDate="2017-02-13T01:54:56.343" Score="24" Body="&lt;p&gt;To add to Uwe's answer –&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Aluminium is a bad material to wind coils around (like conductors in general) because any change in the current will for a while be “shadowed” by &lt;a href=&quot;https://en.wikipedia.org/wiki/Eddy_current&quot; rel=&quot;noreferrer&quot;&gt;eddy currents&lt;/a&gt;. This can to some degree be mitigated by laminating only small sections, but this isn't as easy with aluminium as it is with wood. And even with constant magnetic fields aluminium is not great: it is actually not completely nonmagnetic, but exhibits &lt;a href=&quot;https://en.wikipedia.org/wiki/Paramagnetism&quot; rel=&quot;noreferrer&quot;&gt;paramagnetism&lt;/a&gt;. Ok, but no material is – the absolute susceptibility of Al actually seems to be roughly the same as common organic substances, about 10&lt;sup&gt;-8&lt;/sup&gt; &lt;sup&gt;m³&lt;/sup&gt;⁄&lt;sub&gt;kg&lt;/sub&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Fibreglass could be used, but likely wasn't as cheap to make as wood. Particularly the grooves for the Helmholtz wires could probably be much easier milled into wood than in the refractory glass fibres.&lt;/li&gt;&#xA;&lt;li&gt;Homogeneous plastics like plexiglass can be very cheaply &lt;em&gt;mass&lt;/em&gt;-produced, but this requires molds etc.. Not practical for something you only need once! Today this would be a bit different thanks to 3D printers.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="8609" LastEditorUserId="15906" LastEditDate="2018-10-28T09:01:00.683" LastActivityDate="2018-10-28T09:01:00.683" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20141" PostTypeId="1" AcceptedAnswerId="20155" CreationDate="2017-02-13T05:32:56.900" Score="1" ViewCount="440" Body="&lt;p&gt;The SpaceX Falcon 9 FT is using subcooled liquid oxygen. I understand that LOX gets significantly denser at lower temperatures so more propellant can be filled in the rocket's tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can NASA SLS RS-25s use subcooled liquid oxygen in the first stage to improve mass-to-orbit performance?&lt;/p&gt;&#xA;" OwnerUserId="18487" LastEditorUserId="512" LastEditDate="2017-02-13T19:55:41.753" LastActivityDate="2017-02-13T19:55:41.753" Title="Can SLS use subcooled LOX?" Tags="|sls|ssme|lox|sub-cooled|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20142" PostTypeId="1" CreationDate="2017-02-13T07:06:05.213" Score="2" ViewCount="182" Body="&lt;p&gt;In &lt;a href=&quot;https://astronomy.stackexchange.com/questions/20033/what-could-have-made-a-satellite-appear-to-move-side-to-side-while-near-the-hori#comment33108_20035&quot;&gt;a comment&lt;/a&gt; below &lt;a href=&quot;https://astronomy.stackexchange.com/a/20035/7982&quot;&gt;an answer&lt;/a&gt; to the question &lt;a href=&quot;https://astronomy.stackexchange.com/q/20033/7982&quot;&gt;What could have made a satellite appear to move side to side while near the horizon?&lt;/a&gt; I've said:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Actually a satellite with flat surfaces - especially solar panels - can deviate quite a bit brighter than expected if the angle is just right. But it's an interesting question how a relatively smooth cylinder, covered in diffuse white paint could do that. It could get dimmer, but I'm not sure if a rocket body can get substantially brighter. Even end-on, the end of a tank would likely be rounded rather than flat, but I'll try to find out if a rocket body has the same &quot;flare&quot; vocabulary as a satellite, it's an interesting question.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Approximate visual brightness of orbital &quot;left-overs&quot; is not so much unpredictable as it is just a lot of hard work with little benefit in most cases. However in serious research (object identification, or monitor of changes) it is &lt;a href=&quot;https://space.stackexchange.com/a/16946/12102&quot;&gt;certainly done in special cases&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any observational data, or reliable, citable links that address possible large brightness excursions of rocket bodies? This should not include half-exploded craft or those with big plumes of vented propellant. Just the white tube-like ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm thinking large would be 4 or 5 magnitudes.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2017-03-19T19:35:16.393" Title="How much brighter-than-expected could a spent rocket body become?" Tags="|low-earth-orbit|booster|visibility-of-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20143" PostTypeId="1" AcceptedAnswerId="20145" CreationDate="2017-02-13T07:18:01.313" Score="63" ViewCount="11508" Body="&lt;p&gt;I remember reading several years ago about a mystery or puzzle about a photograph from an Apollo moon landing mission that had to do with why someone's suit was as bright as it was in the photo. There were quantitative questions beyond the normal conspiracy theory-class mysteries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason I remember reading about it is that a new, improved rendering engine that could model complex surfaces and material properties (and substantial computing time) was used to find the answer. While many rendering engines exist that can produce realistically believable images for normal scenes, the result here was essentially photometric rather than just aesthetic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far I can't find this result anywhere. I think I read about it 3 to 5 years ago. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spoiler alert!&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The final conclusion was that...    &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(mouseover the block quote below)&lt;/p&gt;&#xA;&#xA;&lt;blockquote class=&quot;spoiler&quot;&gt;&#xA;  &lt;p&gt; not a fake, we did go to the moon!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="5862" LastEditDate="2017-02-15T12:51:07.527" LastActivityDate="2017-02-15T12:51:07.527" Title="Which Apollo &quot;mystery&quot; was said to be finally solved by a better rendering engine?" Tags="|the-moon|apollo-program|imaging|photography|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20144" PostTypeId="1" AcceptedAnswerId="20206" CreationDate="2017-02-13T08:00:00.100" Score="7" ViewCount="411" Body="&lt;p&gt;I am sure there are a number of safety and engineering considerations, as well as future integration and flexibility to consider, but I can't think of any so far. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I think of an airlock, I think of something with two doors - one at each end. I don't understand how this works - To deploy, does it open like a clam shell, or does it just open the one air-lock that also mates with the ISS, and deploy from there? It seems that it also requires the ISS's Canadarm 2 to grab hold and move it away from the ISS. I can speculate, but I am wondering what the real reason is. Is it too dangerous to just push them out with springs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since everything is in orbit, the usual concept of &quot;away from the spacecraft&quot; is a little more tricky. Does this make it possible to change the direction of the deployment-nudge to something better suited to avoid future collisions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/E5vJy.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/E5vJy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RWHxh.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RWHxh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; from &lt;a href=&quot;http://nanoracks.com/wp-content/uploads/16-Mike-Lewis-Airlock-Presentation.pdf&quot; rel=&quot;noreferrer&quot;&gt;Nanoracks PDF presentation&lt;/a&gt; and &lt;a href=&quot;https://arstechnica.com/science/2017/02/the-international-space-station-will-get-a-new-private-airlock-in-2019/&quot; rel=&quot;noreferrer&quot;&gt;Ars Technica&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflightinsider.com/missions/iss/nanoracks-proposed-airlock-paves-way-for-more-commercial-iss/&quot; rel=&quot;noreferrer&quot;&gt;NanoRacks’ proposed airlock paves way for a more commercial ISS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xb9t6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xb9t6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Php9X.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Php9X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;http://www.spaceflightinsider.com/missions/iss/nanoracks-proposed-airlock-paves-way-for-more-commercial-iss/&quot; rel=&quot;noreferrer&quot;&gt;NanoRacks’ proposed airlock paves way for a more commercial ISS&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-02-18T07:46:55.957" Title="Why would the future Nanoracks airlock be built so that it must be removed from the ISS to deploy cubesats?" Tags="|iss|cubesat|payload-deployment|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20145" PostTypeId="2" ParentId="20143" CreationDate="2017-02-13T08:55:43.547" Score="107" Body="&lt;p&gt;&lt;a href=&quot;https://blogs.nvidia.com/blog/2014/09/18/debunked/&quot;&gt;NVIDIA rendered Aldrin descending&lt;/a&gt; to the surface and discovered that, just as the conspiracies claimed, it couldn't be reproduced with direct light from the sun as the sole light source. Of course, as in photos on Earth, indirect light (reflected, scattered...) is an important source of scene illumination and must be taken into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After adding the indirect, scattered light from the lunar soil and parts of the spacecraft, Aldrin could be seen in the simulation, but still he was darker than in the photograph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following are GIF screen captures from &lt;a href=&quot;https://www.youtube.com/watch?v=O9y_AVYMEUs?t=272&quot;&gt;the NVIDIA video&lt;/a&gt; linked in the article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GpZff.gif&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GpZff.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, once NVIDIA included the the highly reflective spacesuit &lt;em&gt;worn by the photographer himself&lt;/em&gt; (Armstrong), the simulation matched the photograph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oms8a.gif&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oms8a.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key here is the low albedo (reflectivity) of the lunar soil of about 0.12 (12% reflective) compared to the albedo of the white space suit material of about 0.9 (90% reflective). Armstrong's white suit was acting as a reflective &quot;fill light&quot; for Aldrin, who was descending in the shade of the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is very noticeable from the live video feed - the very bright reflection from Armstrong's suit can bee seen at the left side of the ladder, just below Aldrin's waist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vkSX6.png&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vkSX6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NVIDIA also added a star field to the simulation with the correct brightness, and demonstrated that stars would be invisible at the short exposures needed for the brightly lit scenes on the moon. When exposures were long enough to show any stars, anything else in the scene would be highly overexposed and completely washed out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/O9y_AVYMEUs?t=272?start=272&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is an example of photographers using reflected natural lighting from a highly reflective coated fabric, taken from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Photographing_a_model.jpg&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UGj98.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UGj98.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14637" LastEditorUserId="18503" LastEditDate="2017-02-14T19:16:33.863" LastActivityDate="2017-02-14T19:16:33.863" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20146" PostTypeId="2" ParentId="20141" CreationDate="2017-02-13T10:46:16.260" Score="4" Body="&lt;p&gt;For the Space Shuttle, liquid oxygen was used at boiling temperature (95 K). &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970028376.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocketdyne did some successful tests&lt;/a&gt; with LOX subcooled to 88.9 K. In a quick search I've found no indications this was used operationally in the Shuttle, or will be used for the SLS. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-13T10:46:16.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20147" PostTypeId="2" ParentId="1398" CreationDate="2017-02-13T11:14:09.600" Score="26" Body="&lt;p&gt;One of the design drivers for the Space Shuttle was an Air Force requirement for a cross-range of 2000 km. This was laid out in &lt;a href=&quot;http://www.jamesoberg.com/sts-3A_B-DRM.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reference Missions 3A and 3B&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;3A: launch into polar orbit (from Vandenberg), launch a satellite and land at Vandenberg within one orbit. This (specifically, the HEXAGON satellite) was the initial driver for the length of the payload bay and payload weight. The cross-range requirement allowed the landing at Vandenberg even though Earth has rotated a bit during the orbit (a straight-line mission would have ended up in the ocean).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;3B: launch into 100 nautical mile (185.2 km) polar orbit from Vandenberg, retrieve a 25,000 lb (11,340 kg) satellite and land at Vandenberg within one orbit.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The goal of these single-orbit missions was presumably to deploy or retrieve without giving the Soviet Union a chance to establish the Shuttle's orbital parameters over multiple orbits or worse, give them the chance to shoot down the Shuttle.&lt;/p&gt;&#xA;&lt;p&gt;This mission profile was never used in practice. The highest inclination orbit flown by a Shuttle was out of the Cape using a performance-costly &amp;quot;dogleg&amp;quot; maneuver to avoid the coastline. It reached 62 degrees inclination (this was STS-36). Unsurprisingly, perhaps, this was one of the classified DOD missions (thanks to Organic Marble for the addition). A polar orbit would have required a launch from Vandenberg. A Shuttle launch site was prepared there, but was never used.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.thespacereview.com/article/3172/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;This article&lt;/a&gt; has some interesting references to this mission:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Somewhat surprisingly, although the NRO did not choose to redesign the HEXAGON for recovery and refurbishment at this time, the requirement to do so was still incorporated into the space shuttle’s design. A 1973 internal Johnson Space Center document established requirements 3A and 3B for the shuttle. The first requirement was the ability to launch a large payload into polar orbit and return the shuttle to its launch site at Vandenberg Air Force Base in California. The latter required the shuttle to launch into the same orbit and conduct a rapid rendezvous and retrieval of the same payload that would have been launched under requirement 3A. The shuttle then would have returned to the launch site, this time carrying 11,340 kilograms in its payload bay. This is the same mass listed in a declassified document for HEXAGON vehicles 7–12, plus equipment in the shuttle bay for holding the spacecraft. However, a HEXAGON at the end of its mission, with its reentry vehicles and film and fuel expended, would have been considerably lighter.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;HEXAGON was a series of spy satellites that used photographic film. The film was spooled into a reentry capsule, and each satellite carried 4 of these capsules. When full, the capsule would be dropped off for recovery on the ground.&lt;/p&gt;&#xA;&lt;p&gt;That made HEXAGON expensive to use as satellite lifetime was limited by its film supply. You can see why retrieval and relaunch was an attractive option.&lt;/p&gt;&#xA;&lt;p&gt;By the time the Shuttle was operational, HEXAGON was replaced with KENNEN, which used a CCD instead of film and would have less need for servicing missions.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="29381" LastEditDate="2021-01-01T17:02:42.823" LastActivityDate="2021-01-01T17:02:42.823" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="20148" PostTypeId="2" ParentId="1398" CreationDate="2017-02-13T11:27:24.200" Score="7" Body="&lt;p&gt;While the Shuttle served as generic launcher of satellites, and a manned platform for space experiments, including military technology, that was nothing other platforms would not be capable of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one distinguishing task that &lt;em&gt;only&lt;/em&gt; the Shuttle was meant to be capable of &amp;mdash; also, the one justification for its bulky construction, one never exploited in practice either &amp;mdash; was the ability to steal other countries' satellites from orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the Shuttle did retrieve a couple satellites, and even landed four of them, that was always with the owner's full consent, two faulty communication satellites retrieved for repair, refurbishing and relaunch, and two unmanned science platforms for long-term experiments, retrieved to process the results of these experiments. The shuttle was meant to be able to &quot;steal&quot; a satellite though &amp;mdash; without the owner's consent, and, to protect the payload during reentry, carried a bulky cargo bay that had to be shielded for reentry, increasing the dry mass greatly. This was never done, and the huge expense for lifting the cargo bay never appeared justified by practical use.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="6816" LastEditDate="2017-02-13T19:22:24.800" LastActivityDate="2017-02-13T19:22:24.800" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20149" PostTypeId="1" CreationDate="2017-02-13T12:57:15.710" Score="2" ViewCount="375" Body="&lt;p&gt;Given that a spacecraft's purpose is to leave the Solar System as fast as possible, how deep into Jupiter's gravity well would be optimal for an assist in the planet's orbital direction? Jupiter's max orbital speed of 13.7 km/s sets an absolute upper limit for gravity assist, but by how much could the Oberth effect contribute?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Juno now gets to 4,200 km of the cloud tops (by some definition of cloud tops). Would it help much for a high speed spacecraft getting down to 0 km and fire the last instantaneous propulsion stage right there? &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerogravity_assist&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aerogravity assist&lt;/a&gt; would only help change the inclination, not improve the maximum Solar System escape speed, is that right?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-02-13T13:53:29.787" Title="How close to Jupiter does a spacecraft need to get in order to gain maximum gravity assist and Oberth effect bonus for leaving the Solar System?" Tags="|interstellar-travel|gravity-assist|oberth-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20150" PostTypeId="2" ParentId="20149" CreationDate="2017-02-13T13:53:29.787" Score="2" Body="&lt;p&gt;If you are looking for pure speed your spacecraft would need to get as close to Jupiter as possible without interacting with its atmosphere too much. Remember that there is atmosphere above the cloud tops. You are right about aerogravity assists - an aerogravity assist uses aerodynamic lift to achieve the greatest change in direction, however that is at the cost of some velocity due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Drag_(physics)#Drag_in_aerodynamics&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerodynamic drag forces&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lift induced drag: this is the byproduct of the lift you generate. No wing is 100% efficient at creating lift, so you are going to lose more velocity than the change in vector you achieve&lt;/li&gt;&#xA;&lt;li&gt;Parasitic drag: this is the drag on the body of the spacecraft as it passes through the atmosphere&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Essentially, there will be a distance where the atmospheric drag losses will exactly counteract the gains due to the oberth effect, to get the most out of the maneuver you'd want your spacecraft just above that point. To calculate this you'd need to model your spacecraft's body and do some computations on how much drag it would produce. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some other considerations:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If you are going to get into the atmosphere you'd need to consider how it will impact the spacecraft structure in terms of heat and stress&lt;/li&gt;&#xA;&lt;li&gt;Jupiter's intense radiation will require shielding, perhaps adding enough weight that the oberth gains are offset&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2017-02-13T13:53:29.787" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20151" PostTypeId="1" CreationDate="2017-02-13T14:05:57.480" Score="8" ViewCount="2238" Body="&lt;p&gt;I have been trying to simulate a lunar free return trajectory using the state vectors for the Apollo spacecraft provided by JPL after they had performed their trans-lunar injection burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding was that if something had gone wrong, the Apollo spacecrafts would just have swung around the moon and then back to Earth. However, in all of my simulations, disregarding which Apollo mission I'm using for the vectors, the spacecraft does not swing around the moon before coasting back to Earth, but enters a hyperbolic trajectory... Good bye brave astronauts :(&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering whether anyone could shed some light on whether this is because my algorithm is not powerful enough (tried super mega slow time-steps but you always get the same result), or if I have missed something? &lt;/p&gt;&#xA;" OwnerUserId="17423" LastActivityDate="2021-02-06T12:40:56.637" Title="Lunar Free Return Trajectory Simulation" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20153" PostTypeId="1" AcceptedAnswerId="20154" CreationDate="2017-02-13T14:35:53.080" Score="7" ViewCount="921" Body="&lt;p&gt;One of deficit resources on Mars is nitrogen. There's a plenty of carbon dioxide, and a modest amount of water. There's a deficit of nitrogen, but AFAIK plants use nitrogen in compounds found in soil, not from the atmosphere. This could be provided.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While long-term human habitation would likely necessitate atmosphere similar to Earth, would this be necessary for large biodomes? Could plants, given enough light and water, thrive in atmosphere of oxygen and carbon dioxide, without atmospheric nitrogen? Also importantly, could they live with reduced atmospheric pressure - so that the biodomes wouldn't need to withstand a whole 1bar differential, but say, 0.3 bar, similar to what was used in Apollo missions?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2022-05-29T02:20:27.457" Title="Can plants thrive without atmospheric nitrogen?" Tags="|colonization|atmosphere|plants|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20154" PostTypeId="2" ParentId="20153" CreationDate="2017-02-13T14:41:32.620" Score="11" Body="&lt;p&gt;Despite the fact that on Earth, nitrogen is the most abundant element in the atmosphere, plants do not get their nitrogen from the air - it is just too hard to get it. A plant would need to expend lots of energy to break the &lt;span class=&quot;math-container&quot;&gt;$\text{N}_{2}$&lt;/span&gt; bonds.&lt;/p&gt;&#xA;&lt;p&gt;Instead, they get all their nitrogen from the soil, where bacteria in the soil have already converted &lt;span class=&quot;math-container&quot;&gt;$\text{N}_{2}$&lt;/span&gt; to &lt;span class=&quot;math-container&quot;&gt;$\text{NH}_{3}$&lt;/span&gt; (nitrogen fixation), and onwards to &lt;span class=&quot;math-container&quot;&gt;$\text{NH}_{4}^{+}$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$\text{NO}_{2}^{-}$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\text{NO}_{3}^{-}$&lt;/span&gt;, which are much easier for plants to use.&lt;/p&gt;&#xA;&lt;p&gt;So yes, as long as you could supply nitrogen in the soil, plants could thrive without nitrogen in the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="47149" LastEditDate="2022-05-29T02:20:27.457" LastActivityDate="2022-05-29T02:20:27.457" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="20155" PostTypeId="2" ParentId="20141" CreationDate="2017-02-13T16:50:03.500" Score="0" Body="&lt;p&gt;You have an additional problem: SLS fuel is LH2. I am not sure if there is any experience at all using supercooled hydrogen (which is cold enough at boiling temperature).&lt;/p&gt;&#xA;" OwnerUserId="18492" LastActivityDate="2017-02-13T16:50:03.500" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20156" PostTypeId="1" CreationDate="2017-02-13T19:51:48.307" Score="7" ViewCount="235" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/20133/how-is-charge-dissipated-in-ion-propulsed-spacecraft&quot;&gt;this question&lt;/a&gt;, there is discussion of charge buildup on a spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible for a spacecraft to measure its own charge?&lt;/p&gt;&#xA;" OwnerUserId="12742" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2019-12-12T03:24:21.047" Title="Is a spacecraft able to measure it's current electric charge in some way?" Tags="|electronics|spacecraft-charging|" AnswerCount="1" CommentCount="3" ClosedDate="2019-12-12T11:05:06.593" ContentLicense="CC BY-SA 3.0" />
  <row Id="20157" PostTypeId="2" ParentId="20156" CreationDate="2017-02-13T19:54:07.350" Score="5" Body="&lt;p&gt;The ISS has a device called the &lt;a href=&quot;http://www.sdl.usu.edu/programs/fpmu&quot; rel=&quot;nofollow noreferrer&quot;&gt;Floating Potential Measurement Unit&lt;/a&gt; that handles this task. The linked page has a link to a pdf that describes it in more detail.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="12102" LastEditDate="2019-12-12T03:24:21.047" LastActivityDate="2019-12-12T03:24:21.047" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="20158" PostTypeId="1" CreationDate="2017-02-13T21:39:53.803" Score="2" ViewCount="211" Body="&lt;p&gt;I understand that the current use of the Soyuz vehicle is a nice way to get to the ISS in about five or six hours.  I was wondering if there were any plans under development for using another vehicle elsewhere (Florida or wherever) to transport humans to the ISS other than or in addition to Baikanur in the future.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastActivityDate="2017-02-14T14:05:29.353" Title="Are there any plans to reach the ISS by new means?" Tags="|iss|baikonur-cosmodrome|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20159" PostTypeId="2" ParentId="20158" CreationDate="2017-02-13T22:44:35.623" Score="5" Body="&lt;p&gt;Yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll assume we are talking only about crewed vehicles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of the crew capsule, the &lt;a href=&quot;https://en.wikipedia.org/wiki/CST-100_Starliner&quot; rel=&quot;nofollow noreferrer&quot;&gt;StarLiner&lt;/a&gt; by Boeing, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dragon 2&lt;/a&gt; by SpaceX are both being developed but neither has flown yet. Estimates for when these will be available keep being pushed back to 'next year' it seems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of launch vehicle the StarLiner Wiki says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is to be compatible with multiple launch vehicles, including the Atlas V, Delta IV, and Falcon 9, as well as the planned Vulcan. The initial launch vehicle would be the Atlas V, launched from SLC-41 at Cape Canaveral Air Force Station, Florida.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And the Dragon 2 would be launched by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, see the answer to this &lt;a href=&quot;https://space.stackexchange.com/questions/10572/why-do-we-have-so-many-different-spaceships-to-refill-iss?rq=1&quot;&gt;question&lt;/a&gt; for discussion of some of the other cargo carrying vehicles.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-02-14T14:05:29.353" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20160" PostTypeId="1" AcceptedAnswerId="20163" CreationDate="2017-02-14T05:24:44.647" Score="9" ViewCount="1273" Body="&lt;p&gt;The discussion in the NOAA legacy page &lt;a href=&quot;http://legacy-www.swpc.noaa.gov/info/Satellites.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Weather Prediction Center Topic Paper: Satellites and Space Weather&lt;/a&gt; (found &lt;a href=&quot;https://space.stackexchange.com/a/26609/12102&quot;&gt;here&lt;/a&gt;) lists:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Types of Spacecraft Anomalies&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Spacecraft anomalies are grouped into broad categories based upon the effect upon the spacecraft. A list of potential effects follows:&lt;/p&gt;&#xA;  &#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;Surface charging*&lt;/li&gt;&#xA;  &lt;li&gt;Deep dielectric or bulk charging*&lt;/li&gt;&#xA;  &lt;li&gt;Single Event Upset (SEU) * a) Galactic cosmic rays and b) Solar proton events*&lt;/li&gt;&#xA;  &lt;li&gt;Spacecraft drag (&amp;lt;1000 km)*&lt;/li&gt;&#xA;  &lt;li&gt;Total dose effects*&lt;/li&gt;&#xA;  &lt;li&gt;Solar radio frequency interference and telemetry scintillation* &lt;/li&gt;&#xA;  &lt;li&gt;Debris&lt;/li&gt;&#xA;  &lt;li&gt;Spacecraft orientation*&lt;/li&gt;&#xA;  &lt;li&gt;Photonics noise*&lt;/li&gt;&#xA;  &lt;li&gt;Materials degradation&lt;/li&gt;&#xA;  &lt;li&gt;Meteorite impact&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;SWPC has applicable data/alerts/warnings/watches/products&lt;/li&gt;&#xA;  &lt;/ul&gt;&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The two classes of charging listed are further descried: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Surface charging:&lt;/strong&gt; Surface charging to a high voltage does not usually cause immediate problems for a spacecraft. However, electrical discharges resulting from differential charging can damage surface material and create electromagnetic interference that can result in damage to electronic devices. Variations in low energy plasma parameters around the spacecraft, along with the photoelectric effect from sunlight, cause most surface charging. Due to the low energy of the plasma, this type of charging does not penetrate directly into interior components. Surface charging can be largely mitigated through proper materials selection and grounding techniques.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;Deep Dielectric or Bulk Charging:&lt;/strong&gt; This phenomenon is a problem primarily for high altitude spacecraft. At times, when Earth is immersed in a high-speed solar wind stream, the Van Allen belts become populated with high fluxes of relativistic (&gt;~1 MeV) electrons. These electrons easily penetrate spacecraft shielding and can build up charge where they come to rest in dielectrics such as coax cable, circuit boards, electrically floating radiation shields, etc. If the electron flux is high for extended periods, abrupt discharges (tiny &quot;lightening strokes&quot;) deep in the spacecraft can occur.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/20156/12102&quot;&gt;This&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/20133/12102&quot;&gt;this&lt;/a&gt; question ask about spacecraft charging, but I would like to know how the measurement is done quantitatively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drawing below shows an electroscope being used to detect the presence of charge &lt;em&gt;on another object&lt;/em&gt;. It does not measure sign, and the displacement of the gold leaves depends on geometry - both shapes and separation of the rod and top-electrode of the electroscope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft charging must be monitored to prevent mission-threatening issues, anything from perturbing the environment to be measured, to spacecraft arcing and system damage. Electron guns, field ionizers, and other things can be used to reduce spacecraft charge, but certainly the spacecraft needs at least some sign and magnitude information to do this, and for spacecraft that need to measure the local plasma environment, this is even more important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does a spacecraft &lt;em&gt;measure its own charge&lt;/em&gt;, both the &lt;em&gt;magnitude and the sign?&lt;/em&gt; I'm looking to understand the principle behind the metrology, as well as roughly how it is implemented. So &quot;With a charge measuring unit&quot; wouldn't be sufficient here. Thanks!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xiKsp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xiKsp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Illustration of a gold leaf electroscope indicating the presence of charge on the insulating rod held nearby. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Electroscope_showing_induction.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-04-11T22:02:55.597" LastActivityDate="2021-02-05T05:18:56.500" Title="How do spacecraft measure their own charge?" Tags="|failure|spacecraft-charging|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20163" PostTypeId="2" ParentId="20160" CreationDate="2017-02-14T13:35:52.687" Score="7" Body="&lt;p&gt;The ISS measures its charge using the Floating Potential Measurement Unit. This device works as follows (from &lt;a href=&quot;https://ntrs.nasa.gov/citations/20090025959&quot; rel=&quot;nofollow noreferrer&quot;&gt;Data Analysis of the Floating Potential Measurement Unit aboard the International Space Station&lt;/a&gt;):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Floating Potential Measurement Unit (FPMU) was developed by Utah&#xA;State University’s Space Dynamics Laboratory (USU-SDL) to study&#xA;surface charging of the International Space Station (ISS). The surface&#xA;charging of the ISS is a complex problem owing to its large size, its&#xA;variety of conductive/dielectric areas, and the exposed solar cell&#xA;edges on its high voltage solar arrays. Not only is severe charging of&#xA;the ISS a hazard for astronauts on Extra Vehicular Activity, but any&#xA;resultant surface arcing can lead to functional anomalies and surface&#xA;degradation on the ISS. Thus, the FPMU was developed under intense&#xA;oversite and reporting requirements as it was deemed critical for ISS&#xA;safety operations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Although the primary purpose of the FPMU remains to&#xA;monitor charging levels of the ISS and provide a dataset that can be&#xA;used to validate the ISS charging models &lt;a href=&quot;https://i.stack.imgur.com/sHz2K.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;2&lt;/a&gt;, a secondary purpose is&#xA;the measurement of electron density and temperature within the&#xA;F-region of the ionosphere to aid in the understanding of why the ISS&#xA;charges. Unfortunately, the FPMU is not operated continuously. It is&#xA;activated by ground commands and data is recorded only for specific&#xA;data campaign durations. Thus, it is essentially a “snapshot”&#xA;instrument for ionospheric density and temperature measurements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Presented in the remainder of this section is a brief overview of the&#xA;FPMU instrument suite. The next section presents the acquired data&#xA;quality and the steps taken to compensate for noise and errors. This&#xA;is followed by descriptions of the data processing algorithms to&#xA;reduce the acquired data to plasma parameters such as electron and ion&#xA;density (n e and ni) and electron temperature (T e ). We conclude the&#xA;paper with a discussion and comparison between the FPMU derived plasma&#xA;parameters and those derived from the International Reference&#xA;Ionosphere (IRI) model and the Utah State University - Global&#xA;Assimilation of Ionospheric Measurements (USU-GAIM) model.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As&#xA;illustrated in figure 1, the FPMU is an instrument suite comprising of&#xA;four separate instruments [2–4]. Three of the instruments are based on&#xA;Langmuir probe or DC electrical properties, while a fourth instrument&#xA;is based on the radio-frequency (RF) properties of the&#xA;probe.&lt;/p&gt;&#xA;&lt;p&gt;The Floating Potential Probe (FPP) is a gold-plated sphere of radius&#xA;5.08 cm. The sphere is isolated from the chassis ground by a high impedance circuit &amp;gt; 10&amp;quot; ohms. The FPP measures the ISS floating&#xA;potential (φ fISS ) at the FPMU location within a range of&#xA;-180 to +180 V at 128 Hz. The Wide-sweeping Langmuir Probe (WLP) is also a gold plated sphere of radius 5.08 cm and is swept with a&#xA;triangular wave from -20 to +80 V relative to the chassis ground (i.e.&#xA;the ISS structure) in 2048 voltage steps. The up-sweep is followed by&#xA;a down-sweep of equal amplitude and sample length. The current&#xA;resulting from the applied voltage sweep is measured on two different&#xA;12-bit channels: the low-gain channel and the high-gain channel. The&#xA;WLP low gain channel has a resolution of 700 nA and the high gain&#xA;channel a resolution of 3.5 nA per count of ADC. Thus, the high-gain&#xA;channel has sufficient sensitivity to observe both photo emission and&#xA;ion collection currents, and the low-gain channel is optimized for&#xA;observing thermal electron currents. The Narrow-sweeping Langmuir&#xA;Probe (NLP) is a gold-plated cylinder with radius 1.43 cm and length&#xA;5.08 cm. The NLP is placed mid-way on the boom supporting the FPP and is guarded on each side by gold-plated cylinders with radius 1.43 cm&#xA;and length 10.2 cm that are swept in synchrony with the NLP. A sweep&#xA;from -4.9 to +4.9 V, in 512 equal steps, is applied to the NLP during&#xA;one second, followed by a sweep down from +4.9 to -4.9 V the next&#xA;second. This sweep voltage is referenced to the floating potential as&#xA;measured by the FPP. Thus, even this small sweep range should cover&#xA;the electron retardation region and some electron saturation region,&#xA;enabling determination of ne and Te at 1 Hz. The resulting current is&#xA;again measured on two channels with different gains. The NLP low gain&#xA;channel has a resolution of 175 nA and the high gain channel a&#xA;resolution of 0.88 nA per count of ADC. The ground based laboratory&#xA;calibration of the instrument showed that the WLP and NLP instrument&#xA;noise was limited to just quantization errors.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Plasma Impedance&#xA;Probe (PIP) consists of an electrically short dipole antenna that is&#xA;electrically isolated from the ISS. It is operated in two different&#xA;modes. In the Plasma Sweeping Probe (PSP) mode, the instrument&#xA;measures the electrical impedance (magnitude and phase) of the antenna&#xA;at 256 frequencies over a 100 KHz to 20 MHz range. In the Plasma&#xA;Frequency Probe (PFP) mode, the antenna tracks the frequency at which&#xA;an electrical resonance associated with the upper-hybrid frequency&#xA;occurs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To minimize any interference between individual instruments&#xA;the probe surfaces were set at least two Debye lengths apart for a&#xA;worst-case rarified and cold ionospheric plasma. The tip-to-tip&#xA;distance from the WLP to the PIP is 130 cm and the whole instrument&#xA;stands about 150 cm tall. The FPMU interfaces with the ISS through the&#xA;Video Distribution System (VDS) similar to an External TV Camera Group&#xA;on the ISS. Thus, essentially the structural, electrical and&#xA;communication interfaces of the FPMU with the ISS replicate an&#xA;external video camera.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The FPMU was carried to the ISS on STS-121 and&#xA;deployed on August 3, 2006, on the starboard (S1) truss of the ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sHz2K.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sHz2K.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="12102" LastEditDate="2021-02-05T05:18:56.500" LastActivityDate="2021-02-05T05:18:56.500" CommentCount="14" ContentLicense="CC BY-SA 4.0" />
  <row Id="20164" PostTypeId="1" AcceptedAnswerId="28000" CreationDate="2017-02-14T14:04:30.067" Score="3" ViewCount="271" Body="&lt;p&gt;I am developing a software that is capable of simulating the orbit for satellites for my master thesis. The software will not be able to make thermal analysis, but I think it can still help satellite engineers about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I know, there is no thermal analysis software for satellites except for ESATAN which is not easy to reach. There are more common and sofwares that do thermal analysis but they lack orbit simulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can create a data output where code estimates incoming heat flux for each surface of the satellite, based on time (and position). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What format should I use so that people who uses software that are capable of making thermal analysis (lets say Solidworks for instance) can use the output?&lt;/p&gt;&#xA;" OwnerUserId="8902" LastActivityDate="2018-06-21T12:10:19.493" Title="Which format would be useful for satellite engineers to accomplish better thermal analysis?" Tags="|software|thermal|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20165" PostTypeId="2" ParentId="5048" CreationDate="2017-02-14T14:58:58.880" Score="4" Body="&lt;p&gt;There are currently International NASA requirements on near earth and interplanetary/lunar upper stage disposal.  I can't find a database for all the near earth upper stages, but Stratcom (US airforce) is tracking all space junk near earth and warns spacecraft operators of impending conjunctions.  Here is a paper from ULA (atlas and delta launchers) on the topic &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Supporting_Technologies/Orbital_Disposal_of_Launch_Vehicle_Upper_Stages_final.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ulalaunch.com/uploads/docs/Published_Papers/Supporting_Technologies/Orbital_Disposal_of_Launch_Vehicle_Upper_Stages_final.pdf&lt;/a&gt; .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For higher energy orbits the upper stage is disposed into a heliocentric orbit (around the sun).  There are international agreements on &quot;planetary defense&quot; so as to not contaminate planets/moon that may have life.  In the later Apollo missions, they hit the moon on purpose with spent upper stages to cause lunar quakes which were measured by seismographs installed by the astronauts. Apollo 12's upper stage missed it's disposal target  and showed back up near earth decades later (&lt;a href=&quot;https://en.wikipedia.org/wiki/J002E3&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/J002E3&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="18504" LastEditorUserId="18504" LastEditDate="2017-02-14T15:10:39.093" LastActivityDate="2017-02-14T15:10:39.093" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20166" PostTypeId="2" ParentId="18135" CreationDate="2017-02-14T15:03:08.643" Score="3" Body="&lt;p&gt;You can either use a high-speed stepper motor, the kind used in hard disks and CD-ROM drives, or even a brushless DC motor. While you may order or otherwise improvise the flywheel itself, you won't get a better precision than using a stack of hard disk plates; these provide precision in excess of anything you can have custom-made on reasonable budget and provide a decent density; mass easily regulated through changing the number of plates used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wheel speed readout, while helpful, is not essential; in case of brushless DC it's achievable from the same Hall sensor that drives the motor; in case of steppers you just assume the motor follows your control and only monitor current to determine if it's not falling behind or blocked. But these, again, are non-critical. You need to be able to regulate the speed, but not its precise &lt;em&gt;value&lt;/em&gt;, only &lt;em&gt;delta&lt;/em&gt;, &quot;faster/slower than now&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need rotational speed of the satellite, and use that to drive the wheel through a closed-loop algorithm (PID or similar). Accelerometers are helpful in estimating that - after substracting common bias of Earth's gravity (not applicable in space but your major headache when testing) you will have the remaining centrifugal acceleration value. Estabilishing direction of rotation will be a bit more tricky. Likely accelerometer response to changing speed of the reaction wheel will tell you - if you accelerate your wheel clockwise (and craft counterclockwise) increasing centrifugal acceleration means it was spinning clockwise in the first place; decreasing - that you just reduced its angular velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you get the spin in check and reduced to reasonable values (a few rotations per second max) then you may employ a gyroscope to arrest the rotation entirely. (it will be saturated before that, producing no usable output).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for control: Whether using DC motor, or stepper, you'll need a microcontroller, and a driver chip. Microcontroller to read the sensors, provide the calculations and control &lt;em&gt;signal&lt;/em&gt;, the driver to convert &lt;em&gt;signal&lt;/em&gt; into actual driving current. In case of DC, that will be an amplifier, capable of inverting the signal, or a H-bridge, where you drive with PWM (as opposed to voltage from DAC for use with amplifier) and it enables you to switch the direction of the motor; steppers use specialized stepper motor driver chips. The reason is that your microcontroller will be able to produce maybe 50 milliampers of current on its output pins, and your motor needs way more to run; also in case of steppers it's an awful bother to control the sequence of coils activation; the driver lets you output &quot;direction&quot; and &quot;step&quot; instead of requiring which coils need to be on and which ones off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your control program will be a major piece of work. You'll need sensor readout, conversion to useful units, control algorithm, and output, plus external &quot;steering&quot; input so that your reaction wheel can rotate the satellite to desired angle, or perform desaturation operation, instead of just holding the platform immobile in one position forever. You'll need data sheets of the components you use, and a lot of stuff to learn. Not a lesson for a single StackeExchange answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last, the pointing accuracy. Either you derive it by adding expectable errors, or determine it experimentally. Hang your rig on a thin string (fishing line), so it can spin freely and be stabilized in the spin axis by the reaction wheel. Attach a small mirror, shine a laser at that mirror and determine how stable you can keep the dot of the reflected ray. Using trigonometry and determining how much the dot travels while stabilized after angle change or external disturbance, you can determine how accurately you control the angle.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-02-14T15:03:08.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20167" PostTypeId="2" ParentId="20062" CreationDate="2017-02-14T15:39:10.780" Score="2" Body="&lt;p&gt;I'm going to try to answer a more specific question than your title asks, which is &lt;strong&gt;&lt;em&gt;Could OPV solar cells plausibly supply enough energy to maintain a flapping-wing aircraft on Mars?&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The only well-studied Mars flapping-wing concept that I am aware of is the &lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/522Colozza.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ohio Aerospace Institute report&lt;/a&gt; on an entompoter that you had asked about in a &lt;a href=&quot;https://space.stackexchange.com/questions/19988/is-it-possible-for-entomopter-to-fly-on-mars-carrying-payload-of-1-2kg&quot;&gt;previous question&lt;/a&gt;. Unfortunately, that vehicle isn't suited for &lt;em&gt;purely&lt;/em&gt; solar operation because it requires consumables:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Even though positive net lift is obtained from both the CFD and analytical solutions, the force is &#xA;  not sufficient for flight in Mars’ lower atmosphere. Hence, active flow control technology will &#xA;  be used to augment the lift, and thrust along with reducing the drag of the Entomopter...&#xA;  Blowing of the wing in a proper way is critical &#xA;  not only to the stability and control of the Entomopter, but in the Mars application, its ability to &#xA;  fly.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One of the clever part of the entomopter concept is using exhaust gasses from its &quot;muscle&quot; to achieve additional lift, and (by blowing on its own wings) to steer. Without it, the vehicle won't work in the rarefied Martian atmosphere - the craft needs to expel gas to function. And you can't replenish the gas with solar energy alone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I suppose one could imagine a solar-powered condenser light enough to fit on the craft. I'm skeptical that it's possible, but I invite someone to ask a follow-up question to find out!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This study suggests, then, that the answer to the question is no: &lt;strong&gt;&lt;em&gt;any&lt;/em&gt; solar cell will fail to supply enough energy for a flapping-wing aircraft on Mars, because flapping-wing vehicles can't fly on Mars &lt;em&gt;at all&lt;/em&gt; without some sort of expendable propellant.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;That answer isn't very satisfying. Let's play with some numbers and see if we can glean anything else.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In your &lt;a href=&quot;http://www.nature.com/articles/ncomms1772&quot; rel=&quot;nofollow noreferrer&quot;&gt;first source&lt;/a&gt;, Kaltenbrunner et. al. present their results from testing a variety of configurations of Organic photovoltaic (OPV) solar cells, which are extremely thin devices that under investigation because they can be made flexible and lightweight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the article's discussion section:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The OPV devices constructed on PET presented here have a per-area mass of $4 gm^-2$, and 4% efficiency, giving $10 Wg^-1$.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As best I can tell, this figure represents the authors' most successful prototype. It's plausible that the efficiency could actually be improved, but we'll start with these numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The figure of $10W/g$ (roughly) appears to have been computed as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$({1050W/m^2}) / ({4g/m^2}) * 0.04 = {10.5W/g}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $1050W/m^2$ is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_irradiance&quot; rel=&quot;nofollow noreferrer&quot;&gt;common figure&lt;/a&gt; for solar irradiance at the Earth's surface. For our purposes, we need to use solar irradiance on the Martian surface instead. &lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/522Colozza.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This report on Mars entomopters&lt;/a&gt; suggests that this is about $590W/m^2$ on average.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our quick-n-dirty calculations then suggest that (neglecting any and all effects on the OPV's materials by Martian surface conditions) the panels in the study would give us $5.9W/g$ to play with on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this enough to fly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The OAI study's primary design point on page 49 requires $883W$ to fly and appears to have $0.107m^2$ of wing area per wing set (of which it has 2). Coating the wings in OPV panels only costs $0.856g$ - no wonder &lt;em&gt;Nature Communications&lt;/em&gt; was excited about these! If we do that, we buy ourselves:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${0.214m^2}*{590W/m^2}=126.26W$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind you only get this during daylight. When you factor in the power needed to keep the electronics and instruments from freezing (and the fact that you need to store energy so they don't freeze overnight, either), this really isn't much to work with. &lt;em&gt;Maybe&lt;/em&gt; you could make a short flight every once in a while after charging your batteries, if we completely neglect the gas issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, this (admittedly lacking) analysis also suggests that the answer is &lt;strong&gt;no, solar cells cannot plausibly power a flapping-wing vehicle in the Martian atmosphere.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider trying a large fixed wing craft for the greatly increased surface area.&lt;/p&gt;&#xA;" OwnerUserId="5851" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-02-15T01:43:02.313" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20168" PostTypeId="2" ParentId="18135" CreationDate="2017-02-14T19:05:03.347" Score="1" Body="&lt;p&gt;Ad 1: Do not try to implement motor control yourself, unless you have special interest in this aspect. This can easily be a project on its own. A stepper will be way too noisy for accuracy, BLDC with hall-sensor will run very smooth. Look for prebuilt controllers like &lt;a href=&quot;http://vedder.se/2015/01/vesc-open-source-esc/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ad 2: You need specs for moment of inertia of the nanosat. There will be a tradeoff between RPM and wheel-mass. RPM is related to bearings and possibly also to evacuated/pressurized casing. Be sure to consider evaporation of lubricants, this can get nasty in low pressure environments. The machined wheel needs set screws for precision balance adjustment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ad 3: The accuracy will depend on sensors (&lt;a href=&quot;https://en.m.wikipedia.org/wiki/Classical_control_theory#Feedback&quot; rel=&quot;nofollow noreferrer&quot;&gt;feedback&lt;/a&gt;) and disturbance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, with a rate sensor alone, you can not have any pointing at all, because the rate is the derivative of the pointing error (this is different from an &lt;a href=&quot;https://youtu.be/woCdjbsjbPg&quot; rel=&quot;nofollow noreferrer&quot;&gt;inverted pendulum&lt;/a&gt;, where pointing error will result in acceleration and &lt;em&gt;can&lt;/em&gt; be sensed by a rate sensor). So you need a sun tracker for pointing, and pointing accuracy will depend on the trackers performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In LEO, I would expect only very low frequency disturbances, you probably get away with a very low end microcontroller, and you can average sensor readings over a long interval to suppress noise.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17004" LastEditDate="2017-02-14T22:29:54.460" LastActivityDate="2017-02-14T22:29:54.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20169" PostTypeId="1" AcceptedAnswerId="20170" CreationDate="2017-02-14T23:56:09.160" Score="3" ViewCount="485" Body="&lt;p&gt;Is having an engine mounted on a gimbal or having some ailerons better for turning a rocket &lt;strong&gt;that is still in the atmosphere&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I mean is why does SpaceX use some grid fins, which are a type of ailerons, and not simply a engine on a gimbal for their vertical landing??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VD1Xy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VD1Xy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17599" LastActivityDate="2017-02-15T11:30:56.720" Title="Engine on a gimbal vs ailerons" Tags="|spacex|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20170" PostTypeId="2" ParentId="20169" CreationDate="2017-02-15T00:15:47.113" Score="7" Body="&lt;h2&gt;They need to aim the vehicle the whole descent&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The engines only burn three times during landing: once to boost back, once to slow down and once to land at the end.  In between these burns they still need to aim the vehicle, that's where the grid fins are used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RmfsO.jpg&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RmfsO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18506" LastActivityDate="2017-02-15T00:15:47.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20171" PostTypeId="1" AcceptedAnswerId="20173" CreationDate="2017-02-14T22:27:54.657" Score="4" ViewCount="880" Body="&lt;p&gt;I was wondering if anyone can tell me what the composition of the Saturn V's F-1 engine's nozzles are? I have looked online and they have many different statistics but I cannot find out what they were made of, can anyone help?&lt;/p&gt;&#xA;" OwnerUserId="18508" OwnerDisplayName="user140052" LastEditorUserId="6944" LastEditDate="2017-02-15T02:58:13.137" LastActivityDate="2018-10-03T13:34:25.317" Title="F-1 rocket engine composition" Tags="|rockets|propulsion|engines|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20172" PostTypeId="2" ParentId="20169" CreationDate="2017-02-15T00:53:37.573" Score="2" Body="&lt;p&gt;Grid fins are useful over a limited duration of the return trajectory, but are efficient (require no consumables), add to the vehicle's stability, and can be used to make small changes to the trajectory (no &quot;suicide burn&quot;).&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2017-02-15T00:53:37.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20173" PostTypeId="2" ParentId="20171" CreationDate="2017-02-15T02:50:31.927" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;https://history.msfc.nasa.gov/saturn_apollo/documents/F-1_Engine.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;F-1 Engine Fact Sheet&lt;/a&gt; states that the &quot;nozzle extension&quot; (the part of the nozzle below the turbine exhaust manifold) was made of high strength stainless steel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;thrust chamber&quot;, including the portion of the nozzle above the turbine exhaust manifold, was formed from tubes constructed of Inconel X-750, a high-temperature, heat-treatable, nickel base alloy. (&lt;a href=&quot;http://heroicrelics.org/info/f-1/f-1-thrust-chamber.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DFnW3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DFnW3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-02-15T02:52:28.780" LastActivityDate="2017-02-15T02:52:28.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20175" PostTypeId="2" ParentId="20027" CreationDate="2017-02-15T04:58:12.127" Score="-2" Body="&lt;p&gt;Like was said before, the electricity is running through the grids to each other, one being positive one being negative. The negative electrons are attracted to the positive grid, absorbed, and the ions are attracted to the negative grid and accelerated, creating the thrust that the engine produces.&lt;/p&gt;&#xA;" OwnerUserId="18508" LastActivityDate="2017-02-15T04:58:12.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20176" PostTypeId="2" ParentId="19950" CreationDate="2017-02-15T05:04:19.117" Score="4" Body="&lt;p&gt;Yes, the fuel would be used as a monopropellant because as soon as you excite the metallic hydrogen, it will be just H molecules which will quickly bond to each other to Form H2, creating mass amounts of energy. If you convert it back to molecular hydrogen, all the energy required to produce it is released. No burning would be necessary, it would just be the metallic hydrogen, which keep in mind would be denser than regular compressed cryogenic hydrogen, so it depends on how much you carry if it would weigh more or less, but no oxidizer would be needed. This source also explains some other key properties of metallic hydrogen if you are interested. &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/nMfPNUZzG_Q?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; &lt;a href=&quot;https://m.phys.org/news/2017-01-metallic-hydrogen-theory-reality.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://m.phys.org/news/2017-01-metallic-hydrogen-theory-reality.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18508" LastEditorUserId="18508" LastEditDate="2017-02-15T17:20:25.787" LastActivityDate="2017-02-15T17:20:25.787" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="20177" PostTypeId="1" AcceptedAnswerId="20180" CreationDate="2017-02-15T06:05:09.727" Score="14" ViewCount="6217" Body="&lt;p&gt;The Space Shuttle was a fairly compact transportation vehicle in terms of the amount of space available for astronauts to move around. Did they ever attempt to roll a Space Shuttle to induce artificial gravity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, why? It doesn't sound hard, just pump some air through the maneuver thrusters and off you go!&lt;/p&gt;&#xA;" OwnerDisplayName="user14355" LastEditorUserId="7062" LastEditDate="2017-02-15T06:51:34.903" LastActivityDate="2017-02-15T15:39:35.070" Title="Was the Space Shuttle ever rotated to induce artificial gravity?" Tags="|space-shuttle|artificial-gravity|rcs|" AnswerCount="2" CommentCount="20" ContentLicense="CC BY-SA 3.0" />
  <row Id="20179" PostTypeId="2" ParentId="20160" CreationDate="2017-02-15T07:36:36.660" Score="2" Body="&lt;p&gt;You can use a direct approach to the measurements. For sign, you can measure whether electrons are attracted to or repelled by the craft - or ions repelled or attracted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For field strength you can measure the acceleration of electron or ions. Or measure the energy they have after crossing a know distance. For example, see how much light they create when they hit something, or get through a screening grid with some electric potential. Or for acceleration, measure how long it takes for them to cross a known distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For total charge, you need a process, not a simple measurement. Because spacecraft are not all conducting, and they have pointy bits like antenna, the charge distribution is not uniform, and neither is the electric field. You can take measurements all over an imaginary surface that encloses the craft, and develop an empirical formula for calculating charge based on the measurement of the electric field at some agreed on point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These methods are not absolute measurements if you are in a sea of charge, like the solar wind after charges are separated by the Earth's magnetic field.&lt;/p&gt;&#xA;" OwnerUserId="1432" LastActivityDate="2017-02-15T07:36:36.660" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20180" PostTypeId="2" ParentId="20177" CreationDate="2017-02-15T07:47:36.710" Score="28" Body="&lt;p&gt;There are several reasons not to do this: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Artificial gravity in such a small space is not very pleasant. You'll get a noticeable difference in gravity in different places, which makes it difficult to move around without banging into the walls. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You also get &lt;a href=&quot;https://en.wikipedia.org/wiki/Coriolis_force&quot;&gt;coriolis forces&lt;/a&gt; (thrown objects don't move in a straight line) which makes moving around non-intuitive.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Shuttle wasn't designed for it. When rotating around the pitch or yaw axis, the dashboard becomes the floor, and you don't want that. There were no ladders in that direction either so you have to climb on whatever's handy to move between decks. The operator station for the robotic arm becomes inaccessible. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-15T07:47:36.710" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="20181" PostTypeId="2" ParentId="1059" CreationDate="2017-02-15T08:47:35.150" Score="14" Body="&lt;p&gt;As of Feb 15, 2017, it is 104 satellites by India's ISRO.&lt;/p&gt;&#xA;&lt;p&gt;PSLV-C37 launched from Satish Dhawan Space Centre at 09:28 local time (03:58 UTC).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://timesofindia.indiatimes.com/india/isro-sets-history-launches-104-satellites-in-one-go/articleshow/57159734.cms&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISRO-104&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.ndtv.com/india-news/isro-to-launch-a-record-104-satellites-into-earths-orbit-10-points-1659592&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISRO-ndtv&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;&lt;strong&gt;UPDATE: (Jan. 24, 2021)&lt;/strong&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;As of Jan 24, 2021, it is 143 satellites by SpaceX.&lt;/p&gt;&#xA;&lt;p&gt;The Transporter 1 mission launched from Cape Canaveral, Florida at 10:00 local time (15:00 UTC).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://nextspaceflight.com/launches/details/2403&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transporter 1 - nextspaceflight&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasaspaceflight.com/2021/01/transporter-1-rideshare-program-debut/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transporter 1 - nasaspaceflight&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18514" LastEditorUserId="32284" LastEditDate="2021-01-24T18:34:17.050" LastActivityDate="2021-01-24T18:34:17.050" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="20183" PostTypeId="2" ParentId="20169" CreationDate="2017-02-15T11:30:56.720" Score="2" Body="&lt;p&gt;SpaceX uses both. In the landing videos, you can see the center main engine gimbaling to rather large angles during the final approach. In the initial landing attempts, they found that the combination of gimbaled main engine plus nitrogen thrusters at the top of the stage was not sufficient for the last phase of the landing. So they had a choice: add larger thrusters (in groups or gimbaled) to the top of the stage, or add aerodynamic control surfaces.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Eric said, the grid fins offer some advantages over thrusters. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-15T11:30:56.720" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20184" PostTypeId="2" ParentId="20177" CreationDate="2017-02-15T12:44:01.067" Score="17" Body="&lt;p&gt;The highest rotational rate ever achieved by a shuttle in orbit was only 3 degrees/second (approximate).  This was inadvertently caused when Mission Control &lt;a href=&quot;http://hdl.handle.net/2060/19920014000&quot;&gt;uplinked a bad state vector during crew sleep*&lt;/a&gt; and caused the vehicle to go out of control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This rotation rate was not nearly enough to induce artifical gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;*incident is described on pages 2-4 and 7-25 of the pdf&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-02-15T15:39:35.070" LastActivityDate="2017-02-15T15:39:35.070" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20185" PostTypeId="2" ParentId="20054" CreationDate="2017-02-15T12:59:21.487" Score="3" Body="&lt;p&gt;We need a fair number of approximations here...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's ignore atmospheric losses, since according to Russell Borogov's comment these are dwarfed by gravity losses. Let's also assume that the rocket instantly gets to orbital altitude and velocity, so that there is no influence of the trajectory on how efficient we can be (more on that later).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference is the potential energy being higher due to the full rocket being placed at a higher altitude. Let's calculate it with Saturn V's payload weight to LEO: 140 tonnes (to be independent of fuel mass, which we compute later). Let's also assume gravity is constant up to LEO to keep this simple.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;0: We set this potential energy origin to sea level: 0J.&lt;/li&gt;&#xA;&lt;li&gt;1: 10,000 km high: 13,720 TJ (TeraJoule). This is above LEO, so we can't use Saturn V's LEO payload for this though. Anyone got some high Earth orbit payload launcher data?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note that LEO is ~2,000km: 2,74 TJ&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;2: 700km: 960,4 GJ&lt;/li&gt;&#xA;&lt;li&gt;3: 80km: 109,76 GJ&lt;/li&gt;&#xA;&lt;li&gt;4: 50km: 68,6 GJ&lt;/li&gt;&#xA;&lt;li&gt;5: 12km: 16,4 GJ&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;How much fuel is that? Let's compute the total energy of the fuel from the rocket's total dV to LEO. To get there, we burn the entire first and second stages, and 2.5 minutes of the third stage (the rest being used for translunar injection). That is in total 2.58+4.13+1,47 = 8,18km/s delta-V expended (LEO orbital speed being 7.8km/s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much kinetic energy is that delta-V ? That is 4,69 TJ. Note that this is 171% of LEO potential energy, since orbiting is not just about the altitude, but also about the speed, to avoid falling down (see also: Vis-viva equation)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, we can substract the potential energy gained from launching at a higher altitude from that kinetic in order to approximate the required delta-V to get to orbit from a given altitude:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Energy/deltaV still required:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;0: 4,69 TJ, ~8,18km/s i.e.   0 m/s dV saved&lt;/li&gt;&#xA;&lt;li&gt;2: 3,73 TJ, ~7,30km/s i.e. 880 m/s dV saved&lt;/li&gt;&#xA;&lt;li&gt;3: 4,58 TJ, ~8,09km/s i.e.  90 m/s dV saved&lt;/li&gt;&#xA;&lt;li&gt;4: 4,62 TJ, ~8,12km/s i.e.  60 m/s dV saved&lt;/li&gt;&#xA;&lt;li&gt;5: 4,67 TJ, ~8,17km/s i.e.  10 m/s dV saved&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Then, using the rocket equation, we can compute how much fuel is needed, and give the result as a ratio of the original fuel weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To simplify, we'll assume the first stage is entirely expended (makes calculations harder since its specific impulse is lower than the two next stages) and substract its 2.58km/s deltaV.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;0: 5.6km/s to LEO in second + third stages, 2160+456+39=~2660T of fuel in all 3 stages, 456+39=~500T expended fuel from 2nd+3rdstage&lt;/li&gt;&#xA;&lt;li&gt;2: 4,72km/s @421s ISP -&gt; 440T wet mass -&gt; 300T fuel -&gt; 200T saved&lt;/li&gt;&#xA;&lt;li&gt;3: 5,51km/s @421s ISP -&gt; 523T wet mass -&gt; 392T fuel -&gt; 108T saved&lt;/li&gt;&#xA;&lt;li&gt;4: 5,54km/s @421s ISP -&gt; 536T wet mass -&gt; 396T fuel -&gt; 104T saved&lt;/li&gt;&#xA;&lt;li&gt;5: 5,55km/s @421s ISP -&gt; 537T wet mass -&gt; 397T fuel -&gt; 103T saved&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As a ratio to the total 2660T fuel mass:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;0:   0km: 100,0% fuel mass&lt;/li&gt;&#xA;&lt;li&gt;2: 700km: 93%&lt;/li&gt;&#xA;&lt;li&gt;3:  80km: 96%&lt;/li&gt;&#xA;&lt;li&gt;4:  50km: 96%&lt;/li&gt;&#xA;&lt;li&gt;5:  12km: 96%&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Notice that 3-5 should actually be closer to 99%, I think I got the fuel mass wrong on the last stage since I forgot the mass of the lunar modules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's conclude with saying that the fuel savings as a ratio only get significant when you launch from an altitude that gets very close to LEO -- since your energy is expended both to get to orbital speed &lt;strong&gt;and&lt;/strong&gt; orbital altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notes: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;All numbers taken from the Saturn V &#xA;Wikipedia article. &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Saturn_V&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Potential energy calculator:&#xA;&lt;a href=&quot;https://www.easycalculation.com/physics/classical-physics/potential-energy.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.easycalculation.com/physics/classical-physics/potential-energy.php&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="17312" LastEditorUserId="17312" LastEditDate="2017-02-15T14:00:40.543" LastActivityDate="2017-02-15T14:00:40.543" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20186" PostTypeId="1" AcceptedAnswerId="20189" CreationDate="2017-02-15T15:21:00.980" Score="4" ViewCount="405" Body="&lt;p&gt;I was reading the &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920014000.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mission Safety Evaluation Report for STS-32&lt;/a&gt; and I noticed the following passage:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The STS-32 rendezvous was one of the most complex the Space Shuttle&#xA;  had ever attempted, &lt;strong&gt;requiring 11 major firings&lt;/strong&gt; of Columbia's&#xA;  maneuvering engines.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That seems pretty high to me. Now my space flight experience only extends to Kerbal, but through those simulations I found that you can generally get into any rendezvous orbit in just 4-5 burns. Why did it take 11 this time?&lt;/p&gt;&#xA;" OwnerUserId="3141" LastActivityDate="2017-02-15T19:45:12.553" Title="Why did STS-32 require 11 major firings of Columbia's maneuvering engines?" Tags="|orbital-mechanics|space-shuttle|orbital-maneuver|engines|rendezvous|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20188" PostTypeId="2" ParentId="19706" CreationDate="2017-02-15T17:22:49.887" Score="-3" Body="&lt;p&gt;Sputnik is just the Russian word used to describe all satellites.  Below is a photo of a model of the 1958 sputnik in the cosmonautics museum in Moscow  which is a different configuration than the video shown. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rAIGa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rAIGa.jpg&quot; alt=&quot;SPUTNIK&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18504" LastEditorUserId="18504" LastEditDate="2017-02-16T20:42:20.140" LastActivityDate="2017-02-16T20:42:20.140" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20189" PostTypeId="2" ParentId="20186" CreationDate="2017-02-15T19:16:59.917" Score="3" Body="&lt;p&gt;First of all, remember that report is really old. A quick review of the wonderfully informative &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023479.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;History of Space Shuttle Rendezvous&lt;/a&gt; shows that 9 or 10 burns* are not unusual for a rendezvous (see, for example, the writeups on STS-49 and STS-130).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;* depending on whether you include OMS-2 in the count or not&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Iwq1b.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Iwq1b.png&quot; alt=&quot;STS-49 shuttle relative motion profile (May 1992)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were some off-nominal events with the STS-32 rendezvous, however. As detailed in &lt;a href=&quot;http://klabs.org/DEI/lessons_learned/shuttle/cr-2007-2136974.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lessons Learned from 7 Space Shuttle Missions&lt;/a&gt;, problems with the onboard radar resulted in flying a trajectory different from that planned, and additional braking towards the end. This may account for the additional burn(s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6RzIU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6RzIU.png&quot; alt=&quot;Relative motion plot illustrating the planned and actual trajectories&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6816" LastEditDate="2017-02-15T19:45:12.553" LastActivityDate="2017-02-15T19:45:12.553" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20190" PostTypeId="2" ParentId="18657" CreationDate="2017-02-16T04:29:33.223" Score="5" Body="&lt;p&gt;Those falling particles seem to be parts of a protective sheet that shields the opening around the interstage lattice between first and second stages.  Only the Long Marches launched from Taiyuan and Jiuquan during colder times of the year have such cover.&lt;/p&gt;&#xA;&lt;p&gt;The following are a few examples to illustrate the presence and absence of the sheet in different times of the year.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;TanSat (Long March 2D, TSLC, August 2016) &lt;a href=&quot;https://i.stack.imgur.com/qIlXM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qIlXM.jpg&quot; alt=&quot;Long March 2D in August 2016&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://spaceflight101.com/wp-content/uploads/2016/08/Cp63O0OWAAAnW8e.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflight101.com&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;SuperView (Long March 2D, TSLC, December 2016) &lt;a href=&quot;https://i.stack.imgur.com/ggi8b.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ggi8b.jpg&quot; alt=&quot;Long March 2D in December 2016&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://spaceflight101.com/wp-content/uploads/2016/12/GJ1-2-Launch-1-1024x683.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflight101.com&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Tiangong-2 (Long March 2F, JSLC, September 2016) &lt;a href=&quot;https://i.stack.imgur.com/PObGM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PObGM.jpg&quot; alt=&quot;Long March 2F in September 2016&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://spaceflight101.com/wp-content/uploads/2016/10/05-TG02Launch-1-1024x682.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflight101.com&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Shenzhou-11 (Long March 2F, JSLC, October 2016) &lt;a href=&quot;https://i.stack.imgur.com/dzTrG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dzTrG.jpg&quot; alt=&quot;Long March 2F in October 2016&quot; /&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sub&gt;(source: &lt;a href=&quot;https://spaceflight101.com/wp-content/uploads/2016/10/SZ11-Liftoff-Xinhua-5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflight101.com&lt;/a&gt;)&lt;/sub&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;News reports suggest that those are &lt;a href=&quot;http://www.china.com.cn/news/space/2012-11/23/content_27203909.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;insulation sheets&lt;/a&gt; that kept the rocket warm against cold winter.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="340" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-07-20T05:02:37.163" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20191" PostTypeId="1" AcceptedAnswerId="20202" CreationDate="2017-02-16T04:34:48.153" Score="5" ViewCount="396" Body="&lt;p&gt;It's been &lt;s&gt;24 hours&lt;/s&gt; &lt;em&gt;56 hours&lt;/em&gt; since &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Space_Research_Organisation&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISRO&lt;/a&gt;'s &lt;a href=&quot;https://en.wikipedia.org/wiki/PSLV-C37&quot; rel=&quot;nofollow noreferrer&quot;&gt;PSLV-C37&lt;/a&gt; launched from &lt;a href=&quot;https://en.wikipedia.org/wiki/Satish_Dhawan_Space_Centre&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satish Dhawan Space Centre&lt;/a&gt; (SRI) at 09:28 local time (03:58 UTC).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see satcat numbers and object names for 105 total objects; &lt;code&gt;41948 2017-008A&lt;/code&gt; through &lt;code&gt;42052 2017-008DJ&lt;/code&gt;. It looks like only six deployed satellites have TLEs so far &lt;code&gt;A-F&lt;/code&gt; with &lt;code&gt;DJ&lt;/code&gt; appearing to be the fourth stage of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;PSLV&lt;/a&gt; rocket still in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;s&gt;24 hours&lt;/s&gt; &lt;em&gt;56 hours&lt;/em&gt; is longer than I expected for first TLEs to be released. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm pretty sure this is quite a challenge, and I can think up a number of them here:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;n &gt; 100&lt;/li&gt;&#xA;&lt;li&gt;low radar cross-secction&lt;/li&gt;&#xA;&lt;li&gt;low optical cross-section&lt;/li&gt;&#xA;&lt;li&gt;no telemetry or on-board GPS fixes reported from satellite owners&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Do any of these (or other) factors stand out as a primary reason why this would be so challenging? Are any of them not really a reason?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Videos below:  P- and P+ views appear to be nadir and zenith cameras. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are &lt;em&gt;pretty darn cool videos&lt;/em&gt; - but check that your volume is not at maximum before playing. Vids similar but not the same, waiting for a single definitive video to post instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also available as a &lt;a href=&quot;https://twitter.com/planetlabs/status/831906758121512960&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planet Labs Tweet&lt;/a&gt;!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/cvJYSmplzXc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/c0BpjPUT5FE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="25" LastEditDate="2017-02-17T14:11:33.617" LastActivityDate="2017-02-17T14:11:33.617" Title="ISRO's PSLV-C37 with 104 satellites - after 56 hours only six TLEs, which factor is greatest challenge?" Tags="|artificial-satellite|isro|payload-deployment|two-line-elements|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20192" PostTypeId="1" AcceptedAnswerId="20194" CreationDate="2017-02-16T05:27:32.727" Score="4" ViewCount="802" Body="&lt;p&gt;The News Nation (India) article &lt;a href=&quot;http://www.newsnation.in/article/161275-isro-104-satellites-launch-under-single-mission-complete-information-about-the-record-breaking-launch.html&quot; rel=&quot;noreferrer&quot;&gt;ISRO PSLV-C37 launch: Why it is crucial for India? Everything you should know about the 104 satellites mission&lt;/a&gt; includes this photo, with the caption &quot;ISRO's biggest launch: India to script history as 104 satellites set to blast off on PSLV-C37 rocket.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like one could place multiple rockets around a central strongback &quot;core&quot; which seems unusual, and there are so many very long, heavy horizontal platforms extending in various directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;What is happening to the rocket in this ISRO photo - what are these huge platforms?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, photo is also obviously stretched/distorted horizontally. I believe the second photo is a better representation of the pre-launch configuration! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nYwx2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nYwx2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;http://www.newsnation.in/article/161275-isro-104-satellites-launch-under-single-mission-complete-information-about-the-record-breaking-launch.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. Credit ISRO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NfUA5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NfUA5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;http://www.newsnation.in/photos/news/mesmerizing-visuals-of-isro-record-104-satellite-launch-in-single-mission-1980/slide1#ph-19801&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. Credit ISRO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Yad3b.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Yad3b.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Found another one &lt;em&gt;with platforms again.&lt;/em&gt; From &lt;a href=&quot;http://www.theverge.com/2017/2/14/14601938/india-pslv-rocket-launch-satellites-planet-doves&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. Credit ISRO.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-16T05:43:50.063" LastActivityDate="2017-06-24T06:50:37.033" Title="What is happening to the rocket in this ISRO photo - what are these huge platforms?" Tags="|launch-site|isro|launchpad|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20193" PostTypeId="2" ParentId="20142" CreationDate="2017-02-16T06:32:50.097" Score="2" Body="&lt;p&gt;Based on radio-luminescent measurements done over the years (ex- stealth weapons programs), comparatively large reflections from radio-illuminated objects have been attributed from relatively small portions of the objects surfaces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same could be expected in visible light reflections if a certain geometry of a reflective surface concentrates rays toward the observer. &#xA;*The rotation of a reflecting object could create the illusion of cyclic reciprocal motion (side to side wiggle) when viewed at low angles from its doppler / parallax shift, coupled with some image bending from the 'lensing' of atmosphere. (prominent illusion from multiple overlapping optical factors)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good verification of this (to confirm the illusion of side to side wiggle is indeed a product of object rotation, and not actual wiggles) would be to compare the frequency of illumination fluctuation to the object's actual rate of rotation. If congruent, then wiggle is illusion...if not congruent, then there may be more going on up there, lol&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&amp;lt;&amp;lt; hey, Im just swinging at this, stretching any available artifacts and the neurons carrying them in order to synthesize a logical answer... am not liable for misconstrued or intangible conclusions or non-permanent mode corruption...&gt;&gt;&lt;/p&gt;&#xA;" OwnerUserId="16922" LastEditorUserId="16922" LastEditDate="2017-02-17T19:28:23.180" LastActivityDate="2017-02-17T19:28:23.180" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20194" PostTypeId="2" ParentId="20192" CreationDate="2017-02-16T08:44:20.833" Score="8" Body="&lt;p&gt;Those platforms are access walkways for technicians, used for the final inspection of the rocket before launch. The platforms are positioned around the rocket for inspection, then rotated out of the way before launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tower is the &lt;a href=&quot;http://www.isro.gov.in/about-isro/launch-complex&quot; rel=&quot;noreferrer&quot;&gt;Umbilical Tower&lt;/a&gt; of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Satish_Dhawan_Space_Centre_Second_Launch_Pad&quot; rel=&quot;noreferrer&quot;&gt;Second Launch Pad&lt;/a&gt; at Satish Dhawan Space Centre. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As per the “Integrate, Transfer and Launch (ITL)” concept, based on which the second launch pad and the associated facilities are designed, the entire vehicle is assembled and checked-out on a mobile launch pedestal in the Vehicle Assembly Building (VAB) which is 83m tall and then moved in vertical position to the launch pad on a twin rail track. Following the remote fuelling operations and final check out through the Umbilical Tower, the vehicle takes off.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bTzJD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bTzJD.jpg&quot; alt=&quot;Launch tower&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second photo in the question is of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Satish_Dhawan_Space_Centre_First_Launch_Pad&quot; rel=&quot;noreferrer&quot;&gt;First Launch Pad&lt;/a&gt; at Satish Dhawan: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NfUA5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NfUA5.jpg&quot; alt=&quot;First Launch Pad&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This pad has a much smaller umbilical tower without access walkways. Instead, they use the Mobile Service Tower (the big white/green structure in the background), this entire building can be moved on rails to the launch mount. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-02-16T12:00:57.477" LastActivityDate="2017-02-16T12:00:57.477" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20195" PostTypeId="1" AcceptedAnswerId="20203" CreationDate="2017-02-16T23:50:29.410" Score="3" ViewCount="358" Body="&lt;p&gt;Satellite radar cross-sections is a complex function of satellite structure, design, materials, instantaneous attitude, (radar) frequency, polarization, and if has moving parts, configuration, just to name a few.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume I've written a similar paragraph for visible, NIR, and thermal IR, and whatever other electromagnetic probes are used for satellite tracking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I am wondering, for a generic 1U cubesat built from COTS (commercial, off the shelf) cubesat parts, do they end up being sufficiently reliably tracked to get TLEs good enough to keep track of where they are, establish regular communications, and avoid collisions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am assuming the answer would be yes, since the 1U is a popular standard for experimental satellites, and there are several businesses with &lt;s&gt;plans to deploy hundreds more 1U cubesats&lt;/s&gt; collective hopes to deploy hundreds of cubesats in the future. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course one could imagine a &quot;science project&quot; called stealth-cube, 3D printed out of free space impedance matched resistive polymer, with plastic optics to bring sunlight into small, oddly-shaped organic photovoltaics, so let's not include extremes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in general &lt;em&gt;are 1U cubesats sufficiently detectable to get at least minimally usefully predictive public TLEs, updated retularly?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-17T01:32:56.160" LastActivityDate="2017-02-17T14:08:44.340" Title="Are 1U cubesats sufficiently detectable to get at least minimally usefully predictive public TLEs, updated regularly?" Tags="|cubesat|tracking|two-line-elements|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20197" PostTypeId="1" CreationDate="2017-02-17T04:40:43.633" Score="13" ViewCount="1462" Body="&lt;p&gt;I've become curious of this question since my time on KSP, and searches have not rendered much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the simulator, building large stations even without the micro atmosphere being simulated, can cause them to become unstable from even the slightest rotational torque, and start &quot;wiggling&quot; about.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems a little exaggerated in the game, but something tells me their structural stiffness and bendiness values may not be so far off, perhaps? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as they were building the ISS, and the station's length expanded until the solar arrays were installed, were the re-boosts accounting for the extra torque and off-center of thrust, perhaps with throttling? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I just want to see if &lt;strong&gt;structural integrity was ever considered&lt;/strong&gt; for re-boosts at the station's current size, if there's a paper maybe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Extra is for info on off-center &lt;em&gt;center-of-thrust&lt;/em&gt; and &lt;em&gt;center-of-mass&lt;/em&gt; considerations and compensations. (Active systems would add an additional torque, was this considered?)&lt;/p&gt;&#xA;" OwnerUserId="192" LastActivityDate="2017-02-17T08:45:58.673" Title="How do orbital boosts affect the structural integrity of the ISS?" Tags="|iss|orbital-maneuver|orbital-decay|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20198" PostTypeId="1" CreationDate="2017-02-17T06:21:15.993" Score="6" ViewCount="388" Body="&lt;p&gt;I have heard about the Project Gemini flight manuals, but have not been able to locate a copy. Perhaps someone here knows if these exist in some form?&#xA;I am NOT talking about the Familiarization manual, which is often called the flight manual, I have this data already. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am also very interested in other sources of detailed info on the use of the cockpit instrumentation, and especially the flight procedures involving the actual operation of the spacecraft... I have some Flight Plans, but need more background information on WHY and WHAT many of the steps are actually doing. I am interested in building a Gemini flight simulator, if it helps show the kind of info I am after.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="415" LastEditDate="2017-02-17T15:00:08.257" LastActivityDate="2017-02-22T22:43:56.553" Title="Where can I find flight manuals (not familiarization manuals) from Project Gemini?" Tags="|crewed-spaceflight|nasa|project-gemini|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20199" PostTypeId="2" ParentId="20197" CreationDate="2017-02-17T06:34:16.390" Score="18" Body="&lt;p&gt;Yes. It is considered (sorry I don't know exactly how much, but probably a LOT - they track the torque on individual bolts during construction for example). They had an accident a while ago where a thruster started firing and oscillating wildly, causing the whole station to wobble and shake, a bit like they do in KSP. Although the forces applied exceeded design limitations, later analysis showed that no real damage had been done. Still, must have been brown trousers time for everyone involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Onboard footage of the incident: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ENNq69499tA?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here you can listen to Suni Williams talk about the exercise bike. Note that she mentions it is not directly coupled to the walls, because the motions of the cyclist would put energy into the structure, and cause the solar arrays to vibrate up and down: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/doN4t5NKW-k?start=195&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if they are paying attention to the vibrations of the exercise bike, they are surely spending a lot of time calculating all the forces and stresses and oscillations involved in a re-boost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another interesting source of strain on the structure, which is absolutely not modelled in KSP, is that the ISS is so large, the different ends or corners of it are in significantly different orbits, and so want to drift relative to each other. This induces regular, slow compression and expansion on the structure (and no doubt causes a few creaks and pops).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Following a docking, both vehicles are put into a free drift for a while, to prevent unnecessary vibration, torque and stress. The docked vehicle can gently rock back and forth and dissipate energy through friction before being slowly pulled in and firmly latched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sorry I don't have specific data on the re-boost, but the attention paid to the structure is VERY detailed, as seen above, and so no doubt the re-boost gets appropriate attention.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2017-02-17T06:39:53.407" LastActivityDate="2017-02-17T06:39:53.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20200" PostTypeId="2" ParentId="20197" CreationDate="2017-02-17T08:45:58.673" Score="12" Body="&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20120016000&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; describes tests done in 2010, where thrusters on the ISS were fired specifically to analyze the resulting vibrations and compare them with the theoretical models. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Modal analyses, model validations and correlations are performed for the different configurations of the International Space Station (ISS). Three Dedicated Thruster Firings (DTF) tests were conducted during ISS Stage ULF4; this paper will focus on the analysis and results of the DTF S4-1A, which occurred on October 11, 2010. The objective of this analysis is to validate and correlate analytical models used to verify the ISS critical interface dynamic loads.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20110013394&quot; rel=&quot;noreferrer&quot;&gt;Another paper&lt;/a&gt; on structural verification in general:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This paper is intended to give the reader an overview of some of the key decisions made during the structural verification planning for the elements of the U.S. On-Orbit Segment (USOS) as well as to summarize the many structural tests and structural analyses that were performed on its major elements. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-17T08:45:58.673" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20201" PostTypeId="2" ParentId="4608" CreationDate="2017-02-17T09:32:30.880" Score="4" Body="&lt;p&gt;We've seen several failures of reupply missions. When a &lt;a href=&quot;http://spacenews.com/progress-failure-puts-burden-on-upcoming-resupply-missions/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Progress launch failed on 28 April 2015&lt;/a&gt;, this is what NASA said:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In a presentation to a NASA Advisory Council panel here April 8, NASA officials said food supplies on the ISS would reach a threshold called “reserve level” on July 24, and go to zero on Sept. 5. That assumed that the station received no more supplies beyond a SpaceX Dragon cargo mission launched to the station in April.&lt;/p&gt;&#xA;&lt;p&gt;The other major limiting consumable is a solid waste container known by the Russian acronym KTO. Without additional cargo missions beyond the Dragon flight, KTO supplies would reach the reserve level July 20 and be exhausted on Sept. 2. Other consumables, including water, would not reach reserve levels until later in the year or early 2016.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This was with &lt;a href=&quot;https://space.stackexchange.com/questions/20041/lowest-amount-of-people-on-the-iss-at-one-time&quot;&gt;6 crew on board&lt;/a&gt; for most of the time, reducing to 3 for about a month in June-July. Rounding up to 6 people for 4 months, should be enough for 2 people for 1 year.&lt;/p&gt;&#xA;&lt;p&gt;After &lt;a href=&quot;https://web.archive.org/web/20151213035515/http://www.spaceflight101.net/cygnus-orb-3-mission-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;another failure&lt;/a&gt; (Antares Orb-3, on Oct 28, 2014), NASA indicated the station had about 6 months of supplies left.&lt;/p&gt;&#xA;&lt;p&gt;So NASA has a pretty good idea of supply levels at all times, and has planned for the failure of resupply missions.&lt;/p&gt;&#xA;&lt;p&gt;Survival of the ISS in a doomsday scenario is rather a different situation, though. I'm having a hard time imagining a scenario that would render Earth uninhabitable for 1 year, and safe after that. I.e. if the entire Earth becomes uninhabitable, it'll stay that way for much longer.&lt;/p&gt;&#xA;&lt;p&gt;Neal Stephenson's novel Seveneves uses the ISS as a lifeboat in such a scenario. But in his story, there's enough advance warning to make the ISS livable for as long as it needs to be (not being more specific to avoid spoilers).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="11262" LastEditDate="2023-03-19T12:20:14.153" LastActivityDate="2023-03-19T12:20:14.153" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20202" PostTypeId="2" ParentId="20191" CreationDate="2017-02-17T14:04:18.327" Score="4" Body="&lt;p&gt;From experience, I assume that Space Track is in fact tracking them all. The difficulty lies in making positive identifications of the names of all of them, which will happen soon. As the separate further, this identification will be made easier, but I'm confident that in the end, all such objects will be listed, and all of them are being tracked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FYI, this is standard procedure. Falcon 9 launches with less than a dozen satellites typically take days for their TLEs to be released to the public. This is nothing unusual. I don't understand the process completely, but I believe that senior personnel at JSpOC have to review the identification before it is released to the public.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-02-17T14:11:11.453" LastActivityDate="2017-02-17T14:11:11.453" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20203" PostTypeId="2" ParentId="20195" CreationDate="2017-02-17T14:08:44.340" Score="3" Body="&lt;p&gt;1U cubesats are difficult to resolve, but they are in the minimum range of detectability. The difficulty lies not in the detection of said objects, but in the identification of them, as they all look pretty much alike. &lt;a href=&quot;https://www.space-track.org/documents/Recommendations_Optimal_Cubesat_Operations_V2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;JSpOC&lt;/a&gt; has a list of recommendations. &lt;a href=&quot;https://spaceflightnow.com/2015/07/30/nasa-tracking-cubesats-is-easy-but-many-stay-in-orbit-too-long/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; has also stated that tracking the cubesats is easy.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-02-17T14:08:44.340" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20204" PostTypeId="2" ParentId="20198" CreationDate="2017-02-17T17:08:39.423" Score="2" Body="&lt;p&gt;Partial answer on 2ndary question on cockpit instrumentation, like the programming manual for the onboard computers. See &lt;a href=&quot;https://www.ibiblio.org/apollo/links.html#Gemini_spacecraft_computer&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.ibiblio.org/apollo/links.html#Gemini_spacecraft_computer&lt;/a&gt; The site also has source code for computer simulator. (more comment level answer, but don't have the rep). &lt;/p&gt;&#xA;" OwnerUserId="18548" LastActivityDate="2017-02-17T17:08:39.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20205" PostTypeId="2" ParentId="20144" CreationDate="2017-02-17T18:00:42.213" Score="4" Body="&lt;p&gt;Above all else, that design choice is purely for the sake of simplicity.  It uses the existing Common Berthing Mechanism attach hardware and seals to interface with ISS, which eliminates the need to design a custom system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the linked SpaceFlightInsider article: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once attached to Tranquility, the airlock would be pressurized to&#xA;  allow the hatch to be opened. The inside could then be configured by&#xA;  the crew for a variety of tasks. Once ready for deployment, the hatch&#xA;  would be closed and the airlock depressurized.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The robotic Canadarm2 would then grab the airlock and move it to a&#xA;  deployment angle away from the outpost. After satellite deployment,&#xA;  the arm would then return the airlock to its port on Tranquility.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-02-17T18:00:42.213" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20206" PostTypeId="2" ParentId="20144" CreationDate="2017-02-18T07:46:55.957" Score="5" Body="&lt;p&gt;This design minimalizes R&amp;amp;D work and number of active components. At cost of more complex procedure, the module is made very simple. There's A CBM port, a dome, and valves for venting and repressurization, a port for canadarm attachment, and the &quot;nanoracks&quot;, cubesat launchers. That's it - listing in order of device complexity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Airlock hatch is a fairly complex and expensive device, if not as complex as Common Berthing Mechanism port. But since the CBM was obligatory anyway, the designers decided to cut cost and complexity of the rest, doing away with an extra hatch and &quot;abusing&quot; the two sides of CBM as airlock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that the robotic arm can point the launchers in any direction giving a minimal initial orbit adjustment is a minor bonus, but the primary benefit is absolute minimization of the cost of the module itself.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-02-18T07:46:55.957" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20207" PostTypeId="1" AcceptedAnswerId="36345" CreationDate="2017-02-18T14:41:43.717" Score="6" ViewCount="761" Body="&lt;p&gt;From what I understand, Venus's upper atmosphere is much more habitable than its surface, with a temperature and pressure that humans could tolerate.  However, lack of oxygen and the presence of sulphuric acid would likely kill most earth life.  I would think plants could thrive on the abundant CO2 though if the acid did not kill them.  I am curious if it is likely that purely airborne life exists in this zone of Venus's upper atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="3099" LastEditorUserId="19335" LastEditDate="2018-10-05T19:33:17.860" LastActivityDate="2020-09-18T21:44:06.123" Title="Is there likely to be life in Venus's upper atmosphere?" Tags="|atmosphere|venus|life|habitat|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="20208" PostTypeId="1" AcceptedAnswerId="20215" CreationDate="2017-02-19T01:02:20.680" Score="8" ViewCount="461" Body="&lt;p&gt;I didn't realize that either end of the robotic arm could act as both a &quot;shoulder&quot; and a &quot;wrist&quot;, and that it could plug it's &quot;wrist&quot; into a second power-data-grapple fixture and then unplug it's &quot;shoulder&quot;, effectively walking from one fixture to antoher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How may fixtures are there, and how many different walk-off maneuvers can the Canadarm-2 do? For example, if there were three fixtures A, B, and C, the answer could be as many as six (A-&gt;B, B-&gt;A, A-&gt;C, C-&gt;A, C-&gt;B, B-&gt;C) but it could be less if A and C are too far from each other to go directly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there other animations of the arms's various walk-off maneuvers that I could look at &lt;a href=&quot;https://www.youtube.com/watch?v=zbwV0fs5-xU&quot; rel=&quot;noreferrer&quot;&gt;besides the one in this cool video&lt;/a&gt;?  (Walk-off after 01:00, but it's a cool video so watch the whole thing!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=zbwV0fs5-xU&quot; rel=&quot;noreferrer&quot;&gt;Video&lt;/a&gt;'s caption:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Uploaded on Aug 24, 2010&#xA;  This NASA video features astronaut Dottie Metcalf-Lindenburger giving an overview of the International Space Station's Robotic Arm. I received the raw video of Dottie on camera (recorded on ISS during STS-131) and was asked by the NASA client to polish it up. I produced and edited the resulting product.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-19T01:11:00.057" LastActivityDate="2021-11-12T17:20:00.097" Title="How many different walk-off maneuvers can the ISS robotic arm do?" Tags="|iss|robotics|robotic-arm|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20209" PostTypeId="1" AcceptedAnswerId="20212" CreationDate="2017-02-19T14:56:45.317" Score="7" ViewCount="752" Body="&lt;p&gt;During the SpaceX launch of the CRS-10, at T+3:12, there is a circular grey object that passes through the frame.  What is this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q2Aun.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q2Aun.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/rUDLxFUMC9c?t=1065&quot; rel=&quot;noreferrer&quot;&gt;Launch Webcast&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12725" LastEditorUserId="12725" LastEditDate="2017-02-19T15:06:55.470" LastActivityDate="2017-02-19T21:09:23.190" Title="What was the object seen from the stage 2 camera of the CRS-10 launch at T+3:12" Tags="|spacex|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20211" PostTypeId="1" CreationDate="2017-02-19T15:15:20.610" Score="14" ViewCount="1365" Body="&lt;p&gt;When the Apollo spacecraft was en route to the Moon, was it an escape trajectory from the Earth-Moon system? If not for the burn(s) to enter lunar orbit, would it have continued in an independent solar orbit or was it at all times gravitationally bound to the Earth-Moon system?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I ask because of &lt;a href=&quot;https://en.wikipedia.org/wiki/J002E3&quot; rel=&quot;noreferrer&quot;&gt;J002E3&lt;/a&gt;, originally thought to be a near-Earth asteroid, but now believed to be the S-IVB from Apollo 12. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of Apollo 12, it would seem that the spent stage had to be boosted out of the Earth-Moon system, implying that up to the point of the reconfiguration maneuver, the spacecraft was still gravitationally bound to the Earth-Moon system. Was this true for all of the Apollo missions? Was a final burn of the S-IVB post-CMS separation for disposal a part of all flights?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-02-19T23:50:11.553" LastActivityDate="2021-05-26T02:56:33.357" Title="Was the Apollo spacecraft always gravitationally bound to the Earth-Moon system?" Tags="|orbital-mechanics|apollo-program|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20212" PostTypeId="2" ParentId="20209" CreationDate="2017-02-19T15:33:54.423" Score="9" Body="&lt;p&gt;It looks like the Dragon v1 nose cone which protects the CBM port of the spacecraft similarly to payload fairings on satellite launches.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/HHs2a.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HHs2a.jpg&quot; alt=&quot;Dragon with nose cone installed&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;&lt;a href=&quot;http://www.spaceref.com/news/viewnews.html?id=1447&quot; rel=&quot;nofollow noreferrer&quot;&gt;Picture from article by Ken Kremer&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two other covers (which are actually called fairings) on the Dragon spacecraft during launch that protect the solar arrays, but these are not circular and they are only released after orbital insertion right before the arrays deploy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cone is being released at the altitude of ~114 km which is very similar to where &quot;normal&quot; payload fairings are released on other launches (at that altitude the atmospheric drag does not posses danger to the structures anymore). &lt;em&gt;Thanks &lt;a href=&quot;https://space.stackexchange.com/users/12102/uhoh&quot;&gt;@uhoh&lt;/a&gt; for this part&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nPnVm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nPnVm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Credit SpaceX webcast&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nose cone will be a bit different for the Dragon v2 where they designed it to open and close on hinges, so it will not be thrown away but will be brought back to Earth instead.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-02-19T21:09:23.190" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20213" PostTypeId="1" CreationDate="2017-02-19T15:55:26.570" Score="1" ViewCount="633" Body="&lt;p&gt;How does the first stage of the Falcon 9 rocket launch and land at Cape Canaveral, Florida?  I was under the impression that its trajectory sends it about 60 miles off the coast. It makes sense to land on a platform at sea.  It would have to backtrack a long distance to land near the launch location.  Am I missing something or not understanding the SpaceX launch/landing procedure?&lt;/p&gt;&#xA;" OwnerUserId="18554" LastEditorUserId="22581" LastEditDate="2018-01-20T09:00:25.427" LastActivityDate="2018-01-20T09:00:25.427" Title="How does SpaceX launch and land at the same location?" Tags="|spacex|" AnswerCount="1" CommentCount="1" ClosedDate="2017-02-20T11:04:29.090" ContentLicense="CC BY-SA 3.0" />
  <row Id="20214" PostTypeId="2" ParentId="20213" CreationDate="2017-02-19T17:07:28.467" Score="5" Body="&lt;p&gt;Your understanding is correct. The first stage has to come to a halt, then fly back to the Cape.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is done for flights where there's enough margin (fuel left over after stage separation) to fly back.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there's less fuel left, they can land on a barge in the ocean. This takes less fuel than flying back.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX prefers to land the stage on land (recovery is easier, no ships needed), and on many missions they have enough margin to do that. Basically, they made the rocket larger than it needs to be (if you just look at launching the payload) to accommodate the propellant for landing. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-19T17:07:28.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20215" PostTypeId="2" ParentId="20208" CreationDate="2017-02-19T18:32:01.957" Score="9" Body="&lt;p&gt;I'll have to let you count them because there are some ambiguities that arise from the use of the Mobile Transporter (MT)...but here are the possible walk-offs.&lt;/p&gt;&#xA;&lt;p&gt;These are the fixed base-capable Power and Data Grapple Fixtures (PDGFs) on the ISS&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;US Laboratory Nadir-Port Side&lt;/li&gt;&#xA;&lt;li&gt;Node 2 Nadir Side&lt;/li&gt;&#xA;&lt;li&gt;FGB Forward-Port Side&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;This picture shows the Space Station Remote Manipulator System (SSRMS) aka the &amp;quot;big arm&amp;quot; based on the Lab (this is an old ISS config).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iifOO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iifOO.jpg&quot; alt=&quot;photograph of SSRMS based on the Lab with a shuttle orbiter in the background&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This picture shows the SSRMS  based on Node 2.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qQ62r.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qQ62r.jpg&quot; alt=&quot;photograph of the central truss part of the ISS showing the SSRMS based on Node 2&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This is a picture of the PDGF on the FGB.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r3XX4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r3XX4.jpg&quot; alt=&quot;photograph of the PDGF that is mounted on the FGB&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;(note there are many other PDGFs on the ISS that were used for assembly, but they are not base-capable)&lt;/p&gt;&#xA;&lt;p&gt;And the MT has 4 base-capable PDGFs on it.  The MT can move to and site itself at any of eight worksites along the truss.  This diagram shows the worksites.  9 and 10 are not used - they are outboard of the alpha gimbals and it's too much trouble to use them.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MNQNT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MNQNT.jpg&quot; alt=&quot;schematic drawing showing the worksites along the truss where the MT can be parked and their numbers&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This picture shows the MT with its four PDGFs. (in the real world it's covered with white multi-layer insulation).  Technically the top part with the PDGFs is called the MBS (Mobile Based Servicer) but I just call the whole thing the MT.  Saves a letter.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vt9Mf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vt9Mf.jpg&quot; alt=&quot;schematic drawing of the MT with the MBS mounted on it&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here is a picture of the SSRMS based on the MT.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9hRYH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9hRYH.jpg&quot; alt=&quot;photograph showing the MT with the SSRMS based on it&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;So, among these PDGFs, the allowable walkoffs are&lt;/p&gt;&#xA;&lt;p&gt;Walkoffs to/from Lab PDGF&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Lab &amp;lt;-&amp;gt; Worksite 3, MT PDGF 1, 3, 4&lt;/li&gt;&#xA;&lt;li&gt;Lab &amp;lt;-&amp;gt; Worksite 4, Any MT PDGF&lt;/li&gt;&#xA;&lt;li&gt;Lab &amp;lt;-&amp;gt; Worksite 5, Any MT PDGF&lt;/li&gt;&#xA;&lt;li&gt;Lab &amp;lt;-&amp;gt; Worksite 6, Any MT PDGF&lt;/li&gt;&#xA;&lt;li&gt;Lab &amp;lt;-&amp;gt; Worksite 7, MT PDGF 1, 2&lt;/li&gt;&#xA;&lt;li&gt;Lab &amp;lt;-&amp;gt; Node 2 PDGF&lt;/li&gt;&#xA;&lt;li&gt;Lab &amp;lt;-&amp;gt; FGB PDGF&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Walkoffs to/from Node 2 PDGF&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Node 2 &amp;lt;-&amp;gt; Worksite 3, Any MT PDGF&lt;/li&gt;&#xA;&lt;li&gt;Node 2 &amp;lt;-&amp;gt; Worksite 4, Any MT PDGF&lt;/li&gt;&#xA;&lt;li&gt;Node 2 &amp;lt;-&amp;gt; Worksite 5, Any MT PDGF&lt;/li&gt;&#xA;&lt;li&gt;Node 2 &amp;lt;-&amp;gt; Worksite 6, Any MT PDGF&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The FGB can only walk to/from the Lab, as previously stated.  It cannot reach any PDGFs on the MT at any site.&lt;/p&gt;&#xA;&lt;p&gt;Additionally, the SSRMS can walkoff from any MT PDGF to any other.  So, do you count this times 8 because it can happen at any worksite?  I'll let you decide.&lt;/p&gt;&#xA;&lt;p&gt;Source: sadly, personal notes. All photos and schematics credit NASA.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-11-12T17:20:00.097" LastActivityDate="2021-11-12T17:20:00.097" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20216" PostTypeId="1" AcceptedAnswerId="20217" CreationDate="2017-02-19T18:34:12.120" Score="0" ViewCount="412" Body="&lt;p&gt;This picture was shown the CRS-10 launch webcast:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DxwxZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DxwxZ.jpg&quot; alt=&quot;LC-39A during SpaceX CRS-10 countdown&quot;&gt;&lt;/a&gt;&#xA;(red frame added by me)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the marked tunnel-like opening with the white vertical bars? And what is/was its purpose?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same structure can also seen in &lt;a href=&quot;https://www.google.com/maps/@28.6071764,-80.6002037,3a,30y,289.02h,92.87t/data=!3m7!1e1!3m5!1s55FvUSGk78sJ0h1_5MpNkg!2e0!6s%2F%2Fgeo1.ggpht.com%2Fcbk%3Fpanoid%3D55FvUSGk78sJ0h1_5MpNkg%26output%3Dthumbnail%26cb_client%3Dmaps_sv.tactile.gps%26thumb%3D2%26w%3D203%26h%3D100%26yaw%3D277.31677%26pitch%3D0%26thumbfov%3D100!7i13312!8i6656&quot; rel=&quot;nofollow noreferrer&quot;&gt;Google StreetView&lt;/a&gt;:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/nyHZP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nyHZP.png&quot; alt=&quot;LC-39A in 2012&quot;&gt;&lt;/a&gt;&#xA;This is dated Jan 2012, so this is apparently something from the Apollo or Shuttle era.&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2017-02-19T21:14:47.593" Title="What is this tunnel with white vertical structures on LC-39A?" Tags="|spacex|launchpad|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20217" PostTypeId="2" ParentId="20216" CreationDate="2017-02-19T18:46:40.250" Score="3" Body="&lt;p&gt;That is the &quot;high pressure gas storage facility&quot;. Mostly used for N2.  And yes, it dates from the Apollo era.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://apollolaunchcontrol.com/v20test/http___www.apollolaunchcontrol.com_/Launch_Pads.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reference&lt;/a&gt; (search the linked page for the word &quot;exploded diagram&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=14430.0&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reference&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-02-19T18:46:40.250" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20219" PostTypeId="1" CreationDate="2017-02-19T20:23:21.597" Score="5" ViewCount="617" Body="&lt;p&gt;When I see the SpaceX reentry videos I see no flaming ionization&#xA;from the time the reentry starts. I realize things must be getting &#xA;hot at some point but its not the ball of fire I imagined .&#xA;Where should I be seeing this or its probably not as I imagined.&#xA;Anybody got any observations.&lt;/p&gt;&#xA;" OwnerUserId="18558" LastEditorUserId="25" LastEditDate="2017-12-19T13:13:46.187" LastActivityDate="2017-12-19T13:13:46.187" Title="When I see the SpaceX reentry vidos I see no flaming ionizaton" Tags="|spacex|falcon-9|reentry|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20220" PostTypeId="2" ParentId="20216" CreationDate="2017-02-19T21:14:47.593" Score="0" Body="&lt;p&gt;Those are pressurized gas storage tanks. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-02-19T21:14:47.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20221" PostTypeId="2" ParentId="20219" CreationDate="2017-02-19T21:31:19.707" Score="3" Body="&lt;p&gt;They do three burns. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Boostback burn to cancel forward velocity (for ASDS landings) or to head back to the Cape to land at LZ-1/LC-1.&lt;/li&gt;&#xA;&lt;li&gt;Re-entry burn to control the speed and interface with the atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Landing burn.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Boostback burn is too high usually to have much atmospheric effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Re-Entry burn is the one you would likely see it, but it seems they control the speed sufficient to avoid it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-02-19T21:31:19.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20222" PostTypeId="2" ParentId="20219" CreationDate="2017-02-19T22:38:56.803" Score="9" Body="&lt;p&gt;Assuming you are referring to videos that end with a landing, you are only seeing the first stage. Even without the boostback burn it is relatively slow - less than 2 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second stage gets up to actual orbital speeds, around 7 km/s, and that does turn into a fireball on reentry.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-02-19T22:38:56.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20223" PostTypeId="1" CreationDate="2017-02-19T23:57:12.067" Score="18" ViewCount="2494" Body="&lt;p&gt;Stages of the Apollo stack (at least the configuration that landed humans on the Moon) are identified as S-IC, S-II, and S-IVB for the first, second, and third stages. There's no S-III. Why the gap in numbering? Was Saturn going to  be a four-stage rocket at some point in its development?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2018-10-04T21:08:17.473" Title="S-I, S-II, S-IV; what happened to III?" Tags="|apollo-program|saturn-v|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20224" PostTypeId="2" ParentId="20211" CreationDate="2017-02-19T23:58:03.107" Score="1" Body="&lt;p&gt;Yes.  The Apollo stack was always on a free return trajectory between burns.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2017-02-20T01:39:16.297" LastActivityDate="2017-02-20T01:39:16.297" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20225" PostTypeId="2" ParentId="20219" CreationDate="2017-02-19T23:59:16.483" Score="9" Body="&lt;p&gt;Quentin Clarkson's answer is correct; the first stage doesn't get fast enough to create a dramatic reentry fireball. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you look at &lt;a href=&quot;https://www.youtube.com/watch?v=4jEz03Z8azc&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; -- sped-up footage from the top of the first stage, looking down -- you can see at around 0:15-0:20 that the grid fins are getting charred from the heat of reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the real-time feed in &lt;a href=&quot;https://www.youtube.com/watch?v=_uztxi4fE94&amp;amp;t=400s&quot; rel=&quot;nofollow noreferrer&quot;&gt;a different video&lt;/a&gt;, the first reentry burn occurs at 7:00 to 7:19 or so. After the reentry burn shutdown call the fins are a light gray with soot. At 7:26 there's a glint on the fin at the left of the frame -- possibly burning paint. Around 7:30 the frame is starting to gray out, with black flecks hitting the camera, while the engines are not firing. 7:35 is completely grayed out. We're still far above clouds, and around 7:40 the realtime downlink is lost, I assume because of vibration. It's another full minute before the first stage landing burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparing that sequence of events to the (uninterrupted) high-speed video you can see the burn from 0:13-0:14, light gray grid fins, then the gray-out from 0:15-0:16, and blackened grid fins at 0:18 shortly after the live feed dropped out. Something blackened them and it wasn't a main engine burn!&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-02-22T05:52:17.977" LastActivityDate="2017-02-22T05:52:17.977" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20227" PostTypeId="2" ParentId="20223" CreationDate="2017-02-20T00:24:14.097" Score="10" Body="&lt;p&gt;The S-III stage did exist, but only on paper. It was proposed for some Saturn-C configurations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was the original &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_C-2&quot; rel=&quot;noreferrer&quot;&gt;Saturn C-2&lt;/a&gt; variant with the stages S-I, S-III, S-IV and even a S-V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_C-3&quot; rel=&quot;noreferrer&quot;&gt;Saturn C-3&lt;/a&gt; would have been a combination of S-I, S-II and S-III, as well with S-IV and S-V on top, which would have made it a five-stage rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in further development the S-III and the S-V have been dropped, leaving only S-I, S-II and S-IV.&lt;/p&gt;&#xA;" OwnerUserId="10335" LastActivityDate="2017-02-20T00:24:14.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20228" PostTypeId="2" ParentId="20223" CreationDate="2017-02-20T00:26:23.380" Score="18" Body="&lt;p&gt;The planned scope of the Saturn launcher program varied quite a bit before the launchers were finally developed, as &lt;a href=&quot;http://www.spacelaunchreport.com/satstg5.html&quot; rel=&quot;noreferrer&quot;&gt;you can read here&lt;/a&gt;. The basic idea was to progressively develop a larger and larger rocket, re-using upper stage designs while enlarging the lower stages. The exact parameters of the design were altered repeatedly during early development, so the overall picture is complicated and confusing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Five basic stages would be developed, S-I to S-V, with S-V being a modification of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_(rocket_stage)&quot; rel=&quot;noreferrer&quot;&gt;Centaur&lt;/a&gt; stage, using 2 RL10 engines. These stages would be used in different 3- and 4-stage configurations. The &lt;a href=&quot;https://en.wikipedia.org/wiki/S-III&quot; rel=&quot;noreferrer&quot;&gt;S-III stage&lt;/a&gt; would have used 2 J-2 engines and S-IV would have 4 RL10s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea was that the line would begin with a three-stager using a modestly sized S-I stage and the S-IV and S-V stages (eventually this became the two-stage &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_I&quot; rel=&quot;noreferrer&quot;&gt;Saturn I&lt;/a&gt; with a 6-engine &lt;a href=&quot;https://en.wikipedia.org/wiki/S-IV&quot; rel=&quot;noreferrer&quot;&gt;S-IV&lt;/a&gt; stage) and the first stage would be progressively upgraded with more of the intermediate stages used. (S-V never happened; Wernher von Braun was never a fan of the Centaur, possibly because it was primarily developed for the Air Force, while von Braun's fortunes were tied to the Army until his transfer to NASA in 1960.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the Kennedy challenge to land on the moon before the end of the 1960s accelerated the development of the program, and development of the S-III was skipped because the larger &lt;a href=&quot;https://en.wikipedia.org/wiki/S-II&quot; rel=&quot;noreferrer&quot;&gt;S-II&lt;/a&gt; stage was going to be needed sooner. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-06T06:09:11.730" LastActivityDate="2017-05-06T06:09:11.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20229" PostTypeId="1" CreationDate="2017-02-20T02:40:02.983" Score="8" ViewCount="427" Body="&lt;p&gt;Around 2040 EST We viewed an irregularly flashing object flying from WSW to NNE (roughly) from Jacksonville FL. Is the SpaceX 2nd stage still up there and if so, would it be tumbling and visible? Is there somewhere I can go to see where there are known tumbling satellites?&lt;/p&gt;&#xA;" OwnerUserId="18566" LastActivityDate="2021-04-20T07:59:07.090" Title="SpaceX 2nd stage or tumbling satellite? WSW to NE over FL" Tags="|orbital-mechanics|spacex|artificial-satellite|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20230" PostTypeId="2" ParentId="20211" CreationDate="2017-02-20T03:16:56.287" Score="17" Body="&lt;p&gt;Hmm. This question is trickier than it sounds. To know whether a body in a three-body system is always gravitationally &quot;bound&quot;, you have to look into the future. For example, a close-enough flyby of the Moon on the trailing side would put the spacecraft on an Earth-Moon escape trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo 11 was on a free return trajectory for which the lunar flyby (if it didn't go into orbit) would send it back to the Earth. So it was gravitationally bound, right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, what if a spacecraft was on a very similar trajectory that just misses the Earth on the return. (I'm not going to use Apollo 11 in my hypothetical, since I don't want to kill the Apollo 11 astronauts, even hypothetically.) It would continue to orbit Earth in a lunar-crossing orbit, eventually having a close encounter with the Moon again. Then the Moon might fling the spacecraft on an escape trajectory. So was it gravitationally bound?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exactly this happened to the Apollo 12 third stage you mentioned, which was orbiting the Earth for quite some time. It is believed to have left Earth orbit in 2003. Since it escaped three decades later, it must not have been gravitationally bound that whole time. But what if while in solar orbit it encounters the Earth-Moon system some time later, and the Moon does the opposite thing and puts it back in Earth orbit? (That third stage might do just that in the mid-2040's.) Now was it gravitationally bound the whole time? If now you think it is, might it not eventually escape again?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If its wanderings are permanently ended by impacting the Earth or the Moon, then it is now gravitationally bound in pieces on that body. So therefore it was always gravitationally bound, even when it was in solar orbit. Right?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To truly answer your question, you would need to propagate the trajectory from every state between maneuvers, potentially for a very long time, to determine its ultimate fate. There will often not be enough accuracy in the known state, as well as uncertainty in solar pressure perturbations, for that to even be deterministic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for that Apollo 12 third stage, they deliberately tried to have it immediately escape the Earth-Moon system, but failed. Later Apollo third stages were targeted to impact the Moon, which made for nice seismic signals.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-02-20T06:07:41.643" LastActivityDate="2017-02-20T06:07:41.643" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20231" PostTypeId="1" CreationDate="2017-02-20T06:17:34.983" Score="7" ViewCount="1058" Body="&lt;p&gt;Is it possible for conventional x86 based processor/High precision GPU to be used for image processing for space borne &lt;a href=&quot;https://en.wikipedia.org/wiki/Synthetic_aperture_radar&quot; rel=&quot;nofollow noreferrer&quot;&gt;Synthetic Aperture Radar&lt;/a&gt; (SAR) system? If not, are there alternatives using radiation hardened chips?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My primary concern is that the space grade RAD chips aren't powerful enough to process the heavy data output. Can a conventional processor be radiation hardened to be used for space applications?   &lt;/p&gt;&#xA;" OwnerUserId="18567" LastEditorUserId="12102" LastEditDate="2020-05-05T22:35:43.617" LastActivityDate="2020-05-05T22:35:43.617" Title="SAR image processing" Tags="|spacecraft|computing|synthetic-aperture-radar|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20232" PostTypeId="1" AcceptedAnswerId="20238" CreationDate="2017-02-20T07:19:28.660" Score="7" ViewCount="503" Body="&lt;p&gt;Since the 1960s, NASA used both Launchpad 39A and 39B.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They seem fairly equal pads, but 39A is more &quot;famous&quot; for launching almost all Apollo spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet when NASA had a choice which one to lease out, they chose to keep 39B instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are the reasons why documented publicly?&lt;/p&gt;&#xA;" OwnerUserId="18569" LastEditorUserId="10335" LastEditDate="2017-02-20T12:59:55.220" LastActivityDate="2017-02-20T12:59:55.220" Title="Why did NASA take 39B vs 39A?" Tags="|launch-site|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20233" PostTypeId="2" ParentId="20231" CreationDate="2017-02-20T08:49:23.320" Score="11" Body="&lt;p&gt;SAR (Synthetic Aperture RADAR) data is streamed to Earth-based stations for processing. x86 processors can and are used for deconvolving that mass of data. The processing load is high, but a modern desktop and GPU have the processing power of a supercomputer from 20 years ago. And it is easy on Earth to get rid of a couple hundred watts of heat in the electronics, which is a lot harder in space. Collecting and sending the data is much easier, uses fewer parts and less power, and increases reliability and lifetime of a satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An Intel Xeon has a billion transistors.  In rad-hard feature sizes, it would be a physically huge chip.&lt;/p&gt;&#xA;" OwnerUserId="1432" LastActivityDate="2017-02-20T08:49:23.320" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20234" PostTypeId="1" CreationDate="2017-02-20T09:39:48.757" Score="5" ViewCount="1228" Body="&lt;p&gt;Given a cylindrically-shaped space station rotating at 1-2 rpm (Stanford Torus, as an example) what are the engineering considerations regarding docking? The central hub would be the logical docking location, but that would be rotating as well. Is there a way to have the actual docking location not rotate? The problem then would be how to transfer to the rotating spokes for transit out to the 1g habitat section of the station. A related question - if you needed a zero-g environment for space manufacturing (crystals, etc.) how would that be connected to the rotating station? I suppose the manufacturing facility could be built out from the central hub on the side opposite from the docking location. That still returns to the question of how to transfer from a non-rotating location to the rotational portions of the station.&lt;/p&gt;&#xA;" OwnerUserId="18473" LastActivityDate="2018-01-23T14:55:29.390" Title="docking options for a rotating space station" Tags="|design|space-station|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20235" PostTypeId="2" ParentId="20234" CreationDate="2017-02-20T10:08:08.623" Score="5" Body="&lt;p&gt;1-2 RPM is not much - a craft rotating at such a speed would not be very hard to control, so the whole problem could be deferred to docking software/procedure, only requiring the docking port be aligned with the craft's center of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that's not an option, say, the docking craft's rotation axis can't go through the docking port, a smart approach would have a two-phase docking mechanism: a rotating mechanical docking system placed on extending platform or robotic arm, with active motor that counter-rotates the station's port relative to the station so that it's rotation relative to the craft is zero. After docking to it, creating a firm mechanical connection between the craft and the station, the port's motors are de-spun, the craft enters the same rotation as the station (around the common center of mass, not necessarily in line with either of the ports), but is firmly attached to the port, and then it's guided using the port's mechanics to the &quot;second stage docking&quot;, actual classic docking port allowing crew and cargo transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spinning airlocks are currently a stuff of sci-fi novels. If you need a station with non-spinning part and a spinning part, you build two stations.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-02-20T10:08:08.623" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20236" PostTypeId="2" ParentId="20231" CreationDate="2017-02-20T11:26:53.433" Score="2" Body="&lt;p&gt;C. Towne Springer is correct; you can do a LOT more processing on the ground, and most systems do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, SAR processing is a multi-step endeavor, and it would certainly be possible to do some preprocessing onboard before download. The only savings, though, would be processing that results in a smaller, compressed data payload, since downloading is going to be far and away your greatest bottleneck.&lt;/p&gt;&#xA;" OwnerUserId="18572" LastActivityDate="2017-02-20T11:26:53.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20237" PostTypeId="2" ParentId="20231" CreationDate="2017-02-20T11:29:21.190" Score="6" Body="&lt;p&gt;If for some reason you really wanted to do process &lt;a href=&quot;https://en.wikipedia.org/wiki/Synthetic_aperture_radar&quot; rel=&quot;nofollow noreferrer&quot;&gt;Synthetic Aperture Radar&lt;/a&gt; (SAR) data on board a spacecraft, using commercial chips, you could do it. But it would be subject to several constraints and compromises.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Power dissipation:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Supplying kilowatt-ish electrical power would require large, expensive solar panels to keep pointed at the Sun, and a sophisticated cooling system to pull that same kilowatt out of the electronics as heat and then radiate it back into space.  This could be a lot easier if you were operating at low duty cycle - if you could accept a lower duty cycle of measurements or only processing a subset of data to the highest spatial resolution.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Radiation hardness:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;For non-mission-critical applications like this, you would not need the same level of rad-hardness as you would for spacecraft control. Since the calculation is so parallelizable, checkpointing and redundancy could make space-based computation viable with standard-issue Earthling CPUs.&lt;/p&gt;&#xA;&lt;p&gt;(tl;dr if you make a mistake image processing, you can go back and do it again, but if you make a mistake and shut down an engine too soon, rebooting and trying again is a lot trickier.)&lt;/p&gt;&#xA;&lt;p&gt;The mathematical process of analyzing the data is usually parallelizable, which means the problem can be broken into a large number of smaller problems, each solved separately, and then recombined at the end. A simple method to deal with radiation-induced numerical errors might be to simply solve every problem twice in two separate processors and when there are differences, throw them out and do it again. I'm sure there are more sophisticated ways as well.&lt;/p&gt;&#xA;&lt;p&gt;Checkpointing is a systematic saving of intermediate results. Each time a small problem is solved you save it. Each time enough small ones have accumulated to calculate a larger intermediate result, you do it, and then save that. This way you can recover quickly from any radiation-induced crashes basically pick up where you left off.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; I know your question is about x86 processors, the following diverges a little bit from those specifically, but it highlights the push to use more and more off-the-shelf computing power in fault-tolerant architecture to &lt;em&gt;partially replace&lt;/em&gt; super-reliable rad-hardened but slow and expensive processors.&lt;/p&gt;&#xA;&lt;p&gt;FPGA's are discussed within SXSE here: &lt;a href=&quot;https://space.stackexchange.com/q/9555/12102&quot;&gt;1&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/a/9561/12102&quot;&gt;2&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/a/8173/12102&quot;&gt;3&lt;/a&gt;, in comments here; &lt;a href=&quot;https://space.stackexchange.com/a/13934/12102&quot;&gt;4&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;STP-H5 and SpaceCubes:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;(more about SpaceCubes &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1169&amp;amp;context=smallsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://esto.nasa.gov/conferences/estf2011/presentations/Flatley_ESTF20111.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.)&lt;/p&gt;&#xA;&lt;p&gt;Part of the recent &lt;a href=&quot;http://spaceflight101.com/dragon-spx10/stp-h5/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CRS-10 mission delivery to the ISS is STP-H5&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;ISEM – SpaceCube, CHREC Processors&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Validation of new processing technology is one of the most critical areas currently faced in the field of space technology given the rapid advancement of space-based instruments that require more and more processing power from highly reliable processor units. The STP-H5 ISEM (ISS SpaceCube Experiment Mini) will provide validation for hybrid-processing technology to advance that technology’s mission readiness level for future application on small satellites with limited volume for computer equipment.&lt;/p&gt;&#xA;&lt;p&gt;In recent years, a shift has led from ‘radiation hardened systems’ to ‘radiation tolerant technology,’ that is technology &lt;strong&gt;which is susceptible to radiation-induced upsets but not radiation-induced destructive failures. Employing such systems can yield to a ten- to hundredfold increase in processing power at the cost of some science data due to occasional radiation related upsets that simply have to be tolerated&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;p&gt;[...]&lt;/p&gt;&#xA;&lt;p&gt;The v1.0 version has around ten times the processing power of the RAD750 OBC at half the power consumption; v2.0 has twenty times the processing power and requires 75% of RAD’s electrical power.&lt;/p&gt;&#xA;&lt;p&gt;SpaceCube Mini was designed to similar specifications as v2.0 but in a 1U CubeSat form factor – fitting into a 10 x 10 x 10-centimeter volume. &lt;strong&gt;Its core is a multifaceted, hybrid space computer known as CSP, CHREC Space Processor.&lt;/strong&gt; (emphasis added)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;CSP combines fixed and reconfigurable CPU and FPGA processing functions using an algorithm that optimizes the use of the specialized resources of the device&lt;/strong&gt;. A combination of commercial-off-the-shelf and radiation hardened systems creates a processing system with significant processor power and reliability in challenging space environments. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tjHar.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tjHar.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt;  from &lt;a href=&quot;https://en.wikipedia.org/wiki/Synthetic_aperture_radar&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, also described by &lt;a href=&quot;http://www.spacemicro.com/assets/datasheets/digital/slices/CSP.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacemicro&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Field-programmable_gate_array&quot; rel=&quot;nofollow noreferrer&quot;&gt;Field Programmable Gate Arrays&lt;/a&gt; or FPGAs are finding their way into a wide variety of number crunching and signal processing applications. They can be competitive when the tasks have more diversity that would require very large or complex CPU architecture to provide high speed capacity, and especially if there may be a need to adjust or re-optimize signal processing architecture once the unit is in the field (or in orbit in this case).&lt;/p&gt;&#xA;&lt;p&gt;A good example of that (use of FPGA for signal processing, &lt;em&gt;not necessarily spaceflight&lt;/em&gt;) might be the FPGA implemented GPS radio &amp;quot;freely programmable front end and correlator with a custom firmware&amp;quot; mentioned by @Andreas in &lt;a href=&quot;https://electronics.stackexchange.com/a/259174/102305&quot;&gt;this excellent answer&lt;/a&gt;. The &lt;a href=&quot;https://www.swiftnav.com/piksi-multi&quot; rel=&quot;nofollow noreferrer&quot;&gt;Piksi&lt;/a&gt; uses a &lt;a href=&quot;https://www.xilinx.com/products/silicon-devices/soc/zynq-7000.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Xilinx Zynq-7000 All Programmable SoC (AP SoC)&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Zynq-7000 All Programmable SoC Product Advantages&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Zynq®-7000 All Programmable SoC (AP SoC) family integrates the software programmability of an ARM®-based processor with the hardware programmability of an FPGA, enabling key analytics and hardware acceleration while integrating CPU, DSP, ASSP, and mixed signal functionality on a single device. Consisting of single-core Zynq-7000S and dual-core Zynq-7000 devices, the Zynq-7000 family is the best price to performance-per-watt, fully scalable SoC platform for your unique application requirements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Radiation levels:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Radiation in LEO is not nearly as bad as it is in higher orbits or deep space, except of course if there is some occasional unusual solar activity. As long as your application can remain in LEO and is not critical during solar events, this should all work.&lt;/p&gt;&#xA;&lt;p&gt;Again, this is not necessarily competitive with doing the math on the surface, but for certain specific applications it may work.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-28T23:43:53.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20238" PostTypeId="2" ParentId="20232" CreationDate="2017-02-20T12:43:21.937" Score="12" Body="&lt;p&gt;After the end of the Space Shuttle era, NASA planned to convert both launchpads to support launches for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Constellation_program&quot; rel=&quot;noreferrer&quot;&gt;Constellation program&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last few Space Shuttle missions from 2009 until 2011 all launched from 39A. So NASA began rebuilding 39B for the test flight of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I-X&quot; rel=&quot;noreferrer&quot;&gt;Ares I-X&lt;/a&gt; in 2009 as part of the Constellation program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as it happened, the Constellation program got cancelled, and now both launchpads were unused.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since NASA already made upgrades to the 39B pad, they wanted to keep it for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;noreferrer&quot;&gt;Space Launch System&lt;/a&gt; and continued to rebuild it. Thus 39A wasn't needed anymore, so they decided to lease it to commercial users instead.&lt;/p&gt;&#xA;" OwnerUserId="10335" LastActivityDate="2017-02-20T12:43:21.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20239" PostTypeId="1" CreationDate="2017-02-20T16:21:21.497" Score="36" ViewCount="6875" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Climate_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;Mars Climate Orbiter&lt;/a&gt; failed in 1999 due to:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;ground-based computer software which produced output in non-SI units of pound (force)-seconds (lbf·s) instead of the SI units of newton-seconds (N·s) specified in the contract between NASA and Lockheed. The spacecraft encountered Mars on a trajectory that brought it too close to the planet, causing it to pass through the upper atmosphere and disintegrate.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I find this software error to be absolutely shocking.  How was it not found prior to the launch?  I would think some basic integration testing would have caught the error.  And were there no simulations that indicated the spacecraft would have flown on an incorrect path based on the faulty numbers reported by the software?&lt;/p&gt;&#xA;" OwnerUserId="10966" LastEditorUserId="26446" LastEditDate="2020-05-22T21:53:35.827" LastActivityDate="2020-05-22T21:53:35.827" Title="Why wasn't the Mars Climate Orbiter's fatal error caught prior to launch?" Tags="|spacecraft|software|mars-orbiters|unit-systems|" AnswerCount="4" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20240" PostTypeId="1" AcceptedAnswerId="20351" CreationDate="2017-02-20T16:23:43.220" Score="3" ViewCount="1198" Body="&lt;p&gt;In the last discussion on why ion thrusters are so energy hungry(&lt;a href=&quot;https://space.stackexchange.com/questions/20017/why-are-ion-thrusters-so-energy-hungry&quot;&gt;Why are Ion Thrusters so energy hungry?&lt;/a&gt;), people said that the system is not only supposed to ionize the gas but also heat it to sufficient temperatures needed to overcome the potential and enter the acceleration space. This perfectly makes sense.. But then I was wondering, If the heat is getting converted into the kinetic energy (in a particular direction, not random), then the temperature of the plasma is supposed to drop as soon as it gets converted into beam current and its kinetic energy.  Does this happen.. Does the temperature of the plasma drop after being accelerated? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xZo59.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xZo59.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18356" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-02-24T23:57:17.583" Title="In an ion thruster, does the temperature of the plasma drop after acceleration?" Tags="|energy|ion-thruster|temperature|electric-propulsion|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20241" PostTypeId="2" ParentId="20239" CreationDate="2017-02-20T17:34:18.593" Score="48" Body="&lt;p&gt;NASA formed a board to investigate the loss of the spacecraft and reached some &lt;a href=&quot;http://mars.nasa.gov/msp98/news/mco991110.html&quot; rel=&quot;noreferrer&quot;&gt;high level conclusions&lt;/a&gt;. The board cited a number of contributing factors, which I have filtered to include the ones most relevant to the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;errors went undetected within ground-based computer models of how small thruster firings on the spacecraft were predicted and then&#xA;  carried out on the spacecraft during its interplanetary trip to Mars&lt;/li&gt;&#xA;  &lt;li&gt;the systems engineering function within the project that is supposed to track and double-check all interconnected aspects of the mission&#xA;  was not robust enough, exacerbated by the first-time handover of a&#xA;  Mars-bound spacecraft from a group that constructed it and launched it&#xA;  to a new, multi-mission operations team&lt;/li&gt;&#xA;  &lt;li&gt;some communications channels among project engineering groups were too informal&lt;/li&gt;&#xA;  &lt;li&gt;the small mission navigation team was oversubscribed and its work did not receive peer review by independent experts&lt;/li&gt;&#xA;  &lt;li&gt;personnel were not trained sufficiently in areas such as the relationship between the operation of the mission and its detailed&#xA;  navigational characteristics, or the process of filing formal anomaly&#xA;  reports&lt;/li&gt;&#xA;  &lt;li&gt;the process to verify and validate certain engineering requirements and technical interfaces between some project groups, and between the&#xA;  project and its prime mission contractor, was inadequate&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also in the high level report is this quote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These contributing causes include inadequate consideration of the&#xA;  entire mission and its post-launch operation as a total system,&#xA;  inconsistent communications and training within the project, and lack&#xA;  of complete end-to-end verification of navigation software and related&#xA;  computer models.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It sounds like it was a failure of management and quality control at multiple levels. The &lt;a href=&quot;ftp://ftp.hq.nasa.gov/pub/pao/reports/1999/MCO_report.pdf&quot; rel=&quot;noreferrer&quot;&gt;entire report&lt;/a&gt; is also available if you'd like some light bedtime reading. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars Climate Orbiter was one of the probes in administrator Goldin's Faster Better Cheaper (FBC) initiative, which forced tight budgets and very short timelines on projects, which has been controversial ever since as there were several spacecraft failures attributed to failures of management and engineering due to the initiative. The &lt;a href=&quot;https://hbr.org/2004/05/management-lessons-from-mars&quot; rel=&quot;noreferrer&quot;&gt;Harvard Business Review&lt;/a&gt; has a great article summarizing what went wrong:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In shifting to FBC from a slow, reliable, but costly approach to&#xA;  development, NASA forced its project managers to invent radically new&#xA;  processes and procedures. FBC imposed on them budget, schedule, and&#xA;  weight constraints that could not be met using NASA’s traditional&#xA;  approaches to spacecraft development. “The attitude was ‘The book’s&#xA;  not working. So throw out the book, try something different, and then&#xA;  write a new book,’” one NASA manager explained. Implicit in this&#xA;  approach was the need for project managers to learn from the&#xA;  organization’s collective experiences, adopt what worked, and jettison&#xA;  what didn’t. Unfortunately, NASA undermined this learning process in&#xA;  several ways.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;First, with the launch of each FBC mission, NASA demanded ever faster&#xA;  development times and even lower costs. But because it typically takes&#xA;  more than four years for a small spacecraft to go from drawing board&#xA;  to completed mission, managers were forced to meet the tougher demands&#xA;  on new projects while earlier projects were still in progress. So they&#xA;  couldn’t capture all the potential lessons from one mission before&#xA;  moving to the next. In short, NASA was raising the bar before seeing&#xA;  if project managers could clear it where it was. By the time the&#xA;  organization realized it had set the bar too high—around the time the&#xA;  first FBC missions began to fail—the project pipeline was full of&#xA;  missions that were potentially compromised. It’s no surprise that&#xA;  later FBC missions failed more frequently than earlier ones did.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Second, NASA didn’t realize that because the FBC initiative depended&#xA;  so much on shared learning, it would require a more aggressive and&#xA;  systematic approach to knowledge management. Although NASA had&#xA;  implemented a “lessons learned” database in 1995, a 2001 survey found&#xA;  that only one-quarter of its managers contributed to it. A similar&#xA;  number of managers were unaware the system even existed. Furthermore,&#xA;  while “red team reviews”—periodic progress reviews conducted by NASA’s&#xA;  most experienced managers—proved invaluable in early FBC projects,&#xA;  NASA conducted fewer of these assessments in later missions. As a&#xA;  consequence, the transfer of learning across the organization&#xA;  suffered.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Finally, NASA fell prey to “superstitious learning”—the assumption&#xA;  that there is more to be gleaned from failed missions than from&#xA;  successful ones. In the challenging climate of space exploration,&#xA;  however, the difference between what makes one mission succeed and&#xA;  another fail can be subtle. There is no reason to believe that success&#xA;  indicates a flawless process while failure is the result of egregious&#xA;  bad practice. For example, as many mistakes could have been made in&#xA;  the celebrated 1997 Pathfinder mission as were made in the failed 1999&#xA;  Polar Lander mission. But NASA will never know. By not conducting&#xA;  detailed postmortems on its successful missions, the space agency&#xA;  missed the opportunity to identify problems (and solutions) that might&#xA;  have helped avoid later failures.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A summary of this would be that, while there was nothing bad about trying to speed up the pace of missions and cut costs the way it was implemented forced people to cut corners. The strategy depended on shared learning, with newer projects re-using older project's code, equipment, knowledge and lessons learnt but the agency did not put adequate tools in place to do this nor did it foster a sharing culture. Lessons from earlier projects weren't learned because the earlier projects weren't completed before the later projects were started. They didn't review successes as top management didn't think there was anything to be learned. There were several spacecraft failures which are attributed to this strategy backfiring, leading to the phrase &quot;Faster Better Cheaper - you can have any 2 you like&quot;, however there were many notable successes from the strategy including Mars Pathfinder and the NEAR asteroid rendezvous and in the end all of the 10 FBC spacecraft cost about the same as Cassini. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2017-02-21T09:24:43.583" LastActivityDate="2017-02-21T09:24:43.583" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20242" PostTypeId="2" ParentId="20240" CreationDate="2017-02-20T18:55:36.887" Score="2" Body="&lt;p&gt;This post is, strictly, a detailed comment, but one that requires more space than the comment box allows. Please be nice to me on this, its done in a positive spirit and it seems unavoidable as the subject line contains a key assumption that is probably clouding the issue.   (&lt;strong&gt;EDIT&lt;/strong&gt; - the question title has since been changed so this post is more of an orphan now). This same point came up in several answers to the first version of the question. Its not meant to be a complete answer, clearly the detailed questions on the plasma temperature are still of interest to the OP; I aim to be helpful rather than win up-votes.&lt;/p&gt;&#xA;&lt;p&gt;I want to examine the phrase &amp;quot;&lt;em&gt;&lt;strong&gt;so&lt;/strong&gt;&lt;/em&gt; energy hungry&amp;quot;.  My point is that ion thrusters have losses, so do all other forms of propulsion. In the following example the losses are about 35%. That is to say that from the start of the manoeuvre to the end only 35% of the DC electrical energy does not make it into useful beam kinetic energy.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Ion thruster example&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The XIPS thruster given &lt;a href=&quot;https://en.wikipedia.org/wiki/XIPS-25&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; has the following envelope characteristics in low power mode:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Total Input Power (W)    2067&lt;/p&gt;&#xA;&lt;p&gt;Thrust (mN)              79&lt;/p&gt;&#xA;&lt;p&gt;Specific Impulse (s)     3400&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Putting the latter two bits of data together with the &lt;a href=&quot;https://space.stackexchange.com/questions/14201/could-oxygen-be-used-in-a-resistojet-in-a-vacuum-environment/14203#14203&quot;&gt;beam power equation shown in this answer to a different question&lt;/a&gt; ( but the equations are valid for any type of thruster, chemical or electric ) gives a beam power of 1317 W. Thus the efficiency is 1317 W / 2067 W and losses in this case are 36%.&lt;/p&gt;&#xA;&lt;p&gt;Clearly there is ~700 W spent on support processes such as plasma generation. It would indeed be interesting to identify these more clearly though the aim of this post is to put that 36% in context.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Chemical thrusters&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Are chemical rocket motors substantially different? The context there is that the starting point is stored chemical energy and the losses are largely thermal losses trapped the exhaust and radiation from the nozzle.&#xA;This &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket#Energy_efficiency&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt; suggests that the ceiling is commonly 70%, i.e. &amp;gt;=30% losses, from a thermodynamics perspective.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Overall&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Thus the two thruster types may not be &lt;em&gt;&lt;strong&gt;so&lt;/strong&gt;&lt;/em&gt; different overall. Clearly at a system rather than thruster level the trade-off has many factors such as the availability of the power in electrical form and often becomes a trade-off of energy limits vs power limits for chemical and electric respectively.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-24T12:55:59.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20243" PostTypeId="2" ParentId="20239" CreationDate="2017-02-20T19:22:48.010" Score="26" Body="&lt;p&gt;There were a good number of chances to catch the error after launch, which is what most of the reports on the mission focus on. To look specifically at what testing was done before launch this paper from the American Astronautical Society has a decent overview, starting on page 6: &lt;a href=&quot;http://web.mit.edu/16.070/www/readings/Failures_MCO_MPL.pdf&quot; rel=&quot;noreferrer&quot;&gt;The failures of the Mars Climate Orbiter and Mars Polar Lander: a perspective from the people involved&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;They were modifying code from a previous mission, which had the unit conversion factor but buried it in the equations and didn't comment it to make the conversion obvious.&lt;/li&gt;&#xA;&lt;li&gt;Requirement and code walkthroughs failed to notice the missing conversion factor because it wasn't obvious in the original code and they didn't understand the previous equation to the degree necessary to spot the lack.&lt;/li&gt;&#xA;&lt;li&gt;Formal software acceptance testing used a &quot;truth&quot; file produced by manually calculating the equation that was coded, not a data file from an independent source (likely because the navigation team wasn't brought onto the project until very late in development as a cost-saving measure).&lt;/li&gt;&#xA;&lt;li&gt;Integration testing consisted of making sure the file was produced and could be moved to the right server: they didn't do anything with the data in the file.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;What a lot of this came down to was that they were reusing code from a previous mission, and it had gone through all the validation there, so management (who were under pressure to minimize costs) assumed the modifications were low risk. They did enough testing to feel comfortable, but they didn't have experts on hand to do independent testing so it didn't buy them much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, this failure to actually do meaningful integration testing meant that at launch the data files produced by the small forces module (the software with the bug) were in the wrong format and couldn't actually be used. For the first few months of the mission the navigation team was applying/calculating the results of the angular momentum desaturation firings manually based on emails between them and the contractor.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2017-02-20T19:22:48.010" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20244" PostTypeId="2" ParentId="4571" CreationDate="2017-02-20T21:57:49.590" Score="2" Body="&lt;p&gt;I would direct you to a &lt;a href=&quot;http://topdocumentaryfilms.com/thorium-nasa/&quot; rel=&quot;nofollow noreferrer&quot;&gt;documentary by a proponent of LFTR power, Kirk Sorensen&lt;/a&gt;, a nuclear physicist with an engineering degree.  He spends the first part explaining all the ways in which space exploration has been limited by the lack of a safe and efficient power source like LFTR.  He is speaking of enough energy to power a drill to penetrate the thick ice layer on the moon Europa, or heat and greatly extend the life span of probes we might land on cold destinations like Mars, Titan, comets and asteroids which otherwise 'freeze to death', or the ability to power human habitats on Mars or the Moon. As for usefulness for propulsion, it seems the LFTR process actually creates byproducts which can be used as fuel!  The whole presentation is very convincing because he has a good way of explaining science; e.g. in one of the first scenes he shows a salt shaker and explains that salt in that solid state, at room temperature, is actually frozen.  It would be interesting to hear if his approach has application to the recent (2016) idea of sending a featherweight mini camera at one-tenth light speed to one of the nearest star/planet systems.&lt;/p&gt;&#xA;" OwnerUserId="18307" LastEditorUserId="6816" LastEditDate="2017-02-20T22:22:09.887" LastActivityDate="2017-02-20T22:22:09.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20245" PostTypeId="1" AcceptedAnswerId="20247" CreationDate="2017-02-21T00:51:18.700" Score="7" ViewCount="2540" Body="&lt;p&gt;I apologize in advance if this question has already been asked and answered elsewhere, but why do RP-1/LOX rocket systems use helium for propellant tank pressurization?  I understand the fact that O2 and N2 have boiling points in close proximity precludes the use of N2 for pressurizing the oxidizer tank, but couldn't the O2 be pressurized autogenously and the fuel tank with N2?&lt;/p&gt;&#xA;" OwnerUserId="18585" LastEditorUserId="25" LastEditDate="2020-08-23T16:00:12.423" LastActivityDate="2020-08-28T17:24:37.033" Title="Why Use Helium?" Tags="|propulsion|design|helium|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20246" PostTypeId="1" AcceptedAnswerId="20275" CreationDate="2017-02-21T04:08:22.133" Score="9" ViewCount="1803" Body="&lt;p&gt;There are two pieces visible in the video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these some sort of fairing or abalator?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RKnip.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RKnip.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/rUDLxFUMC9c?start=1266&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="18589" LastActivityDate="2017-02-22T18:53:49.313" Title="What is this debris on the CRS-10 reentry at T6:30?" Tags="|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20247" PostTypeId="2" ParentId="20245" CreationDate="2017-02-21T04:21:11.350" Score="10" Body="&lt;p&gt;Bottom line: like so many things in rocketry, it's a design trade, usually at the system level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The book &lt;a href=&quot;https://books.google.com/books?id=7MpQAAAAYAAJ&quot; rel=&quot;noreferrer&quot;&gt;Pressurization Systems for Liquid Rockets&lt;/a&gt; goes into great detail on this.  I highly suggest you read Chapter 2.1.2 &quot;Selection of System Type&quot; which pretty much covers your exact question. There is even a case description on page 33 where the author goes through the trades for selecting helium versus autogenous pressurization on the S-IVB.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's one of the charts from this book describing some of the trade space where weight/compatibility/complexity are considered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VhniC.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VhniC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here is the S-IVB discussion (I am posting it as an image, because whatever format Google Books uses does not allow me to copy the text). Note that the trades were made on weight and reliability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mBSkk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mBSkk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: The linked document is available at &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19760015212&quot; rel=&quot;noreferrer&quot;&gt;NTRS&lt;/a&gt; which is nice because the Google Books format is annoying.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-02-22T02:28:12.510" LastActivityDate="2017-02-22T02:28:12.510" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20248" PostTypeId="2" ParentId="5785" CreationDate="2017-02-21T04:27:16.843" Score="-2" Body="&lt;p&gt;In a documentary, one astronaut mentioned that it took 12 seconds for Apollo to clear the tower; until that point, all 5 engines of the 1st stage needed to function 100%, but after the first 12 seconds or when it cleared the tower, the rocket had used enough fuel and was now light enough that if 1 main engine failed, the rocket could continue to fly with 4 engines running.  I think that may be the reason why it was so critical that it cleared the tower.&lt;/p&gt;&#xA;" OwnerUserId="18590" LastEditorUserId="6816" LastEditDate="2017-02-21T04:55:13.050" LastActivityDate="2017-02-21T04:55:13.050" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20249" PostTypeId="1" AcceptedAnswerId="20659" CreationDate="2017-02-21T04:28:20.280" Score="4" ViewCount="984" Body="&lt;p&gt;I'm trying to &lt;s&gt;make&lt;/s&gt; &lt;em&gt;derive&lt;/em&gt; a concise table of the effects on each Keplerian orbital element for a small impulse in the Radial, Tangential, or Perpendicular direction at periapsis or apoapsis, good to at least 2nd order in $\delta v/v_0$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can think of this as doing the longhand math for the question &quot;&lt;em&gt;What orbit will this cubesat get if I shoot it out of the ISS up, forward, or sideways by 7.7 m/sec?&lt;/em&gt;&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a circular orbit with zero inclination:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_R, \ v_T, \ v_P \ = \ 0, \ v_0, \ 0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$r(t) \ = \ a$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've deduced the following &lt;em&gt;empirical guesses&lt;/em&gt; from number crunching - integrating the ODEs to obtain perturbed orbits, then extracting the orbital elements:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\delta a}{a} \sim \left( \frac{\delta v_R}{v_0}\right)^2 + 2 \frac{\delta v_T}{v_0} + \left( \frac{\delta v_P}{v_0}\right)^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\delta \epsilon \sim  \frac{\delta v_R}{v_0} + 2  \frac{\delta v_R}{v_0} + \left( \frac{\delta v_P}{v_0}\right)^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\delta i \sim  \frac{\delta v_P}{v_0}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $a$, $\epsilon$, and $i$ are the semi-major axis, eccentricity, and inclination, respectively. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can run a bunch of cases including elliptical orbits of varying eccentricity and use the same empirical guessing to make a full set of coefficients including their eccentricity dependence, but I feel bad because I always try to do the math first if possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I'm stumped how to begin those derivations. Ideally the answer would give an example how to do these impulse perturbation expansions with good-old pen and paper, along with hints how to do the rest. A link to the final results would be helpful, &lt;em&gt;but a 'how-to' discussion of the strategy for doing the expansion is what I need most&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;To repeat:&lt;/em&gt;, I'm constraining the problem to an elliptical orbit with zero initial inclination, small impulses, only at periapsis or apoapsis, in only the radial, tangential, or perpendicular directions.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-21T07:28:18.947" LastActivityDate="2017-03-17T17:46:49.667" Title="Deriving the changes in Keplerian Elements induced by small impulses" Tags="|orbital-mechanics|orbital-elements|mathematics|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20251" PostTypeId="1" AcceptedAnswerId="20258" CreationDate="2017-02-21T09:43:49.347" Score="4" ViewCount="514" Body="&lt;p&gt;The &lt;a href=&quot;https://www.nasa.gov/mission_pages/juno/spacecraft/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno Spacecraft&lt;/a&gt; (also &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, launch 05-Aug-2011, Jupiter orbit insertion 05-Jul-2016) was scheduled for a burn to reduce it's period from about 54 days to 14 days, while maintaining a similarly challenging low altitude periJove of only 4000 km above the planet's equator. Among the technical challenges are the intense radiation near Jupiter due to particles trapped in the planet's strong magnetic field. This lead to some extreme measures, including the 1 cm thick Tantalum &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_Radiation_Vault&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno Radiation Vault&lt;/a&gt; to protect roughy 200 kg of electronic equipment.&lt;/p&gt;&#xA;&lt;p&gt;It completed it's first periJove on 27-Aug-2016, and on the second periJove it was scheduled to execute the orbit reducing burn on 19-Oct-2016. Due to potential issues with the propulsion system discovered in telemetry prior to the scheduled burn, it was postponed. The burn was not executed during any subsequent burn, and now it seems the decision has been made to leave Juno in the longer orbit for the duration of the mission, according to Space Daily's &lt;a href=&quot;http://www.spacedaily.com/reports/Juno_to_remain_in_current_orbit_at_Jupiter_999.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno to remain in current orbit at Jupiter&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The article says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Juno's larger 53-day orbit allows for &amp;quot;bonus science&amp;quot; that wasn't part of the original mission design. Juno will further explore the far reaches of the Jovian magnetosphere - the region of space dominated by Jupiter's magnetic field - including the far magnetotail, the southern magnetosphere, and the magnetospheric boundary region called the magnetopause. Understanding magnetospheres and how they interact with the solar wind are key science goals of NASA's Heliophysics Science Division.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Another key advantage of the longer orbit is that Juno will spend less time within the strong radiation belts on each orbit,&amp;quot; said Scott Bolton, Juno principal investigator from Southwest Research Institute in San Antonio. &amp;quot;This is significant because radiation has been the main life-limiting factor for Juno.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What was the purpose of the planned change from 53 days to 14 days? Considering the advantages of staying in the longer orbit, what were the even more compelling advantages of the short orbit?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Plot of Juno's originally planned orbit from historical kernels (&lt;a href=&quot;https://space.stackexchange.com/a/19263/12102&quot;&gt;source and plot described here&lt;/a&gt;)in the &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizions&lt;/a&gt; database. Juno remains in a single plane but with a reduced apoJove, The in-plane &lt;a href=&quot;https://space.stackexchange.com/a/19188/12102&quot;&gt;precession of the apses&lt;/a&gt; is due to Jupiter's substantial oblateness and large &lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt;. &lt;em&gt;Left:&lt;/em&gt; projection into J2000 Ecliptic YZ plane. Right: XZ plane, coincidentally it is edge-on in this view due to mission schedule relative to Jupiter's orbit.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Mxepq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mxepq.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0tDMfm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0tDMfm.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; sketch of a few of Juno's lower 14-day orbits and illustration of the strongest radiation bands near Jupiter. From the &lt;a href=&quot;http://www.latimes.com/local/california/la-sci-sn-juno-jupiter-arrival-updates-why-the-long-1467415666-htmlstory.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Los Angeles Times&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ixhc3.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ixhc3.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Animation of Jupiter's Synchrotron Radiation Torus. Credit: NASA/JPL - Caltech&amp;quot;. From the &lt;a href=&quot;http://www.vofoundation.org/blog/nasas-juno-spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vatican Observatory Blog&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-06-22T10:51:34.553" Title="If Juno's long orbit means a lower rate of radiation damage, why the planned short orbit?" Tags="|radiation|mission-design|jupiter|juno|swri|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20252" PostTypeId="1" CreationDate="2017-02-21T09:44:39.110" Score="1" ViewCount="206" Body="&lt;p&gt;What is the typical amount of radiation levels that a spacecraft would experience when orbiting Mars? is there any formula with which a person could estimate the net radiation experienced at different altitudes and inclination around Mars?&lt;/p&gt;&#xA;" OwnerUserId="18568" LastActivityDate="2017-02-22T04:52:45.237" Title="Radiation levels around Mars" Tags="|mars|radiation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20253" PostTypeId="2" ParentId="18686" CreationDate="2017-02-21T09:48:45.993" Score="1" Body="&lt;p&gt;I am going to advance the answer &quot;&lt;strong&gt;It wasn't.&lt;/strong&gt;&quot; to the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why would Juno's originally planned orbit lowering partially mitigate radiation damage?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to Space Daily's recent article &lt;strong&gt;&lt;a href=&quot;http://www.spacedaily.com/reports/Juno_to_remain_in_current_orbit_at_Jupiter_999.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno to remain in current orbit at Jupiter&lt;/a&gt;&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Juno's larger 53-day orbit allows for &quot;bonus science&quot; that wasn't part of the original mission design. Juno will further explore the far reaches of the Jovian magnetosphere - the region of space dominated by Jupiter's magnetic field - including the far magnetotail, the southern magnetosphere, and the magnetospheric boundary region called the magnetopause. Understanding magnetospheres and how they interact with the solar wind are key science goals of NASA's Heliophysics Science Division.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The operative paragraph:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Another key advantage of the longer orbit is that &lt;strong&gt;Juno will spend less time within the strong radiation belts on each orbit,&lt;/strong&gt;&quot; said Scott Bolton, Juno principal investigator from Southwest Research Institute in San Antonio. &quot;This is significant because radiation has been the main life-limiting factor for Juno.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-02-21T09:48:45.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20256" PostTypeId="2" ParentId="20251" CreationDate="2017-02-21T12:35:36.063" Score="3" Body="&lt;p&gt;I can see several reasons for those shorter orbits.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;first of all, you have to notice that the probe is in a polar orbit around Jupiter. &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Scientific_objectives&quot; rel=&quot;nofollow noreferrer&quot;&gt;One of the goals of the mission&lt;/a&gt; is to make several &quot;maps&quot; of Jupiter (jupiter's gravitational and magnetic fields, variation in atmospheric composition, etc.), so the shorter the orbit the more data you get for a fixed amount of time (eg the lifespan of the probe).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;if you want to make the same amount of orbits, but with the longer one, it would take 34 times 53.5 days = 1819 days (~5 years) instead of 34 x 14 days = 476 (~15 months) for the shorter one. The wikipedia article cited in OP mentioned that the orbits configuration allows the probe's solar panel to be always facing the sun. With a mission time around 5 years, and knowing that Jupiter has an orbital period of 11.86 years, you can deduce that at least a few orbit would have got the probe into Jupiter shadow, and so without the possibility to energize itself through its solar panels.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;last but not least, the said polar orbits let the probe navigate with a minimum contact with Jupiter's dense &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_%28spacecraft%29#Orbit_and_environment&quot; rel=&quot;nofollow noreferrer&quot;&gt;radiation belts&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These orbits minimize contact with Jupiter's dense radiation belts, which can damage spacecraft electronics and solar panels, by exploiting a gap in the radiation envelope near the planet, passing through a region of minimal radiation&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18592" LastActivityDate="2017-02-21T12:35:36.063" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20257" PostTypeId="2" ParentId="4571" CreationDate="2017-02-21T13:09:13.073" Score="-1" Body="&lt;p&gt;We also should consider how practically we can build nuclear powered rockets or space probes which can propel safely. The practical difficulties like radiation from the reactor, the controlled nuclear reaction (either fission or fusion), the metallurgy part which withstand very high temperatures are to be solved. The heat generated by the core engine (nuclear engine) should not spoil the payload. Above all, the engine structure must withstand thermal and mechanical shock produced by the nuclear reactor. The liquid fluorine must be highly efficient to absorb the heat generated to maintain the reaction to proceed further. All these factors plays a important role in developing the nuclear powered rocket.&lt;/p&gt;&#xA;" OwnerUserId="18517" LastEditorUserId="6816" LastEditDate="2017-02-21T19:19:13.473" LastActivityDate="2017-02-21T19:19:13.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20258" PostTypeId="2" ParentId="20251" CreationDate="2017-02-21T16:32:13.397" Score="8" Body="&lt;p&gt;Money. The mission was proposed to a cost cap, which includes operations costs. The science objectives could be achieved with a shorter primary mission and therefore lower cost using tighter orbits.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-02-21T16:32:13.397" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20259" PostTypeId="2" ParentId="20239" CreationDate="2017-02-21T16:34:24.260" Score="-3" Body="&lt;p&gt;I worked on the &lt;em&gt;Midas&lt;/em&gt; and &lt;em&gt;Samos&lt;/em&gt; projects in the 50's and &amp;amp; 60's, and on the Space Shuttle until it's demise in the 80's. On the former, quality assurance was comprehensive, &lt;em&gt;every&lt;/em&gt; nut and solder joint was inspected. There essentially was no QA on the Shuttle, technicians I was told &lt;em&gt;would inspect their own work&lt;/em&gt;! Which would save lots of time and money.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As in most businesses, it's about money. As long as the profits exceeded the losses, it is considered acceptable.&lt;/p&gt;&#xA;" OwnerUserId="18605" LastEditorUserId="1054" LastEditDate="2018-10-06T19:35:07.053" LastActivityDate="2018-10-06T19:35:07.053" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20260" PostTypeId="2" ParentId="19977" CreationDate="2017-02-21T19:57:15.167" Score="0" Body="&lt;p&gt;Not good to use 3/4 empty 2nd stage as a booster. But maybe FH will be able to deliver almost full 2nd stage to LEO and recover all thee cores if used without payload I hope.&lt;/p&gt;&#xA;" OwnerUserId="18609" LastActivityDate="2017-02-21T19:57:15.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20261" PostTypeId="1" AcceptedAnswerId="20263" CreationDate="2017-02-21T23:46:31.647" Score="2" ViewCount="207" Body="&lt;p&gt;Has Kepler (or any other survey) found an exoplanet with a similar orbit as another exoplanet around the same star?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current IAU definition of a planet requires that &quot;a planet has cleared its neighborhood around its orbit&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have there been any exoplanet discoveries that bring such a definition into question?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mathematically, is there anything stopping 2 &quot;planets&quot; from existing in equilibrium in a similar orbit around a single star?&lt;/p&gt;&#xA;" OwnerUserId="8653" LastEditorUserId="13710" LastEditDate="2017-02-22T05:31:11.270" LastActivityDate="2017-02-22T16:24:04.917" Title="Exoplanet(s) that share orbit?" Tags="|pluto|planet|exoplanet|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20262" PostTypeId="2" ParentId="20252" CreationDate="2017-02-22T04:52:45.237" Score="1" Body="&lt;p&gt;After going through many technical papers for the question i myself has poised i have come up with one conclusion.&#xA;As Mars has a very low strength Magnetic field unlike that of Earths,the levels of radiation around the orbit should be similar to the Radiation levels which are present on the surface.&#xA;please correct me if the above statement is wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiation levels as observed by RAD on MSL&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LHRD7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LHRD7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18568" LastActivityDate="2017-02-22T04:52:45.237" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20263" PostTypeId="2" ParentId="20261" CreationDate="2017-02-22T08:38:39.110" Score="6" Body="&lt;p&gt;Two planets sharing their orbit is expected to be a rare configuration. &lt;a href=&quot;https://www.quora.com/Can-two-planets-share-the-same-orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Most configurations&lt;/a&gt; of 2 planets in 1 orbit are unstable. Only when one planet is in a Lagrange point of the other, are the orbits stable.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of now (February 2017), there are &lt;a href=&quot;https://en.wikipedia.org/wiki/Co-orbital_configuration&quot; rel=&quot;nofollow noreferrer&quot;&gt;no known co-orbital planets&lt;/a&gt;. We do know lots of smaller objects co-orbiting with planets (in our own solar system, there are thousands of Trojans in e.g. Jupiter's orbit). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a potential detection (&lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler-223&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler-223&lt;/a&gt;), but on closer examination those planets were unlikely to be co-orbital.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="195" LastEditDate="2017-02-22T16:24:04.917" LastActivityDate="2017-02-22T16:24:04.917" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20264" PostTypeId="2" ParentId="20211" CreationDate="2017-02-22T10:20:50.407" Score="1" Body="&lt;p&gt;The Apollo 13 trajectory looked like this.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y99QB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y99QB.png&quot; alt=&quot;Apollo 13 trajectory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First midcourse correction: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At 030:40:49.65, a 3.49-second midcourse correction lowered the closest point of spacecraft approach to the Moon to an altitude of 60 miles. Before this maneuver, the spacecraft had been on a free-return trajectory, in which the spacecraft would have looped around the Moon and returned to Earth without requiring a major maneuver.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Second midcourse correction: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The spacecraft was then maneuvered back into a free-return trajectory at 061:29:43.49 by firing the LM descent engine for 34.23 seconds. It then looped behind the Moon and was out of contact with the Earth tracking stations between 077:08:35 and 077:33:10, a total of 24 minutes 35 seconds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://history.nasa.gov/SP-4029/Apollo_13a_Summary.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Between these two maneuvers, the spacecraft was not on a free return trajectory. I don't know yet if this section of the trajectory was bound to the Earth-Moon system.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="415" LastEditDate="2017-02-22T16:23:34.667" LastActivityDate="2017-02-22T16:23:34.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20265" PostTypeId="1" AcceptedAnswerId="20271" CreationDate="2017-02-22T10:29:19.407" Score="10" ViewCount="1235" Body="&lt;p&gt;How does the control panel for Space Station Remote Manipulator System (the robotic arm) look like, and where is it located?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it some joysticks, or just a screen+keyboard? What kind of feedback (video?) does it provide? Does it control separate joints, or just position of the end of the arm? Maybe pre-programmed or automatic maneuvers like &quot;walking&quot; to given &lt;em&gt;Power and Data Grapple Fixture&lt;/em&gt; or grappling an indicated PDGF automatically?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="6944" LastEditDate="2020-11-07T00:02:38.467" LastActivityDate="2021-11-12T02:14:47.503" Title="What is the user interface of SSRMS" Tags="|iss|robotics|frames|robotic-arm|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20266" PostTypeId="1" AcceptedAnswerId="20270" CreationDate="2017-02-22T10:39:44.087" Score="4" ViewCount="207" Body="&lt;p&gt;I know the Chinese did some to their own space station, but were there missions like the repair or retrieve missions the Space Shuttle did with for example the Hubble telescope?&lt;/p&gt;&#xA;" OwnerUserId="18534" LastEditorUserId="18534" LastEditDate="2017-02-22T12:01:52.790" LastActivityDate="2017-02-22T12:44:44.890" Title="Were there any manned missions into orbit that went not to the ISS since the end of the Shuttle?" Tags="|crewed-spaceflight|soyuz-spacecraft|shenzhou|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20267" PostTypeId="1" AcceptedAnswerId="31633" CreationDate="2017-02-22T10:41:31.930" Score="8" ViewCount="122" Body="&lt;p&gt;I am interested in videos of Russian space program, especially the older ones (old Soyuz launches, Soyuz docking with MIR, Leonov's spacewalk etc.). While NASA puts all their videos online in good quality, searching for the Russian space program usually yields low-quality rips from TV documentaries and the like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an online repository of Russian space videos in high quality?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My constraints:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;I can somewhat read Russian, but searching in Russian is difficult for me (I cannot efficiently &quot;scan&quot; the text neither write a good query)&lt;/li&gt;&#xA;&lt;li&gt;Free access is good, but I am willing to pay reasonable money&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6917" LastActivityDate="2018-10-26T19:22:29.097" Title="Online availability of high-quality Russian launch/space videos?" Tags="|russia|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20268" PostTypeId="1" AcceptedAnswerId="20272" CreationDate="2017-02-22T11:04:19.947" Score="15" ViewCount="6284" Body="&lt;p&gt;I always though the Space Shuttle needed a combination of its solid rocket boosters &lt;em&gt;and&lt;/em&gt; its main engines to remain stable while launching. The following diagram found on Wikipedia seems to agree: after liftoff, the Shuttle tilts backwards to get its center of mass in line with the direction of thrust of the SSMEs. By the time the SRBs run out, the Shuttle and its drop tank are properly aligned for that configuration. Some time later the SSMEs are cut off, and the drop tank is separated. This concludes the &quot;launch&quot;; what remains are orbit insertion burns, doing what they went up for, and returning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dDqHV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dDqHV.jpg&quot; alt=&quot;https://upload.wikimedia.org/wikipedia/commons/5/5c/Space_shuttle_mission_profile.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I recently saw someone with a T-shirt depicting a Space Shuttle launch with the SRBs firing, but the main engines not showing any exhaust, I thought that was an error in the drawing. However, I've since come across this picture, which shows a launch with the SRBs very much active, but no visible exhaust plume from the main engines:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dwrCZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dwrCZ.jpg&quot; alt=&quot;https://upload.wikimedia.org/wikipedia/commons/d/d3/Atlantis_taking_off_on_STS-27.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this simply an issue with the relative brightness of the exhaust of the &lt;em&gt;big barrels o' fireworks&lt;/em&gt; vs. the exhaust of the liquid fuel engines, or are the engines effectively off in the picture?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For context, pictures in an answer to &lt;a href=&quot;https://space.stackexchange.com/questions/7751/are-rocket-exhaust-flames-ever-opaque&quot;&gt;another question on this site&lt;/a&gt; make it very plausible that the SSMEs simply produce less soot and light. A &lt;a href=&quot;https://www.nasa.gov/images/content/107926main_SRB_sep_sts1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;picture on the NASA website&lt;/a&gt; also seems to show SSMEs which are 'on', but not very bright in comparison with nearly burnt-out SRBs.&lt;/p&gt;&#xA;" OwnerUserId="14602" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2023-01-12T03:34:13.690" Title="Were the Space Shuttle's Main Engines ever off while the Solid Rocket Boosters were firing?" Tags="|launch|space-shuttle|engines|launch-sequence|exhaust|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="20269" PostTypeId="2" ParentId="20268" CreationDate="2017-02-22T11:29:14.993" Score="27" Body="&lt;p&gt;No, they were never off. Here is my explanation:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;have you ever noticed that the top engine is tilted down? This is us to keep the center of thrust more or less aligned with center of mass of the space shuttle. As the SRBs produce &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle#Solid_rocket_boosters&quot; rel=&quot;nofollow noreferrer&quot;&gt;83%&lt;/a&gt; of the thrust at lift-off, a counter-force was needed to go straight-up. With shuttle engines off, I am not sure the whole thing would stay under control and not flip upside-down.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;the SSME exhaust is water vapour as this is an oxygen+hydrogen reaction (but you may &lt;a href=&quot;https://space.stackexchange.com/questions/3358/can-i-drink-the-water-from-the-plumes-of-ssme&quot;&gt;not drink it&lt;/a&gt;). Combine the exhaust speed and pressure and you obtain a hardly visible flow, and the picture in OP can mislead you as the output flame is aligned with SRB exhaust (which is VERY visible and bright). A questionable comparison would be your pressure cooker: the steam is invisible just after the nozzle.&#xA;&lt;a href=&quot;https://web.archive.org/web/20210129012820/https://spaceflight.nasa.gov/gallery/images/shuttle/sts-114/hires/sts114-s-041.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://web.archive.org/web/20210129012820/https://spaceflight.nasa.gov/gallery/images/shuttle/sts-114/hires/sts114-s-041.jpg&quot; alt=&quot;Another lift-off photo&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18592" LastEditorUserId="10273" LastEditDate="2023-01-12T03:34:13.690" LastActivityDate="2023-01-12T03:34:13.690" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="20270" PostTypeId="2" ParentId="20266" CreationDate="2017-02-22T12:44:44.890" Score="4" Body="&lt;p&gt;Since STS-135, landing 21 July 2011, the only non-ISS flights were three Shenzhou missions (9, 10 and 11), two first to Tiangong 1, third to  Tiangong 2. &lt;a href=&quot;https://en.wikipedia.org/wiki/2017_in_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The situation will likely change in upcoming years, with developments of Dragon 2, Starliner, SpaceShipTwo, Blue Shephard, ISRO Orbital Vehicle, and Orion spacecraft on top of sustained China space program and upcoming retirement of ISS.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-02-22T12:44:44.890" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20271" PostTypeId="2" ParentId="20265" CreationDate="2017-02-22T12:47:05.267" Score="15" Body="&lt;p&gt;Like the SSRMS itself, it's rather complicated.&lt;/p&gt;&#xA;&lt;p&gt;The &amp;quot;control panel&amp;quot; is called the Robotics Workstation (RWS).  There are two of them onboard the ISS.  One is in the US Lab, the other in the Cupola. An RWS consists of four main parts.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4FQIQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4FQIQ.png&quot; alt=&quot;Schematic drawing of the Robotics Workstation showing the components described below&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The Display and Control Panel, which has hardware switches for&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Camera routing and control&lt;/li&gt;&#xA;&lt;li&gt;Joint selection for single joint control mode&lt;/li&gt;&#xA;&lt;li&gt;Brakes&lt;/li&gt;&#xA;&lt;li&gt;Safing&lt;/li&gt;&#xA;&lt;li&gt;Auto Sequence control&lt;/li&gt;&#xA;&lt;li&gt;Special Purpose Dextrous Manipulator (aka Dextre) controls&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Some limited feedback is provided here by illumination of the buttons.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZUbAi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZUbAi.jpg&quot; alt=&quot;photograph of the Display and Control Panel&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;The joysticks, one for rotation and one for translation.  The translational control on the left can be pushed in, pulled out, moved left or right, and/or up and down, for +/- directions in all three translations axes. The rotational control on the right can be pitched up or down, rolled left or right, and yawed (twisted) in both directions, for +/- motion in all rotational axes. This is a common setup with Shuttle piloting and Shuttle robotic arm controls working in the same fashion. A trigger on the rotational control allows the user to grapple or release.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Three dedicated CCTV monitors to show camera views.  The RWS generates and displays useful overlays on the monitors showing camera data, joint angles, alignment to grapple target, etc.  Critical feedback to the operator(s) can be provided through these overlays so that the operator(s) does not have to look away from the monitors.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A dedicated Portable Computer System (PCS) laptop used to call up the displays for controlling the SSRMS.  Additional PCSs can be used if desired.  Most of the detailed feedback to the operator(s) is shown on the PCS displays, and detailed control information is entered here.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;This is the RWS in the Lab.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/oLALx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oLALx.jpg&quot; alt=&quot;Photograph of the RWS in the ISS Lab module&quot; /&gt;&lt;/a&gt;&#xA;And this is the RWS in the Cupola.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Kv2kV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kv2kV.jpg&quot; alt=&quot;Photograph of the RWS in the ISS Cupola module&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;As far as modes of operation, as alluded to above, the operator can control one joint at a time, this is called Single Joint mode.  There are many modes of operation besides single joint&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;FOR manual modes, in which the operator controls the motion of a given Frame&#xA;of Reference, often the tip of the arm&lt;/li&gt;&#xA;&lt;li&gt;Pitch Plane mode, which rotates the pitch plane while maintaining&#xA;the chosen FOR&lt;/li&gt;&#xA;&lt;li&gt;FOR Operator Commanded Auto Sequence (OCAS) mode, which moves the&#xA;chosen FOR from a chosen initial to final position and attitude&lt;/li&gt;&#xA;&lt;li&gt;FOR Autosequence, which moves the chosen FOR through a&#xA;preprogrammed sequence of points&lt;/li&gt;&#xA;&lt;li&gt;Joint OCAS, which moves the joints to an operator specified set of&#xA;joint angles&lt;/li&gt;&#xA;&lt;li&gt;Joint Autosequence,which moves the joints through a    preprogrammed&#xA;sequence of joint angles&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Frames of Reference are key to SSRMS operations.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &amp;quot;external&amp;quot; frame is called the ISS Analysis Coordinate System&#xA;(ISSACS). It's a right-handed frame with the origin at the center of&#xA;the S0 Truss. The X axis points to ISS forward, the Z axis points to&#xA;ISS nadir, and the Y axis points ISS starboard. When you need to describe the position and attitude of something relative to the entire ISS, this frame is useful.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Ij8cG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ij8cG.png&quot; alt=&quot;Schematic  drawing of the ISSACS FOR&quot; /&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;A commonly used &amp;quot;internal&amp;quot; frame is the LEE (Latching End Effector,&#xA;i.e. the tip of the arm) frame. This frame's origin is at the center of the LEE cavity.  Its X axis points out of the LEE, the Z axis is away from the LEE mounted camera, and the Y axis completes the right-handed system.  This frame is useful for describing the position and attitude of objects relative to the tip of the SSRMS.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/kwiWH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kwiWH.png&quot; alt=&quot;Schematic drawing of the LEE frame&quot; /&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Each PDGF has its own frame, useful for describing the position and attitude of objects relative to the grapple fixture.&lt;/li&gt;&#xA;&lt;li&gt;Payloads grappled by the SSRMS have their own frames, sometimes&#xA;defined in terms of berthing hardware or cameras.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Final note: Everything can be controlled from Mission Control, and often is.  Usually only highly critical operations like grappling or EVA support are done by the onboard crew.&lt;/p&gt;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=2&amp;amp;ved=0ahUKEwj1gdyv3qPSAhUl7IMKHYtaBmcQFggiMAE&amp;amp;url=https%3A%2F%2Fwww.nasa.gov%2Fcenters%2Fjohnson%2Fpdf%2F486042main_ISSRoboticsHumanFactorsPerspective.pdf&amp;amp;usg=AFQjCNGPiNprFdXV6-CfP0qGfmgWSBhCyQ&amp;amp;sig2=B0AMeLI7N6LV6h2pZJFQ1Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Robotic Systems and Operations&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.asi.org/adb/04/02/00/iss-coordinate-systems.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Coordinate Systems Manual&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Personal notes&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-11-12T02:14:47.503" LastActivityDate="2021-11-12T02:14:47.503" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="20272" PostTypeId="2" ParentId="20268" CreationDate="2017-02-22T13:20:19.680" Score="30" Body="&lt;p&gt;Hydrogen-oxygen engines produce a relatively faint blue flame, with visible blue-white shock discs or diamonds under certain circumstances, particularly low altitudes where air pressure confines the plume. At high altitude the plume is nearly invisible. You can look at a number of shuttle launch photos and see the different appearances the main engine plume can take on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the photo you have there, the exposure is calibrated for the exhaust plumes from the solid rocket boosters, which are a brilliant yellow-white. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exhaust from the main engines is simply too faint to be clearly visible in the shot (though I believe the pinkish tint seen between the right wing and the right SRB is it). You can see the blue-white glow &lt;em&gt;inside&lt;/em&gt; the nozzles; the engines are clearly running. They'd be deeply shadowed otherwise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle's launch profile does include the so-called &quot;throttle bucket&quot;, where the main engines are turned down to about 68% of rated thrust for thirty seconds or so, in order to keep the whole stack from reaching too high a speed while still fighting air, but even at 68% that's still nearly a million pounds of thrust coming from those engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zbtrq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zbtrq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-02-23T08:56:25.123" LastActivityDate="2017-02-23T08:56:25.123" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20273" PostTypeId="2" ParentId="20268" CreationDate="2017-02-22T16:12:49.597" Score="10" Body="&lt;p&gt;Offered as a supplement to the other answers - here's a frame from an SRB mounted camera (post-separation) showing the SSMEs running in the absence of SRB plumes.  As stated by others, there's no real visible SSME plume at this altitude, just a glow inside the nozzles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y7E7Q.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y7E7Q.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-02-22T16:12:49.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20274" PostTypeId="2" ParentId="10829" CreationDate="2017-02-22T17:25:37.510" Score="18" Body="&lt;p&gt;Offered as a supplement to the other answers, here are some data about SRBs thrust profiles and operation.&lt;/p&gt;&#xA;&lt;p&gt;Thrust of a SRB is function of the area of the solid fuel burning, as shown below:&#xA;&lt;img src=&quot;https://i.stack.imgur.com/PzDnY.gif&quot; alt=&quot;several thrust profiles&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;You can find &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#Propellant&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; how the space shuttle SRBs are filled:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The propellant had an 11-point star-shaped perforation in the forward motor segment and a double-truncated-cone perforation in each of the aft segments and aft closure. This configuration provided high thrust at ignition and then reduced the thrust by approximately a third 50 seconds after lift-off to avoid overstressing the vehicle during maximum dynamic pressure (Max Q).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Here is a very interesting GIF about how a SRB is built and burnt:&#xA;&lt;a href=&quot;https://web.archive.org/web/20200228215011/https://giant.gfycat.com/SerpentineIgnorantAmmonite.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://web.archive.org/web/20200228215011/https://giant.gfycat.com/SerpentineIgnorantAmmonite.gif&quot; alt=&quot;SRB gif&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;You can see that once the initial ignition is done you cannot stop it, and all the solid fuel will burn. This is why every space vessel using SRB will always starts its stoppable engine (SSME on NASA space shuttle, Vulcain engine on european Ariane rocket) in order to check if everything is working fine, and only after this verification unleash the power of SRBs:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Space shuttle &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle#Launch&quot; rel=&quot;nofollow noreferrer&quot;&gt;procedure&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The three main engines (SSMEs) started at T-6.6 seconds. The main engines ignited sequentially via the Shuttle's general purpose computers (GPCs) at 120 millisecond intervals. All three SSMEs were required to reach 90% rated thrust within three seconds, otherwise the onboard computers would initiate an RSLS abort. If all three engines indicated nominal performance by T-3 seconds, they were commanded to gimbal to liftoff configuration and the command would be issued to arm the SRBs for ignition at T-0&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;I couldn't find any precise description of an Ariane 5 launch, but any launch video will show you it is the same.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18592" LastEditorUserId="10273" LastEditDate="2021-01-27T20:23:57.230" LastActivityDate="2021-01-27T20:23:57.230" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20275" PostTypeId="2" ParentId="20246" CreationDate="2017-02-22T17:52:31.547" Score="8" Body="&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt;: Small, falcon-shaped debris fell off of the first stage around the time it accelerated upward upon engine re-start. With a much higher drag to weight ratio, they started losing vertical speed more quickly than the rocket, which soon passed them by.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;In order to get a better look at the objects, I extracted frames from the video and made a GIF. It's displayed at 4 frames per second, compared to 30 fps in the original video. The second object seems to have a particularly bird-like shape, and remains visible for over one second.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rmMo4.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rmMo4.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dJ8QP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dJ8QP.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I turned to &lt;a href=&quot;https://www.flightclub.io/&quot; rel=&quot;noreferrer&quot;&gt;Flightclub&lt;/a&gt; to get an idea of the altitude and speed of the CRS-10 Falcon 9 first stage returning for landing. This site is a user-friendly, physics-based simulator, and has a 'library' of missions with careful, best-effort trajectory reconstructions, based on telemetry in the videos, known physical properties of rockets (e.g. mass, ranges of thrust) and simulated aerodynamics.&lt;/p&gt;&#xA;&lt;p&gt;The arrow indicates the estimated location of the start of the entry burn. The separation between the launch pad and landing site is 15 km, so it's already clear this is happening at very high altitude, of order 50km. In fact the nubmer is about 46km as discussed below, and with a scale height of 8.5km that means the atmospheric pressure here was below 5 millibar, or less than half of one percent of a standard atmosphere. This means that at least live birds can be ruled out.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Bf49r.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Bf49r.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here are some more detailed plots. I have labeled the beginning of the entry burn (about T+ of 396 seconds) with an arrow. With a vertical velocity of about 1.2 kilometers per second, the objects would have to be much larger than bird to be visible for 1.5 seconds.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;What's shooting the bird&lt;/strong&gt; theory down is the reasonable assumption that at least a live bird would have essentially zero vertical velocity, compared to that of the rocket. It's that vertical velocity, combined with the 1.5 seconds visibility that would make the bird ridiculously large. (And dying because there's almost no air.)&lt;/p&gt;&#xA;&lt;p&gt;However, &lt;strong&gt;if you give the objects a vertical velocity that's nearly matching the first stage, then they could be quite small&lt;/strong&gt;. In fact, they could be small bits of debris from the rocket that are dislodged by the sudden acceleration due to the engine starting, but they could have fallen off slightly earlier as well.&lt;/p&gt;&#xA;&lt;p&gt;If so, then why would they come floating back up? Drag. The rocket is aligned roughly along the direction of relative motion with respect to the air, and it's very heavy. The objects are randomly shaped, have no attitude control, and probably have a much lower mass per unit area. Even at a half-percent of standard pressure, they can start loosing meters second of velocity quickly, and since the atmospheric pressure is doubling about every four seconds, it's going to happen quickly.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ku9Wt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ku9Wt.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xFYIh.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xFYIh.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I did find a discussion of bird-shaped debris in the Official Launch Discussion Reddit page for CRS-10. While there is little discussion of the objects' trajectory down, and then up again (relative to the rocket), the conclusion there is where I started when writing this answer.&lt;/p&gt;&#xA;&lt;p&gt;From: &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/5uor5r/welcome_to_the_rspacex_crs10_official_launch/&quot; rel=&quot;noreferrer&quot;&gt;Welcome to the r/SpaceX CRS-10 Official Launch Discussion &amp;amp; Updates Thread! (from r/SpaceX)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;sol3tosol4:&lt;/strong&gt; 1265s - Those are pieces of debris, probably from the bottom of the booster when the entry burn was started (above most of the atmosphere).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Bwa_aptos:&lt;/strong&gt; So, you're saying that those black items flapping their wings flying through the sky that look EXACTLY like birds flying are &amp;quot;debris&amp;quot;?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;sol3tosol4:&lt;/strong&gt; Yes. At the timestamp of the link you provided, what the booster is falling through is very nearly vacuum. Just after the &amp;quot;birds&amp;quot; go past the booster, the statement &amp;quot;Stage 1 entry burn has started&amp;quot; can be heard (about T+6:30 in the technical webcast). That's where the booster enters the part of the atmosphere that's thick enough to start being a hazard to the booster, several times as high as the highest bird that's ever been seen flying. The booster is also falling at multiple times the speed of sound, and the &amp;quot;birds&amp;quot; are also falling at multiple times the speed of sound, just not quite as fast as the booster. Finally, if you play the video in slow motion (e.g. 1/4 speed), it can be seen that the &amp;quot;big bird&amp;quot; is tumbling, not flapping.&lt;/p&gt;&#xA;&lt;p&gt;Several people have done flight profiles of reentering boosters, including RTLS - I don't happen to have one handy right now.&lt;/p&gt;&#xA;&lt;p&gt;Edit: According to [these calculations for CRS-10][21] (see &amp;quot;Booster Phasespace&amp;quot;), the booster was at about 60 km altitude and moving about 1250 m/s (about four times the speed of sound) when it encountered the dark tumbling objects. The &lt;a href=&quot;https://en.wikipedia.org/wiki/R%C3%BCppell%27s_vulture&quot; rel=&quot;noreferrer&quot;&gt;greatest altitude a bird was ever encountered at&lt;/a&gt; was 11.3 km, and birds don't move at anywhere near Mach 4.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-22T18:53:49.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20276" PostTypeId="1" AcceptedAnswerId="20277" CreationDate="2017-02-22T20:20:08.410" Score="1" ViewCount="3519" Body="&lt;p&gt;No Shuttle, no Soyuz, and SpaceX not ready for prime time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't get it.&lt;/p&gt;&#xA;" OwnerUserId="18625" LastEditorUserId="58" LastEditDate="2017-02-27T20:23:24.683" LastActivityDate="2017-02-27T20:23:24.683" Title="So with Soyuz being retired, what gets people up to the ISS and back down now?" Tags="|iss|soyuz-spacecraft|human-rating|soyuz-launcher|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20277" PostTypeId="2" ParentId="20276" CreationDate="2017-02-22T20:28:02.680" Score="35" Body="&lt;p&gt;Soyuz the booster and Soyuz the person carrying spacecraft are different. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz the booster is based on the original R-7 ICBM and has seen a series of upgrades. Sometimes to engines, sometimes to computer systems, sometimes to components. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect you are reacting to an article like this one at &lt;a href=&quot;http://spaceflightnow.com/2017/02/22/soyuz-rocket-blasts-off-on-russian-resupply-run-to-international-space-station/&quot; rel=&quot;noreferrer&quot;&gt;SpaceflightNow.com&lt;/a&gt; which is actually a pretty good article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point being made there is that Soyuz the booster, specifically the Soyuz-U version is being retired. To be replaced by the Soyuz booster (There are at least three variants, see &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_(rocket_family)&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Soyuz&lt;/a&gt; for all the details and your head hurting).  Get &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1926837258&quot; rel=&quot;noreferrer&quot;&gt;Anatoly Zak's magnum opus&lt;/a&gt; on the Russian Space program if you care. There is nothing else like it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz spacecraft, on a more modern Soyuz booster will continue carrying crew to the ISS while the Russians maintain participation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US will rely on the SpaceX Dragon 2 launched on a Falcon 9 booster as well as a Boeing CST-100 Starliner launched on a Atlas V booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Dragon and CST-100 are aiming for 2018 launch dates. We shall see if they make it.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="335" LastEditDate="2017-02-23T11:37:36.400" LastActivityDate="2017-02-23T11:37:36.400" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20278" PostTypeId="2" ParentId="20276" CreationDate="2017-02-22T21:15:15.483" Score="5" Body="&lt;p&gt;Soyuz capsule and Progress cargo ship will be launched over Soyuz FG and Soyuz 2.1a boosters - they all from the same R-7 boosters family&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz FG has minor changes of mixing heads comparable to the retired Soyuz-U and Soyuz 2.1a has digital control system and another third stage&lt;/p&gt;&#xA;" OwnerUserId="11046" LastActivityDate="2017-02-22T21:15:15.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20279" PostTypeId="1" AcceptedAnswerId="22298" CreationDate="2017-02-22T21:25:46.423" Score="5" ViewCount="421" Body="&lt;p&gt;The Soyuz series of rockets is famously hardy due to its descent from the R-7 ICBM; the last launch of a Soyuz-U took place &quot;on a launch pad covered in several inches of snow...with a launch time temperature near 15 degrees Fahrenheit / -9 degrees Celsius&quot;. (&lt;a href=&quot;http://spaceflightnow.com/2017/02/22/soyuz-rocket-blasts-off-on-russian-resupply-run-to-international-space-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;) As I understand it, Soyuz 2.1v, is pretty much a clean-sheet redesign of the Soyuz launcher; will it retain the same ability to launch in adverse conditions?&lt;/p&gt;&#xA;" OwnerUserId="13208" LastEditorUserId="58" LastEditDate="2017-02-27T20:22:44.823" LastActivityDate="2017-07-18T05:55:19.187" Title="Is Soyuz-2.1v as hardy as previous Soyuz launchers?" Tags="|rockets|russia|soyuz-launcher|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20280" PostTypeId="2" ParentId="20198" CreationDate="2017-02-22T22:43:56.553" Score="2" Body="&lt;p&gt;The Gemini Program manuals are not searchable via the NASA JSC History or Archive databases, but are currently housed in the National Archives and Records Administration, Fort Worth Regional Archive. The accession number is A-20-64-3. To access these documents, contact the Fort Worth Facility.&lt;/p&gt;&#xA;" OwnerUserId="18628" LastActivityDate="2017-02-22T22:43:56.553" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20281" PostTypeId="1" AcceptedAnswerId="20283" CreationDate="2017-02-22T23:18:02.827" Score="22" ViewCount="26267" Body="&lt;p&gt;According to the NASA Website, the Voyager probes will begin powering down electrical components soon with all electronic functions ceasing some time around 2025.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With no ability to transmit back to earth and thus of 0 value in the Interstellar Medium...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...could either one (or both) of the Voyagers be directed back to Earth at all before electronic shut down?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not; do aerospace engineers have any idea where either (or both) of the probes may end up?&lt;/p&gt;&#xA;" OwnerUserId="18629" LastActivityDate="2017-02-23T15:02:26.080" Title="Can Voyager 1 or 2, theoretically, return to earth if given instructions before their electronic instruments shut down in 2025?" Tags="|voyager|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20282" PostTypeId="2" ParentId="20281" CreationDate="2017-02-22T23:49:20.920" Score="9" Body="&lt;p&gt;Nope. They have small amounts of hydrazine fuel left and have no possible way to slow down and head back. They are traveling very fast (Voyager 1 is at 38,088 mph or 17.027 km/s relative to the sun) and have very little ability to change speed now. The hydrazine is used for attitude control and will outlast their RTG, but it could only change the velocity in a small way.&lt;/p&gt;&#xA;" OwnerUserId="13841" LastEditorDisplayName="user3087" LastEditDate="2017-02-23T15:02:26.080" LastActivityDate="2017-02-23T15:02:26.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20283" PostTypeId="2" ParentId="20281" CreationDate="2017-02-22T23:53:53.023" Score="61" Body="&lt;p&gt;No, not even slightly. Neither Voyager has much fuel left; in fact, they don't have much fuel even for changing their attitude (the way they point), which is orders of magnitude less fuel than for making major course changes, and the course change necessary to return to Earth is so large it would never have been possible in any case: both probes used &lt;a href=&quot;https://space.stackexchange.com/questions/9504/how-can-i-intuitively-understand-gravity-assists&quot;&gt;gravity assists&lt;/a&gt; for the majority of their velocity gain in order to get where they are. The probes would have to have been at least hundreds of times larger than they are to have any chance of returning, meaning we would have to have launched far more into space at one time than has ever been done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We &lt;em&gt;do&lt;/em&gt; &lt;a href=&quot;http://voyager.jpl.nasa.gov/science/hyperbolic.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;know where they're going&lt;/a&gt;, though: &lt;a href=&quot;https://space.stackexchange.com/questions/1621/where-are-pioneer-10-11-and-the-voyagers-ultimately-headed&quot;&gt;nowhere in particular&lt;/a&gt;. Space is a very very big place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if and when they ever get anywhere with intelligent life, they do have one last possible mission. Both probes have diagrams, long-lasting records, and record playing styluses designed to give possible aliens some idea of what we look like, where the probe came from, and what Earth sounds like. It's very unlikely that this will ever be relevant, but it was considered useful enough to justify the extra few hundred grams. (Also, it gave some fodder to e.g. &lt;em&gt;Star Trek&lt;/em&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-02-23T04:38:46.753" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20286" PostTypeId="2" ParentId="20276" CreationDate="2017-02-23T01:11:37.067" Score="1" Body="&lt;p&gt;I believe the Dragon 2 may have a 2017 launch date. Orion is on track for a 2018 launch, and NASA is investigating whether to make the unmanned launch for 2017 a manned launch as well. &lt;/p&gt;&#xA;" OwnerUserId="18632" LastActivityDate="2017-02-23T01:11:37.067" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20287" PostTypeId="1" CreationDate="2017-02-23T01:30:14.813" Score="4" ViewCount="2006" Body="&lt;p&gt;The prevailing theory for how liquid water existed in the distant past of Mars, is that the warmth required was caused by a greenhouse effect caused by high concentrations of CO2 in the atmosphere. This is important because it is believed the Sun was actually weaker in the past than it is today. If high concentrations of CO2 was responsible, there should be high concentrations of carbonate and the Curiosity rover should find be able to find this. So far this has not been the case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This leads me to the question, could Mars have once had an orbit that was much closer to the Sun in the past? What kind of experiment could be done to answer that question? If it did orbit closer, what caused it to move to its current orbit? We do know Mars has received many significant impacts in its past, which may account for the thin atmosphere today. Could an impact give a sufficient nudge to gradually affect the orbit over time?&lt;/p&gt;&#xA;" OwnerUserId="18632" LastActivityDate="2017-02-24T10:20:28.650" Title="Mars and the missing carbonate. Did Mars once orbit closer to the Sun?" Tags="|mars|orbit|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20288" PostTypeId="1" CreationDate="2017-02-23T04:01:59.877" Score="9" ViewCount="490" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/TRAPPIST-1&quot; rel=&quot;noreferrer&quot;&gt;TRAPPIST-1&lt;/a&gt; is described as &quot;Jupiter-sized&quot; (comparable radius), although at 8% of the Sun's mass, must be on the order of 100x greater (Jupiter : Sun is about 1 : 1000). This seems paradoxical and raises several questions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Jupiter is thought to be mostly hydrogen and other light gasses, and as a star, TRAPPIST-1 must be mostly hydrogen (it is fusing hydrogen into helium, isn't it?), how can it be so much hotter and denser at the same time? Shouldn't the high temperature keep the density down?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter has been described as a &quot;failed star&quot; - too small to produce the conditions necessary for nuclear fusion to occur. Does TRAPPIST-1 offer any useful insight as to where that threshold lies - how big is big enough to get the process started? Just how close is Jupiter to that threshold, anyway?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could it just be that some of the facts are wrong, and when corrected, there is really no paradox at all?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="12102" LastEditDate="2017-02-23T07:55:33.353" LastActivityDate="2017-02-23T18:00:11.763" Title="A Jupiter-sized star?" Tags="|jupiter|exoplanet|trappist-1|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20290" PostTypeId="2" ParentId="20288" CreationDate="2017-02-23T05:26:05.160" Score="11" Body="&lt;p&gt;Jupiter is about as large as a planet can get, physically. Suppose we slowly add hydrogen and helium to Jupiter, so slowly so as to keep it at more or less its current temperature. Surprisingly, it would get smaller. Jupiter is about as large as a planet can get. (There are exoplanets that are larger than Jupiter, but that's because they orbit so close to their star that they are very hot.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A very large brown dwarf that is just below the edge of being able to fuse hydrogen and that is as cool as is Jupiter would be smaller in diameter than is Jupiter, despite being about 65 or so times more massive than is Jupiter. A star that is just above the edge of being able to fuse hydrogen would be larger than its just under the edge cousin, but not by that much. The star in question is about 83 times more massive than is Jupiter, which makes it warm enough to be larger than Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for this seemingly paradox is the nature of degenerate matter. Jupiter's core is very slightly degenerate. Adding mass would increase the degeneracy. This is the root of the solution to the paradox. Adding mass to a degenerate object makes the object get smaller.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-02-23T05:26:05.160" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20291" PostTypeId="1" CreationDate="2017-02-23T06:33:11.747" Score="0" ViewCount="43" Body="&lt;p&gt;Has SpaceX published a Mars mission profile, or any detail of what is supposed to happen after people land on Mars? I know that Elon Musk has said that his goal is to ferry people and supplies from Earth to Mars. However, he has also said that his goal is to spark Mars settlement. Does SpaceX have a plan for producing a viable Mars settlement? &lt;/p&gt;&#xA;" OwnerUserId="18638" LastActivityDate="2017-02-23T06:33:11.747" Title="Has SpaceX published a Mars mission profile?" Tags="|spacex|mars|" AnswerCount="0" CommentCount="2" ClosedDate="2017-02-23T16:14:05.303" ContentLicense="CC BY-SA 3.0" />
  <row Id="20292" PostTypeId="1" CreationDate="2017-02-23T07:05:24.030" Score="1" ViewCount="175" Body="&lt;p&gt;NASA has announced the discovery of 7 Earth-like exoplanets 13 hours ago. &lt;strong&gt;What are the chances of life there&lt;/strong&gt;? Is it possible to see the ground of these planets from here? How far are they?&lt;/p&gt;&#xA;" OwnerUserId="18640" LastEditorUserId="6816" LastEditDate="2017-02-23T18:48:55.257" LastActivityDate="2017-02-23T18:48:55.257" Title="What are the chances of life on 7 Earth-like exoplanets discovered by NASA?" Tags="|nasa|exoplanet|trappist-1|" AnswerCount="1" CommentCount="9" ClosedDate="2017-02-23T21:41:43.273" ContentLicense="CC BY-SA 3.0" />
  <row Id="20293" PostTypeId="2" ParentId="20292" CreationDate="2017-02-23T08:32:51.833" Score="3" Body="&lt;p&gt;NASA has said roughly Earth sized. From this, reporters have written &quot;Earth like&quot;. Some publications are even showing what look like photos of planets. These are likely generated with computers used for creating planets in games and films.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for life, we only know of one planet that developed life, and you can't build a statistical model from that. Searching for life could be a complete waste of time.&lt;/p&gt;&#xA;" OwnerUserId="1432" LastActivityDate="2017-02-23T08:32:51.833" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20294" PostTypeId="1" CreationDate="2017-02-23T09:21:12.037" Score="5" ViewCount="470" Body="&lt;p&gt;In last year's question &lt;a href=&quot;https://space.stackexchange.com/q/16926/12102&quot;&gt;Why does the luminous blue shockwave from the RS-25 engine appear to be rectangular?&lt;/a&gt; I showed images of an RS-25 engine test from 2015, and what I believe to be a test of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;nofollow noreferrer&quot;&gt;space shuttle engine&lt;/a&gt; which was much earlier (&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Shuttle_Main_Engine_Test_Firing.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Photo ID: GPN-2000-000543 AND Alternate ID: 81-201-1.&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today I went to look at the new video of the &lt;a href=&quot;https://www.youtube.com/watch?v=u4VRD5cTNE4&quot; rel=&quot;nofollow noreferrer&quot;&gt;RS-25 exhaust rainbow&lt;/a&gt; which was seen during the &quot;&lt;a href=&quot;https://www.nasaspaceflight.com/2017/02/rs-25-test-sls-team-waits-new-controllers/&quot; rel=&quot;nofollow noreferrer&quot;&gt;latest hot-fire test of development engine 0528 at the Stennis Space Center in Mississippi on Wednesday&lt;/a&gt;&quot; (22-Feb-2017) and I still believe that I see the same darn oblique rectangular shape that I saw when asking the last question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here it is as a GIF, and then as the video. The GIF is 30 fps from about 00:55 to 01:02 (frames 1640 to 1860). I still see a &quot;bump&quot;, and from this angle it is about a quarter of the way from the left edge of the shock wave, almost in line with the left vertical railing post.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7C9Bi.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7C9Bi.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is producing this bump??&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The geometry of a cylindrically symmetric engine and exhaust plus the cameras in these three shots should not produce an artifact or optical illusion of an &lt;em&gt;offset bump, clearly to one side of the axis&lt;/em&gt; in all three tests. There is something persistent and reproducible in these tests that creates a bump on one side. What is it?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/u4VRD5cTNE4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1981?:&lt;/strong&gt; (annotation added to GIF)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/14Gls.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/14Gls.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2015:&lt;/strong&gt; (annotation added to GIF)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uHlSI.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uHlSI.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-02-23T10:20:06.850" Title="Why does the shock wave in this new 2017 RS-25 test also have a &quot;bump&quot; on one side?" Tags="|ssme|exhaust|shockwave|" AnswerCount="1" CommentCount="2" ClosedDate="2019-08-09T16:40:49.503" ContentLicense="CC BY-SA 3.0" />
  <row Id="20295" PostTypeId="2" ParentId="20281" CreationDate="2017-02-23T09:25:24.773" Score="8" Body="&lt;p&gt;Return to earth with the remaining fuel is totally impossible. Voyager was started from earth with a very large and heavy rocket tanked with a lot of fuel. Returning to earth would require a rocket of the same or even larger size, but this rocket should be there were Voyager is now. But it is impossible to transport such a gigantic payload like this rocket from earth to the location of voyager. We are not able to build and finance such an incredible large rocket needed for a such a mission with a return ticket. The rocket needed for return should work without problems decades after start from earth. We are not able to build such a complex system that should work almost a century.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-02-23T09:25:24.773" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20296" PostTypeId="2" ParentId="20294" CreationDate="2017-02-23T10:08:28.307" Score="4" Body="&lt;p&gt;First, note that shock diamond formation is dependent on atmospheric pressure. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Shock_diamond&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The distance from the nozzle to the first shock diamond can be approximated by&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;${\displaystyle x=0.67D_{0}{\sqrt {\frac {P_{0}}{P_{1}}}}}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where x is the distance, D0 is the nozzle diameter, P0 is flow pressure, and P1 is atmospheric pressure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Note that at least one side of the test chamber is open to sky, while another is a wall; a vapor plume of some kind is being drawn down in a &lt;em&gt;twisted&lt;/em&gt; path -- I assume it's water vapor condensed out of the atmosphere by cold propellant lines above.&lt;/p&gt;&#xA;&lt;p&gt;This suggests that while the engine and its interior situation may be cylindrically symmetrical, the atmospheric environment it's exhausting into may be significantly &lt;em&gt;asymmetrical&lt;/em&gt;, so it's not surprising that the shock diamond might form slightly earlier on one side of the plume than the other.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-23T10:20:06.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20297" PostTypeId="1" AcceptedAnswerId="20304" CreationDate="2017-02-23T11:19:51.523" Score="1" ViewCount="200" Body="&lt;p&gt;I am not a science guy, So I have couple of questions&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to know what happens to a spacecraft whose mission is to flyby a planet E.G. &lt;strong&gt;Mariner 4&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also &lt;strong&gt;Viking 1&lt;/strong&gt; (lander mission) &lt;strong&gt;is it still on mars&lt;/strong&gt;? &lt;strong&gt;are they still operating it from earth&lt;/strong&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do they operate, are there &lt;strong&gt;already defined algorithms&lt;/strong&gt; by which robot automatically do research &lt;strong&gt;or we do it from there&lt;/strong&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After getting into the space, it does not need to be propelled by engine, right?, but in between if have to change its direction, how can we do that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Changing direction of a spaceship won't work like Air-plane does!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While sending a spacecraft into the space &lt;strong&gt;is it better to send it perpendicular to the earth surface&lt;/strong&gt;(I think the time of resistance should be less).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay last one, have &lt;strong&gt;we ever sent some spaceship&lt;/strong&gt; in to the SUN or &lt;strong&gt;near sun&lt;/strong&gt;(how close man-made spaceship can go to the sun), flyby mission can flyby and go in to the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I also want to know if a spacecraft start revolving on its own axis, can astronauts stop its revolution? &lt;/p&gt;&#xA;" OwnerUserId="18640" LastActivityDate="2017-02-23T13:34:59.120" Title="What happens to a spacecraft in flyby Mission?" Tags="|mars|spacecraft|astronauts|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20300" PostTypeId="1" AcceptedAnswerId="20310" CreationDate="2017-02-23T11:49:49.630" Score="7" ViewCount="916" Body="&lt;p&gt;How had the Saturn V lifting capacity changed throughout the Apollo program? The Apollo 11 mission seems to have been very lightly loaded, enjoying only a short time on the surface, single EVA, and no moon buggy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would longer EVAs, a buggy, and more life support features have been technically possible for the Saturn V to have lifted in July 1969, or did future Saturn V vehicles have higher payload capacities than the one used to send Columbia and Eagle on their way?&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2017-02-23T16:28:16.657" Title="How had the Saturn V lifting capacity changed throughout the Apollo program?" Tags="|saturn-v|" AnswerCount="2" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20301" PostTypeId="2" ParentId="20300" CreationDate="2017-02-23T12:16:56.140" Score="6" Body="&lt;p&gt;The possible payload of Saturn V was carefully planed for the whole mission, but the difference of the payloads of the missions Apollo 11 and 17 was less than 3000 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a nice table on this website here &lt;a href=&quot;https://www.bernd-leitenberger.de/saturn-geschichte.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.bernd-leitenberger.de/saturn-geschichte.shtml&lt;/a&gt; , the page is in german, but the table with the comparison of Apollo 8 to 17 is easy to be found.&lt;br&gt;&#xA;In this table betankt means stage mass with fuel, leer means stage mass without fuel and Nutzlast means payload. All mass values are in kg. The thrust of the rocket engines was equal for all missions with the exception of Apollo 8 with slightly smaller thrust.&lt;br&gt;&#xA;But the propellants mass of stage S II was raised by more than 34000 kg.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-02-23T14:48:15.087" LastActivityDate="2017-02-23T14:48:15.087" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20302" PostTypeId="1" AcceptedAnswerId="20306" CreationDate="2017-02-23T12:57:00.607" Score="25" ViewCount="18922" Body="&lt;p&gt;Over the years, I often heard that we would not be able to build a Saturn V again since its construction plans actually have been destroyed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then again, I heard that there &lt;em&gt;are&lt;/em&gt; copies on microfiche or something, and that the Smithsonian actually has copies of those.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, are the construction plans still available or not? By this I mean plans that are detailed enough that you would be able to rebuild the Saturn V if you had all the tooling and infrastructure (which we don't, but that's another matter).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Sidenote: I know &lt;a href=&quot;https://space.stackexchange.com/questions/6281/why-not-build-saturn-vs-again&quot;&gt;we actually wouldn't &lt;em&gt;want&lt;/em&gt; to build it again&lt;/a&gt; for a variety of reasons. Interestingly, that linked question doesn't seem to touch the topic of the construction plans.)&lt;/p&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2018-09-17T16:17:36.353" Title="Were the Saturn V construction plans destroyed?" Tags="|history|saturn-v|historical-data|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20303" PostTypeId="1" CreationDate="2017-02-23T13:18:59.687" Score="4" ViewCount="93" Body="&lt;p&gt;In reference to the discovery of seven planets around Trappist-1, I'm just wondering if any of these planets are in an orbital resonance and if that is the case, what the implications for habitability on these planets, with reference to Io and Europa around Jupiter, would be? And could such a resonance account for the fact that such a compact system appears to be stable? &lt;/p&gt;&#xA;" OwnerUserId="17423" LastActivityDate="2017-02-23T13:18:59.687" Title="Resonances and the seven sisters recently discovered" Tags="|orbital-mechanics|exoplanet|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20304" PostTypeId="2" ParentId="20297" CreationDate="2017-02-23T13:34:59.120" Score="2" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Depends on the velocity of the spacecraft. Mariner 4 is currently orbiting the sun as it did not have enough velocity to escape its gravitational pull. New Horizon's, which visited Pluto, though, is moving so fast that it will escape our solar system altogether and start orbiting the center of our galaxy.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Yes, it's still there, but it's not operational. The duration over which a lander can operate depends on the energy it has available and the nature of the source providing that energy. Solar power keeps the opportunity rover crawling over Mars, but when the plutonium core in curiosity decays, it will not have the energy necessary to operate. Opportunity could go on for as long as its solar panels manage to avoid getting covered in dust or simply decay. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;It depends. We send commands to the curiosity rover on Mars (hey that looks interesting, tell the rover to head over there) but in the case of New Horizon's the distance to the spacecraft and the time it takes radio signals to travel out there meant that everything it did during the Pluto flyby had to be programmed in advance of the flyby. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Not sure what you are getting at, could you try to explain further?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Noops, not yet. We have spacecraft that observe the sun, but the immense heat and gravity close to the sun mean that as of now we do not have the means to put a spacecraft in a trajectory that will bring it close to the sun and expect it to survive that pass (to understand what I'm getting at, youtube sun grazing comets and have fun watching them get torn to shreds as they swing around or crash into the sun).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Spacecraft can indeed change their rotation using so-called reaction wheels. Reaction wheels on kepler failed a few years ago, which is why it can only observe a limited part of the sky - it can't change its rotation.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="17423" LastActivityDate="2017-02-23T13:34:59.120" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20305" PostTypeId="1" CreationDate="2017-02-23T13:36:11.760" Score="0" ViewCount="1387" Body="&lt;p&gt;Like many laymen I always wonder why scientists only ever talk about life on other planets - and get excited by the prospect - when they find planets that may have water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anyone suggested that other forms of life - like electrical impulse based 'things' might have a form of intelligence and reproductive capability?&lt;/p&gt;&#xA;" OwnerUserId="18654" LastEditorUserId="58" LastEditDate="2017-02-23T13:40:57.067" LastActivityDate="2017-02-23T14:00:09.343" Title="Could 'life' exist on other planets without water - as electrical impulses, for example?" Tags="|life|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20306" PostTypeId="2" ParentId="20302" CreationDate="2017-02-23T13:41:17.260" Score="30" Body="&lt;p&gt;&lt;a href=&quot;http://web.archive.org/web/20100818173517/http://www.space.com/news/spacehistory/saturn_five_000313.html&quot; rel=&quot;noreferrer&quot;&gt;No, the blueprints were not destroyed or lost&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;(This is) a claim John Lewis made in his 1996 book, Mining the Sky, that he went looking for the Saturn 5 blueprints a few years ago and concluded, incredibly, they had been &amp;quot;lost.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Paul Shawcross, from NASA's Office of Inspector General, came to the agency's defense in comments published on CCNet -- a scholarly electronic newsletter covering the threat of asteroids and comets. Shawcross said the Saturn 5 blueprints are held at the Marshall Space Flight Center on microfilm.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;The Federal Archives in East Point, Georgia, also has 2,900 cubic feet of Saturn documents,&amp;quot; he said. &amp;quot;Rocketdyne has in its archives dozens of volumes from its Knowledge Retention Program. This effort was initiated in the late '60s to document every facet of F 1 and J 2 engine production to assist in any future restart.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Shawcross cautioned that rebuilding a Saturn 5 would require more than good blueprints.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;The problem in recreating the Saturn 5 is not finding the drawings, it is finding vendors who can supply mid-1960's vintage hardware,&amp;quot; he wrote, &amp;quot;and the fact that the launch pads and vehicle assembly buildings have been converted to space shuttle use, so you have no place to launch from.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-23T13:41:17.260" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20308" PostTypeId="2" ParentId="20287" CreationDate="2017-02-23T14:00:07.213" Score="0" Body="&lt;p&gt;If there is something the past 20 years of exo planet research has taught us, it is that solar systems, over time periods we humans cannot readily comprehend, are not the clockworks the ancients envisioned them to be, but that planets migrate, sometimes get booted out of the solar system altogether and other times they collide with the sun. So it could very well be the case that Mars's orbit changed in the early years of the solar system, but if it did, it would have been because of the migrations of Jupiter and Saturn, and not because it got knocked into a new orbit by a collision. I say that because the amount of energy it would take to alter the orbit of Mars by flinging a rock at it would be immense. While I'm pulling a rabbit out of a hat now, I don't even think flinging every asteroid in the asteroid belt at Mars would significantly alter its orbit. As for testing this hypothesis, I guess one could measure the isotopes found in Martian rocks with those found in other locations of the solar system to get an idea of where it formed (this is how we know the Moon was created through a collision, because if the object had been captured its set of isotopes would not be identical to those on Earth).&lt;/p&gt;&#xA;" OwnerUserId="17423" LastActivityDate="2017-02-23T14:00:07.213" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20309" PostTypeId="2" ParentId="20305" CreationDate="2017-02-23T14:00:09.343" Score="4" Body="&lt;p&gt;To grossly oversimplify, astrobiologists generally consider three broad prerequisites for life:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a liquid solvent (such as water)&lt;/li&gt;&#xA;&lt;li&gt;chemical building blocks (such as organic molecules)&lt;/li&gt;&#xA;&lt;li&gt;an energy source (such as sunlight, directly as in plants or indirectly as in animals who eat plants or eat other animals that eat plants)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Because water-based, carbon-based life fueled by stellar energy is what we know most about, this is generally what we look for. However, scientists do consider other configurations:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Though more exotic solvents (like ammonia, liquid nitrogen, or supercritical carbon dioxide) may exist in extrasolar systems, the only surface liquids outside of Earth that we know about today occur on Saturn’s smoggy moon Titan.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Titan's seas are made of liquid methane and ethane, which is considered as a possible alternative solvent for life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we've never found life that uses a different solvent or building blocks (even on Earth), it is mostly hypothetical, and so we are limited in how much we can search for them. If we assumed that some sort of electromagnetic-based life existed, how would we go about looking for it? What are the signs that such life is present? We don't even know if such life is present on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Reference:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://nai.nasa.gov/seminars/other-seminar-series/university-of-washington-seminars/2014/10/14/life-jim-but-not-as-we-know-it-prospects-for-life-in-titans-hydrocarbon-seas/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Life, Jim, but Not as We Know It: Prospects for Life in Titan's Hydrocarbon Seas - NASA Astrobiology Institute&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-02-23T14:00:09.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20310" PostTypeId="2" ParentId="20300" CreationDate="2017-02-23T14:10:48.993" Score="7" Body="&lt;p&gt;Small changes were made to the Saturn V launcher over the course of the program -- for example, redundant ullage rockets were deleted, reducing the mass of the interstage between first and second stages &lt;a href=&quot;https://history.nasa.gov/SP-4029.pdf&quot; rel=&quot;noreferrer&quot;&gt;from 12,436 lbs on Apollo 8 to 9975 lbs on Apollo 17&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Uwe notes, the propellant loading on the second stage is the most obvious difference flight-to-flight; the stage would be loaded with enough fuel to perform the mission, plus a safety margin, but no more. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a small revision to the fuel injectors in the first-stage engines that was intended to improve performance, flown on Apollo 15-17, but the difference was a fraction of a percent, in fact less than the variation between individual engines on a single rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Up top, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module#Extended_J-class_missions&quot; rel=&quot;noreferrer&quot;&gt;descent engine nozzle of the lunar module was extended slightly on the later missions&lt;/a&gt;, which yielded slightly more thrust, and the fuel tanks were enlarged. On one mission, the longer nozzle was actually damaged when it touched down on uneven ground, but this was inconsequential, as the descent stage was done with its job at that point. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-02-23T16:28:16.657" LastActivityDate="2017-02-23T16:28:16.657" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20311" PostTypeId="1" CreationDate="2017-02-23T14:38:29.243" Score="1" ViewCount="164" Body="&lt;p&gt;We have some spacecraft that have extended missions outside of the solar system (ie voyager), but are there any plans for new spacecraft that will leave our solar system either by a primary or extended mission?&lt;/p&gt;&#xA;" OwnerUserId="3141" LastActivityDate="2017-02-24T09:05:57.520" Title="Are there any planned missions for spacecraft leaving the solar system?" Tags="|future-missions|solar-system|" AnswerCount="1" CommentCount="11" ClosedDate="2017-02-24T20:07:19.897" ContentLicense="CC BY-SA 3.0" />
  <row Id="20313" PostTypeId="1" AcceptedAnswerId="20314" CreationDate="2017-02-23T16:25:37.597" Score="7" ViewCount="1184" Body="&lt;p&gt;SpaceX just used 39A for CRS-10. Do they have any plans to repair SLC-40 from the September anomaly? The Boca Chica launch center should be ready in 2018, so will they solely use 39A and Boca Chica?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorDisplayName="user8263" LastEditDate="2017-02-24T06:28:47.147" LastActivityDate="2017-02-24T06:28:47.147" Title="Does SpaceX plan to repair SLC-40?" Tags="|spacex|falcon-9|launchpad|falcon-heavy|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20314" PostTypeId="2" ParentId="20313" CreationDate="2017-02-23T16:32:48.903" Score="10" Body="&lt;p&gt;On the prelaunch press conference about LC-39A Gwynne Shotwell (SpaceX COO) said that they plan on repairing the SLC-40 pad now, hoping to finish it in summer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Youtube: &lt;a href=&quot;https://www.youtube.com/watch?v=xjXYSJF-7Cs&amp;amp;index=36&amp;amp;list=PL2aBZuCeDwlT56jTrxQ3FExn-dtchIwsZ&quot; rel=&quot;noreferrer&quot;&gt;NASA Holds Pre-launch Briefing at Historic Pad 39A at Kennedy Space Center&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Preparations for that [Falcon Heavy debut] launch will begin once repairs to SLC-40 are&#xA;  completed, which Shotwell said should be in June, allowing Falcon 9&#xA;  launches to resume there.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacenews.com/falcon-9-lifts-off-on-first-mission-from-kennedy-space-center-pad/#sthash.r5GX2BWF.dpuf&quot; rel=&quot;noreferrer&quot;&gt;SpaceNews&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IIRC &lt;em&gt;June&lt;/em&gt; in the article is not exact, &lt;em&gt;Summer&lt;/em&gt; was used without specifying a month.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="10829" LastEditDate="2017-02-23T17:01:00.450" LastActivityDate="2017-02-23T17:01:00.450" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20315" PostTypeId="2" ParentId="741" CreationDate="2017-02-23T16:47:25.460" Score="2" Body="&lt;p&gt;Very generally speaking, a reaction engine works by throwing energy out the back to get motion towards the front. Now, according to high-school physics, the energy we get for throwing a chunk of mass (Propellant, in our case) is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;Ke = {MV^2 \over 2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the faster we throw out the mass, the more energy per-unit-mass we get. In rocketry, this is called Exhaust Velocity($Ve$), with a derived quantity called Specific Impulse ($Isp$). Higher velocities equate to better efficiency, up to the absolute maximum velocity of $c$, at which point high-school maths break down and the equation starts looking more like&#xA;$$&#xA;Ke = {MC^2 \over sqrt(1-(V/C)^2) } - MC^2&#xA;$$&#xA;Notice how I said &lt;em&gt;Energy&lt;/em&gt; instead of &lt;em&gt;Mass.&lt;/em&gt; Photons move at the absolute maximum velocity the universe will allow, and are therefore the perfect propellant for a rocket. Technically speaking, a simple flashlight is the absolute apex of rocket technology.... Except for one tiny issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While photons pack the maximum possible kinetic energy per unit mass (&lt;em&gt;How photons have kinetic energy without having mass is beyond me, but they do. Let's just  call it mass for simplicity&lt;/em&gt;), their mass is immeasurably tiny. Your flashlight might be the absolute apex of &lt;em&gt;Efficiency,&lt;/em&gt; but its actual thrust is practically nothing. It would take years for you to notice your flashlight had moved at all, which brings us to another issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it would take years for your flashlight rocket to start moving, it would take mere hours for the batteries to die. To give our photon rocket the lifespan and power to do anything at all, we'd have to use a small nuclear powerplant to fuel it. With all that extra mass, our already tiny acceleration is squashed by hundreds of tons of reactor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Technologies like Ion-drives and VASMIR face similar limitations. In order to deposit enough energy into their reaction mass to be &lt;em&gt;effecient&lt;/em&gt;, they have to cut down the propellant flow to a tiny trickle of what it could be, meaning they have very, very low &lt;em&gt;thrust&lt;/em&gt;. In addition to that, they also require large amounts of electrical current, meaning they face the same issue as our nuclear-powered flashlight rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking all that into consideration, the holy-grail of rocketry would be a high thrust, high efficiency engine. There's only a few currently-theoretical contenders for the title, such as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zurbin NSWR&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Orion&lt;/a&gt;. Most if not all of them have some fairly serious drawbacks, and because one of them involves &lt;strong&gt;&lt;em&gt;Using nuclear weapons for propulsion&lt;/em&gt;&lt;/strong&gt; it's unlikely to receive funding any time soon.&lt;/p&gt;&#xA;" OwnerUserId="16440" LastActivityDate="2017-02-23T16:47:25.460" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20316" PostTypeId="1" AcceptedAnswerId="20332" CreationDate="2017-02-23T16:49:07.833" Score="29" ViewCount="4248" Body="&lt;p&gt;We have all heard about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster&quot; rel=&quot;noreferrer&quot;&gt;Challenger disaster&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reading the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster#Vehicle_breakup&quot; rel=&quot;noreferrer&quot;&gt;Vehicle breakup&lt;/a&gt; subsection, this sentence made me very curious:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The external tank at this point suffered a complete structural failure, the LH2 and LOX tanks rupturing, mixing, and igniting, creating a fireball that enveloped the whole stack.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I know it is not the explosion which destroyed the whole vehicle but aerodynamic forces, but I wonder:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How much liquid fuel and oxidizer were left in the external tank at the time of its destruction?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Bonus question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How many seconds were left before SRB separation?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I couldn't find any information, so thank you for your answers!&lt;/p&gt;&#xA;" OwnerUserId="18592" LastEditorUserId="6944" LastEditDate="2020-09-01T17:43:00.390" LastActivityDate="2020-09-01T17:43:00.390" Title="Challenger disaster: how full was the external tank at the time of destruction?" Tags="|space-shuttle|failure|external-tank|shuttle-challenger|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20317" PostTypeId="2" ParentId="20313" CreationDate="2017-02-23T16:54:39.933" Score="6" Body="&lt;p&gt;SpaceX started with a pad at Vandenberg for the Falcon 1, but got kicked out of it because they were considered a risk for a Delta pad nearby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then they opened up at Kwajjelin Atoll. They launched the 5 Falcon 1 launch attempts from there&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They opened SLC-40 at the CCAFS and launched most of their launches (minus 3 at Vandenberg, and one at LC-39A as of this writing) from there.  The AMOS-6 fueling incident did sufficient damage that they switched focus to the LC-39A to buy time to fix LC-40.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan going forward is to launch Polar missions from Vandenberg, Falcon 9 and Falcon Heavy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initially NASA, government, and Manned flights will launch from LC-39A. This is Falcon 9 and Falcon Heavy capable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LC-40 will remain for Falcon 9 flights. (The plan for Falcon Heavy launches from LC-40 essentially means building an entire second pad, on the edge of the current pad and seems unlikely to happen).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Boca Chica, Texas can only launch to a narrow set of orbits due to landmasses underneath the flight path. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So LC-40 will be rebuilt and used. LC-39A will be used. SLC-4E will be used for polar missions. Boca Chica will be used once opened.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX clearly believes they have sufficient demand for all these launch pads. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-02-23T16:54:39.933" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20318" PostTypeId="2" ParentId="20316" CreationDate="2017-02-23T17:05:17.990" Score="26" Body="&lt;p&gt;The breakup of Challenger occurred about 73 seconds into flight. Main engine cutoff normally occurs about 510 seconds into flight, implying that about 86% of the fuel would be remaining. (Many sources give 480 seconds, but I suspect that's a simple division of the tankage mass by the full-throttle consumption rate; looking at actual mission reports supports the ~510 second main engine burn figure.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the throttle schedule of the main engines isn't constant, so it would have been somewhat more than that -- the accident came shortly after the end of the max-Q &quot;throttle bucket&quot;, during which time less fuel would be used, but there's also a scheduled throttle-down toward the end of the main engine burn which partially cancels that out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, at that point, the tank would be holding something more than 540 tons of liquid oxygen and 91 tons of liquid hydrogen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The boosters should burn for about 127 seconds, so they were more than halfway there, with 54 seconds remaining. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-02-23T17:58:00.080" LastActivityDate="2017-02-23T17:58:00.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20319" PostTypeId="2" ParentId="20316" CreationDate="2017-02-23T17:07:17.097" Score="8" Body="&lt;p&gt;A few facts:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SRB Burn Time is 127 seconds&lt;/li&gt;&#xA;&lt;li&gt;Start of Challenger Incident- 64 s&lt;/li&gt;&#xA;&lt;li&gt;Vehicle breakup- 72 s&lt;/li&gt;&#xA;&lt;li&gt;Nominal time to orbit- 510 s.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So the SRBs were about half-way done with their burn time before the vehicle started to break up. The Space Shuttle Main Engine actually produced less than 1 g thrust until about the time of SRB separation. If the SRBs could have somehow been safely released, it is possible the Space Shuttle could have turned around and returned to land at Kennedy Space Center, but the SRBs could not have have been released. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the Main Tank had the vast majority of it's fuel. For much of the time with the SRBs attached, the Main Engine was not throttled at 100%, to not hit too high of resistance. I don't have an exact number, but I would expect that greater than 90% of the fuel in the tank was still there at the time of vehicle breakup.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-02-24T02:00:23.543" LastActivityDate="2017-02-24T02:00:23.543" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20320" PostTypeId="1" AcceptedAnswerId="20323" CreationDate="2017-02-23T17:11:15.837" Score="3" ViewCount="201" Body="&lt;p&gt;Apologies for the long question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With all the speculation surrounding the newly-discovered Earth-sized planets a &quot;mere&quot; 40 ly distant, I'm revisiting earlier musings on ways to send interstellar probes on a timescale that's useful (e.g. less than thousands of years).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we were going to use some sort of linear accelerator or rail gun (I don't understand the differences well enough to know which is best applied) to &quot;shoot&quot; a sub-kilogram scale probe at another solar system, how do you go about calculating the &quot;muzzle velocity&quot; achieved using factors for energy input and length?  I suppose the &quot;sub-kilogram&quot; part isn't that relevant, except to set the scope of the accelerated mass variable, but I wanted to establish the limits around what I'm trying to discover.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Associated questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;For the amount of energy it would take, do we have a good way to use it in orbit?  For instance, if we put a hectare of solar panels in orbit, and use them to charge an ultracapacitor, is that sufficient technology to power our accelerator?&lt;/li&gt;&#xA;&lt;li&gt;Where is a good place to locate such an accelerator?&lt;/li&gt;&#xA;&lt;li&gt;How much acceleration force (gees) is going to be applied to the probe for a given length of &quot;barrel&quot; and &quot;muzzle velocity&quot;?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="14104" LastActivityDate="2017-02-23T18:18:58.660" Title="How to go about calculating the size and energy requirements for a space-based linear accelerator or railgun to accelerate small probes?" Tags="|propulsion|probe|interstellar-travel|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20321" PostTypeId="1" AcceptedAnswerId="20326" CreationDate="2017-02-23T17:12:48.543" Score="2" ViewCount="141" Body="&lt;p&gt;An earlier question asked whether there were any planned missions outside the solar system, which made me think of this question. What would the scientific benefits be in sending a probe outside the solar system? By this I do not mean to other systems, but to explore near the outside of the solar system. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2017-02-23T22:36:01.607" LastActivityDate="2017-02-23T22:36:01.607" Title="What science could be performed by an extra-solar probe?" Tags="|probe|interplanetary|solar-system|instrument|science|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20323" PostTypeId="2" ParentId="20320" CreationDate="2017-02-23T18:18:58.660" Score="4" Body="&lt;p&gt;Start with basic physics. Force = mass times acceleration (F=ma). I find it easiest to start with 1 kg, then scale up or down as necessary. What velocity do you want to achieve? Final velocity is initial velocity plus acceleration times time (v1 = v0 + at). Convert forty light-years into meters (easy to do using online calculators). 40 ly = 3.784292e+17 m. I like to round off to an easy unit, between 3 x 10^17 to 4 x 10^17. Let's take 4 x 10^17, because it will divide nicely with the next value.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time is distance divided by velocity. You want to reach the system in less than 1000 years. Convert years into seconds. Again, that's easy to do with a Web search tool. 1000 years = 3.1556926 x 10^10 seconds. Say 4 x 10^10 seconds for lower velocity (because of more time). So, velocity is 4 x 10^17 / 4 x 10^10, that is, you need to reach a velocity of 1 x 10^7 meters/sec, or 10 x 10^6 m/s in engineering form. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say that you build an accelerator that is 10,000 km long, or 10 x 10^6 meters. Average acceleration is change in velocity over two times the distance, (v1^2 - v0^2)/2s . So, (10 x 10^6 m/s) / 2 x 10 x 10^6 m = 5 x 10^6 m/s^2 . That is about 49 million g. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point, you can see that your accelerator--whether rail gun, coil gun or blow dart--is going have to be much longer than 10 million meters if you want normal materials to survive. If acceleration were at least 1000 times longer, some macroscopic objects might survive. We need a (longer-than) 10 million kilometer accelerator. That's why Stephen Hawking is talking about using laser beams shot into space; he doesn't want to have to build a multi-million kilometer-long tube into space. Theoretical physicists don't make that much money.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration is the change in velocity over the change in time. Rearranging, we find that time is 10 x 10^6 m/s / 5 x 10^6 m/s^2 = 2 seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kinetic energy is 0.5 mass x velocity^2 , so, 5 x 10^12 Joules . Power is the energy divided by time, so 2.5 x 10^12 Joules/sec . Let's say 2x10^12 J/s, or 2 TW. That's a lot of solar panels. If sunlight delivers 1500 Watts/m^2, and our solar panels are 30% efficient, we would need more than 4x10^9 m^2 of solar panels to provide that much power in that much time. Of course, we could use a capacitor and a smaller array. &lt;/p&gt;&#xA;" OwnerUserId="18638" LastActivityDate="2017-02-23T18:18:58.660" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20324" PostTypeId="2" ParentId="20321" CreationDate="2017-02-23T18:53:31.750" Score="1" Body="&lt;p&gt;First, start with the question &quot;how do I convince an organization like the U.S. Congress to allocate funds for the mission?&quot; In other words, &quot;what tangible benefit might I receive from this mission or a future mission for which this one will prepare us?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When talking about interstellar missions, the answer is simple: some day we may need to move to a new home. Where will we go? How will we get there? What would the journey entail?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before sending humans anywhere, it is cheaper and more effective to send probes so we can build a better manned spacecraft. What questions might we need answered before we can send humans out of the solar system?&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What, exactly, is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_medium&quot; rel=&quot;nofollow noreferrer&quot;&gt;interstellar medium&lt;/a&gt; made of and how dense is it?&lt;/li&gt;&#xA;&lt;li&gt;How much ionizing radiation is there outside of the Sun's sphere of influence? Does the Sun have a &lt;a href=&quot;https://en.wikipedia.org/wiki/Heliosphere#Bow_shock&quot; rel=&quot;nofollow noreferrer&quot;&gt;bow shock&lt;/a&gt;? If so, what changes after the boundary? If not, is there some region where harmful radiation changes?&lt;/li&gt;&#xA;&lt;li&gt;How difficult is it to communicate between Earth and a spacecraft in deep space?&lt;/li&gt;&#xA;&lt;li&gt;Is [insert propulsion technology X] viable for propelling a spacecraft to another solar system in a timely manner? Note that &quot;timely&quot; may still be many decades or centuries, e.g. using a &lt;a href=&quot;https://en.wikipedia.org/wiki/Generation_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;generation ship&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;We can always perform calculations and create models: however, we can likely get better results through direct measurements. Perhaps our models are wrong: direct observation may allow us to create more accurate models. This is how science works. All of the ideas I presented above involve models that we should refine and prove to be accurate before we send humans out to the stars. Otherwise, a manned journey may end poorly for those first extra-solar astronauts.&lt;/p&gt;&#xA;" OwnerDisplayName="user5932" LastActivityDate="2017-02-23T18:53:31.750" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20325" PostTypeId="1" AcceptedAnswerId="20331" CreationDate="2017-02-23T19:00:30.137" Score="15" ViewCount="9081" Body="&lt;p&gt;I know it would be awfully weak, but it's present even where all other power sources would be unavailable - could cosmic level radiation be used to power minimal set of systems of a probe, say, a clock to &quot;wake it up&quot; after reaching destination (in a couple hundred years) and activate more efficient energy sources &quot;hibernated&quot; for duration of the travel? What orders of magnitude of power input could be expected, per some unit (mass, surface?) of the &quot;harvesting&quot; apparatus?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, it appears the cosmic background - specifically, CMB - is &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_microwave_background#Primary_anisotropy&quot; rel=&quot;noreferrer&quot;&gt;not uniform&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cIWyL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cIWyL.jpg&quot; alt=&quot;heat map of CMB&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The variance is only ±0.00057 K but the features (hot and cold spots) seem to remain mostly persistent over time, so the device, while needing to pivot to maintain orientation relative to CMB, doesn't have to reconfigure 'absorber' and 'radiator' layout on the fly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question remains, how can one use such tiny variance, especially with total input of 400–500 photons/cm$^3$ - and how much (...how little) energy could be gained that way?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As clarification what I'm asking for - &lt;em&gt;any&lt;/em&gt; external energy source; it doesn't need to be CMB specifically - that is available during intergalactic travel,   where you can't depend on solar wind, light of nearby stars or similar; you must depend on what's available there: thermal cosmic background radiation, cosmic microwave background, high-energy cosmic rays, whatever means are available in intergalactic space, other than what you brought with the craft.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-02-27T09:55:12.153" LastActivityDate="2021-10-14T22:32:21.837" Title="Can cosmic background be harvested for energy?" Tags="|power|cosmic-radiation|" AnswerCount="6" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20326" PostTypeId="2" ParentId="20321" CreationDate="2017-02-23T20:01:25.900" Score="2" Body="&lt;p&gt;This depends very much on how extrasolar the mission is.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With current technology, we can just about send a probe to the edge of our solar system. This allows us to measure the interstellar medium. We've done this with the Voyagers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With enough investment, we could send a probe to the Oort cloud and see what it's made of.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With future technology, we may be able to visit another nearby star, and examine its planetary system. That would broaden our understanding of how stars and planets are formed. It would also enable us to observe distant stars from two vantage points, enabling things like parallax measurements we can't do now.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With more exotic technology, we could travel to distant phenomena like black holes and pulsars and observe them up close. We could look at the sector of sky that's currently obscured by the Milky Way, and see what the Great Attractor consists of.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's before we've found habitable planets or even life. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-23T20:01:25.900" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20327" PostTypeId="2" ParentId="20325" CreationDate="2017-02-23T20:52:00.473" Score="5" Body="&lt;p&gt;You probably want to harvest &lt;em&gt;usable energy&lt;/em&gt;, a physicist would rather call that &lt;em&gt;low entropy&lt;/em&gt;. Harvesting &lt;em&gt;energy&lt;/em&gt; would mean you heat up indefinitely. Accepting that thermodynamics drives deterministic technical processes (as well as life on earth) is is the key to understanding this problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of your field of view, the cosmic microwave background, is almost perfect black body radiation with a temperature of 3.7K. Once your spacecraft completely cools to this point, your entropy balance will be even.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can however use small fluctuations in the spatial distribution to harvest usable energy by exposing the &quot;hot-end&quot; of a harvesting apparatus to the hotter-than-average parts of the sky, the &quot;cold-end&quot; to the other parts. A temperature gradient would build up and can be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any electromagnetic radiation that deviates from a planck spectrum can also be used by reradiating it as thermal radiation with higher entropy (solar panels do exactly that, if you think about it). Visible light and quasar radiation would be a good source. Unfortunately, point sources and sources of narrowband radiation only take up a very small part of your field of view. Quasars are rare and power decays with inverse square law, so visible light from stars may be easier to obtain. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can a clock oscillator be powered from solar panels in outer space? Depends on surface area. Low entropy reservoirs, like RTGs, are far easier solutions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(some of the comments below may refer to the previous version of my answer , which was quite different)&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17004" LastEditDate="2017-02-23T23:56:37.793" LastActivityDate="2017-02-23T23:56:37.793" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20328" PostTypeId="2" ParentId="12495" CreationDate="2017-02-23T21:09:26.457" Score="3" Body="&lt;p&gt;Neutron flux created by cosmic rays would be small and would actually make control of the reactor slightly easier in terms of neutron economy.   The more non-prompt neutrons are involved in a reactor, the farther you can get from prompt criticality while maintaining the reaction, which makes safety easier.   This slight benefit would be outweighed by the risk of electronics failing due to cosmic rays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spallation by high energy protons is commonly used to produce neutrons.  This paper (&lt;a href=&quot;http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/23/015/23015552.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/23/015/23015552.pdf&lt;/a&gt;) has a handy graph of proton energy versus the number of neutrons produced on a tungsten target.  It's almost linear, with about 20 neutrons per GeV.  This will vary a bit depending on the target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most cosmic rays are protons around 1GeV.(&lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_ray&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Cosmic_ray&lt;/a&gt;).  Higher energy cosmic rays exist, but above 1GeV the flux falls off very rapidly, so higher energy cosmic rays contribute a negligible amount of the total energy.  So let's approximate the cosmic ray flux as 10^4 1GeV protons per square meter per second.  Let's suppose your space reactor is large enough to get hit by 10^5 cosmic rays per second that produce an average of 20 neutrons each.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each neutron causes an extra ~0.5 fissions every 10^-4 seconds (because your reactor is thermal and delayed-critical).  Each fission is around 200MeV, so each neutron is worth 100MeV every 10^-4 seconds or 10^6 MeV/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Final calculation:&#xA;10^5 protons/s * 20 neutrons/proton * 10^6MeV/s/neutron =  2*10^12 MeV/s^2 = 0.32 watts/s.   So because of cosmic rays the reactor power level would increase by approximately 0.32 watts/s unless you lowered the control rods slightly to compensate (and thus got a bit farther away from prompt-criticality, which is good for safety)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If cosmic ray flux increased by a billionfold due to flying too close to a supernova, then it might be enough to cause a meltdown, but in that scenario the meltdown is probably the least of your problems.&lt;/p&gt;&#xA;" OwnerUserId="18666" LastEditorUserId="18666" LastEditDate="2017-02-24T10:10:31.667" LastActivityDate="2017-02-24T10:10:31.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20329" PostTypeId="1" AcceptedAnswerId="20333" CreationDate="2017-02-24T00:38:59.490" Score="6" ViewCount="2063" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; Cropped screen capture from &lt;a href=&quot;https://www.youtube.com/watch?v=SHSnOPZAIDI&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX Demo-1 Crew Dragon Docking to ISS - Part 2&lt;/a&gt; after about &lt;code&gt;00:25:00&lt;/code&gt; in the video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jRB0h.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jRB0h.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I am curious &lt;em&gt;if guidelines for the placement of red/green navigation lights on spacecraft been established&lt;/em&gt;. In maritime and most aviation situations, right/left have meaning because up/down does.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;A highly related question would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean that the grapple fixture on Dragon is on the &quot;bottom&quot;? Is this the &lt;em&gt;negative Z direction&lt;/em&gt; of this spacecraft's coordinate system?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bs7GZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bs7GZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From Spaceflight Now's &lt;a href=&quot;http://spaceflightnow.com/2017/02/23/spacex-supply-ship-completes-journey-to-space-station/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX supply ship completes journey to space station&lt;/a&gt; (cropped). Caption: The Dragon spacecraft’s navigation lights illuminate the cargo ship during a nighttime orbital pass. Credit: Thomas Pesquet/ESA/NASA&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;For background on the use of red/green navigation lights in other craft:&lt;/h2&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k8smt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k8smt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;https://i.stack.imgur.com/xHt3p.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;this answer&lt;/a&gt; to the Quora question &lt;a href=&quot;https://www.quora.com/Why-is-the-right-navigation-light-of-an-aircraft-green-and-the-left-one-red&quot; rel=&quot;nofollow noreferrer&quot;&gt;Why is the right navigation light of an aircraft green and the left one red?&lt;/a&gt; There are multiple answers shown there.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-03-04T04:55:27.680" LastActivityDate="2019-03-04T04:55:27.680" Title="Have guidelines for red &amp; green navigation light placement on spacecraft been established?" Tags="|communication|navigation|dragon|crs-10|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20330" PostTypeId="1" AcceptedAnswerId="20402" CreationDate="2017-02-24T00:51:30.977" Score="8" ViewCount="381" Body="&lt;p&gt;After one halted attempt at docking with the ISS, the CRS-10 Dragon capsule &lt;a href=&quot;http://spaceflightnow.com/2017/02/23/spacex-supply-ship-completes-journey-to-space-station/&quot; rel=&quot;noreferrer&quot;&gt;successfully completed&lt;/a&gt; the maneuver. In the time between the two attempts, Dragon must have maintained some kind of orbit with some relationship to ISS's orbit, to make a time for the next attempt available and yet to remain a safe distance away from the ISS without need of active control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What orbit did CRS-10 maintain during the wait for the 2nd attempt, and how did it move relative to the ISS during that time?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-02-28T14:39:40.040" Title="Where/how did the CRS-10 Dragon capsule &quot;wait&quot; before the 2nd, successful docking attempt?" Tags="|iss|dragon|crs-10|" AnswerCount="3" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="20331" PostTypeId="2" ParentId="20325" CreationDate="2017-02-24T01:28:37.433" Score="10" Body="&lt;p&gt;I'm guessing you mean &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_microwave_background&quot; rel=&quot;noreferrer&quot;&gt;cosmic &lt;em&gt;microwave&lt;/em&gt; background&lt;/a&gt; radiation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A radio tuned to an empty frequency hears &quot;static&quot; noise. A small fraction of the static received by an analog television is in fact the CMB. Could an antenna system collect some of this, and with a diode or something similar, convert some of it to a small amount of useful electricity? CMB detectors are usually bolometers - antennas + resistors + temperature sensors, but in this case they could be diodes, as &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law&quot; rel=&quot;noreferrer&quot;&gt;you've pointed out here&lt;/a&gt;, which can provide some DC power from the original AC.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;tl;dr: nope!&lt;/strong&gt; Thermodynamics always wins, unless your spacecraft and electronics are below 2.7 K.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let's see what we lost:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get a rough idea of the amount of power potentially available, without considering the actual functioning circuit which would need to be fairly broadband to make efficient use of the spectrum, here's a quick calculation of the &lt;s&gt;power available&lt;/s&gt; incident microwave power. &lt;strong&gt;It is only an upper limit&lt;/strong&gt; to what would be possible to collect.:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of integrating the thermal distribution, let's just use the full width of 200GHz and a heigh of 400 MJy/sr (see below). The data is available &lt;a href=&quot;https://lambda.gsfc.nasa.gov/product/cobe/firas_monopole_get.cfm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;MJy is 1E+06 Jansy. &lt;a href=&quot;https://lambda.gsfc.nasa.gov/product/cobe/firas_monopole_get.cfm&quot; rel=&quot;noreferrer&quot;&gt;One Jansky&lt;/a&gt; is 1E-26 Watts per square meter per Hertz.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say the antenna has some directionality (they always do unless they play tricks with polarization) and call the acceptance 1/2 of a sphere, or 2$\pi$ sr. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;400E+06 Jy/sr * 2$\pi$ sr * 200E+09 Hz * 1E-26 W/m^2 ~ 1E-06 W/m^2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So that's 1 micro Watt per square meter possible&lt;/strong&gt;. Your milage may vary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To double check that, use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law&quot; rel=&quot;noreferrer&quot;&gt;Stephan-Boltzman equation&lt;/a&gt; $P=\sigma T^4$. With $\sigma$=5.67E-08 watts per square meter per degree Kelvin^4, that gives 3E-06 W/m^2. Remembering the factor 0.5 for the antenna acceptance, it works out nicely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But can a hot (say 273 K) crystal radio (diode rectification) actually extract power from a cold thermal distribution? No, heat flows from hot to cold unless you are using energy to drive a heat pump. From a radio point of view, random electrical fluctuations in your radio's electronics will also be broadcasting microwaves back into the cosmos. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So you'd have to cool your radio down below 2.7K to get any power&lt;/strong&gt;. And at that point, you've just built a thermal collector and a cold, radio-black plate could do roughly as well as the radio. &lt;strong&gt;But you wouldn't be gaining energy because you'd be doing more work cooling it&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in a word, &lt;strong&gt;nope!&lt;/strong&gt; Thermodynamics always wins, unless your spacecraft and electronics are below 2.7 K.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BpvRC.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BpvRC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Spectrum of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_microwave_background&quot; rel=&quot;noreferrer&quot;&gt;CMB&lt;/a&gt; from &lt;a href=&quot;https://i.stack.imgur.com/BpvRC.png&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5WRu7.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5WRu7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Spectrum of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_microwave_background&quot; rel=&quot;noreferrer&quot;&gt;CMB&lt;/a&gt; plotted with x-axis units converted from 1/cm to GHz, &lt;a href=&quot;https://lambda.gsfc.nasa.gov/product/cobe/firas_monopole_get.cfm&quot; rel=&quot;noreferrer&quot;&gt;data from here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-24T11:09:47.863" LastActivityDate="2017-02-24T11:09:47.863" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20332" PostTypeId="2" ParentId="20316" CreationDate="2017-02-24T02:03:24.893" Score="30" Body="&lt;p&gt;I did a crude spreadsheet sim using the &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=3&amp;amp;ved=0ahUKEwjrnN3V0KfSAhUEx2MKHVn0A_UQFggsMAI&amp;amp;url=https%3A%2F%2Fspaceflight.nasa.gov%2Foutreach%2FSignificantIncidents%2Fassets%2Frogers_commission_report.pdf&amp;amp;usg=AFQjCNE-uKH3-qwmmSOMMkfQiiEEZwTVhg&amp;amp;sig2=7RlxXbV0S5Xl7fAoWsUxww&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rogers Commission report&lt;/a&gt; to get throttle times, to wit:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Throttle down to 94% at 24 seconds&lt;/li&gt;&#xA;&lt;li&gt;Throttle down to 65% at 42 seconds&lt;/li&gt;&#xA;&lt;li&gt;Throttle up to 104% at 65 seconds&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I neglected startup propellant consumption and assumed step function throttling.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_external_tank&quot; rel=&quot;nofollow noreferrer&quot;&gt;I took liftoff O2 load to be 1,387,457 lb and H2 load to be 234,265 lb.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pages.erau.edu/~ericksol/courses/sp210/images/ssme_schem.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;I used 154 lb/sec/engine for H2 flow at 104% and 925 lb/sec/engine for O2 flow.&lt;/a&gt; I assumed flow rates scale linearly with throttle setting - this is a decent assumption from what I remember. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that, I got ~ 87.7% O2 load at 73 seconds and ~ 87.9% H2 load at the same time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With my 1 sec step time and googled liftoff loading numbers, it's approximate - real approximate - but you can see that there was a lot of prop left in the ET.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lmTHu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lmTHu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SRB Sep time is ~ 2:03 so they were ~60% burned out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: Calculations in my answer to &lt;a href=&quot;https://space.stackexchange.com/questions/34893/how-much-propellant-is-used-up-until-liftoff&quot;&gt;How much propellant is used up until liftoff?&lt;/a&gt; show that  less than ~1% of the ET propellant is used up pre-liftoff, which would bring the percentages at 73 seconds down a bit. I did not redo the spreadsheet.  I did state right off that &quot;I neglected startup propellant consumption.&quot;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-05-13T17:15:01.133" LastActivityDate="2020-05-13T17:15:01.133" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20333" PostTypeId="2" ParentId="20329" CreationDate="2017-02-24T02:50:02.313" Score="3" Body="&lt;p&gt;Two lights isn't enough to determine orientation of a spacecraft, but I'd put the grapple fixture on the top, on the basis that navigation lights are supposed to point forward, making the pointy end the back. That fits with the orientation for reentry, though the navigation lights appear to be on the trunk rather than the capsule itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wouldn't expect to see any navigation light standards for purely orbital spacecraft - ship/aircraft navigation lights are important for collision avoidance because they show direction of movement, not just orientation. That doesn't work so well when the other vessel is routinely upside down and/or backwards. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cygnus does have navigation lights with the addition of white and yellow for up and down, but that's in the context of docking, where orientation matters and any configuration can be agreed with the single spacecraft you are trying to dock with. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where standardized navigation lights would make sense is after reentry - certainly the space shuttle could reasonably be covered by existing civil aviation rules - but all examples to date have been government/military systems and not required to have lights at all.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-02-24T02:50:02.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20334" PostTypeId="1" AcceptedAnswerId="20429" CreationDate="2017-02-24T07:19:05.547" Score="13" ViewCount="1746" Body="&lt;p&gt;The end of &lt;a href=&quot;https://biology.stackexchange.com/a/56620/27918&quot;&gt;this excellent answer&lt;/a&gt; lead me to &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiotrophic_fungus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; which linked to &lt;a href=&quot;https://earthlingnature.wordpress.com/2016/01/17/the-blacker-the-better-especially-in-chernobyl/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The blacker the better… especially in Chernobyl&lt;/a&gt; and &lt;a href=&quot;http://www.scienceagogo.com/news/20070422222547data_trunc_sys.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chernobyl Fungus Feeds On Radiation&lt;/a&gt;. One passage in 'blacker the better' caught my eye:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Melanized fungi actually seem to thrive in environments with high levels of ionizing radiation (ultraviolet, x and gamma rays), which is usually seen as very dangerous to life. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And then it goes on:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The walls of the damaged nuclear reactor of Chernobyl are covered in melanized fungi and they also are found living very happy on board of the International Space Station. Experiments showed that these melanized species of fungi seem to benefit from radiation, increasing their growth and germination.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Melanized fungi seem to be fungi which can produce a form of melanin, which may be able to derive some biological advantage (energy for synthesis or metabolism perhaps) from light or even radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have melanized fungi been found happy on the ISS? Is there anything written about this I could read? Any idea if they are gaining some biological advantage from higher radiation levels there? Were they there intentionally (part of an experiment) or did did they travel as common contamination or spores from Earth?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-09-30T13:39:37.160" LastActivityDate="2018-09-30T13:39:37.160" Title="Melanized fungi on the ISS - are they 'enjoying' the elevated radiation levels there?" Tags="|iss|radiation|life|astrobiology|" AnswerCount="1" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20335" PostTypeId="2" ParentId="20311" CreationDate="2017-02-24T09:05:57.520" Score="0" Body="&lt;p&gt;There are no &lt;em&gt;official&lt;/em&gt; and &lt;em&gt;founded&lt;/em&gt; plans of &lt;em&gt;new&lt;/em&gt; mission with primary plan to leave the Solar System.&lt;/p&gt;&#xA;" OwnerUserId="4606" LastActivityDate="2017-02-24T09:05:57.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20336" PostTypeId="2" ParentId="20325" CreationDate="2017-02-24T09:58:44.753" Score="1" Body="&lt;p&gt;To be clear: do you actually mean &quot;cosmic microwave background radiation&quot; or simply &quot;cosmic radiation&quot;?  Because the two are two very different things (although the answer is the same for both, but the reason different).  Cosmic radiation consists of GCR (galactic cosmic radiation), a weak flux of very high energy particles, and solar, which is a stronger flux of weaker particles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While such radiation is very damaging to the body, the flux doesn't represent much total power.  Or to put it another way, it can kill you, but you're not going to feel any warmth in the process.  While the power (watts) of the flux is tiny (and no, your immediate reaction of &quot;what about a big...&quot;.... I'll just stop you right there, the answer is &quot;no&quot;), it's damaging because it releases the energy &lt;em&gt;inside of your cells&lt;/em&gt;, not on the protective epidermis layer.  By the time a single particle of GCR, some insignificant number of joules, is full decelerated, the energy has generally been split into many other particles, and each has left trails of destruction through your cells like microscopic bullet paths.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some GCR particles that can have significant energy - see the &quot;Oh My God Particle&quot; (a single particle measured with the energy of a fast-pitch baseball) for an example.  But such things are exceedingly rare. Plus it takes basically a whole atmosphere to stop them.&lt;/p&gt;&#xA;" OwnerUserId="13465" LastActivityDate="2017-02-24T09:58:44.753" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20337" PostTypeId="2" ParentId="20330" CreationDate="2017-02-24T10:02:35.420" Score="-3" Body="&lt;p&gt;The answer is &quot;essentially the same orbit&quot;.  When you're that close to another orbiting object, slight differences in distance from the body being orbited have only a tiny impact on orbital velocity and thus rate of drift away from the rendezvous target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You enter a lower or higher orbit to catch the object you're trying to rendezvous with, then &quot;approximately the same orbit&quot; when you're preparing for docking.&lt;/p&gt;&#xA;" OwnerUserId="13465" LastActivityDate="2017-02-24T10:02:35.420" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20338" PostTypeId="2" ParentId="20287" CreationDate="2017-02-24T10:20:28.650" Score="3" Body="&lt;p&gt;To add more to what Happy Koala wrote (this would be a comment but I don't have the rep. yet to post comments): if you try to model the solar system without planetary migration, you get an Earth-sized (or larger) planet where Mars is and several high-inclination  / eccentricity Mars-sized objects in the asteroid belt. There's now an increasingly high degree of confidence that planetary migration did, in fact, occur.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, Jupiter gorged on the combination of having a high radius before the dropoff in density in the protoplanetary disc, boosted by volatiles being blasted away from the inner solar system, and quickly became a large, dominant body in the solar system.  It slowly migrated inwards (casting icy material from the outer solar system into the inner solar system in the process).  It reached all the way to where Mars is today and cleared the zone between Mars and Earth's present orbit (casting a good chunk of this material into the outer solar system).  However, during this time Saturn had slowly been forming, and the action of Saturn slowed and ultimately reversed Jupiter's inward migration.  Jupiter slowly migrated outward to its current position, casting material into what is now the asteroid belt in the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus, and Earth formed from material in their respective orbits, a combination of original material and material cast in by Jupiter. Mars however had to wait for scraps being cast in by Venus/Earth or Jupiter; there was little to accrete in its zone. As a consequence, it ended up much smaller.  In the process of accretion, all of the inner planets moved around, although not as much as the giants.  This includes not just semimajor axis but inclination and eccentricity as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to how Mars retained liquid water, it used to have a more substantial atmosphere, but its low gravity and lack of a magnetic field led to the atmosphere's loss by the solar wind.  This can be clearly seen in its hydrogen and nitrogen isotopic ratios, both of which are well enriched in their heavier isotopes, as lighter isotopes escape more readily.  Venus, while having lost little nitrogen due to its size, is even more water-depleted than Mars; Mars has a 5-7 times higher D:H ratio than Earth, while Venus has a 150-240 times higher ratio. It's amazing and kind of sad to think about how Earthlike Venus used to be.&lt;/p&gt;&#xA;" OwnerUserId="13465" LastActivityDate="2017-02-24T10:20:28.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20339" PostTypeId="1" AcceptedAnswerId="20341" CreationDate="2017-02-24T10:44:31.733" Score="2" ViewCount="308" Body="&lt;p&gt;Given that everything on earth is rotating at the same speed as Earth, it is simple for an item leaving the earth surface to return to the earth surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, once an object has left the earth's atmosphere and entered space, how can it return to earth and rejoin the rotational speed of the planet?&lt;/p&gt;&#xA;" OwnerUserId="18629" LastActivityDate="2017-02-24T15:04:23.887" Title="At what point in the atmosphere does a returning space object adopt the rotational speed of planet earth and how?" Tags="|gravity|return|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20340" PostTypeId="2" ParentId="20339" CreationDate="2017-02-24T13:36:49.123" Score="3" Body="&lt;p&gt;Oh, &lt;em&gt;at reentry&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no magical change of frame of reference or other weird stuff. The Apollo Command Module returns from its trip to the Moon. It moves in trajectory that makes it move at 11 km/s relative to Earth surface, while rotation of Earth near Equator makes the surface move at about 465.1 m/s, at poles - 0, in between - in between. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how does the craft slow down from 11,000m/s to some 400m/s when it lands somewhere near Hawaii? Air drag. It plunges into the atmosphere at full speed, at a rather shallow angle, air gets compressed under the heatshield, heating immensely and creating huge plasma trails, and reducing the craft's horizontal speed, then, when safe, parachute opens, and the craft begins falling down, its &quot;horizontal speed&quot; bound (through the parachute) to local wind speed which is tightly bound to Earth's rotation speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at suborbital flights, their landing sites are often quite close to launch sites - they don't magically gain or lose speed in horizontal direction when leaving the atmosphere and entering space - if you want to enter the orbit, instead of falling directly where you launched from, like in that ball example, you have to burn lots and lots of fuel in horizontal direction, to get the orbital speed, much higher than Earth rotation speed.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-02-24T13:36:49.123" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20341" PostTypeId="2" ParentId="20339" CreationDate="2017-02-24T13:37:57.223" Score="9" Body="&lt;p&gt;Your question here is more about the frame of reference than relative speeds. To make things easier to understand, let us consider those points:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;you have a launch site exactly on the equator&lt;/li&gt;&#xA;&lt;li&gt;earth equatorial rotation velocity is 1,674.4 km/h, approx 465m/s&lt;/li&gt;&#xA;&lt;li&gt;you want to join the ISS which is in our case also in a equatorial orbit and with a 7.67 km/s speed (7670m/s)&lt;/li&gt;&#xA;&lt;li&gt;there is no wind, never.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As earth is rotating from west to east, you &quot;only&quot; need to add 7205m/s to your vessel, because of ground speed and the orbit you want is west-east oriented (fyi, it would require 2x465 = 930m/s of additional &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;noreferrer&quot;&gt;delta-v&lt;/a&gt; to have an orbit east-west oriented). So:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;when on launchpad, you are already at a speed of 465m/s in the frame of reference of space.&lt;/li&gt;&#xA;&lt;li&gt;when in orbit, you go at 7670m/s in the frame of reference of space, but &quot;only&quot; 7205m/s in the frame of reference of ground.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Returning to earth only means slowing in both frame of reference, and to land safely you want a ground speed of 0m/s. Here is where atmosphere helps a lot. As we previously stated there is no wind, we can consider the atmosphere as a outer layer of earth rotating the same speed as the ground, and so:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;going through this layer will make you &lt;a href=&quot;https://en.wikipedia.org/wiki/Shooting_Star&quot; rel=&quot;noreferrer&quot;&gt;brake and heat&lt;/a&gt;, and you may need some &lt;a href=&quot;https://en.wikipedia.org/wiki/Heat_shield#Space&quot; rel=&quot;noreferrer&quot;&gt;protection&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;in order to low your ground speed to 0, you then use parachute (for example on an &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_%28spacecraft%29#Command_Module&quot; rel=&quot;noreferrer&quot;&gt;Apollo capsule&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;you then only wait to touch down, and voila.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18592" LastEditorUserId="18592" LastEditDate="2017-02-24T15:04:23.887" LastActivityDate="2017-02-24T15:04:23.887" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20342" PostTypeId="1" CreationDate="2017-02-24T17:20:01.163" Score="10" ViewCount="2174" Body="&lt;p&gt;There are a series of questions on this site which are generally &quot;Is there X on Mars?&quot; &quot;Are there sufficient Y to create Z on Mars?&quot; And the answers are generally YES for the most part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is there that we don't expect to be able to find on Mars? I don't mean &quot;we haven't found this yet, but we haven't really looked very hard&quot;, I mean more like &quot;given the soil composition we have from Mars, Mars seems to be very poor in X, which is a pity because that means you can't do Y&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what is there that we would strictly have to import from off-planet to Mars given what we know currently?&lt;/p&gt;&#xA;" OwnerUserId="5965" LastActivityDate="2021-02-22T17:33:24.163" Title="What useful elements are NOT to be found on Mars?" Tags="|mars|colonization|geology|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20344" PostTypeId="1" CreationDate="2017-02-24T19:27:58.127" Score="7" ViewCount="501" Body="&lt;p&gt;The actual Apollo flights 2-6 were test launches of the Saturn rocket, the CM and LM (1 was retired due to the fire). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What were the original plans?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Did they intend to launch people on a Saturn without testing?&lt;/li&gt;&#xA;&lt;li&gt;Was Apollo 1 supposed to do what Apollo 7 did, and onwards (So Apollo 2 was supposed to be a LM test, 3 was an Around the Moon test, 4 was a LM in Lunar Orbit test, and 5 was a Moon landing) or were plans substantially different?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18569" LastEditorUserId="415" LastEditDate="2017-02-24T19:32:23.950" LastActivityDate="2022-09-01T20:24:20.200" Title="Before the Apollo 1 fire, what was the plan for the missions leading up to a lunar landing?" Tags="|apollo-program|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20345" PostTypeId="1" CreationDate="2017-02-24T20:11:09.493" Score="10" ViewCount="1053" Body="&lt;p&gt;Uwe wrote in an &lt;a href=&quot;https://space.stackexchange.com/a/20301/415&quot;&gt;answer&lt;/a&gt; that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The possible payload of Saturn V was carefully planed for the whole mission, but the difference of the payloads of the missions Apollo 11 and 17 was less than 3000 kg.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To me, that feels backwards: Why would you build a huge rocket, with a lead time approaching &lt;em&gt;years&lt;/em&gt;, to the exact payload specifications that you thought you would need for the mission? &lt;strong&gt;Would it not be better to build as powerful an &lt;a href=&quot;https://xkcd.com/1133/&quot; rel=&quot;noreferrer&quot;&gt;Up Goer&lt;/a&gt; as you reasonably could, then bring along as much payload as you could given what it could carry to wherever you were going, in order to get as much as possible useful work done for the money spent?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or put differently, to what extent did Apollo payload mass &lt;em&gt;drive&lt;/em&gt; the capability of the Saturn family of rockets; as opposed to the extent to which the capability of the Saturn family of rockets &lt;em&gt;limited&lt;/em&gt; Apollo payload mass?&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2017-03-08T05:40:38.153" Title="To what extent did Apollo payload mass drive Saturn capability?" Tags="|apollo-program|saturn-v|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20346" PostTypeId="1" CreationDate="2017-02-24T20:15:43.640" Score="14" ViewCount="794" Body="&lt;p&gt;The point of Apollo 10 was to test the LM in Lunar Orbit, but without landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why bother?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the LM had any sort of bugs (say the upper stage wouldn't launch, or they couldn't re-dock), then the astronauts would be just as dead as on Apollo 11.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anything, the in-flight abort added a further risk - if they're on the ground and the takeoff stage doesn't light, they can try to fix something (not much of a chance, but something). In Apollo 10, they'd be dead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what failure modes would be survivable in Apollo 10 that wouldn't be in 11+?&lt;/p&gt;&#xA;" OwnerUserId="18569" LastActivityDate="2023-05-18T21:23:23.857" Title="What was the purpose of an Apollo 10 test?" Tags="|apollo-program|" AnswerCount="4" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20347" PostTypeId="1" CreationDate="2017-02-24T20:18:16.087" Score="3" ViewCount="217" Body="&lt;p&gt;What specifically must be done to Human Rate the Interim Cryogenic Propulsion Stage (&lt;a href=&quot;https://www.nasa.gov/sls/interim_cryogenic_propulsion_stage_141030.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ICPS&lt;/a&gt;) to fly humans on &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_Mission_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Exploration Mission 1 of SLS&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="18682" LastEditorUserId="19946" LastEditDate="2019-11-22T12:07:15.373" LastActivityDate="2019-11-22T12:07:15.373" Title="Human Rate the ICPS on EM-1 of SLS" Tags="|rockets|sls|ula|human-rating|compliance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20348" PostTypeId="2" ParentId="20346" CreationDate="2017-02-24T21:12:04.997" Score="13" Body="&lt;p&gt;One aspect is that the Apollo 10 LM was not lightweight enough to land and return with safe reserves, even if it had been launched with full propellant, and a sufficiently lightweight one wouldn't be ready until later in the year.  So with the end-of-decade deadline looming, Apollo 10 could be used to practice and gather information (e.g. about how the lunar mass concentrations affected a low altitude orbit) that could be applied to Apollo 11, at an earlier date than any landing attempt would have been possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As &lt;em&gt;SF.&lt;/em&gt; mentions, you have the survivability of an ascent engine ignition failure backwards - on Apollo 10, the CSM could have rendezvoused with the LM.  On surface missions the crew would likely have been stuck.  Ascent engine ignition failure isn't a very credible failure mode though compared to other critical systems, because of its hypergolic nature, redundant plumbing etc.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-02-24T21:12:04.997" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20349" PostTypeId="2" ParentId="20347" CreationDate="2017-02-24T22:55:07.783" Score="2" Body="&lt;p&gt;Compliance is mandatory!&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8705_002B_&amp;amp;page_name=Chapter3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Human Rating Requirements for Space Systems&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;It's long, even with the rationale scraped out, but here you go.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;3.2 System Safety Requirements&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;3.2.1 The space system shall provide the capability to sustain a safe, habitable environment for the crew (Requirement 58503).&lt;/p&gt;&#xA;&lt;p&gt;3.2.2 The space system shall meet probabilistic safety criteria derived from the Agency-level safety goals and safety thresholds with&#xA;a specified degree of certainty (Requirement).&lt;/p&gt;&#xA;&lt;p&gt;3.2.3 The space system shall provide failure tolerance to catastrophic events, with specific levels of failure tolerance and&#xA;implementation (similar or dissimilar redundancy) derived via an&#xA;integration of the design and safety analysis (per the requirement in&#xA;paragraph 2.3.7.1) (Requirement).&lt;/p&gt;&#xA;&lt;p&gt;a. Failure of primary structure, structural failure of pressure vessel&#xA;walls, and failure of pressurized lines are excepted from the failure&#xA;tolerance requirement provided the potentially catastrophic failures&#xA;are controlled through a defined process in which approved standards&#xA;and margins are implemented that account for the absence of failure&#xA;tolerance.&lt;/p&gt;&#xA;&lt;p&gt;b. Other potentially catastrophic hazards that cannot be controlled&#xA;using failure tolerance are excepted from the failure tolerance&#xA;requirements with concurrence from the Technical Authorities provided&#xA;the hazards are controlled through a defined process in which approved&#xA;standards and margins are implemented that account for the absence of&#xA;failure tolerance.&lt;/p&gt;&#xA;&lt;p&gt;3.2.4 The space system shall provide the failure tolerance capability in 3.2.3 without the use of emergency equipment and systems&#xA;(Requirement 58557).&lt;/p&gt;&#xA;&lt;p&gt;3.2.5 The space system shall be designed to tolerate inadvertent operator action (minimum of one inadvertent action), as identified by&#xA;the human error analysis (paragraph 2.3.11), without causing a&#xA;catastrophic event (Requirement 58559).&lt;/p&gt;&#xA;&lt;p&gt;3.2.6 The space system shall tolerate inadvertent operator action, as described in 3.2.4, in the presence of any single system failure&#xA;(Requirement 58561).&lt;/p&gt;&#xA;&lt;p&gt;3.2.7 The space system shall provide the capability to mitigate the hazardous behavior of critical software where the hazardous behavior&#xA;would result in a catastrophic event (Requirement 58563).&lt;/p&gt;&#xA;&lt;p&gt;3.2.8 The space system shall provide the capability to detect and annunciate faults that affect critical systems, subsystems, and/or&#xA;crew health (Requirement 58569).&lt;/p&gt;&#xA;&lt;p&gt;3.2.9 The space system shall provide the capability to isolate and/or recover from faults identified during system development that would&#xA;result in a catastrophic event (Requirement 58572).&lt;/p&gt;&#xA;&lt;p&gt;3.2.10 The space system shall provide the capability to utilize health and status data (including system performance data) of critical&#xA;systems and subsystems to facilitate anomaly resolution during and&#xA;after the mission (Requirement 58574).&lt;/p&gt;&#xA;&lt;p&gt;3.2.11 The crewed space system shall provide the capability for autonomous operation of system and subsystem functions which, if lost,&#xA;would result in a catastrophic event (Requirement 58576).&lt;/p&gt;&#xA;&lt;p&gt;3.2.12 The space system shall provide the capability for the crew to readily access equipment involved in the response to emergency&#xA;situations and the capability to gain access to equipment needed for&#xA;follow-up/recovery operations (Requirement 58578).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;3.3 System Control Requirements - General&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;3.3.1 The crewed space system shall provide the capability for the crew to monitor, operate, and control the crewed space system and&#xA;subsystems, where:&lt;/p&gt;&#xA;&lt;p&gt;a. The capability is necessary to execute the mission; or&lt;/p&gt;&#xA;&lt;p&gt;b. The capability would prevent a catastrophic event; or&lt;/p&gt;&#xA;&lt;p&gt;c. The capability would prevent an abort (Requirement).&lt;/p&gt;&#xA;&lt;p&gt;3.3.2 The crewed space system shall provide the capability for the crew to manually override higher level software control/automation&#xA;(such as automated abort initiation, configuration change, and mode&#xA;change) when the transition to manual control of the system will not&#xA;cause a catastrophic event (Requirement 58586).&lt;/p&gt;&#xA;&lt;p&gt;3.3.3 The space system shall provide the capability for humans to remotely monitor, operate, and control the crewed system elements and&#xA;subsystems, where:&lt;/p&gt;&#xA;&lt;p&gt;a. The remote capability is necessary to execute the mission; or&lt;/p&gt;&#xA;&lt;p&gt;b. The remote capability would prevent a catastrophic event; or&lt;/p&gt;&#xA;&lt;p&gt;c. The remote capability would prevent an abort (Requirement 58598).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;3.4 System Control Requirements - Human-Rated Spacecraft&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;3.4.1 The crewed space system shall provide the capability for the crew to manually control the flight path and attitude of their&#xA;spacecraft, with the following exception: during the atmospheric&#xA;portion of Earth ascent when structural and thermal margins have been&#xA;determined to negate the benefits of manual control (Requirement).&lt;/p&gt;&#xA;&lt;p&gt;3.4.2 The crewed spacecraft shall exhibit Level 1 handling qualities (Handling Qualities Rating (HQR) 1, 2 and 3), as defined by the&#xA;Cooper-Harper Rating Scale, during manual control of the spacecraft's&#xA;flight path and attitude (Requirement).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;3.5 System Control Requirements - Proximity Operations with Human-Rated Spacecraft&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;3.5.1 The space system shall provide the capability for the crew to monitor, operate, and control an uncrewed spacecraft during proximity&#xA;operations, where:&lt;/p&gt;&#xA;&lt;p&gt;a. The capability is necessary to execute the mission; or&lt;/p&gt;&#xA;&lt;p&gt;b. The capability would prevent a catastrophic event; or&lt;/p&gt;&#xA;&lt;p&gt;c. The capability would prevent an abort (Requirement 58604).&lt;/p&gt;&#xA;&lt;p&gt;3.5.2 The crewed space system shall provide the capability for direct voice communication between crewed spacecraft (2 or more) during&#xA;proximity operations (Requirement 58607).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;3.6 Crew Survival/Abort Requirements&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;3.6.1 Earth Ascent Systems&lt;/p&gt;&#xA;&lt;p&gt;3.6.1.1 The space system shall provide the capability for unassisted crew emergency egress to a safe haven during Earth prelaunch&#xA;activities (Requirement 58611).&lt;/p&gt;&#xA;&lt;p&gt;3.6.1.2 The space system shall provide abort capability from the launch pad until Earth-orbit insertion to protect for the following&#xA;ascent failure scenarios (minimum list):&lt;/p&gt;&#xA;&lt;p&gt;a. Complete loss of ascent thrust/propulsion (Requirement 58613).&lt;/p&gt;&#xA;&lt;p&gt;b. Loss of attitude or flight path control (Requirement 58614).&lt;/p&gt;&#xA;&lt;p&gt;3.6.1.3 The crewed space system shall monitor the Earth ascent launch vehicle performance and automatically initiate an abort when an&#xA;impending catastrophic failure is detected (Requirement 58616).&lt;/p&gt;&#xA;&lt;p&gt;3.6.1.4 Earth Ascent Abort&lt;/p&gt;&#xA;&lt;p&gt;3.6.1.4.1 The space system shall provide the capability for the crew to initiate the Earth ascent abort sequence (Requirement 58619).&lt;/p&gt;&#xA;&lt;p&gt;3.6.1.4.2 The space system shall provide the capability for the ground control to initiate the Earth ascent abort sequence (Requirement&#xA;58620).&lt;/p&gt;&#xA;&lt;p&gt;3.6.1.5 If a range safety destruct system is incorporated into the design, the space system shall automatically initiate the Earth ascent&#xA;abort sequence when range safety destruct commands are received&#xA;onboard, with an adequate time delay prior to destruction of the&#xA;launch vehicle to allow a successful abort (Requirement 58622).&lt;/p&gt;&#xA;&lt;p&gt;3.6.2 Earth Orbit Systems&lt;/p&gt;&#xA;&lt;p&gt;3.6.2.1 The crewed space system shall provide the capability to autonomously abort the mission from Earth orbit by targeting and&#xA;performing a deorbit to a safe landing on Earth (Requirement 58625).&lt;/p&gt;&#xA;&lt;p&gt;3.6.3 Earth - Lunar Transit and Lunar Orbit Systems&lt;/p&gt;&#xA;&lt;p&gt;3.6.3.1 The crewed space system shall provide the capability to autonomously abort the mission during lunar transit and from lunar&#xA;orbit by executing a safe return to Earth (Requirement 58627).&lt;/p&gt;&#xA;&lt;p&gt;3.6.4 Lunar Descent Systems&lt;/p&gt;&#xA;&lt;p&gt;3.6.4.1 The crewed space system shall provide the capability to autonomously abort the lunar descent and execute all operations&#xA;required for a safe return to Earth (Requirement 58629).&lt;/p&gt;&#xA;&lt;p&gt;3.6.5 Lunar Surface Systems&lt;/p&gt;&#xA;&lt;p&gt;3.6.5.1 The space system shall provide the capability for the crew on the lunar surface to monitor the descent and landing trajectory of an&#xA;uncrewed spacecraft and send commands necessary to prevent a&#xA;catastrophic event (Requirement 58632).&lt;/p&gt;&#xA;&lt;p&gt;3.6.6 Lunar Ascent Systems&lt;/p&gt;&#xA;&lt;p&gt;Reserved for a future version of the NPR.&lt;/p&gt;&#xA;&lt;p&gt;3.6.7 Earth Reentry Systems&lt;/p&gt;&#xA;&lt;p&gt;3.6.7.1 The crewed space system shall provide the capability for unassisted crew emergency egress after Earth landing (Requirement&#xA;58636).&lt;/p&gt;&#xA;&lt;p&gt;3.6.7.2 The crewed space system shall provide a safe haven capability for the crew inside the spacecraft after Earth landing until the&#xA;arrival of the landing recovery team or rescue forces (Requirement&#xA;58638).&lt;/p&gt;&#xA;&lt;p&gt;3.6.7.3 The space system shall provide recovery forces with the location of the spacecraft after return to Earth (Requirement 58640).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-24T23:05:28.583" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20350" PostTypeId="2" ParentId="20346" CreationDate="2017-02-24T23:28:45.560" Score="18" Body="&lt;p&gt;pericynthion is correct that the pacing item for the moon landing was LM weight.&lt;/p&gt;&#xA;&lt;p&gt;In the case of ascent stage failure, CSM had fuel to maneuver to meet the LM. As pericynthion notes, this is an unlikely failure mode.&lt;/p&gt;&#xA;&lt;p&gt;In the case of failure to dock, the LM crew would EVA to the CSM -- not a maneuver you'd want to undertake if you had a better option, but not all that risky.&lt;/p&gt;&#xA;&lt;p&gt;As it happens, making Apollo 10 the landing &lt;a href=&quot;http://www.umiacs.umd.edu/%7Eoard/apollo/poss_moonwalkers.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;was considered&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;According to multiple sources, some consideration was given to assigning the first lunar landing to Apollo 10, which [Tom] Stafford commanded. It was ultimately decided that Apollo 10 could land no more than one month sooner than Apollo 11 could (because Apollo 11's lighter LM was required, and because the Apollo 10 crew would have needed some additional training), and that the additional tracking experience from a second lunar mission would be useful when planning the first landing. Apollo 10 therefore flew a lunar orbital mission as originally planned. Stafford later flew again as CDR of the ASTP mission, his fourth mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I can't find the reference, but I believe Stafford himself felt it would be better to make A10 a dress rehearsal flight rather than the first landing.&lt;/p&gt;&#xA;&lt;p&gt;It's important to remember that Apollo 10 was only the second translunar flight ever, the second crewed flight of the LM, and the third crewed flight of the Saturn V. Every Apollo mission had had some anomalies, and it was by no means clear that the complete system was reliable. Another opportunity to work out the bugs was welcome.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2023-05-17T21:27:57.303" LastActivityDate="2023-05-17T21:27:57.303" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20351" PostTypeId="2" ParentId="20240" CreationDate="2017-02-24T23:57:17.583" Score="1" Body="&lt;p&gt;To a degree. In a typical converging-diverging nozzle, pressure is the force that pushes against the nozzle. This pressure is the result of both the density and temperature of the gas. As the gas flows throughout the nozzle and kinetic energy is recovered, the temperature (and pressure) drop. There are also reactions in the gas which can affect the temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;nozzle&quot; profile of an ion thruster, on the other hand, is quite small. There is very little time for the plasma to exchange heat with the ion thruster during its transit through the acceleration grid. The &quot;optics&quot; of the grid are actually designed to avoid having the ions interact with the grid wall. Moreover, the propulsive force is the Coulomb force, rather than pressure. We rely upon the ion having a charge and interacting with an electric field. There is no attempt to recover the thermal energy of the plasma as there is in a rocket nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is likely that the plasma will cool in the plume as it interacts with other ions or matter, but as to the order of magnitude, it depends on the density of the plasma and the surrounding system. Low density of matter results in little heat transfer (as there is nothing to transfer it to).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are examples of electric propulsion that do attempt to turn thermal energy into kinetic enery: electrothermal and microwave electrothermal thrusters. They use a traditional converging-diverging nozzle to expel a heated gas (either heated by a resistive metal or by a plasma). In the latter case, the gas will be partially ionized as it travels through the nozzle region and the thermal energy will be recovered, assuming the nozzle is well-designed.&lt;/p&gt;&#xA;" OwnerUserId="794" LastActivityDate="2017-02-24T23:57:17.583" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20352" PostTypeId="1" CreationDate="2017-02-25T00:48:32.630" Score="11" ViewCount="396" Body="&lt;p&gt;In what position is the lowest point of microgravity on the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thought the answer was everywhere on the ISS but was told this was not right.&lt;/p&gt;&#xA;" OwnerUserId="18691" LastEditorUserId="6816" LastEditDate="2017-02-25T01:08:36.003" LastActivityDate="2017-02-25T16:07:15.007" Title="Lowest ISS microgravity" Tags="|iss|microgravity|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20353" PostTypeId="1" AcceptedAnswerId="20355" CreationDate="2017-02-25T02:04:22.457" Score="25" ViewCount="4513" Body="&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/l11sW5KOi_g?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;To me this looks like the first instance of space-tourism (humor, pls don't add the tag, see exemplary documentation at end of question). The astronauts landed next to Surveyor-3 on purpose, got out of their spacecraft, walked over to it, took pictures of each other next to it, collected a couple of &lt;a href=&quot;https://space.stackexchange.com/questions/20353/#comment54635_20353&quot;&gt;souvenirs&lt;/a&gt; (for &lt;a href=&quot;https://space.stackexchange.com/questions/20353/#comment54722_20353&quot;&gt;later study&lt;/a&gt;), packed up, got back in their spacecraft, and went home. OK they did do a lot of other things while there as well.&lt;/p&gt;&#xA;&lt;p&gt;How did they know the location of Surveyor-3 with such precision, using 1960's technology? There was precious little extra fuel on the lander to fly around and look for it, so they must have had a pretty good idea how to navigate there and safely land within walking distance (~200m). &lt;em&gt;How was Surveyor-3's location known to such accuracy?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Surveyor_3-Apollo_12.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Charles Conrad Jr., Apollo 12 Commander, examines the unmanned Surveyor III spacecraft during the second extravehicular activity (EVA-2). The Lunar Module (LM) &amp;quot;Intrepid&amp;quot; is in the right background. This picture was taken by astronaut Alan L. Bean, Lunar Module pilot. The &amp;quot;Intrepid&amp;quot; landed on the Moon's Ocean of Storms only 600 feet from Surveyor III. The television camera and several other components were taken from Surveyor III and brought back to earth for scientific analysis. Surveyor III soft-landed on the Moon on April 19, 1967.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Fn8xx.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Fn8xx.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Surveyor-3, cropped copy of From &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Surveyor_3_on_the_Moon.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NASA Image ID number: AS12-H-48-7121, &lt;a href=&quot;http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/a12_h_48_7121.html&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Apollo 12 astronaut Alan Bean took this picture of the Surveyor 3 spacecraft on the second moonwalk EVA on 20 November 1969. Surveyor 3 landed on the Moon inside the edge of a small crater on 20 April 1967 where it took pictures and transmitted them back to Earth and performed soil mechanics experiments. The arm of the soil mechanics sampler is seen extended out to the right. The panels on top of the center mast are the solar arrays. The camera is the white cylinder just to the right of the mast. The arm extending upward to the left is the omnidirectional antenna. The spacecraft is just over 3 meters high. This view is looking to the north.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Pkteq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Pkteq.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Examples of getting out of a transporting craft, walking over to it, taking a picture with it, and (presumably) packing up, getting back into the craft and going home, presumably having done other things as well.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/F617o.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/F617o.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BCFak.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BCFak.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-13T00:10:30.063" LastActivityDate="2021-10-14T11:21:00.430" Title="How did Apollo-12 manage to land next to Surveyor-3? First &quot;Space-Tourists&quot;?" Tags="|lunar-landing|spaceflight-firsts|apollo-12|surveyor-program|surveyor-3|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20354" PostTypeId="2" ParentId="20352" CreationDate="2017-02-25T02:21:11.617" Score="12" Body="&lt;p&gt;Assuming you are mounted on the ISS in some relatively rigid way, the lowest microgravity lies along a &quot;sweet line&quot; (really a very shallow arc) of microgravity which passes through the center of mass in the direction of the ISS' velocity vector.  Isobars of microgravity are ellipses in cross-section.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Keplerian effects refer to the acceleration of any point in a rigid&#xA;  body due to its distance from the center of mass of the body. Every&#xA;  point in an orbiting rigid body, taken as a point mass, wants to&#xA;  travel in a slightly different orbit.  Structural rigidity prevents&#xA;  this from occuring, resulting in a contamination of the microgravity&#xA;  environment. Constant acceleration surfaces are elliptical tubes&#xA;  aligned along the body s velocity vector as shown in Figure 2.1.3.&#xA;  Figure 2.1.4 shows the gravity gradient structure, where the body's&#xA;  velocity vector is perpendicular to the page. These figures show that&#xA;  an orbiting body has a &quot;sweet line&quot; of microgravity which passes&#xA;  through the center of mass in the direction of the body's velocity&#xA;  vector.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v8YlY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v8YlY.png&quot; alt=&quot;Microgravity Envelope&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900015851.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Space Station Based Microacceleration Platform&quot;&lt;/a&gt;, which also provides some definition of microgravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is another view of the microgravity isobars depicted above superimposed on a model of the ISS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ku81t.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ku81t.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pMVbZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pMVbZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.spaceflight.esa.int/impress/text/education/Microgravity/Question_Microgravity_008.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.  Of course, the position of these isobars change relative to the ISS as the ISS' attitude and center of mass changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other effects are important as well -- crew-induced vibration, atmospheric drag (which creates 0.3 micro-gs alone), etc.  This is why microgravity free flyers that depart from the ISS and periodically return have been proposed to support microgravity experiments.  These free flyers could minimize some of these effects.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2017-02-25T16:07:15.007" LastActivityDate="2017-02-25T16:07:15.007" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="20355" PostTypeId="2" ParentId="20353" CreationDate="2017-02-25T02:25:04.510" Score="22" Body="&lt;p&gt;&amp;quot;How was Surveyor-3's location known to such accuracy?&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;The Surveyors were located using Lunar Orbiter photography (not necessarily of the probe themselves, just the surrounding area).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;These coordinates were obtained by determining the position of the&#xA;landed Surveyors on Lunar Orbiter photographs by matching features&#xA;shown in Surveyor pictures with corresponding features in the Lunar&#xA;Orbiter photographs (refs. 1-2, 1-4, 1-5, and 1-19 (see ch. 3)). The&#xA;Lunar Orbiter photographs were, in turn, related to the Orthographic&#xA;Atlas and Mills/Arthur coordinates by matching large features, visible&#xA;from Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;All were locateable in this way except for Surveyor V.  Surveyor I was actually photographed on the surface itself by a Lunar Orbiter.&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiQ9cnomKrSAhUC2WMKHZwfCmoQFggcMAA&amp;amp;url=https%3A%2F%2Fwww.hq.nasa.gov%2Falsj%2FSurveyorProgramResultsNASA-SP-184.pdf&amp;amp;usg=AFQjCNFDDU8VQJL7hNWkZEgt8jVFeOHVFA&amp;amp;sig2=a4zzadaEehzwkiahzGo6oA&quot; rel=&quot;noreferrer&quot;&gt;Surveyor Program Results&lt;/a&gt;, NASA SP-184, page 8.&lt;/p&gt;&#xA;&lt;p&gt;There is more detail in the chapter Location of the Surveyor Spacecraft (page 36):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The landed location of the Surveyor III spacecraft was pinpointed (ref. 3-14) on Lunar Orbiter IV photograph H - 125, as were the&#xA;following eight measured points : Mills 308, Gambart R , Fra Mauro&#xA;B , Saunder 233 , Lansberg beta, Euclides K , Arthur 3402511 , and&#xA;Lansberg N. Adopting a similar procedure to that used for Surveyor I ,&#xA;the following coordinates were determined for Surveyor III:&lt;/p&gt;&#xA;&lt;ol start=&quot;23&quot;&gt;&#xA;&lt;li&gt;34° W ± 0.01°,&#xA;2.97° S ± 0.01°&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;On. the Saunder-Franz system , these would be&lt;/p&gt;&#xA;&lt;p&gt;23.34° W, 2.99° S.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-25T19:01:33.853" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20356" PostTypeId="1" AcceptedAnswerId="20363" CreationDate="2017-02-25T04:45:06.010" Score="3" ViewCount="1085" Body="&lt;p&gt;I'd like to find a fairly precise location of the center of mass of the ISS. I understand it moves for several reasons, including shifting of loads, flexing, and thermal expansion, but to say 1 meter precision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm guessing it falls within the main structural truss, within Truss Segment Zeros &lt;code&gt;S0&lt;/code&gt;, near where the &quot;crew axis&quot; crosses, but I haven't found anything definitive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to plot the microgravity field in 3D using Blender. It is calculated referenced to the ISS orbit, &lt;em&gt;but it is pinned to the ISS at the stations center of mass&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/poszP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/poszP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uC8Pw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uC8Pw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; International Space Station, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:International_Space_Station_after_undocking_of_STS-132.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZxZHP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZxZHP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Handy little 3ds model of ISS imported into Blender, from &lt;a href=&quot;https://nasa3d.arc.nasa.gov/models&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BhcYr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BhcYr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; 2D Vector plot of acceleration relative to the orbit in a given plane.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-02-25T05:25:23.950" LastActivityDate="2017-02-25T11:33:44.180" Title="Where is the center of mass of the ISS relative to it's internal coordinates?" Tags="|iss|spacecraft|design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20358" PostTypeId="1" AcceptedAnswerId="20643" CreationDate="2017-02-25T06:00:10.977" Score="1" ViewCount="1162" Body="&lt;p&gt;Here are screen captures from two sites that seem to be showing approximately live - or at least changing - telemetry from the ISS. They both say the spacecraft attitude is shown in degrees.&lt;/p&gt;&#xA;&lt;p&gt;Looking more closely with fresh data, there is a relationship between the two sets of attitudes, as if they had different definitions of the spacecraft axis. The first one has &lt;code&gt;Roll: -175.93&lt;/code&gt; while the second shows &lt;code&gt;Yaw: -4.08&lt;/code&gt; - they sum to &lt;code&gt;180&lt;/code&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; the situation has been status quo for &lt;em&gt;at least&lt;/em&gt; the past two weeks, it's not a transient.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;25-Feb-2017 06:18 UTC&lt;/p&gt;&#xA;&lt;h2&gt;&lt;strong&gt;below:&lt;/strong&gt; from &lt;a href=&quot;http://www.telemetry.space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.telemetry.space/&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/K8jPE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/K8jPE.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JKXvf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JKXvf.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;25-Feb-2017 06:18 UTC&lt;/p&gt;&#xA;&lt;h2&gt;&lt;strong&gt;below:&lt;/strong&gt; from &lt;a href=&quot;http://isslive.com/displays/adcoDisplay2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://isslive.com/displays/adcoDisplay2.html&lt;/a&gt;&lt;/h2&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sunTH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sunTH.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-14T12:26:08.437" Title="Why are the ISS attitude values shown on these two web sites so different?" Tags="|iss|attitude|attitude-determination-and-control|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20359" PostTypeId="2" ParentId="20353" CreationDate="2017-02-25T06:03:22.830" Score="17" Body="&lt;p&gt;(This answer doesn't directly address the question, but it dovetails well with Organic Marble's on-topic answer.)&lt;/p&gt;&#xA;&lt;p&gt;On the landing approach, Conrad saw and identified &amp;quot;Surveyor crater&amp;quot; very quickly after the &amp;quot;pitchover&amp;quot; maneuver that allowed a surface view out the windows of the LM. This was a 400-foot wide crater inside which Surveyor III was known to be. The crew had trained with both photos and models of the terrain built from Lunar Orbiter imagery, so knew what to look for.&lt;/p&gt;&#xA;&lt;p&gt;Here are some excerpts from the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a12/&quot; rel=&quot;noreferrer&quot;&gt;annotated transcripts&lt;/a&gt;; square brackets are official annotations, curly brackets mine:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;110:29:03 Conrad: I'm trying to cheat and look out there. I think I see my crater.&lt;/p&gt;&#xA;&lt;p&gt;110:29:08 Bean: (Garbled)&lt;/p&gt;&#xA;&lt;p&gt;110:29:09 Conrad: I'm not sure.&lt;/p&gt;&#xA;&lt;p&gt;[Pete is leaning forward, pressing his helmet against the window in an attempt to see the landing site. Figure 4-9 from the Mission Report shows a portion of a view Pete might have had in the LM simulator.]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QKoAr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QKoAr.jpg&quot; alt=&quot;Figure 4-9&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;[Head Crater is the large crater just below the top of the image.&#xA;Surveyor Crater, which is 'his crater', is the larger crater below&#xA;Head Crater. The bottom left image shows the view at pitchover as seen&#xA;by the 16-mm camera mounted in Al's window. The sketch at bottom right&#xA;includes an outline of the Snowman. {the center of which is Surveyor Crater}]&lt;/p&gt;&#xA;&lt;p&gt;110:29:10 Bean: Coming through 7 (thousand feet). (Garbled) P64. P64, Pete. (Garbled)&lt;/p&gt;&#xA;&lt;p&gt;110:29:14 Conrad: P64.&lt;/p&gt;&#xA;&lt;p&gt;110:29:14 Bean: Pitching over&lt;/p&gt;&#xA;&lt;p&gt;110:29:15 Conrad: That's it; there's LPD.&lt;/p&gt;&#xA;&lt;p&gt;[In the Landing Point Designator mode of operation, Pete can use his handcontroller to re-target the LM. If he wants to redesignate the target, he clicks the handcontroller left or right, forward or back to get a small change; or he gives several clicks to get a big change.]&lt;/p&gt;&#xA;&lt;p&gt;110:29:17 Carr {CAPCOM}: Roger. Copy P64.&lt;/p&gt;&#xA;&lt;p&gt;110:29:18 Bean: Okay, there's 6000 update.&lt;/p&gt;&#xA;&lt;p&gt;110:29:20 Conrad: (Very excited) Hey, there it is! There it is! Son-of-a-Gun! Right down the middle of the road!!!&lt;/p&gt;&#xA;&lt;p&gt;110:29:25 Bean: Outstanding! 42 degrees, Pete.&lt;/p&gt;&#xA;&lt;p&gt;110:29:27 Conrad: Hey, it's targeted right...&lt;/p&gt;&#xA;&lt;p&gt;110:29:28 Bean: 42.&lt;/p&gt;&#xA;&lt;p&gt;110:29:28 Conrad: ...for the center of the crater!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://web.mit.edu/digitalapollo/&quot; rel=&quot;noreferrer&quot;&gt;Digital Apollo&lt;/a&gt;, the initial, automatically selected landing target would have been about 600 feet to the southwest of Surveyor itself if Conrad had let the P65 automatic landing program have its way. Apparently he didn't like the look of the terrain there, so made a number of landing point redesignations under the automatic system, before taking over manual control at about 400 feet altitude. He wound up touching down about 600 feet WNW of Surveyor.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uUcHa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uUcHa.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;So Conrad wasn't flying around looking for the probe so much as looking for a good parking spot -- he could see he was in basically the right place.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-26T07:11:28.107" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20361" PostTypeId="1" AcceptedAnswerId="22349" CreationDate="2017-02-25T06:37:42.540" Score="4" ViewCount="657" Body="&lt;p&gt;The nicely and carefully written question &lt;a href=&quot;https://space.stackexchange.com/q/3262/12102&quot;&gt;Have any animals that have been studied onboard the ISS come back alive?&lt;/a&gt; has an affirmative answer that applies to the space shuttle. Now that return to Earth comes in capsule form, have animals ever returned live to the surface — &lt;em&gt;by design?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-05-06T03:02:10.873" LastActivityDate="2024-02-27T04:00:26.120" Title="Post-shuttle, &quot;Have any animals that have been studied onboard the ISS come back alive?&quot;" Tags="|iss|research|animals|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="20362" PostTypeId="2" ParentId="20330" CreationDate="2017-02-25T07:20:17.930" Score="0" Body="&lt;p&gt;Note well: I'm using the terms &quot;chaser&quot; and &quot;target&quot; in this answer. These are technical terms with a long heritage (relatively to the space era). The chaser vehicle acts to change its orbit. The target vehicle keeps its orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose the chaser and target are orbiting an object with no atmosphere, and that the chaser is in the same orbital plane as is the target, and that the chaser has the same semi-major axis as the target. Tweak the chaser's other orbital parameters just right and the chaser will follow a more or less oval shape about the target. Think NASCAR, where drivers only turn left. (I prefer non-oval races. Then again, F1 is boring.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tweak it just right and, ignoring atmospheric drag, the chaser will &quot;orbit&quot; the target in an oval centered on but always missing the target. Atmospheric drag is not something to ignore. The vehicles that bring crew and resupplies to the ISS are bullets. In comparison, the ISS is a huge butterfly. Fortunately, the ISS is a cooperative target. (Other targets: Not so much.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that the Internet is chock full of mentions about racetracks but never mentions the details, I gather that those details are not to be released. But the summary description as an oval where the vehicle only turns in one direction thanks to gravitation: that's just Kepler's laws.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Note well: If the DoD had it's druthers, $F=ma$ and $F=GMm/r^2$ would be classified as TS NOFORN, as would be Kepler's laws. That's one of those details.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="12519" LastEditDate="2017-02-28T14:39:40.040" LastActivityDate="2017-02-28T14:39:40.040" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20363" PostTypeId="2" ParentId="20356" CreationDate="2017-02-25T10:39:35.227" Score="5" Body="&lt;p&gt;The information you need can be found in &lt;a href=&quot;http://athena.ecs.csus.edu/%7Egrandajj/ME296M/RevAB_Volume%20I%20Signed_updated.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;On-Orbit Assembly, Modeling, and Mass Properties Data Book&lt;/strong&gt;, &lt;em&gt;Volume I &amp;amp; II&lt;/em&gt;, &lt;em&gt;International Space Station Program&lt;/em&gt;, January 2008 (JSC 26557)&lt;/a&gt; (you need an updated version for more current data, here it's Revision AB).&lt;/p&gt;&#xA;&lt;p&gt;First, the &lt;strong&gt;Space Station Analysis Coordinate System (SSACS)&lt;/strong&gt; is defined as follows (see page 26):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The origin is located at the &lt;strong&gt;geometric center&lt;/strong&gt; of the mid-ship&#xA;Integrated Truss Segment (ITS) &lt;strong&gt;S0&lt;/strong&gt;. The longitudinal x-axis of multiple core modules, including the Zarya Functionalni Gruzvoi Blok (FGB) and Unity Node 1, is parallel with the analysis coordinate system axis XA, positive in the direction of the velocity vector. Positive YA axis runs parallel with the starboard truss from the center point at S0. Axis ZA completes the triad, pointing to the nadir.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The center of mass positions are then given in (see page 82)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Distance along X/Y/Z-axis from origin in SSACS&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The largest part of this document is then devoted to giving you the information about the different configurations of the ISS. For example, for the configuration after the separation the shuttle during STS-132 (see page 542), you get:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Center of mass:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;X:&lt;/strong&gt;&lt;br /&gt;&#xA;-4.37 m  (-14.34 ft)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Y:&lt;/strong&gt;&lt;br /&gt;&#xA;-.81 m  (-2.66 ft)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Z:&lt;/strong&gt;&lt;br /&gt;&#xA;3.43 m (11.25 ft)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Note that the document I linked above ends after STS-133 (early March 2011). That's after most of the assembly was finished, but about 8 t of equipment was since installed, most of it during STS-134, so you may have to look for a more current version of the document if you need high precision information on the current state of the station.&lt;/p&gt;&#xA;" OwnerUserId="4577" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-25T11:33:44.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20364" PostTypeId="2" ParentId="20358" CreationDate="2017-02-25T14:48:37.360" Score="3" Body="&lt;p&gt;The ISS Live page is using the &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwiP-qLpvKvSAhVmrVQKHQcOA38QFggcMAA&amp;amp;url=http%3A%2F%2Fwww.asi.org%2Fadb%2F04%2F02%2F00%2Fiss-coordinate-systems.pdf&amp;amp;usg=AFQjCNGA_qnoi_2ou_RNoKHVbfuEgY4eFg&amp;amp;sig2=o3FFJ6oADwREX3K-Vl4MCw&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISSACS system&lt;/a&gt; (ISS Analysis frame) which is the normal coordinate frame for this data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just doing the transforms in my head, it looks like the other web site is using a frame where the Y axis is the same as ISSACS, but the frame is pitched so that +X points to nadir and +Z points to ISS aft.  Thus pitch is the same in both frames, but ISSACS +roll becomes -yaw in the new frame, and ISSACS +yaw becomes -roll.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a given data snapshot&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISSACS     /    &quot;Pitched ISSACS&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yaw    -4     /   -.55&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pitch  -1   /      -1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roll   .61   /    -176&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GIMPed picture of the &quot;Pitched Frame&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3xQzt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3xQzt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-02-25T15:03:17.267" LastActivityDate="2017-02-25T15:03:17.267" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20366" PostTypeId="1" CreationDate="2017-02-25T16:03:12.663" Score="-2" ViewCount="386" Body="&lt;p&gt;How do we have telescopes that are powerful enough to see nearby galaxies, such as Andromeda (M31), but are unable to see TRAPPIST-1 or the other exoplanets in its solar system in detail when TRAPPIST-1 is only 39 light years away?&lt;/p&gt;&#xA;" OwnerUserId="18702" LastEditorUserId="13710" LastEditDate="2017-02-25T18:05:49.313" LastActivityDate="2017-02-28T13:25:56.447" Title="How come we can see nearby galaxies but are unable to see TRAPPIST-1?" Tags="|nasa|telescope|trappist-1|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20367" PostTypeId="1" CreationDate="2017-02-25T16:17:46.133" Score="2" ViewCount="600" Body="&lt;p&gt;Super-cold LOX gives advantages in performance for a rocket. But it isn't used in all rockets. SpaceX's Merlin 1D rocket engine uses super-cold LOX. Also NK-33 use sub-cooled LOX. Could super-cold LOX be used in every rocket engine? Does it needs modification with new parts or does it need a new design of the entire engine?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="-1" LastEditDate="2017-03-01T07:05:00.843" LastActivityDate="2017-03-02T16:38:57.447" Title="Could super-cold LOX be used in every rocket engine?" Tags="|fuel|engine-design|liquid-fuel|lox|sub-cooled|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20368" PostTypeId="2" ParentId="20366" CreationDate="2017-02-25T16:24:13.380" Score="4" Body="&lt;p&gt;Trappist-1 is difficult to see because it's not very big.  In fact it's so small (~11% the radius, and ~8% the mass of the Sun), that if it were any less massive, it wouldn't be able to support fusion!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the human eye isn't very good at seeing much of the EM spectrum.  So it's not emitting much visible light, &lt;strong&gt;BUT&lt;/strong&gt; we can build tools to detect the infrared light that Trappist-1 gives off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Galaxies on the other hand are big.  REALLY big. So big in fact that they contain somewhere on the order of 100 thousand million stars!  And of these stars almost all of them are brighter than Trappist-1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we aren't really seeing those galaxies in more absolute detail, but since they are so much more bigger, we can see &quot;intricacies&quot; at the star-level&lt;/p&gt;&#xA;" OwnerUserId="17351" LastEditorUserId="18649" LastEditDate="2017-02-28T13:25:56.447" LastActivityDate="2017-02-28T13:25:56.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20370" PostTypeId="1" CreationDate="2017-02-25T16:59:33.167" Score="-4" ViewCount="104" Body="&lt;p&gt;why Do we have these telescopes to see these galaxy's and stars and we know about the cosmic web way out of our reach we have the technology to see that why not these planets? Is there a way we could use these telescopes or even the satellites to bounce light off of in order to see these exoplanets that are in the TRAPPIST-1?&lt;/p&gt;&#xA;" OwnerUserId="18702" LastEditorUserId="18702" LastEditDate="2017-02-25T18:22:12.537" LastActivityDate="2017-02-25T20:02:24.290" Title="Exploration, and research" Tags="|nasa|" AnswerCount="1" CommentCount="5" ClosedDate="2017-02-25T20:21:00.203" ContentLicense="CC BY-SA 3.0" />
  <row Id="20372" PostTypeId="2" ParentId="20366" CreationDate="2017-02-25T19:32:52.537" Score="3" Body="&lt;p&gt;In recent years, astronomers have been able to see (in their words, 'image directly') a few exoplanets. This is one of those:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6Zhoc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6Zhoc.jpg&quot; alt=&quot;exoplanet&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have found &lt;a href=&quot;https://en.wikipedia.org/wiki/Exoplanet&quot; rel=&quot;nofollow noreferrer&quot;&gt;about 2000 exoplanets&lt;/a&gt; so far, but we've only been able to directly image &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets&quot; rel=&quot;nofollow noreferrer&quot;&gt;22 of them&lt;/a&gt; as of 2017. Most exoplanets are found using indirect methods (e.g. by measuring the star's brightness over time, if there is a regular dip in brightness that may mean a planet has passed in front of the star). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's difficult to directly image an exoplanet.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;planets are very small compared to stars,&lt;/li&gt;&#xA;&lt;li&gt;they are far less bright than stars,&lt;/li&gt;&#xA;&lt;li&gt;and they are (when viewed from far away) very close to their star.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In the image above you can see a 'halo' around the star. This is an optical effect: the star's light gets smeared out to a larger diameter than it should be. For many exoplanets, this means the planet gets lost in this halo.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Angular_resolution&quot; rel=&quot;nofollow noreferrer&quot;&gt;smallest object you can see&lt;/a&gt; in a telescope is determined by the &lt;a href=&quot;https://space.stackexchange.com/questions/9460/were-the-apollo-lunar-activities-observed-from-earth&quot;&gt;angular resolution&lt;/a&gt; of a telescope. Most planets are so close to their stars that they're below the angular resolution of even the largest telescopes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have spectacular images of nearby galaxies because they're really big. This is the &lt;a href=&quot;https://sploid.gizmodo.com/the-incredibly-huge-size-of-andromeda-1493036499&quot; rel=&quot;nofollow noreferrer&quot;&gt;Andromeda galaxy&lt;/a&gt; compared to the Moon: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/B2CFU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/B2CFU.jpg&quot; alt=&quot;Andromeda is huge&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-02-26T09:28:47.443" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20373" PostTypeId="2" ParentId="20370" CreationDate="2017-02-25T19:53:23.173" Score="2" Body="&lt;p&gt;Answering just the last sentence, because the rest is already addressed &lt;a href=&quot;https://space.stackexchange.com/questions/20366/how-come-we-can-see-nearby-galaxies-but-are-unable-to-see-trappist-1&quot;&gt;elsewhere&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That exoplanet is right next to a star, so it is illuminated by a lot of light. But that star is 40 lightyears away from us, so only a few photons of its light reach us, and even fewer photons of starlight reflected by its planets reach us.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any light we could send to that star (in order to better see its planets) is completely negligible compared to the light our sun already sends that way. If you could travel to Trappist-1 and look back at our sun, you'd see a tiny star, which adds a negligible amount of illumination to Trappist-1 and its planets. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-02-25T20:02:24.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20374" PostTypeId="1" CreationDate="2017-02-25T22:08:29.663" Score="7" ViewCount="951" Body="&lt;p&gt;It is my understanding that the Falcon 9 thrust vector control (TVC) actuators use RP-1 as hydraulic fluid and use the fuel turbopump energy to drive the hydraulic pump for the TVC.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do you do a preflight test of actuators since the turbo pumps are not running?&lt;/p&gt;&#xA;" OwnerUserId="18706" LastEditorUserId="6816" LastEditDate="2017-02-25T22:29:35.337" LastActivityDate="2017-03-28T12:49:32.600" Title="Preflighting TVC actuators on Falcon 9" Tags="|launch|falcon-9|launch-sequence|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20376" PostTypeId="1" AcceptedAnswerId="20378" CreationDate="2017-02-26T01:14:25.933" Score="4" ViewCount="230" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/20349/12102&quot;&gt;This answer&lt;/a&gt; details some of the requirements that would need to be met if the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Launch System&lt;/a&gt;'s planned &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_Mission_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Exploration Mission 1&lt;/a&gt; were to be human rated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've further trimmed that truncated selection from the &lt;a href=&quot;https://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8705_002B_&amp;amp;page_name=Chapter3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Human Rating Requirements for Space Systems&lt;/a&gt; document down to four items that mention autonomous operation in the event of some particular catastrophic event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose a contrived, hypothetical catastrophic event occurred which blocked telemetry and any GNSS${}^*$ from Earth and introduced a small but unquantified changes in heading, as the spacecraft was approaching the moon. Here &quot;unquantified&quot; means that for some reason the otherwise excellent inertial guidance system was unable to accurately record some substantial propulsive impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What naturally occurring navigational information might the navigation system use to autonomously and safely go around the moon and return to Earth for a safe and successful reentry and landing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; The scenario is not intended to be a realistic situation, but only to help set the stage for a hypothetical question; if a spacecraft were suddenly to find itself in the Earth-Moon system, could it deduce its state vector and navigate to a landing without &quot;help&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cameras + image processing comes to mind, but they would have to be carefully calibrated in order to allow eventual regeneration of accurate state vectors. Radar timing of the lunar surface during the close passage also comes to mind. Are these accurate enough to be sufficient to execute a safe reentry, or is more needed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm trying to understand what naturally occurring navigational information is available in &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_space#Cislunar_space&quot; rel=&quot;nofollow noreferrer&quot;&gt;cislunar space&lt;/a&gt; that would facilitate a spacecraft's 100% autonomous planning and execution of a safe return if artificial data from earth (telemetry, GNSS, etc.) were not available and inertial guidance historical data were imperfect.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;${}^*$GNSS fixes are potentially available intermittently to spacecraft in cislunar space, see the answers associated with &lt;a href=&quot;https://space.stackexchange.com/q/565/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/5551/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/14739/12102&quot;&gt;this&lt;/a&gt; question.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;3.2 System Safety Requirements (extremely 'cropped' subset thereof)&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.2.11 The crewed space system shall provide the capability for autonomous operation of system and subsystem functions which, if lost,&#xA;  would result in a catastrophic event (Requirement 58576).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;3.6 Crew Survival/Abort Requirements&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.6.2 Earth Orbit Systems&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.6.2.1 The crewed space system shall provide the capability to autonomously abort the mission from Earth orbit by targeting and&#xA;  performing a deorbit to a safe landing on Earth (Requirement 58625).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.6.3 Earth - Lunar Transit and Lunar Orbit Systems&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.6.3.1 The crewed space system shall provide the capability to autonomously abort the mission during lunar transit and from lunar&#xA;  orbit by executing a safe return to Earth (Requirement 58627).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.6.4 Lunar Descent Systems&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.6.4.1 The crewed space system shall provide the capability to autonomously abort the lunar descent and execute all operations&#xA;  required for a safe return to Earth (Requirement 58629).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-02-27T00:30:07.540" Title="What naturally occurring navigational information is available for an autonomous return of SLS's EM-1?" Tags="|navigation|sls|autonomy|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20378" PostTypeId="2" ParentId="20376" CreationDate="2017-02-26T07:04:23.993" Score="5" Body="&lt;p&gt;Optical navigation would be more than sufficient. Images of the Moon and the Earth against key stars, a clock, and a computer is all you need. Apollo had such a system as a backup, which was used to verify the ground tracking results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the Apollo sextant:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QAgo1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QAgo1.jpg&quot; alt=&quot;Apollo sextant&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The definition of &quot;autonomous&quot; in the document is:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Autonomous: Ability of a space system to perform operations&#xA;  independent from any Earth-based systems. This includes no&#xA;  communication with, or real-time support from, mission control or&#xA;  other Earth systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So indeed the orbit determination would need to be possible using only on-board resources. Though you need to be able to &lt;em&gt;look&lt;/em&gt; at the Earth, so hopefully the Earth itself is not considered an Earth system.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-02-26T20:32:30.863" LastActivityDate="2017-02-26T20:32:30.863" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20379" PostTypeId="1" AcceptedAnswerId="20380" CreationDate="2017-02-26T08:07:34.477" Score="0" ViewCount="1098" Body="&lt;p&gt;When looking at moonwalk videos, we can hear bips. What are those bips? Why are there bips? Do this bips still exist in nowaday space missions?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="12102" LastEditDate="2017-02-26T11:13:41.033" LastActivityDate="2017-02-26T11:13:41.033" Title="What purpose serve the audible &quot;bip&quot; on apollo radio communications?" Tags="|nasa|apollo-program|radio-communication|audio|" AnswerCount="1" CommentCount="1" ClosedDate="2017-02-26T14:08:43.813" ContentLicense="CC BY-SA 3.0" />
  <row Id="20380" PostTypeId="2" ParentId="20379" CreationDate="2017-02-26T09:19:10.600" Score="3" Body="&lt;p&gt;These &quot;beeps&quot; are called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Quindar_tones&quot; rel=&quot;nofollow noreferrer&quot;&gt;quindar tones&lt;/a&gt; (named after the manufacter Quindar Electronics, Inc). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These tones were used to simulate the action of the push to talk button. CAPCOM always had to push a button during communication. The first tone (2.525KHz / 250ms) triggers the ground station transmitters to send, the second tone (2.475KHz / 250ms) turns the transmitter off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quindar tones became very famous during the Apollo program but were used also before. Also the first Space Shuttle missions used them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today the quindar tones are not needed anymore because modern communication systems can carry multiple communication channels.&lt;/p&gt;&#xA;" OwnerUserId="17378" LastEditorUserId="17378" LastEditDate="2017-02-26T09:30:07.220" LastActivityDate="2017-02-26T09:30:07.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20381" PostTypeId="1" AcceptedAnswerId="20384" CreationDate="2017-02-26T11:12:17.997" Score="9" ViewCount="2027" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/4746/12102&quot;&gt;This&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/20380/12102&quot;&gt;this&lt;/a&gt; answer explain the function of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Quindar_tones&quot; rel=&quot;noreferrer&quot;&gt;Quindar_tones&lt;/a&gt; within Apollo mission comms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 2525 and 2475 HZ they are only 50Hz or 2% apart in frequency, and the duration is 250 ms or 0.25 seconds. You can &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Quindar_tones.ogg&quot; rel=&quot;noreferrer&quot;&gt;listen to them here&lt;/a&gt; (also included &lt;a href=&quot;https://en.wikipedia.org/wiki/Quindar_tones&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;). Assuming the audio is an accurate representation, the abrupt perceived 'click-like' beginning end ending of the tones suggest that they are not Gaussian-&lt;a href=&quot;https://en.wikipedia.org/wiki/Apodization#Apodization_in_signal_processing&quot; rel=&quot;noreferrer&quot;&gt;apodized&lt;/a&gt; or ramped up or down, but just abruptly turned ON for a quarter second then turned OFF.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; A half-tone, the step between adjacent notes on a piano or other musical instrument with common 'well-tempered' tuning is a twelfth of an octave, or a ratio $2^{1/12} \approx 1.06%$; a difference of 6%. The Quindar tone separation is &lt;a href=&quot;https://en.wikipedia.org/wiki/Microtonal_music&quot; rel=&quot;noreferrer&quot;&gt;microtonal&lt;/a&gt;, with a difference of 2% or only 1/3 of a half-tone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back of the envelope speaking, the tones execute about 631 and 619 cycles during the quarter second, which suggests that if I plotted the spectrum of the two Quindar tones, I'd get two peaks with a separate roughly 12 times larger than their widths. (12 = 631-619) Luckily my life-time supply of envelope backs has not run out (see below). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 2% shift in frequency could be fairly well distinguished with a pair of centered audio filters with a Q in the 50 to 100 range - a valid Quindar tone would pass through both but be much larger in one than the other. But if there is substantial noise due to crossed signals or other interference common in 1960's telephone lines, there could be some problems. Also there is a possibility of thermal or other drift in component values that might cause these filters to drift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Why were the two Quindar tones so close in frequency, and not apodized? There must have been some pressure to keep them only 50Hz apart. This is far closer than necessary to fit through the 300-3000Hz nominal passband of telephone lines. Why so close? Some apodization would have supressed the tailes of the frequency distributions so that each tone would have presented less power in the other tone's filter's passband. Why the hard ON-OFF gating?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Simple FT of computer generated 0.25 second square-&lt;a href=&quot;https://en.wikipedia.org/wiki/Apodization#Apodization_in_signal_processing&quot; rel=&quot;noreferrer&quot;&gt;apodized&lt;/a&gt; tones of 2525 and 2475 Hz, each with 1 second duration and 2${}^{20}$ samples. (Python is in front of me and I'm too lazy to look up the analytical form.) Left: dB relative, Right: linear scale. FWHM of peaks are roughly 12 times the separation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NIK4s.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NIK4s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2022-03-07T04:56:10.490" Title="Why were the two Quindar tones so close in frequency and not apodized?" Tags="|nasa|apollo-program|radio-communication|audio|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20382" PostTypeId="2" ParentId="20374" CreationDate="2017-02-26T12:29:22.187" Score="1" Body="&lt;p&gt;Before every flight, SpaceX do a &lt;a href=&quot;https://spaceflightnow.com/2017/01/06/engine-hotfire-test-completed-for-spacexs-first-launch-since-explosion/&quot; rel=&quot;nofollow noreferrer&quot;&gt;static fire test&lt;/a&gt; where the rocket is placed on the launch pad and the first stage engines are fired. That's an opportunity to test the gimbaling system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9 user manual&lt;/a&gt;, the gimbaling system doesn't seem to be tested during the launch countdown (where you'd have a few seconds to do the tests between ignition and release): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Engine ignition occurs shortly before liftoff, while the vehicle is held down at the base via hydraulic clamps. The flight computer evaluates engine ignition and full-power performance during the prelaunch hold-down, and, if nominal criteria are satisfied, the hydraulic release system is activated at T- 0. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-02-26T12:29:22.187" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20384" PostTypeId="2" ParentId="20381" CreationDate="2017-02-26T19:08:13.660" Score="9" Body="&lt;p&gt;Echo cancellation was by no means perfect, as can often be heard in the Apollo audio tapes. In order to prevent false activation of  the telemetry receiver (located at the tracking station) by echoes from the spacecraft, the inband signaling had to be filtered from the uplink. Having MARK and SPACE so close enabled to filter with a single  notch filter, suppressing only a minor part of the voice signal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How come the tones could be heard at all? Quote from &lt;a href=&quot;https://www.hq.nasa.gov/alsj/quindar.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;While the Quindar filter removed nearly all of the tones, there were cases where you can still hear a little of the unkey tone turned around. This occurred mainly when the audio was distorted a little, causing more of the tone to be uplinked.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It is often told, that the Quindar tones were used to switch the &quot;transmitter&quot;. Apollo used a system called &lt;a href=&quot;https://www.ab9il.net/aviation/apollo-s-band.html&quot; rel=&quot;noreferrer&quot;&gt;Unified S Band&lt;/a&gt; to transmit several channels on subcarriers, and only the voice subcarrier modulation was switched, not the whole uplink signal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Same source as above.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Detection of the tone closed a relay to connect the Net 1 voice circuit to the Unified S-band uplink voice subcarrier.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Quindar Electronics has filed several patents related to telemetry apparel in the sixties/seventies. &lt;a href=&quot;https://patents.google.com/patent/US3681696A/en?q=quindar+electronics&amp;amp;sort=old&quot; rel=&quot;noreferrer&quot;&gt;One of them&lt;/a&gt; suggests, that noise suppression was used after demodulation, requiring fast attack/decay-times on the tones (apodized tones may have been swallowed by the noise suppression). -26dB crosstalk was probably a minor issue, compared to other imperfections due to the poor quality of the leased lines to Guam.&lt;/p&gt;&#xA;" OwnerUserId="17004" LastEditorUserId="17004" LastEditDate="2017-02-27T09:16:21.957" LastActivityDate="2017-02-27T09:16:21.957" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20385" PostTypeId="2" ParentId="20366" CreationDate="2017-02-26T23:35:48.943" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How do we have telescopes that are powerful enough to see nearby galaxies, such as Andromeda (M31), but are unable to see TRAPPIST-1 or the other exoplanets in its solar system in detail when TRAPPIST-1 is only 39 light years away?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You don't need a telescope to see M31. It has an apparent magnitude of 3.4, which means it is easily visible to the naked eye, perhaps even in cities. The reason is that the Andromeda Galaxy comprises about a trillion stars that collectively emit about 26 billion times as much visible radiation as does the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare that to TRAPPIST-1 (the star, not the planets). This is an extremely dim star, about as small and dim as stars can get. TRAPPIST-1 has an apparent magnitude of 18.8, which means that it is inaccessible to all but the most powerful of telescopes. It emits about 1/260000th of the visible radiation emitted by the Sun. Another way to look at it: Andromeda emits about 7 quadrillion ($7\times10^{15}$) more light than does TRAPPIST-1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With regard to seeing the exoplanets that orbit that star, the furthest out exoplanet orbits TRAPPIST-1 at a radius of about 0.063 astronomical units. At 39.5 light years away, that means a maximum separation of 0.005 arc seconds between the star and the exoplanet.  The Hubble Space Telescope's resolution is about 0.1 arc seconds. TRAPPIST-1 and all seven of its exoplanets would resolve as a single pixel.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-02-26T23:35:48.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20386" PostTypeId="1" AcceptedAnswerId="20388" CreationDate="2017-02-27T01:15:32.040" Score="9" ViewCount="1200" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/questions/20353/how-did-apollo-12-manage-to-land-next-to-surveyor-3-first-space-tourists/20355#comment54722_20353&quot;&gt;comment by @Hobbes&lt;/a&gt; on the question &lt;a href=&quot;https://space.stackexchange.com/q/20353/12102&quot;&gt;How did Apollo-12 manage to land next to Surveyor-3? First “Space-Tourists”?&lt;/a&gt; points out that the &quot;souvenirs&quot; removed and retrieved from the Surveyor-3 lander by the astronauts and returned to Earth were studied for &quot;the effects of long-term exposure to space&quot; on moon exploration equipment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was important not only for basic science, but for planning for equipment to be left on the moon by subsequent Apollo missions and further space exploration as well. The components had been exposed to the lunar space environment and solar radiation for 31 and 16 months, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any discussion or reports of &quot;&lt;em&gt;What was learned from study of Surveyor-3's components, retrieved and returned to Earth by Apollo-12?&lt;/em&gt;&quot;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-02-27T16:04:10.437" Title="What was learned from study of Surveyor-3's components, retrieved and returned to Earth by Apollo-12?" Tags="|lunar-landing|apollo-12|surveyor-program|surveyor-3|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20388" PostTypeId="2" ParentId="20386" CreationDate="2017-02-27T02:08:46.353" Score="10" Body="&lt;p&gt;The Apollo 12 &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a12/as12psr.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Preliminary Science Report&lt;/a&gt; contains a section on Surveyor 3. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Among other findings:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;no evidence of &quot;cold welding&quot; of the parts.&lt;/li&gt;&#xA;&lt;li&gt;dust kicked up during the landing of the Apollo 12 LM pitted and &quot;sandblasted&quot; one side of Surveyor &lt;/li&gt;&#xA;&lt;li&gt;micrometeoroid pitting was light and confirmed the estimates used in designing the Apollo spacecraft to withstand same &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There's also a more detailed &lt;a href=&quot;http://www.lpi.usra.edu/lunar/documents/NTRS/collection2/NASA_SP_284.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Analysis of Surveyor 3 material and photographs&lt;/a&gt;, which among other things:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mentions that a &lt;em&gt;streptococcus&lt;/em&gt; bacterium was isolated from a piece of foam in the interior of the camera, presumably a stowaway, but later analysis suggests the possibility that it was contamination occurring after the return from Apollo 12.&lt;/li&gt;&#xA;&lt;li&gt;No living microbes appeared to have survived in a bundle of wiring, but that's inconclusive because they have no way of knowing if there were any living microbes in there when Surveyor was launched.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.lpi.usra.edu/lunar/documents/NTRS/collection2/NASA_SP_284.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;the report's&lt;/a&gt; preface:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Engineering studies of the television camera show that the complex electromechanical components, optics, and solid state electronics were remarkably resistant to the severe lunar surface environment over 32 lunar day/night cycles with their extremes of temperature and long exposure to solar and cosmic radiation. These results indicate that the state of technology, even as it existed some years ago, is capable of producing reliable hardware that makes feasible long-life lunar and planetary installations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastEditorUserId="12102" LastEditDate="2017-02-27T16:04:10.437" LastActivityDate="2017-02-27T16:04:10.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20389" PostTypeId="1" CreationDate="2017-02-27T04:47:30.477" Score="4" ViewCount="430" Body="&lt;p&gt;It appears that amateur groups have launched rockets to space (above the Karman line), but these were flights of commercial (Pegasus) rockets by amateur teams.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I am interested in is record-setting flights using rockets of amateur design and construction - highest altitude attained, highest velocity attained, longest duration under useful thrust. Has any rocket of amateur design/construction launched from essentially sea-level altitude and made it all the way up to or above the Karman line?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-02-28T08:08:19.837" Title="Record-setting flights in amateur rocketry" Tags="|amateur-rocketry|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20392" PostTypeId="1" AcceptedAnswerId="20393" CreationDate="2017-02-27T15:07:13.453" Score="4" ViewCount="1106" Body="&lt;p&gt;Can I start an amateur rocket using nitrous oxide as either oxidizer or monopropellant, using a hypergolic slug of something reasonable for use by an amateur rocketeer? What substance could it be?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="9092" LastEditDate="2021-03-25T12:02:53.730" LastActivityDate="2021-03-25T12:02:53.730" Title="Is nitrous oxide hypergolic with anything reasonably safe and obtainable?" Tags="|liquid-fuel|amateur-rocketry|oxidizer|hypergolics|nitrous-oxide|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20393" PostTypeId="2" ParentId="20392" CreationDate="2017-02-27T15:39:13.453" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://sbir.nasa.gov/SBIR/abstracts/15/sbir/phase1/SBIR-15-1-S3.02-9159.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;This&lt;/a&gt; is a mix that apparently could work.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;When passed over a warm ruthenium catalyst bed, gaseous nitrous oxide and an ethylene-ethane gaseous blend combust instantly&quot;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Supplies could be found for the following:&lt;a href=&quot;https://www.etsy.com/listing/400869491/ruthenium-metal-9995-pure-element-44-ru?utm_campaign=shopping_us_ChemistryCabinet_sfc_osa&amp;amp;utm_medium=cpc&amp;amp;utm_source=google&amp;amp;utm_custom1=0&amp;amp;utm_content=11632179&amp;amp;gclid=CjwKEAiAuc_FBRD7_JCM3NSY92wSJABbVoxB_s32nISkNFI29jfRIHeJWjzs_ZTHFlij0njcpp6EXBoCi87w_wcB&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ruthenium&lt;/a&gt; and &lt;a href=&quot;https://www.shopcross.com/gas/ethane-c2h6-calibration-gas/sku/103L-152N-50?gclid=CjwKEAiAuc_FBRD7_JCM3NSY92wSJABbVoxBJLo8YNj5F8BA2KuuHqoXIfacPioC7wIhZ9J46Ba8WhoChvbw_wcB&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ethane&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I could not find sources for Ethylene, but it seems to be available for industrial uses. This could disqualify this combination depending on the meaning of 'available'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The safety sheets for &lt;a href=&quot;http://www.inchem.org/documents/icsc/icsc/eics0266.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ethane&lt;/a&gt; and &lt;a href=&quot;http://www.inchem.org/documents/icsc/icsc/eics0475.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ethylene&lt;/a&gt; list only that the gases are very flammable, but unlike Triethylaluminium will not combust on contact with air... so thats a step up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The safety data for &lt;a href=&quot;https://www.caymanchem.com/msdss/14339m.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ruthenium&lt;/a&gt; seems to list is as quite poisonous, so that could disqualify this combination depending on the meaning of 'safe'.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2017-02-27T15:47:33.167" LastActivityDate="2017-02-27T15:47:33.167" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20394" PostTypeId="1" CreationDate="2017-02-27T15:49:02.357" Score="12" ViewCount="3305" Body="&lt;p&gt;SpaceX has a plan to &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_satellite_constellation&quot; rel=&quot;noreferrer&quot;&gt;launch up to 4K satellites&lt;/a&gt; to provide global Internet access. Is there a reasonable estimate of how much this constellation would cost to launch? Also, as the minimum number of satellites to start the service is 800, what is that cost?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-02-27T19:58:58.920" Title="How much will SpaceX's constellation of Internet satellites cost?" Tags="|spacex|artificial-satellite|cost|communication-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20395" PostTypeId="2" ParentId="20394" CreationDate="2017-02-27T16:02:30.740" Score="12" Body="&lt;p&gt;First of all, let's figure out how many satellites per launch vehicle. The &lt;a href=&quot;http://www.businessinsider.com/spacex-internet-satellite-constellation-2016-11&quot; rel=&quot;noreferrer&quot;&gt;estimate of the mass&lt;/a&gt; of these satellites is 386 kg. The mass for a launch of a Falcon 9 is 5500 kg. That means one could launch 13-14 satellites per rocket. I suspect 12 is more realistic, because part of the payload mass is the structure to mount all of the satellites, and that is the number that I will assume for future calculations. Let's assume reusability is done heavily, reducing the cost per launch to, say, \$12 million, less than 20% of the actual cost. That comes to about \$1 million per satellite launched for the launch cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the satellites themselves, I suspect that with that many satellites being produced, they could do it for around \$1 million per satellite, possibly even lower cost as time goes on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, a very generous estimate is \$2 million per satellite. Thus, the cost to launch the initial 800 satellites is about \$1.6 billion, plus R&amp;amp;D. The full constellation would be \$8 billion, plus R&amp;amp;D.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that I'm probably a bit underestimating some of these costs. The more industry standard cost (Subtracting estimated profits) would be &lt;a href=&quot;https://www.google.com/search?q=Falcon%209%20cost%20of%20reused%20rocket&amp;amp;ie=utf-8&amp;amp;oe=utf-8&quot; rel=&quot;noreferrer&quot;&gt;\$36 million per launch&lt;/a&gt; , \$10-15 million per satellite. Let's say \$5 million per satellite manufactured, which should be possible with that economy of scale. Thus, the cost could be as much as \$8 million per satellite, increasing the cost by a factor of 4x. But that would be the highest cost that I could see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the comparable satellites in recent history. I suspect the closer comparison for a SpaceX satellite to be the Irridium constellation.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation#Next-generation_constellation&quot; rel=&quot;noreferrer&quot;&gt;Irridium&lt;/a&gt;- \$2.1 billion to manufacture 66 satellites, or \$32 million/ satellite.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/orbcomm-2.htm&quot; rel=&quot;noreferrer&quot;&gt;Orbcomm&lt;/a&gt;- \$117 million for 18 satellites, or \$6.5 million.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also note that a fair amount of ground stations would be required to get this to work. Add in the R&amp;amp;D, licenses, etc, and I suspect the ground costs are \$1- \$2 billion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, SpaceX could hit US internet availability with somewhere between \$3.6 to \$8.4 billion, and have the entire constellation launched for between \$10 to \$37 billion.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-02-27T19:58:58.920" LastActivityDate="2017-02-27T19:58:58.920" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="20396" PostTypeId="2" ParentId="20392" CreationDate="2017-02-27T16:17:04.667" Score="2" Body="&lt;p&gt;Most if not all hypergolic fuels are toxic to varying degrees. Hydrazine, mono-&#xA;methyl hydrazine, and unsymmetrical dimethyl hydrazine are all in this category. Virgin Galactic Space Ship One used nitrous oxide with aluminum grain but needed an igniter. A venting problem on Apollo-Soyuz return knocked out one astronaut and all three had to be hospitalized for a time. I would avoid hypergolic fuels. They are not for the amateur rocketeer. &lt;/p&gt;&#xA;" OwnerUserId="18706" LastActivityDate="2017-02-27T16:17:04.667" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20398" PostTypeId="1" CreationDate="2017-02-27T18:13:52.477" Score="3" ViewCount="228" Body="&lt;p&gt;Based on points raised in &lt;a href=&quot;https://space.stackexchange.com/questions/20392/is-nitrous-oxide-hypergolic-with-anything-reasonably-safe-and-obtainable&quot;&gt;this&lt;/a&gt; question about hypergolic slugs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding of a hypergolic slug is that it is a solid or a liquid on a paper patch placed in the combustion chamber and it will spontaneously combust on contact with the fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can a gaseous hypergol be used as a starter? If so, how?&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-02-27T18:13:52.477" Title="Hypergolic slugs with gaseous reactants" Tags="|fuel|engine-design|hypergolics|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20400" PostTypeId="2" ParentId="658" CreationDate="2017-02-27T19:08:43.307" Score="0" Body="&lt;p&gt;Any method of solar energy production is problematic the farther you get from the sun. If you increase the distance from 1 AU (Astronomical Unit, the distance from sun to earth) to 5 AU, the solar power density decreases to 1/25.&#xA;Solar cells would need a much larger area and all other methods of solar energy generation too. A solar windmill would require moving mechanical parts, that is a problem for long time relaibility. As long as the efficency of a solar windmill is much smaller than that of solar panels, the necessary area is much larger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Rosetta mission sucessfully demonstrated the use of solar panels with an area of 64 square meters in a distance of 3.4 and 5.25 AU.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-02-27T19:08:43.307" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20402" PostTypeId="2" ParentId="20330" CreationDate="2017-02-27T21:29:46.847" Score="2" Body="&lt;p&gt;In the reference frame of the ISS, the course looks approximately like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kxsNf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kxsNf.jpg&quot; alt=&quot;Racetrack mission profile&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note: distances not to scale.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In an earth-bound reference frame, this is simply a pair of &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann transfers&lt;/a&gt; between two orbits below and above the ISS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the spacecraft was below the ISS, its orbital period was slightly shorter than ISS' orbit, so it eventually came out in a safe distance ahead (and still below) the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A slight prograde burn raised the apogee of its orbit above the ISS. In the apogee, a slight prograde burn circularized the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now being in an orbit above the ISS with an orbital period was slightly longer than ISS' orbit, it eventually came out behind (and still above) the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now another pair of burns, this times retrograde, lowered the orbit back below the ISS, still behind but chasing up, ready for another rendezvous attempt.&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2017-02-27T21:29:46.847" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20403" PostTypeId="2" ParentId="18047" CreationDate="2017-02-27T21:59:18.050" Score="6" Body="&lt;p&gt;Yes, there are two major differences between the traditional launch pads at Baikonur and Plesetsk and the modern launch pads at Kourou and Vostochny:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The analog attitude control system of older R-7 / Soyuz launchers&#xA;could only change the pitch angle during flight - yaw and roll were&#xA;fixed (but controlled). This means that to reach different&#xA;inclination orbits, the launch pad has to be rotated into the&#xA;correct roll angle for that inclination. Soyuz 2.1 has a digital&#xA;attitude control system, which can change all three angles, so that&#xA;launch pad rotation is not necessary with pads which are only&#xA;designed for these newer models (Kourou, Vostochny).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;At Baikonur and Plesetsk, the vehicle is completely integrated horizontally in the vehicle assembly building and the complete vehicle is then rolled out to the launch pad. For Soyuz ST at Kourou, ESA requested vertical integration of the payload. The rocket is rolled out to the pad without payload. After the rocket is erected, the payload is integrated in a mobile service tower. The tower is rolled back to a safe distance before launch. Vostochny uses the same design.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="548" LastEditorUserId="3065" LastEditDate="2017-02-27T22:22:57.530" LastActivityDate="2017-02-27T22:22:57.530" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20404" PostTypeId="1" CreationDate="2017-02-28T06:23:50.260" Score="6" ViewCount="585" Body="&lt;p&gt;As part of a project, I am designing a reaction wheel system for a nano-satellite. I am now researching on how to determine the nominal speed of the reaction wheel. I have characterized the deadzone of my motor with the flywheel attached, and found that I would be optimal to not have the nominal speed at zero to avoid the problems cause by the zero-speed crossing. I have tried searching for some research papers on how to determine the nominal speed and I wasn't able to find anything useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are my thoughts on how the nominal speed should be chosen:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My motor has a speed range of 100-4800RPM, if you factor in the deadzone region. I think it should be sufficient to set the nominal to set the speed in the middle, ( (4800-100)/2 + 100  = 2450RPM ). As this would allow the same amount of torque to be generated in either direction, and desaturation in both directions would be required equally as frequently as each other. However, although this logic does sound correct, the justification sounds too weak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any other methods to determine the nominal reaction wheel speed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="16924" LastEditorUserId="18744" LastEditDate="2017-03-02T18:25:40.170" LastActivityDate="2020-09-15T22:05:05.503" Title="On determining nominal speed of reaction wheel" Tags="|cubesat|reaction-wheel|acceleration|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20405" PostTypeId="1" CreationDate="2017-02-28T07:00:56.643" Score="10" ViewCount="865" Body="&lt;p&gt;My question is relatively simple in the essence that it takes few words to ask, but scientifically may be a nightmare. In 2000, the Goldstone Deep Space Communications Complex sent a message to Pioneer 6, a solar observation probe somewhere out there in space, and successfully communicated with it for a few hours. The question is this: Is there any feasible way of communicating with 6 if one acquired the right antenna and sent the right commands to it? If so, how would one do it, or even find out how to do it? &lt;/p&gt;&#xA;" OwnerUserId="18725" LastActivityDate="2017-03-01T15:35:34.553" Title="Can we talk to Pioneer 6?" Tags="|spacecraft|nasa|probe|data-transmission|pioneer|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20406" PostTypeId="1" AcceptedAnswerId="20409" CreationDate="2017-02-28T07:11:22.313" Score="4" ViewCount="2166" Body="&lt;p&gt;I assume (but I may be wrong) that in a communication satellite constellation such as the one &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_satellite_constellation&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceX&lt;/a&gt; plan to put into service:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;each full duplex link requires a dedicated antenna&lt;/li&gt;&#xA;&lt;li&gt;each satellite communicates with a subset of the other satellites in the constellation (some if not all of the sats that are in line of sight)&lt;/li&gt;&#xA;&lt;li&gt;a subset of satellites directly communicate with the ground segment&lt;/li&gt;&#xA;&lt;li&gt;an active antenna requires a substantial amount of energy to work&lt;/li&gt;&#xA;&lt;li&gt;there may be some spare antennas on a satellite&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Given all those assumptions, I wonder how many antennas per satellites are required for the constellation to be able to work as expected?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="897" LastEditDate="2017-02-28T10:54:33.743" LastActivityDate="2017-04-22T22:39:28.073" Title="How many communication antennas does a satellite need if it belongs to a constellation?" Tags="|communication-satellite|satellite-constellation|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20407" PostTypeId="2" ParentId="20389" CreationDate="2017-02-28T08:08:19.837" Score="3" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Amateur_rocketry#Notable_events&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On May 17, 2004, Civilian Space eXploration Team (CSXT) successfully achieved the first officially verified flight of an amateur high-power rocket into space, achieving an altitude of 116 km (72 mi).[7]&lt;/p&gt;&#xA;&lt;p&gt;Prior to that, the Reaction Research Society on November 23, 1996, launched a solid-fuel rocket, designed by longtime member George Garboden, to an altitude of 80 km (50 mi) from the Black Rock Desert in Nevada.[8]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Third place in this list reached an altitude of &lt;a href=&quot;http://www.hobbyspace.com/Rocketry/Advanced/records.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;33 km&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-02-28T08:08:19.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20408" PostTypeId="1" AcceptedAnswerId="20411" CreationDate="2017-02-28T09:35:11.057" Score="7" ViewCount="482" Body="&lt;p&gt;I keep expecting SpaceX to start doing powered landings of the dragon, or at least tests. But to my knowledge they haven't even done a drop test yet. Does anyone know what their plans are in regards to this?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="25" LastEditDate="2017-12-19T13:15:29.373" LastActivityDate="2017-12-19T13:15:29.373" Title="When will they be landing Dragon retropropulsively?" Tags="|spacex|landing|dragon|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20409" PostTypeId="2" ParentId="20406" CreationDate="2017-02-28T10:35:41.970" Score="5" Body="&lt;p&gt;We are living in the 21st century now. One phased array can support a large number of beams, each pointing in a different direction, each moving at a different rate. All the steering can be done electronically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, you don't need one antenna for every link. Just a few spacecraft-fixed, phased arrays is all that's necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a drawing of an &lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation#Next-generation_constellation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Iridium Next&lt;/a&gt; satellite. The SpaceX satellites will look very different, but for the purposes of your question, the Iridium Next is a good enough example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The large (orange) hexagonal array on its belly is the &lt;a href=&quot;https://en.wikipedia.org/wiki/L_band&quot; rel=&quot;nofollow noreferrer&quot;&gt;L-band&lt;/a&gt; (1-2GHz, 30-15cm) is a 48-beam transmit/receive phased array antenna to communicate with multiple ground &quot;customers&quot;. Each beam is &quot;generated&quot; in signal processing electronics, and a different phase and possibly (probably) amplitude would be sent to each of the sub-units with the array.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The higher the frequency, the smaller the array can be and still produce tight, steerable beams.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This excellent, detailed &lt;a href=&quot;https://space.stackexchange.com/a/19864/12102&quot;&gt;answer about GAIA's phased array&lt;/a&gt; describes the situation where the spacecraft is spinning around its axis but needs to keep a continuous data lock with Earth. The phased array is designed to handle this and keep a lock over 360 degrees azimuth and 30 to 60 degrees elevation with respect to the spacecraft's mid-plane. It's all done using math and fast processing to generate the correct signal amplitude and phase in each sub-element so that the beam is always optimized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the answer is &quot;a few&quot; if you count a single fixed array as an antenna. They are made of many small sub-units, but these are not independent antennas, they are all sharing the same signal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IgQ3Q.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IgQ3Q.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Conceptual drawing of Iridium-NEXT from &lt;a href=&quot;https://www.harris.com/solution/hosted-payload-solutions&quot; rel=&quot;nofollow noreferrer&quot;&gt;Harris&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oUsGu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oUsGu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Testing the GAIA phased array, from &lt;a href=&quot;http://web.archive.org/web/20131213013018/http://utopia.duth.gr/~iaitidis/ESA%20conference%202010/Papers/posters/FCXNL-10C09-2004739-1-2004739garcia.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (paywalled &lt;a href=&quot;http://ieeexplore.ieee.org/abstract/document/5505027/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="8693" LastEditDate="2017-04-22T22:39:28.073" LastActivityDate="2017-04-22T22:39:28.073" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20410" PostTypeId="2" ParentId="10004" CreationDate="2017-02-28T11:35:46.760" Score="4" Body="&lt;p&gt;In addition to the logos of the Soviet Space Program and the launch customer, Roskosmos has in fact added non-launch related advertising to their vehicles:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uT8PU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uT8PU.jpg&quot; alt=&quot;Pizza Hut logo on Proton rocket&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I personally have bought exactly one Pizza Hut pizza in the past two decades, and that purchase was specifically to support the advertiser!&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2017-02-28T11:35:46.760" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20411" PostTypeId="2" ParentId="20408" CreationDate="2017-02-28T12:10:04.677" Score="9" Body="&lt;p&gt;SpaceX had a permit/environmental assessment for their Dragonfly test vehicle/series of tests. It included the full gamut of ground level tests (at least one completed, hover test on a crane (&lt;a href=&quot;https://www.youtube.com/watch?v=07Pm8ZY0XJI&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt;) and then helicopter drops, propulsive landings, and then boosted hops. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have been very quiet about performing these tests for some reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the initial Dragon V2 build (apparently based on a Cargo Dragon hull but with Super Dracos added) was tested with the &lt;a href=&quot;https://www.youtube.com/watch?v=1_FXVjf46T8&quot; rel=&quot;noreferrer&quot;&gt;pad abort from LC-40&lt;/a&gt; on May 6, 2015. They have since indicated that the Dragon V2 design changed sufficiently that they do not want to test further with that initial design. Instead, the first unmanned Dragon V2 will do a pad abort as its second mission.&#xA;The initial plan is to parachute into the ocean for the first set of manned flights and then later land propulsively. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-02-28T14:42:53.390" LastActivityDate="2017-02-28T14:42:53.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20412" PostTypeId="1" AcceptedAnswerId="20413" CreationDate="2017-02-28T14:03:40.517" Score="9" ViewCount="2786" Body="&lt;p&gt;I'm coming from a background of Kerbal Space Program, so I'm not &lt;em&gt;100%&lt;/em&gt; sure all of this applies to real-life but I'm asking in the hopes someone might tell me that. Yes, I know, it's not real space travel and it doesn't accurately simulate all physics perfectly, but it's a good approximation and the core concepts are unchanged, so I consider it a good way to get introduced to concepts which one can then go and research further.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing that becomes pretty apparent from the start is that to get to orbit you need engines. To power these engines you need fuel, but fuel has mass, so you need more fuel and eventually things get pretty large and expensive. Each engine has a specific impulse and a craft (as we understand it to be a collection of payload, fuel, and engines) has a certain amount of &lt;em&gt;delta-V&lt;/em&gt;, or the change in velocity possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every maneuver, ascent, or powered landing the craft makes eats away at this budget of velocity change. From playing around it seems to be that making the following changes has the following effects:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;Adding more fuel:&lt;/strong&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The thrust to weight ratio decreases. In orbit this doesn't appear to matter, but when landed it must be greater than 1 to have enough thrust to lift the craft.&lt;/li&gt;&#xA;&lt;li&gt;The amount of time the engines can burn for increases, but does this mean the &lt;em&gt;delta-v&lt;/em&gt; only increases if you assume you are not ascending/landing and therefore do not need a TWR &gt; 1?&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Adding more engines:&lt;/strong&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;s&gt;The TWR increases&lt;/s&gt; The &lt;strong&gt;thrust&lt;/strong&gt; increases (and this usually increases the TWR, but not if the engine is exceptionally heavy in relation to the thrust produced), but the amount of time the engines can burn for decreases as more fuel is burnt in the same amount of time.&lt;/li&gt;&#xA;&lt;li&gt;I am unsure how &lt;em&gt;delta-v&lt;/em&gt; changes here. It seems like adding more engines without adding more fuel does not increase it, it just means the same amount can be expended in a shorter time.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So when constructing a spacefaring vehicle (in reality), is there a mathematical or &quot;proper&quot; way to determine the optimum balance between fuel and engines? Furthermore, is such a method dependant on the celestial body/bodies you are within the Hill sphere of?&lt;/p&gt;&#xA;" OwnerUserId="18523" LastActivityDate="2017-02-28T16:26:41.777" Title="How to find the optimum balance between amount of fuel and number of engines?" Tags="|rockets|propulsion|fuel|construction|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20413" PostTypeId="2" ParentId="20412" CreationDate="2017-02-28T15:31:31.957" Score="10" Body="&lt;p&gt;In general, you want the minimum number/thrust of engines that gets you off the launch pad safely -- in the real world this is typically a TWR of 1.2-1.4. The TWR increases rapidly as fuel is used, so if that minimum thrust level can be achieved it's almost always more effective to carry additional mass in fuel rather than engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're correct that the thrust level doesn't affect delta-v at all -- the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;noreferrer&quot;&gt;rocket equation&lt;/a&gt; (which is not very much math at all, really) doesn't have a thrust term. The only variable factors in it are the mass ratio $ \frac {m0} {mf} $, which is maximized by carrying a lot of fuel, and the exhaust velocity $ v_e $, which is a property of the engine type. In KSP, exhaust velocity is indicated via the &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;noreferrer&quot;&gt;specific impulse&lt;/a&gt; figure for an engine, which when multiplied by 9.81 gives the exhaust velocity in meters per second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other part of the rocket equation is the natural logarithm function $ ln $ which reflects the diminishing returns of carrrying more fuel.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-02-28T16:26:41.777" LastActivityDate="2017-02-28T16:26:41.777" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20414" PostTypeId="2" ParentId="19131" CreationDate="2017-02-28T18:54:18.100" Score="4" Body="&lt;p&gt;Going the other way, NASA has drawn up plans for an inflatable ship to harvest the upper atmosphere of Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first question is, &quot;At this point, how feasible would it be to start exploring the rest of the solar system?&quot; Very feasible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next question is, &quot;And what would be the potential targets for such missions anyway?&quot; Venus is one planet that is already under discussion for human exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus at average closest point is 25 million miles, farthest at 162 million miles. Mars is closest at 34.8 million, farthest at 250 million miles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because I can not figure out why things are voted down put on hold or other things about this forum here are the links.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Colonization_of_Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Colonization_of_Venus&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.popsci.com/nasa-envisions-cloud-city-above-venus-hellish-surface&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.popsci.com/nasa-envisions-cloud-city-above-venus-hellish-surface&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18720" LastEditorUserId="1142" LastEditDate="2017-03-05T11:38:26.113" LastActivityDate="2017-03-05T11:38:26.113" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20415" PostTypeId="1" AcceptedAnswerId="20423" CreationDate="2017-02-28T21:45:32.533" Score="10" ViewCount="725" Body="&lt;p&gt;One of theories behind the origin of the esoteric &lt;a href=&quot;https://en.wikipedia.org/wiki/Oh-My-God_particle&quot; rel=&quot;nofollow noreferrer&quot;&gt;oh-my-god particle&lt;/a&gt; was that it's exhaust of an alien spaceship. There are other explanations, some quite plausible, but let us for the purpose of this question assume that that one is true and try to examine performance of such drive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Known, or plausibly suspected:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the particle energy was 48J.&lt;/li&gt;&#xA;&lt;li&gt;it was a proton. Rest mass: $1.67 \cdot 10^{-27}$kg&lt;/li&gt;&#xA;&lt;li&gt;it was moving extremely close to speed of light. Let's round the speed to $3\cdot10^8$  m/s&lt;/li&gt;&#xA;&lt;li&gt;the source is outside our galaxy, but not very far from its edges. &lt;a href=&quot;http://www.sciencemag.org/news/2014/07/physicists-spot-potential-source-oh-my-god-particles&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;mass equivalent of 48J is $5.28 \cdot 10^{-16}$ kg (calculated using &lt;a href=&quot;https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)#10.E2.88.9218_to_10.E2.88.9213_kg&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; )&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now, for my assumptions about the craft:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it doesn't carry its own energy source for this propulsion &#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mass of any energy source other than antimatter would make propelling particles this close to the speed of light pointless; it's better to pack more propellant and push it to, say, 90% of speed of light instead of pumping so many joules into mass equivalence of energy without gaining extra I&lt;sub&gt;sp&lt;/sub&gt;.&lt;/li&gt;&#xA;&lt;li&gt;if the energy source was antimatter, the trivial, optimal propellant would be photons. There's absolutely no point converting them into energy to propel a proton while they can provide about the same (minimally better) performance directly.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;It abuses energy-mass equivalence to create a large exhaust mass from minimal amount of propellant. Specific impulse is near the absolute maximum $c/g_0$, but thanks to using external energy, it gets far more exhaust gas mass than its fuel flow would provide, circumventing tyranny of the rocket equation &amp;mdash; for every $1.67 \cdot 10^{-27}$kg of fuel spent, $5.28 \cdot 10^{-16}$ kg of fuel is expelled! (Of course also mass equivalent of the energy is lost, but it's soon recuperated by harvesting external energy!)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now, I don't know what kind of energy source would it be. Uhoh ballparked the &lt;a href=&quot;https://space.stackexchange.com/questions/20325/can-cosmic-background-be-harvested-for-energy&quot;&gt;cosmic background non-uniformities&lt;/a&gt; at 1 microwatt per $m^2$ - but it might be a beamed energy from the craft's origin, it might be shine of the (still near) galaxy, or other unknown source.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless &amp;mdash; the point where I'm asking your help is coming up with the new, adjusted rocket equation for this drive. Assuming the rocket still moves at speed pretty far from speed of light (no need to apply relativistic equations to it), what would be the equation for $\Delta v$ where mass of exhaust considerably differs from mass of propellant &amp;mdash; &quot;created&quot;/provided externally?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus question: What fuel mass fraction would such a rocket need to achieve 0.1c?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;This equation might be related to equations for jet engines. In a jet engine, mass of exhaust is much higher than mass of fuel spent to produce it; the extra mass coming from intake air, expelled air continuously recuperated through intakes.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-03-01T21:26:32.113" Title="&quot;Oh-my-god&quot; particle drive performance" Tags="|rocket-equation|relativistic-rocket|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20416" PostTypeId="1" CreationDate="2017-02-28T21:52:31.550" Score="6" ViewCount="466" Body="&lt;p&gt;Most of the Shuttle missions pre-ISS launched to ~28 degrees, due to the latitude of Kennedy Space Center. However, several of the early missions launched to higher inclinations:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-1&quot; rel=&quot;noreferrer&quot;&gt;STS-1&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-2&quot; rel=&quot;noreferrer&quot;&gt;STS-2&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-3&quot; rel=&quot;noreferrer&quot;&gt;STS-3&lt;/a&gt; launched to ~38 degrees.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-9&quot; rel=&quot;noreferrer&quot;&gt;STS-9&lt;/a&gt; launched to 57 degrees.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some later missions also launched to high inclinations, although this may be due to their scientific payloads, most notably Spacelab:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-41-G&quot; rel=&quot;noreferrer&quot;&gt;STS-41-G&lt;/a&gt; launched to 57 degrees.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-51-B&quot; rel=&quot;noreferrer&quot;&gt;STS-51-B&lt;/a&gt; launched to 57 degrees.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-51-F&quot; rel=&quot;noreferrer&quot;&gt;STS-51-F&lt;/a&gt; launched to 49.5 degrees.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-61-A&quot; rel=&quot;noreferrer&quot;&gt;STS-61-A&lt;/a&gt; launched to 57 degrees.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So what motivated the choice of high-inclination orbits on these missions? For the Spacelab missions, was that the deciding factor in launching to 57 degrees?&lt;/p&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2017-02-28T22:58:40.787" Title="Why did some early Shuttle missions launch to high inclinations?" Tags="|orbital-mechanics|space-shuttle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20417" PostTypeId="1" CreationDate="2017-02-28T18:35:24.593" Score="3" ViewCount="327" Body="&lt;p&gt;I'm investigating the idea of transporting gigawatts of electrical power to the first stage booster of an electrical rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first stage booster, like Falcon 9's recoverable first stage, stages at 70km altitude, 60km downrange, for a diagonal distance of 92km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm using 100km for a nice, round figure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we use a 1000 Isp electro-thermal rocket to launch a 10 ton payload with 25 ton upper stage and 20 ton lower stage (ignoring engine and fuel tank masses), and assume a TWR of 1.2 at liftoff, we would need 3.17GW of power at liftoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can this level of power be transported over a wire 100km long? Low mass is critical.&lt;/p&gt;&#xA;" OwnerUserId="35662" OwnerDisplayName="user140629" LastEditorUserId="4660" LastEditDate="2017-02-28T22:12:48.123" LastActivityDate="2017-03-02T16:09:28.330" Title="How to transport gigawatt-level power across a 100km distance?" Tags="|launch|power|" AnswerCount="2" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="20418" PostTypeId="2" ParentId="20416" CreationDate="2017-02-28T22:29:25.697" Score="9" Body="&lt;p&gt;For the early test flights, it was to increase the chance of making it to the abort landing site at Rota, Spain (~36.6 deg latitude).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STS-9 included earth observation objectives, hence the high inclination to increase what was overflown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other flights you list were for payload objectives: 41G, deploying the Earth Radiation Budget Satellite, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I must look for references :)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-02-28T22:58:40.787" LastActivityDate="2017-02-28T22:58:40.787" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20419" PostTypeId="1" AcceptedAnswerId="20420" CreationDate="2017-02-28T22:48:51.720" Score="6" ViewCount="381" Body="&lt;p&gt;I am studying the Apollo Guidance, specifically the derivation of guidance equation. While laying the baseline, a Target-referenced time (T) is chosen as time variable for guidance calculation of the trajectory evolving backward in time from landing target. T remains negative at all points before landing. In general sense, I understand back propagation however w.r.t. Guidance I dont understand this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is convenient to think of the reference trajectory as evolving backwards in time from the target point, with the time variable T reaching zero at the target point and negative prior to that point. Thus target-referenced time (T) is to be distinguished from clock-time (t). Because guidance gains would become unbounded, the target point is never reached. Instead, a guided phase is terminated at a negative time T and the succeeding phase is started.&#xA;  Both the terminus and the target point lie on the reference trajectory, but the target point lies beyond the portion that is actually flown.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Klumpp, A. R., “Apollo Lunar Descent Guidance,” Automatica, Vol. 10, No. 2, 1974, pp. 133–146.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is meant by and how would guidance gains become unbounded? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is meant in the last sentence specifically in &quot; ... target point lies beyond the portion that is actually flown&quot;?&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2017-02-28T22:57:51.100" Title="Apollo guidance" Tags="|apollo-program|trajectory|guidance|apollo-11|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20420" PostTypeId="2" ParentId="20419" CreationDate="2017-02-28T22:57:51.100" Score="8" Body="&lt;p&gt;Because some of the terms in the guidance equations divide by time-to-go and/or the square of time-to-go, relatively small input (sensor) errors can yield large effects, causing large steering oscillations at the end of the run. The early switchover to the next phase just avoids this effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;The part actually flown&quot; means the part flown under that segment of the guidance program.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-02-28T22:57:51.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20421" PostTypeId="1" AcceptedAnswerId="20422" CreationDate="2017-02-28T23:35:12.350" Score="5" ViewCount="401" Body="&lt;p&gt;I was reading about the &lt;a href=&quot;https://esto.nasa.gov/conferences/estf2011/presentations/Flatley_ESTF20111.pdf&quot; rel=&quot;noreferrer&quot;&gt;Advanced Hybrid On-Board Science Data Processor - SpaceCube 2.0 Space Cube 2.0&lt;/a&gt; and I saw maps of computer upsets in orbit recorded by Space Cube 1.0 on slides 8 and 9. What is going on near South America, why so many upsets there? What's the physics behind the phenomenon?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/32Gyj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/32Gyj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8RvPO.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8RvPO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2019-03-09T02:23:00.597" Title="Why would Space Cube 1.0 have so many On-Orbit Upsets over South America?" Tags="|low-earth-orbit|radiation|computing|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20422" PostTypeId="2" ParentId="20421" CreationDate="2017-03-01T01:27:11.370" Score="12" Body="&lt;p&gt;The lower map appears to show the &lt;a href=&quot;https://en.wikipedia.org/wiki/South_Atlantic_Anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;South Atlantic Anomaly&lt;/a&gt; (SAA), an area where the inner Van Allen radiation belt comes closest to the surface of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the linked article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The effect is caused by the non-concentricity of the Earth and its&#xA;  magnetic dipole. The SAA is the near-Earth region where the Earth's&#xA;  magnetic field is weakest relative to an idealized Earth-centered&#xA;  dipole field.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The article gives a good side view showing the asymmetry:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CIKOd.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CIKOd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbiting objects that pass through the SAA are subject to increased radiation flux compared to other portions of their orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;Here's a picture from my personal collection showing the instructor station of the Shuttle Mission Simulator (from long ago).  On the far wall you can see a framed copy of a paper groundtrack map, with the South Atlantic Anomaly marked on it in gray.  Wish I had a closeup! But you can see there is no North Atlantic Anomaly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QAvOw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QAvOw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-03-02T00:32:10.170" LastActivityDate="2017-03-02T00:32:10.170" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="20423" PostTypeId="2" ParentId="20415" CreationDate="2017-03-01T01:44:38.850" Score="6" Body="&lt;p&gt;Rocket equation starts with conservation of momentum:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{dp}{dt} = m\frac{\partial v}{\partial t} + v\frac{\partial m}{\partial t}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But at such a high energy, the rest mass of the proton can be ignored - it's about 1E-11 (as $m_0c^2$) as big as the energy. So drop the second term. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{dp}{dt} \approx m\frac{\partial v}{\partial t}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{dv}{dt} \approx \frac{1}{m}\frac{dp}{dt}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v \approx \frac{\Delta p}{m}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a very relativistic particle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$p = \frac{E}{c}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For $n_{omgp}$ oh-my-god protons:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta p = n_{omgp}\frac{E_{omgp}}{c}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get a rocket of mass $m_R$ to a velocity $0.1c$ velocity (non-relativistic approximation for rocket):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$0.1 c = \frac{n_{omgp} E_{omgp}}{m_Rc}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rearranging&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$n_{omgp} = \frac{0.1 m_Rc^2}{E_{omgp}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$n_{omgp} = \frac{0.1 \times 1.0 kg \ \times \text{9E+16} \ \text{m}^2/\text{s}^2}{48 Joules} = \text{1.9E+14 protons}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or about 30 micro Coulombs, or about 3E-13 kgs of oh-my-god protons per kilogram of rocket.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-03-01T01:44:38.850" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="20424" PostTypeId="2" ParentId="20405" CreationDate="2017-03-01T09:15:33.820" Score="1" Body="&lt;p&gt;Pioneer 6 was started 1965. The spacecraft was constructed for an operation of 6 month only. The last communication was 35 years after the start. The degrading of the solar cells reduces the available power for communication. The chance is very small that everything of the electronics needed for a communication is still functional.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-03-01T09:15:33.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20425" PostTypeId="1" CreationDate="2017-03-01T09:49:37.193" Score="2" ViewCount="246" Body="&lt;p&gt;If I have a spacecraft flying at 90 percent the speed of light and it slams into Jupiter's upper atmosphere, by how much would it slow down?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's for a story idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks.&lt;/p&gt;&#xA;" OwnerUserId="18735" LastActivityDate="2017-03-01T09:49:37.193" Title="Relativistic aerobraking" Tags="|jupiter|aerobraking|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20426" PostTypeId="2" ParentId="14663" CreationDate="2017-03-01T12:01:52.820" Score="2" Body="&lt;p&gt;Another way of estimating the energy transferred to the target?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;10E+13 kg of comet v.s. 500 kT Nuke: where 1kg TNT=4.2 MJoule.&#xA;This looks like a 20 km (13 miles) diameter comet. Looks like 10 times the mass of the K-T event (Dino killer).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy from the bomb will spread in a sphere and less than half will go towards the comet. Lets try 1/3 or 33%.&#xA;From the energy hitting the comet some will be reflected back into space - the value here is a guess - lets try 1/3 reflected back. 67% effective.&#xA;Now to the final step - the energy will heat an area on the comet to different temperatures. Extreme near the blast and less so further away from the blast.&#xA;Lets look at the energy available:&#xA;500.000.000 (kg) * 0.33*0.67*4.200.000=4.6E+14 Joules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When trying to calculate the impulse we have two factors: Mass and temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens to 4x4 km and a depth of 0.2m? Density 1.5g/cm3?&#xA;4000*4000*0.2*1.5=4.8E+6 kg. Temperature (half water half rock 2600J/K)&#xA;We get 35000K temperature - everything is a gas at this temperature - since the speed of sound in a gas is proportional to the square root of the temperature - speed of sound = max possible exhaust speed - aproximately 11 times that of air or 3.7 km/s.  This is spread in a half sphere (not directed as from a rocket engine): 50% useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the Impulse contribution comes out as: 3700*0.5*4.8E6/10E13=8.8E-4 m/s = 1mm/s.  What about the efficiency? (3700*0.5*4.8E6)/4.6E14=0.02%&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It does not look like this is effective enough to make a difference to the path of the comet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand if the comet is 2 km instead of 20 we get about 1 m/s or 2600 km deflection in one month. This is 26.000 km in one year = 2 earth diameters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per&lt;/p&gt;&#xA;" OwnerUserId="18736" LastActivityDate="2017-03-01T12:01:52.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20428" PostTypeId="2" ParentId="20405" CreationDate="2017-03-01T13:27:25.753" Score="4" Body="&lt;p&gt;It's been done before: in &lt;a href=&quot;https://arstechnica.com/science/2014/05/isee-3-spacecraft-makes-first-earth-contact-in-16-years/&quot; rel=&quot;nofollow noreferrer&quot;&gt;2014&lt;/a&gt;, an &lt;a href=&quot;http://spacecollege.org/isee3/&quot; rel=&quot;nofollow noreferrer&quot;&gt;amateur&lt;/a&gt; group &lt;a href=&quot;https://space.stackexchange.com/questions/3748/equipment-needed-to-contact-ice-isee3-spaceraft&quot;&gt;established contact&lt;/a&gt; with ISEE-3, another old space probe (launched in 1978) on a heliocentric trajectory.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main obstacles:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;you need access to a large antenna. There are a few large dishes (25 m) operated by amateurs. The ISEE-3 effort was given time on the (even larger) Arecibo dish. &lt;/li&gt;&#xA;&lt;li&gt;you need to know the communications protocol. For ISEE, NASA assisted by giving information and hardware (a custom transmitter, IIRC). As of 2000, NASA still knew &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19920003902&amp;amp;hterms=Pioneer&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3D%2522Pioneer%25206%2522%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA&quot; rel=&quot;nofollow noreferrer&quot;&gt;how to contact&lt;/a&gt; Pioneer 6, so chances are they'll be able to tell you everything you need to know. &lt;/li&gt;&#xA;&lt;li&gt;you need to know where the spacecraft is. &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; only has ephemerides until 1999, it seems. I don't know if NASA has better information somewhere else.&lt;/li&gt;&#xA;&lt;li&gt;the probe still has to work, obviously. The ISEE example shows it's possible, and these old designs should be pretty robust (no microelectronics to go haywire from a single cosmic ray strike, for example)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Pioneer 6 is in a heliocentric orbit with a period of &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftOrbit.do?id=1965-105A&quot; rel=&quot;nofollow noreferrer&quot;&gt;311 days&lt;/a&gt;.  Periapsis 0.81 AU, Apoapsis 0.98 AU. It had a close approach to Earth in &lt;a href=&quot;http://www.deseretnews.com/article/24774/OLDEST-OPERATING-SPACECRAFT-PASSING-NEAR-MOTHER-EARTH.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;1988&lt;/a&gt;. Every ~year it'll pass us at a distance of less than 0.2 AU, your best timeframe for contact is just before and after closest approach (not during, as it'll be on a straight line between us and the Sun at that time). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-01T15:35:34.553" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20429" PostTypeId="2" ParentId="20334" CreationDate="2017-03-01T15:52:13.160" Score="11" Body="&lt;p&gt;The experiment &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/636.html&quot; rel=&quot;noreferrer&quot;&gt;Growth and Survival of Colored Fungi in Space (CFS-A) - 12.07.16&lt;/a&gt; showed that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;fungi can grow inside the space station and could decompose food and other organic materials in humid conditions. After exposure to air growth stopped, fungal hairs (filaments) developed within the food source and decomposed it by producing digesting enzymes. Submerged filaments don’t make spores, but fungi could still spread through direct contact. Dry spores remained living after 5 months in microgravity, with some fungi living longer than others. The data show that spores would grow well on salted food in space, and some species grow on iron surfaces covered with small quantities of carbohydrates.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One of the species used by the experiment was &lt;em&gt;Ulocladium chartarum&lt;/em&gt;, which is a melanized fungi. You can see more on the results of the experiment at the link above. The experiment found that melanized fungi were indeed resistant to cosmic radiation to the degree that radiation was not found to affect their growth, instead microgravity was the main inhibitor. I did not see anything about radiation providing any &lt;em&gt;advantage&lt;/em&gt;, but it was at least not an obstacle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The experiment page also includes a picture of an actual contamination (of what appears to be melanized fungi) on one of the interior panels of the ISS. The caption says that the crew has weekly sessions to clean such surfaces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QQnagm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QQnagm.jpg&quot; alt=&quot;ISS contaminated interior panels&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2017-03-01T16:26:38.837" LastActivityDate="2017-03-01T16:26:38.837" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20430" PostTypeId="1" AcceptedAnswerId="20444" CreationDate="2017-03-01T16:43:28.747" Score="32" ViewCount="8318" Body="&lt;p&gt;The International Space Station (ISS) has been &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;occupied continuously since 2 November 2000.&lt;/a&gt; we know from a recent answer &lt;a href=&quot;https://space.stackexchange.com/questions/20334/melanized-fungi-on-the-iss-are-they-enjoying-the-elevated-radiation-levels-t/20429#20429&quot;&gt;that it is cleaned weekly&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my home when winter is over, and we are doing spring cleaning we open all the doors and windows.  Doing the same on the ISS would be a bit different, but does raise the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can you open the doors and 'air out' the ISS? If so what would be the impact? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-02T16:06:05.520" Title="Can you vacuum out the ISS by opening the door?" Tags="|iss|cleaning|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20431" PostTypeId="2" ParentId="20430" CreationDate="2017-03-01T17:00:27.620" Score="20" Body="&lt;p&gt;It would be extremely difficult to open the outer airlock hatch if the inner airlock hatch is open because the outer hatch opens inward, and there are tons of pressure force holding it closed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you were suicidal enough, you could open the inner hatch, and then the airlock depressurization valves, and let the station vent down to the point where you could open the hatch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See related answer to &lt;a href=&quot;https://space.stackexchange.com/questions/2387/is-there-no-physical-security-in-space-other-than-being-in-space&quot;&gt;this question&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-03-01T17:00:27.620" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20432" PostTypeId="1" CreationDate="2017-03-01T17:08:36.437" Score="23" ViewCount="1063" Body="&lt;p&gt;SpaceX recently announced they are planning to fly two private citizens around the moon. I'm sure a normal space flight has all sorts of legal complications, but does the addition of 2 private citizens and an orbit around the moon add any significant complications? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm more interested in international laws/treaties and U.S. spaceflight laws and less interested in personal liability/insurance of the travelers.&lt;/p&gt;&#xA;" OwnerUserId="3141" LastEditorUserId="3141" LastEditDate="2017-03-01T17:14:31.947" LastActivityDate="2017-04-06T05:13:54.290" Title="Does SpaceX have any significant additional legal hurdles for flying private citizens around the moon?" Tags="|spacex|the-moon|astronauts|law|tourism|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20433" PostTypeId="2" ParentId="20430" CreationDate="2017-03-01T18:37:59.627" Score="18" Body="&lt;p&gt;Opening the hatch might get rid of most of the loose rubble (minus everything that gets swept along, then slams into a wall), but that's not enough. Any gunk that sticks to a surface is likely to remain stuck.&lt;br&gt;&#xA;Part of the ISS cleaning routine is replacing the filters. Those won't be much cleaner after depressurization either. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-03-01T18:37:59.627" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20434" PostTypeId="1" CreationDate="2017-03-01T19:08:48.423" Score="4" ViewCount="76" Body="&lt;p&gt;And where should I have looked for this information?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass of the avionics rack would also be nice.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-01T19:15:49.973" LastActivityDate="2017-03-01T19:15:49.973" Title="What is the mass of the Cabin Air Rack of the Quest Joint Airlock?" Tags="|life-support|airlock|quest-joint-airlock|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20436" PostTypeId="2" ParentId="20367" CreationDate="2017-03-01T20:23:59.647" Score="3" Body="&lt;p&gt;Short answer: No, lots of them don't use oxygen at any temperature or density&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long answer:&#xA;Less no. If you take an engine that uses liquid oxygen and put subcooled liquid oxygen into it, it will be subject to additional thermal stresses and the turbopumps involved will be pumping a liquid denser than they were designed for. If neither of those problems destroy it or prevent it from starting, it will work but the turbopumps will almost certainly not be delivering the oxygen at the correct rate because of the increased density. &lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-02T16:38:57.447" LastActivityDate="2017-03-02T16:38:57.447" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20437" PostTypeId="1" AcceptedAnswerId="20447" CreationDate="2017-03-01T20:33:32.610" Score="16" ViewCount="12470" Body="&lt;p&gt;Most BBQs use propane. Propane burns quite well and it is quite clean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So why hasn't anyone use it as a rocket fuel??&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="17599" LastEditorUserId="6944" LastEditDate="2017-03-02T01:38:25.243" LastActivityDate="2021-05-25T21:22:11.850" Title="Why don't rockets use propane?" Tags="|fuel|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20438" PostTypeId="2" ParentId="20325" CreationDate="2017-03-01T20:37:02.153" Score="8" Body="&lt;p&gt;Yes! If we assume that you have access to a large black hole and materials that probably can't exist, you can run a heat engine off of the cosmic microwave background radiation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reasonably sized black holes are cold. Very cold. Billionths of a Kelvin for a stellar mass black hole, which is numerous orders of magnitude colder than the CMB. The temperature of a black hole is called the Hawking Temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means you could take a heat engine using helium as working fluid. Your cold sink will be a large radiator facing the black hole that you're orbiting very closely. Your heat source is a similar plate facing away from the black hole. You will need some sort of multi-layer insulation between them for optimal performance. At this point it should be noted that if any matter is falling into the black hole it will be producing radiation and you won't be able to use it as a cold sink. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much energy will you get? Not much, but since the helium would not be a gas I don't know how to calculate it. As an upper limit though, the Stefan-Boltzmann Law limits it to less than 3.1 microwatts per square meter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Honestly though it would probably be easier to use the Penrose Process if you can do real engineering that close to a black hole. &lt;a href=&quot;https://en.wikipedia.org/wiki/Penrose_process&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Penrose_process&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-01T20:47:54.173" LastActivityDate="2017-03-01T20:47:54.173" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20439" PostTypeId="2" ParentId="20437" CreationDate="2017-03-01T20:43:14.070" Score="4" Body="&lt;p&gt;People have used it as rocket fuel, but its density isn't great compared to other hydrocarbons used as rocket fuel, which burn about as well and just as cleanly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/a/airlaunchpropaneengine.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.astronautix.com/a/airlaunchpropaneengine.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-01T21:03:57.497" LastActivityDate="2017-03-01T21:03:57.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20440" PostTypeId="1" AcceptedAnswerId="20442" CreationDate="2017-03-01T21:00:24.770" Score="15" ViewCount="2114" Body="&lt;p&gt;After reading &lt;a href=&quot;https://space.stackexchange.com/q/237/9072&quot;&gt;this question&lt;/a&gt;, I understand that it is simpler to replace a space station than to repair it in order to continue its activity. All modules in a space station may not have the same lifespan (I may be mistaken, but a space station take several years to build, and  is expected to be in service for about 10 years).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, it is possible to add modules to a space station. It may be possible to jettison modules as well. Given this, I imagine an old obsolete module may be dropped and then a new one may take its place in order to maintain its mission and avoid building a new space station from scratch. But I cannot find any historical records of such an operation.&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2023-04-24T20:09:57.923" Title="Have space station modules already been replaced?" Tags="|iss|space-station|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20441" PostTypeId="2" ParentId="20440" CreationDate="2017-03-01T21:03:05.410" Score="1" Body="&lt;p&gt;No, this has never happened. There's no particular reason it couldn't be done though.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-01T21:03:05.410" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20442" PostTypeId="2" ParentId="20440" CreationDate="2017-03-01T21:19:08.577" Score="19" Body="&lt;p&gt;It hasn't happened yet, but ISS docking module &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Pirs_(ISS_module)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pirs&lt;/a&gt; is scheduled to be deorbited and replaced by laboratory module &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Nauka_(ISS_module)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nauka&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Edit: It happened in July 2021 (see links)&lt;/p&gt;&#xA;" OwnerUserId="18207" LastEditorUserId="512" LastEditDate="2023-04-24T20:09:57.923" LastActivityDate="2023-04-24T20:09:57.923" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20443" PostTypeId="2" ParentId="20440" CreationDate="2017-03-01T21:24:25.897" Score="4" Body="&lt;p&gt;The Russian plan for the station, after it is retired by the International partners is to take the new Nauka module (Referenced by Quentin) and its attached docking module (OM) and make those the base of their new station, and grow it from there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in that case it is add new modules, and throw away old ones, sort of what the question asks.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-03-01T21:24:25.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20444" PostTypeId="2" ParentId="20430" CreationDate="2017-03-01T22:41:09.007" Score="34" Body="&lt;p&gt;Because of boundary conditions, despite a massive &quot;woosh&quot; of air from rapid depressurization, the velocity of air near walls and other rigid surfaces approaches zero. The highest velocity of air flowing through the compartments would be where it's &lt;em&gt;furthest&lt;/em&gt; away from the walls, and presumably anything that needs to be cleaned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Boundary_layer&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yJxy4.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="881" LastEditorUserId="881" LastEditDate="2017-03-02T16:06:05.520" LastActivityDate="2017-03-02T16:06:05.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20445" PostTypeId="2" ParentId="3236" CreationDate="2017-03-01T22:59:50.093" Score="1" Body="&lt;p&gt;The current EVA procedure involves the astronauts camping out for several hours in the equipment lock while pressure the air transitions from the standard for the station to what they will be breathing in the suits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During this time, they do all kinds of things. They eat and drink. They perform light exercise to accelerate the removal of nitrogen from their bloodstreams. They take naps. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of time they spend doing this procedure varies quite a bit. See page 80:&lt;br&gt;&#xA;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080021669.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080021669.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-01T22:59:50.093" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20446" PostTypeId="2" ParentId="20440" CreationDate="2017-03-01T23:19:15.970" Score="17" Body="&lt;p&gt;The only module(s) that kinda sorta fits this are/were the MPLMs - the Multi Purpose Logistics Modules. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GWKGY.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GWKGY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These modules were carried up by the Shuttle full of cargo, berthed to the ISS using robotics, emptied out, and then filled up with trash for the return trip.  They were then detached robotically and restowed in the Shuttle bay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MPLMs were built by the Italian Space Agency and were named &lt;em&gt;Leonardo&lt;/em&gt;, &lt;em&gt;Donatello&lt;/em&gt;, and &lt;em&gt;Raffaello&lt;/em&gt;.  For reasons that escape me, &lt;em&gt;Donatello&lt;/em&gt; was never used, but the other two flew 12 round trip missions to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Near the end of the Shuttle program, mission STS-133 (Feb 2011) took &lt;em&gt;Leonardo&lt;/em&gt; up and it was attached permanently to the ISS becoming the Permanent Multipurpose Module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Continuing its odyssey, in 2015 &lt;em&gt;Leonardo&lt;/em&gt;/PMM was moved from its original &quot;permanent&quot; location on Node 1 Nadir, to Node 3 Forward.  &lt;a href=&quot;https://www.nasaspaceflight.com/2015/05/iss-program-station-reconfiguration-future-crew-vehicles/&quot; rel=&quot;noreferrer&quot;&gt;This was done to allow more visiting vehicles to be present at the ISS simultaneously.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a super sped up video of that move:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/pNItvP9PvO0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So one could say that &lt;em&gt;Leonardo&lt;/em&gt; was attached and removed multiple times, (sometimes replaced by &lt;em&gt;Raffaello&lt;/em&gt;) and finally attached and left as an ISS module.  Not sure if that meets your criteria though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-Purpose_Logistics_Module&quot; rel=&quot;noreferrer&quot;&gt;Reference&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-03-01T23:31:28.173" LastActivityDate="2017-03-01T23:31:28.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20447" PostTypeId="2" ParentId="20437" CreationDate="2017-03-01T23:19:22.043" Score="29" Body="&lt;p&gt;If you're going with cryofuels (and don't want to dabble in liquid hydrogen, which opens another can of worms), you're better off with liquid methane for its performance - higher specific impulse thanks to higher hydrogen:carbon ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you don't want cryofuels, you go with something that stays liquid at ambient temperature: RP-1 kerosene.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Propane, with ~5-7 bar vapor pressure at ambient temperatures would require a pressure tank (not acceptable on a rocket due to mass), or needs to be cooled to at least -42&lt;sup&gt;o&lt;/sup&gt;C. This combines disadvantages of cryofuels and complex hydrocarbons, giving a very small specific impulse rise over RP-1, and while rather large, still insufficient boiling point rise above methane.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-01T23:19:22.043" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20448" PostTypeId="1" CreationDate="2017-03-02T00:00:20.203" Score="1" ViewCount="1173" Body="&lt;p&gt;I can't remember where I heard it but I thought that dividing the exhaust velocity by the gravitational pull of a body, it is possible to find the specific impulse of the rocket engine. However, what about when you are in space around another body with a different amount of pull, suppose Mars. Since the surface gravity is 3.711m/s2, when finding specific impulse that would mean the engine is twice as efficient when around Mars than opposed so around Earth... This can't be right &lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2017-03-05T16:19:47.970" Title="Deriving specific impulse from exhaust velocities?" Tags="|gravity|engines|specific-impulse|" AnswerCount="4" CommentCount="0" ClosedDate="2017-03-02T13:59:29.600" ContentLicense="CC BY-SA 3.0" />
  <row Id="20449" PostTypeId="2" ParentId="20448" CreationDate="2017-03-02T00:04:12.250" Score="0" Body="&lt;p&gt;You're right. It's not right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isp is exhaust velocity divided by Earth's surface gravity, no matter where you actually are. It's vaguely useful for launch vehicles but the fact that it gets used in deep space is mind-numbing.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-02T00:04:12.250" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20450" PostTypeId="2" ParentId="20448" CreationDate="2017-03-02T00:22:35.340" Score="7" Body="&lt;p&gt;The conversion factor isn't the gravitational pull of an arbitrary body, but rather the conversion between pounds as a unit of &lt;em&gt;weight&lt;/em&gt; and pounds as a unit of &lt;em&gt;mass&lt;/em&gt; -- which is a convention established on Earth, and hence happens to match Earth's gravitational pull.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some rocket scientists work in feet and pounds, others work in meters and kilograms. Exhaust velocity is likewise measured in feet per second or meters per second, and so can plug directly into the rocket equation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specific impulse is most logically measured in velocity units, but there's another physical interpretation of specific impulse: the length of time that the thrust from a given mass of fuel could be used to cancel the weight &lt;em&gt;on Earth&lt;/em&gt; of that same fuel. This is a weird and contrived metric, and requires dividing velocity by acceleration to yield time, but the result has a very convenient property: it's in the same units whether you're a metric or non-metric rocket scientist. So there's a long tradition of measuring Isp in seconds even though it has that awkward Earth-centric conversion factor in it.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-03-02T00:33:02.410" LastActivityDate="2017-03-02T00:33:02.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20451" PostTypeId="2" ParentId="20448" CreationDate="2017-03-02T00:31:01.663" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;$I_{sp}$ in seconds is equal to the amount of time a rocket must be fired to use a quantity of propellant with weight (measured at one standard gravity) equal to its thrust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The use of strictly earth-bound $g_0$ is the direct result of conversion between units of force (Newtons) and units of mass (Kilograms). Both kilograms and newtons are independent from location, but if you want to compare these apples to oranges, you need &lt;em&gt;some&lt;/em&gt; pretty arbitrarily chosen constant that binds them. $g_0$ is one that gives a rather neat physical interpretation of the result, and reasonable numbers corresponding to the range of quantities encountered in reality, so it was chosen here. But it could have been a constant of 1 $m \over s^2$ or anything else, and all that would change would be something along the lines of&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$I_{sp}$ in seconds is equal to the amount of time a rocket must be fired to use a mass of propellant such that acting upon this mass with force equal to the thrust of the rocket over period of 1 second accelerates it to 1m/s.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You've got to admit the first definition sounds better :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if you're still unhappy, feel free to give $I_{sp}$ in meters per second, as exhaust velocity. Nobody will mind, and with $g_0=9.81 \approx 10$ conversion to get the ballpark value is trivial; AK-47 muzzle velocity is 715 m/s, so its specific impulse is about 70 seconds.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-03-02T00:39:26.453" LastActivityDate="2017-03-02T00:39:26.453" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20452" PostTypeId="1" AcceptedAnswerId="20454" CreationDate="2017-03-02T03:57:25.607" Score="2" ViewCount="667" Body="&lt;p&gt;I recently conducted an experiment using Kerbal Space Program to analyze the effect of burn altitude on orbital properties.  Unfortunately im a bit of a rookie when it comes to orbital dynamics so I'd appreciate if someone could explain the data I've been getting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Details of experiment:&#xA;An anti-radial burn of 50m/s will be conducted at heights of (0%, 20%, 40%, 60%, 80%, and 100%) of orbital altitude.  The starting orbit has a semi-major axis of 30,000,000 m, inclination of 0, and eccentricity of 0.9.  After conducting the burn, changes to the Apogee (main focus of experiment), perigee, inclination, period, and eccentricity will be measured.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that I cannot seem to find a mathematical relationship online between altitude of the burn and apogee and I cannot explain the data I have found.  It is periodic by nature, but the relationship is very odd.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are the resultant apogees from my experiment.  Note: The x-axis labels are as follows (1=0%, 2=20%, 3=40%, 4=60%,5=80%,6=100%,7=80%, 8=60%, 9=40%, 10=20%, 11=0%.  and 1-5 are while ascending and 7-11 are while descending.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CuQUy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CuQUy.png&quot; alt=&quot;Result Apogee Data&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18756" LastActivityDate="2017-03-02T09:55:12.250" Title="The relationship between the altitude of an anti-radial burn on apogee" Tags="|orbital-mechanics|orbital-maneuver|orbital|kerbal-space-program|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20453" PostTypeId="2" ParentId="20404" CreationDate="2017-03-02T04:18:11.873" Score="1" Body="&lt;p&gt;Since you only need 15-20 degrees of precision, you don't need to set a nominal zero for your reaction wheels. You'll need to use pulse width modulation even if the motor is voltage controlled and the movement may be a little jittery, but if you have a halfway decent control algorithm and put minimal effort into characterizing the response of the motors at low speed it should be fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless of course you have other mission needs that require smooth, precise pointing.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-02T04:18:11.873" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20454" PostTypeId="2" ParentId="20452" CreationDate="2017-03-02T09:55:12.250" Score="1" Body="&lt;p&gt;The mathematical relationship is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_equation&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;(Wikipedia)&quot;&gt;orbit equation&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$r={\ell^2\over m^2\gamma}{1\over1+e \cos{\theta}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Decomposing the angular momentum $\ell=mvr\sin{\alpha}$ it can be expressed as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$r={\gamma\,{(1+e \cos{\theta})}\over v^2\sin^2{\alpha}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where the height of the apoapsis will be &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$${\gamma\,{(1-e)}\over v^2\sin^2{\alpha}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to figure out the effects, we can analyze what the burn changes in each component:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;change in the angle of the velocity vector $\alpha$ - if the spaceship is ascending, the sine of the angle increases, if the spaceship is descending, the sine component decreases. The change will be larger when the burn is relatively big compared to orbital velocity.&lt;/li&gt;&#xA;&lt;li&gt;change (increase) in the velocity component $v$ - since an anti-radial burn is perpendicular to the original velocity vector, the lower the initial velocity, the bigger the increase (both absolutely and relatively)&lt;/li&gt;&#xA;&lt;li&gt;eccentricity $e=\sqrt{1+{2E\ell^2\over m^3\gamma^2}}$ will generally increase when burning anti-radial when descending, and decrease when burning while ascending, as the momentum term dominates. If neither ascending nor descending, eccentricity would increase slightly due to increasing speed.&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;angular momentum $\ell=mvr\sin{\alpha}$ will generally increase when the ship moves slowly near the apoapsis or when the ship is ascending, decrease otherwise, based on the analysis of its components above&lt;/li&gt;&#xA;&lt;li&gt;orbital energy $E$ is the sum of potential and kinetic energy, and will increase, as kinetic energy is $mv^2/2$ and will increase due to $v$ increasing (the higher $v$ is, the larger the increase in energy), potential energy remains unchanged at the moment of the burn&lt;/li&gt;&#xA;&lt;li&gt;note that for elliptical orbits $E$ is negative, so increasing $E$ means changing towards zero, increasing $\ell$ reduces $e$&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;$\gamma$ is constant for a specific body&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The angle component will be the dominant one for burns smaller than the orbital velocity, except when the spaceship is moving horizontally (the sine function changes slowly around π/2).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The empirical results seem to support this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;when the spaceship is ascending, an anti-radial burn will lower the apoapsis (divide by a larger sine of the angle of velocity vector)&lt;/li&gt;&#xA;&lt;li&gt;when the spaceship is descending, an anti-radial burn will raise the periapsis (divide by a smaller sine of the angle of velocity vector)&lt;/li&gt;&#xA;&lt;li&gt;when the spaceship is flying level, an anti-radial burn would raise the apoapsis slightly due to increasing eccentricity (through increasing velocity)&lt;/li&gt;&#xA;&lt;li&gt;the first two effects will be larger when the ship moves slower (is higher), due to the angle changing more. This is clearly visible in the graph.&lt;/li&gt;&#xA;&lt;li&gt;I'm not sure if the third will be larger or smaller when moving slower, as, on one hand, the velocity increases more, on the other the energy gains are lower when moving slower. Empirically they look similar.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(Though, for the record, if you care about the apoapsis height, it's most efficient to burn prograde or retrograde at the periapsis.)&lt;/p&gt;&#xA;" OwnerUserId="17667" LastActivityDate="2017-03-02T09:55:12.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20456" PostTypeId="2" ParentId="20417" CreationDate="2017-03-02T11:43:55.473" Score="5" Body="&lt;p&gt;By wire is going to be extremely difficult. Let us first have a look at the required cross section of the wire:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$A = \frac{\rho lP}{V^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can use the resistivity value of graphene for a very optimistic scenario. Even with a voltage over 20kV, the wire is still going to outmass the rocket by far. Not to mention the wire has to tolerate exposure to rocket exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; The above calculation is possibly incorrect, as pointed out by Uwe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simpler argument is that for the wire not to outmass the spacecraft, it can be no more than $0.5kg/m$. Given the size of high voltage wires &lt;a href=&quot;https://electronics.stackexchange.com/questions/111463/high-voltage-cable-cross-section&quot;&gt;carrying an order of magnitude less power&lt;/a&gt;, that is simply not possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;/edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beaming up the power, as others have mentioned, is not likely to be any more successful, as current laser technology simply is not sufficient. Continuous lasers are at the moment only a few kW, roughly five orders of magnitude short.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having the energy source onboard, however, is not as impossible as it sounds. A &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nuclear thermal rocket&lt;/a&gt; may be something you want to look at. Reactor tests at your magnitude of power were conducted during Project Rover.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2017-03-02T12:39:31.460" LastActivityDate="2017-03-02T12:39:31.460" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20457" PostTypeId="2" ParentId="20448" CreationDate="2017-03-02T11:58:23.877" Score="3" Body="&lt;p&gt;I prefer this way of looking at the topic, it may help for some people (&lt;strong&gt;NB 2nd attempt, edited for clarity&lt;/strong&gt;). The answers by SF. and Russell Borogove are no more or less correct. In this view however there is no need to talk about &quot;weight&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specific impulse, as the name suggests is &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ Specific Impulse = \frac{Impulse}{Mass}  =  \frac {Force \cdot time}{Mass}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has the units of velocity, i.e. $  m s^{-1} $ or $  ft s^{-1} $ and so according to one's education its numerical value varies. The next step is to divide velocity by an acceleration to get units of time. Time has the same units in both metric and US standard systems.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnitude of this acceleration must be the same in both sets of units, e.g. g0 ( 9.8 $  m s^{-2} $  or 32 $  ft s^{-2} $) though I don't think it especially matters which acceleration is chosen &lt;strong&gt;&lt;em&gt;as long as everyone uses the same convention&lt;/em&gt;&lt;/strong&gt;. Isp in seconds is always converted back to an exhaust velocity when it is actually used so, in principle, it doesn't matter which value is used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Background context&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One source of confusion, which is not really part of the problem but may help as context is to consider the nature of the two different systems of units: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;In metric the units are&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$   \frac {Newton \cdot second}{kg}$ &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This is nice and tidy because metric, aka SI, is a consistent set of units where a force of one Newton appplied to a one kilogram mass causes an acceleration of 1 $  m s^{-2} $.&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;The US standard units are a bit more complicated as the system has diverged into several groups. The customary use of a pound-force, lbf and pound mass, lbm, together is an inconsistent set of units: a force of one lbf applied to one lbm causes an acceleration of 32 $  ft s^{-2} $.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This single difference causes much confusion. It may help to learn that there are variants of the US system that are consistent, and these replace the force or mass terms with the &lt;em&gt;Poundal&lt;/em&gt; or &lt;em&gt;Slug&lt;/em&gt; which have different magnitudes to the lbf and lbm. If you want to know more about the lb force / lb mass topic try here, &lt;a href=&quot;https://en.wikipedia.org/wiki/Pound_(force)&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Pound_(force)&lt;/a&gt;, though it looks like a rather long-winded explanation to me.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-03-05T16:19:47.970" LastActivityDate="2017-03-05T16:19:47.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20458" PostTypeId="1" CreationDate="2017-03-02T16:00:26.590" Score="-6" ViewCount="398" Body="&lt;p&gt;To use some reference let's take the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS has an orbital speed of 7600m/s. At the altitude of the ISS ~400km the force of the gravity it's about 8m/s2. It will take about 16 minutes to the ISS to achieve a vertical speed of 8000m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, why it doesn't fall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know about orbit is falling but going to fast the you &quot;miss&quot; the earth. But you will be falling faster each time due to the gravity being strong at just 400km.&lt;/p&gt;&#xA;" OwnerUserId="18770" LastActivityDate="2017-03-03T22:45:08.403" Title="Orbit decay, why gravity doesn't increase 'vertical speed'" Tags="|iss|orbit|gravity|" AnswerCount="1" CommentCount="5" ClosedDate="2022-08-20T00:33:21.230" ContentLicense="CC BY-SA 3.0" />
  <row Id="20459" PostTypeId="2" ParentId="20417" CreationDate="2017-03-02T16:09:28.330" Score="0" Body="&lt;p&gt;{disclaimer}&lt;BR&gt;&#xA;&lt;em&gt;I'm going to suggest a method that might work, but is highly speculative and completely untested.&lt;/em&gt;&lt;BR&gt; {/disclaimer}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Simply put use a laser induced air plasma electrical conductor to transmit the power.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In slightly more detail: Use a high power laser to excite a tube of air from the ground to the rocket into a plasma state and then electrically energize the plasma. (You would actually need two plasma tubes for a complete circuit)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most people would easily recognize an atmospheric plasma electrical phenomenon like this as lightning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/jBjqT3AQkH0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: the big light in the video is not the lightning, it is the laser projecting onto the wall, the lightning is the tiny dot of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side note: the laser is likely to use way more than 3 GW to ionize the air and tracking the rocket with a moving plasma cylinder would be very challenging to say the least, and by the time we are good enough at plasma physics to do this we could likely build fusion rockets, but I find there is something inherently awesome about the idea of using artificial lightning to power a rocket.&lt;/p&gt;&#xA;" OwnerUserId="15824" LastActivityDate="2017-03-02T16:09:28.330" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20460" PostTypeId="2" ParentId="19397" CreationDate="2017-03-02T16:42:43.443" Score="-1" Body="&lt;p&gt;Nuclear rocket engines preliminary designs are already there for space operations. There are mainly three types of nuclear fission reactors are proposed by engineers. They are 1. Solid core Nuclear thermal fission reactor engine&#xA;2. Liquid core Nuclear Thermal fission reactor engine&#xA;3. Gas core Nuclear Thermal fission reactor engine&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fusion reactor engines are also considered for designs. However these engines are still in design level. &#xA;These reactor engines works as same as normal rocket engine where the propellant will absorb energy released from nuclear fusion or fission reactors.&#xA; The main practical difficulties in these engines is mainly carrying out controlled fission or fusion reaction at the core of the engine. In future these kind of reactors can be developed.&lt;/p&gt;&#xA;" OwnerUserId="18517" LastActivityDate="2017-03-02T16:42:43.443" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20463" PostTypeId="2" ParentId="10032" CreationDate="2017-03-02T17:04:54.283" Score="1" Body="&lt;p&gt;I think LMP - 103 is Liquid Mono propellant which is a green monopropellant which is ionic salt liquid higher density than commercial hydrazine propellant. It is less toxic and high performance than hydrazine. LMP 103 is a Ammonium Di Nitramide (ADN)  based monopropellant with water and fuel component can be used as better replacement for highly toxic hydrazine propellant. This fuel blend is mostly preferred by ESA and ESA has flown a different type of ADN fuel blend in PRISMA project in 2010 by Swedish Spacy Agency. Theoretical maximum Isp for this kind of propellant blend is about 280s.&lt;/p&gt;&#xA;" OwnerUserId="18517" LastActivityDate="2017-03-02T17:04:54.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20464" PostTypeId="2" ParentId="19397" CreationDate="2017-03-02T17:06:18.010" Score="7" Body="&lt;p&gt;The question is how to convert the heat from a fission reactor into electricity. Most systems that have been either built (Topaz) or proposed (SP-100) rely on &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermionic_converter&quot; rel=&quot;noreferrer&quot;&gt;thermionic conversion&lt;/a&gt;. This is the same approach used by RTGs. It's not very efficient, 5% to maybe 15% on a good day, but thermionic conversion is extremely reliable and requires no moving parts. The Voyager spacecraft are both still today living off of thermionic conversion, almost 40 years after launch. There is no problem with scaling up this conversion approach. For larger reactors, regardless of the conversion approach, you may need to use heat pipes to get the heat efficiently from the reactor to the converter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're ok with moving parts (keep in mind that these things would usually be expected to work at least a decade or two with no maintenance), then Stirling engines are the conversion technology of choice. The fluid would be helium, always in the gas state. (Superheated water is a nasty fluid to use as it likes to destroy, over long periods of time, all materials it comes in contact with.) Efficiencies of 40% to maybe 50% can be achieved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a program to develop an RTG replacement that used a Stirling converter for much more efficient use of the expensive and hard-to-get Plutonium-238. However that program (ASRG) was cancelled a few years ago due to cost overruns and not achieving the technical objectives. Still, with sufficient perseverance, a Stirling engine would be the most sensible conversion technology in the long run for space nuclear power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Brayton Cycle has also been proposed. Here is a &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070038168.pdf&quot; rel=&quot;noreferrer&quot;&gt;good paper on the work on space Stirling and Brayton engines&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-03-02T19:48:13.790" LastActivityDate="2017-03-02T19:48:13.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20465" PostTypeId="2" ParentId="19397" CreationDate="2017-03-02T17:23:15.067" Score="4" Body="&lt;p&gt;At this time thermoelectric and thermionic systems are the only types which have been used in space. However, there are a few other reactor types which have been proposed for use in orbit for the purpose of generating power. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.projectrho.com/public_html/rocket/supplement/Presby_Engineer_Degree_Thesis.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Thermophotovoltaic systems&lt;/a&gt; would essentially use solar cells optimized for the thermal spectrum of the reactor rather than the sun. The radiation hardening of these systems is an obstacle, as is operating them at high temperatures. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Rankine Cycle typically used by reactor power has very low temperature cold sinks, which is going to make radiating waste heat away very difficult thanks to the Stefan-Boltzmann law. However, NASA's project Prometheus proposal featured a fission generator which generated power via the Brayton Cycle. The Brayton Cycle can be used with no intermediate cooling stages or heat exchangers, which is very attractive for conserving mass in spacecraft. Using gaseous helium as the coolant is also a good idea for saving mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While highly speculative, fission fragment reactors may be attractive in the future due to their high efficiencies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fusion reactors at this point are a bit too speculative for me to comment on regarding practicality, though in principle aneutronic fusion could be a very attractive option due to the large amount of energy which can be extracted via direct energy conversion rather than thermodynamic processes.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-02T17:23:15.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20467" PostTypeId="2" ParentId="10007" CreationDate="2017-03-02T21:22:55.030" Score="3" Body="&lt;p&gt;Most ESA and NASA spacecraft have communications systems that are designed to the CCSDS  (Consultative Committee for Space Data Systems) recommendations. For the TM coding there is a specific standard (&lt;a href=&quot;http://webapp1.dlib.indiana.edu/virtual_disk_library/index.cgi/4278621/FID2960/CCSDS/101x0b4.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCSDS 101.0-B-4&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons uses the rate 1/6 Turbo Coding option, as described in this overview: &lt;a href=&quot;http://www.boulder.swri.edu/pkb/ssr/ssr-fountain.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons Spacecraft&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="18180" LastActivityDate="2017-03-02T21:22:55.030" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20469" PostTypeId="2" ParentId="976" CreationDate="2017-03-03T04:03:02.883" Score="10" Body="&lt;p&gt;NASA just released their &lt;strong&gt;&lt;a href=&quot;https://software.nasa.gov/feature/software_cat_2017&quot; rel=&quot;noreferrer&quot;&gt;2017-2018 software catalog&lt;/a&gt;&lt;/strong&gt; which lists many different types of software used by NASA.  Access requirements and restrictions are as follows: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;General Public Release&lt;/strong&gt;—For codes with a broad release and no nondisclosure&#xA;or export control restrictions&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Open Source Release&lt;/strong&gt;—For collaborative efforts in which programmers improve upon codes originally developed by NASA and share the changes&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;U.S. Release Only&lt;/strong&gt;—For codes available to U.S. persons only, with no further transfer of the software allowed without the prior written approval of NASA&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;U.S. and Foreign Release&lt;/strong&gt;—For codes that are available to U.S. persons and&#xA;(under special circumstances) persons outside of the U.S.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;U.S. Government Purpose Release&lt;/strong&gt;—For codes that are to be used on behalf&#xA;of the U.S. government&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Project Release—For use under a contract, grant, or agreement&lt;/li&gt;&#xA;&lt;li&gt;Interagency Release—For use by U.S. government agencies&lt;/li&gt;&#xA;&lt;li&gt;NASA Release—For use only by NASA personnel and contractors&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The fifteen topics are listed below:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/business_systems_and_project_management&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Business Systems and Project Management&lt;/strong&gt;&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/system_testing&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;System Testing&lt;/strong&gt;&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/operations&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Operations&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/design_and_integration_tools&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Design and Integration Tools&lt;/strong&gt;&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/vehicle_management_space_air_ground&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Vehicle Management&lt;/strong&gt;&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/data_servers_processing_and_handling&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Data Servers Processing and Handling&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/propulsion&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Propulsion&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/structures_and_mechanisms&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Structures and Mechanisms&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/crew_and_life_support&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Crew and Life Support&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/data_and_image_processing&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Data and Image Processing&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/materials_and_processes&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Materials and Processes&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/electronics_and_electrical_power&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Electronics and Electrical Power&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/environmental_science_earth_air_space_exoplanet&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Environmental Science&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/autonomous_systems&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Autonomous Systems&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://software.nasa.gov/aeronautics&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;Aeronautics&lt;/strong&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In addition, NASA has a &lt;a href=&quot;https://github.com/nasa/&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;github page&lt;/strong&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="84" LastActivityDate="2017-03-03T04:03:02.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20470" PostTypeId="1" CreationDate="2017-03-03T07:49:08.703" Score="4" ViewCount="362" Body="&lt;p&gt;In a picture of Phobos, it appears as if there are striations which circle it, and orthogonal striations which are reminiscent of topological maps. Are these digital photography effects or real? If they are real, what explanation do we have for them?    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p9lQs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p9lQs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18782" LastEditorUserId="12102" LastEditDate="2018-09-05T03:54:21.690" LastActivityDate="2018-09-05T03:54:21.690" Title="What are the proposed causes of these striations on Phobos?" Tags="|imaging|phobos|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20471" PostTypeId="1" AcceptedAnswerId="20472" CreationDate="2017-03-03T10:07:07.760" Score="34" ViewCount="3801" Body="&lt;p&gt;I'm reading &lt;em&gt;&lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/0062363603&quot; rel=&quot;noreferrer&quot;&gt;Hidden Figures&lt;/a&gt;&lt;/em&gt;, and I'm puzzled by an assertion Shetterly makes in a section describing the first few electronic computers installed at NASA, in the run-up to the Mercury project at around 1959-1960 (p. 209 paperback). Shetterly writes&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The agency set up a third computer, a slightly smaller IBM 709, in a data center in Bermuda. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;with no further details given. This feels much too early for outsourcing and offshoring of computing operations; instead, you'd really want to keep the computer next to its programmers next to the engineers and scientists they supported. What kind of computations did this facility do, why was it so important, and where can I read more about it? &lt;/p&gt;&#xA;" OwnerUserId="653" LastEditorUserId="653" LastEditDate="2018-10-01T14:30:25.010" LastActivityDate="2023-11-15T02:26:28.273" Title="Why did NASA set up a computing facility in Bermuda in the Mercury epoch?" Tags="|nasa|history|computing|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20472" PostTypeId="2" ParentId="20471" CreationDate="2017-03-03T10:51:23.930" Score="47" Body="&lt;p&gt;Bermuda was &lt;a href=&quot;https://history.nasa.gov/SP-45/ch8.htm&quot; rel=&quot;noreferrer&quot;&gt;one of the tracking stations&lt;/a&gt; for the Mercury program. Bermuda was critical as it had a good view of much of the ascent trajectory. In this image, all of the tracking stations are shown along with one of the Mercury flight paths. BDA=Bermuda.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Coe6I.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Coe6I.jpg&quot; alt=&quot;Mercury ground tracks&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Requirements. Early in the design of the Mercury system it was considered mandatory to receive information on a real-time basis and to provide for instantaneous computation and display of mission data from lift-off to landing. To meet these requirements, new data transmission equipment and computer peripheral gear were required. A new concept in large-scale, real-time data processing was required to tailor computations to a computer cycle and to manage the priorities of the computations performed automatically.&lt;/p&gt;&#xA;&lt;p&gt;In all phases of the Mercury mission, it was vital that the many different forms of calculations be performed with exact precision and the data be made available almost instantaneously. For example, in a matter of seconds after launch-vehicle cut-off and spacecraft insertion into orbit, the computers were required to furnish data based on tracking information for evaluating whether or not the mission should be permitted to continue.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;At the start of the Mercury program, there was no real-time computer link available between Bermuda and the USA, or at least none with enough capacity to transfer e.g. radar data to the US. So they had to make some of the flight control decisions in Bermuda instead of running everything from the US.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Before the Bermuda submarine cable was installed, it was decided to supplement the Goddard-Cape Canaveral complex with a secondary computing station at Bermuda. Installed there was an IBM 709 computer that received the inputs of the Bermuda FPS-16 (radar) and Verlort radars. The role of Bermuda was twofold: it served as a backup remote control center during the launch phase and as a tracking site thereafter. Specifically, it performed the following computing tasks:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Provided all the necessary trajectory information to drive the display devices in the Bermuda control center.&lt;/li&gt;&#xA;&lt;li&gt;Computed an independent go-no-go at insertion based on Bermuda data.&lt;/li&gt;&#xA;&lt;li&gt;Computed retrofire times to be used in the event of an abort to land the spacecraft in one of the designated recovery areas.&lt;/li&gt;&#xA;&lt;li&gt;Computed refined landing points for several abort cases.&lt;/li&gt;&#xA;&lt;li&gt;Computed orbital characteristics.&lt;/li&gt;&#xA;&lt;li&gt;Sent postinsertion conditions to Goddard.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;After the submarine cable was installed in April 1962, the Bermuda computer was removed and all the computations listed above were programed in the Goddard computers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-03T12:53:03.357" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20473" PostTypeId="1" AcceptedAnswerId="20506" CreationDate="2017-03-03T11:28:01.363" Score="30" ViewCount="5778" Body="&lt;p&gt;Astronauts are usually prepared to land at a random place on Earth; in case the planned reentry burn fails, but by other means they achieve reentry trajectory, the orbital motion will pretty much randomize their landing place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voskhod 2, due to problems with activating the reentry engine, landed &lt;a href=&quot;http://www.spacefacts.de/mission/english/voskhod-2.htm&quot; rel=&quot;noreferrer&quot;&gt;386km&lt;/a&gt; from the intended landing site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo 13 didn't land at the originally planned site, but within sight distance of the site expected after adjusting the trajectory to the new mission plan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What other missions landed way off target? Which landed farthest?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="10706" LastEditDate="2017-03-06T16:34:07.070" LastActivityDate="2017-03-06T16:34:07.070" Title="What crewed space flight landed farthest off-target?" Tags="|crewed-spaceflight|reentry|" AnswerCount="6" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="20474" PostTypeId="1" CreationDate="2017-03-03T11:35:44.437" Score="5" ViewCount="1455" Body="&lt;p&gt;Outside Earths atmosphere is the vacuum of space and its immense power.  If a cylindrical tube of sufficient strength and size were built tall enough to reach the edge of atmosphere would it not be possible to have a vehicle inside this tube with an air lock and means to open hatch or door to allow the vacuum of space to suck the vehicle up and away?  If the angle were correct and  certain adjustments to tube were done orbit could be obtained.  If my theory is correct.   &lt;/p&gt;&#xA;" OwnerUserId="18784" LastEditorUserId="6944" LastEditDate="2022-03-19T04:06:53.177" LastActivityDate="2022-03-19T04:06:53.177" Title="Vacuum launch, essentially free after system built" Tags="|launch|low-earth-orbit|design-alternative|" AnswerCount="7" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20475" PostTypeId="2" ParentId="20473" CreationDate="2017-03-03T11:41:00.677" Score="27" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Atlas_7&quot; rel=&quot;noreferrer&quot;&gt;Mercury Atlas 7&lt;/a&gt;, Aurora 7, splashed down an estimated 250 miles (402 km) off course, slightly more than Voskhod 2. The error was due to a problem with the automatic attitude controls and a late firing of the retro rockets by astronaut Scott Carpenter.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Passing over Hawaii at the final orbit, Kraft told Carpenter to begin&#xA;  his retrofire countdown and to shift from manual control to the&#xA;  automatic attitude control. Partly because he had been distracted&#xA;  watching the fireflies Carpenter noted that he had began his landing&#xA;  preparations late. As he started to align the spacecraft he found that&#xA;  the automatic stabilization system would not hold the required&#xA;  34-degree pitch and zero-degree yaw attitude. While trying to&#xA;  determine the source of the trouble, he fell behind in his check of&#xA;  other items. When he hurriedly switched to the fly-by-wire control&#xA;  mode, he forgot to switch off the manual system. As a result, both&#xA;  systems were used redundantly together for 10 minutes, and fuel was&#xA;  wasted.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In addition to the attitude error, Carpenter also activated the&#xA;  retrorockets three seconds late, adding another 15 miles or so to the&#xA;  trajectory error. Due to lack of fuel Carpenter overshot his planned&#xA;  reentry mark and splashed down 250 miles (400 km) from target.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;After several hours of frantic searching, Carpenter was located in an&#xA;  area northeast of Puerto Rico and taken aboard the aircraft carrier&#xA;  USS Intrepid. Other than slight exhaustion, he was in good health and&#xA;  spirits and postflight medical exams did not find any significant&#xA;  physical changes or anomalies.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Carpenter never flew another mission. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2017-03-03T11:41:00.677" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20477" PostTypeId="1" CreationDate="2017-03-03T13:12:26.363" Score="3" ViewCount="284" Body="&lt;p&gt;When spasecraft accelerates its center of masses should be on the line of force vector otherwise it will start to spin. How does ISS hold its orientation when accelerating? Do gyros hold station orientation? &lt;/p&gt;&#xA;" OwnerUserId="18787" LastEditorUserId="18592" LastEditDate="2017-03-03T14:52:47.290" LastActivityDate="2017-03-03T16:21:03.023" Title="Why does not the ISS turn when accelerating?" Tags="|iss|acceleration|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20478" PostTypeId="2" ParentId="20477" CreationDate="2017-03-03T13:32:17.667" Score="5" Body="&lt;p&gt;Yes. The ISS has quite large control moment gyroscopes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/f/ff/ISS_gyroscope.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://upload.wikimedia.org/wikipedia/commons/f/ff/ISS_gyroscope.jpg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russian section also has RCS thrusters for desaturating the gyroscopes, and the boosting vehicles have their own RCS thrusters and momentum control systems.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-03T16:21:03.023" LastActivityDate="2017-03-03T16:21:03.023" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20479" PostTypeId="2" ParentId="20473" CreationDate="2017-03-03T13:41:30.407" Score="14" Body="&lt;p&gt;Probably not quite what you had in mind, but &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-T_No.39&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 7K-T No.39&lt;/a&gt; ended up a long way from where they intended to land.  During launch, the second stage failed to separate. The third stage ignition did get rid of the second stage, but damaged the booster and caused an automatic abort.  The capsule landed in an unpopulated mountainous area near the Chinese border, about 1000 miles from the launch site and from the usual Soyuz landing area.&lt;/p&gt;&#xA;" OwnerUserId="11611" LastActivityDate="2017-03-03T13:41:30.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20480" PostTypeId="2" ParentId="15118" CreationDate="2017-03-03T14:44:39.940" Score="6" Body="&lt;p&gt;The question is spot on, the same problem of getting liquid propellant to the exit feed of a propellant tank applies to all liquid propulsion systems. This applies equally to RCS thrusters for settling for a main launch vehicle stage engine and any type of satellite borne propulsion, large or small. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Design solutions that I am aware of:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;An elastomeric diaphragm separating the liquid propellant and&#xA;gaseous pressurant. A common choice has been silicon rubber, which&#xA;works at least for N2H4, hydrazine, though there were concerns in&#xA;the 1980's of the silicon leeching into the propellant and poisoning&#xA;the catalysts of mono-propellant thrusters. As far as I am aware it&#xA;is not compatible with N2O4 or MON.&lt;/li&gt;&#xA;&lt;li&gt;Surface tension devices in the tank that provide a continual&#xA;presence of liquid covering the exit. There are lots of interesting&#xA;designs including channels that run from the top of the tank to the&#xA;bottom and a &quot;sponge&quot; or other trap to guarantee propellant supply&#xA;even under adverse accelerations. The nature of the surface tension&#xA;devices have to be very carefully thought out in conjunction with&#xA;the mass of the satellite and the thrusters and expected manoeuvre&#xA;types.&lt;/li&gt;&#xA;&lt;li&gt;I understand that the Soyuz spacecraft may use a metal diaphragm. I&#xA;imagine there is quite a weight penalty compared to the rubber type&#xA;and also that the expulsion efficiency could be rather poorer. On&#xA;the plus side with careful materials choice it could be long term&#xA;compatible with N204/MON and N2H4/MMH would seem to grant much more&#xA;freedom for arbitrary manoeuvres than a surface tension device.&lt;/li&gt;&#xA;&lt;li&gt;I believe some satellites continue to use settling thrusters, I've&#xA;heard of ion thrusters being used in this way to initiate a good propellant supply for a larger chemical thruster.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;All these designs carry a mass penalty, use-ability limitations for manoeuvres or expulsion efficiency limits and so there has to be a design trade-off for each mission.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-03-03T14:44:39.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20481" PostTypeId="2" ParentId="20473" CreationDate="2017-03-03T14:48:42.073" Score="17" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-11&quot; rel=&quot;noreferrer&quot;&gt;Soyuz TMA-11&lt;/a&gt; landed 475km from it's projected landing site after ballistic re-entry.  Amazingly, no significant injuries were suffered by any of the crew.&lt;/p&gt;&#xA;" OwnerUserId="18789" LastActivityDate="2017-03-03T14:48:42.073" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20482" PostTypeId="5" CreationDate="2017-03-03T14:54:37.993" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2017-03-03T14:54:37.993" LastActivityDate="2017-03-03T14:54:37.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20483" PostTypeId="4" CreationDate="2017-03-03T14:54:37.993" Score="0" Body="A network of satellites carrying out a specific mission or serving a targeted purpose based on an increased area of cover or relative position differences." OwnerUserId="4606" LastEditorUserId="4606" LastEditDate="2017-03-03T16:02:51.973" LastActivityDate="2017-03-03T16:02:51.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20484" PostTypeId="1" CreationDate="2017-03-03T16:32:50.603" Score="4" ViewCount="665" Body="&lt;p&gt;As rocket Science and Space Exploration is becoming more and more popular, I'm currently running some numbers, whether it would be possible to build a rocket engine for showing at schools or universities, without the need to be fit for flight.&lt;/p&gt;&#xA;&lt;p&gt;The idea is to have a &amp;quot;normal&amp;quot; chemical combustion engine, but as performance is not a optimization criterion, but safety and availability for amateurs is I still search for a good fuel:&#xA;The different oxidizers used in rocket motors are either cryogenic(LOX) or highly corrosive (Hydratzine and other Hypergolic Propellants), toxic and cancerous - nothing one would like to handle and which is cheap and easy to buy...&lt;/p&gt;&#xA;&lt;p&gt;Is there any option for an oxidizer to combine with gasoline or ethanol as fuel, or a different combination of fuel and oxidizer?&#xA;They should both be liquid (as pumps for liquids are available) and react strongly enough to build up the pressure in the chamber, but be individually safe enough to be handled without needing to consult a doctor if you accidentally spill a bit of it. I know that for an oxidizer this is a bit contradicting requirements, but maybe there is a good candidate.&lt;/p&gt;&#xA;" OwnerUserId="18790" LastEditorUserId="9092" LastEditDate="2021-03-25T12:06:47.337" LastActivityDate="2021-03-25T12:06:47.337" Title="Rocket Engine demonstrator fuel choice" Tags="|engines|engine-design|nitrous-oxide|" AnswerCount="3" CommentCount="6" ClosedDate="2021-03-25T23:22:34.503" ContentLicense="CC BY-SA 4.0" />
  <row Id="20485" PostTypeId="1" AcceptedAnswerId="20567" CreationDate="2017-03-03T16:43:44.080" Score="11" ViewCount="486" Body="&lt;p&gt;The doppler effect can give a pretty measure of radial speed relative to Earth, and radio transmissions can narrow down the radial position, but what about everything else? I can imagine several ways, but I'm not sure what's most practical.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-05T02:51:53.533" LastActivityDate="2017-03-08T19:07:56.097" Title="How is the trajectory of spacecraft transferring between planets monitored?" Tags="|navigation|astrodynamics|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20486" PostTypeId="2" ParentId="20484" CreationDate="2017-03-03T16:45:16.203" Score="2" Body="&lt;p&gt;A few oxidizer options worth looking at:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;compressed air. Easy to come by, inefficient as it contains so much nitrogen, but that might be an advantage. &lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;compressed gaseous oxygen. Is used in welding, filling a 9 kg cylinder costs around $25. But if you get a good oxygen/gasoline fire going, I suspect your flame temperature will be high enough to melt metals. Having to cool the combustion chamber complicates your design.&#xA;The less efficient combustion you get from compressed air will be easier on the combustion chamber. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrous_oxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;nitrous oxide&lt;/a&gt; is used e.g. in the car tuning world, would fall somewhere between air and oxygen. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-03-03T16:45:16.203" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20487" PostTypeId="2" ParentId="20484" CreationDate="2017-03-03T16:57:52.673" Score="3" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrous_oxide#Rocket_motors&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nitrous oxide&lt;/a&gt; can be used as monopropellant, producing oxygen and nitrogen in ratios not too distant from atmospheric air (about 36%), and by itself is pretty harmless and easy to obtain as &lt;a href=&quot;https://en.wikipedia.org/wiki/Whipped-cream_charger&quot; rel=&quot;nofollow noreferrer&quot;&gt;cartridges for whipped cream maker&lt;/a&gt;. The small, inexpensive, safe, self-contained cartridges are very nice for short &quot;demonstration runs&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding another fuel, be it liquid or solid, or even gaseous will improve specific impulse (and visual entertainment value), but is optional - and with nitrous oxide deflagrating without fuel, it can be prettty much anything - rubber in a hybrid motor, or kerosene for liquid fuel motor, or you could use propane-butane lighter gas as a cryofuel... perfect for a &quot;demonstrator&quot; device if you can showcase different fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also use atmospheric air (out of a compressor) as oxidizer, to provide a performance comparison.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-03-03T17:08:24.137" LastActivityDate="2017-03-03T17:08:24.137" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20488" PostTypeId="2" ParentId="20484" CreationDate="2017-03-03T17:01:08.020" Score="1" Body="&lt;p&gt;Concentrated hydrogen peroxide and ethanol could be a workable combination if you want two liquids. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has a very nice advantage that you can add water to either solution to reduce performance.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-03T17:10:10.413" LastActivityDate="2017-03-03T17:10:10.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20489" PostTypeId="2" ParentId="20474" CreationDate="2017-03-03T18:11:53.300" Score="4" Body="&lt;p&gt;You would have to create a vacuum initially, but as the tube fills with air, the pressure inside becomes equal to the pressure outside, and the flow of air will stop.&lt;/p&gt;&#xA;" OwnerUserId="18793" LastActivityDate="2017-03-03T18:11:53.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20490" PostTypeId="1" CreationDate="2017-03-03T19:25:20.623" Score="5" ViewCount="668" Body="&lt;p&gt;SpaceX has had 8 successful launches in 2016 according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9#Launch_history&quot; rel=&quot;noreferrer&quot;&gt;a wikipedia article&lt;/a&gt;. The &lt;a href=&quot;http://www.spacex.com/missions&quot; rel=&quot;noreferrer&quot;&gt;launch manifest&lt;/a&gt; agrees with that, but leaves out the launch pad failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to another wikipedia article, they are producing around 200 engines per year:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;By June 2015, SpaceX was producing Merlin engines at the rate of four Merlin 1D engines per week, with a total production capacity in the factory of a maximum of five per week.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;9 launches gives 90 engines (9+1 per flight), if I am not mistaken. That leaves 110 unused. There are also 45 recovered engines. In principle, SpaceX has a stockpile of Merlin engines for at least 11 launches, so what are they doing with all these Merlins? &lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="51" LastEditDate="2017-03-03T19:31:39.310" LastActivityDate="2017-03-03T19:36:50.777" Title="What does SpaceX do with all the engines?" Tags="|spacex|falcon-9|merlin-1d|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20491" PostTypeId="2" ParentId="20490" CreationDate="2017-03-03T19:36:50.777" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;... are producing at the RATE of four Merlin 1D engines per week.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That is the key. They have had two stand downs due to loss of vehicles (CRS-7 in flight, AMOS-6 during a test). During that time, they can stop production, when it is clear they have a 3-6 months recovery ahead of themselves. Or else they can assume they will recover next week, continue at the 4/week build rate and only once it is clear their stores are full, pause production.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is clear that SpaceX has decided that they need modifications to the basic Falcon 9 design for easier, better, faster, cheaper reuse after recovering a bunch of stages.  The initial engines recovered first were used for tests. One core is &lt;a href=&quot;http://www.universetoday.com/wp-content/uploads/2016/08/Falcon-9-700x432.jpg&quot; rel=&quot;noreferrer&quot;&gt;on display with 9 engines at the Hawthorne factory&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the cores was fired multiple times at the McGregor test site (JCSAT-14 mission). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the cores is being rebuilt into one of the Falcon Heavy demo flight's side boosters.  (Thaicomm-8 mission). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One is being reused as a launch vehicle for SES-10 satellite (CRS-8 mission).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-03-03T19:36:50.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20492" PostTypeId="2" ParentId="20474" CreationDate="2017-03-03T20:32:52.407" Score="12" Body="&lt;p&gt;It's important to remember that getting into orbit requires not just gaining altitude but achieving horizontal speed of about 7700 m/s, so your vehicle would need an upper stage for circularization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At sea level air pressure against vacuum, a piston about 10 meters wide would develop the same force as the 9 Merlin engines of a Falcon 9 first stage. Unlike a rocket engine, a vacuum piston arrangement would have an absolute upper limit equal to the speed of sound, so the technique would only be usable for an initial launch boost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious problem is that we don't know how to build a structure anywhere big enough to take advantage of this. The world's tallest building is less than 1km in height; an evacuated launch tube that big could save maybe 25-30 seconds of propellant, which would a substantial win. You'd need a seal at the top that was strong enough to keep the air out but weak enough to let the rocket through without damaging it. Of course, you'd have to ignite your rocket engines from on top of that moving piston, and anything that went wrong along the way would be catastrophic.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-03-07T15:34:27.587" LastActivityDate="2017-03-07T15:34:27.587" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20493" PostTypeId="1" AcceptedAnswerId="20494" CreationDate="2017-03-03T20:47:22.627" Score="5" ViewCount="172" Body="&lt;p&gt;For example how long would it take for Atlas V 541 Stage two to accelerate to 7800m/s given it is travelling at 5000m/s. The mass of the rocket is changing over time, how do I find the time required for it to achieve 7800m/s?&lt;/p&gt;&#xA;" OwnerUserId="18270" LastActivityDate="2017-03-03T22:51:34.343" Title="How can I find the time required to achieve certain velocity in orbit with changing mass?" Tags="|orbital-mechanics|rockets|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20494" PostTypeId="2" ParentId="20493" CreationDate="2017-03-03T22:08:25.857" Score="9" Body="&lt;p&gt;What you want to do is to find how much propellant mass that velocity change is going to cost, and divide that by the engine mass flow to get the elapsed time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mass flow is easy, that is just:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{Thrust}{I_{sp} \cdot g}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how can we find the propellant consumption? Consider the rocket equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v = \ln \left({\frac{m_0}{m_f}}\right)\cdot I_{sp} \cdot g$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can use that to get the final mass:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$m_f = \frac{m_0}{e^{\frac{\Delta v}{I_{sp} \cdot g}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Propellant consumption is then $m_0 -m_f$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the combined equation for convenience:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Time = \frac{I_{sp} \cdot g \cdot m_0\left(1 - \frac{1}{e^{\frac{\Delta v}{I_{sp} \cdot g}}}\right)}{Thrust}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, we can formulate it in terms of $m_f$ if we only have the final mass:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Time = \frac{I_{sp} \cdot g \cdot m_f\left(e^{\frac{\Delta v}{I_{sp} \cdot g}}-1\right)}{Thrust}$$&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2017-03-03T22:51:34.343" LastActivityDate="2017-03-03T22:51:34.343" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20495" PostTypeId="2" ParentId="20458" CreationDate="2017-03-03T22:45:08.403" Score="2" Body="&lt;p&gt;The ISS is moving in a circle. Certainly, the Earth is accelerating it &quot;downwards&quot;, but on the other side of the orbit downwards is in the opposite direction. After one orbital revolution, the net acceleration of the space station is zero. &lt;a href=&quot;https://briligg.github.io/moonwards/img/source%20files/anim.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Simple animation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The direction of &quot;vertical velocity&quot; is constantly changing, so to use this as a quantity we would want to use a rotating frame of reference. In such a reference frame, we have an inertial force perfectly balancing the force of gravity.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-03-03T22:45:08.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20496" PostTypeId="2" ParentId="20474" CreationDate="2017-03-04T00:48:07.757" Score="1" Body="&lt;p&gt;To add a little bit to @RussellBorogove's &lt;a href=&quot;https://space.stackexchange.com/a/20492/12102&quot;&gt;definitive answer&lt;/a&gt; there may be disappointing speed limitations due fluid dynamical realities; friction and turbulent flow may severely limit the maximum speed that air can enter the tube. It seems mostly sub-sonic so it's possible basic fluid dynamics texts could be used to estimate this effect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If someone can do this, please go for it and post separately!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may also be some low frequency (sub-Hz) oscillations as air mass in different parts of this multi-kilometer long tube moving at different velocity start bumping into each other.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-03-04T00:48:07.757" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20497" PostTypeId="2" ParentId="20473" CreationDate="2017-03-04T00:49:23.640" Score="12" Body="&lt;p&gt;Probably not what you had in mind, but...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Space Shuttle Challenger&lt;/strong&gt; during &lt;strong&gt;STS-7&lt;/strong&gt; was scheduled to land at the &lt;strong&gt;Kennedy Space Center&lt;/strong&gt; in &lt;strong&gt;Florida&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to weather it ended up landing at &lt;strong&gt;Edwards Air Force&lt;/strong&gt; base in &lt;strong&gt;California&lt;/strong&gt; - about &lt;strong&gt;2,500 miles&lt;/strong&gt; ways (June 24, 1983).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was the first of several shuttle landings that were diverted to Edwards.&lt;/p&gt;&#xA;" OwnerUserId="11819" LastActivityDate="2017-03-04T00:49:23.640" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20499" PostTypeId="2" ParentId="19984" CreationDate="2017-03-04T02:48:06.140" Score="4" Body="&lt;p&gt;From &lt;a href=&quot;http://www.space.com/30008-moon-colony-cost-commercial-space-report.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moon Base Would Be Cheap with Help from Private Industry: Report&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The dream of sending humans back to the moon could become a reality&#xA;  for the relatively low price of \$10 billion — only about 10 percent of&#xA;  one previous cost estimate — as long as NASA is willing to buy from&#xA;  private spaceflight companies, a new report shows.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In 2005, NASA estimated that returning humans to the moon would cost&#xA;  100 billion Dollars (approximately \$122 billion in today's dollars).&#xA;  But if the success of private spaceflight companies like SpaceX and&#xA;  Orbital Sciences continues, NASA could send humans back to the lunar&#xA;  surface in as little as five to seven years, at a highly reduced cost,&#xA;  the new report shows.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;That's not all: 10 to 12 years after that first commercial moon trip,&#xA;  NASA could develop a permanent base on the moon for about \$40 billion&#xA;  in today's dollars, the report said. The proposed permanent moon base&#xA;  would be used to convert lunar ice into hydrogen propellant that could  be sold for use by other spacecraft, including missions headed to Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here is an additional link that seems to support the findings of the above or at the very least be inline with the above (as provided by call2voyage's comment to the OP.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.popsci.com/colonizing-moon-may-be-90-percent-cheaper-we-thought&quot; rel=&quot;nofollow noreferrer&quot;&gt;Colonizing The Moon May Be 90 Percent Cheaper Than We Thought&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As much as I would like to offer you a finite answer to cost, you're going to need to accept a bracket indicating the startup cost. \$10 Billion USD &amp;ndash; \$40 Billion USD. Once the startup phase is concluded, the startup cost is over and the project is profitable from that point on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost will be the cost of shipping people there and a startup habitat and supplies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After that everything will be built from lunar resources by people living on the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delivery to Earth is a piece of cake.  Luna colony builds transfer vehicles from lunar resources and fuels them from lunar resources.  Transfer to low Earth orbit from low lunar orbit (large structures) can be done by nuclear engines (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Rover&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Rover&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/NERVA&quot; rel=&quot;nofollow noreferrer&quot;&gt;NERVA&lt;/a&gt;), or laser powered, solar wind powered, all of which are magnitudes cheaper than any Earth OR low Earth orbit launch cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Profitability (measured sans startup cost) is instant once production of space assets, fuels and transfer vehicles are started.  Earth would, given today's propulsion systems, be incapable of competing in any aspect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic equation is 1/6th G (the Moon) which is infinitely greater gravity than the microgravity of the ISS vs 1G (the Earth).  Mining &amp;hellip; that is not what you asked but it also will be magnitudes cheaper and &amp;hellip; shall we say be a Lunar Colony decision.  With the added bonus of nuclear engines to capture and return near Earth orbit objects for processing or processed in place and the processed products are shipped where needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The greatest problem with space is shedding being Humans: A Planetary Species to become Humans: A Space-faring Species, as in a fundamental change in thinking and how we solve problems. We start by discarding the perception that everything is a problem needing an Earth Hammer solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Habitats: lunar lava tubes are our Habitats, they are already made. We need to seal them, pressurize them and make them habitable.&lt;/p&gt;&#xA;" OwnerUserId="18720" LastEditorUserId="18720" LastEditDate="2017-03-04T18:10:14.937" LastActivityDate="2017-03-04T18:10:14.937" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20500" PostTypeId="2" ParentId="20474" CreationDate="2017-03-04T06:55:44.457" Score="3" Body="&lt;p&gt;Even if you could build a tube from ground level high enough to reach space, the bottom of the tube would not have a vacuum. The weight of the air in the tube is enough to hold it down against the pull from the vacuum above it (really from its own pressure pushing it up). How do we know this? The rest of the air in the atmosphere does not push itself out into space, and it has nothing holding it down but its own weight.&lt;/p&gt;&#xA;" OwnerUserId="6010" LastActivityDate="2017-03-04T06:55:44.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20501" PostTypeId="1" CreationDate="2017-03-04T10:59:55.650" Score="4" ViewCount="938" Body="&lt;p&gt;We have the question &lt;a href=&quot;https://space.stackexchange.com/questions/2318/&quot;&gt;Time to prepare for EVA out of ISS&lt;/a&gt; that has a good and complete answer about the hours to days long process to prepare for an EVA from the ISS.  My question is just about getting into a suit the fastest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a lot of variables, full gravity, micro gravity, suit style, pre-fitted vs first use, available assistance, and probably other I have not considered. While all that the prep for an ISS EVA is important, some people like to compete, I imagine that there has been study and competition for space suit donning time.  This question seeks to identify speed for a &lt;a href=&quot;https://en.wikipedia.org/wiki/24_Hours_of_Le_Mans#Le_Mans_start&quot; rel=&quot;nofollow noreferrer&quot;&gt;Le Mans start&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When the French flag dropped to signify the start, the drivers ran across the track, entered and started their cars without assistance, and drove away. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;When it comes to donning safety gear competition is a frequent tool used for building muscle memory.  Anyone who has been in the military has donned their gas protective gear multiple times in a race against the clock and their peers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in my hypothetical scenario we have two or three astronauts, who are competing for fastest suit donning.  They talk among themselves, each picking the their preferred suit, it is theirs and fits perfect, they agree on a venue (the ISS or Earth, or whatever) and agree to have some number (maybe zero) of assistants for each of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The winner must be safely and fully suited, vacuum ready, and move under their own power some short distance to a finish point (&lt;em&gt;airlock?&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where do they decide to have the contest? What type of suit do they pick? Do they have help? Who wins? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2019-10-20T10:34:59.227" Title="What is the fastest time for donning a space suit?" Tags="|spacesuits|eva|airlock|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20502" PostTypeId="2" ParentId="20501" CreationDate="2017-03-04T13:43:42.150" Score="4" Body="&lt;p&gt;Astronauts universally don their spacesuits during prebreathing procedures, so they cannot accelerate the process by donning the suit faster. There is also no reason to don particularly quickly, unlike the situation when donning BCN protection equiment. In fact, going quickly is discouraged as it is more likely to lead to mistakes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, on the ISS they don their suits while already inside the airlock. &lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-04T13:43:42.150" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20503" PostTypeId="1" CreationDate="2017-03-04T13:50:32.047" Score="-1" ViewCount="112" Body="&lt;p&gt;I'm looking for very detailed information. I read journal articles on a regular basis, so don't pull any punches.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-10-25T02:03:30.530" Title="Where can I find a good source of information on the material properties of space suit fabrics?" Tags="|spacesuits|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20504" PostTypeId="2" ParentId="20503" CreationDate="2017-03-04T15:44:11.433" Score="2" Body="&lt;p&gt;Much of NASA's research is accessible via &lt;a href=&quot;https://ntrs.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/&lt;/a&gt;. A search for '&lt;a href=&quot;https://ntrs.nasa.gov/?N=0&amp;amp;Ntk=All&amp;amp;Ntt=apollo%20spacesuit%20material&amp;amp;Ntx=mode%20matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo spacesuit material&lt;/a&gt;' has more than 1000 matches, so have fun!&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-03-04T15:44:11.433" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20505" PostTypeId="1" CreationDate="2017-03-04T16:02:52.087" Score="1" ViewCount="150" Body="&lt;p&gt;We get onboard a cruise ship 5 to 6 hours prior to the launch, in direction of Nassau, Bahamas. Cruising at about 25 km/hr, we should be about 150 km away from the pad.Do you think that we still have a reasonable chance to see the rocket's trajectory in the sky at this distance?&lt;/p&gt;&#xA;" OwnerUserId="18802" LastEditorUserId="12102" LastEditDate="2017-03-11T17:01:54.737" LastActivityDate="2017-03-11T17:01:54.737" Title="What is the maximal distance to see a rocket's flare in the sky?" Tags="|launch|visibility-of-spacecraft|" AnswerCount="0" CommentCount="3" ClosedDate="2017-03-04T18:35:45.590" ContentLicense="CC BY-SA 3.0" />
  <row Id="20506" PostTypeId="2" ParentId="20473" CreationDate="2017-03-04T18:09:25.180" Score="26" Body="&lt;p&gt;I would like to make my own compilation that includes several missions not mentioned in any other answer so far. (The record is actually 483 km of Soyuz TMA-1, not the 475 km of Soyuz TMA-11.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gemini_5&quot; rel=&quot;noreferrer&quot;&gt;Gemini 5&lt;/a&gt;:&lt;/strong&gt; splashed down 130 km off course due to programming error. Someone put in the rotation rate of Earth for the solar day instead of the sidereal day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Voskhod_2&quot; rel=&quot;noreferrer&quot;&gt;Voskhod 2&lt;/a&gt;:&lt;/strong&gt; landed 382 km off course due to a delay caused by malfunction of the automatic re-entry system. This is probably the most dangerous off-course re-entry for several reasons. It came down over forest (Voskhod had braking rockets under its parachutes, triggered by a contact string hanging under the capsule, but fortunately the string did not hit a tree early). The capsule got wedged between two trees and was stuck in Siberian forest with bears, wolves, and cold weather for a day and a night. Helicopters at that time apparently could not lift them out nor land nearby due to heavy forest, so the crew had to ski out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Atlas_7&quot; rel=&quot;noreferrer&quot;&gt;Mercury-Atlas 7&lt;/a&gt;:&lt;/strong&gt; splashed down 400 km off course due to delays from pilot error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_5&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 5&lt;/a&gt;&lt;/strong&gt; had a ballistic re-entry due to failure to separate from the Orbital Module. &lt;strong&gt;I could not find how far off course it landed.&lt;/strong&gt; &lt;a href=&quot;https://www.youtube.com/watch?v=IkWXQ1hHtOU&quot; rel=&quot;noreferrer&quot;&gt;This YouTube thing&lt;/a&gt; says &quot;over 2,000 km&quot; off course but I find that &lt;strong&gt;physically impossible&lt;/strong&gt; as the other Soyuz missions suffering similar separation problems did not exceed 500 km off course. &lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-5.htm&quot; rel=&quot;noreferrer&quot;&gt;Spacefacts.de&lt;/a&gt; says &quot;hundreds of kilometers short&quot;. &lt;a href=&quot;http://www.astronautix.com/s/soyuz5.html&quot; rel=&quot;noreferrer&quot;&gt;Astronautix&lt;/a&gt; only says &quot;far short of its aim point&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Soyuz 5 landed very rough because of &lt;strong&gt;partial tangling of chute lines and failure of brake rockets.&lt;/strong&gt; Cosmonaut Volynov &lt;strong&gt;broke several teeth&lt;/strong&gt; from this, so it may be tied as the most dangerous off-course landing.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-1&quot; rel=&quot;noreferrer&quot;&gt;Soyuz TMA-1&lt;/a&gt;&lt;/strong&gt; landed 483 km off course according to &lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma1.htm&quot; rel=&quot;noreferrer&quot;&gt;Spacefacts.de&lt;/a&gt;. It had a ballistic re-entry due to failure to separate from the Orbital Module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-10&quot; rel=&quot;noreferrer&quot;&gt;Soyuz TMA-10&lt;/a&gt;&lt;/strong&gt; had a ballistic re-entry for the same reason and landed about 304 km off course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_TMA-11&quot; rel=&quot;noreferrer&quot;&gt;Soyuz TMA-11&lt;/a&gt;:&lt;/strong&gt; landed 475 km off course according to Wikipedia, and 428 km according to &lt;a href=&quot;http://www.spacefacts.de/mission/english/soyuz-tma-11.htm&quot; rel=&quot;noreferrer&quot;&gt;Spacefacts.de&lt;/a&gt;. &lt;a href=&quot;http://www.astronautix.com/s/soyuztma-11.html&quot; rel=&quot;noreferrer&quot;&gt;Astronautix&lt;/a&gt; says &quot;They landed 470 km short of the target point&quot;. The Wiki entry has no cite for its number. It is unclear how Spacefacts and Astronautix came up with their numbers. Soyuz TMA-11 suffered a ballistic re-entry for the same reasons as Soyuz TMA-10.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;strong&gt;There is also &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-T_No.39&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 7K-T No. 39&lt;/a&gt;, also known as Soyuz 18a.&lt;/strong&gt; It never made it to orbit, but it did go above 145 km and thus into space, so I'm not sure if we should count it. A failure in separating stage 1 (core) from stage 2 (highest stage) caused the rocket to go far off course, and the launch was aborted. The capsule came down over 1,000 km from the &lt;strong&gt;launch&lt;/strong&gt; site, though I don't know how far away it was from the intended &lt;strong&gt;landing&lt;/strong&gt; site.)&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;&lt;strong&gt;…Speaking of more separation failures…&lt;/strong&gt;&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_1&quot; rel=&quot;noreferrer&quot;&gt;Vostok 1&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_2&quot; rel=&quot;noreferrer&quot;&gt;Vostok 2&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_5&quot; rel=&quot;noreferrer&quot;&gt;Vostok 5&lt;/a&gt;&lt;/strong&gt; all failed to separate their capsules from the service modules at the beginning of re-entry. Vostok 1 (the very first manned mission in space) landed 280 km off course. I could not find how far Vostok 2 and Vostok 5 landed off course. &lt;strong&gt;Is it possible&lt;/strong&gt; that one of them went farther than 483 km? I don't know. But note that, in all Vostok missions, the lone pilot ejected before the capsule hit the ground. Both landed under parachute and &lt;strong&gt;it is not clear&lt;/strong&gt; whether the Vostok 1 capsule or Yuri Gagarin landed 280 km from the planned site, &lt;strong&gt;nor is it clear&lt;/strong&gt; how far apart the capsule landed from the pilot for any of those Vostok missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for crewed missions that actually reached orbit and landed or splashed down intact, it is &lt;strong&gt;Soyuz TMA-1&lt;/strong&gt; (not 11) that landed the farthest off course… so far. There is still a chance that Soyuz 5 landed even farther off course, since I could not find the exact number.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="595" LastEditDate="2017-03-06T10:44:17.310" LastActivityDate="2017-03-06T10:44:17.310" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20507" PostTypeId="1" AcceptedAnswerId="20508" CreationDate="2017-03-04T18:38:09.533" Score="6" ViewCount="650" Body="&lt;p&gt;Based on &lt;a href=&quot;https://space.stackexchange.com/questions/20473/what-crewed-flight-landed-farthest-off-target&quot;&gt;this question&lt;/a&gt; it sounds like if something goes wrong it's pretty easy for a crewed capsule reentering from space to go way off target. Does that mean that the capsules are equipped to land pretty much anywhere? Obviously landing on the top of mount everest would be a challenge, but can they land on both water and land for example?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To clarify my question a bit, if a capsule is slated to land on land, but gets off course and ends up landing in water, will it be survivable? IE will rafts still deploy? and vice versa.&lt;/p&gt;&#xA;" OwnerUserId="3141" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-03-04T23:33:05.633" Title="Are crewed capsules equiped to land pretty much anywhere in the world?" Tags="|crewed-spaceflight|landing|reentry|capsule|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20508" PostTypeId="2" ParentId="20507" CreationDate="2017-03-04T19:53:36.737" Score="10" Body="&lt;p&gt;The Soyuz capsule can land in water, even if that is by no means an optimal scenario. The last splashdown of a crewed spacecraft was &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_23&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 23&lt;/a&gt; landing in a lake in Kazakhstan. The recovery took a while as locating the spacecraft at night after the communication equipment failed was difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Splashdown tests have later been performed, and a water landing should be more easy to handle now than in 1976.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oLMog.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oLMog.jpg&quot; alt=&quot;Soyuz splashdown test&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for other spacecraft:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo (no longer in use) had a relatively low landing speed (8-10m/s), which should be pretty ok for a landing on land. A splashdown was preferred though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space shuttle (no longer in use) required a runway, an unusually long one. That limits the landing locations a lot, even though there were backup plans for landing in airports in for example Europe in an emergency situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could not find any data on Shenzhou, but the design is very close to Soyuz, and also lands on land.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-03-04T19:53:36.737" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20509" PostTypeId="2" ParentId="20507" CreationDate="2017-03-04T22:24:58.763" Score="6" Body="&lt;p&gt;One environment the capsules definitely can't land on is steep mountain slopes. If the capsule starts rolling down the slope, there's very little the astronauts can do. This will include glaciers, buildings, generally once the capsule makes the touchdown, it better not continue falling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Forests with tall trees (like Amazon jungle) may prove lethal, as the parachute tangles, rips, and the capsule falls last 30-50m like a rock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And of course places where you will die, no matter how you arrived. High voltage power stations, active volcanoes, ISIS training camps, minefields, generally anywhere you wouldn't want to land by any other means.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The astronauts are equipped for survival in even quite harsh environments, and the capsules are quite sturdy, plus most of the world is rather friendly towards space programs, but imagine you've got the best survival gear and a sturdy shelter, but then you appear in a completely random place in the world...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally though, mountains, due to large surface they cover, pose the most danger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(also add atrophy from long time in microgravity; the astronauts won't be likely to walk away on their own, to e.g. find help if they are stuck in the middle of Amazon forest, or Sahara).&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-04T22:24:58.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20510" PostTypeId="1" CreationDate="2017-03-04T21:24:54.840" Score="5" ViewCount="1842" Body="&lt;p&gt;Assume we have a society that has sophisticated enough engineering to build a SSTO chassis that can survive the rigors of repeated launch (MaxQ) and re-entry (thermal stress).  What thrust to weight ratio would they have to achieve from their propulsion systems in order to get the craft into orbit around an Earth-clone planet without the need for external boosters?&lt;/p&gt;&#xA;" OwnerDisplayName="Shalvenay" LastEditorUserId="-1" LastEditDate="2017-03-04T23:15:26.327" LastActivityDate="2019-06-13T15:29:27.353" Title="What thrust-to-weight ratio would be required for a SSTO without any external boosters?" Tags="|ssto|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20512" PostTypeId="2" ParentId="20510" CreationDate="2017-03-04T22:55:27.707" Score="2" Body="&lt;p&gt;The question asked I am answering&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What thrust to weight ratio would they have to achieve from their&#xA;  propulsion systems in order to get the craft into orbit around an&#xA;  Earth-clone planet without the need for external boosters?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I had wanted to use the Delta Clipper X but had a Eureka Moment that it was a scale testing unit and switched to this instead.  To the best of my knowledge, there has not been a &quot;Public&quot; SSTO made or launched. Some may consider Lockheed a credible source.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/VentureStar&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/VentureStar&lt;/a&gt; - VentureStar&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While never built the work presented here seems to be consistent with a Full Scale Ship (SSTO)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Function    Manned Re-usable Spaceplane&#xA;Manufacturer    Lockheed Martin&#xA;Country of origin   United States&#xA;Size&#xA;Height  38.7 m[1] (127 ft)&#xA;Diameter    N/A&#xA;Mass    1,000,000 kg[1] (2,200,000 lb)&#xA;Stages  1&#xA;Capacity&#xA;Payload to LEO  20,412 kg[1] (45,000 lb)&#xA;Launch history&#xA;Status  Cancelled&#xA;Launch sites    Unknown&#xA;Total launches  0&#xA;First stage - VentureStar&#xA;Engines 7 RS2200 Linear Aerospikes[1]&#xA;Thrust  3,010,000 lb[1] (13.39 MN)&#xA;Fuel    LOX/LH2[1]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;It gives both weight and thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following is based on the MASS because I have no clue if the Payload is included or not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0.1643017177 lbs per Newton would be your answer in Newtons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or for my bud SF that likes to groan a lot :) 164.3017177 lbs per kN&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or 164,301.717699776 lbs per MN&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Treat lbs of mass as lbf of weight, divide thrust in lbf by that and&#xA;  you have the clean dimensionless TWR&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;3,010,000 / 2,200,000 gives 1.36818118118182 - Just for you&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For those that want a good and interesting read on Real Life SSTO's go here&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/McDonnell_Douglas_DC-X&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/McDonnell_Douglas_DC-X&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Delta Clipper, VentureStar, SpaceX, Blue Origin Are all linked together. The Delta Clipper was taken over by NASA killed in favor of their choice the VenturStar because of the landing being horizontal rather than vertical then canceled it for the Space Shuttle and guess were everyone went to work. Let us .... say the obvious, The Shuttle, Delta Clipper XA, VentureStar all ran separately and parallel to each other.&lt;/p&gt;&#xA;" OwnerUserId="18720" LastEditorUserId="18720" LastEditDate="2017-03-05T18:07:19.637" LastActivityDate="2017-03-05T18:07:19.637" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="20513" PostTypeId="1" AcceptedAnswerId="20515" CreationDate="2017-03-05T00:39:27.703" Score="1" ViewCount="448" Body="&lt;p&gt;When moving in an elliptical orbit from the Apoapsis toward the Periapsis could you class yourself as being pulled (falling) towards the Periapsis and then your velocity after is then pushing you towards the apoapsis? If this is the case what could the variance of speed be? I always assumed that you inserted yourself into an orbit at one speed and once you accelerated fast enough you could maintain that.&lt;/p&gt;&#xA;" OwnerUserId="18805" LastEditorUserId="6816" LastEditDate="2017-03-05T00:48:28.230" LastActivityDate="2017-03-05T07:09:31.713" Title="Forces acting upon you between Apoapsis and Periapsis" Tags="|orbital-mechanics|orbit|gravity|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20514" PostTypeId="2" ParentId="20325" CreationDate="2017-03-05T00:42:23.697" Score="2" Body="&lt;p&gt;Yes, you can theoretically harvest the CMBR. However...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other answers seem to be describing the (tiny) limits for &quot;naked&quot; antennas. But don't forget you can gather a large area of incoming photons and focus them, say, via a parabolic dish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How big of a dish? That's up to you and your spaceship. You quoted about 450 photons per cubic centimeter so I think you can figure out how much you want. For reference, the CMBR frequency is about 160 GHz, in the microwave range of the spectrum. It is definitely possible to build a smooth enough dish that will reflect microwaves. (We have such dishes on Earth.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Nevermind that. It appears the CMBR is actually a diffuse source of photons (not parallel photons), so a parabolic dish will not focus them.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;However...&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;You mentioned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;any external energy source; it doesn't need to be CMB specifically - that is available during intergalactic travel, where you can't depend on solar wind, light of nearby stars or similar; you must depend on what's available there: thermal cosmic background radiation, cosmic microwave background, high-energy cosmic rays, whatever means are available in intergalactic space, other than what you brought with the craft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The external energy source I think would be best is &lt;strong&gt;nearby starlight&lt;/strong&gt;. Imagine you have a gigantic parabolic dish on your spacecraft, trying to collect the tiny CMBR. Instead, it would collect &lt;strong&gt;a lot more starlight&lt;/strong&gt; from nearby stars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much starlight? Let's just simplify with average stars at average distances. The sunlight near Earth, but not yet hitting the atmosphere, comes in at about &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;1.36 kW per square meter&lt;/a&gt;. The nearest star to our Sun is about &lt;a href=&quot;https://en.wikipedia.org/wiki/Proxima_Centauri&quot; rel=&quot;nofollow noreferrer&quot;&gt;4 lightyears&lt;/a&gt; away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's pretend your spaceship, during its journey, is always within 2 LY of a star. 1 LY is about 63,241.1 AU (1 AU = distance from Sun to Earth). Therefore, the starlight is coming in at 1.36 / 63,241.1^2 = 34.005 nanowatts per square meter. But there are probably several nearby stars. Let's say on average, your ship is surrounded by 8 stars each 2 LY away. So you have about 272 nanowatts per square meter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not a lot but I'm pretty sure it's orders of magnitude above what you could get with the CMBR. It also does not include light from more distant stars. Just from knowing the &lt;a href=&quot;https://en.wikipedia.org/wiki/1/2_%2B_1/4_%2B_1/8_%2B_1/16_%2B_%E2%8B%AF&quot; rel=&quot;nofollow noreferrer&quot;&gt;mathematical series&lt;/a&gt; of 1/2 + 1/4 + 1/8 + 1/16 ..., I know that sums to 1. So I'll just guess that all the other stars farther away add up to another 272 nanowatts per sq meter. Total is 544.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you want 1 watt, you need a solar panel that's about 1.85 million square meters big...or a parabolic reflector taking 1.85 M sq m and focusing it onto a solar panel of just 1 sq m.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind, tho, that you'll hafta point the solar panel or its parabolic focuses at the nearest star. You could have many such things surrounding the ship, each capable of swiveling. There have been some creative ideas about large lightweight reflectors for this purpose, like the unfolding solar sail that reflects and focuses light back to the towed capsule.&lt;/p&gt;&#xA;" OwnerUserId="6605" LastEditorUserId="6605" LastEditDate="2017-03-05T01:52:43.587" LastActivityDate="2017-03-05T01:52:43.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20515" PostTypeId="2" ParentId="20513" CreationDate="2017-03-05T02:06:09.357" Score="3" Body="&lt;p&gt;In an &lt;a href=&quot;https://en.wikipedia.org/wiki/Elliptic_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;elliptical orbit&lt;/a&gt;, velocity varies over the course of the orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equation for speed in elliptical orbit is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ v=\sqrt{\mu\left({2\over{r}}-{1\over{a}}\right)} $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this equation, $\mu$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational parameter&lt;/a&gt; of the body you're orbiting, and $a$ is the semi-major axis of the orbit, both of which are unchanging for a stable orbit. $r$ is the current distance between the orbiting bodies, so it's smallest at periapsis and largest at apoapsis, meaning that speed is fastest at periapsis.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-03-05T03:15:14.350" LastActivityDate="2017-03-05T03:15:14.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20516" PostTypeId="2" ParentId="20485" CreationDate="2017-03-05T02:06:11.167" Score="3" Body="&lt;p&gt;Interplanetary spacecraft monitor their trajectories while in the vicinity of another planet, but not while in transit between planets. That's the job of people and equipment on the surface of the Earth. There's no particular reason to add that capability to the spacecraft's flight software, and many reasons not to do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time spent between planets is a passive phase of flight. There's no need for a fast autonomous reaction by the spacecraft. Compare that to the time in the vicinity of a planet, where some degree of autonomy is essential. Adding autonomous navigation to the spacecraft's flight software would be extremely expensive as flight software is an order of magnitude more costly than is comparable ground-based software.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-03-05T02:06:11.167" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20517" PostTypeId="2" ParentId="20513" CreationDate="2017-03-05T07:09:31.713" Score="2" Body="&lt;p&gt;The model you are looking for is a &lt;strong&gt;co-rotating frame of reference&lt;/strong&gt;. Be careful though! If you thing properly understanding rotating reference frames is easier than properly understanding momentum, you are very likely going to get confused.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the co-rotating model is very useful when working with orbital tethers, solar sail simulation or complex things like the CR3BP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this frame of reference, there are three forces acting on a spacecraft. The first one is gravity, and it works in just the same way as in a normal inertial frame:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\mu}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, you do now have two inertial forces due to momentum being a strange concept in such a frame. The first one is centrifugal force (yes, centrifugal):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{v_h^2}{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $v_h$ is horizontal velocity. The second one is Coriolis force (yes, Coriolis):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$-\frac{2v_vv_h}{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $v_v$ is vertical velocity. This force is normal to the two other forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this frame, a circular orbit is just not moving, with $\frac{\mu}{r^2} - \frac{v_h^2}{r} = 0$. In an elliptical orbit, at periapsis, $\frac{\mu}{r^2} - \frac{v_h^2}{r} &amp;lt; 0$, and at apoapsis $\frac{\mu}{r^2} - \frac{v_h^2}{r} &amp;gt; 0$, just like you said.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-03-05T07:09:31.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20518" PostTypeId="2" ParentId="20510" CreationDate="2017-03-05T09:29:58.820" Score="10" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What thrust to weight ratio would they have to achieve from their propulsion systems in order to get the craft into orbit&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This wording indicates that you think there is a threshold. All designs with a TWR lower than this will fail to reach orbit, and designs with a higher TWR will succeed. &#xA;This premise is incorrect. As SF says in his comment, a winged SSTO can get away with a TWR lower than 1. So depending on body design (large wings, small wings, lifting body or cylindrical rocket) you get a large range of possible TWR figures.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other numbers are more critical than TWR:   &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You need sufficient delta-V to reach orbit. &lt;/li&gt;&#xA;&lt;li&gt;You need a mass fraction that's good enough to bring some payload to orbit. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Those two combined (via the Rocket Equation) give a minimum specific impulse required to get that payload to orbit. &#xA;TWR is a consideration via gravity losses: the longer you take to get to orbital speed, the more gravity losses you'll have and the more delta-V you'll need to account for.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in order to be able to calculate anything, we need 2 parameters:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;required payload  &lt;/li&gt;&#xA;&lt;li&gt;available specific impulse&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Those two are enough to calculate the starting mass of the SSTO. Economics then decides if such an SSTO is worth building. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-03-05T18:16:36.960" LastActivityDate="2017-03-05T18:16:36.960" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="20519" PostTypeId="1" AcceptedAnswerId="20534" CreationDate="2017-03-05T12:12:45.797" Score="6" ViewCount="2264" Body="&lt;p&gt;Could someone explain to me how, during the descent, the pumps pull the oxidizer and propellant?  I could be wrong, but during the burn the propellant should move inside an almost empty tank.&lt;/p&gt;&#xA;" OwnerUserId="18810" LastEditorUserId="1142" LastEditDate="2017-03-05T12:20:46.573" LastActivityDate="2017-03-07T02:57:55.430" Title="How does the propellant flow during descent on SpaceX's Falcon 9 first stage?" Tags="|spacex|reentry|turbopump|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20520" PostTypeId="2" ParentId="20470" CreationDate="2017-03-05T13:17:27.850" Score="3" Body="&lt;p&gt;Yes, they are real, not artifacts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;http://www.msss.com/mars_images/moc/2003/06/23/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Orbiter Camera&lt;/a&gt; website.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The rows of grooves and aligned pits on Phobos are related to, and were probably caused by, a large meteor impact that occurred on the side of Phobos that is not seen here. That large crater, Stickney, was named for the maiden name of the wife of the astronomer that discovered Phobos and the other martian satellite, Deimos, in 1877, Asaph Hall. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That crater was already visible in the &lt;a href=&quot;http://www.planetary.org/multimedia/space-images/mars/phobos-from-mariner-9.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mariner 9 photos&lt;/a&gt; from December 1971.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On this 1998 picture (source &lt;a href=&quot;http://www.msss.com/mars_images/moc/2003/05/16/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.msss.com/mars_images/moc/2003/05/16/&lt;/a&gt;) you can see the grooves radiate outward from Stickney:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/apkJt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/apkJt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user10509" LastActivityDate="2017-03-05T13:17:27.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20521" PostTypeId="2" ParentId="20519" CreationDate="2017-03-05T13:55:38.370" Score="11" Body="&lt;p&gt;During descent, the stage is flying with the rocket nozzle in the flight direction, which means the &quot;bottom&quot; of the tank is also oriented in the flight direction. Since the stage is decelerating due to atmospheric drag (and engine burns) the fuel is &quot;pressed&quot; to the bottom of the tank and can thus be pumped like on ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/csOwN.png&quot; alt=&quot;SpaceX Falcon 9 Flight Profile&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="14549" LastEditDate="2017-03-06T08:48:45.463" LastActivityDate="2017-03-06T08:48:45.463" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20522" PostTypeId="2" ParentId="19006" CreationDate="2017-03-05T15:23:32.173" Score="88" Body="&lt;p&gt;I was a NASA contractor working on the same contract, but with 3 different companies, writing GN&amp;amp;C code for the space shuttles from 1989 until 1995. Initially, it was Ford Aerospace, then Loral, then Lockheed-Martin. IBM was the prime most of that time. It has been a long time since I did any work like that, so don't hold what I write below as gospel.  Memories of a middle-aged guy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have some time this morning, so I'll spew some thoughts ... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle OS isn't general purpose. It is purpose built, designed to be loaded with 1 program, instruction pointer set to 0 and then started running. I honestly don't know how that worked, since we never touched any of the flight computers. I've only seen photos and recently saw one inside a glass case at a museum.  When I worked there, we didn't have access to the computers directly. Because the computing hardware is modified IBM 360-based systems, that is the machine language used by all the main systems. There were 5 GPCs - general processing computers. During ascent and landing, 4 ran the PASS - Primary Avionics System Software and 1 ran the BFS (backup flight software). The BFS was written and maintained by a different contractor, Rockwell. I know nothing about the BFS except it was never used. Heard rumors that eventually that contract was canceled because the PASS was of such high quality there wasn't any need for the BFS. Perhaps someone from Rockwell can answer? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vast majority of programming for the GPCs was written in HAL/S. This is a compiled language similar to PL/1, but with matrix calculation extensions and a multi-priority scheduling system.  Basically, it supported 254 threads, but we only used 3. HFE, MFE, LFE - Hi, Mid, Low Frequency Executives. The priority was H, M, L as well. HFE ran at 25Hz.  MFE ran at 12.5Hz and Low ran at 0.5Hz. Mid-instruction, a higher priority could interrupt a lower one. It was important to leave time so that lower priority tasks would have sufficient time to run.  Some complex guidance code would overrun the allotted time and had to be highly optimized.  Since these were real-time systems, a late answer is just a bad as a wrong answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The PASS software ran in a redundant set with break points along the code paths (managed outside our application code).  Periodically, each computer would get to a checkpoint and compare with the other computers in the &quot;redundant set&quot;. Basically, they would vote. Majority rule. Any computers outside the majority would be dropped and all future values from that one would be ignored. If only 2 systems are in the redundant set and they disagree, the computer which arrived first was deemed correct.  I don't believe they ever had fewer than 3 GPCs running. Basically, only 1 was dropped over code path disagreements (1996 and earlier). I don't know anything after 1996 when I left the NASA area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the poor performance of passing parameters on the stack, we used global variables.  There was a strict naming convention which said which priority the variable could be used on, which subsystem it was part of and what type of variable it was (int, scalar, double, bit, hex). I think we were limited to 32-character variable names, but don't quote me, it wasn't an issue. &#xA;Dynamic memory wasn't allowed to be used. Used pointers once (called &quot;NAMED&quot; variables in HAL/S) and needed to get a variance approved. Since many of the programmers were trained in engineering (including me), not CS, pointers were not well understood.  I was taking advantage of pointer overruns to access contiguous memory variables which were beyond the allow size supported by the compiler for arrays. Basically, I declared multiple arrays back to back and used a pointer to treat their memory as 1 variable. Nothing fancy, but it meant having to ensure that the compiler didn't optimize storage in unexpected ways.  The compilers and linkers were specific to the shuttle program and not bug-proof.  They were fairly high quality, however. Some of our infrastructure guys would perform compiler audits looking for odd issues. One guy, JY, had so much experience and knowledge that asking him questions was frustrating. 10 yrs later, I became just like him at my job and understood that simple questions don't always get simple answers. &quot;It depends&quot; is a valid answer and more info is required to answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Packing bits was common since memory was limited. Much of my code was binary logic based on bit values to decide which calculation would be performed under the vastly different situations possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On orbit, programs were overlayed just like the old MS-DOS &quot;overlay&quot; methods. I didn't work on the Vehicle Utility code, VU, but it handled the arm controls and other things outside of flight control (HiFE), life support (LoFE) and guidance (MedFE).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are interested in the people doing the work, there have been a few articles about us written over the decades.  A coworker left the project to work for AMD. After a year, he returned because AMD was seeking &quot;good enough&quot; engineering, not perfect engineering. His story is in &quot;Fast Company&quot; - &quot;The Write the Right Stuff&quot; &lt;a href=&quot;https://www.fastcompany.com/28121/they-write-right-stuff&quot; rel=&quot;noreferrer&quot;&gt;https://www.fastcompany.com/28121/they-write-right-stuff&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some other &quot;languages used&quot; onboard weren't really languages.&#xA;DFG - Display Format Generator (or something like that). It was the language that drove the DEUs for the astronauts to see status and manage the computers. It was extremely rudimentary with only BE (branch equal) and BNE (branch not-equal) type comparison operations. No label jumping - it only used counts of instructions.  Basically, if X is true, jump 15 instructions and continue. It was very simple, but hard to get a display correct the first time. We had no interactive ways to see the screen output based on our code. Had to wait for printouts of the batch test runs to see the drawings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For initial values of the tens of thousands of parameters, there were 2 different pre-processors - I-Load and K-Loads.  I never touched any K-Load stuff.  The I-load stuff specified initial values which were either copied to a memory location or had calculations performed based on other inputs which were then copied into a memory location.  These locations could be into the RAM or onto tape storage which was loaded during major &quot;OPS&quot; changes.  OPS 1/6 for launch. OPS 3 for Landings. OPS 2/8 for on-orbit stuff.  I-loads happened PRE-mission.  The i-load operator precedence wasn't the normal one taught and used world-wide in math classes everywhere.  PMDAS - Parenthesis, Multiplication, Division, Addition then Subtraction.  Usually, multiplication or division are of the same priority, but not with the i-load tool. Same for addition and subtraction.  Found a bunch of bugs in my mentor's code when I worked there just a few months because of this. I was reading the i-load manual and was new enough NOT to assume anything. My mentor, DC, had been working there at least 5 yrs, perhaps 10. He was EXTREMELY smart. They had a formal code review which I missed, wasn't prepared. A few days later, I had finished reviewing his code and asked about why those statements were ok, referencing the I-load user's guide. He read it carefully, then said I'd found a number of bugs that everyone else missed. The formal review at IBM (we were a sub to them) was delayed and he fixed the issues, re-reviewed it in-house, and sent over new review at IBM. BTW, finding errors in code reviews was extremely rare. Everyone worked extremely hard to help each other out. After the first few months working in the group, most of the programmers wouldn't have any bugs found in their code even at our in-house reviews. A few of us would have 1 or 2 errors found per year in-house. It was extremely odd for IBM reviews to ever find any errors. We used Deming's statistical process control methods to tweak our internal processes as needed to address any errors found. When I left the team, 1 bug was being found every other year in the code. It was generally believed that zero SEV-1 bugs remained in the software. Jim Orr created a report in 2010-ish which showed there were over 100 SEV-1 bugs in the code in 1994 which would be found later in the FSW program.  A SEV-1 bug is loss of crew/vehicle, I believe. &lt;a href=&quot;http://www.slideshare.net/JamesOrr4/space-shuttle-flight-software-pass-loss-of-crew-errors-jk-orr-20150827-52150515&quot; rel=&quot;noreferrer&quot;&gt;http://www.slideshare.net/JamesOrr4/space-shuttle-flight-software-pass-loss-of-crew-errors-jk-orr-20150827-52150515&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We used MVS-based mainframes on the ground to build all the software running TSO/ISPF.  Version control was managed by an IMS database. JCL was used to submit compile, link, testing, printing tasks to the system.  We had a number of different tools to help research the code - basically different menus written in ISPF macro languages or REXX (after I moved on) were used to search the different code bases. Every flight had different code, but most were based off Operational Increments, OIs.  I worked on OI8D-OI24 and a little planning on OI25.  My favorite tool was called HALSTAT. It provided reports about all the variables used in a module (file) throughout the rest of the code. References, assignments and both had different codes. I think the codes were 2, 4, and 6, respectively. I miss HALSTAT in all my coding since then, though c-tags and etags are better than nothing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some systems I worked on in those years were:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Nosewheel steering redesign (JP and JV)&lt;/li&gt;&#xA;&lt;li&gt;2-Engine Out Abort (Don't recall the complete team)&lt;/li&gt;&#xA;&lt;li&gt;3-Engine Out Abort (SM, JP, LP were primary, but EVERYONE helped).&lt;/li&gt;&#xA;&lt;li&gt;Mir Docking (BW and PD) &lt;/li&gt;&#xA;&lt;li&gt;Drag chute Deployment (PD primary)&lt;/li&gt;&#xA;&lt;li&gt;Main Landing Improvements - basically they were blowing out tires, so smoother landings were needed. If you watch landings after 1992, they are all VERY, VERY, smooth thanks to this code.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Just to be clear, I introduced 1 DR in my time there (1 bug that got passed all the internal and IBM reviews) - on the Main Gear Landing Improvements. It was discovered about a year later in level 6/7 testing. I had lots of bugs found in-house by my peers in my first year. After 1 really bad review with about 10 bugs, my boss asked if I was happy with the work. I had read the requirements differently than everyone else based on some indentation. Really it was 1 error, but I was consistent. However, in the code, it was about 10 errors. It is humbling when others catch your mistakes. We were very close members of the same team. Our signatures on review sheets meant something important. We never forgot that lives were at stake and national pride. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were many, many, other projects and fixes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not miss wearing ties. ;)  I do miss the mostly 8-5 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mission control software and payload operations centers around the world are completely different too. Worked on the team that replaced the Apollo era control centers with the Star Trek NG-like systems running mainly DEC Alpha systems with some SunOS, HP-UX, AIX and Irix systems in the back rooms. For reference there were about 600 Alphas and less than 20 other systems.  When we deployed them, they ran OSF/1, a Unix variant, but that name changed to Digital Unix ... this was before Compaq bought DEC.  Visited the MSFC a few years ago and saw their POC for space station support. Couldn't see which OS was running and the tour guide didn't know. The console furniture was all different than what we deployed in the early-mid 1990s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know of any way to run the PASS HAL/S software or use the hardware today (in 2017). The OS isn't interactive. It communicates only with specific hardware which nobody will have or can get at home. Without a DEU and a flight key entry system, control sticks and hundreds of avionics switches/knobs, I don't see how to make this software usable. Someone would need to make an emulation layer for all of that. Suppose it could be done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please recall that all my direct knowledge ended in the mid-1990s. Things were probably updated later in the program which could have changed many things. For example, I think the DEUs were replaced with LCD screens somehow. Any code changes to support that, if they didn't just put in an emulation layer for DEUs is unknown to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back in the 1990s, I did find an IBM 360/370 ASM emulator for Intel CPUs. At the time, it wasn't good enough even to run my beginner-level IBM 360 ASM homework on. It is hard to explain how complex the software development processing was. Everything was cross-compiled and cross-linked from the MVS system for the AP101/s computers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looked through the public NASA code archives today and a search didn't find any &quot;shuttle&quot; related code. Would love to see how GPE (a module name) was changed over the years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Read elsewhere that the early HAL language guide was available, but the HAL/S language is different.  For example, I don't recall seeing any way to &quot;print&quot; anything in HAL/S - doesn't mean that instruction was removed, but there wasn't any device to &quot;print&quot; at. Inputs came from HW devices and were stored into variables. Outputs were written to HW devices. By the time I came along, all the I/O stuff was long written and working, so never needed to learn how it actually happened. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I'm not knowledgeable about any specific missions. My daily work was far removed from current missions. We didn't stop to watch launches/landings during the work day. Our code was generally written a year or two prior to any mission use. I suspect IBM did all the closer per-mission code changes. Around 1995/96, the IBM group was sold to Loral and those two groups were merged into a single team. Some people transferred inside IBM but others loved the work and stayed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A basic HAL language would probably be written with a translation layer in almost any of the popular languages today. Perl, Python, Ruby, Java, C++ could handle it.  At least for simple, 1 module, programs. Perhaps similar to the way C++ began as a C preprocessor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Creating a translation layer that could handle the full multi-priority, interrupt-enabled, shuttle software would be a vastly different, huge, project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sorry for going off topic. Hopefully someone will find this slightly interesting.&lt;/p&gt;&#xA;" OwnerUserId="18834" LastEditorUserId="415" LastEditDate="2017-03-08T18:04:07.380" LastActivityDate="2017-03-08T18:04:07.380" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="20523" PostTypeId="2" ParentId="12006" CreationDate="2017-03-05T16:22:23.113" Score="2" Body="&lt;p&gt;I'd always heard that it was to enable downlink of flight data early in the program when communications weren't as solid.  Antennas don't do well getting through the thicker, bottom, heat shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The antennas on the &quot;top&quot; of the shuttle needed to point down for local ground receivers to get the data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also read that the Soviet Shuttle, Buran, did the same roll because they didn't know why we did it, but they assumed something important required it and didn't want to discover it the hard way.&lt;/p&gt;&#xA;" OwnerUserId="18834" LastActivityDate="2017-03-05T16:22:23.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20524" PostTypeId="2" ParentId="14560" CreationDate="2017-03-05T16:36:24.963" Score="7" Body="&lt;p&gt;The flight computers limited the rates in all directions allowed, however it was possible to disable those limits. This was automatically done for 2 and 3-engine out scenarios.  I suspect the limits were set by K-Load pre-processing. There were not hard limits as numbers typed into the code.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle is a rate control system. Rates of roll, pitch and yaw are requested/introduced, unlike with other aircraft which use deflection-based controls. If you introduce a roll in 1 direction, to stop, you must introduce the exact same force in the opposite direction. The pilots learn to fly the shuttle with this in mind. Even in atmosphere, it is still a rate-control system. They do not pull back on the stick for very long. They pulse the stick to introduce and counteract those rate inputs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: I wrote or reviewed the code that disabled the rate limits on 2/3-EO flight regimes. &lt;/p&gt;&#xA;" OwnerUserId="18834" LastActivityDate="2017-03-05T16:36:24.963" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20527" PostTypeId="1" AcceptedAnswerId="20533" CreationDate="2017-03-05T21:28:04.520" Score="5" ViewCount="6760" Body="&lt;p&gt;Liquid oxygen is cheap to obtain and available, but are there better oxidizers to achieve bigger thrust with JP1?&lt;/p&gt;&#xA;" OwnerUserId="18810" LastEditorUserId="8693" LastEditDate="2017-03-06T06:42:39.957" LastActivityDate="2020-02-09T22:01:57.270" Title="There are better oxidizer than oxygen?" Tags="|propulsion|performance|oxidizer|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20528" PostTypeId="1" CreationDate="2017-03-05T23:50:19.653" Score="5" ViewCount="8596" Body="&lt;p&gt;In the scene where Katherine Johnson/Taraji Henson has to calculate &quot;Go / No-Go,&quot; which I take to mean the &lt;strong&gt;operational go-no-go orbital-insertion criteria.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;She calculates a value for R, using this formula: $R = \dfrac{v^2}{g}\sin (2 \psi)$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking $V = 25731 \ \dfrac{f}{s}$ (note: the digits got transposed in the actual scene -- the value on the board is $25371 \ \dfrac{f}{s}$),  $g = 32.2 \dfrac{f}{s^2},\ \sin (2 \psi) = 0.99848$, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I get the same value that Katherine comes up with: R = 20,530,372 feet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's my question: is this the distance from the launch window to the target in the Bahamas? And why is it important to the orbital insertion?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the movie scene:  &lt;a href=&quot;https://www.youtube.com/watch?v=JAEnv1PvBvw/%22Calculating%20Go%20/%20No-go%20/%20R%22&quot; rel=&quot;noreferrer&quot;&gt;Calculating Go / No-go / R&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18819" LastEditorUserId="25" LastEditDate="2019-05-18T23:41:50.360" LastActivityDate="2019-05-20T14:10:14.710" Title="Formula in &quot;Hidden Figures&quot; Movie" Tags="|orbital-mechanics|launch|trajectory|mathematics|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20531" PostTypeId="2" ParentId="20485" CreationDate="2017-03-06T06:08:33.730" Score="8" Body="&lt;p&gt;In most missions, and especially interplanetary missions, spacecraft position and velocity is determined by calculating the range (i.e. straight-line distance) and range rate between the spacecraft and several ground stations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several techniques for computing the range and range rate. They depend on a number of factors:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Is the satellite responsive? In other words, given a signal, can the spacecraft subsystems interpret this signal and send it back?&lt;/li&gt;&#xA;&lt;li&gt;Will the total time of flight of a signal between the station and the vehicle take so long that the station will no longer be visible by the time that signal reaches it?&lt;/li&gt;&#xA;&lt;li&gt;How precise does the orbit estimation need to be?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If answer 1 is &quot;spacecraft is not responsive&quot; (e.g. you're estimating the position and velocity of spacecraft which is not yours), then you need to rely on pure radar based technologies. Typically, the MIT Lincoln Lab uses that kind of technology to monitor Earth orbit debris.&#xA;If the answer to 2 is &quot;yes&quot;, or if the answer to 3 is &quot;quite precise&quot; you'll want to use multi-way ranging: station 1 will send up a signal (which has encoded time and date information) to the spacecraft whose subsystems will to close to zero processing to that signal before sending it back down to Earth, and another station will pick up that signal, decode it, and check the time of flight and compute the Doppler shift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For interplanetary vehicles, NASA's Deep Space Network (or DSN) is used (at a non-negligible cost). So operators will rent communication time from several stations to a point in space on the DSN. NASA then sends them the range and range rate data. This information is fed into orbital estimation software (such as &lt;a href=&quot;https://github.com/ChristopherRabotin/smd/tree/901e32b8af1bba3c15449fadc38fe7459a409504/examples/statOD/hw3&quot; rel=&quot;nofollow noreferrer&quot;&gt;this extremely basic one I'm developing for a class&lt;/a&gt;), and you can estimate the position and velocity of a spacecraft within a given accuracy. Effectively, you model your spacecraft dynamics (e.g. how the oblateness of the Earth will affect how the spacecraft will move), and your software will spit out an estimate of each of the components of the position and velocity vectors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do note however, that it is getting common for LEO spacecraft to have onboard GPS receivers, but those are only used as an additional measurement points and are not trusted at face value. The spacecraft may use Sun, Earth and (rarely) Moon sensors to have a coarse idea of its position with respect to those bodies, but the accuracy of this method is very bad compared to any ground based measurements. In operations, Sun and Earth sensors are used as backup attitude measurement in order for the spacecraft to switch to &lt;a href=&quot;https://space.stackexchange.com/questions/3633/what-is-safe-mode/3636#3636&quot;&gt;barbecue mode&lt;/a&gt; if ground communication is lost for a much longer delay than planned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following figure shows the difference between the real orbit and estimated orbit (in blue) along with the $3\sigma$ bounds which is the confidence values of the estimator.&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/3633/what-is-safe-mode/3636#3636&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UDMmM.png&quot; alt=&quot;Example of an orbit estimate error&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following two figures show &lt;strong&gt;simulated&lt;/strong&gt; data of range and range rate from three different ground stations (using a two-way ranging, i.e. the same station sends and receives the signal) for a spacecraft which is in a low Earth orbit and a spacecraft which is in an elliptical orbit about the Sun (i.e. hyperbolic orbit about Earth, so an interplanetary spacecraft). Note that the &lt;em&gt;blanks&lt;/em&gt; in the plots represent the time during which the spacecraft is not visible from any ground station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEO range and range rate:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/UDMmM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1zmMa.png&quot; alt=&quot;LEO range and range rate&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interplanetary spacecraft range and range rate:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/1zmMa.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MenNa.png&quot; alt=&quot;Interplanetary spacecraft range and range rate&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-07T04:22:46.313" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="20532" PostTypeId="2" ParentId="12347" CreationDate="2017-03-06T06:22:42.747" Score="4" Body="&lt;p&gt;Using telescopes on Earth, scientists have been confirming for decades that object placed on the moon by Apollo astronauts are still there. (No dogs were harmed in the making of this video.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/VmVxSFnjYCA?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Libration&quot; rel=&quot;noreferrer&quot;&gt;Lunar Libration&lt;/a&gt;&lt;/strong&gt; image from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Lunar_libration_with_phase2.gif&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lLTA5.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lLTA5.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Lunar Laser Ranging&lt;/strong&gt; images &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment&quot; rel=&quot;noreferrer&quot;&gt;from here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j7j38.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j7j38.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/DGsBA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DGsBA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-03-06T06:41:13.767" LastActivityDate="2017-03-06T06:41:13.767" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20533" PostTypeId="2" ParentId="20527" CreationDate="2017-03-06T06:33:22.790" Score="13" Body="&lt;p&gt;This depends on what &quot;better&quot; is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let us talk about fluorine. The main point of using it is that turning your hydrogen into $HF$ gives more energy than getting $H_2O$ for pure hydrogen, that gives a small improvement of $I_{sp}$. However, for fuels containing carbon, like JP1, (or perhaps RP-1 or JP4. JP1 has some unfortunate properties for rocket engine use) the situation is made more complicated by the fact that too much fluorine causes a yield of $CF_4$. That will of course make the performance go down the drain. Also, fluorine is difficult to handle. From &lt;em&gt;&lt;a href=&quot;http://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;noreferrer&quot;&gt;Ignition, an informal history of liquid rocket propellants&lt;/a&gt;, John D. Clark&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Not only did they get a good performance, but there were no ignition&#xA;  problems, liquid fluorine being hypergolic with almost anything that&#xA;  they tried as a fuel. Unfortunately, it was also hypergolic with just&#xA;  about everything else. Fluorine is not only extremely toxic; it is a&#xA;  super-oxidizer, and reacts, under the proper conditions with almost&#xA;  everything but nitrogen, the lighter of the noble gases, and things&#xA;  that have already been fluorinated to the limit. And the reaction is&#xA;  usually violent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(chapter 6, 7, 8, 10 and 12 are highly relevant to this topic)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That oxygen has to be burned into carbon monoxide, $CO$. So a compound like &lt;a href=&quot;https://en.wikipedia.org/wiki/Oxygen_difluoride&quot; rel=&quot;noreferrer&quot;&gt;oxygen difluoride&lt;/a&gt; is a good option. It even has an excellent density. Another approach would be to mix a little liquid fluorine in the LOX. The two compounds can perfectly mixed, in contrast to many other cryogenics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pro: Higher $Isp$ and better density. Con: More expensive, corrosive and toxic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Other compound to check out are $ClF3$, $ClF5$ and $O_2F_2$.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another option would be mixing some $O_3$ into the LOX, again giving slightly more energy and better density. It has the nasty property of sometimes violently decomposing for no apparent reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the other possible definition of &quot;better&quot;, thrust, the solution is generally to use a denser oxidizer and not necessarily one with a good performance. For thrust alone, &lt;a href=&quot;https://en.wikipedia.org/wiki/Dinitrogen_tetroxide&quot; rel=&quot;noreferrer&quot;&gt;nitrogen tetroxide&lt;/a&gt; is an excellent choice.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-03-06T06:33:22.790" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20534" PostTypeId="2" ParentId="20519" CreationDate="2017-03-06T10:03:06.883" Score="10" Body="&lt;p&gt;Falcon first stages also have cold-gas thrusters at the top of the stage; you can see them firing for thrust perpendicular to the stage during some landings. Among other things these thrusters can be fired downwards (parallel to the stage) to provide thrust for &lt;a href=&quot;https://en.wikipedia.org/wiki/Ullage_motor&quot; rel=&quot;noreferrer&quot;&gt;ullage&lt;/a&gt;. A few seconds of thrust will settle propellants at the bottom of the tanks immediately prior to engine ignition (this is probably necessary prior to the boostback burn, for example, as the rocket is clear of the vast majority of the atmosphere by then).&lt;/p&gt;&#xA;" OwnerUserId="11532" LastActivityDate="2017-03-06T10:03:06.883" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20535" PostTypeId="2" ParentId="20473" CreationDate="2017-03-06T13:29:40.560" Score="1" Body="&lt;p&gt;I would argue that Apollo 13 landed farthest off target.  The majority of the crew expected to land on the Moon.  While they did land within site of their objective (&lt;em&gt;you can see the Moon from anyplace on Earth&lt;/em&gt;) they missed their intended destination by a greater distance (238,900 miles) then any other crewed flight to date. &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2017-03-06T13:29:40.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20536" PostTypeId="1" CreationDate="2017-03-06T13:48:45.517" Score="3" ViewCount="355" Body="&lt;p&gt;Suppose a nanosatellite is made with several commercial grade SBCs working together. And we design it so as to give output based on majority voting type of algorithm. I am aware of single event upsets and high energy particles which can destroy circuitry of non radiation hardened devices. How reliable would such a design be compared to a single flight grade board?&lt;/p&gt;&#xA;" OwnerUserId="18825" LastEditorUserId="9060" LastEditDate="2017-03-06T15:45:33.010" LastActivityDate="2017-03-07T07:14:26.230" Title="Commercial grade single board computers (SBCs)" Tags="|artificial-satellite|software|flight-computer|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20537" PostTypeId="2" ParentId="20536" CreationDate="2017-03-06T14:23:05.167" Score="5" Body="&lt;p&gt;For the simple voting algorithm, you need 1 board to generate unverified control (standard), 2 boards to verify control is correct (but offer no backup if it isn't; you can't tell which of the two mismatched votes is correct) and 3 boards to offer the first level of redundancy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 2-board scenario doubles the chance of failure; in the 3-board scenario (any) 2 out of 3 need to fail. So if $p$ is the probability of a single board out of $n$ boards failing, the chance of the system failure will be of order $p^{n-1}$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since other modes of failure - like software errors that repeatably create the same error across all boards, or coronal mass ejections that will fry all the boards, are more probable, going way beyond 3-4 boards doesn't make much sense.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-06T14:23:05.167" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20539" PostTypeId="2" ParentId="10464" CreationDate="2017-03-06T15:24:35.550" Score="0" Body="&lt;p&gt;I first want to comment a bit on the descriptive context given for the question.  (I don’t have enough points to post a comment to the description.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;“Within the context of Aeronautics, …”&lt;/em&gt;  The whole of human aerospace activity (engineering, operations, …) can basically be divided into two domains: aeronautics and astronautics.  The subject matter of the question clearly falls into the latter rather than the former.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I’m going to take the answer by @jah138 as the correct one.  I’ll add a bit of perspective from having taken courses in astrodynamics in the 1970s, followed by some developments after that time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is certainly an overlap between astronomical methods and astronautics when it comes to determining the positions of gravitational bodies in space.  A set of “ephemerides”, i.e., trajectories over time, is kept for such bodies in the solar system.  Astrodynamics encompasses computing trajectories, further refining them based on observational data, and planning/modifying the trajectories of essentially zero-mass (when compared to a planetary body) human-created objects to take advantage of the gravitational field defined by them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some interesting additions have shown up since the 70s. There are now interesting small secondary effects, such as the pressure of particles of solar wind, the continuing photonic pressure from the Sun itself, and heating of the Sun-facing sides of asteroids. From an engineering perspective, a light sail is an attempt to increase the relative impact of the photonic component on the spacecraft, thus adding a non-trivial term in trajectory computation. There is also a proposal to use the mass of a spacecraft to affect the trajectory of an asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In interplanetary space, this leaves the heavily dominating force as gravity from planetary bodies and the Sun.  For satellites in low Earth orbit, particularly, as low as the International Space Station, there is a component of atmospheric drag and the influence of the Moon's gravitational field. Because spacecraft trajectory computation integrates accelerations over long periods of time, the minor components add up, and methods of improving precision become important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have not worked specifically on “space dynamics” problems, but clearly there is a whole area of spacecraft motion within an inertial frame of reference (i.e., a frame experiencing no accelerations), and how the spacecraft reacts in that frame. Within it, you might compute or measure the impact of reaction wheels, articulation of booms, or spinning of a spacecraft on an axis.  All these fall into the kinematics of the spacecraft.  To get to a mid-course correction of a spacecraft, you will need to compute a way to orient the spacecraft to impart a desired delta-V in a certain time window. This is clearly a dynamics problem, but not normally what I include in astrodynamics. Space dynamics seems like a perfectly reasonable descriptive term for this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compared with conventional astrodynamics problems, these feel like short-term effects. But their long-term impact is found in observational data over extended periods of time that go into astrodynamics computation problems.&lt;/p&gt;&#xA;" OwnerUserId="13671" LastActivityDate="2017-03-06T15:24:35.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20540" PostTypeId="1" CreationDate="2017-03-06T16:46:08.293" Score="0" ViewCount="191" Body="&lt;p&gt;So I was watching the Martian and in order to get the rendezvous speed down they let out cabin air to provide a force to slow down. I was wondering if somehow all the air escaped the iss in a retrograde direction, and assume it didn't spin out of control, would the iss dip low enough into the atmosphere to completely deorbit?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2017-03-06T17:34:17.577" Title="If the ISS let out all it's air in retrograde, would the resulting force be enough to deorbit it?" Tags="|iss|" AnswerCount="1" CommentCount="4" ClosedDate="2017-03-06T17:51:18.597" ContentLicense="CC BY-SA 3.0" />
  <row Id="20541" PostTypeId="2" ParentId="20540" CreationDate="2017-03-06T17:34:17.577" Score="10" Body="&lt;p&gt;ISS has approximately 915 cubic meters of pressurized volume.  If we, for the sake of this exercise, ignore the volume of pressurized gases stored in reserve tanks and only focus on what the effects of venting the pressurized volume would be, that gives us a total available propellant mass of roughly 1160 kg of air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To deorbit ISS more or less immediately, its perigee would need to be lowered below 100 km altitude.  At a circular orbit at 400 km, that would require a roughly 90 m/s retrograde change in velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By conservation of momentum, we can explore how fast we much accelerate the ISS atmosphere to effect this change.  The ISS has a mass of approximately 500,000 kg.  Thus, the average velocity of the escaping air must be given by $0 = 1160\,\textrm{kg} \times v_{air}\,\textrm{m/s} + 498840\,\textrm{kg} \times 90\, \textrm{m/s}$, which gives $v_{air} \approx 38700\,\textrm{m/s}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, the escaping air on average would have to self-accelerate to more than three times earth escape velocity.  This is a problem, because the maximum possible exhaust velocity for a supersonic flow nozzle is given by&#xA;$$v_{max} = \sqrt{2\left(\frac{kR}{k-1}\right)T_0}$$&#xA;or&#xA;$$v_{max} = \sqrt{\frac{2}{k-1}}v_{sound}$$&#xA;which for $k=1.4$ and $v_{sound} = 343\,\textrm{m/s}$ gives $v_{max} = 766\,\textrm{m/s}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR:&lt;/strong&gt; No.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-03-06T17:34:17.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20543" PostTypeId="1" CreationDate="2017-03-06T18:37:29.197" Score="7" ViewCount="1654" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Interplanetary_Transport_System&quot; rel=&quot;noreferrer&quot;&gt;SpaceX's Interplanetary Transport System&lt;/a&gt; has a well-defined plan for getting to Mars:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Launch the ITS spaceship into LEO&lt;/li&gt;&#xA;&lt;li&gt;Launch five ITS tankers to fill the spaceship's tanks.&lt;/li&gt;&#xA;&lt;li&gt;Launch the spaceship on a trans-Mars trajectory.&lt;/li&gt;&#xA;&lt;li&gt;Enter Mars's atmosphere, shedding most of the velocity aerodynamically, then landing usings its engines.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;What will the plan for returning from Mars to Earth look like? If the spaceship is fully fueled from ISRU methane/oxygen, will it have the delta-v to get from the Martian surface back to Earth without refueling, or will it require refueling in Mars orbit?&lt;/p&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2017-03-06T21:52:34.880" Title="How will the SpaceX ITS return from Mars?" Tags="|spacex|mars|mission-design|its|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20545" PostTypeId="2" ParentId="20543" CreationDate="2017-03-06T21:07:21.483" Score="5" Body="&lt;p&gt;It takes 3.8 km/s DV to orbit Mars from the surface, 2.5km/s DV to reach an intercept with Earth, and most likely around 1 km/s to vertically land on Earth as proposed by SpaceX. Altogether the ITS ship would need around 7.3 km/s from the Martian surface  to the Earth's surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fully fueled ITS ship weighs 2400 tons, and it lands on Mars weighting around 300 tons. Since the exhaust velocity of a CH4/O2 fueled rocket is around 3538 m/s for the vacuum raptor engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;DV=3538Ln(2400/300)=7.357km/s&#xA;SpaceX plans to do this, but it'll be real tight &#xA;However I didn't take into account gravity losses so my numbers are probably off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon never talked about propellant tankers orbiting mars, and since the ITS ship has enough DV to come all the way back to Earth from the Martian surface, it will most likely be the way they do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX's plan is to generate methane on Mars using the Sabatier reaction using materials on Mars, once it generates the fuel it needs it will take off from Mars and in theory directly return to Earth.&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="18458" LastEditDate="2017-03-06T21:52:34.880" LastActivityDate="2017-03-06T21:52:34.880" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20546" PostTypeId="1" AcceptedAnswerId="23736" CreationDate="2017-03-06T22:26:01.357" Score="20" ViewCount="877" Body="&lt;p&gt;Between power limitations and aperture-limited diffraction I just can't see how it's possible.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="12102" LastEditDate="2021-06-08T17:04:50.387" LastActivityDate="2021-06-08T17:04:50.387" Title="How are the tiny spacecraft proposed by Project Starshot supposed to communicate with Earth?" Tags="|communication|interstellar-travel|breakthrough-starshot|" AnswerCount="4" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="20547" PostTypeId="2" ParentId="20519" CreationDate="2017-03-07T02:57:55.430" Score="-1" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kQwLT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kQwLT.png&quot; alt=&quot;A graphic of a barge landing shows an arc.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing on the drone ship is a ballistic arc with only a reentry burn.&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2017-03-07T02:57:55.430" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20548" PostTypeId="2" ParentId="20536" CreationDate="2017-03-07T07:14:26.230" Score="1" Body="&lt;p&gt;I suspect the answer depends on the mission and how critical the processors or their collective response are to the mission.  In any case, I believe we have no flight data to provide a solid answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have heard of Raspberry Pi being flown or proposed to be flown in space.  I have not heard about them being harnessed as a cluster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A more reliable SBC for flight might be the BeagleBone Black. It also has analog options and real-time support that the RPi apparently does not, and is considerably more rugged. (Then again, the RPi is considered by many to be a better streaming multimedia machine.) However, as noted in other posts, power consumption might be an issue. It is used in rocket and small UAV designs, where the powered duration is relative short.  One can imagine pulling together several BeagleBone machines to improve reliability. But so far, I have not heard of such a specific effort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a project dating back to 2013 to create a &quot;dependable multiprocessor&quot; (DM) which includes several commercial off-the-shelf (COTS) boards.  In this case, the boards are several Gumstix SBCs.  This was a joint project of Honeywell and Morehead State Univ (MSU), in Kentucky. (There seems to be a fair amount of CubeSat activity at Morehead State; if you trace where the originators of the CubeSat concept are, you'll know why.)  The &lt;a href=&quot;http://digitalcommons.usu.edu/smallsat/2013/all2013/10/&quot; rel=&quot;nofollow noreferrer&quot;&gt;DM project&lt;/a&gt; was presented at the SmallSat 2013 Conference. Some prototype hardware exists; I don't know if it has flown yet.&lt;/p&gt;&#xA;" OwnerUserId="13671" LastActivityDate="2017-03-07T07:14:26.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20550" PostTypeId="2" ParentId="20345" CreationDate="2017-03-07T21:52:46.263" Score="13" Body="&lt;p&gt;Both: The capability of the Saturn V set the maximum weight of the Apollo spacecraft, and the development of Apollo forced some changes to the Saturn.&lt;/p&gt;&#xA;&lt;p&gt;Originally, the plan was to develop a large semi-modular family of rockets under the Saturn moniker, at one point ranging from the C-1 for LEO medium-lift (which evolved into the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_I&quot; rel=&quot;noreferrer&quot;&gt;Saturn I&lt;/a&gt;) to the 4800-ton &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_C-8&quot; rel=&quot;noreferrer&quot;&gt;C-8&lt;/a&gt; for lunar &lt;a href=&quot;https://en.wikipedia.org/wiki/Direct_ascent&quot; rel=&quot;noreferrer&quot;&gt;direct ascent&lt;/a&gt;. At the same time, the Apollo program was conceived as a general purpose LEO and lunar spacecraft, with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program#Command.2FService_Module&quot; rel=&quot;noreferrer&quot;&gt;Apollo CSM&lt;/a&gt; being the final lunar-ascent-and-earth-return stage in a direct ascent mission mode.&lt;/p&gt;&#xA;&lt;p&gt;When Kennedy set the end-of-decade goal for lunar landing in his &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Race#Kennedy_directs_the_race_toward_the_Moon&quot; rel=&quot;noreferrer&quot;&gt;May 1961 address&lt;/a&gt; to Congress, waiting for the massive C-8 became unacceptable, so the choice of mission mode came down to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth_orbit_rendezvous&quot; rel=&quot;noreferrer&quot;&gt;Earth orbit rendezvous&lt;/a&gt; (EOR), with two &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_C-4&quot; rel=&quot;noreferrer&quot;&gt;C-4&lt;/a&gt; launches to either assemble or fuel the complete lunar spacecraft, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit_rendezvous&quot; rel=&quot;noreferrer&quot;&gt;lunar orbit rendezvous&lt;/a&gt; (LOR), using a single C-4 launch with a small lunar lander. In the end, LOR was selected as the fastest and cheapest plan. The C-4 design used 4 &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_F-1&quot; rel=&quot;noreferrer&quot;&gt;F-1&lt;/a&gt; engines on the first stage, 4 &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocketdyne_J-2&quot; rel=&quot;noreferrer&quot;&gt;J-2&lt;/a&gt; on the second stage, and the single J-2, S-IVB as the third stage.&lt;/p&gt;&#xA;&lt;p&gt;Before LOR was chosen, the Apollo CSM design had already stabilized to some extent, which is why the engine on the service module is so large: it was intended for lift-off from the moon.&lt;/p&gt;&#xA;&lt;p&gt;By the end of 1961, it appeared the C-4 would not be big enough for the single-launch LOR mission, so the C-5 was developed. This configuration originally had only 4 engines on the first stage, but NASA's Director of Launch Vehicles, Milton Rosen, pushed for the fifth engine. According to &lt;a href=&quot;https://history.nasa.gov/SP-4206/sp4206.htm&quot; rel=&quot;noreferrer&quot;&gt;Stages To Saturn&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Adding the extra power plant really did not call for extensive design changes; this was Rosen's most convincing argument. Marshall [Space Flight Center] engineers had drawn up the first stage to mount the original four engines at the ends of two heavy crossbeams at the base of the rocket. The innate conservatism of the von Braun design team was fortunate here, because the crossbeams were much heavier than required. Their inherent strength meant no real problems in mounting the fifth powerplant at the junction of the crossbeams, and the Saturn thus gained the added thrust to handle the increasingly heavy payloads of the later Apollo missions. &amp;quot;Conservative design,&amp;quot; Rosen declared, &amp;quot;saved Apollo.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;At second glance, MSFC people themselves found no good reason not to add the extra engine, especially with the payload creeping upward all the time. &amp;quot;I had an awfully uneasy feeling, you know,&amp;quot; von Braun remembered; &amp;quot;every time we talked to the Houston people, the damn LEM had gotten heavier again.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;NASA announced plans to build the C-5 at the beginning of 1962, and by early 1963 it was confirmed as the launcher choice for Apollo and renamed &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;noreferrer&quot;&gt;Saturn V&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Late in the project, Grumman had real trouble with weight overruns on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;lunar module&lt;/a&gt;. With the somewhat over-built CSM, the total weight of the spacecraft ran up against the payload limits of the Saturn V, and multiple &amp;quot;campaigns&amp;quot; of weight-stripping engineering effort had to be made to reach the final landing weight goals. As late as Apollo 10, the LM was still a couple hundred pounds overweight, in fact.&lt;/p&gt;&#xA;&lt;p&gt;So early on, the spacecraft drove the launcher payload capability, forcing the 5th engine to be added, and late in the program, the launcher payload limits drove the spacecraft design.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-08T05:40:38.153" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20551" PostTypeId="2" ParentId="19006" CreationDate="2017-03-07T22:19:23.237" Score="7" Body="&lt;p&gt;Reread the question title &quot;&lt;strong&gt;used in the space shuttle&lt;/strong&gt;&quot; again, can add a different answer for the astronaut laptops in the mid-1990s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The astronaut laptops were IBM thinkpads (it was 1996-ish), never knew thinkpad models. They were running some sort of Windows, definitely NOT win95 which was too new for flight use at the time. Could have been Windows for Workgroups 3.12 or NT. Not certain. Nothing really odd about the OS from what I remember.  The laptops had extra batteries attached under the normal laptop.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was part of a team writing a document management system, Hyperman, for all the flight control rooms, and payload operations centers computers as well as shuttle and space station laptops.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was written in C++ for SunOS, Solaris, HP-UX, AIX, Irix, OSF/1 to start, but also ported the code to Windows and MacOS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Borland C++&lt;/strong&gt; was used to compile the code for Windows using the win32s libraries just like any other Windows programmer of the day would for that platform if they wanted 32-bit software.  Don't recall the version used.  So, if you'd like to learn that, just learn Windows programming. &lt;a href=&quot;https://www.theregister.co.uk/2015/09/01/borland_c_now_legacy_as_embarcadero_releases_delphi_and_c_builder_10/&quot; rel=&quot;noreferrer&quot;&gt;Borland's old compiler&lt;/a&gt; is a free download these days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hyperman was built using commercially available cross-platform libraries, &lt;a href=&quot;https://www.faircom.com/products/c-treeace&quot; rel=&quot;noreferrer&quot;&gt;Faircom C-Tree&lt;/a&gt; and &lt;a href=&quot;http://www.ambiencia.com/&quot; rel=&quot;noreferrer&quot;&gt;Visix Galaxy&lt;/a&gt;. We had a requirement for solar-system-wide, royalty free, distribution. ;)  Galaxy was an amazing tool - crazy great, but very expensive. Looks to be about 10x less expensive now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Don't know what happened to Hyperman. Think NASA dropped using it after paper documentation inside the FCRs and POCs was demanded by the flight controllers.&lt;/p&gt;&#xA;" OwnerUserId="18834" LastActivityDate="2017-03-07T22:19:23.237" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20552" PostTypeId="1" CreationDate="2017-03-07T23:12:04.560" Score="4" ViewCount="516" Body="&lt;p&gt;Which launch system, Blue Origin's New Glenn or SpaceX's Falcon 9, has received more government subsidies?  By how much?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a subsidy to be a direct payment, a grant, a procurement where there was only one supplier (a monopoly), or a procurement of an item where the government was the only customer for the product (a monopsony).  Feel free to provide your own, different definition.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2017-03-07T23:52:37.017" LastActivityDate="2017-03-08T00:14:31.413" Title="Which launch system, New Glenn or Falcon 9, has received more government subsidies?" Tags="|spacex|blue-origin|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20553" PostTypeId="2" ParentId="20552" CreationDate="2017-03-08T00:14:31.413" Score="10" Body="&lt;p&gt;I don't care to enter the fray of what is a subsidy, etc. but Parabolic Arc recently published &lt;a href=&quot;http://www.parabolicarc.com/2017/03/07/nasas-commercial-cargo-crew-spending/&quot; rel=&quot;noreferrer&quot;&gt;a nice summary&lt;/a&gt; of what has been spent (or contracted for) by NASA on the commercial contracts to date.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note: this is incomplete, it includes NASA spending on commercial crew and cargo only. DOD contracts with Blue Origin, for example, are not listed.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/y4RSl.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y4RSl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this, SpaceX has been paid (or contracted for) 7.24 &lt;strong&gt;b&lt;/strong&gt;illion dollars, and Blue Origin has been paid (or contracted for) 25.7 &lt;strong&gt;m&lt;/strong&gt;illion dollars.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-03-08T00:14:31.413" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20554" PostTypeId="2" ParentId="18832" CreationDate="2017-03-08T02:43:53.893" Score="3" Body="&lt;p&gt;It appears that Blue Origin's New Glenn 3-stage rocket will be using a variant of the hydrogen-fueled BE-3 for its upper stage engine, which makes a great deal of sense -- enough that we can assume it was the plan from the beginning. Hydrogen is a better choice for upper stage than for boosters, and generally harder to engineer than a methane engine. Having proven out their hydrogen engine in a rocket that, being a suborbital test bed, didn't need maximum performance, they can now put it to better use. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-03-08T02:43:53.893" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20555" PostTypeId="1" CreationDate="2017-03-08T05:21:35.843" Score="0" ViewCount="399" Body="&lt;p&gt;In terms of either thrust or delta v, what is the next big thing?&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-04-06T13:55:26.407" Title="Are there any new breakthrough propulsion systems for nanosatellites?" Tags="|propulsion|cubesat|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20556" PostTypeId="2" ParentId="19936" CreationDate="2017-03-08T08:55:30.087" Score="6" Body="&lt;p&gt;The photos used are not views of the moon as seen from earth, but multiple &lt;a href=&quot;https://astronomy.stackexchange.com/a/19876&quot;&gt;orbital images&lt;/a&gt; that provide a direct overhead view of the entire moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those images are applied as a bitmap texture to a very simple 3d model - a sphere. I'm not sure on the exact shader settings used, but those images look pretty close to the default appearance of a grey sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then all you need is a script that tells you where to put the moon, camera and light source for each frame as per the related question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The smooth cycling comes from careful date selection. Both animations start and end at apogee so that the apparent size is consistent between the first and last frames, and the dates are chosen so that this coincides with a new moon. A dark frame hides any small differences that may exist, and the terminator is not visible, so it doesn't matter if the next orbit would be lit from a slightly different angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The draconic month (time between apogees) is two days shorter than the synodic month (time between new moons), but that shouldn't be enough to cause serious issues - it is the equivalent of three frames that can reasonably be dropped if they occur near new moon when the terminator is not visibly moving much.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastEditorUserId="-1" LastEditDate="2017-04-13T12:59:06.470" LastActivityDate="2017-03-10T02:04:32.530" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20557" PostTypeId="2" ParentId="20555" CreationDate="2017-03-08T09:44:36.140" Score="4" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a bunch of incremental research on adapting different ion thrusters and other pre-existing propulsion systems to cubesats. There are some experimental propulsions (especially if you count braking: deorbiting satellites as a kind of propulsion) - electrodynamic, or utilizing trace atmospheric drag - that are tested with aid of cubesats, although not intended as actual practical propulsion for nanosatellites - as normally propulsion is means to get a satellite into position where it performs its actual tasks - scientific or otherwise - these propulsion systems cost too much and leave too little room for equipment for these tasks. Cubesat becomes a test stand, not the actual target &quot;customer&quot; vessel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one currently being developed, completely novel propulsion system for cubesats, while interesting, is hardly a breakthrough: &lt;a href=&quot;https://space.stackexchange.com/questions/16924/how-does-an-electric-solid-propellant-rocket-actually-work&quot;&gt;Electric Solid Propellant&lt;/a&gt; propulsion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The promise of ESP is bringing fully functional, inexpensive and safe &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_control_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;RCS&lt;/a&gt; capability - and even inexpensive low-performance main propulsion of performance much higher than typical inexpensive alternatives (cold gas, steam), and cost way lower than high-performance alternatives (ion). On top of moderate cost, the solution is really tiny - a single thruster weighing around 6 gram, leaving a plenty of room and mass for the cubesat's primary functions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious disadvantage, making it not-breakthrough, is the poor performance: the single RCS thruster can provide delta-V of order of 10cm/s to your typical 1U cubesat, and all ESP thrusters have a very modest 200-230s of specific impulse. That's better than cold gas or the likes, but worse than about all chemical (solid, mono- and bi-propellant), never mind ion propulsions. Still, the material safety meets the stringent requirements for cubesats, and the device simplicity vastly reduces its dry mass comparing to &quot;competitors&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course RCS is not a highly-sought feature for cubesats, and the puny delta-V is making it hard to to treat as anything but a gimmick.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-03-08T09:44:36.140" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20558" PostTypeId="1" AcceptedAnswerId="20561" CreationDate="2017-03-08T13:40:04.390" Score="11" ViewCount="1995" Body="&lt;p&gt;A plenty has been written and said about reaction wheels on this site, but there's still one thing I'm curious about: what's the &quot;nominal/rest&quot; speed of a reaction wheel? Say, we've just finished the procedure of desaturating the reaction wheel of a satellite, it went exceptionally well, the satellite is currently maintaining its desired orientation very nicely. What is the RPM of the reaction wheel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it at stop / near stop, or does it turn at some nominal rate, say, halfway between stop and saturation speed, or is it in some other state yet?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-09T20:58:34.993" Title="At what RPM does a desaturated reaction wheel run?" Tags="|attitude|reaction-wheel|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20559" PostTypeId="2" ParentId="19936" CreationDate="2017-03-08T14:36:05.513" Score="6" Body="&lt;p&gt;Creating animation like this is fairly simple using rendering tools such as &lt;a href=&quot;http://www.povray.org/&quot; rel=&quot;noreferrer&quot;&gt;POVray&lt;/a&gt;. All you need is a cylindrical projection map of the &lt;a href=&quot;https://www.nhn.ou.edu/~jeffery/astro/moon/map/moon_map_mercator.jpg&quot; rel=&quot;noreferrer&quot;&gt;moon&lt;/a&gt; and appropriate data for the orbit of the moon. The orbital elements available on wikipedia are good enough for most uses. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example here is some of my own work as a still image. Earth from the north pole with all of the known earthquakes mapped, size is intensity, and color is depth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://github.com/blake6489/EarthquakeAnimator&quot; rel=&quot;noreferrer&quot;&gt;Source code&lt;/a&gt; available, though the explanation is nearly non-existent.&#xA;&lt;a href=&quot;https://github.com/blake6489/EarthquakeAnimator&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v13uY.jpg&quot; alt=&quot;Earth from the north pole with all of the known earthquakes mapped&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.f-lohmueller.de/pov_tut/animate/anim112e.htm&quot; rel=&quot;noreferrer&quot;&gt;This&lt;/a&gt; is a reasonable good primer on animations and orbit with POVray.&#xA;The method for putting an &lt;a href=&quot;http://www.povray.org/documentation/view/3.7.0/337/&quot; rel=&quot;noreferrer&quot;&gt;image onto a sphere&lt;/a&gt; is described here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, this site has a &lt;a href=&quot;http://www.astronexus.com/node/93&quot; rel=&quot;noreferrer&quot;&gt;script&lt;/a&gt; to at least produce stills (animations just require the coding from the previously mentioned animations page)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;#include &quot;colors.inc&quot;  &#xA;#include &quot;textures.inc&quot;   &#xA;#include &quot;finish.inc&quot;&#xA;&#xA;//1 POV-Ray unit = 1000 km&#xA;//Earth is approx. 150 e6 km from Sun    &#xA;//Moon is approx. 384 e3 km from Earth&#xA;&#xA;camera {location &amp;lt;-2000,0,149.9E3&amp;gt; &#xA;        look_at &amp;lt;100,0,150E3&amp;gt; angle 1}   &#xA;light_source {&amp;lt;0,0,0&amp;gt; color rgb&amp;lt;1,1,0.9&amp;gt;}  //Sun&#xA;&#xA; #declare Earth = sphere {  &amp;lt;0,0,0&amp;gt;, 6.38&#xA; pigment {image_map {gif &quot;earthmap.gif&quot; map_type 1}} &#xA; finish {ambient 0.01 diffuse 0.85 specular 0 roughness 0.01}  }&#xA;&#xA; #declare Moon = sphere {  &amp;lt;0,0,0&amp;gt;, 1.74  &#xA; pigment {image_map {gif   &quot;moonmap2.gif&quot; map_type 1}}  &#xA; finish {ambient 0 diffuse 0.95 specular 0    roughness 0.5}  }&#xA;&#xA; object {Earth translate &amp;lt;0,0,150E3&amp;gt;} &#xA; object {Moon translate    &amp;lt;384,0,150E3&amp;gt;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2017-03-08T15:16:49.397" LastActivityDate="2017-03-08T15:16:49.397" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20560" PostTypeId="2" ParentId="20558" CreationDate="2017-03-08T15:22:16.563" Score="2" Body="&lt;p&gt;Not quite what you are asking about, but the International Space Station Control Moment Gyros turn at a nominally constant speed of 6600 rpm.  The control function comes about by commanding the inner and outer gimbals to specific locations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7aVIq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7aVIq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100021932.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reference&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-03-08T15:22:16.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20561" PostTypeId="2" ParentId="20558" CreationDate="2017-03-08T15:24:29.107" Score="8" Body="&lt;p&gt;It depends on the system architecture and the needs of the mission. If you're trying to momentum bias the spacecraft then it will be running at a nominal speed. If you really want to conserve energy then you might bring it down all the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're running four reaction wheels arranged tetrahedrally (as is often done to provide failure tolerance) then you can more easily spin reaction wheels all the way down and back up without loss of control authority because you can usually compensate with the other three wheels if going just outside the deadzone would be too much angular momentum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nominal zero solution tends to be used more frequently due to the very precise pointing requirements of many spacecraft, as well as wear issues with stopping and reversing reaction wheels. In this case the nominal zero is put in the middle of the usable angular speed range. This can be difficult for nanosatellites to achieve due to requirements that they be shut down entirely when launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, strictly speaking a reaction wheel that doesn't operate around zero is called a momentum wheel.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-09T20:58:34.993" LastActivityDate="2017-03-09T20:58:34.993" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="20562" PostTypeId="1" AcceptedAnswerId="20565" CreationDate="2017-03-08T16:25:14.440" Score="19" ViewCount="3603" Body="&lt;p&gt;Although the exact trajectory of a rocket is not known before launch, could SpaceX position its landing barge even further down range of the launch pad and eliminate the need for a boostback burn during barge landings. This could allow heavy missions such as Echostar 23 to complete a very low margin barge landing. &lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="51" LastEditDate="2017-03-08T18:28:17.167" LastActivityDate="2017-03-08T19:47:27.387" Title="More efficient SpaceX landings?" Tags="|spacex|falcon-9|vertical-landing|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20563" PostTypeId="2" ParentId="20562" CreationDate="2017-03-08T17:41:02.210" Score="24" Body="&lt;p&gt;The falcon 9 has 3 return modes, depending on the launch profile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first involves barges and has 2 burns. The reentry burn to prevent burn up in the atmosphere and the landing burn, there is no boost back burn as the rocket stage continues on its parabolic trajectory guided by the grid fins.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second involves 3 burns and landing on land. This adds the boost back burn which reverses the direction of travel followed by the reentry burn and the landing burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third also involves 3 burns but lands on a barge. This uses a boost back burn to limit how far down range the rocket travels followed by the reentry burn and the landing burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For heavy launches the first method could be used to allow more fuel to be used in the launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in effect you are exactly right, a barge landing can be more efficient because there may not need to be a boost back burn.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the above does not take into account the points raised by @geoffc, that a rocket at sea is at higher risk for damage after landing than a rocket on land. We can assume Spacex has considered the risks because they continue to use the less fuel efficient boost back to launch zone method when it is possible.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2017-03-08T19:47:27.387" LastActivityDate="2017-03-08T19:47:27.387" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20564" PostTypeId="2" ParentId="19936" CreationDate="2017-03-08T18:25:00.013" Score="24" Body="&lt;p&gt;I made the images on Wikipedia with my own software. The original on Wikipedia is &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Lunar_libration_with_phase_Oct_2007.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The main work needed is getting the proper position of the Moon with its precessing elliptical orbit, while the orientation is based on a constant rotation rate. This book is an older one, but has sufficient terms to predict eclipses.&lt;/p&gt;&#xA;&lt;p&gt;Book: &lt;em&gt;&amp;quot;Astronomical Formulae for Calculators, 4th edition&amp;quot;&lt;/em&gt;, Jean Meeus, 1988, published by &amp;quot;Willmann-Bell, Inc&amp;quot; Chapter 30 &lt;em&gt;&amp;quot;Position of the Moon&amp;quot;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;And yes, just standard diffuse Lambert shading, brightness based on the cosine angle between a surface normal and the Sun's direction. It's actually not an accurate model for the Moon, which is much brighter near full moon. (And the dark side has a constant brightness, while the real Moon's darkside is lit most by earthshine in small phases, and even near new moon, it's actually much, much dimmer.) &lt;a href=&quot;https://en.wikipedia.org/wiki/Lambertian_reflectance&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here is a Wikipedia article about Lambertian reflectance&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="18847" LastEditorUserId="47149" LastEditDate="2022-05-23T00:33:13.200" LastActivityDate="2022-05-23T00:33:13.200" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="20565" PostTypeId="2" ParentId="20562" CreationDate="2017-03-08T18:28:09.860" Score="28" Body="&lt;p&gt;One problem is that towing speed of the ASDS is pretty slow. Around 5-8 mph. This means that to get 500 miles off shore, you need almost 100 hours of towing to get there, and 100 hours to get back. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is a lot of time to pay staff, and ship usage. Also it reduces turnaround time if you need more time to get back and forth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the longer at sea, the more damage the barge takes. On the way back, a short trip with a 140 foot tall structure on top of a bouncing barge is better than a longer trip. The chances of bad seas damaging something or just even big waves causing too much stress on the stage increases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly there is a tradeoff on payload, recovering at all vs expending, and so on. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has been experimenting as discussed with many of these variables and will optimize to what makes sense, if you had all their data.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-03-08T19:11:40.403" LastActivityDate="2017-03-08T19:11:40.403" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20567" PostTypeId="2" ParentId="20485" CreationDate="2017-03-08T19:07:56.097" Score="5" Body="&lt;p&gt;The general term for figuring out the position/velocity of a spacecraft is &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_determination&quot; rel=&quot;noreferrer&quot;&gt;Orbit Determination&lt;/a&gt;.  It's based on &lt;a href=&quot;https://en.wikipedia.org/wiki/Estimation_theory&quot; rel=&quot;noreferrer&quot;&gt;Estimation Theory&lt;/a&gt;, and the most common techniques are &lt;a href=&quot;https://en.wikipedia.org/wiki/Least_squares#Weighted_least_squares&quot; rel=&quot;noreferrer&quot;&gt;Batch Least Squares (BLS)&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Extended_Kalman_filter&quot; rel=&quot;noreferrer&quot;&gt;Extended Kalman Filtering (EKF)&lt;/a&gt;.  A simple description of Orbit Determination is that we predict the motion of the spacecraft as best we can, but then we also take measurements of it's location.  The measurement data is processed through a BLS or EKF to compute a correction that yields our best estimate of the spacecraft's position/velocity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @ChrisR states Range and Range-Rate (Doppler) are two commonly used measurements, but there are many more.  An important one for interplanetary travel is &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-DOR&quot; rel=&quot;noreferrer&quot;&gt;Delta-Differenced-One-Way Range&lt;/a&gt;.  (Abbreviated Delta-DOR, and pronounced Delta-Door)  Delta-DOR is a technique for combining range data from two separate ground antennas at the same time.  It forms a powerful measurement that helps resolve the components of the spacecraft position that are essentially orthogonal to what is measured by Range and Doppler.  As @DavidHammen stated, traditional angular measurements are just not accurate enough over interplanetary distances.  Therefore, the combination of the Range, Doppler, and Delta-DOR measurements are the most common set for interplanetary missions.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;@ChrisR's answer is good for some of the specific details of Orbit Determination, but I wanted to try giving a higher level overview to answer your question.  &lt;/p&gt;&#xA;" OwnerUserId="12483" LastActivityDate="2017-03-08T19:07:56.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20568" PostTypeId="1" CreationDate="2017-03-09T10:49:34.103" Score="2" ViewCount="3820" Body="&lt;p&gt;What is the distance between the Earth's surface and the motorists' satnav satellites?&lt;/p&gt;&#xA;" OwnerUserId="18854" LastEditorUserId="6816" LastEditDate="2017-03-09T11:07:32.640" LastActivityDate="2017-03-09T11:08:17.000" Title="Height of navigation satellites above Earth" Tags="|artificial-satellite|navigation|gps|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20569" PostTypeId="2" ParentId="20568" CreationDate="2017-03-09T11:03:52.873" Score="0" Body="&lt;p&gt;GPS satellites orbit at an altitude of about 12,550 miles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as GPS is controlled by the US Department of Defense, who can (theoretically) turn it off whenever they want there are various other navigation systems:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Glonass (Russian). This is in a highly elliptical orbit, so &quot;altitude&quot; is not constant.&lt;/li&gt;&#xA;&lt;li&gt;Beidou (China). This constellation utilizes satellites at a number of radically different altitudes.&lt;/li&gt;&#xA;&lt;li&gt;Galileo (Europe). These are in a similar, though not identical, orbit to GPS.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some hand-held satnavs can use multiple satellite systems. I have a Garmin cycle computer that can use both GPS and Glonass.&lt;/p&gt;&#xA;" OwnerUserId="18855" LastEditorUserId="6816" LastEditDate="2017-03-09T11:08:17.000" LastActivityDate="2017-03-09T11:08:17.000" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20570" PostTypeId="2" ParentId="20568" CreationDate="2017-03-09T11:05:23.240" Score="12" Body="&lt;p&gt;For satellite navigation, you are most likely using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System&quot; rel=&quot;noreferrer&quot;&gt;Global Positioning System&lt;/a&gt;, which uses six planes of satellites at an altitude of 20,180 km. However, many systems can also use the &lt;a href=&quot;https://en.wikipedia.org/wiki/GLONASS&quot; rel=&quot;noreferrer&quot;&gt;GLONASS&lt;/a&gt; satellite constellation to get more satellites visible, especially near the poles. They orbit in three planes at 19,130 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The less complete systems &lt;a href=&quot;https://en.wikipedia.org/wiki/BeiDou_Navigation_Satellite_System&quot; rel=&quot;noreferrer&quot;&gt;BeiDou&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(satellite_navigation)&quot; rel=&quot;noreferrer&quot;&gt;Galileo&lt;/a&gt; use satellites at 21,150 km and 23,222 km respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can get a real-time view of some of the constellations at &lt;a href=&quot;http://stuffin.space/&quot; rel=&quot;noreferrer&quot;&gt;stuffin.space&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y9fwi.jpg&quot; alt=&quot;Satellite systems&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-03-09T11:05:23.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20571" PostTypeId="2" ParentId="20474" CreationDate="2017-03-09T13:56:07.473" Score="2" Body="&lt;p&gt;Others have mentioned the speed limit of such a system: you can't get above the &lt;a href=&quot;https://en.wikipedia.org/wiki/Light-gas_gun#Design_physics&quot; rel=&quot;nofollow noreferrer&quot;&gt;speed of sound&lt;/a&gt; (around 340 m/s), or 1/20 of orbital speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, you need a really high tower. Even at 500 km altitude, there's still some atmosphere left, which would (slowly) fill your tube if you left the tube open at the top.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 100 km altitude, &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;air pressure&lt;/a&gt; is on the order of 10&lt;sup&gt;-7&lt;/sup&gt; bar, or 100 mg/m&lt;sup&gt;3&lt;/sup&gt;. You should be able to seal the tube with a membrane and have your rocket pierce it when it reaches the end of the tube. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 50 km altitude, &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;air pressure&lt;/a&gt; is on the order of 10&lt;sup&gt;-3&lt;/sup&gt; bar, or 1 kg/m&lt;sup&gt;3&lt;/sup&gt;. Making a cover that will hold against this pressure, yet be easy for the rocket to break through, is going to be difficult. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have no materials strong enough to build a tube this high. The highest building in the world is only 1 km high, and that was easier to build than a structure that has to hold a vacuum. &#xA;Buildings this high are limited by 2 things: the compressive strength of the building materials, and the compressive strength of the ground underneath. Build higher than a few km, and a cylindrical tower collapses under its own weight, so you have to build a pyramid-like structure to spread the load. You end up with a building that's not only 100 km high, but also 100 km wide.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many of the challenges for this scheme are similar to those of a &lt;a href=&quot;https://space.stackexchange.com/questions/2370/what-technological-hurdles-prevent-the-development-of-a-space-gun&quot;&gt;space gun&lt;/a&gt;, with the added problem of an even lower maximum exit speed. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-04-23T14:08:11.427" LastActivityDate="2017-04-23T14:08:11.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20572" PostTypeId="2" ParentId="20474" CreationDate="2017-03-09T15:11:02.357" Score="9" Body="&lt;p&gt;The existing answer all note reasons why it doesn't work, but miss one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The air inside the tube doesn't really behave different than the air outside the tube. Neither is sucked into space. That's because gravity pulls it down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if we had a vacuum tube, and we let air in on the bottom, it would quickly shoot up. And if there's a snug fitting payload, it would go up as well. But as the air molecules below the payload go up, they lose energy to gravity, and near 500 kilometer there's not much energy left.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a practical note, air expanding into vacuum also cools down quickly. This will freeze out the water, so the bottom of your tube will soon be clogged by ice.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2017-03-09T15:11:02.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20574" PostTypeId="1" AcceptedAnswerId="20576" CreationDate="2017-03-09T20:56:29.657" Score="-1" ViewCount="458" Body="&lt;p&gt;I am trying to create an HRE (Hybrid Rocket Engine) and i am trying to find the best oxidizer. Do any of you have experience with using helium?&lt;/p&gt;&#xA;" OwnerUserId="18863" LastEditorUserId="6816" LastEditDate="2017-03-09T21:04:59.067" LastActivityDate="2017-03-09T21:04:59.067" Title="Could you use Helium as an oxidizer?" Tags="|rockets|engines|oxidizer|hybrid-rocket|" AnswerCount="1" CommentCount="4" ClosedDate="2017-03-09T21:11:52.540" ContentLicense="CC BY-SA 3.0" />
  <row Id="20575" PostTypeId="1" AcceptedAnswerId="20578" CreationDate="2017-03-09T20:59:47.193" Score="6" ViewCount="816" Body="&lt;p&gt;After replacing the old refurbished NK33 engines with newer RD-181 engines on Antares, and the successful OA-5 launch with Cygnus to the ISS, why is the next launch of Cygnus occurring on an Atlas V? It seems strange that after investing in new main engines for Antares Orbital just uses an Atlas V instead. Are some Cygnus missions too heavy for Antares?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="18458" LastEditDate="2017-03-09T21:07:25.987" LastActivityDate="2017-03-09T21:18:30.143" Title="Why is OA-7 launching atop an Atlas V?" Tags="|launch|spacecraft|low-earth-orbit|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20576" PostTypeId="2" ParentId="20574" CreationDate="2017-03-09T21:02:25.323" Score="6" Body="&lt;p&gt;Helium is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Noble_gas&quot; rel=&quot;noreferrer&quot;&gt;noble gas&lt;/a&gt;, meaning it does not react with other atoms. In fact, it is the least reactive of them all. No compounds containing helium has ever been found, although some of the heavier noble gasses have formed unstable compounds. As helium can not be part of any molecule, it can not release chemical energy in any way thus failing to be an oxidizer. It is in fact the worst possible oxidizer. Take a look at liquid oxygen or fluorine instead.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-03-09T21:02:25.323" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20577" PostTypeId="2" ParentId="20575" CreationDate="2017-03-09T21:18:22.683" Score="6" Body="&lt;p&gt;Yes, launching on Atlas V allows about 300 kg more payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;https://www.nasaspaceflight.com/2016/11/oa-7-atlas-v-high-praise-antares-rtf/&quot; rel=&quot;noreferrer&quot;&gt;As OA-7 Cygnus moves to Atlas V, high praise noted for Antares RTF&lt;/a&gt; on nasaspaceflight.com&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2017-03-09T21:18:22.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20578" PostTypeId="2" ParentId="20575" CreationDate="2017-03-09T21:18:30.143" Score="11" Body="&lt;p&gt;Cygnus is not as volume limited as Dragon or HTV might be. Rather it is mass limited, for the most part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus an Atlas V can launch a more fully loaded Cygnus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the loss of Dragon CRS-7, Progress MS-04/65P, and the Cygnus Orb3 missions the ISS is running low on supplies. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They had a recent successful HTV mission, and Dragon CRS-10 missions, which took some of the edge off the supply concerns. But they want to rebuild some of the stockpiles they usually maintain. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using an Atlas vs Antares booster, Cygnus can take more mass and has the volume for it, to the station.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-03-09T21:18:30.143" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20579" PostTypeId="1" AcceptedAnswerId="20581" CreationDate="2017-03-10T02:43:00.390" Score="8" ViewCount="1406" Body="&lt;p&gt;On the &lt;a href=&quot;https://en.wikipedia.org/wiki/Voskhod_2&quot; rel=&quot;noreferrer&quot;&gt;Voskhod 2&lt;/a&gt; mission, cosmonaut Alexey Leonov performed a spacewalk which ran into issues when Lenov's spacesuit had some sort of pressure buildup. From the wiki - &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Leonov] was forced to bleed off some of his suit's pressure, in order&#xA;  to be able to bend the joints, eventually going below safety limits.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How did Leonov bleed off the pressure? The way its worded makes me think he punctured his suit like in the final scene of The Martian... but that sounds kinda crazy&lt;/p&gt;&#xA;" OwnerUserId="3141" LastActivityDate="2017-03-10T04:26:18.700" Title="How did Alexey Leonov bleed off the pressure in his space suit?" Tags="|spacesuits|eva|russia|astronauts|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20580" PostTypeId="1" CreationDate="2017-03-10T02:49:48.950" Score="3" ViewCount="179" Body="&lt;p&gt;Every so often the ISS needs to have resupply mission to top off their stockpiles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the full list of items that the ISS regularly resupplies and how much of a stockpile do they try to maintain? IE how much food do they try to keep on the station?&lt;/p&gt;&#xA;" OwnerUserId="3141" LastActivityDate="2017-03-10T12:21:00.623" Title="What is the standard supply target for the ISS?" Tags="|iss|supply-mission|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20581" PostTypeId="2" ParentId="20579" CreationDate="2017-03-10T03:04:46.897" Score="11" Body="&lt;p&gt;According to Astronautix, Leonov's &lt;a href=&quot;http://www.astronautix.com/b/berkut.html&quot; rel=&quot;noreferrer&quot;&gt;Berkut&lt;/a&gt; space suit had an open cycle environmental system with a relief valve, and he simply set the valve to a lower pressure setting:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Oxygen, used for both breathing and cooling, was contained in a metal backpack. A relief valve vented the suit into space, carrying away heat, moisture, exhaled carbon dioxide, and unconsumed oxygen. There were two relief valve pressure settings - 0.27 atmosphere or 0.40 atmosphere. Sufficient oxygen was carried for 45 minutes of depressurized activity. It was only worn on the Voskhod 2 flight, when Leonov had great difficulties in getting back into the airlock due to the suit's stiffness. It was only after switching to the lower relief valve pressure setting that, bathed in sweat, he was able to get the hatch of the inflatable airlock closed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to the Wikipedia article on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sokol_space_suit&quot; rel=&quot;noreferrer&quot;&gt;Sokol pressure suit&lt;/a&gt;  -- the successor to the Berkut, sharing the same open cycle system and same relief valve pressures -- the low pressure setting is intended only for &quot;extreme emergencies&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-03-10T04:26:18.700" LastActivityDate="2017-03-10T04:26:18.700" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20582" PostTypeId="1" CreationDate="2017-03-10T08:45:58.090" Score="12" ViewCount="890" Body="&lt;p&gt;The Phys.org article &lt;a href=&quot;https://phys.org/news/2017-03-nasa-radar-technique-lost-lunar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;New NASA radar technique finds lost lunar spacecraft&lt;/a&gt; describes the use of radar to relocate two spacecraft that were in orbit around the moon but who's orbit had not been actively tracked for a while. (See also the &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?feature=6769&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL version&lt;/a&gt;.)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;We have been able to detect NASA's Lunar Reconnaissance Orbiter [LRO] and the Indian Space Research Organization's Chandrayaan-1 spacecraft in lunar orbit with ground-based radar,&amp;quot; said Marina Brozovic, a radar scientist at JPL and principal investigator for the test project. &amp;quot;Finding LRO was relatively easy, as we were working with the mission's navigators and had precise orbit data where it was located. Finding India's Chandrayaan-1 required a bit more detective work because &lt;strong&gt;the last contact with the spacecraft was in August of 2009&lt;/strong&gt;.&amp;quot; (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The article goes on to mention the use of powerful radar signals broadcast by the Deep Space Network's 70m Goldstone dish and received by the Green Bank 100m dish.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Since the deep space network can perform ranging on spacecraft much farther away (tens of thousands of times farther than the moon) by itself, why was it necessary to use a non-colocated, non-DSN dish to receive signals in this case?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Later in the article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Radar echoes from the spacecraft were obtained seven more times over three months and are in perfect agreement with the new orbital predictions. Some of the follow-up observations were done with the Arecibo Observatory in Puerto Rico, which has the most powerful astronomical radar system on Earth. Arecibo is operated by the National Science Foundation with funding from NASA's Planetary Defense Coordination Office for the radar capability.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;...which suggests to me at least that the Arecibo dish could perform the measurement alone, without the need of a second dish.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; In both cases a pseduo-random coded radio signals are broadcast at the satellite. For spacecraft in deep space it is received, amplified, and simultaneously and coherently rebroadcast back, while for radar ranging the return signal is passively reflected back. Here coherently means that the carrier signal for the transmission is carefully phase-locked with the incoming signal's carrier so that even though it is at a different frequency, the doppler shift can be recovered and analyzed much in the same way as in radar.&lt;/p&gt;&#xA;&lt;p&gt;Due to the &lt;span class=&quot;math-container&quot;&gt;$1/r^4$&lt;/span&gt; loss of signal intensity, &lt;a href=&quot;https://space.stackexchange.com/a/17061/12102&quot;&gt;radar detection of spacecraft can not be used much past a few lunar distances&lt;/a&gt;, so for much longer distances the amplification and coherent rebroadcast is required. From a signal processing point of view, delay and doppler information are recovered by correlating the received signal with the transmitted code. However from an operational point of view there may be substantial differences.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QxaEk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QxaEk.jpg&quot; alt=&quot;&amp;quot;Radar imagery acquired of the Chandrayaan-1 spacecraft as it flew over the moon's south pole on July 3, 2016. The imagery was acquired using NASA's 70-meter (230-foot) antenna at the Goldstone Deep Space Communications Complex in California. This is one of four detections of Chandrayaan-1 from that day.&amp;quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Radar imagery acquired of the Chandrayaan-1 spacecraft as it flew over the moon's south pole on July 3, 2016. The imagery was acquired using NASA's 70-meter (230-foot) antenna at the Goldstone Deep Space Communications Complex in California. This is one of four detections of Chandrayaan-1 from that day.&amp;quot; Credit: NASA/JPL-Caltech.  From &lt;a href=&quot;https://phys.org/news/2017-03-nasa-radar-technique-lost-lunar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cLYeF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cLYeF.jpg&quot; alt=&quot;&amp;quot;This computer-generated image depicts the Chandrayaan-1's location at time it was detected by the Goldstone Solar System radar on July 2, 2016. The 120-mile (200-kilometer) wide purple circle represents the width of the Goldstone radar beam at lunar distance. The white box in the upper-right corner of the animation depicts the strength of echo. Inside the radar beam (purple circle), the echo from the spacecraft alternated between being very strong and very weak, as the radar beam scattered from the flat metal surfaces.&amp;quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;This computer-generated image depicts the Chandrayaan-1's location at time it was detected by the Goldstone Solar System radar on July 2, 2016. The 120-mile (200-kilometer) wide purple circle represents the width of the Goldstone radar beam at lunar distance. The white box in the upper-right corner of the animation depicts the strength of echo. Inside the radar beam (purple circle), the echo from the spacecraft alternated between being very strong and very weak, as the radar beam scattered from the flat metal surfaces.&amp;quot; Credit: NASA/JPL-Caltech.  From &lt;a href=&quot;https://phys.org/news/2017-03-nasa-radar-technique-lost-lunar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LgGK3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LgGK3.png&quot; alt=&quot;Cropped section of the previous figure, with an arrow added to draw attention to &amp;quot;The 120-mile (200-kilometer) wide purple circle represents the width of the Goldstone radar beam at lunar distance.&amp;quot; Credit: NASA/JPL-Caltech.&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped section of the previous figure, with an arrow added to draw attention to &amp;quot;The 120-mile (200-kilometer) wide purple circle represents the width of the Goldstone radar beam at lunar distance.&amp;quot; Credit: NASA/JPL-Caltech.  From &lt;a href=&quot;https://phys.org/news/2017-03-nasa-radar-technique-lost-lunar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nRYUU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nRYUU.png&quot; alt=&quot;Cropped section of the previous figure to draw attention to &amp;quot;The white box in the upper-right corner of the animation depicts the strength of echo.&amp;quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped section of the previous figure to draw attention to &amp;quot;The white box in the upper-right corner of the animation depicts the strength of echo.&amp;quot; Credit: NASA/JPL-Caltech.  From &lt;a href=&quot;https://phys.org/news/2017-03-nasa-radar-technique-lost-lunar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-07-18T01:46:16.230" LastActivityDate="2021-12-20T23:53:29.647" Title="Why was the 100m Green Bank dish needed together with DSN's 70m Goldstone dish to detect Chandrayaan-1 in lunar orbit?" Tags="|radar|chandrayaan-spacecraft|doppler-ranging|lro|green-bank-observatory|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20583" PostTypeId="1" AcceptedAnswerId="24027" CreationDate="2017-03-10T09:07:48.180" Score="3" ViewCount="1148" Body="&lt;p&gt;The Phys.org article &lt;a href=&quot;https://phys.org/news/2017-03-nasa-radar-technique-lost-lunar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;New NASA radar technique finds lost lunar spacecraft&lt;/a&gt; describes the use of radar to relocate two spacecraft that were in orbit around the moon but who's orbit had not been actively tracked for a while. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Finding a derelict spacecraft at lunar distance that has not been tracked for years is tricky because the moon is riddled with mascons (regions with higher-than-average gravitational pull) that can dramatically affect a spacecraft's orbit over time, and even cause it to have crashed into the moon. JPL's orbital calculations indicated that Chandrayaan-1 is still circling some 124 miles (200 kilometers) above the lunar surface, but it was generally considered &quot;lost.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have not found detailed information on Chandrayaan-1's orbit except that it was polar with an altitude of about &lt;a href=&quot;http://www.isro.gov.in/Spacecraft/chandrayaan-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;100 km&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Chandrayaan-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;200 km&lt;/a&gt;. However, the LRO is seems to be in a daring 160 x 20 km orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt; suggests there are &quot;frozen orbits&quot; about the moon with inclinations of  27°, 50°, 76°, and 86°, but the LRO was inserted at about 90°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are low, polar lunar orbits in general relatively stable compared to lower inclination orbits, or is it still necessary to maintain station keeping capability? Do stable polar orbits actively avoid &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)&quot; rel=&quot;nofollow noreferrer&quot;&gt;mascons&lt;/a&gt;, or does the mascon distribution somehow &quot;average out&quot; for polar orbits?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-12-13T23:21:27.933" Title="Are low, polar lunar orbits in general relatively stable?" Tags="|orbital-mechanics|the-moon|lro|mascon|chandrayaan-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20584" PostTypeId="1" AcceptedAnswerId="21545" CreationDate="2017-03-10T09:28:38.493" Score="11" ViewCount="607" Body="&lt;p&gt;I plotted some of the orbital elements of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Reconnaissance Orbiter&lt;/a&gt; from JPL's &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi#top&quot; rel=&quot;nofollow noreferrer&quot;&gt;Horizons&lt;/a&gt; database, and I see that there are constant oscillations. The period of the semi-major axis and eccentricity related parameters appears to be about 27.25 days (see last plot), which matches the orbital period of about 27.32 days. The oscillations in inclination however appear to oscillate with twice that frequency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Why do the LRO's orbital elements appear to constantly oscillate?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, are the sudden episodes of very constant eccentricity &quot;real&quot; or just artifacts of splicing/stitching/pasting different simulations together? Even when the eccentricity appears constant, oscillations are seen in the inclination and to a lesser degree the semi-major axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I've just added the some history information about the various trajectories that have been concatenated. The &quot;period of calm eccentricity&quot; is from &lt;code&gt;2016-Oct-21&lt;/code&gt; to &lt;code&gt;2016-Dec-07&lt;/code&gt; so while it starts in the middle of &lt;code&gt;558day_20160907_01.bsp_V0.2&lt;/code&gt;, it does end on the same infamous date that that segment also ends.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But remember, I'm asking as much about the wiggles themselves as I am their absence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7ITgx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7ITgx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;SPACECRAFT TRAJECTORY: &#xA;  Updated irregularly (on Horizons) or by request. &#xA;  Concatenated historical (reconstructed) trajectories are from PDS. &#xA;&#xA;  Trajectory name                 Start (TDB)         Stop (TDB)&#xA;  ---------------------------  -----------------  -----------------&#xA;  Reconstructed trajectory     2009 Jun 18 22:16  2016 Sep 15 00:01&#xA;  558day_20160907_01.bsp_V0.2  2016 Sep 15 00:01  2016 Dec 07 00:01 predict&#xA;  558day_20161207_01.bsp_V0.2  2016 Dec 07 00:01  2017 Jan 04 00:01 predict&#xA;  558day_20170104_01.bsp_V0.1  2017 Jan 04 00:01  2018 Jul 16 00:01 predict&#xA;  558day_20170216_01.bsp_V0.1  2017 Feb 16 00:01  2018 Aug 28 00:01 predict&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/92uCq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/92uCq.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wb8ZZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wb8ZZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Below:&lt;/strong&gt; Eccentricity plotted versus time (days) for two intervals shifted by 327 days showing a difference of 12 oscillations. The extracted period is 27.25, close to the orbital period of the moon of about 27.32 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/346Mw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/346Mw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-03-20T14:43:56.053" LastActivityDate="2017-05-17T03:59:02.790" Title="Why do the LRO's orbital elements appear to constantly oscillate - except recently?" Tags="|orbital-mechanics|jpl-horizons|lro|" AnswerCount="2" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="20585" PostTypeId="1" AcceptedAnswerId="20586" CreationDate="2017-03-10T10:02:59.913" Score="18" ViewCount="2339" Body="&lt;p&gt;Regarding deep space missions, like the Rosetta mission, how much of the journey to the final destination is pre-programmed vs. recalculated on the fly? For the Rosetta example, was the whole sequence of gravity assists (and burns to align the trajectory in order to do the assists) pre-programmed, or was the trajectory recalculated after every maneuver? Could the whole journey be executed in a &quot;hands off&quot; fashion (not counting the other various stuff that needs to be done during the journey like doing science, telemetry checks etc., just the trajectory planning and maneuver execution)?&lt;/p&gt;&#xA;" OwnerUserId="18868" LastActivityDate="2017-03-10T10:55:38.833" Title="In deep space missions, how much of the journey is pre-programmed and how much is &quot;direct&quot; control?" Tags="|trajectory|deep-space|planning|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20586" PostTypeId="2" ParentId="20585" CreationDate="2017-03-10T10:55:38.833" Score="25" Body="&lt;p&gt;The journey to the destination is about always completely pre-planned. All the gravity assists, close fly-bys, and so on, are planned before launch - and often long before the probe design is completed, as often the requirements of the trajectory influence design considerations: ability to hibernate, delta-V of the engines, tolerance to heat if the probe is to get assists from Venus, equipment to pick science specific from given targets-of-opportunity (asteroid flybys), and all equipment lifetime, as the trajectory means trip time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This doesn't mean the probe can do everything fully autonomously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the mission plan, there are events of &quot;course adjustment&quot;. Each maneuver, like a gravity assist, departure burn, or plane change is performed with a certain error. At these distances, a couple milliseconds of error at departure burn will mean hundreds of kilometers of error at the destination. After a maneuver was performed, the deviation from planned trajectory is determined. Parameters of the corrective burn are established, and the probe performs the maneuver at the pre-planned location and time, but according to parameters that were obtained only after the maneuver that introduced the error.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-10T10:55:38.833" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="20587" PostTypeId="1" AcceptedAnswerId="20627" CreationDate="2017-03-10T11:49:31.873" Score="15" ViewCount="3622" Body="&lt;p&gt;One fundamental problem of spacesuits is stiffnes due to air pressure; at around 0.3atm, with atmosphere of pure oxygen, they grant enough oxygen for breathing, and &lt;em&gt;more than enough&lt;/em&gt; pressure to prevent other problems - blood vessels bursting, tissue bloating, moisture on eyeballs evaporating and so on. Indeed, the Sokol spacesuit &lt;a href=&quot;https://space.stackexchange.com/a/20581/103&quot;&gt;allows to drop the internal pressure&lt;/a&gt; below the 'safe' level in emergency situations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems the primary problems are concentrated in the head area (or areas accessible through head area, like lungs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One idea to overcome the problem is &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_activity_suit&quot; rel=&quot;nofollow noreferrer&quot;&gt;skinsuits&lt;/a&gt; that apply physical pressure instead of atmospheric. And one problem with the skinsuits is that with applying equivalent of 0.3 atmosphere, they are exceptionally difficult to wear, they cause blood circulation problems as stretched areas press too hard, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way around that would be to make them less &quot;tight&quot;. Of course that would require isolating the head area, or the suits will bloat like balloons from the internal pressure. But assuming this is doable (say, hard chest/back plate with internal sealing against the body), how low could we go with the pressure equivalent before it becomes the problem?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-03-12T21:21:58.833" Title="What is the minimum pressure human body can live in, with head protected?" Tags="|spacesuits|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20588" PostTypeId="2" ParentId="20580" CreationDate="2017-03-10T12:21:00.623" Score="3" Body="&lt;p&gt;Source: &lt;a href=&quot;https://www.researchgate.net/publication/267842381_Space_Logistics_Modeling_and_Simulation_Analysis_using_SpaceNet_Four_Application_Cases&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Logistics Modeling and Simulation Analysis using SpaceNet: Four Application Cases&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A mission manifest from September 2010 through December 2015 was created using unofficial launch and&#xA;  mission manifests provided by Orbital, SpaceX, JAXA, and ESA, as well as extrapolating launch rates for Progress&#xA;  and Soyuz as of July 2010&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;[...]&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Figure 4 illustrates the cumulative raw capacity (total up-mass capacity of all vehicles), net capacity (raw&#xA;  capacity less manifested elements, e.g. ELC, AMS, etc.), and estimated demands for the ISS resupply scenario. The&#xA;  total raw capacity to ISS over the simulation is 245 tons, with 225 tons remaining for resources to satisfy demands.&#xA;  The demands over the same time period total 217 tons, of which 80 tons are for crew provision items (COS 2), 77&#xA;  tons are scientific payloads for exploration and research (COS 6), 52 tons are for maintenance and upkeep (COS 4),&#xA;  and 6 tons are for waste and disposal (COS 7). Although not modeled, any pre-positioned resources at ISS would&#xA;  effectively shift the estimated demands curve down by a fixed amount no more than the maximum estimated&#xA;  capacity of 35 tons. Analysis without considering these pre-positioned resources focuses on the steady-state supply&#xA;  and demand. Of note, the demands nearly match the remaining capacity during this time period, indicating the&#xA;  projected resupply operations to the ISS maintain a steady stockpile of resources.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qEgvY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qEgvY.jpg&quot; alt=&quot;fig4&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;Figure 4. ISS resupply cumulative demands and supply capacity. Between September 2010 and December 2015,&#xA;  resource demands nearly match the supply capacity of all resupply vehicles. Note: demands are aggregated for each&#xA;  launch and do not appear perfectly linear.&lt;/em&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Even from a high-level analysis the resupply of the ISS through 2015 warrants significant additional research.&#xA;  As modeled, there is limited supply capacity margin in steady-state, indicating undersupplies of critical resources&#xA;  may be a realistic concern. Steady-state infeasibilities could start to occur with the delay or cancellation of just one&#xA;  of the six resupply spacecraft, however advanced planning given the pre-positioned resources would probably not&#xA;  warrant an emergency. This situation may indeed be realized with the recent (at the time of writing) launch failure of&#xA;  a Progress spacecraft, although immediate focus is on the safety of crew rather than supply of resources for which&#xA;  the existing stockpile is expected to last for many months. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;More detailed analysis should include additional demands for propellant required for orbital re-boost and station&#xA;  keeping and differentiate between pressurized, unpressurized and liquid cargo, including the multiple spacecraft&#xA;  configurations supporting differing capacities of each type.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Unless someone has access to these &lt;em&gt;unofficial launch and mission manifests&lt;/em&gt;, I don't think you're likely to obtain a more accurate data.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-10T12:21:00.623" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20590" PostTypeId="1" AcceptedAnswerId="21952" CreationDate="2017-03-10T17:25:18.713" Score="14" ViewCount="8365" Body="&lt;p&gt;I'll state my mathematical question about the &lt;em&gt;state propagation&lt;/em&gt; and &lt;em&gt;state transition&lt;/em&gt; matrices first, then show you a simple problem for which I would like to use these concepts to generate a densely spaced family of halo orbits.&lt;/p&gt;&#xA;&lt;p&gt;I'll also preface with the statement that I'm looking for an &lt;em&gt;Aha!&lt;/em&gt; type answer. I am not hoping for an explanation as long as this &lt;a href=&quot;https://space.stackexchange.com/a/19390/12102&quot;&gt;excellent, intuitive explanation of quaternions&lt;/a&gt;. I don't need everything worked out, just some explanation how one would go about understanding, getting, and using the State Transition Matrix in this context.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The following is fairly standard, I'm quoting from a paper I happen to have handy at the moment, Juan Senent, Cesar Ocampo, and Antonio Capella; &lt;em&gt;Low-Thrust Variable-Specific-Impulse Transfers and Guidance to Unstable Periodic Orbits.&lt;/em&gt; Journal of Guidance, Control, and Dynamics, &lt;strong&gt;28&lt;/strong&gt; (2) March–April 2005:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For the dynamical system&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{\dot x} = \mathbf{f}(\mathbf{x})$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;evaluated from &lt;span class=&quot;math-container&quot;&gt;$t_0=0$&lt;/span&gt; to some &lt;span class=&quot;math-container&quot;&gt;$t=t_f$&lt;/span&gt;, the final state differential at &lt;span class=&quot;math-container&quot;&gt;$t_f$&lt;/span&gt; is given by&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\text{d} \mathbf{x}_f = \mathbf{\Phi}(t_f, t_0) \delta \mathbf{x}_0 + \mathbf{\dot x}_f \text{d} t_f$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where the &lt;em&gt;state transition matrix&lt;/em&gt; satisfies&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{\dot \Phi} (t,t_0) = \mathbf{F}(\mathbf{x}(t)) \mathbf{\Phi}(t, t_0) $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{\Phi} (t_0, t_0) = \mathbf{I}_{6 \times 6} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;and &lt;span class=&quot;math-container&quot;&gt;$\mathbf{F}$&lt;/span&gt; is the Jacobian of the vector field used as the state propagation matrix,&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{F}(\mathbf{x}(t)) = \frac{\partial\mathbf{f}(\mathbf{x})}{\partial \mathbf{x}}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;I've started with &lt;a href=&quot;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Journals/1984_CM_How.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;the classic paper&lt;/a&gt; written by &lt;a href=&quot;https://engineering.purdue.edu/Engr/People/ptProfile?resource_id=1384&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kathleen Connor Howell&lt;/a&gt; &lt;em&gt;Three-Dimensional, Periodic 'Halo' Orbits&lt;/em&gt; Celestial Mechanics &lt;strong&gt;32&lt;/strong&gt; (1984) 53-71. It describes a technique to find solutions for halo orbits in the the &lt;a href=&quot;https://en.wikipedia.org/wiki/Three-body_problem#Circular_restricted_three-body_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Circular Restricted 3-body Problem&lt;/a&gt; (CR3BP), closely following a technique described by Breakwell, J. V. and Brown, J. V.: 1979, &lt;em&gt;The &amp;quot;Halo&amp;quot; Family of 3-Dimensional Periodic Orbits in the Earth-Moon Restricted 3-Body Problem&lt;/em&gt; Celest. Mech. &lt;strong&gt;20&lt;/strong&gt;, 389.&lt;/p&gt;&#xA;&lt;p&gt;Howell 1984 describes in detail a step-by-step procedure to find members of a family of halo orbits about the Lagrange co-linear libration points which have symmetry about the x-z plane, by taking advantage of the fact that for this group of orbits three of the six components of the state vector should converge to zero at the point where the orbit intersects the plane.&lt;/p&gt;&#xA;&lt;p&gt;The paper tabulates six examples of halo orbits, and with the numbers given there I can integrate the state vectors, verify that the three state vector components &lt;span class=&quot;math-container&quot;&gt;$y, v_x, v_z$&lt;/span&gt; do indeed pass through zero at the midpoint of the orbits, and make a nice plot.&lt;/p&gt;&#xA;&lt;p&gt;What I would like to do is to understand intuitively what is a state propagation vector and a state transition vector, and how to use these to converge faster on a new member of the halo orbit family than if I had just started shooting orbits in a cluster around a starting point and used something simple like steepest descent to find the next orbit with &lt;span class=&quot;math-container&quot;&gt;$y, v_x, v_z$&lt;/span&gt; all equal to zero.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\ddot{x}=x+2\dot{y}-\frac{(1-\mu)(x+\mu)}{r_1^3}-\frac{\mu(x-1+\mu)}{r_2^3}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\ddot{y}=-2\dot{x}+y\left( 1-\frac{1-\mu}{r_1^3} -\frac{\mu}{r_2^3}\right)$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\ddot{z}=-z\left( \frac{1-\mu}{r_1^3} + \frac{\mu}{r_2^3} \right) $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$r_1=\sqrt{(x+\mu)^2 + y^2 + z^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$r_2=\sqrt{(x-1+\mu)^2 + y^2 + z^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;NOTE!&lt;/strong&gt; I believe that &lt;em&gt;the labels&lt;/em&gt; for the positions of L&lt;span class=&quot;math-container&quot;&gt;${}_1$&lt;/span&gt; and  L&lt;span class=&quot;math-container&quot;&gt;${}_2$&lt;/span&gt; in the GIF and script are transposed (incorrect labels/names). I'll update the image soon.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EONpz.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EONpz.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6u0OY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6u0OY.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(X, t):&#xA;    x, y, z, xdot, ydot, zdot = X&#xA;    r1 = np.sqrt((x      + mu)**2 + y**2 + z**2)&#xA;    r2 = np.sqrt((x - 1. + mu)**2 + y**2 + z**2)&#xA;&#xA;    term_1 = x + 2. * ydot&#xA;    term_2 = -(1.-mu) * (x + mu) / r1**3&#xA;    term_3 =     -mu  * (x - 1. + mu) / r2**3&#xA;    xddot  = term_1 + term_2 + term_3&#xA;&#xA;    term_1 = -2. * xdot&#xA;    term_2 = 1. - (1.-mu)/r1**3 - mu/r2**3 &#xA;    yddot  = term_1 + y * term_2&#xA;&#xA;    term_1 = (1. - mu)/r1**3 + mu/r2**3  # should be plus???&#xA;    zddot  = -z * term_1&#xA;&#xA;    return np.array([xdot, ydot, zdot, xddot, yddot, zddot])&#xA;&#xA;&#xA;class Sat(object):&#xA;    def __init__(self, X0, T0, nu12):&#xA;        self.X0 = X0&#xA;        self.pos0 = X0[:3]&#xA;        self.v0   = X0[3:]&#xA;        self.T0 = T0&#xA;        self.nu1, self.nu2 = nu12       &#xA;&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;from mpl_toolkits.mplot3d import Axes3D&#xA;&#xA;# From &amp;quot;Three-Dimensional, Periodic 'Halo' Orbits,&#xA;# Kathleen Connor Howell, Celestial Mechanics 32 (1984) 53-71 &#xA;&#xA;pi, twopi = np.pi, 2*np.pi&#xA;mu = 0.04&#xA;&#xA;# starting points:&#xA;x0     =   [0.723268, 0.729988, 0.753700, 0.777413, 0.801125, 0.817724]&#xA;y0     = 6*[0.0]&#xA;z0     =   [0.040000, 0.215589, 0.267595, 0.284268, 0.299382, 0.313788]&#xA;xdot0  = 6*[0.0]&#xA;ydot0  =   [0.198019, 0.397259, 0.399909, 0.361870, 0.312474, 0.271306]&#xA;zdot0  = 6*[0.0]&#xA;&#xA;# X0s    = np.array(zip(x0, y0, z0, xdot0, ydot0, zdot0)) Nope! &#xA;X0s    = np.array(list(zip(x0, y0, z0, xdot0, ydot0, zdot0)))&#xA;&#xA;Thalf0s = [1.300177, 1.348532, 1.211253, 1.101099, 1.017241, 0.978653]&#xA;T0s     = [2.0*x for x in Thalf0s]&#xA;&#xA;nu1s    = [1181.69,    51.07839,  4.95816,  1.101843,  0.94834,  1.10361]&#xA;nu2s    = [   0.98095, -0.90203, -0.40587, -0.420200, -1.58429, -2.09182]&#xA;nu12s   = zip(nu1s, nu2s)&#xA;&#xA;n_half  = 200&#xA;fractional_times  = np.linspace(0.0, 1.0, 2*n_half+1)&#xA;&#xA;rtol, atol = 1E-12, 1E-12&#xA;&#xA;sats   = []&#xA;for X0, T0, nu12 in zip(X0s, T0s, nu12s):&#xA;    sat = Sat(X0, T0, nu12)&#xA;    sat.n_half  = n_half&#xA;    sat.t = sat.T0 * fractional_times&#xA;    sat.rtol, sat.atol = rtol, atol    &#xA;    sats.append(sat)&#xA;&#xA;for sat in sats:&#xA;    answer, info = ODEint(deriv, sat.X0, sat.t,&#xA;                          rtol=sat.rtol, atol=sat.atol,&#xA;                          full_output = True )&#xA;    sat.answer   = answer&#xA;    sat.mid    = answer[sat.n_half]&#xA;    sat.mid    = answer[sat.n_half]&#xA;    sat.info     = info&#xA;&#xA;if 1 == 1:&#xA;    xL2, xL1 = 0.74091, 1.21643  # lazy!&#xA;    fig = plt.figure(figsize=[10, 8])&#xA;    ax = fig.add_subplot(1, 1, 1, projection='3d')&#xA;&#xA;    for sat in sats:&#xA;        x,  y,  z  = sat.answer.T[:3]&#xA;        ax.plot(x, y, z)&#xA;&#xA;    ax.plot([0.0-mu], [0], [0], 'ob', markersize=20)&#xA;    ax.plot([1.0-mu], [0], [0], 'og', markersize=12)&#xA;    ax.plot([xL2], [0], [0], 'ok', markersize=8)&#xA;    ax.plot([xL1], [0], [0], 'ok', markersize=8)&#xA;&#xA;    ax.set_xlim(0.7, 1.25)&#xA;    ax.set_ylim(-0.225, 0.225)&#xA;    ax.set_zlim(-0.15, 0.40)&#xA;    ax.text(xL1, 0, -0.05, &amp;quot;L1&amp;quot;, fontsize=14, horizontalalignment='center')&#xA;    ax.text(xL2, 0, -0.05, &amp;quot;L2&amp;quot;, fontsize=14, horizontalalignment='center')&#xA;&#xA;    nplot    = 80&#xA;    thetas   = np.linspace(0, twopi, nplot+1)[:-1]&#xA;    azimuths = -90 + 10.0 * np.cos(thetas)&#xA;&#xA;    fnames = []&#xA;    for i, azim in enumerate(azimuths):&#xA;        fname = &amp;quot;haloz_3D_&amp;quot; + str(10000+i)[1:]&#xA;        ax.elev, ax.azim = 0, azim&#xA;        plt.savefig(fname)&#xA;        fnames.append(fname)&#xA;&#xA;    # tight cropping&#xA;    for i in range(len(fnames)):&#xA;        fname_in  = &amp;quot;haloz_3D_&amp;quot; + str(10000+i)[1:]&#xA;        fname_out = &amp;quot;haloz_3D_crop_&amp;quot; + str(10000+i)[1:] + &amp;quot;.png&amp;quot;&#xA;        img = plt.imread(fname_in + &amp;quot;.png&amp;quot;)&#xA;        plt.imsave(fname_out, img[200:-175, 240:-190])&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2022-02-03T00:20:56.433" LastActivityDate="2022-02-03T00:20:56.433" Title="How to best think of the State Transition Matrix, and how to use it to find periodic Halo orbits?" Tags="|orbital-mechanics|mathematics|python|halo-orbit|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="20591" PostTypeId="2" ParentId="12840" CreationDate="2017-03-10T21:29:51.607" Score="2" Body="&lt;p&gt;The original Soyuz was a bit different. This is all true for the Soyuz-TMA, and probably also the Soyuz-T. I don't know anything about the various Soyuz-7Ks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have the sticks switched in your understanding. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the left controller, stick movements produce translation in the same direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the right controller, up and down control pitch, left and right control yaw, and twisting controls roll.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrusters are not throttleable, though vernier control is available by flipping a switch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/press/DA_Schlottke.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://orbit.medphys.ucl.ac.uk/press/DA_Schlottke.pdf&lt;/a&gt;&#xA;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830023190.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830023190.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't yet know whether it accepts multi-directional inputs. I expect not, since it does not use potentiometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no idea what you mean by green and brown plots on the left stick. &lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-13T20:22:44.967" LastActivityDate="2017-03-13T20:22:44.967" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20593" PostTypeId="2" ParentId="20587" CreationDate="2017-03-10T22:27:36.220" Score="-1" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Armstrong_limit&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Armstrong limit&lt;/a&gt; of 0.0618 atm or 6.3 kPa (47 mmHg) is probably a good reference point.&lt;/p&gt;&#xA;" OwnerUserId="17164" LastEditorUserId="17164" LastEditDate="2017-03-10T23:15:00.007" LastActivityDate="2017-03-10T23:15:00.007" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20594" PostTypeId="2" ParentId="20582" CreationDate="2017-03-10T23:18:28.310" Score="4" Body="&lt;p&gt;I know nothing of the activity you're asking about, but I do know something about radar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the radar systems I've worked with used a single antenna to both transmit and receive. The power of the transmitter is very large compared to the return echo that needs to be received, and the very sensitive receiver needs to be disconnected from the antenna when the radar pulse is generated. The receiver is then connected to the antenna until the next pulse. This is performed by a &lt;a href=&quot;https://en.wikipedia.org/wiki/Duplexer&quot; rel=&quot;nofollow noreferrer&quot;&gt;duplexer&lt;/a&gt;, which in many radars consists of a &lt;a href=&quot;http://www.rfcafe.com/references/electrical/NEETS-Modules/NEETS-Module-18-2-21-2-30.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;T/R tube&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Switching from transmit to receive operation takes time. This effects the &lt;a href=&quot;http://electriciantraining.tpub.com/14190/css/Minimum-Range-17.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Minimum range&lt;/a&gt; of the radar. Perhaps this is why a different antenna/receiver was necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, as Phiteros points out in a comment, it could also be the size of the receive antenna was necessary to get the sensitivity needed.&lt;/p&gt;&#xA;" OwnerUserId="12742" LastActivityDate="2017-03-10T23:18:28.310" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20596" PostTypeId="1" AcceptedAnswerId="21533" CreationDate="2017-03-11T02:57:30.097" Score="9" ViewCount="1722" Body="&lt;p&gt;The Phys.org article &lt;a href=&quot;https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA proposes a magnetic shield to protect Mars' atmosphere&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;...Dr. Jim Green – the Director of NASA's Planetary Science Division – and a panel of researchers presented an ambitious idea. In essence, they suggested that by positioning a magnetic dipole shield at the Mars L1 Lagrange Point, an artificial magnetosphere could be formed that would encompass the entire planet, thus shielding it from solar wind and radiation.&lt;/p&gt;&#xA;&lt;p&gt;Naturally, Green and his colleagues acknowledged that the idea might sounds a bit &amp;quot;fanciful&amp;quot;. However, they were quick to emphasize how new research into miniature magnetospheres (for the sake of protecting crews and spacecraft) supports this concept:&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U7HNR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U7HNR.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;The proposed method for creating an artificial magnetic dipole at Mars’ L1 Lagrange Point.&amp;quot; Credit: NASA/J.Green From &lt;a href=&quot;https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;This new research is coming about due to the application of full plasma physics codes and laboratory experiments. In the future it is quite possible that an inflatable structure(s) can generate a magnetic dipole field at a level of perhaps 1 or 2 Tesla (or 10,000 to 20,000 Gauss) as an active shield against the solar wind.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;In addition, the positioning of this magnetic shield would ensure that the two regions where most of Mars' atmosphere is lost would be shielded. In the course of the presentation, Green and the panel indicated that these the major escape channels are located, &amp;quot;over the northern polar cap involving higher energy ionospheric material, and 2) in the equatorial zone involving a seasonal low energy component with as much as 0.1 kg/s escape of oxygen ions.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;While using an artificial magnetosphere to preserve the atmosphere of a terraformed Mars &lt;a href=&quot;https://space.stackexchange.com/a/13842/12102&quot;&gt;may not be completely necessary&lt;/a&gt;, I think the intellectual exercise described in the article still has substantial merit, both for protecting smaller volumes (e.g. spacecraft, space stations, etc.) and perhaps even the Earth should the Sun ever find itself in a bad mood.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is there anything I could read further about NASA's (or anyone else's) interest or research into artificial mini-magnetosphere's for protection of crewed spacecraft or space stations against radiation?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Another side benefit of the exercise is the insight gained in magnetohydrodynamics itself:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To test this idea, the research team – which included scientists from Ames Research Center, the Goddard Space Flight Center, the University of Colorado, Princeton University, and the Rutherford Appleton Laboratory – conducted a series of simulations using their proposed artificial magnetosphere. These were run at the Coordinated Community Modeling Center (CCMC), which specializes in space weather research, to see what the net effect would be.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-16T10:42:37.470" Title="Is NASA doing research on &quot;mini-magnetospheres&quot; to protect crew from radiation in space?" Tags="|crewed-spaceflight|radiation|shielding|magnetosphere|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="20598" PostTypeId="1" CreationDate="2017-03-11T15:00:42.860" Score="8" ViewCount="975" Body="&lt;p&gt;I can't find 1 image, but I've heard there are a lot of geostationary satellites. Hovering in one specific spot in the sky, rotating with the Earth's spin and tilt, rotating with Earth's orbit around the Sun, and all that spinning going around galactic center. Pretty amazing stuff, I would just like to see one amateur or professional telescopic image of 1(one) of these GEOSTATIONARY SATELLITES.&lt;/p&gt;&#xA;" OwnerUserId="14288" LastActivityDate="2017-03-11T15:04:58.123" Title="Has any ground-based telescope taken a picture of a geostationary broadcast satellite?" Tags="|telescope|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20599" PostTypeId="2" ParentId="20598" CreationDate="2017-03-11T15:04:58.123" Score="13" Body="&lt;p&gt;&quot;Resolved images of a geostationary satellite were obtained on October 30,&#xA;2009, with the adaptive optics on the largest telescope on the planet, the 10 m&#xA;Keck-II on the 14000 foot summit of Mauna Kea.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here ya go:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SgPUC.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SgPUC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwjoztu63M7SAhVLxYMKHco0BhUQFggkMAA&amp;amp;url=http%3A%2F%2Fwww.dtic.mil%2Fget-tr-doc%2Fpdf%3FAD%3Dada531722&amp;amp;usg=AFQjCNGnQeO1NRLBGW2RjPGjSK0IjxbFzA&amp;amp;sig2=eBA624hEDzgTglj9EwWLwQ&quot; rel=&quot;noreferrer&quot;&gt;Reference&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-03-11T15:04:58.123" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20600" PostTypeId="1" CreationDate="2017-03-11T16:40:43.033" Score="3" ViewCount="5376" Body="&lt;p&gt;I am working on a college project which is based upon a experimental sounding rocket and it would be really helpful if the community could refer me good softwares to simulate trajectories of a rocket.  &lt;/p&gt;&#xA;" OwnerUserId="18876" LastActivityDate="2023-07-19T16:00:21.200" Title="What are some good softwares to simulate rocket trajectories ?" Tags="|rockets|software|sounding-rocket|trajectory|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20601" PostTypeId="1" CreationDate="2017-03-11T17:01:24.810" Score="11" ViewCount="1170" Body="&lt;p&gt;While a booster on the shuttle had a bit more than twice the thrust of the shuttles engines at take-off, I'd venture a guess that it produced on the order of a thousand time more visible light. In fact, this may be an example of both the brightest and dimmest large rocket engine exhausts for heavy, sea-level launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any tables, or approximate data for the relative visible-light brightness of major rocket engines? I'm not looking for IR or UV or anything of particular military or defense interest, just the ordinary commercial and civilian rockets that people can watch launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What originally got me thinking about this is that &lt;a href=&quot;https://space.stackexchange.com/q/20505/12102&quot;&gt;this question&lt;/a&gt; was marked as a duplicate of &lt;a href=&quot;https://space.stackexchange.com/q/6555/12102&quot;&gt;this question&lt;/a&gt; within 2 hours, apparently with the idea that any old rocket launch can be seen for about the same distance, perhaps about 100 km away. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was originally wondering if there is really a &quot;standard rocket brightness&quot; or if some rockets used today are still far brighter or dimmer than others, but as @RussellBorogove points out in comments a large number of launches include solid fuel boosters which tend to be very bright.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'd like to ask this question about brightness on the basis of individual propellant combinations, not total launch brightness.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that for a large engine at sea level, the brightnesses rank like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SRB &gt; RP-1/LOX &gt; UDMH/N2O4 &gt; LCH4/LOX &gt; LH2/LOX&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I wrong?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Space Shuttle Discovery from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:STS120LaunchHiRes-edit1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. &lt;em&gt;Scroll aaaaaaaall the way down&lt;/em&gt; to get a better estimate of the total brightness. Note that the hydrogen/oxygen flames are so dim that you can still see right through the flames.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Q28ty.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Q28ty.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="23891" LastEditDate="2019-06-29T21:10:39.240" LastActivityDate="2019-06-29T21:10:39.240" Title="How do rocket propellant combinations rank in terms of &quot;brightness&quot;?" Tags="|rockets|fuel|engines|visibility-of-spacecraft|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="20602" PostTypeId="2" ParentId="20587" CreationDate="2017-03-11T18:36:20.187" Score="2" Body="&lt;p&gt;Here's my uneducated opinion. All above is just hypothesis without actual sufficient medical knowledge or links. Hopefully you won't kick me too much for that)))&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The very least you need to have proper helmet, adequate air pressure and oxygen level in the lungs and protection from barotrauma of lungs and gastrointestinal tract (I don't think it's good to continuously pump air from mouth to anus constantly risking rupture if you stop this constant flatulence). So let's assume that you have proper spacesuit for head and torso but not for limbs. I suppose this will lead to the following issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, human body is capable of intaking extra liquid - the blood vessels and cappilars can be stretched and the liquid can fill the cells and intercellular space. I think the extra pressure on head and torso will be squeezing them out of blood and other liquid. On the one hand you will be depriving your vital organs of the blood. On the other hand your limbs will swell up, cappilars rupture producing internal hemorrhaging. Probably the blood will even leak through skin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then as was noted in other answer above Armstrong limit the skin moisture will boil up. This is bad because this will dry out the exposed skin and upper layers killing the cells so at the very least you'll lose your limbs. However combined with the previous point I suspect that even worse will happen - the liquids from below would be pumped to the skin and you'll dry yourself through your unpressurized limbs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most likely the death will take some time but I suspect that even short exposure will already lead to dangerous consequences.&lt;/p&gt;&#xA;" OwnerUserId="18138" LastActivityDate="2017-03-11T18:36:20.187" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20603" PostTypeId="1" AcceptedAnswerId="24491" CreationDate="2017-03-12T02:08:32.700" Score="2" ViewCount="323" Body="&lt;p&gt;Can static cling occurs in space? Can a static charge be amplified laterally to move objects in space similar to Star Trek's tractor beam? Can a static charge could be carried by a sustained particle beam such as a laser or proton beam?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Possible uses are docking, collision avoidance, recovery and gravity via static cling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related: &lt;a href=&quot;https://spectrum.ieee.org/tech-talk/aerospace/space-flight/electrostatic-glider-can-maneuver-around-asteroids-without-expending-fuel&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://spectrum.ieee.org/tech-talk/aerospace/space-flight/electrostatic-glider-can-maneuver-around-asteroids-without-expending-fuel&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MCM1a.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MCM1a.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2018-01-22T17:29:17.273" LastActivityDate="2024-03-30T20:54:21.583" Title="How much can static cling be used to move things in space? Tractor Beam" Tags="|collision-avoidance|flight-control|beamed-propulsion|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20604" PostTypeId="2" ParentId="20603" CreationDate="2017-03-12T02:11:07.777" Score="6" Body="&lt;p&gt;No. Charging of objects in low earth orbit is already a major design issue but it does nothing to prevent collisions. The relative velocities are too high for achievable charges to matter.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-12T02:11:07.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20605" PostTypeId="1" AcceptedAnswerId="20606" CreationDate="2017-03-12T06:22:13.527" Score="13" ViewCount="1711" Body="&lt;p&gt;For example, the Space Shuttle &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Endeavour#Flights&quot; rel=&quot;noreferrer&quot;&gt;Endeavour&lt;/a&gt; did &lt;a href=&quot;https://i.stack.imgur.com/iTF1T.jpg&quot; rel=&quot;noreferrer&quot;&gt;dock with both MIR&lt;/a&gt; (bringing apple seeds) and the ISS. Did all of the shuttles dock with &lt;em&gt;more than one&lt;/em&gt; space station and/or crewed spacecraft?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iTF1T.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iTF1T.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Space Shuttle Endeavour docked with MIR, from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:N6p-024-low.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Cosmic Apple Tree, Dedication Date: October 9, 2009 from &lt;a href=&quot;http://heroicrelics.org/ussrc/cosmic-apple-tree/dsc71267.jpg.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This apple tree was grown from apple seeds provided by the Alabama Aerospace Teachers' Association in conjunction with the Cosmic Apple Seed Project. The seeds were flown aboard Space Shuttle Endeavour (OV-105) by Alabama Astronaut Joe F. Edwards. Launched on January 22, 1998, the seeds traveled over 3.6 million miles in 8 days, 19 hours, 48 minutes, docked with the Mir Space Station and traveled over 18,000 miles per hour. The seeds returned to the Cape on January 31, 1998. Mrs. Diane Watson's 3rd Grade Class at Webb Elementary School, in Webb, Alabama planted the seed during Spring of 1998.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://heroicrelics.org/ussrc/cosmic-apple-tree/dsc71267.jpg.html&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YYfEw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Sts-89-patch.png&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the STS-89 crew insignia, the link between the United States and Russia is symbolically represented by the Space Shuttle Endeavour and Russia's Mir Space Station orbiting above the Bering Strait between Siberia and Alaska. The success of the joint United States-Russian missions is depicted by the Space Shuttle and Mir colored by the rising sun in the background. A shadowed representation of the International Space Station (ISS) rising with the sun represents the future program for which the Shuttle-Mir missions are prototypes. [...]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/n5pid.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/n5pid.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="7062" LastEditDate="2017-03-12T07:34:47.893" LastActivityDate="2020-06-18T20:31:19.993" Title="Did all of the Space Shuttles dock with multiple space stations or crewed spacecraft?" Tags="|iss|space-shuttle|docking|mir|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20606" PostTypeId="2" ParentId="20605" CreationDate="2017-03-12T06:32:02.997" Score="17" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Columbia#Flights&quot; rel=&quot;nofollow noreferrer&quot;&gt;Columbia&lt;/a&gt; did not.&lt;/p&gt;&#xA;&lt;p&gt;Despite being in service during the Shuttle-Mir and International Space Station programs, Columbia did not fly any missions that visited a space station. The other three active orbiters at the time had visited both Mir and the ISS at least once. Columbia was not well-suited for high-inclination missions; it was the heaviest of the orbiters, being the first of the space-rated orbiters produced. It did lose some weight with late modifications, and was scheduled to go to ISS, but was lost before that happened.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Discovery#Flights&quot; rel=&quot;nofollow noreferrer&quot;&gt;Discovery&lt;/a&gt; visited both; it was the first shuttle to visit Mir (but did not dock on that occasion); it docked with Mir on a later flight, and visited ISS 13 times.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Endeavour#Flights&quot; rel=&quot;nofollow noreferrer&quot;&gt;Endeavour&lt;/a&gt; docked with Mir once and ISS 12 times.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Atlantis#Flights_listing&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atlantis&lt;/a&gt; docked with Mir seven times and ISS 12 times.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Challenger&quot; rel=&quot;nofollow noreferrer&quot;&gt;Challenger&lt;/a&gt;, of course, was lost before the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle%E2%80%93Mir_Program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle-Mir program&lt;/a&gt; began.&lt;/p&gt;&#xA;&lt;p&gt;As for other dockings with manned craft, the space shuttle program ended before China's first station went up, and the US and China haven't had that level of cooperation in their space program in any case; likewise, the USSR wouldn't have been likely to invite the shuttle to visit Salyut 7, which was last crewed in 1986.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2020-06-18T20:31:19.993" LastActivityDate="2020-06-18T20:31:19.993" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="20607" PostTypeId="1" CreationDate="2017-03-12T06:48:32.023" Score="9" ViewCount="1852" Body="&lt;p&gt;When a satellite is in geostationary orbit, it is supposed to stay above one spot of the earth and rotate around the earth at the same rate the earth spins. But how exact does that equality need to be? Let's say I pointed a laser such that it hit a geostationary satellite exactly (perhaps a GPS satellite). How long would it take for the satellite to shift out of position such that the laser would no longer hit the satellite? &lt;/p&gt;&#xA;" OwnerUserId="3141" LastActivityDate="2019-01-10T06:03:07.137" Title="How stationary is geostationary?" Tags="|orbital-mechanics|artificial-satellite|orbit|geostationary|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20608" PostTypeId="1" CreationDate="2017-03-12T07:00:40.117" Score="5" ViewCount="250" Body="&lt;p&gt;I am trying to simulate lunar descent and landing as a part of my undergraduate project. For time being, I have included gravity turn guidance scheme and used these equations &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rYR9P.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rYR9P.png&quot; alt=&quot;EOM: Gravity turn&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, I can solve ODEs for flight path angle, velocity and altitude, integrate it with acceleration profile given by the guidance law and get corresponding profiles of altitude, velocity, and flight path angle for the landing sequence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, the program calculates circular orbit velocity at a given input altitude and de-orbits impulsively. It then initiates thrust command and lands unconstrained, controlling only, how and whether or not I land (altitude, velocity, fuel, flight path angle).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the most efficient way to include propagation of elements (Cartesian, Spherical, Keplerian or any one of the others) to determine landing site latitude and longitude in this program? I basically need to visualize how EDL is happening with a celestial spherical body in context.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Background:&#xA; My project is on the investigation of autonomous GNC in EDL (currently for airless body or the Moon), much like (ALHAT, Lander Vision System and other contemporary names given for AGNC with HDA). So, we've used University of Dundee's &lt;a href=&quot;https://www.star-dundee.com/products/pangu-planet-and-asteroid-natural-scene-generation-utility&quot; rel=&quot;noreferrer&quot;&gt;PANGU&lt;/a&gt; for surface and feature generation, DEM synthesis and utilizing slope/altitude maps fo Hazard detection part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the guidance part, I intend to integrate three distinct acceleration command profiles for braking (linear acceleration), approach (quadratic acceleration) and vertical descent as described in &lt;a href=&quot;https://www.google.co.in/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjwr6mIrdDSAhUCTI8KHaftCQwQFggZMAA&amp;amp;url=http%3A%2F%2Fcse.usu.edu%2Fforaffiliates%2F2007%2Fposters_07%2FTravis%2520Moesser%2520poster.pdf&amp;amp;usg=AFQjCNGWykVZNfN14mXI855iv8wZQfZARA&amp;amp;sig2=4M_lAs9HaA-jtxPnZTdUTA&amp;amp;bvm=bv.149397726,d.c2I&quot; rel=&quot;noreferrer&quot;&gt;this work&lt;/a&gt;, taken from this &lt;a href=&quot;https://arc.aiaa.org/doi/abs/10.2514/6.2005-6287&quot; rel=&quot;noreferrer&quot;&gt;reference&lt;/a&gt; by Ronald Sostaric. It seems far-fetched at this point when I'm struggling with above mentioned problem, but I'm trying.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although I'll ask the questions here along the way as and when problems pop up, I would want SE experts to give their opinion or suggestion on anything of  their interest. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the major problems with project is, that review panel is relatively unfamiliar with space technology and exploration. So, visualization is important for expressing the work done. Irrespective of whether panel is able to appreciate, I want to honestly work on the investigation and pull off a relevant study and learning in the remainder of next 3 months as I have done for past two. So, any and every view and response on any part is appreciated.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2017-03-12T07:00:40.117" Title="Lunar Descent and Landing Simulation" Tags="|landing|lunar-landing|guidance|entry-descent-landing|autonomy|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20609" PostTypeId="1" AcceptedAnswerId="20613" CreationDate="2017-03-12T07:12:22.113" Score="33" ViewCount="8298" Body="&lt;p&gt;This is an image of a Saturn on the launch pad on July 20th 1973&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p2FMi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p2FMi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears to be resting on an elevated platform that raises it significantly off the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why was this done? It seems like a lot of effort and risk to raise the rocket so much, and the benefits seem minimal.&lt;/p&gt;&#xA;" OwnerUserId="3141" LastEditorUserId="-1" LastEditDate="2017-03-13T22:04:59.650" LastActivityDate="2017-03-15T15:52:46.593" Title="Why was this Saturn rocket elevated for launch?" Tags="|launch|rockets|crewed-spaceflight|saturn-v|skylab|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20610" PostTypeId="2" ParentId="20607" CreationDate="2017-03-12T07:45:59.853" Score="13" Body="&lt;p&gt;Generally, the GEO satellites are to keep their desired position above ground within +/- 0.05 deg (both N and E) which translates into a 70 km 2D projection corridor at GEO altitude. &#xA;The laser pointer case, is dependent on the beam width. A laser pointer of the kind you get in stores has beamwidth of 1-3 milliradian, i.e. about 0.05 to 0.17 deg. This translates roughly into a circle of diameter 35- 105 km at GEO altitude. The satellite should stay within it for a long time. Calculations can be updated for other cases, like the one for LLCD in &lt;a href=&quot;https://space.stackexchange.com/a/2006/5975&quot;&gt;this answer&lt;/a&gt; which shows 3.5 urad beamwidth for the corresponding laser.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lets look at how it does drift from nominal though. Looking broadly at the perturbation scenario the term geostationary orbit as derived from Kepler's 2BP and its relation with the number 42164.2 km is not as precise in real-life. The real orbit radius is not equal to that number generally. &#xA;The satellite drifts with time, owing to several sources of perturbation including equatorial bulge, SRP, third-body effects and Earth's precession and nutation. Most are long term variation while some short term variation do exist (considerable longitude drift can be seen within 6 months, semi-major axis might drift by tens of kilometers in a matter of few days. &#xA;Here are some trends to give a perception of drifting of a satellite in GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yXeJ1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yXeJ1.png&quot; alt=&quot;SMA vs T&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Graph 1&lt;/em&gt; :&#xA;Shows the drift of semi-major axis for a satellite placed at nominal R of 42164.2 km propagated for about 6 months shows an increase of about 21kms. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P2CDv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P2CDv.png&quot; alt=&quot;Long vs T&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Graph 2&lt;/em&gt;&#xA;:Due to asymmetry of Earth (More elliptical at equator) Longitude drift occurs. Longitude can greatly vary with time.The graph shows how over a propagation period of 160 days, the satellite at a nominal longitude of 125 deg can drift eastward till 105 deg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L8W89.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L8W89.png&quot; alt=&quot;Long Drift Rate vs T&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Graph 3&lt;/em&gt;&#xA;:The change of longitude-drift can be observed with longitude drift-time trend which is essentially linear. The slope depends upon nominal longitude of the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v494T.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v494T.png&quot; alt=&quot;Long vs Long Drift Rate&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Graph 4&lt;/em&gt;&#xA;:The longitudinal drift rate for a nominal longitude at a point, in turn, is given by this graph and varies in form of a parabola.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the asymmetry there are four equillibrium points- two stable (at 75.3°E and 104.7°W) and two unstable (at 165.3°E and 14.7°W) equilibrium points. Any geostationary object placed between the equilibrium points would (without any action) be slowly accelerated towards the stable equilibrium position, causing a periodic longitude variation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inclination changes due to 'wobbling' motion of the Earth with 0.85deg/yr rate between +/-15 deg maximum with a period of  26.6 years.&#xA;Also, eccentricity, argument of perigee and RAAN varies considerably. &#xA;This complex variation can better be perceived by drawing relative motion in cartesian coordinates of the satellite in orbit as well as the observer on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;-&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;[Book] Li, Hengnian. Geostationary satellites collocation. New York: Springer, 2014.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Geostationary Orbit- Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="12102" LastEditDate="2019-01-10T06:03:07.137" LastActivityDate="2019-01-10T06:03:07.137" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20611" PostTypeId="2" ParentId="20609" CreationDate="2017-03-12T08:07:38.530" Score="15" Body="&lt;p&gt;A little digging into KSC Launch Complex 39's Wikipedia page and I found this.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A total of thirteen Saturn Vs were launched for Apollo, and the unmanned launch of the Skylab space station in 1973. The mobile launchers were then modified for the shorter Saturn IB rockets, by adding a &quot;milk-stool&quot; extension platform to the launch pedestal, so that the S-IVB upper stage and Apollo spacecraft swing arms would reach. These were used for three manned Skylab flights and the Apollo-Soyuz Test Project, since the Saturn IB pads 34 and 37 at Cape Canaveral AFB had been decommissioned.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While the unmanned launch of Skylab used a Saturn V, the launch vehicle for crewed missions was the smaller Saturn IB.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/mzNNR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mzNNR.jpg&quot; alt=&quot;Skylab 1&amp;amp;2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;This deliberate double exposure illustrates the comparative sizes and configurations of the Skylab 1 and 2 Space Vehicles at Launch Complex 39 at the Kennedy Space Center, as they sat on the launch pads in&#xA;1973. The double exposure creates an illusion that the rockets are side by side, although actually, they are 1 1/2 miles apart.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/skylab/SkylabImagesCol_archive_1.html&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This photo of Skylab-1 and Skylab-2 vehicles clarifies that the extension platform were in fact used for crewed mission&#xA;So, the picture is from manned skylab mission launch aboard Saturn IB vehicle.&lt;/p&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="195" LastEditDate="2017-03-15T15:52:46.593" LastActivityDate="2017-03-15T15:52:46.593" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20612" PostTypeId="2" ParentId="20609" CreationDate="2017-03-12T08:11:49.500" Score="-6" Body="&lt;p&gt;That upper platform was the crawler used for the transport of the fully assembled Saturn V from the assembly building to the launch pad. The lower platform was necessary for the exhaust of the rocket engines. There should be enough space for the flames to escape to the side and not to damage the bottom of the rocket.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-03-12T08:11:49.500" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20613" PostTypeId="2" ParentId="20609" CreationDate="2017-03-12T10:00:21.040" Score="72" Body="&lt;p&gt;Your picture is not of a Saturn V, it's of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_IB&quot; rel=&quot;noreferrer&quot;&gt;Saturn IB&lt;/a&gt;.  The purpose of the elevated platform (known as the &quot;milkstool&quot;) is to lift the rocket up so that it can be launched from Pad 39B using the same connections to the launch tower that the much taller Saturn V used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The early Saturn IB launches used the shorter Pad 34 and Pad 37, but by 1973, those launch pads had been decommissioned, leaving only Pad 39.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2017-03-12T10:00:21.040" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20614" PostTypeId="1" AcceptedAnswerId="20615" CreationDate="2017-03-12T12:56:10.457" Score="20" ViewCount="1700" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/20606/12102&quot;&gt;this answer&lt;/a&gt; @RussellBorogove discussed the frequency of dockings of each &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot; rel=&quot;noreferrer&quot;&gt;space shuttle&lt;/a&gt; with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mir&quot; rel=&quot;noreferrer&quot;&gt;Mir&lt;/a&gt; space station. I've summarized those frequencies, total number of missions flown, and year of first and last flight below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not counting the Enterprise and the Challenger (lost early in its expected life) there are four shuttles that span the period over which Mir docking missions were carried out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of the 9 dockings, 7 were made by the Atlantis. I've done a rough calculation tossing nine balls into four equal bins repeatedly, and then calculated roughly how many times the maximum number of balls in a given bin would be $N_{max}$. The probability that 7 of 9 balls would end up in any one bin is less than 0.5%. So I think the high frequency of Atlantis-Mir docking is not likely to have been just a statistical fluke, although it's not impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were there any reasons why NASA would have used Atlantis for such a large fraction of all the Shuttle-Mir dockings? &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Enterprise: 0 /  0 (1977-2012, model only)&#xA;Columbia:   0 / 28 (1981-2003, lost)&#xA;Challenger: 0 / 10 (1983-1986, lost)&#xA;Discovery:  1 / 34 (1984-2011)&#xA;Atlantis:   7 / 33 (1985-2011)&#xA;Endeavour:  1 / 25 (1992-2011)&#xA;&#xA;&#xA;With 1,000,000 tries:&#xA;Nmax  %&#xA;---- ----&#xA; 3   37.2&#xA; 4   43.2&#xA; 5   15.6&#xA; 6    3.5&#xA; 7    0.5&#xA; 8   &amp;lt;0.1 &#xA;&#xA;import numpy as np&#xA;from collections import Counter&#xA;&#xA;n_tries = 100000 # I used a million for the question&#xA;&#xA;# shamelessly inefficient way to do this&#xA;bob = np.random.randint(4, size=9*n_tries).reshape(9, -1)&#xA;qq  = [max(Counter(thing).values()) for thing in bob.T]&#xA;a,b = np.histogram(qq, bins=range(11))&#xA;&#xA;for A, B in zip(a, b[:-1]):&#xA;    print B, float(A)/a.sum()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.963" LastActivityDate="2017-03-12T16:40:09.610" Title="Why was Space Shuttle Atlantis selected for Mir docking so frequently?" Tags="|space-shuttle|docking|mir|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20615" PostTypeId="2" ParentId="20614" CreationDate="2017-03-12T14:45:24.860" Score="29" Body="&lt;p&gt;Looking at old manifest documents, it seems pretty clear that Atlantis (OV-104) was outfitted with all the hardware for Shuttle-Mir missions and they used her exclusively for that until they couldn't any more.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rules for orbiter mission scheduling were really byzantine but one major consideration was that every X flights and/or time interval, the orbiter had to go back to Palmdale (later KSC) for an Orbiter Maintenance Down Period (OMDP).  From the attached charts you can see that Atlantis (OV-104) came out of a Palmdale OMDP in Oct 1994 and flew STS-66. Then was down for a long maintenance period, presumably being outfitted with the docking system and external airlock, although this is not explicit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This orbiter then flew Shuttle-Mir missions and nothing else for the next 2+ years.  Then it had to go back to Palmdale in Nov 1997. The final 2 Shuttle-Mir flights were flown by OV-105 and OV-103, presumably setting OV-105 up for the first ISS mission and OV-103 for the 2nd. (Maybe there were 2 docking systems by then?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle-Mir missions are shown on the schedule as SMM-XX&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PJrov.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PJrov.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &#xA;&lt;a href=&quot;https://i.stack.imgur.com/87Yhs.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/87Yhs.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-03-12T14:45:24.860" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20616" PostTypeId="1" AcceptedAnswerId="20626" CreationDate="2017-03-12T15:11:00.083" Score="3" ViewCount="931" Body="&lt;p&gt;I have a question about the gridded electrostatic ion engine.&#xA;In the ionisation chamber there are several magnet rings that create a magnetic field inside the chamber. &lt;a href=&quot;https://i.stack.imgur.com/kjj8q.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kjj8q.png&quot; alt=&quot;gridded electrostatic ion engine&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what is the purpose of that magnetic field?&lt;/p&gt;&#xA;" OwnerUserId="18885" LastActivityDate="2017-08-24T10:57:22.330" Title="Ion engine: purpose of magnet rings" Tags="|ion-thruster|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20617" PostTypeId="1" AcceptedAnswerId="20618" CreationDate="2017-03-12T15:48:30.997" Score="2" ViewCount="113" Body="&lt;p&gt;In the book, Fundamentals of Astrodynamics, the author provides a different form of the Law of Gravitation&#xA;$$\ F_g = - \frac {GMm} {r^2} \frac {\mathbf r}{r} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the book says that the ${\mathbf r} $ is the distance vector, but what does that make non-bold r, the scalar distance between the two masses?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7ff4t.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7ff4t.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18886" LastEditorUserId="6944" LastEditDate="2017-03-12T16:02:50.523" LastActivityDate="2017-03-12T16:47:09.713" Title="Question about variation of Law of Gravitation" Tags="|gravity|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20618" PostTypeId="2" ParentId="20617" CreationDate="2017-03-12T16:01:03.617" Score="2" Body="&lt;p&gt;Yes, $r$ is the scalar distance as usual. The equation that is handwritten into the picture is the scalar formulation.  The printed version is the vector formulation.  Notice the $\mathbf{F}$ is bolded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\mathbf{r}}{r^3}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;is written as &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\mathbf{\hat r}}{r^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mathbf{\hat r}$ is the unit vector in the $\mathbf{r}$ direction.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="12102" LastEditDate="2017-03-12T16:47:09.713" LastActivityDate="2017-03-12T16:47:09.713" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20619" PostTypeId="1" CreationDate="2017-03-12T16:18:13.267" Score="6" ViewCount="58" Body="&lt;p&gt;I am not very experienced with signals, but something that has occurred to me while reading on various ways we are searching for ET signals, is that it seems most people rule out a signal unless it's in a narrow band. Is there any literature about treating signals that span a broad range of frequencies? This came from thinking that perhaps instead of utilizing a narrow band to send out a message, perhaps a theoretical ET might send their messages back and forth in a single burst, where the data, instead of &quot;progressing&quot; by time, progresses by frequency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Possibility or gross concept error?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Who has attempted looking signals in this manner?&lt;/p&gt;&#xA;" OwnerUserId="18870" LastEditorUserId="12102" LastEditDate="2017-03-14T04:11:12.507" LastActivityDate="2017-03-14T04:11:12.507" Title="Frequency progressive signals vs time progressive signals?" Tags="|communication|seti|" AnswerCount="0" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="20620" PostTypeId="1" AcceptedAnswerId="20624" CreationDate="2017-03-12T16:21:03.317" Score="11" ViewCount="1287" Body="&lt;p&gt;In the video clip of this interview, captioned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;General Wayne Monteith, 45th Space Wing Commander, talks about the Autonomous Flight Safety System, or AFSS -- &lt;strong&gt;a new, automated way of having a rocket self-destruct&lt;/strong&gt;. Video by Malcolm Denemark. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;in the Florida Today article &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/2017/03/11/spacex-autonomous-flight-safety-system-afss-kennedy-space-center-florida-falcon9-rocket-air-force-military/98539952/&quot; rel=&quot;noreferrer&quot;&gt;Only on Falcon 9: Automated system can terminate SpaceX rocket launches&lt;/a&gt;, General Monteith says (my attempt at transcription):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Autonomous flight safety system is a game-changer. It increases public safety, it increases our throughput or capacity on the range, and reduces cost to the customers, and eventually cost to the air force.&#xA;  [...] we essentially remove the man or the woman out of the loop, which saves us about three and a half seconds of decision time during flight if there is an anomalous profile. So if the flight is going off-course, it gives the rocket an extra three and a half seconds to get back on course, and correct itself.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Later in the video the self-destruct option is also mentioned. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I really have two questions.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Despite the lengthy list of things described in the interview that have to happen when the decision is made on the ground, is any significant fraction of the three-and-a-half second benefit really coming from moving the decision from ground to rocket, or is essentially all of it really coming from moving the decision from human to computer, and the computer could be on the ground or in the rocket without more than a small fraction of a second difference?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The nature of spaceflight is such that the rocket's flight system is in fact making life-and-death decisions continuously. But with a crewed flight (or perhaps sooner than anticipated a flight with passengers), would the Autonomous Flight Safety System or AFSS now be able to initiate a self-destruct that would include the crew?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-03-12T17:09:55.500" LastActivityDate="2018-11-04T02:05:01.523" Title="Will a computer be allowed to &quot;self-destruct&quot; astronauts (or passengers)?" Tags="|safety|autonomy|self-destruct|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20621" PostTypeId="2" ParentId="20620" CreationDate="2017-03-12T16:43:35.697" Score="7" Body="&lt;p&gt;No, a computer will not be allowed to blow up astronauts. It may activate a launch abort system for them though, after which a flight termination system might be used depending on the situation.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-12T16:49:27.447" LastActivityDate="2017-03-12T16:49:27.447" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20622" PostTypeId="1" AcceptedAnswerId="20625" CreationDate="2017-03-12T17:16:51.557" Score="1" ViewCount="251" Body="&lt;p&gt;There have been a number of paying passengers on orbital missions in the past, and it is likely there will be many, many more in the future, and not only on orbital missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm thinking that passengers and crew are exclusive of each other, the only  other group would be stow-aways.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the past have all of the above been called astronauts (or equivalent in other languages)? Will this change in the future, when passengers may outnumber the crew? &lt;a href=&quot;https://space.stackexchange.com/q/20620/12102&quot;&gt;This question&lt;/a&gt; got me thinking about the distinction.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-03-12T18:11:08.100" Title="Has every passenger been an astronaut? Will they be in the future?" Tags="|crewed-spaceflight|terminology|tourism|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20623" PostTypeId="2" ParentId="20609" CreationDate="2017-03-12T17:28:27.317" Score="30" Body="&lt;p&gt;Offered as a supplement, since no one has posted a direct size comparison:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/niumD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/niumD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The height difference in the Saturn V and Saturn 1-B is clear in this image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: SA-1 through -9 are Saturn 1s, SA-201 through -205 are 1-Bs, the last 2 are Vs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mission payloads, from left to right:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_I_SA-1&quot; rel=&quot;noreferrer&quot;&gt;SA-1&lt;/a&gt;, development flight with dummy second stage and payload&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_I_SA-4&quot; rel=&quot;noreferrer&quot;&gt;SA-4&lt;/a&gt;, more realistic dummy second stage and engine-out test&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_I_SA-5&quot; rel=&quot;noreferrer&quot;&gt;SA-5&lt;/a&gt;, complete launcher, dummy payload&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/AS-101_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;SA-6/AS-101&lt;/a&gt;, &quot;boilerplate&quot; dummy Apollo CSM payload&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/AS-103_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;SA-9/AS-103&lt;/a&gt;, &quot;boilerplate&quot; dummy Apollo CSM payload + &lt;a href=&quot;https://en.wikipedia.org/wiki/Pegasus_(satellite)&quot; rel=&quot;noreferrer&quot;&gt;Pegasus&lt;/a&gt; satellite&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/AS-201&quot; rel=&quot;noreferrer&quot;&gt;SA-201&lt;/a&gt;, unmanned Block 1 Apollo CSM&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/AS-203&quot; rel=&quot;noreferrer&quot;&gt;SA-203&lt;/a&gt;, no payload, restart test of S-IVB upper stage&lt;/li&gt;&#xA;&lt;li&gt;SA-204, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_5&quot; rel=&quot;noreferrer&quot;&gt;Apollo 5&lt;/a&gt;, unmanned Apollo LM test&lt;/li&gt;&#xA;&lt;li&gt;SA-205, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_7&quot; rel=&quot;noreferrer&quot;&gt;Apollo 7&lt;/a&gt;, first manned Apollo CSM flight&lt;/li&gt;&#xA;&lt;li&gt;SA-501, &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_4&quot; rel=&quot;noreferrer&quot;&gt;Apollo 4&lt;/a&gt;, unmanned Apollo CSM + dummy LM&lt;/li&gt;&#xA;&lt;li&gt;SA-513, &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylab&quot; rel=&quot;noreferrer&quot;&gt;Skylab 1&lt;/a&gt;, unmanned space station launch&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All of Apollo 8 through Apollo 17 were similar to SA-501; the Skylab crew flights were CSM on Saturn 1B and were similar to SA-205.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://historicspacecraft.com/Rockets_Saturn.html&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="195" LastEditDate="2017-03-13T02:41:03.100" LastActivityDate="2017-03-13T02:41:03.100" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20624" PostTypeId="2" ParentId="20620" CreationDate="2017-03-12T18:02:14.963" Score="9" Body="&lt;p&gt;AFSS was developed for unmanned launches first. A &lt;a href=&quot;http://commons.erau.edu/cgi/viewcontent.cgi?article=3345&amp;amp;context=space-congress-proceedings&quot; rel=&quot;noreferrer&quot;&gt;2004 NASA paper indicates how manned flight would be handled:&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Autonomous Flight Safety System (AFSS) is an independent flight safety system designed for small to medium sized expendable launch vehicles launching from or needing range safety protection while overlying relatively remote locations. AFSS replaces the need for a man-in-the-loop to make decisions for flight termination. AFSS could also serve as the prototype for an autonomous manned flight crew escape advisory system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;AFSS &lt;a href=&quot;http://21space.info/auto-destruct-system-seen-as-a-key-to-ramping-up-launch-tempos/&quot; rel=&quot;noreferrer&quot;&gt;isn't man-rated yet&lt;/a&gt; (also in &lt;a href=&quot;https://spaceflightnow.com/2017/02/25/auto-destruct-safety-system-seen-as-key-to-ramping-up-launch-tempos/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt;):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Autonomous Flight Safety System could be on crewed flights as soon as next year as commercial companies begin launching astronauts to the International Space Station.&lt;/p&gt;&#xA;&lt;p&gt;The commercial crew missions will be launched under contract to NASA, which will certify the on-board termination computer for piloted flights.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Note that AFSS is not a SpaceX development. This was done at NASA, and it's now available for all launch providers who implement it in their launch vehicles.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-11-04T02:05:01.523" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="20625" PostTypeId="2" ParentId="20622" CreationDate="2017-03-12T18:05:49.410" Score="2" Body="&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_tourism#Terminology&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Many private space travelers have objected to the term &amp;quot;space tourist&amp;quot;, often pointing out that their role went beyond that of an observer, since they also carried out scientific experiments in the course of their journey. Richard Garriott additionally emphasized that his training was identical to the requirements of non-Russian Soyuz crew members, and that teachers and other non-professional astronauts chosen to fly with NASA are called astronauts. He has said that if the distinction has to be made, he would rather be called &amp;quot;private astronaut&amp;quot; than &amp;quot;tourist&amp;quot;.[63] Dennis Tito has asked to be known as an &amp;quot;independent researcher&amp;quot;,[citation needed] and Mark Shuttleworth described himself as a &amp;quot;pioneer of commercial space travel&amp;quot;.[64] Gregory Olsen prefers &amp;quot;private researcher&amp;quot;,[65] and Anousheh Ansari prefers the term &amp;quot;private space explorer&amp;quot;.[18] Other space enthusiasts object to the term on similar grounds. Rick Tumlinson of the Space Frontier Foundation, for example, has said: &amp;quot;I hate the word tourist, and I always will ... 'Tourist' is somebody in a flowered shirt with three cameras around his neck.&amp;quot;[66] Russian cosmonaut Maksim Surayev told the press in 2009 not to describe Guy Laliberté as a tourist: &amp;quot;It's become fashionable to speak of space tourists. He is not a tourist but a participant in the mission.&amp;quot;[67]&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Spaceflight participant&amp;quot; is the official term used by NASA and the Russian Federal Space Agency to distinguish between private space travelers and career astronauts. Tito, Shuttleworth, Olsen, Ansari, and Simonyi were designated as such during their respective space flights. NASA also lists Christa McAuliffe as a spaceflight participant (although she did not pay a fee), apparently due to her non-technical duties aboard the STS-51-L flight.&lt;/p&gt;&#xA;&lt;p&gt;The U.S. Federal Aviation Administration awards the title of &amp;quot;Commercial Astronaut&amp;quot; to trained crew members of privately funded spacecraft. The only people currently holding this title are Mike Melvill and Brian Binnie, the pilots of SpaceShipOne.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-12T18:05:49.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20626" PostTypeId="2" ParentId="20616" CreationDate="2017-03-12T18:36:06.133" Score="4" Body="&lt;p&gt;The goal of the magnetic field is to prevent erosion of the engine itself due to the plumes of the exhaust gas. This is often referred to as &quot;magnetic confinement.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &quot;HIPER: A HIGH POWER 20KW HALL EFFECT THRUSTER FOR EXPLORATION&quot;, a conference paper by Zurbach et al. 2010, the authors discuss many of the design requirements of a 20kW thruster. At the bottom of page 6, they specifically discuss how to implement this confinement in such a way that the temperatures do not surpass the Curie temperature of the magnetic material used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought I had a 2016 SNECMA/Safran conference paper lying around which showed the damage of erosion on a Hall effect thruster they were working on (part of their PPS series) but I can't seem to find it right now. I'll add that information if I do find it.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-03-12T18:36:06.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20627" PostTypeId="2" ParentId="20587" CreationDate="2017-03-12T21:21:58.833" Score="7" Body="&lt;p&gt;The knowledge existing on barotrauma in divers medicin is applicable for those skinsuits. Any larger pressure difference between different parts of the body may cause small or larger injuries. Those injuries caused by pressure differences are called barotrauma. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The air pressure at the mouth should be equal to the pressure on the skin for every part of the body. Pressure differences may impair blood circulation, lymph flow and air flow into and out of the lungs. The minimum blood pressure is at least 80 mbar, any pressure difference should be much smaller. The difference between chest surface and the air at the mouth should be smaller than about 40 mbar, larger differences would be exhausting and may impair the blood flow between both chambers of the heart and the lungs. To high pressure differences may cause severe and dangerous damages to the lungs tissue and to the heart muscle. The muscles of the chest used for breathing are not very strong, if they are exhausted by larger pressure differences, a dangerous situation may occur when less than necessary oxygen gets into the circulatory system and not enough carbon dioxide is removed from the body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this restrictions are also a problem when such a skinsuit is already put on but the helmet not yet, the pressure on the chest would inhibit breathing in. How would you don or doff a skinsuit and the helmet without generating to high pressure differences between chest and mouth during the procedure?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The small pressure differences should be sustained for any phase between full inhalation and full exhalation for any surface part of the torso. For the arms and legs, larger pressure differences are possible than for the torso as longs as blood circulation and lymph distribution is not affected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about cooling of the body? If the sweat could not evaporate and thus cool the body, the body would overheat and the performance of the wearer is substantially reduced, higher temperatures will be dangerous. Either a liquid cooling garment should be worn under the suit or the suit should be permeable for water vapour. But the skin might be impaired if to much water is removed from it. If too much oxygen diffuses from the tissue under the surface of the skin and out of the suit, damage to the tissue may result when the suit is worn for many hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To avoid skin-abrasions near the joints of the body caused by the moving parts of such a skinsuit would be very difficult if the suit is worn for many hours and on many days in sequence.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-03-12T21:21:58.833" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20628" PostTypeId="1" AcceptedAnswerId="26630" CreationDate="2017-03-13T04:42:34.550" Score="16" ViewCount="562" Body="&lt;p&gt;At least one western pilot (Jean-Loup Chrétien) trained to pilot the Buran.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did any other non-soviet pilots train for this shuttle?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="8162" LastEditDate="2018-08-06T14:45:16.673" LastActivityDate="2018-08-06T14:45:16.673" Title="Which western pilots trained on the Buran?" Tags="|buran|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20629" PostTypeId="2" ParentId="4211" CreationDate="2017-03-13T07:10:23.040" Score="6" Body="&lt;p&gt;you can use &lt;a href=&quot;http://rhodesmill.org/pyephem/&quot; rel=&quot;noreferrer&quot;&gt;PyEphem&lt;/a&gt;&#xA;just like this&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;sudo apt-get install python&#xA;&#xA;sudo apt-get install python-dev&#xA;&#xA;sudo apt-get install python-pip&#xA;&#xA;pip install pyephem&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;create test.py:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import ephem&#xA;import datetime&#xA;## [...]&#xA;&#xA;name = &quot;ISS (ZARYA)&quot;;&#xA;line1 = &quot;1 25544U 98067A   12304.22916904  .00016548  00000-0  28330-3 0  5509&quot;;&#xA;line2 = &quot;2 25544  51.6482 170.5822 0016684 224.8813 236.0409 15.51231918798998&quot;;&#xA;&#xA;tle_rec = ephem.readtle(name, line1, line2);&#xA;tle_rec.compute();&#xA;&#xA;print tle_rec.sublong, tle_rec.sublat;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="18891" LastActivityDate="2017-03-13T07:10:23.040" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20630" PostTypeId="1" CreationDate="2017-03-13T08:02:25.780" Score="2" ViewCount="347" Body="&lt;p&gt;I'm trying to find some data concerning current Earth-satellite radio communication systems, namely general characteristics of the equipment currently used on satellites. Specifically, I'm interested in common parameters of satellite transmitters working in Ka and X range such as&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Tx power &lt;/li&gt;&#xA;&lt;li&gt;Antenna gain&lt;/li&gt;&#xA;&lt;li&gt;EIRP (which is basically Tx+Antenna gain)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Is this information public and if it is, how and where can I find it? Thank you very much in advance!&lt;/p&gt;&#xA;" OwnerUserId="17106" LastEditorUserId="10791" LastEditDate="2017-03-13T12:40:05.800" LastActivityDate="2017-03-13T12:40:05.800" Title="Characteristics of the modern satelite radio transmitters" Tags="|artificial-satellite|communication|radio-communication|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20631" PostTypeId="2" ParentId="20630" CreationDate="2017-03-13T10:26:29.057" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I was wondering though whether it's possible to find a table like &lt;strong&gt;sat.name&lt;/strong&gt;, &lt;strong&gt;sat. model&lt;/strong&gt;, &lt;strong&gt;Tx band&lt;/strong&gt;, &lt;strong&gt;Tx power&lt;/strong&gt; and so on f&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't think that table would be overly helpful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You just design that forward. You calculate how much power must reach the satellite, you calculate path loss, you define a receiver operation characteristic (ie. amount of outages you're willing to accept), you derive a security margin from that, this gives you amount of power you need to radiate, and based on your ability to be sure that your satellite is within the beam center, and your amplifier vs antenna cost trade-off, design an antenna/amplifier system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It really doesn't help much to know what others did for their very specific application. Link budget calculations are pretty down to earth (hrhr, a pun) and deterministic – and so are dish and amplifier datasheets. What'd be interesting would be the models for the path loss variations that the operators assume – but you can't read that from the amplifier power and antenna gain, but would need to look into their link budget calculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt;: don't compare your antenna with other people's antenna. It's not about the power density you put out, it's what you do with it, and thus, knowing that a different system uses a certain gain and power is absolutely not helpful for you – a factor of 2 in link requirements could easily be incurred by doing some specific detail differently (e.g. channel coding, which you can't see from the outside), and adding 3dB (=factor of 2) gain to a 40dB dish makes a &lt;em&gt;large&lt;/em&gt; difference. &lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastActivityDate="2017-03-13T10:26:29.057" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20632" PostTypeId="2" ParentId="20600" CreationDate="2017-03-13T15:54:17.713" Score="3" Body="&lt;p&gt;The principal software that I found (and free) is the &lt;a href=&quot;https://software.nasa.gov/software/GSC-16720-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Comprehensive General-Purpose Simulation of Attitude and Trajectory Dynamics and Control of Multiple Spacecraft Composed of Multiple Rigid or Flexible Bodies&quot;&lt;/a&gt; from the NASA Technology Tranfer program. &lt;a href=&quot;https://software.nasa.gov/search/software/trajectory&quot; rel=&quot;nofollow noreferrer&quot;&gt;More alternatives&lt;/a&gt; in the same catalog: &lt;/p&gt;&#xA;" OwnerUserId="18896" LastActivityDate="2017-03-13T15:54:17.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20633" PostTypeId="1" CreationDate="2017-03-13T21:39:46.923" Score="3" ViewCount="222" Body="&lt;p&gt;According to the lists of spacewalks (&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spacewalks_and_moonwalks_1965%E2%80%931999&quot; rel=&quot;nofollow noreferrer&quot;&gt;1965-1999&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_spacewalks_2000%E2%80%932014&quot; rel=&quot;nofollow noreferrer&quot;&gt;2000-2014&lt;/a&gt;), spacewalks have all consisted of only two people.  The only exceptions are the very early space missions, where only one person went out, and one spacewalk on &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-49&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-49&lt;/a&gt; which had 3 people.  It makes sense that you would want more than one person to go out, in case something happens to one of them, and you would still want people inside in case something goes wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a written protocol for spacewalks which determines how many people must go out and how many must stay inside?  Or do the various space agencies just play it by ear and try to minimize the number of people who go out?&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2019-02-02T23:28:00.433" Title="How many people can go on a spacewalk?" Tags="|eva|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20634" PostTypeId="2" ParentId="20633" CreationDate="2017-03-13T22:44:10.850" Score="5" Body="&lt;p&gt;&quot;Is there a written protocol for spacewalks which determines how many people must go out and how many must stay inside? &quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was for Shuttle.  It's in the book that laid down the law for shuttle operations, the &lt;a href=&quot;https://archive.org/details/flight_rules_generic&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flight Rules&lt;/a&gt;. (Warning, giant pdf)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TD1LA.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TD1LA.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/OttRH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OttRH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/OLnBi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OLnBi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-02-02T23:28:00.433" LastActivityDate="2019-02-02T23:28:00.433" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="20635" PostTypeId="1" AcceptedAnswerId="20638" CreationDate="2017-03-13T23:37:17.417" Score="-2" ViewCount="895" Body="&lt;p&gt;Mars has stational liquid water on its surface, a CO2-rich atmosphere and its soil might lack some nutrients that plants need to grow there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what is stopping us from filling a rocket with seeds and fertilizer and crashing it on the surface of Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For one thing, I'd bet it will be way more interesting to see how life brought from Earth evolves in Mars that trying to figure out whether life eventually originated in Mars in the first place. Besides, it might never be proven beyond any reasonable doubt that there is/was life in Mars before we start terraforming the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also if someone, a government, company or individual, would start terraforming Mars on their own, what could be the implications? Are there any laws to prevent them doing that? What could the scientific community do to stop them?&lt;/p&gt;&#xA;" OwnerUserId="962" LastEditorUserId="6816" LastEditDate="2017-03-14T00:17:33.220" LastActivityDate="2023-05-25T04:23:42.213" Title="What is stopping us from starting to terraform Mars right now?" Tags="|mars|law|terraforming|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20637" PostTypeId="1" CreationDate="2017-03-14T00:44:31.510" Score="3" ViewCount="271" Body="&lt;p&gt;Has anybody ever come across any pressure vessels (spacecraft or otherwise) that use more than one bladder; that is, to contain more than two fluids?  Tanks with two fluids -- a pressurant and a propellant -- separated by a single bladder or diaphragm are common, but what about more?&lt;/p&gt;&#xA;" OwnerUserId="18907" LastActivityDate="2017-03-14T20:28:17.287" Title="Any pressure vessels with more than two fluids?" Tags="|propulsion|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20638" PostTypeId="2" ParentId="20635" CreationDate="2017-03-14T01:00:05.387" Score="16" Body="&lt;p&gt;A few of reasons for not mounting such a terraforming mission now are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Cost-effectiveness. It's expensive to go to Mars, and simply throwing some seeds and fertilizer at the place is too unlikely to yield a desirable result to be worth doing. All terrestrial life is adapted to terrestrial environments and ecosystems, especially complex life like plants. Microbial life might stand a better chance, but even then, Mars is a harsh place; radiation could very well sterilize any life left out on its surface, which is one reason the hunt for life there is looking beneath the surface.&lt;/li&gt;&#xA;&lt;li&gt;An actual terraforming mission would require much more than planting seeds. It would require the establishment of a whole web of life including bacteria and other microbes to form a balanced ecosystem.&lt;/li&gt;&#xA;&lt;li&gt;Mars is too dry.&lt;/li&gt;&#xA;&lt;li&gt;Even if the seeds die, there is a good chance the material we might crash on Mars will contaminate the environment to the point of thwarting any meaningful investigation of pre-existing life. To properly investigate and understand any possible past or present Martian ecosystem requires that we disturb it as little as possible. Charging in and turning the place into something else is not how you learn about what was there before you arrived; there is value in that knowledge.&lt;/li&gt;&#xA;&lt;li&gt;A decision to terraform is irrevocable. After the first material is introduced into the Martian environment, there will never be another opportunity to study Mars as it was before human activity because it will never again be as it was.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;We may eventually attempt to terraform Mars, but only after we've given it adequate study to see if it currently harbors indigenous life, or ever had, and what that life is/was like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If someone were to up and throw some seeds and fertilizer at Mars, they would be doing a great dis-service to planetary science. It is possible for someone with enough resources to launch a rogue spaceflight. Post-launch, you'd have to fend off all sorts of agencies tasked with terminating your mission and shutting down your mission control. Of course, this is all rather hypothetical.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-03-14T01:21:50.320" LastActivityDate="2017-03-14T01:21:50.320" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20639" PostTypeId="1" AcceptedAnswerId="20640" CreationDate="2017-03-14T07:00:39.397" Score="6" ViewCount="1251" Body="&lt;p&gt;Rockets are launched vertically and then turn to have an horizontal course. It sems logical to begin vertically to reach less denser atmosphere quickly and to travel horizontally when reaching low Earth orbit, but I am not able to determine what are the parameter to decide when this turn begins and the turn rate, nor if it depends on the launch vehicle or the launch location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the parameters to determine the shape of this turn?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastActivityDate="2017-03-21T04:45:03.407" Title="What does determine when a rocket goes from a vertical trajectory to an horizontal one?" Tags="|rockets|launch-vehicle|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20640" PostTypeId="2" ParentId="20639" CreationDate="2017-03-14T09:45:26.933" Score="12" Body="&lt;p&gt;Atmospheric drag vs gravitational drag curve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The turn is not an instantaneous, or even a short maneuver - it begins either shortly after clearing the tower or in &lt;a href=&quot;http://space.skyrocket.de/doc_lau/ss-520.htm&quot; rel=&quot;noreferrer&quot;&gt;some cases&lt;/a&gt; - even before launch! and lasts until vertical speed is sufficient to clear the atmosphere and reach vicinity of apogeum, quite late into the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The trajectory - and directly related angle of burn - is a result of optimization of the function of gravity losses and atmospheric drag losses: while reaching orbital velocity ASAP (burning horizontally) is the way to minimize loss due to gravitational pull of Earth (&quot;gravity drag&quot;), this is offset by atmospheric drag, which at low altitudes makes it impossible, and at somewhat higher altitudes economically suboptimal - as you lose more to pushing air out of your way than you prevent losing to keeping the rocket from falling down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What decides what the curve should be in case of given, specific rocket? Aerodynamics and engines, and to a lesser degree, payload and structural durability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrust to weight ratio (TWR) of the rocket, dependent on the rocket mass and engine thrust, decides how fast the rocket can climb and accelerate. Aerodynamic profile decides what losses air drag incurs. Structural durability says how it can handle high dynamic pressure (MaxQ) and may impose throttling the engine to reduce risk to the rocket and atmospheric losses. Payload may be rated for certain accelerations, which limits allowable max TWR. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The linked above SS-520 has a very high TWR and very lean aerodynamic profile; it can reach high altitudes really fast, so it starts at an angle, beginning the &quot;gravity turn&quot; (as the curving launch profile is sometimes called) right from the moment of launch. About all other launchers begin the turn at latest after clearing the launch tower, but the initial tilt might be very small. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-03-14T15:23:53.357" LastActivityDate="2017-03-14T15:23:53.357" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20641" PostTypeId="1" AcceptedAnswerId="20642" CreationDate="2017-03-14T11:06:39.287" Score="12" ViewCount="2530" Body="&lt;p&gt;In regenerative cooling the fuel is passed around the nozzle before being pumped into the combustion chamber right? I don't really see how this affects cooling itself, since the heat captured around the nozzle by the fuel/oxidizer seems to be pumped back into the combustion chamber which will have to be captured back by more fuel. So where is my reasoning wrong?&lt;/p&gt;&#xA;" OwnerUserId="16627" LastEditorUserId="11653" LastEditDate="2017-03-14T14:35:06.887" LastActivityDate="2017-03-14T14:35:06.887" Title="The principle behind regenerative cooling?" Tags="|engine-design|liquid-fuel|cryogenics|nozzle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20642" PostTypeId="2" ParentId="20641" CreationDate="2017-03-14T11:21:09.490" Score="30" Body="&lt;p&gt;You are missing how heat is distributed in exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of propellant ejected through the nozzle never makes contact with the nozzle surface or walls of the combustion chamber, and as result never has any chance to transfer its heat into them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exhaust gas primarily cools through adiabatic expansion - high pressure and high temperature both transformed into rapidly increasing volume and velocity of the ejected propellant - high-speed ejection of reaction mass; propulsion. Only a minor part of it is transferred into the walls - but that is still enough to risk damage to them through overheating and compromising durability of the materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, some of the combustion energy is consumed on heating the fuels from their injection temperature to post-combustion temperature (which then transforms into propulsion). The hotter the fuel before injection the less is lost to that heating. So it's beneficial to heat the fuels - especially to boil off cryofuels - before combustion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we both have some excess heat we need to get rid of, as it's dangerous to the structure, and we need the cryofuels turned from liquid to gas for combustion - we can combine the two processes, simultaneously increase the pre-combustion temperature and cool the structure. Most of the heat produced in combustion will be ejected &quot;harmlessly&quot; - and what excess does transfer to the structure, gets recuperated and reused by pre-heating the fuel, that energy which would be otherwise lost, pumped back into the combustion chamber as hot fuel. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-14T11:21:09.490" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20643" PostTypeId="2" ParentId="20358" CreationDate="2017-03-14T12:26:08.437" Score="6" Body="&lt;p&gt;The conversion of the attitude data as it appeared (undocumented) from Lightstreamer to roll, pitch, and yaw was the biggest pile of guesses and assumptions atop assumptions I made back when Matt and I were coding this. It is almost certainly &lt;em&gt;wrong&lt;/em&gt; if it is varying from the ISS Live! website.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've been working on a complete rewrite of the code and it'd be nice to actually understand what the data format is and how to properly convert it, rather than just throwing formulae at the data until it sorta fits; I'll have a look through newly found documentation and review that code again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Feel free to chat with us in Slack or comment on the card: &lt;a href=&quot;https://trello.com/c/NLeP5EXq/57-attitude-values-differ-from-isslive&quot; rel=&quot;noreferrer&quot;&gt;https://trello.com/c/NLeP5EXq/57-attitude-values-differ-from-isslive&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18909" LastActivityDate="2017-03-14T12:26:08.437" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20644" PostTypeId="2" ParentId="20637" CreationDate="2017-03-14T15:21:26.037" Score="1" Body="&lt;p&gt;Normally the propellants are stored separately, especially hypergolics, which spontaneously combust upon contact with one another. However SpaceX currently uses Composite Overwrapped Pressure Vessels (COPV's) filled with liquid helium inside of the liquid oxygen tank. The helium is heated and then released to pressurize the tank. So, technically the helium is stored inside the liquid oxygen tank. Also the S-IVB and I believe the S-II stage of the Saturn V rocket had common bulkheads. This means that the liquid hydrogen and liquid oxygen are just about stored in the same tank, except thy are separated by  2 thin sheets of aluminum and resin. This is all I could think of for 2 propellants/pressurants stored in almost the same tank.&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="18458" LastEditDate="2017-03-14T15:35:26.693" LastActivityDate="2017-03-14T15:35:26.693" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20645" PostTypeId="2" ParentId="20637" CreationDate="2017-03-14T15:38:09.593" Score="0" Body="&lt;p&gt;Safety concerns aside, it would be difficult to ensure that the delivered mix ratio remains correct and constant. Also, if cryogenic fuels are used their temperatures are usually incompatible.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-14T15:38:09.593" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20646" PostTypeId="2" ParentId="20637" CreationDate="2017-03-14T19:49:51.910" Score="2" Body="&lt;p&gt;The only thing I've ever heard of like that is from the petrochemical industry (you did say &quot;otherwise&quot;...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RY0Zi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RY0Zi.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a pressure vessel containing natural gas, oil, and water.  Used at the outlet of wells to separate what comes out of the well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doesn't really have a bladder though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's called a &quot;&lt;a href=&quot;http://www.glossary.oilfield.slb.com/Terms/t/three-phase_separator.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;three-phase separator&lt;/a&gt;&quot; although that is quite a misnomer IMHO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.emersonprocessxperts.com/2014/01/importance-of-flow-measurement-for-separators/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Image source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-03-14T20:28:17.287" LastActivityDate="2017-03-14T20:28:17.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20647" PostTypeId="2" ParentId="14994" CreationDate="2017-03-14T22:51:39.617" Score="2" Body="&lt;p&gt;It is common to describe an orbit using apoapsis and periapsis radii. One reason is that those two parameters are easy to visualize, and provide substantial information about the orbit's shape and energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can, for example, immediately calculate the semimajor axis:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;a = \frac{r_a + r_p}{2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the orbital eccentricity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;e = \frac{r_a - r_p}{r_a + r_p}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will give us a fairly accurate idea about the orbit's shape regardless of the body it is orbiting. But we often know the body and its gravitational parameter, $\mu$. With this information we can calculate the orbital energy:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mathcal{E}=-\frac{\mu}{2a},&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which lets us estimate the orbital velocity at any point in the orbit, because we know that&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mathcal{E} = \frac{v^2}{2}-\frac{\mu}{r}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and we can obtain $r$ from the orbit's equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;r = \frac{p}{1 + e\cos\nu},&#xA;$$&#xA;where &#xA;$$&#xA;p = a(1-e^2)&#xA;$$&#xA;and $v\in[0^{o}, 360^{o})$. At once, we can also calculate the orbital period:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mathcal{P} = 2\pi\sqrt{\frac{a^3}{\mu}},&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the specific angular momentum&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;h^2 =  \mu p.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my experience, most people use &lt;em&gt;altitude&lt;/em&gt; at periapsis and apoapsis (as opposed to &lt;em&gt;radius&lt;/em&gt;). To be sure, it is best to always specify, e.g., &lt;em&gt;this is a 40-by-400 kilometer altitude orbit&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you have described the &lt;em&gt;kinematic properties&lt;/em&gt; of the orbit, you can move on to discuss the &lt;em&gt;orientation properties&lt;/em&gt;, where you would need to know the inclination, nodes, and even current orbital location.&lt;/p&gt;&#xA;" OwnerUserId="18918" LastActivityDate="2017-03-14T22:51:39.617" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20648" PostTypeId="1" AcceptedAnswerId="20650" CreationDate="2017-03-15T01:40:31.463" Score="6" ViewCount="2351" Body="&lt;p&gt;I know the basic premise of tank pressurization. A inert gas (usually Helium) is kept in small tanks in or around the large propellant tanks, and when propellant is burned the helium fills the empty space the burning propellants left. But do both stages need to pressurize the tanks, or only the upper stages? And what's stopping the helium from entering the combustion chamber? &lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="6944" LastEditDate="2021-03-28T18:21:53.073" LastActivityDate="2021-03-28T18:21:53.073" Title="How does tank pressurization work?" Tags="|propulsion|spacecraft|helium|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20649" PostTypeId="2" ParentId="20635" CreationDate="2017-03-15T02:15:52.940" Score="2" Body="&lt;p&gt;I may add also perhaps, the need for it. Today it is not seen by the world as important enough to do. As mentioned in many forms by previous commenters, there are definitely a lot of issues not yet solved, or at least not yet tested. However, I think the main reason at this point can be simplified to survival. It is not in the &quot;collective&quot; mind of humans yet that developing a permanent living space, or simply adding life (human or otherwise) is worth the &lt;strong&gt;current&lt;/strong&gt; cost. Let us find a big rock heading towards earth, or a solar event that exterminated a large portion of life here and humanity may (probably) will make more plans towards permanent colonization beyond earth (mars as a possibility). There are many small to medium sized projects working towards goals similar to what you ask, as well as what I think your overall question might be geared towards (why the hell are we still just on earth?!?). Summarized: it is not (or not yet) perceived worth doing more than what we already are doing. Which I begrudgingly admit is probably a lot given that we've been a space fairing civilization for not even 100 years yet. Let's hope our &quot;patience&quot; and &quot;care&quot; on the issues doesn't take too long.&lt;/p&gt;&#xA;" OwnerUserId="18870" LastActivityDate="2017-03-15T02:15:52.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20650" PostTypeId="2" ParentId="20648" CreationDate="2017-03-15T02:17:35.950" Score="8" Body="&lt;p&gt;1) &quot;do both stages need to pressurize the tanks, or only the upper stages?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general all stages need pressurization. I am unaware of any counterexample. If the engine is pressure-fed, you need higher pressure than the combustion chamber pressure.  If the engine is pump-fed, you need sufficiently high pressure to keep the pumps from &lt;a href=&quot;https://en.wikipedia.org/wiki/Cavitation&quot; rel=&quot;noreferrer&quot;&gt;cavitating&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) &quot;And what's stopping the helium from entering the combustion chamber?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nothing, if you run out of propellant.  But for boosters, acceleration holds the heavier propellants in the bottom of the tank, and the lighter helium stays at the top.  The situation is more complicated for orbital systems, there are questions on the site about that (like this one: &lt;a href=&quot;https://space.stackexchange.com/questions/15118/propellant-settling-for-rcs-thrusters&quot;&gt;Propellant Settling for RCS Thrusters&lt;/a&gt; and this one: &lt;a href=&quot;https://space.stackexchange.com/questions/2446/starting-a-liquid-fuel-engine-in-zero-gravity&quot;&gt;Starting a liquid fuel engine in zero gravity&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="195" LastEditDate="2017-03-15T04:19:31.203" LastActivityDate="2017-03-15T04:19:31.203" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20651" PostTypeId="2" ParentId="20639" CreationDate="2017-03-15T05:30:51.510" Score="7" Body="&lt;p&gt;As a former launch vehicle trajectory analyst the answer isn't very fulfilling: it depends. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trajectories are constrained optimization problems, so at the highest level the answer to what determines when the turn is made is whatever maneuver optimizes the chosen objective function while satisfying the constraints.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be more helpful, constraints are myriad. Keeping aerodynamic forces within limits, maintaining control margin on thrust vector and/or aerodynamic surfaces, ensuring the fairing jettison doesn't occur too early, keeping aero-heating within bounds, relative location of launch site and target orbit, etc. Now that humanity knows how to return first stage boosters there's a whole host of other constraints such as staging so that the first stage can fly to launch site, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The parameters that end up driving the solution and the bounds/values are very rocket dependent, that's why my work as a trajectory analyst always began very early in a program. How the rocket will fly and its basic design have to be done in concert.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those caveats aside, for normal ground launched rockets, they start to turn no later after they are past max dynamic pressure and/or max q-alpha, when the max aero loads on the structure are experienced. Air-launched rockets fly very different trajectories and basically never fly straight up.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="127" LastEditDate="2017-03-21T04:45:03.407" LastActivityDate="2017-03-21T04:45:03.407" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20652" PostTypeId="1" AcceptedAnswerId="20657" CreationDate="2017-03-15T05:45:47.077" Score="8" ViewCount="606" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/20624/12102&quot;&gt;This thorough answer&lt;/a&gt; by @Hobbes links to the item &lt;a href=&quot;https://spaceflightnow.com/2017/02/25/auto-destruct-safety-system-seen-as-key-to-ramping-up-launch-tempos/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Auto-destruct system seen as a key to ramping up launch tempos&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;I'd like to understand &lt;em&gt;Why would autonomous auto-destruct ramp up launch tempo?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;I have tried to read through it, and I think the answer is in here, but I'm not familliar with even the basics of range safety. Could someone help me understand better what the &amp;quot;resources on the range&amp;quot; might be, and which &amp;quot;antenna pointing at the vehicle&amp;quot; is no longer needed? I'm sure there are still some antennas somewhere pointing at it.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;“We have this problem right now where we negotiate dates back and forth, but I think it’s actually going to get better,” Koenigsmann said Feb. 8 at the FAA-sponsored Commercial Space Transportation Conference in Washington. “The reason it’s going to get better is the Autonomous Flight Safety System. It does not need a lot of resources from the range, so the time it takes to reconfigure the resources from a ULA vehicle to a Falcon 9 or some other vehicle, that factor will basically go away.”&lt;/p&gt;&#xA;&lt;p&gt;While the range is still involved in launch operations, “there is not an antenna pointing at the vehicle, so that in itself, I think, will make our lives easier,” Koenigsmann said. “I could easily imagine that we’ll have two launches on the same day because of that.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;According to the article, Hans Koenigsmann is vice president of flight reliability at SpaceX&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-08-05T02:35:37.250" LastActivityDate="2020-08-05T02:35:37.250" Title="Why would autonomous auto-destruct ramp up launch tempo/cadence?" Tags="|range-safety|autonomy|self-destruct|standardization|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20653" PostTypeId="1" CreationDate="2017-03-15T08:12:03.550" Score="26" ViewCount="16885" Body="&lt;p&gt;Why isn't it possible to build a space elevator at the north pole? Why does it have to be built on the equator?&lt;/p&gt;&#xA;" OwnerUserId="18924" LastEditorUserId="6816" LastEditDate="2017-03-15T08:17:55.087" LastActivityDate="2019-08-04T02:13:20.013" Title="Why isn't it possible to build a space elevator at the north pole?" Tags="|space-elevator|" AnswerCount="11" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20654" PostTypeId="2" ParentId="20653" CreationDate="2017-03-15T08:22:43.320" Score="53" Body="&lt;p&gt;A &quot;space tower&quot; could be built at the north pole, but only if materials capable of supporting its weight were available. The &quot;space tower&quot; should be supported by the Earth's crust below it, but the crust will be flexible under the enormous load and over a long time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A space elevator with cables makes use of the centrifugal forces caused by the rotation of the earth. These forces' vertical component is highest at the equator, while it is absent at the poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you did build a tower at the north pole, a payload would only gain height but no speed. If released, it would fall down instantly.&lt;br&gt;&#xA;From a space elevator at the equator, a payload could be lifted to the height of a geostationary orbit. If released, the payload would stay in orbit because the necessary height and speed for an orbit are met.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2019-04-02T09:34:43.953" LastActivityDate="2019-04-02T09:34:43.953" CommentCount="17" ContentLicense="CC BY-SA 4.0" />
  <row Id="20655" PostTypeId="2" ParentId="20653" CreationDate="2017-03-15T08:47:32.547" Score="17" Body="&lt;p&gt;Because a space elevator would connect to a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geosynchronous_satellite&quot; rel=&quot;noreferrer&quot;&gt;geostationary satellite&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Geostationary satellites can only exist above the equator.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2017-03-15T08:47:32.547" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20656" PostTypeId="1" CreationDate="2017-03-15T09:31:16.957" Score="6" ViewCount="492" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/20653/why-isnt-it-possible-to-build-a-space-elevator-at-the-north-pole&quot;&gt;&quot;Why isn't it possible to build a space elevator at the north pole?&quot;&lt;/a&gt; made me think...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like the funny hat with a propeller on a kid's head - replace the head with Earth and propeller blades with space elevators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Build a stubby tower / hill / mountain on one of Earth's poles. Add a huge motor, that spins in the same axis as Earth on top. Extend space elevator (or two, in opposite directions, for balance) from the motor's axis, going initially parallel to Earth surface, then out into space as Earth curves away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Yr1Qf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Yr1Qf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NbDHl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NbDHl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such an elevator could be much shorter than the traditional equatorial space elevator propelled by Earth spin alone - since the rotary speed would be much higher. The huge problem of construction vanishes: lifting the material up to GEO to unroll from there, as segments could be added at the axis instead. There would be the problem of the end moving at hypersonic speeds until there's enough of it extended to slow the spin enough that the part within the atmosphere doesn't exceed speed of sound. And of course the engineering problems of the mighty motor, and continuous energy expenditure to fight atmospheric drag. Never mind logistically awful location. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be possible though, or am I missing something? Say, precession, or maybe Earth's gravity bending it to the ground, or Earth magnetic field, or something like this? &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-15T12:29:29.360" Title="&quot;Propeller-head&quot; polar space elevator?" Tags="|space-elevator|" AnswerCount="1" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="20657" PostTypeId="2" ParentId="20652" CreationDate="2017-03-15T12:29:24.100" Score="8" Body="&lt;p&gt;I haven't found a full list of range activities, but there's a key comment in &lt;a href=&quot;http://21space.info/auto-destruct-system-seen-as-a-key-to-ramping-up-launch-tempos/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Air Force currently requires several days to reconfigure its ranges between Atlas, Delta and Falcon missions. That turnaround time should be reduced with the introduction of auto-destruct mechanisms&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So in the old situation, every user had its own self-destruct system that needed some unique equipment or configuration on the ground (because the system expects information in a unique format, for example). Because the self-destruct system is, well, destructive, they'd want to test the system after every reconfiguration to make sure it works and that no false destruct orders will be sent. That alone could take a few days.&lt;/p&gt;&#xA;&lt;p&gt;During the launch, they'd also need to provide the Range Safety Officer with all the information they need for the decision in real time, again unique information for every launch vehicle. The RSO is the person with the finger on the 'Destroy' button.&lt;/p&gt;&#xA;&lt;p&gt;In the new system, the flight termination rules are decided beforehand, and an automatic system on the rocket checks if the rules have been violated. That means you no longer need a ground-based system to receive and analyze telemetry, and translate the data for human consumption in real time.&lt;/p&gt;&#xA;&lt;p&gt;It turns out, that ground-based system was really &lt;a href=&quot;https://www.nasaspaceflight.com/2017/03/air-force-reveals-48-launches-year-cape/&quot; rel=&quot;nofollow noreferrer&quot;&gt;manpower-intensive&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As the Brig. Gen. explained, “We have now gone completely autonomous with that system.  So with CRS-10 and all others with the AFTS, we’re able to reduce our operational footprint by 60% on day of launch.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;“So we came down 96 people that don’t have to be sitting on console.  And the cost to the customer is cut in half.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Those 96 people &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=42567.msg1656548#msg1656548&quot; rel=&quot;nofollow noreferrer&quot;&gt;were involved in&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Comm, radar, transmitter, receiver, backup power generation, software, tracking cameras, console maintenance, etc  They would be located at the MOCC, JDMTA, Antigua, Cape command antenna site, camera sites, etc.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-21T14:53:43.610" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20658" PostTypeId="2" ParentId="20656" CreationDate="2017-03-15T12:29:29.360" Score="3" Body="&lt;h1&gt;Because of bending&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;You assume that because your carousel is spinning that you do not need to lift it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you do. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;orbit&quot; that this carousel is spinning in is not a real orbit, because it is not centered around the Earth's center of gravity. So the &quot;arms&quot; are subjected to gravity, pulling them down towards the Earth's center of gravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that you have to deal with huge &lt;strong&gt;bending&lt;/strong&gt; forces. And bending is a much harder problem to solve than just compression / tension forces. &lt;/p&gt;&#xA;" OwnerUserId="10493" LastActivityDate="2017-03-15T12:29:29.360" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="20659" PostTypeId="2" ParentId="20249" CreationDate="2017-03-15T16:57:27.200" Score="9" Body="&lt;p&gt;If we assume a perfect two-body problem, absent perturbations from external bodies or non-spherical gravity sources (&lt;em&gt;i.e.&lt;/em&gt;, perfect conic orbits with no precession or variation), your constraints regarding inclination are actually unnecessary, as we may, without loss of generality, examine this problem in a &lt;a href=&quot;https://en.wikipedia.org/wiki/Perifocal_coordinate_system&quot; rel=&quot;noreferrer&quot;&gt;perifocal reference frame&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Perifocal coordinates&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Let the origin of the system be the focus of the orbit, and let the orthogonal unit vectors $\mathbf{\hat{p}}$ and $\mathbf{\hat{q}}$ lie in the orbital plane, such that the periapsis of the orbit is located on a line extending from the origin in the $\mathbf{\hat{p}}$ direction.  The unit vector $\mathbf{\hat{w}} = \mathbf{\hat{p}} \times \mathbf{\hat{q}}$ completes the coordinate system. (The orbital angular momentum vector lies in the $\mathbf{\hat{w}}$ direction).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we further constrain ourselves to only look at periapsis and -- provided $e &amp;lt; 1$ -- apoapsis, our position vector is then strictly in the $\pm \mathbf{\hat{p}}$ direction, and our velocity vector is strictly in the $\pm \mathbf{\hat{q}}$ direction.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Keplerian elements in relation to perifocal coordinates&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The Keplerian elements $i$ (inclination), $\Omega$ (argument of ascending node), and $\omega$ (argument of periapsis) are simply Euler angles used to translate between a perifocal coordinate system and an equatorial coordinate system.  By focusing on a perifocal coordinate system, we can temporarily dispense with these, leaving only $e$ (eccentricity), $a$ (semi-major axis), and $\nu$ (true anomaly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With that established, we need to develop two more scalar terms that remain constant in an unperturbed orbit (note: $\mu$ is the gravitational parameter of the central body):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Specific energy: $$\varepsilon = \frac{v^2}{2} - \frac{\mu}{\|\mathbf{r}\|}$$&lt;/li&gt;&#xA;&lt;li&gt;Specific angular momentum: $$h = \|\mathbf{r} \times \mathbf{v}\|$$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To facilitate the upcoming derivations, we will expand these in terms of components (including ones known to be zero):&#xA;$$\varepsilon = \frac{v^2_p + v^2_q + v^2_w}{2} - \frac{\mu}{\sqrt{r^2_p + r^2_q + r^2_w}}$$&#xA;$$h = \sqrt{(r_q v_w - r_w v_q)^2 + (r_w v_p - r_p v_w)^2 + (r_p v_q - r_q v_p)^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We now note that&#xA;$$a = -\frac{\mu}{2\varepsilon}$$&#xA;and $$e = \sqrt{1 + \frac{2 \varepsilon h^2}{\mu^2}}$$&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Finally, to your question&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;To frame this explicitly, you are looking for how the elements are affected by velocity perturbations.  This means we want some expressions for these as a function of velocity, all other terms (in particular, the position) being constant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, you want to accomplish this through a multivariate Taylor expansion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let $Q : \mathbb{R}^3 \rightarrow \mathbb{R}$ be an arbitrary real-valued continuous and at-least-twice differentiable (since you want second order accuracy) function of its arguments.  Then to second order accuracy, you have&#xA;$$ Q(\mathbf{x}) = Q(\mathbf{a}) + \Big[(\mathbf{x}-\mathbf{a}) \cdot \mathbf{\nabla}Q(\mathbf{a})\Big] + \frac{1}{2}\Big[(\mathbf{x}-\mathbf{a}) \cdot \mathbf{H}(\mathbf{a}) \cdot (\mathbf{x}-\mathbf{a})\Big] + \epsilon(\|\mathbf{x}-\mathbf{a}\|^3)$$&#xA;where $\mathbf{\nabla}Q$ is the gradient of $Q$, &lt;em&gt;i.e.&lt;/em&gt;, $\mathbf{\nabla}Q = [Q_1 \; Q_2 \; Q_3]^T$, (using subscript numerals to denote partial differentiation with respect to that argument), and $\mathbf{H}$ is the Hessian matrix:&#xA;$$H = \begin{bmatrix}Q_{11} &amp;amp; Q_{12} &amp;amp; Q_{13}\\ Q_{21} &amp;amp; Q_{22} &amp;amp; Q_{23}\\ Q_{31} &amp;amp; Q_{32} &amp;amp; Q_{33} \end{bmatrix}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Writing this explicitly gets cumbersome quickly and would not be easily readable, especially since it will involve multiple invocations of the chain rule at higher orders in multiple dimensions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, we will demonstrate one specific case: the semimajor axis (one of the simplest elements to calculate), $a$, to first-order accuracy only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let a prefix $\delta$ denote an infinitesimal perturbation to a quantity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We now have,&#xA;$$\delta a = \frac{\partial a}{\partial \varepsilon}\left(\frac{\partial \varepsilon}{\partial v_p}\delta v_p + \frac{\partial \varepsilon}{\partial v_q}\delta v_q + \frac{\partial \varepsilon}{\partial v_w} \delta v_w\right) + \epsilon(\|\delta \mathbf{v}\|^2)$$&#xA;Expanding this, we have &#xA;$$\delta a = \frac{\mu}{2\varepsilon^2} \Big(v_p \delta v_p + v_q \delta v_q + v_w \delta v_w \Big) + \epsilon(\|\delta \mathbf{v}\|^2)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What can we draw from this? Note that the parenthesized term is identically equal to $\mathbf{v} \cdot \mathbf{\delta v}$.  That is to say, to first-order accuracy, the semimajor axis is affected by the component of the perturbation in the prograde/retrograde direction &lt;strong&gt;only&lt;/strong&gt;.  This is the case regardless of where in the orbit this happens, not just at one of the apsides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we want to explore a second order expansion, it gets ugly quickly.&#xA;$$\delta a = \frac{\partial a}{\partial \varepsilon}\left(\frac{\partial \varepsilon}{\partial v_p}\delta v_p + \frac{\partial \varepsilon}{\partial v_q}\delta v_q + \frac{\partial \varepsilon}{\partial v_w} \delta v_w\right) + \frac{1}{2}\left\{ \frac{\partial a}{\partial \varepsilon} \left(&#xA;\frac{\partial^2 \varepsilon}{\partial v^2_p}(\delta v_p)^2 +&#xA;\frac{\partial^2 \varepsilon}{\partial v_p \partial v_q}(\delta v_p \delta v_q) +&#xA;\frac{\partial^2 \varepsilon}{\partial v_p \partial v_w}(\delta v_p \delta v_w) +&#xA;\frac{\partial^2 \varepsilon}{\partial v_q \partial v_p}(\delta v_q \delta v_p) +&#xA;\frac{\partial^2 \varepsilon}{\partial v^2_q}(\delta v_q)^2 +&#xA;\frac{\partial^2 \varepsilon}{\partial v_q \partial v_w}(\delta v_q \delta v_w) +&#xA;\frac{\partial^2 \varepsilon}{\partial v_w \partial v_p}(\delta v_w \delta v_p) +&#xA;\frac{\partial^2 \varepsilon}{\partial v_w \partial v_q}(\delta v_w \delta v_q) +&#xA;\frac{\partial^2 \varepsilon}{\partial v^2_w}(\delta v_w)^2&#xA;\right) &#xA;+ \frac{\partial^2 a}{\partial \varepsilon^2} \left[&#xA;\left(\frac{\partial \varepsilon}{\partial v_p}\right)^2(\delta v_p)^2 +&#xA;\left(\frac{\partial \varepsilon}{\partial v_p}\right)\left(\frac{\partial \varepsilon}{\partial v_q}\right)(\delta v_p \delta v_q) +&#xA;\left(\frac{\partial \varepsilon}{\partial v_p}\right)\left(\frac{\partial \varepsilon}{\partial v_w}\right)(\delta v_p \delta v_w) +&#xA;\left(\frac{\partial \varepsilon}{\partial v_q}\right)\left(\frac{\partial \varepsilon}{\partial v_p}\right)(\delta v_q \delta v_p) +&#xA;\left(\frac{\partial \varepsilon}{\partial v_q}\right)^2(\delta v_q)^2 +&#xA;\left(\frac{\partial \varepsilon}{\partial v_q}\right)\left(\frac{\partial \varepsilon}{\partial v_w}\right)(\delta v_q \delta v_w) +&#xA;\left(\frac{\partial \varepsilon}{\partial v_w}\right)\left(\frac{\partial \varepsilon}{\partial v_p}\right)(\delta v_w \delta v_p) +&#xA;\left(\frac{\partial \varepsilon}{\partial v_w}\right)\left(\frac{\partial \varepsilon}{\partial v_q}\right)(\delta v_w \delta v_q) +&#xA;\left(\frac{\partial \varepsilon}{\partial v_w}\right)^2(\delta v_w)^2&#xA;\right]\right\} +   \epsilon(\|\delta \mathbf{v}\|^3)$$&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Hoo boy!&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;We now make the following substitutions:&#xA;$$\frac{\partial a}{\partial \varepsilon} = \frac{\mu}{2\varepsilon^2}$$&#xA;$$\frac{\partial^2 a}{\partial \varepsilon^2} = -\frac{\mu}{4\varepsilon^3}$$&#xA;$$\frac{\partial \varepsilon}{\partial v_i} = v_i,\;i=p,q,w$$&#xA;$$\frac{\partial^2 \varepsilon}{\partial v_i \partial v_j} = \delta_{ij} = \left\{\begin{array}{l}1, &amp;amp; i=j\\0, &amp;amp; i \neq j\end{array}\right., \; i,j = p,q,w$$&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Now, finally&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;$$\delta a = \frac{\mu}{2\varepsilon^2} \Big(v_p \delta v_p + v_q \delta v_q + v_w \delta v_w \Big) + &#xA;+ \frac{1}{2}\left\{ \frac{\mu}{2 \varepsilon^2} \Big[&#xA;(\delta v_p)^2 +&#xA;(\delta v_q)^2 +&#xA;(\delta v_w)^2&#xA;\Big]&#xA;- \frac{\mu}{4 \varepsilon^3} \Big[&#xA;v_p^2(\delta v_p)^2 +&#xA;v_q^2(\delta v_q)^2 +&#xA;v_w^2(\delta v_w)^2 +&#xA;2 v_p v_q(\delta v_p \delta v_q) +&#xA;2 v_p v_w(\delta v_p \delta v_w) +&#xA;2 v_q v_w(\delta v_q \delta v_w)&#xA;\Big]\right\} &#xA;+\epsilon(\|\delta \mathbf{v}\|^3)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Writing this more compactly:&#xA;$$\delta a = \frac{\mu}{2\varepsilon^2} \left( \mathbf{v} \cdot \delta \mathbf{v} + \frac{\|\delta \mathbf{v}\|^2}{2} \right) - \frac{\mu}{4 \varepsilon^3} \Big(  \mathbf{v} \cdot \delta \mathbf{v} \Big)^2 +\epsilon(\|\delta \mathbf{v}\|^3)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Important consequences: at second-order accuracy, the previous assertion regarding prograde/retrograde &lt;strong&gt;is not applicable&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Final thoughts&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Though there are a few hiccups involved in applying this to the three orientation elements due to some degeneracy in the rotation formalism defined by Euler angles, this answer should provide all the tools you need to calculate what you are asking.  It is a simple -- though tedious -- matter of simply following the Taylor expansion for each of the remaining elements and applying coordinate transformations where appropriate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This should also provide a healthy illustration for why nobody really does second-order expansions.  The equations get nasty very quickly, with very little benefit to offer from a numerical analysis standpoint.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2017-03-16T02:37:24.063" LastActivityDate="2017-03-16T02:37:24.063" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="20662" PostTypeId="1" AcceptedAnswerId="20693" CreationDate="2017-03-15T20:25:46.963" Score="5" ViewCount="1148" Body="&lt;p&gt;I'm writing code to simulate planetary and lunar orbits in a heliocentric reference plane. I'm using E.M. Standish's &lt;em&gt;&lt;a href=&quot;http://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Keplerian Elements for Approximate Positions of the Major Planets&lt;/a&gt;&lt;/em&gt; as my reference and I think it is a straight path to simulate planetary motions from the equations and data given there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I haven't found is a similar reference that has data for the moons of those planets. I'm assuming that I can use those same formulae to compute the planet-centric positions of its moons and then translate them to heliocentric coordinates by adding the planet's heliocentric location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Additional information:&lt;/strong&gt; The document referred to presents one (time unvarying) set of Keplerian elements which can be applied to the supplied equations to simulate planetary orbits. The Keplerian elements have been best fit to the actual orbits over a given time range. For some of the planets the elements have a time dependent component which has also been fit to an analytic formula. The entire scheme is thus governed by a small set of constants driving a formula with time as the only input variable.&lt;/p&gt;&#xA;" OwnerUserId="16666" LastEditorUserId="16666" LastEditDate="2017-03-16T17:06:35.517" LastActivityDate="2017-05-22T21:16:37.150" Title="Keplerian approximations for moons as well as planets" Tags="|orbital-mechanics|moons|orbital-elements|simulation|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20664" PostTypeId="2" ParentId="20662" CreationDate="2017-03-15T22:39:16.010" Score="3" Body="&lt;p&gt;Use &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;Horizons Web-interface&lt;/a&gt;, with options such as the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/C1UOn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C1UOn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-03-15T22:39:16.010" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20666" PostTypeId="2" ParentId="18251" CreationDate="2017-03-16T01:41:06.723" Score="7" Body="&lt;p&gt;Yet another approach to the answer is to look at the Soviet N-1 rocket, which evolved in fact to attempt a Soviet version of the Lunar Orbit Rendezvous strategy accomplished by Apollo. The Soviets lacking a good developed hydrogen engine, it was all kerlox.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their approach took several measures to greatly lighten the spacecraft involved, a version of Soyuz and a lander called LK that relied on a kerosene-oxygen &quot;crasher&quot; to take the LK down from orbital speed to a half kilometer or so above the landing site--and more importantly than losing altitude, took off most of the orbital speed. Thus the LK itself was a single stage unlike the American LM, basically a smaller analog of the LM ascent module with extra fuel for final descent, hovering and landing. (It also had a complete backup engine in case its main engine failed, of the same thrust but with no throttle ability, for abort or as a backup ascent engine). The LK could only handle one cosmonaut, for a much briefer stay on the lunar surface, and upon returning to the Soyuz in orbit (and to board it to descend) the cosmonaut had to spacewalk--no transfer tunnel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using more efficient pumped kerosene-oxygen engines than the American pressure fed hypergolics, the Soviet approach required a lighter stack to be launched to the Moon, but even so due to the lower efficiency of kerosene compared to hydrogen, the fourth stage that would serve to send that stack to encounter the moon had to be larger in proportion. Counting the kerosene-oxygen &quot;D&quot; block as a fifth stage, the attempt to get the Lunar mission done with a single launch was very marginal despite the fact that the five-stage rocket outmassed the Saturn V by a considerable margin. To try to lighten it enough to enable the mission they took all sorts of dubious weight-saving expedients, such as removing most of the telemetry. The earliest version of N-1 was not expected to boost more than 45 or so tons to low Earth orbit. Between adding engines, raising the dimensions a bit, and these weight trimming and propellant maximizing expedients, it was to put 95 tons into LEO in the Lunar version that was tested several times. Each attempt led to failure, and had they succeeded in getting a manned Lunar stack on the fourth stage, to inject it on to the Moon, I fear the corner cutting would have spelled disaster sooner or later, probably sooner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Had they gone over to a two-launch strategy and made fewer heroic efforts to maximize payload and made it somewhat more robust, I think they could have pulled off a Moon landing with margin to spare--indeed the escape/survival modes enabling the crew to return despite major failures might have been far superior to Apollo with a two launch plan. An Alternate History timeline I much enjoyed was written several years ago on this premise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But looking at the huge size, numerous engines and many stages of the N-1 as built, compared to a much more meagre tonnage to low lunar orbit, illustrates the huge advantage the Americans enjoyed in developing hydrogen upper stages. With kerosene alone, it would have been possible to achieve greater efficiencies than our program accomplished with that fuel mix--the Soviet pumped kerosene engines in their N-1/LK program were all notably more efficient than the best American kerosene engines, though none came close to matching the sheer mighty thrust of the US F-1 engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But even so, with markedly superior ISP kerosene engines, the Soviets, or Americans foregoing development of hydrogen engines, would have had to launch a much greater tonnage off of launch pads to achieve comparable results in terms of a moon landing. If the goal were merely to build space stations or the like, the tradeoff is much more reasonable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But then again, one disadvantage hydrogen burning rockets have is the difficulty of storing the hydrogen for a long time. In space, it is not that difficult to keep LOX liquid and stored for a long time, but hydrogen boils off (or the tanks explode!) Centaur has demonstrated that one can retain enough fuel after several days to be worthwhile, so in principle the Apollo stack might have been braked to low Lunar orbit insertion by such a stage with some savings in weight, but the design was frozen in the early sixties around using nothing but pressure fed hypergolic engines after TLI, and TLI would be initiated just hours after reaching the parking orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the drawback of hydrogen boiling off is not a problem if one is using the hydrogen to put something into low Earth orbit or plans to use it pretty soon after that. Thus to argue ker-lox would be better than using hydrogen upper stages one would have to look closely at the structural fixed mass penalties that hydrogen storage imposes. But with good design these are not terrible; look at the STS fuel tank, that massed 36 tons dry but held something like 750 tons of oxygen and hydrogen. Or, the S-IV third stage of the Saturn V (also the second stage of Saturn 1B) which massed 9 tons or so dry, including the engine, but held 120 or more tons of propellant-these are dry weight fractions well under 1/10, while few ker-lox or hypergolic stages actually used were dramatically lower than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another penalty of using hydrogen is that the ratio of thrust to engine weight is also inferior to what a similar state of the art investment can achieve with less energetic but cooler-burning and denser propellants. The Shuttle SSME had a good thrust/weight ratio compared to first generation kerosene or hypergolic engines, but this was accomplished by heroic and expensive to develop, build and maintain methods involving extremely high pressures and temperatures. The tradeoff between efficiency and thrust is inherent in the basic physics; to get a faster exhaust one operates at higher temperatures; to get decent thrust against high sea level atmospheric pressure one uses high pressure; a lower energy per kilogram propellant mix burns cooler and trades off higher mass flow for lower power for the same thrust. If thrust is the goal, it often makes sense to burn something cooler and use simpler, cheaper machinery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, that initial boosting off the launch pad, the job of a first stage, is a task accomplished by use of massive thrust, and the benefits of using hydrogen are most marginal there, while the advantages of less ambitious fuel mixes in simpler but stronger engines are greatest, especially factoring in easier storage. Vice versa, once raised off the ground, out of the lower atmosphere and boosted to modest speeds that buy time for weaker but more efficient hydrogen based engines to work, at much lower thrusts orbit can be achieved with lower overall upper stage masses and fewer stages. Thus the design of the Saturn V was a good synergy. Note that the first stage is more massive by far than any upper stages--Saturn V had roughly a factor of 5 jump between the third and second stages, but the first stage was something between 3 and 4 times the mass of the entire upper stack, of two stages and a 45 ton Lunar stack all put together. To move this huge total mass engines creating tremendous thrust were required; even if the great challenge of making hydrogen engines that could produce that thrust were met, the drawbacks of storing huge quantities of hydrogen versus compact volumes of kerosene would have offset much of the advantage to be gained by higher efficiency faster exhaust speeds. They economized on these where it counted the most, in the first stage. A mixed stack, despite the problems of managing several types of propellant instead of just two, was the superior way to go. &lt;/p&gt;&#xA;" OwnerUserId="16823" LastActivityDate="2017-03-16T01:41:06.723" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20667" PostTypeId="2" ParentId="10466" CreationDate="2017-03-16T05:51:03.757" Score="0" Body="&lt;p&gt;The answer is simple - robotics are not yet advanced enough to build the required facilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The moon has more going for it than Mars as a base facility.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is close, has large caverns shielded from radiation &amp;amp; small meteors, it has iron, helium-3, possibly water, platinum &amp;amp; rare earths.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to build facilities on the moon that can actually be used for manufacturing we would need an adequate level of technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We would need telemetric &amp;amp; autonomous robots to build space craft, launch facilities and fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is technically feasible today but not comfortably - we are very close.&lt;/p&gt;&#xA;" OwnerUserId="18939" LastActivityDate="2017-03-16T05:51:03.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20669" PostTypeId="1" AcceptedAnswerId="20692" CreationDate="2017-03-16T08:19:25.793" Score="6" ViewCount="1847" Body="&lt;p&gt;I am looking for a free tool which can generates pork chop plots. I'm doing this to compare the output of my tool with one of a third party. Additionally, if anyone has specific test cases on solving Lambert's boundary value problem, I would be more than eager to have a look.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is purely for validation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following figure is an Earth to Mars PCP I generated just earlier.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/02iUN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/02iUN.png&quot; alt=&quot;PCP Earth to Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-03-21T00:04:21.093" Title="Pork chop plot generator and Lambert solver test cases" Tags="|interplanetary|mission-design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20670" PostTypeId="1" AcceptedAnswerId="20675" CreationDate="2017-03-16T13:05:31.640" Score="1" ViewCount="92" Body="&lt;p&gt;What are some good space related websites (or other resources) that have information about past and future missions?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for data such as mission launch date, type of rocket, launch site, type of mission (manned/unmanned) etc.&lt;/p&gt;&#xA;" OwnerUserId="3456" LastActivityDate="2017-03-17T17:49:55.903" Title="Websites about past and future missions" Tags="|launch|rockets|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20672" PostTypeId="1" AcceptedAnswerId="20674" CreationDate="2017-03-16T15:18:06.473" Score="-4" ViewCount="161" Body="&lt;p&gt;I'm Studying 3rd Year Of Computer Science And Engineering. I Planned To Create A Space Application Like Stellarium For Final Year Project. I'm Space Enthusiast.Please Help Me To Choose Which Language Is Best To Create A Space Application.  &lt;/p&gt;&#xA;" OwnerUserId="18946" LastActivityDate="2017-03-16T15:31:36.353" Title="What Languages Need To Create A Space Application Like Stellarium" Tags="|planetary-science|" AnswerCount="1" CommentCount="1" ClosedDate="2017-03-16T16:12:56.603" ContentLicense="CC BY-SA 3.0" />
  <row Id="20673" PostTypeId="2" ParentId="20653" CreationDate="2017-03-16T15:28:15.223" Score="18" Body="&lt;p&gt;As an assist to the current answers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Try imagining an analogy:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First you hold a rope with your arms held out horizontally and you spin. The result would be the rope spinning with you, tending towards the horizontal the faster you go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now imagine spinning around but with the rope above your head. It will only fall down onto your head. You would need a rigid body to stand upright without spinning.&lt;/p&gt;&#xA;" OwnerUserId="18948" LastActivityDate="2017-03-16T15:28:15.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20674" PostTypeId="2" ParentId="20672" CreationDate="2017-03-16T15:31:36.353" Score="4" Body="&lt;p&gt;There's not much philosophy to this. Pretty much any reasonable high-level language will do. You'd need the language to provide graphics libraries, preferably a 3D graphics, so you can perform the drawing easier, and a database access to keep the catalog of stars, plus basic user interface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And about every modern programming language provides these. You might consider C#, Java, Python, C++ - you should base your decision upon which you know best and feel most comfortable with, as they all provide the necessary functionality to a very similar degree. You'll likely need to know SQL as well to interact with the database.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-16T15:31:36.353" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20675" PostTypeId="2" ParentId="20670" CreationDate="2017-03-16T18:51:54.510" Score="2" Body="&lt;p&gt;Past missions: &lt;a href=&quot;http://www.spacelaunchreport.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spacelaunchreport.com/&lt;/a&gt;&#xA;Launch schedule: &lt;a href=&quot;https://spaceflightnow.com/launch-schedule/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://spaceflightnow.com/launch-schedule/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11046" LastActivityDate="2017-03-16T18:51:54.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20676" PostTypeId="1" CreationDate="2017-03-16T21:43:19.783" Score="2" ViewCount="222" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/20624/12102&quot;&gt;This&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/20657/12102&quot;&gt;this&lt;/a&gt; answer have addressed various issues related to the new Autonomous Flight Safety System, and the second answer addresses one potential benefit being a reduction in turn-around time for range safety configuration between two launches of substantially different vehicles or launch providers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interesting &lt;a href=&quot;https://space.stackexchange.com/questions/20652/#comment55868_20657&quot;&gt;comment there&lt;/a&gt; highlights the distinction between the benefits due to automation and the benefits due to standardization. Part of the issue here is that my questions &lt;em&gt;are sourced from less technical sources and enquire about the meaning of quoted statements&lt;/em&gt; rather than ask about official policies or documents, so I think looking into this distinction is really educational and clarifying and does not raise any question of inconsistency or controversy (necessarily):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It sounds like most of the advantages have nothing to do with the system being automatic, but rather with the system being standardized, which apparently it wasn't before. Or from a slightly different perspective: removing the need for it to be standardized since it isn't actually interfacing with the outside world anymore (except for the format of those &quot;data files&quot; mentioned, which can and probably will be converted to a non-standard, flight-computer-specific format before uploading them to the rocket)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is the overall direction of launch range safety &lt;em&gt;both autonomy and standardization&lt;/em&gt;? Will standardization include both &lt;em&gt;hardware&lt;/em&gt; as well as procedure? Will there ultimately be a &lt;strong&gt;standardized&lt;/strong&gt; autonomous black box installed in many/most/all large launch vehicles that can blow them up and presumably the initiate escape of crew first?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; The scope of this and the two previous questions are primarily the US since they are sourced from information about range safety in the US. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-08-09T14:30:19.913" LastActivityDate="2019-08-09T14:30:19.913" Title="Range safety - is the goal both autonomy and standardization - and would it include the &quot;black box&quot; in the rocket?" Tags="|nasa|range-safety|autonomy|self-destruct|standardization|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20677" PostTypeId="2" ParentId="20653" CreationDate="2017-03-16T22:09:01.077" Score="-1" Body="&lt;p&gt;Keep in mind that the earth does not spin on an exact axis but rather has a 'wobbling' effect. You'd need to account for that when setting this up.&lt;/p&gt;&#xA;&lt;p&gt;An excerpt from a &lt;a href=&quot;http://www.cnbc.com/2016/04/09/scientists-just-figured-out-whats-causing-earth-to-wobble.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;CNBC article talking with NASA engineers about the shake that's happening to the earth&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Since 2000, the Earth's axis has jumped eastward by about 7 inches a year, a &amp;quot;massive swing,&amp;quot; Adhikari said. Another pattern of wobbling that occurs every six to 14 years has been vexing scientists for more than a century. The researchers examined both in their study.&lt;/p&gt;&#xA;&lt;p&gt;The movements of the Earth's axis are crucial to understand, since they affect the performance of satellites and global positioning systems. The polar axis also could become a powerful indicator for scientists studying climate change, Adhikari said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="18957" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-16T22:25:46.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20678" PostTypeId="1" CreationDate="2017-03-16T23:47:20.887" Score="5" ViewCount="200" Body="&lt;p&gt;What would you call the method of joining fabric to the glove and helmet rings in a spacesuit by clamping it between two metal rings?&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-18T14:30:34.833" LastActivityDate="2017-03-20T03:48:19.950" Title="What is the method of joining fabric to the glove and helmet rings in a spacesuit called?" Tags="|spacesuits|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20680" PostTypeId="2" ParentId="20653" CreationDate="2017-03-17T11:59:55.740" Score="2" Body="&lt;p&gt;The idea is not as stupid or absurd as it seems at first glance (to me, at least). The elevator's cable would not extend vertically though as in a space elevator on the equator, but instead at an angle, probably almost horizontally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps it helps to imagine moving the base of an existing equatorial space elevator. Moving the base north would pull the counterweight above (not &lt;em&gt;at&lt;/em&gt;) the geostationary orbit north, off the equatorial plane. (Let's suppose a move which is slow enough to neglect Coriolis forces.) Earth's gravity would increasingly pull at a (small) angle to the tether, creating a displacement force parallel to the earth axis. The counter-weight's latitudinal position of equilibrium would therefore be south of the base's position. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When moving closer to the pole, the base would also move closer to the earth's rotational axis, pulling the counter-weight in. The radial distance traveled eventually is the earth's radius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the rotational speed and the centripetal force of the counter-weight are linked to the orbit's radius, the tether would have to be lengthened as a compensation. (As others have mentioned, this probably increases the maximum forces on the tether, which could be a problem given that the load in the optimal equator case is already close to the specs of the best proposed materials.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a result, an ideal, weightless tether would leave the north pole almost, but not quite horizontally, because the counter-weight would be a bit south of the pole. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, in this scenario the space elevator could rotate at any speed (and, coupled to the speed, orbit) the tether can sustain; it is not limited to geo-syncronicity any longer. Other than geostationary orbits would mean that the tether circles slowly around its anchor, which is possible only close to the poles. But this is only a thought-experiment with a weightless tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All realistic tethers would be far from weightless (in fact, the tether's own weight is the ultimate constructive problem); therefore it would bend substantially under earth's gravity and form some kind of catenary, as anybody who has ever flown a kite will easily understand. Consequently a base at the very pole would be quite pointless; the tether would lie on the ground anyway for hundreds or thousands of kilometers. But a base somewhere on the northern or southern hemisphere is not immediately unreasonable or absurd; it will just not be vertical.&lt;/p&gt;&#xA;" OwnerUserId="8922" LastEditorUserId="8922" LastEditDate="2017-03-17T12:39:19.177" LastActivityDate="2017-03-17T12:39:19.177" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20681" PostTypeId="2" ParentId="20653" CreationDate="2017-03-17T13:30:33.293" Score="5" Body="&lt;h2&gt;What is a space elevator, anyway?&lt;/h2&gt;&#xA;&lt;p&gt;The usual form of a 'space elevator' is a thing in geostationary orbit to which a cable is attached between the thing and the Earth surface, along with some adjustment or counterbalance to keep the centre of mass of the structure at the geostationary altitude (or it will no longer be geostationary).&lt;/p&gt;&#xA;&lt;p&gt;The geostationary orbit is the only one in which an orbiting body (like the thing at the top of the elevator) stays a roughly constant distance from a point on the Earth's surface, where the cable would terminate. If the anchor point has to move to account for a non-geostationary orbit, that makes things either a bit harder (a floating base station that drifts around the ocean) or impossibly hard (an aircraft that must be kept continually fuelled and in motion to follow the orbit's groundtrack and counteract the drag on tens of kilometres of cable), and that's for subsonic relative motion on the Earth.&lt;/p&gt;&#xA;&lt;h2&gt;But what about the poles! Well, it would be different.&lt;/h2&gt;&#xA;&lt;p&gt;This would be a very different kind of elevator. Instead of a lump in orbit and a cable hanging down (mostly), this would have to be an entirely ground-supported structure - there is no geostationary orbit over the poles, unless you count plummeting or have a plan for a huge hole through the Earth's core. So this would be a great space tower. On the plus side, it would only need to reach some height in outer atmosphere (maybe 160 km), not geostationary distances (36,000 km). On the down side, you couldn't just release a thing at the top and have it orbit, it would need to be fired off to reach the high LEO orbital speeds (~7.8 km/s).&lt;/p&gt;&#xA;&lt;p&gt;Having started the tower discussion at the pole, it needn't remain there; an edge-of-space tower could be positioned anywhere, albeit the latitude would determine the initially accessible orbits. Thus something closer to the equator would be likely more useful.&lt;/p&gt;&#xA;&lt;h2&gt;So, is it harder to build a 160 km tower or a 36,000 km cable?&lt;/h2&gt;&#xA;&lt;p&gt;In practise the longest continuous cable we've ever made is only 5 km long (&lt;a href=&quot;http://www.kingpin-manufacturing.co.uk/blog/the-worlds-longest-cables/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.kingpin-manufacturing.co.uk/blog/the-worlds-longest-cables/&lt;/a&gt;), and the tallest structure is 0.8 km. Apart from the Burj Khalifa, the next tallest structure was a guyed tower, so it seems more feasible for that type of structure to be extended several times over than to invent cables that can support their own weight over 36,000 km.&lt;/p&gt;&#xA;&lt;h2&gt;We'll need the tower anyway&lt;/h2&gt;&#xA;&lt;p&gt;Even the cable version would need a tower on the ground to provide a counter to the effects of wind, and in fact to avoid dragging the orbital mass off-course (which would be hard to correct and put strain on the whole cable), the sensible (!) thing would be to build a tower quite high in the atmosphere so that the cable doesn't suffer these lateral forces.&lt;/p&gt;&#xA;&lt;p&gt;Which leads us to the conclusion that whichever kind of elevator we want, we have to build a tower most of the way to space.&lt;/p&gt;&#xA;&lt;h1&gt;Just how tall would the tower need to be? About 50 km.&lt;/h1&gt;&#xA;&lt;p&gt;At 160 km, orbits are just about stable. But we could build one to 120 km and only add a relatively small amount of additional drag to overcome with a rocket. For a short elevator like this there is inevitably a tradeoff between the height of the tower and the size of the rocket needed to reach orbit from the top. Given that you need a sizeable rocket anyway, there's a reasonable argument for just building one to 50 km (the top of the ozone layer, to avoid environmental catastrophe).&lt;/p&gt;&#xA;&lt;h1&gt;Step back, look at the balloons.&lt;/h1&gt;&#xA;&lt;p&gt;But wait, all we're doing now is hoiking (technical term) a reasonably large rocket to an altitude of 50 km so we can fire it. So why build a tower? &lt;a href=&quot;https://en.wikipedia.org/wiki/High-altitude_balloon&quot; rel=&quot;nofollow noreferrer&quot;&gt;According to Wikipedia&lt;/a&gt;, we have sent balloons to 53 km before. And expensive though balloons are, they're a lot less expensive than a 50 km steel tower.&lt;/p&gt;&#xA;&lt;h1&gt;So, it's cables or balloons. And the cable version is iffy.&lt;/h1&gt;&#xA;&lt;p&gt;So if you want to build an elevator at the pole, you might as well just use a balloon, and you can use it anywhere. I would add the caveat that the geostationary elevator is no panacea; it only makes geostationary orbits easy, and once you've put up all the cameras and telecoms kit you're going to want different orbits anyway.&lt;/p&gt;&#xA;&lt;h1&gt;What would we do with a space elevator, apart from satellite launches?&lt;/h1&gt;&#xA;&lt;p&gt;One question we can ask is what else we might do with cheaper lifting to space. Some imagine space holidays to the sky anchor in geostationary orbit. But that would turn out to be a slightly underwhelming holiday: If you actually use a vehicle that climbs the cable, you're going to be limited to maybe 150 kph; to permit the length of cable it will be fairly thin and not very robust, and the downside of tearing it is fairly huge. So speeding up it at speeds you wouldn't do on a highway in a car is unrealistic. But at 150 kph, 30,000 km is 200 hours, or over 8 days. The first hour of the holiday would be zipping up through the atmosphere. The next week would be like being on an increasingly remote cruise; the Earth gets smaller and smaller and space gets increasingly vast.&lt;/p&gt;&#xA;&lt;h2&gt;Can you go faster up the cable?&lt;/h2&gt;&#xA;&lt;p&gt;To put people up to the sky anchor really you'd need to send them up in a rocket, or perhaps zip them up near the cable on a rail gun. Either way it's either much more expensive (which was the whole point of the elevator) or requires a more complex (i.e. impossible) cable.&lt;/p&gt;&#xA;&lt;h2&gt;Well what &lt;em&gt;would&lt;/em&gt; be a fun space holiday?&lt;/h2&gt;&#xA;&lt;p&gt;Let's go back to the short 50 km tower version of the elevator (at the pole, if you like). A trip could happily involve being sent at less ridiculous speeds up the tower, and then being gleefully shot into space on an orbital or suborbital path; a few hours to either encircle the globe or be flung to the other side, ending by gliding down to the ground on a winged lander.&lt;/p&gt;&#xA;&lt;h2&gt;Wait, isn't that global space travel?&lt;/h2&gt;&#xA;&lt;p&gt;Basically, yes. A suborbital flight could pitch you from one side of the world to the other in a couple of hours from the top of the tower. You'd probably want another tower (or perhaps just a balloon) at the other end or that's a particularly boring return journey.&lt;/p&gt;&#xA;&lt;p&gt;On the plus side, we've managed to get above the ozone layer before we play with rockets, which is a great improvement on surface-launched orbital and suborbital vehicles. And with technology that is real now, not imagined for the future.&lt;/p&gt;&#xA;" OwnerUserId="519" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-17T13:30:33.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20682" PostTypeId="1" AcceptedAnswerId="20683" CreationDate="2017-03-17T14:38:04.730" Score="7" ViewCount="768" Body="&lt;p&gt;In one BBC article called &quot;Etna escape: 'Pelted with the deadly, hot debris'&quot;, there was an image with the following description: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AHQEz.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AHQEz.png&quot; alt=&quot;Europe&amp;#39;s Sentinel-2a satellite pictured Thursday&amp;#39;s lava flow from space &quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Europe's Sentinel-2a satellite pictured Thursday's lava flow from space &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can somebody explain the blue region of the picture? &#xA;It could be a blend of thermal and visible satellite map?&lt;/p&gt;&#xA;" OwnerUserId="18964" LastActivityDate="2017-03-17T14:50:32.063" Title="Blue region in satellite image" Tags="|artificial-satellite|image-processing|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20683" PostTypeId="2" ParentId="20682" CreationDate="2017-03-17T14:50:32.063" Score="10" Body="&lt;p&gt;According to &lt;a href=&quot;https://baltic.ai/etna-erupts/&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The red hot lava flowing from Mount Etna can be seen clearly in the image from Sentinel-2A. The surrounding snow has been processed in blue to distinguish from the clouds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Sentinel satellites use &lt;a href=&quot;http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-2/Instrument&quot; rel=&quot;noreferrer&quot;&gt;multispectral imagers&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;13 spectral channels incorporating four visible and near-infrared bands at 10 m resolution, six red-edge/shortwave-infrared bands at 20 m and three atmospheric correction bands at 60 m.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it's not so much a blend of visible and infrared channels, but a visible-light image with a false-color overlay based the composition of the soil (obtained by taking spectrograms).&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-03-17T14:50:32.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20684" PostTypeId="1" CreationDate="2017-03-17T14:51:29.480" Score="0" ViewCount="375" Body="&lt;p&gt;I have broken down the process plan to various subsystems( including a Recovery system)but would want to know from the community what are the most important aspects of the rocket that one must focus on...&lt;/p&gt;&#xA;" OwnerUserId="18876" LastActivityDate="2017-03-21T00:13:45.047" Title="What are the most important aspects to look into when building an experimental sounding rocket?" Tags="|rockets|sounding-rocket|" AnswerCount="2" CommentCount="0" ClosedDate="2017-03-17T19:18:26.327" ContentLicense="CC BY-SA 3.0" />
  <row Id="20685" PostTypeId="2" ParentId="20684" CreationDate="2017-03-17T15:27:09.563" Score="2" Body="&lt;h2&gt;&lt;strong&gt;SAFETY&lt;/strong&gt;&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Rockets are dangerous.  Rockets that are big and go high are very dangerous.  You can easily kill yourself or others.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2017-03-17T15:27:09.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20686" PostTypeId="2" ParentId="20684" CreationDate="2017-03-17T15:27:27.227" Score="3" Body="&lt;p&gt;This is getting close-votes for being opinion-based, but I guess most people here will agree about two primary things; both &quot;number one&quot; because neglecting either is the end of the project. Law, and safety. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Law. Countries have different regulations regarding rocketry, starting from complete ban, through necessity to be a member of a registered organization, get permit for any launches and tests, and so on. There is very little technical difference between a sounding rocket and a missile, so building one without proper permits can very easily get qualified as terrorist activity.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Safety. I'd recommend the lecture of &lt;a href=&quot;https://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ignition&lt;/a&gt; on how that kind of work goes. Accidents happen, and will happen. If by the end of your project you still have your eyebrows and all the fingers, count yourself lucky. Regardless, safety should be a priority, because with meticulous safety accidents will be rare and not likely to be serious. With anything less... say good-bye to your fingers. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-03-21T00:13:45.047" LastActivityDate="2017-03-21T00:13:45.047" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20688" PostTypeId="2" ParentId="20027" CreationDate="2017-03-17T16:49:56.653" Score="7" Body="&lt;p&gt;There are many things going on, some of which depend on the exact variety of gridded ion thruster. I'm referencing chapter 6 of Spacecraft Systems Engineering 4th Edition by Fortescue, Swinerd, and Stark. It's a very affordable textbook which gives a good overview of just about any common system found on unmanned spacecraft. It is not a textbook on electric propulsion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first thing to realize is that the ionized propellant is electrically conductive. This is true for the same reason that ion-bearing liquids are electrically conductive. This details of this conductivity are not important to my answer. &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/ctpp.19840240304/abstract&quot; rel=&quot;noreferrer&quot;&gt;http://onlinelibrary.wiley.com/doi/10.1002/ctpp.19840240304/abstract&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The conductivity of the ionized propellant allows current to flow from the negative grid to the positive grid. This is not the whole story though. The propellant becomes ionized within the chamber, and the resulting free electrons are attracted to the positive grid, which creates a source of inflowing current. These electrons need to be continually removed to keep the grid positively charged. There are also electrons which are carried from the negative grid into space by the propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The complete story of how the ions move around is more complicated, but I think this answers your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, the current flow is not a requirement, it's a consequence.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-20T04:08:19.137" LastActivityDate="2017-03-20T04:08:19.137" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20689" PostTypeId="1" AcceptedAnswerId="31867" CreationDate="2017-03-17T17:29:02.350" Score="8" ViewCount="782" Body="&lt;p&gt;In SpaceX's &lt;a href=&quot;https://www.youtube.com/watch?v=lZmqbL-hz7U&quot; rel=&quot;nofollow noreferrer&quot;&gt;Echostar XXIII Hosted Webcast&lt;/a&gt; after &lt;code&gt;T+ 00:03:50&lt;/code&gt; they mentioned the &quot;re-radiation system&quot; as a part of the payload fairing that increased it's value and possibly added to the benefit of &lt;a href=&quot;https://space.stackexchange.com/a/20010/12102&quot;&gt;recovery of the fairing post-launch&lt;/a&gt;, rather than just &quot;ditching it&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is it, why is it so valuable, and &lt;em&gt;why wouldn't&lt;/em&gt; just a low tech, low cost, not-worth-mentioning-in-the-video solution like a little piece of coax with a dipole on each end be enough? Any radios on the satellite would be optimized for strong outgoing and weak incoming signals as they would be in orbit tens of thousands of kilometers away from a ground station, so just some weak coupling would be enough for communication over a distance of few tens of meters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; An example of an RF window (another alternative solution) is described in Section 4.1.3 of &lt;a href=&quot;http://www.ilslaunch.com/sites/default/files/pdf/PMPG%20Section%204.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Proton Launch System Mission Planner’s Guide; SECTION 4; Spacecraft Interfaces&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; The video is set to start at 21:29 (&lt;code&gt;T+ 00:03:30&lt;/code&gt;) so you can see the fairing separate. The re-radiation system of the fairing is mentioned after another 20 seconds or so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/lZmqbL-hz7U?start=1289&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-18T00:19:42.113" LastActivityDate="2018-11-08T07:08:10.747" Title="What is a &quot;re-radiation system&quot;?" Tags="|spacex|falcon-9|recovery|fairings|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20690" PostTypeId="2" ParentId="20249" CreationDate="2017-03-17T17:46:49.667" Score="2" Body="&lt;p&gt;An approach could be based on analyzing energy and angular momentum changes. For example, energy can be expressed as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mathcal{E} = \frac{v^2}{2} - \frac{\mu}{r}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Equivalently:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mathcal{E} = \frac{\boldsymbol{v}^\mathrm{T}\boldsymbol{v}}{2} - \frac{\mu}{r}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means that the energy after an impulsive maneuver $\boldsymbol{\Delta v}$ is given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mathcal{E}_{+} = \frac{\left(\boldsymbol{v}+\boldsymbol{\Delta v}\right)^\mathrm{T}\left(\boldsymbol{v}+\boldsymbol{\Delta v}\right)}{2} - \frac{\mu}{r}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Subtracting the two quantities we can obtain the total energy change&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mathcal{E}_+-\mathcal{E} = \Delta \mathcal{E} = \frac{\left(\boldsymbol{v}+\boldsymbol{\Delta v}\right)^\mathrm{T}\left(\boldsymbol{v}+\boldsymbol{\Delta v}\right)}{2}&#xA;-\frac{\boldsymbol{v}^\mathrm{T}\boldsymbol{v}}{2}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice that the $\mu/r$ term is eliminated because an impulsive maneuver does not change the position. After some simplification we arrive to an expression for the change in orbital energy due to an impulsive maneuver:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta \mathcal{E} = \boldsymbol{v}^\mathrm{T}\boldsymbol{\Delta v} + \frac{\left\|\boldsymbol{\Delta v}\right\|^2}{2}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, how can we decompose this change in radial/tangential/normal (RTN) components? First, we define the principal axes of the RTN frame:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\boldsymbol{i}_r = \frac{\boldsymbol{r}}&#xA;{\left\|\boldsymbol{r}\right\|},\quad&#xA;\boldsymbol{i}_t =  \frac{\boldsymbol{i}_r\times\boldsymbol{i}_n}&#xA;{\left\|\boldsymbol{i}_r\times\boldsymbol{i}_n\right\|},\quad&#xA;\boldsymbol{i}_n = \frac{\boldsymbol{r}\times\boldsymbol{v}}&#xA;{\left\|\boldsymbol{r}\times\boldsymbol{v}\right\|}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and use them to define a rotation matrix from the RTN frame to the inertial frame:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\boldsymbol{Q} = \left[&#xA;\boldsymbol{i}_r\quad &#xA;\boldsymbol{i}_t\quad&#xA;\boldsymbol{i}_n&#xA;\right]&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this infrastructure we can now express the change in energy due to a $\boldsymbol{\delta v}$ maneuver; I use $\delta$ instead of $\Delta$ to denote that the maneuver is expressed in the RTN frame:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\boldsymbol{\delta v} = \left[&#xA;\Delta v_r\quad&#xA;\Delta v_t \quad&#xA;\Delta v_n&#xA;\right].&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our expression is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta \mathcal{E} = \boldsymbol{v}^\mathrm{T}&#xA;\boldsymbol{Q}\boldsymbol{\delta v} + \frac{\left\|\boldsymbol{\delta v}\right\|^2}{2}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can use that expression to express the change in energy due to individual changes in the impulsive maneuver:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta \mathcal{E} = \Delta \mathcal{E}_r + \Delta \mathcal{E}_t + \Delta \mathcal{E}_n&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where, for the subindex $\gamma\in\left\{r, t, n\right\}$, each component is given by: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\Delta \mathcal{E}_\gamma = \boldsymbol{v}^\mathrm{T}\boldsymbol{Q}&#xA;\boldsymbol{\delta v}_\gamma + \frac{\left\|\boldsymbol{\delta v}_\gamma\right\|^2}{2}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vector $\boldsymbol{\delta v}_\gamma$ has all components but $\gamma$ set to zero.  You can work out the matrix product by hand if you want to.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Does this work?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Take an arbitrary orbit (canonical elements; $\mu=1$)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\boldsymbol{r} = \left[1, 2, 3\right],\quad&#xA;\boldsymbol{v} = \left[-0.3, -0.2, -0.1\right]&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;its orbital energy is $\mathcal{E} = -1.9726124191242439\times10^{-1}$. Applying an impulsive maneuver &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\boldsymbol{\delta v} = [1, -1, 2] \times 10^{-3}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;leads to the new energy&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\mathcal{E}_+ = \frac{\left\|\boldsymbol{v} + \boldsymbol{Q}\boldsymbol{\delta v}\right\|}{2} - \frac{\mu}{r} =&#xA;-1.9778736462262000\times10^{-1}. &#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The change is $\Delta\mathcal{E} = -5.2612271019561528\times10^{-4}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Our formula correctly decomposes this change as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\begin{align}&#xA;\Delta \mathcal{E}_r &amp;amp;= -2.6676124191242441\times 10^{-4}\\&#xA;\Delta \mathcal{E}_t &amp;amp;= -2.6136146828319081\times 10^{-4}\\&#xA;\Delta \mathcal{E}_n &amp;amp;= +1.9999999999999321\times 10^{-6}&#xA;\end{align}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which add up to $-5.2612271019561528\times10^{-4}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can use those changes in energy to directly map out the changes in semimajor axis. A similar approach can be used to map out changes in angular momentum, and thus decompose eccentricity and inclination.&lt;/p&gt;&#xA;" OwnerUserId="18918" LastActivityDate="2017-03-17T17:46:49.667" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20691" PostTypeId="2" ParentId="20670" CreationDate="2017-03-17T17:49:55.903" Score="1" Body="&lt;p&gt;You can also check out Blastoff, which allows you to filter upcoming launches by their launch location. It's just a cheap layer that I programmed on-top of the excellent launchlibrary.net API. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Url: &lt;a href=&quot;http://mrhuffman.net/projects/blastoff&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mrhuffman.net/projects/blastoff&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17423" LastActivityDate="2017-03-17T17:49:55.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20692" PostTypeId="2" ParentId="20669" CreationDate="2017-03-17T18:14:01.853" Score="10" Body="&lt;p&gt;I use a Kepler's Problem solver to test a Lambert's Problem solver. For example: if I want to generate a wide variety of test cases where I vary eccentricity and semimajor axis, I simply solve:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;M_0 + \sqrt{\frac{\mu}{a^3}}\Delta t = E - e \sin(E)&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for a wide variety of $(e, a, \Delta t)$ combinations. Having solved a multitude of Kepler's problems (which I can test immediately, because Kepler's equation must hold) I solve the corresponding Lambert's problem and verify that I obtain the same eccentric anomaly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following code explains this approach&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;&#xA;def kepler(a, e, mu=1.0):&#xA;    # EXPLICIT calculation of time of flight given eccentric anomaly&#xA;    # [in 0, 2 pi]&#xA;    n = np.sqrt(mu / pow(a, 3))&#xA;    E = np.linspace(0, 2 * np.pi, 360) # one point per degree&#xA;    t = (E - e * np.sin(E)) / n&#xA;    return (t, E)&#xA;&#xA;def cartesian(sma, ecc, ean, mu=1.0):&#xA;    smp = sma * (1 - ecc**2)&#xA;    energy = -mu / (2 * sma)&#xA;    f = 2 * np.arctan(np.sqrt((1 + ecc) / (1 - ecc)) * np.tan(ean / 2))&#xA;    r = smp / (1 + ecc * np.cos(f))&#xA;    v = np.sqrt(2 * (energy + mu / r))&#xA;    if f &amp;lt; 0:&#xA;        f = 2 * np.pi + f&#xA;    return np.array([r, v, f])&#xA;&#xA;# Let's test our algorithm for a given combination of sma/ecc. In a&#xA;# real test you would add more values, and you would consider multiple&#xA;# rev (you will get two solutions for most of the multirev cases)&#xA;sma = 1.0&#xA;ecc = 0.1&#xA;&#xA;# this computes time of flight and eccentric anomaly NOTICE THIS IS AN&#xA;# EXPLICIT CALCULATION&#xA;tof, ean = kepler(sma, ecc)&#xA;&#xA;# this transforms the quantities to cartesian vector magnitudes and&#xA;# unrestricted true anomaly (to compute transfer angle)&#xA;rvf = np.array([cartesian(sma, ecc, E) for E in ean])&#xA;&#xA;r = rvf[:, 0] &#xA;v = rvf[:, 1] &#xA;f = rvf[:, 2]&#xA;&#xA;# Here I'll just call my own Lambert solver (implemented in C++)&#xA;import ctypes&#xA;lib = ctypes.CDLL(&quot;libsolve.so&quot;)&#xA;vs = (ctypes.c_double * 2)()&#xA;N = len(tof)&#xA;max_diff = 0.0&#xA;&#xA;# This ugly loop simply compares every staged-pairwise combination for&#xA;# SINGLE REV (you can do multiple rev as well, you just need to&#xA;# increase TOF and f to be urestricted). You would attach your own&#xA;# Lambert solver here.&#xA;for n1 in xrange(N):&#xA;    for n2 in xrange(n1 + 1, N):&#xA;        lib.solve(ctypes.c_double(1.0), # gm&#xA;                  ctypes.c_double(r[n1]), # norm of first position&#xA;                  ctypes.c_double(r[n2]), # norm of second position&#xA;                  ctypes.c_double(f[n2] - f[n1]), # angle traveled&#xA;                  ctypes.c_double(tof[n2] - tof[n1]), # time of flight&#xA;                  ctypes.byref(vs)) # output&#xA;&#xA;        # compare the norm of velocities (you could decompose in&#xA;        # radial/tangential)&#xA;        diff = np.sqrt((v[n1] - vs[0])**2 + (v[n2] - vs[1])**2)&#xA;&#xA;        if diff &amp;gt; max_diff:&#xA;           max_diff = diff&#xA;&#xA;# report the maximum difference           &#xA;print max_diff&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Notice how you can calculate the time of flight &lt;em&gt;given&lt;/em&gt; a range of eccentric anomalies, eccentricity, and semimajor axis. In this case I'm considering single-rev only, but the generalization is straightforward. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the eccentric anomaly you can compute true anomaly and time of flight (the later via Kepler's equation, but you &lt;em&gt;do not need to solve iteratively&lt;/em&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That information is used to obtain range and speed at different points in the orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can then use all those combinations of range, speed, time of flight, and transfer angle (difference in true anomaly because the orbit is Keplerian) to test your Lambert solver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I ran a quick test with my own implementation of Gooding's algorithm and obtained a maximum difference of &lt;code&gt;8.76111591946e-14&lt;/code&gt;, perfectly reasonable for double-precision calculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, I generated a similar diagram - it compares well with yours. I would recommend you increase the axes on your arrival time to about 420 days past Jan 01, 2016 to ensure that your Lambert solver detects the transfer node (also, please notice our different baseline epoch for our arrival dates).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/feZ64.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/feZ64.jpg&quot; alt=&quot;Porkchop Earth-Mars Transfer&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18918" LastEditorUserId="18918" LastEditDate="2017-03-21T00:04:21.093" LastActivityDate="2017-03-21T00:04:21.093" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20693" PostTypeId="2" ParentId="20662" CreationDate="2017-03-17T18:26:53.240" Score="1" Body="&lt;p&gt;As you already mention, you cannot &lt;em&gt;directly&lt;/em&gt; propagate the motion of natural satellites in heliocentric coordinates. The reason is that they do not move in conic orbits with respect to the Sun; their motion does not describe a conic, but some sort of helix &lt;em&gt;centered around&lt;/em&gt; a conic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, you got the right approach: obtain the conic elements with respect to their planet, propagate, and finally express the resulting vectors in heliocentric frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now: the analytical description of the motion of natural satellites is orders of magnitude more difficult to describe than the motion of the planets. An osculating orbit (whether taken from Horizons or anywhere else) will describe the motion accurately only for a limited time period. This is the case for planets as well, but it is more marked in satellites.&lt;/p&gt;&#xA;" OwnerUserId="18918" LastActivityDate="2017-03-17T18:26:53.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20694" PostTypeId="1" CreationDate="2017-03-17T19:14:49.023" Score="7" ViewCount="771" Body="&lt;p&gt;Pretty self explanatory: down to what percentage can hybrid rocket engines be throttled to? And what fuel / oxidiser combinations allow for the greatest throttling?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm planning on using hybrids for a school project, which would rely pretty heavily on thrust control. (This is my first time posting a question; I hope this is the right place! If not, just tell me where to post and I'll move there)&lt;/p&gt;&#xA;" OwnerUserId="18688" LastActivityDate="2018-04-04T06:51:25.317" Title="How well can hybrid motors be throttled?" Tags="|design|engines|engineering|thrust|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20695" PostTypeId="2" ParentId="20694" CreationDate="2017-03-17T19:22:16.457" Score="-1" Body="&lt;p&gt;They throttle about as well as liquid propellant rockets. You still have one liquid propellant with which you can adjust the flow rate, and the solid propellant isn't going to react without it. You run into the same issues with nozzle expansion as you do with all-liquid rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One issue you can have is ongoing smoldering from atmospheric oxygen if you try to stop completely, but that's seldom something that you want to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As with liquid-only rockets you can in principle throttle it down as far as you want but run into significant performance losses (reduced exhaust velocity) as you throttle significantly below the optimum thrust for the engine.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-18T14:32:30.307" LastActivityDate="2017-03-18T14:32:30.307" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20696" PostTypeId="1" AcceptedAnswerId="20699" CreationDate="2017-03-17T20:34:37.780" Score="1" ViewCount="450" Body="&lt;p&gt;This is an image of the UK taken by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Suomi_NPP&quot; rel=&quot;nofollow noreferrer&quot;&gt;Suomi NPP&lt;/a&gt; satellite's &lt;a href=&quot;https://jointmission.gsfc.nasa.gov/viirs.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Visible Infrared Imaging Radiometer Suite (VIIRS)&lt;/a&gt; in 2012:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r1Qdm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r1Qdm.jpg&quot; alt=&quot;Sleepless Britain&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are faint lines emanating from Scotland and northern England, with a negative gradient, about 30° to the horizontal. What are these, airplanes? Or artefacts? Why are they so consistent?&lt;/p&gt;&#xA;" OwnerUserId="6473" LastEditorUserId="12102" LastEditDate="2017-03-18T05:45:04.230" LastActivityDate="2017-03-18T09:21:43.700" Title="What causes these faint, straight line artifacts in Suomi NPP images of Earth at night?" Tags="|nasa|imaging|earth-observation|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20698" PostTypeId="2" ParentId="20062" CreationDate="2017-03-18T02:13:31.787" Score="2" Body="&lt;p&gt;I think I found my answer (at least partly) when I found a study about &lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/836Colozza.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Solid-State Aircraft&lt;/em&gt; (SSA)&lt;/a&gt; which is a non-traditional ornithopter that uses a &lt;em&gt;flapping-wing mechanism&lt;/em&gt; and flies like a bird. The study of SSA suggests that solar arrays (which consists of &lt;em&gt;organic photovoltaic solar cells&lt;/em&gt;) which are implanted to the wings of the SSA need lithium battery (&lt;em&gt;thin-film lithium batteries&lt;/em&gt; in this case) to preserve and supply the energy continuously, but the solar power is the only source of power the SSA uses. In the study there were separate sections for Mars purposes and based on the study the SSA should be able to fly there too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That proves that at least this type of ornithopter can actually have its energy for flying from solar power, but as far as I understood it doesn’t consider carrying any payload with it.&lt;/p&gt;&#xA;" OwnerUserId="18363" LastEditorUserId="6816" LastEditDate="2017-03-18T02:38:57.893" LastActivityDate="2017-03-18T02:38:57.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20699" PostTypeId="2" ParentId="20696" CreationDate="2017-03-18T04:00:51.797" Score="4" Body="&lt;p&gt;This is the beginnings of an answer. @called2voyage has &lt;a href=&quot;https://space.stackexchange.com/questions/20696/#comment55919_20696&quot;&gt;commented&lt;/a&gt; that it looks similar to other &lt;a href=&quot;http://www.mdpi.com/2313-433X/2/1/9/pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;striping&lt;/a&gt; artifacts from the same satellite. A quick look through there (I'm still trying to read through more carefully) seems to show the striping as horizontal (East-West). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the image with a bunch of tweaks in color effects shows lines that seem to be tilted at -12 degrees, with a period of about 12 kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have to do more reading/searching now to see if there is a 12 degree tilted instrument on board the satellite, or if the satellite's attitude could have been tilted. Suomi NPP does &lt;a href=&quot;https://landweb.modaps.eosdis.nasa.gov/cgi-bin/NPP_QA/NPPpage.cgi?fileName=NPP_maneuver&amp;amp;subdir=forPage&quot; rel=&quot;nofollow noreferrer&quot;&gt;keep busy doing attitude adjustments&lt;/a&gt;, although these are carefully coordinated with image acquisitions and are not likely to produce &quot;out-of-whack&quot; images without sufficient documentation. &lt;strong&gt;I wish the OP had provided specific information about the date/time of this image or a link to it's source within the database!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you've ever seen any of the &lt;a href=&quot;http://www.nats.aero/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NATS&lt;/a&gt; videos on &lt;a href=&quot;https://www.youtube.com/user/NATSATCO1&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube&lt;/a&gt; or &lt;a href=&quot;https://vimeo.com/natsaero&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vimeo (downloadable)&lt;/a&gt; you can appreciate the OP's comment about air traffic and parallel lines. cf &lt;a href=&quot;https://www.youtube.com/watch?v=a8CQ29yWvZI&quot; rel=&quot;nofollow noreferrer&quot;&gt;UK 24&lt;/a&gt; or see below. Of course the satellite is in a sun-synchronous LEO and would not be generating long or time-lapse exposures that could produce streaks from aircraft, but the similarity of the appearence is worth noting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the mean time if someone more familliar with the satellite can add insight, please feel free to add to this answer or cannibalize it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Below:&lt;/strong&gt; From the &lt;a href=&quot;https://i.stack.imgur.com/l7jjr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;NPP Fact Sheet - Final.pdf&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v1xxI.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v1xxI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MoFvH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MoFvH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0Gyzq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0Gyzq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://www.mdpi.com/2313-433X/2/1/9/pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;VIIRS Day/Night Band—Correcting Striping and Nonuniformity over a Very Large Dynamic Range&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pbmFO.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pbmFO.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RCgkb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RCgkb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/l7jjr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/l7jjr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Watch:&lt;/strong&gt; (watch for the military and the civilian &quot;practice exercises&quot;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/a8CQ29yWvZI?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-18T09:21:43.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20700" PostTypeId="1" AcceptedAnswerId="20706" CreationDate="2017-03-18T04:16:13.287" Score="8" ViewCount="3676" Body="&lt;p&gt;When watching SpaceX launches, around 2-3 seconds after the Merlin Vacuum ignition a ring can be seen breaking in 2 from the bottom of the nozzle and then falling away from the second stage. My question is what is this ring structure and what is its purpose?&lt;a href=&quot;https://i.stack.imgur.com/9qIFO.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9qIFO.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;It can be seen in the top middle portion of the screen above the engine bell&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2021-03-15T23:52:10.197" Title="What is the ring at the bottom of the Merlin vacuum engine?" Tags="|spacex|vacuum|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20701" PostTypeId="1" CreationDate="2017-03-18T07:32:04.307" Score="19" ViewCount="2329" Body="&lt;p&gt;The ISS is a pretty messy place, as in the image here. I have a cable mess under my desk that quickly traps dust and takes some effort to be cleaned. The ISS is much worse. And it has been crewed constantly since November 2000, by almost 100 persons now.&lt;/p&gt;&#xA;&lt;p&gt;I suppose the ventilation and its filters takes care of some of the dust, and I can imagine that it is more efficient as a kind of vacuum cleaner in microgravity. But humans have fatty sticky skin and hair that falls off. And there's a lot of mixed materials exhaled, we exhale about as much water as we deposit in toilettes. The crew cannot walk but has to grab things with their hands in order to move. Door handles and even light switches in unclean homes are disgustingly tainted by filth because they are often touched.&lt;/p&gt;&#xA;&lt;p&gt;What cleaning equipment, chemicals and routines are used on the ISS, and how much time is spent on cleaning? Spray bottles with detergents might not be a good idea in microgravity since it would end up everywhere, including in the lungs and on the skin of the crew. Swabbing the floor isn't even possible. I don't see images of the crew using hair nets and gloves much. Shouldn't the ISS be treated a bit like a clean room?&lt;/p&gt;&#xA;&lt;p&gt;Is filth an important problem as in threatening the astronauts' health or restricting the life time of the hardware and the station itself?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r9rjG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r9rjG.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Columbus laboratory on the ISS.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="17520" LastEditDate="2022-10-07T09:36:38.627" LastActivityDate="2022-10-07T09:36:38.627" Title="How is the space station's interior cleaned?" Tags="|iss|health|hygiene|cleaning|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20702" PostTypeId="1" CreationDate="2017-03-18T08:14:22.783" Score="5" ViewCount="552" Body="&lt;p&gt;As weight was a problem from the beginning (launch) of an Apollo mission, was the LM water tank already filled, or were they filled during the trip to the moon from fuel-cell byproduct water?&lt;/p&gt;&#xA;" OwnerUserId="18974" LastEditorUserId="6816" LastEditDate="2017-03-18T08:23:58.653" LastActivityDate="2019-06-23T16:59:45.940" Title="Lunar Module water supply" Tags="|apollo-program|water|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20703" PostTypeId="2" ParentId="20702" CreationDate="2017-03-18T13:32:51.177" Score="8" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;In the Command Module, water was generated by fuel cell operation; in&#xA;  the Lunar Module, all water supplies were loaded in storage tanks&#xA;  before lift-off.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://history.nasa.gov/SP-368/s6ch4.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LEM did not have fuel cells, it was battery powered only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NXFGO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NXFGO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were asking about product water from the Command Module fuel cells, there was no interface from the CM-&gt;LM to transfer that water. Notice in the water schematic below that the tank fill ports (below each tank) are all labeled &quot;GSE&quot; - Ground Support Equipment. And, the check valves in the tank outlet lines prevent any water from flowing into the tanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All water was, therefore, stored on board prelaunch and not generated. (With the exception of condensed water from the Atmospheric Revitalization System water separators, which entered the system at the middle bottom, and was used for cooling only).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GSE device used to fill the tanks is described in the 1965 Apollo LEM Familiarization Manual LM790-1. (Which I am not finding on the internet right now, except for sale on Amazon...)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;5-62. WATER SUPPLY UNIT, PART NO. 430-94119. The Water Supply Unit is a mobile unit that contains a pump, a reservoir, starters, tubing,&#xA;  valving and controls, remote and manual controls, explosion-proofing&#xA;  and instrumentation. It is used in the LEM Environmental Control&#xA;  Subsystem to fill the water management section with triple-distilled&#xA;  water. This unit is an item of common usage GSE modified for LEM use.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LjNHy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LjNHy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Diagrams from the &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiJ3dG2leDSAhUJ64MKHZAiDCcQFggcMAA&amp;amp;url=https%3A%2F%2Fwww.hq.nasa.gov%2Falsj%2FLM-intro.pdf&amp;amp;usg=AFQjCNFSQsQkjaWzMw9YS33jwzVnQj_1Mw&amp;amp;sig2=4FxtsLGmCUol1Bpz_J1qZg&quot; rel=&quot;nofollow noreferrer&quot;&gt;LM Orientation&lt;/a&gt;, Course No. 30005-012, April 12, 1966&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-06-23T16:59:45.940" LastActivityDate="2019-06-23T16:59:45.940" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="20704" PostTypeId="2" ParentId="20701" CreationDate="2017-03-18T19:26:17.813" Score="9" Body="&lt;p&gt;The Space Station's Environmental Control and Life Support System eliminates much of the dust and other particulates from the breathing atmosphere via HEPA filters. It also removes water vapor, volatiles such as ammonia, and of course carbon dioxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That still leaves dust and hairs that collect onto surfaces. Every Saturday, every cosmonaut and astronaut is assigned two to four hours of weekly cleaning duties. Many of the tasks we do at home are also done in space, such as vacuuming and wiping surfaces.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-03-18T19:26:17.813" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20705" PostTypeId="1" AcceptedAnswerId="20708" CreationDate="2017-03-19T00:40:36.953" Score="5" ViewCount="1100" Body="&lt;p&gt;As we know, during the Space Race, the United States and the Soviet Union were in fierce competition. Each trying to best the other by creating their idea of a moon rocket. While the United States's aprroch was to increase the size of their engines(resulting in the F-1 engine), the Soviet Union simply decided to add more engine to each stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each design choice at first may seem equal, however, the records of both rockets tell otherwise. While the succes to failure rate of the Saturn V was nearly perfect, the N-1 failed on every one of its four launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it may be tempting to simply say the Soviet's rocket design was dumb, it seems that many other prominent factors played a role in the failure of the N-1 program. Such as this excerpt from Wikipedia states: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;N1-L3 was underfunded and rushed, starting development in October 1965, almost four years after the Saturn V. The project was badly derailed by  and the death of its chief designer Sergei Korolev in 1966.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Along with above, there seemed to be strife and disagreement between certain, key project leaders, which also probably caused friction in the project. Furthermore, from what I've read, the engines used on the N-1 were fairly well designed and tested.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, it seems that the failure of the N-1 rocket was a result of many external problems, instead of it simply having a &quot;dumb&quot; design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This brings me to my question. If The problems above were eliminated, and a N-1 rocket design was given proper funding, R &amp;amp; D, etc., then could a working N-1 rocket(or a rocket of similar design)be successfully constructed and flown today? Or does the N-1 simply have a bad design?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;NOTE&lt;/strong&gt;: my use of the word &quot;modern&quot; in the title is a little different from its actual meaning. When I say &quot;a modern N-1 rocket&quot;, I simply mean a rocket which would be designed, built, and tested today, without all of its past design problems.&lt;/p&gt;&#xA;" OwnerUserId="16704" LastEditorUserId="16704" LastEditDate="2017-03-19T00:48:10.073" LastActivityDate="2022-10-20T05:25:15.247" Title="Could a &quot;modern&quot; N-1 rocket be successfully created and flown?" Tags="|rockets|spacecraft|design|" AnswerCount="3" CommentCount="7" ClosedDate="2017-03-20T04:05:14.707" ContentLicense="CC BY-SA 3.0" />
  <row Id="20706" PostTypeId="2" ParentId="20700" CreationDate="2017-03-19T02:48:28.183" Score="14" Body="&lt;p&gt;The nozzle of the Merlin Vacuum is very large, to help get better expansion ratios since it only ever fires in a vacuum, unlike the 9 sea level optimized Merlins that have smaller nozzles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is very obvious when you realize the Falcon 9 first and second stage are the exact same diameter, but on the first stage with normal Merlin engines, 9 engines and bells fit on the base. But on the second stage, only a single Merlin Vaccum fits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The material the nozzle extension is made out of is somewhat flimsy, all things considered and that ring you see is a stiffener, used during launch to provide some strength so that it does not deform under launch stress. It gets blown out as soon as the engine fires, and the nozzle is under uniform pressure outwards and won't deform.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-03-19T02:48:28.183" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20707" PostTypeId="2" ParentId="20694" CreationDate="2017-03-19T02:48:42.043" Score="4" Body="&lt;p&gt;Badly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can throttle hybrid engines quite far down, possibly lower than liquid fuel engines - but it comes at a large cost to efficiency. If you throttle your engine to 50%, you're not getting 50% of thrust at similar ISp. You're getting a hard to determine amount of thrust somewhere around 50% at ISp way worse, likely worse than 50% of max. And at the end of the burn you'll end up with spare oxidizer, running out of solid fuel earlier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, you're unable to throttle fuel flow. You regulate oxidizer supply, but your solid fuel is available in full amount at all times. This results in burn in non-stoichiometric ratio, fuel-rich exhaust with plenty of partially unburnt fuel (soot), and usage of fuel higher than in stoichiometric ratio. Never mind lower chamber pressure and temperature will result in serious loss of specific impulse. Fuel gets used up faster than oxidizer, producing far less energy than it could, and you're getting a really lousy performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hybrid motors can be stopped, unlike solid, and can be restarted somewhat easier than liquid, and if you are desperate, you can throttle one as well. But they are very badly suited for throttling. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-19T02:48:42.043" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="20708" PostTypeId="2" ParentId="20705" CreationDate="2017-03-19T03:23:31.203" Score="15" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/N1_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, the developmental problems with the N-1 were exacerbated by the fact that the NK-15 engines&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;had a number of valves that were activated by pyrotechnics rather than hydraulic or mechanical means, this being a weight-saving measure. Once shut, the valves could not be re-opened.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This meant that individual engines could only be tested by flying them on the N-1. After the first failed flights, while the rocket was being redesigned, the NK-15 was replaced with the &lt;a href=&quot;https://en.wikipedia.org/wiki/NK-33&quot; rel=&quot;nofollow noreferrer&quot;&gt;NK-33/43&lt;/a&gt;, which has since proven to be a successful engine, so we can assume that problems with individual engines could be worked out. Longitudinal &amp;quot;pogo&amp;quot; oscillations of the engines tore plumbing in the test flights, causing various fires and explosions. Anti-pogo measures could be installed, and sensors could be better-calibrated to shut down vibrating engines before they tore something loose.&lt;/p&gt;&#xA;&lt;p&gt;The article also mentions&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;exhaust plume and fluid dynamic problems (causing vehicle roll, vacuum cavitation, and other problems)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It's hard to describe how much more advanced the state of the art in software simulation of fluid dynamics is now than it was in 1969. A typical desktop computer today is something like 1000 times more powerful than the fastest supercomputers of that era. It's very likely that these problems (presumably due to the interaction of the exhaust plumes of 30 engines with the airflow around the body of the rocket) could be identified, but solving them might require extensive redesign of the first-stage thrust structure -- for example, it might turn out that one or two engines at the centerline of the rocket would solve the problems created by the N-1's concentric-ring engine arrangement. Whatever the cause and potential solution of these problems are, they're probably the most &amp;quot;fundamental&amp;quot; of the N1's design faults.&lt;/p&gt;&#xA;&lt;p&gt;Roll control on the N1 was achieved by differential thrusting of canted engines in the outer ring of 24 engines, arranged in 8 groups of 3 engines - each group consisting of 2 outward canted engines, with the middle engine un-canted, as the first stage engines weren't gimbaled; pitch and yaw control was done by differentially throttling the un-canted middle engines of each group of 3 engines. The failure of roll control during the 6L launch would result in 4 dedicated roll control engines being added on the outer axis of Block A, to supplement the system. Aerojet has since added gimbals to their version of the NK-33, which would give far better roll control at the price of additional weight and complexity.&lt;/p&gt;&#xA;&lt;p&gt;A secondary source of problems in the test flights was the KORD engine control system. One of KORD's jobs was, on failure of a single engine, to shut down the opposing engine to maintain balanced thrust. On the first flight, transient signals from pogo-induced damage caused KORD to shut down the entire stage -- not ideal, but the stage was on fire by that time anyway, so it's likely that the rocket would have been lost regardless. More testing, better quality control on sensor installation, and so forth could probably mitigate these problems as well.&lt;/p&gt;&#xA;&lt;p&gt;Because all 4 of the N1 test flights failed during the first stage portion of flight, none of the upper stages were ever tested in realistic conditions, and doubtless they would have had their own problems (though those problems would not include surviving ten &lt;em&gt;million&lt;/em&gt; pounds of thrust). I don't imagine that any such problems would be insurmountable.&lt;/p&gt;&#xA;&lt;p&gt;I think the N1 ultimately failed because not enough time and money was spent, and because the people who could see the engineering problems didn't have the power to set the schedules. In contrast, development of the Saturn was very successful, with an almost flawless first test flight (Apollo 4), and a very good record: I believe 12 of 13 successful launches and no catastrophic failures. Allowing Apollo 4 to be delayed by a full year from its original launch schedule made that possible. As Feynman said in regard to the Challenger disaster, &amp;quot;for a successful technology, reality must take precedence over public relations, for Nature cannot be fooled.&amp;quot;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2022-10-20T05:25:15.247" LastActivityDate="2022-10-20T05:25:15.247" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20709" PostTypeId="1" AcceptedAnswerId="20710" CreationDate="2017-03-19T09:59:30.540" Score="4" ViewCount="1720" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/ATA_100&quot; rel=&quot;nofollow noreferrer&quot;&gt;ATA100 numbering system&lt;/a&gt; is frequently used in aircraft as a common referencing standard.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;The unique aspect of the chapter numbers is its relevance for all aircraft. Thus a chapter reference number for a Boeing 747 will be the same for other Boeing aircraft, a BAe 125 and Airbus Aircraft. Examples of this include Oxygen (Chapter 35), Electrical Power (Chapter 24) and Doors (Chapter 52).&quot;- Wikipedia&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For instance, ATA100 reference code 27-50 implies the chapter Flight Controls, section Flaps.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there an equivalent numbering system for spacecraft and their systems?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="1552" LastEditorUserId="6944" LastEditDate="2020-02-10T16:45:16.273" LastActivityDate="2020-02-10T17:30:07.760" Title="Is there an equivalent to aircraft ATA100 numbering system for Spacecraft?" Tags="|engineering|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="20710" PostTypeId="2" ParentId="20709" CreationDate="2017-03-19T13:29:20.637" Score="1" Body="&lt;p&gt;I am not familiar with ATA100, but shuttle had what seems to be an analogous numbering system. It was used in telemetry IDs, and I think in work packages as well. I doubt it was used in other spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7eATE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7eATE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-03-19T13:50:33.420" LastActivityDate="2017-03-19T13:50:33.420" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20711" PostTypeId="2" ParentId="20705" CreationDate="2017-03-19T13:44:28.180" Score="9" Body="&lt;p&gt;It's worth noting that the N-1 launch failures were &lt;em&gt;not unexpected&lt;/em&gt;. Early on in the program, a decision was made not to invest in facilities for ground-testing complete stages, because building the first-stage test stand would take 4 years and a lot of money.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, the plan was to do 12 test launches, all using boilerplate payloads. This was a common approach in Soviet rocket projects at the time. The first 2 launches were done in quick succession (February and July 1969), but after Apollo 11, the program lost its urgency as the Moon race was lost. Work continued at a slower pace, with the fourth launch occurring in 1972. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1974, Mishin's OBK-1 (in charge of the N-1, and still working on a Moon mission and improvements to the N-1) was combined with Glushko's OKB-456 under Glushko's leadership. He had opposed the N-1 from the beginning and immediately cancelled it. This happened in May 1974. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This termination came at an unfortunate time: the fifth rocket, with restartable NK-33 engines, was being readied for launch, and more rockets were in the pipeline. The fourth launch had come close to doing a full-duration burn of the first stage, so they were hoping to be able to test the second stage on the fifth flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(source: N-1 for the Moon and Mars, N. Stevens and M. Johnson)&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-03-19T19:09:46.393" LastActivityDate="2017-03-19T19:09:46.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20712" PostTypeId="1" AcceptedAnswerId="20716" CreationDate="2017-03-19T14:06:14.523" Score="6" ViewCount="1872" Body="&lt;p&gt;The TRAPPIST-1f is in the TRAPPIST-1 system discovered recently with seven planets. Three of them are said to be in the habitable zone. But the data says that the average temperature on the TRAPPIST-1f planet is -54°C. How is it considered habitable then?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://en.m.wikipedia.org/wiki/TRAPPIST-1f&quot; rel=&quot;noreferrer&quot;&gt;https://en.m.wikipedia.org/wiki/TRAPPIST-1f&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15742" LastActivityDate="2017-03-20T18:14:55.507" Title="Why is TRAPPIST-1f considered habitable?" Tags="|planet|exoplanet|goldilocks-zone|trappist-1|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20713" PostTypeId="2" ParentId="20712" CreationDate="2017-03-19T14:56:36.857" Score="8" Body="&lt;p&gt;The temperature estimate is based on no atmosphere. For instance, Earth, without our atmosphere, would have an average temperature of -20°C . The equator should be warmer, and any atmosphere will also add heat. Also, the objects are most likely tidally locked, which means that a single side of the planet will always face it's star, making it even warmer in some places.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-03-19T14:56:36.857" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20715" PostTypeId="2" ParentId="12506" CreationDate="2017-03-19T16:04:08.353" Score="3" Body="&lt;p&gt;The exact format of the SPICE Toolkit SPK files (also called BSP files) is called &lt;a href=&quot;ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/req/daf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;DAF (&lt;em&gt;Double-precision Array File&lt;/em&gt;)&lt;/a&gt;. It is a binary format made up of consecutive blocks of 1,024 bytes. The first record is always the &lt;em&gt;file record&lt;/em&gt;, then it has some blocks serving as a &lt;em&gt;comment area&lt;/em&gt;, then it has the &lt;em&gt;array records&lt;/em&gt;, and finally a collection of &lt;em&gt;data records&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;&#xA;[FILE RECORD] (1024 bytes)&#xA;[FIRST COMMENT BLOCK] (1024 bytes)&#xA;...&#xA;[LAST COMMENT BLOCK] (1024 bytes)&#xA;[FIRST BLOCK OF SUMMARY RECORDS] (1024 bytes)&#xA;...&#xA;[LAST BLOCK OF SUMMARY RECORDS] (1024 bytes)&#xA;[FIRST BLOCK OF NAME RECORDS] (1024 bytes)&#xA;...&#xA;[LAST BLOCK OF NAME RECORDS] (1024 bytes)&#xA;[FIRST BLOCK OF ELEMENT RECORDS] (1024 bytes)&#xA;...&#xA;[LAST BLOCK OF ELEMENT RECORDS] (1024 bytes)&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All binary SPICE data is stored in DAF format (not only ephemerides). The specific kind of data (that is, the semantics of the records) depend on the info stored in the summary records and the element records. You need to know what kind of data you are reading to correctly interpret the records.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;There are many &lt;em&gt;ephemerides types&lt;/em&gt;&lt;/a&gt;. Planets are often &lt;em&gt;Type II&lt;/em&gt;; satellites are often &lt;em&gt;Type III&lt;/em&gt;; spacecraft are often &lt;em&gt;Type I&lt;/em&gt;. Types II and III represent Chebyshev polynomials, and their difference is whether they use the derivative of the Chebyshev polynomial to calculate velocity (Type II), or whether a separate polynomial is used to calculate velocity (Type III). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words: a Type II SPK file will provide you with one polynomial of certain order whose value represents position, and whose derivative represents speed. A Type III SPK file will provide you with one polynomial for position and a different polynomial for velocity. Both types are Chebyshev polynomials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For SPK files, the summary records will tell you where every record begins and ends, the ephemerides type, the timespan, the frame, the target, and the observer. Each specific records (which will most likely contain more than one block) look as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;&#xA;   +---------------+&#xA;   | Record 1      |&#xA;   +---------------+&#xA;   | Record 2      |&#xA;   +---------------+&#xA;     .&#xA;     .&#xA;     .&#xA;   +---------------+&#xA;   | Record N      |&#xA;   +---------------+&#xA;   | INIT          |&#xA;   +---------------+&#xA;   | INTLEN        |&#xA;   +---------------+&#xA;   | RSIZE         |&#xA;   +---------------+&#xA;   | N             |&#xA;   +---------------+&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and each record looks like&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;&#xA;   +------------------+&#xA;   | MID              |&#xA;   +------------------+&#xA;   | RADIUS           |&#xA;   +------------------+&#xA;   | X  coefficients  |&#xA;   +------------------+&#xA;   | Y  coefficients  |&#xA;   +------------------+&#xA;   | Z  coefficients  |&#xA;   +------------------+&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for Type II and like&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;&#xA;   +------------------+&#xA;   | MID              |&#xA;   +------------------+&#xA;   | RADIUS           |&#xA;   +------------------+&#xA;   | X  coefficients  |&#xA;   +------------------+&#xA;   | Y  coefficients  |&#xA;   +------------------+&#xA;   | Z  coefficients  |&#xA;   +------------------+&#xA;   | X' coefficients  |&#xA;   +------------------+&#xA;   | Y' coefficients  |&#xA;   +------------------+&#xA;   | Z' coefficients  |&#xA;   +------------------+&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;for Type III.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The degree of the polynomial is &lt;code&gt;( RSIZE - 2 ) / 3 - 1&lt;/code&gt;. The &lt;code&gt;MID&lt;/code&gt; tells you the interval mid-point (ephemerides time), and &lt;code&gt;RADIUS&lt;/code&gt; tells you half the size of the interval. So its time span is &lt;code&gt;[MID - RADIUS, MID + RADIUS]&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For obtaining position you simply evaluate the Chebyshev at the corresponding time. For velocity you either evaluate the derivative or a different polynomial. Remember to normalize time before evaluating and de-normalize time after evaluating (this has gotten me many times).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dealing directly with DAF can be a rewarding experience (for example: I have a thread-safe high-performance ephemerides evaluator), but it is easy to get wrong. And after a lot of work you only have a position or velocity in a given frame that you still need to rotate/translate/etc (for which you either need to re-invent SPICE or rely on it). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless you have a very good reason not to, just stick to SPICE.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;A simple Python script&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;I just made a Python script that could help someone get started on the DAF binary format. It will simply dump the file record information and the contents (array summaries) contained in a DAF.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import mmap&#xA;import struct&#xA;&#xA;RECLEN = 1024&#xA;&#xA;def peek_spk(mem):&#xA;    # file record is always the first one&#xA;&#xA;    # string data&#xA;    locidw = mem[0:8]&#xA;    locifn = mem[16:16 + 60]&#xA;    locfmt = mem[88:88+8]&#xA;    ftpstr = mem[699:699+28]&#xA;&#xA;    # endianness&#xA;    fmt     = &quot;&amp;lt;&quot; if locfmt == &quot;LTL-IEEE&quot; else &quot;&amp;gt;&quot;&#xA;    int_fmt = fmt + &quot;I&quot;&#xA;&#xA;    nd,    = struct.unpack_from(int_fmt, mem, offset=8)&#xA;    ni,    = struct.unpack_from(int_fmt, mem, offset=12)&#xA;    fward, = struct.unpack_from(int_fmt, mem, offset=76)&#xA;    bward, = struct.unpack_from(int_fmt, mem, offset=80)&#xA;    free,  = struct.unpack_from(int_fmt, mem, offset=84)&#xA;&#xA;    print &quot;locidw {0}&quot;.format(locidw)&#xA;    print &quot;nd     {0}&quot;.format(nd)&#xA;    print &quot;ni     {0}&quot;.format(ni)&#xA;    print &quot;locifn {0}&quot;.format(locifn)&#xA;    print &quot;fward  {0}&quot;.format(fward)&#xA;    print &quot;bward  {0}&quot;.format(bward)&#xA;    print &quot;free   {0}&quot;.format(free)&#xA;    print &quot;locfmt {0}&quot;.format(locfmt)&#xA;    print &quot;ftpstr {0}&quot;.format(repr(ftpstr))&#xA;&#xA;    # let's do the first summary record only... we need to read nd&#xA;    # doubles and ni integers starting at offset fward&#xA;&#xA;    offset = (fward - 1) * RECLEN&#xA;    sum_fmt = fmt + nd * &quot;d&quot; + ni * &quot;I&quot;&#xA;    size = nd + (ni + 1) / 2 # integer division&#xA;    while True:&#xA;        nxt, prv, nsum = struct.unpack_from(fmt + &quot;ddd&quot;, mem, offset=offset)&#xA;        offset += 24 # skip three doubles&#xA;        for n in range(int(nsum)):&#xA;            print struct.unpack_from(sum_fmt, mem, offset = offset)&#xA;            offset += size * 8&#xA;        if nxt == 0:&#xA;            break&#xA;&#xA;&#xA;def peek(path):&#xA;&#xA;    print &quot;peeking into {0}&quot;.format(path)&#xA;&#xA;    with open(path, &quot;rb&quot;) as fp:&#xA;        mem = mmap.mmap(fp.fileno(), 0, access=mmap.PROT_READ)&#xA;&#xA;    peek_spk(mem)&#xA;&#xA;&#xA;if __name__ == &quot;__main__&quot;:&#xA;    import sys&#xA;    for path in sys.argv[1:]:&#xA;        peek(path)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;For example: running it for jup310.bsp gives&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;&#xA;peeking into /data/spice/jup310.bsp&#xA;locidw DAF/SPK &#xA;nd     2&#xA;ni     6&#xA;locifn NIO2SPK&lt;br&gt;&#xA;fward  6&#xA;bward  6&#xA;free   122077514&#xA;locfmt LTL-IEEE&#xA;ftpstr 'FTPSTR:\r:\n:\r\n:\r\x00:\x81:\x10\xce:ENDFTP'&#xA;(-3155716758.8160305, 3155716867.183885, 501, 5, 1, 3, 897, 7208500)&#xA;(-3155716758.8160305, 3155716867.183885, 502, 5, 1, 3, 7208501, 14367404)&#xA;(-3155716758.8160305, 3155716867.183885, 503, 5, 1, 3, 14367405, 19140106)&#xA;(-3155716758.8160305, 3155716867.183885, 504, 5, 1, 3, 19140107, 22451778)&#xA;(-3155716758.8160305, 3155716867.183885, 505, 5, 1, 3, 22451779, 44074360)&#xA;(-3155716758.8160305, 3155716867.183885, 514, 5, 1, 3, 44074361, 65696942)&#xA;(-3155716758.8160305, 3155716867.183885, 515, 5, 1, 3, 65696943, 90825888)&#xA;(-3155716758.8160305, 3155716867.183885, 516, 5, 1, 3, 90825889, 115954834)&#xA;(-3155716758.8160305, 3155716867.183885, 599, 5, 1, 3, 115954835, 120922238)&#xA;(-3155716758.8160305, 3155716867.183885, 3, 0, 1, 2, 120922239, 121109489)&#xA;(-3155716758.8160305, 3155716867.183885, 5, 0, 1, 2, 121109490, 121168877)&#xA;(-3155716758.8160305, 3155716867.183885, 10, 0, 1, 2, 121168878, 121328726)&#xA;(-3155716758.8160305, 3155716867.183885, 399, 3, 1, 2, 121328727, 122077513)&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means that jup310 contains records spanning ET (-3155716758.8160305, 3155716867.183885) for some moons of Jupiter (501 through 519) relative to Jupiter's Barycenter (5), in J2000 format (1), and are type 3. Other records are for the Barycenters of Earth, Jupiter, and the Sun (3, 5, and 10) relative to the Solar System Barycenter (0) in J200 (1), and are type 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each record tells you where in the file does each array begin and end. You can extend the script to skip to each location and read the entire record for later interpolation.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Available Software&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;I quickly created a simple implementation in Python which can read and work directly with Types II and III. It compares well to CSPICE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://gist.github.com/arrieta/c2b56f1e2277a6fede6d1afbc85095fb&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://gist.github.com/arrieta/c2b56f1e2277a6fede6d1afbc85095fb&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18918" LastEditorUserId="18918" LastEditDate="2017-03-19T20:49:37.247" LastActivityDate="2017-03-19T20:49:37.247" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20716" PostTypeId="2" ParentId="20712" CreationDate="2017-03-19T17:19:11.190" Score="14" Body="&lt;p&gt;None of the TRAPPIST-1 planets are &quot;considered habitable&quot;; we don't know nearly enough about them to say that (any news article which referred to the planets themselves as &quot;habitable&quot; needs firm correction). Three of them are within the &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Circumstellar_habitable_zone&quot; rel=&quot;nofollow noreferrer&quot;&gt;habitable zone&lt;/a&gt;&lt;/em&gt; of the star, which is defined as the region around the star where liquid water could potentially be found on a planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A planet with an average temperature of -55C will not be dominated by liquid water, but -- given that &lt;a href=&quot;https://en.wikipedia.org/wiki/TRAPPIST-1f&quot; rel=&quot;nofollow noreferrer&quot;&gt;TRAPPIST-1f&lt;/a&gt; is likely tidally locked, showing the same face to the star all the time -- on one side of the planet the sun is always in the sky, greatly increasing the temperature on that side, particularly in the region where the sun is most directly overhead. Depending on the atmospheric composition, it's likely that water, if present, would be liquid there.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-03-20T18:14:55.507" LastActivityDate="2017-03-20T18:14:55.507" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20717" PostTypeId="2" ParentId="20705" CreationDate="2017-03-19T20:24:59.130" Score="3" Body="&lt;p&gt;The development of the F-1 rocket engine for the first stage of the Saturn V took a long time and was started years before the development of the Saturn. In 1955 the first studies for the engine were started. In 1958 the first contract for the development was given. The first test of a combustion chamber was 1959, this test was done without the turbine and turbo pumps. Special high pressure tanks were build for this ground test to use only pressure feeding. 1961, before Gagarins flight and Kennedy's speech the combustion chamber was successfully tested. 2471 tests were done with the F-1 and 1110 tests for the full time needed for a flight of the Saurn V. There were 34 tests of a complete S-IC. 1966 the engine was certified for manned flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given enough time for development and tests of the N1 and its engines as well as enough money for numerous ground tests of the engine itself and a complete first stage, a modern rocket like the N1 should be possible. But the interaction of all 30 engines was very complex and difficult.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-03-19T20:24:59.130" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20718" PostTypeId="2" ParentId="20709" CreationDate="2017-03-19T23:59:23.690" Score="2" Body="&lt;p&gt;Spacecraft tracked by NASA's Deep Space Network rely on SPICE, a software library for observation geometry information. SPICE itself maintains a list of &lt;em&gt;semi-official&lt;/em&gt; spacecraft (and natural body) identifiers, the &lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/req/naif_ids.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;NAIF Integer ID codes&lt;/em&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NAIF Integer ID codes do not apply to Earth-orbiting spacecraft. Instead, the North American Aerospace Defense Command (NORAD) a&lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_Catalog_Number&quot; rel=&quot;nofollow noreferrer&quot;&gt;ssigns its own identifiers&lt;/a&gt;. NAIF maps them as &lt;code&gt;SPICE_ID = -100000 - NORAD_ID&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other countries may have their own internal designations, and the &lt;a href=&quot;https://public.ccsds.org/SpacecraftID.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCSDS is trying to standardize the methodology&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer to your question would be &lt;em&gt;no, there is nothing like ATA1000&lt;/em&gt;. Instead, there are many ad hoc numbering systems aiming to consolidate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Within SPICE, specific instruments are numbered according to the spacecraft:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;&#xA;NAIF instrument code = (s/c code)*(1000) - instrument number&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the linked page they cite as example the instruments of Voyager 2:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;&#xA;-32000 -&amp;gt; Instrument Scan Platform &#xA;-32001 -&amp;gt; ISSNA (Imaging science narrow angle camera) &#xA;-32002 -&amp;gt; ISSWA (Imaging science wide angle camera) &#xA;-32003 -&amp;gt; PPS (Photopolarimeter) &#xA;-32004 -&amp;gt; UVSAG (Ultraviolet Spectrometer, Airglow port) &#xA;-32005 -&amp;gt; UVSOCC (Ultraviolet Spectrometer, Occultation port) &#xA;-32006 -&amp;gt; IRIS (Infrared Interferometer Spectrometer and Radiometer)&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;" OwnerUserId="18918" LastActivityDate="2017-03-19T23:59:23.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20719" PostTypeId="1" AcceptedAnswerId="20720" CreationDate="2017-03-20T00:01:14.107" Score="13" ViewCount="4199" Body="&lt;p&gt;I'm trying to grasp what a 2D &amp;quot;side-view&amp;quot; of the solar system looks like. There are many examples of a &amp;quot;top-down&amp;quot; view of the solar system. But the side-view confuses me somewhat.&lt;/p&gt;&#xA;&lt;p&gt;The plane of reference is the ecliptic as far as I can understand and the earth's path viewed from the side follows this line exactly as the &amp;quot;orbital inclination&amp;quot; with the ecliptic is 0 degrees (see blue line on ecliptic plane in screenshot below).&lt;/p&gt;&#xA;&lt;p&gt;Now for the other planets: Mars has an orbital inclination of 1.85 degrees, BUT when I use the Keplerian elements from JPL, it seems the plane is not only inclined at 1.85 degrees, but also tilted? See screenshot. I was expecting a perfectly flat line for the ellipse of Mars (like the one for earth). You can see that Mercury and Venus have the same problem.&lt;/p&gt;&#xA;&lt;p&gt;Does this visualization make sense or am I doing this wrong?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vGpxh.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vGpxh.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18988" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-20T15:49:59.610" Title="What does &quot;side view&quot; of solar system look like?" Tags="|solar-system|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20720" PostTypeId="2" ParentId="20719" CreationDate="2017-03-20T00:26:12.537" Score="22" Body="&lt;p&gt;Your plot looks correct. The behavior you are experiencing stems from the different &lt;em&gt;longitude of the ascending node&lt;/em&gt; of the different planets (the angle between a principal direction and the point where they cross the ecliptic plane). If they all crossed the ecliptic plane at the same node you will see the behavior you expected (lines at different inclinations).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the following example I adjusted the view to make Earth (blue orbit) appear as a line. Other planets (Mercury, Venus, and Mars) appear as long ellipses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MMC1w.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MMC1w.png&quot; alt=&quot;Earth&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I adjust the plot to make Mars appear as a line, now the Earth pops out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EBnZf.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EBnZf.png&quot; alt=&quot;Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even more dramatically if I adjust for Mercury:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fPRDY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fPRDY.png&quot; alt=&quot;Mercury&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18918" LastActivityDate="2017-03-20T00:26:12.537" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20721" PostTypeId="1" AcceptedAnswerId="20724" CreationDate="2017-03-20T02:01:50.440" Score="0" ViewCount="223" Body="&lt;p&gt;As mentioned, I'm following the &lt;a href=&quot;https://en.wikipedia.org/wiki/Copernican_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Copernican principle&lt;/a&gt; for this line of questioning. Yes I've considered and read many other possibilities, but this question is for Copernican-style answers ONLY. Your welcome to ask your own similar non-Copernican question in a separate question, and inform me and I'll give my support as well as my own best answer(s), or you can ask me to formulate my own non-Copernican version.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Estimations are based on various websites, feel free to adjust inaccurate numbers if they change a final formula by more than 5%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, please no government-conspiracy type responses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Summary: why considering these and other numbers, haven't we already found mainstream accepted proof of an ETI?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The numbers: quick search indicates that within 50 Light years (I'll abbreviate LR) of us, 133 stars are visible to the naked Eye, and 1400 stars are found and/or estimated to be in that 50 LR span. As I understand it (introduce some shaky logic with semi-arbitrary numbers and conjectures), we've been sending highly &quot;unnatural&quot; signals to space for about 50 years. Many of them I assume probably aren't strong enough to be detectable past a LR or two (pure conjecture) of space with our own detection instruments, but some I assume probably were strong enough to make it through the noise. I think perhaps our nuclear detonation may also have been able to be detectable by even an earth like civilization. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also read that nearly all of them have (or probably have planets). And 25% have or probably have planets in the habitable zone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's 350 in the habitable zone within 50 LR of us. If 1% have life, that's 3.5 If 1% with life have intelligent life, that's .035... so maybe 1. The number of planets our signals will reach grows exponentially as time goes on. I'll arbitrarily say that in another 100 years we're at something like 160,000 stars reached, 40,000 have planets with life friendly conditions (assuming 25%) thus 400 with life (assuming 1%), thus 4 with intelligent civilizations (assuming 1%, which I will point out that if we applied the copernican principle to this portion of the formula this makes us &quot;special&quot; but I'll make an exception here) Recap: 160,000 stars in 150 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Us plus the other intelligent civilizations at a 25%/1%/1% formula makes 5 intelligent civilizations for every 160,000 stars...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the copernican principle.... if you took it very seriously, and if my math is kind-of accurate, then we should have inadvertently been sending unnatural signals, with various levels of detectability through space for 50 years. If we &quot;imagine&quot; that we are just an average form of life, and that the universe may be teeming with life (since here we are assuming we're not special, wether it's lone survivor, or short-bus special) then also if we are not special, the universe should be teeming with life; and surely a bunch of it would also use signals similar to ours, maybe not decipherable, but at least similar enough to differentiate between natural and unnatural. Yet per-Fermi's paradox, they have not been detected....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question: why the **** aren't we noticeably detecting their signatures if we ourselves have been able to send stuff through space for 50 odd years???&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking for fresh ideas. If you HAVE to, you can point out the great filter or some other already mainstream hypothesis. I'm not discrediting them. Yes many of those types of ideas have merit Imho.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking for fresh thoughts and discussion on this, while preferably being able to hold true to the Copernican principle. Again, I'm not saying they're like us because we're awesome, I'm saying if they're there, then too assume WE are like THEM since we may NOT be awesome nor behind the curve, nor on a separate playing field, just average.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last words: I'm not saying we're not special. Yep, we might be. We might be the only ones or the only ones to make it through a filter(s). What are some other &quot;non-special&quot; possibilities, that might allow ETs and ETIs to exist, while keeping us from seeing them, and perhaps keeping them from seeing us, or each other as well&lt;/p&gt;&#xA;" OwnerUserId="18870" LastEditorUserId="195" LastEditDate="2018-08-10T00:04:32.950" LastActivityDate="2018-08-10T00:04:32.950" Title="Copernican principle based question about ET" Tags="|communication|fermi-paradox|stars|" AnswerCount="2" CommentCount="14" ContentLicense="CC BY-SA 4.0" />
  <row Id="20722" PostTypeId="2" ParentId="20721" CreationDate="2017-03-20T04:03:57.413" Score="0" Body="&lt;p&gt;If we hypothesize that nothing about our state in spacetime is peculiar I would argue that the question has answered itself: we have not detected ET life because &lt;em&gt;why would we?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under that assumption, there is no basis to believe that our neighboring stars, or indeed our entire galaxy, are &lt;em&gt;privileged&lt;/em&gt; in that regard. And even if they were, there is no reason to believe that &lt;em&gt;they are now&lt;/em&gt;. And even if they were, there is no reason to believe that they &lt;em&gt;could contact us in a manner we could understand&lt;/em&gt;. And even if they did, there is no reason to believe that they &lt;em&gt;would like to contact us&lt;/em&gt;. I argue that under that principle there is no basis to the conjecture that we &lt;em&gt;should have&lt;/em&gt; seen anything at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It sounds &lt;em&gt;reasonable&lt;/em&gt; that life must exist elsewhere, and even be common. (I believe so myself.) But there is a big stretch between that conjecture and the &lt;em&gt;expectation&lt;/em&gt; that we will ever find out, or that we somehow &lt;em&gt;should&lt;/em&gt; find out. Our entire history is less than nothing when compared to the time and scale of the universe. Entire civilizations may have existed and may exist in between &lt;em&gt;blinks&lt;/em&gt;; it may be that we are not special, but just so happened to live through a blink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Primitive or even microbial life? Possibly (Europa! Mars!) Intelligent life? My guess is as good as any. It seems to me that the question is undecidable.&lt;/p&gt;&#xA;" OwnerUserId="18918" LastActivityDate="2017-03-20T04:03:57.413" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20723" PostTypeId="2" ParentId="20584" CreationDate="2017-03-20T04:07:00.800" Score="7" Body="&lt;p&gt;JPL HORIZONS features orbits interpolated from actual data rather than pure simulations. I suspect that what happened here is that there is no orbital data for the three sections of data that don't exhibit the oscillation. We know for a fact that there is no raw data for one of those sections because it is in the future. I spoke to a controls expert and he said my suspicion is probably correct. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you use the webtool it says there are several data series with all but the first having the word predict at the end of the line. The dates at the end of those data series correspond to the returned oscillation of the data. I suspect that a new data series is begun when contact with the spacecraft resumes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the oscillation period of ~14 days corresponds to half of the moon's orbital period. Gravitational perturbation by the Earth would then be the main driver of this behavior. The fact that these oscillations occur quite visibly in the absence of real data indicates that the model is doing a decent job of accounting for this perturbation. As for why the amplitude drops considerably, I can't say for sure since I don't know enough about the model used, but I can guess that it's related to the unaccounted for nonuniform density of the moon and non-gravitational perturbations related to its orientation relative to the sun, such as light pressure and outgassing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at the eccentricity data you will see an oscillation with a period of ~14 days on top of an oscillation with a period of ~27 days, with some higher harmonics scattered. There is also a strong ~27 day period oscillation in the periapsis and apoapsis data. It might be interesting to apply a fourier transform to all of this. You could look at the relative magnitudes of perturbation by period and perhaps try to isolate some behavior with a period of one year. &lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-20T16:10:18.457" LastActivityDate="2017-03-20T16:10:18.457" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20724" PostTypeId="2" ParentId="20721" CreationDate="2017-03-20T04:35:26.750" Score="5" Body="&lt;p&gt;This answer will make some assumptions (which I will state explicitly as much as possible) and simplify things, but it should give a rough explanation as to why detecting extraterrestrial life is difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, we can, of course, use the Drake equation (and all of the assumptions therein) to make a rough estimate of how many civilizations there might be in our galaxy.  Answers to the Drake Equation can range from $N\lt \lt1$ to $N\gt 10^{8}$.  Since we're concerned with the Fermi Paradox, let's go ahead and use the high end of the estimates and say that there should be ~1 million civilizations in the Milky Way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's assume that these civilizations are evenly spread throughout the Milky Way.  In this case, we can model the Milky Way as a disk, calculate its volume, and calculate the density of civilizations.  From this, we'll find the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mean_inter-particle_distance&quot; rel=&quot;nofollow noreferrer&quot;&gt;mean inter-particle distance&lt;/a&gt; to estimate the average distance between civilizations.  Mathematically, this looks like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;d = (\frac{\pi R_{MW}^2H_{disk}}{N})^{\frac{1}{3}}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging in our numbers yields an average distance of 0.088 kpc or 287 light years.  This alone is a very large distance.  Assuming that we can't travel faster than the speed of light, this means that it would take a message from an alien civilization 287 years to reach us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, one problem with this is that it takes a lot of energy to transmit a message 287 light years.  EM radiation intensity falls off with the square of the distance, meaning that signals at that distance will be extremely hard to detect if they're radiated in all directions.  In fact, just about the only kind of signal we'd be able to see is one beamed directly at us.  So there would need to be an alien civilization close enough to detect us in some way, they would need to beam a signal straight at us, and we would need to be watching that area of the sky when it arrived.  All in all, the probability of that happening is extremely low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that covers receiving messages from other civilizations.  But what about indirect methods?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, the best method for indirect detection of aliens is called &quot;&lt;a href=&quot;http://spiff.rit.edu/classes/extrasol/lectures/spectra/spectra.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;transit absorption spectroscopy&lt;/a&gt;&quot;.  If you want more information on how this works, you can check out the answer to &lt;a href=&quot;https://space.stackexchange.com/questions/19484/the-gases-of-technology/19488#19488&quot;&gt;this question&lt;/a&gt;.  The problem with this method is that it needs a telescope sensitive enough to detect small changes in the spectrum of a star.  Currently we can try to do this, but we're really waiting for the James Webb Space Telescope to launch in 2018 to give us better sensitivity and higher resolution when looking at exoplanets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TLDR: Distances in space are very large and that makes it very hard to do detection of civilizations.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2018-08-09T23:00:34.973" LastActivityDate="2018-08-09T23:00:34.973" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20725" PostTypeId="1" AcceptedAnswerId="20726" CreationDate="2017-03-20T06:52:33.610" Score="7" ViewCount="1257" Body="&lt;p&gt;I understand telescopes were used to observe and capture images of other galaxies. But, by risking the question sounding pretty ordinary, may I know how did we take the pictures or photographs of our home, the milky way galaxy (the beautiful spiral that we all are familiar with as shown below), given that Voyager mission took 35 years to even reach the heliopause? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/88iSC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/88iSC.jpg&quot; alt=&quot;Artist&amp;#39;s rendition of milky way galaxy&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source : &lt;a href=&quot;http://www.universetoday.com/wp-content/uploads/2013/10/milky_way.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Universe Today&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure there has to be some technique the scientists use to do that. (They don't have galactic selfie cameras yet, do they?) Can someone throw some light on this topic?&lt;/p&gt;&#xA;" OwnerUserId="18992" LastEditorUserId="18992" LastEditDate="2017-03-23T06:27:20.850" LastActivityDate="2017-03-23T06:27:20.850" Title="How did scientists put forth or create the images of milky way galaxy" Tags="|space-telescope|galactic|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20726" PostTypeId="2" ParentId="20725" CreationDate="2017-03-20T09:06:16.890" Score="10" Body="&lt;p&gt;This is a good question and on-topic as far as I'm concerned. As you correctly point out our furthest reaching spacecraft is only at the heliopause, which is no real distance astronomically, and certainly not far enough to get a glimpse of our galaxy from an angle and distance which would allow that self-portrait. The images of the milky way galaxy are all artistic renditions, they are educated guesses of what our galaxy would look like based on observations of other galaxies and our own. Concentrations of matter have been extensively mapped in our galaxy using visible light and spectrum outside the visible range like infrared, x-rays, and others. These concentrations of matter show the structure of our galaxy even though we cannot observe it from an angle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next logical question is how do we know they are right, and the answer is we don't, not with absolute certainty. The map of our galaxy is based on observations from a single point, and interpreted using an understanding of the physical laws of our universe as developed over hundreds (or thousands) of years by some of the finest minds in history. Yet for all of the effort and brainpower it is still within the realm of possibility that this understanding is wrong in which case our map may be wrong.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's going to take a long time before anyone gets far enough away to check that work in any case. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastEditorUserId="2981" LastEditDate="2017-03-20T12:28:13.270" LastActivityDate="2017-03-20T12:28:13.270" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20727" PostTypeId="1" AcceptedAnswerId="20729" CreationDate="2017-03-20T09:56:49.033" Score="15" ViewCount="2887" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Buran_(spacecraft)#Operational_history&quot; rel=&quot;noreferrer&quot;&gt;Soviet/Russian Buran&lt;/a&gt; have launched vertically, and their wings were exposed to the full load of aerodynamic forces during the launch. Stability may come from many factors, but the location of the wings near the bottom of the whole launch assembly certainly must have helped.&lt;/p&gt;&#xA;&lt;p&gt;On the other hand, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_X-37&quot; rel=&quot;noreferrer&quot;&gt;X-37&lt;/a&gt; was launched into space at the top of a long rocket where lift forces could have presented a big stability challenge, so it was launched protected from aerodynamic forces inside a fairing.&lt;/p&gt;&#xA;&lt;p&gt;Are the US shuttle and Buran the only space planes to launch vertically &lt;em&gt;to orbit&lt;/em&gt; with wings exposed?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below x3:&lt;/strong&gt; X-37 with part of fairing, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Boeing_X-37B_inside_payload_fairing_before_launch.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, Buran from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Buran.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, and Space Shuttle Atlantis STS-79 from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:STS-79_rollout.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vB3cD.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vB3cD.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hV87F.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hV87F.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GyC3B.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GyC3B.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;This one doesn't count, as it didn't go into orbit, but it deserves an honorable mention. From the question &lt;a href=&quot;https://space.stackexchange.com/q/20739/12102&quot;&gt;ISRO's space plane on top of of a rocket - how unstable was it?&lt;/a&gt;, here are some photos of ISRO's &lt;a href=&quot;https://en.wikipedia.org/wiki/RLV-TD&quot; rel=&quot;noreferrer&quot;&gt;RLV-TD&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;In this case it is really a passive payload and does not participate in the launch. According to &lt;a href=&quot;https://space.stackexchange.com/a/20764/12102&quot;&gt;this answer&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;From what I have read in journal papers related to RLV-TD so far control surfaces on winged body don't play any role during ascent phase.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iS8UJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iS8UJm.jpg&quot; alt=&quot;ISRO's RLV-TD Space Plane launch&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/f0LEI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f0LEIm.png&quot; alt=&quot;ISRO's RLV-TD Space Plane launch&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above left:&lt;/strong&gt; From &lt;a href=&quot;https://arstechnica.com/science/2016/05/indias-shuttle-like-reusable-spaceplane-makes-its-first-test-flight/&quot; rel=&quot;noreferrer&quot;&gt;Ars Technica&lt;/a&gt;, photo credit: ISRO. &lt;strong&gt;right:&lt;/strong&gt; From &lt;a href=&quot;https://www.newscientist.com/article/2089757-indias-reusable-space-plane-takes-its-first-test-flight/&quot; rel=&quot;noreferrer&quot;&gt;NewScientist&lt;/a&gt;, photo credit: ISRO.&lt;/p&gt;&#xA;&lt;p&gt;asdf THESE ARE ORIGINAL:&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-06-01T17:07:34.960" Title="Are the US shuttle and Buran the only space planes to have launched vertically with wings exposed?" Tags="|space-shuttle|spaceplane|x-37|buran|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="20728" PostTypeId="2" ParentId="20727" CreationDate="2017-03-20T10:03:37.613" Score="4" Body="&lt;p&gt;Buran, the Russian attempt at a Space Shuttle of course also launched the same way. But it was only a single launch ever. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-03-20T10:03:37.613" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20729" PostTypeId="2" ParentId="20727" CreationDate="2017-03-20T10:09:36.107" Score="13" Body="&lt;p&gt;Yes. Buran and the shuttle are the only ones which entered orbit, launched with wings exposed, and launched vertically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other such designs have also been serious considered. Boeing's X-20 got the closest to flying. It was cancelled shortly after assembly began. The MAKS was cancelled by the collapse of the Soviet Union. The MAKS program started the same year Buran flew, so I'm guessing they weren't satisfied with Buran. Incidentally, I would argue that Buran was also cancelled by the collapse of the Soviet Union and would otherwise have likely flown again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several horizontal air launch space planes have flown to suborbital trajectories. If you count Pegasus as a space plane it actually did achieve orbit but was not designed to survive reentry. Several scale models have launched vertically with wings exposed but only went into suborbital trajectories, though in some cases the apogees were high enough to make the reentry speed comparable to the shuttle.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-20T10:38:17.867" LastActivityDate="2017-03-20T10:38:17.867" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20730" PostTypeId="1" AcceptedAnswerId="20734" CreationDate="2017-03-20T11:12:25.877" Score="5" ViewCount="1790" Body="&lt;p&gt;Are there any good sources to understand how to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Cubesat_Space_Protocol&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cubesat Space Protocol&lt;/a&gt; (CSP) and &lt;a href=&quot;https://en.wikipedia.org/wiki/AX.25&quot; rel=&quot;nofollow noreferrer&quot;&gt;AX.25&lt;/a&gt;? I am supposed to use them for the network in an MSP430 microcontroller between a transceiver (which has an SPI interface) and a cubesat's onboard computer (which has an I2C interface).&lt;/p&gt;&#xA;" OwnerUserId="18996" LastEditorUserId="9060" LastEditDate="2017-03-20T15:25:02.400" LastActivityDate="2017-03-20T15:25:37.530" Title="Cubesat Space Protocol and AX.25" Tags="|communication|design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20731" PostTypeId="2" ParentId="20689" CreationDate="2017-03-20T11:12:44.660" Score="1" Body="&lt;p&gt;&lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See page 37, right at the top. It describes a system for post-integration payload antenna testing. This would not be a passive device, as a passive device would not be able to pass on sufficient information about the waveform produced by the payload's antenna. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm extrapolating here, but it is likely a complex system of antenna elements fed into some very expensive interferometers. This data would be used to reconstruct the waveform to verify antenna gain as well as possibly beam steering and side lobe leakage. The antenna's gain is very difficult to test in the fairing because the available distance is nowhere near the focal length of the antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of EchoStar it may be looking for interference between different elements of the payload's array to make sure it will not produce interference which could result in uneven service.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other big possibility I can think of is that the entire fairing is covered in tuned passive elements that absorb the outgoing signal on the inside and re-emit on the outside. The trouble with this is that the re-emited radiation isn't going in the same direction. You get some phasing but it would still create a lot of noise in the waveform. I suppose that does sound more like what they call it.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-20T12:49:07.883" LastActivityDate="2017-03-20T12:49:07.883" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20732" PostTypeId="2" ParentId="20725" CreationDate="2017-03-20T11:23:39.033" Score="5" Body="&lt;p&gt;We can't take photos of our own galaxy that show the beautiful spiral. From astronomical observations (both from the ground and from satellites like Gaia) we do have lots of data on stars inside our galaxy, including their positions. So we can create a visualization that shows the known stars and their positions. Part of the Milky Way is obscured by dust clouds, so in those directions the visualization has to include some guesswork.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This map shows &lt;a href=&quot;http://www.atlasoftheuniverse.com/milkyway.html&quot; rel=&quot;noreferrer&quot;&gt;radio telescope observations&lt;/a&gt; of the Milky Way. Gray and red are observations. The mostly-black cone radiating upward from our position (at +, straight below the center of the image) is an obscured area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BSgny.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BSgny.gif&quot; alt=&quot;Milky Way hydrogen map&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-03-20T11:28:56.407" LastActivityDate="2017-03-20T11:28:56.407" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20733" PostTypeId="1" AcceptedAnswerId="20747" CreationDate="2017-03-20T12:04:52.143" Score="4" ViewCount="821" Body="&lt;p&gt;According to &lt;a href=&quot;http://www.collectorsweekly.com/stories/181391-rare-vintage-boeing-x-20-dyna-soar-usaf&quot; rel=&quot;noreferrer&quot;&gt;this interesting item&lt;/a&gt;, the artist of this auction-found poster is Robert McCall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4a7zj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4a7zj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But who are the space artists who produced these two images of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_X-20_Dyna-Soar&quot; rel=&quot;noreferrer&quot;&gt;Boeing X-20 Dyna-Soar&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/06Ukr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/06Ukr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bnhd3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bnhd3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AjPxC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AjPxC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XFItR.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XFItR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-03-20T21:09:55.017" Title="Who are the space artists behind these images of the Boeing X-20 Dyna-Soar?" Tags="|space-art|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20734" PostTypeId="2" ParentId="20730" CreationDate="2017-03-20T15:12:03.863" Score="3" Body="&lt;p&gt;I must admit I'd have my doubts AX.25 is my &quot;protocol&quot; of choice for anything designed post-1980. However, there's so much amateur radio material out there that I'll leave it up to you to find a project that vaguely resembles what you need to build. Still, if possible, I'd recommend using a modern protocol; AX.25 is suboptimal in many was, not the least being the strange checksum and packet format.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, Cubesat Space protocol is its own network layer protocol. It's fairly versatile, not totally unlike what people designed TCP/IP for. I think you can &lt;em&gt;use&lt;/em&gt; KISS (which is related to AX.25) as physical layer for CSP. I'd, again, recommend not doing that. For a ground/space/ground link, CCSCP 131.0-B is probably the PHY layer you're looking for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CSP's transmission is also specified for I²C. Since your OCB talks I²C, you'll have to teach your MSP430 to do that, too. But that's normally integrated in the hardware, just use the I²C interface instead of SPI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious source for all CSP-related information is the original university's source code and specs: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://github.com/libcsp/libcsp&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://github.com/libcsp/libcsp&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look in the &quot;doc/&quot; directory.&lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastEditorDisplayName="user17550" LastEditDate="2017-03-20T15:25:37.530" LastActivityDate="2017-03-20T15:25:37.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20737" PostTypeId="1" AcceptedAnswerId="20738" CreationDate="2017-03-20T16:08:05.210" Score="13" ViewCount="1228" Body="&lt;p&gt;In an answer to this question &lt;a href=&quot;https://space.stackexchange.com/questions/20727/are-the-us-shuttle-and-buran-the-only-space-planes-to-have-launched-vertically-w&quot;&gt;Are the US shuttle and Buran the only space planes to have launched vertically with wings exposed?&lt;/a&gt; &lt;a href=&quot;https://space.stackexchange.com/users/18744/schlusstein&quot;&gt;Schlusstein&lt;/a&gt; mentioned the proposed MAKS system, which was new to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OMe7s.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OMe7s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;(image by By Titutik at Russian Wikipedia, CC BY-SA 3.0, &lt;a href=&quot;https://commons.wikimedia.org/w/index.php?curid=29868251&quot; rel=&quot;noreferrer&quot;&gt;https://commons.wikimedia.org/w/index.php?curid=29868251&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It sounds pretty interesting but I'm having trouble visualizing how this sat on/was carried by the Antonov 225 carrier plane.  Is there any concept art or photos of how the mated vehicles looked?  My google search has been fruitless - maybe I need some Russian terms.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-03-20T20:56:42.927" Title="Seeking concept art or photo of MAKS on carrier plane" Tags="|conceptual|spaceplane|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20738" PostTypeId="2" ParentId="20737" CreationDate="2017-03-20T16:11:36.527" Score="15" Body="&lt;p&gt;Here you go: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3YKRw.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3YKRw.jpg&quot; alt=&quot;MAKS&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and another &lt;a href=&quot;http://www.buran.ru/htm/molniya6.htm&quot; rel=&quot;noreferrer&quot;&gt;drawing&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aGs3A.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aGs3A.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-03-20T16:11:36.527" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20739" PostTypeId="1" AcceptedAnswerId="20764" CreationDate="2017-03-20T16:22:26.667" Score="13" ViewCount="813" Body="&lt;p&gt;ISRO's space plane was launched on top of a rocket into a sub-orbital trajectory, with separation from the lift vehicle around 45 km and a maximum altitude of about 65 km, based on an analysis of the official ISRO launch video described in the &lt;a href=&quot;https://www.reddit.com/r/ISRO/comments/4ma78y/here_is_what_plotted_time_vs_altitude_data_looks/&quot; rel=&quot;noreferrer&quot;&gt;ISRO fan reddit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With wings at the top of a rocket, was this likely to have been extremely unstable? How was the flight controlled? Were there active surfaces on the space plane's wings that participated in stabilizing the flight? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iS8UJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iS8UJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;https://arstechnica.com/science/2016/05/indias-shuttle-like-reusable-spaceplane-makes-its-first-test-flight/&quot; rel=&quot;noreferrer&quot;&gt;Ars Technica&lt;/a&gt;, photo credit: ISRO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/f0LEI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f0LEI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;https://www.newscientist.com/article/2089757-indias-reusable-space-plane-takes-its-first-test-flight/&quot; rel=&quot;noreferrer&quot;&gt;NewScientist&lt;/a&gt;, photo credit: ISRO.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-03-22T06:13:57.490" LastActivityDate="2017-03-22T06:13:57.490" Title="ISRO's space plane on top of of a rocket - how unstable was it?" Tags="|isro|spaceplane|stability|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20742" PostTypeId="2" ParentId="20739" CreationDate="2017-03-20T18:22:55.637" Score="6" Body="&lt;p&gt;The fins on the HS9 booster are much larger than what you'd normally see on a rocket with a compact payload under a fairing, which contributes to stability passively, and according to this article, they are providing active steering as well:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasaspaceflight.com/2016/05/india-launch-reusable-launch-vehicle-demonstrator-mission/&quot; rel=&quot;noreferrer&quot;&gt;https://www.nasaspaceflight.com/2016/05/india-launch-reusable-launch-vehicle-demonstrator-mission/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-03-20T18:22:55.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20743" PostTypeId="2" ParentId="14161" CreationDate="2017-03-20T18:57:34.447" Score="12" Body="&lt;p&gt;That maneuver was called &quot;fire in the hole&quot;. See the wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Fire_in_the_hole&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt;. According to this NASA &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/a15mrp9.pdf&quot; rel=&quot;noreferrer&quot;&gt;paper&lt;/a&gt; there was an extra &quot;fire-in-the-hole shield&quot;. During Apollo 5, 9 and 10 there was a test of &quot;fire-in-the-hole&quot;, see &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_5&quot; rel=&quot;noreferrer&quot;&gt;1&lt;/a&gt; ,  &lt;a href=&quot;http://www.thespacereview.com/article/2462/1&quot; rel=&quot;noreferrer&quot;&gt;2&lt;/a&gt; and &lt;a href=&quot;https://history.nasa.gov/SP-4029/Apollo_09a_Summary.htm&quot; rel=&quot;noreferrer&quot;&gt;3&lt;/a&gt;. The LM test of Apollo 5 was done unmanned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The insignia of the mission Apollo 5 was designed by Grumman engineers to show the fire in the hole test.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/N9lbz.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/N9lbz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2018-07-25T11:26:05.497" LastActivityDate="2018-07-25T11:26:05.497" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="20744" PostTypeId="2" ParentId="20737" CreationDate="2017-03-20T19:32:24.597" Score="10" Body="&lt;p&gt;There are quite a few drawings in the first link&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.buran-energia.com/documentation/documentation-akc-maks-multipurpose.php&quot; rel=&quot;noreferrer&quot;&gt;http://www.buran-energia.com/documentation/documentation-akc-maks-multipurpose.php&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related development: &lt;a href=&quot;https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-105&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-105&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HtW73.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HtW73.jpg&quot; alt=&quot;https://2ch.hk/spc/src/242910/14514294138961.jpg&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://topwar.ru/uploads/posts/2010-09/1283451927_max005c.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://topwar.ru/uploads/posts/2010-09/1283451927_max005c.jpg&quot; alt=&quot;https://topwar.ru/uploads/posts/2010-09/1283451927_max005c.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://3.bp.blogspot.com/-A7lXnZHTuFA/Vs-mEmXL8EI/AAAAAAAAEVI/1WAlR0huv5U/s1600/55097445.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://3.bp.blogspot.com/-A7lXnZHTuFA/Vs-mEmXL8EI/AAAAAAAAEVI/1WAlR0huv5U/s1600/55097445.jpg&quot; alt=&quot;https://3.bp.blogspot.com/-A7lXnZHTuFA/Vs-mEmXL8EI/AAAAAAAAEVI/1WAlR0huv5U/s1600/55097445.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EwL1E.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EwL1E.jpg&quot; alt=&quot;https://i.stack.imgur.com/EwL1E.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BOR-4 (related development):&lt;br&gt;&#xA;&lt;a href=&quot;https://i.ytimg.com/vi/r4VLRt1fIbA/maxresdefault.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.ytimg.com/vi/r4VLRt1fIbA/maxresdefault.jpg&quot; alt=&quot;https://i.ytimg.com/vi/r4VLRt1fIbA/maxresdefault.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-20T20:04:46.500" LastActivityDate="2017-03-20T20:04:46.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20745" PostTypeId="2" ParentId="10803" CreationDate="2017-03-20T19:41:54.170" Score="4" Body="&lt;p&gt;Yes! I discussed this with a person a few months ago, to include a few of the things mentioned here by others. We also discussed the balloon being solar powered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea here is using a balloon to get as high as possible, might potentially negate a large portion of the fuel and bulk ones of the rocket, lowering cost and complexity, also (potentially, with a lot of friends in the right places, or a disregard for law) the rocket could launch far away from land, over an ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The person I spoke with had (I think) an ingenious idea to recycle the balloon. I.E.: instead of it being a stage that separates, it folds up into a protected portion of the rocket, deployable upon Reentry or a failure to achieve orbit, as well as a possible energy source for a simple camera, or a comms system, potentially allowing the entire system to be used multiple times. This might however, count as cargo.&lt;/p&gt;&#xA;" OwnerUserId="18870" LastActivityDate="2017-03-20T19:41:54.170" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20747" PostTypeId="2" ParentId="20733" CreationDate="2017-03-20T21:00:01.113" Score="6" Body="&lt;p&gt;First image is apparently by Robert McCall&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.collectorsweekly.com/stories/181391-rare-vintage-boeing-x-20-dyna-soar-usaf&quot; rel=&quot;noreferrer&quot;&gt;http://www.collectorsweekly.com/stories/181391-rare-vintage-boeing-x-20-dyna-soar-usaf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second is Frederick E Fahdt (also, better image quality)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://2ch.hk/spc/src/242910/14514290041960.png&quot; rel=&quot;noreferrer&quot;&gt;https://2ch.hk/spc/src/242910/14514290041960.png&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've got nothing on the third one.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-20T21:09:55.017" LastActivityDate="2017-03-20T21:09:55.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20749" PostTypeId="2" ParentId="13473" CreationDate="2017-03-20T21:45:30.830" Score="7" Body="&lt;p&gt;Conformal coatings mostly, just like on Earth. A thin polymer layer impedes problematic whiskers principally by confining them to the inside of the coating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Theses links confirm that NASA has a large interest in the study of conformal coatings and likes to use them.  The first one includes pictures of whiskers that formed under coatings towards the end. &#xA;&lt;a href=&quot;https://nepp.nasa.gov/whisker/reference/tech_papers/2006-Woodrow-Conformal-Coating-PartII.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://nepp.nasa.gov/whisker/reference/tech_papers/2006-Woodrow-Conformal-Coating-PartII.pdf&lt;/a&gt;&lt;br&gt;&#xA;This is a presentation about from NASA about the hazards of whiskers and how they are mitigated.&lt;br&gt;&#xA;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070021449.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070021449.pdf&lt;/a&gt;&lt;br&gt;&#xA;This one just lists some companies that NASA uses for confomal coatings. Possibly of interest.&lt;br&gt;&#xA;&lt;a href=&quot;http://blog.paryleneconformalcoating.com/blog/nasa-conformal-coating-companies&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://blog.paryleneconformalcoating.com/blog/nasa-conformal-coating-companies&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18744" LastEditorUserId="18744" LastEditDate="2017-03-22T15:20:30.677" LastActivityDate="2017-03-22T15:20:30.677" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20750" PostTypeId="2" ParentId="10803" CreationDate="2017-03-20T21:48:57.713" Score="6" Body="&lt;p&gt;Although it stretches the definition of home made, &lt;a href=&quot;http://jpaerospace.com/&quot; rel=&quot;noreferrer&quot;&gt;JP Aerospace&lt;/a&gt; is a very small company run by John Powell.  They started out attempting just that.  Their current plan is to build an airship that is a &quot;V&quot; shaped airfoil.  They plan to spin it out of the atmosphere using both thrust and lift to get to orbital velocities (with the initial workload handled by buoyancy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their plan is to trade time for efficiency, taking far longer to get up there than a rocket but using far less fuel.&lt;/p&gt;&#xA;" OwnerUserId="18842" LastActivityDate="2017-03-20T21:48:57.713" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20752" PostTypeId="2" ParentId="13473" CreationDate="2017-03-21T02:56:21.440" Score="12" Body="&lt;p&gt;Often, electrical systems prone to whisker growth are coated with a 2-3 millimeter layer of polymer (called a conformal coat); this not only prevents growth of whiskers, but also prevents any loose electrical contacts from creating short circuits. There are many benefits that are associated with doing this. Another likely cause of crystalline whisker growth may be residual stress of the circuit after the solder has been electroplated; thus, several strategies, such as using dip-soldering, or hot-oil reflow, are used to release the tension. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the better strategies for mitigating tin-whisker growth, however, is to simply avoid tin-zinc solders; this, however, may not be the most viable option as replacements aren't exactly the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More detail can be found on &lt;a href=&quot;https://www.calce.umd.edu/tin-whiskers/TINWHISKERMITIGATION.pdf&quot; rel=&quot;noreferrer&quot;&gt;this site.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19004" LastEditorUserId="19004" LastEditDate="2018-08-14T18:11:02.097" LastActivityDate="2018-08-14T18:11:02.097" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="20753" PostTypeId="1" CreationDate="2017-03-21T05:57:38.343" Score="3" ViewCount="144" Body="&lt;p&gt;To conduct heat, a medium is required (e.g. Atmosphere)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But at higher altitudes atmosphere is too thin, yet Wikipedia states temperatures in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Thermosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermosphere&lt;/a&gt; can rise to 2,000$^{\circ}$C&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electromagnetism can travel through vacuum, but is thermodynamics that simple? ISS uses radiators for thermal control, how do those &quot;radiate&quot; heat? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it just &quot;radiation&quot;? &lt;/p&gt;&#xA;" OwnerUserId="192" LastEditorUserId="192" LastEditDate="2017-03-21T07:53:50.560" LastActivityDate="2017-03-21T07:53:50.560" Title="How does &quot;temperature&quot; work in vacuum?" Tags="|temperature|thermal|thermal-control|" AnswerCount="0" CommentCount="7" ClosedDate="2017-03-21T11:53:48.613" ContentLicense="CC BY-SA 3.0" />
  <row Id="20754" PostTypeId="1" AcceptedAnswerId="20757" CreationDate="2017-03-21T06:10:26.307" Score="10" ViewCount="6896" Body="&lt;p&gt;I don't really understand why do we use spin stabilisation for rocket launches?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I understand correctly, the idea is to make the rocket spin really fast so that any asymmetry in the thrust/aerodynamics will average out and keep the rocket going straight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus question:&#xA;How can one provide guidance for such launches? Especially how can they achieve a gravity turn...&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6537" LastEditDate="2017-12-19T06:02:24.507" LastActivityDate="2019-07-05T19:47:31.703" Title="Why does spinning help stabilisation?" Tags="|rockets|flight-control|spin-stabilisation|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20755" PostTypeId="1" AcceptedAnswerId="20759" CreationDate="2017-03-21T07:03:35.647" Score="20" ViewCount="3617" Body="&lt;p&gt;The Ares I-X was a test flight from the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Constellation_program&quot; rel=&quot;noreferrer&quot;&gt;Constellation program&lt;/a&gt; for crewed Ares I-launched &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Orion_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Orion capsules.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dT7nX.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dT7nX.jpg&quot; alt=&quot;Ares I-X max q&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If liquid fueled rocket motors can gimbal, how do solid rocket motor-propelled spacecraft control their attitude, especially when performing a &lt;a href=&quot;http://www.spaceflightnow.com/ares1x/091020pad/&quot; rel=&quot;noreferrer&quot;&gt;&quot;pad avoidance maneuver&quot;&lt;/a&gt; directly after ignition? &lt;/p&gt;&#xA;" OwnerUserId="192" LastEditorUserId="192" LastEditDate="2017-03-21T08:01:27.567" LastActivityDate="2023-06-10T16:07:31.607" Title="How does a single SRB control attitude?" Tags="|attitude|srb|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20756" PostTypeId="2" ParentId="20755" CreationDate="2017-03-21T07:57:58.513" Score="12" Body="&lt;p&gt;Solid rocket booster nozzles can gimbal. If you take the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster&quot; rel=&quot;noreferrer&quot;&gt;SRB&lt;/a&gt; of the Space Shuttle:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each SRB had two hydraulic gimbal servoactuators, to move the nozzle up/down and side-to-side. This provided thrust vectoring to help control the vehicle in all three axes (roll, pitch, and yaw).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You will find more explanation on &lt;a href=&quot;https://en.wikipedia.org/wiki/Thrust_vectoring#Rockets_and_ballistic_missiles&quot; rel=&quot;noreferrer&quot;&gt;this Wikipedia article&lt;/a&gt;, where I found this picture:&#xA;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/7/7a/En_Gimbaled_thrust_diagram.svg/220px-En_Gimbaled_thrust_diagram.svg.png&quot; alt=&quot;Moments generated by different thrust gimbal angles&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, rockets can board several devices, as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_control_system&quot; rel=&quot;noreferrer&quot;&gt;Reaction Control System&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_wheel&quot; rel=&quot;noreferrer&quot;&gt;Reaction wheel&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18592" LastActivityDate="2017-03-21T07:57:58.513" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20757" PostTypeId="2" ParentId="20754" CreationDate="2017-03-21T08:46:54.867" Score="5" Body="&lt;p&gt;I made some researchs since my comment, and especially on &lt;a href=&quot;https://en.wikipedia.org/wiki/Rifling&quot; rel=&quot;nofollow noreferrer&quot;&gt;rifles&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In firearms, rifling consists of helical grooves in the internal (bore) surface of a gun's barrel, which impart a spin to a projectile around its long axis. This spin serves to &lt;strong&gt;gyroscopically stabilize the projectile&lt;/strong&gt;, improving its &lt;strong&gt;aerodynamic stability and accuracy&lt;/strong&gt;.&#xA;  &lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/105mm_tank_gun_Rifling.jpg/800px-105mm_tank_gun_Rifling.jpg&quot; alt=&quot;105mm tank run riffling&quot;&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;On the rifling depends the &lt;a href=&quot;https://en.wikipedia.org/wiki/Rifle#Bullet_Rotational_Speed_.28RPM.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;bullet rotational speed&lt;/a&gt;, and figures can go up to 270,000RPM, a rotation speed a rocket will hopefully never achieve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Talking about rockket, &lt;a href=&quot;https://en.wikipedia.org/wiki/Spin-stabilisation&quot; rel=&quot;nofollow noreferrer&quot;&gt;spin stabilization&lt;/a&gt; is not communly used, especially during ascent and it has been superseded by &lt;a href=&quot;https://en.wikipedia.org/wiki/Attitude_control#Types_of_stabilization&quot; rel=&quot;nofollow noreferrer&quot;&gt;three-axis stabilisation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, it seems that this method is more often used while in space, and I found a very interesting video &lt;a href=&quot;https://commons.wikimedia.org/w/index.php?title=File%3APAM-D_1920x1080.ogv&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (but I did not succeed to put it here embedded).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you said in OP, the spacecraft is successively spun, fired, and de-spun (a solution to de-spun for &lt;strong&gt;free&lt;/strong&gt; can be a &lt;a href=&quot;https://en.wikipedia.org/wiki/Yo-yo_de-spin&quot; rel=&quot;nofollow noreferrer&quot;&gt;yo-yo de-spin&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus answer to your bonus question: if spin stabilization is not use during ascent but once the payload reaches space, then there is actually no question about how to manage the gravity turn while spining.&lt;/p&gt;&#xA;" OwnerUserId="18592" LastEditorUserId="18592" LastEditDate="2017-03-21T09:11:18.597" LastActivityDate="2017-03-21T09:11:18.597" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20758" PostTypeId="1" CreationDate="2017-03-21T12:06:44.207" Score="0" ViewCount="1127" Body="&lt;p&gt;My project involves developing a multi-mission modular satellite bus. I have couple of payloads under consideration and have the necessary sensor details. Based on that I am trying to figure the pointing and stability requirements of the satellite.&lt;/p&gt;&#xA;" OwnerUserId="19010" LastActivityDate="2017-03-22T11:17:49.790" Title="How do I calculate pointing and stability requirements for a platform (Cubesat, UAV) if we know sensor details (IFOV, Integration Time etc)?" Tags="|cubesat|imaging|earth-observation|remote-sensing|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20759" PostTypeId="2" ParentId="20755" CreationDate="2017-03-21T12:22:10.693" Score="25" Body="&lt;p&gt;The Ares 1-X used stock Shuttle program SRB thrust vector control (TVC) -  hydrazine fueled power units drove hydraulic pumps which powered actuators that could tilt and rock the nozzle, which incorporated a flexible bearing in its design. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/C0N9U.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C0N9U.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/chlBa.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/chlBa.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;(Pictures from &quot;Space Shuttle&quot;, Jenkins, 1992 edition p.263 and &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwi-ia_AnuvSAhXjy1QKHW6uBvgQFggcMAA&amp;amp;url=https%3A%2F%2Fwww.nasa.gov%2Fcenters%2Fjohnson%2Fpdf%2F390651main_shuttle_crew_operations_manual.pdf&amp;amp;usg=AFQjCNENi1Wfkn12N9uV7BJ3FfCXYBLgag&amp;amp;sig2=slBWyMaEgB2o3GqvlSn_5A&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; - p 2.13-48 of linked PDF)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, unlike the Shuttle system with its 2 SRBs, a single SRB with articulating nozzle cannot provide roll control, only pitch and yaw. So Ares 1-X had 2 unique modules containing hypergolic thrusters (derived from the Peacekeeper missile) mounted on it to provide control in the roll axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9LCi3.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9LCi3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Picture from &lt;a href=&quot;https://blogs.nasa.gov/Ares_I-X/tag/roll-control-system/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I-X#Roll_control_system&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Ares_I-X#Roll_control_system&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="195" LastEditDate="2018-02-17T14:16:56.663" LastActivityDate="2018-02-17T14:16:56.663" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20760" PostTypeId="1" CreationDate="2017-03-21T13:12:32.677" Score="12" ViewCount="682" Body="&lt;p&gt;The next generation crewed spacecrafts is about to fly in the decade of 2020. Not only those in the headline here are trying to make it, but more are being developed privately and internationally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion will be able to support its crew longer than the current commercial contenders, 4 astronauts during 21 days I've heard. First of all, is that basically true? And if so, is that a deep early design choice, or just a formality of the specifications because Orion is planned to fly around the Moon while the commercial contenders are contracted for LEO only?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would it take to extend the crew support lifetime of a spacecraft of this next generation? Say, to make Starliner and Dragon also being able to support 4 crew during 21 days. Would it be enough to extend the size of the service module in order to increase storages of consumables? Or to replace a consumed service module with a fresh one, doubling the life support time?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they just need more luggage or a differently designed ship?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-03-21T14:17:10.627" LastActivityDate="2020-01-12T19:12:30.983" Title="What limits how long spacecrafts like Orion, Dragon, Starliner can support their crews' lives?" Tags="|spacecraft-development|life-support|orion-spacecraft|dragon-v2|cst-100|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20762" PostTypeId="2" ParentId="20760" CreationDate="2017-03-21T16:38:22.217" Score="4" Body="&lt;p&gt;Orion as a capsule will have an actual Service Module attached. It is being provided by ESA, as a highly modified version of the ATV cargo transport. This is a pretty large add on, and provides both fuel for engines for maneuvering and supplies for the crew. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon and Starliner do not have quite the same Service Module. Dragon has a trunk, which in the cargo and crew versions have solar panels on it. Like Orion it is discarded before reentry, but the SpaceX trunk is used for external unpressurized cargo carriage currently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the consumables (Oxygen, Nitrogen, maneuvering fuel) need to be held somewhere. Orion has space in the Service Module. Whereas Starliner and Dragon will need to carry it internally which limits them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, you REALLY want a separate space for the potty. Soyuz when in orbit has two separate modules that can be used. Apollo did not but could use the LM vs the CM during most lunar flights.  Orion is big enough they can probably carve out a corner for this sort of stuff. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-03-21T21:06:07.693" LastActivityDate="2017-03-21T21:06:07.693" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20763" PostTypeId="2" ParentId="20755" CreationDate="2017-03-21T17:35:47.120" Score="14" Body="&lt;p&gt;Apart from having an actuated nozzle, an inert gas or reactive liquid can be injected into the nozzle of an SRB to dynamically deflect or shape the exhaust, thereby vectoring the thrust.&lt;/p&gt;&#xA;&lt;p&gt;ISRO's &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Polar Satellite Launch Vehicle&lt;/a&gt; (PSLV) uses strontium perchlorate from N2 pressure-fed tanks attached on side of core introduced into the SRB exhaust through an array of injectors around the exhaust nozzle. The Secondary Injection Thrust Vector Control (SITVC) system is shown on page 8 of the Oct-92-Mar-93 issue of Space India (&lt;a href=&quot;https://www.isro.gov.in/media_isro/pdf/ResourcesPdf/SpaceIndia/publication(43).pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;PDF&lt;/a&gt;,Pg. 8), and is shown below, where engineers can be seen working on the ring of injectors near the top of the nozzle.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8nMia.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8nMia.png&quot; alt=&quot;SITVC Injectors around exhaust nozzle&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;SITVC is also present on a few of the strapon SRBs of PSLV; two strapons (PSOM-4,5) in PSLV-XL and one in PSLV-G.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oJfov.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oJfov.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Similarly, N2O4 powered Liquid injection thrust vector control (LITVC) was used on Titan IIIC.&lt;/p&gt;&#xA;" OwnerUserId="19014" LastEditorUserId="19014" LastEditDate="2023-06-10T16:07:31.607" LastActivityDate="2023-06-10T16:07:31.607" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="20764" PostTypeId="2" ParentId="20739" CreationDate="2017-03-21T17:58:58.827" Score="8" Body="&lt;p&gt;Just to correct original question in RLV-TD (Reusable Launch Vehicle Technology Demonstration) mission the winged body TDV (Technology Demonstration Vehicle) separated at 56 km altitude and proceeded to reach altitude of about 65 km before descent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes RLV-TD configuration is unsymmetrical and unstable, former chairman of ISRO K Radhakrishnan briefly talks about that aerodynamic instability of RLV-TD, μα being 20 to 25 compared to 1 for PSLV/GSLV and 5 for LVM3 in this talk at 42 min 50 sec mark.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/tmpzUJbYAEo?start=2570&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For control during ascent HS9 booster has RCS + SITVC system(Pitch+Yaw) for first few seconds of ascent, and later &quot;fin tip control&quot; as shown in following slide from presentation by M. V. Dhekane. Those four movable fins are attached to base shroud of HS9 booster. From what I have read in journal papers related to RLV-TD so far control surfaces on winged body don't play any role during ascent phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D7RRn.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D7RRn.jpg&quot; alt=&quot;RLV-TD Stack&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L3oUK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L3oUK.jpg&quot; alt=&quot;HEX 01 Flight Profile&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.sc.iitb.ac.in/~pdsc2014/MVD_talk.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.sc.iitb.ac.in/~pdsc2014/MVD_talk.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Official &lt;a href=&quot;http://www.isro.gov.in/update/23-may-2016/india%E2%80%99s-reusable-launch-vehicle-technology-demonstrator-rlv-td-successfully&quot; rel=&quot;noreferrer&quot;&gt;press release&lt;/a&gt; gives some of flight profile details. They also released a &lt;a href=&quot;http://www.isro.gov.in/sites/default/files/videos/rlvtd2.mp4&quot; rel=&quot;noreferrer&quot;&gt;Time Vs Altitude visualization&lt;/a&gt;[MP4] based on telemetry from RLV-TD during flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.isro.gov.in/launchers/rlv-take-videos&quot; rel=&quot;noreferrer&quot;&gt;http://www.isro.gov.in/launchers/rlv-take-videos&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have plotted it in graph form &lt;a href=&quot;https://www.reddit.com/r/ISRO/comments/4ma78y/here_is_what_plotted_time_vs_altitude_data_looks/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qp60r.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qp60r.png&quot; alt=&quot;RLV-TD Time vs Altitude&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quote from paper titled &quot;Particle Swarm Optimization Based PI Controller Design for Actuation System of Reusable Launch Vehicle&quot;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;RLV consists of a booster stage and a fly back portion. During the ascent phase, it will be controlled by using four fin actuators. During re-entry and return flight, the altitude will be controlled by two primary control surfaces, ie, elevons and rudder.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.irjet.net/archives/V2/i5/IRJET-V2I594.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://www.irjet.net/archives/V2/i5/IRJET-V2I594.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19014" LastEditorUserId="19014" LastEditDate="2017-03-22T05:37:00.553" LastActivityDate="2017-03-22T05:37:00.553" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20765" PostTypeId="1" AcceptedAnswerId="21091" CreationDate="2017-03-21T18:19:32.110" Score="20" ViewCount="1325" Body="&lt;p&gt;Currently everything used by astronauts in space gets there via a state/government sponsored lift. Astronauts can also bring a few personal supplies, but I imagine that most if not all financial considerations occur at government level electronically on earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If an astronaut has a candy bar, or something else of value that they are interested in exchanging with another astronaut, is cash (any countries' money) ever used in that exchange? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="16590" LastEditDate="2017-03-31T09:04:13.807" LastActivityDate="2019-03-20T23:27:35.553" Title="Is there any money in space?" Tags="|iss|" AnswerCount="3" CommentCount="19" ContentLicense="CC BY-SA 3.0" />
  <row Id="20767" PostTypeId="2" ParentId="20760" CreationDate="2017-03-21T20:16:03.337" Score="4" Body="&lt;p&gt;There should be enough supply of everything needed for life support. Oxygen at first, but also the absorbers needed for the removal of carbon dioxide from the atmosphere in the spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water for drinking will be also important, sometimes also used for cooling of the spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spare filters for cleaning the atmosphere, removing dust and odors. Electrical power for the life support system and the space ship, fuel used for maneuvering and attitude control. If an atmosphere containing not only oxygen but also nitrogen is used also a supply of nitrogen to replace leakage loss. Food would be necessary too and some free space for storage of waste.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2020-01-12T19:12:30.983" LastActivityDate="2020-01-12T19:12:30.983" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="20768" PostTypeId="1" AcceptedAnswerId="20776" CreationDate="2017-03-21T21:21:18.067" Score="19" ViewCount="2174" Body="&lt;p&gt;What power sources are viable on a 'long term' Venus lander?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;RTG&lt;/a&gt; relies on a temperature differential, and is not likely to be doable with a 457C 'cold side'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on this &lt;a href=&quot;https://space.stackexchange.com/a/12561/16322&quot;&gt;answer&lt;/a&gt; there is not enough light for a solar panel to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I know this leaves batteries, but NiMH will fail around &lt;a href=&quot;http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures&quot; rel=&quot;nofollow noreferrer&quot;&gt;65C&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And hydrogen fuel cells come in many varieties, but at least one seems to be able to operate in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Fuel_cell#High-temperature_fuel_cells&quot; rel=&quot;nofollow noreferrer&quot;&gt;500-1000C&lt;/a&gt; range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that work is progressing on &lt;a href=&quot;http://www.eetimes.com/document.asp?doc_id=1327322&amp;amp;_mc=NL_EET_EDT_EET_daily_20150803&amp;amp;cid=NL_EET_EDT_EET_daily_20150803&amp;amp;elq=69756d91c25c4d1a82e352046f6dc7f8&amp;amp;elqCampaignId=24193&amp;amp;elqaid=27324&amp;amp;elqat=1&amp;amp;elqTrackId=1282d02ef1c445739ca5599b25089647&quot; rel=&quot;nofollow noreferrer&quot;&gt;high temperature&lt;/a&gt; chips, a high temperature rover seems at least possible.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-03-23T11:24:56.550" Title="Potential high temperature power sources for a Venus lander" Tags="|venus|power|lander|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20770" PostTypeId="2" ParentId="20768" CreationDate="2017-03-21T22:58:06.787" Score="6" Body="&lt;p&gt;For most practical probes, batteries are the only real option. RTGs are for missions that last years, and Venus is an environment where &quot;long duration&quot; translates to &quot;three hours&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For that sort of timeframe you don't need recharging, and keeping the inside of the probe cool can be as simple as insulation and maybe a small supply of ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you do manage to engineer all the other components to survive long term, a wind turbine would be a good choice. Venus surface conditions are no more extreme than the inside of a common jet engine, so the engineering involved is fairly well known.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-03-21T22:58:06.787" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20771" PostTypeId="2" ParentId="20768" CreationDate="2017-03-22T02:50:52.717" Score="13" Body="&lt;p&gt;An RTG certainly can and would work on Venus, since the hot side is about 1200 C. It just wouldn't be as efficient as it would be with a colder cold side. Search for papers by Geoff Landis on this.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-03-22T02:50:52.717" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20772" PostTypeId="2" ParentId="20652" CreationDate="2017-03-22T04:05:09.847" Score="0" Body="&lt;p&gt;In my limited work with the big ranges (e.g. VAFB or CCAFS) a machine is definitely going to be faster. That's not throwing shade at the dedicated people who work there, only that each rocket, nay launch, is treated as a special case. With an autonomous flight safety system (AFSS), all the coordination is done up-front, once, and then not revisited with every launch. It’s really no different than the value of automating any process. There's an up-front investment paid back as an annuity thereafter.&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2017-03-22T04:05:09.847" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20775" PostTypeId="2" ParentId="20758" CreationDate="2017-03-22T11:17:49.790" Score="2" Body="&lt;p&gt;The pointing stability requirement of a platform is &lt;em&gt;dictated by the payload that will use the platform&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What will the payload do? What pointing accuracy and stability does it require to function well and do what it is supposed to do? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just as one particular example, if it has a camera and will photograph the Earth, then you need to look into the magnification - how many pixels per degree, and the exposure time you'll need. Do you need to spin the satellite in order to slew the field of view in order to track the image of the Earth's surface with its apparent motion of 7 kilometers per second, or do you just need to hold steady?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Start with the purpose of the mission and work backwards to find technical requirements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to include some more information about the payload's mission and function, that might help attract a more helpful answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;However:&lt;/strong&gt; you might have meant to ask about the pointing and stability &lt;strong&gt;performance&lt;/strong&gt; rather than requirement. In that case you should revise the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any event, please leave a comment here if/when you revise the question.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-03-22T11:17:49.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20776" PostTypeId="2" ParentId="20768" CreationDate="2017-03-22T12:46:48.983" Score="25" Body="&lt;p&gt;Thanks to @MarkAddler for his &lt;a href=&quot;https://space.stackexchange.com/a/20771/12102&quot;&gt;search suggestions&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've tried to balance length against completeness, and both lost here. However I have included enough material to try to be convincing that RTGs, singe-use storage batteries, and rechargable batteries for higher power events have all been investigated and solutions exist to provide &lt;em&gt;at least the electrical power&lt;/em&gt; for a long-term lander or rover on Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fear about batteries comes from the idea of putting Consumer batteries on a Venus lander at 460C. Of course that's the wrong kind of battery to think about, and there are several kinds already demonstrated to work at these temperatures, some even in use in low volume on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RTGs work fine on Venus. In fact, the high density of the atmosphere ~67 kg/m^3, or 6.7% of liquid water; will cool the sink end of the RTG much much more effectively than the RTGs in space or on the Moon or Mars. With source (hot) end at 1200C, thermodynamic efficiency would be better than half of an RTG in space.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;PAPER I. RTG ON VENUS &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0094576507001208&quot; rel=&quot;nofollow noreferrer&quot;&gt;doi:10.1016/j.actaastro.2006.12.031 paywalled&lt;/a&gt;&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OVtLz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OVtLz.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the analysis case, we assumed thermoelectric&#xA;  converters similar to those used on Cassini [ref].&#xA;  While the high temperature of waste-heat rejection to&#xA;  the Venus atmosphere reduces the theoretical Carnot&#xA;  efficiency of any thermal converter, the density of the&#xA;  atmosphere means that heat transfer is very efficient,&#xA;  and hence the required area of the convective radiators&#xA;  is small.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The assumed hot-side temperature (Th) is 1350 K, and the cold-side temperature (Tc) ejected to the radiator is 870 K. The calculated net thermal to electrical efficiency was 0.05 (5%). A GPHS heat input Qh of 594W was required to produce 30W of output electrical power. The total heat rejected is 564W (Table 2). &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Three such units are required for 100W of electrical power...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Table 1: Power system trade-offs&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Radioisotope power source&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Demonstrated in space&lt;/li&gt;&#xA;&lt;li&gt;Dynamic [14] or thermoelectric [refs] conversion approaches are possible&lt;/li&gt;&#xA;&lt;li&gt;460◦C is a higher heat rejection temperature than conventional dynamic conversion approaches Radioisotope was chosen as the baseline technology for the Venus rover&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Microwave beamed power&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Station in atmosphere produces solar power; power is transmitted to surface by microwaves [ref] Not demonstrated in Venus environment&lt;/li&gt;&#xA;&lt;li&gt;Many technical questions need to be answered&lt;/li&gt;&#xA;&lt;li&gt;Chosen as a backup approach—not analyzed in detail&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Solar power&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Solar power is difficult due to low light levels at surface [ref]&lt;/li&gt;&#xA;&lt;li&gt;High temperature at surface makes photovoltaic conversion inefficient&lt;/li&gt;&#xA;&lt;li&gt;Approach would require new technologies to be developed [ref]&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Chemical (battery or fuel cell) storage&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Requires high-temperature technology&lt;/li&gt;&#xA;&lt;li&gt;Practical approach for short missions or low powers&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0ry1I.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0ry1I.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;PAPER II. BATTERIES ON VENUS &lt;a href=&quot;https://arc.aiaa.org/doi/pdf/10.2514/1.41886&quot; rel=&quot;nofollow noreferrer&quot;&gt;DOI: 10.2514/1.41886 paywalled&lt;/a&gt;.&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0J5vv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0J5vv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;&lt;em&gt;Thermal Batteries (single use):&lt;/em&gt;&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Based on low temperature molten salt eutectics, these are use-once batteries that do not become active (or self-discharge) until they reach operating temperature and the electrolyte melts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H202d.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H202d.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;&lt;em&gt;Sodium Sulphur Battery (rechargeable):&lt;/em&gt;&lt;/h2&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An alternate high-temperature battery technology is the sodium–&#xA;  sulfur battery [refs]. Sodium–sulfur batteries were initially&#xA;  developed as a high-specific-energy rechargeable battery system&#xA;  with a low self-discharge rate for electric vehicles. They are currently&#xA;  being demonstrated in electric utility applications [ref] to serve as an&#xA;  energy storage system to store energy for use during peak demand&#xA;  periods. Such batteries have demonstrated hundreds of charge/&#xA;  discharge cycles with a low decrease in capacity.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[...]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Current sodium–sulfur batteries use a beta-alumina solid oxide&#xA;  electrolyte. This electrolyte also serves as a separator between the&#xA;  liquid sodium anode and the liquid sulfur cathode. At operating&#xA;  temperatures above about 300C, sodium ions are mobile in the&#xA;  solid-electrolyte material. Because of the relative impermeability of&#xA;  the solid electrolyte, the self-discharge rate of the NaS battery is&#xA;  extremely low. A schematic of the battery is shown in Fig. 2.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Because of the high specific energy, there has been some interest in&#xA;  the use of sodium sulfur batteries for space operation [refs], despite&#xA;  the high temperatures. A potential difficulty of the NaS battery&#xA;  technology is the fragility of the beta-alumina electrolyte. &lt;strong&gt;A&#xA;  demonstration test of the sodium–sulfur battery in space was done on&#xA;  space shuttle flight STS-87 in November 1997. This is shown in&#xA;  Fig. 3. The experiment lasted 10 days and showed that the NaS&#xA;  battery could be successfully qualified for space operation and&#xA;  operate in space conditions.&lt;/strong&gt; (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3Lgqk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3Lgqk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;&lt;em&gt;Sodium Metal Chloride Battery:&lt;/em&gt;&lt;/h2&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Both the Na=NiCl2 and Na=FeCl2 chemistries have been&#xA;  demonstrated, although the NiCl2 chemistry is preferred because of&#xA;  the wider temperature range of operation, 200–400C [ref]. The&#xA;  schematic of the Na=NiCl2 battery is virtually identical to that of the&#xA;  NaS battery shown in Fig. 3, except that the Na anode is typically on&#xA;  the outside, whereas the NiCl2 cathode is in the center. The cell&#xA;  voltage Vo is slightly higher than that of the NaS battery. Specific&#xA;  power has been demonstrated up to 143 W  h=kg.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The technology development status of this battery for terrestrial&#xA;  applications is high, with an experience base of thousands of batteries&#xA;  built and many demonstration projects with over 15 years of experience.&#xA;  However, terrestrial batteries typically operate at the lower&#xA;  end of the temperature range, typically 270C, although Pistoia [ref]&#xA;  reported that 450C operation of the Na=NiCl2 battery is possible.&#xA;  Recent work by the U.S. Department of Energy [ref] has tested&#xA;  single-cell Na–FeCl2 batteries at temperatures of 500 and 600C,&#xA;  with no failures after 7 h of operation at 500C, and Na–ZnCl2 for up&#xA;  to 50 h of operation at 425C. However, comparatively little&#xA;  development effort has been done on the higher temperature range of&#xA;  operation, because high-temperature operation is not of interest for&#xA;  terrestrial applications.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;&lt;em&gt;New Battery Technology: Lithium/Lithium Carbonate Battery&lt;/em&gt;&lt;/h2&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the operation at Venus ambient temperature, a molten salt is&#xA;  used for an electrolyte. The obvious choices are either halide salts or&#xA;  carbonate salts.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The optimum electrolyte would be a molten carbonate. This makes&#xA;  the battery structure very much an analog to the molten-carbonate&#xA;  fuel cell, and much of the technology development for molten carbonate&#xA;  fuel cells will be directly applicable. This is a device&#xA;  structure that has undergone considerable technology development&#xA;  for terrestrial applications.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Although pure lithium carbonate, with a melting point of 723C, is&#xA;  not liquid at Venus temperature, a eutectic mixed carbonate is. The&#xA;  ternary eutectic Li0:44Na0:30K0:262CO3 has a melting point of&#xA;  393C [refs], low enough to be liquid at Venus surface&#xA;  temperatures at all locations across the planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-22T16:48:59.230" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20777" PostTypeId="1" AcceptedAnswerId="41127" CreationDate="2017-03-22T15:27:18.567" Score="11" ViewCount="925" Body="&lt;p&gt;I will grant that launching a can of soda pop, or beer into space would be very expensive. Alternately it is not unreasonable to make it in the space station, carbonated drink makers are easily available Earth side for under $100 (US).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming you have your cold carbonated drink, and are floating in the ISS or a commercial space station/craft, would you be able to enjoy it?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not enjoy =&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;without gravity the carbonation boils of explosively&lt;/li&gt;&#xA;&lt;li&gt;without gravity peristalsis is not sufficient to move the carbonated drink to your stomach and keep it there&lt;/li&gt;&#xA;&lt;li&gt;for some reason it is not possible to cool drinks in space and you can only have luke warm beverages&lt;/li&gt;&#xA;&lt;li&gt;something else unpleasant I have not thought of&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Related &lt;a href=&quot;https://space.stackexchange.com/questions/1352&quot;&gt;How do astronauts drink or eat in space?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2019-11-08T00:09:58.913" LastActivityDate="2023-12-17T02:42:10.723" Title="Can you enjoy a cold carbonated drink in space?" Tags="|astronauts|food|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20778" PostTypeId="1" AcceptedAnswerId="23464" CreationDate="2017-03-22T15:55:49.293" Score="8" ViewCount="292" Body="&lt;p&gt;I am exploring some of the properties of a vertical orbiting tether (or &lt;a href=&quot;https://en.wikipedia.org/wiki/Skyhook_(structure)#Non-rotating&quot; rel=&quot;noreferrer&quot;&gt;non-rotating skyhook&lt;/a&gt;). This particular tether has a quite large mass at the location with a circular orbital velocity, with the mass of the tether itself being much smaller and ignored.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BiEST.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BiEST.png&quot; alt=&quot;orbiting tether&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If something, such as a shuttle docks at the tether foot, the mass of the docking object is going to cause a periapsis drop.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yNPQt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yNPQt.png&quot; alt=&quot;periapsis of tether has dropped&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the central mass, the mass of the shuttle, and of course all desirable parameters such as foot altitude etc. how can I calculate the drop in periapsis?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My initial strategy was that both energy and angular momentum in the new orbit is always going to be the same, making it easy to set up an equation with one solution in apopasis and periapsis. But those equations quickly grew out of hand.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-10-20T21:20:18.510" Title="How to calculate drop due to ship docking at orbital tether" Tags="|orbital-mechanics|docking|space-tether|skyhook|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20779" PostTypeId="1" AcceptedAnswerId="35088" CreationDate="2017-03-22T16:32:42.437" Score="5" ViewCount="1139" Body="&lt;p&gt;I would like to know what are the simulation software tools available as of today (free or commercial) to perform some attitude and orbit control (AOC) or guidance and navigation control analysis, I am mostly interested in testing some attitude control algorithms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far I found two that maybe could be useful, do you know any others?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;psatellite: Spacecraft Control Toolbox.&lt;/li&gt;&#xA;&lt;li&gt;42 (open source)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Does anyone had any experience with these software? Can you give me any suggestions?&lt;/p&gt;&#xA;" OwnerUserId="19018" LastEditorUserId="12742" LastEditDate="2017-03-22T20:42:33.157" LastActivityDate="2022-08-12T13:53:35.580" Title="Simulation software for AOCS/GNC?" Tags="|spacecraft|attitude|software|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20782" PostTypeId="1" AcceptedAnswerId="54604" CreationDate="2017-03-22T20:35:42.043" Score="7" ViewCount="542" Body="&lt;p&gt;What is the biggest, by the means of satellite amount, satellite constellation in space right now?&lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="12102" LastEditDate="2021-08-15T05:42:59.087" LastActivityDate="2021-10-15T19:11:00.043" Title="What is the biggest satellite constellation in space right now?" Tags="|record|satellite-constellation|formation-flying|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20783" PostTypeId="2" ParentId="20782" CreationDate="2017-03-22T20:46:40.340" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Planet_Labs#Satellite_constellation&quot; rel=&quot;noreferrer&quot;&gt;Planet Labs' Flock 3p&lt;/a&gt;, a constellation of 88 Dove satellites, was recently launched aboard ISRO's &lt;a href=&quot;https://en.wikipedia.org/wiki/PSLV-C37&quot; rel=&quot;noreferrer&quot;&gt;PSLV-C37&lt;/a&gt;, the record-breaking launch of 104 satellites. Wikipedia says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;They head to a morning crossing time, sun-synchronous orbit (SSO) at an approximate altitude of 500 kilometres (310 mi).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to the Wikipedia Planet Labs article, Flock 3p is the largest fleet of satellites to be launched in history.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-03-22T20:46:40.340" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20784" PostTypeId="1" AcceptedAnswerId="20954" CreationDate="2017-03-22T23:30:47.120" Score="7" ViewCount="400" Body="&lt;p&gt;I have two thruster configurations I'm testing. One is almost symmetric about its center-of-gravity (it's a little bottom-heavy, but the thrusters themselves are the same fore and aft). The other has four additional thrusters aft for more translational delta-v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DgB9B.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DgB9B.png&quot; alt=&quot;asymmetric thruster configuration&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That image shows the asymmetric configuration. The symmetric one is pretty similar, but without the 15cm separation between the second set of translational thrusters. The radius is about 2 and a quarter meters, and the length is around 9 meters; CG is 4 meters from the rear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I get really strange results with the asymmetric configuration.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Candidate Optimal Groups&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, I haven't been able to find the original paper for COGs. But the algorithm itself is described in two places I've found:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Shoemaker, D. 2013. A survey of spacecraft jet selection logic algorithms. In AAS Guidance &amp;amp; Control Conference, Breckenridge, CO.&lt;/li&gt;&#xA;&lt;li&gt;Wie Bong. 2008. Chapter 7: Rotational Maneuvers and Attitude Control. &lt;em&gt;Space Vehicle Dynamics and Control&lt;/em&gt;. (This one seems to be &lt;a href=&quot;https://www.google.com/webhp?sourceid=chrome-instant&amp;amp;ion=1&amp;amp;espv=2&amp;amp;ie=UTF-8#safe=active&amp;amp;q=%22space%20vehicle%20dynamics%20and%20control%22%20pdf&amp;amp;*&quot; rel=&quot;noreferrer&quot;&gt;in PDF form on the Google&lt;/a&gt;. In my copy, the algorithm is given in section 7.6.2, starting on p. 474.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The plots below are my attempt to reproduce those given in the Shoemaker review. The rough idea is that you have a &lt;em&gt;command vector&lt;/em&gt; $\mathbf{b}$ of size $d$ (one entry per degree of freedom), normalized to length 1, and a &lt;em&gt;thruster on-time array&lt;/em&gt; $\mathbf{x}$ of length $N$ (one for each thruster). You're trying to solve&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \mathbf{A}\mathbf{x} = \mathbf{b} $$&#xA;$$\textrm{subject to the constraint that }\mathbf{x} \geq \mathbf{0} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't really solve that directly, since it's under-constrained. So instead we assume that for $d$ degrees of freedom, we need $d$ thrusters to be turned on, and partition $\mathbf{x}$ into $\mathbf{\hat{x}}$ (for selected thrusters) and $\mathbf{\tilde{x}}$ (for thrusters we're definitely keeping off).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can accordingly partition $\mathbf{A}$ into corresponding $\mathbf{\hat{A}}$ (square) and $\mathbf{\tilde{A}}$ for each $\mathbf{x}$ partition. Using that, we can solve&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \mathbf{\hat{A}}\mathbf{\hat{x}} = \mathbf{b}\,\textrm{.}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That gives all the candidate groups of thrusters, only some of which are optimal. The full algorithm describes how you reduce the search space, eliminating groups that are clearly non-optimal &amp;mdash; those where $\mathbf{\hat{A}}$ is singular, where turning on some thruster &lt;em&gt;not&lt;/em&gt; in $\mathbf{\hat{x}}$ produces a better score, or where any value in $\mathbf{\hat{x}}$ is less than $-\epsilon$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final candidate optimal groups are those which maximize the score $\mathbf{b}\cdot(\mathbf{\hat{c}}\mathbf{\hat{A}}^{-1})^\mathrm{T}$, where $\mathbf{c}$ is the cost vector (basically the flow rate for each thruster when turned on) and appropriately partitioned as with $\mathbf{A}$ and $\mathbf{x}$. (Note that most people assume all costs are 1 for simplicity.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So! That brings us to the plots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm only showing 3 DOF here, so I've done the computations for rotation-only or translation-only. Thus, commands $\mathbf{b}$ are of size 3. So, for pure $\hat{x}$ translation, $\mathbf{b}_\textrm{translation} = \left[ 1\,0\,0 \right]$; for pure $\hat{y}$ translation, $\mathbf{b}_\textrm{translation} = \left[0\,1\,0\right]$; etc. I do the same for rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I generate a unit sphere mesh of commands. For each COG, I find the optimal set (or sets) of thrusters to use, and compute the score and the total on-times (the sum of $\mathbf{\hat{x}}$). The on-times figure, then, shows the unit sphere scaled by the sum of computed on-times. The authority figure is supposed to be, according to my reading, the unit sphere points scaled by the reciprocal score. The color map is computed identically.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Results&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7smK0.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7smK0.png&quot; alt=&quot;asymmetric rotational COG analysis&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/LJFTZ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LJFTZ.png&quot; alt=&quot;symmetric rotational COG analysis&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The translation results are identical, as expected:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/TOMqO.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TOMqO.png&quot; alt=&quot;translation COG analysis&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been struggling with this for a few days. I &lt;em&gt;suspect&lt;/em&gt; the issue relates to the LU decomposition I'm using for inverting the candidate authority matrices. Why have I come to this conclusion? In my translational authority matrix, all of my cells have only two magnitudes (0 and a hundred something). But in my rotational authority matrix, I get a couple of magnitudes which are small (~8) and a bunch that are bigger (200 to 600).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anyone else seen anything like this? I've now tried two different LU decomp algorithms, and they produce the same output. (Both use partial pivoting, and they should be stable; one is from GSL. I'm also checking the reciprocal condition number of the result and discarding candidate groups where &lt;code&gt;rcond &amp;lt; eps&lt;/code&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I first wrote the code in Python, I used SVD instead of LU decomposition, and it didn't seem to suffer from this particular problem. But now that I'm writing in a compiled language, SVD's relative algorithmic complexity is of concern, so I'd rather use LU, if possible.&lt;/p&gt;&#xA;" OwnerUserId="869" LastEditorUserId="869" LastEditDate="2017-03-23T15:37:56.543" LastActivityDate="2017-04-03T16:30:05.307" Title="Why does candidate optimal group computation produce strange results when thrusters are too close together?" Tags="|rcs|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20785" PostTypeId="1" CreationDate="2017-03-23T01:28:17.670" Score="1" ViewCount="137" Body="&lt;p&gt;It's been over 50 years since nuclear pulse propulsion using shaped charge fission warheads was first proposed. Have there been any significant technological improvements over the last 50 years that would make this concept work better? Do composites promise to reduce structural masses? Have warheads gotten smaller or cleaner? &lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-23T01:28:17.670" Title="Have prospects for nuclear pulse propulsion improved?" Tags="|nuclear-pulse-propulsion|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20786" PostTypeId="1" AcceptedAnswerId="20792" CreationDate="2017-03-23T04:49:53.907" Score="3" ViewCount="911" Body="&lt;p&gt;The video description says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Published on Apr 15, 2015&lt;br&gt;&#xA;  This footage was taken by U.S. astronaut Terry Virts during a spacewalk on the International Space Station on February 25, 2015. Virts and fellow astronaut Barry “Butch” Wilmore routed a series of cables in preparation for the arrival of two International Docking Adapters later in 2015. Virts also lubricated elements at the latching end of the space station’s Canadarm2 robotic arm while Wilmore prepares the Tranquility module for the relocation of the Permanent Multipurpose Module and the arrival of the Bigelow Expandable Activity Module (BEAM) later this year. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Can anyone identify the source of the sounds recorded by the GoPro?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/19440/12102&quot;&gt;This answer&lt;/a&gt; describes another GoPro video, and why the recorded sounds transmitted through air became more audible as the air density increased, around 30km altitude. But the current video is from the ISS at 400km. I'm guessing the sounds is transmitted mechanically from something in contact with the camera. Is it related to the astronaut's EVA suit, or the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks to @BrianTompsett for &lt;a href=&quot;https://space.stackexchange.com/questions/14327/has-there-ever-been-a-go-pro-satellite#comment37532_14327&quot;&gt;linking to&lt;/a&gt; this GoPro video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/26QWMMT1c8Y?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-03-23T19:19:12.453" Title="What are the sounds in this GoPro-in-orbit ISS video?" Tags="|iss|imaging|sound|robotic-arm|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20787" PostTypeId="1" AcceptedAnswerId="20791" CreationDate="2017-03-23T10:48:33.170" Score="3" ViewCount="539" Body="&lt;p&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2017/pdf/1250.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;A paper&lt;/a&gt; from the 2017 meeting Lunar and Planetary Sciences XLVIII describes a systematic observation of the Martian sky, with a sequence of eight taken every day at zenith and near-horizon angles. They show tenuous clouds, often with a wavy &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravity wave&lt;/a&gt; pattern (that's gravity wave, not gravitational wave).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The images are very grainy compared to the one's I'm used to seeing of the Martian sky or surface. Are they taken before sunrise when it is very dark? Since nothing is moving couldn't they just use a longer exposure? Or does it have to do with noise from some enhancement processing? If so, &lt;em&gt;what is&lt;/em&gt; the enhancement process?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't extensive cloud cover quite rare on Mars, and not a daily occurrence?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: the GIF below is &lt;em&gt;made from only four frames&lt;/em&gt; of the first 8-frame GIF of three, shown in this news item in &lt;a href=&quot;http://www.sciencemag.org/news/2017/03/mars-rover-spots-clouds-shaped-gravity-waves&quot; rel=&quot;nofollow noreferrer&quot;&gt;Science&lt;/a&gt;. They look much better there - the stackexchange image size restriction prevents the direct posting of the 2.7 MB images. You can direct-link to them: &lt;a href=&quot;http://www.sciencemag.org/sites/default/files/styles/inline__699w__no_aspect/public/sol01292_512168867_SHM.gif?itok=rxHxTLwA&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;http://www.sciencemag.org/sites/default/files/styles/inline__699w__no_aspect/public/sol0429_435592502_Zenith.gif?itok=hkG-j8PW&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;http://www.sciencemag.org/sites/default/files/styles/inline__699w__no_aspect/public/sol01302_513061816_SHM.gif?itok=BWmwSRMR&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JgLkG.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JgLkG.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;For contrast&lt;/strong&gt;, here is a &lt;a href=&quot;https://twitter.com/marsrader/status/846886742506590209&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweeted image&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eF1GH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eF1GH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-03T07:10:13.963" LastActivityDate="2017-04-03T07:10:13.963" Title="Why is this &quot;grainy footage&quot; of a Martian (sky) from Curiosity so grainy? Cloud snaps by Opportunity look great!" Tags="|mars|atmosphere|curiosity|mars-science-laboratory|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20788" PostTypeId="1" CreationDate="2017-03-23T11:51:36.713" Score="18" ViewCount="2563" Body="&lt;p&gt;I've heard that the size of Saturn V was pretty much as big as could've been built in the 1960s. And the Soviet N1 did basically fail because it was too large for their infrastructure and engine size at the time. Generally, there's an economic advantage with larger scale, given enough demand for the-bigger-the-better one piece payloads, which I want us to fictionally assume here. What are the technical challenges that limit how large a chemical launcher, from the surface of Earth to orbit, could be built?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-03-23T13:12:51.010" LastActivityDate="2017-03-24T05:18:11.057" Title="How large could launchers become?" Tags="|spacecraft-development|launch-vehicle|" AnswerCount="4" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="20789" PostTypeId="2" ParentId="18563" CreationDate="2017-03-23T11:53:00.280" Score="0" Body="&lt;p&gt;Yes it is possible, no it won't be easy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might already know this, but the higher you go the thinner the atmosphere gets. The higher you want to go the bigger/and or lighter you need to make the balloon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Breaching 50km has been done. I don't know if you can do it for under 600, given current materials. If you're interested, here's a wiki article that discusses high altitude balloons &lt;a href=&quot;https://en.m.wikipedia.org/wiki/High-altitude_balloon&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.m.wikipedia.org/wiki/High-altitude_balloon&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also mentions BU60-1 as a balloon that reached 53km. Check it out for ideas.&lt;/p&gt;&#xA;" OwnerUserId="18870" LastActivityDate="2017-03-23T11:53:00.280" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20790" PostTypeId="2" ParentId="20788" CreationDate="2017-03-23T13:01:23.157" Score="3" Body="&lt;p&gt;Engines haven't changed much, so most improvements to size would likely come from the use of lightweight composite materials. The performance of these isn't as much better than aluminum alloys as is sometimes imagined though, so I doubt that would improve the situation a whole lot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nuclear thermal launch would improve payload fractions. The ability to use purely hydrogen propellant alone would dramatically improve exhaust velocity and thus the payload fraction of the rocket. This is essentially the only benefit, as in all such cases the temperatures and pressures are limited by the properties of chamber materials. It's quite a benefit though. The logarithmic term in the rocket equation makes high exhaust velocity extremely desirable. The key capability tradeoffs of course come from the high cost of nuclear systems and safety issues. It is worth noting though that no practical research has been in this area for several decades, so it's not clear what thrust to weight ratios could be achieved today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-03-23T13:01:23.157" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20791" PostTypeId="2" ParentId="20787" CreationDate="2017-03-23T14:08:55.860" Score="5" Body="&lt;p&gt;The Martian atmosphere is rather thin, and opacity is rather low. The sensitivity (ISO) needs to be cranked up to see the Martian sky at all. This alone will create some amount of graininess. Even with that, from the linked &lt;em&gt;Science&lt;/em&gt; article, &quot;The captured clouds are so thin as to be invisible without painstaking computer enhancement, Kloos said in his presentation Tuesday.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This almost certainly means a good amount of contrast enhancement such as histogram equalization, and perhaps sharpening. Both will enhance contrast at the expense of enhancing noise.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-03-23T14:08:55.860" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="20792" PostTypeId="2" ParentId="20786" CreationDate="2017-03-23T14:09:56.573" Score="2" Body="&lt;p&gt;Those sounds are undoubtedly the microphone simply picking up (via conduction) random vibrations in the camera body due to handling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, the audio track appears to be out of sync from the video -- many of those rustling sounds appear about one or two seconds after the camera is visibly jostled.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-03-23T14:09:56.573" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20793" PostTypeId="1" CreationDate="2017-03-23T14:16:43.020" Score="14" ViewCount="2233" Body="&lt;p&gt;What are the small triangular fins on the lowest part of the boosters of the Soyuz 2.1 rocket? What function do they have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I circled one of them in the upper image here. They are not visible in the lower image, which is a Soyuz launcher in transport almost a year later. Are these fins somehow attached or extended only after the rocket reaches the launch pad?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZIGNB.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZIGNB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A frame I stole from &lt;a href=&quot;https://www.youtube.com/watch?v=q1fihj_ed9w&amp;amp;feature=youtu.be&amp;amp;t=188&quot; rel=&quot;noreferrer&quot;&gt;this youtube clip&lt;/a&gt; A very beautiful filming of a Soyuz launch  28 April 2016!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bDABJ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bDABJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A January 2017 photo I found on &lt;a href=&quot;https://spaceflightnow.com/2017/01/24/soyuz-rolled-to-launch-pad-for-first-geostationary-launch-from-french-guiana/&quot; rel=&quot;noreferrer&quot;&gt;spaceflightnow.com&lt;/a&gt;. The &quot;fins&quot; are not visible there.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6944" LastEditDate="2018-04-09T00:49:57.313" LastActivityDate="2018-11-29T10:46:42.817" Title="What are these tiny triangular fins on the Soyuz launcher?" Tags="|aerodynamics|soyuz-launcher|identify-this-object|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20794" PostTypeId="2" ParentId="20793" CreationDate="2017-03-23T14:27:20.293" Score="17" Body="&lt;p&gt;The &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;sqi=2&amp;amp;ved=0ahUKEwi4j4fn6ezSAhWK7IMKHeL5BLoQFggaMAA&amp;amp;url=http%3A%2F%2Fwww.arianespace.com%2Fwp-content%2Fuploads%2F2015%2F09%2FSoyuz-Users-Manual-March-2012.pdf&amp;amp;usg=AFQjCNE4ZtuZPVwfPXBeygNtMDxxyGKR_Q&amp;amp;sig2=m_Eq_SqKyNkM23paDdmYyA&amp;amp;bvm=bv.150475504,d.cGw&amp;amp;cad=rja&quot; rel=&quot;noreferrer&quot;&gt;Soyuz (booster) User's Manual&lt;/a&gt; from Arianespace calls them &quot;aerofins&quot; and says they are part of the attitude control system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JTbsp.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JTbsp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An additional image I ran across showing the aerofins and stating that they are for &quot;auxiliary course correction in the atmosphere&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/w1QSf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/w1QSf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-09-28T02:02:49.753" LastActivityDate="2018-09-28T02:02:49.753" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="20795" PostTypeId="2" ParentId="20788" CreationDate="2017-03-23T14:39:46.840" Score="13" Body="&lt;p&gt;SpaceX is proposing launch vehicle, as a first stage, and a second stage that would transit to Mars.  Interplanetary Space Transport (ITS). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final size is not really confirmed, and they should actually build it before we compare it, but using chemical rockets, it is quite a bit larger than a Saturn V. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WxaEY.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WxaEY.jpg&quot; alt=&quot;SpaceX ITS booster&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V used 5 F-1 engines with around 1.5 million lbs of thrust. The ITS first stage is planned to use 42 (!!!!) Raptor engines for a total of 29 million lbs of thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly larger than a Saturn V is in the realm of possible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Large engines are hard, as the F-1 engine development showed, and as the Soviet experience that lead to 30 NK-15/NK-33 engines for the N-1 showed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has taken a middle of the road approach, but closer to the N-1 approach with fairly large numbers of medium sized engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving the booster around is quite tricky. NASA solved it by barging the large components. US Rail does not support such large objects. Soviet rail seems like it was able to handle larger as most of their components were moved by rail. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX plans to solve it by flying the vehicle around. (Or at least manufacturing near the launch site, and after that, land and relaunch mostly in place). &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-03-23T14:39:46.840" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20796" PostTypeId="2" ParentId="20784" CreationDate="2017-03-23T15:55:08.130" Score="1" Body="&lt;p&gt;It looks like the problem stems from this layout being &lt;a href=&quot;https://en.wikipedia.org/wiki/Condition_number&quot; rel=&quot;nofollow noreferrer&quot;&gt;ill-conditioned&lt;/a&gt; for numerical calculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you wrote &quot;&lt;em&gt;almost symmetric about its center-of-gravity&lt;/em&gt;&quot;, the word &quot;almost&quot; is the red flag in any computational models.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, the calculation probably attempts to determine pitch and yaw caused by the minuscule offset of the aft thrusters (instead of simply assuming they cancel each other, it calculates torque caused by first from the pair, the second from the pair, and only then sums these). This introduces almost zero torque; almost - but not quite. The value will be within floating point error range, these errors will accumulate and can completely trash your results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Either make the configuration completely symmetric, or strongly asymmetric. Anything &quot;almost&quot; is asking for trouble.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-23T15:55:08.130" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20797" PostTypeId="2" ParentId="20788" CreationDate="2017-03-23T17:17:06.750" Score="7" Body="&lt;p&gt;It's generally thought that the Sea Dragon design was technically feasible; this would be 18000 tons (6 times the mass of Saturn V) at liftoff and deliver 500 tons to LEO, comparable to the SpaceX ITS in expendable mode. Mass fraction and specific impulse would be worse than Saturn V but economy of scale would make it more cost effective per payload ton. It was never produced because the demand for such large payloads never arose, of course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know of any specific hard technical limitation to going bigger than Sea Dragon, though mass fraction probably gets poorer and logistic issues get hairier as you embiggen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/s/seadragon.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.astronautix.com/s/seadragon.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.m.wikipedia.org/wiki/Sea_Dragon_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.m.wikipedia.org/wiki/Sea_Dragon_(rocket)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-03-23T23:48:38.273" LastActivityDate="2017-03-23T23:48:38.273" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20798" PostTypeId="2" ParentId="19522" CreationDate="2017-03-23T17:33:21.853" Score="1" Body="&lt;p&gt;Yes, Santa's sleigh in a semi geosynchronous in 1 long orbit. A sleigh big enough to carry all the presents and each present dropped with GPS controls, airbag heat shielding, parachutes and some form of minor propulsion like an arm. Sure, like clockwork. &lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2018-02-20T18:40:57.330" LastActivityDate="2018-02-20T18:40:57.330" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20799" PostTypeId="2" ParentId="19522" CreationDate="2017-03-23T18:38:50.327" Score="4" Body="&lt;p&gt;Okay, gonna cheat. Launch way early, nearly vertically, into suborbital trajectory with apogee near Hill sphere, that way maybe 10% of an orbit is elapsed, and only one burn is needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Near apoapsis, lob the presents out of the airlock in such a way that each enters suborbital trajectory hitting a different latitude - at these speeds, altitudes and distances, you can change the point of landing to an arbitrary spot on Earth, pretty much with muscle power alone. Keep tossing for 24h to cover all longitudes; time it such that the presents arrive early on  december 24th of local time of the destination; they will differ less than half an hour between latitudes anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's just hope they don't burn up on reentry.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-03-23T18:43:31.117" LastActivityDate="2017-03-23T18:43:31.117" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20801" PostTypeId="2" ParentId="20788" CreationDate="2017-03-24T04:12:02.140" Score="0" Body="&lt;p&gt;The major technical challenges of up-sizing rocket boosters, as I see it, are in two major categories: weight, and risk mitigation.&lt;/p&gt;&#xA;&lt;h3&gt;Weight&lt;/h3&gt;&#xA;&lt;p&gt;The most profitable business is weight reduction. There is money to be made on developing future materials (metals, and fuels) and structural design. These areas are relevant because the rate at which the size of the spacecraft increases, currently, will match and overtake the power efficiency very fast. Which is why we see the SpaceX Interplanetary Transportation System as three smaller craft taped together for added boosters.&lt;/p&gt;&#xA;&lt;h3&gt;Risk mitigation&lt;/h3&gt;&#xA;&lt;p&gt;Therefore the best thing for weight, multiple layered systems, is also the best thing for mitigation of risk for these &amp;quot;glorified explosions&amp;quot;. Realize that not only are you controlling an explosion to propel these things into space, you are also preventing a fuel supply from igniting mid-trajectory. This is a very expensive cargo to be taking from point A to point B, and having a lesser amount of fuel mid-flight and capital loss to control is always prevalent.&lt;/p&gt;&#xA;" OwnerUserId="19048" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-24T05:18:11.057" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20802" PostTypeId="2" ParentId="19558" CreationDate="2017-03-24T16:04:17.380" Score="1" Body="&lt;p&gt;Yes, it is possible for Mars to have a surface pretty much entirely oxidized even with the current low O2 levels (around 0.16 % ) in its atmosphere. Higher O2 levels would be necessary to oxidize surface if O2 sinks continued to show up on Mars's surface, but this does not occur on Mars because it is dry and little erosion occurs (yes there are storms, but these are not powerful enough for strong enough erosion). Therefore low O2 levels can oxidize the surface slowly since itself is very slowly changing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, Mar's O2 levels were higher in the past. Due to Mars' small size, it slowly loses its Oxygen to space via ion recombination reactions in Mar's ionosphere. &#xA;this reaction can be shown as: O2+ + e --&gt; O + O&#xA;these O's then have enough energy to escape Mar's weak gravity. Thus levels of O2 are predicted to be greater in Early Mars. This makes it more likely that the oxidation of the surface occurred in Early Mars. And since the surface sees very little erosion, the surface stays oxidized. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope that helps.&lt;/p&gt;&#xA;" OwnerUserId="19053" LastActivityDate="2017-03-24T16:04:17.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20803" PostTypeId="1" CreationDate="2017-03-24T16:18:57.697" Score="6" ViewCount="247" Body="&lt;p&gt;A recent comment pointed out &lt;a href=&quot;https://space.stackexchange.com/questions/20777/can-you-enjoy-a-cold-carbonated-drink-in-space#comment56328_20777&quot;&gt;you can't burp in zero-g&lt;/a&gt;, while we don't have anything here about it, a Google search seems to support the comment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you spin your body, in theory you could overcome the lack of gravity issue  with centrifugal force. Plausibly causing gas bubbles to gather as desired.  By positioning your arms, legs and or adding extra weight, you might even be able to influence where in your body they gather, in effect controlling which end of the digestive track the gas would exit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this plausible? &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2022-07-27T06:12:26.393" Title="Spinning in space to manage gas?" Tags="|health|food|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20804" PostTypeId="1" AcceptedAnswerId="20806" CreationDate="2017-03-24T17:27:24.653" Score="7" ViewCount="284" Body="&lt;p&gt;I'm familiar with the concept of aerobraking. Consider an object like a star, or a planet with strong magnetic field (Earth, Jupiter, probably all of the gas/ice giants?) Could a probe use some kind of electromagnet (or maybe magnetic induction), to push against the magnetic field as it gets close to the planet, similar to aerobraking, but much farther out?   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Also, is this an old, well known idea?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if there was lower deceleration, the much longer reach of the magnetic field might make it viable for starting to slow down much earlier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was watching a speculative video on YouTube about how tethers push against the earth's magnetic field.  In that video, the idea was for a space station or skyhook or satellite to gain momentum to prevent gradual orbital decay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it occurred to me it might work just fine for slowing down instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some existing work for magnetic sails, to decelerate against a solar wind, but I don't really see anything comparing it to aerobraking, nor anything that considered it at the level of a planetary magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe the idea works just great, except the hardware is simply too heavy to be practical? Or if using induction &amp;mdash; maybe the problem is if you decelerate too much, you build up too much charge and zap your space probe?&lt;/p&gt;&#xA;" OwnerUserId="19054" LastEditorUserId="6816" LastEditDate="2017-03-25T17:51:05.477" LastActivityDate="2017-03-25T17:51:05.477" Title="Magnetic / Inductive braking against planetary magnetic fields?" Tags="|propulsion|probe|magnetic-field|aerobraking|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20805" PostTypeId="2" ParentId="18341" CreationDate="2017-03-24T20:01:40.330" Score="1" Body="&lt;p&gt;For a launch, I'd say no.   About the only scenario where it could be bad, at the planetary level, would be if the mission was to go out, say past pluto, and redirect a comet.   And something goes wrong, and you end up aiming the comet at some planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even that would take a lot.  But if we posit that the original mission plan was to move the comet.  Just that the execution screwed up, and moved it the wrong way -- then maybe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That still wouldn't affect the whole solar system.   It would ruin your day if it hit the planet you were on.   But the rest of the planets would be fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;unless you want to talk about timescales of billions of years.   Then.... maybe ... just possibly... you could get a butterfly effect.   where your ship slightly changes the orbit of hundreds or thousands of pluto-distance comets and over time, we get unlucky enough that several planets get impacted, maybe several times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so, maybe earth (well - Life on Earth - the rock itself wouldn't notice), the moon, maybe some bases on a few other moons, would be toast?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;as far as hurting a gas giant, or the sun.  NO.  Not Gonna Happen.    There's &lt;a href=&quot;https://what-if.xkcd.com/146/&quot; rel=&quot;nofollow noreferrer&quot;&gt;an XKCD &lt;em&gt;What if?&lt;/em&gt; comic about “what are the chances of accidentally affecting Jupiter's orbit?”&lt;/a&gt;   Suffice to say, you cannot accidentally affect a gas giant in any meaningful way.&lt;/p&gt;&#xA;" OwnerUserId="19054" LastEditorUserId="8609" LastEditDate="2019-11-11T00:48:22.877" LastActivityDate="2019-11-11T00:48:22.877" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20806" PostTypeId="2" ParentId="20804" CreationDate="2017-03-24T20:13:46.263" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiauIj7-O_SAhXByFQKHWRVDmoQFggcMAA&amp;amp;url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FElectrodynamic_tether&amp;amp;usg=AFQjCNHBB9k5h9O_5Efk6b81V3bH_H7LIw&amp;amp;sig2=HWUsLCOG9u0JLdML_EcqGg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Electrodynamic Tethers&lt;/a&gt; have the potential to do what you want.  The long wire moving through the planet's magnetic field can either brake or boost the deploying craft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_tether_missions&quot; rel=&quot;nofollow noreferrer&quot;&gt;history of tether experiments&lt;/a&gt; in space is...checkered, to say the least.  Of electrodynamic tethers, I think only OEDIPUS and T-REX are considered unqualified successes.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-03-24T20:13:46.263" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20807" PostTypeId="1" CreationDate="2017-03-24T20:30:01.123" Score="7" ViewCount="16079" Body="&lt;p&gt;I'm acquainting myself with space propulsion and I'd love a good definition of total impulse beyond just a derivation. Why is it important? How does it related to other key metrics for propulsion systems (like thrust, specific impulse, etc.)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, what are examples of total impulse for different kinds of propulsion systems (chemical vs. electric, for instance)?&lt;/p&gt;&#xA;" OwnerUserId="19055" LastEditorUserId="6816" LastEditDate="2017-03-24T20:44:48.007" LastActivityDate="2018-08-29T18:50:50.120" Title="What is total impulse?" Tags="|propulsion|electric-propulsion|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20808" PostTypeId="2" ParentId="20807" CreationDate="2017-03-24T23:04:57.460" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://www.nakka-rocketry.net/impcalc.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Total impulse&lt;/a&gt; is the area under the thrust-time curve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So examples are not real useful.  It's pretty specific to a given launch, or system.  If you want one, though, Sutton, 4th edition, p.356 gives the total impulse of the first-stage Minuteman missile motor as 10,830,000 lb-sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you divide it by the mass of propellant used, you get the specific impulse, which is a lot more useful. For that same motor, the specific impulse was 214 sec.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-03-24T23:26:55.460" LastActivityDate="2017-03-24T23:26:55.460" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20809" PostTypeId="2" ParentId="20807" CreationDate="2017-03-25T00:32:49.290" Score="9" Body="&lt;p&gt;I'll expand on @OrganicMarble's &lt;a href=&quot;https://space.stackexchange.com/a/20808/12102&quot;&gt;answer&lt;/a&gt;, it seems to me that the link there to the page on &lt;em&gt;Richard Nakka's Experimental Rocketry Web Site&lt;/em&gt; titled &lt;a href=&quot;http://www.nakka-rocketry.net/impcalc.html&quot; rel=&quot;noreferrer&quot;&gt;Determining Total Impulse and Specific Impulse from Static Test Data&lt;/a&gt; is really helpful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose you have built a rocket engine and you would like to see how good it is, but you don't have an extra rocket and launch crew to test it. You would do a &lt;em&gt;static test&lt;/em&gt;, meaning you'd mount the engine fixed somewhere safe, and ignite it. The thing you can measure is the force that the engine produces. The plot in &lt;a href=&quot;http://www.nakka-rocketry.net/impcalc.html&quot; rel=&quot;noreferrer&quot;&gt;the link&lt;/a&gt; (75% size) is what you might record writing down an eyeball estimate of the force every few seconds:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rH7UF.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rH7UF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now suppose you'd like to characterize the engine's performance during this test run with a single number. You can calculate the area under the curve my multiplying each force measurement times $\Delta t$, the time between measurements. That total area is called &lt;em&gt;total impulse&lt;/em&gt;, but it depends on the amount of fuel you used for your test. If you use twice as much fuel, the total impulse might be roughly double, but that doesn't mean the engine is better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you can divide total impulse by the mass of the fuel used in the particular test. The result is called &lt;em&gt;specific impulse&lt;/em&gt; or impulse per unit mass. It turns out the specific impulse is the average &lt;em&gt;effective velocity&lt;/em&gt; of the thrust. Traditionally, people divide that by Earth's surface gravity $g$ (about 9.8 m/s^2) and express specific gravity in units of seconds. &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Force (F)                        Newtons                    kg m / s^2&#xA;Total Impulse                    Newton seconds             kg m / s&#xA;Specific Impulse (Isp or Veff)   Newton seconds per kg         m / s&#xA;Specific Impulse (Isp)           Newton seconds per kg / g     s&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Here's a nice example of a plot of force versus time in a static test - nice in the sense of being a realistic example of a test:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/m8wrHH3W7Ss?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-03-27T13:05:43.390" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20810" PostTypeId="1" AcceptedAnswerId="20815" CreationDate="2017-03-25T00:40:56.713" Score="10" ViewCount="308" Body="&lt;p&gt;The ESA has its launchpad in French Guiana. It's not only close to the equator but also has a lot of water to the east, meaning that boosters can fall down without hurting people (probably).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in the Mediterranean Sea, there's quite a lot of water to the east, too. Why didn't the ESA set up their launchpad somewhere around Alicante in Spain such that they don't hit Formentera, Sardegna, or Sicily? The first thing they could hit would be mainland Italy. There's quite some distance to mainland Italy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US seem just fine with having their launchpad on their mainland where all their scientists are anyways and where they can supply materials easily, even though it's pretty far away from the equator. They don't ship their stuff down to like Jarvis Island which almost sits on the equator and has a lot of water to the east (and to the west, too, if someone wants to send a satellite up the other way, again, for a change).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question&lt;/strong&gt;: What is the (size and shape of the) area you need to be uninhabited for a typical launch?&lt;/p&gt;&#xA;" OwnerUserId="19058" LastEditorUserId="12102" LastEditDate="2017-03-25T07:02:02.397" LastActivityDate="2017-03-27T02:02:33.900" Title="How much water to the east is needed?" Tags="|launch|launchpad|range-safety|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20811" PostTypeId="2" ParentId="20807" CreationDate="2017-03-25T02:20:38.120" Score="2" Body="&lt;p&gt;Impulse is defined as force (i.e. thrust) times time. Since the burn time is dependent on the fuel carried, it doesn't make sense to speak of the total impulse for an engine or a propulsion system; it's most often used for a complete launcher or at least a stage or booster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Specific&lt;/em&gt; impulse is shorthand for &quot;mass specific impulse&quot;; that is, impulse per unit of mass, and as such it's the most important figure of merit for a propulsion system. (Sometimes the more obscure metrics of &quot;volume specific impulse&quot; or &quot;cost specific impulse&quot; will come up when mass isn't the main constraint; dense propellants like kerosene have better volume specific impulse than hydrogen, and solids have better cost specific impulse than liquids.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-03-25T02:20:38.120" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20812" PostTypeId="1" CreationDate="2017-03-25T02:42:39.467" Score="3" ViewCount="559" Body="&lt;p&gt;The question is why and how the politics plays into this. Why does NASA and other space agencies usually not (if ever) complain about their minuscule budgets? I can only assume that doing so would be politically foolish. If NASA were to complain about their small budget---which probably seems very high to the general public and probably to the typical politician too---maybe they would seem careless, greedy or incompetent, and get even less support. Is the smart thing to do politically to never talk about failures and problems, and only talk about what positive results are being achieved with what you have the way to go? To be optimistic even when the situation almost couldn't be worse? Is that the reason why they are doing this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this is such an important issue, I wouldn't be surprised if there has been conducted scientific studies on how to manage the public and the politicians regarding the budget. If there are any such literature it would be a great part to include in any good answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some simple justification for budget issues (adjusted down from a lengthy mostly hand-waving rant, and toned down):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For NASA, the budget was around 4.4% of the federal budget [1] during the Apollo days, whereas it is now around 0.5% [1]. It also appears that there do exist resources, but they are allocated for other projects, e.g. the F-35 program at approximately $1,5 trillion [2]. ESA and RFSA both have a fraction of what NASA presently has, and doesn't appear to ever have had big Apollo-like budgets [3][4], although RFSA certainly had more resources at its disposal during the space race.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1] &lt;a href=&quot;https://en.wikipedia.org/wiki/Budget_of_NASA#Annual_budget.2C_1958-2017&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Budget_of_NASA#Annual_budget.2C_1958-2017&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[2] &lt;a href=&quot;https://en.wikipedia.org/wiki/F-35&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/F-35&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[3] &lt;a href=&quot;https://en.wikipedia.org/wiki/European_Space_Agency&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/European_Space_Agency&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[4] &lt;a href=&quot;https://en.wikipedia.org/wiki/Roscosmos_State_Corporation&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Roscosmos_State_Corporation&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8707" LastEditorUserId="8707" LastEditDate="2017-03-25T03:40:21.857" LastActivityDate="2020-08-03T08:11:00.777" Title="Why does NASA and other space agencies seem to rarely if ever complain over the state of the budget?" Tags="|nasa|esa|economics|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20814" PostTypeId="1" AcceptedAnswerId="20816" CreationDate="2017-03-25T08:49:07.840" Score="19" ViewCount="6037" Body="&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/questions/20432/does-spacex-have-any-significant-additional-legal-hurdles-for-flying-private-cit#comment54912_20432&quot;&gt;comment&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a similar vein, it would be interesting to know if going around the moon rather than to the ISS removes any legal hurdles. Do we have any questions covering the legal ramifications (if any) of breaching the ISS's Keep Out Sphere for instance?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;under the question &lt;a href=&quot;https://space.stackexchange.com/q/20432/12102&quot;&gt;Does SpaceX have any significant additional legal hurdles for flying private citizens around the moon?&lt;/a&gt; has inspired me to ask &lt;em&gt;What is the &quot;ISS's Keep Out Sphere&quot; and what is its radius?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have found some references in &lt;a href=&quot;https://nssphoenix.wordpress.com/tag/keep-out-sphere/&quot; rel=&quot;noreferrer&quot;&gt;this blog&lt;/a&gt; but I am not sure I understand it completely.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-03-26T10:49:06.933" Title="What is the &quot;ISS's Keep Out Sphere&quot; and what is its radius?" Tags="|iss|safety|docking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20815" PostTypeId="2" ParentId="20810" CreationDate="2017-03-25T08:51:50.623" Score="8" Body="&lt;p&gt;This depends on the rocket you're launching. The first stage usually lands a few hundred km away from the launch point. Here is an &lt;a href=&quot;https://space.stackexchange.com/questions/9507/asds-position-during-falcon-9r-first-stage-recovery/9534#9534&quot;&gt;example of the exclusion zones&lt;/a&gt; for a SpaceX Falcon 9 launch. The red zone is the first stage landing zone, and stretches from 300 to 600 km outward from Cape Canaveral. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cglVa.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cglVa.png&quot; alt=&quot;F9 landing zone&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/S-II&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V second stages&lt;/a&gt; ended up halfway across the Atlantic (around 34º W). &#xA;The Shuttle external tanks ended up in the Pacific. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last stage (second stage for the Falcon 9, third stage for Saturn V, fourth for Arianespace Vega) goes to orbit with the payload, and generally burns up on reentry.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-03-26T08:57:20.263" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20816" PostTypeId="2" ParentId="20814" CreationDate="2017-03-25T09:59:08.413" Score="21" Body="&lt;p&gt;NASA has made a list of requirements for spacecraft approaching the ISS. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SSP 50808 is an ITAR controlled document that identifies the requirements for rendezvous, proximity operations, and physically meeting the ISS interface. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;An excerpt &lt;a href=&quot;https://prod.nais.nasa.gov/eps/eps_data/126269-OTHER-002-001.ppt&quot; rel=&quot;noreferrer&quot;&gt;is available online&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Trajectories must be designed such that ISS safety is preserved&#xA;  ...&lt;/li&gt;&#xA;  &lt;li&gt;Safe trajectories must be defined for each region near the ISS&lt;/li&gt;&#xA;  &lt;li&gt;Baselined regions defined in concept documents:&lt;/li&gt;&#xA;  &lt;li&gt;Approach Ellipsoid (AE): 4x2x2 km (SSP 50011)&lt;/li&gt;&#xA;  &lt;li&gt;Keep out Sphere (KOS): 200 m radius (SSP 50011)&lt;/li&gt;&#xA;  &lt;li&gt;Omni directional communications disk: 3x1½ km (SSP 50235)&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gDuVZ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gDuVZ.png&quot; alt=&quot;Safety regions around the ISS&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft approaching the ISS must comply with requirements set in SSP 50808 before they're allowed into a safety region, each region has its own requirements. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-03-25T09:59:08.413" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20817" PostTypeId="1" AcceptedAnswerId="20818" CreationDate="2017-03-25T14:26:17.920" Score="14" ViewCount="2805" Body="&lt;p&gt;One of the most important values when talking about rocket engines is the so-called specific impulse. I have read &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;noreferrer&quot;&gt;the Wikipedia article&lt;/a&gt; but still have trouble understanding what it really is and what its value in &lt;em&gt;seconds&lt;/em&gt; &lt;strong&gt;means&lt;/strong&gt;. What does this value really tell us?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's what learned so far:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The specific impulse is the change in &lt;em&gt;momentum&lt;/em&gt; an engine can produce with one &quot;unit&quot; of propellant. (What a &quot;unit of propellant&quot; is in this regard is still unclear to me). Momentum is the potential to change the velocity and direction of a mass, right? Momentum change divided by mass gives us the &lt;em&gt;velocity change&lt;/em&gt;, is that correct?&lt;/li&gt;&#xA;&lt;li&gt;The unit most often used is &lt;em&gt;seconds&lt;/em&gt;, while the &quot;correct&quot; units would be either N·s/kg (my math is rusty, but isn't that equivalent to just m/s?) or lbf·s/lb. If the &quot;correct&quot; value-with-units is multiplied with &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravity&quot; rel=&quot;noreferrer&quot;&gt;standard gravity&lt;/a&gt; you end up with a value in &lt;em&gt;seconds&lt;/em&gt; which is easier to compare.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But what is a useful thought experiment or example to tell me what this &lt;em&gt;seconds&lt;/em&gt; value &lt;strong&gt;means&lt;/strong&gt;? Is it equivalent to letting something fall for &lt;em&gt;x seconds&lt;/em&gt; in a standard gravity field in vacuum?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(The question &lt;a href=&quot;https://space.stackexchange.com/questions/9344/why-do-we-use-the-gravitational-acceleration-at-the-earths-surface-when-calcula&quot;&gt;Why do we use the gravitational acceleration at the Earth's surface when calculating the specific impulse of an engine?&lt;/a&gt;, which this question has been marked as a duplicate of, deals with the specifics for why g&lt;sub&gt;0&lt;/sub&gt; (standard gravity) is used but IMHO doesn't give an explanation about what the meaning of specific impulse actually is in layman's terms.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="14549" LastEditDate="2019-09-12T07:01:55.083" LastActivityDate="2019-09-12T12:52:35.690" Title="What is the &quot;specific impulse&quot;?" Tags="|propulsion|terminology|mathematics|specific-impulse|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="20818" PostTypeId="2" ParentId="20817" CreationDate="2017-03-25T14:34:47.940" Score="9" Body="&lt;p&gt;In simplest terms it is just the thrust produced divided by the propellant flow rate.  &quot;How much thrust am I getting for the propellant I am expending?&quot;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So bigger is better - you are getting more thrust for the same propellant flow rate.  Bigger numbers = more efficient engine = extracting more thrust from the same amount of propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is a key parameter for analysis, and is a useful figure of merit for comparing systems.  Like &quot;gas mileage&quot; for cars. Don't fret over the units.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-03-25T14:40:32.783" LastActivityDate="2017-03-25T14:40:32.783" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20819" PostTypeId="2" ParentId="20817" CreationDate="2017-03-25T16:42:05.280" Score="16" Body="&lt;p&gt;If you want to understand how the 'seconds' value fits the greater image, there's this rather contrived definition (which nobody uses because it's contrived and mostly useless but evocative enough.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$I_{sp}$ in seconds is equal to the amount of time a rocket must be fired to use a quantity of propellant with weight (measured at one standard gravity) equal to its thrust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Imagine a test setup: rocket engine plus dummy weight, such that the total mass (engine+weight) is 100kg (kilogram-force, if you want to nitpick the units).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You drive an external, flexible fuel pipe from a fuel tank which stores 100kg of fuel+oxidizer (the same as the test rig). You start the engine and keep the thrust so that it hovers, without rising or falling. You measure time from the engine start until all fuel is spent, and when it is, the time is your specific impulse. The longer the better obviously, more acceleration from the fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously this is not a very practical test, and this definition is not very helpful - it helps imagine why specific impulse is expressed in seconds, and what these seconds mean... but honestly, beyond &quot;understanding&quot; nobody cares.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What really matters are other, more practical uses of specific impulse:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$I_{sp} = { v_e \over g_0 } $&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is one trivial equation. You have the $g_0$ which is the earth gravitational acceleration, a trivial conversion factor, a constant - and you have $v_e$ - exhaust velocity, speed at which propellant is ejected from the engine. That's it. The only variable - and so you can think of specific impulse as speed of exhaust gases, only multiplied by some constant for convenience. It's not some magical property dependent on a hundred weird and obscure factors - it's just the speed of the exhaust gas. Only 'weirdified' a little by multiplying it by a constant. Really simple.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$F_\text{thrust} = g_0 \cdot I_\text{sp} \cdot \dot m$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's the same thing as that first &quot;useless&quot; definition, only made more useful. That's how you can practically measure specific impulse (measuring velocity of superheated gas or building hovering test rigs with flexible pipes is not really practical). Again, &quot;bang for the buck&quot;, how much thrust is produced - per fuel flow. The more thrust and the less fuel used the better. But you can measure how much force an engine exerts (say, how far the girders of the test cell bend when it blasts at full thrust, if it's something like Apollo's F-1,  or how much the ultra-precise torsion weight turns, if it's something like a colloid thruster), and check how much fuel it uses. This way you can get the specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$\Delta v = I_\text{sp} g_0 \ln \frac {m_0} {m_f}$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is what this whole game is all about - where you make practical use of that painstakingly determined specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delta-v is the actual &lt;em&gt;milleage&lt;/em&gt; of a rocket. Specific impulse is about the engine. But besides the engine, you have fuel, and you have the payload.  This is the Tsiolkovski's Rocket Equation, and this is about &quot;where you can go with your rocket.&quot; Good 9km/s to reach LEO. Another 4km/s or so to escape Earth and start travel over the solar system. 3km/s to capture into Mars orbit. That's the delta-v budget, which is the foundation of any mission plan. And with that you have the $m_f$, dry mass of your rocket - engine, scientific payload, telemetry, tanks, panels, everything else than fuel. And $m_0$ - launch mass. That's the above plus fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That 'ln' plays a nasty trick. Usually your fuel mass will be something of order of 90-95% of the launch mass. There's very little we can do about it, because we need good thrust to overcome earth gravity before we reach orbit, and that is provided by chemical engines which have lousy specific impulse. But then, in orbit, we drop the launch stages (dry mass goes way down!) and switch to efficient engines, like ion. And so, we can produce second as much delta-V as to reach LEO, but without need to haul hundreds of tons of fuel. This is where good ISp rules.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-03-25T16:42:05.280" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20820" PostTypeId="1" CreationDate="2017-03-25T21:10:13.120" Score="0" ViewCount="190" Body="&lt;p&gt;It is believed that the end of the Cretaceous period of the Earth's geology was marked by an asteroid impact which caused a mass extinction event, known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cretaceous-Paleogene Extinction Event&lt;/a&gt;. The causative asteroid is estimated to have been about 10 km in diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/z00PE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/z00PE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If such an asteroid was headed towards the Earth in modern times and we landed a spacecraft on it, attached it, then powered the engines in an effort to deflect the asteroid, how much fuel/energy would be needed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, the requirement changes depending on the distance of the asteroid from earth and the relative speed of the asteroid, so answerers may want to state the answer in terms of angular deviation per unit of fuel, or otherwise express an equation that defines the relationship.&lt;/p&gt;&#xA;" OwnerUserId="6871" LastEditorUserId="238" LastEditDate="2017-03-26T03:22:29.300" LastActivityDate="2017-03-29T19:28:27.417" Title="Spacecraft necessary to deflect extinction event asteroid?" Tags="|asteroid|asteroid-redirect-mission|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20821" PostTypeId="1" AcceptedAnswerId="20822" CreationDate="2017-03-25T21:26:55.800" Score="11" ViewCount="3594" Body="&lt;p&gt;On Earth people's lives are usually dictated by a 24-hour rhythm (with about 8 hours of sleep each night). As far as I know this rhythm is determined by chemical processes in the human body which are influenced by the sun (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Circadian_rhythm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Circadian rhythm&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;However dividing time into 24-hour periods isn't natural for astronauts on ISS (which make a turn around the earth in about 90 minutes, usually but not always with accompanying sunrises and sunsets).&lt;/p&gt;&#xA;&lt;p&gt;Do astronauts aboard the ISS still divide time into 24-hour periods or do they use some other regime? What kind of sleep needs does the human body have in space - e.g. do astronauts still sleep for 8 hours at a time or do they sleep for less/more time more/less often? Are all astronauts sleeping at the same time?&lt;/p&gt;&#xA;" OwnerUserId="8595" LastEditorUserId="12102" LastEditDate="2020-07-21T22:49:34.547" LastActivityDate="2020-07-21T22:49:34.547" Title="What kind of time regiment/schedule do ISS astronauts have?" Tags="|iss|crewed-spaceflight|health|time|astronaut-lifestyle|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="20822" PostTypeId="2" ParentId="20821" CreationDate="2017-03-25T22:46:15.120" Score="12" Body="&lt;p&gt;Like a lot of things that should be readily available, it's frustratingly difficult to find this on any NASA web site. Fortunately ESA came to the rescue in the form of a &lt;a href=&quot;http://blogs.esa.int/alexander-gerst/2014/07/02/planning-down-to-the-minute/&quot; rel=&quot;noreferrer&quot;&gt;blog post&lt;/a&gt; about Alexander Gerst.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew's schedule is shown onboard using a PC tool called &lt;a href=&quot;https://www.nasa.gov/centers/johnson/techtransfer/technology/MSC-24832-1-ostpv.html&quot; rel=&quot;noreferrer&quot;&gt;OSTPV&lt;/a&gt; (Onboard Short Term Plan Viewer).  Gerst was nice enough to post a screenshot of this for one day of his mission.  He was on the ISS in 2014, and it shows GMT day 177, so if I did it right, that would be June 26th.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tv2p2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tv2p2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may wish to view this graphic in a separate window so that you can refer to it as I describe it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Across the top you see three sets of wall clock time: GMT, Houston time, and Moscow time. The next band shows the day/night cycles predicted for the ISS for this time period. For each crewmember, there is a band labeled with their crew position and name.  You can see Gerst's band labeled FE-6 (Flight Engineer 6) and A. Gerst.  Referring to his band, you can see the time alloted for him to arise and eat (POSTSLEEP), then his daily activities start, scheduled down to 5 minute intervals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am no ISS acronym expert, but some of the activities include IFM (In-Flight Maintenance, fixing something on the ISS), midday meal, and exercise.  The day ends with PRESLEEP followed by SLEEP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the bottom are the Russian crewmembers, I believe this is really info-only, as they have their own &lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/111214_tl.pdf&quot; rel=&quot;noreferrer&quot;&gt;scheduling system&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other bands on the OSTPV show comm availability, earth observation opportunities, and special crew constraints.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Summarizing, Gerst awoke at 0600 GMT, went to work at 0730, ate lunch from approximately 1310 to 1410, exercised from 1530 to 1830, and called it a night at 1930.  Presumably this is a fairly typical (non spacewalk) day as the picture label on the blog post is &quot;typical workday for astronauts&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope I addressed all your points, or you can puzzle them out from looking at the OSTPV.  You can see that everyone is scheduled to sleep at the same time.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-03-25T22:57:47.433" LastActivityDate="2017-03-25T22:57:47.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20824" PostTypeId="1" CreationDate="2017-03-18T21:38:11.493" Score="36" ViewCount="986" Body="&lt;p&gt;In the novel &lt;em&gt;Gravity's Rainbow&lt;/em&gt; by Thomas Pynchon, there is a chapter in which Slothrop (the main character) thinks about this equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\theta \frac{d^2\phi}{dt^2} + \delta^* \frac{d\phi}{dt}&#xA;   + \frac{\partial L}{\partial\alpha}(s_1-s_2)\alpha&#xA;   = -\frac{\partial R}{\partial\beta} s_3\beta\,.$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to be something to do with the V2 rocket, and yaw control. Does anyone recognise this equation from a book or research paper? What do the various variables stand for? He may have invented the equation, I suppose. How would one go about solving an equation like this? I find it strange that it contains both partial derivatives and total derivatives. However, there are equations in Hamiltonian mechanics like this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any help would be much appreciated. Thank you.&lt;/p&gt;&#xA;" OwnerDisplayName="user947185" LastEditorUserId="195" LastEditDate="2017-03-26T03:07:07.617" LastActivityDate="2023-09-22T23:48:35.950" Title="Does anyone recognise this differential equation (from Gravity's Rainbow)?" Tags="|reference-request|" AnswerCount="2" CommentCount="9" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20825" PostTypeId="2" ParentId="20824" CreationDate="2017-03-18T22:09:10.867" Score="23" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Schachterle, Lance, and P. K. Aravind. &quot;The three equations in Gravity's Rainbow.&quot; &lt;em&gt;Pynchon Notes&lt;/em&gt; 46-49 (2000): 157-170.&#xA;  &lt;a href=&quot;https://pynchonnotes.openlibhums.org/articles/abstract/10.16995/pn.94/&quot; rel=&quot;noreferrer&quot;&gt;Journal Link&lt;/a&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;In our view, Pynchon inscribes these equations into &lt;em&gt;Gravity's Rainbow&lt;/em&gt; to challenge readers with yet another form of authority within the text.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Sorry&amp;mdash;hit a pay wall...&lt;br /&gt;&#xA;&lt;strong&gt;&lt;em&gt;Later&lt;/em&gt;&lt;/strong&gt;: Got through the pay wall:&#xA;&lt;hr /&gt;&#xA;&amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/JPk4q.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JPk4q.jpg&quot; alt=&quot;Pynchon1&quot;&gt;&lt;/a&gt;&#xA;&lt;br /&gt;&#xA;..........&#xA;&lt;br /&gt;&#xA;&amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp; &amp;nbsp;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/1LWdD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1LWdD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr /&gt;&#xA;" OwnerUserId="10331" OwnerDisplayName="Joseph O'Rourke" LastActivityDate="2017-03-22T12:59:10.157" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="20826" PostTypeId="1" AcceptedAnswerId="31399" CreationDate="2017-03-26T06:13:17.323" Score="15" ViewCount="1004" Body="&lt;h2&gt;Update:&lt;/h2&gt;&#xA;&lt;p&gt;Following a one month loss of launch capability, from Spaceflight Now, April 25, 2017; &lt;a href=&quot;https://spaceflightnow.com/2017/04/25/operations-resume-at-european-run-launch-base-in-french-guiana/&quot; rel=&quot;noreferrer&quot;&gt;Operations resume at European-run launch base in French Guiana&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Protest leaders demanded more 3 billion euros (USD 3.3 billion) in emergency funding to improve conditions in French Guiana. &lt;strong&gt;The final agreement signed Friday calls for 1.1 billion euros (USD 1.2 billion) of immediate aid&lt;/strong&gt;, and a “priority review” of another 2.1 billion euro (USD 2.3 billion) funding package.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;After one or two re-schedulings, Arianespace has &lt;a href=&quot;http://www.arianespace.com/press-release/arianespace-va236-launch-rescheduling-2/&quot; rel=&quot;noreferrer&quot;&gt;postponed&lt;/a&gt; the launch of Flight VA236 with SGDC and KOREASAT-7 &amp;quot;indefinitely&amp;quot;. The rocket is blocked from rolling out to the launchpad a roadblock as part of a protest.&lt;/p&gt;&#xA;&lt;p&gt;According to the BBC's news item &lt;a href=&quot;http://www.bbc.com/news/world-latin-america-39330971&quot; rel=&quot;noreferrer&quot;&gt;Spaceport protest delays rocket launch in French Guiana&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Ariane 5 rocket was due to take a South Korean satellite and a Brazilian satellite into orbit later on Tuesday.&lt;/p&gt;&#xA;&lt;p&gt;But the booster could not be rolled to the launch pad because of the protest at the Guiana Space Center over what workers for energy company EDF said were &amp;quot;deplorable working conditions&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;The launch has been moved to Wednesday.&lt;/p&gt;&#xA;&lt;p&gt;Workers used tyres and wooden pallets to erect barricades at the facility in French Guiana.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The article goes on to say:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Workers used tyres and wooden pallets to erect barricades at the facility in French Guiana.&lt;/p&gt;&#xA;&lt;p&gt;Satellite launch company Arianespace said in a statement that &amp;quot;due to a social movement, it was not possible to carry out today's [Monday's] scheduled transfer of the launch vehicle from the Spaceport's Final Assembly Building to the launch zone.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;It said that the launch vehicle, as well as the satellites had been &amp;quot;placed in a stand-by mode and are being maintained in fully safe conditions&amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;And finally, it says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It is not the first time industrial action has delayed a launch at the Kourou Space Centre.&lt;/p&gt;&#xA;&lt;p&gt;In 2011, another Ariane 5 rocket launch had to be moved back when workers demanded extra pay for working on Sundays.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Further information can be found in Spaceflight Now's &lt;a href=&quot;https://spaceflightnow.com/2017/03/23/launches-from-kourou-suspended-by-social-unrest/&quot; rel=&quot;noreferrer&quot;&gt;Launches from Kourou temporarily suspended by social unrest&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Are these the only two example of launches to space that have been &lt;em&gt;substantially delayed&lt;/em&gt; by protests physically present at the launch site? If not, what are the other cases - how often does such a thing happen?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Background on the most recent protest:&lt;/h2&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NKibQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NKibQ.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;A barricade outside the Guiana Space Center. Credit: France Guyane&amp;quot; From &lt;a href=&quot;https://spaceflightnow.com/2017/03/23/launches-from-kourou-suspended-by-social-unrest/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3uAq4.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3uAq4.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;An Ariane 5 rocket sits on the launch pad at the Kourou Space Center in French Guiana&amp;quot;, from &lt;a href=&quot;https://phys.org/news/2017-03-workers-french-rocket.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ge99W.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ge99W.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; French Guiana, north-east coast, South America. from &lt;a href=&quot;http://www.ouest-france.fr/region-guyane/mouvements-sociaux-en-guyane-lancement-d-ariane-reporte-ecoles-fermees-4878798&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, credit: Ouest-France.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-10-16T23:41:21.087" Title="How often are space launches substantially delayed by protests physically at the launch site?" Tags="|launch|launch-site|international-politics|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20827" PostTypeId="1" AcceptedAnswerId="20828" CreationDate="2017-03-26T06:23:07.537" Score="1" ViewCount="1243" Body="&lt;p&gt;I'm trying to understand how the &lt;em&gt;longitude of the ascending node&lt;/em&gt; &quot;works&quot; for the planets in our solar system.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NASA fact sheet easily gives you the angles for the &quot;longitude of the ascending node&quot;. Let's take Saturn for example, with an angle of 113 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I understand from the book &quot;Fundamentals of Astrodynamics&quot; is that this is 113 degrees counter-clockwise from the point of reference (the vernal equinox direction).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then I looked at this page, which has a really good explanation for the ascending node:&#xA;&lt;a href=&quot;http://astronomy.swin.edu.au/cosmos/A/Ascending+Node&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://astronomy.swin.edu.au/cosmos/A/Ascending+Node&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then I tried to apply that knowledge in the image of Saturn using a &quot;top view&quot; and a &quot;side view&quot; (&lt;a href=&quot;http://calgary.rasc.ca/orbits.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://calgary.rasc.ca/orbits.htm&lt;/a&gt;):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/MDraU.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MDraU.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this image, they have highlighted that part of the orbit of Saturn that is above the ecliptic plane, using a bright white line. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, correct me if I'm wrong, but isn't the vernal equinox represented by the horizontal line pointed towards the east? When I then take that 113 degrees counter-clockwise in the &quot;top view&quot;, I get to the point where the planet reaches above the ecliptic. So far so good. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;BUT, if I look at the &quot;side view&quot;, that point seems to be the point of the descending node, not the point of the ascending node...  &lt;/p&gt;&#xA;" OwnerUserId="18988" LastActivityDate="2017-03-26T06:40:57.310" Title="Understanding Longitude of ascending node (orbit orientation)" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20828" PostTypeId="2" ParentId="20827" CreationDate="2017-03-26T06:40:57.310" Score="1" Body="&lt;p&gt;You are probably confusing front and back in the side view. The way the orbital plane of Saturn is oriented in the side view we are looking at it from below. So the bottom line is in the back and the top line is in the front, that is, Saturn is going clockwise in the side view. Ascending node on the left (back), descending node on the right (front). See it now?&lt;/p&gt;&#xA;" OwnerUserId="10488" LastActivityDate="2017-03-26T06:40:57.310" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20829" PostTypeId="1" AcceptedAnswerId="20832" CreationDate="2017-03-26T06:47:09.233" Score="8" ViewCount="2077" Body="&lt;p&gt;&lt;a href=&quot;http://www.bbc.com/news/science-environment-39355424&quot; rel=&quot;noreferrer&quot;&gt;From the BBC&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A British schoolboy has discovered that a piece of Nasa's top space research &lt;strong&gt;is flawed&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Miles Soloman, a student at Tapton School in Sheffield, was taking part in a project run by the Institute for Research in Schools. He told the World At One how he discovered that equipment used by the International Space Station &lt;strong&gt;was recording false data&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;A further BBC article and interview can be found &lt;a href=&quot;http://www.bbc.com/news/uk-39351833&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. From that article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;During UK astronaut Tim Peake's stay on the station, detectors began recording the radiation levels on the ISS.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;I went straight to the bottom of the list and I went for the lowest bits of energy there were,&amp;quot; Miles explained. Miles's teacher and head of physics, James O'Neill, said: &amp;quot;We were all discussing the data but he just suddenly perked up in one of the sessions and went 'why does it say there's -1 energy here?'&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;What Miles had noticed was that when nothing hit the detector, a negative reading was being recorded. But you cannot get negative energy. So Miles and Mr O'Neill contacted Nasa. &amp;quot;It's pretty cool&amp;quot;, Miles said. &amp;quot;You can tell your friends, I just emailed Nasa and they're looking at the graphs that I've made.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;It turned out that Miles had noticed something no-one else had - including the Nasa experts. Nasa said it was aware of the error, but believed it was only happening once or twice a year. Miles had found it was actually happening multiple times a day.&lt;/p&gt;&#xA;&lt;p&gt;Prof Larry Pinksy, from the University of Houston, told Radio 4: &amp;quot;My colleagues at Nasa thought they had cleaned that up.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Precisely what NASA top space research &amp;quot;is flawed&amp;quot; because it is recording &amp;quot;false data&amp;quot;? What data stream are we talking about, and is the problem catastrophic, or just LSB noise (least significant bit).&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-26T11:31:59.920" Title="&quot;UK schoolboy corrects Nasa data error&quot; - what precisely was the &quot;error&quot;?" Tags="|iss|nasa|research|scientific-data|raw-data|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20830" PostTypeId="1" AcceptedAnswerId="20831" CreationDate="2017-03-26T07:06:13.903" Score="3" ViewCount="562" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_MS-04#Crew&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz MS-04&lt;/a&gt; has the distinction of being the &lt;a href=&quot;http://www.russianspaceweb.com/soyuz-ms-04.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;first manned launch of 2017&lt;/a&gt;, and the first (at least in a while?) to bring only two, rather than three crew members to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an official reason? Is there also a subtext? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;Soyuz MS-04 is being prepared for vacuum testing.&quot; from &lt;a href=&quot;http://www.russianspaceweb.com/soyuz-ms-04.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Credit: RKK Energia&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VtR51.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VtR51.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-03-26T07:14:26.033" LastActivityDate="2017-06-06T08:05:39.903" Title="Why will Soyuz MS-04 bring only two crew to the ISS?" Tags="|iss|soyuz-spacecraft|crew-size|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20831" PostTypeId="2" ParentId="20830" CreationDate="2017-03-26T09:02:56.800" Score="7" Body="&lt;p&gt;&lt;a href=&quot;http://www.space.com/34736-nasa-russia-reduced-space-station-crew.html&quot; rel=&quot;noreferrer&quot;&gt;Official reason&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Beginning with the next Soyuz launch, in March 2017, Russia is cutting one cosmonaut post from the station crew and doesn't plan to restore the job until a new module is launched, in 2018.&lt;/p&gt;&#xA;&lt;p&gt;Cosmonauts typically comprise half the station's six-member crew, but &amp;quot;right now, we don't need it,&amp;quot; Sergei Krikalev, a former cosmonaut who now oversees human spaceflight for the Russian space agency, Roscosmos, said in an interview at the International Astronautical Congress, which was held in late September in Guadalajara, Mexico.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The subtext is budget cuts, I believe.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-26T09:02:56.800" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20832" PostTypeId="2" ParentId="20829" CreationDate="2017-03-26T11:26:04.717" Score="7" Body="&lt;p&gt;The source of the data &lt;a href=&quot;http://www.iflscience.com/space/a-schoolboy-in-the-uk-pointed-out-an-error-to-nasa-on-the-iss/&quot; rel=&quot;noreferrer&quot;&gt;is&lt;/a&gt; the &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/417.html&quot; rel=&quot;noreferrer&quot;&gt;Radiation Environment Monitor&lt;/a&gt;. Lawrence S. Pinsky is listed as co-investigator.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This Radiation Environment Monitor demonstration will provide information that is required to enable the design of an operational active personal space radiation dosimeter.&lt;/p&gt;&#xA;&lt;p&gt;The objectives of the experiment are to demonstrate the viability of this technology in the space radiation environment and the ability to assess crew exposure in near real time (via ground software).&lt;/p&gt;&#xA;&lt;p&gt;In order to effectively manage radiation risks to crewmembers during long-duration space exploration beyond low Earth orbit, there is a strong need to monitor personal radiation exposure in near real time. The current technology uses passive dosimetry techniques and associated logistics that require down-mass and ground analysis which provides radiation dose information several months after return. The REM is a low mass, low volume and low power dosimeter that may meet the near real time exposure monitoring of crew members.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://adsabs.harvard.edu/abs/2015NIMPA.782..143S&quot; rel=&quot;noreferrer&quot;&gt;REM devices&lt;/a&gt; are USB sticks.&lt;/p&gt;&#xA;&lt;p&gt;The error was a &lt;a href=&quot;http://www.iflscience.com/space/a-schoolboy-in-the-uk-pointed-out-an-error-to-nasa-on-the-iss/&quot; rel=&quot;noreferrer&quot;&gt;misunderstanding&lt;/a&gt; of the root cause of the negative values:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NASA had apparently already known about this issue, but they thought it was only occurring once or twice a year due to a problem with an algorithm. But Soloman found it was occurring multiple times a day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Why the REM records negative values remains to be seen.&lt;br /&gt;&#xA;But it is not unusual to use 'impossible values' to convey information, where -1 could mean 'measurement out of range'. There's a name for this phenomenon, but that escapes me at the moment.&lt;/p&gt;&#xA;&lt;p&gt;If you misread the -1 as a literal value, its impact depends on the rest of the data.&#xA;If the data has a range of 0-10000, the impact of having a few instances of -1 that should be 0 is small. If the values are in the range of 0-10, its impact is much larger.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-03-26T11:31:59.920" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20833" PostTypeId="2" ParentId="20812" CreationDate="2017-03-26T11:44:08.777" Score="4" Body="&lt;p&gt;In the US, it is illegal for Federal employees (e.g. NASA civil servants) to take active part in partisan political campaigns, based on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hatch_Act_of_1939&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hatch Act&lt;/a&gt;.&#xA;Other laws restrict lobbying Congress, such as the &lt;a href=&quot;https://www.law.cornell.edu/uscode/text/18/1913&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anti-Lobbying Act&lt;/a&gt;, described as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Act places certain restrictions and limitations on career federal officials lobbying Congress, especially with respect to engaging in certain types of grass roots activities aimed at influencing pending legislation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="19024" LastEditorUserId="6816" LastEditDate="2017-03-26T14:04:06.177" LastActivityDate="2017-03-26T14:04:06.177" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20834" PostTypeId="1" CreationDate="2017-03-26T15:06:02.960" Score="2" ViewCount="297" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Spektr-R&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spektr-R&lt;/a&gt; is a 10 meter diameter radio telescope in Earth's orbit with an apogee of 338,000 km, nearly a Lunar distance. It does interferometry together with the Very-Long-Baseline Interferometry telescopes (&lt;a href=&quot;https://en.wikipedia.org/wiki/Very-long-baseline_interferometry&quot; rel=&quot;nofollow noreferrer&quot;&gt;VLBI&lt;/a&gt; on Earth at wavelengths 92, 18, 6 and 1.3 cm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How hard would it be to build and operate multiple formation flying &lt;strong&gt;infrared&lt;/strong&gt; space telescopes that do interferometry together? Infrared has many astrophysical applications, from small bodies in the Solar system to the earliest universe. JWST will observe in 0.6 to 28.5 µm, about a factor of 1,000 shorter than Spektr-R, while sub-millimeter starts only a factor of 13 shorter. Is there any rule of thumb for how much shorter a baseline would have to be when the wavelength decreases? One kilometer would still be much better than JWST's 6.5 meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from making the interferometry work, it should be cheaper to build multiple smaller identical telescopes, with the same total mirror area as the huge LUVOIR and HDST envisioned. And it could be extended without having to touch existing hardware. Why is this not already on the to-do-list?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RYNmQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RYNmQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Spekr-R/Radioastron - the worlds largest space telescope.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/f9UJu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f9UJu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;Credit: C. Godfrey (STScI). HDST, a proposed High definition Space Telescope. An 11 meter class aperture could be made from 54 peaces of 1.3 meters segments. In infrared, space interferometry seems to be an alternative to huge mirrors like this.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-04-03T11:24:52.187" LastActivityDate="2019-04-18T22:28:38.480" Title="Infrared space interferometry - Why isn't it being done?" Tags="|space-telescope|interferometry|" AnswerCount="0" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20837" PostTypeId="2" ParentId="20432" CreationDate="2017-03-26T18:31:42.657" Score="2" Body="&lt;p&gt;Question being answered: &quot;I'm more interested in international laws/treaties and U.S. spaceflight laws&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks to called2voyage - &lt;a href=&quot;https://www.congress.gov/114/plaws/publ90/PLAW-114publ90.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.congress.gov/114/plaws/publ90/PLAW-114publ90.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only constraint I see is the License to perform the launch. IF SpaceX gets that I do not see anything that constrains a Trip TO the Moon or around it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition I did NOT see anything that constrained the reentry of the HUMAN passengers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From an international perspective as long as no language attempting to show ownership, sovereignty etc exist then International Law does not apply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IF some one can show evidence of an International Law prohibiting an American Corporation Launching Humans on a trip around the Moon I would like to see it and what Roscosmos did to get permission to take Space Tourist to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDITED:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be an exception here, IF SpaceX decides to send the tourist to see the ISS at less than 200 meters then there would be an issue.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If all goes according to plan, at approximately 5:30AM ET, Dragon will be permitted to enter the Keep-Out Sphere (KOS), an imaginary circle drawn 200 meters (656 feet) around the station that prevents the risk of collision, and continue its approach to the capture point&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://nssphoenix.wordpress.com/tag/keep-out-sphere/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://nssphoenix.wordpress.com/tag/keep-out-sphere/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edited: IF the requirement for an answer is what is a Launch License &lt;a href=&quot;https://www.faa.gov/about/office_org/headquarters_offices/ast/licenses_permits/launch_reentry/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.faa.gov/about/office_org/headquarters_offices/ast/licenses_permits/launch_reentry/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A launch- or reentry-specific license authorizes you to conduct one or&#xA;  more launches or reentries having the same operational parameters of&#xA;  one type of launch or reentry vehicle operating at one launch or&#xA;  reentry site. The license identifies, by name or mission, each&#xA;  activity authorized under the license. Your authorization to operate&#xA;  terminates when you complete all launches or reentries authorized by&#xA;  the license or the expiration date stated in the license, whichever&#xA;  occurs first.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Edit: Add History of the LAW ... &lt;a href=&quot;https://en.wikipedia.org/wiki/Private_spaceflight&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Private_spaceflight&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Commercial Space Launch Act of 1984 required encouragement of&#xA;  commercial space ventures, adding a new clause to NASA's mission&#xA;  statement: (c) Commercial Use of Space.--Congress declares that the&#xA;  general welfare of the United States requires that the Administration&#xA;  seek and encourage, to the maximum extent possible, the fullest&#xA;  commercial use of space.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Yet one of NASA's early actions was to effectively ban private space&#xA;  flight through a mountain of red tape. From the beginning, though,&#xA;  this met significant opposition not only by the private sector, but in&#xA;  Congress.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In 1962, Congress passed its first law pushing back the prohibition on&#xA;  private involvement in space, the Communications Satellite Act of&#xA;  1962. While largely focusing on the satellites of its namesake, this was described by both the law's opponents and advocates of private&#xA;  space, as the first step on the road to privatization.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;While launch vehicles were originally bought from private contractors,&#xA;  from the beginning of the Shuttle program until the Challenger&#xA;  disaster in 1986, NASA attempted to position its shuttle as the sole&#xA;  legal space launch option.But with the crash came the suspension of&#xA;  the government-operated shuttle flights, allowing the formation of a&#xA;  commercial launch industry.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;On 30 October 1984, United States President Ronald Reagan signed into&#xA;  law the Commercial Space Launch Act. This enabled an American industry&#xA;  of private operators of expendable launch systems. Prior to the&#xA;  signing of this law, all commercial satellite launches in the United&#xA;  States were restricted by Federal regulation to NASA's Space Shuttle.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;On 5 November 1990, United States President George H. W. Bush signed&#xA;  into law the Launch Services Purchase Act. The Act, in a complete&#xA;  reversal of the earlier Space Shuttle monopoly, ordered NASA to&#xA;  purchase launch services for its primary payloads from commercial&#xA;  providers whenever such services are required in the course of its&#xA;  activities.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Commercial launches outnumbered government launches at the Eastern&#xA;  Test Range in 1997.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The Commercial Space Act was passed in 1998 and implements many of the&#xA;  provisions of the Launch Services Purchase Act of 1990.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Nonetheless, until 2004 NASA kept private space flight effectively&#xA;  illegal. But that year, the Commercial Space Launch Amendments Act of&#xA;  2004 required that NASA and the Federal Aviation Administration&#xA;  legalize private space flight. The 2004 Act also specified a &quot;learning&#xA;  period&quot; which restricted the ability of the FAA to enact regulations&#xA;  regarding the safety of people who might actually fly on commercial&#xA;  spacecraft through 2012, ostensibly because spaceflight participants&#xA;  would share the risk of flight through informed consent procedures of&#xA;  human spaceflight risks, while requiring the launch provider to be&#xA;  legally liable for potential losses to uninvolved persons and&#xA;  structures.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;To the end of 2014, commercial passenger flights in space has remained&#xA;  effectively illegal, as the FAA has refused to give a commercial&#xA;  operator's license to any private space company.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The United States updated US commercial space legislation with the&#xA;  passage of the SPACE Act of 2015 in November 2015. The full name of&#xA;  the act is Spurring  Private Aerospace Competitiveness and&#xA;  Entrepreneurship Act of 2015 The update US law explicitly allows &quot;US&#xA;  citizens to engage in the commercial exploration and exploitation of&#xA;  'space resources' [including ... water and minerals].&quot; The right does&#xA;  not extend to biological life, so anything that is alive may not be&#xA;  exploited commercially. The Act further asserts that &quot;the United&#xA;  States does not [(by this Act)] assert sovereignty, or sovereign or&#xA;  exclusive rights or jurisdiction over, or the ownership of, any&#xA;  celestial body.&quot; &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The SPACE Act includes the extension of indemnification of US launch&#xA;  providers for extraordinary catastrophic third-party losses of a&#xA;  failed launch through 2025, while the previous indemnification law was&#xA;  scheduled to expire in 2016.  The Act also extends, through 2025, the&#xA;  &quot;learning period&quot; restrictions which limit the ability of the FAA to&#xA;  enact regulations regarding the safety of spaceflight participants.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Indemnification for extraordinary third-party losses has, as of 2015,&#xA;  been a component of US space law for over 25 years, and during this&#xA;  time, &quot;has never been invoked in any commercial launch mishap.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="18720" LastEditorUserId="18720" LastEditDate="2017-03-26T20:06:16.903" LastActivityDate="2017-03-26T20:06:16.903" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20838" PostTypeId="1" CreationDate="2017-03-26T19:37:17.167" Score="24" ViewCount="3356" Body="&lt;p&gt;The Merlin 1D engines can restart up to four times, they use the pyrophoric mixture of TEA-TEB (&lt;a href=&quot;http://acronyms.thefreedictionary.com/Triethylaluminum-Triethylborane&quot; rel=&quot;noreferrer&quot;&gt;Triethylaluminum-Triethylborane&lt;/a&gt;) to ignite the engines. My question is where is this stored in the engine so that it can be released several times to restart the engine?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="6537" LastEditDate="2017-12-12T10:23:45.693" LastActivityDate="2020-06-09T22:18:49.037" Title="Where is the TEA-TEB stored for the Merlin engines onboard Falcon 9?" Tags="|spacex|merlin-1d|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20841" PostTypeId="1" CreationDate="2017-03-27T01:27:15.060" Score="0" ViewCount="132" Body="&lt;p&gt;I'm calculating the azimuth and elevation in a ground point to see some satellites, but if for example the error in orbit for a satellite grows 1 km how is that distance (error) represented in radians ( for azimuth/elevation)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE 1:&#xA;For example I'm calculating the position of GPS satellites, that constellation are to 20200 kms of altitude [ECEF]. If the perpendicular error is around 1km in the orbit, how many is the error represented in radians, when I'm seeing azimuth/elevation from an observer?&lt;/p&gt;&#xA;" OwnerUserId="18740" LastEditorUserId="18740" LastEditDate="2017-03-27T13:36:43.177" LastActivityDate="2017-03-29T11:55:32.840" Title="How does the distance in orbit relate to azimuth?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20844" PostTypeId="2" ParentId="18142" CreationDate="2017-03-27T10:21:27.653" Score="6" Body="&lt;p&gt;The key points in thermal design for satellites, which envelopes &amp;quot;to prevent overheating&amp;quot; are:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Have a design that specifically has a heat loss path, this by definition has to be a high conductance path rather than an insulator. Consider that if an item has two similar paths, differing only because of uncertain material properties, it may be very difficult to know which will dominate in any situation.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The high conductance path has to apply at each step from the heat source to the vacuum of space:&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;components bonded to circuit board with thermally conducting adhesive,&lt;/li&gt;&#xA;&lt;li&gt;short lateral paths in circuit boards,&lt;/li&gt;&#xA;&lt;li&gt;thickened floor of electronic boxes (allows spreading of heat for greater access to radiator space)&lt;/li&gt;&#xA;&lt;li&gt;interface filler between equipment box and panel,&lt;/li&gt;&#xA;&lt;li&gt;direct panel view of space with low solar incidence AND / OR heat pipe network in panel AND / OR mirror finish on outside of panel if solar incidence cannot be avoided.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;Methods for the above steps are often chosen to minimise the uncertainty as much as for their maximum performance: it is much less of a headache to have a slightly sub-optimal design that you know will work rather than to be worrying about metal-metal interfaces.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Thereafter consider insulating direct solar input paths. Again do this both a) to reduce the heat load and b) to improve certainty in the controlled space.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Clearly there will be minimum temperature requirements too. There will be compromises in terms of performance and uncertainty to meet both the hot and cold limits in all seasons.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;from a reliability perspective consider heaters before heatpipes, and passive heatpipes before loop heat pipes or pumps,&lt;/li&gt;&#xA;&lt;li&gt;clearly there is another compromise on keeping heater power within the available range,&lt;/li&gt;&#xA;&lt;li&gt;given the interest in keeping uncertainties down, steer away from relying on the thermal mass of components too much. Clearly for sudden changes like eclipse this is important but it means you are relying now on a much longer chain of uncertainties.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;ol start=&quot;6&quot;&gt;&#xA;&lt;li&gt;Assurance&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;make a thermal mathematical model, get someone else to review it&lt;/li&gt;&#xA;&lt;li&gt;test it in a chamber with lots of thermocouples and compare the test with the model, then use this to improve the model links&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2021-04-02T09:15:46.307" LastActivityDate="2021-04-02T09:15:46.307" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="20847" PostTypeId="2" ParentId="20807" CreationDate="2017-03-27T13:48:36.540" Score="3" Body="&lt;p&gt;This answers the part of the question &quot;why is it important&quot; and gives a couple of examples. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;System capability&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is useful where there is no reference to the thrust level or the payload mass to be carried. This is one of many ways of very roughly comparing launch vehicles or individual stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.lr.tudelft.nl/en/organisation/departments/space-engineering/space-systems-engineering/expertise-areas/space-propulsion/propulsion-options/chemical-rockets/liquid/&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; gives the following list of examples for different applications.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;High - space launcher propulsion:  50 MNs to 20,000 MNs &#xA;&#xA;Moderate: 10-50 MNs&#xA;&#xA;Low - missions like for orbit insertion of spacecraft: up to about 15 MNs.&#xA;&#xA;Reaction control systems of spacecraft: up-to to about 1.5 MNs.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;There is a lot more in the article, including data for many specific examples though it is quite dated, several pages long and needs a careful read for context. As a result I don't think its appropriate to paste it in entirely. Its helpful background.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Lifetime capability&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This relates to the qualification lifetime of an engine/thruster; this recognises that any given thruster will have a wearout lifetime rather than just the limit to the propellant actually carried. This view permits comparison between two candidate engines that might have different nominal thrust levels e.g. 2N and 10N. This can help in a trade-off where it is of interest to see if any of the candidate engines are lifetime limited.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2011index/IEPC-2011-041.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; gives the total impulse of a variant of the SPT 100 as 2.67 - 3 MNs. This is based upon a total operating time of 9-10,000 hours and a thrust level of 83mN.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-03-27T14:11:47.897" LastActivityDate="2017-03-27T14:11:47.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20848" PostTypeId="1" AcceptedAnswerId="20851" CreationDate="2017-03-27T19:40:34.977" Score="7" ViewCount="3232" Body="&lt;p&gt;How do I find the delta-v loss of a rocket while ascending to its apogee?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specification of Atlas V 541 rocket:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Specification for Atlas V first stage:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dry Mass: 21054 kg&lt;/li&gt;&#xA;&lt;li&gt;Fuel and Oxidizer(RP1/LOX) Mass:284,089kg&lt;/li&gt;&#xA;&lt;li&gt;(Isp)(SL) = 311 seconds&lt;/li&gt;&#xA;&lt;li&gt;(Isp)(vac) = 338 seconds&lt;/li&gt;&#xA;&lt;li&gt;Thrust(SL) = 3,827 KN&lt;/li&gt;&#xA;&lt;li&gt;Thrust(vac) = 4,152 KN&lt;/li&gt;&#xA;&lt;li&gt;First stage wet mass: 305,143 kg&lt;/li&gt;&#xA;&lt;li&gt;Liftoff TWR: 1.179&lt;/li&gt;&#xA;&lt;li&gt;Maximum TWR: 9.06 (Will be throttled down due to G-Force)&lt;/li&gt;&#xA;&lt;li&gt;Operating time: 253 seconds&lt;br&gt;&lt;br&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Specification for Atlas V second stage:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dry Mass: 2,243 kg&lt;/li&gt;&#xA;&lt;li&gt;Fuel and Oxidizer (Hydrolox) = 20,830 kg&lt;/li&gt;&#xA;&lt;li&gt;Fairing: 1,500 kg&lt;/li&gt;&#xA;&lt;li&gt;Equipment and imaginary payload: 600 kg&lt;/li&gt;&#xA;&lt;li&gt;(Isp)(vac):449.7 seconds&lt;/li&gt;&#xA;&lt;li&gt;Thrust(vac) = 101.8 KN&lt;/li&gt;&#xA;&lt;li&gt;Second stage wet mass: 25,673 kg&lt;/li&gt;&#xA;&lt;li&gt;Minimum TWR : 0.404&lt;/li&gt;&#xA;&lt;li&gt;Maximum TWR : 3.105&lt;/li&gt;&#xA;&lt;li&gt;Operating time: 926 seconds&lt;br&gt;&lt;br&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Rocket Total Mass (Both Stages): 330,816 kg&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18270" LastEditorUserId="8693" LastEditDate="2017-03-28T15:04:06.270" LastActivityDate="2017-03-28T15:04:06.270" Title="How to determine $\Delta v$ loss due to gravitational pull from earth" Tags="|launch|orbital-mechanics|rockets|trajectory|" AnswerCount="1" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="20849" PostTypeId="1" AcceptedAnswerId="20850" CreationDate="2017-03-28T06:27:25.277" Score="11" ViewCount="82639" Body="&lt;p&gt;I'll be in Miami this Thursday 30th of March and there is a SpaceX Falcon 9 rocket launch from LC-39A, Kennedy Space Center, Florida (if it's not delayed).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I won't be able to drive north to be on time at Cape Canaveral to see it. Is it still possible to watch the launch from anywhere along the east coast of Florida, between Miami and the launch site ? And how far north of Miami should I have to go to see something ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that the best I can hope for is to see a distant light and not the actual rocket. But it's better than nothing :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you !&lt;/p&gt;&#xA;" OwnerUserId="19091" LastEditorUserId="19091" LastEditDate="2019-12-15T12:53:41.237" LastActivityDate="2019-12-15T12:53:41.237" Title="How far south can one see a rocket launch from Cape Canaveral, Florida?" Tags="|launch|spacex|kennedy-space-center|cape-canaveral|" AnswerCount="7" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20850" PostTypeId="2" ParentId="20849" CreationDate="2017-03-28T07:49:52.570" Score="7" Body="&lt;p&gt;Yes you can. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't want to link the picture here directly because of license you can see what a STS (Space Shuttle) launch looks like from Miami:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://chicagoboyz.net/archives/6433.html&quot; rel=&quot;noreferrer&quot;&gt;http://chicagoboyz.net/archives/6433.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, the rockets are different, yes the trajectory is different (No it does not go north east as you said, but south east towards the equator), but this is not enough to make a difference for you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The previous question linked shows that antares launches can be seen as far as 700km away. You will be 350 km away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only reason why you would not be able to see it is if the weather is cloudy. &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="13208" LastEditDate="2017-03-28T13:39:31.777" LastActivityDate="2017-03-28T13:39:31.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20851" PostTypeId="2" ParentId="20848" CreationDate="2017-03-28T11:49:00.037" Score="10" Body="&lt;p&gt;This is the problem known as launch trajectory optimization. That is a complicated problem mostly requiring a numerical simulation. That does however not mean we can obtain some useful figures and insights from an analytical approach!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To begin, we can with an inaccurate but usable estimate. The flawed assumption is as follows: &quot;Our rocket is fighting against gravity for a time of $T$, so the lost $\Delta v=Tg$&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Atlas rocket is flying for $253s+926s=1179s$, so the lost $\Delta v=1179s \cdot g=11500\frac{m}{s}$. That is more than the total $\Delta v$ of the rocket... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is the problem? Turns out force is a vector quantity and not a scalar. Our rocket is mostly not accelerating in the direct opposite direction of gravity, in fact, we want to gain velocity normal to gravity to reach an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we draw some force vectors we can clearly see that the total acceleration is larger than the value we get from simple subtraction, depending on the angle. (Accelerating downwards, we even get a small bonus from gravity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TABSPm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TABSPm.png&quot; alt=&quot;force vectors&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Complication 1: &lt;strong&gt;$\Delta v$ loss depends on trajectory&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are more things to consider than this. For instance, why does the rocket not have to fight gravity anymore? Well, it has gained orbital velocity. But &quot;orbital velocity&quot; is not a binary toggle, everything is an orbit, the slower ones just happen to intersect Earth, not desirable for a satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, we are thrusting against a &quot;vertical acceleration&quot; (The direction of course varies while we are orbiting, so we are viewing this in a rotating frame of reference). We can express this from the circular orbital velocity, $v_c$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a_{vertical} = 1 - \frac{v_{horizontal}^2}{v_c^2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Complication 2: &lt;strong&gt;The vertical acceleration depends on velocity&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, an ideal strategy would be to gain horizontal velocity as fast as possible, with just enough vertical acceleration to counteract gravity. This is not very useful though as the drag is going to be extremely large and your rocket will end up crashing into the side of a mountain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A compromise is to start in an almost vertical position, and always accelerate parallel to the current velocity vector. This is efficient as adding together vectors in parallel gives the longest resulting vector. Thir trajectory also has the nice property that the gravitational acceleration is going to slowly turn the rocket horizontal. With some planning, the rocket ends up with orbital velocity at the target altitude. This is known as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_turn&quot; rel=&quot;noreferrer&quot;&gt;gravity turn&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You now have:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Horizontal acceleration, depending on gravity and vertical velocity&lt;/li&gt;&#xA;&lt;li&gt;Gravity, depending on altitude, depending on vertical acceleration over time&lt;/li&gt;&#xA;&lt;li&gt;...&lt;/li&gt;&#xA;&lt;li&gt;And a ton more.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The resulting system of differential equations is way to complicated to solve properly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On top of that, you have to consider the atmosphere, as that affects your trajectory (and $I_{SP}$...)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR&lt;/strong&gt; You must use a numerical simulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So then, &lt;strong&gt;how&lt;/strong&gt; could such a simulation be implemented? Here is a quick sketch: (svg source in answer source for anybody wanting to improve it)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&#xA;    &#xA;        &#xA;            &#xA;            &#xA;        &#xA;    &#xA;    &#xA;    &#xA;    &#xA;    &#xA;    &#xA;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QPRAL.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QPRAL.png&quot; alt=&quot;ascent trajectory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Initial ascent (red). Accelerate vertically, and simulate drag losses. Most of those should be here.&lt;/li&gt;&#xA;&lt;li&gt;Gravity turn (blue). Start to pick up some horizontal velocity, and let gravity turn your rocket over. Repeat this for multiple angles to get your preferred final altitude. Ignore drag for this part.&lt;/li&gt;&#xA;&lt;li&gt;Final orbit (pink). No more acceleration required.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2017-03-28T14:59:01.997" LastActivityDate="2017-03-28T14:59:01.997" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20852" PostTypeId="1" CreationDate="2017-03-28T18:00:50.093" Score="1" ViewCount="167" Body="&lt;p&gt;If a rover on Mercury &quot;roves&quot; only at night when the surface is freezing cold, and before dawn unfolds a Sun shield of a few square meters above itself, under which it stays put all day. Would the surface under the shield still get hot by horizontal thermal conductivity in the surface materials?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-03-29T00:28:19.013" Title="Could a Sun shield keep a rover cool daytime on Mercury?" Tags="|lander|temperature|shielding|mercury|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20853" PostTypeId="1" AcceptedAnswerId="20854" CreationDate="2017-03-28T19:20:05.740" Score="0" ViewCount="1473" Body="&lt;p&gt;What were the chances of the Apollo 11 mission failing? Consider the following scenarios: least one of the crewmembers dying, failing to land on the Moon, unable to return to Earth, the Soviets landing on the Moon first etc. Was it likely or improbable?&lt;/p&gt;&#xA;" OwnerUserId="19097" LastActivityDate="2017-03-28T21:04:51.907" Title="What were the chances of failing the Apollo 11 mission?" Tags="|the-moon|apollo-program|apollo-11|space-race|" AnswerCount="1" CommentCount="0" ClosedDate="2017-03-28T20:44:22.350" ContentLicense="CC BY-SA 3.0" />
  <row Id="20854" PostTypeId="2" ParentId="20853" CreationDate="2017-03-28T20:42:25.603" Score="4" Body="&lt;p&gt;Most of your question is a duplicate;  I'll address the &quot;Soviets land first&quot; question a bit, though that's a more subjective issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think there was ever a significant chance that the USSR would beat the US to a moon landing. Development of the N1 moon rocket started in late '65, over three years behind the Saturn V; the chief designer, Korolev, died the next year. Before the first (failed) flight of the N1 in early '69, the US had already flown Apollo 8 around the moon; at that point it's likely that nothing short of losing a crew in an in-flight accident would have stopped the program, and likely not even that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Kennedy hadn't made his &quot;before the decade is out&quot; speech, things might have been different; NASA had always planned to go to the moon eventually but likely would have taken a slower, less dramatic course to get there, possibly focusing on space stations in Earth orbit in the late 60s and moon landing in the mid 70s. Without that pace, the USSR might have taken a slower and safer approach to the development of the N1, and it could have been closer.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-03-28T21:04:51.907" LastActivityDate="2017-03-28T21:04:51.907" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20855" PostTypeId="1" CreationDate="2017-03-28T23:40:00.170" Score="6" ViewCount="263" Body="&lt;p&gt;SpaceX is slated to launch the SES-10 this coming Thursday from LC-39A at KSC. This is widely seen a pretty major milestone, as this will be the first time a re-used booster will be used to put a payload into orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that the booster's already used, are SpaceX planning on having it return again, either to an autonomous landing barge or to KSC itself?&lt;/p&gt;&#xA;" OwnerUserId="12046" LastEditorUserId="51" LastEditDate="2017-03-29T01:01:44.283" LastActivityDate="2017-03-29T16:23:00.737" Title="Will the SES-10 booster have a secondary mission goal of landing?" Tags="|spacex|falcon-9|landing|reuse|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20856" PostTypeId="2" ParentId="20855" CreationDate="2017-03-29T00:08:23.927" Score="6" Body="&lt;p&gt;According to Florida Today SpaceX plans to land the booster again, on a drone ship.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After launching SES-10, SpaceX will again try to land the booster at&#xA;  sea, though Halliwell said the mission launching the satellite on its&#xA;  way to an orbit more than 22,000 miles up was “right at the limit” of&#xA;  being able to pull off a second landing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/spacex/2017/03/28/ses-confident-spacex-rocket-can-deliver-second-time/99699862/&quot; rel=&quot;noreferrer&quot;&gt;Article&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-03-29T00:08:23.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20857" PostTypeId="2" ParentId="20852" CreationDate="2017-03-29T00:28:19.013" Score="4" Body="&lt;p&gt;As Hobbes pointed out, the ground under the rover will eventually warm up.  Any thermal barrier only slows down the heat transfer.  It would eventually cook from below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing that I can think of would be to put the rover up on stilts and have heat shields both above and below with radiators pointing outward between the two shields.  The radiators are to dissipate the heat coming up the stilts and the heat absorbed by the heat shields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from the mechanical issue of lifting the rover a few meters off the ground every &quot;day,&quot; the rover would have to cart all of that around.  I just don't see that happening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we need a ground mission, I think we will need to drop the rover after &quot;dusk&quot; as soon as the ground is cool enough, run it away from the sun as much as you can and then play taps in a month when it gets cooked.&lt;/p&gt;&#xA;" OwnerUserId="18842" LastActivityDate="2017-03-29T00:28:19.013" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20858" PostTypeId="2" ParentId="20855" CreationDate="2017-03-29T01:16:34.723" Score="2" Body="&lt;p&gt;SpaceX (Fine, Elon Musk, &lt;a href=&quot;https://twitter.com/elonmusk/status/822926184719609856&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweeted&lt;/a&gt;) suggested after the Echostar 23 launch, which was expendable that this would be the last planned expendable launch. Everything else would either land back at base, or on a barge. If it is too heavy to land, then missions would be scheduled to use the more powerful Falcon Heavy booster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This launch is scheduled to land downrange on the ASDS &quot;Of Course I Still Love You&quot; (OCISLY).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another reference, quoting the tweet would be &lt;a href=&quot;http://www.spaceflightinsider.com/organizations/space-exploration-technologies/expendable-falcon-9-launches-indispensable-echostar-23/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight Insider&lt;/a&gt; that adds a twist that the Mod 5 Falcon 9 might have higher payload performance. The version that launched Echostar-23 is considered mod-3 and mod-5 has been talked about as the final build, iterated, to have the final changes needed for greater reuse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After landing many stages (7 as of this writing, more every couple of weeks I suppose) SpaceX has a growing sample of data of components that need to be updated or redesigned for better and simpler reuse. All those changes are being rolled into the design slowly, with a major milestone called Mod 5 being the stake in the ground of all the planned changes (as of this time, no doubt a Mod 6 will follow over time). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engines have followed such a plan with the Merlin 1C used on the Falcon 9 V1.0, then changing to the Merlin 1D for the Falcon 9 V1.1, then an upgraded Full Thrust version, still with Merlin 1D's but at higher performance ratings. The Mod 5 is suggested to have an even higher thrust/performance Merlin 1D for additional margin.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-03-29T16:23:00.737" LastActivityDate="2017-03-29T16:23:00.737" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20859" PostTypeId="1" CreationDate="2017-03-29T01:40:47.360" Score="2" ViewCount="716" Body="&lt;p&gt;Im wondering if it is possible to create an &lt;a href=&quot;https://en.wikipedia.org/wiki/Alcubierre_drive&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alcubierre Drive&lt;/a&gt; without any exotic matter. To clarify: What if you take normal flat space-time and you lower a section of it to far below the standard. Possibly by filling the area with a very heavy homogeneous gas. And to simulate the negative curvature, you take away the gas in those areas. In that perspective it is still all positive curvature but relative to objects in the lowered space-time, it contains negative curvature. I understand that constructing this would be impractical and difficult, but much more possible than negative matter right? Well would that work?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Extraneous Information That May Or May Not Be Grounded In Reality:&#xA;Could that gas, on small scale testing, be a heavy Bose-Einstein condensate? Or just a cloud of super-cold entangled particles?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g&quot; rel=&quot;nofollow noreferrer&quot;&gt;PBS Space Time&lt;/a&gt; informative video on the Alcubierre Drive by astrophysicist Matt O'Dowd:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/94ed4v_T6YM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="19098" LastEditorUserId="12102" LastEditDate="2017-11-06T02:47:50.293" LastActivityDate="2017-11-06T02:47:50.293" Title="Could the use of exotic matter be removed from Alcubierre Drives?" Tags="|alcubierre-drive|spacetime|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20861" PostTypeId="1" AcceptedAnswerId="20874" CreationDate="2017-03-29T02:21:57.870" Score="8" ViewCount="782" Body="&lt;p&gt;SES-10 will be launched in a few days on Space-X's first commercial launch using a recycled rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screenshots from the three links (Brazil&lt;a href=&quot;https://www.satbeams.com/!ajax/embed?beam=8320&quot; rel=&quot;noreferrer&quot;&gt;&lt;/a&gt;, &lt;a href=&quot;https://www.satbeams.com/!ajax/embed?beam=8318&quot; rel=&quot;noreferrer&quot;&gt;Central America&lt;/a&gt;, &lt;a href=&quot;https://www.satbeams.com/!ajax/embed?beam=8319&quot; rel=&quot;noreferrer&quot;&gt;South America&lt;/a&gt;) on &lt;a href=&quot;https://www.satbeams.com/footprints&quot; rel=&quot;noreferrer&quot;&gt;Satbeams&lt;/a&gt; page for &lt;a href=&quot;https://www.satbeams.com/satellites?id=2649&quot; rel=&quot;noreferrer&quot;&gt;SES-10&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;50 high-power Ku-Band transponders to provide excellent coverage of the Gulf of Mexico, serve the Spanish speaking South America in one single beam, and offer complete coverage of Brazil with the ability to support oil and gas exploration off the shores of Brazil. It will replace the capacity currently provided by SES’s AMC-3 and AMC-4 satellites at that location, as well as bring additional capacity to Mexico, Central America, South America and the Caribbean. The high-powered, tailored and flexible beams will provide direct-to-home broadcasting, enterprise and mobility services.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Footprints are also shown on &lt;a href=&quot;https://www.ses.com/network/satellites/363&quot; rel=&quot;noreferrer&quot;&gt;SES's page for SES-10&lt;/a&gt; which seems to indicate there are 55 Ku-band transponders used to form these beams. Compare to &lt;a href=&quot;https://www.ses.com/our-network/satellites&quot; rel=&quot;noreferrer&quot;&gt;other SES footprints&lt;/a&gt;. I believe &quot;HTS&quot; stands for &quot;High Throughput Satellite&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've tried to find images of what SES-10 would actually look like unfolded and deployed in orbit, but it is difficult. I'm wondering, is a flat, phased array used for beam forming, or are there carefully shaped groups of transponders at the focal plane of each of the dishes, and it's the shape of he transponder cluster for each dish that produces the footprint shape? While certain details might be proprietary, shaped footprints for commercial satellites are so well established now that there must be something that describes in at least some technical detail how this is done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below x2:&lt;/strong&gt; screen shots from the three linked pages from &lt;a href=&quot;https://www.satbeams.com/satellites?id=2649&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Z3hdX.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z3hdX.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uD5Pl.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uD5Pl.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From SES-10's &lt;a href=&quot;https://www.satbeams.com/satellites?id=2649&quot; rel=&quot;noreferrer&quot;&gt;image gallery&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RPh2R.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RPh2R.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;a href=&quot;https://spaceflightnow.com/2017/03/18/ses-10-telecom-satellite-fueled-and-readied-for-launch-on-reused-rocket/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DYQT2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DYQT2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2021-04-15T12:13:06.773" Title="How do commercial broadcast satellites in GEO produce such carefully shaped signal footprints?" Tags="|communication-satellite|antenna|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20862" PostTypeId="1" AcceptedAnswerId="43735" CreationDate="2017-03-29T10:55:57.180" Score="10" ViewCount="1085" Body="&lt;p&gt;Usually, when performing trajectory optimizations for chemical rockets, the limiting factor is the $\Delta v$ budget. That means velocity change is the optimization factor, with things like transfer time being a secondary consideration. Some 'perfect' transfers, like an infinite apoapsis bi-elliptic transfer even have infinite transfer times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By contrast, for example when discussing more advanced forms for propulsion, the $\Delta v$ budget is no longer setting a limit and instead minimizing the transfer time is the most important thing to consider. Typically, the spacecraft in question have a high continuous acceleration, and the (subjectively) more intuitive approach is to thrust towards the target and start to slow down half way. This is a brachistochrone transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is what I am thinking: A spacecraft using an ion engine is not usually limited by $\Delta v$ either, and it also uses continuous thrust. However, it has a very &lt;strong&gt;low&lt;/strong&gt; thrust, meaning that the trajectory looks nothing like the simple high-thrust transfers. Here is for instance the trajectory of the spacecraft Dawn:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0bCd6m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0bCd6m.jpg&quot; alt=&quot;The trajectory of dawn.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;&lt;sup&gt;&lt;em&gt;image: JPL&lt;/em&gt;&lt;/sup&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, given the similarities&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Continuous thrust&lt;/li&gt;&#xA;&lt;li&gt;Not limited by $\Delta v$&lt;/li&gt;&#xA;&lt;li&gt;Optimization is for transfer time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Are ion engine trajectories also classified as brachistochrones?&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2021-10-25T14:01:08.040" Title="Are ion thruster trajectories classified as brachistochrones?" Tags="|orbital-mechanics|propulsion|trajectory|terminology|delta-v|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20863" PostTypeId="1" CreationDate="2017-03-29T11:11:22.887" Score="7" ViewCount="1315" Body="&lt;p&gt;We are probably going to create colonies on Mars in few decades or centuries, so which steps have been taken by NASA or other space agencies to find out if child birth is possible in a lower gravity than Earth's?&lt;/p&gt;&#xA;" OwnerUserId="19104" LastEditorUserId="8693" LastEditDate="2017-03-29T11:16:15.727" LastActivityDate="2017-04-06T20:27:09.243" Title="Is child birth possible in lower gravity?" Tags="|mars|colonization|microgravity|life|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20864" PostTypeId="2" ParentId="20841" CreationDate="2017-03-29T11:55:32.840" Score="1" Body="&lt;p&gt;Firstly, this depends on the angle between the orbit and the reference plane (horizon). For instance, a perpendicular error in an orbit perpendicular to the horizon will result in a almost purely azimuthal error, unless close to zenith. In contrast, an orbit with a low maximal elevation will have almost only elevation errors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, it is more convenient to use an angle instead of a distance to represent the error as that makes the calculation independent of altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking  at the problem now, we can see that using the initial orbit as the reference plane, the error is just adding elevation. The problem is now reduced to a simple transform between two spherical coordinate systems. To keep this transform as simple as possible you should use the ascending node of the orbital plane as your reference direction for azimuth in both systems. Getting a final azimuth value can then be done as a final rotational transfer along the polar axis, meaning you are back to your initial system by subtracting the longitude of the ascending node.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-03-29T11:55:32.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20865" PostTypeId="2" ParentId="20849" CreationDate="2017-03-29T12:27:50.053" Score="4" Body="&lt;p&gt;You should be able to.  Considering that I can see every launch from Palm Beach Cty. clearly, you slould be able to.  You said you will be Miami so I reccomend viewing from a beach or where you have an unobstructed view north.  I also reccomend a good pair of binoculars.&lt;/p&gt;&#xA;" OwnerUserId="19105" LastActivityDate="2017-03-29T12:27:50.053" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20866" PostTypeId="1" AcceptedAnswerId="20872" CreationDate="2017-03-29T13:24:24.203" Score="1" ViewCount="785" Body="&lt;p&gt;The Forbes article &lt;a href=&quot;https://www.forbes.com/sites/bridaineparnell/2017/03/29/skyscraper_suspended_on_asteroid/#79b0c56e5194&quot; rel=&quot;nofollow noreferrer&quot;&gt;New York Architects Plan Enormous Skyscraper Hanging From An Asteroid In Space&lt;/a&gt; describes a sky scraper suspended from an asteroid in a geosynchronous (but not geostationary) orbit that does not quite touch the Earth. The orbit is inclined with respect to the Earth's equator, and slightly elliptical. This is the recipe for a figure-eight ground track, and several answers here describe them in more detail (I'll try to find links to a few, if you know a good one, please add it.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The architecture firm announcing the concept is &lt;a href=&quot;http://www.cloudsao.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Clouds AO&lt;/a&gt; and the building is currently referred to as &lt;a href=&quot;http://www.cloudsao.com/ANALEMMA-TOWER&quot; rel=&quot;nofollow noreferrer&quot;&gt;Analemma Tower&lt;/a&gt;, and both of those links are worth exploring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Because the orbit is elliptical, will it only approach the Earth's surface for &quot;boarding&quot; at periapsis in the &lt;em&gt;southern hemisphere&lt;/em&gt;, or &lt;em&gt;could you enter this building when it was over New York City&lt;/em&gt; as well?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note 1:&lt;/strong&gt; As &lt;a href=&quot;https://space.stackexchange.com/a/20872/12102&quot;&gt;pointed out nicely by @sweber&lt;/a&gt;, the path shown can not be the ground track of a compact object in orbit. It goes up to about 41 North in latitude but only down to about 18 South. But the building is something more like a a 2D pendulum with Coriolis(?), so need to look closer at what the plot represents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note 2:&lt;/strong&gt; Remember that the earth is oblate by &lt;em&gt;many kilometers&lt;/em&gt;!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; All images are from the Clouds AO website for the &lt;a href=&quot;http://www.cloudsao.com/ANALEMMA-TOWER&quot; rel=&quot;nofollow noreferrer&quot;&gt;Analemma Tower&lt;/a&gt; concept, first one is cropped from the second one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PusQS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PusQS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/46lrT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/46lrT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iyPZM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iyPZM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RLapY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RLapY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; from Google Street View - anyone who doesn't believe that things from space touch down in NYC hasn't been there, or at least hasn't seen &lt;a href=&quot;https://en.wikipedia.org/wiki/Men_in_black&quot; rel=&quot;nofollow noreferrer&quot;&gt;MIB&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HlDcq.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HlDcq.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-03-30T07:26:50.870" Title="Could you enter this building when it was over New York City?" Tags="|geosynchronous|tourism|space-art|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20867" PostTypeId="2" ParentId="20863" CreationDate="2017-03-29T13:28:48.523" Score="8" Body="&lt;p&gt;Mammal experiments have shown that zero gravity does not affect fetuses in the late stage of gestation. &lt;a href=&quot;http://curious.astro.cornell.edu/physics/153-people-in-astronomy/space-exploration-and-astronauts/the-future-of-human-spaceflight/960-can-a-human-give-birth-in-space-intermediate&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://curious.astro.cornell.edu/physics/153-people-in-astronomy/space-exploration-and-astronauts/the-future-of-human-spaceflight/960-can-a-human-give-birth-in-space-intermediate&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity-friendly birthing postures (e.g. kneeling or squatting) are common in many countries, but most deliveries in the USA have the mother flat on her back with legs up. Gravity is at best neutral to the process, if not an impediment. Babies continue to be born nonetheless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main health concerns for space travel, regardless of age, are atrophy from long term low gravity and cellular damage from increased radiation exposure.&lt;/p&gt;&#xA;" OwnerUserId="3466" LastEditorUserId="58" LastEditDate="2017-04-06T20:27:09.243" LastActivityDate="2017-04-06T20:27:09.243" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20868" PostTypeId="1" CreationDate="2017-03-29T14:11:53.260" Score="14" ViewCount="1282" Body="&lt;p&gt;In the excitement about SpaceX's Falcon recovery and relaunch development program, it's often forgotten in &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/spacex/2017/03/28/ses-confident-spacex-rocket-can-deliver-second-time/99699862/&quot; rel=&quot;noreferrer&quot;&gt;articles like this one&lt;/a&gt; that reusable spacecraft were in regular use far back in the 20th century:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SES this week hopes to see its SES-10 communications satellite become the first payload delivered to orbit by a rocket booster that has already completed an orbital launch. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If we specify that &quot;rocket booster&quot; means, precisely, &lt;em&gt;all the parts of the rocket which are involved in providing propellant and thrust at liftoff&lt;/em&gt;, then this would seem to be correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we relax our definition a bit to mean something like &lt;em&gt;any airframe with engines which is providing thrust at liftoff&lt;/em&gt;, then &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-2&quot; rel=&quot;noreferrer&quot;&gt;STS-2&lt;/a&gt; would be the historic flight; &lt;em&gt;Columbia&lt;/em&gt; carried a number of scientific experiments on its second orbital mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If delivering payload to orbit and &lt;em&gt;leaving it there&lt;/em&gt; is a requirement, then some other shuttle flight would qualify.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the Shuttle external tanks were not reused, one could argue that none of the shuttle flights qualify, though I consider it disingenuous to consider the ET a dealbreaker while ignoring the Falcon 9 second stage entirely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves the shuttle SRBs, which could reasonably be argued to be part of &quot;the rocket booster&quot; in a way that the ET is not. The SRBs, however, were also recovered and rebuilt after launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, my question is: &lt;strong&gt;what was the first shuttle flight in which both the orbiter and the SRB casings were re-used hardware?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For purposes of the question, the SRB wouldn't have to be 100% reused (if, say, nose cones were never reflown) as long as a large majority, say 75% of it was.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="12102" LastEditDate="2021-09-13T00:06:22.680" LastActivityDate="2021-09-13T00:06:22.680" Title="What was the first time a &quot;re-used booster&quot; was used to put a payload into orbit?" Tags="|falcon-9|space-shuttle|reuse|spaceflight-firsts|srb|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20870" PostTypeId="2" ParentId="19839" CreationDate="2017-03-29T15:18:10.523" Score="12" Body="&lt;p&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(This is adapted from my question/answer at &lt;a href=&quot;https://hsm.stackexchange.com/questions/5616/day-to-day-tasks-of-human-computers-ala-hidden-figures-movie/5779#5779&quot;&gt;Day-to-day tasks of human computers, ala Hidden Figures movie - History of Science and Mathematics Stack Exchange&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was also fascinated by the film &lt;a href=&quot;https://en.wikipedia.org/wiki/Hidden_Figures&quot; rel=&quot;noreferrer&quot;&gt;Hidden Figures&lt;/a&gt;, and a related article from New Scientist magazine &quot;&lt;a href=&quot;https://www.newscientist.com/article/mg23331090-400-gifted-and-black-the-incredible-woman-who-got-the-us-into-space/?utm_source=NSNS&quot; rel=&quot;noreferrer&quot;&gt;Gifted and black: The brilliant woman who got the US into space&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Compressible Flow&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the article I found an example of the sort of calculations that these amazing human computers carried out. It is from &lt;a href=&quot;https://crgis.ndc.nasa.gov/crgis/images/b/b7/CompressibleFlow.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://crgis.ndc.nasa.gov/crgis/images/b/b7/CompressibleFlow.pdf&lt;/a&gt;, which is signed at the bottom &quot;nmg - 2-3-53&quot;. &lt;em&gt;Hmmm - I wonder who that was, and how this fit in to the work at the time&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, I put together a modern re-working of it, with a &lt;a href=&quot;http://jupyter.org/&quot; rel=&quot;noreferrer&quot;&gt;Jupyter&lt;/a&gt; notebook and &lt;a href=&quot;https://www.python.org/&quot; rel=&quot;noreferrer&quot;&gt;Python&lt;/a&gt; code. A bit of it is below, and you can see the whole thing, and the IPython notebook behind it, at my gist:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://gist.github.com/nealmcb/a48efd1c4bc1276865e39d22ea69be0b&quot; rel=&quot;noreferrer&quot;&gt;Hidden Figures Example: Compressible Flow Equations and Table in Jupyter, Python&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A bit of googling turned up some relevant background material on the math itself, which includes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/questions/304440/flow-through-a-nozzle&quot;&gt;Flow through a nozzle - Physics Stack Exchange&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19930091974&quot; rel=&quot;noreferrer&quot;&gt;Equations for the Design of Two-Dimensional Supersonic Nozzles - Report No. 907&lt;/a&gt; from 1948&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;$M = { V \over a} $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;${H - p \over q}&#xA; = {F_c}&#xA; = {1 + \eta} &#xA; = { {2 \over {\gamma M^2}} { \left[{( {1 + {{\gamma - 1} \over 2}} M^2)}^{\gamma \over {\gamma - 1} } - 1\right]} } &#xA; = {1.42857 \over M^2} \left[(1 + 0.2 M^2) ^ {3.5} - 1\right] $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;&#xA;M       H−p/q   T/T0    A1/A    rho/rho0 Psonic     p/H     q/H &#xA;0.001   1.0000  1.0000  0.0017  1.0000  -673870.092 1.0000  0.00000&#xA;0.002   1.0000  1.0000  0.0035  1.0000  -168467.127 1.0000  0.00000&#xA;0.003   1.0000  1.0000  0.0052  1.0000  -74873.985  1.0000  0.00001&#xA;...&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Orbits&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first memo, on orbit determination, that &lt;a href=&quot;https://en.wikipedia.org/wiki/Katherine_Johnson&quot; rel=&quot;noreferrer&quot;&gt;Katherine Johnson&lt;/a&gt; did, with a co-worker, is &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980227091.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Determination of Azimuth Angle at Burnout for Placing a Satellite over a Selected Earth Position 1960. T.H. Skopinski, Katherine G. Johnson, NASA TN D-233&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This example from the (most excellent) &lt;a href=&quot;https://en.wikipedia.org/wiki/Hidden_Figures_(book)&quot; rel=&quot;noreferrer&quot;&gt;Hidden Figures book&lt;/a&gt; provides some details of the actual computations that she did, checking the &lt;a href=&quot;https://en.wikipedia.org/wiki/IBM_7090&quot; rel=&quot;noreferrer&quot;&gt;IBM 7090&lt;/a&gt; trajectory calculations for the first US orbital mission. E.g. she was working to 8 significant digits:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[Katherine Johnson] worked thru every minute of what was programmed to be [John Glenn's] three-orbit mission, coming up with numbers for eleven different output variables, each computed to eight significant digits.  It took a day and a half.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Update: See more on Glenn's orbit here:&lt;/em&gt; &lt;a href=&quot;https://space.stackexchange.com/q/19402/5775&quot;&gt;Where to look for historical or reconstructed orbit data for early NASA missions - Mercury-Atlas 6 for example&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Equipment&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The book also notes that the human computers used mechanical calculators. Some great detail on their working environment comes from a 1942 memo reproduced in &lt;a href=&quot;https://airandspace.si.edu/stories/editorial/hidden-figures-and-human-computers&quot; rel=&quot;noreferrer&quot;&gt;Hidden Figures and Human Computers | National Air and Space Museum&lt;/a&gt;, including these brief excerpts: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;... the work of a computer required skill and judgment. Computers gathered data by reading pressure values from manometers placed in the wind tunnel. Depending on the application, the data were smoothed, plotted, and interpolated. Data reduction and analysis were carried out with the help of &lt;a href=&quot;https://en.wikipedia.org/wiki/Mechanical_calculator#1900s_to_1970s&quot; rel=&quot;noreferrer&quot;&gt;calculators&lt;/a&gt;, slide rules, &lt;a href=&quot;https://en.wikipedia.org/wiki/Planimeter&quot; rel=&quot;noreferrer&quot;&gt;planimeters&lt;/a&gt;, drafting tools, and other instruments. The women in these roles knew how to organize computational work and how to do so quickly without making mistakes. This knowledge was unique to them....&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The automatic computing machines and &lt;a href=&quot;https://en.wikipedia.org/wiki/Comptometer&quot; rel=&quot;noreferrer&quot;&gt;Comptometers&lt;/a&gt; cost over $500.00 each....&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The automatic calculator is usually the Friden or Marchant.... The computers were also furnished with 20 inch (log-log duplex) slide rules....&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;... for the type of computer who has some mathematical background the type of work required of her could probably be better done on the calculating machine....&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Each computing section has one light-table for tracing purposes.... A set of five-place logarithmic tables and trigonometric tables is also provided, together with a few scale, triangle and French curves....&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wish the film had had just a bit more on those mechanical calculators. But I found a video of one of the models they used in action, a &lt;a href=&quot;https://www.youtube.com/watch?v=qKm9eM2BuM0&quot; rel=&quot;noreferrer&quot;&gt;1956 Friden STW 10 - Youtube&lt;/a&gt; taking a square root.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Type of Work&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One more excerpt:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is a large amount of simple calculation required in the work here at NACA. Most [wind] tunnels have means for taking photographs of banks of nearly a hundred manometers at a time. The computers read off the liquid levels and complete the analysis. On the other hand, some of the calculations are sent to the computers in the form of complicated formulas which necessitate a knowledge of trigonometry and sometimes of mathematics involving the calculus. In general, however, the group head would reduce this more complicated work down to tabular form requiring rather routine operations before it would  be given to the machine operator. Most of the work coming from the engineers is accompanied by a memo of calculating instructions or word-of-mouth explanations. The computers in any one section soon learn what the usual type of calculation required of them would be.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Many of the calculations which involved ordinary arithmetic could have been handled on the mechanical calculators. But I'm guessing that when it involved trigonometry or logarithms, they would have been using slide rules or looking results up in tables.  Tables which had, most likely, been computed years earlier by other human computers as part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mathematical_Tables_Project&quot; rel=&quot;noreferrer&quot;&gt;Mathematical Tables Project&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, as in the example I started with, they were creating their own tables for things like compressible flow, that others could in turn use in the future.&lt;/p&gt;&#xA;" OwnerUserId="5775" LastEditorUserId="5775" LastEditDate="2017-04-15T18:00:31.357" LastActivityDate="2017-04-15T18:00:31.357" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20871" PostTypeId="1" AcceptedAnswerId="20875" CreationDate="2017-03-29T18:27:19.903" Score="16" ViewCount="627" Body="&lt;p&gt;I have seen it said that because high-energy GCR particles cause a spray of secondary particles when they pass through material, there is more radiation behind even quite thick shielding that there is if there is no shielding at all. &lt;a href=&quot;http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?2000ESASP.462..289A&amp;amp;defaultprint=YES&amp;amp;filetype=.pdf&quot; rel=&quot;noreferrer&quot;&gt;A paper by Vanessa Aulessa&lt;/a&gt; was used to produce the graph below in a &lt;a href=&quot;https://www.youtube.com/watch?v=DI41N-eC7VA&amp;amp;list=PLsPXv7sL0venm-e0XzDde7rOiyWnVjtBD&amp;amp;index=5&quot; rel=&quot;noreferrer&quot;&gt;lecture by Rob Mueller&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/G5NHP.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/G5NHP.jpg&quot; alt=&quot;gcr dose starts at 0.015 Sv, peaks at 0.13 Sv at 180 g/cm2, tails off to 0.02 Sv at 500 g/cm2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I have also been shown graphs that show cosmic radiation starting to drop as soon as it hits shielding. The graph below from &lt;a href=&quot;http://www.marsjournal.org/contents/2006/0004/files/rapp_mars_2006_0004.pdf&quot; rel=&quot;noreferrer&quot;&gt;a paper by Donald Rapp&lt;/a&gt; (7th page) shows that version:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AgeEz.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AgeEz.jpg&quot; alt=&quot;gcr starts at 60 cSv, drops to an average of 40 cSv after 10 g/cm2, tails off to less than 30 cSv after 50 g/cm2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://sites.nationalacademies.org/cs/groups/depssite/documents/webpage/deps_063597.pdf&quot; rel=&quot;noreferrer&quot;&gt;slide presentation by Martha Clowdsley&lt;/a&gt; comes to the same conclusion, and lists the transport models used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have seen both models referred to elsewhere. Why do they apparently contradict each other?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2017-03-30T03:05:39.053" LastActivityDate="2017-03-30T03:05:39.053" Title="Is radiation dose from cosmic rays higher behind 50 cm of shielding, or lower?" Tags="|radiation|cosmic-radiation|3d-modeling|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="20872" PostTypeId="2" ParentId="20866" CreationDate="2017-03-29T18:55:35.630" Score="4" Body="&lt;p&gt;Due to the nature of elliptical orbits, and given that perigee is in the south, the building will fly fast and low in the south and slow and high in the north.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's calculate it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In an elliptical orbit, the earth sits in one of the focal points and the  eccentricity $e$ is the distance between earth and the geometrical center of the ellipse. The eccentricity and the semi-major axis $a$ give the closest distance between earth and satellite as $(a-e)$ and the farthest as $(a+e)$. Interestingly, $a$ is the same for all satellites with same period, so we can just take $a=42\,000km$ from the geostationary ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7PP9Q.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7PP9Q.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The height of the upper &quot;half&quot; of the eight-shaped track is $(a+e)\sin(inclination)$ and that of the lower &quot;half&quot; $(a-e)\sin(inclination)$. From your small drawing with the label &quot;ground trace&quot;, the upper &quot;half&quot; is about 3 times larger than the lower half. So:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{a+e}{a-e}=3$$&#xA;$$a+e=3a-3e$$&#xA;$$e=a/2=21\,000km$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perigee is $a-e=21\,000km$, apogee is $a+e=62\,000km$. This means the satellite / building will be 42000km farther away when over New York than when somewhere over south America. Even if the upper &quot;half&quot; would only be 10% higher than the lower, the altitude of the building would change by 2000km. For comparison: The ISS flies at 400km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another comparison:&#xA;Here is a 2D-plot showing the size of the earth as well as the size and shape of several orbits:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/h59VF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h59VF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's something odd here. The orbit is in the same plane as the center of the earth. Due to this, the most southern latitude of the track equals the most northern one. If the ground track hits New York at 40°N, the most southern position will also be at 40°S, i.e. somewhere in the middle of chile, not be border to Peru at 20°S.&lt;br&gt;&#xA;So the tracks shown are NOT ground tracks.&lt;br&gt;&#xA;If one looks from far, far away, a satellite seems to fly much higher into the north due to its distance. But something geosynchronous would fly higher than the north pole... The shown track could be the orbit of the building, but even then, the shape of the curve doesn't fit. I'll think about this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ah, and if we neglect that we can't build something like this... The building and the asteroid form a nice pendulum, and it definitely will swing heavily. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in general, I'd say this is one of the strange news which pop up everywhere at the moment due to the special date these days. Nevertheless, one could try the math.&lt;/p&gt;&#xA;" OwnerUserId="11749" LastEditorUserId="11749" LastEditDate="2017-03-30T05:38:44.577" LastActivityDate="2017-03-30T05:38:44.577" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20873" PostTypeId="2" ParentId="20820" CreationDate="2017-03-29T19:28:27.417" Score="4" Body="&lt;p&gt;For a first order approximation we can start with the time it will take for the asteroid to impact the Earth, say for example the ship lands on it 1 year before it reaches Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth has a radius of about 6000km, so let's say we need to deflect the course of the asteroid by 6000km to miss Earth. Using the simple distance = velocity / time formula the velocity we would need to impart on the asteroid is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;6000000m / (365 * 86400s) = &lt;strong&gt;0.2m/s&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next we approximate the asteroid's weight: An asteroid with a 5km radius would have a volume of 5.24×10^11, a commonly used density for asteroids is 2g/cm^3, which is somewhere between the density of ice and rock. So we'll take the mass to be 10x10^11. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The momentum we need to impart is 10x10^11 * 0.2m/s = &lt;strong&gt;2x10^11kg·m/s&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for the fuel required: because the ΔV is very small we can ignore the rocket equation and just treat the problem as flinging a large enough lump of mass away fast enough to impart that 0.2m/s. Lets bring some chemical propellant, a generous exhaust velocity would be 4000m/s and we can calculate how much mass we need to fling away at that velocity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2x10^11kg·m/s / 4000m/s = &lt;strong&gt;50,000,000kg&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is equivalent to about 17 fully-fueled Saturn V rockets - &lt;em&gt;parked on the asteroid&lt;/em&gt;. Getting those 17 Saturn V rockets up there would require thousands of Saturn V rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a number of ways that is a worst-case scenario, for example if the asteroid is going to barely graze the Earth it would only need to be deflected by a few hundred km. If there is more time, like a decade, the required deflection is proportionately less. But even landing 5000t on an asteroid would be of formidable difficulty. In reality our only hope would be using the asteroid's own mass as propellant, by vaporizing parts of it with nuclear bombs or giant lasers, or using a mass driver to fling away hundreds of thousands of tons of rock.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2017-03-29T19:28:27.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20874" PostTypeId="2" ParentId="20861" CreationDate="2017-03-29T21:02:21.743" Score="7" Body="&lt;p&gt;I can't speak specifically for the tech on SES-10 since I didn't work with that (yet), but on the previous GEO spacecraft I've worked on, the idea is to alter the radiation pattern. Specifically, heritage antennas on GEO bird are not smooth like you would usually get on a home dish antenna. By adding &quot;bumps&quot; into the antenna, the signal can be directed one way or the other. That allows the signal to be amplified in some areas and to completely cancel out in other (by interfering with itself as it leaves the antenna).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I tried to find a good picture of the bumps on the antennas, but the best public image I've found so far is below. Note how the light of the room does not reflect uniformly on the antenna (this is especially visible on the top part of the antenna).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Vxepv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vxepv.jpg&quot; alt=&quot;space antenna&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-03-29T21:02:21.743" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20875" PostTypeId="2" ParentId="20871" CreationDate="2017-03-29T21:49:26.320" Score="9" Body="&lt;p&gt;Those two graphs are not showing the same thing. The lecture slide is misleadingly labeled, but the original paper makes clear that the table is only showing neutrons, mostly produced by shield spallation. That does go up with added shielding (no shielding means no spallation) but the overall dose including other sources goes down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See figure 7 in the second paper for a graph showing the various components of the radiation dose as shield thickness is increased.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pujky.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pujky.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18207" LastEditorUserId="18207" LastEditDate="2017-03-29T22:38:41.923" LastActivityDate="2017-03-29T22:38:41.923" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20876" PostTypeId="1" AcceptedAnswerId="20877" CreationDate="2017-03-30T01:46:18.780" Score="11" ViewCount="1709" Body="&lt;p&gt;In &lt;a href=&quot;https://www.nasa.gov/images/content/165304main_image_feature_719_ys_full.jpg&quot; rel=&quot;noreferrer&quot;&gt;this image&lt;/a&gt; (NASA &lt;a href=&quot;https://spaceflight.nasa.gov/gallery/images/shuttle/sts-116/html/s116e05983.html&quot; rel=&quot;noreferrer&quot;&gt;original source&lt;/a&gt;) from the International Space Station. what is the &quot;lightning rod&quot;-like instrument at the end of the mast in the upper right of the photo?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iUfzW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iUfzW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U2rM5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U2rM5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19112" LastEditorUserId="6944" LastEditDate="2018-04-09T00:47:13.823" LastActivityDate="2022-08-10T19:02:53.427" Title="What is this instrument on the ISS?" Tags="|iss|instrument|identify-this-object|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20877" PostTypeId="2" ParentId="20876" CreationDate="2017-03-30T02:49:06.543" Score="18" Body="&lt;p&gt;This is the &lt;a href=&quot;http://www.sdl.usu.edu/programs/fpmu&quot; rel=&quot;nofollow noreferrer&quot;&gt;Floating Potential Measurement Unit&lt;/a&gt; (FPMU).&lt;/p&gt;&#xA;&lt;p&gt;Specifically, the ball is the Wide Langmuir Probe.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sHz2K.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sHz2K.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;picture from &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwiSpK3K2I_SAhVhxlQKHcnLCrcQFggcMAA&amp;amp;url=https%3A%2F%2Fntrs.nasa.gov%2Farchive%2Fnasa%2Fcasi.ntrs.nasa.gov%2F20090025959.pdf&amp;amp;usg=AFQjCNETGBTujNh4qO70r35ZANvHfqyzqw&amp;amp;sig2=D6QyVN0qVYB4bpw1yI4R-w&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The FPMU was installed by ISS crewmembers, during an Extra Vehicular&#xA;Activity (EVA), on the starboard (Sl) truss of the ISS in early August&#xA;2006. The system is based on past USU/SDL space instruments and incorporates a Floating Potential Probe, multiple Langmuir Probes, and&#xA;a Plasma Frequency Probe into one sensor suite.&lt;/p&gt;&#xA;&lt;p&gt;The FPMU is used to correct the charging models of the station each&#xA;time a new section is added to the structure. By providing feedback on&#xA;the effectiveness of steps taken to control charging, the FPMU helps&#xA;ensure a safe working environment for the astronauts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;USU = Utah State University&lt;/p&gt;&#xA;&lt;p&gt;SDL = Space Dynamics Laboratory&lt;/p&gt;&#xA;&lt;p&gt;For more info about &lt;em&gt;how it works, not what it is&lt;/em&gt;, see my answer to this question: &lt;a href=&quot;https://space.stackexchange.com/questions/20160/how-do-spacecraft-measure-their-own-charge/20163#20163&quot;&gt;How do spacecraft measure their own charge?&lt;/a&gt; .&lt;/p&gt;&#xA;&lt;p&gt;At some point since the FPMU was originally installed, it has been moved to Camera Port 6 on the P1 truss, as pointed out in this question: &lt;a href=&quot;https://space.stackexchange.com/questions/60019/identifying-antenna-like-object-with-gold-orb-on-iss&quot;&gt;Identifying Antenna-Like Object with Gold Orb on ISS&lt;/a&gt; and as confirmed in this &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/6/61/View_of_the_ISS_taken_during_Crew-2_flyaround_%28ISS066-E-080762%29.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;image&lt;/a&gt; linked from there and annotated by me:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iq9ce.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iq9ce.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2022-08-10T19:02:53.427" LastActivityDate="2022-08-10T19:02:53.427" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="20879" PostTypeId="2" ParentId="20849" CreationDate="2017-03-30T05:23:18.167" Score="6" Body="&lt;p&gt;I can see them from the opposite side of Florida with the naked eye given correct atmospheric conditions. So it is possible to have a LoS of about the entire state of Florida, probably further if you are in an open area.&lt;/p&gt;&#xA;" OwnerUserId="19048" LastActivityDate="2017-03-30T05:23:18.167" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20881" PostTypeId="2" ParentId="14352" CreationDate="2017-03-30T06:26:15.750" Score="5" Body="&lt;p&gt;The very idea was proposed earlier this month at the Planetary Science Vision 2050 Workshop. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The talk was titled: &quot;A Future Mars Environment for Science and Exploration&quot;, J. Green, R. Bamford, et. al.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They combined heliophysics simulation tools with a Mars global climate model to do the simulation. Their abstract suggests placing a 1-2 Tesla dipole at L1 is indeed sufficient to protect and rebuild a Martian atmosphere. However, the scale is not indicated. The full talk specifies 50,000nT measured at one Earth-radius--which is comparable to regenerating Earth's magnetic field and clearly a challenge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Few details were given on how to generate the artificial magnetosphere field, but in was noted that the field lines must remain parallel with the interplanetary field to function well as a shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XyJhb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XyJhb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the full talk &lt;a href=&quot;https://livestream.com/viewnow/vision2050/videos/150701155&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (starts at 1:36:10).&lt;/p&gt;&#xA;" OwnerUserId="19114" LastEditorUserId="19114" LastEditDate="2017-04-01T06:56:21.027" LastActivityDate="2017-04-01T06:56:21.027" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20882" PostTypeId="1" AcceptedAnswerId="20883" CreationDate="2017-03-30T06:41:02.560" Score="8" ViewCount="593" Body="&lt;p&gt;Although I know programs and visions of many space companies, I'd like to ask, does anybody knows if there is any competitor to SpaceX that wants to create a reusable rocket in the future? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's face it - in case SpaceX is successful in this area, the price of launch would decrease and this would lead to a gigantic advantage for SpaceX. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that there are many new companies, such as Blue Origin and Masten Space Systems, but I am concerned about traditional companies (ULA, Arianespace, ..). Or is what we are seeing here a disruption of the spaceflight sector, where old companies would disappear and new companies take over?&lt;/p&gt;&#xA;" OwnerUserId="16590" LastEditorUserId="238" LastEditDate="2017-03-30T07:15:41.210" LastActivityDate="2021-03-15T06:44:44.727" Title="Are SpaceX competitors planning to create reusable vehicles" Tags="|spacex|reuse|uncrewed-spaceflight|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20883" PostTypeId="2" ParentId="20882" CreationDate="2017-03-30T10:34:03.580" Score="11" Body="&lt;p&gt;Jeff Bezos's Blue Origin has flown their New Sheppard vehicle, and reused the boost stage several times now. They intend to build a New Glenn vehicle that would be competitive with the Falcon 9 class payloads; like Falcon 9, it would land the first stage vertically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ULA has discussed the Vulcan launcher where the engine pod would sever itself from the rest of the stage and re-enter under a ballute/parachute style model. They plan on initially replacing Atlas V and Delta 4 with Vulcan and then later enabling engine recovery. The engines would be picked out the air by a helicopter with a hook (Much as spy satellite film canisters were collected regularly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ariane has discussed ABILENE which is similar to Vulcan in that only the engine pod is recovered, but it would fly itself to a landing. As a commenter notes Arianespace is unclear if this is even worth it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But Vulcan and ADELINE seem very pie in the sky compared to New Glenn and an actual flying reusable booster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Russians propose flyback boosters every so often that go nowhere. I could go flip through Anatoly Zak's book or website and list off the various paper only Russian reusable boosters (MAKS, Zenit even, Buran) but why bother they are all paper.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-03-30T13:53:37.373" LastActivityDate="2017-03-30T13:53:37.373" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20885" PostTypeId="1" CreationDate="2017-03-30T12:56:11.083" Score="1" ViewCount="411" Body="&lt;p&gt;I am reading a document on satellite attitude control and the Kalman Filter and it mentions that the satellite is tumbling through free space in a reference frame that has the Sun vector of constant orientation at $\left[\begin{array}{c}-1 \\0\\0\end{array}\right]$ and the Earth's magnetic field at $\left[\begin{array}{c}0\\0\\-1\end{array}\right]$.&lt;br&gt;&#xA;&lt;strong&gt;My question is, is this the same as the ECI (Earth Centred Interial) reference frame?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apologies if this is the wrong place to be asking.&lt;/p&gt;&#xA;" OwnerUserId="18649" LastActivityDate="2017-04-26T16:39:25.573" Title="Is this an ECI frame?" Tags="|orbit|attitude|frames|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20886" PostTypeId="1" AcceptedAnswerId="21548" CreationDate="2017-03-30T15:40:20.747" Score="16" ViewCount="1164" Body="&lt;p&gt;I was just watching a video of two astronauts doing an EVA on ISS to install some things. As I saw how they moved around the outside of the ISS using handrails and other things to grab, I was wondering whether people inside the ISS can actually hear their outside colleagues bumping against the metal or any other sounds that are caused by the astronauts on the outside?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I know, the inside of ISS can be quite noisy so I guess most sounds caused by the spacewalkers are probably drowned by the inside noise. But maybe sounds when using tools (e.g. to tighten or loosen screw s) may be heard inside?&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2017-05-17T14:37:31.050" Title="Can astronauts inside ISS hear their crewmates doing EVAs?" Tags="|iss|eva|sound|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20887" PostTypeId="1" CreationDate="2017-03-30T20:40:33.270" Score="3" ViewCount="775" Body="&lt;p&gt;I want to compare the position calculated with Pyephem and &lt;a href=&quot;https://celestrak.org/NORAD/elements/supplemental/&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE&lt;/a&gt; files, with &lt;a href=&quot;ftp://cddis.gsfc.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;RINEX&lt;/a&gt; files and &lt;a href=&quot;http://www.gpstk.org/bin/view/Documentation/WebHome&quot; rel=&quot;nofollow noreferrer&quot;&gt;GPSTK&lt;/a&gt; library in a specific date and time, but I have not the certainly if the name of the satellite &amp;quot;PRN ##&amp;quot; are the same in both files (TLE and RINEX)?&lt;/p&gt;&#xA;&lt;p&gt;e.g. in &lt;a href=&quot;https://celestrak.org/NORAD/elements/supplemental/gps.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt; (TLE file) for a GPS constellation, the ID or name of satellite is:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;GPS BIIF-1  (PRN 25)&lt;/strong&gt;&lt;br /&gt;&#xA;1 26690C 01004A   17096.69312500 -.00000000&#xA;00000-0  00000-0 0   966 2 26690  53.0252 160.8176 0181123 256.0966&#xA;20.3287  2.00554091    10&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;but in the RINEX files only have a letter joined to a number, like this:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;17 01 12 00 00  0.0000000  0 21G05G16G20G21&lt;strong&gt;G25&lt;/strong&gt;G26G27G29G31R01R02R09&#xA;R15R16R17R18R19E01E26S20S36&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;where the letter is the contellation &lt;strong&gt;G&lt;/strong&gt; for &lt;strong&gt;GPS&lt;/strong&gt; constellation, and &lt;strong&gt;R&lt;/strong&gt; for &lt;strong&gt;GLONASS&lt;/strong&gt; constallation, &lt;strong&gt;S&lt;/strong&gt; is for &lt;strong&gt;SBAS&lt;/strong&gt;, etc.&#xA;Next to the letter is a number, e.g. &lt;strong&gt;G25&lt;/strong&gt;, the number of that satellite is &lt;strong&gt;25&lt;/strong&gt; for the RINEX file.&lt;/p&gt;&#xA;&lt;p&gt;I'm worked with GPS constellation. I'm testing with some code in python, but i'm not sure if the PRN 25 is the same to G25?, because calculating the elevation of satellite with ID 25 in a hole day, is different in the time that RINEX file shows comparating with TLE files... or may be is something wrong in my code.&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;obsHeader, obsData = gpstk.readRinex3Obs(obsfile)&#xA;navHeader, navData = gpstk.readRinex3Nav(navfile)&#xA;# setup ephemeris store to look for satellite positions&#xA;bcestore = gpstk.GPSEphemerisStore()&#xA;for navDataObj in navData:&#xA;    ephem = navDataObj.toGPSEphemeris()&#xA;    bcestore.addEphemeris(ephem)&#xA;bcestore.SearchNear()&#xA;navData.close()&#xA;#look each satellite&#xA;for obsObject in obsData: #round the data&#xA;    for satID, datumList in obsObject.obs.iteritems():&#xA;        if satID.systemGPS == satID.system and satID.id == NumSatellite:&#xA;            time=obsObject.time #satellite time from the observation&#xA;            CivilTime = gpstk.CivilTime(obsObject.time)&#xA;            #break&#xA;            eph   = bcestore.findEphemeris(satID, obsObject.time)&#xA;            svXvt = eph.svXvt(obsObject.time)&#xA;            elev = obsHeader.antennaPosition.elvAngle(svXvt.getPos())&#xA;            azim    = obsHeader.antennaPosition.azAngle(svXvt.getPos())&#xA;            &#xA;            SecondsTime.append(obsObject.time.getSecondOfDay())&#xA;            TimesSatRin.append(CivilTime)&#xA;            AziSatRin.append(azim)&#xA;            ElevSatRin.append(elev)&#xA;#for each observation day.&#xA;DateTimeRinex = CivilTime2datetime(TimesSatRin)&#xA;for t in DateTimeRinex:&#xA;    #for each Time of the rinex (for the specific Satellite)&#xA;    print 'next time: ', t&#xA;&#xA;    names,elevs,azs,types = GetPositionELAZ(t,Longitude_obs,Latitude_obs,1) #just GPS with TLE files&#xA;    print names&#xA;    for i in range (0,len(names)):&#xA;        number = int(filter(str.isdigit, names[i]))&#xA;        #print number,' number sat of the TLE'&#xA;        if number == NumSatellite:&#xA;            print 'GOT',number,' number sat of the TLE'&#xA;            TimeTLE.append(t)&#xA;            ElevTLE.append(elevs)&#xA;            AzimTLE.append(azs)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;The position (lon,Lat) is the same of the ground station of the RINEX file repository.&#xA;Thanks.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE 1:&lt;/strong&gt;&lt;br /&gt;&#xA;I'm testing with my code, one vehicle by one, and I can see the comparison of elevations of each satellite, across the day. For RINEX files I try with 4 days, day of year 101, 100, 80 and 10. and for these day always the &lt;strong&gt;TLS PRN 10&lt;/strong&gt; it results are similar to &lt;strong&gt;RINEX PRN 12&lt;/strong&gt;; I said similar, because it's not the same results. I'm still checking.&lt;/p&gt;&#xA;&lt;p&gt;20th April&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/X59Gk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/X59Gk.png&quot; alt=&quot;20 april&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;19th April&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9so0w.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9so0w.png&quot; alt=&quot;19th April&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;21th March&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SyTZD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SyTZD.png&quot; alt=&quot;21th March&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;10th January&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IUPmO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IUPmO.png&quot; alt=&quot;10th January&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Apparently exists a PRN ID for RINEX by each PRN ID for TLE, in this case RINEX PRN 12 = TLE PRN 10. But this is not the precise answer, so I'll let open the question&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE 2:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The calculate of position with TLE it's very accurate. but I got an error, I got a position in Longitude/Latitude, but was a wrong position, for be sure, you must to take the position for compare, by the RINEX file (the observation file), in the Header; the file have the position in X,Y,Z, so is necessary convert to position longitude-latitude.&lt;a href=&quot;https://i.stack.imgur.com/s6ter.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s6ter.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;for get the Longitude-Latitude:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;def Get_latitude(x, y, z):&#xA;    #radius earth&#xA;    r = 6371000.&#xA;    return np.arcsin(z/r)*(180/np.pi)&#xA;&#xA;def Get_longitude(x, y, z):&#xA;    #radius earth&#xA;    r = 6371000.&#xA;    if (x &amp;gt; 0):&#xA;        return np.arctan(y/x)*(180./np.pi)&#xA;    elif y &amp;gt; 0:&#xA;        return np.arctan(y/x)*(180./np.pi) + 180.&#xA;    else:&#xA;        return np.arctan(y/x)*(180./np.pi) - 180.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="18740" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2021-12-08T22:42:56.860" Title="PRN number in TLE files are the same in RINEX files? (in GPS constellation)" Tags="|gps|ephemeris|two-line-elements|rinex|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20889" PostTypeId="2" ParentId="17046" CreationDate="2017-03-30T22:51:05.667" Score="9" Body="&lt;p&gt;I have been looking into this as well. As I have been told by one NASA employee, there is no &lt;em&gt;easy&lt;/em&gt; way to get old data, but there is a way if you're willing to go through lots of data. NASA saves old &lt;code&gt;dsn.xml&lt;/code&gt; files in an address like so:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://eyes.nasa.gov/dsn/data/1365107113.xml&quot; rel=&quot;noreferrer&quot;&gt;http://eyes.nasa.gov/dsn/data/1365107113.xml&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That number is a unix timestamp, so that you can view the XML data at that time. For a unix to date convert, try e.g. &lt;a href=&quot;https://www.epochconverter.com&quot; rel=&quot;noreferrer&quot;&gt;https://www.epochconverter.com&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The data are stored only every five seconds, so if one unix timestamp gives you a &quot;NoSuchKey&quot; error, try to keep going down by one until you find a good one.&lt;/p&gt;&#xA;" OwnerUserId="19123" LastActivityDate="2017-03-30T22:51:05.667" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20891" PostTypeId="1" AcceptedAnswerId="20899" CreationDate="2017-03-30T23:33:26.410" Score="8" ViewCount="928" Body="&lt;p&gt;According to the table in the Wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Application_to_interplanetary_travel&quot; rel=&quot;noreferrer&quot;&gt;Hohmann transfer orbits&lt;/a&gt;, when traveling to Venus or Mars, the Δv to enter a Hohmann transfer orbit from Earth's orbit is less than the Δv from LEO:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Venus&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Δv to enter Hohmann orbit from Earth's orbit = &lt;strong&gt;2.5 km/s&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Δv from LEO = &lt;strong&gt;3.5 km/s&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Mars&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Δv to enter Hohmann orbit from Earth's orbit = &lt;strong&gt;2.9 km/s&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;li&gt;Δv from LEO = &lt;strong&gt;3.6 km/s&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Would launching a craft &quot;straight up&quot;--directly from the surface into Hohmann transfer (bypassing LEO)--be more efficient than first launching into LEO and then performing a transfer burn?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have any spacecraft launched to either planet in this way?  (If not, why not?)&lt;/p&gt;&#xA;" OwnerUserId="19124" LastActivityDate="2017-04-01T08:38:03.200" Title="Is bypassing LEO more efficient when launching to Venus or Mars?" Tags="|mars|low-earth-orbit|venus|delta-v|hohmann-transfer|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20892" PostTypeId="1" AcceptedAnswerId="20893" CreationDate="2017-03-31T00:05:57.793" Score="1" ViewCount="211" Body="&lt;p&gt;According to a book I am currently reading, space rockets and ballistic rockets fly along an elliptic path. However they differ in where the focus of the ellips describing their path lays. For space rockets on of the foci is placed at the center of the sun (for ballistic rockets it is the center of the earth). Could someone explain, why the center of the sun?&lt;/p&gt;&#xA;" OwnerUserId="13091" LastActivityDate="2017-03-31T02:29:41.233" Title="Why is one of the foci of the ellipse of the trajectory of a space rocket in the center of the sun?" Tags="|orbital-mechanics|rockets|trajectory|the-sun|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20893" PostTypeId="2" ParentId="20892" CreationDate="2017-03-31T02:29:41.233" Score="3" Body="&lt;p&gt;I think you have a misleading picture here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth orbit bounds rockets have the earth as the centre of their ellipse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However; for interplanetary travel, or anything that goes beyond the earth sphere of influence, your rocket will orbit the sun, hence an elliptic sun centred trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first part of the trajectory will however, always be earth centred (that is, until you escape the earth system).&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-03-31T02:29:41.233" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20896" PostTypeId="1" AcceptedAnswerId="20897" CreationDate="2017-03-31T05:51:24.240" Score="22" ViewCount="3593" Body="&lt;p&gt;After a successful reentry burn of the stage, around &lt;a href=&quot;https://youtu.be/xsZSXav4wI8?t=25m55s&quot; rel=&quot;noreferrer&quot;&gt;10 seconds after engine shutdown&lt;/a&gt;, one of the fins glows red, then begins to burn. My question is why would this burn after the stage already completed to reentry burn to slow it down. And if this grid fin began to burn, why didn't the others? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a picture of the fin just as it began to start to burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GLIaN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GLIaN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know there was already a question about whether the grid fins are flammable but I'm just wondering why they burned when they did and what stopped the others from doing so. I read that SpaceX uses ablative paint on their gridfins which would explain why it is burning in the first place, but why would it do so after slowing down for the reentry burn?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reentry burn typically happens at around 70-40 km in altitude. I was unable to find data on the rate of descent after the burn.&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="10894" LastEditDate="2017-03-31T17:50:49.023" LastActivityDate="2019-07-30T02:18:36.480" Title="Why did the grid fin of the CRS-8/SES 10 booster burn?" Tags="|spacex|reentry|reuse|grid-fins|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20897" PostTypeId="2" ParentId="20896" CreationDate="2017-03-31T07:11:39.723" Score="18" Body="&lt;p&gt;According to Elon Musk (during the &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=42544.340&quot; rel=&quot;noreferrer&quot;&gt;post-flight press conference)&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;New design coming for Grid Fin.  Will be largest titanium forging in the world.  Current Grid Fin is aluminum and gets so hot it lights on fire... which isn't good for reuse.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;After the reentry burn, the stage still flies at high speed (I don't have an exact figure, but it's at least supersonic). That means there's a lot of aerodynamic heating, especially on items like the grid fins that don't have an aerodynamic shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The grid fins are indeed covered in ablative paint.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 4 grid fins are controlled independently to steer the trajectory of the stage.  A fin rotated to a large angle interacts less with the air (presents a smaller cross-section to the airstream), so it would experience less heating. &#xA;Fins may also end up in the lee of the rocket body when the rocket isn't pointed exactly along its direction of flight. &#xA;That's why you can get 1 grid fin that's more charred than the others.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-04-01T07:49:44.413" LastActivityDate="2017-04-01T07:49:44.413" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20899" PostTypeId="2" ParentId="20891" CreationDate="2017-03-31T08:52:53.420" Score="9" Body="&lt;p&gt;No. It is not more efficient to bypass LEO. You can't bypass getting out of earth's gravity well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.9 km/s is what it'd take to enter a Mars transfer orbit if the ship were outside of earth's gravity well moving 30 km/s in a circular 1 A.U. orbit. (1 A.U. or one astronomical unit is earth's average distance from the sun.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0njoP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0njoP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, again, that 2.9 km/s is what it takes to get from one sun centered orbit to the sun centered Mars transfer orbit. The planetary gravity wells aren't even considered in that number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From earth's point of view, the exit orbit is a hyperbola and that 2.9 km/s is the hyperbola's &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;noreferrer&quot;&gt;Vinfinity&lt;/a&gt;. There's no getting around delta V to get out of earth's gravity well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And a horizontal burn is the best way to achieve that Vinfinity. Every second you're thrusting vertically, you're losing delta V from gravity loss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atmospheric friction prevents a ship from achieving orbital velocity at earth's surface. So a rocket ascends above the thick atmosphere before turning sideways and doing the major horizontal burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once above the atmosphere, it would be possible to do a single major horizontal burn that achieves TMI (Trans Mars Insertion). But at some period during that burn, the ship will have a velocity of 7.7 km/s with close to a zero flight path angle. At that point the ship could be said to be in LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Virtually every spacecraft passes through LEO during some point of it's flight path. &lt;a href=&quot;http://silverbirdastronautics.com/LaunchMethodology.pdf&quot; rel=&quot;noreferrer&quot;&gt;From Launch Vehicle Performance Estimation by John Schilling&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Townsend technique begins by assuming that all space launches&#xA;  consist of a direct ascent to a low circular parking orbit, followed&#xA;  by a series of on-orbit maneuvers to the final destination orbit. In&#xA;  fact, many launch vehicles fly only a direct-ascent trajectory, even&#xA;  to a high or non- circular orbit. However, an observation of these&#xA;  trajectories almost invariably finds the launch vehicle, at an&#xA;  altitude of a few hundred kilometers, accelerating almost horizontally&#xA;  through the local circular orbit velocity. One may simplify the&#xA;  problem by treating this as an instantaneous &quot;parking orbit&quot;, reached&#xA;  by direct ascent, and with all subsequent powered flight treated as an&#xA;  &quot;on-orbit maneuver&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Antzi and uhoh are correct that spending awhile in LEO gives time to do checks and preparation. Also departing from LEO is the best way to do it when considering delta V.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-03-31T20:02:21.000" LastActivityDate="2017-03-31T20:02:21.000" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20903" PostTypeId="1" AcceptedAnswerId="20905" CreationDate="2017-03-31T14:42:48.200" Score="4" ViewCount="1812" Body="&lt;p&gt;These grid aerobrakes/drag control moving surfaces, have been used for a while on various aerodynamic designs, and presumably they provide a turbulent airflow, which in some way is more predictable than a laminar one, since it never switches between laminar and turbulent. Assuming that a turbulent behaviour is necessary for booster reentry(?), why does it seem like this design is a copy/paste from the past? Are there any other proposals or tests made for this particular aerodynamic part? (maybe with a different grid scale, no grid at all and a different volume/surface ratio, or a totally different shape)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(edit)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for the answer, the idea was also to question the simplicity of the falcon 9 concept, amount of moving parts, landing fuel weight. I was wondering if those fins could be longer, deploy slowly to aerobrake, and when subsonic pitch them in order to land in autorotation, with no fuel at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ePC00.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ePC00.gif&quot; alt=&quot;aerobrake + autorotation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="19132" LastEditDate="2017-04-03T13:47:22.013" LastActivityDate="2017-05-18T13:45:29.347" Title="What's behind the design of SpaceX grid fins?" Tags="|spacex|falcon-9|grid-fins|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20904" PostTypeId="1" CreationDate="2017-03-31T15:18:20.890" Score="5" ViewCount="88" Body="&lt;p&gt;How are extra utility lines and pipes, like data, electrical, ?or gasses and fluids, added to a manned, pressurised and operating vehicle or habitat, in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A spacecraft may be launched with, perhaps, a ?small overcapacity in utilities &#xA;ports to allow for future expansion of services, but once those ports are populated, how does one add more interface ports? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do such techniques exist? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has this been done, e.g. on the ISS? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the techniques used?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is hull and atmospheric integrity maintained?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can such similar techniques developed for terrestrial military submarine rescue be applied in the vacuum of space?&lt;/p&gt;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2017-03-31T17:08:07.133" LastActivityDate="2017-03-31T17:08:07.133" Title="How are electrical and other utilities interfaces added to an operating pressurised spacecraft?" Tags="|spacecraft|habitat|vacuum|service-utilities|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20905" PostTypeId="2" ParentId="20903" CreationDate="2017-03-31T15:39:12.040" Score="8" Body="&lt;p&gt;The issue seems to be the need for a single control surface (for simplicity) to work at hypersonic speeds, down to subsonic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Grid fins have been used high speed missles in this speed regime, so why reinvent the wheel. It works, it is well known, they traded off and decided on this design.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-03-31T15:39:12.040" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20907" PostTypeId="2" ParentId="2095" CreationDate="2017-03-31T15:54:32.603" Score="23" Body="&lt;p&gt;NASA would prefer that you please &lt;a href=&quot;https://www.nasa.gov/about/contact/index.html&quot; rel=&quot;noreferrer&quot;&gt;contact them first&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;h2&gt;NASA-USG_LUNAR_HISTORIC_SITES_RevA-508&lt;/h2&gt;&#xA;  &#xA;  &lt;h2&gt;A4-1 PHYSICAL CONTACT&lt;/h2&gt;&#xA;  &#xA;  &lt;h3&gt;RECOMMENDATION:&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;Visitors should not physically contact any USG lunar hardware. &lt;br /&gt;&#xA;  Exceptions should be pre-coordinated with NASA.&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;RATIONALE:&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;Lunar dust and potential biological contamination may be transferred from the visiting spacecraft onto the historical assets, degrading the historical site and/or impacting the science value of the site. However, physical contact with USG hardware and/or impact debris may provide additional scientific value, which should be balanced with the potential for damage. Coordination with NASA is recommended to ensure acceptance and understanding of all risks and benefits.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.lpi.usra.edu/lunar/strategies/NASA_Protecting_Apollo_Landing_Sites_2011.pdf&quot; rel=&quot;noreferrer&quot;&gt;Source (PDF)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19136" LastEditorUserId="109" LastEditDate="2017-03-31T16:25:15.390" LastActivityDate="2017-03-31T16:25:15.390" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20908" PostTypeId="1" CreationDate="2017-03-31T15:57:36.283" Score="3" ViewCount="650" Body="&lt;p&gt;In droneship landings of SpaceX boosters, many times in live streaming, the video has stopped because of vibration in the droneship caused by the 1st stage landing of a Falcon 9 rocket. Would it be possible to film this event with a rotorcraft drone or quadcopter controlled by satellites? I am speaking for &lt;a href=&quot;http://www.droneomega.com/gps-drone-navigation-works/&quot; rel=&quot;nofollow noreferrer&quot;&gt;GPS drone navigation&lt;/a&gt; for a GPS drone, or using an autopiloted drone using again the GPS satellite, which would give the command for quadcopter lifting from the SpaceX droneship, moving far enough away to not interrupt the landing process, and returning again to the droneship after the booster's landing. Quadcopters are mostly small and it is easier to land in a small spot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.quora.com/What-is-the-maximum-range-of-civilian-drones-UAVs&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here&lt;/a&gt; is a rotorcraft drone that traveled for 2000 km, which is enough for a flight 600 km away from the launch pad of the rocket, where the droneship is stationed, and return back to the launch facility. It could serve as a flyby, but may be difficult to loiter in real time, waiting for the booster landing event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I am focussing more on the GPS drone control, and since is more efficient and could be less costly. They have filmed before using manned chase plane (jkavalik comment) which is expensive. Why isn't SpaceX using these kind of drones? Because it is not possible with these GPS drones, because it would cause interference or interrupt the 1st stage controlled landing, or it is something else? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, could they livestream booster landing event in droneship using a drone, and if not, why?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2017-04-06T21:36:41.747" LastActivityDate="2017-04-06T21:36:41.747" Title="Why not use a rotorcraft drone or quadcopter using GPS navigation to live stream SpaceX booster landings on the droneship?" Tags="|spacex|falcon-9|gps|droneship|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20909" PostTypeId="2" ParentId="20908" CreationDate="2017-03-31T16:43:13.973" Score="3" Body="&lt;p&gt;They have done this with a drone, recording a landing before. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=glEvogjdEVY&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here is an example&lt;/a&gt; of a drone recording the landing at LZ-1, so on land and thus easier. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=RPGUQySBikQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;CRS-8 landing on the drone ship&lt;/a&gt;, from a camera drone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So then the question morphs to, why not do this every time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously weather will make a difference.  On a stormy day, or at night, flying an expensive drone in bad weather is probably not a great idea. Especially since the benefit is mostly for fans like us. There are cameras on deck recording it and storing it for later. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We do not know that they are not doing this every time and just not releasing the footage. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-03-31T16:43:13.973" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="20910" PostTypeId="5" CreationDate="2017-03-31T17:26:50.040" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2017-03-31T17:26:50.040" LastActivityDate="2017-03-31T17:26:50.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20911" PostTypeId="4" CreationDate="2017-03-31T17:26:50.040" Score="0" Body="Tag pertains to utility services and their interface ports in a space craft or habitat, such as data lines, eg, ethernet cables, sensor lines; power busses, or gas and fluid lines, eg $О­_2$, water, propellant, and their housings  and access runs and ports.&#xD;&#xA;" OwnerUserId="11085" LastEditorUserId="11085" LastEditDate="2017-03-31T17:50:58.297" LastActivityDate="2017-03-31T17:50:58.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20912" PostTypeId="5" CreationDate="2017-03-31T17:59:41.617" Score="0" Body="" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2017-03-31T17:59:41.617" LastActivityDate="2017-03-31T17:59:41.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20913" PostTypeId="4" CreationDate="2017-03-31T17:59:41.617" Score="0" Body="A proposed method for orbital launch and transfers. Utilizes a long wire with fixed rotation to manage momentum." OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2017-03-31T17:59:41.617" LastActivityDate="2017-03-31T17:59:41.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20914" PostTypeId="1" AcceptedAnswerId="20984" CreationDate="2017-03-31T21:09:49.437" Score="4" ViewCount="965" Body="&lt;p&gt;Using solid rocket boosters on a manned launch comes with some additional risks because they can't be turned off or throttled down once lit. For the Shuttle this created some additional failures modes that would destroy the stack and kill the crew. Three that I know of:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;If one didn't light at launch.&lt;/li&gt;&#xA;&lt;li&gt;If one didn't jettison at the separation.&lt;/li&gt;&#xA;&lt;li&gt;If launched at temperatures that were too cold for O-Rings to function properly.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It's my understanding the Shuttle could not be man-rated today. How does the SLS deal with these failure modes in order to be man-rated? Are its SRBs improved in ways that can help deal with them (esp. low temperature launches), or is it solely because the SLS will have a crew escape system?&lt;/p&gt;&#xA;" OwnerUserId="19141" LastEditorUserId="198" LastEditDate="2017-04-03T23:54:08.983" LastActivityDate="2017-04-05T14:44:47.293" Title="How are SLS SRBs considered man-rated?" Tags="|nasa|space-shuttle|sls|srb|human-rating|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20915" PostTypeId="1" CreationDate="2017-03-31T22:13:23.643" Score="3" ViewCount="303" Body="&lt;p&gt;Skylon is the frontier of launch vehicle research with literally 1 TRL(Technology Readiness Level), and it is more difficult than the Space Shuttle project. The project would cost estimate of 12 billion dollars, and the development of SABRE is only at the beginning. With heavy competition of other companies, (SpaceX have just launched their reused boosters), is it just a matter of time that this project would be cancelled?&lt;/p&gt;&#xA;" OwnerUserId="18270" LastEditorUserId="12102" LastEditDate="2021-01-01T10:53:22.867" LastActivityDate="2021-01-01T10:53:22.867" Title="How plausible is the Skylon project?" Tags="|propulsion|ssto|sabre-engine|" AnswerCount="1" CommentCount="5" ClosedDate="2017-04-01T18:51:30.853" ContentLicense="CC BY-SA 3.0" />
  <row Id="20916" PostTypeId="1" AcceptedAnswerId="20944" CreationDate="2017-04-01T01:52:07.787" Score="16" ViewCount="1286" Body="&lt;p&gt;While watching &lt;a href=&quot;https://www.youtube.com/watch?v=xfNO571C7Ko&quot; rel=&quot;nofollow noreferrer&quot;&gt;the SES-10 flight&lt;/a&gt;, starting a little after T+1:30, I noticed that the direction of the first-stage exhaust plume started to diverge from the alignment of the rocket body. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jLqvZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jLqvZ.png&quot; alt=&quot;SES-10 at T+1:42, plume deflection slight but definite&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The divergence angle increased continuously until the plume became too tenuous to clearly determine the direction it was pointing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZAWWC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZAWWC.png&quot; alt=&quot;SES-10 at T+1:54, plume deflection much more dramatic&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the 2-min mark, the plume appears something like 30 degrees off-line (quite foreshortened, so the actual divergence angle must be smaller). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7sSSn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7sSSn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought briefly that it might be deflecting to correct for an engine shutdown, but from 2:02 to 2:14, the 8 separate plumes from outer engines are distinctly visible and angled in the expected directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would expect any deliberate angle-of-attack on ascent to be quite small, and that a gimbaling of the engines that substantial would turn the stack very rapidly. At that speed and altitude, dynamic pressure should still be substantial (perhaps 7-8kPa?). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What explains this observation?&lt;/strong&gt; Is this just the effect of a strong crosswind, either pushing the plume aside, or forcing the engines to gimbal to correct for the rocket being pushed off trajectory?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's &lt;a href=&quot;https://i.stack.imgur.com/5kDHb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;a more complete set of images&lt;/a&gt; in the sequence.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-05-12T00:13:20.447" LastActivityDate="2018-05-13T06:03:56.850" Title="Cause of apparent plume deflection on SES-10?" Tags="|launch|spacex|rockets|falcon-9|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="20918" PostTypeId="1" AcceptedAnswerId="24178" CreationDate="2017-04-01T07:29:06.870" Score="6" ViewCount="2042" Body="&lt;p&gt;From Reddit &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/62i6m1/recap_of_the_elon_musk_and_martin_halliwell_press/&quot; rel=&quot;noreferrer&quot;&gt;Recap of the Elon Musk and Martin Halliwell press conference with lots of new info&lt;/a&gt;, March 31, 2017 (presumably not related to April fools day in some time zones?)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Fairing and future second stage recovery&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Payload fairing LANDED SUCCESSFULLY. Fairing has thruster systems and steerable parachute. Was just shown pic of intact fairing floating in ocean.&lt;/li&gt;&#xA;&lt;li&gt;Upper stage reuse is next.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It says &amp;quot;systems&amp;quot; (plural). Does that mean that each half of the fairing use a thruster post-deployment as part of a recovery effort? If so, how are they used?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-12-22T18:50:34.090" Title="Do each of the fairing halves now use thrusters post-deployment? How does that work?" Tags="|spacex|falcon-9|fairings|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20919" PostTypeId="2" ParentId="20915" CreationDate="2017-04-01T07:55:28.087" Score="4" Body="&lt;p&gt;It's plausible enough that companies like BAE Systems and organizations like ESA have invested in the technology. BAE Systems' interest lies in defence applications of the SABRE engine, so we'll probably see those long before Skylon gets off the ground.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-04-01T07:55:28.087" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20920" PostTypeId="2" ParentId="20891" CreationDate="2017-04-01T08:18:28.547" Score="6" Body="&lt;p&gt;Yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a simplifying case of a direct escape vs. a parking orbit departure from an airless, non-rotating body using instantaneous maneuvers. The most efficient way to get to the parking orbit is an initial horizontal $\Delta V$ at the surface to get an orbit with a apoapsis at the parking orbit, followed by a circularization $\Delta V$ at apoapsis to raise the periapsis to that same radius. Then depart from the parking orbit to escape with a third $\Delta V$. Subtract from that an escape to the same velocity at infinity directly from the surface with a single $\Delta V$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I get that difference to be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\sqrt{\frac{\mu }{r}}\left(\sqrt{e-1+\frac{2}{q+1}}-\sqrt{e+1}+\frac{\sqrt{2} q}{\sqrt{(q+1) (q+2)}}\right)&#xA;   $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mu$ is the $GM$ of the body, $r$ is the surface radius, $e$ is the eccentricity of an escape hyperbola tangent to the surface, i.e. $e=1+{C_3 r\over\mu}$, and $q$ is the parking orbit altitude as a fraction of $r$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an escape from Earth to Mars with $C_3\approx 10\,\mathrm{{km}^2/s^2}$, $e\approx 1.16$. For a $100\,\mathrm{nmi}$ parking orbit, $q\approx 0.03$. I then get, using Earth's $\mu$ and $r$, that the cost of going to the parking orbit as opposed to a direct escape is about $74\,\mathrm{m/s}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About $58\,\mathrm{m/s}$ of that is the circularization burn, with the remaining $16\,\mathrm{m/s}$ being effectively a loss of Oberth effect due to an escape from a higher altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A launch vehicle will get to a parking orbit more directly, but it can only cost more $\Delta V$. As for the atmosphere, you can imagine the launch vehicle getting out of the atmosphere in the same way to the same initial conditions at, say, $60\,\mathrm{nmi}$ altitude, and comparing direct escape vs. an intermediate parking orbit starting from there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That $74\,\mathrm{m/s}$ is pretty small compared to the total of $\Delta V\approx 11,600\,\mathrm{m/s}$. The benefits of an intermediate parking orbit easily outweigh this small cost, both in terms of timing flexibility and in terms of lining up the outgoing asymptote with respect to the launch latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only time I am aware of a real consideration of a direct escape was for a mission to Mars that I was working on using an Ariane V before they had qualified upper stage restarts. If you can't restart the upper stage, then you can't use a parking orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-04-01T08:38:03.200" LastActivityDate="2017-04-01T08:38:03.200" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20921" PostTypeId="1" AcceptedAnswerId="20936" CreationDate="2017-04-01T08:29:21.240" Score="2" ViewCount="314" Body="&lt;p&gt;It may just be a handy &quot;material&quot; in the rendering software used to make the images, but it shows up in multiple places.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could there be any possible &lt;em&gt;engineering advantage&lt;/em&gt; of a fairing's outer finish being shiny and/or metallic rather than the more frequent matte white surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Now opening up the question to include any possible &lt;em&gt;engineering DISadvantage&lt;/em&gt; as well.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Images from Ars Technica's &lt;a href=&quot;https://arstechnica.com/science/2017/03/blue-origin-releases-details-of-its-monster-orbital-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Blue Origin releases details of its monster orbital rocket&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JrwU2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JrwU2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/n0x3V.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/n0x3V.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/izb0N.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/izb0N.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-02T07:02:58.677" LastActivityDate="2017-04-02T08:06:19.877" Title="Could a shiny metallic fairing have some engineering advantage? Disadvantage?" Tags="|blue-origin|fairings|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20922" PostTypeId="1" AcceptedAnswerId="20923" CreationDate="2017-04-01T08:47:39.290" Score="0" ViewCount="329" Body="&lt;p&gt;This file photo shows a Falcon 9 first stage being transported by truck after landing. There is some equipment at the end of the truck, which includes dials, knobs, and an exhaust stack, which suggests there may be a diesel generator, although it may just be a compressor for the hydraulics to its immediate right.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, the big box &lt;s&gt;may not necessarily be connected to the rocket&lt;/s&gt; &lt;a href=&quot;https://space.stackexchange.com/a/20923/12102&quot;&gt;&lt;em&gt;is not related  to the rocket at all&lt;/em&gt;&lt;/a&gt;, but it made me question if the rocket does have a need for any connected support equipment for the trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What kinds of support equipment are necessary or at least important to transport the booster by truck? Are there chemicals, materials, or components that need to be kept cool for safety reasons, or things that need to be pressurized, or servos, valves, or actuators that need to be active?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Cropped screenshot from the YouTube video &lt;a href=&quot;https://youtu.be/vzS1Vkpsi5k&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX - Booster Number 4 - Thaicom 8 06-06-2016&lt;/a&gt;. (@Hobbes &lt;a href=&quot;https://space.stackexchange.com/questions/20922/#comment56789_20923&quot;&gt;linked&lt;/a&gt; to this &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=40393.800&quot; rel=&quot;nofollow noreferrer&quot;&gt;discussion&lt;/a&gt; where I found this video.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/sJ7m6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Image from the New York Times article &lt;a href=&quot;https://www.nytimes.com/2017/03/30/science/space-x-reuseable-rockets-launch.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Recycled Rockets Could Drop Costs, Speed Space Travel&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/M1me7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;The recovered booster stage of a SpaceX Falcon 9 rocket that landed on a floating platform in May&lt;/em&gt;. Credit Loren Elliott for The New York Times&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-09-07T14:47:16.893" LastActivityDate="2019-09-07T14:47:16.893" Title="What support equipment is necessary to maintain a falcon 9 booster during transport after recovery?" Tags="|falcon-9|safety|transport|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="20923" PostTypeId="2" ParentId="20922" CreationDate="2017-04-01T10:45:42.793" Score="1" Body="&lt;p&gt;The big gray box attached to the end of the trailer is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Self-propelled_modular_transporter&quot; rel=&quot;nofollow noreferrer&quot;&gt;power pack for the trailer&lt;/a&gt;. It powers the hydraulic jacks that raise/lower and rotate each axle. Depending on the trailer design, it could provide drive power for the trailer too, so it can maneuver at low speeds without a tractor unit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-04-01T13:39:23.007" LastActivityDate="2017-04-01T13:39:23.007" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20925" PostTypeId="1" CreationDate="2017-04-01T17:42:50.760" Score="13" ViewCount="791" Body="&lt;p&gt;Given that the cubic body is of homogeneous mass (say Iron) and we want to start with the best possible orbit and input nothing afterwards, could something orbit this cubic body stably?&lt;/p&gt;&#xA;" OwnerUserId="19146" LastEditorDisplayName="user10509" LastEditDate="2017-12-08T16:15:36.577" LastActivityDate="2021-08-19T01:58:05.350" Title="Could you stably orbit around a square (cubic) body? Would the orbit destabilize automatically if not corrected by input?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20926" PostTypeId="1" CreationDate="2017-04-01T18:17:57.307" Score="4" ViewCount="971" Body="&lt;p&gt;When SpaceX first announced their plans to develop a re-useable first stage, what was the envisioned life span of an individual booster in total launches before retirement (expected typical and maximum potential)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that SpaceX has demonstrated successful reflight of a Falcon booster, what is the currently projected life span in number of launches before retirement (again, expected typical and maximum potential)? Has the flight experience accumulated so far affected the original projections for either better or worse?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="18085" LastEditDate="2017-04-04T16:52:06.277" LastActivityDate="2017-04-06T13:53:37.810" Title="SpaceX Falcon expected life span (how many reflights?)" Tags="|spacex|falcon-9|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20927" PostTypeId="1" AcceptedAnswerId="20930" CreationDate="2017-04-01T18:59:05.910" Score="2" ViewCount="2561" Body="&lt;p&gt;Voyager 1 was launched on Sept 5, 1977 and encountered Jupiter March 5, 1979, a journey of less than 2 years. &#xA;When I try to calculate the trajectory of the probe I assume a Hohmann transfer from Earth to Jupiter even if I'm not 100% about that (the Internet doesn't have a lot of information on that point).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thing is, when I compute the theoretical time of flight with the following formula:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$time = \pi*\sqrt{\frac{a^3}{\mu_{sun}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;with $a$ the semi-major axis and $\mu_{sun}$ the standard gravitational parameter&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a=(149598023+778298361)/2 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mu_{sun} = 1.32712428e11$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I find $time=2.7$ years&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in reality Voyager managed to reach Jupiter in less than 2 years so how is that possible?&#xA;I thought about the velocity gained by the probe from the Titan 3E rocket which certainly played a role. &#xA;But how can I integrate this velocity in the time formula?&lt;/p&gt;&#xA;" OwnerUserId="19147" LastEditorUserId="6816" LastEditDate="2017-04-01T19:57:45.480" LastActivityDate="2017-04-02T02:38:55.043" Title="Voyager Earth to Jupiter journey time" Tags="|voyager|hohmann-transfer|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20929" PostTypeId="2" ParentId="20926" CreationDate="2017-04-01T23:38:37.560" Score="8" Body="&lt;p&gt;Musk was &lt;a href=&quot;http://www.geekwire.com/2017/spacex-falcon-elon-musk-reusability/&quot; rel=&quot;noreferrer&quot;&gt;quoted recently&lt;/a&gt; that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;He expects to get to the point that each Falcon 9 booster could be reused&#xA;  10 times with no refurbishment, or 100 times with moderate&#xA;  refurbishment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-04-01T23:50:18.857" LastActivityDate="2017-04-01T23:50:18.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20930" PostTypeId="2" ParentId="20927" CreationDate="2017-04-01T23:57:09.490" Score="6" Body="&lt;p&gt;It wasn't a Hohmann transfer. The rocket simply boosted it to a higher velocity from Earth in order to enable a Jupiter flyby that would send the spacecraft to Saturn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can get the approximate orbit elements from the &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL HORIZONS Web-Interface&lt;/a&gt;, picking a day in the middle of the Earth to Jupiter transit (which by the way was 1.5 years):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nXIOQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nXIOQ.png&quot; alt=&quot;horizons inputs&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;giving:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;$$SOE&#xA;2443666.500000000 = A.D. 1978-Jun-07 00:00:00.0000 TDB &#xA; EC= 7.969091448846140E-01 QR= 1.003858168243335E+00 IN= 1.036132511480259E+00&#xA; OM= 3.430028790053406E+02 W = 3.591112970337812E+02 Tp=  2443392.027035175357&#xA; N = 8.974748184533540E-02 MA= 2.463325742763491E+01 TA= 1.257125684251016E+02&#xA; A = 4.942901873513668E+00 AD= 8.881945578784002E+00 PR= 4.011254606791074E+03&#xA;$$EOE&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;where:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;  Symbol meaning [1 au= 149597870.700 km, 1 day= 86400.0 s]:&#xA;&#xA;    JDTDB    Julian Day Number, Barycentric Dynamical Time&#xA;      EC     Eccentricity, e                                                   &#xA;      QR     Periapsis distance, q (au)                                        &#xA;      IN     Inclination w.r.t XY-plane, i (degrees)                           &#xA;      OM     Longitude of Ascending Node, OMEGA, (degrees)                     &#xA;      W      Argument of Perifocus, w (degrees)                                &#xA;      Tp     Time of periapsis (Julian Day Number)                             &#xA;      N      Mean motion, n (degrees/day)                                      &#xA;      MA     Mean anomaly, M (degrees)                                         &#xA;      TA     True anomaly, nu (degrees)                                        &#xA;      A      Semi-major axis, a (au)                                           &#xA;      AD     Apoapsis distance (au)                                            &#xA;      PR     Sidereal orbit period (day)                                       &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;There you can see that $a$ is almost $5\,\mathrm{AU}$ — much more than the $\approx 3\,\mathrm{AU}$ for a Hohmann transfer. If it hadn't flown by Jupiter, the orbit would have had an apoapsis of almost $9\,\mathrm{AU}$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-04-02T02:32:56.163" LastActivityDate="2017-04-02T02:32:56.163" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20931" PostTypeId="2" ParentId="18831" CreationDate="2017-04-02T01:01:40.830" Score="-1" Body="&lt;p&gt;Assuming a more permanent human presence, we should have came up with a solution to the atmospheric conditioning of the moon. Maybe something like a magnetic field to hold in place denser gases which provide a building block for a space dome.&lt;/p&gt;&#xA;" OwnerUserId="19048" LastActivityDate="2017-04-02T01:01:40.830" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20932" PostTypeId="2" ParentId="20927" CreationDate="2017-04-02T02:04:27.403" Score="2" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; @MarkAdler's &lt;a href=&quot;https://space.stackexchange.com/a/20930/12102&quot;&gt;answer&lt;/a&gt;'s new edit is conclusive. Here is some additional perspective.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In short - the orbits were elliptical, but already had a semi-major axis much larger than a Hohmann ellipse to Jupiter. The orbits were carefully aimed &lt;em&gt;nearly at Jupiter&lt;/em&gt; but slightly behind it as it moved in its orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The close flybys would boost their speeds even more and give them more range. $\Delta v$ from &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1#Flyby_of_Saturn&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 1's flyby of Saturn&lt;/a&gt; (after Jupiter's flyby) to tilt it's orbital plane far out of the ecliptic. That allowed it to take valuable data at Jupiter first, then to explore the solar system environment &lt;em&gt;out of the ecliptic plane&lt;/em&gt; past Jupiter as well. &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager 2 used four flyby Maneuvers&lt;/a&gt;, at Jupiter, Saturn, Uranus, and Neptune, and used the last one to also move out of the ecliptic plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see a little 3D map-GIF of their orbits below - originally from &lt;a href=&quot;https://space.stackexchange.com/q/14317/12102&quot;&gt;here&lt;/a&gt;. The farthest orbit is that of Pluto, you can see on this scale that Jupiter is relatively close to the Sun. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; data for the Sun, planets, Pluto, Voyager 1 and Voyager 2, from January 1, 1969 (a good year to start things) until now. Dots are now. Data is from &lt;a href=&quot;http://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA JPL Horizons&lt;/a&gt;. Reposted from &lt;a href=&quot;https://space.stackexchange.com/q/14317/12102&quot;&gt;this question&lt;/a&gt;, where additional details are given. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fp1gO.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fp1gO.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &lt;em&gt;A diagram of the trajectories that enabled NASA's twin Voyager spacecraft to tour the four gas giant planets and achieve velocity to escape our solar system&lt;/em&gt;. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Voyager_Path.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/74NGT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/74NGT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-04-02T02:38:55.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20933" PostTypeId="1" CreationDate="2017-04-02T03:56:48.517" Score="1" ViewCount="205" Body="&lt;p&gt;E-sails (&lt;a href=&quot;https://en.wikipedia.org/wiki/Electric_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;Electric Sails&lt;/a&gt;) deflect &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar wind&lt;/a&gt; protons to produce thrust, that's good protection for living things in a permanent centre module that produces and stores food and oxygen. I was thinking a fleet of them, looping around Mars and back as far as Venus in a repeating pattern. Every Earth/Mars alignment there is one available for fast as possible travel to Mars another from Mars to Earth. The others at various stages in the cycle. The Venus gravity assist one gets thrown back to Mars for a contingency flyby at Earth/Mars opposition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question, is the pattern possible with a half dozen of these Oasis transport ships? &lt;/p&gt;&#xA;" OwnerUserId="19155" LastEditorUserId="12102" LastEditDate="2017-11-06T05:52:02.493" LastActivityDate="2017-11-06T16:04:55.143" Title="Could e-sails provide safer space travel" Tags="|mars|radiation|interplanetary|solar-sail|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20936" PostTypeId="2" ParentId="20921" CreationDate="2017-04-02T08:06:19.877" Score="4" Body="&lt;p&gt;One advantage is mass savings if you don't need to coat the surface with something else (or if that &quot;something else&quot; is lighter than the matte white paint would be).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most obvious example where we do have some numbers is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_external_tank&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle External Tank&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first two, used for STS-1 and STS-2, were painted white to protect the tanks from ultraviolet light during the extended time that the shuttle spends on the launch pad prior to launch. Because this did not turn out to be a problem, Lockheed Martin (at that time, Martin Marietta) reduced weight by ceasing to paint the external tanks beginning with STS-3, leaving the rust-colored spray-on insulation bare, saving approximately 272 kg (600 lb).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The surface area of the Blue Origin fairings is much smaller than the ET, but I guess it also saves up to a few dozen kilograms of mass.&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2017-04-02T08:06:19.877" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20937" PostTypeId="1" CreationDate="2017-04-02T08:38:21.463" Score="0" ViewCount="448" Body="&lt;p&gt;ARCA Space's webpage for the &lt;a href=&quot;http://www.arcaspace.com/en/program_stabilo.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stabilo program&lt;/a&gt; has a description and an image. Hydrogen peroxide monopropellant? Reaction closer to the crew than the nozzle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With thrust at the top, it seems that &lt;s&gt;simplification due to inherent stability is a big advantage&lt;/s&gt; no, per @RussellBorogove's &lt;a href=&quot;https://space.stackexchange.com/questions/20937/#comment56848_20937&quot;&gt;comment&lt;/a&gt; that's not right. While the word &quot;Stabilo&quot; appears frequently, the &quot;aerodynamic stability system at the top&quot; does not refer to any passive stabilization due to thrust-at-the-top at all. It's something different entirely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why was the thrust put at the top, and how was the attitude controlled during the phase of descent that used this engine?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, wasn't it tempting to use all that free oxygen to get a boost from combustion? Or was that going in the wrong direction - adding complexity where simplicity was sought?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with steam is that it condenses, shrinking by a factor of roughly 1000 in volume. Was it difficult to ensure that the steam was always above its local boiling point (noncondensing) at each location and pressure, at least until well outside of the nozzle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p9T7k.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p9T7k.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-08-09T20:40:39.527" LastActivityDate="2019-08-09T20:40:39.527" Title="How was the Stabilo crewed re-entry vehicle supposed to work? A tractor engine with four nozzles?" Tags="|engine-design|design|launch-escape-system|monopropellant|" AnswerCount="0" CommentCount="15" ContentLicense="CC BY-SA 4.0" />
  <row Id="20938" PostTypeId="1" AcceptedAnswerId="20941" CreationDate="2017-04-02T10:46:31.390" Score="5" ViewCount="1246" Body="&lt;p&gt;I am reading Heinlein's &lt;a href=&quot;https://en.wikipedia.org/wiki/Time_for_the_Stars&quot; rel=&quot;nofollow noreferrer&quot;&gt;Time for the Stars(1956)&lt;/a&gt; and some of the characters are very excited about confirming &lt;a href=&quot;https://en.wikipedia.org/wiki/Titius%E2%80%93Bode_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bode's law&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The formula suggests that, extending outward, each planet would be approximately twice as far from the Sun as the one before.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Wikipedia essentialy says it is no longer popular but is neither fully supported nor disproved.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The hypothesis correctly anticipated the orbits of Ceres (in the asteroid belt) and Uranus, but failed as a predictor of Neptune's orbit and has eventually been superseded as a theory of solar system formation.&lt;/p&gt;&#xA;&lt;p&gt;Recent astronomical research suggests that planetary systems around some other stars may follow Titius–Bode-like laws.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It seems like we just don't have enough data to say for sure one way or the other.  When will be able to to conclusively confirm or disprove Bode's law?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-05-10T16:13:05.113" Title="When will we be able to conclusively confirm or disprove Bode's law?" Tags="|orbit|planetary-science|planet|exoplanet|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20939" PostTypeId="2" ParentId="20926" CreationDate="2017-04-02T10:58:09.750" Score="5" Body="&lt;p&gt;The expectations of SpaceX have varied greatly over time. Experience has taught them many lessons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This initial reuse had many components replaced. The goal is to understand why those components needed to be replaced, build them more robustly so that they do not need replacement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Mark Adler's answer notes, the first reused booster will not be reflown a third time, but future Mod 5 boosters (the context of Mark's quotation) will be more resuable as that model is meant to be the culmination of the lessons learned from reflight testing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to simple resuse mission (SES-10), SpaceX has declared in the past that rapidity is also important. Thus things like how to reset the legs will need modifications. (This is one of the assumed modifications in Mod 5). If it takes 3 days to unload a stage, remove the legs, swing to horizontal, drive it back to the pad, you cannot relaunch in 24 hours as is the goal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Musk suggested that the Mod 5 design will allow 10 reflights with minor touch work (refurb). Then with more major refurbishment should be good for 100 flights or more. The idea being, the core stage, and most of the major components should be good. Some parts are good for 10 flights and then need to be replaced. Each landing attempt gives them more data on how parts weather the reentry and allow them to test ideas in how to make the weakest link parts, last longer. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-04-06T13:53:37.810" LastActivityDate="2017-04-06T13:53:37.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20940" PostTypeId="2" ParentId="20918" CreationDate="2017-04-02T11:02:18.270" Score="4" Body="&lt;p&gt;In past missions, the second stage camera has caught what look like thrusters firing as the fairing separates. People have speculated that this was early attempts at recovery. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two halves are not connected after separation so would need their own systems regardless. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current assumption seems to be that they have cold gas thrusters to orient the fairing for better survivabilty until the parachute can be deployed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are light (in terms of size) and fluffy, and have a lot of surface area, so the aero forces on reentry should slow it down quite a bit before parachutes become an option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pictures from the recovery ship are starting to appear. An annotated version, of a fairing under a tarp, on the Go Searcher recovery vehicle is interesting to look at.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/W4HjU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/W4HjU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-04-03T15:47:46.490" LastActivityDate="2017-04-03T15:47:46.490" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20941" PostTypeId="2" ParentId="20938" CreationDate="2017-04-02T11:23:18.287" Score="12" Body="&lt;p&gt;I would argue that it is disproven already. There have been &lt;a href=&quot;https://astrobites.org/2014/05/19/testing-the-titius-bode-law-on-exoplanets/&quot; rel=&quot;noreferrer&quot;&gt;studies done with Extrasolar planets&lt;/a&gt;, and it has been found to only apply in a very small number of instances. There might be some that follow the law, but the number is quite small. There are instances where it lines up nicely, but it seems like ultimately it fails most of the time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="2755" LastEditDate="2017-04-02T19:21:09.637" LastActivityDate="2017-04-02T19:21:09.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20943" PostTypeId="2" ParentId="20938" CreationDate="2017-04-02T15:53:23.710" Score="6" Body="&lt;p&gt;Bodes law is a little over stated. It's not a law per se but rather a rule of thumb or a rough guideline &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also doesn't work on our solar system exactly. For it to apply perfectly for our system you have to apply some modifiers for it to work out.&lt;/p&gt;&#xA;" OwnerUserId="19162" LastActivityDate="2017-04-02T15:53:23.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20944" PostTypeId="2" ParentId="20916" CreationDate="2017-04-02T17:57:02.753" Score="12" Body="&lt;p&gt;I believe the visual effect is due to the rocket flying a small positive angle of attack at the end of the first stage burn, combined with a steeply foreshortened camera view. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the simulation data at FlightClub (&lt;a href=&quot;https://www.flightclub.io/results/?code=SS10&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.flightclub.io/results/?code=SS10&lt;/a&gt;), the rocket flies a positive angle-of-attack during the end of the first stage burn, peaking at about 4.6 degrees AoA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that the rocket body is tilted relative to the oncoming airstream. The engine gimbals are nearly in-line with the rocket body; in fact, depending on the distribution of mass within the rocket, they may have to turn very slightly in the &lt;em&gt;opposite&lt;/em&gt; direction of the plume deflection to keep the rocket from flipping! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The oncoming airstream at ~1000m/s then deflects the plume parallel to the direction of motion, up to 4.6 degrees from the line of the rocket body. Despite the thin air, dynamic pressure is still quite high due to the high speeds -- ~10kPa at the 2:00 mark, about 1/3 of Max Q -- so it forces the plume into line.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The camera is at the launch site, and the rocket is far downrange, so the view is severely foreshortened, which increases the apparent angle of the deflection from ~4 degrees to ~30 degrees. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="12102" LastEditDate="2018-05-13T06:03:56.850" LastActivityDate="2018-05-13T06:03:56.850" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="20945" PostTypeId="1" CreationDate="2017-04-02T20:59:41.237" Score="2" ViewCount="1191" Body="&lt;p&gt;A linear particle accelerator permits to accelerate ions to high speed and there is no need to use big, heavy magnets to divert the beam. The space vacuum remove the need of pipes, so it could be delivered compact and then open to reach the right length. An RTG or a bunch of it could power the accelerator for years, maybe the exhausted plutonium could be used as ion source. Is it possible? &lt;/p&gt;&#xA;" OwnerUserId="18810" LastActivityDate="2017-04-03T04:44:06.670" Title="Is it possible to use a linear particle accelerator in deep space as propulsion?" Tags="|ion-thruster|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20946" PostTypeId="2" ParentId="20945" CreationDate="2017-04-03T04:44:06.670" Score="6" Body="&lt;p&gt;A gridded ion thruster &lt;em&gt;is&lt;/em&gt; a linear particle accelerator. So yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're looking for higher exhaust velocity, which is higher Isp, sure, you can make it whatever you like. However you end up with proportionally lower thrust for the same amount of power. You very quickly end up having to take forever to produce a useful amount of $\Delta V$. The sweet spot is where you can consume your propellant in units of years, as opposed to decades or longer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose if we had much longer lifespans and more reliable equipment, we would be tolerant of something like a century-long mission. Then again, humans have spent centuries building cathedrals, so maybe our lack of patience is a cultural thing.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-04-03T04:44:06.670" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20947" PostTypeId="1" CreationDate="2017-04-03T06:34:38.663" Score="2" ViewCount="336" Body="&lt;p&gt;So I recently learned about a project about using lasers on Earth to propel a spacecraft to 20% of light speed for interstellar traveling. So this brings an interesting question, how much shorter would be time for the spacecraft relative to us? For example, you expect it to take 10 years for it to reach its destination. You plan 10 years worth of battery power, relative to it it wouldn't be 10 years, so would it use 10 years of battery power? &lt;/p&gt;&#xA;" OwnerUserId="12420" LastEditorUserId="11611" LastEditDate="2017-04-03T14:35:13.160" LastActivityDate="2017-04-03T14:35:13.160" Title="A laser can propel a spacecraft to 20% of light speed, time shorter on spacecraft?" Tags="|spacecraft|time|laser|relativistic-rocket|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20948" PostTypeId="1" CreationDate="2017-04-03T06:51:01.970" Score="4" ViewCount="1484" Body="&lt;p&gt;Some studies show that there are big '&lt;a href=&quot;https://futurism.com/is-it-raining-diamonds-on-uranus-neptune/&quot; rel=&quot;nofollow noreferrer&quot;&gt;seas&lt;/a&gt;' of diamonds on Uranus and Neptune, there may be loads of it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was thinking, wouldn't it be a great business opportunity to go to Uranus or Neptune (maybe lend some money, build a robotic ship), get some diamonds, and return back (and give back lend money)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How expensive and how realistic would this action be? It seems to me as a really great plan for rich people to multiply their assets.&lt;/p&gt;&#xA;" OwnerUserId="16590" LastEditorUserId="12102" LastEditDate="2019-05-17T11:13:23.727" LastActivityDate="2021-12-17T12:14:42.487" Title="Mining of Uranus or Neptune" Tags="|mining|neptune|uranus|" AnswerCount="5" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="20949" PostTypeId="2" ParentId="20948" CreationDate="2017-04-03T07:11:53.237" Score="4" Body="&lt;p&gt;First of all, Neptune is a gas giant. Meaning there is no place for the spacecraft to land safely. And on top of that Neptune is about 2.26 Billion miles from Earth (When lined up towards the sun).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, but lets say the almighty diamond rush starts and they take off. The last time something visited Neptune was NASAs New Horizon which took 8 years of spacetravel which will probably come down to atleast 18 years of total spacetravel when they come back... The climate on Neptune is kinda harsh compared to Earth. They have calculated that winds of up to 600 m/s is a common thing on Neptune. When the crew goes down for landing they wont only have to account on the 600 m/s winds but also the fact that theres no place to land on. Sure the core of the plants are mostly Ice and stone, however it is beneath miles of water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Realisticly, before the spacecraft can hover safely for days this is not possible. &lt;/p&gt;&#xA;" OwnerUserId="19167" LastActivityDate="2017-04-03T07:11:53.237" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20950" PostTypeId="2" ParentId="20948" CreationDate="2017-04-03T08:04:16.183" Score="11" Body="&lt;p&gt;That sounds like a terrible idea. The article says that:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Lastly, Both planets contain high concentrations of methane. Like&#xA;graphite, it too can actually transform into various forms of complex&#xA;hydrocarbons given enough heat and pressure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Consider this:&lt;/p&gt;&#xA;&lt;p&gt;Pressure is high enough to make diamonds, so any mining equipment would need to sustain even higher pressure. Wouldn't that make your mining equipment more expensive in the first place?&lt;/p&gt;&#xA;&lt;p&gt;Even if you intend to harvest diamonds not for their practical applications, but for their idealistic value, wouldn't it be much cheaper to just manufacture them here at home?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="7062" LastEditDate="2021-12-17T12:14:42.487" LastActivityDate="2021-12-17T12:14:42.487" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="20951" PostTypeId="1" AcceptedAnswerId="22367" CreationDate="2017-04-03T09:43:16.020" Score="3" ViewCount="823" Body="&lt;p&gt;I have a few data points, but not the big picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;19-10-2016, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Giant_Metrewave_Radio_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Giant Metre Wave Telescope&lt;/a&gt; was used to monitor the doppler shift of a beacon signal from the Schiaparelli spacecraft as it descended into the Martian atmosphere. This is a radio telescope for Astronomy research, but was used as an extremely large aperture to collect a signal not necessarily intended for Earth reception. See &lt;a href=&quot;https://space.stackexchange.com/q/18707/12102&quot;&gt;this question&lt;/a&gt; for further discussion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Indian Deep Space Network&lt;/a&gt; in Byalalu supported Chandrayaan-1 in lunar orbit. It then received an upgrade in power and capability, and was used in conjunction with the NASA DSN for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Orbiter Mission&lt;/a&gt;. As explained by Dr K Radhakrishnan, the former Director of ISRO &lt;a href=&quot;https://youtu.be/tmpzUJbYAEo?t=3091&quot; rel=&quot;nofollow noreferrer&quot;&gt;in this video after 51:31&lt;/a&gt; the deep space capability was developed and proven during MOM's mission. It was also tested or used in some way with &lt;a href=&quot;https://en.wikipedia.org/wiki/MAVEN&quot; rel=&quot;nofollow noreferrer&quot;&gt;MAVEN&lt;/a&gt; at Mars as well, but no details are given.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could someone further explain how the Byalalu ground station has been used with MOM, MAVEN? Does it now have full &lt;em&gt;deep space&lt;/em&gt; tracking and communications capability? How in the future it might be integrated as a new, networked deep space ground station?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Indian_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;IDSN&lt;/a&gt;'s 32m dish antenna in Byalalu. Image from &lt;a href=&quot;http://www.vssc.gov.in/VSSC_V4/index.php/ground-segment/82-chandrayaan-1/967-indian-deep-space-network-idsn&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ScdKQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ScdKQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Doppler signal from Schiaparelli, during separation and descent phase, recorded by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Giant_Metrewave_Radio_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Giant Metre Wave Telescope&lt;/a&gt;. Image from &lt;a href=&quot;http://blogs.esa.int/rocketscience/2016/10/18/listening-to-an-alien-landing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, more &lt;a href=&quot;http://www.mso.anu.edu.au/~plah/Home_Page_Stuff/GMRT_2006/GMRT_images_2006.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aGA4x.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aGA4x.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-07-22T04:24:40.277" Title="How have/how will Indian ground stations be used for comms or tracking of deep space spacecraft?" Tags="|deep-space|isro|deep-space-network|doppler-ranging|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20952" PostTypeId="2" ParentId="20947" CreationDate="2017-04-03T13:09:13.747" Score="6" Body="&lt;p&gt;If it took 10 years from our point of view, it would take 9.8 years from the point of view of the spacecraft travelling at 20% of the speed of light.  So a little shorter, but not by much.  A bigger effect would be that the spacecraft would be two light years away by the end of it's journey, so if we were watching though a telescope we wouldn't see it reach it's target until 12 years after launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The time dilation correction is very small until you get quite close to the speed of light.  To get by with five years worth of batteries, you'll need to get the spacecraft to 87% of the speed of light, and to take only one year's worth of batteries you'd want more than 99% of the speed of light.&lt;/p&gt;&#xA;" OwnerUserId="11611" LastEditorUserId="11611" LastEditDate="2017-04-03T13:20:02.333" LastActivityDate="2017-04-03T13:20:02.333" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20953" PostTypeId="1" AcceptedAnswerId="20981" CreationDate="2017-04-03T16:16:42.713" Score="5" ViewCount="345" Body="&lt;p&gt;Supposedly the Progress modules used for ISS reboost, attitude control, and debris avoidance have two large engines and some number of steering jets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know &lt;a href=&quot;http://pages.erau.edu/~ericksol/projects/issa/propul.html&quot; rel=&quot;noreferrer&quot;&gt;Zarya has 24 large and 12 small in addition to its two main engines&lt;/a&gt;, and the same page says that between Zarya and Zvezda there are 32 radial thrusters in two sets of 16.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine Zarya, Zvezda, and Progress all have similarly sized thrusters &amp;mdash; so it seems likely that there are three sizes on Progress. But I don't know what I'm looking for in &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/f/f8/Progress_M-55_undocking_from_ISS.jpg/1200px-Progress_M-55_undocking_from_ISS.jpg&quot; rel=&quot;noreferrer&quot;&gt;pictures like this one&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone in here have an insight into Progress' control capability? Might it be in one of the Space Station Program interface control documents, and if so, can someone point me to the correct one?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="869" LastActivityDate="2017-04-05T14:29:07.870" Title="What are the sizes and locations of Progress's thrusters?" Tags="|iss|attitude|engines|russian-segment|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20954" PostTypeId="2" ParentId="20784" CreationDate="2017-04-03T16:30:05.307" Score="4" Body="&lt;p&gt;It turns out I made a dumb math error (a transposition, specifically), related to &lt;a href=&quot;https://math.stackexchange.com/questions/2201539/left-handed-lup-substitution-instead-of-right-handed-can-i-use-the-same-algorit&quot;&gt;this math.stackexchange question&lt;/a&gt;. In short, I was computing $\mathbf{\lambda} = \mathbf{\hat{A}}^{-1}\mathbf{\hat{c}}$ instead of $\mathbf{\lambda}^\textrm{T} = \mathbf{\hat{c}}^\textrm{T}\mathbf{\hat{A}}^{-1}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It still produced what appeared to be valid answers in many cases &amp;mdash; probably because my $\mathbf{\hat{A}}$ matrices were often symmetric, particularly for translation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do want to note that the values one uses for the different epsilons can make a large difference in some edge cases. So while it didn't solve my problem, it may come in handy for others. Use machine epsilon for your matrix inversion (or don't invert, if you can help it, since forward and back-substitution is more efficient). Use larger negative epsilon values to determine whether $\mathbf{x}$ is non-negative and for the delta-cost computation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this comes in handy for someone.&lt;/p&gt;&#xA;" OwnerUserId="869" LastEditorUserId="-1" LastEditDate="2017-04-13T12:22:09.870" LastActivityDate="2017-04-03T16:30:05.307" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20955" PostTypeId="2" ParentId="17424" CreationDate="2017-04-03T16:32:59.977" Score="0" Body="&lt;p&gt;Helium is used for cryo-storage, and enables freezers that are much colder than those making use of nitrogen.&lt;/p&gt;&#xA;" OwnerUserId="869" LastActivityDate="2017-04-03T16:32:59.977" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20956" PostTypeId="1" AcceptedAnswerId="23638" CreationDate="2017-04-03T19:56:11.087" Score="2" ViewCount="1548" Body="&lt;p&gt;For example, for the Apollo lunar modules, for landing on the Moon the weight of fuel was approximately 60% of the total weight (on Wiki 8 tons of the 15 tons). For departure from the surface, the weight of fuel was about 50% of the total weight (2.5 tons from 5 tons).&lt;/p&gt;&#xA;" OwnerUserId="19175" LastEditorUserId="7062" LastEditDate="2022-06-09T08:25:39.430" LastActivityDate="2022-06-09T08:25:39.430" Title="How much fuel is needed for the motor landing and taking off on Mars?" Tags="|mars|fuel|interplanetary|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="20957" PostTypeId="1" AcceptedAnswerId="20959" CreationDate="2017-04-03T20:35:25.710" Score="14" ViewCount="879" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Jiuquan_Satellite_Launch_Center&quot; rel=&quot;noreferrer&quot;&gt;Jiuquan Satellite Launch Center&lt;/a&gt; is historically China's preeminent spaceport. It's located only about five degrees south of Baikonur Cosmodrome, whose location, I've been led to believe, was selected explicitly for the advantages in takeoff velocity that arise from launching nearer to the equator. NASA and the ESA have also constructed spaceports at the most tropical latitudes available to them (in the ESA's case, even going so far as to place their spaceport on another continent). So why, given that China has plenty of sparsely populated southern reaches, some of which currently host more recent spaceports, would they have located their primary launch facility so far north?&lt;/p&gt;&#xA;" OwnerUserId="19174" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-04-06T10:24:42.517" Title="Why is China's primary launch facility so far north?" Tags="|china|spaceport|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20959" PostTypeId="2" ParentId="20957" CreationDate="2017-04-03T22:03:19.100" Score="7" Body="&lt;p&gt;There are two embodied steps in the question a) &quot;pre-eminent&quot; which raises the question &quot;for what purpose&quot; and b) that &quot;north&quot; does not follow the pattern adopted by other participants. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Expanding a little:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;a) That the crewed Shenzhou missions depart from Jiuquan might make it &quot;pre-eminent&quot; in some points of view.  Please say if this isn't what you meant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;b) An &quot;equatorial&quot; launch site is preferable for the specific task of launching to geostationary transfer orbit (GTO). Cape canaveral and Vandenburg are, I believe historically sited because of existing missile ranges at the dawn of the space age. I believe Kourou was explicitly chosen for GTO access and the wide variety of launch azimuths that also permits polar launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All geostationary launches todate have been from Xichang (28 deg N) except a very recent demonstration mission on the CZ5  first launch. As the new Wenchang site is at 20 deg perhaps the future, with the newer launch vehicles, is that the GTO traffic will go from Wenchang. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Polar missions in general need a clear range North or South and there is a modest energy advantage in being away from the equator.  I don't know the history of Jiquan (~40 N) and why it is that the manned missions have been launched from there though it is the oldest launch site and perhaps there are range and institutional reasons. I'm sure someone else on this forum will be able to expand on this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, &lt;a href=&quot;https://space.stackexchange.com/questions/457/is-there-any-advantage-in-launching-spacecraft-from-a-high-latitude-or-why-was?rq=1&quot;&gt;this&lt;/a&gt; is a helpful related Q&amp;amp;A. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT - Background&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;This &lt;a href=&quot;https://web-beta.archive.org/web/20110613181155/http://www.patrick.af.mil/library/factsheets/factsheet.asp?id=4514&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt; mentions that &lt;strong&gt;Cape Canaveral&lt;/strong&gt; was chosen over El Centro in California as a missile range because of a lack of international objection for the former for missile overflight. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;This &lt;a href=&quot;https://web-beta.archive.org/web/20131104222741/http://www.vandenberg.af.mil/library/factsheets/factsheet_print.asp?fsID=4606&amp;amp;page=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt; suggests that &lt;strong&gt;Vandenburg&lt;/strong&gt; was chosen in 1956 not only because it was remote but also for its access to the Pacific as a missile range and it also hints at some foresight for its polar orbit access.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;This &lt;a href=&quot;http://www.cnes-csg.fr/web/CNES-CSG-fr/9777-implantation.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt; describes the selection process that led to &lt;strong&gt;CSG at Kourou&lt;/strong&gt; amongst 14 candidate sites globally. Its in French and is very interesting but this bit is probably easy enough to follow:&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Critères de sélection :&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;Possibilité de lancements polaires et équatoriaux&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Proximité de l'équateur&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;... and it goes on to list some further criteria and then to say that Kourou  had positive characteristics in these areas.&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;4&quot;&gt;&#xA;&lt;li&gt;These two links &lt;a href=&quot;http://www.nti.org/learn/facilities/71/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NTI&lt;/a&gt; and &lt;a href=&quot;http://www.globalsecurity.org/space/world/china/jiuquan.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Global Security&lt;/a&gt; whilst not explicitly giving the reason for launches being conducted from &lt;strong&gt;Jiquan&lt;/strong&gt; do both describe the origins as a missile range. That leaves inductive reasoning in order to assume therefore that it was chosen as a missile range because of remoteness and that the launches began there simply because that's where the infrastructure already existed.  It would be interesting to understand what launch azimuths have been attempted from this site given the number of near neighbouring countries. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;An aside:&lt;/strong&gt; - an interesting point about the advantage of being closer to the equator is that it is not just the rotation of the Earth that is of benefit but also in reducing the plane change for GEO bound satellites. For the GTO example (the vast majority of GEO bound satellites will start their missions at 0 deg inclination) the reduction in the cost of a plane change when one starts from 5 deg latitude (Kourou) instead of 28 deg (Cape Canaveral) is a better advantage than the assistance given by the tangential velocity at the Earth surface.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.323" LastActivityDate="2017-04-06T10:24:42.517" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20960" PostTypeId="1" CreationDate="2017-04-04T06:07:08.537" Score="5" ViewCount="1712" Body="&lt;p&gt;There's always micrometeorites in orbit, and they constantly hit the ISS. So my question is during EVA, how do astronauts protect themselves? Is their suit designed to take impact? Since there are micrometeorites that impact the ISS windows and can cause a crack, it seems unrealistic that the suit would provide any protection.&lt;/p&gt;&#xA;" OwnerUserId="12420" LastEditorUserId="415" LastEditDate="2017-04-05T11:45:48.923" LastActivityDate="2017-04-05T11:45:48.923" Title="How do astronauts deal with micrometeorites during EVA?" Tags="|spacesuits|eva|meteoroid|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20961" PostTypeId="1" AcceptedAnswerId="20962" CreationDate="2017-04-04T10:09:25.720" Score="17" ViewCount="4883" Body="&lt;p&gt;In the 21st century, why is there is no orbiter for Uranus or Neptune or at least their moons? It would definitely tell us more about the structure and composition of these planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know it must be in the public interest in order to create and finance such mission, but I don't think it would be so expensive (considering the fact that we have created loads of orbiters for other planets). Exploring them would provide us valuable information about their moons as well.&lt;/p&gt;&#xA;" OwnerUserId="16590" LastEditorUserId="16590" LastEditDate="2017-12-18T06:37:08.530" LastActivityDate="2017-12-18T06:37:08.530" Title="Why there is no orbiter for Uranus or Neptune?" Tags="|neptune|uranus|titania|triton|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="20962" PostTypeId="2" ParentId="20961" CreationDate="2017-04-04T11:29:28.220" Score="38" Body="&lt;p&gt;Orbiters are expensive, and the further away its target is, the more expensive it gets. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;you want a high transit speed, to get there in a reasonable time&lt;/li&gt;&#xA;&lt;li&gt;then you need to brake to get into orbit, this takes lots of fuel (more fuel due to the high transit speed)&lt;/li&gt;&#xA;&lt;li&gt;because of the long mission duration, personnel cost is high&lt;/li&gt;&#xA;&lt;li&gt;for the outer planets, solar panels don't generate enough power, so you need a &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;radiothermal generator (RTG)&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;RTGs use Pu-238 as a heat source. Pu-238 is rare and radioactive, so RTGs are very expensive and only available in small numbers. Production of Pu-238 for RTGs has been &lt;a href=&quot;http://spacenews.com/doe-to-crank-out-new-plutonium-238-in-2019/&quot; rel=&quot;noreferrer&quot;&gt;restarted&lt;/a&gt;, but volume will be low: 1.5 kg/year starting in 2019. The New Horizons RTG contains 11 kg of plutonium and produced ~300 W at mission start. At that speed, 2 large probes or 3-4 smaller ones are all you can provide in a decade.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With current rockets, an orbiter to Mars is doable. For planets further out, you quickly run into mass constraints. Cassini required the largest rocket available at the time.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobraking&quot; rel=&quot;noreferrer&quot;&gt;aerobraking&lt;/a&gt;, you need less fuel, but that technique is still experimental, making it less likely to be used on a flagship-class mission. And it's a risky technique to use for capturing a spacecraft into orbit: you only get one shot that has to be just right. Too little braking and you don't achieve orbit but keep going, too much braking and you burn up. Aerobraking depends on detailed knowledge of the atmosphere (and its variability), and we just don't have enough of that for Uranus or Neptune. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbiter missions to Uranus and Neptune &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=33971.0&quot; rel=&quot;noreferrer&quot;&gt;are being considered&lt;/a&gt; (this thread gives lots of detail on tradeoffs, probe size calculations etc.), but they have to compete with other scientific missions. You can only do so much on a given budget.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The selection process for science mission also plays a role, I suspect. Because we've had several missions to Mars already, there is a large, active and experienced community of Mars scientists who can propose new missions with a degree of certainty. For Uranus and Neptune, we have far less data, and a much smaller scientific community with less experience running missions. So during every mission selection process you see lots of focused mission proposals with a high degree of budgetary confidence from the Mars community and one proposal with lots of uncertainty from the Uranus/Neptune community. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question about &lt;a href=&quot;https://space.stackexchange.com/questions/9851/requirements-to-orbit-pluto&quot;&gt;an orbiter for Pluto&lt;/a&gt; goes into more detail, including calculations on how large the spacecraft would need to be. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding &quot;loads of orbiters&quot;:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mercury: 3 orbiters&lt;/li&gt;&#xA;&lt;li&gt;Venus: 5&lt;/li&gt;&#xA;&lt;li&gt;Mars: 14&lt;/li&gt;&#xA;&lt;li&gt;Jupiter: 2 &lt;/li&gt;&#xA;&lt;li&gt;Saturn: 1&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These numbers also give a rough indication of the combination of how expensive a mission is (more expensive/difficult=fewer missions) + how interesting the target is (chance of detecting life = more missions).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-04-06T11:39:37.767" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="20963" PostTypeId="1" AcceptedAnswerId="38838" CreationDate="2017-04-04T12:59:12.433" Score="9" ViewCount="1072" Body="&lt;p&gt;GPS is a Global Positioning System. it uses satellites in orbit to identify precise location on a globe.  Several posts here talk about it being used in Low Earth Orbit (LEO) with success.  About &lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System&quot; rel=&quot;noreferrer&quot;&gt;30 satellites are used in the GPS system&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we had satellites in orbit around the Sun used to identify precise location in the solar system, how many satellites would be required? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If distance impacts the answer, I am interested in locations between the orbits of Mecury and Saturn, locations closer to or farther from the Sun can be ignored. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Locations in the shadow of a planet, can be ignored as well. The current GPS system does not provide coverage in some shadow areas, like deep valleys and large city streets.  &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2019-09-16T04:40:36.800" Title="How many satellites would be required for Solar System GPS?" Tags="|solar-system|gps|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20964" PostTypeId="1" AcceptedAnswerId="20971" CreationDate="2017-04-04T14:28:14.173" Score="47" ViewCount="10384" Body="&lt;p&gt;Somewhere on this site I read that in NASA, the rocket leaves the launchpad (through release of explosive bolts) only after the engines have attained full power.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming in the scarce seconds between SRB ignition and launch a critical failure occurs, say, main engines switch off, could the shuttle just &quot;wait it out&quot; letting the SRBs burn out without leaving the launchpad? Or would that result in yet different disaster?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2020-01-10T13:23:35.873" Title="Could the shuttle wait out the whole burn of SRBs on launchpad in case of a problem on launch?" Tags="|space-shuttle|srb|" AnswerCount="6" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="20965" PostTypeId="1" AcceptedAnswerId="25489" CreationDate="2017-04-04T14:32:50.943" Score="10" ViewCount="410" Body="&lt;p&gt;One of cooler techniques in Kerbal Space Program is safe, easy and cheap separation of side boosters by sending the rocket into a short spin; upon separation centrifugal force ejects the boosters to the sides, so there's no need for separation motors, strong explosive decouplers or other techniques assuring separated boosters don't crash into the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is or was this technique used, or proposed in practice?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2018-02-19T10:14:41.727" Title="Is spin-separation of boosters ever used?" Tags="|booster|separation|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20966" PostTypeId="2" ParentId="20964" CreationDate="2017-04-04T15:04:46.790" Score="52" Body="&lt;p&gt;I don't have an immediate citation handy, but the answer is no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRBs were powerful enough to overcome the hold-down studs.  That said, the same signal was used to blow the nuts on the hold-down studs and ignite the SRBs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the SRBs light, the vehicle is going &lt;em&gt;somewhere&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-04-04T15:04:46.790" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="20967" PostTypeId="2" ParentId="20960" CreationDate="2017-04-04T15:11:33.470" Score="8" Body="&lt;p&gt;Essentially, they just accept the risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The suit is strong enough to withstand strikes from those micrometeoroids and pieces of orbital debris most likely to hit it, and it's small enough (and EVAs are short enough) that larger, more damaging strikes are quite unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The meteoroid and debris environments are only really understood in a broad average sense (weeks to years).  Time scales as short as a single EVA are not particularly useful for determining an accurate level of risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, the limiting condition for dealing with the unlikely-but-probably-inevitable strike is not going to be pressure loss, it's going to be crew injury, as any hole big enough to create a pressure problem is going to basically create a gunshot wound.  As a result, the suit is designed to protect against strikes up to a certain size, beyond which an impact is quite unlikely.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-04-04T15:11:33.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20969" PostTypeId="2" ParentId="20963" CreationDate="2017-04-04T17:30:21.660" Score="0" Body="&lt;p&gt;@szulat is right.  mathematically speaking, 4 spheres cenetered about satellites  with known position, and varying radius will intersect at one point; we can calculate that point and be done with it.  As far as engineering an entire system, you have to provide more constraints&lt;/p&gt;&#xA;&#xA;&lt;p&gt;the radius you want to cover (sun to saturn) is about 80 light minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;if you want to cover the entire volume enclosed by 80 light minutes, that's about 2.14E6 cubic light minutes.  The ratio of the distance to saturn vs the radius of the earth is 1.9E9 : 6.4E3 = .2968 E 6.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;cubing that, it's .0261 E 18 = 2.6 E 16&#xA;that is to say, the volume you have to cover is 10^16 times as big as earth!&#xA;to ensure that each satelite communicates within one minute of the others, you'd &#xA;need about 2 million satelites.  That's 1000 times more satelites than currently orbit the earth&lt;/p&gt;&#xA;&#xA;&lt;p&gt;they'd be orbitting in rings around the sun, much like our planets are.  But in all three dimensions&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so yah, I'd say 2 million is the upper limit.  If you're ok with light-minute accuracy&lt;/p&gt;&#xA;" OwnerUserId="17351" LastActivityDate="2017-04-04T17:30:21.660" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20970" PostTypeId="2" ParentId="8497" CreationDate="2017-04-04T18:31:01.807" Score="0" Body="&lt;p&gt;This question was asked when the only processing facility was the LC-40 hangar, and the rented &lt;a href=&quot;https://space.stackexchange.com/questions/3288/what-is-the-alternate-hangar-spacex-has-available-at-the-cape/5109#5109&quot;&gt;SPIF building&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the AMOS-6 mishap damaging LC-40, LC-39A was pressed into service and the LC-39A hangar (Which sits a top the old crawler path, so no more crawlers at LC-39A) is wide enough to process a Falcon Heavy and a Falcon 9, which means room for 4 and some suggest 5 cores. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been photos of the LC-39A holding several single stick cores, the currently processing stage, but also some of the recovered stages in various states of disarray. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This flexibility was seen when the order of the CRS-10 and Echostar launch were reordered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, the SLC-4 pad at Vandenberg is active for west coast missions. Construction on Boca Chica is still slow and not really progressing.  LC-40 is being refurbished to recover from the AMOS-6 fueling incident. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:51.937" LastActivityDate="2017-04-04T18:31:01.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20971" PostTypeId="2" ParentId="20964" CreationDate="2017-04-04T19:43:22.430" Score="60" Body="&lt;p&gt;No.  Compare &lt;a href=&quot;https://science.ksc.nasa.gov/shuttle/countdown/count.html&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Launch Countdown&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRB fire command and the hold-down fire commands are issued at the exact same time.  The delay of about 6 seconds (note: each SSME is started at a different time, all 0.2 seconds apart) is to ensure that each of &lt;em&gt;those engines&lt;/em&gt; are up to full throttle without errors prior to launch (which is called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Redundant_set_launch_sequencer_.28RSLS.29_abort&quot; rel=&quot;noreferrer&quot;&gt;Redundant Set Launch Sequencer abort&lt;/a&gt;).  This has happened 5 times in the program history:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-41-D#June_launch_attempt&quot; rel=&quot;noreferrer&quot;&gt;STS-41-D&lt;/a&gt; (at T-6 seconds due to a stuck LOX valve)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-51-F#Launch&quot; rel=&quot;noreferrer&quot;&gt;STS-51-F&lt;/a&gt; (at T-3 seconds, due to a coolant valve malfunction....also note the subsequent successful launch of this mission was the first and only Abort-to-Orbit of the Shuttle program)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-51#Launch_Preparations&quot; rel=&quot;noreferrer&quot;&gt;STS-51&lt;/a&gt; (at T-3 seconds, due to a faulty fuel flow sensor)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-55#Launch&quot; rel=&quot;noreferrer&quot;&gt;STS-55&lt;/a&gt; (at T-3 seconds, due to incomplete ignition of #3 because of a LOX check valve leak)&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-68#Launch&quot; rel=&quot;noreferrer&quot;&gt;STS-68&lt;/a&gt; (at T-1.9 seconds, due to a LOX turbopump overtemp reading on #3)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Russell is correct in that the exhaust of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ammonium_perchlorate_composite_propellant&quot; rel=&quot;noreferrer&quot;&gt;APCP&lt;/a&gt; in the SRB is too hot (it &lt;em&gt;boils&lt;/em&gt; steel) for anything to survive it for more than a few seconds.  In fact, &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I-X#Pad_damage&quot; rel=&quot;noreferrer&quot;&gt;the Ares 1-X launch damaged&lt;/a&gt; a &quot;substantial&quot; section of the fixed service structure on Pad 39B when its exhaust plume briefly torched the above-ground structure of the pad after liftoff (due to a pad avoidance maneuver that it had been programmed to execute).  While it didn't matter much since NASA was planning on disassembling this structure anyhow, it's proof that a pad can little survive a continuous SRB burn.&lt;/p&gt;&#xA;" OwnerUserId="19186" LastEditorUserId="1342" LastEditDate="2017-04-05T15:24:39.193" LastActivityDate="2017-04-05T15:24:39.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20972" PostTypeId="1" AcceptedAnswerId="20975" CreationDate="2017-04-04T19:57:50.280" Score="1" ViewCount="514" Body="&lt;p&gt;Can anyone tell me the difference between staged combustion and full flow staged combustion? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does full flow mean there are 2 preburners (fuel- and oxidiser-rich respectively) or does it mean that all of the propellant goes through the preburner before entering the combustion chamber? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've drawn &lt;img src=&quot;https://i.stack.imgur.com/QRR1A.png&quot; alt=&quot;picture&quot;&gt; for reference. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(that red line drawn to FR (fuel rich) on the left is meant to be a label line, not plumbing) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The far right is definitely full flow staged combustion but what are the other two?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I've noticed that the SSME is almost the same as the rightmost but it has some oxidizer go straight to the combustion chamber. It is classified as staged combustion. This suggests full flow means both propellants are flowing through a preburner before getting to the combustion chamber. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what then are the 2 diagrams on the left? &lt;/p&gt;&#xA;" OwnerUserId="19184" LastEditorUserId="6816" LastEditDate="2017-04-04T22:59:18.243" LastActivityDate="2017-04-04T22:59:18.243" Title="Does all of the fuel in a Staged Combustion Cycle go to the preburner?" Tags="|rockets|engine-design|turbomachinery|" AnswerCount="1" CommentCount="2" ClosedDate="2017-04-05T05:52:16.863" ContentLicense="CC BY-SA 3.0" />
  <row Id="20973" PostTypeId="1" CreationDate="2017-04-04T20:02:52.933" Score="3" ViewCount="383" Body="&lt;p&gt;For some purposes it's understandable - we send the probe, get the science it's designed to collect, and we're done. And even in those cases if the mission fails we do not attempt again - we collect the crush data and scrape the mission. For example the schiaparelli lander on mars or the long tether in orbit experiment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why don't we launch few Hubble or Kepler telescopes each costing about 500 million to develop and probably 50 - 100 to build now that we have the design. NASA spends 9 billion on James Webb, why not spend few hundred million to launch more of those observatories?&#xA;Why no more shuttles are built if it's such a great design?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And why don't we ever launch few probes for a backup if one fails?&lt;/p&gt;&#xA;" OwnerUserId="19188" LastActivityDate="2017-04-05T13:19:42.940" Title="Why we always build only one spacecraft?" Tags="|nasa|probe|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="20974" PostTypeId="2" ParentId="20973" CreationDate="2017-04-04T20:40:27.297" Score="7" Body="&lt;p&gt;The Russians often build multiple copies.  The Nauka module, yet to be launched, was the backup for the Zarya module. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Zarya module was the Mir backup. (Might be Zvezda, I always get those two mixed up).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is often an engineering model that is kept back on earth for testing changes in software as a full up simulator before uploading code.  (I.e. Bricking a billion dollar probe orbiting Saturn is VERY VERY VERY VERY bad form).&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-04-04T20:40:27.297" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20975" PostTypeId="2" ParentId="20972" CreationDate="2017-04-04T22:27:11.230" Score="1" Body="&lt;p&gt;Yes. Full Flow Staged Combustion is both a Fuel Rich and Oxidizer Rich Staged Combustion combined together so both parts all go through partial combustion first and then a final combustion later. Your left drawing would be Fuel Rich staged combustion where all of the fuel goes through partial combustion but the Oxidizer is just pumped in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your middle one would be something in-between a Gas Generator Cycle where the part of the fuel that is burned to power the pumps is just dumped overboard and a Staged Combustion Cycle where all the fuel flows through that before being burned. I don't know if there is a name for that or if it is actually used anywhere. &lt;/p&gt;&#xA;" OwnerUserId="16458" LastActivityDate="2017-04-04T22:27:11.230" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20976" PostTypeId="1" CreationDate="2017-04-05T06:49:19.070" Score="8" ViewCount="1678" Body="&lt;p&gt;Looking directly down from the ISS, we see essentially a map.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm working on a project using GIS and would like to use the same scale, e.g. &quot;1:500,000&quot; if someone has done an analysis on it.  I can't seem to get it exactly correct.  I'm having trouble with the curvature and also misleading photos (where zoom was used) on the Internet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anyone has conducted this analysis, &lt;strong&gt;what is the ratio between distance on a 'map' (astronauts view) and the corresponding distance on the ground that they see?&lt;/strong&gt;  A range is acceptable, as there are minor differences all over the place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT for clarification:  Essentially, I'd like to make a map that appears similar to a snapshot of what someone would see or take a direct picture of looking immediately 'down' from the ISS.  I must set my map to Xcm:YYcm My problem is there are many conflicting images on google-search and even on NASA TV, because people are using zoom or looking off to the curvature of the Earth.&lt;/p&gt;&#xA;" OwnerUserId="6712" LastEditorUserId="6712" LastEditDate="2017-04-06T06:40:36.667" LastActivityDate="2017-04-06T06:40:36.667" Title="What is considered the 'map scale' that ISS astronauts view the Earth?" Tags="|iss|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="20977" PostTypeId="2" ParentId="20964" CreationDate="2017-04-05T08:53:55.290" Score="21" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Could the shuttle wait out the whole burn of SRBs on launchpad in case of a problem on launch?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question seems based on the assumtion that something (the hold-down bolts perhaps) could hold the stack down while the SRBs burn out, while being immersed in the &lt;strong&gt;3300 degree Celsius&lt;/strong&gt; exhaust for about &lt;strong&gt;128 seconds&lt;/strong&gt;. Note that during this time, while the hold-down bolts are melting, the stack is getting lighter due to the fuel burned and thus easier for the now-rogue SRBs to lift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, there were &lt;a href=&quot;https://www.nasa.gov/sites/default/files/290339main_8-388221J.pdf&quot; rel=&quot;noreferrer&quot;&gt;two incidents&lt;/a&gt; in which two of the hold-down bolts did not fire, and over twenty more in which a single bolt did not fire. The SRBs tore them apart with no effect on the mission, just as expected. However, the failure of four or more frangible nuts to fire may in fact exceed the space shuttle load-carrying capability, i.e.  hold down the stack. &lt;strong&gt;Thank you to &lt;a href=&quot;https://space.stackexchange.com/users/2900/reirab&quot;&gt;reirab&lt;/a&gt; for linking to the relevant PDF in the comments!&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="472" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.343" LastActivityDate="2017-04-09T06:46:45.420" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20978" PostTypeId="2" ParentId="20976" CreationDate="2017-04-05T12:22:11.027" Score="20" Body="&lt;p&gt;The ISS orbits a 1:1 scale map of Earth at about 400km altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparent visual size changes linearly with viewing distance, so you can simply divide by 400000 to see that this is equivalent to looking at a 1:400000 scale map at 1 meter distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dividing instead by 1000000 yields a 1 miilionth scale map at 40cm.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-04-05T19:55:06.383" LastActivityDate="2017-04-05T19:55:06.383" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="20979" PostTypeId="2" ParentId="20973" CreationDate="2017-04-05T13:19:42.940" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why we always build only one spacecraft?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We don't always build only one spacecraft. A newly launched environmental or communications satellites oftentimes is a carbon copy of existing environmental or communications satellite. For example, 17 Geostationary Operational Environmental Satellites have been built (one suffered a launch faiure), but there have only been five GOES designs. The newest design is expected to be reused at least four more times. That design will be obsolete by the time the satellites using this newest design start failing in the mid 2030s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, building four or five copies does not constitute mass production. Production costs are reduced with those copies, but not by orders of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA spends 9 billion on James Webb, why not spend few hundred million to launch more of those observatories?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I suspect the cost of a carbon copy of the James Webb Telescope would be well over a few hundred million dollars, and more likely over a billion dollars. The mirrors are one of the biggest costs in developing a new large telescope, whether space-based or ground-based. Pseudo mass production doesn't help much here. Moreover, that design will be obsolete by the time the Webb fails in the 2030s (or so we hope).&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-05T13:19:42.940" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20980" PostTypeId="1" AcceptedAnswerId="20982" CreationDate="2017-04-05T14:12:47.953" Score="14" ViewCount="3031" Body="&lt;p&gt;I just saw the video from the YouTube channel &lt;a href=&quot;https://www.youtube.com/channel/UCExKlYHIYQGXnG38XUAnMJg&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Edits&lt;/a&gt; titled &lt;a href=&quot;https://youtu.be/Z7rDXdtyDO8&quot; rel=&quot;noreferrer&quot;&gt;SpaceX B1021: The First Rocket to Fly Twice (a supercut of highlights&lt;/a&gt; mentioned in the &lt;a href=&quot;http://chat.stackexchange.com/rooms/9682/the-pod-bay&quot;&gt;Pod Bay&lt;/a&gt;, and noticed the Falcon 9 for the CRS-8 launch appears to sway back and forth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is happening? Why does the rocket appear to move?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sk4jZ.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sk4jZ.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Z7rDXdtyDO8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="51" LastEditDate="2017-04-05T14:15:37.673" LastActivityDate="2017-04-05T17:21:32.937" Title="Why does the CRS-8 Falcon 9 rocket appear to be swaying before launch?" Tags="|launch|spacex|falcon-9|launchpad|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="20981" PostTypeId="2" ParentId="20953" CreationDate="2017-04-05T14:15:09.553" Score="3" Body="&lt;p&gt;This was an interesting question to research. I did not find one definitive source, but have attempted to synthesize an answer from the list of references at the end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been a number of versions of Progress with slightly different propulsion systems.  AFAIK this information is correct for the latest version currently in use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Progress propulsion system uses as propellants nitrogen tetroxide / unsymmetrical dimethylhydrazine. The main engine and attitude control thrusters use the same propellant tankage. There are four propellant tanks and four helium pressurization tanks. Unused propellant can be transferred to the ISS in addition to propellants specifically carried as cargo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main engine provides axial thrust and its nominal thrust level is 2950 N. It is variously referred to as the S5.80 or KTDU-80 engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 28 attitude control thrusters, each with a thrust of 130 N. The attitude control thrusters are divided into several groupings:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Two clusters of six engines each located 180 degrees apart on the forward part of the Progress service module. These thrusters have two each pointed in the forward, lateral, and tangential directions.  &lt;/li&gt;&#xA;&lt;li&gt;Two clusters of two engines each located 180 degrees apart on the&#xA;forward part of the service module, and 90 degrees away from the&#xA;six-engine clusters. These are pointed in the lateral direction &lt;/li&gt;&#xA;&lt;li&gt;Four clusters of two engines each located 90 degrees apart on the&#xA;flaring aft skirt of the service module. These are pointed radially&#xA;outward. &lt;/li&gt;&#xA;&lt;li&gt;Four aft-firing engines on the aft face of the service module,&#xA;located 90 degrees apart, near the perimeter. These also serve as a backup to the main engine.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Mw3vw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mw3vw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflight101.net/progress-spacecraft-information.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=i&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=images&amp;amp;cd=&amp;amp;ved=0ahUKEwjL0Z6ovI3TAhXl7YMKHSXkDAYQjxwIAw&amp;amp;url=https%3A%2F%2Fspaceodyssey.dmns.org%2Fmedia%2F16235%2Fiss_russian_spacecrafts.ppt&amp;amp;bvm=bv.151426398,d.cGc&amp;amp;psig=AFQjCNGAXukJ7sok3ZcTxk5driNmItEwJw&amp;amp;ust=1491487023302868&amp;amp;cad=rjt&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Russian Spacecrafts&lt;/a&gt; (sic) [powerpoint]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.svengrahn.pp.se/histind/Soyuz1Land/SoyAtti.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Characteristics of the Soyuz attitude control system&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiBoP78v43TAhVG5GMKHZsACHgQFggcMAA&amp;amp;url=https%3A%2F%2Fspaceflight.nasa.gov%2Fhistory%2Fshuttle-mir%2Freferences%2Fdocuments%2Fmirheritage.pdf&amp;amp;usg=AFQjCNG0duTfHkfXfDPnXsXsVbHTBGgvfA&amp;amp;sig2=HqbrUzjrwCxMofoWbq5HvA&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mir Hardware Heritage&lt;/a&gt; [pdf]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mediacast/iss-crew-informed-of-progress-anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS Crew Informed of Progress Anomaly&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additions/corrections welcomed.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-04-05T14:29:07.870" LastActivityDate="2017-04-05T14:29:07.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20982" PostTypeId="2" ParentId="20980" CreationDate="2017-04-05T14:15:16.087" Score="34" Body="&lt;p&gt;&lt;strong&gt;Because it is.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is a very tall structure, first stage alone is 140 feet, plus second stage, plus fairing.  It is considered at the limit of how tall and thin they can make it. Wikipedia says the total height is 230 ft (70m). That is very tall, and 12 feet wide is quite thin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only connection holding it down, is via the Octoweb clamped to the launch mount at the bottom. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is windy, as you can see from the LOX venting and how it is blowing away. The clamp on the upper stage has retracted in the snippet above, so this is normal swaying of a long structure.  Just like buildings also sway in the wind, considering they are only attached at the ground level.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="4703" LastEditDate="2017-04-05T17:21:32.937" LastActivityDate="2017-04-05T17:21:32.937" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="20984" PostTypeId="2" ParentId="20914" CreationDate="2017-04-05T14:44:47.293" Score="4" Body="&lt;p&gt;Human certification has little to do with a concept and more to do with the Factor of Safety in mission-critical design aspects.  SRBs are very reliable and predictable in ignition and burn (if you look at their history, more so than very-complicated liquid-fueled engines), but you have seen some GEM boosters fail in the past because their factors of safety for the casting were so close to 1.  When your FOS is close to 1, there is little margin for flight excursions such as wind shear, or manufacturing excursions such as air bubbles in the epoxy casing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Human rating also considers backups for mission-critical components as well.  The SSMEs had redundant flight computers, so that in the event one fails there are backups to carry on without mission interruption (which has happened before, &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-93&quot; rel=&quot;nofollow noreferrer&quot;&gt;on STS-93&lt;/a&gt;, when two SSME computers failed due to an electrical short and their backups prevented loss of the vehicle and crew).  There may also be mechanical backups, such as redundant ignition pyrotechnics for engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA's declared requirements though aren't a straight FOS number -- it's &lt;a href=&quot;http://www.airspacemag.com/space/certified-safe-281371/&quot; rel=&quot;nofollow noreferrer&quot;&gt;a probability of failure number&lt;/a&gt;.  This is determined differently for different technologies though, but almost always through a combination of simulation and ground testing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also of note: crew escape system is an important component of human-rating a &quot;stack&quot;.  But be aware, you can human-rate a subsystem (such as a liquid-fueled engine or solid rocket motor), but that does not human-rate the entire rocket stack and crew vehicle.&lt;/p&gt;&#xA;" OwnerUserId="19186" LastActivityDate="2017-04-05T14:44:47.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20985" PostTypeId="1" CreationDate="2017-04-05T14:46:29.937" Score="6" ViewCount="253" Body="&lt;p&gt;What's behind the choice of not designing the largest possible folding mirror for the James Webb space telescope, since something like a 13m diameter mirror could also fit in Ariane's fairing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tCu1h.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tCu1h.gif&quot; alt=&quot;folding mirror&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="19132" LastEditDate="2017-04-05T15:14:03.973" LastActivityDate="2017-04-05T15:14:03.973" Title="Why isn't the JWST mirror bigger?" Tags="|payload|space-telescope|payload-deployment|james-webb-telescope|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="20986" PostTypeId="1" AcceptedAnswerId="24246" CreationDate="2017-04-05T15:33:11.223" Score="9" ViewCount="1128" Body="&lt;p&gt;Below is a GIF I prepared and used in an earlier question, and &lt;a href=&quot;https://www.flightclub.io/results/?code=SS10&quot; rel=&quot;nofollow noreferrer&quot;&gt;the answer&lt;/a&gt; seems quite reasonable. With a height to diameter ratio of about 70 m to 3.7 m (nearly 20:1) a weight-conscious design, focused primarily on withstanding axial stresses (e.g. thrust + drag), and allow for some flexing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if it bends this much in a breeze, what about in flight? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/20944/12102&quot;&gt;This answer&lt;/a&gt; references the detailed, knowledgable &lt;a href=&quot;https://www.flightclub.io/results/?code=SS10&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flightclub&lt;/a&gt; simulation of a recent Falcon 9 launch. At one point when the speed of the rocket (wrt Earth's rotating frame presumably) is about 1000 m/s and the pressure must be roughly 0.06 or 0.07 bar at 22km altitude, the angle of attack is estimated to be 4.6 degrees. That would present a crushing-type force, but would it tend to bend the rocket since the fairing is so much wider than the body?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just how much can tall skinny rockets bend? (roughly, safely)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sk4jZ.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sk4jZ.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2018-01-03T00:47:21.237" Title="Just how much can tall skinny rockets bend? (roughly, safely)" Tags="|launch|design|aerodynamics|lift|max-q|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="20987" PostTypeId="1" CreationDate="2017-04-05T15:55:29.967" Score="7" ViewCount="677" Body="&lt;p&gt;I've seen lots of flight profiles that give Max Q during ascent. Around 33 to 35 kilopascals, if memory serves. But I have been having a hard time finding Max Q during descent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the recent reuse of a SpaceX booster there has been some talk of also reusing an upper stage. The re-entry dynamic pressure and temperature seem to me to be the major obstacles barring re-use. But I don't really know the Max Q of an upper stage re-entering the atmosphere. Given that an upper stage will have a delta V budget of around 6 or 7 km/s, the dry mass fraction is less than that of a re-entering capsule. If the dry mass fraction is 5%, it would be difficult to give the re-entering stage robust structure and thermal protection.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-04-05T20:28:49.460" Title="What will Max Q for descent be?" Tags="|reentry|aerobraking|max-q|" AnswerCount="1" CommentCount="2" ClosedDate="2020-02-19T20:56:06.143" ContentLicense="CC BY-SA 3.0" />
  <row Id="20988" PostTypeId="1" CreationDate="2017-04-05T16:12:55.700" Score="3" ViewCount="67" Body="&lt;p&gt;What happen to the LM Ascent stage after it was jettison   I see from the timeline it was while still orbiting the moon. Did it's orbit decay and it crashed onto the moon??&lt;/p&gt;&#xA;" OwnerUserId="18906" LastActivityDate="2017-04-05T16:12:55.700" Title="What happen to the LM ascent stage" Tags="|orbital-mechanics|" AnswerCount="0" CommentCount="1" ClosedDate="2017-04-06T00:13:04.267" ContentLicense="CC BY-SA 3.0" />
  <row Id="20989" PostTypeId="2" ParentId="20961" CreationDate="2017-04-05T16:34:16.423" Score="11" Body="&lt;p&gt;Using my &lt;a href=&quot;http://clowder.net/hop/railroad/Hohmann.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann spreadsheet&lt;/a&gt; you can get an idea of approximate delta V and trip times. The spreadsheet assumes circular, coplanar orbits. So it's ballpark, not exact estimates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEO to Mars capture: 4.3 km/s, .71 years&lt;br&gt;&#xA;LEO to Uranus capture: 8.5 km/s, 16 years&lt;br&gt;&#xA;LEO to Neptune capture: 8.6 km/s, 30.6 years &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a capture orbit I assume a 300 km altitude periapsis and an apo-apsis at the edge of the Sphere of Influence (SOI). This is a loosely bound capture orbit, but apo-apsis can be lowered by passing through the planet's upper atmosphere at periapsis. However the periapsis speed of either Neptune's or Uranus' capture orbit is more than 20 km/s so the periapsis drag passes may be tricky. The scale height of the ice giants is much smaller than Mars' scale height so there is need for greater precision if using their atmospheres to shed velocity. Go a little too low and the orbiter won't come back out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i62C8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i62C8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also the sunlight is much less. At Mars distance, sunlight is 43% what we have on earth. Uranus .3%, Neptune .1%. So we'd need nuke power sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;With Jupiter assist&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the comments LocalFluff brought up the possibility of a Jupiter gravity assist. And in fact this is often used for missions to the outer solar system. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEO to Trans Jupiter Insertion: 6.3 km/s&lt;br&gt;&#xA;Trip time to Jupiter: 2.73 years&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So just the burn to get us on the way to Jupiter is still substantially more than Mars. I'm not sure what the delta V budget would be after the Jupiter gravity assist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 2.73 years it takes to get from earth to Jupiter need to be added to trip times from Jupiter to ice giant. If it's a Hohmann path, trip times from Jupiter are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter to Uranus: 21 years&lt;br&gt;&#xA;Jupiter to Neptune: 37 years  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So total trip time to Uranus would be about 24 years. Total trip time to Neptune would be 40 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while a Jupiter gravity assist may help with the big delta V budgets, it makes the long trip times even longer.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-04-06T14:51:15.977" LastActivityDate="2017-04-06T14:51:15.977" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="20990" PostTypeId="1" AcceptedAnswerId="20992" CreationDate="2017-04-05T18:25:35.103" Score="3" ViewCount="433" Body="&lt;p&gt;According to SpaceX's website, it does not announce a NROL-76 launch in mid-April. However according to other sources such as the NRO and spaceflightnow announce that a Falcon 9 will launch the satellite April 16. Since the NRO satellites are classified, and SpaceX is a private company, will the launch be shown live?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2017-04-05T20:44:24.770" Title="SpaceX NROL-76?" Tags="|spacex|falcon-9|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20991" PostTypeId="2" ParentId="20987" CreationDate="2017-04-05T20:11:30.163" Score="7" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.flightclub.io/results/?code=SS10&quot; rel=&quot;noreferrer&quot;&gt;https://www.flightclub.io/results/?code=SS10&lt;/a&gt; , reentry Q for flights like SES-10 peak at almost 92kPa at T+434 sec, about 3x the ascent max Q. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reentry prep burn brings velocity down to about 1500m/s, about 1/5 of orbital speed, so it doesn't need the kind of thermal protection an orbital capsule would. An orbital capsule also comes in much more shallowly, so spends more time at substantially lower Q.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-04-05T20:28:49.460" LastActivityDate="2017-04-05T20:28:49.460" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20992" PostTypeId="2" ParentId="20990" CreationDate="2017-04-05T20:20:55.293" Score="3" Body="&lt;p&gt;The private company United Launch Alliance live broadcast the launch of NROL-79 on March 1, 2017.  (However, the coverage ended at payload fairing jettison).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/vdaqQ8FaXX0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there would seem to be no reason for SpaceX not to, at least for the same portion of the launch.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-04-05T20:20:55.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20993" PostTypeId="2" ParentId="20990" CreationDate="2017-04-05T20:44:24.770" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=40328.msg1659502#msg1659502&quot; rel=&quot;noreferrer&quot;&gt;According to a journalist and NSF writer ChrisGebhardt&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX confirms this will be an LZ-1 landing.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Webcast will cut off launch coverage as usual for NRO missions (like&#xA;  we see with ULA), but will continue for booster landing coverage.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For the SpaceX website - in the past the &quot;upcoming launch&quot; part of the manifest was often shown/updated just few days before the planned launch date. They seem to not show any info when there is still large probability of date change.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2017-04-05T20:44:24.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="20994" PostTypeId="2" ParentId="4561" CreationDate="2017-04-05T21:48:53.250" Score="6" Body="&lt;p&gt;I was stationed at NAS Keflavik, Iceland in 1994.  I remember reading in one of my &quot;welcome aboard&quot; pamphlets that runway 11 was a designated alternate or emergency landing strip for the STS.  I vaguely remember something about it being highly unlikely because the STS would have to be in a polar orbit versus an equatorial.  I will try to dig through old things to find it.&lt;/p&gt;&#xA;" OwnerUserId="19206" LastEditorUserId="6816" LastEditDate="2017-04-05T21:57:00.150" LastActivityDate="2017-04-05T21:57:00.150" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20997" PostTypeId="1" AcceptedAnswerId="21013" CreationDate="2017-04-06T01:03:00.217" Score="3" ViewCount="1035" Body="&lt;p&gt;I was reading &lt;a href=&quot;https://space.stackexchange.com/questions/20987/what-will-max-q-for-descent-be&quot;&gt;this question about Falcon 9's re-entry Max Q&lt;/a&gt;, which states that the Falcon 9 has a re-entry pressure &lt;strong&gt;3 times higher&lt;/strong&gt; than during launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was shocked, because SpaceX takes precautions against launch &lt;a href=&quot;https://en.wikipedia.org/wiki/Max_Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;Max Q&lt;/a&gt; &amp;mdash; specifically a conical fairing. But during re-entry the leading surface is less than aerodynamic &amp;mdash; the 9 engine bells are basically sails. I'm surprised they don't get torn off during re-entry, but not only are they still there, they're usable!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does SpaceX do to protect the Falcon 9's engines during re-entry?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="6816" LastEditDate="2017-04-06T20:38:04.993" LastActivityDate="2017-04-06T20:38:04.993" Title="How does Falcon 9 mitigate re-entry pressure?" Tags="|spacex|falcon-9|reentry|high-pressure-environment|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="20998" PostTypeId="2" ParentId="20997" CreationDate="2017-04-06T01:55:50.333" Score="3" Body="&lt;p&gt;SpaceX has been using supersonic retro-propulsion to slow down the stages enough to not cause significant damage to the engines. This is the same method that will be used to land the Red Dragon probes on the Martian surface. In order for supersonic retro-propulsion to work you need to have very high chamber pressures to effectively push  the air out of the combustion chamber. Since the engines are in front during reentry, this means the brunt of reentry is in the engines. But they are made to withstand high temperatures and pressures. The reentry burn SpaceX uses slows the stage down enough so that no extensive damage happens to the stage. &lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2017-04-06T01:55:50.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21000" PostTypeId="1" CreationDate="2017-04-06T10:41:07.537" Score="4" ViewCount="562" Body="&lt;p&gt;Probes to the outer Solar system use a Jupiter flyby for gravity assist, all six such missions thus far have. While flying near a gravity well one can enjoy the Oberth effect which multiplies the acceleration per unit propellant consumed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have any spacecrafts using a Jupiter gravity assist fired their (small) rockets in order to profit from the Oberth effect?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does the trade off look like between using all fuel at Earth to get into the Jupiter trajectory, versus saving the upper stage and much of its fuel for a big Oberth bonus blast at perijove? New Horizons passed by Jupiter only 13 months after launch, and took another 101 months to reach Pluto. Would a slower fuel laden first leg of the trip with a big burn at Jupiter have shortened its total travel time to Pluto?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2022-01-26T12:37:35.123" Title="Is it profitable to save fuel for the Oberth effect during a Jupiter gravity assist?" Tags="|mission-design|gravity-assist|oberth-maneuver|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21001" PostTypeId="5" CreationDate="2017-04-06T11:16:01.813" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2017-04-06T11:16:01.813" LastActivityDate="2017-04-06T11:16:01.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21002" PostTypeId="4" CreationDate="2017-04-06T11:16:01.813" Score="0" Body="Titania is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1,578 kilometres (981 mi)." OwnerUserId="16590" LastEditorUserId="16590" LastEditDate="2017-04-06T12:14:09.817" LastActivityDate="2017-04-06T12:14:09.817" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21003" PostTypeId="5" CreationDate="2017-04-06T11:31:04.180" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2017-04-06T11:31:04.180" LastActivityDate="2017-04-06T11:31:04.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21004" PostTypeId="4" CreationDate="2017-04-06T11:31:04.180" Score="0" Body="Triton is the largest natural satellite of the planet Neptune. At 2,700 kilometres (1,700 mi) in diameter, it is the seventh-largest moon in the Solar System." OwnerUserId="16590" LastEditorUserId="16590" LastEditDate="2017-04-06T12:14:14.967" LastActivityDate="2017-04-06T12:14:14.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21005" PostTypeId="1" CreationDate="2017-04-06T11:59:49.067" Score="18" ViewCount="2768" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/users/2752/david-hammen&quot;&gt;David Hammen&lt;/a&gt; wrote in &lt;a href=&quot;https://space.stackexchange.com/questions/20961/why-there-is-not-a-single-orbiter-for-uranus-or-neptune#comment57031_20962&quot;&gt;a comment&lt;/a&gt; on &lt;a href=&quot;https://space.stackexchange.com/a/20962/415&quot;&gt;Why there is not a single orbiter for Uranus or Neptune?&lt;/a&gt; that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The two Voyager satellites still receive multiple Deep Space Network contacts per week, each of which costs several thousands of dollars to tens of thousands of dollars. Then there are people. The most recent Voyager Space Flight Operations Schedule lists eleven JPL employees. I'd be shocked if a fully-loaded FTE JPL engineer costs less than $250K/yr. Plus there are researchers in other NASA centers and in universities who are supported by the Voyager budget.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Let's go with $10,000 per contact for the Voyagers, via the Deep Space Network, to have a figure in the middle of that range. (If someone has a better estimate, by all means feel free to use that for an answer.) At the low bit rates involved, a contact might last for a few hours. I realize that the equipment is awfully expensive and probably takes a small army of people to maintain, but that still boils down to a few thousand dollars per hour of radio time -- pretty close to &lt;em&gt;a dollar per second&lt;/em&gt;. Note that this is &lt;em&gt;in addition to&lt;/em&gt; the people actually working to directly support the Voyager missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What makes using the DSN so expensive?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rough costs breakdown would likely make a perfect answer to this question, but a general reasoning could pass as well if it is well-reasoned enough.&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2018-06-29T16:56:51.350" Title="What makes time on the DSN so expensive?" Tags="|cost|deep-space-network|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21006" PostTypeId="1" CreationDate="2017-04-06T13:33:21.463" Score="2" ViewCount="1468" Body="&lt;p&gt;I am researching satellite technology, and in particular I am looking at the rise and implications of smallsat technology. As far as I know, there are currently no satellites beyond LEO that would be categorized as smallsats. Here's a &lt;a href=&quot;http://www.ucsusa.org/nuclear-weapons/space-weapons/satellite-database#.WOZDFEyLRD8&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt; to a list of all or most satellites currently in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia defines smallsats as anything under 500 kg while &lt;a href=&quot;https://www.nasa.gov/content/what-are-smallsats-and-cubesats&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; puts the cutoff at 180 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Earth-based orbits, it appears that the farther away the satellite, the larger it is. I am just wondering if this will always be the case.&lt;/p&gt;&#xA;" OwnerUserId="19212" LastEditorUserId="19212" LastEditDate="2017-04-06T15:17:41.707" LastActivityDate="2017-04-10T15:03:36.180" Title="Is it possible for small satellites to orbit in MEO or GEO?" Tags="|artificial-satellite|communication-satellite|radio-communication|laser|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21008" PostTypeId="1" CreationDate="2017-04-06T14:08:30.533" Score="0" ViewCount="150" Body="&lt;p&gt;In 40 years why has no country sent a manned mission to the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am aware the US retired the types of rockets that could get a man to the moon but why would they do that and why wouldn't they or any other country in the world develop a similar rocket and send someone to the Moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If finances are the problem, why weren't they a problem 40 years ago?&lt;/p&gt;&#xA;" OwnerUserId="19214" LastEditorUserId="238" LastEditDate="2017-04-06T14:43:15.777" LastActivityDate="2017-04-06T14:43:15.777" Title="Why haven't we gone back to the moon?" Tags="|the-moon|history|" AnswerCount="0" CommentCount="4" ClosedDate="2017-04-06T14:58:39.573" ContentLicense="CC BY-SA 3.0" />
  <row Id="21009" PostTypeId="2" ParentId="21005" CreationDate="2017-04-06T14:37:43.273" Score="23" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I realize that the equipment is awfully expensive and probably takes a small army of people to maintain, but that still boils down to a few thousand dollars per hour of radio time ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The cost in 2017 dollars ranges from \$954 per hour of contact for use of a 34 meter High-Speed Beam waveguide antenna to \$4770 per hour of contact for use of a 70 meter antenna, plus charges for an additional hour for setup and shutdown costs. The specific formulae for standard support (see &lt;a href=&quot;https://deepspace.jpl.nasa.gov/files/6_NASA_MOCS_2014_10_01_14.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA's Mission Operations and Communications Services&lt;/a&gt; for details) is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{aligned}&#xA;A_f &amp;amp;= R_b\, A_w\,(0.9 + F_c/10) \\&#xA;C &amp;amp;= A_f\, (1 + T)&#xA;\end{aligned}$$&#xA;where&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$A_F$ is the hourly aperture fee;&lt;/li&gt;&#xA;&lt;li&gt;$R_b$ is the basic hourly fee, \$1057 (2010 dollars);&lt;/li&gt;&#xA;&lt;li&gt;$A_w$ is the aperture weighting:&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;0.8 for a 34 meter High-Speed Beam waveguide antenna,&lt;/li&gt;&#xA;&lt;li&gt;1.0 for some other 34 meter antenna,&lt;/li&gt;&#xA;&lt;li&gt;2.0 for two 34 meter antennae,&lt;/li&gt;&#xA;&lt;li&gt;3.0 for three 34 meter antennae,&lt;/li&gt;&#xA;&lt;li&gt;4.0 for four 34 meter antennae, and&lt;/li&gt;&#xA;&lt;li&gt;4.0 for a 70 meter antennae;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;$F_c$ is the number of contacts per week;&lt;/li&gt;&#xA;&lt;li&gt;$C$ is the cost per contact; and&lt;/li&gt;&#xA;&lt;li&gt;$T$ is the contact duration in hours.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For example, four two hour long contacts per week with a 70 meter antenna over the course of a year costs \$3.43 million in 2010 dollars, or \$3.87 million in 2017 dollars. Changing those four two hour long contacts per week to one eight hour long contact per week reduces the costs to \$1.98 million in 2010 dollars, or \$2.23 million in 2017 dollars. More frequent short duration contacts are penalized twice, once with the factor $0.9+F_c/10$, and once again with the additional hour (45 minute setup and 15 minute shutdown) per contact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why so high? This is very expensive and highly customized equipment. There aren't very many 70 meter antenna in the world. NASA has switched to full cost accounting, which means that projects that use the DSN have to pay for their share of the use of this very expensive equipment. This sharing includes maintenance and upgrade costs.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="12483" LastEditDate="2018-06-29T16:56:51.350" LastActivityDate="2018-06-29T16:56:51.350" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="21010" PostTypeId="2" ParentId="17885" CreationDate="2017-04-06T15:04:49.837" Score="3" Body="&lt;h2&gt;A multi-part answer to a multi-part question.&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;First, a caveat:&lt;/strong&gt; much of the detailed domain knowledge in this field falls under export control laws, so publicly available citations are not available to support every statement made.  This happens to be my particular line of work, so I will share as much as I feel comfortable.&lt;/p&gt;&#xA;&lt;h3&gt;On paint chips being more lethal than rifle rounds&lt;/h3&gt;&#xA;&lt;p&gt;The truth is more complicated.&lt;/p&gt;&#xA;&lt;p&gt;It very much depends on the configuration of the shield.  The penetrating capability of an incoming projectile does not vary monotonically with its velocity.  For most spacecraft shield configurations, if you were to somehow create a graph of &amp;quot;lethality&amp;quot; vs. velocity for a given particle size, there would be very pronounced peaks at a number of different velocities due to changes in the physical interaction between the projectile and the shield.  For a Whipple shield, a simple paint flake at any velocity is going to be far less lethal than a rifle round.&lt;/p&gt;&#xA;&lt;p&gt;Further complicating this, the behavior of impact at interplanetary velocities is not well characterized.  The best hypervelocity impact test labs in the world still cannot shoot projectiles at velocities anywhere near the expected encounter velocities for a spacecraft in interplanetary space. Extrapolations and engineering judgment are all that is left in these velocity regimes.&lt;/p&gt;&#xA;&lt;h3&gt;On repeat impacts on Whipple shields&lt;/h3&gt;&#xA;&lt;p&gt;The risk exists, but it is generally just accepted.&lt;/p&gt;&#xA;&lt;p&gt;Space vehicle shielding design tends to look at the risk to broad areas of the spacecraft.  Impacts into Whipple shields follow what I call the &amp;quot;Wile E. Coyote&amp;quot; effect: they tend to leave neat holes in the shield of similar-ish size to the incoming particle.  The risk of impact more or less scales linearly with vulnerable area.  If the risk of a lethal strike to a large vehicle is, say, 1 in 10, the risk of a lethal strike based on &amp;quot;threading the needle&amp;quot; through an existing hole is going to be orders of magnitude smaller -- probably tens of thousands of times less likely.&lt;/p&gt;&#xA;&lt;h3&gt;On replaceability of debris shields&lt;/h3&gt;&#xA;&lt;p&gt;If it is a design requirement, then it can be done.&lt;/p&gt;&#xA;&lt;p&gt;For an interplanetary spacecraft, it comes with an added complication of needing to have spares or manufacturing capability on board, as resupply is out of the question, and it would require some way to install it, either through extravehicular activity or robotics.&lt;/p&gt;&#xA;&lt;h3&gt;On radiator design&lt;/h3&gt;&#xA;&lt;p&gt;All of these questions fall under &amp;quot;it depends.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;If protecting for a meteoroid or debris strike is a design priority, then it can be designed as such.  There are multiple design avenues available, but the selection depends on how the risk of a strike is weighed against other factors like heat transfer capability, mass, structural strength, etc.&lt;/p&gt;&#xA;&lt;h3&gt;On solar arrays&lt;/h3&gt;&#xA;&lt;p&gt;This is something I have direct test experience with: in general, individual strikes to solar cells are essentially undetectable at the system level.  Instead, they are a contributing factor (among many) to a slow -- but steady -- loss of power production capability over time.&lt;/p&gt;&#xA;&lt;p&gt;Where power production does present a vulnerability to strikes, it is overwhelmingly in &amp;quot;weak points&amp;quot; in the design: bus lines, power conditioning hardware, etc.  All of these can be protected to keep the risk to an acceptable level.  The degree to which risk exists, and the consequence of a strike to any specific piece of hardware is going to be a function of the architecture and specific detail design of that particular system.&lt;/p&gt;&#xA;&lt;p&gt;The same statements from before apply to the repairability as well: if it's designed for, then yes.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-04-06T15:04:49.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21011" PostTypeId="1" AcceptedAnswerId="21012" CreationDate="2017-04-06T15:37:51.920" Score="9" ViewCount="719" Body="&lt;p&gt;In the &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-68&quot; rel=&quot;noreferrer&quot;&gt;wikipedia article on STS-68&lt;/a&gt;, in the &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-68#Landing&quot; rel=&quot;noreferrer&quot;&gt;landing section&lt;/a&gt;, it says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nose wheel stop at 13:02:21 EDT. Wheel stop at 13:03:08 EDT&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;How can the nosewheel stop time be 47 seconds before wheel stop (which I assume refers to mainwheel stop?) What do these two times refer to?&lt;/p&gt;&#xA;" OwnerUserId="19216" LastActivityDate="2017-04-07T03:01:10.173" Title="What are the &quot;nosewheel stop&quot; and &quot;wheel stop&quot; times in this Wikipedia article?" Tags="|landing|space-shuttle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21012" PostTypeId="2" ParentId="21011" CreationDate="2017-04-06T15:51:31.007" Score="13" Body="&lt;p&gt;It should read &quot;nose wheel &lt;em&gt;touchdown&lt;/em&gt;&quot; instead of nose wheel stop. The Wikipedia article for a different flight, &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-70#Landing&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-70&lt;/a&gt;, says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nose gear touchdown occurred at 8:02:11 am EDT with wheels stop at 8:02:57 am.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That is, 46 seconds from nosewheel down to full stop. I found another flight with 48 seconds nose-down to full-stop time, so the STS-68 time given seems consistent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I presume that different pedants have different definitions of flight duration, hence the calling out of two different time stamps for &quot;landing&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-04-07T03:01:10.173" LastActivityDate="2017-04-07T03:01:10.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21013" PostTypeId="2" ParentId="20997" CreationDate="2017-04-06T19:37:17.363" Score="1" Body="&lt;p&gt;SpaceX does a series of burns during first stage landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a boostback burn, done to initially cancel the forward velocity, and if going back to RTLS at LZ-1, then to actually change direction back to the launch site.  On ASDS landings this is not usually done. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then during reentry itself, they initiate a reentry burn. This is done to slow the stage down, enough that the stage can survive entering the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus they run the engines near the point at which they need to slow down, and as the engines are running they are protected from the air incoming due to the pressure they put out as running engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now it is worth noting only 1 or 3 engines are used at this point, but the other engines are clearly strong enough to survive them.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-04-06T19:37:17.363" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21014" PostTypeId="2" ParentId="21006" CreationDate="2017-04-06T23:59:54.327" Score="3" Body="&lt;p&gt;&lt;strong&gt;This answer&lt;/strong&gt; mainly addresses the last part of the question &quot;&lt;em&gt;For Earth-based orbits, it appears that the farther away the satellite, the larger it is. I am just wondering if this will always be the case.&lt;/em&gt;&quot;  (NB at the time of answering the question title was &quot;&lt;em&gt;Is it possible for small satellites to orbit in MEO or GEO&lt;/em&gt;&quot; which has the simple answer of yes.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simple answer is it is likely but not inevitable that satellites launched to MEO and GEO may be larger. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Options&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The starting premise is that higher orbits require more energy to get to from the Earth's surface. Thereafter, talking in typical terms: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;For &lt;strong&gt;delivery direct&lt;/strong&gt; by the launch vehicle, this will cost more per&#xA;kg than for a LEO delivery, or the small satellite will have to find&#xA;a &quot;ride-share opportunity&quot;.  Small satellites are often small&#xA;because they are budget-constrained, or at least this has often been&#xA;true in the past, so the ride-share approach has been the only real&#xA;option for small satellites.  There is no reason in principle why a 1kg cubesat could not have a direct rideshare delivery to MEO, GTO or GEO.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;For &lt;strong&gt;delivery to an intermediate point&lt;/strong&gt; the small satellite will need&#xA;its own propulsion system. For a small satellite to have some&#xA;potentially significant fraction of the mass is taken by propellant,&#xA;propulsion hardware, or solar arrays means that the satellite will&#xA;have less room for the primary payload.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Presently, plausible targets direct deliveries as rideshares are most likely to be GTO because of the large number of commercial missions there. MEO seems plausible with the traffic to deliver navigation satellites and there are even a small number of direct deliveries to GEO, usually on US and Russian government missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Rideshare examples&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Examples of missions based on direct or near-direct rideshares:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The STRV 1a and 1b satellites (~50kg each) were deliberately launched on a rideshare into GTO as the target orbit so that they could measure the radiation environment and test experimental equipments. See &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/strv-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; for further details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Spirale satellite (120 kg) was also launched into GTO on a rideshare but manoeuvered slightly to increase the perigee from 200km to 600km to prolong the mission life.  The mission was an Early Warning demonstrator and its not entirely clear why it was based in near GTO, it seems quite possible that it was intended to be the forerunner of a GEO mission and used the apogee passes to obtain a GEO like view of the Earth. See &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/spirale&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; for further details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Example: launch to GTO then manoeuvre to GEO&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a rough guide, a mission launched to GTO but targeted to manoeuvre to GEO under the power of the satellite could expect:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;half of its mass to be taken up by propellant if it used a high performance, e.g. 500N, chemical thruster. Such a thruster would probably have a mass of 5kg itself, a 50 - 100 litre propellant tank would be the same again.&lt;/li&gt;&#xA;&lt;li&gt;if a smaller chemical thruster were used, say 10N, its propellant efficiency and its manoeuvre efficiency would be worse still (scaling down of thrusters and longer thrust arcs respectively)&lt;/li&gt;&#xA;&lt;li&gt;if an electric propulsion system were to be used then the propellant mass could be very much reduced but instead a large solar array would be required - 2KW and upwards for the current commercial arcjets and plasma thrusters. Some scaling down is possible at the cost of longer transfer time. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It is entirely possible to do some rough calculations for the manouevres and equipment mass for such a mission or for other orbits though you can see there is a lot to consider; expect there to be a bit of a learning curve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Short of actually stepping through such calculations its hard to see how this could add up looking like a large payload mass fraction for even a 50 - 100kg satellite, let alone a nanosat / cubesat.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-04-10T15:03:36.180" LastActivityDate="2017-04-10T15:03:36.180" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21016" PostTypeId="2" ParentId="21006" CreationDate="2017-04-07T01:10:11.887" Score="3" Body="&lt;p&gt;Addendum: Apollo 15 deployed the Particles and Fields Satellite into lunar orbit.  It returned data from August 1971 to January 1973. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It weighed 36.3 kilograms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TmCxL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TmCxL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1971-063D&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-04-07T01:10:11.887" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21017" PostTypeId="1" CreationDate="2017-04-07T05:01:00.720" Score="1" ViewCount="170" Body="&lt;p&gt;I am a newbie when it comes to orbital mechanics. The velocity of a spacecraft is given by a sum of two types of velocities. But I don't understand the difference between both velocities: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V = V_{кр}+\Delta V$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eIa81.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eIa81.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could somebody help me understand?&lt;/p&gt;&#xA;" OwnerUserId="10215" LastEditorUserId="10215" LastEditDate="2017-04-07T05:06:37.667" LastActivityDate="2017-04-07T14:57:04.963" Title="Why is the velocity of a satellite the result of a sum of two velocities? (clarifying img included)" Tags="|artificial-satellite|physics|moons|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21018" PostTypeId="2" ParentId="21017" CreationDate="2017-04-07T11:05:38.737" Score="4" Body="&lt;p&gt;My translation:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In some cases, one needs to express semi-major axis and eccentricity&#xA;  of an elliptic orbit in terms of parameters (speed and radius) of the&#xA;  circular orbit passing through a certain point of the elliptic orbit&#xA;  (Fig 1.3). Let's represent the spacecraft's velocity as a sum&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$V = V_{circ} + \Delta V$,&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;where $V_{circ}$ is the velocity of movement along the circular orbit&#xA;  of radius $r$, determined by formula (1.4);&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;$\Delta V$ is the velocity relative to the circular, required to&#xA;  attain the actual velocity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, as the text says, for a given point of a given elliptic orbit, $V_{circ}$ or $V_{кр}$ is the velocity that a circular orbit that passes through this point (and lies in the same plane as the elliptic orbit, presumably) has at this point. And $\Delta V$ is the difference between $V$, which is the velocity of the elliptic orbit at this point, and $V_{circ}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's probably explained later in the book what they need to represent $V$ as such a sum for.&lt;/p&gt;&#xA;" OwnerUserId="12448" LastEditorUserId="12448" LastEditDate="2017-04-07T14:57:04.963" LastActivityDate="2017-04-07T14:57:04.963" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21019" PostTypeId="2" ParentId="21000" CreationDate="2017-04-07T12:05:05.063" Score="2" Body="&lt;p&gt;When it comes to the orbital mechanics of this scenario, one thing to note is that when it comes to oberth effect and raising the orbit around the Sun, it's the total velocity relative to the Sun that matters, i.e. the Earth orbits the Sun at 30km/s, and the probe orbits the Earth at 8km/s, giving 38km/s to work with, the bare minimum ejection burn to intercept Jupiter then adds 6.5km/s and the probe is up to 44.5km/s, now the question is are you better off burning on top of that 44.5km/s, or burning at Jupiter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter orbits the Sun at 13km/s and the probe should swing by at about 60km/s if it gets as close to Jupiter as possible, &lt;em&gt;potentially&lt;/em&gt; a combined velocity of 73km/s. The tricky part is that at closest approach to Jupiter the probe probably won't be travelling in exactly the right direction for a prograde burn to enjoy maximum oberth effect with respect to the Sun - at Earth precise timing can arrange this, but when you're swinging by one planet and need to actually encounter a third planet there are constraints on the first encounter which will result in the oberth effect not being maximized, unless you wait a long time for the planets to line up in exactly the right way and when it comes to long period inclined and eccentric orbits like that of Pluto, that's going to be a long wait.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this is not enough to rule out the possibility that it could be more effective to do some of the burn in Jupiter's gravity well, it's just to say you get an awful lot of oberth effect already by virtue of doing the burn deeper in the Sun's gravity well. My hunch is that if all you care about is maximizing heliocentric velocity you'd be slightly better off doing the burn at Jupiter, but if you wish to actually encounter a third planet you'd tend to be equally well served by completing the burn at Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there are also the practical constraints: completing the burn at Earth uses only the one stage for the entire ejection burn. A Jupiter burn would require an additional solid stage or a reliably restartable engine and a no leaks guarantee. And if the burn is in any way messed up you've just lost your best option for cheap course corrections, which is fine-tuning the encounter with Jupiter.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2017-04-07T12:05:05.063" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21020" PostTypeId="2" ParentId="17913" CreationDate="2017-04-07T14:28:33.210" Score="7" Body="&lt;p&gt;To answer the question as it is asked: the &quot;artist&quot; would most likely be one of the SPHERES PIs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the patterns, aesthetically pleasing though they may be, have a specific purpose other than &quot;art&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are part of the &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/869.html&quot; rel=&quot;noreferrer&quot;&gt;VERTIGO&lt;/a&gt; upgrade to the SPHERES payload, which is intended to test computer vision algorithms for maneuvering around an unknown target.  The patterns are &quot;textured stickers&quot; used to aid the computer vision algorithm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the image below (from the link above) for a description.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eihjG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eihjG.jpg&quot; alt=&quot;SPHERES VERTIGO overview&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2017-04-07T14:34:26.993" LastActivityDate="2017-04-07T14:34:26.993" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21021" PostTypeId="1" AcceptedAnswerId="21023" CreationDate="2017-04-07T16:36:38.890" Score="16" ViewCount="3437" Body="&lt;p&gt;That seems a bit counter-intuitive, because in an orbit above the belt, the satellite would eventually &lt;strike&gt;lose&lt;/strike&gt; gain velocity, so its orbit would intersect the orbital plane of the geostationary satellites possibly creating a collision hazard. To be re-orbited, a satellite has to have functional attitude control&lt;a href=&quot;https://en.wikipedia.org/wiki/Graveyard_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt; [Wikipedia]&lt;/a&gt;, which would also allow to &lt;strike&gt;lower&lt;/strike&gt; increase the velocity to decrease the altitude instead. So, why isn't the so-called graveyard orbit below the geostationary altitude?&lt;/p&gt;&#xA;" OwnerUserId="19223" LastEditorUserId="19223" LastEditDate="2017-04-09T11:08:51.893" LastActivityDate="2017-04-09T11:08:51.893" Title="Why are obsolete geostationary satellites re-orbited above the geostationary belt?" Tags="|artificial-satellite|geostationary|communication-satellite|geosynchronous|orbital-decay|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21022" PostTypeId="2" ParentId="20964" CreationDate="2017-04-07T16:37:37.647" Score="11" Body="&lt;p&gt;The existing answers are correct that the same signal that ignites the SRBs also blows the nuts that hold the restraining bolts in place, so the answer is generally, no, it's not possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, for some crazy reason, the nuts didn't blow, but the SRBs did ignite, things are a little different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://www.reddit.com/r/space/comments/jov5g/stephen_colbert_got_the_wrong_nut/&quot; rel=&quot;noreferrer&quot;&gt;this engineer on reddit&lt;/a&gt;, who apparently helped design the aforementioned frangible nuts, the nuts are made of &lt;a href=&quot;https://en.wikipedia.org/wiki/Inconel&quot; rel=&quot;noreferrer&quot;&gt;Inconel-718&lt;/a&gt;. &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster#Hold-down_posts&quot; rel=&quot;noreferrer&quot;&gt;Each SRB is held down by 4 of these.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/space/comments/jov5g/stephen_colbert_got_the_wrong_nut/c2e2tz7/&quot; rel=&quot;noreferrer&quot;&gt;According to the same engineer&lt;/a&gt;, the nuts and studs are actually already preloaded with about 750,000 pounds of force each. About an eighth of the (thrust - weight) of the stack would then be added to this on each post if the SRBs ignited without blowing the nuts. The aforementioned engineer performed a back-of-the-envelope calculation showing that the nuts would have a failure load around 1.65 million pounds each. Since each bolt would only have in the ballpark of 1 million pounds of force on it (including the preloaded tension,) &lt;strong&gt;the bolts should be able to &lt;em&gt;initially&lt;/em&gt; hold the stack down when the SRBs light.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are a couple of remaining problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the weight of the stack is rapidly decreasing due to fuel burn, meaning that the force on the bolts is rapidly increasing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, and probably more importantly, the exhaust of rocket motors is &lt;em&gt;really hot&lt;/em&gt;. While I haven't seen an exact source for the SRB exhaust specifically, &lt;a href=&quot;https://www.nasa.gov/returntoflight/system/system_SSME.html&quot; rel=&quot;noreferrer&quot;&gt;NASA lists the combustion temperature of the main engines&lt;/a&gt; at up to &lt;strong&gt;3,300 C!&lt;/strong&gt; However, &lt;a href=&quot;http://www.hightempmetals.com/techdata/hitempInconel718data.php#3&quot; rel=&quot;noreferrer&quot;&gt;Inconel-718 will melt around 1,300 C&lt;/a&gt;. So, while the nuts and studs should initially hold the stack down, they'll almost certainly start heating up very quickly from the SRB exhaust and fail soon afterwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I'm not sure about Inconel-718 specifically, in general, metals lose a lot of their strength well before hitting their actual melting point, so it's not likely that the bolts would be able to hold the stack down for very long. &lt;strong&gt;Even if they withheld all of their strength prior to melting, given the amount of energy and temperatures involved, it's quite likely that the nuts would melt well before the SRB burnout and the stack would leave the stand.&lt;/strong&gt; No guarantees about &lt;em&gt;how&lt;/em&gt; or &lt;em&gt;in what direction&lt;/em&gt; it leaves the stand, but it will leave it (provided the stack doesn't disintegrate first.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, it's also entirely possible that some other failure mode will destroy the stand and/or stack before the nuts fail. The stand is designed to withstand the energy of several seconds of the SSMEs burning and a very brief encounter with the exhaust of the SRBs, but it's not designed to be able to withstand and quickly dissipate the energy of the entire 2-minute-long SRB burn.&lt;/p&gt;&#xA;" OwnerUserId="2900" LastEditorUserId="2900" LastEditDate="2017-04-07T18:35:16.527" LastActivityDate="2017-04-07T18:35:16.527" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21023" PostTypeId="2" ParentId="21021" CreationDate="2017-04-07T17:50:41.983" Score="25" Body="&lt;p&gt;Orbits at the altitude of GEO are stable for very long times (millions of years). There is no significant decay of the orbital height due to some kind of drag, so the risk of these satellites interfering with working ones is close to zero. On the other hand, there are good reasons to store them above the belt and not below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The region below is used for maneuvers, e.g. to move a satellite from one longitude to another (ok, not a really good argument, because you could do this above as well). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One very important aspect: If satellites are in a lower orbit, they might interfere with communication links between GEO and Earth if the satellite passes by directly in front of another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two ways how satellites typically approach GEO: from a high GTO using conventional thrusters, so they move rather quickly and chance of collisions are low. On the other hand, there are some satellites using ion thrusters to reach their final orbit rather slowly and from lower orbits. Having to move them through the graveyard orbit would be a great risk due to the long travel time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional information: Have a look to this question: &lt;a href=&quot;https://space.stackexchange.com/questions/12528/why-is-the-ribbon-of-decommissioned-geosynchronous-satellites-skewed&quot;&gt;Why is the ribbon of decommissioned geosynchronous satellites skewed?&lt;/a&gt; - the graveyard orbits are not strictly above GEO, but also tilted by several degrees.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.360" LastActivityDate="2017-04-09T10:16:36.633" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21024" PostTypeId="1" AcceptedAnswerId="30785" CreationDate="2017-04-07T17:51:49.267" Score="5" ViewCount="749" Body="&lt;p&gt;From the Wikipedia article &lt;a href=&quot;https://en.wikipedia.org/wiki/Pioneer_P-30&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pioneer P-30&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;At an altitude of about 370 km (230 mi), the first stage separated from the second stage. When the second stage was ignited, telemetry showed abnormal burning, and the stage failed due to a malfunction in the oxidizer system. The vehicle was unable to achieve Earth orbit, re-entered, and was believed to have come down somewhere in the Indian Ocean. Signals were returned by the payload for 1,020 seconds after launch. The mission was designed to reach the Moon approximately 62 hours after launch. Although the mission was a failure, ground controllers fired Able VA's onboard liquid propellant hydrazine rocket engine — the first time that an onboard motor was fired on a space vehicle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The 2010 post &lt;a href=&quot;http://www.collectspace.com/ubb/Forum34/HTML/000140.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Topic: Able IV: World's first space engine (1959)&lt;/a&gt; at &lt;a href=&quot;http://www.collectspace.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Collect Space dot com&lt;/a&gt; give a sighting of an un-flown engine in 1970.&lt;/p&gt;&#xA;&lt;p&gt;Since some were launched, and one even ignited in space successfully, there must be, &lt;em&gt;or have been&lt;/em&gt; at least one on the bottom of the ocean (unless there were heroic recovery efforts).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Where are they now? Are there any currently on public display? Were any of these historical items ever recovered?&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;&lt;strong&gt;Bonus:&lt;/strong&gt;&amp;quot; Why is it notable that this started in space, and why is it different than second stages (including the one in this mission) starting in space? What is the &amp;quot;firstness&amp;quot; here?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In 1960, STL built two more hydrazine monopropellant space engines of the same design and designated Able V-A and Able V-B. These engines were launched September 25, 1960 (Pioneer P-30) and December 15, 1960 (Pioneer P-31). Unfortunately, both launches failed shortly after lift off. However, on the 17 minute Pioneer P-30 mission, the Able V-A engine got to see action. Despite a subpar second stage burn which prevented the Pioneer from reaching escape velocity, ground controllers were still able to fire the Able V-A engine. Therefore, STL’s Able IV/V space engine design “was the first to successfully ignite and operate in space.”&lt;/p&gt;&#xA;&lt;p&gt;Descendants of the Able IV engine are still in use today. Thanks in part to the pioneering efforts of STL, hydrazine engines can be found maneuvering the International Space Station, the Space Shuttle, communication satellites, and more.&lt;/p&gt;&#xA;&lt;p&gt;TRW presented the remaining Able IV space engine to a retiring VP in 1970.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;There are are &lt;a href=&quot;https://web.archive.org/web/20160306124110/http://firstspaceengine.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;archived goodies&lt;/a&gt; for reference material - see &lt;a href=&quot;https://web.archive.org/web/20160323193227/http://firstspaceengine.com/index.php?option=com_content&amp;amp;view=article&amp;amp;id=14&amp;amp;Itemid=15&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; also.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Able_IV_World%27s_First_Space_Engine.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Abel, the world's first space engine&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/glLOb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/glLOb.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-05T23:15:25.917" LastActivityDate="2021-09-05T23:15:25.917" Title="Where are the &quot;World's First Space Engines&quot; now?" Tags="|history|spaceflight-firsts|monopropellant|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="21025" PostTypeId="2" ParentId="21021" CreationDate="2017-04-07T17:52:13.613" Score="20" Body="&lt;p&gt;Actually, it makes a lot of sense to raise the orbit of end-of-life geostationary satellites: Coming from Earth you have to cross through a lower orbit to transfer from low earth orbit to a geostationary orbit but you don't have to go farther out than that (some transfer orbits do, but it's not a requirement). That means that a higher orbit has less risk of collision than a lower orbit with regard to the continuing use of the geostationary orbit belt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the altitude of geostationary satellites (~36,000 km) there is basically nothing to cause orbits to lower on a reasonable time scale. The atmosphere doesn't stop at a set altitude (the &lt;a href=&quot;https://en.wikipedia.org/wiki/Exosphere&quot; rel=&quot;noreferrer&quot;&gt;exosphere&lt;/a&gt; is apparently measurable up to 10,000+ km and estimated to reach half way to the moon), but the effect of drag is minuscule that far out. I didn't find a good reference for the longevity of geostationary orbits specifically, but &lt;a href=&quot;https://en.wikipedia.org/wiki/LAGEOS&quot; rel=&quot;noreferrer&quot;&gt;LAGEOS&lt;/a&gt; is only at 6,000 km and is estimated to not reenter for 8.4 million years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The intended &lt;a href=&quot;https://en.wikipedia.org/wiki/Graveyard_orbit&quot; rel=&quot;noreferrer&quot;&gt;graveyard orbit&lt;/a&gt;, as required by US regulations for geostationary satellites, is 300 km above the geo belt. That's not very far compared to the distance of the geo belt from Earth, but it's more than far enough to ensure that maneuvers in the geo belt of active satellites and perturbations from the sun and moon on the dead satellites don't cause them to cross paths.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="10132" LastEditDate="2017-04-07T17:58:03.690" LastActivityDate="2017-04-07T17:58:03.690" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21026" PostTypeId="1" CreationDate="2017-04-07T20:20:27.360" Score="1" ViewCount="95" Body="&lt;p&gt;My high school is planning to launch a CubeSat to the ISS, but we need a mission. A majority of students involved are interested in the engineering aspect of constructing it, and we don't know what it should accomplish once it's launched. &#xA;I've been looking through &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_CubeSats&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia's page on launched CubeSats&lt;/a&gt; and it seems most are focused on R&amp;amp;D or remote sensing. Any ideas would be greatly appreciated.  &lt;/p&gt;&#xA;" OwnerUserId="14062" LastActivityDate="2017-04-07T20:20:27.360" Title="What are relevant missions for CubeSats?" Tags="|artificial-satellite|mission-design|cubesat|remote-sensing|" AnswerCount="0" CommentCount="2" ClosedDate="2017-04-13T12:12:39.807" ContentLicense="CC BY-SA 3.0" />
  <row Id="21027" PostTypeId="2" ParentId="14101" CreationDate="2017-04-07T21:25:29.947" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://www.heavens-above.com&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Heavens above&lt;/em&gt;&lt;/a&gt; told me there would be an Iridium flare tonight. It would be a bit to the left of Arcturus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a bright point at the approximate location : Okay, that must be Arcturus. And here, to the left, that must be the Iridium satellite. Wait, why are both moving, in the same direction and the exact same speed? Wow, they're both flaring at the same location, a few seconds apart!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I didn't notice it at first, but &lt;em&gt;Heavens above&lt;/em&gt; sent 2 notifications :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vYAmF.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vYAmF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They flared at the exact same location as seen from Earth, and only 7s apart. Both were visible for much longer than 7s and could be seen moving parallel to one another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Iridium 6 and 51 seem to have very &lt;a href=&quot;http://heavens-above.com/orbit.aspx?satid=24794&amp;amp;lat=61.4400&amp;amp;lng=6.23&amp;amp;loc=Reed&amp;amp;alt=0&amp;amp;tz=CET&quot; rel=&quot;noreferrer&quot;&gt;similar&lt;/a&gt; &lt;a href=&quot;http://www.heavens-above.com/orbit.aspx?satid=25262&amp;amp;lat=48.7758&amp;amp;lng=9.1829&amp;amp;loc=Stuttgart&amp;amp;alt=250&amp;amp;tz=CET&quot; rel=&quot;noreferrer&quot;&gt;orbits&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's another &lt;a href=&quot;https://www.youtube.com/watch?v=e45ig0zN2AA&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt; showing them flaring at almost the same time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://skysafariastronomy.com/&quot; rel=&quot;noreferrer&quot;&gt;SkySafari&lt;/a&gt; indicates there were 54km apart from each other during the flares, with a ground speed of approximately 7.5km/s.&lt;/p&gt;&#xA;" OwnerUserId="19229" LastActivityDate="2017-04-07T21:25:29.947" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21030" PostTypeId="1" CreationDate="2017-04-08T13:31:44.887" Score="2" ViewCount="72" Body="&lt;p&gt;Looking at this &lt;a href=&quot;https://www.youtube.com/watch?v=pgj0kSXEtdQ&amp;amp;t=32m30&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5 launch&lt;/a&gt;, I notice that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the speed continously increase (as I expected)&lt;/li&gt;&#xA;&lt;li&gt;the altitude first increase as I expected, then decrease (I was not expecting that) and then increase again.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;When looking at the graph on the right upper corner, it looks like a totally calculated maneuvre. The slight altitude decrease is obvious around 30 minutes in the linked video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do the trajectory of this rocket include a temporary loss of altitude for several minutes before increasing again the altitude? &lt;/p&gt;&#xA;" OwnerUserId="9072" LastActivityDate="2017-04-08T13:31:44.887" Title="Why do rockets sometimes climbs down before reaching orbital velocity?" Tags="|launch|ariane|" AnswerCount="0" CommentCount="0" ClosedDate="2017-04-08T16:52:00.447" ContentLicense="CC BY-SA 3.0" />
  <row Id="21032" PostTypeId="1" CreationDate="2017-04-08T16:51:23.420" Score="1" ViewCount="592" Body="&lt;p&gt;At the beginning of the third part of Seveneves Neal Stephenson describes how the protagonist Kath Two travels from the Earth's surface into the orbit with the aid of a Skyhook-named system. The route between the earth's surface and the skyhook is laid back using a glider, which can reach without any drive only by clever exploitation of thermals up to about 100 to 200 km altitude. My question now would be, is it possible to ascend to such a height using a sail glider?&lt;/p&gt;&#xA;" OwnerUserId="19235" LastEditorUserId="38" LastEditDate="2017-04-09T10:43:02.237" LastActivityDate="2017-04-12T04:13:41.110" Title="How high *could* a glider reach, to catch a skyhook reaching down from orbit?" Tags="|crewed-spaceflight|spacecraft|skyhook|" AnswerCount="2" CommentCount="5" ClosedDate="2017-04-19T13:24:51.397" ContentLicense="CC BY-SA 3.0" />
  <row Id="21033" PostTypeId="1" CreationDate="2017-04-08T20:42:17.330" Score="3" ViewCount="466" Body="&lt;p&gt;I apologize if my question is badly formulated I know it's really specific but here what I would like to know :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I managed to compute the turning angle of the velocity for a body along a hyperbola trajectory thanks to the following equation : &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\delta = 2 \arcsin(1/e)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This equation give me the turning angle at the end of the flyby.&#xA;What I would like to know is the original angle, at start, of that spacecraft with respect to the planet velocity vector. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought using the HORIZONS Web-Interface and put the settings as following :&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Ephemeris Type : VECTORS&lt;/li&gt;&#xA;&lt;li&gt;Target Body : The spacecraft (e.g. Voyager 1)&lt;/li&gt;&#xA;&lt;li&gt;Coordinate origin : The planet (e.g. Jupiter)&lt;/li&gt;&#xA;&lt;li&gt;Time Span : Start of fly-by (e.g. 1979-Mar-4 / 1979-Mar-5 )&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But then I'm completely lost in the generated tables !&#xA;Am I at the right place to find what I'm looking for ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for your help &lt;/p&gt;&#xA;" OwnerUserId="19147" LastActivityDate="2020-12-07T12:06:05.613" Title="Is there a way to get the angle of approach of a spacecraft with respect to a planet velocity vector?" Tags="|gravity-assist|flyby|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21034" PostTypeId="2" ParentId="21032" CreationDate="2017-04-09T08:06:03.120" Score="2" Body="&lt;p&gt;From my memory of the book there's no actual altitude listed, but the suggestion was that the glider had to reach very high. The current glider altitude record is &lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_altitude_record#Gliders&quot; rel=&quot;nofollow noreferrer&quot;&gt;just over 50,000ft&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The highest altitude obtained in an unpowered aircraft is 15,460 m&#xA;  (50,720 ft) on 30 August 2006 by Steve Fossett (pilot) and Einar&#xA;  Enevoldson (co-pilot) in their high performance research glider&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So I would say that currently it is not possible for a glider to reach the kind of altitudes the book alluded to. Seveneyes described a society with much more advanced technology than we have now, the glider itself had the ability to morph its shape considerably, it's hard to say how high a glider like that could get. This is science fiction after all. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2017-04-09T08:06:03.120" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21035" PostTypeId="2" ParentId="8228" CreationDate="2017-04-09T10:53:53.763" Score="5" Body="&lt;p&gt;The issue with solar panels is the mass and complexity of such a system. Firstly, any meaningful solar array is going to need to be big. But you can't launch with solar panels extended, that would result in a stupid looking and unaerodynamic fairing (or in the case of the American Space Shuttles (STS) panels breaking) So, you need somewhere to stow them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In standard spacecraft such as the Apollo CSM, this isn't so much a problem, they can be stowed out of the service module. But for the shuttles, they would have to be stowed inside the shuttle, among the OMS fuel and the OMS oxidiser, reducing the space for said fuel and oxidiser.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's not the only problem, also the batteries needed for the dark side of orbit very heavy for the energy they store, and in the end, for the distance these are going, (to the Moon and back at most) it won't take too much time so therefore the fuel cells and fuel are cheaper and smaller. And they can be put anywhere convenient. Sure, solar power and batteries are best for interplanetary travel but for short flights, fuel cells are lightest, thus needing less fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;THE BOTTOM LINE:&#xA;Fuel cells are the lightest for journeys within Earth's gravitational field, and solar power with batteries is lightest for interplanetary missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the ISS uses solar power because it's up to stay and doesn't need to change its orbit (save for the odd bit of space junk) and fuel cells on the ISS would need resupply missions every day or so.&lt;/p&gt;&#xA;" OwnerUserId="19245" LastEditorUserId="19245" LastEditDate="2017-12-25T17:50:29.920" LastActivityDate="2017-12-25T17:50:29.920" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21037" PostTypeId="1" CreationDate="2017-04-09T18:54:56.933" Score="10" ViewCount="537" Body="&lt;p&gt;I'm &lt;a href=&quot;http://www.planetary.org/blogs/jason-davis/2017/20170407-new-mars-plans-unveiled.html&quot; rel=&quot;noreferrer&quot;&gt;reading&lt;/a&gt; that NASA plans to use a Deep Space Gateway, a space station in some kind of Lunar or Lagrange orbit, in order to send astronauts to Mars. How does such a &quot;gateway&quot; assist human spaceflight to Mars? Wouldn't it be easier to do assembly and tests in low Earth orbit instead? How does the Moon get involved here?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2018-03-06T14:35:42.087" Title="How is a Deep Space Gateway station in Lunar orbit helpful for going to Mars?" Tags="|mars|crewed-spaceflight|nasa|space-station|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21038" PostTypeId="1" CreationDate="2017-04-09T21:39:16.643" Score="20" ViewCount="3571" Body="&lt;p&gt;Could a spacecraft use organic materials, like wood or bone or shell or other tissues, as an interface to the vacuum and radiation of space? Would wood outgas and crack up in space? I wonder if life on Earth has evolved materials, without human technology such as metallurgy, that could work for spaceflight. Something which might have evolved somewhere out there.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6944" LastEditDate="2019-02-11T19:10:31.287" LastActivityDate="2021-08-13T13:48:24.197" Title="Could a spacecraft be made out of wood?" Tags="|life|astrobiology|life-support|design-alternative|" AnswerCount="3" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21040" PostTypeId="1" CreationDate="2017-04-10T08:52:25.003" Score="4" ViewCount="79" Body="&lt;p&gt;A small Pu-238 core with radiator inside the &quot;skirt&quot; to provide sustained heated atmospheric hydrogen supply to the envelope (plus some thermocouples to produce electricity to power the probe), actual scientific payload hanging on a tether long enough that the core radiation wouldn't disturb it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could it work? Would it be possible to build a balloon with enough buoyancy that hydrogen heated with a nuclear core could let it stay afloat in a gas giant atmosphere - break even between mass of heat production structure and buoyant volume? Or are there other problems I missed that would make it impossible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: the question this one was marked as duplicate of, only contains suggestions of &quot;Solar Montgolfier&quot; balloons, heated with sunlight or planet radiation. A linked paper claims these are capable of flight only during the day. Not a problem on a rocky planet where they'd just spend the night on the ground, but not acceptable on a gas giant!&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-04-11T08:35:39.447" LastActivityDate="2017-04-11T08:35:39.447" Title="Would nuclear-heated &quot;hot air&quot; balloon probe be viable for gas giants?" Tags="|aerobot|" AnswerCount="0" CommentCount="2" ClosedDate="2017-04-10T17:07:49.863" ContentLicense="CC BY-SA 3.0" />
  <row Id="21041" PostTypeId="1" AcceptedAnswerId="21045" CreationDate="2017-04-10T11:55:13.223" Score="21" ViewCount="4537" Body="&lt;p&gt;I just watched the live feed of the Soyuz capsule landing in Kazakhstan. All the way down the narrator was describing the expected sequence of parachutes and how retro-rockets would fire approximately 3 seconds before touchdown to soften the landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If these rockets fired it was not visible from the long range camera footage (posted &lt;a href=&quot;https://twitter.com/Space_Station/status/851395305429848064&quot; rel=&quot;noreferrer&quot;&gt;here on Twitter&lt;/a&gt;). If anything there is a &amp;lt;0.2sec window in the clip where they might have fired right &lt;em&gt;at&lt;/em&gt; touchdown. 1 frame is in the air, 2 have a blast ring/glow, and the next is on the ground in what seems like a pretty hard touchdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SGWXm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SGWXm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/VhFTP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VhFTP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/gxEKh.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gxEKh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/spYhk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/spYhk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No mention of this is being made in the rest of the broadcast. (Edit: A narrator just described the replay about 20 minutes later saying the &quot;there we see the soft landing engines firing just a second or two before landing&quot;, but this is neither the 3 seconds that was expected nor is it accurate as what is visible happens only a fraction of a second before touchdown.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was there a problem with the altitude detection or timing of the soft landing rockets firing? Would this have been visible if they had fired earlier? Or might they have fired and we just couldn't see them from this camera show? Is there other footage to compare how this sequence looks?&lt;/p&gt;&#xA;" OwnerUserId="441" LastEditorUserId="441" LastEditDate="2017-04-10T12:26:39.603" LastActivityDate="2017-04-10T19:43:40.643" Title="Did the soft-landing rockets malfunction in the Soyuz (expedition 50 crew) landing?" Tags="|landing|soyuz-spacecraft|" AnswerCount="4" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21042" PostTypeId="1" CreationDate="2017-04-10T12:20:40.700" Score="5" ViewCount="556" Body="&lt;p&gt;The Soyuz launcher can put about a third of the mass in low Earth orbit compared with Proton, Falcon 9, Ariane 5, Atlas V. Is that enough to crew and resupply a space station in Lunar orbit, or at an Earth-Lunar Lagrange point, a so called Deep Space Gateway?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The delta-v from LEO to an EML points seem to be less than 1 km/s, but it took Saturn V about three days to ship three astronauts to the Moon, how long would it take for a Soyuz launcher, at less than a tenth of the capability, to do the same? And could the Soyuz spacecraft support the crew for that long? Could planned commercial spacecrafts like Dragon, CST-100 and Dreamchaser bring 3 (or 7) crews to a DSG? Or would it require the SLS with Orion to reach?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2018-04-08T12:14:23.303" Title="Could the Soyuz/Progress supply and crew a space station in Lunar orbit?" Tags="|space-station|soyuz-spacecraft|commercial|soyuz-launcher|lunar-gateway|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21043" PostTypeId="2" ParentId="21041" CreationDate="2017-04-10T13:59:47.040" Score="33" Body="&lt;p&gt;The &quot;three second&quot; item is incorrect.  In general (and this can be confirmed by watching multiple landings), the rockets fire approximately half a second before landing.  Maybe they meant to say &quot;three feet&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, astronauts who have ridden the Soyuz home invariably describe it as a fairly wild ride.  Terms I've personally heard include &quot;car accident&quot; and &quot;going over Niagara Falls in a barrel.... that's on fire.&quot;&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-04-10T13:59:47.040" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21044" PostTypeId="2" ParentId="21041" CreationDate="2017-04-10T15:33:41.740" Score="-1" Body="&lt;p&gt;They always fire off only a couple of feet from the ground.&lt;/p&gt;&#xA;" OwnerUserId="19254" LastActivityDate="2017-04-10T15:33:41.740" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21045" PostTypeId="2" ParentId="21041" CreationDate="2017-04-10T16:24:06.417" Score="25" Body="&lt;p&gt;ESA has a nice &lt;a href=&quot;https://youtube.com/playlist?list=PLbyvawxScNbsoD_tGlw8kWCw3S5htiVKZ&quot; rel=&quot;noreferrer&quot;&gt;3-part video series&lt;/a&gt; on YouTube that explains on a high-level ascent, rendezvous, and re-entry. These are actual training videos that are shown to ESA astronauts during their training.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The narrator in the video gives the height at which the retro-rockets fire as &quot;70&amp;thinsp;cm&quot;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;70&amp;thinsp;cm above the ground, the six retro-rockets fire to further reduce the capsule's speed to approximately 5&amp;thinsp;km/h.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can see in the video (at about 18:07) that the rockets really &lt;em&gt;do&lt;/em&gt; fire very late and very short. It actually looks more like a small explosion underneath the capsule than sustained thrust from a rocket:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/-l7MM9yoxII?start=1087&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6156" LastActivityDate="2017-04-10T16:24:06.417" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21047" PostTypeId="2" ParentId="21041" CreationDate="2017-04-10T19:43:40.643" Score="7" Body="&lt;p&gt;&lt;a href=&quot;http://www.libros.am/book/read/id/172754/slug/kosmicheskie-tverdotoplivnye-dvigateli&quot; rel=&quot;noreferrer&quot;&gt;I found a book about solid fuel engine design (in Russian) &lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this book, the soft landing rockets are ignited when the Soyuz capsule is about 1 meter from the ground and they work only a fraction of the second, reducing speed from 7-8 m/s to 0.&lt;/p&gt;&#xA;" OwnerUserId="6491" LastActivityDate="2017-04-10T19:43:40.643" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21048" PostTypeId="1" AcceptedAnswerId="21049" CreationDate="2017-04-10T20:15:35.320" Score="8" ViewCount="261" Body="&lt;p&gt;If we consider Mars, with its relatively low gravitational acceleration and low density atmosphere, I had always assumed that if we have a single-stage-to-orbit launch vehicle on the surface of Mars that is required to achieve a stable circular orbit upon engine cutoff, the most fuel efficient method would be to perform a single engine burn in order to minimize exposure to gravitational losses. However, would it somehow be beneficial (in terms of minimum fuel use to get to orbit) to actually have, for example, 2 burns with a coast phase in between them? Does anyone know if any papers have been written regarding the best number of burns for a SSTO vehicle to reach orbit so as to minimze fuel use?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastActivityDate="2017-04-11T16:00:08.220" Title="Number of burns for optimal launch ascent" Tags="|orbital-mechanics|mars|trajectory|ssto|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21049" PostTypeId="2" ParentId="21048" CreationDate="2017-04-10T20:49:46.150" Score="11" Body="&lt;p&gt;Orbits are closed, so you will always return to the last altitude at which you shut down your engines. If that place was in the atmosphere, or close enough to the surface of an airless body that small perturbations would result in an impact, then you are not in orbit. Bottom line: if you want to maximize efficiency by expending as much propellant as possible close to the surface, then you have no choice but to do a second burn at some point to raise the periapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most efficient way to get into orbit off of an airless body (i.e., not Mars) would be a short, high thrust, very close to horizontal burn right at the surface, a coast halfway around the planet, and lastly a periapsis raise maneuver at apoapsis. The periapsis raise maneuver would be quite small compared to the launch maneuver. E.g. ~100 m/s as compared to several km/s.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-04-11T16:00:08.220" LastActivityDate="2017-04-11T16:00:08.220" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21050" PostTypeId="1" AcceptedAnswerId="21053" CreationDate="2017-04-11T05:21:36.950" Score="14" ViewCount="1602" Body="&lt;p&gt;After reading &lt;a href=&quot;https://space.stackexchange.com/a/5286/12102&quot;&gt;this answer&lt;/a&gt; by someone knowledgeable in the field, I realized that I still don't understand even a little bit what &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Traffic_in_Arms_Regulations&quot; rel=&quot;noreferrer&quot;&gt;International Traffic in Arms Regulations (ITAR)&lt;/a&gt; (and its cousins) does and doesn't say what various people can or can't do, and so far I haven't found the &lt;strong&gt;ITAR-tl;dr&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I'm getting rid of the hypothetical scenario and adjusting the question to make sure it doesn't read like a request for legal advice. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've given myself an exercise - launch to orbit in 20 lines of python. It's along the lines of &lt;em&gt;A Neural Network in 11 lines of Python&lt;/em&gt;; &lt;a href=&quot;http://iamtrask.github.io/2015/07/12/basic-python-network/&quot; rel=&quot;noreferrer&quot;&gt;Part 1&lt;/a&gt; and &lt;a href=&quot;https://iamtrask.github.io/2015/07/27/python-network-part2/&quot; rel=&quot;noreferrer&quot;&gt;Part 2&lt;/a&gt;. (Part 2 is 13 lines, a &lt;em&gt;clear case of mission creep&lt;/em&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is purely a deterministic model. ISP is constant or a simple function, thrust is on or off, Earth's atmosphere rotates as a rigid body, drag coefficient is &lt;a href=&quot;https://www.youtube.com/watch?v=cAYDiPizDIs&quot; rel=&quot;noreferrer&quot;&gt;brontosaurus shaped&lt;/a&gt; when plotted vs Mach number, atmosphere is isothermal with scale height behavior, etc. You launch thousands of times and use &lt;a href=&quot;http://scipy-cookbook.readthedocs.io/items/KalmanFiltering.html&quot; rel=&quot;noreferrer&quot;&gt;gradient descent&lt;/a&gt; to eventually find an orbit. Only SciPy and NumPy are allowed. I think fifty lines is realistic and probably straightforward, but it could be shorter if made ugly and/or opaque.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if I allow for realistic wind, or engine fluctuations, or mechanical noise in my thrust vectoring, or turbulence, or other things, then I need to add some simulated measurements (e.g. simulated inertial sensors or GPS) and then feedback, and also raise my limit to a few hundred lines of python and/or import other python packages within the &lt;a href=&quot;https://docs.continuum.io/anaconda/#packages-available-in-anaconda&quot; rel=&quot;noreferrer&quot;&gt;Anaconda distribution&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;I am wondering&lt;/strong&gt; if is there any way to tell when a US citizen sitting with their laptop in the US might be starting to cross into potential ITAR territory?  Is it when feedback is introduced? Or when &lt;a href=&quot;http://scipy-cookbook.readthedocs.io/items/KalmanFiltering.html&quot; rel=&quot;noreferrer&quot;&gt;Kalman filtering&lt;/a&gt; is added? Or is it only when it gets out of the laptop and on to the internet? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not asking for nor do I want a legal opinion. But are there any guidelines, rules of thumb, articles in space related magazines, blogs... anything where one could get some kind of gist of where the gray zone begins to darken noticeably?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see a lot of cautions and warnings peppered in SXSE, but they mostly seemed nuanced for those who should already know but a reminder now an then can't hurt. In this case I'm asking from a responsible noob's point of view.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="25" LastEditDate="2018-12-03T12:51:04.650" LastActivityDate="2023-10-17T04:06:12.317" Title="What computer programs does ITAR say I can or can not write, or share, or use in the US?" Tags="|launch|software|law|python|itar|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21051" PostTypeId="1" CreationDate="2017-04-11T06:08:18.843" Score="6" ViewCount="1400" Body="&lt;p&gt;I thought I would ask what the most used methods in industry are, in order to guide a launch vehicle into a desired circular orbit at engine cutoff. It seems that the use of direction collocation methods in combination with a non-linear programming approach are common means of calculating optimal ascent trajectories, although they seem to be computationally taxing and thus cannot be used in real-time. Another approach that seems to be mentioned often in the literature is the linear-tangent steering (LTS) law which is a near-optimal guidance law developed in the 1960s. So my question is: Is the LTS method still the approach that is most often used for ascent guidance, or are there &quot;more optimal&quot; methods that can be used in real-time. Furthermore, how close to optimal is the LTS law? The literature often mentions the assumptions used in its derivation, but I have not managed to find any comparison of how well it compares to truly optimal solutions.&lt;/p&gt;&#xA;" OwnerUserId="1064" LastActivityDate="2017-11-01T18:49:20.340" Title="Best practices for rocket ascent trajectory guidance" Tags="|orbital-mechanics|rockets|trajectory|guidance|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21052" PostTypeId="1" AcceptedAnswerId="21064" CreationDate="2017-04-11T10:24:19.930" Score="8" ViewCount="357" Body="&lt;p&gt;Subsidiary question: Imagine a sphere 10cm in diameter in low venusian orbit.&#xA;Slow it down a little in order to deorbit it. What's the density of the sphere, in order to touch the ground at 0 vertical m/s, before climbing up again in the venusian sky because it's less dense than venusian atmosphere? (regarding pressure gradient, aerodynamic drag, high speed winds effects on trajectory, and other things i forget.(see images below))(rough approximations and thoughts are welcome)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pressure on surface of Venus is 90 times greater than Earth's sea level pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there studies about some sort of &quot;low &amp;amp; variable density buoyancy braking lander&quot; designed with removable onionlike heatshields -or a single deflatable heatshield- which would provide control over the density -and therefore the speed- of the whole lander during the descent?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea is about bringing multipurpose to parts, in order to minimise the number of parts. Aerobraking starts in high altitude, and stops on the floor, Buoyancybrake should start at a precise altitude, and stop at surface level, &#xA;0 m/s vertical speed, with the separation of the last low density, buoyant-heatshielding onionskin layer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The less onionskins layers needed in the descent, (of low density high temperature resistant &amp;amp; thermical insulant, some sort of aerogel(?)) the better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;links towards atmospheric plots: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT309/HTML/AT30905.HTM&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT309/HTML/AT30905.HTM&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ase.tufts.edu/cosmos/view_picture.asp?id=1103&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ase.tufts.edu/cosmos/view_picture.asp?id=1103&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pMufD.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pMufD.gif&quot; alt=&quot;venus buoyant lander&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/FTfAK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FTfAK.jpg&quot; alt=&quot;venus atmosphere graph pressure temperature altitude&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/fl78q.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fl78q.jpg&quot; alt=&quot;venus winds&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="19132" LastEditDate="2017-04-25T13:52:20.757" LastActivityDate="2019-03-19T03:52:32.767" Title="Archimedean braking for low density Venus lander + subsidiary question" Tags="|venus|lander|heat-shield|entry-descent-landing|venera|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21053" PostTypeId="2" ParentId="21050" CreationDate="2017-04-11T11:30:05.490" Score="19" Body="&lt;p&gt;With the exception of model rockets made of paper, wood, or breakable plastic, that are passively stabilized, that have low thrust and low delta V, and that do not have guidance software, launch vehicles fall under ITAR regulations. The regulations are now much more relaxed once the launch vehicle has achieved its goal and placed its payload into orbit. Certain satellites with limited sensor/effector capabilities fall under the realm of the Export Administration Regulations levied by the Commerce Department rather than the realm of the International Traffic in Arms Regulations levied by the State Department. Satellites with even less capabilities (e.g., some cubesats) are pretty much regulation-free.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether simulating a launch vehicle of interest might trigger the interest of the State Department (ITAR), the Commerce Department (EAR), or the Transportation Department (FAA regulations) -- I am not a lawyer. It's important to keep in mind that &lt;em&gt;F = ma&lt;/em&gt; would be classified TS/NOFORN (top secret / no foreign nationals) if the DoD had its druthers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I've seen, the boundary is rather fuzzy. NASA has publicly released some parts of it's &lt;a href=&quot;https://opensource.gsfc.nasa.gov/projects/cfe/&quot; rel=&quot;noreferrer&quot;&gt;core Flight Executive&lt;/a&gt;, some parts of it's &lt;a href=&quot;https://cfs.gsfc.nasa.gov&quot; rel=&quot;noreferrer&quot;&gt;core Flight System&lt;/a&gt;, and all of it's &lt;a href=&quot;https://github.com/nasa/trick&quot; rel=&quot;noreferrer&quot;&gt;Trick Simulation Environment&lt;/a&gt; as open source. These are highly capable tools. On the other hand, some NASA software that kinda-sorta provides the ability to simulate trajectories and perturbations to those trajectories remains subject to ITAR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Full disclosure: I've written a good chunk of the software used at the NASA Johnson Space Center to model trajectories and perturbations thereof. Much of the software that I have written has been deemed to fall under ITAR. On the other hand, I've written what is to me significantly more complex software that apparently is not subject to ITAR. I am not a lawyer, so I'm rather clueless with regard to the distinction.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-11T11:30:05.490" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21054" PostTypeId="1" CreationDate="2017-04-11T11:44:12.583" Score="8" ViewCount="763" Body="&lt;p&gt;Bundling three or five first stage cores together improves the launch capacity a great deal. But Delta, Falcon and Angara are supposed to use the same upper stage in their multiple common core versions, as their weaker single core versions do. They double or multiply the amount of mass they can put in orbit, so why aren't larger upper stages used for the common core versions of those rockets?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if a Falcon 9 launches 20 tons of fuel to dock in LEO, how can the same upper stage be used to launch 40 tons of fuel to the same orbit? Doesn't it need to be larger and maybe sturdier to support the much heavier payload?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similar with Atlas V that has a pretty wide range of capabilities using zero up to five solid boosters. But it always uses the same single engine Centaur upper stage.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-04-11T11:50:07.243" LastActivityDate="2017-04-11T13:51:50.550" Title="How can the same upper stage be optimal on a common core booster as well as on a single booster?" Tags="|launch-vehicle|falcon-heavy|upper-stage|delta-iv-heavy|angara|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21055" PostTypeId="2" ParentId="21054" CreationDate="2017-04-11T12:06:58.617" Score="9" Body="&lt;p&gt;You haven't defined &quot;optimal&quot; here. The commercial launch business is more concerned with cost optimization than mass optimization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atlas V offers two variants of Centaur, with one or two RL10 engines on it, although the two-engine version hasn't yet been flown on an Atlas V; adding solid rocket boosters seems to be incrementally more cost-effective than using the (very expensive) second engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, the Delta IV has two different second stage options with 4- and 5-meter diameters (DCSS-4 and DCSS-5); the tankage is significantly larger on the latter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Angara uses two entirely different second stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves Falcon Heavy; it wouldn't surprise me to eventually see a (further) stretched upper stage to take advantage of the more powerful booster, but in the near term, I assume that the cost optimization of using a common second stage outweighs the potential performance optimization, particularly since there's almost no market for single payloads that size.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-04-11T13:51:50.550" LastActivityDate="2017-04-11T13:51:50.550" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21056" PostTypeId="1" CreationDate="2017-04-11T15:36:18.220" Score="7" ViewCount="639" Body="&lt;p&gt;In another question, this came up:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For example, if a Falcon 9 launches 20 tons of fuel to dock in LEO, how can the same upper stage be used to launch 40 tons of fuel to the same orbit? Doesn't it need to be larger and maybe sturdier to support the much heavier payload?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And I realized I'm not sure how structural strain on upper stages actually works. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sitting on the ground, the second stage structure is supporting the weight of that 20 or 40 ton payload under Earth gravity, which we can express as a force in Newtons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once out of the atmosphere and staged, though, the operative force is that of the second stage engine - some 95 tons for the Merlin Vac at peak thrust - so is that the force on the structure of the second stage? Or assuming the stage plus payload is accelerating together uniformly, does the force need to be pro-rated between the stage mass and payload mass? Does that mean the effective weight of the payload increases as propellant is used? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would structural strain on the second stage during the early part of ascent be calculated, with aerodynamic drag opposing the thrust from the first stage engines? &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-04-11T21:50:18.377" LastActivityDate="2022-11-29T12:41:53.450" Title="Upper stage structural loads on ascent?" Tags="|launch|rockets|thrust|aerodynamics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21057" PostTypeId="1" CreationDate="2017-04-11T16:35:20.293" Score="2" ViewCount="346" Body="&lt;p&gt;If SpaceX's Red Dragon will ever bring astronauts to the red planet, would it need to connect to a deep space habitat? I'm asking because the red dragon seems to be too small for a seven or eight month trip to Mars.&lt;/p&gt;&#xA;" OwnerUserId="19265" LastEditorUserId="12102" LastEditDate="2017-04-11T23:50:11.747" LastActivityDate="2017-04-11T23:50:11.747" Title="How will astronauts live in Spacex's red dragon?" Tags="|spacex|crewed-spaceflight|deep-space|red-dragon|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21058" PostTypeId="2" ParentId="21057" CreationDate="2017-04-11T16:51:48.123" Score="10" Body="&lt;p&gt;They won't. Red Dragon is an unmanned concept demonstrator and experiment transport. SpaceX wants to use it to test Mars EDL (entry-descent-landing) technology and algorithms they develop for their next generation ship (currently known as &lt;a href=&quot;https://en.wikipedia.org/wiki/ITS_launch_vehicle&quot; rel=&quot;noreferrer&quot;&gt;ITS or BFR&lt;/a&gt;) which they want to use for astronauts.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="12102" LastEditDate="2017-04-11T23:36:34.293" LastActivityDate="2017-04-11T23:36:34.293" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21059" PostTypeId="2" ParentId="21056" CreationDate="2017-04-11T18:33:51.450" Score="8" Body="&lt;p&gt;Edit: this answer was based on a mis-interpretation of the question. Leaving it since it attracted some interest.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;I can explain in general terms how this was done for Shuttle.  From attending interorganizational working group meetings, I can say that this same general process is carried out for expendable launch vehicles, albeit with newer technology.&lt;/p&gt;&#xA;&lt;p&gt;For the shuttle, instead of trying to compute structural loads in real time (considered impractical with 70's/80's technology), constraints were generated ahead of time.  Then, using the mass properties, throttle settings, SRB bulk temperatures, winds, etc, of the day, simulated trajectories were run on the day of launch and steering parameters were generated to ensure that the vehicle would stay within the constraints.&lt;/p&gt;&#xA;&lt;p&gt;Many constraints were checked, but those relating to structural loads were the Q-planes or squatcheloids, a set of three-dimensional volumes in alpha, beta, and q (angle of attack, sideslip angle, and dynamic pressure), indexed by Mach number.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pVAtm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pVAtm.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;If dispersed trajectory lay within the Q-planes at all times, the check was passed.&lt;/p&gt;&#xA;&lt;p&gt;Other constraints were the Structural Load Indicators, &amp;quot;calculations derived from higher fidelity structural analytical limits to determine element structural loadings&amp;quot;. Forty-two Structural Load Indicators were evaluated for each trial trajectory.  All had to have positive margin to pass the check.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dWwN0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dWwN0.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There were many other checks done before a trajectory design was accepted, but these are the major ones dealing with structural loads.&lt;/p&gt;&#xA;&lt;p&gt;For more detail I suggest reading the very accessible and interesting information at these two links.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://archive.org/details/NASA_NTRS_Archive_20110003654&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Day of Launch Trajectory Design Operations&lt;/a&gt; - technical paper&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwjE-OrcgZ3TAhVG02MKHWIwBgYQFggnMAA&amp;amp;url=https%3A%2F%2Fntrs.nasa.gov%2Farchive%2Fnasa%2Fcasi.ntrs.nasa.gov%2F20100015499.pdf&amp;amp;usg=AFQjCNE2kxndl6kJHgyhvK7NVZqoU8lwLg&amp;amp;sig2=0y2aOU3PUFTN51bqPdwrCA&quot; rel=&quot;nofollow noreferrer&quot;&gt;Powerpoint Presentation covering the same material&lt;/a&gt; but better graphics&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2022-11-29T12:41:53.450" LastActivityDate="2022-11-29T12:41:53.450" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="21061" PostTypeId="1" CreationDate="2017-04-12T00:34:56.297" Score="2" ViewCount="788" Body="&lt;p&gt;&lt;strong&gt;Thanks for reopening!:&lt;/strong&gt; &lt;a href=&quot;https://space.stackexchange.com/q/6638/12102&quot;&gt;How fast could a person run on the Moon?&lt;/a&gt; asks for users to make their own conclusions. Here I'm making a reference-request for research and results.&lt;/p&gt;&#xA;&lt;p&gt;Related but different: &lt;a href=&quot;https://space.stackexchange.com/q/33767/12102&quot;&gt;How would you move in low gravity without a suit?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;So I think this &lt;strong&gt;can be reopened.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;https://youtu.be/QmSJlgI04PA&quot; rel=&quot;nofollow noreferrer&quot;&gt;silly video&lt;/a&gt; shows an idiot running across the surface of Mars in a space suit with big boxy boots, constantly looking at his wrist computer instead of obstacles. It is &amp;quot;based on Mars gravity simulations on Earth&amp;quot; and shows a top running speed of 44 kph, or roughly 17% faster than the record sprint on Earth.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Have there been any &lt;em&gt;published&lt;/em&gt; serious biophysics or sports physics attempts to calculate by roughly how much top running speed might change in Mars' or the Moon's gravity? (Indoors, not in a suit.)&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; The issue of running speed in reduced gravity has been introduced and discussed at length in &lt;a href=&quot;https://space.stackexchange.com/q/6638/12102&quot;&gt;How fast could a person run on the Moon?&lt;/a&gt; and its associated answers. Here I'm looking for a formal study, preferably peer reviewed if available, and including Mars if possible. That may be tough — it may be hard to find many peers in reduced-gravity running physics.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; A lot of motion, torque, and work are required to move human legs back and forth to match ground speed and continue to accelerate faster. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Muybridge_runner.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jVHC2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jVHC2.jpg&quot; alt=&quot;Eadweard Muybridge's high-speed photos of a running human&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; This requires substantial muscle at low moment locations within the leg. From &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Nesta_Carter_Tyson_Gay_2010_Memorial_Van_Damme.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lhRe1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lhRe1.jpg&quot; alt=&quot;Photo of athletes mid-sprint&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; GIF from screen shots of the Zaptec video &lt;a href=&quot;https://youtu.be/QmSJlgI04PA&quot; rel=&quot;nofollow noreferrer&quot;&gt;Born to Run&lt;/a&gt;. Appears to be almost effortless — &amp;quot;based on Mars gravity simulations on Earth.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/a3AXH.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/a3AXH.gif&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7oc9V.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7oc9V.png&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-16T05:22:34.740" LastActivityDate="2021-08-16T05:22:34.740" Title="Any published biophysics or sports physics research on how much top running speed might change on Mars or the Moon? (Indoors, not in a suit.)" Tags="|reference-request|reduced-gravity-sports|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="21062" PostTypeId="2" ParentId="21032" CreationDate="2017-04-12T02:35:45.787" Score="1" Body="&lt;p&gt;&lt;a href=&quot;https://www.aerosociety.com/news/surfing-into-the-stratosphere/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Perlan 2&lt;/a&gt; aims to fly a glider up to 90 000 feet. 100 000 feet (30km) also appears possible, but more expensive &amp;mdash; presumably due to complex composite construction to reduce weight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soaring that high requires specific atmospheric conditions, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lee_wave&quot; rel=&quot;nofollow noreferrer&quot;&gt;standing mountain wave&lt;/a&gt; and the right wind conditions to prevent that wave being blocked in the troposphere. As the book is set in the future with advanced materials, presumably the glider could be a lot lighter so you could go a bit higher and maybe have a wider range of conditions in which you could get to altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the required conditions are somewhat rare, so it might not be practical for skyhook(s) as they wouldn't often align with the right weather.&lt;/p&gt;&#xA;" OwnerUserId="18852" LastEditorUserId="6816" LastEditDate="2017-04-12T04:13:41.110" LastActivityDate="2017-04-12T04:13:41.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21063" PostTypeId="2" ParentId="10391" CreationDate="2017-04-12T03:08:02.477" Score="5" Body="&lt;p&gt;One possible thing which might help is to switch to methalox on the second stage to get additional performance to pay for grid fins and a heat shield. The &lt;a href=&quot;https://www.nasaspaceflight.com/2016/10/its-propulsion-evolution-raptor-engine/&quot; rel=&quot;noreferrer&quot;&gt;subscale raptor engine&lt;/a&gt; they tested back in September 2016 has a thrust of 1MN, which is similar to the uprated thrust of the vacuum merlin 1D.  The engine is also of a similar size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dimensions of the second stage can't change much but methane's lower density than kerosene is somewhat offset by the higher O/F ratio and LOX's high density.&lt;/p&gt;&#xA;" OwnerUserId="18852" LastActivityDate="2017-04-12T03:08:02.477" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21064" PostTypeId="2" ParentId="21052" CreationDate="2017-04-12T07:50:54.007" Score="9" Body="&lt;p&gt;Surprisingly the answer is &lt;strong&gt;yes&lt;/strong&gt; there was studies done on that subject.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simple google search could yield this result:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;BUOYANT PLANETARY ENTRY&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://apps.dtic.mil/dtic/tr/fulltext/u2/642361.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://apps.dtic.mil/dtic/tr/fulltext/u2/642361.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In this study, it was assured that the large buoyant volune&#xA;  is deployed prior to atmospheric entry. The effect of buoyancy on the entry&#xA;  dynamics was investigated, using a first-order entry model. That is, a two-&#xA;  dimensional entry trajectory, a perfectly spherical planet, a constant&#xA;  gravity, and no wind were assumed. It was found that the effect of&#xA;  buoyancy on the velocity, maxiimm deceleration, and altitude of&#xA;  maximum deceleration of planetary entryvehiclesisinsignificant.&#xA;  Thisistrueforallentryangles, even if the entry velocity is decreased&#xA;  considerably by rocket braking, and even if the buoyant volune&#xA;  diameter is very large (greater than 500 feet).&#xA;  There is one case, however,for which the buoyant effect is not altogether&#xA;  insignificant, though still small.&#xA;  This is the case of equilibrium-gliding entry. For example, for constant&#xA;  lift-drag ratios of 0.1 and spherical buoyant volune diameters of 300&#xA;  feet, the maxiirain deceleration is decreased by 2.6% for Mars and&#xA;  1.8% for Venus from the value of maximum deceleration for non-buoyant entry vehicles. For constant lift-drag ratios&#xA;  of 1.0 and diameters of 300 feet, the maximum deceleration is&#xA;  decreased by 0.8% for Mars and 0.7% for Venus.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, unsurprisingly, the result is that the &lt;strong&gt;buoyancy effect is insignificant&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6642" LastEditDate="2019-03-19T03:52:32.767" LastActivityDate="2019-03-19T03:52:32.767" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="21065" PostTypeId="1" CreationDate="2017-04-12T08:36:32.823" Score="-1" ViewCount="548" Body="&lt;p&gt;Today communication with different spacecraft is limited to speed-of-light. Around the Earth it is not such a problem, but when you have to work with a spacecraft on the surface of Mars, it becomes an issue one has to contend with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any science or engineering research ongoing in this field?  &lt;/p&gt;&#xA;" OwnerUserId="6269" LastEditorUserId="6816" LastEditDate="2017-04-12T08:46:13.937" LastActivityDate="2017-04-12T10:05:21.747" Title="Faster than light communications in space" Tags="|communication|radio-communication|ftl|" AnswerCount="1" CommentCount="3" ClosedDate="2017-04-12T12:58:14.900" ContentLicense="CC BY-SA 3.0" />
  <row Id="21067" PostTypeId="2" ParentId="21065" CreationDate="2017-04-12T10:05:21.747" Score="6" Body="&lt;p&gt;No, not really.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Faster-than-light communication is currently strongly believed to be impossible. Due to relativity, it would imply time travel (that is, messages could be sent back and forth in such a way as for replies to arrive before the originals were sent), and thus violate causality, leading to situations in which, for example, an action causes a chain of events leading naturally to that action becoming impossible to have happened. Presumably the universe does not allow &lt;a href=&quot;http://www.theculture.org/rich/sharpblue/archives/000089.html&quot; rel=&quot;noreferrer&quot;&gt;paradoxes of this sort&lt;/a&gt; to arise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quantum mechanics describes certain situations (having to do generally with &lt;em&gt;entanglement&lt;/em&gt;) in which certain phenomena occur that a naive observer (well, anyone that isn't fully up on the bizarre and counter-intuitive quirks of quantum physics) might take to mean information is being transmitted faster than light. Unfortunately, this is not actually the case: while there are physical effects that occur instantaneously over very long distances, they cannot be observed in a way that transmits information faster than correlating information can be transmitted at light speed. This is well-known to be quite confusing:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3zIvg.png&quot; rel=&quot;noreferrer&quot; title=&quot;The no-communication theorem states that no communication about the no-communication theorem can clear up the misunderstanding quickly enough to allow faster-than-light signaling.&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3zIvg.png&quot; alt=&quot;The no-communication theorem states that no communication about the no-communication theorem can clear up the misunderstanding quickly enough to allow faster-than-light signaling.&quot; title=&quot;The no-communication theorem states that no communication about the no-communication theorem can clear up the misunderstanding quickly enough to allow faster-than-light signaling.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The few researchers that are looking into things like FTL communication are generally pretty far on the fringes.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2017-04-12T10:05:21.747" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21069" PostTypeId="2" ParentId="27191" CreationDate="2017-04-12T14:03:56.633" Score="4" Body="&lt;p&gt;Linear speed in circular motion depends on angular velocity (here: 1 rotation per day, for all latitudes) and radius of the circle - distance from the axis of rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$ v = \omega r$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XiyIp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XiyIp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If &lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt; is your latitude, the radius of the circle you make with each revolution of Earth, is &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt;. As you can see above, basic trigonometry, &lt;span class=&quot;math-container&quot;&gt;${r \over R} = cos \alpha$&lt;/span&gt;. The closer you are to the pole, the more each degree of latitude decreases the radius of the circle you make.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Indirect consequence is that moderate distance of the space center from the equator diminishes delta-V much less than larger distance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;French Guiana Space Centre is 5 degrees North; Cape Canaveral is 28 degrees North; Baikonur is 45 degrees North, Plesetsk is 63 degrees North.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would superficially seem that the velocity losses between Guiana and Canaveral (23° apart) are higher than between Canaveral and Baikonur (17°) or Baikonur and Plesetsk (18°). But this is not the case - &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\cos 5° - \cos 28° =  0.11;$&lt;/span&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$ \cos 28° - \cos 45°  = 0.17;$&lt;/span&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$ \cos 45° - \cos 63° = 0.25 $&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so the 17 degrees Baikonur loses to Canaveral is way worse than 23&#xA; degrees Canaveral loses to Guiana or the 58 degrees Baikonur loses to Guiana.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2020-04-26T23:50:35.170" LastActivityDate="2020-04-26T23:50:35.170" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21070" PostTypeId="1" CreationDate="2017-04-12T15:31:15.413" Score="26" ViewCount="10962" Body="&lt;p&gt;I found this &lt;a href=&quot;https://space.stackexchange.com/questions/19177/how-often-has-the-iss-been-physically-vacant-un-manned&quot;&gt;question&lt;/a&gt;, and I was wondering &lt;strong&gt;why the ISS is constantly manned&lt;/strong&gt; (Except for some hours during particular situations). Is that a technical problem? Is it to guarantee safety? Or just because it would be a waste of money to leave it empty?&lt;/p&gt;&#xA;" OwnerUserId="16659" LastEditorUserId="-1" LastEditDate="2017-04-13T12:58:52.230" LastActivityDate="2017-04-14T20:48:28.793" Title="Why has the ISS not been left unmanned?" Tags="|iss|crewed-spaceflight|" AnswerCount="4" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21071" PostTypeId="2" ParentId="21070" CreationDate="2017-04-12T17:19:32.547" Score="58" Body="&lt;p&gt;The ISS is a science laboratory -- a &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/nlab/index.html&quot; rel=&quot;noreferrer&quot;&gt;National Lab&lt;/a&gt; in fact.  Leaving it uncrewed would take away a huge portion of the science productivity without substantially impacting the cost to operate it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is it constantly crewed?  Because anything else would be a phenomenal waste of resources.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="5946" LastEditDate="2017-04-12T20:17:40.973" LastActivityDate="2017-04-12T20:17:40.973" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21072" PostTypeId="2" ParentId="14075" CreationDate="2017-04-12T19:08:48.380" Score="5" Body="&lt;p&gt;Supplemental answer time. &lt;a href=&quot;https://en.wikipedia.org/wiki/OK_Go&quot; rel=&quot;nofollow noreferrer&quot;&gt;OK Go!&lt;/a&gt; has just released &lt;a href=&quot;https://youtu.be/YwyXLBQUEC0&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gravity is just a habit&lt;/a&gt;, the &amp;quot;making of&amp;quot; documentary about the original video &lt;a href=&quot;https://youtu.be/LWGJA9i18Co&quot; rel=&quot;nofollow noreferrer&quot;&gt;OK Go - Upside Down &amp;amp; Inside Out&lt;/a&gt;. Not surprisingly, it's essentially the same number that is given in the &lt;a href=&quot;https://space.stackexchange.com/a/14758/12102&quot;&gt;accepted answer&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The new video includes other details about preparing for the series of zero gee periods produced when the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ilyushin_Il-76&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ilyushin Il-76MDK&lt;/a&gt; 4-engine turbofan strategic airlifter at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Yuri_Gagarin_Cosmonaut_Training_Center&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yuri Gagarin Cosmonaut Training Center&lt;/a&gt; in &lt;a href=&quot;https://en.wikipedia.org/wiki/Star_City,_Russia&quot; rel=&quot;nofollow noreferrer&quot;&gt;Star City&lt;/a&gt; executes a series of parabolic trajectories between 6,000 and 9,000 meters. Both are viewable below the screenshot of the trajectory.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ANaDG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ANaDG.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Ilyushin Il-76 from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Russian_Air_Force_Ilyushin_Il-76MD_Dvurekov-16.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SzZH6.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SzZH6.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; The documentary - &lt;em&gt;watch the actual video first (last one)&lt;/em&gt; if you haven't seen it already.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/YwyXLBQUEC0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; OK Go - Upside Down &amp;amp; Inside Out (Behind the Scenes)  - &lt;em&gt;watch the actual video first (last one)&lt;/em&gt; if you haven't seen it already.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Gsnyqu7xq9c?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; The actual video - &lt;em&gt;watch this one first&lt;/em&gt; if you haven't seen it already.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/LWGJA9i18Co?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Ilyushin Il-76 from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:%D0%98%D0%BB%D1%8C%D1%8E%D1%88%D0%B8%D0%BD_%D0%98%D0%BB-76-78-%D0%90-50_0083483510,_%D0%9C%D0%BE%D1%81%D0%BA%D0%B2%D0%B0_-_%D0%92%D0%BD%D1%83%D0%BA%D0%BE%D0%B2%D0%BERP38653.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bsnDc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bsnDc.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-06-25T13:23:58.533" LastActivityDate="2020-06-25T13:23:58.533" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21073" PostTypeId="2" ParentId="21070" CreationDate="2017-04-12T20:53:03.120" Score="22" Body="&lt;p&gt;Because the official United States Government policy reason for funding the International Space Station is to (bold-facing mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Develop and operate the International Space Station to support&#xA;  activities &lt;strong&gt;requiring the unique attributes of humans in space&lt;/strong&gt; and&#xA;  establish a &lt;strong&gt;permanent human presence&lt;/strong&gt; in Earth orbit. The International&#xA;  Space Station will support future decisions on the feasibility and&#xA;  desir­ability of conducting further human exploration activities.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Presidential Decision Directive I National Security&#xA;Council 49, &quot;Fact Sheet: National Space Policy,&quot; White&#xA;House National Science and Technology Council, 19&#xA;September 1996.) quoted in &lt;a href=&quot;https://www.researchgate.net/publication/280157602_The_Changing_Purpose_of_the_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;The Changing Purpose of the Space Station&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the entire rationale for US involvement in the project is directly linked to humans in space.  If the station were to be operated unmanned, the entire reason for its existence would be null and void.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-04-12T21:18:34.403" LastActivityDate="2017-04-12T21:18:34.403" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21075" PostTypeId="2" ParentId="21070" CreationDate="2017-04-13T05:44:42.217" Score="13" Body="&lt;p&gt;Because it might be more costly not to have people in it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The experience from previous space stations was that when a minor issue occur on a space station, it can be easily fixed and detected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If nobody's here, it can in turn become more expensive to fix.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Salyut had such kind of issues: something broke down that prevented the batteries to be charged. It caused the whole station to freeze over, prompting more difficult and extensive repairs that what could've been done if the station was manned at that time.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-04-13T05:44:42.217" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21077" PostTypeId="1" AcceptedAnswerId="21078" CreationDate="2017-04-13T10:36:18.870" Score="12" ViewCount="1881" Body="&lt;p&gt;In Wikipedia they mention that the Cassini spaceship will be destroyed in by sending it into Saturn's atmosphere:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, due to the spacecraft's dwindling fuel resources for further&#xA;  orbital corrections, it is currently planned to be destroyed by diving&#xA;  into the planet's(Saturn) atmosphere in September 2017. This method of&#xA;  disposal was chosen to avoid potential biological contamination of&#xA;  Saturn's moons.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is to make sure that it will never crash into one of the moons (like Titan) where biological material would contaminate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how can a spaceship be a source of potential biological contamination?&lt;/p&gt;&#xA;" OwnerUserId="11002" LastEditorUserId="38" LastEditDate="2017-04-13T23:37:57.227" LastActivityDate="2017-04-14T14:06:28.653" Title="How can Cassini be a source of potential biological contamination?" Tags="|spacecraft|cassini|end-of-life|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21078" PostTypeId="2" ParentId="21077" CreationDate="2017-04-13T10:55:48.260" Score="20" Body="&lt;p&gt;Currently all spacecraft start out on Earth and while fairly stringent measures are taken during manufacture to keep them clean it is pretty much impossible to guarantee that no bacteria, viruses, spores or other biological material get in somewhere. Certainly anything which is exposed to untreated air at any point is quite likely to be contaminated with something. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although it is pretty unlikely it's still a big deal: if the craft was allowed to crash on a moon and evidence of life was subsequently found there there would always be some doubt about where it came from. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously there are cases like landers where things are different but if you are planning for a vehicle to come into contact with a planet or moon you can take steps to mitigate the risk. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Equally as the craft is coming to the end of its life anyway it may well be just as easy to dispose of it in a controlled way while you still can than just leave it without knowing exactly where it will end up, potentially posing a hazard to other missions. &lt;/p&gt;&#xA;" OwnerUserId="13018" LastEditorUserId="19306" LastEditDate="2017-04-14T14:06:28.653" LastActivityDate="2017-04-14T14:06:28.653" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21079" PostTypeId="1" CreationDate="2017-04-13T12:35:22.767" Score="-3" ViewCount="358" Body="&lt;p&gt;Everyone knows that there are tensions with Russia that have existed for years. At what point do they stop cooperating with NASA and the USG in general? Have there never been worries about what cosmonauts could do to our US astronauts in space? Or that the Russian government could do to our astronauts on ground, or in an &quot;anomalous&quot; instance in route to the ISS? I know cosmonauts used to carry a &quot;survival&quot; handgun of some sort… if they still did, and tensions were at their highest, would it not be dangerous to still fly on their rocket, go with their cosmonauts, etc.? Or are the cosmonauts joined in a common mission with the other astronauts, ignoring what the temperamental political leaders of the day are doing? I can see a camaraderie amongst astronauts, so maybe they are ignoring what our leaders are doing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know this is all far-fetched, but if some giant debacle happened with syria/north korea/etc, nukes start flying, would Russia send up a transmission to the cosmonauts to do something in space? Obviously we would stop going to the ISS until Commercial Crew, but how would our astronauts get down?&lt;/p&gt;&#xA;" OwnerUserId="19293" LastEditorUserId="14549" LastEditDate="2017-04-13T13:09:03.677" LastActivityDate="2017-04-13T13:09:03.677" Title="Sharing the skies with Russia, for how long?" Tags="|iss|astronauts|russia|" AnswerCount="1" CommentCount="4" ClosedDate="2017-04-13T13:09:22.087" ContentLicense="CC BY-SA 3.0" />
  <row Id="21080" PostTypeId="2" ParentId="6587" CreationDate="2017-04-13T12:36:53.367" Score="4" Body="&lt;p&gt;That close.....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.parabolicarc.com/2017/04/12/close-video-chinese-rocket-launch/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.parabolicarc.com/2017/04/12/close-video-chinese-rocket-launch/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is pretty insane, the heat must have been immense. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qernI.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qernI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19293" LastEditorUserId="19293" LastEditDate="2017-04-14T12:21:00.090" LastActivityDate="2017-04-14T12:21:00.090" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21082" PostTypeId="2" ParentId="21079" CreationDate="2017-04-13T12:58:02.793" Score="8" Body="&lt;p&gt;The ISS exists because neither Russia nor the US was able/willing to finance a space station on its own at the time. That it exists at all is a testament that large political differences can be overcome to reach a common goal (as is CERN, btw.).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now they are bound by treaties (also with other countries) and as long they don't get into a &lt;em&gt;direct&lt;/em&gt; war, it's unlikely that they will break those as both are still benefitting from their cooperation: neither nation would be able to keep the ISS afloat alone. For example, Russia lacks funding, the US lacks a crew vehicle. Russia does have plans for a &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_Piloted_Assembly_and_Experiment_Complex&quot; rel=&quot;noreferrer&quot;&gt;space station after ISS&lt;/a&gt; (which involves reusing Russian parts of the ISS), but it's still a long way there. It's not possible to undock the Russian section and keep it in space right now, as far as I know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the astronauts, as far as I have heard they are usually very friendly to each other. In Germany, there's a saying: &lt;em&gt;&quot;Sie sitzen alle im selben Boot.&quot;&lt;/em&gt; which translates to &lt;em&gt;&quot;They all sit in the same boat.&quot;&lt;/em&gt; That's quite literally the case with ISS: they live in a small area and cannot reasonably avoid each other, much less leave when they like to. So it's in everyone's best interest to get along. Besides, I have often heard astronauts say you get a different view when being in space: both literally and figuratively. You can't see borders from space (well, only a few… like &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:North_and_South_Korea_at_night.jpg&quot; rel=&quot;noreferrer&quot;&gt;North Korea at night&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2017-04-13T12:58:02.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21085" PostTypeId="1" AcceptedAnswerId="21113" CreationDate="2017-04-14T04:06:55.120" Score="5" ViewCount="633" Body="&lt;p&gt;Almost every single space enthusiast is excited about a Mars mission and when we are sending our scientists there. But from the communications point of view, are we ready to test the recent advancement to the technology? If not, are we looking for an alternate solution (lasers)?&lt;/p&gt;&#xA;" OwnerUserId="19002" LastEditorUserId="6537" LastEditDate="2018-01-10T01:37:11.400" LastActivityDate="2018-01-10T01:37:11.400" Title="Is Deep Space Network (DSN) technology sufficient for a crewed Mars mission?" Tags="|mars|nasa|communication|deep-space-network|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21087" PostTypeId="1" CreationDate="2017-04-14T12:49:06.387" Score="3" ViewCount="564" Body="&lt;p&gt;Immediately after separation, the upper stage of Falcon 9 seems to head 30 or so degrees sideways. What causes that? Is it a problem or a feature?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Separation occurs at 4:40 in &lt;a href=&quot;https://www.youtube.com/watch?v=JEz9uBZOp_c&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; of the Falcon 9 SES-10 launch March 30, 2017. The image below is a frame of it, taken from the first stage about five seconds after separation, showing the upper stage wobbling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ST1u5.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ST1u5.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="12102" LastEditDate="2019-06-07T05:29:48.707" LastActivityDate="2019-06-07T05:29:48.707" Title="Why is the Falcon 9 upper stage tumbling so much after separation? (SES-10)" Tags="|falcon-9|upper-stage|stage-separation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21088" PostTypeId="1" CreationDate="2017-04-14T12:53:55.917" Score="6" ViewCount="1873" Body="&lt;p&gt;When launching stuff into earth orbit like satellites for example often the last stage will perform the burn to achieve stable orbit and will then be jettisoned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now we have a rocket stage in stable orbit. &#xA;This is shown in so many videos, and they never show what happens to that stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it fire retrograde for reentry or prograde to end in some kind of a graveyard   orbit, or stays there as a space junk. &#xA;If so, how do they make sure the payload won't collide with the rocket in the future if it don't have its own propulsion system in order to be re-positioned in a slightly different orbit than the spent stage?&lt;/p&gt;&#xA;" OwnerUserId="19310" LastActivityDate="2017-04-20T08:02:07.990" Title="What happens to the last stage of rockets?" Tags="|rockets|spacecraft|debris|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21089" PostTypeId="2" ParentId="21087" CreationDate="2017-04-14T13:48:41.173" Score="1" Body="&lt;p&gt;Looking at the video, it seems the first stage is tumbling much more than the second.&lt;br&gt;&#xA;You can see the horizon from the second stage cam, the second stage wobbles a bit then straightens out again. The first stage (also in view) rapidly goes sideways.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could be the first stage was moved to get away from the second stage exhaust. Or the separation event itself resulted in some asymmetric force. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-04-14T13:48:41.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21090" PostTypeId="2" ParentId="21087" CreationDate="2017-04-14T14:41:18.103" Score="2" Body="&lt;p&gt;Looking at the exhaust of RCS from the second stage (before ignition), it seems to be exhausting more on one side. In order to rotate. So for me it's on purpose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could be a rotation of the second stage to protect the first stage against the exhaust of the second one.&lt;/p&gt;&#xA;" OwnerUserId="17308" LastActivityDate="2017-04-14T14:41:18.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21091" PostTypeId="2" ParentId="20765" CreationDate="2017-04-14T15:23:05.360" Score="15" Body="&lt;p&gt;I was fortunate enough to sit in on a talk by astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Richard_Mastracchio&quot; rel=&quot;noreferrer&quot;&gt;Rick Mastracchio&lt;/a&gt; this morning. Among other things, he was a flight engineer for &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_38&quot; rel=&quot;noreferrer&quot;&gt;Expedition 38&lt;/a&gt;/&lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_39&quot; rel=&quot;noreferrer&quot;&gt;39&lt;/a&gt;, during which the ISS saw the arrival of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cygnus_CRS_Orb-1&quot; rel=&quot;noreferrer&quot;&gt;first Cygnus resupply&lt;/a&gt; capsule to Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CRS Orb-1 was originally scheduled to arrive on Station just before Christmas, and the crew was looking forward to its arrival like kids on, well, Christmas. Unfortunately, a fault developed with an ISS coolant system that necessitated a series of spacewalks, causing NASA to stand down the launch of the capsule. It eventually arrived and was grappled on January 12th.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before these dates were decided, there was betting amongst the crew about whether the repairs or the delivery would win out in the scheduling - Rick mentioned putting a spicy chicken meal on the line (apparently a favorite).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it's possible this was just a joke, I'd be willing to believe it. Later in the talk, Rick was asked about the food on station - he said that while it was good overall, a 9-day variety cycle across a 6 month stay started to get pretty old. I would imagine that the fresh food right after a delivery is very valuable to the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This doesn't prove that cash is never used in the exchange, of course, but with the crew's basic needs provided for, this sort of wagering is probably about the only time a &quot;money&quot; substitute is needed. And I suspect that the prospect of winning something novel on orbit is much more fun than just putting cash on the line!&lt;/p&gt;&#xA;" OwnerUserId="5851" LastActivityDate="2017-04-14T15:23:05.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21092" PostTypeId="2" ParentId="21070" CreationDate="2017-04-14T20:45:01.447" Score="4" Body="&lt;p&gt;I see a few people have answered that it would be wasteful to have it empty, and that is why. However both the &lt;a href=&quot;https://www.nasa.gov/mission_pages/skylab/missions/skylab_manned.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skylab&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Mir#Expeditions&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mir&lt;/a&gt; space stations had vacant periods between expeditions to them. So, that can't be the only reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I think that the reason it has been manned continuously is that it has been possible to do so. There have been more regular manned flights and supply launches since both the USA and Russia have been launching them. (though manned launches have only been from Russia since the shuttle flights stopped).&lt;/p&gt;&#xA;" OwnerUserId="19314" LastEditorUserId="6816" LastEditDate="2017-04-14T20:48:28.793" LastActivityDate="2017-04-14T20:48:28.793" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21094" PostTypeId="1" AcceptedAnswerId="21103" CreationDate="2017-04-14T23:06:18.913" Score="0" ViewCount="292" Body="&lt;p&gt;Spacecraft and an object are rotating on orbit. The spacecraft is controlled via the velocity vector change, may change its velocity to intersect another object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Needed to find the time and coordinates of meeting, as both of them are dynamic objects. The optimal solution- minimum velocity change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideas?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another question: What is the maximum value of the delta V? It depend on the fuel, according to Cialcovski equation. Obviously, its value should be restricted by something else. Isp? And how?&lt;/p&gt;&#xA;" OwnerUserId="19317" LastEditorDisplayName="user17550" LastEditDate="2017-04-15T14:46:34.593" LastActivityDate="2017-04-15T16:00:22.260" Title="Optimal time and coordinates of intersection of a spacecraft with an object on orbit" Tags="|orbital-mechanics|orbital-maneuver|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21095" PostTypeId="1" AcceptedAnswerId="21096" CreationDate="2017-04-14T23:19:10.327" Score="4" ViewCount="1606" Body="&lt;p&gt;I'm having trouble understanding the current orbit planned for NASA's Space Launch System's &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_Mission_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Exploration Mission 1&lt;/a&gt;. Is it possible to explain how the planned orbit works and what the maximum distance from the Earth, and the Moon will be?&lt;/p&gt;&#xA;&lt;p&gt;The graphic below is from the NASA news item &lt;a href=&quot;https://www.nasa.gov/feature/the-ins-and-outs-of-nasa-s-first-launch-of-sls-and-orion&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Ins and Outs of NASA’s First Launch of SLS and Orion&lt;/a&gt;. The article says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The outbound trip to the moon will take several days, during which time engineers will evaluate the spacecraft’s systems and, as needed, correct its trajectory. Orion will fly about 62 miles (100 km) above the surface of the moon, and then use the moon’s gravitational force to propel Orion into a new deep retrograde, or opposite, orbit about 40,000 miles (70,000 km) from the moon.&lt;/p&gt;&#xA;&lt;p&gt;The spacecraft will stay in that orbit for approximately six days to collect data and allow mission controllers to assess the performance of the spacecraft. During this period, Orion will travel in a direction around the moon retrograde from the direction the moon travels around Earth.&lt;/p&gt;&#xA;&lt;p&gt;For its return trip to Earth, Orion will do another close flyby that takes the spacecraft within about 60 miles of the moon’s surface, the spacecraft will use another precisely timed engine firing of the European-provided service module in conjunction with the moon’s gravity to accelerate back toward Earth. This maneuver will set the spacecraft on its trajectory back toward Earth...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;What is a &amp;quot;new deep retrograde&amp;quot; orbit? How will it be new, deep, and differ from the early Apollo-era free-return orbits? Is it what is roughly suggested between &lt;code&gt;01:30&lt;/code&gt; and &lt;code&gt;01:37&lt;/code&gt; in the following video — also linked from the same NASA article, where the spacecraft is suddenly far from the moon, and the Earth and Moon appear similar size?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L0keO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L0keO.jpg&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Apollo-era drawing of Circumlunar free return trajectory,  from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Circumlunar-free-return-trajectory.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/E34Ci.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/E34Ci.png&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; EM-1 mission path, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Circumlunar-free-return-trajectory.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GPhNL.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GPhNL.png&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Mo8IkHM8fGE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-09-07T00:12:56.997" Title="How does Exploration Mission 1 orbit differ from Apollo-era free-return? How far from Earth (and Moon) does it go?" Tags="|the-moon|sls|free-return|lunar-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21096" PostTypeId="2" ParentId="21095" CreationDate="2017-04-15T00:21:24.293" Score="6" Body="&lt;p&gt;The Apollo missions braked directly into near-circular orbit around the moon at about 100km altitude, with a 2-hour orbital period, and remained there with various small alterations for the duration of the lunar stay (2-6 days, 25-70 orbits). These were always retrograde orbits; the spacecraft passed the &quot;leading edge&quot; of the moon, which slows the orbit in the Earth-centered frame, which is what you want for free-return. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per the &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_Mission_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article&lt;/a&gt;, it appears that the EM-1 mission plan has changed from an 8-day, out-and-back free return mission, to a 3 week mission with six days in lunar orbit. (Your first diagram appears to be for the later, crewed, EM-2 mission which does not enter lunar orbit.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/human_exploration_operations_update.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;presentation&lt;/a&gt; includes a diagram of the updated mission plan; the 70,000km apolune seems to be more or less directly away from the Earth, giving a total distance from Earth approximately 470,000km -- record-setting for a crew-rated craft. The 6-day period given is for a single eccentric orbit from the first 100km approach to 70,000 km and back to 100km. Insertion into that orbit is done with two burns, one at close approach and another later; another two burns symmetric with those return the ship to an Earthbound trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qFch5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qFch5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-09-07T00:12:56.997" LastActivityDate="2018-09-07T00:12:56.997" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="21097" PostTypeId="1" AcceptedAnswerId="21099" CreationDate="2017-04-15T01:13:05.803" Score="4" ViewCount="224" Body="&lt;p&gt;For example, how do the Mars landers communicate when Mars is between them and the Earth? The MRO is in a low orbit; does it store and forward data?&lt;/p&gt;&#xA;" OwnerUserId="759" LastActivityDate="2022-02-08T14:39:02.650" Title="How do landers communicate with Earth when facing away?" Tags="|communication|lander|deep-space-network|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21098" PostTypeId="1" AcceptedAnswerId="21138" CreationDate="2017-04-15T06:29:16.940" Score="6" ViewCount="1505" Body="&lt;p&gt;I need some help in understanding how rate gyros are used for measuring rotation rates about the yaw, roll, and pitch directions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose a gyro is used for measuring roll rate. How should the &lt;em&gt;spin axis&lt;/em&gt; and &lt;em&gt;input axis&lt;/em&gt; be oriented to get the rate in &lt;em&gt;output axis&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="14402" LastEditorUserId="12102" LastEditDate="2017-04-18T07:32:20.540" LastActivityDate="2017-04-19T01:33:17.117" Title="How do rate gyros measure yaw, roll and pitch rates?" Tags="|attitude|attitude-determination-and-control|gyroscope|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21099" PostTypeId="2" ParentId="21097" CreationDate="2017-04-15T09:01:09.247" Score="3" Body="&lt;p&gt;They don't. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a satellite is overhead, it can be used to store the data and replay it to earth, but otherwise the channel is broken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find more details if you look at Schiaparelli landing, most articles explained this issue altho I don't have one right now at hand.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-04-15T09:01:09.247" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21100" PostTypeId="2" ParentId="20765" CreationDate="2017-04-15T12:46:09.117" Score="8" Body="&lt;p&gt;&lt;strong&gt;Another $0.51!&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Currently this money is not in the possession of any astronauts, but it is none the less &lt;em&gt;money in space&lt;/em&gt;. (well, the penny is on Mars, so not exactly &amp;quot;in space&amp;quot;?)&lt;/p&gt;&#xA;&lt;p&gt;Two US state commemorative quarters (Florida and Maryland) were added to New Horizons &amp;quot;for spin balance&amp;quot;. They are mentioned in Coin World &lt;a href=&quot;http://www.coinworld.com/news/us-coins/2014/12/new-horizons-awakens-for-pluto-encounter-with-two-quarters-aboard.all.html#&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://www.coinworld.com/news/us-coins/2015/07/state-quarters-zip-past-dwarf-planet-pluto-aboard-new-horizons.all.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. The Florida quarter is notable because it is space-themed and was &lt;a href=&quot;http://www.collectspace.com/news/news-102808a.html&quot; rel=&quot;noreferrer&quot;&gt;handed to NASA engineers by Florida's Governor Bush&lt;/a&gt;. Actually according to the article it was an entire roll of quarters! The rest were donated as souvenirs to people working on the spacecraft.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;For spin balance, we need to add a number of kilograms to various places [on New Horizons],&amp;quot; explained Stern. &amp;quot;We knew this was the case because the moments of inertia of the spacecraft and the dynamical properties of it, that we would have to trim it out down to literally the grams-level with balance weights. Of course, we had a whole variety of big ones and little ones; you start off with adding a kilogram here and a kilogram there and you end up getting smaller and smaller weights in various places until you're done. We used the coins to that purpose,&amp;quot; he said.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Since we needed a counter balance to [the Florida state quarter], we decided to fly a second state quarter. We picked Maryland because that is where the spacecraft was built. And because we had so many people back in Maryland at the Applied Physics Lab and at Goddard, it was easy for someone to ship us a quarter really quick.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;A 1909 US penny is attached to the Curiosity Rover's test pattern collection. Read more about it at &lt;a href=&quot;http://www.coinnews.net/2013/11/01/1909-lincoln-cent-on-mars-collects-dust/&quot; rel=&quot;noreferrer&quot;&gt;Coin News&lt;/a&gt;. Here is an image of it on Mars:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below x2:&lt;/strong&gt; Martian dust-covered penny on Curiosity Rover. From &lt;a href=&quot;https://photojournal.jpl.nasa.gov/catalog/PIA17365&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k1fkg.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k1fkg.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IYgcY.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IYgcY.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Penny on Curiosity Rover. From &lt;a href=&quot;https://photojournal.jpl.nasa.gov/catalog/PIA17365&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5YvDf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5YvDf.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Penny on Curiosity Rover's eye chart. From &lt;a href=&quot;http://www.coinnews.net/2013/11/01/1909-lincoln-cent-on-mars-collects-dust/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NcWS7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NcWS7.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; State of Florida Commemorative Quarter to be attached to New Horizons spacecraft. Maryland state quarter was also on board, but located at a different position for balance reasons. From &lt;a href=&quot;http://www.collectspace.com/news/news-102808a.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TdYCD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TdYCD.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-04-26T14:19:06.863" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21101" PostTypeId="1" AcceptedAnswerId="30008" CreationDate="2017-04-15T13:18:52.260" Score="12" ViewCount="1021" Body="&lt;p&gt;&lt;a href=&quot;http://www.collectspace.com/news/news-102808a.html&quot; rel=&quot;noreferrer&quot;&gt;This story&lt;/a&gt; about receiving a roll of &lt;em&gt;Space-Themed&lt;/em&gt; Florida State commemorative quarters from the Governor of Florida mentions that the &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;New Horizons&lt;/a&gt; spacecraft had to be &amp;quot;spin-balanced&amp;quot; to grams-level precision.&lt;/p&gt;&#xA;&lt;p&gt;What exactly does this mean &lt;em&gt;mathematically?&lt;/em&gt; What exactly is being balanced?&lt;/p&gt;&#xA;&lt;p&gt;I understand that &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Propulsion_and_attitude_control&quot; rel=&quot;noreferrer&quot;&gt;New Horizons has both spin-stabilized (cruise) and three-axis stabilized (science) modes controlled entirely with hydrazine monopropellant&lt;/a&gt;, but does spin stabilization require careful balancing? I noticed that &lt;a href=&quot;https://space.stackexchange.com/a/19440/12102&quot;&gt;this answer&lt;/a&gt; about another spacecraft also explicitly mentions the need to balance carefully.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Stern accepted the roll from the governor, but explained he could only fly one. The others would be distributed to team members as a souvenir of the mission.&lt;/p&gt;&#xA;&lt;p&gt;Less you think however, that the quarter flew simply as a gesture to the governor, it served a bonafide purpose on the spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;For &lt;strong&gt;spin balance&lt;/strong&gt;, we need to add a number of kilograms to various places [on New Horizons],&amp;quot; explained Stern. &amp;quot;We knew this was the case because the moments of inertia of the spacecraft and the dynamical properties of it, that we would have to trim it out down to literally the grams-level with balance weights. Of course, we had a whole variety of big ones and little ones; you start off with adding a kilogram here and a kilogram there and you end up getting smaller and smaller weights in various places until you're done. We used the coins to that purpose,&amp;quot; he said. (emphasis added)&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Since we needed a counter balance to [the Florida state quarter], we decided to fly a second state quarter. We picked Maryland because that is where the spacecraft was built. And because we had so many people back in Maryland at the Applied Physics Lab and at Goddard, it was easy for someone to ship us a quarter really quick.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; State of Florida Commemorative Quarter to be attached to New Horizons spacecraft. Maryland state quarter was also on board, but located at a different position for balance reasons. From &lt;a href=&quot;http://www.collectspace.com/news/news-102808a.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TdYCD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TdYCD.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; A separate example: LADEE going for a test spin. Looks frightening! Cropped from &lt;a href=&quot;https://www.nasa.gov/content/ladee-spin-test&quot; rel=&quot;noreferrer&quot;&gt;LADEE Spin Test&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;To make sure that the spacecraft is perfectly balanced for flight, engineers mounted it onto a spin table and rotate it at high speeds, approximately one revolution per second. The team measured any offsets during the spinning, and then added small weights to the spacecraft to balance it. Once the spacecraft was balanced dry, the team loaded the propulsion tanks with fuel, oxidizer, and pressurant. The spin testing was performed again &amp;quot;wet,&amp;quot; or with fuel, in order to see if the balance changed with the full fuel tanks.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yJ9Kb.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yJ9Kb.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; A separate example: Curiosity(?) going for a test spin. Looks upside down! From &lt;a href=&quot;https://www.space-electronics.com/Products/poi-spin-balance-series&quot; rel=&quot;noreferrer&quot;&gt;Space Electronics&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Two-Plane Spin Balance Machines with Moment of Inertia Capability.&#xA;Measure product of inertia, dynamic unbalance, center of gravity and moment of inertia.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pZXHs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pZXHs.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2020-07-17T12:28:11.957" Title="Why did New Horizons have to be spin-balanced to grams-level precision? (With quarters!)" Tags="|attitude|new-horizons|space-art|spin-stabilisation|" AnswerCount="3" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21102" PostTypeId="1" CreationDate="2017-04-15T14:03:00.563" Score="2" ViewCount="520" Body="&lt;p&gt;What is the fuel mass nedeed for the 2nd stage of Falcon 9 rocket to land, how much compared with its total fuel mass that it has? Or how much delta-v is necessary? Would it be able for Falcon 9 to send any payload into orbit if they would try to land the 2nd stage, if yes how much?&lt;/p&gt;&#xA;" OwnerUserId="12865" LastEditorUserId="12865" LastEditDate="2017-04-15T14:18:00.643" LastActivityDate="2017-04-15T14:18:00.643" Title="How much fuel is necessary for 2nd stage of Falcon 9 rocket to land?" Tags="|spacex|falcon-9|fuel|reuse|upper-stage|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21103" PostTypeId="2" ParentId="21094" CreationDate="2017-04-15T16:00:22.260" Score="1" Body="&lt;p&gt;As far as I know, no analytical solution to the problem has been found, and it's normally obtained through numerical optimization - like the Simplex method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, let's take care of the Tsiolkovsky's rocket equation: delta-V depends on specific impulse, dry mass of the craft (without fuel), and amount of fuel (or wet mass of the craft.) Since the first two don't change during the flight, only the third is actually a variable, not a constant. Still, delta-V may convert to different change of trajectory, and different change of energy, depending on what maneuver and where is performed - in particular, look up Oberth Effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for your problem: you're asking for minimal burn that creates a fly-by encounter between two arbitrary initial orbits; encounter speed doesn't matter, the crafts don't need to dock to each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will need orbital motion equations of both crafts - for one central body you can use the conic section formulas / &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_mechanics#Formulae_for_free_orbits&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbit equation&lt;/a&gt;, for a more complex system you'll use &lt;a href=&quot;https://en.wikipedia.org/wiki/Hamiltonian_mechanics&quot; rel=&quot;nofollow noreferrer&quot;&gt;hamiltonian mechanics&lt;/a&gt;  to derive equations of motion in the non-uniform gravitational field. You'll also need a third orbit, a generalized parametric equation for one, which will be your transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then you need to bind the three: write a set of equations where:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the position of the target is the same as position of the craft in the transfer orbit, at a certain time (the encounter)&lt;/li&gt;&#xA;&lt;li&gt;the position of the spacecraft in the initial orbit is the same as in the transfer orbit (transfer burn).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Add an equation of delta-V of the departure burn: difference of velocities at the moment of transfer burn between the initial orbit and the transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your set of equations is under-constrained: your free variables are time of departure, and departure delta-V (3 variables, a vector). This set of equations then is the subject to numerical optimization: finding such parameters that the magnitude of the departure burn is least - and where TIME IS REASONABLY CONSTRAINED!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This last part will be a big headache. That departure burn must occur before encounter, this is obvious. But the encounter largely depends on relative position of the two craft, and if their orbits aren't resonant (orbital period ratio isn't a simple fraction) one can always find a more optimal departure burn - if it's to happen enough millions of years away. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply, the best, most optimal transfer exists, which requires the craft to be in certain alignment to each other. Least delta-V to reach nearest point of the target orbit, and the target &lt;em&gt;just&lt;/em&gt; happens to be there at the point of time when you arrive. Usually it won't - it will be in an entirely different place of its orbit. But if you wait long enough, you will be aligned to encounter it pretty close to that optimal point, and you will waste very little fuel vs the optimal for that adjustment. If you wait longer though, it will pass even closer, so you can do this more optimally. And so on, to infinity.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-04-15T16:00:22.260" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21105" PostTypeId="2" ParentId="19402" CreationDate="2017-04-15T17:56:09.553" Score="5" Body="&lt;p&gt;For the record, @Uwe said this in a comment, which @uhoh considers an answer:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The MA-6 Technical Report on &lt;a href=&quot;http://ibiblio.org/mscorbit/document.html&quot; rel=&quot;noreferrer&quot;&gt;http://ibiblio.org/mscorbit/document.html&lt;/a&gt; shows some trajectory parameters in Table II on page 26 to 28 of PDF A nice mixture of nautical and statue miles, velocities in foot per second. Good luck for the orbit plot&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That &lt;a href=&quot;http://prdownloads.sourceforge.net/mscorbaddon/MA6_TechnicalReport.pdf?download&quot; rel=&quot;noreferrer&quot;&gt;MA-6 Technical report&lt;/a&gt; says, for example: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;TABLE II COMPARISON OF PLANNED AND ACTUAL TRAJECTORY PARAMETERS&#xA;| Condition and Quantity              | Planned | Actual  | Difference |&#xA;|----------------------------------------------------------------------|&#xA;| Cutoff conditions (including tail-  |         |         |            |&#xA;|  off):                              |         |         |            |&#xA;| Range time, seconds                 | 303.3   | 302.0   | -1.3       |&#xA;|             min:sec                 | 05.03.3 | 05:02   |            |&#xA;| Geodetic latitude,                  | 30.4273 | 30.4533 |  0.0260    |&#xA;| Longitude, deg west                 | 72.5268 | 72.5865 |  0.0597    |&#xA;| Altitude, feet                      | 528,428 | 528,381 |     -47    |&#xA;|           nautical miles            |   57.00 |   56.96 |   -0.04    |&#xA;| Range, nautical miles               | 436.4   | 433.7   |    -2.7    |&#xA;| Space-fixed velocity, feet per sec  | 25715   | 26708   |    -7.0    |&#xA;| Space-fixed flight-path angle, deg  |     0   | -.0468  |  -.0468    |&#xA;| Space-fixed heading angle, deg      |         |         |            |&#xA;|   east of north                     | 77.4756 | 77.4826 |   0.0070   |&#xA;&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original memo on calculating orbits like Glenn's (by &lt;a href=&quot;https://en.wikipedia.org/wiki/Katherine_Johnson&quot; rel=&quot;noreferrer&quot;&gt;Katherine Johnson&lt;/a&gt;, with Ted Skopinski, is &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980227091.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Determination of Azimuth Angle at Burnout for Placing a Satellite over a Selected Earth Position 1960. T.H. Skopinski, Katherine G. Johnson, NASA TN D-233&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I note there, I'd also love to see recreations of the orbits, and especially comparisons of the orbits calculated according to the original methods (including the IBM 7090 approach), vs modern &lt;a href=&quot;https://naif.jpl.nasa.gov/naif/spiceconcept.html&quot; rel=&quot;noreferrer&quot;&gt;spacecraft kernel approaches&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more on recreations of what those human computers, of Hidden Figures fame, did, see my answer at &lt;a href=&quot;https://space.stackexchange.com/a/20870/5775&quot;&gt;specific examples of the calculations human “computers” did for the Mercury space program?&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These data, showing state vectors for images (position, direction of view) from Apollo may also be of interest.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://apollo.sese.asu.edu/EPHEMERIS/&quot; rel=&quot;noreferrer&quot;&gt;Apollo Image Archive - Ephemeris Data&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="5775" LastActivityDate="2017-04-15T17:56:09.553" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21106" PostTypeId="1" AcceptedAnswerId="21107" CreationDate="2017-04-15T20:53:16.800" Score="4" ViewCount="269" Body="&lt;p&gt;I'm following the slot swaps of the Iridium satellites in plane 6 and cannot make sense of the following: The TLEs of three of the old satellites very often show a marked difference in the mean anomaly to the previous day, while the mean anomaly of the other 7 satellites barely changes. Why could this be, what effect does it have?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example, I attach the TLEs of three consecutive days. Iridium 80, 77 and 98 show the big leaps in mean anomaly while the other satellites seem fairly unchanged. As far as I know, there was no manoeuvering going on with any of them at that time. The IridiumNEXT sats had already been inserted and were being tested.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exhibit 1&#xA;&lt;a href=&quot;https://i.stack.imgur.com/yokUV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yokUV.png&quot; alt=&quot;TLE_1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exhibit 2&#xA;&lt;a href=&quot;https://i.stack.imgur.com/2oJ9f.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2oJ9f.png&quot; alt=&quot;TLE_2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exhibit 3&#xA;&lt;a href=&quot;https://i.stack.imgur.com/5amKx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5amKx.png&quot; alt=&quot;TLE_3&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19326" LastEditorUserId="19326" LastEditDate="2017-04-16T16:22:12.823" LastActivityDate="2017-04-16T16:22:12.823" Title="What does a steep change in mean anomaly indicate?" Tags="|orbital-elements|two-line-elements|iridium|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21107" PostTypeId="2" ParentId="21106" CreationDate="2017-04-15T21:52:58.673" Score="5" Body="&lt;p&gt;Recall that the mean anomaly indicates the position within the orbit of the satellite/object.  In the short term, whilst the other five orbital elements are only very slowly changing, the mean anomaly rotates by 360 degree for every revolution completed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The significance of the mean anomaly in a TLE then is that it is telling us both: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;(trivial answer) that the object was estimated to have that combination of mean anomaly at that given epoch. From this you can calculate forwards or backwards a little way and find the position of the satellite at other times.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;(slightly more meaningful answer) that in the orbit determination effort data was gathered from a number of parts of the orbit, or consecutive orbits, and was decided to publish the TLE with that epoch. You can probably see that this is &lt;em&gt;only slightly&lt;/em&gt; more meaningful in the sense that it could be interpreted as the centroid of that data collection effort though it wouldn't surprise me if the epoch is chosen to obscure any hints about the data collection sensors involved.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TLDR&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In isolation, without the whole orbit information, there is little that can be learned from the mean anomaly alone. The implication, assuming that the Iridium satellites you refer to aren't being wildly manoeuvred, is that the habitual choice of publication epoch chosen for each satellite varies: for some the mean anomaly changes more than for others as a result.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-04-15T21:52:58.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21108" PostTypeId="1" AcceptedAnswerId="21137" CreationDate="2017-04-16T02:30:36.750" Score="2" ViewCount="347" Body="&lt;p&gt;I am trying to plot an orbit and am having some success.  I have directly calculated the orbital parameters so I am able to plot the ellipse.  I am also plotting a path of the orbit as a sanity check. The ellipse from orbital parameters is red and the sanity check is white.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I only apply tangential velocity such that &lt;code&gt;vp.x=0&lt;/code&gt;, things match up exactly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wrY3X.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wrY3X.png&quot; alt=&quot;Pic 1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, when I add some radial velocity, such that &lt;code&gt;vp.x&amp;gt;0&lt;/code&gt;, I have a bit of an issue.  It rotates correctly with the &quot;Argument of Periapsis&quot; and maintains the same shape but I need to shift the ellipse to match what I should be getting in white.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/T1Tol.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/T1Tol.png&quot; alt=&quot;Pic 2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone advise how to calculate the x and y shift required so the red ellipse matches the white?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am making use of the code &lt;em&gt;shown in the following question and answer&lt;/em&gt; to calculate orbital parameters so I can plot the orbital ellipse (in red).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/16891/how%C2%ADto%C2%ADcalculate%C2%ADthe%C2%ADtime%C2%ADto%C2%ADapoapsis%C2%ADperiapsis%C2%ADgiven%C2%ADthe%C2%ADorbital%C2%ADelements&quot;&gt;How to calculate the time to apoapsis &amp;amp; periapsis, given the orbital elements?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1904/how-to-programmatically-calculate-orbital-elements-using-position-velocity-vecto/1919#1919&quot;&gt;How to programmatically calculate orbital elements using position/velocity vectors?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks.&lt;/p&gt;&#xA;" OwnerUserId="19330" LastEditorUserId="12102" LastEditDate="2017-04-16T04:11:27.267" LastActivityDate="2017-04-20T18:12:16.893" Title="How to rotate orbital ellipse when radial velocity is added to the orbital velocity" Tags="|orbital-mechanics|orbit|orbital-elements|mathematics|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21109" PostTypeId="2" ParentId="21085" CreationDate="2017-04-16T05:32:28.697" Score="3" Body="&lt;p&gt;Probably not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back in 2014, NASA &lt;a href=&quot;https://www.fbo.gov/index?s=opportunity&amp;amp;mode=form&amp;amp;id=88ebe03366dd4a884df77caddd46d1fc&amp;amp;tab=core&amp;amp;_cview=0&quot; rel=&quot;nofollow noreferrer&quot;&gt;issued&lt;/a&gt; a Request for Information concerning the use of commercial solutions for Mars relays. One of the key limitations outlined in this RFI was the very limited bandwidth of the current relay infrastructure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, the DSN is used for many other interplanetary missions (if not all, I don't have the data on this). Therefore, there is an inherent scheduling issue if one wants to maintain permanent communication with a crew on Martian soil, or in transit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As &lt;a href=&quot;https://space.stackexchange.com/a/21113/1391&quot;&gt;pointed out&lt;/a&gt; by Mark Adler, the main barrier to laser communication from Earth to Mars may be the lack of money and mission requirement to continue developing this. Briefly looking this up, it seems ESA has &lt;a href=&quot;https://gsp.esa.int/documents/10192/46710/C4000106720ExS.pdf/f645647c-b133-421d-b46c-a4913dff943e&quot; rel=&quot;nofollow noreferrer&quot;&gt;reached&lt;/a&gt; a Technology Readiness Level of 6 for their LEO to Earth systems. NASA was at least &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/tglavich_dsoc.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;planning&lt;/a&gt; to reach a TRL of 6 for their deep space laser communications by Mars 2017.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Concerning the &lt;em&gt;pointing&lt;/em&gt; situation, a GEO spacecraft, which is only about 36,000 km above Earth, would shift its radio-frequency coverage projection by 700 km if its pointing is off by 1 degree. Lasers have a much more narrow beam than a typical reflector (or &lt;em&gt;antenna&lt;/em&gt;). So the pointing constraint is even more stringent. I suspect that the solution we'll end up choosing in a few years is &quot;plain-old&quot; radio communication but at a higher frequency: the higher the frequency, the more power is needed to generate that signal, but the bandwidth is also increased. In any case, a Mars mission would surely have several redundant communication systems.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="6816" LastEditDate="2018-01-09T20:35:50.610" LastActivityDate="2018-01-09T20:35:50.610" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21110" PostTypeId="1" CreationDate="2017-04-16T09:44:10.133" Score="3" ViewCount="302" Body="&lt;p&gt;Most exoplanets making the news are larger than Earth.  I wonder about the smaller worlds, how small is too small? Reword to keep the question narrow enough to be answerable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the Earth was created in it's orbit around our sun, if Earth had been smaller, how small could it have been and still allowed human life to evolve and survive?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine that the ability to maintain an atmosphere is the key.  There are several things impacting atmosphere retention.  If we make the Earth smaller and/or less dense, at what point does life as we know it today became impossible to have evolved and survived to present day? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recognize the Moon is important in the process also, but for simplicity answers may choose to ignore lunar influences. &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="238" LastEditDate="2017-04-16T20:46:20.783" LastActivityDate="2017-04-18T06:29:22.903" Title="Smallest planet where human type life could evolve and survive?" Tags="|gravity|atmosphere|life|earth|exoplanet|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21111" PostTypeId="1" AcceptedAnswerId="21115" CreationDate="2017-04-16T12:58:13.320" Score="2" ViewCount="283" Body="&lt;p&gt;The spacecraft is on a LEO orbit with known keplerian elements. The equation of motion for spacecraft is also known.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have to intersect (just intersect!) the given coordinates $X$, $Y$, $Z$ on near, LEO orbit. Departure and Arrival times are given. $\Delta V$ is applied in an instant.&#xA;X,Y,Z is on different plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How to calculate the required $\Delta V$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I made research about the 'Hohmann transfer orbit' and 'vis-viva equation'. However, I can't understand how to intersect a point on another orbit exactly on time $T$?&lt;/p&gt;&#xA;" OwnerUserId="19219" LastEditorUserId="19219" LastEditDate="2017-04-16T17:19:12.037" LastActivityDate="2017-04-16T22:37:12.027" Title="The delta V required for given cordinates intersection on given time" Tags="|orbital-mechanics|orbital-maneuver|delta-v|" AnswerCount="1" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="21112" PostTypeId="2" ParentId="19564" CreationDate="2017-04-16T13:22:42.300" Score="14" Body="&lt;p&gt;The main 'throat' of the PMA is not slanted -- it is a 31.5 inch passageway that is straight as an arrow. However, the throat has to have a bit more open room on one side because the hatch of the Russian docking port opens outwards. If there was not a space for the hatch to open beyond 90 degrees, it would block the main passageway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NASA document &lt;a href=&quot;http://spacecraft.ssl.umd.edu/design_lib/42120.ISS.APAS.ICD.pdf&quot; rel=&quot;noreferrer&quot;&gt;Space Station Program Androgynous Peripheral Assembly System to Pressurized Mating Adapter Interface Control Document, Part 1 Core (APAS to PMA-2 &amp;amp; 3&lt;/a&gt; (SSP 42120) has a good diagram on page 3-14:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XlGs7.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XlGs7.png&quot; alt=&quot;SSP 42120 page 3-14, Fig. 3.2.1.2.4-1; APAS to PMA stay-out/passageway envelopes&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I can tell, the slant is primarily there to cover the 'lump' required for the Russian hatch to open fully, but this is probably more than an aesthetic choice: a smooth slant provides one less lump for a 29-ton shuttle to bump against, and it is probably also a lot easier to fit the exterior micrometeoroid/orbital debris shield to a flat slope than to an ungainly lump. &lt;/p&gt;&#xA;" OwnerUserId="19332" LastEditorUserId="12102" LastEditDate="2017-04-17T03:09:34.527" LastActivityDate="2017-04-17T03:09:34.527" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21113" PostTypeId="2" ParentId="21085" CreationDate="2017-04-16T13:42:48.863" Score="7" Body="&lt;p&gt;The current DSN could be enough, depending on what the crewed mission requirements are. If you just want to communicate with voice and moderate-rate engineering and science data, then sure. If your requirements are to send back many channels of continuous 8K ultra-high definition video, then no. You would at least need to upgrade the DSN receivers to handle those kinds of data rates. With a big enough antenna and power amplifier on Mars, you could, in principle, transmit those data rates at current DSN radio wavelengths.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it would be much more efficient to use laser communication to meet those sorts of requirements, in terms of power and aperture. It is quite feasible to do so. There is no insurmountable problem with pointing, as speculated in another answer here. We lack only the money and the mission impetus to build such systems. Space and ground systems for Mars-Earth laser communications have been designed and proposed for decades, and a &lt;a href=&quot;https://gameon.nasa.gov/projects-2/archived-projects-2/deep-space-optical-communications-dsoc/&quot; rel=&quot;noreferrer&quot;&gt;technology development effort is currently underway&lt;/a&gt; to prepare for the mission that might someday demand it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the other rather large expenses of crewed missions to Mars, it seems extremely likely that the small investment would be made in laser communications to support those missions and to provide better entertainment value from the mission back here on Earth.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-04-16T13:42:48.863" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21114" PostTypeId="2" ParentId="21101" CreationDate="2017-04-16T14:40:26.557" Score="5" Body="&lt;p&gt;During a spin stabilized cruise, the antenna dish should be pointed to earth precisely. The axis of rotation should be the axis of the antenna. If the space craft would be not precisely spin balanced, the axis of rotation and the antenna axis may be different and there may be a vibration. Both effects would disturb the antenna alignment to earth and impair data transmission in both directions from and to the spacecraft. For a clean rotation without vibrations, the space craft should be both static and dynamic balanced.&lt;br&gt;&#xA;See &lt;a href=&quot;https://en.wikipedia.org/wiki/Tire_balance&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Tire_balance&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Rotating_unbalance&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Rotating_unbalance&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Careful balancing the spacecraft is also important to save the precious fuel for attitude control. For the rotation around the three axes there should be three pairs of thrusters. Each pair should be mounted symmetric to the center of mass. If there is a disbalance and the center of mass is not where it should be, firing one pair of thrusters may cause rotation of the spacecraft around more than one axis only. Stabilizing the attitude would require more fuel than necessary if unwanted rotations have to be controlled too.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-04-18T11:25:31.633" LastActivityDate="2017-04-18T11:25:31.633" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21115" PostTypeId="2" ParentId="21111" CreationDate="2017-04-16T17:50:28.827" Score="6" Body="&lt;p&gt;Finding the orbit that connects two points with a given transit time is known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Lambert%27s_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lambert's Problem&lt;/a&gt;, and has a small set of solutions. Once you have that orbit, you just need to subtract the velocities of the old and new orbit at the starting point of the new orbit.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-04-16T22:37:12.027" LastActivityDate="2017-04-16T22:37:12.027" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21116" PostTypeId="1" CreationDate="2017-04-17T01:50:40.187" Score="6" ViewCount="465" Body="&lt;p&gt;Let's say an evil dictator decide to deliver a payload to a &lt;em&gt;retrograde&lt;/em&gt; &quot;GEO&quot; orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the first inevitable collision would quickly produce a &lt;a href=&quot;https://en.wikipedia.org/wiki/Kessler_syndrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kessler syndrome&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there anything that would allow us to detect it in time, and what could we realistically do about it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NB: Yes, technically a GEO orbit cannot be retrograde, but you get the idea.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6816" LastEditDate="2017-04-17T15:42:15.037" LastActivityDate="2017-04-19T20:45:38.343" Title="Could we prevent a disaster if someone tried to send a rocket in retrograde GEO?" Tags="|debris|collision-avoidance|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21117" PostTypeId="2" ParentId="21116" CreationDate="2017-04-17T04:18:33.513" Score="3" Body="&lt;p&gt;Per &lt;a href=&quot;https://space.stackexchange.com/questions/2914/how-long-does-it-take-for-a-satellite-to-reach-geo&quot;&gt;this question&lt;/a&gt;, it takes about 6 hours from launch to reach that orbit, and it's hard to imagine that another space agency could detect and realize the threat and program an early interception trajectory in time to take it out even if they happened to have a launcher on the pad capable of doing so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the offender was simply injected into equatorial retrograde orbit at geosync altitude, it might not hit anything immediately, but it would now be harder to take it out safely without turning it into Kesslerish debris, effectively giving it the initial collision for free. Carefully ablating one side of it with a great big laser could push it out of position, perhaps. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the offender was a guided, self-propelled spacecraft, I don't think there's a chance of stopping it. Ground-based guidance could put it on a close enough course that onboard radar could finish the job. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;About the only way I see a chance is if the launch trajectory is actively controlled from the ground, with insufficient onboard location capability for the spacecraft to get into position on its own. Then, conceivably, once the retrograde transfer orbit was understood by other nations, a massive military strike against the launching agency's communications uplink could take out the threat, but no half-competent evil genius would neglect that consideration. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-04-17T04:18:33.513" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21121" PostTypeId="2" ParentId="21101" CreationDate="2017-04-17T08:33:22.477" Score="3" Body="&lt;p&gt;As Uwe said, the axis of rotation should coincide with the pointing axis of the high-gain antenna. Exact balancing is required, &lt;a href=&quot;http://www.unmannedspaceflight.com/index.php?showtopic=675&amp;amp;st=283&quot; rel=&quot;nofollow noreferrer&quot;&gt;according to Alan Stern&lt;/a&gt; (principal investigator, New Horizons):  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The s/c has to be balanced for the spin to be stable and without any excessive nutation. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Having the rotation axes in a known location should also make using the thrusters for attitude control easier. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-04-17T08:33:22.477" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21122" PostTypeId="1" AcceptedAnswerId="21127" CreationDate="2017-04-17T09:23:34.777" Score="5" ViewCount="740" Body="&lt;p&gt;Because &lt;a href=&quot;https://en.wikipedia.org/wiki/Phobos_(moon)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phobos&lt;/a&gt; has always &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_locking&quot; rel=&quot;nofollow noreferrer&quot;&gt;the same face turned toward Mars&lt;/a&gt;, an electric propulsion system could be placed at the front to slow down the orbital speed.&lt;/p&gt;&#xA;&lt;p&gt;But what would be the energy needed to supply the force of the propulsion system ?&lt;/p&gt;&#xA;" OwnerUserId="19335" LastEditorUserId="19335" LastEditDate="2021-07-07T16:58:28.710" LastActivityDate="2021-07-07T22:04:14.113" Title="How much energy is needed to bring Phobos closer to Mars?" Tags="|orbital-mechanics|orbital-maneuver|phobos|" AnswerCount="5" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21123" PostTypeId="2" ParentId="21110" CreationDate="2017-04-17T10:29:18.243" Score="2" Body="&lt;p&gt;As far as we know the life only on the Earth, we could only speculate about &lt;em&gt;human-like life&lt;/em&gt;. We don't exactly know what would change depending on one or another condition.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;On ice moons like Enceladus &lt;a href=&quot;https://en.wikipedia.org/wiki/Enceladus#Potential_habitability&quot; rel=&quot;nofollow noreferrer&quot;&gt;life could exist&lt;/a&gt; - despite the size and distance from the Sun are very different. The key factors are hydrothermal activity and shield of ice. You could imagine about some kind of whales which smart enough to build civilisation&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Europa_(moon)#Habitability_potential&quot; rel=&quot;nofollow noreferrer&quot;&gt;Europa&lt;/a&gt; has atmosphere so you could imagine moon like this with life on surface. But without light and having small mass most likely forms of life would be differ from human&lt;/li&gt;&#xA;&lt;li&gt;Mars lost its atmosphere for some reasons and it's possible that human-like life could arise on Mars-like planet&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;So, the atmosphere, energy and chemical elements could be met all together in very different planets and moons. (One could add some factors like magnetic field, ionosphere etc)&#xA;But it's hard to say about the gravity. We could just imagine and read many fantastic books :)&lt;/p&gt;&#xA;" OwnerUserId="19339" LastEditorUserId="19339" LastEditDate="2017-04-18T06:29:22.903" LastActivityDate="2017-04-18T06:29:22.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21124" PostTypeId="1" AcceptedAnswerId="30966" CreationDate="2017-04-17T11:11:30.900" Score="6" ViewCount="696" Body="&lt;p&gt;I was enjoying some 2017 April 13 tweets on robots and reminiscing about shoveling snow after seeing &lt;a href=&quot;https://twitter.com/Rogozin/status/853282603453075458&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt;, but then I scrolled down further and saw a video of a rocket. It's also &lt;a href=&quot;https://www.youtube.com/watch?v=cjaWfn8TwRg&amp;amp;feature=youtu.be&amp;amp;t=89&quot; rel=&quot;nofollow noreferrer&quot;&gt;here in YouTube&lt;/a&gt;, where the information seems to say it is the first launch from &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostochny_Cosmodrome&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vostochny Cosmodrome&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; The time line is not 100% clear - there is dry snow on the ground in the screen shots shown below, but by the time the actual launch happens, there is no snow, nor wet ground where it would have melted recently. The screen shots at the bottom clearly show different weather and different fairing covers - it looks like these are two different days. However, my question is simply about the need for insulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps the screenshots below really represent a &quot;dry run&quot; — testing and preparation for &lt;a href=&quot;http://www.russianspaceweb.com/soyuz2-vostochny-first-launch.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this launch&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is certainly a &quot;cool&quot; video (manage your volume, there is a sound track) but is the Amur Oblast really colder than (a spacecraft might become when it is in) space? Or have I got it wrong, and the payload is pre-cooled cryogenic and the local ambient is actually too hot?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mh8OK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mh8OK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6tIu6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6tIu6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xV0tq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xV0tq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KQt7w.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KQt7w.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/R67Lr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R67Lr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/l8j90.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/l8j90.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-09-27T02:28:39.530" LastActivityDate="2018-09-27T21:47:49.733" Title="Is this fairing covered in thick thermal insulation? Keeping the payload warm? Or Cold?" Tags="|russia|fairings|soyuz-launcher|" AnswerCount="1" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="21125" PostTypeId="1" AcceptedAnswerId="26894" CreationDate="2017-04-17T11:35:56.647" Score="5" ViewCount="738" Body="&lt;p&gt;This solar panel is closer to square than most solar panels I've seen, which tend to be very long - length/width ratio perhaps 4 to 6. This looks like only 1.5, and it seems to be located at the end of a long arm. To me this is an unusual configuration - is there something specific about MetOp-A, B, C's operations that require this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shot from &lt;a href=&quot;http://www.bbc.com/news/science-environment-39632181&quot; rel=&quot;nofollow noreferrer&quot;&gt;2017-April-19 BBC article&lt;/a&gt; about space junk. Presumably this was a random satellite chosen by the author, and does not suggest the configuration is to make it easier to &quot;catch in a big net&quot;!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XiNTo.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XiNTo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt;  from &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/metop.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt; (Astrium).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8u4Yz.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8u4Yz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Astrium Polar Platform (PPF) from &lt;a href=&quot;http://space.skyrocket.de/doc_sat/astrium_ppf.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jXwDF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jXwDF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt;  Artists conception of MetOp-A in orbit including fully deployed solar panel on deployment arm, from &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2006/02/Deployment_of_the_solar_array&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once MetOp is in the correct position the solar array, which is on the zenith side of the satellite, is gently deployed to face the Sun. This procedure takes up to 17 minutes and happens in two stages. Initially, the solar array arm rotates 120° up and over. Then the large eight-section panel opens out to face the Sun.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vbnCt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vbnCt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt;  MetOp-A, including folded solar panel on deployment arm, from &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2006/09/Solar_array_preparation_finished&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QlIiC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QlIiC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/MetOp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; MetOp-A is in a sun-synchronous polar orbit with an altitude of 817 km and a local time of ascending node &lt;code&gt;21:30&lt;/code&gt;. I think this means it is always in sunlight, looking down at (roughly speaking) either 9:30AM or 9:30PM local time on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an &lt;a href=&quot;https://youtu.be/uYI6i2snD3k&quot; rel=&quot;nofollow noreferrer&quot;&gt;excellent video about MetOp Instrumentation&lt;/a&gt; which also includes some animations of the spacecraft's polar orbit and apparently transverse attitude:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b6efy.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b6efy.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yOBvz.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yOBvz.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-26T01:47:38.543" LastActivityDate="2018-04-25T20:12:57.933" Title="MetOp-A; Why this unusual solar panel shape and deployment method?" Tags="|design|solar-power|esa|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21126" PostTypeId="2" ParentId="21116" CreationDate="2017-04-17T12:43:36.230" Score="5" Body="&lt;p&gt;Its not that obvious that placing an object in a retrograde orbit would quickly produce a Kessler syndrome. It may happen in the long run, but could take a while:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;I don't think the first collision is so inevitable. At GEO the chance of a collision is much lower than LEO and the debris threat in GEO presently comes less from large tracked objects as these can be avoided. The aggressor object would have to be actively steered for the first collision. &lt;/li&gt;&#xA;&lt;li&gt;a collision would result in a shower of retrograde fragments but these fragments are themselves now uncontrolled and unable to steer to the next victims&lt;/li&gt;&#xA;&lt;li&gt;there have already been a number of posigrade GEO breakups (perhaps a dozen though I'm sorry I don't have a reference to hand). These have been detected by the existance of clouds of debris.&lt;/li&gt;&#xA;&lt;li&gt;any pre-existing debris clouds already pose a reduced risk compared to a retrograde threat, but only in the sense that &lt;strong&gt;risk =  probability * magnitude&lt;/strong&gt;. Pre-existing posigrade debris clouds would have the same probability &lt;em&gt;each&lt;/em&gt; but lower collision effect magnitude, as their relative velocity is much lower.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This leads us to the conclusion that the event rate (the probability side) should already be apparent if the additional events (say an additional 10%) of a retrograde mission were to be a major problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the same measures that are being advocated for LEO would be appropriate, though &lt;strong&gt;adjusted for the GEO conditions&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;consideration of sensors specific to the dangers&lt;/li&gt;&#xA;&lt;li&gt;removal of decommissioned objects to disposal orbits&lt;/li&gt;&#xA;&lt;li&gt;sharing of orbit data&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If anyone happens to be at the European Space Debris Conference this week, this might be a good time to chip in with thoughts about the event rates of small impacts to date in GEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt; in response to comments:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GEO has a lower probability than LEO because of the lower spatial density of objects. This is relevant to the line of thought here because I am suggesting a hypothesis as follows (apologies for the following rather slack formal logic):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;premise A - we add one retrograde break-up and expect to see a kessler syndrome&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;premise B - a retrograde break-up would mean that each secondary particle could have a much greater impact magnitude but no different an event probability compared to a posigrade particle&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;premise C - there have already clouds of debris in GEO resulting from posigrade break-ups&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;premise D - each cloud has the same distribution of fragments and velocities (this isn't that important it just simplifies things)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Inference E: taken from premises A, B, C, D -  the existing break-up clouds should already have produced lots of impacts, though with magnitude less than a retrograde one&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;premise F (granted, this is like proving black swans don't exist) - we would have become aware of E, if it had happened from civil public satellite owners&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Inference G: A and F are inconsistant with one another and suggests that A is unlikely to be true.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-04-19T20:45:38.343" LastActivityDate="2017-04-19T20:45:38.343" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21127" PostTypeId="2" ParentId="21122" CreationDate="2017-04-17T13:52:28.330" Score="4" Body="&lt;p&gt;It is possible, but too costly!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the orbital energy reduction is done best by retro-thrust, and that means, the exhaust of the propulsion device must be on the face in the direction of orbital motion (so that thrust is retrograde).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy needed would be: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m \left(\frac{v_1^2}{2}-\frac{v_2^2}{2}-GM\left(\frac{1}{r_1}-\frac{1}{r_2}\right)\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this is in the best scenario, the propulsive energy that is not the same as solar energy input (considering solar-electric propulsion), because propulsion efficiency is not 100%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For mining, It is indeed better to keep the Phobos up! Because it is much easier to loft materials off the Phobos' weak gravity and use it for construction in Martian orbit. And also, the debris of Phobos falling on Mars would wreak havoc in Martian atmosphere and climate and might pose danger to Martian colonies. Because when the Phobos falls apart, it's orbital height would be much lower and decays much faster. Also, after falling apart, Kessler syndrome happens and makes low Mars orbit a very dangerous place. And Mars gets its own ring after all! cool ;)&lt;/p&gt;&#xA;" OwnerUserId="16381" LastEditorUserId="10313" LastEditDate="2017-04-17T15:11:53.377" LastActivityDate="2017-04-17T15:11:53.377" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21128" PostTypeId="2" ParentId="21122" CreationDate="2017-04-17T14:10:00.030" Score="7" Body="&lt;p&gt;First of all, the best place to place any such rockets would be at the point where the rocket is facing the direction that Phobos is rotating around Mars. That would give you about 4 x more bang for your buck to deorbiting Phobos than pointing directly at Mars, due to orbital mechanics.&lt;/p&gt;&#xA;&lt;p&gt;Secondly, Phobos is actually being slowly deorbited naturally. It will take about 30 million years, but eventually it will land.&lt;/p&gt;&#xA;&lt;p&gt;If you really wanted to deorbit Phobos, there is one major issue. Phobos is believed to not really be a cohesive body. If one put too much strain on it via a propulsion maneuver, it would probably break apart. Also, bringing it closer to Mars will almost certainly have the same effect.&lt;/p&gt;&#xA;&lt;p&gt;Lastly, anything is in fact possible, if you get a large enough engine. To bring it from it's current 6,000 km orbit to a 3,000 km orbit would require a delta-v of around 660 m/s (I can't find an exact value, but that should be pretty close) The mass is about 10&lt;span class=&quot;math-container&quot;&gt;$^1$$^6$&lt;/span&gt; kg. To have that much change in velocity over 10 years (315360000 s), it would require an average acceleration of 2.08 μm/s&lt;span class=&quot;math-container&quot;&gt;$^2$&lt;/span&gt;. That would require a thrust of about &lt;span class=&quot;math-container&quot;&gt;$2.1*10^{10} N$&lt;/span&gt;, which is a considerable amount. All that really needs to be done is to hook up an engine that can maintain that thrust for 10 years, and you would accomplish your goal.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="7062" LastEditDate="2021-07-07T17:02:54.683" LastActivityDate="2021-07-07T17:02:54.683" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="21129" PostTypeId="1" CreationDate="2017-04-18T05:01:43.910" Score="2" ViewCount="792" Body="&lt;p&gt;I have assignment and part of it involves getting a satellite to orbit both the Earth and the Moon at least once. How ever no matter how I set my initial condition or parameterize my position and time evolve it my satellite won't orbit the moon and comeback. Note both the Moon and Earth lie on the x axis the system is in their co-rotating frame, I am using a fourth order Runge Kutta to simulate the orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update 1: After some suggestion from the comments below my script to explain the problem a bit more when I run my code the satellite spirals out and does not orbit , I have also added a plot of the satellite&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update 2 : For those wondering where I got my acceleration term for the satellite and why it includes Coriolis and the Centrifugal force along with the gravitational force between satellite and Earth and Satellite and moon  see this reference &lt;a href=&quot;http://farside.ph.utexas.edu/teaching/336k/Newtonhtml/node123.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://farside.ph.utexas.edu/teaching/336k/Newtonhtml/node123.html&lt;/a&gt;&#xA;and chapter 7 of this &#xA;&lt;a href=&quot;http://farside.ph.utexas.edu/teaching/336k/Newton.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://farside.ph.utexas.edu/teaching/336k/Newton.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Updated 3: Fixed an error in the position of the satellite, and a typo in the comments of the code,updated the image that corresponded to the correction in the code&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from pylab import plot,show,xlabel,ylabel,cla,clf&#xA;from numpy import linspace,exp,sin,cos,zeros,array,pi ,exp&#xA;&#xA;&#xA;&#xA;M2 = 5.972e24 # M Earth kg&#xA;M1 = 7.34767309e22 # M moon kg&#xA;G= 6.6742e-11&#xA;R=3.844e8 #m Earth-moon distance&#xA;R_e =6371e3 # Earth radius&#xA;R_m = 1737e3 # Moon radius&#xA;M_0 = ((M2 - M1)/(M2 + M1))&#xA;alpha = pi/3&#xA;omega_0 = ((G*(M2 + M1))/(R**3))**(1/2)&#xA;T_a = (2*pi)/omega_0 # System period&#xA;&#xA;def dot_p(a,b):&#xA;    n= len(a)&#xA;    d=0&#xA;    for i in range(n):&#xA;        d = d+ a[i]*b[i]&#xA;    return d&#xA;&#xA;def abs_v(x):&#xA;    return (dot_p(x,x))**(1/2)&#xA;&#xA;r_1 = array([(M2*R)/(M2 + M1),0,0],float) # Position of Moon&#xA;r_2 = array([-(M1*R)/(M2 + M1),0,0],float) #Position of Earth&#xA;omega =  array([0,0,omega_0],float) #Angular velocity of Earth and Moon&#xA;&#xA;def F(r,t):&#xA;    r1 = r[0,:] #Earth position&#xA;    vr1 = r[1,:] #Earth velocity&#xA;    r2 = r[2,:] #Moon position&#xA;    vr2 = r[3,:] #Moon velocity&#xA;    r3 = array([R*cos(t),.5*R*sin(2*t),0],float)  +r[4,:] #Satellite position Figure 8 parameterization&#xA;    vr3 = r[5,:] #Satellite velocity&#xA;    fr1 = -G*M2*(r1 -r2)/(abs_v((r1-r2))**3)&#xA;    fr2 = -G*M1*(r2 -r1)/(abs_v((r2-r1))**3) &#xA;    fr3 = (G*M1*(r3 -r_1)/(abs_v((r3-r_1))**3)) -(G*M2*(r3 -r_2)/(abs_v((r3-r_2))**3)) +2*omega_0*array([vr3[1],-vr3[0],0],float)+ (omega_0**2)*array([r3[0],r3[1],0],float) #Gravitational interaction of Satellite with Earth, Moon, Coriolis, and Centrifugal forces&#xA;    return array((vr1,fr1 ,vr2,fr2,vr3,fr3),float)&#xA;&#xA;&#xA;&#xA;def RungeKutta4o(g,ksi10,vksi10,ksi20,vksi20,ksi30,vksi30,a,b): #Runge Kutta 4th order&#xA;    N=1000&#xA;    M = 6    &#xA;    h = float((b-a)/N)&#xA;    t = linspace(a,b,N)&#xA;    x1 = zeros((N,3),float)&#xA;    x2 = zeros((N,3),float)&#xA;    x3 = zeros((N,3),float)&#xA;    vx1 = zeros((N,3),float)&#xA;    vx2 = zeros((N,3),float)&#xA;    vx3 = zeros((N,3),float)&#xA;    r = array((ksi10,vksi10,ksi20,vksi20,ksi30,vksi30),float)&#xA;    k1 = zeros((M,3),float)&#xA;    k2 = zeros((M,3),float)&#xA;    k3 = zeros((M,3),float)&#xA;    k4 = zeros((M,3),float)&#xA;    for i in range(N):&#xA;        x1[i,:] = r[1,:]&#xA;        vx1[i,:] = r[0,:]&#xA;        x2[i,:] = r[3,:]&#xA;        vx2[i,:] = r[2,:]&#xA;        x3[i,:] = r[5,:]&#xA;        vx3[i,:] = r[4,:]&#xA;        k1 = h*g(r,t[i])&#xA;        k2 = h*g(r +.5*k1,t[i]+.5*h)&#xA;        k3 = h*g(r +.5*k2,t[i]+ .5*h)&#xA;        k4 = h*g(r +k3,t[i]+h)&#xA;        r += (1/6)*(k1 + 2*k2 + 2*k3 + k4)&#xA;    return t,x1,x2,x3,vx1,vx2,vx3 &#xA;&#xA;cla()   # Clear axis&#xA;clf()&#xA;r_i0 = r_2 + array([0,R_e+300e3,0],float) # Satellite is initially in Earth orbit&#xA;vr_i0 = array([11000,0,0],float) # Initial velocity of satellite&#xA;t,R1,R2,R3,vR1,vR2,vR3  = RungeKutta4o(F,r_1,omega,r_2,omega,r_i0,vr_i0,0,10*T_a ) &#xA;plot(R3[:,0],R3[:,1])  #Plots the Satellite orbit R3[:,0] is the x position of the satellite and R3[:,1] is the y position of the satellite&#xA;show() &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wh4z1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wh4z1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19343" LastEditorUserId="19343" LastEditDate="2017-04-21T23:04:04.197" LastActivityDate="2017-05-01T18:51:54.160" Title="Restricted Three body problem need to create an orbit around the two massive bodies" Tags="|orbital-mechanics|celestial-mechanics|python|" AnswerCount="1" CommentCount="25" ContentLicense="CC BY-SA 3.0" />
  <row Id="21130" PostTypeId="2" ParentId="21116" CreationDate="2017-04-18T08:50:44.043" Score="2" Body="&lt;p&gt;If such a weapon is launched, it makes no sense to let it break up naturally. A most effective payload would be 50-100.000 small steel projectiles, each 50-100g (0.1-0.2 pound), painted black.&#xA;It would quickly expand into a cloud 200-300m wide, invisible optically and on radar, with projectiles spaced about 1m apart, guaranteeing destruction of almost any satellite that encounters it at a relative speed of 6Km/s. Each projectile would deliver the equivalent energy of 0.5kg of TNT. The cloud will circle GEO every 12h knocking everything offline in an ever expanding steel net. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The defense would be to move the satellite to a safe orbit for a few days until the natural precession of GEO would move the cloud away. But given the few hours available to react, it seems implausible too many satellites would be saved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kesslerization is not really desirable from a weapon effectiveness point of view since the debris would have significantly different speeds quickly falling away from GEO.&lt;/p&gt;&#xA;" OwnerUserId="19344" LastActivityDate="2017-04-18T08:50:44.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21131" PostTypeId="1" AcceptedAnswerId="21132" CreationDate="2017-04-18T09:17:20.380" Score="-1" ViewCount="506" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Edit: Since i've learned a lot from the excellent answer below, i've changed the content of my question totally to make it more realistic.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The orbital radius of Phobos is gradually decreasing by 2 meters every one hundred years by &lt;a href=&quot;https://en.wikipedia.org/wiki/Tidal_acceleration#Tidal_deceleration&quot; rel=&quot;nofollow noreferrer&quot;&gt;tidal deceleration&lt;/a&gt;.Scientists estimate that &lt;a href=&quot;https://en.wikipedia.org/wiki/Phobos_(moon)#Predicted_destruction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phobos will be destroyed&lt;/a&gt; in approximately 30-50 million years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could an electrical propulsion system on the side of Phobos facing Mars powered by solar energy on the sunny sides be powerful enough to prevent this from happening ?&lt;/p&gt;&#xA;" OwnerUserId="19335" LastEditorUserId="19335" LastEditDate="2018-08-18T16:24:07.510" LastActivityDate="2018-08-18T16:24:07.510" Title="Could Phobos be easily saved from falling apart?" Tags="|orbital-mechanics|phobos|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="21132" PostTypeId="2" ParentId="21131" CreationDate="2017-04-18T11:07:56.280" Score="11" Body="&lt;p&gt;Something is very badly wrong with your calculations. Even putting aside that if you want to raise the orbit of something, you need to thrust in the direction of travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8Asqf.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8Asqf.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to actually calculate the energy requirements you need to first treat the problem in terms of momentum&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You want to change the velocity of Phobos by 1mm/s&lt;/li&gt;&#xA;&lt;li&gt;That requires sending some mass in the opposite direction&lt;/li&gt;&#xA;&lt;li&gt;The required change in momentum is $0.001m/s\times10^{16}kg$ = $1\times10^{13}kg\cdot m/s$&lt;/li&gt;&#xA;&lt;li&gt;Lets say the electric drive has an exhaust velocity of 20km/s&lt;/li&gt;&#xA;&lt;li&gt;The amount of propellant required is thus $\frac{10^{13}kg\cdot m/s}{2\times10^4m/s}=5\times10^8kg$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So a change in velocity of 1mm/s, would require firing 500000t of propellant at 20km/s in the other direction. This happens to be about equal to the mass of a supertanker or the Empire State Building.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that we know we need to fire out 500000t/s at 20km/s we &lt;em&gt;can&lt;/em&gt; calculate the energy requirements using $E=\frac{1}{2}mv^2$ (hint: it's going to be bad). It is simply equal to $$0.5\times5\times10^8\times20000^2 = 10^{16}J$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This amount of energy is roughly equal to the energy unleashed in the detonation of a fairly large thermonuclear warhead. So we actually could impart a change in velocity of 1mm/s on Phobos, altough we would need to nuke it, which would not save it from falling apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Phobos is huge. It's tiny on the scale of moons and planets, but it's still huge relative to things humans build and manipulate. If you take all the coal which is mined every year - which is about 5 billion tonnes, Phobos is 2000 times bigger than that. So even dismantling it into pieces to be flung out of a high powered electric mass driver would be a monumentally huge task.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2017-04-18T11:07:56.280" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21133" PostTypeId="1" CreationDate="2017-04-18T14:37:07.220" Score="9" ViewCount="471" Body="&lt;p&gt;I know there have been some experiments in hydroponics that are addressing issues related to growing significant quantities of food and even fresh vegetables in space in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering; &lt;em&gt;Have there been at least continuous periods of plants growing on the ISS?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the title of this Phys.org article is suggestive:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://phys.org/news/2016-07-decade-biology-space.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;A decade of plant biology in space&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;but I am not sure there was a decade of continuous or nearly plant growth. Let's say watering and waiting for germination counts.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QInv0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QInv0.jpg&quot; alt=&quot;Astronaut Peggy Whitson harvests Tokyo Bekana Chinese cabbage aboard the International Space Station on Feb. 17&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Astronaut Peggy Whitson harvests Tokyo Bekana Chinese cabbage aboard the International Space Station on Feb. 17. From &lt;a href=&quot;https://www.nasa.gov/feature/cabbage-patch-fifth-crop-harvested-aboard-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Credits: NASA TV&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hFTAD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hFTAD.jpg&quot; alt=&quot;NASA astronaut Steve Swanson of Expedition 39 activated the red, blue and green LED lights of the Veggie plant growth system on May 7, 2014.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NASA astronaut Steve Swanson of Expedition 39 activated the red, blue and green LED lights of the Veggie plant growth system on May 7, 2014. From &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/news/flowers&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Credits: NASA&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jS9mK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jS9mK.jpg&quot; alt=&quot;NASA astronaut Scott Kelly took this selfie with the second crop of red romaine lettuce in August of 2015.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NASA astronaut Scott Kelly took this selfie with the second crop of red romaine lettuce in August of 2015. From &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/news/flowers&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Credits: NASA&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pAGCE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pAGCE.jpg&quot; alt=&quot;NASA astronauts Scott Kelly and Kjell Lindgren take a bite of plants harvested for the VEG-01 investigation.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NASA astronauts Scott Kelly and Kjell Lindgren take a bite of plants harvested for the VEG-01 investigation. From &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/news/flowers&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Credits: NASA&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;An earlier experiment on the Mir space station:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PJPNy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PJPNy.jpg&quot; alt=&quot;Shannon Lucid grew wheat stalks on her 1996 mission to the Russian Mir space station&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NASA astronaut Shannon Lucid grew wheat stalks on her 1996 mission to the Russian Mir space station. From &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/news/flowers&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Credits: NASA&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-27T01:18:24.840" LastActivityDate="2019-11-08T22:32:30.360" Title="Have there been at least continuous periods of plants growing on the ISS?" Tags="|food|plants|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21134" PostTypeId="1" CreationDate="2017-04-18T15:25:50.307" Score="2" ViewCount="645" Body="&lt;p&gt;I'm trying to calculate the position of each satellite of the GPS constalation, using &lt;a href=&quot;https://celestrak.org/NORAD/elements/gps-ops.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE&lt;/a&gt; files and library &lt;a href=&quot;http://rhodesmill.org/pyephem&quot; rel=&quot;nofollow noreferrer&quot;&gt;pyephem&lt;/a&gt;, I want to know the position Elevation-Azimuth. But when I compare with the results with the page &lt;a href=&quot;https://www.calsky.com/cs.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;calsky&lt;/a&gt; with the results of my code are diferents, &lt;strong&gt;why&lt;/strong&gt;?&lt;/p&gt;&#xA;&lt;p&gt;The initial information are:&#xA;Bogotá, Colombia&lt;br /&gt;&#xA;&lt;strong&gt;Longitude&lt;/strong&gt; = -74.0817&#xA;&lt;strong&gt;Latitude&lt;/strong&gt;  = 4.60971&#xA;&lt;strong&gt;Alt&lt;/strong&gt;: 2578.0&lt;br /&gt;&#xA;&lt;strong&gt;Date:&lt;/strong&gt; 2017-04-18 &lt;strong&gt;Hour:&lt;/strong&gt; 10:08:51.61&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Results in CALSKY &amp;gt;45º&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;PRN 27/GPS BIIF-(39166 2013-023-A)&lt;/strong&gt;  az: 152.7° SSE  h:  61.1°&lt;br /&gt;&#xA;&lt;strong&gt;PRN 16/GPS BIIR-(27663 2003-005-A)&lt;/strong&gt;  az: 341.8° NNW  h:  52.5°&lt;br /&gt;&#xA;&lt;strong&gt;PRN 14/GPS BIIR-(26605 2000-071-A)&lt;/strong&gt;  az: 109.5° ESE  h:  49.5°&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Results in my code &amp;gt; 45º&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;GPS BIIR-7  (PRN 18)&lt;/strong&gt;  - azim: 27.68  elev: 58.34&lt;br /&gt;&#xA;&lt;strong&gt;GPS BIIR-9  (PRN 21)&lt;/strong&gt;  - azim: 200.54  elev: 55.05&lt;br /&gt;&#xA;&lt;strong&gt;GPS BIIF-1  (PRN 25)&lt;/strong&gt;  - azim: 111.35  elev: 46.92&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;def GetPositionELAZ(Time,Longitude,Latitude,constelation,AllTheLines):&#xA;'''&#xA;- time: Time for look the satellite&#xA;- longitude: observer longitude&#xA;- latitude: observer latitude&#xA;- constelation: &#xA;            1 - just GPS &#xA;            2 - GPS - GLONASS&#xA;            3 - GPS - GLONASS - GALILEO&#xA;            &#xA;RETURN: (Tuble of lists...)&#xA;Satellite Name, Satellite Elevation, Satellite Azimuth, Type Satellite (1:GPS, 2: GLO, 3:GAL)&#xA;'''&#xA;&#xA;contain_GPS = 'GPS'&#xA;contain_GLONASS = 'COSMOS'&#xA;contain_GALILEO = 'GSAT'&#xA;&#xA;sat_alt, sat_az, sat_name, sat_type = [], [], [], []&#xA;original_name = []&#xA;&#xA;longlat = str(Longitude) + &amp;quot;,&amp;quot; + str(Latitude)&#xA;g = geocoder.elevation(longlat) #lon,lat&#xA;#print g.meters&#xA;import ephem&#xA;observer = ephem.Observer()&#xA;observer.long = Longitude&#xA;observer.lat = Latitude&#xA;observer.date = Time&#xA;observer.elevation = 2600. #float(str(g.meters))&#xA;&#xA;GPS_list = 'https://celestrak.org/NORAD/elements/gps-ops.txt'&#xA;GLONASS_list = 'https://celestrak.org/NORAD/elements/glo-ops.txt'&#xA;GALILEO_list = 'https://celestrak.org/NORAD/elements/galileo.txt'&#xA;&#xA;#'http://www.amsat.org/amsat/ftp/keps/current/nasabare.txt').readlines()&#xA;GPS2_list = 'http://www.tle.info/data/gps-ops.txt'&#xA;GLONASS2_list = 'http://www.tle.info/data/glo-ops.txt'&#xA;&#xA;if constelation == 1:&#xA;    list_TLEs = [GPS_list]&#xA;if constelation == 2:&#xA;    list_TLEs = [GPS_list,GLONASS_list]&#xA;if constelation == 3:&#xA;    list_TLEs = [GPS_list,GLONASS_list,GALILEO_list]&#xA;&#xA;for satlist in list_TLEs:&#xA;    tles = AllTheLines #urllib2.urlopen(satlist).readlines() &#xA;    tles = [item.strip() for item in tles]&#xA;    tles = [(tles[i],tles[i+1],tles[i+2]) for i in xrange(0,len(tles)-2,3)]&#xA;    &#xA;    for tle in tles:&#xA;&#xA;        try:&#xA;            sat = ephem.readtle(tle[0], tle[1], tle[2])&#xA;            rt, ra, tt, ta, st, sa = observer.next_pass(sat)&#xA;&#xA;            if rt is not None and st is not None:&#xA;                sat.compute(observer)&#xA;&#xA;                if Time &amp;gt;= ephem.localtime(st) and Time &amp;lt;= ephem.localtime(rt):&#xA;                    Title = tle[0]&#xA;                    sat_alt.append(np.rad2deg(sat.alt))&#xA;                    sat_az.append(np.rad2deg(sat.az))&#xA;                    original_name.append(tle[0])&#xA;&#xA;                    text2 = Title.rsplit(')', 1)[0]&#xA;                    NamePRN = text2.rsplit('(', 1)[1]&#xA;                    sat_name.append(NamePRN)&#xA;&#xA;                    if contain_GPS in Title:&#xA;                        sat_type.append(1)&#xA;                    elif contain_GLONASS in Title:&#xA;                        sat_type.append(2)&#xA;                    elif contain_GALILEO in Title:&#xA;                        sat_type.append(3)&#xA;&#xA;        except ValueError as e:&#xA;            print e&#xA;return sat_name, sat_alt, sat_az, sat_type, original_name&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Calling the function&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;pre&gt;&lt;code&gt;GPS_list = 'https://celestrak.org/NORAD/elements/gps-ops.txt'&#xA;Downloaded = urllib2.urlopen(GPS_list).readlines()&#xA;TimeNow = datetime.datetime.now()&#xA;print Time2SecondsOfDay(TimeNow), 'time: ',TimeNow&#xA;&#xA;names,elevs,azs,types,originalName = &#xA;GetPositionELAZ(TimeNow,Longitude,Latitude,1,Downloaded)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="18740" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:23.330" LastActivityDate="2017-04-20T22:02:56.663" Title="Comparing results with Pyephem" Tags="|orbital-mechanics|ephemeris|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21136" PostTypeId="1" CreationDate="2017-04-18T21:19:06.460" Score="4" ViewCount="119" Body="&lt;p&gt;I have an Orbital model in Equinoctial Elements &lt;a href=&quot;http://adsabs.harvard.edu/full/1985CeMec..36..409W&quot; rel=&quot;nofollow noreferrer&quot;&gt;(paper)&lt;/a&gt;. There is a perturbative acceleration term in the model. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone point me to some relevant papers showing how to include different perturbative effects like the drag, solar radiation and others?&lt;/p&gt;&#xA;" OwnerUserId="19340" LastEditorUserId="19340" LastEditDate="2017-04-20T02:33:35.647" LastActivityDate="2017-04-20T02:33:35.647" Title="Equinoctial Elements and Perturbative Effects" Tags="|tracking|two-line-elements|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21137" PostTypeId="2" ParentId="21108" CreationDate="2017-04-18T22:50:28.993" Score="2" Body="&lt;p&gt;Actually quite a simple fix.  The &quot;Argument of Periapsis&quot; rotates the ellipse around the primary (sun). As the sun is at one focus of the ellipse all you need is a little trigonometry to calculate the shift for the center of the ellipse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RPUw8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RPUw8.png&quot; alt=&quot;Pic 3.1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;var scale = 10000000;&#xA;var rotation = this.ω;&#xA;&#xA;var y = -Math.sin(rotation)*this.x0/scale;&#xA;var x = this.x0/scale-Math.abs(Math.cos(rotation)*this.x0/scale);&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I had actually tried this before but was incorrectly thinking it was rotating around the orbiting body so instead of using x0 I was using a.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see in the below image it is still not perfect.  I seem to have a bit too much energy when calculating the semi-major axis for the ellipse in red.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;//Semi-Major Axis&#xA;var a = 1 / ((2 / math.norm(r)) - (Math.pow(math.norm(v), 2) / µ));&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EaQS1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EaQS1.png&quot; alt=&quot;Pic3.2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have found this spreadsheet that also does the calc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used this to try to figure out where the error was.  Using the input data I used above, m1,m2,v,r it was breaking it so I tried using the input data used in the spreadsheet.  This seems to have worked so I assume my input data was the problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;s&gt;This form will only let me post 2 links because of my reputation so I will post the others for this update as a comment.&lt;/s&gt; Done!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Hopefully a few more people will up vote your question and answer and send you on your way to more rep!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RNDno.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RNDno.png&quot; alt=&quot;Pic 3.3&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19330" LastEditorUserId="12102" LastEditDate="2017-04-20T18:12:16.893" LastActivityDate="2017-04-20T18:12:16.893" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21138" PostTypeId="2" ParentId="21098" CreationDate="2017-04-19T01:33:17.117" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9uAl3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9uAl3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this figure, ζ is the &lt;em&gt;input axis&lt;/em&gt;, η is the &lt;em&gt;output axis&lt;/em&gt;, and the spin axis is obvious.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the vehicle is rotated around the axis ζ (the input axis) with an angular velocity ωζ, the frame will rotate around the axis η (the output axis) to an angle β. The dependence of this angle on ωζ is given by the equation β = ωζH/c, where H is the moment of momentum of the gyroscope and c is a coefficient that depends on the stiffness of the spring and on the location of the point where the spring is attached. The values of β are read from the potentiometer; they define the magnitude of ωζ. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the input axis (when the vehicle is not rotating) should be set up to point along the vehicle axis about which you wish to measure the rotation.  For roll rate, it should point along the vehicle's X axis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://encyclopedia2.thefreedictionary.com/Rate+Gyroscope&quot; rel=&quot;noreferrer&quot;&gt;Reference&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-04-19T01:33:17.117" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21139" PostTypeId="2" ParentId="4003" CreationDate="2017-04-19T03:05:22.270" Score="2" Body="&lt;p&gt;I know the Oscar 7 satellite, launched in 1974 is still working, be it while only in sun. There's reports from December of 2016 that contacts are still being made through it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reports of Amateur satellite contacts are posted here: &lt;a href=&quot;http://www.amsat.org/status/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.amsat.org/status/&lt;/a&gt; and there are frequent &quot;Heard&quot; entries for AO-7 this week.&lt;/p&gt;&#xA;" OwnerUserId="19351" LastEditorUserId="12102" LastEditDate="2017-04-19T10:08:03.823" LastActivityDate="2017-04-19T10:08:03.823" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21140" PostTypeId="1" AcceptedAnswerId="21143" CreationDate="2017-04-19T08:17:57.713" Score="17" ViewCount="2854" Body="&lt;p&gt;I thought about this the other day. We often send messages to outer space hoping that someone picks them up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How could you explain to aliens that we sent this satellite about 13.8 billion years after the big bang? Aliens don't know our concept of years.&lt;/p&gt;&#xA;" OwnerUserId="19352" LastActivityDate="2017-05-12T20:14:45.857" Title="How can we transmit a date to another species?" Tags="|time|spacetime|" AnswerCount="7" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21141" PostTypeId="2" ParentId="21140" CreationDate="2017-04-19T08:36:34.983" Score="8" Body="&lt;p&gt;Our basic unit of time, the second, &lt;a href=&quot;https://en.wikipedia.org/wiki/Second#Based_on_cesium_microwave_atomic_clock&quot; rel=&quot;nofollow noreferrer&quot;&gt;is officially defined in terms of physical phenomena&lt;/a&gt;, so our messages have to get that across. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the aliens and ourselves have a compatible understanding of mathematics, chemistry and physics, which we hope are universal, it's possible to construct a message which builds up the necessary vocabulary from first principles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_Call&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cosmic Call&lt;/a&gt; message designed by &lt;a href=&quot;http://www.plover.com/misc/Dumas-Dutil/messages.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dutil and Dumas&lt;/a&gt; does in fact include an estimate for the age of the universe near the end. Here's &lt;a href=&quot;http://blog.plover.com/aliens/dd/intro.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;a good blog series&lt;/a&gt; describing the messages in detail.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-04-19T14:42:36.713" LastActivityDate="2017-04-19T14:42:36.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21142" PostTypeId="2" ParentId="21140" CreationDate="2017-04-19T09:32:54.647" Score="4" Body="&lt;p&gt;One value for time is common for all species, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Planck_time&quot; rel=&quot;nofollow noreferrer&quot;&gt;planck time&lt;/a&gt;. It depends only on mathemathical/physical concepts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you will have to explain to them the powers of 10 as well because else you would have to draw something like 8*10^60 ticks on the calendar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Age of the universe is  4.32 × 10^17s and planck time is 5.4 × 10^-44s)&lt;/p&gt;&#xA;" OwnerUserId="18312" LastActivityDate="2017-04-19T09:32:54.647" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21143" PostTypeId="2" ParentId="21140" CreationDate="2017-04-19T09:38:06.920" Score="21" Body="&lt;p&gt;The approach taken by &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_Golden_Record&quot; rel=&quot;noreferrer&quot;&gt;Voyager Golden Record&lt;/a&gt; is through use of pulsars. Unlike the insane exponents needed to utilize microscopic physical phenomena or planck time, pulsars operate on ~1 second timescale, which makes them quite convenient. A map specifiying location of Earth in relation to a number of pulsars identified through ratios between their frequencies provides a unique location. Picking frequency of any selected pulsar on that map provides a very decent time base too. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-04-19T09:38:06.920" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21144" PostTypeId="1" AcceptedAnswerId="21145" CreationDate="2017-04-19T10:19:18.630" Score="5" ViewCount="2502" Body="&lt;p&gt;Is there any factual basis for &lt;a href=&quot;https://space.stackexchange.com/a/12882/12102&quot;&gt;this highly up voted/accepted answer&lt;/a&gt; that &quot;Transit 5B-5 a.k.a. Oscar 2 is &quot;still kind of 'active'&quot;?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1964-083D&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transit 5B-5&lt;/a&gt; a.k.a. Oscar 2 (not to confuse with OSCAR ham radio satellites), part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transit_(satellite)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transit / Navsat&lt;/a&gt; navigational satellite program was launched 51 years ago on Dec. 13th 1964 and its telemetry transmitter is still transmitting on 136.650 MHz when it's passing through sunlight.  It's not operational anymore (in fact it quit its job 19 days after launch) but it's most likely the oldest piece of space scrap that's still kind of &quot;active&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1964-083D&quot; rel=&quot;nofollow noreferrer&quot;&gt;this NASA page&lt;/a&gt;, 1964-083D (NORAD 00965, OPS 6582) was nuclear powered, so I don't understand this statement about passing through sunlight.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6816" LastEditDate="2017-04-19T20:30:52.707" LastActivityDate="2021-11-03T01:53:20.853" Title="Is Transit 5B-5 a.k.a. Oscar 2 really &quot;still kind of 'active'&quot;?" Tags="|artificial-satellite|history|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21145" PostTypeId="2" ParentId="21144" CreationDate="2017-04-19T10:35:09.707" Score="10" Body="&lt;p&gt;It appears that the satellite has indeed been transmitting recently; &lt;a href=&quot;https://www.youtube.com/watch?v=qbDPDQdeNaM&quot; rel=&quot;nofollow noreferrer&quot;&gt;here's a video with evidence of signal reception in 2014&lt;/a&gt;. I don't know enough about software-defined radio to understand exactly what's going on there but something seems to be signaling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding power supply, I don't have any authoritative sources, but the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transit_(satellite)#Other_satellites&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transit&lt;/a&gt; page on wikipedia mentions some of the Transit sats having RTGs/&quot;nuclear power sources&quot;. 5B5 isn't among them:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Transit 4A, launched June 29, 1961, was the first satellite to use a radioactive power source (RTG) (a SNAP-3). Transit 4B (1961) also had a SNAP-3 RTG. ...&lt;/li&gt;&#xA;  &lt;li&gt;Transit 5A3 and Transit 5B-1 (1963) each had a SNAP-3 RTG.&lt;/li&gt;&#xA;  &lt;li&gt;Transit 5B-2 (1963) had a SNAP-9A RTG.&lt;/li&gt;&#xA;  &lt;li&gt;Transit-9 and 5B4 (1964) and Transit-5B7 and 5B6 (1965) each had &quot;a nuclear power source&quot;.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note that the illustration on the page shows solar panels but doesn't specify which satellite it is. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Systems_for_Nuclear_Auxiliary_Power#SNAP-9&quot; rel=&quot;nofollow noreferrer&quot;&gt;SNAP&lt;/a&gt; page, however, gives us a big hint:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After SNAP-3 on Transit 4A/B, SNAP-9A units served aboard many of the Transit satellite series. In April 1964 a SNAP-9A failed to achieve orbit and disintegrated, dispersing roughly 1 kilogram (2.2 lb) of plutonium-238 over all continents. Most plutonium fell in the southern hemisphere. Estimated 6300GBq or 2100 man-Sv of radiation was released and led to NASA's development of solar photovoltaic energy technology.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astronautix.com/t/transit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astronautix&lt;/a&gt; says: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There were three series of operational prototypes to finalize other design choices. The Transit 5A and 5C-1 prototypes used solar power, and the 5BN series used nuclear power. The 5E series used solar power but instead of the navigation package had instruments to measure the Transit orbital environment. To accomplish this they were launched piggy-back with the 5B series using Thor Able-Star boosters.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;After the failure of Transit 5BN-3 to achieve orbit, it was decided that operational satellites would be solar powered. Not only was the cost lower, but the need to obtain special approvals to launch each nuclear-powered satellite and the operational and publicity issues in case of a failure to orbit were unacceptable.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it seems that 5B5 was redesigned for solar power after the accident. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-04-20T03:03:26.977" LastActivityDate="2017-04-20T03:03:26.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21146" PostTypeId="1" AcceptedAnswerId="21149" CreationDate="2017-04-19T17:28:23.660" Score="6" ViewCount="301" Body="&lt;p&gt;I'm working on a graphical simulation and I'd like to get the &quot;look and feel&quot; to seem as realistic as possible. Other than Apollo landing videos (which seem pretty limited and low quality obviously) are there any other good sources of high quality moon photos&amp;mdash;specifically at many different altitudes? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I want to be able to compare my real time &quot;descent&quot; rendering visuals to as many actual photos as possible, to ensure that the details visible at any particular altitude appear similar. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd also be interested in other series-of-altitude photo sets (ex. Mars), if anything exists. I figure the moon is probably the best candidate for this. I don't think it matters if it's spacecraft based or land based photography. (Is the Astronomy SE a better place to ask this?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found &lt;a href=&quot;https://www.jpl.nasa.gov/video/details.php?id=1455&quot; rel=&quot;nofollow noreferrer&quot;&gt;one example at JPL&lt;/a&gt;! But Titan is kind of cloudy&amp;hellip;.&lt;/p&gt;&#xA;" OwnerUserId="19355" LastEditorUserId="6816" LastEditDate="2017-04-20T20:05:26.637" LastActivityDate="2017-10-27T18:31:23.720" Title="Where can I find high quality &quot;descent&quot; photos at as many different altitudes as possible?" Tags="|landing|reentry|photography|entry-descent-landing|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21147" PostTypeId="2" ParentId="21140" CreationDate="2017-04-19T17:56:13.310" Score="0" Body="&lt;h1&gt;Use radioactive isotopes&lt;/h1&gt;&#xA;&lt;p&gt;If sending a physical probe that you expect to be found like Voyager, you could create 2 disks, one with say 1 mCi of potassium-40, and one with 0.5 mCi. This is 1.251 billion years independent of any reference frame. Then as long as you can communicate multiplication, you can represent any amount of time.  Something like tritium might be more recognizable as it is more abundant universally, and can more easily represent shorter time periods.&lt;/p&gt;&#xA;&lt;p&gt;If broadcasting, you would need to find a way to transmit which isotope you are using as your base unit of time. The tritium absorption spectrum is shifted enough from deuterium and hydrogen that it might be possible.&lt;/p&gt;&#xA;" OwnerUserId="19357" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-04-19T17:56:13.310" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21148" PostTypeId="1" AcceptedAnswerId="21153" CreationDate="2017-04-19T18:51:10.680" Score="2" ViewCount="369" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/21116/10856&quot;&gt;This post&lt;/a&gt; got me thinking about something I've never quite been able to picture about geostationary orbit. In a perfect model of a spherical earth, there is an infinitesimal circle in the plane of the equator on which the center of mass of a satellite must travel. However, there are obvious imperfections that must be corrected for as mentioned on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit#Orbital_stability&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia page&lt;/a&gt;. Given that there are some imperfections, I assume this means there's some room to play with and still be in a stable, useful GEO orbit. How big is this region.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/2517/10856&quot;&gt;This very good answer&lt;/a&gt; seems to imply that it's tight for practical use, but also notes that the debris mitigation region is considerably larger, at least for GSO. I can't find a more specific number for GTO listed in &lt;a href=&quot;http://www.iadc-online.org/Documents/IADC-2002-01,%20IADC%20Space%20Debris%20Guidelines,%20Revision%201.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this document&lt;/a&gt; which the aforementioned answer references. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So &lt;strong&gt;How big is the usable GEO region? Does it extend beyond the theoretical circular orbit or would that simply be considered GSO?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="10856" LastActivityDate="2017-04-19T20:30:10.270" Title="How big (broad) is GEO orbit?" Tags="|orbital-mechanics|orbit|geostationary|earth|geosynchronous|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21149" PostTypeId="2" ParentId="21146" CreationDate="2017-04-19T19:02:51.980" Score="4" Body="&lt;p&gt;Update: The &lt;a href=&quot;http://quickmap.lroc.asu.edu&quot; rel=&quot;nofollow noreferrer&quot;&gt;QuickMap service&lt;/a&gt; is back up and completely overhauled. For this particular application, there is now a complete 3d model of the Moon that you can navigate freely, and it looks just awesome. To activate that option, click on the globe icon in the top left corner, and then click 'Lunar Globe 3D' in the pop-up menu. Navigation options and instructions are in the top right corner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DatE7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DatE7.png&quot; alt=&quot;looking northwest across the rim of Mare Imbrium in 3d model of Moon&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;There are 3 services right now that have photos by the Lunar Reconnaissance Orbiter (the LRO). It would be possible to create a descent sequence with them as long as the camera looks straight down. The best one is &lt;a href=&quot;http://target.lroc.asu.edu/q3/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Quickmap&lt;/a&gt;, but it is currently unavailable with no ETA for its return (&lt;em&gt;NOOOOO!! Why, oh why!!&lt;/em&gt;). When it is available, it has most of the images taken by the LRO, including the most detailed images ever taken of the Moon. You can even ask to see a feature at different times of day, and see relief maps, and thermal maps. It is a wonderful service. You can see features at a variety of zoom levels and pan around freely. But it is unavailable, which I discovered when I tried to load it to answer this question, and I am very sad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So... there is &lt;a href=&quot;http://webmap.lroc.asu.edu/lunaserv.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunaserv&lt;/a&gt;. It seems to be quite slow at the moment, maybe because Quickmap recently went dark and it is overloaded. It doesn't have images that are as detailed, or under different lighting conditions, or the thermal and topographical maps, but I guess it's nice enough... (sigh...). It also allows you to pull up a list of all the LRO images taken of a particular spot. Go to the menu at left and check the NAC overlay, and then click the eye at top right, and then click where you want images from. With those, you can go browse or download the originals. They are large files. You can do the same thing at the &lt;a href=&quot;http://wms.lroc.asu.edu/lroc&quot; rel=&quot;nofollow noreferrer&quot;&gt;WMS browser&lt;/a&gt;. If you decide to browse footprints, they will both take you to the same place, where you can zoom in on the images and pan around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also the images taken from orbit by the Apollo missions. The &lt;a href=&quot;http://www.lpi.usra.edu/resources/apollo/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo Image Atlas&lt;/a&gt; has all of them. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2017-10-27T03:52:18.970" LastActivityDate="2017-10-27T03:52:18.970" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21150" PostTypeId="2" ParentId="14101" CreationDate="2017-04-19T19:48:57.353" Score="3" Body="&lt;p&gt;Aren't you lucky! I was chasing them yesterday, but thick clouds ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not a coincidence that they flared together. Iridium 6 and 51 were paired several years ago, each of them supposedly providing part of the functionality, with no. 51 officially being labeled a spare satellite. They travel at the same altitude in nearly identical orbits. &#xA;They were as close as 6 seconds apart last summer (when I saw two &quot;massive&quot; double flares), then moved apart to app. 20 seconds and have been moved closer up again recently. &lt;/p&gt;&#xA;" OwnerUserId="19326" LastActivityDate="2017-04-19T19:48:57.353" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21151" PostTypeId="2" ParentId="21140" CreationDate="2017-04-19T19:54:45.923" Score="3" Body="&lt;p&gt;One option nobody else mentioned is cosmic microwave background. If you can encode the current temperature of the universe in a way the receiving aliens understand, they will either have a record of its past development or at least a pretty good model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CMB is the same everywhere in the universe and since the fluctuations are very small and it's relatively simple to measure with high precision, this method may be surprisingly precise.&lt;/p&gt;&#xA;" OwnerUserId="16099" LastActivityDate="2017-04-19T19:54:45.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21152" PostTypeId="2" ParentId="5190" CreationDate="2017-04-19T20:18:06.663" Score="14" Body="&lt;p&gt;SpaceX gives the LEO payload of Falcon Heavy as 63.8 tons (presumably in fully expendable mode). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orion, fully loaded with propellant, is about 26 tons. So LEO flight is doable, possibly even with booster recovery, but probably not with 3-core recovery.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find specs for the &lt;a href=&quot;https://www.nasa.gov/sls/interim_cryogenic_propulsion_stage_141030.html&quot; rel=&quot;noreferrer&quot;&gt;ICPS&lt;/a&gt; stage that's going to be used with SLS, but it's a modification of the DCSS-5 upper stage used with Delta IV:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Those modifications include lengthening the liquid hydrogen tank, adding hydrazine bottles for attitude control and making some minor avionics changes to meet the design parameters and performance characteristics as needed by NASA to meet the flight objectives.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Falcon Heavy can lift a DCSS-5 stacked with Orion to LEO, from&#xA;from which point the DCSS can provide about 2960m/s of delta-v beyond LEO, which is not quite enough to reach the moon. Orion could use its service module to do the rest, but that would leave it with insufficient fuel to enter and exit lunar orbit, so that would be a flyby mission. (ICPS on the Block 1 SLS is supposed to put Orion on a very eccentric lunar orbit for &lt;a href=&quot;https://space.stackexchange.com/questions/21095/how-does-exploration-mission-1-orbit-differ-from-apollo-era-free-return-how-far&quot;&gt;EM-1&lt;/a&gt;; it's unclear to me if that configuration is capable of reaching low lunar orbit and returning, but that larger &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_Upper_Stage&quot; rel=&quot;noreferrer&quot;&gt;EUS&lt;/a&gt; for Block 1B certainly will be able to.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without a hydrogen-fueled upper stage, Falcon Heavy second stage could put the Orion into a GTO, but again Orion wouldn't have quite enough delta-v to both circularize there and return to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point, it's a toss-up whether a suitable upper stage could be adapted for use with a human-rated Falcon Heavy before SLS block 1 is flying.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, Delta IV Heavy can put Orion into LEO for development and shakedown flights, making it analogous to the Saturn 1B to the SLS's Saturn V.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-01-25T22:18:44.037" LastActivityDate="2018-01-25T22:18:44.037" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21153" PostTypeId="2" ParentId="21148" CreationDate="2017-04-19T20:30:10.270" Score="5" Body="&lt;p&gt;There are different answers according to the context.  Firstly though it is worth pointing out that the part of the question &quot;and still be in a stable, useful GEO orbit&quot; is not so clear in terms of the distinction between &quot;stable&quot; and &quot;usable&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One view with much merit is that &lt;strong&gt;nowhere in GEO is stable&lt;/strong&gt;. GEO satellites are kept on station by manoeuvreing to oppose the disturbances. The following topics attempt to identify why not and what the typical usable bounds are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TLDR summary&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is typical that GEO satellites will be found within +-50km of the exact GEO point, with inclinations less than 0.1deg and longitudes within +-0.05 deg of their licence-assigned spot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Luni-Solar (North-South) perturbations&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All satellites are subject to this perturbation which causes the inclination of the orbit to evolve. Left unchallenged the inclination will rise to about 15 degrees and then back to 0 degrees in a 53 year cycle.  Broadcast TV satellites mostly need to station-keep to 0 degrees +-0.05 so that ground antennas don't have to track. Manoeuvres can be anywhere from once a month to every few hours. Mobile telecommunications satellites can tolerate quite a bit of drift as the ground terminals aren't pointed at them in the first place. The limits in this case could be dopplershift or movement of the satellites beam footprint on the ground.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Earth tri-axility (East-West)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are just two stable longitude points and all other places involve some drift. The effect on the satellite is that is orbital period changes and it moves slightly out of GEO and starts drifting in longitide.   The same conditions for broadcast and mobile telephony satellites apply though ITU licence conditions generally also constraint satellites to +-0.05 degree, sometimes +-0.1 degree, slots to allow sharing of the spectrum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Solar radiation pressure (Eccentricity control)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shape of the satellite will determine how much it is affected by solar radiation pressure. The effect on the orbit is to change the eccentricity without changing the period. This causes both doppler shift and a daily East-West libration that is super-posed over the East-West drift. Usable orbits tend to have e = ~0.0005 or less and the manouevres are usually programmed to take place at the same time as East West manoeuvres.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-04-19T20:30:10.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21154" PostTypeId="2" ParentId="287" CreationDate="2017-04-19T20:55:20.633" Score="1" Body="&lt;p&gt;My understanding from basic physics is that when you calculate the requirements for escape velocity from a body's gravitational pull, the math is also able to point out the maximum accumulated velocity possible due to the pull of that gravity toward an object.  In other words, while escape velocity calculates what is necessarily to counteract the body's gravitational force, the terminal impact velocity is the sum of that body's gravitational force as it accumulates over the course of the smaller body's acceleration from the farthest orbital reaches and plunges into the body as an impacting object.  For our solar system, we're talking about the attractive force between the Sun's mass, the impacting body's maximum expected mass, plus whatever components of other solar system bodies (i.e. Jupiter, Earth) can be calculated to have a net boosting pull on that body.  The latter are probably negligible compared to the Sun's pull across the farthest possible reaches.  For asteroids, the upper limit is clearly much smaller than planet size.  We can thus calculate a maximum terminal velocity due to gravity, upon impact, and this works out to approximately 160,000 mph.  Another poster was incorrect in describing the highest value within a range by saying the speeds are &quot;usually&quot; that high.  They are not, at all.  In fact, 64,000 mph is the highest velocity we've directly measured from an asteroid/meteor so far, in our contemporary era of space travel and modern astronomy.  That was the Sutter's Mill meteorite which was seen across the Western U.S. on November 1, 2016.  The fastest possible impact velocity is thankfully seen so rarely on Earth that it probably has not ever occurred within the entire history of human existence.  It's an upper limit - a theoretical maximum.  I should note that comets must also be included in consideration, not just asteroids.  Either one can become an impacting meteorite, and the calculations I've seen are from that perspective--what is the maximum impact velocity we might see from an object in the Solar System?  An object can have its trajectory changed through an encounter with Jupiter, but that isn't going to cause more acceleration than the Sun's gravity is able to exert upon far more distant objects that eventually plunge in like cosmic bullets.&lt;/p&gt;&#xA;" OwnerUserId="19360" LastEditorUserId="19360" LastEditDate="2017-04-19T21:00:46.547" LastActivityDate="2017-04-19T21:00:46.547" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21155" PostTypeId="2" ParentId="21146" CreationDate="2017-04-19T21:48:25.063" Score="4" Body="&lt;p&gt;Curiosity actually had a camera recording when it descended at high enough of a rate to make a video of the decent on Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Esj5juUzhpU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-04-19T21:48:25.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21156" PostTypeId="2" ParentId="13110" CreationDate="2017-04-20T00:18:29.283" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://expanse.wikia.com/wiki/Epstein_Drive&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://expanse.wikia.com/wiki/Epstein_Drive&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See this article for details on these engines.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; Thrust: 1,000,000 N&#xA;Specific Impulse (Isp): 1,100,000 seconds&#xA;Exhaust Velocity (Vex): 11,000,000 m/second (~3.7% of light speed)&#xA;Mass Flow (mdot): 0.09 kg/second&#xA;Thrust Power: 5,500,000 MWatt&#xA;Engine's Thrust to Weight Ratio: ~140&#xA;Propellant mass fraction (yacht's mass fueled / mass empty): ~4 &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="19302" LastEditorUserId="38" LastEditDate="2017-04-20T10:41:28.257" LastActivityDate="2017-04-20T10:41:28.257" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21158" PostTypeId="1" AcceptedAnswerId="38023" CreationDate="2017-04-20T03:32:36.300" Score="4" ViewCount="501" Body="&lt;p&gt;Emily &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2010/2462.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lakdawalla's blog&lt;/a&gt; on the Planetary Society's page explains some of the basic principles in the delay-doppler radar imaging of asteroids, and other astronomical bodies by extension.&lt;/p&gt;&#xA;&lt;p&gt;Radar from a single dish does not (usually!!) have anywhere near enough resolution to image an asteroid. However, the miracle of math allows one to reconstruct an image of the object anyway. A complicatedly modulated (coded) long radar pulse is broadcast toward the asteroid, and each point on it reflects a little of the pulse back to Earth. However each reflection is slightly modified in a unique way.&lt;/p&gt;&#xA;&lt;p&gt;The parts of the asteroid that are farther return reflections that are delayed by the extra distance travelled, and each reflection is doppler shifted by the line-of-sight projection of the rotation velocity of that point around the asteroid's center of mass. The doppler shift is in frequency as well as in phase, and therefore compresses/stretches the reflection in time as well.&lt;/p&gt;&#xA;&lt;p&gt;By correlating the original coding pattern with the received signal, one can build up information about the 3D structure of the asteroid and generate a kind of image. Because you have both absolute time delay differences as well as rotation, you now have a fairly precise physical measurement of the absolute size (and of course shape) of the asteroid as well as its precise distance.&lt;/p&gt;&#xA;&lt;h2&gt;But, &lt;em&gt;What if it wasn't rotating?&lt;/em&gt;&lt;/h2&gt;&#xA;&lt;p&gt;This leads me to the possibly inescapable conclusion that an asteroid that wasn't rotating, or even worse, was rotating very very slowly such that it presented the same face towards Earth as it passed by would be impossible to generate an image or transverse size with radar &lt;strong&gt;using the delay-doppler technique&lt;/strong&gt;. You could still get some kind of size information along the line of sight, and of course precise distance, but the transverse shape and size would be un-image-able.&lt;/p&gt;&#xA;&lt;p&gt;It could be 100x100m or 10x10km (extreme example) in the transverse plane, and except for assumptions on radar albedo, you'd never know which.&lt;/p&gt;&#xA;&lt;p&gt;Am I wrong? Is there something I've missed?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; In order to not get bogged down too deeply in math within the set-up of the question, I've avoided the complications of 3D rotation (tumbling) or that rotation &lt;em&gt;along the line of sight direction&lt;/em&gt; is almost as bad as no rotation at all, or the potential use of polarization or advanced modeling. However that doesn't mean these need to be excluded from answers as well!&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Asteroid 2014 JO25 is passing relatively close to Earth now, and the radar images are already starting to be published!  See for example NPR's &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2017/04/19/524680317/an-asteroid-is-swinging-by-earth-today-for-its-closest-visit-in-400-years&quot; rel=&quot;nofollow noreferrer&quot;&gt;An Asteroid Is Swinging By Earth Today For Its Closest Visit In 400 Years&lt;/a&gt; and the image and video linked there.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Cropped image from NPR's article, with the caption:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This composite of 30 images of asteroid 2014 JO25 was generated with radar data collected using NASA's Goldstone Solar System Radar in California's Mojave Desert.  NASA/JPL-Caltech/GSSR&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WnTrR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WnTrR.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-06T23:47:28.257" LastActivityDate="2021-08-06T23:47:28.257" Title="Is delay-doppler radar imaging of NEO asteroids possible only if it spins fast enough?" Tags="|asteroid|imaging|deep-space-network|radar|delay-doppler|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="21160" PostTypeId="2" ParentId="20042" CreationDate="2017-04-20T04:07:42.907" Score="0" Body="&lt;p&gt;Because if you have a high TWR you might as well put more relatively cheap fuel in and get more deltav. Spacex does exactly that with the falcon 9, they have high TWR engines with a long fuel tank and they even super chill their fuel to get even more in. A high TWR rocket would encounter less gravity drag but the amount of delta v you save wouldn't be nearly as much as more fuel would add. There are other reasons but this is the main one.&lt;/p&gt;&#xA;" OwnerUserId="19364" LastActivityDate="2017-04-20T04:07:42.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21161" PostTypeId="1" AcceptedAnswerId="21163" CreationDate="2017-04-20T06:23:55.340" Score="7" ViewCount="301" Body="&lt;p&gt;Since Russia is going to send only 2 people crew to  the ISS for a while, I wonder how will they adapt the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do they remove the 3rd seat ?&#xA;Can they put extra cargo on theses flights ? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-04-20T10:15:42.747" Title="How are soyuz spacecraft adapted to 2 people crew?" Tags="|iss|soyuz-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21162" PostTypeId="2" ParentId="21088" CreationDate="2017-04-20T08:02:07.990" Score="4" Body="&lt;p&gt;In the early days nobody cared about upper stages, so they occasionally collided with other objects or spontaneously exploded when a pressurized tank failed. That's why they represent a significant source of space debris. See also &lt;a href=&quot;https://books.google.ch/books?hl=en&amp;amp;lr=&amp;amp;id=RehH7b_kmNEC&amp;amp;redir_esc=y&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Debris from Upper Stage Breakup&lt;/a&gt; on Google Books.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the 1990s there are standard practices in place that require &lt;a href=&quot;https://en.wikipedia.org/wiki/Passivation_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;passivation of unused spacecrafts&lt;/a&gt; remaining in orbit. That means propellant and pressurized tanks should be depleted as soon as the deployed payload is out of range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the orbit (LEO, GTO, GEO, etc.) there are different options to dispose an upper stage. The &lt;a href=&quot;https://orbitaldebris.jsc.nasa.gov/library/usg_od_standard_practices.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;U.S. Government Orbital Debris Mitigation Standard Practices&lt;/a&gt; lists them as follows:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;LEO or below: Atmospheric reentry, i.e. maneuver the stage to an orbit that will cause atmospheric drag and it will burn up eventually.&lt;/li&gt;&#xA;&lt;li&gt;Between LEO and MEO: Maneuver the stage to a graveyard orbit between 2,000km and 19,700km.&lt;/li&gt;&#xA;&lt;li&gt;Between MEO and GEO: Maneuver the stage to a graveyard orbit between 20,700km and 35,300km.&lt;/li&gt;&#xA;&lt;li&gt;Above GEO: Maneuver the stage to a graveyard orbit above 36,100km&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The goal is primarily to avoid leaving the stage in an orbit that interferes with any operational orbit in LEO, MEO and GEO. If you cannot achieve reentry, you ideally want to park the stage in an orbit as high as possible to reduce its velocity and thus to minimize energy and debris released in case of a collision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This also applies to other spacecraft, such as satellites at their end-of-life.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The standard practices further mention &quot;direct retrieval&quot;, basically to catch the object and remove it from orbit. Though I don't know of any instance using this method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another option is to leave the upper stage attached to the payload, such that it can provide power and thrust if needed.&lt;/p&gt;&#xA;" OwnerUserId="10335" LastActivityDate="2017-04-20T08:02:07.990" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21163" PostTypeId="2" ParentId="21161" CreationDate="2017-04-20T09:47:50.410" Score="6" Body="&lt;p&gt;MS-04 &lt;a href=&quot;https://www.nasaspaceflight.com/2017/04/roscosmos-soyuz-ms-04-launch-two-member-crew/&quot; rel=&quot;noreferrer&quot;&gt;has been launched with 3 seats&lt;/a&gt;. The 3rd seat will be needed when MS-04 returns to Earth with 3 crewmembers (the third is Expedition 51 Commander Peggy Whitson).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For launch, the empty seat on Soyuz MS-04 will be filled by a cargo container.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6642" LastEditDate="2017-04-20T10:15:42.747" LastActivityDate="2017-04-20T10:15:42.747" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21164" PostTypeId="1" AcceptedAnswerId="21168" CreationDate="2017-04-20T11:33:39.363" Score="17" ViewCount="5408" Body="&lt;p&gt;The Space Daily April 10th Rocket Science item &lt;a href=&quot;http://www.spacedaily.com/reports/Europes_largest_sounding_rocket_launched_from_Esrange_999.html&quot; rel=&quot;noreferrer&quot;&gt;Europe's largest sounding rocket launched from Esrange&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;MAXUS 9, Europe's largest sounding rocket for experiments in microgravity, successfully lifted off from SSC's (Swedish Space Corporation's) launch facility Esrange Space Center in northern Sweden.&lt;/p&gt;&#xA;&lt;p&gt;The rocket was launched at 11:30 local time and carried nine scientific experiments and a technology demonstrator, all together 579 kg, to an altitude of 678 km which enabled slightly more than 12 minutes and of stable microgravity, &lt;span class=&quot;math-container&quot;&gt;$10^{-5}$&lt;/span&gt; g&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;A quick estimate shows that a ballistic trajectory rising to that height and then returning would indeed have a time of about 12 and a half minutes, so I am guessing there was just enough delta-v (about 3600 m/s) to attain vertical velocity quickly, and then the next twelve minutes were free-fall, and there would be no attempt to achieve substantial horizontal velocity to prolong the duration of the zero gee status.&lt;/p&gt;&#xA;&lt;p&gt;Is that roughly correct?&lt;/p&gt;&#xA;&lt;p&gt;Is this a new time and/or altitude record for this kind of trajectory?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-10-17T10:41:28.973" Title="Is 678 km the new altitude record for a rocket shot &quot;straight up&quot; (vertical launch)?" Tags="|trajectory|esa|sub-orbital|sounding-rocket|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="21165" PostTypeId="1" AcceptedAnswerId="21167" CreationDate="2017-04-20T13:07:10.257" Score="5" ViewCount="687" Body="&lt;p&gt;I've literally &lt;a href=&quot;https://space.stackexchange.com/questions/14101/#comment58706_21150&quot;&gt;just heard here&lt;/a&gt; that the days of observing Iridium satellite flares are numbered. Of course it's common sense, but I never thought about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roughly how soon are these going to start diminishing in frequency? In five years they'll be half as often? Or much sooner? Later?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will the ramp-down be uniform over the globe, or is there a chance that it could be noticeable at some latitudes sooner than at others?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-20T13:14:04.717" LastActivityDate="2019-03-15T00:45:30.577" Title="How much longer will we be able to see Iridium satellite flares?" Tags="|iridium|satellite-flare|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21166" PostTypeId="2" ParentId="21164" CreationDate="2017-04-20T13:25:01.527" Score="16" Body="&lt;p&gt;Not sure if this counts, but New Horizons was launched directly into an escape trajectory and did not enter orbit. It made it to Pluto and beyond. From &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;New Horizons was launched from Cape Canaveral Air Force Station&#xA;  directly into an Earth-and-solar escape trajectory with a speed of&#xA;  about 16.26 kilometers per second&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That might be considered &quot;straight up&quot;.&lt;/p&gt;&#xA;" OwnerUserId="12742" LastActivityDate="2017-04-20T13:25:01.527" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21167" PostTypeId="2" ParentId="21165" CreationDate="2017-04-20T15:35:24.457" Score="6" Body="&lt;p&gt;There won't be any privileged locations, we're all in the same sad boat :-)&#xA;The Iridium constellation is not geo-stationary. The sats are in a LEO polar orbit and pass all latitudes every 100 minutes. Due to the rotation of the earth, all longitudes get covered by each satellite as they go along. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two time-schedules are relevant: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;In order to replace the old satellites, the new ones have to be put&#xA;into orbit. This will happen in various sequences of ten satellites.&#xA;The &lt;a href=&quot;http://blog.iridium.com/2017/01/14/success-the-first-ten-iridium-next-satellites-have-arrived-in-low-earth-orbit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;first batch&lt;/a&gt; is already up there (launched in Jan 2017), the next&#xA;one will follow in mid-June, from then onwards app. every 6 weeks&#xA;(if SpaceX can make it happen).    &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;With the new satellites up, there first is a period of manoeuvering&#xA;them to their proper slots, then extensive testing, then the new&#xA;ones take over the functionalities of the old ones. Once the &lt;a href=&quot;https://www.youtube.com/watch?v=TsxWqRiC7m8&quot; rel=&quot;nofollow noreferrer&quot;&gt;slot-swaps&lt;/a&gt; are done, de-orbiting&#xA;can start. From the first batch, de-orbiting started for one of the&#xA;satellites about a week ago. Not exactly sure about that, but I&#xA;assume it will take several months until it burns up. Since most of&#xA;the old satellites are still under control and stable, I guess they&#xA;will be flaring even when on the way down.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So get your fair flare share while you can, it's flarewell soon :-)&lt;/p&gt;&#xA;" OwnerUserId="19326" LastEditorUserId="19326" LastEditDate="2017-04-20T22:29:14.110" LastActivityDate="2017-04-20T22:29:14.110" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21168" PostTypeId="2" ParentId="21164" CreationDate="2017-04-20T15:40:47.057" Score="31" Body="&lt;p&gt;New Horizons went into Earth parking orbit first, so it doesn't count.&#xA;For a suborbital direct ascent trajectory, some early lunar probes (USSR's Luna-1 for example) would hold this record. Otherwise, early vertical research probes included the Blue Scout Junior, one of which reached 44400 km on 1961 Dec 4&#xA;(mission O-2) - another may have reached 225000 km on 1961 Aug 17 (mission O-1) although it wasn't tracked, so we're not sure if it really made it. A Chinese suborbital probe reached over 10000 km and possibly 30000 km on 2013 May 13.&#xA;So the ICBMs, which are optimized for range and not apogee and only get to at&#xA;most 2000 km or so, are not remotely in the running.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The record for a launch from Europe is probably Rubis 04, launched from&#xA;Biscarosse, France, in July 1967 to 2000 km or so. The French navy also launch&#xA;missiles from submarines off the coast of Brittany towards S America, but their apogee is likely closer to 1000 km (and of course, is secret).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find more at my catalog of orbital and suborbital space launches at &lt;a href=&quot;http://planet4589.org/space/lvdb&quot; rel=&quot;noreferrer&quot;&gt;JSR Launch Vehicle Database, 2016 Dec 30 Edition&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="11467" LastEditorUserId="1303" LastEditDate="2017-04-20T20:05:52.177" LastActivityDate="2017-04-20T20:05:52.177" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21169" PostTypeId="1" AcceptedAnswerId="21172" CreationDate="2017-04-20T15:47:59.777" Score="3" ViewCount="214" Body="&lt;p&gt;In watching this video: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/7I78wLrWBmA?start=300&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; of the recent Orbital ATK’s Cygnus launch, I noticed that when the 2nd stage engine fires (after the ullage engine finishes firing), that there does not appear to be any exhaust plume or visual distortion of the 1st stage fading away in the distance, even though the visual goes right through the 2nd stage exhaust plume.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would have expected to see at least a minimal visual distortion of parts the image due to the heat of the exhaust bending light in the exhaust area due to a different coeffecient of refraction due to high exhaust temperatures, but the image does not appear to be distorted across the whole field of view of the camera.  Does light not get bent (in absence of an atmosphere) despite an exhaust?  I would have expected to see at least some heat effects looking through the nozzle.  For comparison, the recent SpaceX launch after 1st stage separation (&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/30hyuVQMXY0?start=225&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;) does show some exhaust presence around the nozzle, but still not a visual distortion looking through the exhaust plume.  Just curious as to why the exhaust plume doesn't visually distort light going through the plume.&lt;/p&gt;&#xA;" OwnerUserId="16735" LastActivityDate="2017-04-20T16:42:40.563" Title="Orbital ATK’s Cygnus 2nd stage exhaust plume" Tags="|rockets|exhaust|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21171" PostTypeId="1" CreationDate="2017-04-20T16:39:58.100" Score="31" ViewCount="3235" Body="&lt;p&gt;I believe that it's common knowledge that Max-Q is the point in which a rocket is undergoing the maximum dynamic stress during a launch and ascent.  But, how often have rockets actually been destroyed or failed at this point (or close to it) during launch &lt;strong&gt;due to Max-Q stresses&lt;/strong&gt;?  I've seen videos of many launches which fail with rapid disassembly before this point, but can't really recall that Max-Q is a statistically large factor in rocket launch failures.  Am I mistaken, or have rocket designs evolved such that Max-Q is a known factor that's been successfully planned into rocket design and manufacture?&lt;/p&gt;&#xA;" OwnerUserId="16735" LastActivityDate="2020-12-18T01:05:34.643" Title="Has Max-Q historically been a common failure point in rocket launches?" Tags="|launch|failure|max-q|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21172" PostTypeId="2" ParentId="21169" CreationDate="2017-04-20T16:42:40.563" Score="4" Body="&lt;p&gt;The visual distortion you see due to heat is due to the &lt;a href=&quot;https://physics.stackexchange.com/a/14948/272&quot;&gt;dependence of the refractive index of air on its temperature&lt;/a&gt;, combined with turbulent mixing which causes the temperature to vary chaotically in space.  For a second stage fired largely in a vacuum, that turbulent mixing is going to be nearly nonexistent, and the variation of optical density within the exhaust plume is going to be far smaller in comparison, to the point that distortion will be nearly invisible.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-04-20T16:42:40.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21173" PostTypeId="1" AcceptedAnswerId="21197" CreationDate="2017-04-20T18:54:09.967" Score="8" ViewCount="396" Body="&lt;p&gt;From only the video footage within the capsule, please estimate the rocket's acceleration (gee's).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bbc.com/news/science-environment-39658947&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.bbc.com/news/science-environment-39658947&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Show all work and assumptions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;There is a different but related video below, also with swinging objects, in case the original BBC link fails at some point in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/fr_hXLDLc38?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-01-12T05:07:01.713" LastActivityDate="2019-01-12T18:32:57.337" Title="Puzzler: What acceleration are these astronauts experiencing?" Tags="|launch|physics|soyuz-launcher|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="21175" PostTypeId="1" AcceptedAnswerId="21176" CreationDate="2017-04-20T19:39:55.807" Score="2" ViewCount="961" Body="&lt;p&gt;What are these capsules made of? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_47&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expedition 47&lt;/a&gt; return capsule that brought Astronauts Yuri Malenchenko, Timothy Peake, and Timothy Kopra back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;em&gt;&lt;a href=&quot;http://www.iflscience.com/space/historic-spacecraft-used-by-british-astronaut-to-go-on-display-at-london-museum/&quot; rel=&quot;nofollow noreferrer&quot;&gt;IFL Science!&lt;/a&gt;&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The display of the Soyuz TMA-19M was possible thanks to a partnership with the Russian space agency, Roscosmos, and the spacecraft’s manufacturer, RSC Energia.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bbc.com/news/science-environment-38755229&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.bbc.com/news/science-environment-38755229&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6RICd.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6RICd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;There is more &quot;char&quot; in this photo, but copper-colored areas are still visible, lower right, and around flanges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bbc.com/news/science-environment-38744822&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.bbc.com/news/science-environment-38744822&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qtoj2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qtoj2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This photo in &lt;em&gt;IFL Science!&lt;/em&gt; shows the char starting to peel off on the left and right edge. 2X crops are shown first, with the first one modified for &quot;cooler&quot; color temperature to color-balance the exposed structural metal, followed by the original image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.iflscience.com/space/historic-spacecraft-used-by-british-astronaut-to-go-on-display-at-london-museum/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.iflscience.com/space/historic-spacecraft-used-by-british-astronaut-to-go-on-display-at-london-museum/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nCaXo.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nCaXo.png&quot; alt=&quot;descr&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/VaLSC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VaLSC.png&quot; alt=&quot;descr&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/WFkqz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WFkqz.png&quot; alt=&quot;descr&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/myWUt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/myWUt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-05-12T01:54:41.763" LastActivityDate="2021-05-12T01:54:41.763" Title="Why does &quot;Tim Peake's capsule&quot; look like it's copper now?" Tags="|reentry|design|soyuz-spacecraft|museum-exhibits|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21176" PostTypeId="2" ParentId="21175" CreationDate="2017-04-20T20:22:47.493" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3234/what-is-the-outside-of-the-soyuz-capsule-covered-in&quot;&gt;The outside of the Soyuz is covered in multilayer vacuum-screen thermal insulation, using layers of metalized film and fiberglass cloth.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The outer layer is black: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rsQeS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rsQeS.jpg&quot; alt=&quot;Soyuz during production, with black blankets wrapped around&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During reentry, some of this burns off, exposing underlying layers, which char and burn off in turn. These layers start off with a lighter color: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CArEL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CArEL.jpg&quot; alt=&quot;Soyuz after reentry, showing lighter layers underneath&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Science Museum has some really weird lighting in places: it's very dark in parts of the museum, and the light is decidedly off-white, which makes the first two pictures in the question difficult to interpret. Here's &lt;a href=&quot;http://www.collectspace.com/news/news-120616a-science-museum-london-soyuz-peake.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;the same capsule&lt;/a&gt; in decent lighting. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jWaph.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jWaph.jpg&quot; alt=&quot;Soyuz capsule&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-06-05T11:18:38.793" LastActivityDate="2017-06-05T11:18:38.793" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21177" PostTypeId="2" ParentId="21134" CreationDate="2017-04-20T22:02:56.663" Score="3" Body="&lt;p&gt;I figured out,It was something wrong in the code:&#xA;The function &lt;code&gt;observer.next_pass(sat)&lt;/code&gt;, return a vector of 5 values&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;0  Rise time&lt;br&gt;&#xA;  1  Rise azimuth&lt;br&gt;&#xA;  2  Maximum altitude time&lt;br&gt;&#xA;  3  Maximum altitude&lt;br&gt;&#xA;  4  Set time&lt;br&gt;&#xA;  5  Set azimuth&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the instruction &lt;code&gt;if Time &amp;gt;= ephem.localtime(st) and Time &amp;lt;= ephem.localtime(rt):&lt;/code&gt; I was using a innecesary comparation because you already compute the time with the instruction &lt;code&gt;observer.date = Time&lt;/code&gt; &#xA;also&#xA;I was comparing with the localtime, the best option is have all in UTC time.&lt;br&gt;&#xA;and a advice with satellites don't use &lt;code&gt;datetime.now()&lt;/code&gt;, use &lt;code&gt;datetime.utcnow()&lt;/code&gt;, and later you can add the GTM of your region.&lt;/p&gt;&#xA;" OwnerUserId="18740" LastActivityDate="2017-04-20T22:02:56.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21178" PostTypeId="1" AcceptedAnswerId="21179" CreationDate="2017-04-21T01:58:50.060" Score="8" ViewCount="1000" Body="&lt;p&gt;I'm solving the Lambert's problem, and already have written a program, which solves the BVP and computes the velocities required to transfer from one point to another in a given time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I discovered, that sometimes it flies in the opposite direction, and therefore requires more Delta-V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;How many solutions (velocities) exist in case of Lambert's problem? &lt;/li&gt;&#xA;&lt;li&gt;How to get the solution, which requires less Delta-V? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="19219" LastEditorUserId="19219" LastEditDate="2017-04-22T06:32:04.153" LastActivityDate="2017-04-22T06:32:04.153" Title="Multiple solutions to Lambert's problem" Tags="|orbital-maneuver|trajectory|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21179" PostTypeId="2" ParentId="21178" CreationDate="2017-04-21T05:43:04.900" Score="8" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How many solutions (velocities) exist in case of Lambert's problem?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Lambert's problem has an infinite number of solutions. For example, there's typically a solution that involves more that 0° but less than 180° on the transfer orbit, another than involves more than 180° but less than 360° on the transfer orbit, yet another than involves more than 360° but less than 540°, and so on. There are also solutions that involve reversing direction, essentially equivalent to less than 0° on the transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The long transfers (anything requiring more than 360° on the transfer orbit) take too much time, are very touchy, and are hard to find. They're not worth looking at. The solutions that require reversing direction are extremely expensive in terms of delta V. There's no point in looking at those solutions, either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves two solutions that are worthy of investigation, the &quot;short way&quot; (more than 0° but less than 180° on the transfer orbit) and the &quot;long way&quot; (more than 180° but less than 360° on the transfer orbit). You need to investigate both.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-21T05:43:04.900" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21181" PostTypeId="2" ParentId="21140" CreationDate="2017-04-21T08:21:40.630" Score="1" Body="&lt;p&gt;Well, the speed of light is never going to change. That being the case, one method could be to use light years as a means to calculate time. &#xA;'x light years from now' could be the way to specify a &lt;strong&gt;relative date&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could also include how much time a second is (via a gap in the audio) and specify that 60 seconds make a minute and 60 minutes make an hour and so on. Using that you can explain our concept of the passing of time and use the same to explain what you need&amp;hellip;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;if they can pick up the signal, they should be able to find where it came from. so you can use the speed of light and the distance between us and them to give them an idea of how long ago the message was sent to them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;I seem to have made a mistake: light year can only be used to measure distance and not time. so my first option can be discarded. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;once i went through the question, i realised one fact: our concept of the big bang and when it happened might not necessary be what they believe in. so using the big bang as the reference might not be the way to go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We could measure the distance travelled by the satellite and use the speed of light as reference to convey the time taken by the satellite to get to the aliens with reference to the launch of the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;that is to say, as long as the satellite had good enough sensors, the satellite could find the distance between local astro bodies and specify that the (satellite) was &lt;strong&gt;launched x units of the time taken to go from one astro body to the other.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="14124" LastEditorUserId="14124" LastEditDate="2017-04-24T06:10:17.763" LastActivityDate="2017-04-24T06:10:17.763" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21182" PostTypeId="1" AcceptedAnswerId="21185" CreationDate="2017-04-21T09:06:19.110" Score="-3" ViewCount="252" Body="&lt;p&gt;And what will then be the speed theoretically at which it hits the surface at one of the poles ? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please, specify your calculations and the trajectory. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Why should this be a bad question, with such a magnificent answer ?&lt;/p&gt;&#xA;" OwnerUserId="19335" LastEditorUserId="19335" LastEditDate="2018-04-09T09:33:44.857" LastActivityDate="2018-04-09T09:33:44.857" Title="Is it possible to get a boulder from Phobos to one of the poles of Mars?" Tags="|orbital-mechanics|mars|terraforming|phobos|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21184" PostTypeId="1" AcceptedAnswerId="21196" CreationDate="2017-04-21T14:20:42.957" Score="5" ViewCount="397" Body="&lt;h2&gt;Empirical Answer Considered!&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;(see the end of the question for details)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Iridium satellite flares are a class of &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satellite Flares&lt;/a&gt; that originate from satellites in the current &lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Iridium constellation&lt;/a&gt;. Unfortunately, according to this &lt;a href=&quot;https://space.stackexchange.com/a/21167/12102&quot;&gt;helpful answer&lt;/a&gt; and as foretold by &lt;a href=&quot;https://space.stackexchange.com/a/14076/12102&quot;&gt;this one&lt;/a&gt; (though schedules have slipped) they are going away relatively soon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reflections come from fairly flat antenna panels on the satellites. The attitude of the spacecraft and especially the antennas are carefully controlled, and so the path of the footprint of the reflected light can be predicted with surprisingly good precision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The web site &lt;a href=&quot;http://www.heavens-above.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heavens Above&lt;/a&gt; will give you predicted visible flares if you give it your specific location. It estimates the brightness based on the your position relative to the position of the maximum brightness point of the flare. You can see it if you are tens of kilometers away, but it will be maximum if you are within only kilometers. Heavens Above has been around quite a while and I believe there are other prediction sites and services.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;The simulation:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;There will be an ephemeris for the position of the sun, satellite, Earth, and the topographic location that you specify. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There will also be an ephemeris or algorithm that handles the attitude of the spacecraft and/or the orientation of the antenna panels themselves.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;However:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;There has to be some kind of model for the shape of the illumination footprint from the panel in order for the prediction to estimate the brightness based on distance from the maximum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there anything written about this? Is it just a fuzzy blob based on empirical or anecdotal reports, built up over time the way meteor counters build up data used to model debris trails in space? Or did someone actually take a specimen of an antenna panel outdoors and just do the experiment - hold it at different angles wrt the sun and an observer, and measure the brightness?&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;I'm wondering:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;What goes into an Iridium Flare prediction model &lt;em&gt;besides the ephemerides?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below&lt;/strong&gt; &quot;View of one of the Main Mission Antenna. The hinged base is on the right side.&quot; Cropped. From &lt;a href=&quot;http://www.satobs.org/iridsat.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hbtAZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hbtAZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below&lt;/strong&gt; cropped (and enlarged) image of a first generation Iridium satellite, showing a bumpy and diffusely reflecting metal antenna panel of the type responsible for producing the flares seen on Earth. From &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Iridium_Satellite.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wV1M8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wV1M8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9elz6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9elz6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Empirical Answer Considered:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;One could conceivably just query a flare simulator a bunch of times, stepping by 1 or 2 km along an East-West line to build up a plot of the cross-track brightness as a function of lateral distance from the track.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curious if it is different for each satellite, or each of the antenna panels, or depends on sun angle, or if its just a fixed width and the same every time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good one of those would definitely be worth &lt;code&gt;+100&lt;/code&gt;!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-28T13:07:38.900" LastActivityDate="2017-04-28T13:07:38.900" Title="What goes into an Iridium Flare prediction model besides the ephemerides?" Tags="|iridium|satellite-flare|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21185" PostTypeId="2" ParentId="21182" CreationDate="2017-04-21T17:58:59.983" Score="5" Body="&lt;p&gt;The inclination change here is large, close to 90 degrees. For major changes like that, pretty much the only alternative is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bi-elliptic transfer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means the optimal manoeuvre here is to accelerate to Mars escape velocity from Phobos. As the gravity of Phobos can be ignored due to its minuscule size, the required velocity change can be expressed as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v=\left(\sqrt{2}-1\right) \cdot \text{Phobos orbital velocity} \approx 885 m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once almost escaped from the Mars system, the spacecraft can do a zero cost &quot;at infinity&quot; burn to change the inclination and fall back towards Mars in a polar orbit intersecting with the atmosphere above the pole. In practice, this manoeuvre costs a few m/s of velocity change as it must be done at a finite distance, but that is a trade off with transfer time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In total, about $900 m/s \Delta v$ is required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for what speed it would hit the poles with, that depends on the trajectory chosen but also the size of the object as the Martian atmosphere can significantly slow down small objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r1inT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r1inT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for why this is more efficient than a more direct trajectory, we can compare it to the simplest approach. That would require us to eliminate all the initial velocity in addition to achieve a polar orbit touching the atmosphere. The total cost of that is $2650 m/s$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jeIEC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jeIEC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2017-04-22T12:23:51.013" LastActivityDate="2017-04-22T12:23:51.013" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21186" PostTypeId="1" CreationDate="2017-04-21T19:39:15.767" Score="5" ViewCount="499" Body="&lt;p&gt;I've been looking at some references of rockets taking off. I came across the Apollo 14 Saturn V launch and noticed that when the rocket was taking off the smoke coming from around the rocket and from underneath twisted around the rocket. I find this very peculiar. Why does this happen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/NK2u1i_bwlE?start=2530&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The effect is very prominent at around 42:10.&lt;/p&gt;&#xA;" OwnerUserId="19361" LastEditorUserId="28871" LastEditDate="2022-07-23T00:36:54.407" LastActivityDate="2022-07-23T00:36:54.407" Title="Why Did Apollo Smoke Twist When Taking Off?" Tags="|rockets|launch|apollo-program|saturn-v|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21188" PostTypeId="1" CreationDate="2017-04-21T21:06:58.163" Score="0" ViewCount="102" Body="&lt;p&gt;Given the poor track record so far for successful unmanned Mars missions (26/55, less than half), and the fact that a manned mission would have to use yet another descent method, let alone a whole lot of newly developed equipment, what would the chances for success of a first mission really look like?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any studies available on how to assure mission success and crew safety? What would alternative scenarios look like for such a mission? What would possible consequences look like in the event of a breakdown?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not looking for vision documents and the like I'm looking for cold hard facts.&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="8693" LastEditDate="2017-04-23T10:46:53.933" LastActivityDate="2017-04-23T11:27:25.367" Title="Are there studies on human Mars missions available?" Tags="|mars|crewed-spaceflight|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21189" PostTypeId="1" CreationDate="2017-04-21T23:16:12.990" Score="7" ViewCount="442" Body="&lt;p&gt;My assumption is that any body in the solar system would have it's own specific geo-synchronous orbit height. But when I took a look at Phobos and calculated the GEO height based on the &lt;a href=&quot;https://space.stackexchange.com/questions/5484/geosynchronous-orbits-around-other-solar-system-objects&quot;&gt;simple formula for synchronous orbits&lt;/a&gt; I got a height that was way above it's L1/L2 points:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GEO = &lt;strong&gt;12.65km&lt;/strong&gt; (above the surface) using the following values&#xA;&lt;br&gt;ME (mass) = 1.07E+16&#xA;&lt;br&gt;T-Rot (rotation period) = 27,552 seconds&#xA;&lt;br&gt;R (radius) = 11.26 km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;L1/L2 = 3.1km  (Mars/Phobos)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Phobos is tidally locked to Mars I assumed that its rotation period would be equal to a single orbit around Mars (7 h 39.2 min), but perhaps this is flawed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to place an object into a stationary orbit around a small mass like Phobos? Does being tidally locked to Mars cancel out it's rotational force? Or perhaps it's possible to have an L1 point below the GEO height, which would be really cool. Since Phobos is already really cool being subsynchronous to Mars...&lt;/p&gt;&#xA;" OwnerUserId="19384" LastActivityDate="2018-09-14T08:14:13.713" Title="Is there a synchronous orbital height for Phobos?" Tags="|orbital-mechanics|mars|lagrangian-points|geostationary|phobos|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21190" PostTypeId="2" ParentId="21189" CreationDate="2017-04-21T23:53:10.043" Score="11" Body="&lt;p&gt;No, or at least there isn't a useful synchronous orbital height.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you pointed out in your question, the mean radius of &lt;a href=&quot;https://en.wikipedia.org/wiki/Phobos_(moon)&quot; rel=&quot;noreferrer&quot;&gt;Phobos&lt;/a&gt; is 11.26 km, but if you look closer the sphere of influence of Phobos is only 7.6 km (from &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=32234.msg1232810#msg1232810&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;). That means that anything in the neighborhood of Phobos is acted on more strongly by the gravity of Mars than of Phobos, so orbiting Phobos directly doesn't really work (because you're really just orbiting Mars).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That doesn't mean there aren't synchronous orbits available: the Mars-Phobos Lagrange points are fixed relative to Phobos (because Phobos is tidally locked: one orbit around Mars = one rotation).&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2017-04-21T23:53:10.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21192" PostTypeId="1" AcceptedAnswerId="21204" CreationDate="2017-04-22T06:37:29.007" Score="3" ViewCount="2659" Body="&lt;p&gt;I'm solving the &lt;code&gt;Lambert's problem&lt;/code&gt;, and already have written a program, which solves the BVP using &lt;code&gt;Shooting method&lt;/code&gt; and computes the velocities of Spacecraft required to transfer to another point in a given transfer time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point is, the solution depends on transfer time, and initial position of the spacecraft. Some solutions are good, some require a lot of Delta-V...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Checking all values one-by-one would be time consuming. Needed to calculate: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The optimal initial position of spacecraft&lt;/li&gt;&#xA;&lt;li&gt;The optimal transfer time &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Probably, I should make attention on &lt;code&gt;Delta True Anomaly&lt;/code&gt;?&lt;/p&gt;&#xA;" OwnerUserId="19219" LastEditorUserId="19219" LastEditDate="2017-04-22T22:28:23.993" LastActivityDate="2017-04-23T03:59:00.413" Title="Lambert's problem solution: optimal transfer time" Tags="|orbital-mechanics|orbital-maneuver|delta-v|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21193" PostTypeId="2" ParentId="21188" CreationDate="2017-04-22T08:05:15.957" Score="2" Body="&lt;p&gt;The poor track record for unmanned Mars missions has good reasons. Mars was only the second planet we tried to explore, back in the 1960s when everything about spaceflight was new and unreliable. &#xA;So there was a process: build a spacecraft, watch it fail, find out why it failed and apply that lesson to the next mission. The same thing happened with &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_missions_to_Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus&lt;/a&gt; missions, by the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That process caused the success rate of later missions &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_missions_to_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;to rise&lt;/a&gt; spectacularly. It's also the process that will be used to prepare for manned landings: every part of the first manned mission will be tested first. There might be an unmanned landing on Mars, or they'll find ways to test the spacecraft here on Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another factor is quality assurance. 60 years of building spacecraft has resulted in an industry that's second to none when it comes to avoiding mistakes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternative scenarios and consequences of a breakdown can only be identified when a mission has been designed (and you need the spacecraft design as well for more detailed work). I don't know of any manned missions that have gone beyond the concept stage, so it's unlikely alternatives and consequences have been defined in any detail yet. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-04-23T11:27:25.367" LastActivityDate="2017-04-23T11:27:25.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21194" PostTypeId="2" ParentId="21189" CreationDate="2017-04-22T17:12:37.523" Score="6" Body="&lt;p&gt;By definition, the only stationary points in a two-body system are the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Langrangian points&lt;/a&gt; This applies as Phobos is tidally locked to Mars, and thus the two periods are equivalent. Even L3, L4 and L5 are by definition stationary points even if not close to Phobos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However that is only the &lt;strong&gt;&lt;em&gt;stationary orbits&lt;/em&gt;&lt;/strong&gt;, many more orbits are going to be &lt;strong&gt;&lt;em&gt;synchronous orbits&lt;/em&gt;&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lissajous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lissajous orbits&lt;/a&gt; around L1, L2 and L3 are also synchronous, and even if they are numerically improbable to be periodic to an exact degree, that is entirely possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More interestingly perhaps are the various &lt;a href=&quot;https://en.wikipedia.org/wiki/Horseshoe_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tadpole and Horseshoe orbits&lt;/a&gt; connecting L4 and L5 as any such orbit &lt;strong&gt;is guaranteed to be synchronous due to its hillbert potential&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A third consideration is that the extremely small mass of Phobos compared with Mars is that any orbit with the same orbital period as Phobos can be considerd a deformed lissajous curve only loosely interacting with Lagrange points. This allows for an infinite family of orbits with two degrees of freedom (eccentricity and inclination) with synchronous orbits of various amplitude.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2018-09-14T08:14:13.713" LastActivityDate="2018-09-14T08:14:13.713" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21195" PostTypeId="2" ParentId="21186" CreationDate="2017-04-22T17:35:07.180" Score="8" Body="&lt;p&gt;The effect you are seeing is simply wind, not a twist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DbLFj.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DbLFj.png&quot; alt=&quot;twist vs wind&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for the diagonal direction is that the smoke is moving horizontally while the rocket is moving upwards, like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ifepN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ifepN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can I be so sure about this? Well, viewing the other side of the rocket is not necessary: If the smoke was indeed ejected sideways, it would continue in the same direction because of inertia, away from the rocket as it curves away. But in the video the smoke follows the curvature of the cylinder, and that can only be explained by the wind causing a lower pressure on the leeward side of the rocket:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tLW06.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tLW06.png&quot; alt=&quot;wind low pressure&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can also confirm this by watching a Saturn launch were the wind speeds were very low, like on the Apollo 8 mission:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/SPBnoj3TKJU?start=48&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="8693" LastEditDate="2017-04-22T17:40:45.410" LastActivityDate="2017-04-22T17:40:45.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21196" PostTypeId="2" ParentId="21184" CreationDate="2017-04-22T17:52:49.463" Score="2" Body="&lt;p&gt;I just happened to come across the answer to one part of your question - the footprint: There is an appendix on Iridium flares in &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/1619565331&quot; rel=&quot;nofollow noreferrer&quot;&gt;Durrell Hillis' book &quot;Creating Iridium&quot;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Flare track on Earth's surface is very narrow:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Solar &quot;image&quot; elliptical spot approximately 5 miles (N-S) by 9 miles&#xA;(E-W) &lt;/li&gt;&#xA;&lt;li&gt;Travels along roughly longitude lines at speed of space vehicle&lt;/li&gt;&#xA;&lt;li&gt;Reflections make space vehicle visble for 5 to 20 seconds&quot;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="19326" LastActivityDate="2017-04-22T17:52:49.463" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21197" PostTypeId="2" ParentId="21173" CreationDate="2017-04-22T18:24:50.000" Score="7" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=3iPuB-Ruzi4&quot; rel=&quot;nofollow noreferrer&quot;&gt;This video&lt;/a&gt; from the launch includes some footage from before liftoff were the pendulum has some momentum. I counted 8 periods over 10.2 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From that, we can find the length of the pendulum:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$L = g\left(\frac{T}{2\pi}\right)^2$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$L \approx 40cm$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;51 seconds after launch, I can start to count again, giving 20 periods over 17.4 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$g = \frac{L}{\left(\frac{T}{2\pi}\right)^2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$g \approx 21 m/s²$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have to subtract gravity, so the rocket is accelerating at a little more than &lt;span class=&quot;math-container&quot;&gt;$1.1g$&lt;/span&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 1 minute 48 seconds (right before the escape tower goes), I can count 8 periods over 6.0 seconds. Using the same logic, that is about &lt;span class=&quot;math-container&quot;&gt;$1.85g$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; I found another way to get a value for the acceleration: The camera angle in the beginning is constant, so we can use the time the rocket uses to accelerate its own height (45.6m). That is 4.9 seconds, so the initial acceleration is &lt;span class=&quot;math-container&quot;&gt;$0.39g$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 2:57 a velocity indicator is visible, but that is cheating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As experienced by the cosmonauts:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; -1s: 1.0g&#xA; +2s: 1.4g&#xA; 51s: 2.1g&#xA;108s: 2.85g&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="15906" LastEditDate="2019-01-12T18:32:57.337" LastActivityDate="2019-01-12T18:32:57.337" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="21198" PostTypeId="1" CreationDate="2017-04-22T21:04:58.480" Score="4" ViewCount="112" Body="&lt;p&gt;What sorts of peculiarities might affect an electromagnetic rocket (VASIMR or Magneto-Inertial Fusion) crossing Jupiter's radiation belts? Could all of that energy fry a magnetic thrust chamber and disable the engine?&lt;/p&gt;&#xA;" OwnerUserId="17089" LastEditorUserId="6816" LastEditDate="2017-04-22T21:58:59.007" LastActivityDate="2017-04-22T21:58:59.007" Title="Electromagnetic engines in heavy radiation regions" Tags="|propulsion|engine-design|advanced-propulsion|magnetoplasmadynamic|vasimr|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21200" PostTypeId="1" AcceptedAnswerId="21201" CreationDate="2017-04-22T22:20:20.470" Score="2" ViewCount="289" Body="&lt;p&gt;What sorts of unique problems are there with planning a trajectory to Proxima Centauri, besides it being out of the plane of the ecliptic? (I'm guessing if you've got the juice to make it there, inclination's the least of your worries)&#xA;So...cumulative errors? Unknown perturbations in gravity (rogue planets, unseen dark matter)?&#xA;Not to mention velocity - I'm assuming an Orion-drive or Magneto-Inertial Fusion of similar power that could eventually (hopefully) accelerate to around 0.10c after an Oberth maneuver either at the Sun or Jupiter.&lt;/p&gt;&#xA;" OwnerUserId="17089" LastEditorUserId="6816" LastEditDate="2017-04-23T21:08:15.187" LastActivityDate="2017-04-23T21:08:15.187" Title="Navigating to Proxima-B" Tags="|orbital-mechanics|interstellar-travel|navigation|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21201" PostTypeId="2" ParentId="21200" CreationDate="2017-04-22T23:21:24.143" Score="1" Body="&lt;p&gt;Only pure $\Delta v$ matters, none of your optimizations are significant:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Inclination: The initial velocity due to our location is about $30,000m/s$. Compare that with $0.1c$, about $300,000,000m/s$, only $0.01\%$.&lt;/li&gt;&#xA;&lt;li&gt;Oberth manoeuvre: The best we can possibly do is burning close to the surface of the Sun (there are some problems with that, but we can ignore them for now). The $\Delta v$ saved, ignoring relativistic effects, is:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;$$0.1c - \sqrt{(0.1c)^2 - v_e^2} \approx 0.00002c \text{ or about 0.02%}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would say the greatest complication is to stop at Proxima Centauri, as you will have a velocity of $0.1c$ when you arrive. You need a similar amount of $\Delta v$ to stop.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perturbation in gravity should be easy to compensate for as you have already shown you have enough boost to escape the gravitational field of an entire star by many orders of magnitude.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-04-22T23:21:24.143" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21202" PostTypeId="2" ParentId="21192" CreationDate="2017-04-22T23:59:21.460" Score="3" Body="&lt;p&gt;Sorry. You need to check a bunch of values. In the example of choosing an Earth escape trajectory to a target, a contour plot of departure and arrival dates is made, where each sample in the plot is a solution to a Lambert problem. This is called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Porkchop_plot&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;porkchop&quot; plot&lt;/a&gt;, due to the typical shape of the contours seen.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-04-22T23:59:21.460" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21203" PostTypeId="2" ParentId="264" CreationDate="2017-04-23T03:00:18.637" Score="3" Body="&lt;p&gt;Many of the figures being offered here for masses of various components seem off, and there is definitely a typo in the Isp given for the Orbiter OMS engines--it is really 316 sec, not 361. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the Orbiter actually massed more than the figures given here, and the ET generally a lot less--116-120 tonnes for Orbiter all up on the pad, 30 to 26 tonnes for a dry tank, containing 725 tonnes of oxygen and hydrogen at launch, attached to two SRBs each massing 88 tonnes empty with 500 tonnes propellant each added up to 2050 on the pad all up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a link to Norbert Brügge's site:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.b14643.de/Spacerockets_2/United_States_1/Space_Shuttle/Description/Frame.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.b14643.de/Spacerockets_2/United_States_1/Space_Shuttle/Description/Frame.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It gives a consistent OMS load maximum of 21.65 tonnes, implying that the load would vary from mission to mission. Liftoff weights excluding payloads vary for various generation of Orbiter and flight but are close to 100 tonnes, between 94.4 and 105.5, I believe that mass includes the OMS propellant and varies largely for that reason. The payloads given raise some serious question marks in some cases, but note how the last decade of Shuttle use brought the loads down below 15 tonnes, because presumably they were mostly missions to ISS, which is significantly higher in orbit than the lower ones that would maximize payload--also ISS is at a 51.64 degree inclination, making reaching it from Canaveral more difficult. The newer the Orbiter model, the lighter it was, and so only Endeavour, Atlantis and perhaps Discovery could be usefully used for ISS missions--Columbia was to be used for alternative, lower-altitude missions until it was lost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the discussion here is putting an ET into orbit, I suppose we should look at the standards for late missions to ISS. Alternative to using an ET as a space station structural element we might want to orbit one to gradually refuel for a heavy deep space mission, but with nothing better than either a Shuttle or perhaps a Titan V heavy launcher capable of perhaps 30 tonnes to LEO, we would not be refilling such a tank fast! Also its propellants would tend to boil away, hydrogen especially,so we'd need some extra tonnage to reliquify the hydrogen (using cold hydrogen, re-condensing oxygen is a snap)--it all points to space station altitude operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the last couple columns of Brügge's second set of &quot;design&quot; tables, we have payloads just under 15 tonnes, all up launch masses almost exactly 2050 tonnes (those are very consistent across the whole range of all STS launches from 1981 to 2011), Endeavour massing 101.5 minus payload (thus 116.5 all up), dry ET 27 tonnes, 726 tonnes of fuel in it, and the SRBs massed 1178.2 tonnes total. I think we can attribute a 3.3 tonne discrepancy to additional fuel in Endeavour's OMS supply, making it mass actually nearly 120 tonnes on the stack. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, per work done by others above, a standard MECO orbit falls short of a circularized target orbit by 150 m/sec, and in this case the target orbit is ISS, at 405 km altitude, then circular orbital speed is 7670 m/sec. Subtracting 150 m/sec would reduce the major axis from 13566 km to 13060 or by 506 km--meaning the perigee would be some 99 km below sea level! I don't know whether Endeavour ever was launched straight to an elliptical MECO orbit of this kind and did a single OMS burn to achieve 150 m/sec delta V requiring about 4.7 percent of on board mass, or 5.67 tonnes. Alternatively of course it could be put into a much lower orbit initially, by means of a MECO orbit falling short of say a nominal 200 km altitude parking orbit, then first circularizing there with a 150 m/sec burn of this same kind, wait for the phasing of its lower faster orbit to line up with the half-period of a transfer orbit up to 405 so it arrives near ISS before doing a second burn to bring it close to synch, followed of course by fussy and slow careful approach maneuvers. I suspect the latter happened, and it is also a conservative assumption it did so. But the energy difference between an orbit with -100 km perigee and 200 km apogee, and one with a 405 km apogee instead, is not tremendous--just under 975 KJ/kg, which at Earth surface gravity would be the potential difference for just under 100 km altitude. Versus the speed of the lower MECO orbit at apogee, it would take just adding 13 m/sec to that to exceed the necessary energy for the higher apogee! (That is not the way to do it of course). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the conservative  assumption is that Endeavour first climbs to a 200 km parking orbit, circularizes there, and then climbs at a calculated moment to the 405 km orbit altitude and circularizes there. This allows for flexible launch times, and to phase into the approach to the actual space station location later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Throw in a tonne of propellant more for frittering around for a safe dock at the established station--but note on the first mission this is not necessary, since wherever the ship stops is the ISS location! Assuming 150 m/sec as given for the first circularization, which requires 5 2/3 tonnes of prop, to go into a 200-405 km transfer orbit requires delta-V of 59.525 m/sec, and then to circularize at 405 requires another 5907, or 118.595 altogether. Over three burns then just under 270 m/sec are required, and these three all apply to the same initial mass, here apparently 120 tonnes, for a total propellant burn of 10 tonnes. Note how this is close to half the maximum installed tankage allows. To return to Earth from there, I estimate 120 m/sec braking is plenty. Note also this must always be applied to a down mass (ofter the burn) less than IIRC 105 tonnes, since this is limited by the lift area and TPS maximum temperatures, and the upper limit for return applies to all models of the Orbiter, though the lighter later ones can make more of that down mass payload. Thus only a bit more than 4 tonnes is needed, and 5 allows a generous safety factor for that burn. I suspect missions to ISS involved maximum OMS propellent load, 21 2/3 tonnes, whereas we see 15 is all that is needed for the nominal mission--implying that 7 tonnes are a safety factor, in this case nearly over 44 percent. This gives the mass of Endeavour, with no payload and no OMS fuel but otherwise loaded with supplies for a nominal mission, as 83.35 tonnes, and a theoretical total delta V of 1130 m/sec, or 880 without touching the 7 tonne OMS reserve, the reserve thus raising total delta V by 28.4 percent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now then, what is the cost of attempting to bring the tank to the ISS orbit by this three stage series of ascent burns, holding 12 tonnes in reserve for nominal descent plus 7 tonnes for emergency contingencies? We can't, if we refuse to touch any of those 7 tonnes because of course I figured the reserve on the basis of the nominal mission. However, we use less than 10 for the ascent phase, and a 26 tonne dry advanced tank raises the nominal 120 tonne Orbiter pre-burn mass, before all three phases of ascent burns, by 21.7 percent. Therefore we can steal less than 2.2 tonnes of OMS prop from the reserve, less than 30 percent of it, and bring the tank to orbit! Future missions that also bring another tank will cost more, because it will be necessary to add some more maneuvering to come to a gentle dock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, given a nominal 725 tonnes fuel load in the tank at launch, if we shave some mass off the SSME burn load, we can save some fuel mass unburned. We are going to want to load the two sections of the tank with air later, and 80 percent of that is nitrogen. The volume of the tank (ignoring the intertank separating the LOX from the LH) holds those 725 tonnes, and at a bit more than 36 percent the density of water on average, when air is 1/800 the density of water, a load of air at 1 atmosphere nominal would be some 2.5 tonnes mass, thus 500 kilograms of it is oxygen. If we want to save half a tonne of oxygen or 1/1243 the total oxygen load in the tank, we would shave that proportion of total mass to OMS burn, or 118 kg, off the payload, and deduct the half tonne from it as well. When fully vaporized the half tonne of oxygen would expand but I believe its pressure would be well under a full atmosphere in the oxygen tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We otherwise do not need to skimp on the nominal payload mass at all, given that we can't trade off any of it to restore the 7 tonne OMS fuel reserve--we could, but it would involve plumbing modifications as well as tankage mass in the payload bay. Eliminating the payload completely would not bring the total mass down to nominal and so we'd need to tap into the reserve in any case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Payload is thus down to 14 tonnes. For the first ET-based ISS assembly I suppose the entire payload should consist of initial outfitting masses, which could comprise a single module meant to be attached to the tank to provide a structural anchor and docking port for a future mission. It has been pointed out the ET is a &quot;fluffy&quot; object of low ballistic coefficient, its orbit will decay more rapidly than a denser object like say Skylab. But I believe ISS fully assembled is also draggy in the same way, so it won't be worse. Still, a top priority is to enable the tank to remain on orbit, and it needs orientation control too. I believe the first module would therefore be a combination of propulsion module and access dock, and much of its mass would be propellant to maintain orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Glancing at the actual ISS historical timeline, the beginning was Zarya, a module of 19 and a third tonnes to which Endeavour attached the almost 12 tonne Unity module. If the tank launch is the second stage of alternate ISS assembly, Endeavour could first dock an expanded Unity module (say carrying supplemental propellant for Zarya) to the Russian beginning, and then, docked to Unity and using its Canadarm, position the tank to a specialized port opposite the Zarya end of Unity. Some of the cargo mass would be devoted to structures such as flanges specially built into the tank itself, so perhaps we can't carry extra fuel for Zarya on this mission after all. Once docked however, subsequent Shuttle missions can bring either modules to dock to Unity's 4 radial ports, or temporarily as cargo modules to be unloaded into the tank. Between the tank orbiting mission and the next visit of a spacecraft to ISS, the vents of the hydrogen tank would be opened to allow the residual hydrogen to boil off into space, while the LOX residual evaporates to fill the oxygen tank as a gas. It would then be possible, perhaps by remote control before the next mission, to close the hydrogen vent, open a special new valve built into the tank between the two tanks to fill the hydrogen tank with oxygen. Just 2 tonnes out of a nominal 15 tonne payload of the next shuttle mission (or perhaps 2 out of the 20 tonne mass of another module equivalent to Zarya also launched on a Proton) would be nitrogen to make up the rest of the air. Upon completion of that (with small mass of water and traces of CO2 added as well) the two tank segments become habitable and crew can move into them with 12 or 18 more tonnes of infrastructure and operational equipment to fit it out with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this goes to show that the Shuttle, as it was, could indeed supply tanks to orbital destinations at a very low cost of dipping into existing fuel reserves. More efficient ways of using the tanks would emerge had we gone ahead and developed &quot;Shuttle-C,&quot; a number of Shuttle-derived vehicle proposals that had in common using the standard equipment provided for Orbiter launches including the tank, SRBs, and a new module meant to recover the SSMEs from orbit. Now I have never been able to get any details on the nature of the engine module, but I would be amazed if a 3 engine module had to mass as much as 60 tonnes all up; more likely in the range of 35-45 I would think. (I actually have my own ideas on this line for a next-generation Space Transport system that would develop 15 tonne or less separate modules for each engine, scaled down Orbiter designs, unmanned, that would enable a very flexible national launch system using various numbers of engines and various sizes of SRB. But to keep it simple the Shuttle-C proposals were all in addition to using Orbiter as the sole crewed vehicle; Shuttle-C designs would all launch unmanned, and some of them proposed using SSMEs up on one launch, presumably old ones near end of life). As unmanned launchers Shuttle-C designs should have been at least somewhat cheaper to launch than an Orbiter, and even if the engine recovery module massed as much as 60 tonnes, half the Orbiter mass, the other 60 tonnes are 3 times the nominal 20 tonne Orbiter payload--4 times the payload to ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With such a system in hand--and I think it clearly could be operational well before 2000--a single Shuttle C mission, with a cargo module permanently attached to the tank and including an OMS engine, could deliver to the orbit a tank pre-fitted for air filling, 5 tonnes of stored liquid air (2 fills of the tank), and 50 more tonnes of other supplies and equipment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I estimate a 9 second burn of a single STS OMS engine once a month would suffice to check orbital decay, assuming similar forces to those on the existing ISS. That would consume under 80 kg of propellant each time, for under a tonne a year. Clearly the propellant reserve does not have to be tremendous! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking up actual figures ISS currently consumes 7.5 tonnes a year; even so, a 10 tonne reserve included in the original launch is only 20 percent of the miscellaneous tonnage available. With 40 tonnes left beyond that, this single launch would be the equivalent of both Zarya and Unity with 10 more tonnes (another Unity, almost) left over. Since Zarya is the propulsion module of the existing station, clearly we could get more utility than these two launches provided aside from the use of the tank itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An Orbiter, arriving after the Shuttle C launch or pre-positioned at the destination before it, coordinated with a Soyuz launch, could provide an international workforce of 10 crew to do the initial docking of a modified Zarya focused on providing power without propulsion to the Shuttle C cargo module, which I envision would have Unity style multiple docking ports built in, one with an adapter (moveable as the station grows) for Soyuz. Whether this crew could finish inflating the tanks with air depends on how fast hydrogen gets flushed out into vacuum from that tank. I'd think they could outfit it to the point of it being immediately habitable for the next crews. At 120 tonnes, it would be almost 30 percent of the mass of the current station. 3 more Shuttle-C launches delivering 3 more tanks, accompanied by 3 more Orbiter visits each bringing 15 tonnes of cargo would surpass the current assembly by 15 tonnes. Crew volume of course would be gigantic, so much so that we probably would not want to do more tank delivery missions, but even so 6 Orbiter visits would round out to the same mass as our current station, presumably a mix of cargo for interior installation and new modules and trusses and solar panel sets and so forth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is then a practical proposal; with Shuttle C it would have been accomplished very quickly with few launches. Whether we would want to develop the 10-tank spinning habitat proposed (an octagon of 8 tanks end to end trussed to a pair along the axis, end to end) is a question of funding, not launch capability. Such an alternative would of course require orders of magnitude more people in space to be worthwhile, and as offered seems incomplete to me--I don't see how Shuttles or any other craft would dock to it once spun up, one or both the axis tanks would need to attach to a de-spin module requiring power and probably reaction mass, attached at the other end to a microgravity station ships could dock to, which would also be the place to put solar panels and radiators I suppose. Such a station would I suspect require an order of magnitude lowering of launch costs, to be remotely fundable, bearing in mind the need to rotate hundreds of crew up every year, and vital supplies along with them. I do think developing Shuttle-C would point the way toward major cost reductions per kilogram, perhaps by a factor of 5, but not 10.&lt;/p&gt;&#xA;" OwnerUserId="16823" LastActivityDate="2017-04-23T03:00:18.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21204" PostTypeId="2" ParentId="21192" CreationDate="2017-04-23T03:59:00.413" Score="4" Body="&lt;p&gt;The shooting method is a general purpose techniques for solving boundary value problems. As a general rule, a solver specialized to the problem at hand will outperform general purpose techniques -- if such a special purpose solver exists. This is most certainly the case for the two-body orbital boundary value problem, aka Lambert's problem because this problem is central to rendezvous and to orbit determination. A large number of special purpose Lambert's problem solvers have been developed over the last two-plus centuries. You will fare much better if you use one of these domain-specific techniques as opposed to a shooting method solver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even then, you'll have to pay attention to delta V. A Lambert's problem solver finds one or more conic sections that intersect the source orbit at time t&lt;sub&gt;0&lt;/sub&gt; and the target orbit at time t&lt;sub&gt;1&lt;/sub&gt;. Just because a solution satisfies the constraints does not mean it is practical. You'll need to check for transfer orbits that are ridiculously expensive in terms of delta V. This includes retrograde transfer orbits, for example, but also some prograde transfer orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this day of open literature / open source, it is far better to look for existing Lambert's solvers as opposed to rolling your own. You can find several in the public literature (but sometimes only as pseudocode), and several on github (but sometimes only as student quality code).&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-23T03:59:00.413" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21205" PostTypeId="1" AcceptedAnswerId="21208" CreationDate="2017-04-23T09:00:54.540" Score="0" ViewCount="242" Body="&lt;p&gt;Of course, research on Mars is first needed to explore if there is life or has been in the past.&lt;br&gt;&#xA;But after that,  boulders could be thrown from Phobos at specific areas on Mars, like Utopia Planitia, to evaporate or liquify enormous amounts of water ice and CO2 ice.&lt;br&gt;&#xA;By dosing the number of impacts, destroying the surface could be minimized.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could Phobos deliver enough boulders to significantly improve the atmosphere?                      &lt;/p&gt;&#xA;" OwnerUserId="19335" LastEditorUserId="19335" LastEditDate="2017-04-23T17:50:40.700" LastActivityDate="2017-04-23T17:50:40.700" Title="Could Mars be made much more habitable by boulders from Phobos?" Tags="|mars|terraforming|phobos|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21206" PostTypeId="1" AcceptedAnswerId="21209" CreationDate="2017-04-23T09:40:03.763" Score="39" ViewCount="4444" Body="&lt;p&gt;Is there a replica of the interior of the space station on the ground? For training astronauts and to try out equipment installations.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="8693" LastEditDate="2017-04-23T10:34:14.953" LastActivityDate="2022-12-05T16:29:57.550" Title="Is there a mockup of the ISS on the ground?" Tags="|iss|crewed-spaceflight|simulation|training|facility|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21207" PostTypeId="1" CreationDate="2017-04-23T09:56:37.173" Score="19" ViewCount="3187" Body="&lt;p&gt;Is it possible to use carbon monoxide as propellant for a rocket? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, carbon dioxide from the atmosphere of Mars could be transformed to carbon monoxide and oxygen via &lt;a href=&quot;https://en.wikipedia.org/wiki/Photochemical_carbon_dioxide_reduction&quot; rel=&quot;noreferrer&quot;&gt;photochemical reduction&lt;/a&gt; using a photocatalytic process, and unlike the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sabatier_reaction&quot; rel=&quot;noreferrer&quot;&gt;Sabatier reaction&lt;/a&gt; (producing methane), precious water is not consumed in the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could the resulting products then be used as a potential bi-propellant?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$2\,{\rm CO}_2 + 2h\nu\ \to\ 2\,{\rm CO} + {\rm O}_2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be issues of storage of LOX or LCO, but provided those are addressed somehow, could a rocket engine or thruster use these effectively?&lt;/p&gt;&#xA;" OwnerUserId="18810" LastEditorUserId="189" LastEditDate="2018-01-09T04:51:20.807" LastActivityDate="2021-11-16T17:43:19.143" Title="Use of carbon monoxide as propellant on Mars" Tags="|mars|fuel|colonization|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21208" PostTypeId="2" ParentId="21205" CreationDate="2017-04-23T09:56:54.267" Score="5" Body="&lt;p&gt;$$\text{Some initial thoughts:}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is Phobos even big enough to make a difference? Slowing Phobos down to hit the Martian atmosphere costs about 540m/s. Reaching escape velocity and then hitting the atmosphere costs more, but the impact energy is greater, corresponding to about [75 days](&lt;a href=&quot;https://www.wolframalpha.com/input/?i=4&quot; rel=&quot;noreferrer&quot;&gt;https://www.wolframalpha.com/input/?i=4&lt;/a&gt;&lt;em&gt;(((mars+escape+velocity)%5E2+&lt;/em&gt;+phobos+mass+%2F2)%2Fmars+area)%2Fmars+solar+flux) of sunlight on Mars ($1.36\cdot10^{23}J$). That is a significant amount of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there are rocks much easier to throw than Phobos. The dwarf planet Vesta (too large to move, of course) is only 920m/s away from hitting Mars, and there are thousands of Phobos-size asteroids only about 50-100m/s away from hitting Mars at slightly more than escape velocity.&#xA;&lt;strong&gt;Many asteroid are closer to Mars than Phobos itself in terms of $\Delta v$.&lt;/strong&gt; Asteroids are also favourable as you can use them to hit the poles without any extra cost for an inclination change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\text{What can we melt?}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Martian polar icecaps contain about &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_polar_ice_caps&quot; rel=&quot;noreferrer&quot;&gt;3.2 million km³&lt;/a&gt; of mixed ice, or about $3.5\cdot10^{18}kg$ of volatiles. That is equivalent to the mass 0.68 Earth atmospheres. Very promising. It only takes about $1\cdot10^{21}J$ to melt that, a lot less than crashing Phobos or an asteroid into Mars.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-04-23T09:56:54.267" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21209" PostTypeId="2" ParentId="21206" CreationDate="2017-04-23T10:22:56.027" Score="48" Body="&lt;p&gt;There are multiple mock-ups of the ISS, for various purposes:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Vehicle_Mockup_Facility&quot; rel=&quot;noreferrer&quot;&gt;Space Vehicle Mockup Facility&lt;/a&gt;&lt;/strong&gt; at Johnston space center contains replicas of many modules of the ISS, some of them assembled together.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wRgLjm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wRgLjm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Neutral_Buoyancy_Laboratory&quot; rel=&quot;noreferrer&quot;&gt;Neutral Buoyancy Laboratory&lt;/a&gt;&lt;/strong&gt; is used for EVA training, and has a modules in the pool&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/POS7lm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/POS7lm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &lt;strong&gt;&lt;a href=&quot;https://crgis.ndc.nasa.gov/historic/Jake_Garn_Simulator_and_Training_Facility&quot; rel=&quot;noreferrer&quot;&gt;Space Station Training Facility&lt;/a&gt;&lt;/strong&gt; does not have full modules, but it has flight electronics used for mission training.  Mockups of the module interiors are provided, including a full Robotics Workstation, but most are of low fidelity.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iEMJnm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iEMJnm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The &lt;strong&gt;&lt;a href=&quot;http://www.gctc.su/main.php?id=133&quot; rel=&quot;noreferrer&quot;&gt;ISS RS Mockup facility&lt;/a&gt;&lt;/strong&gt; at Yuri Gagarin Cosmonaut Training Center is the Russian training model of the ISS (including the US part).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6Y6HIm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6Y6HIm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The Cosmonaut Training Center also has the &lt;strong&gt;GCTC Neutral Buoyancy Pool&lt;/strong&gt;, containing a model of the Zvezda module&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i2Sq6m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i2Sq6m.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="6944" LastEditDate="2017-04-23T16:28:33.173" LastActivityDate="2017-04-23T16:28:33.173" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21210" PostTypeId="2" ParentId="20474" CreationDate="2017-04-23T12:58:30.570" Score="3" Body="&lt;p&gt;So there is the obvious problem of building such a tower (and having it not collapse or burst somewhere along the way). However your question says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;If a cylindrical tube of sufficient strength and size were built tall enough to reach the edge of atmosphere&lt;/strong&gt; would it not be possible to have a vehicle inside this tube with an air lock and means to open hatch or door to allow the vacuum of space to suck the vehicle up and away? If the angle were correct and certain adjustments to tube were done orbit could be obtained.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So we're working off the premise that we have such a tower and those problems can be left for another question.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, just to check I've pictured this right I've drawn it out (in paint so it isn't great but hopefully assists a description).&#xA;&lt;a href=&quot;https://i.stack.imgur.com/L1I70.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L1I70.png&quot; alt=&quot;Space air cannon&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the ball is our ship with a seal around it so air cannot escape past the ship. It would have to be supported at the bottom before the air is added.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We then need to release the pressurised air. In principle yes, this pushes a payload upwards (assuming the tower is fit for coping with these pressures). If we don't use pressurised air then the ship won't get to the top, the heavier the ship the higher the pressure we need.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would need to suck the air back out again afterwards though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Just a little extra:&lt;/strong&gt; I'm not sure of the details but it seems to me that, if a tower like this could be built, then using a mechanical lift would be much safer and use less energy.&lt;/p&gt;&#xA;" OwnerUserId="18948" LastActivityDate="2017-04-23T12:58:30.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21211" PostTypeId="1" AcceptedAnswerId="21220" CreationDate="2017-04-23T14:59:02.030" Score="6" ViewCount="743" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/21207/#comment58871_21207&quot;&gt;This comment&lt;/a&gt; got me thinking...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pure oxygen from a tank of LOX can allow a wide variety of materials to be used as a convenient if not always optimal rocket propellant; things like &lt;em&gt;parafin&lt;/em&gt; and &lt;em&gt;sugar&lt;/em&gt; for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have not seen even a partly-serious demo of wood as fuel. Would wood not work well? Is there some property of wood that would make wood unsuitable, or could wood be used?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-24T02:50:59.230" LastActivityDate="2018-08-12T15:38:11.120" Title="What properties define a good solid propellant for a hybrid engine? (e.g. why not use wood?)" Tags="|hybrid-rocket|sugar-rocket|" AnswerCount="2" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21212" PostTypeId="2" ParentId="12044" CreationDate="2017-04-23T19:42:19.607" Score="2" Body="&lt;pre&gt;&lt;code&gt;!$$SOF&#xA;    COMMAND = 'DES=2010 VQ'&#xA;    CENTER =  '500@0'&#xA;    MAKE_EPHEM = 'YES'&#xA;    TABLE_TYPE = 'VECTORS'&#xA;    START_TIME = '2017-04-01 00:00:00'&#xA;    STOP_TIME = '2017-04-01 00:00:01'&#xA;    STEP_SIZE = '1'&#xA;    OUT_UNITS =  'AU-D'&#xA;    VECT_TABLE =  '3'&#xA;    REF_PLANE =  'ECLIPTIC'&#xA;    REF_SYSTEM =  'J2000'&#xA;    VECT_CORR =  'NONE'&#xA;    VEC_LABELS =  'NO'&#xA;    CSV_FORMAT =  'YES'&#xA;    OBJ_DATA =  'YES'&#xA;    !$$EOF&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;returns heliocentic coordinates:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;$$SOE&#xA;2457844.500000000, A.D. 2017-Apr-01 00:00:00.0000,  7.539123579055962E-01,  7.098001692200981E-01,  1.408370538460704E-03, -1.019002972030220E-02,  1.127277705358026E-02,  9.280758228718769E-05,  5.980387083611182E-03,  1.035471924841630E+00,  3.082270281389151E-04,&#xA;2457844.500011574, A.D. 2017-Apr-01 00:00:01.0000,  7.539122399654240E-01,  7.098002996920419E-01,  1.408371612622512E-03, -1.019003207292571E-02,  1.127277484368998E-02,  9.280757740530809E-05,  5.980387104214990E-03,  1.035471928409068E+00,  3.082263804494916E-04,&#xA;$$EOE&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Here XYZ coordinates is like:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;JDTDB,            Calendar Date (TDB),                      X,                      Y,                      Z,                     VX,                     VY,                     VZ,                     LT,                     RG,                     RR,&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The other asteroid:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;COMMAND = 'DES=2008 LD'&#xA;CENTER =  '500@0'&#xA;MAKE_EPHEM = 'YES'&#xA;TABLE_TYPE = 'VECTORS'&#xA;START_TIME = '2017-04-01 00:00:00'&#xA;STOP_TIME = '2017-04-01 00:00:01'&#xA;STEP_SIZE = '1'&#xA;OUT_UNITS =  'AU-D'&#xA;VECT_TABLE =  '3'&#xA;REF_PLANE =  'ECLIPTIC'&#xA;REF_SYSTEM =  'J2000'&#xA;VECT_CORR =  'NONE'&#xA;VEC_LABELS =  'NO'&#xA;CSV_FORMAT =  'YES'&#xA;OBJ_DATA =  'YES'&#xA;!$$EOF&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;.... &lt;br /&gt;&#xA;then do &lt;code&gt;sqrt(dx^2+dy^2+dz^2)&lt;/code&gt;&lt;/p&gt;&#xA;" OwnerUserId="19399" LastEditorDisplayName="user10509" LastEditDate="2017-04-24T00:42:59.503" LastActivityDate="2017-04-24T00:42:59.503" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21213" PostTypeId="1" AcceptedAnswerId="21226" CreationDate="2017-04-23T21:04:38.593" Score="16" ViewCount="2699" Body="&lt;p&gt;There's something I don't understand:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it comes to de-commissioned satellites, rocket bodies etc. I thought one way of re-entry was via a highly elliptical orbit: The perigee gets low enough (app. 100 km) to reach into the atmosphere and then re-entry happens. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things yet don't seem to be that simple from what my observations of 1976-006D tell me: I started observing it a month ago, when its apogee was at 7767 km, perigee at 99 km. I expected re-entry to happen any day. Four weeks later, apogee has sunk to 3474 km, perigee is still at 100 km. How can this be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2013-062C, on the other hand, re-entered at 3557/105 km. Perigee dropped from 110 to 105 km within four days and the object quickly re-entered. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other recent re-entries happended at 244/228 and 213/167 km orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does this fit into one logic? What else do I need to understand or to factor in? &lt;/p&gt;&#xA;" OwnerUserId="19326" LastEditorUserId="2752" LastEditDate="2017-04-23T23:53:14.763" LastActivityDate="2019-03-21T00:47:02.243" Title="What are the conditions for re-entry of an object in a (highly) elliptical orbit?" Tags="|orbital-mechanics|reentry|debris|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21214" PostTypeId="2" ParentId="21213" CreationDate="2017-04-23T21:43:22.970" Score="13" Body="&lt;p&gt;&lt;strong&gt;Effect of manoeuvring on perigeee/apogee radius&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the issues is that during some instant of any orbital manoeuvre the orbiting object, if the manoeuvre were to end there, would complete one orbit and then pass through the same point on the next rev. Think of this as the idealised situation with no other orbital perturbations. The effect of the manoeuvre is to change the immediate velocity, not the immediate location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Continuing with the idealised situation the effect of an impulsive manoeuvre at the apogee or perigee is to affect the radius at the opposite apps. Thus a retrograde perigee manoeuvre will lower the perigee velocity which in turn mainly reduces the apogee height. The perigee altitude is unaffected. This also happens in reverse for apogee manoeuvres. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of a perigee passage through the upper atmosphere the braking effect of the atmosphere can be thought of as a whole series of retrograde manoeuvres spread out over a short arc. This has the affect of reducing the perigee velocity, which following the previous thought experiment, largely has the effect of dropping the apogee. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Relative drag effects on two objects&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to the second part of the question relating to why two objects have behaved differently - I don't know without looking more closely. It could quite plausibly be explained by a difference in Area/Mass for the two objects, making one behave much more ballistically than the other. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this way it seems plausible that the object with the higher apogee and thus higher perigee velocity and thus more drag, might nevertheless experience less velocity change per perigee pass if it has a lower Area/Mass ratio. It would also have a longer orbital period and thus few perigee passes per day.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-04-23T21:43:22.970" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21215" PostTypeId="1" CreationDate="2017-04-23T22:49:32.600" Score="4" ViewCount="497" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/questions/21213/what-are-the-conditions-for-re-entry-of-an-object-in-a-highly-elliptical-orbit&quot;&gt;&quot;What are the conditions for re-entry of an object in a (highly) elliptical orbit?&quot;&lt;/a&gt; Eerie asks about orbital mechanics of orbital decay of decommissioned satellite: starting with extremely elliptical orbit (apogee 7767 km, perigee at 99 km) the satellite's orbit gradually decays until it's nearly circular, when the satellite finally spirals down burning in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But a fraction of m/s burn at the apogee would be able to drop the perigee into ~30km range, and then the satellite would rapidly burn up, traveling through the atmosphere at way more than 8km/s instead of risking debris surviving the final reentry and causing damage on the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no point saving fuel, because the satellite is going to burn up anyway. The maneuver is really minuscule comparing to what is needed to get the satellite to start aerobraking at all. And the higher reentry speed into the thick atmosphere the less chance any debris will reach the ground. Never mind after so many aerobraking passes the landing point becomes completely unpredictable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why is the slowly decaying orbit through edges of Karman line used, instead of one that would make sure the satellite turns into a cloud of hot plasma over Pacific? Any rationale for keeping the perigee so high?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="2752" LastEditDate="2017-04-23T23:52:58.477" LastActivityDate="2017-04-23T23:52:58.477" Title="Why are deorbited satellites allowed a slow orbit decay instead of burning them up rapidly?" Tags="|artificial-satellite|reentry|debris|end-of-life|orbital-decay|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21216" PostTypeId="2" ParentId="21213" CreationDate="2017-04-23T22:50:11.630" Score="18" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Four weeks later, apogee has sunk to 3474 km, perigee is still at 100 km. How can this be?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The primary effect of drag on a vehicle in a highly elliptical orbit that just touches the atmosphere at perigee is, at least initially, to lower apogee. So long as the vehicle survives its brief encounter with the atmosphere near perigee, that encounter will have very little effect on perigee. As apogee decreases, the vehicle starts spending more than a tiny fraction of it's orbit in the atmosphere. This will eventually reduce perigee. With an apogee at 3474 km and perigee at 100 km, that means most of this vehicle's orbit is still outside the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How does this fit into one logic? What else do I need to understand or to factor in?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You need to factor in coefficient of drag, cross section area, and mass. Acceleration due to drag is proportional to coefficient of drag and cross section area but inversely proportional to mass. Another way to look at this is to combine these three factors into one, the ballistic coefficient: $BC = \frac M {C_d A}$ where $M$ is mass, $C_d$ is the coefficient of drag, and $A$ is the cross section to drag. Drag acceleration is inversely proportional to the ballistic coefficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1976-006D is the upper stage of a rocket. It has a nice aerodynamic shape, making for a smallish coefficient of drag a small cross section. It also has engines, beefy structure to support those engines, and empty fuel tanks, making for a high mass. Putting these together means a high ballistic coefficient. 2013-062C on the other hand is a marked as debris, either a tank or a piece of one. In either case (a tank or part of one), it will have a lousy coefficient of drag, a lousy cross section to drag, and a smallish mass. Putting these together means a much smaller ballistic coefficient that that of 1976-006D.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-23T22:50:11.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21217" PostTypeId="1" CreationDate="2017-04-23T23:12:18.677" Score="2" ViewCount="423" Body="&lt;p&gt;Based on the data about the Dragon 2 that I can find, here are the relevant specs, from &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia on Dragon 2&lt;/a&gt; and on &lt;a href=&quot;https://en.wikipedia.org/wiki/Draco_(rocket_engine_family)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Draco engines&lt;/a&gt;, and their FAA filing for its &lt;a href=&quot;https://www.faa.gov/about/office_org/headquarters_offices/ast/environmental/nepa_docs/review/launch/media/fonsi_dragon_pad_abort.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;pad abort test&lt;/a&gt; (page 8):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dry Mass: 6,400 kg&lt;/li&gt;&#xA;&lt;li&gt;Propellant: 1521 kg&lt;/li&gt;&#xA;&lt;li&gt;Payload: 3,310 kg to ISS (but, as a guesstimate, I lowered the payload to 3 tons)&lt;/li&gt;&#xA;&lt;li&gt;Draco engine I&lt;sub&gt;sp&lt;/sub&gt; : 300 s&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I have no idea what the payload would mass for this mission. I decided to use 3,000 kg, and from that the capsule would be capable of a delta V of about 440 m/s. Maybe the payload to support 2 people for the duration of the mission would be less than 3 tons, if that makes sense then by all means estimate a lower payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like when the &lt;a href=&quot;https://en.wikipedia.org/wiki/Trans-lunar_injection#History&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo missions did their TLI burn&lt;/a&gt;, they increased their speed by about 3.1 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After SpaceX's proposed 'tourist' &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_Lunar_Tourism_Mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;mission to fly around the Moon&lt;/a&gt; did its burn for TLI, would the vessel be capable of aborting to return to Earth early (if there is a system failure a la Apollo 13)?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2017-04-25T13:46:16.193" LastActivityDate="2017-04-25T13:46:16.193" Title="Could the Dragon 2 abort after trans-lunar injection?" Tags="|spacex|the-moon|trajectory|dragon-v2|abort|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21218" PostTypeId="2" ParentId="21215" CreationDate="2017-04-23T23:51:11.717" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why are deorbited satellites allowed a slow orbit decay instead of burning them up rapidly?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The ultimate reason is simple: There are no international rules or regulations placing limitations on orbital debris. The only thing that comes close is the Space Liability Convention. That convention is akin to having no speed limits on roadways, including school zones, but retaining the ability to sue the responsible part of an accident for damages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, one of the two vehicles in the related question was launched in 1976. Orbital debris was not yet a concern in 1976. The other vehicle was launched in 2013, when orbital debris was an unregulated concern. Apparently something happened to one of that vehicle's fuel tanks; 2013-062C is denoted as &quot;debris (tank).&quot; This kind of debris is hard to address even now that debris is a concern.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-23T23:51:11.717" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21219" PostTypeId="1" AcceptedAnswerId="29897" CreationDate="2017-04-24T00:36:16.753" Score="10" ViewCount="644" Body="&lt;p&gt;The BBC News article &lt;a href=&quot;http://www.bbc.com/news/world-us-canada-39686055&quot; rel=&quot;noreferrer&quot;&gt;Aurora photographers find new night sky lights and call them Steve&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;A group of aurora enthusiasts have found a new type of light in the night sky and named it Steve.&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Eric Donovan from the University of Calgary in Canada spotted the feature in photos shared on a Facebook group.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;He did not recognise it as a catalogued phenomenon and although the group were calling it a proton arc, he knew proton auroras were not visible.&#xA;  Testing showed it appeared to be a hot stream of fast-flowing gas in the higher reaches of the atmosphere.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;The European Space Agency (ESA) sent electric field instruments to measure it 300km (190 miles) above the surface of the Earth and found the temperature of the air was 3,000C (5,432F) hotter inside the gas stream than outside it&lt;/strong&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Inside, the 25km-wide ribbon of gas was flowing at 6 km/s (13,000mph), 600 times faster than the air on either side.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Relatively little else is known about the big purple light as yet but &lt;strong&gt;it appears it is not &lt;a href=&quot;http://www.bbc.co.uk/science/space/solarsystem/solar_system_highlights/aurora_(astronomy)&quot; rel=&quot;noreferrer&quot;&gt;an aurora&lt;/a&gt; as it does not stem from the interaction of solar particles with the Earth's magnetic field&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm wondering, what is Steve and why wasn't Steve characterize earlier?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But more importantly:&lt;/strong&gt; Could this 6 km/s flow of glowing gas and plasma have affected the ISS when it was orbiting closer to 300 km, or other low-orbiting spacecraft? According to the article, this is not an aurora, or a 'proton arc'. This seems to be something very different, and the complete phenomenon may not be necessarily limited to an altitude of 300 km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not thinking that the ISS would be &quot;blown off course&quot;, but a sudden transient charging event may be difficult to effectively neutralize, just as an example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Ao8U.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Ao8U.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-25T15:11:39.810" LastActivityDate="2018-08-05T04:17:29.743" Title="Could Steve have hurt the ISS?" Tags="|low-earth-orbit|plasma|thermosphere|steve|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21220" PostTypeId="2" ParentId="21211" CreationDate="2017-04-24T02:25:29.940" Score="10" Body="&lt;p&gt;Wood burns just fine with LOX or gaseous oxygen; it would &quot;work&quot; in the sense that it would produce hot exhaust gas which could be accelerated through a convergent-divergent nozzle to produce thrust. Wood is primarily made up of cellulose and other carbohydrates;  &lt;a href=&quot;http://www.eucass-proceedings.eu/articles/eucass/pdf/2012/01/eucass2p353.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;cellulose has been fired with GOX&lt;/a&gt; for a respectable 247s of specific impulse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are at least a couple of ways it's not ideal, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cellular structure of natural wood is going to burn unevenly, so thrust won't be consistent. Like a fireplace log crackling, pieces of unpredictable size are going to split off from the &quot;grain&quot;; it's possible that splinters could get caught in the nozzle of the throat, causing pressure spikes in the &quot;combustion chamber&quot; and so forth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, &lt;a href=&quot;http://www.engineeringtoolbox.com/wood-density-d_40.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;density of dry wood&lt;/a&gt;, depending on species, runs in the ballpark of about 0.5-1.0 g/cc, as compared with pure &lt;a href=&quot;https://en.wikipedia.org/wiki/Cellulose&quot; rel=&quot;nofollow noreferrer&quot;&gt;cellulose at 1.5 g/cc&lt;/a&gt;. So its density-specific impulse is not great. Additionally, even dry wood also contains a certain amount of moisture, which is dead weight not contributing to combustion (though it would contribute reaction mass, of course). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, while a wood hybrid rocket could be built and flown as a demonstration, there are better options available.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-08-12T15:38:11.120" LastActivityDate="2018-08-12T15:38:11.120" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="21221" PostTypeId="1" CreationDate="2017-04-24T03:19:07.327" Score="5" ViewCount="2214" Body="&lt;p&gt;I was looking at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#Lagrangian_spacecraft_and_missions&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia page on Lagrangian points&lt;/a&gt;, and I noticed that in the list of current and proposed missions, the only mention of the $L_4$ and $L_5$ points is a &quot;this would be a good idea&quot; proposal from the '70s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presuming that Wikipedia's list is &lt;em&gt;actually&lt;/em&gt; comprehensive (which it may easily not be), is the lack of proposed missions due to our inability to get a satellite there?  Or is it just that no one has yet come up with a good reason to send one out there?  Or do we just not have a way to get anything to &lt;em&gt;stop&lt;/em&gt; there, so we could only do a fly-by?&lt;/p&gt;&#xA;" OwnerUserId="898" LastActivityDate="2017-09-26T19:45:09.953" Title="Do we have the capability to place a satellite in the Sun-Earth L4/L5 Lagrange points?" Tags="|lagrangian-points|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21222" PostTypeId="2" ParentId="21221" CreationDate="2017-04-24T07:44:14.857" Score="4" Body="&lt;p&gt;If you are looking for something &quot;at&quot; L4 or L5, the news is that these are not really points (since Earth's orbit is not a perfect circle) as much as they are regions, neighborhoods, or states of mind. It's better to thing of things as orbiting around the Lagrange points (any of them) than being &lt;em&gt;at them&lt;/em&gt;. This means that if there is stuff to look at, but you don't know exactly where it is in those orbits, you'd never get very close to it because these neighborhoods are huge!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Getting to the neighborhood would not be a problem. The &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow noreferrer&quot;&gt;two STEREO spacecraft&lt;/a&gt; passed near the neighborhood of Earth's L4 and L5, and slowing down enough to enter an orbit around the L4/L5 libration points would have required a significant additional delta-v but one that would be small compared to that of getting there from Earth in the first place. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As they passed through Earth's Lagrangian points L4 and L5, in late 2009, they searched for Lagrangian (trojan) asteroids. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here's a plot of data I downloaded from &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt;, showing the approximate distance from the Sun (actually the cm of the solar system) of STEREO-A, STEREO-B, and Earth (lower, upper, and middle (blue, green, red) traces respectively) and the lead/lag angle of the two STEREO spacecraft with respect to the earth (actually the differences in &lt;code&gt;theta = arctan2(y, x)&lt;/code&gt; in J2000 ecliptic coordinates). +/- 60 degrees is indicated by dotted lines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3KuyI.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3KuyI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They did use &lt;a href=&quot;https://space.stackexchange.com/a/18719/12102&quot;&gt;several flyby's of the Moon&lt;/a&gt; in a rather cool maneuver to get into this situation (&lt;a href=&quot;https://i.stack.imgur.com/tH0UC.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;GIF from that answer&lt;/a&gt;) - one orbiting the Sun in a retrograde direction. Doing that without the moon would have been quite difficult = expensive in delta-v!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The canonical answers to &quot;why didn't they...&quot; are always about less-than-limitless funding and scientific prioritization. Without a compelling reason to go to Earth's L4/L5 and &lt;em&gt;not do&lt;/em&gt; something else, it won't happen. See @AndrewThompson's &lt;a href=&quot;https://space.stackexchange.com/questions/21221/#comment58929_21221&quot;&gt;comment&lt;/a&gt; for further insight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now here's a nice video of the two Stereo spacecraft passing through the neighborhood of Earths' L4/L5 (not labeled, at +/- 60 degrees from the Earth); enjoy!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/VzhMvEkK0gA?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-24T08:51:55.047" LastActivityDate="2017-04-24T08:51:55.047" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21223" PostTypeId="1" CreationDate="2017-04-24T10:05:22.037" Score="2" ViewCount="921" Body="&lt;p&gt;As part of a final year project at university I'm currently trying to simulate an interplanetary transfer between Earth and Mars for a manned mission. The ultimate aim is to use &lt;a href=&quot;https://gmat.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's GMAT&lt;/a&gt; to simulate the mission and calculate the detailed orbital elements at each point of the mission as well as $\Delta v$ requirements etc...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've narrowed down our departure window to 26 Jun 2035, taken from the Trajectory Browser webpage, also by NASA. The issue I've now come across is that to simulate the orbit within GMAT we need a lot more detail than the Trajectory Browser provides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other than GMAT, is there any software which I can input my current parameters (departure date, parking orbit, transfer time, dV) and get out the hyperbolic elements of that transfer? For simulation in GMAT I need either SMA/RAAN/INC/ECC/AOP/TA or $\Delta v$ in VNB components. If not, is it possible to calculate them by hand? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have asked a similar question on the GMAT forum, but that seems to have died a bit of a death recently. &lt;/p&gt;&#xA;" OwnerUserId="19407" LastEditorUserId="12712" LastEditDate="2017-04-24T15:09:20.400" LastActivityDate="2017-11-28T10:51:12.933" Title="Calculating hyperbolic orbit elements for interplanetary intercepts" Tags="|orbital-mechanics|mars|interplanetary|simulation|astrodynamics|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21224" PostTypeId="1" AcceptedAnswerId="21225" CreationDate="2017-04-24T11:13:47.127" Score="5" ViewCount="573" Body="&lt;p&gt;I've noticed that the average low temperature at Gale Crater is &lt;a href=&quot;https://en.wikipedia.org/wiki/Climate_of_Mars#Temperature&quot; rel=&quot;noreferrer&quot;&gt;-78.5 degrees&lt;/a&gt;, curiously the freezing temperature of Mars' atmosphere's &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbon_dioxide&quot; rel=&quot;noreferrer&quot;&gt;main component&lt;/a&gt;. &lt;strong&gt;Is that more than a coincidence?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps the freezing of carbon dioxide presents an obstacle which stabilizes, at least for some time, the temperature at that value. As energy is removed from the atmosphere, instead of the temperature dropping the CO2 freezes. Does that sound plausible? And if this is the case, are other areas of the solar system similarly constrained to a temperature that produces a phase-change in some other atmospheric gas?&lt;/p&gt;&#xA;" OwnerUserId="472" LastActivityDate="2017-04-24T12:00:24.683" Title="Does freezing carbon dioxide present a &quot;temperature obstacle&quot; on Mars?" Tags="|mars|planetary-science|atmosphere|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21225" PostTypeId="2" ParentId="21224" CreationDate="2017-04-24T12:00:24.683" Score="3" Body="&lt;p&gt;The mechanism is plausible, especially if you are measuring air temperature. However, that specific case is just a coincidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CO2 does freeze at -78.5 on Earth, but the much lower atmospheric presure on Mars moves the freezing point to below -120.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-04-24T12:00:24.683" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21226" PostTypeId="2" ParentId="21213" CreationDate="2017-04-24T12:20:47.820" Score="22" Body="&lt;p&gt;I'll give you an intuitive way to think about it, then a script to play around with if you like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the spacecraft is at periapsis, it is there, at periapsis. A bit of drag isn't going to change its position by much, but it will change its velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The new velocity determines how high it can reach &lt;em&gt;at apoapsis&lt;/em&gt;. If it has slowed down, it will climb to a lower apoapsis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it's in an elliptical orbit, and there's no drag up there, so it's going to return to nearly the same periapsis whence it came.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;caveat:&lt;/strong&gt; That assumes there is no lift. The term &quot;lift&quot; stands for any aerodynamic force not in the direction of the relative velocity of the spacecraft with respect to the atmosphere. If the spacecraft is not spherical or sufficiently tumbling, or if it has wings and is space-shuttle-shaped and Clint Eastwood is in the &quot;pilot's seat&quot;, then all bets are off.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here's a python script for a simple approximate simulation that demonstrates this. It is not intended to be precise, but it's physical enough to show that this is what tends to happen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Eccentric orbits &quot;touching the atmosphere&quot; at periapsis will tend to first circularize before they burn up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this has at least &lt;strong&gt;one important implication;&lt;/strong&gt; it won't necessarily re-enter and burn up near the original periapsis. It can happen anywhere. &lt;em&gt;Round and round it goes; where it stops, nobody knows&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TqIXT.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TqIXT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def acc_drag(X):&#xA;    x, v  = X.reshape(2, -1)&#xA;&#xA;    alt   = np.sqrt((x**2).sum()) - re&#xA;    rho   = rho0 * np.exp(-alt/hscale)&#xA;&#xA;    Fdrag = -0.5 * rho * CD * area * v * np.sqrt((v**2).sum())&#xA;&#xA;    return Fdrag/mass&#xA;&#xA;def deriv(X, t):&#xA;    x, v  = X.reshape(2, -1)&#xA;&#xA;    acc_g = -GMe * x * ((x**2).sum())**-1.5&#xA;    acc_d = acc_drag(X)&#xA;&#xA;    return np.hstack((v, acc_d + acc_g))&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;halfpi, pi, twopi = [f*np.pi for f in [0.5, 1.0, 2.0]]&#xA;&#xA;GMe  = 3.98600418E+14   # m^3 s^-2&#xA;mass = 5000.   # kg&#xA;area = 5.      # m^2&#xA;CD   = 1.0&#xA;hscale    = 7200.  # meters (ROUGHLY scale height fudged for 100km)&#xA;re   = 6378000. # meters (equatorial radius)&#xA;rho0 = 1.225    # kg/m^3&#xA;&#xA;r_peri = re + 90000. # meters&#xA;v_peri = 10000.  # m/s  &#xA;&#xA;X0 = np.hstack(((r_peri, 0, 0), (0, v_peri, 0)))&#xA;&#xA;time = np.arange(0, 1000000, 100) # sec&#xA;&#xA;answer, info = ODEint(deriv, X0, time, full_output=True)&#xA;&#xA;theta = np.linspace(0, twopi, 360)&#xA;xe, ye = [re*f(theta) for f in [np.cos, np.sin]]&#xA;&#xA;alti = np.sqrt((answer[:,:3]**2).sum(axis=1)) - re&#xA;&#xA;if True:&#xA;&#xA;    plt.figure()&#xA;&#xA;    plt.subplot(2, 1, 1)&#xA;&#xA;    x, y = answer.T[:2]&#xA;    plt.plot(x, y)&#xA;    plt.plot([0], [0], 'ok')&#xA;    plt.plot(xe, ye, '-k', linewidth=1.5)&#xA;&#xA;    plt.subplot(2, 1, 2)&#xA;&#xA;    plt.plot(time/(24.*3600), alti/1000.)&#xA;    plt.ylim(1, None)&#xA;    plt.xlim(0, 10)&#xA;    plt.yscale('log')&#xA;    plt.xlabel('days')&#xA;    plt.ylabel('km')&#xA;&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-25T02:05:19.137" LastActivityDate="2017-04-25T02:05:19.137" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21227" PostTypeId="1" CreationDate="2017-04-24T13:22:25.897" Score="1" ViewCount="134" Body="&lt;p&gt;I noticed that &lt;a href=&quot;https://en.wikipedia.org/w/index.php?title=Climate_of_Mars&amp;amp;oldid=776379487#Temperature&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia lists the climate on Gale Crater per “month”&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r0aDR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r0aDR.png&quot; alt=&quot;Climate on Mars, screenshot&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;em&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/w/index.php?title=Climate_of_Mars&amp;amp;oldid=776379487#Temperature&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of the sources listed contain the information included in the table.  I find it particularly puzzling that the year in this table is divided into 12 months which thus should be an average 668.6/12=55.7 sols each.  Proposals for &lt;a href=&quot;https://en.wikipedia.org/wiki/Timekeeping_on_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;timekeeping on Mars&lt;/a&gt; may divide the year into 24 months, but simply counting the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;668.6 sols&lt;/a&gt; in analogy to day-of-year counting is, as far as I know, more common.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does it mean to state that the average high temperature in Gale Crater in February is -18°C?  Does this have any meaning, or is this just an example flawed original research creeping into Wikipedia?&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2017-04-24T13:52:47.433" Title="What is the meaning of twelve months for climate at Gale Crater, Mars?" Tags="|mars|atmosphere|climate|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21228" PostTypeId="2" ParentId="21227" CreationDate="2017-04-24T13:52:47.433" Score="2" Body="&lt;p&gt;The &lt;a href=&quot;http://cab.inta-csic.es/rems/en/weather-report-mars-year-33-month-10/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centro Astrobiologica&lt;/a&gt; site has temperature and other graphs divided into 360 degrees (=one Mars orbit). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KgzNT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KgzNT.png&quot; alt=&quot;Pressure graph&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Solar longitude is commonly referred to as Ls, this is the angular position of Mars relative to the Sun measured in degrees from the summer equinox (start of northern hemisphere Spring). The Ls year is divided into 360 fractions (degrees).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's possible was misinterpreted into 360 days-&gt;one Earth year. So no original research, but flawed collation of data. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-04-24T13:52:47.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21229" PostTypeId="2" ParentId="21211" CreationDate="2017-04-24T16:47:15.210" Score="4" Body="&lt;p&gt;I'd point out that moisture, and in low enough quantities even minerals, may not add combustion energy but do serve as reaction mass. Unfortunately I suppose minerals would tend to produce abrasive ash which would be bad for the throat and nozzle. The low density of wood also makes it a good insulator so one might have low-temperature tolerant material wrapping the wood block for pressure containment; only as the wood layer gets to be very thin would enough heat leak out to soften or otherwise threaten the structure. But the basic problem of quality control remains, with the wood as pointed out tending to crack, creating hot spots on the pressure containment wrap and terminating the use of the wood early, and at an unpredictable moment, not to mention shedding large embers blocking the nozzle as also mentioned. To make the burn more predictable, one would have to break solid blocks of wood into thin layers for re-wrapping as plywood. It seems to me the main advantage of using wood for fuel would be the simplicity of just finding a suitably shaped block of natural wood, milling it to fit in the pressure containment jacket and drilling a central column for the initial burn, an advantage completely discarded if we have to build the wood up as plywood or particle board. If we have to build it up we might as well use paraffin or some such instead. And the insulation factor is a bit of a red herring; hybrid solid fuel elements insulate by ablation, the outer layers are kept cool enough since the inner layer that is burning is evaporating. Indeed, one limit on development of hybrid rockets is that it is not easy to get the solid fuel to evaporate rapidly enough to deliver thrusts comparable to traditional solid propellants that include the oxidant! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Genetically engineered &quot;stage trees&quot; a la Niven of course are not hybrid since the oxidant is mixed into the fuel to make a more traditional solid propellent, that cannot be controlled. Presumably however the stage trees are very sophisticated in the respect of layering their combustion elements for a prescribed burn, and having both very high thrust (to minimize gravity loss during ascent) and very low propellent/&quot;dry&quot; mass fraction, achieved I suppose by a combination of very light yet strong outer skin (something like spiderweb or Kevlar strong--Kevlar also can tolerate fairly high temperatures) and strong but combustable cell structure. I figure the mass ratio of propellent to spent mass, of which the interstellar &quot;seed&quot; would only be a portion, must be well over 1000. We can't do that with human fabricated rockets but I suppose it might be possible for something biologically grown to achieve it. It has to, to achieve total delta-V from the surface of 20 km/sec or more to achieve system escape velocity and thus send its seed at a decent speed between stars. Once launched I suppose it deploys some kind of solar sail to maneuver in close encounters with stars, to either aim for another candidate star or maneuver to collide with a suitable life-bearing planet. Depending on how big this &quot;seed&quot; needs to be to survive hundreds of thousands to millions of years coasting from star to star--I'm guessing a metric ton, give or take an order of magnitude--I'd suppose the tree itself--the actual rocket trunk, not other vegetation with leaves and roots to obtain the necessary energy and nutrition to grow the trunk and seed--would have to mass from a thousand to a hundred thousand tonnes!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And it would have to &quot;know&quot; the delta-V needed to send its seed on from the host planet. Fortunately this information is available from its entry into the host star system, its detection of and approach to the host planet, and refined by observation of planetary atmospheric entry details. This can all be integrated into a simply benchmark number that can be encoded in genetic material and guide its detailed growth strategy, aiming for a certain size to accomplish another interstellar launch. The tree also has to pick a target star, in the plane of its host system's ecliptic, and time its launch to aim for it. Given the sort of optics it would need for interstellar navigation I suppose this is fairly easy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yeah, stage trees could be a thing, though I don't think much of their chances of evolving by sheer chance. Niven's aren't supposed to have randomly evolved though; they are offspring of genetically engineered products meant to provide cheap launch capability to the &quot;Slaver&quot; species aka Thrintun, and their engineers, the Tnuctipun slave/rebel species, may have secretly installed modules predisposing them to interstellar spread and survival, now honed by a billion years of natural selection after the Tnuctipun took down the Slaver empire in a mutually destructive rebellion.&lt;/p&gt;&#xA;" OwnerUserId="16823" LastActivityDate="2017-04-24T16:47:15.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21230" PostTypeId="1" AcceptedAnswerId="21234" CreationDate="2017-04-24T18:39:24.797" Score="8" ViewCount="1476" Body="&lt;p&gt;What value of M should I use to calculate the speed of a satellite orbiting a Lagrange point where there exists no mass?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1235" LastEditDate="2022-08-22T17:48:29.737" LastActivityDate="2022-08-22T17:55:49.517" Title="What determines the orbital speed around a Lagrange point?" Tags="|orbital-mechanics|lagrangian-points|mass|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="21231" PostTypeId="2" ParentId="21230" CreationDate="2017-04-24T19:33:21.170" Score="10" Body="&lt;p&gt;Closed trajectories around the stable Lagrangian points (L4 and L5) are not elliptical and do not follow Kepler's laws, so there's no value of M you can use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same is true for the quasi-periodic Lissajous &quot;orbits&quot; around the unstable points L1, L2 and L3.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-04-24T19:33:21.170" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21234" PostTypeId="2" ParentId="21230" CreationDate="2017-04-25T00:28:09.900" Score="7" Body="&lt;p&gt;This video shows a telescope in a Halo or Lissajous orbit near the Sun-Earth L2 point. By moving slowly and tracing its path in both inertial and rotating (synodic) frames, you can see that the orbit is primarily around the sun, and the &quot;orbit around&quot; the libration point is a construct that comes from doing the math or thinking in a rotating frame:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Turn your audio &lt;em&gt;down&lt;/em&gt; or &lt;em&gt;off&lt;/em&gt; first!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/IyyQqaF4tNY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a simplified way to look at the situation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbits are the paths that bodies follow in response to forces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Forces are the gradients of potentials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a small planet around a big star, the potential is &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\phi(r) \ = \ GM\frac{1}{r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;so the force felt by the planet is &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F(r) \ = \ -GM\frac{1}{r^2}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;pointed towards the star. When you read about Keplerian orbits, you're reading about only this situation, or a slightly modified version where the planet is bigger and you treat the two as orbiting around their center of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In vector notation, you can change the force to &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{F}(\mathbf{r}) \ = \ -GM\frac{\mathbf{r}}{r^3}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you calculate the trajectory of bodies &lt;em&gt;with various starting conditions in this force field&lt;/em&gt;, they will all be Keplerian orbits.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In a three-body problem, and lets stick with the simple &quot;circular restricted three body problem (CR3BP)&quot; where there are two main bodies (a sun and a large planet for example) &lt;em&gt;in circular orbits&lt;/em&gt; around their center of mass, the potential field and resulting force felt by the third body (so small you can treat its mass as zero) is &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\phi(\mathbf{r}) \ = \ GM_1\frac{1}{r_1}+GM_2\frac{1}{r_2} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{F}(\mathbf{r}) \ = \ -GM_1\frac{\mathbf{r_1}}{r_1^3}-GM_2\frac{\mathbf{r_2}}{r_2^3}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mathbf{r_1}$ and $\mathbf{r_2}$ are the vectors which point from each of the two main bodies to the third body. Now the shape of the potential is constantly changing. Since the problem is kept simple by requiring the two bodies to be in &lt;em&gt;circular orbits&lt;/em&gt; around each other, the potential field is just rotating. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that the force field is also rotating, and the orbit of the third body will be determined by its initial velocity and the acceleration due to that rotating force field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The resulting orbit can be all kinds of crazy things, but it won't be a nice Keplerian orbit, and so none of that math applies.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To &lt;em&gt;mathematically solve&lt;/em&gt; for orbits in this problem, mathematicians and orbital mechanics often switch to doing their math in a rotating frame, one that rotates with the two main bodies. In this frame they create a &lt;em&gt;pseudopotential&lt;/em&gt; with a term called &quot;centrifugal potential&quot; which is really just a way to pretend the frame is not rotating. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, in the rotating frame, along with the real gravitational potential from the two bodies (above) which is not affected by the rotation, and who's gradient produces the force field as shown above, an effective potential term is added such that its gradient produces a fictitious centrifugal force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Resulting orbits can be all kinds of crazy shapes, but since they are not solutions of a simple 1/r potential field, they won't be Keplerian, and so you can't try to represent them with a central mass M.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-02T10:05:03.867" LastActivityDate="2017-05-02T10:05:03.867" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21235" PostTypeId="2" ParentId="21221" CreationDate="2017-04-25T04:35:52.837" Score="-2" Body="&lt;p&gt;At least some discussions of Lagrange points mention that the ratio of primary to secondary body mass must be in a certain range for the points to exist. Now clearly L-1, -2, and -3 all have meaningful existence no matter what the ratio of the small to larger body is, although if it is very small L-3 would be a very subtle effect and scarcely worth bothering with, while -1 and -2 would be very close to the smaller body and weak. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if all planets no matter how small technically have L-4 and -5 points where significant opportunities for less unstable cycling around the points exist than any other random swathe of the orbit, surely the effect is much smaller and more subtle for a planet as small relative to the Sun as Earth is, versus the readily observable capture of significant concentrations of asteroids for Jupiter and Saturn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And aside from the possible astronomical uses, mentioned by Andrew Thompson above, about the only other point of putting something there I can see is as a radio relay for probes or crewed spacecraft going around to the far side of the Sun. Even if a craft happens to go there, it won't be directly behind the Sun for long on most missions. To be sure, the sheer brightness of the Sun might blind or otherwise saturate various signal detectors even with the Sun far off the central axis of a beam, so if Solar noise is powerful enough to drown out clear communications even 20 or 30 degrees away, that's enough to cut Earth off a large percentage of the time, and might justify the effort and extra time lag involved in sending signals roundabout. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway I'm not sure that the -4 and -5 points practically exist. What I mean by that is, if you park a spacecraft at any other random point on the Earth's orbit, we'd expect it to drift due to the fact that initially it is not moving relative to Earth-therefore Earth's pull, weak as it would be across distances of an AU or more, always tugs in the same direction, so the object must accelerate and cease to maintain position. The special thing about Lagrange -4 and -5 points is, due to the combination of potential (in the rotating frame in which primary--here the Sun--and secondary object--here Earth--are holding still and a centrifugal potential exists) being not low, but -highest- along the arc of Earth's orbit, and the Coriolis force (which is mathematically speaking, a &quot;real&quot; force in the rotating frame, just as centrifugal force away from the barycenter is) then &quot;orbits&quot; around the point, which are held to a cyclic path by the Coriolis force, exist, and their characteristic speeds are such that the tendency to fall toward the Earth is offset by the local dynamic--which includes the gravitational slope of Earth's gravitational field at that range in the unique potential form peaking at the point. If the &quot;hill and ridge&quot; of the L -4 and -5 points are not pronounced enough, perhaps these stable closed loops are not possible and the object drifts toward Earth no matter what combination of vector and distance from the L point we give it--perhaps indeed outside of certain ranges of proportion, no stable loops exist. If they don't then the theoretical possibility of putting something there and it remaining the same average distance from Earth forever without maneuvering thrust does not exist and any advantage that comes with going so far away is that perhaps the inevitable drive out of position might be slower than elsewhere, allowing more economical station keeping. &lt;/p&gt;&#xA;" OwnerUserId="16823" LastActivityDate="2017-04-25T04:35:52.837" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21236" PostTypeId="2" ParentId="21217" CreationDate="2017-04-25T05:25:24.220" Score="4" Body="&lt;p&gt;Immediately after a TLI burn, the craft is not moving much radially; an ideal impulse would be tangential to the circular parking orbit. 400 m/sec in the opposite direction to the TLI burn would not return the craft fast, but it would surely cut the apogee of the orbit to a lower altitude than the nominal transfer orbit and return it to low Earth orbit altitudes sooner than that orbit would have.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a catch though. Apollo missions did not use Hohmann orbits, which have an apogee just tangential to Luna's orbit, for two reasons. They used more energetic orbits with apogees far beyond the Moon's orbit, partially because this would speed up the transfer to Lunar distance--the bigger orbit would have a much longer period than the Hohmann orbit would but the Apollo would travel only a fraction of the half-orbit before encountering the Moon. The other and even more important reason was to allow a &quot;free return orbit,&quot; or after Apollo 12 anyway, one near one. For complicated reasons boiling down to the fact that the Moon's gravity, close up to it, changes the orbits, a Hohmann orbit is not free-return. The more energetic Apollo orbits were chosen because on an unpowered close encounter, the effect of the Moon would be to throw it back toward Earth on a similar energetic orbit to the one it approached on, coming back to the same perigee--or actually a bit lower, so it would reenter the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If SpaceX is not going to provide a souped-up super-Dragon with lots of extra propellant for heavy delta-V near the Moon on the nominal mission, but plans a simple flyby, they need to send the Dragon on an extra-energetic transfer orbit similar to those used by Apollo. Thus the TLI speed near Earth is slightly higher than for a Hohmann orbit. If they were planning to use a Hohmann orbit it must be that the Dragon does need have thousands of meters/sec delta V available. I conclude they are doing a free-return flyby. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case, the apogee of the planned transfer orbit is far beyond the Moon's orbit, and while any slowing of the speed immediately after TLI will lower that apogee, it has to be lowered a lot for the return to be sooner than the planned loop around the moon. Otherwise they fall between stools, missing the Moon because the timing is off, flying far out into translunar space--not as far as they would without the braking burn, but then they'd have encountered the Moon as planned. And since the Moon interrupts their outward orbit halfway and sends them back down going fast, they'd do better to just let it ride, unless they had so much delta-V they could lose a lot of speed and get to an orbit with apogee far closer to Earth than the Moon's orbit. &lt;/p&gt;&#xA;" OwnerUserId="16823" LastActivityDate="2017-04-25T05:25:24.220" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21237" PostTypeId="1" CreationDate="2017-04-25T06:43:48.960" Score="7" ViewCount="1246" Body="&lt;p&gt;What type of ground characteristics make it suitable for a satellite ground station?&lt;/p&gt;&#xA;" OwnerUserId="19420" LastActivityDate="2017-04-25T07:46:01.943" Title="Where are earth stations typically located?" Tags="|ground-station|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21238" PostTypeId="2" ParentId="21237" CreationDate="2017-04-25T07:46:01.943" Score="11" Body="&lt;ul&gt;&#xA;&lt;li&gt;Open area, broad horizon, meaning either hilltops or plains; not valleys.&lt;/li&gt;&#xA;&lt;li&gt;good power infrastructure available. Network and other infrastructure a boon.&lt;/li&gt;&#xA;&lt;li&gt;reasonably stable seismically. Also, due to location, landslide risk must be taken into account.&lt;/li&gt;&#xA;&lt;li&gt;reasonable access and servicing logistics, staff availability.&lt;/li&gt;&#xA;&lt;li&gt;distant enough from other ground stations not to be redundant.&lt;/li&gt;&#xA;&lt;li&gt;legal prerequisites fulfilled. Landscape/wildlife protection, rf radiation protection buffer zone, etc.&lt;/li&gt;&#xA;&lt;li&gt;politically available and stable. So that you don't find after next local government change the station now is state property of Republic of Banana or shut down because it's abominable in the eyes of God.&lt;/li&gt;&#xA;&lt;li&gt;calm neighborhood RF-wise. These locations are lucrative not just for space network ground stations, but all kinds of radio broadcasters. A 500KW transmitter of a pop music station is not your desired next-door neighbor.&lt;/li&gt;&#xA;&lt;li&gt;and then there will be small weird environmental factors that may make it harder or impossible... religious significance of the place, a species of lizard finding the dish too welcoming, hurricanes of destructive power prevalent, legal obligation to connect to local sewage network, with nearest pipe 10km away, &quot;at least 80% of staff must be local&quot;, and other weirdnesses you just can't predict - you must survey a prospective area very thoroughly for these.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-04-25T07:46:01.943" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21241" PostTypeId="1" CreationDate="2017-04-25T12:34:53.437" Score="1" ViewCount="196" Body="&lt;p&gt;The Falcon 9 uses a helium system to pressurize the tanks. Does the Heavy first stage also use helium? How big would the explosion be in case of a failure assuming the boosters would go too? &lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="6944" LastEditDate="2021-03-28T18:15:06.577" LastActivityDate="2021-03-28T18:15:06.577" Title="Does the Falcon Heavy also use a helium system?" Tags="|spacex|falcon-heavy|fuel-system|helium|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21242" PostTypeId="2" ParentId="21241" CreationDate="2017-04-25T13:11:25.360" Score="2" Body="&lt;p&gt;The Helium per se does not explode. What happened in the AMOS-6 incident is that when fueling the upper stage which has Helium COPV (Carbon Overwrap Pressurized Vessels) that are submerged in the subcooled LOX. The Helium as it was being pumped in, faster than usual, cooled even further and they think caused oxygen to solidify and work its way into the carbon fiber mix and finally burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then the COPV burst and since it is very pressurized (many thousands of atmospheres of pressure) it blew out the tanks and now you have tens of thousands of pounds of LOX and RP1 mixing in midair, where it is kind of warm which is a recipe for burning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If all three boosters were to burst and burn it would be pretty ugly. Usually a fuel explosion is limited by the amount of oxygen to burn around it. This however brings its own oxygen supply to the party which would burn quite heavily and quickly. Not being contained it is likely more of a conflagartion than an explosion, but at this scale that is just semantics. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-04-25T13:11:25.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21243" PostTypeId="2" ParentId="21241" CreationDate="2017-04-25T13:12:31.640" Score="2" Body="&lt;p&gt;The Falcon Heavy cores are essentially 3 Falcon 9 first stages, with only a few minor differences to distinguish them. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If an explosion somehow resulted from any circumstances, all 3 cores would explode, likely causing an explosion about 3x from the AMOS-6 incident.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-04-25T13:12:31.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21244" PostTypeId="1" AcceptedAnswerId="21456" CreationDate="2017-04-25T15:11:11.070" Score="5" ViewCount="587" Body="&lt;p&gt;How did ESA measure Steve's temperature using the SWARM spacecraft? &lt;em&gt;Please explain how&lt;/em&gt; the measurement is really done, don't just mention an instrument's acronym.&lt;/p&gt;&#xA;&lt;p&gt;I did a quick dump of TLEs for Satcats &lt;code&gt;39451&lt;/code&gt;, &lt;code&gt;39452&lt;/code&gt; and &lt;code&gt;39453&lt;/code&gt;, and estimated their semimajor axes and then altitudes and got the plot below, to double check that SWARM hasn't dipped down to 300km.&lt;/p&gt;&#xA;&lt;p&gt;All of the reports mention that Steve's ion temperature is 3000C.&lt;/p&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.esa.int/Our_Activities/Observing_the_Earth/Swarm/When_Swarm_met_Steve&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA News Release&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Prof. Donovan said, “&lt;strong&gt;As the satellite flew straight though Steve&lt;/strong&gt;, data from the electric field instrument showed very clear changes.&lt;/p&gt;&#xA;&lt;p&gt;“The temperature &lt;strong&gt;300 km above Earth’s surface&lt;/strong&gt; jumped by 3000°C and the data revealed a 25 km-wide ribbon of gas flowing westwards at about 6 km/s compared to a speed of about 10 m/s either side of the ribbon. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;How did one or more of the three SWARM spacecraft above 440km measure the plasma temperature of Steve?&lt;/p&gt;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/q/21219/12102&quot;&gt;more about Steve in this question&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Seven things to know about Steve: &lt;a href=&quot;http://blog.aurorasaurus.org/?p=449&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://blog.aurorasaurus.org/?p=449&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;We are are still learning more about Steve, but here are seven things we think we know so far:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Steve appears ~10-20° (in latitude) closer to the equator (south in the Northern hemisphere) than where the normal green aurora is overhead. This means it could be overhead at latitudes similar to Calgary, Canada.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Steve is a very narrow arc aligned East-West and extending for hundreds or thousands of miles&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Steve emits light in mostly purple-ish colors. It is quite faint but is usually photographed with 5-10 second exposures.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Sometimes, it is accompanied by a rapidly evolving green short-lived picket fence structure.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Steve can last 20 min or even longer.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Steve appears to have a season. For instance, it has not been observed by citizen scientists from October 2016 to February 2017.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;This phenomena has been reported from the UK, Canada, Alaska, northern US states, and even New Zealand.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://gizmodo.com/what-the-hell-is-this-beautiful-thing-1794528895&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://gizmodo.com/what-the-hell-is-this-beautiful-thing-1794528895&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;more? (they used to call it a proton arc for lack of a better name. It isn't one!)  &lt;a href=&quot;http://spaceweathergallery.com/index.php?title=proton&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spaceweathergallery.com/index.php?title=proton&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Opening of SWARM and CryoSat Science Meeting - &lt;strong&gt;Extremely Interesting, Eric Donovan talking&lt;/strong&gt; about how Steve was &amp;quot;discovered by scientists&amp;quot; by listening to amateur astronomers and aurora photographers who had been watching it for a while! Start to talk about Steve after &lt;code&gt;01:16:00&lt;/code&gt; &lt;strong&gt;&lt;a href=&quot;https://livestream.com/ESA/earthexplorer2017/videos/152430872&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://livestream.com/ESA/earthexplorer2017/videos/152430872&lt;/a&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Thanks to @Roger's &lt;a href=&quot;https://space.stackexchange.com/a/21430/12102&quot;&gt;information here&lt;/a&gt; here is an &lt;a href=&quot;http://www.esa.int/spaceinvideos/Videos/2017/03/Opening_of_Swarm_and_CryoSat_Science_Meetings&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;ESA link for the video of the talk&lt;/strong&gt;&lt;/a&gt;. (same time code)&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Also, here is some amazing footage!&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/tH7bVlGNjWU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Mt8WT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mt8WT.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V8LzH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V8LzH.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-06-01T00:17:10.377" Title="If Steve is at 300 km, and SWARM is &gt;440km, how did SWARM measure Steve's temperature?" Tags="|low-earth-orbit|plasma|thermosphere|steve|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21245" PostTypeId="1" AcceptedAnswerId="21248" CreationDate="2017-04-25T15:19:54.100" Score="3" ViewCount="390" Body="&lt;p&gt;If you were looking for, say, a 1m per pixel quality, what focal length/sensor size would you need? Would there be any off the shelf/commercial solutions?&lt;/p&gt;&#xA;" OwnerUserId="19424" LastActivityDate="2017-04-25T21:45:27.397" Title="Could a cubesat in a 500km SSO orbit effectively monitor earth?" Tags="|artificial-satellite|low-earth-orbit|cubesat|imaging|sun-synchronous|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21246" PostTypeId="1" AcceptedAnswerId="21252" CreationDate="2017-04-25T16:34:57.080" Score="9" ViewCount="9748" Body="&lt;p&gt;The Space Shuttle was designed in the seventies and did not require a helium pressurization system. Why does the Falcon 9 require one? What are the advantages/disadvantages?&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="6944" LastEditDate="2021-03-28T18:16:31.540" LastActivityDate="2021-03-28T18:16:31.540" Title="Why does the Falcon 9 require a helium pressurization system?" Tags="|spacex|falcon-9|space-shuttle|helium|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21247" PostTypeId="2" ParentId="21246" CreationDate="2017-04-25T16:39:14.603" Score="4" Body="&lt;p&gt;The Merlin engines use Helium to purge before launch. At least in the Merlin 1C days, and you can see several aborts that were called because of Helium issues. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the upper stage it is used to pressurize the tanks to make sure that the fuel and LOX flow into the engines properly. The pumps suck and the Helium pressurization push. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Helium is used since it is inert and compresses well. So it does not burn with the fuel or LOX (which would be very bad) and so it can be stored in smaller, denser packages.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-04-25T16:39:14.603" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21248" PostTypeId="2" ParentId="21245" CreationDate="2017-04-25T19:29:12.383" Score="5" Body="&lt;p&gt;The &lt;a href=&quot;http://www.planet.com/&quot; rel=&quot;noreferrer&quot;&gt;Planet Labs&lt;/a&gt; &lt;a href=&quot;https://www.planet.com/pulse/planet-launches-satellite-constellation-to-image-the-whole-planet-daily/&quot; rel=&quot;noreferrer&quot;&gt;Flock 3p&lt;/a&gt; constellation is in LEO at 500 km and doing an effective job of imaging all the land area of the Earth daily, albeit at around 5 meter resolution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best angular resolution achievable for a given telescope diameter is determined by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Diffraction-limited_system&quot; rel=&quot;noreferrer&quot;&gt;diffraction limit&lt;/a&gt;.  For visible light at 500 km distance this works out to a minimum aperture diameter of about 40 cm to achieve 1 meter ground resolution.  That's assuming a perfect optical system, so you might need a somewhat larger lens or mirror in reality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atmospheric seeing conditions, solar illumination angle, attitude stability are a few more of the many considerations that go into the overall system design.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2017-04-25T21:45:27.397" LastActivityDate="2017-04-25T21:45:27.397" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21249" PostTypeId="1" CreationDate="2017-04-25T20:19:04.677" Score="3" ViewCount="3199" Body="&lt;p&gt;I am reading sci-history about first steps into space. Before landing cosmonauts had to choose at which &lt;em&gt;loop around Earth&lt;/em&gt; they would start landing procedures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does spacecraft have angular velocity around Earth if it's launched vertically? Why doesn't it follow geostationary orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update.&lt;/strong&gt; To make my question more precise. What is the goal of adding angular velocity to the ship? Does it make a flight more interesting for a cosmonaut? Does it help landing?&lt;/p&gt;&#xA;" OwnerUserId="3184" LastEditorUserId="758" LastEditDate="2017-04-25T22:12:23.907" LastActivityDate="2017-04-26T18:26:46.780" Title="Why does a spacecraft have angular velocity relative to Earth?" Tags="|launch|orbit|spacecraft|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21250" PostTypeId="2" ParentId="21249" CreationDate="2017-04-25T20:37:36.093" Score="25" Body="&lt;p&gt;Launching a rocket into space is really &lt;strong&gt;all&lt;/strong&gt; about getting horizontal velocity: If you go fast enough sideways, you miss the Earth when you fall towards it and stay in a perpetual free fall. That is an orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are correct that a spacecraft is initially launched in a vertical position, but it later turns to gain velocity sideways. And a lot is needed as you have to go at almost 8km/s sideways to not fall back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what a launch trajectory typically looks like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bEw37.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bEw37m.png&quot; alt=&quot;Launch trajectory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for initially launching vertically is to first escape the atmosphere as having a lot of speed inside of it would cause an enormous amount of drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the goal of a launch:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Get high enough to escape the atmosphere&lt;/li&gt;&#xA;&lt;li&gt;Go fast enough sideways to not fall back to Earth.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;A geostationary orbit is not really that different from other orbits, spacecraft are still going sideways to avoid falling back. The angular velocity at that altitude just happens to be the same as the speed the Earth itself rotates, so it just &lt;em&gt;looks&lt;/em&gt; like it is stationary.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-04-25T20:37:36.093" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21251" PostTypeId="2" ParentId="21246" CreationDate="2017-04-25T22:03:39.290" Score="1" Body="&lt;p&gt;The SSMEs were fed by four turbopumps; the fuel and oxidizer each passed through a low-pressure, then high-pressure turbopump. This allowed them to feed from a low-pressure fuel tank into a high-pressure (20.6 MPa) combustion chamber without cavitating. &#xA;For the Falcon series, SpaceX made a design decision to use a single turbopump stage for the fuel and oxidizer. Pumping up to the 6.8 MPa chamber pressure in a single stage without cavitating is more than the pump can handle; incoming fuel and oxidizer must have a minimum pressure greater than that provided by propellant weight and acceleration. I speculate that they could dispense with the helium pressurization by adding a low-pressure turbopump stage similar to the SSME design, but weight and cost considerations favored helium pressurization.&#xA;Also, pressurizing tanks adds mechanical strength by preventing buckling. The Shuttle tanks were fat and therefore less prone to this failure (buckling load is a function of slenderness ratio). The slender Falcon design may require a minimum internal pressure to survive Max-Q.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The advantage to helium in general is that it's literally a perfect gas, which makes it the perfect gas for pressurization. You can fit more helium into a tank of given size and weight (assuming it's built to take the pressure) than any other gas. Helium has been the preferred gas for pressurization since the '60's. The weight, simplicity, and reliability of high pressure helium bottles has so far won out over other proposed pressurization systems such as heating some of your fuel.&#xA;Note that a Delta-IV launch failed because of cavitation in the turbopump inlet. When a rocket first launches it is full of fuel and the pressure at the bottom of the tank is very high; when it is nearing burnout then that fuel stack is only a few feet high so the pressure entering the turbopump is much lower.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastEditorUserId="25" LastEditDate="2017-12-19T13:35:55.177" LastActivityDate="2017-12-19T13:35:55.177" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21252" PostTypeId="2" ParentId="21246" CreationDate="2017-04-26T01:41:24.373" Score="17" Body="&lt;p&gt;The shuttle propellant tanks required pressurization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Prelaunch, the tanks were pressurized by helium, supplied from the ground support equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After Space Shuttle Main Engine ignition, the system used &lt;a href=&quot;https://arc.aiaa.org/doi/abs/10.2514/6.1968-626&quot; rel=&quot;noreferrer&quot;&gt;autogenous pressurization&lt;/a&gt;. Hot gaseous hydrogen and oxygen were tapped from the engines and piped to their respective propellant tank to pressurize it. A &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/stsref-toc.html#sts-mps-et&quot; rel=&quot;noreferrer&quot;&gt;sensor/valve system&lt;/a&gt; measured the tank ullage pressure and increased or decreased the pressurant flow as required. (Late in the program, the gaseous oxygen system was redesigned to remove the valves).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WAelE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WAelE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Helium pressurization may be simpler and cheaper (no sensor/valve control loop needed, just a regulator) and therefore more desirable for Falcon. Some failure modes are avoided. In addition, hot gaseous oxygen is fearsomely reactive and requires extreme cleanliness and material standards (this was the reason for the redesign of the system) and may be desirable to avoid. Helium is the very antithesis of reactive - although it requires extreme care to be taken to prevent leakage. The negatives of helium pressurization could include a weight penalty over an autogenous system, depending on the details of the particular systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also see these related questions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/20648/how-does-tank-pressurization-work&quot;&gt;How does tank pressurization work?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/20245/why-use-helium/20247&quot;&gt;Why Use Helium?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, although I realize it is not what you meant, the shuttle very much did have a helium pressurization system.  Many of them, in fact. The main propulsion system had several dedicated &lt;a href=&quot;http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-mps.html#sts-mps-helium&quot; rel=&quot;noreferrer&quot;&gt;helium tanks&lt;/a&gt; used to purge the system, actuate valves, and pressurize the plumbing for entry.  The orbital maneuvering and attitude control systems used hypergolic propellants which were pressurized with helium.  Other miscellanous systems also used helium pressurization such as the ammonia boilers in the environmental control system.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-04-26T02:02:17.827" LastActivityDate="2017-04-26T02:02:17.827" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21253" PostTypeId="1" AcceptedAnswerId="21260" CreationDate="2017-04-26T06:08:09.627" Score="0" ViewCount="211" Body="&lt;p&gt;What can we do to improve the data rate in outer space? I see that there are some noise sources that affect the signal quality between ground stations &amp;amp; deep space.&lt;/p&gt;&#xA;&lt;p&gt;As I've read so far in various NASA and JPL pages, deep space data rate seem to be in the 500 to 32k bps range, or sometimes even lower, which will not be enough to transmit/receive critical information in the future as spacecraft carry more sophisticated instrumentation and optical and radar imaging as well as larger data buffers.&lt;/p&gt;&#xA;&lt;p&gt;What are possible options for increasing deep space data rates in the future, in the presence of various sources of noise?&lt;/p&gt;&#xA;" OwnerUserId="19002" LastEditorUserId="1235" LastEditDate="2021-02-23T03:42:03.757" LastActivityDate="2021-02-23T03:42:03.757" Title="How can the data rate be increased in noisy outer space?" Tags="|nasa|deep-space|communication-satellite|deep-space-network|jpl-horizons|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="21254" PostTypeId="2" ParentId="21249" CreationDate="2017-04-26T06:18:40.467" Score="6" Body="&lt;p&gt;If a rocket were launched only vertically, it would fall back as soon as its fuel runs out (which happens pretty soon, even the largest rocket we ever used had its main engines on for less than 20 minutes, and more than 90% of a rocket's weight is fuel). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We give them angular velocity so that they can stay up much longer, orbiting the Earth just as the Moon does, without requiring constant thrust. &lt;/p&gt;&#xA;" OwnerUserId="273" LastEditorUserId="-1" LastEditDate="2017-04-26T18:26:46.780" LastActivityDate="2017-04-26T18:26:46.780" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21256" PostTypeId="1" AcceptedAnswerId="21280" CreationDate="2017-04-26T07:41:19.533" Score="7" ViewCount="1556" Body="&lt;p&gt;How do we track the exact location of the spacecraft which is millions or billions of miles away from us? What things do we study for tracking purpose? How exactly do we predict? &lt;/p&gt;&#xA;" OwnerUserId="19002" LastActivityDate="2017-04-28T00:16:48.243" Title="How do we track the exact location of the spacecraft which is millions or billions of miles away from us?" Tags="|nasa|deep-space|communication-satellite|deep-space-network|jpl-horizons|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21257" PostTypeId="1" AcceptedAnswerId="21259" CreationDate="2017-04-26T08:14:26.133" Score="0" ViewCount="615" Body="&lt;p&gt;Let's call &quot;radio&quot; anything electromagnetic below 300 GHz (1 millimeter), unless there is a compelling reason to do otherwise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/14317/12102&quot;&gt;How well can Voyager 1 separate Earth signals from Solar noise these days?&lt;/a&gt; gives some perspective, but there must be several other sources of radio noise and interference that limit either data rates or availability of data links.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A short explanation  describing under what conditions each major source becomes important would be great!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-12-21T05:34:23.407" LastActivityDate="2019-12-21T05:34:23.407" Title="What are the main sources of radio noise &amp; interference limiting deep space data rates and availability?" Tags="|communication|deep-space|radio-communication|deep-space-network|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="21258" PostTypeId="1" AcceptedAnswerId="28220" CreationDate="2017-04-26T09:58:46.023" Score="10" ViewCount="1289" Body="&lt;p&gt;The initial conditions of the thought experiment is (very) LEO / reentry. capsules, space shuttle and other spacecraft can generate lift in upper atmosphere during reentry, in order to reduce deceleration maximum G force loading and maximum heating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Since capsules and space shuttle are not the best lift producing devices ever made, what would happen if we try to deorbit a hypothetical heat resistant/unbreakable high lift-to-weight ratio (let's say) Nimbus4 glider?&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Subsidiaries: At which (maximum) altitude &amp;amp; speed would the variometer tell 0 m/s vertical speed? What would be max temperature reached? Have there ever been real tests of deorbiting high finesse / low wing loaded gliding devices? What would angle of attack be during the whole descent, until subsonic speed? How does Coanda effect work at hypersonic speeds?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Illustrations found here: &lt;a href=&quot;https://www.quora.com/In-regards-to-atmospheric-reentry-what-exactly-is-a-ballistic-reentry&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.quora.com/In-regards-to-atmospheric-reentry-what-exactly-is-a-ballistic-reentry&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BhWNR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BhWNR.png&quot; alt=&quot;capsule lift&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NmuKn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NmuKn.png&quot; alt=&quot;ballistic vs atmospheric&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sXTMp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sXTMp.png&quot; alt=&quot;heating&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nimbus 4, google images&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/W0HZN.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/W0HZN.jpg&quot; alt=&quot;Nimbus 4 glider&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="18879" LastEditDate="2018-12-25T19:37:35.660" LastActivityDate="2022-11-25T16:31:00.867" Title="Challenging the Kármán line from above" Tags="|reentry|space-shuttle|atmosphere|acceleration|lift|" AnswerCount="4" CommentCount="19" ContentLicense="CC BY-SA 4.0" />
  <row Id="21259" PostTypeId="2" ParentId="21257" CreationDate="2017-04-26T11:46:17.230" Score="5" Body="&lt;p&gt;As mentioned in the comments, the Cosmic Microwave Background creates noise. More specifically, cosmic background creates blackbody radiation, which adds kTB noise to any receiver looking at it. It's called kTB noise, or thermal noise, because its intensity is the product of the Boltzmann constant k, the temperature of the blackbody T (around 4K for the cosmic background) and the Bandwidth B of the receiver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good explanation on kTB noise and receiver sensitivity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.highfrequencyelectronics.com/index.php?option=com_content&amp;amp;view=article&amp;amp;id=553:receiver-sensitivity-and-equivalent-noise-bandwidth&amp;amp;catid=94:2014-06-june-articles&amp;amp;Itemid=189&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.highfrequencyelectronics.com/index.php?option=com_content&amp;amp;view=article&amp;amp;id=553:receiver-sensitivity-and-equivalent-noise-bandwidth&amp;amp;catid=94:2014-06-june-articles&amp;amp;Itemid=189&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;This (&lt;a href=&quot;https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_ed.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_ed.pdf&lt;/a&gt;) NASA pdf on Voyager telecommunications gives the following sources of noise for the downlink (Voyager to Earth) (from page 26):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Noise power of each source:             Design value/Favorable Tolerance/Adverse Tolerance        &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Total Noise Spectral Density, dBm/Hz: –185.35/–0.97/0.80&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Total System Noise Temperature, K: 21.12/–4.24/4.24&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Receiver Temperature, K: 13.20/–3.00/3.00&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ground Contribution, K: 2.88/–3.00/3.00&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Galactic Contribution, K: 2.68/0.00/0.00&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atmospheric Contribution, K: 2.36/0.00/0.00&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hot Body Noise, K: 0.00/0.00/0.00&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elev Angle = 58.01 deg&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the major source of noise is the receiver, followed by the blackbody radiation from the ground, from the Milky way and from the atmosphere. The CMB seems to be negligible compared to that.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="4685" LastEditDate="2017-04-29T16:47:21.260" LastActivityDate="2017-04-29T16:47:21.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21260" PostTypeId="2" ParentId="21253" CreationDate="2017-04-26T11:57:51.070" Score="6" Body="&lt;p&gt;Yes we have solutions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Higher-power transmitter&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;More directional emitting antenna (by increasing the frequency for instance)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Larger receiving antenna&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Less noisy electronics&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Electronics with less signal loss&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;More efficient modulation (which may require playing on the bandwidth)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I suggest you look into  what is called the link budget equation to find more about the topic:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.radio-electronics.com/info/propagation/path-loss/link-budget-calculation-formula-equation.php&quot; rel=&quot;noreferrer&quot;&gt;http://www.radio-electronics.com/info/propagation/path-loss/link-budget-calculation-formula-equation.php&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Link_budget&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Link_budget&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The link budget tells you what to improve for analog communications, for ditigal communications you need to combine it to a Eb/N0 calculation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Eb/N0&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Eb/N0&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="4685" LastActivityDate="2017-04-26T11:57:51.070" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21261" PostTypeId="1" AcceptedAnswerId="21279" CreationDate="2017-04-26T13:16:22.510" Score="9" ViewCount="418" Body="&lt;p&gt;Headlines suggest (as do their articles) that as a gravitational result of Cassini's final flyby of Titan, it's fate is sealed and it's orbit is destined to take it into Saturn's atmosphere on September 15, 2017, with 22 complete orbits happening first.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Phys.org: &lt;a href=&quot;https://phys.org/news/2017-04-cassini-finaland-fatefultitan-flyby.html&quot; rel=&quot;noreferrer&quot;&gt;Cassini completes final—and fateful—Titan flyby&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Space.com: &lt;a href=&quot;http://www.space.com/36608-cassini-last-titan-flyby-complete.html&quot; rel=&quot;noreferrer&quot;&gt;No Turning Back: Titan Flyby Assures Cassini's Crash Into Saturn&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What phenomenon is causing this clockwork-like certainty? At Earth, passive drag-induced de-orbit and re-entry is hard to predict in part because the upper atmosphere is constantly changing due to the influence of the sun and solar wind. Saturn's atmospheric scale height &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html&quot; rel=&quot;noreferrer&quot;&gt;is almost 60km&lt;/a&gt; which means that the atmospheric pressure increases much more gradually with decreasing altitude than that it would in Earth's atmosphere, but I don't know if that has anything to do with it or not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it repeated atmospheric drag during each periapsis that will result in the systematic orbit-lowering that will bring Cassini into Saturn's atmosphere on the 23rd periapsis after the Titan flyby, or is it some other force that is causing this to happen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a plot of data extracted from the &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi#top&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; database. Altitude above Titan (thin green line) and Saturn (equatorial radius assumed) (thick blue line) as a function of time relative to an arbitrary &lt;code&gt;2017-sep-15 00:00:00 utc&lt;/code&gt; (&lt;code&gt;JD=2458011.5&lt;/code&gt;) the day of the entry into Saturn's atmosphere. Data were downloaded and plotted at 10 minute intervals. The Titan swingby is indicated by the thin green line reaching down to 1000 km on the left, the planned incineration is the thick blue line approaching 30 km near 2017-sep-15. It looks like the simulation was run longer just to see what would happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NcSd4.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NcSd4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7Ns3h.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7Ns3h.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="8693" LastEditDate="2017-04-26T14:33:26.563" LastActivityDate="2017-04-27T22:31:25.100" Title="What force is bringing Cassini down into Saturn's atmosphere in another 145 days; drag, or...?" Tags="|orbital-mechanics|cassini|drag|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21262" PostTypeId="2" ParentId="21256" CreationDate="2017-04-26T14:11:24.313" Score="4" Body="&lt;p&gt;While &lt;a href=&quot;https://timeandnavigation.si.edu/navigating-space/deep-space/meet-the-navigators&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site&lt;/a&gt; does not give a very detailed answer, it gives a relatively good idea on how this is achieved:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The JPL has five groups, that handle the navigation together.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Ephemerides Group&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculates the positions of astronomical objects at predicted times.&lt;br/&gt;&lt;br/&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Orbit Determination Analysts&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Study radio transmissions and images from cameras to determine the spacecraft’s current location.&lt;br/&gt;&lt;br/&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Maneuver Designers&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plan propulsive maneuvers to keep the spacecraft on the correct flight path.&lt;br/&gt;&lt;br/&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Radiometric Tracking Team&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assesses techniques for acquiring and improving the accuracy of radio-based measurements of the spacecraft’s velocity, location, and angle relative to Earth.&lt;br/&gt;&lt;br/&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Trajectory Designers&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plot the most efficient path for the spacecraft.&lt;br/&gt;&lt;br/&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So it really is a team effort involving large groups of experts employing several different methods to locate a satellite.&lt;/p&gt;&#xA;" OwnerUserId="12712" LastActivityDate="2017-04-26T14:11:24.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21263" PostTypeId="2" ParentId="21249" CreationDate="2017-04-26T14:22:11.583" Score="2" Body="&lt;p&gt;There is a common misconception that spacecraft and satellites stay in orbit by 'escaping gravity', it is not the case. In a low earth orbit the force of gravity is a bit less but s till something like 90% of that on the surface of the earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for the apparent lack of gravity is that the craft and everything inside it are in free-fall. You can achieve the same thing closer to earth by getting in an aircraft which follows a &lt;a href=&quot;https://en.wikipedia.org/wiki/Reduced-gravity_aircraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;parabolic trajectory&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if a satellite is constantly falling why doesn't it fall back to earth. The reason is simple but not very intuitive. To remain in orbit the craft needs a tangential velocity relative to the earth and because the earth is (roughly) spherical  this tangential velocity tends to move it away from the earth's surface. In a stable orbit the movement away is exactly the same as the movement down due to gravity. Because there is virtually no drag in space it requires little or no energy to maintain this tangential speed as there is no overall change in momentum. The sum of the tangential velocity and the free fall towards earth is a circular (or more generally elliptical) orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition the angular velocity associated with a stable orbit is proportional to the altitude or the orbit so a geostationary orbit corresponds to a particular altitude and changing the velocity changes the altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it helps to understand this you can think of the analogy of a tennis ball on a piece of elastic. Here the elastic is equivalent to gravity, if you just stretch the elastic (ie the thrust of a rocket) and let go then the ball will 'fall' back towards your hand. However if you swing the ball around fast enough then the angular velocity of the ball reaches equilibrium with the stretch on the elastic and the faster you spin it the more tension (and stretch) you get in the elastic. Of course there is quite a bit of friction in this system so you need to keep putting energy in to maintain equilibrium but in space these frictional losses pretty much disappear. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However where you do need a lot of energy is in accelerating  the rocket to orbital velocity in the first place and this takes a lot more energy than getting to orbital height. In fact you can get most of the way into space for very little energy using a balloon but at this point you still aren't in orbit. &lt;/p&gt;&#xA;" OwnerUserId="13018" LastActivityDate="2017-04-26T14:22:11.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21265" PostTypeId="1" CreationDate="2017-04-26T16:27:24.280" Score="3" ViewCount="205" Body="&lt;p&gt;As far as I understand, a manned mission to mars would require an unprecedented amount of equipment, most of which would have to be pre-staged in LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is space junk going to affect this? The ISS needs to avoid space junk on a regular basis already, doesn't it?&lt;/p&gt;&#xA;" OwnerUserId="12712" LastActivityDate="2017-04-28T21:10:17.843" Title="How much of an issue is space junk going to be for future manned mars missions?" Tags="|mars|debris|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21266" PostTypeId="2" ParentId="20885" CreationDate="2017-04-26T16:39:25.573" Score="1" Body="&lt;p&gt;It is likely that uhoh is correct, and the frame is just made to provide an example. However, such a frame could exist.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun Synchronous Orbits&lt;/a&gt; are orbits where the satellite is always within LOS of the sun. What happens is the orbital precession is equal to the time it takes for the Earth to complete one orbit around the sun. If a satellite were in such an orbit, one could define a rotating reference frame where one axis was always pointed away from the sun and another was always pointed zenith (away from Earth). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is unlikely this is what the author meant, especially if the satellite is &quot;tumbling through free space&quot;, as you said. In any case, it's definitely not ECI.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastActivityDate="2017-04-26T16:39:25.573" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21267" PostTypeId="2" ParentId="21265" CreationDate="2017-04-26T17:45:33.930" Score="5" Body="&lt;p&gt;The ISS does occasionally maneuver to avoid space debris, but not frequently. From a &lt;a href=&quot;https://www.orbitaldebris.jsc.nasa.gov/faq.html#8&quot; rel=&quot;noreferrer&quot;&gt;NASA FAQ&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Does the International Space Station have to dodge orbital debris?&lt;/p&gt;&#xA;&lt;p&gt;The U.S. Space Surveillance Network regularly examines the trajectories of&#xA;orbital debris to identify possible close encounters. If another&#xA;object is projected to come within a few kilometers of the&#xA;International Space Station (ISS), the ISS will normally maneuver away&#xA;from the object if the chance of a collision exceeds 1 in 10,000. This&#xA;occurs infrequently, about once a year on average.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That's only good for debris large enough to be tracked. Smaller debris is typically mitigated using &lt;a href=&quot;https://en.wikipedia.org/wiki/Whipple_shield&quot; rel=&quot;noreferrer&quot;&gt;Whipple shielding&lt;/a&gt;, which is basically a thin layer of outer shielding spaced away from the skin of the spacecraft. Anything (small) that hits it at orbital speeds is vaporized on impact and the resulting cloud of plasma disperses somewhat as it traverses the spacing distance, reducing the damage to the spacecraft skin.&lt;/p&gt;&#xA;&lt;p&gt;Besides maneuvering or shielding, you could also mitigate risk by staging your equipment in an orbit less affected by orbital debris. The height of the ISS was constrained by the reach of the craft that resupply it, but if you're designing to leave Earth orbit altogether you presumably have the ability to send people beyond LEO if the lower orbits are too risky for your taste. Also remember that you're only staying in LEO for preparation, you don't have to manage anywhere near the longevity of the ISS.&lt;/p&gt;&#xA;&lt;p&gt;Given the options for dealing with orbital debris and the experience we've gained from operating the ISS as long as we have, I'd say that space junk is going to be a consideration when designing and planning manned Mars missions but I doubt it will significantly impact the mission when it's flown.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-04-26T17:45:33.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21268" PostTypeId="1" CreationDate="2017-04-26T22:07:10.827" Score="1" ViewCount="850" Body="&lt;p&gt;I know the Elevation and Azimuth of the a specific GPS satellite (I have the &lt;a href=&quot;https://igscb.jpl.nasa.gov/igscb/data/format/rinex211.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;RINEX files&lt;/a&gt; of the station in a specific Location 4.4801,-73.3433; in order to calculate this), but I want to know the elevation and azimuth of that same GPS satellite at around 200-500 km away (e.g. 7.11392,-73.1198). At the moment I have no idea how do this? any idea?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note:&lt;/strong&gt;&lt;br&gt;&#xA;The RINEX file of a &lt;em&gt;GPS&lt;/em&gt; satellite has some data (ephemeris variables) to re-calculate the position (elevation and azimuth), but the RINEX files for the &lt;em&gt;GLONASS&lt;/em&gt; constellation have the position of the satellite in cartesian coordinates; that will be somewhat difficult (I don`t know yet if impossible).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note 2&lt;/strong&gt;:&lt;br&gt;&#xA;I tried before to &quot;predict&quot; the position using &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE&lt;/a&gt; files with the library &lt;em&gt;&lt;a href=&quot;http://rhodesmill.org/pyephem/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pyephem&lt;/a&gt;&lt;/em&gt;, but the position over time has some significant error. (like this)&lt;a href=&quot;https://i.stack.imgur.com/HmD8w.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HmD8w.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Note 3:&lt;/strong&gt;&#xA;I got a better accuracy with TLE files, the information is &lt;a href=&quot;https://space.stackexchange.com/questions/20887/prn-number-in-tle-files-are-the-same-in-rinex-files-in-gps-constellation&quot;&gt;here&lt;/a&gt;. However, it will be good to get the answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;thanks.&lt;/p&gt;&#xA;" OwnerUserId="18740" LastEditorUserId="12102" LastEditDate="2017-11-03T01:56:05.420" LastActivityDate="2019-04-03T13:45:23.007" Title="Elevation and Azimuth of satellite in different location" Tags="|orbital-mechanics|orbital-elements|ephemeris|two-line-elements|rinex|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21269" PostTypeId="1" AcceptedAnswerId="25744" CreationDate="2017-04-27T03:00:41.633" Score="13" ViewCount="1208" Body="&lt;p&gt;The 70 meter DSN dishes are often used to receive the weakest signals, and so their receiver &lt;a href=&quot;https://en.wikipedia.org/wiki/Low-noise_amplifier&quot; rel=&quot;noreferrer&quot;&gt;LNAs&lt;/a&gt; have cryogenic front-ends in an assembly that also contains a LHe refrigerator and vacuum system. If I understand correctly these are located in the &quot;feed cones&quot; pointed at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassegrain_reflector&quot; rel=&quot;noreferrer&quot;&gt;Cassegrain&lt;/a&gt; secondary reflector which looks at the primary 70 meter mirror, and all of this sits outdoors in the desert air, hot during the day, cooler at night, but definitely warmer than 4K!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why doesn't the blackbody radio emission of the hot dish just swamp the receiver, making the cooling the front-end irrelevant?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not talking about the infrared radiation from the telescope. That is probably removed by managing line-of-sight within the cryogenic waveguide, or a low-temperature window or both. I'm talking about the circa 300K &lt;strong&gt;radio emission&lt;/strong&gt; of the 70 meter primary and 8 meter secondary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below x2:&lt;/strong&gt; Screen shots from an extensive review in &lt;a href=&quot;https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCANSO_sml-110804.pdf&quot; rel=&quot;noreferrer&quot;&gt;Low-Noise Systems in the Deep Space Network&lt;/a&gt; NASA/JPL, Edited by Macgregor S. Reid, February 2008.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GBeZQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GBeZQ.jpg&quot; alt=&quot;Goldstone 70m cooled low noise amplifier&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3YW8G.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3YW8G.jpg&quot; alt=&quot;Goldstone 70m dish tricones&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below are photos of one of the 70 meter &lt;a href=&quot;https://deepspace.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; telescopes for talking to deep space spacecraft. This one is in the Goldstone complex. From the relative sizes in the image, the secondary mirror is close to 8 meters in diameter. Considering the size and mass of the secondary (&lt;strong&gt;those are stairs for humans on each leg&lt;/strong&gt;, and the red lines in the dish are &quot;safe paths for walking&quot;) other, more advanced techniques to scan electronically have been developed, but the concept is the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kdeZg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kdeZg.jpg&quot; alt=&quot;Goldstone 70m dish side view&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Photo credit &lt;a href=&quot;https://www.flickr.com/photos/lightsinthedark/13678357214/in/photostream/&quot; rel=&quot;noreferrer&quot;&gt;JPMajor&lt;/a&gt;, creative commons CC BY-NC-SA 2.0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YbMWG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YbMWG.jpg&quot; alt=&quot;Goldstone 70m dish side view&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:DSN_Antenna_details.jpg&quot; rel=&quot;noreferrer&quot;&gt;commons.wikimedia.org&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-02-28T07:54:19.077" LastActivityDate="2019-04-08T15:41:54.890" Title="Why doesn't thermal radio emission from a DSN &quot;hot dish&quot; completely swamp the benefits of a cold LNA?" Tags="|deep-space|radio-communication|deep-space-network|radio-telescope|" AnswerCount="5" CommentCount="11" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21270" PostTypeId="2" ParentId="21269" CreationDate="2017-04-27T05:22:15.003" Score="12" Body="&lt;p&gt;When discussing radio antennae, radio astronomers usually describe things in terms of temperatures.  We can convert between power and temperature simply by multiplying (or dividing) by Boltzmann's Constant: $P=k_BT$.  We define the antenna's &lt;strong&gt;System Temperature&lt;/strong&gt;, $T_{sys}$, as the sum of all the temperature contributing factors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most important contributor to $T_{sys}$ is the source temperature, $T_{A}$.  Sometimes this temperature is just the temperature of a blackbody, but for non-thermal sources we use brightness temperature:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;T_b=\frac{S_\nu \lambda^2}{2k_B\theta_s^2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $S_\nu$ is the source flux density, $\lambda$ is the wavelength, and $\theta_s^2$ is the angular size of the object.  In the case of a spacecraft, $\theta_s^2$ would be the angular size of the spacecraft's &lt;a href=&quot;https://en.wikipedia.org/wiki/Parabolic_antenna#Beamwidth&quot; rel=&quot;noreferrer&quot;&gt;antenna beam&lt;/a&gt;: $\theta=k\frac{\lambda}{d}$ where $k$ is a coefficient dependent on the geometry of the dish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other sources of temperature come from the spillover from the ground, ambient sky temperature, and the antenna itself.  So how do we get rid of those contributions?  The answer: calibration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we observe a source, be it a spacecraft or a quasar, in order to determine the amplitude of the actual signal we are receiving, we calibrate by looking at a source of known flux density.  Usually this is done in radio astronomy by first observing a bright point source near to our target.  Some telescopes utilize a calibrator device that emits a known flux.  For a spacecraft, it is probably even simpler: the engineers will know the power the signal is being transmitted with, the beam size, and the source distance.  From that they can easily calculate the brightness temperature.  Once we know what kind of flux we should be getting from our source, we can easily subtract away the unwanted temperature components.  They will still add some noise to our signal, as our calibrations can never be perfect.  However, they will not completely swamp our signal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, instead of looking at an absolute flux calibrator, if you're looking at a point-source (like a spacecraft), you can simply calibrate by pointing your beam off-source.  By assuming that your noise is gaussian in nature, it will be the same both on-source and slightly off it.  You can rapidly switch between the beams, and subtract your off-source beam from your on-source beam.  This will leave you with the signal from your source itself.  This is known as &lt;a href=&quot;http://www.cv.nrao.edu/course/astr534/Radiometers.html&quot; rel=&quot;noreferrer&quot;&gt;Dicke Switching&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, why do we cool the receivers so much?  The answer is that we are seeking to decrease the receiver temperature, $T_{R}$.  The antenna's receiver contains amplifiers to gain up the signal.  The receiver temperature is given by:&#xA;$$&#xA;T_R=T_{G,1}+\frac{T_{G,2}}{G_1}+\frac{T_{G,3}}{G_1G_2}+...&#xA;$$&#xA;where $T_{G,n}$ is the temperature of the nth amplifier and $G_n$ is the gain of the nth amplifier.  As long as you are gaining your signal up by a few orders of magnitude at each step, each successive amplifier contributes practically nothing to the overall receiver temperature.  Therefore, your biggest contribution is the first amplifier.  If we were to leave the receiver out in the open, this would add a whopping 300 K to our signal, a contribution that we cannot simply calibrate away.  However, by cooling it to ~4 K, you eliminate the majority of the noise that would come from your receiver.  As the above website notes, when using Dicke switching, you end up doubling your receiver temperature, which is another reason they seek to cool the receivers by such a great amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;em&gt;Tools of Radio Astronomy&lt;/em&gt; by Wilson, Rohlfs, and Huttemeister, 5th ed.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastEditorUserId="13710" LastEditDate="2017-04-27T17:33:10.407" LastActivityDate="2017-04-27T17:33:10.407" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="21271" PostTypeId="2" ParentId="21269" CreationDate="2017-04-27T05:35:47.073" Score="12" Body="&lt;p&gt;Because the dish isn't a black body. At RF it has a very low emissivity, hence the name &quot;reflector&quot;.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-04-27T05:35:47.073" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21272" PostTypeId="1" AcceptedAnswerId="21273" CreationDate="2017-04-27T07:42:24.007" Score="15" ViewCount="2049" Body="&lt;p&gt;Don't treat this question lightly. It's a thing that's been biting me for a pretty long time, and while it's &quot;intuitively obvious&quot;, if you get into finer details, it gets quite convoluted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, a satellite in orbit, means it moves at a speed defined by a bunch of equations, relative to... what exactly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Translationally&quot; the frame of reference is bound to barycenter of the system considered. Usually happily approximated to the planet center, which won't be so simple with binary stars, but that's the lesser problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about rotation of the frame of reference? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously not surface, but then - what?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Direction of equinox&quot; is subject to precession of the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Distant stars&quot; is a common approximation but distant stars belong to the galaxy and rotate with it too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Distant galaxies? But clusters and even superclusters move reative to each other, never mind space expansion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cosmic background radiation? Would seem plausible, except... if our supercluster is spinning (or distorting in some other way), won't frame-dragging make it false? What about orbits in neighborhood of supermassive bodies where relativistic effects like local frame-dragging are non-negligible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is the inertial frame of reference in which orbital motion happens bound to?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="2752" LastEditDate="2017-04-28T00:19:26.880" LastActivityDate="2017-04-28T00:19:26.880" Title="What is the frame of reference for orbital speed?" Tags="|orbital-mechanics|orbit|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21273" PostTypeId="2" ParentId="21272" CreationDate="2017-04-27T10:30:27.207" Score="16" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;So what is the inertial frame of reference in which orbital motion happens bound to?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;TL;DR: Whatever you want. Conceptually, all frames of reference are equally valid. However, some frames are computationally better than others, depending on context. While you could do it, it would be ludicrous to describe the formation of a hurricane or the orbit of a satellite in low Earth orbit from the perspective of a Triton-centered, Triton-fixed frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But you could do it!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, modeling the orbit of an object in low lunar orbit from the perspective of a Neptune-centered inertial frame is one of my favored tests of the orbital mechanics package I developed for the Johnson Space Center. The result is pure garbage after a few dozen orbits, but it does work initially. The object initially orbits the Moon, but numerical issues quickly arise.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;What is this thing that you call an &quot;inertial frame of reference&quot;? As a supervisor said to me almost 40 years ago, &lt;strong&gt;name one&lt;/strong&gt;. The so-called Earth-centered inertial frame obviously isn't inertial; the Earth is accelerating gravitationally toward the Sun, the Moon, the other planets, nearby and remote stars, other galaxies, etc. In addition, the axes of an ECI frame are almost certainly rotating with respect to those of a Newtonian inertial frame of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's one catch, good luck finding a Newtonian inertial frame of reference. Or as my supervisor said almost 40 years ago, &lt;strong&gt;name one&lt;/strong&gt;. To make matters worse, this doesn't even take general relativity into account. Ultimately, the concept of a Newtonian inertial frame of reference is a fiction. That said, it is a very, very useful fiction because our solar system is very close to Newtonian in behavior. Even the motion of Mercury can be approximated extremely accurately as being due to Newtonian gravity plus some very small post-Newtonian accelerations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two challenges with regard to defining a Newtonian frame of reference, the placement of the origin and the placement of the axes. It's important to keep in mind that all frames of reference are equally valid. Using a quasi-inertial solar system barycenter frame to describe the motion of a satellite in low Earth orbit doesn't make any sense. An Earth-centered inertial perspective is a much more sensible perspective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As previously mentioned, an Earth-centered frame is an accelerating frame. This is easily addressed: Add fictitious accelerations due to the Earth's gravitational acceleration toward the Sun, the Moon, and perhaps the other planets. In the space exploration community, the term describing perturbations due to choosing a frame based on the center of some massive body is &quot;third body effects&quot; (or third body accelerations, or third body perturbations).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rotating frame makes for a much messier situation. Up until the mid 20th century, the preference was to use a very slowly rotating frame based on the location of the vernal equinox. This resulted in apparent apsidal precessions of the orbits of the planets about the Sun. As is the case with third body effects, this is not necessarily problematic. The techniques that addressed this apparent precession were sufficient for the 19th century discovery that Mercury suffered a precession that could not be explained by Newtonian mechanics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three key things changed in this regard during the latter half of the 20th century. One was that humanity started putting things into space. Another was drastic improvements to astronomical observations. Both motivated the improvement of the concept of frames of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third key item was the discovery of quasars. Quasars are so remote that their proper motions are are extremely small and are unrelated to anything close by. (A galaxy even remotely connected to the Milky Way qualifies as &quot;close by&quot; compared to quasars.  The current gold standard with regard to the orientation of a frame of reference is the International Celestial Reference Frame (ICRF). This is based on almost 300 quasars, with over 3000 other quasars used as a sanity check.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-04-27T16:51:21.100" LastActivityDate="2017-04-27T16:51:21.100" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21275" PostTypeId="2" ParentId="21272" CreationDate="2017-04-27T15:46:34.567" Score="6" Body="&lt;p&gt;David gave the only answer this question needs. A tl;dr would be this: &lt;strong&gt;There isn't an inertial reference frame&lt;/strong&gt;. Orbital motion isn't bound by anything other than the forces that act on it, which includes every mass in the universe (and maybe some other stuff who really knows). Even if you did as you suggested, and used a reference frame that was centered using the masses of the universe (which would be ridiculous when you include relativistic effects), it wouldn't necessarily be &quot;the inertial reference frame&quot;. Inertial isn't an absolute, it's relative.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastActivityDate="2017-04-27T15:46:34.567" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21277" PostTypeId="2" ParentId="869" CreationDate="2017-04-27T18:52:02.193" Score="2" Body="&lt;p&gt;Venus would appear to be the most practical, not Titan (one HAS to factor the cold). Yes, it's so hot on Venus it melts lead at 621 degrees F. However, lead is a soft metal and it's heavy. We don't build aircraft out of lead.  Aluminum melts at 1,218 degrees Fahrenheit (F) and Titanium melts at 3,200 degrees F.  The Lockheed SR-71, flying at Mach 3+, was designed in the early 1960s with an aluminum structure and titanium skins to protect it from air molecule heat friction created at those speeds.  One could build another &quot;SR-71&quot; type aircraft and fly on Venus albeit the more temperature sensitive materials would all need to be thermally insulated and some would require active cooling. On Titan, at 72 degrees Kelvin that's -330 degrees Fahrenheit and we just don't fly &quot;normal&quot; aircraft in that kind of cold. That's as cold as liquid nitrogen and the metal would become so brittle it would simply break from aerodynamic forces. However, the temperature in space is even colder at 2.7 degrees kelvin (-455 Fahrenheit) but there are no aerodynamic forces since it is a vacuum.  We &quot;fly&quot; there all the time with spacecraft. Boeing just completed a record breaking 674 days in space with it's X-37 space plane.  So, flying on Titan with a space plane similar to the X-37 may be practical after all.   &lt;/p&gt;&#xA;" OwnerUserId="19448" LastEditorUserId="19448" LastEditDate="2017-04-27T19:16:26.617" LastActivityDate="2017-04-27T19:16:26.617" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21278" PostTypeId="1" AcceptedAnswerId="21281" CreationDate="2017-04-27T21:38:15.830" Score="9" ViewCount="525" Body="&lt;p&gt;On Earth, we can use latitude and longitude to fairly accurately describe a single point on the surface of the earth. This works because the earth is, for all intents and purposes, a sphere. It got me thinking about what kind of system, if any, we would use to describe a single point on the surface of a non-spheroid body, like a large asteroid or an oblong moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/T8P9Z.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/T8P9Z.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose you could just &quot;wrap&quot; the object's topology to a spherical projection, though this would result in massive distortions. More importantly, it also raises the question of what the 'fixed points' are of the system (i.e. where are geographical north and south poles)? We've recently witnessed large asteroids and moons &lt;a href=&quot;https://phys.org/news/2011-08-cassini-saturn-moon-hyperion.html&quot; rel=&quot;noreferrer&quot;&gt;tumbling chaotically in their orbits&lt;/a&gt; with no discernible rotation axis.&lt;/p&gt;&#xA;" OwnerUserId="11549" LastEditorUserId="6816" LastEditDate="2017-04-27T21:47:28.663" LastActivityDate="2018-12-03T00:44:08.657" Title="How do we define geographical coordinates on non-spheroid celestial bodies?" Tags="|asteroid|navigation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21279" PostTypeId="2" ParentId="21261" CreationDate="2017-04-27T22:31:25.100" Score="6" Body="&lt;p&gt;It's some other force. In particular, it's that last approach to Titan on September 11 that will send Cassini deep enough into Saturn's atmosphere so as to make the spacecraft burn up on September 15.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice that your graph shows that Cassini will make several somewhat close approaches with Titan during the next several months. Some will raise Cassini's Saturn periapsis, some will lower it. The encounter on August 8 (more or less) will make Cassini just barely dive into Saturn's atmosphere at periapsis. The September 11 encounter seals Cassini's fate. There is no recovery from the deep dive into Saturn's atmosphere that will result from that final encounter.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-27T22:31:25.100" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21280" PostTypeId="2" ParentId="21256" CreationDate="2017-04-28T00:16:48.243" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;How do we track the exact location of the spacecraft which is millions or billions of miles away from us?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We don't track the &lt;strong&gt;exact&lt;/strong&gt; location of spacecraft. There are always errors in measurements, errors in the behaviors of the spacecraft when it makes maneuvers, and errors in our models of the solar system. Exactness (perfection) is unattainable. It is far better to model those errors than it is to pretend they don't exist.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;What things do we study for tracking purpose?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This depends very much on the object of interest. A key distinction is whether the object of interest is a cooperative or uncooperative target. A cooperative target somehow actively cooperates in determining the object's trajectory. The Moon, the Sun, the planets, asteroids, etc., are obviously uncooperative. All objects of interest in space were uncooperative until the middle of the 20th century, and the only available measurements were the object's angular position (azimuth and elevation) as seen by an observer on the surface of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This changed drastically in the latter half of the 20th century when humanity developed radar and when humanity started sending cooperative targets into space. Radar enables the measurement of the range to somewhat nearby objects in space, where &quot;somewhat nearby&quot; means a few astronomical units or so. These range measurements alone are quite precise, enough so that direction measurements (azimuth and elevation) are of lesser importance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cooperative targets that relay a signal sent from the Earth back to the Earth drastically improve the range of this already very precise range measurement and enable an even more precise measurement, range rate (the rate at which the distance to the target changes with respect to time).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With one exception, measurements of where a cooperative target is in the sky are so imprecise compared to range and range rate that those measurements are essentially useless. That one exception is Delta-Differential One Way Ranging, or ΔDOR, for short. This involves simultaneous observations of a deep space probe by two or more ground stations separated by thousands of kilometers. Interferometric techniques yields nanoradian level precision on the angular position of the observed object.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How exactly do we predict?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Even with range, range rate, and ΔDOR measurements, a single set of measurements provides information on only four measurements of an object's state. Moreover, because ΔDOR requires using two ground stations simultaneously, &lt;a href=&quot;https://space.stackexchange.com/questions/21005/what-makes-time-on-the-dsn-so-expensive/21009#21009&quot;&gt;it is rather expensive&lt;/a&gt;. In many cases, there are only one (range) or two observations (range and range rate) at any point in time. Whether it's one, two, or four measurements, that's does not suffice because an object in space has six translational degrees of freedom. Inferring orbital state from a single set of measurements is an underdetermined problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's done is combine multiple sets of measurements gathered over time. This is an overdetermined problem. Each of those measurements has an error associated with it, the propagation from one measurement to another induces additional errors (process noise), and imprecise behaviors of a probe when it performs a correction burn or adjusts its attitude induce yet other errors (plant noise).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Describing the techniques needed to combine those multiple measurements and to account for measurement noise, process noise, and plant noise would require multiple books. If you want to study this on your own, you'll need to understand statistical filtering techniques. One set of keywords of interest is &quot;precision orbit determination&quot;, or POD for short.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-28T00:16:48.243" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21281" PostTypeId="2" ParentId="21278" CreationDate="2017-04-28T03:58:09.183" Score="5" Body="&lt;p&gt;All projections will have some kind of distortion, even on Earth.  Spherical projections may not be ideal for such bowling pin shaped asteroids.  In fact, a cylindrical projection may be applied or even new custom projections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much has been written on the subject including &quot;&lt;a href=&quot;http://adsabs.harvard.edu/full/1987LPI....18..956S&quot; rel=&quot;nofollow noreferrer&quot;&gt;Morphographic Projections for Maps of Non-Spherical Worlds&lt;/a&gt;&quot; and  &quot;&lt;a href=&quot;http://www.utpjournals.press/doi/abs/10.3138/WL61-208J-841J-7R21&quot; rel=&quot;nofollow noreferrer&quot;&gt;Map Projections for Non-spherical Worlds / The Variable-Radius Map Projections&lt;/a&gt;&quot; (paywall). The latter describing an interesting concept applied to Saturn's moon Epimetheus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://sbn.psi.edu/pds/resource/stookemaps.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;This page&lt;/strong&gt;&lt;/a&gt; has a collection of maps of small solar system bodies prepared by Phil Stooke of the University of Western Ontario. Many of the bodies have maps created using a variety of projections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://sbnarchive.psi.edu/pds3/multi_mission/MULTI_SA_MULTI_6_STOOKEMAPS_V2_0/document/433eros/eros_cyl_near.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;This cylindrical projection of Eros&lt;/a&gt; is highly distorted, especially at the poles, but also in the centre.  It's clear that no projection is perfect.&lt;/p&gt;&#xA;" OwnerUserId="84" LastEditorUserId="84" LastEditDate="2018-12-02T22:55:35.503" LastActivityDate="2018-12-02T22:55:35.503" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="21283" PostTypeId="1" AcceptedAnswerId="21284" CreationDate="2017-04-28T06:26:44.727" Score="1" ViewCount="259" Body="&lt;p&gt;In this &lt;a href=&quot;https://youtu.be/IZHw4UKFiHQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;CSA video&lt;/a&gt; CSA Astronaut Chris Hadfield describes the attitude control of the ISS and its importance for things like communication with Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At &lt;code&gt;00:30&lt;/code&gt; here is about a 2 second vignette of these four identical fixed dish antennas all apparently pointing in the same direction -- in the general direction of the Sun, which would be south if this is in North America.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these used to communicate or control the ISS, or is this just commercial stock footage of a cable tv station?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jvPBO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jvPBO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/IZHw4UKFiHQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-04-28T09:24:31.523" Title="What are these dishes looking at?" Tags="|iss|communication|antenna|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21284" PostTypeId="2" ParentId="21283" CreationDate="2017-04-28T09:24:31.523" Score="4" Body="&lt;p&gt;It's stock footage. Here's the source: &lt;a href=&quot;https://www.shutterstock.com/video/clip-3547922-stock-footage-satellite.html?src=search/j9qjEB4D-XcqsNoqfQWZrA:1:4/gg&quot; rel=&quot;noreferrer&quot;&gt;https://www.shutterstock.com/video/clip-3547922-stock-footage-satellite.html?src=search/j9qjEB4D-XcqsNoqfQWZrA:1:4/gg&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-04-28T09:24:31.523" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21285" PostTypeId="1" AcceptedAnswerId="21288" CreationDate="2017-04-28T13:39:06.820" Score="12" ViewCount="1600" Body="&lt;p&gt;On 15th September 2017, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;noreferrer&quot;&gt;Cassini probe&lt;/a&gt; will be destroyed by &lt;a href=&quot;https://saturn.jpl.nasa.gov/mission/grand-finale/overview/&quot; rel=&quot;noreferrer&quot;&gt;crashing it into Saturn&lt;/a&gt;. This will be done in order to eliminate the unlikely risk of it crashing into and forward contaminating one of Saturn's moons with micro-organisms originating from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why have such concerns not prevented us from landing probes on other bodies such as Mars, the Moon, and Titan? Why are we selective about which bodies we don't want to risk contaminating and which we are willing to take the risk with?&lt;/p&gt;&#xA;" OwnerUserId="12941" LastEditorUserId="25" LastEditDate="2017-04-29T01:20:13.173" LastActivityDate="2017-04-29T01:20:13.173" Title="Why do we worry about contaminating some solar bodies but not others?" Tags="|probe|astrobiology|forward-contamination|planetary-protection|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21286" PostTypeId="2" ParentId="21285" CreationDate="2017-04-28T14:07:36.797" Score="20" Body="&lt;p&gt;The deciding factor is if the solar body has a chance of harboring life (or having done so in the past). Scientists don't want to run the risk of false positives: detecting life on another planet, only to find that we're the ones who brought it there.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So probes sent to Mars are sterilized as best we can, while we don't sterilize probes to bodies that are definitely dead (the Moon, or Saturn).   Titan and several other moons have conditions that could possibly have led to life existing there, so we avoid contaminating them.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This has been agreed in the Outer Space Treaty. For NASA's implementation, see the &lt;a href=&quot;https://planetaryprotection.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Planetary Protection&lt;/a&gt; web site. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-04-28T14:21:11.807" LastActivityDate="2017-04-28T14:21:11.807" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21287" PostTypeId="2" ParentId="21265" CreationDate="2017-04-28T21:10:17.843" Score="4" Body="&lt;p&gt;The probability of collision is strongly affected by the &lt;a href=&quot;http://www.duncansteel.com/archives/1425&quot; rel=&quot;nofollow noreferrer&quot;&gt;altitude of the orbit&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/o1MAo.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/o1MAo.png&quot; alt=&quot;graph shows probability of collision is highest at ~700km&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At very low earth orbits, particularly under 300km, debris is removed by drag very quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A low orbit is highly desirable to maximize the oberth effect for the transfer burn, for example when launching a satellite to GTO or probes on an interplanetary trajectory the parking orbit altitude is often just 160-180km. The downside is rapid orbital decay, so it pays to not linger in these low orbits for too long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is thus a tradeoff, departing from a low orbit - the lower the better - would be great for maximizing the oberth effect and is generally safer from debris, but extra fuel needs to be expended to account for drag if the staging time is significant. I have a suspicion that the staging will be done in as low an altitude as practical, if equipment needs to wait for a long time in orbit then it would probably be boosted to a higher initial orbit and allowed to decay down to the departure orbit, for example a component boosted to 350km might be reasonably expected to decay down to a very low orbit in about 6 months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A final advantage of using a low staging orbit is that if disaster does strike and a payload is destroyed it doesn't add to the long term orbital debris problem, this would have to be a consideration when launching large amounts of stuff into orbit.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2017-04-28T21:10:17.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21288" PostTypeId="2" ParentId="21285" CreationDate="2017-04-29T01:18:34.297" Score="10" Body="&lt;p&gt;Early Mars probes were heavily sterilized. Further studies have shown that Mars's surface isn't habitable, at least not in most areas, and thus a low risk approach has been developed, where certain sensitive parts are carefully controlled (Usually robotic arms), and missions to certain interesting places (Such as locations where liquid water is suspected) are more carefully monitored, but overall, the restrictions are much looser.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 4 key bodies (Aside from Earth) that have interesting conditions that might harbor life in our solar system. These are Mars, Europa, Titan, and Enceladus. These are all known to have liquids at or near their surfaces, although how much is debatable. Mars has been studied well enough to know it doesn't possess any risk except underneath the surface, so we aren't as worried about it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, there are &lt;a href=&quot;https://en.wikipedia.org/wiki/Planetary_protection#COSPAR_recommendations_and_categories&quot; rel=&quot;noreferrer&quot;&gt;5 major categories of planetary protection objects of interest&lt;/a&gt;, where Category 1 has very low chance of life, and Category 5 is a sample return mission. The Wikipedia article linked provides further details.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-04-29T01:18:34.297" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21290" PostTypeId="1" CreationDate="2017-04-29T03:23:11.230" Score="-2" ViewCount="3727" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Downrange&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Downrange is the horizontal distance traveled by a spacecraft, or the spacecraft's horizontal distance from the launch site.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Spacecraft don't travel horizontally. I don't even know how the word &quot;horizontal&quot; and the word &quot;spacecraft&quot; can exist in the same sentence. Maybe the word &quot;projected&quot; would be helpful here?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/3287/12102&quot;&gt;Space Exploration Stackexchange&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This is simply the distance across the ground from the launch site.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is the accepted and highly upvoted answer. If I had to write an equation from this explanation, or interpret what 500km downrange means, I'd be hard pressed. It's a great answer to get the general idea, but a bit wanting from a mathematical or orbital-mechanical perspective.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Is there an official or generally accepted, precise definition for how one would calculate downrange distance? I can imagine if I have an orbital plane, then downrange and altitude might be &lt;em&gt;referenced to a sphere or an ellipsoid&lt;/em&gt; (Earth surface model) and thereby &lt;s&gt;could be&lt;/s&gt; &lt;em&gt;could have been&lt;/em&gt; used to define a position &lt;em&gt;fairly precisely&lt;/em&gt;. Does such a definition exist?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To better illustrate why the question is not trivial, let's abstract the mathematics out of history temporarily. Imagine you would like to, or have been asked to calculate a down range distance. What is the first question you might ask yourself:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;If I have ECI coordinates of a launch site and a spacecraft, how would I calculate the downrange distance correctly? What model should I use for the Earth's surface? Or should I project along a sphere with the same altitude as the launch site?&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I'm looking for an authoritative answer, not what it probably means or could or might mean, how to &quot;think about it as...&quot;, or &quot;it doesn't matter&quot; — it has certainly been relevant historically. Thanks!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; To elaborate on the previous sentence defining the scope of the question, a look at NASA's 254 page &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/A11_PressKit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 11 Press Kit&lt;/a&gt; will show a dozen numerical downrange values with single digit nautical mile precision, (and one with decimal precision). &lt;strong&gt;To get a large downrange value to 1 nautical mile precision, one needs to choose a specific model for the shape of the Earth, at least sphere or ellipse&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-04-29T17:41:15.737" LastActivityDate="2018-09-27T20:37:01.827" Title="What precisely is downrange distance - how is it defined mathematically?" Tags="|orbital-mechanics|low-earth-orbit|history|terminology|" AnswerCount="2" CommentCount="26" ContentLicense="CC BY-SA 3.0" />
  <row Id="21293" PostTypeId="1" AcceptedAnswerId="21320" CreationDate="2017-04-29T06:06:44.833" Score="6" ViewCount="1549" Body="&lt;p&gt;I found this &lt;a href=&quot;http://www.honeysucklecreek.net/msfn_missions/ALSEP/hl_alsep.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;really interesting description&lt;/a&gt; of the ALSEP instrumentation placed on the moon during the Apollo 11 landing, written by &lt;a href=&quot;http://www.honeysucklecreek.net/people/lindsay.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hamish Lindsay&lt;/a&gt;, (also, author of &lt;a href=&quot;http://www.springer.com/gp/book/9781852332129&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tracking Apollo to the Moon&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;One of the instruments was a seismometer. The seismometer began transmitting directly to Earth as soon as it was set up, and NASA could begin monitoring and recording it immediately.&lt;/p&gt;&#xA;&lt;p&gt;It was very sensitive to local vibrations, and could even detect activity of astronauts inside the LEM.&lt;/p&gt;&#xA;&lt;p&gt;The full image of the seismogram is shown below, along with a cropped highlight near the end.&lt;/p&gt;&#xA;&lt;p&gt;At first &lt;code&gt;PERSONAL LIFE SUPPORT SYSTEMS (PLSS'S) ARE THROWN OUT OF THE CAPSULE&lt;/code&gt; sounds a bit final to say the least.&lt;/p&gt;&#xA;&lt;p&gt;I understand that obsessive mission weight minimization was the only way to get to the moon and back, so plenty of things were left behind. However it seems that a PLSS might have some utility in some scenarios.&lt;/p&gt;&#xA;&lt;p&gt;Were there any downsides to chucking life support out the door that had to be evaluated as part of the overall risk minimization? Were there any scenarios at least considered where they might have been useful, before being discounted?&lt;/p&gt;&#xA;&lt;p&gt;One example might be a docking difficulty requiring an EVA, though I don't know if that's considered a &amp;quot;thinkaboutable&amp;quot; or not.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; All images from &lt;a href=&quot;http://www.honeysucklecreek.net/msfn_missions/ALSEP/hl_alsep.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ALSEP, Apollo Lunar Surface Experiments Package&lt;/a&gt;, 20 November 1969 – 30 September 1977, by Hamish Lindsay, Click for full size.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WR3v9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WR3v9.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This scan of a copy of the first lunar seismogram – complete with signatures of the Apollo 11 crew, Dr Garry Latham, and other members of his team – is courtesy of Trevor Dalziell at Geoscience Australia.&lt;/p&gt;&#xA;&lt;p&gt;It seems that this copy (now water damaged) was a part of the US display at Expo 88 in Brisbane. Apollo 15 astronaut Jim Irwin’s signature was added at the Expo. Somehow it made its way to the Geoscience Australia field station in Charters Towers, until it was transferred to Canberra in late 2011. Thanks also to Bruce Stewart for first mentioning this plot.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WTK6t.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WTK6t.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Dr. Garry Latham (on left), with the Lamont Geological Observatory, studies seismometer tracings in the Mission Control Center’s ALSEP control room.&lt;/p&gt;&#xA;&lt;p&gt;The electronic data was transmitted from the Passive Seismic Experiments Package which the Apollo 11 astronauts had just deployed on the surface of the moon.&lt;/p&gt;&#xA;&lt;p&gt;NASA image S69-39587.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/N3xd9.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/N3xd9.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-04-07T06:51:39.210" Title="Any potential downside to throwing personal life support out the door on the Moon?" Tags="|lunar-landing|safety|life-support|planning|apollo-11|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21294" PostTypeId="1" AcceptedAnswerId="24478" CreationDate="2017-04-29T12:44:26.163" Score="10" ViewCount="2553" Body="&lt;p&gt;If there are only a few &quot;enemy&quot; radars at known locations that can detect a satellite in LEO, I'd assume there are some radar stealth methods using a spacecraft carefully shaped and oriented such that it does not reflect any radar beam back to any enemy receiver. Is that plausible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And what about the optical, is it possible to make an average sized satellite, without Solar panels, optically invisible from the ground?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2018-01-15T17:58:33.050" LastActivityDate="2018-01-15T17:58:33.050" Title="How well could a satellite hide from Earth detection?" Tags="|artificial-satellite|tracking|security|classified|detection|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21295" PostTypeId="1" CreationDate="2017-04-29T13:02:52.493" Score="9" ViewCount="931" Body="&lt;p&gt;Cassini is scheduled to crash land into Saturn later in 2017. The reason stated was that they did not want to risk contaminating any of Saturn's moons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, during the Cassini mission, we landed the Huygens probe on Titan. If we're concerned about bacteria onboard Cassini surviving, why are we not concerned that the bacteria may have been on the Huygens probe instead?&lt;/p&gt;&#xA;" OwnerUserId="19466" LastEditorUserId="6816" LastEditDate="2017-04-29T17:36:15.183" LastActivityDate="2017-05-03T16:49:07.197" Title="Why are we concerned about Cassini contaminating Saturn's moons when we landed Huygens probe on one of them?" Tags="|cassini|planetary-protection|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21296" PostTypeId="2" ParentId="21295" CreationDate="2017-04-29T13:26:19.953" Score="9" Body="&lt;p&gt;First off, what's done is done. Don't make it worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second the main concern is Enceladus, not Titan. Enceladus is an ocean world whose ocean regularly and significantly interacts with its surface. Titan may be an ocean world, but it is unlikely that that ocean interacts significantly with the surface.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-04-29T13:26:19.953" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21297" PostTypeId="1" AcceptedAnswerId="24671" CreationDate="2017-04-29T13:39:27.303" Score="2" ViewCount="449" Body="&lt;p&gt;I have stumbled across the fascinating page &lt;strong&gt;Photogrammetric Analysis Of Apollo 11 Imagery: New camera-station map with improved locations&lt;/strong&gt; at the following website: &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11Photogrammetry.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.hq.nasa.gov/alsj/a11/a11Photogrammetry.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is extensive analysis and discussion there of the panoramic photographs taken of the Apollo 11 site by Neil Armstrong. While mechanical panoramic cameras did exist at the time, they weren't moon-ready, so instead a series of overlapping photographs were taken. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike a proper mechanical panoramic camera, the astronaut version did not pivot about the pupil of the lens, but instead the astronaut just simply turned in place in some fashion. The resulting translation motion between exposures presented the opportunity to analyze parts of the scene that are overlapped by two exposures with photogrammetric techniques. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Think of this as a prequel to the navigation camera pairs used on martian rovers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I understand correctly, this was not a planned thing, and more modern analysis techniques and computer image processing made this possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I'm wondering, &lt;em&gt;when was stereoscopic photography first used intentionally on the surface of a solar system body?&lt;/em&gt; It doesn't have to be for live navigation, it could be for later analysis. I'm asking when it was done first &lt;em&gt;on purpose&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These include numbered NASA photographs as well as (much) later analysis. Sourced from &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11Photogrammetry.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Right click and open each one in separate tab for full size viewing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oCFTB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oCFTB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ej01d.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ej01d.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UTT4Z.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UTT4Z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-18T01:46:20.300" LastActivityDate="2021-08-18T01:46:20.300" Title="When was stereoscopic photography first used intentionally on solar system body?" Tags="|rovers|photography|navigation|imaging|spaceflight-firsts|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21298" PostTypeId="2" ParentId="21294" CreationDate="2017-04-29T14:19:14.133" Score="7" Body="&lt;p&gt;The answer to your question is classified. What is almost certain is that the USA launched stealth satellite in low Earth Orbit, so presumably it works well enough. These satellites were called &lt;a href=&quot;https://en.wikipedia.org/wiki/Misty_(satellite)&quot; rel=&quot;nofollow noreferrer&quot;&gt;MISTY&lt;/a&gt; by the amateur observers, and were hard to track.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://fas.org/spp/military/program/track/stealth.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Federation of American Scientists has a whole sourcebook dedicated to the issue of stealth in space&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastEditorUserId="6816" LastEditDate="2017-05-01T07:37:39.063" LastActivityDate="2017-05-01T07:37:39.063" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21299" PostTypeId="1" AcceptedAnswerId="21319" CreationDate="2017-04-29T16:25:54.813" Score="1" ViewCount="851" Body="&lt;p&gt;I'm making a spacecraft maneuver from one orbit to another. Firstly, I'm going to change the inclination, therefore, moving to a coplanar orbit. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Where is it better to change the inclination, in terms of less Delta-V?  &lt;/li&gt;&#xA;&lt;li&gt;If I know the current keplerian elements for my spacecraft, how to calculate the time, when the spacecraft will be on the above point, to make an optimal maneuver? &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Found, that the optimal points are the orbital nodes. Are the orbital nodes at 90 and 270 degrees of true anomaly?&lt;/p&gt;&#xA;" OwnerUserId="19219" LastEditorUserId="19219" LastEditDate="2017-04-30T07:31:51.763" LastActivityDate="2017-05-01T10:56:10.713" Title="Inclination change maneuver" Tags="|orbital-maneuver|delta-v|" AnswerCount="1" CommentCount="0" ClosedDate="2017-05-01T10:49:01.173" ContentLicense="CC BY-SA 3.0" />
  <row Id="21300" PostTypeId="2" ParentId="21295" CreationDate="2017-04-29T20:33:43.043" Score="2" Body="&lt;p&gt;Enceladus and some of the other icy moons have the possibility of supporting Earth based life forms.  Titan, on the other hand is fairly toxic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, they are being paranoid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if we can learn something new before Cassini looses contact, it'll have some value. &lt;/p&gt;&#xA;" OwnerUserId="18842" LastActivityDate="2017-04-29T20:33:43.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21302" PostTypeId="2" ParentId="21290" CreationDate="2017-04-29T22:07:01.417" Score="3" Body="&lt;p&gt;The definition would be straightforward: length of arc over Earth surface at sea level (altitude zero sphere, not actual ground), between point of launch, and point directly below ship's nadir (intersection of line connecting craft and Earth center, with altitude zero sphere.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't need to be defined even that precisely (use average ground level instead of zero, or use direct straight line distance instead of arc) because it's not used in actual craft guidance - it's a piece of data helpful for ground observation crew, &quot;range safety&quot;, recovery crew, reporters/photographers, air traffic control, and so on, and for these purposes accuracy of ~100 meters is perfectly sufficient, and at orbital altitudes it loses about all significance for space launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's pretty important for rocket artillery and short range missile launches though. But that's not a space exploration topic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you need to write an equation, being given the coordinates, you normalize the coordinates to the surface (remove the altitude component) and then use whichever distance metric over the surface (as used in aviation, naval navigation, land navigation, artillery balistics) to determine the distance. Which exactly you use doesn't really matter, because, as I mentioned, there is no requirement for this to be precise. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-04-29T22:07:01.417" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="21303" PostTypeId="1" CreationDate="2017-04-29T23:45:05.803" Score="3" ViewCount="248" Body="&lt;p&gt;Title, I tried converting the coordinates from &lt;a href=&quot;http://cosinekitty.com/solar_system.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; to rectangular but it seems off.&lt;/p&gt;&#xA;" OwnerUserId="19472" LastActivityDate="2018-02-18T03:04:20.550" Title="Where to get rectangular coordinates of the solar system planets?" Tags="|coordinates|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21306" PostTypeId="1" AcceptedAnswerId="21308" CreationDate="2017-04-30T05:34:32.483" Score="7" ViewCount="638" Body="&lt;p&gt;How important is secure communication in space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Science-related research is often made public, but does that mean all communications must be unprotected?&lt;/p&gt;&#xA;" OwnerUserId="19002" LastEditorUserId="58" LastEditDate="2017-11-13T14:16:11.967" LastActivityDate="2017-11-13T15:21:35.347" Title="Is secure communication in space important?" Tags="|spacex|mars|deep-space|communication-satellite|mars-orbiter-mission|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21307" PostTypeId="1" CreationDate="2017-04-30T07:16:37.377" Score="1" ViewCount="146" Body="&lt;p&gt;At 24:40 into &lt;a href=&quot;https://connect.arc.nasa.gov/p3vb1u5ijgy/?launcher=false&amp;amp;fcsContent=true&amp;amp;pbMode=normal&quot; rel=&quot;nofollow noreferrer&quot;&gt;this lecture&lt;/a&gt;, technology development scientist Laurent Sibille briefly describes an experiment done with a regolith simulant for Lunar Mare. The process of producing oxygen and reusing methane et cetera doesn't seem to be relevant to the actual melting of regolith to metal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the surface of the Moon really turn into solid metal if one simply concentrates sunlight on it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that is solid metal, it must be easy to heat it again and use a centrifuge, magnetism or chemical reactions to separate the different pure metals. But for a landing pad or a road or most structures, maybe a random iron-magnesium-titanium alloy works very well?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nhsM4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nhsM4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Thermal energy having turned regolith into apparently solid metal lumps. From a slide in Sibille's lecture at &lt;a href=&quot;https://sservi.nasa.gov/in-situ-resource-utilization-isru-a-graduate-seminar/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SSERVI&lt;/a&gt;.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-04-30T07:16:37.377" Title="What happens to Lunar (or asteroidal) regolith if sunlight is concentrated on it?" Tags="|isru|thermal|regolith|sunlight|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21308" PostTypeId="2" ParentId="21306" CreationDate="2017-04-30T09:18:52.143" Score="8" Body="&lt;p&gt;This question is not actually related to space, except insofar as comms in space could be picked up from anywhere on Earth (as opposed to surface comms which may be limited to line of sight or short range)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real question to ask is: what type of comms do you need to protect, either through encryption to keep them private, or through authentication to ensure the right comms are being acted upon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For control of craft, strong security is useful, to ensure the craft is not hijacked in any way, and for satellites with military applications, this is also true. But it costs, in terms of bandwidth and processing power, so especially for low bandwidth comms you are right that for some craft (e.g. Cassini), sending their signal home in the clear makes a lot of sense, as you don't need to protect that data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have a read of &lt;a href=&quot;https://security.stackexchange.com/q/6424/485&quot;&gt;this question on what you would need to hijack a satellite&lt;/a&gt; over on Security Stack Exchange.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="1142" LastEditDate="2017-11-12T23:18:00.140" LastActivityDate="2017-11-12T23:18:00.140" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21309" PostTypeId="1" AcceptedAnswerId="21315" CreationDate="2017-04-30T09:33:10.393" Score="2" ViewCount="173" Body="&lt;p&gt;Given the Keplerian elements for an object for current time. The object is on ellyptic orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How to calculate, at what time the object will  be on one of the orbital nodes? That's required for inclination change maneuver.&lt;/p&gt;&#xA;" OwnerUserId="19219" LastEditorUserId="19219" LastEditDate="2017-05-07T18:11:38.100" LastActivityDate="2017-05-07T18:11:38.100" Title="Flying to orbital nodes" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21310" PostTypeId="1" CreationDate="2017-04-30T09:43:08.497" Score="2" ViewCount="474" Body="&lt;p&gt;There's &lt;a href=&quot;http://www.spacepolicyonline.com/news/space-station-spacesuits-suffer-after-shuttle-shutdown&quot; rel=&quot;nofollow noreferrer&quot;&gt;a problem&lt;/a&gt; with maintaining the EVA suits on the space station. Since the Shuttle was canceled, only recently has Dragon made it possible to bring the suits down to ground for refurbishment, the article says. And the Dragon has had two longer interruptions and even destroyed one spacesuit on CRS-7 in 2015.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't there room for one or more of three astronauts to wear an EVA suit during landing with the Soyuz, in order to bring it home? Are there other problems, like attachment to the seat?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-04-30T13:40:20.573" Title="Could astronauts wear EVA suits in the Soyuz?" Tags="|iss|soyuz-spacecraft|spacesuits|logistics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21312" PostTypeId="2" ParentId="21310" CreationDate="2017-04-30T12:39:50.187" Score="11" Body="&lt;p&gt;The Soyuz seat liners are &lt;a href=&quot;https://books.google.com/books?id=ra4eCAAAQBAJ&amp;amp;pg=PP125&amp;amp;lpg=PP125&amp;amp;dq=soyuz%20seat%20liner%20molding&amp;amp;source=bl&amp;amp;ots=wsSDLBndza&amp;amp;sig=mnBt612pf0mN_gXbyP2PUIKGx2U&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjb0qm8pszTAhUo0oMKHXKGAbAQ6AEISTAJ#v=onepage&amp;amp;q=soyuz%20seat%20liner%20molding&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;carefully molded to each crewperson individually by special craftsmen&lt;/a&gt;; this is supposedly vital to successfully experiencing the shocks and accelerations of landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wearing an EMU in the seat would make the seat liner not fit properly, exposing the crew person to possible injury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.flickr.com/photos/thom_astro/sets/72157650934017941/&quot; rel=&quot;noreferrer&quot;&gt;Photos of the mold process&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-04-30T13:40:20.573" LastActivityDate="2017-04-30T13:40:20.573" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21315" PostTypeId="2" ParentId="21309" CreationDate="2017-04-30T23:13:48.607" Score="3" Body="&lt;p&gt;Using the definition of Keplerian elements:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/EGcGv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EGcGv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then it can be seen that the angle between the ascending- and descending node and the periapsis will be equal to $-\omega + 2\,k\,\pi$ and $\pi-\omega + 2\,k\,\pi$ for all integers $k$ respectively. The first node that you will come across will be the first of these two values which becomes larger than $\nu$ as you increase $k$. The time can be calculated with &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler%27s_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler's equation&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;M = E + e \sin E, \tag{1}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but for this you do first need to the convert these angles (in radians) into the eccentric anomaly. Another way is rewriting $(1)$ into the original angles, which yields,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;t=\sqrt{\frac{a^3}{\mu}}\left[2\tan^{-1}\left(\frac{\sqrt{1-e^2}\tan{\frac{\nu}{2}}}{1+e}\right)-\frac{e\, \sqrt{1 - e^2}\sin{\nu}}{1 + e\cos{\nu}}\right] \tag{2}.&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both equations can give you the time since periapsis passage, which is positive when $E$ or $\nu$ is between $0$ and $\pi$ and negative when between $-\pi$ and $0$. In both cases you need to take the difference between the two times (at the current position and at the node). However you do have to be a bit careful when crossing $\pm\pi$ radians, because you might have to add or subtract the total period of the orbit once to get the actual time till node.&lt;/p&gt;&#xA;" OwnerUserId="1086" LastActivityDate="2017-04-30T23:13:48.607" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21316" PostTypeId="2" ParentId="21294" CreationDate="2017-05-01T05:42:53.443" Score="-2" Body="&lt;p&gt;You could make the argument that the X-37B is a stealth satellite, although technically a drone, and it &lt;a href=&quot;http://www.space.com/36205-x-37b-space-plane-breaks-record.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;spent some 675 days in space recently&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="19464" LastEditorUserId="6816" LastEditDate="2017-05-01T05:58:25.743" LastActivityDate="2017-05-01T05:58:25.743" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21319" PostTypeId="2" ParentId="21299" CreationDate="2017-05-01T10:56:10.713" Score="1" Body="&lt;p&gt;If we neglect various perturbations and assume just Keplerian motion then the motion is planar and it all comes from basic geometry. If you make some idealized maneveur (instanteneous change of velocity) in some point then this point should lie on the initial orbital plane and simultaneously on the final orbital plane. So the point of this idealized maneuver &lt;strong&gt;must&lt;/strong&gt; be at the intersections of the initial and desired orbital planes. If the maneuver is performed anywhere else the resulting orbit will never be coplanar with the desired one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because there are two orbits involved of course you can't say anything general about true anomaly of these intersection points. The periapsis/apoapsis line may have any angle with the desired orbital plane.&lt;/p&gt;&#xA;" OwnerUserId="18138" LastActivityDate="2017-05-01T10:56:10.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21320" PostTypeId="2" ParentId="21293" CreationDate="2017-05-01T11:36:54.547" Score="14" Body="&lt;p&gt;They threw out the PLSS, but that didn't leave them without the use of those space suits entirely. The space suits could be coupled via umbilicals to the LEM ECLSS or to the CM life support system (this was used on Apollo 15-17 for &lt;a href=&quot;http://www.ninfinger.org/karld/My%20Space%20Museum/simbay.htm&quot; rel=&quot;noreferrer&quot;&gt;EVAs to retrieve material from the Service Module&lt;/a&gt; during the flight back to Earth).&lt;/p&gt;&#xA;&lt;p&gt;The PLSS could &lt;a href=&quot;https://en.wikipedia.org/wiki/Primary_Life_Support_System&quot; rel=&quot;noreferrer&quot;&gt;store consumables for 4 hours&lt;/a&gt; of use (8 hours for later flights). After an EVA, the PLSS was &lt;a href=&quot;https://www.hq.nasa.gov/alsj/plss.html&quot; rel=&quot;noreferrer&quot;&gt;recharged&lt;/a&gt; from larger supplies on the LEM descent stage.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Fully charged PLSSs were loaded onto the LM before launch, which saved considerable time before the first EVA. For subsequent EVAs, the crews retrieved fresh batteries and lithium hydroxide canisters during EVA close-out activities for use the next day. Recharging the PLSS was a six-step process. Usually done as part of the EVA prep, a few crews used spare time at the end of their workday to replenish the PLSS consumables. The process took about 30 minutes for each suit, and each crewmember worked on their own equipment. By staggering tasks, the entire process took less than an hour.&lt;/p&gt;&#xA;&lt;p&gt;First, the battery and the lithium hydroxide canister were exchanged for fresh units. Used batteries and canisters were stored in large bags that were thrown under the descent stage at the beginning of the next day's EVA, or jettisoned out the front hatch after the last EVA of the mission. Next, oxygen cylinders in the PLSSs were charged from the descent stage high-pressure oxygen supply in a two step process. First, a charge that filled the PLSS O2 cylinders to about 90% capacity was performed. After a few minutes (to allow the cylinders to cool), the O2 supply was &amp;quot;topped off&amp;quot; to about 95% to 98%. Finally, a three-step procedure was used to service the water management system. Step 1 was to recharge the cooling water supply. Draining the waste water was the second step, and the third step was to vent out the excess gas from the cooling water system. Bubbles formed by such gas could interfere with the flow of cooling water in the suit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;After the last moon EVA, I suppose the astronauts could have recharged the PLSS one more time (if consumables on board the LEM were enough to do that). That would have given 2 astronauts another 8 hours of oxygen each, at a cost of almost 60 kg per PLSS. Parts of the PLSS would not be useful on the flight back (the water cooling system, for instance, was evaporative and would not be pleasant to use inside the CSM/LM).&lt;/p&gt;&#xA;&lt;p&gt;The LEM already had redundant life support systems, and it was a short ride away from the CSM which had more supplies.&#xA;I haven't found a calculation for the weight savings necessary for the LEM to take off successfully. That's maybe a topic to explore in a separate question. Apollo 17 took off with slightly over 100 kg of lunar rock samples on board.&lt;/p&gt;&#xA;&lt;p&gt;Via &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/001160.html&quot; rel=&quot;noreferrer&quot;&gt;this discussion&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;From the Apollo 17 chapter of the Apollo Lunar Surface Journal (ALSJ):&lt;/p&gt;&#xA;&lt;p&gt;[Comm Break. The crew has a small spring scale which gives them terrestrial weights. That is, on Earth, a two-pound (Earth-weight) rock would register twelve pounds on the scale while, on the Moon, it would register two pounds. The actual weight of rocks collected during this EVA is given in the Mission Report as 31 pounds (14 kg). The 48-pound (21.8 kg) total given here includes the weights of the individual sample bags, the two 1.7 pound (0.8 kg) SCBs, and the 14.7 pound (6.7 kg) SRC. Weights are measured so that the flight engineers can adjust the stowage for center-of-mass control, if necessary.]&lt;/p&gt;&#xA;&lt;p&gt;This also would determine how much gear (Backpacks, Overshoes, Food Bags,etc) would be jettisoned prior to launch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So the amount of samples that could be returned, very much depended on the amount of weight they could discard. From the same source:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;1975 figures from NASA give the returned sample weights as 11 - 19.9kgs;&lt;/p&gt;&#xA;&lt;p&gt;After a lot of searching, I found that the Apollo 11 maximum sample allocation was 50 lbs, or about 23 kg. I would assume then that the weights are Earth weights.&lt;/p&gt;&#xA;&lt;p&gt;The ALSJ also states that the &amp;quot;rock sample limit&amp;quot; on Apollo 16 was 215 lbs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Interestingly, the &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=1805.0&quot; rel=&quot;noreferrer&quot;&gt;PLSS was used on Apollo 13&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;I will offer one further clarification, here -- they &lt;em&gt;did&lt;/em&gt; run the suit circuit for a while during Apollo 13, but did so &lt;em&gt;instead&lt;/em&gt; of running the cabin fans.  The hoses that would normally have connected to the suits were arrayed around the cabin to provide circulation -- and the makeshift adapted CSM lithium hydroxide CO2 scrubber was attached to one of the inlet hoses.&lt;/p&gt;&#xA;&lt;p&gt;But, you will note, this setup was run &lt;em&gt;instead&lt;/em&gt; of the cabin fans, and was used not to cool two fully suited crewmen, but to circulate air for all three.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-01T12:35:35.863" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21324" PostTypeId="1" AcceptedAnswerId="21325" CreationDate="2017-05-01T13:55:31.880" Score="14" ViewCount="5904" Body="&lt;p&gt;In SpaceX launch on May 1st, 2017, we get to see the full flight path of the first stage rocket of the falcon 9 from launch to land:&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/EzQpkQ1etdA?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It accelerates to around 1685m/s at an altitude of 68.5km (minute 14:21 in the video), then separates and starts decelerating still going up. It reaches a maximum altitude of 165km before starting to fall back down (at around 16:35 in the video).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is - at the maximum altitude, why is the speed ~300m/s and not 0? Is the speed calculated relative to something else other than the earth's ground?&lt;/p&gt;&#xA;" OwnerUserId="19484" LastEditorUserId="19484" LastEditDate="2017-05-09T13:06:24.990" LastActivityDate="2017-05-09T13:06:24.990" Title="SpaceX rocket strange velocity" Tags="|spacex|rockets|falcon-9|acceleration|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21325" PostTypeId="2" ParentId="21324" CreationDate="2017-05-01T14:18:21.977" Score="20" Body="&lt;p&gt;Two possibilities: The Earth rotates at about 465 m/s at the equator.  This video's measurements could include that speed, though I don't believe so as it starts from 0 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most likely it's because the rocket is traveling in a horizontal arc during its travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets trying to reach orbit need to reach a &lt;strong&gt;horizontal speed of around 7.8 km/s&lt;/strong&gt; in addition to the height.  In fact, the vast majority of a rocket's energy to get into an orbit is horizontal movement, as reaching the height necessary to get into space is comparatively easy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the time the rocket first stage passes its apogee it still has significant horizontal velocity to burn off, which is why it's still traveling at 300 m/s~ at that time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a visual example of the flight profile (not to scale) of the Falcon 9 Drone Ship landing&#xA;&lt;a href=&quot;https://i.stack.imgur.com/uqtNQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uqtNQ.jpg&quot; alt=&quot;Falcon 9 Launch to Drone Landing&quot;&gt;&lt;/a&gt;&#xA;(Source: &lt;a href=&quot;https://spaceflight101.com/falcon-9-ses-10/flight-profile/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight101.com&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And an example of the profile with a return to launch site landing&#xA;&lt;a href=&quot;https://i.stack.imgur.com/FimTp.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FimTp.png&quot; alt=&quot;Falcon 9 Launch to Return Landing&quot;&gt;&lt;/a&gt;&#xA;(Source: &lt;a href=&quot;https://space.stackexchange.com/a/13141/6773&quot;&gt;Space.StackExchange.com post&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some further interesting information, include a GTO Profile (One where the rocket first stage does not return to a landing site) can be found in the &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Falcon 9 User's Guide&lt;/a&gt; on the SpaceX website.&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorUserId="6471" LastEditDate="2017-05-01T20:17:17.683" LastActivityDate="2017-05-01T20:17:17.683" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21327" PostTypeId="2" ParentId="19368" CreationDate="2017-05-01T15:06:03.457" Score="3" Body="&lt;p&gt;In section 3 of your reference, it states &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The EROS-A legacy pass-file contains all metadata in one file, but requires a lot of exceptional processing associated with [sic] customized coordinate system (so-called Q-frame).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Satellites usually have many frames associated with different aspects of the body of the satellite. For instance, there will be at least one star-tracker reference frame, so that star measurements can be easily computed in that frame. If the satellite is pointing a laser towards earth, there will be another frame associated with the direction of the laser. There will be another frame associated with the ACS, and indeed every other portion of the satellite from which calculations need to be made. For sharing data between different components of the satellite, everything is usually converted into one frame that is of primary importance. Going back to the laser, if the primary mission for the satellite dealt with the laser, the laser reference system would probably be the most important frame, and thus most measurements would be converted to that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply put, this &quot;q-frame&quot; is almost certainly just a reference frame associated with an important part of the satellite. All measurements are represented in it to limit ambiguity.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastActivityDate="2017-05-01T15:06:03.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21329" PostTypeId="2" ParentId="11975" CreationDate="2017-05-01T18:06:56.690" Score="6" Body="&lt;p&gt;The target lift-to-drag ratio (L/D) also plays an important role. A high L/D provides greater aerodynamic control authority during hypersonic entry, providing engineers with greater flexibility in terms of controlling the maximum heating rate and maximum g-load, as well as achieving cross range and targeting objectives. Early in the Apollo Program, the capsule designers were targeting L/D ratios as high as 0.5, which requires a significant angle of attack. Thus the sidewall angle of the Apollo capsule was set at roughly 32.5 degrees to minimize direct flow impingement and the associated heating effects on the sidewall, which would impact the thermal protection system (TPS) design and materials. As the Apollo capsule design evolved, engineers found it difficult to achieve the target L/D because it requires a substantial offset to the center of gravity (cg) to achieve the required angle of attack (AoA). In addition, a low cg (closer to the base heat shield) improves aerodynamic stability, but a greater lateral offset is required to achieve the same AoA. This conflicts with the fact that, as designers pack more and more equipment into a capsule, the cg tends to converge toward the centerline. Ballast can be used to influence the cg location, but every extra kilogram in the Apollo capsule flowed through and impacted the mass of the Service Module and the ability of the Saturn V rocket to inject the CSM into the trans-lunar trajectory. In the end, the capsule design represents a compromise across many factors including internal volume/dimensions, ballistic coefficient, L/D, TPS materials and thicknesses, etc. The Apollo capsule ended up flying at a L/D of ~0.32.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soyuz capsule, in contrast, has a very shallow sidewall (I seem to recall a value of 7.5 degrees). Its more cylindrical shape provides greater volume for a given base diameter. But it is also flown at a lower angle of attack, achieving an L/D in the range of 0.18 or so. The Soyuz capsule provides excellent stability, but pulls fairly high g-loads and has limited control. But it is a very efficient design, as well, and has proven itself over a large number of missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a very brief and simplistic treatment of the topic. As noted in other posts, the launch environment and abort scenarios introduce their own design influences to a crew capsule, as does the landing scenario (e.g., land versus water). If you enjoy this type of intellectual exercise, then you might want to consider a career in aerospace engineering.&lt;/p&gt;&#xA;" OwnerUserId="19490" LastActivityDate="2017-05-01T18:06:56.690" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21330" PostTypeId="1" AcceptedAnswerId="21332" CreationDate="2017-05-01T18:46:46.373" Score="14" ViewCount="3968" Body="&lt;p&gt;Why didn't the space shuttle solid rocket boosters have wings and tires to land the same way the orbiter lands? I don't think they haven't thought of that so there must be something that led them not to choose that design, right?&lt;/p&gt;&#xA;" OwnerUserId="19492" LastEditorUserId="6816" LastEditDate="2017-05-01T19:05:53.350" LastActivityDate="2017-05-03T16:41:19.367" Title="Why didn't the space shuttle SRBs have wings and tires?" Tags="|space-shuttle|design|reuse|srb|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21331" PostTypeId="2" ParentId="21129" CreationDate="2017-05-01T18:51:54.160" Score="3" Body="&lt;p&gt;What you are asking for is called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Free_return_trajectory&quot; rel=&quot;nofollow noreferrer&quot;&gt;free return trajectory&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will not be debugging your program or solving the problem for you (that's your homework), but I will pass on some advice given by Bate, Mueller &amp;amp; White*:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;There is more than one solution.&lt;/li&gt;&#xA;&lt;li&gt;Don't expect an exact solution from whatever your initial conditions may be.&lt;/li&gt;&#xA;&lt;li&gt;Start off by using the patched conic method.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You can then refine the approximate answer with a more exact numerical integration. If your integrator gives a wildly different answer from patched conics, try inputting some simple Earth orbits and see if the answers make sense. You can check the orbit against an analytical method for perturbations by the moon. It is likely that the integrator is buggy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Use the patched conic approximation to get a feel for how the evolution of the system is affected by small changes in the initial conditions. Think of what happens at the patch points.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Fundamentals of Astrodynamics, 1971, Dover, exercise 7.12 on p. 354.&lt;/p&gt;&#xA;" OwnerUserId="16733" LastActivityDate="2017-05-01T18:51:54.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21332" PostTypeId="2" ParentId="21330" CreationDate="2017-05-01T19:01:44.790" Score="19" Body="&lt;p&gt;Wings, engines to fly back to the launch site, and landing gear are heavier than parachutes.  Using this design on the SRBs could have resulted in a significant payload to orbit reduction (&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960047493.pdf&quot; rel=&quot;noreferrer&quot;&gt;every 10 pounds added to the SRBs resulted in 1 pound less payload that could be carried to orbit&lt;/a&gt;).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, this was studied for the case of &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980231024.pdf&quot; rel=&quot;noreferrer&quot;&gt;liquid fueled flyback boosters&lt;/a&gt;.  Another interesting shuttle upgrade that never went through.  And, resulting in a crew member comment &quot;I guess we would be 3rd in line for the runway if we had to do an RTLS.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CHilO.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CHilO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tNTgcm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tNTgcm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-05-01T20:17:10.643" LastActivityDate="2017-05-01T20:17:10.643" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="21333" PostTypeId="1" AcceptedAnswerId="40413" CreationDate="2017-05-01T19:37:46.617" Score="13" ViewCount="1120" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/37999/12102&quot;&gt;This answer&lt;/a&gt; links to &lt;a href=&quot;http://lkellogg.vttoth.com/LarryRussellKellogg/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Larry Russell Kellogg&lt;/a&gt; which links to &lt;a href=&quot;http://lkellogg.vttoth.com/LarryRussellKellogg/HybridTest.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Tests Environmentally Friendly Rocket Fuel&lt;/a&gt; where images of &lt;em&gt;Test firing of new Paraffin-Based Fuel at NASA Ames Research Center 1/15/2003&lt;/em&gt; can be seen.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/08n9l.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/08n9l.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;For more on paraffin-based work see &lt;a href=&quot;https://www.nasa.gov/centers/ames/news/releases/2003/03images/paraffin/paraffin.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/centers/ames/news/releases/2003/03images/paraffin/paraffin.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;I've just read the NASA news item &lt;a href=&quot;https://www.nasa.gov/feature/ames/from-pedicures-to-the-peregrine-rocket-paraffin-wax-proves-its-worth&quot; rel=&quot;nofollow noreferrer&quot;&gt;From Pedicures to the Peregrine Rocket, Paraffin Wax Proves Its Worth&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The project started when researchers at Stanford discovered that paraffin fuel burns three times faster than conventional fuels, and therefore can provide more thrust and higher performance than existing hybrid rockets. They approached Ames for assistance in measuring the burn rate at larger scales and to contribute to the maturation of the technology.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I think the &amp;quot;three times faster&amp;quot; means that a given volume (or mass) of the solid wax propellant in a given shape could be consumed in one third the time of a conventional solid propellant for hybrid engines, and so that factor of three speaks directly to thrust-to-weight ratio of the engine.&lt;/p&gt;&#xA;&lt;p&gt;I'm not sure what conventional is, but I'm guessing it's one of the synthetic &amp;quot;rubbers&amp;quot; like &lt;a href=&quot;https://en.wikipedia.org/wiki/Hybrid-propellant_rocket#Common_fuel_choices&quot; rel=&quot;nofollow noreferrer&quot;&gt;HTPB&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Why would wax burn roughly three times faster than other fuels? And why the sudden interest now — is there some specific technical challenge that has been potentially solved allowing for greater thrust/weight, or is there a need for a low temperature robust hybrid rocket on Mars (see below and in link) and the factor of three is a secondary benefit?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The paraffin-based fuel also works under challenging environmental conditions, like the very low temperatures found on the surface of Mars. The Mars Ascent Vehicle, currently under development by NASA’s Jet Propulsion Laboratory, aims to use the paraffin technology to return a sample from the surface of the planet to an orbiting spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;CAUTION:&lt;/strong&gt; lower your volume before playing...&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ZrswPmPQiy8?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;An nice summary of &lt;a href=&quot;http://www.eucass-proceedings.eu/articles/eucass/pdf/2012/01/eucass2p353.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;hybrid fuel combinations&lt;/a&gt; was linked in &lt;a href=&quot;https://space.stackexchange.com/a/21220/12102&quot;&gt;this insightful answer&lt;/a&gt;, and some of the links in &lt;a href=&quot;https://space.stackexchange.com/q/14456/12102&quot;&gt;Does the NK-33 engine require subcooled kerosene so cold that it turns to wax?&lt;/a&gt; helps one appreciate that one can think of wax as being more like solid kerosene than it is like rubber.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="195" LastEditDate="2022-10-02T23:40:18.553" LastActivityDate="2022-10-02T23:40:18.553" Title="Why NASA's sudden renewed (apparently) interest in Wax hybrid engines? Which property is so attractive now?" Tags="|mars|sample-return|hybrid-rocket|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="21335" PostTypeId="2" ParentId="21330" CreationDate="2017-05-01T20:03:16.360" Score="7" Body="&lt;p&gt;In addition to the booster upgrade proposals that Organic Marble mentions, some early shuttle proposals considered using a single large liquid-fueled winged booster that would fly back to the launch site. The development budget for the shuttle didn't allow that strategy to be pursued.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.nss.org/resources/library/shuttledecision/chapter08.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.nss.org/resources/library/shuttledecision/chapter08.htm&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-05-01T20:03:16.360" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21336" PostTypeId="1" AcceptedAnswerId="21337" CreationDate="2017-04-28T18:11:50.730" Score="1" ViewCount="324" Body="&lt;p&gt;The Voyager 1 took one last photograph in 1990 and after that, the camera equipment was shut off for power saving. However, the estimates of where the photo was taken in space vary by many hundreds of thousand of miles - not an exact point in space. The Voyager probe is old, however, and I'm wondering, how well can we estimate a point in 3D space today? Would a spacecraft that was built today be able to say exactly where it was when it took a picture?&lt;/p&gt;&#xA;" OwnerUserId="19506" OwnerDisplayName="Takeru Nakajima" LastEditorUserId="12102" LastEditDate="2017-05-02T02:33:46.623" LastActivityDate="2017-06-06T21:37:22.413" Title="Satellite Position" Tags="|voyager|navigation|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21337" PostTypeId="2" ParentId="21336" CreationDate="2017-04-29T10:55:03.153" Score="2" Body="&lt;p&gt;The precision of position of the space probes depends mostly on the amount of money spent on the navigation sensors and the nav equipment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today at any distance of deep space we can invest in &lt;a href=&quot;https://en.wikipedia.org/wiki/X-ray_pulsar-based_navigation&quot; rel=&quot;nofollow noreferrer&quot;&gt;X ray pulsar based navigation&lt;/a&gt;, for which one test system has been launched on Neutron Star Interior Composition Explorer, NICER. it gives 5km precision using pulsar periods and requires lighter and smaller antenna to capture the x rays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other advanced equipments are &lt;a href=&quot;http://ieeexplore.ieee.org/document/4066860/&quot; rel=&quot;nofollow noreferrer&quot;&gt;self contained on-board nav systems&lt;/a&gt; made of sensors and celestial models that can self correct the course using optical measures and an onboard sky map of given precision. They are used together with a fixed space coordinate system with origin on the solar barycenter and fixed relative to the constellations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Prior to 2017, Deep space position was imprecise, and fine control was done relative to the approached object with cameras, where the approached object is unkown to several km/ 100ds of km. it consists of 1/velocity 2/position 3/movement vector 4/space object map... all of those factors are being improved massively all the time, and the precision only depends on the nav equipment budget.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radio Doppler measurement of the shifted radio signal gives very good precision of radial movement from the terrestrial receivers, to within 1 mm/s, and two radio receivers situated around the planet can determine the movement vector fairly well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The position is more difficult, for example Cassini's position was known relative to Saturn by 1km and Saturn was know relative to earth by 100km. &lt;a href=&quot;https://www.nasa.gov/jpl/cassini/scientists-pinpoint-saturn-with-exquisite-accuracy&quot; rel=&quot;nofollow noreferrer&quot;&gt;(after Cassini this was refined to 2km relative to us).&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are checking all kinds of new equipment to &lt;a href=&quot;http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1241768&quot; rel=&quot;nofollow noreferrer&quot;&gt;increase precision,&lt;/a&gt; for example &lt;a href=&quot;https://www.osapublishing.org/ao/viewmedia.cfm?uri=ao-27-21-4487&amp;amp;seq=0&quot; rel=&quot;nofollow noreferrer&quot;&gt;Laser doppler&lt;/a&gt; between satellites using optical costas loops and helium-neon laser and other precise wavelengths. A test of optical doppler was done on a previous moon mission to improve positioning and data coms up to a distance of &lt;a href=&quot;https://www.nasa.gov/content/goddard/laser-demonstration-reveals-bright-future-for-space-communication&quot; rel=&quot;nofollow noreferrer&quot;&gt;250,000 miles (&gt;earth-moon distance)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If voyager used a 30 megapixel camera and a very good zoom+moving camera, it could determine it's position many many times more finely than the 1970's version. The sky map is better too. the radio Doppler is still used today but is more precise, and the optical Doppler doesn't go that far. It's position relative to visited objects was much higher resolution than the position of the objects visited.&lt;/p&gt;&#xA;" OwnerUserId="17876" OwnerDisplayName="comprehensible" LastEditorUserId="17876" LastEditDate="2017-06-06T21:37:22.413" LastActivityDate="2017-06-06T21:37:22.413" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21338" PostTypeId="2" ParentId="21324" CreationDate="2017-05-01T21:59:56.943" Score="0" Body="&lt;p&gt;The velocity is calculated with reference to a sort of 'Earth's extended atmosphere'. Imagine yourself flying in a small plane, with a handheld GPS. That instrument gives your speed with respect to the 'Earth's absolute space'. If you fly horizontally, the GPS will give a horizontal velocity, and if you dive vertically, you'll also get the (vertical, in this case) velocity. And if the plane climbs at 45º to the horizontal, you'll get the speed along that trajectory, too...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this rocket, it's the same thing...&lt;/p&gt;&#xA;" OwnerUserId="18814" LastActivityDate="2017-05-01T21:59:56.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21339" PostTypeId="1" CreationDate="2017-05-02T01:15:43.343" Score="2" ViewCount="203" Body="&lt;p&gt;I am currently studying DSN related stuff, and for that purpose, I need to know some details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am currently studying the &lt;a href=&quot;https://deepspace.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt;, and I am trying to get a better picture of how DSN communications &quot;traffic&quot; is managed. How is DSN &quot;traffic&quot; prioritized? What happens if there is an emergency? Has routine communication ever been blocked or rescheduled to make way for critical communication?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://ai.jpl.nasa.gov/public/home/bclement/papers/IAAI052ClementB.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Deep Space Network Scheduling Problem &lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19002" LastEditorDisplayName="user10509" LastEditDate="2017-05-03T14:45:04.820" LastActivityDate="2017-05-03T14:45:04.820" Title="What type of comms traffic for deep-space missions needs to be prioritized?" Tags="|communication|deep-space-network|emergency|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21341" PostTypeId="1" CreationDate="2017-05-02T03:18:47.200" Score="7" ViewCount="242" Body="&lt;p&gt;I have found this text in an article:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;The most recent test saw the missile explode about two or three minutes after launch. According to South Korean military experts, &lt;strong&gt;the missile reached an altitude three times higher than &quot;stabilizing height&lt;/strong&gt;, which means the chances that any internal mechanical failure caused the explosion were very low,&quot; the Korea Times reported.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://sputniknews.com/asia/201705011053177882-pyongyang-intentional-missile-test-failure/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What can this &quot;stabilizing height&quot; be? &lt;/p&gt;&#xA;" OwnerDisplayName="user16613" LastEditorUserId="26615" LastEditDate="2018-08-09T13:45:27.147" LastActivityDate="2018-08-09T22:40:54.627" Title="What do they mean by &quot;stabilizing height&quot;" Tags="|stability|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21343" PostTypeId="1" AcceptedAnswerId="21355" CreationDate="2017-05-02T04:06:41.830" Score="10" ViewCount="5387" Body="&lt;p&gt;From NASA Spaceflight's article &lt;a href=&quot;https://www.nasaspaceflight.com/2017/04/spacex-falcon-9-first-nro-mission-nrol-76/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Falcon 9 launches first NRO mission with NROL-76&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The NROL-76 launch was not to be targeting a sun-synchronous orbit, typically used by the NRO’s optical imaging satellites. While it is possible to reach such orbits from the Kennedy Space Center or Cape Canaveral, the rocket would have to perform a series of dogleg maneuvers that would severely limit its payload capacity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What does this mean? Is there a diagram illustrating what these maneuvers might look like? Why a series of them instead of one, presumably longer maneuver? Has this kind of thing been done recently?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; I have found &lt;em&gt;dogleg&lt;/em&gt; mentioned in &lt;a href=&quot;https://sattrackcam.blogspot.tw/2017/04/what-is-nrol-76-and-what-orbit-wil-it.html&quot; rel=&quot;noreferrer&quot;&gt;this blog&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-09T03:07:50.590" LastActivityDate="2021-08-09T03:07:50.590" Title="How does one dogleg from Florida to a sun-synchronous orbit?" Tags="|launch|launch-site|sun-synchronous|dogleg-maneuver|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21344" PostTypeId="1" AcceptedAnswerId="21350" CreationDate="2017-05-02T04:19:19.903" Score="2" ViewCount="260" Body="&lt;p&gt;I came across an article with the following infographic:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/0CnOF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0CnOF.jpg&quot; alt=&quot;GSLV MkII vs. MkIII vs. Ariane V&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;infographic &lt;a href=&quot;http://m.economictimes.com/news/science/gslv-mark-iii-launch-why-isros-biggest-challenge-will-be-at-the-end-of-this-month/articleshow/58466879.cms&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GSLV rockets use solid/liquid engines as core stage and cryogenic engine as upper stage while the Ariane V rocket has an inverted configuration. What are the benefits/drawbacks for such arrangements performance or costwise?&lt;/p&gt;&#xA;" OwnerUserId="17309" LastEditorUserId="17309" LastEditDate="2017-05-02T09:03:31.347" LastActivityDate="2017-05-03T08:18:51.903" Title="Which of these configurations is preferred and why?" Tags="|isro|ariane|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21345" PostTypeId="1" AcceptedAnswerId="21349" CreationDate="2017-05-02T04:30:15.280" Score="11" ViewCount="3439" Body="&lt;p&gt;Here is my understanding, let me know if I've got it right or wrong:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Until recently, most cryogenic propellants were at or close to their boiling points. Each unit of heat leaking into the tank would then boil off a corresponding unit of propellant and the temperature would stay constant. This is roughly like adding near boiling &quot;make-up&quot; water to a pot of boiling water. It might be a little cooler, but not much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in the case of sub-cooled LOX, the temperature is way below the boiling point, somewhere between 10 and 30 degrees C depending on the case. When heat leaks into the tank, it will raise the temperature of the LOX which then expands. A full tank would then overflow - remaining full  but with &lt;em&gt;less mass&lt;/em&gt; of LOX due to the density change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be no boil-off to &quot;make up&quot; by topping it off. It would &lt;em&gt;remain full&lt;/em&gt; but steadily decrease in density as it warmed up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a rocket with first stage tank filled with well sub-cooled LOX, why would it need to be &quot;topped-off&quot; until the last minute or so before launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/EzQpkQ1etdA?t=412&quot; rel=&quot;noreferrer&quot;&gt;This video&lt;/a&gt; at &lt;code&gt;T -00:05:05&lt;/code&gt;, video time 06:52, my approximate transcription of the narration: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Right now we’re just coming up on the five minute mark here, we are concluding the loading of the RP-1 on to the first stage […] and we are topping-off liquid oxygen as well on that first stage, and we’re going to keep topping that off for about two more minutes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The video is already queue'd up at the appropriate time code (in this case I think the video edit is final):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/EzQpkQ1etdA?start=412&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-02T05:28:05.407" LastActivityDate="2021-12-08T23:17:45.940" Title="Why would sub-cooled LOX tanks need to &quot;topped-off&quot; until the last minute or so?" Tags="|launch|lox|sub-cooled|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21346" PostTypeId="1" AcceptedAnswerId="21368" CreationDate="2017-05-02T05:33:34.813" Score="3" ViewCount="1795" Body="&lt;p&gt;Watching the video of the &lt;a href=&quot;https://www.youtube.com/watch?v=EzQpkQ1etdA&quot; rel=&quot;nofollow noreferrer&quot;&gt;1 May NROL launch&lt;/a&gt;, I was surprised at the frequency and apparent size of the cold gas thruster firings. Does anyone know approximately how many kilograms of N2 are required to be loaded to fuel these firings? Is the loading a standard quantity or are there different requirements for a sea or land recovery?&lt;/p&gt;&#xA;" OwnerUserId="14276" LastEditorUserId="8693" LastEditDate="2017-05-02T07:32:50.267" LastActivityDate="2017-05-03T12:56:12.463" Title="How many kilo's of nitrogen does Falcon 9 load for its first stage cold gas thrusters?" Tags="|spacex|falcon-9|engines|first-stage|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21347" PostTypeId="1" AcceptedAnswerId="21352" CreationDate="2017-05-02T08:15:13.293" Score="10" ViewCount="1349" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaster#Re-entry_timeline&quot; rel=&quot;noreferrer&quot;&gt;According to Wikipedia&lt;/a&gt;, the earliest time at which there was a known issue with &lt;em&gt;Columbia&lt;/em&gt;'s reentry was at 08:53:46, when &quot;Various people on the ground saw signs of debris being shed&quot;. Approximately 7 minutes later, the orbiter was lost with all hands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hypothetically, if NASA had immediately been informed of the debris shedding and had immediately known it was due to left wing heat shield damage, what - if any - options would they have been able to provide to the STS-107 crew to prevent the tragic loss of life that ensued? In other words, would it have been possible to abort the reentry and return to orbit to wait for &lt;em&gt;Atlantis&lt;/em&gt;, or somehow alter the reentry trajectory to minimise the impact to the damaged part of the heat shield, or ...?&lt;/p&gt;&#xA;" OwnerUserId="6449" LastEditorUserId="29286" LastEditDate="2020-08-31T18:18:08.307" LastActivityDate="2020-08-31T18:18:08.307" Title="What, if any, abort options would the crew of Columbia had on reentry during STS-107, assuming they became aware of the damage during that phase?" Tags="|space-shuttle|failure|shuttle-columbia|" AnswerCount="1" CommentCount="3" ClosedDate="2017-05-03T13:24:51.287" ContentLicense="CC BY-SA 3.0" />
  <row Id="21348" PostTypeId="1" CreationDate="2017-05-02T08:36:56.927" Score="3" ViewCount="331" Body="&lt;p&gt;I am currently developing a 3U CubeSat and following the &lt;a href=&quot;https://static1.squarespace.com/static/5418c831e4b0fa4ecac1bacd/t/56e9b62337013b6c063a655a/1458157095454/cds_rev13_final2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Design specifications Rev. 13&lt;/a&gt;. there are 3 access ports. Must I provide 3 access ports or 1 access port in one of the locations defined by the specifications?&lt;/p&gt;&#xA;" OwnerUserId="19503" LastEditorUserId="11653" LastEditDate="2017-05-02T15:42:46.793" LastActivityDate="2018-12-09T17:15:39.330" Title="Number of Access Ports" Tags="|cubesat|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21349" PostTypeId="2" ParentId="21345" CreationDate="2017-05-02T08:57:00.537" Score="36" Body="&lt;p&gt;At a pressure of 1 bar, the temperature of liquid boiling oxygen stabilizes at 90 K. For sub-cooling of LOX, the temperature should be lower. It is possible to cool LOX by forced evaporation by a pressure lower than 1 bar. But the LOX tank in a rocket should be as light as possible. If the pressure inside the tank is substantially lower than outside, extra strength and weight is necessary. But according to these papers: &lt;a href=&quot;http://www.enggjournals.com/ijet/docs/IJET14-06-01-055.pdf&quot; rel=&quot;noreferrer&quot;&gt;(1)&lt;/a&gt; &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080047447.pdf&quot; rel=&quot;noreferrer&quot;&gt;(2)&lt;/a&gt; and &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140002998.pdf&quot; rel=&quot;noreferrer&quot;&gt;(3)&lt;/a&gt; there is another method.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cold helium gas is injected at the bottom of the tank and the bubbles raise in the LOX. At the surface of the bubbles, LOX evaporates into the bubble and cools the remaining LOX. But extra space is needed for the bubbles in the LOX and for the gas mixture of helium and oxygen above the liquid level. For topping off, the injection of helium is stopped and the remaining space is filled with LOX. Figure 8 of the first paper shows the effect of different helium gas temperatures. The cooling works best with helium at 85 K, but even helium at 150 K cools the LOX.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A bubble injected into the LOX consists of 100 % helium and 0 % oxygen at first. The LOX around this bubble would boil just like in a vaccum because the partial pressure of oxygen in this bubble is zero. Even a bubble consisting of 50 % helium and 50 % oxygen is able to cool LOX at 90 K. Without sub cooling in a tank with boiling LOX at 90 K, the gas above the liquid is 100 % oxygen and the partial pressure of oxygen is 1 bar. If the partial pressure of oxygen is lower than 1 bar in the gas above the liquid or inside the bubbles, the LOX is cooled by evaporation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the launch pad the LOX may be precooled using a heat exchanger with ground suplied liquid nitrogen boiling at 77.355 K. To save weight of the rocket, this heat exchanger should be outside the rocket but close to it. Liquid nitrogen and oxygen should not be mixed to avoid solving of nitrogen within the LOX. Cooling with helium bubbles may be used within the rocket LOX tank.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2020-06-05T10:57:26.170" LastActivityDate="2020-06-05T10:57:26.170" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="21350" PostTypeId="2" ParentId="21344" CreationDate="2017-05-02T09:35:30.563" Score="3" Body="&lt;p&gt;There are lots of tradeoffs at work here: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;cryogenic stages are more difficult to design than storable liquid stages, so space agencies tend to start building storable liquid stages, then move on to cryogenics when they have some experience. &lt;/li&gt;&#xA;&lt;li&gt;cryogenic stages offer higher performance (Isp) than storable liquid or solid stages. &lt;/li&gt;&#xA;&lt;li&gt;solid stages can be simple, cheap and have very high thrust. They also vibrate more than liquid engines, and cannot be throttled or (easily) shut down early. This makes them less useful as the final stage of a launcher, when you need to achieve an exact orbit. &lt;/li&gt;&#xA;&lt;li&gt;liquid hydrogen (as used in the Ariane 5 core stage) is not dense, so it requires a large tank, which reduces its performance advantage&lt;/li&gt;&#xA;&lt;li&gt;a common compromise is to use a first stage with low Isp but high thrust (i.e. solid or storable propellant), and a second stage with high Isp. Ariane 5 does this, in a way: its solid boosters are the cheap first stage with high thrust. &lt;/li&gt;&#xA;&lt;li&gt;the infographic lists the early Ariane 5G. Most Ariane 5 launches now use the ECA variant, which has a cryogenic upper stage with better performance than the 5G&lt;/li&gt;&#xA;&lt;li&gt;Ariane in particular suffers from some decisions that were made early on to save money: the 5G upper stage used storable propellants because it was cheaper to develop than a cryogenic stage. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Ariane 5 allows us to make a pretty good comparison between storable and cryogenic propellants. The 2 versions of Ariane 5 have rather different performance:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;GS payload to GTO orbit: 6,100 kg (13,400 lb),  &lt;/li&gt;&#xA;&lt;li&gt;ECA payload to the same orbit: 10,500 kg (23,100 lb). &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This is despite the ECA upper stage being much heavier than the GS upper stage. I'll see if I can find development cost of these upper stages. One drawback of the ECA upper stage is that it cannot be restarted (another cost-saving measure), while the GS upper stage is restartable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, all rocket designs are a tradeoff between:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;design cost,&lt;/li&gt;&#xA;&lt;li&gt;production cost,&lt;/li&gt;&#xA;&lt;li&gt;performance. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Some launchers are designed for high performance (using expensive or finicky construction methods to save weight), others are designed for low cost (cheap to launch, despite being heavier than a high-performance design would have).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-03T08:18:51.903" LastActivityDate="2017-05-03T08:18:51.903" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21351" PostTypeId="1" CreationDate="2017-05-02T12:18:18.037" Score="3" ViewCount="498" Body="&lt;p&gt;I am sooooo confused. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer to &lt;a href=&quot;https://space.stackexchange.com/q/12733/12102&quot;&gt;Are sun synchronous orbits possible for any place on Earth?&lt;/a&gt; ends with &lt;a href=&quot;https://space.stackexchange.com/a/12736/12102&quot;&gt;&lt;strong&gt;In short&lt;/strong&gt;, yes, sun-synchronous orbits are possible for any location on Earth.&lt;/a&gt; and mentions that this is true only because they pass over the poles twice a year, based on comments there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had thought that 98° inclination (for example) meant inclination with respect to an equatorial orbit, and therefore &lt;em&gt;never passing beyond&lt;/em&gt; 82° North or South latitude. I had thought that the precession is &lt;em&gt;induced about the symmetry axis of the equatorial bulge&lt;/em&gt; and therefore would never pass through the axis of the Earth. Am I wrong?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What am I missing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Possibly helpful video runs for about six days of simulation time:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4K5FyNbV0nA?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-02T12:59:50.497" LastActivityDate="2017-05-02T12:59:50.497" Title="If a sun-synchronous orbit passes over the poles twice a year, how can it have a constant (e.g. 98°) inclination?" Tags="|orbital-mechanics|low-earth-orbit|sun-synchronous|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21352" PostTypeId="2" ParentId="21347" CreationDate="2017-05-02T15:38:30.947" Score="13" Body="&lt;p&gt;Once the orbiter was committed to entry by executing the deorbit burn, nothing could have been done. The entry trajectory was tightly constrained for structural, thermal, and aerodynamic reasons.  There was no way to abort an entry and return to orbit.  &lt;a href=&quot;https://aviation.stackexchange.com/questions/21981/how-does-the-space-shuttle-slow-down-on-the-re-entry-descent-and-landing/23889&quot;&gt;This answer&lt;/a&gt; on Aviation Stack Exchange gives a good overview of the shuttle entry trajectory constraints.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The entry could be waved off at any time up to the deorbit burn; this was done many times for weather reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only exception would have been that if the damage had been somehow minor enough to keep the vehicle from breaking up or losing control, but major enough to prevent reaching a runway, the crew could have &lt;a href=&quot;https://spaceflight.nasa.gov/shuttle/reference/shutref/escape/inflight.html&quot; rel=&quot;noreferrer&quot;&gt;bailed out&lt;/a&gt; if stable gliding flight was possible. (This was not the case for the actual accident).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For what could have been done prior to entry, see my answer to this question: &#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/10546/what-would-nasa-have-done-if-they-knew-columbia-was-catastrophically-damaged/10547#10547&quot;&gt;What would NASA have done if they knew Columbia was catastrophically damaged?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The document referenced in this answer discusses some entry trajectory modification that might have been considered had the damage been assessed before entry; the consensus is that none can be stated to have any certain appreciable benefit, and both were discussed only in the context of flying the entry with the on-orbit wing repair completed.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;4.7 ADDITIONAL ENTRY OPTIONS – THE “CAIN REPORT ” NASA Flight Director Leroy Cain presented the report from the “Entry Options Tiger&#xA;  Team” to the Orbiter Vehicle Engineering Working Group (OVEWG) on&#xA;  April 22. This report was a very complete analysis of the results of&#xA;  jettisoning most of the payload bay cargo and coldsoaking the wing.&#xA;  Although this report looked at options within the certified entry&#xA;  design envelope, the options presented required some very difficult&#xA;  EVA tasks like cutting power and fluid cables, cutting through a&#xA;  tunnel, and large mass handling. This study does not assess the&#xA;  feasibility of these tasks, but it simply notes that whatever jettison&#xA;  tasks that could be accomplished in any remaining time during the&#xA;  two “repair” EVAs would be performed, as this would decrease the entry &#xA;  heating by a small amount. As there is a very large uncertainty band in the thermal analysis of a wing leading edge repair, it&#xA;  is sufficient to say that jettison of equipment would have occurred&#xA;  during any remaining EVA time, and this may have helped the overall&#xA;  total heat load.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;4.8 UNCERTIFIED OPTIONS - INCREASED ANGLE OF ATTACK /LOW DRAG PROFILE The Entry Options Tiger Team was requested to look at certified options only. The only uncertified entry flight design options&#xA;  that could significantly reduce the wing leading edge temperature&#xA;  would be to change guidance to fly a lower drag profile during entry&#xA;  or to raise the angle of attack (alpha) to a reference of 45 degrees,&#xA;  vice the standard 40 degrees. However, it should be noted that while&#xA;  flying either one of these entry profiles would reduce heating on the&#xA;  leading edge, the heat load would increase on another part of the TPS&#xA;  structure. A simplified analysis that does not account for heating&#xA;  effects due to boundary layer tripping from a damaged area shows that&#xA;  a wing leading edge peak temperature could be decreased from a&#xA;  reference of 2,900 degrees F to 2,578 degrees F. This would be&#xA;  considered as an additional tool in attempting to maintain the spar&#xA;  structural integrity. It should be noted that changing the reference&#xA;  alpha would require a significant software patch to entry guidance&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From the CAIB Report, &quot;&lt;a href=&quot;https://www.nasa.gov/columbia/caib/PDFS/VOL2/D13.PDF&quot; rel=&quot;noreferrer&quot;&gt;STS-107 Inflight Options Assessment&lt;/a&gt;&quot;&#xA;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-05-02T17:53:43.797" LastActivityDate="2017-05-02T17:53:43.797" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21354" PostTypeId="1" CreationDate="2017-05-02T18:21:35.870" Score="5" ViewCount="472" Body="&lt;p&gt;For a landing via &lt;a href=&quot;https://space.stackexchange.com/q/10307/12978&quot;&gt;suicide burn&lt;/a&gt;, I have identified three phases:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Starting at $t_0$, the craft approaches the ground with $v_x$ constant and $v_y$ gradually increasing (due to gravity)&lt;/li&gt;&#xA;&lt;li&gt;At $t_b$, the craft accelerates in retrograde direction until the speed vector $(v_x,v_y)$ is below a certain safety margin. &lt;/li&gt;&#xA;&lt;li&gt;The craft maintains the speed inside the safety margin until it touches down at $t_1$&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I am trying to optimize $t_b$ for fuel consumption. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, there are three cases to consider: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;$t_b$ might be too early, causing the fuel to run out and the craft to crash&lt;/li&gt;&#xA;&lt;li&gt;$t_b$ might be too late, causing the craft to crash due to a high residual velocity&lt;/li&gt;&#xA;&lt;li&gt;$t_b$ might be inside a safe landing corridor&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In order to optimize this, I need a goal function, i.e. some kind of score that I assign to each outcome. The case 3 is trivial: Here we can attempt to maximize the residual fuel when landed. In case 2, I can probably minimize the residual velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about case 1? Intuitively, if the lander has enough power to brake to a standstill (and something), residual velocity should be smaller for larger $t_b$, but is that true?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="12978" LastEditDate="2017-05-02T19:43:48.667" LastActivityDate="2017-05-02T19:43:48.667" Title="Optimizing Suicide Burn" Tags="|landing|simulation|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21355" PostTypeId="2" ParentId="21343" CreationDate="2017-05-02T18:29:49.100" Score="13" Body="&lt;p&gt;A dogleg maneuver is done to change the inclination of a certain payload, and the reason it limits the payload capacity is most likely due to the cosine losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to perform such a maneuver a rocket has to yaw in a certain direction in order to change the direction of flight. However, this yaw comes at a cost, as the more you stray the direction your rocket is pointing in from prograde, the less efficient your rocket will become. For example, if your rocket was only 10 degrees stray from prograde, you would still get 98% if the total Delta V. However, in certain missions a large inclination change is necessary, so if you want to stray 45 degrees away from prograde to achieve a large inclination, you would only get 70% of the total Delta V for that stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason it is called a dogleg maneuver is the resembles the path of travel has to a dogs leg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a picture of a falcon 9 launch, I do not believe it's from NROL-76. The maneuver occurs towards the center of the screen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rCMiq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rCMiq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here's a picture of a dog's leg just to compare :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DYSTf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DYSTf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="12102" LastEditDate="2017-05-02T19:17:49.013" LastActivityDate="2017-05-02T19:17:49.013" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21356" PostTypeId="1" AcceptedAnswerId="21396" CreationDate="2017-05-02T22:00:55.827" Score="3" ViewCount="727" Body="&lt;p&gt;From what I understand, prior to the launch, the relief valves are open to allow some of the boiling LOX to evaporate and be replaced with fresh, cold one (we can see oxygen evaporating - and &quot;bursting&quot;, from time to time -  from the rocket). When some (unknown to me) condition is met, the valves are closed and tanks are pressurized with helium. If someone could confirm or clarify that this is correct, it would be great, because I suspect I'm missing something here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the &lt;a href=&quot;https://www.youtube.com/watch?v=EzQpkQ1etdA&quot; rel=&quot;nofollow noreferrer&quot;&gt;NROL-76&lt;/a&gt; launch, we heard the &quot;pressing for flight&quot; announcement for Stage 2 at T-40s, and for Stage 1 at T-20s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the time of closing these valves vary from launch to launch. For example, during the CRS-7 mission, S1 valves were closed at T-2:20 and S2 at T-50s. During Orbcomm-2 mission, both tanks were said to be pressing for flight at approximately T-1:00. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the reason these times vary from launch to launch? Is it because of the different LOX temperatures? &lt;/p&gt;&#xA;" OwnerUserId="19511" LastEditorUserId="12102" LastEditDate="2021-08-07T00:18:25.303" LastActivityDate="2021-08-07T00:18:25.303" Title="Falcon 9 - relief valves closing times" Tags="|spacex|liquid-fuel|falcon-9|orbcomm-2|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21359" PostTypeId="1" AcceptedAnswerId="21361" CreationDate="2017-05-03T04:35:51.890" Score="2" ViewCount="559" Body="&lt;p&gt;If traveling to distant planets in less time is an issue because of the fuel or speed, why can't we make use of a planet's magnetic field to accelerate ourselves given that almost all planets in our solar system have their own magnetic field?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are they too weak?&lt;/p&gt;&#xA;" OwnerUserId="19513" LastEditorUserId="6816" LastEditDate="2017-05-03T04:51:41.470" LastActivityDate="2018-04-07T19:17:46.430" Title="Accelerating spaceships via planetary magnetic fields" Tags="|spacecraft|magnetic-field|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21360" PostTypeId="1" CreationDate="2017-05-03T05:40:06.970" Score="5" ViewCount="319" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hZRCA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hZRCA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was looking at &lt;a href=&quot;https://saturn.jpl.nasa.gov/galleries/raw-images/&quot; rel=&quot;noreferrer&quot;&gt;the Cassini raw images&lt;/a&gt; taken from Apr. 26, 2017 4:45 AM to 9:09 AM, and I noticed that some of them show a lot of white specks. Sometimes the specks appear to be streaks. What are these things?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know the spacecraft was &quot;diving&quot; through the gap between Saturn and its rings; are these &quot;ring particles&quot; (whatever those are)? If so, then why are some of them streaking and others not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I considered perhaps they were digital noise of some kind, but that would not explain the streaks and the random appearance of al this stuff. If they are particles, why are they so well-lit? Is it ice reflecting the sun or something?&lt;/p&gt;&#xA;" OwnerUserId="18444" LastActivityDate="2017-05-03T05:40:06.970" Title="What are the white streaks and specks in many of the Cassini images of Saturn?" Tags="|saturn|cassini|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21361" PostTypeId="2" ParentId="21359" CreationDate="2017-05-03T08:16:27.497" Score="6" Body="&lt;p&gt;At the distances involved, a magnetic field is much weaker than a gravitational field. So instead of using the magnetic force, we use gravity (i.e. &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;noreferrer&quot;&gt;gravity assists&lt;/a&gt;) to accelerate interplanetary spacecraft. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-05-03T08:16:27.497" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21362" PostTypeId="1" CreationDate="2017-05-03T08:25:59.320" Score="2" ViewCount="125" Body="&lt;p&gt;Helium is a rare and expensive noble gas. When it is loaded into a rocket to be used after launch, it is not possible to recycle it. But the helium used for sub-cooling the LOX before launch (see question &lt;a href=&quot;https://space.stackexchange.com/questions/21345&quot;&gt;Why would sub-cooled LOX tanks need to &amp;quot;topped-off&amp;quot; until the last minute or so?&lt;/a&gt;) could be recycled. The mixture of helium and oxygen should be exhausted from the tank and separated by liquefaction. Is recycling of helium done when using sub-cooled LOX or is it vented into the atmosphere?&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="6944" LastEditDate="2021-03-28T18:20:19.533" LastActivityDate="2021-03-28T18:20:19.533" Title="What about recycling the helium used for sub-cooling the LOX in the tank of a rocket before launch?" Tags="|launch|lox|helium|sub-cooled|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21363" PostTypeId="1" CreationDate="2017-05-03T08:33:11.833" Score="1" ViewCount="3503" Body="&lt;p&gt;From what I understand, the semi-major axis of a circular orbit should be equal to its radius.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, checking Wikipedia's info on Hubble, which is in a nearly circular orbit, I find :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;sMA $= 6,919$ km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;apogee $\approx$ perigee $\approx 540$ km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm having trouble understanding that difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming I approximate an orbit as being circular, how can I get its radius from the semi-major axis? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screenshot from &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/n1db8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/n1db8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19514" LastEditorUserId="12102" LastEditDate="2017-05-03T10:12:40.593" LastActivityDate="2017-05-03T10:12:40.593" Title="Relation between semi-major axis and radius of an orbit?" Tags="|orbital-mechanics|artificial-satellite|orbital-elements|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21364" PostTypeId="2" ParentId="21363" CreationDate="2017-05-03T09:07:56.440" Score="6" Body="&lt;p&gt;The Wikipedia article on Hubble provides the apogee and perigee height with respect to the Earth's surface not its center. As a consequnce the semi-major axis should be equal to the sum of the Earth's radius and the perigee or apogee respectively (of course only for cirular orbits). &lt;/p&gt;&#xA;" OwnerUserId="19518" LastActivityDate="2017-05-03T09:07:56.440" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21365" PostTypeId="2" ParentId="21339" CreationDate="2017-05-03T11:44:22.380" Score="6" Body="&lt;p&gt;Deep space communications are intermittent: the communications link only exists when a dish antenna is aimed at the spacecraft. If a spacecraft has an emergency, nobody knows about it until the next scheduled DSN contact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;DSN traffic is &lt;a href=&quot;https://www.cdscc.nasa.gov/Pages/trackingtoday.html&quot; rel=&quot;noreferrer&quot;&gt;scheduled&lt;/a&gt; by the DSN organization. When a scheduled contact finds a problem, the schedule can be &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;noreferrer&quot;&gt;rearranged&lt;/a&gt;. I assume this involves some negotiation between DSN and the scheduled users, but haven't found data on this process.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The largest antennas of the DSN are often called on during spacecraft emergencies. Almost all spacecraft are designed so normal operation can be conducted on the smaller (and more economical) antennas of the DSN, but during an emergency the use of the largest antennas is crucial. This is because a troubled spacecraft may be forced to use less than its normal transmitter power, attitude control problems may preclude the use of high-gain antennas, and recovering every bit of telemetry is critical to assessing the health of the spacecraft and planning the recovery. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The most famous example is the Apollo 13 mission, where limited battery power and inability to use the spacecraft's high-gain antennas reduced signal levels below the capability of the Manned Space Flight Network, and the use of the biggest DSN antennas (and the Australian Parkes Observatory radio telescope) was critical to saving the lives of the astronauts. While Apollo was also a US mission, DSN provides this emergency service to other space agencies as well, in a spirit of inter-agency and international cooperation. For example, the recovery of the Solar and Heliospheric Observatory (SOHO) mission of the European Space Agency (ESA) would not have been possible without the use of the largest DSN facilities.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One example of an emergency operation is the &lt;a href=&quot;https://sohowww.nascom.nasa.gov/operations/Recovery/recovery_diary.html&quot; rel=&quot;noreferrer&quot;&gt;recovery of SOHO&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In an attempt to recover SOHO as soon as possible, the Flight&#xA;  Operations Team continued uplinking commands to the spacecraft via&#xA;  NASA's Deep Space Network, for at least 12 hours per day (normal pass)&#xA;  plus all supplementary time given by DSN.  The ESA ground stations in&#xA;  Perth, Vilspa and Redu supported the search for a downlink signal.&#xA;  Special equipment was set up at the ground stations to search for&#xA;  spikes in the downlink spectrum and view it in real time at the SOHO&#xA;  operations facilities at Goddard Space Flight Center.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Analysis by attitude experts led to the conclusion that SOHO went into&#xA;  a spin around an axis such that the solar panels were faced nearly&#xA;  edge-on towards the Sun, and thus did not generate any power. Since the&#xA;  spin axis is fixed in space, as the spacecraft progressed in its orbit&#xA;  around the Sun, the orientation of the panels with respect to the Sun&#xA;  gradually changed, resulting in increased solar illumination of the&#xA;  spacecraft solar arrays as time progressed.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;On July 23 researchers at the National Astronomy and Ionosphere Center&#xA;  (NAIC) in Arecibo, Puerto Rico, used the facility's 305-meter diameter&#xA;  radio telescope to transmit a signal toward SOHO while the 70-meter&#xA;  dish of NASA's Deep Space Network in Goldstone (USA) acted as a&#xA;  receiver, locating the spacecraft's echo and tracking it using radar&#xA;  techniques for more than an hour. SOHO was found to be slowly rotating&#xA;  near its expected position in space.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-03T11:50:37.657" LastActivityDate="2017-05-03T11:50:37.657" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21366" PostTypeId="1" AcceptedAnswerId="21369" CreationDate="2017-05-03T12:34:09.937" Score="2" ViewCount="582" Body="&lt;p&gt;I want to find and see &quot;GPS satellites in view&quot; for different orbits missions. For example I want to see the observable GPS satellites for a specific satellite in GEO, for this purpose I need a software which can visualize and make a 3D model of about 33 satellites together(32 GPS satellites and one in GEO) arounding the earth for me. Do you know any special software? I use windows 8.(I couldn't use SaVi because of operating system)&lt;/p&gt;&#xA;" OwnerUserId="19519" LastEditorUserId="12102" LastEditDate="2017-09-01T11:01:36.883" LastActivityDate="2020-02-20T02:26:29.417" Title="3D modelling of satellites around the earth" Tags="|software|gps|gnss|visualization|3d-modeling|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21367" PostTypeId="2" ParentId="21359" CreationDate="2017-05-03T12:55:20.627" Score="4" Body="&lt;p&gt;The Earth's magnetic field is far too weak to provide much force to a spacecraft. At LEO the earth's magnetic field is probably somewhere around 50 microteslas, or 50,000 nanoteslas (it varies greatly by your position above the Earth) in &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth%27s_magnetic_field#Intensity&quot; rel=&quot;nofollow noreferrer&quot;&gt;intensity&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Earth's field ranges between approximately 25,000 and 65,000 nT&#xA;  (0.25–0.65 G). By comparison, a strong refrigerator magnet has a field&#xA;  of about 10,000,000 nanoteslas (100 G).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So a refrigerator magnet is 200 times more powerful than the Earth's magnetic field. In order to convert this force to movement you'd need to interact with the earth's magnetic field with an incredibly powerful magnet field which your spacecraft would have to generate. This would be 1) very heavy and 2) the magnetic field would be hazardous to people and equipment on board. Even with a massively powerful magnetic field on a spacecraft you would not get much propulsion out of it due to the earth's weak field. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, magnetic fields quickly reduce in strength the farther away you get from the source, meaning you would need another form or propulsion for course corrections. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2017-05-03T12:55:20.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21368" PostTypeId="2" ParentId="21346" CreationDate="2017-05-03T12:56:12.463" Score="3" Body="&lt;p&gt;Pretty sure only SpaceX knows. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming ~2 m/s of delta-v, an ISP of say 50 s and we get 0.4% of the mass of the rocket in nitrogen. Now the mass of the rocket is somewhere between its dry mass and its wet mass, which again is unknown but somewhere between ~40 tons and ~400 tons, that would mean 160 kg to 1600 kg of nitrogen gas, probably much closer to the former as the thrusters are only used when the rocket is nearly empty.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your asking your self why don't they use more efficient thrusters like bi-propellant N2O4+MMH well run the numbers: A system with an ISP of say 300 s would weigh 0.07% the mass of the rocket or 27 - 270 kg of fuel, but then you need the mass of the fuel and oxidizer tanks, mass of double the piping, mass of the pressurizing gas, mass of pressurizing gas tanks, mass of control system. The simplicity of cold gas thrusters is probably worthwhile in weight alone when you only need a few delta-v for a few minutes of attitude control consisting of one back-flip and some aiming.    &lt;/p&gt;&#xA;" OwnerUserId="19522" LastActivityDate="2017-05-03T12:56:12.463" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21369" PostTypeId="2" ParentId="21366" CreationDate="2017-05-03T14:38:38.877" Score="4" Body="&lt;p&gt;Maybe Systems Tool Kit could be helpful:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.agi.com/products/engineering-tools&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.agi.com/products/engineering-tools&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a powerful tool, and there is also the possibility to have a free license, although it does not include all capabilities.&#xA;I have used it on Windows 10, but I cannot tell about Windows 8.&lt;/p&gt;&#xA;" OwnerUserId="19290" LastActivityDate="2017-05-03T14:38:38.877" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21370" PostTypeId="2" ParentId="21330" CreationDate="2017-05-03T16:41:19.367" Score="1" Body="&lt;p&gt;It's all about trade-offs. Parachutes are much simpler, lighter, and safer for recovering a &quot;dumb&quot; booster. Wings, landing gear, and control systems add an awful lot of weight (therefore reducing payload) and complexity for pretty much zero return on investment.&#xA;I also imagine there would've been all kinds of stability problems from adding two extra sets of side-mounted wings to the stack. &lt;/p&gt;&#xA;" OwnerUserId="17089" LastActivityDate="2017-05-03T16:41:19.367" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21371" PostTypeId="2" ParentId="21295" CreationDate="2017-05-03T16:49:07.197" Score="3" Body="&lt;p&gt;NASA's &lt;a href=&quot;https://planetaryprotection.nasa.gov/overview&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planetary Protection office&lt;/a&gt; is a good place to start. An excerpt that you might find useful:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;The most stringent requirements are those for Mars, Jupiter’s moons Europa, Ganymede and Callisto, and the Saturn’s moon of Titan and Enceladus due to the great potential for studying the origin, evolution, and distribution of life on these bodies. &quot;&lt;/p&gt;&#xA;" OwnerUserId="17089" LastActivityDate="2017-05-03T16:49:07.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21373" PostTypeId="1" CreationDate="2017-05-04T02:38:52.630" Score="4" ViewCount="1177" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;The polar regions won't be missed as they will pass under the sun-synchronous orbit twice a year. – SF. Nov 19 '15 at 9:56&lt;/p&gt;&#xA;&lt;p&gt;@SF. - Your comment is incorrect. Sun synchronous satellites never pass directly over the poles. The A-train satellites, for example, have an orbital inclination of 98.2 degrees, and hence never go north of 81.8 degrees north latitude. The poles are in view from the northernmost point of the orbit, but are never directly below. – David Hammen 4 hours ago&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Could someone explain it like I was 5?&lt;/p&gt;&#xA;&lt;p&gt;Earth axial tilt of 23.4° sends the poles 23.4° away from the terminator, into the night side, or the day side, at solstices. So, at summer solstice, the north pole is 23.4 degrees away from the terminator.&lt;/p&gt;&#xA;&lt;p&gt;The &amp;quot;terminator-riding&amp;quot; satellites move at  80 to 82 degrees inclination, that means at the time of equinox, they are 10 to 12 degrees from the pole, which is exactly at the terminator line. At summer solstice, if the satellite is to pass over the terminator line over the equator, if it retains the 80 degrees inclination (relative to Earth equator), that would mean it's 20 degrees from the pole + 23.4 degrees between the pole and the terminator = 43 degrees away from the terminator... that's hardly &amp;quot;riding the terminator&amp;quot;, almost halfway to perpendicular to it... (and near winter solstice, they actually ride exactly over the terminator line).&lt;/p&gt;&#xA;&lt;p&gt;or am I missing something? Could someone, say, show a picture of such a satellite's orbit at summer solstice, in relation to north pole and the terminator line? Something like &lt;a href=&quot;http://www.heavens-above.com/orbit.aspx?satid=23710&amp;amp;lat=0&amp;amp;lng=0&amp;amp;loc=Unspecified&amp;amp;alt=0&amp;amp;tz=UCT&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; but for summer solstice date?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-04T11:28:02.520" Title="How are &quot;terminator-riding&quot; sun-synchronous satellites never passing over the pole?" Tags="|orbital-mechanics|artificial-satellite|sun-synchronous|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21374" PostTypeId="1" AcceptedAnswerId="21375" CreationDate="2017-05-04T03:02:08.327" Score="31" ViewCount="6457" Body="&lt;p&gt;As I was watching Tom Mueller describe the Merlin engine, he said that the RP-1 propellant is pumped through ribs in the nozzle to regeneratively cool the engine. Also, a video about Copenhagen suborbital's TM65 rocket engine shows how the alcohol fuel is used to cool the engine. Since both of these engines use liquid oxygen as their oxidizer, why would they use the fuel to cool the engine. Why not use the very cold liquified oxygen instead of the room temperature hydrocarbon fuels? &lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2019-12-12T21:30:53.210" Title="Why cool engines with fuel as opposed to oxidizer?" Tags="|engine-design|temperature|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21375" PostTypeId="2" ParentId="21374" CreationDate="2017-05-04T03:59:04.603" Score="43" Body="&lt;p&gt;Whatever you use as a coolant will become hot. Hot oxygen will &lt;strike&gt;(a) vaporize, making the plumbing somewhat more difficult, and (b)&lt;/strike&gt; react with and erode (or maybe even ignite) the cooling channels, unless they're made of special materials.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2017-05-04T17:55:55.770" LastActivityDate="2017-05-04T17:55:55.770" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21376" PostTypeId="2" ParentId="21373" CreationDate="2017-05-04T09:04:44.607" Score="7" Body="&lt;p&gt;The satellite does not run parallel to the terminator, it crosses the terminator twice on each orbit. So on each orbit, half the path underneath the orbit is in shadow, the other half is in daylight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite's inclination is measured relative to the equator (98.2°) and poles (8.2°), and does not change with the seasons. The northernmost point of the orbit revolves around the pole.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-04T11:28:02.520" LastActivityDate="2017-05-04T11:28:02.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21377" PostTypeId="1" AcceptedAnswerId="21379" CreationDate="2017-05-04T10:27:04.837" Score="-2" ViewCount="175" Body="&lt;ol&gt;&#xA;&lt;li&gt;If there is a super massive black hole at the center of the milky way galaxy and it sucks matter and light especially, how does the center of galaxy look bright?&lt;/li&gt;&#xA;&lt;li&gt;If we are slowly getting sucked into it then does Earth has a specific 'expiration' date?&lt;/li&gt;&#xA;&lt;li&gt;If there really is a black hole at the center of milky way, does that mean we can't escape our galaxy? Since we know lightest matter gets heavily influenced by a stronger force (in this case black hole's gravity).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;EDIT - &#xA;Reference links:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Supermassive_black_hole&quot; rel=&quot;nofollow noreferrer&quot;&gt;Super massive black holes&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Galactic_Center&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galactic center&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19513" LastEditorUserId="19513" LastEditDate="2017-05-04T15:50:02.667" LastActivityDate="2017-05-04T15:50:02.667" Title="Galactic center. Questions about super massive black hole" Tags="|black-hole|" AnswerCount="1" CommentCount="8" ClosedDate="2017-05-04T15:38:14.183" ContentLicense="CC BY-SA 3.0" />
  <row Id="21378" PostTypeId="2" ParentId="21373" CreationDate="2017-05-04T11:13:17.720" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Could someone explain it like I was 5?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;TL;DR: A true &quot;terminator-riding&quot; satellite doesn't exist. The fuel maintenance costs for such an orbit would be ridiculously high.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Sun synchronous satellite orbits are not quite Keplerian. The right ascension of ascending node is constant in a Keplerian orbit. The right ascension of ascending node instead precesses in sun synchronous orbits to match the Earth's orbit about the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This precession is a plane change. Plane changes in Keplerian orbits are rather expensive operations, delta-V wise. These continuous plane changes would make a sun synchronous orbit about a spherical planet be ridiculously expensive. Those continuous plane changes of a sun synchronous orbit about the non-spherical Earth come free of charge because the Earth's oblateness makes any inclined orbit precess. The rate at which an orbit precesses is a function of the orbit's inclination, semi-major axis length, and eccentricity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites in a sun synchronous have orbits with inclination, semi-major axis length, and eccentricity specifically designed to make this natural precession rate match the Earth's orbit about the Sun. Such a satellite with it's orbital plane more or less normal to the direction to the Sun (non-ELI5 term: large solar beta angle) and it will more or less keep that orientation constant. The satellite will more or less ride the terminator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about a true terminator-riding orbit? Such an orbit would require constant plane changes, both in inclination and in right ascension. The Earth's oblateness does not affect inclination. Those plane changes would be the responsibility of the satellite. Moreover, that constantly changing inclination would mean that the natural precession due to oblateness would almost never matches the precession needed to keep the satellite sun-synchronous. A true terminator riding satellite would also have to expend immense amounts of fuel for those plane changes. For these reasons, a satellite in a true terminator-riding orbit does not exist.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-05-04T11:13:17.720" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21379" PostTypeId="2" ParentId="21377" CreationDate="2017-05-04T12:11:17.920" Score="3" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The light is not coming from the black hole. Like you said, it pulls all the light into it so we can't see it. The center of the galaxy looks brighter then the rest of the galaxy, because the star density is very high in the center area. Since the suns are far enough from the black hole then the light can escape the gravitational pull of the black hole. &#xA;The black hole in the center is actually very small when compared to the size of the galaxy. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The bodies in the orbit around the black hole are in stable orbit and they are not sucked into black hole. Like &lt;a href=&quot;https://www.nasa.gov/topics/earth/features/2012-alignment.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth is in stable orbit&lt;/a&gt; with the Sun and the Moon orbits around the Earth.&#xA;Earth has an expiration date, but not due to the black hole in the center of the galaxy. Our Sun runs out of hydrogen in about 4.4 &amp;ndash; 5.5 billion years. Once the hydrogen is exhausted then the Sun will start to expand in size and &lt;a href=&quot;https://phys.org/news/2016-05-earth-survive-sun-red-giant.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;consume Earth in the process&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If you can generate &lt;a href=&quot;https://space.stackexchange.com/a/3949&quot;&gt;enough velocity&lt;/a&gt; then sure, you can escape the Milky Way galaxy.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="19323" LastEditorUserId="19323" LastEditDate="2017-05-04T12:23:13.853" LastActivityDate="2017-05-04T12:23:13.853" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21380" PostTypeId="2" ParentId="21374" CreationDate="2017-05-04T12:48:55.860" Score="17" Body="&lt;p&gt;The specific heat capacity of the fuel is higher than that of the liquid oxygen &lt;em&gt;at the operating temperature/pressure&lt;/em&gt;, hence for the same rate of flow, using the fuel cools better ... and apart from any of the above-mentioned liquid/vapour/pressure issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The operating temperature range when using kerosene / RP-1 is also much greater than if using LOX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As per my comment above, any &quot;coolant&quot; is not used because it's intrinsically cold, but because it can transport heat effectively in the required region at the required temperature range.&lt;/p&gt;&#xA;" OwnerUserId="18620" LastEditorUserId="921" LastEditDate="2018-05-29T14:59:05.963" LastActivityDate="2018-05-29T14:59:05.963" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="21381" PostTypeId="1" AcceptedAnswerId="21395" CreationDate="2017-05-04T13:20:37.603" Score="12" ViewCount="3019" Body="&lt;p&gt;The Ars Technica article &lt;a href=&quot;https://arstechnica.com/science/2017/05/nasa-seeks-industry-help-with-lunar-landings-potentially-sample-return/&quot; rel=&quot;noreferrer&quot;&gt;NASA seeks industry help with lunar landings, potentially sample return&lt;/a&gt; discusses the potential value of ice on the moon as a source for fuel in future space missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ice sublimates. Even if it is very cold hiding in permanent shadows in craters near the Moon's poles, I'd expect it to have a non-zero vapor pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what would keep ice from slowly sublimating over millions of years? Or is it being replenished somehow?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2019-02-13T03:13:37.797" Title="If there is actual ice on the moon, why hasn't it sublimated?" Tags="|the-moon|water|sample-return|ice|" AnswerCount="4" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="21382" PostTypeId="2" ParentId="21381" CreationDate="2017-05-04T13:55:23.517" Score="5" Body="&lt;p&gt;It is of course sublimating. And not only is it cold, but the sublimation cools the remaining ice even further. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since it's in lunar dust, chances are additionally that a sublimated water molecule will hit a speck of dust and re-freeze. This effectively slows the speed at which water moves from deep down to the lunar atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2017-05-04T13:55:23.517" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21383" PostTypeId="1" AcceptedAnswerId="21387" CreationDate="2017-05-04T14:13:24.620" Score="2" ViewCount="165" Body="&lt;p&gt;The Art Technica article &lt;a href=&quot;https://arstechnica.com/science/2017/05/vector-barrels-ahead-with-its-small-satellite-launcher/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vector has completed the first successful flight test of its new micro rocket&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Vector Space Systems successfully launched a full-scale model of its Vector-R rocket on Wednesday in Mojave, California. The test flight, which remained under 50,000 feet for regulatory purposes, allows the company to remain on track to begin providing launch services for small satellites in 2018, said Jim Cantrell, the company’s chief executive and cofounder.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What regulatory purposes might these be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Bonus Question:&lt;/strong&gt; Why is the small-scale launch market like &quot;shooting turkeys in a drum&quot;? For that matter, what is shooting turkeys in a drum like to being with?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Vector launch can be seen in this Tweet (lower your volume): &lt;a href=&quot;https://twitter.com/jamesncantrell/status/859846383158689792&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://twitter.com/jamesncantrell/status/859846383158689792&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-10T02:02:04.980" LastActivityDate="2017-05-10T02:02:04.980" Title="What &quot;regulatory purposes&quot; would limit a launch test of a new vehicle to 50,000 feet?" Tags="|launch|regulatory|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21384" PostTypeId="1" AcceptedAnswerId="21386" CreationDate="2017-05-04T15:33:57.527" Score="1" ViewCount="209" Body="&lt;p&gt;The Ars Technica article &lt;a href=&quot;https://arstechnica.com/science/2017/05/air-force-study-says-us-government-should-get-serious-about-reusable-rockets/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX, Blue Origin have opened a “window of opportunity” for US Air Force&lt;/a&gt; links to an evocatively written Air University report titled: &lt;em&gt;Fast Space: Leveraging Ultra Low-Cost Space Access for 21st Century Challenges&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Available at this dot-mil address: &lt;a href=&quot;http://www.airuniversity.af.mil/Portals/10/Research/documents/Space/Fast%20Space_Public_2017.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.airuniversity.af.mil/Portals/10/Research/documents/Space/Fast%20Space_Public_2017.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The report laments the fact that some countries are just not as afraid of the nuclear deterrent as they used to be, and that the military satellites that make a deterrent possible and effective are also increasingly vulnerable. Of course the report says many other things as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now they can't title the report &lt;em&gt;Cheap Space:&lt;/em&gt;, but I'm wondering, if they chose the title &lt;em&gt;Fast Space:&lt;/em&gt; how fast are they talking about? Is this really about a bunch of low-cost anti-satellite payloads on cheap, commercial rockets ready to go at a moment's notice?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond the compelling urgency, what exactly is the speed argument? How fast is &lt;em&gt;Fast Space&lt;/em&gt;?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-04T15:47:14.377" LastActivityDate="2017-05-04T16:28:00.490" Title="How fast is &quot;Fast Space&quot;?" Tags="|military|weapon|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21385" PostTypeId="2" ParentId="21381" CreationDate="2017-05-04T15:47:28.620" Score="6" Body="&lt;p&gt;You overestimate vapor pressure and underestimate dust power&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Rebed.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Rebed.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;http://www2.bren.ucsb.edu/~dturney/WebResources_13/WaterSteamIceProperties/VaporPressureICEvsTemp.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;0.05 Pa it equivalent to a layer 0.000018382 m of dust which would (if it being a sealant) to prevent the ice from further sublimations if we assume average density of 1700kg/m&lt;sup&gt;3&lt;/sup&gt; and gravity 1.6 m/s&lt;sup&gt;2&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other factors like water being polar solvent and thus adhesion to the dust particles as an example(same way as of why there are gasses He,H, N in regolith), slowing or stopping their travel trough the dust and helping for better sealing on top of the ice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The average velocity of ice molecule at 200K is about 16 m/s, which gives us that there is not enough energy(for most of them) to fly out of the 80m deep crater.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially it is the same thing as with icy asteroids and comets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But overall everything boils down to there was enough water and speed of escaping of the water was not fast enough, so we observe it presence.&lt;/p&gt;&#xA;" OwnerUserId="3039" LastEditorUserId="3039" LastEditDate="2017-05-04T17:39:06.630" LastActivityDate="2017-05-04T17:39:06.630" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21386" PostTypeId="2" ParentId="21384" CreationDate="2017-05-04T16:28:00.490" Score="1" Body="&lt;p&gt;It used to be called Operationally Responsive Access to Space. Fast means faster than now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The faster the better, it is not clear there is a lower limit, faster than launch on demand.  But anything would be better than now, which is multiple months if not years out.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-05-04T16:28:00.490" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21387" PostTypeId="2" ParentId="21383" CreationDate="2017-05-04T16:42:38.437" Score="6" Body="&lt;p&gt;Vector flew their test at the &lt;a href=&quot;http://www.friendsofamateurrocketry.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;Friends of Amateur Rocketry&lt;/a&gt; site, which has permits from the FAA to fly to 50,000 feet on weekends without obtaining additional permits per launch. For early tests of avionics and such, Vector can save time and money that way.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="6816" LastEditDate="2017-05-04T21:11:10.143" LastActivityDate="2017-05-04T21:11:10.143" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21390" PostTypeId="2" ParentId="19924" CreationDate="2017-05-04T19:56:48.147" Score="6" Body="&lt;p&gt;I don't have the reputation to comment on Phiteros's answer, but I would like to add that a huge constraint of solar panels on Mars is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Climate_of_Mars#Effect_of_dust_storms&quot; rel=&quot;noreferrer&quot;&gt;weather&lt;/a&gt;. Mars doesn't have much of an atmosphere, but what it does have is quite turbulent. Dust storms are common, and will block out the sun frequently. On top of this, dust gets onto the solar panels and blocks their efficiency even more. Phiteros kind-of covered this with the performance ratio, but that ratio would change dramatically depending on the weather. So you would need very good batteries to last out storms, particularly since they can last up to a month.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality though, it is very likely that a future Mars base will have some type of reactor, in addition to solar panels.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastActivityDate="2017-05-04T19:56:48.147" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21391" PostTypeId="2" ParentId="19811" CreationDate="2017-05-04T20:01:04.350" Score="2" Body="&lt;p&gt;Two variants of the Globus retained in my collection - in the foreground from Voskhod, in the background installed in the Soyuz TM panel is the later version)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fUBcU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fUBcU.jpg&quot; alt=&quot;Globus Versions&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19473" LastActivityDate="2017-05-04T20:01:04.350" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21392" PostTypeId="1" AcceptedAnswerId="21397" CreationDate="2017-05-04T21:03:08.797" Score="14" ViewCount="1247" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/a/21391/4660&quot;&gt;member today posted this photo&lt;/a&gt; showing a complete instrument panel from a &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz-TM&quot; rel=&quot;noreferrer&quot;&gt;Soyuz TM&lt;/a&gt; capsule, and a globus instrument from a &lt;a href=&quot;https://en.wikipedia.org/wiki/Voskhod_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Voskhod capsule&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fUBcU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fUBcU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both items are in their private collection. Only 5 Voskhod vessels ever flew, in 1964 and 1965. 34 Soyuz TM capsules were launched between 1986 and 2002. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the Voskhod spacecraft, in particular, were of great historical significance. And, like all crew capsules, these things were property of their governments, in this case the Soviet government. (Perhaps the Russian government in the case of the Soyuz panel, although it certainly doesn't look recent.) When I see such an item in a private collection, I must admit my reaction is '...What?...'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do such items end up on the market and get acquired? Is that still happening? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For context, the Voskhod flew during the Gemini Program. The capsules for Gemini flights are all in museums, and the Wikipedia entry for the program even has &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Gemini#Current_location_of_hardware&quot; rel=&quot;noreferrer&quot;&gt;a section listing where each one is&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="26446" LastEditDate="2020-04-18T18:11:15.630" LastActivityDate="2023-07-19T02:32:59.167" Title="How do items from the Russian and Soviet space programs end up in private collections?" Tags="|history|soyuz-spacecraft|russia|museum-exhibits|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21393" PostTypeId="1" CreationDate="2017-05-04T22:58:30.977" Score="2" ViewCount="90" Body="&lt;p&gt;For satellites there's &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=2017-017A&quot; rel=&quot;nofollow noreferrer&quot;&gt;NSSDCA/COSPAR&lt;/a&gt;, but I'm unable to find something similar for &lt;a href=&quot;https://www.flickr.com/photos/spacex/32915199874/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the launches of those objects&lt;/a&gt;. I'm hoping I'm not looking for something that doesn't exist as I'm assuming launch groups cannot use just any name/id they whim to?&lt;/p&gt;&#xA;" OwnerUserId="19539" LastActivityDate="2017-05-04T22:58:30.977" Title="Is there an authoritative catalog for rocket launch identifiers?" Tags="|launch-sequence|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21394" PostTypeId="2" ParentId="19537" CreationDate="2017-05-05T01:15:31.393" Score="8" Body="&lt;h2&gt; - First count of individual birds from space&lt;/h2&gt;&#xA;&#xA;&lt;h2&gt; - First complete count of an entire species population&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The 4-may-2017 BBC News item &lt;a href=&quot;http://www.bbc.com/news/science-environment-39797373&quot; rel=&quot;noreferrer&quot;&gt;Albatrosses counted from space&lt;/a&gt; describes an effort to automate the analysis of WorldView-3 (&lt;a href=&quot;https://en.wikipedia.org/wiki/WorldView-3&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; and &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/v-w-x-y-z/worldview-3&quot; rel=&quot;noreferrer&quot;&gt;EOPortal&lt;/a&gt;) photos in order to count nesting Albatrosses.  It is also described in this week's BBC Podcast of &lt;a href=&quot;http://www.bbc.co.uk/programmes/p05177vf&quot; rel=&quot;noreferrer&quot;&gt;Science in Action&lt;/a&gt; (available for the next 29 days). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the &lt;em&gt;first case of a bird species being counted individually from space&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All six species of &lt;a href=&quot;https://en.wikipedia.org/wiki/Great_albatross&quot; rel=&quot;noreferrer&quot;&gt;Great Albatross&lt;/a&gt; are endangered, these are the largest birds capable of sustained flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the &lt;em&gt;first case of an entire population of a species has been counted&lt;/em&gt;. During mating season the entire population roosts on a few tiny islands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UAyCL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UAyCL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/91iLo.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/91iLo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The project is described in the most recent issue of IBIS:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fretwell, P. T., Scofield, P. and Phillips, R. A. (2017), &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1111/ibi.12482/full&quot; rel=&quot;noreferrer&quot;&gt;Using super-high resolution satellite imagery to census threatened albatrosses&lt;/a&gt;. Ibis. doi:10.1111/ibi.12482&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The study was based on WorldView-3 VHR satellite images, with the visible bands (2/3/5) pan-sharpened to provide a 31-cm resolution colour image using the Gram Schmidt algorithm in ENVI image processing software. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Of the seven visible light channels, 2, 3, and 5 appear to refer to Blue, Green and Red.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ffx2r.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ffx2r.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6nXCZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6nXCZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Illustration of WorldView-3 Earth observing telescope, from &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/v-w-x-y-z/worldview-3&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AG3UH.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AG3UH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.satimagingcorp.com/services/atmospheric-compensation/&quot; rel=&quot;noreferrer&quot;&gt;Satellite Imaging Corporation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/R6Tsw.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R6Tsw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-05T01:44:13.997" LastActivityDate="2017-05-05T01:44:13.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21395" PostTypeId="2" ParentId="21381" CreationDate="2017-05-05T03:30:21.197" Score="11" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Ice sublimates.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So does rock. Yet the planet Mercury is still there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason Mercury still exists is because even though rock does indeed sublimate, the rate at which rock sublimates is extremely low, even at temperatures at the surface of Mercury. The same applies to water ice at the very low temperatures in those permanently shadowed craters on the Moon. Water ice those very low temperatures is essentially rock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The extremely low sublimation rate of an exposed block of water ice at ~100 kelvins would result in that ice thinning by a millimeter per billion years. The temperatures in the permanently shadowed portions of the Shackleton Crater are ten kelvins lower than that. Even exposed water ice can easily survive for billions of years at those low temperatures. Water ice covered by material can survive even longer.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-05-05T03:30:21.197" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21396" PostTypeId="2" ParentId="21356" CreationDate="2017-05-05T08:34:08.550" Score="5" Body="&lt;p&gt;I got an answer to this on Reddit SpaceX community.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Pressing for flight&quot; is indeed closing the pressure relief valve and opening a helium valve to fill the remaining headspace above the LOX with helium. Because the engines are not started at this point this is cold helium but once the engines start the helium is heated in a heat exchanger so the headspace is filled with hot helium to reduce the mass of helium required.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;SpaceX are continuously varying the propellant loading procedures to maximise the mass that can be loaded into fixed size tanks.&#xA;  If they are closing the vent valves later then it implies that they have reduced the head space to fit more propellant in. In that case the evaporating LOX will overpressurise the smaller volume more quickly so less time is available between closing the valve and the point at which the engines fire reducing the propellant load.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;S2 only starts around 140 seconds after the S1 engines so it is pressurised as late as possible consistent with getting the tank to flight pressure to stiffen it up at lift off.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/spacex/comments/68ulnl/rspacex_discusses_may_2017_32/dh54iwc/&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19511" LastActivityDate="2017-05-05T08:34:08.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21397" PostTypeId="2" ParentId="21392" CreationDate="2017-05-05T11:19:18.660" Score="14" Body="&lt;p&gt;There are several paths by which both US and Russian artifacts from the space program make their way into private ownership. These include deassession from the Government via authorized surplus sales, disposal of materials by the contractors who produce the technology at the conclusion of a program; retention of gifted or salvaged items by individuals and their family members who worked on or were affiliated in some way with the program and then subsequently release the artifacts for sale. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that in addition to flight vehicle components that actually flew on mission, multiple examples of each artifact were typically produced for developmental testing, spares ect. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Within the U.S., the National Air and Space Museum has first right of refusal to decline the transfer of government owned artifacts that NASA no longer has an interest in retaining, After that offers may be extended to other museums and institutions. Ultimately if no formal interest expressed, items can be sold via authorized property disposal channels  (GSA auctions for example) to private citizens. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, as a result of &lt;a href=&quot;https://www.congress.gov/bill/112th-congress/house-bill/4158&quot; rel=&quot;noreferrer&quot;&gt;HR 4158&lt;/a&gt;, astronauts who crewed the early US Space missions (Projects Mercury, Gemini, Apollo and Skylab) and who were in possession of certain artifacts from those flights as of the law's enactment, were recognized as legal owners and authorized to subsequently determine those items disposition (to include selling them to other individuals).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have obtained many artifacts through a varienty of paths to include flown and unflown materials from both the US and Russian programs. There is a robust collector market that is sustained by heavy global interest in these items, and major auction houses offer annual (in many cases multiple yearly sales) of Space artifacts and memorabilia. &lt;/p&gt;&#xA;" OwnerUserId="19473" LastEditorUserId="58" LastEditDate="2017-05-05T15:10:27.513" LastActivityDate="2017-05-05T15:10:27.513" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21398" PostTypeId="1" CreationDate="2017-05-05T15:11:26.797" Score="0" ViewCount="170" Body="&lt;p&gt;If we could actually get to the point where technology allows us to accelerate to 1 g for more than a few minutes, would that acceleration still need to be constant? Since a spaceship can't slow down in space because of the laws of motion, always going on until another force stopped it, wouldn't the people on board feel a constant 1g if say....the thrust was stopped and just left to drift? Or would the artificial gravity from the forces of linear acceleration stop until someone started up the thrust again? &lt;/p&gt;&#xA;" OwnerUserId="19543" LastActivityDate="2017-05-05T15:11:26.797" Title="Linear acceleration used for artificial gravity." Tags="|artificial-gravity|" AnswerCount="0" CommentCount="2" ClosedDate="2017-05-07T10:07:57.707" ContentLicense="CC BY-SA 3.0" />
  <row Id="21399" PostTypeId="1" AcceptedAnswerId="28045" CreationDate="2017-05-06T02:17:42.643" Score="6" ViewCount="529" Body="&lt;p&gt;ISRO's GPS Aided GEO Augmented Navigation system (GAGAN) (see &lt;a href=&quot;http://www.isac.gov.in/navigation/gagan.jsp&quot; rel=&quot;noreferrer&quot;&gt;ISRO&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/GPS_Aided_GEO_Augmented_Navigation&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;) is the first GPS augmentation system designed to deal with ionospheric effects near the geomagnetic equator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.isac.gov.in/navigation/gagan.jsp&quot; rel=&quot;noreferrer&quot;&gt;ISRO&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Further, GAGAN is the first system in the world that would be certified to serve the equatorial anomaly region with its unique IONO algorithm (IGM-MLDF: ISRO GIVE Model Multi-Layer Data Fusion) designed and developed by ISAC in collaboration with vendor.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is the equatorial anomaly, and what is unique about the IONO algorithm and how it handles the anomalous ionospheric effects? Why don't other systems have this?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-07T07:54:13.847" LastActivityDate="2018-06-22T22:50:25.673" Title="What is the magnetic equatorial anomaly and how is GAGAN unique in its ability to compensate?" Tags="|isro|gps|gnss|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21400" PostTypeId="1" CreationDate="2017-05-06T02:51:58.570" Score="2" ViewCount="238" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; It's been &lt;s&gt;seven months&lt;/s&gt;. &lt;s&gt;close to two years&lt;/s&gt; &lt;s&gt;close to two years &lt;em&gt;and&lt;/em&gt; seven months&lt;/s&gt; over three years now, have any of the 'gift' transponders been used or at least tested yet by the gift recipient nation?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The BBC news item &lt;a href=&quot;http://www.bbc.com/news/world-asia-india-39814455&quot; rel=&quot;nofollow noreferrer&quot;&gt;India launches 'invaluable' South Asia satellite&lt;/a&gt; describes the 'gifting' or zero-cost licensing of transponders on the recently launched &lt;a href=&quot;https://en.wikipedia.org/wiki/GSAT-9#Payload&quot; rel=&quot;nofollow noreferrer&quot;&gt;GSAT-9&lt;/a&gt; to other South Asian countries.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;So it seems Mr Modi is placing the ISRO in a new orbit by providing this space-based platform that would have cost the participating nations almost $1,500m (£1,158m).&lt;/p&gt;&#xA;&lt;p&gt;According to the government, the satellite will enable a full range of services to India's neighbours in telecommunication and broadcasting areas such as television, direct-to-home (DTH) services, education, telemedicine, weather forecasting and disaster management support.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Is it possible for one transponder to simultaneously support all or many of those services, or would a country have to pick just one of them? I don't know the total bandwidth available from each transponder. I'm wondering if the idea is more that the transponder would allow experimentation and pilot studies, rather than a full range of services?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-06-25T00:17:35.710" LastActivityDate="2020-06-25T00:17:35.710" Title="How have the 'gift' transponders on ISRO's GSAT-9 been used by other countries?" Tags="|communication|geostationary|isro|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="21402" PostTypeId="2" ParentId="13492" CreationDate="2017-05-06T08:30:08.733" Score="18" Body="&lt;p&gt;In the late 70's and early 80's I worked for a company named Unidynamics Defense Systems. We provided this subassembly. We were not the design originators, this design I am quite sure came from NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess as to who the upstream contractor was would land on either Martin Marietta or Thiokol. On this particular design my drawings did not indicate who the actual purchasing authority was.&#xA; My boss held out but finally relented at roughly STS-9 (&quot;need to know&quot; was his normal comment).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as production use my name was on this drawing in the design and draft blocks thru STS-51L, and the title block it stated Unidynamics Defense Systems, who later were purchased by Pacific Scientific. &#xA; This was for the subassembly unit that was mounted on the service masts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From STS-9 thru 51L, I was told this design held. Afterwards I cannot attest as to who next revised and produced the unit, and doubt they would change manufacturers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As nothing is static at NASA from a design and cost standpoint, I would be surprised that it held thru STS-135. &#xA;  I have kept an eye out on its use and it certainly didn't look to me that it changed from a form, fit or function perspective.&#xA; There were not many pieces to the design, but the chemistry and internal disc shapes could easily be modified for a specific output.&lt;/p&gt;&#xA;" OwnerUserId="19550" LastEditorUserId="6816" LastEditDate="2017-05-06T09:29:44.957" LastActivityDate="2017-05-06T09:29:44.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21403" PostTypeId="1" AcceptedAnswerId="24633" CreationDate="2017-05-06T11:12:37.517" Score="1" ViewCount="347" Body="&lt;p&gt;The NASA News item &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2017/nasa-s-new-shape-shifting-radiator-inspired-by-origami&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA’s New Shape-Shifting Radiator Inspired by Origami&lt;/a&gt; describes a technique for spacecraft temperature regulation combining both a shape-changing surface and an emissivity-changing material applied to it.&lt;/p&gt;&#xA;&lt;p&gt;I don't understand what is really happening here. How can one shape be better for cooling (radiating into space) while another be better for warming (presumably from sunlight)? Wouldn't you just want the maximum area? It seems the deeper the folds, the smaller the total area exposed to the environment.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;To reiterate&lt;/em&gt;, the deeper the folds, the smaller the cross-sectional area because the total amount of surface is fixed. Is it really true that a smaller cross-section with deeper folds is better than spread out and planar?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This novel radiator controls the rate of heat loss by performing shape-shifting maneuvers. The resulting topographical changes could be achieved with temperature-sensitive materials like muscle wire or shape-memory alloys. As temperature-sensitive materials experience a change in temperature — caused by spacecraft electronics or the absorption of heat from the Earth or sun — the radiator could automatically change its shape to either shed or conserve heat.&lt;/p&gt;&#xA;&lt;p&gt;The deeper the folds or cavities, the greater the absorption, explained Mulford, adding that scientists have investigated the use of cavities to affect heat loss for nearly 100 years, but no one has approached the challenge in quite this way. “Origami allows you to change the depth of these cavities in real time, thereby changing the heat loss from a surface in real time,” he said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &amp;quot;Brigham Young assistant professor Brian Iverson and doctoral student Rydge Mulford have teamed with NASA technologist Vivek Dwivedi to advance the design of a three-dimensional, foldable radiator, inspired by the art of paper folding. Still early in its development, Iverson and Mulford are experimenting with different shapes to determine which configuration would work best as a radiator.&amp;quot; Credits: Brigham Young University&#xA;&lt;a href=&quot;https://i.stack.imgur.com/08MMT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/08MMT.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-25T22:18:29.313" Title="Why are deeper folds better for absorption?" Tags="|artificial-satellite|design|thermal|thermal-control|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21404" PostTypeId="2" ParentId="21403" CreationDate="2017-05-06T12:06:26.790" Score="3" Body="&lt;p&gt;going on intuition: when a photon hits a surface, it's either absorbed or reflected. On a flat surface, this is easy: if the photon is reflected, it'll travel away from the spacecraft.&lt;br&gt;&#xA;When a photon travels into a cavity, it'll have to be reflected multiple times to get out of the cavity again, so there are multiple chances for the photon to be absorbed. The absorption coefficient is multiplied by the average number of reflections needed to get out of the cavity. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-05-06T12:06:26.790" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21405" PostTypeId="1" CreationDate="2017-05-06T12:53:33.330" Score="0" ViewCount="163" Body="&lt;p&gt;&lt;strong&gt;Launch&lt;/strong&gt; escape systems have been used for half a century and have worked great when tried. But entering Earth's atmosphere from orbit and landing is at least as dangerous to astronauts, and have cause about half of the tragic casualties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would a &lt;strong&gt;landing&lt;/strong&gt; escape system look like? A way for astronauts to abort a failing EDL. Have there been any proposals for this historically? Are there any fresh ideas?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-05-08T16:44:50.467" Title="How would a landing escape system be designed?" Tags="|safety|launch-escape-system|entry-descent-landing|" AnswerCount="2" CommentCount="5" ClosedDate="2017-05-08T00:32:23.050" ContentLicense="CC BY-SA 3.0" />
  <row Id="21406" PostTypeId="1" CreationDate="2017-05-06T17:00:41.343" Score="4" ViewCount="191" Body="&lt;p&gt;You can carve it however you want - it can be purely extra-orbital, it can be NERVA, it can be Orion. What are the specific legal and political barriers that prevent it from being implemented?&lt;/p&gt;&#xA;" OwnerUserId="5965" LastActivityDate="2019-02-13T13:58:45.140" Title="What are the non-technical barriers to creating a practical nuclear propulsion system?" Tags="|nuclear|nuclear-pulse-propulsion|international-politics|policy|" AnswerCount="0" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21407" PostTypeId="2" ParentId="21405" CreationDate="2017-05-06T21:35:07.183" Score="1" Body="&lt;p&gt;I would think it to be a sub module of the craft, to be accommodating weight and size criterion, the escape system would basically be a scalable seating module. With the actual reentry vehicle being an exoskeleton.  This then being able to eject anytime during reentry. This module saving any other reentry attempts from damage caused by any sort of heat shield penetration.&lt;/p&gt;&#xA;" OwnerUserId="19048" LastEditorUserId="19048" LastEditDate="2017-05-06T21:44:57.527" LastActivityDate="2017-05-06T21:44:57.527" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21408" PostTypeId="2" ParentId="18783" CreationDate="2017-05-06T22:54:00.900" Score="0" Body="&lt;p&gt;The big deal with SpaceX's engines is not the $I_{SP}$. &#xA;It's the &lt;strong&gt;Thrust/Mass&lt;/strong&gt; ratio SpaceX is getting that is truly significant.&lt;br&gt;&#xA;T/M is arguably more important for getting payloads to LEO than $I_{SP}$ is, and SpaceX is setting new records for T/M in their designs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Merlin 1D has a T/M $\approx200$.  Raptor is likely to have a T/M $\approx400$.&#xA;Nothing else has ever come close to attaining T/M values like these.&lt;/p&gt;&#xA;" OwnerUserId="18245" LastEditorUserId="6816" LastEditDate="2017-05-06T23:20:30.080" LastActivityDate="2017-05-06T23:20:30.080" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21409" PostTypeId="2" ParentId="5707" CreationDate="2017-05-07T01:41:56.893" Score="1" Body="&lt;p&gt;Supersonic retro propulsion is a method to deaccelerate the space probe safely while reentry on a planet using the retrorockets in supersonic regime.&lt;/p&gt;&#xA;" OwnerUserId="19555" LastActivityDate="2017-05-07T01:41:56.893" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21410" PostTypeId="1" CreationDate="2017-05-07T07:16:39.437" Score="7" ViewCount="464" Body="&lt;p&gt;According to &lt;a href=&quot;http://space.skyrocket.de/doc_sat/hs-702.htm&quot; rel=&quot;noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt; discussion of early versions of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_702&quot; rel=&quot;noreferrer&quot;&gt;Hughes HS-702 Bus&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The first version of the 702 used solar arrays with concentrators. These concentrators tended to early fogging, as due to an inherent design flaw the outgassing of the solar cells was higher than expected. This fogging lead to much reduced lifetime. The flaw was corrected in later versions with higher power triple-junction gallium arsenide solar cells.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Perhaps if it was the only issue the fogging of the reflectors due to outgassing could have been managed, but I can imagine other problems as well:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Additional complexity of unfolding and precise alignment and flatness adds risk (see &lt;a href=&quot;http://www.google.com.gi/patents/US7321095&quot; rel=&quot;noreferrer&quot;&gt;US7321095&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;Deviations in flatness and inclination angle of reflectors can lead to non-uniform illumination of the cells. Unequal illumination of cells can lead to loss of efficiency or other problems.&lt;/li&gt;&#xA;&lt;li&gt;Narrower window of attitudes, the reflectors are designed for a certain attitude, any change can lead to steeper drop-off and loss of power, compared to a flat panels approximately &lt;span class=&quot;math-container&quot;&gt;$cos(\theta)$&lt;/span&gt; behavior.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; I'm wondering if the concentrator reflectors were always intended as a stop-gap measure to deliver a higher level of power in anticipation of improved &lt;a href=&quot;https://space.stackexchange.com/q/17784/12102&quot;&gt;triple-junction cells&lt;/a&gt;, or was the original plan to use the regularly to always achieve maximal power? Has this array-scale concentrator technique every been used again, as opposed to individual per-cell concentrators?&lt;/p&gt;&#xA;&lt;p&gt;From Gunter's Space Page article for &lt;a href=&quot;http://space.skyrocket.de/doc_sat/hs-702.htm&quot; rel=&quot;noreferrer&quot;&gt;HS-702&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Following failures of 702 models have occurred:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Anik F1: - degradation of solar concentrators - long term power loss&lt;/li&gt;&#xA;&lt;li&gt;Galaxy 11: - degradation of solar concentrators - long term power loss&lt;/li&gt;&#xA;&lt;li&gt;PAS 1R: - degradation of solar concentrators - long term power loss&lt;/li&gt;&#xA;&lt;li&gt;Thuraya 1: - degradation of solar concentrators - long term power loss&lt;/li&gt;&#xA;&lt;li&gt;XM 1: - degradation of solar concentrators - long term power loss&lt;/li&gt;&#xA;&lt;li&gt;XM 2: - degradation of solar concentrators - long term power loss&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vP47S.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vP47S.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/zlWaZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zlWaZ.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Drawing of &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/apmt-1.htm&quot; rel=&quot;noreferrer&quot;&gt;APMT 1&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Thuraya#Satellites&quot; rel=&quot;noreferrer&quot;&gt;Thuraya-1&lt;/a&gt;. credit: Hughes, from &lt;a href=&quot;http://www.astronautix.com/h/hs702.html&quot; rel=&quot;noreferrer&quot;&gt;Astronautix&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LtCvT.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LtCvT.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;  &lt;a href=&quot;https://i.stack.imgur.com/X3AoA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/X3AoA.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Video telemetry (first ever!) of a solar panel unfolding, Galaxy-11. credit: Hughes, from &lt;a href=&quot;https://spaceflightnow.com/news/0002/24hs702arrays/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;From the 24-Feb-2000 Spaceflight Now article &lt;a href=&quot;https://spaceflightnow.com/news/0002/24hs702arrays/&quot; rel=&quot;noreferrer&quot;&gt;Camera sees giant Hughes satellite deploying its wings&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The HS 702 solar wings carry angled solar reflector panels along each side that concentrate more of the sun's rays onto the solar cells in order to generate increased power. The gallium arsenide solar cells are among the most efficient available, able to convert nearly 25 percent of the sun's rays into spacecraft power. The satellite, Galaxy 11, was built by HSC for PanAmSat Corporation and was launched on an Ariane rocket on Dec. 21, 1999.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Artist's conception of a Hughes HS-702 bus, credit: Hughes, from &lt;a href=&quot;http://what-when-how.com/space-science-and-technology/communications-satellites-technology-of/&quot; rel=&quot;noreferrer&quot;&gt;What When How&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZZaxC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZZaxC.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-01-20T12:56:40.590" Title="Were solar concentrators on HS-702 just a stop-gap measure until triple-junction PV cells were available?" Tags="|history|design|solar-power|communication-satellite|failure|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21411" PostTypeId="2" ParentId="19886" CreationDate="2017-05-07T07:24:21.043" Score="6" Body="&lt;p&gt;I have just run across the following &amp;quot;first&amp;quot; in satellite selfie history:&lt;/p&gt;&#xA;&lt;p&gt;From a Feb. 24, 2000 Hughes Space and Communications News Release, as found in Spaceflight Now &lt;a href=&quot;https://spaceflightnow.com/news/0002/24hs702arrays/&quot; rel=&quot;noreferrer&quot;&gt;Camera sees giant Hughes satellite deploying its wings&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For the first time from space, video images have been captured of a commercial satellite unfolding its purple-hued solar wings in orbit more than 22,000 miles above Earth.&lt;/p&gt;&#xA;&lt;p&gt;When extended, the wingspan of the satellite approximates that of a Boeing 737 jetliner.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/phN4J.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/phN4J.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/Xb85C.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xb85C.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Black-and-white photo shows fully deployed solar wing with four panels and associated concentrators. These concentrators are innovations for the Hughes HS 702 satellite, to concentrate more of the sun's energy on the highly efficient gallium arsenide solar cells. Photo: Hughes&lt;/p&gt;&#xA;&lt;p&gt;Photo taken by camera on board the Galaxy 11 Hughes HS 702 satellite shows solar wing as it unfolds, and solar concentrators being deployed along the third and fourth solar panels on the wing. Photo: Hughes&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/t2sCO.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t2sCO.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Hughes HS 702 compared to 737 aircraft. Photo: Hughes&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-07T07:24:21.043" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21413" PostTypeId="1" CreationDate="2017-05-07T11:18:24.393" Score="21" ViewCount="6968" Body="&lt;p&gt;I have read that prior to launch all people and tools are fastened down to prevent a rocket from losing the balance. Why doesn't the ISS begin to spin as astronauts are walking inside of it? Or is there some compensation mechanism for it to maintain its orientation?&lt;/p&gt;&#xA;" OwnerUserId="3184" LastEditorUserId="268" LastEditDate="2017-05-08T12:43:50.250" LastActivityDate="2017-05-08T12:43:50.250" Title="Why doesn't the ISS start to spin if people walk inside?" Tags="|iss|astronauts|spin-stabilisation|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21414" PostTypeId="2" ParentId="21413" CreationDate="2017-05-07T12:08:12.513" Score="22" Body="&lt;p&gt;Consider the differences in mass.  A person is say 100 kg to keep it round. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS masses 420,000kg right now. (It changes of course as modules are added, removed, changed, payload arrives, departs). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is a very low effect that any one person can have.  There are only 6 crew on the ISS at any one time, so their torquing effect is quite low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless there are 6 (?) control gyros that manage the orientation of the station. Like the gyros in the Hubble, they seem prone to failure and several have had to be replaced over the years. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="19286" LastEditDate="2017-05-08T00:26:03.140" LastActivityDate="2017-05-08T00:26:03.140" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="21415" PostTypeId="2" ParentId="21413" CreationDate="2017-05-07T13:27:44.710" Score="2" Body="&lt;p&gt;During a launch, everything is fastened because a rocket accelerates really fast (3G or more), so any loose object &quot;falls&quot; really quickly and hits hard, potentially damaging important equipment.&lt;br&gt;&#xA;Changes in the center of gravity similarly have huge effects during launch because of this large acceleration.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In orbit, forces are much smaller by comparison. Because everything is in 0 gravity, you only need small forces to move around. &#xA;Also, forces are generally balanced. You move around the station by pushing off in one spot, then flying to the next wall where you stop by applying the same force in the opposite direction.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite this, the attitude of the ISS is controlled using gyroscopes (and thrusters, if necessary). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-05-07T13:27:44.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21416" PostTypeId="2" ParentId="21413" CreationDate="2017-05-07T16:18:09.473" Score="19" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why doesn't ISS start to spin if people walk inside?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The ISS is already spinning. Its rotational period has been set to 93 minutes in order to match its orbital period and keep one side of the station permanently pointing at the earth. Moving astronauts will not make it start to spin, they will alter its spin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can consider two different effects that an astronaut could have on the station: (1) a permanent change in its state of rotation, or (2) a temporary change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An example of a permanent change would be if an astronaut grabs a massive piece of equipment from a portion of the station that is far from the axis, brings it to a point on the axis, and leaves it there. This reduces the station's moment of inertia, and therefore permanently increases its rate of rotation due to conservation of angular momentum. This change will persist until counteracted by a compensating redistribution of mass away from the axis or possibly a use of CMGs or thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An example of a temporary change would be an astronaut moving their own body closer to the axis and then back out. Because the change is temporary, it only ends up changing the phase of the rotation. That's undesirable, because they want to maintain the same orientation relative to the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible in principle for an astronaut to do something like moving rapidly and continuously in a circle at a fixed distance from the axis, so as steal or donate a significant amount of angular momentum to the station. The effect would be temporary, and the geometry of the space station doesn't seem very helpful in trying to perform such a circuit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronauts' motions could also cause the station's rotation to precess or wobble. This would presumably be corrected by the CMGs.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2017-05-07T16:18:09.473" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21417" PostTypeId="2" ParentId="21413" CreationDate="2017-05-07T18:02:40.603" Score="9" Body="&lt;p&gt;The CMGs (Control Moment Gyros) do have to compensate for astronaut motion.  In the Mission Control Center, it was possible to tell if the astronauts were awake, based on how the CMGs were operating (four of them).  But the motions are very small.&#xA;The CMGs don't require any fuel for general operation, but periodically they get &quot;saturated&quot; and then a desaturation burn is required.  The CMGs can each be pointed independently.  So, initially they are pointed in different directions.  If a rotation is required, they are moved to have a net angular momentum opposite to that, causing the vehicle to rotate (or stop rotating).  Over time they will end up all aligned in the same direction which is &quot;saturation&quot;.  &lt;/p&gt;&#xA;" OwnerUserId="8579" LastActivityDate="2017-05-07T18:02:40.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21418" PostTypeId="1" CreationDate="2017-05-07T18:27:27.400" Score="3" ViewCount="99" Body="&lt;p&gt;Just wondering since we are able to make an object seemingly float or neutrally gravity bouant, is it possible to create a launch pad where electromagnetism is used to &quot;levitate&quot; a vehicle (using repulsive magnetism) to at least decrease the force/energy required to overcome the enertia of liftoff? Thereby significantly decreasing the payload of fuel required to break free of the earths orbit?&lt;/p&gt;&#xA;" OwnerUserId="19562" LastActivityDate="2017-05-07T21:57:49.587" Title="Magnetic opposition for a launch boost" Tags="|magnetic-field|" AnswerCount="1" CommentCount="1" ClosedDate="2017-05-08T02:12:20.483" ContentLicense="CC BY-SA 3.0" />
  <row Id="21419" PostTypeId="2" ParentId="21405" CreationDate="2017-05-07T21:28:31.500" Score="1" Body="&lt;p&gt;Vostok spacecraft effectively had &lt;a href=&quot;http://www.russianspaceweb.com/vostok1_landing.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;a landing escape system&lt;/a&gt;, the cosmonaut simply bailed out of the capsule. This is still possible, up to a certain altitude/velocity envelope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For something at very high altitude and orbital reentry speeds, things similar to &lt;a href=&quot;https://en.wikipedia.org/wiki/MOOSE&quot; rel=&quot;nofollow noreferrer&quot;&gt;MOOSE&lt;/a&gt; have been considered.&lt;/p&gt;&#xA;" OwnerUserId="3140" LastEditorUserId="3140" LastEditDate="2017-05-08T16:44:50.467" LastActivityDate="2017-05-08T16:44:50.467" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21421" PostTypeId="2" ParentId="21418" CreationDate="2017-05-07T21:57:49.587" Score="1" Body="&lt;p&gt;A Magnetic Rail Gun assisted launch or vertical mono rail. Most of rockets would have a Mono-pole positive or negative shell or links if done with solid magnets. Electromagnets are sure to interfere with guidance systems Though not impossible just expensive. Most rockets are build by the lowest bidder.&lt;/p&gt;&#xA;" OwnerUserId="18879" LastActivityDate="2017-05-07T21:57:49.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21422" PostTypeId="1" CreationDate="2017-05-08T00:08:19.650" Score="6" ViewCount="564" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/21413/12102&quot;&gt;Why doesn't ISS start to spin if people walk inside?&lt;/a&gt; is a good one partly because it has attracted several good, informative answers. &lt;a href=&quot;https://space.stackexchange.com/a/21417/12102&quot;&gt;One of them&lt;/a&gt; is particularly intriguing because it contains the following statement:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the Mission Control Center, it was possible to tell if the astronauts were awake, based on how the CMGs were operating (four of them). But the motions are very small. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The physics makes sense of course. As astronauts move (especially) along the forward/backward (roll) axis of the ISS, the movement of mass relative to the center can change the moment of inertia about the left/right (pitch) axis. Assuming angular momentum is conserved this will change the rotation rate about the pitch axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As pointed out in &lt;a href=&quot;https://space.stackexchange.com/a/21416/12102&quot;&gt;this helpful answer&lt;/a&gt; the ISS rotates once around its pitch axis every 93 minutes in order to keep the &quot;bottom&quot; facing towards the nadir.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if everyone &quot;ran&quot; to the front and back ends and waited a while, the rotation rate would slow and the ISS would slowly pitch-up. If they all then &quot;ran&quot; to the center, this drift would reverse and it would pitch down again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, roughly &lt;em&gt;how sensitive&lt;/em&gt; is the attitude control system to this kind of activity? Is it something that can be detected over a period of seconds, or minutes, or more like a day? How well (and how rapidly) can the ISS attitude control system detect and resolve periods of astronaut activity?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-06-07T13:16:15.083" LastActivityDate="2019-06-07T13:16:15.083" Title="How well could the ISS attitude control system detect periods of astronaut activity?" Tags="|iss|nasa|attitude|astronaut-lifestyle|attitude-determination-and-control|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="21423" PostTypeId="1" AcceptedAnswerId="21424" CreationDate="2017-05-08T00:22:32.397" Score="34" ViewCount="5178" Body="&lt;p&gt;Perhaps I'm naive to think that the single large nozzle at the back of the X-37B is a nozzle for an engine, but regardless of the purpose, gas exiting from a nozzle will produce thrust. Is the X-37B's center of mass along this line? I can't think of any other reason why this would be offset.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the most recent flight also tested ion propulsion, and that would not involve a large bell-shaped nozzle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, &lt;em&gt;Why is the X-37B's large nozzle offset to right from the spacecraft's mid-plane?&lt;/em&gt; I suppose an answer should include some suggestion what this nozzle is for to begin with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vF1RP.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vF1RP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Credit: Boeing. From &lt;a href=&quot;http://spaceflight101.com/spacecraft/x-37b-otv/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zhZ2G.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zhZ2G.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Credit: Boeing. From &lt;a href=&quot;http://www.space.com/34633-x-37b-military-space-plane-surprising-facts.html&quot; rel=&quot;noreferrer&quot;&gt;Space dot com&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3Pvz6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3Pvz6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;The military’s mini shuttle on the KSC runway this morning.&quot; Credit: Air Force. From &lt;a href=&quot;https://spaceflightnow.com/2017/05/07/x-37b-spaceplane-returns-to-earth-and-makes-precision-autopilot-landing/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6TwDc.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6TwDc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;http://www.fantastic-plastic.com/X-37B-1-48-CatalogPage.htm&quot; rel=&quot;noreferrer&quot;&gt;Fantastic Plastic&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-08T00:28:41.043" LastActivityDate="2020-05-17T15:54:58.153" Title="Why is the X-37B's large nozzle offset to right from the spacecraft's mid-plane?" Tags="|nozzle|x-37|shuttle-derived|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21424" PostTypeId="2" ParentId="21423" CreationDate="2017-05-08T00:48:15.267" Score="30" Body="&lt;p&gt;First off, it's absolutely an engine nozzle. Some sources say X-37B  has a storable biprop propulsion system with ~900N thrust. The &lt;a href=&quot;http://www.rocket.com/propulsion-systems/bipropellant-rockets&quot; rel=&quot;noreferrer&quot;&gt;Aerojet R-42&lt;/a&gt; is such an engine and the dimensions look about right -- 24&quot; long bell with 15&quot; exit diameter. This engine uses MMH and NTO as propellants, which are toxic, requiring the use of hazmat suits by ground crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The X-37's propulsion system is used for orbital maneuvering and the de-orbit burn only; it rides a two-stage Atlas launcher to orbit, and glides to landing like the space shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an engine only used in space, it's not necessary that it be mounted on the mass centerline of the spacecraft. As long as the engine can thrust directly through the center of mass, it can accelerate the craft without producing unwanted torque. Since the spacecraft will carry a wide variety of possibly-asymmetrical payloads, it likely needs a gimbaling engine anyway, so mounting it off-center with a small &quot;toe-in&quot; angle works as well as putting it on the centerline. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know what drove the off-center placement of the engine in the development of the X-37, but the Boeing document &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060004795.pdf&quot; rel=&quot;noreferrer&quot;&gt;X-37 Storable Propulsion System Design and Operations&lt;/a&gt; shows a symmetrically mounted pair of 450N engines instead of the single. That would point toward the earlier design using the Aerojet R-4, and:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;A pair of R-4s replaced by a single R-42, giving about the same total thrust, with minimal changes to the mounting, or&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A pair of R-4s replaced by a single R-4 with a larger bell -- the 300:1 expansion ratio nozzle for the R-4 is about the same size as the 160:1 nozzle for the R-42, and yields substantially better specific impulse.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I would assume the two-engine layout already had the mountings toed-in by a couple of degrees to minimize off-center thrust if one of the engines failed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternately, since the spacecraft is intended for propulsion tests (among other things), offsetting the engine allows a test-subject propulsion unit to be placed closer to centerline than would be possible if the primary propulsion unit were centered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some illustrations show an asymmetrical solar array deployed on the left side of the spacecraft, but I don't think that's relevant because (a) it would move CoM to the left when deployed, and the engine is on the right, (b) the solar array would likely be retracted when firing for de-orbit, and (c) the illustrations that show the asymmetric solar array also show a centered engine.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-08T13:23:47.703" LastActivityDate="2017-05-08T13:23:47.703" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21425" PostTypeId="2" ParentId="19886" CreationDate="2017-05-08T01:17:05.027" Score="3" Body="&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Shuttle always had payload bay cameras and (if the arm was flown)&#xA;wrist-and-elbow mounted cameras on the robot arm. This allowed&#xA;&quot;selfies&quot; of much of the vehicle, but by no means all.  After the&#xA;STS-107 accident, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbiter_Boom_Sensor_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;boom-mounted camera and sensor system&lt;/a&gt; was&#xA;added, which, when grappled by the arm, allowed all of the vehicle's&#xA;thermal protection system that was critical for entry to be imaged.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XMQ7n.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XMQ7n.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;The Bigelow Aerospace Genesis inflatable test spacecraft included&#xA;many selfie cameras.  Including 'meta' space selfies, taking pictures&#xA;of employee's pictures which were projected onto the exterior of the&#xA;craft.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2VeWv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2VeWv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://bigelowaerospace.com/pages/genesis/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-10-22T16:38:20.703" LastActivityDate="2018-10-22T16:38:20.703" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="21426" PostTypeId="1" CreationDate="2017-05-08T06:28:54.853" Score="21" ViewCount="15315" Body="&lt;p&gt;If ISS was designed this way, couldn't the astronauts have a quick escape if the ISS is destroyed (severe meteor shower, re-entry, uncontrollable fire, explosive decompression, etc.) while they are asleep. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew cabins are also centralized on the station, so they would be quick to access. &lt;/p&gt;&#xA;" OwnerUserId="19566" LastEditorUserId="3029" LastEditDate="2017-05-09T08:02:15.480" LastActivityDate="2017-05-09T08:02:15.480" Title="Why aren't the 'bedrooms' aboard the ISS also the escape pods?" Tags="|iss|orbit|security|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21427" PostTypeId="2" ParentId="21426" CreationDate="2017-05-08T07:57:11.183" Score="63" Body="&lt;p&gt;Primarily, because without a lot of extra equipment, they could at best be space coffins for the astronauts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, they share life support with the rest of the station. Air circulated through them is scrubbed of CO2 in the station's scrubbers. Heat is regulated through station's radiators. This all is powered by station's solar panels. And that's just for breathable atmosphere of reasonable temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now add to this: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;airlock/hatch with undocking mechanism. That's not nearly as simple as it sounds, if you get into details of what that entails.&lt;/li&gt;&#xA;&lt;li&gt;propulsion, to deorbit the pod (or it will float in space until the astronaut dies) - a launch to ISS takes months of planning at least, so no, &quot;rescue mission&quot; is not really an option.&lt;/li&gt;&#xA;&lt;li&gt;attitude control, so that the propulsion deorbits the pod instead of sending it into a higher orbit or into a spin&lt;/li&gt;&#xA;&lt;li&gt;heat shield for reentry. The power output during reentry is of order of gigawatts. Your typical heater for home is 1-2 kilowatts. This is a million times stronger. Shielding against this sort of heat is an enormous challenge&lt;/li&gt;&#xA;&lt;li&gt;parachute&lt;/li&gt;&#xA;&lt;li&gt;survival supplies, and a beacon for locating the landed capsule, because it can land pretty much anywhere on Earth, except polar regions.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;...and probably a bunch of other stuff I missed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a simple box, this turns into a fully featured spacecraft. And you want a bunch of these, one per crew member. The cost becomes... astronomical. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-08T07:57:11.183" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="21428" PostTypeId="2" ParentId="21426" CreationDate="2017-05-08T10:58:34.253" Score="19" Body="&lt;p&gt;Firstly the ISS doesn't have dedicated &quot;escape pods&quot;.  The &lt;a href=&quot;https://en.wikipedia.org/wiki/Crew_Return_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;Crew Return Vehicle&lt;/a&gt; was intended as one, but it was cancelled, so instead the ISS keeps enough Soyuz capsules docked to allow its occupants to escape in an emergency.  This is a case of expedience, not of design from a blank sheet of paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That aside though, all capsules are only big enough to contain the astronauts, and no bigger.  The larger you make it, the more fuel it takes to get it into orbit.  It also costs more with all the heat shielding and other stuff, but that's secondary.  The major cost with getting anything into orbit is the fuel to get it there, and the lighter it is, the less fuel you need.  Also the capsules are designed to safely contain the astronauts during the ridiculously rough rides between Earth and space, but not for comfort when they're up there. So a capsule resembles a race car, not an RV.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of this means that a capsule is just not going to be a comfortable place to sleep.  Conversely, in the main habitation areas you have plenty of space to velcro your sleeping bag to the wall wherever is most convenient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For your list of risks:-&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Meteor showers and other debris risks are known about days or weeks ahead, and the astronauts then do &lt;a href=&quot;https://www.wired.com/2015/07/iss-crew-prepares-evacuate-real-life-gravity-scare/&quot; rel=&quot;noreferrer&quot;&gt;shelter in the Soyuz capsules for safety&lt;/a&gt;, even though it's uncomfortable.&lt;/li&gt;&#xA;&lt;li&gt;ISS re-entry would be known about &lt;strong&gt;months&lt;/strong&gt; in advance, and simply will not happen unexpectedly (because physics). &lt;/li&gt;&#xA;&lt;li&gt;A leak could happen, but that's an all-hands-on-deck emergency, and there is enough reserve air to allow the occupants to don EVA suits for repairs, or if it's not fixable then they can still get to the Soyuz capsules in time.  Contrary to the bad Hollywood films that most likely inform this question, even exposure to space is not immediately fatal, and they've got enough time to get there before that happens anyway.  &lt;a href=&quot;https://space.stackexchange.com/questions/881/are-there-any-safety-procedures-in-place-on-the-iss-in-case-of-puncture&quot;&gt;Related question for this&lt;/a&gt; and &lt;a href=&quot;http://www.spaceref.com/iss/ops/iss.emergency.ops.pdf&quot; rel=&quot;noreferrer&quot;&gt;actual decision tree for what to do&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Uncontrolled_decompression&quot; rel=&quot;noreferrer&quot;&gt;Explosive decompression&lt;/a&gt; needs more than one atmosphere of pressure difference.&lt;/li&gt;&#xA;&lt;li&gt;A fire is an all-hands-on-deck emergency too, and again the crew bail for the Soyuz capsules.  And when fire-fighting, as per the link above: &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Crew actions should provide possible ingress into their spacecraft&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="15976" LastEditorUserId="15976" LastEditDate="2017-05-08T16:19:29.777" LastActivityDate="2017-05-08T16:19:29.777" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="21429" PostTypeId="1" AcceptedAnswerId="21431" CreationDate="2017-05-08T12:56:23.133" Score="6" ViewCount="2043" Body="&lt;p&gt;For a project I have to find select a decoupling mechanism for a spacecraft. I have searched google but I can't find any documentation on decoupling mechanisms. I need to know what types of decoupling mechanisms exist for spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="19570" LastEditorUserId="6944" LastEditDate="2020-09-04T23:24:08.400" LastActivityDate="2020-09-04T23:24:08.400" Title="What types of decoupling mechanisms to separate spacecraft are used in spaceflight?" Tags="|stages|mechanisms|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21430" PostTypeId="2" ParentId="21244" CreationDate="2017-05-08T13:41:13.463" Score="1" Body="&lt;p&gt;&lt;sup&gt;Editor's note: I'm maintaining here information from user &lt;a href=&quot;https://space.stackexchange.com/users/19483/roger&quot;&gt;Roger&lt;/a&gt; until he decides to migrate on his own.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Answer to the question: &quot;Can you add a link or citation that confirms that the temperature of Steve's ions was actually measured by the TII?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have a look at the presentation from Eric Donovan at the 4th Swarm science meeting in Banff: &lt;a href=&quot;http://www.esa.int/spaceinvideos/Videos/2017/03/Opening_of_Swarm_and_CryoSat_Science_Meetings&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.esa.int/spaceinvideos/Videos/2017/03/Opening_of_Swarm_and_CryoSat_Science_Meetings&lt;/a&gt;&#xA;around the time 1:22:14 -&gt; after almost 25 seconds the actual Swarm data are shown (from top to bottom: ion velocity, magnetic anomaly, ion temperature and electron density).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Answer to the question &quot;how did the ESA probe measure Steve's temperature?&quot;: The ion temperature is measured by an Thermal Ion Imager (see &lt;a href=&quot;http://efi.phys.ucalgary.ca/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://efi.phys.ucalgary.ca/&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-05-08T13:41:13.463" CommentCount="3" CommunityOwnedDate="2017-05-08T13:41:18.950" ContentLicense="CC BY-SA 3.0" />
  <row Id="21431" PostTypeId="2" ParentId="21429" CreationDate="2017-05-08T18:44:45.193" Score="13" Body="&lt;p&gt;According to The Logic of Microspace: Technology and Management of Minimum-Cost Space Missions by Rick Fleeter (Microcosm &amp;amp; Kluwer, 2000), p.130, the de-facto standard mechanism for separating spacecraft from launch vehicles is the Marman ring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft and launch vehicle each have a truncated cone structure, the wide side of each fitted against the other. A ring-shaped spring called a Marman band is wrapped around both, forcing them tightly together. There are two explosive bolts on either side of the Marman band, so that if either is blown the band straightens and the two vehicles separate. The release system is set up to blow both bolts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cubesats work differently though I'm not familiar with the standards (I think there are several mechanisms as cubesat is really just a form factor).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Microcosm &amp;amp; Kluwer, 2000, p.131:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Marman or manacle rings are to satellite separation as Mick Jagger is to rock and roll — sort of instant credibiltiy. Nobody doubts Mick is the real thing. Tell any tough reviewer that you’re using a manacle ring separation mechanism, and they’re happy. Unlike Mick, who really is the real thing, the Marman Ring isn’t constant as the Northern star, which Joni Mitchell once pointed out. Like her, over the year it’s been reinterpreted many times, and the real physics behind its operation are somewhat subtle and beyond the current scope.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The text continues on to describe various technical issues that one must consider in a design that needs to have high reliability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screen shots from: &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/e/ee/Guideline_No._GD-Ed-2214_Marman_Clamp_Systems_Design_Guidelines.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://upload.wikimedia.org/wikipedia/commons/e/ee/Guideline_No._GD-Ed-2214_Marman_Clamp_Systems_Design_Guidelines.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zXd68.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zXd68.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tuDwX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tuDwX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16733" LastEditorUserId="12102" LastEditDate="2017-05-09T06:03:57.000" LastActivityDate="2017-05-09T06:03:57.000" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21434" PostTypeId="1" AcceptedAnswerId="21435" CreationDate="2017-05-08T20:18:47.017" Score="6" ViewCount="2365" Body="&lt;p&gt;After many successful launches with the Delta II, ULA  made the Delta IV rocket orange. What's the reason for this strange color change? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also the Atlas V's first stage is orange, is there any specific reason that launch providers like coloring their rockets a specific color? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After all, it seems expensive to paint a whole rocket which will be discarded after a single launch. If the bright color is for tracking the rockets, why not use a paint scheme like the Saturn rockets had? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/h4lWG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h4lWG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This picture shows the Delta rocket evolution, and shows the Delta IV in blue. What's with the sudden color change?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="13208" LastEditDate="2017-05-09T14:50:36.577" LastActivityDate="2017-05-09T14:50:36.577" Title="Why are some rockets orange?" Tags="|rockets|atlas-v|delta-iv-heavy|delta-iv|launch-system|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21435" PostTypeId="2" ParentId="21434" CreationDate="2017-05-08T20:40:17.200" Score="11" Body="&lt;p&gt;From perusing online, it seems there's a consensus that the insulating foam is just that color. They painted the Delta II, but stopped painting due to the added weight. As to why they painted the Delta II (and Saturns), I believe it is for protection against UV. They probably solved the UV issue, and thus opted for the lower weight for the Delta IV.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastActivityDate="2017-05-08T20:40:17.200" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21436" PostTypeId="1" AcceptedAnswerId="21437" CreationDate="2017-05-09T06:25:38.373" Score="1" ViewCount="108" Body="&lt;p&gt;From &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/tips.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The TiPS (Tethers in Space Physics Satellite) satellite is a tether physics experiment consisting of two end masses connected by a 4 km non-conducting tether. The satellite was deployed on 20 June 1996 at an altitude of 1,022 kilometers.&lt;/p&gt;&#xA;&lt;p&gt;This experiment is designed to increase knowledge about gravity-gradient tether dynamics and the survivability of tethers in space. TiPS consist of two end masses separated by a four kilometer tether. The End masses, named &lt;strong&gt;Ralph&lt;/strong&gt; (1a) and &lt;strong&gt;Norton&lt;/strong&gt; (1b), weigh 37.7 and 10.8 kilograms, respectively.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;How or from where did the the two TiPS (Tethers in Space Physics Satellite) components get the names Ralph and Norton?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Naaj.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Naaj.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-09T07:07:25.363" Title="How/from where did the the two TiPS components get the names Ralph and Norton?" Tags="|history|space-tether|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21437" PostTypeId="2" ParentId="21436" CreationDate="2017-05-09T07:07:25.363" Score="5" Body="&lt;p&gt;&lt;strong&gt;Ralph&lt;/strong&gt; Kramden and Ed &lt;strong&gt;Norton&lt;/strong&gt; were two characters in a very early (1951-1955) TV sitcom called &lt;em&gt;The Honeymooners&lt;/em&gt;. Bonus: The &lt;em&gt;New Horizons&lt;/em&gt; satellite that flew past Pluto almost two years ago sported two experiments named &lt;strong&gt;Ralph&lt;/strong&gt; and &lt;strong&gt;Alice&lt;/strong&gt;, named after Ralph and Alice Kramden.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-05-09T07:07:25.363" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21438" PostTypeId="1" CreationDate="2017-05-09T08:19:17.030" Score="3" ViewCount="549" Body="&lt;p&gt;My local engineering newspaper has an article on the X-37B craft, with an image displaying the craft within half of the shell that encapsulates it. The shell has a number of plates arranged on its inside surface, which look decidedly like silvery 'Lego' base plates:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gllRk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gllRk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are those plates for?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image is captioned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This is how the X-37B is encapsulated at the top of an Atlas 5 rocket from the Cape Canaveral Cape Canaveral launch site in Florida. Photo: U.S. Air Force courtesy photo Photo: U.S. Air Force courtesy photo.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="19586" LastEditorUserId="12102" LastEditDate="2017-06-24T12:05:23.907" LastActivityDate="2017-06-24T12:05:23.907" Title="What are these 'Lego' plates inside the Atlas 5 fairing?" Tags="|spacecraft|atlas-v|fairings|identify-this-object|" AnswerCount="1" CommentCount="0" ClosedDate="2017-05-09T09:38:06.207" ContentLicense="CC BY-SA 3.0" />
  <row Id="21439" PostTypeId="2" ParentId="21438" CreationDate="2017-05-09T09:36:39.197" Score="2" Body="&lt;p&gt;As detailed in &lt;a href=&quot;https://space.stackexchange.com/a/9070/19586&quot;&gt;this other answer&lt;/a&gt;, they provide noise dampening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks to Hobbes who is possibly a Helmholtz nut. ;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(...gotta wait 2 days before I can close this question...)&lt;/p&gt;&#xA;" OwnerUserId="19586" LastActivityDate="2017-05-09T09:36:39.197" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21440" PostTypeId="1" AcceptedAnswerId="21441" CreationDate="2017-05-09T11:36:17.787" Score="9" ViewCount="999" Body="&lt;p&gt;The object &lt;a href=&quot;https://en.wikipedia.org/wiki/(486958)_2014_MU69&quot; rel=&quot;nofollow noreferrer&quot;&gt;2014 MU69&lt;/a&gt; is much smaller and dimmer than Pluto. Even as New Horizons approaches, it will be much harder for its 8.2 inch aperture, cooled CCD long range imager &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Long-Range_Reconnaissance_Imager_.28LORRI.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;LORRI&lt;/a&gt; to get a fix on it until it is quite close.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;EDIT:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;New Horizons made it's first sighting of Pluto &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#First_Pluto_sighting&quot; rel=&quot;nofollow noreferrer&quot;&gt;nine years&lt;/a&gt; before it reached Pluto. MU69 is going to be about 100,000 times dimmer, which means it will make its first sighting more like &lt;em&gt;nine days&lt;/em&gt; before it gets there (roughly) (actually it could be a few months, &lt;a href=&quot;https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA21588&quot; rel=&quot;nofollow noreferrer&quot;&gt;see this&lt;/a&gt;). Thus any navigation using the spacecraft's imaging system may have a narrow window of opportunity to be useful. This is why I think this question is particularly interesting - is there a plan to use the spacecraft's imaging data in any way to guide the flyby, or will it be done only with astrometry of MU69 from Earth's Neighborhood and radar range/rate of New Horizons plus data from its last known address, Pluto/Charon?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Pluto's orbit has been monitored for almost a century, while that of 2014 MU69 only about three years, although with three years of diffraction-limited precision astrometry from Hubble that can go a long way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But going from spots on an image to a true three dimensional orbit and six dimensional state vectors is not straightforward. It seems there could be a larger envelope in distance/eccentricity that could generate the same apparent motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How can 2014 MU69's orbit be know well enough for a close flyby by New Horizons?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm also wondering how well has 2014 MU69's actual orbit been plotted, and how close is the flyby expected to be &lt;em&gt;to the target&lt;/em&gt;? Are they just aiming for zero impact parameter (straight-on) and letting the uncertainty determine the distance, or are they aiming a little to one side? But these are secondary and I will ask separately if needed. &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                       Pluto       2014 MU69&#xA;                      -------      ---------&#xA;Diameter              2380 km       ~35 km&#xA;Albedo               ~0.58          ~0.04-0.14?&#xA;distance to sun       5.0E+09 km     6.5E+09 km&#xA;Vis mag from Earth     ~15          ~ 27&#xA;Years of observation    87 yr       ~ 3 yr&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;At the time of flyby:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;relative velocity ~ 50,000 kph&lt;/li&gt;&#xA;&lt;li&gt;distance to earth ~ 6,650,000,000 km&lt;/li&gt;&#xA;&lt;li&gt;round trip radio  ~ 12.3 hours&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;2014 MU69: &lt;a href=&quot;http://www.minorplanetcenter.net/db_search/show_object?object_id=486958&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.minorplanetcenter.net/db_search/show_object?object_id=486958&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-01-01T13:24:43.050" LastActivityDate="2019-01-01T13:28:58.683" Title="How (the heck) can 2014 MU69's orbit be know well enough for a close flyby by New Horizons?" Tags="|orbital-mechanics|new-horizons|planning|2014-mu69|" AnswerCount="2" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="21441" PostTypeId="2" ParentId="21440" CreationDate="2017-05-09T11:48:44.190" Score="5" Body="&lt;p&gt;The long orbital calculations of Pluto's orbit were done primarily to save fuel, and secondarily to target a very narrow region. The mission intended to fly between the Sun and Pluto and Charon (And Earth and Pluto and Charon as well), which could only be done with very careful orbital knowledge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is quite a large amount of fuel reserved for the 2014 MU69 mission. They are still trying to determine how closely they will approach it, but it will be between &lt;a href=&quot;https://blogs.nasa.gov/pluto/2017/04/28/no-sleeping-back-on-earth/&quot; rel=&quot;nofollow noreferrer&quot;&gt;3000 km and 20000 km&lt;/a&gt;. While I don't doubt they would love to be between the Sun and Earth and this object as before, it isn't as high of a priority.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hubble will continue to make observations, as will ground based telescopes, to help refine the mission to come. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, New Horizons will use it's  camera to image it and refine the trajectory, as it did with Pluto.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aiming straight on would be a bad thing, although for this early in the planning, it would be acceptable. I bet they are aiming straight for it now, but will adjust to one side when the date is closer (No source, but it makes sense) Aiming straight at it would mean that you wouldn't know what side the object actually is, making taking pictures very difficult!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I actually did a video on this not remembering that I had answered this here. The short is, lots of observations with Hubble, using stellar occulations, using databases of stars to take in to account the stellar motion that each star makes. Adding in a few observations from New Horizons itself, and we have a trajectory good enough for a flyby!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/nwsyyDV7ReM?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-01-01T13:28:58.683" LastActivityDate="2019-01-01T13:28:58.683" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="21442" PostTypeId="1" CreationDate="2017-05-09T14:28:49.603" Score="5" ViewCount="1525" Body="&lt;p&gt;Popular sources, like Vintage Space and New Scientist, allege that the Chinese experimented with (oak?) wood ablative heat shields for manned reentry vehicles in the 1960s, but I have not been able to find any technical information. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone have translated English papers from the original project, or intelligence reports? Chinese documents would also be valuable, I would be willing to have them translated.&lt;/p&gt;&#xA;" OwnerUserId="19099" LastEditorUserId="12102" LastEditDate="2019-01-10T05:23:36.210" LastActivityDate="2019-01-10T05:23:36.210" Title="White papers on alleged 1960s Chinese wooden ablative reentry heat shield" Tags="|reentry|history|heat-shield|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="21443" PostTypeId="1" AcceptedAnswerId="21455" CreationDate="2017-05-09T19:56:27.083" Score="3" ViewCount="223" Body="&lt;p&gt;A rocket moves under the conservation of linear momentum, where the gases formed inside it, exit through the nozzle, and in turn, the rocket moves forward. What I want to ask about is that: is it true that increasing the velocity of the issued gases would help in letting the gases be unseen by our eyes(or seen much unclearer) when looking at them flying in the atmosphere, as they are being issued in a diverging mode and not in a linear mode ?&lt;/p&gt;&#xA;" OwnerDisplayName="user19597" LastEditorUserId="195" LastEditDate="2017-05-11T14:37:52.713" LastActivityDate="2017-05-11T14:37:52.713" Title="Does velocity of the exit gasses of a rocket affect visibility of the plume?" Tags="|rockets|exhaust|visual-signature|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21444" PostTypeId="1" CreationDate="2017-05-09T20:05:57.827" Score="2" ViewCount="105" Body="&lt;p&gt;3 reaction wheels are used to control the orientation of the satellite. If one wheel fails, how does the redundant wheel take its place?&lt;/p&gt;&#xA;" OwnerUserId="19598" LastActivityDate="2017-05-09T20:05:57.827" Title="CubeSat redundant reaction wheel" Tags="|reaction-wheel|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21445" PostTypeId="1" AcceptedAnswerId="21448" CreationDate="2017-05-10T02:13:18.713" Score="12" ViewCount="522" Body="&lt;p&gt;I see several mentions in recent news of a proposed tax on launches in the state of California, and that it would be 'by the mile'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If someone is familliar with the issue, could they outline how this works? Is it just maximum altitude, or total integrated path length? This distinction is quite significant when comparing launch to orbit and tourist sub-orbital missions. Is there a hard cut-off at the Karman line?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume it applies to rocket propulsion, and not jet engines, but what about balloons and mixed combos?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quartz: &lt;a href=&quot;http://linkis.com/qz.com/977207/urnbQ&quot; rel=&quot;noreferrer&quot;&gt;California’s plan to tax rockets by the mile is exactly what space companies want (what's next, speeding tickets?)&lt;/a&gt; &lt;strong&gt;note&lt;/strong&gt; there are links to pdfs in this article that seem to contain some tax language.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;San Francisco Chronicle: &lt;a href=&quot;http://www.sfchronicle.com/business/article/California-plans-for-collecting-taxes-on-11119631.php&quot; rel=&quot;noreferrer&quot;&gt;California plans for collecting taxes on spaceflight&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ars Technica: &lt;a href=&quot;https://arstechnica.com/science/2017/05/california-may-have-found-a-creative-new-revenue-stream-taxing-rocket-launches/&quot; rel=&quot;noreferrer&quot;&gt;California seeks to tax rocket launches, which are already taxed (The tax will be based upon &quot;mileage&quot; traveled by that spacecraft from California)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parabolic Arc: &lt;a href=&quot;http://www.parabolicarc.com/2017/05/08/california-considers-tax-launches-state/&quot; rel=&quot;noreferrer&quot;&gt;California Considers Tax on Launches Within the State&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-10T04:23:28.317" LastActivityDate="2017-05-10T05:15:03.330" Title="California's rocket tax &quot;by the mile&quot;, how would that work exactly?" Tags="|commercial|tourism|space-taxes|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21446" PostTypeId="2" ParentId="21442" CreationDate="2017-05-10T04:02:22.923" Score="2" Body="&lt;p&gt;Here is the wikipedia entry for the Chinese satellite.&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Fanhui_Shi_Weixing&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Fanhui_Shi_Weixing&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2017-05-10T04:02:22.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21448" PostTypeId="2" ParentId="21445" CreationDate="2017-05-10T05:15:03.330" Score="11" Body="&lt;p&gt;&lt;a href=&quot;https://www.ftb.ca.gov/law/regs/25137-15/06162017-draft-text-for-Notice.pdf&quot; rel=&quot;noreferrer&quot;&gt;The official draft&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears that &quot;milleage&quot; is in fact understood as &quot;altitude&quot;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;if a company can’t reveal the specifics of its mission due to confidentiality concerns — common with contracts with the military — a launch’s mileage will be presumed to be 310 miles under the proposed rules. (For reference, the International Space Station is about 250 miles above the Earth.) &lt;a href=&quot;http://www.sfchronicle.com/business/article/California-plans-for-collecting-taxes-on-11119631.php&quot; rel=&quot;noreferrer&quot;&gt;src&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The project defines which part of receipt from the launch is taxable - it &#xA; applies to portion of altitude &quot;within California aerospace&quot; - 62 miles. If launch brings receipt X(\$), and goes to altitude Y(miles), taxable part of receipt is $X\cdot {62 \over Y}$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then the proposal goes into a lot of detail over how to apply that portion as a portion of receipt globally, in proportion to total sum of altitudes achieved by the company globally, which becomes quite convoluted and entirely tax law, no longer really a &quot;space&quot; subject, so excuse me, I really don't want to get into &lt;em&gt;that&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-10T05:15:03.330" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21449" PostTypeId="1" CreationDate="2017-05-10T06:33:03.407" Score="3" ViewCount="275" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Coherent_backscattering&quot; rel=&quot;nofollow noreferrer&quot;&gt;Coherent backscattering&lt;/a&gt; is an optical phenomenon which is a bit complicated and as far as I know is pretty difficult to demonstrate in &lt;em&gt;visible white light images&lt;/em&gt;. Image below from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Coherent_backscattering.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KrNis.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KrNis.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;According to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Opposition_surge#Coherent_backscatter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Coherent Backscatter section of Wikipedia's article on lunar Opposition Surge&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A theory for an additional effect that increases brightness during opposition is that of coherent backscatter. In the case of coherent backscatter, the reflected light is enhanced at narrow angles if the size of the scatterers in the surface of the body is comparable to the wavelength of light and the distance between scattering particles is greater than a wavelength. The increase in brightness is due to the reflected light combining coherently with the emitted light.&lt;/p&gt;&#xA;&lt;p&gt;Coherent backscatter phenomena have also been observed with radar. In particular, recent observations of Titan at 2.2 cm with Cassini have shown that a strong coherent backscatter effect is required to explain the high albedos at radar wavelengths.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;While the word &amp;quot;light&amp;quot; is mentioned, the references refer to &lt;em&gt;essentially monochromatic, longer wavelength radar measurements&lt;/em&gt;. Opposition surge is probably due to the simpler, well-understood geometrical effects known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Opposition_surge#Shadow_hiding&quot; rel=&quot;nofollow noreferrer&quot;&gt;shadow-hiding&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Heiligenschein&quot; rel=&quot;nofollow noreferrer&quot;&gt;retro-reflection or Heiligenschein&lt;/a&gt;. You can see slightly brighter ground reflections around the shadows of astronaut's helmets in photos on the moon, for example &lt;a href=&quot;https://space.stackexchange.com/q/20071/12102&quot;&gt;in this as-yet unanswered question&lt;/a&gt; and also &lt;a href=&quot;https://space.stackexchange.com/q/21297/12102&quot;&gt;in this as-yet unanswered question&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Coherent backscattering using visible light &lt;em&gt;can be demonstrated in the laboratory&lt;/em&gt; in situations where one has great control over the particle size and index of refraction contrast, for example single sized (&lt;a href=&quot;https://en.wikipedia.org/wiki/Dispersity&quot; rel=&quot;nofollow noreferrer&quot;&gt;monodisperse&lt;/a&gt;) sub-micron transparent plastic spheres suspended in water &lt;a href=&quot;http://repository.kaust.edu.sa/kaust/bitstream/10754/235651/1/Edgar+Morelas_Final+Thesis.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;, &lt;a href=&quot;http://physics.gac.edu/%7Epsaul/cbsweb.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;2&lt;/a&gt;, &lt;a href=&quot;https://www.eng.yale.edu/caolab/papers/Yaron2_2015.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;3&lt;/a&gt;, &lt;a href=&quot;http://laser.physics.sunysb.edu/%7Edbennett/optics4/optics4.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;4&lt;/a&gt;, and with narrow-band light with transparent biological samples &lt;a href=&quot;https://www.equipes.lps.u-psud.fr/Montambaux/polytechnique/PHY560B/papers/appliedoptics.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;5&lt;/a&gt;. At radar wavelengths, rocks and soil particles are not opaque, but instead become dielectric particles, thus the observation of coherent backscattering at radar wavelengths from astronomical bodies.&lt;/p&gt;&#xA;&lt;p&gt;In light of &lt;a href=&quot;https://space.stackexchange.com/q/20072/12102&quot;&gt;this insufficiently-answered question&lt;/a&gt;'s multiple re-bumping by community, I'd like to ask - &lt;em&gt;are there any&lt;/em&gt; visible light images of Earth that contain bright sub-solar spots that are generally agreed to have been produced by coherent backscattering?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-10-14T10:21:59.460" Title="Are there ANY verified satellite images of visible light coherent backscattering from Earth?" Tags="|imaging|photography|earth-observation|image-processing|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21453" PostTypeId="1" AcceptedAnswerId="21469" CreationDate="2017-05-10T15:39:22.217" Score="4" ViewCount="294" Body="&lt;p&gt;I see several mentions in recent news of a proposed tax on launches in the state of California, and that it would be 'by the mile'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nature of the currently proposed tax is described in &lt;a href=&quot;https://space.stackexchange.com/a/21448/12102&quot;&gt;this excellent answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What expenses and risks might the state of California incur that might partially justify a new, specific &quot;rocket tax&quot; beyond how the industry pays taxes already? &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Quartz: &lt;a href=&quot;http://linkis.com/qz.com/977207/urnbQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;California’s plan to tax rockets by the mile is exactly what space companies want (what's next, speeding tickets?)&lt;/a&gt; &lt;strong&gt;note&lt;/strong&gt; there are links to pdfs in this article that seem to contain some tax language.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;San Francisco Chronicle: &lt;a href=&quot;http://www.sfchronicle.com/business/article/California-plans-for-collecting-taxes-on-11119631.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;California plans for collecting taxes on spaceflight&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ars Technica: &lt;a href=&quot;https://arstechnica.com/science/2017/05/california-may-have-found-a-creative-new-revenue-stream-taxing-rocket-launches/&quot; rel=&quot;nofollow noreferrer&quot;&gt;California seeks to tax rocket launches, which are already taxed (The tax will be based upon &quot;mileage&quot; traveled by that spacecraft from California)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Parabolic Arc: &lt;a href=&quot;http://www.parabolicarc.com/2017/05/08/california-considers-tax-launches-state/&quot; rel=&quot;nofollow noreferrer&quot;&gt;California Considers Tax on Launches Within the State&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-05-11T10:22:04.723" Title="What expenses and risks might the state of California incur that might partially justify a new &quot;rocket tax&quot;?" Tags="|commercial|tourism|space-taxes|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21454" PostTypeId="2" ParentId="14313" CreationDate="2017-05-10T15:52:18.413" Score="5" Body="&lt;p&gt;Check out this article and the sources cited: Simulation of LOX reorientation using magnetic positive positioning &lt;a href=&quot;https://link.springer.com/article/10.1007/BF02908417&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://link.springer.com/article/10.1007/BF02908417&lt;/a&gt;&#xA;Looks like it has been studied.&lt;/p&gt;&#xA;&lt;p&gt;Also this:&lt;/p&gt;&#xA;&lt;p&gt;The abstract of the apparently not paywalled &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0895717710000087&quot; rel=&quot;nofollow noreferrer&quot;&gt;Simulation of magnetic positive positioning for space based fluid management systems&lt;/a&gt;  (&lt;a href=&quot;https://doi.org/10.1016/j.mcm.2010.01.002&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://doi.org/10.1016/j.mcm.2010.01.002&lt;/a&gt;) Jeffrey G. Marchetta, Amanda P. Winter, Mathematical and Computer Modelling, 51, (9–10), May 2010, 1202-1212&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Experimental and computational studies have shown that a sufficiently strong magnetic field can influence a magnetically susceptible liquid. An improved simulation integrates an electromagnetic field model and incompressible flow model to predict fluid reorientation using realistic magnetic fields. Flow fields are presented incorporating several realistic magnetic fields to verify and validate the connectivity of the integrated models.&lt;/p&gt;&#xA;&lt;p&gt;Conclusions are drawn about the fidelity of the integrated simulation in modeling magnetically induced fluid flows. The simulation is used to model the application of magnetic positive positioning of LOX in a reduced gravity experiment utilizing a realistic magnetic field. Preflight experiment predictions of the performance of the magnetic field in reorienting LOX are presented and recommendations are made for future design.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The article goes in to significant magnetofluid dynamics calculations and describes &lt;em&gt;a proposed ORBITEC MFM LOX parabolic aircraft flight experiment&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The photo is taken in terrestrial gravity. In mictrogravity the LOX would try to climb all over the magnet, which would probably need to be protected by the quartz.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6d6n4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6d6n4.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Fig. 4. LOX profile for a 0.0962 m length, 0.0354 m height quartz container with a 30% fluid fill influenced by a NdFeB magnet.&amp;quot; From &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0895717710000087&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OxVaP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OxVaP.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Fig. 7. Measured free surface height along the quartz container length.&amp;quot; From &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S0895717710000087&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="19606" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-21T03:01:59.060" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21455" PostTypeId="2" ParentId="21443" CreationDate="2017-05-10T17:48:46.270" Score="5" Body="&lt;p&gt;In the atmosphere, the answer is mostly no, because the exhaust gases slow rapidly when they hit the surrounding air. Increasing the exhaust velocity by a modest factor will diffuse the plume only a very small amount, and increasing it by a large factor will probably put enough energy into the surrounding air to make it incandesce.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside of the atmosphere, the exhaust velocity is high enough to diffuse the plume quite rapidly; you can see the difference in any SpaceX launch video between the first-stage exhaust and second-stage. The chemistry is the same for both stages and the exhaust velocity only modestly higher for the second stage, but the plume from the second stage burn is much fainter. You can also see over the course of the first stage burn that the exhaust goes from a tightly confined bright yellow plume at low altitudes to a fainter, more diffuse and orangey pattern at high altitude.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-05-10T17:48:46.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21456" PostTypeId="2" ParentId="21244" CreationDate="2017-05-10T17:59:12.023" Score="9" Body="&lt;p&gt;I want to offer some input to the above ---&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm thrilled in the level, depth, and knowledge reflected in the questions and discussion. This has been eye opening for me.... Citizen Scientist has a new meaning for me!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding 300 km vs 450 km. In my research I deploy, operate, and use data from scientific cameras. The data I was using for Steve came from a &quot;redline&quot; camera.Those emissions typically peak below 300 km. The Swarm measurements were made at ~450 km, on the magnetic field lines that also go through (below) the glowing region below. The temp/velocity measurements were at 450 km... and I presume that all the way down to 100 km, the velocity is fast and the temperature is high, on those field lines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Swarm data is from the Electric Field Instrument (EFI) on one of the three European Space Agency (ESA) Swarm satellites. These instruments were developed at UCalgary by Professor David Knudsen and his team, and built by Canadian industry (COM DEV which is now Honeywell).&lt;/p&gt;&#xA;" OwnerUserId="19608" LastActivityDate="2017-05-10T17:59:12.023" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21457" PostTypeId="1" CreationDate="2017-05-10T19:01:01.380" Score="6" ViewCount="721" Body="&lt;p&gt;Elon Musk said that the integration of the Falcon Heavy was &lt;a href=&quot;http://spacenews.com/spacex-to-launch-falcon-heavy-with-two-flight-proven-boosters-this-year/&quot; rel=&quot;noreferrer&quot;&gt;&quot;shockingly difficult&quot;&lt;/a&gt;. I can absolutely believe that. But at the same time, SpaceX is reusing two &quot;flight proven&quot; boosters for the Heavy. Presumably, they could be modified rather easily for the job, they would not be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what explains this apparent contradiction? Is the central booster so much different from the Falcon 9 first stage?&lt;/p&gt;&#xA;" OwnerUserId="11256" LastActivityDate="2017-05-10T20:15:11.643" Title="Why is the development of the Falcon Heavy difficult, when two of three cores are reused?" Tags="|spacex|falcon-9|falcon-heavy|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21458" PostTypeId="1" CreationDate="2017-05-10T19:20:12.613" Score="3" ViewCount="1707" Body="&lt;p&gt;This should be a comment to an 2012rcampion's answer to this question: &lt;a href=&quot;https://space.stackexchange.com/questions/8911/determining-orbital-position-at-a-future-point-in-time&quot;&gt;Determining orbital position at a future point in time&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I'm using the &lt;a href=&quot;https://ssd.jpl.nasa.gov/?planet_pos&quot; rel=&quot;nofollow noreferrer&quot;&gt;linked data&lt;/a&gt; of that answer, under &amp;quot;Computing Position from Orbital Elements&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;As each parameter has two values, I have declared the variables as arrays of two values:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;public double[] semimajorAxis = new double[2];&#xA;public double[] eccentricity = new double[2];&#xA;public double[] inclination = new double[2];&#xA;public double[] meanLongitude = new double[2];&#xA;public double[] longitudeOfPerihelion = new double[2]; &#xA;public double[] longitudeOfTheAscendingNode = new double[2];&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;So, for example, in the case of Earth, &lt;code&gt;semimajorAxis[0]&lt;/code&gt; would be 1.00000261, and &lt;code&gt;semimajorAxis[1]&lt;/code&gt; would be 0.00000562.&lt;/p&gt;&#xA;&lt;p&gt;Now, the method is:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;void ComputePosition(){&#xA;    double tMillisFromJ2000 = DateTime.Now.ToUniversalTime().Subtract(new DateTime(2000, 1, 1, 12, 0, 0, DateTimeKind.Utc)).TotalMilliseconds;&#xA;    double tCenturiesFromJ2000 = tMillisFromJ2000 / (1000 * 60 * 60 * 24 * 365.25 * 100);&#xA;    double a = semimajorAxis [0] + semimajorAxis [1] * tCenturiesFromJ2000;&#xA;    double e = eccentricity [0] + eccentricity [1] * tCenturiesFromJ2000;&#xA;    double i = inclination [0] + inclination [1] * tCenturiesFromJ2000;&#xA;    double L = meanLongitude [0] + meanLongitude [1] * tCenturiesFromJ2000;&#xA;&#xA;    Debug.Log (L);&#xA;&#xA;    double p = longitudeOfPerihelion [0] + longitudeOfPerihelion [1] * tCenturiesFromJ2000;&#xA;    double W = longitudeOfTheAscendingNode [0] + longitudeOfTheAscendingNode [1] * tCenturiesFromJ2000;&#xA;&#xA;    double M = L - p;&#xA;    double w = p - W;&#xA;&#xA;    double E = M;&#xA;    while (true) {&#xA;        double dE = (E - e * Mathd.Sin (E) - M) / (1 - e * Mathd.Cos (E));&#xA;        E -= dE;&#xA;        if (Mathd.Abs (dE) &amp;lt; 1e-6)&#xA;            break;&#xA;    }&#xA;&#xA;    double P = a * (Mathd.Cos (E) - e);&#xA;    double Q = a * Mathd.Sin (E) * Math.Sqrt (1 - Mathd.Pow (e, 2d));&#xA;&#xA;    // rotate by argument of periapsis&#xA;    double x = Mathd.Cos(w) * P - Mathd.Sin(w) * Q;&#xA;    double y = Mathd.Sin (w) * P + Mathd.Cos (w) * Q;&#xA;    // rotate by inclination&#xA;    double z = Mathd.Sin(i) * x;&#xA;    x = Mathd.Cos (i * x);&#xA;    // rotate by longitude of ascending node&#xA;    double xTemp = x;&#xA;    x = Mathd.Cos (W) * xTemp - Mathd.Sin (W) * y;&#xA;    y = Mathd.Sin (W) * xTemp + Mathd.Cos (W) * y;&#xA;&#xA;    position = new Vector3d (x * 149597870700d, y * 149597870700d, z * 149597870700d);&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;I've checked the code and orbital parameters three times but it's giving me wrong positions for at least some planets. Neptune, 29 AU above the Sun, totally normal:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mQpjW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mQpjW.png&quot; alt=&quot;Neptune at 29 AU above the Sun?!&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I can't see any sense in these positions. They don't look any similar to what we can see in other simulators.&lt;/p&gt;&#xA;&lt;p&gt;Is there something wrong or missing in the code? should I check the parameters again?&lt;/p&gt;&#xA;" OwnerUserId="19596" LastEditorUserId="8693" LastEditDate="2020-09-24T11:48:17.833" LastActivityDate="2020-09-24T11:48:17.833" Title="Is this C# code to obtain the coordinates of the planets correct?" Tags="|orbital-mechanics|simulation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21460" PostTypeId="2" ParentId="21457" CreationDate="2017-05-10T20:15:11.643" Score="6" Body="&lt;p&gt;In the end, they decided that the existing Falcon 9 cores with minor modifications could be used as the side boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difficulty came in the center booster. This needs to handle the load of the two side boosters, and carries the heaviest load (Second stage and payload). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally seperation is trickier than any they have tried to date. Currently the use a pusher to shove away the second stage. This is a totally different type of separation event.  Have to make sure they fly away, not back into the center core, they have to be pushed away firmly enough, but not with something that can damage the remaining center core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Issues with handling engine out become much more problematic. On a single core, if an engine has trouble, it gets shut down, the computer decides whether to throttle the others up, or else just run longer to compensate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you have two side boosters each with 9 engines, and one engine shutting down you have to balance the thrust difference across the entire stack. That is probably a lot more complex than you would imagine. Especially with 27 engines across three cores.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-05-10T20:15:11.643" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21461" PostTypeId="1" CreationDate="2017-05-08T02:11:10.520" Score="3" ViewCount="1754" Body="&lt;p&gt;I am trying to write a program to be able to &quot;model&quot; a Low Earth Orbit satellite's position at for a given time in the future. However I do not have a TLE for the desired satellite, I simply have a state-vector (Epoch and XYZ Position and Velocity). I am aware that this may not be a accurate projection for any significant amount of time, but this is all I have to work with. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the recommended steps be for working this out? Could I convert the state-vector into orbital elements somehow and then use those elements to calculate the position in respect to time? I am rather new to orbital mechanics so I apologize if this is a simple question. I have done some research and it appears the the orbital elements route seems like the route that most others have taken. However once I get those orbital elements, how do I calculate the satellites position?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once I have an algorithm worked out I have other satellites with TLE's I plan to decompose into state-vectors and then compare the calculated state vector position with the calculated TLE position to ensure I am in the right ballpark.&lt;/p&gt;&#xA;" OwnerDisplayName="Daniel Lecklider" LastActivityDate="2017-05-10T20:56:02.290" Title="How to calculate a Satellite position at a given time" Tags="|orbit|orbital-elements|orbital-mechanics|artificial-satellite|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21462" PostTypeId="2" ParentId="21458" CreationDate="2017-05-10T21:55:04.823" Score="3" Body="&lt;p&gt;The document to which you wished to link is &lt;a href=&quot;https://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Keplerian Elements for Approximate Positions of the Major Planets&lt;/a&gt;. You did not read the instructions! Step 1 says &quot;Compute the value of each of that planet's six elements: $a= a_0 + \dot a T$, &lt;strong&gt;etc.&lt;/strong&gt;&quot; You didn't do the &lt;em&gt;et cetera&lt;/em&gt; part of that step. Be very careful when computing the mean longitude $L*$. You need to bring the result into the range $(-180^\circ,180^\circ]$ using the modulus function (&lt;code&gt;%&lt;/code&gt; in the C family of languages).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another place you didn't read the instructions: The angles in that document are in degrees. I would diverge from that document and convert to radians after computing M (modulo 360 degrees). Then you don't have to do the silly step of computing eccentricity in degrees.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Based on requests, and a closed follow-up question, how to compute the modulus of a real number with respect to another real number is language-dependent. With python, it's easy: Just use the &lt;code&gt;%&lt;/code&gt; operator. You'll get almost what you expect. For example,&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;&amp;gt;&amp;gt;&amp;gt; (10.123456789*math.pi % math.pi)/math.pi&#xA;0.12345678900000093&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;That little bit at the end? That's floating point arithmetic for you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;code&gt;%&lt;/code&gt; operator doesn't work this way in the C family of languages. In C, you'll need to use &lt;code&gt;fmod&lt;/code&gt; or &lt;code&gt;remainder&lt;/code&gt;, prefaced by &lt;code&gt;std::&lt;/code&gt; in C++. The language in question is C#. Here, you'll need to use &lt;a href=&quot;https://msdn.microsoft.com/en-us/library/system.math.ieeeremainder(v=vs.110).aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;code&gt;math.IEEERemainder&lt;/code&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-05-12T23:23:07.100" LastActivityDate="2017-05-12T23:23:07.100" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21464" PostTypeId="1" CreationDate="2017-05-11T05:31:41.497" Score="3" ViewCount="753" Body="&lt;p&gt;How to measure velocity of sounding rocket?? is using a pitot tube an efficient way to measure and how exactly to measure speed of sounding rocket using a pitot tube ???are there any other method to measure speed of sounding rocket&lt;/p&gt;&#xA;" OwnerUserId="19611" LastActivityDate="2017-09-08T12:01:57.827" Title="to measure velocity of sounding rocket" Tags="|design|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21465" PostTypeId="1" CreationDate="2017-05-11T07:23:33.667" Score="5" ViewCount="512" Body="&lt;p&gt;What are the orders of magnitude of disturbing torques on the ISS itself, preferably split into the following categories, but any numbers (with source) will be useful:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Gravity gradient&lt;/li&gt;&#xA;&lt;li&gt;Magnetic&lt;/li&gt;&#xA;&lt;li&gt;Solar Radiation Pressure (SRP)&lt;/li&gt;&#xA;&lt;li&gt;Thrust misalignment &lt;/li&gt;&#xA;&lt;li&gt;Mass movement&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Related:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/21413/why-doesnt-the-iss-start-to-spin-if-people-walk-inside/21416#21416&quot;&gt;Why doesn&amp;#39;t the ISS start to spin if people walk inside?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/21422/how-well-and-how-rapidly-can-the-iss-attitude-control-system-detect-periods-of-a&quot;&gt;How well and how rapidly can the ISS attitude control system detect periods of astronaut activity?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19570" LastEditorDisplayName="user687" LastEditDate="2017-05-12T07:58:10.933" LastActivityDate="2017-05-12T07:58:10.933" Title="What are the disturbing torques on the ISS?" Tags="|iss|low-earth-orbit|attitude|" AnswerCount="0" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21466" PostTypeId="2" ParentId="21440" CreationDate="2017-05-11T09:08:36.460" Score="5" Body="&lt;p&gt;Okay, collecting my comments into an answer: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As soon as you have a visual, and have enough delta-V, you CAN get a fly-by, although precise time and speed of flyby will remain unknown until much later (when changes of size of the object in images become significant).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This works only for when curvature of orbits is not significant - gravitational influence of other bodies is minor and trajectories on the distances involved can be approximated with straight lines. It will work for moderate orbital curvature too, if you repeat the process iteratively, correcting the error whenever curvature introduces it. It's definitely not viable if the two bodies are, say, 90 degrees apart in orbit of a planet though. For New Horizons it's viable though - the ~1-10mln km at which it's expected to spot 2014 MU69 is nothing compared to distance from the sun, and ~10 days is nothing compared to 2014 MU69's orbital period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take a set of coordinates, with Z axis going craft-target; fix it relative to &quot;distant stars&quot; (so it doesn't change as the objects involved move). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orient craft along that Z axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take two images known time apart. The target will have wandered over the field of vision a certain distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perform an arbitrarily chosen delta-V burn perpendicular to the Z axis in the direction of the apparent travel of the target (XY plane), and after known time take another photo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using simple proportions between times and positions you can calculate the value of the burn needed to get a fly-by trajectory - one that brings the relative speed of the target in the XY plane to zero. Since it doesn't move &quot;sideways&quot; and you move towards it, you will get there &lt;em&gt;eventually&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You still don't know when the fly-by will happen, and at what speed, but all you need is to wait - or keep repeating the process to reduce the fly-by distance further.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course if the initial visual contact is way &quot;off&quot;, you'll lack delta-V to perform the correction. Still, in case of New Horizons, about 100,000 km of initial error should be enough to get a very close fly-by, and anything above that will just add to the fly-by distance.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-05-11T09:16:09.460" LastActivityDate="2017-05-11T09:16:09.460" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21467" PostTypeId="1" AcceptedAnswerId="21472" CreationDate="2017-05-11T09:22:59.667" Score="11" ViewCount="884" Body="&lt;p&gt;For fun I'd like to use Hubble Astrometric data &lt;a href=&quot;http://www.minorplanetcenter.net/db_search/show_object?object_id=486958&quot; rel=&quot;noreferrer&quot;&gt;for example the observations listed at the bottom here&lt;/a&gt; to numerically estimate the orbit of 2014 MU69 as well as its uncertainties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My plan is to use some combination of &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizons&lt;/a&gt;, Hubble's TLEs, and &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;noreferrer&quot;&gt;Skyfield&lt;/a&gt; to get the J2000.0 position of the HST at the time of each exposure, and to get the position of the sun and major planets to generate the gravity field in which to integrate MU69's motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand I will have to retard the gravity from each source by its particular light-time, as well as correct for the light time for the HST images.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will not be fast or efficient, it's strictly an exercise. At each time step I'll have to iterate and interpolate to find out, just for example &quot;where would Jupiter have been in it's orbit such that it's gravity would be arriving right now&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd do that and calculate an initial orbit for MU69, then use that to calculate apparent positions for the HST data, calculate an error, then try a different starting state vector for MU69 and see if that's better or worse, and just use steepest descent to find a nominal orbit. From that, I can see how sensitive the fit is to various combinations of deviations from nominal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm aware there may be smarter ways to do this, but to appreciate them it's better to do it brute force at least once. In the age of giga-flop laptops it's a viable option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;My question:&lt;/strong&gt; Are there other things I need to consider? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just for example, do I need to worry about time moving at different speeds at different distances from the sun (general relativity) or forces on MU69 besides gravity from the Sun and outer planets &lt;em&gt;in order to get the level of accuracy relevant to comparison with the HST astrometry?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Again:&lt;/strong&gt; &lt;em&gt;...in order to get the level of accuracy relevant to comparison with the HST astrometry&lt;/em&gt; — so I'm not looking for a list of arbitrarily small effects.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="27793" LastEditDate="2018-12-28T09:22:18.423" LastActivityDate="2019-01-05T10:20:46.010" Title="Besides retarded gravitation, anything else to worry about when calculating MU69's orbit from scratch?" Tags="|gravity|mathematics|software|celestial-mechanics|2014-mu69|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21468" PostTypeId="2" ParentId="21464" CreationDate="2017-05-11T09:48:33.930" Score="2" Body="&lt;p&gt;Pilot tube is an okay method if you're free to put it on exposed parts of the rocket - nose cone, leading fin edges, etc, wherever your reading won't be disturbed by laminar flow along the rocket surface - the tube facing into &quot;inert air&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You perform the readout using a relative pressure sensor (differential barometer) chip. The device has two pipes, you connect one to the pilot tube, and the other to a shielded air intake measuring static pressure (say, a pipe with tip embedded in a piece of sponge). Your control board will read the difference in pressure as either current/resistance/voltage, or a digital signal like SPI, depending on what sensor you use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can either perform calculations of how the airspeed converts to pressure differential, or (preferably, because manufacturing inaccuracies may make your calculations useless) calibrate it, by driving a car (or motorcycle) on a windless day, putting your sensor in airflow (out the window) at specific speeds, and recording the readouts, to get parameters for the pressure-speed function.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course the first limitation: &quot;exposed location&quot; is usually a no-go for this solution, nose cone usually ejected to release a parachute, the fins located near the engine, making the connection a huge hassle, etc. In this case you use alternatives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Andrew suggests GPS. This is a very common solution, and quite precise although slow (most GPS receivers provide readouts once per second) plus needs extra initialization time (&quot;get the fix&quot;). Also, all civilian GPS receivers will cut off at 18,000 meters (this is to prevent unauthorized military use). You need dedicated GPS receiver for aerospace (and to cut through a lot of red tape) to get GPS readout anywhere above that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Uwe suggests a doppler sensor. I don't know of any doppler sensors to measure airspeed with no ground reference. Laser sensors are okay for measuring ground distance up to a couple hundred meters, above that professionals will use aircraft radar systems (ground radar stations), amateurs... will use other means than doppler shift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An absolute pressure sensor (normal barometer chip) with shielded intake can be used to measure air pressure and derive altitude from that - and vertical speed from altitude change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, unless your rocket is meant to go above 18km, a common GPS is probably the most sensible solution. And if you plan to fly it higher... get an airspace GPS. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-11T09:48:33.930" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21469" PostTypeId="2" ParentId="21453" CreationDate="2017-05-11T10:22:04.723" Score="3" Body="&lt;p&gt;SpaceX wanted &lt;em&gt;a sensible tax&lt;/em&gt; because sooner or later space launches would get taxed, and they feared whatever monstrosity would the legislator came up with if left to their own devices. The nice part about this tax is the higher the payload orbit, the lower it is. Suborbital &quot;barely space&quot; launches pay the most.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tax has no specific justifications of specific risks and expenses - it's just a regular extension of standard transport tax applied to trucks, trains, water transport etc - essentially, SpaceX becomes &quot;just another transportation company&quot;. The only &quot;quirk&quot; relative to these taxes is that the distance &quot;over Californian soil&quot; is measured vertically as opposed to horizontally, a fixed 62 miles from launch site to the Kármán line vs payload separation altitude, as opposed to distance over Californian roads vs roads outside California. One might argue the tax for launch sites on &gt;1 mile mountain tops - but then all of downrange distance is tax-free. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-11T10:22:04.723" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21470" PostTypeId="1" CreationDate="2017-05-11T14:17:58.527" Score="9" ViewCount="1768" Body="&lt;p&gt;I want to know how much light is there for a spacecraft that flies from earth to Proxima Centauri. I know there's the sun but what happens after the spacecraft leaves the solar system? Are there stars that provide enough light for solar panels to work?&lt;/p&gt;&#xA;" OwnerUserId="19614" LastActivityDate="2017-05-11T19:52:49.243" Title="How much light is there on the way from Earth to Proxima Centauri?" Tags="|interstellar-travel|solar-power|time|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21471" PostTypeId="2" ParentId="21470" CreationDate="2017-05-11T14:42:01.420" Score="16" Body="&lt;p&gt;No, the power collected by solar panels is reduced by the square of the distance from the light source. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the Earth's distance from the sun, the energy of sunlight is about 1300 watts per square meter, of which something like ~30% can be converted to electricity by solar panels. Once the sun is far enough away to be &quot;just another star&quot;, the total starlight is about 8 orders of magnitude dimmer, something like a microwatt per square meter. If I've done my math right, powering the ISS from interstellar starlight would require solar panels about the size of California.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sunlight doesn't provide enough power for solar panels to be useful even in the outer solar system, let alone interstellar space, so probes going beyond Mars orbit more often use &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;radioisotope thermoelectric generators&lt;/a&gt; (RTGs) for power, which work for decades and do not depend on light. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For interstellar journeys which will take hundreds of years, nuclear fission or fusion reactors would probably be necessary. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-11T19:52:49.243" LastActivityDate="2017-05-11T19:52:49.243" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21472" PostTypeId="2" ParentId="21467" CreationDate="2017-05-11T15:58:02.227" Score="14" Body="&lt;p&gt;This is not a complete answer. It is instead an extended comment to the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I understand I will have to retard the gravity from each source by its particular light-time, as well as correct for the light time for the HST images.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While you do want to correct for light time travel with regard to seeing a moving remote object, you definitely do not want to do the first part (retarded gravity). That is not how Newtonian mechanics works, nor is it not how general relativity works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no lag in Newtonian mechanics; gravitation is instantaneous in Newtonian mechanics. In general relativity, there are some terms that act like lags, but there are other terms that act like leads. Those lag-like and lead-like terms in general relativity nearly cancel for small gravitational sources such as our Sun. That near cancelation is what makes Newtonian mechanics very close to correct. Keep in mind that the even for Mercury, the relativistic effect is very, very small: Only 43 arc seconds per century of precession that is not explainable by Newtonian mechanics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A couple centuries ago, Laplace investigated whether gravitation is instantaneous. He found that adding any significant lag to Newtonian gravity makes the solar system become unstable in short order and this concluded that the speed of gravity had to be very high, at least &lt;span class=&quot;math-container&quot;&gt;$7\times10^{6}$&lt;/span&gt; times the speed of light. A couple of decades ago, another highly respected astronomer published a paper in &lt;em&gt;Physics Letters A&lt;/em&gt; (a highly respectable physics journal) that came to the conclusion that the speed of gravity is at least 20 billion times the speed of light.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both Laplace and this more recent author were wrong. Laplace can be forgiven for not having possession of a time machine that would carry him a century into his future. The latter author cannot. His article has been cited 175 times (per google scholar), but almost all of the citations are essentially &quot;You're wrong. So very, very wrong, and here's why ...&quot; The &quot;here's why&quot; is that that is not the way general relativity works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The easy way to solve what you are trying to do is to ignore relativistic effects. Just assume Newtonian physics, in which gravitation is instantaneous, but the speed of light is not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hard way is to incorporate general relativity, to some extent. You'll need a relativistically-correct time scale (e.g., JPL's T&lt;sub&gt;eph&lt;/sub&gt;) and some kind of post-Newtonian model of gravitation that is, at least to first order, consistent with general relativity. For example, see &lt;a href=&quot;ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/de430_and_de431.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Planetary and Lunar Ephemerides DE430 and DE431&lt;/a&gt;. Do that and you'll be on par with the groups that develop extremely accurate solar system ephemerides.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2019-01-05T10:20:46.010" LastActivityDate="2019-01-05T10:20:46.010" CommentCount="16" ContentLicense="CC BY-SA 4.0" />
  <row Id="21473" PostTypeId="1" AcceptedAnswerId="21474" CreationDate="2017-05-11T16:43:26.253" Score="18" ViewCount="4089" Body="&lt;p&gt;In the Solid Rocket Boosters of the Space Shuttle, the following reaction happened:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathrm{NH_4ClO_4 + Al \rightarrow H_2O + N_2 + Al_2O_3 + AlCl_3}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using carbon instead aluminium, we would have the reaction&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathrm{NH_4ClO_4 + C \rightarrow H_2O + N_2 + CO_2 + CCl_4}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Carbon is &lt;em&gt;much&lt;/em&gt; better in the per-mass number of the free valence electrons (12/4 = 3, while in aluminium it is 27/3 = 9). Furthermore, $\mathrm{CO_2}$ and also $\mathrm{CCl_4}$ are gases (on the temperature of the outgoing product), while the products of the aluminium aren't. Thus, their pressure could have served as additional acceleration of the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My impression is that a rocket using carbon instead aluminium would produce much more specific impulse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why then was aluminium used?&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="19427" LastEditDate="2017-05-11T16:54:59.913" LastActivityDate="2017-05-13T10:15:59.830" Title="Why didn't the SRBs of the Space Shuttle use carbon instead of aluminium?" Tags="|space-shuttle|srb|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21474" PostTypeId="2" ParentId="21473" CreationDate="2017-05-11T16:58:51.783" Score="44" Body="&lt;p&gt;Specific impulse is not the only measure of a rocket.  For the case of the SRBs, high thrust is much more important than high specific impulse.  In this case, you want to look at the heat of formation for carbon dioxide vs aluminum oxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Carbon dioxide has a heat of formation of roughly -390 kJ/mol, compared with roughly -1670 kJ/mol for aluminum oxide.  Even adjusting for the heavier molecular weight, the energy difference is enormous.  Thermal oxidation of aluminum is a vigorous reaction.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-05-11T16:58:51.783" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21476" PostTypeId="5" CreationDate="2017-05-11T18:13:48.770" Score="0" Body="&lt;p&gt;Bright flame, smoke trail or exhaust plume may increase visibility of a rocket and its trajectory. These factors combined are called visual signature, and are an important factor in choice of fuels for military applications as high visual signature may point the enemy to launch location.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-05-11T18:13:48.770" LastActivityDate="2017-05-11T18:13:48.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21477" PostTypeId="4" CreationDate="2017-05-11T18:13:48.770" Score="0" Body="Bright flame, smoke trail or exhaust plume causing increase visibility of a rocket or a missile and its trajectory." OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-05-11T18:13:48.770" LastActivityDate="2017-05-11T18:13:48.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21478" PostTypeId="2" ParentId="21473" CreationDate="2017-05-12T11:23:35.800" Score="22" Body="&lt;p&gt;You might be interested in the tool called &lt;a href=&quot;http://rocketworkbench.sourceforge.net/equil.phtml&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;cpropep&lt;/em&gt;&lt;/a&gt;. This allows us to gain basic performance indications for different fuels by solving the physics behind it: chemical equilibria and expansion/compression.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how does $NH_4ClO_4 + Al$ perform against $NH_4ClO_4 + C$? First we feed it the approximate conditions:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Chamber temperature:    anything (calculated by cpropep for the calculation we're gonna do)&#xA;Chamber pressure:       60 atm (approx chamber pressur of SRB at the start)&#xA;Type of exit condition: pressure&#xA;Exit condition:         1 (sea level performance)&#xA;Units:                  mol (we're gonna calculate at stoichiometric for simplicy)&#xA;Type of problem:        Shifting performance evaluation&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;Shifting performance evaluation&lt;/code&gt; means &lt;em&gt;cpropep&lt;/em&gt; is gonna calculate the chemical equilibrium for chamber, nozzle and exit which is reasonable for big engines. &lt;code&gt;Frozen&lt;/code&gt; would mean to assume chemical equilibrium is achieved in the chamber and then fixed while going down the nozzle (very short nozzles would work like this).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the important numbers for us:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$NH_4ClO_4 + Al$&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                       CHAMBER      THROAT        EXIT&#xA;Pressure (atm)   :      60.000      34.930       1.000&#xA;Temperature (K)  :    3699.861    3533.767    2622.728&#xA;H (kJ/kg)        :   -2049.729   -2483.328   -4817.662&#xA;U (kJ/kg)        :   -2873.367   -3263.109   -5366.296&#xA;G (kJ/kg)        :  -32622.134  -31683.279  -26489.586&#xA;S (kJ/(kg)(K)    :       8.263       8.263       8.263&#xA;M (g/mol)        :      37.350      37.679      39.747&#xA;(dLnV/dLnP)t     :    -1.04313    -1.03830    -1.01314&#xA;(dLnV/dLnT)p     :     1.69444     1.64642     1.29711&#xA;Cp (kJ/(kg)(K))  :     4.42029     4.30003     3.14702&#xA;Cv (kJ/(kg)(K))  :     3.80757     3.72394     2.79963&#xA;Cp/Cv            :     1.16092     1.15470    *1.12408*&#xA;Gamma            :     1.11292     1.11211     1.10950&#xA;Vson (m/s)       :   957.41428   931.23567   780.19846&#xA;&#xA;Ae/At            :                 1.00000    10.26082&#xA;A/dotm (m/s/atm) :                23.97248   233.17936&#xA;C* (m/s)         :              1438.34885  1438.34885&#xA;Cf               :                 0.64743     1.63579&#xA;Ivac (m/s)       :              1768.59728  2586.02157&#xA;Isp (m/s)        :               931.23567 *2352.84221*&#xA;Isp/g (s)        :                94.95961   239.92313&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;$NH_4ClO_4 + C$&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                       CHAMBER      THROAT        EXIT&#xA;Pressure (atm)   :      60.000      34.650       1.000&#xA;Temperature (K)  :    3014.701    2852.316    1876.151&#xA;H (kJ/kg)        :   -2234.870   -2685.717   -4966.844&#xA;U (kJ/kg)        :   -3083.077   -3480.380   -5468.867&#xA;G (kJ/kg)        :  -29392.113  -28380.149  -21867.721&#xA;S (kJ/(kg)(K)    :       9.008       9.008       9.008&#xA;M (g/mol)        :      29.551      29.844      31.073&#xA;(dLnV/dLnP)t     :    -1.01662    -1.01377    -1.00112&#xA;(dLnV/dLnT)p     :     1.34985     1.30609     1.03630&#xA;Cp (kJ/(kg)(K))  :     3.78068     3.58783     1.86674&#xA;Cv (kJ/(kg)(K))  :     3.27640     3.11902     1.57970&#xA;Cp/Cv            :     1.15391     1.15031    *1.18170*&#xA;Gamma            :     1.13504     1.13469     1.18038&#xA;Vson (m/s)       :   981.19852   949.57591   769.79086&#xA;&#xA;Ae/At            :                 1.00000     9.25862&#xA;A/dotm (m/s/atm) :                24.15204   214.76841&#xA;C* (m/s)         :              1449.12231  1449.12231&#xA;Cf               :                 0.65528     1.61305&#xA;Ivac (m/s)       :              1786.43717  2552.27758&#xA;Isp (m/s)        :               949.57591 *2337.50917*&#xA;Isp/g (s)        :                96.82979   238.35960&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;You can see here that Cp/Cv (Ratio of specific heat) is indeed much higher (&lt;em&gt;1.12408&lt;/em&gt; vs &lt;em&gt;1.18170&lt;/em&gt;) which means $NH_4ClO_4 + C$ uses its energy way more effectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what is really interesting for a rocket is in fact (opposed to what @Tristan says) the specific impulse ($I_{SP}$) not the energy stored in it. Even though the energy stored in aluminium is much higher, due to the lower Cp/Cv, its $I_{SP}$ is only a bit higher: &lt;em&gt;2352.84221 m/s&lt;/em&gt; compared to &lt;em&gt;2337.50917 m/s&lt;/em&gt;. Thrust is not based on the energy released, thrust is equal to the mass flow rate (which can be almost freely designed) times the $I_{SP}$ (which is a function of Cp/Cv &lt;em&gt;and&lt;/em&gt; the energy) but Al still results in a bit higher $I_{SP}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway the difference is actually quite low: &lt;em&gt;less than 0.7%&lt;/em&gt;. But there's something that additionally favors $Al$ &amp;mdash; its density is higher: $2,7 g/cm^3$ vs around $2,0 g/cm^3$. That means the same booster can fit a bit more fuel in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You should also note that in reality solid rocket boosters use both: &lt;em&gt;$NH_4ClO_4$&lt;/em&gt;, $Al$ and a resin (usually HTPB) composed of $C$ and $H$ to keep it together. this achieves a higher impulse because $C$ and $H$ (&lt;strong&gt;especially $H$&lt;/strong&gt;) raises the Cp/Cv while $Al$ raises the chamber temperature. From my experience adding Aluminium can improve the $I_{SP}$ by up to 10% for CxH2x fuels like plastics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Results for $NH_4ClO_4 + Al$ (33%) + $C$ (67%):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                       CHAMBER      THROAT        EXIT&#xA;Pressure (atm)   :      60.000      34.781       1.000&#xA;Temperature (K)  :    3250.729    3090.041    2217.389&#xA;H (kJ/kg)        :   -2169.548   -2617.841   -4963.957&#xA;U (kJ/kg)        :   -3016.829   -3415.442   -5510.728&#xA;G (kJ/kg)        :  -30729.859  -29766.380  -24445.532&#xA;S (kJ/(kg)(K)    :       8.786       8.786       8.786&#xA;M (g/mol)        :      31.900      32.212      33.719&#xA;(dLnV/dLnP)t     :    -1.02439    -1.02104    -1.00427&#xA;(dLnV/dLnT)p     :     1.45514     1.41622     1.11496&#xA;Cp (kJ/(kg)(K))  :     4.06798     3.93869     2.38904&#xA;Cv (kJ/(kg)(K))  :     3.52922     3.43165     2.08380&#xA;Cp/Cv            :     1.15266     1.14775    *1.14648*&#xA;Gamma            :     1.12521     1.12410     1.14161&#xA;Vson (m/s)       :   976.40724   946.88257   790.06150&#xA;&#xA;Ae/At            :                 1.00000     9.99677&#xA;A/dotm (m/s/atm) :                24.21831   231.28393&#xA;C* (m/s)         :              1453.09835  1453.09835&#xA;Cf               :                 0.65163     1.62692&#xA;Ivac (m/s)       :              1789.22682  2595.35188&#xA;Isp (m/s)        :               946.88257 *2364.06795*&#xA;Isp/g (s)        :                96.55515   241.06784&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="19531" LastEditorUserId="19531" LastEditDate="2017-05-13T10:15:59.830" LastActivityDate="2017-05-13T10:15:59.830" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21479" PostTypeId="1" AcceptedAnswerId="21490" CreationDate="2017-05-12T11:57:05.593" Score="10" ViewCount="9949" Body="&lt;p&gt;Are astronauts allowed to wear their eyeglasses inside their helmets during launches and landings?? Are there any special considerations or rules?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During spaceflight (launches and landings) there can be a lot of vibrations that could knock eyeglasses off of an astronaut's nose. If contained inside a helmet, it may be difficult to put them back on straight. Improperly positioned might seriously obstruct vision, produce distracting pain, or even pose a potential risk of injury. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I noticed in the photos below that while both astronauts Donald Pettit and Andre Kuipers were wearing glasses while seated in the re-entry vehicle before closing the hatch, only one of them appeared to be wearing glasses upon re-entry, landing, and being removed from the capsule. *Did Astronaut Kuipers' glasses fall off, or did Astronaut Pettit remove his glasses just before descent?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Can eyeglasses just come from the local optician, or are special space-rated astronaut frames and/or lenses required?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sYb8g.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sYb8g.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Astronauts Donald Pettit, Oleg Kononenko and Andre Kuipers preparing to return to Earth. From NASA Letters to Earth &lt;a href=&quot;https://blogs.nasa.gov/letters/2012/06/29/post_1340814534271/&quot; rel=&quot;noreferrer&quot;&gt;June 17-26 – Diary of a Space Zucchini&lt;/a&gt;. If you right-click and open in a separate window for a larger view you can see that two of the three are wearing eyeglasses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/B6S9Q.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/B6S9Q.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P5Mw2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P5Mw2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; Astronaut Pettit seems to have his eyeglasses still on after re-entry and landing. From &lt;a href=&quot;http://www.dailymail.co.uk/news/article-2167538/Soyuz-capsule-Astronauts-Donald-Pettit-Oleg-Kononenko-Andre-Kuipers-Earth.html&quot; rel=&quot;noreferrer&quot;&gt;The Daily Mail&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3pVJX.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3pVJX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; However, astronaut Kuipers does not seem to be wearing his eyeglasses after re-entry and landing. From &lt;a href=&quot;http://www.dailymail.co.uk/news/article-2167538/Soyuz-capsule-Astronauts-Donald-Pettit-Oleg-Kononenko-Andre-Kuipers-Earth.html&quot; rel=&quot;noreferrer&quot;&gt;The Daily Mail&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below x2:&lt;/strong&gt; Astronaut Pettit wearing his eyeglasses during his daily, routine activities on board the ISS. Top: Self Portrait, from &lt;a href=&quot;http://www.popsci.com/10-incredible-space-photos-from-astronaut-don-pettits-new-book#page-10&quot; rel=&quot;noreferrer&quot;&gt;Popular Science&lt;/a&gt;. Credit Don Pettit/NASA, Bottom: From NASA Letters to Earth &lt;a href=&quot;https://blogs.nasa.gov/letters/2012/01/19/post_1326991840179/&quot; rel=&quot;noreferrer&quot;&gt;Lost Chopstick&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XvJFF.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XvJFF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7a3tg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7a3tg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Astronaut Pettit in 2002 on STS-113, wearing a substantially different style of eyeglasses with much larger frames. From &lt;a href=&quot;https://spaceflight.nasa.gov/gallery/images/shuttle/sts-113/html/iss006e05004.html&quot; rel=&quot;noreferrer&quot;&gt;STS-113 Shuttle Mission Imagery (ISS006-E-05004)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Vwn4n.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vwn4n.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-12T17:15:50.793" LastActivityDate="2019-03-07T14:12:55.560" Title="Can astronauts wear eyeglasses inside their helmets during launches and landings?" Tags="|iss|astronauts|safety|astronaut-lifestyle|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21480" PostTypeId="1" AcceptedAnswerId="21482" CreationDate="2017-05-12T12:53:24.937" Score="25" ViewCount="2970" Body="&lt;p&gt;I saw this image in &lt;a href=&quot;https://i.stack.imgur.com/FNvm4.png&quot; rel=&quot;noreferrer&quot;&gt;Popular Science&lt;/a&gt;, one of the example photos from the book by astronaut/photographer/astrophotographer(?) Donald Pettit. It's beautiful and impressive not only from a color/composition perspective, but also from a purely engineering perspective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I noticed there is a fixed dish antenna, so I've included a cropped/magnified/rotated view of it for detailed examination. I'm wondering what it would be for, since it does not appear to have any, or at least much steerability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is it for? Which way does it point? Roughy what wavelength/frequency might it operate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FNvm4.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FNvm4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oEd8F.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oEd8F.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2018-04-09T00:48:19.127" LastActivityDate="2018-04-09T00:48:19.127" Title="What is the function of this (apparently) fixed dish on the ISS?" Tags="|iss|antenna|identify-this-object|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21481" PostTypeId="2" ParentId="21473" CreationDate="2017-05-12T13:04:49.933" Score="8" Body="&lt;p&gt;First off, a link to an existing answer: &lt;a href=&quot;http://www.aerospaceweb.org/question/propulsion/q0246.shtml&quot; rel=&quot;noreferrer&quot;&gt;http://www.aerospaceweb.org/question/propulsion/q0246.shtml&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A little bit of history knowledge goes a long way. Adding aluminum to solid rocket fuels to increase their specific impulse was a breakthrough achieved in 1956 by two of Atlantic Research Corporation's engineers: Keith Rumbel and Charles Henderson. Simplifying for the sake of ITAR, what matters is the optimum size of aluminum particles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please be aware that during World War Two major powers experimented with adding aluminum powder to explosives, which kinda gave superior oxygen balance and yield. It is but one logical step from high-order detonation in explosives to deflagration in solid rockets.&lt;/p&gt;&#xA;" OwnerUserId="19621" LastActivityDate="2017-05-12T13:04:49.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21482" PostTypeId="2" ParentId="21480" CreationDate="2017-05-12T13:25:23.753" Score="34" Body="&lt;p&gt;That's a spare part - a spare Space to Ground Antenna dish for the Ku band. It's mounted on an External Stowage Platform.  It was brought up on STS-127.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;https://www.nasa.gov/pdf/358018main_sts127_press_kit.pdf&quot; rel=&quot;noreferrer&quot;&gt;STS-127 Press Kit&lt;/a&gt; Mission Objectives (p 17) (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Install Integrated Cargo Carrier (ICC) to Payload Orbital Replacement Unit Accommodation (POA)&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;Remove and replace six Port 6 batteries (spacewalk)&lt;/li&gt;&#xA;  &lt;li&gt;Transfer PM-2, Linear Drive Unit (LDU) and &lt;strong&gt;Space-to-Ground Antenna (SGANT)&lt;/strong&gt; to External Stowage Platform-3 (spacewalk)&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Here's a shot of it in the payload bay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/koHKVm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/koHKVm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here are the transfer ops in progress&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ju62p.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ju62p.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All those other ORUs (Orbital Replacement Units) you see mounted on those platforms are spares as well. The last group of shuttle flights brought up many external spares to cover failures of parts that were too large for the remaining logistics vehicles.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2017-05-12T18:16:38.733" LastActivityDate="2017-05-12T18:16:38.733" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="21483" PostTypeId="1" AcceptedAnswerId="21485" CreationDate="2017-05-12T13:48:22.340" Score="5" ViewCount="223" Body="&lt;p&gt;In a recent question, uhoh asked about a dish mounted on the outside of the International Space Station (ISS):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/21480/58&quot;&gt;What is the function of this (apparently) fixed dish on the ISS?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This turned out to be an ORU with a Space-to-Ground Antenna (SGANT) spare. That has me wondering--how good of condition do these spares stay in, while mounted on the outside of the ISS, when they are not in use?&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-05-12T18:03:36.517" Title="What kind of condition do Orbital Replacement Units (ORUs) keep while mounted on the outside of the ISS?" Tags="|iss|maintenance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21484" PostTypeId="1" AcceptedAnswerId="21486" CreationDate="2017-05-12T15:09:33.740" Score="9" ViewCount="1682" Body="&lt;p&gt;I am wondering - what is the nature of the atmosphere in and around the transition to space at the Karman line?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is not difficult to find references using terms like &quot;in the atmosphere&quot; and &quot;out of the atmosphere&quot; as if there is some fairly clear border. For example, &lt;a href=&quot;http://spaceflight101.com/cygnus-oa6/by-the-numbers-how-close-atlas-v-came-to-failure-in-this-weeks-cygnus-launch/&quot; rel=&quot;noreferrer&quot; title=&quot;By the Numbers: How close Atlas V came to Failure in this Week’s Cygnus Launch&quot;&gt;this spaceflight101.com article&lt;/a&gt; about the booster underperformance issue for the Atlas V launch of the Cygnus spacecraft for OA-6 says &quot;...Centaur was holding a pitched-up attitude for much longer in Tuesday’s mission just to keep out of the atmosphere.&quot; This draws the picture in my mind of a fairly clear border.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, the atmosphere is gaseous, so there is no surface tension and no surface - so therefore the boundary cannot be as sharp as being &quot;in the ocean&quot; and &quot;out of the ocean&quot; - but is that actually a fairly reasonable analogy? Do all the heavier gases in the atmosphere sort of tend to pool such that there is a fairly sharp cut-off point, above which the density falls precipitously? E.g., below altitude X, the atmospheric density is approximated by function 1, whereas above altitude X, the density is approximated by function 2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;noreferrer&quot; title=&quot;Karman line&quot;&gt;Wikipedia Article&lt;/a&gt; about the Karman line clearly states &quot;An atmosphere does not abruptly end at any given height, but becomes progressively thinner with altitude.&quot; Again, obviously since it's a gas and there is no surface tension, there is no true &quot;surface&quot; - but for the purposes of spaceflight, is there something relatively close to this? (And is the Karman line a pretty close approximation to where this would happen?)&lt;/p&gt;&#xA;" OwnerUserId="15933" LastEditorUserId="12408" LastEditDate="2017-05-25T14:20:32.960" LastActivityDate="2017-12-27T13:08:10.807" Title="What are the characteristics of the Earth's atmosphere near the Karman line and why?" Tags="|atmosphere|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21485" PostTypeId="2" ParentId="21483" CreationDate="2017-05-12T15:20:07.017" Score="5" Body="&lt;p&gt;In general, pretty good condition.  The ORUs are designed to live outside for their functional life (more less the same as station's), so there's generally no reason they can't live outside while they're waiting to be called upon.  Many of them have flight support equipment (FSE) that provides some support to keep them alive.  These are typically heaters, sometimes telemetry monitors.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are evaluated for environmental risks just the same as the operational ORUs are -- most of them are better protected in their stored locations, by virtue of the location itself or by dedicated covers.  Both of these options generally serve to create a more benign environment for the ORU than it would experience in its deployed, operational state.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not all spare ORUs are stored outside.  Some are small enough (the Sequential Shunt Unit, for example) to be kept indoors and transition through the airlock with the crew.  There's a trade-off to be made, however.  Keeping an ORU outside presents the opportunity to be able to replace it purely robotically, if it was designed with that in mind.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2017-05-12T18:03:36.517" LastActivityDate="2017-05-12T18:03:36.517" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21486" PostTypeId="2" ParentId="21484" CreationDate="2017-05-12T15:52:11.397" Score="12" Body="&lt;p&gt;There is no border at all, just progressively thinner air. The Kármán line is an arbitrary reference point, at which an aircraft can't produce enough lift to stay in the air at less than orbital speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are different layers of atmosphere with somewhat different emergent characteristics, but those don't greatly change the overall progressive thinning of air with altitude, and don't have a large effect on the performance of rockets passing through.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this image you can see there's a bit of a knee in the density curve at 80-90km, but note the log scale on the left: density is practically zero there anyway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/d/de/Atmosphere_model.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The use of the binary distinction of &quot;in the atmosphere&quot; and &quot;out of the atmosphere&quot; in discussion of launchers is informal. In a typical two-stage-to-orbit launcher, the first stage will take you from sea level to high altitude; this has significant effects, so we say the stage must be &quot;designed for flight in the atmosphere&quot;. The upper stage design may not be able to ignore atmosphere completely, but it will do most of its work in microbar pressure or less, so we say it's &quot;designed for flight out of the atmosphere&quot;. The rocket as a whole will spend only a few minutes in the transition. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-12T16:12:25.463" LastActivityDate="2017-05-12T16:12:25.463" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21487" PostTypeId="1" CreationDate="2017-05-12T16:01:11.257" Score="4" ViewCount="269" Body="&lt;p&gt;I am referencing chapter 7 of the book &quot;Spacecraft Navigation and Guidance&quot; by Maxwell Noton. Can anyone tell me how to determine control gains C1 and C2 in equation 7.58  and 7.59 in respective chapter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Constant drag deceleration law(above 50 km altitude)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$(L/D)_v=(L/D)_1+C1(a-a_{nom}(v))+C2(h_{dot}-h_{nom_{dot}})...7.58$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Constant descent rate law (below 50 km altitude)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$(L/D)_v=(L/D)_1++C2(h_{dot}-h_{nom_{dot}})...7.59$$&lt;/p&gt;&#xA;" OwnerUserId="19622" LastEditorUserId="58" LastEditDate="2017-05-12T17:35:34.233" LastActivityDate="2017-05-12T17:35:34.233" Title="Apollo re-entry guidance" Tags="|apollo-program|reentry|guidance|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21488" PostTypeId="2" ParentId="21480" CreationDate="2017-05-12T16:04:30.697" Score="10" Body="&lt;p&gt;To complement: The SGANT is the antenna that transmits high data rate signals, including many up/down video feeds, from/to the ISS. There is a total of 3 antennas: 2 at the end of booms on the Z1 trust, and the spare that you have identified. The active SGANT transmits/receives data by pointing to a TDRS satellite continuously for a portion of an orbit (~120 degrees), then transitions to a different TDRS via ephemeris and a spiral search pattern. They have been designed and built in Canada by SPAR/MDA, same with the ISS S-band antennas, and sold to L3-C (all ISS communications) and then Boeing. As far as I know, the SGANT is the only way to exchange video with the ISS. Hope this helps!&lt;/p&gt;&#xA;" OwnerUserId="19624" LastActivityDate="2017-05-12T16:04:30.697" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21489" PostTypeId="1" AcceptedAnswerId="40699" CreationDate="2017-05-12T17:09:16.913" Score="4" ViewCount="328" Body="&lt;p&gt;As seen in this image below, the cuff checklists have Kapton tape wrapped around their edges.  What is the reason for this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/I3sqN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/I3sqN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="26446" LastEditDate="2021-04-27T20:35:50.860" LastActivityDate="2021-04-27T20:35:50.860" Title="Why are crew EVA cuff checklists wrapped in Kapton tape?" Tags="|eva|materials|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21490" PostTypeId="2" ParentId="21479" CreationDate="2017-05-12T18:28:19.493" Score="6" Body="&lt;p&gt;For shuttle, the answer was yes, they could be worn during ascent and entry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many photographs showing this. Here is Canadian Mission Specialist Julie Payette wearing her glasses whilst suited for ascent on the flightdeck for the STS-127 mission.  This picture was taken very shortly after Main Engine Cutoff; the crew is still strapped in. For a time hack, you can see that Payette is holding the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/567068main_ASC_135_F_1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ascent checklist&lt;/a&gt; open to the Post OMS 1 pages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mMLPo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mMLPo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Picture from &lt;a href=&quot;https://spaceflight.nasa.gov/gallery/images/shuttle/sts-127/hires/s127e005112.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The in-cockpit video taken just prior to destruction of the orbiter Columbia during entry shows MS-4/David Brown wearing his glasses.  The video is not very high quality but there is a fairly clear shot at time 12:11.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/_rIHdk-_UoM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as your other questions embedded in the main question, they had to pass the same safety and materials checks as anything else carried in the crew compartment.  I do not believe the eyewear is provided by NASA but I am not sure on that point.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-03-07T14:12:55.560" LastActivityDate="2019-03-07T14:12:55.560" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="21491" PostTypeId="2" ParentId="21140" CreationDate="2017-05-12T20:07:41.383" Score="1" Body="&lt;p&gt;Time-keeping is a particularly interesting subject for me. There are multiple ways you can communicate a date, and some sort of system of keeping time to reference that to. Some constants in nature can be used to keep time outside our solar system where the relative motion of our planet to our sun become irrelevant. The challenge here is to help this life form understand our definition of time - the second using the various constants in nature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Quantum oscillations:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Defining a second (or some unit of time) to keep time using oscillations of atoms between a ground and excited state when bombarded with microwave radiation. Since atoms of Cesium or Rubidium are found throughout the universe. I should see no reason for this not to be a viable option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Decay:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radioactive decay can be used to define a unit of time. Decay rates are constant for various radioactive atoms. As thegreatemu pointed out, you could alternatively send radioactive isotopes along with the Voyager (or some spacecraft) and point out the constants like rates of decay and the ratio of decayed to undecayed atoms in that sample at the time of launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Speed of light:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This should be pretty obvious for anyone with high-school level physics. We simply define our unit of time (second) using the distance light travels during its interval and expect the life form to understand it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Cosmic Microwave Background (CMB):&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CMB is universal and redshifts at some rate allowing us to communicate a universal definition of time with any extraterrestrial. We could note the z value at the time of launch and expect the life form to read the current z and work out the math to figure out how much time had passed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the life form is smart enough to keep time and know how much time has passed (since some cosmological event such as the Big Bang), it should be possible for us to convey some point in time to the life form and expect it to reference it to the same (or equivalent) time scale.&lt;/p&gt;&#xA;" OwnerUserId="18272" LastEditorUserId="18272" LastEditDate="2017-05-12T20:14:45.857" LastActivityDate="2017-05-12T20:14:45.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21492" PostTypeId="2" ParentId="21484" CreationDate="2017-05-13T00:29:37.083" Score="9" Body="&lt;p&gt;Contrary to Russell Borogove's answer, there is something very significant about the Kármán line. This is more or less where the upper (and more significant) minimum that roughly separates the mesosphere from the thermosphere (the mesopause), and not uncoincidentally, the surface that distinguishes whether turbulent mixing or molecular diffusion is the dominant atmospheric process (the turbopause), and also not uncoincidentally, where temperature once again starts increasing with increasing altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the 100 km mark is a bit arbitrary, that a perhaps fuzzy boundary does exist is not. You can certainly see that boundary in the graph of temperature in Russell Borogove's answer. His graph happens to show a marked temperature change at an altitude of 90 km. (Where this occurs varies, with time of year, latitude, and paper author.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The changes in temperature gradient and in atmospheric behavior both have a marked impact on low-orbiting vehicles. A vehicle is essentially orbiting above this line. (Maybe not for long, but it's orbiting.) Below it, it's entering the atmosphere. The exponential increase in pressure means that drag rather suddenly dominates over gravitation somewhere near this point.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="195" LastEditDate="2017-05-25T15:47:33.727" LastActivityDate="2017-05-25T15:47:33.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21493" PostTypeId="1" AcceptedAnswerId="21888" CreationDate="2017-05-13T02:41:36.493" Score="20" ViewCount="4703" Body="&lt;p&gt;The question &lt;a href=&quot;https://electronics.stackexchange.com/q/304788/102305&quot;&gt;What makes smartphones tilt-sensitive? Will they retain this ability in zero-gravity conditions?&lt;/a&gt; brought to mind the video of astronaut Steven Swanson's detailed tour of the ISS, in which he frequently referred to external images of the ISS using an iPad throughout the tour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the iPads use their camera to judge the orientation of the user and rotate the frame accordingly, or do they rotate randomly, or is the &quot;up&quot; sensing function manually disabled and the orientation locked, or does iOS &lt;em&gt;automatically lock&lt;/em&gt; the display orientation in the absence of a well-defined ~ 1g signal? (In other words, did they already think of this?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3ojUC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3ojUC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/QvTmdIhYnes?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-13T03:07:53.553" LastActivityDate="2020-03-31T02:44:56.457" Title="How do iPads on the ISS know which way is &quot;up&quot; for their users?" Tags="|iss|microgravity|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21494" PostTypeId="2" ParentId="21493" CreationDate="2017-05-13T03:18:38.520" Score="10" Body="&lt;p&gt;It doesn't use the camera. There's no way it can know where is up as someone have already explained in the question you linked. Auto rotation has been manually disabled. You can see the rotation locked icon at 1:25, in the top right corner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yFsej.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yFsej.png&quot; alt=&quot;Rotation locked&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rotation locked icon in google images, for comparison: &lt;a href=&quot;https://www.google.es/search?q=rotation+locked+ios&amp;amp;source=lnms&amp;amp;tbm=isch&amp;amp;sa=X&amp;amp;ved=0ahUKEwih6unh8-vTAhWJoRQKHZofDqEQ_AUICygC&amp;amp;biw=1919&amp;amp;bih=880#imgrc=fQpvfhSyQKxHaM&quot; rel=&quot;noreferrer&quot;&gt;https://www.google.es/search?q=rotation+locked+ios&amp;amp;source=lnms&amp;amp;tbm=isch&amp;amp;sa=X&amp;amp;ved=0ahUKEwih6unh8-vTAhWJoRQKHZofDqEQ_AUICygC&amp;amp;biw=1919&amp;amp;bih=880#imgrc=fQpvfhSyQKxHaM&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interface of the app he is using is in landscape mode because is coded to show only in that way.&lt;/p&gt;&#xA;" OwnerUserId="19596" LastActivityDate="2017-05-13T03:18:38.520" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21495" PostTypeId="1" AcceptedAnswerId="21498" CreationDate="2017-05-13T03:53:13.713" Score="18" ViewCount="2152" Body="&lt;p&gt;What is the lid on the Hubble Space Telescope for?&#xA;&lt;a href=&quot;https://i.stack.imgur.com/rcKDg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rcKDg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18279" LastActivityDate="2023-03-21T14:48:20.577" Title="Why is there a lid on the Hubble Space Telescope?" Tags="|hubble|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21496" PostTypeId="2" ParentId="21493" CreationDate="2017-05-13T07:26:11.843" Score="-4" Body="&lt;p&gt;It won't work in space, but probably uses a component inside like the one listed below.  It uses a tiny enclosed space, where heat rises and then a measurement is made of the variation of conductivity of hot air compared to cold air, and then creates an output telling the circuit which way is up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.parallax.com/product/28036&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.parallax.com/product/28036&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tilt sensor&lt;/p&gt;&#xA;" OwnerUserId="19631" LastActivityDate="2017-05-13T07:26:11.843" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21497" PostTypeId="2" ParentId="21493" CreationDate="2017-05-13T07:29:03.847" Score="4" Body="&lt;p&gt;For excellent technical explanations on how these sensors work see &lt;a href=&quot;https://electronics.stackexchange.com/q/304788/102305&quot;&gt;What makes smartphones tilt-sensitive? Will they retain this ability in zero-gravity conditions?&lt;/a&gt; as is linked in the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your question is more specific by asking about the iPad. The OS/UI framework on iPhone and iPad does distinguish between &lt;em&gt;device orientation&lt;/em&gt; and &lt;em&gt;user interface orientation&lt;/em&gt;. This is required for example to handle laying your iPad flat on a table: which way should the user interface be oriented when &quot;up&quot; is perpendicular to the display? In this case the previous user orientation is retained. Apps can even restrict and thus &quot;enforce&quot; a user interface orientation (for example, they can restrict that an app or even single views may only be shown in horizontal orientation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can lock the user interface orientation on iOS which I guess might be very handy in such an environment: after all, if you move the device it might confuse the acceleration with &quot;gravity&quot; and thus &quot;down&quot; and start to rotate the user interface. This won't happen with the rotation lock. I'm not sure whether the camera is also using the user interface orientation for its orientation, though (the one that gets written to the EXIF metadata and determines how to display the image later on).&lt;/p&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="12102" LastEditDate="2017-05-16T23:14:45.630" LastActivityDate="2017-05-16T23:14:45.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21498" PostTypeId="2" ParentId="21495" CreationDate="2017-05-13T08:14:43.310" Score="23" Body="&lt;p&gt;The aperture door is used to protect the telescope during launch and &lt;a href=&quot;http://hubblesite.org/the_telescope/team_hubble/&quot; rel=&quot;noreferrer&quot;&gt;service&lt;/a&gt; operations, and to prevent the sun from shining down the telescope barrel.&lt;/p&gt;&#xA;&lt;p&gt;Here, the HST is being released from shuttle Discovery.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sVQap.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sVQap.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The door opens soon after the HST has been released:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/udc3n.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/udc3n.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The door &lt;a href=&quot;http://www.pbs.org/deepspace/hubble/diagram.html&quot; rel=&quot;noreferrer&quot;&gt;closes automatically&lt;/a&gt; if the HST is pointed to within 20 degrees of the Sun's direction.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The telescope does not generally observe targets within 50 degrees of the Sun, 15.5 degrees of any illuminated portion of the Earth, 7.6 degrees of the dark limb of the Earth, nor 9 degrees of the Moon. Following the first servicing mission, the telescope is again allowed to point directly away from the Sun.&lt;/p&gt;&#xA;&lt;p&gt;There are exceptions to these rules for HST pointing in certain cases. For instance, the bright Earth is a useful flat-field calibration source. However, there are onboard safety features that cannot be overridden. The most important of these is that the aperture door shown in Fig. 1 will close automatically whenever HST is pointed within 20 degrees of the Sun, in order to prevent direct sunlight from reaching the optics and focal plane.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Direct sunlight would rapidly heat up the optics and focal plane, potentially destroying the instruments, so has to be avoided.&lt;/p&gt;&#xA;&lt;p&gt;The door is also used as a sun shield, it seems:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;HST is, in principle, free to roll about its optical axis. However, this freedom is limited by the need to keep sunlight shining on the solar arrays, and by a thermal design that assumes that the Sun always heats the same side of the telescope.&lt;/p&gt;&#xA;&lt;p&gt;To discuss HST pointing, it is useful to define a coordinate system that is fixed to the telescope. This system consists of three orthogonal axes: V1, V2, and V3. V1 lies along the optical axis, V2 is parallel to the solar-array rotation axis, and V3 is perpendicular to the solar-array axis (see Fig. 1). Power and thermal constraints are satisfied when the telescope is oriented such that the Sun is in the half-plane defined by the +V1 axis and the positive V3 axis. The orientation that optimizes the solar-array positioning with respect to the Sun is called the &amp;quot;nominal orientation.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s3UkP.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s3UkP.gif&quot; alt=&quot;HST diagram with axes&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;note: this diagram is for an older configuration of the HST. Since then, instruments have been replaced during service missions.&lt;/p&gt;&#xA;&lt;p&gt;So there's a sun-facing side of the telescope. I'd be surprised if the aperture door hinge location did not take this into account.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-15T11:19:14.927" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21499" PostTypeId="1" CreationDate="2017-05-13T11:32:56.877" Score="6" ViewCount="513" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_satellite_constellation&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt; talks about launching 4,425 communication satellites into 83 different orbital planes at 1,100 km altitude. That's about 50 satellites per orbital plane. In terms of capacity to LEO, a Falcon 9 has about 22,800/50 = 450 kg to spend on each of them if all 50 are launched on the same rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean that the 50 satellites in the same plane will be spread out 7 degrees apart all with the same orbital speed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the upper stage have to accelerate and decelerate for releasing each of the satellites, or how would it be done? Does it require another kind of upper stage? Or will all satellites be released at once and reach their destinations with their individual perhaps economical low thrust propulsion? Regardless of how SpaceX will actually do this, which I suppose is still not publicly known, what would be a feasible approach? Have for example Iridium or GPS used a similar way to distribute satellites along an orbital plane?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-05-13T13:31:24.400" LastActivityDate="2017-05-27T15:07:06.637" Title="How are multiple low orbit communication satellites per launch distributed along an orbital plane for global coverage?" Tags="|spacex|orbital-maneuver|communication-satellite|satellite-constellation|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21500" PostTypeId="1" CreationDate="2017-05-13T12:24:48.117" Score="17" ViewCount="344" Body="&lt;p&gt;Primary payload customers have traditionally been cautious when allowing secondary payloads on the same launcher. For example, a secondary payload is not trusted to have its own chemical propulsion system since it would be explosive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But how often have secondary payloads caused its primary payload to fail? What are some examples of this happening, or being a strongly suspected cause of failure?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="9092" LastEditDate="2021-03-26T14:27:25.477" LastActivityDate="2021-03-26T14:27:25.477" Title="When have secondary payloads damaged the primary payload of a launcher?" Tags="|launch|failure|payload|secondary-payload|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21501" PostTypeId="1" AcceptedAnswerId="21503" CreationDate="2017-05-13T13:38:31.780" Score="3" ViewCount="666" Body="&lt;p&gt;Almost all the photos I see of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;HST&lt;/a&gt; show the two small dish antennas at the ends of the two long, opposing booms with the &quot;top&quot; one pointed straight ahead at 12-o'clock, and the &quot;bottom&quot; one at 3-o'clock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The HST is in LEO, and probably spends most of its time at an attitude either fixed (or nearly fixed) with respect to the stars so it can do science, or efficiently slewing to the next target. That means that these two antenna booms will spend most of their time without a fixed orientation with respect to ground stations on the Earth, or communications linking satellites in GEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do these dishes actually articulate and independently track targets so they can downlink this constant stream of data and uplink their next instructions? If not, does the telescope stop making measurements, stop tracking the heavens, and slew around to point one or the other at communications links?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Either way (stopping science or extra work for ADCS turning the entire telescope) sounds less optimal than just adding articulation to the little dishes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PhFNQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PhFNQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;An STS-125 crew member aboard the Space Shuttle Atlantis captured this still image of the Hubble Space Telescope as the two spacecraft continue their relative separation on May 19, after having been linked together for the better part of a week.&quot; Cropped, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Hubble_telescope_2009.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ydjp2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ydjp2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;The Hubble Space Telescope as seen from the departing Space Shuttle Atlantis, flying STS-125, HST Servicing Mission 4.&quot; Cropped, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:HST-SM4.jpeg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7OGzc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7OGzc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;The Hubble Space Telescope (HST) begins its separation from Space Shuttle Discovery following its release on mission STS-82.&quot; Cropped and lightened to better show upper antenna. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Hubble_01.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TVsUL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TVsUL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;IMAX Cargo Bay Camera view of the Hubble Space Telescope at the moment of release, mission STS-31.&quot; Cropped, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:1990_s31_IMAX_view_of_HST_release.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-13T13:58:43.200" LastActivityDate="2017-05-13T17:36:17.507" Title="How do those little dish antennas sticking out of the HST track their target independent of telescope pointing?" Tags="|low-earth-orbit|communication|data-transmission|hubble|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21502" PostTypeId="2" ParentId="21499" CreationDate="2017-05-13T15:37:37.020" Score="6" Body="&lt;p&gt;Your arithmetic getting to 50 birds per plane about 7° apart seems reasonable. I would be surprised if 50 satellites could be put on a single launch, if for no other reason that I don't think they'd fit in the fairing. Regardless of how many go up on a single launch, I think we can safely assume it would be more than one, in which case the answer below is still relevant.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;What you're talking about is known as &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_phasing&quot; rel=&quot;nofollow noreferrer&quot;&gt;phasing the orbit&lt;/a&gt; of a spacecraft within its orbital plane, and yes, it’s a common problem that arises when multiple spacecraft are deployed from a single launch vehicle. There are a variety of ways to solve this problem that depend on the capability of the satellite, the launch vehicle, the orbit, and the constellation concept of operations (most specifically how critical the spacing is and how much time you're willing to spend between launch and everything being in the final configuration).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;The Orbital Mechanics of Phasing&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Ideally, phasing only changes a satellite's relative &lt;a href=&quot;https://en.wikipedia.org/wiki/True_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;true anomaly&lt;/a&gt; with respect to the other satellites in the same plane. (I say relative because the TA changes continuously during an orbit.) A very straightforward way to do this is to temporarily change the orbital period by changing the orbit's semi-major axis. For example, consider a satellite in a reference orbit with a 90-minute period: a satellite progresses through 360° of TA in one orbit. If the orbital period was changed to 60 minutes (implausible, but we’re illustrating a concept), after 60 minutes one satellite would have a 360° change in TA, but the reference satellite would have only gone through 60/90*360 = 240°. If the 60-minute satellite was then restored to the original 90-minute orbit, the two satellites would then be permanently 120° out of phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, such an aggressive maneuver would require a prohibitive amount of propellant. So typically the semi-major axis is changed in less dramatic fashion, and the per-orbit drift in relative TA is allowed to integrate up over several orbits. This process can take an arbitrarily long amount of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Luckily, the ΔV required is small if you have some time. A 180° phasing in a 450 km orbit can be done for single-digit m/s of ΔV if you have 3-months to spare. (Exercise to the reader, or perhaps a good topic for another question.)&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Sans Propellant&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Many satellites don't have propulsion but still need to phase. For example, Planet's Dove cubsats do not have propulsion. As presented in &lt;a href=&quot;https://arxiv.org/pdf/1509.03270.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt;, they use drag to induce phasing. By controlling each's satellite's attitude they change its cross-sectional area and drag coefficient, which changes the relative drag. Drag reduces the semi-major axis of an orbit and thus it’s period. They're using the atmosphere like a low-thrust, always anti-velocity-pointing propulsion system.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;All the Fuels&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Obviously the more straightforward approach is to use an on-board propulsion system. In the &lt;a href=&quot;https://youtu.be/AHeZHyOnsm4&quot; rel=&quot;nofollow noreferrer&quot;&gt;Seattle announcement of its communications constellation&lt;/a&gt;, Elon Musk indicated their satellites would have propulsion. (Note: I don't have a time-stamp, perhaps if someone watches the video and finds it they can edit this answer.) Given this, the satellites will likely be deployed by the LV into a baseline insertion orbit and then each satellite will use on-board propulsion to maneuver to a phasing orbit, wait until its relative TA is appropriately phased, and then maneuver back into its operational orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few weeks to get the phasing right should be no big deal because the satellites are likely designed to last for years — a small fraction of their operational life. Also, satellites typically go through on-orbit checkout activities just after launch that probably don't depend on operationally representative relative phasing. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;No Rocket?&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Obviously the rocket upper stage can initially deploy each satellite into a phasing orbit (i.e. an orbit slightly different than what's desired for nominal operations), but the second maneuver that puts the satellite back into the baseline orbit after phasing is complete would still need to be done by the satellite because the rocket's mission would be long over. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd consider deploying an entire plane of satellites into a slightly elliptical phasing orbit and have each satellite circularize itself when it had reached the right relative velocity (so one ASAP, and then each subsequent satellite at a fairly regular frequency thereafter). Or if you had the ability to do at least one restart I'd use two phasing orbits: one with a longer and the other with a shorter period than nominal. That way half the plane's satellites can drift one way and half the other. This would approximately halve the time required to fully deploy a plane.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="127" LastEditDate="2017-05-27T15:07:06.637" LastActivityDate="2017-05-27T15:07:06.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21503" PostTypeId="2" ParentId="21501" CreationDate="2017-05-13T16:21:33.530" Score="6" Body="&lt;p&gt;Those are the High Gain Antennas, and according to &lt;a href=&quot;https://asd.gsfc.nasa.gov/archive/sm3a/downloads/sm3a_media_guide/HST-systems.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this systems overview&lt;/a&gt;, they are steerable:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;High Gain Antennas. Each HGA is a parabolic reflector (dish) mounted on a mast with a two- axis gimbal mechanism and electronics to rotate it 100 degrees in either direction (see Fig. 5-9). &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I believe they're mounted pointing in opposite directions (12 o'clock and 6 o'clock), so I'm not sure why they're parked at 12 and 3 specifically, but it makes sense that they're parked:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The HGAs achieve a much higher RF signal gain, which is required, for example, when transmitting high-data-rate scientific data. These antennas require pointing at the TDRSs because of their characteristically narrow beam widths. On the other hand, the LGAs provide spherical coverage (omnidirectional) but have a much lower signal gain. The LGAs are used for low-rate-data transmission and all commanding of the Telescope.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As any in-situ pictures of HST would have been taken from the space shuttle on a servicing mission, the HGAs likely wouldn't have been in use.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-13T17:36:17.507" LastActivityDate="2017-05-13T17:36:17.507" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21506" PostTypeId="1" CreationDate="2017-05-14T15:46:25.617" Score="8" ViewCount="1049" Body="&lt;p&gt;The Space Transportation System (STS) could bring a fully loaded shuttle orbiter of 109 tons to orbit (or maybe empty+payload 68+25=93 tons). The Space Launch System (SLS) Block I is based on STS, but is said to be able to lift only two thirds to three quarters of that, 70 tons, to LEO. I would have thought that the SLS instead would be more powerful than the shuttle's launch stack.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS has an added fifth segment in its solid boosters and four instead of three of the same main engines. Also, it will have an upper stage with 26 tons of fuel compared with the smaller orbital maneuvering system of the shuttle. The thrust is greater for the SLS on each of the stages. So what does all the extra power go to? If the vague mass-to-LEO measure is useless here, what would a good comparative bottom line measure of their capabilities be? Could SLS Block I carry a fully loaded shuttle orbiter if it could be attached to it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Solid booster (same isp 269 sec):&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STS  15,000 kN&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS  16,000 kN (+6%)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;First stage (same isp 452 sec vacuum):&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STS  5,250 kN&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS  7,440 kN (+40%)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Upper stages OMS versus ICPS:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;STS  2x27 kN and isp 316 sec&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SLS  110 kN and isp 462 sec &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Wikipedia for &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle&quot; rel=&quot;noreferrer&quot;&gt;STS&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Launch_System&quot; rel=&quot;noreferrer&quot;&gt;SLS&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-05-14T16:18:24.360" LastActivityDate="2020-02-13T20:39:40.513" Title="Why will SLS Block I bring less mass to LEO than the STS shuttle system did?" Tags="|space-shuttle|sls|thrust|mass-fraction|shuttle-derived|" AnswerCount="1" CommentCount="12" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21507" PostTypeId="1" CreationDate="2017-05-14T18:53:10.963" Score="1" ViewCount="199" Body="&lt;p&gt;I'm making a videogame/simulator that works with the orbital parameters of &lt;a href=&quot;https://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this document&lt;/a&gt;, but it doesn't have data about moons, dwarf planets, asteroids, comets, or even artificial satellites like the ISS, which I'd like to put to in the simulation. Does anyone know where to find them?&lt;/p&gt;&#xA;" OwnerUserId="19596" LastEditorUserId="34345" LastEditDate="2020-02-26T18:37:40.000" LastActivityDate="2020-02-26T18:37:40.000" Title="Where to find keplerian orbital elements with their time derivatives for all solar system bodies?" Tags="|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21508" PostTypeId="1" CreationDate="2017-05-14T19:46:28.800" Score="6" ViewCount="380" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Arecibo_message&quot; rel=&quot;noreferrer&quot;&gt;Arecibo message&lt;/a&gt; and both forms of the &lt;a href=&quot;http://www.plover.com/misc/Dumas-Dutil/messages.pdf&quot; rel=&quot;noreferrer&quot;&gt;Dumas-Dutil message&lt;/a&gt; are constructed exclusively as pictograms. They are sequences of bits, grouped into &quot;pages&quot; which when viewed as black or white pixels, form low resolution images. These images express their meaning by way of pictograms. My question is why? OK, there are some concepts which require some form of diagramming, but for the first few pages of the Dumas-Dutil message, it's a set of equivalences. Couldn't they be more concisely and perhaps unambiguously expressed using an arbitrary binary code, rather than concocting an alphabet of glyphs to be low-res rasterized, laid out onto a binary canvas as the message, and then serialized into the form needed for transmission? What is the rationale behind the expression of the entire message in this form?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming an alien technological civilization receives the signal, recognizes it to be artificial, and notices that it contains a stream of bits, are there not a whole host of further assumptions about what the message recipient would have to do, what choices they would have to make in order to interpret the message as intended? Can we be sure that they would even think to represent the bits in an array of pixels for further interpretation as images? What cues are built in to the message to facilitate this as the one and only thing to do at this step? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Addendum:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the Dumas-Dutil message was presented to a human team of say, computer engineers with diverse specializations including image processing and cryptography, physicists, mathematicians, chemists, biologists, and astronomers, none of whom have any prior knowledge of the message, would they be able to correctly interpret it? Has this exercise ever been tried? This sort of thing was attempted with the Arecibo message with disappointing results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the pictogram structure really improve the chances of successful interpretation, or do we have a requirement for the recipient to follow a sequence of steps, thinking like a human, each of which introduces the likelihood of an incorrect action and consequent failure to arrive at the intended result?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-05-14T22:52:26.627" LastActivityDate="2017-05-15T00:35:33.737" Title="Why are messages to space constructed as pictograms?" Tags="|seti|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21509" PostTypeId="2" ParentId="21508" CreationDate="2017-05-14T20:41:06.337" Score="5" Body="&lt;p&gt;It would certainly be possible to construct a message that doesn't start with pictorial elements, but certain concepts are much easier to get across if you can &quot;point to a picture&quot;. It's also easier to synthesize information and recognize patterns if you can view a lot of it at once in a structured way; organizing the data in multiple dimensions helps with that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since eventually most of these transmissions do want to include pictures, it's convenient to start with them, but it would be possible to just use something like ASCII encoding to do the initial mathematical education. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first cue to facilitate interpretation as an image is traditionally the length of the sequence: for a single image, it will repeat after a number of symbols which is the product of two primes, say 127 x 131 = 16637. Assuming the recipient has a concept of mathematics comparable to ours, this will immediately suggest 2-dimensionality, and hopefully -- if the idea of a 2-D bitmap is familiar to them -- they'll guess it's an image. Just about any intelligence I can imagine with multiple senses and localized sense receptors ought to have some concept of a 2-D image, but clearly some conceivable intelligences will be left out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Messages like the Dumas-Dutil can be worked out by humans without prior knowledge; it's been done several times (by single persons as well as teams). MJD blogged &lt;a href=&quot;http://blog.plover.com/aliens/dd/intro.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;his decoding of the DD here&lt;/a&gt; (though I believe he started from the bitmaps and had some general high level knowledge of the contents). The Aricebo might be too brief and sketchy to really get much information across.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://puzzling.stackexchange.com/questions/17119/deciphering-an-alien-signal&quot;&gt;Stack Exchange puzzle Q&amp;amp;A&lt;/a&gt; demonstrates the ability of a small group of people to work out such a message &lt;em&gt;from the raw signal&lt;/em&gt;. More significantly, the signal is not a digitally coded bitmap, but a sort of vector display -- not a completely unknown representation to modern techies, but not the most familiar either. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some years ago I also saw a similar puzzle on the internet; in this case the &quot;aliens&quot; started with a linear format, but their numeric and mathematical representations all had an interesting grammatical quirk to them; they eventually graduated to bitmap images, but given in a spiral hexagonal format instead of a rectangular raster grid. Their entire message was likewise decoded completely by a small team. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-15T00:35:33.737" LastActivityDate="2017-05-15T00:35:33.737" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21510" PostTypeId="1" AcceptedAnswerId="21511" CreationDate="2017-05-14T20:42:22.907" Score="2" ViewCount="1279" Body="&lt;p&gt;Today we use audio and video communications (using UDP protocol) over IP networks (VoIP) for communicating (Skype, Whatsapp, FB messenger). This works fine on planet Earth (although there are &lt;a href=&quot;http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/5125-delay-details.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;various factors that affect this&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What technology is used today for communicating in space? How might a person in Mars talk to somebody on Earth?&lt;/p&gt;&#xA;" OwnerUserId="7031" LastEditorUserId="6816" LastEditDate="2017-05-14T21:05:34.210" LastActivityDate="2017-05-14T21:31:30.283" Title="Real time communications in space" Tags="|mars|crewed-spaceflight|communication|radio-communication|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21511" PostTypeId="2" ParentId="21510" CreationDate="2017-05-14T21:31:30.283" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What technology is used today for communicating in space? How might a person in Mars talk to somebody on Earth?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Using radio waves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, first of all, &quot;real time&quot; is a strong word. Depending on the relative position of earth and Mars, light (and thus, electromagnetic waves) take minutes to cross that distance. You can't &quot;have a nice chat&quot; when you say something, and then have to wait minutes until the other end can receive that, and answer it, and for the reply to reach you. Nothing can, physically, be faster than light, so point-to-point low-latency communication is plain impossible in the universe as we know it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;UDP&quot; and things like the differen VoIP methods are rather high-level things that you can do as soon as you have a working IP link. As you can imagine (if you've ever lost reception with your 3G/4G phone when leaving a metropolitan area or going through a tunnel), that's a rather hard thing to achieve if the distance between you and the other end is multiple astronomical units.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the hard part is actually not choosing anything that runs on such a high layer of the protocol stack, but first having contact at all – in fact, if you're on the wrong side of the planet, facing away from the other planet, that becomes a very hard problem, and you'll need a very capable system of satellites orbitting your planets to relay the signals around, so that there's effectively no shadow. The sun potentially being in the way (and being an enormous source of noise) doesn't help, either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, those satellite systems do &lt;em&gt;not&lt;/em&gt; exist; Mars missions lose connectivity for a few hours every Mars-day, when the pure geometry doesn't allow a direct line of sight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, your question starts far too high-leveled; it's like asking &quot;which furniture shops did early humans go to&quot;, when the real question would be &quot;how did early humans build housing at all&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me add that communication technology is a very math-rich field, and we have a lot of equations that describe theoretical limits. So, rest assured, that without a humongous antenna (think: satellite dish with dozens of meters in diameter), you simply mathematically can't achieve sufficient signal reception (and thus, signal-to-noise ratio) to even theoretically allow (not: implement) a intellegible voice call from planet surface to planet surface. (Terms to Google, if you're doing a report: SNR, Shannon's Channel Capacity, Free Space Path Loss, Antenna Gain, Background Radiation, Noise Figure)&lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastActivityDate="2017-05-14T21:31:30.283" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21512" PostTypeId="2" ParentId="21507" CreationDate="2017-05-14T22:09:37.087" Score="3" Body="&lt;p&gt;The JPL Solar System Dynamics site (where the document you mentioned is hosted) has &lt;a href=&quot;https://ssd.jpl.nasa.gov/?bodies#elem&quot; rel=&quot;nofollow noreferrer&quot;&gt;several other resources&lt;/a&gt; for finding orbital elements of moons, asteroids, and comets. These are mainly for natural bodies, but they also recommend using the &lt;a href=&quot;https://celestrak.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CelesTrak&lt;/a&gt; database for artificial spacecraft. The only information they may not have is the time derivatives (at least I didn't see them when taking quick glance through the output).&lt;/p&gt;&#xA;" OwnerUserId="19641" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2017-05-14T22:09:37.087" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21513" PostTypeId="2" ParentId="16604" CreationDate="2017-05-15T08:01:16.130" Score="11" Body="&lt;p&gt;&lt;strong&gt;TL; DR:&lt;/strong&gt; Trajectory optimization for continuous thrust is difficult and this field is very active in research.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;&lt;strong&gt;2021 clarifications:&lt;/strong&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;h3&gt;Methodology&lt;/h3&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;For the least amount of fuel, the best is the thrust the least amount of time as possible and only when it's extremely efficient (&lt;span class=&quot;math-container&quot;&gt;$\eta \geq 0.98$&lt;/span&gt;). But that also implies that it will take an incredibly long amount of time to reach the desired orbit.&lt;/li&gt;&#xA;&lt;li&gt;Conversely, to reach an orbit in the least amount of time, one should thrust as much as possible, even when it isn't &lt;em&gt;that&lt;/em&gt; efficient.&lt;/li&gt;&#xA;&lt;li&gt;Typically, in trajectory optimization, we set a maximum transfer time and optimize for the lowest total thrust over time (i.e. lowest overall &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;) in that maximum time. This allows a fine balance between &amp;quot;least amount of fuel&amp;quot; and &amp;quot;get this spacecraft to its mission orbit and start the science/making money&amp;quot;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;In this &lt;a href=&quot;https://space.stackexchange.com/a/19805/1391&quot;&gt;older answer&lt;/a&gt;, I mention the Ruggiero control laws: these provide the most efficient thrusting angles based on true anomaly to change an orbital element (or set thereof). The paper also provides the partial derivatives of the equations that one may use these to determine the efficiency (&lt;span class=&quot;math-container&quot;&gt;$\eta$&lt;/span&gt;) of modifying an orbital element at a given true anomaly.&lt;/p&gt;&#xA;&lt;h3&gt;Answers&lt;/h3&gt;&#xA;&lt;p&gt;&lt;em&gt;These answers are specific to raising the semi-major axis and reaching an eccentricity of zero. It does &lt;strong&gt;not&lt;/strong&gt; answer the solution for a real GEO orbit, which has additional constraints on inclination (near zero) and other orbital elements specific to a given mission.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;&lt;strong&gt;Q1: In order to use the least amount of ion propellant, should the spacecraft be slowly rotating, or have a fixed attitude, perhaps tangent to apoapsis? If rotating, at a constant, or ramped rate?&lt;/strong&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;If your thrust is always in the direction of velocity vector, you're continuously increasing the velocity of the spacecraft and therefore you'll be increasing your SMA (along with modifying other elements). To circularize an orbit, you need to raise only your periapse until it matches your apoapse. To raise your SMA, you need to thrust all the time... but that will affect your eccentricity. Therefore, the best approach is probably to raise your SMA until GEO and then circularize your orbit. To consume the least amount of fuel, you should only turn on your engine when you're within a very small region of your periapse such that your apoapse is very slightly increased one half-orbit later. Repeat that until you've reached your target SMA. Then circularize.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;&lt;strong&gt;Q2: How would one modulate the thrust and attitude to circularize a standard elliptical GTO using the 0.2N ion propulsion in the least amount of time?&lt;/strong&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;One would be continuously thrusting and varying the in-plane and out-of-plane angles such that both the inclination and the SMA are modified at any given time throughout the orbit, but by weighting one of the orbital elements more then the other based on their efficiency. The high-fidelity &lt;a href=&quot;https://nyxspace.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nyx astrodynamics toolkit&lt;/a&gt; provide the Ruggiero control laws out of the box.&lt;/p&gt;&#xA;&lt;h3&gt;Full details (2017)&lt;/h3&gt;&#xA;&lt;p&gt;&lt;em&gt;&lt;strong&gt;Edit:&lt;/strong&gt;&lt;/em&gt; Concerning the rotation of your spacecraft, you'll want to plot the in-plane and out-of-plane thrusting angles (with respect to the RNC frame of the spacecraft). That will give you an idea of how much the engines needs to gimbal by before thrusting. That angle depends entirely on what orbital parameters you're changing (cf. Naasz, Ruggiero and Petropoulos control laws). I &lt;em&gt;expect&lt;/em&gt; that angle to not vary by much throughout an orbit correction, but I may be wrong (hopefully will have an &lt;a href=&quot;https://github.com/ChristopherRabotin/smd/issues/126&quot; rel=&quot;nofollow noreferrer&quot;&gt;answer soon&lt;/a&gt;). This expectation is based on the equations which perform the instantaneously optimal correction of each parameter, cf. figures 4 and 5 at the bottom.&lt;/p&gt;&#xA;&lt;p&gt;If using continuous thrust, you'll generally want to solve the optimal control problem. When solving an optimal control problem, you're minimizing a functional, i.e. you're searching for a function (the control) which will minimize your cost function at each time step (cf. screenshot of my group's presentation below).&lt;/p&gt;&#xA;&lt;p&gt;There are different ways to solve the optimal control problem for low thrust trajectory design, and (as often, sadly) each group of researchers is somewhat silos.&lt;/p&gt;&#xA;&lt;p&gt;From the (unpublished) research that two colleagues and I did, there are a few strategies to solve this, which I summarize below. All but the first one solve an optimal control problem.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The simplest: use a sub-optimal closed-loop control law (like the Petropoulos Q-Law) which is fed to an optimizer like a genetic algorithm. This leads to a sub-optimal solution for the trajectory but is often very close to the optimal solution. All you need is to code up the Q-Law and the GA and define an initial and final orbit. Other controls worth checking out include Naasz and Ruggiero (but note that the initial formulation for the Naasz control was only validated on cases when you want to increase the value of an orbital parameter, and you need to perform a slight change in the equations to make it work the other way (I can provide the info if needed, I validated the changes in my own research)).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The most theoretical and math-heavy method: the indirect methods, which solve the two point boundary value problem by finding the Hessian. Requires the initial and final conditions desired. There are several such methods, mostly started by Betts in 1998, including Direct Shooting, Indirect Shooting, and used more recently, indirect collocation. One of the issues you'll encounter is the selection of the initial Lagrange multipliers, knowing that a poor choice may prevent convergence. In addition, these methods lead to open loop solutions.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Direct methods: time is discretized and at each step you're trying to find the best control. This is also an open loop method, and because of how the problem is posed, you encounter the curse of dimensionality (cf. Lantoine 2012). Yet, this is considered by some as the state of the art of trajectory optimization (cf. &amp;quot;Spacecraft trajectory optimization&amp;quot;, Conway 2010).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A better way: dynamic programming: the idea here is the split up the problem into simpler independent problems (a &amp;quot;divide and conquer&amp;quot;-like approach) and apply the principle of optimality (Bellman's graph traversing principle where the shortest path from a to c corresponds to the shortest path from a to b and from b to c) (cf. figure 2). The main drawback here is that splitting up the state space into independent sub problems means that you're going to be using a ton of memory to store the ongoing solutions... and then you'll need to traverse this graph. The main advantage though is that you'll always find the global optimal solution, and the returned functional is a closed loop control.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Differential dynamic programming: this isn't a very new method per se, but it's definitely gaining tracking again in recent history. Specifically, it is used in NASA Mystic which does the trajectory optimization for the Dawn mission (which has three Safran PPS-1350 but only one turned on at each time, it's a pretty cool mission). The idea of this method is to pick an initial guess for the control, propagate the (real) dynamics forward, compute the cost, perform a correction of the control by propagating backwards (but using a quadratic approximation of the dynamics (i.e. you'll need the state transition matrix and the Jacobian of the STM too, which is the Hessian of the dynamics)), and finally check if you're minimizing the cost function by propagating forward again (cf. figure 3). The references you'll want to check for this method include Liao 1991, Pantoja 1988, Lantoine 2012, Ozaki 2015, and eventually Aziz 2016 (for an explanation of Hybrid DDP).&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;I hope this helps!&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OoUm0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OoUm0.png&quot; alt=&quot;Figure 1&quot; /&gt;&lt;/a&gt;&#xA;Figure 1&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jrreD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jrreD.png&quot; alt=&quot;Figure 2&quot; /&gt;&lt;/a&gt;&#xA;Figure 2&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S6Q0G.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S6Q0G.png&quot; alt=&quot;Figure 3&quot; /&gt;&lt;/a&gt;&#xA;Figure 3&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ELrQf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ELrQf.png&quot; alt=&quot;Figure 4&quot; /&gt;&lt;/a&gt;&#xA;Figure 4&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WLIj4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WLIj4.png&quot; alt=&quot;Figure 5&quot; /&gt;&lt;/a&gt;&#xA;Figure 5&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="1391" LastEditDate="2021-03-16T03:36:05.070" LastActivityDate="2021-03-16T03:36:05.070" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="21515" PostTypeId="1" AcceptedAnswerId="27777" CreationDate="2017-05-15T13:06:18.930" Score="10" ViewCount="276" Body="&lt;p&gt;The two &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Highwater&quot; rel=&quot;noreferrer&quot;&gt;Project High Water&lt;/a&gt; launches caused the release of 95 tons of water in the ionosphere.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For both of these experiments, the resulting ice clouds expanded to several miles in diameter and lightning-like radio disturbances were recorded.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What caused the lightning-like radio disturbances?&#xA;Are there any images of these ice clouds expanding?&lt;/p&gt;&#xA;" OwnerUserId="16322" LastActivityDate="2018-06-11T06:36:52.503" Title="Project High Water and the lightning-like radio disturbances it caused" Tags="|saturn-v|sounding-rocket|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21517" PostTypeId="1" CreationDate="2017-05-15T14:07:02.523" Score="24" ViewCount="14427" Body="&lt;p&gt;Like why use RTGs (&lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;Radioisotope thermoelectric generator&lt;/a&gt;) to power small spacecraft instead of just placing batteries inside them to power stuff?&lt;/p&gt;&#xA;" OwnerUserId="19617" LastEditorUserId="472" LastEditDate="2017-05-18T05:36:25.737" LastActivityDate="2019-07-14T00:56:07.837" Title="Why aren't batteries used in space?" Tags="|battery|" AnswerCount="4" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="21518" PostTypeId="2" ParentId="21517" CreationDate="2017-05-15T14:15:47.863" Score="7" Body="&lt;p&gt;Simply put, RTGs last a long time. Space probes need a reliable, long-lasting power source, since we can't just change the batteries when they run out. An RTG can run for decades with relatively little reduction in power output, unlike a traditional chemical battery.&lt;/p&gt;&#xA;" OwnerUserId="13714" LastActivityDate="2017-05-15T14:15:47.863" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21519" PostTypeId="2" ParentId="21517" CreationDate="2017-05-15T14:35:30.590" Score="65" Body="&lt;p&gt;Energy density for:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NiMH C battery -&gt; 237,073 Joules per Kilogram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plutonium 238 (used in RTGs) -&gt; 2,239,000,000,000 Joules per Kilogram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if we assume that only 10% of a RTG weight is actually Plutonium, then we  still get about 9,400,000 times as much energy available as heat from an RTG as from the same mass of batteries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In most deep-space missions, landers, and rovers, heat generation is essential to maintain spacecraft function. However, for electrical power, the best conversion efficiency from RTG thermal to electrical power is about 7%, making it &quot;only&quot; 661,000 times as much energy available as electrical power from a RTG as from the same mass of batteries. That's still a pretty huge difference!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;Source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Energy_density&quot; rel=&quot;noreferrer&quot;&gt;Energy Density, &lt;em&gt;wikipedia&lt;/em&gt;&lt;/a&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="11787" LastEditorUserId="6011" LastEditDate="2017-05-18T04:01:54.860" LastActivityDate="2017-05-18T04:01:54.860" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21520" PostTypeId="1" AcceptedAnswerId="48027" CreationDate="2017-05-15T14:44:19.053" Score="5" ViewCount="523" Body="&lt;p&gt;If I don't worry about morale, would daily, average sized meals from the following be sufficient to stay alive for three years in space? I'm assuming only water is recovered from waste.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume enough electricity for cooking, zero gee pots and pans, ISS type kitchen equipment, spare parts for waste treatment hardware, and an routine waste dumps to space as needed. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dried beans &lt;/li&gt;&#xA;&lt;li&gt;Rice&lt;/li&gt;&#xA;&lt;li&gt;Oats&lt;/li&gt;&#xA;&lt;li&gt;Dried meat, fish, squid, etc. (if one partakes)&lt;/li&gt;&#xA;&lt;li&gt;Olive oil&lt;/li&gt;&#xA;&lt;li&gt;Salt&lt;/li&gt;&#xA;&lt;li&gt;Vitamins (+omega 3, 6)&lt;/li&gt;&#xA;&lt;li&gt;Electrolytes&lt;/li&gt;&#xA;&lt;li&gt;Powdered/dried seasonings&lt;/li&gt;&#xA;&lt;li&gt;Water&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; For this mission, dried bulk items were chosen because they can potentially tolerate larger excursions in temperature for long periods of time. There are no freezers or refrigerators, and the mission may go inside Earth's orbit as well as way beyond, so keeping the food &quot;outside&quot; in the cold was not an option on his budget flight.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-10-17T10:16:14.977" LastActivityDate="2020-10-20T23:25:24.997" Title="Dried beans, rice and oats, jerky, olive oil, salt, vitamins, electrolytes, water; anything else?" Tags="|crewed-spaceflight|astronaut-lifestyle|food|reduced-gravity-cooking|nutrition|" AnswerCount="2" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="21521" PostTypeId="2" ParentId="21517" CreationDate="2017-05-15T15:17:43.590" Score="19" Body="&lt;p&gt;RTGs are used in a very small number of spacecraft. They are used only when there are no other options, i.e. for long missions too far away from the Sun to make solar panels feasible.&lt;br&gt;&#xA;Those missions have requirements for a few hundred W of power continuously over a decade or more. If you were to use chemical batteries to supply that much power, your spacecraft would become too heavy to launch. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-05-15T15:17:43.590" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21522" PostTypeId="1" AcceptedAnswerId="30804" CreationDate="2017-05-15T15:29:15.313" Score="2" ViewCount="870" Body="&lt;p&gt;Because of where I'm currently living, I've gotten used to using a small &lt;a href=&quot;https://en.wikipedia.org/wiki/Rice_cooker&quot; rel=&quot;nofollow noreferrer&quot;&gt;rice cooker&lt;/a&gt; regularly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;@Tristan has &lt;a href=&quot;https://space.stackexchange.com/questions/21520/#comment60067_21520&quot;&gt;pointed out&lt;/a&gt; that cooking dried rice, or dried beans in boiling water could not be done in the same way on board a spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a hunch someone has already invented a zero gravity rice cooker, perhaps published, perhaps not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone speculate how one might work on board a spacecraft? You add dried rice or beans, then add water, then close it. On Earth, the water and ingredients stay in the bottom, and the steam goes out the top, gently lifting the loose-fitting lid. Without gravity, how could one keep the boiling water and the ingredients together, and yet have steam come out safely?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CKnkt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CKnkt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Rice cooker (surprisingly, exactly what I have). From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Electronic_rice_cooker_with_scoop.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/R5gOD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R5gOD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Diagram of a rice cooker. This is the older, purely electromechanical kind. After cooking is done, the main heater turns off (related to the magnet?) but the warmer stays on. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Electronic_rice_cooker_with_scoop.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A potential user: &lt;a href=&quot;https://twitter.com/yousuck2020/status/1042209952217985024&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://twitter.com/yousuck2020/status/1042209952217985024&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/zywwk.jpg&quot; width=&quot;400&quot; alt=&quot;wide demo pic&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-09-19T18:23:13.900" LastActivityDate="2018-09-21T05:00:48.657" Title="Zero gee rice cooker for BFR's trip around the Moon; how would that work?" Tags="|spacex|astronaut-lifestyle|food|spacex-starship|reduced-gravity-cooking|" AnswerCount="3" CommentCount="15" ContentLicense="CC BY-SA 4.0" />
  <row Id="21524" PostTypeId="1" AcceptedAnswerId="26665" CreationDate="2017-05-15T17:02:24.447" Score="8" ViewCount="1547" Body="&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What are these very large square panels on Inmarsat 5's? They seem to each have an extremely reflective mirror-like flat/smooth tiled-appearing side, and a nearly black side with a lot of tubing and/or wiring and connections. Also they are oriented oppositely; &lt;em&gt;the shiny side of one is facing the same direction as the black side of the other&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I understand correctly, the dishes all point Earthward (nadir), and the long solar panel axis should be North-South so that they can rotate and receive fairly constant illumination, except during eclipses (similarly to the rotation described in &lt;a href=&quot;https://space.stackexchange.com/q/19064/12102&quot;&gt;this currently-unanswered question&lt;/a&gt;).  So that means that these square panels I'm asking about are roughly co-planar with the Earth's equator, &lt;em&gt;and so are roughly edge-on to both the Earth and the Sun&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QR8Bl.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QR8Bl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped, from &lt;a href=&quot;http://www.defencetalk.com/wp-content/uploads/2015/03/Inmarsat-space-satellite.jpg&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; found at &lt;a href=&quot;http://www.defencetalk.com/wp-content/uploads/2015/03/?SD&quot; rel=&quot;noreferrer&quot;&gt;Defense Talk&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RXxwc.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RXxwc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; . . &lt;a href=&quot;https://i.stack.imgur.com/ssDun.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ssDun.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped sections from image below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/raFOI.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/raFOI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Inmarsat 5-F1 (F4 Identical) – Photo: Boeing. From &lt;a href=&quot;http://spaceflight101.com/falcon-9-inmarsat-5-f4/inmarsat-5-f4/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This screen shot from the video &lt;a href=&quot;https://youtu.be/AiNL7zilS3g?t=6&quot; rel=&quot;noreferrer&quot;&gt;EUTELSAT 8 West B - Testing Solar Array Deployment&lt;/a&gt; shows a different satellite but with a very similar looking shiny tiled-appearing surface (on the left). Perhaps just a coincidence?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7GEH1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7GEH1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-08-20T15:22:45.537" LastActivityDate="2018-08-20T15:22:45.537" Title="What are these very large, square panels on Inmarsat 5?" Tags="|design|communication-satellite|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="21525" PostTypeId="1" AcceptedAnswerId="22113" CreationDate="2017-05-15T17:04:58.307" Score="6" ViewCount="779" Body="&lt;p&gt;Looking at the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Falcon_9_and_Falcon_Heavy_launches&quot; rel=&quot;nofollow noreferrer&quot;&gt;list of Falcon 9 launches&lt;/a&gt; I noticed the booster numbers jump from B1026 to B1028. Anyone know what happened to B1027? Was it scrapped during manufacturing for some reason?&lt;/p&gt;&#xA;" OwnerUserId="19651" LastEditorUserId="10132" LastEditDate="2017-05-15T22:02:37.990" LastActivityDate="2017-06-27T01:17:04.533" Title="Why did Falcon 9 boosters numbers skip B1027? What happened with it?" Tags="|spacex|falcon-9|booster|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21526" PostTypeId="2" ParentId="15012" CreationDate="2017-05-15T17:23:29.493" Score="11" Body="&lt;p&gt;The State Comission report was declassified in 1994. The results were published in &lt;em&gt;Voenno-istorichesky zhurnal&lt;/em&gt; (Military history journal) 1995/5. As well as in some other sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The official lists of military personnel lost (57), industry specialists lost (17) and the list of injured personnel (49) given in the report matches the ones published on &lt;a href=&quot;http://sm.evg-rumjantsev.ru/24.10.1960/nedelin_disaster.htm&quot; rel=&quot;noreferrer&quot;&gt;this site&lt;/a&gt; .&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the immediate death toll is 57+17 = &lt;strong&gt;74&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some &lt;strong&gt;4&lt;/strong&gt; of the 49 injured are marked as deceased in 1960-1961.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorDisplayName="user54" LastEditDate="2017-05-15T18:09:15.563" LastActivityDate="2017-05-15T18:09:15.563" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21527" PostTypeId="2" ParentId="21517" CreationDate="2017-05-15T18:54:51.623" Score="16" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;Nuclear decay is just simply the most energy dense fuel there is. This is enough to overcome huge inefficiencies in power conversion. We can even ignore the inefficiencies of alternate storage methods, and still conclude that fissile material will store more energy per unit mass.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr&gt;&#xA;&lt;h3&gt;RTG&lt;/h3&gt;&#xA;&lt;p&gt;For RTGs I'll refer to &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Space&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia's&lt;/a&gt; article about RTGs that have previously been or are in service. By their nature, the output and efficiency of an RTG is complicated to compute, so I'll refer to the actual measured power output, and assume the output declines along with the nuclear decay (half life of 87 years for &lt;sup&gt;238&lt;/sup&gt;Pu).&lt;/p&gt;&#xA;&lt;p&gt;Evaluating the RTGs used in aerospace applications, the &lt;em&gt;absolute worst performer&lt;/em&gt; was the SNAP-3B generator with a specific power of 1.3 W/kg (at launch). This was used on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transit_(satellite)&quot; rel=&quot;noreferrer&quot;&gt;Transit4B&lt;/a&gt; satellite, which was operational for roughly 1 year (accidentally destroyed by nuclear test detonation). During this time its 2.1kg RTG produced roughly &lt;a href=&quot;https://www.wolframalpha.com/input/?i=integral+from+0+to+1+of+(2.7*24*365)*e%5E(-t%2F(87%2Fln(2)))+dt&quot; rel=&quot;noreferrer&quot;&gt;23.558 kWh&lt;/a&gt; of electricity. This gives a specific storage of &lt;b&gt;11.2 kWh/kg&lt;/b&gt;&lt;/p&gt;&#xA;&lt;hr&gt;&#xA;&lt;h3&gt;Li-Ion&lt;/h3&gt;&#xA;&lt;p&gt;Typical quoted values of Lithium ion specific power capacity are usually around 100 - 200 Wh/kg, however &lt;a href=&quot;https://electronics.stackexchange.com/a/4329/136469&quot;&gt;this&lt;/a&gt; post (linked article no longer accessable; see &lt;a href=&quot;https://en.wikipedia.org/wiki/Lithium%E2%80%93air_battery&quot; rel=&quot;noreferrer&quot;&gt;wikipedia&lt;/a&gt;) from the electronics stackexchange explores the performance of lithium-air batteries (currently have the highest specific energy of any chemistry) with a value of &lt;b&gt;1.7kWh/kg&lt;/b&gt; for a lithium air battery (Li - O&lt;sub&gt;2&lt;/sub&gt;)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;As you can see the absolute worst performing RTG is still several times more energy dense that the best performing chemical battery.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr&gt;&#xA;&lt;h3&gt;Fuel Cell&lt;/h3&gt;&#xA;&lt;p&gt;As far as chemical energy generation goes, fuel cells are much better, as you can more or less ignore the weight of the fuel cell and only consider the fuel (making the assumption that the mass of fuel is much larger than the cell). Hydrogen fuel cells can reach near &lt;a href=&quot;https://en.wikipedia.org/wiki/Fuel_cell#Theoretical_maximum_efficiency&quot; rel=&quot;noreferrer&quot;&gt;85 - 90%&lt;/a&gt; theoretical efficiency from the reaction 2H&lt;sub&gt;2&lt;/sub&gt; + O&lt;sub&gt;2&lt;/sub&gt; -&amp;gt; 2H&lt;sub&gt;2&lt;/sub&gt;O (40 - 60% in practice). Even ignoring the efficiency loss (because it's small and I don't want to add the calculation step) we can &lt;a href=&quot;https://www.wolframalpha.com/input/?i=(241.82+kj%2Fmol)%2F(18.01+g%2Fmol)+to+kwh%2Fkg&quot; rel=&quot;noreferrer&quot;&gt;calculate the specific energy&lt;/a&gt; density to be &lt;b&gt;3.73kWh/kg&lt;/b&gt; using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_enthalpy_of_formation#Miscellaneous_compounds&quot; rel=&quot;noreferrer&quot;&gt;enthalpy of formation&lt;/a&gt; of water (the absolute theoretical amount of energy released when hydrogen and oxygen combine)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;Even fuel cells at above theoretical maximum performance are not quite as good as one of the worst RTGs (keep in mind we're only considering space applications. RTGs for land use have lots of extra radiation shielding and are very heavy)&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="19267" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-07-14T00:56:07.837" CommentCount="13" ContentLicense="CC BY-SA 4.0" />
  <row Id="21529" PostTypeId="1" CreationDate="2017-05-16T00:42:10.827" Score="3" ViewCount="1487" Body="&lt;p&gt;OK we know that due to air pressure a vacuum balloon would not be possible on earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But would it be possible to create one that floated near space with almost no air pressure? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so how much could it lift? what are its limits?&lt;/p&gt;&#xA;" OwnerUserId="19656" LastActivityDate="2022-11-08T13:45:20.087" Title="near space vacuum balloon" Tags="|vacuum|balloons|" AnswerCount="5" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21530" PostTypeId="1" AcceptedAnswerId="21531" CreationDate="2017-05-16T02:56:19.853" Score="6" ViewCount="700" Body="&lt;p&gt;The Inmarsat-5 from SpaceX launched earlier today. I was watching the broadcast and noticed a brief green flame during ignition. What caused this? &lt;a href=&quot;https://i.stack.imgur.com/tiNzD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tiNzD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17532" LastActivityDate="2017-05-16T03:29:37.107" Title="Green fire in Inmarsat-5 Launch" Tags="|launch|spacex|" AnswerCount="1" CommentCount="1" ClosedDate="2017-05-16T12:10:36.313" ContentLicense="CC BY-SA 3.0" />
  <row Id="21531" PostTypeId="2" ParentId="21530" CreationDate="2017-05-16T03:29:37.107" Score="7" Body="&lt;p&gt;That's the TEA-TEB mixture used to start the engines. The boron in the TEB is responsible for the green flame.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-05-16T03:29:37.107" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21532" PostTypeId="1" CreationDate="2017-05-16T07:25:31.003" Score="1" ViewCount="609" Body="&lt;p&gt;How large (mass) are the &lt;a href=&quot;https://en.wikipedia.org/wiki/Control_moment_gyroscope&quot; rel=&quot;nofollow noreferrer&quot;&gt;CMG&lt;/a&gt;s on the ISS?&#xA;What is the minimum torque they apply?&#xA;What is the maximum torque they apply?&lt;/p&gt;&#xA;" OwnerUserId="19570" LastEditorUserId="16322" LastEditDate="2017-05-16T13:20:38.727" LastActivityDate="2017-06-14T16:39:19.380" Title="What are the main properties of the CMGs on the ISS" Tags="|iss|attitude|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21533" PostTypeId="2" ParentId="20596" CreationDate="2017-05-16T10:42:37.470" Score="5" Body="&lt;p&gt;I've been trying to find a link between the mini-magnetosphere actually created at Rutherford Appleton Labs (U.K.) in 2013 to NASA.  So far there's only anecdotal reports and interviews none of which are conclusive as to NASA's involvement.   RAL has a good bit of material published on the proof of concept magnetosphere and as far as I can tell is the only place such work has gone past theoretical models to a practical scaled model (that was very successful) so it would be surprising if there's been no contact with NASA. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.extremetech.com/extreme/159971-scientists-produce-star-trek-like-deflector-device-for-cancer-free-interplanetary-travel&quot; rel=&quot;noreferrer&quot;&gt;https://www.extremetech.com/extreme/159971-scientists-produce-star-trek-like-deflector-device-for-cancer-free-interplanetary-travel&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd also add that NASA did show interest in a group of researchers at CERN in 2014 working to develop an artificial magnetosphere using magnesium diboride superconducting magnets. The magnets are used in the LHC to create ultra-bright particle beams at a low temperature. Used in spacecraft shielding, they'd create a magnetosphere around the craft.  As far as i can tell that research hasn't been publicly updated since mid 2015.&lt;/p&gt;&#xA;" OwnerUserId="19664" LastActivityDate="2017-05-16T10:42:37.470" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21534" PostTypeId="1" CreationDate="2017-05-16T13:14:12.647" Score="1" ViewCount="93" Body="&lt;p&gt;One of the &lt;a href=&quot;http://www.nature.com/news/tiny-chipsat-spacecraft-set-for-first-flight-1.20006&quot; rel=&quot;nofollow noreferrer&quot;&gt;potential uses for chipsats&lt;/a&gt; may be:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Chipsats, which are smaller and cheaper still, are seen as disposable sensors that could be sent on suicide missions to explore hostile environments, such as Saturn’s rings.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It sounds sensible: currently, only near the end of life (such as currently with Cassini) do scientists dare to make measurements that are risky or suicidal.  But with cheap, disposable chipsats in the hold of an interplanetary spacecraft, one could push some into the atmosphere of the giant planets or other hostile environments.  Although the science payload would be naturally limited, they could provide valuable data where no other in-situ measurements are available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any &lt;em&gt;concrete&lt;/em&gt; plans to bring chipsats along on interplanetary space exploration missions?  By concrete plans, I mean chipsats as part of proposals that are under formal consideration by space agencies.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2017-05-16T13:14:12.647" Title="Are there any concrete plans to include chipsats for planetary suicide missions?" Tags="|artificial-satellite|interplanetary|" AnswerCount="0" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21536" PostTypeId="2" ParentId="21529" CreationDate="2017-05-16T13:58:12.083" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;But would it be possible to create one that floated near space with almost no air pressure? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Possibly, depending on what material(s) you use to make the balloon.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If so how much could it lift? what are its limits?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That depends entirely on how big the balloon is and what material you use to create it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A regular balloon uses internal gas pressure (typically helium or hot air) to hold its shape.  A vacuum balloon would require a rigid shell (like a giant Ping-Pong ball) instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since &quot;almost no air pressure&quot; is not the same as &quot;no air pressure&quot;, you will need some rigidity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember that balloons and boats float because they weigh less than the equivalent &quot;medium&quot; (air for balloons or water for boats) that they displace.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have 1 solid cubic meter of something, it'll float only as long as it weighs less than 1 cubic meter of air at that location (considering temp &amp;amp; pressure).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that the higher you go, the less that cubic meter of air weighs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that you won't be able to find a material strong enough to hold its shape against the &quot;almost no air pressure&quot; but still light enough to weigh less than the amount of air it's displacing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to be wrong though - vacuum balloons sound pretty cool.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more info, see the Wikipedia page on &lt;a href=&quot;https://en.wikipedia.org/wiki/Vacuum_airship&quot; rel=&quot;noreferrer&quot;&gt;Vacuum airships&lt;/a&gt; (lots of math there)&lt;/p&gt;&#xA;" OwnerUserId="12978" LastActivityDate="2017-05-16T13:58:12.083" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21538" PostTypeId="2" ParentId="21506" CreationDate="2017-05-16T20:50:22.233" Score="12" Body="&lt;p&gt;It's always difficult to make apples-to-apples comparisons between the space shuttle and other launchers, because the orbiter is ambiguously part launcher and part payload. This is compounded by the broadness of the term &quot;LEO&quot;; shuttle payloads went to a variety of altitudes and inclinations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, since the title of the question specifies &quot;mass to LEO&quot; rather than useful payload, we can start there. At the space shuttle's MECO, it's in an eccentric orbit of perhaps 300km x 70km; this is comparable to a circular orbit of 185km. The total mass on this trajectory, assuming the OMS has not been used for ascent assist, is at most:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;109 tons of &lt;a href=&quot;http://www.braeunig.us/space/specs/orbiter.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbiter&lt;/a&gt; including payload and propellant&lt;/li&gt;&#xA;&lt;li&gt;26.5 tons of external tank (empty &lt;a href=&quot;http://www.braeunig.us/space/specs/shuttle.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;SLWT&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;or 135.5 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.braeunig.us/space/specs/sls.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;SLS&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Exploration_Mission_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;EM-1&lt;/a&gt; mission intends to put an ICPS and Orion spacecraft into a higher orbit than that, 1800km x 93km; this is the trajectory at core stage main engine cutoff (MECO), without the ICPS having fired yet. The low perigee guarantees that the core stage will re-enter Earth's atmosphere and burn up. At this point, we have a total mass in orbit of:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;85 tons empty core stage&lt;/li&gt;&#xA;&lt;li&gt;31 tons loaded ICPS stage&lt;/li&gt;&#xA;&lt;li&gt;5 tons interstage&lt;/li&gt;&#xA;&lt;li&gt;26 tons &lt;a href=&quot;http://www.braeunig.us/space/specs/orion.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion&lt;/a&gt; spacecraft&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For a total of 147 tons in this higher orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My reference says the shuttle payload was reduced by 25kg per km altitude, suggesting the shuttle would have to give up about 19 tons of payload to match the SLS EM-1 trajectory, making the comparison 147 tons to 116.5 tons.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it appears that the larger SRBs and fourth SSME do give greater performance to the Block 1 SLS: roughly&#xA;25% more mass to LEO, which seems intuitively reasonable.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2020-02-13T20:39:40.513" LastActivityDate="2020-02-13T20:39:40.513" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21539" PostTypeId="1" CreationDate="2017-05-16T21:36:56.280" Score="2" ViewCount="284" Body="&lt;p&gt;Sistema Stykovki i Vnutrennego Perekhoda (SSVP) is a probe-and-drogue docking mechanism widely used in Soviet and Russian manned spaceflight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparing to the Androgynous Peripheral Attach System (APAS) and the Common Berthing Mechanism (CBM), SSVP provided neither androgynous docking interface nor a wide passage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the advantage of SSVP?&lt;/p&gt;&#xA;" OwnerUserId="340" LastActivityDate="2022-01-13T14:26:34.557" Title="What is the advantage of SSVP docking mechanism?" Tags="|docking-port|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21541" PostTypeId="1" CreationDate="2017-05-16T23:51:28.423" Score="2" ViewCount="492" Body="&lt;p&gt;Would it be possible to design a tiny satellite, such as a femtosatellite or a chipsat, with such a shape and ballistic coefficient, that it would survive reentry without a heat shield?  A bit like &lt;a href=&quot;https://space.stackexchange.com/q/9527/33&quot;&gt;this feathery question&lt;/a&gt; except that our chipsat wouldn't be made of quite the same material.  Somewhat of the opposite of &lt;a href=&quot;https://space.stackexchange.com/q/9514/33&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="33" LastActivityDate="2017-05-19T09:27:54.880" Title="Could a small enough femtosat/chipsat survive reentry without a heat shield?" Tags="|artificial-satellite|reentry|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21542" PostTypeId="2" ParentId="14530" CreationDate="2017-05-17T00:18:54.150" Score="6" Body="&lt;p&gt;Depends on the definition of usage, there are one or two systems in use as of the time of this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Between November 2011 and June 2013, Tiangong-1 Target Vehicle had been visited by three Shenzhou spacecraft. Those spacecraft used an androgynous docking mechanism. However, it is not clear whether it is compatible with APAS-89/95.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is worth to mention that PMA-1 and Zarya are connected by APAS-95.&lt;/p&gt;&#xA;" OwnerUserId="340" LastActivityDate="2017-05-17T00:18:54.150" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21544" PostTypeId="2" ParentId="20151" CreationDate="2017-05-17T02:46:38.043" Score="7" Body="&lt;p&gt;This is a partial answer, hopefully it will lead to more discussion and a resolution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've found this animation of a free-return trajectory in what looks like earth-fixed &quot;inertial&quot; coordinates, at least the coordinates are not rotating with the Earth-Moon system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Found at Robert A. Braeunig's &lt;a href=&quot;http://www.braeunig.us/apollo/apollo11-TLI.htm&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11's Translunar Trajectory &#xA;and how they avoided the heart of the radiation belts&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does your simulation aim for a spot in front of the leading edge of the moon?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This GIF is SLOW! Keep watching until complete!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/C8Pj2.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C8Pj2.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-17T07:32:55.277" LastActivityDate="2017-05-17T07:32:55.277" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21545" PostTypeId="2" ParentId="20584" CreationDate="2017-05-17T03:49:12.913" Score="1" Body="&lt;p&gt;Based on some very helpful replies from Jon Giorgini at JPL and a bit of additional reading, I can summarize as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual calculation of spacecraft orbits, combining both the effects of gravity and other forces in the Solar System plus orbital maneuvers of the spacecraft itself are combined/reconciled with available radar and telemetry. The result is of course the state vector table. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The osculating elements are derived from the state vectors for convenience only. They are not intended as primary sources of orbit data, and each set of points is only intended to be used to define the position of the spacecraft &lt;em&gt;at that point in time&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a short period of time while waiting for either more data, or a decision about an upcoming maneuver, an alternate algorithm for the osculating elements seems to have been applied to a short span of dates, resulting in a smoother appearance of some of the plots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This does not mean they are necessarily less accurate in position, because the elements are only ever intended to be used to define the position near the moment for which they are calculated. The spacectraft's true orbit is not really a conic section at all to begin with, especially considering the complicated gravity field of the Earth-Moon system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Following the updated calculation, the appearance is back to expected, though this still equally valid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;As for the periodic variations themselves:&lt;/strong&gt; The monthly and bi-monthly periodicities are manifestations of the reality of orbiting the lumpy gravitational field of the moon under the gravitational effects of the Earth. Since the Moon's orbit is significantly elliptical rather than circular, that strongly perturbing effect is periodic. These effects are made particularly visible in these plots because they are not simple Keplerian orbits to begin with, and trying to express them as such will highlight the difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is a quick comparison of the derived osculating elements before and after the regularly scheduled update. All is well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wTmfK.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wTmfK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-17T03:59:02.790" LastActivityDate="2017-05-17T03:59:02.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21547" PostTypeId="2" ParentId="21524" CreationDate="2017-05-17T11:23:32.453" Score="2" Body="&lt;p&gt;This is a partial answer. According to the video animation &lt;a href=&quot;https://www.youtube.com/watch?v=cT2bzzp8P1U&quot; rel=&quot;nofollow noreferrer&quot;&gt;Inmarsat-5 F4 Satellite Launch Simulation&lt;/a&gt; they are radiator panels. However the question still needs to answered how they work. Are they radiating electrical power dissipated by resistors, or cooling off circulated coolants? Why are they not black on both sides so that they radiate twice as effectively?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the way, the launch simulation is really interesting and worth watching!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4vx4t.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4vx4t.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/cT2bzzp8P1U?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-05-17T11:23:32.453" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="21548" PostTypeId="2" ParentId="20886" CreationDate="2017-05-17T14:37:31.050" Score="10" Body="&lt;p&gt;Yes, you can hear the noise of anything that hits the outside of the ISS, because from that point there is no vacuum - vibration is passed to the inside of the ISS where it is audible. This is all reasonably self-evident.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This includes micrometeoroid impacts, as Hobbes mentioned, but also noises from crew members moving on EVA, their tools and anything else.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Garett Reisman, a former NASA Astronaut, &lt;a href=&quot;https://www.quora.com/Astronauts/What-sounds-do-astronauts-hear-during-a-spacewalk&quot; rel=&quot;noreferrer&quot;&gt;commented&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As an EVA is in progress, you can hear the spacewalkers bumping and clanging along as your crewmates translate on the exterior of your module.  It sounds kind of like ghosts in the attic dragging chains&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="38" LastActivityDate="2017-05-17T14:37:31.050" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21549" PostTypeId="1" CreationDate="2017-05-17T16:01:07.027" Score="2" ViewCount="3022" Body="&lt;p&gt;I have been working on calculating all the the delta V needed to fly a rocket from one planet to another. I have worked out the delta V needed to raise the orbit to intercept the second planet, but once there, I cannot figure out how much delta V I would need to then circularize the orbit around the new planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aCGPN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aCGPN.png&quot; alt=&quot;crude MS paint of what I mean. X marks the burn spot&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19683" LastActivityDate="2017-05-19T20:14:02.280" Title="Finding Delta V to circularize orbit after a transfer orbit." Tags="|orbital-mechanics|hohmann-transfer|kerbal-space-program|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21550" PostTypeId="1" AcceptedAnswerId="21551" CreationDate="2017-05-17T16:33:44.783" Score="2" ViewCount="675" Body="&lt;p&gt;We did this expariment at home. It was a lot of fun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But it got me thinking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The water in this picture is atracted to the balloon because of static.&#xA;so couldent a simular principle be used to creat artificial gravity with a large static generator on space ships?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4NxZU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4NxZU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. you should try this at home. it also works with a comb that you hav just ran through your hair.&lt;/p&gt;&#xA;" OwnerUserId="19656" LastActivityDate="2017-05-18T07:55:25.807" Title="static energy artificial gravity" Tags="|artificial-gravity|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21551" PostTypeId="2" ParentId="21550" CreationDate="2017-05-17T17:01:23.950" Score="6" Body="&lt;p&gt;That works only as long as the two charge carriers (water and the balloon, for example) don't touch each other. When they do, the charges equalize and the attracting force drops to 0. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, a static charge strong enough to replace gravity is going to have a huge voltage. Probably high enough to discharge as a spark through the air. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-05-17T17:01:23.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21552" PostTypeId="2" ParentId="8060" CreationDate="2017-05-17T17:08:31.257" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;In reading about SLS, I learned that the reason they won't reuse their rocket is that the weight of the parachutes to enable recovery is too much of a sacrifice.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More like, it's impossible to build parachutes big enough to slow it down enough to survive impact.  The SRBs on STS used the biggest parachutes ever made, and they still hit the water hard enough to cause significant damage.  SRBs are pretty robust vs. liquid fuel tanks and engines.  Even if you &lt;em&gt;could&lt;/em&gt; make parachutes big enough to enable a splashdown of the SLS core, the tanks and engines would be smashed by the impact.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not to mention dunking hot rocket engines in cold salt water would probably render them non-reusable, even if they managed not to be destroyed by the impact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When SpaceX was developing its reusability technology, the first few stages went into the drink, where they promptly broke apart and sank due to wave action.  SLS is a lot fatter for its length than F9, so it might be a little more robust in that respect, but you'd still lose the engines which is the one thing you &lt;em&gt;really want&lt;/em&gt; to recover and reuse.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the simple answer is that reuse was not a design consideration for SLS.  At all.  It's designed to loft very heavy payloads while keeping certain contractors employed.  In fact, with its proposed flight rate (1 every other year), spending the extra money to develop a reusable system doesn't make that much sense.  Reusability makes sense for SpaceX because they have &lt;em&gt;dozens&lt;/em&gt; of missions waiting for a ride &lt;em&gt;right now&lt;/em&gt;, and potentially &lt;em&gt;hundreds&lt;/em&gt; with their satellite constellation plans.  Even if it only saves them a million here or there to use a pre-flown booster instead of building a new one, over the number of missions they have planned that adds up to real money.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2017-05-17T17:08:31.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21553" PostTypeId="2" ParentId="21550" CreationDate="2017-05-17T17:18:22.970" Score="0" Body="&lt;p&gt;Sadly no.&#xA;The attraction by the balloon works, because it has a negative charge. It basically soaks negatively charged electrons and because of that it attracts positively charged things like the hair you rubbed it on (and by that lost the electrons to the balloon). The thing with the water is a bit more complex. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In space, you could charge something, but the astronauts would need to be charged positive aswell. This would cause the ground to discharge into the astronaut and get a shock, like an electronic fence. And even if the feet of the astronaut would touch the charged ground, not even the knees of him would feel the attraction. The electromagnetic force has a pretty short range (in fact it has infinite range, but looses its strenght by 1/r) and can be insulated from by basically anything with a charge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:&#xA;Citation: &lt;a href=&quot;https://en.wikipedia.org/wiki/Fundamental_interaction&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Fundamental_interaction&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19685" LastEditorUserId="19685" LastEditDate="2017-05-18T07:55:25.807" LastActivityDate="2017-05-18T07:55:25.807" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21554" PostTypeId="1" CreationDate="2017-05-17T18:15:19.337" Score="1" ViewCount="292" Body="&lt;p&gt;How much methane would have to be expended (think Raptor engine) to propel Titan into Saturn? It's clearly a humongous amount.&lt;/p&gt;&#xA;" OwnerUserId="13136" LastEditorUserId="58" LastEditDate="2017-05-17T18:32:47.113" LastActivityDate="2017-05-18T19:07:36.340" Title="How much methane to push Titan into Saturn?" Tags="|saturn|titan|raptor|methane|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21555" PostTypeId="2" ParentId="21554" CreationDate="2017-05-17T18:33:26.323" Score="7" Body="&lt;p&gt;Let's do a Fermi estimate: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets bring about 2-5% of their start mass to orbital velocity. To cancel out Titan's orbital velocity, you're looking at two orders of magnitude more fuel and oxidizer than Titan's mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's atmosphere weighs $10^{18}$ kg, or 1/200,000 of Earth's total mass. Titan's is 1.5 times as dense, so if Titan's atmosphere were 100% methane it still wouldn't make a significant difference. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-18T06:43:10.313" LastActivityDate="2017-05-18T06:43:10.313" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21556" PostTypeId="2" ParentId="19064" CreationDate="2017-05-18T10:55:53.827" Score="4" Body="&lt;p&gt;Goes-R uses a &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160012696.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Slip Ring Assembly&lt;/a&gt; to transfer power and communications signals from the rotating solar wing to the stationary spacecraft body. The signals going through the slip ring use the &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceWire&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceWire&lt;/a&gt; network protocol to minimize the number of connections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Slip_ring&quot; rel=&quot;nofollow noreferrer&quot;&gt;Slip rings&lt;/a&gt; come in several different shapes, using brushes, or sliding or rolling contacts to transfer electrical signals. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.diamondroll-ring.com/roll-rings_internal_design.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rolling contacts&lt;/a&gt; are used e.g. for the Alpha joints on the ISS (which support the large solar panels).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slip rings can contain a large number of contacts. This example has 6 contacts, but the same type is available with 70 contacts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ebU2J.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ebU2J.jpg&quot; alt=&quot;Diamond-ring rolling contact slipring&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also get slip rings for hydraulics and radar signals (using waveguides), for example. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slip rings suffer from electrical noise when rotating: it's very difficult to make perfect contact at all times. The circuits connected by the slip ring have to take this into account.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slip rings have some advantages over a radio link: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a slip ring assy can transfer large amounts of power without polluting the local EM environment. &lt;/li&gt;&#xA;&lt;li&gt;a slip ring is a passive component, far less complex than a transmitter/receiver combination.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://goes.gsfc.nasa.gov/text/GOES-P_Databook.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;GOES-N data book&lt;/a&gt; is an extensive description of GOES-P's predecessor. It contains some information on the use of slip rings in that program. GOES-P uses a different design. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-05-18T10:55:53.827" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21557" PostTypeId="1" AcceptedAnswerId="21570" CreationDate="2017-05-18T12:14:40.050" Score="33" ViewCount="5503" Body="&lt;p&gt;On a completely unrelated forum, i came across the following graphic:&#xA;&lt;img src=&quot;https://i.imgur.com/F636xiP.gif&quot; alt=&quot;A joke never quite reaching its intended target&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbit seems wrong to me, especially the first curve. From the initial trajectory, I would expect the orbit to have been in a clockwise direction, or it to have been a slingshot instead of a capture.&#xA;What a wouldn't have expected is what appears to be an attraction by L1.&#xA;As I understand it, a Lagrange point is like a 'hill' in spacetime (as opposed to a gravity well), and so the apparent effect &lt;em&gt;should&lt;/em&gt; be an acceleration away from the point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the orbit shown in the graphic wrong, or is my understanding of orbital mechanics lacking, having only been influenced by KSP?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the trajectory is incorrect, what should it look like?&lt;/p&gt;&#xA;" OwnerUserId="6241" LastEditorUserId="6816" LastEditDate="2017-05-18T15:35:33.053" LastActivityDate="2017-11-01T14:53:17.850" Title="This orbit looks wrong near a Lagrange point. Is it?" Tags="|orbit|lagrangian-points|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21558" PostTypeId="1" AcceptedAnswerId="21559" CreationDate="2017-05-18T13:15:29.033" Score="6" ViewCount="1754" Body="&lt;p&gt;Why does the &lt;a href=&quot;https://en.wikipedia.org/wiki/NEXT_(ion_thruster)&quot; rel=&quot;noreferrer&quot;&gt;NASA NEXT&lt;/a&gt; engine use xenon? Why not use argon or krypton? Does it have to do with the thrust to power ratio or the specific impulse or something else?&lt;/p&gt;&#xA;" OwnerUserId="19570" LastActivityDate="2017-05-18T13:35:30.670" Title="Why does the NASA NEXT engine use xenon?" Tags="|propulsion|electric-propulsion|" AnswerCount="1" CommentCount="0" ClosedDate="2017-05-18T19:37:50.967" ContentLicense="CC BY-SA 3.0" />
  <row Id="21559" PostTypeId="2" ParentId="21558" CreationDate="2017-05-18T13:35:30.670" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=Cb_U_CbQ5sc&quot; rel=&quot;noreferrer&quot;&gt;This video&lt;/a&gt; explains it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, it all boils down to golden middle between energy requirements, thrust and specific impulse. Ionizing lighter noble gases requires much higher potential - much more energy input, larger solar batteries, higher cooling requirements, circuitry and structure toughened against higher voltages and energies. While specific impulse would improve, it's already very high, meanwhile thrust is already low, and it would drop even further.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are secondary advantages - containing xenon is easier, the large particles not as prone to filter through micropores of the tank and more mass storable in the same volume at the same pressure (and while propellant volume itself isn't that much of a concern, the tank for same pressure and higher volume is necessarily bigger - and heavier.) Xenon is not radioactive, like Radon, and being a noble gas is very non-reactive, safe to the structure. But its ionization potential is the primary reason - still reasonably low, while providing excellent performance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely if currently achievable specific impulse appears to be insufficient for our goals, we'll move to lighter noble gases, but for now all destinations within the solar system are reachable on xenon engines, and the advantages of argon or krypton just don't justify the trade-offs.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-18T13:35:30.670" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21560" PostTypeId="2" ParentId="20903" CreationDate="2017-05-18T13:45:29.347" Score="3" Body="&lt;p&gt;To answer the edited addition:&#xA;The mechanics to do something like this would:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Add considerable mass to the stage&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;This would prevent the use of the stage for the non-recoverable (all fuel used) launches that I believe are required for geosyncronous orbit insertion&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Add complexity to the design&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;More points of failure&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;Fail to land safely&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;having read a little about autorotation over on aviation.se, a helicopter performing an autorotation landing needs to pitch up to reduce the vertical speed to within a safe threshold for a landing.  Unpowered rotors aparrently cannot generate enough lift to perform a vertical landing.  This also requires the helicopter to maintain a significant forward speed.  This would make accurate landings more difficult, but more importantly would require the stage to maintain horisontal velocity whilst travelling sideways through the atmosphere, which is not possible due to the large crossection.&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6241" LastActivityDate="2017-05-18T13:45:29.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21562" PostTypeId="2" ParentId="21557" CreationDate="2017-05-18T14:24:50.860" Score="26" Body="&lt;p&gt;As noted by Mys_721tx in comments, this is the trajectory of the object &lt;a href=&quot;https://en.wikipedia.org/wiki/J002E3&quot; rel=&quot;nofollow noreferrer&quot;&gt;J002E3&lt;/a&gt; -- believed to be the S-IVB upper stage launched with the Apollo 12 mission -- and the animation is accurate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As David Hammen explains in his (better) answer, the animation is showing a rotating frame of reference centered on the Earth. Because of the rotation, and the fact that J002E3 is initially in a solar orbit, the initial part of the trajectory is misleading. In the solar frame of reference, Earth is moving at high speed toward the top of the image, and at the start, J002E3 is moving faster than it and so is going to pass above/in front of Earth. As it passes L1, though, Earth's gravity begins to work more strongly on it, pulling it backwards/sideways to its trajectory, which slows it down, so it falls behind Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;KSP experience doesn't help you with this because it uses patched-conic approximation, where only one body at a time influences the trajectory of a spacecraft. You will notice in KSP that your trajectory changes direction abruptly when a new sphere of influence is entered; this is because of the change in reference frame, not the gravity of the body you're approaching. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-19T13:01:18.600" LastActivityDate="2017-05-19T13:01:18.600" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21563" PostTypeId="2" ParentId="14107" CreationDate="2017-05-18T14:37:30.647" Score="4" Body="&lt;p&gt;You might find some answers here: &lt;a href=&quot;http://www.thrustcurve.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.thrustcurve.org/&lt;/a&gt;. It's a reference for hobby rocket motors, but they can get pretty large and the larger ones have a lot of variation in grain geometry (star, moonburners, c-slot, etc).&lt;/p&gt;&#xA;" OwnerUserId="17089" LastActivityDate="2017-05-18T14:37:30.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21564" PostTypeId="2" ParentId="21554" CreationDate="2017-05-18T19:07:36.340" Score="8" Body="&lt;p&gt;Let's turn the question on its head and see what exhaust velocity we need to if Titan's entire (mostly nitrogen) atmosphere were used as a propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v = v_e log(m_i / m_f)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia tells us that the atmosphere of Titan is about &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Titan&quot; rel=&quot;noreferrer&quot;&gt;1.19 times as massive&lt;/a&gt; as &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Earth&quot; rel=&quot;noreferrer&quot;&gt;that of Earth&lt;/a&gt; so we get about 6.13e18 kg of atmosphere (propellant) in a total mass of about 1.35e23 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume that we apply all of our delta-v at apoapsis (1 257 060 km)&amp;mdash;either instantly or over multiple orbits&amp;mdash;to lower periapsis from 1 186 680 km to the equatorial radius of Saturn (60 268 km). The semi-major axis changes from 1 221 870 km to 658 664 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specific energy $\epsilon = v^2/2 - \mu/r = - {\mu \over 2a} = -{3.793e16 m^3/s^2 \over 2a}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus $ \epsilon_{initial} = -1.55e7 J/kg $ and $ \epsilon_{final} = -2.88e7 J/kg $ which we plug back into the specific energy equations to get an initial speed of 5414 m/s and a final speed of 1662 m/s, $ \Delta v = 3752 m/s$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ v_e = {\Delta v \over log(m_i/m_f) } = {3752 m/s \over 4.56e-5 } = 8.24e7 m/s = 0.27c$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(This is a small enough fraction of light speed that we aren't too concerned about relativity.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Raptor or indeed any chemical rocket can not achieve this task by any stretch, as Hobbes has already pointed out. Ion engines don't cut it either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, methane makes up about 1.4% of Titan's atmosphere. If all the methane were antimatter and annihilated with some of the nitrogen in an antimatter rocket, we can calculate our exhaust velocity with (p. 54 of The Starflight Handbook: A Pioneer's Guide to Interstellar Travel by Mallove &amp;amp; Matloff, 1989)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ v_e = \sqrt{\epsilon (2-\epsilon )} c = \sqrt{0.028 \times 1.972} c = 0.23c$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Even if it were antimatter, the methane is not enough.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Numbers are from the Wikipedia pages on the respective bodies and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;noreferrer&quot;&gt;gravitational parameter&lt;/a&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="16733" LastActivityDate="2017-05-18T19:07:36.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21566" PostTypeId="2" ParentId="1380" CreationDate="2017-05-18T20:50:03.730" Score="2" Body="&lt;p&gt;&lt;a href=&quot;http://www.projectrho.com/public_html/rocket/mission.php#id--Hohmann_Transfer_Orbits&quot; rel=&quot;nofollow noreferrer&quot;&gt;Atomic Rockets&lt;/a&gt; is another great source. Plain-language explanations with good graphics and enough math to be useful.&#xA;&lt;a href=&quot;http://www.braeunig.us/space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket and Space Technology&lt;/a&gt; is even better for the technically inclined - basically an online textbook with practice problems.&lt;/p&gt;&#xA;" OwnerUserId="17089" LastActivityDate="2017-05-18T20:50:03.730" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21569" PostTypeId="2" ParentId="21541" CreationDate="2017-05-19T01:51:05.893" Score="-3" Body="&lt;p&gt;I think it's an interesting question,&#xA;The way I see it is your satellite stays in orbit because of it's high speed but it also burns up because of it's high speed. If you can get it traveling slow enough by the time it reaches the atmosphere you should be able to use atmospheric drag without burning up. So you would need a way to slow it between the orbit height and speed and the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think it could be interesting do the math for trying to slow a chipsat by using a collection of tethers using drag generated by geomagnetically induced current, so using the magnetic field of the earth to slow down before hitting the atmosphere. But I wouldn't know where to begin with running those sorts of calculations.&lt;/p&gt;&#xA;" OwnerUserId="10094" LastActivityDate="2017-05-19T01:51:05.893" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21570" PostTypeId="2" ParentId="21557" CreationDate="2017-05-19T03:50:36.920" Score="30" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is the orbit shown in the graphic wrong, or is my understanding of orbital mechanics lacking, having only been influenced by KSP?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's not an either-or question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;The graphic is &quot;wrong&quot; from the perspective of an Earth-centered inertial frame. That graphic instead uses a synodic frame, a frame that rotates with the Earth's orbit about the Sun. You can tell that this is the case by the fact that the L1 point along with the Earth are fixed. In the roughly one year interval shown in that animated graphic, the L1 point would have made a full revolution about the Earth from the perspective of an ECI frame. The rotation of the synodic frame is largely what's responsible for the apparent spirograph behavior of &lt;a href=&quot;https://web.archive.org/web/20170516200620/https://science.nasa.gov/science-news/science-at-nasa/2002/20sep_mysteryobject&quot; rel=&quot;noreferrer&quot;&gt;J002E3&lt;/a&gt; in the graphic. (Other than label changes, the graphic is correct, by the way.) The orbit would look considerably more mundane from the perspective of an ECI frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is where your KSP-understanding of orbital mechanics steers you wrong. While the orbit wouldn't have those neat petals from the perspective of an ECI frame, it would still look markedly non-Keplerian. The object's ECI velocity would make the object appear to be on a hyperbolic trajectory for much of each orbit. Yet it mysteriously turns around and orbits, six times, before escaping. KSP cannot display this behavior because it uses the patched conic approximation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;So why use this weird rotating frame?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the circular restricted three body problem, with the added twist of a fourth body, the Moon. I'll ignore the Moon. The circular restricted three body problem asks about the behavior of a body of negligible mass (e.g., J002E3) under the gravitational influence of two larger bodies orbiting circularly about one another. The term &quot;restricted&quot; means the third body's mass is so small that it essentially doesn't perturb the orbits of the two larger bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The synodic frame turns out to be be very useful in analyzing the three body problem. While the third body's energy and angular momentum are not conserved quantities in this frame, a new quantity, the Jacobi integral, is conserved, in this frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dpVMN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dpVMN.png&quot; alt=&quot;Image showing Lagrange points and forbidden regions for six different values of the Jacobi integral.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Objects with little energy ($C_j&amp;gt;4$), depicted in the upper left sub-image, are restricted to orbiting the smaller mass (e.g., the Earth) or the larger mass (e.g., the Sun). There's a forbidden region surrounding the smaller that objects with a large Jacobi integral cannot enter. Objects in low Earth orbit are stuck there, and objects will sufficiently low energy outside the forbidden zone can't hit us unless perturbed by something else.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something interesting happens with a Jacobi integral of about 3.9: A keyhole opens up around the L1 point. Objects with sufficient energy can enter the vicinity of the smaller mass (e.g., the Earth). This is exactly what happened to J002E3. It changed from orbiting the Sun to temporarily orbiting the Earth by passing through that keyhole.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another keyhole, this one about the L2 point, opens up with an even lower Jacobi integral. You can see this in the graphic that shows J002E3's orbit. In late October 2002, J002E3 came close to the L2 point. It didn't have quite enough energy and it didn't come quite close enough, but if it had, it could have escaped six months prior to when it did. Instead, J002E3 had to wait another six months when it's strange orbit brought it close enough to the L1 point that it could finally escape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's very difficult to see these behaviors from the perspective of an ECI frame. It's quite easy to see them from the perspective of a synodic frame, once you know what to look for.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-05-19T14:21:47.960" LastActivityDate="2017-05-19T14:21:47.960" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21571" PostTypeId="2" ParentId="21541" CreationDate="2017-05-19T09:27:54.880" Score="5" Body="&lt;p&gt;A correctly prepared (shielded) nanosat could survive the reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A satellite the scale of cubesat around Kármán Line would experience about 1 kilowatt of heating. This is not what a cubesat electronics can survive - but this is something a layer of aerogel can stop from conducting in, a mirror-coated foil from radiating in, and a few centimeters of light ablator to dissipate for a couple minutes. So if you package your femtosat of, say, 10cm^3 in a cubesat form factor (1000cm^3) of lightweight thermal shielding, it should survive just fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Locating it on the ground after reentry is an entirely different matter...&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-19T09:27:54.880" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21572" PostTypeId="1" AcceptedAnswerId="21573" CreationDate="2017-05-19T10:53:43.973" Score="5" ViewCount="2309" Body="&lt;p&gt;I just learned about &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceWire&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceWire&lt;/a&gt; in &lt;a href=&quot;https://space.stackexchange.com/a/21556/12102&quot;&gt;this great answer to &lt;em&gt;How will GOES-R simultaneously point some instruments down at Earth and others sunward?&lt;/em&gt;&lt;/a&gt; discussing the slip-ring rotating electrical connection between its solar panel and attached sun-pointing instrumentation, and the body of the craft with its Earth-pointing instrumentation.&lt;/p&gt;&#xA;&lt;p&gt;The Wikipedia article suggests that SpaceWire should have nine conductors; differential pairs each for data in, data out, strobe in, strobe out, and a single ground.&lt;/p&gt;&#xA;&lt;p&gt;It looks like a lot of thought has gone into SpaceWire, and it seems to have been developed specifically for space applications.&lt;/p&gt;&#xA;&lt;p&gt;While Ethernet gets by on a single coaxial cable or (usually shielded) twisted pair, SpaceWire specifies nine conductors. Sometimes the weight of wires is considered a small but significant factor in spacecraft weight minimization. Why were nine wires considered optimal?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2023-10-29T00:14:58.203" LastActivityDate="2023-10-29T00:14:58.203" Title="Why was SpaceWire designed with nine wires?" Tags="|communication|design|spacecraft-development|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="21573" PostTypeId="2" ParentId="21572" CreationDate="2017-05-19T12:44:42.387" Score="15" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ethernet&lt;/a&gt; uses 2 or 4 pairs of wires (2 pairs for lower speeds: 1 pair in each direction; 4 pairs for gigabit Ethernet). &lt;a href=&quot;http://spacewire.esa.int/content/TechPapers/documents/SpaceWire%20CODEC%20ISWS%202003.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceWire&lt;/a&gt; uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Data_strobe_encoding&quot; rel=&quot;nofollow noreferrer&quot;&gt;Data Strobe encoding&lt;/a&gt; for higher reliability: instead of sending a single signal per bit, DSE sends 2 signals (Data and Strobe) over 2 wire pairs. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;These have the property that either Data or Strobe changes its logical value in one clock cycle, but never both. This allows for easy clock recovery with a good jitter tolerance by XORing the two signal line values.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This makes the link &lt;a href=&quot;https://www.star-dundee.com/sites/default/files/SpaceWire%20User%27s%20Guide.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;more reliable&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The reason for using DS encoding is to improve the skew tolerance to&#xA;  almost 1-bit time, compared to 0.5 bit time for simple data and clock&#xA;  encoding.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The tradeoff of twisted pair vs. coax is complex. Twisted pair is simple and cheap, coax can transmit at higher frequencies but is more difficult to use (you have to terminate it correctly to get usable cable characteristics, it's less tolerant of bending than twisted-pair).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Coax links are more difficult to connect through a slip ring, because coax depends on the exact properties of the insulation between the core conductor and the shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacewire uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Low-voltage_differential_signaling&quot; rel=&quot;nofollow noreferrer&quot;&gt;differential signalling&lt;/a&gt;: this means &lt;a href=&quot;https://en.wikipedia.org/wiki/Differential_signaling&quot; rel=&quot;nofollow noreferrer&quot;&gt;both wires&lt;/a&gt; of a pair carry a variable voltage. If you want to add shielding (and you do), you need another pin on the connectors, this is the 9th pin. &#xA;Differential signalling is another technique used to get high reliability and high transfer speeds over twisted-pair links. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-23T16:37:56.480" LastActivityDate="2017-05-23T16:37:56.480" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21574" PostTypeId="2" ParentId="21572" CreationDate="2017-05-19T12:46:21.737" Score="6" Body="&lt;p&gt;9 is a quite straight forward choice considering the features you need:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;all signals should be differential&lt;/li&gt;&#xA;&lt;li&gt;you need one data line for each direction&lt;/li&gt;&#xA;&lt;li&gt;you need to transport a clock along with the data&lt;/li&gt;&#xA;&lt;li&gt;a shield (GND) for the whole assembly&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;about 1) Differential data has advantages over single ended signals - they reject influences from external noise that couples into the wire. To some extent this can be achieved with coaxial cable as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;about 3) There are ways to recover the clock from the data line, but this makes receiving (and sending) data more challenging with respect to the electronics needed on both sides (data encoder, clock recovery...).&lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2017-05-19T12:46:21.737" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21577" PostTypeId="1" CreationDate="2017-05-19T15:40:36.560" Score="5" ViewCount="2560" Body="&lt;p&gt;I'm trying to get some theoretical performance numbers for a cold gas thruster using quasi-one-dimensional flow, and I don't know where to start. For combustion engines you would just take a given chamber pressure and the temperature from the combustion, but with a cold gas thruster you have neither of those. Moreover, due to the quick expansion of the gas from the tank, the temperature will be dropping, so my questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;What would be the pressure for the thrust chamber in a cold gas thruster? I reckon it's not the actual tank pressure, cause then you wouldn't bother having a converging nozzle at all, just diverging. So, given a tank pressure, how would you calculate the pressure inside the engine?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I suppose you have to take into account the cooling of the gas when expanding into the thrust chamber, but for that you'd need the chamber pressure to calculate the temperature drop. Am I missing anything?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;EDIT1: My wording might have been a bit unclear, so as a clarification, the main question is why would you create a thrust chamber and a converging section for a cold gas thruster when you could easily achieve choked flow from the tank outlet itself. Adding to that, in the probable case that I'm missing some important functionality, how would one calculate the chamber pressure if you were to use one?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT2: To anyone that might find this question in the future, here I was assuming the flow from the tank was coming already at mach 1 due to choked flow from the outlet. However, the usage of a regulator removes most, if not all, of that velocity, so you need a nozzle to accelerate the static gas at the outlet pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT3: Looking at it again, why would the inlet pressure be the outlet pressure of the regulator? If you look at that paper, there's an expansion chamber before the actual nozzle where the gas would expand. Then, the pressure would reach an equilibrium dependent on how much gas is leaving the nozzle and how much is being fed, at a pressure lower than the regulator outlet. Am I wrong?&lt;/p&gt;&#xA;" OwnerUserId="19712" LastEditorUserId="19712" LastEditDate="2017-05-21T11:36:07.710" LastActivityDate="2017-05-21T11:36:07.710" Title="How is chamber pressure and temperature determined for cold gas thrusters?" Tags="|engine-design|engines|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21578" PostTypeId="2" ParentId="21549" CreationDate="2017-05-19T20:08:58.647" Score="2" Body="&lt;p&gt;I'll use Earth to Mars as an example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a quick and dirty pic of an Earth to mars Hohmann:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Veyno.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Veyno.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For sake of simplicity I'm rounding quite a bit here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At Mars rendezvous you can see the speed difference is about 2.5 km/s. We'll call this 2.5 km/s &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2013/03/what-heck-is-vinf.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vinfinity&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the ship enters Mars sphere of influence, the path is better modeled by a hyperbola about Mars rather than an ellipse about the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A hyperbola's velocity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V_{hyperbola}=\sqrt{V_{infinity}^2 + V_{escape}^2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good memory device for remembering hyperbola's speed is thinking of Vinf and Vesc as the legs of a right triangle and the hyperbola's speed as the hypotenuse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dEkNC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dEkNC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vescape, a.k.a. escape velocity varies depending on distance from planet. The 11 km/s earth escape velocity near earth perigee. The 5 km/s Mars escape velocity also assumes a periapsis near Mars' surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Low Mars orbit is about 3.5 km/s. So to enter low Mars orbit, you subtract 3.5 from 5.6 km/s. You'd need about a 2 km/s burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the periapsis speed of a 300 x 574,000 km Mars capture orbit is about 4.8 km/s. So if a capture orbit is all you need, .8 km/s would suffice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To calculate speeds at different points of various orbits, use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis viva equation&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$V=\sqrt{\mu (2/r - 1/a)}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where a is semi major axis of an orbit, mu is GM and r is distance from center of gravitating body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you don't want to look up different quantities and work out equations, you can use my &lt;a href=&quot;http://clowder.net/hop/railroad/Hohmann.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hohmann spreadsheet&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-05-19T20:14:02.280" LastActivityDate="2017-05-19T20:14:02.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21579" PostTypeId="1" AcceptedAnswerId="21582" CreationDate="2017-05-19T23:11:22.437" Score="10" ViewCount="1157" Body="&lt;p&gt;SpaceX has unveiled its plans to launch a bunch of satellites into the Low Earth Orbit, and &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_satellite_constellation&quot; rel=&quot;noreferrer&quot;&gt;according to Wikipedia&lt;/a&gt;, the number is either 4500 or 7500 (not sure, could be the sum of both). The &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;noreferrer&quot;&gt;satellite flare page says&lt;/a&gt; that he Iridium constellation (which is in LEO already) causes some of the brightest flares, and it has &lt;a href=&quot;https://en.wikipedia.org/wiki/Iridium_satellite_constellation&quot; rel=&quot;noreferrer&quot;&gt;72 in the sky these days&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: what would the sky look like at night if there were 4 or 7 thousand satellites in LEO reflecting light back to us? And what about obstructing the view of telescopes for scientific researchers? I haven't found any info about whether the light reflection will be reduced, so it could be pretty spectacular.&lt;/p&gt;&#xA;" OwnerUserId="990" LastActivityDate="2017-05-20T12:55:26.240" Title="How will several thousand Low Earth Orbit satellites affect the night sky?" Tags="|low-earth-orbit|satellite-flare|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21580" PostTypeId="2" ParentId="21579" CreationDate="2017-05-19T23:33:24.490" Score="3" Body="&lt;p&gt;Space is big.  Even the little bit that is around the Earth. There would not be enough coverage from the satellites to make any kind of noticeable difference.  Unless they are huge, you probably won't even be able to see them occult stars as they go by.&lt;/p&gt;&#xA;&lt;p&gt;You will see more satellites in the evenings and mornings.  They look very distinctive when you see them move.  You can use the site &lt;a href=&quot;http://www.heavens-above.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heavens Above&lt;/a&gt; to get a list of when and where satellites (and large space debris) will be for your location.  The more satellites and spent boosters we have up there, the more you will be able to see.&lt;/p&gt;&#xA;&lt;p&gt;However, if you look at the list knowing how much is currently up there, doubling or tripling the amount of it won't really be noticeable except to those who like to satellite spot.  The reason is that the object has to pass through the region that is not in the Earth's shadow but is over an area where the Sun has set (or not yet risen).  The brightness of the object depends on a lot of factors including its size and its angle with respect to you and the Sun.&lt;/p&gt;&#xA;&lt;p&gt;When you are looking, if it is flashing, that's an aircraft.  Satellites don't generally come with running lights.&lt;/p&gt;&#xA;" OwnerUserId="18842" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-19T23:33:24.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21582" PostTypeId="2" ParentId="21579" CreationDate="2017-05-20T05:52:22.480" Score="10" Body="&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt; This is a really good question, and the implications for ground-based astronomers of various wavelengths are sobering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an example. The constellation described in &lt;a href=&quot;https://space.stackexchange.com/q/19055/12102&quot;&gt;SpaceX's 4,425 satellite constellation - what's the method to the madness?&lt;/a&gt; might even end up with more satellites now. From Phys.org's discussion of SpaceX's FCC applications in &lt;a href=&quot;https://phys.org/news/2017-05-spacex-thousands-internet-satellites.html&quot; rel=&quot;noreferrer&quot;&gt;SpaceX details plans to launch thousands of internet satellites&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX has also indicated that it plans to deploy an additional 7.500 satellites that will operate at lower altitudes in order to boost broadband capacity in large population centers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The simplest estimate of the number visible would be statistical. Let's say there are 5000 satellites that are uniformly distributed at an altitude of 1200 kilometers. This is not a correct model but it is a starting point for an order-of-magnitude estimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seen from the center of the earth, each would occupy $4\pi/5000$ or about 0.0025 steradians, which rhymes with &quot;square radians&quot;. So that's about 8.25 square degrees (as seen from the Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Standing on the Earth and looking up at a shell of orbits at an altitude of 1200 km, and then only looking at the section within 45 degrees of your zenith (at half-way up or more from the horizon), you are looking at the intersection of a cone drawn from the center of the Earth with a half-angle of about 8.6 degrees, or about 232 square degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means that in that 45 degree half-angle cone you are looking in, there would be, on average, &lt;strong&gt;twenty seven (27) SpaceX satellites looking back at you!&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it's not close to midnight, a good share of them will be sunlit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, they are much farther than most satellites that we actually notice, and that means they may be dimmer and slower than the ones we most often notice and remember. Depending on how they are designed, they might not reflect all that much sunlight most of the time. Those details are very design dependent, and SpaceX is likely to be thinking about this already.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Telescope users:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every one-degree FOV view would have a 10% chance of having one of these in it, on average at any moment, moving at about 0.35 degrees per second at the zenith and about 0.25 degrees per second at 45 degrees from the zenith. slowing down even more closer to the horizon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So unless I'm doing something wrong here, a thirty second exposure through a telescope using low magnification with a 1 degree FOV would probably have a SpaceX satellite in it if you're within +/- 50 degrees latitude!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an average, your milage will vary depending on latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Observatory users:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would be a good question for Astronomy Stackexchange. I can say that for some exposures meant to record both dim and bright objects in the same field or on the same CCD array, CCD frames may be read out many times during the exposure. This might allow for subtraction of satellite streaks. Of course, there are plenty of satellites now, so this must already be done regularly. I think that's quite an interesting question, and it should be asked there if it hasn't been already.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Now, instead of random, let's look at the orbits for this example. Study the drawings and data below, then please &lt;a href=&quot;https://space.stackexchange.com/a/19070/12102&quot;&gt;go read this &lt;strong&gt;excellent answer&lt;/strong&gt;&lt;/a&gt; explaining how this proposed configuration works conceptually!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;concentration&quot; in our 45 degree cones will be lower when we are near the equator, perhaps 15 or 20, but really spike at higher latitudes, with perhaps fifty or even more when we are between 45 degrees and 60 degrees latitude!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eKuGt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eKuGt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pAQYG.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pAQYG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3Db8V.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3Db8V.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wo302.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wo302.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; screen captures from &lt;a href=&quot;http://cdn.geekwire.com/wp-content/uploads/2016/11/Technical-Attachment.pdf&quot; rel=&quot;noreferrer&quot;&gt;SpaceX non-Geostationary Satellite System Attachment A - Technical Information to Supplement Schedule S&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-20T12:55:26.240" LastActivityDate="2017-05-20T12:55:26.240" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21583" PostTypeId="1" CreationDate="2017-05-20T06:48:45.393" Score="2" ViewCount="1640" Body="&lt;p&gt;A description of a &quot;synodic frame&quot; was one of the main ingredients in &lt;a href=&quot;https://space.stackexchange.com/a/21570/12102&quot;&gt;this excellent answer&lt;/a&gt; to a question about an animation of a funky-looking orbit that several different and distinctly interesting aspects to it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As part of the background for &lt;a href=&quot;https://space.stackexchange.com/q/16644/12102&quot;&gt;this question&lt;/a&gt; I plotted historical orbit data for SOHO in a rotating frame centered on a point in space &lt;em&gt;that is associated with&lt;/em&gt; the Sun-Earth L1 libration point. I say associated with because in real-world orbits, and not just circular-restricted ones, L1 is more of a &quot;&lt;a href=&quot;https://space.stackexchange.com/a/21291/12102&quot;&gt;Gestalt&lt;/a&gt;&quot; (or &quot;area&quot;) than a specific, mathematically well-defined point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look at the lengths to which I had to go to explain the frame of this plot, you can see that I could have used a better term, if one exists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So here I'd like to ask both &lt;em&gt;What is a &quot;synodic frame&quot;?&lt;/em&gt; so we could have a great answer and useful definition to link back to in the future. But I'd like to also ask &lt;em&gt;Can one be defined for an elliptical orbit?&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here's the plot from the &lt;a href=&quot;https://space.stackexchange.com/q/16644/12102&quot;&gt;question where the math is described&lt;/a&gt;. It is the orbit of SOHO shown in a home-grown coordinate system. Note that the &quot;Gestalt&quot;, faux-L1, or &quot;instantaneous-L1&quot; red point on the left is fixed in this frame, and the position of the Earth as a function of time, the &lt;em&gt;blue elongated sausage&lt;/em&gt; on the right, &lt;em&gt;defines only the axis&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OL8gz.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OL8gz.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-20T07:03:10.687" LastActivityDate="2017-05-20T07:03:10.687" Title="What is a &quot;synodic frame&quot;? Can one be defined for an elliptical orbit?" Tags="|orbital-mechanics|lagrangian-points|mathematics|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21584" PostTypeId="1" AcceptedAnswerId="25935" CreationDate="2017-05-20T11:46:37.367" Score="3" ViewCount="421" Body="&lt;p&gt;There are several popular news articles that refer to &lt;a href=&quot;https://www.youtube.com/watch?v=cFYoYUBGw4s&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt;, who's notes point to the NASA news item &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2017/nasas-van-allen-probes-spot-man-made-barrier-shrouding-earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Van Allen Probes Spot Man-Made Barrier Shrouding Earth&lt;/a&gt; which points to the interestingly named paper &lt;a href=&quot;https://link.springer.com/article/10.1007/s11214-017-0357-5&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anthropogenic Space Weather&lt;/a&gt; which reviews a wide range of phenomena over the last half-century but does mention VLF.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Abstract&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Anthropogenic effects on the space environment started in the late 19th century and reached their peak in the 1960s when high-altitude nuclear explosions were carried out by the USA and the Soviet Union. These explosions created artificial radiation belts near Earth that resulted in major damages to several satellites. Another, unexpected impact of the high-altitude nuclear tests was the electromagnetic pulse (EMP) that can have devastating effects over a large geographic area (as large as the continental United States). Other anthropogenic impacts on the space environment include chemical release experiments, high-frequency wave heating of the ionosphere and the interaction of VLF waves with the radiation belts. This paper reviews the fundamental physical process behind these phenomena and discusses the observations of their impacts.&lt;/p&gt;&#xA;&lt;p&gt;Gombosi, T.I., Baker, D.N., Balogh, A. et al. Space Sci Rev (2017). doi:10.1007/s11214-017-0357-5&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I don't understand how &lt;a href=&quot;https://en.wikipedia.org/wiki/Very_low_frequency&quot; rel=&quot;nofollow noreferrer&quot;&gt;VLF transmissions&lt;/a&gt; from Earth have formed a &amp;quot;barrier&amp;quot; at all. Is this barrier &lt;em&gt;protecting us from radiation in space&lt;/em&gt;, or does this only mean that certain particles are found to be at lower levels in certain areas of Earth's magnetosphere than otherwise expected? Is this something of significance to people or satellites, or just a deviation or fluctuation that is only being hyped as a barrier against radiation?&lt;/p&gt;&#xA;&lt;p&gt;Are satellites in LEO receiving less radiation damage from the higher energy particles for example because of Human's constant high power VLF transmissions?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;VLF transmissions from Earth are widespread, powerful and steady. According to &lt;a href=&quot;https://www.youtube.com/watch?v=cFYoYUBGw4s&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; most of the transmitted power is between 10 kHz and 100 kHz with wavelengths of kilometers to tens of kilometers. Usually frequencies above 30 kHz are considered &lt;a href=&quot;https://en.wikipedia.org/wiki/Low_frequency&quot; rel=&quot;nofollow noreferrer&quot;&gt;LF&lt;/a&gt; rather than VLF. but there's nothing special about the 30 kHz dividing line. Because of the low frequencies and long wavelengths involved, some signal strength can penetrate the top layer of the otherwise electrically conductive Ocean and can be received by submarines. Other applications include time references.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3urLF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3urLF.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Long-wave transmitter of the Navy of the Bundeswehr, near Saterland-Ramsloh in the district of Cloppenburg (Lower Saxony); Located in the nature reserve&amp;quot; Roughly translated from German. (fyi it's the towers in the background, not the cows!) From &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Marinesender_DHO38_L%C3%A4ngstwellensender_der_Marine.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QXQrX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QXQrX.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Closeup of a few of the antenna towers of the U.S. Navy &lt;a href=&quot;https://en.wikipedia.org/wiki/VLF_Transmitter_Cutler&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cutler VLF transmitter&lt;/a&gt; facility at Cutler, Maine. It transmits operational orders one-way to submerged U.S. submarines worldwide at a frequency of 24 kHz with a power of 1.8 megawatts.&amp;quot; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Cutlervlf2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/N7lQM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/N7lQM.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Radio towers of Varberg radio station at Grimeton (Sweden), which is today listed as a UNESCO World Heritage.&amp;quot; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Grimeton,_Halland,_Sweden_(12498094753).jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/cFYoYUBGw4s?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-04-01T09:09:53.967" LastActivityDate="2021-04-01T09:09:53.967" Title="Is the recently discovered VLF-induced radiation barrier in any way important or useful?" Tags="|low-earth-orbit|radiation|radio-communication|magnetosphere|radio-astronomy|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21585" PostTypeId="2" ParentId="21577" CreationDate="2017-05-20T17:12:01.217" Score="3" Body="&lt;p&gt;&quot;Am I missing anything?&quot; Yes. You have a fundamentally wrong idea of how cold gas thruster systems are designed.  They don't have thrust chambers and they use de Laval nozzles to achieve the maximum practical exhaust velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a conceptual design for a typical cold gas thruster system&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rRLOY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rRLOY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice that the gas is supplied to the thrusters through a pressure regulator.  The outlet pressure of this regulator would be the inlet pressure to the nozzles (assuming that the flow does not exceed the capability of the regulator, and neglecting frictional losses in the plumbing).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given this, I am not sure what you actually want/need to calculate for your theoretical system. The &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=2&amp;amp;ved=0ahUKEwiWi4K5yP3TAhVo9YMKHQSyChsQFgg-MAE&amp;amp;url=http%3A%2F%2Fcdn.intechopen.com%2Fpdfs%2F37528%2Fintech-cold_gas_propulsion_system_an_ideal_choice_for_remote_sensing_small_satellites.pdf&amp;amp;usg=AFQjCNGzBRZYUG7iNUUcrRcnKluMhTJrUQ&amp;amp;sig2=idIu5nsioVoDyUEUgqQwwA&amp;amp;cad=rja&quot; rel=&quot;nofollow noreferrer&quot;&gt;paper&lt;/a&gt; that I extracted this image from gives some calculations that may possibly be of use. I have not included them here since I do not know what your goal is.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-05-20T17:12:01.217" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21586" PostTypeId="1" AcceptedAnswerId="21591" CreationDate="2017-05-21T03:23:07.217" Score="0" ViewCount="191" Body="&lt;p&gt;Images are from the 2012 New Atlas article &lt;a href=&quot;http://newatlas.com/orbitec-vortex-liquid-fuel-rocket-engine/24807/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ORBITEC flight tests new vortex liquid fuel rocket engine&lt;/a&gt;. One looks a bit like a US military or DoD logo, one perhaps is ATK, but I'm not sure. Is this effort a joint research project between all of these organizations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related question: &lt;a href=&quot;https://space.stackexchange.com/q/18413/12102&quot;&gt;Are there any complications of the position of the oxidizer injector on Orbitec's vortex rocket engine?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2tOm5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2tOm5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;The Garvey Prospector P-15 on the launch rail prior to launch&quot; from &lt;a href=&quot;http://newatlas.com/orbitec-vortex-liquid-fuel-rocket-engine/24807/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QojaY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QojaY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Liftoff of the ORBITEC/Garvey Prospector P-15&quot; from &lt;a href=&quot;http://newatlas.com/orbitec-vortex-liquid-fuel-rocket-engine/24807/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="26446" LastEditDate="2020-09-23T04:09:04.877" LastActivityDate="2020-09-23T04:09:04.877" Title="What organizations correspond to the logos on this ORBITEC Vortex engine rocket?" Tags="|rockets|engines|engine-design|insignia|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21587" PostTypeId="1" AcceptedAnswerId="21992" CreationDate="2017-05-21T08:09:14.480" Score="1" ViewCount="281" Body="&lt;p&gt;The discussion section of the very recent, on-line-available &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/2017GL073248/epdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Terrestrial glint seen from deep space: oriented ice crystals detected from the Lagrangian point&lt;/a&gt; Alexander Marshak, Tamás Várnai and Alexander Kostinski, doi: 10.1002/2017GL073248 contains the following text:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Based on in-situ measurements of cirrus clouds, [Korolev et al., 2000; McFarquhar et al., 2002], tiny hexagonal platelets of ice, floating in air in nearly perfect horizontal alignment, are likely responsible for the glints observed by EPIC over land.  Because the EPIC instrument has a field of view of 0.62 degrees (see, &lt;a href=&quot;https://epic.gsfc.nasa.gov/epic&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://epic.gsfc.nasa.gov/epic&lt;/a&gt;) and a 2048x2048 pixels CCD, the specular signal within an angle of only $~3x10^{-4}$ degree (Fig. 2) must either contain smooth large oriented ice plates or smaller oriented platelets sending back diffracted light. Size distribution of such crystals depends greatly on cloud temperature and humidity but the range is from tens of microns to mm. Taking the wavelength of 0.5 micron and ice platelet size of 50 microns, yields the ratio or angular half-width of the diffraction lobe around the specular direction $10^{-2}$ or on the order of a degree [Crawford, 1968, p.486].  This is broader than the angular width of a pixel but narrower than change in the zenith direction over the area covered by a pixel (0.1°).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In other words, if the ice plates are parallel to the local surface of the Earth, the curvature of the Earth itself results in the observed specular reflection being localized to an area tens of kilometers wide at a distance of 1.5 billion kilometers, &lt;em&gt;despite the assumed size of 50 microns and it's large diffraction half-angle of order tenths of a degree&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To roughly summarize the explanation, there are flat plates of ice crystals above 5 km in thin atmospheric clouds that tend to orient themselves &lt;em&gt;very horizontally&lt;/em&gt; - parallel to the local surface of the Earth - &lt;em&gt;most of the individual plates would each need to be horizontal within a small fraction of one degree, not just on average&lt;/em&gt;. Every azimuthal degree along the Earth's surface is about 111 kilometers, so if the glints are just for example 1 pixel or about 10 kilometers wide, it means there is something selecting the angle within about 0.1 degrees. That could be the diffraction limit of something - perhaps a little bigger than the 50 microns mentioned but could certainly be smaller than the limit of order a millimeter as mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;However,&lt;/strong&gt; The &lt;em&gt;source of the illumination&lt;/em&gt; is the Sun, and as seen from Earth the width is about a half of a degree. You have to divide that by a factor of 2 because we're talking about &quot;the mirror tilting&quot;, not the incident angle, but that would still correspond to a width of 111 km times 0.53 degree times one-half, or about 29.5 kilometers minimum size, no matter how large and perfectly horizontal the plates, wouldn't it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the finite apparent size of the sun as seen by DSCOVR reflected in even a perfect mirror in the Earth's atmosphere following the curvature of the Earth, wouldn't the apparent size/shape be a uniform spot with a diameter of ~30 kilometers, and always &lt;em&gt;almost four ~ 8 km pixels wide if not larger&lt;/em&gt; in the un-binned 2048x2048  pixel DSCOVR images?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't forget that there still are the following widening mechanisms that are also contributing at some level:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Diffraction from the finite sizes of the ice plates. Slightly smaller plates would enlarge the spot, while substantially smaller plates would produce a much larger area but with correspondingly lower brightness per unit area and therefore be undetected, so it's only the (roughly speaking) say 20-50 micron sized hexagonal plates that must be absent to avoid widening of the apparent spot.&lt;/li&gt;&#xA;&lt;li&gt;Imperfect &quot;horizontality&quot; or more precisely; plate-to-plate fluctuations from co-planarity.&lt;/li&gt;&#xA;&lt;li&gt;DSCOVR optics? I don't know if the regularly obtained resolution in practice is always below one pixel under these conditions, perhaps it is. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I don't think the cause is a clever &lt;a href=&quot;https://space.stackexchange.com/q/18201/12102&quot;&gt;PrankSat&lt;/a&gt; nor some clever people with a lot of extra frequent flyer miles and aluminum foil, and the upper atmospheric ice plate thesis has a lot of merit, but I still can't reconcile the 0°.53 equatorial width of the sun's rays at the distance of a  reflection from Earth from DSCOVR (e.g. 1 AU + 1.5 million km) with the near-single-pixel observations in the blue channel. Am I just missing something here?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note:&lt;/em&gt; It is only the blue channel (about 443nm) that is routinely transmitted to Earth in full resolution. Pixel size estimates (note the arrow in the annotated cropped image below) should only be applied to images in the full resolution blue channel.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Out of total 4106 images collected, 336 contain land-glint for the &lt;strong&gt;blue channel&lt;/strong&gt;, which was chosen because it has the highest spatial resolution (to reduce the amount of data transmitted from DSCOVR, for all other channels four pixels are averaged onboard the spacecraft).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rlWIy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rlWIy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/f5q4H.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f5q4H.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For anyone who would like to approach the math in a systematic way, the question would be about how large the virtual image of the sun would appear to DSCOVR to be if reflected in a convex spherical mirror centered at the center of the earth with a radius equal to 6378 + 5 kilometers. Would it look like it is about 30 kilometers in diameter if projected to the point of reflection, if you treat it as if it were on the surface of the mirror itself?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FBSxy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FBSxy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From slide #12 &lt;a href=&quot;http://slideplayer.com/slide/4681773/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (after you wait for the fade-in). For this question, imagine the camera shown in this image is closer to the mirror than the candle - &lt;em&gt;the candle, representing the sun, would be behind the camera, representing DSCOVR's camera EPIC&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-16T22:11:20.283" LastActivityDate="2017-06-16T22:14:39.793" Title="How could the recently explained &quot;glints&quot; seen by DSCOVR appear so compact considering the finite size of the sun?" Tags="|imaging|camera|earth-observation|dscovr|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21588" PostTypeId="1" AcceptedAnswerId="21597" CreationDate="2017-05-21T13:28:16.633" Score="5" ViewCount="3226" Body="&lt;p&gt;The NASA Spaceflight article &lt;a href=&quot;https://www.nasaspaceflight.com/2017/05/rocket-labs-electron-inaugural-flight-new-zealand/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Lab’s Electron rocket set for inaugural flight from New Zealand&lt;/a&gt; mentions the upcoming opening of a 10 day launch window:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Rocket Lab, a U.S.-based launch service provider with a New Zealand subsidiary, will become the newest entrant into the world’s launch market with the maiden voyage of its Electron rocket.&lt;/p&gt;&#xA;&lt;p&gt;The 10-day launch window for Electron’s maiden flight opens at 09:00 NZDT (New Zealand Daylight Time) on Monday, 22 May.&lt;/p&gt;&#xA;&lt;p&gt;The time offset for New Zealand is UTC+12, placing the opening of the launch window at 21:00 UTC on Sunday, 21 May (which is 17:00 EDT on the eastern seaboard of the United States on Sunday afternoon).&lt;/p&gt;&#xA;&lt;p&gt;Overall, Rocket Lab’s mission is to offer “lightweight, cost-effective commercial rocket launch services” to the small satellite market.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I noticed that each of the eight engines in the outer ring seems to have two large cylinders attached at the bottom ends to the bottom of the combustion chambers, and the tops to the rocket frame. Initially I'd hoped I'd finally sighted &lt;a href=&quot;https://space.stackexchange.com/q/15463/12102&quot;&gt;shock absorbers&lt;/a&gt; - they look like compressed-gas cylinders, but considering how they are mounted that's not really likely.&lt;/p&gt;&#xA;&lt;p&gt;Since they are mounted at roughly right angles with respect to each engine's axis, I wonder if they could be vectoring actuators, but wouldn't these large cylinders be an unusual method for vectoring engines?&lt;/p&gt;&#xA;&lt;p&gt;One interesting point about the Electron - since the fuel and oxidizer pumps are 50 hp electric motors, there is plenty of electric power available - according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Rutherford_(rocket_engine)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; the Lithium battery can provide up to 1 MW of electric power.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IHQg1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IHQg1.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/flQ88.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/flQ88.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; Illustrations of the nine Rutherford engines on Rocket Lab's Electron rocket. From &lt;a href=&quot;https://www.nasaspaceflight.com/2017/05/rocket-labs-electron-inaugural-flight-new-zealand/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-05-04T08:00:35.227" Title="What are the two very large &quot;blue&quot; cylinders attaching to the combustion chamber of each Rutherford Engine?" Tags="|engine-design|thrust-vectoring|electron|rocketlab|rutherford-engine|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21589" PostTypeId="1" AcceptedAnswerId="31113" CreationDate="2017-05-21T15:38:57.977" Score="1" ViewCount="572" Body="&lt;p&gt;The question &lt;a href=&quot;https://astronomy.stackexchange.com/q/16704/7982&quot;&gt;Why does the Moon appear gray when passing between the Sun and the Earth?&lt;/a&gt; shows the famous &amp;quot;Moon photobombs Earth&amp;quot; image of the moon in front of the Earth taken from the EPIC camera on DSCOVR. It looks fairly neutral color, as does the image below that, which shows a particular section of the lunar soil to be fairly neutral gray, assuming the spacesuit is also neutral (white).&lt;/p&gt;&#xA;&lt;p&gt;But the spectrally resolved albedo of the Moon is not even close to neutral! I've seen other spectral albedo plots of the moon from very different observations and they all look the same, about 0.07 at 400nm and almost doubling to 0.14 at 700nm. In a word &amp;quot;orange&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;But the full or gibbous moon when high in the sky doesn't look orange to me at night, even when there are various terrestrial sources around to get a color reference, even looking through a window from a well lit room, even looking at it during the day. It always looks essentially white.&lt;/p&gt;&#xA;&lt;p&gt;Why doesn't a full/gibbous moon high in the sky ever seem to look orange? Shouldn't it?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;As a non-scientific enquiry, I took the photobomb image as-is from the question (shown below) and used my computer to color analyze the moon. I got this, which does show a definite progression, blue darker than green darker than red:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/biELX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/biELX.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1ulZJ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1ulZJ.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;If I isolate the moon with Python and calculate the average pixel values in the R, G, and B channels, I get [0.33, 0.31, 0,28] which is a pretty good match to the albedo plot if I just look at the plot around 450, 550, and 650nm, (which is what normal people do after spending hours trying and failing to understand the intricacies of human color perception).&lt;/p&gt;&#xA;&lt;p&gt;So is the moon really orange? Does it add a substantial orange tint to the reflected sunlight, and we just don't notice it?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MPCKG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MPCKG.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4wtWq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4wtWq.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Figure 8: Averaged geometrical moon albedos measured by GOME from July 1995, November 1995, and September 1996.&amp;quot; From ESA's &lt;a href=&quot;https://earth.esa.int/workshops/ers97/papers/dobber/index-2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;GOME moon measurements, including instrument characterization and moon albedo&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FNXzZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FNXzZ.png&quot; alt=&quot;moon photobombs earth&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hGZSD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hGZSD.png&quot; alt=&quot;Buzz Aldrin carries the EASEP&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Buzz Aldrin carries the EASEP.&amp;quot; from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Buzz_Aldrin_carries_the_EASEP.jp&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-10T12:42:56.220" LastActivityDate="2021-08-10T12:42:56.220" Title="Why doesn't a full/gibbous moon high in the sky ever seem to look orange? Shouldn't it?" Tags="|the-moon|photography|visualization|" AnswerCount="1" CommentCount="5" FavoriteCount="0" ClosedDate="2018-10-06T12:40:02.893" ContentLicense="CC BY-SA 4.0" />
  <row Id="21590" PostTypeId="1" CreationDate="2017-05-21T16:19:16.103" Score="7" ViewCount="3640" Body="&lt;p&gt;Inspired by the discussion on &lt;a href=&quot;https://space.stackexchange.com/questions/21588/what-are-the-two-very-large-blue-cylinders-attaching-to-the-combustion-chamber&quot;&gt;What are the two very large &amp;quot;blue&amp;quot; cylinders attaching to the combustion chamber of each Rutherford Engine?&lt;/a&gt; and especially &lt;a href=&quot;https://space.stackexchange.com/users/10829/jkavalik&quot;&gt;jkavalik&lt;/a&gt;'s excellent picture of the Merlin engine:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OAr9p.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OAr9p.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the entire Merlin gimbal?  On the SSME, the low pressure turbopumps were mounted to the vehicle structure, with flex hoses between them and the high pressure pumps.  Only the thrust chamber/nozzle/high pressure pump assembly gimbaled. I can't tell where the interface is on the SpaceX vehicle from this picture.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-08-29T23:45:14.767" LastActivityDate="2020-08-29T23:45:14.767" Title="SpaceX Engine Mounting / Gimbaling" Tags="|spacex|thrust-vectoring|gimbaled-engines|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21591" PostTypeId="2" ParentId="21586" CreationDate="2017-05-21T16:26:55.803" Score="2" Body="&lt;p&gt;I found a higher res &lt;a href=&quot;http://www.sae.org/dlymagazineimages/11560_15263_ACT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;picture&lt;/a&gt; which let me ID the logos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pRgIT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pRgIT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The top &quot;patch&quot; is the Air Force Research Lab insignia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/e7yKXm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/e7yKXm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then &lt;a href=&quot;http://www.orbitec.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbitec&lt;/a&gt;, then &lt;a href=&quot;http://www.garvspace.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Garvey Spacecraft Corporation&lt;/a&gt;, then, as you said, &lt;a href=&quot;http://www.atk.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ATK&lt;/a&gt; (ATK logo apparently predates the merger with Orbital).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-05-21T16:26:55.803" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21592" PostTypeId="2" ParentId="21590" CreationDate="2017-05-21T18:02:28.140" Score="4" Body="&lt;p&gt;This image has a better view of the top of the engine: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nxEoV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nxEoV.jpg&quot; alt=&quot;Merlin 1C diagram&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The quadrapod structure is attached rigidly to the rocket. At the bottom of the quadrapod there's a gimbal joint. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact structure of this joint is hard to see in photos, but it looks like the joint sits on top of the thrust chamber. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Merlin 1D structure is more compact, but uses the same principle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the lower half of a Merlin 1D: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tDqEB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tDqEB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-21T19:13:18.813" LastActivityDate="2017-05-21T19:13:18.813" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21593" PostTypeId="1" CreationDate="2017-05-21T19:17:48.933" Score="1" ViewCount="348" Body="&lt;p&gt;This is a question that I sort of view as an intersection between economics/science/space, completely hypothetical in nature, so I'm not sure if this is the right forum for it but, here goes:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I read an article with the headline &lt;a href=&quot;http://fortune.com/2015/07/20/asteroid-precious-metals/&quot; rel=&quot;nofollow noreferrer&quot;&gt;'Asteroid passing close to Earth could contain $5.4 trillion of precious metals'&lt;/a&gt;. I've seen similar articles talking about the value of metals in asteroids in the past.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is, suppose one of these asteroids, on a smaller scale, landed somewhere in the desert, and contained hundreds of billions of dollars worth of precious metals. What would something like that do to the economy? Obviously our money is worth what it is because of the amount of it that is in circulation to some degree, so, if all of a sudden there was an infusion of billions of dollars into the economy (albeit over time), would it have a detrimental impact on the economy? I'm not an economist by any stretch, but in just thinking about it, I've wondered what the effect would be on the economy if suddenly there was hundreds of billions more in precious metals that landed here, not just passing by in space. Would the major impact simply be that the value of those metals would rapidly decrease, or would there be a larger impact on the global economy? Maybe the answer is nobody knows, but theoretically, I've tossed this around in my mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This may be a question more suited for an economics forum, and I'll be happy to move it there if so.&lt;/p&gt;&#xA;" OwnerUserId="19722" LastEditorUserId="2752" LastEditDate="2017-05-24T01:58:59.783" LastActivityDate="2017-05-24T03:09:10.027" Title="Theoretical Asteroid Composition and the economy" Tags="|asteroid|economics|" AnswerCount="5" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21594" PostTypeId="2" ParentId="21593" CreationDate="2017-05-21T21:42:53.093" Score="4" Body="&lt;p&gt;To the extent that these precious metals have actual value in industrial processes it would be a great day.  The cost to make many goods would drop and we all would, to some degree, benefit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To the extent that these precious metals are being used as monetary instruments (people paying for things in, say, gold), it would reduce the value of those monetary instruments dramatically.  For example, there is less than 10 trillion USD of gold that has been mined, so a 5 trillion USD rock sitting on the ground (I know, it won't just be sitting there) would definitely affect the market.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I'm assuming that the &quot;landed&quot; object did so without causing an extinction event -- which I think is your assumption as well.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2017-05-24T03:09:10.027" LastActivityDate="2017-05-24T03:09:10.027" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21595" PostTypeId="2" ParentId="6361" CreationDate="2017-05-21T23:51:24.133" Score="-3" Body="&lt;p&gt;If the manufactured parts for the Apollo spacecraft had a .1% failure rate the mission would have certainly failed.  There were over 6,000,000 parts used.  At .1% failure rate 600 individual parts would have failed.&lt;/p&gt;&#xA;" OwnerUserId="18906" LastActivityDate="2017-05-21T23:51:24.133" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21596" PostTypeId="1" CreationDate="2017-05-22T14:04:30.697" Score="4" ViewCount="1347" Body="&lt;p&gt;What is the lowest altitude that an ion thruster can realistically be used for station keeping before its small amount of thrust is overcome by atmospheric drag? Please state your assumptions such as thrust, spacecraft drag area, mass etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The motivation behind my question is similar to this quote. From &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_decay&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia's article on orbital drag&lt;/a&gt; (even though the quote is about only one revolution):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Due to atmospheric drag, the lowest altitude above the Earth at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 150 km (90 mi).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wonder if ion thrusters in continuous operation can meaningfully extend (or actually reduce) the limit of lowest non-degrading orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related question: &lt;a href=&quot;https://space.stackexchange.com/questions/17941/what-are-the-challenge-to-use-atmosphere-as-a-propellant-for-super-low-altitude&quot;&gt;What are the challenge to use atmosphere as a propellant for super low altitude (180-260 km) satellites?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19725" LastEditorUserId="19725" LastEditDate="2017-05-22T15:30:57.523" LastActivityDate="2021-02-06T05:29:57.970" Title="What is the lowest altitude that an ion thruster can be used for station keeping?" Tags="|ion-thruster|atmospheric-drag|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21597" PostTypeId="2" ParentId="21588" CreationDate="2017-05-22T14:32:28.220" Score="10" Body="&lt;p&gt;The blue cylinders contain &lt;a href=&quot;https://www.rocketlabusa.com/careers/positions/electromechanical-technician-huntington-beach-los-angeles-2707637/&quot; rel=&quot;nofollow noreferrer&quot;&gt;electric motors&lt;/a&gt; that are used to vector the engine assembly. This could be either a linear motor or a conventional motor driving e.g. a thread screw.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The pumps and the thrust vector actuators on the engine have brushless DC motors powered by batteries. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This means they can avoid hydraulics, simplifying the design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this &lt;a href=&quot;https://twitter.com/Peter_J_Beck/status/1026499861019914241&quot; rel=&quot;nofollow noreferrer&quot;&gt;2018 photo&lt;/a&gt; you can see the configuration has been changed a bit from the rendering: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5GV1N.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5GV1N.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2019-05-04T08:00:35.227" LastActivityDate="2019-05-04T08:00:35.227" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="21598" PostTypeId="1" CreationDate="2017-05-22T15:14:35.367" Score="6" ViewCount="2419" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/21597/12102&quot;&gt;This answer&lt;/a&gt; explains that the gimbaling actuators on each of the nine Rutherford engines on the Electron rocket are in fact electrically powered DC brushless motors. Currently it's not known if they are linear motors, or rotary motors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This kind of actuator is able to produce substantial force or torque even at zero linear or angular velocity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering, very roughly, how much force or torque is involved here? When the engine is not running, there is just the inertia of the engine to deal with, and that's a very straightforward mechanical engineering problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take gimbaling the smaller Rutherford engine as a working example for the moment, since it has the electric gimbaling actuators. I'm not sure if there is enough information around to estimate how much force those blue cylinders would need to accelerate an engine by say 1 degree per second squared, but assuming most of the mass is near the pivot point and I = 100 kg m^2, then you'd only need a few Newtons to push sideways, or perhaps 10 Newtons considering the mechanical disadvantage of the angle at which they are mounted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no idea if gimbaling thrust at 1 degree per second squared is fast enough, responsive enough, or too slow to keep a rocket like this flying stable and smoothly. Any ideas on that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other half of the question is what happens when the engine is running and there is thrust? Is it harder to gimbal? There is some more mass in the chamber, pumps and tubing, and if there is flexible propellant feed tubing it will become much stiffer under high pressure. If this dominates, then it would be pretty hard to estimate because of all the specifics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there may be some numbers out there that could be at least scaled for an informal ballpark estimate, and that would be fine in this case!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; In the title of the question I've asked about a &lt;em&gt;mid-sized nine-engine rocket&lt;/em&gt; and I've shown pics of both Merlin and Rutherford engines. Either one could be the focus for an answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IHQg1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IHQg1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/flQ88.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/flQ88.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above x2:&lt;/strong&gt; Illustrations of the nine Rutherford engines on Rocket Lab's Electron rocket. From &lt;a href=&quot;https://www.nasaspaceflight.com/2017/05/rocket-labs-electron-inaugural-flight-new-zealand/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OAr9p.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OAr9p.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://space.stackexchange.com/users/10829/jkavalik&quot;&gt;jkavalik&lt;/a&gt;'s excellent picture of a Merlin engine, from &lt;a href=&quot;https://space.stackexchange.com/users/6944/organic-marble&quot;&gt;OrganicMarble&lt;/a&gt;'s question &lt;a href=&quot;https://space.stackexchange.com/q/21590/12102&quot;&gt;SpaceX Engine Mounting / Gimbaling&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-22T17:11:13.247" LastActivityDate="2017-05-22T17:11:13.247" Title="What kind of forces and response times/accelerations are needed to gimbal a mid-sized nine-engine rocket's engines?" Tags="|engine-design|thrust-vectoring|" AnswerCount="0" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21599" PostTypeId="2" ParentId="21596" CreationDate="2017-05-22T16:08:47.507" Score="3" Body="&lt;p&gt;For a hefty, substantial spacecraft like the 5000 kg, 5 square meter, drag coefficient = 1 simulation &lt;a href=&quot;https://space.stackexchange.com/a/21226/12102&quot;&gt;I did here&lt;/a&gt;, the final complete orbit was circularized at about 95 kilometers. A cube sat of the same average density would have been higher just because of the higher area to mass ratio.&lt;/p&gt;&#xA;&lt;p&gt;For the atmosphere to match other tabulated values around 100km I used the very very rough approximation&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\rho(h) \ = \ \rho_0 \ exp(-h/h_{scale})$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;With the sea-level density and scale height &lt;span class=&quot;math-container&quot;&gt;$\rho_0$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$h_{scale}$&lt;/span&gt; of 1.225 kg/m^3 and 7700 meters. This is just rough.&lt;/p&gt;&#xA;&lt;p&gt;A simple equation for drag from Wikipedia might be&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$F_{drag} \ = \ -\frac{1}{2} \ \rho \ C_D \ A \ v^2$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;and &lt;span class=&quot;math-container&quot;&gt;$C_D$&lt;/span&gt; for a randomly shaped satellite at LEO velocity might be 1, the area might be 5 meters, and the LEO velocity &lt;span class=&quot;math-container&quot;&gt;$v$&lt;/span&gt; might be 7700 m/s.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$F_{drag} \ = \ 0.5 \times 1.225 \times exp(-h/h_{scale}) \times 1.0 \times  5.0 \times 7700^2$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;If I assume the thruster has a force of 0.2 N as &lt;a href=&quot;https://space.stackexchange.com/q/16604/12102&quot;&gt;I did here&lt;/a&gt;, I can solve for the height &lt;span class=&quot;math-container&quot;&gt;$h$&lt;/span&gt;, which in this case turns out to be almost exactly your original number, 146 km.&lt;/p&gt;&#xA;&lt;p&gt;Did I say roughly, and say it enough times?&lt;/p&gt;&#xA;&lt;p&gt;Now the problem is that the upper atmosphere changes a lot in response to activity of the Sun, and so the density is not well approximated by a simple scale height, or is even constant or predictable. This is discussed in @DavidHammon's &lt;a href=&quot;https://space.stackexchange.com/a/21492/12102&quot;&gt;answer here&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/questions/18223/#comment47477_18223&quot;&gt;comment here&lt;/a&gt;.  So if there is a solar event, you may suddenly need maybe even ten times the thrust to keep from burning up all of a sudden. So while estimates can give you a feel for it, your mileage may vary, a lot!&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;People talk about putting spy or Earth monitoring satellites at altitudes between 150 and 200 km because a smaller telescope (like a 10cm aperture inside a 3U cubesat) might get better resolution than one at 400km, or similar to a 50cm aperture at 750 km. Of course there are a lot of other factors that affect resolution, and there is a problem with atomic oxygen eating your satellite, but people still talk about it.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-02-06T05:29:57.970" LastActivityDate="2021-02-06T05:29:57.970" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="21601" PostTypeId="2" ParentId="21593" CreationDate="2017-05-22T19:29:52.240" Score="2" Body="&lt;p&gt;As has been mentioned, it would be much more valuable in Earth or Luna orbit since it would be a lot of resources that we don't have to lift out of the gravity well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the ground, it would all depend on how much there is.  100 pounds of gold, while awesome, will not have much effect on world economy past a short dip in gold prices (which are currently being artificially depressed anyway).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also remember that the more resources in the payload you are dropping on the Earth, the bigger the boom when it hits.  It might be a windfall but not for those nearby.  If it is big enough, we might have to ask that question to whatever evolves intelligence after the extinction event.&lt;/p&gt;&#xA;" OwnerUserId="18842" LastActivityDate="2017-05-22T19:29:52.240" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21602" PostTypeId="2" ParentId="20662" CreationDate="2017-05-22T20:48:01.050" Score="2" Body="&lt;p&gt;You can try &lt;a href=&quot;https://ssd.jpl.nasa.gov/?sat_elem&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's SSD approximations&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Sat.    a   e   w   M   i   node    n   P   Pw  Pnode   Ref.&#xA;(km)        (deg)   (deg)   (deg)   (deg)   (deg/day)   (days)  (yr)    (yr)    &#xA;Moon    384400. 0.0554  318.15  135.27  5.16    125.08  13.176358   27.322  5.997   18.600  1&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="19330" LastEditorUserId="6816" LastEditDate="2017-05-22T21:16:37.150" LastActivityDate="2017-05-22T21:16:37.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21603" PostTypeId="2" ParentId="21593" CreationDate="2017-05-22T23:15:36.187" Score="1" Body="&lt;p&gt;It wouldn't have a lot of effect on currencies, no modern currency is backed by precious metals. If it had a lot of the platinide group of metals, it would definitely benefit industry, since they are used for catalysts in a lot of chemical processes. If it was mostly gold and silver it might drop the price of them for a while, but it would have to have quite a bit to make anything more than short term changes. The total value of the world's assets is about 256 trillion, so that asteroid isn't a big deal by comparison. &lt;/p&gt;&#xA;" OwnerUserId="13841" LastActivityDate="2017-05-22T23:15:36.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21604" PostTypeId="1" AcceptedAnswerId="49678" CreationDate="2017-05-23T06:34:01.247" Score="8" ViewCount="1345" Body="&lt;p&gt;I've used a quick very rough approximation of the drop of atmospheric density with altitude in &lt;a href=&quot;https://space.stackexchange.com/a/21226/12102&quot;&gt;this answer&lt;/a&gt; and in &lt;a href=&quot;https://space.stackexchange.com/a/21599/12102&quot;&gt;this answer&lt;/a&gt; by using a single exponential and scale height parameter, but that's not what happens in reality. I've typed in some numbers from the &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;U.S. Standard Atmosphere 1976&lt;/a&gt; for &lt;span class=&quot;math-container&quot;&gt;$\rho(Z)/\rho_0$&lt;/span&gt; and plotted it below.&lt;/p&gt;&#xA;&lt;p&gt;The Standard Atmosphere document contains a thorough introduction, and I will try to read it, but in the mean time I'd still like to know if there is a very simple way to understand what changes in the atmospheric physics in the neighborhood 100 km that makes the density fall off so much more slowly beyond this region? Why the giant knee?&lt;/p&gt;&#xA;&lt;p&gt;Is there a way to write a nice, smooth, analytic function that better approximates the density that reproduces the general behavior of this knee?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; the three thin lines are simple scale-height plots with &lt;span class=&quot;math-container&quot;&gt;$h_{scale}$&lt;/span&gt; of 6.5, 7, and 7.5 km, bottom to top, just for reference.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CJ9dO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CJ9dO.png&quot; alt=&quot;relative density vs altitude showing &amp;quot;knee&amp;quot; around 100 km&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Python for plot:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;info = &amp;quot;&amp;quot;&amp;quot;80, 1.5068E-05, 85, 6.7099E-06, 90, 2.789E-06, &#xA;95, 1.137E-06, 100, 4.575E-07, 105, 1.898E-07,&#xA;110, 7.925E-08, 115, 3.501E-08, 120, 1.814E-08, &#xA;125, 1.054E-08, 130, 6.655E-09, 135, 4.461E-09, &#xA;140, 3.128E-09, 145, 2.270E-09, 150, 1.694E-09, &#xA;155, 1.294E-09, 160, 1.007E-09, 165, 7.959E-10,&#xA;170, 6.380E-10, 175, 5.174E-10, 180, 4.240E-10,&#xA;190, 2.924E-10, 200, 2.047E-10, 210, 1.507E-10,&#xA;220, 1.116E-10, 230, 8.402E-11, 240, 6.415E-11,&#xA;250, 4.957E-11, 260, 3.871E-11, 280, 2.425E-11,&#xA;300, 1.564E-11, 320, 1.032E-11, 340, 6.941E-12, &#xA;360, 4.739E-12, 380, 3.276E-12, 400, 2.288E-12,&#xA;420, 1.612E-12, 440, 1.144E-12, 460, 8.180E-13,&#xA;480, 5.844E-13, 500, 4.257E-13&amp;quot;&amp;quot;&amp;quot;&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;alti, ratio = [info.strip('/n').split(',')[i::2] for i in range(2)]&#xA;&#xA;alti, ratio = [np.array([float(x) for x in thing]) for thing in [alti, ratio]]&#xA;&#xA;alts = alti[:15]&#xA;&#xA;rats = [np.exp(-alts/hs) for hs in [6.5, 7, 7.5]]&#xA;&#xA;# https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539.pdf&#xA;# U.S. Standard Atmosphere, 1976, NOAA, NASA, USAF&#xA;# Altitude (Z, Geometric Height) is in kilometers above sea level.&#xA;# Data is from Table 4.&#xA;# Here ratio is rho(Z) / rho_0, and rho_0 seems to be 1.225 kg/m^3&#xA;&#xA;plt.figure()&#xA;plt.yscale('log')&#xA;for rat in rats:&#xA;    plt.plot(alts, rat, linewidth=0.6)&#xA;plt.plot(alti, ratio, '-k', linewidth=2)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-10-06T15:26:15.697" LastActivityDate="2021-02-06T02:33:48.023" Title="Why does Earth's atmospheric density have a big &quot;knee&quot; around 100 km? Is there a good analytical approximation?" Tags="|reentry|atmosphere|atmospheric-drag|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="21605" PostTypeId="1" AcceptedAnswerId="21606" CreationDate="2017-05-23T08:13:29.027" Score="3" ViewCount="540" Body="&lt;p&gt;A smoothly running rocket engine will have a center of thrust axis. If the engine is gimbaled, that axis should pass through each of the gimbal axes (if it's a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gimbal&quot; rel=&quot;nofollow noreferrer&quot;&gt;classical gimbal&lt;/a&gt;), or if it is a ball joint, the gimbal point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am sure engines are fabricated carefully to be symmetrical to high tolerances, but nothing is perfect. Any misalignment would result in a static torque, which would have to then be maintained by whatever mechanical system was actuating the gimbal motion, and if there is a mechanical disadvantage such as the one in the way &lt;a href=&quot;https://space.stackexchange.com/q/21598/12102&quot;&gt;these images suggest&lt;/a&gt;, the job is even worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So do they test fire engines and monitor the position of its center of thrust with respect to the gimbaling joints, and shim or adjust the engine mounting or nozzle in order to zero out any major asymmetries, or do manufacturing processes result in engines that are naturally very very well centered?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here is some text from previous comments that help further define why centering might be important: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the thrust axis does not pass through the center of rotation of the gimbaling motion, the engine would have a tendency to &lt;em&gt;&quot;auto-slam&quot;&lt;/em&gt; to one side as soon as it starts, so the actuators would have to deliver substantial force quickly, and track the moment-by-moment increase in thrust as the engine starts. This could potentially be much faster than the rotation of the entire spacecraft since it involves only the moment of inertia of the individual engine, not the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actuator would have to continue to deliver this extra force against the engine's thrust for the entire time the engine is running, (perhaps more of a problem for &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/q/21598/12102&quot;&gt;hot, battery-draining electric actuators&lt;/a&gt; than for hydraulic actuators&lt;/em&gt;) then track the thrust of the engine back to zero as it shuts down. From a dynamics and control system point of view, is a different animal than steering the spacecraft.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-23T13:58:03.973" LastActivityDate="2017-05-23T13:58:06.920" Title="Do gimbaled engines have to be carefully test-fired to ensure the thrust axis intercepts center of rotation?" Tags="|engine-design|thrust-vectoring|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21606" PostTypeId="2" ParentId="21605" CreationDate="2017-05-23T08:40:07.190" Score="2" Body="&lt;p&gt;They are certainly centered for obtaining the exact thrust profile, to be able to control the rocket, but they absolutely don't need to be perfectly centered with the gimbal - the gimbal(s) may be way off-center, and carry a significant torque if other concerns prioritize freeing up the middle - say, you gimbal just the nozzle, and need the throat obstruction-free. Besides, if you have two 1-axis gimbals instead of one 2-axis, at different heights, you're not getting the thrust through the center of the second one. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that case you simply use actuators strong enough. They must be pretty strong anyway, because the engine is quite heavy and the gimbal must be moving fast - that's a considerable acceleration needed, and as result, considerable force. Making them strong enough to carry some extra torque from off-center thrust is not an excessive demand.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Fast, strong actuators used in general engineering often provide different force in different direction - for example, piston-based actuators are usually stronger in the &quot;extending&quot; direction than &quot;retracting&quot; due to higher area of piston exposed to one of the chambers (part of it on the &quot;retracting&quot; side occupied by the piston shaft). In that case a biased load is welcome - you only regulate the level of reaction force without ever needing to change its sign (e.g. only regulate pressure in the piston, without ever toggling the valve to force retraction). Gimbals based on such actuators would welcome a semi-constant torque/force exerted by the engine, since they'd need to provide power only in one direction, just yielding to the engine's torque/force in the opposite direction.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-05-23T13:58:06.920" LastActivityDate="2017-05-23T13:58:06.920" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21609" PostTypeId="1" AcceptedAnswerId="21612" CreationDate="2017-05-23T22:36:55.907" Score="2" ViewCount="278" Body="&lt;p&gt;And what is its life for one dose? I feel like this would be the most feasible cargo for the protection from it.&lt;/p&gt;&#xA;" OwnerUserId="19048" LastActivityDate="2020-08-03T00:39:20.577" Title="Is there in existence a drug, or additive to protect astronauts from solar radiation?" Tags="|radiation|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21610" PostTypeId="2" ParentId="21593" CreationDate="2017-05-23T22:49:22.150" Score="0" Body="&lt;p&gt;That's a good question once expanded to the idea that once we start farming them and towing them to near earth orbit, who is accountable if an asteroid strikes ground and brings more worth in raw materials than GDP of the destroyed area? Also when the question of whose asteroid is it?&lt;/p&gt;&#xA;" OwnerUserId="19048" LastActivityDate="2017-05-23T22:49:22.150" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21611" PostTypeId="2" ParentId="21593" CreationDate="2017-05-24T02:35:11.560" Score="0" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;My question is, suppose one of these asteroids, on a smaller scale, landed somewhere in the desert, and contained hundreds of billions of dollars worth of precious metals. What would something like that do to the economy?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It has already happened, many times over, but about 4 billion years ago, during the late heavy bombardment. Almost all of the gold and platinum we mine today results from &lt;a href=&quot;https://phys.org/news/2011-09-gold.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;asteroids that hit the Earth after its surface had cooled&lt;/a&gt;. This was not a good time for planet Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A half-kilometer asteroid hitting the Earth today also would not be a good time for planet Earth. A bit over 3 years ago, a mere 18 meter diameter asteroid exploded over Chelyabinsk, Russia, releasing the equivalent of about 30 Hiroshima bombs of energy in the process. A half-kilometer diameter asteroid would release the equivalent of a half a million to over a million Hiroshima bombs worth of energy. Needless to say, this would not be a good day for humanity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, that 5.4 trillion dollars is a gross over-exaggeration. A more likely number is 300 billion. Or less. An even more likely number: The value of the precious metals in asteroids is currently negative, and most likely will remain so for a good long time. To quote Chris Lewicki, the president and chief engineer of asteroid mining company Planetary Resources,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“We are only repeating what has been done throughout history, just in a new environment.”  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Source: &lt;a href=&quot;http://www.bbc.com/future/story/20160103-the-truth-about-asteroid-mining&quot; rel=&quot;nofollow noreferrer&quot;&gt;The truth about asteroid mining&lt;/a&gt;, which did not tell the truth about asteroid mining.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, they're lying. Selling snake oil. Without &lt;em&gt;unobtainium&lt;/em&gt;, the numbers just do not add up. What might add up, assuming a nascent industry in space, is mining asteroids for non-precious resources such as water and methane. Most deep pocket investors cannot comprehend how water and methane could possibly be more precious than gold or platinum, so the snake oil salesmen lie to those potential investors about the presence of “gold in them thar asteroids.”&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-05-24T02:35:11.560" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21612" PostTypeId="2" ParentId="21609" CreationDate="2017-05-24T08:18:02.110" Score="10" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar radiation, like any other form of radiation, can be shielded against, and there are medications to relieve the symptoms and aid the organism in healing, but there is no direct cure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike diseases, where the cause is viruses or bacteria developing inside the body, and you need to combat these, stopping the spread before it can cause too much damage, radiation causes direct damage to human body cells - you stop the &quot;spread&quot; by shielding the body against the radiation, and you deal with the damage through normal healing process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are drugs developed, like &lt;a href=&quot;https://en.wikipedia.org/wiki/Ex-Rad&quot; rel=&quot;noreferrer&quot;&gt;Ex-Rad&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Entolimod&quot; rel=&quot;noreferrer&quot;&gt;Entolimod&lt;/a&gt; that aid against radiation by boosting natural healing mechanisms of the body - DNA repair, purging reactive substances that could cause more damage, activating immune response to protect damaged areas from developing infection, trigger improved healing response etc - but that is all just boosting the organism's natural defense and dealing with damage caused by irradiation; the healing process. The only way to prevent that damage though, is not to get irradiated in the first place.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-24T08:18:02.110" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21613" PostTypeId="2" ParentId="1691" CreationDate="2017-05-24T09:45:38.417" Score="1" Body="&lt;p&gt;We are used to see the dust 'floating away', but that's due to the presence of an atmosphere. In a vacuum, the dust particles fall to the ground inmmediatly, even with the relatively low gravitational acceleration of the Moon...&lt;/p&gt;&#xA;" OwnerUserId="18814" LastActivityDate="2017-05-24T09:45:38.417" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21614" PostTypeId="1" CreationDate="2017-05-24T11:32:32.803" Score="111" ViewCount="29468" Body="&lt;p&gt;Say they landed on the moon but discovered the ship was damaged during the landing and they could not launch again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would have been the plan? Would another crew have been sent to rescue them? Would the astronauts just have to wait till they ran out of supplies to die or was there a procedure in place for them to commit suicide in a painless way? How long could they have survived on the moon?&lt;/p&gt;&#xA;" OwnerUserId="10504" LastEditorUserId="19816" LastEditDate="2017-05-28T21:12:27.000" LastActivityDate="2021-04-29T05:51:34.103" Title="If the astronauts on Apollo 11 had landed safely on the moon but could not take off, would there have been a rescue mission?" Tags="|crewed-spaceflight|apollo-program|apollo-11|" AnswerCount="8" CommentCount="22" ContentLicense="CC BY-SA 3.0" />
  <row Id="21615" PostTypeId="1" CreationDate="2017-05-24T12:15:28.860" Score="7" ViewCount="3712" Body="&lt;p&gt;The true anomaly motion as I understand, is not constant in an elliptical orbit. The rate of change of true anomaly is not constant as in the case of circular orbits, in which case the true anomaly is equal to the mean anomaly, and the rate of true anomaly can be taken as the angular rate in the orbit, and the rate of rate of true anomaly can be taken as zero.&#xA;For elliptical orbits, however, there should be a non-constant rate of true anomaly, and some rate of rate of true anomaly. Is there some derivation for the formula that I can refer to for understanding the motion more clearly?&#xA;I have the position and velocity as cartesian state vectors in the orbit in the ECI coordinate frame.&lt;/p&gt;&#xA;" OwnerUserId="19739" LastEditorUserId="19739" LastEditDate="2017-05-25T14:06:19.227" LastActivityDate="2019-03-22T02:11:03.043" Title="How to calculate the complete true anomaly motion in an elliptical orbit?" Tags="|orbital-mechanics|artificial-satellite|orbit|spacecraft|orbital-elements|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21616" PostTypeId="2" ParentId="21614" CreationDate="2017-05-24T12:32:51.963" Score="17" Body="&lt;p&gt;There was no chance to get them back alive - no ready to launch rocket, no ready to launch spaceship, no lunar module capable to take 3 persons (or land automatically) - just nothing.&lt;/p&gt;&#xA;" OwnerUserId="11046" LastEditorUserId="716" LastEditDate="2017-05-25T22:11:43.650" LastActivityDate="2017-05-25T22:11:43.650" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21617" PostTypeId="2" ParentId="21614" CreationDate="2017-05-24T12:43:37.983" Score="156" Body="&lt;p&gt;The following is a &lt;a href=&quot;http://www.space.com/26604-apollo-11-failure-nixon-speech.html&quot; rel=&quot;noreferrer&quot;&gt;speech written for President Nixon&lt;/a&gt;, in the event that the Apollo 11 mission did not succeed.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Fate has ordained that the men who went to the moon to explore in peace will stay on the moon to rest in peace.&lt;/p&gt;&#xA;&lt;p&gt;These brave men, Neil Armstrong and Edwin Aldrin, know that there is no hope for their recovery. But they also know that there is hope for mankind in their sacrifice.&lt;/p&gt;&#xA;&lt;p&gt;These two men are laying down their lives in mankind's most noble goal: the search for truth and understanding.&lt;/p&gt;&#xA;&lt;p&gt;They will be mourned by their families and friends; they will be mourned by their nation; they will be mourned by the people of the world; they will be mourned by a Mother Earth that dared send two of her sons into the unknown.&lt;/p&gt;&#xA;&lt;p&gt;In their exploration, they stirred the people of the world to feel as one; in their sacrifice, they bind more tightly the brotherhood of man.&lt;/p&gt;&#xA;&lt;p&gt;In ancient days, men looked at stars and saw their heroes in the constellations.&lt;/p&gt;&#xA;&lt;p&gt;In modern times, we do much the same, but our heroes are epic men of flesh and blood.&lt;/p&gt;&#xA;&lt;p&gt;Others will follow, and surely find their way home. Man's search will not be denied. But these men were the first, and they will remain the foremost in our hearts.&lt;/p&gt;&#xA;&lt;p&gt;For every human being who looks up at the moon in the nights to come will know that there is some corner of another world that is forever mankind.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This confirms there were no plans for a rescue mission if Apollo 11 wasn't successful.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Edit - 29 April 2021&lt;/p&gt;&#xA;&lt;p&gt;As stated in &lt;a href=&quot;https://www.abc.net.au/news/2021-04-29/michael-collins-apollo-11-mission-secret-terror/100103584&quot; rel=&quot;noreferrer&quot;&gt;this news item&lt;/a&gt;,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The plan had always been that if Armstrong and Aldrin crashed or were unable to leave the moon's surface, Collins was to head back to Earth alone.&lt;/p&gt;&#xA;&lt;p&gt;A prepared contingency statement from President Richard Nixon, to be read out if the worst happened, made no mention of Collins.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="7062" LastEditDate="2021-04-29T05:51:34.103" LastActivityDate="2021-04-29T05:51:34.103" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="21618" PostTypeId="1" CreationDate="2017-05-24T13:40:49.037" Score="4" ViewCount="398" Body="&lt;p&gt;I'm designing a CubeSat mission to the Martian moons, and am having trouble calculating capture delta-V. Once it gets into a circular orbit, I can do the rest, but the capture itself is giving me trouble.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CubeSat would launch with SpaceX's Red Dragon mission, and would be ejected at some point during the cruise to Mars. According to a paper, Red Dragon will hit the Martian atmosphere at 6 km/s, which would mean it enters the Martian system at 1 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most papers on this topic are not especially useful to me, as they assume transfer to Mars using low-thrust propulsion, which enters the Mars system at a much smaller relative velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CubeSat's thruster would provide an acceleration of about 1 mm/s^2. I can give more details if required. Ideally I'd like to capture into a flat, circular orbit at 25,000 &amp;ndash; 30,000 km, but getting the numbers for capture into any orbit would be invaluable.&lt;/p&gt;&#xA;" OwnerUserId="19741" LastEditorUserId="6816" LastEditDate="2017-05-24T14:36:19.437" LastActivityDate="2017-05-25T09:09:06.567" Title="Calculating low-thrust capture delta-V with high $V_{inf}$" Tags="|mars|phobos|deimos|capture|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21619" PostTypeId="2" ParentId="21614" CreationDate="2017-05-24T14:46:23.967" Score="56" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;noreferrer&quot;&gt;Apollo lunar module&lt;/a&gt; was battery powered, so could only maintain a livable environment for a few days (this was a major concern for &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_13&quot; rel=&quot;noreferrer&quot;&gt;Apollo 13&lt;/a&gt;, since the crew was reliant on the LM after the accident which disabled the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot; rel=&quot;noreferrer&quot;&gt;service module&lt;/a&gt;). Once out of power, it would be unable to circulate air or to maintain a comfortable temperature inside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Committing suicide on the moon, fortunately, is trivially easy. Once the LM was nearly out of power, they could depressurize the LM cabin while their suits weren't sealed (or exit the LM and then unseal their suits). They'd lose consciousness in a few seconds, albeit with some discomfort from the rapid depressurization, and die shortly thereafter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As noted in other answers, there was not another rocket stacked and ready to launch in time to effect a rescue. If there had been, time would be very tight; the maximum surface stay of any of the Apollo landers (&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_17&quot; rel=&quot;noreferrer&quot;&gt;Apollo 17&lt;/a&gt;) was shorter than the Earth-moon flight time; I don't know if the LM endurance was longer than that, but it's conceivable that the astronauts could survive long enough to await rescue. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew transfer might be a challenge: the rescue crew would have to be suited up (because the entire cabin depressurizes to open the hatch; there's no airlock) and the rescuees would have to be suited up and wearing the bulky &lt;a href=&quot;https://en.wikipedia.org/wiki/Primary_Life_Support_System#Apollo_PLSS&quot; rel=&quot;noreferrer&quot;&gt;portable life support pack&lt;/a&gt;. They'd have to transfer one at a time, discarding their PLSS after connecting to the cabin environmental system. I'm not certain it would be physically possible to get four suited crew inside. I believe the extra mass would not be a major problem; the LM ascent stage was intended to return with about a human's mass worth of moon rocks, and usually completed its ascent with 150kg or more propellant remaining in the tanks. If it was limited to a lower orbit than usual, the command module could come down to reach it.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The alternative would be to land the rescue ship with a single crew member; this would be hazardous, but likely possible. The LM guidance computer was capable of semi-autonomous landing, and they'd have time on the outbound trip to program the precise location of the disabled lander as a target site. Three people could definitely fit in the LM (as demonstrated by Apollo 13).  &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-30T13:11:03.810" LastActivityDate="2017-05-30T13:11:03.810" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="21620" PostTypeId="1" AcceptedAnswerId="26051" CreationDate="2017-05-24T15:17:56.890" Score="21" ViewCount="3161" Body="&lt;p&gt;I believe that Apollo mission control had the ability to upload data and commands to the LM's guidance computer as long as they had an operational communications link. I'm wondering if, from the point of separation from the CSM, there was any necessary procedure for landing which absolutely had to be done by an astronaut in the cockpit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some points of interest:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;After the braking phase of descent, the guidance computer would default into the P65 program, which was designed to do an automatic touchdown; all the Apollo mission commanders overrode P65 into P66, the semi-automatic mode, so they could choose the exact landing site. (Jim Lovell of Apollo 13 claimed he intended to use the automatic landing if it looked safe to do so, but never got the chance). &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The LM did the Descent Orbit Insertion (DOI) burn while out of contact with mission control; let's assume that either the flight plan changed to do two smaller burns just before and after going out of contact, or that the systems were modified to allow command of the DOI burn from the command module, or on a time-delayed command.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Powered Descent Initiation (PDI), and all subsequent landing maneuvers, occurred while the LM had line-of-sight to Earth.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I know that the lunar module pilot pressed a &quot;Proceed&quot; button to give the guidance computer the go-ahead to start burns of the descent engine; could Mission Control send the &quot;Proceed&quot; signal directly to the computer?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that the crew managed various mode switches on the way down; could the altitude radar mode be switched from Mission Control? Could commands be sent over the low-gain channel to adjust the pointing of the high-gain antenna? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm aware that P65 had no way of knowing if it was landing in a boulder field and there would be no very good reason to land an LM uncrewed, but I'm curious if it was &quot;nearly possible&quot;, particularly in the context of a &lt;a href=&quot;https://space.stackexchange.com/questions/21614/if-the-astronauts-on-apollo-11-had-landed-on-safely-on-the-moon-but-could-not-ta&quot;&gt;rescue landing&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-08-01T02:24:31.367" LastActivityDate="2019-09-22T18:24:36.483" Title="Could an Apollo LM land uncrewed?" Tags="|apollo-program|lunar-landing|uncrewed-spaceflight|" AnswerCount="2" CommentCount="9" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21621" PostTypeId="2" ParentId="19853" CreationDate="2017-05-24T17:35:00.440" Score="9" Body="&lt;p&gt;Generally, well-designed pressure-fed hypergolic engines are very reliable.  However, failures of some designs have certainly occurred.  Here are some recent examples:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.nasa.gov/mission_pages/juno/main/index.html&quot; rel=&quot;noreferrer&quot;&gt;Juno&lt;/a&gt; probe, currently in orbit around Jupiter, had to significantly change its mission plans due to concerns about potential catastrophic (as in explosive) failures due to stuck helium check valves in its main engine, a &lt;a href=&quot;http://www.moog.com/products/thrusters.html&quot; rel=&quot;noreferrer&quot;&gt;Moog LEROS&lt;/a&gt; bipropellant hypergolic thruster.  These concerns arose after  malfunctions of the same engine on the &lt;a href=&quot;http://spaceflight101.com/systemic-propulsion-system-problem-suspect-in-muos-5-intelsat-33e-trouble/&quot; rel=&quot;noreferrer&quot;&gt;MUOS 5 and Intelsat 33e&lt;/a&gt; geostationary satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another example is the &lt;a href=&quot;https://www.nasaspaceflight.com/2015/05/ils-proton-m-launches-mexsat-1/&quot; rel=&quot;noreferrer&quot;&gt;failure&lt;/a&gt; of the 11D458M pressure-fed vernier engine on the third stage of the Proton-M/Briz-M rocket intended to launch the MexSat-1 telecom satellite in 2015.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2017-05-24T17:42:38.337" LastActivityDate="2017-05-24T17:42:38.337" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21622" PostTypeId="2" ParentId="21618" CreationDate="2017-05-24T22:45:22.390" Score="1" Body="&lt;p&gt;&lt;em&gt;Disclaimer:&lt;/em&gt; I am not sure I understand your problem, so let me know if I misunderstood and I'll edit my answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main problem with low thrust engines is that they are ... low thrust: you'll need to solve an optimal control problem (or a sub-optimal one) in order to orbit either moon of Mars. The design of your mission will depend on the following criteria:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;When you are jettisoned from the Red Dragon, are you on a collision course with the Mars system? To know this precisely, you need to calculate the B-Plane parameters of your spacecraft: after jettison, propagate (without thrusting) your orbit until a distance of three times the sphere of influence of Mars and compute the projection. To get a rough idea, look at the mission design of the Red Dragon (supposing that is already done and available to you) and check if you are jettisoned after all deep space trajectory correction maneuvers or not.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Do you need to target one of the Moons directly? My initial hunch is that the easiest would be to target a highly elliptical orbit around Mars. Once in orbit, you can target one of the two moons. The reason here is that it'll be much easier to perform an orbital injection around Mars than around either Moon because of their low mass and size and the precision you'll need to get your ephemerides right. If you inject around Mars, you can more easily compute the injection parameters. The reason why you may want a highly elliptical injection orbit is because you need less $Delta v$ to reach that from a hyperbolic orbit. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How precise of a solution do you want? Low thrust involves solving a control problem. There are a few good sub-optimal solution (cf. Q-Law by Petropoulos or the Naasz control laws) and some global optimal solvers. The latter are much harder to implement (and require knowledge of the math field of convex optimization). However, if you are a US citizen and affiliated to a US university, you may ask for access to NASA Mystic (which uses a &quot;static differential dynamical programming&quot; method, and it is currently used for their Dawn mission). If computer RAM isn't much of a limitation, you should be able to apply to sub-optimal control laws by using simple differential programming where the solution space is split up and you build a graph of all possible solutions from an initial orbit to a final orbit (and then traverse the graph using Bellman's algorithm to find the optimal path).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I hope this helps a bit. Again, I may have misunderstood your question, and if so, let me know so I can update my answer.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-05-24T22:45:22.390" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21623" PostTypeId="1" CreationDate="2017-05-24T22:48:53.297" Score="0" ViewCount="85" Body="&lt;p&gt;To be more specific:&#xA;Suppose a space station is orbiting in a cislunar orbit (about 1000 miles away from the Moon). Now you keep adding more mass (huge cargo) to the cislunar space station. The space station departs for Mars with that huge cargo (much more mass). There will be fuel needed for putting the ship on the trajectory to Mars and initial change of velocity. But will the fuel required be much higher or the same if the mass is added, say, 1000 times more? Thank you for asking for clarification.  &lt;/p&gt;&#xA;" OwnerUserId="19755" LastEditorUserId="19755" LastEditDate="2017-05-25T00:19:49.903" LastActivityDate="2017-05-25T00:19:49.903" Title="in microgravity do you need fuel to continue same velocity?" Tags="|microgravity|" AnswerCount="2" CommentCount="2" ClosedDate="2017-05-25T06:41:04.973" ContentLicense="CC BY-SA 3.0" />
  <row Id="21625" PostTypeId="2" ParentId="8832" CreationDate="2017-05-24T23:19:25.550" Score="2" Body="&lt;p&gt;I'm sure others will provide Hohmann numbers, but if you happen to be using low-thrust propulsion that requires spiraling down, the delta-v approximately equals the difference in Phobos and Deimos's orbital velocities, so about 800 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a &lt;em&gt;theoretical limit&lt;/em&gt; for very low thrust and very long transfer times, many hundreds or thousands of orbits during the spiral, as discussed in a little more detail in &lt;a href=&quot;https://space.stackexchange.com/a/8421/12102&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, while &lt;a href=&quot;https://solarsystem.nasa.gov/planets/deimos/facts&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deimos' orbit is nearly circular&lt;/a&gt;, the &lt;a href=&quot;https://solarsystem.nasa.gov/planets/phobos/facts&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbit of Phobos&lt;/a&gt; has an eccentricity of about 0.015, so some additional details need to be considered - was the starting orbit a circular flyby at Phobos' periapsis, or a matched ellipse for example. These would have a small additional impact on the delta-v needed.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastEditorUserId="12102" LastEditDate="2017-05-25T00:28:23.867" LastActivityDate="2017-05-25T00:28:23.867" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21626" PostTypeId="2" ParentId="21623" CreationDate="2017-05-24T23:23:10.017" Score="1" Body="&lt;p&gt;In short, no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Velocity implies direction. If you're in a circular orbit of a body (planet, star, etc), then you will maintain the same speed, but direction will constantly be changing as the body's gravitational pull curves your path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some cases you want to fight gravity, for instance hovering above an asteroid. In this case, since you're fighting gravity, this would require use of fuel.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-24T23:23:10.017" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21627" PostTypeId="2" ParentId="21623" CreationDate="2017-05-25T00:08:47.330" Score="-1" Body="&lt;p&gt;Although they  made some additional comments, the first 2 answers assume orbital travel - which is of course what we usually think of when we talk about microgravity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Microgravity is the condition we experience when we are under the influence of a constant gravitational field (possibly the earth's) as well as the influence of other minor fields and forces. Examples of these others are the distribution of the mass of the spacecraft we're in, and the friction of any residual atmosphere. In the case of the &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Reduced-gravity_aircraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;vomit comet&lt;/a&gt;&quot; they include the fact that the plane's trajectory never cancels the earth's field exactly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As per Newton's laws of gravitation and motion, under the influence of any gravitational field, micro or not, velocity cannot stay the same; it must change. In a perfect circular orbit, the speed will stay the same, but the direction will change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, if there is any gravity, micro or not, you &lt;strong&gt;always&lt;/strong&gt; need fuel to keep the same velocity. In a circular orbit your speed stays the same, but your velocity does not. In remote space, far from any stars, your velocity may be nearly constant, but it will still change slowly. Hence you still need a force, and thus fuel, to keep it constant.&lt;/p&gt;&#xA;" OwnerUserId="17390" LastActivityDate="2017-05-25T00:08:47.330" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21628" PostTypeId="2" ParentId="21590" CreationDate="2017-05-25T01:53:02.530" Score="2" Body="&lt;p&gt;According to those at Reddit, Falcon 1.0's vacuum Merlin engine did have gimballing turbopump exhaust for roll control. Maybe looking at how that was set up could help?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tHXGn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tHXGn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-25T01:53:02.530" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21629" PostTypeId="1" AcceptedAnswerId="21630" CreationDate="2017-05-25T02:37:11.187" Score="1" ViewCount="468" Body="&lt;p&gt;To be more specific: Suppose a space station is orbiting in a cislunar orbit (about 1000 miles away from the Moon). Now you keep adding more mass (huge cargo) to the cislunar space station. The space station departs for Mars with that huge cargo (much more mass). There will be fuel needed for putting the ship on the trajectory to Mars and initial change of velocity. But will the fuel required be much higher or the same if the mass is added, say, 1000 times more? Thank you for asking for clarification.&lt;/p&gt;&#xA;" OwnerUserId="19755" LastEditorUserId="195" LastEditDate="2017-05-25T13:24:59.087" LastActivityDate="2017-05-26T07:46:38.257" Title="How does fuel requirement change with mass when traveling from cislunar orbit to Mars?" Tags="|mars|cislunar|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21630" PostTypeId="2" ParentId="21629" CreationDate="2017-05-25T03:01:11.033" Score="2" Body="&lt;p&gt;To a first approximation, the amount of fuel required to send a spacecraft on a given trajectory (i.e. at a given velocity) is proportional to the mass of that spacecraft. So if the craft loaded with cargo is 1000 times the mass of the craft without cargo, it will need 1000 times as much fuel to make the same flight to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason for this is the Tsiolkovsky rocket equation,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \Delta v = v_\text{e} \ln \frac {m_0} {m_f} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $\Delta v$ is the required change in velocity, $v_\text{e}$ is the exhaust velocity of the rocket engine, $\ln$ is the natural logarithm operation, ${m_0}$ the initial mass of the spacecraft, and ${m_f}$ the final mass of the spacecraft after the fuel has been burned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shifting a spacecraft's trajectory from lunar orbit to Mars (or from Earth orbit to lunar orbit, or from Jupiter orbit to Saturn orbit, etc) requires a particular total change in the spacecraft's velocity which is independent of the spacecraft's mass; this is $\Delta v$. For a flight from lunar orbit to orbit around Mars the total required value is about 3400 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a given engine design, in vacuum, the exhaust velocity is more or less constant, which is $v_\text{e}$. For high-thrust chemical rockets this will likely be either about 3100 m/s (hypergolic fuels) or 4500 m/s (hydrogen fuel).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves the mass ratio ${m_0} / {m_f}$. Rearranging the equation, we get $$ \frac {m_0} {m_f} = e^{\Delta V\ / v_\text{e}} $$ or, for moon-Mars on hydrogen, &#xA;$$ \frac {m_0} {m_f} = e^{3400 / 4500}  = \frac {m_0} {m_f} = 2.128 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the final mass -- spacecraft plus cargo -- is 100 tons, then the initial mass -- spacecraft, cargo, and fuel -- must be 212.8 tons, i.e. 112.8 tons of fuel must be carried. If the final mass is 1000 tons, then the initial mass must be 2128 tons; 1128 tons of fuel must be carried. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice, it won't be perfectly proportional. If this is the same spacecraft in both cases, probably 90 tons of the final dry mass consists of fuel tanks and structure, so in the first case you'll have about 10 tons of payload, and 910 tons of payload in the second case, but if you're expecting to haul different amounts of payload, you'll design a ship with modular fuel tankage, so you don't have to carry the penalty of huge empty fuel tanks when sending a light load to Mars.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-25T21:06:13.460" LastActivityDate="2017-05-25T21:06:13.460" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21631" PostTypeId="1" CreationDate="2017-05-25T03:29:09.263" Score="1" ViewCount="54" Body="&lt;p&gt;Earth's magnetic field on its surface is 25 to 65 microtesla. Besides earth's atmospheric protection, majority of sun's harmful radiation on the earth is shielded by this small magnetic field. Such a small magnetic field can easily be produced by a permanent dipole magnet or a small current around the spacecrafts or space suits. Then why the space craft or space suites are not designed with magnetic fields?  &lt;/p&gt;&#xA;" OwnerUserId="19755" LastActivityDate="2017-05-25T03:29:09.263" Title="Radiation shield for Mars" Tags="|spacecraft|radiation|spacesuits|shielding|" AnswerCount="0" CommentCount="2" ClosedDate="2017-05-25T14:27:40.170" ContentLicense="CC BY-SA 3.0" />
  <row Id="21633" PostTypeId="2" ParentId="21618" CreationDate="2017-05-25T03:47:03.817" Score="3" Body="&lt;p&gt;It sounds like the Red Dragon is relying on aerobraking to exit the Hohmann transfer ellipse and soft land on Mars. I don't think the cube sat would want to use aerobraking to shed velocity. For one thing to get a good enough &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/the-need-for-better-alpha.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;alpha&lt;/a&gt; for a 1 mm/s^2 acceleration, you would need large and fragile solar arrays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't exploit the Oberth effect with 1 mm/s^2 acceleration. So you would need to gain the full 2.7 km/s Vinfinity before arriving at Mars sphere of influence. At 1 mm/s^2, it would take about 31 days to do 2.7 km/s delta V. So the cube sat would need to separate from the Dragon and start accelerating about a month before Mars rendezvous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the ship is loosely captured at the Sun Mars L1 (SML1) it is moving about .1 km/s wrt to Mars. Deimos is moving about 1.35 km/s wrt to Mars. So I would say about 1.2 km/s to get from SML1 to Deimos.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-05-25T06:09:44.070" LastActivityDate="2017-05-25T06:09:44.070" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21634" PostTypeId="1" AcceptedAnswerId="21646" CreationDate="2017-05-25T06:26:57.073" Score="0" ViewCount="155" Body="&lt;p&gt;The Channel News Asia article &lt;a href=&quot;http://www.channelnewsasia.com/news/technology/new-zealand-launches-into-space-race-with-3d-printed-rocket-8882210&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Zealand launches into space race with 3D-printed rocket&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;New Zealand has created new rocket legislation and set up a space agency in anticipation of becoming a low-cost space hub.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It sounds like space launch is new to New Zealand. What are the main issues addressed, or characteristics of the new body of rocket-related legislation that needed to be created?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-27T22:14:09.090" LastActivityDate="2017-05-27T22:14:09.090" Title="What are the main issues addressed by New Zealand's recent rocket legislation?" Tags="|law|space-agencies|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21635" PostTypeId="1" AcceptedAnswerId="21636" CreationDate="2017-05-25T06:28:56.107" Score="3" ViewCount="1028" Body="&lt;p&gt;The Channel News Asia article &lt;a href=&quot;http://www.channelnewsasia.com/news/technology/new-zealand-launches-into-space-race-with-3d-printed-rocket-8882210&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Zealand launches into space race with 3D-printed rocket&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ships and planes need re-routing every time a rocket is launched, which limits opportunities in crowded U.S. skies, but New Zealand, a country of 4 million people in the South Pacific, has only Antarctica to its south. The country is also well-positioned to send satellites bound for a north-to-south orbit around the poles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why is &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_Lab&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Lab&lt;/a&gt;'s New Zealand launch site good for polar orbits?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-22T13:17:15.950" LastActivityDate="2018-01-22T13:17:15.950" Title="Why is Rocket Lab's New Zealand launch site good for polar orbits?" Tags="|launch|launch-site|polar-orbit|rocketlab|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21636" PostTypeId="2" ParentId="21635" CreationDate="2017-05-25T06:52:58.910" Score="11" Body="&lt;p&gt;Launch sites far from the equator have to cancel out less of the Earth's rotation to get to a polar orbit, the Rocket Lab Launch Complex 1 at Mahia Peninsula is at 39 degrees south giving it an extra ~361 m/s, compared to ~465 m/s at the equator. This is ~104 m/s less velocity to cancel out compared to a launch site like the EAS launch site in Kourou, French Guiana. The fact that there is no one living north and south of the launch site means that they could launch both north and south to reach a polar orbit. The empty ocean to the east also means that they can target a large range of orbits without flying over inhabited land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also convenient islands further north in the Pacific to place ground stations on to control the launcher on ascent and place weather stations on etc (mentioned in passing in &lt;a href=&quot;https://spaceflightnow.com/2017/05/20/small-satellite-launcher-set-for-first-orbital-test-flight/&quot; rel=&quot;noreferrer&quot;&gt;this article on Electrons first test flight&lt;/a&gt;). I was not able to find a reference to which islands Rocket Labs used but island nations like Fiji and Samoa are quite close to a likely flight path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Zealand being a developed nation with good infrastructure and no imminent territorial conflicts is also a plus. This fact makes it easier to accommodate the people working on the project as well as easing the movement of materiel needed for the operation of the launch range.&lt;/p&gt;&#xA;" OwnerUserId="16505" LastEditorUserId="12102" LastEditDate="2017-12-19T09:17:49.293" LastActivityDate="2017-12-19T09:17:49.293" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21637" PostTypeId="2" ParentId="21618" CreationDate="2017-05-25T09:09:06.567" Score="1" Body="&lt;p&gt;Mars SOI radius: 576,000km &lt;a href=&quot;https://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)&quot; rel=&quot;nofollow noreferrer&quot;&gt;src&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars escape velocity: 5.03 km/s &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;src&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're coming in at 6km/s so you need to shed 1km/s to capture. At 1mm/s^2 that's 1mln seconds, or some 11 days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1mln seconds, at 5.5km/s average, you'll travel 5.5mln km, so you definitely can't just happily center your maneuver on Mars. You need to start braking at least 10 days, or 5mln km before closest approach. That way, you will capture into a very long elliptical orbit before exiting Mars SOI through the other side. It's wasteful (not benefiting from Oberth effect) but it's hard to do any other way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in &lt;em&gt;any&lt;/em&gt; orbit, you can try making this into the orbit you want. Perform inclination changes when passing equatorial plane as far from the planet as possible, then reduce periapsis by a burn near apoapsis, finally bring your cubesat into low orbit through multiple burns near periapsis.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-25T09:09:06.567" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21640" PostTypeId="1" CreationDate="2017-05-25T19:06:02.030" Score="4" ViewCount="167" Body="&lt;p&gt;The &lt;a href=&quot;https://en.m.wikipedia.org/wiki/IKAROS&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;IKAROS probe&quot;&gt;IKAROS solar sail&lt;/a&gt; entered its fifth hibernation in spring 2015, with a planned wake up in winter 2015 (Northern Hemisphere). However, there seems to be no news since then regarding the mission status. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any information on if the probe successfully woke up as expected?&lt;/p&gt;&#xA;" OwnerUserId="17395" LastEditorUserId="17395" LastEditDate="2017-05-26T05:12:29.787" LastActivityDate="2017-05-26T05:12:29.787" Title="Did IKAROS wake up from its 5th hibernation in Winter 2015?" Tags="|probe|interplanetary|solar-sail|deep-space-hibernation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21641" PostTypeId="2" ParentId="21640" CreationDate="2017-05-25T21:24:32.677" Score="2" Body="&lt;p&gt;According to JAXA, the fifth wakeup was expected in the winter of 2015, not 2016.&#xA;&lt;a href=&quot;http://global.jaxa.jp/projects/sat/ikaros/topics.html#topics4743&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://global.jaxa.jp/projects/sat/ikaros/topics.html#topics4743&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-25T21:24:32.677" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21642" PostTypeId="1" CreationDate="2017-05-25T21:36:59.007" Score="2" ViewCount="1323" Body="&lt;p&gt;For instance, I read that the Hercules X-265 rocket motor used in the Sprint missile was a solid double base propellant that used nitrocellulose with zirconium metal staples and nitroglycerin as a binder. How would sodium metal staples work instead of zirconium? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or better yet, suppose one were to build an experimental solid/liquid rocket motor that used molten sodium metal with liquid oxygen. Due to extreme temperature difference there might need to be an igniter to get the combustion started. Sodium is abundant, has a melting point close to the boiling point of water (~ 208 degrees Fahrenheit). To my knowledge, such an experimental rocket engine like this has never been tested. The main problem would be that the highly toxic exhaust, NaO, would react with moisture to form NaOH (lye or caustic soda). &lt;/p&gt;&#xA;" OwnerUserId="19775" LastEditorUserId="6816" LastEditDate="2017-05-26T01:07:55.860" LastActivityDate="2017-05-26T01:07:55.860" Title="Could Sodium metal be used in rocket fuel?" Tags="|fuel|solid-fuel|hybrid-rocket|" AnswerCount="0" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21643" PostTypeId="2" ParentId="5387" CreationDate="2017-05-25T21:38:19.037" Score="6" Body="&lt;p&gt;The issue is that the collection mechanism tends to produce more drag than thrust. &#xA;Robert Zubrin and Dana Andrews showed that this makes most Bussard Ramjet-type starship propulsion impractical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, this sort of mechanism is so good at producing drag, it actually &lt;em&gt;does&lt;/em&gt; function well for decelerating a starship once it reaches its destination, meaning that the starship can carry a larger payload or travel faster. This is called the magsail. Bit of history &lt;a href=&quot;https://www.centauri-dreams.org/?p=22138&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-25T21:38:19.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21644" PostTypeId="2" ParentId="21614" CreationDate="2017-05-25T22:16:23.367" Score="4" Body="&lt;p&gt;As others have said, no. There was nothing in place to allow a rescue mission in time to save a stranded crew. This was why the LEM had a separate ascent engine, tucked up in the ascent stage. No matter what happened during landing, it would be basically impossible to disable the ascent engine without killing the crew anyways.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-25T22:16:23.367" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21645" PostTypeId="1" AcceptedAnswerId="21650" CreationDate="2017-05-25T22:23:12.983" Score="2" ViewCount="332" Body="&lt;p&gt;The Channel News Asia article &lt;a href=&quot;http://www.channelnewsasia.com/news/technology/new-zealand-launches-into-space-race-with-3d-printed-rocket-8882210&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Zealand launches into space race with 3D-printed rocket&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ships and planes need re-routing every time a rocket is launched, which limits opportunities in crowded U.S. skies, but New Zealand, a country of 4 million people in the South Pacific, has only Antarctica to its south. The country is also well-positioned to send satellites bound for a north-to-south orbit around the poles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm curious if there is a &lt;em&gt;significant distinction or difference&lt;/em&gt; between launching a polar orbit mission south from New Zealand versus launching it north. Is it only a question of the area of overflight, or are there differences in the spaceflight dynamics or payload weight limits for example?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here &quot;polar orbit&quot; includes of course sun-synchronous LEO orbits and is not limited to exactly 90 degrees inclination.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-05-26T00:53:46.433" Title="Is there a significant difference between launching a polar orbit mission south from New Zealand versus launching it north?" Tags="|launch|low-earth-orbit|mission-design|launch-site|sun-synchronous|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21646" PostTypeId="2" ParentId="21634" CreationDate="2017-05-25T22:25:44.523" Score="1" Body="&lt;p&gt;I can't imagine that New Zealand had any rocket launches before Rocket Labs, especially with this quote - &quot;Nobody thought to get into the space industry (before now)&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It sounds like this legislation is setting up the basic framework required to get involved in the space industry. &lt;a href=&quot;http://www.mbie.govt.nz/info-services/sectors-industries/space&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Zealand's space agency&lt;/a&gt; was created in April 2016, &lt;em&gt;after&lt;/em&gt; ground broke on Launch Complex 1.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-25T22:25:44.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21647" PostTypeId="1" AcceptedAnswerId="21648" CreationDate="2017-05-25T22:40:27.997" Score="2" ViewCount="458" Body="&lt;p&gt;The BBC news article &lt;a href=&quot;http://www.bbc.com/news/science-environment-40053527&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno peers below Jupiter's clouds&lt;/a&gt; includes several very interesting images from the Juno spacecraft. The one below is a photo of Jupiter's faint ring.&lt;/p&gt;&#xA;&lt;p&gt;The article says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;One very smart picture taken by Juno and released on Thursday showed the ring of dust that surrounds Jupiter. It is not well known that Jupiter has a ring, but it does. What was clever was getting the familiar stars of the Orion constellation to be in on the shot as well.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;This is the first image of Jupiter's ring that has ever been collected from the inside of it looking out,&amp;quot; said mission scientist Heidi Becker from Nasa.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Juno is 3,000 miles from the planet when we took this picture. So, what you're looking at is a ring of dust that's 40,000 miles away and stars that are hundreds of light-years away, all in the same picture.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is this the first photo of Jupiter's ring(s) from deep space? (Hubble doesn't count)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OPG9L.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OPG9L.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-18T01:46:36.700" LastActivityDate="2021-08-18T01:46:36.700" Title="Is this the first photo of Jupiter's ring(s) from deep space?" Tags="|photography|imaging|jupiter|spaceflight-firsts|rings|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21648" PostTypeId="2" ParentId="21647" CreationDate="2017-05-25T22:46:59.513" Score="7" Body="&lt;p&gt;It has been photographed before. See this image, &lt;a href=&quot;https://www.nasa.gov/multimedia/imagegallery/image_feature_995.html&quot; rel=&quot;noreferrer&quot;&gt;from NASA/ Galileo&lt;/a&gt;. The difference with the Juno image is it was taken from inside of the orbit of the ring, while the Galileo was taken from further away from Jupiter than the ring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tQuuG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tQuuG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, it was Voyager 1 that first detected the ring system when it did a flyby of Jupiter. And it has been photographed by New Horizons as well. See &lt;a href=&quot;https://en.wikipedia.org/wiki/Rings_of_Jupiter&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; for more, including more pictures.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-05-25T23:13:51.780" LastActivityDate="2017-05-25T23:13:51.780" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21649" PostTypeId="1" CreationDate="2017-05-25T23:08:53.130" Score="3" ViewCount="679" Body="&lt;p&gt;Between &lt;code&gt;03:00&lt;/code&gt; and &lt;code&gt;05:00&lt;/code&gt; in the BBC's radio program &lt;a href=&quot;http://www.bbc.co.uk/programmes/p0537nh9&quot; rel=&quot;nofollow noreferrer&quot;&gt;Giant hurricanes at Jupiter’s poles&lt;/a&gt;, BBC's science correspondent Jonathan Amos summarizes the result of higher than expected multipole intensity of Juno's magnetic field measurements during close passes of the planet, and agrees with the paraphrasing of the potential source of the planet's magnetic field as:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...'rivers of metallic hydrogen' in the atmosphere of Jupiter...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How close of an analogy is this, given what was actually measured? And for that matter, could someone describe what was actually measured? Was it a localized &quot;blip&quot; or does this really show up in the multipole expansion of Jupiter's entire planetary field?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional information:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a bit more in the BBC's &lt;a href=&quot;http://www.bbc.com/news/science-environment-40053527&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno peers below Jupiter's clouds&lt;/a&gt; (&lt;em&gt;written by&lt;/em&gt; Jonathan Amos), and probably much more in the articles just published in Science &lt;a href=&quot;http://science.sciencemag.org/content/356/6340/821&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter’s interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft&lt;/a&gt; and also &lt;a href=&quot;http://science.sciencemag.org/content/356/6340/826&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jupiter’s magnetosphere and aurorae observed by the Juno spacecraft during its first polar orbits&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;a href=&quot;http://www.bbc.com/news/science-environment-40053527&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b5hmg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b5hmg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-08-17T20:22:12.203" LastActivityDate="2018-08-17T21:18:44.140" Title="Better understanding of the &quot;Rivers of metallic hydrogen&quot; in the atmosphere of Jupiter?" Tags="|jupiter|magnetic-field|hydrogen|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="21650" PostTypeId="2" ParentId="21645" CreationDate="2017-05-26T00:53:46.433" Score="4" Body="&lt;p&gt;There shouldn't be a difference in whether the mission is technically feasible. The flight plan would be altered as deflection would be mirrored, but I can't see any reason engineering aspects would differ.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, I can't imagine that it would ever launch north.The reason the launch site was chosen is because there's nothing south of it until Antarctica. The city of Gisborne is about 40 miles to the NNE of Launch Complex 1, and Fiji would be directly under the flight path as well. That's a significant domestic population center and a number of towns belonging to a sovereign nation, none of which you want to drop even a small rocket on. They might not be as &lt;a href=&quot;http://io9.gizmodo.com/5909476/remember-the-time-we-bombed-mexico-with-german-rockets&quot; rel=&quot;nofollow noreferrer&quot;&gt;forgiving as Mexico&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-26T00:53:46.433" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21651" PostTypeId="1" AcceptedAnswerId="21654" CreationDate="2017-05-26T01:37:32.813" Score="8" ViewCount="1927" Body="&lt;p&gt;That is, does NASA still have all the training equipment, computers and so forth that would be required for a new moon program, or would all of that have to be rebuilt as well as building new rockets?&lt;/p&gt;&#xA;" OwnerUserId="19777" LastEditorUserId="19777" LastEditDate="2017-05-26T01:48:44.707" LastActivityDate="2017-05-26T03:34:19.230" Title="Do we still have all the equipment to go back to the moon?" Tags="|the-moon|crewed-spaceflight|nasa|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21652" PostTypeId="2" ParentId="21651" CreationDate="2017-05-26T01:53:46.103" Score="4" Body="&lt;p&gt;First, just about none of the original equipment is in usable condition. It may not be completely impossible to refurbish it, but it would be so far beyond the realm of the achievable that is might as well be. If you really had your heart set on it, some Saturn V's were built for Apollo 18-20 but never flown. Not sure about LEM and capsules though.&#xA;Second, were all that in usable condition, there's no way that it would be politically viable to make people's lives depend, very visibly, on technology 40 years obsolete.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-26T01:53:46.103" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21653" PostTypeId="1" AcceptedAnswerId="21795" CreationDate="2017-05-26T03:24:12.260" Score="5" ViewCount="297" Body="&lt;p&gt;A while back the &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dawn spacecraft&lt;/a&gt; took some photographs of the surface of Ceres while it was first approaching, and there were these surprising, mysterious white dots that nobody could explain. For example, in Sky &amp;amp; Telescope's &lt;a href=&quot;http://www.skyandtelescope.com/astronomy-news/bright-spots-on-ceres-03032015/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bright Spots on Ceres Intrigue Scientists&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NASA's Dawn spacecraft is just a few days from arriving at its second and final objective, asteroid 1 Ceres, on March 6th. Robert Mase, who manages the mission at the Jet Propulsion Laboratory in California, says Dawn will be captured into orbit at 4:20 a.m. (12:20 Universal Time). That's the first critical step in what promises to be an intensive, 16-month-long investigation.&lt;/p&gt;&#xA;&lt;p&gt;But images recently captured by the spacecraft's German-built camera already &lt;strong&gt;have mission scientists scratching their collective heads&lt;/strong&gt;. Tucked inside a 57-mile-wide crater are two tiny spots far brighter than their surroundings. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Unfortunately, after that school commitments started to pile up, and I was unable to continue following closely and watch the story unfold. Has anyone figured out what those white dots are? What are the current theories on it?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MTSSl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MTSSl.jpg&quot; alt=&quot;Early images of the white dots on Ceres&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Figure caption:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NASA's Dawn spacecraft recorded this view of asteroid 1 Ceres on February 19th while still 29,000 miles (46,000 km) from its destination. Scientists were surprised to see the pair of bright spots inside a medium-size crater.&#xA;NASA / JPL / UCLA / MPS / DLR / IDA&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; from &lt;a href=&quot;https://dawn.jpl.nasa.gov/multimedia/images/image-detail.html?id=PIA21405&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://dawn.jpl.nasa.gov/multimedia/images/image-detail.html?id=PIA21405&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BVFYf.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BVFYf.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19777" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-04T16:05:47.517" Title="Those bright spots on Ceres — are scientists still &quot;scratching their heads&quot;?" Tags="|ceres|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21654" PostTypeId="2" ParentId="21651" CreationDate="2017-05-26T03:28:41.537" Score="9" Body="&lt;p&gt;No Apollo training equipment is still functional, at least at Johnson Space Center (JSC).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo Mission Simulators were removed and sent to museums. The facility space they once occupied in Building 5 was then filled with the Shuttle Mission Simulators. They have now been removed and sent to museums.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EmOBIm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EmOBIm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; (personal photo)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sole remaining Lunar Landing Training Vehicle hangs from the ceiling in the lobby of JSC's Teague Auditorium. It is in a safed museum configuration. It is not inconceivable that it could be returned to flight status but it is unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2oxEQ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2oxEQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; (personal photo)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Skylab simulator was removed from Building 5 and placed on display in Space Center Houston.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-05-26T03:34:19.230" LastActivityDate="2017-05-26T03:34:19.230" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21656" PostTypeId="2" ParentId="21629" CreationDate="2017-05-26T07:34:17.733" Score="0" Body="&lt;p&gt;Here is a version of the rocket equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$Mf = 1 - e^{-delta v/v_{exhaust}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mf is the mass fraction that is propellent. The rest of the mass fraction will be rocket and payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;EML2&lt;/a&gt;, delta V for Trans Mars Insertion (TMI) is about 1 km/s. Exhaust velocity of hydrogen/oxygen bi-propellent is about 4.4 km/s. Let's plug those numbers into the rocket equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$Mf = 1 - e^{-1/4.4}$&lt;br&gt;&#xA;$Mf = 1 - .8$&lt;br&gt;&#xA;$Mf = .2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case propellent would be 1/5 the mass and the remaining 4/5 would be rocket and payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If rocket and payload is 4 tonnes, you'd need   1 tonne of propellent.&lt;br&gt;&#xA;If rocket and payload is 400 tonnes, you'd need 100 tonnes of propellent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where mass savings might be realized is what needs to be lifted off from earth's surface. Let's take a look at what it takes to get from earth to low earth orbit. Earth to LEO is about 9 km/s. To minimize gravity loss higher thrust propellents like oxygen/rp-1 is used. This has about 3 km/s exhaust velocity.&lt;br&gt;&#xA;$Mf = 1 - e^{9/3}$&lt;br&gt;&#xA;$Mf = .95$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to get to LEO a rocket and payload of 1 tonne would take 19 tonnes propellent.&lt;br&gt;&#xA;A rocket and payload of 100 tonnes would need 1900 tonnes propellent&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth's surface to EML2 is about 12.5 km/s, only a tad less than earth's surface to TMI. But if we could load up on propellent and life support consumables at EML2, a much lighter vehicle could be launched to EML2 vs luanching a fully stocked and fueled vehicle from earth's surface to TMI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EML2 is about 2.5 km/s from the moon's surface. There may be rich volatile deposits at the lunar poles. A volatile rich asteroid parked in a loose lunar orbit would be even closer to EML2.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-05-26T07:46:38.257" LastActivityDate="2017-05-26T07:46:38.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21657" PostTypeId="1" AcceptedAnswerId="21659" CreationDate="2017-05-26T11:23:45.507" Score="24" ViewCount="5697" Body="&lt;p&gt;The answer to &lt;a href=&quot;https://space.stackexchange.com/questions/21651/do-we-still-have-all-the-equipment-to-go-back-to-the-moon&quot;&gt;Do we still have all the equipment to go back to the moon?&lt;/a&gt; is an unambiguous &quot;no&quot;. Most of the equipment went to museums or got scrapped. But do we still have all the know-how?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say, a political miracle happens, and the Congress decides &quot;Apollo 18 should happen ASAP&quot; and it should be an exact duplicate of Apollo 17. No R&amp;amp;D work is expected, just rebuilding the Apollo 17 from scratch. Rebuild/restart all the manufacturing, reopen all the closed facilities, all the engineering work to get it all up and running basing on existing documentation, training based on old procedures, astronauts training on equipment rebuilt from old blueprints, zero new research. If contractors / 3rd party manufacturers no longer produce something, they'll be paid to reopen the production, rebuilding production facilities and retraining the crew if needed, but not &quot;reinvent&quot; the technology if it was lost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do we have all the data to do this, or was some of it lost, say, passed as word-of-mouth and forgotten, or destroyed along with closure of some facilities? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the duplicate suggestion, the gist of difference between the questions is: the Saturn one asks &quot;Why don't we&quot;, with answer &quot;We don't want to - It's not practical.&quot; Mine asks &quot;But &lt;em&gt;can we&lt;/em&gt;, assuming we'd (somehow) want to and had the budget?&quot;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="415" LastEditDate="2017-05-28T14:22:06.950" LastActivityDate="2017-05-28T19:43:29.063" Title="Do we still have all the blueprints to go to the Moon?" Tags="|the-moon|crewed-spaceflight|nasa|" AnswerCount="5" CommentCount="10" ClosedDate="2017-05-27T15:08:24.123" ContentLicense="CC BY-SA 3.0" />
  <row Id="21658" PostTypeId="2" ParentId="21657" CreationDate="2017-05-26T12:44:12.373" Score="7" Body="&lt;p&gt;This question is possibly too broad because there are many forms of &quot;know-how&quot;. Also, how much of the past do you want to recreate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the key skills that would have been lost are the manufacturing skills - on the shop floor and management of the manufacturing processes. If need be, that could be redeveloped. But would you want to recreate the factories and equipment of the 1950s and 1960s or use modern equipment in existing factories?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other thing is, if you wanted to make a Saturn V rocket and the lunar module and the lunar rover would you want to use the same electrical and electronic systems. Do the transistors and computer chips of that era still exist. Would you want to recreate them from scratch or would you want to modernize the electronic systems with components currently available?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While on the subject of computers, what about computer software? Do you want to use the software of the late 1960s and early 1970s or would you prefer it to be re-written using computer languages that are currently being used? Would you prefer the software to do just want it did during the Apollo era or would you like it to be able to do a more than was possible back then? Do you want to be able to recreate the computer overflow error that occurred during the landing of Apollo 11 because too many devices were left switched on?&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2017-05-26T12:44:12.373" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="21659" PostTypeId="2" ParentId="21657" CreationDate="2017-05-26T12:48:21.520" Score="30" Body="&lt;p&gt;There's a related question on the &lt;a href=&quot;https://space.stackexchange.com/questions/20302/were-the-saturn-v-construction-plans-destroyed/20306#20306&quot;&gt;Saturn V&lt;/a&gt; which addresses some of your question.&lt;br&gt;&#xA;In general, it's difficult to document a complicated system so thoroughly that there will be no surprises at all when you rebuild from the documentation. Blueprints are one thing, but most of the parts of a Saturn V were handbuilt and needed minor adjustments during assembly. I doubt all of those were documented rigorously. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was standard procedure back then. A recent example of how this can resurface and bite you in the ass is the &lt;a href=&quot;http://www.aeroflight.co.uk/aircraft/types/type-details/bae-systems-nimrod.htm&quot; rel=&quot;noreferrer&quot;&gt;BAE Nimrod MRA.4 saga&lt;/a&gt;: the plan was to fit 1960-vintage aircraft with a new wing (in order to accommodate new engines). The new wing was designed in CAD based on measurements from one airframe. When they built the second new wing and tried to mate it to the second airframe, they found it didn't fit. The airframes were different.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo program had tens of thousands of subcontractors (from huge aerospace firms down to tiny mom-and-pop shops), many of which are long gone. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-05-26T12:48:21.520" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21660" PostTypeId="2" ParentId="12927" CreationDate="2017-05-26T15:06:30.550" Score="1" Body="&lt;p&gt;The primary issue is that most missions, such as Cassini, are not particularly useful once consumables such as propellant or RTG lifetime run out. Adding consumables that enable use after the mission extensions are over means redesigning the mission, and in that case you might as well make the main mission longer if that was feasible in the first place...&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-26T15:06:30.550" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21661" PostTypeId="2" ParentId="21657" CreationDate="2017-05-26T16:23:01.597" Score="8" Body="&lt;p&gt;This is, of course, possible. For a very special meaning of the word &quot;possible&quot;: extremely &lt;strong&gt;&lt;em&gt;expensive&lt;/em&gt;&lt;/strong&gt;, &lt;strong&gt;&lt;em&gt;dangerous&lt;/em&gt;&lt;/strong&gt; (to the astronauts), and &lt;strong&gt;&lt;em&gt;pointless&lt;/em&gt;&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question reminded me of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tupolev_Tu-4&quot; rel=&quot;noreferrer&quot;&gt;Tu-4&lt;/a&gt; - the Soviet reverse-engineered copy of the American-made &lt;a href=&quot;https://en.wikipedia.org/wiki/Boeing_B-29_Superfortress&quot; rel=&quot;noreferrer&quot;&gt;Boeing B-29 Superfortress&lt;/a&gt;. Stalin had 3 B-29 which emergency-landed in the USSR during WW2, and he ordered &lt;a href=&quot;https://en.wikipedia.org/wiki/Tupolev&quot; rel=&quot;noreferrer&quot;&gt;Tupolev&lt;/a&gt; to copy them. This turned out to be a &lt;strong&gt;brilliant&lt;/strong&gt; step: copying the plane (as opposed to creating a &quot;better version suitable to the existing Soviet realities&quot;) required creating whole new industries and gave a huge boost the the Soviet aircraft technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Copying Apollo would have the &lt;strong&gt;opposite&lt;/strong&gt; effect: resurrecting obsolete technology instead of creating the new ones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose we want to repeat &lt;a href=&quot;https://en.wikipedia.org/wiki/Ferdinand_Magellan&quot; rel=&quot;noreferrer&quot;&gt;Magellan&lt;/a&gt;'s circumnavigation (yes, I know he died on the way). Would we build a modern ship to do that or re-create his caravels?&lt;/p&gt;&#xA;" OwnerUserId="380" LastEditorUserId="380" LastEditDate="2017-05-26T16:28:40.827" LastActivityDate="2017-05-26T16:28:40.827" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21662" PostTypeId="2" ParentId="21657" CreationDate="2017-05-26T16:52:10.387" Score="5" Body="&lt;p&gt;&lt;strong&gt;No&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo missions were huge, custom built machines made in very small quantity built on the cutting edge of what was possible.  Presumably much of the required infrastructure was shut down and or made obsolete.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if we had the exact specs for every part, and the specs on how to make the machines that make that part (so on and so forth), we no longer have the people that made it possible.  If a single engineer, welder, QA tester, supplier (or so on) has even one slightly different interpretation of a task than anticipated it could end in failure.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX's recent failure due to a strut comes to mind... and that was the same company, building the same rocket with the same team using the same supplier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Think of it as making a cake&lt;/strong&gt;... involving tens of thousands of people... decades later... with different training and tools... where any single minor change could end up as RUD.  I don't care how could good recipe is... I'm not baking that.&lt;/p&gt;&#xA;" OwnerUserId="6250" LastEditorUserId="6250" LastEditDate="2017-05-26T18:03:13.797" LastActivityDate="2017-05-26T18:03:13.797" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21663" PostTypeId="1" AcceptedAnswerId="21664" CreationDate="2017-05-26T18:12:15.987" Score="2" ViewCount="101" Body="&lt;p&gt;With the recent interest in traveling to Mars, it would seem apparent that terraforming would be an integral part of achieving this goal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it a requirement to encompass the whole planet, or is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Closed_ecological_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;CES&lt;/a&gt; considered terraforming as well? What are the current active or proposed projects to accomplish terraforming?&lt;/p&gt;&#xA;" OwnerUserId="14137" LastEditorUserId="12102" LastEditDate="2017-05-27T08:17:36.573" LastActivityDate="2017-05-27T08:17:36.573" Title="Are closed ecological systems considered terraforming?" Tags="|terraforming|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21664" PostTypeId="2" ParentId="21663" CreationDate="2017-05-26T18:55:37.653" Score="4" Body="&lt;p&gt;The distinction between a closed ecosystem and paraterraforming is whether the purpose is to create a livable habitat or to alter the planet's environment. I would argue that domed cities, etc are not terraforming, but if you somehow start doming massive regions, then it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess to me the difference is whether you're creating a livable environment to fill your short-term needs, or whether you're transforming regions that will remain unused for the foreseeable future.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-26T18:55:37.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21665" PostTypeId="2" ParentId="21614" CreationDate="2017-05-26T20:45:21.100" Score="1" Body="&lt;p&gt;Basically, NO.  That is why the LEM Ascent module engine was designed to be extremely simple: Propellants were  pressurized by a single helium tank and a single valve. ( fewer points of failure ).  Fuel and oxidizer were self-igniting.  Once that valve opened, they were going up.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At least that is what I read years ago.&lt;/p&gt;&#xA;" OwnerUserId="19793" LastActivityDate="2017-05-26T20:45:21.100" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21667" PostTypeId="2" ParentId="9273" CreationDate="2017-05-27T02:20:50.257" Score="-3" Body="&lt;p&gt;The thing about a rocket is most of the fuel spent is getting out of the atmosphere and a large portion of the energy spent is wasted on aerodynamic forces rockets are limited to how fast they can go in the atmosphere by this and if they could leave the atmosphere at a faster speed the effects of gravity would be lowered it is all a balancing game and aerodynamics are one of the two major resistances during flight.&lt;/p&gt;&#xA;" OwnerUserId="19799" LastActivityDate="2017-05-27T02:20:50.257" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21669" PostTypeId="1" AcceptedAnswerId="21673" CreationDate="2017-05-27T06:27:38.143" Score="10" ViewCount="1300" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/9274/12102&quot;&gt;This excellent answer&lt;/a&gt; to the question &lt;a href=&quot;https://space.stackexchange.com/q/9273/12102&quot;&gt;Why is Falcon 9's shape so tall and skinny?&lt;/a&gt; mentions California, Florida, Texas and Alabama, within and between-which there must be at least a few limiting underpasses through which Falcon-9 must go, &quot;&lt;a href=&quot;https://youtu.be/IMxWLuOFyZM?t=124&quot; rel=&quot;noreferrer&quot;&gt;ere the other side he see&lt;/a&gt;&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If aspect ratio were not constrained by transportation on fascinating public roads and infrastructure (if for example there were just a bunch of very long, rocket-friendly but otherwise &lt;a href=&quot;http://www.wired.co.uk/article/elon-musk-tunnel-boring&quot; rel=&quot;noreferrer&quot;&gt;boring tunnels&lt;/a&gt; instead), would the Falcon 9 have evolved to be substantially shorter and/or substantially larger in diameter than it's svelte 3.66 meter OD? If the plan to go to Falcon Heavy was still in place, what might Falcon 9's dimensions have been?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that the extra propellants accommodated by  every 10 cm in additional diameter might more than compensate for the additional structural weight and aerodynamic body drag, so I don't see anything that would be preventing it from getting somewhat wider, perhaps a lot wider! But I'm no rocket scientist, so I'm asking here.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Of course, if it weren't so skinny, we might not be able to ask questions like &lt;a href=&quot;https://space.stackexchange.com/q/20980/12102&quot;&gt;Why does the CRS-8 Falcon 9 rocket appear to be swaying before launch?&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The unconfirmed SpaceX adjective &quot;&lt;strong&gt;flying noodle&lt;/strong&gt;&quot; comes from comments below the &lt;em&gt;as yet still unanswered&lt;/em&gt; question &lt;a href=&quot;https://space.stackexchange.com/q/20986/12102&quot;&gt;Just how much can tall skinny rockets bend? (roughly, safely)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; GIF of Falcon 9 faster than normal speed to make the motion more visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sk4jZ.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sk4jZ.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-06-09T17:14:50.177" LastActivityDate="2018-06-09T17:14:50.177" Title="If not constrained by underpasses, etc., would Falcon 9 have been less of a flying noodle?" Tags="|spacex|design|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="21670" PostTypeId="2" ParentId="21614" CreationDate="2017-05-27T06:52:45.223" Score="0" Body="&lt;p&gt;Dig this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Rescue Agreement was considered and negotiated by the Legal&#xA;  Subcommittee from 1962 to 1967. Consensus agreement was reached in the&#xA;  General Assembly in 1967 ( resolution 2345 (XXII)), and the Agreement&#xA;  entered into force in December 1968. The Agreement, elaborating on&#xA;  elements of articles 5 and 8 of the Outer Space Treaty, provides that&#xA;  States shall take all possible steps to rescue and assist astronauts&#xA;  in distress and promptly return them to the launching State, and that&#xA;  States shall, upon request, provide assistance to launching States in&#xA;  recovering space objects that return to Earth outside the territory of&#xA;  the Launching State.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;http://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/introrescueagreement.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="3522" LastEditorUserId="12778" LastEditDate="2018-12-27T05:21:54.643" LastActivityDate="2018-12-27T05:21:54.643" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="21671" PostTypeId="1" AcceptedAnswerId="21672" CreationDate="2017-05-27T07:28:38.537" Score="0" ViewCount="380" Body="&lt;p&gt;As mentioned in &lt;a href=&quot;http://chat.stackexchange.com/transcript/message/37637614#37637614&quot;&gt;The Pod Bay&lt;/a&gt; Nasa Spaceflight has written: &lt;a href=&quot;https://www.nasaspaceflight.com/2017/05/soyuz-2-1b-launches-tundra-missile-detection-spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz 2-1B launches Tundra missile detection spacecraft&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are &lt;em&gt;Tundra missiles&lt;/em&gt;, and why are they so hard to detect that there would have to be a special spacecraft designed and put into orbit to detected them?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-05-27T08:53:21.457" Title="What are &quot;Tundra missiles&quot; and why are they so hard to detect?" Tags="|spacecraft|space-surveillance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21672" PostTypeId="2" ParentId="21671" CreationDate="2017-05-27T08:45:38.510" Score="6" Body="&lt;p&gt;The spacecraft's name is Tundra (or EKS). This is a ballistic missile detection system, designed to detect missiles launched from the US, or China (or their submarines, or insert rogue state here) and heading for Russia with their nuclear warheads.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key to surviving a nuclear attack is early warning. Flight time is only 30 minutes to go halfway across the world, so you want to detect these missiles as early as possible. When the launch site is not visible from your territory, you have to use a satellite to monitor the launch site.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-05-27T08:53:21.457" LastActivityDate="2017-05-27T08:53:21.457" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21673" PostTypeId="2" ParentId="21669" CreationDate="2017-05-27T09:30:20.140" Score="8" Body="&lt;p&gt;Without knowing how SpaceX arrived at the 3.66 m diameter, this has to be speculation.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Most rockets have a lower fineness ratio than the F9v1.2 (i.e. they're fatter). That makes it likely there's an advantage to designing a fatter rocket. A larger frontal area due to a larger diameter (=more air resistance) is compensated for by less weight (fatter tanks have a better volume/surface area ratio) and/or less form drag. &lt;/li&gt;&#xA;&lt;li&gt;The F9 gained ~14 m in length between v1.0 and v1.2. A reasonable fineness ratio became a marginal one. I've no idea how much of the performance increase of v1.2 was foreseen by the time the body diameter of v1.0 was decided upon. Or how constrained the v1.2 is by its length limit: a longer version might have better performance, but has been decided against because it would be too expensive to transport. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What diameter would have been chosen? There are basically 2 choices:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;make everything the same diameter as the payload fairing (5.4 m) &lt;/li&gt;&#xA;&lt;li&gt;choose a smaller diameter than the payload fairing. Depends on the aerodynamics of this solution (I suspect there's some extra drag due to the decrease in diameter).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-05-27T09:30:20.140" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21675" PostTypeId="2" ParentId="21657" CreationDate="2017-05-27T12:02:02.450" Score="2" Body="&lt;p&gt;Every single aspect involved new engineering. I worked on the ablative heat shield, doing just one small corner of the testing. We were constantly trying new combinations of materials. All that work would need to be re-created with newer methods. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It &lt;em&gt;IS&lt;/em&gt; rocket science. Study the history of rocketry, where the difficulty is to get predictable behavior out of hypergolic chemistry. Look up the term hypergolic chemistry. There are some neat videos, including the film of the lunar lander taking off from the moon. &lt;/p&gt;&#xA;" OwnerUserId="19804" LastActivityDate="2017-05-27T12:02:02.450" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21676" PostTypeId="2" ParentId="21669" CreationDate="2017-05-27T14:19:17.693" Score="8" Body="&lt;p&gt;Without the transport constraints, a wider rocket would make sense, but I think SpaceX would still end up with a flying noodle, albeit a much larger one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Changing the diameter of a rocket or aircraft usually ends up being a completely new design - for example the existing landing legs won't fit because the larger rocket is a different shape. You don't do that unless you really have to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, you increase the length - just add another identical fuselage section, and as long as it hasn't gotten too noodly, it will fly much the same way and use all the same parts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The noodle then represents the biggest rocket you can build without either building a new factory or having it fold in half on the launchpad. If the boss has a thing for absurdly large rockets, you will be building a noodle at some point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the real world, the diameter was chosen based on road size. Without that limit, the relevant constraint becomes what you want your largest rocket to achieve. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For SpaceX, that means ITS - a vehicle with a volume of approximately 9000 cubic meters, compared to around 500 for F9.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what diameter would SpaceX choose? Some possibilities:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;4m - Essentially the real world design. Easy to transport, but Falcon Heavy has a bunch of engineering challenges with loads in different directions from a single core and an 800m tall ITS simply won't work, so you'll need to start over with that.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;12m - ITS diameter. at 2m, F9 would resemble a pancake more than a rocket.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;10m - ITS volume is achievable with a 115m vehicle, about the same shape as a real F9. The F9 first stage equivalent is still decidedly stubby at under 10m - not ideal in terms of drag, and possibly impractical to fit standard shaped tanks in, but reusability means that it's ok to use a larger rocket if necessary.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;7m - Possibly a reasonable compromise - F9 would still be quite stubby, but Falcon Heavy capacity could be simply a lengthened rocket, and ITS could be 3 noodly cores together rather than a comppletely new design.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18207" LastEditorUserId="18207" LastEditDate="2017-05-27T23:18:44.820" LastActivityDate="2017-05-27T23:18:44.820" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21677" PostTypeId="1" CreationDate="2017-05-27T18:48:00.307" Score="1" ViewCount="1152" Body="&lt;p&gt;From the book &quot;The Science of Interstellar&quot; by Kip Thorne. On Chapter 4 of the book, there is a diagram on the explanation of moon's gravity influence on Earth's ocean.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/9oenG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9oenG.jpg&quot; alt=&quot;Tidal Gravity&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where diagram A is labeled as the &quot;Usual Viewpoint&quot; and diagram B is labeled as &quot;Earth's Viewpoint&quot; In my mind experiment, I have thought diagram A is what tidal gravity supposed to look like yet I do not understand how scientists come up with diagram B.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the book, it explains as &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;With their[diagram A]'s average subtracted away; that is, it feels a&#xA;  stretch toward and away from the moon, and a squeeze on its lateral sides&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="18270" LastActivityDate="2017-05-27T22:56:55.183" Title="Different perspective of tidal force on Earth's ocean" Tags="|the-moon|gravity|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21678" PostTypeId="2" ParentId="21677" CreationDate="2017-05-27T20:52:50.853" Score="1" Body="&lt;p&gt;Subtract from the above diagram, which is not the 'usual' viewpoint, but the point of view from the moon. It simply derives from the vectorial identity for the force of gravity, that the moon exerts on any fluid particle on Earth&#xA;$\vec F_g = - \frac{GM_{moon}}{r^3} \cdot \vec r$, where $\vec r$ is the moon-centric radius vector.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get to the earth-centric view (B), you subtract the average, which is the central arrow at Earth's center in the moon-centric view (A).&#xA;Subtraction here means vectorial subtraction of course, if you need help with that &lt;a href=&quot;https://www.varsitytutors.com/hotmath/hotmath_help/topics/adding-and-subtracting-vectors&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; link might help.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastEditorUserId="195" LastEditDate="2017-05-27T22:56:55.183" LastActivityDate="2017-05-27T22:56:55.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21680" PostTypeId="1" CreationDate="2017-05-28T03:46:04.837" Score="1" ViewCount="245" Body="&lt;p&gt;Reading about &lt;a href=&quot;https://en.wikipedia.org/wiki/Anemometer#Ping-pong_ball_anemometers&quot; rel=&quot;nofollow noreferrer&quot;&gt;ping pong ball anemometers&lt;/a&gt; I discovered that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Phoenix_(spacecraft)#Meteorological_station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Phoenix Lander&lt;/a&gt; had an anemometer of this design, and there are images and GIFs of it moving. When imaged from one or two different angles, it gives an estimate of wind velocity and direction. But imaging is a lot of bandwidth and memory (especially back then).&lt;/p&gt;&#xA;&lt;p&gt;There was also a LIDAR instrument, but I don't know how demanding it was for power, or bandwidth, and if it could be pointed in at least a few directions fast enough to establish the direction of the wind, nor do I know if the direction was important.&lt;/p&gt;&#xA;&lt;p&gt;After reading Wikipedia's article on &lt;a href=&quot;http://marslab.au.dk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phoenix's Meteorological station&lt;/a&gt; I get the idea that they were looking for times when the wind was minimal, so that they could do sample loading without losing the smallest particles due to wind.&lt;/p&gt;&#xA;&lt;p&gt;Wouldn't a &lt;a href=&quot;https://en.wikipedia.org/wiki/Anemometer#Hot-wire_anemometers&quot; rel=&quot;nofollow noreferrer&quot;&gt;hot-wire anemometer&lt;/a&gt; have used much less bandwidth and been easier to read? I think that as long as the air temperature is measured approximately, and certainly the pressure would be available from the weather station, this should work fine.&lt;/p&gt;&#xA;&lt;p&gt;Also this looks a bit on the heavy side for a payload to Mars, compared to a hot wire, but that's just my perception.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FJbPa.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FJbPa.gif&quot; alt=&quot;GIF from images transmitted from Mars of the telltale on the Phoenix Mars Lander&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF from images transmitted from Mars. From the &lt;a href=&quot;http://marslab.au.dk/research/instrument-development/the-telltale-project/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Telltale project&lt;/a&gt; page on the &lt;a href=&quot;http://marslab.au.dk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Simulation Laboratory&lt;/a&gt;. Also &lt;a href=&quot;https://web.archive.org/web/20120220080017/http://www.marslab.dk/TelltaleProject.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;archived here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xCWAk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xCWAk.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Instrument as delivered to NASA. From the &lt;a href=&quot;http://marslab.au.dk/research/instrument-development/the-telltale-project/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Telltale project&lt;/a&gt; page on the &lt;a href=&quot;http://marslab.au.dk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Simulation Laboratory&lt;/a&gt;. Also &lt;a href=&quot;https://web.archive.org/web/20120220080017/http://www.marslab.dk/TelltaleProject.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;archived here&lt;/a&gt;. That's a 45 degree mirror below the telltale providing a simultaneous view from below.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-06-09T02:32:51.527" LastActivityDate="2021-06-09T02:32:51.527" Title="Was the telltale on the Mars Phoenix Lander used for meteorology? Why not a hot wire anemometer instead?" Tags="|mars|weather|" AnswerCount="0" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="21681" PostTypeId="2" ParentId="21614" CreationDate="2017-05-28T03:48:23.630" Score="5" Body="&lt;p&gt;I've heard about something, but im not sure of its validity because I wasnt paying attention to where I heard it.  Someplace Ive read about some sort of handheld mini ascent stage.  Pretty much, two stranded astronauts outside the lander grab on this thing and activate an engine to go to orbit. Pretty much a rocket stick with two handles. This may have been only in planning, im not sure, but astronauts would then need to survive on suit o2 until the CSM picks them up.  Any confirmation or denial is welcome.&lt;/p&gt;&#xA;" OwnerUserId="19105" LastActivityDate="2017-05-28T03:48:23.630" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21682" PostTypeId="1" AcceptedAnswerId="21683" CreationDate="2017-05-28T04:31:56.283" Score="9" ViewCount="2359" Body="&lt;p&gt;Most rocket engines have their efficiency measured by the specific impulse of the engine, which is just how fast the exhaust of the engine is going. Is there another measure of efficiency that's thrust/mass flow? Or does ISP take that into account, I think I'm missing something.&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="9092" LastEditDate="2021-02-04T22:49:48.487" LastActivityDate="2021-02-04T22:49:48.487" Title="Is ISP all there is to engine efficiency?" Tags="|rockets|engines|engine-design|specific-impulse|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21683" PostTypeId="2" ParentId="21682" CreationDate="2017-05-28T05:00:53.867" Score="14" Body="&lt;p&gt;ISP is really just the effective exhaust velocity of the engine normalized by a gravitational constant.  It does take into account thrust and mass flow and is a first-order measure of an engine's efficiency.  Note that it is effective exhaust velocity and not actual exhaust velocity.  For air-breathing engines, for example, effective will be significantly greater than actual.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISP isn't the whole story though.  Probably the second most important measure for an engine is thrust to weight ratio (TWR) of the engine.  TWR is indirectly a measure of efficiency as it usually dictates the time you will spend fighting gravity drag or thrusting at non-optimal times -- which is inefficient.&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2017-05-28T16:14:34.257" LastActivityDate="2017-05-28T16:14:34.257" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21684" PostTypeId="2" ParentId="21682" CreationDate="2017-05-28T05:18:11.907" Score="8" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Is there another measure of efficiency that's thrust/mass flow? Or does ISP take that into account&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Specific impulse (more fully, mass-specific impulse, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Impulse_(physics)&quot; rel=&quot;noreferrer&quot;&gt;impulse&lt;/a&gt;&#xA;per unit mass) is, in fact, equal to the thrust per mass flow &lt;em&gt;per unit of time&lt;/em&gt; for a pure rocket. This is because momentum is conserved; momentum of the propellant mass ejected out the engine is exactly opposite to the momentum gained by the rocket. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-28T05:24:56.013" LastActivityDate="2017-05-28T05:24:56.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21685" PostTypeId="1" CreationDate="2017-05-28T11:12:00.687" Score="4" ViewCount="486" Body="&lt;p&gt;A typical controller generally gives control torques which should be applied about the axis of a satellite. How exactly is this torque applied using a reaction wheel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example if control torques are a,b,c then will equating a,b,c to $I(\omega_{cur}-\omega_{prev})/step$ (where $I$ is moment of inertia, $\omega_{prev}$ and $\omega_{cur}$ are previous and current angular velocities) give me the new angular rate at which the wheel should rotate, or should I equate it with -a,-b,-c?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And also if I want continuous attitude control while dumping the momentum of reaction wheel using a magnetorquer, is it required that the net torque from magnetorquer and reaction wheel be equal to control torque?&lt;/p&gt;&#xA;" OwnerUserId="18223" LastEditorUserId="18223" LastEditDate="2017-05-30T07:02:29.090" LastActivityDate="2018-04-12T21:51:18.537" Title="Controller to reaction wheel interface" Tags="|attitude|spacecraft-development|reaction-wheel|flight-control|attitude-determination-and-control|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21686" PostTypeId="1" CreationDate="2017-05-28T12:17:25.490" Score="8" ViewCount="2547" Body="&lt;p&gt;The Lunar Module had an abort mode where the crew basically just had to push the ABORT button on the control panel, and the ascent stage would detach from the descent stage, followed by the ascent stage engine igniting to take them away from the descent stage and -- hopefully -- ultimately back into lunar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That would have worked nicely during the descent, when gravity would have done a good job of getting the descent stage out of the way, but &lt;a href=&quot;https://space.stackexchange.com/q/21614/415&quot;&gt;the recent discussion on not being able to take off after landing&lt;/a&gt; made me curious. &lt;strong&gt;What would happen if, &lt;em&gt;after landing&lt;/em&gt;, the crew got into (or remained inside) the LM, performed all relevant pre-launch steps, &lt;em&gt;and pushed the ABORT button&lt;/em&gt;?&lt;/strong&gt; We are of course assuming here that one of the steps performed would be to arm the ascent engine.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Would the LM's computers (PNGS or AGS) have allowed such an &quot;abort&quot;? Or was there some lock-out that prevented aborting after touchdown? (I'd like to think that aborting was one of the options available to the crew with the &lt;em&gt;fewest&lt;/em&gt; things that could go wrong.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How would it be similar or different compared to a normal ascent launch? Are there scenarios where one approach might have worked for getting them off the lunar surface, but not the other?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="415" LastActivityDate="2017-05-29T07:28:10.177" Title="Could the Apollo LM abort mode be engaged after touchdown? What would have happened if it was?" Tags="|crewed-spaceflight|apollo-program|lunar-landing|abort|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21687" PostTypeId="2" ParentId="21686" CreationDate="2017-05-28T12:43:52.627" Score="23" Body="&lt;p&gt;After a nominal landing the crew is waiting for a go from the ground to stay on the Moon. This was called the &quot;Stay/No Stay for T1&quot; and meant that there were no imminent issues requiring an immediate launch. So it was a commitment to stay for at least a few more minutes, until Mission Control could do a more detailed evaluation of the Lunar Module systems. If they didn't receive this Stay, then they would have used the Abort Stage button and the ascent abort programs of the AGC. So a procedure that is essentially identical to an abort before touchdown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After this Stay for T1 was received, the landing confirmation program (Program 68) is called on the AGC. After having started P68 you can't go back to using the ascent abort programs (Programs 70 or 71). Instead the nominal ascent program (Program 12) would have been used. And in fact Program 12 was nominally called directly after P68, in preparation for a launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the more detailed systems inspection I mentioned there was another Stay/No-Stay poll. If they didn't receive a Stay at this time, they would have launched with Program 12, just like a nominal ascent, although with slightly different insertion conditions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some more things about the Abort button. It's actually two buttons, Abort and Abort Stage. The Abort button mostly sends a signal to the AGC (PGNS) and the AGS that an abort with the descent stage engine is desired (using Program 70 of the AGC). The Abort Stage button both causes staging and sends a signal to the AGC and AGS that an abort with the ascent stage engine has to be started (Program 71). The Abort Stage button was also used to arm the staging pyros for a nominal ascent. Alternatively there is a &quot;Staging Fire&quot; switch, which also can separate ascent and descent stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew procedure for this can be found in the LM Timeline book, this one was for Apollo 12: &lt;a href=&quot;https://history.nasa.gov/afj/ap12fj/pdf/a12_lmtb.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://history.nasa.gov/afj/ap12fj/pdf/a12_lmtb.pdf&lt;/a&gt; on page 5 and 6.&lt;/p&gt;&#xA;" OwnerUserId="13716" LastEditorUserId="415" LastEditDate="2017-05-29T07:28:10.177" LastActivityDate="2017-05-29T07:28:10.177" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21688" PostTypeId="1" AcceptedAnswerId="21701" CreationDate="2017-05-28T15:51:29.580" Score="6" ViewCount="1057" Body="&lt;p&gt;Some answers on this site claim that the ISS US Segment solar arrays are double-sided (i.e. cells on both sides of the blanket). I tend to doubt this, but a search of my references did not turn up anything authoritative.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/w9tyr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/w9tyr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image from the solar array repair EVA performed on STS-120 seems to show that the faces of the array are not the same (cropped from &lt;a href=&quot;https://spaceflight.nasa.gov/gallery/images/shuttle/sts-120/hires/iss016e007003.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;).  It is difficult to obtain images that show both sides of the array at once.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I seek an authoritative answer with solid documentation reference(s) that explicitly states one way or the other.  NASA or contractor documentation, preferably.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-05-28T16:04:08.247" LastActivityDate="2017-05-29T18:37:05.723" Title="Are the ISS US Segment solar arrays double-sided?" Tags="|iss|solar-power|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21689" PostTypeId="2" ParentId="20470" CreationDate="2017-05-29T02:21:41.410" Score="1" Body="&lt;p&gt;As the closest someone with no academic experience can be to a Phobos expert...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Crater chains don't especially work, because you'd expect significantly more dispersion even if ejecta from Mars reaches Phobos in minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Paths of rolling boulders matches surprisingly well &lt;a href=&quot;https://www.hou.usra.edu/meetings/lpsc2017/pdf/2084.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.hou.usra.edu/meetings/lpsc2017/pdf/2084.pdf&lt;/a&gt;&#xA;&lt;a href=&quot;http://www.planetary.brown.edu/pdfs/4150.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.planetary.brown.edu/pdfs/4150.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;but I find it hard to believe that every block would escape Phobos, and if you have enough ejecta return to bury boulders, you have enough to bury grooves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dust sifting into fractures is the most popular idea. The most common suggestion for fracture sources is tidal stresses, and that matches most grooves' orientations very well &lt;a href=&quot;http://www.planetary.brown.edu/planetary/geo287/PhobosDeimos/papers/Hurfordetal2015NatureGeoDraft_v2-grooves.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;(source)&lt;/a&gt;, the grooves on Phobos's leading edge don't really match.&#xA;Other causes include outgassing vents and fractures caused by the Stickney impact (the latter becoming increasingly unlikely as Phobos increasingly looks like a reaccreted rubble pile).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Grooves have some different characteristics, so it's likely there's a combination of causes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's a region on Phobos's trailing edge without grooves. My personal theory as to why is that dust from Deimos is primarily deposited on Phobos's trailing edge. If the grooves are old enough, one would expect grooves there to be buried. (&lt;a href=&quot;https://phobos-deimos.arc.nasa.gov/on-demand/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://phobos-deimos.arc.nasa.gov/on-demand/&lt;/a&gt;, scroll to Horanyi)&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-05-29T02:21:41.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21690" PostTypeId="1" AcceptedAnswerId="21700" CreationDate="2017-05-29T12:34:52.973" Score="5" ViewCount="1011" Body="&lt;p&gt;The Iridium constellation is being replaced by new satellites. There are six planes with 11 satellites + one spare each. Since the Falcon 9 launcher can only carry ten satellites per launch, filling the planes is not an easy task. Out of the first launch, eight satellites were inserted into orbit to take the slots of the old satellites. Two vehicles have started drifting to the next plane (westwards). It will take them app. 10 months to get there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next launch will insert only five satellites into service orbit, another five will be drifted west- as well as eastwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to know how the drifting is actually handled. I somewhere read that as far as I understood, westward drifts are helped by precession. I figured out that the drifting satellites move app. 0.11 degrees more to the west than their non-drifting companions' RAAN changes on a daily basis. The drifters are in a lower 629/611 vs. 783/781) and more excentric orbit (0.0013000 vs 0.0002100 acc TLEs) and have a slightly lesser inclination (85.88 vs 86.4). Do I correctly assume that these factors aid the drift? And apart from that, do the vehicles do side-burns on a continuous basis? And if so, does this require less delta v than - hypothetically speaking - putting them in an equatorial orbit and spitting them out at the appropriate RAAN to make a (nearly) right-angle turn (I know it could not work like this, but just to have a simplified model for my brain)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess it also needs to be kept in mind that the drifting satellites have to end up in very precise positions, since they have to perfectly fit into the slots of the constellation. They have to cross-link with the satellites before and behind them as well as to the ones in the planes east and west.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anything else that comes into play that I haven't even touched upon?&lt;/p&gt;&#xA;" OwnerUserId="19326" LastEditorUserId="19326" LastEditDate="2017-05-29T12:42:54.480" LastActivityDate="2017-05-29T18:09:24.833" Title="How exactly are satellites drifted to slots in different orbital planes?" Tags="|orbital-mechanics|artificial-satellite|iridium|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21691" PostTypeId="2" ParentId="21682" CreationDate="2017-05-29T13:36:34.667" Score="3" Body="&lt;p&gt;Erik gave the second &quot;big thing&quot; from the pair - indeed, in space $I_{sp}$ is the king, TWR being secondary, while on ascent the relation is reversed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But these two aren't all. For electric propulsion, &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/the-need-for-better-alpha.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;alpha&lt;/a&gt; is very important - it's a bit of counterpart to TWR but involving electric power, as opposed to raw thrust. This is important, as ion engines are quite power-hungry and still subject to tyranny of rocket equation, especially if dry mass goes considerably up. Whatever gains you get to superior specific impulse, you may lose due to need to haul heavy power sources. That's also why plain old engine &lt;em&gt;mass&lt;/em&gt; matters in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's technology readiness level. The engine may look great on paper and even work promisingly in the laboratory, so what if it's still not &quot;space-worthy&quot; or reliable enough? The 12,000s of specific impulse of VASIMR aren't much of help when it's simply still not ready to leave the laboratory. Reliability and safety fall close on this spectrum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, we have price of the engine and the fuel, both absolute and per unit. Why do so many missions - both commercial and scientific - still fly on these nasty hypergolics, instead of the nice, lean, efficient ion engines? Because both these engines and xenon cost an arm and a leg, and hypergolics, while old, corrosive, dangerous, inefficient, are &lt;em&gt;cheap&lt;/em&gt;. It's often more cost-efficient to pack more cheap fuel and an old, less efficient engine, than the new great thing. And solid rocket boosters are very popular for first stage despite completely lousy specific impulse, simply because they are cheap and give good thrust - you're often better off financially with a couple of big and heavy SRBs than with a better, larger liquid fuel engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's social/political acceptability. Read up on &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;NTR&lt;/a&gt;. An engine of 1600s of specific impulse and enough thrust to propel large manned missions to Mars was scrapped for the sole reason that people don't want nuclear reactors in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Availability and handling properties of fuels are quite important - SpaceX flies on kerosene strictly because liquid hydrogen is difficult to handle. All amateur rocketry flies on nitrous oxide, which, while lousy as an oxidizer, is quite safe and accessible, unlike &quot;nastier&quot; stuff like LOX or RFNA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Military applications have their own requirements. Visual (including smoke) signature is a big thing. Storability of fuels over long periods. Freezing temperature, boiling temperature, outgassing / vapor pressure, stability (insensitivity to shock and similar) are important factors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, $I_{sp}$ is really just a tip of the iceberg...&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-29T13:36:34.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21692" PostTypeId="1" AcceptedAnswerId="21740" CreationDate="2017-05-29T14:39:06.237" Score="1" ViewCount="1196" Body="&lt;p&gt;Last week's &lt;a href=&quot;https://twitter.com/RonBaalke/status/866656898455543809/photo/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweet&lt;/a&gt; by &lt;a href=&quot;https://twitter.com/RonBaalke&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ron Baalke&lt;/a&gt; (of JPL) mentions five near Earth asteroids that will come within five Lunar distances (5 LD) of the Earth in 2017.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of them, 2012 TC4, will pass very close to Earth. Looking at the table, I see a Nominal Closest Approach (CA) Distance of 0.15 LD, but a Minimum CA Distance of only 0.03 LD!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is nominal the most likely value, and minimum the smallest likely, or smallest possible, or 3-sigma closest? Is there any chance it could be even closer?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will these numbers improve as the time of 2012 TC4's pass grows near?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BaEDw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BaEDw.png&quot; alt=&quot;partial tweet&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped from &lt;a href=&quot;https://twitter.com/RonBaalke/status/866656898455543809/photo/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;this tweet&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;Asteroid 2012 TC4 as seen by the Remanzacco Observatory team of Ernesto Guido, Giovanni Sostero, Nick Howes on Oct. 9, 2012&quot; From Phys.org's 2015 article &lt;a href=&quot;https://phys.org/news/2015-04-asteroid-tc4-earth-october.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Will asteroid 2012 TC4 hit Earth in October 2017?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IAfHd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IAfHd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-05-30T06:45:17.743" LastActivityDate="2017-09-24T12:58:03.190" Title="Just how close to Earth will 2012 TC4 pass in 2017?" Tags="|near-earth-asteroid|2012-tc4|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21693" PostTypeId="1" CreationDate="2017-05-29T15:01:16.337" Score="4" ViewCount="715" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/668/12102&quot;&gt;Wheels vs. tracks for rovers&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/17610/12102&quot;&gt;Does new technology make it advantageous to have walking rovers?&lt;/a&gt; ask about the possible use of treads or legs instead of wheels on rovers from a speculative point of view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But which rover already on another solar system body &lt;em&gt;didn't have any wheels?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-01-02T03:20:01.243" LastActivityDate="2021-12-11T11:24:39.787" Title="Which rover(s) didn't have wheels?" Tags="|rovers|" AnswerCount="5" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21694" PostTypeId="2" ParentId="21693" CreationDate="2017-05-29T15:29:17.557" Score="7" Body="&lt;p&gt;The list of successful rover missions is &lt;a href=&quot;https://en.wikipedia.org/wiki/Rover_(space_exploration)&quot; rel=&quot;nofollow noreferrer&quot;&gt;short&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Moon: LRV, Lunokhod, Yutu  &lt;/li&gt;&#xA;&lt;li&gt;Mars: Sojourner, Spirit, Opportunity, Curiosity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All of them have wheels. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soviet Union built two walking rovers (using 2 skis) (&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2#Prop-M_Rover&quot; rel=&quot;nofollow noreferrer&quot;&gt;Prop-M&lt;/a&gt;), for the Mars-2 and Mars-3 missions. Both missions &lt;a href=&quot;https://space.stackexchange.com/questions/21709/is-there-any-chance-mars-3s-prop-m-rover-deployed-automatically-and-roved&quot;&gt;failed&lt;/a&gt; before these rovers could be deployed. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-06-01T14:20:46.660" LastActivityDate="2017-06-01T14:20:46.660" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21695" PostTypeId="2" ParentId="21693" CreationDate="2017-05-29T15:43:00.340" Score="2" Body="&lt;p&gt;Looks like @Hobbes nailed it right off with &lt;a href=&quot;https://space.stackexchange.com/a/21694/12102&quot;&gt;this answer&lt;/a&gt;. There are two Prop-M rovers on Mars. While Mars-2 crash-landed, Mars-3 landed and began transmitting. However, thanks to some excellent research by @called2voyage, we can see that because the rovers would not be deployed without &lt;a href=&quot;https://space.stackexchange.com/a/21734/12102&quot;&gt;explicit radio instruction&lt;/a&gt;, neither rovers were ever able to &amp;quot;rove&amp;quot; on Mars. Nevertheless it seems they are there, in some form or other.&lt;/p&gt;&#xA;&lt;p&gt;I'll have to think of something harder for a puzzler next time.&lt;/p&gt;&#xA;&lt;p&gt;I'll add this cute GIF and photo, both found at the Wikipedia article for &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars 3&lt;/a&gt;. According to the article, the rover was to remain attached by an umbilical to the lander, and do some local exploration while monitored and sometimes filmed by the lander.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9BYUo.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9BYUo.gif&quot; alt=&quot;Prop-1 Mars rover&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF of how the Prop-M rover uses it's &amp;quot;skis&amp;quot; to walk. Linked &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, from &lt;a href=&quot;https://giphy.com/gifs/technology-mars-xXVrw6khBFdIY&quot; rel=&quot;nofollow noreferrer&quot;&gt;Giphy&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sxKr6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sxKr6.jpg&quot; alt=&quot;Prop-1 Mars rover&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Mars Prop-M rover from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Mars_propm_rover.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-12-10T22:26:41.610" LastActivityDate="2021-12-10T22:26:41.610" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21696" PostTypeId="1" CreationDate="2017-05-29T16:03:56.883" Score="1" ViewCount="147" Body="&lt;p&gt;What is the component mass of SpaceX's Falcon 9 or any &#xA;descriptions that could tell the component mass of SpaceX's Falcon 9.&lt;/p&gt;&#xA;" OwnerUserId="19824" LastEditorUserId="19824" LastEditDate="2017-06-01T08:10:36.033" LastActivityDate="2017-06-01T08:10:36.033" Title="Falcon 9's description of component mass" Tags="|falcon-9|" AnswerCount="0" CommentCount="5" FavoriteCount="0" ClosedDate="2017-05-31T18:05:19.590" ContentLicense="CC BY-SA 3.0" />
  <row Id="21697" PostTypeId="1" AcceptedAnswerId="21698" CreationDate="2017-05-29T16:44:02.063" Score="3" ViewCount="245" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/21694/12102&quot;&gt;answer&lt;/a&gt; mentions that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2#Prop-M_Rover&quot; rel=&quot;nofollow noreferrer&quot;&gt;Prop-M&lt;/a&gt; rovers that were carried to the surface of Mars by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars-2&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars-3&lt;/a&gt; missions used &quot;skis&quot; for propulsion, rather than wheels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the GIF below frame by frame suggest that the actuation is likely done by pure rotational motion. Is there anything written or known about the rationale at the time why skis driven by a motor would be better than wheels driven by motors? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not so interested in what one might choose do today. I'm really asking about the state of the art and decision making back then. Why was &quot;walking&quot; on rotary-driven &quot;skis&quot; chosen over putting wheels on those motors, and either a tail (third point of contact dragging, a known good solution for some terrain types) or one or two more passive wheels?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, the rover was to remain attached by an umbilical to the lander, and do some local exploration while monitored and sometimes filmed by the lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9BYUo.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9BYUo.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF of how the Prop-M rover uses it's &quot;skis&quot; to walk. Linked &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, from &lt;a href=&quot;https://giphy.com/gifs/technology-mars-xXVrw6khBFdIY&quot; rel=&quot;nofollow noreferrer&quot;&gt;Giphy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sxKr6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sxKr6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Mars Prop-M rover from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Mars_propm_rover.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-05-30T00:38:03.060" Title="Since the Prob-M Mars Rovers had a motor for locomotion, why not use wheels?" Tags="|rovers|prop-m|mars-3|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21698" PostTypeId="2" ParentId="21697" CreationDate="2017-05-29T16:51:37.207" Score="8" Body="&lt;p&gt;I believe it was because the Soviets at the time did not know the composition of the Martian soil, and how wheels would cope with it. They thought that the soil was too fine for wheels and that they would become easily stuck in the soil. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The robots were sent by the Russians to study Martian soil&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Due to many previous lander failures, they could never know truly what the surface soil was like. Sleds however, work in many different terrains. Although they are slower and more cumbersome, it was the least riskiest method to get movement in the soil. Two more massive wheels would would still have a chance to become stuck in the Martian soil. Based on what the Soviets knew about Mars at the time (which was very little) they decided on this approach.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The traces of movement in the martian soil would also be recorded to determine material properties.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The sleds on the Prop-M rover when in contact with the soil would leave a distinct imprint, and it was hoped that these imprints could be used to determine the properties of the surface soil. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First quote: &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1971-049F&quot; rel=&quot;noreferrer&quot;&gt;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1971-049F&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second quote: &lt;a href=&quot;http://unofficialnetworks.com/2012/11/15/robots-skied-mars/&quot; rel=&quot;noreferrer&quot;&gt;http://unofficialnetworks.com/2012/11/15/robots-skied-mars/&lt;/a&gt;&#xA;(&lt;a href=&quot;https://en.m.wikipedia.org/wiki/Mars_sample_return_mission&quot; rel=&quot;noreferrer&quot;&gt;https://en.m.wikipedia.org/wiki/Mars_sample_return_mission&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="18458" LastEditDate="2017-05-30T00:38:03.060" LastActivityDate="2017-05-30T00:38:03.060" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21699" PostTypeId="2" ParentId="12467" CreationDate="2017-05-29T17:09:08.293" Score="1" Body="&lt;p&gt;I just realized the issue INSIDE the space station is &lt;strong&gt;terminal velocity&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On earth with a gravitational pull of ~9.8 m/s&lt;sup&gt;2&lt;/sup&gt; the &lt;strong&gt;terminal velocity&lt;/strong&gt; of the human body is about 53 m/s or 195 kph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravitational pull between objects on the station is at least a billion times less, so too is the terminal velocity (see the wikipedia article on &quot;free fall&quot;). The &lt;strong&gt;terminal velocity&lt;/strong&gt; on the station is less than 53 nanometers/s or 18 mm/hour.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might say they are gliding, or even flying if they flap something effectively. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Outside the station, no air, terminal velocity not an issue. &lt;/p&gt;&#xA;" OwnerUserId="12131" LastActivityDate="2017-05-29T17:09:08.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21700" PostTypeId="2" ParentId="21690" CreationDate="2017-05-29T18:09:24.833" Score="6" Body="&lt;p&gt;The precession depends on the height of the orbit. The storage orbit for spare satellites is a bit lower than the regular one (620 versus 780 km) and precession is a different one (not going to do the maths here). This means, this satellite drifts slowly with respect to the operational satellites in the higher orbit. The whole process doesn't need any fuel, despite for changing the orbital height when reaching the right plane. This amounts to about 85 m/s using a Hohmann transfer and the two heights you give in your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding your other questions, let me answer them one by one:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And apart from that, do the vehicles do side-burns on a continuous basis?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No, that is not necessary. You don't have to care about precession, as it is the same for all constellation satellites. If they all precess (move westwards) at the same rate, the constellation stays essentially the same for all practical purposes.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And if so, does this require less delta v than - hypothetically speaking - putting them in an equatorial orbit and spitting them out at the appropriate RAAN to make a (nearly) right-angle turn (I know it could not work like this, but just to have a simplified model for my brain)?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can do a simple back-of-the-envelop calculation here: Changing from an equatorial orbit to an almost polar one requires to cancel all &quot;westward&quot; speed and built up &quot;northward&quot; orbital speed. This is a $\Delta v = \sqrt 2 v_{\rm orbit}$, or about 11 km/s. These satellites carry fuel for at most one tenth of this amount. You could do the plane change in a very high and slow orbit (above GEO) and save fuel, but getting there is prohibitively expensive.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I guess it also needs to be kept in mind that the drifting satellites have to end up in very precise positions, since they have to perfectly fit into the slots of the constellation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They only important point is to get the satellite into an orbit with the same RAAN as the others of the destination plane. The precise position along this plane can then easily be reached by varying the orbital height slightly - E.g. if you change to an orbit which is 1 minute (out of 100 minutes orbit) shorter or longer, you can reach any point on the plane within about 50 orbits. For comparison: The lower of the two orbits you mention is 4 minutes shorter. Naturally, you can time raising to the right altitude appropriately to end up in the right spot right away. &lt;/p&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2017-05-29T18:09:24.833" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21701" PostTypeId="2" ParentId="21688" CreationDate="2017-05-29T18:37:05.723" Score="11" Body="&lt;p&gt;Having worked extensively on the solar arrays over the last five years, I can say they do receive about a third of their power from albedo exposure to the back side.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is further confirmed from a &quot;funny&quot; I noticed in 2014 when doing testing on ISS solar cells. When trying to measure a solar cell's illuminated short-circuit current, I measured a value two and a half times what I expected. I soon discovered that I was not controlling the reflection of light behind the cell. Placing the cell in front of some black felt brought the measurements back in line with expectation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately none of the design documentation I am aware of is public. Unless someone here knows of something that has been released, I don't think you'll find authoritative confirmation beyond the assertions of individuals familiar with the hardware.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-05-29T18:37:05.723" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21702" PostTypeId="1" CreationDate="2017-05-29T19:37:48.023" Score="1" ViewCount="386" Body="&lt;p&gt;I am studying Aerospace master and currently on the brim to pursue my master thesis. I would like our input on the new research trends that are going on in the space industry. I would like to do some multidisciplinary work involving mechanical design, space system integration,  mechatronics, communication, nanosatellite, and propulsion. I am also fascinated by the space debris problem. I would really like any feedback on this. I am asking this question to keep myself current so when I get out I will have a career prospect.&lt;/p&gt;&#xA;" OwnerUserId="19828" LastActivityDate="2017-05-29T20:24:38.287" Title="Master thesis - new research topics" Tags="|debris|ion-thruster|" AnswerCount="1" CommentCount="0" ClosedDate="2017-05-30T03:45:51.910" ContentLicense="CC BY-SA 3.0" />
  <row Id="21703" PostTypeId="2" ParentId="21702" CreationDate="2017-05-29T20:24:38.287" Score="1" Body="&lt;p&gt;It's a very broad subject, but let me pick some &quot;current&quot; topics.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;there's very little &quot;new original&quot; ideas involving space travel that are remotely viable (technologically, economically or politically). A lot of entirely new designs will be needed for Mars landing, and there are some unresolved challenges involving Breakthrough Starshot, but the rest of work is pretty much incremental on existing designs. Of course if you do have some entirely original ideas, there's nothing stopping you, but there's likely no &quot;next big thing right out the door&quot; to pick up. (EM Drive might be one, but it might be just a bunch of snake oil as well.)&lt;/li&gt;&#xA;&lt;li&gt;since you're into communications, one of unresolved problems for Breakthrough Starshot is communication at ~4 light years distance. Nobody really has any good idea how to achieve that. The general &quot;hunch&quot; is it should be doable but all current solutions fall woefully short of the goal.&lt;/li&gt;&#xA;&lt;li&gt;and then, the incremental work is important. The focus is on reduction of costs, reduction of power consumption, mass, improving reliability, streamlining production, miniaturization, and turning experimental designs into production equipment.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mechanical design, mechatronics&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Since all larger payloads (and many small) unfold like origami, there's quite a bit of that in spacecraft; especially passive systems (which are usually very fault-proof and inexpensive) are valuable. Reaction wheels and CMGs are still bulky, heavy, expensive, power-hungry and fault-prone. There's a lot of room for improvement in that area. And every device working in vacuum, or on vacuum-air interface is a challenge. Plus of course all robots and rovers, that's a good bit of a current challenge. We &quot;have a general clue&quot; how to do all of that, but getting the details ironed out is an awful lot of work.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;space system integration&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I can't tell you much on that. There's a lot of integration on ad-hoc basis: adapting given booster for given launch stage, given payload for given launcher, given part or piece of software for given purpose. There are some standards and some standarization efforts, but it's still mostly a big zoo where integration is a Sisyphean work, of getting thousands of mismatched available systems to work together. You can certainly find a job with it, but I don't particularly see any master thesis topics. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;communication&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A whole big field with constant room for improvement. I'm not really into it so I can't give you much detail, but if there's a field to find &quot;something completely new&quot;, it's likely here.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;nanosatellite&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Miniaturization, reduction of cost and power consumption. It's a big game because there are a lot of customers for these, and a lot of promising designs that are simply too big. A huge area, pick any system found on big satellites and not on cubesats, and adapt it. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;propulsion.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Besides bringing a few designs out of lab and into space, and un-mothballing several older promising designs that were put on hold by the end of Cold War, the game is the same - improve, miniaturize, reduce inefficiencies, simplify to reduce price, streamline manufacturing etc. Lots of work in that direction, and a lot of good research - replacing active subsystems with passive, better control algorithms, creative material engineering, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for debris, there are a couple projects to combat them, although their practical and economical viability is somewhat dubious. Regardless, even though likely not a single piece of debris will be deorbited as result, that's certainly a subject that can provide ground for quite a few good scientific publications and a stable job at a research institute ;)&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-05-29T20:24:38.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21704" PostTypeId="1" AcceptedAnswerId="21705" CreationDate="2017-05-30T00:50:30.610" Score="42" ViewCount="7252" Body="&lt;p&gt;When an expendable booster rocket stage nears the end of its burn, does the guidance computer shut the engine(s) off at a certain velocity/altitude for the mission, or does the stage completely exhaust its propellants?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2017-05-30T05:55:00.983" Title="Do booster stages run out of fuel, or are they purposefully shut off?" Tags="|launch|rockets|propulsion|fuel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21705" PostTypeId="2" ParentId="21704" CreationDate="2017-05-30T01:46:57.093" Score="84" Body="&lt;p&gt;First stages are generally run to depletion (though not complete depletion - I'll get to that later). First stage ascents often use a preprogrammed, open loop guidance system to get out of the atmosphere with a good chunk of downrange velocity. Because winds aloft and other factors may vary, first stage performance also varies somewhat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Upper stage guidance is almost always closed loop, targeting a specific position and velocity combination. As a result, upper stages of boosters are shut down when that target is reached, with some amount of reserve propellants left to cover dispersions during ascent and a disposal burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Liquid fueled first stages are not run dry. Cutoff is triggered with some amount left in the tank. This is because the turbopumps operate at ridiculous velocities and need something to pump, at a specific inlet pressure. The heat source (gas generator, expander, or preburner) that drives the pump is downstream of the inlet in the flow path, so cutoff has to happen by cutting off the drive force to the pump before the inlet loses pressure or runs dry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that pressure drops below minimum specifications or if the inlet goes dry, you will get cavitation and/or overspeed conditions, both of which would result in a violent reconfiguration of the hardware.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid lower stages are much simpler in this regard. At some point, the thrust will tail off until the booster no longer supports its own weight. At that point, it is jettisoned.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-05-30T01:46:57.093" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21706" PostTypeId="1" CreationDate="2017-05-30T02:48:29.613" Score="2" ViewCount="61" Body="&lt;p&gt;Alcohol seems to have been taken and consumed in space (not so sure about the ISS, but definitely on Mir).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this the only psychotrope substance that has been tried in space ?&#xA;Is there any formal study of the effect of such substances in microgravity ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm especially interested in humans experiments, but animal ones are welcome too.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-05-30T02:48:29.613" Title="Psychotropes study in microgravity" Tags="|microgravity|experiment|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21707" PostTypeId="1" AcceptedAnswerId="27882" CreationDate="2017-05-30T05:42:39.750" Score="9" ViewCount="364" Body="&lt;p&gt;The image below of Mars' surface and text is from the NASA JPL Photojournal Catalog: &lt;a href=&quot;https://photojournal.jpl.nasa.gov/catalog/PIA16920&quot; rel=&quot;noreferrer&quot;&gt;PIA16920: Could This Be the Mars Soviet 3 Lander?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The latest data shown for the Mars 3 site in Wikipedia is from MRO's HiRISE camera in early 2013. According to the caption, the Mars 3 &quot;...candidate descent module is a feature with the right size and shape to be the actual lander, with four open petals.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Have there been any further detections of the Mars 3 Mission and/or landing site? &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;HiRISE camera of the Mars Reconnaissance Orbiter (NASA)&quot;. Cropped, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:MRO_HiRISE.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. Apparently this is &lt;a href=&quot;https://space.stackexchange.com/a/17012/12102&quot;&gt;the largest aperture telescope sent beyond the earth-moon system&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3OXUK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3OXUK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; NASA &lt;a href=&quot;https://photojournal.jpl.nasa.gov/catalog/PIA16920&quot; rel=&quot;noreferrer&quot;&gt;PIA16920&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/X2RgD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/X2RgD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Original Caption Released with Image:&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This set of images shows what might be hardware from the Soviet Union's 1971 Mars 3 lander, seen in a pair of images from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The possible Mars 3 lander hardware was found by an Internet group of Russian citizen enthusiasts who follow news about Mars and NASA's Curiosity rover.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In 1971, the former Soviet Union launched the Mars 2 and Mars 3 missions to Mars. Each consisted of an orbiter plus a lander. Both orbiter missions succeeded, although the surface of Mars was obscured by a planet-encircling dust storm. The Mars 2 lander crashed. Mars 3 became the first successful soft landing on the Red Planet, but stopped transmitting after just 14.5 seconds for unknown reasons.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The predicted landing site was at latitude 45 degrees south, longitude 202 degrees east, in Ptolemaeus Crater. HiRISE acquired a large image at this location in November 2007. This image (HiRISE catalog &lt;a href=&quot;http://www.uahirise.org/PSP_006154_1345&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;PSP_006154_1345&lt;/strong&gt;&lt;/a&gt;) contains 1.8 billion pixels of data, so about 2,500 typical computer screens would be needed to view the entire image at full resolution. Promising candidates for the hardware from Mars 3 were found on Dec. 31, 2012.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Vitali Egorov from St. Petersburg, Russia, heads the largest Russian Internet community about Curiosity, at &lt;strong&gt;&lt;a href=&quot;http://vk.com/curiosity_live&quot; rel=&quot;noreferrer&quot;&gt;http://vk.com/curiosity_live&lt;/a&gt;&lt;/strong&gt;. His subscribers did the preliminary search for Mars 3 via crowdsourcing. Egorov modeled what Mars 3 hardware pieces should look like in a HiRISE image, and the group carefully searched the many small features in this large image, finding what appear to be viable candidates in the southern part of the scene. Each candidate has a size and shape consistent with the expected hardware, and they are arranged on the surface as expected from the entry, descent and landing sequence.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;An advisor to the group, Alexander Basilevsky, of Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, contacted Alfred McEwen, principal investigator for HiRISE, suggesting a follow-up image. HiRISE acquired this image (&lt;a href=&quot;http://www.uahirise.org/ESP_031036_1345&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;ESP_031036_1345&lt;/strong&gt;&lt;/a&gt;) on March 10, 2013. The image was targeted to cover some of the hardware candidates in color and to get a second look with different illumination angles. Meanwhile, Basilevsky and Egorov contacted Russian engineers and scientists who worked on Mars 3 for more information.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The candidate parachute is the most distinctive and unusual feature in the images. It is an especially bright spot for this region, about 8.2 yards (7.5 meters) in diameter. The parachute would have a diameter of 12 yards (11 meters) if fully spread out over the surface, so this is consistent. In the second HiRISE image the parachute appears to have brightened over much of its surface. The brightening is probably due to better illumination over the sloping surface, but it is also possible that dust was removed during the intervening years, resulting in brightening of the parachute. HiRISE recently showed that the parachute of the Mars Science Laboratory mission has shifted in the wind (see &lt;a href=&quot;https://photojournal.jpl.nasa.gov/catalog/PIA16813&quot; rel=&quot;noreferrer&quot;&gt;&lt;strong&gt;PIA16813&lt;/strong&gt;&lt;/a&gt;), which might also kick off dust.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The descent module or retrorocket was attached to the lander container by a chain, and the candidate feature has the right size and even shows a linear extension that could be a chain. Nearby the candidate descent module is a feature with the right size and shape to be the actual lander, with four open petals. The image of the candidate heat shield matches a shield-shaped object with the right size if partly buried.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Together, this set of features and their layout on the ground provide a remarkable match to what is expected from the Mars 3 landing, but alternative explanations for the features cannot be ruled out. Further analysis of the data and future images to better understand the 3-dimensional shapes may help to confirm this interpretation.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;HiRISE is one of six instruments on NASA's Mars Reconnaissance Orbiter. The University of Arizona, Tucson, operates HiRISE, which was built by Ball Aerospace &amp;amp; Technologies Corp., Boulder, Colo. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter Project for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, built the spacecraft.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Image Credit:&#xA;  NASA/JPL-Caltech/Univ. of Arizona&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Image Addition Date:&#xA;  2013-04-11&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I found an interactive google map at the Lunar &amp;amp; Planetary Laboratory's &lt;a href=&quot;http://hirise.lpl.arizona.edu/ESP_031036_1345&quot; rel=&quot;noreferrer&quot;&gt;Could This Be the Soviet Mars 3 Lander? ESP_031036_1345&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Screen shot of the interactive map:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5f3Op.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5f3Op.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6816" LastEditDate="2017-10-05T14:58:50.810" LastActivityDate="2018-06-15T12:22:38.363" Title="Any news about the Mars-3 landing site since early 2013?" Tags="|reconnaissance-satellite|mars-3|mars-reconnaissance-orbiter|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21709" PostTypeId="1" AcceptedAnswerId="21734" CreationDate="2017-05-30T06:39:47.217" Score="3" ViewCount="832" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars-3&lt;/a&gt; landed on the surface of Mars and began transmitting. It carried a &lt;a href=&quot;https://space.stackexchange.com/a/21695/12102&quot;&gt;Prop-M rover&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lander started transmitting from the surface but went silent about 14 seconds later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did rover deployment absolutely require a ground command, or is there some possibility that it could have deployed automatically and &quot;roved&quot; a bit on the Martian surface? Is there anything that can conclusively rule this in or out?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, the rover was to remain attached by an umbilical to the lander, and do some local exploration while monitored and sometimes filmed by the lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9BYUo.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9BYUo.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; GIF of how the Prop-M rover uses it's &quot;skis&quot; to walk. Linked &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, from &lt;a href=&quot;https://giphy.com/gifs/technology-mars-xXVrw6khBFdIY&quot; rel=&quot;nofollow noreferrer&quot;&gt;Giphy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sxKr6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sxKr6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Mars Prop-M rover from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Mars_propm_rover.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2022-08-08T16:50:36.087" Title="Is there any chance Mars-3's Prop-M rover deployed automatically and &quot;roved&quot;?" Tags="|rovers|mars-3|prop-m|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21710" PostTypeId="2" ParentId="9311" CreationDate="2017-05-30T06:46:02.240" Score="0" Body="&lt;p&gt;Use small high  pressure helium tank located inside the tank and well isolated with electric heater, its volume capable of expelling the LOX at a pressure that would be adequate for full thrust of combustion chamber size this saves room.&lt;/p&gt;&#xA;" OwnerUserId="19835" LastEditorUserId="7062" LastEditDate="2017-05-30T09:10:45.373" LastActivityDate="2017-05-30T09:10:45.373" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21712" PostTypeId="1" CreationDate="2017-05-30T14:17:19.813" Score="2" ViewCount="1035" Body="&lt;p&gt;Will there be a privacy partition in the Crew Dragon during &lt;a href=&quot;http://www.spacex.com/news/2017/02/27/spacex-send-privately-crewed-dragon-spacecraft-beyond-moon-next-year&quot; rel=&quot;nofollow noreferrer&quot;&gt;the one week trip around the moon&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="19834" LastEditorUserId="6816" LastEditDate="2017-05-30T14:57:00.550" LastActivityDate="2017-05-30T20:38:13.673" Title="Will there be privacy on the Crew Dragon moon trip?" Tags="|spacex|the-moon|dragon-v2|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21713" PostTypeId="2" ParentId="13044" CreationDate="2017-05-30T15:49:40.970" Score="6" Body="&lt;p&gt;The Voyagers' longevity is not due to luck.&lt;/p&gt;&#xA;&lt;p&gt;The Voyager program really began with the Pioneer 10 and 11 missions. They provided essential data on the conditions the Voyagers were likely to encounter, both in interplanetary space and in the vicinity of Jupiter and Saturn. The Pioneers were a huge jump for NASA, being the first interplanetary craft that had to operate for more than a few months.&lt;/p&gt;&#xA;&lt;p&gt;The environment around Jupiter turned out to be the worst: it alone accounts for about 50% of the radiation dose throughout the Voyager missions.&lt;/p&gt;&#xA;&lt;p&gt;A great deal of reliability analysis was done on the Voyager spacecraft design, leading to (&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20160003534&amp;amp;hterms=voyager%20reliability&amp;amp;qs=Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA%26Ntx%3Dmode%2520matchallpartial%26Ntk%3DAll%26N%3D0%26No%3D20%26Ntt%3Dvoyager%2520reliability&quot; rel=&quot;nofollow noreferrer&quot;&gt;page 55 of long PDF&lt;/a&gt;) ...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Probability of success for the Voyager capsule bus preferred concept is estimated at 0.830.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This reliability was achieved through careful component design and selection, &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_program#Computers_and_data_processing&quot; rel=&quot;nofollow noreferrer&quot;&gt;redundancy&lt;/a&gt;, and &lt;a href=&quot;https://www.wired.com/2013/09/vintage-voyager-probes/&quot; rel=&quot;nofollow noreferrer&quot;&gt;engineers designing the best spacecraft possible&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Still, the Voyager probes have far exceeded expectations. Looking at engineers’ testimonials from when it was built, Dodd said the original designers were told not to worry about reaching interstellar space and focus on making sure the Voyagers could observe Jupiter and Saturn.&lt;/p&gt;&#xA;&lt;p&gt;“Basically they kind of ignored those directions, nodded their heads and did what they wanted to make it capable of getting to interstellar space,” she said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Over the years, there have been failures. The biggest problem was the scan platform getting stuck. That was reversible, though. As the power available drops due to decay, instruments have to be &lt;a href=&quot;https://voyager.jpl.nasa.gov/spacecraft/spacecraftlife.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;switched off&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;There are some &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19990042152&quot; rel=&quot;nofollow noreferrer&quot;&gt;AAIA papers&lt;/a&gt; that provide more information, unfortunately they don't seem to be available online:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Due to reliability requirements placed on the Voyager spacecraft system design and a mission resulting in long two-way, light time communication links, on-board automatic fault detection and correction capabilities are a significant feature of that spacecraft's design. Most of the protection to otherwise mission-catastrophic failures is implemented in the software of the voyager's central computer, while some resides in an attitude control-dedicated processor.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19790041318&amp;amp;hterms=voyager%20reliability&amp;amp;qs=Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA%26Ntx%3Dmode%2520matchallpartial%26Ntk%3DAll%26N%3D0%26No%3D20%26Ntt%3Dvoyager%2520reliability&quot; rel=&quot;nofollow noreferrer&quot;&gt;another&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Tests and analysis indicated the need to protect the Voyager spacecraft from electrostatic discharges in the Jupiter energetic charged particle environment. This paper describes the identified problem areas, the test and analysis methods and results, the resultant design changes, and the final assessment of the design changes and verification methods.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-05-30T16:41:42.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21715" PostTypeId="2" ParentId="21712" CreationDate="2017-05-30T16:42:26.550" Score="6" Body="&lt;p&gt;If the Dragon flies without a service module, which I believe is the current plan as no service module has yet to be unveiled by SpaceX, then one should not expect any kind of privacy during the journey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, the Soyuz toilet is simply an apparatus which is removed from storage when needed, cf. &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/wqRQM5k6uls?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; . Even in the case of the ISS, which has orders of magnitude more volume than the Soyuz, astronauts have a &quot;closet&quot; as a bedroom and the toilet is also barely private: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/XkM_04Ch76E?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; .&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-05-30T16:42:26.550" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21717" PostTypeId="2" ParentId="21712" CreationDate="2017-05-30T20:38:13.673" Score="3" Body="&lt;p&gt;One of the benefits of the Soyuz design is that there are two modules, Orbital module and Descent modules that are habitable and thus, there is 'another room' to get some  privacy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon does not have this option.  Nor did Apollo. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For short term missions, people can deal with it. For longer term some kind of add on modules is usually suggested. A small Bigelow habitat, a hard skinned module, or somesuch that provides extra space, and a second room for some privacy is option.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-05-30T20:38:13.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21718" PostTypeId="1" CreationDate="2017-05-30T21:24:19.303" Score="4" ViewCount="519" Body="&lt;p&gt;This is probably a stupid question, so I apologise in advance, but why is it that you need to achieve a certain speed to leave the planet? I understand that an orbit needs a speed so that when you fall back towards earth you constantly &quot;miss&quot;, but not to simply move away from the planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely if you move away from the planet directly upwards at a steady rate of 1 m/s, after 100,000 seconds (about 28 hours) you would reach 100km from earth and be in space? I've searched a few Escape Velocity questions and couldn't find an answer.&lt;/p&gt;&#xA;" OwnerUserId="19854" LastActivityDate="2017-05-30T21:51:16.897" Title="Why is an escape velocity needed?" Tags="|escape-velocity|" AnswerCount="0" CommentCount="0" FavoriteCount="0" ClosedDate="2017-05-31T06:05:55.837" ContentLicense="CC BY-SA 3.0" />
  <row Id="21719" PostTypeId="2" ParentId="4688" CreationDate="2017-05-30T21:44:08.637" Score="17" Body="&lt;p&gt;Escape velocity is the speed at which you'll leave the Earth and not return &lt;em&gt;if you don't continue to propel your craft&lt;/em&gt;. Below that speed, gravity will pull you back down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to keep propelling your rocket vertically at 1 m/s for 100,000 seconds, you'll need an indescribably vast amount of fuel to do so, because you have to maintain sufficient thrust to cancel out Earth's gravity for that entire time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, merely being in space isn't enough to keep you from falling back to Earth, as has been discussed in many other Q/A here. &lt;a href=&quot;https://what-if.xkcd.com/58/&quot; rel=&quot;noreferrer&quot;&gt;XKCD's got one of the more accessible explanations&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-05-30T21:51:16.897" LastActivityDate="2017-05-30T21:51:16.897" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="21720" PostTypeId="1" CreationDate="2017-05-30T22:58:21.750" Score="2" ViewCount="79" Body="&lt;p&gt;Curiosity has shown that Mars used to have &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;persistent liquid water&lt;/a&gt; on the surface. (~3+ billion years ago.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;IF&lt;/em&gt; life was around at that time, at what depth would we expect to find it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without plate tectonics, would it remain on the surface, covered only by sand from the dust storms?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it matter if water existed 3.8 billion years ago, vs &lt;a href=&quot;https://www.researchgate.net/blog/post/mars-had-water-for-longer-than-previously-thought&quot; rel=&quot;nofollow noreferrer&quot;&gt;3.3 billion&lt;/a&gt; years ago? (From a discovery / drill depth perspective)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How deep would a Mars rover need to dig, to find evidence of life from 3+ billion years ago?&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2017-05-30T22:58:21.750" Title="Mars drill depth to discover Life" Tags="|mars|water|life|geology|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21721" PostTypeId="1" AcceptedAnswerId="24322" CreationDate="2017-05-31T03:09:40.257" Score="8" ViewCount="636" Body="&lt;p&gt;The Parker Solar Probe will experience forces and torques as it passes within about 8.5 solar radii of the Sun's surface that even a &lt;a href=&quot;https://scifi.stackexchange.com/a/128382/51174&quot;&gt;General Products hull&lt;/a&gt; will not protect it from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How large will the tidal acceleration be along the 3 meter length of the spacecraft at perihelion? Will it be detectable in any way?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will there be any (real or effective) torques that will tend to rotate the spacecraft during it's closest flybys? What about effects due to the oblates of the sun?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will any of these be a new record - or have Cassini or Juno experienced greater effects during their closes approaches to Saturn and Jupiter already?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tszch.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tszch.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Parker Solar Probe cropped, from &lt;a href=&quot;http://solarprobeplus.jhuapl.edu/News-Center/Show-Article.php?articleID=24&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ymUaM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ymUaM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; screen shot from NASA's &lt;a href=&quot;http://solarprobe.jhuapl.edu/common/content/SolarProbePlusFactSheet.pdf&quot; rel=&quot;noreferrer&quot;&gt;Solar Probe Plus Fact Sheet&lt;/a&gt; &lt;strong&gt;note:&lt;/strong&gt; the link seems to be dead now, but there is plenty of related information available at &lt;a href=&quot;http://solarprobe.jhuapl.edu/index.php#spacecraft&quot; rel=&quot;noreferrer&quot;&gt;http://solarprobe.jhuapl.edu/index.php#spacecraft&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5SQPR.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5SQPR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Illustration of Parker Solar Probe's orbit, from &lt;a href=&quot;http://solarprobe.jhuapl.edu/index.php#spacecraft&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-25T18:54:46.427" LastActivityDate="2022-01-04T16:17:03.543" Title="How large are the various forces and torques that the Parker Solar Probe will experience during its closest flyby of the Sun?" Tags="|parker-solar-probe|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21723" PostTypeId="1" AcceptedAnswerId="21792" CreationDate="2017-05-31T13:38:42.170" Score="5" ViewCount="187" Body="&lt;p&gt;Let's say a Mars outpost has an inflatable greenhouse. First, it is likely to be high-CO2, as CO2 is easy to acquire and increases plant growth. Second, it may be low-pressure, as plants don't need as much atmosphere as humans, and this would reduce the necessary strength and therefore thickness of the greenhouse walls.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would these mean for a greenhouse that needs to be easily accessible for humans? Namely, regarding CO2, what is the safe limit for inhalation, and is there any limit if the crew uses breather masks? Regarding pressure, what is a reasonable limit for quick pressure changes? The primary habitat would likely be lower-pressure as well. Let's say Skylab's 5 psi if specific numbers are required.&#xA;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-04T08:56:14.860" Title="Mars greenhouse accessibility" Tags="|mars|atmosphere|habitat|plants|pressure-gradient-loss|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21724" PostTypeId="1" CreationDate="2017-05-31T13:47:07.727" Score="2" ViewCount="415" Body="&lt;p&gt;Is ISL used for TV carriers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The information I found is that it is used for IP connectivity. I did not find any information on TV broadcast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, what are the applications of ISL?&lt;/p&gt;&#xA;" OwnerUserId="19869" LastEditorUserId="6816" LastEditDate="2017-05-31T14:54:34.567" LastActivityDate="2017-06-01T17:25:16.330" Title="Inter-Satellite Link (ISL) for TV broadcast" Tags="|communication|communication-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21725" PostTypeId="2" ParentId="21724" CreationDate="2017-05-31T14:43:19.210" Score="2" Body="&lt;p&gt;Television broadcasts will not (generally) use ISL. The ground stations transmit &#xA;signals to the satellites which they rebroadcast back to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Direct-broadcast_satellite_television&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The uplink dish is pointed toward a specific satellite and the uplinked signals are transmitted within a specific frequency range, so as to be received by one of the transponders tuned to that frequency range aboard that satellite.&lt;a href=&quot;https://en.wikipedia.org/wiki/Direct-broadcast_satellite_television&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt; The transponder then converts the signals to Ku band, a process known as &quot;translation,&quot; and transmits them back to earth to be received by home satellite stations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In the case of TV broadcast the source (the ground stations) and the recipients (the large number of customers in the area the satellite broadcasts to) are both prearranged so the satellites and ground stations are placed in the correct orbits and locations to reach these areas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISL becomes more useful when the locations of the sender and receiver do not permit them to see the same satellite in the sky. But this does not (generally) happen in TV broadcast because the broadcaster has the ability to build a new ground station if needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for what ISL is actually used for? I'm not sure. But situations such as a spy satellite on the other side of the globe seem more likely.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2017-06-01T17:25:16.330" LastActivityDate="2017-06-01T17:25:16.330" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21726" PostTypeId="1" CreationDate="2017-05-31T15:02:30.673" Score="4" ViewCount="284" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sun_outage&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun outages&lt;/a&gt; occur when a satellite, usually one in a geostationary orbit, passes between a ground station and the sun. Seen from the point of view of the ground station, the sun looks like it passes behind, or at least close to the satellite. When this happens the weak radio signal received from the satellite is overwhelmed by the loud radio noise generated by the sun, and the link is broken for a short period each day for the period of a week or so, twice a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have noticed that &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun_outage&quot; rel=&quot;nofollow noreferrer&quot;&gt;sun outages&lt;/a&gt; for commercial broadcast are not impacting as often or as severely as for other satellite dishes (telecom dishes for example). Most of the time, there seems to be almost no impact on viewing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am wondering if perhaps this might be due to the fact that TV carriers are more powerful and somehow avoid sun interferences? Or maybe it is related to the design of direct television RX antennas?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks for any insight you could share.&lt;/p&gt;&#xA;" OwnerUserId="19869" LastEditorUserId="12102" LastEditDate="2017-06-01T01:23:10.653" LastActivityDate="2018-03-04T19:06:03.083" Title="Are sun outages having more limited impact in some cases?" Tags="|communication-satellite|geostationary|radio-communication|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21728" PostTypeId="2" ParentId="21614" CreationDate="2017-05-31T18:51:13.637" Score="5" Body="&lt;p&gt;At the time of Apollo 11, there weren't any systems in place for a surface rescue, but several possibilities were explored during the development of the Apollo program. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the more promising was the &lt;a href=&quot;http://www.astronautix.com/g/geminilunarcuespacecraft.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gemini Lunar Rescue Spacecraft&lt;/a&gt;, which would be based on the older 2-man Gemini capsule. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tTVLL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tTVLL.jpg&quot; alt=&quot;Lunar rescue mockup&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/FfEjq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FfEjq.jpg&quot; alt=&quot;Lunar rescue mockup&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would either land uncrewed near the planned landing location for the crew to escape in, or just wait for a rescue mission to be assembled. Alternatively it would act as the rescue craft, with additional seats for the stranded Apollo astronauts as well as its own crew. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oddly, this was vehicle was planned to use two Apollo service modules as upper stages to boost it to the moon. I'm not sure why the SM was chosen though, since a large portion of its internal volume was devoted to life support equipment&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/n5Rjz.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/n5Rjz.jpg&quot; alt=&quot;Apollo SM as upper stages&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19872" LastEditorUserId="19872" LastEditDate="2017-05-31T19:39:41.530" LastActivityDate="2017-05-31T19:39:41.530" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21729" PostTypeId="1" AcceptedAnswerId="21730" CreationDate="2017-05-31T20:00:10.243" Score="45" ViewCount="5682" Body="&lt;p&gt;As I understand it, in order to keep the lunar module ascent engine simple, instead of fuel and oxidant pumps, tanks of helium at pressure were used.   This pressure was used to force the fuel and oxidant through the pipes to the engine, where they would meet and, being hypergolic, spontaneously ignite and produce thrust.   What I cannot understand is how the engineers ensured that only fuel reached the engine and not a mixture of the gas and the fuel; in other words, how did they prevent the helium bubbling through the fuel and a fuel/helium mixture or even just helium reaching the engine.  Having to work in space it could not rely on gravity to help.&lt;/p&gt;&#xA;" OwnerUserId="10441" LastEditorUserId="6944" LastEditDate="2021-03-28T18:22:08.980" LastActivityDate="2021-05-11T15:23:15.137" Title="How did the Apollo Lunar Module ascent engine prevent gas bubbling through fuel?" Tags="|apollo-program|engines|helium|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21730" PostTypeId="2" ParentId="21729" CreationDate="2017-05-31T20:03:23.187" Score="72" Body="&lt;p&gt;The ascent engine was normally fired when the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;LM&lt;/a&gt; was sitting on the moon's surface, so the tankage was subjected to about 1/6 &lt;span class=&quot;math-container&quot;&gt;$g$&lt;/span&gt;, more than sufficient to separate the dense liquid fuel from the helium pressurant. Once the engine was ignited, its own thrust accelerating the spacecraft would maintain the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ullage&quot; rel=&quot;nofollow noreferrer&quot;&gt;ullage&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The same question could well be asked of the LM's &lt;a href=&quot;https://en.wikipedia.org/wiki/Descent_Propulsion_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;descent engine&lt;/a&gt; and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Service_Propulsion_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;main engine&lt;/a&gt; on the Apollo service module, however, which did both need to fire in free-fall. In those cases, the smaller &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_control_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;RCS&lt;/a&gt; thrusters on the LM or &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;CSM&lt;/a&gt; were fired first, to &amp;quot;settle&amp;quot; the tankage and separate the fuel from the helium. In the LM case, this &amp;quot;ullage burn&amp;quot; was about 7.5 seconds. The first couple of service module burns -- typically for mid-course correction while en route to the moon -- generally didn't need an ullage burn prior, as the tanks would be full of propellant with little or no volume of helium. SPS burns later in the mission did require ullage burns. The RCS thrusters produced about 100 lbs of thrust each, and four would be used for the ullage burn, yielding roughly 1/200 &lt;span class=&quot;math-container&quot;&gt;$g$&lt;/span&gt; acceleration.&lt;/p&gt;&#xA;&lt;p&gt;The same RCS ullage burn technique would also apply to a situation where the descent engine failed and the ascent engine needed to be used for abort from free-fall, or in flight testing of the ascent engine.&lt;/p&gt;&#xA;&lt;p&gt;That, in turn, raises the question of how helium ingestion was avoided in the RCS thrusters, since they were also helium pressurized. In those cases, the helium was separated from the propellants by a teflon bladder, so the helium didn't mix with the propellants. This was more practical to do on the smaller scale of the RCS propellant tanks than it would have been for the larger engines.&lt;/p&gt;&#xA;&lt;p&gt;All that said, it was possible for the LM descent engine to ingest helium trapped in the propellant feed lines. On the bright side, even if enough helium was ingested to briefly prevent combustion of propellant, the release of helium through the nozzle would provide a small amount of thrust, which would tend to provide settling force to the propellant tankage. Various helium-ingestion tests were conducted on the engine, detailed in &lt;a href=&quot;https://apps.dtic.mil/dtic/tr/fulltext/u2/822318.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Operating Characteristics of the Apollo LM Descent Engine with Helium Ingestion and Propellant Depletion in a Simulated Space Environment&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-05-11T15:23:15.137" LastActivityDate="2021-05-11T15:23:15.137" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="21731" PostTypeId="1" CreationDate="2017-05-31T21:47:45.653" Score="3" ViewCount="173" Body="&lt;p&gt;I was writing a story and I wanted to write about this topic but I could not find a straight answer from any website.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please help&lt;/p&gt;&#xA;" OwnerUserId="19874" LastEditorUserId="18592" LastEditDate="2017-06-01T08:49:26.417" LastActivityDate="2017-06-01T08:49:26.417" Title="Does a safety team get fined if a rocket launch fails due to the safety of the rocket?" Tags="|launch|failure|safety|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21733" PostTypeId="1" AcceptedAnswerId="28120" CreationDate="2017-06-01T13:55:34.167" Score="8" ViewCount="1343" Body="&lt;p&gt;The upcoming SpaceX launch with the apparently first re-use of a dragon capsule will bring with it the &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/iss-rosa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Roll Out Solar Array&lt;/a&gt; (ROSA).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(See also: &lt;a href=&quot;http://www.spaceflightinsider.com/organizations/space-exploration-technologies/science-reused-dragon-capsule-featured-crs-11/&quot; rel=&quot;nofollow noreferrer&quot;&gt;More science, reused Dragon capsule featured on CRS-11&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do the photovoltaic cells roll up tightly and nicely, and then unroll without any problems with the connections to the silicon? Are these basically small, rigid photovoltaic cells and standard, soldered ohmic contacts, but flexible PC board-like interconnects, or are they made from flexible semiconductor material to begin with? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While I have seen various flexible photovoltaic technology demonstrations in photos and articles, these are mostly R&amp;amp;D and PR, rather than space-rated reliable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All I know so far is that &lt;em&gt;ROSA has a &quot;&lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/iss-rosa&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;innovative flexible blanket!&lt;/strong&gt;&lt;/a&gt;&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZSnTO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZSnTO.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;A large version of the scalable &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/iss-rosa&quot; rel=&quot;nofollow noreferrer&quot;&gt;ROSA technology&lt;/a&gt; undergoes testing in 2014.&quot; Photo Credit: Deployable Space Systems. From &lt;a href=&quot;http://www.spaceflightinsider.com/organizations/space-exploration-technologies/science-reused-dragon-capsule-featured-crs-11/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-05-30T06:03:36.227" LastActivityDate="2021-05-30T06:03:36.227" Title="How does ROSA unroll? (Roll Out Solar Array)" Tags="|solar-power|solar-panel|roll-out-solar-array|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="21734" PostTypeId="2" ParentId="21709" CreationDate="2017-06-01T13:59:47.997" Score="5" Body="&lt;p&gt;An &lt;a href=&quot;http://spectrum.ieee.org/automaton/robotics/industrial-robots/meet-the-very-first-rover-to-land-on-mars&quot; rel=&quot;noreferrer&quot;&gt;IEEE Spectrum article&lt;/a&gt; explains that PROP-M was never deployed:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sadly, PROP-M never got a chance to do any exploring. The Mars-3 lander separated from its spacecraft on December 2, 1971, and entered the Martian atmosphere. After aerobraking with a heat shield, the lander deployed a parachute to slow itself down, and then when within range of the ground fired its retrorockets and made a successful landing on the surface, impacting the ground at about 20 meters per second and cushioning itself with shock-absorbing foam. Four petals on top of the capsule opened and the lander began to transmit data 90 seconds after landing, but all contact was lost just 20 seconds later, &lt;strong&gt;before the PROP-M rover had a chance to deploy.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;emphasis added&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you just read this portion, there might still be some uncertainty as to whether it could have deployed on its own, but the explanation of how the rover was intended to work clears that up:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The way that PROP-M was intended to work was that after landing, &lt;strong&gt;a signal from Earth&lt;/strong&gt; would instruct the Mars-3 lander to place the rover on the surface with a robotic arm&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;emphasis added&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With this, there can no longer be any uncertainty. The rover did not have an automatic deploy function. It was awaiting a signal from Earth, thus since communication was interrupted it could not have deployed.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-06-01T13:59:47.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21735" PostTypeId="1" AcceptedAnswerId="21736" CreationDate="2017-06-01T14:18:14.883" Score="4" ViewCount="189" Body="&lt;p&gt;How thick is the dust layer on Phobos? Is it pretty much constant or are there significant extremes?&lt;/p&gt;&#xA;" OwnerUserId="19570" LastEditorUserId="6816" LastEditDate="2017-06-01T15:20:09.833" LastActivityDate="2017-06-01T15:20:09.833" Title="How thick is the dust layer on Phobos?" Tags="|dust|phobos|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21736" PostTypeId="2" ParentId="21735" CreationDate="2017-06-01T14:25:46.047" Score="6" Body="&lt;p&gt;Nobody knows! We've never done radar sounding of Phobos, and we have no idea what lies beneath its layer of dust.&#xA;We can surmise minimum depth by looking at craters - the estimate range is 5 - 100 meters. There does appear to be significant variation, likely because of the Stickney impact, which laid many (can't remember number) meters of ejecta, and because dust from Deimos predominantly falls on Phobos's trailing edge (which I believe to be the explanation for the lack of grooves there).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two links with details on regolith thickness:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.lpi.usra.edu/meetings/lpsc2012/pdf/1142.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.lpi.usra.edu/meetings/lpsc2012/pdf/1142.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.planetary.brown.edu/pdfs/4180.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www.planetary.brown.edu/pdfs/4180.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-01T14:25:46.047" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21737" PostTypeId="1" AcceptedAnswerId="24532" CreationDate="2017-06-01T14:34:56.233" Score="3" ViewCount="300" Body="&lt;p&gt;The upcoming SpaceX launch with the apparently first re-use of a dragon capsule will bring with it the &lt;a href=&quot;https://www.nasa.gov/nicer&quot; rel=&quot;nofollow noreferrer&quot;&gt;NICER array&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NICER is a directional X-ray spectrometer with very precise timing capability. It can be used to identify X-ray pulses from neutron stars and use the timing information to do basic studies on the possibility of deep space X-ray pulsar navigation for absolute position and time determination. There will also be the possibility of testing &lt;a href=&quot;https://space.stackexchange.com/q/19882/12102&quot;&gt;X-ray communication&lt;/a&gt; as well. It's pretty cool!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a mission patch on the side. I had always thought mission patches were for missions identified with a particular launch, but the NICER-SEXTANT mission is really a research program if I understand correctly. If so, how common is it for research projects to have mission patches?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The patch looks intriguing. Is there a write-up somewhere or some information about the design of the patch? I see some pulses, what looks like a pulsar, and a lot of writing around the edges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zSVJt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zSVJt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; cropped from larger image below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vnRTq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vnRTq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Illustration of the NICER array as it would be deployed on the ISS. Credit: NASA. From &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2017/new-nasa-mission-to-study-mysterious-neutron-stars-aid-in-deep-space-navigation&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/crpch.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/crpch.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; NICER, from &lt;a href=&quot;https://heasarc.gsfc.nasa.gov/docs/nicer/nicer_about.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astrophysics on the International Space Station - &#xA;Understanding ultra-dense matter through soft X-ray timing&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TiRnx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TiRnx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Illustration of X-rays approaching the NICER array as it would be deployed on the ISS. Credit: NASA. Screen shot &lt;a href=&quot;https://www.youtube.com/watch?v=IOEPDf2DYNM&quot; rel=&quot;nofollow noreferrer&quot;&gt;of NASA video&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/IOEPDf2DYNM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="26446" LastEditDate="2020-09-23T04:17:42.743" LastActivityDate="2020-09-23T04:17:42.743" Title="What is the story behind NICER's mission patch?" Tags="|space-art|insignia|nicer|mission-patch|x-ray-communication|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21738" PostTypeId="2" ParentId="21726" CreationDate="2017-06-01T14:35:07.397" Score="3" Body="&lt;p&gt;The sun outages (also called solar interference) you are describing only occur for a few times a year for each geostationary satellite and are very short. They most certainly affect satellite TV carriers.&lt;/p&gt;&#xA;&lt;p&gt;From a &lt;a href=&quot;https://www.timewarnercable.com/en/support/faqs/faqs-tv/tvtroubles/wha&quot; rel=&quot;nofollow noreferrer&quot;&gt;carriers website&lt;/a&gt; it seems there is potential for outages for 15 minutes a day for 6 days.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The effects are seen on most channels and occur during various times of the day, lasting up to 15 minutes.&#xA;You may experience short service interruptions between Friday, March 3, 2017 and Wednesday, March 8, 2017&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.geosats.com/solaroutages.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;site&lt;/a&gt; describes how the outages are caused.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The degree of interference caused by a satellite solar outage varies from slight signal degradation to complete loss of signal as the downlink is swamped by the noise from the Sun.&lt;/p&gt;&#xA;&lt;p&gt;The duration of the sun outage depends on several things: the beam width of the field of view of the receiving ground antenna, the apparent radius of the Sun as seen from the Earth (about 0.25°), the RF energy given off by the Sun, the transmitter power of the satellite, the gain and S/N performance of the ground station receive equipment, and other factors. All of this can be used to determine the outage angle of the receive antenna. The outage angle is defined as the separation angle (measured from the ground station antenna) between the satellite and the Sun at the time when sun outage or signal degradation begins or ends (see diagram).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Differences in outaged between services and carriers can often be related to which satellite is being used. for example a satellite tv installation might be using geostationary satellites in orbital positions 110, 119, and 129 degrees. Only the services on a satellite that you are actually using, and that passes very close to the sun from your point of view will be affected.&lt;/p&gt;&#xA;&lt;p&gt;Sites such as &lt;a href=&quot;https://my.intelsat.com/si/public#/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; can be used to predict outages for particular services and to see how using a different satellite changes the results.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-01T15:21:27.833" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21739" PostTypeId="1" AcceptedAnswerId="21771" CreationDate="2017-06-01T14:48:17.183" Score="5" ViewCount="253" Body="&lt;p&gt;If two stars are very close to each other, a star tracker could identify the pair as a single star, with a magnitude and position different from both stars. This obviously impairs the software's ability to determine which star it's looking at. My question is, how could one figure out if a tracker will distinguish between two stars or blend them together? It must be related to both the distance and the magnitude of each star (as well as the tracker accuracy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding of a star tracker is as follows (please correct if I'm wrong). The tracker records pixel values within a FOV, then it defocuses the image and uses some numerical method to pinpoint where the star centroids are, as well as what their magnitudes are. How could I find out whether this process will identify two stars separately or together, based on star magnitude and distance? Some sort of generic equation like &lt;/p&gt;&#xA;&#xA;&lt;p&gt;stars are blended if: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ M_1*M_2*d_{12}^2 &amp;gt; B$  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;is true,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $M_1$ is the visual magnitude of star 1, $M_2$ is the visual magnitude of star 2, $d_{12}$ is the angular distance between the two stars, and $B$ is some constant (please note I completely made this equation up, but I imagine if there is an answer to this question it might take a similar form).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not looking for a concrete answer, rather an educated guess as to how to determine if it's likely two stars will be blended. Thanks!&lt;/p&gt;&#xA;" OwnerUserId="19427" LastActivityDate="2017-06-03T09:07:04.840" Title="What determines if a star tracker blends two stars that are close to each other?" Tags="|artificial-satellite|attitude|navigation|tracking|stars|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21740" PostTypeId="2" ParentId="21692" CreationDate="2017-06-01T15:56:49.403" Score="4" Body="&lt;p&gt;It looks like the tweet is screenshot of JPL's NEO close approach table (found &lt;a href=&quot;https://cneos.jpl.nasa.gov/ca/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;). JPL has definitions of many terms &lt;a href=&quot;https://cneos.jpl.nasa.gov/ca/neo_ca_info.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.The 'nominal' miss distance is based off the latest/greatest prediction of the asteroid's trajectory, and the 'minimum' is the minimum of all the 3-sigma possibilities for it's range at close approach. This is the 3-sigma error ellipsoid projected onto the B-Plane for the approach, see the diagram below.  The B-Plane is defined as the plane containing the Earth and target object at the time of closest approach, where the plane is perpendicular to the approach velocity of the target.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qs81u.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qs81u.png&quot; alt=&quot;Diagram of miss distances on the B-Plane&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The site below has more information on the close approach, as well as information that goes into the close approach prediction:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2012%20TC4;old=0;orb=0;cov=0;log=0;cad=1#cad&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Small Body Database Browser (2012 TC4)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's maintained by the Solar System Dyanamics group at JPL.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you look, you can see in the Orbit Determination Parameters table that the object was last tracked in 2012, so it's prediction is pretty poor for the 2017 close approach.  I would expect as the object gets closer over the course of this year, there will be additional tracking, and the orbit solution will be re-computed, and get better.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @chirlu pointed out, 2012 TC4 won't be visible until &lt;a href=&quot;http://newton.dm.unipi.it/neodys/index.php?pc=4.1&amp;amp;ots=r&quot; rel=&quot;nofollow noreferrer&quot;&gt;August&lt;/a&gt;, so the solution won't be updated until then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The better solution (paired with shorter prediction time) will likely change the predicted close approach distance, but more importantly, it will reduce the uncertainty in the prediction.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it's impossible to say if the close approach will get closer or further; it will just have less uncertainty.&lt;/p&gt;&#xA;" OwnerUserId="12483" LastEditorUserId="12483" LastEditDate="2017-06-06T14:49:40.663" LastActivityDate="2017-06-06T14:49:40.663" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21741" PostTypeId="2" ParentId="21733" CreationDate="2017-06-01T19:51:08.410" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/content/solar-cells-tested-on-flexible-array-design&quot; rel=&quot;nofollow noreferrer&quot;&gt;This NASA article&lt;/a&gt; explains that the solar arrays are made of standard solar cells on a flexible mesh backing, not unlike the current arrays on the International Space Station (only the ISS panels were stored in an accordion arrangement, not as a roll). A major problem with the arrangement is that individual cells can break while the array is being deployed. At the time they were still deciding the best form of interconnect between the cells.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The test array is also equipped with three different wiring options: open wire, ribbon wire and a combination of the two.  This allows for a variety of solar cell modules to be tested on the array.  “If a cell cracks or malfunctions, we want to know the best way to bypass the failed cell and keep the whole operation intact,” he says.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="19872" LastActivityDate="2017-06-01T19:51:08.410" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21742" PostTypeId="1" AcceptedAnswerId="21744" CreationDate="2017-06-01T20:38:07.350" Score="11" ViewCount="407" Body="&lt;p&gt;In a completely hypothetical scenario, say both of the Soyuz vehicles docked to the station were damaged/destroyed, but there was a Dragon 1.0 on the station left intact. Although i don't believe it has proper life support, if astronauts had spacesuits on with enough oxygen and CO2 scrubbing capability for the several hour journey back to Earth, would Dragon 1.0 be a suitable return vehicle?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="22326" LastEditDate="2020-02-27T14:25:50.720" LastActivityDate="2020-02-27T14:25:50.720" Title="Could Dragon 1.0 be used as an emergency Earth return vehicle" Tags="|crewed-spaceflight|reentry|dragon|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21743" PostTypeId="2" ParentId="18933" CreationDate="2017-06-01T20:44:54.323" Score="8" Body="&lt;p&gt;Here is the list of people (11) I have found so far:  &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;14-Mar-1995&#xA;Norman Thagard,       Soyuz TM-21,   Mir,  STS-71&#xA;Gennady Strekalov,    Soyuz TM-21,   Mir,  STS-71&#xA;Vladimir Dezhurov,    Soyuz TM-21,   Mir,  STS-71&#xA;&#xA;27-Jun-1995:&#xA;Anatoly Solovyev,     STS-71,        Mir,  Soyuz TM-21&#xA;Nikolai Budarin,      STS-71,        Mir,  Soyuz TM-21&#xA;&#xA;31-Oct-2000&#xA;William M. Shepherd,  Soyuz TM-31,   ISS,  STS-102&#xA;Sergei Krikalev,      Soyuz TM-31,   ISS,  STS-102&#xA;Yuri Gidzenko,        Soyuz TM-31,   ISS,  STS-102&#xA;&#xA;23-Nov-2002:&#xA;Kenneth D. Bowersox,  STS-113,       ISS,  Soyuz TMA-1&#xA;Donald R. Pettit,     STS-113,       ISS,  Soyuz TMA-1&#xA;Nikolai Budarin,      STS-113,       ISS,  Soyuz TMA-1&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="58" LastEditorUserId="12102" LastEditDate="2019-07-21T04:55:34.467" LastActivityDate="2019-07-21T04:55:34.467" CommentCount="1" CommunityOwnedDate="2017-06-01T20:44:54.323" ContentLicense="CC BY-SA 4.0" />
  <row Id="21744" PostTypeId="2" ParentId="21742" CreationDate="2017-06-01T23:23:25.317" Score="12" Body="&lt;p&gt;It's better than nothing, of course. I don't think it has carbon scrubbers but other than that it's climate controlled and pressurized (Elon: &quot;If someone were to stowaway on the unmanned version of Dragon, they would have been fine&quot;), and is designed to safely return payloads. It's probably survivable as is, and I'd say they'd have a pretty good shot of making it back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stealing equations from &lt;a href=&quot;https://www.physicsforums.com/threads/how-long-can-one-live-in-a-sealed-room.685796/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, since Dragon has 10 cubic meters of pressurized space, 0.3 cubic meters of CO2 would be toxic. Each person produces about 0.036 cubic meters of CO2 per day. So one person would have a week of breathing air, and even three people could survive over two days. CO2 scrubbers, while desirable (CO2 will reach dangerous levels within a day), are probably not necessary, especially since if a rescue mission isn't viable they're probably returning as fast as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If necessary, spacesuits could be worn adding at least 8.5 man-hours per EMU. Sokol flight suits can be worn for 30 hours in a pressurized environment, but I'm not sure this means 30 hours without external life support. Neither of these have carbon scrubbers, so all crew would have to don these at the same time when CO2 in the capsule starts getting too high. Not sure whether ISS scrubbers could feasible - Those were definitely not designed to fit in a capsule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other big challenge is surviving impact. 3.5 g, while uncomfortable for sure, is not going to be lethal unless something goes really wrong, and if the Soyuz spacecraft are merely disabled, then acceleration couches (among other things) can be salvaged. The impact of hitting the ocean is probably more dangerous, and I frankly can't find figures on that. Without couches, serious injury is likely, especially if the crew is in suits.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-01T23:23:25.317" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21745" PostTypeId="1" AcceptedAnswerId="21746" CreationDate="2017-06-01T23:27:10.100" Score="6" ViewCount="278" Body="&lt;p&gt;Every astronaut launching on a Soyuz has a seat custom-made &lt;a href=&quot;https://twitter.com/Astro_Feustel/status/867796988766695426&quot; rel=&quot;noreferrer&quot;&gt;(source)&lt;/a&gt;. However, I don't believe astronauts typically return on their original Soyuz, and I know that they don't always. In this case, are seats swapped, or are the seats still generic enough that someone else's seat can be used for return?&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-01T23:38:40.283" Title="Soyuz seat - return" Tags="|soyuz-spacecraft|acceleration|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21746" PostTypeId="2" ParentId="21745" CreationDate="2017-06-01T23:38:40.283" Score="7" Body="&lt;p&gt;When the crew are going to return on a different Soyuz, they swap the seat liners beforehand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;One of the first tasks for the Soyuz commanders after arrival was to swap the astronaut’s seat-liners to their new ferry.&quot; - &#xA;&lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/iriss/Spacecraft_swap&quot; rel=&quot;noreferrer&quot;&gt;http://www.esa.int/Our_Activities/Human_Spaceflight/iriss/Spacecraft_swap&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-06-01T23:38:40.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21749" PostTypeId="1" AcceptedAnswerId="23443" CreationDate="2017-06-02T08:16:56.137" Score="3" ViewCount="1021" Body="&lt;p&gt;Now that the protests at &lt;a href=&quot;https://space.stackexchange.com/q/20826/12102&quot;&gt;Kourou Space Center in French Guiana&lt;/a&gt; are over, Arianne has launched both &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/eutelsat-172b.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;EutelSat 172B&lt;/a&gt; and &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/viasat-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ViaSat-2&lt;/a&gt; as &lt;code&gt;2017-029A&lt;/code&gt;. While EutelSat 172B has a photovoltaic power of about 13 kW, &lt;a href=&quot;https://www.viasat.com/news/5-minutes-5-questions-boeing&quot; rel=&quot;nofollow noreferrer&quot;&gt;ViaSat-2 has 18 kW&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;VS: What is the lifespan of the ViaSat-2 satellite and how is it powered?&lt;/p&gt;&#xA;&lt;p&gt;RD: ViaSat-2 has a lifespan of about 14 years and is powered by a solar array that generates nearly 18kW of power as well as Lithium Ion batteries. The vehicle maintains it orbital location for 14 years using xenon ion propulsion (electric propulsion).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This seems to be the top end for the &lt;a href=&quot;http://space.skyrocket.de/doc_sat/hs-702.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Boeing 702HP Bus&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Innovation extends to the Boeing 702 power systems as well. The Boeing 702 offers a range of power up to 18 kW. Dual and triple-junction gallium arsenide solar cells enable such high power levels. Spectrolab, Inc. a Boeing subsidiary, developed the cells.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is an 18 kW solar array the largest to ever be deployed for a commercial telecom satellite?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The article continues with more power-related information on the 702HP bus...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The first version of the 702 used solar arrays with concentrators. These concentrators tended to early fogging, as due to an inherent design flaw the outgassing of the solar cells was higher than expected. This fogging lead to much reduced lifetime. The flaw was corrected in later versions with higher power triple-junction gallium arsenide solar cells.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/q/21410/12102&quot;&gt;Were solar concentrators on HS-702 just a stop-gap measure until triple-junction PV cells were available?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Boeing 702 separates the bus and payload thermal environments and substantially enlarged the heat radiators to achieve a cooler, more stable thermal environment for both bus and payload. This increases unit reliability over service life. Deployable radiators use flexible heat pipes, which increase packageable radiator area. Further thermal control occurs through passive primary rejection via heat pipes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;See &lt;a href=&quot;https://space.stackexchange.com/q/21524/12102&quot;&gt;What are these very large, square panels on Inmarsat 5's?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-19T04:26:45.610" Title="ViaSat's 18 kW solar array - largest ever for a commercial telecom satellite?" Tags="|design|solar-power|geostationary|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21750" PostTypeId="1" AcceptedAnswerId="21753" CreationDate="2017-06-02T09:33:30.380" Score="65" ViewCount="14899" Body="&lt;p&gt;I am watching &lt;a href=&quot;https://www.youtube.com/watch?v=wwMDvPCGeE0&quot; rel=&quot;noreferrer&quot;&gt;live stream from ISS&lt;/a&gt; to follow the return of two ISS crew members, &lt;a href=&quot;https://en.wikipedia.org/wiki/Thomas_Pesquet&quot; rel=&quot;noreferrer&quot;&gt;Thomas Pesquet&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Oleg_Novitskiy&quot; rel=&quot;noreferrer&quot;&gt;Oleg Novitskiy&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At several moments, they crossed their arms in a not really natural way:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://img15.hostingpics.net/pics/914860CrossedArms1.png&quot; alt=&quot;Crossed arms 1&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://img15.hostingpics.net/pics/207840CrossedArms2.png&quot; alt=&quot;Crossed arms 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why do they cross their arms like that? Is that a standard procedure to not touch anything by accident?&lt;/p&gt;&#xA;" OwnerUserId="18592" LastEditorUserId="4660" LastEditDate="2017-06-02T13:38:51.037" LastActivityDate="2017-06-05T15:12:50.960" Title="Why do ISS crew members often cross their arms?" Tags="|iss|crewed-spaceflight|procedure|" AnswerCount="4" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21751" PostTypeId="1" AcceptedAnswerId="21757" CreationDate="2017-06-02T09:39:25.513" Score="10" ViewCount="1385" Body="&lt;p&gt;Soyuz remains docked to ISS for months at a time, then returns the crew to Earth.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;em&gt;The longest the shuttle has stayed in orbit on any single mission is 17.5 days on mission STS-80 in November 1996&lt;/em&gt; &lt;a href=&quot;https://spaceflight.nasa.gov/shuttle/reference/basics/&quot; rel=&quot;noreferrer&quot;&gt;src&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I wonder if it was an inherent limitation of the shuttle, or just some problem of mission logistics and such. Could the shuttle serve as the same kind of &quot;lifeboat&quot; as Soyuz does now?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;on a personal note, I think it would have been nice if they left one shuttle berthed permanently to ISS, as storage space and an emergency shelter, when they were retired.&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-06-02T13:42:08.917" Title="How long could the shuttle remain docked to ISS and still capable of landing?" Tags="|iss|space-shuttle|long-duration|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21753" PostTypeId="2" ParentId="21750" CreationDate="2017-06-02T11:24:11.607" Score="67" Body="&lt;p&gt;It's about as standard procedure as crossing your legs when you're sitting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Arms in microgravity, without conscious effort to keep them by your body, will tend to extend the elbows to the sides - just like holding knees together when sitting, at least for men, requires active (if minor) effort. Holding arms like this simply prevents sticking elbows into the person next-to, and takes less effort than actively squeezing them along your body. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare to these:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;arms loose&#xA;&lt;a href=&quot;https://i.stack.imgur.com/XcVyB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XcVyB.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;people keeping arms along their body, do this consciously - you can see the positions are forced. Others are either spreading their elbows, or holding their arms.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/G6NwH.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/G6NwH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-06-02T11:24:11.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21754" PostTypeId="2" ParentId="21750" CreationDate="2017-06-02T11:29:42.953" Score="5" Body="&lt;p&gt;There is no special procedure there - just microgravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the one on the left looks like a normal relaxed pose, with his arms dropped down as they would be on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other two do look a little &quot;unnatural&quot;, but if you ignore their arms being pointed forward, it is exactly the same pose, and just as comfortable with no gravity pulling their arms down.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-06-02T11:29:42.953" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21755" PostTypeId="2" ParentId="21750" CreationDate="2017-06-02T13:09:24.953" Score="54" Body="&lt;p&gt;Offered as a supplement to &lt;a href=&quot;https://space.stackexchange.com/a/21753&quot;&gt;@SF's answer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This shows the fully relaxed arm position obtained by a sleeping astronaut.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GhYWt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GhYWt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.urban75.org/blog/how-astronauts-sleep-in-space-and-a-fascinating-look-around-the-international-space-station/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2017-06-05T15:12:50.960" LastActivityDate="2017-06-05T15:12:50.960" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21756" PostTypeId="2" ParentId="21751" CreationDate="2017-06-02T13:24:20.623" Score="4" Body="&lt;p&gt;When Shuttle was docked, it was used to do any propulsive maneuvers the station required.  So, eventually you would use up enough prop to hit the propellant quantity redlines for a safe deorbit.  You might be able to stretch it by switching back and forth between ISS and Shuttle control, but I don't know how well ISS motion control would work with a docked Shuttle.  My guess is, not well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A true number would require detailed analysis of the use of Shuttle prop while docked, what the prop quantities at docking were, and what the deorbit redlines were.  Good exercise for the reader.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-06-02T13:24:20.623" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21757" PostTypeId="2" ParentId="21751" CreationDate="2017-06-02T13:42:08.917" Score="8" Body="&lt;p&gt;Shuttle mission length was limited by &lt;a href=&quot;https://space.stackexchange.com/questions/8448/could-the-space-shuttle-be-permanently-left-in-orbit-as-a-space-station/8450#8450&quot;&gt;lots of consumables&lt;/a&gt;, including propellant and canisters for the life support system.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrical_system_of_the_International_Space_Station#Station_to_shuttle_power_transfer_system&quot; rel=&quot;noreferrer&quot;&gt;Station-to-Shuttle Power&lt;/a&gt; Transfer System (SSPTS) could provide electrical power to the Shuttle, but it only allowed the shuttle to &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20110015485&amp;amp;hterms=SSPTS&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3DSSPTS%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA&quot; rel=&quot;noreferrer&quot;&gt;remain docked for an extra 4 days&lt;/a&gt;. Only Discovery and Endeavour were equipped with the SSPTS.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Space Shuttle was most often utilized in its last 10 years of operations as a construction vehicle that hauled pieces of the enormous ISS into orbit. Since much of its time on these missions was spent docked to the station, it seemed logical to design a way for the Orbiter to draw power from the massive solar panels and batteries of the ISS. These solar panels enjoy the renewable resource of sunlight as their power source whereas the shuttle’s fuel cells have a limited amount of cryogenic reactant on board. By being able to run the fuel cells at their minimum power of about 2 kW, the team would be able to increase their cryogenic margins and remain in orbit for 2-3 extra days. These added days allowed critical time for extra-vehicular activities (EVA) or other work on the ISS.&lt;/p&gt;&#xA;&lt;p&gt;The system that connected the Orbiter power system to that of the ISS was called the Station to Shuttle Power Transfer System (SSPTS). This system was able to increase the docked duration from about 6-8 days to approximately 9-12 days by transferring responsibility of up to 8 kW of the Orbiter’s load to the ISS. This power transfer was achieved by adding two power transfer units (PTUs) to the Orbiter which converted the 120VDC input power from the station to 28VDC expected by the main buses (Fig. 27).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;An additional limit is the &lt;a href=&quot;https://space.stackexchange.com/questions/8448/could-the-space-shuttle-be-permanently-left-in-orbit-as-a-space-station/8450#8450&quot;&gt;year-end rollover&lt;/a&gt; on the Shuttle's computers, which was worked around by not having the Shuttles fly on New Year's Eve.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-02T13:42:08.917" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21758" PostTypeId="1" CreationDate="2017-06-02T14:11:59.470" Score="12" ViewCount="11942" Body="&lt;p&gt;I've been looking for this for quite some time now, and I can't find anything other than calculations where they already assume a chamber pressure. So my questions are two:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;What are the equations for calculating the chamber pressure for a bipropellant engine? In case it's the opposite, how do you find out the needed propellant flow rate so that you achieve the design chamber pressure?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;When you have a pressurized gas thruster, that is, with no combustion, what do you have to take into account in order to calculate the parameters of the injector?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="19712" LastActivityDate="2024-02-16T16:02:04.107" Title="How is chamber pressure determined for rocket engines?" Tags="|engine-design|engines|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21759" PostTypeId="1" AcceptedAnswerId="21760" CreationDate="2017-06-02T16:11:47.937" Score="29" ViewCount="11667" Body="&lt;p&gt;I just saw this photo in &lt;a href=&quot;https://space.stackexchange.com/a/21753/12102&quot;&gt;@SF.'s answer&lt;/a&gt; it is &lt;a href=&quot;https://spaceflight.nasa.gov/gallery/images/station/crew-23/html/iss023e023513.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS023-E-023513&lt;/a&gt; &lt;em&gt;STS-131 and Expedition 23&lt;/em&gt;. The full caption is below.&lt;/p&gt;&#xA;&lt;p&gt;Are there really 13 people on the ISS at one time?&lt;/p&gt;&#xA;&lt;p&gt;Is STS-131 the only shuttle mission that resulted in 13 people being inside the ISS?&lt;/p&gt;&#xA;&lt;p&gt;I'm wondering about the load on the station from the extra number of bodies; water vapor, CO2, particulates, movement/vibrations... were any of these at all an issue?&lt;/p&gt;&#xA;&lt;p&gt;How long did this last - just enough for a photo op or did it last a while?&lt;/p&gt;&#xA;&lt;p&gt;I wanna ask if the shuttle crew were allowed to use the facilities on the ISS or if they had to go back to the shuttle, but I'm too shy.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KOx82.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KOx82.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;ISS023-E-023513 (14 April 2010) --- STS-131 and Expedition 23 crew members gather for a group portrait in the Kibo laboratory of the International Space Station while space shuttle Discovery remains docked with the station.&lt;br /&gt;&#xA;STS-131 crew members pictured (light blue shirts) are NASA astronauts Alan Poindexter, commander; James P. Dutton Jr., pilot; Clayton Anderson, Rick Mastracchio, Dorothy Metcalf-Lindenberger, Stephanie Wilson and Japan Aerospace Exploration Agency astronaut Naoko Yamazaki, all mission specialists.&lt;br /&gt;&#xA;Expedition 23 crew members pictured are Russian cosmonauts Oleg Kotov, commander; Mikhail Kornienko and Alexander Skvortsov; Japan Aerospace Exploration Agency astronaut Soichi Noguchi, and NASA astronauts T.J. Creamer and Tracy Caldwell Dyson, all flight engineers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="36360" LastEditDate="2022-03-25T16:00:22.757" LastActivityDate="2022-03-25T16:00:22.757" Title="How many times has there been 13 people inside the ISS and is it hard on the station?" Tags="|iss|crewed-spaceflight|space-shuttle|life-support|crew-size|" AnswerCount="2" CommentCount="6" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21760" PostTypeId="2" ParentId="21759" CreationDate="2017-06-02T17:02:45.593" Score="17" Body="&lt;p&gt;The only 2 missions I am aware of that had 13 people in the ISS at one time are STS-131 (as listed in the question) and STS-127.  Here is the analogous crew photo from that mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/N8G5y.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/N8G5y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which crewperson was in which vehicle can be teased out of the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/438131main_FLT_PLN_131_F_1.pdf&quot; rel=&quot;noreferrer&quot;&gt;Flight Plan&lt;/a&gt; by following their assigned tasks through the Summary Flight Plan section. This does require that one knows where a task is performed, but for the full complement it would pretty much have to be in the ISS (or the interface, for the welcome ceremony).  Once they dock on Flight Day 3, the Plan turns into a two-page format with shuttle crew on one page, ISS crew on the other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have attached a screenshot from the STS-131 Plan showing when their crew photo was planned to happen (Flight Day 09, 01:00 hours Mission Elapsed Time). &lt;a href=&quot;https://i.stack.imgur.com/t18Oo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t18Oo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, just like battle plans don't survive contact with the enemy, Flight Plans don't survive contact with the mission, and updates are sent up daily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am sorry the resolution is so low, I wanted to show both pages.  You can download the Flight Plan .pdf and see it in all its high res glory if you like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quick flip through the Plan showed the welcome ceremony and safety briefing also involving all crewpersons. I could easily have missed something, and there were almost certainly informal occasions (meals, etc) not shown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As being hard on the station, I am sure that all 13 could not stay there for a long time, but all of this was obsessively planned and analyzed.  Please see &lt;a href=&quot;https://space.stackexchange.com/users/897/hobbes&quot;&gt;Hobbes&lt;/a&gt;'s answer for details.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-07-04T01:09:25.970" LastActivityDate="2017-07-04T01:09:25.970" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21761" PostTypeId="2" ParentId="19133" CreationDate="2017-06-02T17:52:04.900" Score="3" Body="&lt;p&gt;Your question &quot;But could those creatures get married rationally?&quot; is a good one. Rationally - sort of. The answers here that refer to thrust forget the solid rocket boosters have to lift themselves. If you run the numbers (try the &quot;Silverbird&quot; tool on the web for ROM purposes) you will get the 70t payload to LEO for an approximate description of the SLS (enter the SRB's and the Core Stage specs). Replace the SRBs with the Falcon 9 (first stage only) and you get about 10% less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dilemma is the SRB and the External Tank derived core stage as a concept. The SRB's support the core stage. That's where the difference in payload occurs. Once I start to change out the SRB's for Falcon 9 1st stages I have to modify the core stage. The natural decision is to increase commonality. Guess what you get? The Falcon heavy at 63.8t to LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The difference in payload becomes more acute with the upper stages - the SLS core stage has a larger diameter, the Falcon 9 less. It's in that final push that the SLS, if it has a saving grace, can add a larger diameter upper stage and get to 105t payload to LEO. Now the Falcon 9 MAYBE can try this, but that gets tricky, and in either case you are quickly creating custom Falcon 9's and upper stages which mean less opportunity for amortizing effort and equipment back at the plant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your question, could Falcon 9's replace SRB's? Not really, unless assuming lots of other &quot;replacements&quot;, like the core stage too! That just takes you on a quick path right back to the Falcon Heavy.&lt;/p&gt;&#xA;" OwnerUserId="19893" LastActivityDate="2017-06-02T17:52:04.900" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21762" PostTypeId="1" AcceptedAnswerId="21794" CreationDate="2017-06-02T19:10:57.580" Score="1" ViewCount="139" Body="&lt;p&gt;The way soldiers meet a wall in an obstacle run; they boost 1 or 2 of the team up to the top and then those up there help pull their colleagues up and over.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just wondered if there might be a way something could be constructed in LEO that could help pull other systems up (not necessarily all the way). I suppose in my head I'm imagining something equivalent to a kind of immensely powerful magnet similar to those used in MRI, but are there any other thoughts or ideas along these lines being, or have been, considered?&lt;/p&gt;&#xA;" OwnerUserId="19894" LastActivityDate="2017-06-04T11:23:33.970" Title="Payloads pulled into orbit/space" Tags="|orbit|escape-velocity|" AnswerCount="3" CommentCount="2" ClosedDate="2017-06-09T08:13:29.367" ContentLicense="CC BY-SA 3.0" />
  <row Id="21763" PostTypeId="2" ParentId="21762" CreationDate="2017-06-02T19:49:32.800" Score="2" Body="&lt;p&gt;The primary issue is that getting into orbit is mostly a matter of velocity/kinetic energy, not height. The craft pulling would be slowed down, and the thrust required to get it and the new craft into the pulling craft's orbit would be the same as the thrust required to get the new craft into orbit without help.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some ideas to use tethers attached to large masses, as a suborbital craft could grab onto a tether moving at zero relative velocity and then climbing up to orbit. This depends on there being a much, much larger mass in orbit. Example: &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/06/phobos-panama-canal-of-inner-solar.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phobos tether&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-02T19:49:32.800" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21764" PostTypeId="2" ParentId="8534" CreationDate="2017-06-02T19:51:28.400" Score="5" Body="&lt;p&gt;Yes, it can. &lt;a href=&quot;https://www.twitter.com/elonmusk/status/669132749500887040&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk himself confirmed it&lt;/a&gt; some time ago, and there are some &lt;a href=&quot;http://www.flightclub.io/results.php?id=42427e8d-2b28-4eee-96b9-9837a59e36b0&amp;amp;code=OG22&quot; rel=&quot;noreferrer&quot;&gt;fan-made simulations&lt;/a&gt; that prove it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/lpDYyOg6sfA?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="19712" LastEditorUserId="6816" LastEditDate="2017-06-02T20:19:20.200" LastActivityDate="2017-06-02T20:19:20.200" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21765" PostTypeId="1" CreationDate="2017-06-02T22:24:19.303" Score="21" ViewCount="4291" Body="&lt;p&gt;Sputnik 1 was the first satellite in space. Did it have a programming language on board or was it purely analog?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;in 1949 Von Neumann released the language Short Code.&#xA;Fortran was invented in 1957.&#xA;Sputnik launched in 1957.&#xA;Dennis Ritchie created the C programming language @ Bell Laboratories in 1972.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: Thanks for the answer and commentary, really appreciate the level of knowledge here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the purposes of my enquiry I'm happy to clarify the question as 'the first machine language in space'. Obviously coding would be done on the ground. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also for the purposes of clarification: will exclude the analog examples provided. ie: only machine code running on semiconductor chips.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another question for those interested.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/21948/what-is-the-most-popular-programming-language-in-space&quot;&gt;What is the most popular programming language in space?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19896" LastEditorUserId="12102" LastEditDate="2021-09-05T23:15:08.623" LastActivityDate="2021-09-06T15:10:21.403" Title="What was the first programming language in space?" Tags="|artificial-satellite|history|flight-computer|spaceflight-firsts|computing|" AnswerCount="2" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="21766" PostTypeId="2" ParentId="21765" CreationDate="2017-06-02T23:22:05.593" Score="46" Body="&lt;p&gt;(I'm interpreting the question as &amp;quot;what programming language was the first spacecraft software written in?&amp;quot;, per discussion in comments above.)&lt;/p&gt;&#xA;&lt;p&gt;This will depend on your definitions of &amp;quot;computer&amp;quot; and &amp;quot;programming language&amp;quot;. Almost certainly the first digital computers on spacecraft were programmed in assembly language or microcode. You can get a lot of historical detail from &lt;a href=&quot;https://www.hq.nasa.gov/office/pao/History/computers/contents.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Computers In Spaceflight: The NASA Experience&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Adoption of high-level languages happened much later in spacecraft than on the ground because, typically, assembly language is more efficient (so you can do equivalent work on a lighter, less power-hungry, and/or more radiation-hardened CPU), and gives finer-grained control to the programmer (thus less likely to produce unexpected results).&lt;/p&gt;&#xA;&lt;p&gt;The earliest digital computer on a spacecraft I can find is &lt;a href=&quot;https://en.wikipedia.org/wiki/Ranger_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ranger 3&lt;/a&gt;, 1962:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Spacecraft control was provided by a solid-state computer and sequencer and an earth-controlled command system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Sequencer&amp;quot; suggests it was programmed via physical switches and/or hard-wired microcode at a quite low level, but I haven't found any real details on the computer itself.&lt;/p&gt;&#xA;&lt;p&gt;Gemini was the &lt;a href=&quot;https://www.scientificcomputing.com/news/2015/10/auction-memory-first-computer-space&quot; rel=&quot;nofollow noreferrer&quot;&gt;first crewed US spacecraft to carry a digital computer&lt;/a&gt;, starting with Gemini 3 in 1965.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;The computer weighed approximately 59 pounds, performed more than 7,000 calculations a second, and needed no more room than a hatbox — 1.35 cubic feet — aboard the Gemini. It had an average power consumption of 94.54 watts, a 500 kc bit rate, a memory cycle time of 250 kc and an add time of 140 microseconds. The computer's memory was a random-access, nondestructive readout design with flexible instruction and data storage organization. Its nominal capacity was 4,096 39-bit words and its operational capacity was 12,288 13-bit words.&amp;quot; (IBM's historical archives).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This almost certainly was programmed in assembly language rather than a high-level language.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo spacecraft&lt;/a&gt; flown in 1989 appears to be the first US uncrewed spacecraft flying &lt;a href=&quot;https://www.hq.nasa.gov/office/pao/History/computers/Ch6-3.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;software written in a high-level language&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;JPL carefully explored memory requirements and software development prospects. Prototype programs were written in HAL/S and FORTRAN for the command computer and the attitude control computer. Ideas for the content of the programs came from Voyager experience and the executive written for the NSSC-1. The project office originally specified that HAL would be used for programming all flight software. When irreducible inefficiencies appeared in the compiler bought for the command and data computers, HAL was abandoned for that system and replaced with &amp;quot;structured macros&amp;quot;. HAL was retained for the other computer system. Although most microprocessors in the scientific experiments are coded in assembler, one is programmed in FORTH, so high-level languages finally appeared on uncrewed spacecraft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The USSR's &lt;a href=&quot;http://web.mit.edu/slava/space/introduction.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;first digital computer&lt;/a&gt; in space was the &lt;a href=&quot;http://www.computer-museum.ru/english/argon11c.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Argon 11c&lt;/a&gt; aboard an uncrewed Zond series spacecraft in 1964. I assume it was programmed in assembly language rather than a high level language.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Number representation - fixed point. Word length - 14 bits, command length - 17 bits. Number of instructions - 15.&lt;/p&gt;&#xA;&lt;p&gt;Execution times: addition - 30 ms, multiplication - 160 ms.&lt;/p&gt;&#xA;&lt;p&gt;RAM capacity - 128 14-bit words, ROM capacity - 4,096 17-bit words.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The early Soyuz spacecraft lacked digital computers; the Soyuz-T in 1980 appears to have been their first crewed spacecraft controlled by an onboard digital computer, &lt;a href=&quot;http://www.computer-museum.ru/english/argon16.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Argon-16&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Representation of numbers - fixed point. Word length: 16 bits (word), 32 bits (double word); instruction length - 16 bits. Number of instructions - 32.&lt;/p&gt;&#xA;&lt;p&gt;Speed: addition - 5 ms, multiplication - 45 ms.&lt;/p&gt;&#xA;&lt;p&gt;RAM capacity: 3 x 2 Kbytes, ROM capacity - 3 x 16 Kbytes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I would guess this was also assembly-programmed, though with the larger ROM and later date, a high-level language wouldn't have been out of the question.&lt;/p&gt;&#xA;&lt;p&gt;The first high-level language on a spacecraft that I know of would be the space shuttle's HAL/S, first &amp;quot;delivered&amp;quot; in 1973 and first flown sometime between 1977 and 1981.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-09-06T15:10:21.403" LastActivityDate="2021-09-06T15:10:21.403" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="21767" PostTypeId="2" ParentId="21762" CreationDate="2017-06-03T00:19:58.797" Score="3" Body="&lt;p&gt;Most such schemes you might envision won't accomplish anything useful. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because of &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton.27s_3rd_Law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's third law&lt;/a&gt;, any assistance given to the launching vehicle would pull down the helper in LEO, reducing the latter's momentum by the same amount of momentum given to the former. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-06-03T00:19:58.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21768" PostTypeId="1" CreationDate="2017-06-03T06:18:08.587" Score="8" ViewCount="1953" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YWa9y.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YWa9y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;https://www.facebook.com/NASATESS/photos/pb.532821116771980.-2207520000.1468552924./1037589306295156/?type=3&quot; rel=&quot;noreferrer&quot;&gt;Facebook&lt;/a&gt;, then annotated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/mpViVEO-ymc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an excellent review of the design of the TESS spacecraft in G. R. Richter et al. Journal of Astronomical Telescopes, Instruments, and Systems 1(1), 014003 (Jan–Mar 2015) &lt;a href=&quot;http://seagerexoplanets.mit.edu/ftp/Papers/TESS2015.pdf&quot; rel=&quot;noreferrer&quot;&gt;PDF available at MIT here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The text of this &lt;a href=&quot;https://twitter.com/NASA_TESS/status/849790829506043904&quot; rel=&quot;noreferrer&quot;&gt;5-Apr-2017 tweet&lt;/a&gt; from &lt;a href=&quot;https://twitter.com/NASA_TESS&quot; rel=&quot;noreferrer&quot;&gt;NASA_TESS&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;.&lt;a href=&quot;https://twitter.com/NASA_TESS&quot; rel=&quot;noreferrer&quot;&gt;NASA_TESS&lt;/a&gt; with solar arrays stowed. &lt;a href=&quot;https://twitter.com/OrbitalATK&quot; rel=&quot;noreferrer&quot;&gt;OrbitalATK&lt;/a&gt; spacecraft will be in this configuration when it is placed in a &lt;a href=&quot;https://twitter.com/SpaceX&quot; rel=&quot;noreferrer&quot;&gt;SpaceX&lt;/a&gt; Falcon9 next year!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and it shows an image of the antenna. Below are cropped sections from that image and from an &lt;a href=&quot;https://twitter.com/NASA_TESS/status/828593769909977089&quot; rel=&quot;noreferrer&quot;&gt;earlier, 06-FEb-2017 tweet&lt;/a&gt;. The largest one highlights the radial undulations in the contour of the reflector as concentric rings. I know that combining diffractive effects with traditional curved surfaces is used in light optics for a variety of 'tricks', but I'm not sure what's going on here. What are these undulations doing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; an effective diffractive pattern on a reflector would need to be of the order of a quarter wavelength tall so that upon reflection the shift would be of order a half-wavelength. For Ka band at 27-40 GHz that's only a few millimeters. These undulations look like the right height, and appear to be spaced by several wavelengths, so this really has at least the ballpark dimensions of a diffractive surface. But &lt;em&gt;why?&lt;/em&gt;, I can't guess.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; A few items;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The wavelength used by the Ka band antenna is roughy 7.5 to 11 millimeters.&lt;/li&gt;&#xA;&lt;li&gt;The spacing of each of those ridges, at least near the edge, is about 1/30th of the 70 centimeter aperture, or about 23 millimeters.&lt;/li&gt;&#xA;&lt;li&gt;The pattern would tend to diffract a Ka band wavelength by about 20° to 30°. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It seems too aggressive to be reshaping the beam from Gaussian to flat-top (analogous to &lt;a href=&quot;https://space.stackexchange.com/a/20874/12102&quot;&gt;footprint engineering by reflector-shaping&lt;/a&gt;), and that's usually done by re-optimizing the shapes of both surfaces (&lt;a href=&quot;https://en.wikipedia.org/wiki/Parabolic_antenna#Dual_reflector_shaping&quot; rel=&quot;noreferrer&quot;&gt;dual reflector shaping&lt;/a&gt;). Could it be some kind of medium gain extension to the beam shape to help with capture? Or a passive rejection filter for a different wavelength?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the surface shown has a distinct fabric pattern. Will there be an additional reflector on top of this (or below it), or is this conductive and smooth enough to be reflective? Perhaps enough for testing at least?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hmmm... &lt;em&gt;non-shiny or optically matte surface finish&lt;/em&gt; and &lt;em&gt;non-specular, diffractive phase surface&lt;/em&gt; on a parabolic reflector which is rigidly fixed to the side of a space telescope that will spend most of its time busy scanning the heavens, pointing in all kinds of directions... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xj3YP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xj3YP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped from image in &lt;a href=&quot;https://twitter.com/NASA_TESS/status/828593769909977089&quot; rel=&quot;noreferrer&quot;&gt;this 5-Apr-2017 tweet&lt;/a&gt; from NASA_TESS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8HnLY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8HnLY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped and modified contrast from image in &lt;a href=&quot;https://twitter.com/NASA_TESS/status/849790829506043904&quot; rel=&quot;noreferrer&quot;&gt;this earlier, 06-FEb-2017 tweet&lt;/a&gt; from NASA_TESS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/beuuG.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/beuuG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped from image in &lt;a href=&quot;https://twitter.com/NASA_TESS/status/849790829506043904&quot; rel=&quot;noreferrer&quot;&gt;this earlier, 06-FEb-2017 tweet&lt;/a&gt; from NASA_TESS.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-21T01:32:03.677" LastActivityDate="2023-02-02T06:47:31.123" Title="Why is TESS' high gain antenna made of undulating BLACK fabric rather than metal?" Tags="|radio-communication|antenna|tess|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21769" PostTypeId="2" ParentId="21759" CreationDate="2017-06-03T08:35:28.587" Score="16" Body="&lt;p&gt;To answer the part 'is is hard on the station': &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/ISS_ECLSS#Air_revitalisation_system&quot; rel=&quot;noreferrer&quot;&gt;air conditioning&lt;/a&gt; system can cope with &lt;a href=&quot;https://www.quora.com/How-do-the-CDRA-and-Vozdukh-systems-scrub-carbon-dioxide-from-the-air-on-the-International-Space-Station-ISS&quot; rel=&quot;noreferrer&quot;&gt;at least 12 people&lt;/a&gt; on board continuously. There are 3 systems for removal of carbon dioxide: 2 Carbon Dioxide Removal Assembly (CDRA) and Vozdukh, each is designed to remove the CO&lt;sub&gt;2&lt;/sub&gt; generated by 6 people.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I get the impression capacity is slightly more than that, to account for experiments that use oxygen (e.g. live animals). &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Vozdukh is primary and can scrub the air for a six person crew.  It uses less power than CDRA.  CDRA serves as a backup and for additional support when the crew complement is higher (such as when there was a visiting shuttle crew).  As a further backup for contingency cases, the Russian Segment also has LiOH canisters that can adsorb contaminants.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Oxygen is supplied from pressure vessels, so that 's a matter of logistics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090033097.p%E2%80%8C%E2%80%8Bdf&quot; rel=&quot;noreferrer&quot;&gt;waste water recovery system&lt;/a&gt; is designed for a load of 6 people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 2 systems that remove the excess humidity (from breathing, perspiration and evaporation, e.g. from wet towels) from the air.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ISS has two water recovery systems. Zvezda contains a water recovery system that processes water vapor from the atmosphere that could be used for drinking in an emergency but is normally fed to the Elektron system to produce oxygen. The American segment has a Water Recovery System installed during STS-126 that can process water vapour collected from the atmosphere and urine into water that is intended for drinking. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.sciencedaily.com/releases/2008/11/081111210838.htm&quot; rel=&quot;noreferrer&quot;&gt;total capacity&lt;/a&gt; of these 2 systems is 12 people.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-06-04T16:18:47.797" LastActivityDate="2017-06-04T16:18:47.797" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21770" PostTypeId="1" AcceptedAnswerId="26383" CreationDate="2017-06-03T08:36:08.253" Score="2" ViewCount="448" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; More information in Phys.org's news item &lt;a href=&quot;https://phys.org/news/2017-08-nasa-transiting-exoplanet-survey-satellite.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;TESS mission to discover new planets moves toward launch&lt;/a&gt;, including a video of the telescope array's final assembly.&lt;/p&gt;&#xA;&lt;p&gt;Also see &lt;a href=&quot;https://tess.gsfc.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;GSFC's TESS page&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;The question:&lt;/strong&gt; &lt;em&gt;Will there be &amp;quot;Near Space&amp;quot; Ka-band allocations for TESS?&lt;/em&gt; is seeking a boolean yes/no plus addressing the item at the end: &lt;em&gt;Are there now approved Near Space Ka-Band frequencies, and does the Wikipedia article need to be updated?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Transiting_Exoplanet_Survey_Satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;TESS&lt;/a&gt; has a group of four wide field cameras each with four large CCD imagers that will collect photometric data at a relatively high cadence, and therefore generate a ton of data.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L91mO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L91mO.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;TESS (Astro-EX 1) [MIT]&amp;quot; From &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/explorer_tess.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Tess will pass near the earth twice a month for a nominal 4 hour 100 Mbit/sec sicence download:&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://eoportal.org/web/eoportal/satellite-missions/content/-/article/tess&quot; rel=&quot;nofollow noreferrer&quot;&gt;TESS (Transiting Exoplanet Survey Satellite)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Table 1: Overview of spacecraft parameters:&#xA;    [...]&#xA;RF communications:     Ka-band 100 Mbit/s science downlink&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;The article goes on to say:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;DHU (Data Handling Unit): The DHU is a Space Micro Image Processing Computer (IPC-7000) which consists of six boards: an IPC (Image Processing Computer), which contains two Virtex-7 FPGAs (Field Programmable Gate Arrays) that serve as interfaces to the four cameras and perform high-speed data processing; a Proton 400 k single board computer, which is responsible for commanding, communicating with the spacecraft master avionics unit, and interfacing with the Ka-band transmitter; two 192 GB SSB (Solid-State Buffer) cards for mass data storage; an analog I/O power switch board to control instrument power; and a power supply board for the DHU.&lt;/p&gt;&#xA;&lt;p&gt;The CCDs (Charge Coupled Devices) produce a continuous stream of images with an exposure time of 2 seconds. These are received by the FPGAs on the IPC, and summed into consecutive groups of 60, giving an effective exposure time of 2 minutes. During science operations, the DHU performs real-time processing of data from the four cameras, converting CCD images into the data products required for ground post-processing. A primary data product is a collection of subarrays (nominally 10 x 10 pixels) centered on preselected target stars. The Proton400 k extracts these subarrays from each 2 min summed image, compresses them and stores them in the SSB prior to encapsulation as CCSDS packets for the Ka-band transmitter. Full frame images are also stacked every 30 minutes and stored in the SSB. Data from the SSB are downlinked every 13.7 days at perigee.&lt;/p&gt;&#xA;&lt;p&gt;At perigee, science operations are interrupted for no more than 16 hours to point TESS 's antenna toward Earth, downlink data, and resume observing. This includes a nominal 4 hr period for Ka-band science data downlink using NASA's DSN (Deep Space Network). In addition, momentum unloading is occasionally needed due to the ~1.5 N m of angular momentum build-up induced by solar radiation pressure. For this purpose TESS uses its hydrazine thrusters.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The use of the higher frequency &lt;a href=&quot;https://en.wikipedia.org/wiki/Ka_band&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ka band&lt;/a&gt; rather than &lt;a href=&quot;https://en.wikipedia.org/wiki/X_band&quot; rel=&quot;nofollow noreferrer&quot;&gt;X band&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Ku_band&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ku band&lt;/a&gt; allows for a combination of higher speed and/or lower power and/or smaller high gain antenna dish diameter.&lt;/p&gt;&#xA;&lt;p&gt;Here are some rounded numbers from those links:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;band    frequency    wavelength&#xA;----    ---------    ----------&#xA; X       8-12 GHz     37-25  mm&#xA; Ku     12-18 GHz     25-17  mm&#xA; Ka     27-40 GHz     11-7.5 mm&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;According to Wikipedia's article &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_space_bands&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Bands&lt;/a&gt;, deep space is defined by the ITU as greater than 2,000,000 km from the Earth's surface. This excludes anything to do with the Moon's orbit and excludes the Sun-Earth L1 and L2 Lagrange points as well. It therefore excludes TESS' Earth orbit with its period of only 13.7 days.&lt;/p&gt;&#xA;&lt;p&gt;In the Wikipedia article, there are no entries for Ka-band approved frequencies for near space; there is an asterisk which indicates &lt;strong&gt;&lt;code&gt;No assignment or not supported by the DSN.&lt;/code&gt;&lt;/strong&gt; And yet TESS will use the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; to download its data - &lt;em&gt;not TDRS&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Are there now approved &lt;em&gt;Near Space&lt;/em&gt; Ka-Band frequencies, and does the Wikipedia article need to be updated?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aGM5W.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aGM5W.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;TESS’s never-before-used P/2 orbit.&amp;quot; Image: GSFC/NASA. From Extreme Tech's &lt;a href=&quot;https://www.extremetech.com/extreme/232963-finally-something-isnt-broken-tess-on-track-to-start-hunting-exoplanets-in-2017&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA’s TESS mission on track to start hunting exoplanets in 2017&lt;/a&gt;. TESS' final orbit is the wider of the two complete ellipses (red) with periapsis near LEO and apoapsis near but less than the Moon's orbit. The period is 13.7 days, or half of the Moon's and phased such that the Moon will be leading or lagging by 90 degrees whenever TESS reaches apoapsis, in order to cancel the strongest of the perturbations from the Lunar gravity and give the orbit a stability lifetime of decades.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-06-01T07:11:49.070" Title="Will there be &quot;Near Space&quot; Ka-band allocations for TESS?" Tags="|communication|radio-communication|antenna|tess|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="21771" PostTypeId="2" ParentId="21739" CreationDate="2017-06-03T09:07:04.840" Score="6" Body="&lt;p&gt;Modern star trackers are intentionally a bit out of focus. This spreads the light from a single star (which would otherwise resolve to a single pixel) across multiple pixels. Close binaries are similarly spread across multiple pixels. This is a feature rather than a misfeature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about stars that happen to appear to be close by one another, resulting in a misshaped blob? The solution is simple: Don't put those confusing objects in the star tracker's star catalog. There are always going to be objects in the field of view that are not in the field of view that are not in the catalog. These range from the wide binaries, apparent binaries, planets, asteroids, chunks of unburnt fuel, and pixels that have been fried due to radiation effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern star trackers don't look at just one object. They instead look for triangles that unambiguously match some triangle of stars in the star tracker's star catalog, which is typically in the tens of thousands. Multiple triangles are even better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is part of why star trackers operating in &quot;Lost In Space&quot; mode take multiple seconds to arrive at a solution. The solution has to be statistically unambiguous. A star tracker in a fictional (think Star Wars) asteroid field won't work because there are too many bright objects in the field of view that aren't in the catalog. A star tracker with a commercial quality CCD array won't work because too many of the pixels in the array will have been fried.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A modern star tracker in a star cluster also won't work. That is not our problem. When humanity reaches the stage where navigation in a star cluster does become a issue, those future star trackers will most likely look at quasars.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-06-03T09:07:04.840" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21772" PostTypeId="1" AcceptedAnswerId="21774" CreationDate="2017-06-03T10:11:03.187" Score="2" ViewCount="118" Body="&lt;p&gt;The book &quot;The Martian&quot; describes a six-people manned mission to Mars. One of the astronauts is both the ship's primary medical doctor/surgeon and primary extravehicular activity specialist. Is such a job division rational? Would you find it reasonable to have in a similar mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that EVA is one of the most dangerous activities occurring on a spacecraft, so I would try to keep the two roles separate so that the surgeon is available to take care of an injured EVA astronaut.&lt;/p&gt;&#xA;" OwnerUserId="18607" LastActivityDate="2017-06-03T14:07:23.600" Title="Doctor and EVA specialist -- good idea?" Tags="|crewed-spaceflight|mission-design|" AnswerCount="1" CommentCount="0" ClosedDate="2017-06-03T17:56:57.037" ContentLicense="CC BY-SA 3.0" />
  <row Id="21773" PostTypeId="1" AcceptedAnswerId="22095" CreationDate="2017-06-03T11:05:35.240" Score="6" ViewCount="665" Body="&lt;p&gt;&lt;strong&gt;update 24-Jun-2017:&lt;/strong&gt; &lt;a href=&quot;https://www.nasa.gov/feature/sofia-arrives-in-new-zealand-to-observe-southern-skies&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOFIA Arrives in New Zealand to Observe Southern Skies&lt;/a&gt; There are plans to fly SOFIA through another &lt;em&gt;predicted&lt;/em&gt; occultation path on July 17. &lt;a href=&quot;https://en.wikipedia.org/wiki/Stratospheric_Observatory_for_Infrared_Astronomy&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOFIA&lt;/a&gt; is a huge (2.5 meter dia.) infrared telescope (1 ~ 250 um) with various cryogenic focal plane arrays that is flown above most of the water in Earth's atmosphere (&lt;a href=&quot;https://youtu.be/g5z6fZKOtP4&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube&lt;/a&gt;). It &lt;a href=&quot;https://www.nasa.gov/ames/nisv-podcast-William-Reach&quot; rel=&quot;nofollow noreferrer&quot;&gt;sound like&lt;/a&gt; they will just use the visible light guide camera (behind the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nasmyth_telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nasmyth&lt;/a&gt; mirror?), rather than the infrared capability, but the portability is certainly handy. This mission might be &amp;quot;An Airplane Hunting for Shadows from the Kuiper Belt&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;update 14-Jun-2017:&lt;/strong&gt; non-committal blurb: &lt;a href=&quot;https://www.nasa.gov/feature/new-horizons-team-digs-into-new-data-on-next-flyby-target&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/feature/new-horizons-team-digs-into-new-data-on-next-flyby-target&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;update 08-Jun-2017:&lt;/strong&gt; There is also this blurb in Sky and Telescope's June 8 2017 article &lt;a href=&quot;http://www.skyandtelescope.com/astronomy-news/solar-system/observers-track-new-horizons-next-target/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Observers Track New Horizons’ Next Target&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;The Results are In (Sort of)&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;According to early reports gleaned by Sky &amp;amp; Telescope, June 3rd's effort couldn't have gone much better. Buie says that every team collected usable data, though some might have been partially impacted by clouds. &amp;quot;That is quite remarkable,&amp;quot; he says, &amp;quot;and it took some heroics on the part of the South African teams.&amp;quot; (Bad weather forced many of them to relocate.)&lt;/p&gt;&#xA;&lt;p&gt;The stellar &amp;quot;shadow&amp;quot; cast by 2014 MU69 took about 11 minutes to sweep across Earth, so from any given location the star would disappear for no more than about 2 seconds. Since the cameras were making ½-second-long exposures, at most four frames will show the star missing. But even with such beefy telescopes, the expected signal-to-noise ratio (even in optimal conditions) won't be high.&lt;/p&gt;&#xA;&lt;p&gt;So did anyone see the star disappear? Anja Genade (SAAO) reports good data but no disappearance with the 74-inch Radcliffe reflector in Sutherland. As for all those mobile teams, Buie isn't saying — at least just yet. Look for him to announce those results early next week.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;update 03-Jun-2017:&lt;/strong&gt; There is a &lt;a href=&quot;https://pbs.twimg.com/media/DBZ9cZOXsAAPckX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;cool image&lt;/a&gt; in this &lt;a href=&quot;https://twitter.com/NASANewHorizons/status/871022145803083776&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweet&lt;/a&gt; from (at)NASANewHorizons:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;@NASANewHorizons&lt;/strong&gt;  Jun 3&lt;/p&gt;&#xA;&lt;p&gt;Clear skies and successful occultation obs in ARG and SA; now to dig into all the data! Credit:J.Jewell  #mu69occ &lt;a href=&quot;http://go.nasa.gov/2rOdJ3x&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://go.nasa.gov/2rOdJ3x&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;which suggests an occultation was successful, but I'm not sure of the definition of success in the tweet yet. Still looking for more info on this amazing group effort. There are two more similar occultations predicted for early July in AsteroidOccultations.com's &lt;a href=&quot;http://www.asteroidoccultation.com/observations/NA/MU69Binzel.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;News &amp;amp; Announcements&lt;/a&gt; for 2014 MU69, I hope we'll hear something more about this one before then.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The predicted time of the first of three occultations of +12 to +14 magnitude stars by asteroid 2014 MU69 has come and passed.&lt;/p&gt;&#xA;&lt;p&gt;People have fanned out over South America and South Africa with telescopes with cameras and GPS clocks and are going to try to &lt;em&gt;watch the shadow of a roughly magnitude +13 star cast by a rock in the KUIPER BELT pass over the Earth!&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;This is of course weather permitting, and early Twitter reports are already showing clear skies and images of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Omega_Centauri&quot; rel=&quot;nofollow noreferrer&quot;&gt;Omega Centauri&lt;/a&gt; star cluster. There is a Twitter hash tag for these events, &lt;strong&gt;#mu69occ&lt;/strong&gt;. Here is a 500 millisecond exposure from a few days ago. The short exposure time is necessary to resolve small scale debris from the main occultation.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/krWZu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/krWZu.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Omega Centari Star Cluster, 500 millisecond exposure. &lt;a href=&quot;https://twitter.com/Alex_Parker/status/870169900970639360&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tweeted by&lt;/a&gt; Alex Partker.&lt;/p&gt;&#xA;&lt;p&gt;The goal is actually to look for shadows cast by &lt;em&gt;even smaller debris, orbiting around the rock in the Kuiper belt.&lt;/em&gt; This is for New Horizons mission planning.&lt;/p&gt;&#xA;&lt;p&gt;When will we hear at least a little news? Did anyone see a dip in a light curve at all? Something?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NSp4X.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NSp4X.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From AsteroidOccultations.com's &lt;a href=&quot;http://www.asteroidoccultation.com/observations/NA/MU69Binzel.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;News &amp;amp; Announcements&lt;/a&gt; for 2014 MU69&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-01-01T13:24:33.493" Title="Timing shadows from the Kuiper belt! Any news? Did it work?" Tags="|orbital-mechanics|asteroid|new-horizons|2014-mu69|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21774" PostTypeId="2" ParentId="21772" CreationDate="2017-06-03T14:07:23.600" Score="1" Body="&lt;p&gt;Every crew member is instructed in first aid. Watney showed us crew members know the basics of surgery. The sort of injury likely to be experienced during EVA is the kind that likely least requires thorough medical expertise. The main medical doctor's duties are probably more subtle, long-term afflictions that one does need thorough experience for.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-03T14:07:23.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21776" PostTypeId="1" AcceptedAnswerId="22153" CreationDate="2017-06-03T15:33:18.887" Score="9" ViewCount="1646" Body="&lt;p&gt;Explosive bolts are often used for stage and payload separation. If only one of the many explosive bolts used fails to break into two pieces, the payload of the rocket may be lost. Are there more than one detonator used to initiate the explosive charges of these bolts, together with separate electric circuits for the control? Very little weight will be added by several initiators and their connections, but the probability of a failure should be much lower.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="26446" LastEditDate="2021-06-18T03:33:59.173" LastActivityDate="2021-06-18T05:55:37.683" Title="Reliability of explosive bolts and redundant initiators" Tags="|stage-separation|pyrotechnic-fasteners|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21777" PostTypeId="1" CreationDate="2017-06-03T18:04:05.397" Score="4" ViewCount="339" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/14290/12102&quot;&gt;@KimHolder's thorough answer to &lt;em&gt;What are the consequences for a person who violates a launch exclusion zone?&lt;/em&gt;&lt;/a&gt; mentioned a fishing tournament. &lt;a href=&quot;https://space.stackexchange.com/questions/14282/#comment37469_14290&quot;&gt;@PearsonArtPhoto's comment&lt;/a&gt; about scheduling launches during fishing tournaments got me thinking about &lt;a href=&quot;http://www.thedenverchannel.com/news/national/east-coast-could-see-colorful-clouds-sunday-morning-thanks-to-nasa&quot; rel=&quot;nofollow noreferrer&quot;&gt;yesterday's launch that was cancelled&lt;/a&gt; due to some &amp;quot;boats in the area&amp;quot;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NASA had hoped to launch the rocket early Saturday, but had to scrub the attempt because of boats in the area where the payload is expected to fall back to Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It turns out &lt;a href=&quot;http://www.americanfishingcontests.com/contest/List.aspx?State=VA&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;there was a fishing tournament&lt;/em&gt;&lt;/a&gt; in Virginia on Saturday, three in fact! One was in Newport News VA, offshore.&lt;/p&gt;&#xA;&lt;p&gt;Coincidence??? (satire, humor)&lt;/p&gt;&#xA;&lt;p&gt;(The other two were state-wide/fresh water so they don't count.)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Xhzob.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xhzob.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/NASA_Wallops/status/870924874382024704&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tweet&lt;/a&gt; from NASA Wallops (&lt;a href=&quot;https://i.stack.imgur.com/OTZsy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;screenshot&lt;/a&gt;):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Launch at @NASA_Wallops scrubbed today because of boats in the impact area.  We will try again between 4:26 and 4:41 a.m., Sunday, June 4. 4:47 PM · Jun 3, 2017&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-06-30T04:43:56.950" LastActivityDate="2021-06-30T04:43:56.950" Title="Was it another Fishing Tournament?" Tags="|launch|range-safety|sounding-rocket|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="21778" PostTypeId="1" AcceptedAnswerId="22048" CreationDate="2017-06-03T18:37:28.100" Score="1" ViewCount="187" Body="&lt;p&gt;For a self-sustaining base on Mars (or elsewhere)... I've read and been told that growing enough plants to eat would produce too much oxygen, but there are many in-depth designs for self-sustaining outposts, and similar things have been achieved here on Earth with Biosphere 2. So, how does one grow enough food to survive on without generating far too much oxygen?&lt;/p&gt;&#xA;" OwnerUserId="19742" LastEditorUserId="19742" LastEditDate="2017-06-19T03:18:32.507" LastActivityDate="2017-06-21T19:41:31.283" Title="Balancing oxygen and food production" Tags="|oxygen|food|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21779" PostTypeId="1" AcceptedAnswerId="22192" CreationDate="2017-06-03T18:46:31.343" Score="3" ViewCount="330" Body="&lt;p&gt;I have a Mars landing simulation, and one of the things it calculates is the dynamic pressure as the spacecraft descends through Mars' thin atmosphere (given by $q=\frac{1}{2}\rho v^{2}$). I know it depends on many different parameters such as the atmospheric model used to find $\rho$ vs. altitude and of course the velocity that you have as you enter Mars, but I was wondering if anyone has any data regarding dynamic pressures experienced by spacecraft entering Mars' atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="1064" LastActivityDate="2017-07-05T05:05:21.437" Title="Maximum dynamic pressure upon Mars entry" Tags="|mars|reentry|landing|entry-descent-landing|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21780" PostTypeId="2" ParentId="21779" CreationDate="2017-06-03T19:34:34.583" Score="3" Body="&lt;p&gt;For &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090007730.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;MSL (Curiosity), the expected peak dynamic pressure was 15.4 kPa&lt;/a&gt;. (table 8)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know how typical of Mars entries that is.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-06-03T19:34:34.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21782" PostTypeId="1" AcceptedAnswerId="21793" CreationDate="2017-06-03T20:53:07.613" Score="3" ViewCount="216" Body="&lt;p&gt;The downmass capability was one of the US Air Force's requirements in the Space Shuttle design. It was planned that the shuttle would retrieve spy satellites from orbit for refurbishing. This ability was used at least once in the &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-51-A&quot; rel=&quot;nofollow noreferrer&quot;&gt;retrieval of Westar 6 during STS-51A&lt;/a&gt;. There was plan for &lt;em&gt;Columbia&lt;/em&gt; to retrieve the Hubble Space Telescope prior to STS-107.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently, programs such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Robotic_Refueling_Mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robotic Refueling Mission&lt;/a&gt; aim to develop robotic means to service satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Energy-wise, in-orbit service will be much cheaper to perform. During the Cold War, this ability could be used to capture satellites of hostile states. However, is satellite retrieval still relevant give the progress of in-orbit service? (For example, other than sentimental value, is it worthwhile to bring back the HST?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know that, as of 2017, after the retirement of the space shuttle, mankind has limited capabilities to perform either task.&lt;/p&gt;&#xA;" OwnerUserId="340" LastEditorUserId="6816" LastEditDate="2017-06-04T21:01:00.877" LastActivityDate="2017-06-04T21:01:00.877" Title="Is satellite retrieval still relevant given the progress of in-orbit service?" Tags="|spacecraft|robotic-missions|in-space-repairs|restore-l|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21783" PostTypeId="1" CreationDate="2017-06-03T21:20:43.283" Score="2" ViewCount="456" Body="&lt;p&gt;So a couple of minutes ago, the Falcon 9 of CRS-11 just landed and I noticed that the &quot;smoke&quot; color was a little different. It is more &quot;red&quot; compared to landings from other missions.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;CRS-11 (weird color):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/glL75.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/glL75.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;CRS-10:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IItQW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IItQW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NROL-76:&lt;a href=&quot;https://i.stack.imgur.com/uR7AW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uR7AW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Is it because of the new paint on LZ-1? What can be the cause of this?&lt;/p&gt;&#xA;" OwnerUserId="17599" LastEditorUserId="17599" LastEditDate="2017-06-03T22:49:00.357" LastActivityDate="2017-06-06T12:58:09.687" Title="SpaceX CRS-11 landing smoke color is weird" Tags="|spacex|falcon-9|landing|crs-10|crs-11|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21784" PostTypeId="1" AcceptedAnswerId="21799" CreationDate="2017-06-03T21:51:52.550" Score="6" ViewCount="598" Body="&lt;p&gt;During the re-entry of Stage 1 of CRS-11 at around &lt;a href=&quot;https://youtu.be/URh-oPqjlM8?t=36m&quot; rel=&quot;noreferrer&quot;&gt;T+6:02&lt;/a&gt; (58.9km up), an object can be seen enter the camera view in the lower left, and pass the camera a second later (58.0km up). (Images below)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first I assumed it to be a bird, but they don't fly this high. The object seemed to be traveling roughly in the direction of the Falcon 9, suggesting it was not space debri in orbit. Was this object something that was detached earlier in the launch, or did it break off from the rocket itself? What was it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZMDo4.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZMDo4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/0pamr.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0pamr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/bTSbn.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bTSbn.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="567" LastEditorUserId="17599" LastEditDate="2017-06-03T22:51:34.730" LastActivityDate="2017-06-05T19:47:07.943" Title="What did CRS-11 stage 1 encounter at T+6:02?" Tags="|spacex|crs-11|" AnswerCount="1" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21785" PostTypeId="2" ParentId="10048" CreationDate="2017-06-03T23:47:20.610" Score="1" Body="&lt;p&gt;Also notice that radar observations have shown that the asteroid contains no more metal than any usual rocky asteroid (see &lt;a href=&quot;https://twitter.com/astrotweeps/status/865105034459189248&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://twitter.com/astrotweeps/status/865105034459189248&lt;/a&gt; for example). The assumption that it's metallic was erroneous.&lt;/p&gt;&#xA;" OwnerUserId="19910" LastActivityDate="2017-06-03T23:47:20.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21786" PostTypeId="1" CreationDate="2017-06-04T00:32:39.953" Score="1" ViewCount="1333" Body="&lt;p&gt;I got the orbital elements of the Moon from this &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi#top&quot; rel=&quot;nofollow noreferrer&quot;&gt;page&lt;/a&gt; but it doesn't use keplerian orbital elements (with time derivatives), for which I've already written a converter. I don't understand why but it seems that they are only used for planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, now I need to write a converter (in C#) that converts the data of the ephemerides to cartesian coordinates (and both position and velocity).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some guidance, please?&lt;/p&gt;&#xA;" OwnerUserId="19596" LastActivityDate="2017-06-04T06:54:00.297" Title="How to convert ephemerides to cartesian coordinates?" Tags="|orbital-elements|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21789" PostTypeId="2" ParentId="21786" CreationDate="2017-06-04T02:56:26.053" Score="2" Body="&lt;p&gt;You do not need something that converts JPL Horizons ephemerides to cartesian coordinates because cartesian coordinates is the system in which the Horizons systems naturally works. Requesting Keplerian elements forces the Horizons system to use a conversion algorithm. That algorithm doesn't quite make sense for the Moon due to the fact that the Moon's mass is over 1% of the Earth's mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The easiest way to programmatically obtain cartesian coordinates of a natural solar system object is to use &lt;a href=&quot;https://naif.jpl.nasa.gov/naif/toolkit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL's SPICE Toolkit&lt;/a&gt;, which is freely available.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-06-04T02:56:26.053" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21790" PostTypeId="2" ParentId="21783" CreationDate="2017-06-04T03:11:54.713" Score="3" Body="&lt;p&gt;My best guess is that it's just simply dust. Florida right now is in somewhat of a drought at the moment and this could cause dry dust to settle on the landing pad. After the Static-fire test of CRS-11 there was a brush fire that had started (demonstrating the dryness in Florida). As for the specific color of the dust, it looks as if the top picture has somewhat of a filter on it which may be contributing to this certain color of the dust. Different times of day may also have affects on the color of it. As launches in the morning (at least where I live) such as NROL-76 and CRS-10 both had lighter colors of smoke/dust, than the evening launch of CRS-11. So unless SpaceX is testing some experimental Nitrogen Tetroxide engines (*joke) It should just be benign dust.&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="18458" LastEditDate="2017-06-04T13:02:13.620" LastActivityDate="2017-06-04T13:02:13.620" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21791" PostTypeId="2" ParentId="21786" CreationDate="2017-06-04T06:54:00.297" Score="2" Body="&lt;p&gt;This is an alternate answer, a list of other possibilities or options. @DavidHammen's answer is the best answer to your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some options: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Caveat, check the &lt;a href=&quot;https://space.stackexchange.com/questions/tagged/jpl-horizons&quot;&gt;jpl-horizions&lt;/a&gt; tag. &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/a/21789/12102&quot;&gt;@DavidHammen's answer&lt;/a&gt; is right. That's the gold. The source. The horses mouth.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;PyEphem&lt;/a&gt; is a python package that gives you some python-wrapped access to many of those same features and math, and is easy to use. &lt;/li&gt;&#xA;&lt;li&gt;You can also just choose to get the state vectors (rather than the orbital elements) directly from Horizons like this: &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AanWH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AanWH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To get the basic position and velocity (state vector) without any light time corrections, make sure it looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EzyIv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EzyIv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get units of kilometers and kilometers per second, and ecliptic coordinates, choose these:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7ZLeP.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7ZLeP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if you want Coordinates compatible with RA and dec, use Earth's Mean Equator (z axis points towards declination of +90 degrees):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k4QFV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k4QFV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;OK, so for the moon's position and velocity in ecliptic coordinates with the solar system barycenter as the origin (a good place to start) and to download a csv directly to your computer, make it look like this (note that z is smaller than x or y):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oXFce.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oXFce.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And 04-Jun-2017 00:00 will be (JD, date, x, y, z, vx, vy, vz):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2457908.500000000, A.D. 2017-Jun-04 00:00:00.0000, &#xA;-4.345736121825012E+07, -1.447334164993358E+08,  8.959531602375209E+03,&#xA; 2.821087875766465E+01, -9.599004577093003E+00,  6.380929713994465E-02,&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If you chose a Geocentric origin, it will look like this (note x, y, z are all of order 1E+05):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gUwNj.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gUwNj.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And 04-Jun-2017 00:00 will be (JD, date, x, y, z, vx, vy, vz):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2457908.500000000, A.D. 2017-Jun-04 00:00:00.0000, &#xA;-3.875221956720588E+05, -8.324494849553939E+04,  2.466711375946739E+04,&#xA; 1.660829018806102E-01, -9.711046752921442E-01,  6.419466109667427E-02,&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;And if you chose the Earth's equator for a reference plane solar system barycenter for origin, it will look like this (note that z is now large):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/C0WjX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C0WjX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And 04-Jun-2017 00:00 will be (JD, date, x, y, z, vx, vy, vz):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2457908.500000000, A.D. 2017-Jun-04 00:00:00.0000, &#xA;-4.345736121825012E+07, -1.327938773171277E+08, -5.756342657388520E+07,  &#xA; 2.821087875766465E+01, -8.832296393941144E+00, -3.759720854934335E+00,&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-06-04T06:54:00.297" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21792" PostTypeId="2" ParentId="21723" CreationDate="2017-06-04T08:56:14.860" Score="2" Body="&lt;p&gt;The question regarding the lowest pressure required for plant growth was asked on the &lt;a href=&quot;https://biology.stackexchange.com/questions/1242/what-is-the-lowest-pressure-at-which-plants-can-survive&quot;&gt;biology stack exchange&lt;/a&gt; The answer given there reports an experiment where vascular plants seemed to be at about their survival limit at 0.1 bar. Unfortunately the quoted paper is paywalled.&#xA;This does not appear to be a particularly well researched area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Regarding CO2 concentrations&lt;/strong&gt;.&#xA;The problem with raised levels of CO2 is that the body needs to dump it's waste CO2 into exhaled air. In order to do this there needs to be a lower partial pressure of CO2 in the inspired air than the partial pressure of CO2 in the blood. This is so that the gas will exchange out of the blood and into the air. If the breathing gas has elevated CO2, the body cannot rid itself, and problems rapidly arise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an article in Wikipedia - &lt;a href=&quot;https://en.wikipedia.org/wiki/Hypercapnia&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hypercapnia&lt;/a&gt; which contains a table of CO2 percentage verses time of exposure. About 6% being the upper limit for even short exposures. But remember this was at 1 bar pressure, so this represents an upper partial pressure limit of about 0.06bar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Regarding decompression.&lt;/strong&gt;&#xA;Decompression is required to allow time for dissolved gases (usually N2) to come out of the blood during respiration. If this time is not adequate, the gas will come out of solution in the blood forming bubbles, these can then block blood vessels causing a whole suite of problems depending where the bubbles end up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The requirement for decompression therefore depends on the pressure change together with the mixture and duration of the gasses being breathed. So there is no simple answer to your question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In SCUBA diving (ie breathing normal air at all stages) it is considered safe to halve your absolute pressure without decompression and without any time limit at depth. A depth of 10m gives 2 bar pressure, and is the depth limit for unlimited time without the requirement to decompress before surfacing. Underwater habitats can exploit this. A habitat at 10m (2 bar absolute pressure) would allow its inhabitants to work in pressures up to 4 bar (30m) for unlimited time and still return to the hab without decompression. However, they cannot return to surface without decompression.&lt;/p&gt;&#xA;" OwnerUserId="14276" LastActivityDate="2017-06-04T08:56:14.860" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21793" PostTypeId="2" ParentId="21782" CreationDate="2017-06-04T09:26:28.037" Score="2" Body="&lt;p&gt;Recovering old satellites for servicing isn't currently worthwhile, but it wasn't particularly relevant forty years ago either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The downmass requirement was never about recovering old satellites. A new satellite is expensive, but after years of micrometeorites, irradiation, vacuum welding and a 500 degree temperature change every hour or so, it isn't worth much more than scrap value even if the design isn't thoroughly obsolete by that point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are two obvious exceptions to that - new satellites in the wrong orbit, and seventies era spy satellites (your own, not someone else's).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Westar 6 was a brand new satellite with a dud upper stage. It didn't really need to come back to earth - the most efficient fix would be sending a small tug to push it into the right orbit. A small tug was not immediately available but a shuttle was, so it got landed and relaunched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle was designed a long time before digital cameras could match the quality of film. Spy satellites of that era had to drop small reentry capsules containing the exposed film. Once the film ran out within a few weeks of launch, the only option was  letting all the expensive optics burn up in the atmosphere. Being able to cheaply recover the whole satellite would have been very useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the shuttle had lived up to its promise as a cheap and reusable vehicle, it might have made sense to launch with one KH-9 in the payload bay and return with an empty one from the same orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, even that sort of satellite recovery wouldn't have been the best way to make use of downmass. Mount a large camera in the payload bay, possibly even permanently, and use the shuttle itself as the spy satellite. Note that satellite recovery only happened once, but Spacelab utilized the downmass capability 20-30 times depending on what you count.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the present day situation, retrieving old satellites remains largely pointless, and digital cameras have removed the spy satellite exception. Spacelab was replaced by delivering equipment to/from ISS. That does require some generic downmass capability, but not a vehicle as big as the shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recovering Hubble would be bringing back a museum piece, just because we can - I certainly wouldn't expect it to be worth relaunching thirty year old hardware.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-06-04T09:26:28.037" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21794" PostTypeId="2" ParentId="21762" CreationDate="2017-06-04T11:23:33.970" Score="2" Body="&lt;p&gt;As was mentioned by NathanTuggy in the comments, there are a number of proposals for ways of getting into orbit without rockets. They include Space Elevators, Sky Hooks, Mass Drivers, and Launch Loops. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I strongly recommend taking a look at the Upward Bound series of YouTube videos which covers these and more. They are produced by Isaac Arthur &lt;a href=&quot;http://www.isaacarthur.net/&quot; rel=&quot;nofollow noreferrer&quot;&gt;(home page)&lt;/a&gt;, and IMHO they are of very good quality. They are all grounded in physics rather than fantasy and are more detailed than anything on cable tv. Each system is given about 30 min and considers how each may be implemented, and what can be done with existing technology and what cannot. (The economic aspect is put aside)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from orbital systems his videos cover a number of other topics for those interested in the far reaches of space exploration. Highly recommended&lt;/p&gt;&#xA;" OwnerUserId="14276" LastActivityDate="2017-06-04T11:23:33.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21795" PostTypeId="2" ParentId="21653" CreationDate="2017-06-04T12:58:32.903" Score="5" Body="&lt;p&gt;Wikipedia has a decent summary of the current state of thinking on the bright spots in &lt;a href=&quot;https://en.wikipedia.org/wiki/Occator_(crater)&quot; rel=&quot;noreferrer&quot;&gt;Occator crater&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;on 29 June 2016, scientists reported the bright spot to be mostly sodium carbonate (Na&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt;), implying that hydrothermal activity was probably involved in creating the bright spots.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Hydrothermal activity may be ongoing, &lt;a href=&quot;https://www.centauri-dreams.org/?p=37277&quot; rel=&quot;noreferrer&quot;&gt;some photos of Occator show a haze&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“The nature of the light scattering at the bottom of Occator differs fundamentally from that at other parts of the Ceres surface. The most likely explanation is that near the crater floor an optically thin, semi-transparent haze is formed.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are other &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/marc-rayman/20151231-dawn-journal-science-on-ceres.html&quot; rel=&quot;noreferrer&quot;&gt;bright spots&lt;/a&gt; on Ceres: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Geologists have located more than 130 bright regions...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;With the data we have, it's &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/marc-rayman/20151231-dawn-journal-science-on-ceres.html&quot; rel=&quot;noreferrer&quot;&gt;difficult to give a definitive answer&lt;/a&gt; on the composition of the bright spots. &#xA;(from a 2015 article, that predates the current 'best fit' of sodium carbonate): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is very difficult to pin down the specific composition with the measurements that have been analyzed so far. Scientists compare how reflective the scene is at different wavelengths with the reflective properties of likely candidate materials studied in laboratories. So far, magnesium sulfate yields the best match (although it is not definitive).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As of June 2017, &lt;a href=&quot;https://dawn.jpl.nasa.gov/mission/journal_05_24_17.html&quot; rel=&quot;noreferrer&quot;&gt;study&lt;/a&gt; of this region is &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/marc-rayman/20170201-dawn-journal-new-perspective-on-ceres.html&quot; rel=&quot;noreferrer&quot;&gt;ongoing&lt;/a&gt;:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If all goes well, on April 29 the new orbit will take Dawn exactly between the sun and the famous bright region at the center of Occator Crater. Named Cerealia Facula, the area is composed largely of salts. (Based on infrared spectra, the strongest candidate for the primary constituent is sodium carbonate). The probe will be at an altitude of about 12,400 miles (20,000 kilometers), or more than 50 times higher than it was in 2016 when it captured its sharpest photos of Occator (as well as the rest of Ceres’ 1.1 million square miles, or 2.8 million square kilometers). But the objective of reaching a position at which the sun and Ceres are in opposite directions, a special alignment known as opposition, is not to take pictures that display more details to our eyes. In fact, however, the pictures will contain intriguing new details that are not readily discerned by visual inspection. Dawn will take pictures as it gets closer and closer to opposition, covering a range of angles. In each image, scientists will scrutinize the handful of pixels on Cerealia Facula to track how the brightness changes as Dawn’s vantage point changes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The latest installment of the &lt;a href=&quot;https://dawn.jpl.nasa.gov/mission/journal.asp&quot; rel=&quot;noreferrer&quot;&gt;Dawn Journal&lt;/a&gt; doesn't include the results of these photos yet. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-04T12:58:32.903" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21796" PostTypeId="1" AcceptedAnswerId="21806" CreationDate="2017-06-04T13:16:40.450" Score="1" ViewCount="4245" Body="&lt;p&gt;Dear Friends,&lt;br&gt;&#xA;             Does any dependency exists for achieving particular combination of &lt;em&gt;&lt;strong&gt;RAAN ( Right Ascension Of Ascending Node)&lt;/strong&gt;&lt;/em&gt; and &lt;em&gt;&lt;strong&gt;argument of perigee&lt;/strong&gt;&lt;/em&gt; with respect to particular launch date and time ? I understand that &lt;em&gt;&lt;strong&gt;RAAN&lt;/strong&gt;&lt;/em&gt; would drift because of the &lt;strong&gt;&lt;em&gt;regression of nodes&lt;/em&gt;&lt;/strong&gt;. So when and how should it matter to launch to achieve particular RAAN ?&lt;/p&gt;&#xA;" OwnerUserId="19925" LastEditorUserId="19925" LastEditDate="2017-06-04T14:51:54.370" LastActivityDate="2021-03-09T14:27:45.543" Title="Relation between launch window and RAAN and Argument Of Perigee" Tags="|launch|orbital-mechanics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21798" PostTypeId="2" ParentId="21750" CreationDate="2017-06-04T18:32:02.030" Score="13" Body="&lt;p&gt;For many astronauts, it is simply a matter of being considerate to your fellow astronaut who is appearing alongside you.  It would not be nice to let your arm drift into contact with the other astronaut - astronauts have &quot;personal space'&quot; too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plus, I think we'd all agree that Earth-bound humans by and large think the natural position that arms want to attain in microgravity conditions looks &quot;sort of dumb.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Astronauts have egos, too!&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="12422" LastEditDate="2017-06-04T18:50:11.930" LastActivityDate="2017-06-04T18:50:11.930" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21799" PostTypeId="2" ParentId="21784" CreationDate="2017-06-04T21:14:22.243" Score="8" Body="&lt;p&gt;This is a patch of ice, coming off the rocket from near the liquid oxygen tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ice builds up there due to that tank's very low temperature.  The patch comes off when the rocket has high velocity and experiences rising air density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally such ice is white but here it's gray because it's polluted by engine exhaust during the boostback burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See e.g. discussion here: &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/6f3lyo/spacex_on_twitter_falcon_9_first_stage_has_landed/dif56e4/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.reddit.com/r/spacex/comments/6f3lyo/spacex_on_twitter_falcon_9_first_stage_has_landed/dif56e4/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6033" LastEditorUserId="6033" LastEditDate="2017-06-05T19:47:07.943" LastActivityDate="2017-06-05T19:47:07.943" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21800" PostTypeId="1" AcceptedAnswerId="21843" CreationDate="2017-06-04T22:51:20.680" Score="1" ViewCount="336" Body="&lt;p&gt;A stirling engine is a type of engine which converts thermal energy into mechanical energy. &#xA;&lt;a href=&quot;https://youtu.be/rGFiaL_gYhU&quot; rel=&quot;nofollow noreferrer&quot;&gt;An example of a rather large one is shown here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know this is pretty much the same principle that an expander cycle engine uses, but could a stirling engine possibly make a rocket engine more efficient, and more reliable? The nozzle of the engine could be attached to the heating part of the stirling engine and the rotational energy could be used to drive the pumps for the engine. Also would a stirling driven engine be confined to the same limited amount of thrust an expander engine has?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2017-06-07T23:46:45.650" Title="Could a Stirling engine be used to pump propellants into engines?" Tags="|propulsion|fuel|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21801" PostTypeId="1" AcceptedAnswerId="21802" CreationDate="2017-06-04T23:29:28.667" Score="1" ViewCount="94" Body="&lt;p&gt;I'm willing to spend a decent amount on a textbook, but please tell me about cheap or free sources as well.&lt;/p&gt;&#xA;" OwnerUserId="18744" LastActivityDate="2017-06-05T16:44:22.693" Title="What's a good source of information on determining spacecraft positions via radio?" Tags="|navigation|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21802" PostTypeId="2" ParentId="21801" CreationDate="2017-06-04T23:51:18.157" Score="4" Body="&lt;p&gt;&quot;Statistical Orbit Determination&quot; by Tapley, Schultz, Born is probably what you'd want if trying to estimate spacecraft orbits in general. Most of what is taught in this book is specific to using range and range-rate measurements in Kalman filters. If you are willing to invest a significant amount of money, you may want to look into the Continuing Education program at CU Boulder and specifically take the &quot;statistical orbital determination&quot; classes.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-06-04T23:51:18.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21803" PostTypeId="2" ParentId="1827" CreationDate="2017-06-05T04:00:44.427" Score="2" Body="&lt;p&gt;The gas flowing out of the combustion chamber and nozzle has mass and is being accelerated to high velocity. So, simple physics tells us there must be a force acting on it in that direction (the engine's thrust). That force comes from pressure in the combustion chamber, acting over some area (at or near the throat). The pressure inside the combustion chamber must be overcome in order for propellants to be continually fed in. So, the pumps have to operate at least a bit above that pressure.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-06-05T04:00:44.427" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21804" PostTypeId="1" CreationDate="2017-06-05T04:02:36.690" Score="4" ViewCount="245" Body="&lt;p&gt;I'd read some stray comment on the internet claiming that ISRO is working on a methane-LOX engine, just as a technology demonstrator. Is this true?&lt;/p&gt;&#xA;" OwnerUserId="19935" LastEditorUserId="58" LastEditDate="2017-06-05T16:43:26.920" LastActivityDate="2017-06-05T16:43:26.920" Title="Is ISRO Making a Methane Engine?" Tags="|isro|cryogenics|methane|development|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21805" PostTypeId="2" ParentId="21765" CreationDate="2017-06-05T08:58:21.943" Score="14" Body="&lt;p&gt;The first vessel that crossed the Kármán line and reached space - the German &lt;a href=&quot;https://en.wikipedia.org/wiki/V-2_rocket#cite_note-27&quot; rel=&quot;noreferrer&quot;&gt;V-2 rocket&lt;/a&gt; - utilized a program for its analog computer, implemented as a rotating irregular disk of specific shape, with radius(angle) encoding output(input) function; the disk rotated by a motor, with angle and radius read by potentiometers, providing calculations for the control system, basing on gyroscopes and accelerometer data, and driving the servos of the control surfaces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dK6WU.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dK6WU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://www.cdvandt.org/Hoelzer%20V4.pdf&quot; rel=&quot;noreferrer&quot;&gt;Helmut  Hoelzer’s  Fully  Electronic  Analog  Computer  used  in  the  German V2 (A4) rockets&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-06-05T08:58:21.943" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21806" PostTypeId="2" ParentId="21796" CreationDate="2017-06-05T09:58:18.293" Score="3" Body="&lt;p&gt;I will try to provide a general relation, assuming a spherical Earth (since my knowledge of the oblate model is not sufficient to produce a good answer).&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Argument of perigee&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/Orbit1.svg/400px-Orbit1.svg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;this image&lt;/a&gt; on wikipedia's article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital elements&lt;/a&gt; we can find that argument of perigee is essentially independent from the time of launch. No matter what time it is (i.e. how is Earth currently rotated), as long as your launch site location is fixed and the ascent trajectory is fixed&lt;sup&gt;1&lt;/sup&gt;, you will end up with the same AoP every time you launch.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Longitude of ascending node&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;For the longitude of ascending node (LAN = RAAN) the time and date does matter - in fact it mostly defines what orbital plane you will end up on. The Earth (or: the body that we're launching from) rotates about the normal to the grey plane (see the above image again). You want to launch &lt;em&gt;a bit&lt;/em&gt;&lt;sup&gt;2&lt;/sup&gt; before the launch site crosses the target plane. Normally you have two such opportunities per day (rotational period) - the northerly and southerly launch windows&lt;sup&gt;3&lt;/sup&gt;. When exactly they happen is quite simple to calculate using spherical trigonometry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, let's assume that at the time of launch $T_0$ the Earth's $0°$ meridian is in line with the reference direction (vernal point $\curlyvee$). Our launch site is given by latitude $\delta$ and longitude $\lambda$, and our desired inclination is given by $i$. We can calculate the resulting longitude of the ascending node from &lt;a href=&quot;https://en.wikipedia.org/wiki/Spherical_trigonometry#Napier.27s_rules_for_right_spherical_triangles&quot; rel=&quot;nofollow noreferrer&quot;&gt;Napier's rules for right spherical triangles&lt;/a&gt;. Let's look at the figure 4.9 below (&lt;a href=&quot;http://www.braeunig.us/space/orbmech.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hhxmp.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hhxmp.gif&quot; alt=&quot;Orbital Mechanics on braeunig.us&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We want to find the angle $\Delta\lambda$, because $LAN=\lambda-\Delta\lambda$. We can use the rule &quot;&lt;em&gt;sine of the middle part = the product of the tangents of the adjacent parts&lt;/em&gt;&quot; for $b$ and write:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$sin(\Delta\lambda) = tan(90°-i)\cdot tan(\delta)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives us LAN as a function of launch site latitude and longitude and orbital inclination:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$LAN = \lambda - arcsin(tan(90°-i)\cdot tan(\delta))$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To introduce time into this, we simply must provide a term to account for Earth's rotation. Let's say the angular velocity is equal to $\omega [{deg\over s}]$. Then, after time $T_0+t$, our launch site will have rotated by $t\omega$ from the initial position, and our final expression for LAN will look like:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$LAN = \lambda + t\omega - arcsin(tan(90°-i)\cdot tan(\delta))$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; - The &quot;fixed trajectory&quot; is a rather unrealistic assumption, but the only other choice is to assume it is unpredictable; answer to that would go beyond orbital mechanics and into ascent mechanics and guidance.&lt;br&gt;&#xA;&lt;sup&gt;2&lt;/sup&gt; - Calculating exactly how much is far from trivial and in general requires simulating the entire launch.&lt;br&gt;&#xA;&lt;sup&gt;3&lt;/sup&gt; - Other constraints like launch-site-specific azimuth limitations can make some opportunities infeasible.&lt;/p&gt;&#xA;" OwnerUserId="17912" LastActivityDate="2017-06-05T09:58:18.293" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21807" PostTypeId="2" ParentId="21336" CreationDate="2017-06-05T13:21:09.373" Score="1" Body="&lt;p&gt;Current interplanetary spacecraft &lt;a href=&quot;https://space.stackexchange.com/questions/5845/how-do-probes-locate-themselves/12138#12138&quot;&gt;have no way to determine&lt;/a&gt; their own position. Their position is &lt;a href=&quot;https://www.scientificamerican.com/article/how-do-space-probes-navig/&quot; rel=&quot;nofollow noreferrer&quot;&gt;measured from Earth&lt;/a&gt; using e.g. the DSN. This gives a position that is very accurate on the Earth-spacecraft axis (&lt;a href=&quot;https://space.stackexchange.com/questions/2358/how-did-nasa-determine-the-distance-and-velocity-of-voyager-1/2362#2362&quot;&gt;~1 m resolution&lt;/a&gt;), but has less resolution in the other two axes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Systems are being investigated to support autonomous positioning, but none of them are in operational use yet. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-05T13:21:09.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21808" PostTypeId="2" ParentId="21801" CreationDate="2017-06-05T16:44:22.693" Score="0" Body="&lt;p&gt;If you're willing to settle for &quot;how it's done&quot; instead of &quot;how to do it&quot;, there's an article &quot;Navigation between the Planets&quot; by William G. Melbourne in the June 1976 Scientific American, pp. 58-74.  The article discusses timekeeping, radiometric range and range-rate measurements, orbit determination, and trajectory-correction maneuvers.  Charts show how accuracy had improved over the years.&lt;/p&gt;&#xA;" OwnerUserId="19941" LastActivityDate="2017-06-05T16:44:22.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21809" PostTypeId="1" CreationDate="2017-06-05T18:03:12.780" Score="3" ViewCount="3366" Body="&lt;p&gt;Dear Friends,&lt;br&gt;&#xA;             Is there any relation between &lt;strong&gt;&lt;em&gt;inclination&lt;/em&gt;&lt;/strong&gt; of injected &lt;strong&gt;&lt;em&gt;orbit&lt;/em&gt;&lt;/strong&gt; and the launch site ?&lt;/p&gt;&#xA;" OwnerUserId="19925" LastEditorUserId="6816" LastEditDate="2017-06-06T14:55:57.283" LastActivityDate="2017-11-30T01:38:12.663" Title="Relation between Orbital Inclination and launch site?" Tags="|orbital-mechanics|launch-site|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21810" PostTypeId="1" CreationDate="2017-06-05T18:22:38.163" Score="4" ViewCount="348" Body="&lt;p&gt;Is it possible to maintain a connection with space probes that are far away by sending other space probes behind it. &#xA;For example - &lt;/p&gt;&#xA;&#xA;&lt;p&gt;[earth] ------1---- 2-------3--------4------5....and so on if required (it depends on the distance from earth) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These 1,2,3,4, 5 are space probes and they are supposed to pass the command given to them to the next probe? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is that, is it possible to navigate or maintain a connection  with the last probe in that way? &lt;/p&gt;&#xA;" OwnerUserId="19945" LastEditorUserId="58" LastEditDate="2017-06-05T21:22:46.350" LastActivityDate="2020-06-06T10:17:10.720" Title="Is a space probe communication relay possible?" Tags="|spacex|artificial-satellite|probe|communication-satellite|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21811" PostTypeId="1" AcceptedAnswerId="21814" CreationDate="2017-06-05T19:26:42.057" Score="4" ViewCount="391" Body="&lt;p&gt;I had never noticed it before, but when I was watching Apollo 13, the path of the rocket looked like &lt;a href=&quot;https://i.stack.imgur.com/oHmdc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;. Obviously since it's going around the moon, it won't actually be elliptical, but that doesn't even look close around the other parts. Is this an accurate model of the orbit? And if so, how can that path be modelled mathematically? Sorry if its a basic question, I'm new to this corner of stack exchange.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/xw10t.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xw10t.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19948" LastEditorUserId="63" LastEditDate="2017-06-07T14:49:56.367" LastActivityDate="2017-06-09T04:52:23.883" Title="Totally Non-elliptic Rocket Path" Tags="|orbital-mechanics|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21812" PostTypeId="2" ParentId="21811" CreationDate="2017-06-05T19:57:18.467" Score="4" Body="&lt;p&gt;The trajectory in your image is actually elliptical. It is just projected in a way that makes it appear differently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How? This is a rotating frame of reference; that is, our point of view is rotating along with the Moon's orbit so both the Earth and the Moon appears stationary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is another example of how this works: Stand in the middle of the room and start spinning. Then place your hand on your face and move it outwards. From your point of view it is moving in a straight line away from you, but for another person in the room that is not spinning it looks like your hand is moving in a spiral.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simplest way of modelling this orbit is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Patched_conic_approximation&quot; rel=&quot;nofollow noreferrer&quot;&gt;patched conic approximation&lt;/a&gt;. It is sufficient in most cases, and assumes we are only affected by the gravity of one object at a time. In this case, the first part of the trajectory is an ellipse with respect to Earth, then it is a hyperbola with respect to the Moon, and then an ellipse again. A more advanced model would be to always consider the gravitational influence of both the Earth and the Moon. This, however is known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Three-body_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;three-body problem&lt;/a&gt; and can only be solved by numerical simulations. (Though restricted versions can have usable analytical approaches.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastEditorUserId="6816" LastEditDate="2017-06-06T14:53:40.050" LastActivityDate="2017-06-06T14:53:40.050" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21813" PostTypeId="2" ParentId="21810" CreationDate="2017-06-05T20:43:58.180" Score="2" Body="&lt;p&gt;That wouldn't be a good approach, for a number of reasons, but I'll focus on power here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The strength of a signal drops off proportionally to the square of the distance, so probe #1, at half the distance, would have to transmit 25% of the power as the Earth to have the two signals be the same strength at probe #2. It's a lot easier to put a big transmitter on Earth, where we can plug it in to mains power, than on a spacecraft where it has to be powered by the solar array or RTG.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting some numbers to this, the Deep Space Network can send 20 kW, compared with a few 10s of Watts on a spacecraft.&lt;a href=&quot;https://space.stackexchange.com/questions/9824/how-much-rf-transmit-power-does-dsn-need-to-send-commands-to-voyager&quot; title=&quot;How much RF transmit power does DSN need to send commands to Voyager?&quot;&gt;1&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The DSN also has 70 m diameter dishes, which allow the power to be concentrated into a much narrower beam than a spacecraft antenna does, giving the ground station an even greater advantage.)&lt;/p&gt;&#xA;" OwnerUserId="12831" LastActivityDate="2017-06-05T20:43:58.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21814" PostTypeId="2" ParentId="21811" CreationDate="2017-06-05T22:56:40.280" Score="5" Body="&lt;p&gt;Orbital paths are elliptical in the case where only one massive body is affecting a spacecraft's path gravitationally. In this case, the Earth and the moon each have significant gravity, so you could approximate the path as  a partial ellipse from Earth to the point where the moon's gravity dominates, then a different partial ellipse going around the moon, then another partial ellipse returning to Earth. As Hohmannfan notes, this is the &quot;patched conic&quot; approximation, because conic trajectories are patched together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's some more info and animations for the &lt;a href=&quot;http://www.braeunig.us/apollo/free-return.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;circumlunar free return&lt;/a&gt; trajectory and the &lt;a href=&quot;http://www.braeunig.us/apollo/hybrid-profile.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;normal Apollo-style&lt;/a&gt; trajectory.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-06-06T16:44:39.440" LastActivityDate="2017-06-06T16:44:39.440" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21816" PostTypeId="1" CreationDate="2017-06-06T06:04:55.470" Score="2" ViewCount="251" Body="&lt;p&gt;Reading through in-situ methods to manufacture oxygen on Mars, I see the following assets:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;in future, 100x scale of MOXIE would produce up to two kilograms of oxygen per hour&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Oxygen_ISRU_Experiment&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Mars_Oxygen_ISRU_Experiment&lt;/a&gt;&#xA;Okay that is, year-demand for one adult will be in place already in 400kg/2=200 hours, or annual production will suffice for a colony of about 40 people. If this oxygen will be just a propellant source before people come, &#xA;Elon Musk is talking about a propellant plant as well - well those 17 tons of LOX per year are not SO much.&#xA;So how could this LOX factory look like on Mars, and maybe it makes sense after MOXIE succeeds indeed to design something self-replicating? &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="19946" LastEditorUserId="19946" LastEditDate="2017-06-06T07:13:51.803" LastActivityDate="2017-08-06T16:54:52.690" Title="In-situ oxygen on Mars - what is MOXIE and SpaceX' ITS relation?" Tags="|mars|oxygen|lox|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21817" PostTypeId="1" AcceptedAnswerId="21821" CreationDate="2017-06-06T07:18:59.787" Score="1" ViewCount="131" Body="&lt;p&gt;What are current known feasible plans to establish self-replicating assets on Mars?&#xA;On Wikipedia, there are references to NASA plans telling that up to 80% could be manufactured, but is there any progress in this direction?&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_mining#Self-replicating_machines&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Asteroid_mining#Self-replicating_machines&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also: &lt;a href=&quot;https://en.wikipedia.org/wiki/Clanking_replicator#Prospects_for_implementation&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Clanking_replicator#Prospects_for_implementation&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19946" LastActivityDate="2017-06-06T11:38:30.417" Title="Schedule for self-replication projects for Mars?" Tags="|mars|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21818" PostTypeId="2" ParentId="20830" CreationDate="2017-06-06T08:05:39.903" Score="4" Body="&lt;p&gt;In addition to Hobbes' answer, I may add this point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not know if this is a cause or a consequence, but the Soyuz MS-04 will bring back to earth, in addition of the two crew members sent upthere, &lt;a href=&quot;https://en.wikipedia.org/wiki/Peggy_Whitson&quot; rel=&quot;nofollow noreferrer&quot;&gt;Peggy Whitson&lt;/a&gt;. She lift off with &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_MS-03&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz MS-03&lt;/a&gt; to be part of &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_50&quot; rel=&quot;nofollow noreferrer&quot;&gt;Expedition 50&lt;/a&gt; but did not return back to Earth with her two fellow travellers, who landed safely June 2, 2017.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is due to an &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-astronaut-peggy-whitson-adds-three-months-to-record-breaking-mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;agreement between NASA and the Russian space agency Roscosmos&lt;/a&gt;, to adds three months to record-breaking mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, she will need an extraseat to land back on Earth, and this seat will be in Soyuz MS-04.&lt;/p&gt;&#xA;" OwnerUserId="18592" LastActivityDate="2017-06-06T08:05:39.903" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21819" PostTypeId="1" AcceptedAnswerId="21827" CreationDate="2017-06-06T08:16:28.613" Score="22" ViewCount="5444" Body="&lt;p&gt;The Wikipedia Article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_MS-04#Crew&quot; rel=&quot;noreferrer&quot;&gt;Soyuz MS-04&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is the first of the Soyuz MS series to rendezvous with the Station in approximately 6 hours, instead of the 2 day orbital rendezvous used for the previous launches.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I have several ideas (like optimal launch window or more &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;noreferrer&quot;&gt;delta-V&lt;/a&gt; due to only &lt;a href=&quot;https://space.stackexchange.com/questions/20830/why-will-soyuz-ms-04-bring-only-two-crew-to-the-iss&quot;&gt;two crew members&lt;/a&gt; at launch, which means more flexibility for orbital maneuvers), but I am not sure at all.&lt;/p&gt;&#xA;" OwnerUserId="18592" LastEditorUserId="12102" LastEditDate="2017-07-06T10:33:39.897" LastActivityDate="2017-07-06T10:33:39.897" Title="How did Soyuz MS-04 reach the ISS in only 6 hours?" Tags="|iss|rendezvous|soyuz-launcher|transfer-time|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21820" PostTypeId="2" ParentId="21819" CreationDate="2017-06-06T09:12:18.373" Score="18" Body="&lt;p&gt;Russia has been using expedited rendezvous since 2012 with the uncrewed Progress and since 2013 with the crewed Soyuz. Expedited rendezvous is a huge bonus for a crewed vehicle. This is particularly the case with the Soyuz capsule, which is a bit cramped when it carries two people and very cramped when it carries three.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The newest revision of the Soyuz fell back to the longer two day rendezvous because there is very little margin for error in the expedited rendezvous. Roscosmos needed to make sure that there wasn't a misfeature hiding amongst the many upgrades made to the Soyuz. The first Soyuz MS flight was planned as a two day rendezvous to allow time for on-orbit checkout. The second flight experienced some difficulties. The third flight used a two day rendezvous for safety. Everything went well (very well) with that flight, giving Roscosmos confidence that they could go back to using an expedited rendezvous for the fourth flight of the Soyuz MS.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-06-06T09:12:18.373" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21821" PostTypeId="2" ParentId="21817" CreationDate="2017-06-06T11:38:30.417" Score="3" Body="&lt;p&gt;There are no concrete plans for off-world self-replicating systems. NASA is &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?N=0&amp;amp;Ntk=All&amp;amp;Ntt=self-replicating%20system&amp;amp;Ntx=mode%20matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;studying the idea&lt;/a&gt; (and has been for decades), but we're too far away from any self-replicating system to start designing one for Mars.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps the group closest to achieving the goal of self-replication is the &lt;a href=&quot;http://www.reprap.org/wiki/RepRap&quot; rel=&quot;nofollow noreferrer&quot;&gt;community trying to build self-replicating 3D printers&lt;/a&gt;, but they're not there yet (can't replicate the electronics, for example). &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-06T11:38:30.417" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21822" PostTypeId="2" ParentId="21783" CreationDate="2017-06-06T12:58:09.687" Score="3" Body="&lt;p&gt;In this launch, LZ-1 was covered with new radio-reflective paint to assist more precise landing. I think the paint could be the source of this red smoke.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://twitter.com/elonmusk/status/871228411494014976&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://twitter.com/elonmusk/status/871228411494014976&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5796" LastActivityDate="2017-06-06T12:58:09.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21825" PostTypeId="1" AcceptedAnswerId="21826" CreationDate="2017-06-06T07:12:54.340" Score="6" ViewCount="1118" Body="&lt;p&gt;As far as I know, CNC machining was being used in the aerospace industry in the 1960s. Were such machines use for the F1 engines? &lt;/p&gt;&#xA;" OwnerUserId="9049" OwnerDisplayName="alkopop79" LastEditorUserId="6816" LastEditDate="2017-06-07T15:25:29.637" LastActivityDate="2019-05-13T09:04:09.700" Title="Was CNC milling used in the 1960's for making the Apollo/Saturn rocket engines?" Tags="|apollo-program|saturn-v|manufacturing|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21826" PostTypeId="2" ParentId="21825" CreationDate="2017-06-06T17:36:14.650" Score="8" Body="&lt;p&gt;At least some parts for the Apollo program were CNC-machined: e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_Launch_Vehicle_Digital_Computer&quot; rel=&quot;noreferrer&quot;&gt;the frame of the flight computer&lt;/a&gt;. &#xA;I haven't found any documents that confirm CNC-machining was used on the F1, though. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On one hand, those engines contain thousands of parts, using all sorts of manufacturing techniques, so it's possible CNC was used on some parts. OTOH, CNC was new enough at the time to be considered newsworthy so if it was used, there'd be a story about it somewhere. My (preliminary) conclusion: CNC was probably not used on the F1 engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some features of the F-1 would have benefited from CNC machining. The nozzle walls consist of a &lt;a href=&quot;http://heroicrelics.org/info/f-1/f-1-thrust-chamber.html&quot; rel=&quot;noreferrer&quot;&gt;series of coolant tubes stacked together&lt;/a&gt;. This was a very labour-intensive process. Modern engines often use nozzle walls that have cooling passages CNC-machined into them, so you have a single part instead of hundreds. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-06-07T08:18:14.923" LastActivityDate="2017-06-07T08:18:14.923" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21827" PostTypeId="2" ParentId="21819" CreationDate="2017-06-06T20:48:25.033" Score="22" Body="&lt;p&gt;On one of the Russian web sites there is &lt;a href=&quot;http://gagarin.energia.ru/past-future/111-2013-03-25-05-13-17.html&quot; rel=&quot;noreferrer&quot;&gt;an interview&lt;/a&gt; with Rafail Murtazin, deputy head of ballistics department of the Energia corporation, who is described as the developer of the expedited rendezvous scheme. In brief, they have achieved this as follows (I apologize for possibly incorrect terms, please feel free to edit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, the ISS orbit is adjusted in advance to ensure that on the launch date the phase angle between the two crafts is within 30-35 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, Soyuz makes its first orbit transfer on the very first orbit, using theoretical orbit parameters computed before launch. This is in contrast with the two-day rendezvous scheme, where they measure actual orbital parameters before the transfer, which takes them two extra orbits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the bottom of the interview page is a link to the presentation on the topic (PowerPoint, in Russian) Murtazin gave at a conference at the Bauman State Technical University in 2013; it has all sorts of diagrams showing in detail the entire launch and rendezvous process. &lt;/p&gt;&#xA;" OwnerUserId="19891" LastEditorUserId="19891" LastEditDate="2017-06-06T22:22:51.470" LastActivityDate="2017-06-06T22:22:51.470" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21828" PostTypeId="1" CreationDate="2017-06-06T21:32:14.130" Score="4" ViewCount="143" Body="&lt;p&gt;Do astronauts beef up their muscle mass before a mission on the ISS? Does it give a buffer of muscle mass they can lose? They don't look like strongmen, although NASA doesn't seem to discriminate size. Maybe they should in order to keep crews healthier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below an image of one of the strongest men in the world. Building an aqueduct or lifting Mars or something between the weightlifting exercises.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qf1Lm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qf1Lm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-06-07T18:18:38.723" LastActivityDate="2017-06-08T11:12:55.243" Title="Does body building help mitigate microgravity health effects? Is it used?" Tags="|microgravity|health|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21829" PostTypeId="2" ParentId="10649" CreationDate="2017-06-07T02:01:53.167" Score="1" Body="&lt;p&gt;CG, Center of Gravity is usually the reason to put LOx ahead of fuel but in the Saturn with the F-1 engine thrust structure you wouldn't want to get cold next to the LOx if the LOx was aft. With the 2nd/3rd stages as aforementioned the LH2 was ahead of the LOX but LH2 being so much lower in density and temperature than the LOx would be the same reason, keeping the thrust structure from the cold, in this case 421 below zero of LH2 vs only 300 below of LOx.&lt;/p&gt;&#xA;" OwnerUserId="17705" LastEditorUserId="6816" LastEditDate="2017-06-07T02:36:59.007" LastActivityDate="2017-06-07T02:36:59.007" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21830" PostTypeId="1" CreationDate="2017-06-07T08:16:38.247" Score="4" ViewCount="391" Body="&lt;p&gt;Chang'e 3 &lt;a href=&quot;http://gbtimes.com/china/chinas-telescope-moon-still-working-and-could-do-30-years&quot; rel=&quot;nofollow noreferrer&quot;&gt;is still operating, 3 years later, sending good science from its LUT (Lunar-based Ultraviolet Telescope) - running on RTG batteries&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA is supposedly scrapping the bottom of the barrel reserving RTG fuel for most critical and valuable missions. ESA can't dream of RTGs even where they would be extremely valuable (like Rosetta/Philae). Meanwhile - yes, a lunar lander is a big step for China, but others have done it a long time ago; if anyone at NASA suggested a robotic, RTG-powered mission to the Moon currently, they'd get laughed off the complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, USA only ever produced about &lt;a href=&quot;https://space.stackexchange.com/a/19986/103&quot;&gt;500 pounds of tritium&lt;/a&gt; while China sells it on Aliexpress in keychains, $7 a piece. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any &lt;em&gt;publicly available&lt;/em&gt; estimates on Chinese stockpile of Pu-238 - their capacity of RTG battery production?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="12102" LastEditDate="2017-11-12T05:56:15.853" LastActivityDate="2017-11-12T05:56:15.853" Title="How much RTG fuel does China have?" Tags="|rtg|china|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21833" PostTypeId="1" AcceptedAnswerId="24385" CreationDate="2017-06-07T13:37:35.607" Score="11" ViewCount="669" Body="&lt;p&gt;Considering we live in an age of unmanned spacecrafts, with computers more than capable of managing everything from attitude control, burns, to docking and insanely precise powered landings; how much manual control, if any is done during today's manned missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is Soyuz flown completely by the computer, or does the crew take the control during critical phases of the flight? Does it even allow for manual control? Can it be flown manually in a case of some major system failure? Did something like that ever occur?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And what about other upcoming manned spacecrafts? Will something like the manned Dragon capsule even have any joysticks or controls or will it likely be &quot;passenger only&quot;?&lt;/p&gt;&#xA;" OwnerUserId="10124" LastEditorUserId="10124" LastEditDate="2017-06-07T14:19:28.600" LastActivityDate="2018-01-10T20:47:00.160" Title="How much manual flying is done in a modern manned spacecraft?" Tags="|crewed-spaceflight|soyuz-spacecraft|dragon-v2|uncrewed-spaceflight|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21834" PostTypeId="1" CreationDate="2017-06-07T13:47:40.540" Score="0" ViewCount="173" Body="&lt;p&gt;NASA has this &quot;planetary protection&quot; scheme. But it seems to apply to non-planets, like Enceladus and Europa. Thus Galileo and Cassini probes are crashed into their gas giants. How does this add up? Is there a misnamed everything-in-space-&quot;protection&quot; policy?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-06-07T17:55:39.450" Title="How does &quot;planetary protection&quot; apply to Europa since Europa is not a planet?" Tags="|planet|planetary-protection|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21835" PostTypeId="2" ParentId="21834" CreationDate="2017-06-07T13:50:15.893" Score="0" Body="&lt;p&gt;Planets, moons, comets it does not really matter.  The goal is to protect an environment where life could have developed or might develop. The name may not be entirely well chosen for accuracy as opposed to emotional impact.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-07T13:50:15.893" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21837" PostTypeId="2" ParentId="21834" CreationDate="2017-06-07T14:08:27.647" Score="4" Body="&lt;p&gt;&quot;Planetary protection&quot; is a minor misnomer. From &lt;a href=&quot;https://planetaryprotection.nasa.gov/overview&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Planetary protection is the term given to the practice of protecting solar system bodies (i.e., planets, moons, comets, and asteroids) &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&quot;Planetary&quot; is simply an easier word to manage than &quot;celestial-body-related&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such misnomers are extremely common in all fields.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-06-07T14:08:27.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21838" PostTypeId="1" CreationDate="2017-06-07T14:39:15.557" Score="5" ViewCount="177" Body="&lt;p&gt;After sending &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_CRS-11&quot; rel=&quot;nofollow noreferrer&quot;&gt;CRS-11&lt;/a&gt; into orbit to rendezvous with the space station, the first stage of the SpaceX rocket Dragon landed on Landing Zone one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=GrP3jHuLQ9o&quot; rel=&quot;nofollow noreferrer&quot;&gt;the video released subsequently&lt;/a&gt; you can see two pieces of debris kicked up by the landing blast to the right of the landing spacecraft:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AQ1O1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AQ1O1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/x5J4s.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/x5J4s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are these, and is this an accident or intentional?  &lt;/p&gt;&#xA;" OwnerUserId="248" LastEditorUserId="51" LastEditDate="2017-06-07T14:48:41.157" LastActivityDate="2017-06-07T14:48:41.157" Title="SpaceX CRS-11 landing pad debris identification" Tags="|spacex|falcon-9|landing|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21839" PostTypeId="1" AcceptedAnswerId="21844" CreationDate="2017-06-07T17:28:12.813" Score="24" ViewCount="4964" Body="&lt;p&gt;There are quite many industrial Linux adaptions around, like in your router's firmware or the Android OS.&#xA;On long term, probably any spacecraft will need to run more or less standard profiles of software packages, like navigation, communication.. the more OSS (Open Source Software) the better I think. We all should know the value delivered by OSS community to Earth's industries.&#xA;(assume also that there will be a rise of avionics open hardware, too).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the question - is there any ongoing approach for a &quot;Linux for Spacecraft&quot; distro?&lt;/p&gt;&#xA;" OwnerUserId="19946" LastEditorUserId="19946" LastEditDate="2017-06-08T05:29:01.900" LastActivityDate="2023-03-23T01:26:40.810" Title="Does anybody work on a &quot;spacecraft linux&quot;?" Tags="|spacecraft|software|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21840" PostTypeId="2" ParentId="21834" CreationDate="2017-06-07T17:55:39.450" Score="2" Body="&lt;p&gt;The term &quot;&lt;a href=&quot;https://planetaryprotection.nasa.gov/history&quot; rel=&quot;nofollow noreferrer&quot;&gt;planetary protection&lt;/a&gt;&quot; predates our current understanding of our solar system. Since 1958, we've made discoveries that led scientists to consider the possibility of life existing on moons like Europa.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the responsibilities of the Office of Planetary Protection expanded, but the name stayed the same. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-07T17:55:39.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21842" PostTypeId="1" CreationDate="2017-06-07T18:49:16.467" Score="3" ViewCount="82" Body="&lt;p&gt;We have seen many community projects towards open hardware.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_open-source_hardware_projects&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/List_of_open-source_hardware_projects&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is state of the art for space avionics?&lt;/p&gt;&#xA;" OwnerUserId="19946" LastActivityDate="2017-06-07T18:49:16.467" Title="What are known space avionics open hardware projects?" Tags="|avionics|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21843" PostTypeId="2" ParentId="21800" CreationDate="2017-06-07T19:21:28.883" Score="2" Body="&lt;p&gt;Probably not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The turbopump in the Space Merlin 1B produces 1900kW and spins at 22000rpm, so thats the number our stirling engine would have to beat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Merlin engine used by SpaceX has a chamber temperature of around 2500 Kelvin, and liquid oxygen has a temperature of around 50 Kelvin. The Carnot efficiency for this is &#xA;$$&#xA;\eta = 1 - \dfrac{T_c}{T_h} = 98\%&#xA;$$&#xA;and stirling engines have an efficiency of 50%, so we have an efficiency of 49%, which is the ratio we have between the hot temperature and the power produced. This means to reach 1900kW of power, our sterling engine would have to move almost 3800kW of power. Depending on the temperature gradient between the thrust chamber and the hot side of the sterling engine, this would require &lt;a href=&quot;https://www.wolframalpha.com/input/?i=3800kW%20%2F%20(thermal%20conductivity%20of%20niobium%20*%2050K%20%2F1cm)&quot; rel=&quot;nofollow noreferrer&quot;&gt;several square meters of surface area&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="19872" LastActivityDate="2017-06-07T19:21:28.883" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21844" PostTypeId="2" ParentId="21839" CreationDate="2017-06-07T20:27:13.290" Score="31" Body="&lt;p&gt;Updates: Added Astro Digital and Planet Labs at bottom.&lt;/p&gt;&#xA;&lt;p&gt;There isn't a specific Linux distribution for spacecraft. At least not yet.  I have heard of groups that basically took Buildroot to build only as much Linux system as they needed for a satellite. Beyond that, they needed to provided the customized middleware or applications.&lt;/p&gt;&#xA;&lt;p&gt;It is tempting to put Linux on a low-cost small satellites (i.e., CubeSats), one needs to remember that these satellites have extremely tight power budgets, dictated by how much power they can muster from photovoltaic panels. That is, this is an environment in which code efficiency and low power consumption far outweigh programmer productivity.&lt;/p&gt;&#xA;&lt;p&gt;As for groups working on Linux in spacecraft, here are a couple that can be publicly discussed -- Tyvak (actively) and Boeing (because NASA and AF require it). (And now I've added Astro Digital below.)&lt;/p&gt;&#xA;&lt;p&gt;Tyvak Nano-Satellite Systems maintains its own Linux distribution for flight software. See: &lt;a href=&quot;http://www.tyvak.com/capabilities/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tyvak capabilities&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;They have built radiation-tolerant boards for CubeSat avionics. In fact, they use to have a specific ARM-based space avionics board on their website, running Linux.  But I don't see it by name anymore. (I recall the name  &amp;quot;Intrepid&amp;quot;; someone might find this in some published research papers.)&lt;/p&gt;&#xA;&lt;p&gt;Boeing recently got the contract to develop a High Performance Spaceflight Computing (HPSC) Processor Chiplet for NASA and the Air Force.  The chiplet is to be a quad-core ARM Cortex-A53 using rad-hard by design (RHBD) standard cell libraries.  The software requirement for the chip is to run a combination of Linux and real-time OS.  The Air Force wants this for their advanced satellite designs. NASA knows it needs advanced processors with greater autonomy capability in deep space robotic as well as human spaceflight missions.  See:&#xA;&lt;a href=&quot;http://www.militaryaerospace.com/articles/2017/04/radiation-hardened-space-processor.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Boeing to develop next-generation radiation-hardened space processor based on the ARM architecture&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There are a couple of other commercial CubeSat companies that I believe use Linux in their satellites, but I can't readily find this on their web pages. For what its worth, the two I'm thinking of both do Earth observation, and have large server farms on the ground.&lt;/p&gt;&#xA;&lt;p&gt;Update: one of the other companies I had in mind is Astro Digital; their former name is Aquila Space.  Their Corvus-BC has (or did have) Linux running on ARM Cortex-A8. &lt;a href=&quot;http://mstl.atl.calpoly.edu/%7Ebklofas/Presentations/DevelopersWorkshop2015/Bertino_Perseus-M_Corvus-BC.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aquila Space slides from 2015&lt;/a&gt; About a year ago, I attended a talk by Astro Digital, where one of them said even the aerospace engineers (non-computer scientists) could program the spacecraft because they were using Python. I raised my hand to check. &amp;quot;Wait. Python on the spacecraft?&amp;quot; Yes. I grinned as any serious Pythonista should at this point. :-)  Note that as CubeSats go, the Astro Digital spacecraft are pretty large. They have 6U and 16U designs, with solar panel arrays geared to feed them.&lt;/p&gt;&#xA;&lt;p&gt;Update: I also had in mind Planet Labs, now simply known as &amp;quot;Planet&amp;quot;. Turns out there is a &lt;a href=&quot;https://space.stackexchange.com/questions/13436/successful-linux-based-cubesat-missions?rq=1&quot;&gt;Space Stack Exchange question&lt;/a&gt; that addresses this. They have their own internal distro that they tweak for their fleet of satellites.&lt;/p&gt;&#xA;&lt;p&gt;Update (Mar 2023): I stated that Boeing got the contract for an HPSC chiplet based on ARM Cortex-A53, as announced by NASA Goddard. That chiplet did not materialize. I don't know why. However, a new award was announced in August 2022, with the contract going to Microchip Technology; the selection was made by JPL. (See &lt;a href=&quot;https://www.nasa.gov/press-release/nasa-awards-next-generation-spaceflight-computing-processor-contract&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA press release&lt;/a&gt;, Aug 2022.) Since then, it has become clear that the Microchip design will be based on RISC-V, using the SiFive X280 multi-core design. (For example, &lt;a href=&quot;https://www.eejournal.com/article/nasa-recruits-microchip-sifive-and-risc-v-to-develop-12-core-processor-soc-for-autonomous-space-missions&quot; rel=&quot;nofollow noreferrer&quot;&gt;EE Journal article&lt;/a&gt;, Jan 2023). The SiFive X280 includes vector extensions and is designed with AI/ML in mind. (See &lt;a href=&quot;https://www.sifive.com/cores/intelligence-x280&quot; rel=&quot;nofollow noreferrer&quot;&gt;SiFive Intelligence X280&lt;/a&gt;.) None of these references mention any operating system -- Linux, VxWorks, or anything else. But if you're running multiple cores with vector processing and vehicle autonomy, Linux would be a nice starting point.&lt;/p&gt;&#xA;" OwnerUserId="13671" LastEditorUserId="13671" LastEditDate="2023-03-23T01:26:40.810" LastActivityDate="2023-03-23T01:26:40.810" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="21846" PostTypeId="2" ParentId="4330" CreationDate="2017-06-07T23:03:48.750" Score="5" Body="&lt;p&gt;32 Megajoules is the energy contained by an object in LEO that has a mass of 1kg, and is traveling at 8000 m/s (with respect to the stationary point around which the earth rotates, and extending a static vector outwards from that point to the orbit)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;105.8 Kilojoules is the kinetic energy possessed by an object sitting on the earth's surface, roughly at the equator, in the same situation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus the theoretical minimum energy required to get an object to 8000 m/s from the earth's surface is 31.89 Megajoules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rounded up, that leaves us with the figure of about 32 Megajoules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everything else about propellants, mass ratios, specific impulse, and so on is salient, but has nothing to do with your original question! This is the lower bound for any 1kg mass. You cannot get it into orbit with &lt;em&gt;less&lt;/em&gt; than this much energy.&lt;/p&gt;&#xA;" OwnerUserId="19973" LastEditorUserId="6816" LastEditDate="2017-06-07T23:25:42.510" LastActivityDate="2017-06-07T23:25:42.510" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21847" PostTypeId="2" ParentId="20009" CreationDate="2017-06-07T23:17:35.150" Score="2" Body="&lt;p&gt;I recall a youtuber who was getting a cubesat into orbit say it was costing him about $10k. You'll have to do some hunting, as I don't recall the specifics of the video, but I do remember that detail, and that he was a student.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly this is a bargain price, as a picosat costs about $8k to get up there. So I would bet you won't see a price like that except as a piggyback, done at the gracious behest of a large organization.&lt;/p&gt;&#xA;" OwnerUserId="19973" LastEditorUserId="19973" LastEditDate="2017-06-07T23:33:36.167" LastActivityDate="2017-06-07T23:33:36.167" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21849" PostTypeId="2" ParentId="10048" CreationDate="2017-06-08T00:07:06.257" Score="0" Body="&lt;p&gt;As a side note, all of this hand waving is based on the idea that we actually know how a particular meteorite/asteroid was formed. We don't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;People are making assumptions based on the accretion theory (and related), which is now coming under attack, quietly mind you, and may fall in our lifetime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I would say the best answer to this is &quot;No one has any serious notion of what the heck is inside any particular asteroid, unless they put sensors physically on top of the thing and prospect it&quot;. (radar observations are valued of course, however many materials could scatter or alter a radar signature form that far away).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you are looking to invest in asteroid mining, don't do it unless they are saying they are going to do prospecting first. You'll be losing your money in a puff of smoke, most likely. You might win.. sure... but personally, I don't like playing the tables. The House always wins (tm).&lt;/p&gt;&#xA;" OwnerUserId="19973" LastActivityDate="2017-06-08T00:07:06.257" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21850" PostTypeId="1" AcceptedAnswerId="21854" CreationDate="2017-06-08T03:46:28.630" Score="7" ViewCount="1012" Body="&lt;p&gt;I thought I'd try to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Patched_conic_approximation&quot; rel=&quot;noreferrer&quot;&gt;patched conics&lt;/a&gt; to see what it's like. I'll go from LEO to LXO (low Planet-X orbit), co-planar all the way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I understand correctly, I'll have (at least) five conics. The initial and final circles around the two planets, the big ellipse in the middle, and two hyperbolae within each planets sphere of influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question is about the second one — the hyperbolic escape from Earth. When I patch it to the interplanetary ellipse, I must give the endpoint of the hyperbola the Earth's velocity first. I can think of several velocities to use, the velocity of the Earth at that moment in its conic orbit around the Sun, or the velocity of a satellite in its own conic orbit around the Sun at the new distance, but there could be other options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia puts Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)&quot; rel=&quot;noreferrer&quot;&gt;sphere of influence&lt;/a&gt; distance at 145 Earth radii, or about 0.01 AU, so the choice would not have a huge effect but it would not be so small either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a generally accepted best way to add a velocity to the endpoint of the first hyperbola to most accurately patch to the following conic?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-12-21T23:16:15.847" LastActivityDate="2019-12-21T23:16:15.847" Title="How to best patch my conics?" Tags="|orbital-mechanics|interplanetary|mission-design|patched-conic|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21851" PostTypeId="2" ParentId="21850" CreationDate="2017-06-08T05:04:44.303" Score="4" Body="&lt;p&gt;The widely used rule is to use the Laplace sphere rather than the Hill sphere, which is what you are using.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I sicced an intern on a rather related problem a decade ago, which was to find the best place to switch the frame of reference in which gravitation is computed in a simulation that integrates the equations of motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had him use an arbitrary precision arithmetic package to develop reference trajectories for a vehicle on an outbound trans-lunar trajectory and an inbound trans-earth trajectory, using a variety of integration techniques. I had him calculate two reference trajectories for each leg, one Earth-centered inertial and the other Moon-centered inertia. The goal here was to get consensus reference trajectories that agreed to over 20 decimal places of accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then I had him use standard IEEE double precision numbers for the integration, switching frames of reference when the vehicle became closer than (trans-lunar trajectory) / further than (trans-earth trajectory) some prescribed distance of the center of the Moon. Rinse and repeat with different distances, rinse and repeat again with different integration techniques.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal: For a given integration technique, find the best place to switch integration frames when using standard IEEE double precision numbers for numerical integration. Not surprisingly, switching to MCI immediately after leaving low Earth orbit yielded lousy results. Waiting to switch to MCI until the vehicle is extremely close to the Moon also yielded lousy results. Also not surprising, switching somewhere near the Hill sphere or Laplace sphere generally yielded the best results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Somewhat surprisingly, it didn't seem to matter all that much which of the two (Hill sphere vs Laplace sphere) was used as a switching point (or some other similar value). Switching near the Laplace sphere gave marginally better results than switching near the Hill sphere for most techniques, but it was the other way around for a few techniques. The distinction is small, not quite how many angels can dance on the point of a pin small, but close to it. Just pick one. I generally pick the Laplace sphere, but that's a bit arbitrary. The space is rather flat in the vicinity of the Hill sphere or Laplace sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are using patched conics as opposed to numerically integrated trajectories, but the same concept applies. Just pick one, and then be consistent. &lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-06-08T05:04:44.303" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21852" PostTypeId="1" CreationDate="2017-06-08T05:31:15.453" Score="20" ViewCount="6365" Body="&lt;p&gt;I see so many discussions about how the main roadblock to terraforming Mars is its lack of a magnetic field. If Mars had an atmosphere as thick as Earth's but no magnetic field, wouldn't it still take geological time scales for the solar wind to strip away the Martian atmosphere? How many years would we be able to live on Mars before we had to replenish it? And would this thick of an atmosphere sufficiently protect people on the surface from radiation?&lt;/p&gt;&#xA;" OwnerUserId="19975" LastEditorUserId="6816" LastEditDate="2017-06-08T05:48:37.677" LastActivityDate="2021-01-15T14:30:21.180" Title="Is a magnetic field really needed to terraform Mars?" Tags="|mars|colonization|terraforming|magnetic-field|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21853" PostTypeId="1" AcceptedAnswerId="21856" CreationDate="2017-06-08T08:28:21.807" Score="8" ViewCount="2130" Body="&lt;p&gt;First of all, sorry for the general context of the question but I’m very inexperienced. I hope I'm not breaking the rules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simplifying, in the short sci-fi novel I’m writing there’s a super-intelligent alien probe that wants to communicate with us. It can intercept all of our communications leaving the Earth so it chooses to send a signal to the &lt;a href=&quot;https://en.wikipedia.org/wiki/ESTRACK&quot; rel=&quot;noreferrer&quot;&gt;ESTRACK&lt;/a&gt; (or one of the other &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Network_(disambiguation)&quot; rel=&quot;noreferrer&quot;&gt;deep space networks&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let’s say that the probe wants to send a simple “text” message, maybe a “do you copy?”. If it’s starting to learn English, to grease the wheels, how can it do it? What kind of... “format” would the signal have to be received and understood?&lt;/p&gt;&#xA;" OwnerUserId="14181" LastEditorUserId="7062" LastEditDate="2017-06-08T09:20:32.483" LastActivityDate="2017-06-09T08:44:24.313" Title="Signals to a deep space network" Tags="|communication|deep-space-network|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21854" PostTypeId="2" ParentId="21850" CreationDate="2017-06-08T08:38:34.063" Score="2" Body="&lt;p&gt;Assuming: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;non-spinning SOIs - they only undergo translational motion along the planet's orbit, but direction of their axes relative to each other is always constant. That's the trivial approach; sun-synchronous orbits are impossible and tidally locked bodies will need spin equal to orbital period, instead of a flat 0, but calculations are made easier.&lt;/li&gt;&#xA;&lt;li&gt;The &quot;root&quot; frame of reference is the Sun's SOI (Sun is assumed to be immobile). &lt;/li&gt;&#xA;&lt;li&gt;We're not concerned about the motion of the &lt;em&gt;craft&lt;/em&gt;, just motion of the &lt;em&gt;conics&lt;/em&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The hyperbola exists in Earth's SOI and is stationary relative to it (speed relative to Earth SOI = 0); its endpoint is its part and so, the endpoint's velocity in the &quot;root&quot; frame of reference will be equal to speed of Earth's SOI at that point - that is, Earth velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would become more complex in case of spinning SOIs, requiring to trace the velocity of the SOI edge relative to Sun, but as I understand this is not our case.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-06-08T10:15:04.577" LastActivityDate="2017-06-08T10:15:04.577" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21855" PostTypeId="2" ParentId="21828" CreationDate="2017-06-08T09:19:26.603" Score="3" Body="&lt;p&gt;Current &lt;a href=&quot;http://www.iceberg.expert/IMG/pdf/iceberg_2012_-_spatial_2_-_bernard_comet.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA standards&lt;/a&gt; for preflight physical training are:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;To be selected it is not necessary to be a high performer in sports activities (some-times it can be even a “handicap” i.e. marathon runner / cardio-vascular status).  &lt;/li&gt;&#xA;  &lt;li&gt;During their whole career they have to maintain a good physical shape (safe sports practice 2 – 3 times per week, = with minimum injury’ risk)  &lt;/li&gt;&#xA;  &lt;li&gt;On NASA side, the sports practices are left on the initiative of each astronaut “self practice”, in Star City the sports’ practices are scheduled and managed in the training time-tables.  &lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Preflight training includes &lt;a href=&quot;http://www.brazilbumbum.com/a-look-at-the-crazy-hardcore-fitness-regimen-nasa-is-using-to-keep-astronauts-healthy-and-impenetrably-strong/&quot; rel=&quot;nofollow noreferrer&quot;&gt;strength training&lt;/a&gt; (using free weight exercises like the squat and deadlift). There doesn't seem to be a fixed regimen, just a &lt;a href=&quot;https://lsda.jsc.nasa.gov/lsda_data/MRID_Docs/MR078LFinal.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;set of tests&lt;/a&gt; for strength and agility the astronauts have to pass before flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During missions in the ISS, training is &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110016265.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;more structured&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;astronauts are scheduled training sessions for the International Space Station (ISS) treadmill (TVIS) and cycle ergometer (CEVIS), as well as the Advanced Resistive Exercise Device (ARED). In-flight programs are designed to maintain or even improve the astronauts’ pre-flight levels of fitness, bone health, muscle strength, power and aerobic capacity. In-flight countermeasure sessions are scheduled in 2.5 h blocks, six days a week, which includes 1.5 h for resistive training and 1 h for aerobic exercise.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Loss of muscle is not the biggest concern. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After flight recovery within 1 - 6 weeks depending of the flight duration and after flight physical training intensity.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;E.g. bone loss is a bigger problem (takes longer to recover from). Strength training helps increase bone density. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In conclusion, astronauts do a fair amount of strength training, just not to the level reached by bodybuilders and other strength athletes. At competition level, those athletes train several hours a day, every day. That would reduce the amount of time available for other aspects of training. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also the question, how much training is necessary? Competition athletes are several times stronger than average, but if there's only 10% muscle loss then you only need to have 110% of average strength at the start of a mission to come out okay. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-06-08T11:12:55.243" LastActivityDate="2017-06-08T11:12:55.243" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21856" PostTypeId="2" ParentId="21853" CreationDate="2017-06-08T10:30:27.360" Score="16" Body="&lt;p&gt;That's a good &amp;amp; fun question; generally, there's a &lt;a href=&quot;https://worldbuilding.stackexchange.com/&quot;&gt;worldbuilding StackExchange sister site&lt;/a&gt; especially meant for people trying to build coherent universes. Check it out!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, your question can be reasonably interpreted as &quot;if we have a sporadic transmitter that ground can't schedule, how would it communicate with ESTRACK&quot;; and that's a serious SX question, so here we go:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's &lt;a href=&quot;http://microelectronics.esa.int/vhdl/pss/PSS-04-105.pdf&quot; rel=&quot;noreferrer&quot;&gt;ESA PSS-04-105, Issue 2.4 (November 1996)&lt;/a&gt; which defines the carrier frequencies, rates, bandwidths and modulation types of transmissions within ESTRACK. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For something like textual messages, that network wasn't actually meant – it's designed to primarily transport telemetry, and do some ranging, and nobody's expecting/decoding/guessing there'd be any English text among these numbers. But: guessing that if you send numbers that clearly aren't from normal operation, you'd be able to actually get an engineer to look for sense in the data received. Then they'd start to guess what it means. Chances are you're best off transmitting it as bits as used in the common (and old) ASCII encoding (which maps letters to binary numbers). At some point, people start recognizing that the numbers representing &lt;code&gt;e&lt;/code&gt; and &lt;code&gt;r&lt;/code&gt; are pretty common...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But: to first get the receiver to actually see data, you'd have to build a transmission that is &quot;sound&quot;. Basically: Radio transmissions are enriched with what us communication engineers/ information theorists call &quot;redundancy&quot;. Think of that as forms of checksums. For example, a common checksum would simply be the sum of all bytes sent (a byte can take values between 0 and 255), modulo 64. You just send your message and attach that checksum. The receiver checks whether it gets the same checksum as you sent, and if it did &lt;em&gt;not&lt;/em&gt;, it knows &lt;em&gt;something&lt;/em&gt; about the transmission is broken (and would ignore it). If you are cleverer than simple checksums, you can build &lt;em&gt;codes&lt;/em&gt; that allow the receiver to &quot;repair&quot; a limited number of errors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space communication engineers &lt;em&gt;are&lt;/em&gt; clever. The problem with that is that the codes can get very long and it can be very hard to guess how to build a valid transmission without their knowledge. So, that would probably pose a reasonable challenge for your aliens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, since that's all grounded in math, and math is the same all over the universe, and your aliens are advanced and definitely will have &lt;em&gt;some&lt;/em&gt; form of codes since they need to communicate using radio waves themselves, that'd basically be a challenge in understanding how this crazy human mind understands number and ring theory, or, more generally, algebra. It will require that they either bring their mathematicians along, or that they have computers onboard that were programmed to solve such hard problems (us earthlings are desperate to find better and more codes than we know – basically, we have a rich, well-proven theory of what great codes &lt;em&gt;must&lt;/em&gt; exist, but we simply know &lt;em&gt;none&lt;/em&gt; of them, although there are very, very many), which makes it likely that they have some kind of quantum computer (just don't quote me on quantum computing – it's just that the computers we know aren't any good at solving that kind of problem, as our lack of code knowledge demonstrates). Which in turn means that it should be relatively easy for them, given enough recording of e.g. TV signals, figure out how to synthesize TV signals containing human speech/imagery. (not the &lt;em&gt;picture&lt;/em&gt;, they've never saw what a TV screen shows, just the signal) Think of these funky deep neural networks transferring e.g. Van Gogh's style to your selfies. You can &quot;deep dream&quot; valid TV signals, too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, maybe they'd much rather directly interact with TV receivers on earth, sending disturbing images first, maybe things that look like strangely combined images that dominate TV transmissions, but get better the more they can actually observe what TV signals the humans reply with, once they figure out there's an extraterrestrial source of those.&lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastEditorDisplayName="user17550" LastEditDate="2017-06-08T11:20:19.360" LastActivityDate="2017-06-08T11:20:19.360" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21857" PostTypeId="2" ParentId="12453" CreationDate="2017-06-08T14:05:36.107" Score="5" Body="&lt;p&gt;Public information on rabbits in space is surprisingly sparse. The earliest instance I have been able to find is the &lt;a href=&quot;http://www.space.com/20648-animals-in-space-history-infographic.html&quot; rel=&quot;noreferrer&quot;&gt;July 2, 1959&lt;/a&gt; launch of the Soviet &lt;a href=&quot;http://www.astronautix.com/r/r-2.html&quot; rel=&quot;noreferrer&quot;&gt;R-2&lt;/a&gt;, which reportedly carried the first rabbit into space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As mentioned in the question, a rabbit was also aboard &lt;a href=&quot;http://io9.gizmodo.com/the-first-animals-in-space-1399364861&quot; rel=&quot;noreferrer&quot;&gt;Korabl-Sputnik 2&lt;/a&gt;, which is more well-known for the dogs Belka and Strelka.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did find a 1999 Japanese study, &lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pubmed/11543026&quot; rel=&quot;noreferrer&quot;&gt;Development of the aortic baroreflex system under conditions of microgravity&lt;/a&gt;, which refers to science performed on rabbits in microgravity. Unfortunately, it is only the abstract, and I was unable to find which mission this was associated with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Searching the NASA domain, I found &lt;em&gt;reference&lt;/em&gt; of lots of incidents of rabbits in space, but none with any detail. I did find one old document (which I've since lost) that mentioned that one of the cons of using rabbits for space experiments was their high susceptibility to stress.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found another Japanese paper, &lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4990110/&quot; rel=&quot;noreferrer&quot;&gt;Life science experiments performed in space in the ISS/Kibo facility and future research plans&lt;/a&gt;, containing proposals for experiments aboard the ISS through 2024. It has one brief mention of rabbits:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mammals such as mice and small rabbits are to be grown and bred in space to analyze fertilization, growth, development, movement and living under µg and 1 g conditions. Centrifugation systems in the ISS will be used to provide different levels of gravity. Potential projects are studies of reproduction, development, birth, care of offspring, behavior and healthcare in gene knockout (mutated) and parental wild-type animals. Multichannel telemetry will be used to monitor body temperatures and behavior with a CCD camera and to download data to the Earth from the ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I speculated in the comments that if they keep sending rabbits up, one might expect them to have at least some survivability. On the other hand, perhaps people just really &lt;em&gt;&lt;a href=&quot;https://space.stackexchange.com/a/2330/58&quot;&gt;want&lt;/a&gt;&lt;/em&gt; rabbits in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also in the comments, Chad pointed out that &quot;[a] living and stressed rabbit will generally not eat or eat very little and will not reproduce&quot; and a 1g centrifuge for rabbits would &quot;require [a lot] of equipment for a few animals on the ISS&quot;.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2017-06-08T14:15:26.000" LastActivityDate="2017-06-08T14:15:26.000" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21858" PostTypeId="2" ParentId="21853" CreationDate="2017-06-08T15:41:07.267" Score="3" Body="&lt;p&gt;Many ESA satellites use the &lt;a href=&quot;http://www.ecss.nl/wp-content/uploads/standards/ecss-e/ECSS-E-70-41A30Jan2003.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Packet Utilization Standard (pdf)&lt;/a&gt; (PUS), published by the &lt;a href=&quot;http://ecss.nl/&quot; rel=&quot;nofollow noreferrer&quot;&gt;European Cooperation for Space Standardization&lt;/a&gt; (ECSS) for sending telecommands and receiving telemetry. This is essentially a transmission protocol, describing packet structure and the semantics of each item. It is based on eight bit octets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Chapter 5.4 you will find the &lt;strong&gt;telemetry packet structure&lt;/strong&gt;. If the aliens are super-intelligent, they should have figured out the structure of PUS telemetry. And yes, there is a string type, usually with ASCII encoding. Knowing this, they should be able to craft telemetry that either appears to come from one of our satellites and be recognizable as &quot;interesting&quot;, or they might craft telemetry triggering frantic activity because it appears to come from an &quot;unknown&quot; satellite or application on a satellite.&lt;/p&gt;&#xA;" OwnerUserId="716" LastActivityDate="2017-06-08T15:41:07.267" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21859" PostTypeId="1" AcceptedAnswerId="21860" CreationDate="2017-06-08T16:06:09.953" Score="3" ViewCount="488" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Abdominal_thrusts&quot; rel=&quot;nofollow noreferrer&quot;&gt;Abdominal Thrusts&lt;/a&gt; (the &quot;Heimlich maneuver&quot;) and &lt;a href=&quot;https://en.wikipedia.org/wiki/Cardiopulmonary_resuscitation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cardiopulmonary Resuscitation&lt;/a&gt; (CPR) are life-saving &lt;a href=&quot;https://en.wikipedia.org/wiki/First_aid&quot; rel=&quot;nofollow noreferrer&quot;&gt;first aid&lt;/a&gt; procedures that involve  carefully targeted manual forcing of a victims body in order to dislodge an obstruction that is preventing breathing, or to move oxygenated blood in the case of heart failure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When done properly, both of these depend upon working against gravity to generate sufficient force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have procedures been developed to do these in weightlessness?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/izWTO.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/izWTO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Illustration of &lt;a href=&quot;https://en.wikipedia.org/wiki/Abdominal_thrusts&quot; rel=&quot;nofollow noreferrer&quot;&gt;Abdominal Thrusts&lt;/a&gt; (the &quot;Heimlich maneuver&quot;), from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:AbdominalThrust_HeimlichManoeuvre.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9E0Ux.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9E0Ux.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Demonstration of &lt;a href=&quot;https://en.wikipedia.org/wiki/Cardiopulmonary_resuscitation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cardiopulmonary Resuscitation&lt;/a&gt; (CPR) on a simulated patient, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:CPR_training-04.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-06-08T17:18:23.293" Title="How would the Heimlich maneuver or CPR be done in weightlessness?" Tags="|crewed-spaceflight|safety|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21860" PostTypeId="2" ParentId="21859" CreationDate="2017-06-08T17:18:23.293" Score="2" Body="&lt;p&gt;Why, by the &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwighJS24K7UAhUO9GMKHdwKDpQQFggnMAA&amp;amp;url=https%3A%2F%2Fwww.nasa.gov%2Fcenters%2Fjohnson%2Fpdf%2F163533main_ISS_Med_CL.pdf&amp;amp;usg=AFQjCNGusAle_m4FVnEM4o6fl2hScb4orA&amp;amp;sig2=Z6GeMzTipXYMQSrbstHLUw&quot; rel=&quot;nofollow noreferrer&quot;&gt;checklist&lt;/a&gt;, of course:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[&lt;img src=&quot;https://i.stack.imgur.com/ZEz33.png&quot; alt=&quot;enter image description here&quot;&gt;]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6S7GS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6S7GS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Heimlich maneuver is referred to, but not described, i.e. &quot;perform Heimlich maneuver&quot;, so it must be standard.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-06-08T17:18:23.293" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21861" PostTypeId="1" CreationDate="2017-06-08T18:25:37.940" Score="6" ViewCount="424" Body="&lt;p&gt;We are used to having hundreds of megabits or more per second of data transfer rates at our fingertips, over the Internet, straight to our desks. For higher-end connections, even multiples of gigabits per second isn't at all uncommon. This makes transferring even large amounts of data basically trivial. Large storage media is also common, allowing us to store the data both prior to and after transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, that's a pretty recent development. It wasn't more than some 30 years ago that a 10 megabit per second connection could be considered a fast local area network, and a few hundred kilobits per second was a respectable inter-network link. Only some 15 years ago, a hard disk on the order of 100 GB was on the large end of the range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In such an environment where what we might today consider high-speed data transfer simply wasn't a thing, &lt;strong&gt;how was radio telescope observation data transferred between science teams, and between locations?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="415" LastActivityDate="2019-01-07T23:26:02.220" Title="How was radio telescope observation data transferred before wide availability of high-speed Internet?" Tags="|observation|radio-telescope|data-storage|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21862" PostTypeId="2" ParentId="21861" CreationDate="2017-06-08T18:56:28.933" Score="6" Body="&lt;p&gt;Transportation of data between observatories was done via magnetic tape using a standard or common format. At least this was the case for the Westerbork Synthesis Radio Telescope and the observatories in Europe that it interacted with in the 1970s, though I imagine that was a pretty common practice. Tape was a good format for storage and transport of large amounts of data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before digital logging, radio telescopes stored data on paper using a &lt;a href=&quot;https://en.wikipedia.org/wiki/Chart_recorder&quot; rel=&quot;nofollow noreferrer&quot;&gt;chart recorder&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As others have pointed out in the comments, transporting data physically (aka the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sneakernet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sneakernet&lt;/a&gt;) is still common for large amounts of data today, and it will probably always have a use as our demand for data storage grows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://books.google.com/books?id=aTbsCAAAQBAJ&amp;amp;pg=PA69&amp;amp;lpg=PA69&amp;amp;dq=tape+transfer+between+observatories&amp;amp;source=bl&amp;amp;ots=-ywlt73cSc&amp;amp;sig=u_Iki01QnhnbuIMn9O6fPDpFPmQ&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjj27zc9a7UAhXm5YMKHa8ZB98Q6AEILjAC#v=onepage&amp;amp;q=tape%20transfer%20between%20observatories&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Use of Standardized Data Formats with the Westerbork Radio Telescope - R.H. Harten and T.A.Th. Spoelstra&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://books.google.com/books?id=_bHDAQAAQBAJ&amp;amp;pg=PP14&amp;amp;lpg=PP14&amp;amp;dq=radio+astronomy+chart+recorders+1940&amp;amp;source=bl&amp;amp;ots=t-J5WIjo5W&amp;amp;sig=va845SAUurZl84AmCcTnBUVErvk&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwiNpc258bDUAhUr7YMKHUAsBQUQ6AEILDAC#v=onepage&amp;amp;q=radio%20astronomy%20chart%20recorders%201940&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Unseen Cosmos: The Universe in Radio&lt;/em&gt; - Francis Graham-Smith&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2017-06-09T13:42:43.623" LastActivityDate="2017-06-09T13:42:43.623" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21863" PostTypeId="1" CreationDate="2017-06-08T20:40:16.683" Score="4" ViewCount="129" Body="&lt;p&gt;On the launch pad, why is the Soyuz launch vehicle supported in the middle by large arms, rather than resting on its base, as with other launch vehicles?&lt;/p&gt;&#xA;" OwnerUserId="19983" LastEditorUserId="6816" LastEditDate="2017-06-08T20:48:33.763" LastActivityDate="2017-06-08T20:48:33.763" Title="Soyuz launcher support" Tags="|soyuz-launcher|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21864" PostTypeId="2" ParentId="21863" CreationDate="2017-06-08T20:44:30.733" Score="2" Body="&lt;p&gt;It is a pretty elegant solution. With the counterweights, as it starts to liftoff the weights drop the arms off, meaning there is no need to build motors for it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since they build their pads on the edge of a cliff, this leaves the bottom sort of hanging out, no need for work to redirect the thrust.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-08T20:44:30.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21866" PostTypeId="1" CreationDate="2017-06-09T00:56:47.873" Score="1" ViewCount="51" Body="&lt;p&gt;I have a friend who swears that he saw the ISS and the arrays sticking out the sides of the bright light.  I'm thinking he has cataracts.  (We're both of that age.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible he really did see the solar panels?&lt;/p&gt;&#xA;" OwnerUserId="19988" LastActivityDate="2017-06-09T00:56:47.873" Title="Is it possible to see ISS solar arrays with naked eye, from earth?" Tags="|iss|visibility-of-spacecraft|" AnswerCount="0" CommentCount="1" ClosedDate="2017-06-09T08:13:19.453" ContentLicense="CC BY-SA 3.0" />
  <row Id="21867" PostTypeId="1" CreationDate="2017-06-09T02:35:55.280" Score="4" ViewCount="791" Body="&lt;p&gt;When you look up right after dusk or before dawn, you are very likely to see passing satellites passing overhead, still illuminated by the sun and reflecting daylight towards you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All theses appear to be yellowish white. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is that so? Is it because they all average out to a similar reflective colour? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What if I sent a huge pink reflective balloon; would it appear pink?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="12102" LastEditDate="2017-06-10T05:03:32.197" LastActivityDate="2017-06-10T05:03:32.197" Title="Why do all satellites seems to have the same color from earth?" Tags="|artificial-satellite|observation|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21868" PostTypeId="2" ParentId="21811" CreationDate="2017-06-09T02:57:04.973" Score="1" Body="&lt;h2&gt;TL;DR Patched Conics solutions look WEIRD when viewed in synodic frames. Since they do not reflect gravitationally correct trajectories, representing them in rotating frames will accentuate their &quot;rough edges.&quot;&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The kernel for this answer is &lt;a href=&quot;https://space.stackexchange.com/questions/21811/totally-non-elliptic-rocket-path/21868#comment61056_21811&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From page 437 of &lt;em&gt;An Introduction to the Mathematics and Methods of Astrodynamics&lt;/em&gt;&#xA;by Richard H. Battin, as seen in &lt;a href=&quot;https://books.google.com.tw/books?id=OjH7aVhiGdcC&amp;amp;lpg=PA440&amp;amp;ots=FfCzQsjhva&amp;amp;dq=patched%20conics%20procedural%20outline&amp;amp;pg=PA438#v=onepage&amp;amp;q=patched%20conics%20procedural%20outline&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;google books&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An adequate approximate trajectory may be had by matching in both position and velocity at the junction points (1) an ellipse from Earth to the sphere of influence of the moon whose focus is at the center of the earth, (2) a hyperbola around the moon, and (3) an ellipse from the sphere of influence back to Earth. The simplified problem, though itself fairly complex, is tractable when then relevant parameters and independent variables are identified.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The faint line in the drawing in the question and the drawing from the linked reference (the line drawings below, not the photos), is shown with the moon fixed. This is a frame rotating with the moon's orbit around the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The hand-drawn yellow-orange arc to the right of the moon in the drawing highlights what bothered me, that there does not seem to be an easy way to smoothly patch a hyperbolic trajectory around the far side of the moon to either of the elliptical conics with the Earth as the focus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The drawing shows corners&lt;/strong&gt; where the elliptical segments are patched to the hyperbola.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;In my opinion&lt;/em&gt; the drawing highlights a problem understanding how this orbit can be approximated with two elliptical segments and one hyperbolic segment that are &lt;strong&gt;smoothly patched&lt;/strong&gt; - preserving both continuity and velocity at the patch points simultaneously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The resolution to this &lt;strong&gt;apparent problem&lt;/strong&gt; is that at each patching, there is another reference frame shift. Another way to say that is that &quot;the moon is moving!&quot; So when you patch back to the 2nd elliptical segment, you have to remember that you've shifted reference frames twice! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The drawing in the question is provocative, it certainly gave me something to think about, but without the accompanying talk, or text, or subtext, it is does not teach us much more than to dig in to the issue more deeply because drawings can be deceiving and those Apollo engineers were really clever!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take a look at &lt;em&gt;Exploration of Lunar Free-return Trajectories&lt;/em&gt; by Brian Michels, ASEN 5050, University Colorado, Aerospace Sciences where there is a nice review of the patched conic analysis of the free-return orbit, and includes a lot of raw, historical data from that analysis:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2012/michels/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ccar.colorado.edu/asen5050/projects/projects_2012/michels/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fuWrY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fuWrY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ToAdd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ToAdd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2AWSw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2AWSw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MQlRz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MQlRz.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/egTLh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/egTLh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-09T04:52:23.883" LastActivityDate="2017-06-09T04:52:23.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21869" PostTypeId="2" ParentId="21867" CreationDate="2017-06-09T03:02:47.697" Score="4" Body="&lt;p&gt;It's possible that most satellites are similar in color, but an additional likely reason for this is for the following physiological fact. The eye has rod cells and cone cells. The rod cells see only black and white, and are extremely sensitive. The cone cells see color, and are much less sensitive. When you look at something that isn't very bright, the result is that you can't see it in color. This is why you can't see color clearly at night when the light is dim. Most satellites are not that bright, comparable to maybe a 3rd magnitude star I would guess. That isn't enough light to see color very clearly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see this effect very clearly when you look at a long-exposure photograph of the night sky, or when you look at stars through a telescope with a large aperture. The colors of the stars are much clearer than with the naked eye.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2017-06-09T03:02:47.697" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21870" PostTypeId="1" CreationDate="2017-06-09T05:14:32.980" Score="4" ViewCount="334" Body="&lt;p&gt;I read that launch pads need to be refurbished after every launch. What stuff on the pad takes damage? What is stopping them from covering the &quot;stuff&quot; in some sort of flame-retardant, explosion-proof material?&lt;/p&gt;&#xA;" OwnerUserId="19991" LastEditorUserId="6816" LastEditDate="2017-06-09T05:20:36.290" LastActivityDate="2017-06-09T07:50:24.300" Title="What type of work goes into refurbishing a launch pad?" Tags="|launch|rockets|reuse|launchpad|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21871" PostTypeId="1" AcceptedAnswerId="21880" CreationDate="2017-06-09T05:39:25.340" Score="2" ViewCount="225" Body="&lt;p&gt;Watching the CRS-11 hosted webcast starting at &lt;code&gt;T -00:08:24&lt;/code&gt; got me thinking about SpaceX launches on both the Atlantic and Pacific coasts of the US. Have there been &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;Autonomous Spaceport Drone Ships&lt;/a&gt; deployed in both oceans? Have landings happened in both? If so, are the ships moved across land, through the Panama Canal, or is one stationed on either side semi-permanently?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; video queued at &lt;code&gt;T -00:08:24&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/JuZBOUMsYws?start=695&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-09T12:57:20.150" LastActivityDate="2017-06-09T14:11:44.657" Title="Has F9 landed on ships in both the Atlantic and Pacific oceans?" Tags="|spacex|falcon-9|landing|asds|droneship|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21872" PostTypeId="2" ParentId="13493" CreationDate="2017-06-09T05:50:57.977" Score="2" Body="&lt;p&gt;I know this is an old question, but since there isn't a single answer... A few years ago, Tyvak was offering its Intrepid System board.  It is no longer in their product offering, but they seem to still offer boards, and still talk about Linux. Start with this page: &lt;a href=&quot;http://www.tyvak.com/platform/microsat-solutions/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.tyvak.com/platform/microsat-solutions/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other CubeSats have flown with Linux, but I haven't heard of those boards be made available for sale.&lt;/p&gt;&#xA;" OwnerUserId="13671" LastActivityDate="2017-06-09T05:50:57.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21873" PostTypeId="2" ParentId="13436" CreationDate="2017-06-09T05:54:46.010" Score="2" Body="&lt;p&gt;Late answer, which I've posted in some other questions.  Tyvak was selling Linux boards for satellites. If the need is still there, it's probably worth talking to them. &lt;a href=&quot;http://www.tyvak.com/platform/microsat-solutions/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.tyvak.com/platform/microsat-solutions/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why them? One of the co-founders of the company, Jordi Puig-Sauri, is one of the co-inventors of the CubeSat standard. It grew out of the work of him and his students at CalPoly SLO.&lt;/p&gt;&#xA;" OwnerUserId="13671" LastActivityDate="2017-06-09T05:54:46.010" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21874" PostTypeId="2" ParentId="21871" CreationDate="2017-06-09T06:07:37.083" Score="2" Body="&lt;p&gt;Yes. Actually the Wikipedia article you &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;cite&lt;/a&gt; has good info. :-)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the successful drone landings have been on OCISLY (Of Course I Still Love You) in the Atlantic. However, on Jan 14, 2017, there was a successful landing on JRtI (Just Read the Instructions).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should answer the other parts of the question:&lt;br&gt;&#xA;* They are semi-permanent, one ship for one ocean.&lt;br&gt;&#xA;* The ASDSs are very large, very wide ships that SpaceX acquired, with extremely large engines to keep the ASDS relatively stable in rough seas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a result, it cannot be shipped over land. I'm not certain if JRtI went through the Panama Canal, or had to go around the tip of South America.&lt;/p&gt;&#xA;" OwnerUserId="13671" LastEditorUserId="13671" LastEditDate="2017-06-09T06:31:34.403" LastActivityDate="2017-06-09T06:31:34.403" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21875" PostTypeId="2" ParentId="20009" CreationDate="2017-06-09T06:18:01.967" Score="5" Body="&lt;p&gt;If the satellite isn't getting an academic free ride from the Air Force or NASA, then it is probably best to work through a payload integrator, who bundles several CubeSats into a single launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One major integration vendor is Spaceflight Industries (formerly Spaceflight Services).  They start with 3U CubeSats for $295K (USD) to LEO. As the satellite gets larger, the price per U goes down.&#xA;&lt;a href=&quot;http://www.spaceflight.com/schedule-pricing/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight schedule and pricing&lt;/a&gt;  They fly using several different boosters.&lt;/p&gt;&#xA;" OwnerUserId="13671" LastActivityDate="2017-06-09T06:18:01.967" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21876" PostTypeId="1" AcceptedAnswerId="41369" CreationDate="2017-06-09T06:55:47.303" Score="4" ViewCount="518" Body="&lt;p&gt;If you use a shallow wedge prism or &lt;em&gt;very low dispersion&lt;/em&gt; diffraction grating (e.g. circa 100 lines/mm) front of a camera, you can photograph the spectra of stars with a modern DSLR camera with a fast, wide field lens fairly easily I believe. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one prepared for a bright satellite pass and oriented the dispersion perpendicular to the expected path, one could capture the evolving spectrum of the satellite as it tumbled or passed in or out of Earth's shadow. You'd have to do a little processing/averaging of the image to get a spectrum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anyone seen an image like this published somewhere, or has made one of them oneself? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One example of the kind of hardware I'm talking about his this, but it would be better to use something with very low dispersion but without blocking the full aperture of the lens. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I don't mean to advocate a commercial product, this is a handy example, and &lt;a href=&quot;http://www.fieldtestedsystems.com/about/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tom Field&lt;/a&gt; is a contributing editor to &lt;em&gt;Sky and Telescope&lt;/em&gt; magazine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/bJbGD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;https://www.rspec-astro.com/star-analyser/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/QflCv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;http://www.fieldtestedsystems.com/poster/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Fv5fFSacVO8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-09-12T04:14:43.977" LastActivityDate="2020-03-12T10:02:40.353" Title="Has anyone seen an actual spectrum of a satellite made by an amateur photographer?" Tags="|artificial-satellite|observation|photography|space-art|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="21877" PostTypeId="1" AcceptedAnswerId="21890" CreationDate="2017-06-09T07:19:26.990" Score="14" ViewCount="2025" Body="&lt;p&gt;&lt;strong&gt;update March 2018:&lt;/strong&gt; I just saw this in Buzzfeed (Google sent me there, I don't normally read it): &lt;a href=&quot;https://www.buzzfeed.com/danvergano/ale-shooting-star-satellite-meteor&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rich People Will Soon Be Able To Buy Fake Meteor Showers On Demand&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/21867/why-does-all-satellites-seems-to-have-the-same-color-from-earth?noredirect=1#comment61269_21867&quot;&gt;@Antzi mentioned&lt;/a&gt; below the question &lt;a href=&quot;https://space.stackexchange.com/q/21867/12102&quot;&gt;Why does all satellites seems to have the same color from earth?&lt;/a&gt; this June 8, 2017 Nikkei business profile &lt;a href=&quot;http://asia.nikkei.com/Business/Companies/On-demand-shooting-stars-Japanese-startup-dreams-big&quot; rel=&quot;nofollow noreferrer&quot;&gt;On-demand shooting stars? Japanese startup dreams big&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article mentions a 50 cm sized nanosatellite at about 500 km altitude that will launch &quot;simulants&quot; or small objects (looks like about 1 cm gum balls) that will then burn up in the atmosphere, likely laced with compounds or elements that will produce brighter emission in the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From a conservation of momentum point of view, how can a reasonably affordable micro or nano satellite in a stable LEO orbit of several hundred kilometers get a small object into the atmosphere at a predictable time. I mean it would be extremely hard to do it in a few minutes, but if it were to be days or years, then the uncertainty in reentry time would be unacceptable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps the idea is to use some serious, expensive hardware, and rely on economies of scale? Millions of shooting-star orders? Or a substantial fraction of the budget for the &quot;opening ceremonies of the 2020 Tokyo Olympics&quot;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this seem possible from the point of view of what is known about objects in stable LEO orbits? Or is it more of a &quot;magic leap&quot; of faith?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is further information from &lt;a href=&quot;http://edition.cnn.com/2016/10/23/asia/on-japan-artificial-meteors/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;CNN&lt;/a&gt;, a nice write-up about the company Ale at &lt;a href=&quot;https://www.f6s.com/ale&quot; rel=&quot;nofollow noreferrer&quot;&gt;f6s.com&lt;/a&gt;, and I found this promotional video that illustrates the idea (little balls &lt;s&gt;pooping&lt;/s&gt; popping out of the rear-end of the satellite) but does not handle momentum conservation that well:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/vHvyz3h-rRo?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-08-26T03:22:59.780" LastActivityDate="2020-08-26T06:41:44.417" Title="Artificial Shooting Stars - how could this possibly work (economically/technologically)?" Tags="|reentry|low-earth-orbit|debris|space-art|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21878" PostTypeId="2" ParentId="21870" CreationDate="2017-06-09T07:50:24.300" Score="2" Body="&lt;p&gt;Rocket launches are extremely violent. Launch pads are already built from &quot;flame-retardant, explosion-proof&quot; materials like steel, concrete and fire brick, and they're often water-cooled during the launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite all that, the pad gets damaged over time. Most of this damage is superficial: paint burns off and has to be replaced to prevent corrosion, small items like light fixtures get blown away, concrete spalls and needs surface repairs, the occasional brick is blown away. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could prevent this, but then you'd need to build the launch pad like a battleship: cover everything in foot-thick steel plates. That would be more expensive than the current solution, and it'd make maintenance more difficult due to reduced access. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A major part of &quot;refurbishing&quot; is just a check: it's a large structure, so this takes some time. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-09T07:50:24.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21879" PostTypeId="2" ParentId="21853" CreationDate="2017-06-09T08:44:24.313" Score="0" Body="&lt;p&gt;I assume you have the probe orbiting the Earth or somehow following the Earth as it orbits the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so the probe is unlikely to be able to send any response it gets from Earthlings, back to its home base, as the physics of Gaussian beams indicates that power spreads out over very large distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The probe would need to transmit Tetrawatts back to its home base for any usable signal to be read by them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the probe is not relatively close to the Earth, no readable signal could be picked-up by us. &lt;/p&gt;&#xA;" OwnerUserId="19993" LastActivityDate="2017-06-09T08:44:24.313" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21880" PostTypeId="2" ParentId="21871" CreationDate="2017-06-09T10:26:42.610" Score="4" Body="&lt;p&gt;There are two ASDS ships. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just Read the Instructions (JRTI) which was first deployed on the Marmac 300 hull, never successfully landed a stage, had its wings removed and moved over to a newer hull (Marmac 303 I think), then shipped through the Panama Canal to the Pacific.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Pacific, JRTI has landed the Jason-3 and Iridium-Next first cores as of Jun 2017. (More Iridium missions in 2017 to come). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Atlantic, Of Course I Still Love You (OCISLY), based on the Marmac 304 hull was deployed and has landed multiple stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The July 17th, 2017 planned launch of BulgariSat on a previously owned (gently used, mission tested, pick your euphemism of choice) Falcon 9 first stage, (was used in the Iridium NEXT 1 launch that landed on JRTI in the Pacific) will try to land on OCISLY in the Atlantic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will make it the first stage to land in both oceans if (when?) successful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moving them ocean to ocean would be a lot of work. The Panama Canal is only wide enough for them if the wings are removed. They tow slowly, so it will take a long time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In all likelihood if they get the launch rate up much higher they will soon need a second ASDS in the Atlantic. If the Mod 5 updates, with the 'fullest thrust' engines allow enough margin for RTLS even on big GEO missions, that prediction could be voided.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have developed an unused (as of this time) Roomba like device to try and automate grabbing and holding a landed stage. Cannot wait to see that in action. But the goal there is to enable quicker turnarounds, and even enable turnarounds in bad ocean weather where it would be unsafe to load a crew to secure the stage. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-06-09T14:11:44.657" LastActivityDate="2017-06-09T14:11:44.657" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21881" PostTypeId="1" AcceptedAnswerId="21882" CreationDate="2017-06-09T11:21:24.773" Score="10" ViewCount="2080" Body="&lt;p&gt;They name launches, usually after the payload, or just launch number, like &quot;Falcon 9 flight 20, also known as Orbcomm OG2 M2&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They named their drone ships some &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;very fancy names&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, are there some kind of identifiers, or other names, that are assigned to individual rockets/boosters? Since they are reusable, the individual launch identifier doesn't apply quite the same as in the case of non-reusable rockets. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2017-06-09T23:55:12.920" LastActivityDate="2017-06-11T15:02:50.323" Title="Does SpaceX name individual boosters?" Tags="|spacex|falcon-9|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21882" PostTypeId="2" ParentId="21881" CreationDate="2017-06-09T11:34:55.607" Score="14" Body="&lt;p&gt;As of the 38th launch, they do not name boosters cores, but they started numbering the cores, and printing that number just around where the legs attach at the base of the booster core. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cores have serial numbers, four digits, so the first reflown booster was 1021 used on CRS-8 then SES-10.  The Bulgariasat booster set to be reflown was first flown as Iridium-1 and is serial number 1029. 1026 is the Thaicomm-8 mission booster and is being used as a Falcon Heavy side core.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first reused Dragon (CRS-11 after flying CRS-4) had an ISS 'kill' logo painted on it. (Much as fighter jets will have logos of various kills marked under the cockpit). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of this writing only one core has reflown, second due in 10 days, and no signs of &quot;kill&quot; markings from previous flights on cores, yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A neat infographic of launches trying to visually capture as much info as possible is nice to add in here as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mkFIC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mkFIC.png&quot; alt=&quot;Infographic of SpaceX Launches&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-06-09T20:54:00.203" LastActivityDate="2017-06-09T20:54:00.203" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21883" PostTypeId="1" CreationDate="2017-06-09T12:32:24.763" Score="1" ViewCount="97" Body="&lt;p&gt;ICBMs are known to be able to carry multiple warheads. The capability to launch against multiple targets in the same time is important when in an all-out war but I can imagine many times that a single strike may be tactically desirable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So are there ICBMs (land or submarine based) loaded with a single warhead? Can an ICBM &quot;unload&quot; warheads relatively quickly according to tactical needs? Or are single nuclear strikes performed exclusively by air assets (bombers, cruise missiles)?&lt;/p&gt;&#xA;" OwnerUserId="15905" LastActivityDate="2017-06-09T12:42:33.593" Title="ICBMs versus number of available warheads" Tags="|icbm|" AnswerCount="1" CommentCount="3" ClosedDate="2017-06-09T13:57:21.983" ContentLicense="CC BY-SA 3.0" />
  <row Id="21884" PostTypeId="2" ParentId="21883" CreationDate="2017-06-09T12:42:33.593" Score="2" Body="&lt;p&gt;Using an ICBM for a single strike is politically unlikely. The receiving state (esp. if it's a nuclear power) will assume incoming ICBM = the beginning of nuclear war and will launch its own missiles in retaliation, before they get annihiliated. &#xA;So for a single strike, other methods are used. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There has been talk of building ICBMs with conventional explosives, but the above scenario keeps that from happening. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-09T12:42:33.593" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21886" PostTypeId="2" ParentId="21839" CreationDate="2017-06-09T14:53:17.120" Score="22" Body="&lt;p&gt;Kubos corporation (&lt;strong&gt;disclosure: I work at Kubos&lt;/strong&gt;) have a cubesat-specific Linux distro and a real-time operating system (RTOS), for cubesats with tighter power and processing budgets, since, as noted by Rick 0xfff, above, smaller cubesats don't get much of a power budget. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kubos have, as suggested in his answer, taken Buildroot and created a lightweight Linux for the Innovative Solutions in Space OBC and the Pumpkin (BeagleBone Black-based) OBC. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RTOS runs on several inexpensive dev boards (MSP430F5529, STM32F407 Discovery, MicroPython), and the ClydeSpace and NanoAvionics OBCs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: our GitHub repository for the software contains &lt;a href=&quot;https://github.com/kubos&quot; rel=&quot;noreferrer&quot;&gt;our open source code for the operating systems.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19997" LastEditorDisplayName="user36497" LastEditDate="2020-06-05T22:41:56.687" LastActivityDate="2020-06-05T22:41:56.687" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="21887" PostTypeId="1" AcceptedAnswerId="21896" CreationDate="2017-06-09T15:22:56.320" Score="7" ViewCount="1215" Body="&lt;p&gt;When performing orbital calculations, under what circumstances should I assume:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a perfectly spherical Earth&lt;/li&gt;&#xA;&lt;li&gt;a &lt;a href=&quot;https://en.wikipedia.org/wiki/Reference_ellipsoid&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference ellipsoid&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geoid&quot; rel=&quot;nofollow noreferrer&quot;&gt;geoid&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;no particular format at all&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="20002" LastEditorUserId="12102" LastEditDate="2017-06-10T05:49:14.047" LastActivityDate="2017-06-10T17:24:15.840" Title="What Earth gravity model is used for different types of calculations?" Tags="|orbital-mechanics|earth|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21888" PostTypeId="2" ParentId="21493" CreationDate="2017-06-09T15:24:58.710" Score="30" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.quora.com/ISS-How-does-an-iPad-pick-its-orientation-on-ISS-in-microgravity/answer/Robert-Frost-1&quot; rel=&quot;noreferrer&quot;&gt;Robert Frost, Flight Controller at NASA&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The onboard iPads are configured with the mute switch acting as the&#xA;  rotation lock. Should a crew member want to change the screen&#xA;  orientation, they toggle the mute switch and then jerk the iPad to&#xA;  impart a force that will change the orientation. They then reactivate&#xA;  the rotation lock.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So not only is it using the lock like the other answers have said, but they can change the orientation by turning off the lock and giving it a quick jerk to simulate gravity.&lt;/p&gt;&#xA;" OwnerUserId="19986" LastActivityDate="2017-06-09T15:24:58.710" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21889" PostTypeId="2" ParentId="21887" CreationDate="2017-06-09T16:00:49.137" Score="4" Body="&lt;p&gt;Called2Voyage is right. Depends on application - and distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For influence on probes far at other planets, point mass is sufficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For spaceflight around the Moon or vicinity of Lagrange points, sphere is okay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'll want a geoid model for LEO satellites in any orbits other than equatorial. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'll need an even more accurate model for sun-synchronous satellites, taking local gravitational anomalies into account.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-06-09T16:00:49.137" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21890" PostTypeId="2" ParentId="21877" CreationDate="2017-06-09T17:46:24.940" Score="16" Body="&lt;p&gt;Now that I have my comment out of the way, I can't envision sending something up to a relatively high LEO would be either effective or efficient.  It would make far more sense to do something on a suborbital trajectory -- this saves you a huge amount of delta-V and provides much finer control over the timing and dispersion of your reentering particles.  It still would present hazards to assets on orbit to the point of coming close to violating international agreements, but it would be far more practical than the concept outlined in the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's probably still a whole lot more bang for one's buck to just use plain old fireworks though.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-06-09T17:46:24.940" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21891" PostTypeId="1" AcceptedAnswerId="21894" CreationDate="2017-06-09T18:15:56.840" Score="32" ViewCount="10886" Body="&lt;p&gt;&lt;strong&gt;update March 2018:&lt;/strong&gt; I just saw this in Buzzfeed (Google sent me there, I don't normally read it): &lt;a href=&quot;https://www.buzzfeed.com/danvergano/ale-shooting-star-satellite-meteor&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rich People Will Soon Be Able To Buy Fake Meteor Showers On Demand&lt;/a&gt;. It seems this might happen in 2018.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;I was watching this &lt;a href=&quot;https://www.youtube.com/watch?v=mpViVEO-ymc&quot; rel=&quot;nofollow noreferrer&quot;&gt;really cool video&lt;/a&gt; about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transiting_Exoplanet_Survey_Satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transiting Exoplanet Survey Satellite&lt;/a&gt; (TESS) spacecraft, and I saw what looks like this &lt;a href=&quot;https://space.stackexchange.com/a/21431/12102&quot;&gt;Marmon ring&lt;/a&gt; fly off and instinctively thought &amp;quot;geez, that could put somebody's eye out, how reckless!&amp;quot; which isn't the right way to think about space debris.&lt;/p&gt;&#xA;&lt;p&gt;Then I asked &lt;a href=&quot;https://space.stackexchange.com/a/21890/12102&quot;&gt;this question&lt;/a&gt; and got quite a rise out of some people. Apparently releasing a lot of ~1cm objects into LEO set off some some &amp;quot;internal alarms&amp;quot; ;)&lt;/p&gt;&#xA;&lt;p&gt;So can someone describe what it is specifically that makes those actually more scary than the standard objects released (=discarded) these days into LEO?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oellg.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oellg.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Apparently this &amp;quot;box full of ball-bearings in LEO&amp;quot; is scarier!&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pzGld.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pzGld.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Here is one of the original videos, the other has since &amp;quot;gone dark&amp;quot; (private):&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/mpViVEO-ymc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2022-09-04T23:51:46.240" LastActivityDate="2022-09-04T23:51:46.240" Title="Why would a box full of 1cm balls released into LEO be so scary to an engineer supporting the ISS?" Tags="|iss|low-earth-orbit|debris|safety|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="21893" PostTypeId="2" ParentId="21891" CreationDate="2017-06-09T18:41:10.747" Score="32" Body="&lt;p&gt;There's a few pieces of information that are needed to explain why one might be wary of 1 cm objects:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Objects as small as 4 inches (about 10 cm) can be seen by radars or optical telescopes on Earth&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Oops, can't see 'em.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;There are also millions of pieces of debris smaller than a third of an inch (1 cm). In Low Earth-orbit, objects travel at 4 miles (7 kilometers) per second. At that speed, a tiny fleck of paint packs the same punch  of a 550 pound object traveling at 60 miles per hour. Not only can such an impact damage critical components such as pressurized items, solar cells, or tethers, they can also create new pieces of potentially threatening debris.&lt;/p&gt;&#xA;&lt;p&gt;Even a salt-sized grain hitting the space shuttle creates orbital debris.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Ouch, it multiplies.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/o6p27.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/o6p27.jpg&quot; alt=&quot;chip in ISS window&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;sup&gt;Image Credit: ESA/NASA&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The chip you see in the image above is 7mm in diameter and orbiting above us embedded in one of the International Space Station’s windows. It’s tiny, and definitely a chip not a crack, but in the vacuum of space you have to take even the smallest of impacts deadly seriously.&lt;/p&gt;&#xA;&lt;p&gt;Luckily, the windows used on the space station are made of some pretty sturdy stuff – borosilicate-glass, fused-silica, and are quadruple-glazed for extra protection.The chip was caused by space debris &lt;strong&gt;only a few thousandths of a millimeter across&lt;/strong&gt;. It could have just been a flake of paint or tiny piece of metal.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;sup&gt;emphasis added&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;Yikes, a little can do a lot of damage!&lt;/p&gt;&#xA;&lt;p&gt;What else do you need to know?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.space.com/16518-space-junk.html&quot; rel=&quot;noreferrer&quot;&gt;Space Junk: Tracking and Removing Orbital Debris - Space.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.geek.com/tech/space-debris-hitting-iss-window-creates-7mm-chip-1655003/&quot; rel=&quot;noreferrer&quot;&gt;Space debris hitting ISS window creates 7mm chip - Geek.com&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2020-03-26T17:34:55.893" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="21894" PostTypeId="2" ParentId="21891" CreationDate="2017-06-09T19:04:23.003" Score="77" Body="&lt;p&gt;Well, gee, this question may as well have my name printed directly on it!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft protection from the orbital debris threat comes in two flavors:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Shield and withstand&lt;/li&gt;&#xA;&lt;li&gt;Detect and avoid&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To start, page 5 of &lt;a href=&quot;http://hdl.handle.net/2060/20140009932&quot; rel=&quot;noreferrer&quot;&gt;this NASA paper&lt;/a&gt; presents a good first-order approximation of the general LEO orbital debris threat.  You'll see some variation with altitude and inclination, but for the most part, the shape of the graph is generally the same.  The line represents the inverse cumulative flux -- that is, the number of particles per square meter per year of a given size or larger from all directions combined.  It's important to note that this flux is on a log scale.  The text near the thick arrows denotes the data source that informs the model for indicated size ranges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the extreme left end of the graph, we have very high numbers of very small particles.  Most of these are operationally inconsequential unless you have particularly sensitive surfaces like telescope or camera optics.  Most surfaces require no or minimal shielding to withstand strikes in these size ranges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As your particle size gets larger, you begin to require dedicated shielding to protect your hardware.  Those shields will typically protect you up to some maximum size for typical impacts.  Larger protection capabilities mean more mass, and it does not scale linearly, so a cutoff has to be made for a given spacecraft to be light enough to fly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Large enough objects are able to be detected and avoided.  Detection and avoidance takes time to plan and execute -- several orbits.  It is nothing like what you would see in a movie.  Of particular importance is the fact that all of the detection and avoidance capability is due to the use of ground-based tracking assets.  In the US, this typically comes in the form of conjunction notices from JSpOC/USSTRATCOM (see &lt;a href=&quot;https://www.space-track.org/documentation#/odr&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; for more info).  This information comes primarily from the Space Surveillance Network, and the typical &quot;tracked object&quot; sizes are denoted by &quot;SSN&quot; in the graph highlighted above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below a certain critical size, objects in space cannot be reliably tracked.  Their presence is observable using radar and telescopes in &quot;stare&quot; mode (&quot;Goldstone&quot;, &quot;HAX&quot;, and &quot;Haystack&quot;), but that just gets you a population count, not tracking.  These objects in general cannot be detected and avoided, and they are in size ranges for which effective shielding is impractically massive for all but the most critical hardware surfaces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As with pretty much any spacecraft application, it is impossible to drive loss-of-mission or loss-of-vehicle risk to zero.  (Indeed, the safest spacecraft is the one that doesn't fly!)  Carefully considered decisions must be made about what levels of risk to accept.  These decisions are predicated on the underlying data that feeds the risk calculations being valid going forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An unexpected release of a large number of objects firmly in the middle of the &quot;too small to detect and too big to practically shield against&quot; region at an altitude for which they could rain down through the orbits of critical assets for months or years to come would be a massive upset in the risk-based decision process.  For hardware already on orbit, this would greatly increase the risk of a catastrophic loss beyond acceptable levels.  At high enough levels, it could render LEO too dangerous to inhabit for years or decades.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-06-09T19:04:23.003" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="21895" PostTypeId="1" CreationDate="2017-06-10T01:34:11.477" Score="4" ViewCount="1177" Body="&lt;p&gt;Jupiter's small moons are likely captured objects. I would've expected half of them to be captured in retrograde orbit, between 90 and 180 degrees inclination. But out of 65 lesser satellites, only 11 are not retrograde. (&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/planetary/factsheet/joviansatfact.html&quot; rel=&quot;noreferrer&quot;&gt;Table&lt;/a&gt;) There's a smaller but similar bias for Saturn's moons, but not for those of Uranus or Neptune.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an orbital mechanical explanation for why objects most often are captured in retrograde orbits? Is there some process that gets rid of small moons that are not retrograde?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Rfn9r.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Rfn9r.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Inclination and distance of &quot;irregular&quot; Jovian moons, Wikipedia&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-06-10T01:47:27.093" LastActivityDate="2017-06-10T02:24:41.543" Title="Why are most of Jupiter's moons retrograde?" Tags="|orbital-mechanics|jupiter|moons|capture|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21896" PostTypeId="2" ParentId="21887" CreationDate="2017-06-10T02:05:50.727" Score="10" Body="&lt;p&gt;Since this answer is a bit longish, a TL;DR is in order. Depending on the application, one should use&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A point mass / spherical mass distribution model.&lt;/li&gt;&#xA;&lt;li&gt;A model that incorporates the effects of the Earth's oblateness. This is subtly different from using the reference ellipsoid.&lt;/li&gt;&#xA;&lt;li&gt;A model that incorporates the effects of Earth's not-quite ellipsoidal shape, based on very detailed analyses of the orbits of existing satellites. For the Earth, there is but one approach, which is to use a spherical harmonics model of the Earth's gravitational potential field.&lt;/li&gt;&#xA;&lt;li&gt;A model that accounts for temporal variations in those spherical harmonics models. For the Earth, the largest temporal variations (by far) are short term variations due to the tides. Longer term variations are observable by specially designed satellites.&lt;/li&gt;&#xA;&lt;li&gt;A very different kind of model for small bodies such as asteroids or comets that look more like lumpy potatoes than distorted beach balls.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When performing orbital calculations, under what circumstances should I assume:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;a perfectly spherical Earth&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A point mass model works quite nice when you are doing rather simplistic mission planning, analyzing an object that is very far from the Earth, or doing undergraduate homework. Beyond that, you'll need to account for the fact that the Earth is not quite spherical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;a reference ellipsoid&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Calculating gravitation from an ellipsoid is nontrivial. There is missing information (e.g., a density model), and even with that information, the calculation will lead elliptical integrals. However, the largest deviation from a point mass / spherical mass distribution model is captured quite nicely by the Earth's second dynamic form factor $J_2$. With this factor, the gravitational potential as a function of orbital radius $r$ and geocentric latitude $\phi$ is&#xA;$$&#xA;U(r,\theta)&#xA;  = -\frac{\mu_E} r&#xA;    \left(&#xA;      1-J_2\left(\frac a r\right)^2 \left(\frac{3\sin^2\phi -1}2\right)&#xA;    \right)&#xA;$$&#xA;where&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$a$ is the Earth's equatorial radius,&lt;/li&gt;&#xA;&lt;li&gt;$J2$ is the Earth's second dynamic form factor, which is caused by the Earth's equatorial bulge, and&lt;/li&gt;&#xA;&lt;li&gt;$\mu_E$ is the Earth's &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational parameter&lt;/a&gt;&lt;sup&gt;1&lt;/sup&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Take the gradient of the potential, negate, and voila! you'll have the gravitational acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's another issue with using the reference ellipsoid: The value of $J_2$ would be exactly described by the Earth's oblateness and its rotation rate if the Earth was in hydrostatic equilibrium. The observed value of $J_2$ and the calculated value based on the reference ellipsoid and the Earth's rotation rate differ slightly. The reason is that the Earth is not quite in hydrostatic equilibrium. It is instead still recovering from the huge masses of ice that covered large tracts of the Northern Hemisphere up until about 12000 years ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;a geoid&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Just as you don't want to use the reference ellipsoid, you don't to use the geoid, either. Calculating gravitation from a geoid model is ridiculously difficult. What you want instead is a spherical harmonics model. Geoid models are calculated from spherical harmonics models. The spherical harmonics model is what you want. The $J_2$ term discussed above is the leading term in the non-spherical part of the Earth's spherical harmonics model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a function of distance to the center of the Earth $r$, geocentric latitude $\phi$, and geocentric longitude $\lambda$&lt;sup&gt;2&lt;/sup&gt;, the spherical harmonics expansion of the Earth's gravitational potential is&#xA;$$U = -\frac{\mu_E}r \left(1 + \sum_{n=2}^N\sum_{m=0}^n \left(\frac a r\right)^n\,\overline{P_{n,m}}(\cos\phi)\left(\overline{C_{n,m}}\cos(m\lambda) + \overline{S_{n,m}}\sin(m\lambda)\right)\right)$$&#xA;where&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\overline{P_{n,m}}(\cos\phi)$ are the fully normalized associated Legendre functions of the first kind, and&lt;/li&gt;&#xA;&lt;li&gt;$\overline{C_{n,m}}$ and $\overline{S_{n,m}}$ are the fully normalized spherical harmonics coefficients for the model.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Take the gradient of the potential, negate, and voila! you'll have the gravitational acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;The fully normalized associated Legendre function and the fully normalized coefficients are used for two reasons. One is that the unnormalized coefficients (which is what physicists tend to use) tend result in all kinds of numerical problems. The other is that the fully normalized coefficient are the &lt;em&gt;de facto&lt;/em&gt; standard. These coefficients are available for the Earth and for many bodies besides the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find detailed descriptions in Vallado and many other texts. A free somewhat dated online paper that describes spherical harmonics as used in gravitation is &lt;a href=&quot;http://www.jhuapl.edu/techdigest/views/pdfs/V15_N4_1994/V15_N4_1994_Vetter.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Evolution of Earth Gravity Models Used in Astrodynamics&lt;/a&gt;. A very recent open paper that describes how one such global gravity model was constructed is &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S1674984717300046&quot; rel=&quot;nofollow noreferrer&quot;&gt;A GOCE only gravity model GOSG01S and the validation of GOCE related satellite gravity models&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find multiple implementations online, in a number of languages, that use spherical harmonics to model gravitation. Regarding the coefficients themselves, the &lt;a href=&quot;http://icgem.gfz-potsdam.de/home&quot; rel=&quot;nofollow noreferrer&quot;&gt;International Center for Global Gravity Field Models&lt;/a&gt; maintains a catalog of static global gravity models at &lt;a href=&quot;http://icgem.gfz-potsdam.de/tom_longtime&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://icgem.gfz-potsdam.de/tom_longtime&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The models listed on the page cited above are static. You'll need to account for temporal variations if you want to be extremely accurate. The largest temporal variations result from how the gravitational forces by the Moon and Sun distort the shape of the Earth. These solid Earth tides subtly affect satellite orbits out to GEO and beyond.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also seasonal effects such as the buildup and melting of snow in Siberia and the buildup and drying of soil moisture in tropical rain forests. These are observable by specially designed satellites. Even longer term, the ice sheets over Greenland and Antarctica are melting, and lands in the far north are still rebounding from the glaciation that ended 12000 years ago. How to model these temporal effects is beyond the scope of this answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;no particular format at all&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's always going to be some format / model. A spherical harmonics model does not work well for a small object whose shape is more like a lumpy potato than distorted ball. (Note well: This does not apply to the Earth.) This is getting into PhD land, quite literally. For example, here's a &lt;a href=&quot;https://ccar.colorado.edu/scheeres/scheeres/assets/Theses%20and%20Abstracts/takahashi_dissertation.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;PhD thesis on how to compute the gravitation in the vicinity of a potato-shaped object&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;h2&gt;Footnotes&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; Regarding the gravitational parameter: Conceptually, this is the product of the gravitational constant $G$ and the Earth's mass $M_E$: $\mu_E = GM_E$. In practice, the Earth's mass is calculated from the gravitational parameter: $M_E = \mu_E/G$. The problem is that $G$ is only known to four or five places of accuracy while $\mu_E$ is known to about nine places of accuracy. This means that almost all of the uncertainty in the estimates of the Earth's mass is due to the uncertainty in the gravitational constant. Never use $GM$ if you know the gravitational parameter to more than five places of accuracy.For the Earth, we know $\mu_E$ to about nine places of accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt; That you need to know longitude means you need a model of the Earth's rotational state. These range from the very simple (the Earth rotates at a constant rate) to the ridiculously complex (thousands and thousands of terms). The ridiculously complex models target the milliarcsecond level of accuracy needed by radio astronomers. A very simple, constant rate model might be good for a few orbits, but you'll still need a good initial rotational state. The &lt;a href=&quot;http://www.iausofa.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;Standards of Fundamental Astronomy&lt;/a&gt; provides a library functions (both in Fortran and C) that calculate the Earth's orientation for you. So does &lt;a href=&quot;https://naif.jpl.nasa.gov/naif/toolkit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL's SPICE Toolkit&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-06-10T17:24:15.840" LastActivityDate="2017-06-10T17:24:15.840" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21897" PostTypeId="2" ParentId="21895" CreationDate="2017-06-10T02:24:41.543" Score="8" Body="&lt;p&gt;According to the Wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Irregular_moon&quot; rel=&quot;noreferrer&quot;&gt;irregular moons&lt;/a&gt;, retrograde orbits further out from a planet are &lt;a href=&quot;https://en.wikipedia.org/wiki/Irregular_moon#Long-term_stability&quot; rel=&quot;noreferrer&quot;&gt;more stable than prograde&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Retrograde satellites can be found further from the planet than prograde ones. Detailed numerical integrations have shown this asymmetry. The limits are a complicated function of the inclination and eccentricity, but in general, prograde orbits with semi-major axes up to 0.47 rH (Hill sphere radius) can be stable, whereas for retrograde orbits stability can extend out to 0.67 rH.&lt;/p&gt;&#xA;&lt;p&gt;The boundary for the semimajor axis is surprisingly sharp for the prograde satellites. A satellite on a prograde, circular orbit (inclination=0°) placed at 0.5 rH would leave Jupiter in as little as forty years. The effect can be explained by so-called evection resonance. The apocenter of the satellite, where the planet’s grip on the moon is at its weakest, gets locked in resonance with the position of the Sun. The effects of the perturbation accumulate at each passage pushing the satellite even further outwards.&lt;/p&gt;&#xA;&lt;p&gt;The asymmetry between the prograde and retrograde satellites can be explained very intuitively by the Coriolis acceleration in the frame rotating with the planet. For the prograde satellites the acceleration points outward and for the retrograde it points inward, stabilising the satellite.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-10T02:24:41.543" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21898" PostTypeId="1" AcceptedAnswerId="21945" CreationDate="2017-06-10T02:59:44.177" Score="2" ViewCount="383" Body="&lt;p&gt;This short BBC News item shows a very large high altitude balloon lifting a science payload to measure high energy cosmic rays for about 100 days, circling the Earth perhaps twice in the process. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately I could not find any reference to the name of the mission or the payload. I'm wondering if there is a website or a map with it's current location as it circles the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm also looking for a short description of the cosmic ray detector itself. The payload looks really small and light, what kind of information can it record? Without a long or deep array, can it measure the angular distribution of the flux?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If each cell in the photovoltaic array is from a 6-inch wafer, I can use that as a scale to estimate the size of the two white &quot;shoe boxes&quot; sticking out on either side of the top of the payload as about 50x50x15 cm. Is this a two-element lateral telescope, looking for coincident detections through both? Can it point itself, or is rotation around the vertical axis random?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.bbc.com/news/av/39720835/up-up-and-away-nasa-s-super-soaring-space-balloon&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;Up, up and away! Nasa's super soaring space balloon&lt;/strong&gt;&lt;/a&gt;&#xA;&quot;This mega balloon - the size of a football stadium - was launched in New Zealand and will float around Earth for 100 days&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternate &lt;a href=&quot;http://www.bbc.co.uk/newsround/39720835&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Click&lt;/strong&gt; (or right-click) images for &lt;strong&gt;large size:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ndm2g.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ndm2gm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lLlV7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lLlV7m.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vZgdh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vZgdhm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-14T00:37:27.860" LastActivityDate="2017-06-14T00:37:27.860" Title="What cosmic ray sensor is attached to this Balloon?" Tags="|nasa|cosmic-radiation|balloons|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21899" PostTypeId="1" CreationDate="2017-06-10T05:26:17.397" Score="1" ViewCount="123" Body="&lt;p&gt;This might be a stupid newbie question, but I really wonder reading that folks use cheap &quot;commodity&quot; hardware for satellites like mentioned here, like boards MSP430F5529 or STM32F407 but for sure there are many others as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking on the vendor sites [2,3] I cannot find anything featuring robustness of specifically these two boards with regard to radiation, high energy particles, high temperature, vibration, pressure.. So do these factor impose far less risks (there would be figures?..) than guys like Boeing, Lockheed Martin &amp;amp; Co. would probably advocate for their less cheap but well-tested &quot;space hardware&quot;, or are many early stage small-scale experimentations just not thought to be sustainable at early stages because the business model/strategy take this into account anyway?.. Or I just miss the point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking forward to learn the objective truth in this context.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1] &lt;a href=&quot;https://space.stackexchange.com/questions/21839/does-anybody-work-on-a-spacecraft-linux&quot;&gt;Does anybody work on a &amp;quot;spacecraft linux&amp;quot;?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[2] &lt;a href=&quot;http://www.ti.com/product/MSP430F5529/datasheet&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.ti.com/product/MSP430F5529/datasheet&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[3] &lt;a href=&quot;http://www.st.com/en/microcontrollers/stm32f407-417.html?querycriteria=productId=LN11&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.st.com/en/microcontrollers/stm32f407-417.html?querycriteria=productId=LN11&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19946" LastEditorUserId="19946" LastEditDate="2017-06-10T06:44:09.050" LastActivityDate="2022-04-08T18:47:42.527" Title="What are the risks of using cheap industrial components for commercial space appliances esp. solution platforms for them?" Tags="|spacecraft|probe|materials|risk-management|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21900" PostTypeId="1" CreationDate="2017-06-10T09:14:53.980" Score="7" ViewCount="1543" Body="&lt;p&gt;If Jupiter is 365 million miles away from earth. Our moon is about 238,900 miles away from earth. that means Jupiter is 1527.8 times farther away from the earth than the moon. Jupiter is 5285.2 times bigger than our moon. now thinking, if the moon was twice as far away as it is now, shouldn't it look twice as small? now thinking of it that way. shouldn't Jupiter look 3.5 times the size of our moon? with Jupiter being 5285.2 times bigger than our moon but 1527.8 times farther away from the earth than the moon. math wise Jupiter should be 3.5 times the size of our moon. so way isn't it? am i over looking something?&lt;/p&gt;&#xA;" OwnerUserId="20012" LastActivityDate="2017-06-10T09:14:53.980" Title="Why doesn't jupiter look bigger than our moon?" Tags="|the-moon|jupiter|earth|" AnswerCount="0" CommentCount="5" ClosedDate="2017-06-10T17:16:31.363" ContentLicense="CC BY-SA 3.0" />
  <row Id="21901" PostTypeId="1" AcceptedAnswerId="21902" CreationDate="2017-06-10T13:34:43.033" Score="1" ViewCount="335" Body="&lt;p&gt;If you switch on a lightbulb in a room its light has a visible range. So how about Sunlight? Does it light more on Mercury and less on Pluto? And what about planets that are not in the Solar System?&lt;/p&gt;&#xA;" OwnerUserId="20014" LastActivityDate="2017-06-10T14:36:59.720" Title="Does Sunlight have a range?" Tags="|planet|solar-system|the-sun|" AnswerCount="1" CommentCount="1" ClosedDate="2017-06-10T20:01:01.590" ContentLicense="CC BY-SA 3.0" />
  <row Id="21902" PostTypeId="2" ParentId="21901" CreationDate="2017-06-10T14:31:50.317" Score="7" Body="&lt;p&gt;Light is electromagnetic energy conveyed by photons. Photons travel in straight lines at approximately 300,000,000 m/s. They do not lose any energy as they travel, so can travel indefinitely, only giving up their energy upon interacting with something. If such an interaction occurs, the photon is absorbed, and a new photon may be emitted by that something, depending on the circumstances of the interaction. Key point here is: barring something to get in the way, light travels indefinitely and without loss until it hits something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A point source (more of an abstraction than a reality) radiates a given amount of power (energy per unit time) equally in all directions. At some distance from the point source (we can think of a spherical shell around it), that power will be evenly distributed over the area of that shell - so we have power per unit area. If you double the distance (the radius of the shell), the area goes up 4 times (math/geometry) so the power per unit area is only one quarter (referred to as the &quot;inverse square&quot; law). Note that we are not talking about a loss of energy with distance, only a &quot;thinning out&quot; because it is distributed over an increasing surface. Note also, that the power per unit area never actually gets to zero, but vanishingly approaches it with extreme distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sun is a huge ball of white hot gas. If you are really close to it, it doesn't look like a point source; it looks like a big wall of light. But, it does have a finite size - about a million miles across. If you get far enough away, say a hundred million miles (about how far away we are from it), it starts to look like a point source, so the further away you get, the more thinly the sunlight is spread, so the dimmer it seems. Because the light can potentially travel an infinite distance, there is no &quot;range&quot;. There will come a point where it is no longer of significance, but that point is subjective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, because of the inverse square law, there is more light per unit area falling on Mercury than on Pluto, and much less on objects outside our solar system. If you are far enough away, it will be too little to measure with any practical instrument, but it will never be zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The light bulb in your room actually has no &quot;range&quot;; its light can continue forever too. We only perceive a limited range because of the limits of our perception relative to the amount of light being given off.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-06-10T14:36:59.720" LastActivityDate="2017-06-10T14:36:59.720" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="21903" PostTypeId="1" AcceptedAnswerId="21904" CreationDate="2017-06-10T17:37:24.310" Score="2" ViewCount="377" Body="&lt;p&gt;Reading this &lt;a href=&quot;https://space.stackexchange.com/a/21896/12102&quot;&gt;tutorial answer&lt;/a&gt; on models of Earth's gravity field I was realized that I'd never thought of it as time dependent. According to it, the biggest short term changes in the shape of Earth's gravity field in LEO space is the movement of the tides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It got me wondering, if that question were relaxed and it asked about the total gravity field of the Earth-Moon system, what gravitational force would tend to perturb LEO satellites more, the modulation of Earth's gravity field due to the movement of the tides, or the gravity field of the Moon (which is also causing those tides)?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-06-10T20:10:01.447" Title="Which perturbs LEO satellites more, the Moon's gravity directly, or the changes in Earth's gravity field due to the Moon?" Tags="|the-moon|gravity|gravity-fields|tides|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21904" PostTypeId="2" ParentId="21903" CreationDate="2017-06-10T20:04:08.647" Score="5" Body="&lt;p&gt;The Moon's gravitational field itself, by far. The perturbations on a satellite due to the reshaping of the Earth due to the Moon and Sun's gravity fields are small effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary impact of this secondary tidal effects is a slight change in $J_2$ ($C_{2,0}$). A description of how the Earth tides affect the gravitational field is in section 6.2 of &lt;a href=&quot;http://iers-conventions.obspm.fr/2010/2010_official/tn36.pdf&quot; rel=&quot;noreferrer&quot;&gt;IERS Technical Note 36&lt;/a&gt;. The next section (6.3) covers the impacts of ocean tides. These are smaller yet.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-06-10T20:10:01.447" LastActivityDate="2017-06-10T20:10:01.447" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21905" PostTypeId="1" AcceptedAnswerId="21933" CreationDate="2017-06-11T00:41:52.373" Score="5" ViewCount="489" Body="&lt;p&gt;In a recent science fiction movie there was a space station similar to the ISS, and it had a &quot;proximity alert&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This made me wonder; &lt;em&gt;in the real world&lt;/em&gt;, does the real ISS have any capability to automatically detect or sense the proximity of unexpected or un-anticipated objects which reach close proximity with a low relative velocity to the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any possibility of something like an automatically generated &quot;proximity alert&quot;?&lt;/p&gt;&#xA;" OwnerUserId="20019" LastEditorUserId="12102" LastEditDate="2017-06-11T02:04:24.217" LastActivityDate="2019-07-02T01:45:40.583" Title="Does the ISS have any ability to detect or sense unexpected/unscheduled objects in close proximity?" Tags="|iss|safety|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21908" PostTypeId="1" AcceptedAnswerId="57317" CreationDate="2017-06-11T05:41:30.157" Score="14" ViewCount="1119" Body="&lt;p&gt;The EurekAlert! &lt;a href=&quot;https://www.eurekalert.org/pub_releases/2017-06/uosf-nmt060917.php&quot; rel=&quot;noreferrer&quot;&gt;NASA mission tests ketogenic diet undersea, &lt;strong&gt;simulating life on Mars&lt;/strong&gt;&lt;/a&gt; &lt;em&gt;University of South Florida researcher will be in nutritional ketosis during NEEMO 22 mission&lt;/em&gt; packs a lot of benefits of ketosis in a small space:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Dr. D'Agostino was selected for his research conducted at the USF Hyperbaric Biomedical Research Laboratory (HBRL) on how extreme environments impact the human body. One of the countermeasures developed is a method to induce and sustain nutritional ketosis with ketone supplement formulations. Nutritional ketosis shifts the body's metabolic state to burn fat rather than glucose as its primary fuel.&lt;/p&gt;&#xA;&lt;p&gt;The USF-patented method will play a pivotal role in advancing the objectives of the NEEMO 22 mission. Dr. D'Agostino &lt;strong&gt;will be in a constant state of nutritional ketosis, which is proven to preserve the genome, protecting DNA. This is beneficial to NASA as it can countermeasure neurological risks that come with space travel such as space radiation, lack of oxygen and stress of small spaces&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So while it may be possible for one to achieve ketosis with a modified diet, in the NEEMO 22 experiment the subject will receive a ketone supplement.&lt;/p&gt;&#xA;&lt;p&gt;Might a low-carb diet on Mars make us more resistant to radiation, better able withstand lack of oxygen, and be more comfortable in small spaces? Will there be ketone-laced chocolates to enhance compliance?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; The short EurekAlert! may have just mixed a few different points together in the two highlighted sentences. Has NASA or any other space agency looked into ketosis as a potentially beneficial state in some situations? Is it possible a low-carb diet might be recommended for extended spaceflight missions?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; Recent related news:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Inverse.com: &lt;a href=&quot;https://www.inverse.com/article/57424-what-happens-in-ketosis&quot; rel=&quot;noreferrer&quot;&gt;What Happens in Ketosis? 5 Scientists Reveal the Risks and Benefits&#xA;Miracle diet or dangerous eating plan? Maybe a ketogenic diet is somewhere in between.&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Yahoo News: &lt;a href=&quot;https://www.yahoo.com/lifestyle/zero-carb-diet-may-simple-184029010.html&quot; rel=&quot;noreferrer&quot;&gt;The Zero Carb Diet May Be a Simple Way to Lose Weight—But Is It Safe?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;I don't really understand what ketosis is but I can copy/paste from &lt;a href=&quot;https://en.wikipedia.org/wiki/Ketosis&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Ketosis is a metabolic state in which some of the body's energy supply comes from ketone bodies in the blood, in contrast to a state of glycolysis in which blood glucose provides most of the energy.&lt;/p&gt;&#xA;&lt;p&gt;[...]Longer-term ketosis may result from fasting or staying on a low-carbohydrate diet (ketogenic diet), and deliberately induced ketosis serves as a medical intervention for various conditions...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;A bit more information about human metabolism and ketosis can be found in &lt;a href=&quot;https://biology.stackexchange.com/a/31239/27918&quot;&gt;this excellent answer&lt;/a&gt;. Here is a reposted graphic from there:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6qnex.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6qnex.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-03T02:24:07.400" LastActivityDate="2022-01-18T17:25:10.570" Title="Will we all be in voluntary ketosis on Mars?" Tags="|mars|radiation|health|medical|nutrition|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="21909" PostTypeId="2" ParentId="3505" CreationDate="2017-06-11T06:27:47.310" Score="2" Body="&lt;p&gt;&quot;AIS 2017 A2 Dr V Narayanan LPSC 14 Feb 17&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/7Jy-fXBTllE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dr. Narayanan has confirmed the built in restart capability for both CUS-7 and C20. (question and answer session from 25:55 onwards ). He also said that the  capability will be tested after the MK-3 flight.&lt;/p&gt;&#xA;" OwnerUserId="1507" LastActivityDate="2017-06-11T06:27:47.310" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21910" PostTypeId="2" ParentId="15268" CreationDate="2017-06-11T10:01:26.870" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I take this to mean that when a halo orbit around an &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$L_2$&lt;/span&gt; point (let's say Sun-Earth) is sufficiently large, it is (at least within the CR3BP model) actually stable to small perturbations, i.e no station keeping is required for sufficiently large halo orbits about &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$L_2$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, this is true but the devil is in what &quot;large&quot; means. The stable orbits are by any means not orbit about the &lt;span class=&quot;math-container&quot;&gt;$L_1$&lt;/span&gt; or the &lt;span class=&quot;math-container&quot;&gt;$L_2$&lt;/span&gt; points. They wander far off the ecliptic plane, and a body on those stable orbits spend little time on the axis Sun-planet. I happen to have given &lt;a href=&quot;https://physics.stackexchange.com/a/338685/154997&quot;&gt;a detail answer to this very issue&lt;/a&gt; in another question a couple of hours ago.&lt;/p&gt;&#xA;" OwnerDisplayName="user20022" LastEditorUserId="12102" LastEditDate="2020-05-01T00:10:40.300" LastActivityDate="2020-05-01T00:10:40.300" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="21911" PostTypeId="1" CreationDate="2017-06-11T11:27:27.050" Score="2" ViewCount="132" Body="&lt;p&gt;When I was looking for previous calculations for JUNO I &lt;a href=&quot;https://space.stackexchange.com/a/19263/12102&quot;&gt;received help&lt;/a&gt; which pointed me &lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/JUNO/kernels/spk/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I'd like to review ISEE-3's orbit and compare it to calculations since receiving &lt;a href=&quot;https://space.stackexchange.com/a/21910/12102&quot;&gt;this welcomed answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I go to Horizons, I can only get circa 2014-2016, the '&lt;a href=&quot;https://www.facebook.com/ISEE3Reboot&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISEE3 Reboot&lt;/a&gt;'. I don't see ICE or ISEE-3 in the NAIF folders.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there some way I can get the 1978-82 timeframe Horizons orbit data (state vectors), either through Horizons, or at least the kernels, as I did &lt;a href=&quot;https://space.stackexchange.com/a/19263/12102&quot;&gt;in this case&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or, is there some standard way to request the data be reposted?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-10-21T10:36:39.967" LastActivityDate="2018-10-21T13:33:20.183" Title="Help with ISEE-3's historic L1 halo orbit's Spice kernels (1978-82); still available somewhere online?" Tags="|jpl-horizons|halo-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21912" PostTypeId="1" AcceptedAnswerId="21913" CreationDate="2017-06-11T13:12:22.733" Score="-4" ViewCount="434" Body="&lt;p&gt;I am seeing Life (2017) movie, movie showed ISS dropped escape pod to the land, when it reaches certain height, escape pod release parachute &amp;amp; near the water surface, thrusters got activated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XD4S1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XD4S1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;you can see here six side smoke is coming out;thrusters, if it is landing on water then Why escape pods activates it thrusters? &lt;/p&gt;&#xA;" OwnerUserId="20019" LastActivityDate="2020-01-30T04:50:34.483" Title="Why escape pods activates thrusters when landing on water ?" Tags="|iss|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21913" PostTypeId="2" ParentId="21912" CreationDate="2017-06-11T14:18:43.297" Score="5" Body="&lt;p&gt;That doesn't look like any landing pod currently in use - at least, I don't recognize the shape. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it's quite common to have soft-landing retrorockets on a capsule. Soyuz fires a brief pulse before landing to take the edge off the g-forces on a ground landing. Using retrorockets on a water landing is a bit more uncommon. The Soyuz hasn't tried water landing, AFAIK, so I don't know what the protocol for that would be. &lt;/p&gt;&#xA;" OwnerUserId="11855" LastActivityDate="2017-06-11T14:18:43.297" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21914" PostTypeId="2" ParentId="21881" CreationDate="2017-06-11T15:02:50.323" Score="2" Body="&lt;p&gt;Wikipedia has &quot;construction numbers&quot; for most of the &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Falcon_9_and_Falcon_Heavy_launches&quot; rel=&quot;nofollow noreferrer&quot;&gt;F9 boosters&lt;/a&gt; since they started using the &quot;Full Thrust&quot; variant -- the model that's been pulling off successful landings. Given that the first to successfully land was numbered B1019 and was the 20th F9 launched, one could extrapolate backwards and theorize that the earlier ones were B1000 - B1018.&lt;/p&gt;&#xA;" OwnerUserId="20023" LastActivityDate="2017-06-11T15:02:50.323" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21915" PostTypeId="2" ParentId="21877" CreationDate="2017-06-11T17:05:32.260" Score="5" Body="&lt;h2&gt;Economically:&lt;/h2&gt;&#xA;&lt;p&gt;Economic arguments and business plans can range in both honesty and subjectivity, but I'll outline a hypothetical estimate.&lt;/p&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;https://www.quora.com/How-much-does-a-20-minutes-decent-quality-fireworks-display-cost&quot; rel=&quot;nofollow noreferrer&quot;&gt;quora post&lt;/a&gt; written by Peter Hand, VP of Western Pyrotechnic Association puts the cost of a 20 minute fireworks display anywhere from $20,000 to a million dollars, depending on the level of firework used:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;For a professional display, you’re probably looking at $10,000 to 20,000. There’s a lot of work for a lot of people in the setup, and a lot of capital tied up in the mortars and firing systems. On the other hand, professional shells are relatively inexpensive compared to the retail consumer kind, with a case of 64 three inch shells often no more expensive than a box of two dozen consumer shells from a retailer. Most professional displays use larger shells, like six inches and higher, and these can cost upward of 300 [dollars] each. If you spend the money, you’ll get these in your display, otherwise it will just be 3 and 4 inch shells. It takes a lot of these small bursts fired at once to fill the sky, which is rather dismissively called “Chinese carpet bombing”. I wouldn’t call that a “decent quality” firework display.&lt;/p&gt;&#xA;&lt;p&gt;Some really outstanding 20 minute displays have cost upward of a million dollars, but these often cover a huge area and use many custom shells, &lt;strong&gt;some 24 inches in diameter and needing 60 pounds of gunpowder to lift&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The website ReactionFireworks.co.uk &lt;a href=&quot;http://www.reactionfireworks.co.uk/faqs/how-much-do-fireworks-cost/&quot; rel=&quot;nofollow noreferrer&quot;&gt;says in its faq (in part)&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As a guide an organisation such as a Round Table or Parish Council typically spends between £3,000 to £6,000 for a 15 to 20 minute pyromusical.&lt;/p&gt;&#xA;&lt;p&gt;Major Guy Fawkes displays as seen in many cities accross the UK over November would typically cost between £10,000 and £16,000 and often have audiences in excess of 20,000.&lt;/p&gt;&#xA;&lt;p&gt;Larger displays seen in such places as Battersea Park, Edinburgh Castle and the London Eye can range from between £30,000 to £200,000.&#xA;The record for the most expensive firework display is held by Kuwait for the 50th anniversary of the ratification of Kuwait’s Constitution in November 2012.  The display cost a reported £10,000,000 !&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So let's say the spacecraft carried enough &amp;quot;stars&amp;quot; to do 100 shows a year for ten years with one hundred 2 gram shooting stars per show. There are certainly that many fireworks displays happening on Earth, but the question of how many would shift to this new technology is... subjective.&lt;/p&gt;&#xA;&lt;p&gt;Then suppose that they grossed &lt;em&gt;on average&lt;/em&gt; $100,000 per show (ranging from personal single shooting stars to million dollar events) that's a gross income of one hundred million dollars, and that could at least conceivably pay for the design, construction, launch, control and orbital maintenance of a ~70 cm, maybe 500 kg nano satellite for ten years, considering the way costs and technology is evolving and beginning to standardize. It's in the right ballpark at least.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;h2&gt;Technologically:&lt;/h2&gt;&#xA;&lt;p&gt;Let's try this using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis-viva equation&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$v^2 = GM_E\left(\frac{2}{r}-\frac{1}{a}\right),$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;the equatorial radius of the Earth 6378 km or &lt;span class=&quot;math-container&quot;&gt;$6.378\times10^6 \ meters$&lt;/span&gt;, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Standard gravitational parameter&lt;/a&gt; of the Earth &lt;span class=&quot;math-container&quot;&gt;$GM_E = 3.986\times 10^{14} \ m^2s^{-3}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The semi-major axis of the satellite in a circular orbit with an altitude of 500 km is &lt;span class=&quot;math-container&quot;&gt;$a_0 = 6.878 \times 10^6 \ meters$&lt;/span&gt;. Using the vis-viva equation, that gives an initial velocity &lt;span class=&quot;math-container&quot;&gt;$v_0 = 7613 \ m/s$&lt;/span&gt;. As a check, the period would be &lt;span class=&quot;math-container&quot;&gt;$2\pi a_0 / v_0 = 5677 \ sec$&lt;/span&gt; or about 94.6 minutes.&lt;/p&gt;&#xA;&lt;p&gt;An educated guess would be that a targeted perigee somewhere between about 80 and 100 km would be guaranteed prompt, localized re-entry. This bars oddly or aerodynamically shaped objects or plane-shaped objects, but small spheres at this perigee and this kind of velocity will re-enter and burn up.&lt;/p&gt;&#xA;&lt;p&gt;What delta-v is needed to do this? If we assume the orbit of the recently ejected ball has its apogee at 500 km altitude and its perigee at 80 km, then the new semi-major axis is now &lt;span class=&quot;math-container&quot;&gt;$a_1 = 0.5 \times (6.878 \times 10^6 + 6.458 \times 10^6) = 6.668  \times 10^6 \ meters$&lt;/span&gt;. Plugging the new semi-major axis &lt;span class=&quot;math-container&quot;&gt;$a_1$&lt;/span&gt; and maintaining the apogee of 500 km into the vis-viva equation, the new velocity required for a re-entry orbit would be &lt;span class=&quot;math-container&quot;&gt;$v_1 = 7492 \ m/s$&lt;/span&gt;, a delta-v of 121 &lt;span class=&quot;math-container&quot;&gt;$m/s$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;This is a minimum.&lt;/strong&gt; You may want substantially greater delta-v for improved re-entry targeting accuracy.&lt;/p&gt;&#xA;&lt;p&gt;Using algebra, one can use &lt;span class=&quot;math-container&quot;&gt;$x=\frac{1}{2}at^2$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$v=at$&lt;/span&gt; to obtain &lt;span class=&quot;math-container&quot;&gt;$a=v^2/2x$&lt;/span&gt;. To get a velocity of 121 meters per second inside a 50 centimeter nano satellite, you need an acceleration of almost 1500 gees.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;The satellite is a gun.&lt;/strong&gt; It fires deer slugs with a muzzle velocity of &lt;em&gt;at least&lt;/em&gt; 400 feet per second, ideally much more. It carries of order ten thousand rounds of ammunition or more, in order to be economically feasable. It can get at you, but it's darn hard for you to get back at it, unless you build a bigger gun, and we know how that story goes...&lt;/p&gt;&#xA;&lt;p&gt;The &lt;em&gt;projectile velocity&lt;/em&gt; relative to you and yours could easily be a few thousand meters per second or ten thousand feet per second depending on the inclinations of the two orbits, and so the impact energy would be enormous if one of these hit anything.&lt;/p&gt;&#xA;&lt;p&gt;But don't worry, it's &amp;quot;not supposed to&amp;quot; do that.&lt;/p&gt;&#xA;&lt;p&gt;Still, the fact that the spacecraft is effectively a gun with ten thousand rounds could conceivably make the whole project moot. If it were &lt;em&gt;struck&lt;/em&gt; by space debris, it could release ten thousand radar-invisible projectiles into short-term stable LEO orbits above the orbit of the ISS and &lt;a href=&quot;https://en.wikipedia.org/wiki/Tiangong-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Taingong-1&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Tiangong-2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tiangong-2&lt;/a&gt;, and this possibility could make this project highly objectionable to any country associated with any current or future manned space mission.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pzGld.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pzGld.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-03-26T02:56:34.533" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21917" PostTypeId="1" CreationDate="2017-06-11T19:43:13.290" Score="2" ViewCount="447" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/21893/6471&quot;&gt;This answer&lt;/a&gt; repeats the fact that something as small as a few micrometers in diameter can make a visible bullseye in a window in orbit.  I heard it with respect to the space shuttle, and now they have one on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we’re talking about a total energy delivered of about a microjoule, even at orbital speeds!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can so little energy make such an impact?&lt;/p&gt;&#xA;" OwnerUserId="6471" LastEditorUserId="6944" LastEditDate="2020-09-09T12:19:47.997" LastActivityDate="2020-09-09T12:19:47.997" Title="How can a paint fleck make a window bullseye?" Tags="|low-earth-orbit|debris|spacecraft-window|" AnswerCount="0" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21919" PostTypeId="1" CreationDate="2017-06-11T22:16:01.060" Score="2" ViewCount="149" Body="&lt;p&gt;SpaceX will be launching &lt;a href=&quot;http://www.ibtimes.co.uk/nasa-says-spacex-may-send-two-red-dragon-spacecraft-mars-2020-case-one-fails-1621053&quot; rel=&quot;nofollow noreferrer&quot;&gt;two Red Dragons&lt;/a&gt; in 2020, one at the beginning of the launch window (July) and one at the end (September).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a CubeSat mission that would have to piggyback on one of the two Red Dragon spacecraft in order to get to Mars orbit, what are the likely pros and cons for the earlier and later launch?&lt;/p&gt;&#xA;" OwnerUserId="19742" LastEditorUserId="415" LastEditDate="2017-06-12T16:24:54.260" LastActivityDate="2017-06-12T16:24:54.260" Title="For a cubesat mission piggybacking on a Red Dragon, what are the tradeoffs for launch early and late in the launch window?" Tags="|launch|spacex|cubesat|red-dragon|" AnswerCount="0" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21920" PostTypeId="1" AcceptedAnswerId="21929" CreationDate="2017-06-12T03:40:35.560" Score="3" ViewCount="431" Body="&lt;p&gt;The video in the interesting question &lt;a href=&quot;https://space.stackexchange.com/q/17089/12102&quot;&gt;How did the GoFast 2014 Rocket de-spin?&lt;/a&gt; shows a de-spin that went well. An initial very fast spin (for stabilization during propulsion) is almost perfectly de-spun during a maneuver that is so fast that it &lt;a href=&quot;https://space.stackexchange.com/a/17091/12102&quot;&gt;is hard to imagine it could be anything else&lt;/a&gt; but a Yo-Yo de-spin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mathematics behind this is summarized &lt;a href=&quot;https://space.stackexchange.com/a/19437/12102&quot;&gt;in this answer&lt;/a&gt; along with links to several original sources. Briefly, $r$ is the ratio of final to initial angular velocity, and will be zero at perfect de-spin but can still be positive or negative for undershoot or overshoot conditions. The remaining terms are described in detail in &lt;a href=&quot;https://space.stackexchange.com/a/19437/12102&quot;&gt;the link&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{m(d+a)^2}{I}=\frac{1-r}{1+r}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's a balance. So if the final angular &lt;em&gt;speed&lt;/em&gt; is much much lower than the initial (near perfect cancellation), it suggests that &lt;em&gt;both&lt;/em&gt; the initial spin rate &lt;em&gt;and&lt;/em&gt; the de-spin were well controlled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another example of careful spin and de-spin cancellation (but not with a Yo-Yo) is discussed in &lt;a href=&quot;https://space.stackexchange.com/a/19437/12102&quot;&gt;this answer&lt;/a&gt; and in &lt;a href=&quot;https://space.stackexchange.com/a/19440/12102&quot;&gt;this answer&lt;/a&gt; about NASA's LSDS test.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what if the initial spin had some uncertainty that could not be controlled?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Has adjustable Yo-Yo de-spin been tested or demonstrated? Reading about a serious but untested proposal would also be interesting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can think of all kinds of possible ways to do this, last-second laser trimming of the wires, or just a clever break-away release, vernier yo-yos (say 10kg plus 160g, 80g, 40g, 20g, 10g independently addressable &quot;fine yo-yos&quot;) but this kind of complexity really defeats the elegance and reliability of the Yo-Yo technique with fixed mass weights and premeasured wires, so maybe backup thrusters to re-adjust the spin before Yo-Yo would be a better way to stay with known technologies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, have variable Yo-Yo's been tested or demonstrated?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-12T05:52:24.690" LastActivityDate="2017-06-12T14:48:03.897" Title="Has adjustable Yo-Yo de-spin been tested or demonstrated?" Tags="|spin-stabilisation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21924" PostTypeId="2" ParentId="21891" CreationDate="2017-06-12T06:22:38.293" Score="10" Body="&lt;p&gt;Original question is not thoroughly answered. As engineer who worked on ISS for seven years during critical design, IMPACTS TO ENGINEER WORKING ON ISS include: (1) design exposed equipment to safely withstand impact using tested techniques, (2) or relocate equipment to more protected location, (3) or in difficult cases ask chief engineer to put your design on the list of equipment so sensitive that entire ISS has to be moved in orbit to avoid incoming debris, (4) or for equipment already on ISS perform damage and hazard assessments. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other impacts specifically bad from ball bearings are that: (a) entire ISS is designed to withstand impact of a 1 cm object, so a hit from these ball bearings puts the entire station exactly on its worst case design condition so that most equipment should survive, (b) steel ball bearings are particularly bad because they are very strong and are likely to survive most initial impacts and keep going.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other answers above are correct about hazard in LEO to ISS or any other spacecraft in orbit in a different inclination or altitude from debris. ISS is inclined at 51° while most launches from the Cape are inclined at 28° and those from Baikonur are at 45°, plus there are lots of spacecraft that change their inclination after launch, so there are plenty of chances for a hit, up to and including head-on collision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last, just to be clear on the TESS animation question above about a booster interface adapter and Marmon clamp being released into an undefined orbit, there are international treaties and requirements for launch licenses and spacecraft operations licenses that PROHIBIT RELEASE OF ANYTHING that won't reenter and burn up, or do anything on orbit that releases any debris whatsoever.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is info about debris - &lt;a href=&quot;https://www.orbitaldebris.jsc.nasa.gov/faq.html&quot; rel=&quot;noreferrer&quot;&gt;https://www.orbitaldebris.jsc.nasa.gov/faq.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is info about designing for a debris environment -&lt;a href=&quot;https://www.nap.edu/read/5532/chapter/6&quot; rel=&quot;noreferrer&quot;&gt;https://www.nap.edu/read/5532/chapter/6&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="20033" LastActivityDate="2017-06-12T06:22:38.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21925" PostTypeId="1" CreationDate="2017-06-12T08:59:04.727" Score="6" ViewCount="1198" Body="&lt;p&gt;We all know that space is about going really fast. We also know that what matters in a collision isn't really absolute speed, but relative velocity. (Two cars with matched speeds on a highway touching each other doesn't necessarily lead to large damage, but if one of them was standing still, it probably would.) A large fraction of the orbiting spacecraft are in prograde orbits, simply because it's easier and, if not actively helpful, at least doesn't hurt; that also reduces the relative velocity between the two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yet people keep saying that in-orbit collisions happen at such extreme velocities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What is the typical &lt;em&gt;relative&lt;/em&gt; impact velocity of a piece of orbital debris to an operational spacecraft&lt;/strong&gt; in low Earth orbit? What are the vector component values of this velocity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus points for answers that include citations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also bonus points for answers that include the data from which the &quot;typical&quot; is derived.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2017-06-12T10:53:09.777" LastActivityDate="2017-06-13T02:14:42.887" Title="What is the typical relative impact velocity of orbital debris in low Earth orbit?" Tags="|low-earth-orbit|debris|impact|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21926" PostTypeId="2" ParentId="21925" CreationDate="2017-06-12T09:26:16.697" Score="2" Body="&lt;p&gt;Not sure how precise you need the answer, but just thinking about the first cosmic velocity and the escape velocity, it can only be a value between that. So Something between ~7.8 km/s - 11.2 km/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course as you mentioned the relative velocity matters. The orbits of the debris could be opposed to the orbit of the spacecraft so the theoretical max relative velocity would be 11.2 km/s + ~7 km/s = ~ 18 km/s (since you're talking about a LEO and not a HEO or something).&#xA;Since most launches take place in a prograde orbit I'd imagine that most of the debris would be in a prograde orbit as well so most impacts probably take place at a relative velocity of the perigee speed of a HEO (9-11 km/s depending on the orbit) and the speed of the spacecraft in LEO (~7-7.5 km/s). Worst case is about 19 km/s as mentioned before though. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All speeds below that are possible though, as the inclinations between the spacecraft can vary, resulting in very different relative velocities.&lt;/p&gt;&#xA;" OwnerUserId="20038" LastActivityDate="2017-06-12T09:26:16.697" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21927" PostTypeId="2" ParentId="21925" CreationDate="2017-06-12T11:01:30.590" Score="0" Body="&lt;p&gt;I calculated a simple example. Two objects are in a circular low orbit but in different planes. The impact velocity depends on the angle between the orbit planes. I use 7.8 km/s for the speed in orbit.&lt;br&gt;&#xA;For an angle of 5° the vectorial velocity difference is 0.68 km/s, for 10 ° 1.36 km/s, for 15 ° 2.04 km/s, for 30 ° 4.04 km/s, for 45 ° 5.96 km/s and for 90 ° 11.04 km/s.&lt;br&gt;&#xA;Two orbits with an angle difference of 45 ° to the equatorial plane in oppositional directions have an angle difference of 90 ° between them.&lt;br&gt;&#xA;Impact velocities of 1 to 11 km/s are possible.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-06-12T19:12:52.650" LastActivityDate="2017-06-12T19:12:52.650" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21928" PostTypeId="1" AcceptedAnswerId="21930" CreationDate="2017-06-12T14:01:23.687" Score="4" ViewCount="824" Body="&lt;p&gt;I am wondering whether the Hubble Space Telescope uses a &quot;simple&quot; PID controller like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9locx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9locx.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is it using another type of control system that may be much more complex?&lt;/p&gt;&#xA;" OwnerUserId="8968" LastEditorUserId="6816" LastEditDate="2017-06-13T14:48:39.370" LastActivityDate="2017-06-13T14:48:39.370" Title="Does the Hubble telescope use a &quot;simple&quot; PID-controller for its pointing control system?" Tags="|attitude|engineering|telescope|hubble|flight-control|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21929" PostTypeId="2" ParentId="21920" CreationDate="2017-06-12T14:37:51.690" Score="6" Body="&lt;p&gt;In the 1960s the Goddard Space Flight Center devoted some study to the topic of  error in yo-yo de-spin. It was acknowledged that spin-up errors and uncertainty of the spin axis moment of inertia were sources of de-spin error. Since de-spin error can be a source of many problems, which may even impact the success of the mission, the stretch yo-yo concept was devised to reduce de-spin error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19630004528.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Analytical Theory of the Stretch Yo-Yo For De-Spin of Satellites&quot; by Joseph V. Fedor&lt;/a&gt; explains:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A device that greatly reduces spin-up errors and errors due to variations in spin moment of inertia is the stretch yo-yo (the stretch concept was first suggested by Henry Cornille of Goddard Space Flight Center (Reference 3). It can consist of a weight, a spring, a wire, and end fittings (Figure 1), or&#xA;  simply a weight, spring, and end fittings (Figure 2). The purpose of the spring is to compensate for errors in initial spin-up. For example, in a given stretch yo-yo application there will be a certain amount of stretching or elongation of the spring during normal operation. If the initial spin is greater than the nominal value, the spring will elongate more than normal during operation and reduce the spin to the desired value. If the initial spin is less than the nominal initial spin, the spring will elongate less&#xA;  than normal during functioning and correct for the under spin. The stretch yo-yo is a simple example of an adaptive control system: it senses the spin environment it is in and corrects accordingly. Also, it will be shown that the stretch yo-yo is relatively insensitive to variations (uncertainties) of the spin moment of inertia.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;After analysis and design, it was decided that the stretch yo-yo despin should be tested on the Ariel I spacecraft, and in April 1962, it was successfully flown (&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19630011465.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Analysis of the Dynamic Tests of the Stretch Yo-Yo De-Spin System&quot; by William R. Mentzer&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Following dynamic analysis and development of design equations for the stretch yo-yo, the decision was made to fabricate and test stretch yo-yos on the Explorer XII (1961 v1) and Ariell (1962 01) type payloads. Three series of tests were conducted beginning with feasibility tests on the two payloads&#xA;  and concluding with flight qualification tests for the Ariel I payload. As a result of these tests, a stretch yo-yo was flown on the successful Ariel I spacecraft in April 1962.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In the record of the experimental results, the effects of the several variables involved in stretch yo-yo design can be readily noted. These variables include optimum spring constant, preload, initial spin rate, spin moment of inertia, and material strength.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can read the full analysis of the tests in that document, but here's one last brief note on the effectiveness of the stretch yo-yo de-spin:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Explorer XII type optimum spring scale tests clearly demonstrated the ability of the stretch yo-yo to compensate for errors in initial spin rate. In this series of tests, spin-up errors of +/-20 percent were reduced to within +/-1.5 percent of the design final spin rate.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2017-06-12T14:48:03.897" LastActivityDate="2017-06-12T14:48:03.897" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21930" PostTypeId="2" ParentId="21928" CreationDate="2017-06-12T15:09:43.830" Score="7" Body="&lt;p&gt;Yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the NASA paper &quot;&lt;a href=&quot;https://arc.aiaa.org/doi/abs/10.2514/3.21369?journalCode=jgcd&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hubble Space Telescope Pointing Control System Design Improvement Study Results&lt;/a&gt;&quot;, in the first full paragraph on page 2, the author states &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nominal control algorithm is a standard proportional-integral-derivative (PID) operating at 40 Hz using the rate gyro assembly measurements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If you want more information on Hubble control, I encourage you to read more of the paper. When Hubble originally launched, there were immediate complications with pointing, stemming from thermal effects caused by the solar arrays. In response to this, NASA looked at five different control algorithms to replace the standard law, which this paper goes into detail about. In the end, these five methods were never used, since &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The troublesome solar arrays were replaced with a more well-behaved set and the world's largest contact lens was installed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So instead of changing the control law they just fixed the problem from its source. Talk about a costly mistake.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastEditorUserId="897" LastEditDate="2017-06-13T12:28:04.593" LastActivityDate="2017-06-13T12:28:04.593" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21931" PostTypeId="1" AcceptedAnswerId="26553" CreationDate="2017-06-12T15:53:59.673" Score="2" ViewCount="1635" Body="&lt;p&gt;A &lt;em&gt;Terrier Improved-Malemute&lt;/em&gt; actually. &lt;a href=&quot;http://www.astronautix.com/t/terrierimprovedmalemute.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;1. Astronautix&lt;/a&gt;, &lt;a href=&quot;http://space.skyrocket.de/doc_lau_det/terrier_imp-malemute.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;2. Gunter's Space Page&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Terrier_Malemute&quot; rel=&quot;nofollow noreferrer&quot;&gt;3. Wikipedia&lt;/a&gt;, &lt;a href=&quot;https://sites.wff.nasa.gov/code810/vehicles/Terrier_Malemute.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;4. NASA pdf&lt;/a&gt;. According to Space.com's &lt;a href=&quot;https://www.space.com/37158-nasa-rocket-launch-visible-tonight-june11.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Watch a NASA Rocket Create Colorful Artificial Clouds Over US East Coast Tonight!&lt;/a&gt; that is the type of sounding rocket that will be used. I went to look up the name because it sounds like a mixed canine breed (Malamute's alternate spelling per &lt;a href=&quot;https://www.merriam-webster.com/dictionary/malemute&quot; rel=&quot;nofollow noreferrer&quot;&gt;Merriam-Webster online&lt;/a&gt;), and at Astronatix I found there is an amazing variety of sounding rocket names.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Terrier Terrier Oriole:&lt;/strong&gt;  &lt;em&gt;Three stage vehicle consisting of 1 x Terrier + 1 x Terrier + 1 x Oriole&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Taurus Nike Tomahawk:&lt;/strong&gt; &lt;em&gt;Three stage vehicle consisting of 1 x Taurus (Honest John motor) + 1 x Nike + 1 x Tomahawk&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Tater:&lt;/strong&gt; &lt;em&gt;Three stage vehicle consisting of 1 x Talos + 1 x Terrier + 1 x Recruit&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How were names like Terrier, Malemute, Nike, Oriole selected for these apparently nearly interchangeable mix-and-match stages for sounding rockets? More importantly, when assembling a rocket for a given launch, how are the individual stages selected for a given mission? Paging through Astronautix it looks like quite a variety of mixes were named and launched over the last 50 years. Are they just stacked to match the mass and altitude required for the payload, or is there more flexibility available for specific mission needs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AdL9g.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AdL9g.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;The ampoule doors on the sounding rocket payload are open during testing at NASA's Wallops Flight Facility in Virginia. The Terrier-Improved Malemute rocket is scheduled to launch at 9:04 p.m. EDT on June 11, 2017.&quot; Cropped from Space.com's &lt;a href=&quot;https://www.space.com/37158-nasa-rocket-launch-visible-tonight-june11.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Watch a NASA Rocket Create Colorful Artificial Clouds Over US East Coast Tonight!&lt;/a&gt;. Credit: Berit Bland/NASA&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-12T17:08:28.910" LastActivityDate="2020-02-04T22:50:50.193" Title="Why a Terrier Malemute?" Tags="|sounding-rocket|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21932" PostTypeId="2" ParentId="21925" CreationDate="2017-06-12T16:12:12.000" Score="5" Body="&lt;p&gt;Take a look at &lt;a href=&quot;https://space.stackexchange.com/a/4861/2848&quot;&gt;this answer&lt;/a&gt; by Mark Adler. As you can see, a small panel over a 15 years endured many impacts. I would expect there have been multitudes of impacts over all. I doubt anyone knows what the average impact velocity has been.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll attempt to give you some tools to examine different scenarios, though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/URKjJ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/URKjJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given a triangle with lengths a, b, c:&lt;br&gt;&#xA;$c^2 = a^2 + b^2 - 2ab * cos(\alpha)$&lt;br&gt;&#xA;where $\alpha$ is the angle between a and b.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Law_of_cosines&quot; rel=&quot;nofollow noreferrer&quot;&gt;Law of Cosines&lt;/a&gt; may look hard. But if you remember cos(90º) is zero, you can see the Pythagorean theorem drop out when alpha is 90º. So if you just memorize the $-2 ab * cos(\alpha)$ part, the rest is the Pythagorean theorem you learned in high school.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And when you do vector subtraction, the third side of the triangle is the delta v between the first two velocity vectors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below is a series of 7.7 km/s velocity vectors forming different angles with the original 7.7 km/s velocity vector. These are vectors from 300 km altitude circular low earth orbits:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BCFpu.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BCFpu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-06-13T02:14:42.887" LastActivityDate="2017-06-13T02:14:42.887" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21933" PostTypeId="2" ParentId="21905" CreationDate="2017-06-12T17:14:20.220" Score="3" Body="&lt;p&gt;The DOD, JPL, NASA, NOAA, and others do track space debris. The debris the ISS is mainly worried about is microdebris, and others of similar sizes. The companies/organizations listed above do track debris using radar, laser ranging, optical, etc... Most large debris (i.e. R/B, S/C, etc...) are well tracked. The debris sizes around 10 cm^3 or or laptop size are slightly more difficult but are still tracked to some degree of uncertainty. All tracking is done from the ground at ground stations. Some objects can be tracked from other spacecrafts in orbit, but it is usually not the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also most debris locations are around an inclination of 71 to 98 degrees. Currently there is about a week notice before a possible known collision with 10^3 cm to larger objects. The ISS does have features that detect incoming debris or smaller objects. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT: Sources&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS Debris Detection: &lt;a href=&quot;http://ieeexplore.ieee.org/document/879836/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ieeexplore.ieee.org/document/879836/&lt;/a&gt; and &lt;a href=&quot;http://www.esa.int/esapub/bulletin/bullet109/chapter16_bul109.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.esa.int/esapub/bulletin/bullet109/chapter16_bul109.pdf&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ISS Collision Maneuvers: &lt;a href=&quot;https://sma.nasa.gov/news/articles/newsitem/2015/11/23/two-more-collision-avoidance-maneuvers-for-the-international-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://sma.nasa.gov/news/articles/newsitem/2015/11/23/two-more-collision-avoidance-maneuvers-for-the-international-space-station&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="20048" LastEditorUserId="195" LastEditDate="2019-07-02T01:45:40.583" LastActivityDate="2019-07-02T01:45:40.583" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="21934" PostTypeId="2" ParentId="20590" CreationDate="2017-06-12T17:17:08.947" Score="5" Body="&lt;p&gt;Let's have a try! To keep it simple, I will consider a one-dimensional equation of motion&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$m \ddot{x(t)} = a(t) x(t) + b(t) \dot{x}(t) \tag{1}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;The applications to halo orbit is actually simpler because the coefficients &lt;span class=&quot;math-container&quot;&gt;$a(t)$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$b(t)$&lt;/span&gt; would not depend on time.&lt;/p&gt;&#xA;&lt;p&gt;The theory of linear differential equations tells us two important results:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Initial conditions &lt;span class=&quot;math-container&quot;&gt;$x(0)=x_0,\ \dot{x}(0)=\dot{x}_0$&lt;/span&gt; completely fix the solution;&lt;/li&gt;&#xA;&lt;li&gt;Any linear combination of two solutions is also a solution.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The first result implies that there must exists a function which maps &lt;span class=&quot;math-container&quot;&gt;$(x_0,\dot{x}_0)$&lt;/span&gt; onto &lt;span class=&quot;math-container&quot;&gt;$x(t)$&lt;/span&gt;. The second result guarantees that this function is linear, i.e.&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ x(t) = \alpha(t)x_0 + \beta(t)\dot{x}_0$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;But then the speed has the same form&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \dot{x}(t) = \dot{\alpha}(t)x_0 + \dot{\beta}(t)\dot{x}_0$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;and we can therefore put everything together&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\begin{pmatrix} x(t) \\ \dot{x}(t) \end{pmatrix} = \underbrace{\begin{pmatrix} \Phi_{11}(t,t_0) &amp;amp; \Phi_{12}(t,t_0) \\ \Phi_{21}(t,t_0) &amp;amp; \Phi_{22}(t,t_0) \end{pmatrix}}_{\displaystyle\Phi(t,t_0)} \begin{pmatrix} x_0 \\ \dot{x}_0 \end{pmatrix} \tag{2}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;And &lt;span class=&quot;math-container&quot;&gt;$\Phi(t,t_0)$&lt;/span&gt; is called the transition matrix from time &lt;span class=&quot;math-container&quot;&gt;$t_0$&lt;/span&gt; to time &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;From this equation, since &lt;span class=&quot;math-container&quot;&gt;$x(t)$&lt;/span&gt; satisfies the differential equation (1) we started from, we can reasonably expect &lt;span class=&quot;math-container&quot;&gt;$\Phi(t,t_0)$&lt;/span&gt; to satisfy one too. To find it, we just need to differentiate (2)&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\begin{pmatrix} \dot{x}(t) \\ \ddot{x}(t) \end{pmatrix} = \dot{\Phi}(t,t_0)\begin{pmatrix} x_0 \\ \dot{x}_0 \end{pmatrix}\tag{3a}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where &lt;span class=&quot;math-container&quot;&gt;$\dot{\Phi}(t,t_0)$&lt;/span&gt; denotes the differentiation with respect to &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt;, keeping &lt;span class=&quot;math-container&quot;&gt;$t_0$&lt;/span&gt; constant. But then the left-hand side reads&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\begin{pmatrix} \dot{x}(t) \\ \ddot{x}(t) \end{pmatrix} = \underbrace{\begin{pmatrix} 0 &amp;amp; 1\\ \frac{1}{m}a(t) &amp;amp; \frac{1}{m}b(t) \end{pmatrix}}_{A(t)}\begin{pmatrix} x(t) \\ \dot{x}(t) \end{pmatrix}$$&lt;/span&gt;&#xA;Then we use (2) to replace &lt;span class=&quot;math-container&quot;&gt;$\begin{pmatrix} x(t) \\ \dot{x}(t) \end{pmatrix}$&lt;/span&gt; on the right-hand side.&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\begin{pmatrix} \dot{x}(t) \\ \ddot{x}(t) \end{pmatrix} =A(t)\Phi(t,t_0)\begin{pmatrix} x_0 \\ \dot{x}_0 \end{pmatrix} \tag{3b}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;By equating the right-hand side of (3a) and (3b), we get&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\dot{\Phi}(t,t_0)\begin{pmatrix} x_0 \\ \dot{x}_0 \end{pmatrix} = A(t)\Phi(t,t_0)\begin{pmatrix} x_0 \\ \dot{x}_0 \end{pmatrix}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;This equality must be true for any &lt;span class=&quot;math-container&quot;&gt;$x_0$&lt;/span&gt; and any &lt;span class=&quot;math-container&quot;&gt;$\dot{x}_0$&lt;/span&gt;. Thus the matrices acting on &lt;span class=&quot;math-container&quot;&gt;$\begin{pmatrix} x_0 \\ \dot{x}_0 \end{pmatrix}$&lt;/span&gt; on both side of the equation shall be equal, and we get the differential equation we sought,&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\dot{\Phi}(t,t_0) = A(t)\Phi(t,t_0). \tag{4}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;After writing all that, I feel I have to explain the last trick in the Howell paper. So we have &lt;span class=&quot;math-container&quot;&gt;$x(t)$&lt;/span&gt; and we want to understand what could make it vary a tiny bit. &lt;span class=&quot;math-container&quot;&gt;$x(t)$&lt;/span&gt; depends on &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt;, so varying &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt; by &lt;span class=&quot;math-container&quot;&gt;$\delta t$&lt;/span&gt; induces a variation, proportional to the derivative: &lt;span class=&quot;math-container&quot;&gt;$\dot{x}(t)\delta t$&lt;/span&gt;. But &lt;span class=&quot;math-container&quot;&gt;$x(t)$&lt;/span&gt; also depends on &lt;span class=&quot;math-container&quot;&gt;$x_0$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\dot{x}_0$&lt;/span&gt; and that dependence is given by (2). The second row of the matrix to be exact, and the variation is &lt;span class=&quot;math-container&quot;&gt;$\Phi_{21}(t,t_0)\delta x_0 + \Phi_{22}(t,t_0)\delta \dot{x}_0$&lt;/span&gt;. Then if we consider only small variations, we can just sum those two contributions and get:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\delta \dot{x}(t) = \Phi_{21}(t,t_0)\delta x_0 + \Phi_{22}(t,t_0)\delta \dot{x}_0 + \dot{x}(t)\delta t$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the problem of interest to you, &lt;span class=&quot;math-container&quot;&gt;$t$&lt;/span&gt; is the half-period &lt;span class=&quot;math-container&quot;&gt;$T/2$&lt;/span&gt;, and the variation &lt;span class=&quot;math-container&quot;&gt;$\delta \dot{x}(T/2)$&lt;/span&gt; comes either from a small variation of &lt;span class=&quot;math-container&quot;&gt;$T/2$&lt;/span&gt;, for the same initial conditions, or from a small variation of the initial conditions, for the same half-period.&lt;/p&gt;&#xA;&lt;p&gt;I hope it bring some enlightenment and I wish you the best for your project!&lt;/p&gt;&#xA;" OwnerDisplayName="user20022" LastEditorUserId="1255" LastEditorDisplayName="user20022" LastEditDate="2021-07-22T22:45:42.920" LastActivityDate="2021-07-22T22:45:42.920" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="21935" PostTypeId="2" ParentId="3505" CreationDate="2017-06-12T17:40:36.613" Score="1" Body="&lt;p&gt;ISRO's Outcome Budget 2013-14 twice mentions restart capability as a design feature for Cryogenic Upper Stage (CUS):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.isro.gov.in/sites/default/files/pdf/budget/Outcome%20budget2013-14.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.isro.gov.in/sites/default/files/pdf/budget/Outcome%20budget2013-14.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Page 18&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Development of a restartable cryogenic engine &amp;amp; stage for GSLV to replace the Russian supplied Cryogenic stage of GSLV.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Page 66&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;4 . Cryogenic Upper Stage (CUS) Project&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;4.1 The objective of the Project is to develop and qualify an indigenous restartable cryogenic stage employing liquid oxygen as oxidizer and liquid hydrogen as fuel for the upper stage of GSLV&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="19014" LastEditorUserId="12102" LastEditDate="2017-10-28T19:47:08.883" LastActivityDate="2017-10-28T19:47:08.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21936" PostTypeId="2" ParentId="20009" CreationDate="2017-06-12T17:47:27.763" Score="-1" Body="&lt;p&gt;The cost of a CubeSat depends on the mass and size of the CubeSat and launch vehicle being used. It can range between on average between 10,000 USD and 500,000 USD. CubeSats sizes range from 0.5U to 27U, with 1U being defined as 10cm x 10cm x 10cm. There are mass requirements that CubeSats must stay under, however, waivers can be signed to allow for a higher mass cubesat. A CubeSat being launched on a Falcon 9 will have a much smaller launch cost when compared to one being launched on a Delta V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also the many companies and NASA is willing to launch certain CubeSats for free depending on the type of research being conducted. Currently CubeSats between 0.5U to 6U have only been launched. There are plans for people to launch larger CubeSats of 6U+.&lt;/p&gt;&#xA;" OwnerUserId="20048" LastActivityDate="2017-06-12T17:47:27.763" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21937" PostTypeId="1" AcceptedAnswerId="21941" CreationDate="2017-06-12T18:31:38.663" Score="5" ViewCount="1289" Body="&lt;p&gt;Let's suppose that the grid fins of the Falcon 9 remain open and the rocket still launches. This is probably never going to happen because the SpaceX team is exceptionally good, but I am trying to see what will happen for a personal project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So what will happen during the flight if the fins are open even before launch?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="17599" LastEditorUserId="51" LastEditDate="2017-06-12T18:55:35.117" LastActivityDate="2020-06-14T05:28:18.113" Title="What happens if the Falcon 9 launches with its grid fins open?" Tags="|spacex|falcon-9|landing|aerodynamics|grid-fins|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21938" PostTypeId="2" ParentId="20590" CreationDate="2017-06-12T18:48:29.890" Score="3" Body="&lt;p&gt;I'll try answering your two questions simply first.  If these responses are too simple or miss the mark, let me know, and I'll edit the response.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1) What are the state propagation vector and State Transition Matrix (STM)?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The state propagation vector is simply the position &amp;amp; velocity at a given time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The STM is a matrix that captures the sensitivity of the propagation to the initial state.  So, it answers the question &quot;If I change my starting x-coordinate by 5 meters, how much will my final position and velocity change?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2) How can I use the STM to improve convergence on new Halo Orbits?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can use the STM to achieve faster convergence on new Halo orbits by mapping the change you need at the Y-axis crossing back to the starting state.  (E.g. if you arrive at the crossing with a +2 Z velocity, you can use the STM to compute a different initial state that will have a Z velocity reduced by about 2.  (subject to linearization errors)  Dr. Davis from CU Boulder (CCAR) provides the following handout in the Interplanetary Mission Design grad-course she teaches:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://ccar.colorado.edu/imd/2015/documents/SingleShootingHandout.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ccar.colorado.edu/imd/2015/documents/SingleShootingHandout.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More over, here is the summary of a project on Halo orbits which includes a number of useful figures:&#xA;&lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2012/dowling/introduction.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ccar.colorado.edu/asen5050/projects/projects_2012/dowling/introduction.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12483" LastEditorUserId="1391" LastEditDate="2017-06-14T09:15:16.140" LastActivityDate="2017-06-14T09:15:16.140" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21939" PostTypeId="2" ParentId="21937" CreationDate="2017-06-12T18:55:21.973" Score="9" Body="&lt;p&gt;If you have noticed, the fins deploy by opening upwards. This makes me wonder if they considered your case. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When in normal use, the force vector upon them pushes upwards.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In your case, the force vector would be forcing them down and closed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So first approximation they would be forced down to a minimum. They would add significant drag in a non-fully open mode.  I.e. Would possibly cause less resistance if fully open and inline vs pushed partially down and more of an angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likely they would be able to continue the mission, but it would probably be a bad thing.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-12T18:55:21.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21940" PostTypeId="2" ParentId="20009" CreationDate="2017-06-12T19:00:21.433" Score="12" Body="&lt;p&gt;For ISRO we have revenue details on few small satellites launched between 2013 to 2015 through an official response in Indian Parliament &lt;a href=&quot;http://164.100.47.234/question/annex/238/Au922.pdf&quot; rel=&quot;noreferrer&quot;&gt;PDF&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/v-w-x-y-z/velox-1&quot; rel=&quot;noreferrer&quot;&gt;VELOX-1&lt;/a&gt; (3U) satellite from Nanyang Technological University of Singapore was launched on 30 June 2014 aboard PSLV-C23 (core alone configuration) to a 660 km Sun-synchronous orbit for €140,000 as a ride-share.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Four LEMUR (3U) satellites from Spire Global were launched on 28 September 2015 aboard PSLV C30 (XL configuration) to a 650 km, 6° inclination orbit for €540,000 as a ride-share.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Details on primary payloads and co-passengers here&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle#Launch_history&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Polar_Satellite_Launch_Vehicle#Launch_history&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19014" LastActivityDate="2017-06-12T19:00:21.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21941" PostTypeId="2" ParentId="21937" CreationDate="2017-06-12T19:07:36.493" Score="10" Body="&lt;p&gt;I can think of three principal issues raised by launching with fins deployed; I would &lt;em&gt;guess&lt;/em&gt; that none of them would endanger a mission, but I don't have any inside information to support that. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Increased drag from the deployed fins. This would hurt performance slightly, but there is some performance margin in the launcher, and the fins aren't designed to create large amounts of drag. I would not expect the additional drag to compare to the performance loss from an engine-out, for example.&lt;/li&gt;&#xA;&lt;li&gt;Aerodynamic stability. Aerodynamic surfaces above the rocket's center of gravity reduce stability; finned things want to fly with the fins toward the back (like a fletched arrow). The second stage and payload are above the fins, so the fins aren't that far ahead of the center of mass, and the gimbaled rocket engines should provide enough control to compensate, but if the rocket is normally flown with minimal stability margin, this could push it over the edge, causing it to flip and break apart. (I suspect the engines have plenty of control authority, but I don't actually know for sure.)&lt;/li&gt;&#xA;&lt;li&gt;Damage to the fins themselves. If I remember correctly, the dynamic pressure sustained during reentry, while the fins are normally deployed, is quite a bit higher than the maximum dynamic pressure on ascent, so the fins should be able to survive easily.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-06-12T19:07:36.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21943" PostTypeId="2" ParentId="21928" CreationDate="2017-06-13T12:19:44.277" Score="2" Body="&lt;p&gt;When taking control theory graduate courses in the 1980's our reference book had an exercise that mentioned that the Saturn V (moon rocket), from a control model point of view, had 22 or 23 orders. And that was designed in the mid sixties. I would be very surprised if the Hubble control model was as simple as a straight PID, due to the complex bending moments and thermal effects that you need to control in an optimized way, from multiple sensors with and fine pointing accuracy requirements.  &lt;/p&gt;&#xA;" OwnerUserId="19624" LastActivityDate="2017-06-13T12:19:44.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21944" PostTypeId="2" ParentId="20868" CreationDate="2017-06-13T17:26:36.287" Score="4" Body="&lt;p&gt;There is two answers to this question depending on the criteria.   If the criteria is first flight of a segment previously used, then the first actual &quot;used&quot; segment to fly was on STS-2 with the mid center segment on both the RH and LH booster from (DM-2) Development Motor 2.  Interestingly, the blow-by referred to on STS-2 occurred between this used segment and the new aft segment.&#xA;The first previously flown segment to make its second flight was STS-11(41B), the LH mid center segment.   The second known occurrence of blow-by in the program was on this joint.  Interesting as well.   By the time of the Challenger accident, 75% of the segments flown were &quot;used&quot;&lt;/p&gt;&#xA;" OwnerUserId="20052" LastActivityDate="2017-06-13T17:26:36.287" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21945" PostTypeId="2" ParentId="21898" CreationDate="2017-06-13T19:38:16.683" Score="1" Body="&lt;p&gt;Sadly this mission that launched May 7 2017 ended in failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BaVQb.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BaVQb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was billed as the third mid-latitude test flight of a Super-Pressure Balloon.  The payload:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Flying on this year’s SPB test flight was the International Extreme&#xA;  Universe Space Observatory-SPB payload. EUSO-SPB is a high-energy&#xA;  cosmic ray particle astrophysics payload testing a fluorescence&#xA;  detector and its supporting technologies under the severe operating&#xA;  conditions of the stratosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The balloon developed a leak and controllers brought it down in a controlled fashion after 12 days, 4 hours and 34 minutes aloft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.nasa.gov/feature/wallops/2017/nasa-completes-balloon-technology-test-flight&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a bit more info on the payload &lt;a href=&quot;http://jem-euso.roma2.infn.it/?page_id=1055&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The instrument is an updated version of the first EUSO-Balloon&#xA;  payload. It includes a full original JEM-EUSO PDM (photon detection&#xA;  module with 2304 pixels), as in EUSO-Balloon, and a Fresnel lens&#xA;  optical system, 1 m each side, covering a field of view of ±6 degree.&#xA;  An improved version of the trigger allows to catch high energy cosmic&#xA;  ray events.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and &lt;a href=&quot;http://astroserve.mines.edu/euso_spb/2017-spb.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The instrument was built by members of the JEM-EUSO international&#xA;  collaboration. It will fly suspended from the super pressure balloon&#xA;  and look down on the atmosphere from suborbital space to record the&#xA;  faint traces of UV light produced by cosmic ray extensive air showers.&#xA;  At its heart is a sensitive Photon Detection Module (PDM) that is&#xA;  mounted at the focus of a custom optics system of UV transmitting&#xA;  fresnel lenses. The lenses are 1x1 m in size. An array of 36 64&#xA;  channel multichannel photomultiplier tubes form the PDM array. The&#xA;  signals of the PDM are digitized with a 2.5 microsecond time exposure.&#xA;  The data acquisition system records &quot;video clips&quot; containing 128 of&#xA;  these images. The field of view of the detector is 11x11 degrees. The&#xA;  recorded “videos clips” will show the trajectories of EASs as they&#xA;  develop in the atmosphere. To improve the contrast of the faint air&#xA;  showers above the night background, each subsection of the PDM is&#xA;  covered by a UV transmission filter. The instrument will operate at&#xA;  night when the moon is down.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Z0bsf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z0bsf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lyCe0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lyCe0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-06-13T20:24:50.813" LastActivityDate="2017-06-13T20:24:50.813" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21946" PostTypeId="1" CreationDate="2017-06-13T19:55:47.927" Score="2" ViewCount="164" Body="&lt;p&gt;This is for a fiction project I'm working on. Assuming it becomes possible to someday land on Pluto, what hazards might be encountered given what we've learned about its surface makeup? &lt;/p&gt;&#xA;" OwnerUserId="17089" LastActivityDate="2018-02-08T13:39:00.017" Title="Landing Hazards on Pluto" Tags="|crewed-spaceflight|spacecraft|planetary-science|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21947" PostTypeId="2" ParentId="21946" CreationDate="2017-06-13T20:27:01.260" Score="4" Body="&lt;p&gt;The primary difficulties I can think of are the sparse atmosphere and the temperature. &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Pluto&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pluto's atmosphere&lt;/a&gt; is incredibly thin, to the point that it is functionally useless for aerobraking or parachutes on descent. This means that any lander would have to burn a lot of fuel on descent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Pluto is &lt;a href=&quot;https://www.space.com/18563-pluto-temperature.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;so cold&lt;/a&gt;, power supply is also an issue. Battery life would be short due to the temperature, so another form of power (like an RTG) would be required for a mission of any serious length.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2018-02-08T13:39:00.017" LastActivityDate="2018-02-08T13:39:00.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21948" PostTypeId="1" CreationDate="2017-06-14T00:48:46.810" Score="13" ViewCount="14510" Body="&lt;p&gt;For the purposes of clarification&lt;br&gt;&#xA; - will include machine code generated by a programming language on earth.&lt;br&gt;&#xA; - 'in space' includes anything in orbit and deep space transit.&lt;br&gt;&#xA; - special mentions of programming language on devices in space appreciated.&lt;br&gt;&#xA; - special mentions of any coding by astronauts in space appreciated.&lt;br&gt;&#xA; - 'popular' = by count of the number of satellites not most 'liked'. although discussion on this is welcome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question follows a reading of the excellent answers to my earlier question &lt;a href=&quot;https://space.stackexchange.com/q/21765/12102&quot;&gt;What was the first programming language in space?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19896" LastEditorUserId="26446" LastEditDate="2018-10-06T17:09:13.750" LastActivityDate="2019-04-29T17:01:47.180" Title="What is the most popular programming language in space?" Tags="|artificial-satellite|communication-satellite|software|computing|" AnswerCount="4" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21949" PostTypeId="1" CreationDate="2017-06-14T04:37:47.943" Score="3" ViewCount="1787" Body="&lt;p&gt;The shuttle had a switch on panel C3 which allowed the crew to trigger a manual separation of the SRB.   This switch had to have a purpose when moved from Auto to manual mode.  What would have happened if the manual separation sequence would have been triggered at 70 seconds into the flight with the SRB at full thrust.   Keep in mind that on the 51L flight both SRBs separated from the stack even without manual separation being triggered.   Would a manual separation at full thrust have any likelihood of success?&lt;/p&gt;&#xA;" OwnerUserId="20052" LastEditorUserId="6944" LastEditDate="2017-06-14T13:05:04.450" LastActivityDate="2022-06-15T12:20:01.203" Title="Could the SRB's have separated successfully at 70 seconds into flight" Tags="|space-shuttle|srb|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21951" PostTypeId="1" AcceptedAnswerId="21969" CreationDate="2017-06-14T05:55:46.263" Score="2" ViewCount="369" Body="&lt;p&gt;I found the following plot in the book &lt;a href=&quot;http://www.springer.com/gp/book/9783540672807&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satellite Orbits; Models, Methods, Applications&lt;/a&gt; by Oliver Montenbruck and Eberhard Gill, Springer, 2000. The figure and description can also be found in &lt;a href=&quot;https://books.google.com.tw/books?id=tLU-wjQktoMC&amp;amp;pg=PA55#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;google books&lt;/a&gt;. It's a low quality snapshot but it's hard to capture a dozen different dependencies over 20 orders of magnitude without showing the whole thing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/21904/12102&quot;&gt;This answer&lt;/a&gt; points out that the orbit-perturbing effects of the Moon on a satellite in LEO are stronger than the orbit-perturbing effects of the changing Earth gravity field &lt;em&gt;due to the Moon&lt;/em&gt; (and Sun, which I'd forgotten about).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After seeing the plot below, I realized that I'd also forgotten about the &lt;em&gt;solid tide&lt;/em&gt;. And then &lt;em&gt;I also noticed&lt;/em&gt; that there is no line for the &lt;em&gt;ocean tide.&lt;/em&gt;  I think these are different - incompressible water flows in order to make a tidal bulge, but can we think of the solid tide similarly - as flowing magma? Does the crustal shell outside just flex and bend to accommodate it? Does the crust significantly restrict it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any idea why ocean tide is not shown, is it's effect on Earth's gravity field so much lower than the solid tide that it wouldn't show up on this twenty orders of magnitude plot?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also notices that the plot for the Dynamic Solid Tide parallels the two &lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt; lines about four orders of magnitude lower than &lt;span class=&quot;math-container&quot;&gt;$J_{2,0}$&lt;/span&gt; (looks like a &lt;span class=&quot;math-container&quot;&gt;$1/r^4$&lt;/span&gt; slope). Does that reflect the relative amplitudes of the Earth's dynamic and static bulges?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lg5CR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lg5CR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="8693" LastEditDate="2020-10-22T21:03:04.827" LastActivityDate="2020-10-22T21:03:04.827" Title="Questions about the Dynamic Solid Tide" Tags="|orbital-mechanics|low-earth-orbit|gravity|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="21952" PostTypeId="2" ParentId="20590" CreationDate="2017-06-14T07:22:32.557" Score="7" Body="&lt;h1&gt;The State Transition Matrix (STM)&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The STM is a &lt;em&gt;linearization&lt;/em&gt; procedure of a dynamical system. It can be used for any non-linear dynamical system and is used to &lt;em&gt;approximate&lt;/em&gt; the dynamics of a system over short period of times. In astrodynamics, it is used especially for statistical orbit determination (stat OD) and the circular restricted third body problem (CRTBP).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Computing the STM for stat OD is explained in depth in &quot;Statistical Orbit Determination&quot; by Tapley, Schultz, Born, Elsevier 2004. Specifically, section 1.2.5 and 4.2.1. From here on in, this reference will be referred to as &quot;(1)&quot;.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;System dynamics&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Let $\boldsymbol{X}$ be the state of your system in a Cartesian frame. In the following, $\bf{r}$ and $\bf{v}$ respectively correspond to the position and the velocity of the spacecraft; $\dot\gamma$ corresponds to the time derivative of the $\gamma$ variable. Choosing position and velocity is &lt;strong&gt;often&lt;/strong&gt; what you'll use for entry-level problems. If doing more &lt;em&gt;serious&lt;/em&gt; stat OD, you'll also want to add in the gravitational parameter, the position of your ground stations, etc. but it's important to note that changing your state vector &lt;strong&gt;will also change&lt;/strong&gt; the STM and the A matrix (cf. below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\boldsymbol{X}=\left[\begin{array}{c}&#xA;\boldsymbol{r}\\&#xA;\boldsymbol{v}&#xA;\end{array}\right]=\left[\begin{array}{c}&#xA;x\\&#xA;y\\&#xA;z\\&#xA;\dot{x}\\&#xA;\dot{y}\\&#xA;\dot{z}&#xA;\end{array}\right]$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can then express the time derivative of the state $\boldsymbol{X}$ as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\boldsymbol{\dot{X}}=\left[\begin{array}{c}&#xA;\boldsymbol{\dot{r}}\\&#xA;\boldsymbol{\dot{v}}&#xA;\end{array}\right]=\left[\begin{array}{c}&#xA;\dot{x}\\&#xA;\dot{y}\\&#xA;\dot{z}\\&#xA;\ddot{x}\\&#xA;\ddot{y}\\&#xA;\ddot{z}&#xA;\end{array}\right]=F\left(\boldsymbol{X}, t\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this formulation, the $F$ function corresponds to the full dynamics of the system: this function is integrated over a period of time if you are computing the real dynamics, i.e. it is a representation of the equations of motion. Assuming the two-body problem, $\boldsymbol{\dot{v}}$ is the acceleration due only to the primary body, i.e. $-\frac{\mu}{r^3}\boldsymbol{X}$. If modeling more complex dynamics, the $F$ function will also include these.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Purpose of the STM&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;As said above, the STM is a linearization of your dynamics. So we start by discretizing the time and &lt;em&gt;assuming&lt;/em&gt; that the system behaves in a linear fashion during that time. This is a very useful approximation. In fact, it allows to simplify the simulation: instead of having to propagate your dynamics (i.e. the $F$ function) over a given integration time, you simply need to multiply the state $X_{i-1}$ with the STM $\Phi$ in order to get $X_i$. Moreover, as per (1), the STM has the following properties (section and page number shown on first line for reference):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/GkcVD.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GkcVD.png&quot; alt=&quot;STM properties&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Computing the STM&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;So as of now, we know that the STM is a linearization of a dynamical system which allows us to consider it as a linear system over a short period of time. So, we need to linearize the dynamics of the system around a given state, herein &lt;em&gt;reference&lt;/em&gt;. This reference is based on the time, and is updated via the STM. In other words, we compute the initial STM, compute the state at the next time, and then re-compute the STM around that new state.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following is an extract from a lecture by Dr. McMahon. What is marked with a star corresponds to the reference state.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vHTLs.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vHTLs.png&quot; alt=&quot;STM computation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can clearly see here that we're simply computing the Taylor series of the $F$ function at the first order! So mathematically this is simple. However, in practice, this corresponds to the derivative of the acceleration, so it is  a bit annoying to compute (but Mathematica or Sage Math (now CoCalc) can help a bunch with their symbolic derivatives, &lt;a href=&quot;https://cocalc.com/projects/1fb6b227-1832-4f82-a05c-7e45614c00a2/files/&quot; rel=&quot;noreferrer&quot;&gt;this might help&lt;/a&gt;). Anyhow, this partial is generally referred to as the $A$ matrix (at least in my experience).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gyO5c.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gyO5c.png&quot; alt=&quot;Relation between A matrix and the STM&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Relation between A matrix and the STM, from &quot;Analysis of the Sun-Earth Lagrangian environment for the New Worlds Observer (NWO)&quot;, Deccia 2017 (&lt;a href=&quot;https://repository.tudelft.nl/islandora/object/uuid%3Ab5b2b94b-3982-4457-a56f-51b5b3574f51?collection=education&quot; rel=&quot;noreferrer&quot;&gt;link&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think a good example is looking at how this can be done in code (these are from my astrodynamics library which is in Golang, sorry... I think/hope it's still relatively readable). Firstly, &lt;a href=&quot;https://github.com/ChristopherRabotin/smd/blob/master/mission.go#L351&quot; rel=&quot;noreferrer&quot;&gt;the computation of the A matrix&lt;/a&gt; with a number of possible perturbations based on the mission configuration. Secondly, &lt;a href=&quot;https://github.com/ChristopherRabotin/smd/blob/master/mission_test.go#L676&quot; rel=&quot;noreferrer&quot;&gt;a series of test cases&lt;/a&gt;. Among other things, the test checks that the norm of the difference between the previous state and the new state (computed via the STM) is within $0.1$ (this is somewhat arbitrary but the state has positions and velocities of a LEO spacecraft, so this is tiny difference). Thirdly, you may wish to check the code source of GMAT (which I have made available &lt;a href=&quot;https://github.com/ChristopherRabotin/GMAT&quot; rel=&quot;noreferrer&quot;&gt;on Github&lt;/a&gt; for convenience -- check their sourceforge repository for the latest updates).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Halo Orbits and the STM&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;From your question, it seems you already know Halo orbits, so I won't dive into these (I'm no expert in them anyway, so I might say wrong things). In short, Halo orbits a quasi-periodic orbits around libration points (they are periodic in the CRTPB). Libration points are equilibrium points between two massive bodies. In effect, an orbit will be periodic for a given time $T$ (and therefore be a Halo orbit) if and only if at half of its period, the motion (i.e. the velocity) of the spacecraft is zero in all but one direction. This &lt;a href=&quot;http://ccar.colorado.edu/imd/2015/documents/SingleShootingHandout.pdf&quot; rel=&quot;noreferrer&quot;&gt;handout&lt;/a&gt; by Dr. Davis (of CCAR at CU Boulder) on finding Halo orbits from an initial guess details how to program this. I will add the following clarifications:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;All computations are done after a normalization between both bodies&lt;/li&gt;&#xA;&lt;li&gt;This solves the Halo orbits only in the circular restricted three body problem. In other problem settings, this method may not apply as such, or at all.&lt;/li&gt;&#xA;&lt;li&gt;$T/2$ corresponds to the half-period time&lt;/li&gt;&#xA;&lt;li&gt;The STM is integrated between time zero and time $T/2$: this is the whole of the discretization period. (If coming from a stat OD background, this time is much much larger than what you would use).&lt;/li&gt;&#xA;&lt;li&gt;The single shooting method allows to find orbits which have at least one period. Halo orbits are unstable by nature, so it is likely that propagating the &quot;final&quot; Halo orbit makes it diverge after more than one orbit (cf. figure below).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xEqQa.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xEqQa.png&quot; alt=&quot;Diverging Halo orbit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Answering your question (hopefully)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why do you want to use the STM to find Halo orbits instead of brute-forcing everything?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Brute forcing is rarely a good idea. It is slow because it looks for all possible solutions. It depends entirely on your dicretization of the solution space. Imagine you set the step size to be 0.5 in the position of the normalized frame but the solution is actually at a 0.2 increment, then your method will never converge.&lt;/li&gt;&#xA;&lt;li&gt;The STM allows to perform several iterations which are getting closer and closer to the Halo orbit. You should expect the algorithm to converge in less than 5-6 iterations (that's nothing compared to a brute force).&lt;/li&gt;&#xA;&lt;li&gt;You refer to a steepest descent. I believe this would involve a gradient descent method for finding global solutions to optimization problems. Gradient descent could be applied to the STM, but it &lt;em&gt;cannot&lt;/em&gt; work with the full dynamics (the system is not linear). In addition, a gradient descent method is applicable to convex problems, but your problem isn't necessarily convex (I don't think it is at all to be honest): you may not find a solution. So you'd have to find a dual problem which is convex, and solve the dual problem. But converting to the dual problem would be very complicated given that you have a non-linear system. Finally, and more importantly than all the math stuff above, what would be the cost function you're minimizing? Where is the optimal problem?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;h2&gt;Code?&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Disclaimer:&lt;/strong&gt; I have &lt;em&gt;not&lt;/em&gt; validated this Matlab code. It &lt;em&gt;may&lt;/em&gt; be buggy, have edge cases, break down in specific cases, etc, etc. But, it may help to get an idea on how to implement this: &lt;a href=&quot;https://gist.github.com/ChristopherRabotin/8293e36daf5454774ce34b5103842273&quot; rel=&quot;noreferrer&quot;&gt;unvalidated code&lt;/a&gt;. (I think I've included all the files needed to run this, but if I haven't, let me know in the comments and I'll add it -- I have no problem sharing my code, quite the contrary)&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="1391" LastEditDate="2017-06-14T07:44:34.213" LastActivityDate="2017-06-14T07:44:34.213" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21954" PostTypeId="2" ParentId="21949" CreationDate="2017-06-14T08:44:36.937" Score="6" Body="&lt;p&gt;The SRB separation sequence is designed to be used &lt;a href=&quot;https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html&quot; rel=&quot;noreferrer&quot;&gt;after thrust drops off enough&lt;/a&gt; that the SRBs will fall behind the orbiter. The separation rockets push the SRB nose out. If you separated the SRB while at full thrust, you'd expose the orbiter and ET to the exhaust of the SRB. At hundreds of tons of thrust, that's a lot of force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760006102.pdf&quot; rel=&quot;noreferrer&quot;&gt;aft attachment points&lt;/a&gt; consist of 3 cylinders with &quot;pistons&quot;, all hinged in a plane perpendicular to the thrust vector. At full thrust, the pistons would be wedged into the cylinders with a large force. This force may be enough to prevent separation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't found a NASA study on early separation. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes&quot; rel=&quot;noreferrer&quot;&gt;standard abort modes&lt;/a&gt; have the SRBs attached to the vehicle until burnout. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If an event requiring an abort happened after SRB ignition, it was not possible to begin the abort until after SRB burnout and separation about two minutes after launch. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-14T08:44:36.937" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21955" PostTypeId="1" AcceptedAnswerId="21956" CreationDate="2017-06-14T09:18:18.687" Score="4" ViewCount="286" Body="&lt;p&gt;The idea behind the Shuttle was &quot;disposable parts are cheap, everything expensive is recovered&quot; - in particular, the three highly advanced engines, plus their systems. The huge but simple fuel tank was discarded. The intent (although not realized) was immense reduction of costs per launch due to heavy reuse of the components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Buran didn't carry the engines - it was strapped to a disposable Energia launcher, with disposable liquid fuel boosters, and I don't really see any obvious advantages (present, or intended but not delivered) over non-recoverable designs. Maybe it would possess the capacity of recovering satellites from orbit (although I never heard of such intent in relation to Buran) but that's a minor advantage and I doubt it would be worth the expense. So - why? &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2018-10-13T07:19:27.813" Title="What intended benefits guided development of Buran?" Tags="|buran|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21956" PostTypeId="2" ParentId="21955" CreationDate="2017-06-14T11:32:44.893" Score="5" Body="&lt;p&gt;The entire Buran programme was &lt;a href=&quot;https://en.wikipedia.org/wiki/Buran_programme#History_of_the_Buran_programme&quot; rel=&quot;nofollow noreferrer&quot;&gt;a response to the US developing the Space Shuttle&lt;/a&gt;. The Soviets saw that a winged orbiter had some unique properties that they wanted to replicate (e.g. the ability to bring back large payloads).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The development of the Buran began in the early 1970s as a response to the U.S. Space Shuttle program. Soviet officials were concerned about a perceived military threat posed by the U.S. Space Shuttle. In their opinion, the Shuttle's 30-ton payload-to-orbit capacity and, more significantly, its 15-ton payload return capacity, were a clear indication that one of its main objectives would be to place massive experimental laser weapons into orbit that could destroy enemy missiles from a distance of several thousands of kilometers. Their reasoning was that such weapons could only be effectively tested in actual space conditions and that to cut their development time and save costs it would be necessary to regularly bring them back to Earth for modifications and fine-tuning.[7] Soviet officials were also concerned that the U.S. Space Shuttle could make a sudden dive into the atmosphere to drop bombs on Moscow.[8]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The book &lt;a href=&quot;https://www.springer.com/gp/book/9780387698489&quot; rel=&quot;nofollow noreferrer&quot;&gt;Energiya-Buran; the Soviet Space Shuttle&lt;/a&gt; (by Hendrickx and Vis) mostly concurs, and adds that the Soviets reasoned, &quot;we don't know what the Shuttle is really intended for but the Americans must be up to &lt;em&gt;something&lt;/em&gt;, so let's replicate the Shuttle's abilities just to be on the safe side.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-10-13T07:19:27.813" LastActivityDate="2018-10-13T07:19:27.813" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="21957" PostTypeId="1" CreationDate="2017-06-14T11:51:09.123" Score="1" ViewCount="241" Body="&lt;p&gt;Maybe this is a (very) stupid question, but anyways, I was wondering about that lately just for fun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So SETI is basically searching (among all the noise coming to use from space) unusual (repeatable) patterns among all the noise from outer-space we capture on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what if this is the wrong way to look for possible alien-signals? What if alien (or us, the same thing) know, that another planet (from &lt;strong&gt;huge&lt;/strong&gt; distances) will receive mostly noise (I know, this already contradicts the stuff below, but anyways) and nothing else (so &lt;em&gt;maybe&lt;/em&gt; quite impossible to detect any useful signal-pattern).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if we consider received noise &lt;strong&gt;as highly random&lt;/strong&gt;, what if aliens transmit &lt;strong&gt;on purpose&lt;/strong&gt; there signal encoded in &lt;strong&gt;super-random-way&lt;/strong&gt;? What I mean to say is what if SETI should look about quite &lt;strong&gt;abnormal-randomness&lt;/strong&gt; (e.g. super random noise) on received signals?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, decoding this super random noise is another question :).&lt;/p&gt;&#xA;" OwnerUserId="11555" LastEditorUserId="415" LastEditDate="2017-06-14T12:20:14.860" LastActivityDate="2017-06-14T12:21:20.283" Title="Why is SETI searching for unusual signal patterns, and not higher levels of randomness than found in background radiation?" Tags="|seti|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21958" PostTypeId="2" ParentId="21957" CreationDate="2017-06-14T12:21:20.283" Score="2" Body="&lt;p&gt;Then we have no way of distinguishing that from natural random noise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even highly compressed data contains patterns, although their complexity is so high, that distinguishing them from natural noise without knowing the algorithm is nearly impossible. A good cryptogram is indistinguishable from random noise without knowing the complete key on top of the algorithm. Good spy techniques hide even the signal level below the background noise level (and provide a lot of error correction to allow extraction of that signal) so that even looking for sources of random noise won't locate the source of the signal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your intent is to be detected, you will create signal as distinct from natural &quot;noise&quot; as possible, a clear order. And it doesn't matter how advanced technology you have at your disposal, you still use primitive, clear means to convey your message. A rescue ship, despite complex radars and advanced radio, will have a mast with powerful light source, to be visible from a long distance by the stranded, so they could signal their presence, e.g. using a flare or lighting a fire. A fighter plane intercepting an unidentified (civilian) airplane illegally entering the airspace, despite all the complex electronics, besides hailing on unencrypted channel of general hailing frequency, will shine a spotlight at the cockpit of the plane, making itself obviously visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because recognizing and distinguishing simple, clear and obvious signals is way easier - and way more likely, than spotting discrepancies in white noise. SETI too doesn't really hope to encounter a signal addressed &lt;em&gt;to us&lt;/em&gt;. It seeks signals exchanged between the aliens, trying to recognize and distinguish them. And since these will most likely be compressed (bandwidth savings!) they hope for spotting patterns.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-06-14T12:21:20.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21959" PostTypeId="1" AcceptedAnswerId="21961" CreationDate="2017-06-14T13:03:50.327" Score="3" ViewCount="1539" Body="&lt;p&gt;Certain people (me) will have hours of fun in the &lt;a href=&quot;http://semiconductormuseum.com/Museum_Index.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transistor Museum&lt;/a&gt; website. Who can resist stuff like this: &lt;a href=&quot;http://semiconductormuseum.com/PhotoGallery/Photogallery_General_Transistor_General_Instrument_Historic_Germanium_Computer_Transistors.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Historic Germanium Transistors; General Transistor/General Instrument (5MB pdf)&lt;/a&gt;?&lt;/p&gt;&#xA;&lt;p&gt;I found there a &lt;a href=&quot;http://semiconductormuseum.com/MuseumLibrary/Final_Transistor_Museum_Book_Review_Opening_Space_Research_by_George_Ludwig.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;book review of &lt;em&gt;Opening Space Research; Dreams, Technology, and Scientific Discovery&lt;/em&gt;&lt;/a&gt;, Ludwig, George H., American Geophysical Union, Washington DC, 2011. (478pp), ISBN: 978-0-87590-733-8.&lt;/p&gt;&#xA;&lt;p&gt;There I saw these photos, including a &lt;em&gt;thanks for the transistors!&lt;/em&gt; note from James Van Allen to Texas Instruments. I believe these particular transistors are silicon, but it got me wondering:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; The first transistors in space: Germanium or Silicon? What about in orbit?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below x2:&lt;/strong&gt; from the Transistor Museum's &lt;a href=&quot;http://semiconductormuseum.com/MuseumLibrary/Final_Transistor_Museum_Book_Review_Opening_Space_Research_by_George_Ludwig.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;book review&lt;/a&gt; of &lt;em&gt;Opening Space Research; Dreams, Technology, and Scientific Discovery&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P81a0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P81a0.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bYquk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bYquk.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-18T01:44:34.173" LastActivityDate="2021-08-18T01:44:34.173" Title="The first transistors in space: Germanium or Silicon? What about in orbit?" Tags="|history|electronics|spaceflight-firsts|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="21960" PostTypeId="2" ParentId="21949" CreationDate="2017-06-14T13:04:48.760" Score="11" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The reason that the SRBs couldn't be &amp;quot;jettisoned&amp;quot; at any desired time&#xA;is because the separation system as designed was not powerful enough&#xA;to safely jettison the boosters if they were delivering a significant&#xA;amount of thrust. The separation system simply severed the bolts&#xA;holding the boosters to the External Tank and fired booster-mounted&#xA;motors to provide clearance between them and the accelerating&#xA;Orbiter/ET stack. If significant thrust loads had been present, then&#xA;significant forces and moments would have been applied to the&#xA;Orbiter/ET stack. It is possible that a more robust system could have&#xA;been designed, but it wasn't incorporated in the system we had.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From my answer to &lt;a href=&quot;https://space.stackexchange.com/questions/10600/what-is-the-problem-with-aborting-a-space-shuttle-launch/10605#10605&quot;&gt;this question&lt;/a&gt; (which see for references, etc).&lt;/p&gt;&#xA;&lt;p&gt;Edit: the final report of the Rogers Commission on the 51-L accident explicitly states that the SRBs cannot be safely separated while producing thrust: (italics mine)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Findings&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The Space Shuttle System was not designed to survive a failure of the Solid Rocket Boosters.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;There are no corrective actions that can be taken if the boosters do&#xA;not operate properly after ignition, i.e., &lt;em&gt;there is no ability to&#xA;separate an Orbiter safely from thrusting boosters&lt;/em&gt; and no ability for&#xA;the crew to escape the vehicle during first-stage ascent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://sma.nasa.gov/SignificantIncidents/assets/rogers_commission_report.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rogers Commission Report&lt;/a&gt;, page 187.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="47506" LastEditDate="2022-06-15T12:20:01.203" LastActivityDate="2022-06-15T12:20:01.203" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="21961" PostTypeId="2" ParentId="21959" CreationDate="2017-06-14T14:45:41.950" Score="7" Body="&lt;p&gt;The first transistors in space (and in orbit) were likely germanium, and they were aboard Sputnik-2 in 1957, though Sputnik-2 did use vacuum tubes as well.&lt;/p&gt;&#xA;&lt;p&gt;The first all-transistor satellite was Explorer 1, as explained by George Ludwig:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Was the Explorer I the first earth satellite to carry transistor electronics?&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;In my original interview I said that it was.  Since then, I researched the question and found that, although the Soviets did not use transistors in Sputnik I, Sputnik II, launched in November 1957, did use a few transistors in one of its instruments.  Of course they had a much larger launch capacity and could carry vacuum tubes and their required batteries.  The Explorer I launch occurred on January 31st local time, which was actually February l Greenwich time, of 1958.  It was the first all-transistor satellite.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Explorer 1 used a combination of germanium and silicon transistors.&lt;/p&gt;&#xA;&lt;p&gt;Sputnik-2 and Explorer 1 were the second and third satellites to orbit Earth, respectively.&lt;/p&gt;&#xA;&lt;p&gt;The first computer in space (and first computer in orbit) was the Gemini digital computer, built by IBM, which first flew in 1965. It was IBM's first completely silicon semiconductor computer.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Sources:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://mentallandscape.com/S_Sputnik2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sputnik-2 - Don P. Mitchell&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://semiconductormuseum.com/Transistors/LectureHall/Ludwig/Ludwig_Page5.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Transistor Museum Interview with Dr. George Ludwig - The First Transistors in Space - Personal Reflections by the Designer of the Cosmic Ray Instrumentation Package for the Explorer I Satellite - Page 5&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://semiconductormuseum.com/Transistors/LectureHall/Ludwig/Ludwig_Index.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Transistor Museum Interview with Dr. George Ludwig - The First Transistors in Space - Personal Reflections by the Designer of the Cosmic Ray Instrumentation Package for the Explorer I Satellite - Index&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://history.nasa.gov/computers/ch1-1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Computers in Spaceflight: The NASA Experience - Chapter One - The Gemini Digital Computer: First Machine in Orbit - Page 1 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://history.nasa.gov/computers/Ch1-2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Computers in Spaceflight: The NASA Experience - Chapter One - The Gemini Digital Computer: First Machine in Orbit - Page 2 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://apennings.com/how-it-came-to-rule-the-world/the-cold-war/subsidizing-silicon-nasa-and-the-computer/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Subsidizing Silicon: NASA and the Computer - Anthony J. Pennings&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://history.nasa.gov/computers/Ch1-5.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Computers in Spaceflight: The NASA Experience - Chapter One - The Gemini Digital Computer: First Machine in Orbit - Page 5 - NASA&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-14T16:34:50.290" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21962" PostTypeId="2" ParentId="21948" CreationDate="2017-06-14T15:53:15.353" Score="4" Body="&lt;p&gt;C would seem like a good guess: C is popular for embedded systems, satellites are embedded systems, therefore C is popular for satellites. There are a lot of holes you can poke in this line of reasoning, and of course many languages have been used to varying degrees of popularity. In the absence of a survey of languages used that would answer this question decisively, the examples here can give some context.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that since a significant number of satellites do not have publicly available information, such as military and spy satellites. One could guess that Ada may be more popular than a general survey would suggest, given it's widespread use in the US military in the past. More information on Ada is provided in this &lt;a href=&quot;https://space.stackexchange.com/q/14605/4686&quot;&gt;related question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A specific example of C's use: NASA's Core Flight System (&lt;a href=&quot;http://coreflightsystem.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;cFS homepage&lt;/a&gt;) is written in C, and is in use on several missions right now (&lt;a href=&quot;https://space.stackexchange.com/q/17353/4686&quot;&gt;related question&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Smaller platforms such as CubeSats may change the languages in space distribution as well: there are many of them and they have a much higher risk tolerance (and lower success rate). From the &lt;a href=&quot;http://digitalcommons.usu.edu/smallsat/2015/all2015/32/&quot; rel=&quot;nofollow noreferrer&quot;&gt;LightSail paper&lt;/a&gt; presented at SmallSat 2015, &quot;LightSail FSW (software and firmware) is written in the &#xA;C programming language.&quot; LightSail also ran a Linux distribution, again written in C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assembly was also a popular language, at least historically, according to Ron Garret who wrote the following about working at JPL in 1988: &quot;Spacecraft were mostly programmed in assembler, or, if you were really being radical, Ada.&quot; The &lt;a href=&quot;http://www.flownet.com/gat/jpl-lisp.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; with this quote documents the work done on Remote Agent, an experimental LISP program that controlled the &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_1&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Deep Space 1&quot;&gt;Deep Space 1&lt;/a&gt; satellite for two days during its mission.&lt;/p&gt;&#xA;" OwnerUserId="4686" LastEditorUserId="4686" LastEditDate="2017-06-16T12:26:15.900" LastActivityDate="2017-06-16T12:26:15.900" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21963" PostTypeId="2" ParentId="21532" CreationDate="2017-06-14T16:39:19.380" Score="2" Body="&lt;p&gt;The L-3 Communications Control Moment Gyroscopes (CMGs) used on the ISS have specifications detailed in the &lt;a href=&quot;http://www2.l-3com.com/spacenav/pdf/datasheets/CMG.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;L-3 CMG Control Moment Gyro datasheet&lt;/a&gt;. Here are the properties you are looking for:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Weight 600 lb., 272 kg&lt;/p&gt;&#xA;&lt;p&gt;Maximum Output&#xA;Torque 258 Newton-meter&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It's not specified, but I see no reason why the minimum torque wouldn't be zero.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-14T16:39:19.380" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21964" PostTypeId="2" ParentId="5405" CreationDate="2017-06-14T18:03:31.563" Score="7" Body="&lt;p&gt;The flight crew had a contingency procedure for this.   On Panel C3 a switch allowed the crew to manually initiate the  SRB separation sequence should the software fail to trigger it automatically.   I don't believe this required a call up from the flight controllers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lCsq9.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lCsq9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;     &lt;/p&gt;&#xA;" OwnerUserId="20052" LastActivityDate="2017-06-14T18:03:31.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21965" PostTypeId="1" AcceptedAnswerId="21967" CreationDate="2017-06-14T18:15:35.723" Score="3" ViewCount="937" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/21961/12102&quot;&gt;The answer&lt;/a&gt; to my question &lt;em&gt;The first transistors in space: Germanium or Silicon? What about LEO?&lt;/em&gt; actually addressed both the first Germanium and Silicon transistor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now I've gotta ask about electron tubes (valves). It sounds like Sputnik-1 and Sputnik-2 had tubes, although 2 had a mix of both. But after that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are types of electron devices are used in RF amplifiers (e.g. klystrons, traveling wave tubes) and of course photomultiplier tubes are &lt;a href=&quot;https://space.stackexchange.com/a/21945/12102&quot;&gt;still being sent to space regularly&lt;/a&gt; (there - the ISS version is scheduled for 2020) and these could count as interesting answers, but what I'm primarily looking for is the small, compact devices like you'd find in an old radio, with the filament, grid(s) and plate all right next to each other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The significance is that even then, transistors required so much less power than vacuum tubes because they didn't require heated cathodes (filaments) for thermionic emission of carriers. By doping a low band-gap semiconductor material, carriers could be generated at 0C rather than cherry-red hot. That meant that you could power a 1 watt transmitter with a few watts of electrical power, something sustainable with photovoltaics, and that was critical for making satellites long-lived.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course if you want to go to Venus, you just choose a &lt;a href=&quot;https://space.stackexchange.com/a/20096/12102&quot;&gt;higher band-gap semiconductor&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; An assortment of electron tubes (vacuum tubes, valves); &quot;Eine Zusammenstellung von Elektronenröhren.&quot; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Elektronenroehren-auswahl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jd9UD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jd9UD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-07-21T16:35:56.977" LastActivityDate="2019-07-21T16:35:56.977" Title="When was the last normal electron tube (valve) for electrical circuitry launched into space?" Tags="|history|electronics|record|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21966" PostTypeId="2" ParentId="9261" CreationDate="2017-06-14T18:21:39.697" Score="3" Body="&lt;p&gt;Production for SRB segments was  as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Tiny\begin{array}{lllllllllll}&#xA;\textbf{Production Batches} &amp;amp; \text{STD FWD} &amp;amp; \text{STD} &amp;amp; \text{newCF} &amp;amp; \text{LWT} &amp;amp; \text{newCF} &amp;amp; \text{STD} &amp;amp; \text{LWT} &amp;amp; \text{STD  } &amp;amp; \text{LWT  } &amp;amp; \text{STD AFT} \\&#xA;                            &amp;amp; \text{Dome   } &amp;amp; \text{CYL} &amp;amp; \text{SCYL } &amp;amp; \text{CYL} &amp;amp; \text{LCYL } &amp;amp; \text{ATT} &amp;amp; \text{ATT} &amp;amp; \text{STIFF} &amp;amp; \text{STIFF} &amp;amp; \text{Dome}    \\&#xA;\text{Actual}               &amp;amp; 61      &amp;amp; 94  &amp;amp; 77    &amp;amp; 124 &amp;amp; 101   &amp;amp; 15  &amp;amp; 62  &amp;amp; 73    &amp;amp; 97    &amp;amp; 65   &#xA;\end{array}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Segments came in several flavors. &#xA;  STD=Standard Weight (original production run)&#xA;  LWT=Lightweight (MTI shaved the case down to reduce weight, with the consequent increase in ovality)&#xA; newCF=new configuration (the RSRM with capture feature as a newly manufactured segment) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All existing segments were remilled to include the capture feature. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;newCf were manufactured in both:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SCYL= Standardweight Cylinder&#xA; LCYL=Lightweight Cylinder. &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; As far as the placement of the cylinders there is 4 segments in a SRB.&#xA;&#xA; FWD segment made up of a fwd dome and two STD cylinders&#xA; FWD CNTR segment made up of two cylinders&#xA; AFT CNTR made up of two cylinders&#xA; AFT segment made up of the ET ATT cylinder, two stiffener cylinders and the AFT Dome&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="20052" LastEditorUserId="20052" LastEditDate="2017-06-30T15:12:49.967" LastActivityDate="2017-06-30T15:12:49.967" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21967" PostTypeId="2" ParentId="21965" CreationDate="2017-06-14T18:55:47.057" Score="7" Body="&lt;p&gt;Electron tubes were still used in the &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1011&amp;amp;context=etd&quot; rel=&quot;noreferrer&quot;&gt;Kurs system&lt;/a&gt; on the Soyuz-MS and Progress-MS until &lt;a href=&quot;http://www.russianspaceweb.com/soyuz-ms-kurs-na.html&quot; rel=&quot;noreferrer&quot;&gt;March 19, 2016&lt;/a&gt;, when the last of the old Kurs was launched. Since then, Soyuz-MS and Progress-MS launches have used the new Kurs-NA system. I have not been able to find if the new system still uses electron tubes, but if it does then today's &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_MS-06&quot; rel=&quot;noreferrer&quot;&gt;Progress-MS launch&lt;/a&gt; [June 14, 2017] would be the most recent electron tube launched.&lt;/p&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2017-06-14T19:08:33.010" LastActivityDate="2017-06-14T19:08:33.010" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21968" PostTypeId="2" ParentId="21965" CreationDate="2017-06-14T20:05:22.953" Score="2" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=6&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiCkq3TkL7UAhXnyoMKHRyUCbcQFggyMAU&amp;amp;url=http%3A%2F%2Fieeexplore.ieee.org%2Fabstract%2Fdocument%2F6697488%2F&amp;amp;usg=AFQjCNGMUey_kqJxHQwpSXLFm55k1JPJgg&amp;amp;sig2=xWNADq6EbbK0aY9lBdcbZA&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt; there is currently a system which includes a traveling wave tube amplifier mounted on an exterior truss of the ISS. Not sure if that is ground-ruled out by your question or not (I am not sure what a Klystron is).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-06-14T20:05:22.953" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21969" PostTypeId="2" ParentId="21951" CreationDate="2017-06-14T22:02:18.267" Score="2" Body="&lt;p&gt;What Montenbruck and Gill call the &quot;dynamic solid tide&quot; is exactly what I was referring to in &lt;a href=&quot;https://space.stackexchange.com/a/21904/2752&quot;&gt;my answer&lt;/a&gt; to your related question.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Can we think of the solid tide similarly - as flowing magma?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Absolutely not. There are isolated pockets of partial melt near the crust/mantle boundary, and a layer of weak material just below (the asthenosphere). However, it's best to look at the Earth's crust and mantle as solid rock rather than flowing magma. On the timescale of half of a day (the timescale of the key tidal components, even the asthenosphere is rock solid. There is no tidal flow in the solid body tides. Just because it's &quot;solid&quot; doesn't mean that it doesn't deform under pressure. It does. As an extreme, look to the Earth's solid inner core. It's most iron and nickel, compressed by 40%.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Any idea why ocean tide is not shown, is it's effect on Earth's gravity field so much lower than the solid tide that it wouldn't show up on this twenty orders of magnitude plot?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Modeling the ocean tides is ridiculously hard. If you're working on model the orbits of the LAGEOS satellites, you'll need to worry about ocean tides. Otherwise, you can pretty much forget about them. They're a couple of orders of magnitude smaller than are the time-varying effects of the solid body tides.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I also notices that the plot for the Dynamic Solid Tide parallels the two $J_2$ lines about four orders of magnitude lower than $J_{2,0}$ (looks like a $1/r^4$ slope). Does that reflect the relative amplitudes of the Earth's dynamic and static bulges?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The primary influence of the solid body tides is to make the supposedly constant $J_2$ terms vary over time. So the effect is roughly $1/r^4$, the same as for the $J_2$ terms. The solid tides also affect the $J_3$ and $J_4$ harmonics, but since those fall as $1/r^5$ and $1/r^6$, it's the changes to the $J_2$ terms that dominates. And yes, that four-plus order of magnitude reduction compared to $J_{2,0}$ is what I meant in my previous answer when I wrote that these effects are small.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-06-14T22:02:18.267" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21971" PostTypeId="1" CreationDate="2017-06-15T14:22:46.550" Score="9" ViewCount="1591" Body="&lt;p&gt;According to the cubesat design specification electrical requirements (3.3.7 in &lt;a href=&quot;https://static1.squarespace.com/static/5418c831e4b0fa4ecac1bacd/t/56e9b62337013b6c063a655a/1458157095454/cds_rev13_final2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;revision 13&lt;/a&gt;) a Cubesat shall include an RBF pin:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;3.3.7 The CubeSat shall include an RBF pin.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;3.3.7.1 The RBF pin shall cut all power to the satellite once it is inserted into the satellite.&lt;/li&gt;&#xA;&lt;li&gt;3.3.7.2 The RBF pin shall be removed from the CubeSat after integration into the P-POD.&lt;/li&gt;&#xA;&lt;li&gt;3.3.7.3 The RBF pin shall protrude no more than 6.5 mm from the rails when it is fully inserted into the satellite.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I'm wondering how these kind of RBF pins are realized according to the cubesat standard, but also whether there are other mechanisms that have been used in other missions.&lt;/p&gt;&#xA;&lt;p&gt;I tried doing extensive internet searches, especially for publications or technical documents, as I am very much interested in the actual electrical solution of a RBF pin and whether anyone ever tried a solution where for example you don't pull something out but put something in.&lt;/p&gt;&#xA;&lt;p&gt;From what I gather even if you put in an RBF pin according to the cubesat standard there are still currents flowing around, as the RBF pin is to short the Base and Collector of a transistor thus preventing it from opening. But this needs pull up resistors and causes current bleed even if the satellite is to be off. It seems kinda silly to have a constant power flow that drains your batteries when you don't want the satellite to do anything.&lt;/p&gt;&#xA;&lt;p&gt;Could anyone tell me of other RBF pin mechanisms, specifically ones where there is a galvanic isolation of the batteries and the rest of the satellite using an RBF pin? I'd be very happy about technical documents or papers that I could look into.&lt;/p&gt;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="20038" LastEditorUserId="6944" LastEditDate="2020-09-04T23:24:23.250" LastActivityDate="2020-09-04T23:24:23.250" Title="How are remove before flight pin mechanisms realized in satellites?" Tags="|artificial-satellite|cubesat|mechanisms|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="21972" PostTypeId="1" CreationDate="2017-06-15T17:50:25.757" Score="17" ViewCount="2935" Body="&lt;p&gt;In a previous thread, I asked the question of the likelihood of the STS stack surviving an early separation of the SRB using the SRB manual separation switch on panel C3:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/21949/could-the-srbs-have-separated-successfully-at-70-seconds-into-flight&quot;&gt;Could the SRB&amp;#39;s have separated successfully at 70 seconds into flight&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/f9ZRN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f9ZRN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Estimates ranged from the &quot;Official&quot; accepted answer of:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Space Shuttle System was not designed to survive a failure of the Solid Rocket Boosters.&#xA;  There are no corrective actions that can be taken if the boosters do not operate properly after ignition, i.e., there is no ability to separate an Orbiter safely from thrusting boosters and no ability for the crew to escape the vehicle during first-stage ascent&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To an estimate I received from a NASA engineer of 75% chance of successful separation.  My educated guess, based upon herculean design efforts made to get maximum separation from the stack, is that a manual separation before booster burnout had a 50/50 chance of destroying the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The question:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose you are in the flight control room on January 28th, 1986.  You are a support person for FIDO, perhaps abort support or range safety.  (I don't want to label any specific party, so lets leave it general)   You look up to John Aaron and have made a point to have lunch with him regularly at JSC.  You have listened to him tell the story of how on the Apollo 12 launch he responded to an unexpected environmental condition (lighting strike).  He used his extensive knowledge of how the systems work and more important how they fail.   As a result of his extra effort, he was able to make a completely unrehearsed call up (SCE to AUX) that saved the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://motherboard.vice.com/en_us/article/john-aaron-apollo-12-curiosity-luck-and-sce-to-aux&quot; rel=&quot;noreferrer&quot;&gt;https://motherboard.vice.com/en_us/article/john-aaron-apollo-12-curiosity-luck-and-sce-to-aux&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to a thousand other details, you have looked into the possible use of the SRB man separation switch in odd failure conditions.   What if a booster suffered a case crack or insulation failure?  What would that look like in the data?   Would I be able to detect it?   Could I respond to it?   You have determined that the best way to detect a case breach would be visually.   SRM engineers tell you that if it occurs it will propagate in seconds, so the response would need to be quick.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your routine at launches has been to watch the data but to also watch the live feed tracking cameras to look for visual anomalies.  ( like your mentor John Aaron, you do odd extra things because you see value, no matter what the &quot;Official&quot; position is).   You don't know what you will do if you think you see an anomaly but you know the SRB manual separation would probably be the only chance, even if the likelihood of success is low.&#xA;On 51L you are watching the following live feed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/GKEDYRtyvQg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 16:39:01  you get the pit in your stomach moment (What the **** is that??).   You know your John Aaron moment has arrived.   Is that really a case breach?? By 16:39:04 you are sure.    Months of thinking about it are upon you.   Make the call,    &quot;FIDO, SRB MAN SEP NOW!!&quot;     &quot;Look at the ROTI video!!!       MAKE IT...&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Within 3 seconds, by 16:39:07, the trust between you and FIDO and FLIGHT has resulted in the call up to the flight crew &quot;Challenger SRB MAN SEP,   Dick, MAN SEP NOW!!&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The response takes another 3 seconds by 16:39:10, the SRB separation process has started. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality it was another 3 seconds before the RH aft ET attachment was destroyed and the RH Booster rotated into the ET and began the destruction of the vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Luckily the high winds aloft on that day assist in the separation.  That and good design are on NASA's side and the SRB's separate and clear the accelerating stack.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did 51L simply need a John Aaron mentor in Flight control that day??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If he had been, would the call up have been made??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or was the official conclusion the correct one, &lt;em&gt;ye who enter here abandon all hope&lt;/em&gt; &lt;strong&gt;..at least till SRB burnout&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="20052" LastEditorUserId="6944" LastEditDate="2020-09-01T17:43:59.267" LastActivityDate="2020-09-01T17:43:59.267" Title="Was a method available to save the Challenger Crew?" Tags="|space-shuttle|failure|abort|srb|shuttle-challenger|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="21973" PostTypeId="2" ParentId="21972" CreationDate="2017-06-15T18:23:55.987" Score="18" Body="&lt;p&gt;My reading of the function of the SRB manual separation control is that it would &lt;a href=&quot;https://spaceflight.nasa.gov/shuttle/reference/shutref/events/1stage/&quot; rel=&quot;noreferrer&quot;&gt;not actually trigger a separation&lt;/a&gt; while the boosters were firing (bold mine):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SRB separation is normally performed automatically by the onboard GPCs; however, the flight crew can command separation through use of the SRB separation switches on panel C3. The SRB separation auto/man (manual) switch is positioned to man and the SRB sep push button depressed.&lt;/p&gt;&#xA;&lt;p&gt;This manual function for SRB separation provides a backup for the automatic function; &lt;strong&gt;however, the manual function uses the same separation logic as the automatic&lt;/strong&gt;. The automatic sequence is initiated by the software in the GPCs when the SRB chamber pressure is below 50 psi.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I haven't found a more detailed confirmation of this interpretation, however.&lt;/p&gt;&#xA;&lt;p&gt;I suspect that even if a manual separation was available and would not have destroyed the spacecraft if attempted, it wouldn't have been possible for a controller to call for the separation and have it relayed quickly enough to make a difference.&lt;/p&gt;&#xA;&lt;p&gt;By 16:39:07 the ET is already compromised and the shuttle's fate is thereby sealed. From the point where the controller makes the call at 16:39:04, all of this has to happen in &lt;em&gt;three seconds&lt;/em&gt;:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;One or two people up the decision chain (FIDO and Flight Director in your scenario) have to hear, comprehend, and &lt;em&gt;unconditionally accept&lt;/em&gt; the call&lt;/li&gt;&#xA;&lt;li&gt;Capcom has to repeat the instruction to the spacecraft&lt;/li&gt;&#xA;&lt;li&gt;Spacecraft commander has to hear and comprehend the instruction&lt;/li&gt;&#xA;&lt;li&gt;Spacecraft commander has to locate the SRB sep switches&lt;/li&gt;&#xA;&lt;li&gt;Spacecraft commander has to operate two switches while under 3G acceleration&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;I don't think this is a possible scenario; just repeating the call from supporting controller to FIDO to Flight to Capcom might take 8 seconds.&lt;/p&gt;&#xA;&lt;p&gt;Compare with the &lt;a href=&quot;https://history.nasa.gov/afj/ap12fj/01launch_to_earth_orbit.html&quot; rel=&quot;noreferrer&quot;&gt;timeline of the &amp;quot;SCE to AUX&amp;quot; incident on Apollo 12&lt;/a&gt; (trimmed down to a few key points):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;000:00:37 Gordon (onboard): What the hell was that?&lt;/p&gt;&#xA;&lt;p&gt;000:00:43 Conrad (onboard): Roger. We had a whole bunch of buses drop out.&lt;/p&gt;&#xA;&lt;p&gt;000:00:56 Conrad (onboard): I just lost the platform.&lt;/p&gt;&#xA;&lt;p&gt;000:01:02 Conrad: Okay, we just lost the platform, gang. I don't know what happened here; we had everything in the world drop out.&lt;/p&gt;&#xA;&lt;p&gt;000:01:08 Carr: Roger.&lt;/p&gt;&#xA;&lt;p&gt;000:01:36 Carr: Apollo 12, Houston. Try SCE to auxiliary. Over.&lt;/p&gt;&#xA;&lt;p&gt;000:01:39 Conrad: Try FCE to Auxiliary. What the hell is that?&lt;/p&gt;&#xA;&lt;p&gt;000:01:41 Conrad: NCE to auxiliary...&lt;/p&gt;&#xA;&lt;p&gt;000:01:42 Gordon (onboard): Fuel cell...&lt;/p&gt;&#xA;&lt;p&gt;000:01:43 Carr: SCE, SCE to auxiliary. [Long pause.]&lt;/p&gt;&#xA;&lt;p&gt;000:01:50 Conrad (onboard): SCE to Aux.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The crew's first report of a problem to the ground comes at 1:02, 25 seconds after Gordon sees a problem; Capcom Carr doesn't even &amp;quot;roger&amp;quot; to Conrad's call for six seconds. In mission control, they should already have seen, by this time, that the telemetry is bad. In particular, we expect that John Aaron is seeing the glitch by this point.&lt;/p&gt;&#xA;&lt;p&gt;It's almost 30 seconds later at 1:36 that the first &amp;quot;SCE to Aux&amp;quot; call goes up to the ship; they have no idea what Carr is talking about and Carr has to repeat the call at 1:43. It's unclear if Conrad's &amp;quot;SCE to Aux&amp;quot; at 1:50 is him confirming he's flipped the switch, but assuming that it is, we're looking at 14 seconds from instruction to operation and probably at least 40 seconds since Aaron saw the problem.&lt;/p&gt;&#xA;&lt;p&gt;In this case, also, &amp;quot;SCE to Aux&amp;quot; is a very low-stakes call, easy to make. If the problem &lt;em&gt;isn't&lt;/em&gt; with the signal conditioning equipment, all Aaron's call does is waste a little time, and at this point the launcher itself seems to be doing fine. An emergency booster sep on the shuttle would be at best a mission failure, and depending on who you ask, anywhere from a 25% to 99% chance of loss of crew.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-15T19:45:59.003" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="21974" PostTypeId="1" AcceptedAnswerId="21987" CreationDate="2017-06-15T19:40:15.920" Score="1" ViewCount="156" Body="&lt;p&gt;If we talk to a person on mars it would take about 30 minutes before they would see the communication, but lets say we stayed in contact 24/7 by video calling starting from Earth, what would happen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the video start to lag? Where would we first notice the difference?&lt;/p&gt;&#xA;" OwnerUserId="20069" LastActivityDate="2017-06-16T11:21:02.113" Title="What would happen to communication if we stayed in contact with a person going to mars?" Tags="|mars|earth|travel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21975" PostTypeId="2" ParentId="18329" CreationDate="2017-06-15T23:02:02.777" Score="-3" Body="&lt;p&gt;The &lt;em&gt;He&lt;/em&gt; is stored either as a very compressed gas (high pressure tank) at &lt;em&gt;LOX&lt;/em&gt; temperature  &lt;strong&gt;OR&lt;/strong&gt; it is stored as a even colder 4K liquid (moderate pressure dewar).   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of &lt;em&gt;He&lt;/em&gt; must be sufficient to expand &lt;strong&gt;or&lt;/strong&gt; vaporise to fill empty &lt;em&gt;LOX&lt;/em&gt; &lt;strong&gt;and&lt;/strong&gt; &lt;em&gt;fuel&lt;/em&gt; tanks to flight pressure.  Remember that &lt;em&gt;liquid He&lt;/em&gt; takes up a lot less volume than compressed &lt;em&gt;He&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Either technique will work:   &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;if using a compressed &lt;em&gt;He&lt;/em&gt; gas tank then service valve is closed until &lt;em&gt;LOX/fuel&lt;/em&gt; tanks need pressure in flight, &lt;em&gt;He&lt;/em&gt; filling can take place at any time and pressure is ready immediately. Some heat transfer may occurs during filling depending of speed and liquid/gas filling method. During flight the pressurised gas is heated in the engine to prevent the introduction of &lt;em&gt;He&lt;/em&gt; into tanks at low temperatures caused by adiabatic (Joule-Thompson) cooling from causing &lt;em&gt;LOX/fuel&lt;/em&gt; to freeze and block pipes.    &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;if &lt;em&gt;liquid He&lt;/em&gt; dewar then filling can occur before, during or after LOX/fuel filling.  There will be no heating when &lt;em&gt;liquid He&lt;/em&gt; is filled, rather some cooling will occur as the inside of the dewar reaches 4K. The boil-off is vented until tank pressure is needed at which point vent is closed and boil-off will pressurise to &lt;em&gt;LOX/fuel&lt;/em&gt; tank to working pressure.  During flight the boil-off speed is  maintained by directing the &lt;em&gt;liquid He&lt;/em&gt; though the engine to boil it and maintain the pressure in the &lt;em&gt;LOX/fuel&lt;/em&gt; tank vapour spaces.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The heating of the &lt;em&gt;He&lt;/em&gt; will provide some welcome cooling of the engine for no additional cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both options gain a benefit from placement in the &lt;em&gt;LOX&lt;/em&gt; tank.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The compressed &lt;em&gt;He&lt;/em&gt; option gains from space saving as the geometric voids around a high pressure tank are filled with &lt;em&gt;LOX&lt;/em&gt; and not wasted space.     &lt;/li&gt;&#xA;&lt;li&gt;The &lt;em&gt;liquid He&lt;/em&gt; option gains from having free primary temperature reduction around the dewar to &lt;em&gt;LOX&lt;/em&gt; temperatures while on the pad and being able to use &lt;em&gt;liquid He&lt;/em&gt; in a thin wall dewar because the differential internal pressure in the dewar is only that which is needed to feed the &lt;em&gt;liquid He&lt;/em&gt; through the engine based heat exchanger.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The alternative of an external tank requires very strong tank for high pressure gas or strong well insulated dewar for liquid.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The low temperature of the expanding &lt;em&gt;He&lt;/em&gt; would be a potential problem in the &lt;em&gt;fuel&lt;/em&gt; tank as it would freeze around the dewar and pipes in flight and the benefits of the cryogenic &lt;em&gt;LOX&lt;/em&gt; temperatures are lost if the &lt;em&gt;liquid He&lt;/em&gt; option is used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many of the comments in this thread point to the fact that the &lt;em&gt;He&lt;/em&gt; in the &lt;em&gt;LOX&lt;/em&gt; tank is liquid and this would have the most benefits from moving the &lt;em&gt;He&lt;/em&gt; storage inside the &lt;em&gt;LOX&lt;/em&gt; tank.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt;&lt;br&gt;&#xA;It is clear that the Helium in the tank is initially colder than the LOX.  It also appears that the &lt;em&gt;He&lt;/em&gt; tanks are of complex structure with an aluminium internal liner surrounded by a composite strength member.  The problem during filling seems to have been the cold rather than heat.  The &lt;em&gt;He&lt;/em&gt; tanks seem to be able to handle high pressures rather than large temperature differences.  It appears that they are filled with &lt;em&gt;liquid He&lt;/em&gt; and then hold pressurised &lt;em&gt;He&lt;/em&gt; as they heat up to &lt;em&gt;LOX&lt;/em&gt; temperature.  The slower filling technique that has been proposed may use pressurised &lt;em&gt;He&lt;/em&gt; to fill them causing MORE heat load rather than less but perhaps avoiding the suspected solid &lt;em&gt;O2&lt;/em&gt; formation.  &lt;a href=&quot;https://www.geekwire.com/2017/spacex-falcon-failed-helium-tank-iridium-jan-8/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.geekwire.com/2017/spacex-falcon-failed-helium-tank-iridium-jan-8/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17484" LastEditorUserId="17484" LastEditDate="2017-06-16T00:32:09.737" LastActivityDate="2017-06-16T00:32:09.737" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="21977" PostTypeId="1" AcceptedAnswerId="21979" CreationDate="2017-06-16T01:06:26.783" Score="34" ViewCount="11138" Body="&lt;p&gt;Jupiter's enormous gravity would turn its atmosphere first into a liquid from a certain depth, and then into a solid further towards its centre.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Jupiter has a solid core, above which is a liquid layer and an atmosphere above that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is Jupiter fundamentally different than the (solid) inner planets? Why is Jupiter called a &lt;em&gt;Gas Giant&lt;/em&gt;?&lt;/p&gt;&#xA;" OwnerUserId="20074" LastEditorUserId="6816" LastEditDate="2017-06-16T01:36:10.810" LastActivityDate="2023-08-23T00:54:42.603" Title="Why is Jupiter called a &quot;Gas Giant&quot;?" Tags="|jupiter|terminology|planet|gas-giant|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21979" PostTypeId="2" ParentId="21977" CreationDate="2017-06-16T02:35:19.673" Score="48" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter&quot; rel=&quot;noreferrer&quot;&gt;Jupiter&lt;/a&gt; being a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gas_giant&quot; rel=&quot;noreferrer&quot;&gt;gas giant&lt;/a&gt; is not about its appearance, as another answer stated. It's only about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_distribution&quot; rel=&quot;noreferrer&quot;&gt;mass distribution&lt;/a&gt; of a planet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jupiter's mass is 320 Earth masses, while we know from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Juno mission&lt;/a&gt; that the rock/ice in the core account for 5–25 of these Earth masses. So the rest of about 300 Earth masses is gas.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus Jupiter is a gas giant. It is really that simple. The more exotic states of matter that the solids in its interior probably exist in, don't affect the definition of a gas giant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put that into perspective:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The same thing goes for &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn&quot; rel=&quot;noreferrer&quot;&gt;Saturn&lt;/a&gt;, 95 Earth masses total, with estimated 20 Earth masses in the core. So 75 Earth masses of gas. Gas giant.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Uranus&quot; rel=&quot;noreferrer&quot;&gt;Uranus&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Neptune&quot; rel=&quot;noreferrer&quot;&gt;Neptune&lt;/a&gt; are different. Their masses are 14 and 17 Earth masses, respectively, with at least half of that in rock/ice. Thus they're not gas giants, but loosely named 'ice giants'.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Another way of inferring what Jupiter is made of, is taking a look at its average density. This is how it was done historically, before we had sophisticated computer models and high-pressure lab experiments.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Knowing Newton's laws, one can measure Jupiter's mass $M$ from its orbiting moons. Its size $r$ is known from the distance. Thus one can calculate its average density $\bar{\rho} = \frac{M}{4/3 \pi r^3}$ and finds a value of $\bar{\rho}_♃ = 1.326~\mathrm{g/cm^3}$. This is significantly lower than what Earth has ($\bar{\rho}_{\oplus} = 5.55~\mathrm{g/cm^3}$) and the other rocky planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Through simple weighted averaging with some rock mass of a few percent, it was then realized early on that one needs a lot of gas to reach such a low average density for such a planet as Jupiter.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interesting detail here is, that one cannot build Jupiter of gas only, the resulting average density would be lower than the real one. This is how it was realized that some very dense material needs to exist somewhere in Jupiter, possibly a core of solid material.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastEditorDisplayName="user10840" LastEditDate="2017-06-18T12:01:28.803" LastActivityDate="2017-06-18T12:01:28.803" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="21980" PostTypeId="1" AcceptedAnswerId="21986" CreationDate="2017-06-16T02:55:14.010" Score="4" ViewCount="5753" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=RtU_mdL2vBM&quot; rel=&quot;nofollow noreferrer&quot;&gt;In this live stream&lt;/a&gt;, we have an astronaut facing the sun (we've seen his hands actually):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OJ0Tf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OJ0Tf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought people doing an EVA should avoid the sun at all cost, but here he's facing it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what kind of deadly things/events does his suit protects him against?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(a rough guess is radiations but a detailed explanation would be appreciated)&lt;/p&gt;&#xA;" OwnerUserId="1368" LastEditorUserId="1368" LastEditDate="2017-06-17T20:39:01.823" LastActivityDate="2020-08-24T08:33:08.917" Title="What dangers are astronauts exposed to when facing the sun?" Tags="|iss|eva|the-sun|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21981" PostTypeId="2" ParentId="17807" CreationDate="2017-06-16T04:54:32.590" Score="3" Body="&lt;p&gt;I should really get back to finishing the other answer. In the mean time here is some recent (today's) news. I've skimmed the Science journal article now and added a summary here. Will improve it further tomorrow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;today's news:&lt;/strong&gt; The current news is about a demonstration of &lt;em&gt;producing entangled pairs in space&lt;/em&gt; and successfully receiving each member of the pair at a separate ground station. This is an important step, but this is not the same as, or close to relaying information from one ground station to another. So far they have confirmed that the photon pairs received at two ground stations are entangled. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;&lt;strong&gt;Fig. 4. Measurement of the received entangled photons after transmission by the two-downlink channel. (A)&lt;/strong&gt; Normalized two-photon coincidence counts in the measurement setting of the jHi=jVi basis. &lt;strong&gt;(B)&lt;/strong&gt; Normalized counts in the diagonal jTi basis. Numbers in parentheses represent the raw coincidence counts of different measurement settings&quot; From Figure 4 in &lt;a href=&quot;http://science.sciencemag.org/content/356/6343/1140&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yin et al. 2017, Science&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, there were polarizers at each receiving station changing polarization states independently using different sequences at each site. When they happened to be in combinations where the entangled photons would both pass, the count rate was much higher than when they had combinations that would not pass &lt;em&gt;photons entangled in this particular way&lt;/em&gt;. In this case the polarizers at the ground stations were (likely synchronized) Pockels cells running different pseudo-random polarization sequences at 4 MHz.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nDk3O.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nDk3O.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;In the future&lt;/strong&gt;, they will try to use this to send an encryption key from one ground station to the other ground station. They would then use the key to transmit data in a conventional way over any high speed link. They could use the internet, dedicated fiber, or even use QUESS in a simpler photorepeater mode for a very small message. You'd use a different satellite with bigger telescopes for high bandwidth, this would be purely a demo. Of course you' really just use fiber on the ground or &quot;the normal internet&quot; once you have successfully sent your key with entangled photons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How?&lt;/strong&gt; Use the think that Einstein didn't like; &quot;Spooky Action at a Distance&quot;. One ground station (the sender) manipulates it's received photons from space in one of various ways, causing the entangled photon received at the other ground station to suddenly attain the complementary state. In many Quantum Key Distribution (QKD) demonstration, the entangled pair generator and one receiving station are located physically much closer together and the only long link is between the pair generator and the other receiving station. Other demonstrations however will use similarly long paths for both photons in the pair. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found the Q&amp;amp;A format in the LA Times particularly transparent and helpful. The &lt;a href=&quot;https://www.youtube.com/watch?v=4QlcKuxDGrs&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; by Science is really worth watching.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Los Angeles Times: &lt;a href=&quot;http://www.latimes.com/science/sciencenow/la-sci-sn-entangled-photons-china-20170615-htmlstory.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Q&amp;amp;A: Chinese satellite breaks a quantum physics record, beams entangled photons from space to Earth&lt;/a&gt;. &lt;/li&gt;&#xA;&lt;li&gt;Science: &lt;a href=&quot;http://www.sciencemag.org/news/2017/06/chinas-quantum-satellite-achieves-spooky-action-record-distance&quot; rel=&quot;nofollow noreferrer&quot;&gt;China's quantum satellite achieves 'spooky action' at record distance&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;BBC: &lt;a href=&quot;http://www.bbc.com/news/science-environment-40294795&quot; rel=&quot;nofollow noreferrer&quot;&gt;China's quantum satellite in big leap&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Science (full article paywalled): &lt;a href=&quot;http://science.sciencemag.org/content/356/6343/1140&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satellite-based entanglement distribution over 1200 kilometers&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;Credits: (Graphic) C. Bickel/Science; (Data) Jian-Wei Pan&quot; From &lt;a href=&quot;http://www.sciencemag.org/news/2017/06/chinas-quantum-satellite-achieves-spooky-action-record-distance&quot; rel=&quot;nofollow noreferrer&quot;&gt;Science&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qfkSu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qfkSu.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4QlcKuxDGrs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;QUESS converts single photons of UV (visible) light from a 405nm photodiode laser (PL in the figure) into pairs of 810nm photons using a &lt;a href=&quot;https://en.wikipedia.org/wiki/Potassium_titanyl_phosphate#Periodically_poled_potassium_titanyl_phosphate_.28PPKTP.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;parametrically poled KPT crystal&lt;/a&gt; (PPKPT in the figure). Similar KPT crystal material is also used in older style &lt;a href=&quot;https://en.wikipedia.org/wiki/Laser_pointer#Green&quot; rel=&quot;nofollow noreferrer&quot;&gt;green laser pointers&lt;/a&gt; to convert pairs of 1064nm photons into single 532nm green photons. QUESS then uses its two telescope systems. They are not identical but one is schematically represented here. One of the two beams of entangled 810nm photons enters from the bottom. It is combined with &quot;helper beams&quot; of 850nm and 532nm (described below) There is also a camera sampling return signals from the Earth station at 671 nm through the same telescope (top left) and directly (bottom). During operation one of these systems is pointed to each of two independent ground stations, and each one tracks the motion of the ground station through several levels of feedback. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;&lt;strong&gt;Fig. 1. Schematic of the spaceborne entangled-photon source and its in-orbit performance. (A)&lt;/strong&gt; The thickness of the KTiOPO4 (PPKTP) crystal is 15 mm. A pair of off-axis concave mirrors focus the pump laser (PL) in the center of the PPKTP crystal. At the output of the Sagnac interferometer, two dichromatic mirrors (DMs) and long-pass filters are used to separate the signal photons from the pump laser. Two additional electrically driven piezo steering mirrors (PIs), remotely controllable  on the ground, are used for fine adjustment of the beam-pointing for an optimal collection efficiency into the single-mode fibers. QWP, quarter-wave plate; HWP, half-wave plate; PBS, polarizing beam splitter.&quot; From Figure 1A in &lt;a href=&quot;http://science.sciencemag.org/content/356/6343/1140&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yin et al. 2017, Science&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/x3nPw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/x3nPw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;&lt;strong&gt;Fig. 2. The transmitters, receivers, and APT performance. (A)&lt;/strong&gt; The entangled-photon beam (810 nm) is combined and co-aligned with a pulsed infrared laser (850 nm) for synchronization and a green laser (532 nm) for tracking by three DMs and sent out from an 8× telescope. For polarization compensation, two motorized QWPs and a HWP are remotely controlled. A fast steering mirror (FSM) and a two-axis turntable are used for closed-loop fine and coarse tracking, based on the 671-nm beacon laser images captured by cameras 1 and 2. BE, beam expander.&quot; From Figure 2A in &lt;a href=&quot;http://science.sciencemag.org/content/356/6343/1140&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yin et al. 2017, Science&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VgVNk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VgVNk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-16T17:34:54.170" LastActivityDate="2017-06-16T17:34:54.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21982" PostTypeId="2" ParentId="19504" CreationDate="2017-06-16T04:56:21.187" Score="4" Body="&lt;p&gt;I'm sure there are several types of standard RBF designs available by vendors and that you can get recommendations and good prices from other CubeSat makers or CubeSat satellite bus makers. But before a specific RBF design is selected, I suggest you think through all the requirements and design constraints on your particular RBF device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have the key functional requirement that an RBF pin system be used and that it shall cut all power to the satellite. I think there's also an accessibility requirement in the design spec you referenced above. I don't know what a non-switch is or a switch with pin inhibit, so before you select make sure you go through how it's going to be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The RBF pin should be designed to fit tightly when inserted and secured so that an accidental pull on the flag doesn't pull it out. It should also fit tightly during transportation and handling. Also note that a pin is often required to protect spacecraft bus flight hardware from damage from your payload hardware as well as personnel. When the pin is removed, it might be by you or perhaps by someone else who is not as careful. The pin should pull out clean and the switch or whatever it was protecting should close firm and clean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the pin has to be reinserted, it has to be accessible and testable. That task also needs to be firm and clean. Once the pin is out, the switch or whatever needs to close and remain closed for the remainder of the mission. I elaborate about firm and clean device operation because of comments I read elsewhere in this stack exchange suggesting use of modified clothes-pins or handmade plastic parts, which would save money and maybe mass but which I would strongly recommend against because of the criticality of this device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My answer is qualified by the fact that I have not done mechanical or electrical design, but I've been in the spacecraft to launch vehicle integration business for 35 years, and have experience on both sides of that interface in high level system engineering and project management roles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Good luck.&lt;/p&gt;&#xA;" OwnerUserId="20033" LastActivityDate="2017-06-16T04:56:21.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21984" PostTypeId="2" ParentId="21980" CreationDate="2017-06-16T06:20:13.067" Score="5" Body="&lt;p&gt;There is no real reason why astronauts should avoid the sun while spacewalking. While it is true that things like seeing what you are doing, dealing with radiation and heat management are somewhat more difficult the suits are designed for this. Remember that ISS revolves around the earth once every 90 minutes (give or take) and spacewalks can take up to 8 hours. It would be exceedingly difficult to perform a spacewalk where they would have to 'run for cover' every 45 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer on &lt;a href=&quot;https://space.stackexchange.com/questions/8296/how-are-evas-in-leo-affected-by-being-at-the-night-side-of-earth&quot;&gt;How are EVA&amp;#39;s in LEO affected by being at the night side of Earth?&lt;/a&gt; has some very good details on how day versus night affects spacewalks.&lt;/p&gt;&#xA;" OwnerUserId="8132" LastActivityDate="2017-06-16T06:20:13.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21985" PostTypeId="2" ParentId="21977" CreationDate="2017-06-16T06:43:55.597" Score="14" Body="&lt;p&gt;One reason they are called gas giants is because they are mostly composed of elements that are gaseous at Earth like temperatures and pressures.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Jupiter is primarily composed of hydrogen with a quarter of its mass being helium, though helium comprises only about a tenth of the number of molecules.&lt;/p&gt;&#xA;&lt;p&gt;Jupiter's upper atmosphere is about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules. A helium atom has about four times as much mass as a hydrogen atom, so the composition changes when described as the proportion of mass contributed by different atoms. Thus, Jupiter's atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass consisting of other elements. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia. The interior contains denser materials - by mass it is roughly 71% hydrogen, 24% helium, and 5% other elements.[21][22] Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Jupiter&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Jupiter&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;So Jupiter and Saturn are almost totally composed of hydrogen and helium, elements that are gaseous at Earth like temperatures and pressures.   Of course the temperatures and pressures deeper inside Jupiter and Saturn are not exactly Earth like!&lt;/p&gt;&#xA;&lt;p&gt;But the elements that they are composed of are commonly called gases even though they might be in exotic conditions such as liquid metallic hydrogen under the immense pressure and temperatures inside the planets.  Most of us think of hydrogen and helium as gaseous elements, not liquids, or solids, or highly exotic forms of matter.&lt;/p&gt;&#xA;&lt;p&gt;Thus Jupiter and Saturn are &amp;quot;gas giants&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;A second reason they are called &amp;quot;gas giants&amp;quot; is historical.  Famed science fiction writer James Blish wrote a science fiction story called &amp;quot;Solar Plexus&amp;quot;, published in &lt;em&gt;Astonishing stories&lt;/em&gt;, September, 1941.  &amp;quot;Solar Plexus&amp;quot; was rewritten and republished in an anthology &lt;em&gt;Beyond Human Ken&lt;/em&gt;, edited by Judith Merrill, 1954.  The &lt;a href=&quot;http://www.jessesword.com/sf/view/52&quot; rel=&quot;noreferrer&quot;&gt;1954 rewritten version&lt;/a&gt; contained the line:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A quick glance over the boards revealed that there was a magnetic field of some strength near by, one that didn't belong to the invisible gas giant revolving half a million miles away.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Science fiction readers who knew anything about the structure of giant planets thought that &amp;quot;gas giant&amp;quot; was a very fitting phrase to describe them.  And some of them were professional astronomers.  Thus the phrase began to be used by astronomers to describe the giant planets in the solar system.&lt;/p&gt;&#xA;" OwnerUserId="20075" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-16T06:50:16.353" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="21986" PostTypeId="2" ParentId="21980" CreationDate="2017-06-16T07:43:17.123" Score="2" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Facing the Sun does not mean looking at it. As there are no air there, you have more light looking directly at the Sun, but LESS light looking somewhat in its direction, but not straightly. Because of that, BTW, you can see stars simultaneously.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Even while looking at the Sun directly, you still could do it for the short time enough for taking a photo, without any real harm.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Edit.&lt;br /&gt;&#xA;The spacesuit protects the wearer from cold, heat, extra visible light, &lt;a href=&quot;https://space.stackexchange.com/questions/2964/how-much-protection-do-space-eva-suits-offer-against-radiation-for-the-wearer&quot;&gt;UV&lt;/a&gt;, soft roentgen, alpha and beta-rays and protons. A bit from gamma-rays. Earth magnetic field is blocking most of Sun alpha, beta rays and protons and also charged energetic cosmic rays.  But not from neutrons and gamma-rays. &lt;a href=&quot;http://source.colostate.edu/space-radiation-csu-studies-the-risks-for-astronauts-journeying-to-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;And the spacesuit and even the spacecraft do not protect from them also&lt;/a&gt;. So, the space travels remain very dangerous. Space radiation is 50-fold more effective than typical radiation on Earth at causing a type of liver cancer called hepatocellular carcinoma in mice.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Space_suit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wiki&lt;/a&gt; has much info on the space suits history.&lt;/p&gt;&#xA;&lt;p&gt;The best fiction literature where space suits are impotant is &lt;a href=&quot;https://en.wikipedia.org/wiki/Have_Space_Suit%E2%80%94Will_Travel&quot; rel=&quot;nofollow noreferrer&quot;&gt;Have Space Suit—Will Travel&lt;/a&gt; of Heinlein.&lt;/p&gt;&#xA;" OwnerUserId="20076" LastEditorDisplayName="user20636" LastEditDate="2020-08-24T08:33:08.917" LastActivityDate="2020-08-24T08:33:08.917" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="21987" PostTypeId="2" ParentId="21974" CreationDate="2017-06-16T11:21:02.113" Score="2" Body="&lt;p&gt;Initially you'll be able to have a fairly normal conversation with the astronauts. The light speed delay in their responses will be noticeable almost immediately though - even the moon is far enough away for a two second delay. Eventually it will increase to somewhere between 6 and 44 minutes round trip, which is clearly not a normal conversation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You aren't likely to notice the increase as it happens though. For a 6 month trip that works out as a maximum of 8 seconds per day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what about the video itself? Ignoring relativistic effects that are on the order of microseconds per year, 24 hours of video transmitted from the spacecraft just arrived on Earth over 24 hours and 4 seconds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In theory an analog signal might play imperceptibly slower, but digital will just assume your internet connection dropped out and will pause briefly until it gets sufficient data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The return trip is a little trickier - with the delay decreasing, you receive video faster than it will naturally play, so you'll need to speed up playback unless you want to maintain the 20 minute delay.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-06-16T11:21:02.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21988" PostTypeId="2" ParentId="21980" CreationDate="2017-06-16T12:59:34.817" Score="4" Body="&lt;p&gt;The shuttle/ISS Extravehicular Mobility Unit (EMU) aka spacesuit incorporates a gold-film plated sun visor to protect the astronaut's vision when looking in the general direction of the sun.  It is called the Extravehicular Visor Assembly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9atbl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9atbl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;https://www.space.com/21987-how-nasa-spacesuits-work-infographic.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this picture you can see the visor totally raised (gold rim at top of bubble)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/yxxwy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yxxwy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;and in this picture you can see the visor half raised.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/chm4G.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/chm4G.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-06-16T12:59:34.817" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="21989" PostTypeId="2" ParentId="15462" CreationDate="2017-06-16T15:48:15.413" Score="2" Body="&lt;p&gt;There is one now! I've just run across the python package callhorizons in &lt;a href=&quot;https://github.com/mommermi/callhorizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;github&lt;/a&gt; and &lt;a href=&quot;https://pypi.python.org/pypi/CALLHORIZONS/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Python Package Index&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is more information in &lt;a href=&quot;http://callhorizons.readthedocs.io/en/latest/use.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;readthedocis.io&lt;/a&gt; which helps explain how to do queries if you are looking for asteroids/minor bodies, for which there are zillions and the names can be people's name, or alphanumerics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll try to test it further and update here. It looks like there is a lot of capability here, but I can't say for sure yet. There is a &lt;a href=&quot;https://michaelmommert.wordpress.com/2016/01/08/a-python-module-to-query-jpl-horizons/&quot; rel=&quot;nofollow noreferrer&quot;&gt;blog post by an astronomer&lt;/a&gt; but it's from January 2016. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was a one-line install, namely &lt;code&gt;pip install callhorizons&lt;/code&gt;. I then typed in the simple demo at the top of the PyPI page and got an answer!&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;&amp;gt;&amp;gt;&amp;gt; import callhorizons&#xA;&amp;gt;&amp;gt;&amp;gt; eros = callhorizons.query('Eros')&#xA;&amp;gt;&amp;gt;&amp;gt; eros.set_discreteepochs([2457446.177083, 2457446.182343])&#xA;&amp;gt;&amp;gt;&amp;gt; eros.get_ephemerides(568)&#xA;2&#xA;&amp;gt;&amp;gt;&amp;gt; print(eros['RA'], eros['DEC'])&#xA;(array([ 292.46003,  292.46332]), array([-27.44392, -27.44335]))&#xA;&amp;gt;&amp;gt;&amp;gt;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Yay python!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current version is a pure python script, and contains the comment:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;CALLHORIZONS - a Python interface to access JPL HORIZONS&#xA;ephemerides and orbital elements.&#xA;&#xA;This module provides a convenient python interface to the JPL&#xA;HORIZONS system by directly accessing and parsing the HORIZONS&#xA;website. Ephemerides can be obtained through get_ephemerides,&#xA;orbital elements through get_elements. Function&#xA;export2pyephem provides an interface to the PyEphem module.&#xA;&#xA;michael.mommert (at) nau.edu, latest version: v1.0.5, 2017-05-05.&#xA;This code is inspired by code created by Alex Hagen.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Currently it only generates the query url for an observer table, not state vectors, but it could certainly be easily customized to get state vectors. Here is a small chunk, additional parameters are added to the url string before it's used.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;### construct URL for HORIZONS query&#xA;url = &quot;http://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=l&quot; \&#xA;      + &quot;&amp;amp;TABLE_TYPE='OBSERVER'&quot; \&#xA;      + &quot;&amp;amp;QUANTITIES='&quot; + str(quantities) + &quot;'&quot; \&#xA;      + &quot;&amp;amp;CSV_FORMAT='YES'&quot; \&#xA;      + &quot;&amp;amp;ANG_FORMAT='DEG'&quot; \&#xA;      + &quot;&amp;amp;CAL_FORMAT='BOTH'&quot; \&#xA;      + &quot;&amp;amp;SOLAR_ELONG='&quot; + str(solar_elongation[0]) + &quot;,&quot; \&#xA;      + str(solar_elongation[1]) + &quot;'&quot; \&#xA;      + &quot;&amp;amp;CENTER='&quot;+str(observatory_code)+&quot;'&quot;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;OK try a spacecraft, also viewed from Mauna Kea. From &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; with the following setup:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bAbQv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bAbQv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;gives (scrolls horizontally):&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;**********************************************************************************************************************************&#xA;Date_________JDUT, , ,R.A._(ICRF/J2000.0), DEC_(ICRF/J2000.0),  APmag, S-brt,            delta,     deldot,    S-O-T,/r,    S-T-O,&#xA;**********************************************************************************************************************************&#xA;$$SOE&#xA;2457754.500000000,*,m,20 01 24.54,-56 59 28.4,   n.a.,  n.a., 1.7047550022E+10, 18.9796569,  36.6514,/T,   0.2976,&#xA;2457784.500000000,*,m,20 03 22.69,-56 58 00.2,   n.a.,  n.a., 1.7080283550E+10,  6.5439542,  40.6419,/L,   0.3218,&#xA;2457814.500000000,*,m,20 05 08.36,-57 01 24.2,   n.a.,  n.a., 1.7082950019E+10, -3.4910462,  59.2751,/L,   0.4270,&#xA;2457844.500000000,*,m,20 06 16.02,-57 09 10.1,   n.a.,  n.a., 1.7065586218E+10, -8.4792450,  82.4692,/L,   0.4975,&#xA;2457874.500000000,*,m,20 06 30.25,-57 19 39.5,   n.a.,  n.a., 1.7043227392E+10, -7.3046878, 106.0860,/L,   0.4870,&#xA;2457904.500000000,*,m,20 05 49.21,-57 30 32.2,   n.a.,  n.a., 1.7031649793E+10, -0.4854246, 127.5819,/L,   0.4048,&#xA;2457934.500000000,*,m,20 04 24.98,-57 39 21.6,   n.a.,  n.a., 1.7043463848E+10, 10.1577911, 141.9907,/L,   0.3144,&#xA;$$EOE&#xA;**********************************************************************************************************************************&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Compare to this script:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import callhorizons&#xA;import numpy as np&#xA;&#xA;JDs = [2457754.5 + 30*i for i in range(7)] # 01-Jan-2017 00:00 UTC + n*30 days&#xA;&#xA;V2  = callhorizons.query(-32, smallbody=False)     # -32 for Voyager 2&#xA;&#xA;V2.set_discreteepochs(JDs)&#xA;&#xA;V2.get_ephemerides(568)    # 568 for Mauna Kea &#xA;&#xA;radecs = np.vstack((V2['RA'], V2['DEC']))&#xA;&#xA;for JD, radec in zip(JDs, radecs.T):&#xA;    print JD, radec&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;A spot check shows that these are the same, great! Fyi the very large apparent motion is due to parallax - the Earth shifting its perspective every month. After one year the position actually only changes by a few tenths of a degree.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2457754.5 [ 300.35226  -56.99121]&#xA;2457784.5 [ 300.84454  -56.96671]&#xA;2457814.5 [ 301.28482  -57.0234 ]&#xA;2457844.5 [ 301.56676  -57.1528 ]&#xA;2457874.5 [ 301.62605  -57.32765]&#xA;2457904.5 [ 301.45504  -57.50895]&#xA;2457934.5 [ 301.10407  -57.65601]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-18T12:07:25.890" LastActivityDate="2017-06-18T12:07:25.890" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="21990" PostTypeId="1" AcceptedAnswerId="21991" CreationDate="2017-06-16T16:24:18.857" Score="3" ViewCount="683" Body="&lt;p&gt;I would like to know, what is the most sophisticated or preferred approach when it comes to connecting a &quot;low earth orbit satellite&quot; to a &quot;low power transmitter devices on the ground&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know there are various factors to it, ranging from antenna design to access channel design to receiver configuration. But I am novice in the technologies to choose to make it one system and the flow that should happen to have an efficient uplink communication from ground to the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If someone who has worked on it can give an idea on this considering the fact that &quot;Ground devices are low power&quot;, it would be a lot of help.&lt;/p&gt;&#xA;" OwnerUserId="20083" LastActivityDate="2017-06-16T19:24:31.473" Title="Radio Link Design of a low power, sensors to satellite communication system" Tags="|artificial-satellite|communication-satellite|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21991" PostTypeId="2" ParentId="21990" CreationDate="2017-06-16T19:11:29.123" Score="4" Body="&lt;p&gt;&lt;em&gt;This is long, but it's still barely scratching the surface. I'll try to take it nice and slow, so bear with me.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, when dealing with radio links, it's a good idea to start with a &lt;a href=&quot;https://ham.stackexchange.com/q/352/29&quot;&gt;link budget&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a link budget, you start with your transmitter's output power, end with the signal strength actually received (most often, this is the minimum required signal strength at the receiver), and in between, list all factors that contribute to either loss or gain of signal strength. This can be everything from antenna gain to attenuation because of absorption in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very broadly speaking, there are three ways you can increase signal strength:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;either by pumping more power into the transmit-side antenna,&lt;/li&gt;&#xA;&lt;li&gt;or by having a bigger antenna at the transmit side,&lt;/li&gt;&#xA;&lt;li&gt;or by having a bigger antenna at the receive side,&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;or of course some combination of those.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Bigger antennas&quot; is a bit of a misnomer, but it's close enough to get a basic understanding of what's involved physically. What you really use in such a case is an antenna with a &lt;em&gt;higher antenna gain&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In radio, one often refers to an &lt;a href=&quot;https://en.wikipedia.org/wiki/Isotropic_antenna&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;isotropic&quot; antenna&lt;/a&gt;. This is a purely theoretical construct, which cannot exist in the real world, but which is said to radiate equally in every direction, and to radiate exactly 100% of the power it receives in the form of radio signals. In physics terms, it's a RF point source. Real-world antennas can then be compared to such a theoretical antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When an antenna &lt;em&gt;focuses&lt;/em&gt; the signal in some particular direction, it accepts &lt;em&gt;the same amount of power&lt;/em&gt; but transmits (or receives) that power &lt;em&gt;in a narrower beam&lt;/em&gt; than the isotropic antenna does. We call that &quot;antenna gain&quot;, but it's important to keep in mind that there is no actual &lt;em&gt;gain&lt;/em&gt; involved; except for losses (which are typically resistive or inductive), the same amount of power is always involved. An antenna can even legitimately have &lt;em&gt;negative&lt;/em&gt; gain compared to an isotropic antenna, but such cases are not relevant here. Look at a satellite dish, or a rooftop TV antenna; those are two different types of the general class of &lt;em&gt;directional&lt;/em&gt; antennas, so called because they concentrate the power into a beam that is much narrower than that of an isotropic antenna. For spacecraft that travel far from Earth, bigger parabolic antennas (like that satellite TV dish) is used; for a real-world example of this, consider the ground stations for NASA's &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is, of course, one more option. Instead of simply pumping more power into the antenna, you can design your signal encoding in such a way that it can be decoded at a lower received power. Very often, this involves reducing the transmission bit rate. Basically, when each bit (&quot;symbol&quot;) is allowed more time, the receiver has more time to determine its value, and it can use this time to metaphorically dig the signal out of the noise. The ultimate limit to this is described by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shannon–Hartley theorem&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Given all this, we can see what can be done given your constraints.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One side of the link (the satellite) is in low Earth orbit, and the other side of the link is on the ground. Thus there is little that can be done about the &lt;em&gt;distance&lt;/em&gt; that needs to be covered by the signal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The frequency needs to be relatively high, so that antennas are physically smaller, because things that are sent into space need to fit inside the spacecraft that are used to launch them. Unfortunately, this means that the signal losses over a given physical path are larger, because free-space path loss scales with distance in terms of wavelengths, and wavelength is the reciprocal of frequency. Thankfully, physical antenna sizes go down with frequency, so we can compensate for this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The transmitted power level is low. This means that we need to be able to pick out and decode a signal that may be close to, or possibly even &lt;em&gt;below&lt;/em&gt;, the ambient noise level at the frequency in use. This benefits from long signal integration times, which reduces our attainable symbol rate. Thankfully, this is a good thing for the power budget of the satellite; you can get away with less on-board power generation capability, and need less cooling. (Believe it or not, but getting rid of waste heat is one of the hardest things in spacecraft design. Reducing excess weight is another.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming that a low bit rate is acceptable, we can choose the signal encoding such that the required received power level is attainable given the other constraints. This is good, because we really don't have much left to play with!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;All this means that&lt;/strong&gt; you would want to pick a frequency that is as high as reasonably possible (while avoiding the atmospheric absorption bands, for example that of water around 2.3 to 2.4 GHz or so) to get more antenna gain with a same-physical-size antenna, and put a relatively large antenna on the satellite in order to listen in the direction of the Earth without listening to too much else. The physical size of the antenna and the frequency would probably be selected together, such that the area seen by the satellite's antenna is no larger than the Earth itself in its intended orbit, and might be smaller. You would choose a signal encoding that ensures that the signal can be decoded at the received power you would be seeing, with a good deal of margin for loss of signal strength. Hopefully you don't need to transmit a lot of data in a short period of time; if you do, you might have to go back to the drawing board.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of those factors (frequency, antenna type and size, signal encoding, bit rate) affect each other, so you would be choosing them together. If one of them is fixed (for example, the physically largest dimension of the satellite will be restricted by, if nothing else, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Payload_fairing&quot; rel=&quot;nofollow noreferrer&quot;&gt;payload fairing&lt;/a&gt; of the chosen launcher), then something else has to give.&lt;/p&gt;&#xA;" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2017-06-16T19:24:31.473" LastActivityDate="2017-06-16T19:24:31.473" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="21992" PostTypeId="2" ParentId="21587" CreationDate="2017-06-16T19:32:34.383" Score="2" Body="&lt;p&gt;An additional factor resulting in small (and even single pixel) glints is that the horizontally oriented ice crystals may cover only a small region. For example an ice cloud may be small, or ice crystals in the rest of a large cloud may be oriented randomly (as opposed to horizontally) as they tumble through the air.  &lt;/p&gt;&#xA;" OwnerUserId="20087" LastActivityDate="2017-06-16T19:32:34.383" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="21993" PostTypeId="1" AcceptedAnswerId="21995" CreationDate="2017-06-16T23:08:13.443" Score="1" ViewCount="332" Body="&lt;p&gt;Does anyone have a table of the delta-V between Titan escape and other moons of Saturn? If not, could someone provide figures for Hyperion, Phoebe, Iapetus, and Enceladus? Or, more simply, what is the approximate delta-V  that would allow you to land on any other moon of Saturn (out to Phoebe) from Titan escape?&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-17T19:39:39.497" Title="Delta-V between Titan and other Saturnian moons" Tags="|delta-v|saturn|titan|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21994" PostTypeId="1" AcceptedAnswerId="21996" CreationDate="2017-06-17T01:45:14.057" Score="4" ViewCount="1408" Body="&lt;p&gt;After the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transposition,_docking,_and_extraction&quot; rel=&quot;nofollow noreferrer&quot;&gt;transposition, docking and extraction maneuver&lt;/a&gt;, it seems that the Apollo spacecraft is &quot;facing the wrong way&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Apollo11-04.png/320px-Apollo11-04.png&quot; alt=&quot;Part 2&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was this &quot;corrected&quot; with another 180° turn after stage separation? Or was there no reason to do so since the spacecraft had already been put on its course towards the moon by the booster during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program#Lunar_mission_profile&quot; rel=&quot;nofollow noreferrer&quot;&gt;trans-lunar injection maneuver&lt;/a&gt; and the Service Module engine would only be used to get back from the moon? &lt;/p&gt;&#xA;" OwnerUserId="20089" LastEditorUserId="20089" LastEditDate="2017-06-17T16:03:29.733" LastActivityDate="2019-07-17T07:54:43.480" Title="Did the combined Command and Service Module and Lunar Module perform another 180° turn after transposition, docking and extraction?" Tags="|apollo-program|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21995" PostTypeId="2" ParentId="21993" CreationDate="2017-06-17T02:29:37.760" Score="2" Body="&lt;p&gt;I've made &lt;a href=&quot;http://clowder.net/hop/TMI/HohmannGasGiantMoons-2.xls&quot; rel=&quot;nofollow noreferrer&quot;&gt;an Excel spreadsheet&lt;/a&gt; of Saturn's moons and Jupiter's moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Type departure and destination moons into blue cells. Type periapsis and apoapsis of parking orbit into orange cells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to go from surface to surface, set periapsis at 0 altitude and look for Hyperbolic periapsis velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes I want to look at Saturn scenarios not covered in the spreadsheet. For example Titan to Hyperion. Hyperion is not in the spreadsheet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I cheat. First go to Wikipedia to get Hyperion's semi major axis, mass and radius. Then put these numbers in the columns of an orbit not used much. Like LSO (Low Saturn Orbit).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HVbPW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HVbPW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then into the blue boxes I type Titan and LSO, pretending that LSO is another name for Hyperion.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-06-17T19:39:39.497" LastActivityDate="2017-06-17T19:39:39.497" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21996" PostTypeId="2" ParentId="21994" CreationDate="2017-06-17T02:39:07.263" Score="12" Body="&lt;p&gt;You're correct that the spacecraft was already on the right course, but on the way to the moon, the CSM/LM stack would make several attitude changes for various purposes.&lt;/p&gt;&#xA;&lt;p&gt;The spacecraft would spend most of the flight aligned roughly perpendicular to its flight path, in a north-south orientation, spinning slowly on its axis to evenly distribute heat from sunlight; this is called the Passive Thermal Control (PTC) roll or informally the &amp;quot;barbecue roll&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;In this attitude, Earth and the Moon would alternate being visible through either the CM or LM windows.&lt;/p&gt;&#xA;&lt;p&gt;The guidance platform has a few different reference orientations programmed into it, called &lt;a href=&quot;https://en.wikipedia.org/wiki/REFSMMAT&quot; rel=&quot;noreferrer&quot;&gt;REFSMMATs&lt;/a&gt;, and after trans-Earth injection, they switch from the &amp;quot;launch orientation&amp;quot; REFSMMAT to the &amp;quot;PTC orientation&amp;quot; REFSMMAT. The orientation of the spacecraft is considered relative to one of these orientations.&lt;/p&gt;&#xA;&lt;p&gt;From the &lt;a href=&quot;https://history.nasa.gov/afj/ap15fj/04troubleshoot_ptc.html&quot; rel=&quot;noreferrer&quot;&gt;Apollo 15 annotated flight journal&lt;/a&gt;, here's how the PTC orientation system is defined (bold mine):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The platform's X-axis is aligned along the ecliptic plane, perpendicular to the line between the Earth and Moon at the time of Trans-Earth Injection (TEI). Its Z-axis is aligned perpendicular to the ecliptic plane, directed southward. This alignment is used for all translunar and trans-Earth coast maneuvers and as the reference for easy alignment of the PTC (Passive Thermal Control) maneuver. &lt;strong&gt;Note that the axes shown do not represent the orientation of the spacecraft during PTC, only that of the [guidance] platform.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Qvrw7.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qvrw7.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;At 11:44:58 in the transcript, they're starting to maneuver into the PTC attitude and we have this annotation:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;[The PTC maneuver requires the spacecraft to be rotated around the X, or longitudinal axis, in a slow roll of 0.35° per second. A complete roll of 360° should take about 17 minutes at that speed.]&lt;/p&gt;&#xA;&lt;p&gt;[The X-axis of the CSM is an imaginary line which runs from the centre of the SPS engine bell, through the centre of the SM and out through the apex of the CM's cone. Incidentally, in the docked configuration, the LM's X-axis is colinear with the CSM's but is in the opposite direction, running from the descent engine to the docking hatch and, because of this, the Y- and Z-axes of each spacecraft are also opposed. The 'front doors' of each spacecraft are aligned 60° around the X-axis from each other. Readers should be careful to distinguish between the spacecraft axes and those of the IMU.]&lt;/p&gt;&#xA;&lt;p&gt;[The last P52 aligned the X-axis of the IMU platform with the ecliptic and at right angles to the Earth-Moon line; its Z-axis was aligned southward, perpendicular to the ecliptic. If the spacecraft were aligned to match the platform, it would have its longitudinal axis aligned with the ecliptic with the Earth and Moon to either side. By pitching 90°, as called for in the Flight Plan, the longitudinal axis of the spacecraft is brought perpendicular to the ecliptic which guarantees that the Sun (which is always in the plane of the ecliptic) will strike the spacecraft side on as it rotates.]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Looking at the image, if you take the X-axis of the spacecraft, and pitch it up (or down) 90 degrees from the IMU X-axis you wind up with the spacecraft aligned north-south, with the sun hitting it on the side, so the roll gets all sides of the craft evenly roasted.&lt;/p&gt;&#xA;&lt;p&gt;The PTC roll was stopped and the spacecraft reoriented to take star sightings along the way, as well as to do short course correction burns.&lt;/p&gt;&#xA;&lt;p&gt;The next major firing of the CSM engine would be to slow down on the far side of the moon to slow down into lunar orbit. This would be done &amp;quot;pointing backwards&amp;quot;, with the engine bell facing in the direction of motion.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-29T01:45:07.513" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="21997" PostTypeId="2" ParentId="3995" CreationDate="2017-06-17T07:21:11.513" Score="5" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;I guess ... during the mission the order had to be switched, so the service module engine could perform the Lunar Orbit Insertion. Is that correct?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's one of the reasons, but not the only one.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before the TDE manuever, the modules are not connected at all. After the TDE maneuver, the CM and LM are connected to each other via an airlock, so the astronauts can move between the two without having to do an EVA. This removes a risky, time-consuming procedure from the mission and makes the journey much more comfortable. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-17T07:21:11.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21998" PostTypeId="2" ParentId="21977" CreationDate="2017-06-17T08:12:29.307" Score="0" Body="&lt;p&gt;Jupiter is composed primarily of gaseous and liquid matter. It is the largest of the gas giants, and like them, is divided between a gaseous outer atmosphere and an interior that is made up of denser materials. It's upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds as well as trace amounts of benzene and other hydrocarbons. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. Crystals of frozen ammonia have also been observed in the outermost layer of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter's core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.&lt;/p&gt;&#xA;" OwnerUserId="20091" LastActivityDate="2017-06-17T08:12:29.307" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="21999" PostTypeId="2" ParentId="18175" CreationDate="2017-06-17T09:43:38.793" Score="1" Body="&lt;p&gt;The 'national security launch costs' refer to launches on Atlas V. The current version uses the Russian RD-180 engine. A few years ago, continued availability of this engine became questionable, and the US government started pushing for a replacement.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One candidate for developing this replacement engine was &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-180&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aerojet Rocketdyne:&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In June 2014, Aerojet Rocketdyne proposed that the US Federal government &quot;fund an all-new, U.S.-sourced rocket propulsion system&quot;, the 2,200-kilonewton-class (500,000 lbf) thrust kerosene/LOX AR-1 rocket engine. As of June 2014, Aerojet's early projection was that the cost of the each engine would be under US$25 million per pair of engines—not including the up to &lt;strong&gt;US\$1 billion estimated development cost to be funded by the US Government&lt;/strong&gt;. Aerojet believed that the AR-1 could replace the RD-180 in the US Evolved Expendable Launch Vehicle fleet, and that it would be more affordable.[7]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Blue Origin, in contrast, says it'll develop the engine using its own funds, and (presumably*) recoup its investment by selling engines, adding a few million to the per-engine cost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm saying &quot;presumably&quot; because Blue Origin can choose to not recoup its investment, and sell the engines at something closer to the marginal manufacturing cost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The AR-1 is designed as a drop-in replacement for the RD-180 and &lt;a href=&quot;http://spacenews.com/be-4-engine-remains-front-runner-for-ulas-next-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;can be used on the current Atlas V&lt;/a&gt;. The BE-4 cannot be used on the Atlas V, but will be used on the Vulcan. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The \$3 billion in additional savings is a more difficult claim to verify. There doesn't seem to be any documentation for it other than the BO website.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the 2000-2009 timeframe, there were &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Atlas_launches_(2000%E2%80%932009)&quot; rel=&quot;nofollow noreferrer&quot;&gt;17 military launches&lt;/a&gt; on Atlas V, in 2010-2019 I count &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Atlas_launches_(2010%E2%80%932019)&quot; rel=&quot;nofollow noreferrer&quot;&gt;43&lt;/a&gt; including GPS satellites, so 300 launches in 20 years sounds plausible. So the savings of \$10M/mission by removing one booster could account for the entire \$3B.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-06-17T09:43:38.793" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22000" PostTypeId="1" AcceptedAnswerId="22005" CreationDate="2017-06-17T16:57:22.413" Score="7" ViewCount="906" Body="&lt;p&gt;So, what do I need, is to have an ability to calculate body position and velocity at given time. At first I have body position and velocity What do I can calculate:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Angular momentum&#xA;$$&#xA;\textbf{h} = \textbf{r}\times\textbf{v}&#xA;$$&#xA;Eccentricity:&#xA;$$&#xA;\textbf{e} = \frac{\textbf{v}\times\textbf{h}}\mu - \frac{\textbf{r}}{r}&#xA;$$&#xA;Also I have found that I can calculate position:&#xA;$$&#xA;x=a\left(e-\cosh\tau\right)&#xA;$$&#xA;$$&#xA;y=a\sqrt{e^2-1}\sinh\tau&#xA;$$&#xA;And also there some relation between eccentricity anomaly and time:&#xA;$$&#xA;t=\sqrt{a^3\over\mu}\left(e\sinh\tau-\tau\right)&#xA;$$&#xA;But I don't understand, how to calculate that eccentricity anomaly? How to calculate velocity? Is there something like mean anomaly and &quot;mean velocity&quot; (like in ecliptic trajectories)? And is there some formula to calculate it?&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Solution&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Okay, guys, thank you all, finally I got it. So what conclusion I came to:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we have some body with velocity and position relative to the body around which first one is going to rotate and time, then we should:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Define orbit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Angular momentum:&#xA;$$&#xA;\bar{h} = \bar{r}\times \bar{v}&#xA;$$&#xA;Eccentricity vector:&#xA;$$&#xA;\bar{e} = \frac{\bar{v}\times \bar{h}}\mu - \frac{\bar{r}}{r}&#xA;$$&#xA;Perigee inclination angle:&#xA;$$&#xA;i=\textrm{atan2}(-\bar{e}_y,-\bar{e}_x)&#xA;$$&#xA;Semi major axis:&#xA;$$&#xA;a=(\frac{2}{r}-\frac{v^2}{\mu})^{-1}&#xA;$$&#xA;If we will rotate position vector by -i, then we will have raw position vector(which not affected by trajectory inclination), this is just inverted 2D vector rotation formula:&#xA;$$&#xA;\bar{p}=\binom{\bar{r}_x\textrm{cos}i+\bar{r}_y\textrm{sin}i}{\bar{r}_y\textrm{cos}i-\bar{r}_x\textrm{sin}i}&#xA;$$&#xA;Then by having raw position and by calculating sinh and cosh we can find creation hyperbolic anomaly:&#xA;$$&#xA;H_0=\textrm{atanh}(\frac{\frac{\bar{p}_y}{a\sqrt{e^2-1}}}{e-\frac{\bar{p}_x}{a}})&#xA;$$&#xA;And then simply find creation mean anomaly:&#xA;$$&#xA;M_0=\textrm{sinh}(H_0)e-H_0&#xA;$$&#xA;Also there is a need of mean velocity:&#xA;$$&#xA;n=\sqrt{\frac{\mu}{|a^3|}}\textrm{sign}(h)&#xA;$$&#xA;And for future we need some creation time, which should be gotten from simulation time, when orbit is created:&#xA;$$&#xA;t_0 = T\textrm{, where T is currect simulation time.}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Any time get orbit status by time&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculate current mean anomaly:&#xA;$$&#xA;M=M_0 + (T-t_0)n&#xA;$$&#xA;Find current hyperbolic anomaly via newton's method:&#xA;$$&#xA;H_{j+1}=H_j+\frac{M-e\textrm{sinh}(H_j)+H_j}{e\textrm{cosh}(H_j)-1}&#xA;$$&#xA;And get current raw position vector:&#xA;$$&#xA;\bar{p}=\binom{a(e-\textrm{cosh}H)}{a\sqrt{e^2-1}\textrm{sinh}H}&#xA;$$&#xA;And finally to get body relative position at time T we should rotate raw position vector by inclination angle:&#xA;$$&#xA;\bar{r}=\binom{\bar{p}_x\textrm{cos}i-\bar{p}_y\textrm{sin}i}{\bar{p}_x\textrm{sin}i+\bar{p}_y\textrm{cos}i}&#xA;$$&#xA;Have fun!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OHSwL.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OHSwL.png&quot; alt=&quot;Reward screen&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19959" LastEditorUserId="19959" LastEditDate="2017-06-25T21:00:44.810" LastActivityDate="2023-11-06T00:19:19.927" Title="Simulate celestial body motion on hyperbolic trajectory (2D)" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22001" PostTypeId="1" AcceptedAnswerId="22002" CreationDate="2017-06-17T20:04:22.483" Score="12" ViewCount="9632" Body="&lt;p&gt;&lt;strong&gt;How are rocket nozzles made??&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it a huge chunk of metal that they cut down or is it a &quot;pipe&quot; that they reshape?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or something else??&lt;/p&gt;&#xA;" OwnerUserId="17599" LastEditorUserId="6944" LastEditDate="2017-06-18T01:53:48.353" LastActivityDate="2018-11-15T17:33:58.133" Title="How are rocket nozzles made?" Tags="|rockets|engine-design|nozzle|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22002" PostTypeId="2" ParentId="22001" CreationDate="2017-06-18T01:27:30.410" Score="21" Body="&lt;p&gt;There are many kinds of nozzles, and many ways to manufacture them. Here is a sampling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actively cooled nozzles such as the the &lt;a href=&quot;https://spaceflightnow.com/news/n0105/31ssmenozzle/&quot; rel=&quot;noreferrer&quot;&gt;SSME&lt;/a&gt; and &lt;a href=&quot;http://heroicrelics.org/info/f-1/f-1-thrust-chamber.html&quot; rel=&quot;noreferrer&quot;&gt;F-1&lt;/a&gt; nozzles were constructed by fabricating the individual tubes that made up the cooling channels (1080 tubes in the case of the SSME) and brazing them together in an autoclave.  Nozzle fabrication was one of the pacing items in building an SSME; at one time I was told it took over a year to complete one nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8QPdU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8QPdU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern actively-cooled engine nozzles may be constructed using &lt;a href=&quot;https://books.google.com/books?id=LQbDOxg3XZcC&amp;amp;pg=PA332&amp;amp;lpg=PA332&amp;amp;dq=channel+wall+rocket+engine&amp;amp;source=bl&amp;amp;ots=TzUkUEyOD_&amp;amp;sig=x_KkOQJ1H_D1EXBM0_w0t5_4qcI&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=2ahUKEwjAtv_19tbeAhUSylMKHbh_DVc4FBDoATAJegQIAxAB#v=onepage&amp;amp;q=channel%20wall%20rocket%20engine&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;channel-wall technology&lt;/a&gt; pioneered in Russia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radiatively cooled nozzles such as the shuttle OMS engines (which have been cannibalized to use on the &lt;a href=&quot;http://spaceflight101.com/spacecraft/orion/&quot; rel=&quot;noreferrer&quot;&gt;Orion service module&lt;/a&gt;) are formed of high temperature metals such as columbium or niobium.  They may be welded or spinformed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7RYAL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7RYAL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This Merlin vacuum nozzle is clearly welded together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sMyPs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sMyPs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nozzles may also be made of composite materials, such as the STS Solid Rocket Booster nozzles.  They were built up of phenolic wraps and included a flexible bearing for thrust vector control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ape1H.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ape1H.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cold gas thruster nozzles may be manufactured of more common materials (stainless steel) using normal manufacturing techniques.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Obun1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Obun1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your question covers a great deal of territory. I have tried to give you a general idea of the range of materials and techniques used to manufacture nozzles.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-11-15T17:33:58.133" LastActivityDate="2018-11-15T17:33:58.133" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="22003" PostTypeId="1" AcceptedAnswerId="22004" CreationDate="2017-06-18T02:23:05.717" Score="5" ViewCount="4824" Body="&lt;p&gt;I just saw @OrganicMarble's &lt;a href=&quot;https://space.stackexchange.com/a/22002/12102&quot;&gt;survey answer&lt;/a&gt; that bravely tackles the wide variety of technologies used to make different types of rocket nozzles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One item caught my eye:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nozzles may also be made of composite materials, such as the STS Solid Rocket Booster nozzles.  They were built up of phenolic wraps and included a flexible bearing for thrust vector control.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;$\hskip4.6cm$ &lt;img src=&quot;https://i.stack.imgur.com/ape1H.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Phenol_formaldehyde_resin&quot; rel=&quot;nofollow noreferrer&quot;&gt;phenolic resins&lt;/a&gt; I'm most familiar with were used in the past to make circuit boards, and to produce that stuff people of a certain age remember - &lt;a href=&quot;https://en.wikipedia.org/wiki/Bakelite&quot; rel=&quot;nofollow noreferrer&quot;&gt;bakelite&lt;/a&gt;. If you've ever noticed that unique &quot;old radio smell&quot;, you've probably smelled phenolic resin in action. There are and have been (I am sure) many other applications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before flow techniques, soldering by hand with soldering irons could expose circuit boards to fairly high temperatures in certain contact points, and old electronics ran hot, so these organic compounds obviously offered some significant temperature resistance. This was important because I think the next step up at the time was probably ceramics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I would not have expected a mid-20th century organic compound to be used to make &lt;em&gt;rocket engine nozzles for the space shuttle!&lt;/em&gt; Yes it's the booster rather than the main engine, but wow! Those plumes look really really hot!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are these nozzles really made from phenolic compounds related to the components of old radios? What keeps them from melting or burning in the oxygen of Earth's atmosphere where they are used during the launch phase?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-18T18:45:06.797" LastActivityDate="2021-06-01T12:05:00.493" Title="How can phenolic (resin?) handle rocket engine nozzle temperatures?" Tags="|space-shuttle|engine-design|materials|nozzle|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22004" PostTypeId="2" ParentId="22003" CreationDate="2017-06-18T02:58:38.647" Score="12" Body="&lt;p&gt;There is a great writeup on this in the wonderful book &quot;Development of the Space Shuttle 1972-1981&quot; by T. R. Heppenheimer.  Highly recommended, as is his prior volume &quot;The Space Shuttle Decision&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;tl;dr: They did melt/burn. That was the whole idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Page 178-179:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nozzle: The ﬂame within a solid motor burned at 5,700 degrees&#xA;  Fahrenheit, which was hot enough to boil iron. What was to prevent it&#xA;  from destroying the booster? A liquid-fuel engine relied on&#xA;  regenerative cooling, as it circulated hydrogen through numerous small&#xA;  tubes or channels, but this was out of the question with solid&#xA;  propellants. Insulation helped, it protected the casing as the ﬂame&#xA;  front approached the wall. The nozzles of large solid motors relied on&#xA;  a third approach, for they were lined with thick slabs of ablative&#xA;  material. Like a reentering nose cone, this ablative layer could&#xA;  slowly decompose, vaporize, and erode as the burning proceeded.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;For the SRB nozzle, the basic ablative material was carbon cloth&#xA;  phenolic, a cloth woven of carbon ﬁber and strongly impregnated with&#xA;  phenolic resin. It cost thirty dollars per pound, and each SRB used&#xA;  it by the ton. Layers of this substance protected the throat as well&#xA;  as other regions that faced the full severity of the hot gas ﬂow.&#xA;  Silica cloth phenolic, woven from silica ﬁbers, protected parts where the&#xA;  thermal environment was less demanding. Glass cloth phenolic served as&#xA;  insulation.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;These materials came from vendors in the form of tape, with widths&#xA;  from three-fourths of an inch to thirteen inches. Rolls of tape fed a&#xA;  wrapping machine that laid the tape in plies on a rotating mandrel. A&#xA;  blast of hot air, at temperatures up to 700 degrees Fahrenheit, softened&#xA;  the resin. A roller pressed the tape against the substrate, with a force of up to three hundred pounds for each inch of tape width. After rotating past the roller, the tape was exposed to a flow of carbon dioxide at -60 degrees. This prevented the resin from curing and produced a hard, solid surface as a substrate for the next ply.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Each nozzle used ﬁve tons of carbon cloth phenolic, two tons of glass&#xA;  phenolic, and one ton of silica phenolic, all tape-wrapped in this&#xA;  fashion. Finished carbon lay-ups were cured in a hydroclave, which&#xA;  used water to apply heat and pressure. Other lay-ups went into an&#xA;  autoclave, which used carbon dioxide. Cured components were machined&#xA;  using diamond cutting tools, achieving tolerances as close as 0.0025&#xA;  inches.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;To achieve thrust vector control, the nozzle was to swivel by up to&#xA;  7.1 degrees in pitch and yaw. Designers avoided the use of sliding surfaces, which could prove difficult to seal against leaks of hot&#xA;  gas. Instead they used a ﬂexible support or bearing, built from ten&#xA;  steel plates interleaved with eleven layers of rubber. Similar ﬂexible&#xA;  bearings had ﬂown previously, but this was the largest ever built.&#xA;  Within the hot gas ﬂow path, the bearing lay in what amounted to a&#xA;  backwater, removed from the full force of this exhaust. Nevertheless,&#xA;  some gas would reach it, which meant that that this ﬂexible support&#xA;  needed ﬂexible thermal protection. It obtained this from a &quot;boot&quot;, a&#xA;  barrier of laminated rubber that eroded or burned away at a calculated&#xA;  rate and that was thick enough to hold out until the motor expended&#xA;  all of its propellant.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-06-18T12:47:30.723" LastActivityDate="2017-06-18T12:47:30.723" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22005" PostTypeId="2" ParentId="22000" CreationDate="2017-06-18T04:01:34.553" Score="4" Body="&lt;p&gt;$\tau$ is your independent variable that runs from some negative value to some positive positive value around the closest approach. Then you can plot the trajectory on an $x$-$y$ plot, and if you like mark times on it using the $t$ formula. $\tau$ and $t$ are both zero at closest approach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find the $\tau$ values corresponding to particular times by solving the $t$ equation using numerical root finding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Formulas for $v_x$ and $v_y$ are easily derived by taking the derivatives of the $x$ and $y$ formulas with respect to $\tau$, and dividing those by the derivative of the $t$ formula with respect to $\tau$.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-06-18T06:09:00.223" LastActivityDate="2017-06-18T06:09:00.223" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22006" PostTypeId="1" AcceptedAnswerId="23508" CreationDate="2017-06-18T11:13:19.670" Score="12" ViewCount="1179" Body="&lt;p&gt;&lt;strong&gt;edit 2:&lt;/strong&gt; There seems to have been some &lt;a href=&quot;https://www.nasa.gov/feature/jpl/dawn-mission-extended-at-ceres&quot; rel=&quot;noreferrer&quot;&gt;announcement/extension&lt;/a&gt; by NASA (20-Oct-2017), possibly this will shed more light on the issues in this question.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit 1:&lt;/strong&gt; Several months have passed since this question was asked, presumably a decision how to best allocate both the xenon and hydrazine propellants has been made. Still, the issues of very different roles each propellant has in an orbit around Ceres is interesting and not straightforward due to the inhomogeneities of Ceres' gravity field at different altitudes.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The Spaceflight Now article &lt;a href=&quot;https://spaceflightnow.com/2017/06/17/dawn-mission-managers-await-nasa-decision-on-spacecrafts-future/&quot; rel=&quot;noreferrer&quot;&gt;Dawn mission managers await NASA decision on spacecraft’s future&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Dawn’s primary mission ended in June 2016, and NASA officials approved a one-year extension that expires June 30. The fate of Dawn after June 30 remains uncertain, but senior managers at NASA Headquarters are expected to soon decide whether the spacecraft should be turned off, continue exploring Ceres, or depart the dwarf planet and perhaps fly by an asteroid.&lt;/p&gt;&#xA;&lt;p&gt;[...]&lt;/p&gt;&#xA;&lt;p&gt;Experts from JPL and Orbital ATK devised a hybrid method of controlling Dawn’s attitude with the two remaining reaction wheels and hydrazine thrusters, the spacecraft now must fully rely on its rocket jets, wrote Marc Rayman, Dawn’s chief engineer at JPL, in a mission update posted on a NASA website.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I can't quite follow it, but according to the article, sequential failure of some some if not of Dawn's &lt;em&gt;initially four&lt;/em&gt; reaction wheels have required the use of hydrazine thrusters for attitude control.&lt;/p&gt;&#xA;&lt;p&gt;I'm wondering if the hydrazine that is used for attitude control is spent mostly in switching between Earth-pointing and Ceres-pointing states, where the latter requires a faster-rotating attitude state? Why would a lower orbit use more hydrazine for attitude control? Or would it be actually for station keeping in an uneven gravity field?&lt;/p&gt;&#xA;&lt;p&gt;Then there is the issue of high versus low orbit.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;“The consequence is that Dawn uses more hydrazine in orbit around Vesta and Ceres than when it is journeying between worlds, orbiting the sun and maneuvering with its ion engine. And it uses more hydrazine in lower orbits than in higher ones,” Rayman wrote.&lt;/p&gt;&#xA;&lt;p&gt;“One day at our low-altitude mapping orbit, which was at 385 kilometers, would be equivalent to about 18 days (of hydrazine fuel) at higher altitude, which is what we’re in now,” Raymond said.&lt;/p&gt;&#xA;&lt;p&gt;“Lifetime at a lower altitude would likely be limited to weeks at this point,” she said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Could someone clarify the issues of hydrazine budget:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;how many reaction wheels are left?&lt;/li&gt;&#xA;&lt;li&gt;why is Dawn's attitude control so much more expensive in low orbit around Ceres?&lt;/li&gt;&#xA;&lt;li&gt;would Dawn's station keeping in low orbit around Ceres actually be a comparable drain to its attitude control?&lt;/li&gt;&#xA;&lt;li&gt;does Dawn's ion thruster have too little thrust for station keeping in low Ceres orbit?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;and how this would affect a decision to &amp;quot;stay high&amp;quot;, &amp;quot;go low&amp;quot;, or just go somewhere else and make better use of the Xenon?&lt;/p&gt;&#xA;&lt;p&gt;I'm looking for an answer specific to this situation - &lt;em&gt;Dawn, in its current state, around Ceres&lt;/em&gt;. General statements about what attitude control and station keeping are for alone aren't enough. I'm asking about the specific aspects in this case that NASA will consider as part of their decision on what Dawn will do next. Thanks!&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; Forward-looking question from 2013 &lt;a href=&quot;https://space.stackexchange.com/q/540/12102&quot;&gt;Does the Dawn spacecraft have the potential for an extended mission?&lt;/a&gt; has a few good answers worth reading.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-05T11:01:34.817" Title="Xenon vs Hydrazine, &quot;Should I Stay or Should I go?&quot; Dawn mission decisions" Tags="|fuel|attitude|station-keeping|dawn-mission|ceres|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22008" PostTypeId="1" AcceptedAnswerId="22009" CreationDate="2017-06-18T17:46:32.660" Score="6" ViewCount="3529" Body="&lt;p&gt;I have an idea for a cool Earth-pointing amateur experiment that could be done through the cupola windows of the ISS. All of them. It would be simple (and not look like the first photo below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there rough drawings of the cupola with approximate dimensions? It looks like it is a round window with six trapezoidal windows around it. I just need an estimate of the sizes of the round and trapezoidal windows, and the 3D offset and inclination of one of the trapezoids. It doesn't need to be very accurate, but right now it's hard to tell from wide-angle photos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've included some reference images below of the cupola as viewed from from inside and outside.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Astronaut Don Pettit and some cameras completely obscuring this view of the cupola aboard the ISS. From the Smithsonian article &lt;a href=&quot;http://www.smithsonianmag.com/smart-news/how-astronauts-take-such-beautiful-photographs-in-space-114708277/&quot; rel=&quot;noreferrer&quot;&gt;How Astronauts Take Such Beautiful Photographs in Space&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FjEvM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FjEvM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Unobstructed but wide-angle distorted view of the cupola, from &lt;a href=&quot;http://www.popsci.com/10-incredible-space-photos-from-astronaut-don-pettits-new-book&quot; rel=&quot;noreferrer&quot;&gt;Popular Science&lt;/a&gt;. There are six, equally spaced trapezoidal windows, although it is difficult to tell in this image due to the fish-eye distortion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/guJ7V.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/guJ7V.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; (From left) Expedition 40/41 crew members Alexander Gerst, Reid Wiseman and Max Suraev peer out of the cupola. From &lt;a href=&quot;https://blogs.nasa.gov/spacestation/2014/11/06/med-studies-as-departing-trio-trains-for-landing/&quot; rel=&quot;noreferrer&quot;&gt;blogs.nasa.gov&lt;/a&gt;. There are more exterior views of the cupola in &lt;a href=&quot;https://space.stackexchange.com/q/19393/12102&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TggV6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TggV6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-18T17:52:44.477" LastActivityDate="2017-06-18T19:34:10.503" Title="Approximate sizes and relative positions of the ISS' cupola windows?" Tags="|iss|design|spacecraft-window|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22009" PostTypeId="2" ParentId="22008" CreationDate="2017-06-18T18:10:12.640" Score="9" Body="&lt;p&gt;&lt;a href=&quot;http://www.capcomespace.net/dossiers/ISS/europe/cupola/fs007_11_cupola.pdf&quot; rel=&quot;noreferrer&quot;&gt;Here's a possibly-final spec sheet&lt;/a&gt;, dimensioned in millimeters:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/46RQx.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/46RQx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found another one on &lt;a href=&quot;https://www.capcomespace.net/dossiers/ISS/europe/cupola/cupola.htm&quot; rel=&quot;noreferrer&quot;&gt;this page&lt;/a&gt;, but it appears to be a quite early design, and while it offers more dimensional callouts, they don't seem to be internally consistent with the diagram.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-06-18T19:28:05.697" LastActivityDate="2017-06-18T19:28:05.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22010" PostTypeId="2" ParentId="22008" CreationDate="2017-06-18T19:34:10.503" Score="7" Body="&lt;p&gt;There's a pdf &lt;a href=&quot;http://wsn.spaceflight.esa.int/docs/Factsheets/4%20Cupola%20HR%20web.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; (may be largely redundant to the other answer).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a crop from it:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/SUupI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SUupI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-06-18T19:34:10.503" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22011" PostTypeId="2" ParentId="17972" CreationDate="2017-06-18T22:07:19.263" Score="13" Body="&lt;p&gt;That's an easy one....   S Band Antenna.....&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MlS04.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MlS04.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image comes from the document &lt;a href=&quot;http://www.klabs.org/DEI/References/general/alt_final_report.pdf&quot; rel=&quot;noreferrer&quot;&gt;JSC-13864 SPACE SHUTTLE ORBITER&#xA;APPROACH AND LANDING TEST FINAL EVALUATION REPORT&lt;/a&gt;.  It is on page 134 of the pdf.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a close up picture of the antenna, from &lt;a href=&quot;http://www.space1.com/Artifacts/Space_Shuttle_Artifacts/Other_Parts/S-band_Antenna/s-band_antenna.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;... &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JywT9.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JywT9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20052" LastEditorUserId="6944" LastEditDate="2017-06-21T20:20:38.777" LastActivityDate="2017-06-21T20:20:38.777" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22012" PostTypeId="1" CreationDate="2017-06-18T22:29:33.677" Score="5" ViewCount="1763" Body="&lt;p&gt;Can I get a rough estimate of the power (in watts) needed to produce light to grow enough food for one person, using vertical farming of the kind proposed for space habitats?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any reasonable outpost/colony concept that grows most of its food and does not use direct light, but has information on the base's energy source (kWe of a nuclear reactor, area of photovoltaic arrays) would be useful for finding conservative estimates.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastEditorUserId="19742" LastEditDate="2017-06-19T00:33:10.907" LastActivityDate="2017-06-21T08:48:03.167" Title="Power to grow food per person" Tags="|energy|food|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22013" PostTypeId="1" CreationDate="2017-06-18T22:31:59.573" Score="2" ViewCount="346" Body="&lt;p&gt;Although the exact usage of the system is classified, NASA has a system called TDRS which is used by other government agencies for relaying data to ground stations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To maintain communications with a ground station, a satellite would be stabilized in some manner and use a directional antenna. If the satellite is in a low earth orbit, the antenna essentially just has gain in the direction towards the earth. A tracking station acquires the satellite and communicates with it while it is in view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do two satellites in low earth orbit establish communications. Do they use directional antennas? For a satellite in typical low earth orbit, what is the radio horizon? How do they avoid interference amongst one another?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia &#xA; link:&lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_data_relay_satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Tracking_and_data_relay_satellite&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14488" LastEditorUserId="14488" LastEditDate="2017-06-18T23:53:21.980" LastActivityDate="2017-06-18T23:53:21.980" Title="How do satellites communicate with a relay satellite?" Tags="|orbit|radio-communication|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22014" PostTypeId="1" AcceptedAnswerId="22015" CreationDate="2017-06-19T01:18:21.210" Score="2" ViewCount="203" Body="&lt;p&gt;This question might be more of a sci-fi question, so forgive me. It was inspired on behalf of the movie Life (2017) where the crew aboard the ISS catch a soil sample from Mars, and discover that it contains a dormant cell lifeform of sorts. This, as you can imagine, ends up being catastrophic for the crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now please humor me for a moment. I understand what I am asking is well beyond any realism, but I think it is still applicable to consider even in realm of 0.00000000001% possibility of happening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; In the event that foreign life is discovered and brought aboard the ISS, and in the event that, somehow, containment of this foreign life fails and is now loose aboard the ISS; &lt;em&gt;(from NASA's perspective)&lt;/em&gt; are there any procedures on what to do in such a situation? Has any such scenario ever been assessed?&lt;/p&gt;&#xA;" OwnerUserId="20109" LastEditorUserId="195" LastEditDate="2017-06-19T02:57:23.020" LastActivityDate="2017-06-19T02:57:55.853" Title="Are there any procedures against biohazard containment failure on ISS?" Tags="|life|failure|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22015" PostTypeId="2" ParentId="22014" CreationDate="2017-06-19T01:26:25.870" Score="6" Body="&lt;p&gt;I don't believe any such procedures have ever been seriously considered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there are vastly greater research capabilities on Earth's surface than on the ISS, and it takes a much greater amount of ∆v to rendezvous with ISS in orbit than to plow into Earth's atmosphere from a Mars return trajectory, there is zero reason to return a sample from Mars to ISS. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-06-19T02:57:55.853" LastActivityDate="2017-06-19T02:57:55.853" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22016" PostTypeId="2" ParentId="22012" CreationDate="2017-06-19T01:50:17.043" Score="2" Body="&lt;p&gt;Looking around for data on indoor agriculture, the most relevant stuff I could find was for marijuana growing in &lt;a href=&quot;http://hightimes.com/grow/how-much-energy-does-indoor-pot-really-use/&quot; rel=&quot;nofollow noreferrer&quot;&gt;High Times magazine&lt;/a&gt;. It appears to be fantastically energy-intensive, possibly using up as much as 1% of the US's electrical energy output. Growing a kilogram of marijuana seems to require about $E=2\times10^{10}$ J of energy. Let's assume we can grow a kilogram of wheat with the same energy input. Wheat has an energy density of about $d=3400$ kcal/kg. A human needs about $c=0.02$ kcal/s of food.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The result is $cE/d\sim100$ kW of electrical power to support one human being with indoor farming. Given all the very rough order-of-magnitude assumptions involved, this seems to be roughly in line with Anthony X's estimate in a comment of 500 kW. No matter what, this is a fantastic amount of energy, dwarfing the approximately 2 kW per person used at Antarctic bases, which ship in their food.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that a self-supporting space colony would do well to use sunlight, not electrical lighting, for their agriculture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison with $E\sim10^{10}$ J to grow a kg of wheat indoors, it only costs something like $5\times10^8$ J to accelerate a kg to the same velocity as the earth's orbital velocity around the sun. This suggests that if you're somewhere that doesn't have enough sunlight (surface of Titan?), you might be smart simply to import your food.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorUserId="12102" LastEditDate="2017-06-19T02:11:44.930" LastActivityDate="2017-06-19T02:11:44.930" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22017" PostTypeId="1" AcceptedAnswerId="22021" CreationDate="2017-06-19T02:39:14.140" Score="5" ViewCount="523" Body="&lt;p&gt;If someone wanted to grow large quantities of food on Mars and they wanted to specify sunlight rather than nuclear or other sources, the two options I can think of are greenhouses and photovoltaic/LEDs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider square kilometer types of areas:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Greenhouse windows would have to support the mechanical pressure of the whatever atmospheric pressure the plants required (let's say 1/3 Earth unless you have more authoritative numbers). Without Earth's warm air, the outside surface of greenhouse windows would be exposed to the infrared temperatures of space and the panes would have to do some thermal insulation also. These plants would suffer if it got too cold inside, and for optimum growth rate would the temperature need to be regulated carefully? Then there are concentrators to take the weaker Martian sunlight and focus it on individual plants for efficient use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compare to photovoltaics - could they be made robust against the low pressure and cold more easily than greenhouse windows?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would stacking plants in 3D with LEDs be easier than spreading them out over kilometers of greenhouse floors? Would humidity and temperature control be easier and more efficient and total kilograms of water needed be lower in one configuration over the other?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an example of what 3D stacking means, see &lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2687674/Now-THATS-power-plant-Indoor-farm-grows-10-000-heads-lettuce-DAY-using-lights-mimic-day-night.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Daily Mail&lt;/a&gt;, and &lt;a href=&quot;http://modernfarmer.com/2014/12/salad-inc/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Modern Farmer&lt;/a&gt; and &lt;a href=&quot;http://news.nationalgeographic.com/news/2014/07/140717-japan-largest-indoor-plant-factory-food/&quot; rel=&quot;nofollow noreferrer&quot;&gt;National Geographic&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several related questions here, but I believe that if an authoritative source comparing greenhouses to PV/LEDs exists, it will likely have considered all of these.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/a2etC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/a2etC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-2687674/Now-THATS-power-plant-Indoor-farm-grows-10-000-heads-lettuce-DAY-using-lights-mimic-day-night.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-12-06T00:21:44.853" LastActivityDate="2018-12-06T00:21:44.853" Title="Which is easier to build on mars per square kilometer; greenhouse windows or photovoltaics/LEDs?" Tags="|mars|colonization|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="22018" PostTypeId="1" AcceptedAnswerId="23904" CreationDate="2017-06-19T04:45:35.077" Score="2" ViewCount="902" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; For the purposes of this exercise, one could consider using three or four identical spacecraft, and using the &quot;best two out of three (or four)&quot; for interferometry at any given moment.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In 1994, &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-59&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-59&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-68&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-68&lt;/a&gt; flew &lt;a href=&quot;https://en.wikipedia.org/wiki/Spaceborne_Imaging_Radar&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceborne Imaging Radar&lt;/a&gt;, a type of &lt;a href=&quot;https://en.wikipedia.org/wiki/Synthetic_aperture_radar&quot; rel=&quot;nofollow noreferrer&quot;&gt;Synthetic Aperture Radar&lt;/a&gt; (SAR). SAR is a type of interferometric radar where phase information (relative to the onboard local oscillator) is recorded and the interference phenomena is calculated/simulated offline during analysis. Instead of multiple antennas, multiple data sets are recorded with the shuttle or spacecraft in different but nearby positions in its orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two sequential measurements milliseconds apart provides the separation in the orbital track direction, but near-coincidental ground-track orbits separated by long periods of time are necessary to get displacements in the perpendicular direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in 2000 &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-99&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-99&lt;/a&gt; was launched with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shuttle Radar Topography Mission&lt;/a&gt;. By extending a second antenna 60 meters away from the shuttle perpendicular to the orbit track, interference in this direction could be implemented with conventional interferometry, while the SAR-like data analysis was still used for the orbit direction. &lt;strong&gt;below:&lt;/strong&gt; from &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission#/media/File:Srtm_1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$\hskip2.4cm$&lt;/span&gt; &lt;a href=&quot;https://i.stack.imgur.com/aDoXo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aDoXo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Without the mechanical extension, if the 2nd antenna was at the same altitude as the shuttle and moving parallel, it would collide with the shuttle in one quarter of an orbit. This is because their orbital planes would intersect. &lt;em&gt;Conventional parallel orbits around a spherically symmetric body do not exist.&lt;/em&gt; (Let's leave discussion of exotic orbits around cigar-shaped bodies to other questions.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an exercise, what kinds of orbital solutions exist if the extension scaffolding were not used? Suppose it wiggled too much for a hypothetical next generation shorter wavelength system, or that a longer baseline was desired. Suppose also you could use more than one antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What orbital &quot;tricks&quot; could be used to provide at least one antenna orbiting roughly parallel to a primary spacecraft at any time? The separation does not have to be constant, it just needs to be known/predictable, and should keep working for a few weeks at least.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note:&lt;/em&gt; Now that we have independently flying antennas, an additional unit could fly ahead of the primary without problem. That may or may not eliminate the need for SAR analysis, there may be other benefits such as the much large effective baselines. Also, assume optical links between spacecraft, so no more coaxial cables.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;Space Shuttle Radar Topography Mission Canister, Antenna&quot; from the Smithsonian National Air and Space Museum &lt;a href=&quot;https://airandspace.si.edu/collection-objects/cannistermast-shuttle-radar-topography-mission-payload&quot; rel=&quot;nofollow noreferrer&quot;&gt;A20040261000d20&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OZSzd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OZSzd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="195" LastEditDate="2019-07-02T01:44:16.620" LastActivityDate="2019-07-02T01:44:16.620" Title="Parallel orbits around the Earth - effectively?" Tags="|orbital-mechanics|imaging|radar|synthetic-aperture-radar|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="22020" PostTypeId="2" ParentId="22017" CreationDate="2017-06-19T08:40:23.247" Score="2" Body="&lt;p&gt;This is a pretty difficult question to solve:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem with using a greenhouse on Mars would be the high levels of harmful solar radiation on the surface due to Mars' thin atmosphere and lack of magnetic field. Using solars panels on the surface of Mars would also be difficult on a large scale due to dust accumulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And like you mentioned, there are many more factors going into this like thermal insulation and the low-pressure environment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A possible solution would be to use solar energy satelites to beam power down to an underground 3D stacked farm using a microwave transmitter and reciever. While such a setup is more viable on Mars due to the low atmospheric pressure, the transmitter would still end up being large and difficult to set up.&lt;/p&gt;&#xA;" OwnerUserId="20112" LastEditorUserId="20112" LastEditDate="2017-06-19T08:52:55.707" LastActivityDate="2017-06-19T08:52:55.707" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22021" PostTypeId="2" ParentId="22017" CreationDate="2017-06-19T09:12:56.080" Score="1" Body="&lt;p&gt;Solar irradiance on Mars is &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2001/benoit/solar_irradiance_on_mars.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;about half that on Earth&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a greenhouse would have to be twice as large as on Earth to get an Earth-equivalent amount of light on the plants, using solar concentrators. If you build the concentrators outside the greenhouse, the greenhouse can be smaller but you'd need a high structure for the concentrator (as high as the greenhouse is wide, at first approximation).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume all the building materials have to be transported from Earth, so weight serves as a proxy for cost. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, greenhouses are easy to build: just a frame and panes of glass. &#xA;On Mars, the structure has to be pressurized, making the structure heavier. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A pane of glass that can contain 0.33 bar is going to be &lt;a href=&quot;http://www.pilkington.com/resources/brglassmechanicalstrengthpdf.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;thick&lt;/a&gt;: a load of 4 kN/m&lt;sup&gt;2&lt;/sup&gt; already needs 2x 25 mm of glass, and 1/3 bar is 33 kN/m&lt;sup&gt;2&lt;/sup&gt;. Plastic may be thinner, but not 100 times thinner. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;PV panels, on the other hand, don't need atmospheric pressure to function. So you can build light frames with panels on top. Even roll-up arrays might work. I can easily see a 10:1 weight advantage for PV panels. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cheapest combination would then be a small pressurized building full of plants using 3D stacking, powered by a field of PV outside. You could even stick the plants inside a tunnel and have them protected from radiation.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-06-19T14:16:18.183" LastActivityDate="2017-06-19T14:16:18.183" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="22022" PostTypeId="1" AcceptedAnswerId="22032" CreationDate="2017-06-19T09:58:33.140" Score="4" ViewCount="415" Body="&lt;p&gt;If we have discovered artificial gravitational force, will we be able to create an artificial habitable planet in our solar system? E.g., if a rotating cylindrical object in space can produce artificial gravity at its perimeter, is it theoretically possible to build such an object in space and by injecting air and other ingredients (soil, water, etc ..., ) into it make it a self-sustaining, life-supporting artificial planet?&lt;/p&gt;&#xA;" OwnerUserId="20114" LastEditorUserId="6816" LastEditDate="2018-01-07T07:03:17.343" LastActivityDate="2018-01-07T07:03:17.343" Title="Artificial planet" Tags="|life-support|artificial-gravity|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22024" PostTypeId="1" AcceptedAnswerId="22026" CreationDate="2017-06-19T18:10:26.060" Score="11" ViewCount="1359" Body="&lt;p&gt;In the &lt;a href=&quot;https://www.abebooks.com/Man-Moon-Voyages-Apollo-Astronauts-Andrew/22429031488/bd?cm_mmc=gmc-_-sig-_-PLA-_-v01&amp;amp;product=COM9780670814466SIG&quot; rel=&quot;noreferrer&quot;&gt;first edition of Andrew Chaikin's 1994 book, &lt;em&gt;&quot;A Man on the Moon&lt;/em&gt;&lt;/a&gt;, there is a paragraph in chapter 1 which talks about troubles the Apollo 1 testing crew were going through. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chaikin states that on January 27, 1967 around 1 pm just after testing had started, Grissom smelled something like sour milk. It took the crew about an hour to find the source of the smell, and the testing continued.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the specific excerpt from the book:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Today's simulated countdown was nothing new; it wasn't considered dangerous-The Saturn booster was not fueled-or even difficult. But there was trouble almost from the time Grissom and his crew climbed into the command module cabin, around 1 pm. First there was an unidentified odor in the breathing oxygen that reminded Grissom of sour milk; that alone held up the test for an hour. Finally the problem was solved, and at 2:45 P.M. the pad crew installed the command module's heavy, two-piece hatch and sealed it shut.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As you can see, Chaikin never really mentioned what exactly was causing the smell or &lt;em&gt;how&lt;/em&gt; they fixed it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I am curious about this. Does anyone know what exactly was causing the smell that Grissom smelled, and how they fixed the problem?&lt;/p&gt;&#xA;" OwnerUserId="16704" LastEditorUserId="10363" LastEditDate="2019-02-18T05:16:31.250" LastActivityDate="2019-02-18T05:16:31.250" Title="What was the &quot;sour milk smell&quot; during the 1967 Apollo 1 testing?" Tags="|apollo-program|failure|testing|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22025" PostTypeId="2" ParentId="9661" CreationDate="2017-06-19T18:26:58.857" Score="12" Body="&lt;p&gt;There are windows on Zvezda from which you can see zenith, but these aren't facing exactly zenith. It doesn't seem as though any other segment on the ISS have zenith windows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://suzymchale.com/ruspace/sm.html&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; website, at the bottom there are attachments that show the various cross-sections for Zvezda. It's a little confusing, however, since this image&lt;a href=&quot;https://i.stack.imgur.com/LUCgp.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LUCgp.gif&quot; alt=&quot;zenith cross section&quot;&gt;&lt;/a&gt; has the +x and +z labelled as though you were looking at it from the +y direction, not from zenith. Based on actual photos of Zvezda, however, this image is indeed zenith. As you can see, windows 12 and 13 on the neck of the craft are pointed at a 45 degree angle towards zenith.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/lXHwA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lXHwA.jpg&quot; alt=&quot;3D model of Zvezda&quot;&gt;&lt;/a&gt;&#xA;This 3D model of Zvezda shows the two closed windows, on the neck near the orange circle. You can clearly see window 12 facing at an angle towards space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FHlD5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FHlD5.jpg&quot; alt=&quot;Zvezda&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the best actual image I could find of Zvezda in space (it proves we are looking at the correct side), but unfortunately the view of the windows are obstructed by the body of the craft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found this picture&#xA;&lt;a href=&quot;https://i.stack.imgur.com/SPGgc.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SPGgc.jpg&quot; alt=&quot;Far away&quot;&gt;&lt;/a&gt;&#xA;which is the correct angle, but it's difficult to see the window. If you zoom in, you can, however, barely make it out. It's covered and just looks like a circle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in conclusion, it doesn't look like there are any windows facing &lt;em&gt;exactly&lt;/em&gt; zenith, but there are two that angle in that direction.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastEditorUserId="19427" LastEditDate="2017-06-20T18:09:26.767" LastActivityDate="2017-06-20T18:09:26.767" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="22026" PostTypeId="2" ParentId="22024" CreationDate="2017-06-19T18:42:09.477" Score="8" Body="&lt;p&gt;The only further information I've ever seen on the odor is in the paper &quot;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=18&amp;amp;ved=0ahUKEwjrjL-Vx8rUAhUF0oMKHQPoAIY4ChAWCEIwBw&amp;amp;url=https%3A%2F%2Fdspace.mit.edu%2Fbitstream%2Fhandle%2F1721.1%2F55162%2F607570236-MIT.pdf%3Fsequence%3D2&amp;amp;usg=AFQjCNF-_4FgYXyGKjWceSRbQNgy2pbHGg&amp;amp;sig2=PZ8QdSe7IvZhVNA3ml6YdA&quot; rel=&quot;noreferrer&quot;&gt;Accidents, Engineering, and History at NASA&lt;/a&gt;&quot; which states in a footnote that&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sour milk smell is characteristic of butyric acid, which was a&#xA;  constituent element of a number of plastic products in the 1960s.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I can not find any evidence that anything was done to resolve the problem, just that air samples were taken for future analysis.  The &lt;a href=&quot;https://history.nasa.gov/SP-4029/Apollo_01c_Timeline.htm&quot; rel=&quot;noreferrer&quot;&gt;Timeline&lt;/a&gt; states that the cabin was purged with oxygen, but does not make it clear whether this was because of the odor, or a planned event.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-06-19T18:42:09.477" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22027" PostTypeId="1" CreationDate="2017-06-19T20:31:43.803" Score="2" ViewCount="234" Body="&lt;p&gt;How will the Red Dragon take off from Mars and get back to earth? All the drawings I've seen just show the capsule on the surface and no rocket to take off again or power the return flight.  Could they really build that all into the Dragon?  &lt;/p&gt;&#xA;" OwnerUserId="19834" LastEditorUserId="51" LastEditDate="2017-06-19T23:06:53.937" LastActivityDate="2017-06-21T14:58:16.287" Title="Red Dragon sample return" Tags="|spacex|sample-return|red-dragon|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22028" PostTypeId="2" ParentId="22024" CreationDate="2017-06-19T22:05:06.973" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://books.google.com/books?id=xwtrDQAAQBAJ&amp;amp;pg=PA245&amp;amp;lpg=PA245&amp;amp;dq=sour%20milk%20smell%20oxygen%20supply&amp;amp;source=bl&amp;amp;ots=BjS947LcxF&amp;amp;sig=m78qIV7g-9hKA1mYbFToC4aMHb4&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwi8tqyY9MrUAhVQ1GMKHV_fB1UQ6AEIKzAC#v=onepage&amp;amp;q=sour%20milk%20smell%20oxygen%20supply&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Calculated Risk: The Supersonic Life and Times of Gus Grissom&lt;/a&gt; says: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Donald Babbitt, the North American Aviation pad leader who arrived in the White Room for his normal shift about 3:30 pm, later told investigators the smell &quot;reminded [him] of a potting compound&quot; used in spacecraft assemblies for waterproofing and to withstand shock and vibrations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The book also backs up the statement in Organic Marble's answer that an air sample was taken and there was no actual solution to the problem at the time.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-06-19T22:05:06.973" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22029" PostTypeId="2" ParentId="22027" CreationDate="2017-06-19T23:06:36.513" Score="2" Body="&lt;p&gt;At this early stage, it is unclear what SpaceX is really planning.  They have passed up on the 2018 window, and now planning for the 2020 transfer window. This means they will have two more years to change their plans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Early suggestions have been to treat the inside as a missile silo with a one or two stage booster to launch from inside the Dragon capsule to return a payload. It is still very early in the planning.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-19T23:06:36.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22030" PostTypeId="2" ParentId="22027" CreationDate="2017-06-19T23:49:38.210" Score="1" Body="&lt;p&gt;Everything about Red Dragon is in flux, including whether it's doing a sample return at all, as the main purpose is to demonstrate landing. Chances are sample return depends heavily on how SpaceX does in the next few years, and where they focus, as it will be a challenging mission to engineer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't seen a single proposal to have the capsule itself take off again after landing, only small rockets launched from inside the capsule. This makes sense, as Red Dragon is far more useful staying on the surface than being sacrificed for maybe a few hundred m/s, max. It would also require less re-engineering, as most of the changes would be restricted to the interior and top hatch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In situ resource utilization (ISRU) to enable lower launch mass and a higher return payload is unlikely, as the Dragon 2 does not use methalox.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastEditorUserId="18085" LastEditDate="2017-06-21T14:58:16.287" LastActivityDate="2017-06-21T14:58:16.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22031" PostTypeId="1" AcceptedAnswerId="32512" CreationDate="2017-06-20T06:17:09.447" Score="7" ViewCount="988" Body="&lt;p&gt;Comments below &lt;a href=&quot;https://space.stackexchange.com/a/22021/12102&quot;&gt;this answer&lt;/a&gt; to the question &lt;a href=&quot;https://space.stackexchange.com/q/22017/12102&quot;&gt;Which is easier to build on mars per square kilometer; greenhouse windows or photovoltaics/LEDs?&lt;/a&gt; discuss the possibility of using plastic instead of glass for transparent greenhouse roofs on Mars in square kilometer quantities for a contributing role in food production there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one is shipping then from Earth, mass is important. The densities of common transparent glass and plastic materials useful for windows are around 2.4 and 1.2 g/cm^3, so you could ship twice as many panes of plastic as glass if the dimensions were equal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But they won't be. The completed window assembly must support some pressure differential, and insulate against an infrared radiative heat loss to space through the thin Martian atmosphere in order to keep plants at their ideal temperature for maximum growth. At night you can use opaque insulating covers, but while the windows are letting in sunlight, you're going to be radiating into space. If you don't believe me, point an IR thermometer up into a blue sky during the day, but away from the sun. It will register c-c-c-cold!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even in the UV received on Earth at sea level, many plastics will age in direct sunlight. The UV breaks the weaker of the organic bonds in the plastic. Results are solarization, crazing, and structural weakening. In the much stronger solar UV flux on the surface of Mars, this will be accelerated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any known transparent plastic candidates for an optimized &quot;Mars plastic&quot; that could replace glass as 15-year greenhouse windows on Mars? Would they be strong enough so that the total shipping weight per square kilometer would be lower than that for panes of optimized &quot;Mars glass&quot;?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;Lexan window after 7 years&quot; (on Earth, of course), from &lt;a href=&quot;http://windborneinpugetsound.blogspot.tw/2016/07/lexan-vs-plexiglass.html&quot; rel=&quot;noreferrer&quot;&gt;Lexan vs. Plexiglass&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xNInt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xNInt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below $\times$3:&lt;/strong&gt; &quot;This Plexiglass port is 38 years old.&quot; (on Earth, of course), from &lt;a href=&quot;http://windborneinpugetsound.blogspot.tw/2016/07/lexan-vs-plexiglass.html&quot; rel=&quot;noreferrer&quot;&gt;Lexan vs. Plexiglass&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\hskip1.6cm$&lt;a href=&quot;https://i.stack.imgur.com/mqeCG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mqeCG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sqLUX.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sqLUX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$\hskip1.6cm$&lt;a href=&quot;https://i.stack.imgur.com/1i2QW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1i2QW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-20T09:50:55.900" LastActivityDate="2021-08-07T13:30:46.410" Title="Sunroofs on Mars - durable transparent structural plastics lighter than glass to ship from Earth?" Tags="|mars|colonization|habitat|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="22032" PostTypeId="2" ParentId="22022" CreationDate="2017-06-20T08:21:53.237" Score="3" Body="&lt;p&gt;It is theoretically possible to build large artificial structures in place, including ones capable of supporting life. You wouldn't need artificial gravity if you use centripetal acceleration as you mention in your post, or if the object is massive enough it will have gravity of its own sufficient for humans.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What it would take to build structures like that are propulsion, energy and manufacturing technologies which are far, far more advanced than what we have currently. You'd have to be able to move a vast amount of material to one place, then transform it into into something else using heat or some sort of manufacturing process. Our propulsion systems are inadequate to move large objects quickly enough to use them, they are barely enough to get small probes to other planets in our solar system. We can power spacecraft for life support and to do a bit of work, not melt down an asteroid, which we don't have the technology to do anyway. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while its certainly possible, its going to take a long time. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2017-06-20T08:21:53.237" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22033" PostTypeId="1" AcceptedAnswerId="22035" CreationDate="2017-06-20T08:40:16.020" Score="1" ViewCount="436" Body="&lt;p&gt;I spotted this tiny, articulated dish antenna-like thing on the Zvezda module of the ISS while looking around for images &lt;a href=&quot;https://space.stackexchange.com/questions/9661/does-the-iss-have-zenith-facing-windows#comment61946_22025&quot;&gt;for here&lt;/a&gt;. It looks like it's associated with and/or pointing in the same direction as the big shiny concave cone below it and the black antireflection-coated cone to the right (camera?).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideas?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Original, full size, uncropped image: &lt;a href=&quot;http://iss.jaxa.jp/spacerad/images/img_dos01_e.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://iss.jaxa.jp/spacerad/images/img_dos01_e.jpg&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source/Location: &lt;a href=&quot;http://iss.jaxa.jp/spacerad/EI004.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://iss.jaxa.jp/spacerad/EI004.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U1zDq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U1zDq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-06-21T06:50:15.307" Title="What is this tiny articulated dish antenna-like thing on Zvezda?" Tags="|iss|identify-this-object|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22034" PostTypeId="1" AcceptedAnswerId="22037" CreationDate="2017-06-20T11:02:05.647" Score="2" ViewCount="266" Body="&lt;p&gt;Say we have a LEO satellite. There are 5000 low power devices on earth (They are in the close vicinity, can be served by same satellite) which needs to be communicated with the satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now my question is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) With the number of devices on earth, the number of channels changes so does the bandwidth of operation ? so what is the band of operation in this case ? If after few years the number of devices double then how will satellite expert go on expanding their system?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) How does the frequency allocation actually works in this case ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) How does the satellite respond to the simultaneous queries ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4) Can we use combination of FDM and TDM for this operation ? or DS-CDMA ? or which is the most suitable standard (Access Method) for this case ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5) What is the lowest power with which the devices on earth can operate on ?&lt;/p&gt;&#xA;" OwnerUserId="20083" LastEditorUserId="20083" LastEditDate="2017-06-20T11:10:30.270" LastActivityDate="2017-06-20T16:29:25.153" Title="How can satellites serve a large number of devices on earth?" Tags="|artificial-satellite|low-earth-orbit|power|communication-satellite|standards|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22035" PostTypeId="2" ParentId="22033" CreationDate="2017-06-20T11:39:14.730" Score="4" Body="&lt;p&gt;It seems to be the &lt;a href=&quot;https://en.wikipedia.org/wiki/Kurs_(docking_navigation_system)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kurs Antenna&lt;/a&gt; used to align Soyuz and Progress while docking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russian vehicles have the same antenna as depicted in this picture, we clearly see the alignment of those on the right of the picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0jimT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0jimT.jpg&quot; alt=&quot;Progress docking with the ISS&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source of the picture: &lt;a href=&quot;https://en.wikipedia.org/wiki/Kurs_(docking_navigation_system)#/media/File:Progress_M-13M_docking.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17308" LastEditorUserId="12102" LastEditDate="2017-06-21T06:50:15.307" LastActivityDate="2017-06-21T06:50:15.307" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22036" PostTypeId="2" ParentId="22033" CreationDate="2017-06-20T11:44:25.277" Score="5" Body="&lt;p&gt;That antenna is actually on the Docking Compartment (Pirs), not the Service Module, and is part of the Kurs automated docking system.  Note how it lines up with the antenna on the Progress in this picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;shiny concave cone&quot; is the docking system interface for the &quot;probe&quot; on the Soyuz/Progress vehicles, and the black cone is a window in the Service Module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OdgiA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OdgiA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-06-20T11:51:12.170" LastActivityDate="2017-06-20T11:51:12.170" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22037" PostTypeId="2" ParentId="22034" CreationDate="2017-06-20T16:01:14.757" Score="1" Body="&lt;p&gt;for question 1: &#xA;bandwidth required depends on what kind of communications you want. Telephone can be done in something like 9.6 kbit/s for one device, video needs hundreds of kbit/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite (and its ground station) is designed with a certain load in mind (X devices * Y kbit/s), and the satellite owner buys/negotiates enough bandwidth to use this capacity.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you exceed the design load, there's not much you can do. You can't transmit more data inside your allocated bandwidth without replacing all the devices in the chain ground station-satellite-end user devices. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's some information on bandwidth allocation here: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/5943/who-what-is-the-international-authority-to-assign-refuse-orbit-to-a-satellite-sp/5948#5948&quot;&gt;Who/What is the international authority to assign/refuse orbit to a satellite/space-craft?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/1264/what-frequency-wavebands-are-used-for-communication-in-leo/1267#1267&quot;&gt;What frequency wavebands are used for communication in LEO?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not a full answer, but some starting points on the relationship between bandwidth, data load and number of devices: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shannon-Hartley&lt;/a&gt; theorem gives an upper bound to the amount of data that can be sent over a given bandwidth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Techniques like &lt;a href=&quot;https://en.wikipedia.org/wiki/Quadrature_amplitude_modulation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Quadrature Amplitude Modulation&lt;/a&gt; can be used to transmit more information in a single clock pulse. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-06-20T16:29:25.153" LastActivityDate="2017-06-20T16:29:25.153" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22038" PostTypeId="1" CreationDate="2017-06-20T16:27:52.197" Score="2" ViewCount="325" Body="&lt;p&gt;Part of the problem is that the definition of robot can be a bit of a slippery slope. Can a spacecraft which has autonomous docking capability be considered a potential robot for example? If you are counting spacecraft, assume one of each kind you are counting is present.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And for this question, a robot does not necessarily need to be autonomous. A spacecraft remotely controlled could be a robot for this question for example. If something is officially robotic, it's a robot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To further narrow it down, for this question a robot should have some mobility. It should be able to move or orient itself — or at least appendages or parts of itself — without someone (a human) grabbing it and moving it, or d be able to move/orient another substantial object in a complex manner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Science packages may have little gizmos that feed animals or mix, aliquot, or titrate liquids for example. If you know of some cool or prominent ones, count 'em!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Routine movement like pumps to periodically circulate fluids (e.g. water) or fans or magnetic pickups on old-style hard drives would not be robots for example.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;And don't forget to count the ones that just plain look like robots!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/roWle.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/roWle.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-23T01:43:01.190" LastActivityDate="2018-02-21T17:32:36.843" Title="How many robots have been at, in, or on the ISS?" Tags="|robotic-missions|robotics|aerobot|robotic-arm|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22039" PostTypeId="2" ParentId="22038" CreationDate="2017-06-20T19:05:52.197" Score="3" Body="&lt;p&gt;Here is a selection of robotic equipment that has been at/in/on the ISS, feel free to add additional references:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/CBEF.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cell Biology Experiment Facility (CBEF)&lt;/a&gt; can autonomously control a centrifuge, among other functions. While holding at 1G may fall under the &quot;routine movement&quot; section of the question, the centrifuge can alternate between a range of gees, which seems to qualify it to be a robot under the constraints of the question (dynamic movement of an appendage).&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/380.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Avian Development Facility (ADF)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/269.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Microscope Observation System&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/221.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aquatic Habitat (AQH)&lt;/a&gt; has &quot;[a]n automatic feeding system [that] can be programmed to support different feeding sequences as needed.&quot;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/338.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Commercial Generic Bioprocessing Apparatus (CGBA)&lt;/a&gt; may be fitted with Gas Exchange - Group Activation Packs (GE-GAPs), &quot;a piece of hardware similar to a small test tube&quot;, which can be programmed or remotely operated to &quot;activate, mix, grow, and terminate biological experiments.&quot;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/378.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced Space Experiment Processor (ADSEP)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/350.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Microgravity Science Glovebox (MSG)&lt;/a&gt; &quot;allow[s] experiments to be controlled from the ground.&quot;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/187.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dynamically Controlled Protein Crystal Growth (DCPCG) apparatus&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/541.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Light Microscopy Module (LMM)&lt;/a&gt; &quot;[can begin] the remote operation of the microscope, and processing of the samples&quot; &quot;[a]fter the sample has been positioned [by ISS crew]&quot;.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/277.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Microencapsulation Electrostatic Processing System (MEPS)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/185.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced Astroculture (ADVASC)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/345.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;European Modular Cultivation System (EMCS)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/273.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Plant Generic Bioprocessing Apparatus (PGBA)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/2302.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced Plant Habitat Facility (PHF)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/121.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robot Experiment on JEM (REXJ)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/structure/elements/mss.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mobile Servicing System (MSS) / Canadarm2&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://iss.jaxa.jp/en/kibo/about/kibo/rms/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Japanese Experiment Module Remote Manipulator System (JEMRMS)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/1358.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NanoRocks&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/1004.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stratospheric Aerosol and Gas Experiment III (SAGE III)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/407.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun Monitoring on the External Payload Facility of Columbus - SOLar Auto-Calibrating EUV/UV Spectrophotometers (Solar-SOLACES)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/2421.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Optical Fiber Production in Microgravity (OFPIM)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/1210.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NanoRacks-ArduSat-1&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/690.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Synchronized Position Hold, Engage, Reorient, Experimental Satellites-Zero-Robotics (SPHERES-Zero-Robotics)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Honorable Mention:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/853.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Buzz Lightyear&lt;/a&gt; fails the robot test because he was neither autonomous nor remote-controlled.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="58" LastEditorUserId="58" LastEditDate="2018-02-21T17:32:36.843" LastActivityDate="2018-02-21T17:32:36.843" CommentCount="13" CommunityOwnedDate="2017-06-20T19:05:52.197" ContentLicense="CC BY-SA 3.0" />
  <row Id="22040" PostTypeId="2" ParentId="22012" CreationDate="2017-06-21T06:15:57.203" Score="2" Body="&lt;p&gt;If you accept the numbers at &lt;a href=&quot;http://www.theoildrum.com/node/6252&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.theoildrum.com/node/6252&lt;/a&gt; (which are sourced, but that doesn't make them necessarily accurate), it takes 0.430 kwh of energy to produce 390 food calories (390 kcal) worth of corn, for an efficiency rating of 102%. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought this was impossible at first, since it would seemingly violate the Seecond Law of Thermodynamics, but the calculation only includes the light energy required: the additional caloric energy comes from the physical matter that makes up corn, primarily carbon, hydrogen, and oxygen. In other words, creating corn requires both energy and raw materials, and the raw materials themselves contain stored energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming you could live primarily (though not entirely) on corn, this number is close enough to 100% that we can assume roughly 1-to-1 transfer of light energy to caloric energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The average healthy person burns at least 2000 calories a day (right around 100 watts), though this assumes very little activity. With a reasonable amount of activity, this is closer to 3000 calories (about 150 watts).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, somewhat uninterestingly, the amount of energy you need is, assuming you have sufficient raw materials, is pretty much the amount of energy an average person burns.&lt;/p&gt;&#xA;" OwnerDisplayName="user7073" LastActivityDate="2017-06-21T06:15:57.203" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22041" PostTypeId="1" AcceptedAnswerId="23585" CreationDate="2017-06-21T07:09:06.303" Score="6" ViewCount="534" Body="&lt;p&gt;I learned about the Kurs docking system in &lt;a href=&quot;https://space.stackexchange.com/a/21967/12102&quot;&gt;this answer&lt;/a&gt;, then found out what they looked like in &lt;a href=&quot;https://space.stackexchange.com/a/22035/12102&quot;&gt;this&lt;/a&gt; and in &lt;a href=&quot;https://space.stackexchange.com/a/22036/12102&quot;&gt;this&lt;/a&gt; answer. Those images show several tiny dish antenna structures sticking out from various ISS modules and spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;I've shown an example image below, where I've indicated what looks like two (slightly) different types of antenna-pairs, plus an unpaired object that might be an antenna as well.&lt;/p&gt;&#xA;&lt;p&gt;Reading &lt;strong&gt;Section 3.2.2&lt;/strong&gt; of David C. Woffinden's 2008 Ph.D. thesis &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1011&amp;amp;context=etd&quot; rel=&quot;nofollow noreferrer&quot;&gt;Angles-Only Navigation for Autonomous Orbital&#xA;Rendezvous&lt;/a&gt; (originally linked &lt;a href=&quot;https://space.stackexchange.com/a/21967/12102&quot;&gt;here&lt;/a&gt;) I can see that there are several radar different radar signals used by Kurs, possibly both linked and one-way signals.&lt;/p&gt;&#xA;&lt;p&gt;How many Kurs antennas does a spacecraft or ISS module have &lt;em&gt;in toto&lt;/em&gt;, and how do the different functions and signals map on to them?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Cropped and annotated, from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Progress_M-13M_docking.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. &amp;quot;ISS029-E-036167 (2 Nov. 2011) --- An unpiloted ISS Progress resupply vehicle approaches the International Space Station, carrying 1,653 pounds of propellant, 110 pounds of oxygen, 926 pounds of water and 3,108 pounds of maintenance gear, spare parts, experiment hardware and resupply items for the residents of the space station. Progress 45 docked to the station's Pirs docking compartment at 7:41 a.m. (EDT) on Nov. 2, 2011.&amp;quot; Original from &lt;a href=&quot;https://spaceflight.nasa.gov/gallery/images/station/crew-29/html/iss029e036167.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ar7n4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ar7n4.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-03-20T12:25:07.487" LastActivityDate="2021-03-20T12:25:07.487" Title="How many antennas does Kurs use in toto, and what are each of their functions?" Tags="|communication|docking|radar|kurs|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22042" PostTypeId="2" ParentId="22012" CreationDate="2017-06-21T08:48:03.167" Score="5" Body="&lt;p&gt;The 3kw/person figure in &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070032685.pdf&quot; rel=&quot;noreferrer&quot;&gt;the original report&lt;/a&gt; may just be for general life support - it's caveated as &quot;in the habitat&quot; and the report goes on to explicitly say that the manufacturing unit will have extra power. The farm is shown as a third module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://barryontheice.wordpress.com/2013/02/05/south-pole-semi-automated-hydroponic-growth-chamber/&quot; rel=&quot;noreferrer&quot;&gt;South Pole greenhouse&lt;/a&gt; (entirely indoors) is estimated to run at an efficiency of &quot;...assuming that fuel costs about \$4.50 per litre, greens cost about \$50 per pound&quot;. A litre of diesel seems to produce approximately 10kwh of power, so you're looking at 100kwh to produce a pound of vegetables. That could be anything from 100 calories (spinach) upwards; rice and wheat have much higher energy densities (500-1500 cal/lb) but they also have a lot of additional inedible weight, eg in the stalk. Let's say you can get 500 cal/lb with really intensive farming approaches and clever reuse of heat, probably a generous estimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means you need enough energy input for about four and a bit pounds of growth per person per day, or 450 kwh. Assuming 24-hour farming, you're then looking at ~20kw of solar power on a continual basis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems challenging. The whole ISS (assuming the arrays work about 50% of the time) would produce just enough power to feed a three-person crew with nothing left over for life support...&lt;/p&gt;&#xA;" OwnerUserId="11259" LastActivityDate="2017-06-21T08:48:03.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22043" PostTypeId="2" ParentId="21125" CreationDate="2017-06-21T14:10:53.503" Score="6" Body="&lt;p&gt;We get eclipses every orbit on Metop - between about 22 and 28 minutes depending on the season. These will disappear as the LTAN starts to drift (&lt;a href=&quot;https://scienceblog.eumetsat.int/2017/06/extending-the-working-lifetime-of-the-metop-a-weather-satellite/&quot; rel=&quot;noreferrer&quot;&gt;https://scienceblog.eumetsat.int/2017/06/extending-the-working-lifetime-of-the-metop-a-weather-satellite/&lt;/a&gt;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no single reason why the array stowage and deployment is so complex, but you have a SPOT heritage platform (the smaller cube where the Solar Array is physically attached) and the Envisat heritage payload module (the longer bit where the array is stowed). The satellite must also fit inside a launcher fairing (typically taller than it is wide), so the array must stow on one of the 4 longer sides. One of the shorter sides is taken up by the launcher interface ring, and the other is where the fairing will taper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Metop-SG, the satellite body is sideways compared to the first generation, so the array deployment is much simpler - it just pops out of the side.&lt;/p&gt;&#xA;" OwnerUserId="20129" LastActivityDate="2017-06-21T14:10:53.503" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22044" PostTypeId="1" CreationDate="2017-06-21T15:05:41.813" Score="2" ViewCount="4677" Body="&lt;p&gt;If the &lt;a href=&quot;http://xtar.com/look-angle/&quot; rel=&quot;nofollow noreferrer&quot;&gt;look angles&lt;/a&gt; for a pair of satellites are $Az_1, El_1$ and $Az_2, El_2$, what would be the angular separation between them?&#xA;An associated problem is how do we determine whether and when the two satellites, in slightly different orbits, i.e. slightly different COE (Clasical Orbital Elements), are in radio frequency interference with respect to a given ground antenna. Again a way to determine their angular separation is through the angle of their respective topocentric vectors&lt;/p&gt;&#xA;" OwnerUserId="12133" LastEditorUserId="12133" LastEditDate="2017-06-26T16:59:12.867" LastActivityDate="2024-02-06T12:51:09.930" Title="Angular distance between two satellites given azimuth and elevation angles for each?" Tags="|antenna|" AnswerCount="2" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="22045" PostTypeId="1" AcceptedAnswerId="22046" CreationDate="2017-06-21T17:11:15.093" Score="37" ViewCount="9725" Body="&lt;p&gt;Note that this is somewhat related to &lt;a href=&quot;https://space.stackexchange.com/questions/797/radiation-shielding-magnetic-or-mass-which-is-more-efficient&quot;&gt;this question&lt;/a&gt; but here I am specifically asking about the suitability of using existing shielding for Mars travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is about this article and video titled &quot;Scientists overlooked a major problem with going to Mars — and they fear it could be a suicide mission&quot; &lt;a href=&quot;http://www.businessinsider.com/scientists-overlooked-high-radiation-mars-suicide-missions-2017-6&quot; rel=&quot;noreferrer&quot;&gt;here at Business Insider&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article says that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Going to Mars may be more dangerous than we thought. The major problem&#xA;  is high-energy space radiation. Scientists know that cosmic rays can&#xA;  damage DNA. They had just overlooked how bad it could get. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;A team re-examined how damaged DNA can cause cancer. They then&#xA;  estimated levels of radiation exposure in space and on Mars. Their&#xA;  results are devastating. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The risk of cancer on Mars is twice as high as previously thought. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;It comes down to how damaged DNA spreads throughout the body. A&#xA;  detailed study in mice reveals a sinister side to radiation. Damaged&#xA;  DNA doesn't just keep to itself.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;It sends signals to nearby healthy cells, which triggers the healthy&#xA;  cells to mutate, which could cause more cancer.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The protection provided by Earth's magnetic field, referred to in the article, is through the Van Allen belts (held in place by Earth's magnetic field) that trap cosmic rays and high energy particles from the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that the Moon and Mars are both completely outside of the Van Allen belts (as opposed to ISS which is below the belts), we must have had some (relatively lightweight) radiation shielding when traveling to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus I was thinking that this shielding could for example be sent to Mars in advance (e.g. on some kind of supply ship manned by robots).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Admittedly this would require that the astronauts on Mars would always have to stay inside structures built with such shielding (except for relatively brief moments as was done during the moon landing). This does not sound very unusual to me however.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So the question I would like to ask:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is radiation a problem when traveling to or living on Mars when we can use the same shielding as has been used before? I.e. what makes the existing shield technologies unsuitable?&lt;/p&gt;&#xA;" OwnerUserId="7089" LastEditorUserId="7089" LastEditDate="2017-06-21T20:33:04.497" LastActivityDate="2017-12-08T03:37:32.347" Title="Why can't we use the same radiation shielding in Mars that we used when going to the moon?" Tags="|mars|crewed-spaceflight|colonization|radiation|shielding|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22046" PostTypeId="2" ParentId="22045" CreationDate="2017-06-21T17:27:16.347" Score="66" Body="&lt;p&gt;Radiation exposure is a cumulative risk. The more radiation you receive, the more likely you are to develop cancers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo missions took no more than two weeks to complete; the astronauts flying those missions accepted that dose of radiation with the health risks that come with it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A manned Mars mission will take, at minimum, months of travel. For the most fuel-efficient mission plans, the total time including the stay on Mars is about 32 months. So we're considering about 50-100 times the amount of radiation exposure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, a &lt;a href=&quot;https://space.stackexchange.com/a/18425/195&quot;&gt;solar flare occurring during the trip could be immediately debilitating or lethal to the crew&lt;/a&gt;. Flares of that kind are infrequent, so the risk was accepted for Apollo, but again, with the longer travel window of a Mars mission, the chances of encountering such a flare are much higher.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-06-21T18:37:54.283" LastActivityDate="2017-06-21T18:37:54.283" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22048" PostTypeId="2" ParentId="21778" CreationDate="2017-06-21T19:41:31.283" Score="1" Body="&lt;p&gt;This question is based on something of a misunderstanding: the study which made excess oxygen a problem was not about a &lt;em&gt;self sustaining&lt;/em&gt; mars colony but rather an initial stay relying mainly on stuff brought from Earth. When plants perform photosynthesis they consume CO2 (exhaled by the astronauts and/or from the martian atmosphere) and release O2, which would build up inside the habitat until reaching dangerous levels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem wasn't too much oxygen per-se, but rather that nitrogen would be vented at the same time as the excess oxygen - resulting in the very limited nitrogen reserves running out - since nitrogen is only a small fraction of the Martian atmosphere it is difficult to build up large reserves of it without heavy machinery, and in the study the proposed base could neither prevent nor accommodate the nitrogen losses due to venting excess oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A well-designed long term colony would either have a way to isolate and eliminate just oxygen, or alternatively have a plentiful source of nitrogen. Either way internal atmospheric pressure and N2/O2 balance could be maintained safely.&lt;/p&gt;&#xA;" OwnerUserId="6599" LastActivityDate="2017-06-21T19:41:31.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22049" PostTypeId="2" ParentId="22045" CreationDate="2017-06-21T22:36:01.013" Score="66" Body="&lt;p&gt;In addition to what Russell Borogove says about cumulative risk you're operating under a false assumption--that there was shielding on the Apollo capsules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not only did the Apollo capsules not have shielding but shielding was considered &lt;strong&gt;undesirable&lt;/strong&gt;.  There are two main radiation threats in space:  cosmic rays and solar flares.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Their &quot;defense&quot; against solar flares was to launch when they weren't expected.  Had a flare nailed an Apollo capsule we would have lost the crew.  The boosters simply weren't powerful enough to lift the necessary mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cosmic rays are at a much lower dose but they are very, very high energy particles and shielding against them is difficult.  Doing a poor job of it is actually &lt;strong&gt;worse&lt;/strong&gt; than not doing anything.  The thing is they are coming in hot enough they go right through you, most of the energy remains in the particle.  Put an inadequate shield in the way and that one particle knocks others loose, which knock others loose, you get a whole shower of much lower energy particles and your body actually does absorb the energy that gets through the shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shielding against them is far harder than shielding against solar flares.  Against solar flares we can at least arrange things so all the extra mass they are carrying (food, fuel etc.) is between them and the sun.  A shield that can stop cosmic rays, though, is simply impractical to lift on any chemical rocket, period.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="12408" LastEditDate="2017-06-25T04:57:32.107" LastActivityDate="2017-06-25T04:57:32.107" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="22050" PostTypeId="1" AcceptedAnswerId="22053" CreationDate="2017-06-21T23:25:18.977" Score="7" ViewCount="531" Body="&lt;p&gt;It looks like &lt;a href=&quot;https://space.stackexchange.com/a/22025/12102&quot;&gt;this answer&lt;/a&gt; is converging on a conclusive answer about a zenith-facing window on the ISS, and possibly also a window which might at least be able to offer a bit of a view of the zenith.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be really really nice to find a photograph of the top side (zenith size) of the ISS - the side that sees the stars, opposite from the side facing Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were there any shuttle missions that passed slowly over the top of the ISS, or at least had a view of the top? If so that might help narrow down a search for photographs of the top of the ISS by limiting to a few missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An actual high resolution photo of the top would be really great, but just the name of one or a few most likely missions would be good enough. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These would need to be after the Zvezda module was added and up and running. There is one window that is interesting there, (discussed &lt;a href=&quot;https://space.stackexchange.com/a/22025/12102&quot;&gt;here&lt;/a&gt;) so it would be great to see it!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Ultimately I'd like to be able to verify the existence, some more details, and angle of &quot;window #13&quot; as described &lt;a href=&quot;https://space.stackexchange.com/a/22025/12102&quot;&gt;here&lt;/a&gt; as well as check for any others. This gives an idea of the kind of resolution I'd like to find.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1X6BH.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1X6BH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6944" LastEditDate="2017-06-22T13:23:24.520" LastActivityDate="2021-07-26T19:05:54.737" Title="Were there any STS missions that included maneuvers that might have photographed ISS's top side?" Tags="|iss|space-shuttle|spacecraft-window|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22051" PostTypeId="2" ParentId="22044" CreationDate="2017-06-22T03:50:12.963" Score="5" Body="&lt;p&gt;Indeed, the solution to the problem is found in solving a spherical triangle, as @uhoh states.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider this diagram for a general spherical triangle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6oYm1.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6oYm1.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this problem, $C$ is the zenith, and the two points are $A$ and $B$;  the co-altitudes of the two points are the arc-lengths $A-C$ and $B-C$.  The difference in the two azimuths is the angle $ACB$, and the desired value is the arc-length $A-B$.  The sphere portrayed is the celestial sphere;  the observer is on the tiny, invisible terrestrial sphere, located at the centre of the celestial sphere, down where the three straight lines $A-A'$, $B-B'$ and $C-C'$ intersect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you use the Law of Cosines for spherical triangles...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT as suggested by @Uhoh&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we use lower-case letters to denote the arc-length of the sides of the spherical triangles, and Greek letters to denote the angles at each vertex of the spherical triangle, the Law of Cosines gives:&#xA;$$\cos(c)=\cos(a)\times \cos(b)+\sin(a) \times \sin(b) \times \cos(\gamma)$$Note:  error correctd!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The co-altitude of a point is the altitude measured &lt;em&gt;down&lt;/em&gt; from the zenith, rather than up from the horizon.  The co-altitude is just 90 degrees minus the altitude..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Find $b$ and $c$ by subtracting the altitudes of each point from $90$ degrees&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Find $\gamma$ by subtracting the smaller azimuth from the larger.( If $\gamma$ is bigger than 180 degrees, then use 360-$\gamma$)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Substitute these values into the right-hand -side of the formula above to find $\cos(c)$&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Take the inverse cosine (arcos) of this value to find $c$, the angular separation of the two points&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;EDITED AGAIN&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Writing a small Excel spreadsheet to demonstrate an example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6tpcN.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6tpcN.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the angles are in degrees.  Excel's trig functions insist on radians, so the RADIANS function changes degrees to radians, and the DEGREES function, naturally, changes radians to degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the actual values for one case:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OwQrE.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OwQrE.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can mimic all of these calculations with a scientific calculator (and even skip the degrees/radians conversions.&lt;/p&gt;&#xA;" OwnerUserId="895" LastEditorUserId="895" LastEditDate="2017-06-22T23:18:20.260" LastActivityDate="2017-06-22T23:18:20.260" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22052" PostTypeId="1" AcceptedAnswerId="25231" CreationDate="2017-06-22T13:06:05.277" Score="15" ViewCount="541" Body="&lt;p&gt;During the old space race days, launching double or multiple scientific probe copies seems to have been the standard. It should have the advantage of sharing the one time fixed development costs while less than doubling the manufacturing and operational costs given diminishing marginal costs of reiteration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on Pioneer, Viking, Voyager and other duplicate missions (even to some degree Galileo/Cassini and MSL/Mars2020 and the Soviet Venus missions), how should one estimate the marginal cost today of duplicating a Cassini class orbiter to Uranus with a copy to Neptune at the same time (albeit on separate launchers a year apart)? Would major design differences of the spacecrafts and their instruments be worthwhile to turn it into two unrelated missions?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-06-22T14:44:31.747" LastActivityDate="2018-02-12T14:38:51.113" Title="What's the marginal cost of a second interplanetary space probe copy, like Voyager 2 or Viking 2?" Tags="|probe|interplanetary|mission-design|cost|" AnswerCount="2" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22053" PostTypeId="2" ParentId="22050" CreationDate="2017-06-22T13:23:06.303" Score="9" Body="&lt;p&gt;Yes.&lt;/p&gt;&#xA;&lt;p&gt;Many of the shuttle missions late in the program performed a flyaround of the ISS after undocking.  (STS trivia fact: This maneuver was traditionally flown by the pilot (the docking was traditionally flown by the commander)).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NhKCMm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NhKCMm.png&quot; alt=&quot;a graph showing a plot of a  shuttle flyaround of the ISS in a LVLH frame&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here is a partial list obtained by googling &amp;quot;which shuttle missions performed flyarounds of the ISS&amp;quot;:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;STS-96&lt;/li&gt;&#xA;&lt;li&gt;STS-100&lt;/li&gt;&#xA;&lt;li&gt;STS-108&lt;/li&gt;&#xA;&lt;li&gt;STS-119&lt;/li&gt;&#xA;&lt;li&gt;STS-131&lt;/li&gt;&#xA;&lt;li&gt;STS-133&lt;/li&gt;&#xA;&lt;li&gt;STS-135&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Videos of these flyarounds are often available on youtube although the viewing distance is pretty far:&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/bXNH7whveGk?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;The astronauts took quality photography during the flyaround; much of it was available on the now-deleted site &lt;a href=&quot;https://spaceflight.nasa.gov/home/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflight.nasa.gov&lt;/a&gt; in the Galleries/Shuttle area.  The pictures can be found on the newer NASA image website &lt;a href=&quot;https://images.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;images.nasa.gov&lt;/a&gt; but the search interface is very rudimentary.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1Eba3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1Eba3.jpg&quot; alt=&quot;a picture of the ISS taken during a flyaround executed on STS-119&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-07-26T19:05:54.737" LastActivityDate="2021-07-26T19:05:54.737" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22054" PostTypeId="1" AcceptedAnswerId="22055" CreationDate="2017-06-22T15:38:25.337" Score="8" ViewCount="1599" Body="&lt;p&gt;I found the image below in a &lt;a href=&quot;https://i.stack.imgur.com/UOIOE.jpg&quot; rel=&quot;noreferrer&quot;&gt;NASA image gallery for 2014&lt;/a&gt;. There, the text only says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This unprecedented view of the space shuttle Atlantis, appearing like a bean sprout against clouds and city lights, on its way home, was photographed by the Expedition 28 crew of the International Space Station. Airglow over Earth can be seen in the background.&lt;/p&gt;&#xA;&lt;p&gt;Image Credit: NASA, Last Updated: July 31, 2015, Editor: NASA Administrator&lt;/p&gt;&#xA;&lt;p&gt;Tags:  Expedition 28, Landings, Shuttle Atlantis, Space Shuttle, STS-135&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Is the visible glow from a de-orbit burn, or heat from re-entry, or something else? Since it's moving away from the camera, why would the light be visible in either case? There's a bump or knee in the trail, highlighted in the second, annotated/cropped image. What probably caused it?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UOIOE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UOIOE.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DZYCJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DZYCJ.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-23T14:52:37.120" Title="What is making the glow from this image of STS-135 going home, de-orbit burn or re-entry?" Tags="|reentry|space-shuttle|photography|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22055" PostTypeId="2" ParentId="22054" CreationDate="2017-06-22T16:22:01.143" Score="10" Body="&lt;p&gt;Your picture shows the plasma trail behind STS-135, the last space shuttle entry in history.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The trail you’re seeing is therefore not actually the Orbiter&#xA;  streaking across the Earth! The plasma trail behind it fades with&#xA;  time, so the trail is brightest near the Orbiter’s position and&#xA;  fainter as you backtrack along its path. Think of it as an afterglow&#xA;  of the passing of Atlantis.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Why does this happen? The air gets heated by the Orbiter’s ramming the&#xA;  atmosphere at 20+ times the speed of sound. And contrary to popular&#xA;  belief, it’s not friction that heats the air, but compression. When&#xA;  you compress a gas it heats up (like when a bicycle pump gets hot when&#xA;  you use it a lot), and the Orbiter is screaming through the atmosphere&#xA;  at hypersonic speeds. That compresses the air a lot. A shock wave&#xA;  forms in front of the Orbiter, and the air begins to glow as it gets&#xA;  heated up to temperatures as high as 1260° C (2300° F).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;That’s what you’re seeing above: the shocked, rammed, and glowing air&#xA;  as Atlantis pounded through it at several kilometers per second. And&#xA;  it did this many, many times over its life… until this one final time,&#xA;  caught on camera by astronauts high above the Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://blogs.discovermagazine.com/badastronomy/2011/07/22/the-fiery-descent-of-atlantis-seen-from-space/&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plasma trails were also visible from the ground if one was in the right place at the right time, as in this incredible picture of the STS-93 entry over Johnson Space Center.  I was standing outside watching this in person, but I did not take this picture, sadly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/axhJT.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/axhJT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a picture of the STS-107 plasma trail over California, before things went visibly wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qZOlC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qZOlC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not know what caused the bump in the trail, but possible causes are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a maneuver by the shuttle (roll reversal)&lt;/li&gt;&#xA;&lt;li&gt;high altitude winds or other meteorology&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-06-22T16:37:13.817" LastActivityDate="2017-06-22T16:37:13.817" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22056" PostTypeId="1" AcceptedAnswerId="22300" CreationDate="2017-06-22T18:26:14.477" Score="9" ViewCount="804" Body="&lt;p&gt;Below &lt;a href=&quot;https://space.stackexchange.com/a/22049/90&quot;&gt;an answer to a recent question about shielding against radiation on a trip to Mars&lt;/a&gt;, there was this comment from dubu (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[..] For high-energy particles entering dense matter, Bremsstrahlung is joined by &lt;a href=&quot;https://en.wikipedia.org/wiki/Pair_production&quot; rel=&quot;noreferrer&quot;&gt;pair production&lt;/a&gt;, and both together create &lt;a href=&quot;https://en.wikipedia.org/wiki/Particle_shower&quot; rel=&quot;noreferrer&quot;&gt;particle showers&lt;/a&gt;. [..] A known problem in radiation shielding (&lt;strong&gt;thin shielding might be worse than no shielding at all&lt;/strong&gt;).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Thus,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How thick must a shield be to be better than no shield at all?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the shield is used on a trip between planets of our solar system and is intended to protect the human crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for both theoretical papers / calculations as well as practical studies (if there are any).&lt;/p&gt;&#xA;" OwnerUserId="90" LastEditorUserId="12102" LastEditDate="2017-06-23T00:10:54.363" LastActivityDate="2017-07-16T03:11:24.653" Title="How thick must a shield be to be better than no shield at all?" Tags="|crewed-spaceflight|radiation|interplanetary|shielding|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22057" PostTypeId="2" ParentId="22044" CreationDate="2017-06-22T19:59:49.333" Score="6" Body="&lt;p&gt;From &lt;a href=&quot;https://books.google.com/books?id=MTGYxQyW998C&amp;amp;pg=PA66&amp;amp;lpg=PA66&amp;amp;dq=%22the+angle+between+two+celestial+objects%22&amp;amp;source=bl&amp;amp;ots=UMhTOyKjZG&amp;amp;sig=X3M3S3h7M-EHDsF6BNBJHNBIvZ4&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwi90-GVmtLUAhVs04MKHb0gBOYQ6AEIKzAB#v=onepage&amp;amp;q=%22the%20angle%20between%20two%20celestial%20objects%22&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Practical Astronomy with your Calculator or Spreadsheet&quot;&lt;/a&gt; by Peter Duffett-Smith, Jonathan Zwart:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sometimes it is of interest to know what is the angle between two objects in the sky, and this can be calculated very easily provided their equatorial coordinates (&lt;span class=&quot;math-container&quot;&gt;$\alpha$&lt;/span&gt;,&lt;span class=&quot;math-container&quot;&gt;$\delta$&lt;/span&gt;) or ecliptic coordinates (&lt;span class=&quot;math-container&quot;&gt;$\lambda, \beta$&lt;/span&gt;) are known. The formulae are:&lt;br&gt;&#xA;    cos &lt;span class=&quot;math-container&quot;&gt;$d$&lt;/span&gt; = sin &lt;span class=&quot;math-container&quot;&gt;$\delta_1$&lt;/span&gt; sin &lt;span class=&quot;math-container&quot;&gt;$\delta_2$&lt;/span&gt; + cos &lt;span class=&quot;math-container&quot;&gt;$\delta_1$&lt;/span&gt; cos &lt;span class=&quot;math-container&quot;&gt;$\delta_2$&lt;/span&gt; cos (&lt;span class=&quot;math-container&quot;&gt;$\alpha_1 - \alpha_2$&lt;/span&gt;),&lt;br&gt;&#xA;  or&lt;br&gt;&#xA;    cos &lt;span class=&quot;math-container&quot;&gt;$d$&lt;/span&gt; = sin &lt;span class=&quot;math-container&quot;&gt;$\beta_1$&lt;/span&gt; sin &lt;span class=&quot;math-container&quot;&gt;$\beta_2$&lt;/span&gt; + cos &lt;span class=&quot;math-container&quot;&gt;$\beta_1$&lt;/span&gt; cos &lt;span class=&quot;math-container&quot;&gt;$\beta_2$&lt;/span&gt; cos (&lt;span class=&quot;math-container&quot;&gt;$\lambda_1 - \lambda_2$&lt;/span&gt;),&lt;br&gt;&#xA;  where &lt;span class=&quot;math-container&quot;&gt;$d$&lt;/span&gt; is the angle between the objects whose coordinates are &lt;span class=&quot;math-container&quot;&gt;$\alpha_1, \delta_1$&lt;/span&gt; (or &lt;span class=&quot;math-container&quot;&gt;$\lambda_1, \beta_1$&lt;/span&gt;) and &lt;span class=&quot;math-container&quot;&gt;$\alpha_2, \delta_2$&lt;/span&gt; (or &lt;span class=&quot;math-container&quot;&gt;$\lambda_2, \beta_2$&lt;/span&gt;). Those formulae are exact and mathematically correct for any values of &lt;span class=&quot;math-container&quot;&gt;$\alpha, \delta$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$\lambda, \beta$&lt;/span&gt;. However, when &lt;span class=&quot;math-container&quot;&gt;$d$&lt;/span&gt; becomes either very small, or close to 180 degrees, your calculator may not have enough precision to return the correct answer, in which case better expressions are&lt;br&gt;&#xA;  &lt;span class=&quot;math-container&quot;&gt;$ d = \sqrt{cos^{2}\delta * \Delta \alpha^{2} + \Delta\delta^2} $&lt;/span&gt;&lt;br&gt;&#xA;  or&lt;br&gt;&#xA;  &lt;span class=&quot;math-container&quot;&gt;$ d = \sqrt{cos^{2}\beta* \Delta \lambda^{2} + \Delta\beta^2} $&lt;/span&gt;&lt;br&gt;&#xA;  where &lt;span class=&quot;math-container&quot;&gt;$\Delta\alpha, \Delta\delta$&lt;/span&gt; (or &lt;span class=&quot;math-container&quot;&gt;$\Delta\lambda, \Delta\beta$&lt;/span&gt;) are the differences in the two coordinates (i.e. &lt;span class=&quot;math-container&quot;&gt;$\Delta\alpha = \alpha_1 - \alpha_2$&lt;/span&gt;, etc.). These expressions may be used for values of &lt;span class=&quot;math-container&quot;&gt;$d$&lt;/span&gt; within about 10 arcminutes of 0 degrees or 180 degrees. Both &lt;span class=&quot;math-container&quot;&gt;$\Delta\alpha$&lt;/span&gt; &lt;span class=&quot;math-container&quot;&gt;$(\Delta\lambda) $&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\Delta\delta$&lt;/span&gt; &lt;span class=&quot;math-container&quot;&gt;$(\Delta\beta)$&lt;/span&gt; must be expressed in the same units (e.g. arcseconds) and &lt;span class=&quot;math-container&quot;&gt;$d$&lt;/span&gt; will then be returned in those units.  &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;span class=&quot;math-container&quot;&gt;$\beta$&lt;/span&gt; in the &lt;span class=&quot;math-container&quot;&gt;$cos^{2}\beta$&lt;/span&gt; above can be either &lt;span class=&quot;math-container&quot;&gt;$\beta_1$&lt;/span&gt; or &lt;span class=&quot;math-container&quot;&gt;$\beta_2$&lt;/span&gt;.  It makes little difference, since this formula applies only when d is nearly 0, or nearly 180.  Eg, the two points are either very close together, or almost antipodal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This method should work for RA/Declination, Elev/Azimuth, Lat/Lon, or any similar coordinate system with north and south &quot;poles.&quot;  (unsure of the general term for these)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe this is what you are asking for, as it's generally the most useful in astronomy, attitude determination, etc.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastEditorUserId="35548" LastEditDate="2020-04-15T23:08:04.467" LastActivityDate="2020-04-15T23:08:04.467" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22059" PostTypeId="1" AcceptedAnswerId="22064" CreationDate="2017-06-23T00:12:47.520" Score="1" ViewCount="778" Body="&lt;p&gt;Space is pretty expensive to get to, and there's a shocking lack of things in it. A planet made of gold sounds like a treasure worth the trip! How far would such a thing have to be away from Earth in order for it not to be economical to retrieve it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;current technology. Developing is fine as long as it's not blatantly unphysical (no EmDrive)&lt;/li&gt;&#xA;&lt;li&gt;a single launch&lt;/li&gt;&#xA;&lt;li&gt;a solid gold planet of equivalent mass to the Moon.&lt;/li&gt;&#xA;&lt;li&gt;a fixed price for gold&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A tolerably exact answer to this question may be difficult; alternately a reasonable upper bound may be presented. I believe the most appropriate measure of &quot;distance&quot; would be in terms of delta-v.&lt;/p&gt;&#xA;" OwnerUserId="18408" LastEditorUserId="18408" LastEditDate="2017-06-23T16:24:09.873" LastActivityDate="2022-07-16T14:39:34.930" Title="If the Moon were solid gold, how far away would it have to be to not be economical to mine it?" Tags="|mining|economics|" AnswerCount="3" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="22060" PostTypeId="1" AcceptedAnswerId="22062" CreationDate="2017-06-23T01:55:10.117" Score="4" ViewCount="2533" Body="&lt;p&gt;If we are using lots of toxic fuels like hydrazine for rockets, wouldn't this cause pollution due to the burnt leftovers ? I mean, just consider the amount of rockets and missiles launched in world so far and all the jet fuel that's being burnt by jet engines everyday. And then comes the huge debate about wood burning by people in poor countries when even the most developed countries pollute the air everyday as mentioned above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I always have this doubt within me whenever I read about various scientific experiments like rockets, missiles, jet engines, etc. Why don't someone study and emphasize these effects just like ozone depletion or deforestation....&lt;/p&gt;&#xA;" OwnerUserId="20151" LastEditorUserId="58" LastEditDate="2018-09-27T14:39:58.480" LastActivityDate="2018-09-27T14:39:58.480" Title="Pollution by rockets and missiles" Tags="|rockets|pollution|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22061" PostTypeId="1" AcceptedAnswerId="22104" CreationDate="2017-06-23T02:03:38.327" Score="3" ViewCount="441" Body="&lt;p&gt;I'm plotting the position of satellites in the sky using &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt;. If I project the &lt;code&gt;.altaz()&lt;/code&gt; data on to a rectangle (in a similar fashion to the way ground track lat/lon plots are made) I get two big &quot;holes&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a way to understand, explain, and articulate the cause of these holes in alt/az space beyond &quot;the ISS can't go there&quot;? I don't think it's anything fundamental or profound, but I can't wrap my mind around why there can be places in the sky of a given location where the ISS can't go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The python script below automatically grabs a TLE from the internet and then calculates 40,000 positions over +/- 2 days from the TLE's epoch, so it takes a few seconds. The two dots are the north and south poles of the celestial sphere. &lt;em&gt;They are not really related to the question&lt;/em&gt; but were added to address questions in comments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/T2jYW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/T2jYW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;from skyfield.api import load, Loader, Topos&#xA;&#xA;degs = 180./np.pi&#xA;&#xA;stations_url = 'http://celestrak.com/NORAD/elements/stations.txt'&#xA;&#xA;loader = Loader('~/Documents/foldername/SkyData')&#xA;data   = loader('de421.bsp')&#xA;ts     = loader.timescale()&#xA;&#xA;earth = data['earth']&#xA;loc   = Topos(-39.2617, 177.8652, elevation_m=20)&#xA;&#xA;sats  = load.tle(stations_url)&#xA;ISS   = sats['ISS (ZARYA)']&#xA;print(ISS)&#xA;&#xA;tstep = np.arange(-2, 2, 0.0001)   # range of +/- 2 days from TLE epoch&#xA;time  = ts.tt(jd=ISS.epoch.tt + tstep)&#xA;&#xA;alt, az = [x.radians for x in (ISS - loc).at(time).altaz()[:2]]&#xA;snipit = np.abs(az[1:] - az[:-1]) &amp;gt; 2  # snip the plotting at wrap-around&#xA;dalt, daz = degs*alt, degs*az&#xA;daz[:-1][snipit] = np.nan&#xA;&#xA;plt.figure()&#xA;plt.plot(daz[:-1], dalt[:-1], linewidth=1)&#xA;plt.xlabel('azimuth (degs)')&#xA;plt.ylabel('altitude (degs)')&#xA;plt.plot(0, degs*loc.latitude.radians, 'or', markersize=8)&#xA;plt.plot(180, -degs*loc.latitude.radians, 'or', markersize=8)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-23T03:04:29.363" LastActivityDate="2017-06-26T11:38:46.793" Title="Trying to understand the two &quot;holes&quot; in this alt/az plot of the ISS" Tags="|low-earth-orbit|mathematics|visualization|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22062" PostTypeId="2" ParentId="22060" CreationDate="2017-06-23T02:16:08.227" Score="13" Body="&lt;p&gt;Hydrazine burns very nicely to water and ammonia. It is the unburnt stuff that is dangerous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;LH/LOX engines burn products are water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RP1/LOX engines burn much the same as car engines, just hotter so some weird nitrogen compounds form.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SRB's are really quite polluting. But consider that in 30 years 135 pairs were launched. (Not sure if that should be 136 and include Challenger or not, or 137 and include Ares-1X).  Over something as large as a planet that is a negligble amount.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The number of rocket and missile launches, again, over the scale of a planet is surprisingly low.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-23T02:16:08.227" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22063" PostTypeId="2" ParentId="22060" CreationDate="2017-06-23T02:46:03.203" Score="13" Body="&lt;p&gt;About 150 thousand kg of kerosene (RP-1) is used in a single Falcon 9 launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;About 500 billion kg of gasoline is used in the US per year. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, launching 80 rockets a day would increase pollution in the US by about 1%. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-06-23T02:53:24.537" LastActivityDate="2017-06-23T02:53:24.537" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22064" PostTypeId="2" ParentId="22059" CreationDate="2017-06-23T03:24:28.707" Score="12" Body="&lt;p&gt;Given your constraints I can't see it being worthwhile, period, even if it were our own moon.  Lets throw some numbers at it:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Current cost to deliver a kilogram of payload to the moon: \$1.2 million.  Price of a kilogram of gold:  \$40k.  In other words, for every kilogram you land on the target you need to bring back 30kg of gold just to pay your launch costs, not even considering the cost of what you landed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oops, look at the Apollo landers.  Their ascent stages were mostly fuel.  In other words, you're paying well over $1.2 million to lift a kilogram of anything off the moon. In reality it's a lot more brutal than that as the density means you're closer in.  The escape velocity goes up by 1/3, increasing both the cost of landing and takeoff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, bringing gold home the moon by rocket isn't remotely worthwhile even if it's free for the taking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, I hear squawking about proposed lunar and asteroid mining&amp;mdash;surely the numbers can't be right??  The key is &lt;strong&gt;by rocket&lt;/strong&gt;&amp;mdash;if you want to make money mining extraterrestrial bodies you'll have to come up with some better means of bringing the stuff home.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, such methods have been known for some time.  Take an electric motor, unwrap it and make it very long.  Instead of turning something round and round it sends it off at a high speed.  While I am not aware of anyone building one powerful enough to toss stuff off your gold moon it's simply a case of building bigger.  (Note:  We do not use such a system for launch from Earth due to the atmosphere.  Scaling the motor is fine, building a system that can survive the shockwaves and heating is quite problematic.)  Your crew lands, and starts tossing packages of gold somewhere.  Since the launch only costs electricity the cost is quite cheap. (After the package has been boosted to the required speed it's released from the booster. The rest of the track has reversed polarity and stops the booster.  Take it back to the start and throw another package.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, that's a lot more equipment than you're going to deliver with a single launch&amp;mdash;which is why I said your constraints mean it's not worth doing.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="6816" LastEditDate="2017-06-23T04:00:28.010" LastActivityDate="2017-06-23T04:00:28.010" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22065" PostTypeId="1" AcceptedAnswerId="22078" CreationDate="2017-06-23T11:44:40.110" Score="2" ViewCount="609" Body="&lt;p&gt;What I mean is that, when the fuel and oxidiser are mixed, at what state (solid - liquid - gaseous) is the oxygen and the fuel. To be more precise, I would like to know how it is for the Merlin engine of the Falcon 9 and the F-1 engine from the Saturn V.&lt;/p&gt;&#xA;" OwnerUserId="17599" LastEditorUserId="195" LastEditDate="2017-06-23T22:52:46.447" LastActivityDate="2017-12-19T13:26:54.920" Title="In what state are the propellants entering a (RP-1) rocket engine's combustion chamber?" Tags="|fuel|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22069" PostTypeId="2" ParentId="22059" CreationDate="2017-06-23T19:26:41.090" Score="3" Body="&lt;p&gt;I just realized that your question does not specify to where the gold must be retrieved (sure, &quot;retrieve&quot; could mean to physically bring back to Earth, but it could also simply mean &quot;carry away and into [the people of] Earth's possession&quot;). One of the biggest advantages of space mining is for use of the resources &lt;em&gt;in situ&lt;/em&gt; or elsewhere in space. Therefore, I am going to argue that from that perspective the only way that the distance of the gold matters is in whether or not we need to be that far out in the first place. If we have a reason to be out that far, then it is economical to mine it.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-06-23T19:26:41.090" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22070" PostTypeId="1" CreationDate="2017-06-23T22:17:07.750" Score="3" ViewCount="349" Body="&lt;p&gt;In watching today's BulgariaSat-1 broadcast:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Y8mLi-rRTh8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was mentioned that the drone ship had acquired the 1st stage as it was returning.  I had always been under the impression that the landing was &quot;controlled&quot; by computers on the 1st stage rocket as it was re-entering, but based on the video, it seemed to indicate that it was the drone ship that was doing the controlling?  So, where is the autonomous landing actually controlled from?  The 1st stage, or the landing platform (whether on a barge or on land)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that the control is done from the rocket side, but with information provided to the rocket from the ground in regards to positioning, distance, wind speed, etc., but I am curious to the actual role each &quot;side&quot; has in the landing procedure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="16735" LastEditorUserId="6537" LastEditDate="2017-12-18T20:02:56.697" LastActivityDate="2017-12-18T20:02:56.697" Title="SpaceX stage 1 reentry control?" Tags="|spacex|landing|asds|flight-control|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22071" PostTypeId="2" ParentId="22070" CreationDate="2017-06-23T22:38:44.147" Score="6" Body="&lt;p&gt;I don't have any citation, but the first stage is doing all its own guidance and control; it would be insane to do it any other way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;acquisition&quot; called out at about 80km altitude is acquisition of signal, not of control. What exactly is being signalled and how the ASDS makes use of it is unclear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've heard (again, without citation) that both the ASDS and returning stage are trying to navigate to the same GPS coordinates simultaneously; it's possible that the stage is telling the ASDS what its current trajectory error is, but the stage is so much more maneuverable than the ASDS that I can't believe the latter would try and move underneath it. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-06-23T22:38:44.147" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22072" PostTypeId="1" CreationDate="2017-06-24T02:23:27.863" Score="1" ViewCount="378" Body="&lt;p&gt;Say a spacecraft is arriving at Saturn, can aerobrake arbitrarily, and wishes to arrive at Hyperion. How much should it aerobrake (i.e. what should the SMA be after aerocapture) to minimize the Delta-V required afterwards?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I initially had the spacecraft aerobrake until its apochron matches Hyperion's SMA, but it takes over 3,600 m/s of Delta-V to match Hyperion's velocity. I realized that aerobraking was reducing my kinetic energy by so much, I had to increase it afterwards to match velocity with Hyperion. However, if I barely brake enough to capture (apochrone reaches out to Phoebe), the burns raising perichrone and matching velocity with Hyperion total to under 2,400 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, my question is, how much should I aerobrake (i.e. what should the SMA be after aerocapture) to minimize the Delta-V needed to intercept Hyperion? I think it's likely between my two extremes, and I have a feeling it should be Hyperion's SMA, but that's just intuition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, Hyperion's SMA is 1,481,009 km and its orbital velocity is 5,060.8 m/s, Saturn's equatorial radius is 60,268 km, and there's a handy vis-viva solver &lt;a href=&quot;http://www.1728.org/kepler3a.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. I'm positive this has lots of similarities to a bi-elliptic transfer orbit, with the exception that the first 'burn' is free.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-24T08:10:14.790" Title="Amount of aerobraking to minimize Delta-V" Tags="|orbital-mechanics|delta-v|saturn|aerobraking|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22074" PostTypeId="2" ParentId="22072" CreationDate="2017-06-24T08:10:14.790" Score="3" Body="&lt;p&gt;Coming from an earth to Saturn Hohmann, it would take .435 km/s to brake into a&#xA;0 x 54104061 km capture orbit. 54104061 km is the altitude of Saturn's sphere of influence. Some or maybe all of that .435 km/s could be achieved by aerobraking. But the ship would enter Saturn's atmosphere at 36 km/s. That is pretty tough aerobraking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the 54104061 km apoapsis it will take a .154 km/s burn to raise periapsis to altitude of Hyperion's orbit (1420741 km).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then it would take 2 km/s to circularize at Hyperion's orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is assuming circular, coplanar orbits.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-06-24T08:10:14.790" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22075" PostTypeId="1" AcceptedAnswerId="22077" CreationDate="2017-06-24T11:40:00.777" Score="28" ViewCount="8703" Body="&lt;p&gt;In his  &lt;a href=&quot;https://twitter.com/elonmusk/status/878334920141135872&quot; rel=&quot;noreferrer&quot;&gt;tweet&lt;/a&gt; Elon Musk said: &quot;Rocket is extra toasty and hit the deck hard (used almost all of the emergency crush core)&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is he talking about?&lt;/p&gt;&#xA;" OwnerUserId="20128" LastEditorUserId="51" LastEditDate="2017-06-25T01:53:00.043" LastActivityDate="2017-06-25T01:53:00.043" Title="What is the &quot;emergency crush core&quot;?" Tags="|spacex|falcon-9|landing|design|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22076" PostTypeId="1" AcceptedAnswerId="22081" CreationDate="2017-06-24T12:03:20.650" Score="0" ViewCount="1629" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0SDeX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0SDeX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not sure this is a view of the inside of a liquid propellant tank, and even if was, not sure if it belongs to a Falcon 9 &lt;em&gt;per se&lt;/em&gt;. But there are places for fourteen round objects at the far end, and thirteen of them are present and purple/grape-like.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are they? And what is this liquid propellant tank-like structure we are looking in to?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lower your volume before watching this really cool video, there is a lot of ambient sound:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/875849793204928512&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://twitter.com/elonmusk/status/875849793204928512&lt;/a&gt; (can go full screen)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.instagram.com/p/BVarZZSgfIP/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.instagram.com/p/BVarZZSgfIP/&lt;/a&gt; (can't go full screen, at least for me)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="51" LastEditDate="2017-06-25T01:56:09.163" LastActivityDate="2017-06-25T01:56:09.163" Title="What are these thirteen purple grape-like things inside this tank?" Tags="|spacex|falcon-9|liquid-fuel|identify-this-object|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22077" PostTypeId="2" ParentId="22075" CreationDate="2017-06-24T13:25:31.923" Score="30" Body="&lt;p&gt;The landing legs have a honey-comb filler that can compress to absorb significant energy of a hot landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8CPos.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8CPos.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;https://twitter.com/spacex/status/738453366649430016&quot; rel=&quot;noreferrer&quot;&gt;Source: @SpaceX&lt;/a&gt;)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="20166" LastEditorUserId="6816" LastEditDate="2017-06-24T14:37:41.213" LastActivityDate="2017-06-24T14:37:41.213" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22078" PostTypeId="2" ParentId="22065" CreationDate="2017-06-24T13:57:08.633" Score="3" Body="&lt;p&gt;I haven't been able to find a complete set of properties for the propellants entering the F-1 combustion chamber.  You can see from the schematic, however, that after leaving the pump, the fuel and oxidizer pass directly into the combustion chamber through a couple of valves.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ZP3w4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZP3w4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel and lox pumps wouldn't work if the propellants were gaseous (they were designed to pump liquids), so I think it's safe to conclude the propellants were liquids on entry to the combustion chamber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://history.msfc.nasa.gov/saturn_apollo/documents/F-1_Engine.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-06-24T13:57:08.633" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22079" PostTypeId="1" AcceptedAnswerId="22082" CreationDate="2017-06-24T15:42:40.760" Score="6" ViewCount="2012" Body="&lt;p&gt;As pointed out in the question &lt;a href=&quot;https://space.stackexchange.com/q/22075/12102&quot;&gt;What is the “emergency crush core”?&lt;/a&gt;, an  &lt;a href=&quot;https://twitter.com/elonmusk/status/878334920141135872&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elon Musk tweet&lt;/a&gt; about the Falcon 9 1st stage landing following the &lt;a href=&quot;https://en.wikipedia.org/wiki/BulgariaSat-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;BulgariaSat-1&lt;/a&gt; launch says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Rocket is extra toasty and hit the deck hard (used almost all of the emergency crush core), but otherwise good&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;An &lt;a href=&quot;https://twitter.com/elonmusk/status/878313692986621953&quot; rel=&quot;nofollow noreferrer&quot;&gt;earlier tweet&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Falcon 9 will experience its highest ever reentry force and heat in today's launch. Good chance rocket booster doesn't make it back.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The narrator in the &lt;a href=&quot;https://www.youtube.com/watch?v=Y8mLi-rRTh8&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX BulgariaSat-1 webcast&lt;/a&gt; says at around &lt;code&gt;T -00:11:32&lt;/code&gt; (about 04:56 into the video currently) that this is an extra-challenging, &lt;em&gt;three-engine&lt;/em&gt; landing:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Our landing today involves some of the highest heating and structural loads on the first stage that we have seen to date, and it includes a three-engine landing burn. While it is still a secondary goal, this landing is going to prove to be extra challenging for us. But if we are successful, this will be the first rocket to land on both our East- and West-coast drone ships.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm fairly sure that burning three roughly ~650 kN &lt;a href=&quot;https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#Merlin_1D&quot; rel=&quot;nofollow noreferrer&quot;&gt;Merlin 1D (throttle-able) engines&lt;/a&gt; to land a nearly empty rocket has something to do with the hot landing, and there must have been something about this launch that made the use of three engines necessary or at least the best option, but I don't know what it is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Why were three engines used for the F9 1st stage landing burn for BulgariaSat-1?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A short description of why the use of three engines for the landing burn makes the landing so challenging would be great as well.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-24T18:49:22.467" LastActivityDate="2018-02-23T15:55:52.230" Title="Why were three engines used for the F9 1st stage landing burn (BulgariaSat-1)?" Tags="|spacex|landing|mission-design|bulgariasat-1|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22080" PostTypeId="1" AcceptedAnswerId="22111" CreationDate="2017-06-24T15:46:28.547" Score="11" ViewCount="1772" Body="&lt;p&gt;GPS receivers &lt;a href=&quot;https://space.stackexchange.com/questions/5423/how-does-gps-receiver-synchronize-time-with-gps-satellites&quot;&gt;need four satellites&lt;/a&gt; to determine their position and time. They provide a receiver with data that is fed into four equations. Three equations are solved (simultaneous with the fourth) to get position. The fourth equation solves for the exact time. The best explanation I've found for the solution of time is &lt;a href=&quot;http://electronics.howstuffworks.com/gadgets/travel/gps3.htm&quot; rel=&quot;noreferrer&quot;&gt;here (Paragraph starting with &quot;The Global...&quot;)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If your GPS receiver had an onboard atomic clock (imagine a \$50,600 cell phone, \$600 for the iPhone and \$50,000 for the atomic clock), &lt;strong&gt;could you do away with the fourth satellite with the atomic clock on board the receiver?&lt;/strong&gt; You would directly know the exact time of flight from each satellite. This would mean you also know your distance from each of the three satellites directly (no system of linear equations or need to solve for time).&lt;/p&gt;&#xA;" OwnerUserId="12320" LastEditorUserId="12320" LastEditDate="2017-06-26T19:12:06.737" LastActivityDate="2017-06-27T13:41:37.660" Title="Would a GPS receiver with an onboard atomic clock only need 3 satellites to determine position?" Tags="|gps|" AnswerCount="2" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="22081" PostTypeId="2" ParentId="22076" CreationDate="2017-06-24T16:28:06.713" Score="4" Body="&lt;p&gt;The pattern silvery staple-like features around the interior match the slightly earlier overhead shot with people working inside a stage; I believe from the dimensions the earlier shot is an F9 second stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a longer tank, so a preponderance of evidence says it's the F9 first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Organic Marble surmises, the &quot;grapes&quot; are almost certainly the COPV helium tanks; we know those live in the LOX tank and there's not much else you'd want to tank-in-tank like that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LOX tank lives above the kerosene tank, and I thought at first the helium tanks should be at the top of that, making this a shot up into the top of the LOX tank. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Someone in this &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/6hprri/elon_on_instagram_flying_through_the_falcon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Reddit thread&lt;/a&gt; says opposite, though, that the center hole is the LOX feed and the COPVs are at the bottom of the tank so they stay in a consistent LOX-submerged environment throughout the stage burn, which is probably right. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thread sports several assertions that they're the COPV tanks without any support, but one of them also asserts there are 9 rather than 13. ;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each and every rocket has the option to be fitted with a different COPV configuration. It depends entirely on the mission, so they've flown various configurations over the years.&#xA;  After Amos-6, they didn't really &quot;add more&quot; COPVs as much as they simply eliminated the more &quot;cutting edge&quot; configuration which stuffed as much Helium into as few COPVs as possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The thread also says the tanks are black with a (translucent?) red liner.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-06-24T16:36:45.030" LastActivityDate="2017-06-24T16:36:45.030" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22082" PostTypeId="2" ParentId="22079" CreationDate="2017-06-24T17:07:58.250" Score="12" Body="&lt;p&gt;Falcon 9 FT has demonstrated the ability to do a drone ship landing after launching a 5.3 ton payload into geosynchronous transfer orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;BulgariaSat-1 is only 3.7 tons, but was launched into a higher &lt;a href=&quot;https://en.wikipedia.org/wiki/Supersynchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;super-synchronous&lt;/a&gt; transfer orbit. (This allows it to do a plane change to an equatorial orbit using less fuel than it would at geosynchronous altitude.) This higher orbit requires more fuel expenditure from the first stage, and thus less fuel remains to execute the landing maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the final landing approach, every second in flight accelerates the stage by another 9.8 m/s due to gravity's pull. &lt;a href=&quot;https://space.stackexchange.com/questions/10307/what-is-a-suicide-burn&quot;&gt;You spend the least fuel by waiting until the very last instant to decelerate.&lt;/a&gt; If you decelerate on three engines, you can wait longer than if you decelerate on one engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The downside is, doing the terminal burn on three engines means everything is happening faster; there's less time to correct for any measurement errors or irregularities in throttle response.  Note that hitting slightly faster than planned and expending the &quot;crush core&quot; is preferable to the alternative, which is to run out of fuel while still above the pad. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a one-engine landing, things are a little more leisurely and SpaceX has no trouble hitting the bullseye.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(We went over some of this &lt;a href=&quot;https://space.stackexchange.com/questions/17663/can-falcon-9-first-stage-landing-final-burn-using-one-two-and-three-engines-t&quot;&gt;last year&lt;/a&gt;.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-06-24T17:07:58.250" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22083" PostTypeId="1" AcceptedAnswerId="22091" CreationDate="2017-06-24T17:24:34.197" Score="0" ViewCount="493" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wrL2q.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wrL2q.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TMdxa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TMdxa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at this cropped section of the screenshot described below, this is what I think I see:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A: Shadow from the leg support on the right, but not the left.&lt;/li&gt;&#xA;&lt;li&gt;B: Scorch marks?&lt;/li&gt;&#xA;&lt;li&gt;C: Scorch marks?&lt;/li&gt;&#xA;&lt;li&gt;D: Shadow of the Falcon 9 first stage?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;At an estimated time of &lt;code&gt;UTC 2017-06-23 19:19&lt;/code&gt; and a roughly estimated location of &lt;code&gt;29°N, 76°W&lt;/code&gt;, the sun would have had an elevation of about 60° above the horizon, and an azimuth of about 267°, or nearly due West.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering that the seas look incredibly calm, and the engines (nacelles?) should be well below the surface, the sudden appearance of rough water and spray on the left side moments before landing is probably the result of exhaust from the rocket just outside the camera's view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can imagine two possibilities.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The last few seconds of the final approach was roughly from the &lt;strong&gt;East&lt;/strong&gt;, not the West. If so, was this a planned maneuver or is this just a tiny overshoot in an otherwise heroic &lt;a href=&quot;https://space.stackexchange.com/q/22079/12102&quot;&gt;three engine landing&lt;/a&gt;? &lt;/li&gt;&#xA;&lt;li&gt;The last few seconds of the final approach was indeed roughly from the &lt;strong&gt;West&lt;/strong&gt; as one might expect, or was vertical, but the stage was tilted substantially so that the bottom swung out towards the west and stirred up the water and scorched the west side of the landing surface.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Is it known which is a better understanding of the last few seconds, or if a different description is needed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Qx5gA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qx5gA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; cropped/annotated sections from screen shots &lt;strong&gt;below&lt;/strong&gt; from the &lt;a href=&quot;https://youtu.be/Y8mLi-rRTh8&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX BulgariaSat-1 Launch Webcast&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HEosV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HEosV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9IjP3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9IjP3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wx1Cl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wx1Cl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-06-24T19:17:29.530" LastActivityDate="2017-06-25T09:43:36.953" Title="From which direction was the Falcon 9 1st stage's landing approach (BulgariaSat-1)?" Tags="|spacex|landing|asds|bulgariasat-1|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22084" PostTypeId="1" AcceptedAnswerId="22085" CreationDate="2017-06-24T17:57:44.080" Score="2" ViewCount="408" Body="&lt;p&gt;In the &lt;a href=&quot;https://youtu.be/Y8mLi-rRTh8&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX BulgariaSat-1 Launch Webcast&lt;/a&gt; there is a 3D animation of the orbits/trajectories of the first and second stage showing most of the Earth. There is a blue line that starts out over Florida, bur later &quot;goes south&quot; both literally and figuratively. At &lt;code&gt;T +00:27:20&lt;/code&gt; it starts sliding south faster and then disappears.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does this line represent?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y7EEs.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y7EEs.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Qx5gA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Qx5gA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9IjP3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9IjP3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-06-24T22:14:55.667" Title="What is the extra moving and disappearing line in the animation in the BulgariaSat-1 webcast?" Tags="|spacex|visualization|bulgariasat-1|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22085" PostTypeId="2" ParentId="22084" CreationDate="2017-06-24T22:03:11.223" Score="9" Body="&lt;p&gt;The blue line represents the part of the satellite's trajectory after it traveled once around the earth, so it's basically the continuation of the 2nd stage track (except that the payload will have been deployed by then). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;If you don't mind my paint skills, I've &lt;a href=&quot;https://i.stack.imgur.com/osAVE.jpg&quot; rel=&quot;noreferrer&quot;&gt;connected the two tracks in this image&lt;/a&gt;.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the stage/satellite is still being accelerated the track is at a higher altitude and because the earth rotates beneath it, it has shifted to the west from the earth's frame of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Why does it suddenly go south and disappears?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't really go south, it just appears to do so, because the virtual camera is moving. As the track approaches the camera the faster it seems to be moving, until the camera flies right over it and it gets out of sight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shortly before it disappears you can even see the track being cut off. This is caused by the &quot;near clipping plane&quot; during the 3D rendering process, in which elements that are too close (in front of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Viewing_frustum&quot; rel=&quot;noreferrer&quot;&gt;viewing frustum&lt;/a&gt;) get discarded. In the animation more and more of the track exits the frustum until it's gone completely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AE1Ba.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AE1Ba.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10335" LastActivityDate="2017-06-24T22:03:11.223" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22086" PostTypeId="2" ParentId="22084" CreationDate="2017-06-24T22:14:55.667" Score="3" Body="&lt;p&gt;That should be the projected future path on the second orbit. It looks like it is drawn relative to Earth surface (otherwise it would connect directly to current position of the second stage) and that only limited part of it is computed (it ends abruptly even before it vanishes). Unfortunately the ground track (the shadow) is only drawn for past (real) path as can be seen so this second end looks weird because without the shadow we are not able to properly assess it's actual position in the perspective projection from that 2d video (and because it does not help that is an &quot;infinitely thin&quot; line drawn with constant width line). It imho does not actually go so much south during the second stage2 burn as it goes up and at the same time gets shorter because the limited number of computed steps is taken by the long way to the raised apogee. I remember on some previous launch you could clearly see the inclination change the second stage performed during second burn by watching this line. Will try to locate it when I am on PC.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2017-06-24T22:14:55.667" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22087" PostTypeId="2" ParentId="22075" CreationDate="2017-06-25T01:52:46.187" Score="8" Body="&lt;p&gt;The legs are designed to bounce and take some impact on landing. For further absorption of shock they include non-reusable crush cores, that deform and absorb the energy but cannot bounce back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are made replaceable and it is preferable to use them up than to have the stage hit the deck.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-25T01:52:46.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22088" PostTypeId="2" ParentId="22076" CreationDate="2017-06-25T01:55:51.933" Score="4" Body="&lt;p&gt;The grapes are Helium storage tanks, called COPV's (Carbon Overwrapped Pressure Vessels). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are immersed in the LOX tank of the first and second stages (for the extra cooling) to make them able to take more pressure than at STP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The AMOS-6 refueling incident and the CRS-7 mission loss were due to COPV's venting.  AMOS-6 is thought that the loading order of the Helium caused it to cool more than expected and form solid oxygen around the piping, which then burnt at the slightest touch and the bursting of the COPV burst the stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CRS-7 in flight RUD was thought to be a strut holding the COPV failed and it broke off a pipe which vented the pressure into the tank, bursting the LOX tank.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-25T01:55:51.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22090" PostTypeId="1" CreationDate="2017-06-25T07:14:37.027" Score="24" ViewCount="2260" Body="&lt;p&gt;Why did the Apollo spacecraft use both DC and AC power, requiring heavy inverters? Was it impractical to design everything to run on DC power only? Why? &#xA;Are both AC and DC power still used in present day and future spacecraft designs?&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="12102" LastEditDate="2018-05-29T23:47:00.727" LastActivityDate="2020-01-16T09:23:48.130" Title="Why did Apollo spacecraft use both AC and DC equipment? Still used in present and future designs?" Tags="|spacecraft|apollo-program|power|spacecraft-development|electronics|" AnswerCount="1" CommentCount="15" ContentLicense="CC BY-SA 4.0" />
  <row Id="22091" PostTypeId="2" ParentId="22083" CreationDate="2017-06-25T09:43:36.953" Score="7" Body="&lt;p&gt;The landing had tighter timing than usual (see &lt;a href=&quot;https://space.stackexchange.com/questions/22079/why-were-three-engines-used-for-the-f9-1st-stage-landing-burn-bulgariasat-1&quot;&gt;&lt;em&gt;Why were three engines used for the F9 1st stage landing burn (BulgariaSat-1)?&lt;/em&gt;&lt;/a&gt;), which means less room for corrections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/878828849722671104&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk tweeted&lt;/a&gt;: &lt;em&gt;Rocket was suddenly slammed sideways right before landing. Heavy gust or something malfunctioned onboard. Reviewing telemetry.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the booster did work correctly, it seems the first stage was offset in the last moments of landing by a wind gust and due to the faster landing there was less room to compensate.&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2017-06-25T09:43:36.953" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22092" PostTypeId="1" AcceptedAnswerId="22093" CreationDate="2017-06-25T15:18:38.753" Score="2" ViewCount="523" Body="&lt;p&gt;The SpaceX &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;Autonomous spaceport drone ships&lt;/a&gt; have longer and shorter direction, both for the flat, apparently land-able area, and total footprint in the water. The ships have vertical projections at the extreme ends.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several potentially meaningful directions in the problem, including:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;General direction of the waves; affects pitching of the ship&lt;/li&gt;&#xA;&lt;li&gt;General direction of the winds; affects rocket's trajectory&lt;/li&gt;&#xA;&lt;li&gt;Direction of rocket's return orbit, often &lt;a href=&quot;https://space.stackexchange.com/q/22083/12102&quot;&gt;from the West&lt;/a&gt; from Florida, but not necessarily so, especially from California (more polar orbits).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;For landings on Mars there are elongated landing ellipses. But on Earth there is so much more live, constantly updating information available (e.g. GPS, meteorology) and of course some significant propulsive control at certain moments. Still, for Earth propulsive landing, wind direction and speed will vary along the path back to the surface and there can be gusting. I don't know if the ASDS had them, but laser doppler anemometers capable of profiling wind speed exist. When all is said and done, Is there anything like a landing ellipse for the Falcon 9? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the time of landing, are the ASDS ships generally oriented with their longer direction parallel to the direction with the largest residual uncertainties in the landing location, or parallel to the direction of oncoming waves to minimize the amplitude of wave-induced pitching, or does it not really matter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; A SpaceX &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;Anonymous Spaceport Drone Ship &lt;/a&gt; (ASDS), from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:CRS-8_(26239020092).jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sgf0g.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sgf0g.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="51" LastEditDate="2017-06-25T15:48:16.550" LastActivityDate="2017-06-25T15:48:16.550" Title="Do the SpaceX ASDS orient themselves in a particular direction before a landing? Is there a &quot;landing ellipse&quot;?" Tags="|spacex|falcon-9|landing|weather|asds|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22093" PostTypeId="2" ParentId="22092" CreationDate="2017-06-25T15:48:02.927" Score="1" Body="&lt;p&gt;The general consensus amongst the barge-stalkers in Reddit and the NSF forums is that the ASDS aligns itself long wise into the waves. This allows it to minimize rockings as it is longer than the wavelength of most waves. If the waves are that far apart, odds are good they cannot land in that weather. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They do apparently attempt to land on the X, as the BulgariaSat-1 landing, which came in quite a bit off center showed, as Musk tweeted that a sudden gust (or some malfunction) hit at the very end of the landing burn to push it off the center. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-25T15:48:02.927" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22094" PostTypeId="2" ParentId="13333" CreationDate="2017-06-25T16:25:44.867" Score="1" Body="&lt;p&gt;As explained in the Wikipedia articles on &lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;GPS&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/Trilateration&quot; rel=&quot;nofollow noreferrer&quot;&gt;trilateration&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Multilateration&quot; rel=&quot;nofollow noreferrer&quot;&gt;multilateration&lt;/a&gt;, it basically boils down to this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mathematically, your position is described by three unknowns (three dimensions). You could solve for it using three simultaneous equations incorporating distances from three known positions. However, you only have values for differences in arrival time; to compute distances, you need to know actual time-of-flight which means you need to know what time it is - a fourth unknown. So, solution requires four simultaneous equations in four unknowns (x,y,z, and time). When a GPS receiver picks up signals from four satellites, the four relative arrival times together with the position of each satellite at the emission time of each signal allows the receiver to compute both its own position and the actual time.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-06-25T17:31:47.610" LastActivityDate="2017-06-25T17:31:47.610" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22095" PostTypeId="2" ParentId="21773" CreationDate="2017-06-25T18:59:01.547" Score="6" Body="&lt;p&gt;One of the scientists involved wrote a &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2017/20160616-mu69-occulation-campaign.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;blog post for the Planetary Society&lt;/a&gt; dated June 16, 2017:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;“What about the results? Did you see the occultation?” you ask. Everyone asks this. Right now, we don’t know yet. This was an extremely challenging occultation to observe. MU69 occulted a very dim star, which always makes the observation tough. On top of that, we need to combine data from 24 deployed sites, plus some fixed sites, that cover a wide range of conditions, including seeing and illumination. Therefore, the data analysis will take awhile. It is likely that only a couple of the telescopes actually observed the occultation. All of the data, however, will be useful for the New Horizons mission. All of us—astronomers and the general public—just need to wait until the New Horizons team finishes the analysis and announces the results. I am not a New Horizons team member, so I am eagerly awaiting the results like everyone else.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;MU69 has three stellar occultations this summer—June 3rd, July 10th, and July 17th.&lt;/p&gt;&#xA;&lt;h2&gt;Update July 6th:&lt;/h2&gt;&#xA;&lt;p&gt;On July 5th (so before the second stellar occultation opportunity), the New Horizons team has shared a &lt;a href=&quot;https://www.nasa.gov/feature/new-mysteries-surround-new-horizons-next-flyby-target&quot; rel=&quot;nofollow noreferrer&quot;&gt;first look&lt;/a&gt; at the results:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Combined, the pre-positioned mobile telescopes captured more than 100,000 images of the occultation star that can be used to assess the environment around this Kuiper Belt object (KBO). While MU69 itself eluded direct detection, the June 3 data provided valuable and unexpected insights that have already helped New Horizons.&lt;/p&gt;&#xA;&lt;p&gt;“These data show that MU69 might not be as dark or as large as some expected,” said occultation team leader Marc Buie, a New Horizons science team member from Southwest Research Institute (SwRI) in Boulder, Colorado.&lt;/p&gt;&#xA;&lt;p&gt;Initial estimates of MU69’s diameter, based primarily on data taken by the Hubble Space Telescope since the KBO’s discovery in 2014, fall in the 12-25-mile (20-40-kilometer) range – though data from this summer’s ground-based occultation observations might imply it’s at or even below the smallest sizes expected before the June 3 occultation.&lt;/p&gt;&#xA;&lt;p&gt;Besides MU69’s size, the readings offer details on other aspects of the Kuiper Belt object.&lt;/p&gt;&#xA;&lt;p&gt;“These results are telling us something really interesting,” said New Horizons Principal Investigator Alan Stern, of SwRI. “The fact that we accomplished the occultation observations from every planned observing site but didn’t detect the object itself likely means that either MU69 is highly reflective and smaller than some expected, or it may be a binary or even a swarm of smaller bodies left from the time when the planets in our solar system formed.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;Update July 20&lt;/h2&gt;&#xA;&lt;p&gt;At least &lt;a href=&quot;https://www.sciencenews.org/blog/science-ticker/new-horizons-next-target-caught-making-star-blink?tgt=nr&quot; rel=&quot;nofollow noreferrer&quot;&gt;5 telescopes&lt;/a&gt; have captured the occultation during the &lt;a href=&quot;https://www.nasa.gov/feature/nasa-s-new-horizons-team-strikes-gold-in-argentina&quot; rel=&quot;nofollow noreferrer&quot;&gt;July 17 observation campaign&lt;/a&gt; (animated GIF):&lt;br /&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Cpg4r.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Cpg4r.gif&quot; alt=&quot;blink&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Analysis of the data is still ongoing.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-07-21T12:28:27.323" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22096" PostTypeId="1" AcceptedAnswerId="22097" CreationDate="2017-06-25T23:48:30.900" Score="24" ViewCount="16380" Body="&lt;p&gt;Elon Musk &lt;a href=&quot;https://twitter.com/elonmusk/status/878821062326198272&quot; rel=&quot;noreferrer&quot;&gt;tweeted&lt;/a&gt; &quot;Flying with larger &amp;amp; significantly upgraded hypersonic grid fins. Single piece cast &amp;amp; cut titanium. Can take reentry heat with no shielding.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These have been commonly known as a large visible element of the Mod 4 Falcon 9 design.  (Falcon 9 1.0 has been retconned to Mod 1, V1.1 to Mod 2, 1.1 Full Thrust/1.2 to Mod 3). The other is fuller thrust running the Merlin 1D at even higher thrust than the current 190,000 lbs thrust. (The Merlin Vacuum has been tweeted to have run at 240Klbs, instead of its current 205Klbs, but that was a test run). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What do we know has changed on these grid fins? &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-30T13:29:42.880" Title="How are the SpaceX Falcon 9 Mod 3 and Mod 4 grid fins different?" Tags="|spacex|falcon-9|landing|guidance|grid-fins|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22097" PostTypeId="2" ParentId="22096" CreationDate="2017-06-25T23:55:23.867" Score="42" Body="&lt;p&gt;Ok, I asked so I could use these awesome photos in an answer...  I confess.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The material has changed from Aluminium with an ablative paint to bare titanium. The specific shape, size, and mount points have changed as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's start with a nice shot of the Mod 3 design on a Falcon 1.1 Full Thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MkUHY.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MkUHY.jpg&quot; alt=&quot;Mod 3 design&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the size, shape, and design differences in these side by side photos, or as commonly known rocket porn. (Admit it, you just come here for the articles).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UdpKo.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UdpKo.jpg&quot; alt=&quot;Comparing grid fins&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Showing some of the structural differences highlighted. You can see that two support columns within the fin are no longer needed. The new fins are a bit bigger. They are unpainted titanium. The mount mechanism uses the same connectors but a different design on the fin components. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AJXoW.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AJXoW.jpg&quot; alt=&quot;Showing structural differences&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for a profile shot, look at the teeth on that baby... Betcha they got some bite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r9aQ8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r9aQ8.png&quot; alt=&quot;Profile view of Mod 4 grid fins&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Elon &lt;a href=&quot;https://twitter.com/elonmusk/status/879080848359686144&quot; rel=&quot;noreferrer&quot;&gt;tweeted&lt;/a&gt; after the landing: &quot;New titanium grid fins worked even better than expected. Should be capable of an indefinite number of flights with no service.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The previous fins were clearly ablating hard, and getting quite hot during previous landings. Thus these new ones should make reuse one step easier. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I saw a comment that the camera lens on the descent cam did not get covered in goop as usual, and wondered if it was the absence of the ablative paint; in fact in previous recoveries, it was not atmosphere (read: water) getting on the lens cover, but rather the older-model grid fins shedding material. In other words, all boosters with the new titanium fins should be less vulnerable to ablation and thus the camera feed should remain intact.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="6449" LastEditDate="2017-06-26T10:22:01.540" LastActivityDate="2017-06-26T10:22:01.540" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22098" PostTypeId="1" CreationDate="2017-06-26T01:23:34.193" Score="1" ViewCount="344" Body="&lt;p&gt;How would I estimate the transfer time between Mars and Saturn if I have more Delta-V than necessary for a Hohmann transfer?&lt;/p&gt;&#xA;" OwnerUserId="19742" LastActivityDate="2017-06-26T04:25:30.557" Title="Reducing transfer time between Mars and Saturn" Tags="|delta-v|hohmann-transfer|transfer-time|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22099" PostTypeId="1" AcceptedAnswerId="22106" CreationDate="2017-06-26T03:57:22.640" Score="16" ViewCount="1457" Body="&lt;p&gt;Can someone explain the purpose of the 'teeth' seen in the last picture of this &lt;a href=&quot;https://space.stackexchange.com/a/22097/20191&quot;&gt;answer&lt;/a&gt;?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r9aQ8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r9aQ8.png&quot; alt=&quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Do they have a specific name? They look too far up to be used to grab anything since the rockets land vertically on their base.&lt;/p&gt;&#xA;" OwnerUserId="20191" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-06-26T13:10:35.883" Title="What do the Teeth on the SpaceX Falcon Mod4 do?" Tags="|spacex|falcon-9|landing|grid-fins|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22100" PostTypeId="2" ParentId="22098" CreationDate="2017-06-26T04:25:30.557" Score="3" Body="&lt;p&gt;I whomped up &lt;a href=&quot;http://clowder.net/hop/TMI/NonHohmannMarsToSaturn.xlsx&quot; rel=&quot;nofollow noreferrer&quot;&gt;a spreadsheet&lt;/a&gt; where you can set perihelion and aphelion of transfer orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a screen capture from the spreadsheet where I set perihelion of transfer orbit at 1 A.U. and aphelion at 12 A.U.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S7hAL.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S7hAL.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course SMA is (perihelion+aphelion)/2. Where the transfer orbit crosses Mars orbit, r is 1.524 A.U.. At Saturn, r is 9.537 A.U.. Knowing a, mu and r you can figure speed using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis viva equation&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides speeds you must also take angle between velocity vectors into account. Incorrectly assuming Mars and Saturn are in circular orbits, that angle would be transfer orbit's &lt;a href=&quot;https://en.wikipedia.org/wiki/Elliptic_orbit#Flight_path_angle&quot; rel=&quot;nofollow noreferrer&quot;&gt;flight path angle&lt;/a&gt;. Then you can use the law of cosines to get the delta V between vectors. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am hoping you will examine the cells in this spreadsheet and try to reverse engineer what I did. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a screen capture with some of the relevant numbers emphasized:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EtTlz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EtTlz.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would do well to proofread this spreadsheet. I hastily adapted it from a NonHohmann Earth To Mars spreadsheet. I tried to relabel and replace different quantities like planet radius, gravitational parameter, and so on. But I do make mistakes. I would be grateful if anyone reviewed my efforts.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-06-26T04:25:30.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22101" PostTypeId="2" ParentId="22070" CreationDate="2017-06-26T06:19:09.850" Score="1" Body="&lt;p&gt;The drone ship acts as a relay for telemetry from the rocket to ground control, that's the signal being acquired.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It probably also feeds location and weather information to the rocket for last minute adjustments (I'd do that were I designing such a system, and I'm not as smart as the guys at SpaceX).&lt;/p&gt;&#xA;" OwnerUserId="3333" LastActivityDate="2017-06-26T06:19:09.850" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22102" PostTypeId="2" ParentId="22060" CreationDate="2017-06-26T07:09:53.137" Score="1" Body="&lt;p&gt;For launching rockets and now with the newest high bypass turbofan engines, the engines are tuned and controlled to the point that the chemical reaction is stoichiometric. For the most part, the chemical reactions in the engines operating in the atmospheric layers produce byproducts of either just water (for a liquid hydrogen/liquid oxygen engine) or water and carbon dioxide (Jet Fuel/Liquid Oxygen). The efficiency of these engines is so vitally important that companies and governments cannot afford to have reactions less than perfect. Neither of those sets of byproducts shall alter the homeostasis of our atmosphere. What does make a difference is the example of a wood fire. That is a burn that is incredibly dirty with incomplete reactions and many large particulate molecules coming from the combustion reaction. The incentive to change habits for someone using a wood stove just do not exist. &lt;/p&gt;&#xA;" OwnerUserId="20193" LastActivityDate="2017-06-26T07:09:53.137" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22104" PostTypeId="2" ParentId="22061" CreationDate="2017-06-26T11:38:46.793" Score="7" Body="&lt;p&gt;Let's try with pictures...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, the frame of reference: Earth. Frame bound to Earth center, spinning with Earth spin. This is the frame of reference which is &quot;immobile&quot; for you and me as we stand on Earth and look at the sky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I try to draw all possible positions of the satellite in this frame of reference - first, in 3D:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm creating a shell - a hollow sphere with its top and bottom &quot;caps&quot; cut off. This shell is the area the satellite reaches, ever.  This shell radius is Earth radius + ~400km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2dxyW.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2dxyW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PnbXD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PnbXD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's move the camera to the observer on the surface of Earth - inside that shell - and make Earth vanish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is about the image we'll see if we gaze towards where the center of Earth used to be. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZgQhz.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZgQhz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And if we squeeze the focal length really, really short - an extreme &quot;fisheye&quot; -  we're getting this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yFgeU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yFgeU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two distinct features are two holes, distorted by perspective, one closer, one farther from you. One due north, mostly above us, though partially behind the horizon, one due south, all &quot;below the ground&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now try to take the 360 degrees panorama from your point of view - map your surroundings onto a cylindrical projection - horizon being the equator, poles being your local zenith and nadir. You'll arrive at your original image.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-06-26T11:38:46.793" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22105" PostTypeId="2" ParentId="22080" CreationDate="2017-06-26T12:09:48.067" Score="4" Body="&lt;h1&gt;Yes&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;There was in fact the belief that future GPS receivers would contain a tiny atomic clock.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.wired.com/insights/2013/09/how-chip-scale-atomic-clocks-will-change-the-future-of-time/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.wired.com/insights/2013/09/how-chip-scale-atomic-clocks-will-change-the-future-of-time/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the opposite occured. Atomic clocks became redundant in many applications as the number of Navigation satilites increased.&lt;/p&gt;&#xA;" OwnerUserId="6157" LastActivityDate="2017-06-26T12:09:48.067" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22106" PostTypeId="2" ParentId="22099" CreationDate="2017-06-26T12:20:12.407" Score="15" Body="&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt; - Better aerodynamics at high speeds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a paper that compares the plain grid fins with locally swept &quot;toothed&quot; grid fins:&lt;br&gt;&#xA;&lt;a href=&quot;https://www.sto.nato.int/publications/STO%20Meeting%20Proceedings/RTO-MP-AVT-135/MP-AVT-135-35.pdf&quot; rel=&quot;noreferrer&quot;&gt;Novel High-Performance Grid Fins for Missile Control at High Speeds (PDF)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To quote a part of its conclusion (emphasis mine):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The results show an essential reduction of the drag for the locally&#xA;  swept configurations. The most important results can be summarized as&#xA;  follows:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;strong&gt;The gained profit for the investigated realistic locally swept lattice wing configurations in comparison to the conventional ones in&#xA;  the zero-lift total drag amounts up to 38% and in the lift-to-drag&#xA;  ratio up to over 20%.&lt;/strong&gt;&lt;/li&gt;&#xA;  &lt;li&gt;The lowest zero-lift wave drag is shown by the lattice wing construction with the peak-type of locally swept lattice wings (LSLWp&#xA;  and LSLWp2). On the contrary, better lift-to-drag performance at wing&#xA;  incidence is shown by the valley-type of LSLW (LSLWv and LSLWv2).&lt;/li&gt;&#xA;  &lt;li&gt;The effect of the LSE increases above all with the free stream Mach number, the local sweep angle, and the relative thickness of the&#xA;  members and the bluntness of their leading edges. It decreases with&#xA;  the incidence angle and for bigger relative tooth-sizes.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Or, in non-scientific-paper-speak: The toothed grid fins are better than the flat grid fins as they have better &lt;a href=&quot;https://en.wikipedia.org/wiki/Zero-lift_drag_coefficient&quot; rel=&quot;noreferrer&quot;&gt;zero-lift drag&lt;/a&gt; and a better &lt;a href=&quot;https://en.wikipedia.org/wiki/Lift-to-drag_ratio&quot; rel=&quot;noreferrer&quot;&gt;lift-to-drag ratio&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially it's for the same reason fighter jets have a swept delta wing design.&lt;/p&gt;&#xA;" OwnerUserId="10335" LastEditorUserId="10335" LastEditDate="2017-06-26T13:10:35.883" LastActivityDate="2017-06-26T13:10:35.883" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22107" PostTypeId="1" AcceptedAnswerId="22112" CreationDate="2017-06-26T16:05:42.610" Score="3" ViewCount="265" Body="&lt;p&gt;The ramjet in the novel &quot;Tau Zero&quot; required a separate decelerator which, spoilers, breaks, and causes the ship to career endlessly through the universe. Is this accurate from a real-world engineering standpoint? Could they not simply have adjusted the collector to reduce the efficiency and tip the balance between thrust and drag?&lt;/p&gt;&#xA;" OwnerUserId="5965" LastEditorUserId="13867" LastEditDate="2017-06-26T22:21:19.453" LastActivityDate="2017-06-26T23:33:07.630" Title="Is the behavior of the Bussard ramjet as described in Poul Anderson's novel Tau Zero accurate?" Tags="|engines|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22108" PostTypeId="1" AcceptedAnswerId="22109" CreationDate="2017-06-26T16:53:50.217" Score="15" ViewCount="2618" Body="&lt;p&gt;In &lt;a href=&quot;https://youtu.be/P5_GlAOCHyE?t=16m25s&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; we can see an astronaut on the payload bay of the Space Shuttle, placing what looks like a &lt;a href=&quot;http://www.google.com/images?q=radar%20reflector&quot; rel=&quot;noreferrer&quot;&gt;radar reflector&lt;/a&gt;, during an &lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_activity&quot; rel=&quot;noreferrer&quot;&gt;EVA&lt;/a&gt;. The following image is a frame from the video in question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UVSlB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UVSlB.jpg&quot; alt=&quot;astronaut placing radar reflector&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Corner_reflector&quot; rel=&quot;noreferrer&quot;&gt;Radar reflectors&lt;/a&gt; have many applications. They are usually mounted on small boats to increase their radar fingerprint (&lt;a href=&quot;http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/25224/SGNO411978.pdf?seq&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;), and they are part of the emergency equipment on board a lifeboat.(&lt;a href=&quot;https://en.wikipedia.org/wiki/Lifeboat_(shipboard)&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I had no idea that they were used on board spacecraft, so I find it very intriguing.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Why is this astronaut placing a radar reflector?&lt;/li&gt;&#xA;&lt;li&gt;Are radar reflectors used consistently during&#xA;space missions or was this just part of an experiment?&lt;/li&gt;&#xA;&lt;li&gt;Can anybody identify which &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Space_Shuttle_missions#Launches_and_orbital_flights&quot; rel=&quot;noreferrer&quot;&gt;STS mission&lt;/a&gt; this was?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="20201" LastEditorUserId="20201" LastEditDate="2017-06-26T23:02:25.530" LastActivityDate="2018-08-12T21:01:47.273" Title="Why is this astronaut placing a radar reflector?" Tags="|space-shuttle|eva|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22109" PostTypeId="2" ParentId="22108" CreationDate="2017-06-26T17:29:03.250" Score="24" Body="&lt;p&gt;This was mission &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-61-B&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-61B&lt;/a&gt;. The giveaways are the ACCESS payload box and the Mexico logo on the PAM-D sunshield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The last task of the first EVA was to deploy a small satellite  (the radar reflector) to be used for station-keeping experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ironically, the shuttle radar was failed, so the targetting was all done visually.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was definitely part of an experiment; I am not aware of radar reflectors being used &quot;consistently&quot;, at least in the shuttle program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is &lt;a href=&quot;https://en.wikipedia.org/wiki/Sherwood_C._Spring&quot; rel=&quot;nofollow noreferrer&quot;&gt;Woody Spring&lt;/a&gt; deploying the satellite/reflector (a few frames after the picture in the question)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UmcLX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UmcLX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=28098.180&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt; (scroll down past the pictures to get to the text)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="25224" LastEditDate="2018-08-12T21:01:47.273" LastActivityDate="2018-08-12T21:01:47.273" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="22110" PostTypeId="2" ParentId="19297" CreationDate="2017-06-26T18:41:10.510" Score="27" Body="&lt;p&gt;China has nothing to prove in this case, as it &lt;em&gt;asked&lt;/em&gt; to be part of the International Space Station. This request was shut down by the US Government, which now prevents NASA from working with any Chinese citizen affiliated with the government (e.g. CNSA). This is entirely on the US and its decision to bar the CNSA from any cooperation with NASA, and not on a Chinese reluctance to be part of the CNSA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.reuters.com/article/us-space-usa-china-idUSKCN0S61SU20151012&quot; rel=&quot;noreferrer&quot;&gt;NASA chief says ban on Chinese partnerships is temporary&lt;/a&gt; (Reuters, 12 Oct 2015):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The United States should include China in its human space projects or face being left out of new ventures to send people beyond the International Space Station, NASA chief Charles Bolden said on Monday.&lt;br /&gt;&#xA;  Since 2011, the U.S. space agency has been banned by Congress from collaborating with China, due to human rights issues and national security concerns. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/Politics_of_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;Politics of the International Space Station&lt;/a&gt; (Wikipedia):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In 2007, Chinese vice-minister of science and technology Li Xueyong said that China would like to participate in the ISS.&lt;br /&gt;&#xA;  In 2010, ESA Director-General Jean-Jacques Dordain stated his agency was ready to propose to the other 4 partners that China be invited to join the partnership, but that this needs to be a collective decision by all the current partners. While ESA is open to China's inclusion, the US is against it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://time.com/3901419/space-station-no-chinese/&quot; rel=&quot;noreferrer&quot;&gt;The Silly Reason the Chinese Aren't Allowed on the Space Station&lt;/a&gt; (Time, 29 May 2015):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;China has been barred from the ISS since 2011, when Congress passed a law prohibiting official American contact with the Chinese space program due to concerns about national security.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;More info e.g. &lt;a href=&quot;https://www.quora.com/Why-is-China-not-part-of-the-ISS&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20206" LastEditorUserId="20206" LastEditDate="2018-05-31T17:29:07.210" LastActivityDate="2018-05-31T17:29:07.210" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="22111" PostTypeId="2" ParentId="22080" CreationDate="2017-06-26T22:01:40.000" Score="7" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;If your GPS receiver had an onboard atomic clock (imagine a \$50,600 cell phone, \$600 for the iPhone and \$50,000 for the atomic clock), could you do away with the fourth satellite with the atomic clock on board the receiver?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, in theory. Actually in normal GPS, four signals will normally not give a unique solution, even with zero error bars, but usually only one solution will be close enough to the surface of the earth to make sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that chip-scale atomic clocks (CSAC) are now down to &lt;a href=&quot;https://forum.cosmoquest.org/archive/index.php/t-162153.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;about \$1500&lt;/a&gt;. However, their precision is not good enough for them to work for long periods of time without recalibration, and that recalibration would normally require that you get four GPS satellites again. The &lt;a href=&quot;https://www.microsemi.com/products/timing-synchronization-systems/embedded-timing-solutions/components/sa-45s-chip-scale-atomic-clock&quot; rel=&quot;nofollow noreferrer&quot;&gt;spec sheet&lt;/a&gt; for a current model says &quot;&amp;lt;9E-10 /mo Aging Rate (Typical),&quot; which suggests that if you ran the thing for a month without reacquiring signals from four satellites, you would be off by the end of the month by a few milliseconds, which is an error of hundreds of kilometers.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To go one step further, if you assume you are in the surface of the earth I think you might be able to get away with only TWO satellites (although in this case there are two solutions on the surface of the earth that are both reasonable and choosing the correct one is harder). This wouldn't work for a plane and would require a very precise knowledge of the Earth's topography.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't think this works very well. The intersection of three spheres is typically two points. With normal GPS, you get multiple solutions with 4 satellites, but you can usually tell which one is real because it's near the earth's surface. With only two satellites plus the CSAC, you would get two solutions with no obvious way to tell which was correct.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The problem with a GPS satellite is it literally tells an adversary where it is at any time making it inherently easy to attack and destroy. An advantage with the on-board atomic time piece approach is one could work with a minimal GPS constellation [...]  you could determine a receivers position with only three entities revealing their position as opposed to the conventional four.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't really see how this helps. A GPS satellite that doesn't reveal its position isn't functioning as a GPS satellite, so it might as well have been put out of commission. Also, the most likely method that's been discussed for destroying GPS satellites is as a result of a nuclear explosion in space. You can get EMP or the trapping of high-energy charged particles in the earth's magnetic field. The latter has already happened unintentionally in 1962 with the Starfish Prime nuclear test. In this type of scenario, an attacker doesn't need to know the satellite's position to high precision -- they may not need to know its position at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe a more compelling application is that you don't want to lose GPS tracking if you're in a deep canyon where you only have a limited view of the sky. This happens to me fairly often when I'm hiking. If a limited view of the sky cuts you from 4 satellites to 3 for 15 minutes, it would be nice to still be able to use GPS. However, we now have so many different satellites up there (GPS, Galileo, Beidou, and GLONASS) that with a multi-constellation receiver you should probably always have plenty of satellites visible. I think in the future consumer-grade GPS units will all start to be multi-constellation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Aron pointed out in a comment, it may be easier to jam a GPS signal than to destroy a GPS satellite.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2017-06-27T13:41:37.660" LastActivityDate="2017-06-27T13:41:37.660" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22112" PostTypeId="2" ParentId="22107" CreationDate="2017-06-26T23:33:07.630" Score="6" Body="&lt;p&gt;This makes me sad because I grew up reading Niven's novels about ramscoops, but as early as 1978 T. A. Heppenheimer showed that the power required to compress protons enough to fuse exceeds the power that can be produced. Other reactions have been proposed but they have problems as well. There is a good writeup here: &lt;a href=&quot;https://www.centauri-dreams.org/?p=22274&quot; rel=&quot;noreferrer&quot;&gt;https://www.centauri-dreams.org/?p=22274&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-06-26T23:33:07.630" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22113" PostTypeId="2" ParentId="21525" CreationDate="2017-06-27T01:17:04.533" Score="2" Body="&lt;p&gt;1027 was a Falcon Heavy structural test article that spent some time in the structural stand at McGregor before disappearing. It had no engines or fueling quick disconnects, so it couldn't have done propellant loading tests. After showing up in the stand, it disappeared.&lt;/p&gt;&#xA;" OwnerUserId="12375" LastActivityDate="2017-06-27T01:17:04.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22115" PostTypeId="1" AcceptedAnswerId="22117" CreationDate="2017-06-27T07:10:27.360" Score="10" ViewCount="14544" Body="&lt;p&gt;Well I'm new to Space SE and have gone through many questions but I couldn't help but notice that is it even remotely possible&#xA;that one can touch the stars without burning him/her self ? Is there any kind of possibility ?&lt;/p&gt;&#xA;" OwnerUserId="20210" LastEditorUserId="20210" LastEditDate="2017-06-27T09:11:11.087" LastActivityDate="2022-11-16T13:25:45.880" Title="Is it possible to touch stars with our bare hands?" Tags="|earth|stars|" AnswerCount="4" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22116" PostTypeId="2" ParentId="19297" CreationDate="2017-06-27T08:24:37.030" Score="12" Body="&lt;p&gt;This is will be a very humble answer and an overview of events that caused the complication. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been more of a political issue rather than technical issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;U.S. congress always have wanted to ban the Chinese from launching U.S. satellites due to security and financial reasons. (So the money goes to ULA, not Chinese)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back in 1996, Intelsat 708 was aboard Long March 3B rocket, seconds after launch, the rocket tilts and exploded in the mountains.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Intelsat_708&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Intelsat_708&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SSL(Owner of Intelsat) immediately identified the problem and fixed the problem for the Chinese.(Gyro sensor had problem) Then, the congress was not happy with what SSL did, and one of the republican senator used this incident and urged to ban US satellites launching from Chinese rockets, and the congress approved. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Further from that, Congress feared the Chinese might modify their communication satellites or steal technologies from US satellites when the satellite is in the Chinese hands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The event was catastrophic for space cooperation between US and China, and this complication is still in effect today.&lt;/p&gt;&#xA;" OwnerUserId="18270" LastActivityDate="2017-06-27T08:24:37.030" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22117" PostTypeId="2" ParentId="22115" CreationDate="2017-06-27T08:42:10.617" Score="16" Body="&lt;p&gt;Surprisingly, yes, for some of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Small, old stars can be at room temperature ex: &lt;a href=&quot;https://en.wikipedia.org/wiki/WISE_1828%2B2650&quot; rel=&quot;noreferrer&quot;&gt;WISE 1828+2650&lt;/a&gt;, so you could touch the surface without getting burned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any star you can see in the sky with the naked eye, however, would be hot enough to destroy your body instantaneously if you came anywhere near them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An article about cold stars: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.universetoday.com/129154/can-stars-cold/&quot; rel=&quot;noreferrer&quot;&gt;https://www.universetoday.com/129154/can-stars-cold/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="5965" LastEditDate="2017-06-27T20:56:59.303" LastActivityDate="2017-06-27T20:56:59.303" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22118" PostTypeId="2" ParentId="22115" CreationDate="2017-06-27T10:10:03.237" Score="4" Body="&lt;p&gt;You might be touching a piece of one right now. Heavier elements, like gold, are formed when massive stars undergo a supernova at the end of their lives. During this gigantic explosion they fuse elements together to form heavier ones, which then get scattered out into space as dust, which eventually coalesces under gravity, forms a planet, and with a bit of luck, evolves life forms which mine and smelt that gold and turn it into jewelry.  So if you have a gold ring or chain on you, spend a minute to think how it was made in the core of a dying star.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2017-06-27T10:10:03.237" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22121" PostTypeId="2" ParentId="22115" CreationDate="2017-06-27T20:51:45.957" Score="13" Body="&lt;p&gt;No.  While none exist to date it would be possible for a dead star to have cooled to a safe temperature.  However, such objects are inherently supported by degeneracy pressure--they're very dense.  Very heavy &amp;amp; very dense = very high surface gravity (typically 300,000+g.)  While you're not burned you're squashed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The low mass limit for a brown dwarf is 13 times Jupiter's mass.  However, Jupiter is about as big as such things get, piling on more mass increases the pressure enough the size stays about constant.  Jupiter's gravity is already 2.5g, for a constant size surface gravity scales linearly with mass, so a brown dwarf should have a surface gravity upwards of 30g.  You're not quite so squashed as you were on the dead star but you're still squashed.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-06-27T20:51:45.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22122" PostTypeId="1" AcceptedAnswerId="22123" CreationDate="2017-06-27T23:28:40.357" Score="10" ViewCount="2172" Body="&lt;p&gt;As evidenced by the bright exhaust plume at launch, the billowing black smoke rolling out of the flame trench during static tests, the wispy grey effluent from the Merlin Vacuum engine, and the layer of soot on the recovered F9 first stages, it seems that there is substantial unburned fuel in the exhaust of the Merlin engines and kerolox engines more generally.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question: is this due to incomplete fuel/oxidizer mixing, or is the engine deliberately operated with fuel-rich stoichiometry in order to moderate combustion temperatures? &lt;/p&gt;&#xA;" OwnerUserId="18585" LastEditorUserId="23891" LastEditDate="2020-06-02T20:54:29.970" LastActivityDate="2022-02-09T08:34:25.017" Title="Merlin engine stoichiometry" Tags="|spacex|falcon-9|engines|merlin-1d|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22123" PostTypeId="2" ParentId="22122" CreationDate="2017-06-27T23:56:49.660" Score="15" Body="&lt;p&gt;Fuel-rich operation is common in hydrocarbon engines and improves specific impulse, although there's some confusion about the mechanism by which that occurs.&lt;/p&gt;&#xA;&lt;p&gt;According to Sutton &amp;amp; Biblarz' &lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/com/1118753658&quot; rel=&quot;nofollow noreferrer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Propulsion Elements&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Rocket propulsion systems usually do not operate with the proportion of their oxidizer and fuel in the stoichiometric mixture ratio. Instead, they usually operate fuel rich because this allows lightweight molecules such as hydrogen to remain unreacted; this reduces the average molecular mass of the reaction products, which in turn increases the specific impulse.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://yarchive.net/space/rocket/fuels/fuel_rich.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Henry Spencer&lt;/a&gt;, however, says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In a chemical rocket, where the reaction mass and&#xA;energy source are one and the same, trying to lower the molecular weight&#xA;by adding an excess of one propellant also reduces the flame temperature,&#xA;and if you actually &lt;em&gt;do the math&lt;/em&gt;, it's always a net loss.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;So why do they run fuel-rich?  Well, partly there are some simplifying&#xA;assumptions in that math which aren't strictly correct.  More important,&#xA;though, is that the textbooks look at the wrong part of the equation.&#xA;They skip over the nozzle efficiency, which is &lt;em&gt;not&lt;/em&gt; independent of the&#xA;gas composition.  In particular, the subexpression (gamma-1)/gamma, &lt;em&gt;[(k-1)/k in Sutton; the &amp;quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Heat_capacity_ratio&quot; rel=&quot;nofollow noreferrer&quot;&gt;ratio of specific heats&lt;/a&gt;&amp;quot;]&lt;/em&gt; which&#xA;appears as an &lt;em&gt;exponent&lt;/em&gt; in the nozzle efficiency, is a strong function of&#xA;gas composition, and to a first approximation, it's inversely proportional&#xA;to the number of atoms per molecule.  So an excess of fuel, meaning that&#xA;some of the fuel ends up as CO or H&lt;sub&gt;2&lt;/sub&gt; rather than CO&lt;sub&gt;2&lt;/sub&gt; and H&lt;sub&gt;2&lt;/sub&gt;O, can make a big difference to nozzle efficiency, and that can more than make up for&#xA;the reduced energy release.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In complex molecules, apparently, a lot of thermal energy can be bound up in flexing of interatomic bonds  &lt;a href=&quot;https://yarchive.net/space/rocket/fuels/fuel_ratio.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;(as discussed in another yarchive thread)&lt;/a&gt;, which doesn't contribute to the momentum of the escaping exhaust, which determines the thrust.&lt;/p&gt;&#xA;&lt;p&gt;Here's Clark's &lt;em&gt;Ignition!&lt;/em&gt; (he uses &lt;span class=&quot;math-container&quot;&gt;$\frac {R} {C_p}$&lt;/span&gt; in place of Spencer's &lt;span class=&quot;math-container&quot;&gt;$\frac {\gamma - 1} {\gamma})$&lt;/span&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;If we consider specific exhaust products, this is what we find: N&lt;sub&gt;2&lt;/sub&gt; and solid C are practically useless as energy producers. HCl, H&lt;sub&gt;2&lt;/sub&gt;, and CO are fair. CO&lt;sub&gt;2&lt;/sub&gt; is good, while B&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;, HBO&lt;sub&gt;2&lt;/sub&gt;, OBF, BF&lt;sub&gt;3&lt;/sub&gt;, H&lt;sub&gt;2&lt;/sub&gt;O, and HF, as well as solid B&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; and Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;, are excellent. When we consider the R/Cp term, the order is quite different. The diatomic gases, with an R/Cp above 0.2, are excellent. They include HF, H&lt;sub&gt;2&lt;/sub&gt;, CO, HCl, and N&lt;sub&gt;2&lt;/sub&gt;. (Of course a monatomic gas has an R/Cp of 0.4, but finding a chemical reaction which will produce large quantities of hot helium is out of the range of practical politics.) The triatomic gases, H&lt;sub&gt;2&lt;/sub&gt;O, OBF, and CO&lt;sub&gt;2&lt;/sub&gt;, with an R/Cp between 0.12 and 0.15 are fair. The tetratomic HBO&lt;sub&gt;2&lt;/sub&gt; and BF&lt;sub&gt;3&lt;/sub&gt;, at about 0.1, are poor, and B&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; -- well, perhaps it should be passed over in silence. As for the solids, C, Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;, and B&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;, their R/Cp is precisely zero, as would be the thermal efficiency if they were ever the sole exhaust products.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Note the focus on the number of atoms in the exhaust products.&lt;/p&gt;&#xA;&lt;p&gt;Hydrogen-oxygen engines do the same; here's Spencer again:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Yes, the SSMEs run&#xA;fuel-rich -- all hydrogen engines are run very fuel-rich, because that&#xA;improves performance considerably.  Having a fair bit of unreacted&#xA;hydrogen in the exhaust turns out to be good for performance in several&#xA;ways, and hydrogen is so light that the mass penalty for this is small.&#xA;Ideally, hydrogen engines would run at about 4:1, with half the hydrogen&#xA;unburned; in practice, because hydrogen is so bulky, considerations of&#xA;tank mass usually force the engine people to compromise on about 6:1.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In addition to putting simpler and lighter molecules into the exhaust, running fuel-rich also keeps the combustion temperature down, making it more practical to cool the chamber and nozzle. Fuel-rich operation is preferred over oxidizer-rich for cooling because hot oxidizer-rich exhaust tends to attack the metal of the combustion chamber and nozzle, which quickly leads to &amp;quot;engine-rich combustion&amp;quot; -- destructive burn-through of parts of the engine.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2020-12-31T18:57:07.437" LastActivityDate="2020-12-31T18:57:07.437" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="22124" PostTypeId="2" ParentId="21776" CreationDate="2017-06-28T09:29:14.310" Score="9" Body="&lt;p&gt;I found a document about that theme:&lt;br /&gt;&#xA;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090015395.pdf&quot; rel=&quot;noreferrer&quot;&gt;Apollo Spacecraft &amp;amp; Saturn V Launch Vehicle Pyrotechnics / Explosive Devices&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some cites:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;More than 210 pyrotechnic devices per Apollo Mission.  &lt;/li&gt;&#xA;&lt;li&gt;All devices required high reliability and safety&lt;/li&gt;&#xA;&lt;li&gt;Most devices were classified as either crew safety critical or mission critical.  &lt;/li&gt;&#xA;&lt;li&gt;When complete system redundancy was not possible, redundant cartridges or single cartridges with dual initiators were used.  &lt;/li&gt;&#xA;&lt;li&gt;Typically, two separate and electrically independent &#xA;systems operated in parallel and provided complete &#xA;redundancy in the firing circuitry.  &lt;/li&gt;&#xA;&lt;li&gt;No failures of any pyrotechnic device were ever detected during&#xA;any of the Apollo missions.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="15906" LastEditorDisplayName="user10509" LastEditDate="2017-06-30T14:14:53.803" LastActivityDate="2017-06-30T14:14:53.803" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22125" PostTypeId="1" AcceptedAnswerId="22132" CreationDate="2017-06-28T16:42:24.880" Score="10" ViewCount="13255" Body="&lt;p&gt;My question may sound naive and hence would like to apologise if this question is already answered in same or other form in this forum.&lt;br&gt;&#xA;How does &lt;code&gt;Right Ascension Of Ascending Node&lt;/code&gt; ( RAAN ) of a &lt;code&gt;sun synchronous orbit&lt;/code&gt; &lt;code&gt;earth&lt;/code&gt;  &lt;code&gt;orbit&lt;/code&gt;  is related to its &lt;code&gt;local time&lt;/code&gt;  ? Suppose a &lt;code&gt;spacecraft&lt;/code&gt; needs to be injected into a &lt;code&gt;sun synchronous orbit&lt;/code&gt; of certain &lt;code&gt;a,e,i&lt;/code&gt; and &lt;code&gt;local time t&lt;/code&gt; combination from a particular &lt;code&gt;launch site&lt;/code&gt;. Now for the same combination of &lt;code&gt;a,e,i and local time t&lt;/code&gt;, if the satellite is injected on any other day from the same launch site, the &lt;code&gt;RAAN&lt;/code&gt; would differ. Still the desired &lt;code&gt;local time&lt;/code&gt; is achieved. As long as rate of change of &lt;code&gt;nodal regression&lt;/code&gt; is maintained, does it matter whether the &lt;code&gt;injection RAAN&lt;/code&gt; should have any  particular value ?&lt;/p&gt;&#xA;" OwnerUserId="19925" LastActivityDate="2017-07-18T09:05:43.683" Title="How Local time of a sun synchronous orbit is related to Right ascension of ascending node?" Tags="|orbit|orbital-elements|sun-synchronous|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22126" PostTypeId="1" CreationDate="2017-06-28T16:56:47.977" Score="0" ViewCount="87" Body="&lt;p&gt;Awesome place!&#xA;I wonder, can you get a cold in space that needs antibiotics?&#xA;Specifically, after some time at the ISS and not carried from earth?&#xA;If so,do the rest of the crew has to avoid you?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&#xA;jdiez&lt;/p&gt;&#xA;" OwnerUserId="20224" LastActivityDate="2017-06-28T16:56:47.977" Title="Can you get a cold in space?" Tags="|iss|" AnswerCount="0" CommentCount="5" ClosedDate="2017-06-29T06:54:42.480" ContentLicense="CC BY-SA 3.0" />
  <row Id="22127" PostTypeId="2" ParentId="22065" CreationDate="2017-06-29T00:33:27.943" Score="2" Body="&lt;p&gt;At the risk of damaging my reputation here, I'm going to answer this question without citing sources. I'll instead buttress my argument with a sequence of logical inferences.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, pintle injectors (as are used in SpaceX's Falcon 9 launch vehicles) depend on the kinetics of liquid-phase propellants to achieve adequate atomization and mixing. Secondly, regenerative cooling in all orbital-class rocket engines (to my knowledge) is entirely in the liquid phase.  The only gaseous-phase fluid conduits that are visible in orbital rocket engines is the turbopump exhaust ducts (which are obviously much more voluminous than the cooling channels in the combustion chamber).  Also, the least volatile/most thermally stable constituent of bi-propellant rocket propellants is almost always selected as the regenerative coolant.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taken together, these clues strongly suggest that both propellants in bi-propellant rocket engines (full-flow staged combustion engines excluded) are in the liquid phase as they enter the injectors.  &lt;/p&gt;&#xA;" OwnerUserId="18585" LastEditorUserId="25" LastEditDate="2017-12-19T13:26:54.920" LastActivityDate="2017-12-19T13:26:54.920" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22129" PostTypeId="1" CreationDate="2017-06-29T08:58:29.207" Score="11" ViewCount="943" Body="&lt;p&gt;I am wondering what kind of ability NASA had during the Gemini or Apollo program era to control the vehicle from the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From reading some capcom transcripts and looking through some random procedures, the ground is always relaying commands to the astronauts by voice, and expecting the astronauts to respond by flipping switches, e.g. requesting &quot;O2 tank fan heater to OFF&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But say the astronauts were asleep, did the ground controllers have any kind of remote control of the vehicle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was telemetry always a passive reading of vehicle state?&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2017-06-29T10:02:33.397" LastActivityDate="2018-02-23T11:14:50.773" Title="Apollo era remote control of vehicle" Tags="|crewed-spaceflight|spacecraft|apollo-program|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22132" PostTypeId="2" ParentId="22125" CreationDate="2017-06-29T12:14:41.697" Score="6" Body="&lt;p&gt;The special property of sun-synchronous satellites is the precession, at pace equal to Earth's orbital period around the Sun (1 year). That means, the satellite's RAAN drifts by a full 360 degrees over the year, and as result, local solar time at the moment of crossing the equator remains constant.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;if the satellite is injected on any other day from the same launch site, the RAAN would differ. Still the desired local time is achieved.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No - the RAAN would not differ. The two RAAN values &lt;strong&gt;from moments of respective injections&lt;/strong&gt; would, but if you want the second satellite to achieve the same solar time as the first, your injection RAAN must be equal to RAAN of the first satellite &lt;strong&gt;at the moment of the second injection&lt;/strong&gt; - significantly different from what it was at its own injection time, having drifted to the new value with precession since.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pictured below are two sun-synchronous orbits (or thereabouts...) differing only by Right Ascension of Ascending Node values - by 90 degrees. This picture remains &quot;true&quot; - unchanged - regardless of time of day or time of year. There's the sunlit side and the night side, and one satellite passes the celestial equator at noon and midnight, the other - at sunrise and sunset (6AM, 6PM) - regardless of whichever country or ocean happens to be below at that moment, and regardless of date.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IF the date was 20th March, the day of Vernal Equinox, then RAAN of first satellite would be 0, second - 90 degrees. At any other day, the angle will differ - at autumnal equinox these values will be 180 and 270 degrees, respectively. That's because the sunlit side of Earth will be facing in exactly the opposite direction of the universe, being on the other side of the Sun. But the orbits will remain in their orientation relative to the direction of Sun and the terminator line. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dOCrA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dOCrA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Thtub.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Thtub.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/POTU3.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/POTU3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-06-30T14:25:57.457" LastActivityDate="2017-06-30T14:25:57.457" CommentCount="30" ContentLicense="CC BY-SA 3.0" />
  <row Id="22133" PostTypeId="2" ParentId="614" CreationDate="2017-06-29T13:11:28.290" Score="1" Body="&lt;p&gt;The real question is where would all the power come from to extract, refine and smelt these materials?  Obviously no organic fuel sources are available, it would all have to be solar.  Which would mean shipping thousands of tons of solar panels to Mars, until the colony developed its own solar panel manufacturing capability.&lt;/p&gt;&#xA;" OwnerUserId="20232" LastActivityDate="2017-06-29T13:11:28.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22134" PostTypeId="1" AcceptedAnswerId="22142" CreationDate="2017-06-29T16:34:31.450" Score="2" ViewCount="687" Body="&lt;p&gt;The Dnepr rocket was initially developed for use as a ballistic missile. Therefore it was developed with an explosive charge that fired the rocket out of the ground and into the air, followed by the main engine ignition right above ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oD7e5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oD7e5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;br /&gt;&#xA;&lt;sup&gt;Graphic showing the launch sequence&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did the Dnepr rocket still use the silo tube even after it became a commercial launcher? Wouldn't it be cheaper to stop using the ejection charges and just launch it from Baikonur?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="18085" LastEditDate="2017-07-01T19:55:53.943" LastActivityDate="2017-07-01T19:55:53.943" Title="Ejection charges on Dnepr Rocket tube?" Tags="|launch|rockets|propulsion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22135" PostTypeId="1" AcceptedAnswerId="22136" CreationDate="2017-06-29T17:02:50.923" Score="7" ViewCount="1943" Body="&lt;p&gt;SpaceX has been caught developing a thin flat robot to drive out on the ASDS deck and grab a landed stage. Some are calling it the SpaceX Roomba (Xoomba?) others are campaigning for the name Attitude Adjuster (Since that is a Ian M Banks ship name like OCISLY and JRTI). Octograbber may end up being closer to real, but I like Xoomba.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;People watching the barge from shore have caught lots of photos of it in testing on the OCISLY ASDS barge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zDsnJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zDsnJ.jpg&quot; alt=&quot;Xoomba on deck&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They even built a hangar for it on OCISLY with a garage door to protect it during landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AkJlH.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AkJlH.jpg&quot; alt=&quot;Xoomba to scale&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Musk had tweeted usage was a few months out.  When will we see the first use of the rocket grabber?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-06-30T01:52:48.987" LastActivityDate="2017-06-30T01:52:48.987" Title="When will the first use of the SpaceX rocket grabber be on an ASDS?" Tags="|spacex|falcon-9|landing|asds|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22136" PostTypeId="2" ParentId="22135" CreationDate="2017-06-29T17:05:06.260" Score="8" Body="&lt;p&gt;Seems like on the BulgariaSat-1 launch, the booster landed hard and used up the emergency crush core in the landing legs. This is the design case for the Xoomba, to provide extra support to a rocket that might need it. (Also to allow securing the stage, before crew get on board, perhaps in bad weather).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photos of OCISLY coming into Port Canaveral with the stage on deck clearly show the white Xoomba at the base. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FSHF7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FSHF7.jpg&quot; alt=&quot;Bottom white thing is the Xoomba&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P1R7S.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P1R7S.jpg&quot; alt=&quot;Xoomba holding a stage&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-06-29T17:05:06.260" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22138" PostTypeId="2" ParentId="22129" CreationDate="2017-06-29T18:18:28.097" Score="6" Body="&lt;p&gt;Backing up a bit to give design comments from &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Mercury#Pilot_accommodations&quot; rel=&quot;noreferrer&quot;&gt;Project Mercury&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Mercury astronauts had taken part in the development of their spacecraft, and insisted that manual control, and a window, be elements of its design. As a result, spacecraft movement and other functions could be controlled three ways: remotely from the ground when passing over a ground station, automatically guided by onboard instruments, or manually by the astronaut, who could replace or override the two other methods. Experience validated the astronauts' insistence on manual controls. Without them, Gordon Cooper's manual reentry during the last flight would not have been possible&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Furthermore, astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury_Seven#Selection_process&quot; rel=&quot;noreferrer&quot;&gt;group 1&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_2&quot; rel=&quot;noreferrer&quot;&gt;group 2&lt;/a&gt; both required test pilot experience, and &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_3&quot; rel=&quot;noreferrer&quot;&gt;group 3&lt;/a&gt; allowed military fighter jet experience instead of test pilot experience. &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Astronaut_Group_4&quot; rel=&quot;noreferrer&quot;&gt;Group 4&lt;/a&gt; brought in scientists and gave them pilot training if needed, so overall the astronaut pool for Gemini and Apollo was made up of people that were expected to be able to fly the craft, and considering the astronauts were leaning on the design of the craft to make it user-pilotable they expected to be doing the flying: &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Gemini#Equipment_module&quot; rel=&quot;noreferrer&quot;&gt;Project Gemini&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It became known as a &quot;pilot's spacecraft&quot; due to its assortment of jet fighter-like features, in no small part due to Gus Grissom's influence over the design...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Given the features included for manually piloting and the fact that the person flying the craft was always a pilot, I'd guess that the pilot doing all the flying was the method preferred by the people on the craft. I'd have sworn I'd read that somewhere too, but it's been a while and I'm not finding that reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the Apollo unmanned test flights demonstrate that everything necessary could be controlled from the ground:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_4&quot; rel=&quot;noreferrer&quot;&gt;Apollo 4&lt;/a&gt;: Launched unmanned on a Saturn V into a circular parking orbit, tested re-igniting the S-IVB to get to an elliptical orbit, separated the command module and used the service module engine to adjust the orbit twice. Landed 16 km from the target landing site.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_5&quot; rel=&quot;noreferrer&quot;&gt;Apollo 5&lt;/a&gt;: Launched the lunar module unmanned aboard a Saturn IB. The computer aborted the planned maneuver after 4 seconds due to a miscommunication about launch configuration, so &#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ground controllers moved to an alternate plan to fire the descent engine manually two more times. They then performed the &quot;fire in the hole&quot; test and another ascent engine burn.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_6&quot; rel=&quot;noreferrer&quot;&gt;Apollo 6&lt;/a&gt;: Launched aboard a Saturn V but had performance issues and ended up in an elliptical orbit instead of the planned circular one. The S-IVB failed to restart so they used the service module to raise the orbit, which left them short on fuel for finishing out the planned tests. Landed 80 km from the planned touchdown point.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Furthermore, &lt;a href=&quot;https://space.stackexchange.com/questions/9521/why-choose-to-crash-the-apollo-lunar-lander-ascent-stage-after-it-ascended/9522#9522&quot;&gt;this answer&lt;/a&gt; discusses remotely controlling the lunar module to crash it into the moon on most lunar missions after transferring the astronauts back to the command module.&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2017-06-29T18:18:28.097" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22140" PostTypeId="1" CreationDate="2017-06-30T00:22:34.997" Score="5" ViewCount="12497" Body="&lt;p&gt;I read that with our current ion propulsion technology it is possible we could send a craft moving the maximum of 100km per second, or around 62 miles.&#xA;However we seem to be limited by how strong of a power generator we are able to put aboard the craft.. Is it possible that with enough power we could accelerate a spacecraft beyond this 100km per second limit? What is the maximum speed we could send a craft moving in space at with our current ion engine technology?&lt;/p&gt;&#xA;" OwnerUserId="20235" LastActivityDate="2021-04-03T08:22:58.320" Title="What is the maximum speed an ion engine can propel a spacecraft at?" Tags="|spacecraft|propulsion|probe|power|" AnswerCount="2" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="22141" PostTypeId="2" ParentId="22140" CreationDate="2017-06-30T01:06:42.543" Score="20" Body="&lt;p&gt;The limit isn't due to power, but to engine lifetime and fuel limits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ion engines produce very little thrust, so in order to reach speeds of 100km/s they must accelerate continuously for months or years. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Dawn&lt;/a&gt; spacecraft, for example, was built with three redundant &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness&quot; rel=&quot;noreferrer&quot;&gt;ion thrusters&lt;/a&gt; to extend its lifetime, and got nowhere near 100km/s; it carried enough propellant to change its speed by about 10km/s, and took &lt;a href=&quot;https://www.space.com/8579-nasa-spacecraft-breaks-speed-boost-record.html&quot; rel=&quot;noreferrer&quot;&gt;15000 hours (~1.7 years) of continuous thrust to change its speed by 4.3km/s&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To achieve a &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;noreferrer&quot;&gt;∆v&lt;/a&gt; of 100km/s with an ion engine like those on Dawn (with a specific impulse of 3100 s (exhaust velocity of 30400 m/s)) we turn to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;noreferrer&quot;&gt;Tsiolkovsky rocket equation&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v = v_\text{e} \ln \frac {m_0} {m_f}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $v_\text{e}$ is the rocket exhaust velocity and ${m_0}$ and ${m_f}$ are the mass of the craft full of fuel and empty respectively.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$100000 = 30400 \ln \frac {m_0} {m_f} \Rightarrow 3.29 = \ln \frac {m_0} {m_f} \Rightarrow  \frac  {m_0} {m_f} = e^{3.29} = 26.8$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So an ion-thruster spacecraft intending to reach 100km/s must carry 26 times its dry mass in xenon propellant. This is an extremely high mass ratio, not generally achievable in a single-stage spacecraft. A two- or three-stage spacecraft could do it with reasonable mass ratios.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ion engines are already very power-hungry, and need to be paired with a lot of mass in solar panels or &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;RTGs&lt;/a&gt;. More powerful thrusters could increase the exhaust velocity by a small amount, driving down the mass ratio, but at the cost of greatly increasing the dry mass with the larger power supply. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that I'm referring to a &lt;em&gt;change in speed&lt;/em&gt; or ∆v throughout this answer, rather than a maximum speed. This is because the speed of objects in space must be described relative to a particular reference point, and speed limits don't work the way they do for Earth-bound vehicles. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-17T12:35:59.753" LastActivityDate="2017-08-17T12:35:59.753" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22142" PostTypeId="2" ParentId="22134" CreationDate="2017-06-30T04:01:33.273" Score="7" Body="&lt;p&gt;In order to launch it from Baikonur you need to build launch facility. Launch pad, even for simple rockets is expensive thing. It is much simpler and cheaper to use existing infrastructure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also - almost certainly, Dnepr will not be launched more, because Kosmotras company was Russian-Ukrainian joint, and now there are too much tensions between Russia and Ukraine &lt;/p&gt;&#xA;" OwnerUserId="11046" LastActivityDate="2017-06-30T04:01:33.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22143" PostTypeId="2" ParentId="22096" CreationDate="2017-06-30T13:25:43.323" Score="6" Body="&lt;p&gt;On official spacex website I found dimensions:&#xA;&lt;a href=&quot;http://www.spacex.com/news/2015/08/31/grid-fins&quot; rel=&quot;noreferrer&quot;&gt;http://www.spacex.com/news/2015/08/31/grid-fins&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Falcon 9’s first stage is equipped with hypersonic grid fins which manipulate the direction of the stage’s lift during reentry. The fins are placed in an X-wing configuration and are stowed on ascent and deployed during reentry. While the fins are relatively small – they measure just 4 feet by 5 feet – they can roll, pitch, and yaw the 14-story stage up to 20 degrees in order to target a precision landing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="18561" LastEditorUserId="6773" LastEditDate="2017-06-30T13:29:42.880" LastActivityDate="2017-06-30T13:29:42.880" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22144" PostTypeId="1" AcceptedAnswerId="22145" CreationDate="2017-06-30T15:36:44.043" Score="9" ViewCount="19145" Body="&lt;p&gt;What is the difference between true anomaly and mean anomaly in orbital mechanics?&#xA;I want to understand the physical meaning of mean anomaly, for example true anomaly is the angular position of a satellite/object when viewed from earth/focus but what is the corresponding mean anomaly for that true anomaly. I don't want formulas, but the physical explanation.&lt;/p&gt;&#xA;" OwnerUserId="20244" LastEditorUserId="20244" LastEditDate="2017-07-02T02:39:37.570" LastActivityDate="2020-06-14T23:56:17.033" Title="Difference between true anomaly and mean anomaly" Tags="|orbital-mechanics|artificial-satellite|orbit|spacecraft|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22145" PostTypeId="2" ParentId="22144" CreationDate="2017-06-30T16:54:16.610" Score="9" Body="&lt;p&gt;True anomaly is the actual measured angle in the orbital plane between the vector extending from the focus to the point of periapsis and the vector extending from the focus to the object's actual position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mean anomaly is the angle between the periapsis point and the imagined position of an object for the same elapsed time since periapsis for a circular orbit around the same body with the same orbital period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key difference is that the mean anomaly always increases linearly with time.  The true anomaly, in general, does not, except if the orbit is circular, in which case the mean anomaly and true anomaly are identical (well, not really, because circular orbits don't have a periapsis, but in the limiting case, this is true).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two are related through the &lt;a href=&quot;https://en.wikipedia.org/wiki/Eccentric_anomaly&quot; rel=&quot;noreferrer&quot;&gt;eccentric anomaly&lt;/a&gt;: let &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; be the mean anomaly, &lt;span class=&quot;math-container&quot;&gt;$E$&lt;/span&gt; be the eccentric anomaly, &lt;span class=&quot;math-container&quot;&gt;$e$&lt;/span&gt; be the eccentricity, and &lt;span class=&quot;math-container&quot;&gt;$\nu$&lt;/span&gt; be the true anomaly.  Then,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$M = E - e \sin E$$&lt;/span&gt;&#xA;and&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\cos \nu = \frac{\cos E - e}{1 - e \cos E}$$&lt;/span&gt;&#xA;or perhaps more clearly (thanks to David Hammen)&#xA;&lt;span class=&quot;math-container&quot;&gt;$$\tan\frac{\nu}{2}=\sqrt{\frac{1+e}{1-e}}\tan\frac{E}{2}$$&lt;/span&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2018-11-08T16:26:28.697" LastActivityDate="2018-11-08T16:26:28.697" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22146" PostTypeId="1" AcceptedAnswerId="22249" CreationDate="2017-06-30T18:02:10.280" Score="7" ViewCount="1547" Body="&lt;p&gt;In a question about the &lt;a href=&quot;https://space.stackexchange.com/questions/21819/how-did-soyuz-ms-04-reach-the-iss-in-only-6-hours&quot;&gt;6-hour Soyuz / ISS transfer&lt;/a&gt; the concept of a High Energy Transfer orbit is discussed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've heard of Low Energy Transfer Orbits before, which are commonly used on extended missions to save as much &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;noreferrer&quot;&gt;delta-V&lt;/a&gt; as possible.  One of the most common examples is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;noreferrer&quot;&gt;Hohmann-transfer orbit&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding is that a High Energy Transfer Orbit would be a path where time is the biggest concern over delta-V economy.  Is this a fair assessment?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are some examples of High Energy Transfer Orbits that have been used in missions?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastActivityDate="2017-07-11T09:46:39.103" Title="What is a High Energy Transfer Orbit and what are some examples of their usage?" Tags="|delta-v|transfer-time|" AnswerCount="2" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22147" PostTypeId="2" ParentId="22129" CreationDate="2017-06-30T20:11:16.637" Score="2" Body="&lt;p&gt;As you can read here &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Apollo_program&lt;/a&gt; there were six unmanned missions before the manned missions. The remote control ability was necessary to do enough unmanned test flights before the risk of a manned flight could be rated.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-06-30T20:11:16.637" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22148" PostTypeId="1" AcceptedAnswerId="22149" CreationDate="2017-07-01T01:16:40.093" Score="7" ViewCount="6405" Body="&lt;p&gt;A question ive been thinking about:&#xA;If an engine works of a liquid propellant(Hydrogen in this case) and LOX, is there a specific proportion required for the two chemicals  to burn completely?  If so what is it?  Otherwise, explain how it may be different, say, if it depends on the specific engine.&#xA;Thanks&lt;/p&gt;&#xA;" OwnerUserId="19105" LastEditorUserId="195" LastEditDate="2017-07-01T03:23:48.600" LastActivityDate="2017-07-01T08:03:16.067" Title="Liquid fuel / Oxygen proportions" Tags="|propulsion|engines|lox|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22149" PostTypeId="2" ParentId="22148" CreationDate="2017-07-01T03:04:15.040" Score="13" Body="&lt;p&gt;There is indeed a ratio of fuel to oxidizer that yields complete combustion, called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stoichiometry&quot; rel=&quot;noreferrer&quot;&gt;stoichiometric ratio&lt;/a&gt;. This produces the most thermal energy from combustion, but for a few different reasons, specific engines run at different ratios, most often on the fuel-rich side. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As it happens, &lt;a href=&quot;https://space.stackexchange.com/a/22123/195&quot;&gt;a very similar question came up a few days ago which explained some of this&lt;/a&gt;; a fuel-rich combustion can produce better efficiency in spite of the lower thermal energy release. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You wouldn't want to run oxidizer-rich, because the oxidizer that doesn't combust with fuel will react with the metal of the combustion chamber and nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since RP-1 kerosene is a blend of hydrocarbons, the stoichiometric ratio is difficult to calculate; the &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_rocket_propellant#Bipropellants&quot; rel=&quot;noreferrer&quot;&gt;optimum performance ratio is about 2.58-2.77:1&lt;/a&gt; (ratios are normally given in form of oxidizer-mass-to-fuel-mass).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For hydrogen engines, the stoichiometric ratio is 8:1 -- oxygen atoms are 16 times as massive as hydrogen atoms, and we're trying to combine two hydrogens with one oxygen to make H&lt;sub&gt;2&lt;/sub&gt;O. The optimum ratio for engine performance, however, is 4.13-4.83:1, with nearly half the hydrogen uncombusted, but because hydrogen's density is so low, going with a higher oxidizer ratio yields savings in fuel tank volume, so the overall best performance for a hydrolox rocket is somewhere in the 5.5-6.0:1 range.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-07-01T08:03:16.067" LastActivityDate="2017-07-01T08:03:16.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22150" PostTypeId="1" AcceptedAnswerId="22170" CreationDate="2017-07-01T09:05:16.290" Score="6" ViewCount="1682" Body="&lt;p&gt;I'm using the software &lt;a href=&quot;https://en.wikipedia.org/wiki/General_Mission_Analysis_Tool&quot; rel=&quot;nofollow noreferrer&quot;&gt;GMAT&lt;/a&gt; (General Mission Analysis Tool) (&lt;a href=&quot;http://gmatcentral.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;website&lt;/a&gt;, &lt;a href=&quot;http://www.jhuapl.edu/techdigest/views/pdfs/V15_N4_1994/V15_N4_1994_Vetter.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube&lt;/a&gt;) for propagating spacecraft and calculating their &lt;a href=&quot;http://gmat.sourceforge.net/doc/R2014a/html/SpacecraftOrbitState.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Brouwer-Lyddane (BL) mean semi-major axis (SMA)&lt;/a&gt;. I found out something interesting that I don't understand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I propagate a spacecraft, with the initial orbital element:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;BL mean: [a,e,i,RAAN,AOP,MA] = [7000 km,0,45 deg,0,0,0]&lt;/li&gt;&#xA;&lt;li&gt;osculating elements: [a,e,i,RAAN,AOP,MA] = [7004.718 km , 0.000898, 45.019 deg, 0, 0,&#xA;0]&lt;/li&gt;&#xA;&lt;li&gt;and the epoch: 01 Jan 2000 12:00:00.000&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Where the aforementioned osculating elements and the BL mean elements correspond to each other, by using the Brouwer-Lyddane Short transformation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The propagator uses a Runge-Kutta 8th order integrator, with a force model of the Earth’s gravitational field &lt;a href=&quot;http://www.jhuapl.edu/techdigest/views/pdfs/V15_N4_1994/V15_N4_1994_Vetter.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;JGM-2&lt;/a&gt; with zonal and tesseral harmonics up to degree 21 and order 21. I don't use drag, sun radiation pressure, or third body gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The produced BL mean SMA is:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/XLsBI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XLsBI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where the y axis is the BL SMA in [km], and the x axis is the ElapsedDays&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I propagate the same spacecraft with the same initial orbital elements, same propagator, but with a different epoch: 01 Jan 2000 00:00:00.000, I get the following BL SMA:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/fItgQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fItgQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice that while the first epoch produces a significant bias value of approximately 130 meters, the second epoch does not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does any one has any explanation for this bias? Why is it epoch-depended? Why is it constant in time?&lt;/p&gt;&#xA;" OwnerUserId="20246" LastEditorUserId="33864" LastEditDate="2020-11-18T15:20:33.073" LastActivityDate="2020-11-18T15:20:33.073" Title="Brouwer-Lyddane mean semi major axis bias" Tags="|orbital-mechanics|orbital-elements|gmat|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22151" PostTypeId="1" AcceptedAnswerId="22157" CreationDate="2017-07-01T17:23:48.690" Score="7" ViewCount="2736" Body="&lt;p&gt;The really nicely written question &lt;a href=&quot;https://space.stackexchange.com/q/22150/12102&quot;&gt;Brouwer-Lyddane mean semi major axis bias&lt;/a&gt; describes the use of the software &lt;a href=&quot;https://en.wikipedia.org/wiki/General_Mission_Analysis_Tool&quot; rel=&quot;noreferrer&quot;&gt;GMAT&lt;/a&gt; (General Mission Analysis Tool) (&lt;a href=&quot;http://gmatcentral.org/&quot; rel=&quot;noreferrer&quot;&gt;website&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/channel/UCt-REODJNr2mB3t-xH6kbjg/videos&quot; rel=&quot;noreferrer&quot;&gt;YouTube&lt;/a&gt;, &lt;a href=&quot;https://indico.esa.int/indico/event/111/session/2/material/0/0.pdf&quot; rel=&quot;noreferrer&quot;&gt;giant PDF for TESS&lt;/a&gt;) for propagating the orbit of a spacecraft in Earth's &lt;a href=&quot;http://www.jhuapl.edu/techdigest/views/pdfs/V15_N4_1994/V15_N4_1994_Vetter.pdf&quot; rel=&quot;noreferrer&quot;&gt;realistic, lumpy gravity field&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question discussed the calculation of the a parameter called the &lt;a href=&quot;http://gmat.sourceforge.net/doc/R2014a/html/SpacecraftOrbitState.html&quot; rel=&quot;noreferrer&quot;&gt;Brouwer-Lyddane (BL) mean semi-major axis (SMA)&lt;/a&gt;. I've linked to the documentation there, because I'm not really sure where to find a better link because I don't really know what it means.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I'm really interested! For a simple, non-relativistic orbit around a single, spherically symmetric gravitational potential with no other perturbations, the Keplerian orbital elements would be well defined and stable. If the body was oblate, there would be precession, but there would be a well defined period that could be defined as the interval between successive apoapses or equatorial plane crossings, or if it were freakishly in a perfectly circular and zero inclination, some &quot;inertial&quot; reference point could be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But once &quot;rotating, lumpy gravity&quot; is turned on and the orbit never quite repeats itself and would actually be slightly unpredictable with anything less than a full numerical integration. In fact I think it would meet the mathematical definition of a chaotic system at that point. The period and semi-major axis would be approximate, but one could not describe the orbit with any fixed values. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; So what is this thing called the  Brouwer-Lyddane (BL) mean semi-major axis (SMA), how is it defined, how is it used, and what can be learned from it? Are there other kinds or definitions of mean semi-major axes as well?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-01T19:03:38.430" LastActivityDate="2017-07-02T19:15:35.013" Title="What's a Brouwer-Lyddane mean semi major axis, or any other, for an orbit in a lumpy gravity field?" Tags="|orbital-mechanics|orbital-elements|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22152" PostTypeId="1" CreationDate="2017-07-01T17:26:01.987" Score="11" ViewCount="3814" Body="&lt;p&gt;How does the SpaceX Merlin engine control the mass flow rate to throttle its thrust?&lt;br&gt;&#xA;Do the pintle injectors move?&lt;br&gt;&#xA;Does the mass flow rate to the gas generator change? &lt;/p&gt;&#xA;" OwnerUserId="20225" LastEditorUserId="18085" LastEditDate="2017-07-02T19:23:15.017" LastActivityDate="2017-07-03T14:35:02.707" Title="Merlin 1D Engine Throttling" Tags="|spacex|engine-design|engines|thrust|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22153" PostTypeId="2" ParentId="21776" CreationDate="2017-07-01T17:26:54.567" Score="7" Body="&lt;p&gt;Sometimes they can do their job a little too well, or blow out some stuff when they're not supposed to, but I've never heard of one failing. They are normally used in a redundant pair fired close to simultaneously where either one will do the trick. The electronics firing them are also redundant. If your pyro fails to do its job, I'll be willing to bet 20:1 odds it was because either a) there was no power, or b) the software or controller made a boo boo and didn't try to fire it.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-07-01T17:26:54.567" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22154" PostTypeId="1" AcceptedAnswerId="22156" CreationDate="2017-07-01T18:14:01.150" Score="5" ViewCount="1131" Body="&lt;p&gt;The Washington Post's &lt;a href=&quot;https://www.washingtonpost.com/news/speaking-of-science/wp/2017/06/30/trump-relaunches-the-national-space-council/?utm_term=.9a8342d0a6a7&quot; rel=&quot;noreferrer&quot;&gt;President Trump relaunches the National Space Council&lt;/a&gt;, and Quartz's &lt;a href=&quot;https://qz.com/1019410/donald-trumps-new-mike-pence-led-national-space-council-offers-a-baffling-signal-to-the-space-community/&quot; rel=&quot;noreferrer&quot;&gt;Donald Trump’s new National Space Council is already baffling the space industry&lt;/a&gt; present the news that the US president has &quot;re-activated&quot; the US National Space Council. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is/will be the function of the United States National Space Council? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will it have an advisory capacity on NASA's science missions for example, or commercial satellite constellations, bandwidth allocations, space junk clean-up, etc?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For that matter, what &lt;em&gt;was&lt;/em&gt; it's function when it was active?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; US former astronaut Buzz Aldrin and Secretary of Commerce Wilbur Ross listen to the announcement. The NASA Administrator, and the Director of Office of Science and Technology Policy were not present because the positions have yet to be filled. From &lt;a href=&quot;https://www.washingtonpost.com/news/speaking-of-science/wp/2017/06/30/trump-relaunches-the-national-space-council/?utm_term=.9a8342d0a6a7&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.  (Brendan Smialowski/AFP/Getty Images)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UDNtf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UDNtf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; See also the next day's WP's Speaking of Science: &lt;a href=&quot;https://www.washingtonpost.com/news/speaking-of-science/wp/2017/07/01/it-has-to-be-something-but-it-could-be-infinity-trump-ponders-space-in-strange-ceremony/?utm_term=.c2af016e1ebd&quot; rel=&quot;noreferrer&quot;&gt;‘It has to be something, but it could be infinity’: Trump ponders space in strange ceremony&lt;/a&gt; for additional details/reporting.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-01T19:30:58.823" LastActivityDate="2017-07-05T23:27:52.207" Title="What is/will be the function of the United States National Space Council?" Tags="|policy|" AnswerCount="1" CommentCount="7" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22155" PostTypeId="2" ParentId="22151" CreationDate="2017-07-01T18:27:40.697" Score="10" Body="&lt;p&gt;The &lt;a href=&quot;http://help.agi.com/stk/index.htm#stk/vehSat_meanElements.htm&quot; rel=&quot;noreferrer&quot;&gt;Brouwer-Lyddane Transformation&lt;/a&gt; is based on two articles:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;Solution of the Problem of Artificial Satellite Theory Without Drag,&quot; D. Brouwer, The Astronomical Journal, Nov. 1959, pp.378-396&lt;/li&gt;&#xA;&lt;li&gt;&quot;Small Eccentricities or Inclinations in the Brouwer Theory of the Artificial Satellite,&quot; R. H. Lyddane, The Astronomical Journal, Oct. 1963, pp.555-558&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;There are several commonly used versions of this transformation. All version have the same purpose: to average the osculating orbital elements for receiving mean orbital elements by excluding the zonal harmonic oscillations. The main advantage of these methods over simply using a moving average is that they are analytical and do not require data from several orbital periods. More precisely, they can even perform on one osculating element measurement and produce the corresponding mean elements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually, orbital elements are described using three components: secular (non-periodic), short-period (oscillations with a equal or smaller period than the orbital period, and long-period (oscillations with a period much larger than orbital period).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most software distinct between Brouwer-Lyddane Short Transformation (BLST) and the Brouwer-Lyddane Long Transformation (BLLT). The BLST includes averaging the osculating elements using the short-period terms, while the BLLT uses both the short- and long-period terms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The programs STK, FreeFlyer, and GMAT include BLST and BLLT that include the $J_2$, $J_2^2$, $J_3$, $J_4$, and $J_5$ zonal harmonics.&#xA;Notice that it is necessary to use $J_2^2$ when using higher orders, because it is in the same order of magnitude as $J_3$, $J_4$, and $J_5$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simple formulation of the BLST that includes only the $J_2$ zonal harmonic can be found in the book &lt;a href=&quot;http://www.fisica.net/mecanicaclassica/Analytical%20Mechanics%20of%20Aerospace%20Systems.pdf&quot; rel=&quot;noreferrer&quot;&gt;Analytical Mechanics of Aerospace Systems&lt;/a&gt; in appendix G, pp. 573 - 576.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Couple of points:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;While using mean elements, it is necessary to know which zonal harmonics are considered. If the propagator uses a lower set of zonal harmonics than the BL transformation, the BL transformation tries to average suppressed elements and consequently introduces errors.&lt;/li&gt;&#xA;&lt;li&gt;The first Brouwer formulation from 1959 included singularities at zero eccentricity and zero inclination. Lyddane solved that problem at 1963.&lt;/li&gt;&#xA;&lt;li&gt;The current formulation has a singularity near critical inclinations (63.4 [deg], and 116.6 [deg]). The book &lt;a href=&quot;http://rads.stackoverflow.com/amzn/click/0750685336&quot; rel=&quot;noreferrer&quot;&gt;Spacecraft Formation Flying: Dynamics, Control and Navigation&lt;/a&gt; offers a solution that requires replacing all the singular expressions: $ (1 - 5\cos^2{i})^{-1} $ with some small value, such as $ \varepsilon = 0.05$&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="20246" LastActivityDate="2017-07-01T18:27:40.697" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22156" PostTypeId="2" ParentId="22154" CreationDate="2017-07-01T18:30:04.477" Score="4" Body="&lt;p&gt;We can look at the Executive Order that established it in the first place for the definitive answer. Section 2 has been &lt;strong&gt;highlighted&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;By the authority vested in me as President by the Constitution and&#xA;  laws of the United States of America, and in order to provide a&#xA;  coordinated process for developing a national space policy and&#xA;  strategy and for monitoring its implementation, it is hereby ordered&#xA;  as follows:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Section 1. Establishment and Composition of the National Space&#xA;  Council.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(a) There is established the National Space Council (&quot;the Council&quot;).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(b) The Council shall be composed of the following members:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(1) The Vice President, who shall be Chairman of the Council;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(2) The Secretary of State;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(3) The Secretary of the Treasury;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(4) The Secretary of Defense;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(5) The Secretary of Commerce;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(6) The Secretary of Transportation;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(7) The Director of the Office of Management and Budget;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(8) The Chief of Staff to the President;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(9) The Assistant to the President for National Security Affairs;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(10) The Assistant to the President for Science and Technology;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(11) The Director of Central Intelligence; and&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(12) The Administrator of the National Aeronautics and Space&#xA;  Administration.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(c) The Chairman shall, from time to time, invite the following to&#xA;  participate in meetings of the Council:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(1) The Chairman of the Joint Chiefs of Staff; and&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(2) The heads of other executive departments and agencies and other&#xA;  senior officials in the Executive Office of the President.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While there are several other activities described later on, perhaps the core function of the council is described in Section 2 (highlighted here in bold):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Sec. 2. Functions of the Council. (a) &lt;strong&gt;The Council shall advise and&#xA;  assist the President on national space policy and strategy, and&#xA;  perform such other duties as the President may from time to time&#xA;  prescribe.&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(b) In addition, the Council is directed to:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(1) &lt;strong&gt;review United States Government space policy, including long-range&#xA;  goals, and develop a strategy for national space activities;&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(2) &lt;strong&gt;develop recommendations for the President on space policy and&#xA;  space- related issues;&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(3) &lt;strong&gt;monitor and coordinate implementation of the objectives of the&#xA;  President's national space policy by executive departments and&#xA;  agencies; and&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(4) &lt;strong&gt;foster close coordination, cooperation, and technology and&#xA;  information exchange among the civil, national security, and&#xA;  commercial space sectors, and facilitate resolution of differences&#xA;  concerning major space and space-related policy issues.&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(c) The creation and operation of the Council shall not interfere with&#xA;  existing lines of authority and responsibilities in the departments&#xA;  and agencies.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Sec. 3. Responsibilities of the Chairman. (a) The Chairman shall serve&#xA;  as the President's principal advisor on national space policy and&#xA;  strategy.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(b) The Chairman shall, in consultation with the members of the&#xA;  Council, establish procedures for the Council and establish the agenda&#xA;  for Council activities.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(c) The Chairman shall report to the President on the activities and&#xA;  recommendations of the Council. The Chairman shall advise the Council&#xA;  as appropriate regarding the President's directions with respect to&#xA;  the Council's activities and national space policy generally.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(d) The Chairman shall authorize the establishment of such committees&#xA;  of the Co uncil, including an executive committee, and of such working&#xA;  groups, composed o f senior designees of the Council members and of&#xA;  other officials invited to par ticipate in Council meetings, as he&#xA;  deems necessary or appropriate for the effi cient conduct of Council&#xA;  functions.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Sec. 4. National Space Policy Planning Process. (a) The Council will&#xA;  establish a process for developing and monitoring the implementation&#xA;  of national space policy and strategy.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(b) To implement this process, each agency represented on the Council&#xA;  shall provide such information regarding its current and planned space&#xA;  activities as the Chairman shall request.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(c) The head of each executive department and agency shall ensure that&#xA;  its space-related activities conform to national space policy and&#xA;  strategy.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Sec. 5. Establishment of Vice President's Space Policy Advisory Board.&#xA;  (a) The Vice President shall establish, in accordance with the&#xA;  provisions of the Federal Advisory Committee Act, as amended (5 U.S.C.&#xA;  App. 2), governing Presidential advisory committees, an advisory&#xA;  committee of private citizens to advise the Vice President on the&#xA;  space policy of the United States (&quot;the Board&quot;).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(b) The Board shall be composed and function as follows:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(1) The Board shall be composed of members appointed by the Vice&#xA;  President.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(2) The Vice President shall designate a Chairman from among the&#xA;  members of the Board. The Executive Secretary of the National Space&#xA;  Council shall serve as the Secretary to the Board.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(3) Members of the Board shall serve without any compensation for&#xA;  their work on the Board. However, they shall be entitled to travel&#xA;  expenses, including per diem in lieu of subsistence, as authorized by&#xA;  law, for persons serving intermittently in the Government service (5&#xA;  U.S.C. 5701-5707), to the extent funds are available for that purpose.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(4) Necessary expenses of the Board shall be paid from funds available&#xA;  for the expenses of the National Space Council.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(5) Notwithstanding the provisions of any other Executive order, the&#xA;  responsibilities of the President under the Federal Advisory Committee&#xA;  Act, as amended, except that of reporting annually to the Congress,&#xA;  which are applicable to the Board established by this order, shall be&#xA;  performed on a reimbursable basis by the Director of the Office of&#xA;  Administration in the Executive Office of the President, in accordance&#xA;  with the guidelines and procedures established by the Administrator of&#xA;  General Services.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Sec. 6. Microgravity Research Board. Section 1(c) of Executive Order&#xA;  No. 12660 is amended by deleting &quot;Economic Policy Council&quot; and&#xA;  inserting in lieu thereof &quot;National Space Council.&quot; Sec. 7.&#xA;  Administrative Provisions. (a) The Office of Administration in the&#xA;  Executive Office of the President shall provide the Council with such&#xA;  administrative support on a reimbursable basis as may be necessary for&#xA;  the performance of the functions of the Council.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(b) The President shall appoint an Executive Secretary who shall&#xA;  appoint such staff as may be necessary to assist in the performance of&#xA;  the Council's functions.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(c) All Federal departments, agencies, and interagency councils and&#xA;  committees having an impact on space policy shall extend, as&#xA;  appropriate, such cooperation and assistance to the Council as is&#xA;  necessary to carry out its responsibilities under this order.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;(d) The head of each agency serving on the Council or represented on&#xA;  any working group or committee of the Council shall provide such&#xA;  administrative support as may be necessary, in accordance with law and&#xA;  subject to the availability of appropriations, to enable the agency&#xA;  head or its representative to carry out his responsibilities.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Sec. 8. Report. The Council shall submit an annual report setting&#xA;  forth its assessment of and recommendations for the space policy and&#xA;  strategy of the United States Government.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;GEORGE BUSH THE WHITE HOUSE, April 20, 1989.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.presidency.ucsb.edu/ws/?pid=60450&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="12102" LastEditDate="2017-07-02T15:58:53.527" LastActivityDate="2017-07-02T15:58:53.527" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22157" PostTypeId="2" ParentId="22151" CreationDate="2017-07-01T19:10:47.567" Score="6" Body="&lt;h2&gt;The Brouwer-Lyddane Transformation (BLT)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Secular and Periodic Changes&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perturbations on the osculating elements result in &lt;em&gt;secular&lt;/em&gt; and &lt;em&gt;periodic&lt;/em&gt; variations. The following figure and three paragraphs explain this: (from Fundamentals of Astrodynamics and Applications, 1st Edition, D. Vallado, pp. 545)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/OfQFd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OfQFd.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&#xA;&quot;$c$ represents a general orbital element. The straight line shows secular effects. The large oscillating line shows the secular plus long-periodic effects, and the small oscillatory line, which combines all three, shows the short-periodic effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Secular&lt;/strong&gt; change in a particular element vary linearly over time, or in some cases, proportionally to some power of time, such as a quadratic. The important point is that secular terms grow with time, and errors in secular terms produce unbounded error growth. Secular terms are the primary contributor to the degradation of analytical theories over long time intervals.&#xA;&lt;strong&gt;Short-periodic&lt;/strong&gt; effects typically repeat on the order of the satellite's period or less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Long-periodic&lt;/strong&gt; effects have cycles considerably longer than one orbital period—typically one or two orders of magnitude longer. These long-periodic effects are often seen in the motion of the node and perigee and can last from a few weeks to a month or more.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Applications&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Sometimes, the periodic changes are very significant and consequently it is difficult to quantify the secular components. Because the BLT eliminates most of the oscillations, the secular components are easier to spot.&lt;/li&gt;&#xA;&lt;li&gt;The following example shows one use of the mean elements. Assume a  tangential thrust is applied in a circular orbit for increasing the SMA. According to the Gauss Variational Equations (GVEs), the SMA derivative is: $$ \dot{a} = \frac{2}{n}f_t = \frac{2a^3}{\mu}f_t$$&#xA;where $\mu$ is the gravitational parameter, and $f_t$ is the tangential acceleration. The problem with this equation is that $a$ changes all the time during the flight, so which $a$ should we choose? The solution is to rewrite this equation using mean elements, which are denoted using $\bar{x}$.&#xA;$$ \dot{\bar{a}} = \frac{2\bar{a}^3}{\mu}f_t$$&#xA;Now, a single $\bar{a}$ value can be used, and as a result, this equation makes sense.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="20246" LastEditorUserId="20246" LastEditDate="2017-07-02T19:15:35.013" LastActivityDate="2017-07-02T19:15:35.013" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22158" PostTypeId="2" ParentId="22152" CreationDate="2017-07-01T21:02:07.103" Score="14" Body="&lt;p&gt;SpaceX is not exactly forthcoming with detailed information about the Merlin's workings but we can figure some things out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tom Mueller, designer of the Merlin, gave a brief description of the engine in this &lt;a href=&quot;https://www.youtube.com/watch?v=vVQyZn-VtXU&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt;.  He drew a sketch of the engine schematic which I have screen-captured for your use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s5xVy.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s5xVy.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only valves shown on this sketch were described by Mueller as (the LOX valve) &quot;the valve that we use to turn it on and off&quot; and (the fuel valve) as &quot;the little valve&quot;.  Not much help there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, &lt;a href=&quot;http://spaceflight101.com/wp-content/uploads/2015/09/SpaceX-Concept1-1024x315.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;spaceflight101.com&lt;/a&gt; has this image of the conceptual Merlin 2 engine.  You will notice that it is remarkably similar to the Mueller sketch except with some more details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ie64V.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ie64V.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this, we can infer that the valves Mueller drew on his sketch were the Main Oxidizer Valve and the Main Fuel Valve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This more detailed schematic shows gas generator control valves which would certainly be necessary to throttle the engine - increasing or decreasing the turbopump power to pump more or less propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The remaining question is about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pintle_injector&quot; rel=&quot;nofollow noreferrer&quot;&gt;pintle injector&lt;/a&gt;, does it move?  This &lt;a href=&quot;https://pintleinjector.blogspot.com/2016/12/pintle-injector.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;writeup&lt;/a&gt; states that it moves, but only in the sense that it is spring loaded closed, and increasing pressure causes it to open more.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Merlin 1D is famous pintle engine from SpaceX. This pintle engine uses&#xA;  face shut off type pintle injector. Pintle is spring loaded and&#xA;  tightly pressed againist the pintle post. High pressure from LOX side&#xA;  pushes the pintle to open.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Putting all this together, I would infer that the Merlin throttles by controlling the propellant flow to the gas generator.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="12102" LastEditDate="2017-07-03T14:35:02.707" LastActivityDate="2017-07-03T14:35:02.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22159" PostTypeId="2" ParentId="17738" CreationDate="2017-07-02T04:04:35.480" Score="5" Body="&lt;p&gt;ITAR regulations restrict the defense-related information you can share with non-U.S. persons. There is no distinction between the rules applicable to a government agency like NASA or private contractors like SpaceX, Lockheed Martin etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Abiding by ITAR regulations is crucial for businesses in order to maintain a continual trust with their government customers. There are entire departments within private companies devoted to enforcing adherence to these regulations. A company has to carefully evaluate which of its physical components are restricted for public display. For example, looking at the outer casing of the F9 first stage may not provide any pertinent information that is not publicly available elsewhere (i.e. spacex.com). However, there are some components which may be removed prior to placing the first stage on public display. This can range from electronics to sensitive proprietary components on the engines. U.S. regulations will typically target anything that shows adversaries (and even allies) the full extent of our space launch capabilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For in-depth information about ITAR regulations, I'll refer you to the &lt;a href=&quot;https://www.pmddtc.state.gov/regulations_laws/itar.html&quot; rel=&quot;noreferrer&quot;&gt;US State Department regulation depository&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="20250" LastEditorUserId="6816" LastEditDate="2017-07-02T04:17:28.113" LastActivityDate="2017-07-02T04:17:28.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22160" PostTypeId="2" ParentId="21038" CreationDate="2017-07-02T04:29:07.930" Score="8" Body="&lt;p&gt;Yes, and it has been used in the past (See: &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1962-001A&quot; rel=&quot;noreferrer&quot;&gt;NASA - balsawood&lt;/a&gt;). It was not a significant portion of the spacecraft but it has occurred. You can, in theory, use any material to build a space vehicle, however, mechanical constraints will arise. A mechanical designer would have to evaluate the cost-benefit of utilizing wood and determine the risks of using a non-traditional material. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space is a brutal environment with extreme temperatures, significant radiation exposure, and vacuum environment. All of these are factors analyzed in FEA simulations by engineers prior to making such a design decision. Since you mentioned 'spacecraft' I'm assuming you're referring to some kind of orbiter, satellite etc. Aside from the space environments, there are significant stress loads (i.e. vibrations, g-forces) that are experienced by the vehicle during launch. Lastly, there may be cases where manufacturing wooden parts prove to be more difficult than more widely used metals such as aluminum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like you mentioned, outgassing can be in issue in porous materials because a vacuum environment can cause the expansion of gas pockets. This can lead to material failure where the structural integrity of entire spacecraft is destroyed. As far as evolved materials are concerned, I'd find it difficult for it to naturally occur because the space environment is not immediately present during the evolution process of plants and mammals due to our atmosphere and gravitational pull. &lt;/p&gt;&#xA;" OwnerUserId="20250" LastActivityDate="2017-07-02T04:29:07.930" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22161" PostTypeId="1" CreationDate="2017-07-02T07:26:23.833" Score="6" ViewCount="218" Body="&lt;p&gt;The NASA News item &lt;a href=&quot;https://www.nasa.gov/feature/nasa-s-first-asteroid-deflection-mission-enters-next-design-phase&quot; rel=&quot;noreferrer&quot;&gt;NASA’S First Asteroid Deflection Mission Enters Next Design Phase&lt;/a&gt; describes a test the kinetic impactor technique that could someday be used to adjust the orbit of an asteroid in order to avoid a collision with Earth in the future by making a small change at a time much earlier than the predicted event.&lt;/p&gt;&#xA;&lt;p&gt;The article says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The target for DART is an asteroid that will have a distant approach to Earth in October 2022, and then again in 2024. The asteroid is called Didymos -- Greek for “twin” -- because it’s an asteroid binary system that consists of two bodies: Didymos A, about one-half mile (780 meters) in size, and a smaller asteroid orbiting it called Didymos B, about 530 feet (160 meters) in size. DART would impact only the smaller of the two bodies, Didymos B.&lt;/p&gt;&#xA;&lt;p&gt;The Didymos system has been closely studied since 2003. The primary body is a rocky S-type object, with composition similar to that of many asteroids. The composition of its small companion, Didymos B, is unknown, but the size is typical of asteroids that could potentially create regional effects should they impact Earth.&lt;/p&gt;&#xA;&lt;p&gt;[...]“A binary asteroid is the perfect natural laboratory for this test,” said Tom Statler, program scientist for DART at NASA Headquarters. “The fact that Didymos B is in orbit around Didymos A makes it easier to see the results of the impact, and ensures that the experiment doesn’t change the orbit of the pair around the sun.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Can someone explain the physics of this explanation to me more completely? Is it a small change in the orbital period of the pair that will be detected? Wouldn't this be predictable from just conservation of linear and angular momentum and Energy? Radar would only work during close Earth approach, so can the rotation be phased by optical means? What if the impact changes the &lt;em&gt;rotation&lt;/em&gt; rate of the primary about its own axis and brings it out of tidal lock. Wouldn't this distort the brightness curve unpredictably?&lt;/p&gt;&#xA;&lt;p&gt;Or is is the details of how the impact process itself proceeds, how efficiently momentum is transferred. In this case wouldn't the unknown composition of the B component make this more difficult to interpret? Will the spacecraft release an &amp;quot;observer&amp;quot; spacecraft to record the collision from a safe distance, say a few hours behind the impactor?&lt;/p&gt;&#xA;&lt;p&gt;I don't understand what the observables are here, and what can be learned from them.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YYEJO.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YYEJO.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Delay-doppler data from Arecebo in 2003, found &lt;a href=&quot;http://www.infobservador.com/2015/10/desviando-un-asteroide-didymos-en-2022/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. The small line above the primary is caused by the relative orbital motion of the secondary.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-07-12T03:10:35.367" LastActivityDate="2021-07-12T03:10:35.367" Title="What can be learned from NASA's choice of Didymos B for the Double Asteroid Redirection Test" Tags="|near-earth-asteroid|delay-doppler|doppler-ranging|dart|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22162" PostTypeId="1" CreationDate="2017-07-02T15:48:18.603" Score="-4" ViewCount="1208" Body="&lt;p&gt;Take a well documented case like the Saturn V. The Saturn V had about 3 million parts. What is the total number of part failures at all stages of testing? If this is in any way unclear, describe how it is unclear. This is a simple quantitative question. &lt;/p&gt;&#xA;" OwnerUserId="20252" LastActivityDate="2017-07-03T01:25:16.387" Title="What is the failure rate of rocket components?" Tags="|rockets|" AnswerCount="3" CommentCount="3" ClosedDate="2017-07-03T13:34:23.900" ContentLicense="CC BY-SA 3.0" />
  <row Id="22163" PostTypeId="2" ParentId="22162" CreationDate="2017-07-02T16:14:23.343" Score="-4" Body="&lt;p&gt;There is no documentation of the kind you are seeking. Almost nothing of the Apollo program documentation survives, if it existed in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.theorsociety.com/Pages/ORWiki/Reliability_1.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;This&lt;/a&gt; source states about 1 failure per hour in an operational fighter jet. So extrapolate that to a rocket and you would likely see one or two failures per launch without testing, which would likely be fatal given how sensitive rockets are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;source=web&amp;amp;rct=j&amp;amp;url=https://www.tuv.com/media/usa/aboutus_1/pressreleases/press_release_pdf/PTL_Magazine_Article.pdf&amp;amp;ved=0ahUKEwi1jK7J_urUAhUmjlQKHYi6CT8QFggeMAA&amp;amp;usg=AFQjCNGMVavkk1FN35pJezE-xqthq2Vqxg&quot; rel=&quot;nofollow noreferrer&quot;&gt;This&lt;/a&gt; source gives extremely high failure rates for PV modules. Failures exceed 10% for many components and 1% of components fail even the basic out of the box tests. So in a rocket with millions of parts that would mean thousands of failures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer is somewhere in between these.&lt;/p&gt;&#xA;" OwnerUserId="20253" LastActivityDate="2017-07-02T16:14:23.343" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22164" PostTypeId="2" ParentId="22162" CreationDate="2017-07-02T17:58:37.980" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19720005767&amp;amp;hterms=saturn%20failure%20rate&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3Dsaturn%2520failure%2520rate%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA&quot; rel=&quot;nofollow noreferrer&quot;&gt;This document&lt;/a&gt; contains failure rate information for several classes of components used in the Apollo program. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's lots of information to be found on the &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?N=0&amp;amp;Ntk=All&amp;amp;Ntt=%22reliability%20analysis%22%20Saturn&amp;amp;Ntx=mode%20matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;NTRS&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19660023504&amp;amp;hterms=reliability%20analysis%20Saturn&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3D%2522reliability%2520analysis%2522%2520Saturn%26Ntx%3Dmode%2520matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;This document&lt;/a&gt; has a reliability analysis of some of the Saturn stages (i.e. a cumulative failure rate for the entire stage). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A report that lists all individual failures in all tests for the Saturn program may not exist, or may not be accessible on the internet. A gigantic pile of tests were done on individual components, assemblies, subsystems and systems, across 500,000 subcontractors. The important data was the aggregate ('this subsystem has an n% failure rate') and the aggregate would be communicated up the chain, leading to reports like the third link, which lists a probability of mission success for the S-IVB stage of 96.3% assuming one engine-out capability.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-07-02T17:58:37.980" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22165" PostTypeId="1" CreationDate="2017-07-02T19:26:08.157" Score="9" ViewCount="9917" Body="&lt;p&gt;I wonder how much a cordless space screwdriver costs and what its mass would be? I guess it would be a lithium-ion screwdriver, like on Earth, but would it have other specifications?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anybody know who manufactures screwdrivers for space?&lt;/p&gt;&#xA;" OwnerUserId="20251" LastEditorUserId="7062" LastEditDate="2017-07-03T03:06:05.090" LastActivityDate="2022-03-19T17:50:55.777" Title="How much does a cordless screwdriver for space cost?" Tags="|iss|space-shuttle|cost-to-orbit|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22167" PostTypeId="2" ParentId="22165" CreationDate="2017-07-02T22:39:46.773" Score="16" Body="&lt;p&gt;The EVA &amp;quot;power screwdriver&amp;quot; is called the Pistol Grip Tool.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Pistol-Grip Tool The main tool used by spacewalkers is this 21st&#xA;century hand-drill, built by Swales Aerospace Inc. Designed for use in&#xA;the thick-gloved hands of spacewalkers, it features a pistol-style&#xA;handle and large information screen. Astronauts can program the speed&#xA;and torque, and the settings show up on the screen. The torque can&#xA;range from less than 1 to 38 foot-pounds of force (1.36 - 49 N⋅m), and the drill can&#xA;run at anywhere between 5 and 60 rotations per minute. According to&#xA;NASA, this is the first hand-held electronic power tool to include all&#xA;the features of a cabinet-mounted tool, courtesy of the configurable&#xA;design.&lt;/p&gt;&#xA;&lt;p&gt;A rechargeable battery slots into the handle like the clip of a&#xA;handgun. Its metal hydride batteries can hold more charge at extreme&#xA;temperatures—perfect for the cycles of shadow and sunlight the station&#xA;experiences in orbit. The body is made of a durable, glass-infused&#xA;plastic called Lexan. But you won’t see it; the whole thing is covered&#xA;with aluminum tape for durability. NASA began developing the&#xA;requirements for the three-pound tool in 1993 to make repairing the&#xA;Hubble Space Telescope easier. It was first used in space in 1997.&#xA;Engineers are not eager to design another one—the pistol-grip tool is&#xA;modular so improvements can be added later.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.airspacemag.com/space/tools-of-the-astronaut-trade-15273242/&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p3wYR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p3wYR.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;According to the &lt;a href=&quot;https://archive.org/details/eva-tools-and-equipment-reference-book-nov-83-2&quot; rel=&quot;nofollow noreferrer&quot;&gt;EVA Tool Catalog&lt;/a&gt; it weighs 2.09 lb without battery (page 270 of the pdf) (This catalog is pretty old, and lists several other power tools.  The current incarnation of the tool may be somewhat different. You might have fun browsing the pages near page 270 to read about other power tools.)&lt;/p&gt;&#xA;&lt;p&gt;It would be difficult to find out what it cost.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7Ci9L.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7Ci9L.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The source of &lt;a href=&quot;http://toolmonger.com/2008/04/10/tool-pr0n-tools-in-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this image&lt;/a&gt; says the tool was made by Alliant Techsystems. The associated article says &amp;quot;If you find a price, let us know.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;The book &lt;a href=&quot;https://books.google.com/books?id=QczRqXWSWwMC&amp;amp;pg=PA122&amp;amp;lpg=PA122&amp;amp;dq=space%20shuttle%20pistol%20grip%20tool%20cost&amp;amp;source=bl&amp;amp;ots=TpqSOpZ1rr&amp;amp;sig=uXPIuNTXDviijAQTJwXz5il49qw&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwiEsLqz1-vUAhXDxYMKHePSCQAQ6AEIbDAN#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wings in Orbit&lt;/a&gt; describes the difficulty of adapting the Shuttle PGT to ISS use.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Pistol Grip Tool Some of the biggest problems with tools came from&#xA;attempting to expand their use beyond the original purpose. Sometimes&#xA;new uses were very similar to the original use, but the details were&#xA;different—like trying to use a hacksaw to perform surgery. The saw is&#xA;designed for cutting, but the precision required is extremely&#xA;different. An example is the computerized Pistol Grip Tool, which was&#xA;developed to actuate bolts while providing fairly precise torque&#xA;information. This battery-operated tool was similar to a powered&#xA;screwdriver, but had some sophisticated features to allow flexibility&#xA;in applying and measuring different levels of torque or angular&#xA;rotation. The tool was designed for Hubble, and the accuracy was more&#xA;than adequate for Hubble. When ISS required a similar tool, the&#xA;program chose to purchase several units of the Hubble power tool&#xA;rather than design a new tool specific to ISS requirements. The&#xA;standards for certification and documentation were different for&#xA;Hubble. ISS had to reanalyze bolts, provide for additional ground and&#xA;on-orbit processing of the Pistol Grip Tool to meet ISS accuracy&#xA;needs, and provide additional units on orbit to meet fault tolerance&#xA;requirements and maintain calibration.&lt;/p&gt;&#xA;&lt;p&gt;The use of the Pistol Grip Tool&#xA;for ISS assembly also uncovered another shortcoming with regard to&#xA;using a tool developed for a different spacecraft. The Pistol Grip&#xA;Tool was advertised as having an accuracy of 10% around the selected&#xA;torque setting. This accuracy was verified by setting the Pistol Grip&#xA;Tool in a fixed test stand on the ground where it was held rigidly in&#xA;place. This was a valid characterization when used on Hubble where EVA&#xA;worksites were designed to be easily accessible and where the Pistol&#xA;Grip Tool was used directly on the bolts. It was relatively easy for&#xA;crew members to center the tool and hold it steady on any bolt. ISS&#xA;worksites were not as elegant as Hubble worksites, however, since ISS&#xA;is such a large vehicle and the Pistol Grip Tool  often had to be used&#xA;with socket extensions and other attachments that had inaccuracies of&#xA;their own. Crew members often had to hold the tool off to the side&#xA;with several attachments, and the resulting side forces could cause&#xA;the torque measured by the tool to be very different than the torque&#xA;actually applied. Unfortunately, ISS bolts were designed and analyzed&#xA;to the advertised torque accuracy for Hubble and they didn’t account&#xA;for this “man-in-the-loop” effect. The result was a long test program&#xA;to characterize the accuracy of the Pistol Grip Tool when used in&#xA;representative ISS worksites, followed by analysis of the ISS bolts to&#xA;this new accuracy.&lt;/p&gt;&#xA;&lt;p&gt;To focus only on tool problems, however, is a&#xA;disservice. It’s like winning the Super Bowl and only talking about&#xA;the fumbles. While use of the Pistol Grip Tool caused some problems as&#xA;NASA learned about its properties, it was still the most sophisticated&#xA;tool ever designed for EVA. It provided a way to deliver a variety of&#xA;torque settings and accurately measure the torque delivered. Without&#xA;this tool, the assembly and maintenance of the ISS would not have been&#xA;possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2022-03-19T17:50:55.777" LastActivityDate="2022-03-19T17:50:55.777" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22168" PostTypeId="2" ParentId="22162" CreationDate="2017-07-03T01:25:16.387" Score="0" Body="&lt;p&gt;Very, very high. Lets do some math.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://reliabilityanalyticstoolkit.appspot.com/mechanical_reliability_data&quot; rel=&quot;nofollow noreferrer&quot;&gt;Failure rates of common parts&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.quora.com/What-are-the-major-causes-of-rocket-failures&quot; rel=&quot;nofollow noreferrer&quot;&gt;Causes of rocket failure&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;USA&#xA;Atlas V in 2007, due to a fuel leak from a faulty valve.&lt;/p&gt;&#xA;&lt;p&gt;Delta III in 2000, due to a guidance issue. Two previous Delta III launches in the late 1990s failed due to a software issue and an upper stage issue respectively.&lt;/p&gt;&#xA;&lt;p&gt;Delta IV in 2004, due to premature engine cutoff.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Etc...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Nevertheless, the above list of launch failures has a few trends. Some common problems are&lt;/p&gt;&#xA;&lt;p&gt;Third/upper/vacuum stage problems. The vacuum stage is difficult to test on Earth, often runs on cryogenic fuel, is the last thing to go wrong, and has to operate in a bizarre environment. Some organizations have jealously guarded expertise in this area, others obviously do not.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As you can see, the software and fuel pumps are key sources of error. The riveting, structure, other mechanical parts etc are rarely involved. We also see from the first link that fuel seals and valves have relatively high failure rates in the range of 10 per million hours.&lt;/p&gt;&#xA;&lt;p&gt;Software is not technically a part, so we will disregard that. But the fueling components are definitely important and have a high failure rate. Assuming at least 10% of the million parts in a rocket are fueling related, that's 100,000 parts that fail at a high rate. So at a 0.99999 reliability per part, and using Lussers Law relationships, that's a 36% reliability per hour.&lt;/p&gt;&#xA;&lt;p&gt;[Rocket launch processnasps://www.nasa.gov/mission_pages/shuttle/launch/countd own101.html&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;T-27 hours and counting&#xA;Begin operations to load cryogenic reactants into the orbiter's fuel cell storage tanks&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;However, it takes at least 24 hours to fuel a rocket. So those parts are under stress potentially the entire time, resulting in a 2e-11 reliability over that period with dozens of failures. This is why you need testing.&lt;/p&gt;&#xA;&lt;p&gt;In conclusion, a good testing program is needed to avoid having a minuscule probability of ever taking off. The failure risk is concentrated in the fuel valves and gaskets, which are weak components put under prolonged load.&lt;/p&gt;&#xA;" OwnerUserId="20257" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-07-03T01:25:16.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22170" PostTypeId="2" ParentId="22150" CreationDate="2017-07-03T08:31:27.180" Score="4" Body="&lt;p&gt;I'll expand on my &lt;a href=&quot;https://space.stackexchange.com/questions/22150/#comment62506_22150&quot;&gt;comment&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I'm not familliar with the software, but if you start the satellite at the &lt;em&gt;same everything&lt;/em&gt; except time (12 hours difference) then the satellite will be starting in a different gravitational potentials for the two cases. I am guessing that if you ran it 100 times spacing your starting Epochs evenly over 24 hours, you'd see their &quot;altitude biases&quot; would be spread above and below, depending on if you started in a gravity peak or valley. There should be no expectation that the mean should be equal to the instantaneous starting value, you're starting randomly.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;To keep it simple, I'll use only the $J_2$ multipole component of the Earth's gravitational potential in addition to the monopole term $GM_E$. I've written a short python script using numbers and equations from Wikipedia (links shown within the script).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've constructed a 90 degree inclination orbit, at an altitude of about 901 kilometers and propagated for 14 orbits or about 1 day. (I chose this altitude so I could set the inclination to at 99 degrees and confirm it was &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit#Technical_details&quot; rel=&quot;nofollow noreferrer&quot;&gt;sun-synchronous&lt;/a&gt; by propagating for 91 days and watching the ascending node move from the x-axis to the y-axis, thereby checking that the gradient of the $J_2$ potential term is OK.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I computed a semi-major axis and orbital speed $a_0$ and $v_0$ for a circular orbit in the monopole field $GM_E/r$ and used those for starting conditions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first plot shows $x(t), y(t), z(t)$ and $r(t)-a_0$ for 24 hours (14 orbits) starting on the equator with &lt;code&gt;phase = 0&lt;/code&gt;. The calculation was repeated for &lt;code&gt;phase = 30, 60, 90&lt;/code&gt; degrees, such that the last one started above the pole, also at the same altitude of 901 kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the second plot, I show the four plots of $r(t)-a_0$ which start at the same altitude, but different locations over the Earth. Right away you can see that the &quot;bias&quot; you speak of depends on the starting location and the particular gravitational potential that exists there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starting over the equator with &lt;code&gt;phase = 0&lt;/code&gt; degrees, where one is sitting lower in the Earth's gravity well, the speed is a little low for a circular orbit, and so the satellite is actually at apoapsis, dropping to a slightly lower altitude a half-period later. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, starting over the pole with &lt;code&gt;phase = 90&lt;/code&gt; degrees, higher in Earth's gravity well, the speed is a little high for circular orbit, and so this is now periapsis, and the altitude rises a half-period later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;summary:&lt;/strong&gt; By choosing an easier to understand gravitational field, the behavior of the &quot;bias&quot; is more systematic here. In your question, you're using a very complex, bumpy-looking gravity field which is constantly rotating. &lt;strong&gt;By starting at the same place in space, but different epochs, you were actually rotating the earth, moving different gravitational potentials to the starting location of your satellite&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;I now predict&lt;/strong&gt; that if you turn off the multipole field in JGM-2 and use a spherically symmetric potential, your calculations will show a flat altitude, no variations and no biases. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/h4cle.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h4cle.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sE287.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sE287.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;closing thoughts:&lt;/strong&gt; Realistic orbits are not perfect conics, and so they and their Keplerian elements &lt;em&gt;do not represent realistic orbits&lt;/em&gt;. They are only approximations to reality, and so are not right even though they are close. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keplerian elements were used when people were writing with feathers using light from burning animal fat (if they were not busy being burned at the stake themselves). They are a mixed blessing in the 21st century when everything has so many more digits.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(X, t):&#xA;&#xA;    rr, vel = X.reshape(2, -1)&#xA;&#xA;    acc0, acc2 = accs(rr)&#xA;&#xA;    return np.hstack([vel, acc0 + acc2])&#xA;&#xA;&#xA;def accs(rr):&#xA;&#xA;    x,   y,   z   = rr&#xA;    xsq, ysq, zsq = rr**2&#xA;&#xA;    rsq = (rr**2).sum()&#xA;    rm3 = rsq**-1.5&#xA;    rm7 = rsq**-3.5&#xA;&#xA;    acc0  = -GM_earth * rr * rm3&#xA;&#xA;    # https://en.wikipedia.org/wiki/Geopotential_model#The_deviations_of_Earth.27s_gravitational_field_from_that_of_a_homogeneous_sphere&#xA;    acc2x = x * rm7 * (6*zsq - 1.5*(xsq + ysq))&#xA;    acc2y = y * rm7 * (6*zsq - 1.5*(xsq + ysq))&#xA;    acc2z = z * rm7 * (3*zsq - 4.5*(xsq + ysq))&#xA;&#xA;    acc2  = J2_earth * np.hstack((acc2x, acc2y, acc2z))&#xA;&#xA;    return acc0, acc2&#xA;&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;halfpi, pi, twopi = [f*np.pi for f in [0.5, 1, 2]]&#xA;degs, rads        = 180./pi, pi/180.&#xA;&#xA;GM_earth = 3.986004418E+14 #  m^3/s^2  https://en.wikipedia.org/wiki/Standard_gravitational_parameter&#xA;J2_earth = 1.7555E+25      #  m^5/s^2  https://en.wikipedia.org/wiki/Geopotential_model&#xA;&#xA;a0  = (901 + 6378) * 1E+03 # meters (roughly) for a n=14 sun-synchronous orbit&#xA;&#xA;v0  = np.sqrt(GM_earth/a0)  # m/s  (vis-viva)&#xA;T   = twopi * a0 / v0       # sec&#xA;&#xA;print T/60, ' minutes'&#xA;&#xA;phases = [rads*d for d in [0, 30, 60, 90]]&#xA;&#xA;X0s = []&#xA;for phase in phases:&#xA;    sp, cp = np.sin(phase), np.cos(phase)&#xA;&#xA;    X0 = np.hstack(( [ a0*cp, 0, a0*sp],   # initial positions&#xA;                     [-v0*sp, 0, v0*cp] )) # initial velocitys&#xA;    X0s.append(X0)&#xA;&#xA;&#xA;n, days = 14, 1.  # orbits/day, days&#xA;&#xA;time = np.linspace(0, n*days*T, int(n*days*100)+1)  &#xA;&#xA;answers = []&#xA;for X0 in X0s:&#xA;&#xA;    answer, info = ODEint(deriv, X0, time, full_output=True)&#xA;&#xA;    answers.append(answer)&#xA;&#xA;if 1 == 1:&#xA;&#xA;    names  = 'x(t)', 'y(t)', 'z(t)', 'r(t)-a0'&#xA;    answer = answers[1]   # just choose one for the plot&#xA;    posn   = answer.T[:3]&#xA;    rdiff  = np.sqrt((posn**2).sum(axis=0)) - a0&#xA;    things = [thing for thing in posn] + [rdiff]&#xA;&#xA;    plt.figure()&#xA;    for i, (thing, name) in enumerate(zip(things, names)):&#xA;        plt.subplot(4, 1, i+1)&#xA;        plt.plot(time/(24.*3600), thing)   # x-axis in days, not seconds&#xA;        plt.title(name, fontsize=16)&#xA;        plt.xlim(0, days+0.01)  # a little more than n days&#xA;    plt.show()&#xA;&#xA;if 1 == 1:&#xA;&#xA;    plt.figure()&#xA;&#xA;    for answer in answers:&#xA;&#xA;        posn   = answer.T[:3]&#xA;        rdif   = np.sqrt((posn**2).sum(axis=0)) - a0&#xA;&#xA;        plt.plot(time/(24.*3600), rdif)   # x-axis in days, not seconds&#xA;&#xA;    plt.xlim(0, days+0.1)  # a little more than n days&#xA;    plt.text(1.01, -6000, u' 0\u00B0', fontsize=15 )&#xA;    plt.text(1.01,  2000, u'30\u00B0', fontsize=15 )&#xA;    plt.text(1.01, 16000, u'60\u00B0', fontsize=15 )&#xA;    plt.text(1.01, 22000, u'90\u00B0', fontsize=15 )&#xA;    plt.title('r(t)-a0', fontsize=16)&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; a quick check running for 91 days with an inclination of 99 degrees to confirm the orbit is roughly sun-synchronous in order to make sure there are at least no gross errors with the way I typed the equations for the gradient of the $J_2$ component, or the numerical values. The script above does not allow for inclination, but you'd include it by mixing the $v_y$ and $v_z$ values when &lt;code&gt;X0&lt;/code&gt; is calculated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/orDp0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/orDp0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-03T09:38:16.030" LastActivityDate="2017-07-03T09:38:16.030" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22172" PostTypeId="1" AcceptedAnswerId="22179" CreationDate="2017-07-03T20:51:45.253" Score="5" ViewCount="2837" Body="&lt;p&gt;I'm writing a little project that simulates an orbit by converting from initial state vectors &lt;span class=&quot;math-container&quot;&gt;$\vec{r}$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\vec{v}$&lt;/span&gt; to Keplerian elements, then converting back to &lt;span class=&quot;math-container&quot;&gt;$\vec{r}$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$\vec{v}$&lt;/span&gt; from those Keplerian elements, but stepped one frame ahead. I'm having some trouble with that last part. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been using &lt;a href=&quot;https://downloads.rene-schwarz.com/download/M001-Keplerian_Orbit_Elements_to_Cartesian_State_Vectors.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;René Schwarz's conversion guide&lt;/a&gt; quite a lot, but the transformations and rotations at the end honestly just confuse me, so I ended up just using the True Anomaly and &lt;span class=&quot;math-container&quot;&gt;$|\vec{r}|$&lt;/span&gt; to calculate the new &lt;span class=&quot;math-container&quot;&gt;$\vec{r}$&lt;/span&gt;, which does seem to work, but I'm stumped on how to calculate &lt;span class=&quot;math-container&quot;&gt;$\vec{v}$&lt;/span&gt;, I have the equation for the velocity at any point on an orbit with&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$v^2 = \mu \left(\frac{2}{|\vec{r}|}-\frac{1}{a}\right)$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where I just plug in the new &lt;span class=&quot;math-container&quot;&gt;$|\vec{r}|$&lt;/span&gt; and I get &lt;span class=&quot;math-container&quot;&gt;$|\vec{v}|$&lt;/span&gt;, but trying to use this equation for the tangent vector to any point on an ellipse given the angle,&#xA; &lt;span class=&quot;math-container&quot;&gt;$$-a \cos\theta i + b \cos\theta j$$&lt;/span&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt; is the semi major axis, &lt;span class=&quot;math-container&quot;&gt;$b$&lt;/span&gt; is the semi minor axis, and &lt;span class=&quot;math-container&quot;&gt;$\theta$&lt;/span&gt; is the angle around the ellipse (I've been using the True Anomaly as input for that angle, is that correct?) doesn't seem to work to find the vector tangent to the ellipse. So my questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;How do I find the vector tangent to a 2D orbital ellipse given &lt;span class=&quot;math-container&quot;&gt;$e$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$a$&lt;/span&gt;, as well as &lt;span class=&quot;math-container&quot;&gt;$|\vec{r}|$&lt;/span&gt;, &lt;span class=&quot;math-container&quot;&gt;$|\vec{v}|$&lt;/span&gt;, and the True, Eccentric, and Mean anomaly at that point? (I also have some additional elements that I haven't used yet such as the argument of periapsis if that's useful.)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How do I determine which direction that vector should face (so it doesn't point toward retrograde when it should point towards prograde) given the same values as above?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="20260" LastEditorUserId="12102" LastEditDate="2018-12-15T10:39:36.567" LastActivityDate="2018-12-15T10:39:36.567" Title="Calculating velocity state vector with orbital elements in 2D" Tags="|orbital-mechanics|orbit|orbital-elements|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="22173" PostTypeId="1" CreationDate="2017-07-03T23:47:25.133" Score="0" ViewCount="126" Body="&lt;p&gt;What would the best way be to crash an impactor into Mars with greatest topological effect? And how deep a crater could it excavate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars missions to date have been designed to approach the planet at low speed to land or enter orbit. How would a mission to hit it at maximum speed and mass be designed? Would a Krafft-Ehricke style gravity assist turnaround about Jupiter be the best way? What should the proportions of fuel to impact mass be in the launched payload? Does it matter whether the payload consists out of solid tungsten or a fusion bomb? Assume a single of today's launchers or a Saturn V at most.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The motivation could be to then land science probes in that fresh crater of the underworld of Mars, but that's not the issue here. Just how it could be done if it were assumed to be wished for as the next mission to Mars today.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-07-04T06:35:07.150" Title="How deep a crater could an impactor spacecraft cause on Mars?" Tags="|orbital-mechanics|mars|mission-design|impact|crater|" AnswerCount="2" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22174" PostTypeId="1" CreationDate="2017-07-04T02:24:40.477" Score="5" ViewCount="795" Body="&lt;p&gt;What is the reason for the square shape of the cells on grid fins such as those on the Falcon 9 landed by SpaceX, on missiles, or other places?&lt;/p&gt;&#xA;" OwnerUserId="20265" LastEditorUserId="20265" LastEditDate="2017-07-04T04:08:40.703" LastActivityDate="2017-07-05T04:41:48.777" Title="Why are the cells on grid fins square in shape?" Tags="|spacex|falcon-9|aerodynamics|grid-fins|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22175" PostTypeId="1" CreationDate="2017-07-04T03:53:28.453" Score="12" ViewCount="3088" Body="&lt;p&gt;SpaceX (among others) does a series of final automated checks 10 seconds before launch, and aborts the launch if any of them fail.  Yesterday and today, one of them did (each time), causing the launch to be scrubbed for the day.  (Today, it almost had to be scrubbed due to weather, but they were able to have the attempt at the end of the launch window).  At the end of the webcast, they were sure to thank range support, the FAA, and all the others who worked to support the launch, which probably includes a lot of SpaceX engineers/managers as well as civilian and military authorities outside the company.  Presumably, there is some significant cost to doing all this and then having an abort within the last 10 seconds.  What is that cost? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems clear that the cost of a failed launch is much higher, but it'd be interesting to know the (approximate) number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Answers should ideally be sourced, but do not have to be specific to SpaceX. &lt;/p&gt;&#xA;" OwnerUserId="11181" LastEditorUserId="6944" LastEditDate="2017-07-04T13:11:43.970" LastActivityDate="2017-07-04T13:17:32.830" Title="How much does a launch scrub cost?" Tags="|cost|abort|range-safety|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22176" PostTypeId="2" ParentId="22173" CreationDate="2017-07-04T04:31:12.867" Score="1" Body="&lt;p&gt;It comes down to how much spacecraft you want to throw at it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Newton tells us that a high speed impactor will be basically stopped by displacing it's own mass.  (Although if what it hits is hard enough it may stop sooner.)  Also, an object which is very long and thin may punch a hole but not have the energy to clear the rubble away adequately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bigger your spacecraft, the deeper the hole.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-07-04T04:31:12.867" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22177" PostTypeId="2" ParentId="21171" CreationDate="2017-07-04T04:39:21.323" Score="9" Body="&lt;p&gt;Rockets usually fail early in flight, within a minute of liftoff, or in the vacuum stage since that is hard to simulate on Earth. You can see a list of rocket failures &lt;a href=&quot;http://www.astronautix.com/t/thewrongstuhiclefailures.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. The structure is one of the most reliable rocket parts, the failures are mainly related to fuel pumping and bad design decisions. &lt;/p&gt;&#xA;" OwnerUserId="20267" LastActivityDate="2017-07-04T04:39:21.323" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22178" PostTypeId="2" ParentId="22173" CreationDate="2017-07-04T06:35:07.150" Score="0" Body="&lt;p&gt;The largest possible rocket is something like a Delta V. So a roughly 10 ton payload going at Mach 10 or so would have 50 gj of energy. That's 5% of a WW2 A bomb. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the crater would be less than 100x100 feet or so and it would not penetrate into deep layers of the crust. For that you would be better off sending multiple spacecraft to gradually drill deep over time. This would be a major logistical feat because drill heads would constantly melt or wear out at extreme depth.&lt;/p&gt;&#xA;" OwnerUserId="20269" LastActivityDate="2017-07-04T06:35:07.150" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22179" PostTypeId="2" ParentId="22172" CreationDate="2017-07-04T08:57:01.740" Score="3" Body="&lt;p&gt;Assuming that the direction of the periapsis is the positive direction of the x-axis, then the vector $-a\sin E\cdot \mathbb{i} + b\cos E\cdot \mathbb{j}$ is tangent to the orbit at the point with the eccentric anomaly $E$. The velocity is codirectional with this vector if the orbit goes counter-clockwise, and contradirectional if the orbit goes clockwise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the angle between the positive direction of the x-axis and the direction of the periapsis is $\omega$ (measured counter-clockwise), then you need to rotate the vector by this angle, and the result is $(-a\sin E\cos \omega - b\cos E\sin \omega)\mathbb{i} + (b\cos E\cos\omega - a\sin E\sin\omega)\mathbb{j}$.&lt;/p&gt;&#xA;" OwnerUserId="12448" LastActivityDate="2017-07-04T08:57:01.740" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22180" PostTypeId="2" ParentId="22172" CreationDate="2017-07-04T10:46:51.250" Score="1" Body="&lt;p&gt;To calculate the velocity vector in a two-body problem, you can use the flight path angle $\phi_{fpa}$ (from Fundamental of Astrodynamics and Applications, D. Vallado). $\phi_{fpa}$ is the angle measured from the local horizon to the velocity vector.&#xA;For this end, calculate the angular momentum, which is constant:&#xA;$$ \vec{h} = \vec{r}_0\times \vec{v}_0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The position vector norm $||r||$ is:&#xA;$$||r|| = \frac{a\sqrt{1-e^2}}{1+e\cos{f}}$$&#xA;where $f$ is the true anomaly. As you previously showed, the velocity norm as a function of $r$ is&#xA;$$v^2 = \mu \left(\frac{2}{||r||}-\frac{1}{a} \right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, $\phi_{fpa}$ is calculated is as such:&#xA;$$ \cos{\phi_{fpa}} = \frac{||h||}{||r||||v||} $$&#xA;$$ \sin{\phi_{fpa}} = \frac{||h||}{||r||||v||}\times\frac{e\sin{f}}{1 + e\cos{f}} $$&#xA;$$ \phi_{fpa} = \text{atan2}(\sin{\phi_{fpa}}, \cos{\phi_{fpa}}) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before I show you the velocity vector expression, we need to define the following direction vectors:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mdt92.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mdt92.jpg&quot; alt=&quot;Directions&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $\hat{r}$ is the direction of $\vec{r}$, $\hat{t}$ is in the velocity direction, and $\hat{n}$ is normal to the orbit and to $\hat{t}$. Furthermore, the directions $\hat{r}$ and $\hat{s}$ are perpendicular.&#xA;Thus, the velocity vector $\vec{v}$ can be expressed as:&#xA;$$\vec{v} = ||v||\hat{t} = ||v||(\hat{s}\cos{\phi_{fpa}} + \hat{r}\sin{\phi_{fpa}})$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The calculation of $\vec{r}$ and $\hat{s}$ as a function of some inertial direction is an easy task, using the true anomaly angle $f$.&lt;/p&gt;&#xA;" OwnerUserId="20246" LastEditorUserId="20246" LastEditDate="2017-07-05T12:47:13.393" LastActivityDate="2017-07-05T12:47:13.393" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22181" PostTypeId="2" ParentId="22175" CreationDate="2017-07-04T13:11:20.073" Score="12" Body="&lt;p&gt;It will be difficult to find information about SpaceX costs, but Space Shuttle launch scrub costs have been published.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA estimates every launch cancelled after fuel tanking has begun can&#xA;  cost as much as 1.2 million dollars. Endeavour endured five liftoff&#xA;  scrubs before successfully launching Wednesday at 6:03 p.m. (2203&#xA;  GMT), though some of these cancellations occurred before ground crews&#xA;  started loading propellant into the shuttle's external tank. The total&#xA;  price tag for this mission's postponements, which began in mid-June&#xA;  and ended with yesterday's liftoff, was less than $5 million, NASA&#xA;  spokesman Allard Beutel said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;........&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA estimates that each scrub costs 500,000 dollars in lost fuel, and&#xA;  700,000 dollars to pay for the extra workforce needed for launch attempts.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(The mission referenced is STS-127)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.space.com/6991-5-shuttle-launch-scrubs-cost-millions.html&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-07-04T13:17:32.830" LastActivityDate="2017-07-04T13:17:32.830" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22182" PostTypeId="2" ParentId="22174" CreationDate="2017-07-04T15:32:57.113" Score="6" Body="&lt;p&gt;There are 3 regular geometric patterns you can use: triangle, rectangle and hexagon. All others have irregularities (e.g. very small cells next to larger cells) or space lost (circles). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the 2004 study, '&lt;a href=&quot;http://www.icas.org/ICAS_ARCHIVE/ICAS2004/PAPERS/092.PDF&quot; rel=&quot;noreferrer&quot;&gt;Grid pattern effects on aerodynamic characteristics of grid fins&lt;/a&gt;': &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The results of the wind tunnel test indicate that the aerodynamic characteristics of grid fin depend much more on the area of each and total grid cell than that of lifting surface. In other words, the triangle pattern could be better choice for the grid fin because of higher structural strength compared to a square and a hexagonal pattern. That makes it possible to have a thinner web, which reduces the drag of the grid fin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The size of the cells is driven by the shockwaves that come off the cell walls when supersonic: you don't want the shockwave to hit the opposite cell wall. Cells with more corners have trouble in the transonic region (where the study found the hexagonal cells had choked flow, while the triangular and rectangular cells didn't).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why SpaceX chose a rectangular grid is a more difficult question to answer, as SpaceX doesn't publish much of its research. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-07-04T15:32:57.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22184" PostTypeId="1" AcceptedAnswerId="22186" CreationDate="2017-07-04T16:34:55.420" Score="8" ViewCount="668" Body="&lt;p&gt;I am referring to &lt;a href=&quot;http://online.liebertpub.com/doi/full/10.1089/space.2017.29009.emu&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; recently published document.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Producing propellant on Mars is a key part of Elon Musk's plan. What would happen if the first crew got to Mars and found out that, for whatever reason, it is not feasible to produce (enough) propellant (deep-cryo methalox) there? I know that this would be a big blow to the economics of the colonization of Mars as envisioned by Elon Musk, but more practically, what happens with that first crew?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I guess that there will be unmanned missions to (i) verify that the required raw materials are available in proximity to the site of the planned colony and (ii) perhaps even to attempt conducting the relevant chemical processes on a small scale. However, I presume that it would be beyond the capabilities of any unmanned mission to actually set up a production-grade refinery on Mars. If my assumption in the previous sentence is wrong, then that would obviously render my question moot.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some options that I have thought about:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The first mission carries the components necessary to build the refinery as well as enough propellant to be able to make it home in case the refinery doesn't work out (if that is the plan, would one ship be enough?)&lt;/li&gt;&#xA;&lt;li&gt;The first mission carries the components necessary to build the refinery as well as enough supplies for the crew to be able to wait for a re-supply or rescue mission launched during the next transfer window&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="20089" LastActivityDate="2017-07-22T19:47:56.980" Title="Is there a contingency option in Elon Musk's plan to colonize Mars in case the in-situ propellant production does not work out?" Tags="|spacex|mars|colonization|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22186" PostTypeId="2" ParentId="22184" CreationDate="2017-07-05T02:29:41.177" Score="6" Body="&lt;p&gt;If Elon Musk's plan is similar to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Direct&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Direct plan&lt;/a&gt;, which was the first to consider in situ propellant production, an unmanned return vehicle is launched in advance with a small chemical plant. That chemical plant would then produce the propellant and fill the tanks, so to speak. After being verified, the crewed vehicle launches on the next window to Mars.  &lt;/p&gt;&#xA;" OwnerUserId="20276" LastEditorUserId="20089" LastEditDate="2017-07-22T19:47:56.980" LastActivityDate="2017-07-22T19:47:56.980" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22188" PostTypeId="1" CreationDate="2017-07-05T04:35:00.070" Score="4" ViewCount="449" Body="&lt;p&gt;The point of a boilerplate is to test out the features of a system at a reduced cost. I would like to know what it cost to build the Apollo boilerplates. There was a series of such rockets so I would appreciate any cost data available on any of them.&lt;/p&gt;&#xA;" OwnerUserId="20275" LastEditorUserId="18592" LastEditDate="2017-07-06T08:41:49.713" LastActivityDate="2017-10-05T11:07:50.293" Title="How much did the Apollo boilerplate rockets cost?" Tags="|apollo-program|cost|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22190" PostTypeId="2" ParentId="22165" CreationDate="2017-07-05T04:55:21.477" Score="3" Body="&lt;p&gt;CORRECTION: $1.5M&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/amp/s/xtraterrestrial.wordpress.com/2013/04/26/gadgets-for-and-from-out-of-this-world-part-1/amp/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.google.com/amp/s/xtraterrestrial.wordpress.com/2013/04/26/gadgets-for-and-from-out-of-this-world-part-1/amp/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;To replace damaged parts, NASA created its own version of a cordless drill called a pistol-grip tool. Five years and $1.5M to develop each one of them, the drill was widely used during repair programs for the Hubble Telescope and later on was used for the International Space Station. The time and production cost raises the question that why couldn’t astronauts use a drill from a hardware store? The answer is because their work environment is unlike any environment that those tools have been designed for.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Old answer about the mini power tool:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are hacked together from stock materials.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.consumerreports.org/cro/news/2009/05/humble-cordless-screwdriver-helps-nasa-with-hubble-space-telescope-repair/index.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.consumerreports.org/cro/news/2009/05/humble-cordless-screwdriver-helps-nasa-with-hubble-space-telescope-repair/index.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During an extended EVA—that's NASA-speak for extravehicular activity or a space walk—Atlantis space-shuttle astronaut Mike Massimino used the Mini Power Tool, whose handle design was based on the grip on a DeWalt cordless screwdriver. And in a case of NASA tapping NASCAR, the innards of this 450-rpm driver were based on tools used by stock-car-racing pit crews. The photo shows Massimino practicing with the Mini Power Tool.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/search?q=dewalt+cordless+screwdriver&amp;amp;oq=dewalt+cordless+sc&amp;amp;aqs=chrome.1.69i57j0l3.6362j0j4&amp;amp;client=ms-android-hms-tmobile-us&amp;amp;sourceid=chrome-mobile&amp;amp;ie=UTF-8&quot; rel=&quot;nofollow noreferrer&quot;&gt;You can buy a DeWalt for $80-300&lt;/a&gt; and that will be pretty close.&lt;/p&gt;&#xA;" OwnerUserId="20275" LastEditorUserId="20275" LastEditDate="2017-07-06T03:15:00.413" LastActivityDate="2017-07-06T03:15:00.413" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22191" PostTypeId="1" CreationDate="2017-07-05T05:00:55.753" Score="23" ViewCount="4849" Body="&lt;p&gt;Why does Titan have an atmosphere given its relatively small mass (comparable to our moon)? Why is it not lost to space or is Titan's atmosphere constantly being replenished?&lt;/p&gt;&#xA;" OwnerUserId="18990" LastEditorUserId="20286" LastEditDate="2017-07-05T17:13:22.517" LastActivityDate="2019-02-10T21:54:41.773" Title="Why does Titan have an atmosphere?" Tags="|atmosphere|titan|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22192" PostTypeId="2" ParentId="21779" CreationDate="2017-07-05T05:05:21.437" Score="4" Body="&lt;p&gt;A typical rigid mid L/D aeroshell indicated in Mars exploration architecture encounters a maximum dynamic pressure of 11 kPa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UwWxT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UwWxT.png&quot; alt=&quot;Rigid Mid L/D aeroshell of Ares V type&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While, Hypersonic Inflatable Aerodynamic Decelerator would encounter a maximum entry dynamic pressure of 4.24 kPa. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Hlrtt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Hlrtt.png&quot; alt=&quot;HIAD&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, for mid L/D aeroshell based entry vehicles of Ares V kind, (Pg. 14)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The aerodynamic model covers Mach 1.3 through 50, angles of attack of 0 through 90o, and dynamic pressures of 1.E-7 through 0.75 bars&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm not entirely sure if it is reasonable to infer that max dynamic pressures of 0.1 - 0.75 bar are involved. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100026676.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;&lt;em&gt;Entry, Descent and Landing Systems Analysis Study: Phase 1 Report&lt;/em&gt;&lt;/strong&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2017-07-05T05:05:21.437" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22193" PostTypeId="2" ParentId="22191" CreationDate="2017-07-05T07:32:01.720" Score="22" Body="&lt;p&gt;The mass of Titan is 1,345 · 10&lt;sup&gt;23&lt;/sup&gt; kg, but the mass of the Moon is 7,349 · 10&lt;sup&gt;22&lt;/sup&gt; kg. The gravity at the surface is 1,35 m/s² for Titan and 1,62 m/s² for the the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the surface temperatures are very different, 94 K for Titan but the mean surface temperature of the Moon is 218 K and the peak about 300 K. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to the high surface temperature of the Moon, a small part (due to the Bolzman distribution) of the gas atoms have a velocity bigger than the escape velocity of the Moon. Over millions of years, this small part is enough to let escape most of the gas. &lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2019-02-10T21:54:41.773" LastActivityDate="2019-02-10T21:54:41.773" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="22194" PostTypeId="2" ParentId="1909" CreationDate="2017-07-05T08:11:03.693" Score="-2" Body="&lt;p&gt;There would be great benefit and could be the objective proof of whether the dominating theories about the nature of a black hole are correct or incorrect.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can 'prove' any number of things with logic but it doesn't make them necessarily true because you could easily be missing a crucial element of evidence in your deduction/induction.  We see this happen all of the time, even though mathematical proofs are logically flawless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As such, sending a probe in would be able to demonstrate the validity of proofs or lack thereof.  If we never hear from it again then we have objective and tangible evidence to weigh in support of the singularity theory, as well as the theory of infinite compression of dimensional matter and space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, you can posit with equations that jumping off your bed will pull you to the floor but you won't know for an absolute certainty until you do it.&#xA;Theoretically your bed is the only place in the universe where gravity exists.&lt;/p&gt;&#xA;" OwnerUserId="20278" LastEditorUserId="6773" LastEditDate="2017-07-05T17:11:46.977" LastActivityDate="2017-07-05T17:11:46.977" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22195" PostTypeId="1" AcceptedAnswerId="22196" CreationDate="2017-07-04T23:02:11.063" Score="3" ViewCount="348" Body="&lt;p&gt;Is it possible that a multi-rotor polycopter of some kind could escape Mars' atmosphere from its own propulsion?&lt;/p&gt;&#xA;" OwnerDisplayName="user20289" LastEditorDisplayName="user20289" LastEditDate="2017-07-05T18:35:24.243" LastActivityDate="2017-07-05T19:01:56.470" Title="Could a drone escape Mars' atmosphere?" Tags="|orbit|gravity|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22196" PostTypeId="2" ParentId="22195" CreationDate="2017-07-05T14:31:28.267" Score="7" Body="&lt;p&gt;Reaching orbit using aerodynamic lift from a rotor like an Earth-based drone is likely impossible.  The atmosphere on Mars is very thin compared to Earth (by about a factor of 100!) so lift is much harder to achieve.  At best, you'd be able to do short hops, but nowhere near orbital speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, a powerful enough launch vehicle could be equipped on a lander for a sample return mission.  However, it would have to use conventional rocket engines to have enough thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A sample return is in fact one of the proposed requirements for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Mars 2020 Rover&lt;/a&gt; and has been discussed for &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_sample_return_mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;other missions&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The launch vehicle for sample return would need about 4.1 km/s of delta V to reach orbit from the Martian surface.  It's possible it would then perform a rendezvous with another transfer vehicle, which would occur once in orbit for the retrieval capsule. &#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Related question with more information: &lt;a href=&quot;https://space.stackexchange.com/questions/17176/could-a-helicopter-fly-on-mars&quot;&gt;Could a helicopter fly on Mars?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check out this video for helicopter-like test in a simulated Martian atmosphere:&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/vpBsFzjyRO8?start=103&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6773" LastEditorDisplayName="user20289" LastEditDate="2017-07-05T19:01:56.470" LastActivityDate="2017-07-05T19:01:56.470" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22197" PostTypeId="1" CreationDate="2017-07-05T20:08:26.200" Score="3" ViewCount="785" Body="&lt;p&gt;Many exoplanets, even small ones, have densities so low that it indicates that they have hydrogen atmospheres. Is a hydrogen gas atmosphere of a terrestrial planet somehow detrimental to biology as we know it? Does it for example cause a flow of free protons that prevent the formation of larger molecules. Are such planets discarded from the list of habitable candidates?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since oxygen is the second most common chemically reactive element in the universe, and with all of that hydrogen around, shouldn't water oceans form, below the surface of which the composition of the atmosphere doesn't matter much?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-07-06T09:17:14.490" Title="Could a terrestrial planet with a hydrogen atmosphere be habitable?" Tags="|atmosphere|astrobiology|exoplanet|hydrogen|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22198" PostTypeId="2" ParentId="10895" CreationDate="2017-07-05T22:05:31.997" Score="9" Body="&lt;p&gt;The maximum pressure for long term survival in an atmosphere of 79 % nitrogen and 21 % oxygen is limited by oxygen toxicity. The limit of the partial pressure of oxygen is about 0.5 bar, the maximum pressure therefore is about 2.5 bar. For only some hours the pressure may be 4 to 5 bar, but nitrogen narcosis might be a problem. For higher pressures the amount of oxygen and nitrogen in the mix must be reduced and replaced by helium. For a total pressure of 5 bar, the amount of oxygen should be not more than 4.2 % but not less 3.2 %. The partial pressure of oxygen is then again 0.21 bar like that of normal air.&lt;br&gt;&#xA;For deep sea saturation diving, a pressure of about 60 bar was survived for weeks in an atmosphere of oxygen and helium.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-07-07T14:24:17.393" LastActivityDate="2017-07-07T14:24:17.393" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22199" PostTypeId="1" CreationDate="2017-07-05T22:28:49.040" Score="11" ViewCount="4737" Body="&lt;p&gt;The information and schematic are from the web site &lt;a href=&quot;http://pintleinjector.blogspot.com/2016/12/pintle-injector.html&quot; rel=&quot;noreferrer&quot;&gt;Pintle Injector Blog&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A pintle injector is a variable area injector consists of a movable pintle, an annular nozzle and the central pintle nozzle. Propellants from the center and annular nozzles collide near the pintle tip.&lt;/p&gt;&#xA;&lt;p&gt;Either fuel centered or oxidizer centered approaches can be used in design. In oxidizer centered approach, there is a moving pintle at the center that controls the oxidizer and an annular gap on the outside of the inner body that controls the fuel.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;All the pintle injector diagrams I have found show that moving the pintle tip could close &lt;em&gt;the central pintle nozzle&lt;/em&gt;, but wouldn’t affect &lt;em&gt;annular gap on the outside of the inner body&lt;/em&gt;. The pintle tip does not cover the annular gap. By enlarging the pintle tip and extending the outer edge of the annular gap, moving the pintle could regulate and shut off the flow of both the fuel and oxidizer.&lt;/p&gt;&#xA;&lt;p&gt;Is this what is done in the Merlin engine?&lt;/p&gt;&#xA;&lt;p&gt;How many pintle injectors are in the Merlin combustion chamber?&lt;/p&gt;&#xA;&lt;p&gt;Does the Merlin gas generator use pintle injectors?&lt;/p&gt;&#xA;" OwnerUserId="20225" LastEditorUserId="-1" LastEditorDisplayName="user10509" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-10T12:33:10.343" Title="Pintle Injector Face Shut Off - Merlin" Tags="|spacex|merlin-1d|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22201" PostTypeId="2" ParentId="22197" CreationDate="2017-07-05T23:58:36.617" Score="1" Body="&lt;p&gt;We would not find it habitable.  There would be very little free oxygen since it would tend to fuse with the hydrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I disagree that low density planets would have a hydrogen atmosphere.  It would take a significant gravity well to keep hydrogen from &quot;evaporating off&quot; from the planet.  At best, you would end up with a trace atmosphere like the Mars.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A look at the chart in the upper right on &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_escape&quot; rel=&quot;nofollow noreferrer&quot;&gt;this wiki&lt;/a&gt; (which I've seen here before) shows that only massive planets can retain a hydrogen atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="18842" LastActivityDate="2017-07-05T23:58:36.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22202" PostTypeId="1" CreationDate="2017-07-06T00:36:08.293" Score="3" ViewCount="319" Body="&lt;p&gt;The question is pretty much stated in the title.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there anything other than &quot;human caused&quot; reasons for this (such as clearing airspaces, etc.)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm only looking for possible natural reasons (other than weather, assume good weather and clear range). General possibilities that may apply to any aircraft are welcome, even though the SpaceX launch of Intelsat 35e prompted this question.&lt;/p&gt;&#xA;" OwnerUserId="12742" LastEditorUserId="25" LastEditDate="2017-12-19T13:15:48.367" LastActivityDate="2017-12-19T13:15:48.367" Title="Why would there be a launch window for a Geosynchronous orbit?" Tags="|launch|launch-window|" AnswerCount="1" CommentCount="2" FavoriteCount="0" ClosedDate="2017-07-06T08:09:41.480" ContentLicense="CC BY-SA 3.0" />
  <row Id="22203" PostTypeId="2" ParentId="22202" CreationDate="2017-07-06T00:54:21.467" Score="1" Body="&lt;p&gt;Without knowing the specifics for this launch considerations for airline trafic and shipping could play a part. There might be reasons not to close the range to everyone else indefinetly.&lt;/p&gt;&#xA;" OwnerUserId="16505" LastActivityDate="2017-07-06T00:54:21.467" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22205" PostTypeId="2" ParentId="22191" CreationDate="2017-07-06T03:35:39.370" Score="2" Body="&lt;p&gt;The primary factor is the lower temperature of Titan that results in the highest speed of the gas molecules in Titan's atmosphere being less than the escape velocity.  Hypothetically, the incoming solar wind bombardment also increases the kinetic energy of the gas molecules over the escape velocity so that over time an atmosphere not protected by a magnetic field would lose its atmosphere due to the solar wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flux of the solar wind decreases as the square of the distance from the sun.  Being that its host planet, Saturn, is 10 Astronomical Units from the sun.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn&quot; rel=&quot;nofollow noreferrer&quot;&gt;It is 9 times farther&lt;/a&gt; from the sun than our moon.  So the solar wind flux reaching Titan is 9 squared or 81 times weaker than at our moon.  Since, unlike our moon, Titan has a significant magnetic field, plus it &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_(moon)&quot; rel=&quot;nofollow noreferrer&quot;&gt;spends 95%&lt;/a&gt; of its orbit within Saturn's very strong magnetic field, the little solar wind that does get in the vicinity of Titan is totally deflected or weakened by these magnetic fields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, one can assume that the plasma particles of the solar wind would be so deflected and weakened that their average speed would be much less than Titan's escape velocity by the time they penetrate Titan's atmosphere at its poles, thus the solar wind actually is increasing Titan's atmosphere rather than blow it away as what happened to our moon.&lt;/p&gt;&#xA;" OwnerUserId="19730" LastEditorUserId="6816" LastEditDate="2017-07-06T03:45:23.297" LastActivityDate="2017-07-06T03:45:23.297" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22206" PostTypeId="2" ParentId="22197" CreationDate="2017-07-06T04:31:22.090" Score="-2" Body="&lt;p&gt;The radiation is by far the main problem. A cold, airless planet actually could be &lt;a href=&quot;https://www.google.com/amp/www.newyorker.com/tech/elements/meet-the-martians/amp&quot; rel=&quot;nofollow noreferrer&quot;&gt;habitable&lt;/a&gt; to extremophiles. However practically everywhere in the visible universe besides Earth is bathed in massive radiation, and it is a freak coincidence that the Earth has a sufficiently strong radiation defense- how it works is unknown- for anything to survive at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if there is sufficiently low radiation, great, you can put life there. If it has some oxygen and sufficient gravity to retain it, and is at least -50 degrees c or so warm, then advanced life forms might even survive. You could even find a warm air pocket underground, shielded from radiation. But realistically these conditions are rare to nonexistent outside of Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore &lt;a href=&quot;http://www.express.co.uk/news/science/728799/Solar-storm-magnetic-shield-WEAKENING-radiation&quot; rel=&quot;nofollow noreferrer&quot;&gt;even the earths&lt;/a&gt; magnetic field is buffeted all the time and it will not last very long.&lt;/p&gt;&#xA;" OwnerUserId="20275" LastActivityDate="2017-07-06T04:31:22.090" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22207" PostTypeId="1" CreationDate="2017-07-06T04:56:57.887" Score="2" ViewCount="324" Body="&lt;p&gt;James M Beggs was NASA administrator from 1981 to 1985.   He was accused of and later completely exonerated of contract fraud. What were the details of the charges and why were they later completely dropped?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some background:&#xA;&lt;a href=&quot;http://articles.latimes.com/1988-07-07/news/mn-8324_1_profound-apology&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://articles.latimes.com/1988-07-07/news/mn-8324_1_profound-apology&lt;/a&gt;  &lt;/p&gt;&#xA;" OwnerUserId="20052" LastEditorDisplayName="user10509" LastEditDate="2017-07-06T19:41:47.957" LastActivityDate="2017-07-06T19:41:47.957" Title="Charges against NASA Administrator James Beggs?" Tags="|nasa|space-shuttle|history|" AnswerCount="0" CommentCount="5" ClosedDate="2017-07-06T13:48:06.047" ContentLicense="CC BY-SA 3.0" />
  <row Id="22208" PostTypeId="2" ParentId="18851" CreationDate="2017-07-06T07:22:06.310" Score="2" Body="&lt;p&gt;It had a ~ 100 x 100 km orbit around the moon originally, but a 100 x 15 km for more precise imaging.&#xA;On 25 August 2011, it reached L2. 15 April of the next year, it departed for 'asteroid 4197'. Beijing time on December 13, 2012 16:30, 09 seconds, it was only 3.2 km away from it (7 000 000 km from earth), flying speed of up to 10.73 km/s.&lt;/p&gt;&#xA;&lt;p&gt;A translated version of &lt;a href=&quot;http://news.china.com.cn/2013-11/21/content_30660437.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://news.china.com.cn/2013-11/21/content_30660437.htm&lt;/a&gt; and the characters '轨道 的 嫦娥二号' may help more.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZlzED.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZlzED.jpg&quot; alt=&quot;low quality pic&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20295" LastEditorUserId="37597" LastEditDate="2023-08-01T00:48:20.117" LastActivityDate="2023-08-01T00:48:20.117" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="22209" PostTypeId="2" ParentId="22197" CreationDate="2017-07-06T09:17:14.490" Score="0" Body="&lt;p&gt;I would worry that your Hydrogen atmosphere would wither away. Simplifying a bit, there are two mechanisms at play.&lt;/p&gt;&#xA;&lt;h1&gt;Jean's escape&lt;/h1&gt;&#xA;&lt;p&gt;The molecules making up the atmosphere have their velocities distributed according to the so-called Mawell-Boltzmann distribution. The lighter the mass of the molecules, the broader is this distribution. By that I mean that the lighter the mass, the more molecule will be found in the tail of velocities higher than the escape velocity. The width of the distribution if proportional to the square root of the mass of the molecule, which brings a factor 4 between Hydrogen and our atmosphere.&lt;/p&gt;&#xA;&lt;h1&gt;Hydrodynamic escape&lt;/h1&gt;&#xA;&lt;p&gt;If the gas in the upper atmosphere is a good absorber of ultra-violet light, then it will be heated up and then this will build up pressure which will push the outmost layers out. Rinse and repeat. If I remember correctly, this effect is actually the dominant one. Unfortunately, I don't have time to double-check, sorry about that.&lt;/p&gt;&#xA;&lt;p&gt;Just to be clear about the difference with the previous mechanism. Jean's escape considers an atmosphere in thermodynamical equilibrium, i.e. the temperature at any given altitude is constant, and this is a pure effect of the thermal agitation of the molecules. Hydrodynamic escape on the contrary is a dynamic phenomenon.&lt;/p&gt;&#xA;&lt;h1&gt;A nice illustration: &lt;a href=&quot;https://en.wikipedia.org/wiki/HD_209458_b&quot; rel=&quot;nofollow noreferrer&quot;&gt;HD 209458 b&lt;/a&gt;&lt;/h1&gt;&#xA;&lt;p&gt;This is a Jupiter-like planet which is estimated to loose from 100 to 500 million kg of Hydrogen per seconds! The Wikipedia article has more details.&lt;/p&gt;&#xA;" OwnerDisplayName="user20022" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-07-06T09:17:14.490" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22210" PostTypeId="1" AcceptedAnswerId="22227" CreationDate="2017-07-06T10:41:00.843" Score="6" ViewCount="449" Body="&lt;p&gt;According to &lt;a href=&quot;https://space.stackexchange.com/a/20606/12102&quot;&gt;this answer&lt;/a&gt;, Columbia is the only Space Shuttle not to have docked with more than one space station. For the other shuttles, I think at least most of the time they only docked with one space station per mission. But I'm not sure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In total, how many times has any spacecraft docked with two or more different space stations during a single mission?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-07-07T13:07:09.183" Title="How many times has a spacecraft docked with two or more different space stations during a single mission?" Tags="|crewed-spaceflight|mission-design|space-station|docking|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22211" PostTypeId="1" AcceptedAnswerId="22234" CreationDate="2017-07-06T10:50:08.473" Score="15" ViewCount="1760" Body="&lt;p&gt;Has a spacecraft ever docked with the same space station twice in one mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the &quot;corollary&quot; to the question &lt;a href=&quot;https://space.stackexchange.com/q/22210/12102&quot;&gt;How many times has a spacecraft docked with two or more different space stations during a single mission?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-07-09T04:56:49.623" Title="Has a spacecraft ever docked with the same space station twice in one mission?" Tags="|crewed-spaceflight|mission-design|space-station|docking|" AnswerCount="3" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22212" PostTypeId="2" ParentId="22211" CreationDate="2017-07-06T11:06:49.730" Score="20" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_29#Soyuz_31_crew_visits.2C_first_Soyuz_redock&quot; rel=&quot;noreferrer&quot;&gt;first, experimental redocking&lt;/a&gt; was performed on Soyuz 29 (though by crew of Soyuz 31) with the Salyut 6 station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Afterwards, the maneuver was repeated several times, usually between different ports of a station, &lt;a href=&quot;https://9gag.com/gag/anB9OZ0/soyuz-un-docking-and-re-docking-with-the-iss-x50-speed&quot; rel=&quot;noreferrer&quot;&gt;moving a docked craft&lt;/a&gt; from one port to another, in order to make room for a new arrival, although other purposes happened too - e.g. &lt;a href=&quot;https://www.c-span.org/video/?89459-1/soyuz-redocking-mir&quot; rel=&quot;noreferrer&quot;&gt;visual inspection of Mir&lt;/a&gt; by a Soyuz, after &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_M-34&quot; rel=&quot;noreferrer&quot;&gt;Progress M-34&lt;/a&gt; failed to redock (testing a new docking procedure), crashing into Mir instead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another notable case is &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_T-15#Mission_highlights&quot; rel=&quot;noreferrer&quot;&gt;Soyuz T-15&lt;/a&gt;, which after 55 day stay on Mir, went to (obsolete) Salyut 7, primarily to retrieve experiments and equipment, which would be useful on Mir. Their absence on Mir laster 28 days (with another 20 days spent on Mir afterwards before return), which makes it the longest period spent between undocking and redocking to the same station in the same mission. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-07-06T17:35:14.280" LastActivityDate="2017-07-06T17:35:14.280" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22213" PostTypeId="2" ParentId="22210" CreationDate="2017-07-06T11:25:19.500" Score="8" Body="&lt;p&gt;The transition between the Salyut 7 station to Mir, had &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_T-15&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz T-15&lt;/a&gt; transfer between Mir and Salyut 7.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They launched and docked with Mir, then departed and docked with Salyut-7, picking up some equipment, and then departed and returned to the nascent Mir space station.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They had to adjust the orbits of Salyut 7 to make the transfer less fuel intensive, and thus make it possible. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-07-06T17:31:02.433" LastActivityDate="2017-07-06T17:31:02.433" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22214" PostTypeId="1" AcceptedAnswerId="22219" CreationDate="2017-07-06T14:18:36.167" Score="-2" ViewCount="457" Body="&lt;p&gt;Let's assume the aliens that come to visit our planet are friendly and somewhat like humans. Who will be the one to tell them about our habits and technology? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Very curious if there are any laws that cover this if there might ever be an Alien meeting.&lt;/p&gt;&#xA;" OwnerUserId="20302" LastActivityDate="2017-07-06T18:35:41.063" Title="Who gets to represent Earth in case of an Alien meeting?" Tags="|life|astrobiology|" AnswerCount="1" CommentCount="3" ClosedDate="2017-07-06T22:00:09.990" ContentLicense="CC BY-SA 3.0" />
  <row Id="22216" PostTypeId="1" AcceptedAnswerId="22218" CreationDate="2017-07-06T16:16:38.077" Score="13" ViewCount="1692" Body="&lt;p&gt;I watched the latest SpaceX launch of Intelsat 35e on 05 July 2017 carried by their Falcon 9 rocket into geostationary orbit, and noticed some confusing telemetry data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Telemetry just before the second burn was as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Velocity: 26503 km/hr&#xA;Altitude: 248 km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using the vis-viva equation, I calculated the semi-major axis (a) of this low Earth orbit and found a = 6017.6 km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that the semi-major axis of this orbit is SMALLER than the radius of the Earth (R = 6371 km), and therefore, will have a perigee that is BENEATH the Earth's surface....i.e. the orbit is sub-orbital.  If the 2nd stage were to continue in this orbit (and not perform the second burn to get it into higher orbit) then doesn't this mean the 2nd stage would crash back into the Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that the 2nd stage sat in this sub-orbital low Earth orbit for only about 20 minutes before performing the second burn, it wasn't an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But my question is this:  Is parking a 2nd stage in a sub-orbital low Earth orbit safe, even if it is eventually going to get boosted to a higher altitude orbit?  Is this routine or common?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you very much!  I appreciate it!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;-Will&lt;/p&gt;&#xA;" OwnerUserId="20305" LastActivityDate="2017-07-07T08:47:54.717" Title="Does SpaceX put Falcon 9 2nd stages carrying geostationary satellites into a temporary sub-orbital trajectory before the second burn?" Tags="|spacex|rockets|falcon-9|orbit|science|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22218" PostTypeId="2" ParentId="22216" CreationDate="2017-07-06T17:12:31.110" Score="21" Body="&lt;p&gt;Excellent question!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's probably ground velocity or more likely velocity in an &lt;a href=&quot;https://en.wikipedia.org/wiki/ECEF&quot; rel=&quot;noreferrer&quot;&gt;ECEF (earth-centered, earth-fixed), also known as ECR (earth-centered rotational)&lt;/a&gt; frame that they are displaying, referenced to the rotating frame of the Earth. A hint would be that &lt;em&gt;it is zero before lift-off&lt;/em&gt; even though the Earth is rotating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia gives the value of $3.986004418\times 10^{14}$ for $GM_E$ the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;noreferrer&quot;&gt;standard gravitational parameter&lt;/a&gt; for Earth. Using $(6378 + 248) \times 1000$ meters for the semi-major axis, the orbital velocity from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;noreferrer&quot;&gt;vis-viva equation&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v^2 = GM_E/a$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;would be 7756 m/s. The difference between that an your value of 7362 m/s is only 394 m/s, which is a little less than the Earth's equatorial rotation velocity of &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{2 \pi \ 6378\times1000 \ m}{36484 \ s} = 464 \ m/s$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the ratio is probably close to the cosine of the inclination of the orbit. It's not going to be exact because we're not doing anything exactly, but it should be close:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$cos^{-1}\left(\frac{394}{464}\right) \approx 32° $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I think the answer is that it is &lt;em&gt;not&lt;/em&gt; sub-orbital. The velocity in the video is probably referenced to the rotating frame of the Earth. The calm state of the altitude accurately reflects an orbit that will stay outside the atmosphere for at least days or weeks.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-07T08:47:54.717" LastActivityDate="2017-07-07T08:47:54.717" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22219" PostTypeId="2" ParentId="22214" CreationDate="2017-07-06T18:35:41.063" Score="2" Body="&lt;p&gt;This is one of the cases where facts are made.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lets assume the aliens &lt;em&gt;do&lt;/em&gt; land somewhere. Then of course the state in which they landed will deal with it. They will probably send their army to secure the landing site, and then send representatives to speak to the aliens once its been established that those aliens are peaceful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, whoever gets their hand on them determines what they hear, first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of couse this might not sit well with the neighboruing states. If another state has superior military power and thinks he could benefit from it, he could try to invade and seize the opportunity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the dust settles, its politics and diplomacy. The aliens are a precious commodity, everyone will want to talk to them. So there will be deals and treaties signed that govern who gets access and when. rest assured, the country where they landed will want something in return when they let foreigners speak to the aliens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thats assuming the aliens stay there. We don't know. They might just take off, find another spot to land on the Earth - rinse and repeat all of the above.&#xA;If they +do* tour the world on their own, at some point demand will drop, and it will get harder for each subsequent hosting country to get favorable deals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact of the matter is that it would be solved how all international disputes get solved. Either by force (militaristic) or through politics and diplomacy in the form of deals and treaties. everything else is mere speculation. There is no single government that rules all of Earth, no comitee that &lt;em&gt;could&lt;/em&gt; represent the whole planet.&lt;/p&gt;&#xA;" OwnerUserId="13867" LastActivityDate="2017-07-06T18:35:41.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22220" PostTypeId="2" ParentId="19942" CreationDate="2017-07-06T19:12:55.727" Score="1" Body="&lt;p&gt;No, no and no.  Though you will get a stronger magnetic field with increased rotation, if you increased the magnetic field through other means it would not increase the rotation rate.  An active magnetic field would have no effect on the orbit of Mars, in fact you could say the larger magnetic field would create a slightly great 'push' from the solar wind on Mars.  But as that effect is extremely small in comparison to the planets mass and orbit, it would not likely even be measureable.  It definitely not would pose a danger to Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also this question has been asked before within the stack exchange so browsing some of those should help you as well.  &lt;/p&gt;&#xA;" OwnerUserId="20287" LastActivityDate="2017-07-06T19:12:55.727" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22221" PostTypeId="1" CreationDate="2017-07-06T19:41:50.450" Score="1" ViewCount="2608" Body="&lt;p&gt;How much more stress does a rocket experience during max-q compared to other parts of the flight (such as when it is at rest on the pad)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Use data for any 'cylindrical' rocket if a specific example is needed. By cylindrical I mean to exclude the Space Shuttle, Space Ship One and other exotic designs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that I am not interested in the value of q. Only the stress on the rocket.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2017-07-07T02:21:32.103" LastActivityDate="2017-10-18T23:31:13.943" Title="How great is the stress on a rocket at max-q" Tags="|launch|aerodynamics|" AnswerCount="2" CommentCount="6" FavoriteCount="0" ClosedDate="2017-10-25T19:28:18.810" ContentLicense="CC BY-SA 3.0" />
  <row Id="22222" PostTypeId="2" ParentId="2911" CreationDate="2017-07-07T00:58:58.627" Score="2" Body="&lt;p&gt;The proposed, but never completed and flown, &lt;a href=&quot;http://astrocryptotriviology.blogspot.com/2015/10/a-jones-for-mol-11-retroactivity-of-mol.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gemini B spacecraft might have used its solid-fuel retrorocket package, normally used for de-orbiting the spacecraft, for early launch aborts.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NASA Gemini flights launched on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_II_GLV&quot; rel=&quot;nofollow noreferrer&quot;&gt;Titan II&lt;/a&gt;, using only hypergolic liquid fuels, which can't explode quite as dramatically as non-hypergolics or solids, &lt;a href=&quot;https://space.stackexchange.com/questions/14690/why-were-ejection-seats-used-in-project-gemini-instead-of-a-tower-escape-system?rq=1&quot;&gt;so ejection seats were an acceptable escape route&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The USAF Gemini B, on the other hand, would have launched along with the &lt;a href=&quot;https://en.wikipedia.org/wiki/Manned_Orbiting_Laboratory&quot; rel=&quot;nofollow noreferrer&quot;&gt;Manned Orbiting Laboratory&lt;/a&gt; on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_IIIM&quot; rel=&quot;nofollow noreferrer&quot;&gt;Titan IIIM&lt;/a&gt; with solid rocket boosters. Ejection seats might not get the crew to a safe distance from a SRB accident, so two additional retrorockets were added to the capsule (totaling 6), and the retros would be fired simultaneously in the event of an abort. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-07-07T00:58:58.627" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22226" PostTypeId="2" ParentId="22211" CreationDate="2017-07-07T06:12:49.940" Score="9" Body="&lt;p&gt;When USSR were in the final stages of putting Mir in to orbit, Salyut 7 was still in orbit containing many devices and equipment that the USSR wanted on Mir.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So on 13th March 1986 the Soyuz T-15 launched, docking with Mir. On the 5th May 1986 it then undocked for the first time from Mir, and travelled to the orbiting Salyut 7 space station and docked successfully on the 6th May. It then undocked from Salyut 7 on the 25th June, docking with Mir again on the 26th June. Soyuz T-15 undocked from Mir for the final time on the 16th July.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also remains the first and only time where a space craft has travelled back and forth between two places in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information can be found &lt;a href=&quot;http://www.svengrahn.pp.se/histind/MirtoSal7flyover/MirToSal7Flyover.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; about the mission.&lt;/p&gt;&#xA;" OwnerUserId="19791" LastActivityDate="2017-07-07T06:12:49.940" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22227" PostTypeId="2" ParentId="22210" CreationDate="2017-07-07T13:07:09.183" Score="12" Body="&lt;p&gt;When USSR were in the final stages of putting Mir in to orbit, Salyut 7 was still in orbit containing many devices and equipment that the USSR wanted on Mir.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So on 13th March 1986 the Soyuz T-15 launched, docking with Mir. On the 5th May 1986 it then undocked for the first time from Mir, and travelled to the orbiting Salyut 7 space station and docked successfully on the 6th May. It then undocked from Salyut 7 on the 25th June, docking with Mir again on the 26th June. Soyuz T-15 undocked from Mir for the final time on the 16th July.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also remains the first and only time where a space craft has travelled back and forth between two places in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information can be found &lt;a href=&quot;http://www.svengrahn.pp.se/histind/MirtoSal7flyover/MirToSal7Flyover.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; about the mission.&lt;/p&gt;&#xA;" OwnerUserId="19791" LastActivityDate="2017-07-07T13:07:09.183" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22228" PostTypeId="2" ParentId="2541" CreationDate="2017-07-07T14:47:16.607" Score="0" Body="&lt;p&gt;As carbon balloons block more sunlight, the temperature would slowly drop, if you could block the sun all at once it is estimated the CO2 would totally freeze out in just 200 years.  Though the 3 atm of N2 would still be in the atmosphere.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a CO2 ocean, which would cause very intense weathering of the Venetian surface during the 'big rain', plate tectonics would possibly be restarted due to the surface liquid ocean and with it volcanism.  The planet would not expand as the same amount of mass is being applied to the surface whether in liquid or gaseous form.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Working over such big time scales maintaining the balloons/planet shield will be necessary, dealing with the acid in atmosphere wearing down the shield, the solar wind directly pummeling the shield, occasional meteor impacts, adjusting the altitude and shape as the pressure drops underneath the shield, etc.  &lt;/p&gt;&#xA;" OwnerUserId="20287" LastActivityDate="2017-07-07T14:47:16.607" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22229" PostTypeId="2" ParentId="21816" CreationDate="2017-07-07T16:05:32.837" Score="2" Body="&lt;p&gt;Considering the challenges of self-replicating it is more likely that more MOXIEs would be shipped in, or a Sabatier reactor if CH4 and LOX is preferred to just LOX and CO.  In parallel for a colony though greenhouses would be started to produce food and oxygen which could supplement the colony and eventually be relied on to add to the breathing supply without importing more hardware for the colony from Earth.  &lt;/p&gt;&#xA;" OwnerUserId="20287" LastActivityDate="2017-07-07T16:05:32.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22231" PostTypeId="1" AcceptedAnswerId="22232" CreationDate="2017-07-07T20:53:50.443" Score="7" ViewCount="2281" Body="&lt;p&gt;I don't have an exact citation or reference but I remember hearing that in some specific instances, that a bielliptic transfer can be more efficient than Hohmann transfers. The only possible instance I can think of where it would be more efficient would be changing inclinations to a target. Are there any other instances which make a bielliptic transfer maneuver more efficient than a Hohmann?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2017-07-08T17:24:56.190" Title="How can Bielliptic transfers be more efficient than Hohmann transfers?" Tags="|orbital-mechanics|hohmann-transfer|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22232" PostTypeId="2" ParentId="22231" CreationDate="2017-07-07T23:10:15.040" Score="8" Body="&lt;p&gt;Citing &lt;a href=&quot;https://en.wikipedia.org/wiki/Bi-elliptic_transfer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;some bi-elliptic transfers require a lower amount of total delta-v than a Hohmann transfer when the ratio of final to initial semi-major axis is 11.94 or greater, depending on the intermediate semi-major axis chosen.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;An intuitive approach to understanding it, is when you look at delta-V of insertion after Hohmann transfer. For near orbits, it's quite low, but as they grow apart, eccentricity of the transfer orbit increases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's look at a bi-elliptic transfer &quot;outwards&quot; (from a lower to higher orbit) where it's more intuitive. Say, you want to go from LEO (7000km) to the Earth-Moon L5 point, almost 400,000 km. 400:7 is way more than 12:1 so it's a definite candidate for a bi-elliptic transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your transfer orbit will be a narrow spike, an very elongated ellipse. Speed near apogeum will be a crawl, a scarce couple meters per second. Meanwhile, the Moon in its circular orbit, goes at over 1000m/s, and so does it's L5 Lagrangian point. You'd need to accelerate your probe by nearly 1000m/s on arrival.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And while making that &quot;spike&quot; reach Moon orbit is costly, we can't skip that cost - but extending it much farther, to near Hill sphere. is nearly free - that's just a couple meters per second more. Circularizing an orbit with perigee near Moon orbit, from apogee near Earth's Hill sphere will still cost  a bit - but considerably less than the 1000m/s needed to enter Moon orbit; not only motion at apogee this high is way slower, you're not performing a full circularization, leaving a considerable eccentricity. Then you'll still need to perform third burn, to match Moon orbit at L5, dropping that apogee, but even adding it, you're still spending less than you spent on bringing the perigee from LEO to Moon altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way to think of it is: Hohmann orbit leaves you at apogee with speed lower than the target by &lt;em&gt;x&lt;/em&gt;. Arrival &quot;from infinity&quot; / &quot;from outside the system&quot; into a flyby trajectory at altitude of target orbit leaves you with speed higher than the target by &lt;em&gt;y&lt;/em&gt;. If the radii are much different, the difference between speed for Hohmann transfer and between escape is &lt;em&gt;peanuts$_1$.&lt;/em&gt;  If you arrive from outside the system, and want to choose altitude of your fly-by, when you're still very far, that costs &lt;em&gt;peanuts$_2$&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now if &lt;em&gt;y&lt;/em&gt; + &lt;em&gt;peanuts$_1$&lt;/em&gt; + &lt;em&gt;peanuts$_2$&lt;/em&gt; &amp;lt; &lt;em&gt;x&lt;/em&gt;, Bi-elliptic transfer takes less delta-V. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And while Hohmann transfer takes less time to transfer, finding the optimal time to perform it, especially with elliptic, non-coplanar orbits can get quite tricky. With bi-elliptic, all maneuvers at the apogee cost peanuts, and you can take significant liberties as to where that apogee will be. As result, the bi-elliptic transfer may take less than Hohmann transfer - if you count time until optimal alignment of departure and arrival objects towards the transfer time.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-07-08T17:24:56.190" LastActivityDate="2017-07-08T17:24:56.190" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22233" PostTypeId="1" CreationDate="2017-07-08T13:07:17.503" Score="8" ViewCount="1377" Body="&lt;p&gt;If I open the valve on my nitrogen bottle there is a terrible noise but the gas jet is totally invisible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On some Falcon 9 launches, the gas jets are very visible both on ground and in the low pressure environment immediately after second stage separation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is it we see? In the normal atmospere on ground I could guess it is air moisture condensation, but in vacuum?&lt;/p&gt;&#xA;" OwnerUserId="8278" LastEditorUserId="6537" LastEditDate="2017-12-19T19:49:02.797" LastActivityDate="2017-12-28T01:40:31.983" Title="Why is the gas jet from Falcon 9's cold gas thrusters so clearly visible?" Tags="|falcon-9|engines|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22234" PostTypeId="2" ParentId="22211" CreationDate="2017-07-08T20:49:22.447" Score="5" Body="&lt;p&gt;In short, the answer to your question is &quot;Yes&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft, especially cargo transports, are frequently moved between docking ports on space stations.  This was done on Mir, and is being done on the ISS.  It was first done on a Salyut station, but I would have to dig through my Space History News database to find the reference(s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To test the new manual docking system, Progress 62 (Progress MS) cargo spacecraft automatically undocked from the Pirs Docking Compartment of the International Space Station and manually re-docked. The undocking occurred at 05:36 UTC (1:36 a.m EDT) and re-docking at 06:05 UTC (2:05 a.m EDT), on 1 July 2016.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Video at &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Rhw2JQTZeZQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-07-09T04:56:49.623" LastActivityDate="2017-07-09T04:56:49.623" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22235" PostTypeId="1" AcceptedAnswerId="22236" CreationDate="2017-07-08T21:57:46.660" Score="6" ViewCount="11907" Body="&lt;p&gt;Is it possible for me to buy kerosene from a local store and then try to refine it to the extent where it would be considered RP1. If so how would I do it? For those who are wondering me and a group of friends is trying to make a model rocket with an actual engine(aka not store bought), but want to learn about the burning process first. In addition, what is the best oxidizer? We think liquid hydrogen is the best. &lt;/p&gt;&#xA;" OwnerUserId="20332" LastActivityDate="2017-07-08T23:57:22.543" Title="Can I make my own RP1?" Tags="|rockets|fuel|chemistry|amateur-rocketry|combustion|" AnswerCount="1" CommentCount="6" ClosedDate="2018-07-18T02:22:50.907" ContentLicense="CC BY-SA 3.0" />
  <row Id="22236" PostTypeId="2" ParentId="22235" CreationDate="2017-07-08T23:57:22.543" Score="10" Body="&lt;p&gt;I'm going to start off with a warning: if you want to get started with model rocketry, it is about a thousand times safer and easier to start with off-the-shelf solid rockets. Don't even consider trying to build your own liquid fueled engine until you've learned a lot more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, reaching the &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;noreferrer&quot;&gt;RP-1&lt;/a&gt; specification from commercial kerosene is probably beyond what a novice chemist can reasonably attempt at home, but for simple liquid rocket engines you don't need RP-1. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normal kerosene is problematic in regeneratively cooled engines (which pass the fuel through channels around the combustion chamber to remove heat) because some of the components vaporize at relatively low temperatures, while others polymerize; the deposits and gas bubbles so produced impair heat transfer, leading to overheating of the chamber walls, burn-through, and rapid failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's much simpler to build a non-regeneratively cooled engine, which requires much lower combustion temperature; the ugly way to achieve this is to run the engine very fuel-rich, leaving much of the fuel uncombusted, which gives you a very sooty (and somewhat hazardous) exhaust. The better way is to use a different fuel entirely: the &lt;a href=&quot;https://en.wikipedia.org/wiki/V-2_rocket&quot; rel=&quot;noreferrer&quot;&gt;V-2&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Redstone_Launch_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;Redstone&lt;/a&gt; rockets used 75% ethyl alcohol, 25% water, which is available from several sources -- perhaps most amusingly (if not the cheapest or purest) your local liquor store (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Bacardi_151&quot; rel=&quot;noreferrer&quot;&gt;Bacardi 151&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Everclear_(alcohol)&quot; rel=&quot;noreferrer&quot;&gt;Everclear&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Organic Marble alludes, the oxidizer of choice is &lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_oxygen&quot; rel=&quot;noreferrer&quot;&gt;liquid &lt;em&gt;oxygen&lt;/em&gt;&lt;/a&gt; rather than hydrogen. This is also tricky stuff to handle; nearly anything you spill it on will become highly flammable if you're lucky and explosive if you're not. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-07-08T23:57:22.543" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22237" PostTypeId="1" AcceptedAnswerId="22238" CreationDate="2017-07-09T03:08:39.563" Score="22" ViewCount="4608" Body="&lt;p&gt;A short comment in the BBC Crowd Science audio program &lt;a href=&quot;http://www.bbc.co.uk/programmes/p057b7wl&quot; rel=&quot;nofollow noreferrer&quot;&gt;Does Time really Exist?&lt;/a&gt;'s discussion of the slow divergence between &lt;a href=&quot;https://en.wikipedia.org/wiki/Coordinated_Universal_Time&quot; rel=&quot;nofollow noreferrer&quot;&gt;UTC&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Atomic_Time&quot; rel=&quot;nofollow noreferrer&quot;&gt;TAI (&lt;s&gt;IAT&lt;/s&gt;)&lt;/a&gt; (coordinated time and international atomic time) says that NASA and ESA for example avoid launches around &lt;a href=&quot;https://en.wikipedia.org/wiki/Leap_second&quot; rel=&quot;nofollow noreferrer&quot;&gt;leap seconds&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Is this true? Is it done out of an abundance of caution, or even an overabundance, or are there mixed clocks, some running UTC and some running IAT and offsets hard-coded in for each launch? Currently the difference is 37 seconds and there have been 27 leap seconds since 1970.&lt;/p&gt;&#xA;&lt;p&gt;In the past it would not have been much of a burden to avoid them if a launch is considered a few minute or few hour event, but the duration of a mission is often years or decades, so an entire mission can almost never completely avoid a leap second.&lt;/p&gt;&#xA;&lt;p&gt;So how do launches avoid leap seconds? And why?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-12-24T12:42:11.513" LastActivityDate="2021-12-26T22:02:08.340" Title="How do launches avoid leap seconds? Why?" Tags="|launch|time|" AnswerCount="4" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="22238" PostTypeId="2" ParentId="22237" CreationDate="2017-07-09T07:55:49.350" Score="18" Body="&lt;p&gt;Avoiding leap seconds is easy, don't launch at June 30th or December 31st when a leap second is announced, see &lt;a href=&quot;https://en.wikipedia.org/wiki/Leap_second&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Leap_second&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;It is difficult to test if leap seconds during a launch may cause problems: leap seconds occur only once or rarely twice a year, but there have been a lot of years without leap seconds.&lt;/p&gt;&#xA;&lt;p&gt;In this century, leap seconds were inserted only in 2005, 2008, 2012, 2015 and 2016. Only 2012 and 2015 there were leap seconds in June. Since the introduction of leap seconds, only the points of time in June and December have been used.&lt;/p&gt;&#xA;&lt;p&gt;There were no leap seconds in the years 2017, 2018, 2019, 2020 and none announced for 2021.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2021-12-26T22:02:08.340" LastActivityDate="2021-12-26T22:02:08.340" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="22239" PostTypeId="2" ParentId="22237" CreationDate="2017-07-09T09:05:36.337" Score="21" Body="&lt;p&gt;It's not just launches. It's, well, everything. It drives me nuts!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft flight software almost always have the capability to execute uplinked commands based on time. But what time scale? The operational control systems for spacecraft almost always have the ability to issue timed commands to spacecraft. But what time scale? The various mission planning and analysis systems also are time dependent. But yet again, what time scale?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some systems support TAI as well as UTC. Others support GPS time as well as UTC. Yet others only support GMT (Greenwich Mean Time), which hasn't existed since 1973. A tiny few support relativistically-correct time scales as well as UTC. The ubiquitous answer is UTC (or GMT in the case of legacy systems from some previous millennium). So, rather sadly, this is the one time scale that is almost always used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This sadly means that spacecraft should not be doing anything critical near a leap second boundary.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-07-09T09:16:56.607" LastActivityDate="2017-07-09T09:16:56.607" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="22240" PostTypeId="1" AcceptedAnswerId="22244" CreationDate="2017-07-09T09:57:30.070" Score="9" ViewCount="2744" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/22239/12102&quot;&gt;@DavidHammen&lt;/a&gt;'s answer got me thinking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System#Timekeeping&quot; rel=&quot;noreferrer&quot;&gt;GPS time&lt;/a&gt; &quot;comes from&quot; (&lt;a href=&quot;https://en.wikipedia.org/wiki/United_States_Naval_Observatory&quot; rel=&quot;noreferrer&quot;&gt;US Naval Observatory&lt;/a&gt;) a whole bunch of atomic clocks, in space and on Earth. &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Atomic_Time&quot; rel=&quot;noreferrer&quot;&gt;TAI or International Atomic Time&lt;/a&gt; also comes from a whole bunch of atomic clocks, distributed around the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm guessing they are not strictly coupled, but are they &quot;really close&quot; to each other, say on the order of microseconds?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="2752" LastEditDate="2017-07-09T23:45:01.813" LastActivityDate="2017-07-10T00:24:03.177" Title="Is GPS time at least &quot;really close&quot; to TAI (International Atomic Time)?" Tags="|time|gps|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22241" PostTypeId="2" ParentId="22240" CreationDate="2017-07-09T10:50:11.127" Score="6" Body="&lt;p&gt;From the book GPS Satellitennavigation by Frank Schröder ISBN 3-7723-6692-1: &quot;GPS has it own time scale, it starts with the week 0 at 6th January of 1980. The long term difference between GPS and UTC time should be held less than 1 microsecond (disregarding leap seconds). To acheive this, the atomic clocks of GPS satellites and ground stations are synchronized to less than 20 nanoseconds and the GPS time to UTC time to less than 100 nanoseconds. The GPS ground stations monitors all atomic clocks of the GPS system and UTC for that purpose.&quot;  &quot;Atomic clocks on earth could be synchronized using GPS to less than 10 nanoseconds difference.&quot;&lt;br&gt;&#xA;So GPS and TIA clocks are not strictly coupled, but the differences are kept very small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The differences between atomic clocks on earth on different gravitational potentials are compensated since 1977. After that TAI corresponds to the time at mean sea level.&#xA;&quot;Due to several relativistic effects, the atomic clocks in the GPS satellites run a little faster, they gain 445 picoseconds per second. To compensate this a master clock frequency of nominal 10.23 MHz has actual 10.229999995453 MHz&quot;&lt;br&gt;&#xA;GPS: &lt;a href=&quot;https://en.wikipedia.org/wiki/Global_Positioning_System#Timekeeping&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Global_Positioning_System#Timekeeping&lt;/a&gt;&lt;br&gt;&#xA;TIA or TAI: &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Atomic_Time&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/International_Atomic_Time&lt;/a&gt;&lt;br&gt;&#xA;UTC: &lt;a href=&quot;https://en.wikipedia.org/wiki/Coordinated_Universal_Time&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Coordinated_Universal_Time&lt;/a&gt;&lt;br&gt;&#xA;Atomic clocks: &lt;a href=&quot;https://en.wikipedia.org/wiki/Atomic_clock&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Atomic_clock&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-07-09T13:34:23.963" LastActivityDate="2017-07-09T13:34:23.963" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22242" PostTypeId="2" ParentId="22237" CreationDate="2017-07-09T11:41:53.567" Score="6" Body="&lt;p&gt;Most satellites I'm familiar with work in GPS time internally, but correct for the time by some kind of a constant that is periodically updated for leap seconds to UTC. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the specific instance of a launch, it would almost certainly be either a mission clock, or GPS time, that is used internally. Most rockets have some kind of a GPS clock on them, which will enable them to find their position, and keep extremely precise time measurements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For systems that might require launching at any time (And those do exist), they will specifically check for both positive and negative leap seconds to see how the system reacts to them. But generally internally they use either time since launch or GPS time to keep track, and not UTC time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-07-09T11:41:53.567" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22243" PostTypeId="1" AcceptedAnswerId="22301" CreationDate="2017-07-09T12:04:11.990" Score="5" ViewCount="1074" Body="&lt;p&gt;In the question &lt;a href=&quot;https://space.stackexchange.com/questions/22235/can-i-make-my-own-rp1&quot;&gt;Can I make my own RP1?&lt;/a&gt; the comments talk about using ethanol and water as rocket fuel. That made me wonder how well easily availible ethanol fuel for cars such as E85 (85% ethanol 15% gasoline) would work. Would such a rocket run into the same problems as a pure kerosen rocket or would it work well for a simple launcher?&lt;/p&gt;&#xA;" OwnerUserId="16505" LastActivityDate="2017-07-18T17:56:13.907" Title="How would E85 work as rocket fuel?" Tags="|fuel|amateur-rocketry|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22244" PostTypeId="2" ParentId="22240" CreationDate="2017-07-09T13:04:33.033" Score="17" Body="&lt;p&gt;TL;DR: For all practical purposes except milliarcsecond radio astronomy, TAI and GPS time are separated by exactly 19 seconds.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;TAI and GPS time both attempt to represent time as ticked by an ideal clock at mean sea level on the surface of the Earth. Both differ from UT1, time as ticked by the Earth's rotation (this is the source of leap seconds), and because of how they represent time, they also differ very slightly from one another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GPS time is an operational time scale. Each GPS satellite contains an atomic clock, but with a tick rate that deviates slightly from the standard definition of &quot;9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atomic clocks on GPS satellites are subject to both special relativistic and gravitational effects. The special relativistic effects result from the relative motion of the GPS satellites and a clock on the surface of the Earth. The gravitational (general relativistic) effects result from the altitude at which GPS satellites orbit. To compensate for these effects, the GPS satellites' atomic clocks are intentionally made to tick at a slightly slower rate than is standard (the standard being &quot;9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal is to have the tick rate of a GPS satellite as observed from a receiver at sea level be equal to the tick rate of the best atomic clocks, adjusted to sea level. While this is the goal, it is not the reality. The smallish atomic clocks on the GPS satellites aren't as accurate as are the very large atomic clocks maintained by the US Naval Observatory. The GPS satellite atomic clocks are updated regularly, roughly once per day, to satisfy the promised accuracy of GPS receivers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TAI is an ideal time scale as opposed to an operational time scale. To best represent this ideal, time as ticked by atomic clocks worldwide are averaged over the course of a year, weighted by the accuracies of those clocks. This addresses issues such as tidal effects and subtle changes in elevation. As the most accurate atomic clocks currently are those maintained by the US Naval Observatory, those clocks get more weight as to what constitutes TAI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The end result is that GPS time and TAI are not separated by exactly 19 seconds. However, except for those who need milliarcsecond pointing accuracy, treating GPS time as being offset from TAI by exactly 19 seconds is more than good enough.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="6816" LastEditDate="2017-07-10T00:24:03.177" LastActivityDate="2017-07-10T00:24:03.177" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22245" PostTypeId="1" CreationDate="2017-07-09T13:48:41.453" Score="1" ViewCount="263" Body="&lt;p&gt;There is a science fiction idea called terraforming. Assuming that it is possible to change the orbit of Venus, is there an orbit that would give Venus the same climate as the Earth (i.e. the same level of radiation from the sun), but which would have a zero risk of collision?&lt;/p&gt;&#xA;" OwnerUserId="20334" LastActivityDate="2017-07-13T03:22:56.080" Title="A Safe Orbit for a Terraformed Venus?" Tags="|colonization|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22246" PostTypeId="1" CreationDate="2017-07-09T14:42:54.970" Score="1" ViewCount="99" Body="&lt;p&gt;After the Apollo 13 explosion, the service module was retained almost until re-entry. Why was this if it was just dead weight?&lt;/p&gt;&#xA;" OwnerUserId="20335" LastActivityDate="2017-07-09T14:42:54.970" Title="Why was the Service Module Retained?" Tags="|apollo-program|" AnswerCount="0" CommentCount="3" ClosedDate="2017-07-09T19:12:58.857" ContentLicense="CC BY-SA 3.0" />
  <row Id="22248" PostTypeId="1" CreationDate="2017-07-09T23:41:43.473" Score="2" ViewCount="533" Body="&lt;p&gt;Say I want to make a reverse hybrid rocket motor, that runs on a gaseous/liquid fuel which is passed by a solid oxidizer. Would KNO3 make a good oxidizer?  Or does this concept not work with Liquid fuels?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="18458" LastEditDate="2017-10-17T02:39:15.040" LastActivityDate="2017-10-17T02:43:12.363" Title="KNO3 as reverse hybrid oxidizer?" Tags="|rockets|propulsion|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22249" PostTypeId="2" ParentId="22146" CreationDate="2017-07-10T00:42:04.000" Score="4" Body="&lt;p&gt;First off a definition of a low energy transfer orbit;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A low energy transfer orbit is...&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;An orbit that allows transfers between two central bodies (e.g. earth - moon).&lt;/li&gt;&#xA;&lt;li&gt;An orbit that does so using energy from a source other than the spacecraft (most commonly perturbations due to the sun).&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;A high energy transfer orbit basically doesn't satisfy point #2 above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason the these types of orbits are typically discussed in reference to transfers between two central bodies is due to the requirement of the second central body to &lt;em&gt;capture&lt;/em&gt; the satellite. This is different to a simple change in semi-major axis around a single body as the changes due to perturbations typically act over a much, much longer period of time (think years+ for significant differences in most cases*). When there is a small perturbation as you're approaching a second body things get a little more complicated. This might help:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.xkcd.com/681/&quot; rel=&quot;nofollow noreferrer&quot;&gt;XKCD Gravity Wells&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FCbgL.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FCbgL.png&quot; alt=&quot;XKCD Gravity Wells&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at gravity wells, you can see there are peaks between all objects. If you imagine your satellite as a marble, you could roll up a peak with a given energy &lt;em&gt;X&lt;/em&gt; but not quite make it over the peak - you would fall back toward the planet you left. But if there was a small amount more energy, let's call it &lt;em&gt;w&lt;/em&gt;, then you might just make it over the peak.once you're over the peak you fall into the gravity well of the next body. Now if we say we need 100 energy (let's forget about units for now) to get to from body 1 to body 2, clearly &lt;em&gt;X&lt;/em&gt; was less than 100. Maybe it was 99, maybe it was 95, but either way it wasn't enough. &lt;em&gt;w&lt;/em&gt; however was tiny, maybe ~1% of &lt;em&gt;X&lt;/em&gt;... so roughly 1 energy. Luckily 99+1=100 so we made it. That is a stripped down version of how low energy transfers work - it gets more complicated when you're talking about 3 body systems (earth - moon transfer in a sun system for example), since the specifics of the orbit need to play to the strengths of solar perturbations, but you get the idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Hohmann transfer orbit is actually a high energy transfer orbit, in that it is higher energy then the low energy transfer orbits. It is the most energy efficient transfer orbit &lt;strong&gt;if&lt;/strong&gt; you ignore perturbations and outside sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For further reading (and as a reference to this answer) I would suggest: &lt;a href=&quot;https://www.google.co.uk/url?sa=t&amp;amp;source=web&amp;amp;rct=j&amp;amp;url=http://www.gg.caltech.edu/~mwl/publications/papers/lowEnergy.pdf&amp;amp;ved=0ahUKEwi74JzNvP3UAhWlJcAKHX43ClgQFgg9MAM&amp;amp;usg=AFQjCNGM-ugxWlYSB795dq47Vz2ACl6J-A&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low Energy Transfer to the Moon&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*This can depend on what types or changes you're looking for. Drag will be exponentially more significant the lower your perigee altitude; solar radiation pressure is less altitude dependant etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT #1&lt;/strong&gt;&#xA;Named transfer orbits are hard to come by. Hohmann transfers are reasonably low energy &lt;strong&gt;but&lt;/strong&gt; in this context they are considered high energy - to be clear here;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any transfer orbit that doesn't benefit from energy from external sources (e.g. solar perturbations, etc.) is considered high energy. A Hohmann transfer is simply the lowest energy of the high energy transfers (best of the worst).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Transfers that include areobreaking could also be considered low energy, as you gain the thrust to retard the spacecraft motion from a source other than the spacecraft (drag) but practically speaking you can't reliably areobreak into a stable orbit (once you start areobreaking, you're on a collision course with the planet one way or another unless you do another burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't heard of this being done, but in theory I would imagine you could use a gravity gradient stabilisation system to reduce the energy required. The oscillation due to the stabilisation would provide a source of energy assuming you moving toward/away from your central body. This is important as a change in inclination (ignoring zonal harmonic effects) would not cause there to be a change in the stabilisation oscillation.&lt;/p&gt;&#xA;" OwnerUserId="17507" LastEditorUserId="17507" LastEditDate="2017-07-11T09:46:39.103" LastActivityDate="2017-07-11T09:46:39.103" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22250" PostTypeId="2" ParentId="22237" CreationDate="2017-07-10T03:45:33.387" Score="2" Body="&lt;p&gt;I suspect that one of the reasons is for the reference frame synchronization. In fact, if doing statistical orbital determination, even a one error second could throw off your state estimation by a significant amount for a short period of time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any deviation larger than a few seconds will have significant effects on simulations too. For example, I am currently working on a project where switching to the True of Date frame from the J2000 corrected significant orbital drift errors when all other variables of the high fidelity simulation were kept identical. &lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-07-10T03:45:33.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22251" PostTypeId="2" ParentId="6523" CreationDate="2017-07-10T04:55:19.527" Score="2" Body="&lt;p&gt;This will heavily depend on the re-entry speed. The typical solutions of fluid mechanics available to us via the Navier-Stokes equation are only applicable in the continuum flow regime (where the molecules of gas are dense enough such that their mean free path is quite short, i.e. they collide with each other rather than just ping off into the distance).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can compute whether the flow around a spacecraft is in the continuum flow regime by the Knudsen number:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$Kn = \lambda / L  = Ma / Re \sqrt{\gamma\pi / 2} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where lambda is the mean free path and L is a characteristic length scale (length of your spacecraft, or your fin).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively it can be defined in terms of the Mach number, the Reynolds number, and the ratio of specific heats. So, you've got to know how fast they are entering the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When $Kn$ is greater than 10, it is safe to say you're now in the land of rarefied gas dynamics. Each molecule must now be treated individually, with its own speed, energy, temperature, etc. instead of as a continuous mass of fluid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this mean that compressible gas effects go away when your gas is rarefied? No, it doesn't. Using DSMC methods, this research paper (on the characteristics of a Brazilian satellite during re-entry) shows that there's still a significant drag coefficient when you're in rarefied flow: &lt;a href=&quot;http://www.scielo.br/scielo.php?script=sci_arttext&amp;amp;pid=S0103-97332003000200044&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.scielo.br/scielo.php?script=sci_arttext&amp;amp;pid=S0103-97332003000200044&lt;/a&gt; (Hypersonic flow of rarefied gas near the Brazilian satellite during its reentry into atmosphere, Felix Sharipov). Shock waves are still quite possible but they have a much smaller standoff distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hate to be glib, but the question is a difficult one. When we say &quot;non-issue&quot;, what do we mean? Can we quantify the strength of the shock that will form at a Mach number close to 1? Can we say what drag force the shock will cause, and whether this matters as to the controllability of the vehicle? The altitude at which rarefied flow exists will also change depending on the atmospheric density - and that can vary with the energy output of the sun, getting you into the whole category of space weather. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the sort of question that good instrumentation around your spacecraft helps to answer, as well as thoughtful analysis and simulation - and it's still hard then. You'd need the CAD of the vehicle, the flight profile, a powerful particle solver, a good computer cluster, and a month or two of your time to get a reasonably good answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may also just be that the vehicle quickly passes through transonic to subsonic (where the grid fins again will behave predictably). If you're not in that regime for too long, you can perhaps just ride out the transient effects of the shock - like your car tires slipping on ice for just a fraction of a second. This is hard to know without seeing the flight profile with Mach numbers vs. time (and I'd note that the speed at which the vehicle is moving is will not necessarily give you a correct idea of the Mach number, which is influenced by temperature).&lt;/p&gt;&#xA;" OwnerUserId="794" LastActivityDate="2017-07-10T04:55:19.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22252" PostTypeId="2" ParentId="6523" CreationDate="2017-07-10T06:59:17.437" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;given their potential to create drag at hyper-sonic speeds, thus possibly lowering requirements for heat shielding &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 booster does not use a heat shield. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Grid fins are designed to be in the Q zone AFTER that which require a heat shield. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the titanium used by SpaceX for the grid fins melt at 1650°C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A Soyuz heat shield encounter temperatures of up to 3000°C. (Close to the &lt;strong&gt;Ebullition&lt;/strong&gt; point of titanium).&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="25" LastEditDate="2017-12-19T13:19:45.167" LastActivityDate="2017-12-19T13:19:45.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22253" PostTypeId="1" CreationDate="2017-07-10T09:21:17.297" Score="2" ViewCount="255" Body="&lt;p&gt;I'm curious if there is a statistically demonstrable &lt;em&gt;tendency not to&lt;/em&gt; schedule launches on certain days, such as those that contain leap seconds (see answers below &lt;a href=&quot;https://space.stackexchange.com/q/22237/12102&quot;&gt;this question&lt;/a&gt;), &lt;a href=&quot;https://space.stackexchange.com/questions/22237/how-do-launches-avoid-leap-seconds-why/22250#comment62770_22238&quot;&gt;year changes&lt;/a&gt;, or superstitious connotations like &lt;a href=&quot;https://en.wikipedia.org/wiki/Friday_the_13th&quot; rel=&quot;nofollow noreferrer&quot;&gt;Friday the thirteenth&lt;/a&gt; for example.&lt;/p&gt;&#xA;&lt;p&gt;The three sources where I believe I can find or from where I can at least assemble a fairly complete list of orbital launch attempts are:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.crummy.com/software/BeautifulSoup/bs4/doc/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scrape&lt;/a&gt; the &lt;a href=&quot;http://www.spacelaunchreport.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Launch Report&lt;/a&gt; site, (found in &lt;a href=&quot;https://space.stackexchange.com/a/8737/12102&quot;&gt;this answer&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;li&gt;Download the complete &lt;a href=&quot;https://celestrak.org/satcat/search.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;SatCat from Celestrak&lt;/a&gt; and then do some sorting/filtering.&lt;/li&gt;&#xA;&lt;li&gt;Go to the &lt;a href=&quot;http://planet4589.org/space/lvdb/&quot; rel=&quot;nofollow noreferrer&quot;&gt;JSR Launch Vehicle Database&lt;/a&gt; as mentioned &lt;a href=&quot;https://space.stackexchange.com/a/21168/12102&quot;&gt;here&lt;/a&gt;, and download potential orbital launches and launch attempts in lists &lt;a href=&quot;http://planet4589.org/space/lvdb/lis/G.lis&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;G&lt;/strong&gt;&lt;/a&gt; (Harvard 1957-62), &lt;a href=&quot;http://planet4589.org/space/lvdb/lis/O.lis&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;O&lt;/strong&gt;&lt;/a&gt; (COSPAR 1963-), &lt;a href=&quot;http://planet4589.org/space/lvdb/lis/U.lis&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;U&lt;/strong&gt;&lt;/a&gt; (Uncataloged (sic)), &lt;a href=&quot;http://planet4589.org/space/lvdb/lis/F.lis&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;F&lt;/strong&gt;&lt;/a&gt; (Orbital Failures), and &lt;a href=&quot;http://planet4589.org/space/lvdb/lis/E.lis&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;E&lt;/strong&gt;&lt;/a&gt;  (Orbital pre-launch Explosions) all found in the list of lists on &lt;a href=&quot;http://planet4589.org/space/lvdb/list2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this page&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;But even if I did &lt;em&gt;all three&lt;/em&gt; of these, then filtered and merged, I would be missing most/all scrubbed launches; launches that were indeed scheduled, but didn't happen and were rescheduled for the next launch window.&lt;/p&gt;&#xA;&lt;p&gt;Are there any, even partial lists that would include scrubbed launches for reasons like faulty non-critical sensor data, weather, downrange interference (e.g. &lt;a href=&quot;https://space.stackexchange.com/q/21777/12102&quot;&gt;fishing contests&lt;/a&gt;) etc? Even if not exhaustive, it may still be helpful and informative.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2017-08-19T03:56:25.303" Title="Databases of &quot;scrubbed&quot; launch attempts?" Tags="|launch|failure|launch-sequence|planning|" AnswerCount="0" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="22254" PostTypeId="2" ParentId="22146" CreationDate="2017-07-10T10:53:51.563" Score="0" Body="&lt;p&gt;A High Energy Transfer Orbit is one where minimizing time to complete the maneuver is less important than economzing the propellant burn, hence it has a higher energy consumption than would a Low Energy Transfer Orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As was mentioned in the discussion of the question you cited, HETOs were often used in the Shuttle program to maximize utilization of the manned space flight.  Unlike LETOs which tend to be studied and tuned to reach an optimal solution, often by a particular researcher who they are then named for, HETOs are likely to be one-off calculations for a particular circumstance, and therefore shouldn't be expected to be named.  Obviously there are exceptions to this rule, e.g., a direct intercept ballistic collision course, but generally a &quot;let's get there as quickly as we can today&quot; orbit isn't going to be something that has a name tomorrow.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastActivityDate="2017-07-10T10:53:51.563" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22256" PostTypeId="1" AcceptedAnswerId="22622" CreationDate="2017-07-10T22:12:26.807" Score="5" ViewCount="228" Body="&lt;p&gt;Martial artists who do thing like Muay Thai, kickboxing, and many others strengthen their bones by hitting them in various ways. Generally by kicking a hard object like a tree, by vigorously rolling something hard like a bottle over their shins, or by standing still while someone else whacks them with a stick. You get the idea. The strikes cause microfractures and the bones respond by healing stronger than they were originally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has NASA (or any other space agency) ever tried something similar? What were the results of the tests? If they've never tried anything along these lines, why not?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ETA: Also, the experiments need not have been solely whacking someone with a stick. Any high tech method of causing microfractures to bones would count. Or, really, I'm interested in any tested interventions that wasn't either chemical/supplement based or exercise based.&lt;/p&gt;&#xA;" OwnerUserId="11719" LastEditorUserId="11719" LastEditDate="2017-07-12T16:41:26.450" LastActivityDate="2017-08-11T22:34:34.773" Title="Has NASA ever tested the effects of martial training to alleviate spaceflight osteopenia?" Tags="|microgravity|health|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22257" PostTypeId="1" AcceptedAnswerId="22258" CreationDate="2017-07-11T05:26:46.700" Score="9" ViewCount="1426" Body="&lt;p&gt;Should there be a visible steam plume from the Apollo astronauts sublimation cooler in the lunar EVA videos?&lt;/p&gt;&#xA;" OwnerUserId="20346" LastEditorUserId="6816" LastEditDate="2017-07-11T06:10:42.303" LastActivityDate="2017-07-11T14:40:24.827" Title="Apollo Suit sublimation coolers" Tags="|apollo-program|spacesuits|eva|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22258" PostTypeId="2" ParentId="22257" CreationDate="2017-07-11T07:47:39.620" Score="18" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Liquid_Cooling_and_Ventilation_Garment&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia says&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As an example, during the Apollo 12 commander's first EVA (of 3 hrs, 44 minutes), 4.75 lb of feedwater were sublimated, and this dissipated 894.4 Btu / hr.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That's 2.154 kg of water over 224 minutes, or 10g/min, or 0.16g/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Water vapor is invisible to the naked eye (&lt;a href=&quot;http://vsgc.odu.edu/src/Conf09/UnderGrad%20Papers/Ojo%20-%20Paper.pdf&quot; rel=&quot;noreferrer&quot;&gt;2&lt;/a&gt;, &lt;a href=&quot;https://physics.stackexchange.com/questions/303804/how-are-water-vapors-not-visible&quot;&gt;3&lt;/a&gt;). What you see e.g. when boiling water is not the water vapor itself, it's small droplets of liquid water (fog, mist, steam) which are suspended in the air and carried by the air flow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With only so little mass escaping the suits, those droplets do not form in any substantial amount, and there is no air that could carry them around. Thus, there is nothing to see.&lt;/p&gt;&#xA;" OwnerUserId="13867" LastEditorUserId="6816" LastEditDate="2017-07-11T14:40:24.827" LastActivityDate="2017-07-11T14:40:24.827" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="22259" PostTypeId="1" CreationDate="2017-07-11T14:11:05.050" Score="0" ViewCount="185" Body="&lt;p&gt;There are many sites around the world apart from USA, that have such great locations to launch from. Saving hundreds of thousands of dollars in fuel costs.&#xA;Can someone assist me with a model that proves the effect of a geographical location such as higher field elevation above sea level? Or point me in the right direction&lt;/p&gt;&#xA;" OwnerUserId="20348" LastActivityDate="2017-07-11T14:11:05.050" Title="What is the effect of launching from higher altitude on fuel burn and rocket performance?" Tags="|launch|rockets|performance|" AnswerCount="0" CommentCount="3" ClosedDate="2017-07-11T18:02:21.367" ContentLicense="CC BY-SA 3.0" />
  <row Id="22260" PostTypeId="1" AcceptedAnswerId="22261" CreationDate="2017-07-11T15:05:06.693" Score="7" ViewCount="2981" Body="&lt;p&gt;I'll accept the largest defensible number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;LEGO Building At KSC&quot;. Click image for full size - go ahead! From &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:LEGO_Building_At_KSC.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rv4qf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4PIb7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6816" LastEditDate="2017-07-12T02:58:17.503" LastActivityDate="2017-07-12T02:58:17.503" Title="PUZZLER: Around how many solar system bodies are there Legos in orbit?" Tags="|orbit|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22261" PostTypeId="2" ParentId="22260" CreationDate="2017-07-11T15:20:26.497" Score="23" Body="&lt;p&gt;&lt;strong&gt;Three Bodies&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;Earth&lt;/strong&gt;: Legos have made their way to the space station multiple times, and it doesn't appear as though they have left the station.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Jupiter&lt;/strong&gt;: Lego figures were sent aboard the Juno spacecraft, and are currently orbiting Jupiter.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Sun&lt;/strong&gt;: All legos in existence are currently orbiting the Sun, as both the Earth and Jupiter do :)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I do not believe they have ever orbited the moon, and I can't see any information for them being anywhere else in the solar system. &lt;a href=&quot;http://www.popsci.com/lego-figurines-on-juno-are-latest-in-long-line-bricks-in-space&quot; rel=&quot;noreferrer&quot;&gt;This&lt;/a&gt; is my source.&lt;/p&gt;&#xA;" OwnerUserId="19427" LastEditorUserId="19427" LastEditDate="2017-07-11T16:17:13.250" LastActivityDate="2017-07-11T16:17:13.250" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="22262" PostTypeId="2" ParentId="22245" CreationDate="2017-07-11T15:24:38.717" Score="3" Body="&lt;p&gt;Our planet is right in the middle of the sweet spot or Goldilocks zone for orbital distance from the sun for habitable temperature.  While the orbit of Venus is too close to the sun, the distance from the sun is not the main reason for the extreme heat.  In fact, by studying Venus, scientists were able to better gauge the greenhouse affect of CO2 and thus extrapolate what could happen on earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://answers.yahoo.com/question/index?qid=20101007094209AAmddCT&quot; rel=&quot;nofollow noreferrer&quot;&gt;The formula&lt;/a&gt; for approximating the mean surface temperature with albedo factor (surface reflectivity), greenhouse factor, and orbital distance included would be &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T_p = T_s * \sqrt{R_s / 2R_o} \times (1 – A)^{0.25} \times (1 + G)^{0.25}$$&#xA;where&lt;br&gt;&#xA;$T_p$ is the mean surface temperature of the planet&lt;br&gt;&#xA;$T_s$ is the temperature of the Sun (approx. 6000 K)&lt;br&gt;&#xA;$R_s$ is the radius of the Sun (taken as 700,000 km)&lt;br&gt;&#xA;$R_o$ is the radius of the planet's orbit (in km)&lt;br&gt;&#xA;$A$ is albedo factor (0.76 for Venus), and&lt;br&gt;&#xA;$G$ is the greenhouse factor (120 for Venus)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia, the mean surface temperature on the earth is 15 deg C (59 deg F, 288 deg K).&lt;br&gt;&#xA;So if we re-arrange the above equation to figure out what $R_o$ needs to be to obtain a $T_p$ of 288 deg K, we have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$288 = 6000 \times \sqrt{350000 / R_o} \times 0.7 \times 3.317$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$R_o = 818,978,589 \text{ km or } 5.4745 \text{ AU}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put things in perspective Mars is at 1.52 AU and Jupiter goes between 4.94 AU at perihelion and 5.454 at aphelion.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we are talking just outside Jupiter's orbit, and too close to avoid collisions with its moons or diversion by its gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we revise the above calculation to not include the greenhouse and albedo factors we would have &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T_p = T_s \times\sqrt{R_s / 2R_o}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and solving for $R_o$ would give us &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$288 = 6000 \times\sqrt{350000 / R_o}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$R_o$ = 151,909,722 km or 1.015 AU, right where Earth is!!!!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would rather put it at about 2.5 to 3.0 AUs in the middle of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Asteroid_belt&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kirkwood gap&lt;/a&gt; in the asteroid belt.  Venus would still be too hot for us, but can possibly be terraformed by hitting it with a comet of water ice and then introducing specially engineered phytoplankton that would bloom into the water-filled and $CO_2$-dense planet.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The planet would turn green, and maybe in a few million years the $CO_2$ would be converted to Oxygen as on earth, the greenhouse factor would be much lower, so the surface temperature would lower to something more inhabitable. We would be able to tune the greenhouse effect to get the ideal temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my humble opinion, rather than trying to shift the orbit of Venus, it would be much easier relatively speaking to screen the planet from most of the sun's energy either by putting into the shadow of a large adjustable screen at ideal location between it and the Sun, and/or by putting the planet into permanent winter mode by sending an asteroid or comet into it causing a highly obscuring dust cloud in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can also focus on reforming an earth sized planet in the asteroid belt by clustering the asteroids back together. Then sending in a few comets and other objects from the Kuiper belt.  We can custom engineer the planet from core up.&lt;/p&gt;&#xA;" OwnerUserId="19730" LastEditorUserId="6816" LastEditDate="2017-07-13T03:22:56.080" LastActivityDate="2017-07-13T03:22:56.080" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22263" PostTypeId="2" ParentId="20896" CreationDate="2017-07-11T15:28:43.020" Score="1" Body="&lt;p&gt;It slows down at 70-40 km in altitude before the atmosphere pressure increased.  As it descends the atmospheric density will increase and heating will increase if speed is unchanged.  See &lt;a href=&quot;http://www.npl.co.uk/reference/faqs/why-does-air-density-and-pressure-change-with-altitude-(faq-mass-and-density)&quot; rel=&quot;nofollow noreferrer&quot;&gt;air density vs. altitude&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20349" LastActivityDate="2017-07-11T15:28:43.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22264" PostTypeId="1" CreationDate="2017-07-11T17:02:25.170" Score="1" ViewCount="71" Body="&lt;p&gt;Cassini's &lt;a href=&quot;https://saturn.jpl.nasa.gov/mission/grand-finale/overview/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Grand Finale&lt;/a&gt; is underway. What special observations will be taken during this time that were not possible before the Grand Finale?&lt;/p&gt;&#xA;" OwnerUserId="11264" LastActivityDate="2017-07-11T17:02:25.170" Title="What observations will Cassini take during its &quot;Grand Finale&quot;?" Tags="|saturn|cassini|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22266" PostTypeId="1" CreationDate="2017-07-11T17:29:59.960" Score="6" ViewCount="2118" Body="&lt;p&gt;As a continuation of my previous &lt;a href=&quot;https://space.stackexchange.com/questions/22150/brouwer-lyddane-mean-semi-major-axis-bias&quot;&gt;post&lt;/a&gt; regarding the use of the software &lt;a href=&quot;https://en.wikipedia.org/wiki/General_Mission_Analysis_Tool&quot; rel=&quot;nofollow noreferrer&quot;&gt;GMAT&lt;/a&gt; (General Mission Analysis Tool) (&lt;a href=&quot;http://gmatcentral.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;website&lt;/a&gt;, &lt;a href=&quot;https://www.youtube.com/channel/UCt-REODJNr2mB3t-xH6kbjg/videos&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube&lt;/a&gt;) for propagating spacecraft, I have an additional question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This time I propagated 2 spacecraft in a $J_2$ gravity field (gravity degree 2 and gravity order 0, no higher gravitational harmonics, no drag, no sun radiation pressure, no third bodies). Both spacecraft have the same initial mean orbital elements:&#xA;$$[\bar{a}_0,\bar{e}_0,\bar{i}_0,\bar{\Omega}_0,\bar{\omega}_0,\bar{M}_0] = [7000\text{ km}, 0, 45 \text{ deg}, 0, 0, 0]$$&#xA;The only difference between the spacecraft is their epoch. The first spacecraft epoch is $$1/1/2005 \quad \quad 00:00:00$$&#xA;The second spacecraft epoch is  $$1/1/2015 \quad \quad 00:00:00$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, I plotted their osculating inclinations in the J2000 frame (The regular Earth Centered Inertial (ECI) frame):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JAlyM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JAlyM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $J_2$ induced long-period inclination oscillation (period of about 70 days) does not surprise me. What is surprising is that &lt;strong&gt;the year governs the oscillation amplitude&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two points:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;To the best of my knowledge, the utilized gravity field is axisymmetric and time-constant, so no time-varying perturbation should occur.&lt;/li&gt;&#xA;&lt;li&gt;In order to verify this result, I also used &lt;a href=&quot;https://www.agi.com/products/engineering-tools&quot; rel=&quot;nofollow noreferrer&quot;&gt;STK&lt;/a&gt; with the same scenarios and got the same results.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I think that this phenomenon (year governing the oscillation amplitude) stems perhaps from precession and nutation effects. What do you think? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update:&#xA;Here is an output from &lt;a href=&quot;https://www.agi.com/products/engineering-tools&quot; rel=&quot;nofollow noreferrer&quot;&gt;STK&lt;/a&gt; where I defined some initial conditions with the aforementioned force model:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/VEmfw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VEmfw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plots depict the osculating inclination as a function of time.&#xA;We can clearly see that on long time periods, the oscillation amplitude varies.&#xA;Can anyone explain this dependency?&lt;/p&gt;&#xA;" OwnerUserId="20246" LastEditorUserId="20246" LastEditDate="2017-07-12T13:47:19.197" LastActivityDate="2017-07-14T16:31:49.090" Title="J2 long-period perturbations in the inclination" Tags="|orbital-mechanics|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22267" PostTypeId="1" CreationDate="2017-07-11T18:53:29.643" Score="2" ViewCount="119" Body="&lt;p&gt;If not for the earth's magnetic field our atmosphere would be bombarded by high energy plasma (ionized gas) from the sun and other space sources. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much of these particles have &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_ray&quot; rel=&quot;nofollow noreferrer&quot;&gt;more energy&lt;/a&gt; than the protons accelerated in the large hadron collider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At very high altitudes and orbital altitudes this protection is less and when space travel takes astronauts out of range of earth's magnetic field there is no shielding or protection.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does the International Space Station or other craft have an artificial magnetic field to protect its astronauts? If not are there plans for implementation?&lt;/p&gt;&#xA;" OwnerUserId="19730" LastActivityDate="2017-07-11T18:53:29.643" Title="Are artificial magnetic fields used or proposed to be used to shield astronauts from charged, high energy radiation?" Tags="|magnetic-field|shielding|cosmic-radiation|magnetosphere|" AnswerCount="0" CommentCount="6" ClosedDate="2017-07-12T16:11:22.950" ContentLicense="CC BY-SA 3.0" />
  <row Id="22268" PostTypeId="2" ParentId="22245" CreationDate="2017-07-12T00:40:31.707" Score="1" Body="&lt;p&gt;Not in any reasonable way, if you want to have the same radiation from the sun then it has to be the same orbit.  An L3 or L4 position would keep you in the same orbit but would not be stable.  The exact opposite position from Earth in Earth's orbit is possible and would likely remain safe for a long time in human terms, say for thousands of years or more but would not be stable for long geologically ie millions of years.  So someone would need to readjust it back to its original position or it would very much risk collision or slinging both planets into new and likely unfavorable orbits.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shading Venus in its current orbit is the easiest and cheapest way to get it cooled down and begin the terraforming process.  (Unless you don't mind blasting off huge amounts of atmosphere, and some of the surface, with asteroids which would likely be even cheaper still.)  &lt;/p&gt;&#xA;" OwnerUserId="20287" LastActivityDate="2017-07-12T00:40:31.707" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22269" PostTypeId="1" CreationDate="2017-07-12T17:01:22.973" Score="1" ViewCount="79" Body="&lt;p&gt;I was wondering how the electric field in an electrospray thruster works to ionize particles and propel a micro spacecraft. I was thinking of possible circuits but I wasn't sure what it could look like. &#xA;I am familiar with the design of the needles and the fuel container but I don't know how the extractor electrode fits in.&lt;/p&gt;&#xA;" OwnerUserId="20359" LastEditorUserId="12102" LastEditDate="2021-04-22T04:19:19.213" LastActivityDate="2021-04-22T04:19:19.213" Title="Generating an electric field for a colloid thruster" Tags="|propulsion|engines|ion-thruster|electric-propulsion|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22270" PostTypeId="1" CreationDate="2017-07-12T18:17:59.910" Score="0" ViewCount="387" Body="&lt;p&gt;Let me try ask it this way. How efficient is current power generation thermodynamic cycles in space? Is it even used? My info is that most spacecraft use PV solar panels, is the heat turned to energy via a standard Rankine cycle?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If engineers developed a thermodynamic heat recovery cycle that's 92% efficient at converting heat to energy / electricity, would it therefore be better than current tech and have applications in space exploration and for example the international space station? &lt;/p&gt;&#xA;" OwnerUserId="20351" LastEditorUserId="6816" LastEditDate="2017-07-13T03:05:22.500" LastActivityDate="2017-07-13T19:26:49.417" Title="heat recovery thermodynamics in space" Tags="|power|heat|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22271" PostTypeId="2" ParentId="22270" CreationDate="2017-07-12T18:29:29.307" Score="2" Body="&lt;p&gt;Most spacecraft I know of use PV panels (so direct conversion of sunlight into electricity). Waste heat is not recovered, but rejected using radiators. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The biggest use of heat recovery I know of is in RTGs, where the heat from Pu-238 decay is converted to electricity using &lt;a href=&quot;https://en.wikipedia.org/wiki/Multi-Mission_Radioisotope_Thermoelectric_Generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;thermocouples&lt;/a&gt;. These are inefficient (less than 10% of the heat energy is converted to electricity). At one point, NASA had a project for an RTG that used a &lt;a href=&quot;https://tec.grc.nasa.gov/rps/stirling-research-lab/advanced-stirling-convertor/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stirling engine&lt;/a&gt; to drive a generator, with an efficiency in the region of 30%. The &lt;a href=&quot;https://futureplanets.blogspot.nl/2013/12/the-asrg-cancellation-in-context.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ASRG&lt;/a&gt; project was cancelled as a cost-reduction measure, though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-07-13T19:26:49.417" LastActivityDate="2017-07-13T19:26:49.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22272" PostTypeId="2" ParentId="22256" CreationDate="2017-07-12T18:34:28.877" Score="1" Body="&lt;p&gt;A quick &lt;a href=&quot;https://ntrs.nasa.gov/?N=0&amp;amp;Ntk=All&amp;amp;Ntt=%22martial%20arts%22&amp;amp;Ntx=mode%20matchallpartial&quot; rel=&quot;nofollow noreferrer&quot;&gt;NTRS search&lt;/a&gt; yields no results, so unlikely NASA's studied it seriously. The current training regimen uses weight training, which also strengthens the bones and seems to me to carry a lower risk of training injuries. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-07-12T18:34:28.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22273" PostTypeId="1" CreationDate="2017-07-12T23:06:32.807" Score="5" ViewCount="853" Body="&lt;p&gt;I haven't seen much about &lt;a href=&quot;https://en.wikipedia.org/wiki/Colloid_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;colloidal or electrospray thrusters&lt;/a&gt; besides that Wikipedia article and a few questions here. The idea is similar to ion thrusters in that propellant mass is broken into small bits, ionized, and accelerated electrostatically. The difference is in the nature of the small bits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ion thrusters can achieve high ionization efficiencies, most propellant atoms that leave are successfully ionized and accelerated. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However If I understand correctly, a colloidal thruster only breaks the propellant into tiny droplets and ionized and accelerates those. The maximum charge you can put on a tiny droplet or solid particle is only a small fraction of the number of electrons because the Coulomb force is so great. I don't know exactly, but perhaps on the order of one charge per one thousand atoms give or take a power of ten, though it will depend on the droplet's or particle's size and nature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With such a low ionization efficiency, would colloidal thrusters have any advantage over ion thrusters? Simplicity perhaps? Lower weight? Or is my understanding of the technology flawed? Perhaps electrospray thrusters and colloidal thrusters not the same thing?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Related questions: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/5838/12102&quot;&gt;Relationship between part dimensions and performance of ion electrospray thruster&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/q/22269/12102&quot;&gt;Generating an electric field for a colloid thruster&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;NASA News: &lt;a href=&quot;https://www.nasa.gov/feature/mit-spl-delivers-the-scalable-ion-electrospray-propulsion-system-s-ieps-for-cubesats-to-nasa&quot; rel=&quot;nofollow noreferrer&quot;&gt;MIT SPL delivers the Scalable ion Electrospray Propulsion System (S-iEPS) for CubeSats to NASA&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-14T09:37:00.277" LastActivityDate="2021-04-22T04:20:45.113" Title="Colloidal or electrospray thrusters - any advantages over ion thrusters?" Tags="|ion-thruster|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22274" PostTypeId="1" CreationDate="2017-07-12T23:53:31.630" Score="8" ViewCount="557" Body="&lt;p&gt;Can the ISS achieve any lift by using it's solar panels as 'wings' against the sparse atmosphere?  My understand is they started moving the solar panels into a 'low drag' orientation when on the shadowed side of Earth to save on the not insignificant drag the panels created.  But if you tilted them even slightly from horizontal would you get a slight lift?  &lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="12102" LastEditDate="2017-07-13T01:00:15.003" LastActivityDate="2017-07-15T12:05:16.840" Title="ISS achieving lift by using it's solar panels" Tags="|iss|orbital-maneuver|atmospheric-drag|" AnswerCount="5" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="22275" PostTypeId="2" ParentId="22274" CreationDate="2017-07-13T09:55:41.080" Score="12" Body="&lt;p&gt;I can see your reasoning, however it's not going to work. Aerodynamically the byproduct of lift is drag, meaning when you create lift you also create drag as well. An efficient wing creates less drag per unit of lift, but the drag is still there. A wing works because the pressure of the air above it lower than the pressure below, pushing the wing in the direction of the lower pressure (ok, that's a simplification), to do this with any efficiency wings must be a certain shape to create that pressure and to allow smooth airflow. Solar panels are not wings and will not have smooth airflow or a shape designed to produce lift (just the opposite), so if you tried to use the ISS solar panels to create aerodynamic lift you would create infinitesimal lift for the drag produced. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case lift is not what you want, even if it was for free. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2017-07-13T09:55:41.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22276" PostTypeId="2" ParentId="22274" CreationDate="2017-07-13T12:18:47.297" Score="4" Body="&lt;p&gt;No, but to some sense, the opposite can be done. If you want to lower the orbit of a satellite for some reason, you set the panels to be in the &quot;drag&quot; position, where they are perpendicular to the direction of motion as much as you can. If you want to keep the drag minimized, as the ISS does, you move the solar panels such that they are aligned with the direction of velocity, to lower your drag. Usually satellites only will do this when in eclipse mode, as they can't receive sunlight anyways during that period of time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-07-13T12:18:47.297" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22277" PostTypeId="1" CreationDate="2017-07-13T15:01:58.047" Score="0" ViewCount="220" Body="&lt;p&gt;Writing a near-future SF novel here. One character has to rescue another character from an enemy space shuttle (in orbit). Most hull breach scenarios sound really dangerous-- is there a method that would be non-catastrophic? Somehow containing the breach, or surrounding the shuttle with a giant bag before breaching it? &lt;/p&gt;&#xA;" OwnerUserId="20367" LastActivityDate="2017-07-13T19:15:34.837" Title="How could you safely break onto a space shuttle?" Tags="|spacecraft|" AnswerCount="1" CommentCount="7" ClosedDate="2017-07-16T05:45:54.660" ContentLicense="CC BY-SA 3.0" />
  <row Id="22278" PostTypeId="1" AcceptedAnswerId="22281" CreationDate="2017-07-13T16:49:46.983" Score="18" ViewCount="5633" Body="&lt;p&gt;On all of the launch videos and recordings for the Space Shuttle, the mention is made from Houston (after getting through Max-Q) of &quot;go at throttle up&quot;, indicating that the launch is ok for going back to full thrust.  Is there something that the astronauts on the shuttle needed to do manually at this point, or is this just courtesy information from the ground to let the on-board astronauts know what's going on (similar to other information from the ground later in the launch in regards to being able to make it to orbit if an engine fails, etc.)?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd always been under the impression that for launch that the astronauts were pretty much just along for the ride and computers controlled everything in regards to controls and actions until at least the main tank is released (assuming there are no issues).  So, I'm curious the purpose of this instruction from the ground.  &lt;/p&gt;&#xA;" OwnerUserId="16735" LastEditorUserId="195" LastEditDate="2017-07-13T20:33:03.663" LastActivityDate="2018-02-25T23:53:51.083" Title="Space Shuttle &quot;go at throttle up&quot;" Tags="|launch|space-shuttle|communication|" AnswerCount="2" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22279" PostTypeId="1" CreationDate="2017-07-13T18:29:24.950" Score="1" ViewCount="357" Body="&lt;p&gt;Ammonia can be used in the production of fertilizer, as a source for nitrogen in soil for plants, in this case for food.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Synthesis of ammonia from nitrogen&lt;/em&gt; can be done with the help of bacteria using the process of &lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrogen_fixation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nitrogen Fixation&lt;/a&gt; in a bioreactor, the classic &lt;a href=&quot;https://en.wikipedia.org/wiki/Haber_process&quot; rel=&quot;nofollow noreferrer&quot;&gt;Haber process&lt;/a&gt;, and by other &lt;a href=&quot;https://en.wikipedia.org/wiki/Nitrogen_fixation#Ambient_nitrogen_reduction&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ambient nitrogen reduction&lt;/a&gt; process based on catalysis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been any discussion or research into the synthesis of ammonia from nitrogen on Mars, possibly addressing some of the limitations of doing it there? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's understood that one obstacle would of course be the sourcing of the nitrogen to begin with.&lt;/p&gt;&#xA;" OwnerUserId="20368" LastEditorUserId="12102" LastEditDate="2021-08-09T01:33:03.140" LastActivityDate="2021-08-09T01:33:03.140" Title="Ammonia synthesis on Mars — has there been any discussion or research?" Tags="|mars|nasa|ammonia|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22280" PostTypeId="2" ParentId="22277" CreationDate="2017-07-13T19:15:34.837" Score="2" Body="&lt;p&gt;Depends on the Shuttle mission. If it's carrying an airlock, wait for someone to go out the hatch and then sneak in. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-07-13T19:15:34.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22281" PostTypeId="2" ParentId="22278" CreationDate="2017-07-13T19:51:48.430" Score="14" Body="&lt;p&gt;The only source I could find on the issue was a &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=1781.0&quot; rel=&quot;noreferrer&quot;&gt;forum&lt;/a&gt; post, which itself did not provide any references. It seems that there is nothing the astronauts actually need to do, it is just a status check between the crew and mission control.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“throttle up” is usually complete by about 56 (for station trajectories) seconds into the flight…the call made by Houston, “Go At Throttle Up” is not a command but rather a status check/comm check performed at that point…think of it as “you are go at (this point in time)”…The engines are already throttled back up and Houston has looked at all of the data an they like what they see.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The text is a little ways down, a search &quot;Go at throttle up&quot; in your browser should quickly pin point its location.&lt;/p&gt;&#xA;" OwnerUserId="20309" LastActivityDate="2017-07-13T19:51:48.430" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22283" PostTypeId="1" AcceptedAnswerId="22285" CreationDate="2017-07-14T00:52:39.843" Score="7" ViewCount="338" Body="&lt;p&gt;&quot;The Eagle Has Landed: The Flight of Apollo 11&quot; is the title of an 18 minute NASA public relations film from 1969, available through the &lt;a href=&quot;https://catalog.archives.gov/id/45017&quot; rel=&quot;noreferrer&quot;&gt;National Archives Catalog&lt;/a&gt;. One section shows the view from the Eagle approaching the landing, accompanied by what appears to be genuine voice transmissions. However the timescale is apparently highly  compressed - here's an altitude loss of 16,800 feet in 25 seconds (transcript taken from &lt;a href=&quot;https://www.archives.gov/files/social-media/transcripts/transcript-eagle-has-landed-1969-45017.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; and agrees with my listening) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(video offset time 09:56)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ed Aldrin: &#xA;    Altitude now 21,000 feet.  Still looking very good.  Velocity down now to 1200 feet per second. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CAPCOM:&#xA;    You’re looking great to us Eagle. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Neil Armstrong: &#xA;    Give us a reading on the 1202 program alarm. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CAPCOM:&#xA;    Roger we got – we’re a Go on that alarm.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Ed Aldrin: &#xA;    Good radar data.  We’re now in the approach phase.  Everything looking good. Altitude 4,200... &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(video offset time 10:22)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not surprising that the landing is truncated to meet the needs of a short educational film, but some of the text such as &quot;we're now in the approach phase&quot; doesn't appear in the Apollo 11 transcripts I've found elsewhere, such as &lt;a href=&quot;http://apollo11.spacelog.org/page/04:04:35:26/&quot; rel=&quot;noreferrer&quot;&gt;http://apollo11.spacelog.org/page/04:04:35:26/&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's the provenance of this film - e.g. did NASA have the astronauts record additional dialog to &quot;make a long story short&quot;, or use recordings from training sessions, etc?&lt;/p&gt;&#xA;" OwnerUserId="14637" LastActivityDate="2017-07-14T03:07:35.880" Title="Provenance of the Apollo 11 landing presented in a NASA PR film" Tags="|apollo-11|historical-data|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22284" PostTypeId="2" ParentId="22274" CreationDate="2017-07-14T01:21:13.783" Score="7" Body="&lt;p&gt;GdD correctly answered the question you asked but I see a misunderstanding that needs correction:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lift is of no value to a spacecraft.  Spacecraft stay aloft because the outward &quot;force&quot; from their speed matches the downward pull of gravity.  In the time it takes the spacecraft to fall a foot it's moved forward enough that it's a foot farther away from the Earth.  The two movements cancel, the spacecraft stays up.  (Note that a spacecraft in a non-circular orbit does not have such a perfect balance and there's an eternal cycle of trading speed for height and the trading the height back for speed.  They stay up for the same reason, though.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose you push that spacecraft up, what happens?  It of course heads up--but as it heads up it loses speed climbing away from Earth's gravity.  Soon this loss of speed overcomes the shove you gave it, it ceases heading out and heads back in again.  Now it speeds up, eventually that overcomes the inwards velocity and it heads out again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the end effect of pushing the spacecraft up is to change it from a circular orbit to an elliptical one.  The higher you pushed it the lower it goes on the other side of the orbit--and the atmosphere isn't linear.  You have increased the drag on the spacecraft.  Your attempt to push it up (even if it didn't cost you speed due to drag) has actually accelerated the orbital decline, if you push hard enough you'll actually cause it to re-enter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice the only way to counteract orbital decay is to push the craft forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyone who is serious about learning about such things should play &lt;a href=&quot;https://kerbalspaceprogram.com/en/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kerbal Space Program&lt;/a&gt;.  While the orbital mechanics aren't perfect they're good enough to give a good grounding in how things will actually behave when you move them.  And, yes, I have seen the effect of burning outward sending you into the planet many a time, although I've always corrected the orbit before impact.  A burn straight out is the fastest, easiest (but not most fuel efficient!) way to get to something following you not too far behind in your orbit.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2017-07-15T03:57:02.957" LastActivityDate="2017-07-15T03:57:02.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22285" PostTypeId="2" ParentId="22283" CreationDate="2017-07-14T02:52:12.100" Score="9" Body="&lt;p&gt;The lines:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Altitude now 21 thousand feet. Still looking very good. Velocity down now to 12 hundred feet per second.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;We're now in the approach phase of it, looking good. Altitude 52 hundred feet&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;were spoken, verbatim, during the Apollo 11 landing, but not by Aldrin; according to the &lt;a href=&quot;https://www.jsc.nasa.gov/history/mission_trans/AS11_PAO.PDF&quot; rel=&quot;noreferrer&quot;&gt;PAO Mission Commentary transcript&lt;/a&gt; both were spoken by the NASA Public Affairs Office commentator.&lt;/p&gt;&#xA;&lt;p&gt;Interpolating from the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11.landing.html&quot; rel=&quot;noreferrer&quot;&gt;Apollo Flight Journal transcript&lt;/a&gt;, which doesn't include PAO, these calls must have been at about mission time 102:40 and 102:42.&lt;/p&gt;&#xA;&lt;p&gt;Combining the two sources, we get something like:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;102:39:48 Aldrin: AGS and PGNS look real close.&lt;/p&gt;&#xA;&lt;p&gt;102:39:50 Armstrong (onboard): Okay. (Pause) (Garbled) No flags. RCS is good. DPS is good. Pressure...Okay. [Cabin recording, not air-to-ground, so not in PAO transcript]&lt;/p&gt;&#xA;&lt;p&gt;[PAO &amp;quot;Altitude now 21 thousand feet. Still looking very good. Velocity down now to 12 hundred feet per second.&amp;quot;]&lt;/p&gt;&#xA;&lt;p&gt;102:40:08 Duke: At 7 minutes, you're looking great to us, Eagle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;and&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;102:41:51 Duke: Eagle, you're looking great. Coming up 9 minutes. (Pause)&lt;/p&gt;&#xA;&lt;p&gt;[PAO &amp;quot;We're now in the approach phase of it, looking good. Altitude 52 hundred feet&amp;quot;]&lt;/p&gt;&#xA;&lt;p&gt;102:42:05 Armstrong: Manual attitude control is good.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So that's a 16800 foot drop in roughly 120 seconds; the 1200 fps figure in the first call is mostly horizontal velocity.&lt;/p&gt;&#xA;&lt;p&gt;Armstrong's &amp;quot;give us a reading on the 1202&amp;quot; and Capcom's response are out of sequence in your transcript, coming at 102:38:42-102:38:53.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-07-14T03:07:35.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22286" PostTypeId="2" ParentId="22274" CreationDate="2017-07-14T07:58:36.120" Score="4" Body="&lt;p&gt;&lt;strong&gt;Short Answer&lt;/strong&gt;: No.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Long Answer&lt;/strong&gt;:&lt;br&gt;&#xA;Once you pass the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;nofollow noreferrer&quot;&gt;Karman-line&lt;/a&gt; air becomes so thin, that individual particles are not collisionally well-coupled to each other any more.&lt;br&gt;&#xA;One can also express this as the particle mean-free path $\lambda_{mfp}$ being greater than the mean particle distance $d$.&lt;br&gt;&#xA;Below the Karman-line you have approximately $\lambda_{mfp} \sim d$, which keeps all particles well-coupled to each other. This then allows to describe the gas as a fluid, which is where the notion of &lt;strong&gt;lift&lt;/strong&gt; comes from. Because a fluid can obey &lt;a href=&quot;https://en.wikipedia.org/wiki/Bernoulli%27s_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bernoulli's principle&lt;/a&gt;, lift can be generated.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However above the Karman-line the Bernoulli principle cannot act, because the gas there is not a fluid, or $\lambda_{mfp} &amp;gt; d$. This means &lt;strong&gt;lift in space doesn't exist&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drag, however is a more universal phenomenon, related to how entropy loves to eat up slowly all structure in the universe. This means that random collisions with particles will always nibble away your momentum, no matter how dense the surrounding medium is.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastActivityDate="2017-07-14T07:58:36.120" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22287" PostTypeId="2" ParentId="22273" CreationDate="2017-07-14T14:45:17.690" Score="5" Body="&lt;p&gt;One significant advantage, or disadvantage, depending on the way you look at it, is the minuscule thrust, way lower than in case of classic ion engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another advantage, is the electrospray process produces ionized droplets out of liquid without the need for fairly complex electromechanical plumbing that first releases xenon from a pressurized tank, then ionizes the stream of it. This allows both for miniaturization (no moving parts, reduced number of parts) and assures 100% of the spray is charged (and to a fairly constant charge) - it's the charging process of the liquid surface that causes the separation, overcoming the surface tension, and the threshold of separation is quite stable (per propellant), and rapid, fine control over the rate of the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as power consumption, similarly to ion engines, is proportional to specific impulse and mass flow, with mass flow this minuscule, it's quite moderate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means, that electrospray thrusters don't seem to be possible to scale up in performance like, say, VASIMR, nor will they allow near the same level of thrust, they are compact, they allow for very precise (and lossless) control of thrust (you just shut down the electrodes that cause the electrospray process and the droplets cease to separate), they don't put much of a strain on the craft's power source, and in effect they make wonderful RCS thrusters of extreme precision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, electrospray thrusters are not a very good idea for main propulsion - they could work with nanosatellites in a pinch (considering craft life time and mission duration makes them rather unattractive) they are THE option for station-keeping of ultra-precisely aligned constellations. &lt;a href=&quot;https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna&quot; rel=&quot;noreferrer&quot;&gt;eLISA&lt;/a&gt;, the gravitational wave observatory with 2.5 million km separation between the satellites, will maintain a couple millimeter, or smaller scale of positioning imprecision of the satellites (the remainder of required precision will be achieved through mechanical alignment of sensors withing the craft - more out of need to conserve the electrospray RCS fuel than out of need for better alignment precision using the RCS) but other proposed projects involve particle accelerators utilizing vacuum of space as medium of the &quot;corridor&quot;, or telescopes of extreme focal length, where separate lenses or mirrors are mounted on separate satellites aligned to the optical path.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, they are a completely different kind of animal than typical ion thrusters - they are the designated ultra-precise RCS thrusters, and trying to compare their performance to dedicated main engines is a misguided endeavor focusing on entirely wrong set of properties.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-07-14T14:45:17.690" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22288" PostTypeId="2" ParentId="22266" CreationDate="2017-07-14T15:11:20.263" Score="3" Body="&lt;p&gt;OK, I think I figured it out. In &lt;a href=&quot;http://www.sciencedirect.com/science/book/9780750685337&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacecraft Formation Flying p. 79&lt;/a&gt;, they write the equations of motion for the $J_2$ problem as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pkqj4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pkqj4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $X$,$Y$,$Z$ are in inertial frame ECI.&#xA;In &lt;a href=&quot;https://arc.aiaa.org/doi/book/10.2514/4.102400&quot; rel=&quot;nofollow noreferrer&quot;&gt;Analytical Mechanics of Space Systems pp. 379-390&lt;/a&gt; there is an expression for the axi-symmetric spherical harmonic gravity perturbation $J_2 - J_6$:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LCL3K.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LCL3K.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also here, $x$,$y$,$z$ are in inertial frame ECI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In these examples, the expression of the coordinates in ECI is an approximation. If I would have used these expressions (in many application they are used), I would not see the phenomenon discussed above,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because the gravity is related to the Earth and not to an arbitrary inertial coordinate frame, the coordinates should be expressed in ECEF. So, in these approximations, they assumed that $z_{ECEF} = z_{ECI}$. This is true only if you neglect nutation, precession, and polar motion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, an accurate expression for the gravity can be found in &lt;a href=&quot;http://www.springer.com/gp/book/9780387718316&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fundamentals of Astrodynamics and Applications, First Edition, pp. 496-497&lt;/a&gt;. They write the general spherical harmonic gravity perturbation as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uoPxn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uoPxn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $U$ is the gravity potential, $\lambda$ is the longitude, $\phi_{gc}$ is the geocentric latitude, and $r_I$,$r_J$,$r_K$ are expressed in ECEF.&#xA;Professional programs like STK and GMAT use the latter expression. So, once the user defines the orbital elements, they are transformed to ECI coordinates, then the gravity perturbations are calculated in ECEF and transformed to ECI, and the integration is in ECI.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, although I disabled all the perturbations but $J_2$, there is still precession, nutation, and polar motion, which changed slightly the ECEF attitude with respect to ECI. So, precession, nutation, and polar motion vary the gravity forces over time.&lt;/p&gt;&#xA;" OwnerUserId="20246" LastEditorUserId="20246" LastEditDate="2017-07-14T16:31:49.090" LastActivityDate="2017-07-14T16:31:49.090" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22289" PostTypeId="2" ParentId="8779" CreationDate="2017-07-14T19:37:48.733" Score="7" Body="&lt;p&gt;The stage and ship both navigate to a &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=36527.msg1361027#msg1361027&quot; rel=&quot;noreferrer&quot;&gt;preprogrammed GPS&lt;/a&gt; coordinate. It seems that GPS datum cannot be changed in-flight. Last-minute corrections are done via a radar on the stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;AOS by the drone ship is one-way communication (stage telemetry going to the ship). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I know, using a forum post as a source. It's corroborated by another &lt;a href=&quot;http://forum.nasaspaceflight.com/index.php?topic=36967.msg1650305#msg1650305&quot; rel=&quot;noreferrer&quot;&gt;post in the L2&lt;/a&gt; subscription-only section of NasaSpaceflight, by someone with good contacts in SpaceX) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The use of Radar was mentioned by Elon Musk in the &lt;a href=&quot;https://www.youtube.com/watch?v=SKXSHZUIFBo&quot; rel=&quot;noreferrer&quot;&gt;post flight interview for the Falcon Heavy Launch&lt;/a&gt;. He discusses that the returned side stages were slightly staggered to avoid radar interference between the two. There is likely some use of Radar altimeter or homing on final approach.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2018-02-07T08:36:49.697" LastActivityDate="2018-02-07T08:36:49.697" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22290" PostTypeId="1" CreationDate="2017-07-15T07:49:28.970" Score="15" ViewCount="857" Body="&lt;p&gt;Does atmospheric entry shake the craft or does the process force the craft to fly in a straight line without causing vibrations?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sci-fi movies and all have the craft shake very slightly. It would seem it would shake but why then?&lt;/p&gt;&#xA;" OwnerUserId="20377" LastActivityDate="2021-02-16T08:12:11.497" Title="Does atmospheric entry shake the craft?" Tags="|reentry|atmosphere|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22291" PostTypeId="2" ParentId="22274" CreationDate="2017-07-15T12:05:16.840" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;But if you tilted them even slightly from horizontal would you get a slight lift?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Lift is ultimate caused by directing the flow of oncoming air downward. However, even if a spacecraft does just that, it's not going to accomplish much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming Earth acts as a point mass, the specific energy (energy divided by mass) of a spacecraft orbiting the Earth is&#xA;$$E = \frac{v^2}2 - \frac{\mu_E}{r}$$&#xA;where $v$ is the magnitude of the spacecraft's velocity, $\mu_E$ is the Earth's gravitational parameter, and $r$ is the distance between the center of the Earth and the spacecraft's center of mass. Differentiating with respect to time results in&#xA;$$\dot E = \vec v \cdot \dot {\vec v} + \frac{\mu_E}{r^2}\dot r$$&#xA;Assuming the vehicle is subject only to gravitational and drag forces results in $\dot{\vec v} = \vec a_{\text{grav}} + \vec a_{\text{drag}}$, which in turn results in&#xA;$$\dot E = \vec v \cdot \vec a_{\text{drag}}$$&#xA;While the component of acceleration normal to the velocity vector can change the orbital eccentricity or the orbital plane, it has zero effect on the orbital energy. Only the component of velocity that is parallel or anti-parellel to the velocity vector changes the orbital energy, and in the case of drag, that component is always anti-parallel (directed against the velocity vector).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While tilting the arrays with respect to horizontal can increase lift, this has no effect on orbital energy because lift is orthogonal to the velocity vector. On the other hand, tilting the arrays with respect to horizontal markedly increases the cross section to drag. This in turn increases the component of drag acceleration that is anti-parallel to the velocity vector.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-07-15T12:05:16.840" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22294" PostTypeId="1" CreationDate="2017-07-15T19:14:29.840" Score="1" ViewCount="162" Body="&lt;p&gt;In our space missions, we use gravity assists by planets to change the speed relative to the sun. The speed relative to planets remains the same. &lt;a href=&quot;https://space.stackexchange.com/questions/829/could-the-sun-be-used-as-a-gravity-assist-outside-the-solar-system-with-current&quot;&gt;We cannot use the sun for gravity assists&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Interstellar movie, the black hole was the center of their system and their velocities were relative to it. So using it as means to get higher velocity correct? Am I missing something in this?&lt;/p&gt;&#xA;" OwnerUserId="20380" LastActivityDate="2017-07-15T19:14:29.840" Title="Is gravity assist provided by black hole as shown in Interstellar possible?" Tags="|gravity-assist|black-hole|" AnswerCount="0" CommentCount="4" ClosedDate="2017-07-17T08:14:02.630" ContentLicense="CC BY-SA 3.0" />
  <row Id="22295" PostTypeId="2" ParentId="20279" CreationDate="2017-07-15T19:25:18.350" Score="1" Body="&lt;p&gt;&lt;strong&gt;TL;DR:&lt;/strong&gt; All the Russian rockets are obviously made to withstand the common winter weather conditions up to -20C/-4F at least. So speaking to &quot;...as hardy as...&quot;, the simple answer should be &quot;Yes&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let's review in more detail:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.1a is basically the good old Soyuz but the digital guidance system replacing the analog one&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.1b is basically 2.1a with a better 3rd stage engine&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.1v (v being third letter in Cyrillic alphabet а,б,в) has the main engine replaced on top of that&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soyuz 2.1v has a few issues:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NK-33 engine on the 1st stage. It has a very high performance, on par with RD-180, allowing to get rid of the side boosters. However, being developed for the Soviet moon mission in the 60s, it has a very bad reputation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you noticed, Russian engines are usually called RD but this one has prefix NK being developed by the maker of the turbojet engines, but not jet engines. It's a long story, but in short, making a jet engine is not the same as making a turbojet engine even though they are both have words &quot;engine&quot; and &quot;jet&quot; in it. It's like a maker of golf balls was asked to develop a ball for the football, they are both round, both called &quot;ball&quot; and used to play a game but that's about it what they have in common.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, NK-33 relabeled as Aerojet AJ26 it is considered responsible for the 2014 Antares crash. There is a well-known meme how Mr Glushkov, (the developer of the original RD-107 Soyuz engine as well as RD-170/180 family) used to call NK-33 as &quot;the rotten engine&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So having the main engine replaced, it does not really allow to apply the R7 ~2000-and-counting successful launch stats to 2.1v version. Which was kind of &quot;confirmed&quot; during 2016 &quot;Canopus&quot; 2.1v crash. It is not human-rated and will never be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the supply for NK-33 is limited, the engine is no longer in production, the leftovers from the Soviet moon program is about 20 engines are kept at the warehouse, after using all of them, the engine is supposed to be replaced with RD-193 which will mean, basically, starting over.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in my opinion, 2.1v does not have much of a bright future in its current shape. The rocket will be used to utilize the NK-33 free leftovers. Thanks to the &quot;rotten engine&quot;, most likely, the only consumers who are brave enough to use 2.1v is going to be Russian military/govt-related who don't care much about the insurance cost etc.&lt;/p&gt;&#xA;" OwnerUserId="14118" LastEditorUserId="12102" LastEditDate="2017-07-18T05:55:19.187" LastActivityDate="2017-07-18T05:55:19.187" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22296" PostTypeId="2" ParentId="20510" CreationDate="2017-07-15T20:42:58.153" Score="3" Body="&lt;p&gt;There is a chart and the explanation at &lt;a href=&quot;https://en.wikipedia.org/wiki/Single-stage-to-orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Being limited by the performance of the existing chemical fuels and being limited by the sane take-off mass of less than 3K metric tonns (the weight of a Saturn V, the biggest rocket ever) then dry weight (everything but the fuel itself - the body, the payload itself, engines, re-entry fuel, safety margin fuel, landing gears, wings etc - everything) must fit into less than &lt;strong&gt;~10%&lt;/strong&gt; of the total mass, close to 5%, which is very challenging task to achieve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usage of better fuels like hydrogen improves the thrust to weight ratio but affects the dry weight because of the decreased density and insulating material for the cryogenic fuel tanks, so overall it does not make a big difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to SSTO to be technically possible we need the next generation of fuels / engines. Something like metallic hydrogen, thermonuclear fusion engine, photon engine etc.&lt;/p&gt;&#xA;" OwnerUserId="14118" LastEditorUserId="14118" LastEditDate="2019-06-13T15:29:27.353" LastActivityDate="2019-06-13T15:29:27.353" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="22297" PostTypeId="2" ParentId="829" CreationDate="2017-07-15T21:18:22.837" Score="6" Body="&lt;p&gt;OK, we've got three problems here:  First, in the title of your question, you are asking about &quot;a gravity assist outside the solar system&quot; but in the body of the question you ask about passing close to the Sun from &lt;em&gt;within&lt;/em&gt; the Solar system.  I think the title of this question should be edited to be &quot;Could the Sun be used as a gravity assist to acheive an escape from the Solar system (with current tech)?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second problem is with PearsonArtPhoto's accepted answer wherein the Sun is assumed to be a &lt;em&gt;stationary object&lt;/em&gt; but in fact it is in orbit around the center of the galaxy.  Therefore, swinging around the Sun &lt;em&gt;in the direction of its orbit&lt;/em&gt; &lt;strong&gt;will&lt;/strong&gt; produce exactly the same sort of gravity assist as going past Jupiter (or any other relatively massive object).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third problem is that a gravity assist from the Sun is &lt;em&gt;not&lt;/em&gt; required to attain escape velocity from the Solar system - we already have &lt;strong&gt;&lt;em&gt;FOUR&lt;/em&gt;&lt;/strong&gt; probes that are leaving, never to return - two &lt;a href=&quot;https://www.nasa.gov/centers/ames/missions/archive/pioneer.html&quot; rel=&quot;noreferrer&quot;&gt;Pioneers&lt;/a&gt; (10 and 11, launched in 1972 and 1973), and two &lt;a href=&quot;https://voyager.jpl.nasa.gov/&quot; rel=&quot;noreferrer&quot;&gt;Voyagers&lt;/a&gt; (launched in 1977).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pioneer 10's last, very weak signal was received on 23 January 2003. NASA engineers calculated its radioisotope power source has decayed to where it does not have enough power to send additional transmissions to Earth.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Pioneer 10 will continue to coast silently as a ghost ship through deep space into interstellar space, heading generally for the red star Aldebaran, which forms the eye of Taurus (The Bull). Aldebaran is about 68 light years away and it will take Pioneer over 2 million years to reach it.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Pioneer 11 studied energetic particles in the outer heliosphere.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Pioneer 11 Mission ended on 30 September 1995, when the last transmission from the spacecraft was received. There have been no communications with Pioneer 11 since. The Earth's motion has carried it out of the view of the spacecraft antenna. The spacecraft cannot be maneuvered to point back at the Earth. It is not known whether the spacecraft is still transmitting a signal. No further tracks of Pioneer 11 are scheduled. The spacecraft is headed toward the constellation of Aquila (The Eagle), Northwest of the constellation of Sagittarius. Pioneer 11 will pass near one of the stars in the constellation in about 4 million years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Voyager 1 is already in interstellar space, and Voyager 2 is in the heliosheath, and both spacecraft are still sending scientific information about their surroundings through the Deep Space Network.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastActivityDate="2017-07-15T21:18:22.837" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22298" PostTypeId="2" ParentId="20279" CreationDate="2017-07-15T21:37:29.830" Score="5" Body="&lt;p&gt;It has been launched from Plesetsk at 62°46′N, a latitude higher than Anchorage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two launches happened so far on December 28 2013 and December 5 2015. Right in the middle of the northern winter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9jYv0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9jYv0.jpg&quot; alt=&quot;Soyuz 2.1v in Plesetsk&quot;&gt;&lt;/a&gt;&#xA;&lt;em&gt;Photo from the Defence Ministry of Russian Federation website.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Redesign or not, there are lots of launches from Plesetsk all year round. That includes all kinds of Soyuz, Rokot, Angara and other rockets.&lt;/p&gt;&#xA;" OwnerDisplayName="user54" LastEditorUserId="6816" LastEditDate="2017-07-16T19:27:49.877" LastActivityDate="2017-07-16T19:27:49.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22299" PostTypeId="1" CreationDate="2017-07-16T01:34:32.990" Score="6" ViewCount="783" Body="&lt;p&gt;The Soviet space shuttle Buran launched and landed (unmanned, because it could) once very successfully. I would think it would be a very valuable industrial asset for the bankrupt Russian Republic that came just after Buran's success. But it never flew again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were there decisive technical issues with refurbishing it for a second flight? Aside from the political and economic obstacles. Since it didn't carry its main engines, I'd think it would be by design easier to reuse than the STS shuttle orbiter.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2018-03-03T19:53:39.047" Title="Were there problems with the Russian Buran space shuttle's reusability?" Tags="|space-shuttle|history|reuse|buran|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22300" PostTypeId="2" ParentId="22056" CreationDate="2017-07-16T03:05:50.177" Score="5" Body="&lt;p&gt;First of all let's define what the radiation is:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;alpha (helium nucleus i.e. two protons and two neutrons)&lt;/li&gt;&#xA;&lt;li&gt;beta (electrons)&lt;/li&gt;&#xA;&lt;li&gt;gamma (high energy photons)&lt;/li&gt;&#xA;&lt;li&gt;neutrons&lt;/li&gt;&#xA;&lt;li&gt;ions (bare stripped nucleus)&lt;/li&gt;&#xA;&lt;li&gt;other stuff like visible light, IR, UF. X-RAY is not considered here, X-RAY is gamma.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The only thing that is important to consider is a &lt;strong&gt;nuclear cross section&lt;/strong&gt; which is different for every type and every protection material.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Protection:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;alpha is easy, it's a heavy and well-charged particle. And there is not much of them in the space. The energy of the ones from the space is low. 1 mm of a kitchen tin foil will be a good protection.&lt;/li&gt;&#xA;&lt;li&gt;beta. There are lots of electrons in the space and they can have &lt;strong&gt;wild&lt;/strong&gt; energy, ranging from 10 to 10^20, and this is huge. For example, the large hadron collider at CERN is able to produce just 10^13 tops. There is nothing in the world that can stop such a high energy stuff. However, the good news is that the nuclear cross section of such a high energy particles is relatively small. 3mm of the kitchen foil is enough just as a placebo shield and in order to stop some low energy ones.&lt;/li&gt;&#xA;&lt;li&gt;gamma. this is simple. the heavier the stuff the better. The earth is protected by 10km of air, which has the same level of protection as 10m of water, or 1.4m of steel, or 0.9m of lead, or 0.1m of depleted uranium. Sounds scary but 0.1m of U-238 can be the best option to protect from this kind of radiation.&lt;/li&gt;&#xA;&lt;li&gt;neutron. this is the most difficult one. The best shielding is the light stuff - water, hydrogen, oxygen, graphite, etc. Ideally - by other neutrons. However, the nuclear cross section raises as its energy drops! The slower neutron is the more dangerous it is! Also when a neutron is stopped by some materials like cadmium, the neutron absorption is accompanied by a strong emission of gamma which requires more gamma protection. You can easily google for neutron protection articles &lt;a href=&quot;http://www.nuclear-power.net/nuclear-power/reactor-physics/atomic-nuclear-physics/fundamental-particles/neutron/shielding-neutron-radiation/&quot; rel=&quot;noreferrer&quot;&gt;like this&lt;/a&gt;, but overall, either not protect at all or establish an earth-like level of protection. The earth is protected by ~10 km of air which has about 25% of oxygen, hydrogen, and water. I would estimate that 3m of water will be a good earth-like protection from neutrons. Water is better than other material - it is rich in hydrogen.&lt;/li&gt;&#xA;&lt;li&gt;Ions, UF, IR - the same stuff as alpha&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Overall, 3mm of tin foil + 3m of water + 0.1m U-238 and the Martian astronauts will be just fine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a scan from one work, it's in Russian but you should get the idea,&#xA;the legend here is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;X is the aluminum shield thickness gram/cm^2&lt;/li&gt;&#xA;&lt;li&gt;Y is the radiation absorbed dose mRad/24hours&lt;/li&gt;&#xA;&lt;li&gt;1: total dose&lt;/li&gt;&#xA;&lt;li&gt;2: primary protons&lt;/li&gt;&#xA;&lt;li&gt;3: cascade protons&lt;/li&gt;&#xA;&lt;li&gt;4: cascade neutrons&lt;/li&gt;&#xA;&lt;li&gt;5: evaporative neutrons&lt;/li&gt;&#xA;&lt;li&gt;6: evaporative protons&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qGwsO.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qGwsO.jpg&quot; alt=&quot;Dependency between the cosmic radiation power and shield thickness&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="14118" LastEditorUserId="14118" LastEditDate="2017-07-16T03:11:24.653" LastActivityDate="2017-07-16T03:11:24.653" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="22301" PostTypeId="2" ParentId="22243" CreationDate="2017-07-16T03:32:04.387" Score="7" Body="&lt;p&gt;Which one? Which supplier, which brand, which set of additives, what content of impurities?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That matters. This is why rockets fly on RP-1, not on JP-1 kerosene - because the standard for JP-1 is crap, and JP-1 can be pretty much anything, while RP-1 restricts contents of substances that adversely affect combustion stability and regulates contents of substances that affect combustion process, so that it's repeatable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ethanol is not a very desirable fuel due to high combustion temperature accompanied by moderate pressure of the exhaust - the nozzle runs very hot while not producing all that much thrust. This can be managed by other additives and E85 could be better than pure ethanol (though likely worse than RP-1) - providing it's standarized to a regime as tight as RP-1 kerosene; it doesn't matter so much &lt;em&gt;what&lt;/em&gt; the standard is, only that it's &lt;em&gt;tight&lt;/em&gt; so that if a test article engine is made to work reliably on one batch on the fuel, the production engine will work exactly the same on another batch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, with existing E-85 you'll run into the same kind of problems as the early rocket engineers ran into with JP-1 jet fuel. There are detergents, there are denaturing additives, pigment, all that crap that's mostly neutral for a car engine, but your rocket - even if you don't clog the injectors or regenerative cooling pipes - will simply vary in parameters from batch to batch. And so, even if your rocket flies, you'll have a lot of headaches assuring it flies where you want it to!&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="415" LastEditDate="2017-07-18T14:02:10.080" LastActivityDate="2017-07-18T14:02:10.080" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22303" PostTypeId="2" ParentId="19852" CreationDate="2017-07-16T07:14:13.200" Score="18" Body="&lt;p&gt;From the same &quot;Ignition&quot; book&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;«It happened at their Shreveport, Louisiana, installation, while they were preparing to ship out, for the first time, a one-ton steel cylinder of CTF (Chlorine trifluoride,). The cylinder had been cooled with dry ice to make it easier to load the material into it, and the cold had apparently embrittled the steel. For as they were maneuvering the cylinder onto a dolly, it split and dumped one ton of chlorine trifluoride onto the floor. It chewed its way through twelve inches of concrete and dug a threefoot hole in the gravel underneath, filled the place with fumes which corroded everything in sight, and, in general, made one hell of a mess. Civil Defense turned out, and started to evacuate the neighborhood, and to put it mildly, there was quite a brouhaha before things quieted down. Miraculously, nobody was killed, but there was one casualty — the man who had been steadying the cylinder when it split. He was found some five hundred feet away, where he had reached Mach 2 and was still picking up speed when he was stopped by a heart attack.»&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You must imagine how will the launch pad look like after the launch since F2 + H2 → HF &#xA;= Hydrogen fluoride, in other words, hydrofluoric acid. It is highly corrosive and dissolves almost everything, even glass (watch &quot;Breaking Bad&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now imagine a rocket containing 1000 tons of fluorine crashing in Florida. After that event, even enriched Uranium will look like eco-friendly fuel.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Fluorine is toxic starting from 0.1 part of fluorine per million parts&#xA;  of air (ppm).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Xenon Ion thruster engine has even higher specific impulse - about 3000 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specific impulse is important, but there are also other aspects to consider, such as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;thrust (if the engine is used at the ground level)&lt;/li&gt;&#xA;&lt;li&gt;fuel cost&lt;/li&gt;&#xA;&lt;li&gt;cost of engine R&amp;amp;D&lt;/li&gt;&#xA;&lt;li&gt;cost of handling the fuel on earth&lt;/li&gt;&#xA;&lt;li&gt;environmental impact&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Fluorine was R&amp;amp;D heavily in the 60s.  &lt;a href=&quot;http://www.astronautix.com/r/rd-301.html&quot; rel=&quot;noreferrer&quot;&gt;RD-301, RD-350&lt;/a&gt; and &lt;a href=&quot;https://www.flightglobal.com/pdfarchive/view/1967/1967%20-%200069.html&quot; rel=&quot;noreferrer&quot;&gt;RRC&lt;/a&gt; studies It was canceled for obvious reasons.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The highest specific impulse chemistry ever test-fired in a rocket engine was lithium and fluorine, with hydrogen added to improve the exhaust thermodynamics (making this a tripropellant) [ARBIT, H. A., CLAPP, S. D., DICKERSON, R. A., NAGAI, C. K., &lt;a href=&quot;https://web.archive.org/web/20110724231717/https://www.aiaa.org/content.cfm?pageid=406&amp;amp;gTable=mtgpaper&amp;amp;gID=40999&quot; rel=&quot;noreferrer&quot;&gt;Combustion characteristics of the fluorine-lithium/hydrogen tripropellant combination&lt;/a&gt;. AMERICAN INST OF AERONAUTICS AND ASTRONAUTICS, PROPULSION JOINT SPECIALIST CONFERENCE, 4TH, CLEVELAND, OHIO, Jun 10-14, 1968. ] . The combination delivered 542 seconds (5.32 kN·s/kg, 5320 m/s) specific impulse in a vacuum. The impracticality of this chemistry highlights why exotic propellants are not actually used: to make all three components liquids, the hydrogen must be kept below -252 °C (just 21 K) and the lithium must be kept above 180 °C (453 K). Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, and hydrogen, while not hypergolic, is an explosive hazard. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which damages the environment, makes work around the launch pad difficult, and makes getting a launch license that much more difficult. The rocket exhaust is also ionized, which would interfere with radio communication with the rocket.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="14118" LastEditorUserId="14118" LastEditDate="2019-06-22T05:06:40.467" LastActivityDate="2019-06-22T05:06:40.467" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="22305" PostTypeId="1" AcceptedAnswerId="22309" CreationDate="2017-07-16T09:28:16.757" Score="5" ViewCount="452" Body="&lt;p&gt;I see something interesting the webcast of the NROL-76 launch. At around &lt;code&gt;T+00:00:55&lt;/code&gt; it looks like flames from the exhaust start jumping around the bases of the ring of eight nozles in a circular sweep. Until this point the exhaust is only below the nozzles, after this point it seems everywhere. The process takes place between 4 and 6 km of altitude, at a speed 250 to 300 m/sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/EzQpkQ1etdA?t=763&quot; rel=&quot;nofollow noreferrer&quot;&gt;The video is keyed&lt;/a&gt; to start ten seconds earlier, where it looks to me like the exhaust on the left side of the image starts getting &quot;jittery&quot; before it begins to jump around the bases of the nozzles where they connect to the combustion chambers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image samples are for convenience only. If you can, view the video itself. If you need more screen shots, explain what you need and I&quot;ll try to add it for you. Also, remember that YouTube allows you to play at half and quarter speed, and in HD.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is happening here? What causes this change in the shape and location of the engine plume?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the GIF the images are spaced by about 1 second, so it is approximately normal speed (though under-sampled)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kOk6b.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kOk6b.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vNk9n.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vNk9n.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/bOeSt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bOeSt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rA8F3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rA8F3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/86XC2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/86XC2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HopQd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HopQd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/EzQpkQ1etdA?start=763&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-10-05T15:02:50.837" LastActivityDate="2018-10-05T15:02:50.837" Title="Why are exhaust flames &quot;jumping around&quot; the bases of the Falcon-9 engine nozzles; NROL-76?" Tags="|spacex|engines|nozzle|exhaust|" AnswerCount="1" CommentCount="2" ClosedDate="2018-12-06T08:50:00.140" ContentLicense="CC BY-SA 4.0" />
  <row Id="22307" PostTypeId="1" AcceptedAnswerId="22308" CreationDate="2017-07-16T12:45:41.927" Score="3" ViewCount="776" Body="&lt;p&gt;In the Wikipedia page for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Goldstone_Deep_Space_Communications_Complex&quot; rel=&quot;nofollow noreferrer&quot;&gt;Goldstone DSN complex&lt;/a&gt; there are coordinates. If you click them it takes you to GeoHack. I copies the decimal coordinates there &lt;code&gt;35.426667, -116.89&lt;/code&gt; into my browser and it took me to the Goldstone complex. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does the map show a landmark labeled &quot;Mars&quot; next to one of the large dishes?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this an easter egg, or some name for the location?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0vYKl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0vYKl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/whJmw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/whJmw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GKX80.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GKX80.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-17T02:45:32.863" LastActivityDate="2018-09-01T08:09:05.803" Title="Why does the word &quot;Mars&quot; shows up in google maps when viewing the Goldstone DSN complex?" Tags="|mars|deep-space-network|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22308" PostTypeId="2" ParentId="22307" CreationDate="2017-07-16T14:25:48.770" Score="10" Body="&lt;p&gt;The answer is in the Wikipedia article: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The DSS 14, &quot;Mars&quot; telescope &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So it's the name for one of the antennas of the Goldstone complex.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the &lt;a href=&quot;https://www.gdscc.nasa.gov/?page_id=40&quot; rel=&quot;noreferrer&quot;&gt;antennas&lt;/a&gt; and their names: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HzixV.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HzixV.jpg&quot; alt=&quot;Goldstone names&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-07-16T15:35:01.983" LastActivityDate="2017-07-16T15:35:01.983" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22309" PostTypeId="2" ParentId="22305" CreationDate="2017-07-16T14:31:16.140" Score="5" Body="&lt;p&gt;So, rocket nozzle geometry is designed differently for different ranges of atmospheric pressure because as pressure decreases, the direction of rocket gasses escaping moves from downward vertical toward more and more horizontal. By the time rockets designed to operate in the atmosphere reach the vacuum of space, the exhaust is exiting pretty much perpendicular to the engine bell.&#xA;The jittery-ness you see, is atmospheric drag on the nose cone slowing the overall craft down a little more than the lighter upward moving exhaust gasses. The gasses are actually passing the rocket nozzle in the upward direction. This phenomenon is really visible watching Saturn V launches. Views from ground camera's really capture how far up the first stage the engine plume is travelling while the overall rocket is experiencing atmospheric drag.&#xA;In the movie Apollo 13, Tom Hanks say's, &quot;Get ready for a little jolt boys.&quot; When the first stage shuts down. Pressure/drag on the nose cone RAPIDLY slows the craft and from the outside you can see the engine plume appear to completely pass the vehicle.&#xA;Pretty cool stuff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Apollo(s) 15, 16 and 17 the geometry of the engine bell on the Lunar Module was widened to accommodate for plume direction in a vacuum. (Also to minimize engine bell blow back inches from the Lunar ground, which can rupture the engine.)&#xA;(Hmm, Apollo 15 performing new engine shape flight testing on the first operational all up science mission? Pretty gutsy move.)&lt;/p&gt;&#xA;" OwnerUserId="20342" LastActivityDate="2017-07-16T14:31:16.140" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22310" PostTypeId="1" CreationDate="2017-07-16T17:14:41.553" Score="4" ViewCount="123" Body="&lt;p&gt;What is a rough estimate for the bitrate needed for &lt;a href=&quot;https://en.wikipedia.org/wiki/Guidance,_navigation,_and_control&quot; rel=&quot;nofollow noreferrer&quot;&gt;GNC&lt;/a&gt; (guidance, navigation, and control) of an interplanetary spacecraft from Earth? I know it varies a great deal from case to case, but I want an order-of-magnitude estimate so I can plan conservatively.&lt;/p&gt;&#xA;" OwnerUserId="19742" LastEditorUserId="415" LastEditDate="2017-07-18T13:54:51.407" LastActivityDate="2017-07-18T13:54:51.407" Title="What approximate bitrate is required for interplanetary spacecraft GNC?" Tags="|deep-space|data-transmission|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22311" PostTypeId="1" AcceptedAnswerId="22480" CreationDate="2017-07-16T17:17:00.817" Score="2" ViewCount="479" Body="&lt;p&gt;The July 14, 2017 NASA Spaceflight article &lt;a href=&quot;https://www.nasaspaceflight.com/2017/07/soyuz-2-1a-launches-kanopus-v-ik-70-satellites/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz 2-1A launches with Kanopus-V-IK and over 70 satellites&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Mayak is a three-unit CubeSat which was built by Tvoii Sektor Kosmosa – or “Your Sector of Space” – an independent, crowd-funded team of engineers in conjunction with the Moscow State University of Mechanical Engineering. Mayak – meaning Lighthouse – will deploy a highly reflective tetrahedral structure.&lt;/p&gt;&#xA;&lt;p&gt;Each side of this structure has an area of four square meters, or 43 square feet. To ground observers, the satellite is expected to have an apparent magnitude of up to -10, making it one of the brightest objects in the night sky. The structure will double as a deorbit mechanism, hastening the decay of the satellite’s orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Right now the orbit of the primary payload Kanopus-V-IK &lt;code&gt;2017-042A, 42825&lt;/code&gt; should be a good guess for the rough predictor of Mayak passes, but once it deploys that giant reflector, things should change quickly.&lt;/p&gt;&#xA;&lt;p&gt;How is this large reflector made from thin polymer film carefully expanded to and maintained in its full size and shape so that its faces are at least somewhat flat?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shot from the YouTube video &lt;a href=&quot;https://youtu.be/IBtI_pV4Q_I&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mayak. No more space debris!&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Bm6Xj.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Bm6Xj.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Mayak Reflector – Photo: CosmoMayak, From &lt;a href=&quot;http://spaceflight101.com/soyuz-kanopus-v-ik/mayak/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PJEf7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PJEf7.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Mayak Artists conception, From &lt;a href=&quot;https://www.nasaspaceflight.com/2017/07/soyuz-2-1a-launches-kanopus-v-ik-70-satellites/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Spaceflight&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WkIbo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WkIbo.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-15T16:04:19.110" Title="Mayak, a magnitude -10 (minus ten) satellite; how is the large, delicate reflector expanded?" Tags="|cubesat|space-art|deorbit|" AnswerCount="1" CommentCount="9" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22312" PostTypeId="1" CreationDate="2017-07-17T06:43:45.287" Score="1" ViewCount="165" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; Answers to &lt;a href=&quot;https://space.stackexchange.com/q/22476/12102&quot;&gt;Two week mark; has Mayak (Маяк) been spotted yet? Reflector deployed? Astronomy “ruined”?&lt;/a&gt; indicate that the reflector did not deploy successfully (more at &lt;a href=&quot;https://space.stackexchange.com/q/22311/12102&quot;&gt;Mayak, a magnitude -10 (minus ten) satellite; how is the large, delicate reflector expanded?&lt;/a&gt;), so I've changed the tense from future to conditional.&lt;/p&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;https://ham.stackexchange.com/q/8897/8643&quot;&gt;this question&lt;/a&gt; I've asked about the possible reflection of UHF radio signals from the recently launched Mayak cubesat once it's large metallized polymer solar reflector would be deployed. However it would certainly be detectable by reflected sunlight and this was one of the primary purposes. The other is to study the large, light-weight expanded structure as a de-orbiter.&lt;/p&gt;&#xA;&lt;p&gt;You can learn more on the YouTube videos &lt;a href=&quot;https://youtu.be/IBtI_pV4Q_I&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mayak. No more space debris!&lt;/a&gt; and also &lt;a href=&quot;https://youtu.be/K91z2rT_4Xw&quot; rel=&quot;nofollow noreferrer&quot;&gt;A tour through the Mayak project&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;I thought I'd try to calculate what the reflections might look like and which positions relative to the ground and the Sun are most likely to make it visible. When fully deployed it would have been a very large, very light weight tetrahedron with a 3U cubesat body attached to one corner. See details of the deployed shape below.&lt;/p&gt;&#xA;&lt;p&gt;The primary satellite from this July 14, 2017 launch is Kanopus-V-IK; &lt;code&gt;42825U 17042A&lt;/code&gt; and the one TLE I can find so far shows 15.194 orbits per day, which corresponds to an average altitude of roughly 500 km. However the cubesat may have been deployed at a different altitude.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; Now there are many more objects, 2017-042A through 042BZ; almost 80 of them. A is still at 500 km, but there is a group at 460 and another group near 590 km. If these are close to correct, this must have been an interesting launch!&lt;/p&gt;&#xA;&lt;p&gt;Sooner or later through, even small atmospheric drag on the large surface area to mass ratio spacecraft would have an effect.&lt;/p&gt;&#xA;&lt;p&gt;My &lt;s&gt;first&lt;/s&gt; &lt;strong&gt;second&lt;/strong&gt; guess is that the mass concentration at one corner would lead and the large-area, low-mass reflector would be dragging directly behind it. (Thanks to @Damien_The_Unbeliever for not believing what I originally wrote, which was exactly the opposite!) But I don't know how dense the gas particles in space would have to be for this orientation to establish itself. It would have to be enough to damp any residual rotation other than that necessary to rotate once every 90 or 100 minutes in inertial space.&lt;/p&gt;&#xA;&lt;p&gt;Even once the symmetry axis were moving tangent to the direction of motion, it could rotate around its axis until that too were damped. What would the final equilibrium orientation be, and how long before it would settle down?&lt;/p&gt;&#xA;&lt;p&gt;Is this configuration inherently stable, or inherently unstable? Would it have settled down in with the attitude I natively believe, or start oscillating or rotating at some angle, or perhaps tumble wildly and unpredictably?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The  NASA Spaceflight article &lt;a href=&quot;https://www.nasaspaceflight.com/2017/07/soyuz-2-1a-launches-kanopus-v-ik-70-satellites/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz 2-1A launches with  and over 70 satellites&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Mayak is a three-unit CubeSat which was built by Tvoii Sektor Kosmosa – or “Your Sector of Space” – an independent, crowd-funded team of engineers in conjunction with the Moscow State University of Mechanical Engineering. Mayak – meaning Lighthouse – will deploy a highly reflective tetrahedral structure.&lt;/p&gt;&#xA;&lt;p&gt;Each side of this structure has an area of four square meters, or 43 square feet. To ground observers, the satellite is expected to have an apparent magnitude of up to -10, making it one of the brightest objects in the night sky. The structure will double as a deorbit mechanism, hastening the decay of the satellite’s orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Right now the orbit of the primary payload Kanopus-V-IK &lt;code&gt;2017-042A, 42825&lt;/code&gt; should be a good guess for the rough predictor of Mayak passes, but once it deploys that giant reflector, things should change quickly.&lt;/p&gt;&#xA;&lt;p&gt;How is this large reflector made from thin polymer film carefully expanded to and maintained in its full size and shape so that its faces are at least somewhat flat?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PJEf7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PJEf7.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Mayak Reflector – Photo: CosmoMayak, From &lt;a href=&quot;http://spaceflight101.com/soyuz-kanopus-v-ik/mayak/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WkIbo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WkIbo.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Mayak Artists conception, From &lt;a href=&quot;https://www.nasaspaceflight.com/2017/07/soyuz-2-1a-launches-kanopus-v-ik-70-satellites/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Spaceflight&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-01-24T00:02:02.093" Title="Would Mayak's large solar reflector have produced a stable attitude, or more likely start tumbling or rotating?" Tags="|attitude|atmospheric-drag|attitude-determination-and-control|deorbit|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="22313" PostTypeId="1" AcceptedAnswerId="22320" CreationDate="2017-07-17T08:10:13.863" Score="7" ViewCount="402" Body="&lt;p&gt;The Wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pressure&quot; rel=&quot;noreferrer&quot;&gt;Radiation Pressure&lt;/a&gt; gives an equation for the pressure on a perfect reflector produced by a beam of photons with an energy flux $E_f$ (in units of power/area) as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$P=\frac{2E_f}{c}cos^2(\theta)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\theta$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Angle_of_incidence_(optics)&quot; rel=&quot;noreferrer&quot;&gt;angle of incidence&lt;/a&gt; between the incoming photon beam and the surface normal of the reflector, and $c$ is the speed of light. Maximum pressure is at normal incidence, and it drops to zero at 90° when the incidence is perpendicular to the surface normal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $cos^2(\theta)$ behavior can be understood as follows; the component of the photon's momentum perpendicular to the surface scales as $cos(\theta)$, and the projected cross-sectional area of a flat surface exposed to the incoming light also varies as $cos(\theta)$. Together they result in the $cos^2(\theta)$ behavior.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, Wikipedia's article &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;noreferrer&quot;&gt;Solar Sail&lt;/a&gt; mentions both the simple $cos^2(\theta)$ shape and also a more complicated, &quot;realistic&quot; shape with the form&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$0.349 + 0.662cos(2\theta) - 0.11cos(4\theta)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and states that realistically the force drops to zero &lt;em&gt;at around 60°&lt;/em&gt; rather than at 90°. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An actual square sail can be modeled as: F = F0 (0.349 + 0.662 cos 2θ − 0.011 cos 4θ) / R2 Note that the force and acceleration approach zero generally around θ = 60° rather than 90° as one might expect with an ideal sail.(18)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I don't understand how the square shape of the sail has anything to do with the pressure, and I don't understand why it would go to zero near 60°. Reference (18) is given as &lt;em&gt;Space Sailing&lt;/em&gt;, Jerome Wright (1992), Gordon and Breach Science Publishers, Appendix B.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've pasted that expression into an internet search and I see that it is repeated in many places, with similarly worded text, but so far I have not found any explanation for why it is supposed to better represent the performance of a solar sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is the physics behind this approximate expression? Why would the pressure drop from maximum to zero at about 60°? Would a square shape have anything to do with it at all? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond 60° the expression goes negative; are we supposed to clip it at zero, rather than there being an &lt;em&gt;attractive force&lt;/em&gt; due to &lt;em&gt;negative photon pressure&lt;/em&gt;, or does this represent a known physical phenomenon? Under some conditions light can be used as an attractive force as in the case of a laser trap, but that usually involves either a strong gradient, or tuning to a specific resonance. In this case the light is uniform and the interaction is non-resonant.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In the plot below, the simple $cos^2(\theta)$ behavior is shown by the thick solid (blue) curve as a function of incident angle $\theta$, and the more complex expansion is shown by the thinner solid (green) curve. Zero is indicated by the dashed (black) line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gVbWk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gVbWk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-17T11:03:12.817" LastActivityDate="2017-07-17T19:13:00.580" Title="Negative photon pressure? What is the physics behind this expression?" Tags="|physics|mathematics|solar-sail|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22314" PostTypeId="1" CreationDate="2017-07-17T11:02:52.957" Score="5" ViewCount="336" Body="&lt;p&gt;I'm looking for a cheap and cheerful, but ultimately &lt;em&gt;working&lt;/em&gt; utility or library for tracking a cubesat and predicting link windows. I've been looking at using Orekit. I'm a student, so a lot of its capability is just over my knowledge horizon, but its authors' assertion that it has been used and benchmarked by CNES looks like a good bet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been asked to look at PREDICT (&lt;a href=&quot;http://www.qsl.net/kd2bd/predict.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.qsl.net/kd2bd/predict.html&lt;/a&gt;), which seems to have a lot of use by amateurs and a little use by NASA and other organisations. It uses the same SGP4 propagation model as Orekit, but has no apparent facility for updating reference data. Its last release was in 2006.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can anyone offer their experience of PREDICT and tell me whether this is going to be up to scratch for a satellite we definitely want to communicate with on each pass?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many thanks for the attention.&lt;/p&gt;&#xA;" OwnerUserId="20391" LastActivityDate="2017-09-21T18:39:27.417" Title="Is the utility PREDICT good enough for operational orbital prediction?" Tags="|orbital-mechanics|cubesat|communication-satellite|radio-communication|prediction|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22315" PostTypeId="1" CreationDate="2017-07-17T11:58:11.537" Score="6" ViewCount="1415" Body="&lt;p&gt;Does the gravity assist get affected by any of the forces by other planets as they proceed towards the required orbit? Moreover what can be the other setbacks of gravity assist flyby?&lt;/p&gt;&#xA;" OwnerUserId="20383" LastEditorUserId="6816" LastEditDate="2017-07-27T05:28:06.650" LastActivityDate="2018-04-08T19:22:35.340" Title="What are the disadvantages of gravity flyby assist?" Tags="|gravity-assist|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22316" PostTypeId="1" CreationDate="2017-07-17T17:04:12.223" Score="4" ViewCount="1015" Body="&lt;p&gt;How do the F9 grid fins extend and rotate?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the vehicle launches they are flat against the surface of the vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/STWgd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/STWgd.jpg&quot; alt=&quot;Old and new grid fins with mount points&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the first stage begins reentry, the fins extend and start 'flying' the vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the mechanism that extends and controls them?  Hydraulics? Electric? &lt;/p&gt;&#xA;" OwnerUserId="20392" LastEditorUserId="51" LastEditDate="2017-07-17T18:18:15.753" LastActivityDate="2017-07-17T19:26:33.820" Title="How do the F9 grid fins extend and rotate?" Tags="|spacex|falcon-9|landing|grid-fins|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22317" PostTypeId="2" ParentId="22315" CreationDate="2017-07-17T17:37:14.467" Score="13" Body="&lt;p&gt;Arthur Dent caught the basic problem with gravity assists - you can't decide where and when you can use them - you must plan your mission around opportunities for gravity assists, and that means a very complex and often very long trajectory. It takes a lot of computational power to plan. It sets a rigid launch window; the planets won't wait for new budget for your probe or time you need for extra testing. The time spent maintaining the probe from the ground station while waiting for the assists is not free - the ground station and crew need to be funded. With each day in space the risk that something fails on the craft increases. And botching an assist will send your probe into a trajectory which is very unlikely to give you another chance to encounter the target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You gain &quot;free&quot; delta-V. That's one huge advantage. But the two primary disadvantages of rigid mission plan, and increased mission duration lead to so many small secondary disadvantages many missions are designed with no gravity assists simply to save the headaches. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-07-17T17:37:14.467" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22318" PostTypeId="1" CreationDate="2017-07-17T17:45:37.107" Score="24" ViewCount="4365" Body="&lt;p&gt;We know that just before reentry started, there was concern over a signal which indicated that the heat shield which would protect the capsule from the heat of reentry had become detached. This would have meant catastrophe, and orders were given to keep the retrorocket packs intact to hold the heat shield in place. However, it turned out that it was merely a faulty signal and there was no problem with the heat shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question, or questions are:  Why would the heat shield possibly come loose? Wasn’t it permanently attached to the rest of the capsule? Since there was a sensor that indicated that it had possibly come loose, this implies that in some circumstance, the heat shield was, for lack of a better term, detachable, or at least at risk of coming loose. Why would this be?&lt;/p&gt;&#xA;" OwnerUserId="20393" LastEditorUserId="6816" LastEditDate="2017-07-17T17:50:11.810" LastActivityDate="2017-11-13T04:35:21.890" Title="Why was the Friendship 7 Mercury heat shield detachable?" Tags="|heat-shield|project-mercury|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22319" PostTypeId="2" ParentId="22316" CreationDate="2017-07-17T18:19:27.097" Score="5" Body="&lt;p&gt;The earliest use of the grid fins failed to land successfully because it ran out of hydraulic fluid. They use the RP1 from the main tank to run the hydraulics and they were initially open loop (the hydraulics). This meant they would use the fluid, and throw it away as they used it.  Thus they ran out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They later switched to a closed loop system so they do not run out of hydraulic fluid during landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We see the fins start against the side of the vehicle. They pop out during landing, and then after the vehicle lands, they safe themselves and retract back to the closed position. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-07-17T19:26:33.820" LastActivityDate="2017-07-17T19:26:33.820" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22320" PostTypeId="2" ParentId="22313" CreationDate="2017-07-17T18:47:16.527" Score="4" Body="&lt;p&gt;I doubt that we are going to answer this without finding a copy of the book by Wright or contacting him. The following is therefore not a complete answer, but it's too long to fit in a comment. The WP article's history shows that Wright was one of the article's main early authors, and that he wrote the skimpy text introducing the equation.  Wright has a &lt;a href=&quot;http://sail.quarkweb.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;web site&lt;/a&gt;, where he has a very brief presentation of some information that is presumably given in more detail in his book. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ideal function can be written as&#xA;$$f(\theta)=\frac{1}{2}+\frac{1}{2}\cos2\theta,$$&#xA;while the numerical expression given in the WP article is of the form&#xA;$$g(\theta)=A_0+A_2\cos2\theta+A_4\cos4\theta,$$&#xA;where they seem to have chosen the normalization such that $A_0+A_2+A_4=1$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On &lt;a href=&quot;http://sail.quarkweb.com/light.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;one page&lt;/a&gt; on Wright's web site, he has the following somewhat more detailed description of how $g$ was obtained: &quot;A ship can have multiple sails, each with different orientations.  Each sail has some curvature in it.  Using finite element analysis, the local pressure on parts of sails can be integrated to determine the total force and moments acting on the ship.  A resulting approximation of the force versus its angle theta, valid for a square sail, is: [...]&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is theoretically possible to have $g&amp;lt;0$ without any exotic physics. For example, one could imagine a system that accepted light coming in at an oblique angle, sent it through fiber optic cables, and threw it back out along the normal to the surface, on the dark side of the surface. However, it's not clear to me at all how anything like this can happen with a solar sail. In particular, it seems very difficult to explain the fact that $g$ attains a large negative value at $\theta=90$. At this angle, a non-billowing sail will intercept zero sunglight, so there should be zero force. Even if the sail is billowing, the amount of sunlight intercepted should be very small. Furthermore, at 90 degrees you would expect the component of the force along the sail's average normal to be zero by symmetry. The only way I can think of to break this symmetry would be if the sail had unequal absorbance and emittance on its two sides. Even so, this result seems very implausible.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2017-07-17T19:13:00.580" LastActivityDate="2017-07-17T19:13:00.580" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22321" PostTypeId="2" ParentId="22318" CreationDate="2017-07-17T19:48:14.227" Score="41" Body="&lt;p&gt;To reduce the force of the impact on landing the heat shield was designed to separate and be held on by a skirt that acted as an airbag. From the &lt;a href=&quot;https://history.nasa.gov/SP-4001/figures.htm&quot; rel=&quot;noreferrer&quot;&gt;NASA list of Mercury illustrations&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DENRZ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DENRZ.jpg&quot; alt=&quot;Impact attenuation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://history.nasa.gov/SP-4001/images/fig46.jpg&quot; rel=&quot;noreferrer&quot;&gt;Figure 46&lt;/a&gt;:  Impact attenuation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When the heat shield was released the impact skirt would fill with air, but when the heat shield hit the water the air being forced out the holes at the bottom (which would be under water) would provide a bit of softer deceleration before the capsule was fully landed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More detail on how the system worked here (from the same source):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mEuCd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mEuCd.jpg&quot; alt=&quot;Landing shock attenuation system&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://history.nasa.gov/SP-4001/images/fig33.jpg&quot; rel=&quot;noreferrer&quot;&gt;Figure 33&lt;/a&gt;:  Landing shock attenuation system&lt;/p&gt;&#xA;" OwnerUserId="10132" LastActivityDate="2017-07-17T19:48:14.227" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22322" PostTypeId="2" ParentId="108" CreationDate="2017-07-18T00:11:20.030" Score="7" Body="&lt;p&gt;All of the Apollo Command Modules are on display as follows:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Apollo  6 - Fernbank Science Center, Atlanta, Georgia&lt;br&gt;&#xA;  Apollo  7 - Frontiers of Flight Museum, Dallas, Texas&lt;br&gt;&#xA;  Apollo  8 - Chicago Museum of Science and Industry, Chicago, Illinois&lt;br&gt;&#xA;  Apollo  9 - San Diego Air and Space Museum, San Diego, California&lt;br&gt;&#xA;  Apollo 10 - Science Museum, London, England&lt;br&gt;&#xA;  Apollo 11 - The National Air and Space Museum, Washington, D.C.&lt;br&gt;&#xA;  Apollo 12 - Virginia Air and Space Center, Hampton, Virginia&lt;br&gt;&#xA;  Apollo 13 - Kansas Cosmosphere and Space Center, Hutchinson, Kansas&lt;br&gt;&#xA;  Apollo 14 - Visitor's Center, Kennedy Space Center, Florida&lt;br&gt;&#xA;  Apollo 15 - USAF Museum, Wright-Patterson Air Force Base, Dayton, Ohio&lt;br&gt;&#xA;  Apollo 16 - U.S. Space and Rocket Center, Huntsville, Alabama&lt;br&gt;&#xA;  Apollo 17 - NASA Johnson Space Center, Houston, Texas&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;(&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/planetary/lunar/apolloloc.html&quot; rel=&quot;noreferrer&quot;&gt;Thanks to NASA's list of Apollo locations&lt;/a&gt;.)&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="13648" LastEditorUserId="6816" LastEditDate="2017-07-18T00:21:38.823" LastActivityDate="2017-07-18T00:21:38.823" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22324" PostTypeId="1" AcceptedAnswerId="35882" CreationDate="2017-07-18T09:47:56.007" Score="11" ViewCount="1140" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/O9DIa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/O9DIa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From the BBC's &lt;a href=&quot;http://www.bbc.com/news/world-asia-40640039&quot; rel=&quot;noreferrer&quot;&gt;Japan's zero-gravity space drone sends first pictures from ISS&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is &lt;a href=&quot;http://iss.jaxa.jp/en/kiboexp/news/170714_int_ball_en.html&quot; rel=&quot;noreferrer&quot;&gt;JAXA's GEM internal ball camera&lt;/a&gt;, or &quot;Int-Ball&quot;. &lt;em&gt;Kawaii!&lt;/em&gt; It looks like it uses a &lt;a href=&quot;http://www.kenkai.jaxa.jp/eng/research/electrical/triaxial.html&quot; rel=&quot;noreferrer&quot;&gt;three axis attitude control system developed for a ~31mm small cube satellite&lt;/a&gt; (see screen shots below), but for maneuvering around in space there are twelve electric micro-fans or $\mu$Fans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any information about how the airflow of these fans is managed? Do they work in adjacent pairs to avoid torque, or are they all pointing back toward the center of mass? Do the fans on the opposite side of the blowing fans actively suck in air, and transfer it via internal manifolds to the blowing fans? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Start watching &lt;a href=&quot;https://youtu.be/D_A-yb8rlac?t=63&quot; rel=&quot;noreferrer&quot;&gt;the video&lt;/a&gt; at &lt;code&gt;01:00&lt;/code&gt; to see attitude control and simultaneous live camera view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shot from &lt;a href=&quot;https://youtu.be/ZtIARUS7Lqc?t=40&quot; rel=&quot;noreferrer&quot;&gt;this helpful JAXA video&lt;/a&gt; at &lt;code&gt;00:40&lt;/code&gt;. The glowing lines are data/power lines from the control electronics, which sits above the cube with gyros. They to not represent airflow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/m3IeC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/m3IeC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shot from &lt;a href=&quot;https://youtu.be/VamXKnQnrPg?t=42&quot; rel=&quot;noreferrer&quot;&gt;this helpful JAXA video&lt;/a&gt; at &lt;code&gt;00:42&lt;/code&gt;. The&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AuPy9.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AuPy9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/D_A-yb8rlac?start=63&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-18T15:21:26.080" LastActivityDate="2019-05-02T11:23:08.397" Title="How does JAXA's &quot;Int-Ball&quot; manage airflow to optimally navigate inside the ISS?" Tags="|attitude-determination-and-control|robotics|aerobot|jaxa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22325" PostTypeId="1" CreationDate="2017-07-18T10:38:54.693" Score="5" ViewCount="523" Body="&lt;p&gt;I’m trying to understand what could happen when the Tiangong-1 will reenter the atmosphere (probably on march 2018).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starting from an initial state obtained from a TLE, I integrate the position and velocity vectors of the Tiangong-1 until the reentry date.&#xA;I do the same for each TLE I downloaded so far and here is the result:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2DTZ1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2DTZ1.png&quot; alt=&quot;Tiangong-1 reentry&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graph shows 65 possible trajectories based on 65 TLEs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, I integrated few trajectories down to the ground to see the so called max q (maximum dynamic pressure) encountered during reentry.&lt;br&gt;&#xA;According to my simulation, the max q should not exceed 11 kPa (usually I get 5 to 8 kPa). Just for comparison, the max q for the Saturn V was 35 kPa and for the Space Shuttle it was 28 kPa.&lt;br&gt;&#xA;I also compared the max q for the TMA-M reentry module in an hypothetical ballistic reentry trajectory (not used during normal reentry), here is what I get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0GBxm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0GBxm.png&quot; alt=&quot;Reentry comparison&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the Tiangong-1 max q is very small, I think that some &quot;hard&quot; components (like engine nozzles) will survive to the reentry. What do you think? Any comment?&lt;/p&gt;&#xA;" OwnerUserId="20397" LastEditorUserId="415" LastEditDate="2017-07-18T13:53:21.953" LastActivityDate="2017-07-18T13:53:21.953" Title="Tiangong-1 reentry" Tags="|reentry|tiangong-1|" AnswerCount="0" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="22326" PostTypeId="1" CreationDate="2017-07-18T12:04:17.423" Score="2" ViewCount="160" Body="&lt;p&gt;The Apollo missions, like all missions really, were limited in what they could bring into space. How did they decide what would make the trip? Were there any tests or failures that they learned from?&lt;/p&gt;&#xA;" OwnerUserId="20398" LastEditorUserId="20398" LastEditDate="2017-07-18T12:11:44.113" LastActivityDate="2017-07-18T12:11:44.113" Title="How did the Apollo engineers decide what to include in their cargo?" Tags="|apollo-program|payload|" AnswerCount="0" CommentCount="2" ClosedDate="2017-07-19T12:36:29.843" ContentLicense="CC BY-SA 3.0" />
  <row Id="22327" PostTypeId="5" CreationDate="2017-07-18T16:47:18.807" Score="0" Body="" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2017-07-18T16:47:18.807" LastActivityDate="2017-07-18T16:47:18.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22328" PostTypeId="4" CreationDate="2017-07-18T16:47:18.807" Score="0" Body="For questions specifically about the Chinese Tiangong-1 space laboratory, which was launched in 2011." OwnerUserId="415" LastEditorUserId="415" LastEditDate="2017-07-18T16:47:18.807" LastActivityDate="2017-07-18T16:47:18.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22329" PostTypeId="5" CreationDate="2017-07-18T16:48:42.670" Score="0" Body="" OwnerUserId="415" LastEditorUserId="415" LastEditDate="2017-07-18T16:48:42.670" LastActivityDate="2017-07-18T16:48:42.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22330" PostTypeId="4" CreationDate="2017-07-18T16:48:42.670" Score="0" Body="For questions specifically about the Chinese Tiangong-2 space laboratory, which was launched in 2016." OwnerUserId="415" LastEditorUserId="415" LastEditDate="2017-07-18T16:48:42.670" LastActivityDate="2017-07-18T16:48:42.670" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22331" PostTypeId="1" AcceptedAnswerId="22339" CreationDate="2017-07-18T17:31:34.733" Score="1" ViewCount="242" Body="&lt;p&gt;When a &lt;code&gt;SpaceCraft&lt;/code&gt; is injected into an &lt;code&gt;orbit&lt;/code&gt;( either LEO/GTO ), does the &lt;code&gt;point of injection&lt;/code&gt; coincide with that of the &lt;code&gt;perigee&lt;/code&gt; of the resultant &lt;code&gt;orbit&lt;/code&gt; ?&lt;/p&gt;&#xA;" OwnerUserId="19925" LastActivityDate="2017-07-18T21:28:38.710" Title="Does spacecraft injection point and perigee coincide?" Tags="|launch|orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22332" PostTypeId="1" AcceptedAnswerId="22337" CreationDate="2017-07-18T17:44:29.263" Score="20" ViewCount="5808" Body="&lt;p&gt;The last launch of a Falcon 9 (Intelsat 35e) took place on July 5, but the next launch of a falcon 9 (CRS-12) won't take place until August 10. Elon said that SpaceX still had 12 launches to go this year to finish their manifest for this year. So what's the reason for the month delay between falcon 9 launches? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found a &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Falcon_9#Falcon_9_Block_4&quot; rel=&quot;noreferrer&quot;&gt;wikipedia article&lt;/a&gt; that stated that CRS-12 was supposed to use the transitional falcon 9 block 4, so is SpaceX putting the final touches on it? I also thought maybe SpaceX  was fixing SLC-40 for the next falcon 9 launch but Spaceflightnow lists CRS-12 for 39A. &lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="51" LastEditDate="2017-07-18T19:27:07.817" LastActivityDate="2017-07-19T20:50:50.063" Title="Why won't SpaceX launch a rocket for a month?" Tags="|launch|spacex|falcon-9|launch-site|" AnswerCount="2" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22333" PostTypeId="2" ParentId="22243" CreationDate="2017-07-18T17:56:13.907" Score="2" Body="&lt;p&gt;It &lt;em&gt;could&lt;/em&gt; work, but for commercial purposes, it's not really economical.  The energy density of E85 is much lower (34.7% lower per unit volume, and 26.9% lower per unit mass) than diesel (which is basically kerosene, which is basically RP-1....sorry I couldn't find reliable energy numbers for RP-1).  Which basically means that you need a larger (and therefore heavier) launch vehicle to carry the extra fuel you'll need to get the needed delta-V for orbit.  Engineers continually aim for the best tradeoff of energy density vs. reliability, and complexity of the engine, tanks, and plumbing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And even though ethanol burns hot (1920&amp;deg;C), the previous answer's assertion that it's hotter than RP-1 is incorrect based on my research, since the RP-1 combustion temperature is 3396&amp;deg;C; which is actually cooler than hydrogen at stochiometric mixture -- 2800&amp;deg;C.&lt;/p&gt;&#xA;" OwnerUserId="19186" LastActivityDate="2017-07-18T17:56:13.907" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22334" PostTypeId="1" CreationDate="2017-07-18T18:23:25.773" Score="0" ViewCount="1919" Body="&lt;p&gt;What is the logic behind spacecraft spinning to increase stability? How does angular momentum play a role in this? &lt;/p&gt;&#xA;" OwnerUserId="20383" LastEditorUserId="38" LastEditDate="2017-07-19T11:49:14.907" LastActivityDate="2017-07-19T11:49:14.907" Title="Why does a spacecraft's attitude become stable when it rotates?" Tags="|spacecraft|nasa|juno|spin-stabilisation|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22336" PostTypeId="2" ParentId="22332" CreationDate="2017-07-18T19:22:44.557" Score="5" Body="&lt;p&gt;The range is closed for maintenance during most of July. The next planned launch is ULA Atlas V with TDRS-M on August 3, but that one is now &lt;a href=&quot;https://www.nasaspaceflight.com/2017/07/tdrs-m-under-review-pre-launch-incident/&quot; rel=&quot;noreferrer&quot;&gt;under review after an incident&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2017-07-18T19:22:44.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22337" PostTypeId="2" ParentId="22332" CreationDate="2017-07-18T19:26:25.150" Score="39" Body="&lt;p&gt;The Eastern Range run by the Air Force has a 2 or 3 week maintenance window. Thus no one is launching from either Cape Canaveral (Where LC-39A is located), nor the CCAFS (Cape Caneveral Air Force Station, where LC-40 is located) locations. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX is using the time the range is down to modify the RSS (Rotating Support Structure) that is left over from Shuttle days, to remove more of it, and to work towards installing the Crew Access Arm, needed for manned Dragon V2 flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has two (at least) major modifications on the books for LC-39A.  Install the Crew Access Arm, and add the Falcon Heavy hold downs and equipment. It is unclear how much of the work they will get finished in this window. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They have said the Falcon Heavy modifications with take 60 days, and will start once LC-40 is reactivated and flying. I.e. They need to keep a pad open and launching. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is why they were in a hurry to get the Intelsat 35e launch off before the range shut down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then once the range is ready again, they have to find a launch time, and the CRS-12 mission, which is next, has ISS requirements for launch dates. The ISS has a lot of Visiting Vehicles (Soyuz, Progress, Dragon, Cygnus, and soon CST-100 and Dragon V2) and scheduling crew time to unload and reload is a complex ballet. Thus the date of CRS-12 is set by the ISS schedule and there may not have been time to slide another launch in beforehand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a mostly &lt;a href=&quot;https://www.inverse.com/article/34339-spacex-next-mission-august-10-range-downtime&quot; rel=&quot;noreferrer&quot;&gt;information null article&lt;/a&gt; on this very issue. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-07-19T20:50:50.063" LastActivityDate="2017-07-19T20:50:50.063" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22338" PostTypeId="2" ParentId="22334" CreationDate="2017-07-18T19:42:08.837" Score="0" Body="&lt;p&gt;A symmetric body with no torques applied with even a slight bit of internal damping (as all real object have) will eventually rotate about its principal axis with the lowest moment of inertia.  The faster the spin (= higher angular momentum), the more effort it takes to alter the axis of the spin (= greater stability).&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2017-07-19T04:32:31.277" LastActivityDate="2017-07-19T04:32:31.277" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22339" PostTypeId="2" ParentId="22331" CreationDate="2017-07-18T21:28:38.710" Score="3" Body="&lt;p&gt;No, the point of injection may be any point along the ellipse of the orbit. It is &lt;em&gt;common&lt;/em&gt; for it to be at or near the perigee, but there's no physical requirement that it be so. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-07-18T21:28:38.710" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22340" PostTypeId="1" CreationDate="2017-07-18T21:34:22.840" Score="7" ViewCount="508" Body="&lt;p&gt;I know Elon Musk is shooting for a crewed mission to Mars sometime in the 2020s but what will happen from there?&lt;/p&gt;&#xA;" OwnerUserId="20407" LastEditorUserId="11902" LastEditDate="2022-09-21T05:59:22.020" LastActivityDate="2022-09-21T09:02:07.890" Title="What steps is Elon Musk taking to colonize Mars?" Tags="|spacex|mars|colonization|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22341" PostTypeId="2" ParentId="22334" CreationDate="2017-07-18T22:19:13.197" Score="1" Body="&lt;p&gt;The process is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Spin-stabilisation&quot; rel=&quot;nofollow noreferrer&quot;&gt;spin stabilization&lt;/a&gt; and is not used on every spacecraft, but some. Most notably, it is not used on &lt;em&gt;any&lt;/em&gt; manned craft since it would be detrimental to the health of the passengers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conservation of angular momentum applies. A body always spins about its principal axis. If the rocket already spins at a high RPM, it is much more diffcult to alter the axis of rotation - the rocket will be much more stable. See it that way: if you add just a little bit of rotation to a body at rest, it will slowly rotate. If you apply the same tiny bit of rotation to a fast-spinning object, its axis of rotation will barely even change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, a rotating rocket smoothes out any individual disturbance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its pretty much the same effect effect as a gyroscope or momentum wheel (which &quot;absorb&quot; angular momentum on demand), just with the whole rocket body and only on one axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not that the rocket has to be de-spun from its usually high RPM (50 - 600) once it reaches its target orbit in order to release its payload (typical satellites can handle at most 2-5 RPM with their own attitude control). Various techniques are available, e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Yo-yo_de-spin&quot; rel=&quot;nofollow noreferrer&quot;&gt;yoyo-despin&lt;/a&gt;, but this technique is not always seen as desirable because of the debris it generates.&lt;/p&gt;&#xA;" OwnerUserId="13867" LastActivityDate="2017-07-18T22:19:13.197" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22342" PostTypeId="2" ParentId="22340" CreationDate="2017-07-18T23:09:21.803" Score="10" Body="&lt;p&gt;Elon Musk is doing a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gnomes_(South_Park)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Underpants Gnome&lt;/a&gt; style mission.&lt;/p&gt;&#xA;&lt;p&gt;He plans on building the railroad to colonize the West, and others will come along and sell/build other needed infrastructure. Without the Conestoga wagons or railroad, it won't happen.&lt;/p&gt;&#xA;&lt;p&gt;So he is doing Step 1: Steal Underpants. Ooops, I mean, build a transport system to Mars.&lt;/p&gt;&#xA;&lt;p&gt;Step 2: ????&lt;/p&gt;&#xA;&lt;p&gt;Step 3: Profit!&lt;/p&gt;&#xA;&lt;p&gt;But more seriously, someone has to build the way there; that is his focus. He expects others to step up and create other stuff once they get there.&lt;/p&gt;&#xA;&lt;p&gt;It does not seem to be seriously more considered than that, at least in the public literature.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="40302" LastEditDate="2022-09-21T09:02:07.890" LastActivityDate="2022-09-21T09:02:07.890" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="22343" PostTypeId="1" CreationDate="2017-07-19T03:18:03.363" Score="15" ViewCount="424" Body="&lt;p&gt;As discussed in &lt;a href=&quot;https://space.stackexchange.com/questions/22332/why-wont-spacex-launch-a-rocket-for-a-month&quot;&gt;this question&lt;/a&gt;, there will be no launches from Cape Canaveral for several weeks during the summer of 2017 because Eastern Range is closed for maintenance. I assume it includes some work on the tracking equipment and such. Could someone shed more light on what range maintenance includes?&lt;/p&gt;&#xA;" OwnerUserId="12080" LastActivityDate="2017-07-24T05:20:09.450" Title="What is Range maintenance?" Tags="|launch|launch-site|range-safety|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22344" PostTypeId="2" ParentId="22334" CreationDate="2017-07-19T03:24:09.783" Score="4" Body="&lt;p&gt;Rotations about a principal axis for an object with three distinct principal moments of inertia are stable if the rotation is about the axis with the least or greatest moment of inertia but unstable if the rotation is about the axis with the intermediate moment of inertia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Showing that this is the case is one of the torture tests for physics majors. One name for this phenomenon is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tennis_racket_theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;tennis racket theorem&lt;/a&gt;. You can easily see this by wrapping a rubber band about a book and tossing it with a bit of a spin. Give the book a flip about either the smallest and largest principal axes and you'll see a nice and stable rotation. Give it a flip about the intermediate axis and you'll see something a bit chaotic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some satellites take advantage of this phenomenon and establish a rotation that is more or less about either the smallest or largest principal axis. The satellite's control system can detect and correct deviations precisely because these rotations are stable.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-07-19T03:29:46.983" LastActivityDate="2017-07-19T03:29:46.983" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22345" PostTypeId="1" CreationDate="2017-07-19T07:10:46.673" Score="9" ViewCount="9905" Body="&lt;p&gt;I just want to ask if someone does possess the formula linking the local hour to the right ascension of the ascending node (RAAN).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will try to explain myself: I know that I want my satellite to be on a sun-synchronous orbit 10h30-22h30 with 10h30 corresponding to my local time of ascending node (LTAN) as my entrance parameter; and I want to know the RAAN because in every software simulating an orbit the entrance parameter is the RAAN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how can I pass from the LTAN to the RAAN?&lt;/p&gt;&#xA;" OwnerUserId="20396" LastEditorUserId="9060" LastEditDate="2017-07-19T15:47:57.393" LastActivityDate="2022-06-10T00:06:25.637" Title="formula linking RAAN and LTAN" Tags="|orbit|sun-synchronous|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22347" PostTypeId="1" CreationDate="2017-07-19T15:17:46.360" Score="1" ViewCount="122" Body="&lt;p&gt;I started learning &lt;a href=&quot;https://en.wikipedia.org/wiki/SolidWorks&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solidworks&lt;/a&gt; in  hopes of being able to run simulation  to study  rocket chambers.  Currently, I am studying the flow of plasma and its  interaction with the magnetic  field,  but found that,  Solid works does  not have that  feature. or, can I just use Matlab  for  this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you  are  aware  of this  field, could  you please recommend me the  software to use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some names I came across were &lt;a href=&quot;https://en.wikipedia.org/wiki/Ansys&quot; rel=&quot;nofollow noreferrer&quot;&gt;ANSYS&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/COMSOL_Multiphysics&quot; rel=&quot;nofollow noreferrer&quot;&gt;COMSOL Multiphysics&lt;/a&gt;, but would like to know if there is something better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you.&lt;/p&gt;&#xA;" OwnerUserId="20418" LastEditorUserId="20418" LastEditDate="2017-07-19T19:35:11.950" LastActivityDate="2017-07-19T19:35:11.950" Title="Simulation of Rocket chambers like in Hall Effect Thrusters" Tags="|propulsion|engine-design|software|ion-thruster|" AnswerCount="0" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22349" PostTypeId="2" ParentId="20361" CreationDate="2017-07-19T19:13:00.763" Score="6" Body="&lt;p&gt;Yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A nice example would be the recent splashdown of the first SpaceX Dragon capsule to have been re-used after a previous visit to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See for example Spaceflight Now's July 3, 2017 article &lt;a href=&quot;https://spaceflightnow.com/2017/07/03/dragon-capsule-returns-home-with-animals-and-station-equipment/&quot; rel=&quot;noreferrer&quot;&gt;Dragon capsule returns home with animals and station equipment&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Items stowed for Dragon’s return included live mice from an experiment to investigate the effectiveness of a therapeutic drug to promote bone growth, combating atrophy in astronauts in space and osteoporosis patients on Earth. The mice will be euthanized and examined after landing.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;The Dragon spacecraft was lifted onto a boat for a trip back to port in Southern California following Monday’s predawn splashdown. Credit: SpaceX&quot; From &lt;a href=&quot;https://spaceflightnow.com/2017/07/03/dragon-capsule-returns-home-with-animals-and-station-equipment/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/epS1q.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/epS1q.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-07-19T19:13:00.763" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22351" PostTypeId="1" AcceptedAnswerId="22355" CreationDate="2017-07-20T05:31:07.527" Score="8" ViewCount="424" Body="&lt;p&gt;I was looking at pictures of a test fire of a TR-106 engine, and it appears to be fairly under-expanded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/h6fXB.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h6fXB.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's the reason for this? Does this somehow increase efficiency with this specific engine?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="6816" LastEditDate="2017-07-20T14:50:52.703" LastActivityDate="2017-07-20T15:52:40.423" Title="TR-106 under-expansion in sea-level test?" Tags="|engine-design|nozzle|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22352" PostTypeId="1" AcceptedAnswerId="22357" CreationDate="2017-07-20T08:52:07.820" Score="5" ViewCount="256" Body="&lt;p&gt;The Reuters article &lt;a href=&quot;https://www.reuters.com/article/us-northkorea-missile-un-idUSKBN1A50AA&quot; rel=&quot;nofollow noreferrer&quot;&gt;Russia, U.S. duel at U.N. over whether North Korea fired long-range missile&lt;/a&gt; describes some ongoing &lt;a href=&quot;https://en.wikipedia.org/wiki/United_Nations_Security_Council&quot; rel=&quot;nofollow noreferrer&quot;&gt;United Nations Security Council&lt;/a&gt; discussion related to the classification of the recent test. I am not sure if it is the launch vehicle's potential capability or demonstrated capability that is used in order to distinguish between medium-range and long-range classifications, as well as unfamilliar what those classifications actually mean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone clarify these terms, and how they are applied in difficult-to-verify situations?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &lt;em&gt;Simplistic&lt;/em&gt; simulation of a ballistic trajectory with initial velocities of 6,500 to 6,700 m/s at a small angle, with the dots representing location after 40 minutes (first plot) and 28 minutes (second plot). This is just to get a rough feeling for the scale of the trajectory and not an accurate representation of the real event, but it illustrates that &lt;strong&gt;the potential range of the launch is much much farther than the range of the test&lt;/strong&gt; because the launch was nearly vertical&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/I7tC3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/I7tC3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/skTlG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/skTlG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(X, t):&#xA;    x, v = X.reshape(2, -1)&#xA;    acc  = -GMe * x * ((x**2).sum())**-1.5&#xA;    return np.hstack((v, acc))&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;eightthpi, quarterpi, halfpi, pi, twopi = [2**n*np.pi for n in range(-3, 2)]&#xA;degs, rads = 180/pi, pi/180&#xA;&#xA;GMe = 3.98600418E+14     # m^3 s^-2&#xA;re  = 6378000.&#xA;kms = 1E-03&#xA;&#xA;time = 60 * np.arange(0, 41)&#xA;&#xA;answers = []&#xA;&#xA;# v0s = np.linspace(6500, 6700, 5)&#xA;#     for v0 in v0s:&#xA;&#xA;v0 = 6600.&#xA;thetas = rads * np.arange(5, 61, 5)&#xA;&#xA;for theta in thetas:&#xA;    X0 = np.hstack(([0, re], [v0*f(theta) for f in [np.sin, np.cos]]))&#xA;&#xA;    answer, info = ODEint(deriv, X0, time, full_output=True)&#xA;    answers.append(answer)&#xA;&#xA;if 1 == 1:&#xA;    plt.figure()&#xA;    for answer in answers:&#xA;        stop = np.argmax(np.sqrt((answer.T[:2]**2).sum(axis=0)) &amp;lt; re-1000)&#xA;        x, y = kms*answer[:stop+1].T[:2]&#xA;        print stop,&#xA;        plt.plot(x, y-kms*re)&#xA;        plt.plot(x[::28], y[::28]-kms*re, '.k')&#xA;    theta = np.linspace(0, halfpi+eightthpi, 100)&#xA;    x, y = [kms*re*f(theta) for f in [np.cos, np.sin]]&#xA;    plt.plot(x, y-kms*re, '-k')&#xA;    plt.xlim(-2000, 7999)&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-20T14:46:09.163" LastActivityDate="2017-07-28T09:44:55.290" Title="How to decide if the rocket most recently tested by North Korean is classified as long-range or medium-range launch?" Tags="|military|icbm|international-politics|policy|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22353" PostTypeId="1" CreationDate="2017-07-20T09:14:38.103" Score="2" ViewCount="779" Body="&lt;p&gt;It seems that the mass of the Space Shuttle at launch was around 2,000 tons, and it could deliver a total payload of around 122 tons to LEO. This would give it a launch/LEO mass ratio of around 16.4. Is this the lowest ratio among all orbital launchers in history?&lt;/p&gt;&#xA;" OwnerUserId="11820" LastActivityDate="2017-07-20T16:06:14.163" Title="Was the Space Shuttle the most efficient orbital launcher, based on the launch/LEO mass ratio?" Tags="|launch|low-earth-orbit|space-shuttle|payload|" AnswerCount="1" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22354" PostTypeId="2" ParentId="22353" CreationDate="2017-07-20T14:32:04.353" Score="13" Body="&lt;p&gt;The shuttle was certainly one of the more mass-efficient orbital launchers, but it's a more complicated question than it might seem at first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle orbiter is ambiguously part of the launcher and part of the payload, so it's simply not possible to compare it apples-to-apples against other launchers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For &quot;pure&quot; payload that's going to stay in orbit after the flight, the ratio is very poor, as it delivers about 25-27.5 tons. This considers the orbiter and crew to be dead weight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the crew are going to be doing useful science then it's reasonable to consider the 68-ton orbiter as payload, bringing the delivered payload to 93-95.5 tons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ET isn't useful and doesn't reach stable orbit, so it's not reasonable to count it as delivered payload. Building space stations or  fuel depots out of ETs has been proposed, but these plans were never developed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives us a practical payload ratio of about 21.2:1. This is just about on par with Saturn V's demonstrated 21.7:1 (Apollo 11; later flights may have performed slightly better). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 Full Thrust has a ratio of about 26; Atlas V 401 has a ratio of about 32.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will also take this opportunity to note that defining efficiency by the ratio of liftoff mass to LEO mass is only of academic interest; liftoff &lt;em&gt;cost&lt;/em&gt; to LEO mass is a much more significant figure of merit.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-07-20T16:06:14.163" LastActivityDate="2017-07-20T16:06:14.163" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22355" PostTypeId="2" ParentId="22351" CreationDate="2017-07-20T15:39:48.033" Score="9" Body="&lt;p&gt;There's no good reason to run a &lt;em&gt;production&lt;/em&gt; rocket engine underexpanded at ground level; it loses efficiency and will only become more underexpanded as it gains altitude.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you compare the shape of that nozzle and the size of the combustion chamber to pretty much any other rocket engine, it's clear that this is a very short nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The curve of the expanding exhaust plume gives a pretty good idea of what the nozzle should actually look like -- 3-4 times the length of the existing nozzle stub.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, I believe this is is a test of the combustion chamber only. Since the TR-106 is ablatively cooled, there's no point in fabricating a disposable nozzle extension for early tests, and the more compact engine is easier to transport and install. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-07-20T15:52:40.423" LastActivityDate="2017-07-20T15:52:40.423" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22357" PostTypeId="2" ParentId="22352" CreationDate="2017-07-20T17:10:47.767" Score="4" Body="&lt;p&gt;Ballistic missiles are usually divided into groups based on range. These groups were originally defined by the &lt;a href=&quot;https://www.mda.mil/&quot; rel=&quot;nofollow noreferrer&quot;&gt;US military&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Intercontinental (ICBM), with a range of more than &lt;a href=&quot;https://fas.org/nuke/intro/missile/icbm.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;5500&lt;/a&gt; km, i.e. able to reach the US from Russia v.v. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Intermediate-range (IRBM), with a range of 3,000–5,500 km. Able to reach Moscow from the UK. Still a strategic asset.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Medium and short range ballistic missiles, anything with a shorter range than an IRBM. Generally meant to be used in the area (theatre) where war is being waged, but some countries use them for strategic purposes too (deterrent).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;A country like the USA can verify some aspects of the launch. With a ballistic missile tracking radar stationed at sea or in South Korea (or at the altitudes we're talking about, much further away), they can monitor the trajectory. They can monitor the missile's radio transmissions (they might be encrypted though). They could to spectroscopy on the exhaust plume. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you noted, there's some guesswork involved too. Espionage assets can give a pretty good idea of how large the missile is, which gives an upper bound to performance. North Korea may have programmed the flight to be ambiguous (range near the IRBM/ICBM threshold) on purpose. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-07-28T09:44:55.290" LastActivityDate="2017-07-28T09:44:55.290" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22359" PostTypeId="1" CreationDate="2017-07-21T01:41:11.080" Score="3" ViewCount="755" Body="&lt;p&gt;I know in the lower levels of the stratosphere we can still have 100 mph winds albeit low density winds.  Could I possibly go high enough in a balloon or imaginary vertical craft that I would go above the atmosphere and remain relatively stationary at that altitude and the earth rotates away underneath me?  IOW, 24 hours would go by and I can return (descend vertically) to around same lat long I launched from just one day later.  What altitude would that be?  Or would earth's gravity always tug me along to some degree as it rotates? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the physics that describe this?&lt;/p&gt;&#xA;" OwnerUserId="19730" LastEditorUserId="19730" LastEditDate="2017-07-21T05:52:12.053" LastActivityDate="2020-05-01T20:26:36.050" Title="At what altitude would I have to go in a lighter than air balloon to be above all wind and just have the earth rotate underneath me?" Tags="|balloons|rotation|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22360" PostTypeId="2" ParentId="22359" CreationDate="2017-07-21T05:06:35.773" Score="10" Body="&lt;p&gt;The question contains a misconception - wind is the movement of the atmosphere relative to the surface.  So if you are at an altitude with no wind, you'll be stationary relative to the surface - the earth won't be rotating underneath you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if the objective is to go up, move around the world and come back close to where you started, you might be interested in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Polar_vortex&quot; rel=&quot;noreferrer&quot;&gt;circumpolar vortices&lt;/a&gt;.  These are regularly used by (unpiloted) scientific high altitude balloon missions such as &lt;a href=&quot;https://en.wikipedia.org/wiki/BLAST_(telescope)&quot; rel=&quot;noreferrer&quot;&gt;BLAST&lt;/a&gt;, about which an enjoyable &lt;a href=&quot;https://en.wikipedia.org/wiki/BLAST!_(2008_film)&quot; rel=&quot;noreferrer&quot;&gt;documentary&lt;/a&gt; was made.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-07-21T05:06:35.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22361" PostTypeId="1" AcceptedAnswerId="22362" CreationDate="2017-07-21T09:17:29.263" Score="2" ViewCount="439" Body="&lt;p&gt;I understant that these six different Earth-observing satellite platforms measure and image Earth data differenty, but there is just such a wide variety of different ways to build solar panels and attach them shown here. Are the shapes and attachment methods of the panels so extremely sensitive to they type of Earth observation being done? Or was a different contractor used each time and everybody wanted to avoid everybody else's patents?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Why do the solar panels on the seven Sentinel configurations look so completely different from each other?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.copernicus.eu/main/overview&quot; rel=&quot;nofollow noreferrer&quot;&gt;What is Copernicus?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Copernicus is a European Union Programme aimed at developing European information services based on satellite Earth Observation and in situ (non-space) data.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.copernicus.eu/main/sentinels&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sentinel Satellites&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Sentinel Satellites are developed for the specific needs of the Copernicus programme. They provide a unique set of observations for Copernicus. &lt;a href=&quot;https://sentinels.copernicus.eu/web/sentinel/home&quot; rel=&quot;nofollow noreferrer&quot;&gt;Visit Sentinel Online!&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;They consist of six different families, presented below.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sentinel-1:&lt;/strong&gt; &lt;a href=&quot;https://i.stack.imgur.com/IBWKx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IBWKx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sentinel-2:&lt;/strong&gt; &lt;a href=&quot;https://i.stack.imgur.com/J7yzC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J7yzC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sentinel-3:&lt;/strong&gt; &lt;a href=&quot;https://i.stack.imgur.com/t0ju8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/t0ju8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sentinel-4:&lt;/strong&gt; &lt;a href=&quot;https://i.stack.imgur.com/hWD5v.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hWD5v.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sentinel-5:&lt;/strong&gt; &lt;a href=&quot;https://i.stack.imgur.com/eaB7b.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eaB7b.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Sentinel-5 configuration looks like &lt;a href=&quot;https://space.stackexchange.com/q/21125/12102&quot;&gt;MetOp-A&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sentinel-5P:&lt;/strong&gt; &lt;a href=&quot;https://i.stack.imgur.com/7aLmt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7aLmt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sentinel-6:&lt;/strong&gt; &lt;a href=&quot;https://i.stack.imgur.com/unbmX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/unbmX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-21T11:46:52.037" LastActivityDate="2017-07-21T11:46:52.037" Title="Why do the solar panels on the seven Sentinel configurations look so completely different from each other?" Tags="|design|solar-power|earth-observation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22362" PostTypeId="2" ParentId="22361" CreationDate="2017-07-21T10:49:51.827" Score="3" Body="&lt;p&gt;You are right that the difference stems in part from the contractor teams and then through that because of the different solutions pursued. There is always scope for different solutions when considering different ways of obtaining two axis solar array orientation. However the industry is also quite conservative and its quite possible that a given solution would be used, not because it is the optimum, but because it worked for a similar previous mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sentinel 1 and 3 are both led by Thales Alenia Space (Italy) and, I think, are both based on their Prima platform. However Sentinel 1 are radar satellites and have a much higher power requirement than Sentinel 3. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sentinel 2 and 5 are led, I think, both by Airbus (Germany) and 5P by Airbus UK.  Sentinel 4 is led by OHB and will go to Geostationary orbit. Most GEO satellites use only single axis array orientation.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-07-21T10:49:51.827" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22363" PostTypeId="2" ParentId="8378" CreationDate="2017-07-21T11:31:05.293" Score="2" Body="&lt;p&gt;The delays in Falcon Heavy launching has led to SpaceX having contracts to launch payloads that are too big to land the first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several payloads that needed more performance than is available from a reusable Falcon 9. The Falcon Heavy was meant to be used but was not ready.  Thus to fullfill the contracts they launched on Falcon 9 in expendable mode.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This provides the 30% of payload lost for recovering the stage to the payloads mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally the absence of legs, grid fins, cold gas thrusters mean the first stage is a bit lighter and thus more performance again.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-07-21T11:31:05.293" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22364" PostTypeId="1" AcceptedAnswerId="22398" CreationDate="2017-07-21T13:35:47.773" Score="5" ViewCount="3113" Body="&lt;p&gt;I have been building some &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_candy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket-candy&lt;/a&gt; engines for fun, and informally experimenting with different designs or techniques. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now I'm interested in figuring out a way to compare the performance of the different engines I make. What kind of hobby-level experiments might I do, and what simple metrics might I use to try to decide if one engine is &quot;better&quot; than another, or what &quot;better&quot; would even mean?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've looked at some videos of engine tests and have been experimenting with trying to use a weight scale (or my own spring+ruler) to measure the thrust, but I'm not sure the best way to record the data, or use it to try to calculate standard things like impulse or rocket acceleration. What about going a step further and trying to predict which rocket might go higher or lift a heavier payload to the same height?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And are there other instruments I should look into buying to do other measurements on this kind of hobby rocket engine? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where to begin? Thanks!&lt;/p&gt;&#xA;" OwnerUserId="20332" LastEditorUserId="12102" LastEditDate="2017-07-21T17:18:19.023" LastActivityDate="2017-07-25T05:15:30.817" Title="How can I measure the thrust of a model rocket and get other statistics" Tags="|rockets|engines|model-rocketry|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22365" PostTypeId="2" ParentId="22359" CreationDate="2017-07-21T17:50:53.727" Score="1" Body="&lt;p&gt;Well the Earth is rotating around 1000 mph. (A rough estimate) You, in your balloon are moving with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you rise into almost no atmosphere you are still moving East at 1000 mph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An object set in motion stays in motion unless acted upon by some opposite force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since there are very few molecules to slow you down and these few molecules are moving East with you, you would continue eastward with the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After reaching altitude, you would have to use some type of propulsion to slow yourself in the westward direction by 1000 mph.&lt;/p&gt;&#xA;" OwnerUserId="20342" LastActivityDate="2017-07-21T17:50:53.727" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22366" PostTypeId="1" CreationDate="2017-07-21T20:02:28.940" Score="9" ViewCount="1549" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3ZZ6f.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3ZZ6f.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.reactionengines.co.uk/sabre&quot; rel=&quot;nofollow noreferrer&quot;&gt;SABRE engine&lt;/a&gt; is designed to power the Skylon SSTO vehicle. It reduces the amount of oxygen that needs to be carried by cooling and compressing air in the early part of the flight. More hydrogen is needed for cooling than is used in the engine, so it is sent to ramjets surrounding the main nozzle. Presumably the ramjets would generate no thrust at engine start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why have ramjets been used? Wouldn't it be more efficient to use slush hydrogen and condense the hydrogen for use later in the flight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, wouldn't it be more efficient to design the SABRE as an air-augmented rocket? Wouldn't that mean that the spare hydrogen would generate thrust from engine start? Could it also improve specific impulse after transition to rocket mode?&lt;/p&gt;&#xA;" OwnerUserId="20335" LastEditorDisplayName="user20636" LastEditDate="2018-12-13T14:50:44.483" LastActivityDate="2018-12-13T14:50:44.483" Title="Why does the SABRE engine have ramjets?" Tags="|propulsion|hydrogen|sabre-engine|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22367" PostTypeId="2" ParentId="20951" CreationDate="2017-07-22T04:24:40.277" Score="2" Body="&lt;p&gt;32 meter antenna of IDSN is being used by JAXA and ISRO to study Venusian atmosphere using signals from Akatsuki probe and Radio occultation technique. ISRO is also planning a Venus mission so these results are of mutual interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Excerpt from &lt;a href=&quot;http://www.isro.gov.in/annual-report-2016-2017-english&quot; rel=&quot;nofollow noreferrer&quot;&gt;Annual Report 2016-17 of Dept. of Space (India)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The space agencies of India and Japan have agreed to further enhance cooperation by signing a new MoU, which enables inclusion of satellite navigation and planetary exploration as additional areas of cooperation. &lt;strong&gt;Both agencies have also agreed to conduct a joint experiment to study Venus atmosphere by collecting signals from JAXA’s Akatsuki mission by ISRO’s ground stations (IDSN).&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A recent paper on results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Initial results of the radio occultation experiment in the&#xA;Venus orbiter mission Akatsuki&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://meetingorganizer.copernicus.org/EPSC2017/EPSC2017-403.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://meetingorganizer.copernicus.org/EPSC2017/EPSC2017-403.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The primary ground station used so far is the 64-m antenna of Usuda Deep Space Center (UDSC) of JAXA. In addition to UDSC, for increasing the number of observation opportunities, we started to use the 32-m antenna of Indian Deep Space Network (IDSN) of Indian Space Research Organization (ISRO) in 2017.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="19014" LastActivityDate="2017-07-22T04:24:40.277" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22370" PostTypeId="2" ParentId="22359" CreationDate="2017-07-22T12:01:11.757" Score="6" Body="&lt;p&gt;A balloon cannot ascend above the entire atmosphere. The balloon's buoyancy decreases with height until you're at an equilibrium. The current record is 53 km. At that altitude, &lt;a href=&quot;http://www.intercomms.net/AUG03/content/struzak1.php&quot; rel=&quot;noreferrer&quot;&gt;you still get wind&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8R1Ny.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8R1Ny.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wind speeds at 50 km are low, indicating that at that altitude, the atmosphere still corotates with the surface. This means your balloon will stay near your takeoff site, drifting slowly due to wind. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd need a craft that can fly fast enough to cancel out Earth's rotation speed. Depending on your latitude, an aircraft can do this. A famous example is Concorde, which did London-New York while flying faster than Earth's rotation, so that it would land &quot;before it took off&quot; (local time of landing was before local time of takeoff).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also launch a spacecraft into an orbit that cancels out Earth's rotation. This class of orbits is called a polar orbit. Polar orbits are in common use for Earth observation satellites. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-07-22T12:09:22.020" LastActivityDate="2017-07-22T12:09:22.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22371" PostTypeId="1" AcceptedAnswerId="22391" CreationDate="2017-07-22T15:38:02.587" Score="2" ViewCount="686" Body="&lt;p&gt;I am currently designing specifications for a human-inhabited station on Mars. Said station would require communications with Earth, to do so requires power.&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Arecibo_message&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apparently&lt;/a&gt;, a radio telescope is capable of transmission of data. But power is the main concern at the moment; I have not decided on a power source yet. To do so I need to know a radio telescope's peak power consumption to make sure that the lights on the rest of the station don't go out, or, more importantly, the lithium-ion cells are not overloaded (they are apparently &lt;a href=&quot;https://en.wikipedia.org/wiki/Lithium-ion_battery#Safety&quot; rel=&quot;nofollow noreferrer&quot;&gt;combustible&lt;/a&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to have communication speeds close to 256kbit/s (the average uplink speed for satellite internet), and a dish diameter of 30 to 50 meters (which should work for both Earth-Mars comms and some radio astronomy).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, if you have suggestions about more effective ways of communication with the Earth from Mars, please let me know and I'll edit the question accordingly.&lt;/p&gt;&#xA;" OwnerUserId="20450" LastEditorUserId="20450" LastEditDate="2017-07-24T12:28:50.900" LastActivityDate="2017-07-24T18:59:41.400" Title="What is a transmitting radio telescope's peak power consumption?" Tags="|mars|communication|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22372" PostTypeId="2" ParentId="22366" CreationDate="2017-07-22T18:53:08.657" Score="11" Body="&lt;p&gt;At engine start the ramjets would provide no thrust - but at this stage of flight there are not excessive cooling needs, so there's no &quot;excess hydrogen&quot; to burn in them anyway.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the middle of the flight regime the air intake is decelerating more air than is needed, by reheating this extra air with the excess hydrogen in the ramjet burners, this extra ram drag is mitigated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the end of the flight, when the incoming air is hottest almost no air is available for the bypass burners so they again produce little thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a happy coincidence that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Ramjet#Performance_and_control&quot; rel=&quot;nofollow noreferrer&quot;&gt;conditions under which a ramjet are most effective&lt;/a&gt; are also the conditions that the SABRE nacelles have spare hydrogen and air.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using slush hydrogen (directly) would reduce the amount of fuel needed to cool the incoming air, and so reduce the wastage at the final part of the flight, but potentially removing the advantage given by the ramjets in the middle part of the flight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If slush hydrogen was used as you suggest it would increase the system complexity - hot hydrogen would have to be brought back into the tank area, and held in tanks which would need to be able to cope with increasing pressure throughout the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using slush hydrogen would increase the complexity of the fuelling - One of the Skylon design goals is to have simple, fast and automated fuelling procedures &lt;a href=&quot;https://web.archive.org/web/20160328220308/http://www.reactionengines.co.uk/tech_docs/SKYLON_Users_Manual_Rev_2.1.pdf#page=35&quot; rel=&quot;nofollow noreferrer&quot;&gt;(40 minutes to connect, fill and disconnect)&lt;/a&gt;.&#xA;It would add additional untested technologies into the mix another Skylon design goal is to minimise development risk by using well understood techniques and materials - at least for the first batch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For solid fuel, an air augmented rocket has about double the ISP. Apparently a study into an &lt;a href=&quot;http://www.astronautix.com/n/n1-mokstage.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;air augmented LH2/LOx engine&lt;/a&gt; expected an ISP of around 830 - but I haven't found a source I trust for that. SABRE is expected to deliver an ISP &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=36826.0;attach=1106832;image&quot; rel=&quot;nofollow noreferrer&quot;&gt;peaking&lt;/a&gt; at 3600 in it's air-breathing phase&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A further reason for there being little benefit in using slush hydrogen is the latent heat of fusion is low - about 58 j/g and the specific heat capacity of liquid hydrogen below the planned temperature 18K is also low - about 8j/gK, so using slush hydrogen would gain around 80j/g. When compared to 447j/g for the latent heat of fission and the 10-15 j/gK SHC over the 18-500K operation range of the precooler giving around 6400j/g using slush hydrogen would represent a saving of around 1.25% of the wasted fuel at the high altitude &amp;amp; Mach portion of the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OrangePeel52 rightly points out hydrogen embrittlement is an issue; to minimise it in the SABRE design helium is used as a working fluid in a &quot;loop&quot; to cool the incoming air but is itself in turn cooled (and effectively compressed in a Brayton cycle) by the hydrogen, so it is the hydrogen the heat is dumped into.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately, they've designed the engine this way because they think it's the most achievable solution to the problem they have in mind. Other engines may be better for other scenarios.&lt;/p&gt;&#xA;" OwnerDisplayName="user20636" LastEditorDisplayName="user20636" LastEditDate="2018-06-09T22:33:58.693" LastActivityDate="2018-06-09T22:33:58.693" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22373" PostTypeId="1" AcceptedAnswerId="22377" CreationDate="2017-07-22T19:33:13.787" Score="17" ViewCount="2124" Body="&lt;p&gt;The NASA Goddard video &lt;a href=&quot;https://www.youtube.com/watch?v=MJgXaqW3md8&quot; rel=&quot;noreferrer&quot;&gt;Tracing the 2017 Solar Eclipse&lt;/a&gt; is a visualization of the calculated shape of the umbra during the upcoming August 2017 total solar eclipse, as it moves across North America.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simulation takes into account the actual measured 3D shape of the Moon from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;LRO&lt;/a&gt;'s laser altimeter, the shape of the Earth is from &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission&quot; rel=&quot;noreferrer&quot;&gt;SRTM&lt;/a&gt; (radar map from the space shuttle), and the locations of the Sun, Earth and Moon from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris&quot; rel=&quot;noreferrer&quot;&gt;JPL Development Ephemeris&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is confusing me are the big &quot;corners&quot; on the Moon's shadow. I had thought that the moon was close to an ellipsoid. There are large mountains, mare, and craters for sure, but have these deviations been &lt;em&gt;greatly&lt;/em&gt; exaggerated in the animation? The largest fat spot shows an &quot;altitude deficit&quot; of roughly 4% of the Moon's diameter, or about 140 kilometers! Does this turn out to be in fact far less accurate than projecting a simple ellipsoidal moon's umbra?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shots from the linked NASA Goddard video &lt;a href=&quot;https://www.youtube.com/watch?v=MJgXaqW3md8&quot; rel=&quot;noreferrer&quot;&gt;Tracing the 2017 Solar Eclipse&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/WqM1Q.png&quot; alt=&quot;Screenshot of NASA Goddard video Tracing the 2017 Solar Eclipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/MXWtd.jpg&quot; alt=&quot;Screenshot of NASA Goddard video Tracing the 2017 Solar Eclipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/2YncK.jpg&quot; alt=&quot;Screenshot of NASA Goddard video Tracing the 2017 Solar Eclipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/C734L.jpg&quot; alt=&quot;Screenshot of NASA Goddard video Tracing the 2017 Solar Eclipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/Nn3eP.jpg&quot; alt=&quot;Screenshot of NASA Goddard video Tracing the 2017 Solar Eclipse&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-09-16T08:42:05.940" LastActivityDate="2019-09-16T08:42:05.940" Title="The Moon's shadow could not possibly look like this — could it?" Tags="|the-moon|nasa|ephemeris|lro|solar-eclipse|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="22374" PostTypeId="1" CreationDate="2017-07-22T22:01:34.153" Score="2" ViewCount="128" Body="&lt;p&gt;I'm writing an SF book and want to know if a 400 by 1000 meter tubular habitat at Lagrange point 4 (or L5) would be easily detected from earth?&lt;/p&gt;&#xA;" OwnerUserId="20452" LastActivityDate="2017-07-22T22:01:34.153" Title="How easy would it be to detect a large 400x1000 meters tubular habitat at L4" Tags="|lagrangian-points|habitat|detection|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22375" PostTypeId="1" AcceptedAnswerId="27357" CreationDate="2017-07-23T06:00:01.937" Score="8" ViewCount="528" Body="&lt;p&gt;The NASA Goddard video &lt;a href=&quot;https://www.youtube.com/watch?v=MJgXaqW3md8&quot; rel=&quot;noreferrer&quot;&gt;Tracing the 2017 Solar Eclipse&lt;/a&gt; is a visualization of the calculated shape of the umbra during the upcoming August 2017 total solar eclipse, as it moves across North America.&lt;/p&gt;&#xA;&lt;p&gt;The simulation takes into account the actual measured 3D shape of the Moon from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;LRO&lt;/a&gt;'s laser altimeter, the shape of the Earth is from &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle_Radar_Topography_Mission&quot; rel=&quot;noreferrer&quot;&gt;SRTM&lt;/a&gt; (radar map from the space shuttle), and the locations of the Sun, Earth and Moon from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris&quot; rel=&quot;noreferrer&quot;&gt;JPL Development Ephemeris&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What are the &amp;quot;Moon L, B, C&amp;quot; angles shown in this solar eclipse simulation?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; If you haven't seen the video, it's worth watching! The realistic shape of the umbra gives plenty to think about, as discussed in the question &lt;a href=&quot;https://space.stackexchange.com/q/22373/12102&quot;&gt;The Moon's shadow could not possibly look like this — could it?&lt;/a&gt; and the (likely to be) excellent answer(s) there.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Ku7a.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Ku7a.jpg&quot; alt=&quot;Tracing the 2017 Solar Eclipse screenshot&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/k83qt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/k83qt.png&quot; alt=&quot;Tracing the 2017 Solar Eclipse screenshot&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0s8vX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0s8vX.png&quot; alt=&quot;Tracing the 2017 Solar Eclipse screenshot&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/MJgXaqW3md8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-05-20T19:47:40.707" Title="What are the &quot;Moon L, B, C&quot; angles shown in this solar eclipse simulation?" Tags="|orbital-mechanics|the-moon|solar-eclipse|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22376" PostTypeId="1" AcceptedAnswerId="22656" CreationDate="2017-07-23T08:48:29.077" Score="2" ViewCount="501" Body="&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=5GIjfkFucSs&quot; rel=&quot;nofollow noreferrer&quot;&gt;this nice video&lt;/a&gt; musing about (among other things) the US Naval Observatory's $3,500 expedition to &lt;a href=&quot;https://en.wikipedia.org/wiki/Baker_City,_Oregon&quot; rel=&quot;nofollow noreferrer&quot;&gt;Baker City&lt;/a&gt; or at least &lt;a href=&quot;https://en.wikipedia.org/wiki/Baker_County,_Oregon&quot; rel=&quot;nofollow noreferrer&quot;&gt;Baker County&lt;/a&gt; Oregon to observe the June 18, 1918 solar eclipse, there is also one of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_12#Solar_eclipse&quot; rel=&quot;nofollow noreferrer&quot;&gt;photos of a penumbral shadow taken by the Expedition 12 crew&lt;/a&gt; during the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_eclipse_of_March_29,_2006&quot; rel=&quot;nofollow noreferrer&quot;&gt;March 29, 2006 solar eclipse&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have there been others? One additional example would be sufficient for an answer. I'm curious how many different solar eclipse penumbral shadows on the Earth have been photographed by people from space, but one more is good enough for this question. Thanks!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screenshot from the &lt;a href=&quot;https://www.youtube.com/watch?v=5GIjfkFucSs&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt;, photo credits available at the end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AIeKd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AIeKd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-20T08:04:01.913" LastActivityDate="2017-08-21T20:23:23.180" Title="Is this the only eclipse where Moon's shadow on the Earth (umbra) has been photographed by a person in space?" Tags="|astronauts|space-station|photography|solar-eclipse|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22377" PostTypeId="2" ParentId="22373" CreationDate="2017-07-23T15:26:44.763" Score="14" Body="&lt;p&gt;The deviations are real and directly related to the size of valleys on the Moon. The simple calculation you do here does only work for a point-like light source that projects the shape of the Moon onto Earth. In case of an eclipse, the light source has almost the same apparent size as the object. For this eclipse, the magnitude is given as 1.03. This is the ratio between the apparent diameters of Moon and Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can easily imagine what is going on by assuming the Moon to be the same apparent size as the Sun. That is, the Moon can just cover the Sun, but not more. Now, the slightest valley on the Moon would make totality vanish - there would be a (small) amount of Sun still visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking the actual magnitude of 1.03 into account and putting our observer in the middle of the eclipse (on the red path), there is a ring of 1.5% of the diameter of the Moon &quot;around&quot; the Sun. In the screen shot above uhoh measured a deviation of 4% from a perfect ellipse. These 4% we have to put in relation with this ring of 1.5% of the Moon's diameter, resulting in a valley depth of about 2 km, which is absolutely reasonable. To put it in other words: If there was a valley of 4% / 140 km depth on the Moon, you wouldn't have any totality at all, because Moon wouldn't be able to cover the Sun at any moment.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2017-07-29T15:55:42.257" LastActivityDate="2017-07-29T15:55:42.257" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22379" PostTypeId="2" ParentId="22373" CreationDate="2017-07-23T18:48:06.820" Score="4" Body="&lt;p&gt;The outline of the lunar umbra on Earth (shadow of totality) will be defined by those rays of light emanating from the very edge of the solar disk as seen from Earth which just miss the surface features at the lunar terminator (line between day and night on the Moon's surface). Because the Moon is some 500 times smaller than the Sun (by diameter), those rays are more-or-less converging to a point about 1/500 the distance between Sun and Moon (the ratio of their actual diameters). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As it happens, the Earth-Moon distance is about 1/500 the distance between the Sun and the Earth/Moon system. In the upcoming eclipse, the Moon will be a little closer than &quot;normal&quot; and the &quot;point&quot; of convergence will be &quot;behind&quot; the Earth's surface. The umbra on Earth's surface will be on the order of 100km across.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you consider a point source at the Sun, as rays pass by objects on the Moon, they will form a shadow on the Earth slightly larger than their true size - if we use the approximation of sizes and distances as 500:1, the shadows will be about 501:500 of their true size. But, the umbra is only a small fraction of the Moon's size. So, as the rays converge down on their way to the Earth's surface, the shadows of lunar surface features at the terminator will overlap and &quot;blur&quot; together to form the odd shape shown in the video.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-07-24T23:23:12.283" LastActivityDate="2017-07-24T23:23:12.283" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22380" PostTypeId="2" ParentId="2110" CreationDate="2017-07-23T23:43:32.520" Score="2" Body="&lt;p&gt;I thought everything that could carry people to orbit used cryogenic propellants.  Those can't be stored on board, the rocket must be fueled just before launch.  I have been corrected, the Gemini missions were based off a modified missile and used nasty chemicals instead.  There haven't been any Gemini birds around for quite some time, though, and anything modern uses cryogenic stuff because it gives your energy.  Thus the only time a rocket is flyable is just before it's flown.  Thus you can't just make off with a space rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your only hope of getting to space is to stow away on something already heading there.  The Russians had the supplies out of one of their Progress rockets stolen so it might be possible to slip into one.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2017-07-27T03:03:11.263" LastActivityDate="2017-07-27T03:03:11.263" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22381" PostTypeId="2" ParentId="22340" CreationDate="2017-07-24T05:00:46.893" Score="6" Body="&lt;p&gt;If you want to look at the outlined plan I'd recommend watching &lt;a href=&quot;https://www.youtube.com/watch?v=H7Uyfqi_TE8&quot; rel=&quot;noreferrer&quot;&gt;Musk's presentation at 67th International Astronautical Congress in 2016.&lt;/a&gt; You can find the slides presented &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/mars_presentation.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd continue to outline his plan but the video covers all the bases. Some things to note is that SpaceX is currently moving toward the Mars mission by reducing launch costs, improving and leading the way on rocket reusability, developing new crew capsules, improving propulsion systems etc. It is worth noting that SpaceX is currently using its own capital made through its launch services to develop some of this technology. However, the brunt of the Mars colonization expenses will be dependent on the US government whether initially or sustainably throughout the colonization.&lt;/p&gt;&#xA;" OwnerUserId="20250" LastActivityDate="2017-07-24T05:00:46.893" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22382" PostTypeId="2" ParentId="22343" CreationDate="2017-07-24T05:20:09.450" Score="4" Body="&lt;p&gt;This Is Wikipedia describing the Eastern Range &lt;a href=&quot;https://en.wikipedia.org/wiki/Eastern_Range&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Eastern_Range&lt;/a&gt;. For US spaceflight, the range supports tracking, comm, data, and range security for Cape Canaveral AAFS, KSC, Wallops Flight Facility, Patrick AFB, and even Arianne launches from Guiana Space Center. It also includes all the ground structures, bunkers, piping and storage equipment for fuel and other liquids and gases, and ground support equipment. The range starts at KSC and extends eastward to 90° East (Bangladesh) where it meets the Western range. There is comm, data, and tracking equipment on most of the Caribbean islands heading southeast from KSC down below the equator to Ascension Island and beyond. There are also airborne and sea-based tracking assets that support launches when available. Much of the sea-based tracking assets and many of the land base stations have been replaced by space-based tracking primarily using TDRSS. The same equipment also supports US missile testing.&#xA;The current maintenance activity is &lt;a href=&quot;https://thespaceport.us/forum/topic/45546-eastern-range-hiatus-in-july/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://thespaceport.us/forum/topic/45546-eastern-range-hiatus-in-july/&lt;/a&gt;. They seem to be mostly disassembling and safing the large buildings and equipment that supported the shuttle.&lt;/p&gt;&#xA;" OwnerUserId="20033" LastActivityDate="2017-07-24T05:20:09.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22383" PostTypeId="1" AcceptedAnswerId="22396" CreationDate="2017-07-24T09:11:26.380" Score="5" ViewCount="698" Body="&lt;p&gt;Apologies if this has been asked before using terminology for somebody more familiar with the subject matter. I've searched but cannot find an answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming space travel was possible (to other galaxies way beyond ours, for example), how would the location of Earth be represented so that pilots could find their way back?  Or another example, to describe how to find Earth to passing craft in interstellar travel?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm thinking that technologies like GPS use satellities with the Earth itself as an abolute object to calculate from. In space, Earth obviously has to be &quot;located&quot; within a wider scope.&lt;/p&gt;&#xA;" OwnerUserId="20461" LastEditorUserId="20461" LastEditDate="2017-07-24T11:08:44.367" LastActivityDate="2017-07-25T08:23:01.480" Title="How might Earth's location be referenced in stellar terms?" Tags="|interstellar-travel|navigation|travel|" AnswerCount="5" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22384" PostTypeId="1" CreationDate="2017-07-24T09:30:42.587" Score="4" ViewCount="285" Body="&lt;p&gt;Is this the ISS from Antalya, Elmali, 23.7.2017 morning around 03:00 hours? &#xA;&lt;a href=&quot;https://i.stack.imgur.com/0BsVA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0BsVA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;Photo 4 frames of 30 seconds each, with 1 second in between each frame, all images superimposed with star movement taken out.&lt;/p&gt;&#xA;" OwnerUserId="20462" LastActivityDate="2017-07-24T12:15:13.143" Title="Observing ISS from the earth" Tags="|iss|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22385" PostTypeId="2" ParentId="22384" CreationDate="2017-07-24T11:51:22.723" Score="5" Body="&lt;p&gt;There are several easy-to-use websites that can help answer your question. One example is &lt;a href=&quot;http://heavens-above.com/&quot; rel=&quot;noreferrer&quot;&gt;heavens-above.com&lt;/a&gt; and another is &lt;a href=&quot;https://in-the-sky.org/&quot; rel=&quot;noreferrer&quot;&gt;in-the-sky.org&lt;/a&gt;. You can try both. In-the-sky is a recently developed site and works very well on mobile devices as well as computers, and hand has a planetarium mode and a helpful &quot;What's in the sky?&quot; button.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I clicked on your location on the location page, and it gave me your time zone as UTC+03:00. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure what &quot;around 03:00 hours&quot; means. It's really really important for you to keep track of the exact times of your exposures, and to make sure the clock in your camera is set correctly!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the home page, you can select &lt;code&gt;Spacecraft&lt;/code&gt; from the menu and there are options for sky maps or tables. Start with the table.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the table shown, there are no noted passes of the ISS on the morning of July 23. It's a little bit early in the morning to have a good view of things in such a low orbit, about ~400km. At this time, most of the sky at that altitude is still in darkness (shadowed by Earth) and is not exposed to sunlight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, around this time there are several old rocket bodies passing by, and these seem to be at a much higher altitude. They may be slowly tumbling and that would definitely account for the pulsed (modulated) brightness as they passed, causing the dashed lines. I would not expect the ISS to produce so many cycles of bright and dim during part of one pass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you know the exact time of the photograph, you can choose one of the sky viewing options and dial in the exact time and watch the calculated motion of the satellites across the sky and compare directly to the photographs. I used the feature to make the GIF shown below. Incidentally the GIF proves that those astronomers did not observe the ISS either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GOUP1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GOUP1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kSR6X.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kSR6X.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; GIF made from in-the-sky.org showing a spy satellite and the ISS both passing close to the moon. From &lt;a href=&quot;https://space.stackexchange.com/a/14846/12102&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oIL0N.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oIL0N.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-24T12:15:13.143" LastActivityDate="2017-07-24T12:15:13.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22386" PostTypeId="2" ParentId="22383" CreationDate="2017-07-24T12:44:30.787" Score="1" Body="&lt;p&gt;The problem of navigation in space is, at least for now, going back to basics: Navigate by the stars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First let's stick to the interstellar variant. GPS works by triangulating your position from at least 3 satellites, while in air, as an aircraft or spaceship, you need at least 4 satellites you pinpoint your position. As Uwe in the comments stated, even in space above Geostationary Orbit, so above the GPS satellites, it is possible to get your position from those. But the further the distance to the satellites, it gets harder and harder, as the GPS signal is directed towards the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In interstellar space in the milky way, the receiver would need to know, where the sender was at the time the signal was send and how long the signal traveled. Then you have signal jamming and wrong signals from stars and so on. Also you would need signals from many different directions, so many from across the galaxy. &#xA;If you assume traveling at relativistic speeds, you have redshift and blueshift that complicate things even more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only reliable option would be the stars you see, most likely pulsars, to orientate, but as you navigate through them also other galaxies, as they are the fixed stars for you, while stars and pulsars would work as some kind of lighthouse replacement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Intergalactic travel is even worse, as you got only the galaxies as reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Stars would be the highway drive-off, as you can't detect planets by themselves. So you would aim for their star and then look for the planets it orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On thing we can hope for is a mass detector, something to detect gravitational masses directly at a distance. That would allow to find planets, black holes and maybe even dark matter cluster. And so you would be able to navigate through them aswell.&lt;/p&gt;&#xA;" OwnerUserId="19685" LastEditorUserId="19685" LastEditDate="2017-07-25T06:41:39.950" LastActivityDate="2017-07-25T06:41:39.950" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22387" PostTypeId="2" ParentId="22373" CreationDate="2017-07-24T13:50:05.537" Score="6" Body="&lt;p&gt;I'll add this to the other answers, which do a very good job of explaining why this very irregular shape is not inconsistent with the Moon's original much &quot;rounder&quot; shape.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This could be done with some geometry and math, but I was lazy and did a rendering in &lt;a href=&quot;https://blender.stackexchange.com/&quot;&gt;Blender&lt;/a&gt;. I made a spheroid and added some bumps, then added a Sun lamp above and screen far below. I can adjust the half-angle of the sun lamp. This is not intended to be an exact simulation, but once you see the GIF the principle described in words in the other two answers is reinforced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note the appearance of sharp corners on the shape of the &quot;umbra&quot; that do not exist on the original shape!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to delineate the &quot;umbra&quot; where the light reaches zero, I just turned up the brightness of the Sun to &lt;code&gt;100&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QNIhy.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QNIhy.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; The original shape, moved down into the plane of the screen so that the cross-section shape is clear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NbjIp.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NbjIp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Selected frames, showing a nearly square shaped &quot;umbra&quot; &lt;em&gt;with the appearance of sharp corners that do not exist on the original shape!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yBJ1g.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yBJ1g.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p7iH4.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p7iH4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the Python script I used in Blender to make the shape. Animation is easily scripted also, I just did it manually this time because it was quicker.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import bpy&#xA;import numpy as np&#xA;&#xA;halfpi, pi, twopi, fourpi = [f*np.pi for f in [0.5, 1, 2, 4]]&#xA;&#xA;half_nth = 50&#xA;nth, nph = 2*half_nth + 1, 200&#xA;&#xA;th  = np.linspace(-halfpi, halfpi, nth+2)[1:-1]&#xA;&#xA;phi = np.linspace(0, twopi, nph+1)[:-1]&#xA;&#xA;n1, n2 = 8, 32&#xA;&#xA;thing = 0.5*(1.+np.cos(np.linspace(0, pi, n2+1-n1)))[:,None]&#xA;ramp   = np.zeros_like(th)[:,None]&#xA;&#xA;ramp[half_nth+n1   : half_nth+(n2+1)] = thing&#xA;ramp[half_nth-n2   : half_nth-n1+1  ] = thing[::-1]&#xA;ramp[half_nth-n1+1 : half_nth+n1    ] = 1.0&#xA;&#xA;nwiggle = 4&#xA;awiggle = 0.05&#xA;&#xA;wiggle = 1. + awiggle * np.cos(nwiggle*phi) * ramp&#xA;&#xA;X = wiggle * np.cos(phi)[None,:]       * np.cos(th)[:,None]&#xA;Y = wiggle * np.sin(phi)[None,:]       * np.cos(th)[:,None]&#xA;Z =          np.ones_like(phi)[None,:] * np.sin(th)[:,None]&#xA;&#xA;verts = [tuple(thing) for thing in zip(X.flatten(), Y.flatten(), Z.flatten())]&#xA;&#xA;faces = []&#xA;for ith in range(nth-1):&#xA;    for iph in range(nph):&#xA;        v1 =  iph        + nph*ith&#xA;        v2 = (iph+1)%nph + nph*ith&#xA;        v3 = (iph+1)%nph + nph*(ith+1)&#xA;        v4 =  iph        + nph*(ith+1)&#xA;        faces.append((v1, v2, v3, v4))&#xA;&#xA;bot = [tuple([iph               for iph in range(nph)])]&#xA;top = [tuple([iph + nph*(nth-1) for iph in range(nph)])]&#xA;&#xA;faces += bot + top&#xA;&#xA;me = bpy.data.meshes.new('wow')&#xA;ob = bpy.data.objects.new('wow', me)&#xA;&#xA;ob.location = (0.0, 0.0, 102.0) &#xA;bpy.context.scene.objects.link(ob)&#xA;me.from_pydata(verts,[],faces)&#xA;&#xA;bpy.data.objects['wow'].select = False&#xA;bpy.data.objects['wow'].select = True&#xA;&#xA;bpy.ops.object.shade_smooth()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-24T13:55:12.653" LastActivityDate="2017-07-24T13:55:12.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22388" PostTypeId="1" CreationDate="2017-07-24T14:20:35.613" Score="5" ViewCount="629" Body="&lt;p&gt;Are there examples of Components Off The Shelf (COTS) used in the Space Transportation System?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And is NASA considering using COTS in future manned space flight systems like Orion and the SLS?&lt;/p&gt;&#xA;" OwnerUserId="20251" LastEditorUserId="12102" LastEditDate="2021-03-25T23:15:11.857" LastActivityDate="2021-04-08T12:44:54.733" Title="Were any Components Off The Shelf (COTS) used in the Space Shuttle?" Tags="|space-shuttle|commercial|sls|orion-spacecraft|cots|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22389" PostTypeId="2" ParentId="22371" CreationDate="2017-07-24T15:30:22.610" Score="1" Body="&lt;p&gt;DSN dishes (the largest of which are 70 and 36 m in diamter) use up to 20 kW to communicate with deep-space missions including Voyager and various Mars missions. On the DSN Now site, &lt;a href=&quot;https://space.stackexchange.com/questions/9824/how-much-rf-transmit-power-does-dsn-need-to-send-commands-to-voyager/18955#18955&quot;&gt;you can see how much power&lt;/a&gt; a DSN dish uses for each contact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that should give sort of an upper bound: if you use a 36-m dish, you can get away with using a tiny antenna on Earth. In practice, you want the biggest dish to be on Earth (construction is much cheaper here). For a simple manned mission, a 5-m parabola with a low-powered transmitter (100 W or less) should be feasible: you can build a folding construction that fits inside a reasonably-small lander. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-07-24T16:41:23.433" LastActivityDate="2017-07-24T16:41:23.433" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22390" PostTypeId="1" CreationDate="2017-07-24T16:11:14.607" Score="6" ViewCount="281" Body="&lt;p&gt;I've read that derivatives of the original &lt;a href=&quot;http://heroicrelics.org/info/surveyor/surveyor-vernier.html&quot; rel=&quot;noreferrer&quot;&gt;Thiokol TD-339&lt;/a&gt; model of &lt;a href=&quot;https://en.wikipedia.org/wiki/Vernier_thruster&quot; rel=&quot;noreferrer&quot;&gt;vernier engine&lt;/a&gt; for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_program&quot; rel=&quot;noreferrer&quot;&gt;Surveyor program&lt;/a&gt; was reused in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Viking_program&quot; rel=&quot;noreferrer&quot;&gt;Viking program&lt;/a&gt;, and seen a hint that &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Science_Laboratory&quot; rel=&quot;noreferrer&quot;&gt;MSL&lt;/a&gt; was using the same heritage design, but I'm not sure if this is correct. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What about the Mars landers &lt;a href=&quot;https://en.wikipedia.org/wiki/Phoenix_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Phoenix&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/InSight&quot; rel=&quot;noreferrer&quot;&gt;InSight&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More info and photos: &lt;a href=&quot;https://airandspace.si.edu/collection-objects/space-propulsion-reaction-motors-td-339-surveyor-vernier-rocket-system-surveyor&quot; rel=&quot;noreferrer&quot;&gt;1&lt;/a&gt;, &lt;a href=&quot;https://airandspace.si.edu/collection-objects/rocket-engine-liquid-fuel-surveyor-vernier-also-designated-td-339&quot; rel=&quot;noreferrer&quot;&gt;2&lt;/a&gt;, &lt;a href=&quot;https://airandspace.si.edu/collection-objects/rocket-engine-liquid-fuel-surveyor-vernier-also-designated-td-339-0&quot; rel=&quot;noreferrer&quot;&gt;3&lt;/a&gt;, &lt;a href=&quot;https://airandspace.si.edu/collection-objects/rocket-engine-vernier-surveyor-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;4&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZGzTvm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZGzTvm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Photo from &lt;a href=&quot;http://heroicrelics.org/info/surveyor/surveyor-vernier.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="3140" LastEditorUserId="12102" LastEditDate="2017-07-25T13:32:48.320" LastActivityDate="2019-07-22T23:32:11.773" Title="Surveyor TD-339 engines - used on all US powered robotic landers?" Tags="|curiosity|lander|surveyor-program|" AnswerCount="1" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22391" PostTypeId="2" ParentId="22371" CreationDate="2017-07-24T17:51:14.760" Score="3" Body="&lt;p&gt;NASA's &lt;a href=&quot;https://mars.nasa.gov/mro/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Reconnaissance Orbiter&lt;/a&gt; currently transmits data at &lt;a href=&quot;https://mars.nasa.gov/mro/mission/spacecraft/parts/telecommunications/&quot; rel=&quot;nofollow noreferrer&quot;&gt;up to &lt;strong&gt;6 megabits per second&lt;/strong&gt;&lt;/a&gt; using a &lt;a href=&quot;https://mars.nasa.gov/mro/mission/spacecraft/parts/antennas/&quot; rel=&quot;nofollow noreferrer&quot;&gt;3 meter (10 foot) diameter dish antenna&lt;/a&gt; deployed from its &lt;em&gt;stowed&lt;/em&gt; (not &lt;em&gt;folded&lt;/em&gt;) position shortly after launch, and Traveling Wave Tube (TWT) amplifiers that transmit radio signals at a power of 100 watts, when communicating with the &lt;a href=&quot;https://deepspace.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; antennas back on Earth.  MRO gets all of its electrical power from &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter#Power_systems&quot; rel=&quot;nofollow noreferrer&quot;&gt;two solar panels&lt;/a&gt; (folded &lt;em&gt;and&lt;/em&gt; stowed for launch), each of which produces more than 1,000 watts of power at Mars.  Where the &lt;em&gt;entire&lt;/em&gt; spacecraft power budget is approximately 2kW, the transmitters have to be consuming considerably less than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;MRO launch configuration&lt;/strong&gt;&lt;br&gt;&#xA;&lt;img src=&quot;https://mars.nasa.gov/mro/images/mro/spacecraft/config_cruise3_th200.jpg&quot; alt=&quot;MRO launch configuration illustration&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to have a radio &lt;em&gt;telescope&lt;/em&gt; to use for radio &lt;em&gt;astronomy&lt;/em&gt; work, you would probably want to keep that separate from your &lt;em&gt;communications&lt;/em&gt; antenna if you can (so they can be pointed independently), and have the two antennas as widely separated as possible, to minimize transmitter interference from affecting the observational work.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-07-24T18:59:41.400" LastActivityDate="2017-07-24T18:59:41.400" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22392" PostTypeId="2" ParentId="22383" CreationDate="2017-07-24T21:53:38.853" Score="0" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Assuming space travel was possible (to other galaxies way beyond ours, for example), how would the location of Earth be represented so that pilots could find their way back?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Andromeda Galaxy is 2.5 million light years from earth, so a round trip, even at speeds approaching the speed of light, will take at least 5 million years of earth time. Proper time aboard the ship, taking relativistic time dilation into account, will still be millions of years. If the pilots are human beings, then it seems unlikely that they themselves will be finding their way back. It would have to be their distant descendants, or an automated probe, that would come back.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbits of stars in our galaxy are &lt;a href=&quot;https://arxiv.org/abs/1012.3606&quot; rel=&quot;nofollow noreferrer&quot;&gt;chaotic on time scales of about $10^9$ years&lt;/a&gt;. If the spaceship is able to travel close enough to the speed of light, and the galaxy being visited is not too distant, then in theory the ship ought to be able to come back in less than this amount of time. So it seems to me that the simplest method of finding our solar system again would be to carry along a stellar atlas of the orbital parameters of stars in our galaxy, and run a simulation. If you've returned home to the Milky Way after 73.088 million years, then you run a simulation and figure out what the galaxy should look like. Match that up with what you're seeing, and navigate to where the sun is simulated to have ended up.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2017-07-24T21:53:38.853" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22393" PostTypeId="2" ParentId="22388" CreationDate="2017-07-24T22:34:45.107" Score="7" Body="&lt;p&gt;Yes, Velcro is one example. Wheat Thins are another. Did you have some specific type of component in mind?&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-07-24T22:34:45.107" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22394" PostTypeId="2" ParentId="22383" CreationDate="2017-07-25T00:38:05.850" Score="2" Body="&lt;p&gt;Earth navigation involves two determinations of location: location of your destination and your current location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To determine location you need a coordinate system. The coordinate system we made up for Earth navigation uses latitude and longitude. The origin of Earth latitude is set at the equator while the origin of Earth longitude is set at Greenwich observatory in the UK. Space travel will likely involve some similar made up origin system, depending on the scale of the travel. For interplanetary spaceflight, NASA uses several different coordinate systems described &lt;a href=&quot;https://omniweb.gsfc.nasa.gov/coho/helios/plan_des.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; Interstellar spaceflight, travel within the galaxy, may use the same &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Galactic_coordinate_system?wprov=sfla1&quot; rel=&quot;nofollow noreferrer&quot;&gt;galactic coordinate system&lt;/a&gt; that astronomers use, or it may use an adjusted coordinate system with the galactic center as the origin. Likewise, intergalactic spaceflight, may use the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Supergalactic_coordinate_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;supergalactic coordinate system&lt;/a&gt; that astronomers use, or it may use a modified version.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we navigate on Earth, the locations of most destinations are determined using search software, we really only look up GPS coordinates ourselves and enter them into our GPS if the destination is unnamed. Determination of the location of destinations in long-distance spaceflight will likely be similar. A destination will be entered into the navigation computer, which will find the location in its database.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth-orbiting satellites often use star trackers to determine the satellite's orientation (attitude). It's possible interstellar spacecraft will be equipped with telescopes and computers that determine location based on the arrangement of stars in our galaxy and other galaxies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whatever the coordinate system, spaceflight navigation will require more information than GPS systems. On Earth, coordinates are relatively fixed (tectonic movement is usually negligible on human timescales). In space, the various orbital speeds of different bodies cause the locations of each destination to change with respect to others over time. Because of this, spaceflight navigation computers will have to periodically recalculate the locations of the destinations in their databases and take further orbital mechanics into account as it charts a course to the requested destination.&lt;/p&gt;&#xA;" OwnerUserId="6570" LastEditorUserId="6570" LastEditDate="2017-07-25T06:08:35.733" LastActivityDate="2017-07-25T06:08:35.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22395" PostTypeId="2" ParentId="22383" CreationDate="2017-07-25T01:21:35.777" Score="5" Body="&lt;p&gt;I suspect that navigating to a galaxy would be done by knowing the characteristics of the galaxy. There are a few ways to do so, but depending on the orbital resolution, the shape, pulsar characteristics, center of the galaxy black hole characteristics, or even more generic items like &lt;a href=&quot;https://en.wikipedia.org/wiki/Galaxy_morphological_classification&quot; rel=&quot;noreferrer&quot;&gt;this Wikipedia&lt;/a&gt; article hints at would characterize a galaxy. This would work for relatively far away to identify the Milky Way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Within a galaxy the problem is much easier. Pulsars have a unique frequency, and there are enough of them that one can use them to find where one is in space, and as a result, where Earth is. Essentially you take a hundred of these markers, and plot the angles vs each other, run through a linear solver to determine where you are. I suspect the practical accuracy of these is such that once you have used them, you can find where the Sun is close enough to send a ship there and spot Earth through visual means.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-07-25T01:21:35.777" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22396" PostTypeId="2" ParentId="22383" CreationDate="2017-07-25T01:57:14.603" Score="13" Body="&lt;p&gt;Like the lower left part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_Golden_Record&quot; rel=&quot;noreferrer&quot;&gt;image below&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OFVCX.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OFVCX.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Define a set of pulsars through describing proportions between their frequencies. Define distances from each of them. This uniquely identifies a point in space, and the pulsars are very efficient &quot;beacons&quot; recognizable from very far.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would determine position of Sun. As for Earth, third planet, easy.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-07-25T08:23:01.480" LastActivityDate="2017-07-25T08:23:01.480" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22397" PostTypeId="1" CreationDate="2017-07-25T02:36:43.683" Score="5" ViewCount="327" Body="&lt;p&gt;I was very interested to know what the black lake on Titan would look like. Are there any plans or proposals for a future mission to land on and/or in one of the lakes of black liquid on Titan, or even to land nearby and approach it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would there be any unique technical challenges to this that have not been encountered and solved for previous lander missions?&lt;/p&gt;&#xA;" OwnerUserId="11436" LastEditorUserId="12102" LastEditDate="2017-07-25T06:12:06.333" LastActivityDate="2019-05-16T21:58:13.197" Title="Any proposed missions to explore the black liquid on Titan? Technical challenges?" Tags="|future-missions|titan|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22398" PostTypeId="2" ParentId="22364" CreationDate="2017-07-25T05:15:30.817" Score="3" Body="&lt;p&gt;First, I'd like to say props on pursuing your curiosity. Please make sure you're trying to adhere to some &lt;a href=&quot;http://www.jamesyawn.net/rcandy/safety.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;safety guidelines&lt;/a&gt; while pursuing this - use your best judgment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, it sounds like you're willing to do some learning/already have some engineering hobby experience. I will be assuming that you're willing to make some purchases for this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Experiment:&lt;/strong&gt;&#xA;This experiment builds off your spring-ruler idea. It can be tedious to measure the spring constant of the ruler and the subsequent deflections due to the rocket thrust. My suggestion is that you pursue the cantilever beam strain gauge experiment with slight modifications. &lt;a href=&quot;https://i.stack.imgur.com/htx6B.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/htx6B.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;em&gt;Image Source: Chegg&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above you see the general setup of the experiment. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Strain_gauge&quot; rel=&quot;nofollow noreferrer&quot;&gt;strain gauge&lt;/a&gt; provides you an instrument for measuring the deflections by correlating beam deflection, induced strain and the force experienced by the beam. In the image above, you see a force vector applied to the end. This is where you will modify the beam setup so you can install your rocket motors to this end, allowing you to measure thrust (force) by measuring the strain and subsequent deflection of the cantilever beam. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Attached to the strain gauge will be some type of data acquisition device (i.e. DAQ, microcontroller) that will read and transfer your data to your computer. This part gets a little difficult because you may need to tune your controller to be customized to the beams material properties, dimensions etc. I suggest looking at some basic mechanics of materials topics to help with this portion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the data phase, you will only be able to measure voltages so you will need to dive in depth into the workings of a strain gauge. From this experiment, you'll be able to get thrust, burn rate and other &lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_engine#Overall_performance&quot; rel=&quot;nofollow noreferrer&quot;&gt;performance parameters&lt;/a&gt; - most importantly &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse#Energy_efficiency&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific impulse&lt;/a&gt;. You may use the linked performance parameters to determine which rocket motor is better. Of course, in a 'real-life' scenario the most powerful engine may not be universally desired, however, generally speaking, the most efficient is. This is a &lt;em&gt;very&lt;/em&gt; on the surface explanation of the entire experiment but I think it provides the best path for what you would like to learn&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Required Materials:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Strain gauge (Wheatstone Bridge) &lt;/li&gt;&#xA;&lt;li&gt;Metal Beam (metal ruler)&lt;/li&gt;&#xA;&lt;li&gt;Data Acquisition Device (DAQ)&lt;/li&gt;&#xA;&lt;li&gt;Amplifier (for mV signals from strain gauge)&lt;/li&gt;&#xA;&lt;li&gt;List item&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Required Experience:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Programming&lt;/li&gt;&#xA;&lt;li&gt;Basic understanding of electronics (i.e. strain gauges, microcontrollers)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As far as predicting which motor will go further, you can make that deduction from the experiment listed above, particularly from the performance criteria. Hope this helps!&lt;/p&gt;&#xA;" OwnerUserId="20250" LastActivityDate="2017-07-25T05:15:30.817" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22399" PostTypeId="1" CreationDate="2017-07-25T09:56:50.813" Score="2" ViewCount="210" Body="&lt;p&gt;Do orbit design/mission analysis have equivalent documentation to the Preliminary/Critical Design Reviews you would use in systems engineering?&lt;/p&gt;&#xA;" OwnerUserId="1562" LastActivityDate="2017-07-26T15:18:32.210" Title="Orbit Design Process" Tags="|orbit|design|mission-design|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22401" PostTypeId="1" AcceptedAnswerId="22402" CreationDate="2017-07-24T14:53:36.193" Score="4" ViewCount="699" Body="&lt;p&gt;Given an O'Neill &quot;Island Three&quot; style cylindrical habitat located at Mercury–Sun L5 orbit, how much radiation shielding would be required to protect the occupants, and how would it need to be structured?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Island Three &lt;a href=&quot;http://www.nss.org/settlement/space/oneillcylinder.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;O’Neill Cylinder&lt;/a&gt; design has two counter-rotating cylinders aligned with the axes pointed towards the sun, with a sun shade/solar power facility at the sunward end, and mirrors to reflect light into the the habitats. A sun-shade and radiator seemed to be enough for the &lt;a href=&quot;https://solarsystem.nasa.gov/missions/messenger/indepth&quot; rel=&quot;nofollow noreferrer&quot;&gt;recent MESSENGER mission&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't found a source for how much radiation to expect in Mercury orbit, which comes within 46 million kilometers of the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;http://www.nss.org/settlement/space/GlobusRadiationPaper.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital Space Settlement Radiation Shielding (Globus 2016)&lt;/a&gt;, Globus recommends a limit of 20 mSv/year exposure for the general population, and 6.6 mGy/year exposure for pregnant women.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Globus recommends water as the most effective shielding material, but his calculations are for the LEO environment, where Earth's radiation belts are more of a concern than GCR.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring the cost, assuming all the materials needed, and the technology to construct them, what radiation shielding would be required to limit the risk of life-long occupants, especially pregnant women, of such a habitat, and how would the shielding need to be configured?&lt;/p&gt;&#xA;" OwnerUserId="20474" OwnerDisplayName="David" LastEditorUserId="12102" LastEditDate="2017-07-25T14:11:33.697" LastActivityDate="2017-07-25T14:11:33.697" Title="How much radiation shielding would be required for a habitat at Mercury–Sun L5?" Tags="|radiation|space-station|habitat|solar-wind|the-sun|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22402" PostTypeId="2" ParentId="22401" CreationDate="2017-07-24T17:00:53.767" Score="5" Body="&lt;p&gt;You have to worry about the solar wind - the charged particles that are emitted by the Sun, and which are quite effectively shielded by the Earth's magnetic field. The magnetic field of Mercury is very weak (150x weaker than Earth's), and probably wouldn't do a very good job deflecting the solar wind. See details &lt;a href=&quot;https://www.space.com/14026-mercury-magnetic-field-mystery-solar-wind.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. It also has negligible atmosphere (low gravity and strong solar wind stripped away any semblance of atmosphere as we would recognize it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fortunately, charged particles are stopped quite efficiently by water. But you also need to worry about neutrons (byproducts of the fusion process) and of course the intense heat and gamma radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not a bad start to look at the radiation dose that astronauts receive in the ISS - see for example this summary that tells us that they might get 160 mSv of radiation over a 6 month period. We need to scale that number because Mercury is closer... with an eccentricity of 0.21, it is sometimes &quot;much closer&quot; to the Sun than at other times (from 4.6E10 to 7.0E10 m, compared to Earth at 1.5E11 m).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it is closest to the sun, its orbit is 3x closer, and the radiation intensity would be 9x greater. To get back to the levels of the ISS, you need shielding that cuts the radiation dose by about 10x. But that ignores the fact that the ISS is operating &quot;within Earth's magnetic field&quot;, and thus is still benefiting somewhat of the shielding that that provides.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lot of detail can be found in &lt;a href=&quot;https://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; - although it is specifically talking about the issues for a Mars mission, the principles are the same; it's just that the radiation levels on Mercury will be much greater, as the orbital radius is about 1/4 of that of Mars... note also that this means that when there is a significant solar flare, you don't have much time to hide...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &quot;lot of water&quot; should do the trick for most of the radiation. &lt;/p&gt;&#xA;" OwnerUserId="10738" OwnerDisplayName="Floris" LastActivityDate="2017-07-24T17:00:53.767" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22404" PostTypeId="2" ParentId="22388" CreationDate="2017-07-25T19:00:07.497" Score="5" Body="&lt;p&gt;&lt;a href=&quot;http://www.nytimes.com/1998/11/05/laptops-on-the-shuttle-age-does-does-matter.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;www.nytimes.com/1998/11/05/laptops-on-the-shuttle-age-does-does-matter.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;To keep costs down, NASA has shifted to using off-the-shelf products as much as possible. So they bought dozens of I.B.M. Thinkpad computers and one Epson Stylus Color 800 printer to send into space with the shuttle fleet and the new international space station.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.forbes.com/sites/quora/2016/05/14/what-kind-of-laptop-are-on-the-iss/#664b7d227295&quot; rel=&quot;nofollow noreferrer&quot;&gt;www.forbes.com/sites/quora/2016/05/14/what-kind-of-laptop-are-on-the-iss/#664b7d227295&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;There are, last time I checked, about 80 laptops deployed throughout the ISS.  Most of them are Lenovo T61P laptops, but there are still a few old Lenovo A31p Thinkpads floating around.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;So NASA does take off-the-shelf, the first is an old quote but state they are trying to generally to save money.  The second is very recent and shows they went through with it in regards to the ISS.&lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="20287" LastEditDate="2021-04-08T12:44:54.733" LastActivityDate="2021-04-08T12:44:54.733" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22405" PostTypeId="1" AcceptedAnswerId="22417" CreationDate="2017-07-26T03:19:36.660" Score="8" ViewCount="1531" Body="&lt;p&gt;The recent question &lt;a href=&quot;https://space.stackexchange.com/q/22401/12102&quot;&gt;How much radiation shielding would be required for a habitat at Mercury–Sun L5?&lt;/a&gt; got me thinking. There are a large number of disadvantages and challenges to building or putting a large artificial structure so much closer to the Sun than at the distance of Earth's orbit, and the only advantage I can think of is that you'd have a whole lot of solar power available to deal with those challenges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I also wondered if there is even any expectation that there is any meaningful benefit to putting something at one of Mercury's triangular libration points Sun-Mercury L4 or L5 as compared to just putting it in a heliocentric orbit and ignoring Mercury altogether, except of course for not getting hit by it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I chose a point in space that follows Mercury's orbit, except that it trails it by 1/6 of a period, the approximate temporal equivalent of trailing by 60° would be for a circular orbit. I then calculated the acceleration it would experience from Mercury, Venus, and Earth for five years, and it turns out that the &quot;perturbations&quot; from Venus and Earth are always stronger, and &lt;em&gt;often much stronger&lt;/em&gt; than any guiding or stabilizing effects from Mercury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; So I'm wondering, does it make sense to talk about Mercury's triangular libration points (L4, L5)? Besides the linked question, has there ever been any proposed missions or even serious discussion about these locations? Or are they really best thought of as orbital mechanical red herrings?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; python script and results using the package &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;noreferrer&quot;&gt;Skyfield&lt;/a&gt;. Dots in last plot are at &lt;code&gt;years = 2024.14&lt;/code&gt; when Venus comes closer than Mercury.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hjluE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hjluE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6u6Og.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6u6Og.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;class Ob(object):&#xA;    def __init__(self, name):&#xA;        self.name = name&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from skyfield.api import Loader, Topos, EarthSatellite&#xA;&#xA;load = Loader('~/Documents/YourNameHere/SkyData')&#xA;&#xA;data = load('de421.bsp')&#xA;ts   = load.timescale()&#xA;&#xA;days  = np.arange(365.2564*5)&#xA;times = ts.utc(2020, 1, days)&#xA;times_trailing = ts.utc(2020, 1, days-88./6)&#xA;years = 2020 + days/365.2564&#xA;&#xA;names = ['sun', 'mercury', 'venus',&#xA;         'earth barycenter', 'mars barycenter',&#xA;         'jupiter barycenter', 'saturn barycenter',&#xA;         'uranus barycenter', 'neptune barycenter']&#xA;&#xA;obs = []&#xA;for name in names:&#xA;    ob = Ob(name.split()[0])&#xA;    obs.append(ob)&#xA;    ob.ob = data[name]&#xA;&#xA;&#xA;for ob in obs:&#xA;    ob.pos = ob.ob.at(times).ecliptic_position().km&#xA;    if ob.name == 'mercury':&#xA;        ob.pos_trailing = ob.ob.at(times_trailing).ecliptic_position().km&#xA;&#xA;sun, mercury, venus, earth, mars = obs[:5]&#xA;jupiter, saturn, uranus, neptune = obs[5:]&#xA;&#xA;GMs = [1.32712440018E+20, 2.2032E+13, 3.24859E+14,&#xA;       3.986004418E+14 + 4.9048695E+12,&#xA;       4.282837E+13, 1.26686534E+17, 3.7931187E+16,&#xA;       5.793939E+15, 6.836529E+15]&#xA;&#xA;for ob, GM in zip(obs, GMs):&#xA;    ob.GM = GM&#xA;&#xA;for ob in obs:&#xA;    rsq = ((ob.pos - mercury.pos_trailing)**2).sum(axis=0)&#xA;    ob.F = ob.GM / rsq&#xA;    ob.r = np.sqrt(rsq)&#xA;&#xA;if 1 == 1:&#xA;    fig = plt.figure()&#xA;&#xA;    ax = fig.add_subplot(2, 1, 1)&#xA;    for ob in obs[1:4]:&#xA;        ax.plot(years, ob.F, label=ob.name)&#xA;    # ax.legend()&#xA;    ax.set_title('acceleration (m/s^2)', fontsize=16)&#xA;    ax.get_xaxis().get_major_formatter().set_useOffset(False)&#xA;    ax.text(2020.2, 0.01, 'Mercury')&#xA;    ax.text(2020.2, 0.04, 'Earth')&#xA;    ax.text(2020.2, 0.13, 'Venus')&#xA;&#xA;    ax = fig.add_subplot(2, 1, 2)&#xA;    for ob in obs[1:4]:&#xA;        ax.plot(years, ob.r, label=ob.name)&#xA;    # ax.legend()&#xA;    ax.set_title('distance (m)', fontsize=16)&#xA;    ax.get_xaxis().get_major_formatter().set_useOffset(False)&#xA;    ax.text(2020.2, 0.4E+08, 'Mercury')&#xA;    ax.text(2020.2, 0.7E+08, 'Venus')&#xA;    ax.text(2020.2, 2.2E+08, 'Earth')&#xA;&#xA;    fig.suptitle(&quot;in Mercury's orbit trailing by 88/6 days&quot;, fontsize=16)&#xA;&#xA;    plt.show()&#xA;&#xA;if 1 == 1:&#xA;    fig = plt.figure()&#xA;    ax = fig.add_subplot(1, 1, 1)&#xA;    for ob in obs[1:4]:&#xA;        x, y, z = ob.pos&#xA;        ax.plot(x, y)&#xA;    i = np.argmax(venus.F)&#xA;    for ob in obs[1:4]:&#xA;        x, y, z = ob.pos&#xA;        ax.plot(x[i:i+1], y[i:i+1], 'ok')&#xA;    x, y, z = mercury.pos_trailing&#xA;    ax.plot(x[i:i+1], y[i:i+1], 'or')&#xA;    x, y, z = sun.pos&#xA;    ax.plot(x, y, '-k', linewidth=4)&#xA;    ax.set_xlim(-2E+08, 2E+08)&#xA;    ax.set_ylim(-2E+08, 2E+08)&#xA;&#xA;    # ax.legend()&#xA;    ax.set_title('ecliptic projection (m)', fontsize=16)&#xA;&#xA;    fig.suptitle(&quot;in Mercury's orbit trailing by 88/6 days&quot;, fontsize=16)&#xA;&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-26T04:01:24.543" LastActivityDate="2021-05-06T23:39:30.853" Title="Does it even make sense to talk about Mercury's triangular libration points (L4, L5)?" Tags="|orbital-mechanics|orbit|lagrangian-points|mercury|skyfield|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22406" PostTypeId="1" AcceptedAnswerId="22409" CreationDate="2017-07-26T06:11:26.623" Score="5" ViewCount="272" Body="&lt;p&gt;The Southern California Public Radio news item &lt;a href=&quot;http://www.scpr.org/news/2017/07/25/74094/what-cassini-saw-on-titan-dunes-of-the-arabian-des/&quot; rel=&quot;noreferrer&quot;&gt;What Cassini saw on Titan: 'Dunes of the Arabian desert' but made of water chips, not sand&lt;/a&gt; explains: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Little chips of water ice come off (the mountains) and get washed downstream, and then they get pushed up onto the beaches,&quot; Ray said. &quot;They get dried out and then they get picked up by the wind and blown into the deserts. And around the entire equator of Titan is nothing but dunes as far as the eye can see -- just like the dunes of the Arabian desert. But they're made out of little chips of water covered in hydrocarbon goo.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to the article, Trina Ray is the Titan orbiter science co-chair at JPL.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is the evidence that the dunes of Titan are made of &quot;water chips&quot;?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Dune patterns on Titan, &lt;a href=&quot;https://www.jpl.nasa.gov/spaceimages/details.php?id=pia15225&quot; rel=&quot;noreferrer&quot;&gt;pia15225 from NASA JPL&lt;/a&gt;. Click for full size. Despite the names, these represent various imaging techniques by Cassini of dunes on Titan. There is a much longer description in the pia15225 link. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xfdc0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xfdc0m.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-26T09:39:06.460" LastActivityDate="2017-07-26T14:27:54.267" Title="What is the evidence that the dunes of Titan are made of &quot;water chips&quot;?" Tags="|saturn|cassini|titan|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22409" PostTypeId="2" ParentId="22406" CreationDate="2017-07-26T14:27:54.267" Score="6" Body="&lt;p&gt;It is modeled that the dunes must form from chips of Titan's bedrock. Whether those chips are primarily hydrocarbons or primarily water ice, depends on what Titan's crust is presumed to be composed of. There are varying models, depending on what is known of Titan (such as the moment of inertia), and one model with a reasonable level of support involves a substantial water ice crust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A.D. Fortes explains in &lt;a href=&quot;http://ac.els-cdn.com/S0032063311001401/1-s2.0-S0032063311001401-main.pdf?_tid=762a73ba-720d-11e7-860e-00000aab0f6c&amp;amp;acdnat=1501078961_bde9892b02ae34225732d729acb7688f&quot; rel=&quot;noreferrer&quot;&gt;&quot;Titan’s internal structure and the evolutionary consequences&quot;&lt;/a&gt; that Titan's moment of inertia was estimated from measurements taken by Cassini of Titan's quadrupole gravity field. From these observations, an internal structure with a shell of water ice can be derived.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, to answer your question, there is no direct evidence that the dunes are made of water chips, rather what the dunes are made of is assumed based on what model of Titan's composition is considered most likely. Further, modeling the geological effects, depending on the assumed composition, may help to refine our understanding of Titan's dunes.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-07-26T14:27:54.267" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22410" PostTypeId="2" ParentId="22399" CreationDate="2017-07-26T14:42:08.873" Score="3" Body="&lt;p&gt;It depends on the requirements. The PDR/ CDR process is common to government projects. You can see stuff like Orion that had a &lt;a href=&quot;https://www.nasa.gov/feature/orion-s-service-module-completes-critical-design-review/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CDR&lt;/a&gt; last year. I have also seen evidence for many commercial satellite services, like &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwi7lcXClqfVAhUMdj4KHT_GBysQFggyMAA&amp;amp;url=http%3A%2F%2Flicensing.fcc.gov%2Fmyibfs%2Fdownload.do%3Fattachment_key%3D703485&amp;amp;usg=AFQjCNFr-MWKzoL-QiShk2Jsb1RW-Sh0Zg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbcomm&lt;/a&gt;, &lt;a href=&quot;http://investor.iridium.com/releasedetail.cfm?ReleaseID=797759&quot; rel=&quot;nofollow noreferrer&quot;&gt;Irridium&lt;/a&gt;, &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=5&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiSh77_lqfVAhVGeT4KHXX-CaQQFgg9MAQ&amp;amp;url=https%3A%2F%2Fapps.fcc.gov%2Fedocs_public%2Fattachmatch%2FDA-15-391A1.txt&amp;amp;usg=AFQjCNGZkIH3MWjd7NcbL1I-aQnz9col0g&quot; rel=&quot;nofollow noreferrer&quot;&gt;Globalstar&lt;/a&gt;, and no doubt many others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the orbit itself, that is a small piece of the overall design, and is often done quite late in the process. A range of orbits will be proposed from the initial request usually, but the specific orbit will only be done a few months before the rocket launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as interplanetary trajectories, there will be one (Or a few) people picking such paths, and they will be reviewed by some group, but they won't have a full blown PDR/CDR. Those area large scale reviews that review a large program all at once. The trajectory or orbit might be a piece of those reviews, but they won't be the center of the views.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-07-26T15:18:32.210" LastActivityDate="2017-07-26T15:18:32.210" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22411" PostTypeId="1" AcceptedAnswerId="22424" CreationDate="2017-07-26T15:42:14.717" Score="9" ViewCount="819" Body="&lt;p&gt;The NASA News Feature &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2017/chasing-the-total-solar-eclipse-from-nasa-s-wb-57f-jets&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chasing the Total Solar Eclipse from NASA’s WB-57F Jets&lt;/a&gt; describes two telescope-equipped NASA jets that will fly fast enough to spend about 7 minutes in the Umbra (totality) of the upcoming  August 21, 2017 solar eclipse.&lt;/p&gt;&#xA;&lt;p&gt;In addition to using the moon as a sort-of natural coronagraph to look for never-directly-observed-previously &lt;em&gt;nanoflares&lt;/em&gt; in the Solar corona, they will try to image the surface of the planet Mercury in the infrared in order to generate surface temperature maps.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;These images, taken in the infrared, will be the first attempt to map the variation of temperature across the surface of the planet.&lt;/p&gt;&#xA;&lt;p&gt;Mercury rotates much slower than Earth — one Mercurial day is approximately 59 Earth days — so the night side cools to a few hundred degrees below zero while the dayside bakes at a toasty 800 F. The images will show how quickly the surface cools, allowing scientists to know what the soil is made of and how dense it is. These results will give scientists insight into how Mercury and other rocky planets may have formed.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Why is this a type of observation that requires such an heroic effort - flying a telescope through a total solar eclipse, using the Moon as a coronagraph? There is no way to do this with NASA's airborne infrared telescope, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stratospheric_Observatory_for_Infrared_Astronomy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stratospheric Observatory for Infrared Astronomy (SOFIA)&lt;/a&gt;? Nothing (civilian at least) in space has ever had this capability?&lt;/p&gt;&#xA;&lt;p&gt;I'm looking for a fairly technical answer. If this &lt;em&gt;were possible&lt;/em&gt; with SOFIA it seems it would have been addressed long ago. So there must be something specific to the eclipse.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Screen shots from the NASA Goddard video &lt;a href=&quot;https://youtu.be/R0GNqlGNZkI&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Jets Chase The Total Solar Eclipse&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3lEHu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3lEHu.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UZGBx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UZGBx.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-11-20T01:06:32.120" LastActivityDate="2021-12-31T21:15:59.687" Title="Why the thermal imaging of Mercury's surface requires a telescope on a jet flying through an eclipse?" Tags="|imaging|space-telescope|thermal|mercury|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22412" PostTypeId="2" ParentId="22411" CreationDate="2017-07-26T16:01:27.607" Score="2" Body="&lt;p&gt;It could be done by SOFIA, but it would be rather limited. There's actually a really &lt;a href=&quot;https://www.hao.ucar.edu/people/phil-judge/collaborators/eclipse2017/eclipse2017.html#sofia&quot; rel=&quot;nofollow noreferrer&quot;&gt;great explanation of why SOFIA&lt;/a&gt; isn't the best choice. Basically, it comes down to the location of SOFIA's telescope. It points out to the left. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KIE4u.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KIE4u.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the observatory were to make an eclipse observation, it would have to fly perpendicular to the Sun, which would shorten the observation period to under 2 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, this would be a very high risk activity, if the timing was even slightly off the telescope could be damaged. This applies to any telescope making such observations, but Sofia, being a full sized jet, lacks the maneuverability that other telescopes might have.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any satellite making such an observation would have the same risk as Sofia, and even less time to take advantage of it, thus limiting any satellite capability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other options. &lt;a href=&quot;https://en.wikipedia.org/wiki/High-performance_Instrumented_Airborne_Platform_for_Environmental_Research&quot; rel=&quot;nofollow noreferrer&quot;&gt;HIAPER&lt;/a&gt; would be one such plane, for instance. But these types of aircraft are in high demand, mostly to study the sun itself, my guess is they found a way to make a jet work for them for this particular solar eclipse to do the Mercury research they desire. This is primarily a radar plane, but can be configured for other instruments if required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for why during an eclipse, there's a few reasons. The main is that Mercury, at is maximum elevation, is only 28 degrees from the Sun! But that applies to one on Earth's surface, it is somewhat distorted if you are higher up. And that is required to do any such observations. Basically, the Sun has to be low enough that there isn't any stray light coming from it at all, which due to atmospheric effects could happen for some time. I'm not sure about the refraction piece, but the horizon angle at 13 km is about 15 degrees. Given that the Sun would have to be below 15 degrees, and the SOFIA points slightly up, it seems that if it could do it, it would require the perfect alignment, and even then only barely.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-07-26T21:27:48.560" LastActivityDate="2017-07-26T21:27:48.560" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22413" PostTypeId="1" AcceptedAnswerId="22423" CreationDate="2017-07-26T16:06:22.100" Score="4" ViewCount="594" Body="&lt;p&gt;NASA, &lt;a href=&quot;http://blog.lightstreamer.com/2014/02/how-nasa-is-using-lightstreamer.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;via Lightstreamer SRL&lt;/a&gt;, provides four &quot;quaternion components&quot; for the International Space Station's current attitude. These are labelled USLAB000018–21 and described as &quot;US Current Local Vertical Local Horizontal (LVLH) Attitude Quaternion Component [0-3]&quot; respectively. For example, at roughly 15:01:30 2017-07-26 UTC we saw: &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;TIME_000001:   17938885000&#xA;USLAB000018:   0.999387085437775    &quot;[...] Component 0&quot;&#xA;USLAB000019:   0.00414502713829279  &quot;[...] Component 1&quot;&#xA;USLAB000020:  -0.00237915641628206  &quot;[...] Component 2&quot;&#xA;USLAB000021:  -0.0346784666180611   &quot;[...] Component 3&quot;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Lightstreamer has a demo website that displays the raw values as received here: &lt;a href=&quot;http://demos.lightstreamer.com/ISSLive/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://demos.lightstreamer.com/ISSLive/&lt;/a&gt; - to get to those attitude values, click ADCO on the left, then click on the four USLAB000018–21 subheadings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we built &lt;a href=&quot;http://telemetry.space&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://telemetry.space&lt;/a&gt;, I converted those to roll, pitch, and yaw using the &lt;a href=&quot;http://threejs.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;threejs library&lt;/a&gt; by guessing components 0, 1, 2, and 3 correspond to x, y, z, w, like so:&lt;/p&gt;&#xA;&#xA;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;const quaternion = new THREE.Quaternion();&#xA;quaternion.x = USLAB000018;&#xA;quaternion.y = USLAB000019;&#xA;quaternion.z = USLAB000020;&#xA;quaternion.w = USLAB000021;&#xA;const euler = new THREE.Euler();&#xA;euler.setFromQuaternion(quaternion);&#xA;&#xA;const roll = THREE.Math.radToDeg(this.euler.x);&#xA;const pitch = THREE.Math.radToDeg(this.euler.y);&#xA;const yaw = THREE.Math.radToDeg(this.euler.z);&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Producing this output via &lt;a href=&quot;https://jsfiddle.net/n9tuuzbs/2/&quot; rel=&quot;nofollow noreferrer&quot;&gt;JSFiddle&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Roll   -176.0264711321606&#xA;Pitch    -0.28893712359948726&#xA;Yaw      -0.46525059186533985&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Short of getting a HAM license, how can I verify this is right or wrong, and if wrong—as was reasonably suspected by others and myself in &lt;a href=&quot;https://space.stackexchange.com/questions/20358&quot;&gt;another query&lt;/a&gt;—how &lt;em&gt;should&lt;/em&gt; I convert those USLAB values to roll, pitch, yaw for telemetry.space?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;I've used Javascript and threejs in the examples above only as it's what I &lt;a href=&quot;https://github.com/sensedata/space-telemetry&quot; rel=&quot;nofollow noreferrer&quot;&gt;originally used&lt;/a&gt; and any help in any programming language, plain old English, or just the maths will be very much appreciated.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="18909" LastEditorUserId="12102" LastEditDate="2018-02-20T00:02:32.407" LastActivityDate="2021-04-05T10:05:47.720" Title="How should the ISS telemetry for attitude be converted to roll, pitch, and yaw?" Tags="|iss|quaternion|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22414" PostTypeId="1" AcceptedAnswerId="22416" CreationDate="2017-07-26T16:10:52.377" Score="15" ViewCount="1508" Body="&lt;p&gt;&lt;a href=&quot;https://twitter.com/Astro2fish/status/889975065315377152&quot; rel=&quot;noreferrer&quot;&gt;This tweet&lt;/a&gt; from &lt;a href=&quot;https://en.wikipedia.org/wiki/Expedition_51&quot; rel=&quot;noreferrer&quot;&gt;Expedition 51&lt;/a&gt;/52 astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Jack_D._Fischer&quot; rel=&quot;noreferrer&quot;&gt;Jack Fischer&lt;/a&gt; admires the great view straight down from the ginormous Russian porthole of the Progress cargo vehicle on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't really understand where this porthole is. Is it the porthole part of a docked spacecraft, or is it a permanent part of the ISS?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this the same direction that the center, round window of the ISS's Cupola faces?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j4lm0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j4lm0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dA0cs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dA0cs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J7z3q.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J7z3q.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-07-26T20:15:58.857" Title="Is this ginormous porthole a permanent feature of the ISS or &quot;just passing through&quot;?" Tags="|iss|progress|spacecraft-window|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22415" PostTypeId="1" CreationDate="2017-07-26T18:24:47.410" Score="0" ViewCount="1178" Body="&lt;p&gt;How fast would ISS need to travel at the current altitude (LEO 250mi) to be geosynchronous with say 36 degree N lattitude?&lt;/p&gt;&#xA;" OwnerUserId="20480" LastActivityDate="2017-07-26T20:21:05.960" Title="ISS Speed required for true geosynchronous orbit" Tags="|iss|geosynchronous|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22416" PostTypeId="2" ParentId="22414" CreationDate="2017-07-26T18:37:42.070" Score="15" Body="&lt;p&gt;It's on one of the Russian modules of the ISS, that's why it's a ginormous &lt;em&gt;Russian&lt;/em&gt; porthole.  Since the porthole is facing Earth, I'd say yes, it's also facing the same &lt;em&gt;general&lt;/em&gt; direction as the cupola, but not necessarily the same &lt;em&gt;exact&lt;/em&gt; direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;@Steve the first cover is part of the pressure containment vessel of the station, note the mating seals around its rim and the seat of its closed position.  If the glass of the porthole cracks or otherwise starts leaking, the first cover keeps the air in the station from all rushing out.  The other cover is the access hatch in the inner walls of the ISS that define the bounds of normal day-to-day operation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The opinions expressed in this analysis are my own, YMMV.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's window number 9 (of 14) in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_(ISS_module)#Design&quot; rel=&quot;noreferrer&quot;&gt;Zvezda&lt;/a&gt; module, a 16-inch-diameter (410 mm) window in the main Working Compartment.  (&lt;em&gt;N.B.&lt;/em&gt;  Window No. 11 is &quot;unaccounted for in all available sources&quot; - so it's quite possible there are only &lt;em&gt;13&lt;/em&gt; windows on the module.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Russian Space Segment windows&lt;/strong&gt;&lt;br&gt;&#xA;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/2/23/ROS_Windows_0114_complete.jpg/639px-ROS_Windows_0114_complete.jpg&quot; alt=&quot;Russian Space Segment windows, from Wikipedia&quot;&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/2/23/ROS_Windows_0114_complete.jpg&quot; rel=&quot;noreferrer&quot;&gt;link to full size (1096 x 823) image&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess &quot;ginormous&quot; is a matter of opinion - a 16 inch window doesn't seem to be that big - from here on the ground.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-07-26T20:15:58.857" LastActivityDate="2017-07-26T20:15:58.857" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22417" PostTypeId="2" ParentId="22405" CreationDate="2017-07-26T18:59:29.143" Score="7" Body="&lt;p&gt;You're right, the Sun-Mercury libration points (all five of them) are merely mathematical curiosities of a hypothetical two-body system.  As you've calculated, the actual gravitational effects of Venus and Earth (both much larger than Mercury, but also farther away) make the two-body approximation &amp;quot;less than useful&amp;quot; for any real world systems.&lt;/p&gt;&#xA;&lt;p&gt;Even the stable Earth-Moon libration points (L5 being the most widely known) are not &lt;em&gt;exactly&lt;/em&gt; useful:  If you read discussions of O'Neill's &lt;em&gt;High Frontier&lt;/em&gt; space colonies, you will note that they don't sit &lt;em&gt;at&lt;/em&gt; (e.g.) L5, but rather follow dumbell-shaped orbits around it because of the Sun's gravity.  Even &lt;em&gt;that&lt;/em&gt; is an approximation because it doesn't take into account the gravity of the other planets, but their effect is largely swamped by the Sun's gravity and is generally left out of current discussions where the colonies are (presently) only design studies.  Once there are actually multiple colonies &lt;em&gt;at&lt;/em&gt; L5, though, you can bet their management is going to be taking &lt;em&gt;every&lt;/em&gt; effect into account they can measure to ensure there aren't any collisions!&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The calculations Lagrange derived that bear his name are for a &lt;em&gt;two body&lt;/em&gt; system - and as your own results indicate, Mercury and the Sun cannot be considered a two body system because of other nearby massive objects - Venus and the Earth.  Hence, Lagrange's calculations aren't applicable, and as a result of &lt;em&gt;that&lt;/em&gt;, L1 - L5 basically don't exist with respect to Mercury.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2021-05-06T23:39:30.853" LastActivityDate="2021-05-06T23:39:30.853" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="22418" PostTypeId="2" ParentId="22415" CreationDate="2017-07-26T19:19:09.543" Score="3" Body="&lt;p&gt;If the ISS were to decrease speed to try to be geosynchronous it would start to fall in it's orbit.  The only way to keep it in its current orbit and be moving slow enough to be geosynchronous you would have to apply thrust backward against its current direction and constantly apply thrust up away from the Earth to counter the ISS from falling in altitude.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speed at geosynchronous orbit is 3 km/sec, the speed of the rotation of the Earth at the equator is 0.46 km/sec, geosynchronous orbit is at 35,786 km and the ISS is at 400 km, the current speed of the ISS is 7.66 km/sec so the speed the ISS would need to travel to match the ground would only be slightly greater than 0.46 km/sec.   &lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="20287" LastEditDate="2017-07-26T20:20:42.247" LastActivityDate="2017-07-26T20:20:42.247" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22419" PostTypeId="2" ParentId="22415" CreationDate="2017-07-26T19:19:10.253" Score="-2" Body="&lt;p&gt;AFAIK it's not possible for it to be Geosynchronous at it's current altitude. Geosynchronous orbits are only possible at a much higher altitude. At any given altitude there is only one speed which produces a stable orbit. Any faster and you'll spiral outward and any slower and you'll spiral inward. There's only one altitude where the correct orbital speed results in a 24 hour orbital period which is what produces a Geosynchronous orbit.&lt;/p&gt;&#xA;" OwnerUserId="16458" LastActivityDate="2017-07-26T19:19:10.253" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22420" PostTypeId="1" AcceptedAnswerId="22430" CreationDate="2017-07-26T19:22:03.187" Score="19" ViewCount="714" Body="&lt;p&gt;In NPR's news item &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2017/07/26/532610193/after-a-year-in-space-the-air-hasnt-gone-out-of-nasas-inflated-module&quot; rel=&quot;noreferrer&quot;&gt;After A Year In Space, The Air Hasn't Gone Out Of NASA's Inflated Module&lt;/a&gt; there is a quote that's interesting:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;We do believe we've taken at least one hit,&amp;quot; says Crusan. &amp;quot;Very small in nature, and actually we can't even visually see where it's at.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Crusan says there was no loss of pressure from the hit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Without a leak, or a visible site, what is it that makes them think they've &amp;quot;taken a hit&amp;quot;? I assume it's a meteorite or particle impact they are referring to, right?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ns7yk.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ns7yk.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Photo credit: NASA, from &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2017/07/26/532610193/after-a-year-in-space-the-air-hasnt-gone-out-of-nasas-inflated-module&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ljngK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ljngK.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Astronauts Peggy Whitson and Thomas Pesquet are photographed inside BEAM, which has an interior roughly the size of a medium school bus.&amp;quot; Photo credit: NASA, from &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2017/07/26/532610193/after-a-year-in-space-the-air-hasnt-gone-out-of-nasas-inflated-module&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jJCsb.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jJCsb.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Flight engineer Kate Rubins checks out the Bigelow Expandable Activity Module, which is attached to the International Space Station.&amp;quot; Photo credit: NASA, from &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2017/07/26/532610193/after-a-year-in-space-the-air-hasnt-gone-out-of-nasas-inflated-module&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2021-05-08T04:30:36.340" Title="Why do they believe that the Bigelow Expandable Activity Module has &quot;taken a hit&quot;?" Tags="|iss|bigelow|bigelow-expandable-module|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22421" PostTypeId="2" ParentId="22415" CreationDate="2017-07-26T19:48:15.663" Score="2" Body="&lt;p&gt;Actually, the ISS would have to &lt;strong&gt;slow down&lt;/strong&gt; by an order of magnitude in order to take 24 hours to orbit the Earth &lt;strong&gt;&lt;em&gt;at its current altitude&lt;/em&gt;&lt;/strong&gt; where it completes an orbit every 1.5 hours - 16 times a day - with an orbital velocity of 4.76 mi/sec.  Thus, to keep it &quot;geosynchronous&quot; at its current altitude, you would have to slow it to &lt;strong&gt;1071 mph&lt;/strong&gt; (4.76 mi/sec / 16 * 3600 - current speed divided by orbits per day times seconds per hour).  At that speed, it &lt;em&gt;wouldn't be in orbit&lt;/em&gt; any more, so you'd have to be expending a &lt;strong&gt;&lt;em&gt;LOT&lt;/em&gt;&lt;/strong&gt; of energy as upward thrust to keep it from falling straight down from the sky like a brick.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-07-26T20:21:05.960" LastActivityDate="2017-07-26T20:21:05.960" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22423" PostTypeId="2" ParentId="22413" CreationDate="2017-07-26T21:08:55.600" Score="3" Body="&lt;p&gt;It's not clear from the documentation what the four parameters of quaternion represent explicitly -- specifically, whether &lt;code&gt;w&lt;/code&gt; is the real part.  From my experience, NASA likes to specify quaternions with the real part first, so you may move &lt;code&gt;quaternion.w&lt;/code&gt; to &lt;code&gt;USLAB000018&lt;/code&gt; and shift the remaining three accordingly.  ISS also reports its Tait-Bryan angles in Yaw-Pitch-Roll order, so you would need to set the &lt;code&gt;euler.order&lt;/code&gt; option to &lt;code&gt;'ZYX'&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under nominal operation, all three angles (YPR) should be within a few degrees of zero.  180 degrees roll is definitely not correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See my modifications in action &lt;a href=&quot;https://jsfiddle.net/n9tuuzbs/3/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-07-26T21:08:55.600" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22424" PostTypeId="2" ParentId="22411" CreationDate="2017-07-26T21:37:49.720" Score="15" Body="&lt;p&gt;&lt;em&gt;Why not a satellite-based telescope to observe Mercury in the thermal infrared?&lt;/em&gt;&lt;br /&gt;&#xA;Space-borne satellites that are designed to look at the Sun (e.g., SOHO) aren't instrumented to look in the thermal infrared, while satellite-based telescopes that are instrumented to look in the thermal infrared in general don't point anywhere close to the Sun.&lt;/p&gt;&#xA;&lt;p&gt;One issue with imaging Mercury from roughly one AU is accidentally imaging the Sun. Hubble was rarely allowed to &lt;a href=&quot;https://www.stsci.edu/hst/phase2-public/13047.pro&quot; rel=&quot;nofollow noreferrer&quot;&gt;point to Venus&lt;/a&gt;, but never Mercury. Another issue is cooling. Even if the telescope isn't directly imaging the Sun, key unshielded portions of the satellite will inevitably be facing the Sun while imaging Mercury. Thermal imaging satellites need to be cryogenically cooled. Pointing a space-based thermal imager at Mercury would reduce the vehicle's usable lifespan.&lt;/p&gt;&#xA;&lt;p&gt;Neither of those issues (accidentally imaging the Sun and heating problems) is an issue in the case of satellites sent to Mercury. To date, only two satellites have been sent to Mercury (Mariner 10 and MESSENGER) and one is on the way (BepiColombo). MESSENGER was not instrumented to see in the thermal infrared. Mariner 10 was and BepiColombo is. Mariner 10 made three flybys of Mercury in 1974 and 1975. These included observations in the thermal infrared, but the received data were limited.&lt;/p&gt;&#xA;&lt;br&gt;&#xA;*Why not a ground-based telescope to observe Mercury in the thermal infrared?*  &#xA;There would be so much thermal infrared airglow when the Sun is below the horizon so as to make observations of Mercury worthless. There would be too much of a chance of accidentally imaging the Sun when the Sun is high in the sky. Cooling is of course an issue as well.&#xA;&lt;p&gt;The above do not apply if path of a solar eclipse happens to pass over a thermal infrared telescope. Exactly that happened in July 1991, when Mauna Kea observatory was almost exactly in the center of the path of totality. The infrared telescope at Mauna Kea was most definitely put to use during this eclipse, but not to look at Mercury. Apparently observing Mercury wasn't given a high enough priority.&lt;/p&gt;&#xA;&lt;br&gt;&#xA;*Why not SOFIA?*  &#xA;For one thing, it's construction is suboptimal for this usage; see the other answer. For another, it's too valuable. SOFIA's predecessor, the Kuiper Airborne Observatory (KAO), was used twice to observe Mercury in the infrared. The aircraft's fuselage protected the telescope against seeing the Sun. However, because of the possibility of pointing errors that might result in imaging the Sun, these observations were made at the end of the KAO's life. (SOFIA was about to replace the KAO.) SOFIA isn't yet at the end of its life.&#xA;&lt;br&gt;&#xA;*Why during an eclipse?*  &#xA;Solar eclipses provide unique opportunities for observing the Sun, and also apparently for observing Mercury. Solar eclipses have long been accompanied by numerous scientific observations specialized to take advantage of the eclipse.&#xA;&lt;p&gt;It doesn't have to be done during an eclipse. Mercury does need to be fairly high in the sky to be able to see it in thermal infrared, even at the high altitude at which NASA's WB-57s fly. A total eclipse is not essential for this experiment. The personnel and aircraft are being used to observe the Sun during totality, and this apparently conflicts with the Mercury observation experiment. The Mercury observations instead are being performed 30 minutes prior to and after the total eclipse.&lt;/p&gt;&#xA;&lt;p&gt;Being a one-off experiment, and being a secondary experiment, (the primary experiment is observing the Sun), the possibility of accidentally imaging the Sun is not a complete disaster. The Sun will still be partially eclipsed by the Moon during that period. This will reduce the amount of secondary light (solar thermal infrared absorbed and reemitted by the atmosphere, eventually reaching the instrument) compared to that which results from an uneclipsed Sun.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="44662" LastEditDate="2021-12-31T21:15:59.687" LastActivityDate="2021-12-31T21:15:59.687" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="22425" PostTypeId="1" AcceptedAnswerId="22426" CreationDate="2017-07-26T23:32:36.067" Score="16" ViewCount="3876" Body="&lt;p&gt;Why does the crewed version of the Soyuz rocket use grid fins on its launch escape system? It seems like extra weight to carry along during a portion of the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a rendering of the upper portion of the Soyuz rocket with the grid fins.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Ik2vS.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ik2vS.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2021-02-21T23:37:28.063" Title="What are the Grid fins on the Soyuz escape system used for?" Tags="|launch|crewed-spaceflight|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22426" PostTypeId="2" ParentId="22425" CreationDate="2017-07-27T02:00:44.880" Score="31" Body="&lt;p&gt;The grid fins passively stabilize the spacecraft while the LES is firing during an abort. They deploy by pivoting outward but aren't otherwise movable.&lt;/p&gt;&#xA;&lt;p&gt;In an abort situation, the booster may be in the process of tumbling or exploding or both when the spacecraft separates. Having draggy fins toward the back of the package quickly straightens out its flight, providing the maximum separation from the booster during the powered portion of the escape, and allowing the fairing to separate cleanly from the spacecraft after the escape tower burns out.&lt;/p&gt;&#xA;&lt;p&gt;Stable flight of a rocket requires that the center of aerodynamic pressure be behind the center of mass. The Soyuz LES abort has to carry both the orbital module (uncrewed during launch) and the reentry module (crewed) with it because the orbital module is stacked above the reentry module. The service module below that is left behind during an LES abort.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zfQy9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zfQy9.png&quot; alt=&quot;diagram of Soyuz launch escape configuration&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Without the fins, most of the aero pressure would manifest at the point where the base of the LES tower meets the top of the spacecraft fairing, above the orbital module. I estimate the center of mass is near the top of the orbital module when the LES is ignited, moving backwards as the LES rocket propellant is expended. With the center of mass behind the center of pressure, the stack would be unstable. Adding the fins moves the center of aerodynamic pressure back, yielding a stable configuration, much like a badminton shuttlecock.&lt;/p&gt;&#xA;&lt;p&gt;This is in contrast to Apollo, which has no equivalent to the Soyuz orbital module, and separates just the conical command module with its LES. Because there's no fairing to separate, it's not critical to keep the capsule stable after the first few seconds of separation, and in fact the CM would normally begin to tumble after separation. Depending on the abort altitude, either a drogue parachute or canard vanes on the top of the tower would eventually stabilize it in a blunt-end-first attitude.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hqVOF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hqVOF.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-02-21T23:37:28.063" LastActivityDate="2021-02-21T23:37:28.063" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22427" PostTypeId="1" CreationDate="2017-07-27T04:07:51.540" Score="-7" ViewCount="203" Body="&lt;ol&gt;&#xA;&lt;li&gt;Would it be possible? &lt;/li&gt;&#xA;&lt;li&gt;Could you survive? &lt;/li&gt;&#xA;&lt;li&gt;Practical? &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="20485" LastActivityDate="2017-07-27T09:15:32.553" Title="Could you use two (or more) weather balloons, and an oxygen tank to get a human to low orbit?" Tags="|crewed-spaceflight|" AnswerCount="2" CommentCount="3" ClosedDate="2017-07-27T16:59:04.443" ContentLicense="CC BY-SA 3.0" />
  <row Id="22428" PostTypeId="2" ParentId="22427" CreationDate="2017-07-27T05:10:47.057" Score="7" Body="&lt;p&gt;No. Reaching low orbit requires a horizontal velocity of almost 8 km/s (17500 mph).  The balloons won't provide any velocity aside from that of the atmosphere they're floating in, which tops out at about 70 m/s (150 mph) in a strong jetstream.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-07-27T05:10:47.057" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22429" PostTypeId="2" ParentId="22427" CreationDate="2017-07-27T09:15:32.553" Score="1" Body="&lt;p&gt;No. A low orbit must be above the atmosphere, but you could not leave the atmosphere with a balloon. A low orbit needs a high horizontal velocity but a balloon just moves with the wind but does not accelerate on its own. The height of a low orbits is at least 200 km, but a balloon only about 30 km. As Pericynthion wrote, a velocity of about 8 km/s is necessary, but a balloon is very, very slow in comparison. Besides that, the direction of the velocity is important.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-07-27T09:15:32.553" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22430" PostTypeId="2" ParentId="22420" CreationDate="2017-07-27T09:41:13.163" Score="28" Body="&lt;p&gt;Since the module is a prototype the main purpose is to test and verify the concept. Primarily goal is to test the module's ability to withstand space debris. For this purpose the module is equipped with sensors to detect impacts. There are a few articles you can find some information about this at&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/feature/first-year-of-beam-demo-offers-valuable-data-on-expandable-habitats&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Researchers at NASA’s Langley Research Center in Hampton, Virginia, continually analyze data from internal sensors designed to monitor and locate external impacts by orbital debris, and, as expected, have recorded a few probable micrometeoroid debris impacts so far. BEAM has performed as designed in preventing debris penetration with multiple outer protective layers exceeding space station shielding requirements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;or &lt;a href=&quot;http://www.3ders.org/articles/20170529-iss-bigelow-expandable-activity-module-gets-3d-printed-radiation-shields-after-passing-space-debris-tests.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;3ders&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The first year of the expandable module’s life on the ISS has been mostly about resisting space debris. Sensors in the module, which are monitored by staff at NASA’s Langley Research Center in Hampton, Virginia, indicate that the blow-up structure has taken a few hits. Fortunately, the kevlar-like weave of the module has done its bit, preventing any penetration from outside forces.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;After some research &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/content/-/article/beam&quot; rel=&quot;nofollow noreferrer&quot;&gt;and this article&lt;/a&gt; I found out that they are using a system called Distributed Impact Detection System (short DIDS) to detect these impacts. &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/news/ubnt.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; describes the system like:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;DIDS units are high-speed, four-channel digitizers that record ultrasonic noises. Instead of listening for the hiss of air, these units detect the high-frequency sounds moving through the metal itself&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b67cE.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b67cE.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There are a few of these systems installed in different modules at the ISS&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L9IYN.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L9IYN.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Detailed information about the system can be found in this technical report of NASA &lt;a href=&quot;https://ntrs.nasa.gov/citations/20100021981&quot; rel=&quot;nofollow noreferrer&quot;&gt;Distributed Impact Detector System (DIDS) Health Monitoring System Evaluation&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="17378" LastEditorUserId="12102" LastEditDate="2021-05-08T04:30:36.340" LastActivityDate="2021-05-08T04:30:36.340" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22432" PostTypeId="1" CreationDate="2017-07-27T13:53:40.497" Score="3" ViewCount="1099" Body="&lt;p&gt;I'm struggling with orbital elements...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm trying, at the same time as finding a clear definition to them, to write a piece of code to switch from osculator elements to mean equinoctial elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've found &lt;a href=&quot;https://space.stackexchange.com/questions/14731/nuances-of-the-terms-mean-osculating-keplerian-orbital-elements&quot;&gt;this topic&lt;/a&gt; but it did not really help me to distinguish the two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could somebody give me a hint?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And is it possible the other way around?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Benoist&lt;/p&gt;&#xA;" OwnerUserId="20489" LastActivityDate="2017-07-27T20:11:39.547" Title="From osculating to mean equinoctial elements and back" Tags="|orbit|orbital-elements|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22434" PostTypeId="2" ParentId="22432" CreationDate="2017-07-27T16:08:10.663" Score="1" Body="&lt;p&gt;I hadn't heard about equinoctial elements until just now, but this is too long for a comment.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;http://www.kpubs.org/article/articleMain.kpubs?articleANo=OJOOBS_2011_v28n1_37&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.kpubs.org/article/articleMain.kpubs?articleANo=OJOOBS_2011_v28n1_37&lt;/a&gt; may or may not be helpful.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The open source CSPICE library (&lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/index.html&lt;/a&gt;) doesn't have a function that converts between osculating and equinoctial elements directly, but:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/eqncpv_c.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/eqncpv_c.html&lt;/a&gt; converts equinoctial elements to a state vector (position and velocity).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/oscelt_c.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/oscelt_c.html&lt;/a&gt; and &lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/oscltx_c.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/oscltx_c.html&lt;/a&gt; convert a state vector to osculating elements&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/conics_c.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/conics_c.html&lt;/a&gt; converts osculating elements to a state vector.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;This doesn't quite do everything you want, and there are several caveats (in some cases, converting a state vector to osculating elements yields a nonsense results, particularly if one body doesn't actually orbit the other -- eg, if you pretend Charon revolves around Earth, for example), but, if you look at the source code, it might help.&lt;/p&gt;&#xA;" OwnerDisplayName="user7073" LastActivityDate="2017-07-27T16:08:10.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22435" PostTypeId="2" ParentId="22432" CreationDate="2017-07-27T20:11:39.547" Score="3" Body="&lt;p&gt;According to Analytical Mechanics of Space Systems, Osculating Elements are the one containing different perturbations in the Orbit. When you omit the short time perturbation (J2) you gain the mean orbital element. The mean orbital elements change not much during the short time so they are a good choice for long term orbital maneuvers (when you have small thruster and are interested to see the changes in orbit in a long term basis).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The whole concept of osculating to mean came with two paper from 1959:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The motion of a close earth satellite &lt;em&gt;Kozai&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;Solution of the problem of artificial satellite theory without drag &lt;em&gt;Brouwer&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;They did not cover all the orbits, so Lyddane done some correction to them:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Numerical comparison between Brouwer's theory and solution by Cowell's method for the orbit of an artificial satellite &lt;em&gt;Lyddane&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are relevant for Keplerian Elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Books and Papers which can help you for Equinoctial Elements:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Analytical Mechanics of Space Systems &lt;em&gt;Schaub and Jenkins&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;An Introduction to Astrodynamics &lt;em&gt;Battin&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;On the Equinoctial Orbit Elements &lt;em&gt;Burke and Cefola&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;The Long-Term prediction of artificial Satellite Orbit  &lt;em&gt;Cefola&lt;/em&gt;&lt;/li&gt;&#xA;&lt;li&gt;A computerized algebraic utility for the construction of nonsingular satellite theories &lt;em&gt;Zeis&lt;/em&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;If you want to test your implementation or the mathematics use &lt;a href=&quot;http://orekit.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orekit&lt;/a&gt;, it has most of these orbit types implemented.&lt;/p&gt;&#xA;" OwnerUserId="19601" LastActivityDate="2017-07-27T20:11:39.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22437" PostTypeId="1" CreationDate="2017-07-27T21:32:14.550" Score="3" ViewCount="1403" Body="&lt;p&gt;Are the reported satellite velocities, such as for the ISS at 17,450 mph relative to the surface of the earth, which is turning, or to some fixed point without the rotational velocity.  In other words, is the ISS moving at the above velocity relative to a point on the earth's surface?&lt;/p&gt;&#xA;" OwnerUserId="20494" LastEditorUserId="12102" LastEditDate="2017-07-29T02:29:32.650" LastActivityDate="2017-07-29T02:29:32.650" Title="Satellite Velocities: Relative or Absolute?" Tags="|orbit|frames|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22438" PostTypeId="2" ParentId="22437" CreationDate="2017-07-27T21:44:22.767" Score="8" Body="&lt;p&gt;All velocities are relative.  In this case, the orbital velocity is in an Earth-Centered Inertial (ECI) framework, so it's relative to a non-rotating set of axes whose orientation is fixed with respect to the stars, and with it's center at the center of the earth.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="12102" LastEditDate="2017-07-28T02:54:25.143" LastActivityDate="2017-07-28T02:54:25.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22439" PostTypeId="2" ParentId="22437" CreationDate="2017-07-28T09:39:06.343" Score="3" Body="&lt;p&gt;The OP's question can be restated as &quot;&lt;em&gt;Are satellite velocities stated in an ECEF (Earth Centered, Earth Fixed) frame, or an ECI (Earth Centered Inertial) frame?&lt;/em&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As indicated by David Hammen's answer to &lt;a href=&quot;https://space.stackexchange.com/questions/21272/what-is-the-frame-of-reference-for-orbital-speed&quot;&gt;What is the frame of reference for orbital speed?&lt;/a&gt;, the choice of frame of reference is somewhat arbitrary.  However, partly &lt;em&gt;by convention&lt;/em&gt;, orbital velocity for a satellite in orbit around the Earth is defined in an Earth-Centered Inertial (ECI) frame of reference, relative to a non-rotating set of axes whose orientation is nominally fixed with respect to the stars, and with it's origin at the center of the Earth.  (For reference, see &lt;em&gt;Frame of reference&lt;/em&gt; section of the Wikipedia article about &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_state_vectors#Frame_of_reference&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital state vectors&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that using an ECI frame for orbital parameters, rather than an ECEF frame, makes figuring out how to transition to another frame of reference &lt;em&gt;much&lt;/em&gt; less mathematically complicated, e.g., going to the Moon where an LCI (Lunar Centered Inertial) frame makes more sense.  This is most likely why the &lt;em&gt;convention&lt;/em&gt; of using the ECI frame for orbital velocities was chosen over the more obvious (to an observer on the ground) ECEF one.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-07-29T02:09:09.907" LastActivityDate="2017-07-29T02:09:09.907" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22440" PostTypeId="1" AcceptedAnswerId="22442" CreationDate="2017-07-28T12:50:41.000" Score="35" ViewCount="4993" Body="&lt;p&gt;There is some news that I've just read about in the Pod Bay SE chat room:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Wired: &lt;a href=&quot;https://www.wired.com/story/spacexs-mars-plans-hit-a-pothole-up-next-the-moon/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX's Mars Plan Hits a Pothole. Up Next: The Moon?&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Ars Technica: &lt;a href=&quot;https://arstechnica.com/science/2017/07/elon-musk-drops-an-important-hint-about-his-revised-mars-rocket/&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk’s Mars rocket may be about to lose half of its engines&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;YouTube: &lt;a href=&quot;https://www.youtube.com/watch?v=BqvBhhTtUm4&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk, ISS R&amp;amp;D Conference, July 19, 2017&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The news items include a lot of background and interpretation and speculation and future-predicting and it's hard for me to roll all of that back and identify just the bare technical facts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is propulsive landing of a &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_Red_Dragon&quot; rel=&quot;noreferrer&quot;&gt;Red Dragon capsule&lt;/a&gt; on Mars &lt;em&gt;more difficult than originally thought for some technical reason&lt;/em&gt;, or is it just that &lt;em&gt;the goal is less attractive&lt;/em&gt; for logistic, business, or long-range planning types of reasons?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-28T13:18:36.597" LastActivityDate="2017-07-28T20:09:14.697" Title="SpaceX and propulsive landing on Mars -- what just happened? (and why?)" Tags="|spacex|red-dragon|propulsive-deceleration|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22441" PostTypeId="2" ParentId="22440" CreationDate="2017-07-28T13:13:49.793" Score="13" Body="&lt;p&gt;Red Dragon was an attempt to get an early mission to Mars using off the shelf hardware.  That is, assuming Dragon V2 was developed and off the shelf. Sure needed modifications but the core functionality was there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA and SpaceX decided in the development of Dragon V2 to use only parachute landings, and not powered landings. Thus no landing legs popping through the heat shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus there no longer is an off the shelf vehicle that has most of the core functionality&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why did they decide?  That is a different and complicated question to answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The NASA standards for human rating were so difficult and it was not clear that for the number of possible landings they would ever do with Dragon V2 that it was worth the costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was some debate as to whether there was an issue with legs going through the heat shield, or if it was just the overall NASA standards.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-07-28T13:57:40.577" LastActivityDate="2017-07-28T13:57:40.577" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22442" PostTypeId="2" ParentId="22440" CreationDate="2017-07-28T13:30:18.800" Score="57" Body="&lt;p&gt;Red Dragon was intended to piggyback on the effort to develop &lt;em&gt;and qualify&lt;/em&gt; Dragon for propulsive landings on Earth. &#xA;The qualification hit a snag: NASA requirements for human-rating the propulsive landing system were so stringent that &lt;a href=&quot;https://spaceflightnow.com/2017/07/19/propulsive-landings-nixed-from-spacexs-dragon-spaceship/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX decided&lt;/a&gt; they didn't want to spend the time and money to meet those requirements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That left development of the landing system &lt;a href=&quot;http://www.latimes.com/business/la-fi-spacex-red-dragon-20170721-story.html&quot; rel=&quot;noreferrer&quot;&gt;incomplete&lt;/a&gt;. They'd have to finish the landing system for Red Dragon alone, which would make Red Dragon a much more expensive mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reading between the lines, SpaceX wants to focus its development resources on their Interplanetary Transport System (ITS/BFR) as soon as possible, and not spend time on one-off missions like Red Dragon.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-07-28T15:25:49.073" LastActivityDate="2017-07-28T15:25:49.073" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22443" PostTypeId="1" CreationDate="2017-07-28T14:20:04.637" Score="1" ViewCount="573" Body="&lt;p&gt;I know others have touched on this, but I'm looking for the LEO to HEO case specifically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the parameters I have: LEO 454 km circular orbit at 61 degree inclination; HEO 1100 x 9000 km at 64 degree inclination (I know, two problems, orbit &quot;height&quot; and inclination change)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a choice of two engines, one produces 16680 kN vacuum thrust with isp of 328 sec while the other produces 3870 N with isp of 281 sec.&lt;/p&gt;&#xA;" OwnerUserId="20501" LastActivityDate="2017-07-28T17:59:47.813" Title="What's delta V and propellant required to go from LEO to HEO?" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22444" PostTypeId="1" AcceptedAnswerId="24914" CreationDate="2017-07-28T15:24:28.210" Score="15" ViewCount="1430" Body="&lt;p&gt;In Wikipedia's article SpaceX Falcon-Heavy the section titled &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_Heavy#Capabilities&quot; rel=&quot;noreferrer&quot;&gt;Capabilities&lt;/a&gt; has a table comparing maximum payload masses to Falcon 9 for different payload destination. The current numbers there are shown below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is probably some fundamental, easy to grasp reason why Heavy can bring 4.2 times as much mass to Mars than F9, but only 3.2 times as much to GTO and only 2.7 times as much to LEO, but I don't know what that would be.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Payload            Falcon Heavy   Falcon 9    Ratio&#xA;LEO (28,5°)          63,800 kg   22,800 kg    2.68&#xA;GTO (27°)            26,700 kg    8,300 kg    3.22&#xA;GTO (27°) Reusable    8,000 kg    5,500 kg    1.45  -  does not apply here&#xA;Mars                 16,800 kg    4,020 kg    4.18&#xA;Pluto                 3,500 kg      - - &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; rough plot of ratio versus &quot;altitude&quot; of the three (presumably non-reusable) data points, just to show there is a trend. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bHp8W.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bHp8W.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Artist conception of a Falcon Heavy, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Pad_39_A_Falcon_Heavy_Artist_Cropped.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SOsGa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SOsGa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-29T12:45:20.273" LastActivityDate="2018-02-07T03:17:11.743" Title="Why can Falcon Heavy bring 4.2 times as much mass to Mars than F9, but only 2.7 times as much to LEO?" Tags="|spacex|falcon-9|falcon-heavy|performance|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22445" PostTypeId="2" ParentId="22443" CreationDate="2017-07-28T15:59:55.630" Score="2" Body="&lt;p&gt;This problem is a good exercise in orbital mechanics. For my calculations, I assume a spherical earth of radius 6371km. A bunch of orbital mechanics terminology will be used and I make assumptions that all burns are done optimally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking the parameters specified, we first calculate the LEO semi-major axis to be 6825km. The HEO semi-major axis is 11421km. For this calculation, I will assume a Hohmann transfer with a transfer orbit semi-major axis of 11098km. Using the vis-viva equation, we can calculate the delta-V for the initial burn.&lt;br&gt;&#xA;$$v_{LEO,Pe}=\sqrt{GM_{earth}(\frac{2}{r}-\frac{1}{a})}=7642\mathrm{m/s}$$&lt;br&gt;&#xA;Same for HEO:&#xA;$v_{Transfer,Pe}=8994\mathrm{m/s}$&lt;br&gt;&#xA;This gives a delta-V of $1352\mathrm{m/s}$ for the first burn.&#xA;A similar calculation for the second burn at apogee of the transfer orbit yields a delta-V of $4119\mathrm{m/s}-3993\mathrm{m/s}=126\mathrm{m/s}$.&lt;br&gt;&lt;br&gt;&#xA;Now for the inclination change: Inclination changes are not as simple as you might assume. It is possible to have an orbit at 61 degrees inclination be completely, horribly misaligned with an orbit at 64 degrees inclination. But I will assume best case scenario, where either the Ascending or Descending nodes are at Apogee and the relative inclination is the difference between the two orbits' inclinations (i.e 3 degrees). In this case, the inclination change burn is given by&lt;br&gt;&#xA;$v_{IC}=2v \cdot sin(\theta/2)$ with $\theta=3^o$. At apogee, before the prograde burn, $v=3993\mathrm{m/s}$ and so $v_{IC}=209\mathrm{m/s}$.&lt;br&gt;&#xA;Note as well that the inclination change burn does not change our total orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now we have a total delta-V of $1352+209+126=1687\mathrm{m/s}$. In designing a spacecraft, we need to have a small amount extra since burns are never optimal. With a 5% buffer, our total delta-V is $1770\mathrm{m/s}$.&#xA;&lt;br&gt;&lt;br&gt;&#xA;Finally, we can calculate the mass fraction of your spacecraft in LEO:&lt;br&gt;&#xA;$\frac{m_i}{m_f}=e^{\Delta V/(I_{SP}\cdot g_0)}$&lt;br&gt;&#xA;For 328 sec Isp, $\frac{m_i}{m_f}=1.73$&lt;br&gt;&#xA;For 281 sec Isp, $\frac{m_i}{m_f}=1.90$&lt;br&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this helps in your calculations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using a spacecraft initial mass of 30,000lbs, we can calculate the total burn time. $30000\mathrm{lb}=13600\mathrm{kg}$, and so this means there is $5379\mathrm{kg}$ of fuel given a mass fraction of 1.73. If there is $16680\mathrm{N}$ of thrust, then:&#xA;$$\dot m=\frac{T}{I_{SP}*g_0}=5.2\mathrm{kg/sec}$$&#xA;This gives a total burn time of $\frac{5379\mathrm{kg}}{5.2\mathrm{kg/sec}}\approx 1000\mathrm{sec}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For engine 2, I get a burn time of 4600 sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The times for the individual burns will not be exactly in proportion to their delta-V out of total, but it will be pretty close.&lt;/p&gt;&#xA;" OwnerUserId="20309" LastEditorUserId="20309" LastEditDate="2017-07-28T17:59:47.813" LastActivityDate="2017-07-28T17:59:47.813" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22446" PostTypeId="1" CreationDate="2017-07-28T17:27:39.657" Score="8" ViewCount="274" Body="&lt;p&gt;Anybody have a copy of the specifications for the Interim Control Module?  Particularly interested in mass, either dry or fully loaded...or both!&lt;/p&gt;&#xA;" OwnerUserId="20501" LastActivityDate="2017-07-28T17:59:48.043" Title="ISS Interim Control Module Specifications" Tags="|iss|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22447" PostTypeId="1" AcceptedAnswerId="25380" CreationDate="2017-07-28T17:42:16.170" Score="5" ViewCount="646" Body="&lt;p&gt;This question has been bugging me for some time. First a little background. We know that a space elevator is essentially a (very) long rope with a heavy asteroid attached at the other end well beyond geostationary orbit. This orbit has some angular momentum associated with it, notably $L=M_{ast}\omega h$ where $h$ is the height above the center of the earth and $\omega$ is the angular velocity of the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But wait! If we want to send something to geostationary orbit via the space elevator, we increase the angular momentum of the system by $L_{new}=M_{ast}\omega h + M_{obj}\omega h_{GSO}$. So clearly the angular momentum must be supplied by something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if it was supplied by the asteroid, its angular momentum must decrease and this would lead to instability in the system as $\omega$ would have to decrease. It can't be supplied only by earth (unless we use a very long rigid pole).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is thus: &lt;strong&gt;What methods have been proposed to preserve the angular momentum of a space elevator?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Final addendum: I found on &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt; that the GSO orbit velocity is 3.03km/s. Even with LH2/LOX fuel, this works out to a mass fraction of $\approx 2$. So if we wanted to send a 20 ton payload to GSO, we would need a minimum of 20 tons of fuel, and that neglects any equipment to store and use this fuel or to lift 40 tons of mass off the surface of the earth. I also sense that packing 20 tons of explosives next to our (very expensive!) space elevator is not the most appealing of solutions.&lt;/p&gt;&#xA;" OwnerUserId="20309" LastActivityDate="2018-02-15T12:41:01.043" Title="How will the angular momentum of a Space Elevator be preserved?" Tags="|orbital-mechanics|space-elevator|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22448" PostTypeId="2" ParentId="22446" CreationDate="2017-07-28T17:54:24.087" Score="7" Body="&lt;p&gt;A problem with getting detailed info on the ICM is that it was based on a classified spacecraft. Even when I worked at JSC, details were scant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the best info I've seen in the wild, from the January 20, 1997 issue of &lt;a href=&quot;http://archive.aviationweek.com&quot; rel=&quot;noreferrer&quot;&gt;Aviation Week&lt;/a&gt;.  I can't give a direct link since it's behind a paywall, but here is an excerpt (sorry that it's an image, that is how the archives are stored).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uM7p3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uM7p3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The April 14, 1997 issue contained a schematic drawing of the ICM docked to the proto-ISS. At this point the ISS (exclusive of the ICM) consisted of Node 1, the FGB, three conical Pressurized Mating Adaptors, and the Z1 truss segment.  A Soyuz is shown docked.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xpgcd.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xpgcd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-07-28T17:59:48.043" LastActivityDate="2017-07-28T17:59:48.043" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22449" PostTypeId="1" AcceptedAnswerId="22454" CreationDate="2017-07-28T18:07:56.213" Score="2" ViewCount="113" Body="&lt;p&gt;After looking into the experienced radiation for certain space journeys, I was wondering wether it was correct to state that the amount of biological damage is a function of the exposure time for a certain predetermined radiation amount expressed in mSv.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where a shorter exposure time would imply higher damage due to the accute deterministic radiation damage as described in: &lt;a href=&quot;https://parker.bidmc.harvard.edu/BiologicalEffectsRadiation.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://parker.bidmc.harvard.edu/BiologicalEffectsRadiation.html&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;*I understand the stoichiometric damage would increase because the increased chance of e.g. cancer occurs earlier in a specific non-infinite life in the case of a shorter exposure period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I am not sure of is, how the pre-determined radiation would influence the actual consistency of the radation; I assumed for this question that the particle mass, charge and velocity would remain constant, where simply the amount of particles/flux would double.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there are assumptions required to answer this question, or my understanding incorrect, please let me know. And if you care to explain, if true, how that actually happends I'd find it most interesting. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kind regards,&lt;/p&gt;&#xA;" OwnerUserId="18246" LastActivityDate="2017-07-29T13:42:23.620" Title="Is the amount of damage to humans due to x Sievert a function of exposure time?" Tags="|radiation|astrobiology|shielding|" AnswerCount="2" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22450" PostTypeId="1" AcceptedAnswerId="22451" CreationDate="2017-07-28T18:19:13.493" Score="2" ViewCount="3043" Body="&lt;p&gt;Looking at a video from Roscosmos on instagram, Exp. 52 Cosmonaut Sergey Ryazansky carried a blue box with him during his walk to the Soyuz rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y5WHq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y5WHq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Upon looking at more videos of Astronauts boarding into rockets. I found &lt;a href=&quot;https://youtu.be/YdyL-GVpWR4&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; of Apollo 10 Astronauts walking towards the rocket with similar looking boxes that they were each holding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i8v1L.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i8v1L.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what are these boxes that Astronauts/Cosmonauts carry into the rockets they board?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="6944" LastEditDate="2018-04-09T00:53:09.290" LastActivityDate="2018-04-09T00:53:09.290" Title="What is the blue box that the Exp. 53 Astronuats/Cosmonauts carried with them?" Tags="|launch|crewed-spaceflight|spacesuits|identify-this-object|sokol-spacesuit|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22451" PostTypeId="2" ParentId="22450" CreationDate="2017-07-28T18:53:05.957" Score="9" Body="&lt;p&gt;Yes, the Sokol suits are provided with a ventilation unit for use prior to ingress into the spacecraft. &quot;When worn on the ground, the suit is attached to a portable ventilation unit —a hand-held device that supplies air to the suit, cooling it first with an ice filled heat exchanger.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a training version of the unit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/R3Zih.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R3Zih.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sokol_space_suit&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-07-28T18:53:05.957" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22452" PostTypeId="2" ParentId="17256" CreationDate="2017-07-28T19:18:26.843" Score="-5" Body="&lt;p&gt;Pogo oscillation could be dealt with by negatively vibrating opposing engines. Like sound cancellation this would result in a total, or a net vibration of zero.&#xA;The trick is to synchronize all the engines to cancel the effect of Pogo.&#xA;  I am not a rocket scientist, but a lateral thinker. When we ( lateral thinkers ) hit a brick wall we go either left or right. That is always better then trying to break through the wall.&#xA;  The system could be used on all Falcon 9 block fives and the modified falcon heavy cores. The negative effects of Pogo are cumulative, so if each of the three sections of the Heavy have no net vibration, then the Pogo effect for the entire vehicle would be within tolerances. &#xA;  Aerodynamically, I see no real issues with the Heavy. You treat it as an ascending lifting body design. Using the widest part of the vehicle to create additional lift on ascent. The benefit would be small, but might off-set other issues, including max-Q. &lt;/p&gt;&#xA;" OwnerUserId="20502" LastActivityDate="2017-07-28T19:18:26.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22454" PostTypeId="2" ParentId="22449" CreationDate="2017-07-28T22:50:33.127" Score="4" Body="&lt;p&gt;As your link already points out, the &lt;em&gt;regulatory&lt;/em&gt; assumption is that there's a &quot;linear, no-threshold assumption&quot;. This means that doubling the time and halving the radiation intensity cancels out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We &lt;em&gt;know&lt;/em&gt; this assumption is wrong. In particular, we know this both empirically (statistically) and theoretically. We know the DNA double helix can be repaired if radiation damages one half, but the linear no threshold model presupposes that no repair can happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, yes, spreading that x Sievert over several years will cause just as much direct DNA damage, but less damage after DNA repair is taken into account.&lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2017-07-28T22:50:33.127" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22456" PostTypeId="2" ParentId="22449" CreationDate="2017-07-29T13:42:23.620" Score="4" Body="&lt;p&gt;The linear no-threshold (LNT) hypothesis is certainly wrong in general, but the details are complicated and sometimes controversial. The following two articles are fairly recent summaries of the science, with conflicting interpretations. (They appeared back to back in the same issue of the same journal.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tubiana et al., &quot;The Linear No-Threshold Relationship Is Inconsistent with Radiation Biologic and Experimental Data,&quot; doi: 10.1148/radiol.2511080671, April 2009  Radiology,   251,  13-22. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Little et al., 2009, &quot;Risks Associated with Low Doses and Low Dose Rates of Ionizing Radiation: Why Linearity May Be (Almost) the Best We Can Do,&quot; doi: 10.1148/radiol.2511081686, April 2009  Radiology,   251,  6-12.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is quite a bit of evidence that low doses of radiation are good for various lab animals. This is called radiation hormesis. The empirical evidence in humans is not good enough to say anything about linearity, except in a few special cases (e.g., leukemia, which is clearly nonlinear).&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2017-07-29T13:42:23.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22458" PostTypeId="1" AcceptedAnswerId="22460" CreationDate="2017-07-30T00:09:59.103" Score="45" ViewCount="10845" Body="&lt;p&gt;One thing I've always wondered is why don't astronauts sleep in a kind of rotating bed that spins creating force? This would allow them to sleep and would be able to simulate earths gravity. Why don't they do this considering the impacts zero g has on the human body? &lt;/p&gt;&#xA;" OwnerUserId="18805" LastEditorUserId="58" LastEditDate="2018-04-05T16:31:43.750" LastActivityDate="2018-10-10T18:25:51.003" Title="Why isn't a centrifuge used for astronauts on the space station?" Tags="|iss|mission-design|sleep|" AnswerCount="10" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="22459" PostTypeId="2" ParentId="22458" CreationDate="2017-07-30T00:43:23.883" Score="6" Body="&lt;p&gt;A rotating structure big enough to accomplish that would be bulky, heavy, and very power-hungry to operate. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space, mass, and power are all at a sharp premium on spacecraft and space stations such as the ISS, so a centrifugal bed isn't remotely within the budget.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-07-30T00:43:23.883" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22460" PostTypeId="2" ParentId="22458" CreationDate="2017-07-30T00:45:42.287" Score="51" Body="&lt;p&gt;The short answer is it would cost a lot of money.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to get a 1G force, you'd either need something really big, or rotating very fast.  For example, the &lt;a href=&quot;http://l5development.com/d_space_sys/d_colony/b_Design/DesignTheory.php&quot; rel=&quot;noreferrer&quot;&gt;reference design&lt;/a&gt; for the space colonies I'm working on calls for a structure with a 900 meter radius rotating once a minute.  For something the size of the ISS, it would have to be rotating &lt;em&gt;much&lt;/em&gt; faster.  (I will get the actual numbers in a bit, when I'm not in the middle of another project.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the rotational speed problem, you have to also take into account that the structure would have &lt;em&gt;a lot&lt;/em&gt; of mass in order to be strong enough to support all that (centrifugal) weight - and the more mass you put into orbit, the more it costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On top of that, since you probably don't want to make the entire ISS rotating that fast (to keep the mass - and costs - down), you'd need to have a set of bearings between the rotating and the non-rotating parts of the station, preferably one that's big enough to provide a passageway for the crew to go through (so they don't have to put on their space suits to get to bed) - and that bearing is going to - guess what - have a lot of mass that needs to be launched - which means it would cost more money.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oh, and you'd also have to make sure that bearing isn't leaking, or you'd have to send up more air to replace what got lost - which would cost more money.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a host of other issues, but I'm guessing the list I've given already made the ISS designers realize a centrifugal gravity sleeping chamber was probably not something that would fit the project's budget.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK, I did some calculations.  If your centrifuge is 5 meters in diameter, it's got to be rotating at 18.9 RPM for a 1G acceleration at the rim which will be moving at 17.82 kph (11 mph).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you don't want the centrifuge to be torquing the station around it, you're actually going to need &lt;em&gt;two&lt;/em&gt; counter-rotating centrifuges of equal mass, and both arms of each centrifuge must have the same mass being rotated so everything is in balance.  That's not impossible, you could, for example, have a system that pumps a balancing amount of water into each of the four ends - but that adds system complexity, weight and cost.  I'm open to suggestions for a better solution.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As &lt;a href=&quot;https://space.stackexchange.com/users/195/russell-borogove&quot;&gt;Russell Borogove&lt;/a&gt; has pointed out, this &lt;em&gt;could&lt;/em&gt; be done in an enclosing compartment to eliminate the seal issue, but now you've got to build a vessel 5.5 meters or so in diameter that's twice the width of a centrifuge pod, plus clearances, in length, figure 3 meters.  That's a larger diameter but about half the length of the Unity module (4.57 m dia x 5.47 m long), so it's not entirely out of the question.  The noise of the equipment and the pods passing each other at 22 mph relative speed would be pretty substantial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speaking of the equipment, the centrifuges are going to need motors to start and stop them every time an astronaut goes to bed or gets up.  If you don't want to spend all night getting up to speed, you will need a bigger motor, along with a beefier power system to run it.  Then, when you're slowing the centrifuge down so the astronauts can get in or out, you don't want to throw away all the energy that was used to accelerate it, so you need an energy storage system.  Batteries may come to mind first, but rapid-cycle batteries that could repeatedly store and release enough energy across &lt;em&gt;many&lt;/em&gt; cycles would be really heavy and expensive.  An alternative would be winding up a flywheel for energy storage, but again, that's going to be heavy and expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oh, and if you're going to have more than one of those four sleeping pods occupied at a time, make sure the astronauts all have the same sleep cycles:  We wouldn't want an early riser having to lie in bed awake waiting for the other guy to get back from dreamland, or astronauts cranky from having been awakened too soon because the centrifuge stopped to let the other one off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;...and make sure there aren't any emergencies that require getting out of bed on short notice - yeah, you &lt;em&gt;could&lt;/em&gt; jump out of a pod moving at 11 mph without &lt;em&gt;too&lt;/em&gt; much danger of hurting yourself - but make sure you get out of the way before the next one comes along a second and a half later and bops you in the head!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The math:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\begin{align}&#xA;a &amp;amp;= v^2 / r = 1G = 9.8\:\mathrm{m/s^2}\\&#xA;d &amp;amp;= 5\:\mathrm m\\&#xA;r &amp;amp;= 2.5\:\mathrm m\\&#xA;\end{align}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\begin{align}&#xA;v^2 &amp;amp;= 9.8\:\mathrm{m/s^2} \cdot 2.5\:\mathrm{m} = 24.5\:\mathrm{m^2/s^2} \\&#xA;v &amp;amp;= \sqrt{24.5\:\mathrm{m^2/s^2}} = 4.95\:\mathrm{m/s} = 17.82\:\mathrm{kph} = 11\:\mathrm{mph}&#xA;\end{align}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\text{circumference}&#xA;= \pi \cdot d&#xA;= \pi \cdot 5\:\mathrm{m} = 15.71\:\mathrm{m}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;{15.71\:\mathrm{m} \over 4.95\:\mathrm{m/s}} = 3.17\:\text{sec per rotation} = 18.9\:\text{RPM}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://physics.tutorvista.com/motion/centrifugal-acceleration.html&quot; rel=&quot;noreferrer&quot;&gt;Centrifugal Acceleration&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="2712" LastEditDate="2017-08-01T12:19:32.793" LastActivityDate="2017-08-01T12:19:32.793" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="22461" PostTypeId="2" ParentId="22458" CreationDate="2017-07-30T07:30:03.473" Score="15" Body="&lt;p&gt;The point of the ISS is to study 0G. 1G sleeping bags defeat the purpose...&#xA;The humans are experiment subjects too:)&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6642" LastEditDate="2017-07-30T14:19:15.583" LastActivityDate="2017-07-30T14:19:15.583" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22463" PostTypeId="2" ParentId="22458" CreationDate="2017-07-30T08:33:13.930" Score="9" Body="&lt;p&gt;In addition to the other answers: a small structure (like a single module on the ISS) needs to rotate really fast to create 1G. This has undesirable side effects: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Coriolis forces make moving around inside the module non-intuitive. There's an old Soviet experiment where people lived inside a centrifuge for a while, on the film (haven't found this online, it's in the BBC documentary '&lt;a href=&quot;http://www.bbc.co.uk/programmes/b04lcxms&quot; rel=&quot;noreferrer&quot;&gt;Cosmonauts: How Russia won the space race&lt;/a&gt;') you can see them stagger and lurch along a corridor as if they're drunk. Another segment has someone throw darts at a dartboard, with the darts flying in a 90º horizontal arc.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;in a small centrifuge, there's a significant difference in the gravity levels between your head and feet, again making movement inside this module non-intuitive. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;if you use the centrifuge module only to sleep in, the astronauts have to get used to 0 G every morning. This would mean adapting fully to 0 G (takes about 2 weeks in the current situation) takes much longer, and you lose valuable time to space sickness. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-07-30T12:44:18.680" LastActivityDate="2017-07-30T12:44:18.680" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22464" PostTypeId="1" AcceptedAnswerId="22472" CreationDate="2017-07-30T09:16:49.140" Score="3" ViewCount="2353" Body="&lt;p&gt;I am new to the site and am unaware of the correct formation of my question. That being said, I wanted to know if this is plausable and what would be the correct way of tackling this idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I had a mass of 100 - 500 grams and I wanted to get it into orbit I was thinking the best way for me to do it would be to use a weather balloon for the first leg of the journey and launch from there?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The balloon could be taurus shaped with a circular platform below to ensure it was a vertical launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to get my mass of 100 - 500 grams into orbit what sort of size rocket would I be looking at for such a task.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks&lt;/p&gt;&#xA;" OwnerUserId="20521" LastEditorUserId="897" LastEditDate="2017-07-30T11:39:30.070" LastActivityDate="2017-07-31T11:08:08.293" Title="Launching a rocket from a weather balloon for a very small payload" Tags="|rockets|orbit|earth|balloons|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22465" PostTypeId="2" ParentId="22458" CreationDate="2017-07-30T09:45:48.357" Score="4" Body="&lt;p&gt;Even if a 1g sleeping compartment were feasible, which the other posts have demonstrated it is not, the health issues associate with micro gravity would not be alleviated. Just sleeping in full gravity but working and waking in micro gravity would still have significant health effects. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular &lt;a href=&quot;https://www.nasa.gov/audience/foreducators/postsecondary/features/F_Bones_in_Space.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;calcium depletion would still occur&lt;/a&gt;. Skeletons grow in response to compression stress (on the bones), which is usually caused by weight which is the effect of gravity on body mass. In the ISS this compression stress is &lt;a href=&quot;https://www.nasa.gov/audience/foreducators/stem-on-station/ditl_exercising&quot; rel=&quot;nofollow noreferrer&quot;&gt;simulated with ample exercise&lt;/a&gt; which when combined with a calcium and vitamin D enriched diet provides an element of compensation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are many &lt;a href=&quot;https://scholar.google.co.uk/scholar?q=health+effects+of+microgravity&amp;amp;hl=en&amp;amp;as_sdt=0&amp;amp;as_vis=1&amp;amp;oi=scholart&amp;amp;sa=X&amp;amp;ved=0ahUKEwiuiaqM3LDVAhXIKsAKHYO5ANoQgQMIJjAA&quot; rel=&quot;nofollow noreferrer&quot;&gt;other health issues associated with microgravity&lt;/a&gt; and I have only focussed on one to illustrate the issue, but many of the others would also not be solved by a specially engineered sleeping chamber.&lt;/p&gt;&#xA;" OwnerUserId="8162" LastActivityDate="2017-07-30T09:45:48.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22466" PostTypeId="2" ParentId="22458" CreationDate="2017-07-30T09:47:13.103" Score="15" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/APdUV.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/APdUV.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;OK let's build a hypothetical cylindrical sleeper system that could fit inside the crewed area of the current ISS for example, and look at some of the issues you would need to address. We'll name it after the famous Bill Haley and the Comets song: &lt;strong&gt;&lt;a href=&quot;https://youtu.be/FFxAEQgN_ao&quot; rel=&quot;noreferrer&quot;&gt;Shake, Rattle and Roll&lt;/a&gt;.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;You can also apply what's learned here to a futuristic, much larger structure for a higher g-force system to produce skeletal stress with hopes of reducing calcium loss.&lt;/p&gt;&#xA;&lt;p&gt;Find a spare, or currently empty, unused module on the ISS and build a 2 meter diameter, 2 meter long rotating cylindrical &amp;quot;&lt;strong&gt;astronaut tumbler&lt;/strong&gt;&amp;quot;. The astronauts sleep along the inside walls, laying parallel to the cylinder's axis, around which it rotates.&lt;/p&gt;&#xA;&lt;p&gt;Using &lt;a href=&quot;https://en.wikipedia.org/wiki/Acceleration#Circular_motion&quot; rel=&quot;noreferrer&quot;&gt;&lt;span class=&quot;math-container&quot;&gt;$\mathbf{a}=-\omega^2 / \mathbf{r}$&lt;/span&gt;&lt;/a&gt; the speed required to obtain a modest 1/6 of Earth gravity in order to provide a small but meaningful experience of &amp;quot;laying down&amp;quot; rather than floating is &lt;span class=&quot;math-container&quot;&gt;$\omega=1.3 \text{s}^{-1}$&lt;/span&gt; which works out to &lt;strong&gt;one revolution every 5 seconds, or a rotation frequency of 0.2 Hz&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;There may not be room for six of these, so it's going to be a shared space and astronauts will still need their cubby-holes for personal space, and a separate allotment of time to spend in it. Alternatively, they could pick up and move their personal cubby-holes and either attach them to this rotating frame, or move it back to the wall.&lt;/p&gt;&#xA;&lt;p&gt;No matter how you look it it, it's more stuff shipped from Earth, which is OK if it offers a significant enhancement to the astronauts' well being or contribution to the science of living in space.&lt;/p&gt;&#xA;&lt;p&gt;Balance is critical. If one astronaut wants to sleep, a &amp;quot;&lt;strong&gt;dummy astronaut&lt;/strong&gt;&amp;quot; needs to be placed opposite in order not to unduly shake the ISS with 0.2 Hz mechanical oscillation. If the sleeping astronaut moves, the dummy needs to move accordingly, or a servo mechanism at each end of the cylinder must automatically and constantly translate the rotation axis of the cylinder back to the center of mass. More stuff to break and mass to ship from Earth. If there are two people arranged opposite each other and a third wants to join, one person must &lt;strong&gt;&amp;quot;re-azimuth&amp;quot; themselves&lt;/strong&gt; by 60 degrees (or if they are sleeping soundly, be re-azimuthed by their fellow astronaut), or the dummy astronaut could be added opposite the third person.&lt;/p&gt;&#xA;&lt;p&gt;If someone wants to &amp;quot;get on&amp;quot; or &amp;quot;get off&amp;quot; the whole thing must be stopped and started. That may wake up anyone already &amp;quot;on&amp;quot;. Where does that angular momentum come from? If it stopped and started on a regular schedule with a fixed duty cycle, perhaps it could be balanced out by a tiny counter-rotation of the ISS, and each major stop/start cycle would alternate direction so that the net rotation of the ISS were minimal.&lt;/p&gt;&#xA;&lt;p&gt;The alternative is to build a counter-rotating flywheel either coaxially, or at least nearby. As the load (number of real + dummy astronauts) on the astronauts' cylinder changed, the load on the flywheel would have to be adjusted as well. The flywheel could also have servos to better null out some components of the structural vibrations as long as it were spinning synchronously. You could null out angular momentum at any frequency, so you wouldn't have to change the mass, but if it's not synchronous you are now adding a &lt;em&gt;second exciting frequency&lt;/em&gt; to your vibrations, doubling the chances you might hit a particularly dangerous one!&lt;/p&gt;&#xA;&lt;p&gt;The ISS is in no need for a periodic source of vibration. Unless that servo-system that constantly re-aligned the rotation axis of the cylinder to pass through the instantaneous center of mass of the astronauts in the drum, a cyclic vibration will be transmitted to the ISS frame. This is a problem that must constantly be battled, and has to be dealt with every time an astronaut begins or ends a sleep period, or rolls around too much.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Low frequency periodic vibrations&lt;/strong&gt; are the bane of large mechanical structures not previously designed for them.&lt;/p&gt;&#xA;&lt;p&gt;From The International Space Station (ISS) Researcher’s Guide &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/acceleration-environment-iss-mini-book_detail-508c.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Space Station Acceleration Environment&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Vehicle Structural Modes&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Vehicle structural modes reside at the low-frequency end of the vibratory portion of the acceleration spectrum. These vibrations fall within the frequency range &lt;strong&gt;from about 0.1 hertz to about 5 hertz&lt;/strong&gt;. These vibrations arise from the excitation of natural frequencies associated with large components of the space station structure, such as the main truss, and with fundamental appendage modes, such as solar arrays. These structures are typically excited by relatively large magnitude, relatively brief impulsive events like during a reboost or by crew locomotive events like push-offs. The driving excitation of such events results in response vibrations as &lt;strong&gt;structural ringing&lt;/strong&gt; damps out. Also, &lt;strong&gt;relatively small magnitude vibrations at just the right frequency will give rise to structural resonance&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qWTI6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qWTI6.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Cropped from Figure 4 of The International Space Station (ISS) Researcher’s Guide &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/acceleration-environment-iss-mini-book_detail-508c.pdf&quot; rel=&quot;noreferrer&quot;&gt;International Space Station Acceleration Environment&lt;/a&gt;. &lt;em&gt;&amp;quot;Figure 4. Spectrogram Showing Mode One with Crew Slow Transition to Sleep.&amp;quot;&lt;/em&gt; This suggests there are several structural resonances in the 0.1 to 1.0 Hz region. See original document for further discussion, and a list of &lt;strong&gt;about 20 different known resonant frequencies&lt;/strong&gt; on page 12.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;A &lt;a href=&quot;http://www.nbcnews.com/id/28998876/ns/technology_and_science-space/t/shaking-space-station-rattles-nasa/#.WX2X5dOGP_Q&quot; rel=&quot;noreferrer&quot;&gt;very frightening and dangerous event happened&lt;/a&gt;&lt;/strong&gt; on board the ISS in 2009 when a mis-programmed servo on a booster engine started tweaking the booster engine's thrust direction &lt;strong&gt;at about 0.5 Hz&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But during the Jan. 14 firing, something went seriously awry. The station’s solar power wings began swaying back and forth alarmingly. More dramatically, an interior camera captured views of wall-mounted equipment and cables &lt;strong&gt;flopping back and forth to a two-second beat&lt;/strong&gt;, as the camera itself swayed on its mounting bracket.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Buildup of gyrations&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;It was quickly apparent that some periodic force had excited the space station’s structure at one of its resonant frequencies, leading to a buildup of gyrations rather than a damping down. &lt;strong&gt;As with the traditional &amp;quot;soldiers marching across a bridge&amp;quot; story, and the all-too-real Tacoma Narrows bridge collapse in 1940, resonance buildup in a large structure can quickly lead to serious consequences&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;See also Space.com's &lt;a href=&quot;https://www.space.com/4432-nasa-weighs-excessive-vibrations-space-station.html&quot; rel=&quot;noreferrer&quot;&gt;NASA Weighs Excessive Vibrations on Space Station&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/CF_hRKqFicQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-07-30T16:59:37.167" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22467" PostTypeId="2" ParentId="22458" CreationDate="2017-07-30T12:37:12.213" Score="8" Body="&lt;p&gt;Offered as an addendum: a Centrifuge Module was planned for the ISS, and was partially built.  Its centrifuge was for science experiments, not sleeping though.  Budget problems doomed it, and it now sits in a parking lot in Japan.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Centrifuge_Accommodations_Module&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nTFAq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nTFAq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Cp8KP.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Cp8KP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/2NQA3.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2NQA3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;last image from &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=14987.0&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-07-30T13:13:44.957" LastActivityDate="2017-07-30T13:13:44.957" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22468" PostTypeId="1" CreationDate="2017-07-31T02:00:20.123" Score="1" ViewCount="349" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=V0GiQA_lh6Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lissajous Orbit Demonstration&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the demonstration, I do not get how is it possible for any object to orbit around the L2 point if there is no massive celestial object at the L2 point?&lt;/p&gt;&#xA;" OwnerUserId="18270" LastActivityDate="2017-07-31T02:00:20.123" Title="Orbital motion of Lissajous orbit?" Tags="|orbital-mechanics|orbital|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22469" PostTypeId="2" ParentId="22464" CreationDate="2017-07-31T02:01:59.063" Score="1" Body="&lt;p&gt;Using a balloon to elevate your launch location will only really be beneficial for getting small payloads into orbit where the launching rocket would otherwise suffer significant aerodynamic losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As indicated on page 7 of &lt;a href=&quot;http://www-pw.physics.uiowa.edu/van90/VanAllenBio_LudwigOct2004.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this biography of James van Allen &lt;/a&gt;, the rockoon (balloon/rocket combination) was introduced because it made it possible to reach high altitudes with small but useful payloads at very low cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike a sounding rocket where the primary goal is to reach the maximum &lt;em&gt;altitude&lt;/em&gt; possible flying more or less straight up and down, an &lt;strong&gt;&lt;em&gt;orbital launch&lt;/em&gt;&lt;/strong&gt; requires a large change in &lt;em&gt;angular velocity&lt;/em&gt; around the center of mass you're launching from - the Earth, in the case of all of the current human readers of these pages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your balloon launch platform could get you past most of the aerodynamic resistance on your journey to orbit, but most (larger) rockets spend &lt;em&gt;much&lt;/em&gt; more of their fuel fighting gravity than pushing air out of the way.  As your rocket gets smaller, though, atmospheric drag becomes an increasingly important factor, until you start getting to the limits of &lt;a href=&quot;https://space.stackexchange.com/questions/18789/how-small-could-an-orbital-rocket-be&quot;&gt;how small you can make an orbital launcher&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-07-31T06:19:54.967" LastActivityDate="2017-07-31T06:19:54.967" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22470" PostTypeId="2" ParentId="22444" CreationDate="2017-07-31T02:49:47.577" Score="-1" Body="&lt;p&gt;Moving around once in orbit is a lot easier than getting something into orbit. The only issue with this is bringing either another stage or even just a larger stage into orbit first to allow for fuel amounts to get to mars. The extra 40 tons to LEO of the Falcon Heavy allows for it to get this extra stage/ larger stage to orbit which it can then use to get to Mars. For the Falcon 9 to get something to Mars it would also need this extra/larger stage to get to orbit however the lifting capability of only 9 Merlin engines would result in an extremely inefficient launch to orbit due to the first stage burning so much extra fuel to get the extra/larger stage to orbit. This would result in the extra/larger stage using up more of it's own fuel getting to orbit and therefore would have less net force left to push the payload to Mars.&lt;/p&gt;&#xA;" OwnerUserId="20528" LastActivityDate="2017-07-31T02:49:47.577" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22471" PostTypeId="1" AcceptedAnswerId="24753" CreationDate="2017-07-31T03:58:28.907" Score="4" ViewCount="965" Body="&lt;p&gt;From the Spaceflight 101 article &lt;a href=&quot;http://spaceflight101.com/ss-520-4-rocket-launches-on-experimental-mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Experimental Launch of World’s Smallest Orbital Space Rocket ends in Failure&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SS-520-4 lifted off from the Uchinoura Space Center in Japan’s Kagoshima prefecture at 8:33 a.m. local time on Sunday, 23:33 UTC on Saturday on a one-off demonstration mission, aiming to put the TRICOM-1 CubeSat into an elliptical orbit around Earth. The small launcher quickly vanished from view after an on-time blastoff with a thrust eclipsing the rocket’s initial mass by a factor of seven.&lt;/p&gt;&#xA;&lt;p&gt;The rocket’s climb to orbit was expected to take seven and a half minutes, however, all telemetry from the ascending launch vehicle was lost around 20 seconds into the planned 31-second firing of the rocket’s first stage. Tracking of the rocket showed the first stage separated from the second stage, reaching a peak altitude just shy of 200 Kilometers before both fell into the Pacific Ocean in a closed zone south-east of the launch site.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Are there plans to try again?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yt04y.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yt04y.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; SS-520-4 rocket ready for launch. From &lt;a href=&quot;http://spaceflight101.com/ss-520-4-rocket-launches-on-experimental-mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, Photo: JAXA&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dNKD1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dNKD1.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; SS-520-4 rocket. From &lt;a href=&quot;http://spaceflight101.com/ss-520-4-rocket-launches-on-experimental-mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, Image: JAXA&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SS-520-4 is a three-stage solid-fueled rocket standing 9.54 meters tall, measuring 52 centimeters in diameter and weighing in at 2,600 Kilograms – smaller and lighter than any previous ground-based orbital launch vehicle. It is based on the SS-520 sounding rocket design, modified with a small third stage tasked with injecting a payload into Low Earth Orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NgdHr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NgdHr.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; TRICOM-1 in Launch Configuration. From &lt;a href=&quot;http://spaceflight101.com/ss-520-4-rocket-launches-on-experimental-mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, Photo: JAXA&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/A7wCC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/A7wCC.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Person with TRICOM-1 for scale - in this case Professor Hiroto Habu of the Japan Aerospace Exploration Agency. Image from &lt;a href=&quot;https://english.kyodonews.jp/photos/2016/11/445500.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-08-03T08:07:55.390" Title="Will JAXA try again to launch TRICOM-1 with the &quot;world's smallest orbital rocket&quot; SS-520-4 again?" Tags="|cubesat|sounding-rocket|jaxa|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22472" PostTypeId="2" ParentId="22464" CreationDate="2017-07-31T04:36:03.690" Score="4" Body="&lt;p&gt;You have the right idea. For very small payloads launched by very small rockets, it becomes increasingly attractive to get the rocket above as much of the atmosphere as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And you are in good company, Virgin Galactic's &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceShipTwo&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Ship Two&lt;/a&gt;, Orbital ATK's &lt;a href=&quot;https://en.wikipedia.org/wiki/Pegasus_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pegasus&lt;/a&gt;, and Scaled Composites' &lt;a href=&quot;https://en.wikipedia.org/wiki/Scaled_Composites#Stratolaunch_Carrier_Aircraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stratolaunch&lt;/a&gt; are all current or near future air-launch platforms. Incidentally, Wired Magazine's article about &lt;a href=&quot;https://www.wired.com/2017/06/paul-allen-built-worlds-largest-plane-fling-satellites-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Microsoft co-founder Paul Allen founded Stratolaunch&lt;/a&gt; points out that it is going to be the largest airplane in the world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several helpful answers to the question &lt;a href=&quot;https://space.stackexchange.com/q/13729/12102&quot;&gt;Can a miniature Saturn V get to the moon and back?&lt;/a&gt; and you should read them all, but I'll summarize the issues as aerodynamic drag and &quot;gravity-drag&quot;, and just say here that as you scale a rocket down in size, the thrust decreases faster than the drag. This ends up requiring the rocket to accelerate more slowly in the dense part of the atmosphere to decrease the drag, which causes a secondary &quot;gravity drag&quot; penalty, the fraction of the thrust that is needed to simply keep the rocket from falling back to Earth. It's a little like having to push down constantly on the pedal of a bike just to keep from rolling backwards down a hill, except that rockets have to push by spending active thrust because they aren't touching the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;This is why small rockets tend to be very long and skinny&lt;/em&gt;, to pack more thrust behind them while minimizing area. This only works to some extent because the sides of the rocket also generate drag, otherwise we'd have ridiculously long, skinny needle-rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a 500 or 100 gram payload and a small rocket (which seems like it might be affordable until you do the numbers), getting above the air is a great idea!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The big problem is that when you make rockets smaller, you end up making other compromises by making it cheaper and simpler. This means the performance is lower. It won't be a &quot;mini Saturn-V&quot;. &lt;a href=&quot;https://space.stackexchange.com/questions/22464/#comment63544_22464&quot;&gt;This comment&lt;/a&gt; by @Deimophobia links to the analysis done by Dorin Patru, Jeffrey D. Kozak, and Robert J. Bowman at The Rochester Institute of Technology, and presented at the 20th annual AIAA/USU Conference on Small Satellites as &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1599&amp;amp;context=smallsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Custom Launch System for Satellites Smaller Than 1 kg&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The LEO orbital velocity, i.e. 200-300 km, is $v_{LEO}$ = 7600 m/s. A total $\Delta v_{LOSS}$  of 1600 m/s is assumed, due to (1) thrust-atmospheric loss, (2) drag loss, (3) gravity loss, and (4) maneuvering and launch window allowance. The thrust-atmospheric and drag losses will be much smaller compared to a sea-level launch, due to the very high altitude of the entire powered flight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Also you end up making it &lt;em&gt;relatively&lt;/em&gt; heavier. What does that mean? A really high performance big rocket can have well over 90% of its mass as pure propellant. I think the record is 94% or 95% but I'm still looking for a source. But when you build a small rocket, this gets really difficult. Making all the structural components, containers, and the engine as small and lightweight becomes a real challenge, and that means expensive materials and manufacturing techniques not available to hobbyists. The RIT work explains that the test stage they built never aimed for the &quot;structure to propellant mass ratio of 1/10&quot; since it was for ground tests. This would have been even harder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They show that for a smaller scale rocket, using a fairly safe hybrid $I_{SP} = 230 \text{s}$ engine design that is still way beyond &quot;hobby level&quot;, you'll need at least $100,000, a team of people, some radio equipment, and about a 200 kg, &lt;em&gt;four stage&lt;/em&gt; rocket to put 1kg into LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space-capable rockets are much more difficult and expensive to make than hobby-rockets, as illustrated by the engine comparison in &lt;a href=&quot;https://space.stackexchange.com/a/22481/12102&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You also need to file several documents and receive approval from some serious government agencies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same rocket would have to be much much bigger to launch from Earth, I'm guessing 500 to 1000 kg, but the point is it's still not going to be a &quot;hobby rocket&quot; to get to orbit from a balloon. It is still quite a technical challenge, and if you can build this kind of challenging rocket, then when you are faced with the choice of the hassles of the balloon versus just making the rocket bigger, growing the rocket becomes more and more attractive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you'd like more explained about the RIT paper, leave a comment and I'll expand the discussion.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shots from &lt;a href=&quot;http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1599&amp;amp;context=smallsat&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Custom Launch System for Satellites Smaller Than 1 kg&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6d5vV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6d5vV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Vrz5h.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vrz5h.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ARXo0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ARXo0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-07-31T11:08:08.293" LastActivityDate="2017-07-31T11:08:08.293" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22473" PostTypeId="1" AcceptedAnswerId="22481" CreationDate="2017-07-31T05:15:36.473" Score="3" ViewCount="4059" Body="&lt;p&gt;Now that I've learned a bit about real rockets, I wondered how a model rocket D-size engine fits in quantitatively. The Wikipedia article &lt;a href=&quot;https://en.wikipedia.org/wiki/Model_rocket#Performance&quot; rel=&quot;nofollow noreferrer&quot;&gt;Model rocket&lt;/a&gt; lists thrust in Newtons, and Total Impulse in Newton-seconds, but one would need to know the mass of the propellant to calculate mass-specific impulse $I_{SP}$ and that's not listed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A good, quantitatively reasoned educated guess or a number for a similar propellant type and design would be sufficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And of course this kind of engine will probably not get your rocket to the Karman line, but I'm looking for a number &lt;em&gt;to better understand how rockets that do make it to space compare&lt;/em&gt; to a handy, historical reference point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sI8vZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sI8vZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Douxp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Douxp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2018-11-27T08:04:56.353" Title="What's the Isp of a model rocket &quot;D-size&quot; engine, compared to the Isp of engines that do reach space?" Tags="|engines|specific-impulse|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22474" PostTypeId="1" CreationDate="2017-07-31T05:48:02.057" Score="5" ViewCount="1122" Body="&lt;p&gt;As noted in &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;noreferrer&quot;&gt;this Wikipedia article&lt;/a&gt;, &lt;em&gt;delta-v&lt;/em&gt;, used in &lt;em&gt;spacecraft flight dynamics&lt;/em&gt;, is a measure of &lt;strong&gt;the impulse that is needed to perform a maneuver&lt;/strong&gt;.  However, in &lt;em&gt;general physics&lt;/em&gt; (a &lt;em&gt;much&lt;/em&gt; longer standing branch of study, i.e., with seniority), &lt;em&gt;delta-v&lt;/em&gt; is simply a &lt;strong&gt;change in velocity&lt;/strong&gt;.  Therefore, as used in the context of space flight dynamics, &lt;em&gt;delta-v&lt;/em&gt; is not the same as the &lt;em&gt;physical change in velocity&lt;/em&gt; of the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, for a delta-v (physical) of 7.8 km/s needed to reach low Earth orbit, a delta-v (impulse) of between 9 and 10 km/s is required, the additional 1.5 to 2 km/s being due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_drag&quot; rel=&quot;noreferrer&quot;&gt;gravity losses&lt;/a&gt; (the amount of energy spent fighting gravity rather than on gaining horizontal (orbital) speed) and atmospheric drag (the amount of energy spent pushing the air out of the way).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is the symbol &quot;delta-v&quot; &lt;strong&gt;repurposed&lt;/strong&gt; in spacecraft flight dynamics with a &lt;em&gt;significantly&lt;/em&gt; different meaning than the previously well established one?  Wouldn't it be less confusing to use a different symbol to identify the clearly different quantity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this part of why &quot;rocket science&quot; is generally considered to be so hard - because &lt;em&gt;rocket scientists&lt;/em&gt; use &lt;em&gt;previously standardized symbols&lt;/em&gt; for &lt;em&gt;significantly different meanings&lt;/em&gt;?&lt;/p&gt;&#xA;" OwnerUserId="19916" LastActivityDate="2017-07-31T08:52:11.850" Title="Why do delta-v and delta-v both use the same term?" Tags="|orbital-maneuver|terminology|delta-v|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22475" PostTypeId="1" AcceptedAnswerId="22515" CreationDate="2017-07-31T06:47:00.837" Score="8" ViewCount="792" Body="&lt;p&gt;The recent July 14, 2017 launch of KANOPUS-V-IK also brought along many other payloads, including some smaller satellites and a bunch of cubesats, and deployed them at different altitudes between about 400 and 600 km. The cube sats included the &lt;a href=&quot;https://space.stackexchange.com/q/22311/12102&quot;&gt;giant shiny balloon satellite Mayak&lt;/a&gt; and latest flock for Planet Labs. See:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.planet.com/pulse/when-doves-fly-48-flock-2k-satellites-successfully-launched-and-deployed/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;When Doves Fly:&lt;/strong&gt; 48 Flock 2K Satellites Successfully Launched and Deployed&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But looking back in the list a little bit, I saw:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Int. Desig.  NORAD    Name            T(min)   inc   apo  peri&#xA;2017-042F    42830 *+ OBJECT F         96.6   97.6   606   586      &#xA;2017-042G    42831 *+ FLYING LAPTOP    96.6   97.6   606   586      &#xA;2017-042H    42832 *+ OBJECT H         96.6   97.6   605   586 &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is there really a laptop in an &lt;code&gt;n = 15&lt;/code&gt; sun-synchronous orbit? If so, what model?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; &lt;em&gt;If it isn't&lt;/em&gt; a laptop, please give a short description of &lt;em&gt;what it is then&lt;/em&gt;, and why it's called a &quot;flying laptop.&quot;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-01T18:28:20.773" LastActivityDate="2018-03-30T07:19:33.003" Title="Flying Laptop? 2017-042G; 42831" Tags="|artificial-satellite|cubesat|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22476" PostTypeId="1" AcceptedAnswerId="22661" CreationDate="2017-07-31T07:35:51.500" Score="16" ViewCount="948" Body="&lt;p&gt;Deborah Byrd's article &lt;a href=&quot;http://earthsky.org/space/mayak-bright-russian-satellite-july-august-2017&quot; rel=&quot;noreferrer&quot;&gt;Have you seen the Mayak satellite?&lt;/a&gt; explains Mayak with a bit of humor:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A team of young Russians – led by Moscow State Mechanical Engineering University (MAMI) – managed to raise more than $30,000 on Russian crowdfunding website &lt;a href=&quot;https://boomstarter.ru/projects/shaenko/kosmicheskiy_sputnik_mayak#content&quot; rel=&quot;noreferrer&quot;&gt;Boomstarter&lt;/a&gt;, in order to launch their own small satellite. The satellite is called Mayak, which means beacon in English. It’s a cubesat, roughly the size of a loaf of bread. And it’s up there. Mayak went into space on July 14, 2017, as part of a &lt;a href=&quot;https://spaceflightnow.com/2017/07/14/soyuz-rideshare-launch/&quot; rel=&quot;noreferrer&quot;&gt;secondary payload&lt;/a&gt;, launched on a Soyuz 2.1v vehicle from Baikonur Cosmodrome in Kazakhstan. It’ll be orbiting Earth, about 370 miles (600 km) high, for the coming month. It’s supposed to be very, very bright, so bright that it would, supposedly, &lt;strong&gt;&lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-4654084/Russia-set-launch-brightest-star-night-sky.html&quot; rel=&quot;noreferrer&quot;&gt;ruin night skies&lt;/a&gt;&lt;/strong&gt; and &lt;strong&gt;&lt;a href=&quot;http://www.iflscience.com/space/russian-scientists-just-launched-a-controversial-artificial-star-that-might-anger-astronomers/&quot; rel=&quot;noreferrer&quot;&gt;threaten astronomy&lt;/a&gt;&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The article continues and it worth the read!&lt;/p&gt;&#xA;&lt;p&gt;Mayak has been receiving roughly daily TLE updates, so it's definitely up there and being tracked. However, I do not know if the 3U cubesat has deployed it's giant metallized polymer film reflector yet.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Have there been any reported sightings or photos of Mayak yet? Is it known if the reflector has even successfully deployed?&lt;/p&gt;&#xA;&lt;p&gt;It's supposed to be as bright as -8 or -10 magnitude if the sun catches one of the faces of the 2 meter tetrahedral reflector.&lt;/p&gt;&#xA;&lt;p&gt;You can learn more on the YouTube videos &lt;a href=&quot;https://youtu.be/IBtI_pV4Q_I&quot; rel=&quot;noreferrer&quot;&gt;Mayak. No more space debris!&lt;/a&gt; and also &lt;a href=&quot;https://youtu.be/K91z2rT_4Xw&quot; rel=&quot;noreferrer&quot;&gt;A tour through the Mayak project&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;More background:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The July 14, 2017 NASA Spaceflight article &lt;a href=&quot;https://www.nasaspaceflight.com/2017/07/soyuz-2-1a-launches-kanopus-v-ik-70-satellites/&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 2-1A launches with Kanopus-V-IK and over 70 satellites&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Mayak is a three-unit CubeSat which was built by Tvoii Sektor Kosmosa – or “Your Sector of Space” – an independent, crowd-funded team of engineers in conjunction with the Moscow State University of Mechanical Engineering. Mayak – meaning Lighthouse – will deploy a highly reflective tetrahedral structure.&lt;/p&gt;&#xA;&lt;p&gt;Each side of this structure has an area of four square meters, or 43 square feet. To ground observers, the satellite is expected to have an apparent magnitude of up to -10, making it one of the brightest objects in the night sky. The structure will double as a deorbit mechanism, hastening the decay of the satellite’s orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PJEf7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PJEf7.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Mayak Reflector – Photo: CosmoMayak, From &lt;a href=&quot;http://spaceflight101.com/soyuz-kanopus-v-ik/mayak/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WkIbo.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WkIbo.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Mayak Artists conception, From &lt;a href=&quot;https://www.nasaspaceflight.com/2017/07/soyuz-2-1a-launches-kanopus-v-ik-70-satellites/&quot; rel=&quot;noreferrer&quot;&gt;NASA Spaceflight&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-15T15:51:04.350" Title="Two week mark; has Mayak (Маяк) been spotted yet? Reflector deployed? Astronomy &quot;ruined&quot;?" Tags="|artificial-satellite|photography|space-art|visibility-of-spacecraft|deorbit|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22477" PostTypeId="2" ParentId="22474" CreationDate="2017-07-31T08:46:41.993" Score="17" Body="&lt;p&gt;There's no conflict here. Because delta-v is a scalar figure, the result of applying velocity changes in different directions to a single object in 3 dimensional space over a period of time doesn't necessarily add up linearly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If a spacecraft applies a delta v of 100 m/s in one direction, then another maneuver of 100 m/s in a perpendicular direction, it has &lt;em&gt;applied&lt;/em&gt; 200 m/s of delta v for a &lt;em&gt;net change in velocity&lt;/em&gt; of 141 m/s. If the second maneuver is in the opposite direction, 200 m/s of delta v is applied for a net delta v of zero!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likewise, for an ascent from Earth's surface to orbit, some 9400 m/s is applied, in various directions and with some modification from gravity and drag loss, for a net of ~7800 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have a car whose fuel tank provides 500 km of linear range and you start driving on real world roads with bends and turns and keep going until the tank is empty, then you measure on a map that you're only 400 km from your origin, is your car's range 400 km or 500 km? Is it inappropriate to use the term &quot;range&quot; for both figures?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do object to Wikipedia's use of the term &lt;em&gt;impulse&lt;/em&gt;, though, since that technically means &quot;force times time&quot;, incompatible with velocity, and I'd strike that word from your question.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-07-31T08:52:11.850" LastActivityDate="2017-07-31T08:52:11.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22479" PostTypeId="1" CreationDate="2017-07-31T09:31:12.810" Score="1" ViewCount="452" Body="&lt;p&gt;I have bachelor's degree in computer science and engineering. And thinking about learning masters degree in aerospace engineering. Will that help me in becoming an astronaut?&lt;/p&gt;&#xA;" OwnerUserId="20530" LastEditorUserId="1054" LastEditDate="2020-12-23T22:58:31.787" LastActivityDate="2020-12-23T22:58:31.787" Title="Can I become an astronaut by having bachelor's degree in computer science and engineering?" Tags="|spacecraft|" AnswerCount="0" CommentCount="0" ClosedDate="2017-07-31T14:08:23.563" ContentLicense="CC BY-SA 4.0" />
  <row Id="22480" PostTypeId="2" ParentId="22311" CreationDate="2017-07-31T10:33:40.050" Score="1" Body="&lt;p&gt;I've posted this answer to make sure there is some information here. If someone would like to use this as a starting point for an additional answer, that's great!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/questions/22311/#comment63012_22311&quot;&gt;comment by @Hobbes&lt;/a&gt; seems to be correct. I've just found the YouTube video &lt;a href=&quot;https://youtu.be/K91z2rT_4Xw&quot; rel=&quot;nofollow noreferrer&quot;&gt;A tour through the Mayak project&lt;/a&gt; and I show some screenshots below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original plan was to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Ammonium_carbonate&quot; rel=&quot;nofollow noreferrer&quot;&gt;ammonium carbonate&lt;/a&gt; or &lt;em&gt;baker's ammonia&lt;/em&gt; that produces both ammonia and carbon dioxide gas when reacted with water. However, the video points out that the ammonia will tend to refreeze out on the inside of the film (bp: -33°C, mp: -78°C) and the ultralight PET film is too porous to hold pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So motor-driven tape measures provide stiff extensors that can be rolled out with an electric motor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's hope it works!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6SZDV.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6SZDV.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1NAUJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1NAUJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/w7rni.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/w7rni.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5nEEg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5nEEg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-15T16:04:19.110" LastActivityDate="2017-08-15T16:04:19.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22481" PostTypeId="2" ParentId="22473" CreationDate="2017-07-31T11:01:21.400" Score="4" Body="&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/questions/22473/#comment63566_22473&quot;&gt;comment&lt;/a&gt; by @RussellBorogove is correct and while the information is not found in Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Model_rocket#Performance&quot; rel=&quot;nofollow noreferrer&quot;&gt;model rocket&lt;/a&gt; page, it can be found on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse#Model_rocketry&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific impulse&lt;/a&gt; page, where &lt;span class=&quot;math-container&quot;&gt;$I_{SP}$&lt;/span&gt;'s for space-worthy rocket engines are also kept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qeyQf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qeyQf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a D12-3 engine, &lt;span class=&quot;math-container&quot;&gt;$I_{SP}$&lt;/span&gt; is about 82 seconds. The &quot;12&quot; in D12-3 &lt;a href=&quot;https://en.wikipedia.org/wiki/Model_rocket#Performance&quot; rel=&quot;nofollow noreferrer&quot;&gt;means an average thrust of about 12N&lt;/a&gt;, so the total burn time is only about 1.7 seconds. After that, you have to eject the engine or the whole stage and ignite a new one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can compare that to the following data in the same article:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aK8uN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aK8uN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle-type SRB's have an &lt;span class=&quot;math-container&quot;&gt;$I_{SP}$&lt;/span&gt; of about 250 seconds, and a good LOX/LH2 engine can reach 450 seconds. Stages using those tend to be about 90% propellant mass and burn for on the order of 100 or 200 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This puts a better perspective on the difference between model rocket engines and the real thing, and if one thought it would ever be possible to launch to orbit with D-engines, better think again!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-11-27T08:04:56.353" LastActivityDate="2018-11-27T08:04:56.353" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22487" PostTypeId="1" CreationDate="2017-07-31T17:14:36.097" Score="5" ViewCount="384" Body="&lt;p&gt;In other words, why was the F1 rocket engine programme stopped? Is there a sense that solid motors can take care of high thrust cases in a better way? Or is it an unfortunate fallout of the end of the space race?&lt;/p&gt;&#xA;" OwnerUserId="20537" LastEditorUserId="6816" LastEditDate="2017-07-31T17:34:20.307" LastActivityDate="2018-09-25T05:03:32.100" Title="Is there a demand for large liquid engines (&gt; 5 MN)?" Tags="|apollo-program|saturn-v|liquid-fuel|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22488" PostTypeId="2" ParentId="22487" CreationDate="2017-07-31T18:35:07.697" Score="6" Body="&lt;p&gt;It's likely a mix of both factors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, there's little demand at present for 100-or-more-ton-to-orbit launchers like the Saturn V. Robotic exploration of the solar system is cheaper and safer than crewed missions, and requires payloads less than a tenth the size. New Horizons, for example, massed only half a ton. For the vast majority of space launches, the payload is under 20 tons. Very large engines like F-1 aren't needed for rockets that size. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, solid rockets provide much more thrust per dollar in expendable systems than liquids, so the large solid booster + medium liquid sustainer first-stage strategy is preferred for large uncrewed launchers, and even some crewed ones (shuttle and SLS), though many people challenge the wisdom of using unstoppable solids for crewed flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX believes that reusability can reduce the cost of large liquid-fueled launchers to a point where latent demand manifests, but they haven't proven this. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-09-24T02:09:22.920" LastActivityDate="2018-09-24T02:09:22.920" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22489" PostTypeId="1" AcceptedAnswerId="22490" CreationDate="2017-07-31T20:19:47.973" Score="15" ViewCount="1206" Body="&lt;p&gt;Why are launch escape towers usually jettisoned fairly early in the flight? What if a problem went wrong during an upper stage burn and the escape tower had already been jettisoned? I know that carrying a heavy escape tower for a majority of the flight will certainly degrade the orbital capabilities of the rocket, but isn't crew safety more important?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="26446" LastEditDate="2018-11-06T03:56:32.857" LastActivityDate="2018-11-06T03:56:32.857" Title="Why are launch escape towers jettisoned early?" Tags="|launch|crewed-spaceflight|launch-escape-system|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22490" PostTypeId="2" ParentId="22489" CreationDate="2017-07-31T21:13:28.327" Score="20" Body="&lt;p&gt;Escape towers get jettisoned once they're no longer necessary, usually when the spacecraft can abort under its own power or using an intact upper stage. Using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_abort_modes&quot; rel=&quot;noreferrer&quot;&gt;Apollo abort modes&lt;/a&gt; as an example:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The pad abort (before launch) and modes IA/IB/IC (between launch and escape tower jettison) would use the escape tower.&lt;/li&gt;&#xA;&lt;li&gt;Mode II (after escape tower jettison, early in the second stage burn) would use the Service Module's engine to pull away from the rest of the rocket.&lt;/li&gt;&#xA;&lt;li&gt;Mode III (later in the second stage burn) would use the third stage (and Service Module engine, if necessary) to abort to Earth orbit.&lt;/li&gt;&#xA;&lt;li&gt;Mode IV (during the third stage burn) would use the Service Module engine to abort to Earth orbit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2017-07-31T21:13:28.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22491" PostTypeId="1" AcceptedAnswerId="22517" CreationDate="2017-07-31T22:18:58.203" Score="2" ViewCount="1539" Body="&lt;p&gt;I am  trying to figure out how you would navigate on Venus. On Earth you can use Polaris as a North Star to find north. Would there be a star on the Venus sky which could function as either a north star or south star?&lt;/p&gt;&#xA;" OwnerUserId="11308" LastActivityDate="2018-06-03T01:50:12.033" Title="Does Venus have a north star or south star?" Tags="|navigation|venus|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22492" PostTypeId="2" ParentId="22458" CreationDate="2017-07-31T22:41:11.953" Score="3" Body="&lt;p&gt;One reason is that rotation at the kind of speeds practical in a spacecraft would cause nausea and dizziness, if not vomiting.   Not conducive to rest.&#xA;Also rotating machinery would cause a large number of risks of various sorts, and the need for a maintenance program.  The more I think about this&#xA;the more reasons I can think of.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rotating space station, or one with a rotating gallery, is practical, if large enough that the rotation rate doesn't cause nausea (after all that's what Earth is).&lt;/p&gt;&#xA;" OwnerUserId="20539" LastActivityDate="2017-07-31T22:41:11.953" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22493" PostTypeId="1" AcceptedAnswerId="22512" CreationDate="2017-07-31T23:54:06.313" Score="10" ViewCount="1085" Body="&lt;p&gt;The visually opaque clouds of Venus still pass radio and some microwaves, and so powerful radar signals can be used to map the surface. This is probably done using some kind of delay-Doppler technique, the Arecibo is big but it's not going to resolve features this small, and even if it could (which it can't), building a map pixel-by-pixel would not be a reasonable use of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I would assume the gap is somehow related to the delay-Doppler geometry, but I can't figure out how.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The caption for the color map shown below from the &lt;a href=&quot;https://airandspace.si.edu/multimedia-gallery/6138hjpg&quot; rel=&quot;noreferrer&quot;&gt;National Air and Space Museum&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A radar map of Venus collected by CEPS scientists. This shows most of one hemisphere of the planet, &lt;strong&gt;with a thin band near the center for which the mapping technique cannot produce an image&lt;/strong&gt;. Dark areas are smooth or covered by fine material from impact craters, while bright areas are very rough. These maps are being used to study geologic processes beneath the dense atmosphere, and to search for changes in the surface over time. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Why can the mapping technique not form an image for this slice?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The monochrome image is from the Lights in the Dark item &lt;a href=&quot;https://lightsinthedark.com/2015/03/12/ground-based-radar-reveals-the-surface-of-venus/&quot; rel=&quot;noreferrer&quot;&gt;Ground-Based Radar Reveals the Surface of Venus&lt;/a&gt; and carries the following caption:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Radar map of Venus’ surface made from signals sent from Puerto Rico and received in West Virginia (Credits: B. Campbell, Smithsonian, et al., NRAO/AUI/NSF, Arecibo)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The last monochrome image comparing two nearly-identical images from 1988 and 2012 show the band in the same place for both observations. I can't figure out why this is so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for a technical explanation, not just some guesses, or &quot;it's related to...&quot; type answers. Thanks!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FuoKP.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FuoKP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;https://airandspace.si.edu/multimedia-gallery/6138hjpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6H0g7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6H0g7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;https://lightsinthedark.com/2015/03/12/ground-based-radar-reveals-the-surface-of-venus/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NUE8C.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NUE8C.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from &lt;a href=&quot;https://airandspace.si.edu/multimedia-gallery/9776hjpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mmdpI.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mmdpI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Some geographical reference points on Venus, found &lt;a href=&quot;http://www.physast.uga.edu/~jss/1010/ch9/ovhd09.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-04T17:16:00.143" LastActivityDate="2017-08-04T17:16:00.143" Title="Why do radar maps of the surface of Venus have missing slices?" Tags="|venus|radar|delay-doppler|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22494" PostTypeId="2" ParentId="22458" CreationDate="2017-08-01T03:02:28.023" Score="8" Body="&lt;p&gt;I'd like to add the words of &lt;a href=&quot;https://en.wikipedia.org/wiki/Chris_Hadfield&quot; rel=&quot;noreferrer&quot;&gt;Chris Hadfield&lt;/a&gt; about this, from the FAQ appendix of his book &lt;em&gt;An astronaut's guide to life on earth&lt;/em&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;h3&gt;Is it comfortable to sleep on the ISS?&lt;/h3&gt;&#xA;&lt;p&gt;It is a whole new type of comfortable to sleep in weightlessness. Even&#xA;on the most expensive Earth mattress you occasionally have to roll&#xA;over or adjust your pillow. In orbit, you can relax every muscle in&#xA;your body. At bedtime you ﬂoat into your sleeping bag (which is&#xA;loosely tethered to the wall with a couple of shoelaces), do up the&#xA;long zipper and shut off the light. Because there's no effect of&#xA;gravity pushing you into your mattress, you are perfectly relaxed and&#xA;your whole body can go deliciously limp. Your arms and leg joints bend&#xA;a bit and ﬂoat up, your neck slumps forward like a napping passenger&#xA;on a plane; every muscle takes a rest. You can feel the slow pulse of&#xA;your heartbeat, moving you slightly against the nothingness. When&#xA;space travel eventually becomes inexpensive enough, it may well be the&#xA;“space sleep spa” that attracts the largest crowd.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;sup&gt;Chris Hadfield. An astronaut's guide to life on earth. --- Pan Books Ltd. 2015&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;So astronauts would probably vote for sleeping in 0 G.&lt;/p&gt;&#xA;" OwnerUserId="20542" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-01T03:46:28.603" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22496" PostTypeId="2" ParentId="22444" CreationDate="2017-08-01T04:28:37.437" Score="6" Body="&lt;p&gt;All else being equal (e.g. propellant choice), higher-energy trajectories favor a rocket with more stages.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thought experiment that can help you understand this is to consider two missions launched on identical rockets, the first to LEO and the second to an interplanetary trajectory.  Both missions have payloads sized to &quot;max out&quot; the capacity of the launch vehicle.  Obviously the second payload will have to be much lighter than the first, but notice that the second payload is therefore a much smaller fraction of the dry mass of the upper stage.  Recall that the upper stage must end up on the same trajectory as the payload, so that second mission is less &quot;efficient&quot; because most of the work the upper stage does is just accelerating its own mass rather than the payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were constrained to the same overall vehicle mass, you'd be better off with a smaller second stage and adding a third stage.  That way the upper stage mass will be better matched to the payload mass, so less of the work it does will be wasted.  It's almost like &lt;a href=&quot;https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem&quot; rel=&quot;noreferrer&quot;&gt;impedance-matched power transfer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how does this relate to the Falcon Heavy?  Essentially the addition of the extra boosters make the system into a 3-stage rocket.  The center core is throttled down during first-stage flight, so it still has quite a bit of propellant remaining when the side boosters empty themselves and separate.  The center core burns for a while longer (effectively 2nd-stage flight, because you've thrown away the dead mass of the empty side boosters), then it too separates and the upper stage takes over.  This means that the upper stage of the Heavy is responsible for a smaller fraction of the overall energy delivered (or the velocity added) compared to the upper stage of the F9.  So for the reasons described earlier, the Heavy as a vehicle is more efficient at delivering payloads to high energy trajectories than the F9.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-08-01T04:28:37.437" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22498" PostTypeId="1" CreationDate="2017-08-01T07:28:35.357" Score="3" ViewCount="348" Body="&lt;p&gt;If a space craft is to travel at 0.6 c, how must one design a protection system such that any and all debris collisions with the sail do not render it damaged for further use? Is it possible to employ a &lt;a href=&quot;http://en.wikipedia.org/wiki/Whipple_shield&quot; rel=&quot;nofollow noreferrer&quot;&gt;Whipple shield&lt;/a&gt; mechanism?&lt;/p&gt;&#xA;" OwnerUserId="20546" LastEditorUserId="1142" LastEditDate="2018-10-20T15:42:17.087" LastActivityDate="2018-10-22T17:56:55.267" Title="Solar sail and debris shielding" Tags="|interstellar-travel|debris|light-sail|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22499" PostTypeId="1" AcceptedAnswerId="22502" CreationDate="2017-08-01T08:49:17.340" Score="1" ViewCount="340" Body="&lt;p&gt;I've read a space elevator at GSO 35,800 km (actually above GSO to create the needed counterweight) is not feasible due to lack of current available strength of materials.  Also that a tether at low Earth orbit 200 km is not feasible due to extremely high speeds in the atmosphere leading to very high drag and impossible speeds at which to 'catch' the tether for delivery of payloads/people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an orbit in between these two extremes that would allow a tether to be short enough to be feasible from a strength and from an atmospheric speed/drag standpoint and what is that orbit?&lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="20287" LastEditDate="2017-08-01T11:48:07.377" LastActivityDate="2017-08-01T22:43:52.207" Title="Space elevator orbit below GSO but slow enough to be caught by an airplane" Tags="|space-elevator|space-tether|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22502" PostTypeId="2" ParentId="22499" CreationDate="2017-08-01T13:39:56.930" Score="4" Body="&lt;p&gt;Firstly, while the idea sounds good, it turns out it can't work at all even if we did have the materials for the tether and a plane. The reason is that a space elevator doesn't just have one counterweight! It has two. The first one is the obvious one, out beyond GSO. The second one is the earth! In order for our space elevator to be effective with a plane, the plane is going to have to weigh a very large amount if we want to send reasonable amounts of mass to the top. But that is the boring answer, with lots of ifs and buts. Anything is possible if you drop enough constraints. But say we did have a big enough plane, and could build your space elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lets do the math to find out what altitude this orbit could be at!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume the airplane travels over the surface at 240 m/s along the equator. Also at the equator, the earth rotates at $2\pi r_{earth}/24\mathrm{hr}=460\mathrm{m/s}$. So now our airplane is travelling around the earth at 700m/s in an earth-centered-inertial frame. Our airplane pretty close to the surface of the earth, lets say 10km altitude. This gives a radius from the center of 6,381km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for the orbit. First, we calculate the angular velocity:&#xA;$$\omega=\frac{v}{2\pi r_{plane}}=1.097\cdot10^{-4}\mathrm{rad/sec}$$&#xA;And finally we use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;formula&lt;/a&gt; for geostationary orbit:&#xA;$$r_{GSO}=\sqrt[3]{\frac{GM}{\omega^2}}-r_{earth}=25744\mathrm{km}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sadly, this is only 30% less than the original value, and still vastly out to space compared to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To bring the orbit down to say, 3629km (still much further than the ISS!), the plane would have to travel at&#xA;$$\sqrt{\frac{GM}{r^3}}\cdot r_{plane}-v_{earth}=3570\mathrm{m/s}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ouch. And like Baldrickk said, the counterweight has to be much further out than station at the top of the space elevator.&lt;/p&gt;&#xA;" OwnerUserId="20309" LastActivityDate="2017-08-01T13:39:56.930" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22504" PostTypeId="1" AcceptedAnswerId="22509" CreationDate="2017-08-01T19:31:44.673" Score="1" ViewCount="878" Body="&lt;p&gt;No particular emphasis is placed on a particular rocket but rather if a rocket capable of carrying a human payload exists.&lt;/p&gt;&#xA;" OwnerUserId="20407" LastActivityDate="2017-08-02T19:33:14.730" Title="What rockets are currently capable of carrying humans to Mars?" Tags="|mars|rockets|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22505" PostTypeId="2" ParentId="22475" CreationDate="2017-08-01T20:26:14.537" Score="8" Body="&lt;p&gt;Per &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/flying-laptop.htm&quot; rel=&quot;noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;, &quot;Flying Laptop&quot; is a 120 kg smallsat with a variety of technology demonstration payloads, including near-infrared optical links.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cFbVj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cFbVj.jpg&quot; alt=&quot;Flying Laptop [University of Stuttgart]&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gunter links to &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/f/flying-laptop&quot; rel=&quot;noreferrer&quot;&gt;this Earth Observation Portal page&lt;/a&gt; with a lot more info.  It looks like the relation to a laptop is in name only, aside from some vague references to use of commercial off-the-shelf components where possible.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2017-08-02T06:29:07.933" LastActivityDate="2017-08-02T06:29:07.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22506" PostTypeId="2" ParentId="22504" CreationDate="2017-08-01T20:43:25.877" Score="2" Body="&lt;p&gt;Any rocket that can carry humans (NASA would say Falcon 9, Atlas V in the near future, China would say Long March 2F, ESA would say Ariane, Russia would say Soyuz) can carry them to Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality most plans for Mars missions involve launching the transit vehicle on one booster and then the crew on another. SpaceX with ITS/BFR is actually the opposite and suggesting doing everything in one launch (well maybe multiple refueling launches of other vehicles, and then crew on a single vehicle).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it is almost immaterial which rocket could take them to Mars, since it is really the first step of getting into orbit that matters.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or you could have meant, does any rocket active today have the ability to launch from the surface of the earth direct to Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe SLS will one day, maybe Falcon Heavy, maybe ITS/BFR but nothing really available today.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-08-01T20:43:25.877" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22507" PostTypeId="1" AcceptedAnswerId="22513" CreationDate="2017-08-01T21:48:58.530" Score="4" ViewCount="110" Body="&lt;p&gt;I'm looking at a &lt;a href=&quot;https://deepspace.jpl.nasa.gov/files/dsn/Brochure_IrisV2_201507.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;brochure for the Iris V2 Deep Space Transponder by JPL&lt;/a&gt;. At the end of the brochure there is some data on suggested antennas. For each antenna the bps at approximately 1 AU is shown for opposition, arrive, and conjunction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What do opposition, arrive, and conjunction mean with respect to telecommunications?&lt;/p&gt;&#xA;" OwnerUserId="20557" LastEditorUserId="6816" LastEditDate="2017-08-02T03:12:42.817" LastActivityDate="2017-08-03T04:05:35.723" Title="Terminology for CubeSat telecommunications" Tags="|communication|terminology|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22508" PostTypeId="2" ParentId="22499" CreationDate="2017-08-01T22:43:52.207" Score="2" Body="&lt;p&gt;It can be done if you back up and change some of your ideas about what you're building.  Forget the space elevator per se, Quietghost demolished that.  Space elevators must have a net upwards pull or they fall down and that upward pull is only generated by mass beyond synchronous orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, there is also the concept called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Momentum_exchange_tether#Rotovator&quot; rel=&quot;nofollow noreferrer&quot;&gt;rotovator&lt;/a&gt;.  Take a cable and put it in orbit.  It is spun such that the top of the cable is moving forward and the bottom is moving backwards &lt;em&gt;relative to it's orbit&lt;/em&gt;.  If you build it big enough you can make the bottom part move backwards as fast as the center of mass is going forward--thus to an observer on Earth it would appear to slow as it dipped down, come to a momentary rest and then climb back up and speed up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, if you have god-like ability to make things precise it could come down and pick up a payload that's sitting on surface.  Since getting it ever so slightly wrong would be catastrophic, though, in practice you are going to intercept the cable somewhere in flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this system is not tethered to produce a net outward force.  This means there's no free lunch in lifting payloads into space, for every Newton you use to speed up your payload you must also exert a Newton slowing something down.  You also must manage the rotational speed of the rotovator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A lifter that must bring down as much as it brings up at first glance might seem to not have much value but that's not the case--what comes down need not be what goes up.  Put a mining base on the moon, pack up your refinery slag and use that as the mass to be brought back down.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-08-01T22:43:52.207" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="22509" PostTypeId="2" ParentId="22504" CreationDate="2017-08-01T23:20:40.800" Score="6" Body="&lt;p&gt;The answer is either &quot;none&quot; or &quot;all&quot;, depending on how you look at it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The limiting factor is how you intend to launch the spacecraft that's actually going to carry the people to Mars.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For reference, the Apollo CSM massed about 12000 kg dry (no propellants or consumables).  The CSM could keep 3 people alive for a period of a week or so; figure a spacecraft to Mars would be considerably bigger.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to send the whole shebang off in a &lt;em&gt;single launch&lt;/em&gt; (as with the Apollo missions), then you're going to need &lt;em&gt;at least&lt;/em&gt; an SLS Block II class rocket (50,000 kg to TMI), and even then I'm not sure that's adequate (Orion is not going to be the transfer vehicle - it doesn't have the lifetime for a trip to Mars).  The ITS as originally envisioned by SpaceX is insanely, stupidly huge, making the SLS look like a bottle rocket, and even then the concept requires 2 launches (one to launch the transfer vehicle, one to launch the fuel for the transfer vehicle).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that SpaceX is scaling back on the ITS a bit, from 12 m diameter to 9 m, which is still huge.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;No commercial launcher currently in production will be able to do it.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, if you send your transfer vehicle up as a series of smaller modules that get stacked and fueled in Earth orbit (sort of like the ISS), then pretty much any launcher is up to the task, you just have to size the individual modules appropriately.  A FH (or similar class launcher) could lift the transfer spacecraft modules (along with propellant and consumables), and an F9 (or Atlas) could launch a Dragon (or a Starliner) as a taxi.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would not be &lt;em&gt;easy&lt;/em&gt;, but it would be more tractable than trying to do it all in a single launch.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If all you're concerned about is sending the equivalent mass of a person (or several people, say several hundred kg in total), then almost any launcher in use today can get that mass to Mars - a good chunk of the Mars exploration program was launched on an Atlas V.  If your human payload doesn't have to survive the trip, you're golden.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, the issue isn't the people themselves, but the spacecraft required to keep them alive and healthy for months at a time in space.  That's going to be your limiting factor for &lt;em&gt;any&lt;/em&gt; human spaceflight.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastEditorUserId="12922" LastEditDate="2017-08-02T19:33:14.730" LastActivityDate="2017-08-02T19:33:14.730" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22510" PostTypeId="1" AcceptedAnswerId="22511" CreationDate="2017-08-02T00:45:11.263" Score="7" ViewCount="319" Body="&lt;p&gt;Why is Orbital Science's &lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Minotaur V&lt;/a&gt; rocket a five stage design? I am aware this is derived from an ICBM design, but why would anyone want a five stage design for launching non-lethal payloads? It seems to me that additional stages only open more opportunity for problems and thus mission failure.&lt;/p&gt;&#xA;" OwnerUserId="14488" LastEditorUserId="6816" LastEditDate="2017-08-02T17:28:05.673" LastActivityDate="2017-08-02T17:28:05.673" Title="Why was the Minotaur V built as a 5-stage design?" Tags="|rockets|solid-fuel|stages|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22511" PostTypeId="2" ParentId="22510" CreationDate="2017-08-02T01:03:42.873" Score="13" Body="&lt;p&gt;It's always easier to create a new launcher by making a small modification to a reliable existing launcher. Small solid rocket stages are relatively simple and reliable, so building up a launcher from solids is less risky than the stage count might indicate. Small solid stages are also extremely cheap. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/LGM-118_Peacekeeper&quot; rel=&quot;noreferrer&quot;&gt;Peacekeeper ICBM&lt;/a&gt; is a three-stage suborbital rocket. By replacing its warhead payload with a fourth stage which could insert a payload into low Earth orbit, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Minotaur_IV&quot; rel=&quot;noreferrer&quot;&gt;Minotaur IV&lt;/a&gt; was created. Two different solid rocket motors were used for the upper stage, the Orion 38 and the Star 48. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Star_(rocket_stage)#Star_48&quot; rel=&quot;noreferrer&quot;&gt;Star 48&lt;/a&gt; in particular has a very long operational history going back to the 1980s; &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_(rocket_stage)&quot; rel=&quot;noreferrer&quot;&gt;Orion 38&lt;/a&gt; was 20 years old when Minotaur IV was first flown, so both of these are proven stages. Minotaur IV has 5 successful launches under its belt -- not a long track record, but validating the principle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Minotaur V extended Minotaur IV by adding a Star 37 to propel a payload out of low Earth orbit to GTO or to the moon. The Star 37 has an even longer history than the Star 48, so again this is a low-risk modification to the launcher.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-02T02:39:34.020" LastActivityDate="2017-08-02T02:39:34.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22512" PostTypeId="2" ParentId="22493" CreationDate="2017-08-02T01:17:34.653" Score="9" Body="&lt;p&gt;These maps were produced with doppler-delay imaging, which uses a combination of range and motion to identify the radar return from each specific point on the surface and build a map.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Longitude is determined from doppler shift - as Venus rotates, one side of the planet is moving towards Earth while the other is moving away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Latitude is calculated from the time taken for the signal to return - the equator is closer to Earth than the poles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This method has two limitations that prevent imaging the doppler equator. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Equivalent North and South latitudes are the same distance from Earth, and cannot be distinguished if the beam is aimed at the equator. This can be mitigated by aiming the beam at the northern hemisphere so that the return from the south is much weaker, but near the equator the ambiguous points are too close together to distinguish that way.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Latitude resolution is much lower near the equator. Near the poles, the view is edge on and two close points will have a significantly different distance from Earth. The equator appears flat and two points must be a lot further apart to show a measurably different range.  &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-08-02T01:17:34.653" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22513" PostTypeId="2" ParentId="22507" CreationDate="2017-08-02T03:20:31.837" Score="3" Body="&lt;p&gt;Conjunction and opposition are terms used in positional astronomy, see the Wikipedia article &lt;a href=&quot;https://en.wikipedia.org/wiki/Opposition_(planets)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Opposition (planets)&lt;/a&gt; for an illustration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think what we've got in the brochure you refer to is an example of poor presentation design.  I &lt;em&gt;believe&lt;/em&gt; the expression &lt;code&gt;Mars ~1 AU (bps)&lt;/code&gt; in the 3-column heading of the tables you are referring to is supposed to be associated with the &quot;arrive&quot; column, i.e., the data rate is for &quot;arrival at Mars when the distance to Earth is approximately 1 AU.&quot;  This would be consistent with the other two columns where the data rates are higher at opposition (Mars less than 1 AU distant) and significantly lower at opposition where Mars is about 2.5 AU away.  (The distances vary, as illustrated at &lt;a href=&quot;https://www.universetoday.com/14824/distance-from-earth-to-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;How Far is Mars from Earth?&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I will have to write to Courtney Duncan to ensure my interpretation is correct.  Once I get a response, I will update this answer.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Date: Wed, 02 Aug 2017 03:53:03 -0400&lt;br&gt;&#xA;From: Fred Koschara &lt;br&gt;&#xA;To: Courtney Duncan &lt;br&gt;&#xA;Subject: Puzzling specs in Iris V2 CubeSat Deep Space Transponder data sheet&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hello Courtney --&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;I happened to look at the Iris V2 CubeSat Deep Space Transponder data&#xA;  sheet&#xA;  (&lt;a href=&quot;https://deepspace.jpl.nasa.gov/files/dsn/Brochure_IrisV2_201507.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://deepspace.jpl.nasa.gov/files/dsn/Brochure_IrisV2_201507.pdf&lt;/a&gt;)&#xA;  and found the data rate tables on page 4 rather puzzling.  After&#xA;  pondering the matter for a while, I came to the conclusion that the&#xA;  expression &quot;Mars ~1 AU (bps)&quot; in the 3-column heading of the tables on&#xA;  that page is supposed to be associated with the &quot;Arrive&quot; column, i.e.,&#xA;  the data rate shown is for &quot;arrival at Mars when the distance to Earth&#xA;  is approximately 1 AU.&quot;  This would be consistent with the other two&#xA;  columns where the data rates are higher at opposition (Mars less than&#xA;  1 AU distant) and significantly lower at opposition where Mars is&#xA;  about 2.5 AU away.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Is this interpretation correct?&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Thank you for your time and information.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;-- Fred Koschara, President&lt;br&gt;    The L5 Development Group&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Date: Thu, 3 Aug 2017 02:49:00 +0000&lt;br&gt;&#xA;From: &quot;Duncan, Courtney B (337G)&quot; &lt;br&gt;&#xA;To: Fred Koschara &lt;br&gt;&#xA;Subject: Re: Puzzling specs in Iris V2 CubeSat Deep Space Transponder data sheet&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hi Fred,&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Yes, that's correct.  1 A.U. is a nominal distance for a Mars arrival&#xA;  (or departure) from (to) earth which is a likely time for things to be&#xA;  happening with SmallSat payloads.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;And at opposition, it's about half an AU (in round numbers, average is&#xA;  0.524), and at conjunction 2.5 (2.524).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;It just so happens that Mars is just leaving conjunction right now. &#xA;  We usually stand down missions for a couple of weeks in order to avoid&#xA;  communications issues that can occur due to lots of sun noise in this&#xA;  period.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;That table is further confused by the fact that all data rates were&#xA;  rounded down to the next DSN supported data rate and so are, on&#xA;  average, root two pessimistic.  DSN data rates go up and down from&#xA;  1000 bits per second in factors of two.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Courtney&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Courtney Duncan&lt;br&gt; Supervisor, Reprogrammable Signal Processing Group&#xA;  337G&lt;br&gt; Flight Communications Systems Section&lt;br&gt; M/S 161-260&lt;br&gt; Jet&#xA;  Propulsion Laboratory, California Institute of Technology&lt;br&gt; 4800 Oak&#xA;  Grove&lt;br&gt; Pasadena, CA 91109&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-08-03T04:05:35.723" LastActivityDate="2017-08-03T04:05:35.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22514" PostTypeId="2" ParentId="22458" CreationDate="2017-08-02T08:09:47.273" Score="5" Body="&lt;p&gt;There is no reason to do this for the sleeping quarters, as lying in bed at normal Earth gravity does not reduce the effects of weightlessness on the human body - in fact, it is has been used in multiple experiments to study the effects of weightlessness on bone and muscle loss:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a recent review on bed rest studies of the past 20 years, it was concluded that head-down bed rest has proved its usefulness as a reliable simulation model for most physiological effects of spaceflight. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2577398/&quot; rel=&quot;noreferrer&quot;&gt;Simulating human space physiology with bed rest&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the difference would be between sleeping in a level bed and sleeping with your head a few degrees down. It would be better to use centrifugal gravity for zones where the astronauts where performing load bearing activities.&lt;/p&gt;&#xA;" OwnerUserId="1286" LastActivityDate="2017-08-02T08:09:47.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22515" PostTypeId="2" ParentId="22475" CreationDate="2017-08-02T08:34:15.217" Score="3" Body="&lt;p&gt;The &lt;em&gt;Flying Laptop&lt;/em&gt; is likely to be a metaphor for a fantastic 21st century education.&lt;/p&gt;&#xA;&lt;p&gt;From the University of Stuttgart Institute of Space Systems fact sheet &lt;a href=&quot;http://www.kleinsatelliten.de/img/news/fact_sheet.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Academic Small Satellite Flying Laptop&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Mission Objectives: The development of the Flying Laptop has been conducted by students in the frame of Ph.D., diploma, master, bachelor, and study theses, and internships. The project is used to improve teaching quality by providing hands-on project experience.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From Research-in-Germany.org's article &lt;a href=&quot;https://www.research-in-germany.org/en/research-landscape/news/2017/07/2017-07-14-the-small-satellites--technosat--and--flying-laptop--are-successfully-launched-into-space.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;The small satellites 'TechnoSat' and 'Flying Laptop' are successfully launched into space&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Flying Laptop – a small satellite as a training and test mission&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;The 'Flying Laptop' project offers both undergraduate and doctoral students a fantastic opportunity to put learned theory into practice and gain project experience in a real space mission. &lt;strong&gt;So far, more than 150 student dissertations and over 20 doctoral papers have been written in connection with this project&lt;/strong&gt;,&amp;quot; reports Sabine Klinkner, project manager at the University of Stuttgart. The 110-kilogram ‘Flying Laptop’ small satellite was developed and constructed by post-graduate and undergraduate students at the university's Institute of Space Systems. The necessary infrastructure for the construction, qualification and operation of small satellites in general was also created as part of the development of the satellite. In addition to a large clean room for the integration of satellites, an optics laboratory and a thermal-vacuum chamber, the ground station with a control segment at the University of Stuttgart was also set up and a satellite simulation environment was developed. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/f1ePl.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f1ePl.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Flyling Laptop Satellite from &lt;a href=&quot;http://spaceflight101.com/soyuz-kanopus-v-ik/flying-laptop/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight101&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The University of Stuttgart Institute of Space Systems also has a &lt;strong&gt;dedicated ground station&lt;/strong&gt; to manage both the &lt;strong&gt;Flying Laptop&lt;/strong&gt; and &lt;strong&gt;TechnoSat&lt;/strong&gt; spacecraft!&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AS1Gv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AS1Gv.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From a &lt;a href=&quot;https://i.stack.imgur.com/4Ww9D.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flying Laptop tweet&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Specs for the Flying Laptop can be found in Spaceflight 101's article &lt;a href=&quot;http://spaceflight101.com/soyuz-kanopus-v-ik/flying-laptop/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flying Laptop&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;120 kg&lt;/li&gt;&#xA;&lt;li&gt;60 x 70 x 90 cm&lt;/li&gt;&#xA;&lt;li&gt;tripple-junction GaAs cells with a maximum power generation of 269 Watts. Also included is a test of next generation ultra-thin 100 µm cells.&lt;/li&gt;&#xA;&lt;li&gt;Three lithium iron phosphate batteries with a capacity of 35 Amp-hours and a nominal operating voltage of 23.1V&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/f/flying-laptop&quot; rel=&quot;nofollow noreferrer&quot;&gt;On Board Computer&lt;/a&gt;: &lt;strong&gt;Core:&lt;/strong&gt; UT699 LEON3 with a fault tolerant 32 bit SPARC V8, &lt;strong&gt;I/O:&lt;/strong&gt; radiation-tolerant flash FPGA w/ non-volatile memory, incorporates all digital interfaces to the satellite components, except the payload components, &lt;strong&gt;CCSDS TM/TC:&lt;/strong&gt; similar technical composition as the I/O board, &lt;strong&gt;PCDU:&lt;/strong&gt; reconfigurable Power Control and Distribution Unit.&lt;/li&gt;&#xA;&lt;li&gt;Attitude determination: autonomous star tracker, three-axis magnetometer, eight coarse sun sensors, inertial sensors and GPS receivers&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Attitude determination via GPS&lt;/strong&gt; (not a typo): GENIUS (GPS Enhanced NavIgation system for the University of Stuttgart micro-satellite). GENIUS uses three GPS antennas arranged in L-shape configuration on the central, body-mounted solar array to deliver real time position with ten-meter accuracy, velocity accurate to 0.1m/s and timing with 1µs accuracy. Additionally, &lt;strong&gt;the arrangement of the three antennas, spaced at 44 and 61 centimeters, enables an experimental attitude determination algorithm to be run using the phase and Doppler shift of the GPS carrier signal&lt;/strong&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Primary attitude sensor:  µASC (micro Advanced Stellar Compass) developed by the University of Denmark comprising a pair of camera heads and a data processing unit and providing a pointing knowledge of 2 arc seconds and supporting slew rates of up to 10 degrees per second for rapid repointing of the platform.&lt;/li&gt;&#xA;&lt;li&gt;Magnetometer: Anisotropic-Magneto-Resistive Magnetometer provides magnetic field vector and strength for actuation of the toque rods&lt;/li&gt;&#xA;&lt;li&gt;Fiber Optic Gyro: four single-axis optic rate gyros provides body rate data.&lt;/li&gt;&#xA;&lt;li&gt;Sun sensors: 6° pointing accuracy in case of a satellite safe mode to ensure proper power generation via solar-pointing of the arrays.&lt;/li&gt;&#xA;&lt;li&gt;S-Band communications system: low- and high-gain antennas Command uplink at 2.068 GHz, telemetry downlink at 2.245 GHz, science data downlink at 2.425 GHz - achieves 10Mbps using QPSK modulation.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Optical HighSpeed Infrared Link System&lt;/strong&gt; (OSIRIS): laser communications terminal developed by German Aerospace Center for high-data rate downlink for data-intensive payloads on small satellites. OSIRIS comprises two optical transmitter units, each with a laser source, modulators and optical fibers that connect to the collimator units installed on the satellite’s optical bench. One laser source employs a high power laser diode while the other uses an erbium doped fiber amplifier (EDFA). Both operate in the infrared band, at a wavelength of 1550 nanometers and OSIRIS is expected to reach a data rate of 100 Mbit/s. All in all, OSIRIS weighs around 1.5 Kilograms and requires 25 Watts of power during operation.&lt;/li&gt;&#xA;&lt;li&gt;Receiver for Automatic Identification System (AIS): Developed at DLR Bremen. AIS is used by sea vessels that send and receive VHF messages containing identification, position, course and speed information to allow the monitoring of vessel movements and collision avoidance as well as alerting in the event of sudden speed changes. Deploying space-based AIS terminals allows a broad coverage and data relay to ground stations for monitoring of large sea areas. However, due to the large footprint of satellites, overlapping and signal collisions become a problem, especially for frequented traffic routes, requiring a steady improvement in reception technology to separate the different signals. The AIS Receiver was a late addition to the Flying Laptop and will be employed in conjunction with the satellite’s cameras to examine the accuracy of the position data transmitted by ships.&lt;/li&gt;&#xA;&lt;li&gt;Multispectral Imaging Camera System (MICS): Comprises three single cameras with CCD array detectors for snapshot observation in the visible and near infrared range; 10 x 9 x 10 cm, ~ 4 kg. The three optical systems employ identical double Gauss telescopes with interference filters placed in front to set the wavelength passband. They achieves a 21.5-meter ground resolution across a 22-Kilometer swath with spectral channels of 530-580nm (green), 620-670nm (red) and 820-870nm (near infrared).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mJeIS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mJeIS.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;MICS works in close cooperation with the satellite’s attitude control system to establish three different modes of image acquisition. In Inertial Pointing, the star tracker delivers high-precision attitude knowledge and the satellite will remain inertially stabilized for stellar and lunar observations. Nadir Pointing Mode aligns the z-vector of the satellite perpendicular to the Earth’s surface, keeping the cameras pointing directly downward as the satellite makes its orbit, allowing for Earth imaging. A dedicated Spotlight Mode points the spacecraft at a fixed target on Earth, slewing the satellite to compensate for its orbital motion to keep pointing to the target to achieve the required coverage/resolution of the planned scientific observations. The spotlight mode will be used to capture images of the same target at different observation angles for the measurement of the Bidirectional Reflectance Distribution Function (BRDF) – a fundamental radiometric parameter that is dependent on the vectors of solar incidence, outgoing reflection and surface normal as well as the properties of the surface itself.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Ww9D.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Ww9D.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-02T09:32:50.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22516" PostTypeId="1" AcceptedAnswerId="49918" CreationDate="2017-08-02T08:59:58.300" Score="0" ViewCount="203" Body="&lt;p&gt;The &lt;a href=&quot;https://space.stackexchange.com/a/20181/12102&quot;&gt;current answer&lt;/a&gt; to the question &lt;a href=&quot;https://space.stackexchange.com/q/1059/12102&quot;&gt;Highest number of satellites launched on a single rocket&lt;/a&gt; is 104, on ISRO's PSLV-C37. One of the runner-ups is KANOPUS-V-IK with about 73 satellites. The trick is to land an order from Planet Laps for a flock of doves. The Doves should be considered mostly identical here, and would count as one satellite design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; So far there are no answers, so I'll make another adjustment. The whole concept of the cubesat standard is that they should all deploy the same. A cubesat &quot;dispenser&quot; should not care who's cube it is or how it was designed internally. Therefore:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1U, 2U, 3U, 6U, etc. Each cubesat size counts as &lt;strong&gt;one&lt;/strong&gt; design for the purposes of this question. I'm trying to get a handle on the variety of satellite shapes and sizes that a single rocket must handle. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Cropped from Spaceflight Now's &lt;a href=&quot;https://i.stack.imgur.com/66ufZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz rolled out for launch of multinational satellite cluster&lt;/a&gt; &lt;em&gt;Some of the 73 satellites slated to launch on a Russian Soyuz rocket Friday are pictured before encapsulation inside the launcher’s payload fairing at the Baikonur Cosmodrome in Kazakhstan.&lt;/em&gt; Credit: Glavkosmos&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/66ufZ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/66ufZ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-03T18:44:49.600" LastActivityDate="2021-02-03T16:52:38.137" Title="Highest number of distinctly different satellite designs on a single rocket" Tags="|launch|payload|record|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22517" PostTypeId="2" ParentId="22491" CreationDate="2017-08-02T10:05:19.953" Score="6" Body="&lt;p&gt;A &lt;em&gt;pole star&lt;/em&gt; for a given planet would be a star that coincidentally just happens to be in the direction that the planet's axis of rotation is pointed. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, that direction for Venus is &lt;code&gt;RA=18h11m, Dec=67.16 deg&lt;/code&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see on the screenshot that the Earth's pole star &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Polaris&quot; rel=&quot;nofollow noreferrer&quot;&gt;Polaris&lt;/a&gt;&lt;/em&gt; is not exactly at the pole position, and if you watched it over centuries or eons you'd see that its always moving around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An interesting question might be how to do navigation on Venus without a view of the night sky due to the (very) opaque clouds. An antenna large enough to receive radio signals from the Sun, Jupiter, or a variety of bodies outside of the solar system may not be so portable, and the very thick atmosphere would tend to block X-rays and any directional components that might be found in cosmic rays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, a robotic solar power plane flying somewhere around 60 - 80 km above the surface where sunlight can recharge its batteries might venture near the darker parts of the terminator to spot some stars and planets for additional location cues (assuming it had an on-board ephemeris). It wouldn't necessarily need a pole star to do this though.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shots from a planetarium view at &lt;a href=&quot;https://in-the-sky.org/skymap.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;in-the-sky.org&lt;/a&gt;, but with a &quot;star&quot; shape added at the approximate position of a hypothetical north or south pole star for Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; But stating which one is which requires some careful definition as was pointed out by @TomSpilker's &lt;a href=&quot;https://space.stackexchange.com/questions/27591/what-is-the-science-behind-the-variation-of-mars-effective-sky-temperature-with/27595?noredirect=1#comment82301_27595&quot;&gt;comment&lt;/a&gt;. If you look down at the solar system &quot;from above&quot; and looked at each planet, you'd see that all planets rotate around the Sun in the same direction, counter-clock-wise. The direction of this rotation is called &lt;em&gt;prograde&lt;/em&gt;. If you then look at the direction of the rotation of each planet &lt;em&gt;around its own axis&lt;/em&gt; you'd see that &lt;em&gt;Venus&lt;/em&gt; and &lt;em&gt;Uranus&lt;/em&gt; spinning was &lt;em&gt;retrograde&lt;/em&gt;.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Retrograde_and_prograde_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Retrograde_and_prograde_motion&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you use the &lt;em&gt;right hand rule&lt;/em&gt; to define a &lt;em&gt;North&lt;/em&gt; for each planet, Venus' North points to the same hemisphere as Earth's South. So I've left the plots of the locations of Venus' hypothetical &lt;em&gt;pole stars&lt;/em&gt; unlabeled.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Definition of rotation       Axial Tilt&#xA;     &quot;retrograde&quot;                2.64°&#xA;    right-hand-rule            177.36°&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J0ev4.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J0ev4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SgH3J.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SgH3J.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-06-03T01:50:12.033" LastActivityDate="2018-06-03T01:50:12.033" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="22518" PostTypeId="1" CreationDate="2017-08-02T10:44:33.137" Score="2" ViewCount="172" Body="&lt;p&gt;The graph shows that 36 km/s is the escape velocity of the solar system at the Earth. But if we do some calculations, we will get 42.1 km/s as the right answer. I have been checking all other escape velocities of the solar system at the different planets from the graph, and they match with the real values. But the Earth escapes velocities do not match. Am I missing something or is it wrong? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculations:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s3wLC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s3wLC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;sqrt((6.674E-11*1.989E30 *2)/(149.6E9)) = 42126.918623693244 km/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yzfeX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yzfeX.png&quot; alt=&quot;Plot of Voyager 2&amp;#39;s heliocentric velocity against its distance from the Sun&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="20563" LastEditDate="2017-08-03T09:23:18.357" LastActivityDate="2017-08-03T09:23:18.357" Title="Isn't the escape velocity for the solar system at the Earth 42.1 km/s? Then why does the graph show 36 km/s for the same value? Is the plot wrong?" Tags="|voyager|gravity-assist|escape-velocity|" AnswerCount="0" CommentCount="7" FavoriteCount="0" ClosedDate="2017-08-02T16:06:10.430" ContentLicense="CC BY-SA 3.0" />
  <row Id="22520" PostTypeId="1" AcceptedAnswerId="22529" CreationDate="2017-08-02T19:51:31.840" Score="12" ViewCount="2149" Body="&lt;p&gt;It's well known that the near side of the Moon and the far side have different topographical features and that if the Moon had a core it might have been shifted to the near side that faces Earth due to a large impact it might have sustained in the past, but do we know which side is heavier?  I could guess that if the core was shifted that might make the near side heavier, but I haven't found a definite answer.&#xA;Please attach any evidence that supports your answer!&lt;/p&gt;&#xA;" OwnerUserId="13032" LastEditorDisplayName="user687" LastEditDate="2017-08-03T20:14:15.473" LastActivityDate="2023-11-05T16:44:33.923" Title="Which side of the Moon faces Earth, the heavy side or the lighter side?" Tags="|the-moon|" AnswerCount="3" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22521" PostTypeId="2" ParentId="22520" CreationDate="2017-08-02T21:54:45.333" Score="19" Body="&lt;p&gt;According to &lt;a href=&quot;https://spaceflightsystems.grc.nasa.gov/education/rocket/moon.html&quot; rel=&quot;noreferrer&quot;&gt;this NASA page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The mass of the Moon is not evenly distributed; mass concentrations, called Mascons, lie beneath many of the lunar basins, and the center of mass of the Moon is displaced several kilometers towards the Earth. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This indicates that the near side is slightly heavier.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-08-02T21:54:45.333" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22524" PostTypeId="1" AcceptedAnswerId="22530" CreationDate="2017-08-03T11:57:11.943" Score="6" ViewCount="2201" Body="&lt;p&gt;Wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Rutherford_(rocket_engine)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rutherford rocket engine&lt;/a&gt; states that it uses electric pump feed cycle and:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The pumps (one for the fuel and one for the oxidizer) in electric pump&#xA;  feed engines are driven by an electric motor.  The Rutherford engine&#xA;  uses dual brushless DC electric motors and a lithium polymer battery. &#xA;  This improves efficiency &lt;strong&gt;from the 50% of a typical gas-generator cycle&#xA;  to 95%&lt;/strong&gt;. However, the battery pack increases the weight of the complete&#xA;  engine.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What does the efficiency improvement refer to? Engine feed cycle (propellant injection) improvement, or the whole engine improvement?&lt;/p&gt;&#xA;" OwnerUserId="20571" LastEditorUserId="20571" LastEditDate="2017-08-03T19:03:30.160" LastActivityDate="2017-08-03T19:03:30.160" Title="What is efficiency improvement in Rutherford rocket engine?" Tags="|engines|rutherford-engine|electron|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22525" PostTypeId="1" AcceptedAnswerId="22526" CreationDate="2017-08-03T12:15:07.997" Score="32" ViewCount="7978" Body="&lt;p&gt;NASA is hiring a new 'planetary protection officer' to defend Earth from alien matter, and the pay is a six-figure salary: as much as $187,000 a year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we are not sure whether aliens exist, why are we still hiring staff for protecting Earth? I do understand we have to take precautions. But when we don't have any proof why spend &lt;strong&gt;$187,000&lt;/strong&gt; a year?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://timesofindia.indiatimes.com/home/science/nasa-hiring-new-planetary-protection-officer-to-defend-earth-from-alien-matter/articleshow/59891893.cms&quot; rel=&quot;noreferrer&quot;&gt;Nasa [sic] hiring new 'planetary protection officer' to defend Earth from alien matter - &lt;em&gt;Times of India&lt;/em&gt;, Aug 3, 2017&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11765" LastEditorUserId="6816" LastEditDate="2017-08-04T04:36:59.817" LastActivityDate="2017-08-08T07:02:08.533" Title="Is it true that NASA is hiring a new 'planetary protection officer'?" Tags="|nasa|planetary-science|earth|planetary-protection|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22526" PostTypeId="2" ParentId="22525" CreationDate="2017-08-03T12:28:00.827" Score="81" Body="&lt;p&gt;You're observing shamefully bad journalism. The &amp;quot;protect Earth from aliens&amp;quot; bullet point in the &amp;quot;Highlights&amp;quot; section of the article was put there by an editor who either ignorantly or willfully distorted the actual role of the Planetary Protection Officer.&lt;/p&gt;&#xA;&lt;p&gt;The first paragraph of the story gets a little closer:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The full-time role of &amp;quot;planetary protection officer&amp;quot; will involve &lt;strong&gt;ensuring that humans in space do not contaminate planets and moons&lt;/strong&gt;, as well as ensuring that alien matter does not infect Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Planetary Protection Officer&amp;quot; isn't a new position; the current one is retiring. The primary focus of the planetary protection office is sterilizing probes that are going to other planets and moons. This is done both to preserve any existing life elsewhere in the solar system from Terran competition, and so that if and when we find life on other bodies, we will know we didn't bring it ourselves.&lt;/p&gt;&#xA;&lt;p&gt;Preventing microbes from other worlds reaching Earth is also a concern, for similar reasons.&lt;/p&gt;&#xA;&lt;p&gt;Developing and implementing the policies of the planetary protection office is a complex job requiring a broad array of both technical and human skills, and so a $187K salary is not remarkable for it:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Candidates will be required to travel frequently — but like any job, there will be a significant amount of emails, proposals and other reading.&lt;/p&gt;&#xA;&lt;p&gt;Candidates must have at least one year's experience as a top-level civilian government employee, and an advanced degree in physical science, engineering or mathematics. They must also have &amp;quot;advanced knowledge&amp;quot; of planetary protection.&#xA;The position also requires &amp;quot;demonstrated skills in diplomacy that resulted in win-win solutions during extremely difficult and complex multilateral discussions&amp;quot;. The new hire will also receive &amp;quot;secret&amp;quot; security clearance. Only US citizens and US nationals can apply.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-03T21:30:25.280" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22527" PostTypeId="2" ParentId="22525" CreationDate="2017-08-03T12:48:16.057" Score="13" Body="&lt;p&gt;The planetary protection officer has 2 major functions: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Protect other planets from contamination by our space probes&lt;/li&gt;&#xA;&lt;li&gt;Protect the Earth from contamination by alien matter&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The press have for some reason sensationalized this. Alien matter in this case does not mean little green men with laser pistols, it means matter from any celestial body or object other than Earth. There are good reasons to do both: we don't want Earth organisms interfering with our search for life on other planets, and we don't potentially disruptive extra-terrestrial organisms or matter in our biosphere. If you wait until it's already happened to think about it then it's too late.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the salary the requirements are for someone with scientific background and people management skills, that amount of money doesn't sound unreasonable to me. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planetary protection was first raised as a concern in 1956, when missions to the moon were being discussed, so it's nothing new. Article IX of the &lt;a href=&quot;http://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/outerspacetreaty.html&quot; rel=&quot;noreferrer&quot;&gt;outer space treaty&lt;/a&gt; states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;States Parties to the Treaty shall pursue studies of outer space,&#xA;  including the moon and other celestial bodies, and conduct exploration&#xA;  of them so as to avoid their harmful contamination and also adverse&#xA;  changes in the environment of the Earth resulting from the&#xA;  introduction of extraterrestrial matter and, where necessary, shall&#xA;  adopt appropriate measures for this purpose.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The office of planetary protection (and therefore this role) are required to be compliant with the treaty, as well as government policy and good practice. &lt;/p&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2017-08-03T12:48:16.057" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22529" PostTypeId="2" ParentId="22520" CreationDate="2017-08-03T17:46:14.713" Score="13" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;[...] current understanding of the structure of the Moon involves an iron-rich, aluminum-poor mantle whose center of mass is offset about 10 km from the center of mass of an aluminum-rich, iron-poor crust. The direction of offset is toward the Earth, about 14 degrees to the east of the Earth-Moon line. Such a model accounts for the basaltic maria which face the Earth, and the aluminum-rich highlands on the Moon's far side, and for a 2 km offset between the observed center of mass and center of figure for the Moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Will, &lt;a href=&quot;https://arxiv.org/abs/1403.7377&quot; rel=&quot;noreferrer&quot;&gt;https://arxiv.org/abs/1403.7377&lt;/a&gt; , p. 51&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this description, the average density of the half of the sphere that is closer to the earth would be greater, because it would contain more than half the volume of the denser mantle, and less than half of the less dense crust.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastActivityDate="2017-08-03T17:46:14.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22530" PostTypeId="2" ParentId="22524" CreationDate="2017-08-03T18:54:20.840" Score="18" Body="&lt;p&gt;This claim of &quot;efficiency&quot; is factually accurate but nearly meaningless as a measure of a rocket's performance. It's similar to pointing out that a car has high-efficiency LED headlights. That's great but it doesn't necessarily mean the car will have better performance or economy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a gas generator cycle, a small amount of fuel and oxidizer are burned to create hot, high-pressure gas which is then through a turbine. The turbine converts the energy in the gas into work, which it uses to run a fuel or oxidizer pump. Turbines have practical and theoretical limits to their efficiency; the turbine exhaust gas is cooler and lower-pressure than the entering gas but still carries about half the energy of the entering gas. Hence, 50% efficiency (roughly, typically). An electric motor is not constrained by the same thermodynamic law as the turbine. Modern brushless DC motors can turn 95% of the incoming electrical power into mechanical power, losing only 5% to waste heat. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the overall driving metric for launch rockets isn't the efficiency of the fuel pump, it's the dollars/kg cost of payload. Things that factor into that are the efficiency of the rocket, the cost of construction, and costs of testing and operation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Turbopumps driven by a gas-generator cycle are ubiquitous in rockets primarily because of their amazing power-to-weight ratio. The gas generator itself (burner) is basically an assembly of highly-engineered sheet metal. It's light. The turbines themselves are engineered to operate at very high temperatures and speeds and extract an ungodly amount of power for their weight. For example, the SSME high-pressure oxidizer turbopump had a power output of 23,260 hp and measured only 600 by 900 mm (24 by 35 in). That's for the turbine AND the pump. The fuel they burn has weight, but it's already on board and depletes as the rocket climbs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electric motors have great power-to-weight, but not as good as gas turbines. The battery is an added system that weighs the same at the end of the rocket's burn as it did at the start. A detailed analysis would probably show that a gas generator cycle would save your rocket a lot of pounds, and those saved pounds would go straight into payload capacity, bringing down the $/kg substantially.&#xA;However, the turbines used in rocket turbopumps are among the most highly-engineered, exhaustively tested, longest development time, and overall expensive parts of the entire rocket. Replacing them with high efficiency electric motors and batteries represents a substantial cost and time savings in both development and final production.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What Rutherford has done is made the engineering decision that given the specifics of their rocket - its size, anticipated volumes, their development budget and schedule - they are better off using &quot;cheap but heavy&quot; electric motors and batteries than a &quot;light but expensive&quot; gas generator and turbine. The dollars/kg increase due to losing payload to the heavier electric pump system will be more than offset by the (presumably) faster time-to-market and lower development and production costs. Pointing out that an electric motor is more efficient than a gas turbine is someone's idea of a good marketing point but it's meaningless from the standpoint of rocket performance.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2017-08-03T18:54:20.840" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22531" PostTypeId="2" ParentId="5739" CreationDate="2017-08-03T23:58:48.677" Score="-2" Body="&lt;p&gt;Orion on the top. Cygnus on the bottom left. Bottom right cannot be determined without a supercomputer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking up into the sky from a planet not our own is what makes this alleged &quot;artistic license&quot; appear as such. The &quot;meatball&quot; logo is perhaps the largest pink elephant in the room of which there has been no real research to pinpoint exactly where in our cosmic backyard these stars appear as such.&lt;/p&gt;&#xA;" OwnerUserId="20579" LastActivityDate="2017-08-03T23:58:48.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22532" PostTypeId="1" CreationDate="2017-08-04T02:20:54.907" Score="4" ViewCount="164" Body="&lt;p&gt;Between the years of 1980 and 1990 the McDonnell Douglas company supplied at least 10 different variants of the Delta rocket family. Some of these were considered to be Delta rockets while some were Delta II rockets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This became so complex that they invented &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_(rocket_family)#Delta_numbering_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta Numbering System&lt;/a&gt; just to try and keep track of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this variety gave way to United State's Air Force Evolved Expendable Launch Vehicle program in 1995, with the goal of developing a more reliable platform for reaching space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is there such a large variety in the members of the Delta rocket family? Some flew large numbers of missions, like the Delta 2000 series with 44 flights. Later on some became total disasters, like the Delta III family.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were all Delta rockets essentially made to order for a specific mission? Did each launch come with some degree of customization?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only alternative theory I have been able to come up with is that START treaty was put forward as early as 1982 by Ronald Reagan. Although it would not enter law until much later, did the discussion of this treaty leave McDonnell Douglas looking at the prospect of having far too many parts sitting around? For example, Wikipedia even states that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_5000&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta 5000&lt;/a&gt; was produced from spare parts&lt;/p&gt;&#xA;" OwnerUserId="14488" LastEditorUserId="14488" LastEditDate="2017-08-05T12:44:26.283" LastActivityDate="2017-08-05T12:44:26.283" Title="Is the variety in the Delta rocket family the result of an artisanal rocket industry?" Tags="|rockets|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22534" PostTypeId="1" CreationDate="2017-08-04T14:43:05.710" Score="7" ViewCount="394" Body="&lt;p&gt;In response to the United States announcement of manned moon landing plans, the Soviet Union announced the &lt;a href=&quot;https://en.wikipedia.org/wiki/TMK&quot; rel=&quot;nofollow noreferrer&quot;&gt;TMK&lt;/a&gt; program. This program would have sent manned missions to Venus and Mars. The original mission was just a flyby of the two planets. Several possible variations were considered, some of which added manned landing components on the surface of Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the purposes of this we can just assume the N1 rocket was a total success. There is no reason why it couldn't have been completed, it is only a failure in history because of poor management and that changes in the political attitudes of the USSR made it un-needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the technology and knowledge of the era was there any chance the crew even surviving? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the announcement of this program, multiple manned missions had been made to low earth orbit. But it was not until the manned moon landings of the Apollo program that we learned how harsh even the environment on the moon could be. There are more details about this experiment &lt;a href=&quot;http://www.lpi.usra.edu/lunar/missions/apollo/apollo_16/experiments/crd/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Did the Soviets possess the correct knowledge to adequately shield the crew from radiation encountered during the mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the missions had durations measured in years. Even if you could put the crew on a calorie-restricted diet and somehow carry enough food, you would have to reuse water and oxygen while in transit. Did the Soviets posses an adequate knowledge of life support systems that can support such a long duration mission?&lt;/p&gt;&#xA;" OwnerUserId="14488" LastEditorUserId="14488" LastEditDate="2017-08-08T21:10:10.683" LastActivityDate="2017-08-09T12:09:34.470" Title="Did the Soviet TMK crew have any chance of survival with the knowledge and technology of its era?" Tags="|interplanetary|life-support|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22537" PostTypeId="1" AcceptedAnswerId="22538" CreationDate="2017-08-04T23:29:59.610" Score="1" ViewCount="447" Body="&lt;p&gt;I was reading &lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/87967/how-plausible-is-a-tilted-axis-planet&quot;&gt;this world building post&lt;/a&gt; and it got me thinking - every body eventually tidally locks with the body it rotates with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say for a moment a planet near the sun had a Uranus style axis that was in line with the solar plane. Over time the Sun would 'lock on' to this planet as it is doing with Venus, and apparently will do to us in 40 billion years (if, of course, we weren't going to get destroyed before then...) Would it simply stop spinning on that sideways axis? Or would the Sun torque it upright?&lt;/p&gt;&#xA;" OwnerUserId="1537" LastActivityDate="2017-09-04T00:52:11.190" Title="Could a Uranus-style planet tidally lock?" Tags="|planet|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22538" PostTypeId="2" ParentId="22537" CreationDate="2017-08-05T00:17:28.837" Score="3" Body="&lt;p&gt;If it was near enough, it definitely could.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember, that the planet's axis, beyond slow (centuries) precession, points towards the same point in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's take a planet with exactly 90 degrees axial tilt. As the planet circles the star, at one point, the north pole points towards the star, and the daily tidal forces are nearly constant, not affecting it. But 1/4 of the year later, the axis points in the same direction but the planet is 90 degrees away in the orbit - essentially, the star is right above its equator. At that time the tidal forces act at full strength arresting the spin with maximum efficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the spin is slowed enough that the rotary momentum doesn't force the axis to remain in place, and tidal forces over the year (orbital period) begin dominating the forces over the diurnal period, the spin axis will begin shifting towards the standard 0 degrees, perpendicular to the orbital plane - until the planet spins at 1 spin per orbit - meaning, is tidally locked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would take longer than with planets with spin axis near-perpendicular to their orbital plane, but that's not an excessive difference - something of order of twice as long or so. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-08-05T00:17:28.837" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22539" PostTypeId="1" AcceptedAnswerId="22662" CreationDate="2017-08-05T03:18:07.057" Score="7" ViewCount="753" Body="&lt;p&gt;I know that Elon Musk has stated that SpaceX will try to launch the &quot;silliest thing imaginable&quot; on the maiden flight of the Falcon Heavy, but will it be a Dragon capsule on top or a special SpaceX satellite?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="6816" LastEditDate="2017-08-15T15:51:11.007" LastActivityDate="2018-01-05T20:27:45.070" Title="What will the Falcon Heavy maiden flight launch as its payload?" Tags="|launch|spacex|testing|falcon-heavy|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22542" PostTypeId="1" CreationDate="2017-08-05T12:18:23.323" Score="3" ViewCount="222" Body="&lt;p&gt;In the 1960s the USSR developed the &lt;a href=&quot;https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fractional Orbital Bombardment System&lt;/a&gt;. We can call this system FOBS for short. This system permitted the launch of a payload of nuclear warheads which briefly remained in orbit. The warheads could deorbit as required to attack targets on the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RswIT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RswIT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's unclear if this system was ever fully operational or how reliable it would have been. Later, this system was withdrawn as the result of international treaties. There is a possibility that newer Russian built systems still have FOBS capability, although this is unconfirmed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/R8m3W.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R8m3W.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jiffy Pop was developed in the same time frame by the Frederick C. Mennen. It is a packaged food product that is ready to cook. It uses an aluminum container to allow stovetop cooking of popcorn. Really, any heat source would work. It allows fresh popcorn to be quickly produced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can we modify the Jiffy Pop container to survive re-entry and still properly pop the popcorn? It shouldn't arrive undercooked or burnt up, but properly cooked and ready to eat. The speed on touchdown should be low enough not to cause damage to simple structures like huts. If we needed to add a parachute that would be OK.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea being we can load our modified Jiffy Pop containers into a FOBS capable missile. Then we could deliver freshly popped popcorn to anyone under the orbital path.&lt;/p&gt;&#xA;" OwnerUserId="14488" LastEditorUserId="12102" LastEditDate="2017-08-15T16:11:52.187" LastActivityDate="2017-08-15T16:11:52.187" Title="Could a modified Jiffy Pop container survive atmospheric re-entry to deliver fresh popcorn anywhere in the world?" Tags="|reentry|space-art|parachute|" AnswerCount="1" CommentCount="10" FavoriteCount="0" ClosedDate="2017-08-15T19:08:29.973" ContentLicense="CC BY-SA 3.0" />
  <row Id="22543" PostTypeId="1" CreationDate="2017-08-05T15:12:24.630" Score="5" ViewCount="233" Body="&lt;p&gt;I am trying to understand RAIM. My teacher explained three RAIM methods, I will just quote all of them because I think that the jargon might not be consistent.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Maximum separation / Position comparison method (PCM)&lt;/strong&gt;: Different combinations of satellites calculate the position of the receiver and these results are compared with statistical methods&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Pseudo-Range residual / Range comparison method (RCM)&lt;/strong&gt;: All satellites calculate one solution. The residuals of the pseudo ranges are then investigated with statistical methods.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Parity space&lt;/strong&gt;: The teacher said that the system of equations of the satellites is transformed orthogonally with constraint equations. And these constraint equations are used to detect statistical inconsistencies.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;My question: Could someone explain me in normal words what the parity space method does? If there is a simple/short mathematical explanation that would be great. Did I explain the first and second method correctly?&lt;/p&gt;&#xA;" OwnerUserId="20601" LastActivityDate="2017-08-05T15:12:24.630" Title="GPS Receiver Autonomous Integrity Monitoring (RAIM) - parity space method" Tags="|navigation|gps|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22544" PostTypeId="1" CreationDate="2017-08-05T15:38:41.330" Score="0" ViewCount="158" Body="&lt;p&gt;With the equipment development timelines we know about, what is the soonest date a manned lunar landing could take place?&lt;/p&gt;&#xA;" OwnerUserId="19834" LastActivityDate="2017-12-19T13:16:03.217" Title="What is the soonest manned lunar landing date?" Tags="|the-moon|landing|" AnswerCount="3" CommentCount="2" ClosedDate="2017-08-06T12:11:28.343" ContentLicense="CC BY-SA 3.0" />
  <row Id="22545" PostTypeId="2" ParentId="22544" CreationDate="2017-08-05T18:24:54.250" Score="1" Body="&lt;p&gt;We could conceivably have a manned mission anytime (given a few years heads up) if it wasn't for the lack of interest in going to the Moon. &lt;a href=&quot;https://www.space.com/10601-apollo-moon-program-public-support-myth.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lack of Public Interest in the Moon&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="20407" LastActivityDate="2017-08-05T18:24:54.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22547" PostTypeId="2" ParentId="22544" CreationDate="2017-08-05T18:36:50.893" Score="1" Body="&lt;p&gt;The Apollo program gives a baseline: from May 1961 (Kennedy's speech) to July 1969 (Apollo 11 landing), so 8 years. That included inventing a lot of the technology and procedures needed, cranking up the industrial base, and the most expensive peacetime development program to date.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Design of the LEM started in &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Lunar_Module&quot; rel=&quot;nofollow noreferrer&quot;&gt;1963&lt;/a&gt; (after 2 years of deciding on what configuration to use), LM-1 flew in 1968.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These days, it might be quicker than that, given that we already have rockets capable of reaching the moon, although you'd have to use 2-3 launches and Earth Orbit Rendezvous. &#xA;You would need to design the CSM and LM, plus probably an upper stage. How much time that takes depends on how much time pressure you're under (lots of overtime during the Space Race), and how big your budget is. It's hard to imagine it taking less than the 5 years it took to get the LM ready. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-08-05T18:36:50.893" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22548" PostTypeId="1" AcceptedAnswerId="24026" CreationDate="2017-08-05T20:01:39.837" Score="5" ViewCount="799" Body="&lt;p&gt;Does the orbital period of halo/Lissajous orbits around L1/2 points increase, decrease or stay the same when increasing the (average) orbital radius?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A function period(radius,m1,m2) would be cool, although I guess for Lissajous orbits it would require additional parameters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some large radii the orbits would probably become unstable.&lt;/p&gt;&#xA;" OwnerUserId="5858" LastActivityDate="2017-12-13T22:12:23.140" Title="What is the period of halo/Lissajous orbits?" Tags="|lagrangian-points|halo-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22549" PostTypeId="1" CreationDate="2017-08-06T00:53:01.163" Score="11" ViewCount="696" Body="&lt;p&gt;Before 2.5 to 2Gya, Earth's surface was a chemically reducing environment (evidence includes reduced forms of iron, sulfur, and uranium in rocks before that time). It is believed to have become oxidized as a direct result of photosynthetic life, leading to the precipitation of oxidized iron in BIFs, sulfate deposition, etc., etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The surface of Mars is now oxidized (evidence includes sulfates and iron-oxides).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the mechanism? Why doesn't the oxidized state require life?&lt;/p&gt;&#xA;" OwnerUserId="20606" LastEditorUserId="6816" LastEditDate="2017-08-06T01:30:26.950" LastActivityDate="2019-07-12T23:16:58.957" Title="Why is the surface of Mars oxidized?" Tags="|mars|astrobiology|earth|oxygen|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22550" PostTypeId="2" ParentId="22544" CreationDate="2017-08-06T02:36:54.430" Score="1" Body="&lt;p&gt;&lt;a href=&quot;https://en.m.wikipedia.org/wiki/List_of_missions_to_the_Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.m.wikipedia.org/wiki/List_of_missions_to_the_Moon&lt;/a&gt; Russia and Japan have current plans for a manned mission in 2030, China in 2036.  The US and SpaceX have a manned mission planned for lunar orbit in 2023.&lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="25" LastEditDate="2017-12-19T13:16:03.217" LastActivityDate="2017-12-19T13:16:03.217" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22551" PostTypeId="1" CreationDate="2017-08-06T06:59:24.127" Score="8" ViewCount="508" Body="&lt;p&gt;I've been playing around linear tangent based guidance laws (specifically the old Atlas-Centaur Surveyor guidance, Apollo IGM and Space Shuttle PEG code) and while PEG has an option for a coast phase before the final burn, I do not there's any modes where the predictor-corrector will optimize that coast.  It seems to be an external parameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've found papers like, e.g.:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ping Lu, Brian J. Griffin, Gregory A. Dukeman, and Frank R. Chavez.  &quot;Rapid Optimal Multiburn Ascent Planning and Guidance&quot;, Journal of Guidance, Control, and Dynamics, Vol. 31, No. 6 (2008), pp. 1656-1664.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But ripping out the guts of the PEG algorithm and replacing it with an approach like that is going to be fairly aggressive (and I'm not sure I'd be capable of that right now).  I also don't necessarily require optimality down to the last dV of fuel and simplicity and convergence would be more important to me -- so the algorithms that NASA currently uses are likely more extensive than what I'm asking for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect there's some simple late-60s/early-70s era algorithms for producing at least good-enough answers to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;coast-phase before final burn to orbital insertion&lt;/li&gt;&#xA;&lt;li&gt;time of ignition before an on-orbit finite-thrust manuever&lt;/li&gt;&#xA;&lt;li&gt;time of ignition before a descent burn to land on a body (airless)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And this is ultimately a Kerbal Space Program question.  And unfortunately my last calculus of variations course was about 20 years ago, so I'm a bit rusty on math.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I vaguely understand primer vector theory, and it certainly seems like the bang-bang switching function is the thing I'd like to get to, but I don't know how to get it out of PEG, or how to bolt it onto PEG.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some possible clarification:  I don't think I'm concerned with closed-loop optimization.  I suspect what I want is closer to the OPGUID algorithm and the SWITCH algorithm defined in:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Brown, K.R., Harrold, E.F., and Johnson, G.W. 1969. Rapid optimization of multiple-burn rocket flights. Technical Report NASA CR-1430, NASA, Marshall Space Flight Center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I don't know is if there's simpler approaches that I could use which could leverage more recent off-the-shelf algorithms (more specifically I already link against &lt;a href=&quot;http://www.alglib.net/optimization/&quot; rel=&quot;noreferrer&quot;&gt;http://www.alglib.net/optimization/&lt;/a&gt; for the Levenberg-Marquardt optimizer there, and it has a lot of other optimizers that might make solving the trajectory optimization problem much simpler, but I don't have the background to know what might be useful).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I think my question comes down to if I should dive into the OPGUID and SWITCH algorithms -- and into dozens of pages of Fortran code which are older than I am -- or if there's a simpler more modern way to go about this which I'm missing?  Subject to the external constraint that so far I've been more successful at implementing old NASA TDs than algorithms in the literature.&lt;/p&gt;&#xA;" OwnerUserId="14420" LastEditorUserId="14420" LastEditDate="2017-08-07T03:14:50.247" LastActivityDate="2018-01-11T00:18:49.027" Title="How to optimize time of ignition for an on-orbit burn and/or coast phase on ascent?" Tags="|orbital-mechanics|landing|orbital-maneuver|trajectory|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22554" PostTypeId="1" AcceptedAnswerId="24469" CreationDate="2017-08-06T13:33:21.377" Score="15" ViewCount="2071" Body="&lt;p&gt;Sorry if this has been asked a lot, I couldn't find anything by searching the site. I'm a software engineer looking for an open-source space exploration project to contribute to in my spare time. No real restrictions, I'm happy to learn new languages or theory if it's something I find interesting. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't necessarily need to involve satellites or probes, it could be astronomy or even websites and support systems but it needs to be supporting space exploration to keep my interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks in advance!&lt;/p&gt;&#xA;" OwnerUserId="13369" LastEditorUserId="25" LastEditDate="2018-01-12T12:46:34.577" LastActivityDate="2018-02-06T17:03:40.543" Title="Open source space exploration software projects accepting contributions" Tags="|software|public-involvement|open-source|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22555" PostTypeId="1" CreationDate="2017-08-06T20:08:43.027" Score="51" ViewCount="9053" Body="&lt;p&gt;&lt;a href=&quot;http://www.newsweek.com/solar-eclipse-space-national-disaster-solar-eclipse-prep-643766&quot; rel=&quot;noreferrer&quot;&gt;This Newsweek article&lt;/a&gt; states that &quot;GPS...will likely be nonexistent near the eclipse zone&quot; because of all the people going to that area.  While it makes sense that cell towers might be overloaded and it might be impossible to download maps or conduct Internet searches that find coordinates for an input address, and one shouldn't plan to be streaming music/videos over mobile connections during that time, why would GPS be affected?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I thought GPS didn't require communication from devices to satellites, it only required the devices to listen and compute, and thus the system was scalable to a very large number of devices, even when those devices are relatively close together. &lt;/p&gt;&#xA;" OwnerUserId="11181" LastEditorUserId="25" LastEditDate="2017-08-07T16:29:28.740" LastActivityDate="2017-08-08T18:36:50.953" Title="Why would GPS availability be reduced by high demand (or solar eclipse)?" Tags="|radio-communication|gps|myth-busting|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22556" PostTypeId="1" CreationDate="2017-08-06T20:20:37.643" Score="1" ViewCount="188" Body="&lt;p&gt;Are there any fusion thrusters with a twr &gt; 1 and an isp of a couple thousand seconds? &lt;/p&gt;&#xA;" OwnerUserId="17259" LastActivityDate="2017-08-06T20:20:37.643" Title="Are there any designs for fusion engines with similar performance to a nuclear lightbulb?" Tags="|rockets|nuclear|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22557" PostTypeId="2" ParentId="22555" CreationDate="2017-08-06T20:32:48.903" Score="62" Body="&lt;p&gt;GPS isn't affected by demand, as it is transmitting only from satellites, and the receivers only receive, they do no transmit to the satellite at all. At best, there is a slight degradation by having antennas in really close proximity. A million man march, each with a GPS device, might cause some degradation as each device will absorb a bit of the energy that others might. If it were to happen, it would only absorb the signals at low angles, which aren't really required, although they do help to get a better solution. There could also be some noise generated, as receivers can transmit a very small amount of signal in the range of the intended signal as generated by their oscillators. But in large part, it won't be degraded, and at worst case, would only be reducing the number of satellites slightly. And even that effect is probably more due to the large number of people around you, and not the electronics which they possess. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Differential GPS, on the other hand, might have some degradation. This requires that GPS corrections be sent out, and the network traffic that these are sent out on might not work as well. Still, that would only lead to uncertainty in the highest order, it would still be able to figure out which street you are on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the article comes the following quote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;print out directions since GPS (especially Google Maps) likely won’t be an option&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Many people associate GPS with turn by turn directions from something like Google Maps. And that has the potential to be really affected. I suspect the author made the same mistake.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-08-07T12:04:55.153" LastActivityDate="2017-08-07T12:04:55.153" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="22558" PostTypeId="2" ParentId="22555" CreationDate="2017-08-06T20:56:14.653" Score="39" Body="&lt;p&gt;As PearsonArtPhoto says, it's not the GPS protocol itself that causes the problem.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cell phones use &lt;a href=&quot;https://en.wikipedia.org/wiki/Assisted_GPS&quot; rel=&quot;noreferrer&quot;&gt;Assisted GPS&lt;/a&gt;, where cellular data is used to speed up obtaining a GPS fix. This should be just a few kb per session though.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many mapping applications also download map data as you go along, again causing lots of network traffic. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This may be an aberration, but I'm leaving it here for future reference: &#xA;in my own tests an iPhone went through 100 Mb of data just for AGPS in 1 hour. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-08-07T16:37:50.907" LastActivityDate="2017-08-07T16:37:50.907" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="22559" PostTypeId="1" CreationDate="2017-08-06T21:04:21.137" Score="2" ViewCount="146" Body="&lt;p&gt;I know that LRO photographed some landing sites. Did they get all of them? Are there still missing landers?&lt;/p&gt;&#xA;" OwnerUserId="20614" LastActivityDate="2017-08-06T21:04:21.137" Title="Have all the lunar landers been photographed?" Tags="|apollo-program|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22562" PostTypeId="1" CreationDate="2017-08-07T06:40:56.697" Score="5" ViewCount="1188" Body="&lt;p&gt;I am designing a liquid fueled amateur rocket with my brother, and settled on a gas generator engine. We understand the concept and process, however we are still confused on how the engine is initially ignited. Does a battery/other means of energy kick-start the turbopumps? Thank you for the help&lt;/p&gt;&#xA;" OwnerUserId="20624" LastActivityDate="2017-08-07T15:55:50.873" Title="How does a gas generator engine begin ignition?" Tags="|liquid-fuel|" AnswerCount="1" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22563" PostTypeId="1" CreationDate="2017-08-07T15:36:03.380" Score="1" ViewCount="185" Body="&lt;p&gt;T. J. Fill, “PEG Prediction Corrections,” Shuttle Memo 10E-80-11, The Charles Stark Draper Laboratory, Inc., Cambridge, MA, March 1980.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And this isn't a request to violate copyright, if that's an issue.  I would think it would be accessible via NTRS or some other online library like that.  I can't see anywhere that I could pay CSDL to obtain access to it.  I don't know of any library where these CSDL memos would be accessible.&lt;/p&gt;&#xA;" OwnerUserId="14420" LastActivityDate="2017-08-07T15:36:03.380" Title="Where can I find this Space Shuttle Memo?" Tags="|orbital-maneuver|reference-request|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22564" PostTypeId="2" ParentId="22562" CreationDate="2017-08-07T15:39:48.867" Score="10" Body="&lt;p&gt;One common solution is what is referred to as a &quot;start bottle&quot; (see the J-2 flow schematic diagram below -- look for the item labeled &quot;start tank&quot;) -- a small tank of highly compressed gas (e.g., helium or hydrogen) with either a nozzle that impinges directly on the turbine of the turbopump or a line that pressurizes the gas generator exhaust lines to spin the turbopump up to pump the propellants into the gas generator.  When you hear of an engine that cannot be restarted or can only be restarted once or twice, this is a likely reason why -- one start bottle per start.  When it's used up, that's it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/K0f7I.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/K0f7I.png&quot; alt=&quot;J-2 engine flow schematic&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2017-08-07T15:55:50.873" LastActivityDate="2017-08-07T15:55:50.873" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22565" PostTypeId="2" ParentId="22476" CreationDate="2017-08-07T15:50:43.003" Score="12" Body="&lt;h1&gt;No, the reflector hasn't been deployed.&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Project head, Alexander Shayenko &lt;a href=&quot;https://geektimes.ru/post/291835/&quot; rel=&quot;noreferrer&quot;&gt;reported about it (RUS)&lt;/a&gt; (&lt;a href=&quot;https://translate.google.com/translate?sl=ru&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=ru&amp;amp;ie=UTF-8&amp;amp;u=https%3A%2F%2Fgeektimes.ru%2Fpost%2F291835%2F&amp;amp;edit-text=&amp;amp;act=url&quot; rel=&quot;noreferrer&quot;&gt;ENG&lt;/a&gt;) today.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Early report information&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;On July 17th the  team reported about &lt;a href=&quot;https://geektimes.ru/post/291189/&quot; rel=&quot;noreferrer&quot;&gt;possible success (RUS)&lt;/a&gt; (&lt;a href=&quot;https://translate.google.com/translate?sl=ru&amp;amp;tl=en&amp;amp;js=y&amp;amp;prev=_t&amp;amp;hl=ru&amp;amp;ie=UTF-8&amp;amp;u=https%3A%2F%2Fgeektimes.ru%2Fpost%2F291189%2F&amp;amp;edit-text=&amp;amp;act=url&quot; rel=&quot;noreferrer&quot;&gt;ENG&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They analyzed the &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;noreferrer&quot;&gt;TLE&lt;/a&gt; of orbits of 73 satellites in cluster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Based on braking factor(#9 element) and ballistic ratio(#11 element) they found &lt;code&gt;2017-042F&lt;/code&gt;, satellite with extreme values.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ballistic ratio for Mayak is about $1\space m^2/kg$, and for other satellites is not greater $0,01\space m^2/kg$. That means, it is most of all slowing down.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Reality&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Space amateurs (including myself) have seen another satellites, not Mayak. For example  &lt;code&gt;GLOBALSTAR M042&lt;/code&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/M8t40.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M8t40.jpg&quot; alt=&quot;Another satellite&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Team checked their calculations on Indian &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/i/inflatesail&quot; rel=&quot;noreferrer&quot;&gt;InflateSail&lt;/a&gt; satellite with aerodynamics brake. Its area is about $10\space sq\space m$, that's about 2 times greater than Mayak.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mayak mistake calculations&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ioFYi.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ioFYi.png&quot; alt=&quot;mistake calculations&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;InflateSail calculations&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/u3hNy.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/u3hNy.png&quot; alt=&quot;InflateSail calculations&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20623" LastEditorUserId="58" LastEditDate="2017-08-07T17:11:15.730" LastActivityDate="2017-08-07T17:11:15.730" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22566" PostTypeId="2" ParentId="5246" CreationDate="2017-08-07T18:57:16.977" Score="15" Body="&lt;h2&gt;Astronaut spacesuits &lt;a href=&quot;https://www.nesdis.noaa.gov/content/good-gold-are-satellites-covered-gold-foil&quot; rel=&quot;noreferrer&quot;&gt;use gold&lt;/a&gt; (thin layer)&lt;/h2&gt;&#xA;&lt;p&gt;Gold coating protects eyes from harmful sunlight&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Gold is also used by NASA in the construction of spacesuits. Because of its excellent ability to reflect infrared light while letting in visible light, astronauts’ visors have a thin layer of gold on them to protect their eyes from unfiltered sunlight.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rwbFa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rwbFa.jpg&quot; alt=&quot;Gold coating protects eyes from harmful sunlight&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h2&gt;Satellites don't&lt;/h2&gt;&#xA;&lt;p&gt;They use Multi-layer insulation, see &lt;a href=&quot;https://space.stackexchange.com/a/5248/20623&quot;&gt;&lt;em&gt;Rikki-Tikki-Tavi&lt;/em&gt;'s answer&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20623" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-08T17:39:39.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22568" PostTypeId="1" AcceptedAnswerId="22569" CreationDate="2017-08-07T20:10:47.250" Score="3" ViewCount="279" Body="&lt;p&gt;Many current outlines for trips to mars, such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Base_Camp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Base Camp&lt;/a&gt;, propose a large ship capable of transporting astronauts to Mars. However this plan only proposes to send humans to Mars' orbit and not the surface. So what is the purpose of a manned flyby/orbit of Mars but not actually landing on the surface? It seems like a ton of infrastructure just to do something that can be done by robotic probes fairly easily. &lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="195" LastEditDate="2017-08-07T21:00:39.223" LastActivityDate="2017-08-07T21:12:53.207" Title="What's the purpose of a manned flyby/orbit of Mars/Venus?" Tags="|mars|crewed-spaceflight|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22569" PostTypeId="2" ParentId="22568" CreationDate="2017-08-07T21:12:53.207" Score="6" Body="&lt;p&gt;The wikipedia article on Mars Base Camp describes its purposes. Lightweight  robots can be operated by humans from orbit with only milliseconds of latency, as compared with the several minutes of latency they'd have if controlled from Earth; MBC can also be used as a staging area for Phobos/Deimos missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As @called2voyage points out, Mars landing and return is a significantly greater challenge than reaching Mars orbit; you have to give up about 13 kg of payload in Mars orbit for each kg of payload you want to land and return to orbit. Sending robots on one-way trips without the additional mass of life support equipment is one way to minimize this penalty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, it's worth noting that Lockheed Martin doesn't have a space program; their goal with the Mars Base Camp proposal is to sell hardware to NASA. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-08-07T21:12:53.207" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22570" PostTypeId="1" CreationDate="2017-08-07T23:50:52.807" Score="3" ViewCount="132" Body="&lt;p&gt;What uncrewed interplanetary probes have launched on heavy/superheavy-lift rockets? I'm using &lt;a href=&quot;https://www.nasa.gov/pdf/500393main_TA01-LaunchPropulsion-DRAFT-Nov2010-A.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's definitions of heavy and super-heavy&lt;/a&gt; vehicles; heavy-lift is 20-50 metric tons to LEO, super-heavy is 50+ metric tons. &lt;/p&gt;&#xA;" OwnerUserId="13208" LastEditorUserId="13208" LastEditDate="2017-08-08T10:52:59.483" LastActivityDate="2017-08-08T10:52:59.483" Title="What uncrewed interplanetary exploration probes have launched on heavy/superheavy-lift vehicles?" Tags="|probe|launch-vehicle|hllv|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22572" PostTypeId="2" ParentId="22570" CreationDate="2017-08-08T01:39:04.587" Score="6" Body="&lt;p&gt;If you count Shuttle as a heavy lift vehicle:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Galileo (to Jupiter) on STS-34&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hNE1X.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hNE1X.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Magellan (to Venus) on STS-30&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/V4Tz9.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/V4Tz9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ulysses (to the Sun) on STS-41&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xvSgM.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xvSgM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-08-08T01:44:26.170" LastActivityDate="2017-08-08T01:44:26.170" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22573" PostTypeId="1" CreationDate="2017-08-08T02:39:09.797" Score="5" ViewCount="392" Body="&lt;p&gt;As we know the rocket equation, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$ \Delta v = v_e \ln\left(\frac{m_i}{m_f}\right) = I_{sp} \ g \ \ln\left(\frac{m_i}{m_f}\right) $$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So do &lt;span class=&quot;math-container&quot;&gt;$I_{sp}$&lt;/span&gt; and the mass ratio have an inverse relation, or is it that &lt;span class=&quot;math-container&quot;&gt;$I_{sp}$&lt;/span&gt; is inversely related to natural log of the mass ratio?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it mean that to have logarithmic relation implies in the short interval (early phase) the difference is very large?&lt;/p&gt;&#xA;" OwnerUserId="20418" LastEditorUserId="19335" LastEditDate="2022-01-29T12:26:17.853" LastActivityDate="2022-01-29T12:26:17.853" Title="Tsiolkovsky's Rocket Equation" Tags="|rockets|propulsion|rocket-equation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22574" PostTypeId="1" CreationDate="2017-08-08T03:00:32.747" Score="3" ViewCount="549" Body="&lt;p&gt;I've been reading of stories such as &lt;a href=&quot;https://www.space.com/11804-nasa-moon-rock-sting-apollo17.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;one&lt;/a&gt; where a woman was detained after trying to sell a moon rock. This made me wonder how do Moon rocks get away from NASA and eventually find there way into the public?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastActivityDate="2018-03-22T00:15:38.920" Title="How did the Apollo Moon rocks get away from NASA and into the public?" Tags="|the-moon|nasa|" AnswerCount="2" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22575" PostTypeId="2" ParentId="22574" CreationDate="2017-08-08T03:32:49.703" Score="-3" Body="&lt;p&gt;The moon rocks are indistinguishable from everyday lunar metereorites. Any claim of this or that clump of dirt being an Apollo moon rock is solely based on whatever the person making the claim is able to present.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.telegraph.co.uk/news/science/space/6105902/Moon-rock-given-to-Holland-by-Neil-Armstrong-and-Buzz-Aldrin-is-fake.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.telegraph.co.uk/news/science/space/6105902/Moon-rock-given-to-Holland-by-Neil-Armstrong-and-Buzz-Aldrin-is-fake.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20638" LastActivityDate="2017-08-08T03:32:49.703" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22576" PostTypeId="2" ParentId="22573" CreationDate="2017-08-08T03:51:45.903" Score="14" Body="&lt;p&gt;$I_{sp}$ is inversely related to log of mass ratio &lt;em&gt;if delta v is held constant&lt;/em&gt;, yes, but that's not how the rocket equation is usually applied. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way the rocket equation is usually applied is that you have a delta-v requirement given by a particular mission -- for example, the 4100 m/s needed to get from low Earth orbit to lunar orbit. Your $v_e$ will be constrained by your available choices of rocket engine, and $m_f$ will be constrained by the payload you're trying to move. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rearranging the equation to solve for $m_i$ is left as an algebraic exercise.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-08T04:24:19.987" LastActivityDate="2017-08-08T04:24:19.987" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22578" PostTypeId="2" ParentId="22570" CreationDate="2017-08-08T04:35:02.167" Score="4" Body="&lt;p&gt;In addition to the shuttle-based missions, I see:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/ExoMars_Trace_Gas_Orbiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;ExoMars-Schiaparelli&lt;/a&gt; was launched on a Proton-M&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini-Huygens&lt;/a&gt; on a Titan IV (h/t @DylanSP)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Both launchers are at the low end of &quot;heavy lift&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-08-08T04:35:02.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22580" PostTypeId="2" ParentId="22570" CreationDate="2017-08-08T08:22:23.093" Score="2" Body="&lt;p&gt;Energia launching the military Polyus satellite in 1987. 80 tons of payload delivered to orbit. &lt;a href=&quot;https://en.wikipedia.org/wiki/Energia#First_launch_.28Energia-Polyus.29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wiki&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-08T08:22:23.093" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22581" PostTypeId="2" ParentId="22549" CreationDate="2017-08-08T12:24:33.080" Score="3" Body="&lt;p&gt;Oxidizing metals may cost a bit of energy, but the net oxidation process is still exothermic. That means that given enough time and oxygen, oxidation is unavoidable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Life (plants in particular) will make oxygen readily available which accelerates the process, though. &lt;/p&gt;&#xA;" OwnerUserId="1447" LastActivityDate="2017-08-08T12:24:33.080" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22582" PostTypeId="1" AcceptedAnswerId="22589" CreationDate="2017-08-08T16:04:26.987" Score="11" ViewCount="2408" Body="&lt;p&gt;In the movie The Martian, a giant spacecraft known as the Hermes is used to ferry astronauts from Earth orbit to Mars' orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aFEhx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aFEhx.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in Mars orbit, the astronauts take a Mars descent vehicle to a pre-assembled hab, pre-fueled MAV, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Gl9Hu.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Gl9Hu.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and two large rovers waiting on the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gowpe.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gowpe.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I remember that Mars Direct called for a Earth Return Vehicle to be fueled on the surface of Mars via a series of chemical reactions, similar to that in the Martian. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting all economic obstacles aside with building the Hermes and sending tens of tons to Mars, are there any major flaws in the Martian Mars Architecture that would prohibit a manned mission to Mars from happening? Are there any design flaws with any of the vehicles used to transport the humans, or with any life support systems?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="198" LastEditDate="2017-08-14T23:59:13.730" LastActivityDate="2017-08-14T23:59:13.730" Title="Are there any major flaws with the Mars Architecture used in the Martian?" Tags="|mars|crewed-spaceflight|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22583" PostTypeId="2" ParentId="22555" CreationDate="2017-08-08T18:36:50.953" Score="2" Body="&lt;p&gt;It isn't the GPS, but the applications using this data.  On your phone, you generally wouldn't interact with GPS, but with a Map, in some form.  This map data is not stored on your phone.  The applications using the GPS are what puts the load on the network.&lt;/p&gt;&#xA;" OwnerUserId="20649" LastActivityDate="2017-08-08T18:36:50.953" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22584" PostTypeId="2" ParentId="22290" CreationDate="2017-08-09T00:03:32.937" Score="1" Body="&lt;p&gt;It really depends on a few possible variables. Firstly, the atmosphere isn't 100% the same density over a certain area which in combination with atmospheric movement can cause vibrations especially once the atmosphere begins to thickens (same as plane turbulence). Secondly, the design of the re-entry craft will change its characteristics; a plate will experience more vibrations and wobbles than say a spear because of the difference in how the air flows around it. Thirdly, when entering the atmosphere large amounts of energy cause the craft to slow down. This gets input as just compression of air and some friction against the craft and released as a mix of heat, sound and some kinetic energy. This kinetic energy will also vary based on the craft but it could cause anything from slight vibrations to larger wobbles.&lt;/p&gt;&#xA;" OwnerUserId="20529" LastActivityDate="2017-08-09T00:03:32.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22585" PostTypeId="2" ParentId="22290" CreationDate="2017-08-09T01:35:41.913" Score="7" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=j1vmVJKqUFE&quot; rel=&quot;noreferrer&quot;&gt;This onboard video&lt;/a&gt; from the 2014 Orion spacecraft reentry test shows what looks like a pretty smooth ride. During the peak heating period there's a dramatic plasma tail visible, but starting around 1:45 in the video you can see the motion of the horizon, indicating that the capsule is rocking smoothly rather than shuddering at all. Early, smaller capsules like Gemini and Mercury may have had a slightly rougher ride, but I would guess nowhere near what movies like The Right Stuff or Apollo 13 suggest.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-08-09T01:35:41.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22586" PostTypeId="2" ParentId="22582" CreationDate="2017-08-09T02:29:45.270" Score="9" Body="&lt;p&gt;One key piece of technology that Andy Weir &amp;quot;created&amp;quot; was radiation-blocking material. In the book they have a really thin, light, flexible material that blocks all radiation.&lt;/p&gt;&#xA;&lt;p&gt;There's nothing close to that in the real world. &lt;a href=&quot;https://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars&quot; rel=&quot;noreferrer&quot;&gt;NASA is working on solving the problem&lt;/a&gt;, but it's probably not going to be anything like it was in the book.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.universetoday.com/14979/mars-radiation1/&quot; rel=&quot;noreferrer&quot;&gt;Why we need much better radiation blocking technology then we have today.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Over the course of about 18 months, the Mars Odyssey probe detected ongoing radiation levels which are 2.5 times higher than what astronauts experience on the International Space Station – 22 millirads per day, which works out to 8000 millirads (8 rads) per year. The spacecraft also detected 2 solar proton events, where radiation levels peaked at about 2,000 millirads in a day, and a few other events that got up to about 100 millirads.&lt;/p&gt;&#xA;&lt;p&gt;For comparison, human beings in developed nations are exposed to (on&#xA;average) 0.62 rads per year. And while studies have shown that the&#xA;human body can withstand a dose of up to 200 rads without permanent&#xA;damage, prolonged exposure to the kinds of levels detected on Mars&#xA;could lead to all kinds of health problems – like acute radiation&#xA;sickness, increased risk of cancer, genetic damage, and even death.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="19323" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-09T02:29:45.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22587" PostTypeId="1" CreationDate="2017-08-09T04:31:00.353" Score="1" ViewCount="501" Body="&lt;p&gt;In the eoPortal's spacecraft Mission profile for the &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/f/flying-laptop&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flying Laptop&lt;/a&gt;, the term FPGA (&lt;a href=&quot;https://en.wikipedia.org/wiki/Field-programmable_gate_array&quot; rel=&quot;nofollow noreferrer&quot;&gt;Field Programmable Gate Array&lt;/a&gt;) appears 12 times, including the paragraph below. I'm still having trouble understanding how many FPGAs are used &lt;em&gt;in toto&lt;/em&gt; and what precisely is each of their functions. Could someone more fluent in spacecraft computer systems architecture help clarify this?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;FPGAs: The introduction and reliance of onboard computing with an FPGA system represents a new approach to conventional system architectures. It provides the capability to directly generate the logical configuration of FPGA gates from a C-like high level language without producing the machine code for a processor (hence, massive parallel processing is possible). Using an onboard computer architecture with several reciprocative checking FPGAs, a safe system is obtained that even exceeds the performance of current PCs through its ability of parallel real-time processing. An inherent advantage of FPGA architectures is the capability of reconfiguration within milliseconds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I've taken the time to &lt;a href=&quot;https://space.stackexchange.com/a/22515/12102&quot;&gt;profile some of the specifications and innovations of the Flying Laptop&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also see the question &lt;a href=&quot;https://space.stackexchange.com/q/9555/12102&quot;&gt;How do Field Programmable Gate Arrays affect the capabilities of probes?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; I had a false start when I was reading about the &lt;a href=&quot;https://en.wikipedia.org/wiki/LEON#LEON3FT_processor_core&quot; rel=&quot;nofollow noreferrer&quot;&gt;LEON 3 FT processor core&lt;/a&gt;. According to Wikipedia:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An FPGA implementation called LEON3FT-RTAX is proposed for critical space applications.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, &lt;em&gt;I don't think&lt;/em&gt; that the FPGA implementation is used here; The &lt;a href=&quot;http://mil-embedded.com/eletter-products/aeroflex-your-one-stop-source-to-create-your-custom-leon-3ft-microprocessor/&quot; rel=&quot;nofollow noreferrer&quot;&gt;UT 699 LEON 3 FT&lt;/a&gt; is an &lt;a href=&quot;https://en.wikipedia.org/wiki/Application-specific_integrated_circuit&quot; rel=&quot;nofollow noreferrer&quot;&gt;ASIC&lt;/a&gt; implementation.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-04-20T22:38:57.320" LastActivityDate="2020-04-28T22:38:56.123" Title="What are all the ways that FPGAs are used in the Flying Laptop spacecraft?" Tags="|flight-computer|computing|data-systems|" AnswerCount="0" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="22589" PostTypeId="2" ParentId="22582" CreationDate="2017-08-09T17:17:46.170" Score="10" Body="&lt;p&gt;There were some major flaws in the backstory. I believe these were the movie studio's fault and not Andy Weir.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Tyson's trailer for the Martian &lt;a href=&quot;https://www.youtube.com/watch?feature=player_detailpage&amp;amp;v=-fdKyszL1Zo#t=77&quot; rel=&quot;noreferrer&quot;&gt;The Hermes is harbored in low earth orbit between missions&lt;/a&gt;. The Hermes departs from LEO and &lt;a href=&quot;https://youtu.be/-fdKyszL1Zo?t=136&quot; rel=&quot;noreferrer&quot;&gt;arrives in Mars orbit 124 days later&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hermes &lt;a href=&quot;https://www.youtube.com/watch?v=khIHZp_GTEI&quot; rel=&quot;noreferrer&quot;&gt;accelerates at 2 millimeters/second^2&lt;/a&gt;. With this acceleration it would take Hermes 40 days to climb out of earth's gravity well. That completely wrecks the 124 day trajectory Weir had so painstakingly calculated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Moreover, most of the slow 40 day spiral would be in the highly radioactive Van Allen Belts. The slow climb would kill the astronauts as well as Weir's trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I like that Weir used a solar powered, ion propelled interplanetary vehicle. Ion engines get amazing ISP and are very reliable and low maintenance. However they suck at climbing in and out of deep planetary gravity wells. Climbing deep planetary gravity wells is time consuming for ion engines and it is hard to enjoy an Oberth benefit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The movie would have been more believable if Hermes were harbored closer to the edge of planetary gravity wells. Myself, I'd park Hermes at &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;noreferrer&quot;&gt;EML2&lt;/a&gt; and &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2016/01/deimos-tether.html&quot; rel=&quot;noreferrer&quot;&gt;Deimos&lt;/a&gt;. This way the 124 day trajectory would have been believable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H8H7d.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H8H7d.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above is a cartoon illustration of my dream infrastructure. It's from my blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2016/09/xenon.html&quot; rel=&quot;noreferrer&quot;&gt;Xenon&lt;/a&gt;. Solar powered on engines would do the trips between various heliocentric orbits. Climbing in and out of planetary gravity wells would be done by orbital tethers and chemical rockets.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-08-09T18:54:47.453" LastActivityDate="2017-08-09T18:54:47.453" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22590" PostTypeId="1" CreationDate="2017-08-09T18:33:47.233" Score="63" ViewCount="15512" Body="&lt;p&gt;How did the early designers of spacecrafts have any idea what space was actually like?    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How was a vessel ever constructed that actually flew through Earth's atmosphere without burning up, and then was able to travel around once it reached space without just freezing up?   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did they ever speculate no gravity and train astronauts according?&lt;/p&gt;&#xA;" OwnerUserId="20658" OwnerDisplayName="korrinab" LastEditorUserId="20661" LastEditDate="2017-08-10T12:09:27.680" LastActivityDate="2019-05-16T04:54:55.977" Title="How did people know how to build the first space ship?" Tags="|spacecraft|history|" AnswerCount="4" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="22591" PostTypeId="2" ParentId="22590" CreationDate="2017-08-09T19:47:19.023" Score="92" Body="&lt;p&gt;This is a very broad question, but I'll take a stab at it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was understood that gravity pulled the Earth into a spherical shape, with dense solids and liquids below less-dense gases, and it was expected as &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_space#Discovery&quot; rel=&quot;noreferrer&quot;&gt;early as the 17th century&lt;/a&gt; that the atmosphere would get steadily thinner with increasing altitude, giving way to vacuum. These expectations were confirmed with e.g. balloon observations as @ACAC notes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vacuum chambers on the ground were straightforward to make, so predictions about heating and cooling of a vehicle in the vacuum of space could be confirmed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry#History&quot; rel=&quot;noreferrer&quot;&gt;Heating due to high speeds in air was also understood from the 1920s&lt;/a&gt;, and the extremely high temperatures encountered during reentry into the atmosphere from near-orbital speeds were studied with ballistic missiles through the 1950s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Flying &lt;a href=&quot;https://en.wikipedia.org/wiki/Reduced-gravity_aircraft&quot; rel=&quot;noreferrer&quot;&gt;ballistic trajectories in aircraft&lt;/a&gt; simulated weightlessness for short periods, allowing astronauts to prepare somewhat for the experience. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="7034" LastEditDate="2017-08-10T13:29:47.593" LastActivityDate="2017-08-10T13:29:47.593" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="22592" PostTypeId="2" ParentId="6556" CreationDate="2017-08-09T22:16:50.783" Score="2" Body="&lt;p&gt;The mylar is yellow on one side and silver on the other. Today I was cleaning out the effects of my father who for 30-some years was security for Cape Canaveral I came across an envelope for he had saved a small piece of it and a piece of the original velcro strips used on the bottom of the astronauts shoes so they could maneuver in the capsule  the inside of the capsule being lined with the soft portion of Velcro &lt;/p&gt;&#xA;" OwnerUserId="20659" LastActivityDate="2017-08-09T22:16:50.783" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22593" PostTypeId="2" ParentId="22590" CreationDate="2017-08-09T22:59:38.140" Score="62" Body="&lt;p&gt;One important note is that rocketry predates space travel - by a lot. &lt;a href=&quot;https://en.wikipedia.org/wiki/V-2_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;The V-2 rocket christened &quot;MW 18014&quot;, the very first suborbital flight,&lt;/a&gt; (meaning the rocket flew to space, but didn't go fast enough to orbit the earth, so came back down) was in 1944. This rocket was designed to launch, fly in a specific trajectory, and land in a (relatively) specific location - and it worked. It didn't &quot;do&quot; anything, it had no practical payload. But it worked.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By 1944 our knowledge of physics/astrophysics was fairly advanced - we understood quite well that orbits are just a matter of going &lt;em&gt;really fast&lt;/em&gt; around a celestial body. So, armed with a rocket that can fly into space, and knowledge of how orbits are made, spaceflight was a very natural next step. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It took quite a few tests like the one linked above to understand the engineering behind the first rockets, but for the most part rockets' trajectories, the physics of space, and the nature of vacuums were already known. It was just that these things were mostly known by theoretical scientists - not by engineers. So the first spaceflights were mostly just applying the theoretical understanding we already had of how an object might go from the ground to an orbit around earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted that while I make it sound simple, the &lt;em&gt;actual engineering&lt;/em&gt; that went into making real spacecraft was very, very hard. To more directly answer your question about how humans found out how to deal with the unique challenges of space travel; we iterated for a long time. It took 13 years between MW 18014 and the successful flight of &lt;a href=&quot;https://en.wikipedia.org/wiki/Sputnik_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sputnik 1&lt;/a&gt;, which was the first manmade object to orbit the Earth. 13 years is a long time to iterate on a design - even accounting for the chaotic political effects of the end of WW2 and the diffusion of German knowledge after it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way we learned about how to make a craft that could take a &lt;em&gt;person&lt;/em&gt; into orbit was by actually sending objects up to space; rocket scientists were simply launching more and more satellites into the atmosphere to study the environment. By the time Yuri Gagarin became the &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;first man in space&lt;/a&gt; in 1961 - a good four years after Sputnik - scientists already had a lot of experience sending rockets into orbit and dealing with the crazy effects of space travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt; the way people learned how to deal with space was by shooting rockets into it until they figured out how it worked. Then they shot people into it.&lt;/p&gt;&#xA;" OwnerUserId="4659" LastEditorUserId="4659" LastEditDate="2019-05-16T04:54:55.977" LastActivityDate="2019-05-16T04:54:55.977" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="22596" PostTypeId="1" AcceptedAnswerId="22601" CreationDate="2017-08-10T06:45:55.680" Score="2" ViewCount="250" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/21740/12102&quot;&gt;excellent and under-up-voted answer&lt;/a&gt; describes the calculation of the probabilities of closest approach to Earth of the NEO &lt;a href=&quot;https://en.wikipedia.org/wiki/2012_TC4&quot; rel=&quot;nofollow noreferrer&quot;&gt;2012 TC4&lt;/a&gt; in October 2017. &lt;a href=&quot;https://space.stackexchange.com/questions/21692/#comment63970_21692&quot;&gt;This comment&lt;/a&gt; noted that TC4 has recently been re-acquired (observed) and its orbit and predicted closest approach have been updated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I downloaded the new data in &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; and plotted it below. I also looked at the &lt;a href=&quot;https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2012%20TC4;old=0;orb=0;cov=0;log=0;cad=1#cad&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Small Body Data Browser for 2012 TC4&lt;/a&gt; and I can see that while the predicted nominal distance of closest approach is now a bit closer, the &lt;em&gt;minimum predicted distance&lt;/em&gt; of closest approach is now much closer to the nominal. In other words, with the new, August 2017 observations the uncertainty has been greatly reduced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How was this observation made? Horizons shows that the most recent observational data is from August 5, 2017. On that day 2012 TC4 was about 52 million km from Earth. This is much too far to be radar, so it must be an observation from a visible or infrared telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How was this observation made and what instrument was used? Did it turn up in a routine survey by accident, or was there a specific, directed effort to look for it? I ask that because for this object the uncertainty in the orbit brought it potentially quite close to Earth!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;(km)   nominal   minimum&#xA;Old:   58,300    13,200&#xA;New:   50,100    49,800&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Predicted paths for 2012 TC4 (two red, nearly straight lines), Moon (green, arc segment on opposite side) and the Earth (blue dot in center) plotted centered on the Earth-Moon Barycenter in ecliptic coordinates. Dashed line is the Horizons data back in May 2017, and shows a closest approach of about 90,000 kilometers, so it's even older than the &quot;Old&quot; closest approach data in the Small Body Data Browser. Dots on the new TC4 prediction (solid, red) are spaced every 2 hours.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I set up the Horizons web page like this, downloaded the state vectors for each body (and the Earth-Moon barycenter) and then subtracted each from the barycenter before plotting:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/aKG9T.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/aKG9T.png&quot; alt=&quot;Horizons screenshot&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;OLD (dashed line):&#xA;Rec #:620538 (+COV)   Soln.date: 2017-Jul-14_11:14:21       # obs: 223 (7 days)&#xA;ASTEROID comments: &#xA;1: soln ref.= JPL#26, OCC=5&#xA;2: source=ORB&#xA;&#xA;NEW (solid line):&#xA;Rec #:620541 (+COV)   Soln.date: 2017-Aug-06_07:17:54    # obs: 228 (2012-2017)&#xA;ASTEROID comments: &#xA;1: soln ref.= JPL#27, OCC=1&#xA;2: source=ORB&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lUsND.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lUsND.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lLEJ3.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lLEJ3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-12T18:03:42.093" LastActivityDate="2017-08-12T18:03:42.093" Title="Here it comes! 2012 TC4 has been re-acquired and its orbit revised; at over 50 million km, how was this done?" Tags="|near-earth-asteroid|2012-tc4|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22597" PostTypeId="2" ParentId="22590" CreationDate="2017-08-10T06:53:26.677" Score="17" Body="&lt;p&gt;I think this part of your question wasn't addressed very well so far:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;How did they ever speculate &lt;em&gt;no gravity&lt;/em&gt; and train astronauts according?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Russell Borogove already said a word on the second part so I'm gonna add something to the first part.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actually there &lt;em&gt;is&lt;/em&gt; quite a lot of gravity near Earth. For example gravity on the ISS is still around 0.89g meaning that it accelerates towards earth at 89% of the rate we have down on earth's surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The laws of gravity were known as early as 1687 from Newton's law of universal gravitation and Einstein's famous general theory of relativity gave additional insight in 1915 (See &lt;a href=&quot;https://space.stackexchange.com/questions/22590/how-did-people-know-how-to-build-the-first-space-ship#comment63992_22597&quot;&gt;@vsz comment&lt;/a&gt;). So we knew that as soon as we shoot a rocket to &lt;em&gt;~400km&lt;/em&gt; above ground it would still accelerate downward at &lt;em&gt;~8.7 m/s^2&lt;/em&gt; (increasing with reduced altitude). So in order to stay at constant height (=orbit) we need to account for that somehow. This means we knew it would be zero acceleration towards earth because that is exactly how we need to design the orbit in order to stay in space and not crash into the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem now is: how to cancel out gravity. One explanation is that on observer on earth would see the space ship moving side ways so fast that it literally &lt;em&gt;falls&lt;/em&gt; around earth (and this observer wouldn't say there's no acceleration on the space ship !). For an observer in the space ship we could calculate that (for a circular orbit) a centrifugal force is accelerating us away from earth at the same rate gravity attracts us which means there is an effective &lt;em&gt;0g&lt;/em&gt; acting on us &lt;em&gt;but only in this rotating reference system&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;TL;DR:&lt;/strong&gt; Gravity still pulls us towards earth but we must counteract it in order to not fall down. On a space ship that would not rotate around earth but fire its engine to keep a constant low altitude we would feel almost as heavy as on earth.&lt;/p&gt;&#xA;" OwnerUserId="19531" LastEditorUserId="5862" LastEditDate="2017-08-14T17:31:06.260" LastActivityDate="2017-08-14T17:31:06.260" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22598" PostTypeId="2" ParentId="22199" CreationDate="2017-08-10T07:03:33.810" Score="8" Body="&lt;p&gt;Here is an image on how this can be done. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PBoZ3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PBoZ3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It can be done using a sleeve that covers the central pintle. The sleeve also has a &quot;bump&quot; in it that controls the gap around the annular gap. So the sleeve covers the center flow along with the annular gap. The Apollo Lunar Module Decent Engine did something similar, but cant find a good cross-section view that shows this. It had a throttling capability of 10/1. Not sure if it was face shut-off.The problem you run into is the when using LOX you're dealing with cryogenics. Sealing is tough since you have a metal/metal surface used as a seal. Not sure if this is how they do it on the Merlin engine, but the image was taken from a Tom Mueller talk. I'm pretty sure that each engine has has just one injector since that is what past designs at TRW have done. Two injectors would change the mixing characteristics. Hope this helps.&lt;/p&gt;&#xA;" OwnerUserId="20610" LastEditorUserId="6944" LastEditDate="2017-08-10T12:33:10.343" LastActivityDate="2017-08-10T12:33:10.343" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22600" PostTypeId="1" AcceptedAnswerId="22671" CreationDate="2017-08-10T14:50:47.663" Score="5" ViewCount="1387" Body="&lt;p&gt;I cannot confirm the eccentricity value of a TLE using corresponding position and velocity vectors.&#xA;Let me go through an example, ISS, to explain the situation.&lt;/p&gt;&#xA;&lt;p&gt;Using this TLE for ISS,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;1 25544U 98067A   17198.89938657  .00000988  00000-0  22167-4 0  9998&lt;/p&gt;&#xA;&lt;p&gt;2 25544  51.6416 245.2318 0005849  47.2823 302.7554 15.54170925 66526&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Tle data says eccentricity is 0.0005849. Propagating this tle for 0 minutes to get TEME vectors yield,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;r=-3.468881045420031e+03, -5.752706429352231e+03, -9.339830044476138e+02&lt;/p&gt;&#xA;&lt;p&gt;v= 3.693246416348322 , -3.17757480606644, 5.92375001851741&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Using ordinary orbital mechanics formulations to obtain eccentricity from those vectors finds eccentricity as 0.0016. The error is 168% compared to TLE data.&lt;/p&gt;&#xA;&lt;p&gt;Then, I tried converting to ECI since an inertial system is required to use usual orbital mechanics formulations. Converting TEME to ECI with a lot of assumptions about polar motion and nutation finds,&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;r = -3492.98975522683,    -5739.01656212581,  -928.333286976148&lt;/p&gt;&#xA;&lt;p&gt;v = 3.69073985425235, -3.19228499069521,  5.91739984184932&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;eccentricity is again 0.0016.&lt;/p&gt;&#xA;&lt;p&gt;What is the reason for this big difference in TLE and orbital calculations?&lt;/p&gt;&#xA;&lt;p&gt;(eccentiricy is found as 0.0750 when calculated with ECEF vectors).&lt;/p&gt;&#xA;&lt;p&gt;I also found this website (&lt;a href=&quot;http://www.tle.info/data/ISS_DATA.TXT&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.tle.info/data/ISS_DATA.TXT&lt;/a&gt;), it gives information about ISS orbit. First, it lists some orbital properties for ISS. It says eccentricity is 0.0005425, however, on the same page it also lists keplerian elements and says that eccentricity is 0.0018464. They have the same inconsistency that I'm trying to solve.&lt;/p&gt;&#xA;&lt;p&gt;I'm aware TLE elements and coordinate transformations I used are not very precise but I don't think they would cause this much error.&lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-15T14:03:07.500" Title="Recreating the eccentricity value of TLE" Tags="|two-line-elements|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22601" PostTypeId="2" ParentId="22596" CreationDate="2017-08-10T18:22:21.057" Score="4" Body="&lt;p&gt;As reported on the &lt;a href=&quot;https://groups.yahoo.com/neo/groups/mpml/conversations/messages/32874&quot; rel=&quot;nofollow noreferrer&quot;&gt;Minor Planets Mailing List&lt;/a&gt;, 2012 TC4 was recovered by the European Southern Observatory's &lt;a href=&quot;https://en.wikipedia.org/wiki/Very_Large_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;Very Large Telescope&lt;/a&gt; in Chile. Here's their reported &lt;a href=&quot;http://www.minorplanetcenter.net/mpec/K17/K17P26.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;astrometry data&lt;/a&gt;.  According to the ESO's &lt;a href=&quot;http://www.eso.org/public/images/ann17052b/&quot; rel=&quot;nofollow noreferrer&quot;&gt;announcement&lt;/a&gt;, the observation at apparent magnitude 27 is the faintest near-earth asteroid so far measured.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/omq76.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/omq76.jpg&quot; alt=&quot;Near Earth Asteroid 2012 TC4 appears as a dot at the centre of this composite of 37 individual 50-second exposures obtained with the FORS2 instrument on ESO&amp;#39;s Very Large Telescope.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the above stack of (37) 50-second exposures, 2012 TC4 is the dot at the center.  The stack tracks the asteroid's motion, leaving the background stars etc as trails.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/fors/&quot; rel=&quot;nofollow noreferrer&quot;&gt;FORS 2&lt;/a&gt; (FOcal Reducer and low dispersion Spectrograph) instrument was used.  Although I can't find an explicit statement saying so, it is almost certain that this was a directed search since this observation is close to the limiting magnitude of the instrument, and furthermore FORS2's field of view is only 6.8 x 6.8 arc minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Image credit: ESO / ESA NEOCC / Olivier Hainaut (ESO), Marco Micheli (ESA) and Detlef Koschny (ESA); &lt;a href=&quot;http://creativecommons.org/licenses/by/4.0/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CC-BY&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="12408" LastEditDate="2017-08-10T20:55:55.257" LastActivityDate="2017-08-10T20:55:55.257" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22602" PostTypeId="1" AcceptedAnswerId="22603" CreationDate="2017-08-10T20:00:07.927" Score="3" ViewCount="374" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5y94g.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5y94g.gif&quot; alt=&quot;Electric pump rocket engine&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am trying to build a rocket with liquid fuel and oxidizer that is being pumped to the injectors/combustion chamber using electric motors/pumps. Assuming that the oxidizer and fuel are liquid, wouldn't the tanks need an inert gas to be put back in? I don't believe this diagram shows this. If anyone can explain how this process would take place, or can provide another diagram it would be greatly appreciated. Thank you!&lt;/p&gt;&#xA;" OwnerUserId="20624" LastActivityDate="2017-08-14T02:28:44.933" Title="How is gas put back into the tanks once liquid has begun being pumped out?" Tags="|liquid-fuel|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22603" PostTypeId="2" ParentId="22602" CreationDate="2017-08-10T20:14:00.813" Score="5" Body="&lt;p&gt;There are a few ways to do this: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Using an inert gas (helium is often used). This is carried in a separate high-pressure tank and fed into the oxidizer/fuel tanks as they are depleted. The ox/fuel tanks are kept at a pressure of a few bar to help the pumps draw in the liquids. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Autogenous pressurization: some of the fuel/ox is warmed up (by running it through a hot part of the engine) and evaporated, and fed back into the tank as a gas. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-08-10T20:14:00.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22604" PostTypeId="1" CreationDate="2017-08-10T22:54:42.217" Score="1" ViewCount="882" Body="&lt;p&gt;Apollo 11&lt;/p&gt;&#xA;&lt;p&gt;Landing: 102.75 GET&#xA;EVA&lt;/p&gt;&#xA;&lt;p&gt;Start: 109.00 GET, 14.0 deg.&lt;/p&gt;&#xA;&lt;p&gt;Finis: 111.75 GET, 15.4 deg.&lt;/p&gt;&#xA;&lt;p&gt;According to NASA's website the sun angles during the Apollo 11 mission are the ones stated above. At 15 deg an object of 1 meter would cast a shadow length of 3,7 meters. Yet, the images from the Apollo 11 mission show objects casting a shadow of roughly 2 meters for every 1 meter of height. To achieve the latter, the sun would have to be at 26,5 deg.&lt;/p&gt;&#xA;&lt;p&gt;Why this discrepancy?&lt;/p&gt;&#xA;&lt;p&gt;A fitting Apollo 11 image:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NNEpP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NNEpP.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20678" LastEditorUserId="7062" LastEditDate="2023-12-26T07:19:15.950" LastActivityDate="2023-12-26T07:19:15.950" Title="Length of shadows Apollo 11" Tags="|apollo-11|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22605" PostTypeId="2" ParentId="22604" CreationDate="2017-08-10T23:21:26.383" Score="12" Body="&lt;p&gt;The major factor here is foreshortening. Unless a photo is taken exactly perpendicular to the shadow cast by an upright object, the apparent length of the shadow will be shortened by the perspective. In your example, the photo seems to be taken somewhere around 45-60 degrees off perpendicular. Taking the cosine of that tells you the shadows appear at somewhere between 0.5 and 0.7 times their actual length, which gets you near enough to your expected 3.7:1 stretch factor.&lt;/p&gt;&#xA;&lt;p&gt;Here's a top-view diagram of how that works.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VhrRU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VhrRU.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;A smaller factor is the unevenness of the lunar surface. It's notoriously difficult to estimate the elevation changes from the Apollo photos. Shadows falling on an uphill slope will be shorter than expected.&lt;/p&gt;&#xA;&lt;p&gt;You also reference a near-perpendicular-shadow picture:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iltzG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iltzG.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I am able to nearly reproduce the geometry of the image in a 3D tool using a &lt;a href=&quot;http://www.clavius.org/photlens.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;53.5º field of view&lt;/a&gt; and a 15.3º sun angle. It's not a perfect match, but I don't know the exact dimensions of the box or geometry of the terrain:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fdFsF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fdFsF.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;(Clearly, The Conspiracy has reached &lt;a href=&quot;https://unity3d.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;Unity Technologies&lt;/a&gt;.)&lt;/p&gt;&#xA;&lt;p&gt;Additionally, the debunking site Clavius.org has &lt;a href=&quot;http://www.clavius.org/shad45.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;some nice sets of photos&lt;/a&gt; showing how the same shadow can look very different from different directions. You may enjoy some of the &lt;a href=&quot;http://www.clavius.org/analyze.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;other photo analysis pages&lt;/a&gt; on that site.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="7062" LastEditDate="2023-12-25T07:54:30.210" LastActivityDate="2023-12-25T07:54:30.210" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="22606" PostTypeId="1" CreationDate="2017-08-10T23:28:36.243" Score="3" ViewCount="177" Body="&lt;p&gt;If two people are in a box (no windows)   One is on the earth at 1G gravity and the other one is Accelerating in space with only the acceleration (inertial equal to 1G gravity) is there a way for the man inside the box to tell if he's on the earth or accelerating in space&lt;/p&gt;&#xA;" OwnerUserId="20679" LastActivityDate="2017-08-11T17:09:01.837" Title="Acceleration / inertial / astrophysics" Tags="|orbital|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22608" PostTypeId="1" AcceptedAnswerId="23774" CreationDate="2017-08-11T00:17:54.707" Score="32" ViewCount="3593" Body="&lt;p&gt;It's frequently claimed without citation that &lt;a href=&quot;https://en.wikipedia.org/wiki/Alexey_Leonov&quot; rel=&quot;noreferrer&quot;&gt;Alexey Leonov&lt;/a&gt; carried a poison pill of some kind during his &lt;a href=&quot;https://en.wikipedia.org/wiki/Voskhod_2&quot; rel=&quot;noreferrer&quot;&gt;Voskhod 2&lt;/a&gt; spacewalk mission, to commit suicide should he be unable to re-enter the spacecraft. Examples: &lt;a href=&quot;http://www.realclearscience.com/blog/2013/10/astronauts-and-suicide-pills.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;https://books.google.com/books?id=EJi1BwAAQBAJ&amp;amp;pg=PA138&amp;amp;lpg=PA138&amp;amp;dq=leonov%20cyanide%20pill&amp;amp;source=bl&amp;amp;ots=gpZmGaEBnJ&amp;amp;sig=6cqU-6EhCMrMIVXnYzCV2bhPj0w&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjB3obq5s_VAhXiwFQKHRf4CCcQ6AEIQjAE#v=onepage&amp;amp;q=leonov%20cyanide%20pill&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This seems unlikely: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;As American astronauts such as Jim Lovell have stated, simply depressurizing a suit or spacecraft would be a quicker and more comfortable method of suicide. &lt;/li&gt;&#xA;&lt;li&gt;Any poison pill intended for use in a spacewalk would have to be accessible &lt;em&gt;by mouth alone&lt;/em&gt; inside the helmet, which presents a possibility of accidental ingestion. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I'm taking Lovell at his word that &lt;a href=&quot;https://books.google.com/books?id=iZu8AQAAQBAJ&amp;amp;pg=PA247&amp;amp;lpg=PA247&amp;amp;dq=lovell%20suicide%20pill&amp;amp;source=bl&amp;amp;ots=-PXmh-DqD8&amp;amp;sig=f8e8rOlulBqtomxJiZYNo_JW3y0&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjzxKOS_M3VAhWCqFQKHeyfCmc4ChDoAQguMAI#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;the American program never included poison pills&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Since Apollo 13 many people have asked me, &quot;Did you have suicide pills on board?&quot; We didn't, and I never heard of such a thing in the eleven years I spent as an astronaut and NASA executive. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is no mention of suicide pills in Asif Siddiqi's extremely thorough history of the USSR's early space program (&lt;a href=&quot;https://history.nasa.gov/SP-4408pt1.pdf&quot; rel=&quot;noreferrer&quot;&gt;part 1&lt;/a&gt;, &lt;a href=&quot;https://history.nasa.gov/SP-4408pt2.pdf&quot; rel=&quot;noreferrer&quot;&gt;part 2&lt;/a&gt;). A search of the text for key terms &quot;poison&quot;, &quot;cyanide&quot;, and &quot;suicide&quot; yield references to an automated poison injector for &lt;a href=&quot;https://en.wikipedia.org/wiki/Laika&quot; rel=&quot;noreferrer&quot;&gt;Laika&lt;/a&gt;'s benefit, but nothing related to human cosmonauts. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mary Roach, in &lt;a href=&quot;https://books.google.com/books?id=SUbFAQAAQBAJ&amp;amp;pg=PT39&amp;amp;lpg=PT39&amp;amp;dq=leonov%20cyanide%20pill&amp;amp;source=bl&amp;amp;ots=FzZsL92n3v&amp;amp;sig=C4D5lnzAQT0DYgN4AfKzPSeJbYA&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjB3obq5s_VAhXiwFQKHRf4CCcQ6AEIUTAH#v=onepage&amp;amp;q=leonov%20cyanide%20pill&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Packing For Mars&lt;/a&gt;, casts doubt on it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Space physiology expert Jon Clark told me the [Leonov] suicide pill story is most likely untrue. I had emailed Clark at his office at the National Space Biomedical Research Institute regarding the perplexing logistics of pill-popping in a spacesuit, and he did some asking around. His Russian sources also dismissed another rumour, that Belyayev was under orders to shoot Leonov if he couldn't get back in...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The 1997 document &lt;a href=&quot;https://history.nasa.gov/monograph7.pdf&quot; rel=&quot;noreferrer&quot;&gt;Walking To Olympus&lt;/a&gt; states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Leonov recently revealed that he had a suicide pill he could have swallowed if he had been unable to ingress Voskhod 2 and Belyayev had been forced to leave him in orbit. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I followed up the citations from that article; the most likely source for this statement is “Orbital Castling: Life of Mir Station Will Be Prolonged Three More Years,” &lt;em&gt;Segodnya&lt;/em&gt; in Russian, May 11, 1995, p. 9. Translated in &lt;em&gt;JPRS Report, Central Eurasia: Space&lt;/em&gt;, August 2, 1995 (FBIS-UST-95-030), p. 20, but I can't find the article online. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Is there any credible evidence that any cosmonaut carried poison pills, or for that matter any other device primarily intended for suicide?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-11T19:18:02.430" LastActivityDate="2023-05-16T20:59:38.123" Title="Were cosmonauts equipped with poison pills?" Tags="|astronauts|eva|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22610" PostTypeId="2" ParentId="22606" CreationDate="2017-08-11T00:41:13.723" Score="3" Body="&lt;p&gt;With sufficiently precise instruments* you can tell which box you are in because the Earth-based box gravity vectors will point to the center of the earth as you move about in the box (box on the left) whereas the space box will have parallel acceleration vectors (box on the right)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Chqaj.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Chqaj.png&quot; alt=&quot;&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;* probably not achievable in practice for a reasonably sized box but this is a thought experiment after all.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6816" LastEditDate="2017-08-11T07:23:40.347" LastActivityDate="2017-08-11T07:23:40.347" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22611" PostTypeId="1" CreationDate="2017-08-11T02:32:53.187" Score="7" ViewCount="167" Body="&lt;p&gt;In the history of astronomy we usually come up with the name of European astronomers like Kepler and Galileo. Earlier Indian astronomers, for example &lt;a href=&quot;https://en.wikipedia.org/wiki/Aryabhata&quot; rel=&quot;noreferrer&quot;&gt;Aryabhata&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Bh%C4%81skara_II&quot; rel=&quot;noreferrer&quot;&gt;Bhaskaracharya&lt;/a&gt;, are not usually mentioned although it was them that first proved that Earth moves round the Sun and also identified and named galaxies. Why it is thus?&lt;/p&gt;&#xA;" OwnerUserId="20685" LastEditorUserId="12102" LastEditDate="2017-08-11T03:34:27.650" LastActivityDate="2017-08-11T03:34:27.650" Title="What are the contributions of ancient Indian astronomers in mapping of the solar system and galaxies?" Tags="|planetary-science|history|mathematics|astronomy|" AnswerCount="0" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22614" PostTypeId="2" ParentId="22606" CreationDate="2017-08-11T06:46:38.270" Score="4" Body="&lt;p&gt;@OrganicMarble's &lt;a href=&quot;https://space.stackexchange.com/a/22610/12102&quot;&gt;answer&lt;/a&gt; is correct. An absolutely uniform gravity field would require infinite mass at infinite distance and so realistic gravity fields will usually have local gradients that can be measured with sufficiently sensitive equipment. There might be some pathological situations where higher terms also cancel and gravity appears extremely uniform, but that's best left as &lt;a href=&quot;https://physics.stackexchange.com/q/351361/83380&quot;&gt;another question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Although it is not named explicitly, the question refers to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Equivalence_principle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Equivalence Principle&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In the theory of &lt;a href=&quot;https://en.wikipedia.org/wiki/General_relativity&quot; rel=&quot;nofollow noreferrer&quot;&gt;general relativity&lt;/a&gt;, the equivalence principle is any of several related concepts dealing with the equivalence of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_mass&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational&lt;/a&gt; (mass) and &lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_mass&quot; rel=&quot;nofollow noreferrer&quot;&gt;inertial mass&lt;/a&gt;, and to Albert Einstein's observation that the gravitational &quot;force&quot; as experienced locally while standing on a massive body (such as the Earth) is the same as the [pseudo-force][] experienced by an observer in a non-&lt;a href=&quot;https://en.wikipedia.org/wiki/Inertial_frame_of_reference&quot; rel=&quot;nofollow noreferrer&quot;&gt;inertial&lt;/a&gt; (accelerated) frame of reference.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The problem here is that the Earth is rotating, and so &lt;strong&gt;the Earth-box is not in an inertial frame&lt;/strong&gt; to begin with. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;One way&lt;/em&gt; this would be noticeable would be to look for non-inertial effects such as the precession of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Foucault_pendulum&quot; rel=&quot;nofollow noreferrer&quot;&gt;Foucault pendulum&lt;/a&gt;. However, at the equator that precession goes to zero. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way to notice it would be in the motion of a simple gyroscope. For example, at the equator, a gyroscope initially spinning with a vertical axis would rotate in a vertical plane once a day. If it pointed &quot;up&quot; at noon, it would point &quot;west&quot; about six hours later, and &quot;down&quot; another six hours after that. Of course if you were not on the equator, or the initial direction were not &quot;up&quot;, the motion would be more complex, but still indicate clearly that you were not in an inertial frame, except in the particular situation that you were at the North Pole and the initial direction were &quot;up&quot;, or that it were at some orientation that coincided with the Earth's axis no matter where you were.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to be absolutely sure, you probably should use the gyroscope with several initial directions, or combine the gyroscope with the Foucault pendulum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wq7tH.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wq7tH.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Foucault Pendulum animation, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Foucault-rotz.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gp7Vp.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gp7Vp.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Gyroscope animation, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Gyroscope_wheel_animation.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-11T07:43:41.220" LastActivityDate="2017-08-11T07:43:41.220" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22615" PostTypeId="2" ParentId="22590" CreationDate="2017-08-11T09:54:00.877" Score="4" Body="&lt;p&gt;By using science to make educated guesses, by performing tests on the ground as much as possible, then by attempting actual flights and learning what works and what doesn't, and then improving and taking on more ambitious goals. The other answers have gone into more detail but I want to share the following link which I think is extremely illustrative of the incremental nature of space flight and engineering.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mentallandscape.com/V_Venus.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://mentallandscape.com/V_Venus.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It covers the many Soviet missions to Venus, starting with missions that went as far as the moon, then some more missions getting into interplanetary space, then ones that actually make it near to Venus, then ones that actually hit Venus, then missions which try to make it through the atmosphere, finally cumulating in a landing and a photo from the surface. It is long but an &lt;strong&gt;excellent&lt;/strong&gt; read, and shows how each mission builds on the last, and become increasingly more complicated and ambitious.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastActivityDate="2017-08-11T09:54:00.877" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22616" PostTypeId="2" ParentId="22606" CreationDate="2017-08-11T13:02:14.117" Score="2" Body="&lt;p&gt;Both gravity fields are not totally equal. In the box on Earth, gravity at top is a little smaller than at the bottom. But in the other box gravity is the same at bottom and top.&lt;br&gt;&#xA;There are ultra precise atomic clocks that show a very small difference in frequency when mounted at different heights and there are gravimeters with very high sensitivity. The relative time difference is -1,09 · 10&lt;sup&gt;-16&lt;/sup&gt; per meter of height.&lt;br&gt;&#xA;On Earth the gravity decreases with every meter of increased height by 3.1 µm/s² (&lt;a href=&quot;https://de.wikipedia.org/wiki/Schwerefeld&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://de.wikipedia.org/wiki/Schwerefeld&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_of_Earth#Free_air_correction&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Gravity_of_Earth#Free_air_correction&lt;/a&gt;). The best superconducting gravimeters have a resoultion of 0.1 nm/s² (&lt;a href=&quot;https://www.bkg.bund.de/DE/Observatorium-Wettzell/Messsysteme-Wettzell/Lokale-Messsysteme/Gravimeter/gravimeter_cont.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.bkg.bund.de/DE/Observatorium-Wettzell/Messsysteme-Wettzell/Lokale-Messsysteme/Gravimeter/gravimeter_cont.html&lt;/a&gt;). Such gravimeters are also able to measure the influence of tidal effects of Moon and Sun to the Earth, but only in the box on earth tidal effects will be measureable.&lt;br&gt;&#xA;A modern seismometer will show if there are the typical seismic waves of the Earth or completely different vibrations of a rocket in space.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-08-11T17:09:01.837" LastActivityDate="2017-08-11T17:09:01.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22617" PostTypeId="1" AcceptedAnswerId="31872" CreationDate="2017-08-11T17:23:26.693" Score="7" ViewCount="1886" Body="&lt;p&gt;It seems like four earth-like planets were found near Tau Ceti as per this &lt;a href=&quot;https://news.ucsc.edu/2017/08/tau-ceti-planets.html&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A new study by an international team of astronomers reveals that four Earth-sized planets orbit the nearest sun-like star, tau Ceti, which is about 12 light years away and visible to the naked eye. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Assuming that the planets have a civilization as advanced as ours, let's say an exact analogue (not implying anything, purely hypothetical), could we make our presence known to them? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is this simply to great a distance, no matter how much energy we spent on the communication?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, how would it be done?&lt;/p&gt;&#xA;" OwnerUserId="20690" LastActivityDate="2018-11-07T17:06:41.447" Title="Can we &quot;Communicate&quot; with Tau Ceti?" Tags="|communication|" AnswerCount="5" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22618" PostTypeId="2" ParentId="22617" CreationDate="2017-08-11T17:36:26.123" Score="4" Body="&lt;p&gt;It's too great a distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Look at it this way: the sun to that system &lt;em&gt;barely&lt;/em&gt; emits enough energy to make that planet visible to our technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if that planet wanted to send us data, they would have to send as much energy as gets reflected by that planet in our direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you said &quot;as advanced as ours&quot;: We're not capable of producing that much power. Not even remotely. So they're not capable of doing that, either. A star is &lt;em&gt;amazingly&lt;/em&gt; powerful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's look at this way. The estimated power of the sun is something like 10&lt;sup&gt;26&lt;/sup&gt; W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Humanity's electrical production is something like 10&lt;sup&gt;12&lt;/sup&gt; W.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, one sun is roughly 100 trillion earth electricity productions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heck, we don't freaking matter on a stellar scale.&lt;/p&gt;&#xA;" OwnerDisplayName="user17550" LastEditorDisplayName="user17550" LastEditDate="2017-08-11T20:58:37.227" LastActivityDate="2017-08-11T20:58:37.227" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="22619" PostTypeId="2" ParentId="22617" CreationDate="2017-08-11T17:57:24.287" Score="7" Body="&lt;p&gt;Could we make our presence known to them? We have definitely tried our hardest. In the 1950s and 60s, we broadcast TV and radio directly into space in the hope that one day the signals will reach a civilization capable of receiving them. You describe such a situation, but alas it is more complicated than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main problem with any civilization receiving signals is that those signals will be incredibly weak. Think about it this way: our sun is an incredibly powerful signal emitter, and there are millions of other stars just as bright and far, far more luminous. But they are also far, far away, just like Tau Ceti. Take &lt;a href=&quot;https://en.wikipedia.org/wiki/Sirius&quot; rel=&quot;noreferrer&quot;&gt;Sirius A&lt;/a&gt;, the brightest star in the night sky. It is only 8.6 lyr away and a far more luminous star than our sun, and yet Jupiter outshines it by about double. So you can imagine that the signals that have reached Tau Ceti are incredibly weak. Given our current technology, it is hard to say whether they would have detected our broadcasts or no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we knew to broadcast in that specific direction, it is conceivable* that communication would be possible. But the roundtrip time of 24 years is not exactly productive for conversation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;*Just some numbers: Lets say we broadcast a megawatt (gonna fry birds) of radio waves from a dish 100 meters in diameter. This gives a signal strength at Tau Ceti of around $10^{-23}\mathrm{W/m^2}$. I would venture to guess that even our most sensitive equipment would struggle to pick up this signal.&lt;/p&gt;&#xA;" OwnerUserId="20309" LastActivityDate="2017-08-11T17:57:24.287" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22620" PostTypeId="1" AcceptedAnswerId="22629" CreationDate="2017-08-11T19:18:08.307" Score="2" ViewCount="547" Body="&lt;p&gt;After reading an &lt;a href=&quot;http://www.world-nuclear-news.org/ON-NASA-boosts-nuclear-thermal-propulsion-with-BWXT-contract-04081701.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;announcement&lt;/a&gt; about NASA restarting NTR (nuclear thermal rocket) research I was wondering about how a working fluid would be chosen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For clarity, the working fluid is the gas that is heated by the nuclear reactor, and the radioactive material in the reactor is the fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hydrogen is the fluid considered in most articles on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_thermal_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;topic&lt;/a&gt;. This is because of the (often discussed on this &lt;a href=&quot;https://space.stackexchange.com/questions/22148/liquid-fuel-oxygen-proportions/22149#22149&quot;&gt;site&lt;/a&gt;) advantage of having lightweight high speed combustion products accelerated out of the rocket nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the efficiencies be for other gasses given the same total energy input?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am specifically curious about comparing H2, argon, N2, and O2.&lt;/p&gt;&#xA;" OwnerUserId="16322" LastEditorUserId="16322" LastEditDate="2017-08-12T12:35:56.477" LastActivityDate="2017-08-12T20:31:05.550" Title="Nuclear thermal rocket working fluid performance" Tags="|fuel|nuclear|ntr|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22622" PostTypeId="2" ParentId="22256" CreationDate="2017-08-11T22:34:34.773" Score="3" Body="&lt;p&gt;I talked with someone over ten years ago at an ISDC about a project that he was working on for NASA to solve this.  It involved a vibration plate that one strapped themselves to that would cause micro fractures in the bones.  They had done Earthbound tests and it was safe and it was effective (on Earth) in increasing bone density (I don't know/remember how much).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I don't know if it ever flew.  The problem that he was working on was how to isolate the vibrations from the rest of the station.  Apparently, vibrations travel very well through the station and can cause all kinds of havoc with instruments and experiments.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Quite frankly, I don't think that I'd like having a vibration running through the station.  Aside from it likely producing an audible buzz in places, it might find a harmonic in something important.&lt;/p&gt;&#xA;" OwnerUserId="18842" LastActivityDate="2017-08-11T22:34:34.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22623" PostTypeId="2" ParentId="22617" CreationDate="2017-08-11T23:05:01.573" Score="10" Body="&lt;p&gt;Yes, such communication is unequivocally possible given our current technology!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Michael Busch (from the SETI institute) has calculated that an Arecibo-class receiver could detect TV carrier wave leakage at a range of about 16 light years, slightly further than the distance to Tau Ceti, assuming favorable geometry for the receiving antenna.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's for &lt;em&gt;leakage&lt;/em&gt; -- not an intentional directed signal, but RF energy that happens to go over the horizon from routine television broadcasts.  The situation would be &lt;em&gt;much&lt;/em&gt; more favorable for a deliberately beamed signal using the full power of an Arecibo-class transmitter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that you don't have to come close to matching the total energy output of the Sun to still be detectable.  The Sun's energy, being close to a blackbody spectrum, is spread over an enormous range of wavelengths from radio, through infrared and visible, to X-ray and beyond, while the energy of a directed radio or optical transmission would be extremely tightly concentrated around a single frequency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://www.quora.com/Could-Earth-pick-up-signals-from-a-hypothetical-clone-of-Earth-12-lightyears-away/answer/Michael-Busch-4&quot; rel=&quot;noreferrer&quot;&gt;Quora&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17791" LastEditorUserId="17791" LastEditDate="2017-08-11T23:15:40.373" LastActivityDate="2017-08-11T23:15:40.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22625" PostTypeId="1" AcceptedAnswerId="22628" CreationDate="2017-08-12T10:17:23.437" Score="6" ViewCount="1822" Body="&lt;p&gt;I would like to know the operating points of the fuel-turbo-pumps used in the Saturn rocket engine. (From what I could learn via google, the turbo pump went under the name Mark III) &#xA;Is there any information on the pressure ratio of these pumps?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Xj6Gt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xj6Gt.png&quot; alt=&quot;markIII&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11838" LastEditorUserId="6944" LastEditDate="2017-08-12T12:26:41.270" LastActivityDate="2021-02-14T00:57:33.407" Title="At what pressure ratio did the fuel turbopumps of the F1 and J2 operate?" Tags="|rockets|engine-design|engines|turbomachinery|turbopump|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22626" PostTypeId="1" CreationDate="2017-08-12T10:32:43.620" Score="2" ViewCount="207" Body="&lt;p&gt;If you were to build your own satellite (say, like a CubeSat), is it required to build it in sterile conditions? The satellite would never reach another planet/the Moon so I wouldn't have thought that contamination is an issue.&lt;/p&gt;&#xA;" OwnerUserId="5880" LastActivityDate="2017-08-12T11:23:06.233" Title="Do you have to build your own satellite in sterile conditions?" Tags="|artificial-satellite|contamination|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22627" PostTypeId="2" ParentId="22626" CreationDate="2017-08-12T11:23:06.233" Score="5" Body="&lt;p&gt;No, sterile conditions aren't necessary for most spacecraft. Satellites are usually built in a cleanroom to prevent dust and dirt getting in and causing problems like electrical shorts, dirt in moving parts causing extra wear, and contamination of optical systems.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-08-12T11:23:06.233" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22628" PostTypeId="2" ParentId="22625" CreationDate="2017-08-12T12:24:59.593" Score="4" Body="&lt;p&gt;According to the monograph &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750012398.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA SP-8107 Turbopump Systems for Liquid Rocket Engines&lt;/a&gt; (table II) :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The F1 fuel (RP-1) pump had an inlet pressure of 45 psi and an outlet pressure of 1856 psi.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The J2 fuel (Hydrogen) pump had an inlet pressure of 30 psi and an outlet pressure of 1238 psi.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-08-12T18:28:57.827" LastActivityDate="2017-08-12T18:28:57.827" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22629" PostTypeId="2" ParentId="22620" CreationDate="2017-08-12T14:55:15.673" Score="4" Body="&lt;p&gt;The relation is quite straightforward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The energy injected into a unit of propellant before it evaporates and spent on evaporating it is minuscule comparing to energy applied to the propellant as gas; and for gas of similar pressure and temperature the number of particles per unit of volume varies very little. As result, approximation that assumes each particle of propellant, no matter what propellant it is, receives about the same amount of energy from the reactor core, is quite close to accurate. So let's take the equations for kinetic energy and specific impulse as function of exhaust velocity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$E = {1 \over 2} m v^2  \\ &#xA;I_{sp} = {v \over g_0} \\&#xA;v = \sqrt{ 2E \over m } \\ &#xA;I_{sp} = \sqrt{ 2E \over m {g_0}^2}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$g_0$ is a constant. We assume each particle receives about the same amount of energy. Therefore, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ I_{sp} ∝ m^{-1/2}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $m$ is the mass of a particle of the propellant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If $ I_{sp}$ for monoatomic hydrogen (not really doable, but just to establish baseline) was 1 (of some unit, it's not quite important what), then:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;One $H_2$ particle weighs about 2u, $\sqrt{1/2} = 0.7$  &lt;/li&gt;&#xA;&lt;li&gt;A helium atom is about 4u, $\sqrt{1/4} = 0.5$&lt;/li&gt;&#xA;&lt;li&gt;A lithium atom is about 7u, $\sqrt{1/4} = 0.37$&lt;/li&gt;&#xA;&lt;li&gt;One $H_2O$ particle is about 18u, $\sqrt{1/18} = 0.23$&lt;/li&gt;&#xA;&lt;li&gt;Zinc was proposed in one of questions on this site. One $Zn$ atom is about 65u, $\sqrt{1/65} = 0.12$&lt;/li&gt;&#xA;&lt;li&gt;If for some perverse reason you wanted to use xenon, 131u, $\sqrt{1/131} = 0.08$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As you can see, hydrogen leaves the competition in the dirt. Helium is &lt;em&gt;somewhat&lt;/em&gt; comparable, but there are no big advantages using it over hydrogen, and for any hydrogen-rich compounds (say, methane), the savings are mostly lost in the mass of the binding atom ruining the total performance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: adding other gases you're asking about: $Ar$: 0.16; $N_2$: 0.19; $O_2$: 0.18. Hydrogen is more than three times better than the best of them (Nitrogen). &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-08-12T20:31:05.550" LastActivityDate="2017-08-12T20:31:05.550" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22630" PostTypeId="1" CreationDate="2017-08-12T16:14:18.107" Score="8" ViewCount="8600" Body="&lt;p&gt;From the Wikipedia article, &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_artificial_objects_leaving_the_Solar_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;List of artificial objects leaving the Solar System&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Although other probes were launched first, Voyager 1 was able to achieve a higher speed and has overtaken all others. Voyager 1 overtook Voyager 2 a few months after launch, on 19 December 1977. It overtook Pioneer 11 some time in the late 1980s, and then Pioneer 10—becoming the probe farthest from Earth—on February 17, 1998.&lt;/p&gt;&#xA;&lt;p&gt;Voyager 2 is also moving faster than the probes launched before it, and thus overtook Pioneer 11 in 1997 and will overtake Pioneer 10 sometime in 2021.&lt;/p&gt;&#xA;&lt;p&gt;New Horizons will also likely pass the Pioneer probes, but will need many years to do so. It will not overtake Pioneer 11 until ~2082 and will not overtake Pioneer 10 until ~2130. Barring some unforeseen gravity assist it will never overtake the Voyagers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;My query is why Voyager 1 is faster than all the other probes? Even New Horizons would never be able to overtake it, even with its more modern technology.&lt;/p&gt;&#xA;" OwnerUserId="20699" LastEditorUserId="7062" LastEditDate="2023-07-13T21:25:25.310" LastActivityDate="2023-07-13T21:25:25.310" Title="Why is Voyager 1 faster than all other space probes?" Tags="|spacecraft|propulsion|voyager|" AnswerCount="3" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="22632" PostTypeId="2" ParentId="22630" CreationDate="2017-08-12T21:35:41.150" Score="9" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why Voyager 1 is faster than everybody else ?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Five vehicles have been sent on a solar system escape trajectory: Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizons. These five missions were launched on different dates, had different objectives, and hence had trajectories and different levels of gravity assist. One of the five was bound to be moving the fastest. This turned out to be Voyager 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;None of these five vehicles was sent on a trajectory with the goal of making it the fastest. Each of the escaping vehicles instead had mission goals. The mission goals of Voyager 1 (flybys of Jupiter, Saturn, and Titan) happened to make it the vehicle of those five with the greatest hyperbolic excess speed.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-08-12T21:35:41.150" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22633" PostTypeId="1" AcceptedAnswerId="22638" CreationDate="2017-08-12T22:12:29.043" Score="0" ViewCount="56" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/22601/12102&quot;&gt;nifty answer&lt;/a&gt; describes and provides links to the data of the recovery of near earth object 2012 TC4, and the answer also shows the first actual image of the body for this pass. At apparent magnitude +27 (at about 52 million km) it may be the faintest asteroid to have been imaged to date. It's worth visiting that answer first before reading on here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/22596/#comment64062_22601&quot;&gt;This comment&lt;/a&gt; points out that 2012 TC4 appears on the &lt;a href=&quot;https://echo.jpl.nasa.gov/asteroids/goldstone_asteroid_schedule.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Goldstone Asteroid Schedule&lt;/a&gt;, a list of upcoming planned observations of asteroids by antennas at the &lt;a href=&quot;https://deepspace.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt;. Here are the entries for 2012 TC4 in the &lt;code&gt;Upcoming Observations&lt;/code&gt; and &lt;code&gt;Past Observations&lt;/code&gt; tables:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Upcoming Goldstone Observations&#xA;DATES           TARGET    H     Notes:&#xA;2017 Oct  9-14  2012 TC4  26.7  Close approach = 7.9 Earth radii on Oct. 12. NHATS&#xA;&#xA;Past Goldstone Observations&#xA;DATES           TARGET          Notes:&#xA;2012 Oct        2012 TC4        Echoes not detected. Likely equipment issues.&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I noticed that the 2012 TC4 observation is labeled as a NHATS, or &lt;a href=&quot;https://cneos.jpl.nasa.gov/nhats/intro.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Near-Earth Object Human Space Flight Accessible Targets Study&lt;/a&gt;. I tried to find it in &lt;a href=&quot;https://cneos.jpl.nasa.gov/nhats/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the DATA TABLE of NHATS objects&lt;/a&gt; but I don't find it there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is one of these more up-to-date than the other? Is 2012 TC4 part of the Near-Earth Object Human Space Flight Accessible Targets Study? &lt;em&gt;What does it really mean&lt;/em&gt; if 2012 TC4 is or is not part of NHATS anyway?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-12T22:49:49.680" LastActivityDate="2017-08-13T12:57:48.640" Title="Is 2012 TC4 part of the Near-Earth Object Human Space Flight Accessible Targets Study?" Tags="|radar|radio-telescope|near-earth-asteroid|delay-doppler|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22635" PostTypeId="2" ParentId="9128" CreationDate="2017-08-13T08:09:23.000" Score="2" Body="&lt;p&gt;It's about 140 degrees, varying a little as the ISS altitude varies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get an idea of what that 140 should look like, your fist held out at arms length occupies about 10 degrees. So, look to your left, and measure two fists in. And the same on the right. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this exceeds the available FoV of all VR headsets at the time of writing, apart from the HTC Vive which does 145 on the widest setting, but the edges look blurry due to the fresnel lenses. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also exceeds the eyes natural vertical FOV, of about 135 degrees (and 120 horizontal binocular vision).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you won't see the full Earth disk in VR, and you likely &lt;strong&gt;won't see it if you were in space using your real eyes either&lt;/strong&gt;, unlike what the above &quot;answer&quot; claims. You will have to move your head or eyes slightly from side to side and up and down to see the full disk. And that's if the spacesuit helmet didn't get in the way (a quick google didn't show any numbers, but just by looking at a few pictures it looks nearly 180)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was a bit surprised when I calculated how it looks from geosynchronous orbit. The Earth only occupies about 20 degrees of the sky. That's pretty high altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For VR rendering you will not need to render a curved planetary surface. Unreal Engine will by default switch from stereo to mono rendering for objects further than 750m (i think). The parallax visible at that range starts to be too small to bother about, so everything further than that is rendered in mono, and could just as easily be projected onto a flat skybox, or the inside surface of a giant sphere. Obviously lighting, texturing, atmospheric effects and so on contribute to a curved look, but the curved planetary surface itself doesn't need to be explicitly rendered as one in VR. Just as a full moon looks flat to us on Earth, the difference between the distance from the astronaut to the center of Earth and the distance from the astronaut to the edge is indistinguishable.&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorUserId="16727" LastEditDate="2017-08-13T08:23:45.277" LastActivityDate="2017-08-13T08:23:45.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22636" PostTypeId="5" CreationDate="2017-08-13T08:36:16.680" Score="0" Body="&lt;p&gt;Questions about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transiting_Exoplanet_Survey_Satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transiting Exoplanet Survey Satellite&lt;/a&gt; (&lt;a href=&quot;https://heasarc.gsfc.nasa.gov/docs/tess/&quot; rel=&quot;nofollow noreferrer&quot;&gt;TESS&lt;/a&gt;) - NASA's space telescope, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the Kepler mission.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="33864" LastEditDate="2020-12-17T10:05:32.380" LastActivityDate="2020-12-17T10:05:32.380" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22637" PostTypeId="4" CreationDate="2017-08-13T08:36:16.680" Score="0" Body="Questions about the Transiting Exoplanet Survey Satellite (TESS)" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-13T08:36:16.680" LastActivityDate="2017-08-13T08:36:16.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22638" PostTypeId="2" ParentId="22633" CreationDate="2017-08-13T12:11:48.833" Score="1" Body="&lt;p&gt;It is an NHATS object. I found it on there but first I had to select 'Use Unconstrained Settings'. The default view restricts on some parameters.&#xA;DeltaV is over 10 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wk11K.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wk11K.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On question over the Goldstone planning info, H=26.7 is the absolute magnitude of the object. This relates to the size and reflectivity (albedo) of the object itself and is not specific to the 2017 apparition. Suggests size is in the range of 9 to 30 meters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The table can be found at &lt;a href=&quot;http://www.minorplanetcenter.net/iau/lists/Sizes.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.minorplanetcenter.net/iau/lists/Sizes.html&lt;/a&gt; and a subset of the table is shown below. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The line bracket by arrows applies to H=26.5. There are three columns of diameters for three assumed albedos of 0.5, 0.25 and 0.05, and the units are km for H(a), meters for H(b) which applies here, and millimeters for H(c). This is because fifteen magnitudes represents a factor of $10^6$ in brightness or area, which would be a factor of $10^3$ in diameter.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;                  Albedos              &#xA;    H(a)   0.50    0.25    0.05    H(b)  H(c)&#xA;&#xA;   -2.0    4700 -  6700 - 14900    13.0  28.0&#xA;&#xA;    0.0    1900 -  2600 -  5900    15.0  30.0&#xA;&#xA;    5.0     190 -   260 -   590    20.0  35.0&#xA;&#xA;    0.0      19 -    25 -    60    25.0  40.0&#xA;   10.5      15 -    20 -    50    25.5  40.5&#xA;   11.0      12 -    17 -    37    26.0  41.0&#xA;-&amp;gt; 11.5       9 -    13 -    30    26.5  41.5 &amp;lt;- this line&#xA;   12.0       7 -    11 -    24    27.0  42.0&#xA;   12.5       6 -     8 -    19    27.5  42.5&#xA;   13.0       5 -     7 -    15    28.0  43.0&#xA;   13.5       4 -     5 -    12    28.5  43.5&#xA;   14.0       3 -     4 -     9    29.0  44.0&#xA;   14.5       2 -     3 -     7    29.5  44.5&#xA;   15.0       2 -     3 -     6    30.0  45.0&#xA;&#xA;   17.5       1 -     1 -     2    32.5  47.5&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="20706" LastEditorUserId="12102" LastEditDate="2017-08-13T12:57:48.640" LastActivityDate="2017-08-13T12:57:48.640" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22639" PostTypeId="2" ParentId="22630" CreationDate="2017-08-13T13:18:37.353" Score="15" Body="&lt;p&gt;I can add one interesting bit about why Voyager 1 is going faster than Voyager 2. Voyager 1 had assists from Jupiter and Saturn, but Voyager 2 had assists from those &lt;em&gt;and&lt;/em&gt; Uranus and Neptune. So you might think that Voyager 2 should have been accelerated more by the assists, and so should be going faster. Yet it's not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason is that in order to get a close flyby of Neptune's moon Triton, Voyager 2 had to fly by the leading side of Neptune, which &lt;em&gt;reduced&lt;/em&gt; its heliocentric energy instead of increasing it. After its Uranus flyby, Voyager 2 had a higher heliocentric energy than Voyager 1. Had they left it at that, Voyager 2 would now be going faster than Voyager 1. But Neptune slowed it down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are plots of the heliocentric specific energy of the Voyagers 1 and 2:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ARqRr.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ARqRr.png&quot; alt=&quot;Voyager 1&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/1NK66.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1NK66.png&quot; alt=&quot;Voyager 2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-08-13T13:59:10.113" LastActivityDate="2017-08-13T13:59:10.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22640" PostTypeId="1" AcceptedAnswerId="22641" CreationDate="2017-08-13T13:46:31.830" Score="10" ViewCount="6217" Body="&lt;p&gt;The era of the space shuttle program was one of the highlights in the history of space exploration. Why did NASA decide to retire the space shuttles, or why aren't they planning to start a new program with a more reliable and efficient design like the current Dream Chaser spacecraft? Why is it that NASA relies on commercial firms to complete their tasks?&lt;/p&gt;&#xA;" OwnerUserId="20707" LastEditorUserId="63" LastEditDate="2017-09-06T05:34:44.673" LastActivityDate="2017-09-06T05:34:44.673" Title="Why did NASA close the space shuttle program?" Tags="|nasa|space-shuttle|commercial|" AnswerCount="2" CommentCount="1" ClosedDate="2017-08-13T21:02:52.210" ContentLicense="CC BY-SA 3.0" />
  <row Id="22641" PostTypeId="2" ParentId="22640" CreationDate="2017-08-13T13:58:58.910" Score="27" Body="&lt;p&gt;This could be a book, but here's the tl;dr version:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1) The space shuttle system never lived up to its cost and performance goals. (See &lt;a href=&quot;https://space.stackexchange.com/questions/10350/why-didnt-nasa-use-the-shuttle-to-make-a-profit/10352#10352&quot;&gt;this question&lt;/a&gt; for background info.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2) NASA shuttle management became complacent to the point that they ignored hazardous foam shedding from the external tank. This resulted in the loss of the orbiter Columbia and its crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3) The accident investigation board recommended either shutting down the shuttle program or re-certifying the system.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Recertification&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;R9.2-1 Prior  to  operating  the  Shuttle  beyond &#xA;  2010,  develop and conduct a vehicle recertification at  the material,&#xA;  component, subsystem, and system  levels.  Recertification &#xA;  requirements  should  be  included in the Service Life Extension&#xA;  Program.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;4) The 2nd Bush administration did not want to spend the money to re-certify the system (given reason #1), so they directed NASA management to make only critical safety fixes, shut down the logistic chain, and fly the shuttle only long enough to complete the International Space Station. At the last minute, a contentious decision was made to also include a final Hubble Space Telescope servicing mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;5) The Obama administration did not reverse these decisions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source; &lt;a href=&quot;https://www.nasa.gov/columbia/home/CAIB_Vol1.html&quot; rel=&quot;noreferrer&quot;&gt;Report of Columbia Accident Investigation Board, Volume I&lt;/a&gt; (it is a book)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the why didn't NASA develop a better replacement shuttle part of your question, billions were spent on this to no avail (see &lt;a href=&quot;https://www.nasaspaceflight.com/2006/01/x-33venturestar-what-really-happened/&quot; rel=&quot;noreferrer&quot;&gt;X-33/VentureStar – What really happened&lt;/a&gt;, which could be another book). And while it is possible that Dream Chaser may, if it ever flies, be &quot;more reliable and efficient&quot;, it will be far less capable than the STS orbiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also see &lt;a href=&quot;https://space.stackexchange.com/questions/12156/technology-how-would-a-state-of-the-art-space-shuttle-be-built/12157#12157&quot;&gt;this question&lt;/a&gt; for some ideas about a follow-on shuttle.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-09-06T01:30:06.133" LastActivityDate="2017-09-06T01:30:06.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22642" PostTypeId="2" ParentId="22640" CreationDate="2017-08-13T14:39:45.173" Score="24" Body="&lt;p&gt;The vision which initiated the shuttle program was economic, routine, schedulable access to space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All other space launch systems involved single-use essentially hand-crafted vehicles making each launch very expensive. Aside from SpaceX, this is still largely true today. The Shuttle was intended to change all that by being highly re-usable. It was also envisioned to be capable of quick turnaround so that a small fleet of say 5 or 6 vehicles could be operated on a weekly launch schedule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reality turned out to be very different.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A key feature of the shuttle - the &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system&quot; rel=&quot;noreferrer&quot;&gt;heat resistant tiles&lt;/a&gt; - were critical to its re-usability. Unlike &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmospheric_entry#Ablative&quot; rel=&quot;noreferrer&quot;&gt;ablative heat shields&lt;/a&gt; which work by slowly vaporizing, the shuttle's heat tiles are &quot;durable&quot;; they don't sacrifice material during re-entry. The problem is that they are difficult and expensive to make, and somewhat fragile; any damage can compromise their thermal protection ability. They tended to suffer some degree of damage during every launch. Among other things, inspection and repair of the heat tiles drove up the cost and cycle time to prepare each shuttle for its next launch. Damage repair aside, the tiles needed a waterproofing treatment which re-entry conditions removed, requiring careful re-application for each launch - yet another deviation from the original &quot;quick turn-around&quot; concept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another &quot;signature&quot; feature of the shuttle was the SRBs. They weren't part of the initial concept, but were incorporated into its design in order to make the whole thing work - to provide enough thrust during the initial boost phase while adding a minimum of weight. SRBs are typically expendable; the shuttle's are recovered and re-used, but it's a costly process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shuttle also has inherent safety issues. A big one is the absence of an &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_abort_modes&quot; rel=&quot;noreferrer&quot;&gt;abort mode&lt;/a&gt; (crew escape) between SRB ignition and separation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two loss-of-vehicle events in the shuttle's career underscore these issues. Challenger revealed issues with the SRBs and lack of an escape option during SRB burn. Columbia showed how vulnerable the TPS was to damage during launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite all its amazing qualities, the shuttle proved to be too expensive in relation to other launch systems and involved compromises in crew safety which don't exist in other launch systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Dream Chaser to pick up where the Shuttle left off, it would have to prove to be a meaningful step in the direction of the Shuttle's original concept goals - greater/cheaper/more reliable access to space. At the very least, it would have to be immune to the problems unique to a re-useable spacecraft which affected the Shuttle. Not having cryo-propellant tankage higher in the stack than your exposed re-entry shielding helps; it avoids the risk of damage from material (ice or insulation) which could be shed during ascent, both improving safety and mitigating wear and tear. Placing it atop the man-rated (and expendable) Atlas booster may avoid some of the other safety concerns associated with the shuttle, but won't be a game changer where cost is concerned.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-08-13T20:43:55.790" LastActivityDate="2017-08-13T20:43:55.790" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22643" PostTypeId="1" AcceptedAnswerId="22646" CreationDate="2017-08-13T16:27:15.857" Score="1" ViewCount="140" Body="&lt;p&gt;In the beginning of space exploration, there was sort of a Space Race between the United States and Soviets in order to achieve something bigger to dominate over one another. But after a matter of time, both the countries decided to end the competition and co-operate together for better results and the most magnificent example is the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since, nearly all the space agencies are setting up their slingshot for Mars, I can feel a similar upcoming race or competition in the Martian Exploration e.g. Elon Musk. Are there any possible chances that all the organisations will again co-operate together to add better results in the Martian Exploration, will it be economically and scientifically better to do so?&lt;/p&gt;&#xA;" OwnerUserId="20707" LastActivityDate="2017-08-23T03:30:57.723" Title="Will the countries ever co-operate again for the next upcoming era of Martian exploration?" Tags="|mars|nasa|roscosmos|space-agencies|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ClosedDate="2017-08-14T07:35:39.960" ContentLicense="CC BY-SA 3.0" />
  <row Id="22644" PostTypeId="2" ParentId="22630" CreationDate="2017-08-13T19:02:23.937" Score="2" Body="&lt;p&gt;There are two factors at play here. The dominant factor is the gravity assists, see Mark Adler's answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other factor is the &lt;a href=&quot;http://www.daviddarling.info/encyclopedia/F/fastest_spacecraft.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;launch speed&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;New Horizons was launched with a heliocentric speed of 16 km/s. It had one gravity assist, at Jupiter. That would have added in the region of 10-12 km/s to its speed. &lt;/li&gt;&#xA;&lt;li&gt;the Pioneers were launched at 14.47 km/s. Had gravity assists at Jupiter, not sure about any speed gained at Saturn.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I haven't found launch speeds for the Voyagers. Voyager 1 had 2 gravity assists, Voyager 2 had 4 assists, but one of those reduced Voyager's speed to below that of Voyager 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speed graph after launch looks like this. Speed drops off as the craft gets further away from the Sun (=has to climb out of the Sun's gravitational field). Lots of speed can be added by gravity assists, if you approach the planet the right way round (pass behind the planet). If you pass in front of the planet, you're slowed down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NLM2W.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NLM2W.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-08-13T19:02:23.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22646" PostTypeId="2" ParentId="22643" CreationDate="2017-08-13T21:51:17.640" Score="4" Body="&lt;p&gt;Most likely the countries will eventually cooperate when it comes to Mars, &lt;em&gt;after the Mars race&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Plans to go to Mars.&lt;/strong&gt;&#xA;The details of when and where are not clear. China wants to go to Mars by the end of &lt;em&gt;2020&lt;/em&gt;. Whilst Obama stated that the United States will go to Mars in the 2030s. As for private companies, SpaceX wants to go there and are making the most headway. Blue Origin wants to put a human on both the Moon and Mars. &lt;em&gt;It's just a more passive space race.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The big competitors.&lt;/strong&gt; For right now, the main competitors are &lt;em&gt;NASA&lt;/em&gt;, &lt;em&gt;SpaceX&lt;/em&gt; and &lt;em&gt;Blue Origin&lt;/em&gt;. In the 1960s when the USSR and the USA were competing, it was between nation-states. Many experts say that the Mars race is going to be between private companies and NASA. &lt;em&gt;Considering the private companies need money from contracts, they're competing, not joining forces.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Who's further?&lt;/strong&gt; This is debatable. &lt;em&gt;In my opinion,&lt;/em&gt; NASA is ahead. NASA has manned flight (and manned landings) experience from Gemini, Mercury, and of course, Apollo. NASA also still has the powerful rockets, with the Saturn V rocket and the SLS Block 1. NASA also has also traversed to Mars and planted rovers on the surface.&lt;/p&gt;&#xA;" OwnerUserId="20596" LastEditorUserId="6816" LastEditDate="2017-08-23T03:30:57.723" LastActivityDate="2017-08-23T03:30:57.723" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22647" PostTypeId="2" ParentId="22602" CreationDate="2017-08-14T02:28:44.933" Score="1" Body="&lt;p&gt;Before a rocket takes off you can see gas coming off the side. This is excess fuel and oxidiser from the tank which has evaporated and must be let out to ensure stable pressure. If you balance it right, you can let out just enough gas whilst still keeping pressure in the tanks. While use of something like a helium pressurisation system may work, you could get away with using the natural fuel evaporation to provide pressure. &lt;/p&gt;&#xA;" OwnerUserId="20529" LastActivityDate="2017-08-14T02:28:44.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22650" PostTypeId="2" ParentId="14744" CreationDate="2017-08-14T02:47:12.433" Score="1" Body="&lt;p&gt;It all depends on the rocket, the ultimate goal is to essentially get as much sideways velocity as possible, as fast as possible. The time and rate at which the rocket turns will depend on the design (stages and size mainly due to the fact it has to avoid heating up too much trying to leave the atmosphere at high speeds). In most cases it will tend to get just enough velocity upwards to stay away from the ground while it gains horizontal velocity and that's why rockets tend to turn quite early.&lt;/p&gt;&#xA;" OwnerUserId="20529" LastActivityDate="2017-08-14T02:47:12.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22651" PostTypeId="2" ParentId="14744" CreationDate="2017-08-14T04:26:20.287" Score="5" Body="&lt;p&gt;Yet another question where the answer becomes obvious if you play Kerbal Space Program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is going on here is gravity loss.  As you climb into the sky on your tail of fire some of your energy is going to simply holding your rocket in the sky.  While there is no way to get around this on a body with an atmosphere it's energy expended for no progress, you want to minimize it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, consider what orbit is:  For a circular orbit you are moving fast enough that your path climbs as fast as gravity pulls you down.  (If your orbit is elliptical you keep trading back and forth between speed and height but it averages out the same.)  This does not magically switch on as you reach orbital velocity, any horizontal velocity produces some effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, for the purposes of minimizing gravity loss you want to get as close to horizontal as you can (consistent with not falling down) as soon as you can.  Doing this too soon on a body with an atmosphere will cause too much loss due to atmospheric drag, though.  In practice you pick a path that minimizes the sum of gravity loss and drag loss.  This ends up being a path that goes almost horizontal while still in the outer edges of the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(The reason I say it can be avoided on airless bodies is that if you build a big enough base you can launch your spacecraft with a maglev train.  It's riding on rails as it builds velocity, thus you are not expending any energy countering gravity.  With a properly designed track you can boost to your desired velocity without even lighting your rocket.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not know how big an effect it has on Earth.  I have played with the numbers in Kerbal Space Program, though, and I've seen going up almost straight costing about 10% more velocity (and remember that the rocket equation is exponential--that's a lot more than 10% more fuel.)  I suspect it's worse in the real world as KSP rocket engines are cheap and thus KSP rockets tend to be a lot more powerful than real world engines and thus suffer less gravity loss.  In KSP I find a thrust to weight ratio of about 2 (on the pad) to be ideal, using a less powerful engine makes for a more expensive rocket.  (Going higher gets one going too fast while the atmosphere is still too thick which causes problems with trying to steer.  Even at 2 I lose a bit more due to gravity because it can't tip over fast enough without having the nose pointed dangerously far from the direction of travel.)  In the real world the engines are a much bigger portion of the total cost, the Space Shuttle has a thrust to weight ratio of 1.5 on the pad.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-08-14T04:26:20.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22653" PostTypeId="1" CreationDate="2017-08-14T08:09:13.797" Score="5" ViewCount="126" Body="&lt;p&gt;An article in The Next Platform, &lt;a href=&quot;https://www.nextplatform.com/2017/08/11/one-small-step-toward-supercomputers-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;One Small Step Toward Supercomputers in Space&lt;/a&gt; describes a small High Performance Computing system (HPC) built by &lt;a href=&quot;https://www.hpe.com/us/en/solutions/hpc-high-performance-computing.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hewlett Packard Enterprise&lt;/a&gt; (HPE), to be brought to the ISS soon on CRS-12 for extensive testing in a space environment.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As HPE’s Mark Fernandez tells The Next Platform, the duo of HPE Apollo 50 machines is direct from the factory—in other words, no hardware hardening against radiation and magnetic disturbances has happened. Most of his team’ work has focused on the many tunable parameters for the CPU, memory, and solid state disk drives that are aboard ISS. What is different are the “lockers” that HPE built and tested for flight against the over 140 safety certifications required for on-board ISS gear.&lt;/p&gt;&#xA;&lt;p&gt;[...]&lt;/p&gt;&#xA;&lt;p&gt;The system itself is running standard RHEL 6.8 across its benchmark suite and has features common to much larger supercomputers, including the Infiniband connections.  “We went with the 56Gb/s optical interconnect because we imagined with copper, we would get more of a reaction &lt;strong&gt;from the radiation and magnetic fields&lt;/strong&gt;. We also eliminated the spinning rust—there is no traditional hard disk because it would be affected by the same conditions. On each node there are eight solid state disks; four of those are small but fast, the others are large but slow so we can see what effects there might be on one versus the other,” Fernandez explains.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I'm wondering why a high speed copper interconnect between the (in this case two) nodes would be affected by radiation. There are many tradeoffs between copper and optical node-to-node connections (which I am assuming is what's being discussed here, rather than board-level or below) but I've never heard of radiation hardness being one of them.&lt;/p&gt;&#xA;&lt;p&gt;Also, what would be the issue(s) related to magnetic fields? Are the fields any different in orbit than they are on Earth? Wouldn't any &lt;span class=&quot;math-container&quot;&gt;$\partial \mathbf{B} / \partial t$&lt;/span&gt; effects be shielded by the computers housing? Or is that something not worth assuming?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-14T14:44:17.423" Title="HPE's demo HPC system; why choose optical and not copper interconnects because of magnetic fields and radiation?" Tags="|magnetic-field|computing|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22654" PostTypeId="1" AcceptedAnswerId="22655" CreationDate="2017-08-14T08:58:39.743" Score="0" ViewCount="1025" Body="&lt;p&gt;At one point there were formal specifications for cubesat dimensions. I know it's a bit larger than precisely &lt;span class=&quot;math-container&quot;&gt;$10m \times 10n \times 10$&lt;/span&gt; centimeters,  where &lt;span class=&quot;math-container&quot;&gt;$m=1, 2$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$n=1, 2, 3$&lt;/span&gt; but I am not sure how much.&lt;/p&gt;&#xA;&lt;p&gt;The reason I've asked is that I was looking at this photo from the JLP/Caltech page &lt;a href=&quot;https://www.jpl.nasa.gov/cubesat/missions/asteria.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;Arcsecond Space Telescope Enabling Research in Astrophysics (ASTERIA); Mission Information&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BHShv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BHShv.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The article continues:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;ASTERIA is a 6U CubeSat (roughly 10 x 20 x 30 cm, 12 kg) that will operate in low-Earth orbit. The payload consists of a lens and baffle assembly, a CMOS imager, and a two-axis piezoelectric positioning stage on which the focal plane is mounted. A set of commercial reaction wheels provides coarse attitude control. Fine pointing control is achieved by tracking a set of guide stars on the CMOS sensor and moving the piezoelectric stage to compensate for residual pointing errors. Precision thermal control is achieved by isolating the payload from the spacecraft bus, passively cooling the detector, and using trim heaters to perform small temperature corrections over the course of an observation.&lt;/p&gt;&#xA;&lt;p&gt;In May 2017, the flight spacecraft was delivered for integration into the Nanoracks Cubesat Deployer. ASTERIA is currently scheduled to launch to the International Space Station (ISS) with the SpaceX Falcon-9 Crew Resupply Services – 12 (CRS-12) mission in August 2017. Deployment from ISS is planned for October 2017 to begin the 90-day ASTERIA technology demonstration mission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;So if JPL calls it a cubesat, and it will be accommodated by the &amp;quot;Nanoracks Cubesat Deployer&amp;quot; &lt;em&gt;then by gosh it certainly sounds like it's a cubesat!&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;But, when I look at the photo, there is an optical table breadboard with its grid of holes. These are traditionally either 25.0mm or 25.4mm spacing (you can order either one). While the camera is at an oblique angle, the satellite appears fairly parallel to the grid in both directions. From that I can estimate a size, and it does not appear to me to have a footprint anywhere near 20 x 30 cm, nor a height of 10 cm.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; so I'd like to ask; What are the absolute maximum dimensions of a proper 6U cubesat? Does ASTERIA comply?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-04-20T06:57:58.137" Title="What are the absolute maximum dimensions of a proper 6U cubesat? Does ASTERIA comply?" Tags="|cubesat|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22655" PostTypeId="2" ParentId="22654" CreationDate="2017-08-14T13:37:55.753" Score="1" Body="&lt;p&gt;There are a number of definitions out there as to what a cubesat is. 4 inches and 10 cm are the two most common size constraints for a 1U, but there are others out there. The best definition, in my opinion, comes from &lt;a href=&quot;https://static1.squarespace.com/static/5418c831e4b0fa4ecac1bacd/t/573fa2fee321400346075f01/1463788288448/6U_CDS_2016-05-19_Provisional.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cal Poly&lt;/a&gt;, as they are responsible for launching many cubesats. The dimensions that they list are 100 x 226.3 x 386.0, which I assume is mm. The reason why a larger cubesat is allowed more space is that the main body of the spacecraft is limited to 10 cm, but there are poles and other things that have to extend a bit beyond the base frame. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that those dimensions are for when it is in the container. You can have deployable items, like antennas and solar panels, that extend further than that after deployment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can only assume that Asteria complies with those dimensions, but I don't have any way to verify that.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-08-14T13:37:55.753" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22656" PostTypeId="2" ParentId="22376" CreationDate="2017-08-14T16:00:10.760" Score="2" Body="&lt;p&gt;Here is another, not as nice, taken March 20, 2015 by Samantha Cristoforetti.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b149V.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b149V.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.flickr.com/photos/astrosamantha/16686549769/in/album-72157651047516588/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a whole &lt;a href=&quot;http://www.collectspace.com/news/news-081617a-total-solar-eclipse-from-space-astronauts.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; on this on collectspace with other examples. Here's one from Mir in 1999.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/C0ifj.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/C0ifj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;And one from the ISS in 2002&#xA;&lt;a href=&quot;https://i.stack.imgur.com/L5rOO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L5rOO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Added one taken by ISS crewmember Paolo Nespoli on August 21 2017.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/H0Fm5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H0Fm5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-08-21T20:23:23.180" LastActivityDate="2017-08-21T20:23:23.180" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22657" PostTypeId="1" CreationDate="2017-08-14T16:42:36.107" Score="19" ViewCount="6414" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZhV3z.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZhV3z.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the recent &lt;a href=&quot;https://www.youtube.com/watch?v=vLxWsYx8dbo&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX CRS-12&lt;/a&gt; launch which was broadcast live on YouTube, it appeared at one point as if the horizon of the earth was completely flat.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What caused this to happen?&lt;/p&gt;&#xA;" OwnerUserId="20721" OwnerDisplayName="Tyler Faile" LastEditorUserId="6537" LastEditDate="2017-12-12T22:31:01.790" LastActivityDate="2017-12-12T22:31:01.790" Title="What made the Earth appear to be flat during the recent SpaceX launch?" Tags="|spacex|falcon-9|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22658" PostTypeId="2" ParentId="22657" CreationDate="2017-08-14T18:50:10.253" Score="33" Body="&lt;p&gt;I suspect it is a combination of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Fisheye_lens&quot; rel=&quot;noreferrer&quot;&gt;fisheye effect&lt;/a&gt; of the camera lens and the lack of altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fisheye effect is well known for making the earth appear more round than it should be. It does so by curving seemingly straight lines towards the center of the image, with greater effect the further from the center. Take the opposing camera angle from the same launch:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/dHVPW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dHVPW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, the fisheye effect makes the earth more curved than usual as the curvature of the earth is added to the curvature due to the wide-angle lensing.&#xA;Since in your screenshots, the earth is in the corner away from the centerpoint, the fisheye effect tries to reverse what curvature there was away from the center, the net effect making the earth flat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, at an altitude of ~200km, the curvature of the earth is not very apparent. Even with the naked eye, you would only notice the curvature due to your wide angle of vision.&lt;/p&gt;&#xA;" OwnerUserId="20309" LastEditorUserId="20309" LastEditDate="2017-08-14T20:51:29.820" LastActivityDate="2017-08-14T20:51:29.820" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22660" PostTypeId="1" CreationDate="2017-08-14T20:37:41.580" Score="12" ViewCount="675" Body="&lt;p&gt;Does the KSC Range Safety Officer (RSO) have the ability to abort (destruct) the Falcon 9 first stage on its return to launch site?  If so, at what point(s) in the trajectory does this ability exist?&lt;/p&gt;&#xA;" OwnerUserId="63" LastEditorUserId="63" LastEditDate="2017-08-18T03:53:46.457" LastActivityDate="2017-08-20T19:35:07.470" Title="Falcon 9 Range Safety" Tags="|spacex|falcon-9|range-safety|self-destruct|" AnswerCount="2" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22661" PostTypeId="2" ParentId="22476" CreationDate="2017-08-14T22:35:19.527" Score="5" Body="&lt;p&gt;Some breaking news on this, &lt;a href=&quot;http://www.iflscience.com/space/russian-artificial-star-fails-to-deploy-in-orbit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;according to this source (not the most reputable, to be sure)&lt;/a&gt; the reflectors have failed to deploy.  They link to &lt;a href=&quot;https://geektimes.ru/post/291835/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this source&lt;/a&gt; which appears to be from the creators of Mayak, saying that technical failures have occurred and they are still trying to diagnose them.  I don't speak Russian and Google Translate only makes it partially readable, so hopefully someone who can read the source can elaborate.&lt;/p&gt;&#xA;" OwnerUserId="16599" LastEditorUserId="16599" LastEditDate="2017-08-15T15:51:04.350" LastActivityDate="2017-08-15T15:51:04.350" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22662" PostTypeId="2" ParentId="22539" CreationDate="2017-08-15T00:02:41.457" Score="4" Body="&lt;p&gt;Elon Musk has stated that it is highly unlikely that the Falcon Heavy will actually make it to orbit on the maiden flight. This is due to the fact that it will be the first time that they can test out how the fuel pumps between the boosters work and also the separation and stage strengthening. Due to this, it is highly likely that the first Falcon Heavy launch will be with a fairing containing some form of test payload. A Dragon capsule would likely not be used due to pricing given that they would basically be throwing it away. One situation I can think of however where they would use a Dragon would be if it is an early Dragon 2 as it would allow them to practice launch escape with the capsule if something was to go wrong. In terms of what they will actually launch, it will likely be something like a test payload. Musk did say it would be something silly so you never know, his mind works in mysterious ways.&lt;/p&gt;&#xA;" OwnerUserId="20529" LastEditorUserId="6816" LastEditDate="2017-08-15T15:52:07.367" LastActivityDate="2017-08-15T15:52:07.367" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22663" PostTypeId="2" ParentId="22542" CreationDate="2017-08-15T00:20:40.113" Score="2" Body="&lt;p&gt;The first problem here is guidance, the container has none on its own. To solve this, a small extra stage would need to be placed in the rocket fairing with the container which would be able to orientate and de-orbit the container. This is very similar to how nuclear warheads are delivered however even the most accurate nuclear missile couldn't aim at a specific house due to wind, atmospheric pressure etc.&#xA;A method of making it more accurate would be the curiosity rover's landing process. You could place the container inside a small capsule which has a heat shield, its own parachute and fins / reaction control and that would allow for a much more accurate re-entry.&#xA;After the main parachute had deployed the heat shield would separate and the capsule would slow whilst also course correcting, the container would then drop out the bottom of the capsule with its own small parachute which would then allow it to land relatively close to your house.&#xA;As for making sure the capsule and heat shield don't hit your house; you could add small solid fuel boosters onto them to launch them over to your neighbours house and make it their issue.&#xA;The second issue is heating. this can be easily achieved now that the container is in a capsule which could act like a microwave however using re-entry heat instead of radiation. You could introduce heat vents on the capsule to ensure and optimal temperature is kept.&#xA;After all that you should have your fresh popcorn wherever you are!  &lt;/p&gt;&#xA;" OwnerUserId="20529" LastActivityDate="2017-08-15T00:20:40.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22664" PostTypeId="1" CreationDate="2017-08-15T00:28:42.570" Score="2" ViewCount="412" Body="&lt;p&gt;How plausible would it be to build and use an upper stage with little to no avionics and RCS, just an engine and fuel tanks? Would it be possible to use a satellite's/probe's RCS and guidance for the duration of a burn?&lt;/p&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2019-06-26T07:02:03.883" Title="How plausible is an upper stage with minimal/no avionics and RCS?" Tags="|design|upper-stage|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22665" PostTypeId="1" AcceptedAnswerId="22675" CreationDate="2017-08-15T01:39:09.027" Score="11" ViewCount="636" Body="&lt;p&gt;Looking at the question &lt;a href=&quot;https://space.stackexchange.com/questions/10145/has-in-space-refueling-been-done&quot;&gt;Has in-space refueling been done?&lt;/a&gt;, it looks like all fuel transfer operations in orbit have been done with room-temperature hypergolics. Has on-orbit transfer of cryogenic fuels ever been demonstrated, even as an experiment?&lt;/p&gt;&#xA;" OwnerUserId="13208" LastEditorUserId="12102" LastEditDate="2017-08-15T16:21:25.373" LastActivityDate="2017-08-16T03:34:06.880" Title="Has on-orbit refueling ever been demonstrated with cryogenic fuels?" Tags="|propulsion|docking|rendezvous|refueling|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22667" PostTypeId="1" AcceptedAnswerId="23123" CreationDate="2017-08-15T05:05:50.673" Score="8" ViewCount="1338" Body="&lt;p&gt;In their (misleadingly titled) article &lt;a href=&quot;http://www.cnn.com/2017/08/14/health/cassini-probe/index.html&quot; rel=&quot;noreferrer&quot;&gt;NASA's Cassini probe dives into Saturn's atmosphere&lt;/a&gt;, CNN reports&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Once the final five orbits are complete, Cassini will plunge towards Saturn, with seven instruments turned on and reporting real-time measurements before its thrusters fail against Saturn's atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This leaves the impression there will still be fuel in its tanks when Cassini burns up in Saturn's atmosphere, which &lt;em&gt;does&lt;/em&gt; make sense:  We want a &lt;em&gt;controlled&lt;/em&gt; descent into the atmosphere in order to maximize the scientific data collected by the effort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is - why make the final plunge on 15 September, in particular?  Where Cassini was launched 15 October 1997, it's mission is going to end a few hours more than a month less than 20 years after it was launched.  Is the fuel level so low that the mission couldn't have been extended one more orbit, so that the team could say their probe spent 20 full years in space before it was terminated?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I realize the mission lasted well over half a billion seconds, but gee whiz, couldn't we have made it to 20 years?  Maybe I'm just being sentimental, &quot;but to deny our own impulses is to deny the very thing that makes us human...&quot;&lt;/p&gt;&#xA;" OwnerUserId="19916" LastActivityDate="2019-09-09T04:25:59.277" Title="Is Cassini running out of fuel?" Tags="|saturn|cassini|fuel-system|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22668" PostTypeId="2" ParentId="22664" CreationDate="2017-08-15T06:14:59.477" Score="1" Body="&lt;p&gt;There are actually two questions here:  1) Would it make sense to have [upper] stages controlled by the spacecraft being launched (a &quot;dumb&quot; booster)? and 2) Would it make sense for upper stages to have no directional control and just be an &quot;inactive&quot; propulsion system?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first question's answer is yes - in fact, the Apollo Guidance Computer could be used to control the &lt;em&gt;entire&lt;/em&gt; Saturn V stack, not just the upper stages.  Adding a few more command sequences to an existing piece of hardware to more fully utilize its capabilities makes a &lt;em&gt;lot&lt;/em&gt; of sense:  Unless you end up having to add more memory, computer commands have &lt;em&gt;zero&lt;/em&gt; mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, while it would be &lt;em&gt;plausible&lt;/em&gt; to build a completely &quot;inactive&quot; upper stage, it would not be &lt;em&gt;reasonable&lt;/em&gt; to do so:  In order to have the payload engines control the upper stage, you would have to add mass both for the maneuvering fuel and for beefier engines to have a effective control over the larger vehicle.  This means your satellite/probe (launch payload) would be larger and more expensive than an &quot;active&quot; upper stage design would require.  You would then either need a larger booster to carry the same mission payload into space, or reduce the size of the mission payload to compensate for the increased propulsion system overhead.  In addition, an &quot;active&quot; upper stage isn't going all the way to the target orbit or trajectory, so when staging occurs and it's left behind, the extra mass of its control mechanisms is removed from the component being propelled toward the final destination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, RCS engines are not the only option for providing directional control for an upper stage:  The stage's rocket motor can also be gimballed to control the direction of the thrust vector, and thereby the direction the vehicle is going.  Using an RCS to point the stack while the upper stage engine &lt;em&gt;is not firing&lt;/em&gt; would not require substantially larger RCS engines because the change of heading could be done more slowly, but to change direction while the booster engine &lt;em&gt;is&lt;/em&gt; firing, gimballing the engine makes a lot more sense.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-08-18T21:59:48.247" LastActivityDate="2017-08-18T21:59:48.247" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22669" PostTypeId="2" ParentId="22664" CreationDate="2017-08-15T08:23:49.763" Score="4" Body="&lt;p&gt;It's not only plausible, it has been done. On the ATK &lt;a href=&quot;https://en.wikipedia.org/wiki/Star_(rocket_stage)#Star_48&quot; rel=&quot;nofollow noreferrer&quot;&gt;STAR&lt;/a&gt; series of solid upper stages, a reaction control system is optional. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the more common situation is to have the avionics and RCS for a rocket installed on its upper stage, so the first (and maybe second) stage is the one without RCS and avionics. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the &lt;a href=&quot;https://www.capcomespace.net/dossiers/espace_europeen/ariane/ariane5/caracteristiques.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ariane 5&lt;/a&gt;, for example, the avionics and RCS are integrated in a structure called the Vehicle Equipment Bay. &#xA;On the Ariane 5G, the second stage (EPS) sits inside the VEB. &#xA;On the Ariane 5 ECA, the VEB sits on top of the second stage (ESC-A), and the RCS has been moved from the VEB to the ESC-A.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gFSih.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gFSih.jpg&quot; alt=&quot;Ariane 5 VEB&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are three reasons not to have the payload control the entire rocket: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The rocket's RCS has been sized to control a 1000-ton rocket instead of a payload that weighs a few tons. Such an RCS is relatively heavy, so it makes sense to install it on the rocket instead of having it eat up space and weight on the payload. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You need an RCS and avionics on the upper stage to deorbit it after payload separation. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Putting the RCS and avionics on the satellite makes the satellite and rocket designs more dependent on each other. It becomes more difficult to switch a payload to a different launcher. It becomes more difficult to test and qualify the avionics and RCS (because it exists in a different environment for every launch, instead of being a constant). The rocket designer has to give information to the satellite designer (information of the type ITAR was designed to protect), meaning you get lots more paperwork and it becomes almost impossible to lunch satellites made in foreign countries.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2019-06-26T07:02:03.883" LastActivityDate="2019-06-26T07:02:03.883" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22670" PostTypeId="2" ParentId="22667" CreationDate="2017-08-15T09:44:48.560" Score="4" Body="&lt;p&gt;&lt;strong&gt;tl;dr:&lt;/strong&gt; The September 15, 2017 end for Cassini is part of the second mission extension (XXM, or Solstice Mission) proposed in 2009. The plan was approved.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/7VnRV.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Choosing the XXM Tour: Cassini project manager Bob Mitchell demonstrates the features of the Cassini spacecraft to the XXM senior review panel at JPL on February 10th 2009, watched by project scientist Bob Pappalardo (left), lead mission planner David Seal (right), and members of the review panel.&amp;quot; Credit: John Spencer. From the 2009 blog post &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/1856.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini's Proposed Extended-Extended Mission Tour&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/21279/12102&quot;&gt;excellent, concise answer&lt;/a&gt; to the question &lt;a href=&quot;https://space.stackexchange.com/q/21261/12102&quot;&gt;What force is bringing Cassini down into Saturn's atmosphere in another 145 days; drag, or…?&lt;/a&gt; describes how the whole thing is going down, both literally and idiomatically.&lt;/p&gt;&#xA;&lt;p&gt;Cassini's August 8 gravitational encounter with Titan sets it up for its September 11 encounter with Titan, which then sets it up for its deep-dive into Saturn's atmosphere on September 15.&lt;/p&gt;&#xA;&lt;p&gt;This is not like a de-orbit burn you might be imagining, it is a pair of highly choreographed gravitational assists from Titan flybys, lowering the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apsis&quot; rel=&quot;nofollow noreferrer&quot;&gt;periapsis&lt;/a&gt; enough to assure annihilation.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The Cassini mission has been incredibly successful in terms of both observation of Saturn and many of its moons at close range, and in a demonstration of orbital acrobatics.&lt;/p&gt;&#xA;&lt;p&gt;Because of this amazing success, the Cassini mission &lt;em&gt;has been extended twice already&lt;/em&gt;, at significant cost, in 2008 and again in 2010. See below.&lt;/p&gt;&#xA;&lt;p&gt;The internet search term would be &lt;em&gt;Cassini Ball of Yarn&lt;/em&gt;; see also &lt;a href=&quot;https://space.stackexchange.com/q/19515/12102&quot;&gt;Unravelling Cassini's “ball of yarn” orbit around Saturn, tabulation of propulsive maneuvers?&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/qcnxG.gif&quot; alt=&quot;Cassini ball of yarn&quot; /&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://saturn.jpl.nasa.gov/resources/7393/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Full video here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;More insight can be found in the 2009 John Spencer blog on The Planetary Society's website &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/1856.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini's Proposed Extended-Extended Mission Tour&lt;/a&gt;. The double encounters with Titan and burn up in Saturn's atmosphere on September 15, 2017 &lt;em&gt;had already been planned and proposed and waiting for approval back in 2009:&lt;/em&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It seems like no time since we selected Cassini's extended mission tour of the Saturn system, in early 2007. Now we're flying that tour, which extends Cassini's original four years in Saturn orbit for another 27 months, until September 2010. So now we're looking into the future- far into the future. If NASA approves the mission plan that was recommended by the Cassini project in late January, and if the spacecraft remains healthy, we hope to continue Cassini's exploration of Saturn for another seven years, until 2017. The ungainly working title of this planned new mission phase is the extended-extended mission, or XXM.&lt;/p&gt;&#xA;&lt;p&gt;The year 2017 is special because May 2017 marks northern midsummer in the Saturn system, and the more elegant alternative name for the XXM is the Solstice Mission. A Saturn year is 29.4 Earth years, and the considerable 27 degree tilt of Saturn's pole (which, incidentally and bizarrely, can probably be blamed on the planet Neptune), means that the slow progression of the seasons has major effects, especially on Saturn itself and on its giant moon Titan.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The blog is extensive and informative and definitely worth a read. It further explains that the final orbits of Cassini started back in April of 2017. The difference between April 22 and September 15 2017 is 146 days, and the basis of the title of the question &lt;a href=&quot;https://space.stackexchange.com/q/21261/12102&quot;&gt;What force is bringing Cassini down into Saturn's atmosphere in another 145 days; drag, or…?&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Cassini will execute 20 of these close &amp;quot;F-ring&amp;quot; orbits before setting up for a final close Titan flyby on April 22, 2017. This flyby will do something astonishing: it will perturb the orbit so that Saturn closest approach jumps, in a single leap, from just outside the main ring system into the narrow zone of safety between the inner edge of the innermost ring (the D ring) and the planet itself, just 3,800 kilometers above Saturn's cloud tops. Cassini will continue to thread this needle for 23 orbits (called, with some understatement, the &amp;quot;proximal&amp;quot; orbits) until a final distant nudge from Titan on September 11, 2017 delivers the death blow, altering the orbit just enough to drop Cassini into Saturn on September 15.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Later in the article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;There remains one major hurdle before we can put these plans in motion: money&lt;/strong&gt;. Cassini will need another seven years of funding, and NASA has many other claims on its budget. To sweeten the deal the Cassini project has found ways to simplify operations by concentrating efforts on the highest-priority science, and it looks like we can run each year of the XXM on considerably less money than we need to maintain the more hectic pace of the prime and extended missions. A couple of weeks ago I participated in a &amp;quot;Senior Review&amp;quot; at JPL, where we made the scientific case for the XXM to an external panel of scientists. The panel will pass on their recommendations to NASA headquarters, and we will wait more-or-less patiently for the final decision. In the meantime, we have lots of preparatory work to do to flesh out the details of the XXM. If and when we get the go-ahead from headquarters, we will be ready.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;And for balance, from &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#Mission_extension&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Mission extension&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;On April 15, 2008, Cassini received funding for a 27-month extended mission. It consisted of 60 more orbits of Saturn, with 21 more close Titan flybys, seven of Enceladus, six of Mimas, eight of Tethys, and one targeted flyby each of Dione, Rhea, and Helene. The extended mission began on July 1, 2008, and was renamed the Cassini Equinox Mission as the mission coincided with Saturn's equinox.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Second mission extension&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;A proposal was submitted to NASA for a second mission extension (Sept 2010 – May 2017), provisionally named the extended-extended mission or XXM. This ($60M pa) was approved in Feb 2010 and renamed the Cassini Solstice Mission. It includes Cassini orbiting Saturn 155 more times, conducting 54 additional flybys of Titan and 11 more of Enceladus.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-09-09T04:25:59.277" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="22671" PostTypeId="2" ParentId="22600" CreationDate="2017-08-15T13:48:17.110" Score="7" Body="&lt;p&gt;Orbital values provided by TLEs are mean elements. They change through the orbit and propagation. Therefore, a direct calculation of eccentricity from position and velocity vectors would not produce the same eccentricity value, it gives the eccentricity of the satellite for one moment in orbit only. That is why calculated eccentricity and the TLE value are different. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally,  the value of eccentricity changes through orbit because the model TLEs are propagated with, SGP4, does consider various perturbations that changes eccentricity, it is not a simple two body motion propagation.&lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="8902" LastEditDate="2017-08-15T14:03:07.500" LastActivityDate="2017-08-15T14:03:07.500" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22672" PostTypeId="1" AcceptedAnswerId="22673" CreationDate="2017-08-15T17:17:36.587" Score="5" ViewCount="555" Body="&lt;p&gt;Voyager 1 has gone far enough probing the interstellar spaces outside the Solar System, after its Radioisotope Thermoelectric Generators would completely decay the radioactive material inside, the Voyager would then have no option but to shut down, how is the space-craft going to end up in the future when it will run out of its electric supply. Are there any plans to safely dispose off the probe when it will reach its end like the Cassini spacecraft or will they allow it to bump freely into anything? &lt;/p&gt;&#xA;" OwnerUserId="20707" LastActivityDate="2017-08-16T20:43:36.520" Title="How will the Voyager 1 manage to generate electricity in the future or even now?" Tags="|nasa|voyager|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22673" PostTypeId="2" ParentId="22672" CreationDate="2017-08-15T17:47:50.027" Score="15" Body="&lt;p&gt;NASA currently expects that the RTG on Voyager 2 will no longer provide enough power to run a single instrument by &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_2#Future_of_the_probe&quot; rel=&quot;noreferrer&quot;&gt;2025&lt;/a&gt;. Voyager 1 might last a bit longer. That will be the end of the mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both Voyagers are in a very empty part of space. There is nowhere to crash into, and not enough fuel to change the trajectory noticeably.&lt;br&gt;&#xA;Both Voyagers will pass stars at a distance of 1.6-1.7 light years in 40,000 years, and calculations have been made to much longer timescales with no collisions predicted. I don't know specifically if this was deliberate choice, or if it was a happy coincidence of the trajectories chosen for the planetary encounters, but that seems to be 'safe disposal' to me. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should the Voyagers run into anything, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;RTGs&lt;/a&gt; were built to survive reentry on Earth, they're pretty robust. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6773" LastEditDate="2017-08-16T20:43:36.520" LastActivityDate="2017-08-16T20:43:36.520" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22674" PostTypeId="2" ParentId="22660" CreationDate="2017-08-15T19:16:06.737" Score="-4" Body="&lt;p&gt;I think the self-destruct can be performed before the first burn after the separation. After that the potential debree can hit urban areas and even cause deaths.&lt;/p&gt;&#xA;" OwnerUserId="20730" LastActivityDate="2017-08-15T19:16:06.737" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22675" PostTypeId="2" ParentId="22665" CreationDate="2017-08-16T03:34:06.880" Score="9" Body="&lt;p&gt;No. (Unless it has been done as part of a &lt;a href=&quot;http://www.airspacemag.com/space/spaceplane-x-37-180957777/&quot; rel=&quot;noreferrer&quot;&gt;secret mission&lt;/a&gt;.) NASA's Space Technology Mission Directorate had plans for just that called &lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/CPST_Fact_Sheet(1).pdf&quot; rel=&quot;noreferrer&quot;&gt;Cryogenic Propellant Storage &amp;amp; Transfer (CPST)&lt;/a&gt;, which would have launched a mission to demonstrate both the transfer of cryogenic propellent, and its storage in space for a long period of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately it was cancelled for cost reasons, and replaced with a ground-based investigation.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-08-16T03:34:06.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22676" PostTypeId="1" AcceptedAnswerId="22683" CreationDate="2017-08-16T09:28:28.443" Score="4" ViewCount="636" Body="&lt;p&gt;I don't understand the speed reading for the CRS-12 misson's stage 1. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While being around an altitude of ~100km the minimum speed that is shown in &lt;a href=&quot;https://youtu.be/vLxWsYx8dbo&quot; rel=&quot;nofollow noreferrer&quot;&gt;the webcast&lt;/a&gt; for stage 1 is ~&lt;strong&gt;1600km&lt;/strong&gt;/h.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the speed shown were the &lt;strong&gt;horizontal speed&lt;/strong&gt; measured relative to earth it should reach zero at some point, as it turns around and flies back to the Landing Zone in Cape Caneveral.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It can't be the &lt;strong&gt;vertical speed&lt;/strong&gt; as it is still a couple of thousand km/h while being on a constant altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;strong&gt;combination of the two&lt;/strong&gt; couldn't be much more than 100km/h at some point during the turnaround / minimal climb period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how am I to read that velocity data, does anyone know?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a nice plot of the trajectory in the image shown below, which seems plausible because of the fact that the stage 1 actually lands only a few km away from the lift-off site, but the speed data shown in the video simply does not match with that plotted trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what am I missing here?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gV9JM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gV9JM.png&quot; alt=&quot;this thread&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After looking at the great answers below i now realize that even with the 'not to scale' image above you can understand whats happening (once the nice ppl on stackoverflow explain it to you ;) ), the turnaround is 'roughly' circular, so imagening it as almost circular explains why the speed does not drop to zero, the combination of vertical + horizontal movement keeps the total speed high.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks guys :)&lt;/p&gt;&#xA;" OwnerUserId="20733" LastEditorUserId="20733" LastEditDate="2017-08-18T11:12:43.820" LastActivityDate="2017-08-20T16:38:10.530" Title="I don't understand the speed readings for stage 1 on CRS-12" Tags="|spacex|falcon-9|first-stage|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22677" PostTypeId="2" ParentId="22667" CreationDate="2017-08-16T10:49:31.540" Score="9" Body="&lt;p&gt;To answer the question, 'is Cassini running out of fuel':&lt;/p&gt;&#xA;&lt;p&gt;As you'd expect, NASA has monitored Cassini's fuel levels. In 2014 JPL published a study, '&lt;a href=&quot;https://trs.jpl.nasa.gov/bitstream/handle/2014/45501/14-5307_A1b.pdf?sequence=1&amp;amp;isAllowed=y&quot; rel=&quot;noreferrer&quot;&gt;Ensuring Cassini’s End-of-Mission Propellant Margins&lt;/a&gt;':&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;With three years left and only 2.5% of its loaded bipropellant and 37% of its loaded monopropellant remaining, the Cassini project actively manages the predicted end-of- mission propellant margins to maintain a high confidence in the spacecraft’s ability to complete the CSM as designed.&lt;/p&gt;&#xA;&lt;p&gt;Accurate spacecraft navigation, rigorous remaining-propellant estimation, and frequent future propellant consumption prediction have resulted in efficient propellant use and a probability of sufficient propellant margin greater than 99%.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Cassini project maintains contingency plans that seek to minimize impacts to future science activities and mission requirements in the event of bipropellant depletion. The contingency response are dependent on the date of bipropellant depletion. Using the results from the October 2014 prediction, of the 11% of cases that depleted the bipropellant, more than 95% of them would have little to no impact on future science activities or mission requirements.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The study contains graphs like the following, which show the amount of delta-V remaining:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JzCcw.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JzCcw.png&quot; alt=&quot;100 m/s of Delta-V remaining ± 20 m/s&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;By October 2014, there was 55 m/s of Delta-V from the bipropellant thrusters remaining ± 15 m/s, so there's a significant error bar here. The same goes for the monopropellant system (which has larger reserves remaining).&lt;/p&gt;&#xA;&lt;p&gt;The mission extension was designed to run nominal operations until after the next Saturn solstice (which happened on &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?feature=6855&quot; rel=&quot;noreferrer&quot;&gt;May 24, 2017&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The goal of CSM trajectory design was to maintain science observation opportunities commensurate with those in the PM and EM, while extending the mission to the next Saturn solstice in 2017, seven years away [4].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The solstice was the &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?feature=6855&quot; rel=&quot;noreferrer&quot;&gt;starting point for the Grand Finale&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As Saturn's solstice arrives, Cassini is currently in the final phase of its long mission, called its Grand Finale. Over the course of 22 weeks from April 26 to Sept. 15, the spacecraft is making a series of dramatic dives between the planet and its icy rings. The mission is returning new insights about the interior of the planet and the origins of the rings, along with images from closer to Saturn than ever before. The mission will end with a final plunge into Saturn's atmosphere on Sept. 15.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So the milestone NASA was looking for, was the Saturn solstice than a round number of Earth years since launch.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-16T10:49:31.540" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22678" PostTypeId="1" AcceptedAnswerId="22679" CreationDate="2017-08-16T11:05:33.847" Score="2" ViewCount="8251" Body="&lt;p&gt;I know so far that escape velocity on the surface of Earth is 11.2 km/s Wikipedia says that escape velocity is the minimum speed needed for an object to escape from the gravitational influence of a massive body. Isn't it the initial speed, which should be equal or overcome Ve in order to escape the gravitational pull? Rockets or other spaceships use fuel in order to propel out of the planet, but they are constantly burning fuel and accelerating. As far as I know rockets do not get launched at a speed which equals or overcomes escape velocity. Do they later have to achieve it in order to escape? Or can they just constantly be accelerating but at a speed slower than Ve?  &lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="6773" LastEditDate="2017-08-16T14:15:50.217" LastActivityDate="2017-08-16T14:15:50.217" Title="Does escape velocity apply to rockets?" Tags="|rockets|earth|escape-velocity|" AnswerCount="1" CommentCount="3" ClosedDate="2017-08-16T17:05:54.340" ContentLicense="CC BY-SA 3.0" />
  <row Id="22679" PostTypeId="2" ParentId="22678" CreationDate="2017-08-16T12:02:17.270" Score="7" Body="&lt;p&gt;The escape velocity is a bit of abstraction, an idealized situation. It applies, to a degree, but about never as the exact value of velocity of any object.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off, a rocket launched at that speed from the surface would burn up in the atmosphere long before reaching space - but in practice the segment where an interplanetary rocket accelerates is short enough comparing to its total trajectory, you can approximate the &quot;initial speed&quot; with &quot;speed at the end of the escape burn&quot;; it's near enough and short enough the error is rather small.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, a rocket given exactly the escape speed will escape Earth's gravity and... fly nowhere interesting. Just orbiting the Sun on an elliptic trajectory reaching maybe halfway towards Mars or Venus (depending on direction of escape). You want to give the rocket enough of initial velocity to reach the actual target, not just escape (due to Oberth effect, it's best done early on, combining escape and transfer burn. It's massively wasteful to do this only &lt;em&gt;after&lt;/em&gt; the rocket escaped Earth's gravity.) So the rocket at the end of the burn will possibly move slightly faster than Ve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next - we're not alone in empty space. The escape velocity is usually an abstraction for a two-body system, rocket and planet, and nothing else. No gravitational field of the Moon. No Sun gravity and its lagrangian points. So, the actual velocity will account for these.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next - issues of low-thrust transfers. If the probe uses a low-thrust engine, like the ion engine, the burn duration will no longer be negligibly short. Loss of velocity due to climbing out of gravity well, vs gain due to thrust; the probe never achieves escape velocity, it just &quot;climbs out&quot; of the gravity well, instead of being ejected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then delta-V, Oberth effect and gravity losses. If your rocket packs 11.2 km/s of delta-V in its tanks, you're getting somewhere halfway to the Moon, or thereabouts. Nowhere near escaping. Atmospheric drag, gravitational losses, loss of velocity due to climbing to LEO from surface altitude, that all needs to be accounted for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The escape velocity is nice as a &quot;milestone&quot;, a marker to compare requirements and actual velocities against, and to train/play/learn with simulated, idealized models, but it never applies &lt;em&gt;directly&lt;/em&gt; - it's just an arbitrary, if evocative data point to compare &lt;em&gt;actual&lt;/em&gt; velocities to. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-08-16T12:02:17.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22680" PostTypeId="1" AcceptedAnswerId="22681" CreationDate="2017-08-16T12:45:15.220" Score="6" ViewCount="2313" Body="&lt;p&gt;I'm watching the &lt;a href=&quot;https://www.nasa.gov/multimedia/nasatv/index.html#public&quot; rel=&quot;noreferrer&quot;&gt;live TV from NASA&lt;/a&gt; broadcasting the ISS / SpaceX Dragon docking and I've seen a red light and a white flashing light. See them at the bottom of this image &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gTXMa.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gTXMa.png&quot; alt=&quot;screenshot from NASA live TV&quot;&gt;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are they for?&lt;/p&gt;&#xA;" OwnerUserId="298" LastActivityDate="2017-08-16T14:37:29.793" Title="What is the purpose of red / white lights in ISS / SpaceX Dragon docking?" Tags="|spacex|iss|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22681" PostTypeId="2" ParentId="22680" CreationDate="2017-08-16T14:37:29.793" Score="10" Body="&lt;p&gt;Navigation lights. Red on port, green on starboard, white on top, and yellow on bottom. The white light is also usually used as a tracking light from distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Navigation_light&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Navigation_light&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20736" LastActivityDate="2017-08-16T14:37:29.793" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22682" PostTypeId="2" ParentId="19290" CreationDate="2017-08-16T14:39:21.117" Score="0" Body="&lt;p&gt;An OV-103 or also called Space Shuttle Discovery has an average lift-off mass of 2,050,000 kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;2,050,000 kg x 9.81 = 20,110,500 N is the total weight at lift-off&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Space Shuttle Discovery has two big white solid rocket boosters on the sides and three main engines at the tail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thrust of both boosters is equal to 2,404,040 kg x 9.81 = 23,583,632 N &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thrust of each main engines equals to 179,097 kg x 9.81 = 1,756,942 N &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thrust of all three equals to 1,756,942 N x 3 = 5,270,826 N&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overall thrust is 23,583,632 N + 5,270,826 N = 28,854,458 N&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Resultant force = thrust – weight = 28,854,458 N – 20,110,500 N  = 8,743,958 N&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Acceleration = resultant force ÷ mass = 8,743,958 N ÷ 2,050,000 kg = 4.27 m/s^2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spaceship is at rest before launched, so its initial speed is 0. After one second of the launch, its speed is going to be 4.26 m/s which is going to increase as the ship accelerates. We could say that the plume from a rocket launch could be happening at the first second after the launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;Final Answer: The energy released at such moment is (1/2) x 2,050,000 kg x (4.27 m/s)^2 = 18,688,722.5 J&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: &lt;em&gt;The amount of joules in that second equals 18,688,722.5 Watts, because J/s = Watt&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe 18,688,722.5 J is the energy released at such moment, but it would not all convert into visible light because most of the energy is lost as heat.&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="20563" LastEditDate="2017-08-18T12:15:29.440" LastActivityDate="2017-08-18T12:15:29.440" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22683" PostTypeId="2" ParentId="22676" CreationDate="2017-08-16T15:58:39.487" Score="10" Body="&lt;p&gt;Your intuition about the combination of the two is leading you astray. In fact, the speed &lt;strong&gt;is&lt;/strong&gt; a combination of the two. 1600km/h is 450m/s, which is practically stationary in terms of orbital mechanics. Also, remember that any graphics you see are not to scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the more detailed answer: The boostback burn has to negate all the horizontal velocity and &lt;strong&gt;add reverse horizontal&lt;/strong&gt; velocity to come back. Given that it has probably flown downrange on the order of 50km by stage separation, it needs to supply enough horizonal velocity to come back all of those 50km. It can be done quickly or slowly. If it is done quickly, a lot of fuel will be used in adding horizontal reverse velocity and then cancelling it out again. If it is done slowly, then the rocket might fall back to earth before it returns close enough to the pad. But the magic of the boostback burn is that it preserves vertical upwards velocity, and so a large amount of time is gained for the horizontal trip to the pad. But this time gained is on the order of a minute or two, and so the horizontal velocity reflects that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just some rough numbers:&#xA;$$50\mathrm{km} / 120\mathrm{sec} \approx 420\mathrm{m/s} \approx 1500 \mathrm{km/h}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the speeds shown in the webcast at highest point are in fact spot on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: uhoh has put together some awesome graphics, showing just how aggressive the horizontal trajectory is. Check uhoh's answer out!&lt;/p&gt;&#xA;" OwnerUserId="20309" LastEditorUserId="20309" LastEditDate="2017-08-20T16:38:10.530" LastActivityDate="2017-08-20T16:38:10.530" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22684" PostTypeId="1" CreationDate="2017-08-16T16:58:17.977" Score="1" ViewCount="283" Body="&lt;p&gt;Like the Soyuz or Apollo spacecrafts, the crew module detaches itself from the service module during re-entry, is it possible to cover the total space-craft with a sleek-aerodynamic heat shield at the time of re-entry thus preventing the addition of space-debris and making the spacecraft reusable up to a certain extent.&lt;/p&gt;&#xA;" OwnerUserId="20707" LastEditorUserId="8162" LastEditDate="2017-08-16T17:29:18.753" LastActivityDate="2017-08-16T17:29:18.753" Title="Why can't spacecrafts create a complete heatshield cover around them during re-entry?" Tags="|reentry|reuse|heat-shield|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22685" PostTypeId="2" ParentId="22684" CreationDate="2017-08-16T17:04:21.893" Score="6" Body="&lt;p&gt;It certainly is possible to have enough of the spacecraft shielded to have it all land; that's what the Space Shuttle was designed for.  However, if you want to be able to land with a heat shield, you must first get it into space to begin with.  The &lt;a href=&quot;https://en.wikipedia.org/wiki/AVCOAT&quot; rel=&quot;noreferrer&quot;&gt;heat shielding&lt;/a&gt; for the Apollo Command Module has a density of 32 pounds per cubic foot.  In total, the heat shielding alone weighed about &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Command/Service_Module#Command_Module_.28CM.29&quot; rel=&quot;noreferrer&quot;&gt;3,000 pounds&lt;/a&gt;.  Now, if you want to reuse your entire spacecraft, the whole thing would have to be covered in heat shielding, increasing your weight by several thousand pounds, depending on its size.  That means that you're going to need more fuel in order to get it up there, greatly increasing the mission's cost.  In space exploration, every gram counts and is carefully considered.  It's much easier to leave a part of your spacecraft in orbit, to be &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_debris#Dealing_with_debris&quot; rel=&quot;noreferrer&quot;&gt;dealt with later&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2017-08-16T17:04:21.893" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22688" PostTypeId="1" AcceptedAnswerId="22721" CreationDate="2017-08-17T14:35:30.800" Score="11" ViewCount="3252" Body="&lt;p&gt;There is one mission by GSFC/NASA - Shields-1 to be launched into GTO. Apart from that I couldn't find any other mission to GTO/GEO. It would be of great help if there is any information on this.&lt;/p&gt;&#xA;" OwnerUserId="20750" LastActivityDate="2019-09-01T04:34:45.397" Title="Are there any CubeSats that have been launched into Geostationary (GEO/GSO) or Geostationary Transfer Orbit (GTO)?" Tags="|spacecraft|cubesat|geostationary|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22689" PostTypeId="1" CreationDate="2017-08-17T16:34:47.157" Score="2" ViewCount="1441" Body="&lt;p&gt;The reaction wheels rotate the satellite only about the centre of mass.If the reaction wheels are not placed about the principal axes,Will the amount of torque produced be the same as when they are placed about the principal axes?&lt;/p&gt;&#xA;" OwnerUserId="18223" LastActivityDate="2017-08-17T17:34:35.210" Title="Placement of Reaction Wheels in a nano satellite" Tags="|cubesat|reaction-wheel|attitude-determination-and-control|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22690" PostTypeId="2" ParentId="22689" CreationDate="2017-08-17T17:34:35.210" Score="3" Body="&lt;p&gt;The torque will be the same - application of point torque to an object (in this case the point is the reaction wheel motor axis) results in the same torque exerted regardless of the point. But the moment of inertia won't be - because the layout of mass will differ. As the reaction wheels are not massles, they are a part of the system, and while mass won't change, its distribution - distances from center of mass - will differ. And as result, the cubesat will react differently than one with the RW axes through its center of mass; in particular with mass distributed broader (farther from center of mass) the same torque will result in lower angular acceleration.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-08-17T17:34:35.210" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22691" PostTypeId="1" CreationDate="2017-08-17T17:54:08.987" Score="9" ViewCount="10453" Body="&lt;p&gt;Satellites are somewhat closer to the Moon than the Earth is (Or rather, can be), and might well have the sun blocked from them. Furthermore, they can move somewhat more than the Earth, and thus have the potential for more eclipses. Are solar eclipse a significant concern for the design of satellites (Specifically, the Moon blocking the Sun)?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2019-06-09T18:59:02.250" Title="How does a solar eclipse affect satellites?" Tags="|artificial-satellite|power|thermal|solar-eclipse|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22692" PostTypeId="1" CreationDate="2017-08-17T18:26:39.087" Score="5" ViewCount="312" Body="&lt;p&gt;I'm making a space exploration game. It will all take place within a single 2D plane (no inclination) and use patched-conics approximation so that a spacecraft is only gravitationally affected by one celestial body at a time. I have orbit simulations working perfectly given a set of parameters. These are the variables that orbits have in the program:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;semi-major axis (meters)&lt;/li&gt;&#xA;&lt;li&gt;eccentricity&lt;/li&gt;&#xA;&lt;li&gt;argument of periapsis (radians)&lt;/li&gt;&#xA;&lt;li&gt;whether the orbit is prograde or retrograde&lt;/li&gt;&#xA;&lt;li&gt;standard gravitational parameter of the celestial body being orbited (m^3 s^-2)&lt;/li&gt;&#xA;&lt;li&gt;the eccentric anomaly at t = 0 (radians)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And an object's current position is stored as its current eccentric anomaly in radians. Using these six parameters, I can define any elliptical orbit in the plane, and get the position and velocity of the object for any given time or eccentric anomaly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I &lt;em&gt;can't&lt;/em&gt; do yet is make the orbit change based on the spacecraft's acceleration. Given a current eccentric anomaly, all six orbital parameters for an object, and a 2D vector for change in velocity, how can I compute the new orbital parameters?&lt;/p&gt;&#xA;" OwnerUserId="20751" LastActivityDate="2017-08-18T17:21:02.227" Title="How does a change in velocity affect the properties of an orbit in a 2D plane?" Tags="|orbital-mechanics|orbit|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22693" PostTypeId="2" ParentId="22691" CreationDate="2017-08-17T18:45:36.397" Score="10" Body="&lt;p&gt;Orbiting satellites have to deal with darkness all the time when Earth is between the satellite and the sun, and these periods last for much longer than an eclipse does for the satellite. So although I'm no expert, I expect that any satellite designed to function above the dark side of the planet would have no problems with an eclipse.&lt;/p&gt;&#xA;" OwnerUserId="20751" LastActivityDate="2017-08-17T18:45:36.397" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22694" PostTypeId="2" ParentId="22692" CreationDate="2017-08-17T19:39:49.817" Score="5" Body="&lt;p&gt;So you know $\mu$ r and v, right? And you want to find a and e?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vis Viva&lt;/a&gt; equation tells us:&lt;br&gt;&#xA;$v^2=\mu(2/r-1/a)$&lt;br&gt;&#xA;$\mu/a=(2\mu/r)-v^2$&lt;br&gt;&#xA;$a=\mu/((2\mu/r)-v^2)$&lt;br&gt;&#xA;There's your expression for a.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I make mistakes so it's a good idea to check my algebra.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now to find e.&#xA;Denote specific angular momentum as h.&lt;br&gt;&#xA;$h=\sqrt{a(1-e^2)\mu}$&lt;br&gt;&#xA;$h^2=a(1-e^2)\mu$&lt;br&gt;&#xA;$h^2/(a*\mu)=1-e^2$&lt;br&gt;&#xA;$e=\sqrt{1-(h^2/(a*\mu))}$&lt;br&gt;&#xA;Now $h=|\vec{h}|=|\vec{r}\times\vec{v}|$. Substitute this value into the above and you get e.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, it's a good idea to check my algebra.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="195" LastEditDate="2017-08-17T22:25:52.270" LastActivityDate="2017-08-17T22:25:52.270" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22695" PostTypeId="1" CreationDate="2017-08-17T20:15:14.773" Score="2" ViewCount="246" Body="&lt;p&gt;Does the jacket of tubes used to cool the nozzle and combustion chamber in a rocket engine also give it some structural strength? Does it have any function relating to  withstanding the pressure of the byproduct gases, or is it for cooling purposes only?&lt;/p&gt;&#xA;" OwnerUserId="20418" LastEditorUserId="34345" LastEditDate="2020-02-26T18:37:04.763" LastActivityDate="2020-02-26T18:37:04.763" Title="The structures in regenerative cooling method and structural strength?" Tags="|rockets|propulsion|engines|liquid-fuel|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="22696" PostTypeId="2" ParentId="22695" CreationDate="2017-08-17T20:20:10.920" Score="4" Body="&lt;p&gt;Take a look at this question: &lt;a href=&quot;https://space.stackexchange.com/questions/22001/how-are-rocket-nozzles-made/22002#22002&quot;&gt;How are rocket nozzles made?&lt;/a&gt; In pictures there, you can see that the cooling jacket IS the nozzle in many cases.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this picture I took of a flown, but scrapped SSME nozzle, you can see that the inside of the nozzle is constructed solely of the cooling tubes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZBCLr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZBCLr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a close-up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oEJVm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oEJVm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-08-17T20:27:23.167" LastActivityDate="2017-08-17T20:27:23.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22697" PostTypeId="2" ParentId="22676" CreationDate="2017-08-17T23:16:54.527" Score="5" Body="&lt;p&gt;I like the approach and explanation in &lt;a href=&quot;https://space.stackexchange.com/a/22683/12102&quot;&gt;Quietghost's answer&lt;/a&gt; and just want to add a screen shot and link to some helpful resources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I took a screen shot at around &lt;code&gt;T+ 00:04:10&lt;/code&gt; or about 250 seconds after launch. At an altitude of 118 km the speed relative to the Earth's rotating frame is 1658 km/h or 461 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember that these numbers are in the rotating frame of the Earth, and that you have to tack on roughly 70 m/s to translate that back into an Earth-centered inertial frame. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.flightclub.io/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flightclub.io&lt;/a&gt; is a really fun site to run simulations of particular launches. I re-ran the the CRS-12 simulation that is already set up on the website and plotted velocity and the altitude/range profile below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also don't forget that the &lt;code&gt;Downrange distance&lt;/code&gt; plot is a 2D projection of a three-dimensional orbit in the Earth's rotation frame. The x-axis is distance from the launch site to the ground-track of the orbit, the point on the Earth below each point in the orbit. But that distance along the surface can be in any direction. While it is mostly East in the beginning, the final landing site is more south-southwest (shown below in a screnshot from Google maps. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wHQEf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wHQEf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XS0JB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XS0JB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bG21a.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bG21a.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7WFkE.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7WFkE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4Pfqd.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4Pfqd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CeAqk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CeAqk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-18T16:32:32.733" LastActivityDate="2017-08-18T16:32:32.733" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22698" PostTypeId="2" ParentId="22692" CreationDate="2017-08-18T00:26:57.340" Score="2" Body="&lt;p&gt;If you already have your zenith angle $\gamma$, between velocity &lt;strong&gt;v&lt;/strong&gt; and radius &lt;strong&gt;r&lt;/strong&gt;, then &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$e^2 = 1-\frac{v^2r}{\mu} \ (2 - \frac{v^2r}{\mu}) \ sin^2(\gamma)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Derive this by substituting $|\mathbf{h}| = |\mathbf{r}||\mathbf{v}|sin(\gamma)$ and &lt;em&gt;vis viva&lt;/em&gt; for $1/a$ into &lt;a href=&quot;https://space.stackexchange.com/a/22694/12102&quot;&gt;HopDavid's equation&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12999" LastEditorUserId="12999" LastEditDate="2017-08-18T17:21:02.227" LastActivityDate="2017-08-18T17:21:02.227" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22699" PostTypeId="1" AcceptedAnswerId="22701" CreationDate="2017-08-18T00:49:36.700" Score="3" ViewCount="207" Body="&lt;p&gt;I &lt;a href=&quot;https://aviation.stackexchange.com/questions/43033/what-is-this-flying-boat-being-restored-at-the-udvar-hazy-center&quot;&gt;recently requested help identifying&lt;/a&gt; what turned out to be a Sikorsky JRS-1 on Aviation.SE.  One of the comments in my post led to this follow up. In the same picture there is what I'm pretty certain is an Apollo command module:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6L6EO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6L6EO.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I mentioned in my previous post, I took this picture from the glass enclosed walk surrounding the restoration area at the Smithsonian Udvar Hazy Center in Dulles, Virginia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've seen the Apollo 11 Command module at the Washington D.C. Air and Space museum, so I'd have to guess this is a different one.  &lt;em&gt;Does anyone know?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="20755" LastEditorUserId="26446" LastEditDate="2020-04-18T17:29:01.443" LastActivityDate="2020-04-18T17:29:01.443" Title="Which Apollo Command Module is pictured here?" Tags="|identify-this-object|capsule|museum-exhibits|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22700" PostTypeId="1" CreationDate="2017-08-18T00:55:24.987" Score="6" ViewCount="1197" Body="&lt;p&gt;The Apollo Guidance Computer had only 2k RAM and 32kb storage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also what O.S. did they use, and who made it?&lt;/p&gt;&#xA;" OwnerUserId="20757" LastEditorUserId="6816" LastEditDate="2017-08-18T02:54:47.143" LastActivityDate="2017-08-18T17:59:35.113" Title="How were these NASA engineers able to take men to the moon and bring them back with this limited computing capability?" Tags="|apollo-program|flight-computer|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22701" PostTypeId="2" ParentId="22699" CreationDate="2017-08-18T02:10:01.950" Score="5" Body="&lt;p&gt;According to &lt;a href=&quot;https://airandspace.si.edu/columbia-moves-virginia&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;, the Apollo 11 CM was moved to Udvar-Hazy at the end of 2016. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As none of the other Apollo CMs seem to be there &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/planetary/lunar/apolloloc.html&quot; rel=&quot;noreferrer&quot;&gt;according to this page&lt;/a&gt;, I would guess that is in fact &lt;a href=&quot;https://airandspace.si.edu/collection-objects/command-module-apollo-11&quot; rel=&quot;noreferrer&quot;&gt;Columbia&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-08-18T02:10:01.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22702" PostTypeId="2" ParentId="22700" CreationDate="2017-08-18T02:21:29.087" Score="25" Body="&lt;p&gt;The equations required for flight to the moon are not all that complicated, particularly given that much of the work was done by larger computers based on the Earth. Some of the computer programs (e.g. P40-P42) run on the Apollo guidance computer simply calculated what direction to point the ship and how long to fire the engine in order to achieve a change in course called for by Mission Control. Others (e.g. P51, P52) computed what the current orientation of the ship was based on what direction various stars were found. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://history.nasa.gov/afj/compessay.html&quot; rel=&quot;noreferrer&quot;&gt;hardest real-time problem solved by the Apollo computers was probably the landing guidance itself&lt;/a&gt; -- pointing and throttling the engine on the Lunar Module over the course of several minutes in order to reach the surface of the moon at a low speed. The central equations for this guidance are not complex at all: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NIocJ.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NIocJ.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, you could compute one iteration of the guidance loop by hand in a few minutes. The computer was fast enough to run the loop ten times a second, which was more than sufficient to control the lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#Software&quot; rel=&quot;noreferrer&quot;&gt;software was developed by a team at MIT&lt;/a&gt; led by &lt;a href=&quot;https://en.wikipedia.org/wiki/Margaret_Hamilton_(scientist)&quot; rel=&quot;noreferrer&quot;&gt;Margaret Hamilton&lt;/a&gt;, whose work advanced the state of the art of software engineering significantly. The software was based around a simple real-time scheduling kernel for which &quot;operating system&quot; might be too grand a term. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern operating systems provide a vast array of complex facilities to software, most of which aren't needed by any single program. With no graphical display, audio output, network connection, disk I/O, support for software written for legacy systems, etc. it's not that difficult to keep a complete system in a few kilobytes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read the assembly &lt;a href=&quot;https://github.com/chrislgarry/Apollo-11&quot; rel=&quot;noreferrer&quot;&gt;source code itself&lt;/a&gt; here. The &lt;a href=&quot;https://github.com/chrislgarry/Apollo-11/blob/master/Comanche055/EXECUTIVE.agc&quot; rel=&quot;noreferrer&quot;&gt;EXECUTIVE&lt;/a&gt; module, for example, comes to about 500 lines of heavily-commented assembly, and the &lt;a href=&quot;https://github.com/chrislgarry/Apollo-11/blob/master/Comanche055/P40-P47.agc&quot; rel=&quot;noreferrer&quot;&gt;maneuvering subroutines P40-P47&lt;/a&gt; come to about five times that. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I personally find the source code very difficult to follow, as the &lt;a href=&quot;https://www.ibiblio.org/apollo/assembly_language_manual.html&quot; rel=&quot;noreferrer&quot;&gt;instruction set of the AGC&lt;/a&gt; has a very different flavor from that of the modern CPUs I'm more familiar with. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-18T17:59:35.113" LastActivityDate="2017-08-18T17:59:35.113" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22703" PostTypeId="2" ParentId="22691" CreationDate="2017-08-18T02:39:03.983" Score="6" Body="&lt;p&gt;During non-eclipse times it is normal for satellites to experience &quot;solar outage&quot;, where the signal is cut or degraded, but not for more than a few minutes a day. This is caused by the Sun lining up directly behind the satellite (from Earth's perspective).The solar energy interferes with the signal/our ability to &quot;read&quot; that signal. In an eclipse, the source of that interference (the Sun) would actually be blocked by the Moon). If there is any kind of outage caused by the eclipse it would also only last a few minutes (the length of the eclipse time). So, likely, if the eclipse causes any loss/degradation of signal, it would no worse than during the normal solar outages, if there's any effect at all. And the solar outages really are not a big deal. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.radio-electronics.com/info/satellite/solar-effects/sun-solar-outages-basics-tutorial.php&quot; rel=&quot;noreferrer&quot;&gt;Reference: Radio-Electronics.Com&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="20741" LastEditorUserId="6816" LastEditDate="2017-08-18T02:51:52.620" LastActivityDate="2017-08-18T02:51:52.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22704" PostTypeId="1" AcceptedAnswerId="22707" CreationDate="2017-08-18T02:54:23.540" Score="8" ViewCount="811" Body="&lt;p&gt;I've just read Wernher Von Braun's &lt;em&gt;Project Mars: A Technical Tale,&lt;/em&gt; and I'm trying to understand the math in the back. I can follow some of what he's doing, but other parts I just can't tell where his numbers come from.  In particular, I can't figure out his equation for theoretical I&lt;sub&gt;sp&lt;/sub&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've searched all over the internet, and found several equations relating I&lt;sub&gt;sp&lt;/sub&gt; to the temperature, pressure, and geometry of the chamber, along with gas constants, specific heat ratios and molar masses of the propellant, as well as ambient pressure.  However, Von Braun doesn't use any of these numbers (or he doesn't mention that he does); his equation is simply:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;$I_{sp,th} = 9.323 * \sqrt{H_0 - H_e}$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Does anyone have any idea how that was derived?  Or at the very least, what the significance of 9.323 is?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/upekj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/upekj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://www.goodreads.com/book/show/1279669.Project_MARS&quot; rel=&quot;noreferrer&quot;&gt;Goodreads&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="20756" LastEditorUserId="8162" LastEditDate="2017-08-18T06:34:39.007" LastActivityDate="2019-05-18T23:42:36.520" Title="Understanding Von Braun's Math: Theoretical Isp" Tags="|mars|rockets|engine-design|specific-impulse|rocket-equation|" AnswerCount="1" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22707" PostTypeId="2" ParentId="22704" CreationDate="2017-08-18T08:20:04.377" Score="13" Body="&lt;p&gt;I guess that it goes approximately like this: assume that the enthalpy change (I'll denote it $\Delta H$) is fully converted to the kinetic energy of the exhaust and the exhaust moves with velocity $v$ relative to the engine. Then we have:&#xA;$$ mv^2/2 = m \Delta H;$$&#xA;$$ v = \sqrt{2\Delta H};$$&#xA;$$I_{sp} = \sqrt{2\Delta H}/g.$$&#xA;But that is assuming that all values are expressed in, for example, SI units ($I_{sp}$ in seconds, $\Delta H$ in J/kg, $g$ in m/s2). Von Braun uses the value of $\Delta H$ in cal/g instead. Using the value of calorie as 4.184 J, we have 1 cal/g = 4.184 J/g = 4184 J/kg, so&#xA;$$I_{sp}[s] = \sqrt{2\cdot 4184\Delta H[cal/g]}/g[m/s^2] = 91.48\sqrt{\Delta H[cal/g]}/g[m/s^2].$$&#xA;Using the value $g = 981~ \text{cm/s}^2 = 9.81~ \text{m/s}^2$, which is used earlier in the page, we get&#xA;$$I_{sp}[s] = 91.48\sqrt{\Delta H[cal/g]}/9.81 = 9.325\sqrt{\Delta H[cal/g]}.$$&#xA;Well, close enough. Von Braun probably used slightly different values for calorie or $g$.&lt;/p&gt;&#xA;" OwnerUserId="12448" LastActivityDate="2017-08-18T08:20:04.377" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22708" PostTypeId="2" ParentId="21852" CreationDate="2017-08-18T08:34:27.633" Score="8" Body="&lt;p&gt;Mars has no magnetic field to protect any atmosphere we might add to the planet, but fortunately it would be lost very slowly, over geological time. Fortunately, both protecting and creating a Martian atmosphere can be achieved by parking one giant powerful space station at the Lagrange 1 point which would broadcast a magnetic field which would protect the Martian atmosphere. Surprisingly, there is a little volcanic activity on Mar which releases enough gas to eventually generate a runaway greenhouse effect which would give Mars about a third Earth's atmosphere.&#xA;&lt;a href=&quot;https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html&quot; rel=&quot;noreferrer&quot;&gt;https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But we can speed that up. Zubrin says a city of a million people would have the economy to manufacture super-greenhouse gases 17,000 times more powerful than CO2. In just 100 years a city of 1 million people would get the following:-&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The Martian CO2 poles warm and melt.&lt;/li&gt;&#xA;&lt;li&gt;Mars would then have a CO2 atmosphere about a third the pressure of sea-level air here on Earth. That gives us radiation protection from the sun and cosmic rays.&lt;/li&gt;&#xA;&lt;li&gt;It protects all colony domes across the whole of Mars from tinier micrometeorites which would just burn up in the atmosphere. They're just not an issue ever again! (There would of course be a minimum size at which they did still bust through that 0.3 bar atmosphere, and domes would still require urgent repair mechanisms).&lt;/li&gt;&#xA;&lt;li&gt;The atmosphere would make every dome on the planet easier to build because it would be less prone to explosive decompression, increasing the size and viability of some of the valley-covering dome habitats.&lt;/li&gt;&#xA;&lt;li&gt;It lets us walk on Mars in plain clothes (with an oxygen breather mask of course), meaning you could cycle a bike long distances out in the open with a light breather pack and mask, repair your domes easier, mine resources, and basically live on the surface of an alien world far, far easier.&lt;/li&gt;&#xA;&lt;li&gt;It melts water ice, creating oceans and a rain cycle, which will eventually wash out the perchlorates in the regolith. &lt;/li&gt;&#xA;&lt;li&gt;This in turn allows plants and trees and farming on the surface and plankton in the oceans. All that about 100 years after the million-strong colony starts releasing super-greenhouse gases.&#xA;Why move nitrogen from other planets if Mars has enough that 'just' needs baking out of its rocks? Green Mars doesn't immediately require a breathable atmosphere, but a giant space lense could focus sunlight down on the Martian surface concentrated to one point to melt rock and free  volatiles.&lt;a href=&quot;https://www.universetoday.com/9730/zubrin-on-terraforming-mars/&quot; rel=&quot;noreferrer&quot;&gt;https://www.universetoday.com/9730/zubrin-on-terraforming-mars/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="20758" LastActivityDate="2017-08-18T08:34:27.633" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22709" PostTypeId="1" AcceptedAnswerId="22717" CreationDate="2017-08-18T08:57:16.183" Score="11" ViewCount="671" Body="&lt;p&gt;Why is there a push to get humans on Mars in the next few decades? Wouldn't it be wiser to send only robots first to set up an entire colony; do research; try to establish self-sustainability, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Human lives would not be at risk, and the benefits of using robots are manifold: cheaper, no food or oxygen needed, can work longer hours, do not suffer psychologically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And then once the hardest work is done, humans could go...&lt;/p&gt;&#xA;" OwnerUserId="20704" LastActivityDate="2017-08-18T12:56:11.677" Title="Why would humans travel to Mars before machines have built a colony?" Tags="|mars|colonization|robotic-missions|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22710" PostTypeId="1" CreationDate="2017-08-18T11:22:26.423" Score="12" ViewCount="5938" Body="&lt;p&gt;If there is no oxygen in the sun, then how is it burning?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question may be weird but I need a realistic answer for that.&lt;/p&gt;&#xA;" OwnerUserId="20760" LastEditorUserId="2784" LastEditDate="2017-08-18T16:29:21.250" LastActivityDate="2018-07-12T06:58:10.070" Title="Is oxygen present in the sun?" Tags="|oxygen|the-sun|" AnswerCount="4" CommentCount="9" ClosedDate="2017-08-19T00:30:59.657" ContentLicense="CC BY-SA 3.0" />
  <row Id="22711" PostTypeId="2" ParentId="22691" CreationDate="2017-08-18T11:28:51.420" Score="5" Body="&lt;p&gt;Satellites commonly go through the shadow of the Earth, and thus are able to deal with those outages. The question of being in the Moon's shadow really is how can that affect the ability to recover from being in the Earth's shadow. The question really is, how long will such an eclipse affect the satellite? Let's look at a few orbits, and compare the Earth/ Moon distances. I'm going to use the max width of a lunar eclipse, 267 km, as a reference. I'm also going to double that, as the penumbra will have effects. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEO (800 km)- 100 minute orbit, The Earth eclipse time max is around 34 minutes. Eclipse max is less than 2 minutes. Probably not a significant effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;GEO- 72 minutes eclipse time, at least 3 minutes of eclipse time possible. Could be a bit higher if at the closest point towards the Moon, as the altitude will make a big difference, but only another minute or two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, unless a satellite is already at margin, and has an absolute worst case experience in an eclipse, it will not have a signficant effect.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-08-18T11:28:51.420" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22713" PostTypeId="1" CreationDate="2017-08-18T12:07:42.810" Score="2" ViewCount="107" Body="&lt;p&gt;The recent article in The Register &lt;a href=&quot;http://www.theregister.co.uk/2017/08/17/single_cme_onbserved_by_12_spacecraft/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ten spacecraft – from Venus Express to Voyager 2 – all tracked same solar flare&lt;/a&gt; talks about ten spacecraft noticing the same coronal mass ejection (CME) as it expanded through the system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It mentions:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Other craft reported marked decreases in cosmic rays for a day or more&#xA;  as the CME passed&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One of the big problems with manned space flight is the exposure to radiation caused by cosmic rays. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be viable to &quot;ride&quot; a CME to reduce the radiation exposure or would this cause all sorts of other issues? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess all the electronics on the craft would need serious protection at the very least, but I have little knowledge of CMEs. Keeping up with it's speed is probably something beyond current technology.&lt;/p&gt;&#xA;" OwnerUserId="20761" LastEditorUserId="12102" LastEditDate="2017-08-20T05:46:55.707" LastActivityDate="2017-08-20T05:46:55.707" Title="Could riding a coronal mass ejection to avoid radiation be viable?" Tags="|crewed-spaceflight|cosmic-radiation|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22714" PostTypeId="1" CreationDate="2017-08-18T12:18:33.280" Score="6" ViewCount="301" Body="&lt;p&gt;A few years ago I read about a decision methodology that NASA uses to reduce the number of options/variables for future missions. Essentially, they make decisions that will remove the largest number of options earlier in the process. This means they can take thousands of variables and chop them down to a handful by making just a few decisions rather than judge each on its own merit. What is the name of this decision-making model?&lt;/p&gt;&#xA;" OwnerUserId="18805" LastEditorUserId="6816" LastEditDate="2017-08-19T03:54:56.103" LastActivityDate="2017-08-21T15:11:32.770" Title="Decision making for NASA missions" Tags="|nasa|planning|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22715" PostTypeId="2" ParentId="22710" CreationDate="2017-08-18T12:23:47.393" Score="6" Body="&lt;p&gt;So, the Sun isn't actually burning, hydrogen is fused to helium (without the need for oxygen). It should be noted that in the presence of carbon, nitrogen and oxygen, stars heavier than the Sun may burn hydrogen to helium by using the C, N and O as catalysts. Even in these stars, however, an absence of oxygen does not prevent nuclear burning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to read more, try &lt;a href=&quot;http://curious.astro.cornell.edu/physics/84-the-universe/stars-and-star-clusters/nuclear-burning/403-how-can-a-star-burn-with-no-oxygen-beginner&quot; rel=&quot;nofollow noreferrer&quot;&gt;How can a star burn with no oxygen? (Beginner)&lt;/a&gt; by Kristine Speakers.&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="20563" LastEditDate="2017-08-18T20:25:24.823" LastActivityDate="2017-08-18T20:25:24.823" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22716" PostTypeId="2" ParentId="22709" CreationDate="2017-08-18T12:42:45.450" Score="0" Body="&lt;p&gt;In the past 50 something years robots have been providing us with a wealth of information and incredible discoveries. There are many experts that agree with the idea that it will probably take human explorers to determine whether there ever was or even still life on Mars and therefor the conduct of many other scientific investigations that are not possible with robots alone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a very well written post with some strong points about that &lt;a href=&quot;http://www.foxnews.com/opinion/2017/01/17/six-essential-reasons-why-need-to-send-humans-to-mars.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Six essential reasons why we need to send humans to Mars&quot;&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="20563" LastActivityDate="2017-08-18T12:42:45.450" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22717" PostTypeId="2" ParentId="22709" CreationDate="2017-08-18T12:56:11.677" Score="13" Body="&lt;p&gt;One reason is that we don't have robots as versatile as humans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have machines capable of high speed, high precision, reliability and endurance not possible for a human, but no robot is good enough to look at a hole in a sheet of plastic covering the base, take a roll of duct tape and glue the hole closed within 30 seconds since the breach, unless you design and program that robot for that specific task, taking at least a year of work of a large team of engineers. And there are millions of such various easy, simple, but extremely varied tasks - a large number of them unknown, unforseen - where human intelligence, agility and dexterity allow to complete the &lt;em&gt;unplanned&lt;/em&gt; task within seconds or minutes from occurrence of condition that forced the task.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In particular, we don't have robots that can autonomously repair themselves or each other. And this is not going to happen in at least a couple decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we want to reach Mars beforehand, we will need humans to assist construction of the base. Robots can complete a large portion of the work, and about everything that was foreseen, but for the unforeseen humans will be a necessity.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-08-18T12:56:11.677" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22718" PostTypeId="2" ParentId="22710" CreationDate="2017-08-18T13:07:59.723" Score="23" Body="&lt;p&gt;The sun is not &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Combustion&quot; rel=&quot;noreferrer&quot;&gt;burning&lt;/a&gt;&quot; in the sense you are used to: there is no chemical reaction going on. Instead, there is a &lt;em&gt;very&lt;/em&gt; high pressure in the core of a star (like our sun) due to the high mass that starts/sustains a &lt;a href=&quot;https://en.wikipedia.org/wiki/Stellar_nucleosynthesis&quot; rel=&quot;noreferrer&quot;&gt;nuclear fusion process&lt;/a&gt;. In our sun, hydrogen is fused to helium and the energy that's released in this process is what makes the sun &quot;glow&quot; and emit energy.&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2017-08-18T13:07:59.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22719" PostTypeId="2" ParentId="22688" CreationDate="2017-08-18T14:40:45.463" Score="3" Body="&lt;p&gt;I have done some research, and I believe that there have been 5 CubeSats that have been launched into Geostationary Orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have put all those 5 CubeSats together with all the information I found.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qytz0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qytz0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source is &lt;a href=&quot;http://www.nanosats.eu&quot; rel=&quot;nofollow noreferrer&quot;&gt;World's largest database of nano satellites&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;I hope this is what you were asking for :D&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="20563" LastEditDate="2017-08-18T14:44:31.677" LastActivityDate="2017-08-18T14:44:31.677" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22720" PostTypeId="2" ParentId="22713" CreationDate="2017-08-18T15:45:16.370" Score="3" Body="&lt;p&gt;A coronal mass ejection travels at a speed of &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot; rel=&quot;nofollow noreferrer&quot;&gt;between 20 and 3200 km/s&lt;/a&gt;. The fastest launch ever (New Horizons) reached 16 km/s, so we can't do this at the moment.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, if you were to do this, you'd have to fly through the 'bow wave' of the CME to reach the low-radiation zone. The bow wave is a high-radiation environment. It remains to be seen if the net effect is positive or not. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-08-18T15:45:16.370" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22721" PostTypeId="2" ParentId="22688" CreationDate="2017-08-18T16:19:38.223" Score="7" Body="&lt;p&gt;No cubesats have yet been launched into GEO or GTO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Used the database suggested by Matthew, which is a wonderful database - &lt;a href=&quot;http://www.nanosats.eu/&quot; rel=&quot;nofollow noreferrer&quot;&gt;World's largest database of nano satellites&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the lists of CubeSat missions planned for GEO and GTO.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;strong&gt;SpectroCube&lt;/strong&gt; - 3U CubeSat by ESA, planned to be launched in Dec. 2020.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;GTOSat&lt;/strong&gt; - 6U CubeSat by GSFC, NASA planned to be launched in Dec. 2020.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Orbital Factory II&lt;/strong&gt; - 3U CubeSat by Uni. of Texas at El Paso&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;LACCE&lt;/strong&gt; (LousianA Coronal mass ejection Correlation Experiment) - 3U CubeSat by Uni. of Lousiana, Lafayette.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;ADE&lt;/strong&gt; (Aerodynamic Deorbit Experiment) - 3U CubeSat by Purdue University.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;DSM-BRAC&lt;/strong&gt; - 2U CubeSat by Uni. of Michigan.&#xA;The last four CubeSats are the winning entries of ULA's STEM education program. They get to hitch-hike to GTO for free!&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Cheers to Matthew again! :D&lt;/p&gt;&#xA;" OwnerUserId="20750" LastEditorUserId="20750" LastEditDate="2017-10-10T17:37:14.773" LastActivityDate="2017-10-10T17:37:14.773" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22722" PostTypeId="2" ParentId="22710" CreationDate="2017-08-18T16:21:40.550" Score="10" Body="&lt;p&gt;There is oxygen and many other elements on the Sun. The &quot;burning&quot; you refer to is fusion, it does not require oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oxygen is created by: 12 C + 4 He ------&gt; 16 O + energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case you are wondering how the carbon for that reaction is created it comes from: 3 (4He) ------&gt; 12C + energy, with the helium derived from: 4 (1H) ------&gt; 4 He + 2 e+ + 2 neutrinos + energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/activity-fusion.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/activity-fusion.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20766" LastEditorUserId="-1" LastEditDate="2017-08-18T22:47:07.027" LastActivityDate="2017-08-18T22:47:07.027" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22723" PostTypeId="2" ParentId="22710" CreationDate="2017-08-18T20:24:43.100" Score="4" Body="&lt;p&gt;The sun is so hot that if any steam were present, it would break down into a &quot;plasma&quot; of parts of hydrogen atoms, parts of oxygen atoms, and electrons.  Thus, the sun is not being heated by hydrogen and oxygen chemically burning to form steam.  Other answers mention the thermonuclear reactions (between especially fast-moving individual nuclei at the center of the sun) that heat the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Andrew M. Davis, Ko Hashizume, Marc Chaussidon, Trevor R. Ireland, Carlos Allende Prieto, and David L. Lambert's 2008 &lt;a href=&quot;http://rimg.geoscienceworld.org/content/68/1/73&quot; rel=&quot;nofollow noreferrer&quot;&gt;article&lt;/a&gt; on &quot;Oxygen in the Sun&quot;, the sun has about 468 atoms of oxygen per million atoms of hydrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article was published in &lt;em&gt;&lt;a href=&quot;http://rimg.geoscienceworld.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reviews in Mineralogy and Geochemistry&lt;/a&gt;&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="17891" LastEditorUserId="17891" LastEditDate="2018-07-12T06:58:10.070" LastActivityDate="2018-07-12T06:58:10.070" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22724" PostTypeId="1" AcceptedAnswerId="25486" CreationDate="2017-08-19T15:26:20.740" Score="7" ViewCount="2207" Body="&lt;p&gt;Propagation of AM and FM radio signals up into the sky would behave differently than propagation along the surface of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a &lt;a href=&quot;https://en.wikipedia.org/wiki/Thought_experiment&quot; rel=&quot;noreferrer&quot;&gt;Gedankenexperiment&lt;/a&gt;, if you were on the ISS and had an AM/FM radio sitting at the ISS' cupola window, what would it &lt;em&gt;sound like&lt;/em&gt; as you slowly tuned through one of the bands? Would it be packed full of stations competing at each frequency, or would it be nearly empty with only an unusually strong station making it through. Naturally AM and FM should be discussed separately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were hoping to catch a particularly weak AM or FM station, would you arrange to listen when you passed overhead, or would a different pass be best?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r3nps.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r3nps.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; from the Radio Jove Project's exercise &lt;a href=&quot;https://radiojove.gsfc.nasa.gov/education/educ/radio/tran-rec/exerc/iono.htm&quot; rel=&quot;noreferrer&quot;&gt;The Effects of Earth's Upper Atmosphere on Radio Signals&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gfp7B.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gfp7B.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;em&gt;Vintage Singer 9-Transistor Radio, AM-FM Bands, Model HE-229, Made in Japan.&lt;/em&gt; From &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Vintage_Singer_9-Transistor_Radio,_AM-FM_Bands,_Model_HE-229,_Made_in_Japan_(11000971123).jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="26446" LastEditDate="2021-04-06T05:35:13.667" LastActivityDate="2021-04-06T05:35:13.667" Title="How many stations could one hear with an AM/FM radio in front of the ISS' cupola window?" Tags="|iss|radio-communication|audio|" AnswerCount="4" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22729" PostTypeId="1" CreationDate="2017-08-19T18:01:44.383" Score="1" ViewCount="104" Body="&lt;p&gt;How to orient the ground station antenna and that of the satellite just after the launch?&lt;/p&gt;&#xA;" OwnerUserId="20773" LastActivityDate="2017-08-19T18:01:44.383" Title="Calibration of Antennae in satellite and in ground station" Tags="|artificial-satellite|antenna|telemetry|" AnswerCount="0" CommentCount="4" ClosedDate="2017-08-21T09:42:29.093" ContentLicense="CC BY-SA 3.0" />
  <row Id="22730" PostTypeId="1" CreationDate="2017-08-20T03:21:30.820" Score="1" ViewCount="5794" Body="&lt;p&gt;The characteristic length of a engine's combustion chamber $L^*$ is defined as the ratio of the chamber volume to nozzle throat area:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$L^*=\frac{V_{chamber}}{A_{throat}} $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be correct to say that it simply refers to the length of the  combustion chamber or, should we say that it is the minimum length over which fuel and oxidizer can undergo stoichiometric combustion?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you for sharing your knowledge.&lt;/p&gt;&#xA;" OwnerUserId="20418" LastEditorUserId="12102" LastEditDate="2017-08-20T05:28:01.557" LastActivityDate="2017-08-20T12:40:22.127" Title="What is Characteristic Length in a rocket engine?" Tags="|rockets|propulsion|rocket-equation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22731" PostTypeId="1" CreationDate="2017-08-20T07:13:50.410" Score="6" ViewCount="1331" Body="&lt;p&gt;This question has not been answered. This is different from the blueprints because it is a more general question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Off the top of my head there exists:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The testing data available on the NASA website&lt;/li&gt;&#xA;&lt;li&gt;Lunar module manuals &lt;/li&gt;&#xA;&lt;li&gt;Module source code&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What I have not seen is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Specifications&lt;/li&gt;&#xA;&lt;li&gt;Component level data&lt;/li&gt;&#xA;&lt;li&gt;Launch instructions&lt;/li&gt;&#xA;&lt;li&gt;Mission telemetry&lt;/li&gt;&#xA;&lt;li&gt;Incident reports/mission evaluations&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It would be interesting to know if any other documentation exists of Mankind's greatest achievement.&lt;/p&gt;&#xA;" OwnerUserId="20775" LastActivityDate="2017-08-20T22:36:16.023" Title="What documentation remains from Apollo?" Tags="|apollo-program|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22732" PostTypeId="1" AcceptedAnswerId="22746" CreationDate="2017-08-20T08:15:55.090" Score="7" ViewCount="679" Body="&lt;p&gt;The NASA Spaceflight article &lt;a href=&quot;https://www.nasaspaceflight.com/2017/08/russian-eva-begins-satellite-deployments/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Extended Russian EVA complete – conducts satellite deployments&lt;/a&gt; says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The first order of the day was for Ryazanskiy to head into the &lt;strong&gt;manual deployment of five nanosatellites from a ladder outside the airlock&lt;/strong&gt;, following set up tasks including the hosting of a Go-Pro camera. (emphasis added)&lt;/p&gt;&#xA;&lt;p&gt;The satellites, each of which has a mass of about 11 pounds, have a variety of purposes.&lt;/p&gt;&#xA;&lt;p&gt;One of the satellites, with casings made using 3-D printing technology, will test the effect of the low-Earth-orbit environment on the composition of 3-D printed materials. Another satellite contains recorded greetings to the people of Earth in 11 languages.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It sounds like they were just &lt;a href=&quot;https://space.stackexchange.com/q/17739/12102&quot;&gt;tossed&lt;/a&gt; by hand into space. What are the orbital mechanical consideration behind &amp;quot;hand-launching&amp;quot; things from the ISS? What keeps it from coming back and hitting you in the back of the head 93 minutes later? (or from the side 46.5 minutes later for that matter)&lt;/p&gt;&#xA;&lt;p&gt;Seriously, are there rules and constraints applied to launching satellites by hand from the ISS so that there is very little risk of it intercepting the ISS at some point later in time? Certain directions and/or speeds only?&lt;/p&gt;&#xA;&lt;p&gt;More background on the nanosatellites:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A third satellite &lt;strong&gt;commemorates the 60th anniversary of the Sputnik 1 launch and the 160th anniversary of the birth of Russian scientist Konstantin Tsiolkovsky&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-09-20T18:39:08.477" Title="What are the orbital mechanical consideration behind hand-launched nanosatellites from the ISS?" Tags="|iss|cubesat|payload-deployment|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22733" PostTypeId="1" CreationDate="2017-08-20T08:33:34.617" Score="4" ViewCount="229" Body="&lt;p&gt;The NASA Spaceflight article &lt;a href=&quot;https://www.nasaspaceflight.com/2017/08/russian-eva-begins-satellite-deployments/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Extended Russian EVA complete – conducts satellite deployments&lt;/a&gt; contains a very long discussion about the timing of the CRS-12 launch. Had it missed the &quot;single shot launch attempt&quot; it would have to be delayed by several days in deference to the upcoming Atlas V launch of TDRS-M.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Incidentally, there was an additional item of interest for Dragon’s ability to arrive within this short window of opportunity, relating to the communication systems on the ISS.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;which is followed by a long, nine-paragraph discussion of redundancy of communications, the space walk, Dragon's arrival, Field Effect Transistor (FET) Hybrid failure and the Canadian robot Dextre. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't follow this article very well at all. There are issues of timing, redundancy and a hint of a possibly impressive number of things that could have been going on at once. But I still don't really understand the point of this. What happened, or what could have happened, that would be so notable?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2023-09-24T12:33:01.293" Title="What is this NASA Spaceflight article trying to explain?" Tags="|iss|communication|eva|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22734" PostTypeId="2" ParentId="22731" CreationDate="2017-08-20T09:09:05.843" Score="11" Body="&lt;p&gt;As @xxavier notes, there's a large gap between existing and internet-accessible documentation. It seems likely that over time, more and more Apollo-era documentation will be digitized in order to reduce warehousing costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A vast amount of digitized material -- hundreds of thousands of documents -- is available on NASA's Technical Report Server. The best way to find it is with a google search for key words using &lt;code&gt;site:ntrs.nasa.gov&lt;/code&gt; to filter out other sources. NTRS has its &lt;a href=&quot;https://ntrs.nasa.gov/advSearch.jsp&quot; rel=&quot;noreferrer&quot;&gt;own search facility&lt;/a&gt;, but it's not of much use unless you know a lot about a specific document you're looking for. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Specifications &amp;amp; Component Level Data&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;I haven't seen much of this. You might have some luck searching for the name of a particular subsystem plus &quot;specification&quot;, but I see a lot more &quot;experience reports&quot; which mention specifications than the specifications themselves. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't found anything at all on the level of &quot;this thing is held together with #8 bolts&quot;, probably because those details are proprietary to the private companies that built the stages and spacecraft. This material is in more danger of being lost, because it's held by for-profit organizations but has only historical value. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find information at the level of an electrical schematic for the &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700018941.pdf&quot; rel=&quot;noreferrer&quot;&gt;Block II IMU Temperature Control Subsystem&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Adding the name of the contractor company might get you a little deeper. If you're having trouble sleeping, I recommend something like North American Rockwell's &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690023263.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Apollo Command and Service Module Stabilization and Control System Design Survey&quot;&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Launch Instructions&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;As launch of any major rocket is a complex choreographed operation involving hundreds of people, there's nothing like a single document with the launch instructions. Googling &lt;code&gt;saturn v launch manual&lt;/code&gt; yields documents like &lt;a href=&quot;https://history.nasa.gov/afj/ap08fj/pdf/sa503-flightmanual.pdf&quot; rel=&quot;noreferrer&quot;&gt;this one for Apollo 8 (SA-503)&lt;/a&gt;, but it's necessarily kind of a sketchy overview. While ostensibly intended as an orientation for the astronaut crews, it contains things of questionable value to that audience, such as detailed diagrams of the actuators for the F-1 engine gimbals.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Mission Telemetry&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Some of this has been digitized; a quick Googling turned up a doozy, &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160014527.pdf&quot; rel=&quot;noreferrer&quot;&gt;Apollo 15 PLSS Telemetry&lt;/a&gt;: raw images of strip charts of data from the portable life support backpacks showing things like suit pressure, battery performance, sublimator gas outlet temperature, etc. over the course of each EVA. &lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Incident Reports/Mission Evaluations&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Google &lt;code&gt;apollo # mission report&lt;/code&gt;. As an example, the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a15/ap15mr.pdf&quot; rel=&quot;noreferrer&quot;&gt;mission report for Apollo 15&lt;/a&gt; is 238 pages and includes among other details a table of each propulsive maneuver performed on the flight, results of scientific experiments, and dozens of minor equipment problems from &quot;loose object in cabin fans&quot; (a quarter-inch washer) to &quot;difficult to obtain water from in-suit drinking device&quot;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-20T22:36:16.023" LastActivityDate="2017-08-20T22:36:16.023" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22735" PostTypeId="1" AcceptedAnswerId="22739" CreationDate="2017-08-20T11:48:23.523" Score="3" ViewCount="2754" Body="&lt;p&gt;I know that different rockets are built with different a range of different sizes or purposes, but there must be some kind of range of typical speeds for those things when leaving the Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how fast are rockets or any other human made ship when leaving the Earth, after the engines shut down?&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="195" LastEditDate="2017-08-20T17:51:34.653" LastActivityDate="2017-08-21T10:26:34.720" Title="Roughly, how fast do rockets travel?" Tags="|rockets|earth|escape-velocity|" AnswerCount="1" CommentCount="7" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22736" PostTypeId="2" ParentId="22730" CreationDate="2017-08-20T12:40:22.127" Score="3" Body="&lt;p&gt;From Sutton, Rocket Propulsion Elements, 4th edition, page 277:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The &lt;em&gt;characteristic chamber length&lt;/em&gt; is defined as the length of a&#xA;rocket of the same volume would have if it were a straight tube and&#xA;had no converging section...&lt;/p&gt;&#xA;&lt;p&gt;(He then shows the same equation you do in&#xA;the question).&lt;/p&gt;&#xA;&lt;p&gt;...Here the chamber is considered to include all the&#xA;volume up to the throat area....Because this parameter does not&#xA;consider any variables except the throat area, it is useful only for a&#xA;particular propellant combination and a narrow range of mixture ratio&#xA;and chamber pressure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-20T12:40:22.127" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22737" PostTypeId="1" AcceptedAnswerId="22740" CreationDate="2017-08-20T15:05:13.187" Score="8" ViewCount="3123" Body="&lt;p&gt;The slingshot effect or gravity assist maneuver produces a boost on the speed of the object which attempts to approach a planet head-on at a speed V while the planet is moving directly toward us at a speed U (both speeds defined relative to the &amp;quot;fixed&amp;quot; Solar frame).&lt;/p&gt;&#xA;&lt;p&gt;Moon slingshot effect (if possible):&lt;/p&gt;&#xA;&lt;p&gt;A spaceship is traveling at a speed of v1 at an angle θ towards the moon, which is traveling an an orbital velocity (U) of 1 km/s. Then the spaceship gets away at a much higher speed of v2.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3UCN5.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3UCN5.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Could we actually use the moon for a gravity assist maneuver in order to boost up the speed of a spaceship?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-09-27T20:35:45.430" Title="Could we use the moon for a slingshot effect?" Tags="|rockets|the-moon|gravity-assist|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22738" PostTypeId="1" CreationDate="2017-08-20T16:30:03.540" Score="6" ViewCount="384" Body="&lt;p&gt;If there is a satellite or the ISS passing by over a point during a total solar eclipse in the darkest region, will the satellite be visible by naked eye or a pair of binoculars, since it will be dark on Earth and the slight sunlight from the uncovered part of sun might light the satellites or ISS up.&lt;/p&gt;&#xA;" OwnerUserId="20707" LastActivityDate="2017-08-22T14:29:51.320" Title="Can satellites or the ISS be seen from Earth during a Total Solar Eclipse?" Tags="|iss|artificial-satellite|visibility-of-spacecraft|solar-eclipse|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22739" PostTypeId="2" ParentId="22735" CreationDate="2017-08-20T16:55:25.807" Score="8" Body="&lt;p&gt;Your ballpark estimate, a velocity you can compare everything else to is: 8 km/s. This is the orbital speed in Low Earth Orbit (LEO); you'll see this number quite frequently. It's something to get started with. You need that much of &quot;horizontal&quot; speed, on top of whatever's needed to lift the rocket above the atmosphere to enter the orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Circular_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Circular orbit&lt;/a&gt; velocity: $v_o = \sqrt{{GM} \over r}$. &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;Escape velocity&lt;/a&gt; $v_e = \sqrt{{2GM} \over r}$. $ {v_e \over v_o} = \sqrt{2}$. So, multiply LEO speed by $\sqrt{2}$ (resulting in 11.3km/s) for escape speed of Earth. That multiplier applies to any body and any circular orbit, giving the speed needed for leaving given body. The speed for interplanetary transfers when escaping Earth will usually be higher - usually only slightly so (for Mars and Venus transfers), so that the probe reaches the target instead of escaping Earth gravity and ending up nowhere in particular.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then more advanced orbital mechanics kicks in, varying the speed wildly, regardless of what you measure the speed relative to. Earth moves around the Sun at ~30km/s so once you escape Earth gravity, this is the baseline to which you add (or subtract) your travel speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In strongly eccentric elliptical orbits, the speed will vary depending on where in the orbit you are: near the body it will be close to escape speed; at apoapsis you'll crawl at scarce meters per second, taking weeks until gravity pulls you back in for something of like half an hour dashing past the planet. Low gravity bodies, like comets or asteroids will have very slow orbits, you could enter orbit of Cruithne, a 10km asteroid, by &lt;a href=&quot;https://physics.stackexchange.com/questions/46318/is-there-a-small-enough-planet-or-asteroid-you-can-orbit-by-jumping/46325#46325&quot;&gt;running&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;High circular orbits are much slower than low orbits; Pluto's about 1km/s (vs Earth's 30km/s). Moon is also 1km/s (relative to Earth; vs LEO 8km/s.) I'd recommend playing &lt;a href=&quot;https://kerbalspaceprogram.com/en/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kerbal Space Program&lt;/a&gt; to understand how the speed varies in orbital mechanics - e.g. how your rocket blasting at 11km/s past Earth's upper atmosphere ends up crawling at speed of a bicycle once it approaches the edges of Earth sphere of influence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But your ballparks are 8km/s for entering LEO, 1km/s for Moon, or trans-Neptunian objects, 30km/s for solar orbit around Earth altitude. And a scarce meter per second or less for orbiting asteroids and the likes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember, multiply circular orbit by $\sqrt{2}$ for escape speed; to escape Earth, $30\sqrt{2} = 42$ km/s to escape the solar system. &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-08-21T10:26:34.720" LastActivityDate="2017-08-21T10:26:34.720" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22740" PostTypeId="2" ParentId="22737" CreationDate="2017-08-20T17:30:05.607" Score="9" Body="&lt;p&gt;Your equation is correct, but it does a few assumptions that are seldom applicable to reality. For instance, it ignores the mass of the planet and also how close the fly by is. The later parameter is limited by the radius of the body, or the outer edge of its atmosphere. If we indeed treat the planet as a point mass like that, we can always achieve a &quot;perfect turning angle&quot;. That is, the hyperbola of the flyby can always turn completely around to change the direction of the $\vec{v_{\infty}}$ vector. This would a speed boost once the coordinate system is transformed back (for the head-on case, this simplifies to $v_2=v_1+2u).$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The turning angle is however often very limited! It can be calculated in the following way:&#xA;$$\delta=2\sin^{-1}{\left(\frac{1}{1+\frac{r_p v_{\infty}^2}{\mu}}\right)}$$&#xA;We can see directly from this that the turning angle diminishes with low planet mass, high flyby altitude and especially high entry velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Moon example&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Head-on, a Hohmann transfer towards the Moon has a relative velocity of around 850m/s. Using the radius of the Moon as the lowest possible flyby radius, the turning angle is then 106°. Not bad, but definitely not a complete 180°.&#xA;Transforming the coordinate system back, that means the velocity is now ~1500m/s. That is approximately the escape velocity of the Earth system at that altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But&lt;/strong&gt;. The difference between a Moon transfer and a complete escape is only 90m/s in LEO. That is not much saved. To make matters worse, this is for barely escaping. Usually, you would like to have an injection into a interplanetary transfer from this manoeuvre. That would require a higher initial velocity and therefore a lower turn angle, ultimately decreasing the benefit further. A delta-v saving of less than 90m/s is not worth substantially narrowing launch windows for.&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-08-20T17:30:05.607" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22741" PostTypeId="2" ParentId="22660" CreationDate="2017-08-20T18:29:26.940" Score="3" Body="&lt;p&gt;The Range Safety Officer? Nowadays no, because Falcon 9 uses the &lt;a href=&quot;http://www.spaceflightinsider.com/space-centers/ccafs/automated-flight-safety-improving-space-access/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Automated Flight Safety System (AFSS)&lt;/a&gt; (spaceflightinsider.com link)&lt;/p&gt;&#xA;&lt;p&gt;However, AFSS can destruct the Falcon during landing. In fact this is crucial for Falcon heavy. From the spaceflightinsider.com article linked above:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Additionally, AFSS can support multiple objects in simultaneous flight, which is crucial for companies like SpaceX, which plans to land multiple first stage cores for its Falcon Heavy vehicle at nearly the same time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From the Florida Today article &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/2017/03/11/spacex-autonomous-flight-safety-system-afss-kennedy-space-center-florida-falcon9-rocket-air-force-military/98539952/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Only on Falcon 9: Automated system can terminate SpaceX rocket launches&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SpaceX wants to land the two side boosters back at Cape Canaveral, while the middle booster flies on to a ship at sea, so all that three can be recovered and potentially reused. But current Range systems can’t track more than one returning booster.&lt;/p&gt;&#xA;&lt;p&gt;“If you want to fly multiple boosters back, they have got to be autonomous,” said Monteith. “Otherwise, they’ve got to put them in the ocean.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Of course, this implies that before AFSS was introduced, the Range Safety Officer had the ability to destruct the Falcon 9 during landing if necessary.&lt;/p&gt;&#xA;" OwnerUserId="548" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-20T19:35:07.470" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22742" PostTypeId="2" ParentId="4797" CreationDate="2017-08-21T01:58:21.810" Score="10" Body="&lt;p&gt;The accepted answer describes the general Apollo mission plan, but as the question references Apollo 11 specifically, we can check &lt;a href=&quot;https://history.nasa.gov/SP-4029/Apollo_11i_Timeline.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;the mission timeline&lt;/a&gt; to see all the maneuvers done during the mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In later mission timelines the engine used for midcourse correction is consistently called out, but for Apollo 11 I had to check the transcripts to verify.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was one SPS midcourse correction burn on the outbound leg of the trip. Four optional corrections were scheduled in the flight plan, but three were canceled as unnecessary. (Only one or two are ever needed, but by scheduling four they gain a lot of flexibility.) The sole midcourse correction on the return leg &lt;a href=&quot;https://history.nasa.gov/afj/ap11fj/22day7-leave-lsi.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;was done on RCS according to the transcripts&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;0:00 S-IC ascent&lt;/li&gt;&#xA;&lt;li&gt;0:02 S-II ascent&lt;/li&gt;&#xA;&lt;li&gt;0:09 1st S-IVB Earth orbit insertion/circularization&lt;/li&gt;&#xA;&lt;li&gt;2:44 2nd S-IVB trans-lunar injection&lt;/li&gt;&#xA;&lt;li&gt;26:44 SPS midcourse correction &lt;/li&gt;&#xA;&lt;li&gt;75:49 SPS lunar orbit insertion&lt;/li&gt;&#xA;&lt;li&gt;80:11 SPS lunar orbit circularization&lt;/li&gt;&#xA;&lt;li&gt;101:36 LM DPS descent orbit insertion&lt;/li&gt;&#xA;&lt;li&gt;102:33 LM DPS descent&lt;/li&gt;&#xA;&lt;li&gt;124:22 LM APS ascent&lt;/li&gt;&#xA;&lt;li&gt;135:23 SPS trans-Earth injection&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So that's 11 total burns if you count each stage of the ascent separately. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is one more burn that's not strictly necessary to complete the mission; the S-IVB stage used its auxiliary propulsion system to fine-tune its trajectory for a lunar slingshot maneuver, or, on some later missions, to crash into the moon.  &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-12-17T15:08:22.190" LastActivityDate="2019-12-17T15:08:22.190" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22743" PostTypeId="1" CreationDate="2017-08-21T11:15:42.817" Score="5" ViewCount="902" Body="&lt;p&gt;Government space agencies and some of the largest space companies are talking about building a space station somewhere in cis-Lunar space, the so called &quot;Gateway&quot;. As far as I've heard, the (still very preliminary) plans are to assemble its modules in the intended final orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't it be safer and cheaper to assemble it in Low Earth Orbit instead?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Safer given the experiences from the ISS and with much better radiation protection for the crew and the option for quick evacuation to Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cheaper given that chemical rockets would only need to launch the modules of the space station to LEO instead of to a higher delta-v orbit at the Moon. The space station, when ready to commence operations, could then be towed outwards to its intended orbit by a Solar electric propulsion spacecraft. Since the now canceled Asteroid Retrieval Mission was formulated for over half a decade ago, it has been a priority for NASA to find some kind of mission for a powerful SEP-tow to prove the abilities of this propulsion technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a recent Boeing presentation of a Gateway station concept:&#xA;&lt;a href=&quot;http://spirit.as.utexas.edu/~fiso/telecon/Duggan_8-9-17/&quot; rel=&quot;noreferrer&quot;&gt;http://spirit.as.utexas.edu/~fiso/telecon/Duggan_8-9-17/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2018-04-24T22:57:40.970" Title="What are the advantages and disadvantages of assembling a space station in LEO before sending it to its intended Lunar orbit?" Tags="|low-earth-orbit|orbital-assembly|cislunar|lunar-gateway|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22744" PostTypeId="2" ParentId="22743" CreationDate="2017-08-21T13:13:02.800" Score="5" Body="&lt;p&gt;Making this a community wiki answer so others may add to it as they see fit:&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Advantages&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;LEO assembly allows less mission risk if assembly does not go completely as planned.  It's much easier to schedule and launch replacements or corrective hardware.&lt;/li&gt;&#xA;&lt;li&gt;LEO assembly allows for checkout and stress testing of hardware in a location where failure allows a safe return of crew in a matter of hours, rather than days.&lt;/li&gt;&#xA;&lt;li&gt;&quot;Once you're in orbit, you're halfway to anywhere.&quot; -- Keeping the upmass of individual launches small allows the use of existing, proven launch technology to get components up for assembly, lowering the program risk posture and reducing the consequences of a single launch failure.  As the question implies, transport to higher orbits can then be accomplished over a longer period of time with less mechanical stress to the vehicle using a low-thrust, high-$I_{sp}$ electric thrust system.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Disadvantages&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Raising the apogee of a structure in the mass class of the ISS to lunar orbit levels with electric propulsion will have to happen over the course of months.  During this time, the vehicle will spend an unprecedented amount of time in two particularly dangerous regions: the Van Allen belts, which could severely damage the avionics on board, and a region of LEO between roughly 800 and 1400 km altitude, in which the orbital debris threat is roughly 100 times worse than the ISS altitude.  While it's impossible to get to lunar orbit without passing through these regions, all missions to date have spent as little time as possible exactly to minimize the risk from passing through them.&lt;/li&gt;&#xA;&lt;li&gt;Multiple launches to LEO for assembly vs. fewer launches to lunar orbit means more wasted upmass in terms of launch vehicles.  While the consequences of any one launch failure are greater, the probability of having at least one launch failure does depend on the number of launches.  Though the probability per launch will not be the same for different architectures, the aggregate difference may sufficiently offset the program level risk.&lt;/li&gt;&#xA;&lt;li&gt;The space environments in LEO and lunar orbit are sufficiently different that an optimal design for one area is far from optimal for the other.  Among the environments that are drastically different between LEO and lunar orbit: thermal (earth albedo mellowing things out in LEO, not so much in lunar orbit), radiation (galactic cosmic rays are mitigated by the Van Allen belts in LEO, no such protection in lunar orbit), MMOD (basically no orbital debris in lunar orbit, and if we're smart, it will stay that way), atomic oxygen (LEO still has wispy tendrils of earth's atmosphere, and AO is insidious enough that it is always a design consideration, lunar orbit has basically none).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-08-21T13:13:02.800" CommentCount="3" CommunityOwnedDate="2017-08-21T13:13:02.800" ContentLicense="CC BY-SA 3.0" />
  <row Id="22745" PostTypeId="2" ParentId="22714" CreationDate="2017-08-21T15:11:32.770" Score="2" Body="&lt;p&gt;Generally NASA responds to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Decadal_survey&quot; rel=&quot;nofollow noreferrer&quot;&gt;decadal surveys&lt;/a&gt; put together by the &lt;a href=&quot;https://en.wikipedia.org/wiki/National_Academies_of_Sciences,_Engineering,_and_Medicine&quot; rel=&quot;nofollow noreferrer&quot;&gt;National Academies of Science&lt;/a&gt; for each major science branch (i.e., Earth, Planetary, Astrophysics, and Heliophysics).  The decadal surveys are constructed by a panel of experts, at the request of NASA, from the respective field and presented as a sort of suggestion for the path the field should take over the next decade.  It generally tries to frame the current, big unanswered questions that could be addressed by new/future missions.  In those reports, the panels often make suggestions for new missions including size/scale (e.g., &lt;a href=&quot;https://en.wikipedia.org/wiki/Explorers_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;MIDEX&lt;/a&gt;), goals, and timelines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no law stating that NASA must obey the recommendations of these surveys but there is also no reason to ignore them since they are put together by the very people NASA supports and people supporting NASA.  So far as I know, there has never been a decadal survey that was ignored by NASA but there is some &quot;wiggle room&quot; for interpretation.  The panels are given a little guidance regarding limitations on (expected) funding and scope of potential future missions but not much else.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2017-08-21T15:11:32.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22746" PostTypeId="2" ParentId="22732" CreationDate="2017-08-21T18:35:47.413" Score="4" Body="&lt;p&gt;If you launch nadir and retrograde, you will put the object into a lower energy orbit such that, barring ISS deboosts (they do happen, but they are rare), it will never again intersect ISS's orbit.  Launching prograde, zenith, or out of plane will set you up for a potential recontact scenario.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These objects are also large enough that they can be independently tracked once they've gained enough separation from ISS, so any unlikely potential recontact scenario can be averted.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-08-21T18:35:47.413" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22747" PostTypeId="1" CreationDate="2017-08-21T19:40:22.980" Score="7" ViewCount="445" Body="&lt;p&gt;Have any space stations experienced a total solar eclipse [edit: with the moon blocking out the sun, and seen from the vicinity of the Earth *sigh*]? Otherwise, will any manned satellites pass through one in the foreseeable future?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, are there any real video clips shot from space of a satellite or spacecraft moving into or out of totality, showing the moon's shadow moving on the limb of the Earth, and even better, with the sun and moon in the frame?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All I could find is a partial one from the ISS:&#xA;&lt;a href=&quot;http://time.com/4908629/total-solar-eclipse-international-space-station/&quot; rel=&quot;noreferrer&quot;&gt;http://time.com/4908629/total-solar-eclipse-international-space-station/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="555" LastEditorUserId="555" LastEditDate="2017-08-22T11:08:46.880" LastActivityDate="2021-04-06T22:13:51.117" Title="Have any space stations experienced a total solar eclipse?" Tags="|artificial-satellite|crewed-spaceflight|space-station|solar-eclipse|" AnswerCount="2" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22750" PostTypeId="1" CreationDate="2017-08-21T21:05:57.443" Score="3" ViewCount="226" Body="&lt;p&gt;Found &lt;a href=&quot;http://www.breakthroughinitiatives.org/News/4&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; (italics added, bold in source):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Nanocrafts are gram-scale robotic spacecrafts comprising two main parts:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;strong&gt;StarChip&lt;/strong&gt;: Moore’s law has allowed a dramatic decrease in the size of microelectronic components. This creates the possibility of a &lt;strong&gt;gram-scale wafer&lt;/strong&gt;, carrying cameras, &lt;em&gt;photon thrusters&lt;/em&gt;, power supply, navigation and communication equipment, and constituting a fully functional space probe.&lt;/li&gt;&#xA;  &lt;li&gt;&lt;strong&gt;Lightsail&lt;/strong&gt;: Advances in nanotechnology are producing increasingly thin and light-weight metamaterials, promising to enable the fabrication of &lt;strong&gt;meter-scale&lt;/strong&gt; sails no more than &lt;strong&gt;a few hundred atoms thick&lt;/strong&gt; and at &lt;strong&gt;gram-scale mass&lt;/strong&gt;.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What are those photon thrusters? Why would it need thrusters, since its sail is pushed by an earth-based huge laser?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think some man-made device able to reach 0.2c would already be an amazing achievement, even if it doesn't send back any picture from another stellar system, and even if it misses its destination. (Sputnik only said &quot;bip&quot; to tell everyone &quot;I'm orbiting.&quot;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What could be the mass of a probe saying &quot;I'm going 0.145c&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Probe's components would be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;self aligning sail (with dihedral)&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;thin photovoltaic sheet powering an emitter with antenna&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;no photonic thruster or navigation system.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;no camera or any optical device&lt;/li&gt;&#xA;&lt;li&gt;no battery&lt;/li&gt;&#xA;&lt;li&gt;no reaction wheel or gyro  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="6816" LastEditDate="2017-08-22T16:17:50.080" LastActivityDate="2017-08-22T16:17:50.080" Title="Why would &quot;Breakthrough Starshot&quot; need photon thrusters?" Tags="|engines|laser|light-sail|breakthrough-starshot|" AnswerCount="0" CommentCount="12" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22751" PostTypeId="2" ParentId="22738" CreationDate="2017-08-21T23:22:20.943" Score="7" Body="&lt;p&gt;As an answer to the question in your title, NASA photographer &lt;a href=&quot;https://www.flickr.com/photos/gsfc/36548067642/in/photostream/&quot; rel=&quot;noreferrer&quot;&gt;Joel Kowsky&lt;/a&gt; captured this stunning composite earlier today in Banner, Wyoming. The positioning required to perfectly frame Station during this solar transit took months of planning, and the window of opportunity was brief enough that it was shot at 1,500 frames per second with a high-speed camera.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rribf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rribf.jpg&quot; alt=&quot;Composite image of seven frames showing the International Space Station transit the surface of the sun during partial eclipse conditions.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Goddard Space Flight Center has a &lt;a href=&quot;https://www.flickr.com/photos/gsfc/36717505815/in/photostream/&quot; rel=&quot;noreferrer&quot;&gt;flickr album&lt;/a&gt; that includes video of the transit, as well as a few other details of its capture.&lt;/p&gt;&#xA;" OwnerUserId="5851" LastActivityDate="2017-08-21T23:22:20.943" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22752" PostTypeId="1" CreationDate="2017-08-22T05:24:58.237" Score="2" ViewCount="669" Body="&lt;p&gt;An alien intelligence, after observing Voyagers trajectory to work out where it came from, would know that there is intelligent life on Earth.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How could they use that information to start communicating with us? &lt;/p&gt;&#xA;" OwnerUserId="20714" LastEditorUserId="20714" LastEditDate="2017-08-25T07:48:25.520" LastActivityDate="2017-08-25T07:48:25.520" Title="Can aliens use Voyager to communicate with us?" Tags="|voyager|" AnswerCount="1" CommentCount="4" ClosedDate="2017-08-23T00:45:10.397" ContentLicense="CC BY-SA 3.0" />
  <row Id="22753" PostTypeId="1" CreationDate="2017-08-22T06:25:58.417" Score="1" ViewCount="94" Body="&lt;p&gt;Imagine we want to go to Alpha Centauri,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume we are at position &lt;code&gt;{100, 100, 100}&lt;/code&gt; and the star is at position &lt;code&gt;{1000, 1000, 1000}&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this way of positioning (cartesian coordinates) sufficient for such journey ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I mean by that is, in real life generally things are more complex than a simple vector, there are certain roads you must take, things you'd want to avoid and so on. And here you have GPS which while does the positioning it also has a database of places/routes etc ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Like when going to the moon, while it's clear on where to go, it is not a matter of going on a straight line, you have to avoid stuff like space debris around earth and so on (simplified example).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would one need when travelling far in space, beside destination coordinates ?&lt;/p&gt;&#xA;" OwnerUserId="1368" LastEditorUserId="1368" LastEditDate="2017-08-22T14:43:35.707" LastActivityDate="2017-08-22T14:43:35.707" Title="Are destination coordinates sufficient to travel to a far place?" Tags="|travel|trajectory|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22754" PostTypeId="2" ParentId="22752" CreationDate="2017-08-22T07:26:36.863" Score="2" Body="&lt;p&gt;The aliens wouldn't be able to use Voyager's systems directly - by the time it reaches them, they would have deteriorated beyond use, never mind even now we're near the limit of Voyager's antenna power capabilities - there's zero chance to hear it from a different system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the aliens should be able to reverse-engineer the system. They might find out what frequencies we use; on the off-chance scraps of memory of the computer are recoverable (likely by taking it apart atom by atom and performing statistical analysis on the memory cells to find traces of data that used to be there) they might even reverse-engineer the encodings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, whether they'd be able to build a transmitter of power (or beam narrow enough) to reach the Solar System? We know for a fact, we can't - but that's likely a couple million years from now and we can't foresee advances in technology on such timescales. So they might use the &lt;em&gt;information&lt;/em&gt; to build a system to send signal we'd be able to decode and understand. But no, using the actual electronics of Voyager as the radio would be completely implausible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Never mind we keep making advances in space technology; it's likely by the time Voyagers cover 5% of the way to the next star systems, our newer probes will have long reached it, overtaking the old Voyagers, traveling at much higher speed... the only plausible alternative to that is that the aliens send the signal towards a dark, heavily irradiated, dead husk of Earth. The disks were very evocative, a wonderful publicity stunt, but Voyagers are simply too slow to get anywhere before either we get there by other means, or destroy ourselves.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-08-22T07:26:36.863" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22755" PostTypeId="1" AcceptedAnswerId="22756" CreationDate="2017-08-22T07:39:30.330" Score="2" ViewCount="157" Body="&lt;p&gt;Sending a signal at interstellar distance requires enormous power - something of order not too far from energies of the stars. Even if we harness the power of stars, modulating that signal would take a lot of energy - though the actual &lt;em&gt;power&lt;/em&gt; required could be reduced - at cost of bandwidth. Imagine a swarm of statites near Oort Cloud, designed for such task: crafts with solar sails large enough to eclipse the Sun as seen from other star to a visible degree, able to turn the sails to reveal it or obscure it - modulate Sun brightness as seen from the distant destination - but the process of turning the statite takes a week. So our bandwidth is of order of 1 bit per week.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does SETI search signals modulated as slowly? Transmissions of order of bit per week, month, year? &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-08-22T08:50:24.633" Title="Does SETI perform search of extremely low bandwidth signals?" Tags="|communication|interstellar-travel|seti|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22756" PostTypeId="2" ParentId="22755" CreationDate="2017-08-22T08:50:24.633" Score="3" Body="&lt;p&gt;SETI analyses signals in the 1000MHz to 9000MHz range. So you would expect communications modulated on these frequencies to utilise the high frequency to include more data rather than less.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the data on, say, the hydrogen line at 1420 MHz and the methanol line at 6667 MHz are stored, and analyses are run on these over short, medium and long timeframes and yes, cycles or variations of this length are detected, and in fact have been used to identify interesting astronomical objects (variations in signal can imply occultation by something like a moon or other orbiting body)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This does present a problem - if low frequency variation could be occultation or other natural process, why would an alien civilisation use it to communicate? Rather than a large, slow solar sail, why not thousands of small sails acting in concert, able to change position rapidly and give you changes sub 1-second?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TL;dr - yes, SETI does consider low frequency variations, but it is not an ideal channel for communication.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2017-08-22T08:50:24.633" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22757" PostTypeId="1" AcceptedAnswerId="22879" CreationDate="2017-08-22T09:08:00.247" Score="5" ViewCount="190" Body="&lt;p&gt;Does there exist two orbits synchronized so that a telescope in one would line up with a &lt;a href=&quot;https://science.nasa.gov/technology/technology-stories/starshade-enable-first-images-earth-sized-exoplanets&quot; rel=&quot;nofollow noreferrer&quot;&gt;star shade&lt;/a&gt; in the other on a line pointing to a certain star? And this during a period of time relevant for great astronomical imaging, such as hours or days. And in the Solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any deeper meaning to why it is so hard or impossible to put three points on a straight line in this spacetime?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="12102" LastEditDate="2017-08-26T13:42:27.763" LastActivityDate="2017-09-02T16:23:27.547" Title="Are there orbits for a starshade and a telescope to stay linear to an interstellar object?" Tags="|exoplanet|space-telescope|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22758" PostTypeId="1" AcceptedAnswerId="22761" CreationDate="2017-08-22T13:31:01.050" Score="24" ViewCount="10042" Body="&lt;p&gt;Both &lt;a href=&quot;https://en.wikipedia.org/wiki/X-15_Flight_90&quot; rel=&quot;noreferrer&quot;&gt;Flight 90&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/X-15_Flight_91&quot; rel=&quot;noreferrer&quot;&gt;Flight 91&lt;/a&gt; of the &lt;a href=&quot;https://en.wikipedia.org/wiki/North_American_X-15&quot; rel=&quot;noreferrer&quot;&gt;North American X-15&lt;/a&gt; crossed the &lt;a href=&quot;https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line&quot; rel=&quot;noreferrer&quot;&gt;Kármán line&lt;/a&gt;, reaching altitudes of 106.01 and 107.96 km respectively. Both flights were piloted by &lt;a href=&quot;https://en.wikipedia.org/wiki/Joseph_A._Walker&quot; rel=&quot;noreferrer&quot;&gt;Joseph A. Walker&lt;/a&gt;, who became in 1963 the &quot;United States' seventh man in space&quot; and &quot;qualified him as an astronaut under the rules of the U.S. Air Force and the &lt;a href=&quot;https://en.wikipedia.org/wiki/F%C3%A9d%C3%A9ration_A%C3%A9ronautique_Internationale&quot; rel=&quot;noreferrer&quot;&gt;Fédération Aéronautique Internationale&lt;/a&gt; (FAI)&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://en.wikipedia.org/wiki/North_American_X-15&quot; rel=&quot;noreferrer&quot;&gt;X-15&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;By November 1960, Reaction Motors was able to deliver the XLR99 rocket engine, generating 57,000 pounds-force (250 kN) of thrust. The remaining 175 flights of the X-15 used XLR99 engines, in a single engine configuration. The XLR99 used anhydrous ammonia and liquid oxygen as propellant, and hydrogen peroxide to drive the high-speed turbopump that delivered propellants to the engine. It could burn 15,000 pounds (6,804 kg) of propellant in 80 seconds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I hadn't heard of anhydrous ammonia as a fuel before reading this. It needs to be either pressurized or cryogenic (about -33C) to remain as a liquid, unlike organic fuels like alcohols or the heavier hydrocarbons. What were the various reasonings behind developing an ammonia burning engine in this case? Has ammonia been used again after the X-15? Or before for that matter!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SJHYG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SJHYG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Test pilot and astronaut &lt;a href=&quot;https://en.wikipedia.org/wiki/Joseph_A._Walker&quot; rel=&quot;noreferrer&quot;&gt;Joseph A. Walker&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lQl65.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lQl65.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_Motors_XLR99&quot; rel=&quot;noreferrer&quot;&gt;Reaction Motors XLR99&lt;/a&gt; rocket engine from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:XLR-99_Rocket_Engine_USAF.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xHR71.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xHR71.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above&lt;/strong&gt; X-15 from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:X-15.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-09T01:24:02.350" LastActivityDate="2021-08-09T01:24:02.350" Title="Reasons why liquid anhydrous ammonia fuel chosen for the X-15? Has it been used in other rocket engines?" Tags="|engines|fuel|liquid-fuel|x-15|ammonia|" AnswerCount="5" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22759" PostTypeId="2" ParentId="22738" CreationDate="2017-08-22T13:48:30.137" Score="2" Body="&lt;p&gt;If the ISS was overhead during the Total Solar Eclipse, you could have seen it. During Totality, it isn't nearly as dark as during actual night, only a few bright planets were visible. But the ISS is bright enough that it could be seen, if it happened to be overhead at the right time. You might be able to see a few other really bright satellites as well, but not many. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-08-22T14:29:51.320" LastActivityDate="2017-08-22T14:29:51.320" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22760" PostTypeId="2" ParentId="22758" CreationDate="2017-08-22T14:35:44.503" Score="14" Body="&lt;p&gt;The book &lt;em&gt;Aerofax Datagraph 2 / North American X-15/X-15A-2&lt;/em&gt; by Ben Guenther, Jay Miller, and Terry Panopalis has some more info on the history of the propellant choice (page 27):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Eventually it was determined that the two most important requirements&#xA;  from a safety standpoint concerned the propellant combination, and the&#xA;  means of achieving combustion safety during starting and shut-down.&#xA;  Seven propellant combinations were explored in depth, these eventually&#xA;  being narrowed to liquid oxygen as the oxidizer and anhydrous ammonia&#xA;  as the fuel. The choice was based primarily on the fact that Reaction&#xA;  Motors had significant experience with liquid oxygen/ammonia&#xA;  propellant systems, and also on the fact that this propellant&#xA;  combination had much less critical starting characteristics. &#xA;  Additionally, the liquid oxygen/ammmonia combination was an ideal&#xA;  coolant for the regenerative cooling of the proposed engine's thrust&#xA;  chamber.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;FWIW, this book also has a good writeup on the developmental history of the XLR-99 and specifics on its design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;https://nh3fuel.files.wordpress.com/2013/01/2011-seaman-huson.pdf&quot; rel=&quot;noreferrer&quot;&gt;pdf of a powerpoint presentation&lt;/a&gt; the choice was made because one of the chief designers at Reaction Motors Dr. Paul F. Winternitz was an advocate of NH3/LOX engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rationale for propellant choice:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The directive to use NH 3 came from Dr. Paul F. Winternitz, a&#xA;propellant scientist from Austria.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Dr. Winternitz had to find a fuel that would be stable, would be easy&#xA;to keep, show a good volumetric energy density, density, would work in&#xA;in the the temp range and allow conclusions for a later H2 fuel&#xA;system.&lt;/li&gt;&#xA;&lt;li&gt;NH 3 /LOX fit the bill and it worked!&lt;/li&gt;&#xA;&lt;li&gt;Later, when gravimetric energy density was more important than&#xA;volumetric energy density (for the Shuttle) H 2 was preferred&lt;/li&gt;&#xA;&lt;li&gt;Good heat transfer properties&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The presentation is a bit sketchy (and incorrectly states that an earlier Reaction Motors built engine, the XLR-10 was NH3 fueled), but includes this wonderful schematic of the XLR-99.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1tXSW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1tXSW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Interestingly, this presentation was given to the &lt;a href=&quot;https://nh3fuelassociation.org&quot; rel=&quot;noreferrer&quot;&gt;NH3 Fuel Association&lt;/a&gt; whose goals are &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;to promote NH3 as an affordable, sustainable, carbon-free fuel for&#xA;  stationary power, transportation, and energy storage applications,&#xA;  thereby reducing dependence on fossil fuels and enabling the&#xA;  transition to a low-carbon economy.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-08-22T19:42:58.730" LastActivityDate="2017-08-22T19:42:58.730" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22761" PostTypeId="2" ParentId="22758" CreationDate="2017-08-22T14:47:19.347" Score="38" Body="&lt;p&gt;According to Clark's &lt;a href=&quot;https://library.sciencemadness.org/library/books/ignition.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Ignition!&quot;&lt;/a&gt;, German rocket scientists in WW2 had done the math on ammonia, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory&quot; rel=&quot;noreferrer&quot;&gt;JPL&lt;/a&gt; had burned it with &lt;a href=&quot;https://en.wikipedia.org/wiki/Red_fuming_nitric_acid&quot; rel=&quot;noreferrer&quot;&gt;RFNA&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/White_fuming_nitric_acid&quot; rel=&quot;noreferrer&quot;&gt;WFNA&lt;/a&gt; oxidizers in 1949-1951.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regarding the &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_Motors_XLR99&quot; rel=&quot;noreferrer&quot;&gt;XLR99&lt;/a&gt;, Clark says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But something more potent than alcohol was needed for the X-15 rocket-driven supersonic research plane. Hydrazine was the first choice, but it sometimes exploded when used for regenerative cooling, and in 1949, when the program was conceived, there wasn't enough of it around, anyway. Bob Truax of the Navy, along with Winternitz of Reaction Motors, which was to develop the 50,000 pounds thrust motor, settled on ammonia as a reasonably satisfactory second best. The oxygen-ammonia combination had been fired by JPL, but RMI really worked it out in the early 50's. The great stability of the ammonia molecule made it a tough customer to burn and from the beginning they were plagued with rough running and combustion instability. All sorts of additives to the fuel were tried in the hope of alleviating the condition, among them methylamine and acetylene. Twenty-two percent of the latter gave smooth combustion, but was dangerously unstable, and the mixture wasn't used long. The combustion problems were eventually cured by improving the injector design, but it was a long and noisy process.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;At that point in time, state of the art in big rockets was the &lt;a href=&quot;https://en.wikipedia.org/wiki/PGM-11_Redstone&quot; rel=&quot;noreferrer&quot;&gt;Redstone&lt;/a&gt;, which was burning 75/25 ethyl alcohol/water with LOX in an engine which was largely copied from the &lt;a href=&quot;https://en.wikipedia.org/wiki/V-2_rocket&quot; rel=&quot;noreferrer&quot;&gt;V-2&lt;/a&gt;; watering the fuel down was necessary to moderate combustion temperature. While regeneratively cooled, the design of the cooling tubes was not as complexly efficient at that time as it would be in later engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kerosene would &lt;a href=&quot;https://en.wikipedia.org/wiki/Polymerization&quot; rel=&quot;noreferrer&quot;&gt;&quot;coke up&quot; (polymerize)&lt;/a&gt; in regeneratively cooled engines, potentially catastrophically clogging coolant tubes. That problem would eventually be solved by the development of the &lt;a href=&quot;https://en.wikipedia.org/wiki/RP-1&quot; rel=&quot;noreferrer&quot;&gt;RP-1&lt;/a&gt; kerosene specification in the mid-50s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So during the development of the XLR99, ammonia had a useful niche -- it gave better performance than 75% alcohol, was more suitable for regenerative cooling than cheap kerosene, and was safer than the hydrazine fuels.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-02-21T05:30:37.937" LastActivityDate="2019-02-21T05:30:37.937" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="22762" PostTypeId="2" ParentId="22757" CreationDate="2017-08-22T15:53:50.050" Score="3" Body="&lt;p&gt;I do not think the problem is the orbit here. What you really need is the capacity for both satellites (starshade and telescope) to fly in formation (at least until they are properly positioned) in order to align themselves over the telescope-star axis, and point in the right direction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An example would be the &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/p/proba-3&quot; rel=&quot;nofollow noreferrer&quot;&gt;Proba3 mission of the ESA&lt;/a&gt;. Proba3 is a formation flying mission (to be launched) comprised of two satellites, an occulter and a coronograph. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the names indicate the occulter will occult the sun and the coronograph will be used to study the solar corona. However, because it is mainly a demonstration mission (i.e. meant to test new technologies), the orbit of the system formed by the two satellites will be HEO, not an orbit around the Lagrangian L1 as many other sun observation missions).&lt;/p&gt;&#xA;" OwnerUserId="20764" LastEditorUserId="6816" LastEditDate="2017-08-22T16:26:13.790" LastActivityDate="2017-08-22T16:26:13.790" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22763" PostTypeId="1" CreationDate="2017-08-22T18:36:04.567" Score="2" ViewCount="538" Body="&lt;p&gt;I'm curious as to the typical manufacturing (or gathering) methodology of H2 and O2 used in modern rockets for fuel and oxidizer.  It would seem that the most straightforward process would seem to be electrolysis of water into its H2 and O2 components, but rarely does the most straightforward process seem to be what is actually used.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other option I can come up with is to just pull H2 and O2 off as just other gases during a standard liquification air cooling process that can be used to pull any number of gasses out of standard breathing air by carefully cooling and gathering the condensed liquids as the gasses in the air liquify at different temperatures as they cool. It just seems that this way would take a long time and a huge abundance of liquid N2 (based on volume of gasses in the air) just due to the huge volumes of H2 and O2 needed (especially O2 since O2 is used for just about all liquid fueled rockets, no matter the fuel used).&lt;/p&gt;&#xA;" OwnerUserId="16735" LastActivityDate="2017-08-24T14:42:22.200" Title="Manufacturing H2 and O2 rocket fuel components?" Tags="|fuel|oxygen|hydrogen|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22764" PostTypeId="2" ParentId="22763" CreationDate="2017-08-22T18:56:30.500" Score="3" Body="&lt;p&gt;LOX is produced by &lt;a href=&quot;http://www.madehow.com/Volume-4/Oxygen.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;cryogenic distillation&lt;/a&gt;. Hydrogen is mainly (∼95%) produced from fossil fuels by &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrogen_production&quot; rel=&quot;nofollow noreferrer&quot;&gt;steam reforming or partial oxidation of methane and coal gasification&lt;/a&gt;, with only 4% coming from electrolysis. Electrolysis is inefficient compared to other methods. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-08-24T14:42:22.200" LastActivityDate="2017-08-24T14:42:22.200" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22766" PostTypeId="2" ParentId="12830" CreationDate="2017-08-22T23:12:37.940" Score="3" Body="&lt;p&gt;The only potential differences I can see in this would be in gimbal range and the ability to turn off certain engines. The falcon 9 uses 9 individual engines which allows it to turn off as many as it needs to in flight to either land, re-enter the atmosphere or in case one engine breaks. A multiple chamber single engine could do this with some modification however at the point where you are putting separate on the fuel lines it would essentially become individual engines. For gimballing purposes, using one pump system would only really work if all of the nozzles are connected and gimbal together. Again; if they were to gimbal separately the pump system would likely need modification which would essentially result in individual engines.&lt;/p&gt;&#xA;&lt;p&gt;The only real advantage of using a single pump system is that it would ensure a stable fuel flow to each chamber.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Several Soviet and Russian rocket engines use the approach of clustering small combustion chambers around a single turbine and pump. During the early 1950s, many Soviet engine designers, including Valentin P. Glushko, faced problems of combustion instability while designing bigger thrust chambers. At that time they solved the problem by using a cluster of smaller thrust chambers.&amp;quot; (Wikipedia, 2017). Due to the size of large thrust chambers, combustion may happen unevenly within a single chamber due to the ability for pressure fronts to more significantly move. The use of a single pump ensures that all chambers can be operated as one, and decreases weight, complexity, and parts.&lt;/p&gt;&#xA;&lt;p&gt;Using a single large pump system over multiple chambers also likely helps keep the cost and complication down in the rd-170. This design however is not widely used today in currently planned and used launch systems. The only system currently using something like this to my knowledge (please correct me if I'm wrong) is the RD-180 used on the Atlas launch vehicle (&lt;a href=&quot;https://en.wikipedia.org/wiki/File:Rd180schematic.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/File:Rd180schematic.png&lt;/a&gt;).&lt;/p&gt;&#xA;" OwnerUserId="20529" LastEditorUserId="-1" LastEditDate="2023-07-30T21:16:22.290" LastActivityDate="2023-07-30T21:16:22.290" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="22768" PostTypeId="1" CreationDate="2017-08-23T02:18:48.783" Score="4" ViewCount="294" Body="&lt;p&gt;If we placed and maintained a comet at Mars L1 point would we get a coma and tail that would add volatiles to the Martian atmosphere?  I've also read comets create a bow shock with the solar wind, slowing it down, so would the comet's coma and tail at L1 provide Mars's atmosphere some protection from the solar wind?&lt;/p&gt;&#xA;&lt;p&gt;The coma can grow to the size of Jupiter or even bigger than the sun. The hydrogen gas halo can be as large as 1 AU in radius!  But by the time a comet gets to within 1.5 AU of the sun, ie the orbit of Mars, the size decreases and the tail increases as the solar wind gets strong enough to blow the gas and dust away from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Coma_(cometary)&quot; rel=&quot;nofollow noreferrer&quot;&gt;coma&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The particles given off by the comet slow down and deflect the solar wind.  So would this be enough to protect Mars from the solar wind?    spacephysics.ucr.edu/index.php?content=solar_wind/sw/swq5.html&lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="23891" LastEditDate="2021-06-19T20:24:26.140" LastActivityDate="2021-06-19T20:24:26.140" Title="Comet at Mars L1" Tags="|mars|terraforming|comet|solar-wind|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22769" PostTypeId="2" ParentId="18989" CreationDate="2017-08-23T03:49:21.407" Score="-4" Body="&lt;p&gt;There are magnetic fields everywhere in space. If it were possible to use them for propulsion the achievable speeds would be incredible. Place two magnets close to each other and see how fast they come together. &lt;/p&gt;&#xA;" OwnerUserId="20784" LastActivityDate="2017-08-23T03:49:21.407" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22770" PostTypeId="1" AcceptedAnswerId="22773" CreationDate="2017-08-23T12:05:39.273" Score="27" ViewCount="15395" Body="&lt;p&gt;There was news of the new SpaceX space suit &lt;a href=&quot;https://www.reddit.com/r/Damnthatsinteresting/comments/4cgajk/spacex_suits_look_like_they_come_straight_from_a/&quot; rel=&quot;noreferrer&quot;&gt;in Reddit&lt;/a&gt; and now a mention in &lt;a href=&quot;https://www.instagram.com/p/BYIPmEFAIIn/&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk's Instagram&lt;/a&gt; where he says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;First picture of SpaceX spacesuit. More in days to follow. Worth noting that this actually works (not a mockup). Already tested to double vacuum pressure. Was incredibly hard to balance esthetics and function. Easy to do either separately.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What does &quot;double vacuum pressure&quot; mean in space-suit testing? How does one test a space suit to double vacuum pressure?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From The Verge's 08-Sep-2017 article &lt;a href=&quot;https://www.theverge.com/2017/9/8/16278404/spacex-nasa-space-suits-crew-dragon-elon-musk-commercial-crew&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk shares another photo of SpaceX’s future space suits; &lt;strong&gt;Full-body this time&lt;/strong&gt;&lt;/a&gt;. Credit: SpaceX/Elon Musk, original &lt;a href=&quot;https://www.instagram.com/p/BYyvO2WA3Ra/&quot; rel=&quot;noreferrer&quot;&gt;Instagram post&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The suits that SpaceX has designed are pressure suits, meant to be worn by astronauts while riding inside the Crew Dragon. They’re primarily needed in case there’s an emergency scenario during flight; if the spacecraft suddenly depressurizes, the suits will keep the astronauts alive until they can get to safety.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0w36J.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0w36J.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;em&gt;last year's reddit post&lt;/em&gt; &lt;a href=&quot;https://www.reddit.com/r/Damnthatsinteresting/comments/4cgajk/spacex_suits_look_like_they_come_straight_from_a/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX suits look like they come straight from a scifi movie&lt;/a&gt;, appears similar to image in Musk's instagram post.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zXnKI.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zXnKI.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-09T02:56:03.597" LastActivityDate="2022-06-01T06:33:30.940" Title="What does &quot;double vacuum pressure&quot; mean in space-suit testing?" Tags="|spacesuits|" AnswerCount="5" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="22772" PostTypeId="2" ParentId="22770" CreationDate="2017-08-23T14:14:31.400" Score="9" Body="&lt;p&gt;Pressure here refers to a relative measurement, not absolute, as we're comparing things (inside and out). &quot;Vacuum pressure&quot; here refers to the pressure generated by having a vacuum on one side, and 1 atmosphere of pressure on the other.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This suit has been tested to double that, meaning it had its internal pressure increased to 2 atmospheres, so that double the pressure was applied to it. Thus it was tested at double vacuum pressure.&lt;/p&gt;&#xA;" OwnerUserId="20792" LastEditorUserId="6816" LastEditDate="2017-08-24T16:27:03.900" LastActivityDate="2017-08-24T16:27:03.900" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="22773" PostTypeId="2" ParentId="22770" CreationDate="2017-08-23T14:36:41.167" Score="30" Body="&lt;p&gt;A space suit could be tested to double the operating pressure. This test can be done in a vacuum or in air. For the stability of the suit, only the pressure difference between inside and outside is essential.&lt;/p&gt;&#xA;&lt;p&gt;To allow enough flexibility, the pressure difference (inside to outside) should be not greater than 0.2 to 0.4 bar. Breathing pure oxygen in the suit at such a low pressure is possible and healthy to the user of the suit. At higher internal pressure it would be difficult for the astronaut to bend a knee or an elbow.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="47149" LastEditDate="2022-06-01T06:33:30.940" LastActivityDate="2022-06-01T06:33:30.940" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="22774" PostTypeId="2" ParentId="22770" CreationDate="2017-08-23T18:05:05.050" Score="-2" Body="&lt;p&gt;Double vacuum pressure would mean two atm of difference from inside to outside.  Others have remarked &quot;you can't close your hands or bend a knee or elbow against that pressure difference&quot;: that's why space suits are normally designed with joints that operate without changing the inside volume and thus are not susceptible to stiffening under the large pressure difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's what makes them bulky.  The picture shown here is nothing like that.  I wonder why.&lt;/p&gt;&#xA;" OwnerDisplayName="user20802" LastActivityDate="2017-08-23T18:05:05.050" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22776" PostTypeId="2" ParentId="4743" CreationDate="2017-08-23T20:12:44.260" Score="5" Body="&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;: I'm retracting this answer because I'm pretty sure I was abusing the higher-level statistical models for calculating things like vapor pressure—I now doubt that they scale well with such low temperatures and absurdly long timeframes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm leaving my original answer below in case anyone finds it interesting:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Assuming the spacecraft isn't destroyed by a collision or radiant energy from closely passing a star, it isn't immediately clear to me if the maximum theoretical lifetime would be limited by the theoretical lifetime of protons (&gt;&lt;span class=&quot;math-container&quot;&gt;$10^{25}$&lt;/span&gt; years) or the process of sublimation. I was curious about the answer so I started to do some digging (I would post more reference links, but my lack of reputation limits the number of links I can include)...&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In a vacuum over along enough period of time, anything above 0K tends to eventually become a gas via sublimation. The practical effect of this is that the entire spacecraft will eventually deteriorate into a rubble pile. The period of time that takes is extremely long (which is also an extreme understatement).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;But how long? Well... We know the record was gold-plated copper so we can first start with how long it would take for the gold plating to sublimate. First we would need to calculate the vapor pressure of gold. We use the temperature of the interstellar medium (&lt;a href=&quot;https://physics.stackexchange.com/questions/133985/how-can-interstellar-space-have-a-temperature-of-2-3k&quot;&gt;2.73K&lt;/a&gt;) as our temperature. By my calculations¹, the vapor pressure is around &lt;span class=&quot;math-container&quot;&gt;$1.1725×10^{-40939}$&lt;/span&gt; atmospheres or &lt;span class=&quot;math-container&quot;&gt;$1.1572×10^{-40944}$&lt;/span&gt; pascals. That pressure is exerted over the surface area of the record, which is &lt;span class=&quot;math-container&quot;&gt;$2827cm^{2}$&lt;/span&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;At this point I need to stop because I need to re-familiarize myself with the appropriate physics equations to calculate the actual material loss rate. I will update this post when I do that.&lt;/p&gt;&#xA;  &#xA;  &lt;hr&gt;&#xA;  &#xA;  &lt;p&gt;¹ Using data from &lt;a href=&quot;http://mmrc.caltech.edu/PVD/manuals/Metals%20Vapor%20pressure.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Will post links to my calculations once I have more reputation points.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="20805" LastEditorUserId="20805" LastEditDate="2019-12-11T20:36:16.547" LastActivityDate="2019-12-11T20:36:16.547" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="22777" PostTypeId="2" ParentId="22770" CreationDate="2017-08-23T20:48:24.280" Score="-1" Body="&lt;p&gt;I concur the Double vacuum testing is probably testing to twice bursting strength in vacuum. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &lt;a href=&quot;http://www.astronautix.com/s/spacesuits.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.astronautix.com/s/spacesuits.html&lt;/a&gt; for a fascinating look at various spacesuits.  They have been made out of pure metal, fabric, mixed materials, at 1 atm, at 0.2 atm, or at no atm simply mechanical counter-pressure. They have used &quot;tomato worm&quot; limbs (constant volume convoluted tubes - the better with to bend at pressure) among other solutions.  All of the solutions work. The question is what question are you asking; &quot;get me down&quot; suit, survival suit, open space/outside vehicle suit, lunar surface, martian surface?  What the constraint of this particular suit is will be fascinating. &lt;/p&gt;&#xA;" OwnerUserId="20807" LastActivityDate="2017-08-23T20:48:24.280" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22778" PostTypeId="1" CreationDate="2017-08-23T23:05:49.013" Score="7" ViewCount="460" Body="&lt;p&gt;Is there something that tracks the locations of space debris impacts, and how big the debris or damage is?&lt;/p&gt;&#xA;" OwnerUserId="20808" LastActivityDate="2018-06-13T14:11:22.147" Title="ISS impacts by space debris" Tags="|iss|debris|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22779" PostTypeId="1" CreationDate="2017-08-24T03:01:12.380" Score="4" ViewCount="363" Body="&lt;p&gt;Following &lt;a href=&quot;https://www.instagram.com/p/BYIPmEFAIIn/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elon Musk&lt;/a&gt;'s and &lt;a href=&quot;https://www.reddit.com/r/Damnthatsinteresting/comments/4cgajk/spacex_suits_look_like_they_come_straight_from_a/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt;'s recent announcements of their spacesuits being tested to &quot;&lt;a href=&quot;https://space.stackexchange.com/questions/22770/what-does-double-vacuum-pressure-mean-in-space-suit-testing&quot;&gt;double vacuum pressure&lt;/a&gt;&quot;, there's been some speculation on what this meant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It got me wondering - do NASA, ESA, or other agencies have a standard test protocol? And could someone post a short description of it?&lt;/p&gt;&#xA;" OwnerUserId="2972" LastEditorUserId="12102" LastEditDate="2017-08-24T03:13:41.783" LastActivityDate="2017-08-24T15:01:06.177" Title="Is there a standard in spacesuit pressure testing protocol?" Tags="|spacesuits|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22780" PostTypeId="2" ParentId="20616" CreationDate="2017-08-24T10:57:22.330" Score="1" Body="&lt;p&gt;The electrons emitted by the hollow cathode are accelerated towards the anode, i.e. the positive grids. The magnetic field forces the electrons onto a helical path (additional circular motion), which is much longer than a straight one. By this, the electrons are more likely to collide with the neutral background gas, which is a necessity to sustain a plasma (electron-impact ionization).&lt;/p&gt;&#xA;" OwnerUserId="20812" LastActivityDate="2017-08-24T10:57:22.330" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22781" PostTypeId="2" ParentId="22778" CreationDate="2017-08-24T13:24:42.713" Score="11" Body="&lt;p&gt;To the extent that there is something that tracks the locations of impacts, that &quot;something&quot; would be me, along with a small handful of other people spread across a few teams: MMOD, EVA, Imagery analysis, and a few dedicated hardware and system owners.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally speaking, our only way of tracking debris strikes that are too small to cause significant anomalies or system failures (i.e., nearly all of the strikes) is downlinked imagery.  A technique that was common during shuttle but is all-but-nonexistent these days is direct returned surface inspection -- in my five years in the field, I've examined one piece of returned hardware.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the most part, we only track the location of strikes for which there exists a decent potential of affecting station operations or if the location of the strikes will help us fingerprint individual hardware elements that are otherwise virtually indistinguishable in imagery.  Strikes into &quot;dumb structure&quot; are usually ignored unless they are very large.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know this isn't a super adequate answer right now, but I'd be willing to expand on any followup questions you may add in comments.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-08-24T13:24:42.713" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22782" PostTypeId="1" CreationDate="2017-08-24T14:03:43.177" Score="1" ViewCount="71" Body="&lt;p&gt;I had a thought about what the operating air pressure of the ISS would be. I was surprised to find it is &lt;a href=&quot;https://en.m.wikipedia.org/wiki/ISS_ECLSS#atmosphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;1 atmosphere&lt;/a&gt; or 14.7 psi.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that airliners fly in the lower atmosphere but pressurise cabins at about &lt;a href=&quot;http://aerosavvy.com/aircraft-pressurization/&quot; rel=&quot;nofollow noreferrer&quot;&gt;11 or 12 psi&lt;/a&gt; (and fly at altitudes producing a pressure differential of a half atmosphere), what is the rationale for the ISS to maintain a whole atmosphere difference against the vacuum?&lt;/p&gt;&#xA;" OwnerUserId="2972" LastActivityDate="2017-08-24T14:03:43.177" Title="Why is the ISS pressurised to 1 atmosphere?" Tags="|iss|" AnswerCount="0" CommentCount="2" ClosedDate="2017-08-24T14:05:17.893" ContentLicense="CC BY-SA 3.0" />
  <row Id="22783" PostTypeId="2" ParentId="22779" CreationDate="2017-08-24T15:01:06.177" Score="2" Body="&lt;p&gt;Pressure tests are done on-orbit as part of the EMU CHECKOUT procedure, and during airlock depressurization prior to extra-vehicular activity (EVA).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the EMU CHECKOUT procedure, the suit is pressurized to a delta pressure of 4.3 psia (its normal working pressure) and the suit caution and warning system monitors for a drop in pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OK2S1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OK2S1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the airlock depressurization prior to an EVA, the depressurization is halted when the airlock pressure reaches 5 psia, and again, the suit caution and warning system monitors for a drop in pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lTC5s.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lTC5s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: the shuttle-era EVA checklist is online &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/492872main_EVA_G_H_20.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-08-24T15:01:06.177" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22784" PostTypeId="1" AcceptedAnswerId="22785" CreationDate="2017-08-24T16:38:15.820" Score="10" ViewCount="2084" Body="&lt;p&gt;Today SpaceX will (presumably) finally launch &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/formosat-5.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Formosat-5&lt;/a&gt;. However, while it was originally &lt;a href=&quot;https://space.stackexchange.com/a/16666/12102&quot;&gt;planned to share the launch with SHERPA&lt;/a&gt; this did not happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What ever happened to SHERPA and all the cubesats that it was planned to deploy? I believe there was supposed to be room for both, and I thought the whole thing had been ready and waiting for launch for a while.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a better description (and a nicer image), read &lt;a href=&quot;https://space.stackexchange.com/a/16666/12102&quot;&gt;this excellent answer&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/16672/12102&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/s/sherpa&quot; rel=&quot;nofollow noreferrer&quot;&gt;EOPortal&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The 2015 SHERPA mission addresses the limitations of current launch configurations by maximizing the number of payloads integrated to the structure. The SHERPA concept is well suited to support small spacecraft that range from CubeSats to 300 kg minisatellites. By developing unique adapters for the standard ports, Spaceflight has enabled many different sizes of spacecraft to deploy from SHERPA, including the DARPA eXCITe microsatellite, built by NovaWurks Inc. of Alamitos, CA &lt;s&gt;(NovaWurks is a wholly owned subsidiary of Spaceflight Industries, Inc., Seattle)&lt;/s&gt; and two BlackSky Global Pathfinder satellites. Future planned configurations involve multiple SHERPA rings on a single launch, as well as support for much larger spacecraft on the forward section of the SHERPA ring.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;NOTE:&lt;/strong&gt; It now seems that NovaWurks is now a privately held, autonomous company and not a subsidiary as the block quote suggested at the time it was published.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FFbxt.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FFbxt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-10-29T17:40:53.793" LastActivityDate="2018-10-29T17:40:53.793" Title="What ever happened to SHERPA?" Tags="|sherpa|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="22785" PostTypeId="2" ParentId="22784" CreationDate="2017-08-24T17:27:35.690" Score="6" Body="&lt;p&gt;According to this &lt;a href=&quot;http://spacenews.com/spacex-delays-force-spaceflight-to-find-alternative-launches/&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt;, it's because of the delays&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In a March 2 message, Curt Blake, president of Seattle-based&#xA;  Spaceflight, said that “significant” delays in the planned launch of&#xA;  the Formosat-5 mission on a Falcon 9 from Vandenberg Air Force Base in&#xA;  California forced the company to find alternative rides for nearly 90&#xA;  satellites that were to launch as secondary payloads on a payload&#xA;  adapter called Sherpa.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;“We learned our launch would occur potentially much later than&#xA;  expected,” Blake wrote, not giving a specific launch date. “While&#xA;  delays are inevitable in the launch business, we made the decision to&#xA;  rebook all our customers slated to launch on the FormoSat-5 mission.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and the cubesats are being moved off to various different launches,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Spaceflight spokeswoman Jodi Sorensen said March 2 that most of the&#xA;  satellites that had been flying on Sherpa will be rebooked on one of&#xA;  two launches. One is on the company’s own dedicated Falcon 9 mission,&#xA;  dubbed SSO-A, scheduled to launch from Vandenberg later this year. The&#xA;  other is an unspecified “international launch” scheduled for this&#xA;  summer or fall.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As for Sherpa itself, Sorensen said it could fly on the SSO-A mission&#xA;  or another launch with a different set of satellite payloads. “It’s&#xA;  completely flight ready, so if we can, we’ll definitely use it,” she&#xA;  said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-08-24T17:27:35.690" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22786" PostTypeId="2" ParentId="22784" CreationDate="2017-08-24T17:35:48.063" Score="0" Body="&lt;p&gt;There were reports that initially Sherpa had issues getting all the payloads ready at the same time, causing some delays in the launch, beyond SpaceX manifest issues. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then once the SpaceX manifest issues and delays got to much for them to accommodate they moved the payloads off to various other launchers doing small sat launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In some ways, the ability to handle delays and find alternate launchers is actually a good sign. It means there is flexibility in the market which is good news. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-08-24T17:35:48.063" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22787" PostTypeId="2" ParentId="22784" CreationDate="2017-08-24T17:52:24.880" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;http://spacenews.com/spacex-delays-force-spaceflight-to-find-alternative-launches/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, SpaceX's schedule delays motivated Spaceflight to find other launches for the payloads that would've flown on SHERPA. One launch is a dedicated Falcon 9 launch titled SSO-A, &lt;a href=&quot;https://twitter.com/jeff_foust/status/874980790983163904&quot; rel=&quot;nofollow noreferrer&quot;&gt;currently scheduled&lt;/a&gt; for early 2018. The other launch is an unspecified international launch; I don't believe any details have been announced since that article was written.&lt;/p&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2017-08-24T17:52:24.880" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22788" PostTypeId="2" ParentId="22770" CreationDate="2017-08-24T18:24:37.243" Score="3" Body="&lt;p&gt;The suit is not designed for space walks. It is meant to protect the user in case they are exposed to vacuum. Therefore I find it strange that the accepted answer assumes the suit would be depressurized. It is not intended to be operated in a vacuum. In fact, the suit will be worn inside the capsule at 1 atm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, seems unlikely that pressurizing a suit to 1.8 atm would be considered rigorous testing (0.8 differential, double max operating pressure). I find it far more reasonable to assume that double vacuum pressure means they pressurized the suit such that the differential was double what it would be if exposed to vacuum at 1 atm. This would be a 2 atm differential, as many downvoted answers have suggested.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: &lt;a href=&quot;https://spacex.com/sites/spacex/files/pdf/DragonLabFactSheet.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here&lt;/a&gt; is a fact sheet which has the Dragon capsule's internal pressure at 13.9-14.9 psi&lt;/p&gt;&#xA;" OwnerUserId="20822" LastEditorUserId="20822" LastEditDate="2017-08-24T19:31:49.680" LastActivityDate="2017-08-24T19:31:49.680" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="22789" PostTypeId="1" AcceptedAnswerId="22790" CreationDate="2017-08-24T19:03:31.597" Score="4" ViewCount="383" Body="&lt;p&gt;Did I see the F9 2nd stage nozzle vibrate at about 2Hz at SECO-1? It looked like the bottom of the bell vibrated in an elliptical shape. Did I imagine that? Has this happened before?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\hskip 3.4 cm$It's just after &lt;code&gt;T+ 09:17&lt;/code&gt;, playing at half speed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\hskip 3.5 cm$&lt;a href=&quot;https://i.stack.imgur.com/QPpNG.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QPpNG.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-24T20:10:37.837" LastActivityDate="2017-08-24T20:10:37.837" Title="Did I see the Formosat-5 F9 2nd stage nozzle vibrate at about 2Hz at SECO-1?" Tags="|spacex|falcon-9|nozzle|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22790" PostTypeId="2" ParentId="22789" CreationDate="2017-08-24T19:30:46.510" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=ei3nGWD4d5A&quot; rel=&quot;noreferrer&quot;&gt;This video from the Iridium-2&lt;/a&gt; launch definitely shows a squash-and-stretch oscillation at SECO-1, after 23:45 into the video. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see a &lt;a href=&quot;https://www.youtube.com/watch?v=JuZBOUMsYws&quot; rel=&quot;noreferrer&quot;&gt;less distinct version of the same oscillation in CRS-11&lt;/a&gt; after 29:00.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Merlin vacuum engine nozzle is a quite thin shell (getting down to 1/64&quot; thickness according to one member of this SE) and subject to substantial forces from the exhaust; it would be surprising if it didn't vibrate and flutter a bit depending on the throttle state. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/YC9rw.jpg&quot; alt=&quot;Merlin Vac nozzle extension&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-24T20:01:54.110" LastActivityDate="2017-08-24T20:01:54.110" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22792" PostTypeId="1" CreationDate="2017-08-24T20:23:24.093" Score="0" ViewCount="79" Body="&lt;p&gt;when we put two magnets together they push each other apart. So why can't we use the same concept with the magnetic field and a man made magnet?&lt;/p&gt;&#xA;" OwnerUserId="20824" LastActivityDate="2017-08-24T20:23:24.093" Title="I'm confused on why we can't magnet idea to propel ship to space" Tags="|magnetorquer|" AnswerCount="0" CommentCount="7" ClosedDate="2017-08-25T06:36:27.470" ContentLicense="CC BY-SA 3.0" />
  <row Id="22793" PostTypeId="1" CreationDate="2017-08-24T21:18:05.907" Score="4" ViewCount="217" Body="&lt;p&gt;I'm curious about the pressurization of a full pressure flight suit (&lt;a href=&quot;https://en.wikipedia.org/wiki/Advanced_Crew_Escape_Suit&quot; rel=&quot;noreferrer&quot;&gt;such as&lt;/a&gt;). For example, the linked suit has an operating pressure of 3.5 psi, but my assumption would be that during normal flight operation the suit and cabin would be at the same pressure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this accurate, and if so, what happens if the suit is exposed to a vacuum? Or perhaps more to the point, what is the maximum pressure differential that the suit may be subjected to, assuming the cabin is at 1 atm and suddenly depressurizes?&lt;/p&gt;&#xA;" OwnerUserId="20822" LastActivityDate="2017-08-25T12:08:16.067" Title="Flight Suit Pressurization" Tags="|spacesuits|vacuum|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22795" PostTypeId="2" ParentId="22793" CreationDate="2017-08-24T21:48:43.747" Score="3" Body="&lt;p&gt;For the ACES suit that you show in the question, there is an integral pressure controller that determines what level the suit is pressurized to.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...pressurization begins when the cabin pressure altitude rises to&#xA;  around 35,000 feet and is “hard pressed” (3.67 psia) above 60,000&#xA;  feet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Note that your assumption that when the cabin altitude is sea level, there is no pressurization, is correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjQiPv66_DVAhWjj1QKHTHpDqQQFggnMAA&amp;amp;url=https%3A%2F%2Fwww.nasa.gov%2Fcenters%2Fjohnson%2Fpdf%2F390651main_shuttle_crew_operations_manual.pdf&amp;amp;usg=AFQjCNENi1Wfkn12N9uV7BJ3FfCXYBLgag&quot; rel=&quot;nofollow noreferrer&quot;&gt;SCOM&lt;/a&gt;, page 2.10-4&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rough equivalent pressure for the cabin altitudes given are 3.5 psi (35,000 ft) and 1 psi (60,000 ft).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://www.mide.com/pages/air-pressure-at-altitude-calculator&quot; rel=&quot;nofollow noreferrer&quot;&gt;Air pressure at altitude calculator&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The maximum delta pressure between the suit and ambient pressure is controlled by a positive pressure relief valve (PPRV) that is part of the suit controller.  This valve cracks at 5.5 psid and closes at 3.5 psid.  This valve keeps the suit from becoming too rigid if, for example, the suit is closed up with a 14.7 environment inside and the cabin pressure subsequently falls. The PPRV vents gas to maintain the proper delta pressure and prevent the suit from swelling up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't have a great reference for this (personal notes) but it's at least referred to in &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140013134.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt;, page 20.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-08-25T12:08:16.067" LastActivityDate="2017-08-25T12:08:16.067" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22797" PostTypeId="1" CreationDate="2017-08-25T03:55:33.077" Score="8" ViewCount="1024" Body="&lt;p&gt;For reference, you can hear the distinct 'whoop' sound at T minus 3 seconds mark of the &lt;a href=&quot;https://youtu.be/JwfojQKh-1E&quot; rel=&quot;noreferrer&quot;&gt;recent TDRS-M launch&lt;/a&gt;. This sound has been distinctive for most previous Atlas-V launches as far as I can remember.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have came across a similar observation during the Apollo-era Titan II launches. The reason for the pre-liftoff sound here is the flow of hot combustion products from a starter cartridge into turbine inlet nozzle under high pressure. The turbine drove the turbopump to supply the propellants to the combustion chamber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume that the sound in Atlas-V has also to do with the engine startup. I found a explanation of the startup method of the Atlas-V RD-180 engine (&lt;a href=&quot;http://lpre.de/energomash/RD-180/index.htm#flow_schematic_operation&quot; rel=&quot;noreferrer&quot;&gt;Russian article here&lt;/a&gt;). It also involves a startup cartridge which contains the staring fuel. During startup, the cartridge breaks under the action of pressurized fuel released by a reservoir and the fuel contained in the cartridge flows to a gas generator. I am not sure if this pressurised flow of the starting fuel is reason behind the startup sound. If yes, what is the exactly causing the sound? If no, what may be the other sources of the startup sound?&lt;/p&gt;&#xA;" OwnerUserId="20834" LastEditorUserId="20834" LastEditDate="2017-08-25T19:25:02.147" LastActivityDate="2020-03-26T22:29:01.623" Title="What is the source of the 'whoop' sound made before the liftoff (T-3s) of Atlas-V rocket?" Tags="|launch|engines|ula|atlas-v|rd-180|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22800" PostTypeId="1" CreationDate="2017-08-25T11:54:48.143" Score="0" ViewCount="556" Body="&lt;p&gt;If I have the Altitude and the Azimuth of a given lat/long and I have to launch a satellite from that given point on Earth. What value do I put in GMAT NASA software to get a simulated orbit? &lt;/p&gt;&#xA;" OwnerUserId="20840" LastEditorUserId="36360" LastEditDate="2022-04-04T11:27:04.787" LastActivityDate="2022-04-04T11:27:04.787" Title="NASA GMAT values to get a simulated orbit" Tags="|nasa|gmat|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22801" PostTypeId="1" CreationDate="2017-08-25T13:51:24.147" Score="5" ViewCount="2303" Body="&lt;p&gt;The common conception of satellite reentry is that they just burn up and are &quot;gone&quot;. But the total mass must still be somewhere, right? What forms does it all end up as, and where?&lt;/p&gt;&#xA;" OwnerUserId="20841" LastActivityDate="2019-02-15T16:51:37.630" Title="What happens to materials which burn up during reentry?" Tags="|reentry|mass|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22802" PostTypeId="2" ParentId="22801" CreationDate="2017-08-25T14:49:29.017" Score="8" Body="&lt;p&gt;The products of combustion, apart from gases, are small solid particles, 'ash', that slowly floats down to the lower atmosphere and, eventually, to the surface.&lt;/p&gt;&#xA;" OwnerUserId="18814" LastActivityDate="2017-08-25T14:49:29.017" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22803" PostTypeId="2" ParentId="22800" CreationDate="2017-08-25T15:17:46.067" Score="2" Body="&lt;p&gt;I believe the information you are providing is not enough. Do you intend to simulate the launch? Do you intend to simulate launch and the orbit, or just the orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to &quot;observe&quot; the evolution of your satellite for a given orbit you require enough information to position your satellite (Epoch and Keplerian elements - your altitude and azimuth are included here but not enough-) in time and space, and then you need to define how to simulate the orbit (propagator) in terms of masses attracting your satellite, and mathematical model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess a good starting point would be the &lt;a href=&quot;http://gmat.sourceforge.net/doc/R2016a/help.html#SimulatingAnOrbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;GMAT help&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to undertand Keplerian orbital elements I recommend the book &quot;Satellite Technology Principles and Applications&quot; from Maini and Agrawal.&lt;/p&gt;&#xA;" OwnerUserId="20764" LastActivityDate="2017-08-25T15:17:46.067" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22804" PostTypeId="1" AcceptedAnswerId="22815" CreationDate="2017-08-25T15:26:47.907" Score="9" ViewCount="1913" Body="&lt;p&gt;I have been looking at a space station that orbits Earth at an altitude of 3500 km above the equator, and has tethers that extend from it downwards to 250 km above the surface, and outwards to 6500 km altitude - a non-rotating skyhook. Right now I'm considering what would happen if a Dragon or Soyuz type re-entry capsule was dropped from the foot of the lower tether and returned on a ballistic trajectory.&lt;/p&gt;&#xA;&lt;p&gt;I used the &lt;a href=&quot;https://www.moonwards.com/teth.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tether Tool&lt;/a&gt; (written by our own Hohmannfan) to find the velocity of the foot. It was written first for the Moon and the rest of the solar system has recently been added to it. Don't forget to change the selection to &lt;code&gt;Earth&lt;/code&gt; at the top.&lt;/p&gt;&#xA;&lt;p&gt;The station at 3500 km altitude rotates with a period of 2.711 hours. At the foot (altitude = 250 km), this corresponds to a speed of 4.26 km/sec, or about half of the re-entry speed of a satellite from LEO. Since it's moving in the same general direction that the atmosphere is rotating, the speed relative to it is about 0.5 km/s less than that. Assume this is all in the plane of the equator.&lt;/p&gt;&#xA;&lt;p&gt;A Dragon 2 weighs &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;6400 kg&lt;/a&gt; empty. If it was returning one person perhaps it's fair to increase that to 6500 kg. On such a steep descent, would it need to have a larger area facing into the wind, (perhaps an &lt;a href=&quot;https://en.wikipedia.org/wiki/Low-Density_Supersonic_Decelerator&quot; rel=&quot;nofollow noreferrer&quot;&gt;inflatable decelerator&lt;/a&gt;)? What sort of descent time would be involved before the parachutes deploy, if it was set up to slow properly? What would be the peak deceleration?&lt;/p&gt;&#xA;&lt;p&gt;Together, that is maybe a lot to ask. It seems like it all belongs together. All input appreciated.&lt;/p&gt;&#xA;&lt;p&gt;Related question: &lt;a href=&quot;https://space.stackexchange.com/q/22847/4660&quot;&gt;Roughly what is the rate of energy loss of survivable capsule re-entries?&lt;/a&gt;. (New)&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-09-02T15:48:25.810" Title="If there was a non-rotating skyhook in Earth orbit, what would re-entry be like after dropping from its foot?" Tags="|reentry|space-tether|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22805" PostTypeId="1" CreationDate="2017-08-25T16:03:14.283" Score="6" ViewCount="165" Body="&lt;p&gt;On the last few webcasts, SpaceX has shown video from both ground tracking cameras and the on-board camera side-by-side, and there's a noticable delay between the two, with the on-board camera being at least a couple of seconds behind the ground camera.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not a signals processing engineer, but I suspect there may be two issues at play:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Longer signal path - I'm assuming that the on-board signal is being bounced off of a satellite or routed through multiple receivers, as opposed to the ground cameras which are &quot;right there&quot;, so to speak;&lt;/li&gt;&#xA;&lt;li&gt;Highly compressed signal - I'm also assuming that the video signal is highly compressed, which would add to the delay on each end.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Is that even in the ballpark?  What else could account for the delay?  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2017-08-25T16:03:14.283" Title="What accounts for the signal delay from cameras on the Falcon 9?" Tags="|spacex|falcon-9|camera|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22807" PostTypeId="2" ParentId="22801" CreationDate="2017-08-25T19:39:27.783" Score="4" Body="&lt;p&gt;What is the destination for mass of a candle when you light it up? Same things will happen if some inbound spacecraft speed towards earth surface hitting air molecules. Only difference is the outer shell of spacecraft is too hard to burnout totally like a candle but remains mostly intact losing very few amount of mass by various forms including combustion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;During reentry on earth it collide with air molecules and heated up high enough to produce bright light visible. If same thing was not spacecraft but asteroid made by soft material it would burn out leaving gas and dust of various particles based on material it made on. Same thing would happen if any satellite fall from orbit and enter earth atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally spacecraft do reentry process to return earth surface but satellite do not have such entry process in normal operation planing. Earth returning spacecraft designed with high temperature in mind and &quot;gone&quot; material during reentering process is relatively small which turned to gas if not fine dust. These materials ultimately falls on surface of earth if not light enough to float on air. But amount is really negligible to count.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Links: &lt;a href=&quot;http://www.slate.com/articles/news_and_politics/explainer/2005/04/where_satellites_go_when_they_die.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.slate.com/articles/news_and_politics/explainer/2005/04/where_satellites_go_when_they_die.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18788" LastEditorUserId="18788" LastEditDate="2017-08-26T22:08:20.990" LastActivityDate="2017-08-26T22:08:20.990" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22808" PostTypeId="2" ParentId="17615" CreationDate="2017-08-25T22:12:24.967" Score="-2" Body="&lt;p&gt;Those are bubbles of air, trapped underneath the reflective mylar sheeting that makes up the outermost layer of heat shield for the forward sufaces of the CM. After splashdown, that's the same material that appears ragged, having been shredded by reentry.&lt;/p&gt;&#xA;" OwnerUserId="20845" LastActivityDate="2017-08-25T22:12:24.967" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22809" PostTypeId="1" AcceptedAnswerId="22810" CreationDate="2017-08-26T12:11:44.840" Score="0" ViewCount="299" Body="&lt;p&gt;The screen shot and block quote below are from Oribal ATK's &lt;a href=&quot;https://www.orbitalatk.com/news-room/feature-stories/MinotaurIV_ORS5_Mission-Page/default.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mission Update: Minotaur IV ORS-5 Launch&lt;/a&gt;. ORS-5 or &quot;SensorSat&quot; is a technology demonstration for a class of satellites that will sit in LEO and keep an eye on the GEO belt for any &quot;debris&quot; which I suspect means &lt;a href=&quot;https://space.stackexchange.com/q/18201/12102&quot;&gt;pranksats&lt;/a&gt; or worse. ORS-5 will sit in a 600 x 600 km equatorial LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graphic suggests that after stage 5 (Orion 38) burnout and payload separation, the velocity will be 7.05 km/s and the altitude 600 km and that checks out; the sidereal rotation speed of the Earth-Fixed frame at this altitude is another 0.51 km/s. That means the speed of the orbit &lt;em&gt;in inertial coordinates&lt;/em&gt; is 7.05 + 0.51 = 7.56 km/s.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Orbital ATK's Minotaur IV space launch vehicle will launch the ORS-5 mission for the U.S. Air Force as a part of the Orbital/Suborbital Program-3 (OSP-3) contract. ORS-5, also known as SensorSat, is designed to scan for other satellites and debris to aid the U.S. military’s tracking of objects in geosynchronous orbit. For the ORS-5 launch, the Minotaur IV will use two Orion 38 upper stage motors. &lt;strong&gt;The final Orion 38 motor burn will reduce the angle of the ORS-5 satellite’s orbit, redirecting the spacecraft to equatorial orbit.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If I understand correctly, &quot;The final Orion 38 motor burn&quot; does not suggest that the solid fuel motor can burn twice, but that both the 4th and 5th stages are Orion 38 motors, and &quot;the final burn&quot; really means the 5th stage, or the burn of &quot;the final Orion 38 motor&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm curious what &quot;angle&quot; means in last sentence, and guessing that it means inclination. For such a light satellite (something like 110 - 140 kg) would the standard thrust of the Orion 38 happen to provide just the right amount of delta-v to change the inclination to equatorial? Since the table shows the velocity unchanged during the 5th stage burn, it seems all of the thrust should be perpendicular to the direction of motion and purely for inclination change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Could someone help to estimate the change in inclination angle that an Orion 38 can provide this satellite, starting with the specs in English units :-( and see if it's even in the right ballpark for this launch? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Approximate mass of the payload:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://spaceflight101.com/ors-5/sensorsat/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;: 113 kg&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/o/ors-5&quot; rel=&quot;nofollow noreferrer&quot;&gt;EOPoartal&lt;/a&gt;: ~120 kg&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://space.skyrocket.de/doc_sdat/ors-5.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;: 140 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Approximate mass of the Orion 38:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.orbitalatk.com/flight-systems/propulsion-systems/docs/2016%20OA%20Motor%20Catalog.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Oribal ATK&lt;/a&gt;: ~875 kg&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UmOeW.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UmOeW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Image of Orion 38 motor from Orbital ATK's &lt;a href=&quot;https://www.orbitalatk.com/flight-systems/propulsion-systems/docs/2016%20OA%20Motor%20Catalog.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Propulsion Products Catalog&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/g6Bya.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/g6Bya.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Orion 38 motor specifications from Orbital ATK's &lt;a href=&quot;https://www.orbitalatk.com/flight-systems/propulsion-systems/docs/2016%20OA%20Motor%20Catalog.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Propulsion Products Catalog&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ej7FQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ej7FQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; ORS-5 during assembly from &lt;a href=&quot;https://twitter.com/OrbitalATK/status/899628721367392256&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital ATK tweet&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7KwQ7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7KwQ7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-27T11:51:45.883" LastActivityDate="2017-08-27T14:22:57.913" Title="What inclination change could the 5th stage Orion 38 motor provide ORS-5 (SensorSat)?" Tags="|orbital-maneuver|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22810" PostTypeId="2" ParentId="22809" CreationDate="2017-08-26T13:20:41.053" Score="4" Body="&lt;p&gt;For &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_inclination_change&quot; rel=&quot;nofollow noreferrer&quot;&gt;inclination change in circular orbit&lt;/a&gt;, $$\Delta v = {2v\, \sin \left(\frac{\Delta{i}}{2} \right)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Delta v from a burn done all in one direction is given by our old friend the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tsiolkovsky equation&lt;/a&gt;: $$\Delta v = v_\text{e} \ln \frac {m_0} {m_f}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here, $m_0$ is the mass at the start of the burn, combining the total motor mass of the Orion 38 (873 kg) and the mass of the payload (I took the 113 kg figure as the most accurate) for 986 kg. $m_1$ is the mass at the end, 94 kg from the motor + 113 kg payload for 207 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_e$ is the effective exhaust velocity, which is 9.81 m/s&lt;sup&gt;2&lt;/sup&gt; times the effective $I_{sp}$ given in seconds: 2815 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the motor can deliver to this payload $$2815 \ln \frac {986} {207}$$ or a very respectable 4391 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If all that ∆v is applied perpendicular to the circular orbit, we get a pure inclination change maneuver; our circular orbit velocity at 600km is 7558 m/s, so:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ 4391 = {2} \times {7558} \sin \left(\frac{\Delta{i}}{2} \right)$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rearranging:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ 0.290 = \sin \left(\frac{\Delta{i}}{2} \right)$$&#xA;$$\Delta{i}  = 2 \arcsin 0.290$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or 33.77º.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you throw in a little extra unaccounted mass, e.g. a payload adapter between the Orion 38 and the satellite, the rocket equation figure might come out closer to 4000 m/s; this still gets you about a 30º inclination change. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As either of these figures is greater than &lt;a href=&quot;https://en.wikipedia.org/wiki/Spaceport_Florida_Launch_Complex_46&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cape Canaveral's 28º latitude&lt;/a&gt;, it's sufficient to push the satellite into equatorial inclination. I assume the rocket was initially launched to just the right inclination for the Orion stage to cancel, and did the correction burn while crossing the equator early in its first orbit. Launching to an initially higher inclination effectively sacrifices a tiny bit of the &quot;free&quot; velocity you get from Earth's rotation in exchange for not having to do anything excessively clever with the Orion 38.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I note also that while the velocity figure is unchanged across the 5th stage burn, the altitude goes from 578 km at 4th stage burnout to 600 km at 5th stage burnout; if the final orbit of the ORS-5 is in fact 600km circular, then the Orion 38 stage must be doing some circularization as well as plane change. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-08-27T14:22:57.913" LastActivityDate="2017-08-27T14:22:57.913" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22811" PostTypeId="1" AcceptedAnswerId="22812" CreationDate="2017-08-27T09:33:41.257" Score="1" ViewCount="1339" Body="&lt;p&gt;New concept: &lt;strong&gt;What about if we are to depend from external forces?&lt;/strong&gt; So we cannot produce any kind of propulsion from our ship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Previous question:&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;I know that there are two main ways to gain speed in the emptiness of space. One is by burning fuel and therefor accelerating the spaceship. The other one is by using slingshot effects (gravity assists) where the spaceship gets a boost in speed. Are there any other ways to gain speed in empty space?&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="20563" LastEditDate="2017-08-27T13:46:36.217" LastActivityDate="2017-10-14T02:10:00.577" Title="How to gain speed in space?" Tags="|fuel|gravity-assist|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22812" PostTypeId="2" ParentId="22811" CreationDate="2017-08-27T10:05:44.457" Score="3" Body="&lt;p&gt;By &quot;burning fuel&quot; you probably mean usage of the law of conservation of momentum. So you should just throw away something - not necessary gases of burning fuel, but probably radiation, or accelerate gas by electro magnetic field like in &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ion thruster&lt;/a&gt; or just explode atomic bombs like in &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nuclear pulse propulsion&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another option is &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;Solar Sail&lt;/a&gt; - by using Sun radiation pressure&lt;/p&gt;&#xA;" OwnerUserId="11046" LastActivityDate="2017-08-27T10:05:44.457" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22813" PostTypeId="2" ParentId="22811" CreationDate="2017-08-27T13:22:34.193" Score="1" Body="&lt;p&gt;Along with burning fuel and slingshot you mention there are a few more ways for increasing speed, either used already or in theory for consideration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the methods used is &lt;a href=&quot;https://en.wikipedia.org/wiki/Ion_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;ionized discharge&lt;/a&gt;. The way it differs from conventional propulsion system where chemical reaction of combustion process pushes the vehicle in the opposite direction is that the ion exhaust is created from electric power moving things forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other methods could be the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hall-effect_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hall Effect&lt;/a&gt;. Getting electrons in space is easier than carrying chemical fuel for further use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are &lt;a href=&quot;https://en.wikipedia.org/wiki/Spacecraft_propulsion&quot; rel=&quot;nofollow noreferrer&quot;&gt;other more exciting way of getting momentum in space currently under research that Wikipedia lists&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="18788" LastEditorUserId="6816" LastEditDate="2017-10-12T17:18:00.030" LastActivityDate="2017-10-12T17:18:00.030" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22815" PostTypeId="2" ParentId="22804" CreationDate="2017-08-28T03:02:47.740" Score="8" Body="&lt;p&gt;I recreated your scenario using &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/12/how-wolfes-tether-spreadsheet-works.html&quot; rel=&quot;noreferrer&quot;&gt;Wolfe's Spreadsheet&lt;/a&gt;. Here's a screen capture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MxLpV.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MxLpV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The numbers you cited match mine fairly closely. A very nice tool Hohmann fan has made available. I intend to link to his page from several of my tether blog posts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;s&gt;The bottom of the spreadsheet answers some of the your questions. I called 70 kilometers the altitude where the craft starts serious aerobraking. Which is just a guess, to be honest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ship would be moving 4.6 km/s at that altitude. It would 7.3e-1 hours to reach this altitude which comes out to 43 minutes.&lt;/s&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Scratch that - it is a mistake. Thank you for calling that to my attention, uhoh. The bottom of the spreadsheet was a transfer orbit from a 250 km apogee to a 70 km perigee. But in this scenario perigee is well below earth's surface and the ship re-enters well before perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a pic of Kim's scenario:&lt;a href=&quot;https://i.stack.imgur.com/NXRTw.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NXRTw.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The eccentricity of the suborbital ellipse is .698. Semi major axis of that ellipse is about 3902 kilometers. Given that semi major axis, the period would be 2425 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But only a fraction of that period is swept out from release at apogee to atmospheric entry at 70 km. That fraction of the ellipse is shaded blue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get area of the fraction of an ellipse swept out, Kepler scaled the ellipse to make it a circle. Which is what I did. I also scaled radius of our circle to 1 to eliminate some arithmetic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b2BJv.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b2BJv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get area swept out, Kepler would add area of a triangle and a wedge. Which is what I did above. In this case a little more than 1/10 of the ellipse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;.104*2425 seconds is about 252 seconds. Or about 4 minutes and 12 seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After release from a 250 km altitude, the spacecraft would start serious aerobraking in about 4 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 70 km altitude I get a flight path angle of 19.2º. Horizontal velocity would be be about 4.38 km/s. Vertical velocity would be 1.53 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The short path from a 70 kilometer altitude to earth's surface doesn't give much time to shed the 4.64 km/s velocity.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;As the capsule descends gravity will increase slightly, centrifugal acceleration decreases rapidly while deceleration from dynamic pressure increases rapidly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I whomped up a first order Runge Kutta model of this drop from the tether foot at a 250 km altitude to earth's surface. A few different scenarios:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Dropping in a Vacuum&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time to Impact:&#xA;279 seconds&lt;br&gt;&#xA;Impact Velocity: 4.63 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is within about 4% of what I got using Kepler's method to get time of flight and the Vis Viva equation to get speed at 6378 km (earth's radius).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Dropping into earth's atmosphere&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get atmosphere density at an altitude I use $exp(-altitude/scale height)*1.225 kg/m^3$. I call 8500 meters the scale height. Dynamic pressure is $.5*\rho*v^2$). Force newtons is dynamic pressure x drag coefficient x cross sectional area capsule.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Scenario 1&lt;/strong&gt;: 6500 kg, 1.85 meter radius, drag coefficient .5&lt;br&gt;&#xA;Time to impact: 354 seconds&lt;br&gt;&#xA;Impact velocity: 150 meters/sec which is about 333 mph.&lt;br&gt;&#xA;Max Q: 217 kilo pascals.&lt;br&gt;&#xA;Max aerobraking acceleration: 18 g's  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've been told 90 kilo pascals has been a Max Q for descent. I am guessing 217 kilo pascals is unacceptable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Scenario 2&lt;/strong&gt;: 6500 kg, 2.9 meter radius, drag coefficient .5&lt;br&gt;&#xA;Time to impact: 416 seconds&lt;br&gt;&#xA;Impact velocity: 92 meters/sec which is about 205 mph.&lt;br&gt;&#xA;Max Q: 87 kilo pascals.&lt;br&gt;&#xA;Max aerobraking acceleration: 18 g's  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here Max Q is less than 90 kilo pascals. This may be acceptable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know what how to figure heating.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;If made of Zylon with a safety factor of 3, the lower tether would have a taper ratio of about 1090. Tether mass to payload ratio would about about 6,670.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think with that great length it would have a large enough cross sectional ratio that impacts would be inevitable in the LEO volume which has lots of debris and satellites. As much as I love vertical tethers, I don't believe they could survive close to earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A rotovator might be a way to catch suborbital flights as well as release re entering craft into the atmosphere at suborbital speeds. John Carmack has already tweeted to Musk if he can land a booster on an ocean platform, he should be able to rendezvous with a rotovator. &lt;a href=&quot;https://twitter.com/elonmusk/status/757600711714701312&quot; rel=&quot;noreferrer&quot;&gt;Musk agreed&lt;/a&gt;, sort of.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-09-01T22:00:24.147" LastActivityDate="2017-09-01T22:00:24.147" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22816" PostTypeId="1" CreationDate="2017-08-28T04:35:08.783" Score="9" ViewCount="1650" Body="&lt;p&gt;Looking at an ISS transit picture:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L6odr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L6odr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I started wondering wether we could see the ISS shadow passing on the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My intuition tells me that it would be impossible with the naked eye, but do we have any instrument able to detect it ? &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="12102" LastEditDate="2020-05-06T03:01:51.583" LastActivityDate="2020-05-06T03:01:51.583" Title="Can we detect the projected ISS shadow on the earth surface?" Tags="|iss|sensors|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="22817" PostTypeId="2" ParentId="22816" CreationDate="2017-08-28T12:34:01.980" Score="4" Body="&lt;p&gt;In terms of detecting the shadow there are a few lengthy answers to, essentially, the same question in &lt;a href=&quot;https://www.google.be/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=2&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjo_oPo9fnVAhVDPFAKHTijAjYQFgguMAE&amp;amp;url=https%3A%2F%2Fwww.reddit.com%2Fr%2Faskscience%2Fcomments%2F2ie8v8%2Fdoes_the_international_space_station_cast_an%2F&amp;amp;usg=AFQjCNGZb03T6HvJMhi1G5PTA9cTDHxBdg&quot; rel=&quot;nofollow noreferrer&quot;&gt;reddit&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most concise answer being that the ISS is too small to cast a shadow on the surface of the earth. The earth being in fact in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Umbra,_penumbra_and_antumbra&quot; rel=&quot;nofollow noreferrer&quot;&gt;antumbra&lt;/a&gt; of the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, we have means of knowing actively where the ISS is. So what would be the point?&lt;/p&gt;&#xA;" OwnerUserId="20764" LastActivityDate="2017-08-28T12:34:01.980" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22819" PostTypeId="2" ParentId="22816" CreationDate="2017-08-28T22:21:07.250" Score="9" Body="&lt;p&gt;&lt;strong&gt;TL;DR; Yes!&lt;/strong&gt; Use a Raspberry Pi, a square millimeter photodiode, a low cost ADC and some DC and high frequency rejection with a quad op-amp and passive components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Best possible case the ISS presents about 3200 square meters of occulting area. Since the solar panels move in various ways, let's also consider half of that, or 1600 square meters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shortest distance an observer on earth can be from the ISS is 400 km. Let's use 600 as a typical value (within 45 degrees of the zenith).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solid angle of the ISS is area/distance^2. That's between 4E-09 and 2E-08 sr. The sun's radius is about 696,000 km and distance is about 1.5E+08 km. The solid angle is pi r^2 / R^2 or about 6.8E-05 sr. That puts the fraction of the sun covered by the ISS between 7E-05 and 3E-04. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's call the optical power of the sun reaching the surface that can be easily detected as only 500 W/m^2. That's 500 microwatts per square millimeter. Let's use a photodiode of 1 square millimeter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a quantum efficiency of 0.5 and average photon energy of 2.5 eV, that's 6E+14 photoelectrons per second or about 100 microamperes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The transit takes at least 0.5 seconds, so that's at least 3E+14 photoelectrons. The statistical variation on that is 1/sqrt(N) or only about 6E-08; in other words with a shaping time of the order of a half-second, the statistical noise will be about 0.1 parts per million, or &lt;em&gt;far below the expected size of the signal in a 1 square millimeter photodiode!&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the task is to detect a rectangular drop in sunlight of the order of 7E-05 to 3E-04 for about a half second. This would be on a 100 uA signal from a photodiode, which would be 10,000 time bigger than shot noise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could do this with a photodiode and a Raspberry Pi, but you would have to reject signals with frequency lower than say 0.5 Hz (including rejecting DC) so that you could use a hobby-quality ADC. Take some time to design your filter to make sure the high frequency gain (above several tens of Hz) is also restricted - don't just use a differentiator or all you will be digitizing is high frequency noise. Make sure to sample at fairly high frequency, &lt;em&gt;but not at a multiple of 60 Hz&lt;/em&gt;, so you can digitally remove any 60Hz noise or it's harmonics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note 1:&lt;/strong&gt; &lt;em&gt;If you want to measure the shadow by looking at it:&lt;/em&gt; you need to point your photodiode at a diffuse reflector like a white sheet of paper. In this case the job is a bit more difficult; the total light on the detector from a large diffuse surface will be roughly three to ten times lower. Each square millimeter's contribution is small, but they add up again as you integrate over the surface. If you need this worked out mathematically, leave a note.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note 2:&lt;/strong&gt; &lt;em&gt;If you want to do this from space, looking down at the shadow on the Earth&lt;/em&gt;, you'd need to use a small telescope and point it at a small patch on the Earth to look for the small drop. &lt;em&gt;This would be much more difficult&lt;/em&gt; because any drift in FOV across the surface would produce plenty of noise as brighter or darker areas passed through the FOV.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note 3:&lt;/strong&gt; You can find the shadow path using this &lt;a href=&quot;https://transit-finder.com&quot; rel=&quot;noreferrer&quot; title=&quot;ISS Transit calculator&quot;&gt;ISS Transit calculator&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Full sized image below &lt;a href=&quot;https://www.livescience.com/32583-how-big-is-the-international-space-station.html&quot; rel=&quot;noreferrer&quot;&gt;is from here&lt;/a&gt;. Python was used to estimate maximum possible projected size of the ISS by counting orange pixels and using 30cm/pixel, as estimated from the American Football field width of 300 feet = 300 pixels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dX0qQ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dX0qQ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ammPm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ammPm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6642" LastEditDate="2017-08-29T02:45:25.083" LastActivityDate="2017-08-29T02:45:25.083" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22820" PostTypeId="1" CreationDate="2017-08-29T10:55:37.940" Score="2" ViewCount="2528" Body="&lt;p&gt;My dream is to be a NASA astronaut. I am a 16 year old girl from India, my height is currently 5 feet, and my weight is 55kg. Can my height and weight create problems in fulfilling my dream?&lt;/p&gt;&#xA;" OwnerDisplayName="user20863" LastEditorDisplayName="user10509" LastEditDate="2017-08-29T14:51:46.143" LastActivityDate="2017-09-03T17:02:28.250" Title="My height is 5 feet and weight is 55 kg. Can I be a NASA astronaut?" Tags="|space-station|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22821" PostTypeId="1" CreationDate="2017-08-29T11:10:10.900" Score="0" ViewCount="72" Body="&lt;p&gt;What is the mechanism principle of the grid fins in F9?&#xA;or how to design the mechanism for the electricity grid fins?&#xA;or how to design a electricity folding wing like the grid fins in F9?&lt;/p&gt;&#xA;" OwnerUserId="20392" LastEditorUserId="897" LastEditDate="2017-08-30T12:48:07.260" LastActivityDate="2017-08-30T12:48:07.260" Title="What is the mechanism principle of the grid fins in F9?" Tags="|spacex|falcon-9|design|engine-design|engines|" AnswerCount="0" CommentCount="4" ClosedDate="2017-08-29T15:00:20.693" ContentLicense="CC BY-SA 3.0" />
  <row Id="22822" PostTypeId="2" ParentId="22820" CreationDate="2017-08-29T13:19:18.463" Score="4" Body="&lt;p&gt;Besides potential height requirements: you are still young. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Focus on what you can achieve towards reaching your goal, and by the time you'll be old enough, the requirements might have changed. &#xA;Especially with the booming space commercial industry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Namely:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Study hard&lt;/li&gt;&#xA;&lt;li&gt;Aim for the top; you want to be the number 1 student in &lt;strong&gt;every&lt;/strong&gt; topics. &lt;/li&gt;&#xA;&lt;li&gt;Reach for higher education, especially if your local school isn't one of the best. Websites like kan academy can help a lot. &lt;/li&gt;&#xA;&lt;li&gt;Keep a fit shape, exercising regularly is good. &lt;/li&gt;&#xA;&lt;li&gt;Try playing KSP: it's fun and will teach you something useful. &lt;/li&gt;&#xA;&lt;li&gt;Learning foreign languages. Currently Russian and Japanese are the most useful but I suspect Chinese will supplant them soon.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In the end, it will payoff. You might not become an astronaut but you'll accomplish other great things by aiming high regardless. &lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6642" LastEditDate="2017-08-30T01:57:31.127" LastActivityDate="2017-08-30T01:57:31.127" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22823" PostTypeId="2" ParentId="22820" CreationDate="2017-08-29T15:01:00.597" Score="1" Body="&lt;p&gt;To complement Antzi's answer, here is some advice by the one and only Chris Hadfield on &lt;a href=&quot;https://www.youtube.com/watch?v=IWrTwwLZm4Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;how to become an Astronaut&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Paraphrasing: In order to become anything first you have to decide what you want to be -it should be exciting to you-, once decided other decisions should aim at turning yourself into want you want to be. In order to become an Astronaut, you need to keep your body in shape, learn things -expose yourself to the right stuff- and learn to work (at home or somewhere else).&lt;/p&gt;&#xA;" OwnerUserId="20764" LastEditorUserId="20764" LastEditDate="2017-09-03T17:02:28.250" LastActivityDate="2017-09-03T17:02:28.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22824" PostTypeId="2" ParentId="22820" CreationDate="2017-08-29T15:17:33.590" Score="1" Body="&lt;p&gt;As far as physical size, it will all depend on what the design requirements were/are for the vehicle you plan to ride in.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example:&#xA;Since the Space Shuttle was designed to serve a large and diverse crewmember population, it had generous height limitations. (I'd love to quote them, but the situation in Houston is...interesting right now, and it would take a while for me to find them.) Memory says it was 5%-95% percentile of the American population in the 70s when it was being designed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Future vehicles designed for tourism may have even more generous size requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;http://www.ilcdover.com/sites/default/files/system_considerations_for_an_exploration_spacesuit_upper_torso_architecture_1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;paper&lt;/a&gt; quotes the same 5%-95% percentile requirements for an advanced space suit. Perhaps that is the de facto standard.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-08-30T10:02:36.617" LastActivityDate="2017-08-30T10:02:36.617" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22825" PostTypeId="1" AcceptedAnswerId="22913" CreationDate="2017-08-29T15:37:17.330" Score="16" ViewCount="1537" Body="&lt;p&gt;I suppose there's a launch window to Neptune by using Jupiter as often as there are Jupiter/Neptune conjunctions, every 13½ years. But which years are Jupiter and Neptune best aligned for gravity assist? How long does such a launch window last (three months, to take advantage of Earth's orbital speed?)&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="25" LastEditDate="2017-09-09T14:18:55.977" LastActivityDate="2017-09-09T14:18:55.977" Title="When are there launch windows to Neptune via Jupiter?" Tags="|orbital-mechanics|jupiter|gravity-assist|neptune|launch-window|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22826" PostTypeId="1" CreationDate="2017-08-29T15:53:00.467" Score="3" ViewCount="214" Body="&lt;p&gt;Could a spacecraft orbiting Saturn, like Cassini, use a Titan gravity assist to leave the Saturn system (e.g. to bring a sample back to Earth), without using fuel other than to reach Titan?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="1457" LastEditDate="2017-08-29T16:00:16.433" LastActivityDate="2022-07-08T19:56:36.333" Title="Does a Titan flyby guarantee that a spacecraft can escape Saturn?" Tags="|gravity-assist|cassini|titan|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22827" PostTypeId="1" AcceptedAnswerId="22828" CreationDate="2017-08-29T16:17:08.677" Score="7" ViewCount="983" Body="&lt;p&gt;I don't know why Gizmodo says &lt;a href=&quot;http://gizmodo.com/nasas-latest-venus-probe-concept-looks-like-a-tim-burto-1798501354&quot; rel=&quot;noreferrer&quot;&gt;NASA's Latest Venus Probe Concept Looks Like a Tim Burton Creation&lt;/a&gt;, because it looks far more like a &lt;a href=&quot;http://www.strandbeest.com/&quot; rel=&quot;noreferrer&quot;&gt;Theo Jansen creation&lt;/a&gt;! Compare videos below as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can understand looking into wind to power strictly mechanical instead of electromechanical locomotion, but I don't see how meaningful computing could be done mechanically. Roughly, what kind of mechanical computing hardware is being considered for a future Venusian rover? Is it like 1 FLOP and 1 kilobit, or something small and dense or even &lt;a href=&quot;https://en.wikipedia.org/wiki/Microelectromechanical_systems&quot; rel=&quot;noreferrer&quot;&gt;MEMS&lt;/a&gt;-like?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested because the &lt;a href=&quot;https://i.stack.imgur.com/j8YMx.jpg&quot; rel=&quot;noreferrer&quot;&gt;10,000 year satellite&lt;/a&gt; could really benefit from non-electronic computers to survive in space that long!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also NASA's &lt;a href=&quot;https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Automaton_Rover_Extreme_Environments&quot; rel=&quot;noreferrer&quot;&gt;Automaton Rover for Extreme Environments (AREE)&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j8YMx.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j8YMx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/LewVEF2B_pM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/u9fki7dligU?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-06-06T23:06:48.213" LastActivityDate="2021-06-06T23:06:48.213" Title="Mechanical computing hardware for a future Venusian rover? How does it work?" Tags="|venus|lander|temperature|high-pressure-environment|" AnswerCount="4" CommentCount="8" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22828" PostTypeId="2" ParentId="22827" CreationDate="2017-08-29T17:13:59.087" Score="5" Body="&lt;p&gt;Don't underestimate what you can do with mechanical components. In the &lt;strong&gt;18&lt;/strong&gt;40s, Charles Babbage worked on a programmable general-purpose computer (the &lt;a href=&quot;https://en.wikipedia.org/wiki/Analytical_Engine&quot; rel=&quot;noreferrer&quot;&gt;Analytical Engine&lt;/a&gt;). Sadly, it was never completed, although the idea is sound. An earlier design of his, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Difference_engine&quot; rel=&quot;noreferrer&quot;&gt;Difference Engine&lt;/a&gt; (which was not Turing-complete), was built in the 1990s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/niac_2016_phasei_saunder_aree_tagged.pdf&quot; rel=&quot;noreferrer&quot;&gt;AREE study&lt;/a&gt; hasn't advanced to the point of designing hardware. From page 19, the study summarizes the possible solutions (mechanical, pneumatics/fluidics, vacuum tubes and other electronics), then shows some possible implementations for elements like power storage, and navigation using a method for obstacle avoidance that does not need computation.  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;While Phase 1 demonstrated feasibility, a number of areas remain that require further refinement and demonstration to establish concept credibility. Continued work on a Venus rover concept that does not require yet to be developed technologies with unknown costs and timescales significantly changes the conversation with regards to Venus missions and achievable science. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-08-29T17:13:59.087" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22829" PostTypeId="2" ParentId="22804" CreationDate="2017-08-29T17:53:06.917" Score="5" Body="&lt;p&gt;&lt;strong&gt;tl;dr&lt;/strong&gt; Everyone will die within five minutes. Your plan is a non-starter. You must consider letting go from a &lt;em&gt;much lower altitude&lt;/em&gt;, or higher forward velocity, or convert your capsule into a spaceplane the likes of which do not yet exist.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here is some physics. To get an approximate idea what the re-entry would be like, I've used a simple scale-height model of atmospheric density (7, 7.5 and 8 km), constant drag coefficient (0.2, 0.5, and 0.8) and a constant lift-to-drag ratio (0, 0.15, and 0.3). With all 27 combinations you get a peak heat production between 3 and 4 gigawatts, and a peak deceleration of between 15 and 20 gees. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These should be compared to values from &lt;em&gt;survivable reentries.&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In all cases the capsule hits the ground in roughly five minutes. If the Earth were flat that would be a bit less; the curvature buys you some extra seconds. You could even ballpark the fall time with:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ x = \frac{1}{2} a t^2$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is that it's dropping for nearly 200 km before it hits enough air to really start losing energy. It would be better to let it go in the ballpark of 100 to 80 km so it can get rid of a lot of energy before it starts sinking too fast and the air gets too dense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9SuuF.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9SuuF.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(X, t):&#xA;&#xA;    x, v = X.reshape(2, -1)&#xA;&#xA;    r, speed = [np.sqrt((thing**2).sum()) for thing in x, v]&#xA;&#xA;    acc_g = -x * GMe *((x**2).sum())**-1.5&#xA;&#xA;    alt = r - re&#xA;&#xA;    rho    = rho0 * np.exp(-alt/hscale)&#xA;    Fdrag  = -0.5 * v * speed * CD * Area * rho&#xA;    n_lift = np.hstack((-v[1], v[0]))/speed   # definition of lift&#xA;    Flift  = LDR * 0.5 * n_lift * speed**2 * CD * Area * rho&#xA;&#xA;    acc_d = (Fdrag + Flift)/m0&#xA;&#xA;    return np.hstack((v, acc_g + acc_d))&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;pi  = np.pi&#xA;GMe = 3.986E+14&#xA;&#xA;alt = 250000.   # meters&#xA;re  = 6378000.  # meters&#xA;v0  = 4260.     # m/s&#xA;hscales = [7000., 7500., 8000.]   # meters&#xA;CDs     = [0.2, 0.5, 0.8]&#xA;LDRs    = [0, 0.15, 0.3]&#xA;Area    = pi * 1.1**2             # m^2&#xA;m0      = 6800. # kg&#xA;rho0    = 1.25  # kg/m^3&#xA;&#xA;X0   = np.array([0, re+alt, v0, 0])&#xA;dt   = 1.0      # seconds per reported value by the solver (internally variable timesteps)&#xA;time = np.arange(0, 301, dt)&#xA;&#xA;answers = []&#xA;&#xA;for CD in CDs:&#xA;    for hscale in hscales:&#xA;        for LDR in LDRs:&#xA;&#xA;            answer, info = ODEint(deriv, X0, time, full_output = True)&#xA;            answers.append(answer)&#xA;&#xA;km = 1E-03&#xA;&#xA;gee = 9.8  # m/s^2&#xA;&#xA;plt.figure()&#xA;&#xA;for answer in answers:&#xA;    x, y, vx, vy = answer.T&#xA;    r = np.sqrt( x**2 +  y**2 )&#xA;    v = np.sqrt(vx**2 + vy**2)&#xA;    KE = 0.5 *m0 * v**2&#xA;&#xA;    plt.subplot(5, 1, 1)&#xA;    plt.plot(time, km*vx)&#xA;    plt.plot(time, km*vy)&#xA;    plt.plot(time, km*v )&#xA;    plt.title('vx, vy, vtot (km/s) versus time (seconds)', fontsize=16)&#xA;    plt.subplot(5, 1, 2)&#xA;    plt.plot(time, km*(r-re))&#xA;    plt.title('altitude (km) versus time (seconds)', fontsize=16)&#xA;    plt.subplot(5, 1, 3)&#xA;    plt.plot(time[:-1], KE[:-1] - KE[1:])&#xA;    plt.title('Watts dissipated', fontsize=16)&#xA;    plt.subplot(5, 1, 4)&#xA;    plt.plot(time[:-1], ((v[:-1] - v[1:])/dt)/gee)&#xA;    plt.title('gees', fontsize=16)&#xA;    plt.subplot(5, 1, 5)&#xA;    plt.plot(km*x, km*(y-re))&#xA;    plt.plot(km*x, km*(np.sqrt(re**2 - x**2)-re), '-k', linewidth=2)&#xA;    plt.title('y versus x (km)', fontsize=16)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-31T06:15:52.510" LastActivityDate="2017-08-31T06:15:52.510" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22830" PostTypeId="2" ParentId="22827" CreationDate="2017-08-29T17:55:13.070" Score="3" Body="&lt;p&gt;Also don't forget that we don't have to do everything digitally.  &lt;a href=&quot;https://en.wikipedia.org/wiki/Analog_computer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Analog computers&lt;/a&gt; were common until fading out in the 60's and 70's, but can be useful in some areas.&lt;/p&gt;&#xA;" OwnerUserId="17059" LastActivityDate="2017-08-29T17:55:13.070" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22831" PostTypeId="2" ParentId="22827" CreationDate="2017-08-29T19:27:50.813" Score="8" Body="&lt;p&gt;This isn't a computer as the term is usually understood today, and it certainly isn't using morse code.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What the system is actually doing is taking the output of an instrument, probably a low voltage electrical signal, amplifying it to something mechanically usable, and converting it into rotation of the four disks on top of the rover (with 4 positions each, it's the equivalent of an 8 bit signal). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The value shown on those disks can be seen from an orbiter or high altitude baloon that can use conventional computers for all the more complex tasks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Physically, the 'computer' part would be a bunch of gears that move the disks. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As shown in the previous answers, you can calculate a surprising with such systems, but due to weight concerns the complexity would probably be kept to a minimum - for example using relative positioning of the disks rather than resetting to an absolute value because the orbiter can easily subtract the previous value with no weight penalty.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-08-29T19:27:50.813" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22832" PostTypeId="2" ParentId="22826" CreationDate="2017-08-29T20:15:19.887" Score="6" Body="&lt;p&gt;You'd need to get to Jupiter to get an assist to Earth. It might be possible. &lt;a href=&quot;https://arc.aiaa.org/doi/abs/10.2514/1.42893?journalCode=jsr&quot; rel=&quot;nofollow noreferrer&quot;&gt;See Saturn Escape Options for Cassini Encore Missions&lt;/a&gt; and &lt;a href=&quot;https://yellowdragonblogdotcom.files.wordpress.com/2015/08/okutsu_cassini_saturn_escape.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini End-of-Life Escape Trajectories to the Outer Planets&lt;/a&gt;. You can certainly escape Saturn with multiple Titan flybys. Getting to Jupiter is trickier, and requires the right timing and a lot of patience. Low-energy Saturn-Jupiter transits are available about every 20 Earth years.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="10273" LastEditDate="2022-07-08T19:56:36.333" LastActivityDate="2022-07-08T19:56:36.333" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22833" PostTypeId="1" AcceptedAnswerId="22840" CreationDate="2017-08-30T03:42:14.337" Score="4" ViewCount="605" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/22656/12102&quot;&gt;Here is a great answer&lt;/a&gt; with photos of the shadow of the moon on the Earth's surface (umbra) during a total solar eclipse, taken by &lt;em&gt;real people in space&lt;/em&gt;, and links to even more of them. I think there's been about seven in total (see &lt;a href=&quot;http://www.collectspace.com/news/news-081617a-total-solar-eclipse-from-space-astronauts.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://www.collectspace.com/news/news-082117b-solar-eclipse-space-station-astronauts.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But has the Moon's shadow on the Earth (solar eclipse umbra) ever been photographed from beyond Earth orbit? For this question photos by satellites are of course fine!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2023-10-11T15:38:28.567" Title="Has the Moon's shadow on the Earth (solar eclipse umbra) ever been photographed from beyond Earth orbit?" Tags="|spacecraft|photography|solar-eclipse|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22834" PostTypeId="1" CreationDate="2017-08-30T03:57:50.943" Score="85" ViewCount="23754" Body="&lt;p&gt;I have seen at least several different users mention that they've found &lt;a href=&quot;https://en.wikipedia.org/wiki/Kerbal_Space_Program&quot; rel=&quot;noreferrer&quot;&gt;Kerbal Space Program&lt;/a&gt; helpful to understand issues of spaceflight and orbital mechanics.&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Kerbal_Space_Program#Physics&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;While the game is not a perfect simulation of reality, it has been praised for its accurate orbital mechanics; all objects in the game except the celestial bodies are simulated using Newtonian dynamics. For instance, rocket thrust is applied to a vehicle's frame based on the placement of force-generating elements, and joints between parts have limited strength, allowing vehicles to be torn apart by excessive or misdirected forces.&lt;/p&gt;&#xA;&lt;p&gt;The game simulates trajectories and orbits &lt;strong&gt;using patched conic approximation instead of a full n-body simulation; thus, it does not support Lagrange points, perturbations, Lissajous orbits, halo orbits or tidal forces&lt;/strong&gt;. According to the developers, implementing full n-body physics would require the entire physics engine to be rewritten.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;But that last bit is only because it was written with simplifying mathematical approximations in the first place, and just changing to 3-body Newtonian physics instead of patched conics would then allow &amp;quot;&lt;em&gt;Lagrange points, perturbations, Lissajous orbits, (and) halo orbits&lt;/em&gt;...&amp;quot;; there would not be any need to invoke the extreme-sounding n-body limit. Within that framework, tidal forces would be straightforward to include as well.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;disclaimer:&lt;/strong&gt; KSP is a commercial product and I don't meant to advertise it by asking about it; personally I'd recommend starting with what you can get for free here in Stack Exchange, in books, on line, at various NASA, university, and private blog sites. But since it has been mentioned several times in this site I decide to learn more.&lt;/p&gt;&#xA;&lt;p&gt;I am not sure if KSP teaches you any of the &lt;em&gt;actual mathematics of spaceflight&lt;/em&gt; (e.g. equations) or if it just shows you what happens and gives you a false sense of &amp;quot;understanding&amp;quot; because after a while you start to know what's going to happen on the KSP screen, which is the whole idea behind getting better at a video game.&lt;/p&gt;&#xA;&lt;p&gt;I'm interested in answers that &lt;em&gt;explain&lt;/em&gt; (rather than just listing) how one can learn about spaceflight and orbital mechanics by paying money for and then playing this game. I am biased and highly skeptical that this game teaches you anything more than how to keep playing the game and posting comments here telling people to buy it; can you demonstrate otherwise?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-31T17:51:33.993" Title="What can the KSP game actually teach about spaceflight and orbital mechanics, and what are its limitations?" Tags="|orbital-mechanics|crewed-spaceflight|uncrewed-spaceflight|kerbal-space-program|education|" AnswerCount="7" CommentCount="20" ContentLicense="CC BY-SA 3.0" />
  <row Id="22835" PostTypeId="2" ParentId="22834" CreationDate="2017-08-30T04:25:54.167" Score="26" Body="&lt;p&gt;In a sense, you're correct; KSP does not teach you any of the math involved in orbital dynamics.  But it gives you an incredibly good intuitive sense of how they work.  For example, one of the first things you learn is just how much fuel you need to get to orbit.  You quickly have to come to grips with the tyranny of the rocket equation if you want to get anywhere at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you've gotten above the atmosphere, the next thing you learn is where the appropriate burn points are.  You need to burn at apogee to raise your perigee.  Then, you burn at perigee to circularize your orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After you're in orbit, you then have to tackle all sorts of things, like changing orbital inclination, docking, and gravity assists to other planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So no, it doesn't provide you with equations and how to solve them.  And yes, the physics engine is off (but can you blame them?  It would be quite a task to program a full n-body simulation for a simple video game).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But ask yourself: what does the average person want?  Do they want to learn all of the mathematics behind spaceflight?  No, they usually don't.  And if they do, they don't turn to KSP.  But thanks to KSP, people who play it are more familiar with how spacecraft orbits work.  It presents something normally complex and out-of-reach in an entertaining and intuitive way.  And in the end, KSP isn't meant to teach a student how to calculate whether or not a rocket will crash.  It's a game, meant for entertainment.  The fact that it also helps you develop an intuitive sense of orbital mechanics is a bonus.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2017-08-30T04:25:54.167" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="22836" PostTypeId="2" ParentId="22834" CreationDate="2017-08-30T04:33:58.453" Score="118" Body="&lt;p&gt;In my former job I was writing educational software.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, it's exactly what you described: we offered a paid version of what you could get for free by looking out on the internet, going to class, going to the library, ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And yet, I'm still incredibly proud of it, knowing I made a difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the difference between well written software and a book?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's all about pedagogy. Not only did our apps offer knowledge, they did so in a &lt;strong&gt;fun&lt;/strong&gt; way. This is not about making the knowledge available, it's about making it desirable. With our apps, the kids not only learned (exactly the same content as they would've with books), but &lt;em&gt;they actually wanted to!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That made all the difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure, maybe knowledge is enough of a motivation for you to learn about something, and thus you don't actually need this kind of software, but for many kids they need just a little bit more to get interested into learning, doing exercises, ...&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Let me explain with some real life KSP examples:&lt;/h2&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;KSP can be used as a simulator.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Got an interesting idea about creating an orbital refueling station around the moon? Try it out and see the difference!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Want to try fuel saving by using a gravitational assist? Go ahead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will more boosters suffice to double your payload? Not even close.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure you could take a sheet of paper and do the computations, but seeing it live on your screen, with the rocket and station you build and put in orbit yourself is a whole different thing.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;It's a driving force to make you research things you wouldn't have searched otherwise.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How to calculate if my ship has enough fuel to reach Saturn?&lt;br&gt;&#xA;=&gt; Learn about ISPs, rocket equation, ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the optimal trajectory to go to orbit?&lt;br&gt;&#xA;=&gt; Learn about gravity drag, atmospheric drag, ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This game builds up your interest in space. This is why they got a partnership with NASA and built a part of the game around it.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;It allows you to &lt;em&gt;experience&lt;/em&gt; things&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the concepts of orbital mechanics are hard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure you can read 10 times about why you need to decelerate in orbit to overtake another spaceship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sure you can learn that rendezvous is not aiming for where the other spaceship is, but where it &lt;em&gt;will&lt;/em&gt; be.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;It makes you ponder engineering challenges&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did I forget the ladder? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are 3 landing legs better than 4?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is my CG (center of gravity) too high? Is it aligned with my vector of thrust?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should my aerodynamic surfaces go on the top or bottom?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should I put this booster further apart to facilitate separation, or will the induced stability issues, pilot induced oscillation destroy/slow down my rocket?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;KSP lets you experience all this, making it much easier to understand and internalise rather than just reading about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Let me conclude with a quote from a NASA engineer, about the failed rendezvous of Gemini 4:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There is a good explanation for what went wrong with rendezvous. The crew, like everyone else at MSC, &quot;just didn't understand or reason out the orbital mechanics involved[...].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes that's right. Kids playing KSP now have a better understanding of orbital mechanics than NASA engineers and astronauts from 1965.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="19857" LastEditDate="2017-08-31T08:27:11.710" LastActivityDate="2017-08-31T08:27:11.710" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="22837" PostTypeId="1" AcceptedAnswerId="22841" CreationDate="2017-08-30T05:22:26.843" Score="0" ViewCount="146" Body="&lt;p&gt;&lt;a href=&quot;https://twitter.com/Tungsten_Flight/status/902582635502870528&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here is a tweet&lt;/a&gt; from NASA flight director Royce Renfrew's personal twitter account. I believe at least some of the Hurricane Harvey OPS team is on-site at &lt;a href=&quot;https://www.nasa.gov/centers/johnson/home/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Johnson Space Center&lt;/a&gt; but the work might be distributed. What kind of tasks and challenges does the Hurricane Harvey OPS team have? What's been going on there during the hurricane and its aftermath?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also &lt;a href=&quot;https://www.nasa.gov/feature/nasa-s-johnson-space-center-closes-for-hurricane-harvey&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA’s Johnson Space Center Closes for Hurricane Harvey&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zhoXp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zhoXp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-31T01:15:36.447" LastActivityDate="2017-08-31T01:15:36.447" Title="What responsibilities does the Hurricane Harvey OPS team have, and what challenges?" Tags="|nasa|weather|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22838" PostTypeId="1" AcceptedAnswerId="22846" CreationDate="2017-08-30T05:52:47.153" Score="7" ViewCount="749" Body="&lt;p&gt;How many spacecraft have taken a &quot;Pale Blue Dot&quot; type photo of the Earth from deep space? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &quot;Pale Blue Dot&quot; type photo would be a photo of the earth from far away; for the purposes of this question it can be anything outside of cis-lunar space. See for example the NASA news article &lt;a href=&quot;https://www.nasa.gov/jpl/voyager/pale-blue-dot-images-turn-25&quot; rel=&quot;noreferrer&quot;&gt;'Pale Blue Dot' Images Turn 25&lt;/a&gt; and the question&lt;a href=&quot;https://space.stackexchange.com/q/20125/12102&quot;&gt;What is the brown band in which we find the pale blue dot that is Earth?&lt;/a&gt; for the original Pale Blue Dot image, and &lt;a href=&quot;https://space.stackexchange.com/a/3758/12102&quot;&gt;this great answer&lt;/a&gt; for more cool photos!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a random collection, there are quite a few in fact. If someone has or know of a complete list and can post it, that would be great. If this needs to be changed to a community wiki, that's fine. Ideally one sample image from each spacecraft would be super, maybe this has been done already somewhere?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-30T05:58:10.097" LastActivityDate="2024-01-16T16:07:48.693" Title="How many spacecraft have taken a &quot;Pale Blue Dot&quot; type photo of the Earth from beyond cis-lunar space?" Tags="|deep-space|photography|pale-blue-dot|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22839" PostTypeId="2" ParentId="22834" CreationDate="2017-08-30T08:52:28.420" Score="99" Body="&lt;p&gt;In several press conferences, employees of NASA or private space firms have been asked if they played KSP, and some answered with &amp;quot;Yes&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;NASA used patched conics to find candidate orbits for Apollo back in the days.&lt;/p&gt;&#xA;&lt;p&gt;With that being said, KSP strikes the balance between &lt;strong&gt;accuracy and simplicity&lt;/strong&gt;. Patched conics give a good idea how space works, without being so overly complicated that it's no longer fun to deal with. And that means it makes people learn stuff they otherwise wouldn't. The mantra &lt;a href=&quot;https://what-if.xkcd.com/58/&quot; rel=&quot;noreferrer&quot;&gt;&amp;quot;Space isn't high up, it's about going sideways fast&amp;quot;&lt;/a&gt; can be repeated thousands of times, and people won't get it. But when they actually try it out and see for themselves why it is how it is, they suddenly understand. When they fall back onto the planet after launching straight up, and then finally learn to pitch over and do a &lt;strong&gt;gravity turn&lt;/strong&gt; to achieve orbit.&lt;/p&gt;&#xA;&lt;p&gt;The fact that orbits are always influenced at the opposite side of the burn. That deceleration (&lt;strong&gt;retrograde&lt;/strong&gt; burns) lowers the orbit and so forth. What &lt;strong&gt;radial burns&lt;/strong&gt; do, what &lt;strong&gt;normal burns&lt;/strong&gt; do, what &lt;strong&gt;inclination&lt;/strong&gt; is and why it is important, how &lt;strong&gt;ascending and descending nodes&lt;/strong&gt; work and why they are important, why &lt;strong&gt;rendezvous&lt;/strong&gt; takes so long and what you need to do to actually make it. All those things are taught by KSP and make spaceflight accessible to people that otherwise wouldn't &amp;quot;get&amp;quot; it.&lt;/p&gt;&#xA;&lt;p&gt;The Mun (the equivalent to the Moon in KSP) is in an equatorial orbit to make it easier for new players to get there (no launch window, no inclination change), but those things can be used to get to the second, much smaller moon, Minmus.&lt;/p&gt;&#xA;&lt;p&gt;The concept of &lt;strong&gt;Hohmann transfers&lt;/strong&gt;, &lt;strong&gt;bi-elliptical transfers&lt;/strong&gt; and other transfers is explained. The difference between fuel-efficiency and time efficiency can be learned by KSP. &lt;strong&gt;Ejection angles&lt;/strong&gt; and &lt;strong&gt;launch windows&lt;/strong&gt; for interplanetary missions can be learned with KSP. &lt;strong&gt;Gravity assists&lt;/strong&gt; work, and can be very useful. That's a concept a lot of people struggle with, but in KSP you can try it out and see how that works.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Re-entry heating&lt;/strong&gt; exists and matters somewhat, depending on the settings. You learn that you should neither plow too fast too deep into the atmosphere neither too low so that you skip it. The concept of re-entry is hard to grasp for some people, but with KSP, you can easily see why it works how it works and what's important about it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;The rocket equation&lt;/strong&gt;, and especially the &lt;em&gt;tyranny of the rocket equation&lt;/em&gt;, is important in KSP. You learn &lt;strong&gt;good engineering techniques&lt;/strong&gt;. You learn that you can't bring what you want, that rockets grow exponentially. The concept of staging and why it is important comes naturally with KSP. Some &lt;em&gt;basic aerodynamics&lt;/em&gt; — how &lt;strong&gt;CoM&lt;/strong&gt; and &lt;strong&gt;CoL&lt;/strong&gt; work, how &lt;strong&gt;drag&lt;/strong&gt; affects the path of your rocket — are explained.&lt;/p&gt;&#xA;&lt;p&gt;What &lt;strong&gt;Isp&lt;/strong&gt; is and does is explained. That &lt;em&gt;Isp at sea level&lt;/em&gt; and &lt;em&gt;Isp in vacuum&lt;/em&gt; are different things and are important is explained. That you can steer a rocket with &lt;strong&gt;gimbaling&lt;/strong&gt; engines, &lt;strong&gt;reaction wheels&lt;/strong&gt; or &lt;strong&gt;RCS&lt;/strong&gt; is explained, and that some of those are better in some situations then others (RCS for ascent? Not the best idea!).&lt;/p&gt;&#xA;&lt;p&gt;In short: KSP teaches &lt;em&gt;a lot&lt;/em&gt;, while using physics and orbital mechanics that are not so overly complicated that it gets too frustrating. It stays in the area that most people can still grasp.&lt;/p&gt;&#xA;&lt;p&gt;And for those who want the extra challenge, the game can be modded. N-Body physics are provided by a mod called &amp;quot;Principia&amp;quot;, and the star system can be transformed to the Real Solar System by the mod of the same name (RSS). Historic engines and other parts are provided by the mod called &amp;quot;Realism Overhaul&amp;quot; (RO). Signal delay is provided by RemoteTech, life support by one of the various LS mods (Kerbalism, USI-LS etc).&lt;/p&gt;&#xA;&lt;p&gt;So apart from the base experience that already teaches lots of the &lt;em&gt;concepts of spaceflight&lt;/em&gt; (I forgot about &lt;strong&gt;heating and radiators&lt;/strong&gt; in my list above) while maintaining accessible and approachable (if you don't want to learn too much, trial and error will get you somewhat far), the modding community has created a lot of mods that bring even more important concepts to the player (FAR has a more realistic aerodynamic simulation, for example).&lt;/p&gt;&#xA;&lt;p&gt;And the game lets you deal with it how you want. Don't want to learn about &lt;strong&gt;launch azimuth&lt;/strong&gt;? Fine, don't. The parts have enough &lt;strong&gt;delta-v&lt;/strong&gt; so that you &lt;em&gt;can&lt;/em&gt; be sloppy sometimes. Want to learn? Good! You can learn about it, and then apply what you have learned and see it actually work. See your orbit end up exactly what you wanted (or not). The community has created lots of maths tools and helpers for that as well.&lt;/p&gt;&#xA;&lt;p&gt;Finally, Squad (the developer) already has/had a deal with NASA. NASA helped them make some parts, the so called &amp;quot;NASA parts&amp;quot; that are still in the game today (I believe they were originally for the &amp;quot;Asteroid day&amp;quot; official mod, but are now in the base game).&lt;/p&gt;&#xA;&lt;p&gt;So yeah, it's not a 100% accurate simulator. But its close enough to teach the concepts, and simple enough so that its accessible for a very wide audience. In that regard, I think it offers a great example for how educational games should work in the future. Because it's actually fun to play &lt;em&gt;and&lt;/em&gt; makes stuff easy to understand.&lt;/p&gt;&#xA;&lt;p&gt;Some random things I haven't mentioned: &lt;strong&gt;Aerodynamics&lt;/strong&gt; (&lt;em&gt;Rudder&lt;/em&gt;, &lt;em&gt;Elevons&lt;/em&gt;, &lt;em&gt;Trim&lt;/em&gt;, placement of wings, CoM, CoL and concept of lift for planes), how an &lt;strong&gt;FDAI&lt;/strong&gt; (called &lt;em&gt;Navball&lt;/em&gt; in KSP) works, &lt;strong&gt;reference frames&lt;/strong&gt; (the FDAI has &amp;quot;surface&amp;quot; mode [rotating reference frame of the body] and &amp;quot;orbital&amp;quot; reference frame [non-rotating frame of the body], as well as &amp;quot;target&amp;quot; frame [frame centered around the own vessel]), &lt;em&gt;connectivity of antennas&lt;/em&gt; and &lt;strong&gt;signal strength&lt;/strong&gt; (&lt;em&gt;inverse square law&lt;/em&gt;), &lt;strong&gt;solar power output&lt;/strong&gt; (again, inverse square law, you learn why solar panels don't work well for outer planets), you learn about the &lt;strong&gt;pendulum rocket fallacy&lt;/strong&gt; and a lot more things.&lt;/p&gt;&#xA;&lt;h3&gt;On accuracy and computing power:&lt;/h3&gt;&#xA;&lt;p&gt;Being a game, the simulation has a hard &lt;strong&gt;&lt;em&gt;realtime&lt;/em&gt; requirement&lt;/strong&gt;. Patched conics are not only &lt;em&gt;far easier to understand for a wide audience&lt;/em&gt;, but they are also &lt;em&gt;much&lt;/em&gt; faster. Patched conics have an &lt;em&gt;analytical solution&lt;/em&gt;. That means it's cheap to calculate positions. Furthermore, finding closest approach / intercepts is cheap, which is needed for maneuver planning. The added benefit is that it makes finding a rendezvous something accessible for a wide audience. It's not fun to try to find a close approach for two vessels when you need a simulator to make that decision. The base concepts apply whether you use patched conics or not. A more realistic model does not add much educational value, but a lot of frustrations for players — who mostly have no background whatsoever in orbital mechanics. Furthermore, the game needs to be able to simulate literally hundreds of vessels at the same time — something that is only possible with patched conics in real time. The aforementioned mod for n-body physics quickly breaks down after collisions of vessels that generate lots of debris.&#xA;Furthermore, it doesn't matter whether a vessel is in atmosphere or not from a physics point of view. The physics engine still needs to simulate all parts and all joints between all parts to figure out how the vessel behaves, especially under thrust.&#xA;Those hard realtime requirements put a hard cap on what you can realistically achieve on the average &lt;em&gt;home computer&lt;/em&gt;. Again, it's a game, it needs quick results, &lt;em&gt;it's not an academic simulation&lt;/em&gt;. &lt;strong&gt;It's close enough to teach the concepts&lt;/strong&gt;. It doesn't claim to have 100% scientific accuracy.&lt;/p&gt;&#xA;&lt;p&gt;To the point raised in other comments, that it uses &amp;quot;17th century maths&amp;quot;. Sure it does. But so do a lot of people. You don't start teaching people physics by jumping into quantum mechanics. You don't even start with GR/SR. You start with classical, Newtonian physics. Those are still around, and still valid as ever, and still have their applications. Just because we nowadays know that they aren't accurate on certain scales doesn't mean those physics are useless.&#xA;Again, KSP has to strike a balance between accuracy and simplicity. Be accurate enough to actually use the real concepts of spaceflight, while being simple enough to appeal to a wide audience.&lt;/p&gt;&#xA;&lt;p&gt;Have you ever been at a planetarium? When they show the orbits of planets in the solar system, they use patched conics. Because that's enough to give people an idea how that stuff works, without being too over-the-top. Have you ever looked at orbits plotted in a Jacobian frame? Those look extremely confusing, while arguably being more &amp;quot;accurate&amp;quot;.&lt;/p&gt;&#xA;" OwnerUserId="13867" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-09-02T03:53:37.613" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="22840" PostTypeId="2" ParentId="22833" CreationDate="2017-08-30T09:20:32.293" Score="12" Body="&lt;p&gt;The &lt;a href=&quot;https://epic.gsfc.nasa.gov/epic&quot; rel=&quot;noreferrer&quot;&gt;Earth Polychromatic Imaging Camera (EPIC)&lt;/a&gt; on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Climate Observatory (DSCOVR)&lt;/a&gt; spacecraft has observed both the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_eclipse_of_March_9,_2016&quot; rel=&quot;noreferrer&quot;&gt;2016&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_eclipse_of_August_21,_2017&quot; rel=&quot;noreferrer&quot;&gt;2017&lt;/a&gt; eclipses and probably every solar eclipse since it's arrived, as EPIC is always looking at the sunlit part of Earth.  DSCOVR is at the Sun-Earth &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;L₁ point&lt;/a&gt;, roughly 4× farther out than the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;2016 (edited and zoomed in):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://eoimages.gsfc.nasa.gov/images/imagerecords/87000/87675/eclipse_epc_2016068_zoom.jpg&quot; alt=&quot;DSCOVR eclipse photo&quot;&gt;&lt;br&gt;&#xA;Source: &lt;a href=&quot;https://earthobservatory.nasa.gov/NaturalHazards/view.php?id=87675&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2017 (entire disk, straight from image browser):&#xA;&lt;img src=&quot;https://i.stack.imgur.com/OH95T.jpg&quot; alt=&quot;DSCOVER eclipse photo 2017&quot;&gt;&lt;br&gt;&#xA;Source: &lt;a href=&quot;https://epic.gsfc.nasa.gov/?date=2017-08-21&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can browse photos for yourself at the &lt;a href=&quot;https://epic.gsfc.nasa.gov/?date=2017-08-21&quot; rel=&quot;noreferrer&quot;&gt;EPIC browser&lt;/a&gt;, searching for 2017-08-21 and you'll see the so-called “Great American Eclipse” soon enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA also produced an animation at of the 2016 eclipse at:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://earthobservatory.nasa.gov/NaturalHazards/view.php?id=87675&quot; rel=&quot;noreferrer&quot;&gt;https://earthobservatory.nasa.gov/NaturalHazards/view.php?id=87675&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sun-Earth L₁ is 1.5 million km from Earth, which is farther than the Moon, which has a semi-major axis of 384,000 km.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Although it didn't seem to happen during the eclipse, during the solar eclipse or indeed any new Moon, the Moon can't be very far from the field of view of EPIC.  Indeed, sometimes the Moon &quot;photo bombs&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.nasa.gov/sites/default/files/thumbnails/image/epicearthmoonstill.png&quot; alt=&quot;not an eclipse&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Not quite as far away is the Lunar Reconnaissance Orbiter (LRO), which took the following photo during the 2017 total eclipse.  Note that during a solar eclipse (and any new moon) the entire disk of the Earth is sunlit as seen from the moon:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://www.nasa.gov/sites/default/files/thumbnails/image/untitled-1_4.jpg&quot; alt=&quot;LRO eclipse photo&quot;&gt;&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="2752" LastEditDate="2019-03-02T17:31:58.883" LastActivityDate="2019-03-02T17:31:58.883" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="22841" PostTypeId="2" ParentId="22837" CreationDate="2017-08-30T12:22:53.237" Score="2" Body="&lt;p&gt;The 'Hurricane Harvey ops team' &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/2017/08/28/mission-control-monitors-iss-closed-jsc/609211001/&quot; rel=&quot;nofollow noreferrer&quot;&gt;is a subset of the flight operations team&lt;/a&gt;. They've kept working in the Mission Control Center while all other personnel has been sent home.&lt;/p&gt;&#xA;&lt;p&gt;Challenge: being cooped up in the office for several days with a hurricane going on outside, possible utility interruptions due to storm damage.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Despite the crisis unfolding flooding outside, ISS flight directors and more than 100 other “essential” personnel have ridden out the storm inside JSC’s iconic Mission Control Center to maintain watch over the ISS.&lt;/p&gt;&#xA;&lt;p&gt;Another flight director described the scene as surreal, with off-duty teams sleeping in a backup flight control room. Pictures showed cots lined up in front of a Flight Dynamics Officer’s console.&lt;/p&gt;&#xA;&lt;p&gt;Thanks in part to the team’s sacrifice, Harvey has caused no disruption to space station operations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There's a backup flight control center:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In one precautionary measure, downlinks of science research video has been rerouted to NASA’s Marshall Space Flight Center in Huntsville, Alabama, which can serve as a backup Mission Control Center in an emergency.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;h2&gt;Diversion:&lt;/h2&gt;&#xA;&lt;p&gt;(this was part of my initial answer, leaving it here because it's interesting)&lt;/p&gt;&#xA;&lt;p&gt;For hurricanes etc., NASA assigns a rideout team to various locations. The rideout team watches the site for damage, tries to mitigate it and decides when it's safe for other personnel to enter the base.&lt;/p&gt;&#xA;&lt;p&gt;Here's a story about how &lt;a href=&quot;https://blogs.nasa.gov/kennedy/2016/10/06/ksc-prepares-to-ride-out-matthew/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt; handled &lt;a href=&quot;https://www.nasaspaceflight.com/2016/10/ksccape-hurcon-alert-matthew/&quot; rel=&quot;nofollow noreferrer&quot;&gt;hurricane Matthew&lt;/a&gt; at &lt;a href=&quot;https://www.cbsnews.com/news/hurricane-matthew-florida-kennedy-space-center-braces-for-storm/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cape Canaveral in 2016&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Members of a 139-member “rideout” team will be stationed at various facilities across the space center to monitor critical systems “and report any significant events” to emergency operations personnel in the Complex 39 Launch Control Center where space shuttle launchings were once managed.&lt;/p&gt;&#xA;&lt;p&gt;“After the hurricane has passed, and winds have dropped below 50 knots, areas around KSC will be assessed and the damage assessment and recovery team will report for duty,” Diller said.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-08-30T13:51:34.200" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22842" PostTypeId="2" ParentId="22834" CreationDate="2017-08-30T13:38:37.963" Score="12" Body="&lt;p&gt;You concentrate on the unrealistic points which have to be present in the game and ignore strong realistic points. The Kerbal Space Program is not a perfect spacecraft simulator however it successfully teaches the basics. Most importantly even if it doesn't full N-body simulation it's perfectly capable of simulating Kepler's orbits that give you good first approximation for many spacecraft missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because other answers already emphasized many strong points let me put a personal perspective. Because of my background as a physicist and general interest in the space exploration I knew all the equations and had all the knowledge required long before playing KSP. However when I tried to perform a docking between two vessels it took me one hour of constant trajectory corrections because even with all this knowledge I did a lot of really stupid stuff. Which exactly what was happenning during the first real docking missions. The reason is that even when you have the knowledge without experience your mind doesn't apply it the right way. This is what games like KSP are very good in - they give you very nice intuition about basic orbital mechanics.&lt;/p&gt;&#xA;" OwnerUserId="18138" LastActivityDate="2017-08-30T13:38:37.963" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22844" PostTypeId="1" AcceptedAnswerId="22880" CreationDate="2017-08-30T18:44:07.277" Score="10" ViewCount="664" Body="&lt;p&gt;Recently, I came across &lt;a href=&quot;https://mars.jpl.nasa.gov/msl/mission/mars-rover-curiosity-mission-updates/?mu=sol-1796-elevation-4220-meters&quot; rel=&quot;noreferrer&quot;&gt;information that Curiosity is currently at -4220 m of altitude&lt;/a&gt;. I have three questions: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Since there is no sea level on Mars, what is the level of reference for altitude measuring? &lt;/li&gt;&#xA;&lt;li&gt;How much elevation has Curiosity climbed so far from the base of Mount Sharp? &lt;/li&gt;&#xA;&lt;li&gt;How much more elevation of Mount Sharp can Curiosity be reasonably expected to climb until the end of its mission?  &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="20879" LastEditorUserId="6816" LastEditDate="2017-08-30T18:53:50.517" LastActivityDate="2017-09-02T16:57:01.537" Title="How much elevation has Mars Curiosity climbed so far from the bottom of Gale crater?" Tags="|mars|curiosity|" AnswerCount="2" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22845" PostTypeId="2" ParentId="22834" CreationDate="2017-08-30T19:27:27.397" Score="18" Body="&lt;p&gt;The other 4 answers here do a wonderful job of articulating what I love about KSP, but I'd like to point out one more thing from my perspective as a game designer who has logged over 600 hours in the game.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While much of the game is simplistic relative to the real world, it is accurate enough to be intuitive why things failed.  Rocket flipped over during launch? Gotta adjust aerodynamics or try a different launch profile.  Got partway to my destination and ran out of fuel?  Maybe I should look into more efficient engines or miniaturizing my rocket. Trying to get back to Kerbin from one of its moons, but you're low on fuel?  Better make sure to exit the moon's orbit the opposite direction as its motion.  Made it back to Kerbin but all my Kerbalnauts died from hitting the side of the mountain? Add more parachutes.  KSP is more than the orbital dynamics, it is engineering with rapid iterations on design.  Obviously guess-and-check development doesn't work for NASA, but with access to a &quot;Revert to launch&quot; and &quot;Revert to vehicle assembly&quot; buttons always available to you (unless you turn them off or a mod does it for you), learning why things don't work and how to fix them is much easier than just crunching numbers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ability to try, fail, attempt to fix the problem and try again in rapid cycles is part of what makes KSP a great learning tool. Once you've been in the game enough, you build a good intuition for how it works in real life, and the mods mentioned in &lt;a href=&quot;https://space.stackexchange.com/a/22839/16599&quot;&gt;Polygnome's answer&lt;/a&gt; can add in those extra levels of realism if you want to up the challenge. While the vanilla game hides most of the numbers from you (total delta V, current ISP, distance above the ground rather than distance above sea level), there are mods that make those accessible too. I personally can't play without Kerbal Engineer showing me some of those.  Same goes for launch windows, eyeballing when to do a planetary transfer is hard, but using Kerbal Alarm Clock to calculate it down to the second helps a lot too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, KSP does a great job of making some of the challenges NASA faces very intuitive, and the ability to iterate rapidly on plane and spacecraft designs makes learning them fun and engaging. If you are the type that wants to dig into the math, optimizing every part of the mission, free mods are easily available to give you that option.  If you want it to be much closer to realism, that can be done easily.  Other mods give you new solar systems to explore, so you never run out of challenges.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as an example, &lt;a href=&quot;https://www.youtube.com/watch?v=sF8roW6Cxm0&quot; rel=&quot;noreferrer&quot;&gt;here is a YouTube video&lt;/a&gt; from a very skilled KSP player explaining how the Oberth Effect and gravity assists are useful to optimize your fuel consumption. Note that this video is old and things have improved greatly since then. Even with the game's simplified understanding of orbital mechanics, they still matter.&lt;/p&gt;&#xA;" OwnerUserId="16599" LastActivityDate="2017-08-30T19:27:27.397" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22846" PostTypeId="2" ParentId="22838" CreationDate="2017-08-31T02:08:31.210" Score="10" Body="&lt;p&gt;&lt;a href=&quot;https://science.nasa.gov/science-research/earth-science/23jul_palebluedot/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini&lt;/a&gt; and Voyager 1 are the only ones to take a photo from the outer solar system.&lt;/p&gt;&#xA;&lt;p&gt;From Mars, &lt;a href=&quot;https://www.nasa.gov/feature/jpl/earth-and-its-moon-as-seen-from-mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;MRO&lt;/a&gt;, &lt;a href=&quot;https://www.nasa.gov/jpl/msl/earth-view-from-mars-pia17936&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity&lt;/a&gt;, and &lt;a href=&quot;https://mars.nasa.gov/mer/gallery/press/spirit/20040311a.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spirit&lt;/a&gt; all have.&lt;/p&gt;&#xA;&lt;p&gt;From Mercury, &lt;a href=&quot;http://www.planetary.org/multimedia/space-images/earth/earth-and-the-moon-from-mercury.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;MESSENGER&lt;/a&gt; did.&lt;/p&gt;&#xA;&lt;p&gt;Deep Space- &lt;a href=&quot;http://www.planetary.org/multimedia/space-images/earth/moon-transiting-earth-deep-impact-sequence.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Impact&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;If there are any others, I can't find them. Of course, many have taken them from the Moon, or in Earth lagrange points, or during flybys of the Earth, but for those, take a look at this &lt;a href=&quot;http://planetary.org/explore/space-topics/earth/pics-of-earth-by-planetary-spacecraft.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planetary Society&lt;/a&gt; list.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2024-01-16T14:42:29.637" LastActivityDate="2024-01-16T14:42:29.637" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22847" PostTypeId="1" CreationDate="2017-08-31T06:41:22.810" Score="2" ViewCount="211" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/22829/12102&quot;&gt;this answer&lt;/a&gt; I've estimated the peak rate of energy loss of a particularly unwise re-entry as 3 to 4 gigawatts. Never mind that it's 15-20 gees of acceleration, that's a lot of power injected into the gas and plasma directly in front of the heat-shield.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that for survivable capsule re-entries with existing heat shields, the power generation is way lower than that, even on a log scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any ballpark, order of magnitude-type numbers out there for the rate of energy loss to the atmosphere in manned capsule reentries?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-08-31T07:35:11.150" LastActivityDate="2017-09-02T14:47:56.933" Title="Roughly what is the rate of energy loss of survivable capsule re-entries?" Tags="|apollo-program|reentry|soyuz-spacecraft|capsule|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22849" PostTypeId="1" CreationDate="2017-08-31T11:32:52.327" Score="17" ViewCount="3204" Body="&lt;p&gt;What would happen if the astronauts on the ISS stopped maintaining it, and no further vehicles or remote control commands were sent up from the ground? I'd like to know what systems would stop working, what would burn out, fail, jam, freeze, overflow, etc, at what point the astronauts would die, and in what order the station would break down into a heap of unrecoverable junk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To prolong this experiment, let's assume it was in a much higher orbit so orbital decay wouldn't become terminal for hundreds of years. I am just interested in the process of decay, and if/when a steady state eventually gets reached, what would be salvageable and what would be ruined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can, please format an answer as a rough timeline or chain of events, with guesses as to the durations, and comments on interesting events where appropriate. Answer should include much info on the main life support failures, but also cover the other subsystems, and can span hundreds of years if necessary. Please include: power generation, truss bearings, cooling, stations attitude, pressure, water and o2 content, long term viability of electrical systems should power ever be restored.. What is no longer salvageable and repairable after 10years, 20, 50..&lt;/p&gt;&#xA;" OwnerUserId="16727" LastEditorDisplayName="user20636" LastEditDate="2018-02-02T07:10:36.040" LastActivityDate="2018-03-21T10:37:05.387" Title="What would happen to the ISS if we stopped maintaining it?" Tags="|iss|" AnswerCount="1" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="22850" PostTypeId="1" AcceptedAnswerId="22887" CreationDate="2017-08-31T13:13:09.207" Score="4" ViewCount="2773" Body="&lt;p&gt;This question is a bit of a mix between satellite knowledge and math. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suppose I have the &quot;look angles&quot; (elevation and azimuth) for an otherwise unidentified &lt;em&gt;geostationary satellite&lt;/em&gt;, and I would like to use that, plus my ground latitude and longitude to calculate the longitude coordinate of the satellite. How would I go about doing that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solution might be written as the following function:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$lon_{Sat} = f(lon, \ lat, \ el, \ az)$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally I'd like an equation that I could use for this. If not, at least a procedure for solving for the geostationary satellite's longitude.&lt;/p&gt;&#xA;" OwnerUserId="20889" LastEditorUserId="12102" LastEditDate="2017-09-02T19:26:26.627" LastActivityDate="2017-10-14T18:30:10.263" Title="Computing GEO satellite's longitude from elevation/azimuth from a given latitude/longitude?" Tags="|geostationary|coordinates|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22851" PostTypeId="2" ParentId="22834" CreationDate="2017-08-31T13:32:03.167" Score="2" Body="&lt;p&gt;KSP teaches an intuitive understanding of orbital mechanics in a way that reading text books, watching videos or any other non-interactive method of learning fails to achieve. By displaying a real-time prediction of future orbits as manoeuvres are planned and executed, it instils a visual representation of orbital changes that can be used to to reason about arbitrary situations while not even playing the game.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example, I posed a question about orbital rendezvous to someone who supposedly knew about spaceflight, but he was unable to answer sensibly, whereas any moderately experienced KSP player would know what to do. In the same way NASA knew about the mathematics of orbital rendezvous but had to build simulators to teach astronauts how to it, KSP makes orbital mechanics a matter of practical application rather than a theoretical knowledge.&lt;/p&gt;&#xA;" OwnerUserId="20869" LastActivityDate="2017-08-31T13:32:03.167" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22852" PostTypeId="1" CreationDate="2017-08-30T18:45:43.150" Score="1" ViewCount="44" Body="&lt;p&gt;Why is Cassini self destructing in Saturn's atmosphere while the Huygens Probe was sent to land on Titan?&lt;/p&gt;&#xA;" OwnerUserId="20883" OwnerDisplayName="Richard Wales" LastActivityDate="2017-08-31T13:43:40.437" Title="Cassini and the Huygens Probe" Tags="|saturn|" AnswerCount="0" CommentCount="2" ClosedDate="2017-08-31T17:32:36.983" ContentLicense="CC BY-SA 3.0" />
  <row Id="22853" PostTypeId="2" ParentId="22850" CreationDate="2017-08-31T15:10:13.783" Score="4" Body="&lt;p&gt;No, you would need to know the range to the satellite as well.  Think of it this way - if you draw a diagram of what you describe above, the vector representing the line of sight from your ground position to the satellite (az, el) would cross a swath of longitudes, except for the trivial case where el=90&lt;/p&gt;&#xA;" OwnerUserId="4709" LastActivityDate="2017-08-31T15:10:13.783" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22854" PostTypeId="1" CreationDate="2017-08-31T16:04:58.080" Score="8" ViewCount="544" Body="&lt;p&gt;The Starshade is a space telescope with a large occulter that can fly off and block the light of a star so its telescope can picture the surrounding exoplanets: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Hn0VtQ3FqCw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I sort of understand the optics behind its shape; the light that bleeds around the edges their waves cancel each other out. But I can't get the math of the distance right. Say it wants to image the Trappist system, which is 4e17m away and its star is 7e7m in diameter and the shade is 10m. So the distance/size ratio would be 5e9, thus the shade would have to be 5e10m (50,000,000km) from the telescope right? Or is my math wrong?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't that way too far?! Why is it so big? The orbital-mechanics behind it are also problematic. Will they orbit the Earth? Getting in an orbit 5e9m from the Earth is rather energetically expensive, not to mention the difference in orbital period they'd have to deal with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess these are a lot of questions, but I can't find a proper explanation of the New Worlds mission (it's real name). So I'm hoping someone can explain the gist of it to me.&lt;/p&gt;&#xA;" OwnerUserId="20892" LastEditorUserId="20892" LastEditDate="2017-09-05T09:17:11.610" LastActivityDate="2017-09-05T09:17:11.610" Title="The physics behind the Starshade" Tags="|orbital-mechanics|imaging|exoplanet|" AnswerCount="1" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="22856" PostTypeId="1" CreationDate="2017-08-31T20:41:15.673" Score="29" ViewCount="19122" Body="&lt;p&gt;The pressure on Venus is 1334 PSI (9.1976 MPa) or 92 times the pressure at sea level on Earth, which is 14.5 PSI (99.9734 kPa). Now, in our oceans the pressure increases by 14.5 PSI (99.9734 kPa) every 33 feet (10.058 m), or 9997.734 kPa every meter. So, we're told that the atmospheric pressure on Venus is equal to the pressure on Earth at 3036 feet (925 m) under the ocean? Do I have this right? If so what is causing that much pressure in a gas?&lt;/p&gt;&#xA;" OwnerUserId="20883" LastEditorUserId="7062" LastEditDate="2023-04-01T05:04:05.840" LastActivityDate="2023-11-08T04:43:44.963" Title="Why is the atmospheric pressure on Venus so high?" Tags="|venus|" AnswerCount="6" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="22857" PostTypeId="1" AcceptedAnswerId="22864" CreationDate="2017-08-31T20:58:12.147" Score="1" ViewCount="299" Body="&lt;p&gt;How big is a human's (space) bubble - in other words: how far could we (as mankind) transport a human being from Earth in such a way that the being would be still alive? (Let's not consider returning.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm a novice in space travel but I suspect the most difficult part is sustaining life in space for a non-trivial period without external support; the second most difficult might be accelerating the spacecraft in such a way that it covers the most distance in the available timeframe.  (Colloquially: The bigger the spacecraft the easier I'd believe to support living on it; however it makes it harder to move around.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since it's a fully speculative exercise, everything that's realistically available currently or could be built in a relatively short time based on what we know and possess now can be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess it's a much-discussed scenario (by beers or otherwise) but I don't know how to search for it.  Any pointers are welcome.&lt;/p&gt;&#xA;" OwnerUserId="20896" LastActivityDate="2017-09-03T23:01:55.313" Title="How big is a human's space bubble?" Tags="|crewed-spaceflight|life-support|calculation|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22858" PostTypeId="1" AcceptedAnswerId="22859" CreationDate="2017-09-01T00:29:58.070" Score="10" ViewCount="2394" Body="&lt;p&gt;Part of &lt;a href=&quot;https://space.stackexchange.com/a/22851/12102&quot;&gt;this answer&lt;/a&gt; was inspired &lt;em&gt;if I remember correctly&lt;/em&gt; from a question about how to reverse the direction of a satellite in LEO with the minimum delta-v. Whether this was the original question or not, I'm still curious about the answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently I would know this if I were an active KSP player, but I'm not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I would probably have to either do an internet search, or write a python script and do some hunting in maneuver space. If I did that I'd explore using 3-body effects, getting assistance from the Moon's or Sun's gravity by first rising to a higher, elliptical orbit, or maybe even an Apollo-style free-return. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, it seems that the lowest delta-v maneuver is already known by many, just not me :-(&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the lowest delta-v maneuver to reverse the direction of a satellite in LEO? For a satellite in a circular, equatorial LEO with an altitude of 600 km, &lt;em&gt;and what would the value of that delta-v be?&lt;/em&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The initial velocity is $\sqrt{GM_E/a}=$7558 m/s and so the worst case delta-v is double that. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2020-04-10T17:18:28.413" Title="Lowest delta-v maneuver to reverse the direction of a satellite in LEO?" Tags="|orbital-maneuver|kerbal-space-program|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22859" PostTypeId="2" ParentId="22858" CreationDate="2017-09-01T00:52:56.837" Score="23" Body="&lt;p&gt;Reversing direction is a special case of inclination- / plane-change.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's one way to find a better upper bound on the necessary delta-V for a 180 degree plane change:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assume a 2-body system (i.e. ignoring the sun and moon).  Burn once to increase your orbital energy to C3 = 0 (i.e. escape velocity), coast out to an infinite distance where your velocity is 0, make an infinitesimal velocity change, fall back to 600 km altitude and burn again to recircularize.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Escape velocity at 600 km altitude is 10690 m/s.  Therefore the total delta-V required by this technique is $ 2*(10690 - 7558) = 6264 \mathrm{m}/\mathrm{s} $ - only 41% of that required by the single-burn &quot;brute force&quot; method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course as given above it requires infinite distance and time, but you can still do pretty well by raising your apogee to something merely &quot;very high&quot;, reversing direction and subsequently recircularizing at perigee.  This is known as a &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/476156.pdf&quot; rel=&quot;noreferrer&quot;&gt;bi-elliptic transfer with plane change.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you suggest, you can do better still by making use of &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2012/reid/&quot; rel=&quot;noreferrer&quot;&gt;lunar flybys&lt;/a&gt; or &lt;a href=&quot;http://google.ne/patents/WO1998047765A1?cl=en&quot; rel=&quot;noreferrer&quot;&gt;Weak-Stability Boundary transfers&lt;/a&gt;.  Those don't lend themselves to clean analytic solutions though.  I'll also point out that the &quot;lowest delta-V maneuver&quot; may not be a practical one - in the real world there will definitely be time and distance constraints, and the optimal maneuver is also likely to depend on the starting conditions (e.g. the inclination of the 600 km circular orbit).&lt;/p&gt;&#xA;" OwnerUserId="3065" LastEditorUserId="3065" LastEditDate="2017-09-01T19:36:51.447" LastActivityDate="2017-09-01T19:36:51.447" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22861" PostTypeId="2" ParentId="22844" CreationDate="2017-09-01T08:20:45.847" Score="1" Body="&lt;p&gt;I have found a &lt;a href=&quot;https://mars.jpl.nasa.gov/multimedia/images/?ImageID=7149&quot; rel=&quot;nofollow noreferrer&quot;&gt;map from 2015&lt;/a&gt;, when Curiosity was still close to the floor of Gale crater. The map shows altitude of -4456 m. That means that the rover has climbed only ~ 230 m so far. I still don't know how much higher is he expected to get.  &lt;/p&gt;&#xA;" OwnerUserId="20879" LastActivityDate="2017-09-01T08:20:45.847" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22862" PostTypeId="1" AcceptedAnswerId="22866" CreationDate="2017-09-01T11:28:48.830" Score="17" ViewCount="579" Body="&lt;p&gt;I understand that Newton's Law of Gravitation is still used today to calculate the paths of probes, since it is such an accurate theory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the Law of Gravitation does not accurately describe Mercury's orbit, so when sending a probe to Mercury, do you need to use General Relativity instead? How large is the potential error in Newton's Law?&lt;/p&gt;&#xA;" OwnerUserId="5880" LastActivityDate="2017-09-01T20:15:48.580" Title="Is GR required to send a probe to Mercury?" Tags="|probe|gravity|mercury|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22864" PostTypeId="2" ParentId="22857" CreationDate="2017-09-01T13:34:23.657" Score="2" Body="&lt;p&gt;Assuming unlimited funding and limited concern for astronaut safety, anywhere in the solar system is reachable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming you aren't building a space farm, which is entirely new technology, the liniting factor is probably food - air and water are fairly easy to store indefinitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons got to Pluto in about 8 years, and a quick search suggests that although canned goods are typically expected to last two to five years, they may be nutritious (if unappetizing) for decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given more reasonable constraints, Mars is definitely doable, with Jupiter and Saturn possible albeit extremely expensive because the efficient transfer would take longer than the food will last.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-09-01T13:34:23.657" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22865" PostTypeId="2" ParentId="22856" CreationDate="2017-09-01T15:47:57.717" Score="-4" Body="&lt;p&gt;Yes you are right. There is a lot more and heavier gas on Venus than on Earth, and since Venus is smaller than Earth, the pressure that the gas exerts per square inch is much higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://pages.uoregon.edu/soper/Venus/atmosphere.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here is a good comparison between the two planets&lt;/a&gt;. Notice that according to Dr Soper's table, there are 66 molecules per square meter on Venus, versus only 1 on Earth.&lt;/p&gt;&#xA;" OwnerUserId="12318" LastActivityDate="2017-09-01T15:47:57.717" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22866" PostTypeId="2" ParentId="22862" CreationDate="2017-09-01T20:09:29.073" Score="19" Body="&lt;p&gt;In any interplanetary mission, it's impossible to perfectly accurately measure the position and velocity of a spacecraft or a planet, so mission planners generally schedule a number of opportunities to measure the accumulated error and make a course correction. Therefore, there's no reason a Mercury probe mission would have been a failure if &lt;a href=&quot;https://en.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_of_Mercury&quot; rel=&quot;noreferrer&quot;&gt;General Relativity&lt;/a&gt; wasn't understood. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of precession of Mercury's orbit due to GR is, according to Wikipedia, 43 arcseconds per century. Let's see how that would impact a mission like &lt;a href=&quot;https://en.wikipedia.org/wiki/MESSENGER&quot; rel=&quot;noreferrer&quot;&gt;MESSENGER&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;MESSENGER took about four years to make its first encounter with Mercury. If, for some reason, you didn't notice that Mercury's actual precession didn't match up with your theory, and planned to go where you thought Mercury should be according to theory, Mercury's orbit would be rotated 1.72 arcseconds out of position. At Mercury's distance from the Sun, that amounts to an error of about 500km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, MESSENGER did a flyby of Earth and two of Venus to reach its Mercury-bound trajectory. After the second Venus flyby, &lt;a href=&quot;http://messenger.jhuapl.edu/About/Mission-Design.html&quot; rel=&quot;noreferrer&quot;&gt;it made a fairly large course correction (called DSM-2)&lt;/a&gt; to properly line up the Mercury encounter, and this occurred only three months before reaching Mercury. So if you measured Mercury's position at that point, you'd be just 0.1 arcseconds off target when aiming three months out, missing your target by only 30km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;MESSENGER's first flyby of Mercury had a closest approach of 200km, so 30km wouldn't risk a crash or a complete miss; uncorrected, though, it would mess up the timeline of the orbital capture and insertion. As it happens, MESSENGER did a small correction a month after DSM-2. Assuming, again, our Relativity-ignorant mission controllers just corrected for Mercury's observed position at that time, the error would come down to about 20km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Between that point and the first Mercury encounter, MESSENGER used a different strategy for the very small course corrections needed, in order to save fuel: it angled its solar panels slightly to use them as sails on the solar wind! The remaining error due to General Relativity at this point could be corrected with a delta-V of only about 4mm/s; if solar panel sailing wasn't sufficient, the small attitude thrusters on the spacecraft would be. So I don't see any reason MESSENGER would have failed given an ignorance of GR; at worst it would have spent a negligible amount of additional propellant in corrections, possibly shortening its 4-year lifespan in Mercury orbit by a couple of months. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only other mission to reach Mercury was &lt;a href=&quot;https://en.wikipedia.org/wiki/Mariner_10&quot; rel=&quot;noreferrer&quot;&gt;Mariner 10&lt;/a&gt;; it also did a single Venus flyby for gravitional assist. It did have maneuvering thrusters, and it was nearly two months between its Venus flyby and first Mercury flyby (at 700km), so similar assumptions would hold. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally: in 1859, &lt;a href=&quot;https://en.wikipedia.org/wiki/Urbain_Le_Verrier#Precession_of_Mercury&quot; rel=&quot;noreferrer&quot;&gt;Le Verrier&lt;/a&gt; &lt;em&gt;measured&lt;/em&gt; the precession effects of GR, without knowing what the cause was; he got a figure of 38 arc-seconds per century instead of 43. If our mission plan took that figure into account, the GR error figures I've given here would be much reduced: ~60km at 4 years, ~3.5km at 3 months, ~2.5km at 2 months. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-01T20:15:48.580" LastActivityDate="2017-09-01T20:15:48.580" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22867" PostTypeId="2" ParentId="22856" CreationDate="2017-09-01T20:26:22.897" Score="5" Body="&lt;p&gt;Atmospheric pressure (or water pressure, for that matter) is simply the weight of the atmosphere that one square inch (or any square unit) supports. The atmospheric pressure on earth is, for the sake of argument, 14.5 PSI. That simply means that there is 14.5 pounds of atmosphere above one square inch of surface at sea level on Earth. (To calculate the pressure at a given depth in the ocean, you only need to calculate the weight of the ocean above that depth.) As @Uwe pointed out in comments, the mass of the atmosphere on Venus is 93 times that of Earth's, so one would expect that the surface pressure would be that much greater, as well.&lt;/p&gt;&#xA;" OwnerUserId="13307" LastActivityDate="2017-09-01T20:26:22.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22868" PostTypeId="2" ParentId="22856" CreationDate="2017-09-01T22:10:39.177" Score="36" Body="&lt;p&gt;The pressure exerted on a surface area under the ideal gas law is $P = \frac{\rho k_B T}{\mu}$ with the following notation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$\rho$ being the volume mass density (or simply density)&lt;/li&gt;&#xA;&lt;li&gt;$k_B$ is the Boltzmann constant&lt;/li&gt;&#xA;&lt;li&gt;$T$ is the temperature in Kelvin&lt;/li&gt;&#xA;&lt;li&gt;$\mu$ is the mean molecular mass in g/mol&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;On Earth $\mu_E \approx 29$, while on Venus it is $\mu_V \approx 44$. Furthermore $T_E \approx 300$ and $T_V \approx 750$, so the factor of $\frac{T}{\mu}$ accounts for only a factor of $1.76$ between those pressures.&lt;br&gt;&#xA;The main difference here comes from the density: $\rho_E = 1.2 kg/m^3$ while $\rho_V=67 kg/m^3$.&lt;br&gt;&#xA;Together you get $P_V = P_E \cdot 1.76 \cdot 55.8 \approx 98 \cdot P_E$. The difference from this calculation compared with the factor of $92$ that is actually true, come from the fact that Venus atmosphere on the surface mostly is in a state of supercriticality, which means it is somewhere between the state of gas and fluid. This makes it necessary to apply corrections to the ideal gas law which I've not used above, for simplicity. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the main difference in pressure comes from the higher density of Venus atmosphere. What does that mean?&lt;br&gt;&#xA;&lt;strong&gt;There is simply so much more 'stuff'&lt;/strong&gt; in Venus' atmosphere. Earth's total atmosphere has a mass of around $5 \cdot 10^{18}$kg, while the Venusian one has $5 \cdot 10^{20}$kg, so this is 100 times more than what Earth has.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How could this happen?&lt;/strong&gt;&#xA;Or you could ask &quot;How can a planet nearly the same mass as Earth have an atmosphere 100 times more massive?&quot;. This is one of the big mysteries of Planet formation &amp;amp; evolution. There are two halfway compelling scenarios that come to my head now, both involve Earth and Venus having similar atmospheres at a relatively young age of $\sim 100$ Million years.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Venus might have gotten less accretional heat in the phase as the solar system formed. Together with it's smaller size, it was able to radiate this heat away even faster than Earth was. Also smaller size implies less radiogenic heat production throughout the ages.&lt;br&gt;&#xA;All those factors might have conspired to make plate tectonics much weaker/nonexistent on Venus. This prevents burying of the atmosphere, as it happens in Earth's mantle. But it is hard / impossible to estimate how strong this effect was in the past.&lt;/li&gt;&#xA;&lt;li&gt;Venus went early into a stage of runaway greenhouse effect. We know this, because we find it today in this state. Somehow this protected the atmosphere from escaping (I don't want to specify any mechanism, as there are many possibilities), while Earth lost a significant portion of its atmosphere over time.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If you're more interested in this topic I recommend as introductory read Imke de Pater and Jack Lissauer's book &quot;Planetary Science&quot;.&lt;/p&gt;&#xA;" OwnerUserId="2839" LastEditorUserId="2839" LastEditDate="2017-09-02T03:31:35.130" LastActivityDate="2017-09-02T03:31:35.130" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22869" PostTypeId="2" ParentId="22758" CreationDate="2017-09-01T22:13:15.750" Score="8" Body="&lt;p&gt;Chiming in as a long-time member of the NH3 Fuel Association.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, a small correction. &quot;Cryogenic&quot; refers to gases that liquefy below -150 degrees C. As you state, ammonia liquefies at -33 degrees ... which is fairly close to ambient conditions in the industry. &quot;Refrigerated&quot; would be a more accurate term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, you ask why ammonia was used as the rocket fuel for the X-15. I'm no engine expert, and others have addressed this here with more knowledge than I can, but I understand that the &quot;coking&quot; issue was vital: hydrocarbon fuels formed soot that affected performance, but ammonia contains no carbon and thus formed no soot. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, you ask if ammonia was used before or after the X-15. Yes and yes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the past ... Belgium, municipal buses in 1940s; Norway, demonstration truck in 1930s; Louisiana, public trolley car in 1870s (ammonia steam engine, working fluid not fuel). See &lt;a href=&quot;https://nh3fuelassociation.org/introduction/&quot; rel=&quot;noreferrer&quot;&gt;https://nh3fuelassociation.org/introduction/&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now underway: dozens of projects for carbon-free power and fuel, from island energy projects to national grid-scale import/export projects. See &lt;a href=&quot;http://ammoniaenergy.org&quot; rel=&quot;noreferrer&quot;&gt;my website&lt;/a&gt; if you'd like to search for more information on projects around the world.&lt;/p&gt;&#xA;" OwnerUserId="20912" LastEditorUserId="6816" LastEditDate="2017-09-02T02:28:04.547" LastActivityDate="2017-09-02T02:28:04.547" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22870" PostTypeId="2" ParentId="14704" CreationDate="2017-09-02T00:54:39.200" Score="2" Body="&lt;p&gt;Males produce new sperm cells continually, so it's hard to see how radiation exposure could make them permanently sterile. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Females draw from the same stock of egg cells for their whole lives. The empirical evidence, from Hiroshima survivors, seems to be that even with a pretty hefty radiation dose, there is no epidemiologically detectable additional risk of birth defects. This seems to me to make sense, because radiation has the biggest effect on cells undergoing rapid division -- that's why it works as a cancer treatment. Eggs cells aren't undergoing division at all until fertilization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some empirical evidence that low doses of radiation increase fertility in female humans, trout, and mice. According to &lt;a href=&quot;https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2477686/&quot; rel=&quot;nofollow noreferrer&quot;&gt;one study&lt;/a&gt;,&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Lightly irradiated rodents were more fertile than controls through several generations (increased ovulation in dams, increased number, viability and growth rates of young, and faster physical development of young) with no evidence of mutations in the young exposed in utero. Irradiated colonies were maintained in good health through 21 generations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is an example of a more general type of effect known as radiation hormesis, which is hypothesized to occur because radiation triggers cells' mechanisms for damage control. The doses at which measurable hormesis occurs are usually fairly large, so I would guess that any pro-fertility effect on ISS astronauts is negligible, and in any case effects such as increased ovulation, even if significant, would be temporary.&lt;/p&gt;&#xA;" OwnerDisplayName="user687" LastEditorDisplayName="user687" LastEditDate="2017-09-02T01:07:35.850" LastActivityDate="2017-09-02T01:07:35.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22871" PostTypeId="1" AcceptedAnswerId="22872" CreationDate="2017-09-02T01:02:01.460" Score="3" ViewCount="131" Body="&lt;p&gt;I would like to know the meaning of a sentence below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;While docked, the ATV will perform ISS attitude control, debris avoidance manoeuvres and raising of the 183 tonne station's orbit to overcome the effects of atmospheric drag.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(From: &lt;a href=&quot;http://www.space-propulsion.com/spacecraft-propulsion/bipropellant-thrusters/200n-bipropellant-thrusters.html#moved&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.space-propulsion.com/spacecraft-propulsion/bipropellant-thrusters/200n-bipropellant-thrusters.html#moved&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot understand &quot;raising of the 183 tonne station's orbit...&quot;.&#xA;The weight of ISS is not 183 tonne, right?&#xA;What does this &quot;183 tonne&quot; refer to?&lt;/p&gt;&#xA;" OwnerUserId="20903" LastActivityDate="2017-09-02T01:27:03.220" Title="What does &quot;183 tonne&quot; in this sentence refer to?" Tags="|iss|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22872" PostTypeId="2" ParentId="22871" CreationDate="2017-09-02T01:21:51.477" Score="6" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Assembly_of_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;According to WP&lt;/a&gt;, by summing up the masses of the modules in the order they were added to the station, the mass of the ISS was quite close to 183 tons after the installation of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Integrated_Truss_Structure#P3.2FP4.2C_S3.2FS4_truss_assemblies&quot; rel=&quot;noreferrer&quot;&gt;P3/P4 truss and solar arrays&lt;/a&gt; by &lt;a href=&quot;https://en.wikipedia.org/wiki/STS-115&quot; rel=&quot;noreferrer&quot;&gt;STS-115&lt;/a&gt; in September 2006. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Automated_Transfer_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;ATV&lt;/a&gt; was in development at that time, but didn't go into service until 2008. Most likely the information on your page is dated from 2006.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-02T01:27:03.220" LastActivityDate="2017-09-02T01:27:03.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22874" PostTypeId="2" ParentId="22856" CreationDate="2017-09-02T08:07:31.753" Score="19" Body="&lt;p&gt;The question you did ask:&lt;br&gt;&#xA;&lt;strong&gt;Why is the atmospheric pressure on Venus so high?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer is that the mass of the atmosphere of Venus is very high compared to that of the Earth, by almost a factor of 100. On the other hand, the two planets have similar surface areas and similar gravitational accelerations at the surface. Since surface pressure is approximately the weight (mass times gravitational acceleration) of the atmosphere divided by the surface area, this means that surface pressure on Venus is very high compared to that of the Earth, by almost a factor of 100.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;This leads to the question you didn't ask:&lt;br&gt;&#xA;&lt;strong&gt;Why is the atmosphere of Venus so massive (and why is it almost all CO&lt;sub&gt;2&lt;/sub&gt;)?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer to this question is that the Earth has water on the surface, plate tectonics, and abundant life. Venus has none of these.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The combination of the above means that Earth has a vigorous &lt;a href=&quot;https://en.wikipedia.org/wiki/Carbonate%E2%80%93silicate_cycle&quot; rel=&quot;noreferrer&quot;&gt;carbonate-silicate cycle&lt;/a&gt; that sequesters vast amounts of carbon underground. This cycle does not exist on Venus. Venus has instead expelled a large amount of its crustal carbon into its atmosphere, mostly in the form of carbon dioxide.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-09-02T08:07:31.753" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22875" PostTypeId="2" ParentId="22858" CreationDate="2017-09-02T11:26:33.500" Score="2" Body="&lt;p&gt;A seasoned KSP player with foreknowledge of need of reversal of orbit direction would give the craft good heat shielding, lean aerodynamic profile and good hypersonic wings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perform a mild reentry burn. Reenter. When the air density is sufficient, pull up to level the flight, bank left or right and perform a 180 degrees turn slow enough that G-forces won't be too destructive. Then perform a burn that sets your apoapsis above the atmosphere and then circularize at the apoapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of delta-V will be lost to atmospheric drag, and this number is difficult to determine, depending on the craft and its aerodynamics a lot; a ballpark of my KSP intuition tells me about 10-20% of delta-V needed to start. The obligatory reentry and circularization burns would be minuscule, of order of 100m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, it might be possible to use scramjet engines for the most expensive part of the maneuver.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-09-02T11:26:33.500" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22876" PostTypeId="1" CreationDate="2017-09-02T12:47:26.130" Score="15" ViewCount="2956" Body="&lt;p&gt;The Space.com article &lt;a href=&quot;https://www.space.com/38026-space-station-crew-landing-expedition-52-webcast.html&quot; rel=&quot;noreferrer&quot;&gt;Space Station Crew Returns to Earth Tonight: Watch It Live&lt;/a&gt; leads with a photo of three astronauts on the ISS that are backlit with bright green light, which can also be seen spilling out onto the walls in various patterns. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither the article nor the photo caption (shown below) mention it or explain the origin or purpose of this bright green lighting, but there must be some reason for it. Is this explained elsewhere?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;NASA astronaut Peggy Whitson, Russian cosmonaut Fyodor Yurchikhin and NASA astronaut Jack Fischer float through the Harmony module of the International Space Station. All three will return to Earth on Sept. 2, 2017.&quot; Credit: NASA, from &lt;a href=&quot;https://www.space.com/38026-space-station-crew-landing-expedition-52-webcast.html&quot; rel=&quot;noreferrer&quot;&gt;Space.com&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/twEri.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/twEri.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2021-06-21T04:40:50.273" Title="Why are these astronauts green?" Tags="|iss|astronauts|" AnswerCount="2" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22877" PostTypeId="2" ParentId="22847" CreationDate="2017-09-02T14:47:56.933" Score="4" Body="&lt;p&gt;&lt;a href=&quot;http://spaceflight101.com/soyuz-tma-19m/landing-profile/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here&lt;/a&gt; is the descent profile for Soyuz.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 08:53:30 the speed is 7.62km/s and touchdown is at 09:14:39.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Over 1269 seconds the object sheds 7.62 km/s. Kinetic energy is $.5mv^2$. So that's 29,032,200 joules per kilogram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;29,032,200 joules/1269 seconds = 22878 watts.  Over that 21 minute interval I get about 23 kilowatts per kilogram.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the descent profile, Soyuz descent maximum g load is around 4 g's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am trying to find the descent profiles of the Apollo capsules. They would enter the earth's atmosphere at almost 11 km/s. But so far I haven't been able to find descent profiles that give altitudes and speeds at different times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the scenario you link to I get max speed of 4.52 km/s at about 71 km altitude. Impact is 117 seconds later. For this I get 87 kilowatts per kilogram. More than triple of the Soyuz capsule. This is with a 1.85 meter radius, 6500 kilograms and a drag coefficient of .5&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Increasing the radius to 2.9 meters, max speed of 4.5 km/s is reached at about 79 km altitude. Impact is 185 seconds later. Over that 185 seconds the capsule endures 54 kilowatts per kilogram. More than double the Soyuz.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-09-02T14:47:56.933" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22878" PostTypeId="2" ParentId="22876" CreationDate="2017-09-02T16:10:22.267" Score="25" Body="&lt;p&gt;New &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/research/experiments/651.html&quot; rel=&quot;noreferrer&quot;&gt;Solid State Lighting Modules&lt;/a&gt; being installed on the ISS have the ability to be set to &lt;a href=&quot;https://www.space.com/35997-led-lights-help-astronauts-sleep-better.html&quot; rel=&quot;noreferrer&quot;&gt;millions of different colors&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The crew probably enjoys playing around with the settings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ucOtn.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ucOtn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/yywx9.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yywx9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These pictures are from ISS crew member Jack Fischer's &lt;a href=&quot;https://twitter.com/astro2fish?lang=en&quot; rel=&quot;noreferrer&quot;&gt;twitter&lt;/a&gt; account.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-09-02T16:10:22.267" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22879" PostTypeId="2" ParentId="22757" CreationDate="2017-09-02T16:23:27.547" Score="3" Body="&lt;p&gt;Well, in theory you could launch a telescope to some orbit and a starshade to the orbit obtained by reflection in the plane that passes through the central body perpendicularly to the direction to the object you want to observe:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/g49nj.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/g49nj.png&quot; alt=&quot;Two symmetric orbits&quot;&gt;&lt;/a&gt;&#xA;Then, assuming perfectly Keplerian orbits, if you synchronize the telescope and the starshade to move perfectly symmetrically, then the line which passes through them will always point to the observed object. (Of course, if they move perfectly symmetrically, then they will collide in the points where their orbits inntersect, so there's that.) Part of the time, the telescope will be in front of the starshade, though, but you can make this fraction small by making orbits highly elliptical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These two orbits don't have to lie in the same plane, they just have to be symmetric to each other w.r.t. the plane perpendicular to the direction to the star. If you make the orbits non-coplanar, then the central body won't block the view when the telescope and the starshade are in apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I doubt that this would work in practice, though. Influences of other celestial bodies and irregularities in the gravity of the central body would affect the orbits, and the direction of the line connecting the telescope and the starshade will change. But I don't know how quickly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I can tell, this is the only solution under the assumption of perfectly Keplerian orbits. If the direction of the line that passes through two orbiting objects stays constant over some period of time, then the components of their accelerations perpendicular to that direction must be the same. This is possible only if they are at the same distance from the central body, i.e., they are symmetric to each other w.r.t. to the plane passing through the central body and perpendicular to the line connecting the two objects.&lt;/p&gt;&#xA;" OwnerUserId="12448" LastActivityDate="2017-09-02T16:23:27.547" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22880" PostTypeId="2" ParentId="22844" CreationDate="2017-09-02T16:34:13.167" Score="13" Body="&lt;p&gt;As of Sol 1802, a few days ago, Curiosity was &lt;a href=&quot;https://mars.nasa.gov/imgs/2017/09/MSL_TraverseMap_Sol1802-full.jpg&quot; rel=&quot;noreferrer&quot;&gt;at -4206 m MOLA&lt;/a&gt;. It landed &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/2014JE004622/full&quot; rel=&quot;noreferrer&quot;&gt;at -4501 m MOLA&lt;/a&gt;, so the net climb so far is 295 m. Of course, it's gone up and down over that time, so the total amount of climbing is more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars Global Surveyor spacecraft (1997-2006) carried an instrument called MOLA (1997-2001), the Mars Orbiter Laser Altimeter. It generated what is still today the most detailed map of Mars' elevation. Along with the radio science from that orbiter and two others, we have a correlated gravity map to high order.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MOLA science team arbitrarily but sensibly defined the zero altitude level (&quot;sea level&quot;) for Mars using the MOLA data. To first order, it's the average radius at the equator, extrapolating an equipotential surface (taking into account both the high-order gravity field and the rotation of the planet) from the equator to the rest of the planet. More specifically, from the paper &lt;a href=&quot;http://www-geodyn.mit.edu/mola.summary.pdf&quot; rel=&quot;noreferrer&quot;&gt;&quot;Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars&quot;&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Zero elevation on Mars from MOLA is defined as the equipotential&#xA;  surface (gravitational plus rotational) whose average value at the&#xA;  equator is equal to the mean radius as determined by MOLA (cf. Table&#xA;  4). The planetary radius and a gravity model derived from MGS Doppler&#xA;  tracking data [Lemoine et al., this issue] with the IAU91 coordinate&#xA;  system parameters for Mars [Davies et al., 1992a] collectively&#xA;  provided the geopotential of Mars' mean equatorial radius. This&#xA;  equipotential surface was then extended to all latitudes as the&#xA;  zero-level reference for topography.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Prior to MOLA, a pressure reference was used. The average surface pressure of Mars is about 6.1 mbar, which happens to be close to the triple point of water. It was then almost poetic to use that as &quot;sea level&quot; for Mars, which we did. However there were huge errors in that approach, which was fine when we didn't know the elevation of anything very well anyway (except for the two Viking landing sites). Once we had MOLA data, a pressure reference was not usable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The MOLA reference ended up being about 1.6 km higher than the 6.1 mbar reference. I remember a time when we had to keep asking if that altitude was a 6.1 mbar altitude or a MOLA altitude, and having to convert between the two.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for how far it can or will go, no one knows. I would have never guessed that Opportunity ever could or would have driven 45 km, which it has. The only limit then would be the tippy top of Mount Sharp.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-09-02T16:57:01.537" LastActivityDate="2017-09-02T16:57:01.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22881" PostTypeId="2" ParentId="22747" CreationDate="2017-09-02T17:46:36.023" Score="10" Body="&lt;p&gt;Although not really a &amp;quot;space station,&amp;quot; the crew inside the Gemini XXII spacecraft capsule (James A. Lovell, Jr., and Edwin E. &amp;quot;Buzz&amp;quot; Aldrin, Jr.) witnessed a total solar eclipse in November of 1966 (&lt;a href=&quot;https://web.archive.org/web/20120925134954/http://astronautix.com/flights/gemini12.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gemini 12&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Although the pilots missed the ride to high altitude, Lunney soon found something for them to do with their spare time. The flight plan had originally called for them to photograph a solar eclipse, if it did not conflict with the rest of the mission. This task fell by the wayside when the two-day launch delay - from 9 to 11 November - meant that the eclipse would occur during their high-altitude excursion. Canceling the main engine burn inspired two of the mission planners to thoughts of reinstating the eclipse photography. Schneider and Lunney conferred with James R. Bates, Experiments Advisory Officer for Gemini XII, on the effect this might have on the rest of the experiments. Since the flight plan had to be changed anyway, Bates said, why not include the eclipse?&lt;/p&gt;&#xA;&lt;p&gt;This conference with Bates marked a significant change in mission control operations. Formerly working out of an adjacent staff support room, the experimenters' representative was now allowed by the engineers in charge to operate as a part of the flight control team in the main control room. Although there had been an experiments console in the control room by Gemini X, it had been only occasionally manned. Bates, on Gemini XII, was the first full-time experiments officer. This experience worked out so well that the custom was continued in Apollo.&lt;/p&gt;&#xA;&lt;p&gt;Even after the eclipse became a flight-plan casualty, planners continued to plot its path. Now there was a chance to work this experiment back into the mission. The Agena's secondary propulsion system had enough power to get the spacecraft into position for an eight-second photographic pass at the proper time. Schneider and Lunney agreed that this piece of realtime planning would give an added fillip to the mission.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;The eclipse got to us after all,&amp;quot; Lovell remarked. &amp;quot;Yes, it looks like it,&amp;quot; Conrad answered. Although the crew had wanted to do the experiment when it was first planned, these sudden preparations came at an inconvenient time. They were still working with the Agena and were scheduled to begin such activities as eating, sleeping, and working on other experiments.&lt;/p&gt;&#xA;&lt;p&gt;Nevertheless, at 7:05 hours after launch, Jim Lovell fired the Agena's smaller engines to slow his speed 13 meters per second. Agena still had its doubters - Conrad had told them, &amp;quot;If It gets away from . . . take it over with the (spacecraft).&amp;quot; But the target vehicle performed splendidly, and the crew then bedded down for the night.&lt;/p&gt;&#xA;&lt;p&gt;The Canary Island controller greeted the crew in the morning with the news that there would be a second maneuver - 5 meters forward - to line the vehicles up properly. The prospects panned out richly, and the crew reported seeing the eclipse &amp;quot;right on the money at 16:01:44 g.e.t.&amp;quot; The path of the eclipse cut a swath across South America from north of Lima, Peru, nearly to the southernmost tip of Brazil. Although they thought for a moment, they were slightly off track, their aim had been accurate.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Interestingly, Jim Lovell's oral description of the event (as can be seen on disc 3 of &lt;a href=&quot;http://www.collectspace.com/resources/reviews/dvd/project_gemini.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Project Gemini: A Bold Leap Forward&lt;/a&gt;) coincides closely with my own recollections of totality (having witnessed same on 21 August, 2017, in Central City, Nebraska, USA).  However, I'm sure it went by much more quickly for the Gemini XXII crew than it did for me...&lt;/p&gt;&#xA;" OwnerUserId="12422" LastEditorUserId="12422" LastEditDate="2021-04-06T22:13:51.117" LastActivityDate="2021-04-06T22:13:51.117" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22882" PostTypeId="1" AcceptedAnswerId="39570" CreationDate="2017-09-02T18:38:30.950" Score="15" ViewCount="2238" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/22880/12102&quot;&gt;This great answer&lt;/a&gt; suggests the limiting high altitude to which the Curiosity rover on Mars could drive would be the top of &lt;a href=&quot;https://i.stack.imgur.com/Oyah2.jpg&quot; rel=&quot;noreferrer&quot;&gt;Mt. Sharp (Aeolis Mons)&lt;/a&gt; - in the center of &lt;a href=&quot;https://en.wikipedia.org/wiki/Gale_(crater)&quot; rel=&quot;noreferrer&quot;&gt;Gale crater&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering, is there enough imaging data from satellite images from Mars orbit to say something about the terrain and grades (slopes) that Curiosity would encounter if it tried to drive to the top? If there are boulder fields or sloping sand, the trip might actually be very unlikely for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, are there any problems operating Curiosity at higher altitude? Is it colder up there, or are there other weather issues that might affect it? Can it run in a complete vacuum or are there some localized heating issues that do take advantage of the low pressure atmosphere so far below &quot;sea level&quot; on Mars? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the drive took 10 years, would the RTG still have enough oomph to keep it moving up safely and communicating? Do comms with Earth rely on any satellite who's lifetime is limited? Could it even make it to the top in 10 years, or would it take much longer? Anything else?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;This image taken by NASA's Curiosity shows what lies ahead for the rover -- its main science target, Mount Sharp. The rover's shadow can be seen in the foreground, and the dark bands beyond are dunes. Rising up in the distance is the highest peak Mount Sharp at a height of about 3.4 miles, taller than Mt. Whitney in California. The Curiosity team hopes to drive the rover to the mountain to investigate its lower layers, which scientists think hold clues to past environmental change. This image was captured by the rover's front left Hazard-Avoidance camera at full resolution shortly after it landed. It has been linearized to remove the distorted appearance that results from its fisheye lens.&quot; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:673885main_PIA15986-full_full.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, original description at &lt;a href=&quot;https://en.wikipedia.org/wiki/File:673885main_PIA15986-full_full.jpg&quot; rel=&quot;noreferrer&quot;&gt;NASA&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Oyah2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Oyah2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2020-03-24T20:24:47.697" Title="Does the Curiosity rover really have a chance of driving to the top of Mt. Sharp?" Tags="|mars|rovers|curiosity|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22883" PostTypeId="2" ParentId="22834" CreationDate="2017-09-02T21:34:47.297" Score="5" Body="&lt;p&gt;I'd like to first address this part:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But that last bit is only because it was written with simplifying&#xA;  mathematical approximations in the first place, and just changing to&#xA;  3-body Newtonioan physics instead of patched conics would then allow&#xA;  &quot;Lagrange points, perturbations, Lissajous orbits, (and) halo&#xA;  orbits...&quot;; there would not be any need to invoke the extreme-sounding&#xA;  n-body limit. Within that framework, tidal forces would be&#xA;  straightforward to include as well.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;n-body isn't essentially different from 3-body. If you have a 2-body problem, You can immediately calculate the configuration for any future moment in time. But 3-body and above is chaotic, which means that you have to do step-by-step integration, and the further into the future you go, the more precise the calculations have to be to remain accurate. This means smaller time steps and custom-precision numerics - and it means the computational effort is not linear in the time elapsed (as would be the case for step-by-step integration of a non-chaotic system), it is theoretically &lt;em&gt;exponential&lt;/em&gt;. Compared to 2-body which is logarithmic (I would want to say constant since you just plug in a formula, but just representing the time elapsed is logarithmic in it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that you will not be able to timewarp x1000000 as you can currently do in the game, and you can't visually see the entire orbit. This means that to have 3-body you don't just need to rewrite the engine, you would have to rethink what is possible in the game and how the UI and UX works.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not to say that it cannot be done - since nobody will be die if the calculations are a bit off, you can do approximations to make the computation tractable - but it is something best left for a completely new installment of the game. On top of that, while the effects you get from 3-body are nice, there's a ton to explore that does not involve them.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;On to more general considerations. I think that pretty much anything you can learn from KSP, you can learn by studying from books and using less-gamified simulators. But what makes KSP unique is the visceral, emotional feedback you get. Doing calculations on paper is one thing. But getting an audiovisual show, with rockets burning, air ebbing, and the blue marble shrinking, for a rocket that &lt;em&gt;you&lt;/em&gt; designed, is quite another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's also quite different to have game-centric goals and objectives that you have a stake in succeeding to accomplish. Suppose you're trying to complete a simple mission, getting a ship to orbit. You design a ship and, if you're inclined, you do some calculations and estimate that it can succeed. It probably won't be very accurate since there are a lot of factors to take into account, which are time consuming at best. So you do a rough estimate and test it out in practice. And so you find yourself flying at suborbital speeds, with a bit of fuel left that should take you to orbit. You hope it will be sufficient, and as you watch your speed going up and your fuel supply dwindling, you think &quot;come on come on come on...&quot;, and then either rejoice at your success or mope about your failure, knowing that at the very least you'll have to redo the whole thing again. Of course, the same goes for more complicated missions where it's even harder to plan everything in advance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this kind of visceral feedback is important, as it makes the learning much more vivid and memorable, and allows you to have some fun in the process.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Another feature of KSP is that you have a lot of flexibility in how you choose to plan ahead. You can ignore all formulas and computations entirely and just wing it, making use of the game's visual aids as needed. But then you will probably not be very efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or you can do only very rudimentary calculations when the situation calls for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or you can make very sophisticated calculations and planning, requiring good understanding of rocket dynamics and orbital mechanics, complete with your custom-made simulations that you go through before trying out anything in the actual game.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also many plugins you can use which either provide additional aids, or give you data you can use in your calculations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Myself, I had a launch simulator I used in earlier versions of the game to maximize the potential of my designs. But in recent versions they have modified the aerodynamics model to be more realistic but also much, much more complicated. It would take me a lot of effort to rewrite the simulator accordingly to give me actionable information, so for now I've just dropped it and let the game simulate everything for me.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;As for the kind of things you can specifically learn about, these include things such as launches (complete with staging, TWR, air resistance and gravity turns), reentries (slowing down to reentry trajectory, dealing with heat, braking using parachutes or rockets depending on atmosphere, and landing in one piece at the location you wanted), orbital maneuvers (including reaching the specific orbit you want and delta-v economy), orbital rendezvous (with celestial bodies or misplaced crew members), and gravitational slingshots.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As mentioned earlier, the audiovisual feedback and gamified objectives make all these things quite different from learning them on paper.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;All in all, I don't know if I would recommend paying for the game just to learn more about orbital mechanics. But if you want to enjoy the game itself while learning along the way, it's definitely worth trying out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, a case can be made that you can learn a lot by just watching videos of others playing the game and doing all kinds of stuff, there's plenty of that. And it will give you a better idea of what to expect from the game and whether you are interested in experiencing it yourself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Disclaimer: I am not in any way affiliated with the makers of the game, and I do not receive any sort of compensation for convincing people to buy it. I play it, I enjoy it, and I believe that by doing so I have gained a better understanding of, and just as importantly, appreciation for, orbital mechanics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By way of example, I have recently taken an interest in the topic of non-rocket launches. You can't do any of that in KSP, but thinking intelligently about these things requires a good understanding of concepts like TWR, Isp, delta-v requirements for touching space and for reaching LEO and GEO, etc. These things wouldn't have made as much sense to me if I hadn't had experience playing KSP.&lt;/p&gt;&#xA;" OwnerUserId="20918" LastActivityDate="2017-09-02T21:34:47.297" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="22884" PostTypeId="1" CreationDate="2017-09-02T23:14:17.043" Score="1" ViewCount="134" Body="&lt;p&gt;If a planet passed near the moon and caused its orbit to decay down to earth- how long could it take and how would that effect the visible phases of the moon on earth?&lt;/p&gt;&#xA;" OwnerUserId="20919" LastActivityDate="2017-09-02T23:14:17.043" Title="If the moon's orbit decays- how would it effect the moon's phases?" Tags="|orbital-mechanics|orbit|moons|orbital-decay|" AnswerCount="0" CommentCount="1" ClosedDate="2017-09-03T10:50:50.433" ContentLicense="CC BY-SA 3.0" />
  <row Id="22885" PostTypeId="1" AcceptedAnswerId="22888" CreationDate="2017-09-03T08:21:12.243" Score="5" ViewCount="155" Body="&lt;p&gt;Was the booster of the interplanetary transport system ever considered as a super heavy launch vehicle for commercial payloads onto LEO and GEO? (with an expendable or reusable second stage) Or its sole purpose is to enable Mars and outer SolSys exploration? &lt;/p&gt;&#xA;" OwnerUserId="20924" LastActivityDate="2017-09-03T23:27:14.400" Title="SpaceX's ITS as heavy lifter for Earth orbit" Tags="|launch|spacex|commercial|its|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22886" PostTypeId="1" CreationDate="2017-09-03T08:31:21.000" Score="0" ViewCount="762" Body="&lt;p&gt;Okay, so assume you have an electric propulsion (no Oberth effect) spacecraft leaving from Earth escape (for example) to Mars (for example). You want to get there much faster than a Hohmann transfer would allow, say 90 days.&#xA;1. How do you calculate delta V for the stated trip time?&#xA;2. How do you calculate thrust requirement assuming you want a constant thrust for the entire duration?&#xA;3. How do you calculate round trip duration including stay time at destination before the next return window opens?&#xA;4. Turning the problem backwards, assuming you want a round trip time to Mars of 1 year (for example), how do you determine the one way time and stay time?&lt;/p&gt;&#xA;" OwnerUserId="20921" LastActivityDate="2017-09-03T18:16:34.587" Title="How do you calculate delta v and thrust for a given time?" Tags="|mars|thrust|electric-propulsion|" AnswerCount="1" CommentCount="3" ClosedDate="2017-09-04T14:18:37.727" ContentLicense="CC BY-SA 3.0" />
  <row Id="22887" PostTypeId="2" ParentId="22850" CreationDate="2017-09-03T08:38:13.017" Score="3" Body="&lt;p&gt;$lon_{Sat}=f(lon, lat, ht, el, az)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$lon$ Receiver longitude&lt;/li&gt;&#xA;&lt;li&gt;$lat$ Receiver latitude&lt;/li&gt;&#xA;&lt;li&gt;$ht$ Receiver height in metres (does not have a major effect but adding for completeness)&lt;/li&gt;&#xA;&lt;li&gt;$el$ Satellite elevation in degrees&lt;/li&gt;&#xA;&lt;li&gt;$az$ Satellite azimuth in degrees&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;$a = 6377.301243$ (Semi-Major Axis of Earth in Kilometres)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$f = \frac{1}{ 298.257223563}$ (Flattening of Earth)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\rho = \frac{a \times (1 - f)}{\sqrt{1 - (2 - f) \times f \times \cos^{2}lat}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$x = 90 - el - \arcsin\left(\frac{(\rho + \frac{ht}{1000})}{42164^*}\right) \times \cos(el)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: * The orbital radius in Kilometres&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$y = \arccos\bigl(\cos(x) \times \cos(90 - lat) + \sin(90 - lat) \times \sin(x) \times \cos(az)\bigl)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$z = \arcsin\left(\sin(x) \times \frac{\sin(az)}{\sin(y)}\right)$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$lon_{Sat} = (z + lon + 540) \bmod 360 - 180$ (Can be simplified to '$z + lon$', but this normalizes the values of longitude)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gives me correct location of GNSS satellites (with the azimuth/elevation of satellites reported by Android GNSS sensor) when checked from &lt;a href=&quot;https://in-the-sky.org/satmap.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://in-the-sky.org/satmap.php&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Web based calculator using these formulas available here:&#xA;&lt;a href=&quot;https://deeppradhan.heliohost.org/misc/satellite-calculator.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://deeppradhan.heliohost.org/misc/satellite-calculator.htm&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16787" LastEditorUserId="16787" LastEditDate="2017-10-14T18:30:10.263" LastActivityDate="2017-10-14T18:30:10.263" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22888" PostTypeId="2" ParentId="22885" CreationDate="2017-09-03T12:02:17.610" Score="6" Body="&lt;p&gt;It is unclear at this point since everything has been only notional so far.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Musk has said that they will try a 9 meter diameter, 21 engine interim ITS first now, (original was 12 meters, 42 engines).  This is clearly aimed at maturing the technology and being cheaper to develop and test. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, SpaceX has a 4425 satellite constellation (CommX) they want to launch. Falcon, (9 or Heavy) are volume limited in their fairings, and even though the Falcon Heavy could launch a lot of mass, the number of satellites is still going to be volume limited.  A hammerhead fairing can only be so wide, before it stops working aerodynamically, and at 5m wide the fairing is already sticking out a meter on either side of the Falcon core booster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The speculation (Hoping to be confirmed Sept 28th in Australia when Musk is scheduled to speak at the IAC) is that they would test the baby/mini ITS as a satellite launcher and fund it, through launching their CommX constellation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CommX FCC application PDF is &lt;a href=&quot;https://licensing.fcc.gov/myibfs/download.do?attachment_key=1190019&quot; rel=&quot;nofollow noreferrer&quot;&gt;here.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-09-03T23:27:14.400" LastActivityDate="2017-09-03T23:27:14.400" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22889" PostTypeId="2" ParentId="22886" CreationDate="2017-09-03T18:16:34.587" Score="1" Body="&lt;p&gt;The unsatisfactory answer is: numerically. You take a numeric model of the solar system, numeric, parametric model of the spacecraft, create a draft of the mission keypoints as parametric entries (departure burn time, value, insertion burn time, value, waiting for return window, departure burn from Mars, reentry prerequisites) - and then you apply an optimization algorithm and let the computer come up with answers nearest to optimal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Analytic approach, where you calculate these values &quot;by hand&quot; would be far too complex to be practically usable - there are dozens of variables, some extremely non-linear equations (gravitational field of the solar system over time) and while &lt;em&gt;theoretically&lt;/em&gt; possible, there's no mathematician on Earth who would dare to brave such a task. Instead, the answer is brute-forced through a supercomputer, calculating millions of simulations of the mission differing by parameters a little, obtaining a solution that meets the prerequisites best.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you insist on analytic solution, you can model it using Lagrangian or Hamiltonian Mechanics, where the gravitational field of the solar system is the potential field, and each segment of flight is a separate equation of motion, with restraints of start and end speed set to be equal to these of neighboring segments. But for anything more complex than 2-body model, you'll end up with such a mess of equations nobody would dare to challenge solving them.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-09-03T18:16:34.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22891" PostTypeId="1" CreationDate="2017-09-03T21:26:59.643" Score="-3" ViewCount="78" Body="&lt;p&gt;What are the sources of energy for a Space Transit Habitat(manned) in terms of both propulsion( after passing over the Low Earth Orbit) and required electric facilities)through a 6-8 month voyage to Mars? &lt;/p&gt;&#xA;" OwnerUserId="20628" LastActivityDate="2017-09-03T22:28:39.193" Title="'Space Transit Habitat' source(s) of energy" Tags="|mars|propulsion|solar-power|power|energy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22893" PostTypeId="2" ParentId="22891" CreationDate="2017-09-03T22:28:39.193" Score="1" Body="&lt;p&gt;Your options are pretty much chemical, solar and nuclear. Solar isn't as bad at Mars as farther out, as it is only half at Mars what it is on Earth. However, you have the problem of having to point a large flat structure at the sun, the thrust in a second direction and the communication dish in a third direction. Not very elegant. &#xA;Chemical is doable for only 6-8 months but would be a bit massive - on the order of a few hundred tons. You still have the problem of what to do when you land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nuclear is the future for space travel as it is over a million times as energy dense as chemical. A steel can the size of kitchen trash can might be enough do the trick. The only problem is that anti-nuclear sentiment might delay your departure until the year 9999. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you go nuclear, your options are nuclear thermal and nuclear electric. For nuclear thermal, the thruster was fully developed by 1971 (NERVA) but no longer exists. The onboard electric would be generated by a 'bimodal' approach in which some of the nuclear heat also drives a generator. It's advantage would be that it could 'refuel' on any fluid (gas or liquid) available in space or transferable from a destination (water, hydrogen, methane, CO2, etc). However, you would have to build expensive scrubbing facilities in which to test it during (re)development.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nuclear electric is easier in the sense that you just have to use a small nuclear reactor of a type similar to many already tested at national labs and universities. Electric thrusters of at least a half dozen architectures have been lifetime tested and shown usable. The overall weight of the reactor is much greater for the energy output as compared to nuclear thermal, but exhaust velocities of electric thrusters are much higher, offsetting this. There is also a better chance of having a president or black ops project push it through quickly and fly it secretly before the lobbyists can whine about it. &lt;/p&gt;&#xA;" OwnerUserId="20921" LastActivityDate="2017-09-03T22:28:39.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22894" PostTypeId="1" CreationDate="2017-09-03T22:52:53.950" Score="1" ViewCount="124" Body="&lt;p&gt;I have bolded the part that I am most interested in checking, because I am not sure if the formula is valid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have been using Excel to find the thrust, power, etc for a NEP mission. I have been using a delta-V map I found online which states that for low thrust, I should multiply by 1.5. So, in SI units:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Required deltaV = 1.5x stated on chart &#xA;Required thrust time = Hohmann transfer time&#xA;Wet mass (kg) is payload+reactor+structure+propellant&#xA;Propellant (kg) is determined by Isp and rocket equation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;Thrust in N is determined as (wet mass + dry mass)/2 x delta V / thrust time&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Power in kW is thrust x Isp / 125 (based on VASIMR needing 200kW for 5N, 5000s)&#xA;Structure (kg) = 1/10 x propellant&#xA;Reactor mass (kg) = 1500 + 12.5 x power&#xA;Solver in Excel optimizes Isp for maximum payload and wet mass is 8300 kg based on Falcon 9 to GTO. (I am calling GTO and Earth Escape close enough to the same.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So am I on track or am I completely off?&lt;/p&gt;&#xA;" OwnerUserId="20921" LastActivityDate="2017-09-03T22:52:53.950" Title="Is this equation for NEP thrust correct?" Tags="|falcon-9|thrust|rocket-equation|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22896" PostTypeId="2" ParentId="14704" CreationDate="2017-09-03T23:06:59.967" Score="0" Body="&lt;p&gt;The carcinogenic risk of a year of spaceflight has been determined to be on a par with smoking a (very) few cigarettes over the same time frame. Not something to be done for no reason, but not something to change your choice of career over, either.&lt;/p&gt;&#xA;" OwnerUserId="20921" LastActivityDate="2017-09-03T23:06:59.967" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22898" PostTypeId="1" AcceptedAnswerId="22908" CreationDate="2017-09-04T00:51:58.177" Score="6" ViewCount="542" Body="&lt;p&gt;Which spacecraft will be the first for which the Sun would become the second brightest object in the sky?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When that will happen and how far?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the most efficient direction to fly in order to achieve that goal?&lt;/p&gt;&#xA;" OwnerUserId="14191" LastEditorUserId="12102" LastEditDate="2021-09-13T00:06:42.760" LastActivityDate="2021-09-13T00:06:42.760" Title="Which spacecraft will be the first for which the Sun would become the second brightest object in the sky?" Tags="|interstellar-travel|the-sun|spaceflight-firsts|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22899" PostTypeId="1" CreationDate="2017-09-04T01:59:52.933" Score="8" ViewCount="6537" Body="&lt;p&gt;Accompanying a newsreader's report of North Korea's launch of a new long-range (claimed up to 10,000km) missile is footage of a launch in which the rocket produces clouds of orange/brown smoke. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Korean_People%27s_Army_Strategic_Force&quot; rel=&quot;noreferrer&quot;&gt;this Wikipedia page&lt;/a&gt;, it would seem to be the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hwasong-14&quot; rel=&quot;noreferrer&quot;&gt;Hwasong-14&lt;/a&gt;, said to be based on a Russian design burning dinitrogen tetroxide and UDMH.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the preceding is correct, what are the combustion products or by-products producing that nasty-looking and presumably toxic smoke? One might make the simplistic assumption that the fuel components would react to produce carbon dioxide, water, and nitrogen. Even if those are the primary reaction products, there is clearly other stuff being emitted. What is it, and in what sort of quantity? Would it be a chemical hazard for anyone downwind of the launch outside the range of explosion hazard?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2022-05-29T19:46:18.623" Title="What is producing the orange-brown smoke in the exhaust of some North Korean missiles?" Tags="|rockets|liquid-fuel|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22900" PostTypeId="2" ParentId="22899" CreationDate="2017-09-04T03:22:07.950" Score="15" Body="&lt;p&gt;The visible reddish-brown component is mostly nitrogen dioxide, produced by breakdown of the nitrogen tetroxide oxidizer used in the engine.&lt;/p&gt;&#xA;&lt;p&gt;Ideal combustion of N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; with hydrazine-family fuels like UDMH indeed produces &lt;a href=&quot;http://yarchive.net/space/rocket/fuels/hydrazine.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;mostly water, nitrogen, and H&lt;sub&gt;2&lt;/sub&gt;&lt;/a&gt;, plus ammonia if run fuel-rich, but it's not unusual for the engine to run with incomplete combustion at startup, as Mark Adler describes in a comment &lt;a href=&quot;https://space.stackexchange.com/a/2983/195&quot;&gt;here&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Yes, whenever rocket engines start up, it takes a little bit for them to get to equilibrium chamber pressure, temperature, and flow rates. Until they do, you're bound to get all kinds of stuff coming out, including unburned oxidizer or unburned fuel, partial combustion products, etc. If a storable-propellant biprop engine is deliberately running oxidizer rich, then yes, you will see some red all the time. However I am not aware of any like that. I'd expect that they are run even or a little fuel rich.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;You can see it in launches of the Russian &lt;a href=&quot;https://www.youtube.com/watch?v=LMwDASkWPJU&quot; rel=&quot;nofollow noreferrer&quot;&gt;Proton&lt;/a&gt; launcher or US &lt;a href=&quot;https://www.youtube.com/watch?v=E4vz4xM_4Ok&quot; rel=&quot;nofollow noreferrer&quot;&gt;Titan II&lt;/a&gt; as well.&lt;/p&gt;&#xA;&lt;p&gt;Both the fuel and the oxidizer in these hypergolic motors is toxic and nasty stuff. When burned completely the resultants aren't bad, but the plume at startup is definitely hazardous.&lt;/p&gt;&#xA;&lt;p&gt;You can conveniently compare the size of the initial cloud to the size of the explosion hazard in &lt;a href=&quot;https://www.youtube.com/watch?v=OngbHZ2fsbA&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video of a failed Proton launch&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2022-05-29T19:46:18.623" LastActivityDate="2022-05-29T19:46:18.623" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22901" PostTypeId="1" AcceptedAnswerId="22902" CreationDate="2017-09-04T08:00:50.250" Score="3" ViewCount="497" Body="&lt;p&gt;I am trying to reproduce the output of the &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;HORIZONS&lt;/a&gt; system using the SPICE toolkit from NASA, but I am not getting &lt;em&gt;exactly&lt;/em&gt; the same results. I am using this script to compute the position and velocity of the Venus barycenter in the &lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/frames.html#Appendix.%20%60%60Built%20in%27%27%20Inertial%20Reference%20Frames&quot; rel=&quot;nofollow noreferrer&quot;&gt;J2000 frame&lt;/a&gt;, using the &lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/spkez_c.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;spkez function&lt;/a&gt; (&lt;a href=&quot;http://spiceypy.readthedocs.io/en/master/documentation.html?highlight=spkez#spiceypy.spiceypy.spkez&quot; rel=&quot;nofollow noreferrer&quot;&gt;Python docs&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import spiceypy as spice&#xA;&#xA;SSB_ID = 0&#xA;VENUS_ID = 299&#xA;&#xA;FRAME = 'J2000'  # Earth mean equator and equinox of J2000&#xA;#FRAME = 'ECLIPJ2000'  # Mean ecliptic and equinox of J2000&#xA;&#xA;print(spice.tkvrsn(&quot;TOOLKIT&quot;))&#xA;&#xA;spice.furnsh(&quot;./cassMetaK.txt&quot;)&#xA;&#xA;utc = '2017-09-01 12:05:50'&#xA;et = spice.str2et(utc)&#xA;&#xA;state, lightTimes = spice.spkez(VENUS_ID, et, 'J2000', 'NONE', SSB_ID)&#xA;&#xA;position = state[:3]&#xA;velocity = state[3:]&#xA;&#xA;print(position)&#xA;print(velocity)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;My kernels:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;$ cat cassMetaK.txt &#xA;\begindata&#xA;KERNELS_TO_LOAD=(&#xA;   'naif0009.tls',&#xA;   'de431.bsp',&#xA;)&#xA;\begintext&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;These are the values of position and velocity that I am getting:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;CSPICE_N0066&#xA;[ 19217297.26117753  97944611.05090818  42842518.49871409]&#xA;[-34.60894367   4.64339943   4.27861046]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;And notice the slight differences with the HORIZONS output:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;*******************************************************************************&#xA;Ephemeris / WWW_USER Mon Sep  4 00:46:19 2017 Pasadena, USA      / Horizons    &#xA;*******************************************************************************&#xA;Target body name: Venus (299)                     {source: DE431mx}&#xA;Center body name: Solar System Barycenter (0)     {source: DE431mx}&#xA;Center-site name: BODY CENTER&#xA;*******************************************************************************&#xA;Start time      : A.D. 2017-Sep-01 12:05:50.0000 TDB&#xA;Stop  time      : A.D. 2017-Sep-02 00:00:00.0000 TDB&#xA;Step-size       : 1440 minutes&#xA;*******************************************************************************&#xA;Center geodetic : 0.00000000,0.00000000,0.0000000 {E-lon(deg),Lat(deg),Alt(km)}&#xA;Center cylindric: 0.00000000,0.00000000,0.0000000 {E-lon(deg),Dxy(km),Dz(km)}&#xA;Center radii    : (undefined)                                                  &#xA;Output units    : KM-S                                                         &#xA;Output type     : GEOMETRIC cartesian states&#xA;Output format   : 3 (position, velocity, LT, range, range-rate)&#xA;Reference frame : ICRF/J2000.0                                                 &#xA;Coordinate systm: Earth Mean Equator and Equinox of Reference Epoch            &#xA;*******************************************************************************&#xA;JDTDB&#xA;   X     Y     Z&#xA;   VX    VY    VZ&#xA;   LT    RG    RR&#xA;*******************************************************************************&#xA;$$SOE&#xA;2457998.004050926 = A.D. 2017-Sep-01 12:05:50.0000 TDB &#xA; X = 1.921958776713952E+07 Y = 9.794430371601710E+07 Z = 4.284223531920820E+07&#xA; VX=-3.460881129595693E+01 VY= 4.644081825248464E+00 VZ= 4.278909130085439E+00&#xA; LT= 3.623116682955181E+02 RG= 1.086183056003940E+08 RR=-2.484635847985155E-01&#xA;$$EOE&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I &lt;em&gt;know&lt;/em&gt; the errors are under 0.1 %, but it is very important to me to know how &lt;em&gt;exactly&lt;/em&gt; is HORIZONS computing those values, if not with the code I am using. What can be the source of those discrepancies?&lt;/p&gt;&#xA;" OwnerUserId="10716" LastActivityDate="2017-09-04T08:11:07.190" Title="Discrepancy between HORIZONS and SPICE" Tags="|ephemeris|jpl-horizons|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22902" PostTypeId="2" ParentId="22901" CreationDate="2017-09-04T08:11:07.190" Score="5" Body="&lt;p&gt;Based on &lt;a href=&quot;https://naif.jpl.nasa.gov/pipermail/spice_discussion/2013-February/000373.html&quot; rel=&quot;noreferrer&quot;&gt;this answer in spice-discussion&lt;/a&gt;, one must input the time explicitly as TDB, like this:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;tdb = '2017-09-01 12:05:50 TDB'&#xA;et = spice.str2et(tdb)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Then the results match exactly:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;CSPICE_N0066&#xA;[ 19219587.76713952  97944303.7160171   42842235.3192082 ]&#xA;[-34.6088113    4.64408183   4.27890913]&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="10716" LastActivityDate="2017-09-04T08:11:07.190" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22903" PostTypeId="2" ParentId="22898" CreationDate="2017-09-04T08:51:48.970" Score="8" Body="&lt;p&gt;To answer the second part of your question, we have to look for a bright and close star. Close, because the closer you are, the faster it changes its apparent brightness when approaching it. I guess (didn't prove it), Sirius and Alpha Centauri are the only candidates here as Sirius is the brightest star and only 8.6 ly away, while Alpha Centauri is 4.4 ly away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The brightness of a star scales inversely with the square of the distance, i.e. $I \sim 1/x^2$. Sirius has an absolute magnitude of 1.42 while the sun has 4.83. That means, the ratio of their absolute brightness is $100^{(4.83-1.42)/5} = 23$. So, the brightness of the two stars at any point between them is&#xA;$$I_{Sun} = \frac{1}{x^2} = \frac{23}{(8.6-x)^2} = I_{Sirius}$$&#xA;Solving for x gives us a distance of 1.48 ly from the Sun where Sirius starts to get brighter. Even when travelling in the opposite direction, i.e. straight away from Sirius, there is a point where Sirius gets brighter, namely at -2.26 ly from Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's look at Alpha Centauri: It has an absolute magnitude of 4.38, very similar to the Sun's. It will seem brighter than the Sun at about halfway between the stars, precisely at a distance of 1.98 ly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, the closest point to us where any star seems brighter than the Sun itself is about 1.98 ly in the direction of Alpha Centauri, or 1.48 ly in the direction of Sirius. Regardless of direction, the furthest possible distance where Sun is still the brightest object in the sky is 2.26 ly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, all these distances are so huge and travel times are long so that we have to take the relative movement of stars with respect to Sun into account. At the moment, Alpha Centauri is approaching us faster than Sirius, so it could be the &quot;most efficient&quot; way to go is in the direction of Alpha Centauri, depending on the speed of the probe.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="6816" LastEditDate="2017-09-05T18:24:31.620" LastActivityDate="2017-09-05T18:24:31.620" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22904" PostTypeId="1" AcceptedAnswerId="22905" CreationDate="2017-09-04T11:28:51.157" Score="7" ViewCount="855" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/22837/what-responsibilities-does-the-hurricane-harvey-ops-team-have-and-what-challeng/22841?noredirect=1#comment65239_22841&quot;&gt;This related question&lt;/a&gt; asks about Mission Control's responsibilities during a hurricane. But there's a more general question underneath: what do the people in Mission Control do, what are their responsibilities? &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="26446" LastEditDate="2018-11-21T04:44:47.253" LastActivityDate="2018-11-21T04:44:47.253" Title="What does the ISS Mission control center do?" Tags="|iss|communication|mission-control|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22905" PostTypeId="2" ParentId="22904" CreationDate="2017-09-04T11:48:11.320" Score="7" Body="&lt;p&gt;The ISS is controlled from Flight Control Room 1 of the ISS Mission Control Center, located at NASA Johnson Space Center, Houston, TX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160012370.pdf&quot; rel=&quot;noreferrer&quot;&gt;layout of the room&lt;/a&gt; with the positions of each person/team: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/1xlAO.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1xlAO.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The yellow &lt;a href=&quot;https://en.wikipedia.org/wiki/Flight_controller&quot; rel=&quot;noreferrer&quot;&gt;positions&lt;/a&gt; are manned 24/7:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Flight Director:  Flight has overall operational responsibility for missions and payload operations and for all decisions regarding safe, expedient flight. This person monitors the other flight controllers&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;CapCom – Capsule Communicator (legacy name from the 60s).  This is the person that talks directly to the crew.  It is often another astronaut.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;ADCO – Attitude Determination and Control Officer. Works in partnership with Russian controllers to determine and manage the station’s orientation, controlled by the onboard Motion Control Systems. This position also plans and calculates future orientations and maneuvers for the station and is responsible for docking the ISS with other vehicles.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;SPARTAN – Station Power, Articulation, Thermal and ANalysis&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;ETHOS – Environmental and Thermal Operating Systems&lt;/li&gt;&#xA;&lt;li&gt;CRONUS – Communications RF Onboard Networks Utilization Specialist&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The remaining consoles are manned as needed.  For some that is every day during crew wake hours.  For others it is only when a particular operation requires that individual.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;OSO – Operations Support Officer&lt;/li&gt;&#xA;&lt;li&gt;ISO – Inventory Stowage Officer&lt;/li&gt;&#xA;&lt;li&gt;OPS PLAN – Operations Planner&lt;/li&gt;&#xA;&lt;li&gt;PLUTO - Plug–in Plan Logistics and Utilization Officer&lt;/li&gt;&#xA;&lt;li&gt;TOPO – Trajectory Operations Officer&lt;/li&gt;&#xA;&lt;li&gt;RIO – Remote Interface Officer (Interface with MCC-M and ATC-CC)&lt;/li&gt;&#xA;&lt;li&gt;SURGEON – Flight surgeon&lt;/li&gt;&#xA;&lt;li&gt;BME – Biomedical Engineer&lt;/li&gt;&#xA;&lt;li&gt;GC – Ground Controller (responsible for MCC facility equipment)&lt;/li&gt;&#xA;&lt;li&gt;PAO – Public Affairs Officer&lt;/li&gt;&#xA;&lt;li&gt;MOD - Mission Operations Directorate (Management position)&lt;/li&gt;&#xA;&lt;li&gt;ROBO – Robotics Officer&lt;/li&gt;&#xA;&lt;li&gt;VVO – Visiting Vehicle Officer&lt;/li&gt;&#xA;&lt;li&gt;EVA – Extra Vehicular Activity Officer&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So &lt;a href=&quot;https://www.quora.com/What-are-all-these-people-doing-in-the-ISS-Mission-Control-room&quot; rel=&quot;noreferrer&quot;&gt;what&lt;/a&gt; do &lt;a href=&quot;http://mentalfloss.com/article/92769/13-behind-scenes-secrets-nasa-mission-controllers&quot; rel=&quot;noreferrer&quot;&gt;these&lt;/a&gt; people &lt;a href=&quot;https://www.waypoint2space.com/news/inside-international-space-station-control-room&quot; rel=&quot;noreferrer&quot;&gt;do&lt;/a&gt;? &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;they monitor the station's health&lt;/li&gt;&#xA;&lt;li&gt;they monitor the crew's health&lt;/li&gt;&#xA;&lt;li&gt;they support the crew in carrying out their scheduled tasks (in addition to Mission Control, a lot of work goes into building the daily schedule for the astronauts, planning experiments and maintenance, etc.).  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Specialists are available to assist the astronauts for EVA, station maneuvering, robotics operations (manipulator arms, etc.), vehicle docking, logistics, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, the ISS needs far more than the 6 people inside it to keep it running smoothly. The MCC offloads lots of functions to the ground, allowing the astronauts to focus on the core tasks (science) and non-offloadable tasks (maintenance).&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-09-04T11:48:11.320" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22906" PostTypeId="2" ParentId="22854" CreationDate="2017-09-04T11:49:33.053" Score="2" Body="&lt;p&gt;Your math is wrong, the starshade operates at 50 000 km from the telescope. See the &lt;a href=&quot;https://www.google.pl/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;ved=0ahUKEwj85v-TuovWAhXMJFAKHT9XCSwQFggmMAA&amp;amp;url=https%3A%2F%2Fexoplanets.nasa.gov%2Fstdt%2FExo-S_Starshade_Probe_Class_Final_Report_150312_URS250118.pdf&amp;amp;usg=AFQjCNFkjI4rpWM2H0ZQ12g5zghK-8qcCQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;Exo-S starshade report&lt;/a&gt;, executive summary, page 4.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The physics behind the starshade are the following:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there were no diffraction, putting the starshade in front of the star would compeletely block the light of the star, because when seen from the telescope it has a larger angular diameter than the star.&#xA;However, due to diffraction, the light from the star is not completely blocked: it forms an &lt;a href=&quot;https://en.wikipedia.org/wiki/Arago_spot&quot; rel=&quot;nofollow noreferrer&quot;&gt;Arago spot&lt;/a&gt;. This means the telescope still sees some light from the star, and that can hide the signal from the planets orbiting the star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shape of the starshade is designed to minimize the intensity of this Arago spot. The petals are a way to do that, and still use a completely opaque material (i.e. not using partially transparent materials). &lt;a href=&quot;https://www.princeton.edu/~hcil/papers/Cady07a.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here&lt;/a&gt;'s an article deriving their shape.&lt;/p&gt;&#xA;" OwnerUserId="4685" LastActivityDate="2017-09-04T11:49:33.053" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22907" PostTypeId="1" CreationDate="2017-09-04T13:28:17.887" Score="3" ViewCount="979" Body="&lt;p&gt;I've been trying to find data in the literature that would provide analytical expressions for the relationship between the lift and drag of an object similar in shape and size to the Falcon 9 first-stage (i.e. a large, high-speed cylinder), under different angle of attack and Mach number values (that is, $C_{L}=C_{L}(\alpha,M)$ and $C_{D}=C_{D}(\alpha,M)$). One paper I found had a fairly simple relationship given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$C_{L}=-0.041065+0.016292\alpha+0.0002602\alpha^{2}$$&#xA;$$C_{D}=0.080505-0.03026C_{L}+0.86495C_{L}^{2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where the angle of attack, $\alpha$, was scheduled with respect to velocity (i.e. acting as a passive control input) by&#xA;$$\alpha = 40$$&#xA;for $V&amp;gt;4570 m/s$&#xA;$$\alpha = 40-0.20705(V-4570)^{2}/340^{2}$$&#xA;for $V\leq4570 m/s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, since it seems like SpaceX use angle-of-attack as an active aerodynamic control input, I wanted to try and incorporate this into an optimal control simulation that already takes thrust control into account, in order to give the simulation a bit more fidelity. Are there any possible models that were developed for, say, ICBMs re-entering the Earth's atmosphere that could be used as rough models for the Falcon 9's lift and drag with respect to angle-of-attack and Mach number?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="12102" LastEditDate="2021-09-04T00:31:45.840" LastActivityDate="2021-09-04T00:31:45.840" Title="Modeling SpaceX's lift and drag versus angle of attack and Mach number" Tags="|spacex|atmospheric-drag|lift|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="22908" PostTypeId="2" ParentId="22898" CreationDate="2017-09-04T15:23:02.067" Score="12" Body="&lt;p&gt;As a reference, part of this question &lt;a href=&quot;https://astronomy.stackexchange.com/a/1064/182&quot;&gt;I answered at Astronomy.SE&lt;/a&gt;. The closest point at which the Sun would not be the brightest object in the sky is if we headed directly towards Sirius A, at a distance of 1.46 light years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fastest object leaving the Solar System right now is Voyager 1. It's speed right now is about 38026 miles per hour, or about 5.7534471e-5 light years/ year. If it was heading in the right direction (It's not), the time would be about 25,000 years. Of course, Voyager will be slowing down over that period of time, but not significantly. In &lt;a href=&quot;https://www.space.com/22783-voyager-1-interstellar-space-star-flyby.html&quot; rel=&quot;noreferrer&quot;&gt;40,000&lt;/a&gt; years it will pass fairly close to a star, mostly because the star is moving towards us, called AC +79 3888.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 2 is actually heading &lt;a href=&quot;http://www.express.co.uk/news/world/567957/NASA-s-Voyager-2-sets-course-for-star-Sirius-by-time-it-arrives-human-race-will-be-dead&quot; rel=&quot;noreferrer&quot;&gt;vaguely in Sirius&lt;/a&gt;'s direction, so it might be the first. It will take about 30K years for Sirius to be the brighest star in it's view, however.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, it is HIGHLY likely that a spacecraft will be sent to a neighbor sooner than that, that will be much quicker. How long will it take? Well, that depends on technology of the future, which I can't fathom to guess at this point in time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it's going to take quite a while, but we will eventually have a spacecraft that the Sun isn't the brightest star in the sky.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-09-04T15:33:42.900" LastActivityDate="2017-09-04T15:33:42.900" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22909" PostTypeId="1" AcceptedAnswerId="22914" CreationDate="2017-09-04T20:14:16.080" Score="14" ViewCount="716" Body="&lt;p&gt;NASA has &lt;a href=&quot;https://www.nasa.gov/feature/new-horizons-receives-mission-extension-to-kuiper-belt-dawn-to-remain-at-ceres&quot; rel=&quot;noreferrer&quot;&gt;approved the New Horizons mission to 2014MU69&lt;/a&gt; on 1 January 2019 that will require &lt;a href=&quot;https://spaceflightnow.com/2017/01/22/new-horizons-to-continue-mission-of-discovery-with-kuiper-belt-encounter/&quot; rel=&quot;noreferrer&quot;&gt;operations at least into 2021&lt;/a&gt; to download the data.  However, after that, things get a bit fuzzy - New Horizons will still have a substantial amount of fuel left, so it could &lt;em&gt;theoretically&lt;/em&gt; visit another KBO - if one was found close to its trajectory.  Even if another object in the Kuiper Belt doesn't turn up to visit, NH is on a trajectory that will take it out of the Solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://voyager.jpl.nasa.gov/mission/status/#where_are_they_now&quot; rel=&quot;noreferrer&quot;&gt;Voyager&lt;/a&gt; probes are currently (4 September 2017) traveling 17kps and 15.4kps away from the Sun, in interstellar space (beyond the heliopause), and transmitting scientific data.  Although it was the &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;fastest vehicle ever launched&lt;/a&gt;, New Horizons is &quot;only&quot; going &lt;a href=&quot;https://theskylive.com/newhorizons-tracker&quot; rel=&quot;noreferrer&quot;&gt;14.2kps&lt;/a&gt;, so it will take quite a bit longer to reach the heliopause than the Voyagers did - but it will get there, and go beyond.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming it doesn't have a fatal crash within the Kuiper Belt, how long &lt;em&gt;could&lt;/em&gt; New Horizons continue to return scientific data?  Is it limited by its data rate at distance, or by the power available in its RTGs?&lt;/p&gt;&#xA;" OwnerUserId="19916" LastActivityDate="2017-09-29T18:33:59.103" Title="What's after the Kuiper Belt for New Horizons?" Tags="|mission-design|new-horizons|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22911" PostTypeId="1" AcceptedAnswerId="22912" CreationDate="2017-09-05T01:00:54.473" Score="15" ViewCount="1786" Body="&lt;p&gt;The Zenit-3SL has a slightly higher thrust for both 1st and 2nd stage and is launched from the Equator. However, the Falcon FT(expendable) has a higher payload to GTO and its diameter is 3.7m compared to the Zenit's 3.9m.&#xA;You can also explain why the LEO capacity for Falcon 9 FT is higher than for Zenit-2SLB.&lt;/p&gt;&#xA;" OwnerUserId="20936" LastEditorUserId="20936" LastEditDate="2017-09-08T16:16:21.260" LastActivityDate="2017-09-08T16:16:21.260" Title="Why does the Falcon 9 FT have a higher payload capacity to GTO than a Zenit-3SL?" Tags="|spacex|rockets|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22912" PostTypeId="2" ParentId="22911" CreationDate="2017-09-05T01:42:27.997" Score="30" Body="&lt;p&gt;The most recent iteration of the &lt;a href=&quot;http://spaceflight101.com/spacerockets/falcon-9-ft/&quot; rel=&quot;noreferrer&quot;&gt;Falcon 9 FT&lt;/a&gt; has slightly &lt;em&gt;more&lt;/em&gt; thrust than &lt;a href=&quot;http://spaceflight101.com/spacerockets/zenit-3sl/&quot; rel=&quot;noreferrer&quot;&gt;Zenit&lt;/a&gt; on both the first and second stages (according to current Spaceflight101 figures), but thrust is actually a relatively small factor in the performance of an orbital launcher (beyond the minimum required to get off the launch pad, of course).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Zenit has a much better engine specific impulse on the first stage due to the &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-170#RD-171&quot; rel=&quot;noreferrer&quot;&gt;RD-171&lt;/a&gt;'s staged-combustion engine cycle, but that's about the only place where it outdoes the Falcon 9. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dominant difference between the two launchers is the ratio of fuel mass to dry mass, particularly on the second stage. Falcon just carries less stage mass to orbit. Here's a breakdown of the important performance specs:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KqRIo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KqRIo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dry first stage of the Zenit masses almost 25% more than Falcon's despite carrying 20% less fuel. Zenit's second stage is carrying a 4-ton handicap relative to Falcon's, and that mass, flying all the way to second stage cutoff, comes directly off the payload capability. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both rockets use a similar construction style, a thin aluminum skin with stiffeners welded in (Zenit's design was a major influence on Falcon 9). You would think that their mass ratios would be more similar. My guess is that improvements in structural-engineering modeling software in the 25 years between the development of the two rockets allowed SpaceX to shave a lot of weight while maintaining the necessary strength.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-06T15:52:22.117" LastActivityDate="2017-09-06T15:52:22.117" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22913" PostTypeId="2" ParentId="22825" CreationDate="2017-09-05T02:19:41.777" Score="7" Body="&lt;p&gt;Using the &lt;a href=&quot;https://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&amp;amp;NECs=on&amp;amp;chk_maxMag=on&amp;amp;maxMag=25&amp;amp;chk_maxOCC=on&amp;amp;maxOCC=4&amp;amp;chk_target_list=on&amp;amp;target_list=Neptune&amp;amp;mission_class=oneway&amp;amp;mission_type=flyby&amp;amp;LD1=2010&amp;amp;LD2=2040&amp;amp;maxDT=21&amp;amp;DTunit=yrs&amp;amp;maxDV=20&amp;amp;min=DV&amp;amp;wdw_width=365&amp;amp;submit=Search#a_load_results&quot; rel=&quot;noreferrer&quot;&gt;Trajectory Browser&lt;/a&gt;, I was able to figure out the following trajectories that are available. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/loRek.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/loRek.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dates are Jan-18-2018, Feb-22-2019, Feb-23-2031, and Mar-29-2032.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So basically there are two good launch windows roughly every 13 years To show out the path a bit more, I've included more than just the optimal paths.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hJs5H.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hJs5H.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately the tool doesn't show how long the optimal window lasts, but it can be inferred that it lasts for some time. The total mission time is about 10 years for such a mission, depending on exactly how much one wishes to optimize the fuel expenditure. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-09-05T02:19:41.777" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22914" PostTypeId="2" ParentId="22909" CreationDate="2017-09-05T08:56:21.287" Score="10" Body="&lt;p&gt;RTG power will drop below the level needed to run the transmitter &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Power&quot; rel=&quot;noreferrer&quot;&gt;sometime in the 2030s&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can still communicate with the Voyagers, which are 3 times further away than New Horizons. NH is more flexible than the Voyagers in what data rates it can use and how much data it can store for later transmission, so it's unlikely that will be the bottleneck. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bigger question is, what science can New Horizons do outside of planetary encounters? It carries some particles experiments (SWAP, PEPPSI and SDC) that could be useful. &#xA;SWAP &lt;a href=&quot;https://www.researchgate.net/publication/300371665_The_new_horizons_solar_wind_around_pluto_swap_observations_of_the_solar_wind_from_11-33_au&quot; rel=&quot;noreferrer&quot;&gt;has been used en route to Pluto&lt;/a&gt; to monitor the solar wind, this would be useful to keep running. &#xA;PEPPSI was designed to measure Pluto's atmosphere, I don't know if it can be used in interplanetary space. Same for SDC.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, all of this is guesswork. The New Horizons extended mission is funded until 2021. Operations after that will need another &lt;a href=&quot;https://www.nasa.gov/feature/new-horizons-receives-mission-extension-to-kuiper-belt-dawn-to-remain-at-ceres&quot; rel=&quot;noreferrer&quot;&gt;mission extension&lt;/a&gt;, for which the NH team will need to compete with other interplanetary missions. The NH team has indicated it's looking for possible targets for flybys after 2014MU69.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-09-29T18:33:59.103" LastActivityDate="2017-09-29T18:33:59.103" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22915" PostTypeId="1" CreationDate="2017-09-05T08:56:31.237" Score="6" ViewCount="1513" Body="&lt;p&gt;I am trying to understand how to calculate the orbits of solar system bodies in an n-body framework, based on the pair-wise gravitational interaction between the objects. At present, I am considering 44 objects (sun, planets, major moons and major asteroids).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am starting with the state vectors (position and velocity) of each of the objects with Sun as centre obtained from &lt;code&gt;telnet ssd.jpl.nasa.gov 6775&lt;/code&gt; (JPL Horizons) 01-Jan-2017 at 00:00 UTC and would like to let the system evolve for 4344h,  01-July-2017 at 00:00h.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have written a program to do this in Java, and so far the results do not seem to be even reasonably close to what they should be, comparing with the state vectors obtained from Horizons. After every 2 second time step the net gravitational forces on each body from all of the others is calculated, and then in one shot all the velocities and positions are updated based on the accelerations from those net forces. Then I compare the final updated position vectors from the application with data obtained from Horizons after correcting for the Sun's updated position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The comparison shows that the positions of Earth and the outer planets are have position error of less than 50km (In fact, the farther planets it is less then 10km). Where as for Mercury the error is 250km. And the moons of Jupiter and Saturn are off by 50,000 to 300,000 km!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my application, I am not differentiating between the Sun, planets and moons, so I am not sure why there should be so much more error for the moons. I have tried decreasing the step size, from 2 seconds down to 0.25 seconds, but there is not significant improvement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What might be the problems that I should investigate here? Are there things that clearly need improvement right away? Or perhaps there are tests I can to do help diagagnose the primary sources of error?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; Here is the gist of the calculation method &lt;em&gt;as requested in comments:&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;while (nowT &amp;lt; endT) {&#xA;    doOneStep(step, nowT)&#xA;    nowT += stepT&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;allItemLinks&lt;/code&gt; is collections of &lt;code&gt;ItemLink&lt;/code&gt; - links between objects. in this case Gravity link between all object pairs. For &lt;code&gt;n&lt;/code&gt; objects, there will be &lt;code&gt;n.(n+1)/2&lt;/code&gt; links&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;doOneStep(double deltaT, double nowT){&#xA;    initForces fo all items to 0,0,0&#xA;    for each ItemLink **allItemLinks**)&#xA;        inf.evalForce(deltaT, false)&#xA;    updatePosAndVel(deltaT, nowT, true)&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;In ItemLink:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;evalForce(double deltaT, boolean bFinal) {&#xA;    addGravityEffect(deltaT);&#xA;}&#xA;&#xA;boolean addGravityEffect(double deltaT) {&#xA;    rVector = item2.pos - item1.pos&#xA;    double gF = G.mq.m2/r2&#xA;    fVector = gF in rVector direction&#xA;    item1.addForce(fVector)&#xA;    similarly for item2 to item1&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;allItems&lt;/code&gt; is a collection of Item objects (Sun, planets and moons)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;void updatePosAndVel(double deltaT, double nowT) {&#xA;    for each Item of **allItems** updatePandV(deltaT, nowT);&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;In Item:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;code&gt;netForce, nowAcc, effectiveAcc, deltaV, newPos etc.&lt;/code&gt; are all Vector3d&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;updatePAndV(double deltaT, double nowT, boolean bFinal){&#xA;        nowAcc = netForce / mass&#xA;        effectiveAcc = mean of lastAcc and nowAcc&#xA;        deltaV = effectiveAcc * deltaT&#xA;        meanV ...&#xA;        newPos = oldPos + meanV * deltaT&#xA;}&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Not working on gravitation fields but with direct forces due to inter-object gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the above code, I am able to get stable orbits. Even the orbit times of the moons are good. Get nice Saturn set with the Cycloidal motions of moons and the Uranus set with helical motion of the moons around Uranus. &#xA;I do not know as to how to send files or images for this discussion&lt;/p&gt;&#xA;" OwnerUserId="20941" LastEditorUserId="12102" LastEditDate="2017-09-06T11:38:58.627" LastActivityDate="2017-09-10T11:59:31.890" Title="Calculating the planets and moons based on Newtons's gravitational force" Tags="|orbit|mathematics|calculation|ephemeris|" AnswerCount="2" CommentCount="22" ContentLicense="CC BY-SA 3.0" />
  <row Id="22916" PostTypeId="1" CreationDate="2017-09-05T11:30:03.983" Score="5" ViewCount="775" Body="&lt;p&gt;Cassini is going to crash into Saturn later this month to avoid contaminating one of its moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why isn't anyone worried that Cassini will contaminate Saturn itself?&#xA;Life might exist in Saturn's atmosphere.&#xA;Different altitudes, pressures &amp;amp; temperatures may have environments where life has evolved. It's closed minded to rule this out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we REALLY wanted to not contaminate anything, we should sling it out of the plane of the solar system entirely.&lt;/p&gt;&#xA;" OwnerUserId="20942" LastEditorUserId="1235" LastEditDate="2019-08-14T15:40:30.760" LastActivityDate="2019-08-15T05:07:53.267" Title="Will Cassini contaminate Saturn?" Tags="|saturn|cassini|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="22917" PostTypeId="1" AcceptedAnswerId="22920" CreationDate="2017-09-05T15:02:45.990" Score="14" ViewCount="4221" Body="&lt;p&gt;It seems the standard approach to control of rockets during launch is either vernier thrusters, or gimbals on the main engines. Sure that works, and is quite efficient, but I wonder about a simpler solution- with multiple engines, throttle engines on the side you want the rocket lean to, and simply allow the summary thrust vector to wander a little off-center while retaining the same direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was such a solution ever used?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="9092" LastEditDate="2021-02-19T14:31:42.440" LastActivityDate="2021-02-19T14:31:42.440" Title="Did any rockets use differential throttling instead of gimbal?" Tags="|engine-design|attitude|thrust-vectoring|gimbaled-engines|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22919" PostTypeId="2" ParentId="22917" CreationDate="2017-09-05T16:37:38.313" Score="13" Body="&lt;p&gt;Differential thrust of a set of axially aligned engines can't provide roll control by itself; either dedicated roll-control thrusters or at least one off-center and movable engine is needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket development literature frequently mentions differential throttle as a possibility, but it seems like it hasn't been used in practice very often. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Surveyor_program&quot; rel=&quot;noreferrer&quot;&gt;Surveyor&lt;/a&gt; lunar landers had three thrusters for landing, and used differential thrust for pitch and yaw control, but one of the thrusters was movable for roll control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe &lt;a href=&quot;https://en.wikipedia.org/wiki/Dragon_2&quot; rel=&quot;noreferrer&quot;&gt;Dragon 2&lt;/a&gt; was designed to use differential throttle for landing control, but it may wind up not doing propulsive landings at all.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know of any large launchers (apart from the N-1 as described in Josh King's answer) that rely on differential thrust for maneuvering. Attitude control during the high-Q portion of ascent needs to be fast; it may be that throttle control of big engines isn't fast enough to do the job.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-05T19:50:03.293" LastActivityDate="2017-09-05T19:50:03.293" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22920" PostTypeId="2" ParentId="22917" CreationDate="2017-09-05T17:50:33.307" Score="26" Body="&lt;p&gt;The first stage of &lt;a href=&quot;https://en.wikipedia.org/wiki/N1_(rocket)&quot; rel=&quot;noreferrer&quot;&gt;the Soviet N-1 moon rocket&lt;/a&gt; (Block A) used this type of differential thrust system. It had 30 engines in 2 rings. The outer ring of 24 engines used differential thrust control to control pitch and yaw, and was set up to shut off opposing engines in case of a single engine failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Four launches were attempted and all failed in the first stage; the first and third failure were directly due to problems in the electronic control system of the first stage engines.  No further launches occurred, so it never really worked.&lt;/p&gt;&#xA;" OwnerUserId="15824" LastEditorUserId="6816" LastEditDate="2017-09-06T06:20:02.097" LastActivityDate="2017-09-06T06:20:02.097" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22921" PostTypeId="2" ParentId="22916" CreationDate="2017-09-05T22:22:59.427" Score="14" Body="&lt;p&gt;If we were going to send a probe into Saturn's atmosphere and were concerned about contamination of a potential ecosphere there, we would sterilize the probe first, e.g. with dry heat microbial reduction, to make sure that nothing viable was on the probe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cassini has no protection from the entry heat like a probe would, and will be entering at an incredible velocity, so &lt;em&gt;every tiny bit of Cassini will be massively sterilized&lt;/em&gt; in the fiery entry, far beyond puny humans ability to do so.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-09-05T22:22:59.427" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22922" PostTypeId="2" ParentId="22915" CreationDate="2017-09-05T22:27:14.020" Score="7" Body="&lt;p&gt;Aside from numerical issues, &quot;With Sun as centre&quot; may be part of your problem. Get all the data from Horizons relative to the Solar System Barycenter, not the Sun, which moves relative to the barycenter. That barycenter is an inertial frame of reference, whereas the center of the Sun is not. Also make sure that you are putting in the initial position and velocity of the Sun and letting it move, if you aren't already.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-09-06T01:18:16.617" LastActivityDate="2017-09-06T01:18:16.617" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="22923" PostTypeId="1" CreationDate="2017-09-05T23:22:17.700" Score="3" ViewCount="777" Body="&lt;p&gt;I've heard that Osama Bin Laden was first found via satellite. I was wondering if it is possible to have a satellite that can scan an entire continent and find a single person or terrorists without having any specific location to look for? Can sattelites be that powerful?&lt;/p&gt;&#xA;" OwnerUserId="20945" LastActivityDate="2017-09-08T17:29:40.870" Title="Can a satellite scan an entire continent to search for someone?" Tags="|artificial-satellite|communication-satellite|photography|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22924" PostTypeId="1" AcceptedAnswerId="22986" CreationDate="2017-09-06T08:20:15.527" Score="6" ViewCount="453" Body="&lt;p&gt;Apparent temperature is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. There are several ways of measuring it, personally I like the  wet-bulb globe temperature as it combines the effects of radiation, humidity, temperature and wind speed on the perception of temperature. But there are other ways, such as heat index and wind chill factor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is very cold, with a mean temperature of −63 °C. But it's atmosphere is also very thin, at 0.00628 atm. This would strongly reduce the effects of wind chill but increase evaporation. But evaporation is easily countered with a suit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what would be the apparent temperature on Mars?&lt;/p&gt;&#xA;" OwnerUserId="20892" LastActivityDate="2017-09-11T06:38:22.117" Title="Apparent temperature Mars" Tags="|mars|atmosphere|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22927" PostTypeId="1" AcceptedAnswerId="24094" CreationDate="2017-09-06T12:32:19.617" Score="6" ViewCount="392" Body="&lt;p&gt;Starting with George W. Bush, the presidents of the United States have presented an ambitious plan to the public to attain human travel of deep space. Bush's &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_policy_of_the_George_W._Bush_administration&quot; rel=&quot;noreferrer&quot;&gt;Vision for Space Exploration&lt;/a&gt;, announced in 2004, called for a Moon-first path to deep space. The plan was to return to the Moon by the year 2020, then reach for Mars and other deep space destinations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2010, Obama announced &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_policy_of_the_Barack_Obama_administration&quot; rel=&quot;noreferrer&quot;&gt;his path&lt;/a&gt; to deep space. Instead of Moon-first, Obama called for a crewed mission to an asteroid by 2025, followed by a mission to Mars in the mid-2030s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now that Trump has nominated Jim Bridenstine as the new NASA Administrator, has Trump or someone in his administration intimated what his vision for reaching deep space is?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Note: For those confused about the usage of &quot;deep space&quot; here, please see &lt;a href=&quot;https://astronomy.stackexchange.com/a/14402/6&quot;&gt;this post&lt;/a&gt; about NASA's usage of the term in relation to human exploration of space. Also, see &lt;a href=&quot;https://www.nasa.gov/feature/deep-space-gateway-to-open-opportunities-for-distant-destinations&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2019-06-08T02:00:14.277" Title="What is the Trump-era path to deep space?" Tags="|nasa|deep-space|policy|" AnswerCount="1" CommentCount="13" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22930" PostTypeId="1" AcceptedAnswerId="22932" CreationDate="2017-09-06T15:11:07.173" Score="5" ViewCount="1295" Body="&lt;p&gt;Questions like &lt;a href=&quot;https://space.stackexchange.com/questions/22917/did-any-rockets-use-differential-throttling-instead-of-gimbal&quot;&gt;this one&lt;/a&gt; emphazise the role of gimbal to control the attitude of the first stage. But once in orbit, satellites usually uses reaction control systems. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is it possible to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Reaction_control_system&quot; rel=&quot;nofollow noreferrer&quot;&gt;RCS&lt;/a&gt; to control the first stage ? &lt;/li&gt;&#xA;&lt;li&gt;Has it already been done? &lt;/li&gt;&#xA;&lt;li&gt;Why do we use gimbal instead of RCS?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="693" LastEditDate="2017-09-06T15:44:07.560" LastActivityDate="2017-09-07T21:52:13.313" Title="Is it possible to use RCS instead of gimbal to control first stage attitude?" Tags="|first-stage|rcs|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22931" PostTypeId="2" ParentId="22930" CreationDate="2017-09-06T15:24:01.473" Score="5" Body="&lt;p&gt;Although perhaps larger than what would normally be considered RCS, the Ares-1X solid rocket booster did use jets to control its &lt;strong&gt;roll&lt;/strong&gt; attitude in first stage (its only stage, for this test).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9LCi3.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9LCi3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pitch and yaw were controlled by gimbaling the nozzle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://en.wikipedia.org/wiki/Ares_I-X#Roll_control_system&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Ares_I-X#Roll_control_system&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-09-06T15:24:01.473" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22932" PostTypeId="2" ParentId="22930" CreationDate="2017-09-06T15:37:46.083" Score="11" Body="&lt;p&gt;As Organic Marble notes, some single-engine stages do use RCS for roll control since the requirements for roll on ascent are less demanding than pitch and yaw. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 first stage also uses RCS to control its attitude during the early phases of its reentry trajectory, where the grid fins are less effective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible in principle to use RCS for pitch/yaw steering on ascent, but gimbaling is more efficient. To steer a massive launcher, you need a fairly large amount of thrust, and thus fairly massive engines; they'd outweigh the gimbals and actuators. If they're fixed perpendicular to the longitudinal axis of the vehicle, then using them is somewhat wasteful: the force from the main engines and the RCS are the two legs of a right triangle, and the net force is the hypotenuse. With gimbals, the main engines just steer directly to provide the desired net force.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a given deflection angle $\phi$ the total force needed from a longitudinal engine and a perpendicular engine is $\sin \phi + \cos \phi$ times the desired net force if the perpendicular engine is the same distance from the center of gravity as the gimbaled engine. For an average steering angle of 1º, this isn't too bad: only a 1.7% penalty. CG moves downward on the vehicle during flight, though, so it might be possible to do a little better by putting the perpendicular thruster at the top end of the stage; CG will still probably be a third of the way up at max Q, where you need the most steering authority, but you might be able to double the &quot;lever arm&quot; of the perpendicular force, amplifying torque, so the penalty might be under a percent. These lateral engines probably can't be as fuel-efficient as the mains, but we'll ignore that because 1º average deflection is probably a gross overestimate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to produce the necessary steering force in any perpendicular direction, you need at least 4 engines mounted perpendicularly. Even if the &lt;em&gt;average&lt;/em&gt; deflection over the course of the flight is infinitesimal, the engines need to produce enough thrust for the &lt;em&gt;maximum&lt;/em&gt; thrust needed at any point in the flight. For a 12º maximum, the perpendicular thrusters would have to each produce about 20% of the thrust of the main engines! That's big -- for a Falcon 9, each of the lateral engines would be nearly as powerful as two of the main Merlins. For rapid response you'd need pressure-fed, hypergolic-fuel engines, which means separate and heavier propellant tankage; if you bring too little fuel for the laterals, you lose control mid-flight, and if you bring too much, you waste mass. With gimbaled engines, almost all your fuel goes toward acceleration; cosine loss at 5 degrees is less than half a percent. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Engines of that size don't have much in common with what's usually thought of as RCS. For rapid control you have to start and stop the engine quite quickly; doing pulsed hard starts like that on a &lt;a href=&quot;https://en.wikipedia.org/wiki/R-4D&quot; rel=&quot;noreferrer&quot;&gt;490N thruster&lt;/a&gt; is one thing, doing so on an engine 1500 times more powerful is going to be positively destructive -- and I'm not aware of any existing pressure-fed engines of that size.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-07T21:52:13.313" LastActivityDate="2017-09-07T21:52:13.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22933" PostTypeId="2" ParentId="1212" CreationDate="2017-09-06T16:47:27.960" Score="2" Body="&lt;p&gt;GEO satellites tend to be large satellites, because they have to be larger to support their missions. When you have to make them larger, then they are built with additional redundancy. In addition, GEO satellites do not have as many eclipses, they have them at most once a day for 90 minutes during &lt;a href=&quot;http://www.intelsat.com/tools-resources/library/satellite-101/eclipse-seasons/&quot; rel=&quot;nofollow noreferrer&quot;&gt;eclipse season&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For LEO satellites, their death tends to be due to poor batteries. The batteries are very stressed due to the constant use that they require. GEO satellites tend to end their lives when the fuel runs out. Fuel requirements are usually easier than batteries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LEO satellites tend to require a constellation to work effective. The redundancy in many ways is that there are usually other satellites in orbit that can &quot;pick up the slack&quot; if one of them fails. GEO satellites tend to require being in a specific location, and cannot support that kind of redundancy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, LEO satellites are built smaller, cheaper, and with lower lifetimes, and have more battery stress than GEO satellites.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-09-06T16:47:27.960" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22934" PostTypeId="1" CreationDate="2017-09-06T17:10:11.097" Score="7" ViewCount="1650" Body="&lt;p&gt;I've noticed that maneuvers to leave a body's sphere of influence on the way to another body are invariably referred to as &lt;em&gt;injection&lt;/em&gt; maneuvers, for instance, &lt;a href=&quot;https://en.wikipedia.org/wiki/Trans-lunar_injection&quot; rel=&quot;nofollow noreferrer&quot;&gt;translunar injection&lt;/a&gt;, while maneuvers to achieve a closed orbit around a body from either a hyperbolic or a suborbital trajectory are referred to as &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbit_insertion&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;insertion&lt;/em&gt;&lt;/a&gt; maneuvers, such as lunar orbit insertion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The etymology makes reasonable sense: &lt;a href=&quot;http://www.etymonline.com/index.php?term=insert&quot; rel=&quot;nofollow noreferrer&quot;&gt;insert means &quot;place into&quot;&lt;/a&gt;, and &lt;a href=&quot;http://www.etymonline.com/index.php?term=inject&quot; rel=&quot;nofollow noreferrer&quot;&gt;inject means &quot;throw into&quot;&lt;/a&gt;, suggesting a more energetic action.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;When and how did these terms become standardized in the spaceflight business?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="6816" LastEditDate="2017-09-07T20:43:08.557" LastActivityDate="2018-12-19T04:33:19.897" Title="Injection versus insertion" Tags="|orbit|orbital-maneuver|terminology|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22938" PostTypeId="1" AcceptedAnswerId="22939" CreationDate="2017-09-07T14:14:00.453" Score="3" ViewCount="505" Body="&lt;p&gt;Why do SpaceX rockets' re-entries seem to burn (as in heat the hull) less than the space shuttle ? Is it because of less friction surface ?&#xA;Thanks !&lt;/p&gt;&#xA;" OwnerUserId="20957" LastActivityDate="2017-09-10T01:45:52.363" Title="Why do SpaceX rockets' re-entries seem to burn (as in heat the hull) less than the space shuttle?" Tags="|launch|spacex|rockets|reentry|space-shuttle|" AnswerCount="2" CommentCount="1" ClosedDate="2017-09-07T16:40:19.183" ContentLicense="CC BY-SA 3.0" />
  <row Id="22939" PostTypeId="2" ParentId="22938" CreationDate="2017-09-07T14:18:26.137" Score="12" Body="&lt;p&gt;The Space Shuttle re-enters from full orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 first stage cuts off after only 2 minutes of flight. Much more comparable to the SRB flight times.  Velocity and height are much lower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They do a reentry burn to slow down as they enter the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this suffices to keep the heating down to the level below the melting point of the Aluminum hull (The material it is mostly made of).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-09-07T18:55:47.407" LastActivityDate="2017-09-07T18:55:47.407" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22940" PostTypeId="2" ParentId="22938" CreationDate="2017-09-07T16:35:41.347" Score="1" Body="&lt;p&gt;The answer of geoffc is only partially true, as some parts of the Falcon 9 are forged out of titanium to sustain more heat than aluminium.&lt;br&gt;&#xA;And considering material science the heating has to be kept even lower than to the level of the melting point to avoid that the aluminium alloys undergo a phase change of the crystal structure.&lt;/p&gt;&#xA;" OwnerUserId="20960" LastEditorUserId="195" LastEditDate="2017-09-10T01:45:52.363" LastActivityDate="2017-09-10T01:45:52.363" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22941" PostTypeId="1" CreationDate="2017-09-07T17:45:12.330" Score="4" ViewCount="1702" Body="&lt;p&gt;I'm trying to understand the equations of motion for a rocket launched from earth, through the atmosphere, and into LEO.  I found a set of ordinary differential equations which describe the motion of the rocket (&lt;a href=&quot;http://personal.ee.surrey.ac.uk/Personal/A.Pechev/lnlvp/AP-lec3.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;see this reference, slide 3&lt;/a&gt;).  Below is a diagram of which should help to describe the variables in the equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JJ6aZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JJ6aZ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The differential equations are:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\dot{v}=-\frac{ksin(\gamma)}{r^2}-\frac{D}{m}+\frac{Tcos(\phi_T)}{m}$$&#xA;$$\dot{\gamma}=-\frac{kcos(\gamma)}{vr^2}+\frac{L}{mv}+\frac{Tsin(\phi_T)}{mv}+\frac{vcos(\gamma)}{r}$$&#xA;$$\dot{\nu}=v\frac{cos(\gamma)}{r}$$&#xA;$$\dot{r}=vsin(\gamma)$$&#xA;$$\dot{m}=-T/I_{sp}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recognize the first equation as the equation for linear acceleration, where I think $k=\mu=GM$ is the gravitational constant times the mass of earth (please correct me if I'm wrong).  I don't know what the second equation represents or how it was obtained.  Can anyone give me some insight into how this equation is derived?  Is it the statement of the conservation of some forces? If so, what forces in particular?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not even entirely sure that this is the proper form of the dynamic model for the rocket's trajectory.  I tried cross-validating this model with other models to see if this is correct.  In this process, &lt;a href=&quot;https://mathematica.stackexchange.com/search?tab=votes&amp;amp;q=combining%20gravity%20turn%20and%20orbit%20models&quot;&gt;I found an answer on another stack exchange question which almost describes the same equation&lt;/a&gt;, but with an additional $\frac{v}{r}$ term in the $\dot{\gamma}$ equation.  If I were to add this term to the $\dot{\gamma}$ equation, what would it mean?  What force does it describe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I'm having a little bit of trouble understanding the notation, since the variables are not explicitly defined anywhere in the document.  I think the D and L represent drag and lift forces. If so, I'm not sure why lift forces are not included in the $\dot{v}$ equation or why they appear only in the $\dot{\gamma}$ equation.  The term T seems to represent the thrust of the rocket, which acts at an angle $\phi_T$ from the normal direction to the surface of the (perfectly circular) earth. The angle $\gamma$ seems to be the angle from the velocity vector to the local horizon.  Have I correctly summarized these variables?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any help understanding this equation would be greatly appreciated.&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="6494" LastEditDate="2017-09-08T12:25:45.370" LastActivityDate="2017-09-08T12:25:45.370" Title="Differential Equations for Rocket Launch in Polar Coordinates" Tags="|launch|orbital-mechanics|trajectory|" AnswerCount="0" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22942" PostTypeId="2" ParentId="3004" CreationDate="2017-09-07T18:25:45.743" Score="5" Body="&lt;p&gt;Having worked at Rocketdyne for many years my understanding was that although they wanted to put it on the Space Shuttle, the politics of a single source caused NASA to write a proposal that required usual rocket nozzles.&lt;br&gt;&#xA;At that point there was no funding to finish full scale development and flight testing.&lt;/p&gt;&#xA;" OwnerUserId="20963" LastActivityDate="2017-09-07T18:25:45.743" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22943" PostTypeId="1" CreationDate="2017-09-07T23:36:44.137" Score="11" ViewCount="1501" Body="&lt;p&gt;This is something I'm wondering for a science book I'm writing (not science fiction) where I cover the various forms of life that could exist especially in our solar system - such as life on Mars of course - but also the more exotic types - life in supercritical CO&lt;sub&gt;2&lt;/sub&gt; on Mars or even deep in the Earth oceans, life using sulfur on Io, life in the ethane / methane seas of Triton (able to live in a non polar liquid), life living in the sulfuric acid clouds of Venus. All of these are serious suggestions by astrobiologists.&lt;/p&gt;&#xA;&lt;p&gt;The book is mainly focused on Planetary Protection - protection from Earth microbes - but this is about places in our solar system where there may be life that can't be adversely affected by Earth microbes. E.g. no way that Earth life could survive in a liquid neon or nitrogen or hydrogen ocean or most of these other places, unlike Mars where it could and could seriously interfere with scientific study of the planet.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;POSSIBILITY OF FINDING NEW WORLDS IN THE OUTER KUIPER BELT AND OORT CLOUD&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Mike Brown arguing on the basis of the discovery of Sedna close to the sun thinks there may well be many Mercury, Mars and even Earth sized objects out there. See &lt;a href=&quot;https://youtu.be/WHNO079G1i8?t=3044&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;We haven't discovered any of these yet, but here for instance is &lt;a href=&quot;https://arxiv.org/pdf/1704.02444.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;a paper from 2017, by Kathryn Volk and Renu Malhotra&lt;/a&gt;, building on earlier work by Lykawka&amp;amp;Mukai in 2008, suggesting the possibility of a Mars sized object at a distance of 65–80 AU to explain the “Curiously warped mean plane of the Kuiper belt”. That’s based on the observation that the Kuiper belt bodies are in a plane offset by about eight degrees from the “Invariable plane” of our planets. For non technical account of their research by the astrophysicist Brian Koberlein, see &lt;a href=&quot;https://www.forbes.com/sites/briankoberlein/2017/06/22/goodbye-planet-nine-hello-planet-ten/#6b94868aaa15&quot; rel=&quot;nofollow noreferrer&quot;&gt;Goodbye Planet Nine, Hello Planet Ten&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There's also the possibility that &amp;quot;Planet 9&amp;quot; consists of not just one but several objects: Worlds Beyond Pluto --&lt;a href=&quot;http://www.dailygalaxy.com/my_weblog/2016/08/worlds-beyond-pluto-theres-not-just-planet-9-but-several-planets-beyond-pluto.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&amp;quot;There's Not Just Planet 9, But Several Planets Beyond&amp;quot;&lt;/a&gt; and then you also have the possibility that while searching for it, they turn up smaller objects as happened for Pluto with Percival Lowell and Clyde Tombaugh's search.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;ASTROBIOLOGY POTENTIAL OF LIQUID NITROGEN / NEON / ARGON / HYDROGEN ETC&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Moving outwards in the solar system, I have got as far as life in liquid neon which is another hypothesis by an astrobiologist William Baine. But there may be many worlds that are too cold for liquid neon.&lt;/p&gt;&#xA;&lt;p&gt;The nearby objects could have liquid neon lakes or oceans (a very speculative hypothesis but possible) - but further out - it would be too cold even for liquid neon. But what about liquid hydrogen?&lt;/p&gt;&#xA;&lt;p&gt;If we discover a distant Kuiper belt or even Oort cloud object the size of Mars or Earth - which we could easily do in the next decade or two, might find one any time indeed - could it have a liquid hydrogen ocean?&lt;/p&gt;&#xA;&lt;p&gt;The idea is to use chemical reactions that are too fast for our temperatures. &lt;a href=&quot;http://www.ifa.hawaii.edu/%7Emeech/a281/handouts/Baines_astrobio04.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;William Baine suggests silanols&lt;/a&gt;, a silicon version of alcohol. The silicon - hydrogen bond is destroyed within hours at mammalian temperatures in presence of water, not nearly as stable as the carbon - hydrogen bond, but it's just what you want at such low temperatures and in non polar solvents. He writes:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;As a barrier to using silicon for living systems, however, this is&#xA;only a problem if we assume that strongly protonating solvents (water,&#xA;ammonia, hydrogen sulfide, and sulfuric acid in a cosmological&#xA;context) are the solvent base. Oceans of cold, non-protonating fluids,&#xA;and specifically liquid nitrogen, would be more favorable for silicon&#xA;life. In such environments, the inherently greater reactivity of&#xA;silicon-based chemicals could be an advantage, enabling “living”&#xA;processes to occur at greater than geological speeds at the relatively&#xA;lower temperatures where such fluids are stable. Such fluids could&#xA;also include liquid methane, ethane, neon, or argon.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So astrobiologist have looked at life in liquid neon and liquid nitrogen - maybe even life in the Triton geysers. He mentions liquid hydrogen as another possible non polar solvent but he doesn't take that any further. He rules out Helium as needing temperatures too cold for the planets he discusses (2000 km diameter or larger) but he doesn't rule out Hydrogen, just drops the topic as far as I can see. His paper is &lt;a href=&quot;http://www.ifa.hawaii.edu/%7Emeech/a281/handouts/Baines_astrobio04.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Astrobiologists have also looked at the possibility of life in the liquid hydrogen layer inside a gas giant - but there it's based on supercriticality which makes it a solvent in a region above 12.8 atmospheres and 33.3 K&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;WORLDS WITH SURFACE LIQUID HYDROGEN OCEANS&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;What I'm wondering about here though is, what about a non gas giant that has an ocean  of liquid hydrogen - &lt;strong&gt;ON ITS SURFACE&lt;/strong&gt; just like our oceans - and perhaps an atmosphere above that - maybe hydrogen again or perhaps helium -at very low temperatures where everything else except hydrogen and helium are solid.&lt;/p&gt;&#xA;&lt;p&gt;If such was possible - well - it might even be the most common type of ocean in our galaxy, and indeed universe -given the size of the Oort cloud and the number of Mars or Earth sized objects that might be in it - not trillions - but dozens or hundreds seem not impossible. There could easily be more of them than there are in the inner solar system, and if you go down to the smaller Moon sized or smaller planets then there are many more of them already than in the inner solar system -and if so - then if that is repeated elsewhere they could be by far the most common type of planet in the galaxy (I'm using the geophysical definition of a planet here).&lt;/p&gt;&#xA;&lt;p&gt;New Horizons found that Pluto was losing Nitrogen only at the Jeans' escape rate, not at the higher rates of thermohydrodynamic escape. If an Earth mass world had a hydrogen atmosphere and it only lost it at a similarly low escape rate - well it might easily retain a liquid hydrogen ocean - if such could form in the first place.&lt;/p&gt;&#xA;&lt;p&gt;So - I've searched the scholarly literature and have not found any cites on this idea yet. Anyone know if it is possible? Anyone studied it?&lt;/p&gt;&#xA;&lt;p&gt;I can imagine that the main problem might be, how does the hydrogen collect in the first place? Without the gravity of a gas giant? A Jupiter sized object seems the minimum size for a sub brown dwarf so it couldn't form just by a condensing gas cloud for smaller objects.&lt;/p&gt;&#xA;&lt;p&gt;But you only need a small amount of hydrogen by percent in an entire Earth sized planet to lead to oceans or at least lakes.&lt;/p&gt;&#xA;&lt;p&gt;There's another way you could get a surface liquid hydrogen ocean too. I suppose also, maybe you find a gas giant that is like Jupiter say - but so cold it is covered in a very deep liquid hydrogen ocean. That also would be interesting as it could have life there too.&lt;/p&gt;&#xA;&lt;p&gt;Incidentally such a planet might be hard to spot if the ocean is smooth like a mirror and global, or nearly so, reflecting only a small amount of light back to Earth. Liquid hydrogen is transparent however, much more so than water - not sure what that does to its albedo for a deep ocean of liquid hydrogen.&lt;/p&gt;&#xA;&lt;p&gt;This was &lt;a href=&quot;http://iopscience.iop.org/article/10.1086/382513/fulltext&quot; rel=&quot;nofollow noreferrer&quot;&gt;a question for instance for Quaoar after it was discovered&lt;/a&gt;. &amp;quot; If Quaoar instead has a surface with a strong specular reflection, it could have a much smaller half-total light diameter and actually be larger. The true limb-darkening function of such an icy outer solar system body is unknown.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;This is &lt;a href=&quot;http://robertinventor.com/booklets/If_humans_touch_Mars.htm#life_in_neon_seas&quot; rel=&quot;nofollow noreferrer&quot;&gt;my section about life in liquid neon seas and lakes with cites&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;And I have now written up the idea of life in liquid hydrogen seas as &amp;quot;work in progress&amp;quot; &lt;a href=&quot;http://robertinventor.com/booklets/If_humans_touch_Mars.htm#life_in_liquid_helium&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;RETAINING A HYDROGEN ATMOSPHERE&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/R3rIH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/R3rIH.jpg&quot; alt=&quot;This shows the gases prone to escape from atmospheres of worlds of various sizes through &amp;quot;Jeans' escape&amp;quot; - just through loss of individual velocities whose velocity due to temperature is enough to reach escape velocity&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;There if you draw a line up to the Hydrogen line through the Pluto dot and then across it passes through the Mars dot (note, this is from 2009, before the New Horizons results). Though the exobase could be warmer than the planet surface, still, it looks as if Mars sized objects or larger, further out than Pluto could hold onto a hydrogen atmosphere - if they can get one in the first place - at least, if they lose it only by Jeans' escape.&lt;/p&gt;&#xA;&lt;p&gt;Gases can also escape via thermodynamic escape, a heating up of gas through absorption of UV from sunlight, leading to bulk flows of the gas into space starting from below the exobase, which makes it more complex. However Pluto is losing nitrogen at the Jeans' escape rate, not the thermodynamic escape rate, corresponding to a loss of about 6 cm of nitrogen ice over the age of the solar system. See the &lt;a href=&quot;https://www.astro.umd.edu/%7Edphamil/research/reprints/GlaSteEnn16.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;section on Escape towards the end of this paper&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;I've found one mention of the idea so far, in &lt;a href=&quot;https://books.google.co.uk/books?id=ZECsAAAAIAAJ&amp;amp;dq=Life%20Beyond%20Earth%20-%20Gerald%20Feinberg&amp;amp;focus=searchwithinvolume&amp;amp;q=%22liquid%20hydrogen%22&quot; rel=&quot;nofollow noreferrer&quot;&gt;a 1980 book &amp;quot;Life Beyond Earth&amp;quot; by Gerald Feinberg&lt;/a&gt;. I haven't read it yet but have ordered it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;UPDATE 2&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;I've found many more sources on this now, there are two main possibilities. Supercritical hydrogen, which has the advantage it can dissolve organics, in gas giant atmospheres it first becomes possible at  above 12.8 atmospheres (1.297 MPa) and 33.3 K. I work out that alternatively for less massive bodies, hydrogen trapped below 147 meters of low density amorphous ice could be under enough pressure for super criticality.&lt;/p&gt;&#xA;&lt;p&gt;The other possibility is liquid hydrogen. If the gas is lost only at the Jeans escape rate not the thermodynamic escape rate (a heating up of gas through absorption of UV from sunlight, leading to bulk flows of the gas into space starting from below the exobase).&lt;/p&gt;&#xA;&lt;p&gt;Pluto loses its atmosphere only at the Jeans escape rate, so it's not impossible. If so, then it could just about be possible for a Mars or Earth sized body in the distant Oort cloud to retain a liquid hydrogen ocean.&lt;/p&gt;&#xA;&lt;p&gt;William Bains &lt;a href=&quot;http://www.ifa.hawaii.edu/%7Emeech/a281/handouts/Baines_astrobio04.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;writes&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;The surface temperature of such cold worlds as predicted here is also most likely incorrect, as heating from cosmic rays and internal radioactivity will be a significant part of their heat budget. Thus the frequency calculations for liquid hydrogen or neon must be regarded as even more speculative than those for other liquids. The uncertainties in all aspects of this calculation as applied to hydrogen are too large to consider liquid hydrogen further in this paper.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;David Stevenson &lt;a href=&quot;http://hoffman.cm.utexas.edu/courses/rogue_planet_nature_letter.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;writes&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;For sufficiently low masses, an alternative (collapsed atmosphere) solution exists with a molecular hydrogen ocean overlain by a thin vapor pressure-equilibrium atmosphere.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So - it may be marginally possible.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://robertinventor.com/booklets/If_humans_touch_Mars.htm#life_in_liquid_helium&quot; rel=&quot;nofollow noreferrer&quot;&gt;Life in liquid hydrogen or liquid helium as far away as the Oort Cloud&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://robertinventor.com/booklets/If_humans_touch_Mars.htm#distant_liquid_water_oceans&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rogue planets and very distant planets - liquid water or liquid hydrogen surface oceans&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="7062" LastEditDate="2022-06-09T08:15:54.210" LastActivityDate="2022-07-10T10:58:23.517" Title="Could there be liquid hydrogen oceans on the surfaces of worlds (including frozen gas giants) in the distant Kuiper belt or Oort cloud?" Tags="|life|astrobiology|oort-cloud|" AnswerCount="1" CommentCount="19" ContentLicense="CC BY-SA 4.0" />
  <row Id="22947" PostTypeId="1" AcceptedAnswerId="22950" CreationDate="2017-09-08T13:17:28.130" Score="17" ViewCount="7670" Body="&lt;p&gt;In the context of staged combustion, I understand that burning ox-rich creates complications associated with having hot, high pressure, oxidizer. Some engines, notably the RD-170 and RD-180 family run ox-rich. The &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-180#Design_and_specifications&quot; rel=&quot;noreferrer&quot;&gt;wikipedia page for the RD-180&lt;/a&gt; explains this by saying:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The engine runs with an oxidizer-to-fuel ratio of 2.72 and employs an oxygen-rich preburner, unlike typical fuel-rich US designs. The &#xA;  thermodynamics of the cycle allow an oxygen-rich preburner to give a greater &lt;strong&gt;power-to-weight ratio&lt;/strong&gt;, but with the drawback that high-pressure, high-temperature gaseous oxygen must be transported throughout the engine.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However this block isn't cited and I haven't been able to find much in the way of corroborating sources. I have however found some &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=18646.msg674763#msg674763&quot; rel=&quot;noreferrer&quot;&gt;message boards&lt;/a&gt; that mention &lt;strong&gt;soot buildup&lt;/strong&gt; offhand, but again, not a lot of further reading that I can find.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone provide a conclusive answer on either the soot buildup, an &lt;strong&gt;explanation of where a power-to-weight ratio advantage comes from&lt;/strong&gt;, or some combination of the two?&lt;/p&gt;&#xA;" OwnerUserId="10856" LastActivityDate="2023-10-10T17:43:04.233" Title="What are the advantages of Ox-rich staged combustion?" Tags="|propulsion|engine-design|engines|oxidizer|combustion|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22948" PostTypeId="1" AcceptedAnswerId="29585" CreationDate="2017-09-08T13:54:08.407" Score="16" ViewCount="2770" Body="&lt;p&gt;Where can I find the best values to use for standard gravitational parameters of solar system bodies for numerical simulation experiments?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are two sets I've put together. The first set is from Wikipedia, the second set is from the a JPL progress report listing the masses used for the calculation of the DE421 ephemeris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to the Sun and planets, I've included Earth's Moon, and the three largest asteroids. I am not sure how I should handle the masses of the larger moons of the large planets; I assume &lt;em&gt;they are not&lt;/em&gt; counted here and I should deal with them separately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a significantly better &quot;master table&quot; than the DE430/431 values, or is that about as good as it gets? &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;NOTE: units are MKS&#xA;&#xA;            Wikipedia:        JPL: DE421          JPL: DE430/431&#xA;           (1) unless         (2) unless            (6) unless &#xA;         otherwise noted     otherwise noted       otherwise noted&#xA;        -----------------  --------------------  --------------------&#xA;Sun     1.32712440018E+20  1.32712440040944E+20  1.32712440040944E+20&#xA;Mercury 2.2032E+13         2.203209E+13          2.203178E+13&#xA;Venus   3.24859E+14        3.24858592E+14        3.24858592E+14&#xA;Earth   3.986004418E+14    3.98600436233E+14     3.98600435436E+14&#xA;Moon    4.9048695E+12      4.902800076E+12       4.902800066E+12&#xA;Mars    4.282837E+13       4.2828375214E+13      4.2828375214E+13&#xA;Ceres   6.26325E+10        6.2178E+10            6.28093938E+10&#xA;Pallas  2.11E+20*G(4,5)    1.3402E+10            1.3923011E+10&#xA;Vesta   2.59076E+20*G(3,5) 1.7630E+10            1.7288009E+10&#xA;Jupiter 1.26686534E+17     1.267127648E+17       1.267127648E+17&#xA;Saturn  3.7931187E+16      3.79405852E+16        3.79405852E+16&#xA;Uranus  5.793939E+15       5.7945486E+15         5.7945486E+15&#xA;Neptune 6.836529E+15       6.836535E+15          6.83652719958E+15&#xA;&#xA;G = 6.67408E-11 (5)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;(1) &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia Standard Gravitational Parameter&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;(2) &lt;a href=&quot;https://ipnpr.jpl.nasa.gov/progress_report/42-178/178C.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Planetary and Lunar Ephemeris DE 421, IPN Progress Report 42-178 (August 2009)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;(3) &lt;a href=&quot;https://en.wikipedia.org/wiki/4_Vesta&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia 4 Vesta&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;(4) &lt;a href=&quot;https://en.wikipedia.org/wiki/2_Pallas&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia 2 Pallas&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;(5) &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia Gravitational Constant&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;(6) &lt;a href=&quot;https://ipnpr.jpl.nasa.gov/progress_report/42-196/196C.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Planetary and Lunar Ephemeris DE 430 and 431, IPN Progress Report 42-196 (February 2014)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-10T09:26:53.843" LastActivityDate="2021-12-13T19:34:46.123" Title="Where to find the best values for standard gravitational parameters of solar system bodies?" Tags="|orbital-mechanics|planetary-science|mass|jpl-horizons|" AnswerCount="1" CommentCount="17" ContentLicense="CC BY-SA 3.0" />
  <row Id="22949" PostTypeId="1" CreationDate="2017-09-08T14:50:51.917" Score="2" ViewCount="131" Body="&lt;p&gt;I am making a small &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Magnetoplasmadynamic thruster&lt;/a&gt; (MPT) which uses compressed air as propellant. I want to know how many volts of electricity would be needed such that the exhaust plasma velocity is around 50 to 100m/s? What apparatus would I need to generate the high voltage at home?&lt;/p&gt;&#xA;" OwnerUserId="8608" LastEditorUserId="12102" LastEditDate="2017-09-11T17:11:48.327" LastActivityDate="2017-09-11T17:11:48.327" Title="How much electricity is needed to run a magnetoplasmadynamic thruster?" Tags="|magnetoplasmadynamic|plasma|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22950" PostTypeId="2" ParentId="22947" CreationDate="2017-09-08T15:41:40.800" Score="13" Body="&lt;p&gt;According to the super-detailed and very informative &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19820002372&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA-CR 165404 Fuel/Oxidizer-Rich High-Pressure Preburners&lt;/a&gt;, the primary advantages of hydrocarbon-fueled oxidizer-rich staged-combustion engines is that &amp;quot;carbon formation, coking, and the attainment of ignition are no longer issues.&amp;quot; (page 133)&lt;/p&gt;&#xA;&lt;p&gt;This useful rocket engine cycle overview &lt;a href=&quot;https://archive.org/download/engine-cycles-2/EngineCycles-2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;presentation&lt;/a&gt; from Purdue University's AAE 539: Advanced Rocket Propulsion class mentions only the carbon formation (sooting) issue as an advantage (pdf page 17).&lt;/p&gt;&#xA;&lt;p&gt;The book &amp;quot;Progress In Astronautics and Aeronautics: Liquid Rocket Thrust Chambers&amp;quot;, excerpted at Google Books &lt;a href=&quot;https://books.google.com/books?id=0HWotm1k40QC&amp;amp;pg=PA632&amp;amp;lpg=PA632&amp;amp;dq=comparison%20of%20fuel-rich%20and%20oxidizer-rich%20staged%20combustion%20engines&amp;amp;source=bl&amp;amp;ots=MXD1VbdCyt&amp;amp;sig=ojQTzyVnwnz5b2eToQUhsU0AQio&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjUjqrh-ZXWAhXnqlQKHZXwBcg4ChDoAQg_MAQ#v=onepage&amp;amp;q=comparison%20of%20fuel-rich%20and%20oxidizer-rich%20staged%20combustion%20engines&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, may provide a clue as to the claimed power-to-weight advantage.  It shows an analysis that concludes for equal turbine temperatures and pump discharge pressures, an oxidizer-rich cycle has an 87% increase in chamber pressure over a fuel-rich cycle.  This would seem to imply that a smaller oxidizer-rich engine could produce the same thrust as a larger fuel-rich engine.  Unfortunately, the book is only excerpted there, and I don't own a copy.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2023-10-10T17:43:04.233" LastActivityDate="2023-10-10T17:43:04.233" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22951" PostTypeId="2" ParentId="22923" CreationDate="2017-09-08T17:29:40.870" Score="7" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;I've heard that Osama Bin Laden was first found via satellite.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That isn't the case. Bin Laden was instead first found by human intelligence (interrogation, spy work). Detainees at Guantanamo revealed the name of bin Laden's most trusted courier. This eventually led to that courier being found and then discretely tailed to the compound in Abbottabad. It was suspected that someone of greater importance than a courier lived in the compound. Satellite recon was used in the process, but to develop a 3D map of the compound rather than to recognize bin Laden. Photographs of someone suspected of being bin Laden were taken, but by drones, not satellites.&lt;/p&gt;&#xA;&lt;br&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Can a satellite scan an entire continent to search for someone?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;No, for many reasons.&lt;/p&gt;&#xA;&lt;p&gt;One reason is the resolution limitations of a satellite due to physics. Geostationary satellites can see almost half of the globe, but the large distance between those geostationary satellites and the surface of the Earth places severe limitations on resolution. Reconnaissance satellites (aka spy satellites) operate in low Earth orbit to overcome these limitations, but even then, the resolution is estimated to be about ten centimeters. That's not enough resolution to recognize that a face is present, let alone who's face it is.&lt;/p&gt;&#xA;&lt;p&gt;Another issue is that those reconnaissance satellites can only see a rather small part of the globe. It takes multiple days for a reconnaissance satellite to fully map a continent. The person subject to the search can hide indoors (or simply not look up) during the very brief intervals when the satellite is in view.&lt;/p&gt;&#xA;&lt;p&gt;Yet another issue is the huge amount of data needed to accomplish this task. Assuming good conditions, assuming the person subject to the search walks around looking up, and assuming a satellite with 0.4 centimeter resolution exists (the resolution needed for facial identification under good conditions), a continent's worth of monocolor (grey scale) imagery represents over an exabyte of data. Many such images will need to be scanned to find the elusive person of interest. Even the most powerful supercomputer is not up to this task.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-09-08T17:29:40.870" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22952" PostTypeId="1" AcceptedAnswerId="22956" CreationDate="2017-09-08T17:39:40.433" Score="2" ViewCount="1255" Body="&lt;p&gt;I may be mistaken but it seems to me that to increase the payload a rocket can put into orbit, manufacturer add booster but do not modify upper stage(s).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;falcon heavy is a falcon 9 with additional booster strapped to the first stage&lt;/li&gt;&#xA;&lt;li&gt;ariane 6 variants differs only by the number of boosters&lt;/li&gt;&#xA;&lt;li&gt;the same happen with the delta IV variants.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;To me it seems more logical to increase power of both the first stage (by adding boosters) and the upper stages. By only modifying the first stage, this stage may reach at greater acceleration, speed and altitude and thus operate in a non optimal flight environement (assuming the variant with less boosters operate at optimal acceleration, speed and altitude).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not quite sure I'm understandable, so feel free to edit/suggest edit&lt;/p&gt;&#xA;" OwnerUserId="9072" LastActivityDate="2017-09-08T19:48:12.757" Title="Why do we only add boosters to increase payload launch by a rocket?" Tags="|propulsion|first-stage|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22953" PostTypeId="1" CreationDate="2017-09-08T17:47:30.620" Score="6" ViewCount="838" Body="&lt;p&gt;I was recently listening to a talk about &lt;a href=&quot;https://www.jwst.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;JWST&lt;/a&gt; and other future astrophysics missions when the speaker talked about many of the size limitations associated with mirrors etc.  I then thought perhaps one could solve this problem similar to how JWST approached their limitation, i.e., by &quot;folding up&quot; the mirror.  However, weight is also an issue so then I started to wonder if there were any materials &lt;strong&gt;similar to a fabric&lt;/strong&gt; (not necessarily a fabric) that could used for a mirror.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume there is or are materials that are fabric-like (i.e., can be folded without kinking/wrinkling) and reflective like glass or polished metal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could one stretch such a material accurately and precisely enough to form a useable mirror with competitive resolution and clarity (i.e., good enough for new research, not blurry results by today's standards)?&lt;/p&gt;&#xA;" OwnerUserId="12508" LastEditorUserId="12508" LastEditDate="2017-10-10T13:15:17.430" LastActivityDate="2023-04-19T09:19:06.393" Title="Can mirrors be made from fabric?" Tags="|materials|space-telescope|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22954" PostTypeId="2" ParentId="22953" CreationDate="2017-09-08T18:03:52.037" Score="4" Body="&lt;p&gt;The STS-77 mission had a related experiment, the Inflatable Antenna Experiment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sad5E.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sad5E.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/r8VuC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/r8VuC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While intended for RF wavelengths, conceivably something could be done for other wavelengths.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, the experiment was not much of a success. It was quickly punctured by micro-meteroids and lost its shape.  It was made of reflective mylar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information &lt;a href=&quot;http://www.lgarde.com/assets/content/files/publications/207.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-09-08T18:03:52.037" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22956" PostTypeId="2" ParentId="22952" CreationDate="2017-09-08T18:54:44.230" Score="11" Body="&lt;p&gt;Rocket manufacturers are optimizing for cost, not for total mass at launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's cheaper and easier to modify one thing to improve overall performance, even if the result isn't mass-optimal, than to modify two things. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Delta IV medium, Atlas V, and Ariane 6 cases in particular, changing just the number of installed SRBs is both cheap and flexible; you can start building an inventory of launcher cores and SRBs well in advance of knowing what payloads you're launching! They don't have to design different boosters, different cores, or different upper stages, they just pick the right number of boosters to do the job. Manufacturing a bunch of modular boosters is cheaper than developing several different kinds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, upper stages may also get small customizations for different configurations. Falcon Heavy may need some structural work on the upper stage for heavier payloads. Atlas V has a two-engine version of the Centaur upper stage (though that hasn't flown yet, and the single engine version is more mass-efficient). Ariane 5 refined the boosters, core, and the upper stage over various iterations.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-09-08T18:54:44.230" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22957" PostTypeId="2" ParentId="22953" CreationDate="2017-09-08T19:14:34.837" Score="8" Body="&lt;p&gt;Mirrors have to have a very smooth surface. The &lt;a href=&quot;https://jwst.nasa.gov/mirrors.html&quot; rel=&quot;noreferrer&quot;&gt;JWST mirrors&lt;/a&gt; have surface imperfections on the order of 10 nm. They also have to be very rigid, in order to stay accurate through the rigors of launch. Despite this, the JWST mirror segments weigh only 20 kg each (for a diameter of 1.3 m).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's possible to use a very thin mirror, and use electric actuators to bend the mirror to the required shape. But that just shifts the weight to the supporting structure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hubble telescope used flexible &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Engineering_Technology/How_Hubble_got_its_wings&quot; rel=&quot;noreferrer&quot;&gt;solar arrays initially&lt;/a&gt;. The thermal shock when entering/leaving Earth's shadow meant the arrays would flex, and the vibrations would disturb the observation. Without an effective active control mechanism, flexible mirrors would have similar behavior. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="12422" LastEditDate="2017-10-09T18:09:02.490" LastActivityDate="2017-10-09T18:09:02.490" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22958" PostTypeId="2" ParentId="22952" CreationDate="2017-09-08T19:48:12.757" Score="6" Body="&lt;p&gt;Frankly, it's just plain easier and cheaper to slap on additional boosters to the lower stage than to add capability to the upper stage.  Every Kg of mass you add to the upper stage translates into N Kg of additional propellant in the booster, and N can get impractically large in a hurry. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, where thrust (power) matters more for the booster, specific impulse (efficiency) matters more for the upper stage.  You're better off using a lower-thrust, high-Isp engine on your upper stage than slapping more high-thrust, low-Isp engines to it.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Comparing the Falcon 9 and Atlas V vehicles:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Rocket    Mass to LEO    Mass to GEO    &#xA;------    -----------    -----------    &#xA;Falcon 9  22800          8300           &#xA;Atlas V   18850          8900           &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;F9 can lift more to LEO, but the Atlas can lift more to GEO.  That's because the Atlas uses the hydrolox Centaur upper stage with the RL-10 engine, which has much lower thrust than the kerolox F9 upper stage with the MVac (99,100 N vs. 934,000 N, per Wikipedia) but a much higher specific impulse (451 s vs. 348 s, same source).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, practical engineering considerations come into play. Hydrolox stages are expensive - because of the low density of LH2, the tanks have to be a lot bigger to hold the same mass of fuel, they have to be kept much colder, etc.  The F9 is as wide as it can get (~3.6 m diameter) and still be road-transportable.  They can't stretch either stage to be any longer because the whole stack is as tall as it can get for its width (otherwise it can bend due to wind shear).  SpaceX chose to use common materials, tooling, and propellant for both stages to keep costs down.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2017-09-08T19:48:12.757" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22959" PostTypeId="1" AcceptedAnswerId="22961" CreationDate="2017-09-09T03:43:11.587" Score="3" ViewCount="361" Body="&lt;p&gt;I've been learning about rocket propulsion and had an idea for an engine cycle, maybe its been done before but I haven't been able to find anything this online. So the full flow staged combustion cycle has many benefits over other cycles such as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;smaller, lower power turbopumps needed&lt;/li&gt;&#xA;&lt;li&gt;no need for the fuel/oxidizer seal that can cause engine failure&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;My idea is to do a hybrid between staged combustion and FFSC. By this I mean having two preburners to get the benefits of FFSC, but running them both fuel rich for simplicity. So it would essentially be a fuel rich staged combustion cycle, but with two separate preburners/turbopumps to eliminate the shaft seal, lighter weight, etc. Good idea / bad idea?&lt;/p&gt;&#xA;" OwnerUserId="20961" LastActivityDate="2017-09-09T10:38:38.433" Title="Hybrid staged combustion engine cycle" Tags="|propulsion|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22960" PostTypeId="2" ParentId="22915" CreationDate="2017-09-09T08:42:46.803" Score="2" Body="&lt;p&gt;I'll post the numerical methods in this answer, and the full-blown solar system calculation (including relativity and possible effects from the Sun's oblate shape) as a second answer. There's just too much to put it all in one answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The method you describe using looks like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Euler_method&quot; rel=&quot;nofollow noreferrer&quot;&gt;Euler Method&lt;/a&gt; or what can be called &lt;a href=&quot;http://mathworld.wolfram.com/EulerForwardMethod.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Euler Forward method&lt;/a&gt;. At each time step $dt$ you calculate all of the forces and their resulting net accelerations $\mathbf{a}$, and then just increment the velocities $\mathbf{v}$ by $\mathbf{a} dt$ and all positions $\mathbf{x}$ by $\mathbf{v} dt$. You need absurdly tiny time steps to get even close with this. You mentioned 2 seconds and 250 milliseconds, a bit short for solar system timescales.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the script below, I've written the Euler Forward method and two more &lt;a href=&quot;https://en.wikipedia.org/wiki/Runge%E2%80%93Kutta_methods&quot; rel=&quot;nofollow noreferrer&quot;&gt;Runge-Kutta&lt;/a&gt; low order methods usually called RK2 and RK4. For each method a simplified (fake) Mercury orbit around a fixed Sun is calculated for 100 days, with the number of iterations varied from 10 to 10,000. Also, for each, I use the &lt;a href=&quot;https://scipy.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SciPy&lt;/a&gt; library solver &lt;code&gt;odeint()&lt;/code&gt; with a relative precision tolerance of 1E-12 per step. &lt;a href=&quot;https://docs.scipy.org/doc/scipy/reference/generated/scipy.integrate.odeint.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;odeint&lt;/a&gt; is a python wrapper for &lt;code&gt;lsoda&lt;/code&gt; from the FORTRAN library &lt;code&gt;odepack&lt;/code&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that even RK4 agrees with &lt;code&gt;odeint&lt;/code&gt; &lt;em&gt;to the level of meters after 100 days&lt;/em&gt; if you use a time step of about 15 minutes, and the Euler Forward method (what you use) will need an absurd number of steps to even approach this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Numerical techniques are not the only issue here, I will post a second answer in a few days with the rest of what you need. &lt;em&gt;I may do it under a separate question rather than post two answers to the same question&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this should get you started either coding RK4 in java, or looking for a java numerical library like the Apache one mentioned in comments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The simplest standard way to solve an orbital problem using an ODE solver is to put all of the cartesian coordinates and velocities in one long vector, call it $y$, and write a single function for the time derivative:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\dot{y} = f(t, y)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you had two bodies and three dimensions, the vector $y$ would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ y=(x_1, y_1, z_1, x_2, y_2, z_2, v_{x1}, v_{y1}, v_{z1}, v_{x2}, v_{y2}, v_{z2})$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;having six elements per body. The derivative of $x_1$ is $v_{x1}$, and the derivative of $v_{x1}$ is the acceleration $a_{x1}$ due to all of the other bodies. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say you have one body in a central force with a standard gravitational parameter $GM$. The rate of change of position is just the velocity,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{d\mathbf{x}}{dt} = \mathbf{v}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the rate of change of velocity is the acceleration, due to the force; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{d\mathbf{v}}{dt} = \mathbf{a} = -GM \frac{\mathbf{r}}{r^3}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you had multiple bodies, the $\mathbf{r}$ would be the distance between pairs of bodies and for each body you would sum over all of the others as you've described already.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you wrote out $f$, it would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ f=(v_{x}, v_{y}, v_{z}, -GMx/r^3, -GMy/r^3, -GMz/r^3).$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;strong&gt;Euler Forward method&lt;/strong&gt; that I believe you are using just iterates&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$y_{i+1} = h \ f(t, y_i) $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;with time steps of $h$. The improved &lt;strong&gt;RK2 method&lt;/strong&gt; (shown below) would be written:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$k_1 = f(t, y_i)$$&#xA;$$k_2 = f(t+\frac{h}{2}, y_i+\frac{h}{2}k_1)$$&#xA;$$y_{i+1} = y_n + h k_2 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the ubiquitous &lt;strong&gt;RK4 method&lt;/strong&gt; is written as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$k_1 = f(t, y_i)$$&#xA;$$k_2 = f(t+\frac{h}{2}, y_i+\frac{h}{2}k_1)$$&#xA;$$k_3 = f(t+\frac{h}{2}, y_i+\frac{h}{2}k_2)$$&#xA;$$k_4 = f(t+h, y_i+k_3)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$y_{i+1} = y_n + h (k_1 + 2k_2 + 2k_3 + k_4)/6 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the section that shows the similarity and differences between the Euler Forward method (the simplest Runge-Kutta method) and two higher order RK methods. This is written for clarity, not speed obviously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uhnqK.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uhnqK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3Tof2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3Tof2.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ysV8k.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ysV8k.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(t, X):&#xA;&#xA;    x, v = X.reshape(2, -1)&#xA;    acc  = -GMs * x * ((x**2).sum())**-1.5&#xA;&#xA;    return np.hstack((v, acc))&#xA;&#xA;def derivv(X, t):&#xA;    return deriv(t, X) # historical reasons&#xA;&#xA;def RK_all(F, t0, X0, n, h, method=None):&#xA;&#xA;    hov2, hov6   = h/2.0, h/6.0&#xA;    t, X         = t0, X0&#xA;    answer, time = [], []&#xA;&#xA;    answer.append(X)&#xA;    time.append(t)&#xA;&#xA;    for i in range(n):&#xA;        k1 = F(t,        X            )&#xA;        k2 = F(t + hov2, X + hov2 * k1)&#xA;        k3 = F(t + hov2, X + hov2 * k2)&#xA;        k4 = F(t + h,    X + h    * k3)&#xA;&#xA;        if   method == 'EulerFwd':&#xA;            X = X + h*k1        # X + h*F(t, X)&#xA;&#xA;        elif method == 'RK2':&#xA;            X = X + h*k2&#xA;&#xA;        elif method == 'RK4':&#xA;            X = X + hov6*(k1 + 2.*(k2+k3) + k4)&#xA;&#xA;        else:&#xA;            pass&#xA;&#xA;        t += h&#xA;&#xA;        answer.append(X)&#xA;        time.append(t)&#xA;&#xA;    return np.array(time), np.array(answer)&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;GMs = 1.327E+20 # approx&#xA;&#xA;X0 = np.array([-2.094E+10,  4.303E+10,  5.412E+09,&#xA;               -5.328E+04, -2.011E+04,  3.243E+03]) # approx&#xA;&#xA;methodnames = ['RK4', 'RK2', 'EulerFwd']&#xA;niterations = [10, 20, 50, 100, 200, 500, 1000, 2000, 5000, 10000]&#xA;&#xA;Time = 100*24*3600.  # total time&#xA;&#xA;methdict = dict()&#xA;&#xA;for methodname in methodnames:&#xA;&#xA;    times, answers, ODEanswers, posdevs = [], [], [], []&#xA;&#xA;    for n in niterations:&#xA;&#xA;        h  = Time/float(n)&#xA;        t0 = 0.0&#xA;&#xA;        time, answer    = RK_all(deriv, t0, X0, n, h, method=methodname)&#xA;        # recalculate using library ODE solver for same times, to compare&#xA;        ODEanswer, info = ODEint(derivv, X0, time,&#xA;                                 rtol=1E-12, full_output=True)&#xA;        posdev = np.sqrt((((answer - ODEanswer)[:,:3])**2).sum(axis=1))&#xA;&#xA;&#xA;        times.append(time)&#xA;        answers.append(answer)&#xA;        ODEanswers.append(ODEanswer)&#xA;        posdevs.append(posdev)&#xA;&#xA;    methdict[methodname] = (times, answers, ODEanswers, posdevs)&#xA;&#xA;if 1 == 1:&#xA;    plt.figure()&#xA;    for i, meth in enumerate(methodnames):&#xA;        plt.subplot(1, 3, i+1)&#xA;        for time, answer, ODEanswer, posdev in zip(*methdict[meth]):&#xA;            x, y, z = answer.T[:3]&#xA;            plt.plot(x, y)&#xA;        plt.ylim(-2.8E+11, 0.8E+11)&#xA;        plt.xlim(-1.2E+11, 0.8E+11)&#xA;        plt.title(meth, fontsize=16)&#xA;        plt.plot([0],[0], 'ok')&#xA;    plt.show()&#xA;&#xA;if 1 == 1:&#xA;    plt.figure()&#xA;    for i, meth in enumerate(methodnames):&#xA;        plt.subplot(1, 3, i+1)&#xA;        for time, answer, ODEanswer, posdev in zip(*methdict[meth]):&#xA;            plt.plot(time/(24*3600.), posdev)&#xA;        plt.yscale('log')&#xA;        plt.ylim(1E-01, 1E+12)&#xA;        plt.title(meth+' vs odeint', fontsize=16)&#xA;    plt.suptitle('RKmethod - odeint (meters) vs time (days)', fontsize=18)&#xA;    plt.xticks([0, 20, 40, 60, 80, 100])&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-10T11:59:31.890" LastActivityDate="2017-09-10T11:59:31.890" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22961" PostTypeId="2" ParentId="22959" CreationDate="2017-09-09T10:29:48.737" Score="1" Body="&lt;p&gt;You are describing the SSME. It is (was) a staged combustion engine with two fuel-rich preburners.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RTaou.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RTaou.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your concept may differ, of course, but your question is not detailed enough to determine that.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-09-09T10:38:38.433" LastActivityDate="2017-09-09T10:38:38.433" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22962" PostTypeId="1" AcceptedAnswerId="22963" CreationDate="2017-09-09T12:41:48.993" Score="9" ViewCount="425" Body="&lt;p&gt;Have there been individual satellites that have been deployed into Earth orbit (or beyond) and recovered safely back to Earth's surface more than twice?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Deployed&lt;/em&gt; means it's not a launch vehicle itself, and has no substantial propulsive capability beyond attitude control and minor adjustments. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Recovered&lt;/em&gt; also rules out anything that can re-enter the atmosphere by itself, so no space planes or shuttles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Satellite&lt;/em&gt; means that it should pass the &quot;Does it look like a satellite and quack like a satellite?&quot; test. It has a battery and/or solar panels, at least a small computer, and can actively control its own attitude (ADCS), and does something useful. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-09T13:50:05.970" LastActivityDate="2021-05-14T17:31:34.913" Title="Have there been individual satellites that have been deployed and recovered more than twice?" Tags="|artificial-satellite|payload-deployment|recovery|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="22963" PostTypeId="2" ParentId="22962" CreationDate="2017-09-09T13:12:03.427" Score="10" Body="&lt;p&gt;There have been a few. One notable one was &lt;a href=&quot;http://www.deutsches-museum.de/en/collections/transport/astronautics/satellites/astro-spas/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ASTRO-SPAS&lt;/a&gt;.  This satellite was deployed from a space shuttle, flew independently making astronomical observations, and was retrieved on four separate missions.&lt;/p&gt;&#xA;&lt;p&gt;For two missions it carried the primary instrument &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/orfeus-spas.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;ORFEUS&lt;/a&gt; (Orbiting &amp;amp; Retrievable Far &amp;amp; Extreme UV Spectrometer), on STS-51 and STS-80.&lt;/p&gt;&#xA;&lt;p&gt;For the other two missions the primary instrument was &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/crista-spas.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;CRISTA&lt;/a&gt; (Cryogenic Infrared Spectrometers &amp;amp; Telescopes for the Atmosphere), on STS-66 and STS-85.&lt;/p&gt;&#xA;&lt;p&gt;Here is a picture of the ASTRO-SPAS being deployed on STS-66.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ekT0S.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ekT0S.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Here it is flying free&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0olTK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0olTK.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Link to a video of the SPAS being grappled on STS-85.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://web.archive.org/web/20080328134116/http://spaceflight.nasa.gov/gallery/video/shuttle/sts-85/mpg/sts85fdh09-1.mpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://web.archive.org/web/20080328134116/http://spaceflight.nasa.gov/gallery/video/shuttle/sts-85/mpg/sts85fdh09-1.mpg&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The ASTRO-SPAS is now displayed in the Deutsches Museum in Munich.&lt;/p&gt;&#xA;&lt;p&gt;Other shuttle-deployed examples include the &lt;a href=&quot;https://en.wikipedia.org/wiki/Wake_Shield_Facility&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wakeshield&lt;/a&gt; (two free flights, a third attempt remained grappled due to a failure)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8jaHA.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8jaHA.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;and, as pointed out by &lt;a href=&quot;https://space.stackexchange.com/users/12102/uhoh&quot;&gt;uhoh&lt;/a&gt;, probably the all time champion, &lt;a href=&quot;https://science.nasa.gov/missions/spartan&quot; rel=&quot;nofollow noreferrer&quot;&gt;SPARTAN-201&lt;/a&gt; with five free flights.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HjzYI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HjzYI.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Because these satellites flew so often, they were used as generic payloads in the Space Shuttle Remote Manipulator (i.e. robot arm) simulator training flows.  For the sake of nostalgia, here's a cheat sheet I made when I was learning to grapple free-flying SPASes and SPARTANs.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4v8GD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4v8GD.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-05-14T17:31:34.913" LastActivityDate="2021-05-14T17:31:34.913" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="22964" PostTypeId="1" AcceptedAnswerId="22967" CreationDate="2017-09-09T13:46:46.037" Score="6" ViewCount="1241" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/22954/12102&quot;&gt;this answer&lt;/a&gt; I read about the Inflatable Antenna Experiment that flew with &lt;a href=&quot;http://www.americaspace.com/2014/05/24/mission-for-science-and-technology-18-years-since-sts-77-part-1/&quot; rel=&quot;noreferrer&quot;&gt;STS-77&lt;/a&gt;. According to &lt;a href=&quot;https://spinoff.nasa.gov/spinoff1996/13.html&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Packed into the Spartan, the IAE weighed only 132 pounds; on deployment (inflation), it would expand to an antenna &lt;strong&gt;50-feet in diameter mounted on three 92-foot struts&lt;/strong&gt;. The IAE was developed by L'Garde Inc., Tustin, California and Jet Propulsion Laboratory under NASA's In-Space Technology Experiment Program. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The box at the focal point of the antenna is the satellite &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/spartan-201.htm&quot; rel=&quot;noreferrer&quot;&gt;Spartan-201&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am having difficulty figuring out what I'm even looking at in these images. How is the structure of this expanding? It seems to consist of three long struts and a circular ring. Is the whole thing one giant balloon? If so, what is it made of and how was it inflated? If not, then what is going on here?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Images from Spacefacts.de's article &lt;a href=&quot;http://www.spacefacts.de/mission/english/sts-77.htm&quot; rel=&quot;noreferrer&quot;&gt;International Flight No. 186; STS-77, Endeavour (11)&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZBwuA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZBwuA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/U8Lnc.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/U8Lnc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jpJ5W.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jpJ5W.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-09T13:56:04.317" LastActivityDate="2017-09-09T15:06:16.897" Title="How did this structure fit into the Shuttle and then expand to 28 meters?" Tags="|artificial-satellite|experiment|antenna|expandable|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22965" PostTypeId="2" ParentId="22964" CreationDate="2017-09-09T14:20:22.813" Score="2" Body="&lt;p&gt;You're initial suspicions are correct. It is basically a big balloon. Here is why.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although information is sparse, let's establish one thing: Spartan was just a carrier platform. It's sort of an interesting platform in that it has absolutely zero command and control. After release it executes its mission, is recovered, and the results are pulled off it after it is on the ground. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spartan was just used to carry the inflatable antenna experiment. Antennas in space generally have an incentive to be physically large. Roughly speaking, a physically larger antenna will have higher gain. Gain is a phenomenon of antenna design. An ideal isotropic antenna radiates power equally in all directions. For a spacecraft communicating with Earth this is incredibly poor. Even a simple antenna design that radiates energy only in the 180 degrees toward the Earth would be twice as effective as an ideal antenna. You could use a poor antenna with a powerful transmitter, but this requires larger solar panels and a bigger battery. This increase the payload launch weight even more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Real antennas with real gain have a huge number of variables to deal with. A television satellite antenna is often designed to radiate energy only towards the part of the Earth where viewers are expected to be. A deep space probe uses a high gain antenna, but just points at the Earth. The apparent size of Earth is extremely small, but by keeping pointed at Earth it can radiate more energy back towards Earth. This raises the Signal-to-Noise ratio. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shannon-Hartley theorem&lt;/a&gt; establishes that a higher Signal to Noise ratio enables more information to be communicated in the same communications channel. It doesn't matter if you're 10 feet from the receiver or 10 light years away, this theorem holds true.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;High gain antennas onboard a satellite usually have a complicated system that very slowly expands a tightly packed high gain antenna to its final shape (usually a parabola) once the satellite is in orbit. The antenna is packed because it is physically large, but not very dense. Trying to launch a high gain antenna onboard a satellite that was already in its normal parabolic shape would require an unrealistically payload fairing on the final stage of the rocket. &lt;strong&gt;A failure of the deployment system can render the satellite useless&lt;/strong&gt;. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Galileo spacecraft&lt;/a&gt; high gain antenna never deployed properly. It got stuck while trying to assume a parabolic shape. As a result the engineers opted to use the low-gain antenna for all communications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cfyGB.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cfyGB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find some more information about this type of antenna on one manufacturer's website. &lt;a href=&quot;https://www.harris.com/solution/unfurlable-mesh-reflector-antennas&quot; rel=&quot;nofollow noreferrer&quot;&gt;Unfurlable Mesh Reflector Antennas&lt;/a&gt; is the term that Harris Corporation uses to refer to their product. There are many suppliers, I just have seen one of their antennas in person before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An inflatable antenna could be much simpler! A small gas generator could slowly raise the pressure inside the antenna until the antenna assumes the desired shape. I am assuming this experiment used a &lt;a href=&quot;https://en.wikipedia.org/wiki/Gas_generator&quot; rel=&quot;nofollow noreferrer&quot;&gt;gas generator&lt;/a&gt;, I don't actually know this.  for This gas generator is probably lighter than the mechanical means. For the same weight, a satellite could carry a much larger inflatable antenna. This is very important if you are developing a surveillance satellite. A surveillance satellite needs as much gain as possible to detect faint signals that are radiated from transmitters on Earth. One example case for this type of system is &lt;a href=&quot;https://en.wikipedia.org/wiki/ECHELON&quot; rel=&quot;nofollow noreferrer&quot;&gt;ECHELON&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The antenna you see unfurling in the images is a common parabolic design. You can think of it is as a cut out section of a sphere. It's no different than the small dishes used on peoples home to receive satellite television&lt;a href=&quot;https://en.wikipedia.org/wiki/Parabolic_antenna&quot; rel=&quot;nofollow noreferrer&quot;&gt;There is more information here on Wikipedia about parabolic antennas.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in theory inflatable antennas should be used by almost every communication satellite. What I can't find information on is the long term practicality of such an antenna. There are a number of issues with such an antenna. Primarily, micrometeorites and space debris could puncture the antenna's structure. A few tiny holes might not matter. Most satellite carry some sort of propellant anyways, so they could use some of that to keep the antenna inflated in the presence of small leaks. Eventually you'd have so many leaks that you couldn't keep it inflated no matter how much gas you have. So the actual lifespan of such an antenna might be much lower than the commercially available products on the market at present.&lt;/p&gt;&#xA;" OwnerUserId="14488" LastActivityDate="2017-09-09T14:20:22.813" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22966" PostTypeId="2" ParentId="21971" CreationDate="2017-09-09T14:29:53.327" Score="3" Body="&lt;p&gt;Most of the time Remove Before Flight mechanisms disconnect the batteries from everything else. The system might still obtain some power though the solar panels, or via an external connector (Still on the ground), but will not be able to power themselves via the batteries. The ones that I have seen are purely mechanical in nature. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-09-09T14:29:53.327" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22967" PostTypeId="2" ParentId="22964" CreationDate="2017-09-09T14:58:26.213" Score="10" Body="&lt;p&gt;Yes, the whole strut-and-lens system was a giant balloon. There is good information in &lt;a href=&quot;http://www.lgarde.com/assets/content/files/publications/aiaa-98-2104.pdf&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Folded up and deflated, it looked like this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/g3H9R.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/g3H9R.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This fit into a box mounted on the side of the SPARTAN-201 carrier bus. The &quot;balloon&quot; was made of Mylar.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The basic deployment scheme for IAE was based on ejecting the stowed&#xA;  reflector structure, as a package, away from its launch container&#xA;  prior to initiation of its own deployment.  This way the&#xA;  inertially-loaded reflector structure would essentially “pull” the&#xA;  struts out of their launch containers in a near uniform manner. When&#xA;  the struts were stretched to about 80% of their deployed length, a gas&#xA;  flow path would be developed and at that time inflation gas was to be&#xA;  introduced to all three struts at their intersection with the canister&#xA;  structure. While the struts were being pulled from their pods, a low&#xA;  rate of deployment of the lenticular structure would be initiated from&#xA;  the release of strain energy from the inflatable materials and a small&#xA;  amount of residual gas in both the torus and lenticular structure. The&#xA;  plan was that when the struts were nearly completely deployed,&#xA;  inflation gas would be introduced to the already partially deployed&#xA;  reflector structure in order to complete its deployment.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There were problems encountered:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On orbit the magnitude of residual gas and the strain energy in the&#xA;  stowed structure was significantly more than  anticipated.  As  a &#xA;  consequence,  the   planned deployment sequence did not materialize.&#xA;  Instead, the reflector  structure  deployed  prematurely  so  that &#xA;  its planned ejection away from the canister did not take place. By the&#xA;  time the struts migrated from their pods as a consequence of residual&#xA;  gas and material strain energy, the reflector structure was over half&#xA;  deployed. However, due to the robust nature of this type of space&#xA;  structure, the torus and two of the struts completed deployment at&#xA;  about the same time and complete deployment of the third strut&#xA;  followed within a minute or two.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All quotes and image are from the linked paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Former Flight Director and Shuttle Program Manager Wayne Hale has a interesting &lt;a href=&quot;https://blogs.nasa.gov/waynehalesblog/2010/05/26/post_1274899149308/&quot; rel=&quot;noreferrer&quot;&gt;blog post&lt;/a&gt; describing the travails encountered by this payload.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-09-09T15:06:16.897" LastActivityDate="2017-09-09T15:06:16.897" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22968" PostTypeId="1" AcceptedAnswerId="22978" CreationDate="2017-09-09T15:03:09.940" Score="2" ViewCount="335" Body="&lt;p&gt;When was the idea behind the &lt;a href=&quot;https://descanso.jpl.nasa.gov/monograph/mono.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;DESCANSO Book Series&lt;/a&gt; first conceived, by whom, and what is its &amp;quot;mission&amp;quot;?&lt;/p&gt;&#xA;&lt;p&gt;When I'm looking for information on deep space issues including communications and orbital mechanics (especially Volume 2), I often run across troves of goddies in an on-line copy of some article from a DESCANSO Book.&lt;/p&gt;&#xA;&lt;p&gt;Putting all of this scholarly and practical information together is a significant amount of work, and for many organizations publishing the &amp;quot;company goodies&amp;quot; would be the last thing anyone would consider. But NASA has always taken the high road here.&lt;/p&gt;&#xA;&lt;p&gt;In this case, what was the origin of this project? Who decided to do it (person? committee? mandate?), when, and why?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Deep Space Communications and Navigation Series&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Joseph H. Yuen, Editor-in-Chief&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Deep Space Communications and Navigation Series, authored by scientists and engineers with many years of experience in their respective fields, lays a foundation for innovation by communicating state-of-the-art knowledge in key technologies. The series also captures fundamental principles and practices developed during decades of deep-space exploration at JPL. In addition, it celebrates successes and imparts lessons learned. Finally, the series will serve to guide a new generation of scientists and engineers.&lt;/p&gt;&#xA;&lt;p&gt;*Hard copies of the DESCANSO books are now published by John Wiley &amp;amp; Sons.&lt;/p&gt;&#xA;&lt;p&gt;*This series has been translated into Chinese by Tsinghua University, Beijing, China.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;pre&gt;&lt;code&gt;Volume 1&#xA;    Radiometric Tracking Techniques for Deep-Space Navigation&#xA;        Catherine L. Thornton  (About the Author)&#xA;        James S. Border  (About the Author)&#xA;            Full Version (pdf - 2.5 MB)&#xA;Volume 2&#xA;    Formulation for Observed and Computed Values of Deep Space Network Data Types for Navigation&#xA;        Theodore D. Moyer  (About the Author)&#xA;            Full Version (pdf - 3.0 MB)&#xA;Volume 3&#xA;    Bandwidth-Efficient Digital Modulation with Application to Deep-Space Communications&#xA;        Marvin K. Simon  (About the Author)&#xA;            Full Version (pdf - 2.3 MB)&#xA;Volume 4&#xA;    Large Antennas of the Deep Space Network&#xA;        William A. Imbriale (About the Author)&#xA;            Full Version (pdf - 23.9 MB)&#xA;Volume 5&#xA;    Antenna Arraying Techniques in the Deep Space Network&#xA;        David H. Rogstad (About the Author)&#xA;        Alexander Mileant (About the Author)&#xA;        Timothy T. Pham (About the Author)&#xA;            Full Version (pdf - 1.56 MB)&#xA;Volume 6&#xA;    Radio Occultations Using Earth Satellites: A Wave Theory Treatment&#xA;        William G. Melbourne&#xA;            Full Version (pdf - 7.87 MB)&#xA;Volume 7&#xA;    Deep Space Optical Communications&#xA;        Hamid Hemmati, Editor (About the Editor)&#xA;            Full Version (pdf - 22 MB)&#xA;Volume 8&#xA;    Spaceborne Antennas for Planetary Exploration&#xA;        William A. Imbriale, Editor (About the Editor)&#xA;            Full Version (pdf - 24 MB)&#xA;Volume 9&#xA;    Autonomous Software-Defined Radio Receivers for Deep Space Applications&#xA;        Jon Hamkins, Editor (About the Editor)&#xA;        Marvin K. Simon, Editor (About the Editor)&#xA;            Full Version (pdf - 3.5 MB)&#xA;Volume 10&#xA;    Low-Noise Systems in the Deep Space Network&#xA;        Macgregor S. Reid, Editor (About the Editor)&#xA;            Full Version (pdf - 14.7 MB)&#xA;Volume 11&#xA;    Coupled-Oscillator Based Active-Array Antennas&#xA;        Ronald J. Pogorzelski (About the Author)&#xA;        Apostolos Georgiadis (About the Author)&#xA;            Full Version (pdf - 6.96 MB)&#xA;Volume 12&#xA;    Low-Energy Lunar Trajectory Design&#xA;        Jeffrey S. Parker (About the Author)&#xA;        Rodney L. Anderson(About the Author)&#xA;            Full Version (pdf - 7.7 MB)&#xA;Volume 13&#xA;    Deep Space Communications&#xA;        Jim Taylor, Editor (About the Editor)&#xA;            Full Version (pdf - 20.45 MB)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-13T00:14:15.970" LastActivityDate="2021-09-13T00:14:15.970" Title="When was the idea behind the DESCANSO Book Series first conceived, by whom, and what is its &quot;mission&quot;?" Tags="|nasa|history|spaceflight-firsts|jpl|" AnswerCount="1" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="22970" PostTypeId="2" ParentId="20229" CreationDate="2017-09-09T16:30:07.340" Score="1" Body="&lt;p&gt;You wouldn't see a 2nd stage tumbling and visible. Odds are pretty high that what you saw was some kind of an aircraft. The best site to see visible satellites, or at least the one that I use, is &lt;a href=&quot;http://heavens-above.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://heavens-above.com/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-09-09T16:30:07.340" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="22971" PostTypeId="1" CreationDate="2017-09-09T19:18:07.740" Score="1" ViewCount="519" Body="&lt;p&gt;What is required for a satellite to remain positioned 400 km above the north pole, thus allowing it to appear essentially stationary relative to the surface of the earth? Could this best be achieved by placing the satellite in a solar orbit with the same period as the earth, only 400 km above the pole?&lt;/p&gt;&#xA;" OwnerUserId="20982" LastActivityDate="2017-09-11T03:52:51.910" Title="Traveling in a circle while in orbit" Tags="|orbital-mechanics|orbit|orbital-maneuver|" AnswerCount="2" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22972" PostTypeId="2" ParentId="22971" CreationDate="2017-09-09T19:37:28.213" Score="15" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Could this best be achieved by placing the satellite in a solar orbit with the same period as the earth, only 400 km above the pole?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;No. At least, not without it continuously applying thrust against Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At a distance of only 400km above the surface, Earth's gravity (acceleration toward Earth) will still be nearly that of the value at Earth's surface (9.8 m/s/s). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you could &quot;magically&quot; place an object geostationary to Earth, 400km straight up from the North Pole and release it there, it won't stay there. Yes, technically, it will be in orbit around the Sun, just like Earth is. But, it won't be in orbit around the Earth, so will respond to Earth's gravity field with a downward acceleration of pretty much 1g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no way for an object to orbit the Earth such that it would appear to be stationary over a pole. There are orbits which allow an object to &quot;linger&quot; at high latitudes, but they are actually in highly inclined, highly elliptical orbits (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Molniya orbit&lt;/a&gt;); they can be geosynchronous - appearing in the same region of the sky most of the time, and repeatedly describing the same path in the sky (from the point of view of a &quot;fixed&quot; ground-based observer), but they are by no means geostationary.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;About Geostationary Orbits&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Gravity follows the inverse-square law; the greater the distance, the smaller the force. At low altitudes such as where the ISS orbits, an object must travel at about 7.6km/s to remain in orbit (completing one orbit about every 90 minutes). The Moon, circling the Earth once a month at a distance of about 239,000 miles, travels in its orbit at about 1km/s. At about 22,000 miles, an object would have to travel about 3km/s, completing one orbit every 24 hours. At just the right speed and distance, the orbit will be synchronous with Earth's rotation. If the orbit is also circular and at zero inclination to the equator, an object in that orbit will appear to be at a fixed point in the sky, always directly over some specific point on the Earth's surface. For more on this, see this question: &lt;a href=&quot;https://space.stackexchange.com/questions/13872/how-do-communication-satellites-remain-positioned-above-a-particular-region&quot;&gt;How do communication satellites remain positioned above a particular region?&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;this Wikipedia article&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-09-10T14:40:05.157" LastActivityDate="2017-09-10T14:40:05.157" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22973" PostTypeId="1" AcceptedAnswerId="22974" CreationDate="2017-09-10T00:42:57.663" Score="35" ViewCount="22924" Body="&lt;p&gt;I'd like to ask if anyone can, in layman's terms, &lt;em&gt;&lt;strong&gt;describe the procedure by which the Apollo 11 astronauts were able to get back to their ship?&lt;/strong&gt;&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I ask because the most accessible literature on this subject is exceptionally detailed; what seems lacking is a (relatively) plain-text explanation of how the astronauts got off the moon, and back on their ship to return to earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand the process of returning to their space-craft was multi-stage, and that it involved a craft known as the &quot;Command/Service Module.&quot; &lt;/p&gt;&#xA;" OwnerUserId="20986" LastEditorUserId="6816" LastEditDate="2017-09-10T04:02:30.250" LastActivityDate="2020-02-29T09:59:52.940" Title="How did the Lunar Module dock with the rest of Apollo 11 and what is the &quot;CSM&quot;?" Tags="|orbital-mechanics|the-moon|apollo-program|history|docking|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22974" PostTypeId="2" ParentId="22973" CreationDate="2017-09-10T01:08:43.497" Score="81" Body="&lt;p&gt;The &lt;strong&gt;Apollo spacecraft&lt;/strong&gt; consists of three major parts:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;strong&gt;Command Module (CM)&lt;/strong&gt;, a conical module where the three crew members live during launch from Earth and travel to and from the moon, and which re-enters Earth's atmosphere alone at the end of the trip;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;strong&gt;Service Module (SM)&lt;/strong&gt;, a cylindrical section containing fuel, power, life support, communications, a big rocket engine, and other components; &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &lt;strong&gt;Lunar Module (LM)&lt;/strong&gt;, a &quot;spidery-looking&quot; craft which lands on the moon carrying two of the crew members. The LM in turn is made up of a lower section called the descent stage and an upper section, the ascent stage. Both stages land on the moon, and only the ascent stage returns. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The Command and Service Modules remain mated for all but the very last minutes of the mission, so the combination is referred to as the &lt;strong&gt;Command/Service Module&lt;/strong&gt; or &lt;strong&gt;CSM&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/seK73.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/seK73.jpg&quot; alt=&quot;Diagram showing scale and capacity of the separate Apollo modules with the astronauts each carries&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During launch (which I won't detail) the LM is tucked behind the CSM under a conical fairing attached to the Saturn V 3rd stage (the &lt;strong&gt;S-IVB&lt;/strong&gt;). The S-IVB is the part that propels the spacecraft out of low Earth orbit and on its way to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OAZ8H.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OAZ8H.jpg&quot; alt=&quot;Schematic of CSM, LM, and S-IVB configuration in LEO&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fairing opens up and the CSM detaches, turns around, and docks to the LM. The front of the CM has a hatch that mates with a hatch on the top of the LM. After docking, the CSM pulls the LM away from the S-IVB. This sequence is called the &lt;a href=&quot;https://space.stackexchange.com/questions/3995/reasons-behind-the-transposition-docking-and-extraction-maneuver&quot;&gt;transposition, docking, and extraction&lt;/a&gt; maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YjYys.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YjYys.gif&quot; alt=&quot;Transposition-docking-extraction maneuver&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The S-IVB and spacecraft then go their separate ways and the docked CSM/LM coasts to the moon, over a flight of about 3 days. During the flight to and from the moon, the spacecraft would be rotating about its long axis to maintain even heating from sunlight (this is the PTC, &quot;passive thermal control&quot; mode, sometimes known as the &quot;barbecue roll.&quot;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WSxhI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WSxhI.png&quot; alt=&quot;Photo of lego model of docked CSM and LM&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the spacecraft reaches the moon, the large engine on the Service Module is used to enter a circular orbit around the moon. The commander and LM pilot enter the LM, undock, and use the engine on the descent stage to reach the surface of the moon. The CM pilot remains in orbit around the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/c3wVG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/c3wVG.jpg&quot; alt=&quot;Diagram showing angle, thrust, and altitude at various points before touchdown&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LM stays on the lunar surface for between one and three days, with the commander and LMP generally doing one &lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_activity#Apollo_lunar_EVA&quot; rel=&quot;noreferrer&quot;&gt;EVA&lt;/a&gt; per day, while the CSM in orbit overhead circles the moon every 2 hours. At the end of the stay, they discard unnecessary equipment (like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Primary_Life_Support_System&quot; rel=&quot;noreferrer&quot;&gt;big life-support backpacks&lt;/a&gt;). To return, the LM ascent stage lifts off, using the descent stage as a launch pad, and arcs into orbit around the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0el91.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0el91.gif&quot; alt=&quot;Video from prepositioned camera as ascent stage lifts off&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once in lunar orbit, the ascent stage meets up with the CSM (&lt;a href=&quot;https://space.stackexchange.com/questions/14172/how-did-the-apollo-11s-eagle-ascent-stage-find-the-csm-after-leaving-the-moon&quot;&gt;discussed a bit here&lt;/a&gt;), and docks again, and the crew return to the Command Module. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NzXCG.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NzXCG.jpg&quot; alt=&quot;Schematic of docked CSM and LM&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The LM ascent stage is discarded, and the CSM fires the big engine again to return to Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The back side of the Command Module sports a thick heat shield; detaching from the Service Module, the CM turns blunt-end-first and hits Earth's atmosphere, using the resistance of the atmosphere to slow down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6SCmL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6SCmL.jpg&quot; alt=&quot;Artist&amp;#39;s rendition of reentry&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And finally deploys parachutes to slow down for a safe splash-down in the ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Kay7s.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Kay7s.jpg&quot; alt=&quot;Picture from above of two out of three parachutes and the capsule at the moment of splashdown&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is of course a rather loose overview of a very complex process! Several &lt;a href=&quot;/questions/tagged/apollo-program&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;apollo-program&amp;#39;&quot; rel=&quot;tag&quot;&gt;apollo-program&lt;/a&gt; Q/As here go into various aspects in more detail, so you might enjoy perusing them.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-04-25T22:05:39.193" LastActivityDate="2018-04-25T22:05:39.193" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="22975" PostTypeId="2" ParentId="22971" CreationDate="2017-09-10T02:58:55.373" Score="2" Body="&lt;p&gt;4,000,000km but not 400km.  Even if you ignore Earth there's no solar orbit that stays 400km above the North pole.  It's going to swing back and forth over the course of a year, from 400km above the North Pole to 400km above the South Pole.  In practice Earth's gravity would rapidly pull it down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A million km up it's possible to have a pole-hovering satellite but it's &lt;strong&gt;not&lt;/strong&gt; actually in orbit, but using a solar sail to hover.  That can only be done when you're far enough out that the minuscule force of a solar sail is enough to counteract the gravity of the body you are trying to hover over.  This actually has a practical use--you could put a TV satellite over the pole to serve customers too far north to be served by the equatorial satellites.  The patent has expired, you're free to build one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit:  The patent:  &lt;a href=&quot;https://www.google.com/patents/US5183225&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.google.com/patents/US5183225&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2017-09-11T03:52:51.910" LastActivityDate="2017-09-11T03:52:51.910" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22976" PostTypeId="1" CreationDate="2017-09-10T06:14:51.527" Score="8" ViewCount="2604" Body="&lt;p&gt;The term &lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt; seen in discussions of nodal precession of orbits (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;sun-synchronous&lt;/a&gt;) appears to come in two flavors. For example, in the Wikipedia article &lt;a href=&quot;https://en.wikipedia.org/wiki/Geopotential_model&quot; rel=&quot;noreferrer&quot;&gt;Geopotential model&lt;/a&gt; it has a value of &lt;span class=&quot;math-container&quot;&gt;$1.7555 \times 10^{10} km^5s^{-2}$&lt;/span&gt;; in MKS units that's &lt;span class=&quot;math-container&quot;&gt;$1.7555 \times 10^{25} m^{5}s^{-2}$&lt;/span&gt;. It is also listed in the alternate, unitless form as &lt;span class=&quot;math-container&quot;&gt;$\tilde{J_2}=1.0826 \times 10^{-3}$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;The article says that when many coefficients in the spherical harmonic expansion are used, they are normalized to the representative radius of the body &lt;em&gt;and&lt;/em&gt; its standard gravitational parameter, in this case the Earth. It gives the expression:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\tilde{J_n}=-\frac{J_n}{\mu R^n}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where I believe &lt;span class=&quot;math-container&quot;&gt;$\mu$&lt;/span&gt; is the standard gravitational parameter also written as the product &lt;span class=&quot;math-container&quot;&gt;$GM$&lt;/span&gt;, and &lt;span class=&quot;math-container&quot;&gt;$R$&lt;/span&gt; is the normalizing radius.&lt;/p&gt;&#xA;&lt;p&gt;I also see Earth's &lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt; written in normalized form, as in this article about the &lt;a href=&quot;https://ipnpr.jpl.nasa.gov/progress_report/42-196/196C.pdf&quot; rel=&quot;noreferrer&quot;&gt;JPL Ephemerides DE430 &amp;amp; DE431&lt;/a&gt;, Tables 9 and 10 for the Sun and Earth parameters are shown below.&lt;/p&gt;&#xA;&lt;p&gt;Since conversion between these two representation requires two separate numerical values (the characteristic radius defined by convention and the standard gravitational parameter) one of them can be used without &lt;span class=&quot;math-container&quot;&gt;$R$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$GM$&lt;/span&gt;, but the other requires the value for both to be stated explicitly or implied.&lt;/p&gt;&#xA;&lt;p&gt;Which value stands on its own, and which one is the &lt;em&gt;derived&lt;/em&gt; value? This question becomes more important when considering less familiar bodies than the Earth or the Sun (or Jupiter or Saturn), especially &lt;em&gt;irregular rocky bodies such as some asteroids&lt;/em&gt; where the definition of a radius may be difficult.&lt;/p&gt;&#xA;&lt;p&gt;The answer may depend on the method of measurement. I am not sure if &lt;span class=&quot;math-container&quot;&gt;$GM$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt; (the one with units) can be measured separately, or if it is a radiometric determination.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;extra credit:&lt;/em&gt; To make sure I understand, is &lt;span class=&quot;math-container&quot;&gt;$\tilde{J}_{2 \ ☉} \approx -1.36 \times 10^{31} m^5s^{-2}$&lt;/span&gt;? Or is the minus sign in the equation in Wikipedia erroneous (shown above, and below, and just before Equation 10 in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Geopotential_model&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia article&lt;/a&gt;)?&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below x2:&lt;/strong&gt; From &lt;a href=&quot;https://en.wikipedia.org/wiki/Geopotential_model&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6Bkdh.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6Bkdh.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tcCMA.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tcCMA.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;a href=&quot;https://ipnpr.jpl.nasa.gov/progress_report/42-196/196C.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Zy8m3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Zy8m3.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-10-08T08:22:15.223" Title="For the mathematical relationship between J2 (km^5/s^2) and dimensionless J2 - which one is derived from the other?" Tags="|orbital-mechanics|gravity|mathematics|measurement|gravity-fields|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22978" PostTypeId="2" ParentId="22968" CreationDate="2017-09-10T15:34:23.883" Score="4" Body="&lt;p&gt;From the &lt;a href=&quot;https://descanso.jpl.nasa.gov/monograph/series1/Descanso1_all.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;foreword of Volume 1&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Deep Space Communications and Navigation Systems Center of Excellence (DESCANSO) was recently established for the National Aeronautics and Space Administration (NASA) at the California Institute of Technology’s Jet Propulsion Laboratory (JPL). DESCANSO is chartered to harness and promote excellence and innovation to meet the communications and navigation needs of future deep-space exploration.&lt;/p&gt;&#xA;&lt;p&gt;DESCANSO’s vision is to achieve continuous communications and precise navigation—any time, anywhere. In support of that vision, DESCANSO aims to seek out and advocate new concepts, systems, and technologies; foster key scientific and technical talents; and sponsor seminars, workshops, and symposia to facilitate interaction and idea exchange.&lt;/p&gt;&#xA;&lt;p&gt;The Deep Space Communications and Navigation Series, authored by scientists and engineers with many years of experience in their respective fields, lays a foundation for innovation by communicating state-of-the-art knowledge in key technologies. The series also captures fundamental principles and practices developed during decades of deep-space exploration at JPL. In addition, it celebrates successes and imparts lessons learned. Finally, the series will serve to guide a new generation of scientists and engineers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So the book series is a consequence of the &lt;a href=&quot;https://descanso.jpl.nasa.gov/function/function.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;charter&lt;/a&gt; of the Deep Space Communications and Navigation Systems Center of Excellence, which was established at JPL around 2000.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-09-10T15:34:23.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22980" PostTypeId="1" AcceptedAnswerId="24095" CreationDate="2017-09-10T19:14:57.437" Score="12" ViewCount="256" Body="&lt;p&gt;I’m aware of NOTAMS (notice to airmen) which are issued for launches of rockets, as for example mentioned in this &lt;a href=&quot;https://space.stackexchange.com/a/2936&quot;&gt;answer&lt;/a&gt;. The same seems to be the case for deorbiting second stages (see &lt;a href=&quot;https://space.stackexchange.com/a/7821&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/13174&quot;&gt;here&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On September 8, a video taken by the pilots of flight LX178 from Zurich to Singapore was released by Swiss Radio and Television (&lt;a href=&quot;https://twitter.com/Wgcdr_Enriquez/status/906266919833137158&quot; rel=&quot;noreferrer&quot;&gt;twitter&lt;/a&gt;). It appears that the pilots were surprised (first thinking of a meteor) and thus were not aware of the re-entry of Soyuz MS-04 on September 3. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are scheduled re-entries such as for Soyuz announced in advance and does it have consequences on commercial aviation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;--&#xA;external link to &lt;a href=&quot;https://www.srf.ch/news/panorama/swiss-piloten-filmen-raumschiff-2&quot; rel=&quot;noreferrer&quot;&gt;news article in German&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20992" LastActivityDate="2017-12-19T04:15:48.350" Title="Is the location and time of a scheduled re-entry (e.g. Soyuz) announced in advance?" Tags="|reentry|soyuz-spacecraft|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22981" PostTypeId="1" AcceptedAnswerId="23002" CreationDate="2017-09-10T19:55:41.190" Score="2" ViewCount="940" Body="&lt;p&gt;For example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider a prograde Earth orbit with $\mu = 3.98588738 \times 10^{14} m^3s^{-2}$ semi-major axis $a= 4 \times 10^6 m$ and an eccentricity $\epsilon = 0.3$. Let's ignore the argument of periapsis $\omega$ since the we can just rotate the input coordinates by it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a given point along that orbit, say $(2.418542, 1.626223) \times 10^6 m$ (about 126 seconds after periapsis), the eccentric anomaly is about 0.440273 radians. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can I compute the eccentric anomaly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get position from eccentric anomaly, I'm using the first two equations that I found in &lt;a href=&quot;https://space.stackexchange.com/a/4098/12102&quot;&gt;this answer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$x=a\left(\cos\tau-e\right)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$y=a\sqrt{1-e^2}\sin\tau$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But rearranging either of those equations to isolate $\tau$ gives me a value totally off from where it should be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm working in 2D if that matters. Greatly appreciate any help.&lt;/p&gt;&#xA;" OwnerUserId="20751" LastEditorUserId="12102" LastEditDate="2017-09-11T17:07:04.123" LastActivityDate="2017-09-12T08:12:19.160" Title="Given orbital parameters and position vector along that orbit, how can I determine the eccentric anomaly at that point?" Tags="|orbital-mechanics|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22982" PostTypeId="2" ParentId="22981" CreationDate="2017-09-10T22:55:38.723" Score="1" Body="&lt;p&gt;Use the &lt;a href=&quot;https://en.wikipedia.org/wiki/Eccentricity_vector&quot; rel=&quot;nofollow noreferrer&quot;&gt;eccentricity vector&lt;/a&gt;,&#xA;$$ \vec e = \frac{\vec v \times (\vec r \times \vec v)}{\mu} - \frac{\vec r}{||\vec r||} = \left( \frac1r - \frac1a \right) \vec r - \frac{\vec r \cdot \vec v}{\mu}\,\vec v$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This vector is a constant of motion. Its magnitude is the eccentricity of the orbit and its direction is along the line from the center of the central body to the periapsis point. The eccentricity vector is thus useful for determining both the eccentricity and the argument of periapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You need the argument of periapsis to compute the true anomaly given the position of some orbiting body, and you need the eccentricity to compute the eccentric anomaly given the true anomaly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To compute the true anomaly, first compute the argument of latitude, which is the sum of the true anomaly and the argument of periapsis. In 2D, the relationship between argument of latitude and the position of an orbiting body in a coordinate system that has the central body at the origin is given by $\tan u = r_y/r_x$, where $r_x$ and $r_y$ are the $x$ and $y$ coordinates of the orbiting body. Use a two argument inverse tangent (e.g., $u = \mathrm{atan2}(y, x)$) to compute $u$. Then subtract the argument of periapsis to yield the true anomaly: $\nu = u - \omega$. Finally, to compute the eccentric anomaly, use $$\tan \frac\nu2 = \sqrt\frac{1+e}{1-e} \tan\frac E2$$&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-09-11T09:04:09.967" LastActivityDate="2017-09-11T09:04:09.967" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22983" PostTypeId="1" AcceptedAnswerId="22984" CreationDate="2017-09-11T00:39:31.250" Score="4" ViewCount="888" Body="&lt;p&gt;Would it be efficient to use water as a coolant in a hydrolox rocket, then feed the steam after the water has boiled (due to heat transfer from the engine nozzle) into the combustion chamber, so as to add mass to the exhaust plume? Correct me if I'm wrong but if the thrust to weight ratio of this coolant/thrust system is higher than 1 then it allows for an improvement in the efficiency of the rocket.&lt;/p&gt;&#xA;" OwnerUserId="20995" LastEditorUserId="6816" LastEditDate="2017-09-11T01:11:25.047" LastActivityDate="2017-09-11T01:11:25.047" Title="Cooling a hydrolox rocket with Water" Tags="|rockets|engine-design|nozzle|combustion|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22984" PostTypeId="2" ParentId="22983" CreationDate="2017-09-11T01:07:13.043" Score="6" Body="&lt;p&gt;It would work, but it would be far from optimal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat recovered from the nozzle would result in steam much colder than combustion chamber temperature (after all, we never allow the nozzle to reach these temperatures, that's what cooling is all about, so the steam wouldn't heat to temperatures higher than the nozzle). That would cool the combustion products, resulting in lower performance. Additionally, we'd need to use extra energy to bring the liquid hydrogen up to temperatures of combustion, which again reduces performance per unit of mass of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using hydrogen as coolant we kill two birds with one stone - we keep the nozzle cool, and we heat the hydrogen so it performs better during combustion. The combustion creates far more than enough energy to evaporate all the liquid hydrogen, oxygen, and heat the resulting steam to the desired temperatures, but every little bit helps - the fuel being the coolant returns the energy harvested from the nozzle to the combustion chamber, then releases more, burning. If you used water, it would return the energy from the nozzle, but then it would absorb more from combustion as it's inert, and can't provide more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Simply put, if you replace water with LH2 as coolant, you're replacing the inert propellant that can't produce any extra energy, just transfer it from place to place, with propellant that not only can transfer the energy from the nozzle but also produce a lot more of it.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-09-11T01:07:13.043" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="22985" PostTypeId="1" CreationDate="2017-09-11T04:17:52.180" Score="11" ViewCount="1481" Body="&lt;p&gt;I'm especially thinking of the loss of the Columbia due to damage from chunks of insulation that came off.  I got to wondering, why is it there in the first place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course it's needed while you are preparing for launch or you would get insane boil-off.  However, why does it fly?  Why isn't it designed to peel off as the rocket lifts off the pad?  For the few minutes of flight the boil-off wouldn't be a big loss, especially since a lot of fuel is going to the engines which will negate at least some of the boil-off.  I have a hard time picturing the boil-off being anywhere near as much of a penalty as the cost of lifting that insulation.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2019-08-15T01:17:12.827" Title="Insulation on rockets--why take it to space?" Tags="|thermal|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="22986" PostTypeId="2" ParentId="22924" CreationDate="2017-09-11T06:38:22.117" Score="4" Body="&lt;p&gt;A Google search for &quot;mars thermal losses&quot; brought me to  &lt;a href=&quot;http://www.sciencemag.org/news/2014/06/no-wind-chill-mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;a Science article&lt;/a&gt;  introducing an American Meteorological Society paper: &quot;&lt;a href=&quot;http://journals.ametsoc.org/doi/full/10.1175/BAMS-D-12-00158.1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Martian Windchill in Terrestrial Terms&lt;/a&gt;&quot; by R. Osczevski.  The idea appears to be the same one that Uwe's comment hints at:  &quot;wind chill&quot; temperatures are reference temperatures that would cause the same heat loss, without winds, as the conditions you want to understand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Table 2 in the article is a table of Earth Equivalent Temperatures.  Based on a quick skim, the paper also discusses a couple of the model assumptions (which I believe to be made intentionally the same as Earthly windchill model assumptions): emissivity and internal thermal resistance, and how varying those parameters effects the Earth Equivalent Temperature.&lt;/p&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2017-09-11T06:38:22.117" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="22987" PostTypeId="2" ParentId="22981" CreationDate="2017-09-11T06:49:49.693" Score="1" Body="&lt;p&gt;This may be a little thin for an answer but maybe I or someone else can expand it from a start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Take note of the first figure in &lt;a href=&quot;https://en.wikipedia.org/wiki/Eccentric_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Wikipedia article on Eccentric Anomaly&lt;/a&gt;.  You have &lt;strong&gt;r&lt;/strong&gt; (position to point P in the diagram) as a vector.  You have &lt;em&gt;a&lt;/em&gt; and &lt;em&gt;e&lt;/em&gt; as orbital parameters, from which you can locate C and F in the figure (the center of the orbital ellipse and the focus for the central body).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you know all of those you can compute the eccentric anomaly with standard trigonometric methods.&lt;/p&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2017-09-11T06:49:49.693" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22988" PostTypeId="1" AcceptedAnswerId="22990" CreationDate="2017-09-11T10:22:54.143" Score="7" ViewCount="1365" Body="&lt;p&gt;HURCON II sounds like &lt;code&gt;DEFCON 2&lt;/code&gt;, the &lt;em&gt;2nd highest level&lt;/em&gt; of five levels of &lt;a href=&quot;https://en.wikipedia.org/wiki/DEFCON&quot; rel=&quot;noreferrer&quot;&gt;The DEFense readiness CONdition (DEFCON); an alert state used by the United States Armed Forces&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The DEFCON system was developed by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Joint_Chiefs_of_Staff&quot; rel=&quot;noreferrer&quot;&gt;Joint Chiefs of Staff&lt;/a&gt; (JCS) and &lt;a href=&quot;https://en.wikipedia.org/wiki/Unified_Combatant_Command&quot; rel=&quot;noreferrer&quot;&gt;unified and specified combatant commands&lt;/a&gt;. It prescribes five graduated levels of readiness (or states of alert) for the U.S. military. It increases in severity from DEFCON 5 (least severe) to DEFCON 1 (most severe) to match varying military situations.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Does the Kennedy Space Center have formal levels of readiness for hurricanes that match the DEFCON's 5 down to 1 escalation, or is that a bit of humor mixed with the very serious message?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NPR's &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2017/09/10/550003276/riding-out-irma-on-floridas-space-coast-and-keeping-on-eye-on-the-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;Riding Out Irma On Florida's Space Coast — And Keeping On Eye On The Spacecraft&lt;/a&gt; is worth a quick read.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;NASA's Kennedy Space Center, shown on Saturday, closed ahead of the arrival of Hurricane Irma as a crew of 120 arrived to ride out the storm.&quot; From &lt;a href=&quot;http://www.npr.org/sections/thetwo-way/2017/09/10/550003276/riding-out-irma-on-floridas-space-coast-and-keeping-on-eye-on-the-spacecraft&quot; rel=&quot;noreferrer&quot;&gt;NPR&lt;/a&gt;.&lt;br&gt;&#xA;Bruce Weaver/AFP/Getty Images&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YmPkt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YmPkt.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; DEFCON levels, with 1 being the highest level of alert. From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Dc_all_five.svg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/e8MBd.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/e8MBd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-11T11:59:13.907" LastActivityDate="2019-09-02T11:29:54.613" Title="What is KSC's HURCON II?" Tags="|nasa|safety|weather|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22989" PostTypeId="2" ParentId="22985" CreationDate="2017-09-11T10:29:25.777" Score="12" Body="&lt;p&gt;The insulation's job is also to prevent ice from forming on the rocket's skin (and breaking off during ascent). Shards of ice are more dangerous than chunks of foam. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if ice were no problem, for the Space Shuttle it'd be very difficult to design insulation that comes away cleanly from the tank without hitting the orbiter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For classic rocket designs, this is much easier. The second stage of &lt;a href=&quot;https://space.stackexchange.com/questions/2532/ariane-4-shroud-around-stage&quot;&gt;Ariane 4&lt;/a&gt;, for example, used insulation that's rigged to fall away at liftoff. This stage had precooled, but non-cryogenic propellants, so no icing risk but they wanted to avoid the propellants warming up (and expanding) before launch.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KhY78.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KhY78.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third stage (LOX/LH) used non-removable insulation tiles, though (the dark-gray/brown area). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that this rocket used both solutions, the tradeoff of permanent insulation for the third stage vs. removal on liftoff and accepting the ice buildup is likely that permanent insulation gives better performance.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-09-11T10:29:25.777" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="22990" PostTypeId="2" ParentId="22988" CreationDate="2017-09-11T12:04:00.407" Score="10" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/HURCON&quot; rel=&quot;nofollow noreferrer&quot;&gt;HURCON levels range from I to V and are properly defined by the USAF.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The KSC uses these levels as well, as outlined by the &lt;a href=&quot;https://prod.nais.nasa.gov/eps/eps_data/128659-SOL-001-011.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;KENNEDY SPACE CENTER&#xA;AND CAPE CANAVERAL AIR FORCE STATION CONSOLIDATED COMPREHENSIVE EMERGENCY MANAGEMENT PLAN (CCEMP)&quot;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan establishes that both the space center and air force base are subject to very similar threats and presents a joint plan to combat them.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The CCEMP combines NASA and CCAFS requirements into a single document.  [...] The CCEMP applies to all NASA and CCAFS contractor and subcontractor&#xA;  organizations, USAF, and NASA personnel and other Federal agencies and contractors operating within the boundaries of CCAFS and KSC.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Hurricanes are identified as a major threat to both bases in Section 4 of the Executive summary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Section 4.D of the Plan deals with mitigation of Hurricanes (page 27). 4.D (2) reads:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The KSC Director and the 45th Space Wing Commander will jointly establish the&#xA;  hurricane condition (HURCON) &lt;strong&gt;applicable to the Spaceport and the Florida Annexes&lt;/strong&gt; to maximum extent possible. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Highlight mine).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;HURCON is applied to the spaceport.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.patrick.af.mil/Portals/14/documents/45SW%20Hurricane%20Survival%20Guide%20(Apr%202018).pdf?ver=2018-05-22-170835-280&quot; rel=&quot;nofollow noreferrer&quot;&gt;Patrick AFB offers a &quot;Hurricane Survival Guide&quot; outlining the HURCON levels as of 2018&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Hurricane Conditions (HURCONs).&lt;/strong&gt;&#xA;  A tool utilized by 45th Space Wing to ensure pre-disaster information and actions are disseminated and implemented throughout the installations. These conditions provide sufficient preparation time to safeguard personnel, aircraft, equipment, and facilities. HURCONs are based upon forecast on-set of destructive winds 58 mph (50 kts) or more at a specific time. The timeframes listed below are defined in AFMAN 10-2504, Air Force Incident Management Guidance for Major Accidents and Natural Disasters, and 45th Space Wing Installation Emergency Management Plan (IEMP) 10-2. The 45th Space Wing Command or designated representative declares HURCONs as appropriate. HURCONs descend from HURCON 5 to HURCON 1.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The tables on page 4 &amp;amp; 5 provide further information on the conditions and activities:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RciIm.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RciIm.png&quot; alt=&quot;HURCON levels&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QUu7M.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QUu7M.png&quot; alt=&quot;HURCON activities&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13867" LastEditorUserId="13867" LastEditDate="2019-09-02T11:29:54.613" LastActivityDate="2019-09-02T11:29:54.613" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="22991" PostTypeId="2" ParentId="22988" CreationDate="2017-09-11T12:45:34.937" Score="7" Body="&lt;p&gt;Just FYI, Johnson Space Center also has hurricane readiness levels.  But, because no 2 NASA centers can do anything the same, they use a different system.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;h3&gt;Level 5 - Awareness&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;Hurricane and tropical storm preparation is a continuing activity.  There should be a heightened state of awareness before and during hurricane season.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[…]&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;Level 4 - Concern&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;JSC will initiate Level 4 activities when a &lt;strong&gt;storm poses a threat to JSC within 72 hours or a storm enters the Gulf of Mexico&lt;/strong&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[…]&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;Level 3 - Preparation&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;Level 3 activities prepare JSC to be in a position to be able to close down operations in a timely and orderly manner if necessary by notifying and &#xA;  assembling the appropriate personnel and groups.&lt;br&gt;&#xA;  The Director, Center Operations (JA) will authorize &#xA;  Level 3 activities when there is a chance that a &lt;strong&gt;severe weather condition will pose a significant threat to the Center within 48 hours&lt;/strong&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[…]&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;Level 2 - Center Closure&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;Level 2 activities consist of completing all protective measures that will place the Center in a final state of preparedness.&lt;br&gt;&#xA;  Upon authorization by the Center Director, the Director of Center Operations will start Level 2 Activities to the HIMT when there is a possibility that a &#xA;  &lt;strong&gt;severe weather condition will pose a significant threat to the Center within 36 hours&lt;/strong&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;[…]&lt;/p&gt;&#xA;  &#xA;  &lt;h3&gt;Level 1 Rideout, Assessment, and Recovery (post-storm) Activities&lt;/h3&gt;&#xA;  &#xA;  &lt;p&gt;JSC will start Level 1 activities on authorization by the Center Director when severe weather conditions poses an imminent threat to the Center &#xA;  within 24 hours.&lt;br&gt;&#xA;  Activities during the storm period will be limited to:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;HIMT activities. &lt;/li&gt;&#xA;  &lt;li&gt;Essential emergency repairs only if they can be done without endangering the health and safety of assigned employees.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;Activities after the storm:&lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;The HIMT assessing storm damage and arranging for necessary repairs on a &#xA;  priority basis. &lt;/li&gt;&#xA;  &lt;li&gt;Recovery operations initiated by the Director, Center Operations.&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I &lt;em&gt;think&lt;/em&gt; HIMT is Hurricane Impact Management Team.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://jschandbook.jsc.nasa.gov/sma_hurricane_prep/docs/Hurricane_Checklist_MGT.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;JSC S&amp;amp;MA Hurricane Checklist&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6816" LastEditDate="2017-09-11T17:44:50.527" LastActivityDate="2017-09-11T17:44:50.527" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22992" PostTypeId="1" CreationDate="2017-09-11T17:18:03.107" Score="0" ViewCount="522" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/7780/9072&quot;&gt;This answer&lt;/a&gt; states that solar sails may deflect asteroids. I understand that the goal is to deflect an incoming asteroid enough to avoid collision. I assume painting the asteroid white may also deflect it. Let's say we discover this 10km wide asteroid on year before impact. How much can we deflect it during this year using white painting and solar sails? Is it enough to avoid collision?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT&lt;/strong&gt;: This question assumes all raw materials to build and put in place solar sails and painting are available and ready to use as I don't want to take into account all the inherent preparation steps to space missions.&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="18206" LastEditDate="2017-09-14T17:54:31.510" LastActivityDate="2018-06-28T22:04:48.357" Title="How effective are solar sails to deflect an asteroid?" Tags="|asteroid|solar-sail|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="22993" PostTypeId="2" ParentId="22992" CreationDate="2017-09-11T19:37:46.767" Score="5" Body="&lt;p&gt;Simply white-painting an asteroid probably isn't enough to avoid a predicted collision in typical cases according to my back-of-the-envelope math.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 10km diameter asteroid has a mass of around 1e15kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Radiation_pressure#Pressures_of_absorption_and_reflection&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, at 1 AU from the sun (i.e. in an orbit near that of Earth) you get about 9 µN/m^2 of radiation pressure via perfect perpendicular reflection. The cross section of the asteroid is about 8e7 m^2; so if this was a perfect flat mirror facing the sun you'd get 720 N of force. I'm too lazy to work out the corresponding pressure for a sphere, so let's say it's 100 N. That force divided by the mass of the asteroid gets you 1e-13 m/s^2 of acceleration. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 year lead time is about 3e7 seconds; d = 1/2 at^2 gives you a displacement of around 45 meters. This is well inside the error bars for prediction of an asteroid's trajectory that far out; you wouldn't even know if you'd made things better or worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The smaller the asteroid, the bigger the impact of solar pressure -- mass varies as the cube of the diameter, but cross section and thus pressure varies as the square, so the displacement goes inverse-linear with the size of the asteroid. By the time you reach a possibly-significant ~500km displacement, your asteroid is a non-threatening 1m across. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'd need a solar sail vastly larger than the asteroid, say 1000km x 1000km, to be able to deflect it within a year. You'd probably do better by using nuclear warheads to nudge the asteroid.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-09-11T19:37:46.767" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="22994" PostTypeId="1" AcceptedAnswerId="22995" CreationDate="2017-09-11T20:13:15.293" Score="10" ViewCount="928" Body="&lt;p&gt;Watching the &lt;a href=&quot;https://youtu.be/QOzzdG9u7Dk?t=42m30s&quot; rel=&quot;noreferrer&quot;&gt;Amazonas5 launch&lt;/a&gt;, and at 1st stage separation, the commentator noted that the 2nd stage would ignite &lt;strong&gt;before&lt;/strong&gt; the separation in order to &quot;maintain constant acceleration.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering how this works - surely the 1st stage is still in the way, so the rocket will be pointed directly at the top of the 1st stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, if this is beneficial to the launch in some way, why don't all launches do this?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/19374/why-do-the-russians-use-these-fence-like-interstage-fairings&quot;&gt;Why do the Russians use these fence-like interstage fairings?&lt;/a&gt; is related, but not really a duplicate.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2017-09-11T21:10:13.227" LastActivityDate="2017-09-11T21:10:13.227" Title="Why did Amazonas5 2nd stage ignite before 1st stage separation?" Tags="|launch|stage-separation|" AnswerCount="1" CommentCount="1" ClosedDate="2017-09-12T07:31:05.630" ContentLicense="CC BY-SA 3.0" />
  <row Id="22995" PostTypeId="2" ParentId="22994" CreationDate="2017-09-11T20:26:18.737" Score="16" Body="&lt;p&gt;The initial exhaust from the upper stage fires at the first stage and is deflected and exits through the openings in the interstitial fairings, as shown in the markup of a Proton rocket below:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YKQoy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YKQoy.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The reason for this is to maintain the ullage of the upper stage tanks (if the fuel/oxidizer slosh forward at separation they won't flow into the pumps/engine evenly and things tend to end badly).  Starting the upper motor while still under thrust is one way to ensure proper ullage, another method is to use small solid rockets to separate the stages and push the fluids back in the tanks.&lt;/p&gt;&#xA;&lt;p&gt;A related question:&#xA;&lt;a href=&quot;https://space.stackexchange.com/questions/19374/why-do-the-russians-use-these-fence-like-interstage-fairings&quot;&gt;Why do the Russians use these fence-like interstage fairings?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15824" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-09-11T20:26:18.737" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="22996" PostTypeId="2" ParentId="3868" CreationDate="2017-09-11T21:57:53.220" Score="2" Body="&lt;p&gt;This is of course theory-craft since I'm sure if satellites do exist that can resolve your face, read a license plate, or make out the time on your watch or a phone number on your iphone, we (the public) would be the last to hear about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe its worth noting that you can achieve the same with lower diameter lens simply because of how fast the satellite is moving or by quantity or both.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Explanation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2 x 13 meter telescopes taking a photo of the same object at the same time equates to the same as 1 x 18 meter telescope. 4 x 9 meter telescopes can do the same as 1 x 18 meter telescopes. I am sure there are diminishing returns, but you get the point...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 satellite at 1/2 meter lens moving at 25,400 feet per second can take 160 unique (different location) photos in 1/100th of a second. Since atmosphere, heat, and other contributors to atmospheric distortion do not change (enough) in 1/100th of a second, it's not really any different than taking 1 photo with a much larger diameter telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One may argue the technology &quot;doesn't exist to do it&quot; but the truth is it does. Consumer technology available (like the camera they use on slow mo guys) is a high resolution 10,000 fps camera. I'm gonna go out on a limb and say technology available to the Defense Departments / NSA / Etc are better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another thing to note is the same technology used for atmospheric correction (the laser that draws a false star) can be used to take satellite photos. The satellite simply needs a landmark, or something to which the shape is known to be able to adjust and remove atmospheric effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure it'd be expensive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The  NRO satellites can coast as much as $9 billion though.&lt;/p&gt;&#xA;" OwnerUserId="21005" LastActivityDate="2017-09-11T21:57:53.220" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22997" PostTypeId="1" AcceptedAnswerId="22998" CreationDate="2017-09-11T22:19:24.033" Score="25" ViewCount="5561" Body="&lt;p&gt;As SpaceX has launched both NROL-76 and OTV-5, does anyone at SpaceX know the trajectory of the vehicle, or is the falcon upper stage under the direct control of the Air Force? &lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="25" LastEditDate="2017-09-11T23:45:11.867" LastActivityDate="2017-09-12T08:00:45.537" Title="Does anyone who works at SpaceX know the paths of classified payloads?" Tags="|launch|spacex|artificial-satellite|millitary|classified|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="22998" PostTypeId="2" ParentId="22997" CreationDate="2017-09-11T23:44:50.223" Score="39" Body="&lt;p&gt;The way something like this typically works is that the customer, in this case the Air Force, calls the path classified, to some level. In the contract, the customer declares what parts are classified, and how such classified data should be handled. That will include the fact that any SpaceX employee that has access to the classified data must have the right security clearance, and the classified data must be kept on an isolated computer network. SpaceX then determines how best to meet the contract requirements, including any requirements to secure their launch permission. I believe for classified payloads they don't require FAA approval. SpaceX then reports on their findings, including simulations and such. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the launch is very classified, they might even compartmentalize the data, such that those who know the trajectory might not be able to see the satellite they are launching, to keep things even more secure. It is very likely that those who actually loaded the satellite on to the payload require a security clearance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, some employees at SpaceX must have a clearance to know what the trajectory was, as they would be too involved not to know.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2017-09-12T08:00:45.537" LastActivityDate="2017-09-12T08:00:45.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="22999" PostTypeId="1" CreationDate="2017-09-12T05:50:39.533" Score="7" ViewCount="328" Body="&lt;p&gt;When I was writing &lt;a href=&quot;https://astronomy.stackexchange.com/a/22645/7982&quot;&gt;this answer&lt;/a&gt; about the &lt;a href=&quot;https://en.wikipedia.org/wiki/LAGEOS&quot; rel=&quot;nofollow noreferrer&quot;&gt;LAGEOS&lt;/a&gt; satellites, I notices that four of the over four hundred corner reflectors are made of optical germanium rather than fused silica. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Fused silica (FS) is transparent to some wavelengths in the infrared, but it's limited. For a wider transparency window, especially at longer wavelengths, optical-grad silicon or germanium is used (e.g. FLIR cameras). Also, there are likely to be antireflection coatings which further limits the use of the FS elements away from their optimized wavelength range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was probably done &quot;just in case&quot; someone would like to try IR, and it's good thinking, but I'm wondering if these have every been used in orbit, even for testing purposes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question has a new companion: &lt;a href=&quot;https://space.stackexchange.com/q/30000/12102&quot;&gt;Why do the LAGEOS' satellites have four germanium corner cube reflectors out of over 400?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Sample output from &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Revised: Sep 11, 2017                 LAGEOS-2                         -122195&#xA;&#xA;LAGEOS-2 (1992-070B, &quot;Laser Geodynamics Satellite&quot;, NASA/ASI)&#xA;60-cm diameter sphere, 426 corner cube reflectors (VIS), 4 germanium (IR)&#xA;Deployed: 1993-Oct-22 from Space Shuttle Columbia (STS 52)&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:LAGEOS-NASA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OFHer.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OFHer.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-08-09T01:18:57.873" LastActivityDate="2018-08-09T04:39:49.997" Title="Have LAGEOS' germanium corner cube reflectors ever been used?" Tags="|earth-observation|measurement|retroreflector|" AnswerCount="0" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="23000" PostTypeId="1" CreationDate="2017-09-12T06:12:44.677" Score="5" ViewCount="169" Body="&lt;p&gt;Let us presume that we have a scenario with ballistic objects.  As an example, one object $m_1$ is launched at time $t_0$ from Earth Location $a$ towards Earth Location $b$, both of which are in the same hemisphere, positioned so that a minimum time trajectory must go over one polar region, and are between 60° and 120° apart. Then at some time $t_1$ between the first launch and its impact at $t_2$, a ballistic object $m_2$ is launched from $b$ toward the first object's launch site $a$, to impact at $t_3$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both ballistic objects are on flight paths with approximately the same eccentricity, chosen to minimise flight time as far as practical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the objects will both pass roughly over the pole, the trajectories are not superimposed since the Earth is rotating, and at closest approach the objects are a considerable distance apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, while launched from $a$ in such a manner that it should not have to use it to reach $b$, $m_1$ has an on-board capability to generate delta-V, and if it has &lt;em&gt;sufficient&lt;/em&gt; delta-V, it has the capability of targeting and manoeuvring to generate a collision with $m_2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How can I calculate the minimum amount of delta-V required to alter the trajectory of $m_1$ starting at $t_1$ so that it will impact $m_2$ at some time between $m_2$'s launch at $t_1$ and $m_1$'s impact at $m_2$'s launch site $b$ at $t_2$ or $m_2$'s impact at $m_1$'s launch site $a$ at $t_3$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_2$ should be considered to have no way of having its trajectory altered once launched.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;My thinking so far:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously, if $t_1$=$t_2$, then $m_1$'s delta-V required will be minimal.  I am more concerned with solutions in the range $t_1 = 0$ to $t_1 = t_0 +(t_2-t_0)/2$, i.e. $m_2$'s launch at $t_1$ occurring with $m_1$ half-way along its trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this is a missile counter-battery question, this is not a zero-velocity rendezvous, but should be an impact that takes place at as high a closing velocity as practical, effectively meaning that no deceleration or course-matching by $m_1$ to meet $m_2$ is necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have access to AGI's free STK 11.2 software package, but I have none of the paid add-ons that would make this question unnecessary, since I am not only short of funds, but I am also not American, and hence prohibited from receiving the most relevant of AGI's packages even as a trial due to US export restrictions, and I have also exhausted the use of trial licenses for the paid packages that I am able to receive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, I can write my own code, either stand-alone or interop with STK, but I lack knowledge of the math involved.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, how can I calculate the impact velocity and the maximum extent of the debris field in the event of a successful intercept?&lt;/p&gt;&#xA;" OwnerUserId="21009" LastEditorUserId="21009" LastEditDate="2017-09-12T16:48:35.740" LastActivityDate="2017-09-12T16:48:35.740" Title="How to calculate if a co-location intercept is possible in a ballistic counter-battery scenario" Tags="|orbital-mechanics|delta-v|rendezvous|ballistics|" AnswerCount="0" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23002" PostTypeId="2" ParentId="22981" CreationDate="2017-09-12T08:12:19.160" Score="1" Body="&lt;p&gt;From the first equation, we get $\cos\tau = \frac x a + e$. This on its own is not quite enough to find $\tau$, since a given value of $\cos\tau$ usually corresponds to two values of $\tau$ in $[0,2\pi)$. To find the correct value, we can use the fact that for a prograde orbit with the argument of periapsis $0$, $0\leq \tau\leq \pi$ if $y\geq 0$, and $\pi&amp;lt;\tau&amp;lt;2\pi$ if $y&amp;lt;0$. So we have&#xA;$$&#xA;\tau = \begin{cases}&#xA;\arccos\left(\frac x a + e\right)&amp;amp;\text{if $y\geq 0$};\\&#xA;2\pi - \arccos\left(\frac x a + e\right)&amp;amp;\text{if $y &amp;lt; 0$}.&#xA;\end{cases}&#xA;$$&#xA;Substituting the values from your example, we have $\tau = \arccos(2.418542/4 +0.3) = 0.440273$, just as it should be.&#xA;Alternatively, you can express $\tau$ in terms of $y$ from the second equation. It's better to use this approach for positions very close to the periapsis or the apoapsis as it give better precision in these cases:&#xA;$$&#xA;\tau = \begin{cases}&#xA;\arcsin\left(\frac y {a\sqrt{1-e^2}}\right)&amp;amp; \text{if $y\geq 0$ and $x\geq -ae$};\\&#xA;\pi - \arcsin\left(\frac y {a\sqrt{1-e^2}}\right)&amp;amp; \text{if $x &amp;lt; -ae$};\\&#xA;2\pi + \arcsin\left(\frac y {a\sqrt{1-e^2}}\right)&amp;amp; \text{if $y &amp;lt; 0$ and $x\geq -ae$}.&#xA;\end{cases}&#xA;$$&#xA;Again, substituting your values, we get $\tau = \arcsin(1.626223/(4\sqrt{0.91})) = 0.440273$.&lt;/p&gt;&#xA;" OwnerUserId="12448" LastActivityDate="2017-09-12T08:12:19.160" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23005" PostTypeId="1" CreationDate="2017-09-12T11:19:25.810" Score="5" ViewCount="681" Body="&lt;p&gt;How feasible would a rocket engine that ignited fuel in fast, high pressure pulses be.&#xA;This type of engine would be especially applicable for low thrust burns such as for the raising of a small satellite into a higher orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engine could use bi-propellants such as methane/oxygen and an igniter or hypergolics or a mono-propellant sprayed onto a catalyst. Such a system would inject a spray of propellant into the combustion chamber followed by the closing of the propellant valves leading into the chamber and then the ignition of the propellant. The hot gas would rush out of the chamber in a high speed pulse and then the cycle would repeat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The advantages I can see with such a system are be:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Very high possible chamber pressures and thus high exhaust velocity and ISP without heavy high propellant tanks or the impracticability of tiny high pressure gas turbines/pumps because the propellant lines into the chamber would be sealed off before each pulse cycle.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The required chamber pressure can be easily changed by increasing/decreasing the throat area and the strength of the combustion chamber walls without the need for modifying the rest of the propulsion system(Tanks, propellant feed lines etc.).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;EDIT: A simple diagram for clarification&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Bd3an.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Bd3an.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20715" LastEditorUserId="6944" LastEditDate="2017-09-13T21:16:47.817" LastActivityDate="2017-09-13T21:16:47.817" Title="Pulse Rocket Engine" Tags="|propulsion|engine-design|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23006" PostTypeId="1" AcceptedAnswerId="43312" CreationDate="2017-09-12T14:04:10.753" Score="4" ViewCount="271" Body="&lt;p&gt;Thw news.com.au article &lt;a href=&quot;https://www.news.com.au/technology/science/space/nasa-spacecraft-to-fly-over-australia-during-earth-gravity-assist-manoeuvre/news-story/6fcf512f5a613f085b84745289dc462f&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA spacecraft to fly over Australia during Earth Gravity Assist manoeuvre&lt;/a&gt; has some interesting graphics (shown below).&lt;/p&gt;&#xA;&lt;p&gt;In the article is discussion of meteor cameras, and quotes from Professor Phil Bland of Curtin University in Western Australia, who:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;is a member of the OSIRIS-REx science team and also leads an Australian group called the Desert Fireball Network, which, as he puts it, &amp;quot;tracks fireballs coming through the atmosphere.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question(s):&lt;/strong&gt; How close will OSIRIS-REx get to Australia? Will it be bright enough to be seen by cameras used to record meteors? Is there any way quantitatively (gu)estimate the brightness as a sanity check here?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I'll reiterate that. Can the satellite be &lt;em&gt;be seen by cameras used to record meteors?&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &amp;quot;The route OSIRIS-REx will take over Australia&amp;quot; From news.com.au's  &lt;a href=&quot;http://www.news.com.au/technology/science/space/nasa-spacecraft-to-fly-over-australia-during-earth-gravity-assist-manoeuvre/news-story/6fcf512f5a613f085b84745289dc462f&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA spacecraft to fly over Australia during Earth Gravity Assist manoeuvre&lt;/a&gt;. Source:Twitter (likely &lt;a href=&quot;https://twitter.com/OSIRISREx/status/905579360127557632&quot; rel=&quot;nofollow noreferrer&quot;&gt;this OSIRISREx tweet&lt;/a&gt; with the classic meme theme)&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;em&gt;I don't always make a close approach of Earth, but when I do, this is the path I fly over.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/UtDvJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/UtDvJ.jpg&quot; alt=&quot;The route OSIRIS-REx will take over Australia&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2020-04-08T02:44:32.903" Title="Will they really be able to &quot;see&quot; OSIRIS-REx from Australia? With meteor cameras?" Tags="|nasa|photography|visibility-of-spacecraft|osiris-rex|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23007" PostTypeId="1" AcceptedAnswerId="23008" CreationDate="2017-09-12T16:09:28.123" Score="6" ViewCount="928" Body="&lt;p&gt;Long time ago, I was wondering which orbital element should I use to differentiate between the blue and red orientation of an otherwise same elliptical orbit on this image (all the three orbits are in an identical ecliptic plane; Sun in the center is, of course, at the foci.):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2mZJq.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2mZJq.png&quot; alt=&quot;Two orbit orientations&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After some studies I realized Euler angles are used for these purposes in the form of &lt;em&gt;inclination&lt;/em&gt; and &lt;em&gt;longitude of ascending node&lt;/em&gt;. However, longitude of a. n. is useful only when inclination is non-zero so that the nodes exist. For theoretical purposes this does not pose a problem since we are free to choose any other plane so the parameters can have definite values.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice there could be a problem for the used methods of simulation and orbit-element communication when inclination is near- or true-zero and &lt;em&gt;we are bound to use the same plane of reference&lt;/em&gt;. As it is stated in this &lt;a href=&quot;https://space.stackexchange.com/questions/9652/why-is-argument-of-periapsis-defined-in-terms-of-longitude-of-ascending-node&quot;&gt;question&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For a low-inclination, high-eccentricity orbit, the longitude of ascending node may be very uncertain (becoming undefined in the purely theoretical zero-inclination case) even though the orientation of the ellipse as projected to the reference plane is clear.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;My questions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is there a recommended method for how to avoid non-ambiguous zero-inclination cases in computer simulation? If we solve the motion equations numerically we might like to know the present orbital parameters of an object without any possibility of errors. (You can answer this also with your own experience.)&lt;/li&gt;&#xA;&lt;li&gt;How is the longitude of a. n. communicated in practice through the &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;noreferrer&quot;&gt;two-line element sets&lt;/a&gt; in zero-inclination cases?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="21014" LastActivityDate="2017-09-12T16:57:51.803" Title="Ascending node with zero inclination" Tags="|orbital-mechanics|orbital-elements|simulation|raw-data|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23008" PostTypeId="2" ParentId="23007" CreationDate="2017-09-12T16:57:51.803" Score="3" Body="&lt;p&gt;In planar orbital mechanics, the &lt;strong&gt;&lt;em&gt;argument of periapsis&lt;/em&gt;&lt;/strong&gt; is measured directly from the reference direction used. To avoid confusion with the 3D usage it is sometimes called the &lt;em&gt;orbital argument&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your questions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1.&#xA;This is not actually an issue when implementing keplerian elements, as it should be very numerically robust against floating point artefacts. A relatively large change in longitude of ascending node means close to no change in the actual orbit when the inclination is close to zero. The opposite can be a problem though, like working with eccentricity values close to 1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One issue you can encounter is that numerical comparison of orbits &quot;are these orbits approximately the same?&quot; is &lt;em&gt;very&lt;/em&gt; touchy to corner cases with angular elements. A naive implementation here (+-margins or multiplicative error margins) will perform poorly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may consider using an internal representation using state vectors, and just use the keplerian elements as a front-end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2.&#xA;You &lt;em&gt;can't&lt;/em&gt; make any assumptions about how an implementation of two-line elements handles this particular corner case. It is in principle allowed to give you any number between 0 and 360 depending on how it internally handles the numbers. You should however always expect the orbital argument to be accurate, given as a sum of line 2 field 4 and 6. (for cases with a noticeable inclination, you would have to project the argument of periapsis down on the reference plane first. This is however not useful for anything in practice as it only communicates the relationship with the reference plane, not their mutual angular separation. A single angular parameter is never sufficient.)&lt;/p&gt;&#xA;" OwnerUserId="8693" LastActivityDate="2017-09-12T16:57:51.803" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23010" PostTypeId="2" ParentId="23006" CreationDate="2017-09-12T20:14:10.580" Score="3" Body="&lt;p&gt;Yes, if you have a telescope or really good set of binoculars. The article you linked even states this:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“It’s not going to be visible to the naked eye, but really any amateur astronomer with a half decent telescope should be able to do it,” the professor said. “Or a big lens on a regular camera like a telephonic lens, they should be able to pick it up and see it themselves.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://spaceflight101.com/osiris-rex/osiris-rex-mission-profile/&quot; rel=&quot;nofollow noreferrer&quot;&gt;altitude is 20,000 km&lt;/a&gt;, give or take. That is about how high the GPS satellite constellation is, and is a bit bigger. The brightness will be  about 10, per &lt;a href=&quot;http://www.heavens-above.com/OsirisFlyby.aspx?lat=-26.7456&amp;amp;lng=120.5859&amp;amp;loc=Unspecified&amp;amp;alt=-4262&amp;amp;tz=WAST&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.heavens-above.com/OsirisFlyby.aspx?lat=-26.7456&amp;amp;lng=120.5859&amp;amp;loc=Unspecified&amp;amp;alt=-4262&amp;amp;tz=WAST&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, if you have a telescope and know exactly where to point it, you can see it. Otherwise, it's not that likely.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-09-13T01:01:12.820" LastActivityDate="2017-09-13T01:01:12.820" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="23011" PostTypeId="2" ParentId="6135" CreationDate="2017-09-12T23:32:49.913" Score="7" Body="&lt;p&gt;In addition to the points Deer Hunter makes, in reading &lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/0887307833&quot; rel=&quot;noreferrer&quot;&gt;a history of the Shuttle-Mir exchange program&lt;/a&gt;, it's evident that Mir was built with very different quality and safety standards than any American space project would accept. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, when modules were connected in Mir, electrical and ventilation conduits were routed through hatchways to connect them, making it dangerously difficult to seal off a leaking module after the &lt;a href=&quot;https://en.wikipedia.org/wiki/Progress_M-34#Collision&quot; rel=&quot;noreferrer&quot;&gt;Progress M-34 collision&lt;/a&gt; in 1997.  This is an incredibly, shamefully obvious design oversight, which could be straightforwardly addressed with more engineering (e.g. a smaller conduit interface adjacent to the hatch, where cables can be quickly disconnected in an air-tight manner), but that engineering would increase the costs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, the station's coolant loop was almost continuously leaking toxic &lt;a href=&quot;https://en.wikipedia.org/wiki/Ethylene_glycol&quot; rel=&quot;noreferrer&quot;&gt;ethylene glycol&lt;/a&gt;, and cosmonaut crews spent an absurd amount of time and effort chasing down these leaks. This suggests that the materials selected for plumbing were inappropriate, or the design or assembly process was shoddy, or all of the above. Proper design reviews, quality control, and pre-flight testing are all expensive, but would likely catch this sort of problem.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-09-12T23:32:49.913" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23012" PostTypeId="1" AcceptedAnswerId="23673" CreationDate="2017-09-13T02:42:38.390" Score="4" ViewCount="692" Body="&lt;p&gt;In preparation for another question, I typed &quot;keeping track of satellites in orbit&quot; into a search engine, and found a website called &lt;code&gt;keeptrack.space&lt;/code&gt; which has a bunch of features that look cool but I don't understand.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are dropdown menus that list some locations where I assume there is equipment that keeps track of Earth satellites. They may be a mix of radar and optical (vis and IR) but I don't know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact I don't even know what the official term for keeping track of (all of) Earth satellites is. I always thought there was some giant farm of megaWatt radar antennas in some government facility in the US and something similar in a few other big countries, but it's a vague notion, and that may be more for total awareness of space debris including the very small objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What would be a &quot;big picture&quot; understanding of how the orbits of Earth satellites are monitored? Are there a few main facilities that do this, or is it a distributed effort? Is it mostly optical, or radar, or are both roughly equally important?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For some background, a discussion of an optical vis-NIR telescope for satellite monitoring can be found in Aerospace.org's article &lt;a href=&quot;http://www.aerospace.org/crosslinkmag/web-exclusive/ground-based-telescopes-for-space-situational-awareness/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ground-Based Telescopes for Space Awareness&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; screen shots of pull-down menus from &lt;a href=&quot;http://keeptrack.space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://keeptrack.space/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9qbkw.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9qbkw.png&quot; alt=&quot;SSN Sensors&quot;&gt;&lt;/a&gt; . &lt;a href=&quot;https://i.stack.imgur.com/3JUdS.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3JUdS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;Aerotel captured this image of three actively maintained geostationary satellites (center) with another satellite nearby (lower left). For this observation, the telescope was staring at one spot, with no tracking movement. Thus, the stationary satellites appear as dots, while the background stars, which are moving at the natural sidereal rate, appear as streaks.&quot; From &lt;a href=&quot;http://www.aerospace.org/crosslinkmag/web-exclusive/ground-based-telescopes-for-space-situational-awareness/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cX2Fv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cX2Fv.jpg&quot; alt=&quot;geostationary satellites&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-01-30T07:05:42.173" LastActivityDate="2020-01-30T07:05:42.173" Title="What would be a &quot;big picture&quot; understanding of how the orbits of Earth satellites are monitored?" Tags="|artificial-satellite|tracking|two-line-elements|space-surveillance|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23013" PostTypeId="1" AcceptedAnswerId="23018" CreationDate="2017-09-13T03:47:47.763" Score="5" ViewCount="1181" Body="&lt;p&gt;My understanding is that the ISS's solar panels are silicon and double sided to maximize bang-for-the-pound (average power per kilogram transported to orbit). See &lt;a href=&quot;https://space.stackexchange.com/q/21688/12102&quot;&gt;Are the ISS US Segment solar arrays double-sided?&lt;/a&gt; and the &lt;a href=&quot;https://i.stack.imgur.com/w9tyr.jpg&quot; rel=&quot;noreferrer&quot;&gt;image&lt;/a&gt; there, and &lt;a href=&quot;https://space.stackexchange.com/q/9714/12102&quot;&gt;Why does the ISS not use the most efficient solar panels available?&lt;/a&gt; and associated images and answers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But I don't understand the appearance of this panel in the photo below. I'm assuming the dark stuff is silicon, but there are a lot of silver/gray dots and for every four of those there is a larger interstitial red dot. And the whole think looks flexible as if it were a film rather than a crystal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the structure of these devices? Are they made from amorphous flexible films of silicon, or lots of small little thin crystals of silicon on a flexible matrix of some kind? How does sunlight reach both sides - does it pass through a backing on one side?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Cropped and original image &quot;Scott Kelly fixing a cooling pump during a spacewalk.&quot; from Gizomodo's &lt;a href=&quot;http://gizmodo.com/astronaut-scott-kelly-on-liquid-salt-a-stinky-station-1755015842&quot; rel=&quot;noreferrer&quot;&gt;Astronaut Scott Kelly on Liquid Salt, a Stinky Station, and Sleeping in Freefall&lt;/a&gt;. Image credit: NASA/Kjell Lindgren&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fj28S.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fj28S.jpg&quot; alt=&quot;expanded and cropped&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JOV3F.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JOV3F.jpg&quot; alt=&quot;cropped&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; original image shown in reduced size. (right) click for full size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ugz7x.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ugz7xm.jpg&quot; alt=&quot;original&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2020-05-10T18:56:38.643" Title="How are the silicon PV cells constructed in the ISS's solar panels? Are they as flexible as they appear here?" Tags="|iss|solar-power|solar-panel|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23015" PostTypeId="2" ParentId="12634" CreationDate="2017-09-13T08:45:01.390" Score="7" Body="&lt;p&gt;First of all, the heat shields aren't there just for the dense atmosphere with high deceleration, the upper atmosphere part of reentry also generates heat. If you had a craft that could fly in the upper atmosphere it would still heat up just from its forward speed. Hypersonic vehicles also heat up considerably even though they are just normal aircraft. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird&quot; rel=&quot;noreferrer&quot;&gt;SR71 Blackbird&lt;/a&gt; reached speeds of 'only' mach 3.3 and had heated up to 300°C, so it had to be made from titanium instead of regular aluminium. Heating also increases with velocity squared. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secondly, the density of the atmosphere decreases exponentially with height. So the speed an 'aircraft' needs to fly also increases exponentially with height. On the other hand orbital speed (though not total orbital energy) &lt;em&gt;decreases&lt;/em&gt; the higher your orbit is. So the faster/higher you go the less lift you need as the closer you are to orbiting. At the Karman line at 100 km altitude both effects are roughly equal, and thus above the Karman line the speed you need to fly is higher than your orbital speed, so 'flying' there would mean shooting away from the earth. Or another way around, a stable altitude above the Karman line would imply an almost-orbit with only a very little bit contributed by lift. However there is still enough atmosphere that you need an engine to overcome the drag, so you would need to continually spend rocket fuel (no airbreathing engine is going to work at that altitude) to not drop down/descend slowly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I guess you could just use bigger wings, increasing the lift, but with this exponential thinning of the atmosphere your wing size needs to be exponential with height, so that gives problems with total drag and with weight and getting up there in the first place. Weight is also the main reason the Space Shuttle did not have bigger wings than it did, with its rather poor lift to drag ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, a shallower entry actually produces &lt;em&gt;more&lt;/em&gt; total heat for an entry vehicle to absorb. A very steep entry produces higher &lt;em&gt;peak&lt;/em&gt; heating and higher G forces, but lower total heating on the vehicle. For manned reentry vehicles shallowest possible angle of entry is limited by the total heating and the steepest angle by the maximum allowable G forces on the crew. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite all of the above, your idea works (but in the lower atmosphere) and is used. Though not to eliminate heat shields (as mentioned before shallower angle = more total heating), but to limit G forces experienced by crews. The Soyuz in normal entry mode actively controls its attitude to generate lift, giving a relatively shallow trajectory with a maximum of 5G deceleration or so. It also has an emergency ballistic mode in which the reentry vehicle is spin stabilised. In that case it does not generate lift, its descent is much steeper, and the crew experiences up to 9G deceleration. The space shuttle also made use of lift, with a maximum deceleration of 3G.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short, heating is caused by high speed, not (only) by low altitude. Entry can be done without a heat shield, but then you need to make sure you slow down enough before hitting the dense atmosphere. So that means using rockets. Reentering from orbit using rockets to break requires as much fuel as launching in the first place, so that is not practical. However the SpaceX Falcon 9 booster reaches much lower top speeds and uses this approach (the 'entry burn') to slow down and limit reentry heating before it lands. &lt;/p&gt;&#xA;" OwnerUserId="16969" LastActivityDate="2017-09-13T08:45:01.390" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23016" PostTypeId="2" ParentId="12634" CreationDate="2017-09-13T10:07:16.733" Score="8" Body="&lt;h2&gt;You can't just slow it down over many orbits&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;I think the question is suggesting letting a little bit of drag slow the Cessna down until it's at a normal speed before gliding through the atmosphere. That's nice, but it won't work. Orbiting is ballistic flight. Ballistic flight without lateral speed (i.e. once you've started to really slow the Cessna down) is just falling - your ballistic trajectory intersects the Earth's surface. There is no low-speed way to miss the Earth without lots of thrusters, which is not gliding.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Orbit isn't high, it's fast&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Orbit is moving sideways so fast you miss the Earth. Something like 7.8 km/s, which is 17,500 mph. A Cessna flies at rounding error speeds in comparison.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So instead of thinking that it would be possible if the Cessna were dropped low enough, think of it being possible if the Cessna is dropped at a slow enough air speed.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Kinetic Energy is your enemy&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;We have a Cessna strapped to an orbital craft doing 7.8 km/s (or it's not orbiting), and by the time it gets into the atmosphere to avoid burning up it needs to be going about 130 mph, or 58 m/s, which is zero by comparison to the initial speed. Our 1,000 kg Cessna thus has a relative KE of 0.5*1000*(7.8e3^2) = 30.4 Gigajoules which we need to lose.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Decelerockets!&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;There is only one way at present to transfer 30 GJ of energy to a Cessna before it reaches the atmosphere (i.e. before needing heatshields) and that is a rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially you are going to drop the Cessna off the orbital vehicle and fire a large booster rocket with a delta-v of 7.8 km/s. That is energetically similar to launching a LEO rocket from Earth (LEO has a delta-v of 9 or so from Earth), because again it's about the speed, not the height, and you're just accelerating backwards instead of forwards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An appropriately sized rocket might be the &lt;a href=&quot;https://en.wikipedia.org/wiki/Epsilon_(rocket)&quot; rel=&quot;noreferrer&quot;&gt;Japanese Epsilon rocket&lt;/a&gt;; it has a payload of 1200 kg for LEO which should get the Cessna and a pilot from +7.8 km/s to 0.06 km/s over the course of a couple of minutes.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;The Tyranny&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;There's one snag; the Epsilon is 91,000 kg because of the Tyranny Of The Rocket Equation - to accelerate a thing you have to stick it on a rocket, which you also have to accelerate, which takes more fuel, which also has to be accelerated, etc. So 1,200 kg payload requires a 91,000 kg rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That rocket is now a payload we have to put in orbit in the first place, on the top of a much bigger rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But wait, we've lifted more before; Saturn V managed 140,000 kg. The smallest previous launch capable of that scale is the Russian &lt;a href=&quot;https://en.wikipedia.org/wiki/Energia&quot; rel=&quot;noreferrer&quot;&gt;Energia rocket&lt;/a&gt;, a follow-on from the unsuccessful N1 project (Russia's Saturn V equivalent). It is about 2,400,000 kg (2.4 kt), about the size of Big Ben's tower and would cost something over $1.5bn.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Or just put a coat on&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;I've let it get a bit silly. It should hopefully be clear that a heatshield is rather cheaper than a rocket to decelerate with, which is essentially why Shuttle was designed the way it was - decelerating by airbrake only costs you the weight of a heatshield.&lt;/p&gt;&#xA;" OwnerUserId="519" LastEditorUserId="519" LastEditDate="2017-09-18T08:07:53.783" LastActivityDate="2017-09-18T08:07:53.783" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23018" PostTypeId="2" ParentId="23013" CreationDate="2017-09-13T14:42:31.997" Score="8" Body="&lt;p&gt;The cells themselves are not flexible -- indeed, they are fairly thin and delicate (I should know, I've broken quite a few of them myself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flexibility you see comes from the fact that the cells are individually welded and glued to a very thin, flexible blanket made from Kapton, silicone, and fiberglass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find a public datasheet for the solar cells (if I find one, I'll edit the post), but I can do my best to describe what you're seeing, with the help of this image I found online (rehosted from &lt;a href=&quot;http://pages.erau.edu/~ericksol/courses/sp300/images/iss_cells.jpg&quot; rel=&quot;noreferrer&quot;&gt;http://pages.erau.edu/~ericksol/courses/sp300/images/iss_cells.jpg&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mGt0B.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mGt0B.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The inset image on the lower right is a closeup of the back side of a single cell as seen through the blanket substrate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, the four dots are &quot;wrapthrough&quot; connections that take the collector grid from the front face and feed it to a set of contacts on the back face.  The ISS solar cells are a little bit unconventional in that rather than having the interconnect contacts on opposite edges of the cells, all of the contacts are on the back face of the cell.  Those dots are actually silver-plated holes in the silicon cell, much like a via in a printed circuit board.  Those holes are covered by the coverglass, which is a thin sheet of glass with UV-blocking and anti-reflective coatings that protect the exposed face of the cell.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interstitial red dots you see are actually the blanket material behind the cells.  The cells are shaped like squares with the corners lopped off, so the dots are actually where the missing corners of four adjacent cells come together. You can see this somewhat in the image, though the edges of the cell are obscured by the copper traces on the blanket layup.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sunlight (when the back of the blanket faces the sun -- earth albedo light otherwise) reaches the back of the cell by nature of the fact that the blanket material is largely transparent, which is plainly apparent in the image.  The orange color is due to the Kapton on the blanket.  The back face of an ISS solar cell is actually green.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Side note, the numbers you see printed on the cell are the lot number (on top) and the grade number (on bottom).  The cell depicted is on an engineering test article (hence the extra wiring for what appears to be thermocouples) and is not a flight-grade article, but the physical appearance is identical.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-09-13T14:42:31.997" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23019" PostTypeId="2" ParentId="16876" CreationDate="2017-09-13T14:48:56.780" Score="0" Body="&lt;p&gt;For shorter life time you save fuel  and you might save on EEE components (COTS).&#xA;You might also propose not redundant functions (e.g. cubesats).&#xA;Lead time might be shorter as well as launch cost (reduced mass).&lt;/p&gt;&#xA;" OwnerUserId="21026" LastEditorUserId="6816" LastEditDate="2017-09-13T15:23:05.313" LastActivityDate="2017-09-13T15:23:05.313" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23020" PostTypeId="2" ParentId="6135" CreationDate="2017-09-13T14:53:46.200" Score="5" Body="&lt;p&gt;Supplementary answer: the ISS is a lot bigger than Mir was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v13FW.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v13FW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-09-13T14:53:46.200" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23021" PostTypeId="1" AcceptedAnswerId="23022" CreationDate="2017-09-13T23:59:17.450" Score="102" ViewCount="29593" Body="&lt;p&gt;I found this image online, but is it a real message?&#xA;&lt;a href=&quot;https://i.stack.imgur.com/vaPH0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vaPH0.jpg&quot; alt=&quot;Space Shuttle on 747 transport plane, showing the mounting on the 747, on which the words are stencilled &amp;#39;Attach Orbiter Here, Black Side Down&amp;#39; &quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6241" LastEditorDisplayName="user20636" LastEditDate="2017-09-23T14:37:13.820" LastActivityDate="2018-02-15T17:59:33.410" Title="Is this &quot;Attach Orbiter Here / Note: Black Side Down&quot; sign on the Shuttle Carrier Aircraft real?" Tags="|space-shuttle|shuttle-carrier-aircraft|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23022" PostTypeId="2" ParentId="23021" CreationDate="2017-09-14T00:14:09.067" Score="105" Body="&lt;p&gt;I believe it's real, just a little bit of fun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Wikipedia article on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle_Carrier_Aircraft&quot; rel=&quot;noreferrer&quot;&gt;SCA&lt;/a&gt; states it as fact, as do other historical articles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Search engines will lead you to &lt;a href=&quot;https://www.google.com/search?q=attach%20orbiter%20here&amp;amp;safe=off&amp;amp;source=lnms&amp;amp;tbm=isch&amp;amp;sa=X&amp;amp;ved=0ahUKEwjS_4yUs6PWAhVqllQKHcybDg8Q_AUICygC&quot; rel=&quot;noreferrer&quot;&gt;multiple pictures of the attach points&lt;/a&gt; from different angles, all showing the label, for example:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P0g0p.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P0g0p.jpg&quot; alt=&quot;shuttle carrier attachment pylon photographed from a low angle&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like an earlier version of the label, since painted over and replaced, was even sillier:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L8sP5.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L8sP5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Lefty Loosy, Righty Tighty&quot; being a well-known mnemonic for which way to turn standard-threaded mechanical screws and bolts.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-14T00:19:58.907" LastActivityDate="2017-09-14T00:19:58.907" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23023" PostTypeId="2" ParentId="23021" CreationDate="2017-09-14T01:03:53.207" Score="70" Body="&lt;p&gt;It's real!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some pictures I took on April 29, 2014 as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Shuttle_Carrier_Aircraft&quot; rel=&quot;noreferrer&quot;&gt;Shuttle Carrier Aircraft (N905NA)&lt;/a&gt; was being moved from Ellington Field to its final resting place at Space Center Houston.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the label as I zoom in (and walk down the railroad tracks) in the series of pictures, especially if you view them full size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yuxFx.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yuxFx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DwxKJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DwxKJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KVMKs.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KVMKs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="12519" LastEditDate="2017-09-14T10:21:04.723" LastActivityDate="2017-09-14T10:21:04.723" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23025" PostTypeId="1" AcceptedAnswerId="23028" CreationDate="2017-09-14T10:59:47.253" Score="1" ViewCount="384" Body="&lt;p&gt;Would it be possible to move the ISS to an orbit of the sun, say 120 million miles away from the sun, putting it halfway between Earth and Mars? If it was purged of oxygen there would be little or no oxidation. Heating and cooling might be tricky! Perhaps it could be a staging/emergency post for Mars?&lt;/p&gt;&#xA;" OwnerUserId="21036" LastEditorUserId="25" LastEditDate="2017-09-14T16:07:42.213" LastActivityDate="2017-09-14T16:07:42.213" Title="ISS as a Mars safe house?" Tags="|mars|iss|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23026" PostTypeId="1" AcceptedAnswerId="23031" CreationDate="2017-09-14T15:09:38.603" Score="14" ViewCount="1397" Body="&lt;p&gt;The Los Angeles Times' Deborah Netburn just wrote a really nice retrospective; &lt;a href=&quot;http://www.latimes.com/science/la-sci-cassini-oral-history-20170912-htmlstory.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;‘OK. Let’s do it!’ An oral history of how NASA’s Cassini mission to Saturn came to be&lt;/a&gt; with plenty of photos and quotes from contributors to the spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;Some of the images really surprised me! Everyone views a spacecraft differently, but from a perspective of size, Cassini is two big things; a giant fuel tank to go far away from home, and a giant dish to stay in touch with home, and possibly to protect the spacecraft from direct impacts from ring particles during the more daring maneuvers.&lt;/p&gt;&#xA;&lt;p&gt;This made me look up the wet and dry masses of Cassini. From the Wikipedia page for &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini-Huygens&lt;/a&gt; I see a launch mass of 5,712 kg and a dry mass of 2,523 kg. I am not sure if these both include the Huygens lander or not.&lt;/p&gt;&#xA;&lt;p&gt;If I just use those two numbers, I get about 56% of the mass as fuel, but I am not sure if this is the correct usage of the numbers because of how Huygens factors in, so I thought I would pose the question to the experts, and let you decide how to handle the math:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Which deep-space spacecraft had the largest fuel mass fraction?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sKT3e.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sKT3e.jpg&quot; alt=&quot;Cassini 1&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Engineers and technicians at JPL work to complete the stacking assembly of the Cassini spacecraft in 1996.&amp;quot; From &lt;a href=&quot;http://www.latimes.com/science/la-sci-cassini-oral-history-20170912-htmlstory.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Ken Lubas / Los Angeles Times&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1T5RY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1T5RY.jpg&quot; alt=&quot;Cassini 4&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;Parts of the Cassini spacecraft are assembled at JPL in 1996.&amp;quot;&#xA;Cropped, from &lt;a href=&quot;http://www.latimes.com/science/la-sci-cassini-oral-history-20170912-htmlstory.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Iris Schneider / Los Angeles Times&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D1OEm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D1OEm.jpg&quot; alt=&quot;Cassini 2&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &amp;quot;A diagram of the Cassini spacecraft and Huygens probe.&amp;quot; From &lt;a href=&quot;http://www.latimes.com/science/la-sci-cassini-oral-history-20170912-htmlstory.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. NASA Jet Propulsion Laboratory. &lt;em&gt;Open in a new window for full size so you can see the detail and read the descriptions!&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-18T04:32:35.510" LastActivityDate="2021-09-18T04:32:35.510" Title="Flying fuel tanks! Which deep-space spacecraft had the largest fuel mass fraction?" Tags="|fuel|deep-space|cassini|mass-fraction|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="23027" PostTypeId="1" AcceptedAnswerId="23034" CreationDate="2017-09-14T15:35:51.367" Score="4" ViewCount="558" Body="&lt;p&gt;Cassini has quite a complex &lt;a href=&quot;https://saturn.jpl.nasa.gov/radio-science-subsystem/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Radio Science Subsystem&lt;/a&gt;. Accorting to that article (and confirmed in &lt;a href=&quot;https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#Description&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Cassini is the only deep space mission to transmit to Earth at three radio wavelengths (approximately 14 cm wavelength, designated S-band; 4 cm, designated X-band; and 1 cm, designated Ka-band) simultaneously. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That says &quot;simultaneously&quot;, quite a dish-full.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are also a bunch of non-axially located rectangular-waveguide-like objects that are also near the focus of the high-gain antenna system. What are these things for, and are they actually pointed towards the secondary mirror and using it for collimation? I am wondering if this gives some spatial resolution like a giant compound-eye of an insect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below x2:&lt;/strong&gt; Screen shots from the animated/simulated part of the montage video &lt;a href=&quot;https://youtu.be/Lie5-rTcCmk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Video file: Saturn Plunge Nears for NASA Cassini Spacecraft&lt;/a&gt; found on the apparently official &lt;a href=&quot;https://www.youtube.com/channel/UCYt2JN2z3ZXQpgPHt_eKcTw&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPLraw&lt;/a&gt; YouTube channel. The video contains a variety of Cassini-related material, images, and interviews.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZRnIw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZRnIw.jpg&quot; alt=&quot;Cassini multiple waveguide feeds&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/W0cYO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/W0cYO.jpg&quot; alt=&quot;Cassini multiple waveguide feeds and low-gain antenna&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Lie5-rTcCmk?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D1OEm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D1OEm.jpg&quot; alt=&quot;Cassini 2&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;A diagram of the Cassini spacecraft and Huygens probe.&quot; From &lt;a href=&quot;http://www.latimes.com/science/la-sci-cassini-oral-history-20170912-htmlstory.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. NASA Jet Propulsion Laboratory. &lt;em&gt;Open in a new window for full size so you can see the detail and read the descriptions!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From Handbook of Reflector Antennas and Feed Systems Volume III: Applications of... edited by Sudhakar Rao, Lotfollah Shafai , Satish K. Sharma, screenshot from google books:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VPhzU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VPhzU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-05-08T06:22:53.907" LastActivityDate="2019-05-08T06:22:53.907" Title="Why are there so many waveguide feeds (?) near the focus of Cassini's dish antenna?" Tags="|radio-communication|cassini|radar|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23028" PostTypeId="2" ParentId="23025" CreationDate="2017-09-14T16:07:30.173" Score="4" Body="&lt;p&gt;Putting it half-way between Earth and Mars doesn't make as much sense as you think. It's something akin to putting a ship half-way between two other ships. It's a moving target, and doesn't really make sense as a result. The closest is something like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Gateway&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Gateway&lt;/a&gt;, which orbits the Moon, although that is of questionable value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some discussion of moving the ISS to be a &lt;a href=&quot;https://www.space.com/8927-part-space-station-carry-crew-asteroid-nasa.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;deep space space station&lt;/a&gt;. At best, a small part of the station could be used. Simply put, the radiation, thermal, and power systems are all designed to be in low Earth orbit, and wouldn't function well at anything else. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A purposely build mission to store fuel could quickly and easily be developed, and would be far easier than trying to bring the entire Space Station somewhere. It would also probably do the job better.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-09-14T16:07:30.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23029" PostTypeId="1" AcceptedAnswerId="23045" CreationDate="2017-09-14T16:33:54.073" Score="3" ViewCount="4813" Body="&lt;p&gt;I want to write a SIMULINK model that calculates the position visible of satellites for the current time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As input for the model I have the coordinates of a location, the current time and the ephemeris/TLE data of satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As output I want the angle and the distance to the satellite, from the given location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far, I have found the SGP4 model which is able to calculate predictions, for positions using TLE data and I also found a &lt;a href=&quot;http://www.calsky.com/cs.cgi/Satellites/12&quot; rel=&quot;nofollow noreferrer&quot;&gt;Website&lt;/a&gt; that does what I need and even more. Unfortunately I can't find any information on how they calculate that.&lt;/p&gt;&#xA;" OwnerUserId="21043" LastActivityDate="2017-09-18T13:55:03.130" Title="How to calculate the current position of a satellite?" Tags="|orbital-mechanics|artificial-satellite|orbit|mathematics|coordinates|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23031" PostTypeId="2" ParentId="23026" CreationDate="2017-09-14T19:03:31.070" Score="14" Body="&lt;p&gt;I'm not up to a complete exhaustive survey of every possible contestant, so I've focused on relatively recent orbiters. I found a few that beat Cassini. The figures I've found so far are occasionally a bit contradictory or squirrelly -- masses are rounded differently here and there, some sources give the design tankage while others give the actual flown amount of propellant, for instance, or include pressurant or other consumables mass in propellant mass --  so any of these may be off by a couple of percent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had assumed that the winner was going to be a Jupiter or Saturn orbiter, since they have the greatest ∆v requirement for orbital insertion, but multiple Venusian and Martian orbiters seem to beat Cassini. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Venus orbiter &lt;a href=&quot;https://en.wikipedia.org/wiki/Magellan_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Magellan&lt;/a&gt; is the leader at a hair under 70% -- 2414 kg of propellant on a total launch mass of 3449 kg. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spaceflight101.com/maven/spacecraft-information/&quot; rel=&quot;noreferrer&quot;&gt;MAVEN&lt;/a&gt; tops the list of Mars orbiters at 66%. It will need to expend some fuel to accomplish several &quot;deep dips&quot; into the upper Martian atmosphere, and the intent is for it to move into a high circular orbit to serve as a communications relay once its science mission is complete. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission#Spacecraft_design&quot; rel=&quot;noreferrer&quot;&gt;Mangalyaan/MOM&lt;/a&gt; had to do its own trans-Mars injection burn (!), so it required a propellant fraction of 63%, with 852 kg propellant on 1337 kg launch mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://weebau.com/satplan/tgo.htm&quot; rel=&quot;noreferrer&quot;&gt;ExoMars&lt;/a&gt; is 63% sans &lt;a href=&quot;https://en.wikipedia.org/wiki/Schiaparelli_EDM_lander&quot; rel=&quot;noreferrer&quot;&gt;Schiaparelli&lt;/a&gt; or 54% with the lander included.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.jpl.nasa.gov/news/press_kits/JunoLaunch.pdf&quot; rel=&quot;noreferrer&quot;&gt;Juno&lt;/a&gt; appears to tie with Cassini at 56% (2032 kg propellants, 3625 kg launch mass). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Strangely, I can only find the combined propellant + pressurant mass for &lt;a href=&quot;https://en.wikipedia.org/wiki/MESSENGER&quot; rel=&quot;noreferrer&quot;&gt;MESSENGER&lt;/a&gt;, putting it something less than 54%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;Mars Reconnaissance Orbiter&lt;/a&gt; is close behind at 52%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/2001_Mars_Odyssey&quot; rel=&quot;noreferrer&quot;&gt;Mars Odyssey&lt;/a&gt; is about 46%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Venus_Express&quot; rel=&quot;noreferrer&quot;&gt;Venus Express&lt;/a&gt; is about 44%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Galileo&lt;/a&gt; is about 41%; like Cassini-Huygens, it's complicated by being an orbiter-plus-probe configuration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Express#Propulsion&quot; rel=&quot;noreferrer&quot;&gt;Mars Express&lt;/a&gt;, 40%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Akatsuki_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Aktsuki&lt;/a&gt;, 38%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Global_Surveyor&quot; rel=&quot;noreferrer&quot;&gt;Mars Global Surveyor&lt;/a&gt;, 34%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even though &lt;a href=&quot;https://en.wikipedia.org/wiki/Dawn_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Dawn's&lt;/a&gt; ion propulsion system achieves 6-10 times the specific impulse of storable bipropellant thrusters, it still has a respectable 35% mass fraction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Propulsion_and_attitude_control&quot; rel=&quot;noreferrer&quot;&gt;New Horizons&lt;/a&gt; only needed to make small course corrections, no orbit insertions, so only carried 16% of its mass in fuel.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-15T02:37:25.840" LastActivityDate="2017-09-15T02:37:25.840" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="23032" PostTypeId="1" CreationDate="2017-09-14T19:11:38.890" Score="-1" ViewCount="162" Body="&lt;p&gt;Just watched Elon's video on &lt;a href=&quot;https://www.youtube.com/watch?v=bvim4rsNHkQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;How Not to Land an Orbital Rocket Booster&lt;/a&gt; (ORB), and I just look at that and wonder why they don't build a large metal cylindrical cage on the landing platform so if it falls the (padded) rim of the cage will keep it propped up. The cylindrical cage could even be telescopic and motorized, so there is no fear of accidentally tapping the rim in the landing process, then, once it's in the center, engage the motorized telescopic cylindrical cage to expand up to capture the ORB safely.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Another idea, inspired from the net idea &lt;a href=&quot;https://space.stackexchange.com/questions/14338/spacex-first-stage-landing-why-not-use-an-elevated-net-above-the-landing-platfo&quot;&gt;here&lt;/a&gt;, would be instead of a net, just two clotheslines,&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;----- ----- &amp;lt;--- wires&#xA;|   | |   |&#xA;|   | |   | &amp;lt;--- poles&#xA;|   | |   |,&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;on either side of the landing area, and then when the rocket's in the landing area, clasp the two wires around the circumference of the rocket.&lt;/p&gt;&#xA;" OwnerDisplayName="user20289" LastEditorDisplayName="user20289" LastEditDate="2017-09-14T20:02:45.683" LastActivityDate="2017-09-14T20:27:00.400" Title="Is there some reason why ORBs don't land in a cylindrical cage?" Tags="|falcon-9|landing|" AnswerCount="1" CommentCount="8" FavoriteCount="0" ClosedDate="2017-09-15T03:50:47.567" ContentLicense="CC BY-SA 3.0" />
  <row Id="23033" PostTypeId="2" ParentId="23032" CreationDate="2017-09-14T20:27:00.400" Score="6" Body="&lt;p&gt;What you're proposing is a complex and expensive engineering solution to a problem which is rapidly becoming irrelevant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Falcon_9_and_Falcon_Heavy_launches&quot; rel=&quot;noreferrer&quot;&gt;SpaceX's 12 most recent landing attempts as of September 2017 have been successes&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only two of their failures were due to tip-over after touchdown; the causes of both those tip-overs have been addressed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any sort of net, funnel, cage, or other capture mechanism is going to be an extremely complex piece of engineering -- it has to stay out of the way prior to touchdown (otherwise it will cause a disaster in a case where the stage has a high horizontal velocity component), and then snap into place very suddenly to catch the stage before it falls, but not snap into place so hard that it damages the stage. Building, testing, perfecting, and maintaining a rig like that would be more expensive than simply improving the stage itself.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-09-14T20:27:00.400" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23034" PostTypeId="2" ParentId="23027" CreationDate="2017-09-14T20:28:04.513" Score="5" Body="&lt;p&gt;Some of those are the Ku-band antenna feeds for the Radar. The high-gain antenna also serves (served) as part of the Radar science instrument. The HGA was pointed at Titan many times to get synthetic-aperture radar images of the surface of Titan, otherwise obscured by the haze in the atmosphere to visible light instruments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gsw76.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gsw76.png&quot; alt=&quot;lots of feeds&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/7bcwb.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7bcwb.jpg&quot; alt=&quot;image of HGA&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-09-14T20:45:43.957" LastActivityDate="2017-09-14T20:45:43.957" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23035" PostTypeId="2" ParentId="19515" CreationDate="2017-09-14T20:52:52.257" Score="1" Body="&lt;p&gt;I can't unravel the ball of yarn for you, but maneuvers less than about 1 m/s were done with the ACS thrusters, not one of the main engines. While the main engine is more efficient, it is not accurate enough for very small burns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be able to extract information about Cassini's maneuvers from its &lt;a href=&quot;https://naif.jpl.nasa.gov/pub/naif/pds/data/co-s_j_e_v-spice-6-v1.0/cosp_1000/data/ek/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Event Kernel (EK) files&lt;/a&gt;, publicly available as part of the Planetary Data System distributions.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-09-14T21:35:42.353" LastActivityDate="2017-09-14T21:35:42.353" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23036" PostTypeId="1" AcceptedAnswerId="23037" CreationDate="2017-09-14T21:00:49.723" Score="28" ViewCount="3366" Body="&lt;p&gt;In a graphic describing Cassini's final approach to Saturn NASA lists the timestamps as 4:55 am PDT (ERT). What does this mean? I am assuming that PDT mean pacific time, but I can't find out what  ERT means.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also when talking about time, is it the time when something &lt;em&gt;actually&lt;/em&gt; happens, or, is it when the light actually gets to us?&#xA;&lt;a href=&quot;https://i.stack.imgur.com/8HHYL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8HHYL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11719" LastEditorUserId="6944" LastEditDate="2017-09-16T16:45:48.560" LastActivityDate="2017-09-17T03:26:17.883" Title="What does NASA mean when they use the term 'PDT (ERT)'" Tags="|deep-space|time|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23037" PostTypeId="2" ParentId="23036" CreationDate="2017-09-14T21:03:45.937" Score="42" Body="&lt;p&gt;ERT is Earth Received Time. I.e., when we find out about the event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://saturn.jpl.nasa.gov/mission/grand-finale/grand-finale-orbit-guide/&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-09-14T21:03:45.937" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23038" PostTypeId="2" ParentId="23036" CreationDate="2017-09-15T01:47:50.593" Score="-5" Body="&lt;p&gt;PDT is Pacific Daylight Time. PST is Pacific Standard Time. A lot of the communication with space probes is done at JPL (Jet Propulsion Labratory), which is in California, so that's probably why that time zone was chosen. Presumably the probe itself doesn't have much use for Daylight Savings Time.&lt;/p&gt;&#xA;" OwnerUserId="21050" LastActivityDate="2017-09-15T01:47:50.593" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23039" PostTypeId="1" AcceptedAnswerId="23044" CreationDate="2017-09-15T07:36:43.277" Score="4" ViewCount="211" Body="&lt;p&gt;Laser and other beamed energy weapons, in orbit or from Earth, could potentially make their targets malfunction from overheating and perhaps even partial melting. Would this be a way of space warfare that is safe from causing much hazardous debris in orbit, compared to kinetic anti satellite weapons? Or would for example fuel tanks and batteries explode anyway?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-09-16T03:36:44.700" Title="Would laser and beamed energy anti satellite weapons cause much space debris?" Tags="|debris|laser|military|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23040" PostTypeId="1" AcceptedAnswerId="35091" CreationDate="2017-09-15T12:24:37.430" Score="8" ViewCount="288" Body="&lt;p&gt;I decided that I wanted to see DSN receive the last bits of data from &lt;a href=&quot;https://eyes.nasa.gov/dsn/dsn.html&quot; rel=&quot;noreferrer&quot;&gt;DSN Now&lt;/a&gt;. The last data was supposed to be received at about 7:55. The signal seemed to continue strong until about 10 minutes after, when I started seeing the signal seem to be received, and then disconnected, and then received again, in cycles of about 20 seconds. Any idea what was actually happening?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DvZjT.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DvZjT.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what I've heard, the actual signal lasted about 30-40 seconds longer then expected, but still should have stopped well before this happened. Also, the signal did finally end about an hour later.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-09-15T13:18:29.923" LastActivityDate="2019-03-27T13:04:58.210" Title="What was happening with the DSN/ Cassini?" Tags="|deep-space-network|cassini|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23041" PostTypeId="1" CreationDate="2017-09-13T13:09:39.500" Score="1" ViewCount="57" Body="&lt;p&gt;I've been looking all over the internet and I cannot find the specific details on the engine of the Curiosity rover. I would need the specifics such as the type of engine, torque, force, rpm, and so on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anybody know where I can find this information?&lt;/p&gt;&#xA;" OwnerDisplayName="Viktor" LastActivityDate="2017-09-15T13:12:23.857" Title="Where can I find the specs for the engines of the Curiosity rover?" Tags="|astronomy|nasa|" AnswerCount="0" CommentCount="2" ClosedDate="2017-09-15T13:46:53.490" ContentLicense="CC BY-SA 3.0" />
  <row Id="23043" PostTypeId="2" ParentId="22992" CreationDate="2017-09-15T14:05:48.243" Score="2" Body="&lt;p&gt;The National Academy of Sciences undertook a study, funded by NASA, regarding near-Earth object detection and mitigation. This study looked at four mitigation techniques: nuclear, kinetic impactors, gravity tractors, and civil defense. The study did not address paint or solar sails due to the extremely low level of technology readiness of those techniques.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The results of this study were published in 2010 at &lt;a href=&quot;https://www.nap.edu/catalog/12842/defending-planet-earth-near-earth-object-surveys-and-hazard-mitigation&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nap.edu/catalog/12842/defending-planet-earth-near-earth-object-surveys-and-hazard-mitigation&lt;/a&gt; . The graph below portrays key aspects of their findings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/hkXtR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hkXtR.jpg&quot; alt=&quot;Graph depicting applicability and effectiveness of the four mitigation techniques described above as a function of warning time and diameter. Level of shading indicates effectiveness. The nuclear options are depicted in red, kinetic impactors in yellow, gravity tractors in green, and civil defense in blue. The level of shading indicates effectiveness.&quot;&gt;&lt;/a&gt;&lt;br&gt;&#xA;&lt;sub&gt;&lt;sup&gt;Source: Chapter 5 of the previously cited study.&lt;/sup&gt;&lt;/sub&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nuclear option comes in two forms: Disruption and standoff explosions. The intent of the disruptive explosions is to cause the asteroid to break into tiny parts, almost all of which will be placed on a trajectory that avoids collision with the Earth. Unfortunately, this approach will most likely leave at least one large mass that remains on a collision course.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The intent of the standoff approach is to have the weapon explode some distance from the asteroid, thereby avoiding disruption. The gamma rays and energetic neutrons produced by the explosions will cause surface material on the asteroid to vaporize, generating an impulsive change in the velocity of the asteroid body, which remains intact. The disadvantage of the standoff compared to disruptive explosions is that a standoff explosion delivers less than half of the weapon's energy to the asteroid while the disruptive approach delivers almost all of the weapon's energy to the asteroid. The advantage of the standoff approach is that it isn't disruptive. A disruptive explosion is the last resort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kinetic impactors are non-nuclear devices that collide with the threatening object at a very high relative velocity, thereby imparting an impulsive change in the velocity of the threatening object. These are orders of magnitude less effective but orders of magnitude more acceptable than the nuclear options.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity tractors are devices that maintain a somewhat largish mass close to the threatening object by means of a low-level thrust. Gravitation makes the threatening object accelerate toward the tractor. The low-level thrust keeps the tractor and its massive payload from falling into the asteroid. This is a very slow but steady approach. Multiple decades of operational time are needed to be effective. In addition to the long time span, another downside of this and related approaches is that these approaches require rendezvous, significantly increasing the delta V requirements of such missions compared to the nuclear options or kinetic impactors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;Note that in the above graph, the level of shading indicates the effectiveness of the technique. For an object 10 km across, the only effective technique is the nuclear option, and then only if there is a decade or more advance warning. The question asks about using paint or solar sails for an object that is 10 km across, with only one year of advance warning. &lt;strong&gt;Nothing&lt;/strong&gt; is effective in this extreme case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;What about using paint or solar sails? These techniques (along with a myriad of others) were dismissed out of hand by the National Academy of Sciences study due to impracticality, lack of knowledge, or lack of technology readiness. The Yarkovsky effect, which is what using paint relies upon, is something that suffers from lack of knowledge. Scientists don't quite know the magnitude of the Yarkovsky effect on asteroids. Studying this is one of the key aspects of the ongoing OSIRIS-REx mission. The extremely low technology readiness of this approach also contributes to making this a technique that, at least currently, can be dismissed out of hand from an engineering perspective.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solar sails &lt;em&gt;might&lt;/em&gt; be effective as an alternative to using thrusters in the gravity tractor. An extremely large solar sail to accomplish this, making this a TRL 1 (idea on paper) kind of approach. Tethering the solar sail to the threatening object falls into the sub TRL 1 territory.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-09-15T14:05:48.243" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23044" PostTypeId="2" ParentId="23039" CreationDate="2017-09-15T14:28:56.590" Score="4" Body="&lt;p&gt;This would still cause debris, either immediately or eventually.  Technically, an unresponsive satellite is 1 piece of space debris.  But the real question is lots of small pieces....  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Overheating could certainly cause the tanks to explode at the time.  If left disabled, a satellite with non-empty tanks and charged batteries does pose a threat to explode/breakup.  That's why the accepted decommissioning practice is to empty the tanks and drain the batteries.  An Air Force Weather satellite recently broke up because of a battery explosion:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacenews.com/battery-likely-the-culprit-in-military-weather-satellite-explosion/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spacenews.com/battery-likely-the-culprit-in-military-weather-satellite-explosion/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12483" LastActivityDate="2017-09-15T14:28:56.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23045" PostTypeId="2" ParentId="23029" CreationDate="2017-09-15T14:45:52.257" Score="5" Body="&lt;p&gt;Here is information on an open source Java package that has code for TLE propagation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.orekit.org/forge/projects/orekit/wiki/Tle&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.orekit.org/forge/projects/orekit/wiki/Tle&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also contains classes for all the components you need to get your range/angles to the satellite.  Look at the topocentric frame interfaces in the javadoc; it's got interfaces to get the azimuth/elevation/range:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.orekit.org/site-orekit-9.0/apidocs/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.orekit.org/site-orekit-9.0/apidocs/index.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might be a bit easier to integrate your Matlab/Simulink with Java over python.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(full disclosure, I haven't used the package, but I believe it to be reliable)&lt;/p&gt;&#xA;" OwnerUserId="12483" LastEditorUserId="12483" LastEditDate="2017-09-18T13:55:03.130" LastActivityDate="2017-09-18T13:55:03.130" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23047" PostTypeId="2" ParentId="3895" CreationDate="2017-09-15T17:26:50.383" Score="9" Body="&lt;p&gt;The Lagrange 1 and Lagrange 2 regions are the hubs of the trajectories that Ross, Lo, Martin, and Belbruno advocate. Szebehely developed equations to find the L1 and L2 regions. Using his equations I made a &lt;a href=&quot;http://clowder.net/hop/railroad/L1-2_distances.xls&quot; rel=&quot;noreferrer&quot;&gt;L1 and L2 spreadsheet&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of interest is the 3 body mass parameter, usually denoted $\mu$.&lt;br&gt;&#xA;$\mu=mass_{orbiting body}/(mass_{orbiting body} + mass_{central body})$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As central and orbiting bodies go, the Earth Moon system has a hefty $\mu$. But Pluto and Charon is even larger.&lt;br&gt;&#xA;Pluto/Charon    1.043E-01&lt;br&gt;&#xA;Earth/Moon  1.216E-02&lt;br&gt;&#xA;Sun/Jupiter 9.545E-04&lt;br&gt;&#xA;Sun/Saturn  2.856E-04&lt;br&gt;&#xA;Saturn/Titan    2.374E-04&lt;br&gt;&#xA;Jupiter/Ganymede    7.789E-05&lt;br&gt;&#xA;Jupiter/Callisto    5.684E-05&lt;br&gt;&#xA;Sun/Neptune 5.153E-05&lt;br&gt;&#xA;Jupiter/Io  4.700E-05&lt;br&gt;&#xA;Sun/Uranus  4.366E-05&lt;br&gt;&#xA;Jupiter/Europa  2.526E-05&lt;br&gt;&#xA;Saturn/Rhea 4.046E-06&lt;br&gt;&#xA;Sun/Earth   3.039E-06&lt;br&gt;&#xA;Sun/Venus   2.448E-06&lt;br&gt;&#xA;Saturn/Dione    1.935E-06&lt;br&gt;&#xA;Saturn/Tethys   1.091E-06&lt;br&gt;&#xA;Sun/Mars    3.229E-07&lt;br&gt;&#xA;Saturn/Enceladus    1.935E-07&lt;br&gt;&#xA;Sun/Mercury 1.659E-07&lt;br&gt;&#xA;Saturn/Mimas    7.037E-08&lt;br&gt;&#xA;Mars/Phobos 1.682E-08&lt;br&gt;&#xA;Sun/Pluto&amp;amp; Charon   7.149E-09&lt;br&gt;&#xA;Mars/Deimos 2.803E-09&lt;br&gt;&#xA;Sun/Ceres   4.741E-10  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Big $\mu$'s make for a very interesting set of trajectories emanating from the L1 and L2 necks. Something nudged outward from the EML2 would fly to a 1.7 million kilometer apogee in a simple earth moon system. And the Sun Earth L1 and L2 is only 1.5 million kilometers from Earth. Here's a variety of trajectories from &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html&quot; rel=&quot;noreferrer&quot;&gt;EML2&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1WNFG.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1WNFG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see some pellets find their way to near Earth perigees and others are thrown into heliocentric orbits outside of Earth's sphere of influence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/om6V4.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/om6V4.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above a ship is launched at a certain phase of the moon, set by inputing Barycentric Longitude of Sun. As it passes by the moon, a lunar assist raises apogee. At apogee the sun's tidal influence raises perigee allowing a ballistic slide to EML2. LEO to EML2 is accomplished with 3.1 km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is somewhat like the path Belbruno used to save the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hiten&quot; rel=&quot;noreferrer&quot;&gt;Hiten Mission&lt;/a&gt;. Note that Hiten had already achieved a 290,000 km apogee. 3.1 km/s had already been invested when Belbruno stepped in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is 3.1 km/s from LEO to a loosely bound lunar orbit ground breaking and revolutionary? No. &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/05/eml2.html#Farquhar&quot; rel=&quot;noreferrer&quot;&gt;Farquhar's route from LEO to EML2&lt;/a&gt; takes only 3.4 km/s. And Farquhar's route is about 8 days whereas the the trips out to the edge of Earth's Hill Sphere and back take months. A .3 km/s savings is nice but not a major game changer in my opinion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When it comes to the sun and rocky planets of the inner solar system, the 3 body mass parameter is miniscule. The trajectories from the Sun Earth L1 and L2 regions are much less varied and dramatic thatn the paths from EML1 and EML2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y5JdT.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y5JdT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above are the heliocentric orbits payloads would follow if nudged from the Sun Earth L1 and L2. These orbits have a semi-major axis little different from Earth's. That means the synodic period is quite large, about 20 years for the payload nudged from SEL2. And when the payload finally gets lapped by the Earth 20 years later, it's about 43º from perihelion. Not being close to Earth, Earth's gravity makes no dramatic changes to the payload path. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The payload released from SEL2 would not come back to a near Earth neighborhood until 160 years later.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did a blog post &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/04/potholes-on-interplanetary-superhighway.html&quot; rel=&quot;noreferrer&quot;&gt;Pot Holes On the Interplanetary Super Highway&lt;/a&gt;. Shane Ross, one of the architects of the ITN, left &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2015/04/potholes-on-interplanetary-superhighway.html?showComment=1430401243662#c6047172815618707790&quot; rel=&quot;noreferrer&quot;&gt;a comment&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Hollister, yes, the superhighway definitely has its limitations. It&#xA;  was inspired by comets, which have plenty of time, but of course we&#xA;  don't for planning space missions (unless we take Deep Time approach).&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;I suspect that gravitational corridors which naturally connect&#xA;  Earth-bound orbits with Mars-bound orbits using no-fuel do exist, but&#xA;  might take a long time to achieve &lt;strong&gt;(I'm thinking thousands of years of&#xA;  flight time between the two planets)&lt;/strong&gt;. But using some propulsion, we&#xA;  may be able to reduce that time to several decades. This is probably&#xA;  still too much time for a mission, especially a manned mission, so for&#xA;  now, conventional approaches will probably continue to be used. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;It would be an interesting study to try to map out the Earth-Mars&#xA;  natural connections. It would have to involve the concept of multiple&#xA;  gravity assists, or consecutive gravity assists, as discussed in&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;http://www2.esm.vt.edu/~sdross/talks/DS07_2007_Keplerian_Map_Talk.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www2.esm.vt.edu/~sdross/talks/DS07_2007_Keplerian_Map_Talk.pdf&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;a href=&quot;http://www2.esm.vt.edu/~sdross/papers/multiple_gravity_assists.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://www2.esm.vt.edu/~sdross/papers/multiple_gravity_assists.pdf&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Shane Ross, 'evangelist' :)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Belbruno and Toppotu have found a path using some propulsion. It entails a 2 km/s aphelion burn to slide into a ballistic path into Mars capture. But a .7 km/s burn at a near Mars periapsis will also capture a payload into a loosely bound Mars orbit. So it is actualy 1.3 km/s &lt;em&gt;more&lt;/em&gt; expensive than a straightforward Hohmann orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These pathways are interesting when it comes to systems with a large $\mu$. That includes the Earth and moon, gas giants and their larger moons, as well as the sun and gas giants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However it is not much use for departing Earth for other regions of the solar system. Departing from Earth, it's helpful to reach different parts of the Earth Moon neighborhood including SEL1 and SEL2, but that's about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you might guess, I was once a huge fan of the possibilities of ITN. I've invested some time and effort studying it hoping to find just the sort of map you're asking for. I have pretty much discarded that hope.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="1142" LastEditDate="2017-09-20T22:30:59.607" LastActivityDate="2017-09-20T22:30:59.607" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23048" PostTypeId="1" CreationDate="2017-09-15T20:24:27.460" Score="4" ViewCount="137" Body="&lt;p&gt;There are many images of Cassini with Saturn in the background on the internet. How are these photos taken? Is the video of Cassini disintegrating in Saturn's Atmosphere real or made using graphics? &lt;/p&gt;&#xA;" OwnerUserId="21066" LastActivityDate="2017-09-15T23:35:40.453" Title="Photographs and Videos &quot;of&quot; Cassini" Tags="|saturn|cassini|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23049" PostTypeId="2" ParentId="23048" CreationDate="2017-09-15T21:00:52.270" Score="5" Body="&lt;p&gt;Any picture of Cassini in space is an artist's conception, probably a computer rendering, although the Saturn backdrop may come from real Cassini photographs. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had thought it possible that the Huygens probe carried by Cassini to Titan might have taken one or more pictures of Cassini as it departed, but it's not possible, as &lt;a href=&quot;https://space.stackexchange.com/questions/23050/did-huygens-take-a-picture-of-cassini&quot;&gt;the camera was enclosed under the probe's heat shield&lt;/a&gt; until it was well into its descent. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-15T23:35:40.453" LastActivityDate="2017-09-15T23:35:40.453" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23050" PostTypeId="1" AcceptedAnswerId="23051" CreationDate="2017-09-15T21:08:44.587" Score="9" ViewCount="414" Body="&lt;p&gt;In reference to a &lt;a href=&quot;https://space.stackexchange.com/a/23049/195&quot;&gt;question about pictures of Cassini&lt;/a&gt;, I stated that it was possible that Huygens could have tested out its camera immediately after separation by taking a picture of Cassini. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was this actually done? Did Huygens take any pictures of its mothership prior to its descent to Titan's surface?&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-08-14T15:50:41.197" LastActivityDate="2019-08-14T15:50:41.197" Title="Did Huygens take a picture of Cassini?" Tags="|cassini|camera|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23051" PostTypeId="2" ParentId="23050" CreationDate="2017-09-15T23:01:05.237" Score="9" Body="&lt;p&gt;As far as I can tell, Huygens would not have been able to take a picture of Cassini.  Huygen's camera was part of the Descent Imager/Spectral Radiometer &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Huygens_instruments&quot; rel=&quot;noreferrer&quot;&gt;instrument package&lt;/a&gt;.  As seen in this cutaway from &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/image/spacecraft/huygens_cutaway.jpg&quot; rel=&quot;noreferrer&quot;&gt;GSFC&lt;/a&gt;, the DISR port is on the side of the spacecraft.  This port would have been entirely covered by the &lt;a href=&quot;http://www.imperial.ac.uk/ImageCropToolT4/imageTool/uploaded-images/Huygens-5--tojpeg_1446038171732_x2.jpg&quot; rel=&quot;noreferrer&quot;&gt;heat shield&lt;/a&gt;.  Therefore, the very design of Huygens would have prevented it from being able to take any images until after the heat shield had been jettisoned.  Plus, Huygens had no directional controls, so it could not have pointed itself back at Cassini to take a picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0ZAlR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0ZAlR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2017-09-15T23:01:05.237" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23053" PostTypeId="1" AcceptedAnswerId="23055" CreationDate="2017-09-16T02:07:58.833" Score="2" ViewCount="1028" Body="&lt;p&gt;I understand how one could be heated, but how does the space suit cool the water that is noodled throughout it exchanging heat with the body?&#xA;Why is this method used over traditional methods like a heat pump?&lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2017-09-17T04:40:14.493" LastActivityDate="2019-06-02T23:10:27.487" Title="How is a space suit climate controlled?" Tags="|spacesuits|technology|climate|" AnswerCount="1" CommentCount="1" ClosedDate="2017-09-17T08:01:14.023" ContentLicense="CC BY-SA 3.0" />
  <row Id="23055" PostTypeId="2" ParentId="23053" CreationDate="2017-09-16T03:28:39.420" Score="4" Body="&lt;p&gt;Apollo EVA suits and the shuttle/International Space Station Extravehicular Mobility Units (suits) use(d) a sublimator for heat rejection. This device is a sort of heat exchanger that the suit's water cooling loop passes through. Water from storage tanks &amp;mdash; a consumable &amp;mdash; is supplied to the sublimator and becomes ice in the low ambient pressure environment. This ice then transforms directly to water vapor &amp;mdash; &quot;sublimes&quot; &amp;mdash; and is vented to space, carrying heat with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As the reference puts it:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;However, the primary purpose of the water tanks is to feed water to the sublimator. The sublimator works on the principle of sublimation, that is, the process by which a solid turns directly into a vapor, bypassing the liquid phase. In this case, ice is formed on the sublimator evaporator sieve and is allowed to vaporize to space, removing heat with it. Air and cooling water are passed through fins in the sublimator, which extracts heat from each system.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eclss/emu.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I could not find a picture of the shuttle sublimator, but here is a drawing showing the Apollo one. From &lt;a href=&quot;http://www.therebreathersite.nl/03_Historical/apollo_rebreathers.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/StD0m.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/StD0m.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="10273" LastEditDate="2019-06-02T23:10:27.487" LastActivityDate="2019-06-02T23:10:27.487" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23056" PostTypeId="2" ParentId="23039" CreationDate="2017-09-16T03:36:44.700" Score="0" Body="&lt;p&gt;I would think that it would be very unlikely. The beam from an earth based system, even if very powerful would be large and spread out by the time it has reached the satellite especially if its in a high orbit like geostationary orbit. A space based system although potentially much closer to it's target would be much more power and heat limited than a ground based system. Satellites are already designed to last for many years bathing in the intense solar radiation of of space(~1.3 kw/m^2) without any damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A beam for example, 10 meters by to meters(I'll pretend the beam is square to simplify things) when it reaches the satellite would would have to be provided by a 130 kw laser to even equal the intensity of sunlight. So any weapon able to destroy the satellite would likely be megawatt level.&#xA;Because of the difficulties of keeping a several megawatt laser(or many smaller lasers) focused on a very small area after thousands of kilometers, a more conservative and much less power hungry approach then destroying the satellite itself  would probably be taken such as frying the satellites communication or camera systems, essentially making it useless.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the laser were powerful enough to cause significant physical damage other than just frying sensitive components and sensors then explosion might actually be a risk. The biggest risk of a large explosion would be the fuel tanks especially if they contain mono-propellant. A tank rupture could be caused by it heating up and causing the walls to lose strength or by the boiling of propellant causing the tank to over-pressure.  A tank rupture in and of itself poses a relatively low risk for producing debris as it is likely to just tear apart into only a few pieces with not a huge amount of energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the tank is filled with a hypergolic propellant then as long as only one tank is ruptured at a time and the two components do not mix then the outcome shouldn't be much different from a normal over-pressure tank rupture. If the satellite uses mono-propellant, Then the outcome could be much worse likely resulting in an ignition of the mono-propellant and a very large explosion. Batteries could also pose an explosion risk, however this would be relatively low energy and would require the battery be inside a pressure vessel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that the majority of the satellite would be covered in &#xA;reflective foil and/or polished which would make it much harder to damage the internal systems.&lt;/p&gt;&#xA;" OwnerUserId="20715" LastActivityDate="2017-09-16T03:36:44.700" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23057" PostTypeId="2" ParentId="10022" CreationDate="2017-09-16T06:48:37.393" Score="11" Body="&lt;p&gt;There is a number of expressions which convey much the same meaning. And it is hard, if not impossible, to find the originator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The expression I heard first was 'spontaneous rapid disassembly event' which according to New &lt;a href=&quot;https://www.newscientist.com/article/mg18324556-900-feedback/&quot; rel=&quot;noreferrer&quot;&gt;Scientist Feedback column&lt;/a&gt; (July 2004) is used to describe explosions in scientific papers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other expressions &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fire     : uncontrolled thermal event &lt;/li&gt;&#xA;&lt;li&gt;Bursting : unplanned loss of containment&lt;/li&gt;&#xA;&lt;li&gt;Crash    : deconstructive deceleration&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="21071" LastEditorUserId="21071" LastEditDate="2017-09-16T10:22:08.850" LastActivityDate="2017-09-16T10:22:08.850" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23058" PostTypeId="1" CreationDate="2017-09-16T08:25:50.353" Score="1" ViewCount="108" Body="&lt;p&gt;I always loved the part in 2001 where Jupiter becomes as star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has anyone ever suggested placing power generating stations high in the atmosphere of gas giants, and beaming the energy to colonies on the moons? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much energy would a laser on one of the gas giants need to put out to heat up an entire moon to Earth-like temperatures? Would it be conceivable to generate enough energy in-situ on gas giants that it would make economic sense to send it by laser to other parts of the Solar System?&lt;/p&gt;&#xA;" OwnerUserId="10979" LastActivityDate="2017-09-16T08:25:50.353" Title="Could the moons of Jupiter or Saturn be heated to human-friendly temperatures?" Tags="|colonization|terraforming|laser|gas-giant|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23059" PostTypeId="1" CreationDate="2017-09-16T14:36:50.777" Score="3" ViewCount="485" Body="&lt;p&gt;I have two objects each one moves in elliptic orbit I calculated the distance of closest approach using (the minimum intersection disatnce).Knowing the true anomalies for both objects corresponding to the minimum distance and knowing the orbital period for both objects, I have to find the time of closest approach.&#xA;As a beginning, i calculated the required time for each object to be at the point that corresponds to the minimum distance. that is to say, the time T1 and T2(when the two objects at the position q1 and q2 that corresponding to the minimum distance),So my question now I have the time that the first object needs to be in the position q1 and the time that the second object need to be in the position q2 where q1 and q2 are  the true anomalies that corresponding to the minimum distance how can i find the same time where the two objects be at the minimum distance?&lt;/p&gt;&#xA;" OwnerUserId="21073" LastActivityDate="2017-09-16T14:36:50.777" Title="How to calculate the time of closest approach between two orbiting objects" Tags="|orbital-mechanics|celestial-mechanics|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23060" PostTypeId="1" CreationDate="2017-09-16T18:54:35.280" Score="2" ViewCount="2540" Body="&lt;p&gt;Suppose I know all of the orbital elements of a particular orbital trajectory (not necessarily an elliptical one).  Suppose I know the position and true anomalies at two different points in this trajectory, but not the time it takes to get from one to the other.  Assuming a prograde trajectory from point 1 to point 2, how can I determine the time of flight between it takes to get from the first position to the second position?  Is there a single algorithm I can use to do this independently of the shape of the trajectory?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="6494" LastEditDate="2017-09-21T21:04:15.673" LastActivityDate="2017-09-21T22:40:56.553" Title="Time of flight between two anomalies in a known orbital trajectory" Tags="|orbital-mechanics|trajectory|time-of-flight|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23061" PostTypeId="1" CreationDate="2017-09-17T02:35:36.550" Score="6" ViewCount="675" Body="&lt;p&gt;I am wondering if any astronauts took a copy of the Holy Bible into space.  I know the crew of Apollo 8 read part of Genesis 1 in space, but I am wondering if anyone took a full Holy Bible into space.  &lt;/p&gt;&#xA;" OwnerUserId="21077" LastEditorUserId="6816" LastEditDate="2017-09-17T02:40:26.323" LastActivityDate="2023-05-25T22:14:06.663" Title="Did any astronaut take a Bible into space?" Tags="|crewed-spaceflight|history|astronauts|space-station|astronaut-lifestyle|" AnswerCount="3" CommentCount="1" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23062" PostTypeId="2" ParentId="23036" CreationDate="2017-09-17T03:26:17.883" Score="0" Body="&lt;p&gt;As answered before, ERT is when something is retrieved/received at Earth, which may be a while after its occurrence when talking about interplanetary/beyond communication (low Earth orbital data transfers are pretty fast). PDT is Pacific Daylight Time, which is UTC-7. PST is UTC-8, which is used at different times of the year because of daylight savings (despite my opinion of it being redundant for most people in today's world).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the second question, time is confusing. But for the most part, we use when light (or other energy) is available to our senses, or often more preferably, our data collectors. Not only is it annoying to subtract the small amount of time (relative to Earthly sources) from when light reaches the eyes, but when talking about spacebound objects, such as stars, we don't have any means of knowing what is occurring at the spot, as a star can die and we can still see its light from the past traveling to us, due to our location being many lightyears away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hope this helps with your curiosities! (Also correct me if I somehow got something wrong here, which I don't believe I did)&lt;/p&gt;&#xA;" OwnerDisplayName="user21078" LastActivityDate="2017-09-17T03:26:17.883" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23063" PostTypeId="2" ParentId="7768" CreationDate="2017-09-17T03:53:07.667" Score="3" Body="&lt;p&gt;Actually, there is &lt;a href=&quot;https://forum.kerbalspaceprogram.com/index.php?/topic/50471-12-real-solar-system-v120-dec-8/&quot; rel=&quot;nofollow noreferrer&quot;&gt;a mod for KSP called Real Solar System&lt;/a&gt; which replaces the default with our real one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Aside from that, the Universe Sandbox game might be an alternative, a very good simulator.&lt;/p&gt;&#xA;" OwnerUserId="21079" LastEditorUserId="6816" LastEditDate="2017-09-17T04:20:16.440" LastActivityDate="2017-09-17T04:20:16.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23064" PostTypeId="2" ParentId="23061" CreationDate="2017-09-17T04:06:57.760" Score="7" Body="&lt;p&gt;The astronauts' personal preference kit allowances were small enough that carrying a large format bible would be inconvenient. However, several &lt;a href=&quot;https://airandspace.si.edu/collection-objects/bible-microform-apollo-13/nasm_A19720199000&quot; rel=&quot;nofollow noreferrer&quot;&gt;microfilm copies of the Bible were taken on Apollo missions&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2023-05-25T22:14:06.663" LastActivityDate="2023-05-25T22:14:06.663" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23067" PostTypeId="2" ParentId="23061" CreationDate="2017-09-17T15:58:12.387" Score="4" Body="&lt;p&gt;Ilan Ramon, an Israeli Air Force pilot took a very small Torah scroll (So complete first 5 books, Pentateuch in English/Greek I guess) on the Columbia mission that burned up on re-entry. (Columbia and crew burned up, I mean).&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-09-17T21:48:27.470" LastActivityDate="2017-09-17T21:48:27.470" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23068" PostTypeId="1" CreationDate="2017-09-17T21:37:36.850" Score="4" ViewCount="825" Body="&lt;p&gt;What would happen to humans if we set foot on &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler-442b&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler-442b&lt;/a&gt;, where the surface gravity is predicted to be about 30% percent stronger than that of earth? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If humans colonized the planet, would it then begin to alter our descendants' bodies significantly over time? &lt;/p&gt;&#xA;" OwnerUserId="21084" LastEditorUserId="12102" LastEditDate="2017-09-18T02:11:23.247" LastActivityDate="2017-09-20T09:26:42.143" Title="How does hypergravity affect the human body?" Tags="|gravity|astrobiology|hypergravity|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23069" PostTypeId="1" AcceptedAnswerId="23070" CreationDate="2017-09-17T22:14:30.420" Score="3" ViewCount="1576" Body="&lt;p&gt;I'm creating a mars re-entry simulation and i need a model for density variation w.r.t. altitude on mars.  I know that earth can often be modeled using an exponential density model with a &quot;sea level&quot; reference density and a scaling height $h0$.  Can mars be modeled similarly? If so, what would the reference density and scale height be for mars?  If not, what model is typically used for martian atmospheric density variations with altitude?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastActivityDate="2017-09-18T15:26:03.890" Title="How to model the martian atmospheric density variations with respect to altitude?" Tags="|mars|reentry|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23070" PostTypeId="2" ParentId="23069" CreationDate="2017-09-17T22:28:37.097" Score="5" Body="&lt;p&gt;According to &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html&quot; rel=&quot;noreferrer&quot;&gt;NASA's fact sheet&lt;/a&gt;, scale height is 11.1km, surface density 0.020 kg/m&lt;sup&gt;3&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.agi.com/news/blog/november-2010/a-better-mars-atmosphere-model?feed=AGIBlogsFeed&quot; rel=&quot;noreferrer&quot;&gt;Here's some additional information&lt;/a&gt; if you want to incorporate a more detailed model than the simple exponential. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-09-17T22:28:37.097" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23072" PostTypeId="2" ParentId="23068" CreationDate="2017-09-18T01:34:24.707" Score="5" Body="&lt;p&gt;We don't have test results for exceeding gravity by little, for exceedingly long periods. Longest tests were of order of a week (see this &lt;a href=&quot;https://space.stackexchange.com/a/6157/12102&quot;&gt;substantial answer with links and data&lt;/a&gt;), at 1.5g acceleration (which, for all practical purposes simulates increased gravity) - that means a typical expedition, like the Lunar expeditions could operate in these conditions just fine, but a colony could encounter unforeseen health problems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be techniques that could mitigate some of the problems of prolonged exposure - e.g. remaining immersed in water causes external pressure to mitigate a large part of the stress on cardiovascular system. Although high-G worlds are definitely low-priority when planning a manned landing; tyranny of the Rocket Equation makes launch/return from these exceptionally expensive.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="12102" LastEditDate="2017-09-20T09:26:42.143" LastActivityDate="2017-09-20T09:26:42.143" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23073" PostTypeId="1" CreationDate="2017-09-18T06:06:26.420" Score="3" ViewCount="129" Body="&lt;p&gt;Suppose I am executing an interplanetary rendevous (Hohmann transfer) between two coplanar, perfectly spherical planets, each with circular orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal is to execute a hyperbolic escape from a circular parking orbit on planet 1 and arrive from a hyperbolic trajectory into a circular parking orbit on planet 2. Both hyperbolic trajectories are aimed in the direction of each planet's velocity. But due to the hyperbolic trajectory, the spacecraft will exit/enter each planet's sphere of influence some distance $\Delta$ (i.e. the aiming radius) away from the line of orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How can I increase or decrease $\Delta$ while still maintaining the same velocity required to execute the interplanetary transfer? Also, is $\Delta$ is the same both for departure and arrival of the two planets in this ideal case? If not, how can I shape the two aiming radii independently of each other?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="1054" LastEditDate="2017-09-19T00:07:50.933" LastActivityDate="2017-09-19T00:07:50.933" Title="How can I control the departure and arrival aiming radii in an ideal interplanetary trajectory?" Tags="|orbital-mechanics|interplanetary|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23074" PostTypeId="1" CreationDate="2017-09-18T08:51:31.463" Score="14" ViewCount="5255" Body="&lt;p&gt;A &lt;a href=&quot;https://skeptics.stackexchange.com/questions/39437/did-yuri-gagarin-end-up-in-critical-condition-after-coming-back-to-earth-due-to&quot;&gt;currently trending question on Skeptics.se&lt;/a&gt; references a website that makes the claim that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In Yuri Gagarin’s historic flight to space, he returned in near critical condition after only one hour and forty-eight minutes in space. Clearly, there was some vital element missing in space that we receive on earth. Yuri had plenty of food, water and oxygen and since the flight was less than 2 hours, he only needed oxygen. The critical missing element appears to be the earth’s magnetic field. &lt;strong&gt;Since that first flight, pulsed magnetic devices have been used in every space suit and space station.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Emphasis mine).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ignoring the statements on Yuri Gagarin's health and just focusing on the question of &quot;pulsed magnetic devices&quot;:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Has there been an instance of a space suit / space station that does not generate a magnetic field?&lt;/li&gt;&#xA;&lt;li&gt;If this is not the case, and a magnetic field is generated by the suit / station, is this generated by a specific device (that could be called a &quot;pulsed magnetic device&quot;) with the intention of correcting some medical condition?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="14543" LastActivityDate="2017-09-18T15:43:05.913" Title="Have any space suits (not) used &quot;pulsed magnetic devices&quot;?" Tags="|spacesuits|magnetic-field|" AnswerCount="2" CommentCount="20" ContentLicense="CC BY-SA 3.0" />
  <row Id="23075" PostTypeId="1" AcceptedAnswerId="25830" CreationDate="2017-09-18T08:53:39.093" Score="6" ViewCount="478" Body="&lt;p&gt;I was reading about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Robot_Operating_System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Robot Operating System&lt;/a&gt; ROS, and in particular the 2014 ROS.org article &lt;a href=&quot;http://www.ros.org/news/2014/09/ros-running-on-iss.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ROS running on ISS&lt;/a&gt; and wondering if the Robonaut is being actively tested on the ISS.&lt;/p&gt;&#xA;&lt;p&gt;Do they take it out, turn it on, and let it roam around from time to time?&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://space.stackexchange.com/a/10887/12102&quot;&gt;this answer&lt;/a&gt;, there were some initial tests and some problems about two years ago. Has there been any change of status since then?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;ROS was delivered to ISS aboard a SpaceX rocket as part of a recent resupply mission that also included a set of robotic legs that will be attached to R2 soon.&lt;/p&gt;&#xA;&lt;p&gt;NASA was able to upgrade the R2 torso earlier this month after on-orbit surgery to remove old processors and electronics and install new ones. On August 12, R2 powered up using ROS for the first time.&lt;/p&gt;&#xA;&lt;p&gt;As we mentioned in a &lt;a href=&quot;http://www.ros.org/news/2014/03/roslaunch.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;prior post&lt;/a&gt;, the R2 team at JSC has been using ROS for R2 development on Earth for the last couple of years. They combine ROS with &lt;a href=&quot;http://www.orocos.org/rtt&quot; rel=&quot;nofollow noreferrer&quot;&gt;OROCOS RTT&lt;/a&gt; to produce a flexible development environment with real-time capabilities.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As a side note, there was a &lt;a href=&quot;http://wiki.ros.org/Robots/Robonaut2&quot; rel=&quot;nofollow noreferrer&quot;&gt;ROS Robonaut Simulator package&lt;/a&gt; at one point (see also &lt;a href=&quot;http://www.ros.org/news/2014/09/ros-running-on-iss.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;) that you could run at home, but I am not sure this is still an active project.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From &lt;a href=&quot;http://www.ros.org/news/2014/09/ros-running-on-iss.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/z15Nm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/z15Nm.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-08-18T05:44:38.833" Title="ROS is running on the Robonaut, but is the Robonaut running on the ISS (post 2015)?" Tags="|iss|robotics|ros|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23076" PostTypeId="2" ParentId="23074" CreationDate="2017-09-18T09:12:07.370" Score="49" Body="&lt;p&gt;Spacecraft and space suits &lt;strong&gt;do NOT&lt;/strong&gt; generate a magnetic field for medical reasons.   Any magnetic fields generated are side effects of using electric motors etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The linked question on Skeptics thoroughly debunks the idea, noting that:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Gagarin was not in &quot;critical condition&quot; after his flight; and,&lt;/li&gt;&#xA;&lt;li&gt;Any flights in LEO are well within Earth's magnetic field.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/15191/how-actually-have-space-suits-dissipated-the-heat-removed-from-astronauts&quot;&gt;This question&lt;/a&gt; has a schematic of the Apollo life support system. The schematic does not contain a pulsed magnetic device. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-09-18T15:43:05.913" LastActivityDate="2017-09-18T15:43:05.913" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23077" PostTypeId="2" ParentId="23074" CreationDate="2017-09-18T12:02:13.267" Score="22" Body="&lt;p&gt;Well, an electric motor is arguably a 'pulsed magnetic device', as is a solenoid valve, but good ones try very hard to contain the fields to where they will do useful work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose one could design a suit without any motors, and it may even have been done, but pumps, blowers and valves and such seem a sort of natural feature of a space craft life support package. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this sense, yes space vehicles do contain such things, and they ARE necessary for the crews continued well being. If you turn off the electrical power to the life support system and the crew will tend to end up in critical condition fairly quickly!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can almost always come up with sciency sounding names for mundane technology, &quot;Pulsed magnetic device&quot; = electrical machine of almost any type, and yes you need motors and solenoid control valves to keep the crew alive....    &lt;/p&gt;&#xA;" OwnerUserId="21091" LastActivityDate="2017-09-18T12:02:13.267" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23078" PostTypeId="2" ParentId="13022" CreationDate="2017-09-18T12:56:31.060" Score="8" Body="&lt;p&gt;The way to solve this problem has recently been proposed by NASA. Put a magnet on the L1 Legrange point (between Mars and the Sun) of about 2 Tesla. This is what it looks like: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DhxA7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DhxA7.jpg&quot; alt=&quot;Magnetic Mars shield&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At present, atmospheric loss on Mars is balanced to some degree by volcanic outgassing from Mars interior and crust. This contributes to a surface atmosphere that is about 6 mbar in air pressure (less than 1% that at sea level on Earth). As a result of the magnet, Mars atmosphere would naturally thicken over time, this would include an average increase of about 4 °C (~7 °F), which would be enough to melt the carbon dioxide ice in the northern polar ice cap. This would trigger a greenhouse effect, warming the atmosphere further and causing the water ice in the polar caps to melt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is what it looks like now:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/SR9l8.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SR9l8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html#jCp&quot; rel=&quot;noreferrer&quot;&gt;https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html#jCp&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="20892" LastActivityDate="2017-09-18T12:56:31.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23079" PostTypeId="1" CreationDate="2017-09-17T14:44:25.400" Score="28" ViewCount="4036" Body="&lt;p&gt;My preemptive apologies for asking a question about a movie, and the spoilers within said question, but considering the &lt;a href=&quot;http://www.businessinsider.com/the-martian-reviewed-by-astronauts-scientists-2015-9&quot; rel=&quot;noreferrer&quot;&gt;widespread support for its scientific plausibility&lt;/a&gt;, I'm hoping you'll let it slide :)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the movie &lt;em&gt;The Martian&lt;/em&gt;, the character Rich Purnell is shown using the Pleiades supercomputer at the NASA Ames Research Center to confirm the calculations for his maneuver designed to safely redirect the Hermes spacecraft back to Mars, and then to Earth. Why?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space is just about the most ideal place possible for predictable physics. Little in the way of air or external forces, short of gravity which can be calculated between the spacecraft and the Sun and planets and little else, centrifugal force only during the maneuver itself, almost none of the fluid mechanics that otherwise make simple calculations complicated... Basically, with so few moving parts and sources of complexity, does it really take a &lt;a href=&quot;https://www.nas.nasa.gov/hecc/resources/pleiades.html&quot; rel=&quot;noreferrer&quot;&gt;250,000-core supercomputer&lt;/a&gt; to run those calculations, or could Rich use, say, his Macbook instead?&lt;/p&gt;&#xA;" OwnerUserId="21107" OwnerDisplayName="TheEnvironmentalist" LastEditorUserId="653" LastEditDate="2017-09-18T17:58:12.707" LastActivityDate="2020-09-13T15:30:35.337" Title="The Martian: Does it really take a supercomputer to calculate spaceflight maneuvers?" Tags="|orbital-mechanics|rockets|simulation|computing|" AnswerCount="7" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23080" PostTypeId="2" ParentId="23079" CreationDate="2017-09-18T07:45:40.777" Score="-4" Body="&lt;p&gt;When it comes to maneuvering in space I suppose you do need super computer. Consider this: when rocket is lunched, as it leaves the atmosphere Earth is not the only massive object influencing the motion of the rocket, it is true that effect of Sun or other planets are not as big as Earth's but as rocket goes further we can say that it is influenced by more than 1 or two moving massive objects in space and we face sort of three-body problem (&lt;a href=&quot;https://en.wikipedia.org/wiki/Three-body_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Three-body_problem&lt;/a&gt;). And in &lt;em&gt;Martian&lt;/em&gt; they were not simply launching a rocket, they were &amp;quot;slingshooting&amp;quot; it back to Mars and trajectory needed to be calculated precisely as Earth and Mars were moving themselves around Sun and changing their coordinates as well. This is not the case where you simply write Newton's second law and and use Matlab for other calculations, here you will have many variables that will definitely need huge computing power...&lt;/p&gt;&#xA;" OwnerUserId="21100" OwnerDisplayName="Niko" LastEditorUserId="33864" LastEditDate="2020-09-13T15:30:35.337" LastActivityDate="2020-09-13T15:30:35.337" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23081" PostTypeId="2" ParentId="23079" CreationDate="2017-09-18T15:15:41.743" Score="31" Body="&lt;p&gt;Well, developing low-thrust trajectories does take more computation than impulsive trajectories (e.g. like Voyager, which was done with rather primitive computers). You have no choice but to run many fully integrated trajectories. However it would not take a supercomputer of the future, or even a supercomputer of the present to search for and find trajectories like that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, Andy Weir used his home computer (whatever that was, but likely not even a small cluster) to find and check his trajectories. You can find &lt;a href=&quot;https://github.com/the-martian-trajectory-app/trajectory&quot; rel=&quot;noreferrer&quot;&gt;his code here&lt;/a&gt;, and there is also &lt;a href=&quot;https://opensource.gsfc.nasa.gov/projects/GMAT/index.php&quot; rel=&quot;noreferrer&quot;&gt;publicly available NASA code&lt;/a&gt; you can use to propagate trajectories to try this yourself.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-09-18T15:15:41.743" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23082" PostTypeId="2" ParentId="23069" CreationDate="2017-09-18T15:26:03.890" Score="4" Body="&lt;p&gt;What you're looking for is called &lt;a href=&quot;https://software.nasa.gov/software/MFS-33158-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;MarsGRAM&lt;/a&gt;. You can request it there. It is quite sophisticated and is based on a large amount of data and global atmosphere modeling. MarsGRAM will generate Mars atmosphere profiles for a given time of the Martian year, location, and atmospheric dust loading, and will also generate randomly varied atmospheres given the relevant uncertainties, for use in Monte Carlo simulations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to do something simpler, you would at a minimum have to account for the seasonal variation in the total mass of the Martian atmosphere. It is quite significant. Also the dust loading can significantly change the distribution of density with altitude, annoyingly making the atmosphere more dense higher up where you don't care, and less dense nearer to the surface where you do care.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to land something on Mars that costs millions or billions of dollars, you should not use MarsGRAM either. You should consult with Mars atmosphere experts to construct an atmosphere model for your case.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-09-18T15:26:03.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23083" PostTypeId="2" ParentId="23079" CreationDate="2017-09-18T15:40:17.803" Score="1" Body="&lt;p&gt;No. You do not need a supercomputer. But in the movie it would not look so impressive. Iron Sky seems to be more realistic in this case. :-) And yet - very impressive. (spoiler)&lt;/p&gt;&#xA;" OwnerUserId="19946" LastEditorUserId="19946" LastEditDate="2017-09-18T18:16:26.533" LastActivityDate="2017-09-18T18:16:26.533" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23084" PostTypeId="1" AcceptedAnswerId="23085" CreationDate="2017-09-18T16:29:16.767" Score="6" ViewCount="1053" Body="&lt;p&gt;I saw &lt;a href=&quot;https://space.stackexchange.com/a/23082/12102&quot;&gt;this answer&lt;/a&gt; about MarsGRAM, and when I clicked on the &lt;a href=&quot;https://software.nasa.gov/software/MFS-33158-1&quot; rel=&quot;noreferrer&quot;&gt;NASA software link&lt;/a&gt; to read about it I saw at the bottom of the page a blurb for &lt;a href=&quot;https://software.nasa.gov/software/MFS-32780-1&quot; rel=&quot;noreferrer&quot;&gt;EarthGRAM 2010&lt;/a&gt;, an atmospheric model for Earth with likely &lt;a href=&quot;https://see.msfc.nasa.gov/model-gram&quot; rel=&quot;noreferrer&quot;&gt;all kinds of insight&lt;/a&gt; beyond a simple scale heigh or scale height modified by temperature spherical model.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was happy to see &quot;open-source&quot;, as well as the easy-to-read and transcribe &quot;FORTRAN&quot; language.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But at the bottom it says &quot;U.S. Release Only&quot; and I am living overseas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; &lt;em&gt;What does it mean when a software is called open-source for US-release only?&lt;/em&gt; What are the rules exactly? Does this refer to the location of download, or of use, or to the citizenship of the person requesting the software?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far I have not been able to find out what the restrictions really are. I've cycled through a few links but so far without initiating a request for the software, I am not sure how to view the restrictions on eligibility. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jXxjC.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jXxjC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-18T17:44:10.187" LastActivityDate="2017-09-18T17:44:10.187" Title="What does it mean when a software is called open-source for US-release only?" Tags="|nasa|software|regulatory|classified|open-source|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23085" PostTypeId="2" ParentId="23084" CreationDate="2017-09-18T16:50:32.187" Score="6" Body="&lt;p&gt;Here is what the &lt;a href=&quot;https://software.nasa.gov/faq&quot; rel=&quot;noreferrer&quot;&gt;NASA Software FAQ&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The release type determines who can have a NASA software code. If you meet the access criteria for the code (as defined below), NASA can transfer the software to you.&#xA;  Release types:&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;General Public Release: For codes with a broad release and no nondisclosure or export control restrictions&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Open Source Release: For collaborative efforts in which programmers improve upon codes originally developed by NASA and share the changes&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;strong&gt;U.S. Release Only: For codes available to U.S. persons only&lt;/strong&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;U.S. and Foreign Release: For codes that are available to U.S. persons and persons outside of the U.S. (who meet certain export control restrictions)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;U.S. Government Purpose Release: For codes that are to be used on behalf of the U.S. government by a federal agency or business/university under a federal contract/grant/agreement.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;emphasis added&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case that is not clear, here is the definition of a U.S. person from the &lt;a href=&quot;http://www.research.pitt.edu/exco-us-person-vs-foreign-person&quot; rel=&quot;noreferrer&quot;&gt;University of Pittsburgh Office of Research&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;U.S. Person (EAR Part 772 and ITAR 120.15)&#xA;  Pursuant to the EAR and the ITAR, a U.S. Person includes :&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;any individual who is granted U.S. citizenship; or&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;any individual who is granted U.S. permanent residence (&quot;Green Card&quot; holder); or&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;any individual who is granted status as a &quot;protected person&quot; under 8 U.S.C. 1324b(a)(3);&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;any corporation/business/organization/group incorporated in the United States under U.S. law;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;any part of U.S. government.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You will note that the page for Earth-GRAM has a request link instead of a download link. That is how they check to ensure you qualify to receive the release.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-09-18T16:50:32.187" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23086" PostTypeId="2" ParentId="23079" CreationDate="2017-09-18T18:38:02.653" Score="-1" Body="&lt;p&gt;The answer can be yes and no, it really depends on what degree of accuracy or how much resolution you want in your simulated data. You can always reduce the floating points or use averaged terms or reduce the upper limit of some loop that tests convergence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can simulate something on your home PC or a supercomputer in sufficient times given you don't use extensive recursive methods or switch to more efficient iterative methods or what not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again it's about the kind of math that you employ for the task, not the task itself that would require a super computer.&lt;/p&gt;&#xA;" OwnerUserId="18272" LastEditorUserId="18272" LastEditDate="2017-09-18T18:45:21.277" LastActivityDate="2017-09-18T18:45:21.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23088" PostTypeId="1" CreationDate="2017-09-18T22:58:12.023" Score="6" ViewCount="6002" Body="&lt;p&gt;Wouldn't a &lt;a href=&quot;https://en.wikipedia.org/wiki/Brachistochrone_curve&quot; rel=&quot;nofollow noreferrer&quot;&gt;Brachistochrone curve&lt;/a&gt; to Mars orbit be a much faster and more efficient way to send personnel and equipment to mars as compared to the Hohmann transfer if we used an Ion engine?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The below picture is from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Inspiration_Mars_trajectory.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, but with the addition of the blue line to indicate a trajectory that seems more efficient to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6gQye.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6gQye.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21097" LastEditorUserId="195" LastEditDate="2017-09-19T01:59:28.227" LastActivityDate="2020-03-15T00:55:42.457" Title="Brachistochrone variation for Earth-to-Mars Orbit" Tags="|orbital-mechanics|mars|crewed-spaceflight|orion-spacecraft|hohmann-transfer|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23089" PostTypeId="2" ParentId="20546" CreationDate="2017-09-18T23:15:03.013" Score="1" Body="&lt;p&gt;Either we would need to launch waves of spacecraft to form a web of relay signals, or we would have to manually retrieve spacecraft sometime in the future. Power could easily be solved by using the same principle of 'torch ships' (that is, breaking down the materials of the ship into energy). However, any communication would be next to impossible.&lt;/p&gt;&#xA;" OwnerUserId="21097" LastActivityDate="2017-09-18T23:15:03.013" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23090" PostTypeId="2" ParentId="20037" CreationDate="2017-09-18T23:23:45.797" Score="1" Body="&lt;p&gt;I'm not quite sure about the first part of your question, however NASA's Eyes program should have a list of all the manuevers. I know that Juno had manuevers and that you can watch all spacecraft at any moment in time back to 1976, so that should help you.&lt;/p&gt;&#xA;" OwnerUserId="21097" LastActivityDate="2017-09-18T23:23:45.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23091" PostTypeId="1" AcceptedAnswerId="23152" CreationDate="2017-09-19T00:12:58.613" Score="6" ViewCount="429" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/questions/20546/how-are-the-tiny-spacecraft-proposed-by-project-starshot-supposed-to-communicate&quot;&gt;recent question&lt;/a&gt; about Project Starshot's communication asks about one of the seemingly insurmountable problems with this project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is however another problem that has bugged me ever since I first heard about the project. We are told the craft will consist of an electronics module with a mass of about 1g, attached to a sail of about 1 m^2 which is just a few atoms thick. Then we accelerate the entire craft with a laser, pushing it up to 20% of the speed of light in a few minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quick calculation shows that we need of the order of 60,000 G to reach that speed. Hence, during acceleration our 1 g mass will weigh 60 kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What kind of technology do they envisage to produce a sail that will hold that kind of weight, without tearing or just collapsing under the strain?&lt;/p&gt;&#xA;" OwnerUserId="17390" LastActivityDate="2019-06-05T05:25:16.927" Title="Project Starshot acceleration" Tags="|interstellar-travel|technology|breakthrough-starshot|" AnswerCount="1" CommentCount="7" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23092" PostTypeId="2" ParentId="23088" CreationDate="2017-09-19T02:35:32.510" Score="3" Body="&lt;h2&gt;There can't be a &amp;quot;Brachistochrone curve to Mars orbit&amp;quot;&lt;/h2&gt;&#xA;&lt;p&gt;A Brachistochrone curve or something analogous would be a &lt;em&gt;non sequitur&lt;/em&gt; in orbital mechanics as it is defined by a constraint, such as a track or rail whose shape is varied. Without a constraint to optimize, you only have the orbits that respond to gravity and propulsive forces.&lt;/p&gt;&#xA;&lt;p&gt;If there were a physical track in space, you might optimize its shape to minimize time in an analagous way by perhaps falling towards the Sun and then veering later, but you would have to attach the track to something much bigger and immovable, which is impractical.&lt;/p&gt;&#xA;&lt;h2&gt;The use of the term is relegated to fiction on Earth and science on Kerbin&lt;/h2&gt;&#xA;&lt;p&gt;&lt;strong&gt;2020 update:&lt;/strong&gt; Comments point to the use of  &lt;a href=&quot;http://www.projectrho.com/public_html/rocket/torchships.php#brachistochrone&quot; rel=&quot;nofollow noreferrer&quot;&gt;Torchships; What exactly is a Brachistochrone anyway?&lt;/a&gt; which while it doesn't give any useful math or science, says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A &amp;quot;Brachistochrone&amp;quot; is a minimum transit time / maximum deltaV mission. Torchships use this because they have lots of deltaV to spare.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5J3Cr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5J3Cr.png&quot; alt=&quot;Detail of artwork by Philippe Bouchet AKA &amp;quot;Manchu&amp;quot; for Robert Heinlein's Time For the Stars, Torchship &amp;quot;Lewis &amp;amp; Clark&amp;quot;&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Detail of artwork by &lt;a href=&quot;http://manchu-sf.blogspot.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Philippe Bouchet&lt;/a&gt; AKA &amp;quot;Manchu&amp;quot; for Robert Heinlein's Time For the Stars, Torchship &amp;quot;Lewis &amp;amp; Clark&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Likewise Scott Manley's video &lt;a href=&quot;https://www.youtube.com/watch?v=toMnjO8aJDI&quot; rel=&quot;nofollow noreferrer&quot;&gt;Brachistochrone Trajectories For Spaceships Explained&lt;/a&gt; talks about torchships but without anything more than &lt;span class=&quot;math-container&quot;&gt;$x = \frac{1}{2} a t^2$&lt;/span&gt; in the math or science departments.&lt;/p&gt;&#xA;&lt;p&gt;So while &amp;quot;Brachistochroneness&amp;quot; might be invoked on Kerbin, I don't see how it can be applied to propulsive spaceflight.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FHmVP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FHmVP.jpg&quot; alt=&quot;Manley's lame discussion of orbital mechanics based on x=1/2 at^2&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2020-03-14T03:41:55.680" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="23093" PostTypeId="1" CreationDate="2017-09-19T04:36:24.660" Score="7" ViewCount="384" Body="&lt;p&gt;I know that one can determine the approximate wait time for a window of opportunity to open up for an interplanetary transfer based on the relative positions and velocities of the two planets in question.  But once a window opens up, how long does it remain open?  Assuming the simplest case of a Hohmann transfer with a hyperbolic escape trajectory toward the direction of the planet's velocity, how does one compute/control the amount of time in which one can depart?  What factors limit it and how can expand it as much as possible?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastActivityDate="2017-09-19T19:19:01.787" Title="What factors determine how long a window of opportunity opens for an interplanetary hohmann transfer?" Tags="|orbital-mechanics|interplanetary|escape-velocity|launch-window|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23095" PostTypeId="2" ParentId="23079" CreationDate="2017-09-19T11:04:17.330" Score="4" Body="&lt;p&gt;I think the answer is probably no, but not for the reasons other answers give.  First of all we can ignore the whole multi-body problem: it's a really good approximation that the planets &amp;amp; Sun  run on rails since they are hugely more massive than the spacecraft.  let's also assume that modelling a trajectory between two points is tractable, whether or not you use continuous thrust or not (this could well be a reasonably hard optimisation problem to minimise fuel &amp;amp;c but I suspect that's very doable on a modern personal computer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's not what makes it hard: what makes it hard is that this is a search problem merely dressed up as a physics problem, and search problems, famously, have combinatorial explosions.  Search problems require machines like &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Blue_%28chess_computer%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Blue&lt;/a&gt; to solve them, and these things are definitely supercomputers (albeit specialised ones).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is is a search problem?  Well, because the way you get around the Solar System isn't in fact by computing a trajectory between two points, it's by computing a bunch of gravitational slingshots around other bodies in the Solar System.  And there are a large number of such possible trajectories, and the number increases, possibly exponentially, as you increase the number of slingshots.  And you can't deform the trajectories into each other to use any nice numerical solving approach because you keep crashing into planets since all these trajectories go rather close to planets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Checking&lt;/em&gt; a proposed trajectory is much easier: if I tell you the plan is to do a couple of assists around Venus, a course correction burn in deep space then an assist around Earth and one around Jupiter on the way to Saturn (this is what Cassini did) then you can pretty easily check the trajectory is OK and compute its fine details.  But &lt;em&gt;arriving&lt;/em&gt; at such a trajectory is a different question.  This smells strongly of P and NP: given a solution it is easy to check, but arriving at a solution might be hard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So this might actually be a computationally seriously demanding problem.  I think it probably &lt;em&gt;isn't&lt;/em&gt; in fact, for a few reasons: there aren't very many objects you can use for slingshots so the search space doesn't explode too badly, and the mission duration is constrained as is fuel for course adjustments &amp;amp;c so you can prune solutions which take more time than you have or may need more fuel than you have.  I suspect that keeps the computation sane.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;[Note I'm posting this answer as a guest: I started writing it on the physics SE last night but the question got migrated &amp;amp; I don't belong to this SE.]&lt;/p&gt;&#xA;" OwnerUserId="21102" LastActivityDate="2017-09-19T11:04:17.330" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23096" PostTypeId="2" ParentId="22907" CreationDate="2017-09-19T16:47:04.433" Score="3" Body="&lt;p&gt;Unfortunately you'll need to eyeball the graphs yourself to try and create any analytical functions - but &lt;a href=&quot;https://www.ewp.rpi.edu/hartford/~ernesto/S2013/EP/MaterialsforStudents/Lee/Sutton-Biblarz-Rocket_Propulsion_Elements.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Propulsion Elements (7th ed.) by Sutton and Biblarz&lt;/a&gt; has some graphs of $C_{L}(\alpha,M)$ and $C_{D}(\alpha,M)$ for the German V-2 in Section 4.2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These plots assume no interactions with the exhaust plume, and the V-2 isn't quite cylindrical, but it should get you started. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're looking to model drag during supersonic retropropulsion as well, &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/42g25l&quot; rel=&quot;nofollow noreferrer&quot;&gt;there's a fantastic thread on r/SpaceX&lt;/a&gt; discussing that, specifically with the Falcon 9 in mind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZLqDe.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZLqDe.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21025" LastEditorUserId="12102" LastEditDate="2017-09-20T00:41:51.277" LastActivityDate="2017-09-20T00:41:51.277" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23097" PostTypeId="2" ParentId="23093" CreationDate="2017-09-19T18:06:05.477" Score="11" Body="&lt;p&gt;The factor which determines the duration of a transfer window is the performance margin of the launcher or spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hohmann transfer is the minimum delta-v for transfer between two circular orbits; in the strict sense the transfer window that achieves the minimum delta-v is instantaneous, but if a small amount of delta-v margin above the absolute minimum is available, the window becomes longer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're looking at a &lt;a href=&quot;https://en.wikipedia.org/wiki/Porkchop_plot&quot; rel=&quot;noreferrer&quot;&gt;porkchop plot&lt;/a&gt;, the point at the center of the innermost ring of the porkchop is the instantaneous window. Each contour line represents constant C&lt;sub&gt;3&lt;/sub&gt;, which is closely related to the ∆v in the injection maneuver; if you have enough performance margin to reach a given contour line, the duration of the window corresponds to the width of the contour on the plot. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want a longer window, lighten the payload or improve the launcher. Note that if you have performance margin, you can also use it to reduce the flight time at the cost of narrowing the window; it's not unusual to fly a faster-than-Hohmann trajectory for interplanetary missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launching from Earth, the rotation of the Earth also sets constraints; you need to maneuver your spacecraft from the available inclinations at your launch site to the inclination of the destination. To minimize the cost of that inclination change, you need to launch at a certain time of day. Once again, the more performance margin your launcher has, the more you can stretch that window.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-19T19:19:01.787" LastActivityDate="2017-09-19T19:19:01.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23098" PostTypeId="2" ParentId="23079" CreationDate="2017-09-19T19:31:36.677" Score="8" Body="&lt;p&gt;The short answer is no, the calculations do not require a supercomputer.  Any modern laptop has the computing power to handle the scenario.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The long answer is that the particular orbit depicted in the movie is what's called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_cycler&quot; rel=&quot;noreferrer&quot;&gt;Planetary Cycler&lt;/a&gt;, which cycles between two bodies (Earth/Mars).  You can use gravity assists at each body to put your spacecraft on a return trajectory to the other body.  These gravity assists can be augmented with maneuvers as needed.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a &lt;a href=&quot;https://repositories.lib.utexas.edu/bitstream/handle/2152/1253/russellr74662.pdf&quot; rel=&quot;noreferrer&quot;&gt;PhD dissertation&lt;/a&gt; on Planetary Cyclers where an optimization routine was developed to find cyclers for Earth/Mars in the real solar system, without any simplifying assumptions.  While the dissertation doesn't discuss computational load, it was done on a single machine.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, things like low-thrust maneuvers or trajectory optimization do increase the computational burden, and for certain applications compute clusters are used, but usually not required.  The usual approach is to do a wide search for viable trajectories with a simplified (low computational cost) model, and then when a smaller viable set has been found, apply higher-fidelity models that have a higher computational cost.  In this way you can avoid the need to use a supercomputer.&lt;/p&gt;&#xA;" OwnerUserId="12483" LastActivityDate="2017-09-19T19:31:36.677" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23101" PostTypeId="1" CreationDate="2017-09-19T22:59:56.973" Score="1" ViewCount="1045" Body="&lt;p&gt;When departing a planet using a hyperbolic trajectory, one should ideally reach the sphere of influence boundary with an excess velocity needed for interplanetary transfer.  Of course, the true excess velocity is only reached after infinite time has passed.  But how much time does it require for the spacecraft's velocity to decay to within a reasonable threshold relative error of the excess velocity (within its sphere of influence)?  What percent error should one expect when reaching the boundary of a planet's sphere of influence?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="6494" LastEditDate="2017-09-20T02:23:27.497" LastActivityDate="2017-09-20T04:21:14.617" Title="What is the characteristic time to reach a hyperbolic excess velocity within a sphere of influence?" Tags="|orbital-mechanics|interplanetary|escape-velocity|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23102" PostTypeId="1" CreationDate="2017-09-19T23:27:03.823" Score="5" ViewCount="2025" Body="&lt;p&gt;An impulse thrust maneuver is a good approximation of a true orbital maneuver. Of course, a true impulse maneuver in reality would require an infinite amount of force in an infinitely small amount of time (a dirac delta function). In reality, thruster apply finite force in finite time, generating a continuous trajectory change which I think should asymptotically acheive the same result, probably by continuously applying a force starting some time before until sometime after the ideal point where the impulse maneuver would be applied.&lt;/p&gt;&#xA;&lt;p&gt;My question is: for a given velocity change &lt;span class=&quot;math-container&quot;&gt;$\Delta v$&lt;/span&gt;, how do I determine the required finite force and when exactly to start and stop applying the force? Would I need to account for mass losses during the burn? If so, how do I account for them?&lt;/p&gt;&#xA;&lt;p&gt;I'm looking for equations that can help me determine this.&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="1054" LastEditDate="2020-12-23T22:58:11.703" LastActivityDate="2020-12-23T22:58:11.703" Title="How can I design a finite thrust maneuver to approximate an impulse thrust maneuver?" Tags="|orbital-mechanics|specific-impulse|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23103" PostTypeId="2" ParentId="23101" CreationDate="2017-09-19T23:43:07.803" Score="-1" Body="&lt;p&gt;Zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual excess velocity is reached right upon leaving the planet's sphere of influence. Since then the velocity starts changing (in an arbitrary direction) primarily according to influence of the body whose sphere of influence the craft entered. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course that's a simplification - in fact both (as well as all other) bodies influence the velocity at all times, it's just that the strongest influence comes from the body whose SOI you're in, but for purpose of this explanation, let's assume Patched Conics instead of N-body model; the difference is moot for this purpose. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since you're always in &lt;em&gt;some&lt;/em&gt; SOI, it's pointless to think of &quot;true excess velocity&quot; relative to one SOI after traveling ways through another SOI. You have the (one, actual, true) excess velocity upon reaching the edge, then you just have the velocity in the other SOI, and it may differ a lot, and you may still get the relative velocity to the first body if you're inclined to, but it won't be any special &quot;true&quot; one, it will just be a different one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say, you escape Moon SOI on the Free Return trajectory. What will be the &quot;true excess velocity&quot; if you're on an Earth reentry trajectory? What velocity are we supposed to calculate the error relative to?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And even if you go into deep space, eject out of Solar System towards Proxima Centauri, you'll remain in Sun's SOI until roughly halfway through, when you'll enter Proxima's SOI - and at that point you'll cease to lose velocity as you'll start getting pulled towards Proxima. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-09-19T23:43:07.803" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23104" PostTypeId="2" ParentId="23101" CreationDate="2017-09-20T01:18:04.870" Score="1" Body="&lt;p&gt;&lt;strong&gt;tl;dr:&lt;/strong&gt; For a trip from Earth to Mars, I get a velocity 1% greater than $v_{\infty}$ at point about 70% to the way to the Earth's SOI.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The question is about departing a planet and proceeding to interplanetary transfer, and since the term &quot;sphere of influence&quot; is invoked, presumably the patched conics approximation is assumed, so we can think of the Earth as the only source of gravity, and work in the rest frame of the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Vis-viva_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;vis-viva equation&lt;/a&gt; gives everything we need.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v^2=GM_E\left(\frac{2}{r}-\frac{1}{a}\right).$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we start from a circular orbit in LEO with a radius $a_0$, the velocity is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{LEO}=\sqrt{\frac{GM_E}{a_0}.}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After a propulsive $\Delta v$ the velocity is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_0 = v_{LEO} + \Delta v.$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rearrange the vis-viva to solve for the new semi-major axis of the hyperbola:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a_{hyp}=1/\left(\frac{2}{a_0}-\frac{v_0^2}{GM_E}\right).$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a hyperbolic orbit, the semi-major axis is negative.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now you can get the velocity (speed really) at any distance r:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v^2=GM_E\left(\frac{2}{r}-\frac{1}{a_{hyp}}\right),$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v(r)=\sqrt{GM_E\left(\frac{2}{r}-\frac{1}{a_{hyp}}\right)},$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and taking the limit of $r \rightarrow \infty$ we get $v_{\infty}$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_{\infty}= \sqrt{\frac{GM_E}{-a_{hyp}}}.$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, if you want to calculate kinetic and potential energy of a spacecraft with mass $m$ at any distance $r$ you can just use:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T(r)=\frac{1}{2}mv^2(r),$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$U(r)=\frac{-GM_E m}{r},$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and the sum of the two should stay constant. That's actually the soul of the vis-viva equation!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;OK, now let's put in some numbers; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$GM_E$ = 3.986E+14 m^3/s^2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$GM_S$ = 1.327E+20 m^3/s^2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$a_0$ = 6,378,000 + 250,000 meters&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta v$ = 5,7000 m/s&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$AU$ = 1.5E+11 meters&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$R_{SOI} = AU\left(\frac{GM_E}{GM_S}  \right)^{2/5}$ ~ 9.27E+08 meters&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I get $v_{\infty}$ of about 7,795 m/s, and a velocity of 1% above that at a distance of 655,000 km from earth, or about 70% of the way to the edge of the sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Fyujx.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Fyujx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-20T04:21:14.617" LastActivityDate="2017-09-20T04:21:14.617" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23105" PostTypeId="1" CreationDate="2017-09-20T03:30:19.580" Score="3" ViewCount="391" Body="&lt;p&gt;According to Wikipedia, &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Independence&quot; rel=&quot;nofollow noreferrer&quot;&gt;Independence&lt;/a&gt; is a high-fidelity replica of the space shuttle. The timing of its construction would seem to be &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Challenger_disaster&quot; rel=&quot;nofollow noreferrer&quot;&gt;post-Challenger&lt;/a&gt;, but before &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Columbia_disaster&quot; rel=&quot;nofollow noreferrer&quot;&gt;Columbia&lt;/a&gt; and the subsequent winding down of the program. What motivated the construction of such a thing at that point in time? Was it built solely as a full-scale display model, or did it have utility in the shuttle program as a mock-up?&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-09-20T13:55:59.810" Title="What was the purpose for which the shuttle mockup Explorer aka Independence was constructed?" Tags="|space-shuttle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23106" PostTypeId="2" ParentId="23105" CreationDate="2017-09-20T03:59:33.600" Score="7" Body="&lt;p&gt;It was built solely for display at KSC's visitor center, as described in the Wikipedia article you reference, and in this &lt;a href=&quot;http://www.tested.com/science/space/44264-nasas-many-mock-space-shuttle-orbiter-models/&quot; rel=&quot;noreferrer&quot;&gt;article&lt;/a&gt; about Orbiter mockups.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Explorer was built primarily for display and educational&#xA;  purposes--visitors could actually go inside the replica to tour the&#xA;  payload bay and crew cabin.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Explorer was renamed Independence after its move to Houston.  Other, sarcastic, names suggested by JSC employees such as Fauxlumbia/Ainterprise/Notlantis weren't accepted...)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-09-20T13:55:59.810" LastActivityDate="2017-09-20T13:55:59.810" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23107" PostTypeId="2" ParentId="23102" CreationDate="2017-09-20T04:20:53.783" Score="3" Body="&lt;p&gt;Warning: the following is KSP-level discussion; professional trajectory design probably doesn't allow this much hand-waving.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a sufficiently short but finite impulse, the difference between the idealized impulse and the actual is small enough that it will be lost in the measurement error of other uncertainties -- altitude measurement, nonuniformity of the planet's gravitational field, thrust irregularity, and so forth, and so you can expect to correct the error after the fact with a midcourse adjustment of some sort. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a longer impulse, I see two options:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Time the burn to end at the ideal point, but adjust length of the burn and continually alter the direction so that you do not leave the initial orbital path, just increasing your velocity on it, so that the exit condition -- position and velocity -- is exactly the same as it would be if the impulse was instantaneous. If, for example, your ideal maneuver is a tangential Hohmann out of circular orbit, you would need to initially aim below the horizon a bit, to get some downward force to stay on the circular path while exceeding orbital velocity -- a form of &quot;forced orbit&quot;. You need to very slightly extend the length of the burn because your burn direction is turning along an arc instead of in a straight line. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Balance the duration of the burn evenly around the ideal point, holding the direction of the ideal burn constantly throughout the whole period. This means your altitude at exit will be higher than you intended (because you exceeded orbital velocity when you started the burn). You can compensate by iteratively computing the error in final position induced by the finite duration burn and adding a correction to the ideal burn to compensate, repeating until the error is small enough to be dealt with in later midcourse correction. There's probably an analytical solution to short-cut the iterative correction, but my calculus isn't strong, and a complex gravitational model might make that too much trouble. :) &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I don't know off the top of my head what the relation between burn duration and trajectory error would be; I'm sure trajectory designers have some &quot;don't worry about it&quot; threshold as a rule of thumb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electric propulsion such as ion thrusters will of course have such a long burn duration that more comprehensive handling is required -- the total burn would need to be divided into repeated short burns at perigee, to begin with. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're expensing a sizable fraction of your mass in fuel, you'll want to account for that as well; you can use the Tsiolkovsky equation to work out the mass drop given the delta-v. The falling mass implies rising acceleration, which I think breaks the symmetry of centering the burn duration on the ideal. Not sure how to handle that.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-20T04:49:37.153" LastActivityDate="2017-09-20T04:49:37.153" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23108" PostTypeId="1" AcceptedAnswerId="27113" CreationDate="2017-09-20T06:44:23.777" Score="11" ViewCount="2268" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/MHW-RTG&quot; rel=&quot;noreferrer&quot;&gt;MHW-RTG&lt;/a&gt; model &lt;a href=&quot;https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator&quot; rel=&quot;noreferrer&quot;&gt;Radioisotope Thermoelectric Generators&lt;/a&gt; on the two &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_program&quot; rel=&quot;noreferrer&quot;&gt;Voyager spacecraft&lt;/a&gt; started out at about 1000C and 300C on the hot and cold sides, but will have dropped somewhat due to slow decay of the radioisotope thermal output.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the computer is still working, presumably the electronics is warmer than the two spacecrafts' extremities by self-heating or other sources of heat within the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But roughly how cold are the extremities; parts of the spacecraft that are warmed mostly by the distant Sun and a bit by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Cosmic_microwave_background&quot; rel=&quot;noreferrer&quot;&gt;cosmic microwave background&lt;/a&gt; (CMB)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would there be some extremities of each spacecraft that are now permanently in the shadow of the high-gain antenna dishes pointed at Earth and Sun, and so not even warmed directly by sunlight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there is some telemetry, that would be great to know. If there is a way to calculate by first principles, that's fine also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it colder than LOX? Than solid oxygen (SOX)? Than LH2? Will gas stick to it and start building up hydrogen ice?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-20T08:53:39.920" LastActivityDate="2018-05-06T07:31:04.207" Title="How cold are the Voyagers now? Colder than LOX? Colder than SOX?" Tags="|voyager|temperature|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23109" PostTypeId="2" ParentId="18802" CreationDate="2017-09-20T08:59:31.500" Score="4" Body="&lt;p&gt;Here is a supplementary answer addressing the last part of the OP's question about the possibility of miniaturization and use specifically in cubesats.&lt;/p&gt;&#xA;&lt;p&gt;Cubesats have several restrictions on chemical propellants and storage of chemical energy. One step towards addressing this problem has been discussed in &lt;a href=&quot;http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2013index/968x932g.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Performance Characteristics of Low-Power Arcjet Thrusters Using Low Toxicity Propellant HAN Decomposed Gas&lt;/a&gt;, Matsumoto et al. IEPC-2013-095, (33st International Electric Propulsion Conference, The George Washington University, USA, October 6 – 10, 2013).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Abstract:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Although hydrazine (N2H4) is used as a propellant of spacecraft thrusters, it is high toxicity liquid. Spacecraft researchers need a low toxicity propellant. Hydroxyl Ammonium Nitrate (HAN: NH3OHNO3) is proposed as a low toxicity propellant. In this study, the possibility of HAN as a propellant of direct-current arcjet thrusters is examined. Using the HAN-simulated-decomposed gas of H2O, CO2 and N2 mixture, the hydrazine-simulated-decomposed gas of N2 and H2 mixture, and N2 itself, an arcjet thruster was&#xA;operated, and the basic characteristics were obtained and compared. The thrust and the thrust efficiency with the HAN decomposed gas were 182.6 mN and 10.3 %, respectively, at a specific impulse of 156.4 sec with 1.36 kW input power, although severely-eroded electrodes was observed, and their performance was slightly low compared with the hydrazine decomposed gas.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="21116" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-09-20T09:21:33.103" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23110" PostTypeId="2" ParentId="23108" CreationDate="2017-09-20T09:08:07.050" Score="8" Body="&lt;p&gt;Voyager was designed to operate at -35 °C. As the RTG output drops, heaters have been switched off. The &lt;a href=&quot;https://www.nasa.gov/mission_pages/voyager/voyager20120117.html&quot; rel=&quot;noreferrer&quot;&gt;spectrometer currently runs below -79 °C&lt;/a&gt;. Its temperature is not known accurately, as it's dropped below the operating range of the temperature sensor.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The magnetometer on its long boom is heated by a &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810001583.pdf&quot; rel=&quot;noreferrer&quot;&gt;Pu-238 heater&lt;/a&gt; (page 20). The other extremity is the camera platform, which was switched off years ago. Haven't found temperature data for either though (no engineering data in the &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/datasetSearch.do&quot; rel=&quot;noreferrer&quot;&gt;Voyager data collections&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Radioisotope heating units, small non-power-using heat elements that generate one watt of thermal energy, are located on the magnetometer sensors and the Sun sensors. No radioisotope heating units are used near instruments that detect charged particles. Electric heaters are located throughout the spacecraft to provide additional heat during portions of the mission. Many&#xA;  of the heaters are turned off when their respective valves, instruments or subassemblies are on and dissipating power.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The vidicon tubes in the imaging system were designed to operate at temperatures above +10 °C. The hydrazine tanks need to be kept above +2 °C. I suspect the tape recorder needs to be kept fairly warm as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The main body of the spacecraft is shaded by the HGA these days (after the last planetary encounter there was no reason anymore to point the HGA away from Earth), only the RTG, camera platform and magnetometer boom are sunlit.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-09-20T11:26:34.760" LastActivityDate="2017-09-20T11:26:34.760" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23111" PostTypeId="1" CreationDate="2017-09-20T10:53:02.803" Score="3" ViewCount="344" Body="&lt;p&gt;I am very confused with regards the simulation and fitting of satellite orbits. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, how does one compare the results of a numerically integrated force model with an orbit that has been fitted to laser ranging data??&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you numerically integrate a force model how do you compare with one of the many many many reference frames that exist that your fitted orbit will be subjected to! Any suggestions? &#xA;My understanding is: when you simulate an orbit you will generate a whole host of positions. Then to compare it to an actual orbit you must select a time epoch which will orient the coordinate system ( is this correct??). If this is correct what follows? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The model I would like to numerically integrate is only Newtonian gravity plus relativistic corrections (assuming point particles) as recommended by the IERS given by Eq. (10.12) in the &lt;a href=&quot;https://www.iers.org/IERS/EN/DataProducts/Conventions/conventions.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;most current&lt;/a&gt; technical document/convention. &lt;/p&gt;&#xA;" OwnerUserId="21031" LastEditorUserId="21031" LastEditDate="2017-09-20T22:26:41.823" LastActivityDate="2017-09-21T14:26:07.713" Title="How do you compare a simulation with a fitted orbit?" Tags="|orbital-mechanics|artificial-satellite|low-earth-orbit|orbit-determination|prediction|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23112" PostTypeId="1" CreationDate="2017-09-20T14:12:08.480" Score="1" ViewCount="731" Body="&lt;p&gt;Are there open source solutions (preferable Python) to calculate the ground track of a spacecraft (i.e. in latitute/longitude) from &lt;a href=&quot;https://public.ccsds.org/Pubs/502x0b2c1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCSDS OEM&lt;/a&gt; (orbit ephemeris message) format (which is a dataset of epochs and associated position vectors)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternatively, one could get lat/lon from TLE via pyehpem or other libraries. Then the question would be: how to convert OEM to TLE?&lt;/p&gt;&#xA;" OwnerUserId="21118" LastEditorUserId="12483" LastEditDate="2017-09-23T03:21:16.547" LastActivityDate="2017-09-23T03:21:16.547" Title="Calculate ground track from CCSDS OEM" Tags="|orbital-elements|two-line-elements|" AnswerCount="0" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23113" PostTypeId="2" ParentId="23111" CreationDate="2017-09-20T14:38:39.383" Score="5" Body="&lt;p&gt;Comparisons between predicted orbits (numerically integrated) and fitted orbits (from tracking data) are usually done in an inertial reference frame, such as &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Celestial_Reference_Frame&quot; rel=&quot;noreferrer&quot;&gt;ICRF&lt;/a&gt; or &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth-centered_inertial&quot; rel=&quot;noreferrer&quot;&gt;Mean of J2000&lt;/a&gt;.  Within these frames, the orbit comparison can take several forms, such as point by point comparisons, or comparisons of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;noreferrer&quot;&gt;orbital elements&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12483" LastActivityDate="2017-09-20T14:38:39.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23114" PostTypeId="2" ParentId="23060" CreationDate="2017-09-20T18:06:33.400" Score="3" Body="&lt;p&gt;Try Kepler's method, which works for orbits with a (1/r$^2$) central force.  See Nathaniel Grossman, &lt;em&gt;The Sheer Joy Of Celestial Mechanics&lt;/em&gt;,  &lt;a href=&quot;http://spiff.rit.edu/classes/phys440/lectures/ellipse/ellipse.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spiff.rit.edu/classes/phys440/lectures/ellipse/ellipse.html&lt;/a&gt;, or&#xA;&lt;a href=&quot;http://www.bogan.ca/orbits/kepler/orbteqtn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.bogan.ca/orbits/kepler/orbteqtn.html&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example.  For an ellipse around Sol, suppose you know perihelion r$_P$ and aphelion r$_A$.  You are given the true anomaly f$_1$, the angle between perihelion and the object's position as measured from the focus occupied by Sol.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then eccentricity  e = (r$_A$-r$_P$)/(r$_A$+r$_P$), &lt;/p&gt;&#xA;&#xA;&lt;p&gt;semi-major axis  a = (r$_A$+r$_P$)/2, and   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;period T = 2$\pi$ sqrt(a$^3$/(MG)), &lt;/p&gt;&#xA;&#xA;&lt;p&gt;where M is the mass of Sol and G is Newton's constant.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compute the eccentric anomaly E$_1$, the angle between perihelion and the object's position as measured from the center of the ellipse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;cos(E$_1$) = (e+cos(f$_1$))/(1+ecos(f$_1$))&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The travel time deltaT$_1$ from perihelion to f$_1$ is &lt;/p&gt;&#xA;&#xA;&lt;p&gt;deltaT$_1$ = (E$_1$-esin(E$_1$))T/(2$\pi$)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Find deltaT$_2$ for the second position.  Then the travel time from f$_1$ to f$_2$ = deltaT$_2$ - deltaT$_1$.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third reference above tabulates the variations needed for circles, parabolae, and hyperbolae.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;-MBMelcon&lt;/p&gt;&#xA;" OwnerUserId="12999" LastEditorUserId="1142" LastEditDate="2017-09-21T22:40:56.553" LastActivityDate="2017-09-21T22:40:56.553" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23115" PostTypeId="2" ParentId="15463" CreationDate="2017-09-20T19:48:58.537" Score="-3" Body="&lt;p&gt;Yes they do use shock absorbers. In this case it's called an &quot;accumulator.&quot; The problem arose during the Apollo 6 launch.&quot; The &quot;resonance effect&quot; or &quot;pogp oscillations&quot; abruptly appeared during the first 2 minutes of flight. Pogo arises fundamentally because you have thrust fluctuations in the engines. Those are normal characteristics of engines. All engines have what you might call noise in their output because the combustion is not quite uniform, so you have this fluctuation in thrust of the first stage as a normal characteristic of all engine burning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, in turn, the engine is fed through a pipe that takes the fuel out of the tanks and feeds it into the engine. That pipe's length is something like an organ pipe so it has a certain resonance frequency of its own and it really turns out that it will oscillate just like an organ pipe does.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The structure of the vehicle is much like a tuning fork, so if you strike it right, it will oscillate up and down longitudinally. In a gross sense it is the interaction between the various frequencies that causes the vehicle to oscillate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_6&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Apollo_6&lt;/a&gt;&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/pOOrXWLLza0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="20970" LastActivityDate="2017-09-20T19:48:58.537" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23118" PostTypeId="2" ParentId="15463" CreationDate="2017-09-20T22:24:39.747" Score="-2" Body="&lt;p&gt;Marshall and North American engineers devised three changes to the second stage. They installed a helium gas accumulator in the LOX line of the center engine. This reservoir served to dampen fluid pressure oscillations, keeping them out of phase with the vibrations of the thrust structure and engines.&lt;/p&gt;&#xA;" OwnerUserId="20970" LastActivityDate="2017-09-20T22:24:39.747" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23120" PostTypeId="1" CreationDate="2017-09-21T11:33:49.110" Score="4" ViewCount="349" Body="&lt;p&gt;&lt;strong&gt;EDIT October 15, 2017:&lt;/strong&gt; the problem in this question is cast as an optimal control problem and reformulated &lt;a href=&quot;https://space.stackexchange.com/questions/23392/how-to-numerically-solve-an-optimal-control-problem&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft I am designing is flying in an elliptical orbit (8 x 3000 km) around the Moon. At periapsis, the non-impulsive bi-propellant retromotor mounted upon the spacecraft will cancel the orbital velocity while at the same time increase the altitude from 8 to 28 km (the spacecraft will then start its free-fall phase to the surface, but that's not part of this problem). The Moon is assumed to be a perfect sphere with $r_{Moon}=1737.100$ $km$ and non-rotating.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The state vector consists of 7 components (position and velocity vectors, plus the mass). The initial state vector is defined as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\textbf{y}_0 = \begin{bmatrix}x \\ y \\ z \\ \dot{x} \\ \dot{y} \\ \dot{z} \\ m \end{bmatrix}_{t=0} = \begin{bmatrix}1745100 \enspace m\\ 0 \\ 0 \\ 0 \\ 2026.813 \enspace m/s\\ 0 \\ 70 \enspace kg \end{bmatrix}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dynamics are given by:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\ddot{\textbf{r}}=-\mu\frac{\textbf{r}}{r^{3}}+\frac{\textbf{T}}{m}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\dot{\textbf{y}} = \begin{bmatrix}\dot{x} \\ \dot{y} \\ \dot{z} \\ \ddot{x} \\ \ddot{y} \\ \ddot{z} \\ \dot{m} \end{bmatrix} \quad \textbf{f}(t,\textbf{y}) =  \begin{bmatrix}y_4\\ y_5 \\ y_6 \\ -\mu\frac{y_1}{r^{3}}+\frac{T}{y_7}\cdot (k_x ?) \\ -\mu\frac{y_2}{r^{3}}+\frac{T}{y_7}\cdot (k_y?)\\ -\mu\frac{y_3}{r^{3}}+\frac{T}{y_7}\cdot (k_z?) \\ -\frac{T}{I_{sp}g_{0}} \end{bmatrix}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With $r=\sqrt{y_1^{2}+y_2^{2}+y_3^{2}}$ and constants:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mu = 4.90487E(+12) \enspace m^3/s^2$; $T = 200 \enspace N$; $I_{sp} = 300 \enspace s$; $g_{0} = 9.81 \enspace m/s^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The $k$'s in the set of equations should determine the thrust magnitude (and sign) in that particular direction ($x$, $y$ or $z$) at each integration step: $\textbf{k}$ is the unit vector of the thrust vector. The solution I'm seeking should be in the form of a vector set containing thrust vectors at each integration step of the breaking/climbing phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The final state vector should be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;\textbf{y}_n = \begin{bmatrix}x_n \\ y_n \\ z_n \\ 0 \\ 0 \\ 0 \\ m_n \end{bmatrix}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $r_n=\sqrt{x_n^{2}+y_n^{2}+z_n^{2}}=1737100+28000=1765100 \enspace m$ and $m_n$ must be maximum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any ideas on how to find an optimal solution (read: optimal thrust-vectoring strategy, final mass $m_n$ should be maximum) for this boundary value problem?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks a lot!&lt;/p&gt;&#xA;" OwnerUserId="21124" LastEditorUserId="21124" LastEditDate="2017-10-15T11:45:23.320" LastActivityDate="2017-10-15T11:45:23.320" Title="Retromotor thrust-vectoring strategy (boundary value problem)" Tags="|thrust-vectoring|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23121" PostTypeId="1" CreationDate="2017-09-21T12:15:48.083" Score="3" ViewCount="204" Body="&lt;p&gt;This is a continuation of a &lt;a href=&quot;https://space.stackexchange.com/questions/21048/number-of-burns-for-optimal-launch-ascent&quot;&gt;previous&lt;/a&gt; question posted a few months ago. If we consider a SSTO vehicle launching from a planet with an atmosphere, would it be fuel optimal to have a period of lowered thrust whilst going through the thickest part of the atmosphere (even if we &lt;em&gt;ignore&lt;/em&gt; dynamic pressure constraints), in order to minimize the largest losses from drag? If so, would a fuel-optimal thrust profile for an ascent into orbit consist of an initial period of maximum thrust, a second period of lowered thrust, a third period of maximum thrust, a fourth period of zero thrust (coast phase), and a fifth and final period of maximum thrust in order to circularize the orbit?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastActivityDate="2017-09-21T12:55:16.120" Title="Fuel-optimal thrust profile for an orbital ascent on a planet with an atmosphere" Tags="|launch|orbital-mechanics|orbital-maneuver|launch-sequence|launch-vehicle|" AnswerCount="1" CommentCount="0" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23122" PostTypeId="2" ParentId="23121" CreationDate="2017-09-21T12:55:16.120" Score="4" Body="&lt;p&gt;Essentially, yes. Providing you can output enough thrust and the atmosphere is so dense that your atmospheric drag losses during that relatively short period would outpace gravitational drag losses over the whole ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Earth, with the kind of propulsion we have, this isn't the case: MaxQ is primarily a structural durability concern, air pressure drops rapidly over the first few kilometers, air drag becoming far less of an issue rapidly, and gravitational drag swallows nearly 1000m/s of delta-V, never mind higher pressure reduces performance of engines making such rapid acceleration difficult to achieve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, essentially, on our planet, with engines as they are, and rockets as durable against dynamic pressure as they are, the &quot;reduce thrust to reduce atmospheric drag&quot; constraint is never encountered. It might become an issue with ground-aided launches (e.g. a &quot;space gun&quot; that gives the rocket first 2km/s at ground level) or if trying to launch from Venus or such. Not with current propulsions though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wouldn't be entirely sure about &quot;fifth and final period of maximum thrust in order to circularize the orbit&quot;. You need very little delta-V to circularize, you operate at very high TWR (engine acting at maximum vacuum thrust, fuel tanks mostly empty not weighing it down!) and you want to achieve your orbit with a decent precision. Therefore throttling down may be desirable for practical/safety purposes (completely apart from fuel-optimality). &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-09-21T12:55:16.120" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23123" PostTypeId="2" ParentId="22667" CreationDate="2017-09-21T13:01:32.507" Score="2" Body="&lt;p&gt;Ultimately, the answer is no, Cassini did not run out of fuel:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://saturn.jpl.nasa.gov/mission/grand-finale/cassini-quick-facts/&quot; rel=&quot;nofollow noreferrer&quot;&gt;At the end of its mission&lt;/a&gt;, Cassini had used 6504 pounds out of 6565 pounds (2950 kilograms out of 2978 kilograms) that it had started with.  Thus, there was about 61 pounds (28 kg) of fuel still on board when Cassini &lt;a href=&quot;https://saturn.jpl.nasa.gov/news/3122/cassinis-family-takes-the-plunge/&quot; rel=&quot;nofollow noreferrer&quot;&gt;entered Saturn's atmosphere&lt;/a&gt; on 15 September 2017.  Although the referenced page doesn't indicate the distribution of the available propellant between the various systems that use it, there was probably enough fuel available so the mission &lt;strong&gt;&lt;em&gt;could&lt;/em&gt;&lt;/strong&gt; have gone on for another orbit or so.  However, the amount of science that could have been performed in &quot;a few more orbits&quot; before the fuel ran out &lt;em&gt;vs.&lt;/em&gt; the unique perspective achieved through the end-of-life &quot;Grand Finale&quot; that was followed made the latter choice clearly advantageous.  In addition, leaving the probe in orbit around Saturn would have risked contaminating the environment much more than burning it up in the atmosphere contaminated Saturn.  Thus, spending fuel to crash Cassini into Saturn rather than using orbital mechanics would have limited the amount of time the mission could have been extended, further adding to the attractiveness of the selected option.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As others have pointed out in their excellent and extremely informative answers, orbital mechanics were a much more dominant factor in deciding when the Cassini mission would end, rather than the available fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, it's much better to have some propellant left over when a mission ends, rather than running out before the [extended] goals were achieved:  From this distance, topping off the tanks just &lt;em&gt;isn't&lt;/em&gt; an option.  It's &lt;em&gt;far&lt;/em&gt; better to have &quot;too much&quot; than too little.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastActivityDate="2017-09-21T13:01:32.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23124" PostTypeId="2" ParentId="23111" CreationDate="2017-09-21T14:26:07.713" Score="1" Body="&lt;p&gt;First, you need to make sure both results (simulation results and laser ranging data) are converted into the same coordinate system. This is usually chosen as an inertial frame, like ECI. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can compare two orbits in ECI frame but you will only have one meaningful information from that: the magnitude of the error. However, in most papers, you will see another coordinate transformation to a satellite-based coordinate system is done to get more meaningful results. RSW and NTW systems are most common for this practice since most modeling differences create visible biases in those coordinate elements. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do not forget to take into account of the differences that are caused by coordinate transformations.&lt;/p&gt;&#xA;" OwnerUserId="8902" LastActivityDate="2017-09-21T14:26:07.713" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23125" PostTypeId="2" ParentId="22314" CreationDate="2017-09-21T16:03:25.443" Score="3" Body="&lt;p&gt;PREDICT uses legacy SGP4 code. SGP4 code is usually referred to a 1km/day rule of error increase. (However that actually strictly depends on the orbit type and satellite.)&lt;/p&gt;&#xA;&lt;p&gt;The second thing to note is SGP4 was updated by Vallado in 2006 with some error fixes and PREDICT does not see to have implemented those fixes.&lt;/p&gt;&#xA;&lt;p&gt;Therefore, using an updated SGP4 propagator like STK or &lt;a href=&quot;https://celestrak.org/software/vallado-sw.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vallado's code&lt;/a&gt; would be better in my opinion. But to be fair, as far as I remember, none of the fixes of Vallado was about usual cubesat orbits' ranges.&lt;/p&gt;&#xA;&lt;p&gt;If you are looking for a much better method than SGP4 but also if you do not have another source of tracking except TLEs, orbit determination is your answer.&lt;/p&gt;&#xA;" OwnerUserId="8902" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2017-09-21T16:03:25.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23126" PostTypeId="2" ParentId="22314" CreationDate="2017-09-21T18:39:27.417" Score="4" Body="&lt;p&gt;I used PREDICT operationally for the first few Planet Labs tech demo missions (Dove 1 / 2 / 3).  It's fine. The quality of your ephemeris data is going to be the limiting factor, not the software you use to propagate it.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-09-21T18:39:27.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23127" PostTypeId="1" CreationDate="2017-09-21T21:02:59.970" Score="3" ViewCount="522" Body="&lt;p&gt;I'm simulating an interplanetary trajectory between two spheres of influence.  I have already simulated a hyperbolic escape within the sphere of influence and have arrived at the boundary of the sphere of influence of the starting planet, where I'm trying to patch the escape hyperbola conic to an hohmann transfer ellipse and determine how long it will take to reach the other planet's sphere of influence (where I would need to patch from the hohmann transfer ellipse to another hyperbola within the destination planet's sphere of influence).  In the simulation I can simply do this by trial and error (i.e. simulate the trajectories of the transfer ellipse and see when it falls within the sphere of influence's radius).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I was wondering if there is a way to do this analytically using orbital mechanics principles.  Given that I have the spacecraft's initial position and velocity with respect to the sun (and therefore can derive all other orbital elements as required), as well as the destination planet's initial position and velocity w.r.t. the sun, how can I find the time of flight to reach the sphere of influence of the destination planet?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the key to solving this problem lies in parameterizing the true anomalies of the spacecraft's trajectory and the planet's trajectory as a function of time and starting anomaly.  Is there a way to do this?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastActivityDate="2018-02-04T12:07:59.713" Title="How can I find the time of flight between spheres of influence?" Tags="|orbital-mechanics|soi|time-of-flight|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23128" PostTypeId="1" CreationDate="2017-09-21T23:48:36.183" Score="3" ViewCount="1043" Body="&lt;p&gt;I was working an example in Charles Brown's &quot;Elements of Spacecraft Design&quot; but I can not figure out how the Time of Flight was calculated. Here is the problem &lt;a href=&quot;https://books.google.com/books?id=vpilMLP7OHQC&amp;amp;pg=PA106&amp;amp;lpg=PA106&amp;amp;dq=design%20transfer%20ellipse%20based%20on%20time&amp;amp;source=bl&amp;amp;ots=8LXxSpOx9i&amp;amp;sig=qHWxN4SjXLz4_OTHkjTB1zHemn8&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwj6m9_irrfWAhVQ9mMKHZKrAC0Q6AEIQjAE#v=onepage&amp;amp;q=design%20transfer%20ellipse%20based%20on%20time&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Example 6.5&lt;/a&gt;. I got the correct answer when calculating a, rP, and e, but after that I haven't been successful. Could anyone help?&lt;/p&gt;&#xA;" OwnerUserId="21121" LastEditorUserId="2752" LastEditDate="2017-09-23T12:02:53.907" LastActivityDate="2018-01-28T05:00:11.383" Title="Design of an elliptical transfer orbit" Tags="|orbital-mechanics|orbital-maneuver|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23129" PostTypeId="2" ParentId="23128" CreationDate="2017-09-22T03:37:47.020" Score="2" Body="&lt;p&gt;&lt;strong&gt;How to compute time of flight&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's easy if you know the gravitational parameter $\mu\equiv GM$, the semi-major axis $a$, the eccentricity $e$, the initial true anomaly $\theta_0$, and the final true anomaly $\theta_1$ (or equivalently, the change in true anomaly $\Delta \theta \equiv \theta_1 - \theta_0$).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Step 1: Compute the initial and final eccentric anomalies from the eccentricity and the initial and final true anomalies via&#xA;$$\tan \frac E2 = \sqrt{\frac{1-e}{1+e}} \tan\frac\theta2$$&#xA;Step 2: Compute the initial and final mean anomalies from the eccentricity and the initial and final eccentric anomalies via Kepler's equation&#xA;$$M = E - e\sin E$$&#xA;Step 3: Compute the change in mean anomaly via&#xA;$$\Delta M = M_2 - M_1$$&#xA;Step 4: Compute the mean motion from the gravitational parameter and semi-major axis length via&#xA;$$n = \sqrt{\frac \mu {a^3}}$$&#xA;Step 5: Compute the time of flight from the mean motion and the change in mean anomaly via&#xA;$$\Delta t = \frac {\Delta M} n$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;strong&gt;How to design a transfer orbit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is considerably more challenging. Here the goal is to find the transfer orbit given the initial and final position of the object. This is Lambert's problem. The underlying equations are transcendental; there is no nice closed form solution. Instead you need to make a guess and then iteratively refine the guess. There are a number of ways to iteratively solve Lambert's problem.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2017-09-22T11:32:44.787" LastActivityDate="2017-09-22T11:32:44.787" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23130" PostTypeId="2" ParentId="23128" CreationDate="2017-09-22T06:23:29.093" Score="2" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; There's more about the math for hyperbolic orbits in  @DavidHammon's other &lt;a href=&quot;https://space.stackexchange.com/a/24649/12102&quot;&gt;answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll add to @DavidHammon's &lt;a href=&quot;https://space.stackexchange.com/a/23129/12102&quot;&gt;clear answer&lt;/a&gt; by including the parabolic and hyperbolic equivalents just to have everything in one place. I've transcribed from &lt;a href=&quot;http://www.bogan.ca/orbits/kepler/orbteqtn.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this handy table&lt;/a&gt; and added alternate forms for $E$ and $F$ from Wikipedia. I did some spot checking with numerical integration just to make sure the results agreed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ellipse, Circle $(0 \le \epsilon &amp;lt; 1)$:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\tan \frac E2 = \sqrt{\frac{1-e}{1+e}} \tan\frac\theta2$$&#xA;$$\text{or}$$&#xA;$$\cos E = \frac{e+\cos\theta}{1+e\cos\theta} \tan\frac\theta2,$$&#xA;$$M = E - e\sin E,$$&#xA;$$\Delta M = M_2 - M_1,$$&#xA;$$\Delta t = \sqrt{\frac{a^3}{\mu}} \Delta M,$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Hyperbola ($\epsilon &amp;gt; 1)$:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\tanh \frac F2 = \sqrt{\frac{e-1}{e+1}} \tan\frac\theta2$$&#xA;$$\text{or}$$&#xA;$$\cosh F = \frac{e+\cos\theta}{1+e\cos\theta} \tan\frac\theta2,$$&#xA;$$M = e\sinh F-F,$$&#xA;$$\Delta M = M_2 - M_1,$$&#xA;$$\Delta t = \sqrt{\frac{(-a)^3}{\mu}} \Delta M,$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Parabola ($\epsilon = 1)$:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$D = \tan\frac\theta2,$$&#xA;$$M = D + \frac{D^3}{3},$$&#xA;$$\Delta M = M_2 - M_1,$$&#xA;$$\Delta t = \sqrt{\frac{q^3}{\mu}} \Delta M,$$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;To get the semi-major axis $a$ or to get $q$, use the following (don't worry that $a$ is negative for the hyperbola):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ellipse, Hyperbola:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a=\frac{r_{peri}}{1-e}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ellipse:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a=\frac{r_{peri}+r_{apo}}{2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Circle:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a=r$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Parabola:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$q=r_{peri}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quick check with $\mu=1$ and $r_{peri}=1$:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; e      theta      a     v_peri    E/D/F       M         t&#xA;1.5   90.000000  -2.0   1.581139  55.14281  40.94513  2.021271&#xA;1.0   90.000000   n/a   1.414214  57.29578  76.39437  1.885618&#xA;0.5   90.000000   2.0   1.224745  60.00000  35.19020  1.737177&#xA;0.0   90.000000   1.0   1.000000  90.00000  90.00000  1.570796&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;If you want to try it in Python:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(X, t):&#xA;    x, v = X.reshape(2, -1)&#xA;    acc  = -mu * x * ((x**2).sum())**-1.5&#xA;    return np.hstack((v, acc))&#xA;&#xA;def get_D(theta, e):&#xA;    if e == 1.0:&#xA;        D    = np.tan(0.5*theta)&#xA;    else:&#xA;        D    = np.nan&#xA;    return D&#xA;&#xA;def get_E(theta, e):&#xA;    if e &amp;lt; 1.0:&#xA;        term = np.sqrt((1.-e)/(1.+e)) * np.tan(0.5*theta)&#xA;        E    = 2.*np.arctan(term)&#xA;    else:&#xA;        E    = np.nan&#xA;    return E&#xA;&#xA;def get_E_alt(theta, e):&#xA;    if e &amp;lt; 1.0:&#xA;        term = (e + np.cos(theta)) / (1. + e*np.cos(theta))&#xA;        E    = np.arccos(term)&#xA;    else:&#xA;        E    = np.nan&#xA;    return E&#xA;&#xA;def get_F(theta, e):&#xA;    if e &amp;gt; 1.0:&#xA;        term = np.sqrt((e-1.)/(e+1.)) * np.tan(0.5*theta)&#xA;        F    = 2.*np.arctanh(term)&#xA;    else:&#xA;        F    = np.nan&#xA;    return F&#xA;&#xA;def get_F_alt(theta, e):&#xA;    if e &amp;gt; 1.0:&#xA;        term = (e + np.cos(theta)) / (1. + e*np.cos(theta))&#xA;        F    = np.arccosh(term)&#xA;    else:&#xA;        F    = np.nan&#xA;    return F&#xA;&#xA;def get_M_from_E(E, e):&#xA;    if e &amp;lt; 1.0:&#xA;        M = E - e*np.sin(E)&#xA;    else: &#xA;        M = np.nan&#xA;    return M&#xA;&#xA;def get_M_from_F(F, e):&#xA;    if e &amp;gt; 1.0:&#xA;        M = e*np.sinh(F) - F&#xA;    else: &#xA;        M = np.nan&#xA;    return M&#xA;&#xA;def get_M_from_D(D, e):&#xA;    if e == 1.0:&#xA;        M = D + D**3/3.&#xA;    else: &#xA;        M = np.nan&#xA;    return M&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;# http://www.bogan.ca/orbits/kepler/orbteqtn.html&#xA;&#xA;quarterpi, halfpi, pi, twopi = [f*np.pi for f in [0.25, 0.5, 1, 2]]&#xA;rads, degs = pi/180, 180/pi&#xA;&#xA;mu = 1.0&#xA;&#xA;th0, th1 = 0.0, halfpi&#xA;print &quot;th0, th1 (degs): &quot;, degs*th0, degs*th1&#xA;&#xA;eccs = [1.5, 1.0, 0.5, 0.0]&#xA;&#xA;for e in eccs:&#xA;&#xA;    print &quot;e: &quot;, e&#xA;&#xA;    rp =  1.0  # periapsis&#xA;&#xA;    if e &amp;lt; 1.0:&#xA;        print &quot;     is ellipse!&quot;&#xA;&#xA;        ra = rp * (1+e)/(1-e)&#xA;        print &quot;rp, ra: &quot;, rp, ra&#xA;&#xA;        a0 = 0.5*(rp + ra)&#xA;        v0 = np.sqrt(mu * (2./rp - 1./a0))&#xA;        print &quot;a0, v0: &quot;, a0, v0&#xA;&#xA;        E0,  E1  = [get_E(th, e) for th in [th0, th1]]&#xA;        M0,  M1  = [get_M_from_E(E, e)  for E  in [E0,  E1 ]]&#xA;        print &quot;E0, E1 (degs): &quot;, degs*E0, degs*E1&#xA;        print &quot;M0, M1 (degs): &quot;, degs*M0, degs*M1&#xA;&#xA;        print &quot;E0, E1: &quot;, E0, E1&#xA;        print &quot;M0, M1: &quot;, M0, M1&#xA;&#xA;        dt = np.sqrt(a0**3/mu) * (M1-M0)&#xA;&#xA;        print &quot;dt (sec): &quot;, dt&#xA;&#xA;    elif e &amp;gt; 1.0:&#xA;        print &quot;     is hyperbola!&quot;&#xA;&#xA;        ra = rp * (1+e)/(1-e)&#xA;        print &quot;rp, ra: &quot;, rp, ra&#xA;&#xA;        a0 = 0.5*(rp + ra)&#xA;        v0 = np.sqrt(mu * (2./rp - 1./a0))&#xA;        print &quot;a0, v0: &quot;, a0, v0&#xA;&#xA;        F0,  F1  = [get_F(th, e) for th in [th0, th1]]&#xA;        M0,  M1  = [get_M_from_F(F, e)  for F  in [F0,  F1 ]]&#xA;        print &quot;F0, F1 (degs): &quot;, degs*F0, degs*F1&#xA;        print &quot;M0, M1 (degs): &quot;, degs*M0, degs*M1&#xA;&#xA;        print &quot;F0, F1: &quot;, F0, F1&#xA;        print &quot;M0, M1: &quot;, M0, M1&#xA;&#xA;        dt = np.sqrt((-a0)**3/mu) * (M1-M0)&#xA;&#xA;        print &quot;dt (sec): &quot;, dt&#xA;&#xA;    elif e == 1.0:&#xA;        print &quot;     is parabola!&quot;&#xA;&#xA;        print &quot;rp: &quot;, rp&#xA;&#xA;        v0 = np.sqrt(mu * (2./rp))&#xA;        print &quot;v0: &quot;, v0&#xA;&#xA;        D0,  D1  = [get_D(th, e) for th in [th0, th1]]&#xA;        M0,  M1  = [get_M_from_D(D, e)  for D  in [D0,  D1 ]]&#xA;        print &quot;D0, D1 (degs): &quot;, degs*D0, degs*D1&#xA;        print &quot;M0, M1 (degs): &quot;, degs*M0, degs*M1&#xA;&#xA;        print &quot;D0, D1: &quot;, D0, D1&#xA;        print &quot;M0, M1: &quot;, M0, M1&#xA;&#xA;        q = rp&#xA;&#xA;        dt = np.sqrt(2.*q**3/mu) * (M1-M0)&#xA;&#xA;        print &quot;dt (sec): &quot;, dt&#xA;&#xA;    time = np.array([0, dt])&#xA;    X0   = np.array([rp, 0, 0, v0])&#xA;&#xA;    answer, info = ODEint(deriv, X0, time, atol=1E-13, rtol=1E-13, full_output=True)&#xA;&#xA;    x, y, vx, vy = answer.T&#xA;    theta = np.arctan2(y, x)&#xA;&#xA;    print degs*theta[0], degs*theta[-1], &quot; should be &quot;, degs*th0, degs*th1&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-28T05:00:11.383" LastActivityDate="2018-01-28T05:00:11.383" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23131" PostTypeId="1" CreationDate="2017-09-22T08:08:54.637" Score="5" ViewCount="1594" Body="&lt;p&gt;A while ago, I did a Saturn V ascent simulation in Matlab. The catering for drag, gravity and thrust vectors and mass depletion worked well, but I struggled to implement ascent guidance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I duplicated roughly what I saw in the ascent profile - a gravity turn a little while after liftoff (I tinkered with this, but I recall it being about 1.5km-2.5km alt).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where I ran into problems was trying to guide it to orbit, as I didn't have a good set of rules for this. Typically it would either not reach orbit at all, or it would have too steep an angle of attack on SIVB cutoff and it would fall back to earth...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand it isn't a simple question but I appreciate any pointers. I'm happy to share the Matlab code if this helps (as long as you don't laugh too hard).&lt;/p&gt;&#xA;" OwnerUserId="21114" LastEditorDisplayName="user10509" LastEditDate="2017-09-22T13:59:36.587" LastActivityDate="2019-08-26T16:20:39.823" Title="Saturn V ascent guidance (simulation)" Tags="|saturn-v|guidance|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23132" PostTypeId="2" ParentId="23131" CreationDate="2017-09-22T08:43:00.623" Score="5" Body="&lt;p&gt;Try comparing your simulation to the flight data at &lt;a href=&quot;https://history.nasa.gov/SP-4029/Apollo_11d_Ascent_Phase.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://history.nasa.gov/SP-4029/Apollo_11d_Ascent_Phase.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Robert A. Braeunig used this data to &lt;a href=&quot;http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;generate a pitch program&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;+------------+----------------+------------+&#xA;|    Time    |     Pitch      | Pitch Rate |&#xA;| (seconds)  |   (degrees)    | (degree/s) |&#xA;+------------+----------------+------------+&#xA;| 0.3 to 30  | 0              | 0          |&#xA;| 30 to 80   | 0 to 36.40     | 0.7280000  |&#xA;| 80 to 135  | 36.40 to 62.23 | 0.4696364  |&#xA;| 135 to 165 | 62.23 to 71.14 | 0.2970000  |&#xA;| 165 to 185 | 71.14 to 60.57 | -0.5285000 |&#xA;| 185 to 320 | 60.57 to 64.75 | 0.0309630  |&#xA;| 320 to 460 | 64.75 to 77.35 | 0.0900000  |&#xA;| 460 to 480 | 77.35 to 74.59 | -0.1380000 |&#xA;| 480 to 550 | 74.59 to 81.39 | 0.0971429  |&#xA;| 550 to 570 | 81.39 to 77.25 | -0.2070000 |&#xA;| 570 to 640 | 77.25 to 85.07 | 0.1117143  |&#xA;| 640 to 705 | 85.07 to 88.23 | 0.0486154  |&#xA;| over 705   | 88.23          | 0          |&#xA;+------------+----------------+------------+&#xA;&#xA;&lt;/pre&gt;&#xA;" OwnerDisplayName="user20636" LastEditorUserId="195" LastEditDate="2017-09-22T15:08:52.563" LastActivityDate="2017-09-22T15:08:52.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23133" PostTypeId="1" CreationDate="2017-09-22T11:43:44.217" Score="7" ViewCount="357" Body="&lt;p&gt;Assume that Doctor Evil insists on dancing outdoors although he has been forbidden by the UN to do so. And that spy satellites can observe this against Doctor Evil's volition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could Doctor Evil keep on dancing outdoors invisibly if he dances right around a search light, or some other light source, pointed towards any spy satellite that passes over him?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(If you wonder, I'm asking on behalf of someone else.)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-09-22T21:56:19.237" Title="Could spy satellites be blinded? For example by search lights?" Tags="|earth-observation|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23135" PostTypeId="2" ParentId="23133" CreationDate="2017-09-22T14:33:13.247" Score="6" Body="&lt;p&gt;There have been many reports of spy satellites being &quot;blinded&quot; by lasers over the years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I do not vouch for the accuracy of any of these reports. As you might imagine, hard data is difficult to come by.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the Soviet Union: &lt;a href=&quot;https://books.google.com/books?id=AGecCQAAQBAJ&amp;amp;pg=PA34&amp;amp;lpg=PA34&amp;amp;dq=aviation%20week%20spy%20satellite%20blinded&amp;amp;source=bl&amp;amp;ots=4ljklvLIVw&amp;amp;sig=_1ne1msYtznIsmCh0Yj5pjan1_I&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwi557_LgLnWAhUS0mMKHQKiDlAQ6AEIQjAE#v=onepage&amp;amp;q=aviation%20week%20spy%20satellite%20blinded&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;&quot;Late 1975&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the US: &lt;a href=&quot;https://en.wikipedia.org/wiki/MIRACL&quot; rel=&quot;noreferrer&quot;&gt;1997&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By China: &lt;a href=&quot;http://www.telegraph.co.uk/news/worldnews/1529864/Beijing-secretly-fires-lasers-to-disable-US-satellites.html&quot; rel=&quot;noreferrer&quot;&gt;2006&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By Iran: &lt;a href=&quot;https://www.csmonitor.com/World/Middle-East/2011/1215/Exclusive-Iran-hijacked-US-drone-says-Iranian-engineer&quot; rel=&quot;noreferrer&quot;&gt;2011&lt;/a&gt; (search article for &quot;laser burst&quot;)&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-09-22T21:56:19.237" LastActivityDate="2017-09-22T21:56:19.237" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23137" PostTypeId="1" CreationDate="2017-09-22T19:39:48.263" Score="4" ViewCount="1412" Body="&lt;p&gt;I'm trying to simulate a gravity turn using a gimbaled thruster.  I'm using the dynamical system described in a &lt;a href=&quot;https://space.stackexchange.com/questions/22941/differential-equations-for-rocket-launch-in-polar-coordinates&quot;&gt;previous post of mine&lt;/a&gt; to execute a gravity turn by changing the angle $\phi_T$ between the thrust vector and the velocity vector by some number of degrees once the rocket reaches a certain height $h_{turn}$ and leaving the gimbal angle constant until burnout.  After experimentation with a couple of rocket designs, I've found that I've needed quite large angles to make it happen (almost 45 degrees), and I don't think this is really feasible in reality.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I'm curious how the pitch-over program is actually executed in reality.  For example, take the space shuttles.  How large of a gimbal angle was needed to execute the gravity turn?  Was it constant or did the angle change over time (like with an automatic control loop)?  For how much time were the gimbals angled to produce the turn?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a database showing the gimbal controls over time for different spacecraft to execute a gravity turn?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="6494" LastEditDate="2017-09-23T01:40:04.393" LastActivityDate="2018-04-16T06:10:13.567" Title="How are rockets gimballed to produce a gravity turn for the space shuttle?" Tags="|space-shuttle|thrust|gimbaled-engines|gravity-turn|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23138" PostTypeId="2" ParentId="23137" CreationDate="2017-09-22T19:49:19.203" Score="6" Body="&lt;p&gt;Gimbaling the engines off the line intersecting the center of mass produces torque, which yields a rotation &lt;em&gt;rate&lt;/em&gt;; if the vehicle is stable then a very small deflection should eventually bring you to the desired pitch angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic logic is an automatic control loop using something like a &lt;a href=&quot;https://en.wikipedia.org/wiki/PID_controller&quot; rel=&quot;noreferrer&quot;&gt;PID controller&lt;/a&gt;, usually with several additional constraints -- like not exceeding a certain angle of attack limit. The actual gimbal angle would be changing constantly throughout the flight, since it has to compensate for all the real-world things happening to the vehicle like gusts of wind, uneven thrust, propellant slosh, and so on. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nice features of a well-tuned PID controller are that it produces a larger control signal (i.e. larger gimbal angle) when it's further off target (the P-for-proportional part), increases its effort if the proportional control isn't keeping up for one reason or another (the I-for-integral part), and avoids overcompensation and unwanted oscillation around the target (the D-for-derivative part). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the shuttle's update loop &lt;a href=&quot;https://space.stackexchange.com/questions/12254/what-is-the-maximum-acceptable-latency-in-rocket-control-computers/12255#12255&quot;&gt;executed every 40 milliseconds&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V's guidance system &lt;a href=&quot;https://space.stackexchange.com/questions/12769/how-does-rocket-guidance-deal-with-changing-mass&quot;&gt;adjusted the gain of the proportional and derivative portions of its controller at scheduled times during flight&lt;/a&gt; to adjust for the vehicle getting lighter and into less dense air. (I don't know if this implies that its control was PD rather than PID). This document on the &lt;a href=&quot;http://www.ibiblio.org/apollo/Documents/tn_d-5869_1970023342.pdf&quot; rel=&quot;noreferrer&quot;&gt;Saturn V guidance system&lt;/a&gt; might be an informative read. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure where your sim is going wrong; I think turning gimbaled main engines to 45 degrees and holding them there will flip the rocket pretty quickly. (Are you modeling the distance between the engines and the center of mass correctly and getting reasonable torque values therefrom?)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-22T20:00:35.047" LastActivityDate="2017-09-22T20:00:35.047" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23139" PostTypeId="2" ParentId="23137" CreationDate="2017-09-22T20:06:47.350" Score="5" Body="&lt;p&gt;Here are some shuttle ascent guidance basics so you can interpret my answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During first stage, the shuttle flew a predesigned (therefore open-loop) pitch-yaw-roll profile based, among other things, on the measured winds of the day. (Note that only the attitude targets were open-loop. The shuttle used a closed-loop control system to fly to those targets.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After solid rocket booster separation, a closed-loop guidance scheme called Powered Explicit Guidance was used, which steered the vehicle to its Main Engine Cutoff (MECO) targets. During this phase, the commanded attitude was continuously calculated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, given that, even if the shuttle was flying a constant attitude, the engines would be continually gimbaling to keep the thrust vector pointed through the center of mass as the propellant was used up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that during first stage, attitude control was primarily controlled via the SRB thrust vector control, due to the enormous SRB thrust and long moment arms from the nozzles to the c.g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have some notes that show the center SSME pitch gimbal started out at about 4.25 degrees at liftoff, ramped up slowly to slightly over 5 degrees at SRB sep, then moved sharply to a little over 1 degree at sep.  During second stage, the gimbal moved up slowly and wound up around 4.25 degrees again at MECO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Please note that these gimbal angles are measured from the null mounting position. As you can see in this side view, there was a built-in pitch angle in the engine installed position so that the null mounting position pointed roughly through the average c.g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WB0zy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WB0zy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The installed null position for the left and right main engines is 10°&#xA;  up from the X axis and 3.5° outboard from an engine centerline&#xA;  parallel to the X axis. The center engine's installed null position is&#xA;  16° above the X axis centerline for pitch and on the X axis centerline&#xA;  for yaw.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the maximum angular deflection from the centerline of the External Tank that I know of would be about 5 degrees plus the 16 degree null mounting angle, for approximately 21 degrees. Given the rather strange shuttle side-mounted engine configuration, I'd have to say that your 45 degree gimbal angle is excessive, especially if you have a more symmetrical vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100015499.pdf&quot; rel=&quot;noreferrer&quot;&gt;DOLILU overview presentation&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=1&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwj1x6ye0rnWAhWEv1QKHcjNBC4QFggnMAA&amp;amp;url=https%3A%2F%2Fwww.nasa.gov%2Fcenters%2Fjohnson%2Fpdf%2F390651main_shuttle_crew_operations_manual.pdf&amp;amp;usg=AFQjCNENi1Wfkn12N9uV7BJ3FfCXYBLgag&quot; rel=&quot;noreferrer&quot;&gt;Shuttle Crew Operations Manual&lt;/a&gt; 2-13.47&lt;/p&gt;&#xA;&#xA;&lt;p&gt;personal notes&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-09-22T21:58:56.357" LastActivityDate="2017-09-22T21:58:56.357" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23140" PostTypeId="1" AcceptedAnswerId="23141" CreationDate="2017-09-23T03:40:10.503" Score="6" ViewCount="797" Body="&lt;p&gt;I've been reading about accelerometers. On earth, in a lecture demonstration, in a table-top experiment, they will generate a signal equal to the sum of the acceleration you'd like to measure plus the local acceleration due to gravity and all of the other accelerations of our laboratory frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Local gravity on the Earth's surface varies a few tenths of a percent; strongest at the North pole and weakest in equatorial mountains, due to variations in the distance from the center of the Earth and other mass-distribution effects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a variation in the centripetal acceleration due to the Earth's rotation from pole to equator, which points away from the Earth and further decreases local acceleration near the Equator. All together, it's about a 0.5% maximum difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a great inventory of these variations in &lt;a href=&quot;https://physics.stackexchange.com/a/141981/83380&quot;&gt;this answer&lt;/a&gt; and I'd recommend a visit there if you are interested.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But my question is about daily variation in local acceleration due to the rotation of the Earth with respect to the Sun and Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was reading the document &lt;a href=&quot;http://a.moirier.free.fr/Vibrations/Documentation/Precise_Measurement_of_Mass.pdf&quot; rel=&quot;noreferrer&quot;&gt;Precise Measurement of Mass&lt;/a&gt;, presented at the  60th Annual Conference of the Society of Allied Weight Engineers&#xA;Arlington, Texas May 21-23, 2001 by Richard Boynton of Space-Electronics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One statement on page 8 surprised me:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;1.5 Tidal variations An object on the surface of the earth is attracted to every celestial body. Most of these masses are too far away to have any significance on weight, but the sun and the moon do have a significance. &lt;strong&gt;If you have a scale whose accuracy is 0.003 % or better, you will notice that the weight of an object varies as a function of the time of day.&lt;/strong&gt; This effect is most pronounced during spring and fall when the sun and moon align. This produces the “neap tides” that often cause flooding of marinas at these critical dates. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The figure of 0.003% is repeated twice more in other sections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have tried to reproduce this number, but it seems high by about two orders of magnitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The tidal effect I know how to calculate goes as follows. I like to imagine being an astronaut in the ISS, orbiting the Earth. At that low altitude Earth's gravity is almost as strong as it is on the surface, but an astronaut in the center of the ISS will not experience any acceleration with respect to the ISS since they are in the same orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However if an astronaut moves to a point toward or away from the Earth, they will experience a tidal acceleration that points away from the center of the ISS, and which will be proportional in 1st order to the distance from the center. For a shift of $\pm \Delta r$ from the orbital distance $r_0$, the change in acceleration is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta a = \frac{-GM_E}{(r_0-\Delta r)^2} - \frac{-GM_E}{(r_0+\Delta r)^2}.$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For $\pm$ 1 meter at the ISS's $r_0$ of say 6378+400 km, that's a $\Delta a$ of about 5E-06 m/s^2, or 5E-07 g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, using the standard gravitational parameter $GM$ from the Sun and Moon of 1.327E+20 and 4.905E+12 and distances $r_0$ of 1.5E+11 and 3.8E+08 meters, and now using the radius of the Earth for $\Delta r$, I get $\Delta a$'s of 1E-06 and 2E-06 m/s^2, or 1E-07 and 2E-07 g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When those add together, it's 3E-07 g, or 0.00003%, one hundred times smaller than the amount stated in the paper.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Have I done my calculation correctly? Is the figure in the paper a factor of 100 too high?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note 1:&lt;/em&gt; 0.003% results if you make the change to percent (by multiplying by 100) twice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note 2:&lt;/em&gt; FWIW, the paper points out that the company is selling scales with a precision of 0.002%.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VVWiu.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VVWiu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-09-23T04:48:08.860" LastActivityDate="2017-09-27T12:41:57.800" Title="Could daily variations of weight on Earth really be 0.003%?" Tags="|microgravity|mass|measurement|tides|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23141" PostTypeId="2" ParentId="23140" CreationDate="2017-09-23T09:44:53.470" Score="4" Body="&lt;p&gt;The total gravitational pull of the Moon is $g_M = 3.3\cdot10^{-5} \frac{m}{s^2}$ or $0.0003\%$ of Earth's gravity. This is one order of magnitude smaller than the stated value.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At all times, most of this force is counteracted by the centrifugal force of the rotation of Earth around the barycenter of the Earth-Moon system, resulting in the small tidal force calculated by @uhoh. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, even if we ignore the revolution around the barycenter of the system and make the Moon vanish instantaneously, the felt change in gravitational force would be $0.0003\%$ at most.&lt;/p&gt;&#xA;" OwnerUserId="12024" LastEditorUserId="12024" LastEditDate="2017-09-23T10:53:16.140" LastActivityDate="2017-09-23T10:53:16.140" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23142" PostTypeId="1" CreationDate="2017-09-23T16:55:35.747" Score="4" ViewCount="420" Body="&lt;p&gt;Let us suppose we manage to mine water on the Moon and return lots of it to LEO. Would it be hard to design an upper stage that could refuel at that depot, and uses that fuel to return to the surface and be reused? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was reading about ULA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Advanced_Cryogenic_Evolved_Stage&quot; rel=&quot;nofollow noreferrer&quot;&gt;Advanced Cryogenic Evolved Stage&lt;/a&gt;, and in a Defense News article Tony Bruno said &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;We had the idea, well, why do you have to bring it back to Earth just&#xA;  to reuse it?&quot; Bruno said &quot;Why don't we just leave it in space?&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...Yeah, but... why not bring it back to Earth and reuse it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it just a matter of how to have enough fuel up there so that its reasonable to propose refueling a bunch of specialized upper stages with enough fuel to propulsively decelerate and land? Are there other complications?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe I should mention that I'm thinking about this in terms of long-range lunar development. Jkavalik mentioned in comments that a big complication is that things are launched to many different orbits and so it would be rare for an upper stage to be in range of a fuel depot (though they mentioned an architecture change where payloads are transferred to space tugs that complete the orbit insertion process at the depot could get around that). In the particular case I'm thinking of, the payloads are destined for one staging area before continuing on to the Moon, probably a space station.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="4660" LastEditDate="2017-09-25T18:44:53.357" LastActivityDate="2017-09-26T19:43:41.280" Title="Could refueled upper stages re-enter by retro-propulsion?" Tags="|reuse|upper-stage|fuel-depots|" AnswerCount="2" CommentCount="18" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23144" PostTypeId="1" CreationDate="2017-09-24T05:29:02.903" Score="5" ViewCount="1366" Body="&lt;p&gt;Has there been any research on what lunar in-situ processes would have to be developed to mine and process thorium to build nuclear thermal rockets on the Moon?&lt;/p&gt;&#xA;" OwnerUserId="21145" LastEditorUserId="4660" LastEditDate="2017-09-25T21:30:50.130" LastActivityDate="2018-02-16T22:44:03.873" Title="Mining lunar thorium for building nuclear rockets on the Moon" Tags="|rockets|lunar-landing|nuclear|research|isru|" AnswerCount="4" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="23146" PostTypeId="1" AcceptedAnswerId="23149" CreationDate="2017-09-24T20:26:29.053" Score="43" ViewCount="8009" Body="&lt;p&gt;Referring to the Apollo missions,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As many know, during Apollo 11, Michael Collins didn't descend with Neil Armstrong and Buzz Aldrin towards the moon. I can only begin to imagine the sadness of not being able to walk on the majestic satellite. I understand that someone did need to be in the CSM, and only two people could fit in the LEM. This begins to spark the question,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How did NASA choose who walked on the moon?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="20596" LastEditorUserId="21158" LastEditDate="2017-09-25T19:34:21.043" LastActivityDate="2019-01-12T20:54:04.463" Title="How did NASA choose who descended in the LEM during the Apollo missions?" Tags="|apollo-program|apollo-11|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23147" PostTypeId="2" ParentId="23127" CreationDate="2017-09-24T21:40:42.770" Score="3" Body="&lt;p&gt;Sorry, I can't promise you a perfect answer.&#xA;Firstly, I found this page helpful - &lt;a href=&quot;http://www.bogan.ca/orbits/gravasst/gravasst.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.bogan.ca/orbits/gravasst/gravasst.html&lt;/a&gt;. Unfortunately the structure has changed a bit since I used it, but useful info is still there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I made a tool to model 'grand Tour' scenarios. I did this by modelling a closed ellipse intersecting with the orbit path of the target 'slingshot' planet. At the point the planet became the dominant gravitational influence the model switched to hyperbolic transition. Once the sun became the dominant gravitational influence again I switched to a closed ellipse with the next planet (for slingshot) as target.&#xA;I can share the matlab code for the transition and ellipse models if this helps.&#xA;I would say it wasn't a perfect solution, but it did verify the Voyager trajectories. &#xA;Best wishes&#xA;Stephen&lt;/p&gt;&#xA;" OwnerUserId="21114" LastActivityDate="2017-09-24T21:40:42.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23148" PostTypeId="2" ParentId="5835" CreationDate="2017-09-24T22:28:04.677" Score="2" Body="&lt;p&gt;The Wikipedia article on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_15#Implications&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soviet Luna 15 mission&lt;/a&gt;, which ran simultaneous to the Apollo 11 moon landing, claims this to be &quot;one of the first instances of Soviet-American space cooperation&quot;, though it doesn't provide reference for this. I believe I've read about it elsewhere (possibly &lt;em&gt;In the Shadow of the Moon&lt;/em&gt; or one of the other French/Burgess books). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two teams shared their flight paths and landing coordinates in order to avoid conflict/catastrophe, though the Soviets didn't overtly divulge the purpose/details of their mission. Interestingly, Aldrin was recorded in the CSM asking Houston for updates on the Soviet mission in the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11transcript_tec.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo 11 radio transcripts&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="21149" LastActivityDate="2017-09-24T22:28:04.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23149" PostTypeId="2" ParentId="23146" CreationDate="2017-09-24T23:46:04.687" Score="69" Body="&lt;p&gt;During the Apollo era, &lt;a href=&quot;https://en.wikipedia.org/wiki/Deke_Slayton&quot; rel=&quot;noreferrer&quot;&gt;Deke Slayton&lt;/a&gt;, as chief of the astronaut office, was the primary decision maker when it came to choosing who was assigned to which crews. Slayton was chosen for the Mercury program but was grounded due to a heart issue. Since he had been through astronaut training, but wasn't in competition for a mission assignment himself, he was widely trusted as an impartial judge of astronaut qualifications. (Slayton was restored to flight status in 1972 and flew on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo%E2%80%93Soyuz_Test_Project&quot; rel=&quot;noreferrer&quot;&gt;Apollo-Soyuz Test Project&lt;/a&gt; mission.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slayton actually started this process at the beginning of the Gemini program, taking into account the plans for Apollo in deciding how many astronauts to hire and how to train them. During the program, Slayton didn't reveal all the details of his selection process; he wanted the astronauts to excel overall rather than giving them ways to game the system. Some observers have suggested the opaque evaluation process was discouraging to some of the astronauts, or that it gave Slayton cover to play favorites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since then, in his autobiography, &lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/B005VDK0AQ&quot; rel=&quot;noreferrer&quot;&gt;Deke!&lt;/a&gt;, he's given somewhat more insight into the process. Regarding how candidates were selected to join the space program, he writes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I had already developed a point system that we used in making the final evaluations on astronaut candidates. There were three parts: academic, pilot performance, and character/motivation, ten points for each part, with thirty being the highest possible score. Some of it was cut-and-dried: you got points for a certain amount of flying time and for education. Some of it, by design, was subjective and based on face-to-face interviews.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As for crew assignments, he says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;ol&gt;&#xA;  &lt;li&gt;Everybody was considered to be qualified and acceptable for any mission when brought into NASA. That is, if I hired the guy and kept him around, he was eligible to fly.&lt;/li&gt;&#xA;  &lt;li&gt;But some are more qualified than others for specific seats on specific missions. That is, guys with command or management or test pilot experience were more likely to be handed the more challenging assignments.&lt;/li&gt;&#xA;  &lt;li&gt;I would try to match people in a crew based on individual talents and, when possible, personal compatibility. But I didn’t give nearly as much weight to the compatibility issue as everybody thought, because it wasn’t necessary. Everybody in the astronaut group was talented and motivated, or they wouldn’t have gotten there in the first place. They were likely to get along no matter how they were matched up. On the other hand, matching people where possible made life easier all around, and added some fun.&lt;/li&gt;&#xA;  &lt;li&gt;I always kept future requirements and training in mind. When I wanted somebody for an assignment in Gemini, I had to think how that would affect Apollo. So I had a long-term plan that was updated regularly.&lt;/li&gt;&#xA;  &lt;li&gt;I assumed a ten percent annual attrition rate for all causes—accidents or resignation. (This was a wild-assed guess I made in 1963 that turned out to be pretty close.)&lt;/li&gt;&#xA;  &lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;He and others in the astronaut office evaluated every astronaut's aptitude and dedication to the program. From this information, and the constraints of the missions, Slayton assembled crews that he thought were likely to perform well. In late 1964, after having already put together a tentative slate, Slayton also asked for a round of peer reviews:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;I got the thirteen remaining guys in the third group together and gave them a little job. Each guy was to exclude himself and rank the others in the group in the order he thought they should go into space. We had done the same thing in Mercury.&#xA;  ...&#xA;  These guys knew each other’s abilities better than anyone, and they were pretty objective. The ones who came to the top of the peer ratings were pretty much the ones I had assigned to the later Gemini missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Each Apollo crew consisted of a commander, a lunar module pilot, and a command module pilot; the commander and LMP would land while the CMP stayed behind with the CSM in orbit. Commanders for Apollo were all selected from astronauts who had already flown at least once before during the Mercury or Gemini programs. The early Apollo flights had experienced astronauts as CMP as well, but for Apollos 13 through 17, the CMP and LMP were all rookie astronauts on their first space flights.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Each crew of 3 was assigned at the same time, and to the extent possible the crews were kept together. If a problem cropped up before a crew had begun intensive training for their next mission, the whole crew might be pushed back to a later mission (for example, Alan Shepard's crew got pushed from Apollo 13 to Apollo 14 to give Shepard, recently restored to flight status, more time to train). Each Apollo mission had both a prime crew and a backup crew. The backup crew would train for the mission alongside the prime crew and an individual from the backup crew might move up to the prime crew if a prime crewman was taken out of the running later in training (as happened during the run-up to Apollo 13, when &lt;a href=&quot;https://en.wikipedia.org/wiki/Ken_Mattingly&quot; rel=&quot;noreferrer&quot;&gt;Ken Mattingly&lt;/a&gt; was exposed to measles, and backup CMP &lt;a href=&quot;https://en.wikipedia.org/wiki/Jack_Swigert&quot; rel=&quot;noreferrer&quot;&gt;Jack Swigert&lt;/a&gt; took his place). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally, the backup crew for a given mission would expect to skip the next two missions and be assigned to the following mission as the prime crew, although as often as not some change would be made over that time. The backup crew for Apollo 11, for example, was Lovell/Anders/Haise, so Lovell's crew was in line for Apollo 14; the assignments for Apollo 13 and 14 got switched, and Anders left NASA, so Ken Mattingly wound up in the prime CMP slot on 13. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It might seem that command module pilot would be the less desirable role, but a good performance as CMP would put an astronaut in line to be a commander on a later mission, so there was plenty of incentive to excel in the CMP role. The pattern was that a good CMP would be assigned as the commander of the backup crew on the third following mission, and the commander of the prime crew on the third mission after that. For example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Jim_Lovell&quot; rel=&quot;noreferrer&quot;&gt;Jim Lovell&lt;/a&gt; was CMP on Apollo 8, backup commander for 11, and selected as commander of 14 (before that crew was shifted to 13), &lt;a href=&quot;https://en.wikipedia.org/wiki/David_Scott&quot; rel=&quot;noreferrer&quot;&gt;David Scott&lt;/a&gt; was CMP on 9, backup commander on 12, and commander on 15, and &lt;a href=&quot;https://en.wikipedia.org/wiki/John_Young_(astronaut)&quot; rel=&quot;noreferrer&quot;&gt;John Young&lt;/a&gt; was CMP on 10, backup commander on 13, and commander on 16. Apollo 12 CMP &lt;a href=&quot;https://en.wikipedia.org/wiki/Richard_F._Gordon_Jr.&quot; rel=&quot;noreferrer&quot;&gt;Dick Gordon&lt;/a&gt; was backup commander for 15, but the program was cancelled and he lost his chance to command 18.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It wasn't until quite late in the process that it was clear that Armstrong and Aldrin would be the first to land. Prior to his death in the Apollo 1 fire, &lt;a href=&quot;https://en.wikipedia.org/wiki/Gus_Grissom&quot; rel=&quot;noreferrer&quot;&gt;Gus Grissom&lt;/a&gt; probably had the best chance at it; Slayton and &lt;a href=&quot;https://en.wikipedia.org/wiki/Johnson_Space_Center&quot; rel=&quot;noreferrer&quot;&gt;MSC&lt;/a&gt; director &lt;a href=&quot;https://en.wikipedia.org/wiki/Robert_R._Gilruth&quot; rel=&quot;noreferrer&quot;&gt;Bob Gilruth&lt;/a&gt; agreed that &quot;if possible&quot;, one of the Mercury astronauts should get the opportunity. The Armstrong/Aldrin/Collins crew was assigned to Apollo 11 after Apollo 8 had flown; while it appeared at that point that 11 could be the first landing, any severe problems that cropped up with the spacecraft or the process on Apollo 9 or 10 could have delayed the landing to Apollo 12 or later. Some people in the program argued for attempting a landing with Apollo 10, but others, including the commander of that mission, &lt;a href=&quot;https://en.wikipedia.org/wiki/Thomas_P._Stafford&quot; rel=&quot;noreferrer&quot;&gt;Tom Stafford&lt;/a&gt;, felt that a &quot;dress rehearsal&quot; flight was the better option. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the end of the program, there was pressure to put a scientist on an Apollo crew, so test pilot &lt;a href=&quot;https://en.wikipedia.org/wiki/Joe_Engle&quot; rel=&quot;noreferrer&quot;&gt;Joe Engle&lt;/a&gt; was bumped from the LMP slot on Apollo 17 in favor of geologist &lt;a href=&quot;https://en.wikipedia.org/wiki/Harrison_Schmitt&quot; rel=&quot;noreferrer&quot;&gt;Harrison Schmitt&lt;/a&gt;. Schmitt became a qualified jet pilot after joining the program, but it's worth noting that the role of the lunar module pilot is more of a systems-operator or engineer position than a hands-on-joysticks job. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2019-01-12T20:54:04.463" LastActivityDate="2019-01-12T20:54:04.463" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="23150" PostTypeId="2" ParentId="23079" CreationDate="2017-09-25T02:37:37.017" Score="20" Body="&lt;p&gt;Calculating an energy-optimal orbit for a simple thing like Earth-&gt;Mars is no big deal.  Even using low-thrust engines doesn't add much to it.  It would be tedious but you could do it on a calculator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, that's not what they needed in the book &amp;amp; movie.  The objective wasn't to get there as cheap as possible, the objective was to get there as fast as possible given the available energy.  There's no standard solution for this, you're simply going to have to try a huge range of possible orbits and see what's best.  That's why you want a supercomputer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modern deep space craft often use paths that take some pretty extreme calculating.  Consider Messenger:  6 planetary encounters and 5 deep space burns.  They obviously simulated a &lt;strong&gt;huge&lt;/strong&gt; number of possible paths to find the one that got them there the cheapest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=Ownzbb1mKxs&quot; rel=&quot;noreferrer&quot;&gt;You-Tube video of the path&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-09-25T02:37:37.017" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23151" PostTypeId="2" ParentId="23142" CreationDate="2017-09-25T03:23:08.307" Score="3" Body="&lt;p&gt;It's close.  Plugging in the numbers for the Falcon 9 upper stage I get 11,300m/s of delta-v.  Since 9000m/s will get you to LEO that's enough to get you back down with something to spare.  &lt;strong&gt;However&lt;/strong&gt; the upper stage doesn't have landing gear.  Since you have 5000kg of payload capacity (brings the delta-v down to 9111m/s) you could land it.  The weight of the legs and other landing equipment would come directly off the weight of the payload, though, on a 1:1 ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Landing this will be a nightmare.  The upper stage has only one engine and it doesn't throttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking for some numbers on the Falcon 9 I find the landing burn is 267 m/s but that includes 117 m/s of gravity loss.  The upper stage will be landing very, very hot, though&amp;mdash;if we figure the max payload weight of 5000kg the engine still is going to be putting out 103m/s^2 at exhaustion.  The actual velocity to kill is only 150 m/s.  Figure 1.5 seconds for the burn which adds another 15 m/s of gravity loss, so figure 1.6 seconds for the burn.  The accuracy required is going to be incredible.  If the engine ignites 1 millisecond early it's going to shut down while still 66 cm in the air above the pad and the rocket will have to fall the rest of the way.  If it ignites 1ms late it hits &lt;strong&gt;much&lt;/strong&gt; harder, my gut says the same as a fall from 7 meters up and it's late enough I don't want to spend the time to confirm this.  Note that the engines do not start with anything like 1ms precision!!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Note that I am using the terminal velocity of the first stage.  Nobody cares about the terminal velocity of the second stage as it doesn't fall anyway so the number isn't to be found.  I would assume it's a bit lower.  This will change the duration of the landing burn but will not change the touchdown numbers.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="6816" LastEditDate="2017-09-26T19:07:31.907" LastActivityDate="2017-09-26T19:07:31.907" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23152" PostTypeId="2" ParentId="23091" CreationDate="2017-09-25T03:37:26.560" Score="2" Body="&lt;p&gt;You're looking at it wrong.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, pulling 1g payload with a 60,000g acceleration would be a tremendous engineering challenge.  (I wouldn't call it impossible, though&amp;mdash;that's not too far beyond gun accelerations and we routinely fire electronics out of guns.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, that's not how you do something like this.  You do &lt;strong&gt;not&lt;/strong&gt; want to pull a payload!!  Rather, your probe needs to be spread over the sail surface.  If you could somehow spread the probe totally evenly across the sail the only force would be compressive and since it would be incredibly thin that's not that much force even given the acceleration in question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In practice the limit is going to be how thin you can spread the probe.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="6816" LastEditDate="2017-09-25T19:03:37.600" LastActivityDate="2017-09-25T19:03:37.600" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23153" PostTypeId="1" AcceptedAnswerId="23159" CreationDate="2017-09-25T07:41:49.950" Score="4" ViewCount="606" Body="&lt;p&gt;After reading the BBC article &lt;a href=&quot;http://www.bbc.com/news/world-australia-41382609&quot; rel=&quot;nofollow noreferrer&quot;&gt;Australia to create national space agency&lt;/a&gt; and clicking around to identify the dish antenna shown in the photo in the article, I came across an image of an interesting reflector configuration called the &lt;a href=&quot;https://www.honeysucklecreek.net/other_stations/carnarvon_OTC/42ft.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;42 foot Cassegrain Horn&lt;/a&gt; at Australia's historic &lt;a href=&quot;https://en.wikipedia.org/wiki/Honeysuckle_Creek_Tracking_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Honeysuckle Creek Tracking Station&lt;/a&gt; where the famous video of the first footsteps on the moon were received among of course plenty of other things.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The horn looks to me like a big dead-end, but of course it isn't. How does it work optically? How does it collect microwaves entering its large aperture and concentrate them into a single small feed horn? Which way is it actually pointed as shown in the photo? What are the advantages of this unusual design?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;Honeysuckle Creek Tracking Station&quot; from &lt;a href=&quot;http://museum.wa.gov.au/city-lights&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/x4J0t.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/x4J0t.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-02-17T22:15:49.130" LastActivityDate="2019-02-17T22:15:49.130" Title="How does this strangely-shaped horn at Honeysuckle Creek Tracking Station work?" Tags="|radio-communication|radio-telescope|dish|honeysuckle-creek|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23154" PostTypeId="1" CreationDate="2017-09-25T13:32:57.410" Score="3" ViewCount="165" Body="&lt;p&gt;3 Apollo missions brought their lunar rovers (&lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunar Roving Vehicle&lt;/a&gt;) LRV with them, and they remained on the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of them have broken parts, and their batteries seem to be emtpy, according to &lt;a href=&quot;https://space.stackexchange.com/a/6268/21153&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, &lt;strong&gt;would they still be able to drive today, given new batteries?&lt;/strong&gt; Would a solar cell provide enough power to either load those batteries or allow to drive directly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Background: I am wondering whether any of the current Lunar-X-Prize-Missions would be able to re-use those LRV's? Or is this ruled out by the competition anyways?&lt;/p&gt;&#xA;" OwnerUserId="21153" LastEditorUserId="6944" LastEditDate="2020-10-09T00:46:07.297" LastActivityDate="2020-10-09T00:46:07.297" Title="Would the lunar rovers work (again) today? What would be needed?" Tags="|the-moon|reuse|lunar-rover|xprize|" AnswerCount="0" CommentCount="1" ClosedDate="2017-09-25T19:09:06.367" ContentLicense="CC BY-SA 3.0" />
  <row Id="23155" PostTypeId="2" ParentId="4173" CreationDate="2017-09-25T18:06:26.187" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;What would be the positive and negative affects of using other planets and/or space itself for storing trash - instead of wasting place for that on Earth?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'll focus on the positive since others have covered the negative already and quite well.  First, as you stated, it would remove nuclear material from Earth where human and other life exists.  Second you could use that nuclear material to add mass and heat to another body that could in some very small way use it.  Deep within Mars for example or a gas giant like Jupiter or Saturn.  Adding mass and heat to these bodies would be an advantage to a future spacefaring species.  Unfortunately the amount of mass and heat added to any of these bodies is extremely small to say the least.  70,000 tons of spent fuel waste buried very deep on Mars, which weighs 631,710,000,000,000,065,536 tons is 1.1 X 10^-11, in other words 100 billionth of the mass of the planet.  Adding other nuclear waste materials listed previously would barely change this and so would moving it to Jupiter or Saturn instead.  But technically these bodies can be argued would be more useful if they were more massive and so that is an extremely tiny positive.  A more massive (and dense) Mars would have a stronger gravity for human habitation, atmospheric losses, and a potentially active internal magnetic field.  Again, extremely tiny amount of mass and heat added.  Jupiter and Saturn are contracting and giving off IR radiation to their moons, this would technically increase those effects, but again so monumentally small that it would be like removing a single tiny microbe from your skin to try and lose weight.  Most other bodies would not see any gains at all that would be useful and in fact the sun would be an extremely tiny negative since it would add mass and get us an extremely tiny amount closer to the red giant phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all still not worth the risks or costs but part of your question was what positive effects would there be for other planets or space.             &lt;/p&gt;&#xA;" OwnerUserId="20287" LastActivityDate="2017-09-25T18:06:26.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23156" PostTypeId="2" ParentId="23142" CreationDate="2017-09-25T18:20:02.573" Score="3" Body="&lt;p&gt;You need to keep in mind that every kilogram you add for a recovery system on the upper stage robs an equivalent kilogram from the payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a refueled stage that can shed most of the 9000 m/s ΔV in the deorbit burn, we don't need a heat shield (or at least as robust a heat shield). We could probably get away with something like the inflatable heat shield NASA has been testing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure about an engine-first re-entry, though; the end of the MVac nozzle is a little thicker than a soda can (remember they were able to manually trim four feet off of it using a pair of tin snips for COTS-1), and I'm not sure it wouldn't be damaged from the buffeting as the atmosphere got thicker.  Then again, I'm not sure a nose-first re-entry would protect it all that much, either (not an aerospace engineer, so feel free to ignore that).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You won't be able to use the MVac for propulsive landing &amp;mdash; it's optimized for vacuum operation and would be ridiculously unstable at near-sea-level pressure.  You'd need something like the SuperDraco for an actual landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A parachute drop coupled with some &lt;em&gt;serious&lt;/em&gt; shock-absorbing legs and a quick Soyuz-style retro-fire from a Draco or SuperDraco might work, but that's a lot of payload mass to sacrifice.  The on-orbit use case makes a &lt;em&gt;lot&lt;/em&gt; more sense.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're still robbing a non-trivial amount of payload mass to pay for those legs, parachutes, and thrusters.  &lt;/p&gt;&#xA;" OwnerUserId="12922" LastEditorUserId="12922" LastEditDate="2017-09-26T19:43:41.280" LastActivityDate="2017-09-26T19:43:41.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23158" PostTypeId="2" ParentId="15389" CreationDate="2017-09-26T12:17:44.713" Score="2" Body="&lt;p&gt;I am assuming that the point of the question is Soviet imaging of the Moon, and not strictly lunar orbiters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were, in fact, two Soviet programs which included spacecraft with the task of imaging Moon's surface. First was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_programme&quot; rel=&quot;nofollow noreferrer&quot;&gt;Luna program&lt;/a&gt;, which you found yourself. In this program, the stations that managed to get images of the Moon from the orbit were Luna-3 (quite low-quality images of the far side), Luna-12 (the ones you saw), Luna-16 (it was a sample-retrieval mission, but it sent several photos taken during the flight) and Luna-22. Several other craft, for example, Luna-11, failed to achieve the right orientation and ended up with their cameras pointing away from the Moon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Zond_program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zond program&lt;/a&gt;. Spacecraft of this series were testbeds for several other programs, but several of them did a flyby of the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can find photos of the Moon taken by Luna- and Zond-series Soviet spacecraft &lt;a href=&quot;http://galspace.spb.ru/index142.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here (most photos need to be clicked to get the full image)&lt;/a&gt;. Sadly, most of Soviet archives are still hardcopy-only and there are no official online repositories.&lt;/p&gt;&#xA;" OwnerUserId="21168" LastEditorUserId="21168" LastEditDate="2017-09-26T15:14:34.773" LastActivityDate="2017-09-26T15:14:34.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23159" PostTypeId="2" ParentId="23153" CreationDate="2017-09-26T15:50:30.160" Score="7" Body="&lt;p&gt;That antenna is a Cassegrain horn (or 'casshorn'). This is &lt;a href=&quot;https://www.honeysucklecreek.net/images/other_stations/OTC_Carnarvon_images/42/KSK_Antenna.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;an evolution of the earlier horn-reflector antenna&lt;/a&gt; (paper that describes the design), where a horn radiates into a parabolic surface: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/drnJ1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/drnJ1.png&quot; alt=&quot;Horn-reflector antenna&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Cassegrain horn reduces the antenna size (long distance between the horn and parabolic dish) by introducing a hyperbolic reflector: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/d7eKc.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d7eKc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The feed is now at position F', but the hyperboloid is shaped to produce a virtual source at F.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The result is a beam that looks like this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LRmu0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LRmu0.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The photo is upside down relative to the drawings in the paper: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/rHXQk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rHXQk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to reduced antenna size, the Cassegrain horn has these advantages over a normal Cassegrain antenna: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;no blockage by the subreflector in front of the main reflector&lt;/li&gt;&#xA;&lt;li&gt;less feed spillover (leakage), both in transmission (less interference with other systems) and reception&lt;/li&gt;&#xA;&lt;li&gt;good rejection of ground noise&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Horn_antenna#Horn-reflector_antenna&quot; rel=&quot;nofollow noreferrer&quot;&gt;disadvantages&lt;/a&gt; are obvious from the photo:&lt;br&gt;&#xA; - the antenna needs more structure. Instead of a few trusses holding up the secondary, you get 3 full &quot;walls&quot;. It's cheaper to build a Cassegrain with a slightly larger primary to compensate for the blockage of the subreflector.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The book &lt;a href=&quot;https://books.google.nl/books?id=pHxTQTiYkGYC&amp;amp;pg=PA50&amp;amp;redir_esc=y#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Practical Conic Sections: The Geometric Properties of Ellipses, Parabolas and hyperbolas&lt;/a&gt; also describes the geometry of this antenna.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-09-27T09:18:10.877" LastActivityDate="2017-09-27T09:18:10.877" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23161" PostTypeId="2" ParentId="6460" CreationDate="2017-09-26T22:51:57.117" Score="2" Body="&lt;p&gt;As others have said pretty much every piece of this ends up being challenging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fuel reserves needed for both managing to accomplish the rendezvous initially and then for moving it are far from trivial. It's the reason GEO sats are pushed out a little bit to a graveyard orbit instead of trying to deorbit them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Intercepting a quickly moving uncontrolled object is also far from trivial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Grappling a quickly moving and potentially tumbling object that wasn't designed specifically to be grappled is far from trivial.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Modifying a second stage for extended operation in space is far from trivial. You have to worry about fuel storage assuming cryogenic fuels are used. A simple battery may no longer be enough which now means solar arrays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's all ignoring the fact that most launches are to GTO (a transfer orbit) and not directly to GEO so the second stage isn't even making it to GEO in the first place which further increases the fuel requirements.&lt;/p&gt;&#xA;" OwnerUserId="16458" LastActivityDate="2017-09-26T22:51:57.117" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23162" PostTypeId="1" CreationDate="2017-09-27T02:24:56.340" Score="3" ViewCount="103" Body="&lt;p&gt;In Wiesel's Spaceflight Dynamics, 3rd Edition (2010) Chapter 7.5 (pg 220), there is a table of mass values for Von Braun's 1951 design for a three-stage space station supply rocket. In the book, the author defines several quantities of interest with respect to stage masses:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_{0k}$ is the initial mass of the kth stage, which includes structural and propellant mass of the current stage and everything else after it.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_{fk}$ is the final mass after burnout of all the propellant of the kth stage, which includes the structural mass of the current stage and everything else after it (i.e. it omits the propellant of the kth stage).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_{sk}$ is the structural mass of the current stage only (not including structural masses of all the other stages after it).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$m_{pk}$ is the propellant mass of the current stage only.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The author provides values for each of the stages of the rocket as follows:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;           Stage 1    Stage 2  Stage 3   Payload  &#xA; m_{0k}  6,349,000   897,930   129,700    35,400   &#xA; m_{fk}  1,587,250   199,540    60,225   &#xA; m_{sk}    698,390    69,840    21,950   &#xA; m_{pk}  4,761,750   698,400    69,480   &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;However, when analyzing this number, it seems like they are off quite significantly.  If we analyze Stage 1, for example, the difference between the total initial mass $m_{01}$ and the first stage propellant mass $m_{p1}$ is exactly equal to the final 1st stage mass $m_{f1}$ as expected; that is,  $m_{01}-m_{p1} = m_{f1}$.  However, if we take the difference between the final stage 1 mass $m_{f1}$ and the initial mass of stage 2 $m_{02}$, we should expect this to equal the structural mass of stage 1 $m_{s1}$.  Instead, when we subtract, we get an absolute error of 9070 kg.  This seems to be a rather large error in the calculation and I'm wondering if I'm missing something fundamental or if this is just a simple typo or something else that I'm not factoring into the mass calculations.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the event that this was just a transcription error of some sort on the part of the author, I looked further to another similar table (same book, pg 222) for the space shuttle, where the following (nominal) values are provided:  &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;           Stage 1    Stage 2  Stage 3   Payload  &#xA; m_{0k}  2,015,600   670,500   108,300    29,500   &#xA;        (1,169,200) (846,400)&#xA; m_{fk}    934,300   143,300   104,700   &#xA; m_{sk}    163,800    35,000    68,000   &#xA; m_{pk}  1,181,300   527,200     3,600   &#xA;        (1,005,400) (703,100)  &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;where the values in parentheses refer to components of the shuttle and are not equivalent parallel stages.  In this table, the values seem more consistent. &#xA; The values satisfy the equation $m_{fk}-m_{0k+1}=m_{sk}$ for interstages 1-2 and 2-3, but not for 3-payload.  Is there a reason why the stage 3 structural mass is not &lt;em&gt;supposed&lt;/em&gt; to be equal to the payload mass?   On a side note, what exactly is meant by &quot;components of the shuttle&quot; as opposed to the equivalent parallel stages?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="6494" LastEditDate="2017-09-27T14:24:52.920" LastActivityDate="2017-09-27T14:24:52.920" Title="Understanding the multi-stage mass tables in Wiesel's Spaceflight Dynamics" Tags="|stages|mass-fraction|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23163" PostTypeId="2" ParentId="23140" CreationDate="2017-09-27T10:41:33.350" Score="3" Body="&lt;p&gt;The reported difference of $0.003\%$ would be plausible, if the difference in measured weight (acceleration) was caused by the fact that the gravity of the sun acts in roughly the same direction as the Earth's gravity at midnight and roughly in the opposite direction at noon. &lt;strong&gt;But this is wrong.&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the exact same accelaration affects the scale, the reading on the scale is unaffected by the alignment of the gravity of Earth and Sun (thx @asdfex for pointing it out).&lt;/p&gt;&#xA;" OwnerUserId="6917" LastEditorUserId="6917" LastEditDate="2017-09-27T11:32:09.263" LastActivityDate="2017-09-27T11:32:09.263" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23165" PostTypeId="2" ParentId="23140" CreationDate="2017-09-27T12:41:57.800" Score="4" Body="&lt;p&gt;From a book in french about tides: If a man with a mass of 102 kg is standing on earth, the constant gravitational force of Earth to the man is 1000 N, of the Sun 0.61 N or 610 mN and of the Moon 0.0034 N or 3.4 mN.&lt;br&gt;&#xA;The maximal tidal variable force of the Moon is 0.000112 N or 112 µN and of the Sun 0.000052 N or 52 µN. The relation of 3.4 mN to 1000 N is 3.4 ppm (part per million) or 0.00034 %. But the tidal forces are much smaller: 112 ppb (part per billion) for the Moon and 52 ppb for the Sun.&lt;br&gt;&#xA;The effect of the tides to the ocean is also very small far away from the coasts, about 0.7 m height variation. Compared to the Earth diameter of 6378 km it is only 110 ppb. Only the very small tidal forces would affect the scale. There are superconducting gravimeters able to measure such small variations. If the constant force of 3.4 mN were responsible for oceanic tides, the variations of water height would be incredible strong, about 31 times higher.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-09-27T12:41:57.800" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23166" PostTypeId="1" CreationDate="2017-09-27T13:30:19.210" Score="7" ViewCount="64" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/directorates/spacetech/niac/index.html&quot; rel=&quot;noreferrer&quot;&gt;NIAC&lt;/a&gt; as in &lt;em&gt;the NASA Institute for Advanced Concepts&lt;/em&gt; wasn't founded until 1998, so such futuristic concepts might not have come to fruition much yet. But I'd like to hear of any, or of what the expectations and purposes of NIAC is. Is it for example intended to stimulate the creativity of the staff? To attract creative people who want a serious shot at designing a real space mission one day? To be a part of NASA's organizational culture?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://livestream.com/viewnow/NIAC2017&quot; rel=&quot;noreferrer&quot;&gt;Here&lt;/a&gt; are videos of the NIAC 2017 presentations, with schedule at the bottom of the video list.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-09-27T13:30:19.210" Title="What NIAC investigations were implemented in real spaceflight? (NASA Innovative Advanced Concepts)" Tags="|nasa|technology|conceptual|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23167" PostTypeId="1" AcceptedAnswerId="23168" CreationDate="2017-09-27T16:09:21.567" Score="6" ViewCount="2594" Body="&lt;p&gt;Reading this question &lt;a href=&quot;https://space.stackexchange.com/questions/5744/why-are-rocket-engines-operating-above-100-often-considered-nominal&quot;&gt;Why are rocket engines operating above 100% often considered nominal?&lt;/a&gt; leads me to a question about throttling back the SSMEs in the later stages of a Shuttle Launch to limit G forces on the astronauts.  If the SSMEs were not throttled back, what would have been the anticipated G Forces on the astronauts at the end of the main SSME burn time?&lt;/p&gt;&#xA;" OwnerUserId="16735" LastEditorUserId="6944" LastEditDate="2017-09-27T16:23:15.440" LastActivityDate="2017-09-27T20:44:53.157" Title="Theoretical Max G forces on Shuttle Launch?" Tags="|space-shuttle|ssme|acceleration|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23168" PostTypeId="2" ParentId="23167" CreationDate="2017-09-27T16:19:06.183" Score="9" Body="&lt;p&gt;Stealing the mass-at-MECO number from this &lt;a href=&quot;https://space.stackexchange.com/questions/264/energy-cost-of-bringing-the-space-shuttle-external-tank-to-orbit&quot;&gt;answer&lt;/a&gt;, and using the 104.5% thrust numbers for 3 SSMEs from &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_main_engine&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, I get&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(3 x (490,000) lbf / 308650 lbm ) = &lt;strong&gt;~ 4.75 g's&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Sanity check&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;(3 x (.65 x 490,000) lbf / 308650 lbm ) = ~ 3.1 g's&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Using the approximate 65% power level at shutdown, assuming that the thrust varies linearly with throttle percent (and ignoring that they normally only throttled to 67%) gives the expected approximate 3 g's&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Info on nominal throttle settings in the answer to this question: &lt;a href=&quot;https://space.stackexchange.com/questions/16971/which-sts-mission-raised-the-normal-engine-throttle-above-100-and-what-change&quot;&gt;Which STS mission raised the normal engine throttle above 100%, and what change to the SSME made that possible?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-09-27T20:44:53.157" LastActivityDate="2017-09-27T20:44:53.157" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23170" PostTypeId="1" AcceptedAnswerId="23351" CreationDate="2017-09-28T09:53:02.290" Score="6" ViewCount="2991" Body="&lt;p&gt;As part of an optimal control problem (see &lt;a href=&quot;https://space.stackexchange.com/questions/23120/retromotor-thrust-vectoring-strategy-boundary-value-problem&quot;&gt;linked problem&lt;/a&gt;), I need the polar form of the equations of motion (EOM) defining the orbit of a spacecraft. In e.g. Bryson and Ho (1969) and Vallado (2007) the following EOM are presented:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\dot{r}=v_r$$&#xA;$$\dot{\theta}=\frac{v_{\theta}}{r}$$&#xA;$$\dot{v}_r=\frac{v_{\theta}^2}{r}-\frac{\mu}{r^2}+\frac{T\sin{\beta}}{m_0-|\dot{m}|t}$$&#xA;$$\dot{v}_{\theta}=-\frac{v_{r}v_{\theta}}{r}+\frac{T\cos{\beta}}{m_0-|\dot{m}|t}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $r$ is the radial distance of the spacecraft from the attracting center, $v_r$ is the radial-velocity component, $v_{\theta}$ is the tangential/transverse-velocity component, $\mu$ is the gravitational constant of the attracting center, $T$ is thrust (constant), $\beta$ is the in-plane control angle (measured from the local horizontal to the thrust vector), $m$ is the mass of the spacecraft and:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\dot{m}=-\frac{T}{I_{sp}g_0}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;is the (constant) fuel consumption rate or mass flow rate in $kg/s$, with $I_{sp}$ being the (constant) specific impulse and $g_0$ the standard gravity (Earth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately the full derivations for the latter two equations, radial ($\dot{v}_{r}$) and tangential acceleration ($\dot{v}_{\theta}$), are not given. I don't necessarily need these derivations for my calculations, but I just like to know how they were done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The equation for the radial acceleration ($\dot{v}_{r}$) can be derived by starting with:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\ddot{r}-r\dot{\theta}^2=-\frac{\mu}{r^2}+\frac{T\sin{\beta}}{m_0-|\dot{m}|t}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However for the derivation of the equation for tangential acceleration ($\dot{v}_{\theta}$), I'm not completely sure where to start. &lt;strong&gt;Probably&lt;/strong&gt; with:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$r\ddot{\theta}+2\dot{r}\dot{\theta}=\frac{T\cos{\beta}}{m_0-|\dot{m}|t}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this correct or am I missing a term? Does anyone have an idea how derive the equation for $\dot{v}_{\theta}$ as stated at the top of this post?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Extra edit:&lt;/strong&gt; Maybe the product rule offers a solution here?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$[r\dot{\theta}]^{'}=\dot{r}\dot{\theta}+r\ddot{\theta}$$&#xA;$$r\ddot{\theta}=[r\dot{\theta}]^{'}-\dot{r}\dot{\theta}$$&#xA;$$r\ddot{\theta}=[v_{\theta}]^{'}-\dot{r}\dot{\theta}$$&#xA;$$r\ddot{\theta}=\dot{v}_{\theta}-\dot{r}\dot{\theta}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting this in the equation above would yield the same result presented in the literature. But if this is true, can someone explain me why $r\ddot{\theta}\neq\dot{v}_{\theta}$ (see e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Equations_of_motion#General_planar_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;)?&lt;/p&gt;&#xA;" OwnerUserId="21124" LastEditorUserId="21124" LastEditDate="2017-10-13T16:46:44.803" LastActivityDate="2017-10-13T16:46:44.803" Title="How to derive the polar form of the equations of motion?" Tags="|mathematics|astrodynamics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23171" PostTypeId="1" AcceptedAnswerId="24394" CreationDate="2017-09-28T15:16:35.253" Score="8" ViewCount="209" Body="&lt;p&gt;I wonder if a cryogenically cooled probe could fly very fast tangentially pass the Sun and through its &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_transition_region&quot; rel=&quot;noreferrer&quot;&gt;transition zone&lt;/a&gt; where the temperature drops to below 6 000 K, from millions in the corona. What maximum speed would a spacecraft achieve from the Sun's gravity and how long would it be within, say, one Solar radius of the surface?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the plasma in the lower corona too dense even for an ablative shield? The spacecraft could be shaped like a sphere and tumble to spread the heat. It should still be able to detect temperature, density, mass concentrations and magnetic field.&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2018-01-10T21:25:59.590" Title="Could a probe fly through the Sun's transition zone between chromosphere and corona?" Tags="|mission-design|flyby|temperature|the-sun|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23172" PostTypeId="2" ParentId="23144" CreationDate="2017-09-28T17:09:47.167" Score="1" Body="&lt;p&gt;It might be worth it to mine thorium on the Moon.  The comments have indicated the political costs of shipping it from the Earth (and we'll likely need it down here anyway). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The risks of launch can be minimized by increasing the mass you are launching which starts to explode the cost of launching out of our gravity well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have no information about the cost of mining thorium on the Moon but it will all depend on how much you need up there.  At some point, shipping the proper refining equipment will cost less than shipping continuous quantities of thorium.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice that I said &quot;refining&quot; in the above paragraph.  It makes sense to mine once and refine as many minerals out as you can.&lt;/p&gt;&#xA;" OwnerUserId="18842" LastActivityDate="2017-09-28T17:09:47.167" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23173" PostTypeId="1" CreationDate="2017-09-28T19:59:56.913" Score="1" ViewCount="597" Body="&lt;p&gt;I understand how to use lagrange multipliers to obtain the optimal stage masses given the payload mass, the burnout velocity, the structural ratios of each stage and the specific impulses of each stage.   However, the &quot;optimal staging&quot; formulation seems to be independent of the mass flow rate $\dot{m}$, which is an essential ingredient into computing the overall thrust T that a stage needs to produce:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$T=I_{sp}g_0\dot{m}$$  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I understand, the classical optimal staging problem can tell you how much fuel you need in each stage to give the payload a given velocity, but it doesn't tell you how quickly it needs to burn to achieve this.  How can I find the (optimal) mass flow rate needed to send the optimally staged rocket's payload to its desired velocity?  Is there another formulation of the optimization problem that can help me yield the optimal mass flow rates of each stage in addition to the stage masses?  Or is there a way that I can calculate the necessary (optimal) thrusts of each stage first so that I can then derive the optimal mass flow rates?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="6494" LastEditDate="2017-09-28T20:13:07.917" LastActivityDate="2017-09-30T03:36:07.390" Title="How can I obtain the necessary thrusts / mass flow rates to achieve a given burnout velocity for an optimally staged rocket?" Tags="|mass|thrust|stages|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23174" PostTypeId="2" ParentId="6250" CreationDate="2017-09-28T22:53:57.143" Score="1" Body="&lt;p&gt;take everything I'm going to say with a huge grain of salt. I'm just a guy who had the same question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As stated, there is no medium in space against which to measure temperature, however you can measure incident radiation which is a kind of temperature in that too little of it and you will freeze and too much and you will get baked, and not in a good way.&#xA;But that's only half the story, you also need to consider things like surface area, spin (which would allow a more even distribution of heat energy on the surface) and insulation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you want is to find a region around the sun where the outgoing heat energy through black body radiation of the object in question is balanced out by the incoming heat energy from the sun at the given temperature.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just a warning, I'm going to be really ballparking the numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance a human being produces about 100 watts of heat energy at rest. It also radiates away roughly 1000 watts in vacuum assuming a surface area of 2m^2 and body temperature of 300Kelvin (found using the Stefan-Boltzmann law. (This law works best if you assume the human being is a sphere.)) That means if you were butt naked in space, apart from the fact that you have much bigger things to worry about, you would want to be in a region of space where the incident solar radiation is about 900 watts to balance out what your body is radiating away plus what it is generating so there is zero net flow of energy from your body.&#xA;That would put you somewhere between the orbits of Mars (700 watts) and Earth(1400 watts).&#xA;You would want to be spinning too, otherwise the side of you facing the sun might be overheating as the side facing away freezes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were in a space suit without thermal regulation, you would probably need to be even further away since they probably radiate much less heat, however you could make it so the suit reflected away almost all of the radiation making you both shiny and able to maintain ideal body temperature even closer to the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, if you are in the shadow of something, you will freeze no matter how close to the sun. Even the night side of Mercury for instance which is very close to the sun goes down to nearly absolute zero since there is nothing to insulate it and hold on to the heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, take everything I say with a grain of salt.&lt;/p&gt;&#xA;" OwnerUserId="21193" LastEditorUserId="21193" LastEditDate="2017-09-28T23:18:44.297" LastActivityDate="2017-09-28T23:18:44.297" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23175" PostTypeId="1" AcceptedAnswerId="23176" CreationDate="2017-09-29T09:28:36.947" Score="14" ViewCount="4227" Body="&lt;p&gt;When we use our rockets in space, are we using up a fraction of the finite oxygen here on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm assuming that it's a negligible amount at our current rate of space travel, but could this potentially become an issue in the long run, assuming we increase our space travel frequency?&lt;/p&gt;&#xA;" OwnerUserId="21194" LastActivityDate="2022-05-28T16:54:01.257" Title="Do rockets deplete the oxygen on earth?" Tags="|fuel|oxygen|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23176" PostTypeId="2" ParentId="23175" CreationDate="2017-09-29T10:37:14.323" Score="34" Body="&lt;p&gt;Let's start with a Fermi estimate: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The atmosphere has a mass of about &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Earth&quot; rel=&quot;noreferrer&quot;&gt;5.15×10&lt;sup&gt;18&lt;/sup&gt; kg&lt;/a&gt;, 20% of that is oxygen. A rocket launch uses on the order of 10&lt;sup&gt;6&lt;/sup&gt; kg of oxygen. To use up all the oxygen (and assuming no oxygen is replenished by plants) requires 10&lt;sup&gt;12&lt;/sup&gt; rocket launches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, rockets are only a tiny part of all oxygen consumption. We burn &lt;a href=&quot;http://www.worldwatch.org/global-fossil-fuel-consumption-surges&quot; rel=&quot;noreferrer&quot;&gt;4×10&lt;sup&gt;12&lt;/sup&gt; kg of oil&lt;/a&gt; every year, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Coal#Market_trends&quot; rel=&quot;noreferrer&quot;&gt;7×10&lt;sup&gt;12&lt;/sup&gt; kg of coal&lt;/a&gt;. This requires on the order of 10&lt;sup&gt;13&lt;/sup&gt; kg of oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oil/gas/coal/wood burning has a measurable effect on our atmosphere. CO&lt;sub&gt;2&lt;/sub&gt; concentration was &lt;a href=&quot;http://www.scientificpsychic.com/etc/timeline/atmosphere-composition.html&quot; rel=&quot;noreferrer&quot;&gt;stable at 280 ppm&lt;/a&gt; until the year 1800, then it began to rise to the 400 ppm we have today. The total plant mass on Earth is no longer able to convert all the CO&lt;sub&gt;2&lt;/sub&gt; we create back to oxygen. This also means we have to consider the total oxygen use by the rocket, not just the fraction that is used outside the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming all oxygen is converted to CO&lt;sub&gt;2&lt;/sub&gt;, the oxygen content has changed by the same amount, from 209,580 ppm in 1800 to 209,460 ppm now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the rocket industry to have a similar effect on our atmosphere would require 10&lt;sup&gt;6&lt;/sup&gt; rocket launches per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.scientificpsychic.com/etc/timeline/atmosphere-composition.html&quot; rel=&quot;noreferrer&quot;&gt;history of our atmosphere&lt;/a&gt; gives some hints of what might happen when the oxygen content drops. The O&lt;sub&gt;2&lt;/sub&gt; fraction was held at 10% until the soil and oceans were saturated. I suspect this means oxygen trapped in the soil and oceans will start to be released once the O&lt;sub&gt;2&lt;/sub&gt; fraction drops below 10%.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-09-30T08:36:52.270" LastActivityDate="2017-09-30T08:36:52.270" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="23177" PostTypeId="1" CreationDate="2017-09-29T17:04:54.113" Score="10" ViewCount="1978" Body="&lt;p&gt;Space exploration pessimists say that there's nothing to import from space to Earth. So why not begin with doing exactly that?! :-)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's commonly claimed that the vacuum of space is harder (emptier) than any vacuum created in a lab on Earth. So what about bringing a container to orbit, evacuate it to empty space, seal it and take it home as a perfect vacuum chamber? Would such a thing be in demand?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2020-05-21T21:13:19.973" Title="Could there be any use with importing vacuum from space to Earth?" Tags="|vacuum|natural-resources|sample-return|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23178" PostTypeId="2" ParentId="23177" CreationDate="2017-09-29T17:42:15.073" Score="17" Body="&lt;p&gt;According to WP, at the upper end of Earth thermosphere -- that's LEO, somewhere above the orbit of ISS -- the pressure goes to around $1 \times 10^{-7}$ Pa. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ultra-high_vacuum&quot; rel=&quot;noreferrer&quot;&gt;This level of vacuum&lt;/a&gt; is regularly achieved on Earth, with &lt;a href=&quot;https://en.wikipedia.org/wiki/Molecular_beam_epitaxy&quot; rel=&quot;noreferrer&quot;&gt;MBE&lt;/a&gt; chambers going down to around $1 \times 10^{-10}$ Pa. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get a better vacuum you'd need to get quite a bit farther from Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, there's no practical use for &quot;bringing vacuum back&quot;. If you bring back a perfectly evacuated chamber of volume $v$ and connect it to your vacuum-requiring-experimental system of equal volume, you only drop the pressure by 50%. To make a significant difference, you'd need to bring back an enormous pressure vessel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would make much more sense to send your experimental rig or your MBE or &lt;a href=&quot;https://en.wikipedia.org/wiki/Electron-beam_lithography&quot; rel=&quot;noreferrer&quot;&gt;EBL&lt;/a&gt; system up into space and do your experiments or production in orbit. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-09-29T17:42:15.073" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23179" PostTypeId="2" ParentId="23177" CreationDate="2017-09-29T17:42:35.183" Score="36" Body="&lt;p&gt;There are multiple problems with this idea.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first and most obvious problem is that the sealed container which has a perfect vacuum inside (seller claims so at least) is not of much use until you can put something inside (experiment, material, ...). And when you do that, you unavoidably get some gas inside with it, which ruins your perfect vacuum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could avoid that problem by putting things into the box first and then open it in space. But there is usually hardly any advantage compared to running the whole experiment in space and only bringing back the results (which will often be just immaterial data which you do not need to land softly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But let's stick to the idea for a while. Let's connect some airlock to the perfect vacuum chamber to put things inside later. This can work... sort of. The problem is that a better vacuum usually means that the pressure is lower by at least an order of magnitude than what we can create on Earth. If your vacuum is not at least an order of magnitude better than the one we have, then usually nobody would care. So, if you can pump down the airlock to your laboratory's ultimate vacuum pressure, and you want to gain at least one additional order of magnitude, the perfect vacuum container needs to have at least ten times the volume of the airlock. And you can only use it that way once. That's not very practical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anyway, the real show-stopper is the fact &lt;em&gt;why&lt;/em&gt; we can not create a perfect vacuum in laboratories on earth. It turns out that the problem does not lay in the vacuum pumps. The problem is that the container itself ruins the vacuum. You can build an almost perfect vacuum pump, quite similar in effect to connecting an infinitely large container with a perfect vacuum to your experimental chamber. You just need to cool down the chamber wall to a really low temperature (think liquid helium or below). Then any particle hitting this wall just sticks there -- effectively equivalent to the situation when it would continue into an infinitely large empty container.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But such a perfect pump would still not create a perfect vacuum. Every piece of material you use to build your chamber, experiment etc. is continuously outgassing lots of gas (&quot;lots&quot; in terms of an ultra high vacuum where a few thousand particles per cm³ is considered a lot). So the resulting pressure is kind of an equilibrium between outgassing flow and pumping flow. In order to get a better vacuum, you usually need to pump faster, not &quot;better&quot;. (And use containers and materials releasing less gas, of course.) And you need to pump out lots of gas (again, &quot;lots&quot; in the terms of an ultra high vacuum, but still a lot compared to the amount of remaining gas present in one moment inside the evacuated chamber), especially for a sustained vacuum. And a container with a perfect vacuum is not a good vacuum pump in these terms.&lt;/p&gt;&#xA;" OwnerUserId="18835" LastEditorUserId="335" LastEditDate="2017-09-30T23:09:01.823" LastActivityDate="2017-09-30T23:09:01.823" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23180" PostTypeId="1" AcceptedAnswerId="23183" CreationDate="2017-09-29T18:57:59.980" Score="-3" ViewCount="980" Body="&lt;p&gt;Is there any indication throttling would be used instead?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The underbelly of the BFR v2017 has not been shown in the IAC presentation, we only know it has 31 engines for a 9 meter diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, we know the BFR v2016 had planned for gimballing, with 8 of them used for the landing part: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/4iRdk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4iRdk.png&quot; alt=&quot;42 engines, BFR v2016&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6852" LastEditorUserId="897" LastEditDate="2017-09-30T08:40:37.410" LastActivityDate="2017-09-30T08:40:37.410" Title="BFR: will there be no more gimballing of the first stage?" Tags="|spacex|its|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23181" PostTypeId="1" AcceptedAnswerId="23190" CreationDate="2017-09-29T19:15:33.083" Score="3" ViewCount="6145" Body="&lt;p&gt;The 2016 Mars vehicle design had a payload to LEO of 500 tons in expendable mode.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its replacement, the 2017 &quot;BFR&quot; design, is planned to have a payload to LEO of 150 tons with reuse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would the payload of the 2016 version have been be if reused?&#xA;And what would the payload of the 2017 version be in expendable mode?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we would know how much weight penalty the downsizing cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bdiIZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bdiIZ.png&quot; alt=&quot;payload diff between BFR v2016 and v2017&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6852" LastEditorUserId="6816" LastEditDate="2017-10-01T20:59:09.693" LastActivityDate="2017-10-01T20:59:09.693" Title="Payload to LEO of the BFR 2017 compared to the 2016 version" Tags="|spacex|its|" AnswerCount="1" CommentCount="0" ClosedDate="2017-10-02T15:29:39.807" ContentLicense="CC BY-SA 3.0" />
  <row Id="23182" PostTypeId="1" AcceptedAnswerId="29767" CreationDate="2017-09-29T19:26:30.483" Score="11" ViewCount="930" Body="&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?v=XcPtQYalkcs&quot; rel=&quot;noreferrer&quot;&gt;this video overview of NASA's proposed EM-1 mission&lt;/a&gt;, the solar panels on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orion_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;Orion spacecraft&lt;/a&gt; are initially shown extending perpendicular to the spacecraft (at around 3:46).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AYJGe.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AYJGe.jpg&quot; alt=&quot;Orion spacecraft with ICPS and perpendicular panels&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shortly after that, as the perigee raise maneuver begins, the panels are shown dramatically swept back (at 3:52):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0qrTC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0qrTC.jpg&quot; alt=&quot;Orion spacecraft with ICPS and swept panels&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 5:24 in the video, while flying by the moon, the panels once again extend perpendicularly, but are rotated to face the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9rZeW.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9rZeW.jpg&quot; alt=&quot;Orion spacecraft seen against the moon with perpendicular rotated solar panels&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 6:18, during a propulsive maneuver using the service module engines, the panels are now swept far &lt;em&gt;forward&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cdkOk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cdkOk.png&quot; alt=&quot;Orion spacecraft firing service module engine with solar panels swept forward&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What's going on here? Why do the panels sweep both forward and back?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously the sweep isn't for aerodynamic purposes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only explanation that makes any sense to me is:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The forward sweep helps get the panels out of the way of the expanding exhaust plume from the service module's engine (an &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_Orbital_Maneuvering_System&quot; rel=&quot;noreferrer&quot;&gt;AJ-10-190 repurposed from space shuttle OMS&lt;/a&gt;);&lt;/li&gt;&#xA;&lt;li&gt;Having paid the weight and complexity cost of a forward sweep mechanism, sweeping backward using the same mechanism reduces mechanical stress on the mountings during ICPS burns.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;However, the &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10&quot; rel=&quot;noreferrer&quot;&gt;RL10&lt;/a&gt; engine on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_Cryogenic_Second_Stage#ICPS&quot; rel=&quot;noreferrer&quot;&gt;ICPS&lt;/a&gt; develops about 110kN; against the 40 tons of stage and spacecraft at the end of the burn, this is only about 1/3g acceleration. It seems wrong that the extended solar panels wouldn't be able to support their own weight in Earth's gravity, if only for convenience in assembly of the spacecraft. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-09-29T22:26:17.453" LastActivityDate="2018-08-02T17:09:43.153" Title="Do Orion's solar panels have adjustable sweep?" Tags="|solar-power|orion-spacecraft|sls|rl-10|" AnswerCount="1" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23183" PostTypeId="2" ParentId="23180" CreationDate="2017-09-29T20:35:59.027" Score="2" Body="&lt;p&gt;Gimballing has significantly quicker response than throttling so most probably no, differential throttling won't replace gimballed engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We just did not get to see the &quot;business end&quot; of the first stage because there was nothing &quot;new&quot; compared to last year. Just a different number of engines.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2017-09-29T20:35:59.027" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23184" PostTypeId="1" CreationDate="2017-09-29T23:18:31.253" Score="1" ViewCount="1712" Body="&lt;p&gt;I would like to pose a fact-checking question about this article, which is about the Musk/SpaceX idea of using rockets for travel between terrestrial locations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.theverge.com/2017/9/29/16385026/elon-musk-spacex-rocket-transportation-point-to-point&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.theverge.com/2017/9/29/16385026/elon-musk-spacex-rocket-transportation-point-to-point&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“You can’t fly humans on that same kind of orbit,” Brian Weeden, director of program planning for Secure World Foundation, told The Verge. “For one, the acceleration and the G-forces for both the launch and the reentry would kill people. I don’t have it right in front of me, but &lt;strong&gt;it’s a lot more than the G-forces on an astronaut we see today going up into space and coming back down&lt;/strong&gt;, and that’s not inconsiderable.”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is a surprising claim, but is it disingenuous? Firstly, what's the implied physical mechanism underlying the bolded text? Given that you start out with an orbital-capable rocket in the first place (as Musk's argument goes), how hard would it be to keep g-forces within tolerable range?&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2017-10-08T02:59:33.360" Title="g-forces of suborbital versus orbital passenger travel" Tags="|reentry|sub-orbital|g-force|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23185" PostTypeId="2" ParentId="23184" CreationDate="2017-09-30T00:55:50.223" Score="4" Body="&lt;p&gt;Back-of-the-envelope:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle kept ascent acceleration at or below about 3g - tolerable for most people; other human-rated launches have gone higher. Even under the 3g constraint, burn duration to LEO is only about 8 minutes. LEO period is about 90 minutes. The furthest you would need to travel to get from anywhere to anywhere is equivalent to one half orbit (45 min coast). Assume similar descent acceleration as ascent (not more than 3g). This gives us a worst-case trip time of about 8+45+8 minutes or 61 minutes total. The claim is to get from anywhere to anywhere in not more than an hour, which is reasonable. Another claim is that typical trips may be no more than 30 minutes (because the distances involved are considerably less); this would also be reasonable.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="429" LastEditDate="2017-09-30T14:26:00.610" LastActivityDate="2017-09-30T14:26:00.610" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23186" PostTypeId="2" ParentId="23184" CreationDate="2017-09-30T01:05:33.463" Score="3" Body="&lt;p&gt;The paper &lt;a href=&quot;https://isulibrary.isunet.edu/opac/doc_num.php?explnum_id=95&quot; rel=&quot;nofollow noreferrer&quot;&gt;Great Expectations: Assessing the Potential for Suborbital Transportation&lt;/a&gt; says&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Suborbital tourism operators expect g-forces of +3g at the start of &#xA;  the ballistic phase and up to +6g during reentry.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Based on this, stating that the forces would kill people seems hyperbolic (for some values of &quot;people&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the paper goes on to state&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;it  seems  reasonable  to  suggest  that  the  average  passenger &#xA;  should  not  be   subjected to g-loads greater than 3.0 +Gx and 2.0&#xA;  +Gz, and that the period of exposure to these  maximum  g-loads  should  not  exceed  thirty  minutes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-09-30T01:05:33.463" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23187" PostTypeId="2" ParentId="6460" CreationDate="2017-09-30T02:54:27.937" Score="1" Body="&lt;p&gt;There's a very big problem here:  Rendezvous.  Sure, it's a pretty routine maneuver with the ISS but that's because the rocket is launched into the correct orbit in the first place.  If the rocket has to match orbits with it's target that gets very expensive (in terms of delta-v) very fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, suppose there was something in the right place they could match orbits with it.  What are you going to do?  Politely ask it to come along for a ride down to the fire?  You don't have any way to couple the booster to the target.  If you align it to the exact center of mass (is it even known?) and light your booster how do you ensure it doesn't fall off?  (Take a baseball bat and balance it on your hand--vertically.  Hard, isn't it?  Now take another bat and sit it on top of the bat you're balancing.  Finally, give it a good shove up into the air--without the second bat falling off.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, for a dedicated de-orbit tug you could do something like harpoon the target and drag it along.  Said tug would have a very small engine which would keep the force down to something for which this would work.  However, you're talking about the upper stage.  Lets take the Falcon 9 upper stage since I just had occasion to do some math on it a few days ago.  It's engine puts out 210,000 pounds of thrust.  Assuming it harpooned something it's own size that means the tether is going to experience 105,000 pounds of force--that tether had better be anchored awfully well!  And what's the geometry?  If the booster is pushing the target the coupling must be solid enough to withstand that kind of force without shifting (not merely a harpoon!)  If the booster is pulling that avoids the big balance problem but means the tether cables go awfully close to the rocket exhaust.  Can you say &quot;melt&quot;?&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-09-30T02:54:27.937" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23188" PostTypeId="2" ParentId="23173" CreationDate="2017-09-30T03:36:07.390" Score="1" Body="&lt;p&gt;This question inherently can't be answered as the problem is incompletely defined.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You have three very different problems based on where you are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1)  In space:  Here it's simple:  You want the tiniest engine you can get that doesn't sacrifice ISP.  A bigger engine will complete the burn faster but so what?  Using the same mass as fuel will almost certainly get you there faster by cutting travel time.  Look at some of the burn times NASA lists for maneuvers by it's deep space probes and you'll see their engines have to be tiny.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2)  On an airless body:  Now it gets a lot more complex and there is no optimal answer without considering the whole mission.  The higher the thrust of your engine the less delta-v you'll use but you'll be carrying more engine weight--and you'll also have had to get that engine weight there in the first place.  Hauling that bigger engine might cost you more fuel than you save by reducing your gravity loss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3)  In atmosphere:  Now it's even more complex as you have to consider air drag--now the bigger engine also carries an increased drag penalty as well as an increased weight penalty.  Not only must you consider the whole mission but you must consider the shape of your rocket also.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since cases #2 and #3 are optimizations between competing variables you have to optimize for each mission, there are no simple equations that give you an overall best answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, unless you want to design an engine for the mission you're limited to what's on the shelf--and that may not be optimum for what you want to do.   I play Kerbal Space Program and I've sent out plenty of overpowered spacecraft because the available parts don't match up perfectly with my objectives and because engines might be called upon to operate in multiple realms.  Consider a simple mission:  Land a probe on the Mun (the closest moon).  The first part of the mission is atmospheric flight.  The second part of the mission is space flight (transfer orbit, orbital insertion burn).  The third part is airless body flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the boosters used to get off Kerbin will generally fall back.  Space flight is in my category #1--which would say to use the smallest engine.  However, I'm going to land which means I must carry enough thrust to land.  Why not simply use the landing engine for the space flight also?  Thus I end up doing the space mission on engines capable of landing on the Mun, not merely getting there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, consider a more expensive mission:  Land on Ike.  It's the moon of the Mars-analogue, a bit weaker gravity than the Mun.  However, you have to do the interplanetary voyage.  If it's a simple lander I would use the same system, just with a bit more fuel.  However, if I'm taking something big (say, a manned rover capable of exploring the moon) the economics change: You get a smaller, cheaper rocket by doing all the space maneuvering with a nuclear engine.  It doesn't come in a really small size, though, so it doesn't get used for the small stuff.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-09-30T03:36:07.390" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23190" PostTypeId="2" ParentId="23181" CreationDate="2017-09-30T12:29:38.267" Score="8" Body="&lt;p&gt;We can make a base assumption first about how much $\Delta V$ we need to get to LEO and if you wanna update that the rest of the calculations are super easy to redo. Let's say we need $10km/s$ for now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A stage has a $\Delta V$ given by Tsiolkovsky's Rocket Equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V = g_0 \cdot I_{sp} \cdot ln \Bigl( \frac{m_{dry} + m_{prop} + m_{payload}}{m_{dry} + m_{payload}} \Bigr) $&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In 2016, Elon Musk's presentation in Guadalajara mentioned that the booster uses 7% of its propellant to perform the boostback and landing maneouvres, and this gives us almost all of the information we need. The one quantity which we are unsure of is the value of $I_{sp}$ to use for the first stage, since it is constantly changing as the ambient atmospheric pressure changes around it during ascent. A quick and dirty (but surprisingly accurate) approximation is to take the mid-point of the sea-level and vacuum values, and round up a bit. In this case we get &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$I_{sp} = (334 + 361)/2 = 347.5s$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's just take 350s. Now, our $\Delta V$ is given by&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V = 9.81 \cdot 350 \cdot ln \Bigl( \frac{275 + 6700 + 2100 + m_{payload}}{275 + 0.7\cdot6700 + 2100 + m_{payload}} \Bigr) + 9.81 \cdot 382 \cdot ln \Bigl( \frac{2100 + m_{payload}}{150 + m_{payload}} \Bigr) $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.wolframalpha.com/input/?i=350*9.81*ln((9075%2Bx)%2F(2844%2Bx))%20%2B%20382*9.81*ln((2100%2Bx)%2F(150%2Bx))%20%3D%2010000&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wolfram Alpha can solve this for us, for $\Delta V = 10km/s$&lt;/a&gt;. We get a payload mass of ~303t.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All figures are taken from the IAC presentation slides here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7DjSH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7DjSH.png&quot; alt=&quot;Raptor&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/wqW1B.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wqW1B.png&quot; alt=&quot;ITS Booster&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/jIEET.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jIEET.png&quot; alt=&quot;ITS Spaceship&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;This year, Musk didn't go into detail about the downsized booster so we don't know its dry mass or propellant mass, which is a problem. But to get some idea, we could assume that the booster has the same propellant mass fraction. (It's a bit hand-wavy, but you can make different assumptions and follow the same methodology.) Then we can derive the dry mass and propellant mass, modify our expression to use 100% of propellant, and then solve for our new payload mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, the 2016 booster had a propellant mass fraction of &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$pmf_{2016} = \frac{6700}{6700+275} = 0.961 $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then our 2017 booster gives us&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$4400 - 1335 = 3065 = \frac{m_{prop}}{0.961}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ m_{prop} = 2944.2t$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ m_{dry} = 3065 - 2944.2 = 120.8t$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plugging these numbers in actually gives us $ \Delta V = 9.685km/s$ but we can live with that. In reality, the $pmf_{2017}$ will be higher since the stage has a lower dry mass than in 2016. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for an expendable version, we get&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\Delta V = 9.81 \cdot 345 \cdot ln \Bigl( \frac{2944.2 + 120.8 + 1185 + m_{payload}}{120.8 + 1185 + m_{payload}} \Bigr) + 9.81 \cdot 375 \cdot ln \Bigl( \frac{1185 + m_{payload}}{85 + m_{payload}} \Bigr) = 9685 $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Solving this using Wolfram Alpha gives us an &lt;a href=&quot;http://www.wolframalpha.com/input/?i=345*9.81*ln((4250%2Bx)%2F(1305.8%2Bx))%20%2B%20375*9.81*ln((1185%2Bx)%2F(85%2Bx))%20%3D%209685&quot; rel=&quot;nofollow noreferrer&quot;&gt;expendable payload mass of 183t&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All figures are taken from the IAC presentation slides here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Mkxal.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Mkxal.png&quot; alt=&quot;Raptor&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/fXDxr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fXDxr.png&quot; alt=&quot;BFR Booster&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/1TAON.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1TAON.png&quot; alt=&quot;BFR Spaceship&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21025" LastEditorUserId="21025" LastEditDate="2017-10-01T14:49:34.320" LastActivityDate="2017-10-01T14:49:34.320" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23191" PostTypeId="2" ParentId="23175" CreationDate="2017-09-30T13:22:53.397" Score="-6" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;LOX-LH2 (liquid oxygen-liquid hidrogen) fuel is used mainly to leave the orbit of the Earth. Its main reason is that liquid hydrogen boils on 40K, and there is no simple way to hold it on this temperature too long, not even in the space.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The hydrogen is created mainly by two ways for that:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;burning methane with cathalitic reactions&lt;/li&gt;&#xA;&lt;li&gt;or by electrolysing water&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;As the H2 burns, essentially these reactions are going only into the opposite way.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="1054" LastEditDate="2019-05-10T22:50:44.730" LastActivityDate="2019-05-10T22:50:44.730" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23193" PostTypeId="2" ParentId="20049" CreationDate="2017-09-30T19:19:34.960" Score="0" Body="&lt;p&gt;To reach a given orbit, anything starting out on the surface (presumably starting at zero speed relative to the surface) has to be accelerated to the same final speed, regardless of method used. Whether it has to accelerate itself (like rockets do), or hitches a ride on something already moving that fast, that acceleration has to happen somehow. Some methods (like rockets) are most efficient when they apply a hefty kick to get the payload above the atmosphere and up to orbital velocity quickly. Whatever the method, a generally bad thing is to be traveling at or near orbital velocity within any substantial atmosphere. For one thing, it can be inefficient because of the drag produced - wasting energy. The other is heat; all that energy lost in atmospheric drag becomes heat which has to go somewhere, like into the skin of the vehicle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space elevators side-step some of the problems. A geostationary orbit still requires acceleration, but to a lower final speed, though it requires the input of more total energy than does a low Earth orbit. The elevator adds this energy mostly by lifting the payload up, and imparts the required acceleration by keeping the payload aligned with the cable. Most of the required orbital acceleration takes place along the portion of the cable which is above the atmosphere (which is most of it).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rail guns on the other hand, face the atmospheric drag problem head-on. The payload is accelerated to extreme speed and as soon as it leaves the gun, becomes ballistic, except that it encounters atmospheric drag, immediately  slowing it down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any other method would have to either deal with hypersonic drag, lift the payload out of the atmosphere before accelerating, or simultaneously accelerate the payload and raise it up out of the atmosphere while keeping it enclosed in a vacuum.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-09-30T19:19:34.960" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23194" PostTypeId="1" AcceptedAnswerId="23198" CreationDate="2017-09-30T22:04:57.060" Score="3" ViewCount="221" Body="&lt;p&gt;Suppose a spacecraft is on an interplanetary Hohmann transfer trajectory from an inner planet to an outer planet.  When the spacecraft finally enters the target planet's sphere of influence, which direction does it typically enter from?  From behind the target planet's orbit (i.e. the spacecraft flies into the SOI) or ahead of it (i.e. the planet's SOI moves into the spacecraft's orbit?  To clarify, this is a question about the relative motion of the frames of reference of the target planet and the spacecraft, not about which side of the planet the spacecraft ends up in.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm developing a simulation of such a transfer trajectory and I just want to know which direction I should expect to see the two meet for debugging purposes only.  For simplicity, we can assume the planets and the Hohmann transfer are coplanar.&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="6494" LastEditDate="2017-10-01T00:26:16.433" LastActivityDate="2017-10-01T01:56:16.193" Title="From which direction should a spacecraft intercept a target planet's sphere of influence for an inner-to-outer planet trajectory?" Tags="|trajectory|interplanetary|hohmann-transfer|soi|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23198" PostTypeId="2" ParentId="23194" CreationDate="2017-10-01T01:56:16.193" Score="3" Body="&lt;p&gt;When going from an inner to outer body in a Hohmann -- Earth to Mars, for example -- the spacecraft will be nearing the aphelion of its elliptical orbit as it enters the destination SOI, so will be going substantially slower than the outer body. From a fixed solar frame, the spacecraft will be ahead of and inside the planet, and the planet will overtake it from behind. The spacecraft is not going fast enough to have a near-circular orbit, so it would simply fall back to perihelion if not for the gravity of the outer body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going from outer to inner, the spacecraft will be near perihelion, going faster than the inner body, overtaking it from behind. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-10-01T01:56:16.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23200" PostTypeId="2" ParentId="18442" CreationDate="2017-10-02T01:10:15.483" Score="3" Body="&lt;p&gt;Besides the points raised, that a cluster of engines would need space to gimbal, and the fact that the outer engines can throttle which can also “turn” your rocket, an engine with the apparatus is slightly heavier and signiicantly more complex than a fixed one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another thing I have the impression might be true is that “gimballed” engine closer to the sides would create a less sound structure.&lt;/p&gt;&#xA;" OwnerUserId="21214" LastActivityDate="2017-10-02T01:10:15.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23201" PostTypeId="2" ParentId="23184" CreationDate="2017-10-02T02:01:52.463" Score="1" Body="&lt;p&gt;Actually considering an acceleration of 3G you would be less than 8 minutes to accelerate,. On more a more realistic approach and for “engineering” reasons the upper stage has lower accelerations.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to simplify on what I managed to find and some back of the envelope calculations the passenger would receive some 1,5 G’s on launch up to 3G’s just before the separation. And on the second stage 1G at the start to 2/3 G’s on final burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This acceleration are not that high with the higher end at 3G’s,  it is however constant acceleration so someone with “bad circulation” or other cardio circulatory deficiency might have trouble getting blood where it should be.  That said it is not really constant acceleration given vectoring and time for acceleration the average acceleration should be at about 1,5, 1,7G’s . sqrt((750/480)^2+1^2)*”Earth gravity effect”/”vertical time”=1,6 &lt;em&gt;(e/v) . For information “e&lt;/em&gt;v” shouldn’t be more than 1 ( and if anyone tells me this formula is wrong I know I just “simplified this last division”, I welcome you to do the whole calculation though that would take Excel spreadsheets or  c++/java, etc. I’m lazy just back of the envelope calculations today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still for comparison when you jump some 2 fett high the ground receive some 300kgf if you have an avg height and some 75kg weight. Aceleration of the bady is about 4 g’s and acceleration of the head of the person jumping should be the second highest pasrt of the body so I would guess  5G’s at least. Droping from 5 feet maybe some 6 ~ 8 G’s of deceleration.  Both of those are things anyone can do without problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore a 3G’s maximum acceleration should be ok.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;About the time a trip to the most distant point in the world would take with an acceleration of 8 minutes and considering constant acceleration and equivalent deceleration it would take 45+8min.(45min trip plus 8 minute of acele + 8 minutes decal minus 8minutefrom the trip because the 8min acceleration +8min decel = traveled distance equal to 8min at top speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the acceleration should be ok for someone healthy limited to 3 Gs and they might even trade off a bit of extra delta v for lower acceleration. Travel time at 53 min for the farthest reaches of earth.(Longer range than any commercial airplane by about 45 % from 14000km to 20000km).&#xA;Travel time for the longest trips from 14000km at 16 hours to 39,5minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any doubts ask away.(Btw I’m not English native, just learned the language a couple years ago.)&lt;/p&gt;&#xA;" OwnerUserId="21214" LastActivityDate="2017-10-02T02:01:52.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23202" PostTypeId="2" ParentId="23144" CreationDate="2017-10-02T07:42:07.427" Score="2" Body="&lt;p&gt;There are other considerations like the fact that you might take thorium from  the slag after reefing aluminum, magnesium, iron, oxygen, silicon and hydrogen(if you go polar). There are other materials but these are the main ones I remember. And that there maybe someplace with better thorium concentration than earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said at the start of actual mining on the moon, a theoretical process that we are only beginning to explore would be very expensive. Mine 1000 ton for 5 kg of thorium. With very limited resources on people, and chemicals to separate the components. After all even aluminum is not so easy as iron to melt and most of the unwanted things go away. And aluminum is not even the most difficult metal to extract from regolith, or any natural resource. That is usually not something you really think about. You just imagine melting everything, but industrial process are rarely so simple and we just begun to actually experiment to try doing it differently than how it works on earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we manage to get a facility running that main consideration  would be the capital cost and the people working there. 1 astronaut send to the moon should cost about USD 50milion at 10kUSD/ kg along why a year of supplies considering bulk cost. At 5PPm if this person alone can maintain a station that refines 1000 tons a day the Gross profit from selling the 5 kg of thorium should be about 25k USD or 9 million USD a year. So 18% of the cost of sending the astronaut there. And this is considering this single person can manage from extraction to refinement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a very high level I suppose that those would not be impossible just very hard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sending thorium up is not all that expensive in the grand scheme of things even if you would need a special container that can survive reentry massively overbuild made of pure metal so even if the whole thing exploded or were directly hit by a small Kinect penetrator it would survive. You don’t really need to worry that much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also a kg of thorium has about 80.000.000 MJ(+-10% I doing from memory) or 1,02MW for 2,5 years. Much more than the ISS, enough for an ion/plasma engine going somewhere in the solar system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So extract a metal that we at the moment we have a surplus on earth and would need to send in small amounts in the foreseeable future from a far place. Not really smart. Little to no demand, high cost, high initial investment. (machines than can mine and refine 1000 tons of ore a day. Plus massive amounts of power for the refinement.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the future when we are mining the moon for Oxygen, silicon, aluminum. It may make sense to develop technics to also extract thorium, but at the moment there is absolutely no reason except maybe to learn how to do it and develop better new thecnics of refinement suitable for space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(I’m not native so my English is a bit skewed)&lt;/p&gt;&#xA;" OwnerUserId="21214" LastActivityDate="2017-10-02T07:42:07.427" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23203" PostTypeId="1" CreationDate="2017-10-02T08:37:22.487" Score="3" ViewCount="371" Body="&lt;p&gt;SpaceX recently announced their plans for creating a 1-hour flight service using BFR rockets which would fly higher than commercial airliners but lower than ISS. It implies that within 1 hour it would be able to reach any point on the globe (with a suitable launchpad).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm curious and a bit worried about the potential of intersecting trajectories with airliners and satellites on lower orbits. There are also plans from SpaceX to launch around &lt;a href=&quot;https://en.wikipedia.org/wiki/SpaceX_satellite_constellation&quot; rel=&quot;nofollow noreferrer&quot;&gt;12 thousand satellites starting in 2020&lt;/a&gt; to serve internet connection. All of this makes me wonder what the chances are of colliding with so many flying objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's &lt;a href=&quot;https://what-if.xkcd.com/imgs/a/24/model_space.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;the famous XKCD illustration for &quot;going to space&quot;&lt;/a&gt;, and to me it seems like BFR is going to fly in a similar manner. What are the actual trajectories going to look like? How will they compare to airline trajectories?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I'd like to see a visual representation of intersecting airspaces of airliners, existing satellites and the planned internet satellites, to get a general idea of it all.&lt;/p&gt;&#xA;" OwnerUserId="990" LastEditorUserId="990" LastEditDate="2017-10-02T08:54:37.987" LastActivityDate="2017-10-02T10:39:03.313" Title="What would the SpaceX's BFR flight trajectories look like?" Tags="|spacex|trajectory|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23204" PostTypeId="2" ParentId="23203" CreationDate="2017-10-02T10:39:03.313" Score="4" Body="&lt;p&gt;Satellites in LEO are between 100-1240 miles, and the BFR is certainly intending to reach LEO to deliver satellites. For global transport, the BFR would enter this region - but that doesn't make a huge chance of colliding with &quot;so many objects&quot; as space is still really empty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In terms of collision risk with aeroplanes - no, this really isn't an issue. Airspace is also very empty except around major airports, and the location of aircraft is well known. A rocket will only be at the same altitude as aircraft (up to 40000ft or so) for a very short period of time, and while planes fly horizontally, a ballistic trajectory is almost vertical at the same altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you shouldn't be worried about this as an issue at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(There are many other reasons why this is unlikely to work, unfortunately)&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2017-10-02T10:39:03.313" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23205" PostTypeId="1" AcceptedAnswerId="23207" CreationDate="2017-10-02T11:07:41.800" Score="45" ViewCount="8406" Body="&lt;p&gt;Does the ISS keep a standing watch with one person always up and awake and monitoring while the others sleep, or do all the astronauts sleep and work and they all get alerted in case of issues beyond the normal scheduled stuff?&lt;/p&gt;&#xA;" OwnerUserId="21218" LastActivityDate="2017-10-02T11:26:53.507" Title="Does the ISS have a watch system with at least one person monitoring at all times" Tags="|iss|procedure|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23206" PostTypeId="1" AcceptedAnswerId="23256" CreationDate="2017-10-02T11:09:12.587" Score="7" ViewCount="512" Body="&lt;p&gt;So far, it appears Mars One will be one-way mission to Mars, to establish a colony with no options to return to Earth, astronauts staying on Mars to the end of their lives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But does that mean the mission ends with death of the last astronaut, or is it planned to be expanded indefinitely, with continued shipments, colony expansion and new arrivals?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2019-10-04T01:45:08.977" Title="Is there an end-of-mission for Mars One?" Tags="|mars|colonization|mars-one|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23207" PostTypeId="2" ParentId="23205" CreationDate="2017-10-02T11:26:53.507" Score="56" Body="&lt;p&gt;It definitely does, and not just one person - except these people are ground-based.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The astronaut crew is only a small fraction of the number of people operating, controlling and maintaining ISS. While astronauts operate in shifts, and usually (though not always) at the same time, periods when everyone on the station is asleep aren't anything infrequent. Especially after long EVA, where two astronauts work outside the station, while others take care of the robotic arm, systems, maintenance, and aid these on EVA in preparations and return, you'll have everyone asleep after a long period of joint activity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Meanwhile, the control center operates 24/7, and save for brief periods where ISS is outside reach of all ground stations, they perform maneuvers, perform IT maintenance, control experiments that are remotely operable, monitor station status, apply regulation of life support systems, and so forth, and so on. There's way too much work just for the astronaut crew - the astronauts only do what can't be done remotely, or doing what remotely would be impossible, dangerous or troublesome, e.g. due to transmission delay (ping), or lack of remote control infrastructure. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-10-02T11:26:53.507" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23208" PostTypeId="1" AcceptedAnswerId="23209" CreationDate="2017-10-02T12:07:16.737" Score="10" ViewCount="3883" Body="&lt;p&gt;Can an asteroid pass near Earth at a right velocity and angle to bend its trajectory and make an orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: I know about objects such as 2016 HO3, which are quasi-satellites of Earth due to Lagrange points. I'm asking whether an asteroid can become an actual satellite of Earth.&lt;/p&gt;&#xA;" OwnerUserId="19223" LastActivityDate="2021-08-17T03:50:11.913" Title="Can an asteroid enter Earth orbit?" Tags="|orbit|asteroid|near-earth-asteroid|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23209" PostTypeId="2" ParentId="23208" CreationDate="2017-10-02T12:41:39.903" Score="16" Body="&lt;p&gt;Our Moon makes this possible. An asteroid with a low &lt;span class=&quot;math-container&quot;&gt;$V_\infty$&lt;/span&gt; with respect to Earth making a close flyby of the Moon in the right direction could get into a distant orbit around the Earth. That object would likely continue to encounter the Moon, and could be ejected again.&lt;/p&gt;&#xA;&lt;p&gt;It is important to realize that things cannot just drift into orbit without something to slow them down. This results from conservation of energy. An object moving towards Earth will gain momentum due to gravity as it approaches. Unless something else interferes to slow the object down (like a gravitational assist by the Moon), that kinetic energy will also necessarily be enough to allow the object to escape Earth's gravitational pull.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="10259" LastEditDate="2021-08-16T17:16:47.153" LastActivityDate="2021-08-16T17:16:47.153" CommentCount="16" ContentLicense="CC BY-SA 4.0" />
  <row Id="23211" PostTypeId="1" AcceptedAnswerId="23212" CreationDate="2017-10-02T13:31:37.467" Score="16" ViewCount="2491" Body="&lt;p&gt;In the event of something accidently being placed into the same orbital path as the ISS and on an imminent collision course, assuming it would be an object with the potential to cause some damage or loss of hull integrity, what would the procedure be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the crew have to get suited up, are there enough suits? and assuming the object was fast approaching how would that work as for more than one person at a time it would be a slow process?  Is there some sort of emergency suit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or would they climb into an attached resupply ship and undock?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is there a secure refuge somewhere in the structure where they could seek shelter?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or would they try to move the ISS to a slightly different orbit given enough time to get out of the objects path?&lt;/p&gt;&#xA;" OwnerUserId="21218" LastActivityDate="2017-10-02T14:33:37.593" Title="What would the procedure be in the event of a collision warning on the ISS" Tags="|iss|procedure|emergency|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23212" PostTypeId="2" ParentId="23211" CreationDate="2017-10-02T14:33:37.593" Score="25" Body="&lt;p&gt;This kind of situation, while not exactly frequent, has happened on ISS multiple times.  With the large amount of debris out there, tracked objects do intersect the ISS orbit from time to time.  There are procedures in place on precisely how this is handled.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally speaking, this is how it works:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;ISS flight controllers get regular conjunction updates from USSTRATCOM (United States Strategic Command), which is the group at Vandenberg Air Force Base that is responsible for tracking all in-orbit objects.  USSTRATCOM notifies NASA if a tracked object is projected to enter the &quot;pizza box&quot;, a 50 km by 50 km by 4 km zone centered around ISS.  From &lt;a href=&quot;https://arstechnica.com/science/2013/07/how-nasa-steers-the-international-space-station-around-space-junk/&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt;, it seems that the ISS team gets one of these notifications about once a week on average.  In general (but not always), these notifications provide a roughly three day warning.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The TOPO flight controller takes these notifications and computes a probability of collision for each one and gives it a label.  A collision probability between 0.001% and 0.01% results in a &quot;yellow&quot; label, and a probability higher than 0.01% results in a &quot;red&quot; label.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What happens next depends on several factors.  Mission rules specify that an avoidance maneuver is to be taken for &quot;yellow&quot; threats when the maneuver would not have a mission impact, &lt;em&gt;e.g.&lt;/em&gt;, affecting an upcoming Soyuz rendezvous.  Action &lt;strong&gt;must&lt;/strong&gt; be taken for &quot;red&quot; threats unless a maneuver would itself endanger station, &lt;em&gt;e.g.&lt;/em&gt;, if something is damaged on ISS and firing the thrusters would cause further damage.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;If the ISS team gets warning more than 28.5 hours in advance (per flight rule -- the odd time period is necessary to coordinate between US and Russian flight control teams, as the Russian segment is what executes the maneuver), a custom debris avoidance maneuver (typically a 0.5 - 1.0 m/s nudge) is planned and executed that best alleviates the threat while minimizing the effects on mission success, vehicle consumables, and structural health.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If the warning gives less than 28.5 hours notice, the team can fall back to what is known as a PDAM -- a Predetermined Debris Avoidance Maneuver.  This is a canned, 0.5 m/s maneuver that has been vetted ahead of time by the various engineering teams and can be executed very quickly.  It may not be the most ideal option, but it's available quickly.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;If warning comes within that 28.5 hour window and a PDAM cannot alleviate the threat, or if a PDAM will create a different conjunction, simply swapping one threat for another, the crew will &quot;shelter in place&quot; in whichever vehicle is their ride home.  This means they will retreat to their return vehicle (Soyuz at present, soon to include Dragon and Starliner), seal up the hatches, and wait until the threat passes.  In the unlikely event of a loss of station atmosphere, they will be protected by the closed hatches, and -- provided that station still has stable attitude control -- they can undock and go home if a full evacuation is necessary.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;All of these options have been exercised multiple times in ISS's flight history.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-10-02T14:33:37.593" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23213" PostTypeId="2" ParentId="23208" CreationDate="2017-10-02T14:40:27.377" Score="8" Body="&lt;p&gt;Aerobraking would not work. If the asteroid shaved off enough speed to put it into orbit, the orbit would be highly eccentric resulting in repeated encounters with the atmosphere and eventual impact.&lt;/p&gt;&#xA;" OwnerUserId="21221" LastActivityDate="2017-10-02T14:40:27.377" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23214" PostTypeId="2" ParentId="23208" CreationDate="2017-10-02T16:02:25.850" Score="1" Body="&lt;p&gt;It really depends whether you're willing to consider collisions, and how large an asteroid. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An object of sufficient size impacting the Earth would shear off a good amount of material and disturb the orbit of the Earth. This could potentially slow down the impacting object enough while still keeping it distinct from the Earth, causing it to orbit around the Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the currently predominant theory is that &lt;a href=&quot;https://en.wikipedia.org/wiki/Giant-impact_hypothesis&quot; rel=&quot;nofollow noreferrer&quot;&gt;this is how the Moon was formed.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21222" LastActivityDate="2017-10-02T16:02:25.850" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23215" PostTypeId="1" AcceptedAnswerId="23216" CreationDate="2017-10-02T17:05:13.527" Score="1" ViewCount="428" Body="&lt;p&gt;I cannot find references if the Delta IV heavy 7 except on &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/5/5a/Delta4_family_future.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;this wikipedia image&lt;/a&gt;. It consists of a Delta IV heavy with 6 boosters. There may be a need for such powerfull launcher (otherwise the SLS' developpement would not have been started) but no reference to it on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Super_heavy-lift_launch_vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia list of current and futur super heavy launcher&lt;/a&gt;. What is the current status of this launcher?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="58" LastEditDate="2020-03-18T18:35:53.050" LastActivityDate="2020-03-18T18:35:53.050" Title="What is the current status of delta IV heavy 7 (in 2017)?" Tags="|delta-iv-heavy|development|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23216" PostTypeId="2" ParentId="23215" CreationDate="2017-10-02T17:34:07.813" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Evolution/DeltaIVLaunchVehicle_GrowthOptionstoSupportNASAsSpaceExplorationVision.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;ULA presented the 6-booster &quot;Delta Superheavy&quot; in 2004&lt;/a&gt; (&lt;a href=&quot;https://web.archive.org/web/20170722094125/https://www.ulalaunch.com/uploads/docs/Published_Papers/Evolution/DeltaIVLaunchVehicle_GrowthOptionstoSupportNASAsSpaceExplorationVision.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;archive link&lt;/a&gt;) as a possible upgrade path for the Delta IV. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as I can tell, it's only a proposed design. Actually building it would likely require structural changes to the core stage to handle the additional stresses (as we've seen with Falcon Heavy), and the launch pad infrastructure would have to be changed to support the new configuration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Delta growth options paper is attributed to staff at Boeing who work closely with NASA, so my assumption is that they would like NASA to take an interest in the options. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the 6-booster Delta is in roughly the same payload class as Block 1 and Block 1B SLS, it's unlikely that it will get support from NASA at present. If SLS fails completely, ULA might try to push the proposal again, but it couldn't be evolved to support payloads in the Block 2 SLS class. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2020-03-02T03:39:21.350" LastActivityDate="2020-03-02T03:39:21.350" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23217" PostTypeId="1" CreationDate="2017-10-02T20:17:20.227" Score="51" ViewCount="2837" Body="&lt;p&gt;This is a followup to a recent &lt;a href=&quot;https://space.stackexchange.com/a/23207/21219&quot;&gt;question and answer&lt;/a&gt; discussing ground-based monitoring of the International Space Station, specifically in the context of an astronaut crew that is all asleep.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How often is the ISS outside of reach of all ground stations?  How long is a typical such period, and how long is the longest possible such period?&lt;/p&gt;&#xA;" OwnerUserId="21219" LastActivityDate="2019-03-13T20:40:34.440" Title="How long is the ISS out of reach of all ground stations?" Tags="|iss|ground-station|mission-control|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23218" PostTypeId="2" ParentId="21244" CreationDate="2017-10-02T20:57:59.527" Score="2" Body="&lt;p&gt;In a later (independent) interview with Eric Donovan, he states the height was 450km. I suspect that in the presentation to the ESA meeting he should have said &amp;quot;300 miles&amp;quot;, which is 482 km; that is very close to the altitude of the &lt;em&gt;SWARM A&lt;/em&gt; satellite at the time, rather than 300 km. All the online articles, including Wikipedia, appear to be clones of the ESA presentation and 'press release', so may all reflect the same error if I am correct.&lt;/p&gt;&#xA;" OwnerUserId="21225" LastEditorUserId="47149" LastEditDate="2022-06-01T00:17:10.377" LastActivityDate="2022-06-01T00:17:10.377" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23221" PostTypeId="2" ParentId="23144" CreationDate="2017-10-02T22:23:59.627" Score="5" Body="&lt;p&gt;Mining lunar thorium gains you nothing. Thorium is not fissile and cannot be used to fuel a nuclear rocket, power plant, or RTG. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Designs for &quot;Thorium&quot; nuclear power plants use thorium as a fertile material to breed fissile U-233 using the neutrons from a fission reaction fueled by fissile U-235 or U-233. You start with a fairly conventional U-235 fueled fission reaction, use surplus neutrons to breed U-233 from a fertile thorium blanket, and eventually you have enough U-233 to power your core reactor and you are no longer reliant on U-235. It's a breeder scenario that is attractive to many because it ends our dependence on U-235 among other reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So to use lunar thorium for a NTR, you need to ship enough fissile (and radioactive) U-235 to the moon to fuel a fission reactor, which you must also build there, operate the reactor with a thorium blanket, chemically separate out the resultant U-233, and then process THAT into NTR fuel elements. You're launching a large quantity of radioactive U-235 as the first step in a very expensive process in making something very similar to U-235.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastActivityDate="2017-10-02T22:23:59.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23223" PostTypeId="2" ParentId="8289" CreationDate="2017-10-02T22:29:55.890" Score="5" Body="&lt;p&gt;Yes, you can but like it was said it would need some changes. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Better insulation&lt;/strong&gt;. You would need to keep the internal temperature the same receiving only 43% the solar heat ((149km/228km)²=0,43).&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Bigger solar panels&lt;/strong&gt;.  232% the size from earth ones to achieve equivalent power.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;A bigger fuel tank&lt;/strong&gt;, with 1 to 4 times the current ISS weight added as fuel  (depending  on $I_{SP}$ and how long the trip will take.) A simple Hoffman transfer orbit (the bare minimum)  needs 2,9km/s, with hydrogen/oxygen fuel (the best traditional fuel) and the ISS would need to add 90,4% of its weight as fuel with no margin for error or losses.&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Acceleration&lt;/strong&gt;. With small acceleration you could avoid the structural problems but you would also need to add fuel to compensate for a non-optimal burn. I believe a 0,01G acceleration would be 10 times less than the ISS should be able to handle with retracted solar panels. (So the initial boost would take about 3 hours and 6 hours on Mars.)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="21214" LastEditorUserId="6816" LastEditDate="2017-10-03T06:18:27.453" LastActivityDate="2017-10-03T06:18:27.453" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23224" PostTypeId="2" ParentId="19124" CreationDate="2017-10-02T23:01:30.967" Score="3" Body="&lt;p&gt;The difficulties would be significant after all you would need either the high wing surface area per weight or high landing speed that would be needed but I believe with proper care and lot of extra mass to make the needed structure it would be possible. Not sure why you would do that  on a module with such weight penalties. The powered option would be my choice.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something that might make more sense and contradict what people are saying is using a shuttle like shaped reentry vehicle with a small area to weight ratio. It would not have enough lift to actually land and therefore would need to have a powered landing. You would suffer a small weight penalty because of the shape versus ballistic but the advantages would be lower temperatures on reentry, lower G loads and a greater precision on landing place. it would not be able to correct gross reentry mistakes but would provide a far greater precision. And to correct the misconception presented on one other comment. A properly designed winged reentry vehicle would have no trouble reducing drag while increasing lift so undershooting would not be a problem. The space shuttle has this capability.&lt;/p&gt;&#xA;" OwnerUserId="21214" LastEditorUserId="6816" LastEditDate="2017-10-03T06:16:18.723" LastActivityDate="2017-10-03T06:16:18.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23225" PostTypeId="1" AcceptedAnswerId="26255" CreationDate="2017-10-03T00:59:51.320" Score="4" ViewCount="388" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Status 2018-Jan-01 inclination still about 7 degrees, location still not listed in Wikipedia.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/23217/12102&quot;&gt;This question&lt;/a&gt; got me thinking about the location of active TDRS satellites, and the newest addition &lt;a href=&quot;https://en.wikipedia.org/wiki/TDRS-M&quot; rel=&quot;nofollow noreferrer&quot;&gt;TDRS-M&lt;/a&gt; which does not yet have a fixed GEO longitude listed in Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_TDRS_satellites&quot; rel=&quot;nofollow noreferrer&quot;&gt;List of TDRS satellites&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;N2YO &lt;a href=&quot;https://www.n2yo.com/satellite/?s=42915&quot; rel=&quot;nofollow noreferrer&quot;&gt;shows it over the pacific&lt;/a&gt; at about 150° W in an apparently close-to-geosynchronous orbit with an inclination of about 7 degrees, which is also reflected in the TLE shown in &lt;a href=&quot;https://celestrak.org/cgi-bin/TLE.pl?CATNR=42915&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Is this the final planned location, or will it continue to move to a target longitude? Is there a plan to reduce the inclination closer to zero?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/X2C4j.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/X2C4j.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;TDRS 13                 &#xA;1 42915U 17047A   17275.30862482  .00000108  00000-0  00000+0 0  9993&#xA;2 42915   6.9726 332.4220 0005466 324.9709  34.9480  1.00272650   498&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5gHm9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5gHm9.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2018-04-27T15:25:21.037" Title="Is TDRS-M (TDRS-13) where it's supposed to be?" Tags="|artificial-satellite|communication-satellite|geosynchronous|tdrss|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23226" PostTypeId="2" ParentId="19124" CreationDate="2017-10-03T01:12:22.483" Score="3" Body="&lt;p&gt;The weight reduction would not be nearly as much as 2/3. Carbon fiber has about twice the yield strength for 2/3 weight compared to aluminum, so theoretically this would mean a 2/3 weight reduction. In practice buckling becomes more likely, and carbon needs a higher safety margin for safe use. If you still don’t believe me compare weight in bike frames. Good high end aluminum frames usually weighs between 2.5 and 3 pounds while carbon is between 2 and 2.4 pounds. Carbon usually uses some 5 times safety margin vs 2 for metals. (Those vary wildly but they tend to be around that.)&lt;/p&gt;&#xA;" OwnerUserId="21228" LastEditorUserId="6816" LastEditDate="2017-10-03T06:06:45.187" LastActivityDate="2017-10-03T06:06:45.187" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23231" PostTypeId="2" ParentId="5400" CreationDate="2017-10-03T05:00:12.423" Score="1" Body="&lt;p&gt;They might have been going for the advantage of you manufacturing a greater amount of individual engines, similar to what SpaceX is going for. SpaceX instead of manufacturing rockets with 1 or two engines goes for now with 9 per rocket and can improve on the manufacture using the methods that made so many industrialized things so much cheaper today. That is what I read about SpaceX take on it and it might have influenced the decision of this older rocket as well.&lt;/p&gt;&#xA;" OwnerUserId="21214" LastEditorUserId="25" LastEditDate="2017-12-19T13:20:55.713" LastActivityDate="2017-12-19T13:20:55.713" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23232" PostTypeId="1" AcceptedAnswerId="23234" CreationDate="2017-10-03T07:02:41.687" Score="5" ViewCount="2813" Body="&lt;p&gt;Would adding satellites along the path from Earth to Mars decrease the amount of time it takes to send and receive messages between them? &lt;/p&gt;&#xA;" OwnerUserId="15681" LastActivityDate="2020-06-07T20:21:53.850" Title="Would adding satellites between Earth and Mars improve communications latency?" Tags="|artificial-satellite|communication|communication-satellite|radio-communication|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23234" PostTypeId="2" ParentId="23232" CreationDate="2017-10-03T07:24:33.097" Score="22" Body="&lt;p&gt;No. Communication latency (the time between sending a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bit&quot; rel=&quot;noreferrer&quot;&gt;bit&lt;/a&gt; and receiving it on the other end) between Earth and Mars probes is limited almost entirely by the speed of light, as they are radio waves on a direct path in vacuum. (There are also plans for optical communications, but as far as I know no Martian orbiter has yet been launched with that capacity. It wouldn't change the latency, though, as all &lt;a href=&quot;https://en.wikipedia.org/wiki/Electromagnetic_radiation&quot; rel=&quot;noreferrer&quot;&gt;EM waves&lt;/a&gt; of whatever frequency/wavelength travel at the speed of light in vacuum by definition.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, communication &lt;em&gt;bandwidth&lt;/em&gt;, the amount of data transmitted per second, would probably increase significantly, since modern satellite communications are limited in bandwidth mostly by the fundamental limitations of signal-noise differentiation with a given transmitter power level, beam width, distance, and receiver size. So simply reducing the distance and using repeaters would greatly increase the bandwidth: every doubling of distance between links decreases the power loss to one quarter, and therefore improves the bandwidth by the corresponding four times or even better. You'd need some extra repeaters to compensate for smaller antennas and lower power budgets, however; as the largest dishes on Earth are around 70 m, and satellites' dishes have trouble reaching even a tenth of that, a back of the envelope calculation shows that you'd need at least ten intermediaries in sequence to match the current state of the art. Beyond that, every additional intermediate increases bandwidth, as long as they're distributed evenly. Since they would not likely be in a perfectly straight line, and because of the time for signal processing internally, latency would get slightly worse, probably a fraction of a second over the trip out of a total of 200&amp;ndash;1200 seconds, which is pretty negligible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, putting satellites between Earth and Mars isn't especially practical at the moment; intermediate orbits inevitably carry the satellites out of alignment, requiring either large solar sails to partially hover in place (and maneuver back and forth to a certain extent), or a large constellation of similar satellites (probably tens of thousands) to keep moving into position as each moves out of alignment. Solar sails have only been deployed a few times, and not for anything this adventurous, so this would require some new engineering work.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastEditorUserId="6816" LastEditDate="2017-11-06T05:05:11.637" LastActivityDate="2017-11-06T05:05:11.637" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23238" PostTypeId="1" AcceptedAnswerId="23249" CreationDate="2017-10-03T11:37:06.467" Score="18" ViewCount="6826" Body="&lt;p&gt;JWST will be orbiting L2 which is &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point#Lagrange_points&quot; rel=&quot;noreferrer&quot;&gt;unstable equilibrium&lt;/a&gt; thus will require propellant for station keeping. JWST will carry propellant for &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Orbit&quot; rel=&quot;noreferrer&quot;&gt;5-10 years&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What will happen once it runs out of propellant? Will it drift in a random direction? Is some direction more probable than others?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;PS:&lt;/strong&gt; why is JWST not parked at L4/L5 instead? They are stable equilibriums and JWST would not require much propellant to stay there indefinitely. Yes, it would require &lt;a href=&quot;https://en.wikipedia.org/wiki/Sunshield_(JWST)&quot; rel=&quot;noreferrer&quot;&gt;extra shielding from Earth&lt;/a&gt; but to a layperson that seems negligible compared to the sun shield it already carries.&lt;/p&gt;&#xA;" OwnerUserId="21200" LastEditorUserId="12102" LastEditorDisplayName="user20636" LastEditDate="2021-02-23T03:01:12.037" LastActivityDate="2021-10-11T00:24:36.973" Title="What happens to JWST after it runs out of propellant?" Tags="|orbital-mechanics|orbital-maneuver|james-webb-telescope|halo-orbit|lissajous-orbit|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="23240" PostTypeId="1" CreationDate="2017-10-03T14:44:31.997" Score="2" ViewCount="108" Body="&lt;p&gt;If we assume that we have a given launcher that is able to produce a &#xA;$\Delta v$ of 10km/s at sea level takeoff, and is able to reach an LEO of 200km above sea level, what LEO altitude would that launcher achieve if if the launchpad was located at an altitude X above sea level? Say 15km above sea level?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I stumbled over this function,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{align}&#xA;\Delta v = \sqrt {2GM_{\oplus} \left( \frac{1}{r_{\oplus}}- \frac{1}{r_{target}} \right)}&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;which is a solution to the underlying work integral with a lot of simplifications&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{align}&#xA;W = \int \vec F \cdot d\vec s&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given $\Delta v$ being constant, with all simplifications I do doubt solving the equation&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{align}&#xA;\sqrt {2GM_{\oplus} \left( \frac{1}{r_{\oplus}}- \frac{1}{r_{target}} \right)} = \sqrt {2GM_{\oplus} \left( \frac{1}{r_{\oplus}+x}- \frac{1}{r_{target}} \right)}&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;towards $r_{target}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;will do the job.&lt;/p&gt;&#xA;" OwnerUserId="21240" LastActivityDate="2017-10-03T14:44:31.997" Title="How can the delta-V be calculated depending on the launch altitude above sea level?" Tags="|rockets|low-earth-orbit|delta-v|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23241" PostTypeId="1" AcceptedAnswerId="24380" CreationDate="2017-10-03T16:17:11.887" Score="7" ViewCount="417" Body="&lt;p&gt;The current design of all orbital spacecraft capable of re-entry is cold-structure, using ablative materials, insulating tiles other approaches to keep the structure of the spacecraft relatively low during re-entry. The X-15 and Spaceship One are hot-structure spacecraft, where the materials absorb the heat of friction. The X-15 used special alloys to withstand heating while Spaceship One is composite, but distributes heat by continually exposing different parts of the structure. Neither the X-15 or Spaceship one got to orbital speeds, however. A hot-structure design would seem to have significant advantages over thermal protection systems in re-usability and maintenance, however the heat generated from re-entry is far more than suborbital flight, even at high mach speeds. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Are there true advantages in a hot-structure design in cost or reuse of the spacecraft, or would the costs of the materials and manufacturing outweigh the benefits?&lt;/li&gt;&#xA;&lt;li&gt;Are there any viable hot-structure spacecraft designs that could survive re-entry using current materials? &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="2981" LastActivityDate="2018-01-10T17:30:56.670" Title="Are there any viable hot structure orbital spacecraft designs capable of re-entry?" Tags="|reentry|reuse|thermal-control|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23243" PostTypeId="2" ParentId="8289" CreationDate="2017-10-03T19:14:23.457" Score="0" Body="&lt;p&gt;FWIW, escape velocity from the orbit of ISS is almost the same as Earth's surface, 24,258 mph (10.844 km/s) for ISS as opposed to 25,022 (11.186 km/s) at Earth's mean radius. &lt;/p&gt;&#xA;" OwnerUserId="13307" LastEditorUserId="13307" LastEditDate="2017-10-04T18:05:47.350" LastActivityDate="2017-10-04T18:05:47.350" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23244" PostTypeId="1" CreationDate="2017-10-03T20:18:38.027" Score="-1" ViewCount="410" Body="&lt;p&gt;There was another question that was excluded that opened up this can of warms in my head. I have some ideas on why go and what to do there but I wanted to open up and have other ideas coming in for a properly formulated question. (I hope)&lt;/p&gt;&#xA;" OwnerUserId="21214" LastActivityDate="2017-10-04T03:21:20.290" Title="Reasons to colonize Mars and what would we do there?" Tags="|mars|" AnswerCount="1" CommentCount="1" ClosedDate="2017-10-04T06:36:14.900" ContentLicense="CC BY-SA 3.0" />
  <row Id="23245" PostTypeId="2" ParentId="8289" CreationDate="2017-10-03T21:07:54.917" Score="5" Body="&lt;p&gt;Yes.  If you added a slow ion thruster you could slowly raise the orbit, also you can use the moon to help pull you out of the current low earth orbit.  Eventually gravity assist passed the moon and on to Mars where you'll need to enter orbit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Things you'll need:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A large amount of fuel (at least 400 tons (Mangela)), storage, and an ion thruster for the ISS. &lt;/li&gt;&#xA;&lt;li&gt;Significantly increased radiation shielding for ISS, since it was built for LEO which is well shielded by Earth's magnetic field.&lt;/li&gt;&#xA;&lt;li&gt;232% more solar panels, (Mangela).&lt;/li&gt;&#xA;&lt;li&gt;Regular resupply from earth, though if you start redesigning you can find ways to reduce those needs.  (Stable long term orbit without need for regular orbit boosts, food production, simple manufacture with 3D printing for parts, etc.)  &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;So it would be very costly but not impossible.  Honestly leaving it in LEO and letting private industry get a crack at using it makes more sense, or only boosting it to lunar orbit with some of the above upgrades.  The single shot or Orion style would be cheaper than the large number of launches and assembly that ISS took.   &lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="20287" LastEditDate="2017-10-04T00:55:04.283" LastActivityDate="2017-10-04T00:55:04.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23246" PostTypeId="1" AcceptedAnswerId="23267" CreationDate="2017-10-03T22:07:28.933" Score="9" ViewCount="1063" Body="&lt;p&gt;Has anyone here ever done enough back-of-the-envelope math to come up with a rough estimate of the maximum landing range of the F9 first stage?  I'm talking unladen-swallow-quality math here -- trying to think through just how far F9 is from BFR-like point-to-point ops today, assuming no payload and from a technical standpoint only, ignoring permitting, landing zone availability, etc.  (Seems like someone would have discussed this in public already, but I'm not finding it.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm trying to get my head around the math, and I'm realizing that (a) it's been a long time, and (b) it's all about managing re-entry energy, I think...&lt;/p&gt;&#xA;" OwnerUserId="21250" LastEditorUserId="21250" LastEditDate="2017-10-03T22:54:34.097" LastActivityDate="2017-10-07T14:58:11.123" Title="Falcon 9 maximum point-to-point landing range" Tags="|orbital-mechanics|spacex|falcon-9|sub-orbital|" AnswerCount="2" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23247" PostTypeId="2" ParentId="23244" CreationDate="2017-10-04T03:21:20.290" Score="3" Body="&lt;p&gt;Mars colonization idea suffers from the lack of economic case for it. It's not clear how to make it profitable. If there was an economic case for colonization, things would've been moving much faster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several non-economic reasons for Mars colonization:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;aspirational - humanity has a drive to explore and expand&lt;/li&gt;&#xA;&lt;li&gt;survival - a hypothetical planetary scale disaster wouldn't extinguish humankind if we were dispersed among planets&lt;/li&gt;&#xA;&lt;li&gt;romantic - &quot;a new frontier&quot;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One of the most forceful and persistent advocates of Mars colonization is Robert Zubrin. He published his first book, &lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/0684827573&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Case for Mars&lt;/a&gt;, more than 20 years ago, in which he attempted to answer these questions. His argument was not simple. In my opinion, he didn't provide a clear economic case for colonization. He builds the case essentially by analogy with colonization of America. I understood his reasoning this way:&#xA;In the very long term, there will be self-sustaining human civilization on Mars because Mars has sufficient and diverse resources needed to sustain it (think USA of 19-20 centuries). In the long but a bit shorter term, Mars could provide agricultural and other commodities to Asteroid Belt mining in a triangle trade arrangement (hi-tech from Earth to Mars, commodities from Mars to the Belt, raw materials from the Belt to Earth). This is equivalent to Europe-Americas-Caribbean triangle of 18th century. Note that this assumes that Belt mining operations will be profitable - so far, a big IF. Medium term - big unknown. Short term - Earth has enough resources to start the colonization program. This will probably be proven true at least as far as first steps go - it's highly likely that manned expeditions to Mars are going to happen within decades, not centuries.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This still leaves open the question about what will sustain the Mars colony economically (what will be the equivalent of gold, silver and furs that Americas exported to Europe). Malthusian fears that various resources are running out on Earth are not as popular any more, so there is no pressing need to go to other planets to mine their mineral wealth. Martian rocks as novelties might pay for about 1 crewed expedition, but not for sustained colony. There may be other ideas out there, but I haven't seen enough yet. Maybe cheaper launch and space travel costs will help create unexpected innovative ideas.&lt;/p&gt;&#xA;" OwnerUserId="12080" LastActivityDate="2017-10-04T03:21:20.290" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23248" PostTypeId="1" CreationDate="2017-10-04T04:05:20.440" Score="4" ViewCount="714" Body="&lt;p&gt;What would be required to have a vehicle like the space shuttle in that it could launch from earth and travel through space and was reusable except that it also possessed the capability to land on the moon and return to earth. I am writing a science fiction novel where this occurs...just need a simplistic theoretical idea to make the journey in the book somewhat plausible even if it's not possible yet.&lt;/p&gt;&#xA;" OwnerUserId="21254" LastActivityDate="2023-06-28T17:37:59.410" Title="Space Ship capable of travelling from Earth to Moon and back" Tags="|the-moon|spacecraft|space-shuttle|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23249" PostTypeId="2" ParentId="23238" CreationDate="2017-10-04T04:34:56.163" Score="20" Body="&lt;p&gt;This &lt;a href=&quot;https://youtu.be/v6ihVeEoUdo?t=571&quot; rel=&quot;noreferrer&quot;&gt;Northrop Grumman video&lt;/a&gt; (starting at &lt;code&gt;09:31&lt;/code&gt;) illustrates JWST's orbit in a non-rotating (normal) frame. It's really in an orbit around the Sun about 1% farther than Earth's, but the weak tug of Earth pulls it along a bit faster so that it remains in 1:1 resonance with the Earth. The orbit is called a &amp;quot;Halo orbit&amp;quot; because in a rotating frame it looks like it's a ring around the L2 point.&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/v6ihVeEoUdo?start=571&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&lt;p&gt;Regular, but very small station keeping propulsive maneuvers keep it in this otherwise unstable configurations. The more frequent the adjustments, the lower overall fuel consumption per year. According to &lt;a href=&quot;https://www.stsci.edu/jwst/documentation&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://ntrs.nasa.gov/citations/20140008868&quot; rel=&quot;noreferrer&quot;&gt;James Webb Space Telescope Initial Mid-Course Correction Monte Carlo Implementation using Task Parallelism&lt;/a&gt; and &lt;a href=&quot;https://ntrs.nasa.gov/citations/20140007519&quot; rel=&quot;noreferrer&quot;&gt;Station Keeping Monte Carlo Simulation for the James Webb Space Telescope&lt;/a&gt; there will be a small propulsive station-keeping event every 21 days. &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Orbit&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia says&lt;/a&gt; this will consume 2-4 m/s of delta-v per year from a budget of 150 m/s, &lt;em&gt;so the lifetime could conceivably be much longer than 5-10 years&lt;/em&gt;, although I believe about &lt;a href=&quot;https://ntrs.nasa.gov/citations/20140008868&quot; rel=&quot;noreferrer&quot;&gt;half of that (~67 m/s) will be used in mid-course corrections&lt;/a&gt; on it's way out to the Halo orbit.&lt;/p&gt;&#xA;&lt;p&gt;If/when station-keeping maneuvers were to stop, JWST will wander away from this resonance and go into a more (or less) chaotic orbit around the sun. It will not be unpredictable though. The understanding of the orbital mechanics is well developed, and if there is no refueling, the final orbit will be chosen and the final propulsive maneuvers will be carefully executed to put the JWST in it's chosen final path into Heliocentricity.&lt;/p&gt;&#xA;&lt;p&gt;However, &lt;a href=&quot;https://space.stackexchange.com/q/16845/12102&quot;&gt;there is talk&lt;/a&gt; about making the JWST approachable for service missions, and this might make refueling possible. See for example &lt;a href=&quot;https://space.stackexchange.com/q/16845/12102&quot;&gt;Besides HST, JWST and stations, are there any examples of satellites designed for service in space?&lt;/a&gt;, refueling technology for spacecraft is also discussed in &lt;a href=&quot;https://space.stackexchange.com/q/16840/12102&quot;&gt;Is Landsat-7's propellant resupply port “robot-ready”? (Restore-L mission)&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;NASA's &lt;a href=&quot;https://sspd.gsfc.nasa.gov/propellant_transfer_technologies.html&quot; rel=&quot;noreferrer&quot;&gt;Propellant Transfer Technologies&lt;/a&gt;, &lt;a href=&quot;https://sspd.gsfc.nasa.gov/robotic_refueling_mission.html&quot; rel=&quot;noreferrer&quot;&gt;Robotic Refueling Mission&lt;/a&gt;, and &lt;a href=&quot;https://sspd.gsfc.nasa.gov/rrm_refueling_task.html&quot; rel=&quot;noreferrer&quot;&gt;RRM Task : Refueling&lt;/a&gt; webpages have more photos and information.&lt;/p&gt;&#xA;&lt;p&gt;Considering that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hubble_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Hubble Space Telescope&lt;/a&gt; is still highly productive after 27 years, it's hard to imagine that there is no contingency for extending the JWST's lifetime beyond ten years, even though it many not be a formal part of it's current mission. Throwing away a billions-of-dollar science instrument simply for want of some propellant (or other consumables) is hard to imagine.&lt;/p&gt;&#xA;&lt;p&gt;In &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; there is a sample calculation of a Halo orbit for JWST that was done in 2014. It orbits for about eleven years, and presumably includes small station-keeping maneuvers to do so. This real-world orbit is more complex than a pure CR3BP (Circular Restricted Three-body problem) halo orbit because the Earth is in an elliptical orbit around the Sun, and the Moon adds further perturbations.&lt;/p&gt;&#xA;&lt;p&gt;The first figure below shows the complete, eleven-year orbit with it's final wander away from L2 when station keeping ends. This is a &amp;quot;top-down&amp;quot; view of the Ecliptic plane, with the Sun to the left.&lt;/p&gt;&#xA;&lt;p&gt;The next three images are just one year's worth, showing the two loops of the six-month period halo orbit. The Blue dot is the Earth, the green torus is the Moon's orbit, and the red line is the orbit of JWST. The coordinates are in the rotating frame of the Earth-Moon barycenter around the Sun.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vu82S.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vu82Sm.png&quot; alt=&quot;image&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/M4erN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M4erNm.png&quot; alt=&quot;image&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above left:&lt;/strong&gt; Top-down view, eleven years. &lt;strong&gt;right:&lt;/strong&gt; Top-down view, one year. (Sun to the left)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rcx7c.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rcx7cm.png&quot; alt=&quot;image&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/rSuD8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rSuD8m.png&quot; alt=&quot;image&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above left:&lt;/strong&gt; View from the Sun, one year. &lt;strong&gt;right:&lt;/strong&gt; View from the side, one year. (Sun to the left)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-10-11T00:24:36.973" LastActivityDate="2021-10-11T00:24:36.973" CommentCount="18" ContentLicense="CC BY-SA 4.0" />
  <row Id="23250" PostTypeId="2" ParentId="23248" CreationDate="2017-10-04T06:02:59.973" Score="4" Body="&lt;p&gt;Being very minimalist you could achieve this trip with the following Delta V’s.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;LEO&lt;/strong&gt; with 9.2 km/s Delta V&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;LLO&lt;/strong&gt; with 4,04 km/s&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Lunar surface&lt;/strong&gt; 1,7 km/s&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Lift to LLO&lt;/strong&gt; 1,7 km/s&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;Back to LEO&lt;/strong&gt; 1,31km/s (without the burn to slow down the craft therefore stressing the heat shield more)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Adding all that up gives us about 18km/s of required Delta V. That however is a very minimalist Delta V budget, absolutely no room for mistakes or non-optimal paths. Not only is that but the launch from earth uses metrics for dense launch systems which is very optimistic. The landing on the moon is also very tight so if distances are judged incorrectly the retrograde burn would waste more fuel. Assuming we managed to solve those two problems the very minimum Delta V would be the mentioned 18Km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using &lt;strong&gt;HydroLox&lt;/strong&gt; Rockets the highest ISP of chemical rockets ever produced.&#xA;At about &lt;strong&gt;450s&lt;/strong&gt; you would need e^(18/4,5)= ~55 . That means that your rocket would need a craft that could carry 55 times its own weight while inside earth atmosphere with at the very least 1,5 G’s of acceleration. The best rockets that can deliver the required acceleration with fuel easier to handle than hydrogen are three times heavier than that and current normal hydrogen rockets are about 6 to 8 times the target weight so rocket fuels are out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we use &lt;strong&gt;NTR&lt;/strong&gt; (Thermal Nuclear Rocket) engines with the target specs of project Timberwind and considering that the hydrogen tanks had equivalent mass per volume of the External Shuttle Tank entirely for H2. We would them have about 150 Tons H2, 25 Tons for Tank, 9 tons engines. This is a realistic rocket up to the point where we notice that we ignored all other needed things like navigation, payload fairings, all the wiring, heat shields, landing struts, control structures, Etc. that said the Vac DeltaV is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;DeltaV&lt;/strong&gt;=1000s*ln(184/34)=1,688*1000s=16,88 Km/s &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the bare minimum weight is not quite slim enough. With a different design things start to look better. The High thrust portions are dealt with by the NTR and Ion engines for Lunar transfer and return. So you would need:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;High thrust&lt;/strong&gt; 9,2+1,7+1,7=12,6 Km/s = 72% weight&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And another 12 Km/s for &lt;strong&gt;low thrust&lt;/strong&gt; Lunar Transfer. (for some low thrust maneuvers you need more Delta V)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 40 Km/s Exhaust speed you would further need=33% weight&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Total fuel weight=1-0,28*0,67=0,79&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The payload + ion engine weight is limited to 3% (100%-18%-79%=3%)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we assume 1 % payload and 2 % “engine”&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let’s assume a high-power density of 500 W/kg. For every ton of land craft we have 20 kg of solar panels+engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;10 kw/ton. Modest 50% Efficiency = 40mg exhaust / ton. Total exhausted fuel = 70 kg&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1,75 Mega seconds=24 days thrusting for the moon and back (Actual time should be 2 or 3 times higher because the order in witch it was calculated didn’t take into account the fuel to burn on the moon. For all other purposes the calculations are correct.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some approximations like 1000 sec is not 10 km/s but 9,82 km/s but the results are pretty accurate in the context they were used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conclusion, with a mix of NTR engines and Ion engines, ignoring all other things needed except propulsion (good enough for a book with significant but quite sci-fi improvements on current technology). Or you stop mid way to refuel. Leo before going to moon sound good and with current thecnology should be achievable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are of course other technology open cycle that with enough money and a decade we could “easily” make, but releasing nuclear fuel in the atmosphere is not such a good idea. &lt;/p&gt;&#xA;" OwnerUserId="21214" LastActivityDate="2017-10-04T06:02:59.973" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23251" PostTypeId="2" ParentId="23246" CreationDate="2017-10-04T06:26:46.437" Score="3" Body="&lt;p&gt;I can't remenber the entire math/base values and am rounding the values abit. We have a rocket with about 300(SL) to 340sec(vac) and a dry mass of 5 or 6% Can't remenber for sure. lets assume 6% that gives an &quot;avg&quot; of 327sec in a relativelly shallow launch profile.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;ln 16.6*327&lt;/strong&gt;=9km/s of delta v(calculator says 9.2,LOL)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A single stage migth be able to reach orbit or near orbit, though without any payload. Not very good for sensing cargo to LEO but plenty reach anywhere on earth even shooting for a retrograde suborbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The minimun to reach anywhere on Earth is no more tham 8km/s if I'm not mistaken.&lt;/p&gt;&#xA;" OwnerUserId="21214" LastEditorUserId="21214" LastEditDate="2017-10-04T22:01:12.143" LastActivityDate="2017-10-04T22:01:12.143" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23252" PostTypeId="1" CreationDate="2017-10-04T09:09:11.610" Score="10" ViewCount="314" Body="&lt;p&gt;April 12 has &lt;a href=&quot;https://yurisnight.net/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Yuri's Night&lt;/a&gt; to commemorate Yuri Gagarin's first orbital flight by a human.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://l5development.com/i_resource/history/DayInHistory.php?d=1004&quot; rel=&quot;nofollow noreferrer&quot;&gt;launch of Sputnik 1&lt;/a&gt;, the first satellite humanity put into orbit, on 4 October 1957 at 10:58pm Moscow time is at least as important event, but I'm unaware of any organized events to commemorate the occasion.&lt;/p&gt;&#xA;&lt;p&gt;In any case, Happy Sputnik Day!  (It’s a &lt;a href=&quot;https://l5nation.com/supt/holidays.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;national holiday&lt;/a&gt; for those of us who are citizens of the #L5Nation, celebrate it appropriately!)&lt;/p&gt;&#xA;&lt;p&gt;This is picture of the day # &lt;a href=&quot;https://photobyfred.com/SlideShow.php/ID=20171004&quot; rel=&quot;nofollow noreferrer&quot;&gt;4727&lt;/a&gt; on &lt;a href=&quot;https://PhotoByFred.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;PhotoByFred.com&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;img src=&quot;https://photobyfred.com/photos/ImgPhoto.php/q=201710044&quot; alt=&quot;Happy Sputnik Day! photo for 4 October 2017 at PhotoByFred.com&quot; /&gt;&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2023-02-02T13:58:35.070" LastActivityDate="2023-02-02T13:58:35.070" Title="Are there any events planned to celebrate Sputnik's 60th anniversary today?" Tags="|launch|history|sputnik|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23253" PostTypeId="1" CreationDate="2017-10-04T10:06:00.357" Score="13" ViewCount="1173" Body="&lt;p&gt;Is there any basis for the ordering of the L-points? Specifically, is there any particular reason for choosing L1 as the first L-point?&lt;/p&gt;&#xA;" OwnerUserId="21257" LastEditorUserId="12102" LastEditDate="2019-03-18T05:06:47.167" LastActivityDate="2019-03-20T13:18:53.627" Title="Ordering of the Lagrange points" Tags="|orbital-mechanics|history|lagrangian-points|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23254" PostTypeId="2" ParentId="23253" CreationDate="2017-10-04T12:04:00.920" Score="20" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, L1-L2-L3 were discovered first, by &lt;a href=&quot;https://en.wikipedia.org/wiki/Leonhard_Euler&quot; rel=&quot;noreferrer&quot;&gt;Euler&lt;/a&gt;, prior to &lt;a href=&quot;https://en.wikipedia.org/wiki/Joseph-Louis_Lagrange&quot; rel=&quot;noreferrer&quot;&gt;Lagrange's&lt;/a&gt; work, and L1 is &quot;the most intuitively understood&quot; of them. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-10-04T12:04:00.920" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23255" PostTypeId="1" AcceptedAnswerId="23257" CreationDate="2017-10-04T15:23:07.240" Score="7" ViewCount="1379" Body="&lt;p&gt;I'm starting to study orbital perturbations and I can't find any physical explanation about the perigee advance (&lt;a href=&quot;https://en.wikipedia.org/wiki/Apsidal_precession&quot; rel=&quot;noreferrer&quot;&gt;apsidal precession&lt;/a&gt;) when considering Earth oblateness effects. Can someone give a physical explanation or recommend a book/article to read?.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS: From the equations it is quite clear that it must happen, but I am looking for some sort of physical meaning similar to the change in angular momentum due to equatorial bulges when studying nodes regression.&lt;/p&gt;&#xA;" OwnerUserId="21262" LastEditorUserId="12102" LastEditDate="2017-10-04T15:49:54.483" LastActivityDate="2018-03-06T08:26:21.550" Title="Physical meaning of perigee advance" Tags="|orbital-mechanics|spacecraft|earth|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23256" PostTypeId="2" ParentId="23206" CreationDate="2017-10-04T18:00:22.330" Score="3" Body="&lt;p&gt;&lt;a href=&quot;https://www.mars-one.com/mission/roadmap&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars One's Mission Roadmap&lt;/a&gt; on their website states that the first crews living quarters arrive, 26 months later the first crew arrives, a few weeks later the second crew living quarters arrive, 26 months later the second crew arrive, and on and on as additional living quarters and crews continue to arrive.  Also eventually martian colonists will start having children in the colony.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Supplies will be needed, and in particular spare parts, as the base grows. In fact until Mars based in-situ manufacturing is able to cover most of the spare parts, the base is unlikely to be able to grow very much, if at all, in the beginning. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://gizmodo.com/mars-one-is-still-completely-full-of-shit-1724032400&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gizmodo: Mars One is Still Completely Full of Shit&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="11721" LastEditDate="2019-10-03T17:40:07.387" LastActivityDate="2019-10-03T17:40:07.387" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23257" PostTypeId="2" ParentId="23255" CreationDate="2017-10-04T18:04:39.473" Score="4" Body="&lt;p&gt;In the subsection &lt;a href=&quot;https://en.wikipedia.org/wiki/Geopotential_model#The_deviations_of_Earth.27s_gravitational_field_from_that_of_a_homogeneous_sphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;The deviations of Earth's gravitational field from that of a homogeneous sphere&lt;/a&gt; of the Wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Geopotential_model&quot; rel=&quot;nofollow noreferrer&quot;&gt;Geopotential model&lt;/a&gt; you can see that the $J_2$ or quadrupole moment of the Earth's gravitational potential falls off much more rapidly with distance than the monopole term. In the Earth's equatorial plane, the acceleration due to the monopole and quadrupole moments are given as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a_0 = -\frac{GM_E}{r^2},$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a_2 = -\frac{3}{2} J_2 \frac{GM_E R_E^2}{r^4},$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where the &lt;a href=&quot;https://space.stackexchange.com/q/22976/12102&quot;&gt;unit-less value of Earth's $J_2$&lt;/a&gt; is about 0.0010825 and $R_E$ is the normalizing radius of the Earth of 6378136.3 meters, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;standard gravitational parameter&lt;/a&gt; of the Earth $GM_E$ is about 3.986E+14 m^3/s^2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read a little more about Earth's $J_2$ and it's effect on gravity at the equator and poles in &lt;a href=&quot;https://physics.stackexchange.com/a/141981/83380&quot;&gt;David Hammen's nice table&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the Earth's surface, at the equator, the values for these two are 9.7983 and 0.0159 m/s^2 respectively, but remember that they fall of with distance as $1/r^2$ and $1/r^4$ respectively as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a satellite orbiting in Earth's equatorial plane in an elliptical orbit will &quot;think&quot; that the Earth's gravity is stronger at periapsis than at apoapsis, even taking $1/r^2$ into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the Earth (or any oblate spheroid) &quot;pulls harder&quot; as the satellite swings closest to the planet, it sort-of wraps the orbit tighter. The following apoapsis will come a bit later and advance around the planet, as will the periapsis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a Python simulation run for a satellite in a very elliptical LEO orbit with a periapsis altitude of about 400km and apoapsis altitude of about 32,000 km. I've run it for Earths normal $J_2$, and again for ten times larger $J_2$ to magnify the effect so that each orbit clearly advances. In addition to the advancement you can see that the semimajor axis is slightly smaller for the larger $J_2$ because the average gravitational force is slightly larger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1DBIb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1DBIb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;def deriv(X, t):&#xA;&#xA;    x, v = X.reshape(2, -1)&#xA;&#xA;    acc0  = -GMe * x * ((x**2).sum())**-1.5&#xA;    acc2  = -1.5 * GMe * J2 * Re**2 * x * ((x**2).sum())**-2.5&#xA;&#xA;    return np.hstack([v, acc0 + acc2])&#xA;&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;# David Hammen's nice table https://physics.stackexchange.com/a/141981/83380&#xA;# See http://www.iag-aig.org/attach/e354a3264d1e420ea0a9920fe762f2a0/51-groten.pdf&#xA;# https://en.wikipedia.org/wiki/Geopotential_model#The_deviations_of_Earth.27s_gravitational_field_from_that_of_a_homogeneous_sphere&#xA;&#xA;GMe = 3.98600418E+14  # m^3 s^-2&#xA;J2e = 1.08262545E-03  # unitless&#xA;Re  = 6378136.3 # meters&#xA;&#xA;X0 = np.hstack([6778000.0, 0.0, 0.0, 10000.])  # x, y, vx, vy&#xA;&#xA;time = np.arange(0, 300001, 100)&#xA;&#xA;J2 = J2e  # correct J2&#xA;answerJ2, info = ODEint(deriv, X0, time, full_output=True)&#xA;&#xA;J2 = 10*J2e # 10x larger J2&#xA;answer10xJ2, info = ODEint(deriv, X0, time, full_output=True)&#xA;&#xA;if 1 == 1:&#xA;    plt.figure()&#xA;    x, y = answerJ2.T[:2]&#xA;    plt.plot(x, y, '-b')&#xA;    x, y = answer10xJ2.T[:2]&#xA;    plt.plot(x, y, '-r')&#xA;    plt.plot([0], [0], 'or')&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-03-06T08:26:21.550" LastActivityDate="2018-03-06T08:26:21.550" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23258" PostTypeId="2" ParentId="23232" CreationDate="2017-10-04T18:15:52.410" Score="6" Body="&lt;p&gt;Only for about two weeks every 26 months when Earth and Mars are in conjunction. The Sun blocks communications, so the latency is effectively that two weeks, or however many days remain. In that case a relay satellite in some intermediate orbit around the Sun not on that Earth-Sun-Mars line would reduce the effective latency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hardly seems worth the effort though. The conjunction problem could instead be greatly reduced or even eliminated much of the time with laser communications.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-10-04T18:15:52.410" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23259" PostTypeId="1" CreationDate="2017-10-04T19:43:37.610" Score="9" ViewCount="1038" Body="&lt;p&gt;I've read that on Mars you lose 30% of your heat through convection and 70% through black body radiation.  (The much lower amount from convection due to the tenous atmosphere vs. Earth.)  &lt;a href=&quot;http://www.sciencemag.org/news/2014/06/no-wind-chill-mars&quot; rel=&quot;noreferrer&quot;&gt;http://www.sciencemag.org/news/2014/06/no-wind-chill-mars&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If so could the heat be used in greenhouses attached to the base?  It seems wasteful to not use it elsewhere. The greenhouses would have a higher pressure than the atmosphere which should also increase convection from the radiators of the habitat.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One final question, what pressure would be high enough on Mars that you would not need to heat or cool?  I understand Mars is too low in pressure, as is obviously the ISS or the moon, and Earth pressure is too much so that you quickly become cold due to convection.  30,000 feet also appears to have too much pressure and cause excessive cooling due to convection.  The paper below appears to be a study of altitude, pressure and convection rates in avionic equipment but the data is not easily extracted.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://dspace.mit.edu/bitstream/handle/1721.1/39011/20404614-MIT.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://dspace.mit.edu/bitstream/handle/1721.1/39011/20404614-MIT.pdf&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="6944" LastEditDate="2017-12-09T23:54:50.293" LastActivityDate="2018-01-16T00:10:01.067" Title="Would a mars base need to expel excess heat?" Tags="|mars|planetary-science|atmosphere|temperature|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23260" PostTypeId="2" ParentId="2158" CreationDate="2017-10-04T20:41:27.403" Score="4" Body="&lt;p&gt;&lt;a href=&quot;https://arc.aiaa.org/doi/abs/10.2514/6.2017-5372&quot; rel=&quot;nofollow noreferrer&quot;&gt;recent paper I wrote on the lunar space elevator&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a Mars elevator, the Moon Phobos gets in the way ..... a more practical solution on Mars is to use Phobos as an attach point for a partial elevator, called PAMSE &quot;Phobos And Mars Space Elevator&quot;.   If you drop a tether down from Phobos it will drag across the atmosphere of Mars a few hundred km / hour, such that an aircraft could rendezvous with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=8&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwj196ru6tfWAhUR92MKHYklDvsQFghCMAc&amp;amp;url=http%3A%2F%2Fwww.nss.org%2Fsettlement%2Fmars%2F2003-SpaceColonizationUsingSpaceElevatorsFromPhobos.pdf&amp;amp;usg=AOvVaw1jAj6u4B2XuhCM7giqJa0j&quot; rel=&quot;nofollow noreferrer&quot;&gt;See this paper.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21264" LastActivityDate="2017-10-04T20:41:27.403" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23261" PostTypeId="2" ParentId="23255" CreationDate="2017-10-04T23:41:19.583" Score="3" Body="&lt;p&gt;&lt;a href=&quot;http://www.wiley.com/WileyCDA/WileyTitle/productCd-1118753658.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sutton&lt;/a&gt;, (note - 4th edition!), page 156, has this to say:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[the figure] shows an exaggerated shift of the apsidal line with the&#xA;  center of the earth remaining as a focus point. This perturbation may&#xA;  be visualized as the movement of the prescribed elliptical orbit in a&#xA;  ﬁxed plane. Obviously, both the apogee and perigee points change in&#xA;  position, the rate of change being a function of the satellite&#xA;  altitude and plane inclination angle. At inclinations of 63.4° and&#xA;  116.6°, the rate of shifting of the apsidal line, also called apsidal drift, is zero. At an apogee altitude of 1000 nautical miles (n.m.)&#xA;  and a perigee of 100 n.m. in an equatorial orbit, the apsidal drift is&#xA;  approximately 10°/day.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TakwG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TakwG.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More descriptive than explanatory but perhaps it's of interest.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-10-04T23:41:19.583" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23262" PostTypeId="1" AcceptedAnswerId="23264" CreationDate="2017-10-05T01:38:08.760" Score="3" ViewCount="650" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, the MRO has an orbital period of 112 minutes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In September 2006 MRO fired its thrusters twice more to fine-tune its final, nearly circular orbit to approximately 250 to 316 km (155 to 196 mi) above the Martian surface, with a period of about 112 minutes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However, it also says that the MRO completed a second burn in 2015 which lasted 75 seconds.  Did this burn change its orbital period at all?&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2017-10-05T16:53:40.687" Title="What is the Mars Reconnaissance Orbiter's orbital period?" Tags="|orbit|mars-reconnaissance-orbiter|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23263" PostTypeId="2" ParentId="23262" CreationDate="2017-10-05T03:30:21.033" Score="3" Body="&lt;p&gt;That burn used the &quot;intermediate thrusters&quot;, 6 units producing 22 N force each, for a total of around 10000 newton-seconds of impulse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.wolframalpha.com/input/?i=(132%20newton%20x%2077%20seconds%20%2F%201300%20kg)%20in%20m%2Fs&quot; rel=&quot;nofollow noreferrer&quot;&gt;I was able to coax Wolfram Alpha into admitting that, against a mass of about 1300 kg, this corresponds to a delta-v of about 7.8 meters per second&lt;/a&gt;, which is about 0.2% of the 3415 m/s average orbital velocity expected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on how this delta-v is applied, it could increase or decrease the period, or leave it unchanged; if it was used to put the spacecraft into a slightly higher, slower orbit, the period could increase from 112.6 minutes to 113.4 minutes. (I see from @uhoh's answer that the actual period is a bit shorter; I'm using spherical-cow figures here.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-10-05T16:53:40.687" LastActivityDate="2017-10-05T16:53:40.687" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23264" PostTypeId="2" ParentId="23262" CreationDate="2017-10-05T04:37:39.983" Score="5" Body="&lt;p&gt;I went to &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizions&lt;/a&gt; and extracted the osculating elements for MRO about Mars at 1 day intervals over the last ten years or so. Osculating elements are not exactly Keplerian elements, but they are close. They are really the instantaneous fit to the cartesian state vectors $x, y, z, v_x, v_y, v_z$ produced in the integration for the ephemeris. Every little perturbation due to gravity from all solar system bodies, Mars' uneven gravity field, and all other effects cause tiny departures from simple Keplerian orbits and so look like noise in the osculating elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've plotted the daily period and a 100-day running average of the period as a function of year. It looks pretty darn steady!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't see any evidence of a permanent change in the period after the 2015 even that you mention.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've also included a dump of all twelve osculation elements (days since 01-Jan-2017) for kicks. You can see a roughly 700 day oscillation in MRO's inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've also included the header at the end, it may have some interesting information for you. It's fairly long, don't forget to scroll up and down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ylyx6.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ylyx6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sjwyl.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sjwyl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ssDfW.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ssDfW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;Revised: Jul 10, 2017    Mars Reconnaisance Orbiter (MRO)  / (Sun)         -74&#xA;                               http://marsprogram.jpl.nasa.gov/mro/&#xA; UPDATE (2014-Jun-10):&#xA;  C/2013 A1 (Siding Spring) will have its closest approach to Mars on Oct 19,&#xA;  2014 at a distance of about 135,000 km.&#xA;&#xA;  Location of predicted MRO minimum risk from comet particles&#xA;  (occultation mid-point):&#xA;&#xA;  Latitude: 7.6083 deg&#xA;  Time:     19-OCT-2014 20:08:07.182392952 ET (CSS CA + 98 min)&#xA;&#xA; MISSION LAUNCH DATE:                                         MARS ARRIVAL DATE:&#xA;  Aug 12, 2005 11:43 UTC (it's up and away)                   March 10, 2006&#xA;  Notes:  Aug 10, 2005: Original launch was scrubbed (gyros&#xA;          Aug 11, 2005: Second attemped was launch scrubbed (fueling sensors)&#xA; LAUNCH VEHICLE:&#xA;  Atlas V two-stage. Separation from Centaur: 2005-Aug-12 12:40:51.68 UTC&#xA;&#xA; BACKGROUND:&#xA;  MRO aerobraked for 6 months from the post-insertion 35 hour orbit, saving&#xA;  450 kg of fuel. Primary science phase starts in November of 2006 and&#xA;  nominally operates for 2 Earth years from a 112 minute, near circular polar &#xA;  orbit. &#xA; SPACECRAFT PHYSICAL PROPERTIES:&#xA;  Height=  6.5 m                          Solar panels =   20 m^2&#xA;  Width = 13.6 m (tip-to-tip)             launch weight= 2180 kg&#xA;&#xA; SPACECRAFT TRAJECTORY:&#xA;  Trajectory starts after Centaur launch stage separation (cruise phase).&#xA;  Trajectory after 2017-Jul-03 is PREDICTION.&#xA;&#xA;  Trajectory files (from MRO Nav., JPL)      Start (TDB)         End (TDB)&#xA;  --------------------------------------  -----------------  -----------------&#xA;  mro_cruise                              2005-Aug-12 12:42  2006-Mar-10 22:06&#xA;  mro_ab                                  2006-Mar-10 22:06  2006-Sep-12 06:40&#xA;  misc reconstruction (mro_psp1-42)       2006-Sep-12 06:40  2017-Apr-01 01:01&#xA;  mro_psp_rec                             2017-Apr-01 01:01  2017-Jul-03 23:16&#xA;  mro_psp                                 2017-Jul-03 23:16  2017-Aug-07 14:12&#xA;&#xA;*******************************************************************************&#xA;Ephemeris / WWW_USER Wed Oct  4 20:17:49 2017 Pasadena, USA      / Horizons    &#xA;*******************************************************************************&#xA;Target body name: Mars Reconnaissance Orbiter (spacecraft) (-74) {source: MRO_MERGED}&#xA;Center body name: Mars (499)                      {source: mar097}&#xA;Center-site name: BODY CENTER&#xA;*******************************************************************************&#xA;Start time      : A.D. 2005-Aug-13 00:00:00.0000 TDB&#xA;Stop  time      : A.D. 2017-Aug-07 00:00:00.0000 TDB&#xA;Step-size       : 1440 minutes&#xA;*******************************************************************************&#xA;Center geodetic : 0.00000000,0.00000000,0.0000000 {E-lon(deg),Lat(deg),Alt(km)}&#xA;Center cylindric: 0.00000000,0.00000000,0.0000000 {E-lon(deg),Dxy(km),Dz(km)}&#xA;Center radii    : 3396.2 x 3396.2 x 3376.2 km     {Equator, meridian, pole}    &#xA;Keplerian GM    : 4.2828373620699087E+04 km^3/s^2&#xA;Output units    : KM-S, deg, Julian Day Number (Tp)                            &#xA;Output type     : GEOMETRIC osculating elements&#xA;Output format   : 10&#xA;Reference frame : ICRF/J2000.0                                                 &#xA;Coordinate systm: Ecliptic and Mean Equinox of Reference Epoch                 &#xA;*******************************************************************************&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-05T04:44:02.100" LastActivityDate="2017-10-05T04:44:02.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23265" PostTypeId="1" AcceptedAnswerId="23274" CreationDate="2017-10-05T05:02:28.193" Score="1" ViewCount="336" Body="&lt;p&gt;What elements travel at the speed of light. I was under the impression it was light and gravity but apparently electromagnetic waves (radio waves) travel that fast as well. And is element the right word? Is gravity an element? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;FYI Feel free to update the tags. Light, photons and speed were not listed&lt;/p&gt;&#xA;" OwnerUserId="15681" LastEditorUserId="15681" LastEditDate="2017-10-05T05:07:41.337" LastActivityDate="2017-10-06T01:33:57.327" Title="What objects travel at the speed of light?" Tags="|gravity|interstellar-travel|ftl|" AnswerCount="1" CommentCount="1" ClosedDate="2017-10-07T22:18:56.263" ContentLicense="CC BY-SA 3.0" />
  <row Id="23267" PostTypeId="2" ParentId="23246" CreationDate="2017-10-05T09:13:22.750" Score="10" Body="&lt;p&gt;I agree with @OuNelson Mangela on the 4,000km figure. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I got 4,052km on &lt;a href=&quot;https://www.flightclub.io&quot; rel=&quot;nofollow noreferrer&quot;&gt;flightclub.io&lt;/a&gt;, but it was a bit of a struggle. I had to glide quite a bit between entry and landing burns to bleed off velocity that I didn't have the fuel to remove propulsively, and that extended my range quite a bit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That said, there may be flight profiles that are more efficient than mine - for example my profile stays quite low in the atmosphere, as I needed to execute the pitch kick quite early to get as much downrange velocity as I could before MECO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe there is a profile that executes a high-thrust, quite vertical burn to get out of the atmosphere, coasts to near-apogee and then executes a second burn, free from atmospheric and gravity drag to get the downrange distance. Feel free to play around yourself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To glide between entry and landing, remember that you're travelling engines first, so you want to make sure your pitch angle has a lower magnitude than your velocity angle (i.e you have a positive angle of attack). The larger that angle of attack (up to ~15$^{\circ}$), the more lift you'll experience and the more velocity you'll bleed off before your landing burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flight profile I built can be viewed &lt;a href=&quot;https://www.flightclub.io/build/# &quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and the simulation results are &lt;a href=&quot;https://www.flightclub.io/results/?id=9d0d3088-9e6e-4607-9f44-60200dd0511f&amp;amp;code=CR10&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. I've included some plots of my results below.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Flight Profile (note the axes are not to scale)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PTkDV.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PTkDV.png&quot; alt=&quot;Flight Profile (note the axis scales)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Altitude vs. Time (Booster maintains altitude for longer thanks to the glide)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oefB0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oefB0.png&quot; alt=&quot;Altitude vs. Time&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Angle of Attack vs. Time (note the positive angle of attack to create aerodynamic lift between entry and landing burns)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Rwh63.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Rwh63.png&quot; alt=&quot;Angle of Attack vs. Time&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Booster Phasespace&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tPFV7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tPFV7.png&quot; alt=&quot;Booster Phasespace&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Updated Answer to keep within Falcon 9 acceleration limit. I've assumed that limit is 6g here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was actually a killer. I only made it to ~1,750km downrange. 5km/s of my $\Delta V$ was used in the initial burn which began to heavily throttle about 30s before MECO to keep within 6gs acceleration. This incurred pretty heavy gravity losses so our MECO velocity was only 3.6km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The entry burn needed to be extremely long for us to calm down to a manageable speed entering the atmosphere. Specifically, I used ~2.3km/s of $\Delta V$ for the entry burn but the final 10 seconds of this were also throttled to stay within limits so extra gravity losses were incurred here too. After entry burn cutoff, the atmosphere burned off ~1.3km/s of velocity and then the landing burn had enough propellant remaining to finish the job.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The flight profile I used can be viewed &lt;a href=&quot;https://www.flightclub.io/build/# &quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and the results of the simulation can be viewed &lt;a href=&quot;https://www.flightclub.io/results/?id=2ab9c4b2-5a90-4e62-aca7-87621319c79f&amp;amp;code=CR10&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are the corresponding plots to the previous simulation:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Flight Profile (note the axes are not to scale)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IWr3L.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IWr3L.png&quot; alt=&quot;Flight Profile&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Altitude vs. Time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9b0kQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9b0kQ.png&quot; alt=&quot;Altitude vs. Time&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Angle of Attack vs. Time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sO2D7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sO2D7.png&quot; alt=&quot;Angle of Attack&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Booster Phasespace&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/w8AYR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/w8AYR.png&quot; alt=&quot;Booster Phasespace&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21025" LastEditorUserId="21025" LastEditDate="2017-10-07T14:58:11.123" LastActivityDate="2017-10-07T14:58:11.123" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23268" PostTypeId="1" CreationDate="2017-10-05T11:01:33.757" Score="0" ViewCount="103" Body="&lt;p&gt;Is there something inherent in the technology of a fission thermal rocket engine that makes it a more expensive endeavour than developing yet another chemical engine? For example because of the handling of radioactive matter, or if it would have to be of much larger scale than what is in demand in the near term. There are some experiences of &lt;a href=&quot;https://en.wikipedia.org/wiki/NERVA&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nerva&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Timberwind&quot; rel=&quot;nofollow noreferrer&quot;&gt;Timberwind&lt;/a&gt;. Were those developments markedly more expensive than &quot;ordinary&quot; rocket engine development projects?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-10-05T11:01:33.757" Title="Are fission thermal engines more expensive to develop than chemical engines?" Tags="|engine-design|cost|nuclear|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23269" PostTypeId="2" ParentId="22188" CreationDate="2017-10-05T11:07:50.293" Score="5" Body="&lt;p&gt;Regarding NASA boiler plates for the Apollo program, &lt;a href=&quot;http://www.jonessite.net/upload/LRD/stories/1227withpics.pdf&quot; rel=&quot;noreferrer&quot;&gt;one source&lt;/a&gt; claims, &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The 1200 series were manufactured by a firm called Ace Fabrications&#xA;   in Clute, Texas, each costing some $10 – 15K.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2017-10-05T11:07:50.293" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23270" PostTypeId="1" CreationDate="2017-10-05T12:41:28.897" Score="0" ViewCount="75" Body="&lt;pre&gt;&lt;code&gt;                       MESSAGE FORMATTING&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Ground station networks exist that invite registered satellite clients to utilize their provided fee based  download and upload services. &#xA;Example:&#xA;One University Ground station in USA offers pre setup message capture and delivery services to clients for the 15 minute pass overhead window at their location, using a good size dish that has to track (using latest Keplerian elements) the satellite passing overhead.  Ground station services costs can be reduced by maximizing automation, so that perhaps each ground reception transaction, can be done with a minimum of ground station staff time.&#xA;I think a fee to cover their communications infrastructure, for the say 15 minute pass is, maybe ~$300 USD.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an existing /Should there be a standardized message format that most or all the ground stations can count on to easily capture a download message and send to their client?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something like:&lt;br&gt;&#xA;TIME DATE SAT ID  MESSAGE NUMBER TITLE STATUS REPORT  SENSOR 1 SENSOR 2 SENSOR 3, etc  etc...&#xA;And, how to do that, if  messages are encrypted?  How to set up modulation schemes, error detection codes, etc.&#xA;Can a ground station/ are there any that exist that have a web interface for a client to set up a coming overhead pass?   &lt;/p&gt;&#xA;" OwnerUserId="17558" LastActivityDate="2017-10-07T00:06:25.997" Title="Standardized download message formatting" Tags="|ground-station|" AnswerCount="1" CommentCount="1" ClosedDate="2017-10-14T15:53:31.773" ContentLicense="CC BY-SA 3.0" />
  <row Id="23271" PostTypeId="2" ParentId="2158" CreationDate="2017-10-05T12:53:18.807" Score="4" Body="&lt;p&gt;The Moon is considerably less. A Lunar Elevator tether would require the strength of Kevlar. From what I've been able to tell, Kevlar would just barely work for a Mars elevator, but wouldn't be practical as it would be at the safety margin, any small defect could cause it to stop working. The thought is that a Mars elevator would be easier than Earth, as it could work with either M5 or Zylon, both of which are much easier to manufacture then carbon nanotubes, which is the only substance that is known to work for Earth.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-10-05T12:53:18.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23272" PostTypeId="1" AcceptedAnswerId="23276" CreationDate="2017-10-05T13:08:38.167" Score="4" ViewCount="237" Body="&lt;p&gt;A launcher upper stage may not stay in orbit for long, as it does not need to go as high as the payload and thus travel at an altitude at which although it is thin, the atmosphere is still dense enough to slow down the launcher stage. I don't know how long it typically stays in orbit, but I believe it is a few months for LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, for GTO, the energy involved is higher: when releasing the payload, the upper stage is fast enough so the payload reaches the desired altitude without significant trajectory correction, and thus the upper stage may stay in orbit longer even if it is slowed down (unwanted aerobrake) when at apoapsis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For how long does it stay in this orbit? Is there any action taken to decrease this duration?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="897" LastEditDate="2017-10-05T14:16:48.387" LastActivityDate="2017-10-05T14:16:48.387" Title="How long does the upper stage stay in orbit if the payload is released on GTO?" Tags="|debris|geostationary|launch-vehicle|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23274" PostTypeId="2" ParentId="23265" CreationDate="2017-10-05T13:23:37.797" Score="8" Body="&lt;p&gt;Firstly &quot;element&quot; is probably the wrong word to use here since in scientific language that would refer to a &lt;em&gt;chemical&lt;/em&gt; element (think hydrogen, helium, etc).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for what travels at the speed of light, all electromagnetic waves (including light itself) will travel at the same speed with the exact value dependent upon the medium through which they are traveling. In a vacuum, this is approximately 300,000km per second and is often denoted by the term &lt;em&gt;c&lt;/em&gt; (of &lt;em&gt;E=mc²&lt;/em&gt; fame!).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity is a tiny bit more complicated&amp;hellip; according to Einstein's theory of gravity what objects experience as gravity is the distortion to the &quot;normal&quot; curvature of spacetime caused by the presence of a large amount of energy (typically a mass). It's a bit of a simplified metaphor but if you think of spacetime as like a trampoline then put a bowling ball on the trampoline it causes the trampoline's surface to distort downwards around the weight of the bowling ball, and the extent of this distortion decreases the further you get from the bowling ball. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's kind of what gravity is, so it's not really &lt;em&gt;traveling&lt;/em&gt; per se. What travels is any &lt;em&gt;changes&lt;/em&gt; to this distortion. If the source of that distortion were to be moved (you come along and pick the bowling ball up off the trampoline or roll it around to a different section etc) then the distortion to the surface changes accordingly and these changes aren't instantaneous. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These changes to the curvature propagate in a wave-like manner and are referred to as &quot;gravity waves&quot; or &quot;gravitational radiation&quot;. We don't know that much about gravity waves yet, but it's theorized that they are carried by a hypothetical particle called a &quot;graviton&quot; which is the gravitational version of a photon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lots and lots of complicated maths later and the headline is that according to Einstein the propagation of changes to this distortion (i.e. the speed of gravity) is expected to be the same as the speed of light and &lt;a href=&quot;https://medium.com/starts-with-a-bang/what-is-the-speed-of-gravity-8ada2eb08430&quot; rel=&quot;nofollow noreferrer&quot;&gt;recent experiments&lt;/a&gt; seem to show that this is indeed the case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So anything that has no mass when it's not moving will travel at the speed of light and basically, everything with a resting mass &gt; 0 is slower (yes neutrinos, &lt;a href=&quot;https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;I'm looking at you!&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="19967" LastEditorUserId="18663" LastEditDate="2017-10-06T01:33:57.327" LastActivityDate="2017-10-06T01:33:57.327" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23276" PostTypeId="2" ParentId="23272" CreationDate="2017-10-05T14:12:58.887" Score="5" Body="&lt;p&gt;The upper-most stage of a launch vehicle is in essentially the same orbit as the payload. The stage burns out, and then the payload is separated from the stage. This is typically at a very low relative velocity on the order of meters per second, several orders-of-magnitude below orbital speed, which is measured in km per second. The separation velocity, and usually a post-separation collision avoidance maneuver performed by the stage, are just enough to prevent re-contact on future orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An obvious exception to this are satellites that significantly change their orbit after deployment. The most obvious example is a satellite that is separated in geosynchronous transfer orbit (GTO) and then maneuvers into geosynchronous earth orbit (GEO). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So upper stages, if not intentionally de-orbited (which some are) can stay in orbit a long time. Stages have low &lt;a href=&quot;https://en.wikipedia.org/wiki/Ballistic_coefficient&quot; rel=&quot;noreferrer&quot;&gt;ballistic coefficients&lt;/a&gt; because of the empty tanks, which will cause them to re-enter sooner than a more-dense satellite, but they can still stay in orbit for years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Exactly how long depends a lot on the details: the orbit, the ballistic coefficient of the stage, and the solar cycle (high solar activity &quot;puffs up&quot; the atmosphere, which increases drag). Part of obtaining a launch license (in the US) is running a government provided code to calculate the de-orbit time for the payload and any spent stages. The best practice is to ensure everything is de-orbited (actively or passively) within 25 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See related answer to &lt;a href=&quot;https://space.stackexchange.com/a/2768/127&quot;&gt;Why do malfunctioning satellites come back to Earth?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="127" LastActivityDate="2017-10-05T14:12:58.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23277" PostTypeId="1" AcceptedAnswerId="23281" CreationDate="2017-10-05T14:31:06.577" Score="2" ViewCount="230" Body="&lt;p&gt;If there were a Coronal mass ejection would having a satellite closer to the sun give us an ability to detect and warn us or would all communications be knocked out and/or the solar flare would be the same speed as the communications?&lt;/p&gt;&#xA;" OwnerUserId="15681" LastActivityDate="2017-10-07T15:06:08.957" Title="Would there be any advantage to set satellites closer to the sun?" Tags="|artificial-satellite|communication-satellite|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23279" PostTypeId="2" ParentId="23277" CreationDate="2017-10-05T15:01:25.190" Score="2" Body="&lt;p&gt;As Wikipedia states, an astronomical unit is roughly the distance from the Sun to the Earth. Roughly because the earth's orbit is slightly elliptical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One astronomical unit is defined as 149,597,870,700 metres.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any visual information about the Sun travels at the speed of light, which is 299,792,485 m/s. The time taken for light to travel from the Sun to the Earth, even the light from a coronal mass ejection will be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;149,597,870,700 / 299,792,485 = 499 s = 8 minutes &amp;amp; 19 seconds&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By placing a probe closer to the Sun, we won't be gaining much time.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2017-10-05T15:01:25.190" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23280" PostTypeId="2" ParentId="23217" CreationDate="2017-10-05T16:44:54.063" Score="35" Body="&lt;p&gt;In order to answer this question, I tracked down Michael Lammers who the Flight Director for the ISS and he kindly offered to speak to me on the phone about ISS communications. (He is also a super nice guy)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;He said that almost all of the ISS Command and Control, Voice Communications, as well as science downlink, goes through TDRS with the Ku-Band handling HDEV, Voice channels 3 and 4 and other high bandwidth communications. The S-Band handles critical systems, command uplink, and voice channels 1 and 2. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;TDRS is a shared resource and is mostly computer scheduled and he said that HDEV is a good proxy for when the S-Band is up (S-Band has a little higher availability), they share it with Hubble and other resources as well as some launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nominally TDRS is available 80-85% of the time with 10-15min dropouts due to random blockages by station parts, and other users (each TDRS satellite can support 2 high bandwidth users). The longest such dropout in normal operation is around 15 mins. This depends on scheduling (i.e. you could schedule no comms for a whole day if you wanted)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During critical times (such as the spacewalk on 5th October 2017 during  &lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/exp-53-summary.pdf&quot; rel=&quot;noreferrer&quot;&gt;Expedition 53&lt;/a&gt;) they will look at the outage schedule that is computer generated and if something lines up poorly then they can request that the TDRS schedule be readjusted; on that occasion they only had one outage all day which was 5 mins long.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other communications platforms on the ISS, but they are all there to support TDRS in Science downlink not providing primary or (as far as he knew) contingency communications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other Comm systems onboard:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Russian Luch SDRN (Satellite Data Relay Network) He was unsure how much it was used, mostly the Russians share TDRS&lt;/li&gt;&#xA;&lt;li&gt;ICS (Interorbital Communications System), on Kibō used for additional science downlink capability&lt;/li&gt;&#xA;&lt;li&gt;Ham radio, I didn't ask specifically but I assume this is only for its outreach purpose and is not a primary backup system&lt;/li&gt;&#xA;&lt;li&gt;EDRS (European Data Relay System) Mostly just for technology development, not seriously used for Science downlink, definitely not for Command and Control&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Cool picture from &lt;a href=&quot;https://www.nasa.gov/pdf/167129main_Systems.pdf&quot; rel=&quot;noreferrer&quot;&gt;This PDF&lt;/a&gt; of ISS info showing all the Comms systems onboard (I only talked about the ISS to ground ones here):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/WmedE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WmedE.jpg&quot; alt=&quot;ISS Comms&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station#Station_systems&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia ISS article&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Phone Call and an email or two with Michael Lammers (on 10/5/17), who kindly took some time out of his day to answer my questions.&lt;/li&gt;&#xA;&lt;li&gt;Many other odd cool NASA resources on the web&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="18663" LastEditorUserId="18663" LastEditDate="2019-03-13T20:40:34.440" LastActivityDate="2019-03-13T20:40:34.440" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23281" PostTypeId="2" ParentId="23277" CreationDate="2017-10-05T21:21:17.363" Score="4" Body="&lt;p&gt;This answer addresses the overall effects of coronal mass ejection and solar flare events rather than simply the e-m radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The answer is yes, it would help.&lt;/strong&gt; If budget were not a concern it would be most helpful to have a belt of solar orbiting satellites close to the sun and further out. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some background terminology: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;A CME is a release of plasma - charged particles and also involves variation of magnetic fields. A solar flare is broad band e-m radiation, though people may concentrate on visible or x-ray components. The two phenomena are closely enough linked to cause some sloppiness in usage and so when one is mentioned it can usually be assumed that the discussion could be about the release of particles or e-m radiation or field disturbance.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Communications are not necessarily &quot;knocked out&quot; by the phenomena travelling at light speed. Causes of disturbances to satellites or ground infrastructure could variously be the solar ejected particles or field disturbances that have in turn stirred up particles resident in the Earth's belts.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The answer - more detail.&lt;/strong&gt; I believe the causes of communications loss on Earth are thus mostly at risk from the slower moving phenomena taking 1-3 days to arrive at Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites have been positioned at the Earth Sun L1 point, such as SOHO, ACE and DSCOVR. These are able to sample interplanetary physical parameters beyond the magnetosphere though this point is actually relatively close to Earth and so its benefit as an Early warning is modest in purely temporal terms (no disrespect intended here to folks working on these programmes - I think they are very interesting and helpful in scientific terms).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Lastly, why would it help?&lt;/strong&gt;  It would enable operators of satellites, polar flying aircraft (and their passengers) and power stations to make better preparations to reduce their vulnerabilities. &lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-10-05T21:21:17.363" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23283" PostTypeId="1" AcceptedAnswerId="26061" CreationDate="2017-10-06T06:52:30.197" Score="10" ViewCount="908" Body="&lt;p&gt;On-orbit refueling of the SpaceX BFR upper stage is necessary for trips to the Moon and Mars. In the presentation at the &amp;quot;&lt;a href=&quot;http://www.spacex.com/mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;International Astronautical Congress (IAC) in Adelaide, Australia, SpaceX CEO and Lead Designer Elon Musk (provided) an update to his 2016 presentation regarding the long-term technical challenges that need to be solved to support the creation of a permanent, self-sustaining human presence on Mars.&lt;/a&gt;&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;In the SpaceX video &lt;a href=&quot;https://youtu.be/S5V7R_se1Xc?t=1423&quot; rel=&quot;nofollow noreferrer&quot;&gt;Making Life Multiplanetary&lt;/a&gt; after &lt;code&gt;23:40&lt;/code&gt; the fuel transfer process between similar-looking upper stages is illustrated, with the tanker on the left and the ship on the right. Musk explains:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;They would use the same mating interface that they use to connect to the booster on liftoff… and reuse the propellant fill lines that are used when… the ship is on the booster. […] And then to transfer propellant it becomes very simple, use control thrusters to accelerate in the direction that you want to empty…  transfer propellant very easily… from the tanker to the ship.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This is analogous to &amp;quot;gravity feed&amp;quot; replacing gravitational acceleration with propulsive acceleration, but I'm guessing that the magnitude of the acceleration would be an order of magnitude or more lower.&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Have I understood this concept correctly?&lt;/li&gt;&#xA;&lt;li&gt;Using some estimate of a control thruster's acceleration, how long would this pump-less, pressure-driven-less milli-gravity feed take? An hour? A day? Filling rocket tanks before liftoff takes perhaps tens of minutes with all the benefits of a refueling infrastructure on the ground. With only milli-gravity and no pressure assist, wouldn't this take a lot longer?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/50OUn.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/50OUn.gif&quot; alt=&quot;from https://youtu.be/S5V7R_se1Xc?t=1420&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/S5V7R_se1Xc?start=1420&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-11-17T04:53:14.903" LastActivityDate="2021-11-17T04:53:14.903" Title="Is BFR refilling in space purely milli-g &quot;gravity&quot; feed rather than using pumps and/or pressure?" Tags="|spacex|refueling|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="23284" PostTypeId="1" CreationDate="2017-10-06T07:11:32.733" Score="9" ViewCount="2466" Body="&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; Here are some BF numbers from the video:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;total vehicle mass  :  4,400 ton  (2nd stage propellant fill fraction unclear)&#xA;booster thrust:            52,700 kN&#xA;max ascent payload:           150 ton  (to LEO)&#xA;&#xA;2nd stage dry mass:            85 ton&#xA;max propellant mass:        1,100 ton (240/860 CH4/LOX)&#xA;&#xA;Merlin SL engine ISP  (x2):   330 sec (SL)&#xA;                              356 sec (vac)&#xA;Merlin vac engine ISP (x4):   375 sec&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;On-orbit refueling of the SpaceX BFR upper stage is necessary for trips to the Moon and Mars. In the presentation at the &quot;&lt;a href=&quot;http://www.spacex.com/mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;International Astronautical Congress (IAC) in Adelaide, Australia, SpaceX CEO and Lead Designer Elon Musk (provided) an update to his 2016 presentation regarding the long-term technical challenges that need to be solved to support the creation of a permanent, self-sustaining human presence on Mars.&lt;/a&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the &lt;a href=&quot;https://youtu.be/S5V7R_se1Xc?t=1637&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube video&lt;/a&gt; after &lt;code&gt;27:17&lt;/code&gt; the value of orbital refueling is discussed, along with a calculation of payload mass versus delta-v for what looks like different numbers of tankers and refillings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At a later time &lt;code&gt;32:06&lt;/code&gt; the task of bringing cargo to the Moon for a base station is described, and the refilling(s) is(are) shown to happen in elliptical Earth orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If &lt;em&gt;five refillings&lt;/em&gt; are used as shown earlier, how much cargo can be brought to the moon, roughly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could &lt;em&gt;five refillings&lt;/em&gt; be used to get to Mars? Would the last one or two actually have to take place beyond Earth orbit to be useful? &lt;strong&gt;note:&lt;/strong&gt; unlike the proposed Moon mission, the proposed Mars mission includes a refueling at Mars with propellent synthesized from Martial water and carbon dioxide.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/STUMb.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/STUMb.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2ZAOG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2ZAOG.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3liHh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3liHh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/S5V7R_se1Xc?start=1637&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-12-20T13:17:25.473" LastActivityDate="2019-01-11T08:39:35.323" Title="Can five refillings of the BFR second stage be useful to get to the Moon? To Mars? All five in Earth orbit?" Tags="|spacex|the-moon|refueling|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23285" PostTypeId="2" ParentId="23284" CreationDate="2017-10-06T12:37:46.477" Score="2" Body="&lt;p&gt;First of all, let's look at what the delta V is for a fully loaded BFR upper stage in orbit. Given the Delta V tables shown, and assuming 150 tons, a delta V of 6.2 km/s is shown. 150 tons is assumed because that is the mass to LEO provided. As I understand it, these numbers come from a tanker orbiting Earth approaching the payload, and refueling it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems to me that the Moon mission would involve at least some refilling in LEO, and some in an elliptical orbit. I'm not sure of the number of refillings, but I would suspect there is more than the standard (5?) that is required for a deep space mission, say to Mars. The Delta V to land on the Moon from LEO is 5.9 km/s or so. Thus, a BFR could get from LEO to the Moon's surface, but couldn't return without refueling. I can't find the exact delta V required to get to Earth from the Moon's surface, but it is around 2.74 km/s. Therefore, the elliptical orbit must use around 2.5 km/s of delta V, and be fully loaded, in order to get to the Moon without refueling. That is entirely possible, but it will be a significantly elliptical orbit, something on the order of to the GEO belt, so far as I can tell.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume that what will happen is that the tankers will fuel the lunar bound BFR in LEO, and then top off the rocket when it is in the correct transfer orbit. That at least is how I understood it. I suspect it will take about as many missions to refuel the BFR in the elliptical orbit as it will take to refuel it in LEO, although there will be less fuel per load, the rest is lost to match the orbit of the payload rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for Mars, the Delta V required to get to Mars is about 4.3 km/s. I'm not sure what it takes to land the BFR on Mars, but it seems perfectly reasonable that a fully loaded BFR can take 150 tons to Mars, from LEO.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-10-06T13:31:55.110" LastActivityDate="2017-10-06T13:31:55.110" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="23286" PostTypeId="1" AcceptedAnswerId="23287" CreationDate="2017-10-06T12:38:12.363" Score="16" ViewCount="2197" Body="&lt;p&gt;The BBC news article &lt;a href=&quot;http://www.bbc.com/future/story/20171005-the-worlds-oldest-scientific-satellite-is-still-in-orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;The World's Oldest Scientific Satellite is Still in Orbit&lt;/a&gt; talks about the Vanguard program and that Vanguard-1 is still in orbit around the Earth, and near the end shows a photo of Vanguard-3, which survived a launchpad explosion, was recovered, and is now (possibly) at the Smithsonian Air and Space Museum:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Cy50Q.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Cy50Q.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1958-002B&quot; rel=&quot;nofollow noreferrer&quot;&gt;NSSDC GSFC NASA Page&lt;/a&gt; Vanguard-1 looks like this engineering model:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zL2Eh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zL2Eh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DLZTE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DLZTE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7cnhm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7cnhm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In both cases, and in the two screen shots above from &lt;a href=&quot;https://www.youtube.com/watch?time_continue=9&amp;amp;v=R6BHwvKZjnk&quot; rel=&quot;nofollow noreferrer&quot;&gt;this old video&lt;/a&gt; and &lt;a href=&quot;https://www.youtube.com/watch?v=2EoAeJMPSr8&amp;amp;feature=youtu.be&amp;amp;t=6m31s&quot; rel=&quot;nofollow noreferrer&quot;&gt;this old video&lt;/a&gt; it seems to have six antennas pointing in orthogonal  directions like the normals to faces of a cube standing on its corner. However, the header for the BBC article shows this instead. What satellite with four antennas protruding from its equator is this, or of what satellite is it a model? Does it represent a later Vanguard design?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7fQSD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7fQSD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-05-12T01:54:28.277" LastActivityDate="2021-05-12T01:54:28.277" Title="This BBC photo does not show a replica of Vanguard-1, what might it be?" Tags="|artificial-satellite|nasa|history|museum-exhibits|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23287" PostTypeId="2" ParentId="23286" CreationDate="2017-10-06T13:46:21.950" Score="19" Body="&lt;p&gt;The photo is a NASA photo, and is displayed on their web site &lt;a href=&quot;https://www.nasa.gov/content/vanguard-satellite-1958&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The satellite itself is Vanguard 2.  It can be seen mounted on its booster in this photo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eUEWI.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eUEWI.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was successfully launched into Earth orbit on February 17, 1959, and was NASA's (not the US's) first successful satellite launch.  However, the mission was a partial failure because separation issues caused the satellite to wobble after it was released from the third stage of the booster, which prevented its primary imaging payload from returning usable data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vanguard 2 was not solar powered but subsisted solely on battery power. Its mission ended when the batteries ran down 26 days after launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A supplemental note on Vanguard satellites and their confusing names:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only the three satellites that made it to orbit were officially named &lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_1&quot; rel=&quot;noreferrer&quot;&gt;Vanguard 1&lt;/a&gt;,  &lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_2&quot; rel=&quot;noreferrer&quot;&gt;Vanguard 2&lt;/a&gt;, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_3&quot; rel=&quot;noreferrer&quot;&gt;Vanguard 3&lt;/a&gt;. The eight satellites that were lost due to launch failures had other designations, although contemporary sources may have called them Vanguard-x based on launch sequence.  The satellite payload from the infamous first launch attempt, which basically exploded on the launch pad, was recovered from the debris and now exists in the &lt;a href=&quot;https://airandspace.si.edu/collection-objects/satellite-vanguard-tv-3&quot; rel=&quot;noreferrer&quot;&gt;Smithsonian&lt;/a&gt;. It is most correctly referred to as Vanguard TV-3, where TV-3 stands for Test Vehicle 3 and was the name of the booster. It looked basically identical to Vanguard-1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/y0AtR.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y0AtR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Vanguard: A History, &lt;a href=&quot;https://history.nasa.gov/SP-4202.pdf&quot; rel=&quot;noreferrer&quot;&gt;NASA SP-4202&lt;/a&gt; (picture is truncated in official NASA pdf)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources: &lt;a href=&quot;http://www.drewexmachina.com/download-pdf/SV_1999_02_01.pdf&quot; rel=&quot;noreferrer&quot;&gt;www.drewexmachina.com&lt;/a&gt;,&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1959-001A&quot; rel=&quot;noreferrer&quot;&gt;Goddard Space Flight Center&lt;/a&gt;,&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_2&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-10-06T21:34:16.327" LastActivityDate="2017-10-06T21:34:16.327" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23288" PostTypeId="1" CreationDate="2017-10-06T14:09:32.040" Score="6" ViewCount="103" Body="&lt;p&gt;The ISS' SATCAT ID is 1998-067A.&#xA;The CubeSats and other objects released from the space station are considered its sub-units, thus using different suffixes: 1998-067C, etc.&#xA;The same principle applies to satellites which break up in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apparently, the suffix might be up to 3 letters long, thus giving us 17576 sub-units per one launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What happens when that space gets exhausted? How are IDs assigned to the objects outside of that range?&lt;/p&gt;&#xA;" OwnerUserId="19223" LastActivityDate="2017-10-06T14:09:32.040" Title="What happens when the 3-letter suffix space of a SATCAT ID gets exhausted?" Tags="|two-line-elements|satcat|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23289" PostTypeId="5" CreationDate="2017-10-06T14:10:26.683" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2017-10-06T14:10:26.683" LastActivityDate="2017-10-06T14:10:26.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23290" PostTypeId="4" CreationDate="2017-10-06T14:10:26.683" Score="0" Body="SATCAT is a catalog of satellites orbiting Earth." OwnerUserId="19223" LastEditorUserId="19223" LastEditDate="2017-10-07T01:10:04.807" LastActivityDate="2017-10-07T01:10:04.807" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23291" PostTypeId="2" ParentId="19706" CreationDate="2017-10-06T15:16:46.927" Score="2" Body="&lt;p&gt;I am going to venture that what is shown is a model of a Vanguard Project satellite such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vanguard-2&lt;/a&gt;. Roughly 50cm sphere with four antennas protruding in a plane spaced at 90 degrees. The similarity was also pointed out by Sarah Bailey in &lt;a href=&quot;https://space.stackexchange.com/questions/19706/#comment54143_19706&quot;&gt;this comment&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vanguard was a popular image in the West and there were several models of it available, and there are even some now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example see all the cool images in the Scale Model News page for September 8, 2015 &lt;a href=&quot;http://www.scalemodelnews.com/2015/09/space-technology-from-six-decades-ago.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Technology from Six Decades Ago - 1:5 Scale Hawk Project Vanguard Satellite&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sgBJp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sgBJp.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mbiOT.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mbiOT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8i8PD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8i8PD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The GIF image in the question for comparison:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lFyIe.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lFyIe.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-06T16:00:05.347" LastActivityDate="2017-10-06T16:00:05.347" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23292" PostTypeId="2" ParentId="23270" CreationDate="2017-10-06T22:08:31.023" Score="4" Body="&lt;p&gt;The relevant standards body is CCSDS (the Consultative Committee for Space Data Systems).  They publish a set of &lt;a href=&quot;https://public.ccsds.org/Publications/BlueBooks.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;Blue Books&quot;&lt;/a&gt; containing recommended&#xA;interfaces and protocols for telemetry formatting, data compression, encryption, and so forth.  &lt;/p&gt;&#xA;" OwnerUserId="17791" LastEditorUserId="17791" LastEditDate="2017-10-07T00:06:25.997" LastActivityDate="2017-10-07T00:06:25.997" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23293" PostTypeId="1" CreationDate="2017-10-07T04:09:27.003" Score="3" ViewCount="4655" Body="&lt;p&gt;I need help creating my own custom model rocket nozzle. I'm making a model rocket, it will have many intricate parts, but the most intricate maybe the nozzle. My question is where do I start if I want to create my own custom rocket nozzles. I have specifically chosen the bell nozzle that is widely used. Though I don't know how to make the blueprints with the specifications. Hence are there any sites, books, journals that outline how am I able to do so? Or are there any example that I can modify to my liking? &lt;/p&gt;&#xA;" OwnerUserId="20332" LastActivityDate="2017-10-07T04:46:55.540" Title="How to design a bell nozzle from scratch" Tags="|rockets|nozzle|" AnswerCount="1" CommentCount="5" ClosedDate="2017-10-25T19:28:24.960" ContentLicense="CC BY-SA 3.0" />
  <row Id="23294" PostTypeId="2" ParentId="23293" CreationDate="2017-10-07T04:46:55.540" Score="2" Body="&lt;p&gt;It is an interesting question and I am afraid there is no simple answer to this question as there are many variables in it, for starters design will change with the chamber pressure, fuel used, flume profile needed, exit velocity. Make sure you make a proper design as nozzles can blast from the throat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basics of designing CONDI nozzle can be found here:&#xA;&lt;a href=&quot;https://www.grc.nasa.gov/www/k-12/airplane/nozzled.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.grc.nasa.gov/www/k-12/airplane/nozzled.html&lt;/a&gt;&#xA;Another tool that will help you could be:&#xA;&lt;a href=&quot;http://www.engapplets.vt.edu/fluids/CDnozzle/cdinfo.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.engapplets.vt.edu/fluids/CDnozzle/cdinfo.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know this is brief but this could be a starting point.&lt;/p&gt;&#xA;" OwnerUserId="21282" LastActivityDate="2017-10-07T04:46:55.540" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23295" PostTypeId="1" AcceptedAnswerId="23307" CreationDate="2017-10-07T09:51:23.223" Score="9" ViewCount="10036" Body="&lt;p&gt;When you search up in google gravity assist formula, it comes up with the simplest version: &lt;a href=&quot;https://i.stack.imgur.com/L1KSN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L1KSN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which means that: Final velocity = Initial velocity + 2 (velocity of the planet)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then if we do some more research we get a formula that takes into account the angle of approach. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GRrRu.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GRrRu.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But how do we come up to this formulas?&lt;/strong&gt; What is the reasoning behind it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&lt;em&gt;And is there any formula that takes into account the mass of both bodies and the distance between those (altitude)?&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The source: &lt;a href=&quot;http://www.mathpages.com/home/kmath114/kmath114.htm&quot; rel=&quot;noreferrer&quot;&gt;http://www.mathpages.com/home/kmath114/kmath114.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After trying for different values of v and U, this is what we usually get (a V shape). This graph shows the final velocities for different angles of approach.&lt;a href=&quot;https://i.stack.imgur.com/Ws5E8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ws5E8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="20563" LastEditDate="2017-10-07T16:50:28.170" LastActivityDate="2021-05-15T12:12:14.080" Title="How do we come up with the gravity assist (or slingshot) formula?" Tags="|history|gravity-assist|escape-velocity|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23296" PostTypeId="1" AcceptedAnswerId="23311" CreationDate="2017-10-07T09:59:09.673" Score="8" ViewCount="819" Body="&lt;p&gt;When looking at the operational altitude of satellites, it seems that the highest one is GEO (36000km), which is an equatorial orbit. Non-equatorial orbits are less high (from LEO to 20000km for GPS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there an operational satellite orbiting Earth in a &lt;em&gt;higher&lt;/em&gt; non-equatorial orbit than the GPS satellites? If so, what is its altitude?&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="14095" LastEditDate="2017-10-07T16:05:44.407" LastActivityDate="2019-01-02T16:59:30.880" Title="What is the highest non-equatorial earth orbit used?" Tags="|artificial-satellite|orbit|record|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23297" PostTypeId="2" ParentId="23296" CreationDate="2017-10-07T10:45:30.400" Score="12" Body="&lt;p&gt;The highest that I can find record of ever was the &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1966-058A&quot; rel=&quot;noreferrer&quot;&gt;AIMP-D&lt;/a&gt;, which &quot;perigee varied between 6 and 44 earth radii&quot;. Currently it's perigee is about 265,000 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The furthest operation is probably &lt;a href=&quot;https://en.wikipedia.org/wiki/Spektr-R&quot; rel=&quot;noreferrer&quot;&gt;Spektr-R&lt;/a&gt;, with a perigee of 10,000 km and an apogee of 390,000 km initially, and I now understand it to have a perigee of 64,000 km, due to gravitational interaction with the Moon.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-10-08T14:55:40.750" LastActivityDate="2017-10-08T14:55:40.750" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23299" PostTypeId="2" ParentId="23277" CreationDate="2017-10-07T15:06:08.957" Score="2" Body="&lt;p&gt;Yes, and in fact, it has been done. &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHO&lt;/a&gt;, for one, was placed at the L1 point, and studied CMEs, partially to give advanced warning of particularly strong ones. &lt;a href=&quot;https://en.wikipedia.org/wiki/STEREO&quot; rel=&quot;nofollow noreferrer&quot;&gt;STEREO&lt;/a&gt;, which wasn't much closer to the Sun, expanded the ability to see CMEs that were beyond the view of Earth, allowing for better predictions. Also, being able to see them in 3-D assists with this view.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-10-07T15:06:08.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23301" PostTypeId="1" AcceptedAnswerId="23302" CreationDate="2017-10-07T17:54:02.020" Score="3" ViewCount="979" Body="&lt;p&gt;I know the trajectory the parabolic flight takes, just not how it works. I believe it works because the floor is literally falling below you. I have heard that astronauts have been trained in water, is there like a plane that is filled with water that we can't see or what?&lt;/p&gt;&#xA;" OwnerUserId="21286" LastEditorUserId="6816" LastEditDate="2017-10-07T18:12:56.430" LastActivityDate="2017-10-09T17:26:04.373" Title="How do reduced gravity aircraft work?" Tags="|microgravity|training|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23302" PostTypeId="2" ParentId="23301" CreationDate="2017-10-07T18:21:48.050" Score="11" Body="&lt;p&gt;The &quot;Vomit Comet&quot; and similar zero gravity aircraft, as you say, fly a parabolic trajectory.  Here's a description from &lt;a href=&quot;https://en.wikipedia.org/wiki/Reduced-gravity_aircraft&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Initially, the aircraft climbs with a pitch angle of 45 degrees using&#xA;  engine thrust and elevator controls. The sensation of weightlessness&#xA;  is achieved by reducing thrust and lowering the nose to maintain a&#xA;  neutral, or &quot;zero lift&quot;, configuration such that the aircraft follows&#xA;  a ballistic trajectory, with engine thrust exactly compensating for&#xA;  drag. Weightlessness begins while ascending and lasts all the way&#xA;  &quot;up-and-over the hump&quot;, until the craft reaches a downward pitch angle&#xA;  of around 30 degrees. At this point, the craft is pointing downward at&#xA;  high speed and must begin to pull back into the nose-up attitude to&#xA;  repeat the maneuver. The forces are then roughly twice that of gravity&#xA;  on the way down, at the bottom, and up again. This lasts all the way&#xA;  until the aircraft is again halfway up its upward trajectory, and the&#xA;  pilot again reduces the thrust and lowers the nose.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lxRdC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lxRdC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as the water goes, you have probably heard of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Neutral_Buoyancy_Laboratory&quot; rel=&quot;noreferrer&quot;&gt;Neutral Buoyancy Lab&lt;/a&gt; or similar facility, where astronauts practice spacewalk tasks while immersed in water to give them a simulated free-fall environment. It is unrelated to the aircraft except that it is a different means of simulating free-fall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gM6Uz.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gM6Uz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Neither is a fully satisfactory free-fall simulation but no one has yet come up with a better one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drawbacks of the plane: very short free-fall duration, alternating with periods of high acceleration. High cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drawbacks of the water facility: crewmembers are not really in free-fall at all. Their suit, tools and other items that they manipulate are specially made or modified to be neutrally buoyant but the internal suit environment is a normal 1-g field. Extra drag while moving through the water compared to vacuum.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-10-07T18:59:51.440" LastActivityDate="2017-10-07T18:59:51.440" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23303" PostTypeId="1" CreationDate="2017-10-07T18:46:32.087" Score="3" ViewCount="271" Body="&lt;p&gt;I think that the new Mars rover in 2020 will make oxygen, from thin Martian atmosphere. I think Mars' atmosphere is 90% &lt;span class=&quot;math-container&quot;&gt;$CO_2$&lt;/span&gt;. Carbon dioxide is made of &lt;span class=&quot;math-container&quot;&gt;$C$&lt;/span&gt; and &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt;. &lt;span class=&quot;math-container&quot;&gt;$O_2$&lt;/span&gt; is oxygen, so can't we separate oxygen from carbon dioxide? We only need to release the carbon, don't we?&lt;/p&gt;&#xA;" OwnerUserId="21286" LastEditorUserId="47149" LastEditDate="2022-05-14T00:13:45.910" LastActivityDate="2022-05-14T00:13:45.910" Title="Could we get free O₂ on Mars?" Tags="|mars|technology|chemistry|oxygen|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23306" PostTypeId="2" ParentId="23301" CreationDate="2017-10-07T23:59:43.237" Score="8" Body="&lt;p&gt;Imagine that you are falling. You would perceive gravity exactly the same way as you would if there were none.  (This is one of the reasons that the sensation, even in space, is known as free-fall) Now imagine that as you fall, there is an airplane around you, matching your fall exactly.  This is how planes such as the 'vomit comet' work.  The role of the airplane is to get you high enough, to block the wind as you fall, and to catch you at the bottom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: As Organic Marble points out, it does start while you (with the plane) are going up, so perhaps being thrown through the air with a plane around you is more accurate, but the concept is the same in either case.&lt;/p&gt;&#xA;" OwnerUserId="21293" LastEditorUserId="21293" LastEditDate="2017-10-09T17:26:04.373" LastActivityDate="2017-10-09T17:26:04.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23307" PostTypeId="2" ParentId="23295" CreationDate="2017-10-08T00:44:27.057" Score="13" Body="&lt;p&gt;You get that formula simply using the Pythagorean theorem, also known as vector addition. The link already provides $v_{2x}=v_1\cos\theta+2u$ and $v_{2y}=v_1\sin\theta$. Then you simply compute the magnitude $v_2=\sqrt{v_{2x}^2+v_{2y}^2}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually the body shown as a red disk in the diagram is much, much, much more massive than the thing following the black curved trajectory. In that case, the mass of the smaller object has no bearing on the trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the &quot;altitude&quot;, the closest approach distance from the center of the body, along with the approach velocity and mass of the large body, determines $\theta$. See &lt;a href=&quot;https://space.stackexchange.com/a/6510/265&quot;&gt;this answer&lt;/a&gt; for how to calculate it. ($\delta$ there is $\pi-2\theta$ here). It is of course important that the closest approach distance from the center of the body be greater than the radius of the body, lest you have a very violent and extremely short-lived swingby.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-10-08T04:22:36.533" LastActivityDate="2017-10-08T04:22:36.533" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23308" PostTypeId="2" ParentId="23184" CreationDate="2017-10-08T02:59:33.360" Score="1" Body="&lt;p&gt;This had still been eating at me, because I felt like it still wasn't a fully specified problem. I think that people either assumed a ballistic trajectory (parabolic/elliptical), or that the trip would start out by achieving full orbital velocity. The options are not constrained to these two possibilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I investigated one option where a small amount of constant upward thrust is maintained throughout the duration of the trip. This allows the trip to proceed at a speed less than full orbital velocity... or at least it does for &lt;em&gt;some&lt;/em&gt; trips.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://medium.com/@AlanSE/suborbital-transit-with-hovering-a516dc8bc383&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://medium.com/@AlanSE/suborbital-transit-with-hovering-a516dc8bc383&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found that A-&gt;B trips that covered less than 0.5 radians of the Earth could benefit from some of this (I call &quot;hovering&quot;). Distances greater than that, you would still need to achieve basically full orbital velocity. As it happens, basically all of the possible trips mentioned by Musk in the talk were further than this minimum range.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I feel like this is still an important consideration, although ultimately it proves to be irrelevant in the vast majority of practical cases. There may be some other options (that I have not thought of) that similarly reduce propellant use (relative to an orbital trip), while still keeping g-forces in the tolerable range.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2017-10-08T02:59:33.360" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23310" PostTypeId="2" ParentId="23303" CreationDate="2017-10-08T04:15:07.947" Score="-1" Body="&lt;p&gt;I believe your concept is a bit off. I don't think that new Mars rover in 2020 will make oxygen... In order to break apart CO2, into C and O2, energy is required and it isn't as simple as getting a knife and cutting a CO2 molecule in half. Just bring a bunch of plants into Mars where they are in contact with their surroundings but they don't die due to freezing, warm them and give them some light so they will transform CO2 into C and O2. This could work!&lt;/p&gt;&#xA;" OwnerUserId="20563" LastActivityDate="2017-10-08T04:15:07.947" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23311" PostTypeId="2" ParentId="23296" CreationDate="2017-10-08T04:26:10.467" Score="7" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Spektr-R&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spektr-R&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_Boundary_Explorer&quot; rel=&quot;nofollow noreferrer&quot;&gt;IBEX&lt;/a&gt; are both dancing with the Moon. They are both in high Earth orbits that are strongly affected by perturbations from the Moon and possibly the Sun. According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_Boundary_Explorer#Orbit_adjusted&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In June 2011 IBEX shifted to a new orbit that raised its perigee to more than 30,000 kilometres (19,000 mi). The new orbit avoids taking the spacecraft too close to the Moon, whose gravity can negatively affect IBEX's orbit. Now the spacecraft uses less fuel to maintain a stable orbit, increasing its useful lifespan to more than 40 years.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can see by the plots below that both Spektr-R and IBEX have semi-major axes that vary between about 170,000 and 200,000 kilometers. But while IBEX's eccentricity varies between about 0.4 and 0.65, Spektr-R's eccentricity varies betwen about 0.6 and 0.95! Recently it seems to be the case that IBEX reaches a higher apoapsis than Spektr-R, and IBEX has &lt;strong&gt;historically&lt;/strong&gt; had the higher semi-major axis and longer period of the two. Recently however (2016), Spektr-R's semi-major axis has spiked from about 170,000 to 200,000.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to a reduction in TLEs for both spacecraft starting in 2017, it's going to be a difficult job to say which is in the &quot;higher&quot; orbit definitively in the future.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital parameters deduced from TLE dumps:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ARoRD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ARoRD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="11653" LastEditDate="2019-01-02T16:59:30.880" LastActivityDate="2019-01-02T16:59:30.880" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23312" PostTypeId="1" AcceptedAnswerId="23315" CreationDate="2017-10-08T04:47:46.527" Score="4" ViewCount="148" Body="&lt;p&gt;I'm having a difficult time finding information on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Interstellar_Boundary_Explorer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Interstellar Boundary Explorer&lt;/a&gt;'s (IBEX) orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unless I've made some kind of error, the TLE's have stopped at TLE epoch   &lt;code&gt;17162.83&lt;/code&gt; which is 11-Jun-2017. I was doing a quick search for &lt;a href=&quot;https://space.stackexchange.com/a/23311/12102&quot;&gt;this answer comparing the orbits&lt;/a&gt; of IBEX and &lt;a href=&quot;https://en.wikipedia.org/wiki/Spektr-R&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spektr-R&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I noticed while writing &lt;a href=&quot;https://space.stackexchange.com/a/23311/12102&quot;&gt;this answer&lt;/a&gt; that while I can easily find Spektr-R's orbit in &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL Horizons&lt;/a&gt;, I can't find IBEX in Horizons under any of its aliases (IBEX, Explorer 91, SMEX-10). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia lists IBEX's web site as &lt;a href=&quot;https://www.spaceops.swri.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.spaceops.swri.org/&lt;/a&gt; but that page is no help. I found this page: &lt;a href=&quot;http://ibex.swri.org/archive/index.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ibex.swri.org/archive/index.shtml&lt;/a&gt; but the last update is in 2014.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd like to see IBEX's orbital data, but with no TLEs since June and no Horizons, it's &quot;gone dark&quot; in terms of publicly accessible data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it on a secret mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Orbital parameters from TLE for IBEX (33401) and Spektr-R (37755) since mid 2016. IBEX has &quot;gone dark&quot; and Spektr-R is petering out as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AvLy1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AvLy1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Historical Orbital parameters from TLEs for IBEX (33401) and Spektr-R (37755) since orbits were established for each. From &lt;a href=&quot;https://space.stackexchange.com/a/23311/12102&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/69N4s.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/69N4s.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-03-12T12:54:36.490" LastActivityDate="2019-03-12T14:07:24.427" Title="Has IBEX &quot;gone dark&quot;?" Tags="|artificial-satellite|two-line-elements|jpl-horizons|ibex|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23314" PostTypeId="2" ParentId="23303" CreationDate="2017-10-08T10:34:30.573" Score="4" Body="&lt;p&gt;The experiment in question, &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Oxygen_ISRU_Experiment&quot; rel=&quot;nofollow noreferrer&quot;&gt;MOXIE&lt;/a&gt;, &quot;Mars Oxygen ISRU Experiment&quot; uses a process of solid oxide electrolysis, to produce oxygen from Martian atmospheric $CO_2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, as reversal of the process of burning coal, production of oxygen from carbon dioxide requires significant amount of energy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, due to specifics of the process, and problems of solid residue the produced carbon would create, the actual reaction performs operation of splitting carbon dioxide into oxygen and carbon monoxide, and separating them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While technologically much simpler than detaching both oxygen atoms from a $CO_2$ particle, this leaves the risk of mixing highly poisonous carbon monoxide with the oxygen produced. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Experiments with utilizing photosynthesis of plants or cyanobacteria were performed; these release oxygen while using carbon as a building material of the organism (and using sunlight energy to do the splitting); this comes with a range of different problems though, and isn't to be tested in the 2020 rover.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-10-08T10:34:30.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23315" PostTypeId="2" ParentId="23312" CreationDate="2017-10-08T15:13:06.180" Score="2" Body="&lt;p&gt;IBEX is a small satellite,  with less than 1 square meter of area visible. In addition it is very far out, at the closest point is is 59,000 km or so away from Earth. &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/news/orbital_debris.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Track&lt;/a&gt; has stated that they can track an object at Geosynchronous orbit that is about the side of IBEX. It seems that it can only be tracked when it is at it's closest, and there are definitely some holes in that tracking even at the closest approaches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, it is unusual to have no TLE updates for such a long time. From the very data you showed, however, this isn't the first time that such a long gap has occurred. It happened between 2012-2013. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I had to speculate, I would guess that JSPOC isn't tracking these objects are carefully for some reason, which could be that the closest approach is happening over the Pacific ocean or some other very remote location, and it isn't able to be tracked when it is at that closest time period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA not releasing status of a spacecraft that has been dead for 4 months would be unprecedented, and I don't see any reason why they would hold back. The mission is well beyond it's planned lifetime. In fact, IBEX has released data as recently as &lt;a href=&quot;https://heliophysicsdata.sci.gsfc.nasa.gov/websearch/dispatcher?dataStart=1997-08-25&amp;amp;dataEnd=2019-02-11&amp;amp;name=ACE%2027-day%20Survey%20Plots&amp;amp;pID=131&amp;amp;accessor_type=7&amp;amp;aID=0&amp;amp;action=DatashopAccessorResultAction&amp;amp;start=2019-01-01&amp;amp;end=2019-02-11&amp;amp;vspoid=spase%3A%2F%2FVSPO%2FDisplayData%2FACE%2FMAG%2F27-Day&amp;amp;action=DatashopAccessorResultAction&amp;amp;vspo_subindex=0%20&quot; rel=&quot;nofollow noreferrer&quot;&gt;2019&lt;/a&gt;, so while there seems to be some difficulty tracking it from JSPOC, it is there and operating.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-03-12T13:06:58.313" LastActivityDate="2019-03-12T13:06:58.313" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23316" PostTypeId="1" CreationDate="2017-10-08T18:43:27.407" Score="5" ViewCount="130" Body="&lt;p&gt;One of NASA's plans for future mars entry, descent and landing missions includes an ambitious deceleration process involving retropropulsion in a supersonic airflow environment.  I know that previous missions made use of subsonic retropropulsion.  I'm curious to know what challenges are expected from extending this technique to the supersonic regime.  Particularly, I'm curious how supersonic retropropulsion affects the aerodynamics and heat transfer differently from the subsonic case, especially in the presence of shock waves.&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="12102" LastEditDate="2017-10-09T01:15:34.757" LastActivityDate="2017-10-09T01:15:34.757" Title="What aerothermal effects present significant challenges in supersonic retropropulsion?" Tags="|advanced-propulsion|aerodynamics|thermodynamics|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23318" PostTypeId="2" ParentId="12737" CreationDate="2017-10-08T23:20:53.163" Score="1" Body="&lt;p&gt;I am not very comm literate but hopefully I can parrot this information about TDRSS Ku-band modulation I found and point you to the source so that you can confirm whether or not it actually answers your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://calhoun.nps.edu/bitstream/handle/10945/23145/useoftrackingdat00topp.pdf?sequence=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;document&lt;/a&gt; states&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The K-band SA forward link is the only service for command data&#xA;  greater than 300 kbs and its carrier is &lt;em&gt;BPSK modulated&lt;/em&gt; without range&#xA;  channel transmission.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Where SA stands for Single Access and BPSK stands for BiPhase Shift Key. (Italics mine).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Multiple Access (MA) service this table is provided:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OAOvv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OAOvv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-10-08T23:42:49.733" LastActivityDate="2017-10-08T23:42:49.733" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23319" PostTypeId="1" CreationDate="2017-10-09T02:06:36.927" Score="4" ViewCount="268" Body="&lt;p&gt;Also do we have any idea how deep some of the ore is?&lt;br&gt;&#xA;Edit: &#xA;I can't believe I left this that vague.  Iron.  I'm talking about iron.  &lt;/p&gt;&#xA;" OwnerUserId="21303" LastEditorUserId="21303" LastEditDate="2017-10-09T03:19:21.463" LastActivityDate="2017-10-09T11:44:08.530" Title="Would a metal detector on Mars be usable or would the &quot;soil&quot; return too many false positives?" Tags="|mars|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="23320" PostTypeId="2" ParentId="23319" CreationDate="2017-10-09T11:44:08.530" Score="3" Body="&lt;p&gt;It may depend on how the metal detector has been &quot;tuned&quot; or calibrated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When looking for gold in a region with a lot red soil that contains iron, metal detectors work better if they are &quot;tuned&quot;/calibrated to find gold.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to look for iron deposits the metal detector may need to be calibrated for iron, but that would be difficult because there are many different types of iron deposits for bog iron ore, which contains about 30% Fe to &lt;a href=&quot;https://www.britannica.com/science/hematite&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hematite&lt;/a&gt;, which is 70% Fe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From what is currently in the public domain, calibrating for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hematite&quot; rel=&quot;nofollow noreferrer&quot;&gt;hematite &quot;blueberries&quot;&lt;/a&gt; on Mars may be the best option. This could be done on Earth by creating spherules from local hematite, similar in size to those on Mars, and mixing it with various types of iron rich soil and test how various metal detectors respond.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastActivityDate="2017-10-09T11:44:08.530" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23321" PostTypeId="2" ParentId="1599" CreationDate="2017-10-09T18:40:35.447" Score="4" Body="&lt;p&gt;If you try this, be sure to coordinate with the other cubesat teams sharing your deployer pod.  In 2011 the MCubed and HRBE teams found out the hard way that there can be &lt;a href=&quot;https://pdfs.semanticscholar.org/2f70/6d36f8d26f5bf35b9a0ab8e3e0fcec9c900c.pdf&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Investigation of the on-orbit conjunction between the MCubed and HRBE CubeSats&quot;&gt;unintentional interactions&lt;/a&gt; between cubesats employing passive magnetic attitude stabilization.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-10-09T18:40:35.447" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23323" PostTypeId="2" ParentId="19807" CreationDate="2017-10-10T01:14:17.470" Score="3" Body="&lt;p&gt;SpaceX knows from the time they are getting in to the detailed mission plan the number of satellites, and where they will be on the spacecraft. The stack looks like below (&lt;a href=&quot;https://spaceflightnow.com/2016/12/30/iridium-satellites-closed-up-for-launch-on-falcon-9-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/DxH6S.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DxH6S.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that there were 10 satellites, I imagine they are in a pentagon formation. Each of these satellites is mounted by a few points, usually 4 or more, with springs loaded to them. There is also a cable that is attached together such that it can be detached without any damage, to connect it to the satellite. You can see an example from the very similarly mounted Orbcomm satellites. There are a number of such pictures &lt;a href=&quot;http://www.spaceflight101.net/orbcomm-g2-launch-1---falcon-9-launch-updates.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oWApJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oWApJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also a photo that shows the satellite attached at 4 points, most likely for a vibration test, although that isn't clear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mLFgA.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mLFgA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically what happened is the rocket commanded each of the satellites to be released at a certain time. I'm not sure of the exact latching mechanism, likely it was a kind of exploding bolt. When it was released, the satellite would be accelerated by the spring and leave the rocket behind. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX will have done an analysis to figure out the proper orientation of the rocket when deployed. There is a particularly funny phenomena in orbital mechanics where something that is moved faster actually will orbit slower, so all of the objects will switch in orbit very early on, within the first orbit usually. SpaceX will have done the simulation to show that there is no chance that these objects will &quot;recontact&quot; during that period of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rocket will try to remain as still as possible during the deployment, at it's assigned orientation. There may be a slight &quot;tip off&quot; rotation that is induced during the deployment, but this is taken in to account during the planning of the sequence.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-10-10T01:20:28.340" LastActivityDate="2017-10-10T01:20:28.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23324" PostTypeId="1" CreationDate="2017-10-10T07:40:45.700" Score="-4" ViewCount="281" Body="&lt;p&gt;I want to know if this is possible and any limitations it has.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Time Dilation is an arse to say the least. It is one the things that ought to stop humankind from ascending to sci-fi badassness and I am not ok with it, BUT, I have been told by a very reputable scientist (&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/J2QsbFOCeD4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;  6:55-9:09 mins in)  that there are two types of time dilation, so to speak.. The infamous one is the one where time runs slower for a moving clock than a stationary one (mechanical, biological, etc.) but the slightly less known one is the one I am interested in. I will get to that later. If you go at any speed under the speed of light, time will run slower for you as seen by a stationary observer. Say you went at the speed of light for 10 minutes, you would experience no travel time and would instantly teleport to your location but the world around you would be 10 minutes in the future. Once you exceed lightspeed, external time would run backwards instead of forwards. . So, say you have your hypothetical super fast magical ship that can somehow go at speeds up to 1000 times the speed of light. If that ship goes at several times the speed of light until external time, is at, say, 5 years in the past, would the ship be able to escape the clawing grasp of time dilation by lowering its speed to just below lightspeed so that external time runs forward again until it is normal, at which point the ship would go faster than light and so on until it reaches its destination, free of T.D because of playing jump rope with the laws of the universe.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(this is kind of a weird question, but I am going to write a sci-fi book series about the end of the universe in around a decade and I want it to be as scientifically accurate as possible and there is a depressing void of this sort of info on the internet so thought I would ask you guys. plz don't get salty about this question)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/O3tkJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/O3tkJ.jpg&quot; alt=&quot;Limits of google&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20954" LastActivityDate="2017-10-10T12:30:38.647" Title="Beating Time Dilation With Time Dilation?" Tags="|spacetime|" AnswerCount="1" CommentCount="6" ClosedDate="2017-10-10T13:23:27.440" ContentLicense="CC BY-SA 3.0" />
  <row Id="23326" PostTypeId="1" AcceptedAnswerId="23328" CreationDate="2017-10-10T11:20:24.860" Score="3" ViewCount="1674" Body="&lt;p&gt;Would it make sense to use &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid_oxygen#Red_oxygen&quot; rel=&quot;nofollow noreferrer&quot;&gt;solid (aka red) oxygen&lt;/a&gt; as oxidizer instead of liquid oxygen? &#xA;(Red oxygen: An allotrope of solid oxygen (O8))&lt;/p&gt;&#xA;" OwnerUserId="19946" LastEditorUserId="19946" LastEditDate="2017-10-10T12:11:04.933" LastActivityDate="2017-10-10T16:53:19.297" Title="Red oxygen as solid oxidizer?" Tags="|fuel|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="23327" PostTypeId="2" ParentId="23324" CreationDate="2017-10-10T12:30:38.647" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Once you exceed lightspeed, external time would run backwards instead of forwards.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This is where it all falls down. You're applying the laws of relativity to FTL travel, but the laws of relativity, specifically the Lorentz factors, don't make sense for velocities above the speed of light, $c$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just look at the mathematics - the Lorentz Factor is given by &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$ \gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} $&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If $v &amp;gt; c$, then you have the square root of a negative number which doesn't make sense.&lt;/p&gt;&#xA;" OwnerUserId="21025" LastActivityDate="2017-10-10T12:30:38.647" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23328" PostTypeId="2" ParentId="23326" CreationDate="2017-10-10T12:34:17.270" Score="7" Body="&lt;p&gt;No. The pressure required to use red oxygen are insane, 10 GPa at room temperature, and still &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Phase_diagram_of_solid_oxygen.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;quite high otherwise&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S685a.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S685a.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to use a solid part as a fuel, there are a few options. You can either burn it in place, or convert it to a liquid to pump. Burning it in place would mean that you would have to maintain that extreme pressure with a system designed to allow gas to flow through, which is very difficult, to say the least. Converting it to a liquid would be very difficult to do in the short term that a rocket needs to launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only use case in rockets that I can imagine would be to store oxygen for a longish duration mission, something like a lunar mission, where you could convert a small amount of the oxygen to a liquid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted that there is some hope that it can be stable at lower pressures once created. To my knowledge, this has never been demonstrated, however, it could still happen.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-10-10T16:53:19.297" LastActivityDate="2017-10-10T16:53:19.297" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23329" PostTypeId="2" ParentId="22953" CreationDate="2017-10-10T13:00:18.527" Score="2" Body="&lt;p&gt;Mirrors can be made of thin, flexible metal-coated foil, and they work very well in space... except don't expect any considerable precision from them. Certainly not something like a telescope; maybe in a pinch a mirror for beaming power (sun light) to specific area of a planet or a central target not too far. Problems of wrinkling, creasing etc also cause extra engineering headaches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regardless, the mirror finish, reflecting photons, reacts to light pressure twice as efficiently as absorptive material, so the lightweight, thin foil mirrors are the primary material for solar sails.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/giXZ5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/giXZ5.jpg&quot; alt=&quot;solar sail craft&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-10-10T13:00:18.527" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23331" PostTypeId="2" ParentId="4153" CreationDate="2017-10-10T14:44:31.190" Score="1" Body="&lt;p&gt;First, binder Jetting is the most appropriate 3D printing technology for solid fuel modules, allowing complicated 3D structures without additional heat. The propellant, properly powdered, would be layered in thin layers on top of the previous layer, then an inkjet type head would selectively spray the appropriate binder to secure loose powder to the layer below, then the process starts again with another layer.  It would be self supporting, but could not support voids since the voids would contain loose powder.  In most solid rocket designs this would not be an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, solid rocket motors have limited uses.  Their primary benefit is low cost, but you make a number of tradeoffs for that, including less real-time control and fewer safety options. Quite frankly they are of limited use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The grain structures already designed provide sufficient variation in impulse over time, so that most mission profiles can be met without more specialized cores, so there simply isn't a need for a more complex grain structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once a mission gets to the point where a peculiar profile is required, the decision usually starts to tip towards liquid propellant designs, which offer a host of other benefits. 3D printed engines aren't going to drastically alter the landscape, particularly as we are now approaching 100% reusable launch vehicles.&lt;/p&gt;&#xA;" OwnerUserId="248" LastActivityDate="2017-10-10T14:44:31.190" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23332" PostTypeId="2" ParentId="23153" CreationDate="2017-10-10T15:41:18.287" Score="2" Body="&lt;p&gt;I'll add a little bit to Hobbes' &lt;a href=&quot;https://space.stackexchange.com/a/23159/12102&quot;&gt;excellent answer&lt;/a&gt; and detective work. I looked up a physical copy of the book linked there; Practical Conic Sections: The Geometrical Properties of Ellipses, Parabolas and Hyperbolas by J. W. Downs, Dover, NY, 1993 and found it a short but incredibly interesting and informative little book, if you like reading about conic sections.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From reading and enjoying the illustrations, I've learned that the hyperboloidal secondary does not have to be at all coaxial with the primary paraboloid. All sections of the paraboloid focus to a single point, so you can orient they hyperboloid any way you choose, so long as one of its foci coincide with the focus of the paraboloid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, you can use a positive or negative hyperboloid, whichever suits your purpose. Hyperbolae come in pairs and are associated with two foci. Concave or convex, it will redirect the paraboloids rays converging to one of its focal points to a focus at the other point. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, as discussed in the &lt;a href=&quot;https://www.honeysucklecreek.net/images/other_stations/OTC_Carnarvon_images/42/KSK_Antenna.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;linked paper&lt;/a&gt; in Hobbes' answer, a motivation for building this unusual-looking reflector is that it prevents the feed horn from &quot;seeing&quot; the ground or atmosphere near the horizon where thermal noise is a serious problem. According to the abstract:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A new antenna type is described which combines the low noise temperature characteristics of the horn -ref lector antenna with the more attractive mechanical features associated with the paraboloidal reflector. Cassegrain optics used in an off-set feed configuration enables a virtual source to be formed without sub-reflector blockage. An extremely compact structure is realized with a concave hyperboloid which mirrors the actual feed located on the paraboloidal surface. &lt;strong&gt;Except for the aperture, the antenna is completely shielded. The design approach is outlined and measurements on an experimental model are presented. Ground noise contribution from minor lobes is about 2°K&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From: &lt;em&gt;A New Low Noise, High Gain Antenna&lt;/em&gt; S. R. Jones and K. S. Kelleher&#xA;Aero Geo Astro Corporation, Alexandria, Virginia, Reprinted from 1963 IEEE International Convention Record.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Figures 6.4 and 6.5 on pages 49-50 of Practical Conic Sections: The Geometrical Properties of Ellipses, Parabolas and Hyperbolas by J. W. Downs, Dover. 1993&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VG3yn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VG3yn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wlLzw.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wlLzw.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-11T02:01:58.443" LastActivityDate="2017-10-11T02:01:58.443" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23335" PostTypeId="1" AcceptedAnswerId="26389" CreationDate="2017-10-11T02:26:07.363" Score="1" ViewCount="943" Body="&lt;p&gt;For dish antennas used to transmit a signal over large distances, the on-axis gain scales as $D^2/ \lambda^2$ or $D^2 f^2 / c^2$ where $\lambda$ and $f$ are the wavelength or frequency and $c$ is the speed of light. All else equal, doubling the frequency means four times the power received by a distant receiver with a fixed receive antenna area.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Excluding optical frequencies, what is the highest frequency that has been used or even tested with spacecraft &lt;em&gt;that could be used&lt;/em&gt; beyond Earth orbit? I'm looking for an approximate frequency, not just a letter-code for a band designation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note 1:&lt;/em&gt; reception in Earth orbit (as well as on Earth's surface) counts for this question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;note 2:&lt;/em&gt; I've adjusted this question a bit to allow for testing that wasn't necessarily done beyond Earth orbit, as long as the results are applicable and informative toward potential use beyond Earth orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Microwave band names and approximate frequencies, from Skyware Technology's &lt;a href=&quot;http://www.skywaretechnologies.com/news/item/84-ka-vs-ku-an-unbiased-review&quot; rel=&quot;nofollow noreferrer&quot;&gt;Ka vs. Ku - An Unbiased Review&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/taQuW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/taQuW.png&quot; alt=&quot;microwave bands&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; 70 meter dish at the DSN's Goldstone complex. For scale, note that red paint denotes walkways and stairs. From &lt;a href=&quot;https://gizmodo.com/keeping-in-touch-with-our-space-explorers-1558539151&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gizmodo&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gtWb5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gtWb5.jpg&quot; alt=&quot;Goldstone 70 meter dish&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-04-02T06:29:56.857" LastActivityDate="2018-09-01T08:03:57.707" Title="What is the highest non-optical frequency used or tested for use in deep space communication?" Tags="|deep-space|radio-communication|deep-space-network|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23336" PostTypeId="1" AcceptedAnswerId="23345" CreationDate="2017-10-11T03:08:13.207" Score="6" ViewCount="780" Body="&lt;p&gt;I was looking at &lt;a href=&quot;https://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201B.pdf&quot; rel=&quot;noreferrer&quot;&gt;this document&lt;/a&gt; who's name I can only speculate. Its title and designation includes:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;810-005, Rev. E, 201, Rev. B, DSN Telecommunications Link, Design Handbook, 201, Rev. B, Frequency and Channel Assignments, Released December 15, 2009.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The following section caught my eye:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;2.2 Spacecraft Transponder Turnaround Ratios&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;To measure two-way or three-way Doppler shift, the spacecraft must transmit a downlink signal that is phase coherent with the uplink signal. Table 1 provides the recommended spacecraft transponder turnaround ratios for various uplink and downlink frequency bands. The tracking equipment at the DSN 34-m and 70-m stations can accommodate other turnaround ratios but this support must be negotiated through the JPL Frequency Manager, who is resident in the Plans and Commitments Program Office.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Doppler shift is an essential technique for measuring the spacecraft trajectories. Along with absolute range from delay measurements, the rate of change of range provides a precise one-dimensional projection of the spacecraft's velocity. Combined with sophisticated orbital mechanics calculations, these can be used to locate a spacecraft in 3D within the solar system.&lt;/p&gt;&#xA;&lt;p&gt;For two-way doppler shift measurements, A signal of known frequency is (usually) transmitted from Earth to a spacecraft. When it is received, the uplink frequency is shifted by a stable internally generated frequency or by a specific PLL-controlled divide-by ratio to shift it to a new, downlink frequency and transmitted back toward Earth. The spacecraft often (always?) uses two different bands so that it's sensitive receiver is not saturated by the simultaneous high-power broadcast of the shifted frequency.&lt;/p&gt;&#xA;&lt;p&gt;While range-rate can be done on passive objects (e.g. NEO asteroids or very large planets) using standard radar techniques and picking up the weak, passively reflected signal, spacecraft at deep-space distances are too small to reflect a useful amount of power, so a boost in power by the spacecraft's sensitive receiver and powered retransmission is necessary.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How does a three-way Doppler shift measurement work in practice? I'm looking for some level of technical specificity here, not just hypothesizing, generalizations, or hand-waving.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-12T04:07:24.493" Title="How does a three-way doppler shift measurement work?" Tags="|radar|doppler-ranging|delay-doppler|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23337" PostTypeId="1" AcceptedAnswerId="23343" CreationDate="2017-10-11T03:26:50.493" Score="2" ViewCount="240" Body="&lt;p&gt;Table 1 of &lt;a href=&quot;https://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201B.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSN Telecommunications Link Design Handbook, 201, Rev. B, Frequency and Channel Assignments&lt;/a&gt; gives Spacecraft Transponder Turnaround Ratios that are supported by the Deep Space Network. These are the PLL controlled divide-by ratios that a spacecraft uses when receiving a carrier frequency from the DSN and retransmitting a different frequency back to Earth which is still phase-coherent, for the purposes of precise doppler shift measurements as part of a determination of a (usually deep space) spacecraft's trajectory. Read more about this in &lt;a href=&quot;https://space.stackexchange.com/q/23336/12102&quot;&gt;How does a three-way doppler shift measurement work?&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;Some of the supported ratios are within-band and close to unity; X to X-band, Ka to Ka-band and S to S-band. This means that the spacecraft must simultaneously receive a very weak signal and transmit a very strong signal at frequencies that are very close, and this requires more frequency rejection in the spacecraft's receive circuit.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Are there any deep-space spacecraft which actually support within-band turn-around for Earth-based doppler measurements? Are these spacecraft using one dish for simultaneous receive and transmit within the same band?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZIw9e.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZIw9e.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-10-13T15:42:56.183" LastActivityDate="2021-10-13T15:42:56.183" Title="Have any deep-space spacecraft supported same-band turnaround of Earth-based doppler measurements?" Tags="|spacecraft|doppler-ranging|delay-doppler|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23338" PostTypeId="1" CreationDate="2017-10-10T22:33:26.670" Score="-4" ViewCount="1195" Body="&lt;p&gt;for shipbuilding, above 10 meters length, a reinforced concrete wall of 70 mm thickness is competitive in buoyancy to hull weight with wooden or metal construction. ('Les Bateaux en Ciment Armé', Charles Vireton; La Science &amp;amp; la Vie, 1917, an article in French; 'Concrete Boatbuilding', 1971. &#xA;Gainor W. Jackson, W. Morley Sutherland; FAO fisheries documents, of open and free download, and much more...) Concrete made car: &lt;a href=&quot;https://www4.uwm.edu/cbu/Concrete.Car.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www4.uwm.edu/cbu/Concrete.Car.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;General data about materials appear in the Bosch: 'Manual of Automobile Technik'&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Heat pipes were shown being able to keep satellite temperatures quite uniform and cold (Dornier), and a concrete hull would eliminate the need for silica tiles. About 'Heat Pipes', SAE held a webinar, by ACT (Advanced Cooling Technologies), focused mainly in electronics cooling. A classical text is: 'Heat Pipes. Theory, Design and Applications', 6th ed:  David Reay, Ryan McGlen and Peter Kew. ISBN: 978-0-08-098266-3&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When hearing that the construction of a 14.1 MeV neutron generating machine, in order of testing materials to be used in ITER, was just funded, I remembered the Dyna Soar case, that after expending lots of money in design, they realized temperatures from friction with air on re-entry would melt the machine. (this may not be 100% exact, pls search for Dyna Soar in the web)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reinforced concrete columns in the Windsor building fire in Madrid, Spain,  resisted without problems hours of temperatures above 900º and 1'000º C. &lt;a href=&quot;https://www.ieca.es/show_doc.asp?id_doc=31&quot; rel=&quot;nofollow noreferrer&quot;&gt;'Fire resistance of concrete structures', article in Spanish&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, a fully concrete made Space Shuttle won't need special isolation tiles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would the weight to volume ratio and the payload of such a concrete machine be acceptable for a Spaceship? Thanks, regards. Salut +&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;13&quot;&gt;&#xA;&lt;li&gt;&lt;ol start=&quot;6&quot;&gt;&#xA;&lt;li&gt;CEMENTS&#xA;Several familiar ceramic materials are classified as inorganic cements: cement, plaster of Paris, and lime, which as a group, are produced in extremely large quantities. The characteristic feature of these materials is that when mixed with water, it forms a paste that subsequently sets and hardens. This trait is especially useful in that solid and rigid structures having just about any shape may be expeditiously formed. Also, some of these materials act as a bonding phase that chemically binds particulate aggregates into a single cohesive structure. Under these circumstances, the role of the cement is similar to that of the glassy bonding phase that forms when clay products and some refractory bricks are fired. One important difference, however, is that the cementitious bond develops at room temperature. &#xA;Of this group of materials, portland cement is consumed in the largest tonnages. It is produced by grinding and intimately mixing clay and lime-bearing minerals in the proper proportions, and the heating the mixture to about 1’400º C (2550º F),  in a rotary kiln; this process, sometimes called calcination, produces physical and chemical changes in the raw materials. The resulting ‘clinker’ product is then ground into a very fine powder to which is added a small amount of gypsum (CaSO4-2H2O) to retard the setting process. This product is portland cement. The properties of portland cement, including setting time and final strength, to a large degree, depend on its composition.&#xA;Several different constituents are found in Portland cement, the principal ones being tricalcium silicate (3CaO-SiO2), and dicalcium silicate (2CaO-SiO2). The setting and hardening of this material result from relatively complicated hydration reactions that occur between the various cement constituents and the water that is added. For example, one hydration reaction involving dicalcium silicate is as follows:&#xA; 2CaO-SiO2 + xH2O = 2caO-Si&gt;O2-xH2O &#xA;where x is variable and depends on how much wáter is available. These hydrated products are in the form of complex gel or crystalline substances that form the cementitious bond. Hydration reactions begin just as soon as water is added to the cement. These are first manifested as setting (i.e, the stiffening of the once plastic paste), which takes place soon after mixing, usually within several hours. Hardening of the mass follows as a result of further hydration, a relatively slow process that may continue for as long as several years. It should be emphasized that the process by which cement hardens is not one of drying, but rather, of hydration in which water actually participates in a chemical bonding reaction. &#xA;Portland cement is termed a hydraulic cement because its hardness develops by chemical reactions with water. It is used primarily in mortar and concrete to bind, into a cohesive mass, aggregates of inert particles (sand and/or gravel); these are considered to be composite materials (see section 16.2 in Particle reinforced composites:  Large particle composites). Other cement materials, such as lime, are nonhydraulic; that is compounds other than water (e.g., CO2) are involved in the hardening reaction. &lt;/li&gt;&#xA;&lt;/ol&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;CONCRETE&#xA;Concrete is a common large-particle composite in which both matrix and dispersed phases are ceramic materials. Since the terms ‘concrete’ and ‘cement’ are sometimes incorrectly interchanged, perhaps it is appropriate to make a distinction between it. In a broad sense, concrete implies a composite material consisting of an aggregate of particles that are bound together in a solid body by some type of binding medium, that is, a cement. The two most familiar concretes are those made with Portland and asphaltic cements, where the aggregate is gravel and sand. Asphaltic concrete is widely used primarily as a paving material, whereas Portland cement concrete is employed extensively as a structural building material. Only the latter is treated in this discussion.&#xA;Portland cement concrete &#xA;The ingredients for this concrete are portland cement, a fine aggregate (sand), a coarse aggregate (gravel), and water. The process by which Portland cement is produced and the mechanism of setting and hardening were discussed very briefly in Section 13.6. The aggregate particles act as a filler material to reduce the overall cost of the concrete product because it is cheap, whereas cement is relatively expensive. To achieve the optimum strength and workability of a concrete mixture, the ingredients must be added in the correct proportions. Dense packing of the aggregate and good interfacial contact are achieved by having particles of two different sizes; the fine particles of sand should fill the void spaces between the gravel particles. Ordinarily, these aggregates comprise between 60% and 80% of the total volume.  The amount of cement-water paste should be sufficient to coat all the sand and gravel particles, otherwise, the cementitious bond will be incomplete. Furthermore, all constituents should be thoroughly mixed. Complete bonding between cement and the aggregate particles is contingent upon the addition of the correct quantity of water. Too Little water leads to incomplete bonding, and too much results in excessive porosity; in either case, the final strength is less than the optimum.&#xA;      The character of the aggregate particles is an important consideration. In particular, the size distribution of the aggregates influences the amount of the cement-water paste required. Also, the surfaces should be clean and free from clay and silt, which prevent the formation of a sound bond at the particle surface.&#xA;      Portland cement concrete is a major material of construction, primarily because it can be poured in place and hardened at room temperature, and even when submerged in wáter. However, as a structural material, there are some limitations and disadvantages. Like most ceramics, portland cement concrete is relatively weak and extremely brittle; its tensile strength is approximately 10 to 15 times smaller than its compressive strength. Also, large concrete structures can experience considerable thermal expansion and contraction with temperature fluctuations. In addition, water penetrates into external pores, which can cause severe cracking in cold weather as a consequence of freeze-thaw cycles. Most of these inadequacies may be eliminated or at least improved by reinforcement and/or the incorporation of additives. &#xA;&lt;strong&gt;Reinforced concrete&lt;/strong&gt; &#xA;The strength of Portland cement concrete may be increased by additional reinforcement. This is usually accomplished by means of Steel rods, wires, bars (rebar), or mesh, which are embedded into the fresh and uncured concrete. Thus, the reinforcement renders the hardened structure capable of supporting greater tensile, compressive, and shear stress. Even if cracks develop in the concrete, considerable reinforcement is maintained. &#xA;Steel serves as a suitable reinforcement material because its coefficient of thermal expansion is nearly the same as that of concrete. In addition, Steel is not rapidly corroded in the cement environment, and a relatively strong adhesive bond is formed between it and the cured concrete. This adhesion may be enhanced by the incorporation of contours into the Surface of Steel member, which permits a greater degree of mechanical interlocking.&lt;br&gt;&#xA;       Portland cement concrete may also be reinforced by mixing into the fresh concrete fibers of a high-modulus material such as glass, Steel, nylon, and polyethylene. Care must be exercised in utilizing this type of reinforcement, since some fiber materials experience rapid deterioration when exposed to the cement environment.&lt;br&gt;&#xA;     Still another reinforcement technique for strengthening concrete involves the introduction of residual compressive stresses into the structural member; the resulting material is called prestressed concrete.  This method utilizes one characteristic of brittle ceramics, namely, that it is stronger in compression than in tension. Thus, to fracture a prestressed concrete member, the magnitude of the pre-compressive stress must be exceeded by an applied tensile stress. &#xA;     In one such prestressing technique, high-strength Steel wires are positioned inside the empty molds and stretched with a high tensile force, which is maintained constant. After the concrete has been placed and allowed to harden, the tension is released. As the wires contract, it put the structure in a state of compression because the stress is transmitted to the concrete via the concrete-wire bond that is formed. &#xA;     Another technique is also utilized in which stress are applied after the concrete hardens; it is appropriately called post-tensioning. Sheet metal or rubber tubes are situated inside and pass through the concrete forms, around which the concrete is cast. After the cement has hardened, Steel wires are fed through the resulting holes, and tension is applied to the wires by means of jacks attached and abutted to the faces of the structure. Again, a compressive stress is imposed on the concrete piece, this time by the jacks. Finally, the empty spaces inside the tubing are filled with a grout to protect the wire from corrosion. &#xA;     Concrete that is prestressed should be of a high quality, with a low shrinkage and a low creep rate. Prestressed concretes, usually prefabricated, are commonly used for highway and railway bridges.&#xA;MATERIALS SCIENCE AND ENGINEERING. AN INTRODUCTION. &#xA;William D. Callister Jr.  6th edition, 2002.  Wiley. ISBN 0-471-13576-3&lt;/p&gt;&#xA;" OwnerUserId="20053" OwnerDisplayName="Urquiola" LastEditorUserId="20053" LastEditDate="2017-12-12T09:22:46.917" LastActivityDate="2017-12-12T09:28:43.740" Title="Is it possible building an Space Shuttle in reinforced concrete?" Tags="|materials|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23339" PostTypeId="2" ParentId="23338" CreationDate="2017-10-11T13:20:13.337" Score="15" Body="&lt;p&gt;The short answer is no. Hulls of airplanes are much much thinner than those of ships, spacecraft hulls are even thinner. Think somewhere between a sheet of paper and an eggshell, though I can't find a number for the Space Shuttle atm. Add to that a myriad problems, that this approach would bring...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Concrete is porous, so air would escape, unless specially sealed&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Concrete can crack under uneven thermal loads&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Concrete would probably crack in micrometeorite impacts&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Concrete would probably not show observable crack growth, but instead fail catastrophically all at once, because of its brittleness&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Stiffness is an issue, distinct from mechanical strength&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;...&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;And the most important part is plausibility: If concrete could take the heat of re-entry they wouldn't have used the tile system that they have used. It wouldn't. Not for thermal considerations, not for mechanical considerations and most importantly not for thermo-mechanical considerations.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2017-10-20T12:29:07.690" LastActivityDate="2017-10-20T12:29:07.690" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23341" PostTypeId="2" ParentId="23338" CreationDate="2017-10-11T17:53:47.330" Score="4" Body="&lt;p&gt;As an addition to the answer from Rikki-Tikki-Tavi; the silica tiles were not used solely for their (lack of) weight, but also because they were great at insulating the aluminium fuselage from the heat of reentry.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Concrete is a lot worse at this; it would heat up, and while it might withstand the heat, the internals of the shuttle would heat to an untolerable level, and the shuttle would disintegrate as a result of the control systems melting. &lt;/p&gt;&#xA;" OwnerUserId="21325" LastActivityDate="2017-10-11T17:53:47.330" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23342" PostTypeId="2" ParentId="17041" CreationDate="2017-10-11T20:40:54.767" Score="6" Body="&lt;p&gt;The boots of the Apollo astronauts experienced temperature swings from 355 K in the sun to 166 K in the shade.  There were no reports of temperature problems in regards to the suits and/or footwear, though at one point Aldrin reported being cool and adjusted the thermal control on his suit.  Given the boots handled these temperature swings without issue I'd extrapolate the insulation is very good. &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/A11_PAOMissionReport.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.hq.nasa.gov/alsj/a11/A11_PAOMissionReport.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Though even with very good insulation if it wasn't designed for those very low temperatures the boots would start dropping in temperature over time and so would their feet.  Eventually it would cause frostbite though I do not think hypothermia since the rest of the suit would keep the astronaut relatively comfortable.  The suits had cooling systems on them to keep them comfortable which would likely need to be turned off sooner with such weak sunlight.  In fact since conduction works by physical contact if they switched from one foot to the other or better jumped periodically in 0.063 g (~1/15 Earth's gravity) their feet would spend far less time on the ground to reach the very low temperatures.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The degassing N2 and other volatiles would definitely occur, their boots would be at a much higher temperature having come from the room temperature conditions of the inside of the space craft.  Unless of course the exhaust from the landing itself had already cleared the nearby area of any low boiling point volatiles.   &lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorDisplayName="user35272" LastEditDate="2020-04-10T20:34:35.220" LastActivityDate="2020-04-10T20:34:35.220" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23343" PostTypeId="2" ParentId="23337" CreationDate="2017-10-11T23:29:52.250" Score="3" Body="&lt;p&gt;Yes, and yes. The standard Doppler data type on deep-space spacecraft is X-band to X-band, and transmit and receive are on the same antenna. Most deep-space spacecraft only &lt;em&gt;have&lt;/em&gt; X-band transceivers. A few have more than one band, usually for either radio science reasons and/or higher data rate on Ka (accepting some added risk of weather interference). S-band is passé.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-10-11T23:29:52.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23344" PostTypeId="1" CreationDate="2017-10-12T00:38:01.523" Score="7" ViewCount="1734" Body="&lt;p&gt;Just after T+3 minutes during today's SES-11 launch I noticed a small chunk fall off the end of a &quot;boom&quot; on the Falcon 9 second stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/iv1zeGSvhIw?start=1022&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qs6e3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qs6e3.jpg&quot; alt=&quot;Video frames&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What's the boom and what came off?  Just a chunk of ice?&lt;/p&gt;&#xA;" OwnerUserId="21328" LastEditorUserId="21328" LastEditDate="2017-10-19T05:41:51.640" LastActivityDate="2020-04-10T23:56:38.917" Title="What's this chunk that fell off Falcon 9 second stage?" Tags="|spacex|falcon-9|debris|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23345" PostTypeId="2" ParentId="23336" CreationDate="2017-10-12T00:50:37.227" Score="4" Body="&lt;p&gt;Three-way Doppler tracking was developed for when the spacecraft (specifically Voyager) is so danged far away, that by the time the round trip signal gets back to Earth the original transmitting complex has rotated out of view. Then the signal is being received at a different complex. Then there needs to be a third link between the two complexes for a phase reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to confuse things, I've seen the same term referring to something else entirely: two-way Doppler on one complex simultaneous with one-way Doppler to another complex, e.g. while handing off from one complex to another during the overlap.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;https://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201B.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201B.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YwhNH.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YwhNH.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="12102" LastEditDate="2017-10-12T04:07:24.493" LastActivityDate="2017-10-12T04:07:24.493" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23346" PostTypeId="2" ParentId="23344" CreationDate="2017-10-12T01:37:05.163" Score="6" Body="&lt;p&gt;It's hard to say for sure, but here's what I suspect happened:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The small boom was the deployment of the grid fins. It happened within a few seconds, and it's not uncommon for audio to be off for such items. Note there was a small amount of applause right after that boom, indicating it was something expected.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The chunk was certainly something small and inconsequential. It can be seen that it comes from some kind of a small pipe near the bell. I believe it is the Nozzle Fuel Return Line, seen in the below diagram. It could also be a feed line that takes the fuel and pre-warms it before entering the ignition chamber.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/e8uV4.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/e8uV4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, it is almost certainly a small chunk of ice. It seems to completely disappear when it enters the exhaust, likely sublimating completely.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-10-12T01:37:05.163" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23347" PostTypeId="1" AcceptedAnswerId="33174" CreationDate="2017-10-12T02:04:38.223" Score="17" ViewCount="1663" Body="&lt;p&gt;SpaceX has re-used a first stage once, several times now. Has it re-used the same first stage twice yet? If not, has a particular mission been named that will be the first second re-use?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-13T00:19:05.583" LastActivityDate="2021-09-13T00:19:05.583" Title="Has SpaceX re-used a first stage twice yet?" Tags="|spacex|falcon-9|reuse|launch-vehicle|spaceflight-firsts|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23348" PostTypeId="1" CreationDate="2017-10-12T02:38:53.747" Score="2" ViewCount="3141" Body="&lt;p&gt;It is said that we do not have such advanced technology in order to send a spacecraft out of the solar system. (Source: &lt;em&gt;&lt;a href=&quot;http://www.sunherald.com/latest-news/6ncb1u/picture70280707/alternates/FREE_640/0407_BILO_Greenhouse%201.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Everyone around&lt;/a&gt;&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When a spacecraft is launched from Earth and aims to leave the solar system, it would get slowed down due to the Sun's gravitational pull. And at some point, it will have a speed of 0 and would start falling into the Sun (if it only uses fuel to propel itself).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if an object is traveling faster than the escape velocity at that certain location, then it would end up escaping from the gravitational pull at some point, but its speed would get slowed down. $v_\infty=\sqrt{V^2-v_e^2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbital velocities of planets in the solar system are used to gain speed in interstellar trips. The voyagers are good examples, they both used Jupiter for a slingshot so they could boost their speed in order to overcome escape velocity at that certain location and be able to leave the solar system. (Source: &lt;em&gt;&lt;a href=&quot;https://solarsystem.nasa.gov/basics/primer&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most interstellar ships (so far) use Jupiter for gravitational slingshots. Would humans be able to launch a &lt;strong&gt;spacecraft&lt;/strong&gt; and make it go beyond Jupiter or if possible leave the solar system without making any gravitational assist?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Such spacecraft has an IMPORTANT launch mass, it is not like the New Horizons which was about 500kg. A big rocket such as the Apollo 11 has a launch mass of 45,000kg. (Source: &lt;em&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo11&lt;/a&gt;&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How far would it go beyond Jupiter, could it reach Saturn or go even farther?&lt;/p&gt;&#xA;" OwnerUserId="20563" LastEditorUserId="25" LastEditDate="2017-10-12T18:14:53.653" LastActivityDate="2018-01-12T20:49:06.557" Title="Would it be possible to go beyond Jupiter without making any slingshot?" Tags="|rockets|trajectory|gravity-assist|escape-velocity|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23349" PostTypeId="2" ParentId="23347" CreationDate="2017-10-12T02:41:39.043" Score="12" Body="&lt;p&gt;No. The cores used are tracked at &lt;a href=&quot;https://www.reddit.com/r/spacex/wiki/cores&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reddit&lt;/a&gt;, among other places. There are no plans to reuse a booster for a third time. There are 3 booster now that have been reused&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;B1021- Will be displayed at cape, retired.&lt;/li&gt;&#xA;&lt;li&gt;B1029- Likely mothballed.&lt;/li&gt;&#xA;&lt;li&gt;B1031- This one just landed today, not sure of it's fate yet.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It will likely be a while before a second reuse is done, but they are slowly inching forward in that direction. I would guess one of the Block IVs, maybe B1042.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current plan is to only reuse more then once the Block 5s. Currently SpaceX is working on expending all of the Block 3 and 4s in existence, to focus on Block 5, for a number of reasons. One of the most important is NASA wants to use Block 5 for the crewed missions, and needs 7 successful missions in a row before they will allow it to be used for manned missions.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-04-03T12:57:32.830" LastActivityDate="2018-04-03T12:57:32.830" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23350" PostTypeId="2" ParentId="23348" CreationDate="2017-10-12T02:52:49.963" Score="18" Body="&lt;p&gt;It is absolutely possible, just not advised. New Horizons was &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;launched at Solar System Escape Velocity&lt;/a&gt;, meaning it could have visited anywhere beyond Earth without stopping. It did visit Jupiter, however, that was to allow it to leave even faster, the Jupiter stop was purely optional.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the issue of human capable spacecraft, again, it could be done, if one is willing to use a large enough rocket. The proposed BFR could certainly launch a satellite in to Earth orbit that could leave the solar system without a gravity assist required. The Delta V requirements for such can be see at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Interplanetary&quot; rel=&quot;nofollow noreferrer&quot;&gt;Delta-V Budget Wikipedia stage&lt;/a&gt;. It requires 8.8 km/s from Earth orbit to flyby Jupiter. It requires 12.3 km/s to escape the Sun's gravity. So it saves quite a bit of delta-V, which allows for more payload and/or quicker times, but overall it can be done. But it hardly makes sense to talk about outer solar system manned spacecraft when we haven't even sent a manned mission to Mars.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-10-12T18:20:13.027" LastActivityDate="2017-10-12T18:20:13.027" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23351" PostTypeId="2" ParentId="23170" CreationDate="2017-10-12T08:33:54.290" Score="6" Body="&lt;p&gt;There's probably a simpler way to do it, but here is how you can obtain these equations from the basics. Let's start with equations in Cartesian coordinates:&#xA;\begin{align}&#xA;\dot{x} &amp;amp; = v_x;\\&#xA;\dot{y} &amp;amp; = v_y;\\&#xA;\dot{v_x} &amp;amp; = \frac{F_x}{m};\\&#xA;\dot{v_y} &amp;amp; = \frac{F_y}{m}.&#xA;\end{align}&#xA;The relations between Cartesian and the polar coordinates are&#xA;\begin{align}&#xA;x &amp;amp; = r\cos\theta;\\&#xA;y &amp;amp; = r\sin\theta,&#xA;\end{align}&#xA;or&#xA;\begin{align}&#xA;r &amp;amp; = \sqrt{x^2 + y^2};\\&#xA;\theta &amp;amp; = \arctan \frac yx.&#xA;\end{align}&#xA;(Actually, this equation for $\theta$ is valid only if $x&amp;gt;0$, but let's assume that it is so. The final result is the same anyway.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides, in the polar coordinates, instead of $v_x$ and $v_y$, we would like to use $v_r$ and $v_\theta$, the projections of the velocity on the vertical and the horizontal axes. These axes are rotated by angle $\theta$ relative to $x$- and $y$-axis, so&#xA;\begin{align}&#xA;v_r &amp;amp; = v_x\cos\theta + v_y\sin\theta;\\&#xA;v_\theta &amp;amp; = v_y\cos\theta - v_x\sin\theta.&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Similarly, instead of $F_x$ and $F_y$, we would rather use the vertical and the horizontal components of the force:&#xA;\begin{align}&#xA;F_r &amp;amp; = F_x\cos\theta + F_y\sin\theta;\\&#xA;F_\theta &amp;amp; = F_y\cos\theta - F_x\sin\theta.&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking the derivative of the expressions for $r$ and $\theta$ by time, we get&#xA;\begin{align}&#xA;\dot{r} &amp;amp; = \frac{2\dot{x}x + 2\dot{y}y}{2\sqrt{x^2 + y^2}} = \frac{v_x r\cos\theta + v_y r\sin\theta}{r} = v_x\cos\theta + v_y\sin\theta = v_r;\\&#xA;\dot{\theta} &amp;amp; = \frac{\dot{y}x - \dot{x}y}{x^2}\cdot\frac{1}{1+(y/x)^2} = \frac{v_y\cos\theta - v_x\sin\theta}{r} = \frac{v_\theta}{r}.&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Taking the derivative of the expressions for $v_r$ and $v_\theta$ by time, we get&#xA;\begin{align}&#xA;\dot{v_r} &amp;amp; = \dot{v_x}\cos\theta - v_x\dot{\theta}\sin\theta + \dot{v_y}\sin\theta + v_y\dot{\theta}\cos\theta = \frac{F_x}{m}\cos\theta + \frac{F_y}{m}\sin\theta + \dot{\theta}(v_y\cos\theta - v_x\sin\theta) = \frac{F_r}{m} + \frac{v_\theta^2}{r};\\&#xA;\dot{v_\theta} &amp;amp; = \dot{v_y}\cos\theta - v_y\dot{\theta}\sin\theta - \dot{v_x}\sin\theta - v_x\dot{\theta}\cos\theta = \frac{F_y}{m}\cos\theta - \frac{F_x}{m}\sin\theta - \dot{\theta}(v_y\sin\theta + v_x\cos\theta) = \frac{F_\theta}{m} -\frac{v_\theta v_r}{r}.&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we leave out the thrust for now, then $F_r = -\frac{\mu m}{r^2}$, $F_\theta = 0$, and substituting these values, we get&#xA;\begin{align}&#xA;\dot{v_r} &amp;amp;= -\frac{\mu}{r^2} + \frac{v_\theta^2}{r};\\&#xA;\dot{v_\theta} &amp;amp; = -\frac{v_\theta v_r}{r}.&#xA;\end{align}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You say that $\beta$ is the angle between the velocity vector and the thrust vector; however, to obtain the formulas you have given we need to assume that $\beta$ is the angle between the horizontal positive direction and the thrust vector. Then $F_r = -\frac{\mu m}{r^2} + T\sin\beta$, $F_\theta = T\cos\beta$, and (substituting $m = m_0 - |\dot{m}|t$) we get the formulas in the original post.&lt;/p&gt;&#xA;" OwnerUserId="12448" LastActivityDate="2017-10-12T08:33:54.290" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23353" PostTypeId="2" ParentId="23348" CreationDate="2017-10-12T13:00:13.070" Score="2" Body="&lt;p&gt;With a light enough spacecraft and large enough rockets, it is possible to achieve solar escape velocity directly.  Currently this has only been done with the &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;nofollow noreferrer&quot;&gt;New Horizons&lt;/a&gt; probe.  So it's possible - just very hard to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hence the slingshot orbit, which gives you &quot;free&quot; acceleration without needing extra rocket fuel.  It's not that it's 100% required, simply that it's the most practical way of doing it.  An analogy would be whether you choose to walk along a smooth path around the outside of a mountain, or whether you go over the top - both will get you to the same place, and the smooth path covers a longer distance, but doing it the hard way is slower and takes a lot more effort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the surface of the Earth, the gravitational attraction of the Sun is &lt;a href=&quot;https://van.physics.illinois.edu/qa/listing.php?id=184&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.0006&lt;/a&gt; times Earth's gravity.  G=9.8N/kg, so this gives a gravitational attraction of approximately 6mN/kg.  An unpowered spacecraft which does not have solar escape velocity would eventually be pulled back towards the Sun unless there were other factors controlling its trajectory.  It would not necessarily fall &quot;into&quot; the Sun, but it would most likely end up on an elliptical orbit around the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However gravity is not the only game in town here.  The Sun produces a radiation pressure which allows &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar sails&lt;/a&gt; to propel a spacecraft.  Although this radiation pressure decreases with distance from the Sun, it decreases with an inverse-square characteristic, the same as gravity does.  A solar sail which can exceed gravity in Earth's orbit will therefore also exceed gravity for the remainder of its journey out of the solar system, and the spacecraft will always be accelerating away from the Sun.  Currently we have &lt;a href=&quot;https://en.wikipedia.org/wiki/IKAROS&quot; rel=&quot;nofollow noreferrer&quot;&gt;one successful solar sail craft&lt;/a&gt; demonstrating that this theory does actually work in practise, but we have not yet had a serious spacecraft using this as a primary means of propulsion.  It's still worth mentioning though.&lt;/p&gt;&#xA;" OwnerUserId="15976" LastEditorUserId="15976" LastEditDate="2017-10-13T10:17:36.973" LastActivityDate="2017-10-13T10:17:36.973" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23354" PostTypeId="1" AcceptedAnswerId="23356" CreationDate="2017-10-12T14:29:33.153" Score="1" ViewCount="142" Body="&lt;p&gt;How long would it take to orbit the earth at a medium altitude (with equation). I need a equation that would help me figure out how long it would take to orbit at that altitude.&lt;/p&gt;&#xA;" OwnerUserId="21332" LastActivityDate="2017-10-12T16:45:09.400" Title="Apollo 11 CSM spacecraft orbit" Tags="|orbital-maneuver|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23355" PostTypeId="2" ParentId="23335" CreationDate="2017-10-12T15:47:53.030" Score="2" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Since I'm also the OP, after accepting @BobJacobsen's answer I've slightly revised the question to allow for another, &lt;a href=&quot;https://space.stackexchange.com/a/26389/12102&quot;&gt;very informative answer&lt;/a&gt;, which addresses testing of a higher frequency band through the Earth's atmosphere, though the testing was done from LEO and not deep space. This answer here is about the highest frequency used or tested &lt;em&gt;from deep space&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I'll post this as a tentative answer. I'm not 100% sure a higher frequency hasn't been tested but &lt;strong&gt;it looks like the Deep Space Ka-band frequencies around 32 GHz is the highest that's received substantial, systematic testing and use&lt;/strong&gt; in deep-space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should also point out that the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transiting_Exoplanet_Survey_Satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;Transiting Exoplanet Survey Satellite&lt;/a&gt; (TESS) will be using Ka-band from cis-lunar space (roughly speaking) as discussed in the question &lt;a href=&quot;https://space.stackexchange.com/questions/21770/will-there-be-near-space-ka-band-allocations-for-tess&quot;&gt;Will there be “Near Space” Ka band allocations for TESS?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the DSN Telecommunications Link Design Handbook &lt;a href=&quot;https://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201B.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;201, Rev. B, Frequency and Channel Assignments, Released December 15, 2009&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pK9lD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pK9lD.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should not that all of these seem to be downlink. I haven't found any corresponding Ka-band uplink information.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Kepler:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler_(spacecraft)#Operations&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler Telescope&lt;/a&gt; has been using Ka-band to downlink processed data regularly over the distance of roughy 1 AU.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Mars Reconnaissance Orbiter (MRO):&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;According to the abstract of this paper &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.499.9247&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Reconnaissance Orbiter Ka-band (32 GHz) Demonstration: Cruise Phase Operations&lt;/a&gt; (also this &lt;a href=&quot;http://csse.usc.edu/GSAW/gsaw2006/s10d/shambayati.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;presentation&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Among the highlights of these was setting a single day record for data return from a deep space spacecraft (133 Gbits) achieved during one 10-hour pass; achieving the highest data rate ever from a planetary mission (6 Mbps) and successfully demonstrating Ka-band DDOR.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm not sure of the distances, the experiments seem to have been done during the cruise-phase of the mission before orbital insertion.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Juno:&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The Ka-band transmissions from the Juno Spacecraft have been systematically studied for an extended period of time. In the IPN Progress Report &lt;a href=&quot;https://ipnpr.jpl.nasa.gov/progress_report/42-208/208B.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cassini Downlink Ka-Band Carrier Signal Analysis&lt;/a&gt;. Here's the complete abstract, there is plenty to read in this report:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Lower frequency telemetry bands are becoming more limited in bandwidth with more competition between flight projects and other entities. Higher frequency bands offer significantly more bandwidth and hence the prospect of much higher data rates. Future or prospective flight projects considering Ka-band (32-GHz) telemetry data links are interested in past flight experience with received Ka-band data. Over 10 years of Cassini closed-loop received Ka-band carrier data involving over 2 million individual measurements were acquired at all three Deep Space Network (DSN) sites. We analyzed these data to characterize link performance over a wide range of weather conditions and as a function of elevation angle. Based on this analysis, we have derived a recommendation for telecommunications&#xA;  link margin for preflight planning purposes. These results suggest that a 4-dB margin will ensure a ~94 percent data return at a minimum 20-deg elevation angle under 90 percent weather conditions at 32 GHz (Ka-band).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-09-01T08:03:57.707" LastActivityDate="2018-09-01T08:03:57.707" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23356" PostTypeId="2" ParentId="23354" CreationDate="2017-10-12T16:09:48.250" Score="3" Body="&lt;p&gt;According to wikipedia, the equation for &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_period#Small_body_orbiting_a_central_body&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital period of a small body&lt;/a&gt; is $$T = 2\pi\sqrt{\frac{a^3}{\mu}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where $µ$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational parameter&lt;/a&gt; of the body you're orbiting -- the product of the mass of the body and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational constant&lt;/a&gt; $G$ -- and $a$ is the length of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Semi-major_and_semi-minor_axes&quot; rel=&quot;nofollow noreferrer&quot;&gt;semi-major axis&lt;/a&gt; of the orbit, that is, the radius of &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth&lt;/a&gt; plus the altitude of the orbit above the Earth's surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Earth, $µ$ is $3.986004419×10^{14} m^{3}/s^{2}$. For the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISS&lt;/a&gt; orbiting at around 400km altitude, $a$ is $6371000 + 400000 = 6771000 m$. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Make sure to be careful of the units; the $µ$ figure I've given assumes meters, so you need the radius and altitude to be in meters as well, with the result in seconds. Low Earth orbits (100km to 2000km) should yield periods from about 90 minutes to 2 hours. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-10-12T16:45:09.400" LastActivityDate="2017-10-12T16:45:09.400" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23357" PostTypeId="1" CreationDate="2017-10-12T16:47:04.330" Score="2" ViewCount="343" Body="&lt;p&gt;I wonder if there are any calculations that estimate the percentage of star systems with planets that potentially can support life?&lt;/p&gt;&#xA;" OwnerUserId="21333" LastActivityDate="2017-10-12T23:46:39.890" Title="What percent of star systems has planets that could potentially support life?" Tags="|life|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23358" PostTypeId="2" ParentId="22811" CreationDate="2017-10-12T17:11:50.837" Score="2" Body="&lt;p&gt;You could go around planets in order to gain in speed. This is called gravitational assist or the slingshot effect, where the orbital speed of the moving planet boosts up your final velocity and modifies your direction.&lt;/p&gt;&#xA;" OwnerUserId="21335" LastActivityDate="2017-10-12T17:11:50.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23359" PostTypeId="2" ParentId="23295" CreationDate="2017-10-12T17:28:42.460" Score="2" Body="&lt;p&gt;We need to have in mind the term relative velocity. Where Vi is Initial relative velocity and Vf is final relative velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vi = V1 + U&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vf = -V2 + U&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since Vi = -Vf then V1 + U = V2 - U ; after we do some simple algebra we get that V2 = V1 + 2U&#xA;Where V2 is the final velocity of the space shuttle, V1 is the initial velocity and U is the orbital velocity of the planet that the space shuttle has approached to do a slingshot.  &lt;/p&gt;&#xA;" OwnerUserId="21335" LastActivityDate="2017-10-12T17:28:42.460" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23361" PostTypeId="1" AcceptedAnswerId="23364" CreationDate="2017-10-12T20:34:16.803" Score="13" ViewCount="3030" Body="&lt;p&gt;I'm curious as to what rocket engine in a launched rocket burned the longest in a continuous fashion.  I'm not interested in testing burns or inter-planetary engines which are designed for low-thrust long-duration use (such as ion engines), but rather launched chemical-powered engines which require an oxidizer of some sort to allow burning of the fuel (whether hydrogen, RP1, solid fuel, etc.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In doing some casual searching, I've pulled together the following numbers for well-known launches:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Saturn V first stage: ~ 168 seconds&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 first stage: ~ 170 seconds&lt;/li&gt;&#xA;&lt;li&gt;Space Shuttle deorbit burn: ~ 240 seconds&lt;/li&gt;&#xA;&lt;li&gt;Friendship 7: ~ 300 seconds&lt;/li&gt;&#xA;&lt;li&gt;Saturn V third stage: ~ 347 seconds&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9 second stage: ~ 376 seconds&lt;/li&gt;&#xA;&lt;li&gt;Saturn V second stage: ~ 384 seconds&lt;/li&gt;&#xA;&lt;li&gt;Energia: ~ 500 seconds&lt;/li&gt;&#xA;&lt;li&gt;Space Shuttle Main Engines: ~ 513 seconds&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Based on these numbers it looks like the Space Shuttle main engines (then Energia) are the ones on my list which have the longest burn time.  I would attribute this to their continuous burn from ground launch to orbit, where most other rockets are multi-stage.  Am I missing other rockets which had a longer continuous burn time, whether from a ground launch, or in-orbit?&lt;/p&gt;&#xA;" OwnerUserId="16735" LastEditorUserId="21562" LastEditDate="2020-12-10T19:31:53.543" LastActivityDate="2021-03-15T18:57:49.650" Title="Longest continuous burning chemical rocket engine?" Tags="|rockets|fuel|record|burn|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23362" PostTypeId="1" AcceptedAnswerId="23366" CreationDate="2017-10-12T21:01:14.993" Score="17" ViewCount="2760" Body="&lt;p&gt;A recent &lt;a href=&quot;https://space.stackexchange.com/a/23350/103&quot;&gt;answer&lt;/a&gt; by PearsonArtPhoto claims:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;New Horizons was launched at Solar System Escape Velocity, meaning it could have visited anywhere beyond Earth without stopping. It did visit Jupiter, however, that was to allow it to leave even faster, the Jupiter stop was purely optional.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;In one hand, this operation shortened the wait time, reduced risk associated with deterioration of the probe over time, and reduced cost of upkeep of the mission (through making it shorter).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, it made the flyby of Pluto awfully fast and short, resulting in only so much data from the brief closest approach. A slower flyby would give the probe more time to collect and send data. Slow enough, it might even allow for a capture into Pluto orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Weighing costs and benefits - how much longer would skipping the Jupiter assist make the period to reach Pluto? And how much slower would the flyby be?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-10-15T12:51:43.977" Title="How much longer would New Horizons take to reach Pluto without the Jupiter slingshot?" Tags="|jupiter|new-horizons|gravity-assist|pluto|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23363" PostTypeId="2" ParentId="23361" CreationDate="2017-10-12T21:16:32.933" Score="6" Body="&lt;p&gt;I'm going to go with the SSME, specifically SSME serial numbers 2020 and 2021. These were the engines installed in the left and right positions on Challenger for mission STS-51F.&lt;/p&gt;&#xA;&lt;p&gt;When the center engine failed at 5:43 after liftoff, the other two engines had to burn longer to make it to the MECO (Main Engine Cutoff) target.  So they burned for ~ 587 seconds (MECO was 9:41 and the engines started ~ 6 seconds before liftoff).&lt;/p&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://ntrs.nasa.gov/api/citations/20110001406/downloads/20110001406.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Shuttle Missions Summary&lt;/a&gt;, page 2-20.&lt;/p&gt;&#xA;&lt;p&gt;h/t to &lt;a href=&quot;https://space.stackexchange.com/users/32044/ikrase&quot;&gt;ikrase&lt;/a&gt; who asked in a comment about ground testing: In August 1988 an SSME was fired on the ground for 2017 seconds; over 33 minutes.&lt;/p&gt;&#xA;&lt;p&gt;Source: &lt;a href=&quot;http://history.nasa.gov/presrep1988.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;1988 Aeronautics and Space Report of the President&lt;/a&gt; p. 25&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2021-03-15T14:44:34.117" LastActivityDate="2021-03-15T14:44:34.117" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23364" PostTypeId="2" ParentId="23361" CreationDate="2017-10-12T22:04:45.690" Score="14" Body="&lt;p&gt;As @OON mentions, some small spacecraft maneuvering engines do really long burns. Juno's orbital insertion burn at Jupiter is 35 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are some notable launcher/bus stage burns:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ariane 5 upper stages &lt;a href=&quot;https://spaceflightnow.com/2017/06/01/launch-timeline-for-ariane-5s-flight-with-viasat-2-and-eutelsat-172b/&quot; rel=&quot;noreferrer&quot;&gt;routinely burn for 16 minutes&lt;/a&gt; for GTO insertion missions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't find a precise timeline for the New Horizons launch; that would be a likely candidate for a really long Centaur burn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ExoMars 2016, &lt;a href=&quot;http://spaceflight101.com/exomars/exomars-2016-launch-timeline/&quot; rel=&quot;noreferrer&quot;&gt;Briz-M second burn was ~18 minutes&lt;/a&gt;, in addition to ~5, ~12, and ~11 minute burns.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-10-13T04:20:12.600" LastActivityDate="2017-10-13T04:20:12.600" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23365" PostTypeId="2" ParentId="23357" CreationDate="2017-10-12T23:46:39.890" Score="5" Body="&lt;p&gt;We don't have a good estimate of this yet. Our ability to detect planets around other stars is very limited, and biased toward finding very large planets in very close orbits around stars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drake's &lt;a href=&quot;https://en.wikipedia.org/wiki/Drake_equation#Original_estimates&quot; rel=&quot;noreferrer&quot;&gt;original estimate&lt;/a&gt; was that a star with planets would average between one and 5 planets capable of developing life, so the percentage would be quite high. &lt;a href=&quot;https://en.wikipedia.org/wiki/Drake_equation#Average_number_of_planets_per_star_having_planets_that_might_support_life.2C_ne&quot; rel=&quot;noreferrer&quot;&gt;Other estimates range as low as ~40%.&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-10-12T23:46:39.890" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23366" PostTypeId="2" ParentId="23362" CreationDate="2017-10-13T00:37:28.027" Score="24" Body="&lt;p&gt;It was said in &lt;a href=&quot;http://pluto.jhuapl.edu/news_center/news/022807.php&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; that the time to reach Pluto was shortened by 3 years. It's also said that after the Jupiter flyby the probe gained ~ 4 km/s accelerating to the speed of 23 km/s relative to the Sun. We can use simple Keplerian estimate (ignoring all complexities of the actual orbital mechanics) to obtain the speed when the probe approaches Pluto,&#xA;\begin{equation}&#xA;\frac{v_1^2}{2}-\frac{\mu}{R_1}=\frac{v_2^2}{2}-\frac{\mu}{R_2}&#xA;\end{equation}&#xA;That gives ~8 km/s without flyby and ~15 km/s with flyby. This simple estimate doesn't tell you what the relative velocity to the Pluto would be without the Jupiter flyby but considering it's current orbital velocity of ~5.5 km/s and escape velocity ~1.2 km/s one may safely assume that even in the best case scenario it would still be ~4 km/s instead of ~13.8 km/s, so we talk about 3 times longer Pluto flyby at best.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To orbit the Pluto without the Jupiter flyby the probe would still have to perform an insertion maneveur losing even in the best case ~3 km/s whereas it had only 290 m/s delta-v budget. Of course the probe could be launched in such a way that it would approach Pluto at very low relative speed but then it's orbital period would be about half of the orbital period of Pluto, so we talk about ~50 year journey to Pluto. The orbiter should have been a specially designed for that mission, much heavier because of the booster required for insertion and therefore would require a different launch rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another point is that the Pluto orbit has rather significant inclination. Much of the delta-v at the launch would be spent to put the probe on sufficiently inclined orbit resulting in much lower speed than that naive estimate gives. The Jupiter flyby inclined its orbit for free.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The higher speed also means that the New Horizons will be able to reach other Kuiper belt objects in much shorter time. For example it's expected to pass by just 3500 km from (486958) 2014 MU$_{69}$ already in 2019.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And of course Jupiter is interesting by itself. Its flyby allowed to obtain some extra scientific data.&lt;/p&gt;&#xA;" OwnerUserId="18138" LastActivityDate="2017-10-13T00:37:28.027" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23367" PostTypeId="2" ParentId="23362" CreationDate="2017-10-13T01:51:52.647" Score="14" Body="&lt;p&gt;Depending on the time of launch, there were 4 different major plans for the NH mission profile. The first 3 involved Jupiter flybys, and would have an arrival date of 2015, 2016, or 2017. The last was a direct flyby, which I believe would have been 2019-2020, per &lt;a href=&quot;https://www.nasa.gov/home/hqnews/2005/dec/HQ_05550_New_Horizons.html&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; (. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the scientific value, going slower through the system would have allowed for a few more photographs, at more risk to the spacecraft and missing the Jupiter flyby. But the biggest thing is something that actually proved to not be a worry, but only after the mission launched. The thought when NH launched was that Pluto's atmosphere &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Pluto#Seasonal_changes&quot; rel=&quot;noreferrer&quot;&gt;would freeze&lt;/a&gt;. Pluto is actually near its closest approach to the Sun, which was in 1999, and it was thought when it launched that the atmosphere would freeze as it got further from the sun. The earlier timeline had more of a chance of seeing that atmosphere, along with a clearer landscape (Not covered in frozen nitrogen). Further modeling determined this wasn't likely, but only in 2014. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the Jupiter flyby approach path was the best path. The benefit from a slower approach would have been negligible, and the scientific bounty was much higher to do the Jupiter flyby.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-10-15T12:51:43.977" LastActivityDate="2017-10-15T12:51:43.977" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23369" PostTypeId="1" CreationDate="2017-10-13T15:20:52.550" Score="6" ViewCount="2966" Body="&lt;p&gt;Voyager launched in the 70s. At that time, modern CCD or CMOS image sensor may not have been available.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how does Voyager take pictures in space, digitize the image and send it back to us?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1977-084A-01&quot; rel=&quot;noreferrer&quot;&gt;The details of Voyager's cameras don't seem to be public at NASA&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="21340" LastEditorUserId="6816" LastEditDate="2017-10-13T15:49:23.080" LastActivityDate="2017-10-14T08:06:22.443" Title="How does Voyager take pictures in space?" Tags="|voyager|camera|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23370" PostTypeId="2" ParentId="23369" CreationDate="2017-10-13T16:46:03.573" Score="10" Body="&lt;p&gt;Voyager's camera used a &lt;a href=&quot;https://en.wikipedia.org/wiki/Video_camera_tube#Vidicon&quot; rel=&quot;noreferrer&quot;&gt;vidicon tube&lt;/a&gt;, which is sort of a vacuum tube precursor to a CCD. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't 100% understand the principle, but the basic idea is that the image is captured on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Photoconductivity&quot; rel=&quot;noreferrer&quot;&gt;photoconductive&lt;/a&gt; surface, thus converting it to a pattern of electrical charge; an electron beam is &lt;a href=&quot;https://en.wikipedia.org/wiki/Raster_scan&quot; rel=&quot;noreferrer&quot;&gt;raster-scanned&lt;/a&gt; across the photoconductor producing a varying voltage out of the system, which can be amplified and converted to a digital data stream. It's something like a television operating in reverse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get color, you take multiple pictures with different filters in front of the camera for each one. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-10-14T08:06:22.443" LastActivityDate="2017-10-14T08:06:22.443" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23371" PostTypeId="1" CreationDate="2017-10-13T17:32:36.393" Score="4" ViewCount="311" Body="&lt;p&gt;Corona discharges were observed on the Corona satellites which carried film cameras, negatively affecting the image quality.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was it a coincidence that the name of this phenomenon is the same as the series of photography satellites where it used to occur?&lt;/p&gt;&#xA;" OwnerUserId="19223" LastActivityDate="2017-10-15T08:44:38.370" Title="Does the name &quot;corona discharge&quot; bear any relation to the Corona surveillance satellites or is this a coincidence?" Tags="|artificial-satellite|reconnaissance-satellite|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23372" PostTypeId="2" ParentId="23371" CreationDate="2017-10-13T17:52:21.537" Score="5" Body="&lt;p&gt;It was a coincidence. The program was named Corona before the launch of the first satellite, so before the corona effect was ever seen on the recovered films.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Secret projects like the Corona program usually have a deliberately nondescriptive name. Apparently, the name Corona was derived from the &lt;a href=&quot;http://www.laserfocusworld.com/articles/print/volume-40/issue-3/features/back-to-basics-optical-surveillance-satellites/surveillance-satellites-adapt-to-a-changing-world.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;typewriter company&lt;/a&gt;. The contemporary CIA project to replace the U-2 airplane was named Oxcart. In the UK, this practice was standardized as &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_Rainbow_Codes&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rainbow codes&lt;/a&gt;, where two lists of random words were combined, leading to e.g. &quot;Green cheese&quot; (a missile with nuclear warhead). &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-10-15T08:44:38.370" LastActivityDate="2017-10-15T08:44:38.370" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23373" PostTypeId="2" ParentId="22811" CreationDate="2017-10-14T02:10:00.577" Score="1" Body="&lt;p&gt;In addition to solar sails already mentioned, you could also shine a strong laser on a solar sail from the point of departure ( or any other point in space, really ), thus being able to go in a different direction than Solar pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also depending on if there is a magnetosphere nearby, electrodynamic tethers work basically in a similar way as an electric motor does. Magnetic torque rods are routinely used for attitude control on small spacecraft, but the principle can be used also to gain kinetic energy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Generally, most spacecraft propulsion methods are listed in &lt;a href=&quot;https://en.wikipedia.org/wiki/Spacecraft_propulsion#Table_of_methods&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3140" LastActivityDate="2017-10-14T02:10:00.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23374" PostTypeId="1" AcceptedAnswerId="23381" CreationDate="2017-10-14T06:09:25.160" Score="19" ViewCount="4889" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_15&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, Luna 15 crashed a few hours before Eagle lifted off from the moon, and since it crashed North-east of the landing site, it must have overflown them. Would any of the crew have been able to see Luna 15, perhaps the deorbit burn or debris ejected into space from the crash itself?&lt;/p&gt;&#xA;" OwnerUserId="19872" LastEditorUserId="19872" LastEditDate="2017-10-14T22:50:00.873" LastActivityDate="2020-04-30T04:24:57.547" Title="Would Luna 15 have been visible to any of the members of the Apollo 11 crew?" Tags="|the-moon|crewed-spaceflight|apollo-11|luna|" AnswerCount="5" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23375" PostTypeId="2" ParentId="23374" CreationDate="2017-10-14T06:27:27.420" Score="20" Body="&lt;p&gt;It's unlikely. &lt;a href=&quot;https://space.stackexchange.com/questions/10342/were-the-apollo-11-astronauts-aware-of-any-parts-of-the-luna-15-mission&quot;&gt;They made no report of seeing it, although it was mentioned as a news item at several points during the mission.&lt;/a&gt; The sky is big, the moon's surface is big, rockets are fast, hypergolic propellant rocket plumes are very faint.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-10-14T08:06:55.933" LastActivityDate="2017-10-14T08:06:55.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23377" PostTypeId="2" ParentId="23374" CreationDate="2017-10-14T12:24:11.933" Score="11" Body="&lt;p&gt;Unlikely. Luna 15 left earth a little before Apollo 11 did.  Although the &quot;Cold War&quot; was in full swing with intense rivalry, NASA took the then very unusual step of asking its enemy the Soviet Union if Luna 15 was going to interfere with the Apollo 11 trajectory.  The Soviets, equally unusually for that time, replied it would not. Possibly the recordings of the Apollo 11 mission and the news items that went with that, will confirm what I recall from being there and hearing that mission &quot;live&quot;, that the astronauts reported that they never saw the Luna 15 at any time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So of course my reference for the above information is &quot;me&quot;, young adult alive at the time. &lt;/p&gt;&#xA;" OwnerUserId="21344" LastActivityDate="2017-10-14T12:24:11.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23378" PostTypeId="2" ParentId="8453" CreationDate="2017-10-14T13:22:18.057" Score="18" Body="&lt;p&gt;For whatever reasons and before the Apollo 16 flight, NASA had already made a rule limiting the farthest distance that the rover could be from the LM, to five miles.  Apollo 16 astronauts tried to persuade NASA that should the rover break down, they could get back to the LM from a distance of 10 miles, and demonstrated the fact on earth at a NASA facility by jogging 10 miles in simulated one-sixth gravity and wearing their space suits.  NASA wouldn't budge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Duke, C. &amp;amp; D., &lt;em&gt;Moonwalker&lt;/em&gt;. Thomas Nelson Publishers, Nashville, Tennessee. 1990. pp.148-149)&lt;/p&gt;&#xA;" OwnerUserId="21344" LastActivityDate="2017-10-14T13:22:18.057" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23379" PostTypeId="2" ParentId="23371" CreationDate="2017-10-14T13:46:30.403" Score="1" Body="&lt;p&gt;Word &quot;corona&quot; evolved from Latin , which is same as Crown in English. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even I can't give any reference, as far as I know corona discharge of Sun and spy satellite project both having common name 'corona' is not intentional. But both naming was targeted for meaning Crown. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Corona discharge is bearing this name for beautiful shining compare with crown.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellite project corona took name from model of typewriter sitting on desk during name selection. This project was considered as having greater power over something and naming it with something like crown seem well justified.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Two separate events, unique in own, crown reference is perfect and both have no relation in term of naming connection...&lt;/p&gt;&#xA;" OwnerUserId="18788" LastEditorUserId="18788" LastEditDate="2017-10-15T07:51:40.260" LastActivityDate="2017-10-15T07:51:40.260" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23381" PostTypeId="2" ParentId="23374" CreationDate="2017-10-14T14:15:59.387" Score="17" Body="&lt;p&gt;On Earth, most of visibility of objects coming from space is created by interaction with the atmosphere. As Moon has no atmosphere, an approaching space object would appear in a similar way to how on Earth we perceive orbiting objects: light reflected from the Sun at best. That is, under conditions that the object is large, reflective and properly positioned relative to the Sun and the observer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can't notice Soyuz boosting ISS so the deorbiting burn would be similarly unnoticeable. As for ejected debris, without an atmosphere to burn in, they were small and some of them charred, so even less visible than Luna in one piece.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo crew would be able to notice Luna 15 only if they knew when and where exactly to look for it. Even then, if the Sun was in &quot;wrong&quot; position, they'd have very little chance of seeing it coming.&lt;/p&gt;&#xA;" OwnerUserId="16410" LastActivityDate="2017-10-14T14:15:59.387" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23382" PostTypeId="2" ParentId="1691" CreationDate="2017-10-14T14:29:43.950" Score="3" Body="&lt;p&gt;It was well publicised on radio, television, newspapers and manybooks on the subject in 1969 when I, as a young adult and along with most of the Western World, took in every bit of information about the Apollo 11 mission. That is one of my references: I was alive and heard it all happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the last minute or so before the landing, Buzz Aldrin was heard to say, &quot;Picking up &lt;em&gt;some&lt;/em&gt; dust&quot; (accent mine). This comment about &quot;some&quot; dust contrasts with Apollo 16 Charlie Duke's comment about the large amounts of dust his mission kicked up close to their landing, and why that should be so (this ref. below).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the Apollo 11 LM was on the moon and the astronauts outside the spacecraft, they reported to NASA a small scar in the lunar surface quite some distance from the LM. Given that they and the LM were the only &quot;entities&quot; that could have put the scar there, the told NASA about it and reasoned that the significant sideways motion of the LM  at landing meant that the 5 foot probe attached to a leg of the LM touched the surface and left the scar.  NASA concurred (I was hearing this live, BTW) And this fact is also relevant to the question: The scar was absolute proof that when the LM was 5 feet above the surface, it had quite horizontal distance to go before it touched down.  And the significance of that is, the purpose of the long probe was so that an accurate point close to the surface would be indicated and that was the point when mission rules demanded that the descent motor be turned off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Newtons Laws of Motion and lunar gravity then coasted the LM sideways into the landing.  This explains why there wasn't much dust over the landing pads (such dust as had been kicked up got &quot;left behind&quot;, falling vertically in a vacuum), and it also explains why there wasn't a deep blast hole under the descent engine exhaust (it wasn't firing then, by mission rules).  And this provides the real reason why the LM didn't &quot;blow the dust away&quot; - it can't if it isn't firing where and when the LM is on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Apollo 16 by contrast did kick up thick dust that visibility became difficult in the final landing phase (Duke, 1990, pp. 161-162).  Apollo 16 was landing vertically in its final phase (ibid.) so of course it was landing right into the dust it was kicking up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally, and obtusely related to the dust question, Charlie Duke's stated reason why the descent engine was turned off before landing (ibid.) was that if it was going and if the engine landed right where a rock was which blocked the engine, it could blow up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Duke, C. &amp;amp; D. &lt;em&gt;Moonwalker&lt;/em&gt;. Thomas Nelson Publishers, Nashville, Tennessee. 1990) &lt;/p&gt;&#xA;" OwnerUserId="21344" LastActivityDate="2017-10-14T14:29:43.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23383" PostTypeId="2" ParentId="9487" CreationDate="2017-10-14T15:17:27.857" Score="8" Body="&lt;p&gt;Yes, they could have been killed if a solar flare had erupted. My 1st-year university essay which I wrote in 2009 as a 64 year old mature student, refuting the dilly claims of the 2001 made-for-television documentary, &lt;em&gt;&quot;Conspiracy Theory: Did we land on the moon?&quot;&lt;/em&gt;, cites a reference revealing this. Unfortunately that website I referenced no longer exists.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have read other serious sources which reveal how fatal a strong solar flare could be, but naturally didn't keep their details because &quot;I already had a reference&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However you can read my essay which cites the information, at: &lt;a href=&quot;https://stanhoodnz.weebly.com/essay-1-apollo.html&quot; rel=&quot;noreferrer&quot;&gt;https://stanhoodnz.weebly.com/essay-1-apollo.html&lt;/a&gt;&#xA;The university gave me an A+ for it, so I guess you can trust my research of the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And anyway, even the idiots who made that conspiracy theory doco support the belief that a strong solar flare can be fatal. They claimed in the doco that the Apollo 16 trip was a lie because the astronauts would have been &quot;fried&quot; by the strongest solar flare of the century which happened while Apollo 16 was in space.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Er, yes and no.  The solar flare really did happen and it really was the strongest in the century, but it happened in August 1972, months after Apollo 16 was finished and months before Apollo 17 in December that year.  &lt;/p&gt;&#xA;" OwnerUserId="21344" LastActivityDate="2017-10-14T15:17:27.857" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23384" PostTypeId="2" ParentId="19707" CreationDate="2017-10-14T15:23:39.607" Score="1" Body="&lt;p&gt;Consider a huge, capped drum floating in space, rotating on its axis of radial symmetry. If you are free-floating inside it, no matter where you are, you experience no forces/accelerations due to the drum's rotation, because you are not moving with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, lets add a ladder attached to the inside of one of its end caps leading radially out from the center. At the &quot;foot&quot; of the ladder, you are standing on the interior surface of the drum and feel the artificial gravity created by the drum's rotation because you are moving with it. As you climb the ladder, you will feel lighter and lighter. While you may stop at any point and be &quot;stationary&quot; on the ladder, you are still moving with the drum's rotation, traveling along a circular path. When you get to the &quot;top&quot; (the center of the drum), you will feel no net acceleration (assuming your center of mass has arrived at the drum's axis of rotation). For the formulas to calculate the acceleration/force felt at any point up the ladder, see: &lt;a href=&quot;https://en.wikipedia.org/wiki/Acceleration#Circular_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Acceleration#Circular_motion&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-10-14T15:23:39.607" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23385" PostTypeId="2" ParentId="4325" CreationDate="2017-10-14T16:20:34.923" Score="1" Body="&lt;p&gt;I have mentioned in other answers that I was around as an adult when the Apollo programme was happening.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following was &quot;common knowledge' around the time of Apollo 11, as read in newspapers and books, so common I never thought that I would grow old enough to require references for what were &quot;current affairs&quot; for me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA was formed with the early part-reason to reduce the sometimes bitter inter-service rivalries over which service would or could put a satellite in orbit.  America needed all of its experts to work together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was always reported over the years when the Apollo programme was still current affairs for me, that Neil Armstrong was a civilian and that, alongside his technical abilities and academic training, was a prime reason why he was chosen to be the first man to set foot on the moon (and thus prevent an apparently ever-present worry of inter-service rivalry rearing its head sufficiently via &quot;bragging rights&quot; to compromise the high level of teamwork needed for NASA to operate efficiently).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I never heard of any other major reason why Neil Armstrong was chosen, and I did hear the &quot;civilian&quot; reason touted many times, for his choice among many others of equal competence. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, although not publicised as such as a factor for his being chosen for his role, Neil Armstrong was known at NASA to be cool under threat of imminent death. He was seen - and the live video of the crash probably still exists on youtube although I haven't looked for a while - bailing out about one second before the crash of the out-of-control LLRV (lunar landing research vehicle) he was flying, impacted the ground.  Later in the day, he apparently astonished his fellow astronauts by working quietly away at his normal tasks in his office like nothing had gone wrong with his day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question does raise the issue that by virtue of existing aeronautical tradition if nothing else, Buzz Aldrin should have been the lunar pilot and therefore should have been the first man to set foot on the moon.  But he was a military man from a military family, and that would have counted against him, nothing to do with ability to do the job. Many articles were written around late 1960s-early 1970s of his prolonged efforts to be the LM pilot and thus be the first man to walk on the moon.  Never once was his competency questioned, as I recall. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; Addition to clarify that I answered the question &quot;Why?&quot; Point taken that there is more than one &quot;why&quot; about the choice of Neil Armstrong in the pilot's place in the LM. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I did dwell, above, on the &quot;civilian&quot; aspect why Neil Armstrong was chosen to fly the LM.  It was consistently and widely publicised in international media.  Could every reporter have gotten it so wrong for decades, without picking up the phone to NASA or having NASA call the major news feeds and say &quot;you got the (civilian) reason wrong&quot;?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the question as put here here was fairly asked, and I was fairly reminded in a comment below, that my &quot;why&quot; answer, above, was maybe a bit limited in its breadth. Question: why was Neil Armstrong the LM pilot?  Answer, worded in a different way so it covers more aspects of &quot;why&quot;:  because Neil Armstrong was in the pilot position, because he was to be first out the spacecraft hatch, because he and Buzz Aldrin couldn't swap places when suited up in the LM therefore Neil had to be nearest the hatch, because both Buzz and Neil were competent fliers of the LM so the question of who was the better pilot didn't arise, and in this most critical procedure of all time - the first landing on another world with no chance of rescue if things went wrong - it didn't hurt to know down at NASA that Neil was the one who had tangibly displayed incredible coolness under life-threatening danger with the LLRV crash.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So that is why Neil Armstrong flew the LM when Buzz Aldrin was the LM pilot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I see that Deke Slayton has been quoted from his autobiography in a comment below, what he had felt in 1964 about how he chose astronauts. It's his autobio and one should respect that.  However, the international news reports of US military rivalries and the damaging effect of a lack of co-operation between the services on a matter of international prestige, dated back to shortly after 1957 and Sputnik 1 - long before Deke's memories of his feelings in 1964.  This was also a time of embarrassing American failures from different arms of the military around that time to get a &quot;face-saving&quot; satellite into orbit.  NASA had to demonstrate that there was no place for egos of any kind as it settled down and focused on doing a job well, with eventual awe-inspiring success. That &quot;unspoken&quot; awareness had to have been in the NASA &quot;psyche&quot;.  The plentiful evidence of history strongly suggests why Neil Armstrong was to be given the ultimate prestige of all time rather than a military man, as he went on that journey carrying President Nixon's message, &quot;We come in peace [not war] for all mankind.&quot;  &lt;/p&gt;&#xA;" OwnerUserId="21344" LastEditorUserId="21344" LastEditDate="2017-10-15T11:38:29.203" LastActivityDate="2017-10-15T11:38:29.203" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23387" PostTypeId="2" ParentId="10860" CreationDate="2017-10-14T18:29:32.183" Score="0" Body="&lt;h3&gt;(No-math answer)&lt;/h3&gt;&#xA;&lt;p&gt;You're falling around the earth at any altitude at any speed.  Even if you throw a ball, it is falling around the earth.  It just doesn't have enough velocity to keep from hitting it.  So the sweet spot is for an orbit that you travel far enough that the curvature of the earth is equal to how far you fell.  The closer you are the more gravity, the less distance you have to fall before you hit, the faster you have to go for the earth to curve away from / out of your fall.  The higher you are the slower you can go as the earth curves out of your way — less gravity.  This way you don't have to add any energy — you just keep falling.  At a certain altitude, your speed exactly matches the rotation of the earth.  This is great because we can point our satellite dish at it. If you want to be geosync at any other altitude, you can be — but you will need fuel/energy and a lot of it to do it and you won't be weightless.  You are only weightless because you are falling.  If there was a tower built up that high, you would stand on it with gravity just as you would down here.  A little less gravity — but still gravity.  Hence the falling.  You are weightless when you fall down here too.  You're just too worried about sticking the landing to notice.&lt;/p&gt;&#xA;" OwnerUserId="21348" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-15T02:34:37.677" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23389" PostTypeId="1" CreationDate="2017-10-15T01:31:49.817" Score="13" ViewCount="1487" Body="&lt;p&gt;Is there a limit to the size (mass, structural issues) of boosters that can be launched either to orbital or escape velocity? I am asking about such constraints as the required thrust to weight ratio with existing technology, square-cube scaling, and the ability of the vehicle to physically support itself. I am not asking about the size of the infrastructure needed to operate the vehicle such as pad size, etc.&lt;/p&gt;&#xA;" OwnerUserId="21351" LastEditorUserId="20563" LastEditDate="2017-10-20T16:40:06.467" LastActivityDate="2017-10-21T23:12:05.290" Title="Is there a theoretical limit to the size of launch vehicles?" Tags="|launch|escape-velocity|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23391" PostTypeId="1" AcceptedAnswerId="23395" CreationDate="2017-10-15T09:14:20.130" Score="13" ViewCount="1971" Body="&lt;p&gt;Tooth ache is one of the more stupid and annoying disruptions to one's week. Dental hygiene can reduce the risk a lot, but only goes so far - for example, inflammation of roots of the tooth, with pus gathering underneath is something that &lt;em&gt;just happens&lt;/em&gt; and you have about zero control over it, and without a dentist either releasing the pus through opening the root canal or removing the tooth altogether, the pain is simply debilitating and won't &quot;go away&quot; all on its own. At times old fillings fail, teeth can break, and generally that's one &quot;mode of failure&quot; of our bodies that can't be completely prevented or foreseen, but is fixable by a rather short and not overly complex medical procedure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides obligatory dental hygiene (the edible toothpaste used by astronauts daily) how can astronauts deal with it? Aborting a mission due to tooth ache would be an awful waste. Do they have tools and training to extract one another's tooth? Or maybe some 'industrial strength' painkillers and antibiotics that would carry them through the remainder of the mission? Something else?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-10-15T22:51:50.703" Title="How can astronauts on ISS deal with dental emergency?" Tags="|iss|crewed-spaceflight|health|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23392" PostTypeId="1" AcceptedAnswerId="23539" CreationDate="2017-10-15T11:36:18.130" Score="7" ViewCount="598" Body="&lt;p&gt;This question is a follow-up to &lt;a href=&quot;https://space.stackexchange.com/questions/23170/how-to-derive-the-polar-form-of-the-equations-of-motion&quot;&gt;this question&lt;/a&gt; and is linked to &lt;a href=&quot;https://space.stackexchange.com/questions/23120/retromotor-thrust-vectoring-strategy-boundary-value-problem&quot;&gt;this question&lt;/a&gt;, where if needed more details can be found on the parameters used in the following.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an in-plane non-impulsive orbital maneuver, I'd like to find the thrust-direction history $\beta(t)$ to &lt;em&gt;minimize the time&lt;/em&gt; needed to transfer the spacecraft from a specified initial state to a specified terminal state, subject to the following &lt;strong&gt;state equations&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\dot{\textbf{x}}(\textbf{x},\beta,t)=\begin{bmatrix}\dot{r} &amp;amp; \dot{V}_r &amp;amp; \dot{V}_{\theta}\end{bmatrix}$$&#xA;$$\dot{r}=v_r$$&#xA;$$\dot{v}_r=\frac{v_{\theta}^2}{r}-\frac{\mu}{r^2}+\frac{T\sin{\beta}}{m_0-|\dot{m}|t}$$&#xA;$$\dot{v}_{\theta}=-\frac{v_{r}v_{\theta}}{r}+\frac{T\cos{\beta}}{m_0-|\dot{m}|t}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mu$, $T$, $m_0$ and $\dot{m}$ are constants, with &lt;strong&gt;state variables&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\textbf{x}(t)=\begin{bmatrix}r &amp;amp; V_r &amp;amp; V_{\theta}\end{bmatrix}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and initial and terminal conditions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align*}&#xA;\textbf{x}(t_0) &amp;amp;=\begin{bmatrix}1745100 &amp;amp; 0 &amp;amp; 2027\end{bmatrix}\\&#xA;\textbf{x}(t_f) &amp;amp;=\begin{bmatrix}1765100 &amp;amp; 0 &amp;amp; 0\end{bmatrix}\\&#xA;\end{align*}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;i.e. we deal with a two-point boundary value problem where a spacecraft transfers from an initial position to a higher position, while decreasing its initial velocity to zero terminal velocity. The problem is cast as an optimal control problem for which I have formulated the following &lt;em&gt;minimum-time&lt;/em&gt; cost functional:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$J=\min_{\beta}\int_{t_0}^{t_f}L(\textbf{x},\beta,t)dt=\min_{\beta}\int_{t_0}^{t_f}(1)dt$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $L$ denotes the Lagrangian. The &lt;strong&gt;Hamiltonian&lt;/strong&gt; is, therefore,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align*}&#xA;  H(\textbf{x},\beta,t) &amp;amp;= L(\textbf{x},\beta,t)+\lambda^T\dot{\textbf{x}}(\textbf{x},\beta,t) \\&#xA;                        &amp;amp;= 1+\lambda_{r}\dot{r}+\lambda_{v_r}\dot{v}_r+\lambda_{v_{\theta}}\dot{v_{\theta}} \\&#xA;                        &amp;amp;= 1+\lambda_{r}\cdot v_r+\lambda_{v_r}\cdot (\frac{v_{\theta}^2}{r}-\frac{\mu}{r^2}+\frac{T\sin{\beta}}{m_0-|\dot{m}|t})+\lambda_{v_{\theta}}\cdot (-\frac{v_{r}v_{\theta}}{r}+\frac{T\cos{\beta}}{m_0-|\dot{m}|t})&#xA;\end{align*}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mathbf{\lambda}(t)$ is the Lagrange multiplier vector, whose elements are the &lt;strong&gt;costate variables&lt;/strong&gt;. The &lt;em&gt;$\lambda$-dynamics&lt;/em&gt; that capture the behavior of the Lagrange multipliers, also known as the &lt;strong&gt;costate equations&lt;/strong&gt;, are given by: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\begin{align*}&#xA;  \dot{\lambda}_{r} =-\frac{\delta H}{\delta r} &amp;amp;= -\lambda_{v_r} \cdot (-\frac{v_{\theta}^2}{r^2}+\frac{2\mu}{r^3}) - \lambda_{v_{\theta}} \cdot (\frac{v_{r}v_{\theta}}{r^2})\\&#xA;  \dot{\lambda}_{v_r} =-\frac{\delta H}{\delta v_r} &amp;amp;= -\lambda_{r} + \lambda_{v_{\theta}} \cdot (\frac{v_{\theta}}{r})\\&#xA;  \dot{\lambda}_{v_{\theta}} =-\frac{\delta H}{\delta v_{\theta}} &amp;amp;= -\lambda_{v_r} \cdot (\frac{2v_{\theta}}{r}) + \lambda_{v_{\theta}} \cdot (\frac{v_r}{r})\\&#xA;\end{align*}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The partial of $H$ with respect to $\beta$ must equal zero, so&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{\delta H}{\delta \beta}=\lambda_{v_r} \cdot (\frac{T\cos{\beta}}{m_0-|\dot{m}|t}) - \lambda_{v_{\theta}} \cdot (\frac{T\sin{\beta}}{m_0-|\dot{m}|t})=0$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;leading to the following &lt;strong&gt;control law&lt;/strong&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\tan{\beta}=\frac{-\lambda_{v_r}}{-\lambda_{v_{\theta}}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;or,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\sin{\beta}=\frac{-\lambda_{v_r}}{\sqrt{\lambda_{v_r}^2+\lambda_{v_{\theta}}^2}} \qquad \cos{\beta}=\frac{-\lambda_{v_{\theta}}}{\sqrt{\lambda_{v_r}^2+\lambda_{v_{\theta}}^2}}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: the signs in the ratio are negative because we are looking for a minimum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the first time I'm working with an optimal control problem, and I have some trouble understanding how to proceed from here and solve the problem with MATLAB. Is there perhaps someone out here who knows how to do this and could explain what the next steps would be?&lt;/p&gt;&#xA;" OwnerUserId="21124" LastEditorUserId="21124" LastEditDate="2017-10-18T07:43:07.280" LastActivityDate="2017-10-30T11:39:01.730" Title="How to numerically solve an optimal thrust-vectoring problem?" Tags="|mathematics|astrodynamics|thrust-vectoring|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23393" PostTypeId="2" ParentId="10860" CreationDate="2017-10-15T17:39:22.940" Score="0" Body="&lt;p&gt;There is no magical number 22,000. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, as you say, you could achieve geostationary orbit at any altitude, then you could go to any location on Earth's equator, hold an object at arm's length, release it, and it expect it to remain in place, essentially hovering in the air. After all, you and the object are travelling some 1,000 miles per hour around Earth's axis. We all know the object would simply fall to ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also know that objects in low-Earth orbit must travel at some 17,000 miles per hour to remain in orbit, taking some 90 minutes to complete one orbit. We also know that the Moon is in orbit around the Earth (strictly speaking, the Earth-Moon barycenter), is about 240,000 miles away, and completes one orbit in about 27 days, travelling something like 2,500 miles per hour. We also know that gravity follows the inverse-square law, diminishing in proportion to the square of the distance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does this tell us about orbits in general? For one thing, the closer an object to the body it is orbiting, the more it must oppose gravity, which it can only do by traveling faster, which requires greater acceleration to remain on the closed, curved path we call an orbit. Given the two examples of low Earth orbit and the Moon, there must be an infinite range of orbital distances, each of which has an associated velocity and period. There must therefore be an orbit where the period coincides with the rotation of the Earth, and it will have its own specific distance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given the above, knowing Earth's gravitational acceleration (~9.8 m/s/s at the surface), the radius of the Earth (the point at which gravity has that value), the inverse-square law, and the formula for circular motion relating radius and period to acceleration, we can calculate the distance at which an orbit will have a desired period. It turns out that the orbital distance at which the period coincides with the Earth's rotation occurs some 22,000 mile up.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-10-15T17:39:22.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23395" PostTypeId="2" ParentId="23391" CreationDate="2017-10-15T22:51:50.703" Score="16" Body="&lt;p&gt;As in all things ISS, the crew would turn to the appropriate checklist.  In this case, the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/163533main_ISS_Med_CL.pdf&quot; rel=&quot;noreferrer&quot;&gt;Medical Checklist&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its table of contents contains these dentistry-related items:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QXwWt.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QXwWt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the first page of TOOTH EXTRACTION to give you an idea:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FpPdB.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FpPdB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-10-15T22:51:50.703" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="23396" PostTypeId="1" CreationDate="2017-10-15T23:52:18.073" Score="6" ViewCount="4787" Body="&lt;p&gt;I'm trying to see how much thrust in needed to move 6,000 kg (1 surplus LK lander) from low earth orbit to lunar orbit?  Even better would be to know what is currently available (off the shelf) to do the job knowing that there will have to be extensive modification to attach it in orbit.  I'm thinking of one or two surplus stage &quot;D&quot; that are available but don't know if they would have enough thrust.&lt;/p&gt;&#xA;" OwnerUserId="21356" LastEditorDisplayName="user10840" LastEditDate="2017-10-16T16:28:51.360" LastActivityDate="2017-10-17T10:21:07.547" Title="How much thrust needed to move 6,000 kg from LEO to Lunar Orbit?" Tags="|rocket-equation|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23397" PostTypeId="2" ParentId="17089" CreationDate="2017-10-16T00:31:13.523" Score="4" Body="&lt;p&gt;Around &lt;code&gt;03:45&lt;/code&gt; in the &lt;a href=&quot;https://youtu.be/001IXnp0ogc?t=223&quot; rel=&quot;nofollow noreferrer&quot;&gt;linked video&lt;/a&gt; you can see (both?) Yo-Yo(s) deployed and move away from the spacecraft. The frame with the red box shows a small object which may be some retainer, followed by the Yo-Yo masses themselves.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is another object fixed near the horizons, which is likely to be the just-past-full Moon on July 14, 2014 at 07:32 local time in New Mexico as shown by a screen shot from &lt;a href=&quot;https://in-the-sky.org/skymap.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;in-the-sky.org&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RN7dR.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RN7dR.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XZdPM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XZdPM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OlVnJ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OlVnJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LaNdd.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LaNdd.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jZ2Mr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jZ2Mr.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bP0hk.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bP0hk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-10-16T00:31:13.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23398" PostTypeId="2" ParentId="23396" CreationDate="2017-10-16T00:32:04.233" Score="18" Body="&lt;p&gt;What is needed is not thrust (above a certain basic amount), but &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;noreferrer&quot;&gt;delta-v&lt;/a&gt;, a function of the type of engine and the ratio between fueled mass and dry mass, according to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;noreferrer&quot;&gt;Tsiolkovsky equation&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The required delta-v from low earth orbit to low lunar orbit is &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space.E2.80.94high_thrust&quot; rel=&quot;noreferrer&quot;&gt;about 4040 meters per second&lt;/a&gt;. If I remember rightly, the plan for &lt;a href=&quot;https://en.wikipedia.org/wiki/LK_(spacecraft)&quot; rel=&quot;noreferrer&quot;&gt;LK&lt;/a&gt; was to have the &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-LOK&quot; rel=&quot;noreferrer&quot;&gt;Soyuz 7K-LOK&lt;/a&gt; spacecraft carry it into a lunar-suborbital descent trajectory, release the lander, then return to a safe orbit; the LK doesn't have enough fuel to descend from and ascend back to low lunar orbit on its own -- so you'd need something more than 4040 m/s to do the job; I'm not sure exactly how much, possibly 4500 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can evaluate whether the &lt;a href=&quot;https://en.wikipedia.org/wiki/Blok_D&quot; rel=&quot;noreferrer&quot;&gt;Blok-D&lt;/a&gt; would have this capability. Dry mass would be 2.5 tons (Blok D) + 6 tons (LK) = 8.5 tons; propellant is ~15 tons for a total of 23.5 tons loaded. Exhaust velocity for the &lt;a href=&quot;https://en.wikipedia.org/wiki/RD-58&quot; rel=&quot;noreferrer&quot;&gt;RD-58&lt;/a&gt; engine is ~3460 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the equation is:&#xA;$$\Delta v = 3460\ \mathrm{m/s} \ln \frac {23.5} {8.5} = 3518\ \mathrm{m/s}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is enough to reach the moon, but not to insert into lunar orbit; using a Blok D for such a mission would raise another problem anyway: the stage uses liquid oxygen as an oxidizer, and would boil off a substantial amount of it on the 3 day journey. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally, you'd want a larger stage than Blok D, using &lt;a href=&quot;https://en.wikipedia.org/wiki/Hypergolic_propellant&quot; rel=&quot;noreferrer&quot;&gt;storable propellants (typically UDMH and NTO in Russian designs)&lt;/a&gt;, to carry out the mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Briz_(rocket_stage)&quot; rel=&quot;noreferrer&quot;&gt;Briz-M&lt;/a&gt; comes closer; it carries 20 tons of propellant in a 2.5 ton stage, burning storable UDMH/NTO with an exhaust velocity of 3198 m/s. Adding in the mass of the LK, we come to: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta v = 3198\ \mathrm{m/s} \ln \frac {28.5} {8.5} = 3869\ \mathrm{m/s}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is still well short of what's needed; it could insert the LK in a high lunar orbit but not get it into the required trajectory for landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What might work would be a Briz-M - &lt;a href=&quot;https://en.wikipedia.org/wiki/Fregat&quot; rel=&quot;noreferrer&quot;&gt;Fregat-MT&lt;/a&gt; - LK stack. The Briz-M provides 2308 m/s of ∆v, the Fregat provides 2275 m/s, and the total of 4584 m/s may well be enough to get on the proper descent trajectory for the LK to finish the job. Fregat also probably isn't designed for a three day trip, so some payload or propellant mass would be lost to larger batteries, fuel cells, or some other power supply. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're going to need something to catch it on the ascent, though!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike the Blok D, this configuration wouldn't be available off the shelf prior to about year 2000. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorDisplayName="user10840" LastEditDate="2017-10-16T17:01:28.863" LastActivityDate="2017-10-16T17:01:28.863" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23399" PostTypeId="2" ParentId="23396" CreationDate="2017-10-16T01:29:41.223" Score="2" Body="&lt;p&gt;Russell Borogove is correct that the parameter more useful to you is likely to be delta-V, which you need a lot of to get to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer the thrust question, instead of the available-stages question:  if you have sufficiently sophisticated navigation options, there are very low-thrust trajectories available for lunar insertion because of the nature of the Earth-Moon system.  You can keyhole through the Earth-Moon L1 point and into a predominantly lunar trajectory with very small accelerations (as opposed to being on a short timeline from a hyperbolic transfer as you'd see on interplanetary patched-conic trajectories, which would require a minimum acceleration to provide enough delta-V to close the orbit in the appropriate window).  My understanding is that the limit would truly be down to your ability to control the spacecraft, and your ability to wait.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no shortage of articles to peruse on &quot;low-thrust transfers&quot; to the moon; it's a favorite topic in space-navigation circles.&lt;/p&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2017-10-16T01:29:41.223" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23400" PostTypeId="2" ParentId="23396" CreationDate="2017-10-16T04:13:38.430" Score="16" Body="&lt;p&gt;While Russell Borogove mentioned it I think it deserves more focus:  Thrust is the wrong thing to measure here.  Thrust is how hard you can push--akin horsepower in your car.  Your question is thus akin to asking how much horsepower does it take to drive from New York to Los Angeles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rather, you are interested in whether it has the fuel to get there.  With your car you measure miles per gallon * gallons = how many miles you can go.  In space it doesn't cost any fuel to keep going, what you care about is how much you must speed up or slow down.  Here's the map:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(You'll need to expand this to read it decently.)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Y0wEV.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y0wEV.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Follow the lines to where you want to go (to/from Earth, it will overstate all other paths) and add up the numbers.  That's how much total velocity change you need.  See Russell Borogove's post for what the rockets can actually do.  Note that his numbers are slightly higher--this chart shows the mathematical minimums, not what you can actually do in the real world.  It also does not consider gravity assists.  If you're heading beyond our nearest neighbors you can get there for less fuel than this shows at the cost of time.  Look at MESSENGER.  Spend the 11,580 m/s and it could have gotten there in half a year.  I'm not finding how much it used on the ejection burn but after that it used 2,212 m/s--but took 7 years to make the trip.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2017-10-16T04:28:28.597" LastActivityDate="2017-10-16T04:28:28.597" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23403" PostTypeId="1" AcceptedAnswerId="24283" CreationDate="2017-10-16T05:59:48.360" Score="8" ViewCount="590" Body="&lt;p&gt;After seeing Elon Musk's proposal of &lt;a href=&quot;https://www.theverge.com/2017/9/29/16383048/elon-musk-spacex-rocket-transport-earth-travel&quot; rel=&quot;noreferrer&quot;&gt;using rockets to travel from city to city&lt;/a&gt; I was wondering how much fuel such a rocket would use, and how would that compare to traditional flights in terms of environmental impact (CO2 emissions).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it possible to provide an estimate based on current technology?&lt;/p&gt;&#xA;" OwnerUserId="21360" LastEditorUserId="21360" LastEditDate="2017-10-16T16:54:21.687" LastActivityDate="2018-01-06T02:10:28.840" Title="How much CO2 would city-to-city rocket flight produce compared to airliners?" Tags="|rockets|fuel|future-missions|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23408" PostTypeId="1" AcceptedAnswerId="23409" CreationDate="2017-10-16T16:11:39.160" Score="11" ViewCount="3128" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/22915/12102&quot;&gt;Calculating the planets and moons based on Newtons&amp;#39;s gravitational force&lt;/a&gt; was &lt;a href=&quot;https://space.stackexchange.com/a/22960/12102&quot;&gt;pretty much answered&lt;/a&gt; with two items:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Use a reasonable ODE solver; &lt;em&gt;at least&lt;/em&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Runge%E2%80%93Kutta_methods&quot; rel=&quot;nofollow noreferrer&quot;&gt;RK4&lt;/a&gt; (the classic Runge-Kutta method) or better. Do not use just the &lt;a href=&quot;https://en.wikipedia.org/wiki/Euler_method&quot; rel=&quot;nofollow noreferrer&quot;&gt;Euler Method&lt;/a&gt;,&lt;/li&gt;&#xA;&lt;li&gt;Express all position and velocity vectors of all &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; bodies as a single vector of length &lt;span class=&quot;math-container&quot;&gt;$6n$&lt;/span&gt; and solve these simultaneously.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;But that's not good enough to match something like &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL's Horizons&lt;/a&gt; because reality is harder than simple Newtonian gravity between point particles.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How to calculate the planets and moons beyond Newtons's gravitational force?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2022-01-03T00:05:13.337" LastActivityDate="2022-01-03T00:05:13.337" Title="How to calculate the planets and moons beyond Newtons's gravitational force?" Tags="|orbital-mechanics|jpl-horizons|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23409" PostTypeId="2" ParentId="23408" CreationDate="2017-10-16T16:12:36.177" Score="13" Body="&lt;p&gt;Let's add an approximations to take into account some of the &lt;a href=&quot;https://en.wikipedia.org/wiki/General_relativity&quot; rel=&quot;nofollow noreferrer&quot;&gt;General Relativity&lt;/a&gt; (GR) effects — at least for bodies orbiting close to the massive Sun — and start to look at &lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt; the lowest order multipole term for a body's gravitational potential beyond the monopole term &lt;span class=&quot;math-container&quot;&gt;$-GM/r$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Newton:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The acceleration of a body in the gravitation field of another body of &lt;a href=&quot;https://en.wikipedia.org/wiki/Standard_gravitational_parameter&quot; rel=&quot;nofollow noreferrer&quot;&gt;standard gravitational parameter&lt;/a&gt; &lt;span class=&quot;math-container&quot;&gt;$GM$&lt;/span&gt; can be written:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{a_{Newton}} = -GM \frac{\mathbf{r}}{|r|^3},$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where &lt;span class=&quot;math-container&quot;&gt;$r$&lt;/span&gt; is the vector from the body &lt;span class=&quot;math-container&quot;&gt;$M$&lt;/span&gt; to the body who's acceleration is being calculated. Remember that in Newtonian mechanics the acceleration of each body depends only on the mass of &lt;em&gt;the other body&lt;/em&gt;, even though the force depends on both masses, because the first mass cancels out by &lt;span class=&quot;math-container&quot;&gt;$a=F/m$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;General Relativity (approximate):&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Although I'm not familliar with GR, I'm going to recommend an equation that seems to work well and seems to be supported by several links. It is an approximate relativistic correction to Newtonian gravity that is used in orbital mechanics simulations. You will see various forms in the following links, most with additional terms not shown here:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://physics.stackexchange.com/q/313146/83380&quot;&gt;https://physics.stackexchange.com/q/313146/83380&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Eq.    1 in &lt;a href=&quot;https://www.lpi.usra.edu/books/CometsII/7009.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.lpi.usra.edu/books/CometsII/7009.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Eq.   27 in &lt;a href=&quot;https://ipnpr.jpl.nasa.gov/progress_report/42-196/196C.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ipnpr.jpl.nasa.gov/progress_report/42-196/196C.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Eq. 4-26 in &lt;a href=&quot;https://descanso.jpl.nasa.gov/monograph/series2/Descanso2_all.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://descanso.jpl.nasa.gov/monograph/series2/Descanso2_all.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;also see David Hammen's discussion/advice in &lt;a href=&quot;https://space.stackexchange.com/a/21472/12102&quot;&gt;this answer&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;li&gt;Eq. 3.11 in &lt;a href=&quot;http://adsabs.harvard.edu/full/1994AJ....107.1885S&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://adsabs.harvard.edu/full/1994AJ....107.1885S&lt;/a&gt; (click &amp;quot;print&amp;quot;) see &lt;a href=&quot;https://astronomy.stackexchange.com/a/25304/7982&quot;&gt;this answer&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Eq. A.8 in G. F. Rubilar1 &amp;amp; A. Eckart 2001 &lt;a href=&quot;https://www.aanda.org/articles/aa/abs/2001/28/aah2613/aah2613.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Periastron shifts of stellar orbits near the Galactic Center&lt;/a&gt;, except that the &lt;span class=&quot;math-container&quot;&gt;$\mathbf{r}/r^3$&lt;/span&gt; Newtonian is included rather than treated separately.&lt;/li&gt;&#xA;&lt;li&gt;Eq. 2 in M. Parsa et al. 2017 &lt;a href=&quot;https://www.eso.org/public/archives/announcements/pdf/ann17051a.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Investigating the Relativistic Motion of the Stars Near the Supermassive Black Hole in the Galactic Center&lt;/a&gt;, except that the &lt;span class=&quot;math-container&quot;&gt;$\mathbf{r}/r^3$&lt;/span&gt; Newtonian is included rather than treated separately.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The following approximation should be &lt;em&gt;added to&lt;/em&gt; the Newtonian term:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{a_{GR}} = GM \frac{1}{c^2 |r|^3}\left(4 GM \frac{\mathbf{r}}{|r|} - (\mathbf{v} \cdot \mathbf{v}) \mathbf{r} + 4 (\mathbf{r} \cdot \mathbf{v}) \mathbf{v} \right),$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Oblateness (&lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt; only):&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;I'm just using the math from &lt;a href=&quot;https://en.wikipedia.org/wiki/Geopotential_model#The_deviations_of_Earth.27s_gravitational_field_from_that_of_a_homogeneous_sphere&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia's article on the Geopotential Model&lt;/a&gt; with a very important-to-remember approximation; I am assuming that the oblateness is in the plane of the ecliptic — that the rotational axis of the oblate body is in the &lt;span class=&quot;math-container&quot;&gt;$\mathbf{\hat{z}}$&lt;/span&gt; direction, perpendicular to the ecliptic. &lt;strong&gt;Don't forget that this is an approximation!&lt;/strong&gt; The full vector equations are messier than this, I'll try to come back and update this once I'm sure I've got it correct. In the mean time, here is an approximation:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{r} = x \mathbf{\hat{x}} + y \mathbf{\hat{y}} + z \mathbf{\hat{z}} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$a_x = J_2 \frac{x}{|r|^7} (6z^2 - 1.5(x^2+y^2)) $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$a_y = J_2 \frac{y}{|r|^7} (6z^2 - 1.5(x^2+y^2)) $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$a_z = J_2 \frac{z}{|r|^7} (3z^2 - 4.5(x^2+y^2)) $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;The following should be &lt;em&gt;added to&lt;/em&gt; the Newtonian term:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\mathbf{a_{J2}} = a_x \mathbf{\hat{x}} + a_y \mathbf{\hat{y}} + a_z \mathbf{\hat{z}} $$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Tidal Forces:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;It gets even more complicated when looking at terms that involve non-spherical mass distributions in both bodies at the same time, whether they are static or not. At this point it's probably necessary to hit the books.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Here's a test run. I've compared to downloaded data from &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL's Horizons&lt;/a&gt;. For the outer planets I use the Horizons data for each planet's barycenter, which makes sure it's the velocity for the center of mass of the planet plus all of its moons. I haven't added the correction to the planet's masses, but that's a much smaller effect since it only affects the movement of other, distant bodies.&lt;/p&gt;&#xA;&lt;p&gt;For the Earth data, make sure to download the Earth's geocenter and the Moon separately (not the Earth-Moon barycenter). For the outer planets remember to download the barycenters.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/d1KPF.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d1KPF.png&quot; alt=&quot;Screenshot of JPL Horizons - Earth&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Lx7b0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Lx7b0.png&quot; alt=&quot;Screenshot of JPL Horizons - Jupiter&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I've integrated for a year, and everything is within about one kilometer of the Horizons data except for Earth's Moon. That's not a surprise considering all the intimate tidal effects between these two. Adding Earth's &lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt; to the potential felt by the Moon only helps partially, it's really not the right way to do it, especially considering that the Earth's axis (and therefore oblateness) is so far away from the ecliptic.&lt;/p&gt;&#xA;&lt;p&gt;So this is overall an illustration that the more physics you put in, the closer you can get to a really serious JPL simulation! This is the absolute distance between the simulated positions here and the Horizons output from &lt;code&gt;2017-01-01 00:00&lt;/code&gt; to &lt;code&gt;2018-01-01 00:00&lt;/code&gt;. Following that is the Python script I used to generate it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/l32NY.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/l32NY.png&quot; alt=&quot;Screenshot of Python Output&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Based on discussion of stiffness in comments below, here's a quick plot of step size versus time. I think the stiffness is coming from the Earth-Moon system, these frequent excursions look a bit like the Earth and Moon's error excursions. I think I am going to try to rescale the problem to dimensionless units, right now the numerical tolerance applies to all velocities and positions equally, not a good idea.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FVVS9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FVVS9.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;def deriv_Newton_Only(X, t):&#xA;    x,  v  = X.reshape(2, -1)&#xA;    xs, vs = x.reshape(-1, 3), v.reshape(-1, 3)&#xA;    things = zip(bodies, xs, vs)&#xA;&#xA;    accs, vels = [], []&#xA;    for a, xa, va in things:&#xA;        acc_a = np.zeros(3)&#xA;        for b, xb, vb in things:&#xA;            if b != a:&#xA;                r = xa - xb&#xA;                acc_a += -b.GM * r * ((r**2).sum())**-1.5&#xA;        accs.append(acc_a)&#xA;        vels.append(va)&#xA;&#xA;    return np.hstack((np.hstack(vels), np.hstack(accs)))&#xA;&#xA;def deriv_sunGRJ2EarthJ2(X, t):&#xA;    x,  v  = X.reshape(2, -1)&#xA;    xs, vs = x.reshape(-1, 3), v.reshape(-1, 3)&#xA;    things = zip(bodies, xs, vs)&#xA;&#xA;    accs, vels = [], []&#xA;    for a, xa, va in things:&#xA;        acc_a = np.zeros(3)&#xA;        for b, xb, vb in things:&#xA;            if b != a:&#xA;                r = xa - xb&#xA;                acc_a += -b.GM * r * ((r**2).sum())**-1.5&#xA;&#xA;        if a.do_SunGR and not a.name == 'Sun':&#xA;&#xA;            a.flag0 = True&#xA;&#xA;            r   = xa - xs[0]&#xA;            v   = va - vs[0]&#xA;            rsq = (r**2).sum()&#xA;            rm3 = rsq**-1.5&#xA;            rm1 = rsq**-0.5&#xA;&#xA;            # https://physics.stackexchange.com/q/313146/83380&#xA;            # Eq.    1 in https://www.lpi.usra.edu/books/CometsII/7009.pdf&#xA;            # Eq.   27 in https://ipnpr.jpl.nasa.gov/progress_report/42-196/196C.pdf&#xA;            # Eq. 4-26 in https://descanso.jpl.nasa.gov/monograph/series2/Descanso2_all.pdf&#xA;            # Eq. 3.11 in http://adsabs.harvard.edu/full/1994AJ....107.1885S&#xA;            &#xA;            term_0 = Sun.GM / (clight**2) * rm3&#xA;            term_1 = 4.*Sun.GM * r * rm1&#xA;            term_2 =   -np.dot(v, v) * r&#xA;            term_3 = 4.*np.dot(r, v) * v&#xA;&#xA;            accGR = term_0*(term_1 + term_2 + term_3)&#xA;            acc_a += accGR&#xA;            &#xA;        if a.do_SunJ2 and not a.name == 'Sun':&#xA;&#xA;            a.flag1 = True&#xA;&#xA;            r = xa - xs[0] # position relative to Sun&#xA;            x,   y,   z   = r&#xA;            xsq, ysq, zsq = r**2&#xA;&#xA;            rsq = (r**2).sum()&#xA;            rm7 = rsq**-3.5&#xA;&#xA;            # https://en.wikipedia.org/wiki/Geopotential_model#The_deviations_of_Earth.27s_gravitational_field_from_that_of_a_homogeneous_sphere&#xA;            accJ2x = x * rm7 * (6*zsq - 1.5*(xsq + ysq))&#xA;            accJ2y = y * rm7 * (6*zsq - 1.5*(xsq + ysq))&#xA;            accJ2z = z * rm7 * (3*zsq - 4.5*(xsq + ysq))&#xA;&#xA;            accJ2  = J2s * np.hstack((accJ2x, accJ2y, accJ2z))&#xA;            acc_a += accJ2&#xA;            &#xA;        if a.do_EarthJ2 and not a.name == 'Earth':&#xA;&#xA;            a.flag2 = True&#xA;&#xA;            r = xa - xs[3] # position relative to Earth&#xA;            &#xA;            x,   y,   z   = r&#xA;            xsq, ysq, zsq = r**2&#xA;&#xA;            rsq = (r**2).sum()&#xA;            rm7 = rsq**-3.5&#xA;&#xA;            # https://en.wikipedia.org/wiki/Geopotential_model#The_deviations_of_Earth.27s_gravitational_field_from_that_of_a_homogeneous_sphere&#xA;            accJ2x = x * rm7 * (6*zsq - 1.5*(xsq + ysq))&#xA;            accJ2y = y * rm7 * (6*zsq - 1.5*(xsq + ysq))&#xA;            accJ2z = z * rm7 * (3*zsq - 4.5*(xsq + ysq))&#xA;&#xA;            accJ2  = J2e * np.hstack((accJ2x, accJ2y, accJ2z))&#xA;            acc_a += accJ2&#xA;            &#xA;        accs.append(acc_a)&#xA;        vels.append(va)&#xA;&#xA;    return np.hstack((np.hstack(vels), np.hstack(accs)))&#xA;&#xA;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;from scipy.integrate import odeint as ODEint&#xA;&#xA;names = ['Sun', 'Mercury', 'Venus',&#xA;         'Earth', 'Moon', 'Mars',&#xA;         'Ceres', 'Pallas', 'Vesta',&#xA;         'Jupiter', 'Saturn', 'Uranus',&#xA;         'Neptune']&#xA;&#xA;GMsDE430 = [1.32712440040944E+20, 2.203178E+13,  3.24858592E+14,&#xA;        3.98600435436E+14,    4.902800066E+12,  4.2828375214E+13,&#xA;        6.28093938E+10,       1.3923011E+10,    1.7288009E+10, &#xA;        1.267127648E+17,      3.79405852E+16,   5.7945486E+15,&#xA;        6.83652719958E+15 ] # https://ipnpr.jpl.nasa.gov/progress_report/42-178/178C.pdf&#xA;&#xA;# for masses also see ftp://ssd.jpl.nasa.gov/pub/xfr/gm_Horizons.pck&#xA;&#xA;# https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/satellites/&#xA;# https://naif.jpl.nasa.gov/pub/naif/JUNO/kernels/spk/de436s.bsp.lbl&#xA;# https://astronomy.stackexchange.com/questions/13488/where-can-i-find-visualize-planets-stars-moons-etc-positions&#xA;# https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/satellites/jup310.cmt&#xA;# ftp://ssd.jpl.nasa.gov/pub/xfr/gm_Horizons.pck&#xA;&#xA;GMs = GMsDE430&#xA;&#xA;clight = 2.9979E+08 # m/s&#xA;&#xA;halfpi, pi, twopi = [f*np.pi for f in [0.5, 1, 2]]&#xA;&#xA;# J2 values&#xA;J2_sun = 2.110608853272684E-07  # unitless&#xA;R_sun  = 6.96E+08 # meters&#xA;J2s    = J2_sun * (GMs[0] * R_sun**2)   # is there a minus sign?&#xA;&#xA;J2_earth = 1.08262545E-03  # unitless&#xA;R_earth  = 6378136.3 # meters&#xA;J2e      = J2_earth * (GMs[3] * R_earth**2)   # is there a minus sign?&#xA;&#xA;JDs, positions, velocities, linez = [], [], [], []&#xA;&#xA;use_outer_barycenters = True&#xA;&#xA;for name in names:&#xA;&#xA;    fname = name + ' horizons_results.txt'&#xA;&#xA;    if use_outer_barycenters:&#xA;        if name in ['Jupiter', 'Saturn', 'Uranus', 'Neptune']:&#xA;            fname = name + ' barycenter horizons_results.txt'&#xA;&#xA;    with open(fname, 'r') as infile:&#xA;&#xA;        lines = infile.read().splitlines()&#xA;&#xA;        iSOE = [i for i, line in enumerate(lines) if &amp;quot;&lt;span class=&quot;math-container&quot;&gt;$$SOE&quot; in line][0]&#xA;        iEOE = [i for i, line in enumerate(lines) if &quot;$$&lt;/span&gt;EOE&amp;quot; in line][0]&#xA;&#xA;        # print name, iSOE, iEOE, lines[iSOE], lines[iEOE]&#xA;&#xA;        lines = lines[iSOE+1:iEOE]&#xA;&#xA;        lines = [line.split(',') for line in lines]&#xA;        JD  = np.array([float(line[0]) for line in lines])&#xA;        pos = np.array([[float(item) for item in line[2:5]] for line in lines])&#xA;        vel = np.array([[float(item) for item in line[5:8]] for line in lines])&#xA;&#xA;        JDs.append(JD)&#xA;        positions.append(pos * 1000.)   # km   to m&#xA;        velocities.append(vel * 1000.)  # km/s to m/s&#xA;        linez.append(lines)&#xA;&#xA;JD = JDs[0] # assume they are identical&#xA;&#xA;class Body(object):&#xA;    def __init__(self, name):&#xA;        self.name = name&#xA;&#xA;bodies = []&#xA;for name, GM, pos, vel in zip(names, GMs, positions, velocities):&#xA;    &#xA;    body = Body(name)&#xA;    bodies.append(body)&#xA;    &#xA;    body.GM = GM&#xA;    &#xA;    body.daily_positions  = pos&#xA;    body.daily_velocities = vel&#xA;    &#xA;    body.initial_position = pos[0]&#xA;    body.initial_velocity = vel[0]&#xA;&#xA;x0s = np.hstack([b.initial_position for b in bodies])&#xA;v0s = np.hstack([b.initial_velocity for b in bodies])&#xA;&#xA;X0  = np.hstack((x0s, v0s))&#xA;&#xA;ndays   = 365&#xA;nperday = 144&#xA;&#xA;time = np.arange(0, ndays*24*3600+1, 24*3600./nperday)&#xA;days = time[::nperday]/(24*3600.)&#xA;&#xA;for body in bodies:&#xA;    body.do_SunGR   = False&#xA;    body.do_SunJ2   = False&#xA;    body.do_EarthJ2 = False&#xA;    body.flag0      = False&#xA;    body.flag1      = False&#xA;    body.flag2      = False&#xA;&#xA;Sun, Mercury, Venus, Earth, Moon, Mars = bodies[:6]&#xA;Ceres, Pallas, Vesta = bodies[6:9]&#xA;Jupiter, Saturn, Uranus, Neptune = bodies[9:]&#xA;&#xA;Mercury.do_SunGR = True&#xA;Venus.do_SunGR   = True&#xA;Earth.do_SunGR   = True&#xA;Moon.do_SunGR    = True&#xA;Mars.do_SunGR    = True&#xA;Ceres.do_SunGR   = True&#xA;Pallas.do_SunGR  = True&#xA;Vesta.do_SunGR   = True&#xA;&#xA;Mercury.do_SunJ2 = True&#xA;&#xA;Moon.do_EarthJ2  = True&#xA;&#xA;rtol = 1E-12   # this is important!!!&#xA;&#xA;answer, info = ODEint(deriv_sunGRJ2EarthJ2, X0, time,&#xA;                      rtol = rtol, full_output=True)&#xA;&#xA;print answer.shape&#xA;&#xA;nbodies = len(bodies)&#xA;&#xA;xs, vs = answer.T.reshape(2, nbodies, 3, -1)&#xA;&#xA;for body, x, v in zip(bodies, xs, vs):&#xA;    body.x = x&#xA;    body.v = v&#xA;    body.x_daily = body.x[:, ::nperday]&#xA;    body.v_daily = body.v[:, ::nperday]&#xA;    body.difference = np.sqrt(((body.x_daily - body.daily_positions.T)**2).sum(axis=0))&#xA;&#xA;if True:&#xA;    for body in bodies[:6]:&#xA;        print body.name, body.flag0, body.flag1, body.flag2&#xA;&#xA;if True:&#xA;    plt.figure()&#xA;    for i, body in enumerate(bodies[:12]):  # Sorry Neptune!!!&#xA;        plt.subplot(4, 3, i+1)&#xA;        plt.plot(days, 0.001*body.difference)&#xA;        plt.title(body.name, fontsize=14)&#xA;        plt.xlim(0, 365)&#xA;    plt.suptitle(&amp;quot;calc vs JPL Horizons (km vs days)&amp;quot;, fontsize=16)&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-07-28T02:00:36.203" LastActivityDate="2021-07-28T02:00:36.203" CommentCount="16" ContentLicense="CC BY-SA 4.0" />
  <row Id="23411" PostTypeId="1" CreationDate="2017-10-16T18:01:45.600" Score="-2" ViewCount="490" Body="&lt;p&gt;What is the real cost (all costs) in to send a freight craft  to ISS?  Which launch system is the most cost effective?&lt;/p&gt;&#xA;" OwnerUserId="21358" LastActivityDate="2017-10-18T09:26:40.483" Title="What is the &quot;true&quot; cost to send a freighter (spaceX or SoyuZ) to the ISS?" Tags="|spacecraft|soyuz-spacecraft|" AnswerCount="1" CommentCount="10" ClosedDate="2017-10-24T12:59:35.123" ContentLicense="CC BY-SA 3.0" />
  <row Id="23412" PostTypeId="1" CreationDate="2017-10-16T18:51:23.693" Score="5" ViewCount="122" Body="&lt;p&gt;The Japanese H-IIB seems like an interesting cost effective system.  I see that it can carry a hefty payload into Earth orbit.  Could it send a smaller payload to the Moon?&lt;/p&gt;&#xA;" OwnerUserId="21358" LastEditorUserId="1142" LastEditDate="2017-10-17T03:51:56.117" LastActivityDate="2017-10-17T03:51:56.117" Title="Could the Japanese H-IIB carry a payload to the moon?" Tags="|jaxa|japan|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23413" PostTypeId="1" AcceptedAnswerId="23414" CreationDate="2017-10-16T19:01:23.437" Score="4" ViewCount="578" Body="&lt;p&gt;How much higher is a rocket following a gravity turn going to be able to reach compared to a rocket following a vertical flight ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that it depends on which type of rocket and which type of gravity turn but just a rough estimate for a real rocket (Saturn V type) not necessarily based on any calculations just from your empirical knowledge ? 50% higher ? Twice as high ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm just trying to get an idea of how important the change is.&lt;/p&gt;&#xA;" OwnerUserId="21272" LastActivityDate="2017-10-17T00:19:12.880" Title="Estimate of effectiveness of gravity turn compared to vertical flight" Tags="|rockets|gravity|gravity-assist|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23414" PostTypeId="2" ParentId="23413" CreationDate="2017-10-16T19:43:01.347" Score="6" Body="&lt;p&gt;A rocket flying vertically will reach a much &lt;em&gt;higher&lt;/em&gt; altitude than one flying a gravity turn; it just won’t &lt;em&gt;stay&lt;/em&gt; at that altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A vehicle flying sideways at sufficient speed will continuously “miss” the Earth as it falls, yielding a circular or elliptical closed path. If instead all the velocity the rocket can produce is applied vertically, the result will be like a baseball thrown straight up instead of sideways; it will reach a quite high altitude before falling straight back to earth and crashing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact altitude reached will vary a lot with the design of the rocket; a low-thrust upper stage as used on Ariane 5 will not reach as high an altitude going straight up as would something like Falcon 9. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an example, an Atlas V 401 might be capable of putting a given payload into a circular orbit at 250km altitude via a gravity turn trajectory. The same stack going straight up would reach an apogee of perhaps 4000km according to my crude simulations. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-10-16T20:42:10.390" LastActivityDate="2017-10-16T20:42:10.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23415" PostTypeId="2" ParentId="23412" CreationDate="2017-10-16T20:22:23.250" Score="6" Body="&lt;p&gt;Almost certainly. The predecessor &lt;a href=&quot;https://en.wikipedia.org/wiki/H-IIA&quot; rel=&quot;noreferrer&quot;&gt;H-IIA&lt;/a&gt; rocket delivered the orbiter &lt;a href=&quot;https://en.wikipedia.org/wiki/SELENE&quot; rel=&quot;noreferrer&quot;&gt;SELENE&lt;/a&gt; to the moon in 2007. The &lt;a href=&quot;https://en.wikipedia.org/wiki/SELENE-2&quot; rel=&quot;noreferrer&quot;&gt;SELENE-2&lt;/a&gt; mission slated for 2018 or 2019 will use either an H-IIA or an H-IIB launcher -- it's not entirely clear which from the references I've found, but I think it will be the H-IIA, as the payload is not significantly heavier than that of SELENE.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-10-16T20:22:23.250" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23416" PostTypeId="2" ParentId="23413" CreationDate="2017-10-17T00:19:12.880" Score="4" Body="&lt;p&gt;As Russell Borogove says, if you just go straight up you fall back.  Thus I'm looking at this from a standpoint of going straight up and then burning horizontal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know what the numbers look like in the real world but I have played with trajectories almost like this in Kerbal Space Program when hauling drag monsters.  (The game has no good answer for lifting large rovers, thus some very ugly rockets go up at times.)  Going nearly vertically until I'm out of the atmosphere costs about 10% more delta-v than a proper flight profile.  Note that this is with a thrust to weight ratio well above what is normally used for real world space launches and thus understates the penalty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternately, you're looking at an interplanetary trajectory.  That way you will not fall back, it's simply a matter of efficiency.  My memory of the one time I tried it was that the penalty was over 20%--and that with an even higher thrust to weight ratio as I was trying to approximate a vertical cannon launch + minimal fuel to go into orbit.  (Go out to the edge of the sphere of influence, a small burn to put my periapsis in the upper atmosphere, aerobrake until my apoapsis was in low orbit, the a final burn to circularize.)  Note that this is even more sensitive to the thrust to weight ratio as you're spending even longer without horizontal velocity.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-10-17T00:19:12.880" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23417" PostTypeId="2" ParentId="22248" CreationDate="2017-10-17T02:43:12.363" Score="2" Body="&lt;p&gt;In theory this should work, You may run into problems regarding keeping the potassium nitrate solid as it may tend to fall apart. Finding a binder that would keep the potassium nitrate solid but not turn itself into a solid fuel may be a challenge. You could also maybe just compact the potassium nitrate onto itself to hold its shape. Either way it will be a challenge but again in theory this should work.&lt;/p&gt;&#xA;" OwnerUserId="21369" LastActivityDate="2017-10-17T02:43:12.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23420" PostTypeId="2" ParentId="23396" CreationDate="2017-10-17T10:21:07.547" Score="2" Body="&lt;p&gt;Lifting a spaceship from launchpad to a low orbit is very different to increasing the height of an orbit from LEO to GEO. For a launch from ground, there must be enough thrust to fight gravity and to accelerate to the necessary velocity for a LEO. Increasing the height of an orbit can be done with very low thrust if there is enough time for the maneuver. But for both cases a strategy to use a minimal amount of fuel is essential.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-10-17T10:21:07.547" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23424" PostTypeId="1" CreationDate="2017-10-17T21:13:14.993" Score="2" ViewCount="106" Body="&lt;p&gt;What are the state of the art balloon materials used for high altitudes ascending (top of troposphere, approximately 50 km)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there new material improvements likely in the next future? If so which are likely candidates?&lt;/p&gt;&#xA;" OwnerUserId="16049" LastActivityDate="2017-10-17T23:33:48.827" Title="State of the art balloon materials" Tags="|materials|balloons|" AnswerCount="1" CommentCount="2" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23425" PostTypeId="2" ParentId="23424" CreationDate="2017-10-17T23:33:48.827" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Standard NASA scientific balloons are constructed of polyethylene&#xA;  film; the same type material used for plastic bags. This material is&#xA;  only 0.002 centimeters (0.0008 inches) thick, about the same as an&#xA;  ordinary sandwich wrap.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.csbf.nasa.gov/balloons.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-10-17T23:33:48.827" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23426" PostTypeId="1" CreationDate="2017-10-18T01:30:37.490" Score="9" ViewCount="372" Body="&lt;p&gt;I read recently about the huge volumes of water that is sprayed on a launch pad during lift off. I remember it to be about four times as much water as fuel burned. This is done to reduce noise (energy that would have become acoustic, instead taken up in change-of-state i.e. water to steam) and to help the launch pad survive the heat, so it can be used again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I occurred to me that in a vacuum, like on the moon, a landing and launch facility cannot spray water on the launch pad to protect it. &#xA;Is anyone aware of studies of what a lunar launch pad would look like and how it would survive the heat of launch?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="6816" LastEditDate="2017-10-18T22:07:03.690" LastActivityDate="2017-10-18T22:07:03.690" Title="Lunar launch pad cooling" Tags="|the-moon|launchpad|thermal|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23427" PostTypeId="2" ParentId="23426" CreationDate="2017-10-18T07:42:44.750" Score="5" Body="&lt;p&gt;I haven't seen studies, but there are ways to build a launch pad without water cooling. The Soyuz launch pads for instance are built over the edge of a &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Soyuz_TMA-05M_rocket_at_the_launch_pad_at_the_Baikonur_Cosmodrome.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;large hole in the ground&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bqfgo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bqfgo.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would work on the Moon as well. The main issue would be loose rubble at the bottom of the pit being launched across the Moon or even into orbit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-10-18T10:20:13.990" LastActivityDate="2017-10-18T10:20:13.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23428" PostTypeId="2" ParentId="23411" CreationDate="2017-10-18T09:26:40.483" Score="2" Body="&lt;p&gt;The linked question on launch &lt;a href=&quot;https://space.stackexchange.com/questions/1989/what-is-the-current-cost-per-pound-to-send-something-into-leo&quot;&gt;cost per pound&lt;/a&gt; is a good starting point. Multiply this with a rocket's maximum payload and you have the true cost of one launch. This is what you'd pay to the launch provider to have your payload launched to LEO. The launch provider uses this to pay for building the rocket, transporting it to the launch site, launch services etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launching to the ISS is more complicated. SpaceX advertises the launch price for a &lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon 9&lt;/a&gt; at $62M. The NASA contract with SpaceX for ISS supply runs was more expensive at ~90M/launch, because NASA wanted extra certification work, and because the launch now includes a Dragon capsule. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For manned Soyuz launches to the ISS, &lt;a href=&quot;https://space.stackexchange.com/questions/9238/what-is-the-flyaway-cost-of-a-soyuz-and-proton-rocket&quot;&gt;NASA paid \$70 million per seat in 2013&lt;/a&gt;. This seems to include a certain amount of price gouging and/or subsidies, because you can buy a generic Soyuz launch &lt;a href=&quot;https://space.stackexchange.com/questions/9238/what-is-the-flyaway-cost-of-a-soyuz-and-proton-rocket&quot;&gt;for in the region of $60M&lt;/a&gt;.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch costs have always been difficult to calculate. SpaceX publishing them on their website has made it a bit easier, but I don't see Roscosmos putting the sticker price for a Soyuz on their site quite yet. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-10-18T09:26:40.483" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23429" PostTypeId="1" CreationDate="2017-10-18T10:08:56.963" Score="7" ViewCount="2854" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/23427/12102&quot;&gt;This answer&lt;/a&gt; which mentions blasting on the moon for excavation got me thinking. There may be many ways that blasting could potentially be useful on the Moon. Excavation for habitats or other structures, mining for things besides water, perhaps leveling for roads, or even for burying the occasional sub-surface 1:4:9 black monolith. There is more discussion of the use of explosives in the answers to the question &lt;a href=&quot;https://space.stackexchange.com/q/6606/12102&quot;&gt;Could explosives replace drilling for science and construction in space?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would storage and use of explosives like TNT (or something more modern) and electronic detonators or detonator cord have any unique challenges, difficulties or safety issues on the Moon? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For background, here's the Periodic Table of Videos epsiode for Dynamite and TNT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/pWuYtvzxuLE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-18T10:17:04.573" LastActivityDate="2017-10-18T13:00:34.297" Title="Lunar excavation by blasting - would there be unique challenges, difficulties or safety issues?" Tags="|the-moon|explosives|lunar-base|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23430" PostTypeId="1" CreationDate="2017-10-18T10:22:01.870" Score="2" ViewCount="1555" Body="&lt;p&gt;How are the velocity and range of a spacecraft measured from ground based stations? For example we can go on NASA's website and look at the real time data for the mission status of their past spacecrafts, like Voyager. How do they obtain this data?&lt;/p&gt;&#xA;" OwnerUserId="21383" LastActivityDate="2020-05-04T20:43:42.220" Title="How are the velocity and range of a spacecraft measured from ground based stations?" Tags="|spacecraft|nasa|deep-space|deep-space-network|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23431" PostTypeId="1" AcceptedAnswerId="23436" CreationDate="2017-10-18T10:28:59.757" Score="3" ViewCount="318" Body="&lt;p&gt;Breaking Space news! I just heard a short item on the BBC news broadcast (ok, podcast). It said that a JAXA lunar orbiter has determined that a previously identified cave is quite extensive - perhaps 50 km long!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What spacecraft could they be talking about, and how does a spacecraft determine the length of sub-surface caves?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far I've only found holes. See for example &lt;a href=&quot;https://www.universetoday.com/73659/lro-takes-closer-look-at-moon-caves/&quot; rel=&quot;nofollow noreferrer&quot;&gt;LRO Takes a Closer Look at Moon Caves&lt;/a&gt; and also &lt;a href=&quot;https://www.universetoday.com/90031/russia-eyes-caves-on-moon-for-setting-up-a-lunar-base/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Russia Eyes Caves on Moon for Setting Up a Lunar Base&lt;/a&gt; where dimensions of 50 to 400 &lt;em&gt;meters&lt;/em&gt; (not kilometers) are mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I have found a ~50 km collapsed cave identified by surface features in this &lt;a href=&quot;https://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/lroc-20110217-chain.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA LRO page&lt;/a&gt;, but I think the breaking news is that a &lt;em&gt;new, different, not-collapsed sub-surface cave&lt;/em&gt; has been identified.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Previously identified collapsed cave from &lt;a href=&quot;https://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/lroc-20110217-chain.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;LRO imagery&lt;/a&gt;, captioned:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This section of WAC frame M117773324 shows the area where the feature transitions from a chain of collapse pits to a continuous uncollapsed segment. A large depression at the northern tip of the chain maybe a possible source region for the flow of lava across this region. The chain is approximately 50 km long. Image resolution 58.9 m/pixel [NASA/GSFC/Arizona State University].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0UK92.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0UK92.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-18T11:00:31.943" LastActivityDate="2017-10-18T12:05:03.253" Title="How did the JAXA lunar probe discover that there is a 50 km long cave on the Moon?" Tags="|the-moon|caves|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23434" PostTypeId="2" ParentId="23429" CreationDate="2017-10-18T11:13:17.373" Score="20" Body="&lt;p&gt;Besides problems of chemistry of explosives exposed to space (temperature/heating, electric charge due to ionizing radiation, evaporation/degradation) one major risk would be lack of air slowing down the debris.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An explosive blast ejects material at huge velocity. For most of that material, on Earth, air drag causes loss of that velocity within a very short distance - dust and small gravel falls within a hundred meters or so, of a typical quarry blast; only larger chunks fly farther, but they are relatively sparse and gravity brings them down to the surface, impacts removing their kinetic energy quickly - and due to the mass, initially they are usually slower than the smaller grains.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With lack of atmosphere and 1/6 Earth gravity, you have a lot of tiny shrapnel that can fly for many kilometers before impact, and hit whatever it hits at the same speed as when it was adjacent to the explosion. Particles thrown vertically upwards, will fall at the same speed they were ejected, possibly many minutes later. Pebbles bouncing off rock surfaces will lose some energy per bounce, but will continue to fly at speed from unexpected directions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means without massive shielding no blasting can occur near any infrastructure. Underground explosions like tunneling should be relatively doable, but all surface work, like leveling for roads would become very risky.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2017-10-18T12:24:42.270" LastActivityDate="2017-10-18T12:24:42.270" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="23435" PostTypeId="1" AcceptedAnswerId="23440" CreationDate="2017-10-18T11:22:18.393" Score="0" ViewCount="214" Body="&lt;p&gt;From &lt;a href=&quot;http://www.minister.industry.gov.au/ministers/sinodinos/media-releases/australia-and-united-states-sign-historic-space-treaty&quot; rel=&quot;nofollow noreferrer&quot;&gt;Australia and the United States sign historic space treaty&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In Washington this morning the Turnbull Government signed a treaty with the National Aeronautics and Space Administration (NASA), reaffirming a commitment made by Australia and the United States of America more than 50 years ago to collaborate on space exploration.&lt;/p&gt;&#xA;&lt;p&gt;Ambassador Joe Hockey signed the historic Space Tracking Treaty on behalf of Australia, with the Acting Administrator of NASA, Robert Lightfoot Jr, signing on behalf of the USA at a ceremony at the Australian Embassy in Washington D.C.&lt;/p&gt;&#xA;&lt;p&gt;The Space Tracking Treaty covers civil space facilities owned by NASA and located within Australia, including the Canberra Deep Space Communication Complex (CDSCC) at Tidbinbilla, as well as facilities in Western Australia and the Northern Territory.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There is of course a large, critical, and historic collaboration between Australia and the US on space matters, including the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt;. Is this a renewal, or a change in understanding? Is it related to the imminent formation of the Australian Space Agency (see &lt;a href=&quot;http://www.fleet.space/open-letter/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Open letter to the Australian Government: why Australia can no longer afford not to have a space agency&lt;/a&gt; and &lt;a href=&quot;http://www.spaceindustry.com.au/Documents/SIAA%20White%20Paper%20-%20Advancing%20Australia%20in%20Space.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;SIAA White Paper: Advancing Australia in Space&lt;/a&gt;), or perhaps the 50th anniversary of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_Space_Treaty&quot; rel=&quot;nofollow noreferrer&quot;&gt;Outer Space Treaty&lt;/a&gt;?&lt;/p&gt;&#xA;&lt;p&gt;I'm guessing that the signatories mentioned; director of NASA and Australian ambassador are symbolic - that the &lt;em&gt;treaty proper&lt;/em&gt; is between two countries, not two space agencies, and other agreements in the form of contracts are used between the agencies.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-18T17:06:06.173" Title="What is the nature and purpose of the treaty that US (NASA) has signed with Australia?" Tags="|nasa|space-agencies|treaty|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23436" PostTypeId="2" ParentId="23431" CreationDate="2017-10-18T12:05:03.253" Score="3" Body="&lt;p&gt;The mission is SELENE (Kaguya), and they &lt;a href=&quot;http://www.asahi.com/ajw/articles/AJ201710180045.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;used radar&lt;/a&gt; to detect a void under the surface:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In 2009, the Kaguya probe found a large shaft with an opening about 50 meters in diameter in the Marius Hills area. The shaft descends about 50 meters beneath the surface.&lt;/p&gt;&#xA;&lt;p&gt;The JAXA team analyzed data obtained from a &lt;a href=&quot;http://www.selene.jaxa.jp/en/equipment/lrs_e.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;lunar radar sounder&lt;/a&gt; on the probe that indicated an underground structure extended west from the shaft.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;LRS is the radar instrument:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;LRS is designed for sounding the surface and subsurface structures of the Moon by using HF radar technique with the frequency of 5 MHz. The low frequency radar method makes it possible to realize the mapping of the subsurface structure within a depth of several km with a range resolution of less than 100 m for a region with a horizontal scale of several tens of km.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-18T12:05:03.253" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23437" PostTypeId="2" ParentId="23429" CreationDate="2017-10-18T12:13:33.810" Score="5" Body="&lt;p&gt;This is reviving some long dormant brain cells ...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Electric detonators can unexpectedly explode because of stray electrical currents. Also, they may even be sensitive to certain frequencies of radio waves. For this reason, electric detonators are transported in metal cases/boxes. Non electrical detonators are transported in wooden boxes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The use of certain types of plastics, particularly conventional polyethylene, is prohibited with electrical detonators due to the potential for static electricity to develop and set off a detonator. Low static polyethylene can be used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because detonators, for what ever reason may explode, detonators and explosives are never transported in the same vehicles, nor in vehicles in close proximity to one another. Likewise, they are never stored in the same magazine, nor in magazines close to another, in case the explosives are initiated by the shock wave produced by the detonators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would not use dynamite, or any other nitroglycerine base explosives because they are shock sensitive. The sensitivity increases when the dynamite stick sweats - beads of nitroglycerine form on the outside of the stick. The sensitivity further increases if the beads of nitroglycerine crystallize.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nitroglycerine based explosives are not used as much as they used to be for these reasons. They have been replaced by other types of explosives. Unfortunately, most people only know about dynamite and TNT because of movies and old cartoons. They're not aware of other types of explosives.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because anyone handling explosives on the Moon would be wearing gloves, it would unlikely be an issue on the Moon, but handling nitroglycerine explosives and then wiping one's brow can cause nitroglycerine head aches, which are very unpleasant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Blasting with electric detonators can be fiddly. The detonators have to be wired up correctly, have no earth (in this case Moon) leakages/short-circuits and the whole circuit of detonators need to be tested with a special tester, similar to a resistivity meter, for short-circuits, prior to initiation of the blast.&lt;/p&gt;&#xA;" OwnerUserId="7062" LastEditorUserId="7062" LastEditDate="2017-10-18T13:00:34.297" LastActivityDate="2017-10-18T13:00:34.297" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23438" PostTypeId="2" ParentId="23429" CreationDate="2017-10-18T12:14:17.740" Score="8" Body="&lt;p&gt;We have some data, in the form of &lt;a href=&quot;https://science.nasa.gov/science-news/science-at-nasa/2007/23nov_flyingmoondust/&quot; rel=&quot;noreferrer&quot;&gt;high velocity debris from the Apollo LM descent engine&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;... Dust particles accelerated by a rocket's exhaust could theoretically travel all the way around the Moon!&lt;/p&gt;&#xA;&lt;p&gt;Metzger's team has analyzed how the impact craters formed on Surveyor 3 and finds that the particles must have been traveling at least 400 to 1,000 meters per second. &amp;quot;In fact, they may have been traveling as fast as the exhaust gases of the lunar lander - that is, at 1 or 2 kilometers per second.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Particles speeding horizontally at 1.7 kilometers per second will travel literally halfway around the Moon. Boost that speed to 2 kilometers per second, and the projectiles can completely circle the Moon. If no mountains got in the way, grit accelerated by a rocket landing could zip entirely around the Moon &amp;quot;and land back at the rocket's feet,&amp;quot; says Metzger.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This isn't much of an issue if you're alone on the Moon, but when a serious colonisation effort is under way, you'll have to take precautions not to puncture every habitat in the entire hemisphere. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Table_of_explosive_detonation_velocities&quot; rel=&quot;noreferrer&quot;&gt;detonation velocity&lt;/a&gt; of modern explosives easily exceeds 2 km/s.&lt;/p&gt;&#xA;&lt;p&gt;Some explosives are unsuitable for use on the Moon. Dynamite, for instance, &lt;a href=&quot;https://en.wikipedia.org/wiki/Dynamite&quot; rel=&quot;noreferrer&quot;&gt;sweats&lt;/a&gt; even on Earth (nitroglycerin migrates to the outside of the stick, and would evaporate in a vacuum).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-18T12:14:17.740" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="23439" PostTypeId="2" ParentId="22221" CreationDate="2017-10-18T12:17:16.833" Score="2" Body="&lt;p&gt;According to &lt;a href=&quot;https://space.stackexchange.com/a/15461/6117&quot;&gt;this answer&lt;/a&gt; Max q for the saturn V is 34kPa. Note that it is very easy to calculate q, as it depends only on velocity and atmospheric density. The aerodynamic stress depends on the shape of the rocket so is more difficult to calculate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We can get a ball-park figure for the force as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Force &#xA;= pressure x area&lt;/p&gt;&#xA;&#xA;&lt;p&gt;= 34kPa x (10m)^2 * PI / 4&lt;/p&gt;&#xA;&#xA;&lt;p&gt;= 2670kN&lt;/p&gt;&#xA;&#xA;&lt;p&gt;= 267000 kgf approx or 587000lbf approx.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is about a tenth of the launch weight of the saturn V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However it is likely a significant overestimate as the rocket does not stop the air that hits it completely, but rather pushes it out of the way. In order to get the actual drag force acting on the rocket, you have to multiply by the coefficient of drag which is a function of the shape of the vehicle, and is usually somewhat less than 1 (though it can be greater than 1.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope that helps.&lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="6117" LastEditDate="2017-10-18T23:31:13.943" LastActivityDate="2017-10-18T23:31:13.943" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23440" PostTypeId="2" ParentId="23435" CreationDate="2017-10-18T14:38:27.383" Score="4" Body="&lt;p&gt;The treaty appears to be called &amp;quot;The Agreement concerning Space Vehicle Tracking and Communications Facilities&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;Here's a summary page from &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=2&amp;amp;ved=0ahUKEwiOn5KBtfrWAhUl54MKHbOHDVgQFggsMAE&amp;amp;url=http%3A%2F%2Fwww.aph.gov.au%2Fparliamentary_business%2Fcommittees%2Fhouse_of_representatives_committees%3Furl%3Djsct%2F23august2011%2Freport%2Fchapter7.pdf&amp;amp;usg=AOvVaw23CnsoVEpJwYTMe8nKnk50&quot; rel=&quot;nofollow noreferrer&quot;&gt;this pdf&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ol start=&quot;3&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;2010 will mark the 50 th anniversary of treaty-level cooperation&#xA;between the US and Australia in space vehicle tracking. Australia's&#xA;cooperation with the US on space-related activities began in 1957 with&#xA;the establishment of facilities at Woomera in South Australia, to&#xA;track US satellites. This was broadened to include additional&#xA;scientific facilities set up by the US National Aeronautics and Space&#xA;Administration (NASA) in 1960.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Since then, the space vehicle tracking and communication relationship between Australia and the US has been the subject of a&#xA;succession of Agreements and Exchanges of Notes between the two&#xA;countries. Under these treaties, NASA has spent in excess of $A610&#xA;million on space-related activities in Australia since 1960.&#xA;Significant scientific and economic benefits have been derived from&#xA;activities conducted under the Agreement, especially through&#xA;encouraging collaboration between Australian and NASA scientists. The&#xA;treaty action under consideration further amends and extends the 1980&#xA;Program Agreement, which provides for the establishment, operation and&#xA;maintenance of NASA facilities in Australia that will provide&#xA;commensurate benefits for Australia.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;From what I can tell, the original agreement has been modified over the decades by the exchange of &amp;quot;diplomatic notes&amp;quot; between the countries. (An example of these exchanges can be found &lt;a href=&quot;https://www.state.gov/documents/organization/244967.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.)&lt;/p&gt;&#xA;&lt;p&gt;If there is a current document containing the complete, updated agreement with all the changes, I was unable to find it.  The State Department's Jan 1, 2017 &lt;a href=&quot;https://www.state.gov/documents/organization/273494.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Treaties in Force&lt;/a&gt; document lists this about the treaty:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Agreement concerning space vehicle tracking and communications facilities.&lt;/p&gt;&#xA;&lt;p&gt;Exchange of notes at Canberra  May 29, 1980.  Entered into&#xA;force May 29, 1980; effective February 26, 1980.&lt;br /&gt;&#xA;32 UST 1417; TIAS 9781; 1217 UNTS 237&lt;/p&gt;&#xA;&lt;p&gt;Amendment and Extensions:&lt;/p&gt;&#xA;&lt;p&gt;July 21, 1981 (TIAS 10198)&lt;/p&gt;&#xA;&lt;p&gt;January 17 and May 2, 1990 (TIAS 12413)&lt;/p&gt;&#xA;&lt;p&gt;August 4, 2000 (2171 UNTS 139)&lt;/p&gt;&#xA;&lt;p&gt;February 25, 2010&lt;/p&gt;&#xA;&lt;p&gt;August 19, 2011 and January 11, 2012&lt;/p&gt;&#xA;&lt;p&gt;November 18 and 21,  2013&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-10-18T17:06:06.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23441" PostTypeId="1" CreationDate="2017-10-18T17:45:37.163" Score="2" ViewCount="43" Body="&lt;p&gt;I know L1-3 are saddle points in the potential, and thus small perturbations would cause an object at one of these points to accelerate away from the point, but where would that object end up? For the purposes of this problem assume a 3 body system with M1&gt;&gt;M3 and M2&gt;&gt;M3. If it helps to assume M1&gt;&gt;M2 as well, I am fine with that assumption.&lt;/p&gt;&#xA;" OwnerUserId="21389" LastActivityDate="2017-10-18T17:45:37.163" Title="When an object in an unstable Lagrange point falls away from that point, where does it end up?" Tags="|orbital-mechanics|lagrangian-points|" AnswerCount="0" CommentCount="1" ClosedDate="2017-10-19T07:23:08.313" ContentLicense="CC BY-SA 3.0" />
  <row Id="23442" PostTypeId="2" ParentId="22221" CreationDate="2017-10-18T19:42:22.317" Score="0" Body="&lt;p&gt;Here's a graphic that shows &lt;a href=&quot;http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml&quot; rel=&quot;nofollow noreferrer&quot;&gt;dynamic pressure vs. mission time&lt;/a&gt; for the Shuttle: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0kCo8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0kCo8.jpg&quot; alt=&quot;Shuttle max-Q is about 700 lb/ft^2&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(apologies for the awful units, this was the first graph I found)&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-10-18T19:42:22.317" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23443" PostTypeId="2" ParentId="21749" CreationDate="2017-10-18T23:59:00.277" Score="3" Body="&lt;p&gt;EchoStar XV, EchoStar XXIII and XM-5 are all cited to have 18 to 20kW arrays, launched in 2010 and 2017. These are based on 20kW version of SSL 1300 bus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The exact numbers are hard to find, as some sources use maximum design capacity of the bus, some sources use launch-time power, and some end of life power.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;EchoStar-23 is one of SpaceX’s heaviest payloads to date, weighing&#xA;  around 12,000 pounds (5,500 kg). It carries a pair of X-shaped solar&#xA;  array “paddles” and can support an operational life span at&#xA;  geostationary altitude of 15 years. It is expected that the&#xA;  satellite’s End-of-Life (EOL) power will be in the region of 20&#xA;  kilowatts&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.americaspace.com/2017/03/12/pad-39a-ready-for-first-night-time-launch-in-seven-years-with-heavyweight-echostar-23-mission/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reference&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3140" LastEditorUserId="3140" LastEditDate="2017-10-19T04:26:45.610" LastActivityDate="2017-10-19T04:26:45.610" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23445" PostTypeId="1" CreationDate="2017-10-19T02:31:13.390" Score="38" ViewCount="1367" Body="&lt;p&gt;Mine shafts on earth are limited in how deep they can go by two things, internal heat and rock pressure. The deepest mine on Earth currently is just under 4 kilometers.&lt;/p&gt;&#xA;&lt;p&gt;There is internal heat on the moon caused by tidal pressures, but it is very deep. The motion core is a tiny portion of the whole compared to the earth. With ⅙&lt;sup&gt;th&lt;/sup&gt; G gravity, I wonder how deep you could go before rock pressure got higher than the compressive strength of the rock? I'm guessing pretty far.&#xA;My question is; how deep could a lunar mine go before heat or rock pressure made it impractical?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="7062" LastEditDate="2023-07-04T02:14:48.567" LastActivityDate="2023-07-04T02:14:48.567" Title="How deep could a lunar mine go?" Tags="|the-moon|planetary-science|mining|natural-resources|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="23447" PostTypeId="1" CreationDate="2017-10-19T11:43:02.990" Score="2" ViewCount="410" Body="&lt;p&gt;Skydivers normally wear a backup parachute. &lt;a href=&quot;https://space.stackexchange.com/questions/2521/did-any-apollo-missions-have-backup-parachutes&quot;&gt;Apollo 15&lt;/a&gt; survived with two out of three parachutes working properly. But wouldn't it be great to have a second deployment option, like skydivers do? The deployment maneuver itself must be a huge part of the risks with parachute landings from space. AFAIK reserve parachutes have never been used in space travel. Is the mass penalty of an independent secondary parachute system so big that it is easier to instead make sure it works every time?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="6816" LastEditDate="2017-10-19T16:08:24.060" LastActivityDate="2022-10-26T20:34:03.080" Title="Why haven't backup parachutes been used for spacecraft?" Tags="|landing|safety|parachute|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="23448" PostTypeId="1" CreationDate="2017-10-19T19:14:25.900" Score="6" ViewCount="1289" Body="&lt;p&gt;Can you use an aircraft to simulate Mars gravity on Earth, similar to the way weightlessness is simulated by fling in a parabolic arc?&#xA;What would the flight profile have to look like to achieve mars gravity? I know the acceleration required to achieve weightlessness, 9.8 m/s&lt;sup&gt;2&lt;/sup&gt;. &lt;/p&gt;&#xA;" OwnerUserId="21394" LastEditorUserId="897" LastEditDate="2017-10-20T07:35:51.650" LastActivityDate="2017-10-29T04:23:11.907" Title="Is it possible to simulate Mars gravity on Earth?" Tags="|microgravity|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23449" PostTypeId="1" CreationDate="2017-10-19T20:26:59.727" Score="3" ViewCount="94" Body="&lt;p&gt;On Earth we are thinking about reducing the cirrus clouds to cool off the planet Earth, but can cirrus clouds be created to add to Mars' atmosphere and keep the heat from escaping into space?&lt;/p&gt;&#xA;" OwnerUserId="21402" LastEditorUserId="6816" LastEditDate="2017-10-20T03:24:35.987" LastActivityDate="2021-08-31T04:45:30.377" Title="Can cirrus clouds be created on Mars?" Tags="|mars|terraforming|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23450" PostTypeId="1" AcceptedAnswerId="23451" CreationDate="2017-10-19T21:28:42.770" Score="6" ViewCount="3941" Body="&lt;p&gt;I was wondering if, and how, one can tell from the values in a TLE whether an orbit is LEO, MEO, HEO, or GEO. I'd like to do this in code eventually, so any formulas or threshold values would be much appreciated.&lt;/p&gt;&#xA;" OwnerUserId="21403" LastEditorUserId="6816" LastEditDate="2017-10-20T03:23:34.663" LastActivityDate="2020-11-23T23:34:57.613" Title="Determine orbit type from TLE" Tags="|orbit|low-earth-orbit|geostationary|orbital-elements|two-line-elements|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23451" PostTypeId="2" ParentId="23450" CreationDate="2017-10-19T21:46:39.857" Score="9" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set#Format&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, field 8 of TLE line 2 is the &amp;quot;mean motion in revolutions per day&amp;quot;; you can determine the orbital period from this.&lt;/p&gt;&#xA;&lt;p&gt;For geosynchronous orbit, you should expect 1.0 revolutions per day (in fact, due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Rotation_period#Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;complexity in the definition of &amp;quot;day&amp;quot;&lt;/a&gt;, they're very close to &lt;a href=&quot;https://celestrak.org/NORAD/elements/geo.txt&quot; rel=&quot;nofollow noreferrer&quot;&gt;1.0027&lt;/a&gt; as a rule).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Low_Earth_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;LEO&lt;/a&gt; defined as &amp;lt; 2000 km altitude should get periods between 84 and 127 minutes (i.e. ~11 to ~17 revolutions per day).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Medium_Earth_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;MEO&lt;/a&gt; defined as 2000 km to geostationary would then be 1-11 revs a day.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/High_Earth_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;HEO&lt;/a&gt; defined as higher than geostationary would be less than 1 rev per day.&lt;/p&gt;&#xA;&lt;p&gt;Highly elliptical orbits complicate this, obviously; it's up to you how you want to classify a satellite with perigee in LEO and apogee in MEO. You can &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_period#Small_body_orbiting_a_central_body&quot; rel=&quot;nofollow noreferrer&quot;&gt;derive the SMA from the period&lt;/a&gt;, and the &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_eccentricity&quot; rel=&quot;nofollow noreferrer&quot;&gt;eccentricity&lt;/a&gt; is in line 2 field 5, if you want the actual perigee and apogee.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2017-10-20T15:12:27.700" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23452" PostTypeId="1" CreationDate="2017-10-19T23:25:01.357" Score="16" ViewCount="885" Body="&lt;p&gt;Assuming the recently &lt;a href=&quot;https://www.theguardian.com/science/2017/oct/19/lunar-cave-discovery-raises-hopes-for-human-colonisation-of-moon&quot; rel=&quot;noreferrer&quot;&gt;discovered lava tube&lt;/a&gt; is exactly what scientists think it might be, and assuming you could build 2 walls and a door inside the lava tube, and assuming you filled it with a breathable atmosphere...&lt;/p&gt;&#xA;&lt;p&gt;How livable would such a place be?&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Would people need to wear protective clothing?&lt;/li&gt;&#xA;&lt;li&gt;Would the air pressure blow the roof off?&lt;/li&gt;&#xA;&lt;li&gt;Would the natural gravity be high enough to support life?&lt;/li&gt;&#xA;&lt;li&gt;Would the enclosure retain the right amount of heat to sustain a livable temperature?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Etc.  (These are sample questions, not necessarily relevant to the answer)&lt;/p&gt;&#xA;&lt;p&gt;In general, would the lunar lava tube, with nothing more than 2 new walls, and a breathable air pressure be able to sustain a livable environment?&lt;/p&gt;&#xA;" OwnerUserId="8653" LastEditorUserId="7062" LastEditDate="2021-10-06T15:58:14.597" LastActivityDate="2021-10-06T16:01:28.430" Title="How livable would a lunar lava tube be?" Tags="|the-moon|habitat|lava-tubes|lunar-base|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="23454" PostTypeId="2" ParentId="23430" CreationDate="2017-10-20T09:41:26.727" Score="5" Body="&lt;p&gt;The speed of a spacecraft may be measured from ground using the doppler effect. If the spacecraft sends a signal with a known and very precise frequency, the velocity may be determined by measuring the frequency shift caused by the movement of the spacecraft relative to ground.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Measuring the distance is possible by measuring the delay of a signal from ground to spaceship and back. The known speed of light is used to calculate the distance from the delay time.&lt;br&gt;&#xA;See &lt;a href=&quot;https://en.wikipedia.org/wiki/Doppler_effect&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Doppler_effect&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Radar&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Radar&lt;/a&gt; .  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The measurement of distance is done in the same way as using Radar, but when the spacecraft sends a signal back using its transmitter, the method may be used for very long distances too which could not be measured using passive Radar. This way the speed and distance of the Voyager probes may be measured.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2020-05-04T20:43:42.220" LastActivityDate="2020-05-04T20:43:42.220" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23457" PostTypeId="1" AcceptedAnswerId="23459" CreationDate="2017-10-20T14:44:20.677" Score="3" ViewCount="191" Body="&lt;p&gt;I'm very interested to see first person what Saturn's upper atmosphere looked like as Cassini began its descent into it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What footage exists of this, an where can it be viewed?&#xA;(I'm not interested in CGI simulations of 'what it would have looked like'.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If none exists, why not?&lt;/p&gt;&#xA;" OwnerUserId="21408" LastActivityDate="2017-10-20T16:45:09.937" Title="What video or imagery was captured onboard Cassini during its descent into Saturn's atmosphere?" Tags="|saturn|cassini|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23458" PostTypeId="2" ParentId="23448" CreationDate="2017-10-20T15:52:35.660" Score="11" Body="&lt;p&gt;The short answer is &lt;strong&gt;&quot;Yes, it's possible&quot;&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The typical technique to fly a zero-G parabola is to put the plane into a steep climb and when the airspeed is low enough, push forward on the yoke until the G-meter reads &quot;0&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars gravity is roughly (3.71 / 9.8 = 0.38G).  Instead of pushing to 0, the pilot just pushes to 0.38.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would result in a somewhat flatter arc and the experience would be a little bit longer than a comparable zero-G flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: personal experience.  I'm an aerobatic pilot and have flown similar profiles (but in a single seat plane, not a jet)&lt;/p&gt;&#xA;" OwnerUserId="12978" LastActivityDate="2017-10-20T15:52:35.660" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23459" PostTypeId="2" ParentId="23457" CreationDate="2017-10-20T16:27:01.210" Score="5" Body="&lt;p&gt;The raw images of Cassini can be viewed online in reverse order &lt;a href=&quot;https://saturn.jpl.nasa.gov/galleries/raw-images?order=earth_date+desc&amp;amp;per_page=50&amp;amp;page=0&quot; rel=&quot;noreferrer&quot;&gt;https://saturn.jpl.nasa.gov/galleries/raw-images?order=earth_date+desc&amp;amp;per_page=50&amp;amp;page=0&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="11397" LastActivityDate="2017-10-20T16:27:01.210" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23460" PostTypeId="2" ParentId="23457" CreationDate="2017-10-20T16:45:09.937" Score="2" Body="&lt;p&gt;As for the question, &quot;why not (why no film/more photos etc.)&quot;, the &lt;a href=&quot;https://www.space.com/38154-cassini-saturn-spacecraft-final-hours.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;limited data rate available&lt;/a&gt; for Cassini meant they had to limit the amount of photos taken in the last few hours.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-10-20T16:45:09.937" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23461" PostTypeId="1" CreationDate="2017-10-20T17:40:28.900" Score="3" ViewCount="141" Body="&lt;p&gt;Images from SOHO, SDO and other sun observatories are often coloured differently for different wavelengths or temperatures:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/sXxLa.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sXxLa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/WUxuv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WUxuv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;http://www.nasa.gov/mission_pages/sunearth/science/Sun-Wavelength-Chart.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nasa.gov/mission_pages/sunearth/science/Sun-Wavelength-Chart.html&lt;/a&gt; has statements like &quot;This light is emitted from the upper transition region and the chromosphere and are typically colorized in red.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is the convention of which temperature or wavelength gets which colour established?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it simply so that scientists can immediately tell what temperature or wavelength a picture shows, or are the colours optimized to show particular features?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And why is there a white ring or circle in the sunshade in some SOHO imagery, and why different sizes compared to the disc in the two photos above?&lt;/p&gt;&#xA;" OwnerUserId="555" LastEditorUserId="555" LastEditDate="2017-10-20T17:51:55.123" LastActivityDate="2017-10-20T17:51:55.123" Title="What's the rationale behind the false colours in solar observation photographs?" Tags="|observation|photography|the-sun|scientific-data|solar-dynamic-observatory|" AnswerCount="0" CommentCount="4" ClosedDate="2017-10-21T13:32:33.437" ContentLicense="CC BY-SA 3.0" />
  <row Id="23462" PostTypeId="2" ParentId="23448" CreationDate="2017-10-20T17:44:06.303" Score="5" Body="&lt;p&gt;Yes, and it is done in such parabolic flights as explained and showed in &lt;a href=&quot;https://www.youtube.com/watch?v=lVTTpKShVtE&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt;, (in French but with English closed-captions available) especially the cockpit scene between about &lt;code&gt;24:30&lt;/code&gt; and &lt;code&gt;26:30&lt;/code&gt; where you can watch the instruments and exterior view of the plane. Basically, you adjust the trajectory and speed of the aircraft to fit the required acceleration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MvoMJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MvoMJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="9072" LastEditorUserId="12102" LastEditDate="2017-10-21T00:57:23.210" LastActivityDate="2017-10-21T00:57:23.210" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23464" PostTypeId="2" ParentId="20778" CreationDate="2017-10-20T21:20:18.510" Score="3" Body="&lt;p&gt;On the right are the equations.&lt;br&gt;&#xA;  On the left is an example based on LEO around Terra.  &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; 5.97E24 kg    Mc = Central mass  &#xA;  900000 kg    Mb = mass of tether base  &#xA;  100000 kg    Ms = mass of shuttle  &#xA;                       Assume tether mass is negligible &#xA;6.67E-11 m^3/(s^2*kg)  G = Newton’s constant &#xA;3.986E14 m^3/(s^2)    mu = G*Mc&#xA; 7000000 m     Rb = distance from center of Mc to center of Mb&#xA;  100000 m     Lt = length of tether  &#xA;&#xA;                    With Mb in circular orbit it has speed &#xA;    7546 m/s   Vb = sqrt[mu/Rb]  and flight angle zero&#xA;&#xA;                    If tether hangs straight down, speed at foot is &#xA;    7438 m/s   Vf = Vb *((Rb-Lt)/Rb)  &#xA;&#xA; The term “dock” indicates that the shuttle matches velocity with the foot &#xA; of the tether, so that it is moving horizontally at Vf, and it stays on &#xA; the line between Mc and Mb.  &#xA;&#xA;                    The center of mass of the combination sits at &#xA; 6990000 m     Ca = Rb – Lt*(Ms/(Ms+Mb)) and has speed less than circular&#xA;    7535 m/s   Va = Vb * Ca/R , moving horizontally.  &#xA;&#xA;                    Find the eccentricity of the center of mass from &#xA;0.004280        e = sqrt[1–(Va^2*Ca/mu)*(2-Va^2*Ca/mu)*(cos(0))^2],&#xA;                    and the argument of the cosine is zero since the &#xA;                    velocity is horizontal.  &#xA;&#xA;                    Then the periapsis of the center of mass is &#xA; 6930426 m     Cp = Ca*(1-e)/(1+e) and the shuttle hangs below at&#xA; 6840426 m     Ds = Cp - Lt*(Mb/(Ms+Mb)) hwile the base is above the &#xA;                    center of mass at  &#xA; 6940426 m     Db = Cp + Lt*(Ms/(Ms+Mb))&#xA;&#xA;                    The speed of the center of mass at periapsis is&#xA;    7600 m/s   Vp = sqrt[(mu/Cp)*(1+e)] &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Caveat, this assumes that Mc, Mb, and Ms always remain in a line.  I think there is an oscillation about the center of mass, as the angular motion of the center of mass around Mc is not a constant.     - MBM&lt;/p&gt;&#xA;" OwnerUserId="12999" LastActivityDate="2017-10-20T21:20:18.510" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23466" PostTypeId="2" ParentId="23389" CreationDate="2017-10-21T06:14:02.353" Score="5" Body="&lt;p&gt;I believe the answer is yes--there should be a theoretical maximum rocket size for a given set of rocket technology (available structural materials, propellants, and engine design), but I don't currently have the resources at hand to give a specific answer.  Here are the places I would start looking:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Engine size.  Unless the Technology Readiness Level (TRL) of aerospikes gets improved by more testing, bell nozzles will probably get size-constrained by their cooling needs, which generally require plumbing the nozzle with a jacket of propellant.  More pipe requires more turbopump pressure, which certainly has some practical limit, though whether it gets dictated by pump size or the plumbing itself I don't know.  Once engines hit their size limit they have to multiply, requiring supporting structure to distribute their thrust into the rocket, driving down the propellant mass fraction, decreasing available delta-V and thus the capability to reach orbit.&lt;/li&gt;&#xA;&lt;li&gt;Propellant tank sizes would probably ultimately be limited by hoop stress, which increases in proportion to the radius of the tank.  For a given material and propellant choice there should be an ultimate tank size, after which there need to be multiple tanks with connecting structures, which begins driving the propellant mass fraction down, which directly affects the delta-V and thus the capability to reach orbit.&lt;/li&gt;&#xA;&lt;li&gt;As you get into REALLY big rockets, you may start to have structural torsion problems (essentially, the tail steers one way, but the rest of the rocket...doesn't) if you try to do steering without spreading the controlling forces along the structure.  This multiplication of steering motors, associated structure, plumbing, and controlling electronics should also eat into the propellant mass fraction.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="11262" LastActivityDate="2017-10-21T06:14:02.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23469" PostTypeId="2" ParentId="23389" CreationDate="2017-10-21T23:12:05.290" Score="3" Body="&lt;p&gt;Obviously.  There are limits to how tall a building can be because of the weight of the materials used.  A rocket is in a sense a building, you'll reach the structural limits a lot sooner with a rocket because such a low fraction of it's mass is capable of bearing a load at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As your rocket gets taller and taller more and more of it's mass will have to go into it's own strength, eventually you will reach a point that making it bigger reduces performance instead of increases it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no such limit on getting wider, however.  In practice, though, the wider it gets the more torsion will matter and the more precisely things will have to be balanced.  Somewhere down that road will lie a limit to what you can actually build without the rocket tearing itself apart but there is no theoretical limit in this aspect.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-10-21T23:12:05.290" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23470" PostTypeId="1" CreationDate="2017-10-22T08:47:53.107" Score="6" ViewCount="499" Body="&lt;p&gt;Were I able to build a space elevator (ignoring for the moment that I can't due to current material science etc), what issues would there be relating to atmospheric conditions? Presumably it would be a very good lightning conductor. Would this be an issue? Could it actually be used for the capture of energy? My thoughts are that it might prevent the occurrence of lightning as any electrical buildup in its vicinity would be dissipated to earth by the conductor itself.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, it would provide a direct conductor from the solar wind down to the earth, so would there be a way of collecting energy from this? Could it be protected from this with a self-generated magnetic field?&lt;/p&gt;&#xA;" OwnerUserId="21423" LastEditorUserId="6816" LastEditDate="2017-10-22T20:19:30.003" LastActivityDate="2017-10-27T02:20:21.283" Title="Atmospheric effects on a space elevator" Tags="|space-elevator|solar-wind|lightning|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23471" PostTypeId="1" AcceptedAnswerId="43693" CreationDate="2017-10-22T12:33:44.473" Score="10" ViewCount="356" Body="&lt;p&gt;I'm interested in studying where in Lunar orbit a station could be deployed.  Due to mascons, Lunar orbits below about 200km will eventually decay according to &lt;a href=&quot;https://space.stackexchange.com/questions/19944/moon-orbit-station-keeping-delta-v-budget&quot;&gt;this answer&lt;/a&gt;.  Presumably, if the goal of such a station were to just be in Lunar orbit, then an orbit above 200km is preferred.  However, if the goal were also to land, or conduct closer surface reconnaissance, lower orbits might be preferred.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the relationship between orbital altitude and delta V per day to maintain orbital stability?  For simplicity I'm happy to assume a circular, equatorial orbit (although a polar orbit would make more sense for landing).&lt;/p&gt;&#xA;" OwnerUserId="21424" LastActivityDate="2020-04-25T07:10:52.573" Title="How much delta v is required to keep a low Lunar orbital station stable?" Tags="|lunar-orbit|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23472" PostTypeId="2" ParentId="23338" CreationDate="2017-10-22T14:14:34.870" Score="1" Body="&lt;p&gt;A cite from &lt;a href=&quot;https://aviation.stackexchange.com/questions/12648&quot;&gt;https://aviation.stackexchange.com/questions/12648&lt;/a&gt; about air turbulences : &quot;The wings act as a structural dampener to disturbances such as turbulence, similar to the suspension on a car. As they flex, they absorb the sudden energy changes.&quot;&lt;br&gt;&#xA;A space shuttle must sustain air turbulences too and needs very flexible wings, but wings made from reinforced concrete are stiff and not flexible. Not every landing would be influenced by turbulences, but it is not possible to totallay avoid them. Decades ago I experienced a flight through air turbulences as a passenger and saw how flexible the wings are.&lt;br&gt;&#xA;But not only during the atmospheric flight back heavy vibrations are possible, also during the burn of the boosters vibrations are frequent and strong. A reusable shuttle should be constructed for a very high number of flex cycles.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-10-22T14:14:34.870" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23473" PostTypeId="2" ParentId="23470" CreationDate="2017-10-22T22:02:17.427" Score="9" Body="&lt;p&gt;According to &quot;Space Elevator Technology and Research&quot;, John Knapman, International space Elevator Consortium, JBIS Vol 69 issue 6/7 pp.211-219; the major issues are ice, wind pressure (particularly in the troposphere where it can exceed 1kPascal) with electrical storms only being mentioned in respect of protecting payload and the climber solar panels. (This paper strongly references &quot;Space Elevators: An Assessment of the Technological Feasibility and the Way Forward&quot;, Peter Swan et al; which may be a better source of information, but I don't have access to it)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tether material is presumably NOT a good conductor, as beamed power is the &quot;usual&quot; method of powering the climbers - although a carbon nanotube cable to supply a self climbing fairing for the first 40km is considered.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;IAC-13-D4.3.7 &quot;How do Intense Magnetic Storms affect a Space Elevator&quot; A.M. Jorgsen and S.E. Patmia may also be of interest&lt;/p&gt;&#xA;" OwnerDisplayName="user20636" LastEditorDisplayName="user20636" LastEditDate="2017-10-22T22:16:06.733" LastActivityDate="2017-10-22T22:16:06.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23475" PostTypeId="2" ParentId="23448" CreationDate="2017-10-23T00:36:13.783" Score="2" Body="&lt;p&gt;Suppose at the start of the parabola the aircraft has&#xA;initial altitude $H$ meters,&#xA;speed $V_o$ m/s,&#xA;and is moving upward at an angle to the horizontal $M ^{\circ}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the desired gravitational force is $G m/s^2$&#xA;then the aircraft must accelerate vertically at $(9.81-G) m/s^2$.&#xA;The horizontal component of the speed is constant at $V_x(t) = V_o\times \cos M$,&#xA;so that the horizontal co-ordinate after $t$ seconds is $X(t) = V_o\times \cos M\times t$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initially the vertical component of the speed is $V_y(t=0) = V_o\times\sin M$.&#xA;The vertical component of the speed is $V_y(t) =  V_o\times\sin M -(9.81-G)\times t$.&#xA;At the peak vertical speed is zero, and occurs at $t_{\text{max h}} = V_o\times\sin M/(9.81-G) $.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The vertical co-ordinate is $$Y(t) =  H + V_o\times\sin M\times t - {(9.81-G)\times(t^2)\over 2}$$&#xA;The maximum height attained is&#xA;$$Y(t_{\text{max h}}) = H+V_o\times\sin M\times[V_o\times\sin M/(9.81-G)] - (9.81-G)\times{[V_o\times\sin M/(9.81-G)]^2\over 2}$$&#xA;By symmetry it takes $t_{\text{max h}}$ seconds to go from the peak to the base altitude $H$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Greatest altitude and time at reduced gravity occur if the initial angle is $90 ^{\circ}$, but steep angles introduce other problems. For a given initial airspeed and initial angle, a larger perceived gravity (as of Mars) has longer reduced gravity time than a smaller perceived gravity (Luna or freefall). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There must be sufficient airspace below the base height to accelerate and maneuver into the parabola, and recover from the parabola. At the peak there must be sufficient air density and indicated air speed for the control surfaces to function. The indicated air speed may be significantly less than the true air speed, and the pilot must compensate for winds that vary with altitude. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am not sure that the perceived gravity will be perpendicular to the aircraft floor. For free fall that would not matter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example:&#xA;$$\begin{array}{lll}&#xA;  H                 &amp;amp; 6000 m      &amp;amp; \text{base height}                   \\&#xA;  V_o               &amp;amp; 200 m/s     &amp;amp; \text{initial speed}                 \\&#xA;  M                 &amp;amp; 30 ^{\circ} &amp;amp; \text{initial flight angle}          \\&#xA;                    &amp;amp; 3.71 m/s^2  &amp;amp; \text{desired perceived gravity}     \\&#xA;                    &amp;amp; 6.10 m/s^2  &amp;amp; \text{needed vertical acceleration}  \\&#xA; X(t)               &amp;amp; 173 m/s     &amp;amp; \text{horizontal component of speed} \\&#xA;  t_{\text{max h}}  &amp;amp; 16.4 s      &amp;amp; \text{time to max height}            \\&#xA;Y(t_{\text{max h}}) &amp;amp; 6820 m      &amp;amp; \text{max height}&#xA;\end{array}$$&lt;/p&gt;&#xA;" OwnerUserId="12999" LastEditorUserId="6816" LastEditDate="2017-10-29T04:23:11.907" LastActivityDate="2017-10-29T04:23:11.907" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23476" PostTypeId="1" AcceptedAnswerId="23477" CreationDate="2017-10-23T02:27:39.630" Score="5" ViewCount="428" Body="&lt;p&gt;After a Falcon 9 has its static fire several days prior to the actual launch, are the TEA-TEB ignitors inside the engine replenished? Or do they have enough to ignite the engines for the static fire and the actual mission?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="6816" LastEditDate="2017-10-23T19:15:40.193" LastActivityDate="2017-10-23T19:16:33.653" Title="Do Falcon 9 preflight static fire ignitions use up the engines' ignitors?" Tags="|launch|spacex|engines|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23477" PostTypeId="2" ParentId="23476" CreationDate="2017-10-23T05:13:06.207" Score="10" Body="&lt;p&gt;Only the in-flight restarts are supplied from onboard TEA-TEB storage. The launch and static fire ignition uses fluids supplied from the ground tanks. There was &lt;a href=&quot;http://www.wacotrib.com/blogs/joe_science/the-tea-teb-glitch-can-t-light-a-falcon-without/article_1b7c4ae6-5a16-11e3-afbb-0019bb2963f4.html&quot; rel=&quot;noreferrer&quot;&gt;a problem on the ground supply&lt;/a&gt; on the SES-8 launch &amp;mdash; &lt;a href=&quot;https://twitter.com/elonmusk/statuses/406806983023820800&quot; rel=&quot;noreferrer&quot;&gt;the tank was contaminated&lt;/a&gt; and the flight computer aborted the launch after detecting non-standard readings during ignition.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Only three engines have the onboard ignitor available on the first stage as others do not restart after launch. There has to be another engine which can start mid-flight &amp;mdash; the second stage one (capable of multiple restarts too).&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="6816" LastEditDate="2017-10-23T19:16:33.653" LastActivityDate="2017-10-23T19:16:33.653" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23478" PostTypeId="1" AcceptedAnswerId="23479" CreationDate="2017-10-23T12:28:10.417" Score="3" ViewCount="1832" Body="&lt;p&gt;How much energy would it require to get the ISS around or on the moon? And how much fuel would that require? And how many cargo missions would that require? And if that is not worth it, would it be beneficial to disassemble the ISS and transport the useful parts to the moon?&lt;/p&gt;&#xA;" OwnerUserId="21429" LastEditorUserId="512" LastEditDate="2017-10-23T14:40:01.870" LastActivityDate="2019-07-06T22:40:49.333" Title="How much energy would it require to get the ISS to the moon?" Tags="|iss|the-moon|" AnswerCount="1" CommentCount="1" ClosedDate="2017-10-23T20:55:18.087" ContentLicense="CC BY-SA 3.0" />
  <row Id="23479" PostTypeId="2" ParentId="23478" CreationDate="2017-10-23T18:44:36.407" Score="3" Body="&lt;p&gt;Since you ask several progressive questions, ill try to give you a progressive answer. Going to the moon with the ISS and landing it are 2 completely different topics, therefore I will focus in getting in a Lower Lunar Orbit (LLO)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How much energy would it require to get te ISS around the moon&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To go from a Lower Earth Orbit (LEO), in which the ISS resides now, to a LLO requires a delta-V budget of roughly 4.04 KM/s. This consists of 2 parts, accelerating until we reach an lunar transfer orbit and then decelerate unit we are in an orbit around the moon. The latter of the 2 cost roughly 840 m/s, whereas the transfer burn is roughly 3200 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How much fuel would that require&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket engines efficiency is rated in Isp. In short, the higher the Isp the more efficient a rocket will propel you. The amount of fuel required is therefore related to the Isp of the rocket used to propel the ISS. In the next section I will give some Isp examples and the required amount of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simple calculator to try for yourself can be found &lt;a href=&quot;http://www.quantumg.net/rocketeq.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How many cargo missions would that require&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get to the moon we want to accelerate the ISS, but in doing so we also accelerate the fuel that it is caring. Since the amounts of fuel are massive, it would be more probable that the ISS would be supplied with fuel, burn on its periapsis and then be resupplied to do another round. It does need to be able to decelerate when it reaches the moon. To incorporate this in the calculations I have taken the weight of the ISS as &lt;a href=&quot;https://www.nasa.gov/feature/facts-and-figures&quot; rel=&quot;nofollow noreferrer&quot;&gt;420000 kg&lt;/a&gt;, except for the last LEO burn, when it also carries the weight of 840 m/s worth of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Currently the ISS requires some delta-V to maintain its orbit, which is being provided by an &lt;a href=&quot;http://www.space-propulsion.com/spacecraft-propulsion/bipropellant-thrusters/200n-bipropellant-thrusters.html#moved&quot; rel=&quot;nofollow noreferrer&quot;&gt;ATV&lt;/a&gt;. To give you an idea, we will first look what happens if we would use those.&#xA;The ATVs used have 4.7 tonnes of propellant and an Isp of 270s. This gives it a delta-v budget of a whopping 29 m/s. This would require 110 trips with a burn in LEO and another 33 that have to be used to provide fuel to decelerate at the moon, giving a total of 143 trips. Of course a these also carry a lot of supplies and with 143 trips instead of the normal annual trip we could expect that it would be entirely filled with fuel.&#xA;If we look fill the entire cargo area this would provide 12 tonnes of propellant. &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/ATV/Automated_Transfer_Vehicle&quot; rel=&quot;nofollow noreferrer&quot;&gt;(cite)&lt;/a&gt;&#xA;This would reduce the burns in LEO to 43 with 13 to supply the ISS with braking propellant. But with a total of 56, this is still a lot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead of using the existing propulsion system one with a better Isp could be designed. Where the ATVs have only an Isp of 270, an higher Isp engine could be used. For example the &lt;a href=&quot;https://en.wikipedia.org/wiki/Centaur_(rocket_stage)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Centaur &lt;/a&gt;with an Isp of 450s, would require a lot less fuel. With the largest payload to LEO at this moment being &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Launch_Vehicles/Delta_IV_Users_Guide_June_2013.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;28790&lt;/a&gt; kg using the Delta IV, we would only need 18 burns in LEO and only 3 cargo ships to supply the braking propellant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note however that I did not include the weight of the fuel tanks required to store the braking fuel in the ISS, so this will increase the amount of propellant needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;If it is worth it?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course this question also has a financial side, therefore I calculated the amount of trips it would take to use &lt;a href=&quot;http://www.spacex.com/falcon-heavy&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX Falcon Heavy&lt;/a&gt;, which with a price of 2200 $/kg is, according to &lt;a href=&quot;https://space.stackexchange.com/questions/1989/what-is-the-current-cost-per-pound-to-send-something-into-leo&quot;&gt;this&lt;/a&gt; topic is the cheapest per Kg to LEO. (Although it isn't available yet). For the record, with 63800 kg to LEO and a Isp of 311, it would require 7.5 burns in LEO and 2 launches to supply the braking propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Summing up to a total of roughly 607.000 kg of propellant to be supplied to the ISS at a total cost of 1.3 billion dollars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my own opinion I would say no. &#xA;Although this is only 1% of the cost of the ISS as is, a lot of the compartments that we have now are simply not needed at the moon. It might be worth it to take one or 2 single modules, but with the technological advancement it might even be better to take new and improved equipment. &lt;/p&gt;&#xA;" OwnerUserId="17153" LastEditorUserId="10273" LastEditDate="2019-07-06T22:40:49.333" LastActivityDate="2019-07-06T22:40:49.333" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23480" PostTypeId="2" ParentId="834" CreationDate="2017-10-24T02:04:40.630" Score="-2" Body="&lt;p&gt;We might answer this question with greater clarity had we a knowledge of every speck of matter in our star system.  In addition to gravity assists, velocity transfers via impacts (guided or not) could impinge.  Thus with accurate knowledge of the orbits of every asteroid, we might with little effort, engender a chain of gravity assists and/or body impact events which would lead to maximum interstellar velocity attainable with current means.&lt;/p&gt;&#xA;" OwnerUserId="21433" LastActivityDate="2017-10-24T02:04:40.630" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23481" PostTypeId="2" ParentId="14075" CreationDate="2017-10-24T02:15:36.503" Score="5" Body="&lt;p&gt;50 seconds is actually possible but AFIAK there's no commercial jet with the performance to do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every second of zero g produces a downward velocity of 9.8 meters per second.  Thus 50 seconds is 490 meters per second.  If you were heading up at 245 m/s when you start the zero g maneuver you would be heading down at 245 m/s when you do your pullout.  Since this is only about 70% of the speed of sound you can do this while staying subsonic.  However, you will have to be going pretty close to vertical at the start of the parabola to keep your plane subsonic--and that means you need a plane that can do a near vertical climb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the specs for the &lt;em&gt;Zero G&lt;/em&gt; airplane I find it's a 727-200.  While I don't know what engines it has the best performance for the class is a thrust to weight ratio of .26 at max takeoff weight, up to .46 completely empty (if you could somehow get it into the sky with no fuel.)  That's nowhere near what it would take to make the climb.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only aircraft I find that can do it are fighters--and they're not limited by having to stay subsonic anyway.  You will also use 10,000 feet of altitude on the loop--and if you do the pullout at 2g you'll use another 10,000 doing it.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-10-24T02:15:36.503" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23482" PostTypeId="2" ParentId="834" CreationDate="2017-10-24T06:52:24.667" Score="2" Body="&lt;p&gt;The ecliptic plane of our solar system is at an angle of &lt;a href=&quot;https://www.physicsforums.com/threads/ecliptic-planes-relation-to-galactic-plane.9666/&quot; rel=&quot;nofollow noreferrer&quot;&gt;about 60º&lt;/a&gt; relative to the Galactic plane of our galaxy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Milky_Way&quot; rel=&quot;nofollow noreferrer&quot;&gt;Milky Way&lt;/a&gt; is about 100,000 ly (30 kpc) in diameter, and, on average, approximately 1,000 ly (0.3 kpc) thick. This means that when you leave the solar system along our ecliptic plane, you can visit the stars in our neighborhood, but you 'quickly' leave the galaxy. &#xA;You can use a gravity assist from one of the gas giants to aim your trajectory into the Galactic plane, allowing you to visit far more stars. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-10-24T06:52:24.667" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23483" PostTypeId="1" AcceptedAnswerId="23485" CreationDate="2017-10-24T08:23:26.937" Score="16" ViewCount="954" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/J002E3&quot; rel=&quot;noreferrer&quot;&gt;J002E3&lt;/a&gt; is believed to be the S-IVB third stage of the Apollo 12 rocket.  It is expected to return to temporary Earth orbit in the 2040's.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If some enterprising soul decided to build a rocket capable of capturing it and returning it to Earth (not going to happen, I know) for study/to put in a museum, what state would we expect it to be in?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would expect that it's less likely to have taken physical blows than objects in a stable orbit closer to the Earth, but equally likely, it would probably have been subject to more radiation etc.  Is this correct, and how would it have affected it?&lt;/p&gt;&#xA;" OwnerUserId="6241" LastEditorUserId="25" LastEditDate="2017-10-24T14:18:23.753" LastActivityDate="2018-02-21T11:00:19.083" Title="What will J002E3 be like on its return?" Tags="|rockets|apollo-program|radiation|debris|long-duration|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23485" PostTypeId="2" ParentId="23483" CreationDate="2017-10-24T14:47:32.613" Score="8" Body="&lt;p&gt;We don't know exactly what kind of shape it will be in. There are a few similar objects that have been returned, however, that we can get at least an idea of what it might look like. Let's look at the comparisons:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Surveyor 3- Landed on the Moon, was visited (Ironically) by the crew of Apollo 12. &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720019081.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;A report&lt;/a&gt; was done on the damage to this craft. 2 years on moon.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Long_Duration_Exposure_Facility&quot; rel=&quot;nofollow noreferrer&quot;&gt;LDEF&lt;/a&gt;- Basically this was a satellite placed in to orbit by the space shuttle, and recovered years later, to see what would happen. 6 years in LEO&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/European_Retrievable_Carrier&quot; rel=&quot;nofollow noreferrer&quot;&gt;EURECA&lt;/a&gt;- Same thing as LDEF, but from ESA, not NASA. 1 year LEO&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Genesis_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;GENESIS&lt;/a&gt;- Solar sample return, crashed on Earth after 3.5 years.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Stardust_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stardust&lt;/a&gt;- Comet Dust- small capsule returned to Earth. Deep space, 7 years.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Hayabusa&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hayabusa&lt;/a&gt;- Asteroid sample return, 7 years Deep Space.&lt;/li&gt;&#xA;&lt;li&gt;Many of the missions to deep space have included a dust counter. I'm going to look at the &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1029/2010GL043300/full&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venetia Student Dust Counter&lt;/a&gt;, on New Horizons.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A few other things. The dust environment in interplanetary space is quite a bit less than that around Earth. I would expect there to be some small impacts, but nothing compared to the LDEF mission, for instance. The dust levels seen in space have been measured, look at the SDC from New Horizons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jlVJf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jlVJf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dust levels are quite low, so we can assume that there will have been some, but not a significant number of small impacts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radiation is of some note. There is a &lt;a href=&quot;http://www.ijche.com/article_10361_3e7db599f47c97944b269467141aeb1a.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;good article&lt;/a&gt; about the effects of paint in space. Specifically, it states:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Ultraviolet radiation is particularly damaging  to white paints used on the external surfaces of satellites. UV radiation causes significant increase  in  the  solar  absorptance  of  white paints,  while  it  decreases  the  absorptance  of black paints due to bleaching effects.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bottom line, the black paint will become lighter, and the white paint darker.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, there is some indication that the paint, might bubble, which would lead to some chipping in the paint over time. This is based on the bubbles seen on &lt;a href=&quot;https://space.stackexchange.com/q/17615/25&quot;&gt;Apollo 17&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-02-21T11:00:19.083" LastActivityDate="2018-02-21T11:00:19.083" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23487" PostTypeId="1" AcceptedAnswerId="23488" CreationDate="2017-10-24T23:47:47.193" Score="4" ViewCount="320" Body="&lt;p&gt;Molecular clouds in deep space are fascinating objects. Various organic molecules have been found there (including alcohol, heh heh).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if one of these clouds contained about 20% of oxygen, would it be possible to breathe it in the inner regions of the cloud?&lt;/p&gt;&#xA;" OwnerUserId="1477" LastActivityDate="2017-10-25T12:25:42.060" Title="Could a molecular cloud be breathable?" Tags="|life|deep-space|interstellar-medium|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23488" PostTypeId="2" ParentId="23487" CreationDate="2017-10-24T23:59:22.400" Score="11" Body="&lt;p&gt;Not a chance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Those &quot;clouds&quot; are a vacuum, but just a vacuum that has a few more particles than outside those &quot;clouds&quot;.  The density of those &quot;clouds&quot; around &lt;a href=&quot;https://en.wikipedia.org/wiki/Molecular_cloud#Giant_molecular_clouds&quot; rel=&quot;noreferrer&quot;&gt;1–100 particles per cm³&lt;/a&gt;.  You can't breathe it at all.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may want to look at this Scaling in Density page at &lt;a href=&quot;https://www.astro.princeton.edu/~dns/teachersguide/ScDensAct.html&quot; rel=&quot;noreferrer&quot;&gt;Princeton University&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth: 5.5 gram/cm³, $1.8 \cdot 10^{23}$ molecules per cm³,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Earth atmosphere: 0.0012 g/cm³ (&lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Earth&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;),&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Molecular Clouds: $2 \cdot 10^{-22}$ gram/cm³, 10 molecules per cm³.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2017-10-25T00:04:56.613" LastActivityDate="2017-10-25T00:04:56.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23489" PostTypeId="2" ParentId="21246" CreationDate="2017-10-25T02:27:41.037" Score="12" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Thanks, that much is clear; the question is more why can they not make&#xA;  do with out the helium? Because the shuttle didn't need it and the MCT&#xA;  reportedly also won't need it, so why the falcon?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;All rockets need to keep the tank pressurized during flight.  Some use hot gases from the engines to do that.  Others don't.  The fuel used in the rocket is the main reason why many use helium. The the falcon 9 uses RP1 which is basically Kerosene. When you heat Kerosene you don't get enough hot gas to maintain tank pressure. So you need fuels that expand greatly in volume when heated.   You could get enough hot oxygen from the engine to do that job but if you mix hot oxygens with kerosene you could have an  internal tank fire and explosion.  So oxygen is not compatible with any fuel.  Helium and nitrogen are two good choices.  I believe The Saturn V used nitrogen to maintain tank pressure in the first stage.&lt;/p&gt;&#xA;" OwnerUserId="21444" LastActivityDate="2017-10-25T02:27:41.037" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23490" PostTypeId="1" CreationDate="2017-10-25T07:47:38.283" Score="10" ViewCount="847" Body="&lt;p&gt;Would a large ceramic golfball absorb less heat on reentry than a standard sphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From &lt;a href=&quot;http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml&quot; rel=&quot;noreferrer&quot;&gt;aerospaceweb.org&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The difference in the flowfields around a smooth sphere and a rough, or dimpled, sphere can be seen above. Since the laminar boundary layer around the smooth sphere separates so rapidly, it creates a very large wake over the entire rear face. This large wake maximizes the region of low pressure and, therefore, results in the maximum difference in pressure between the front and rear faces. As we have seen, this difference creates a large drag like that seen below the transition Reynolds number.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Would the reduction in heat absorbed be significant enough to warrant a larger payload via lessening the weight of the heat shield?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nPxXD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nPxXD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="7075" LastEditorUserId="12102" LastEditDate="2017-10-25T09:06:33.930" LastActivityDate="2023-03-20T13:40:09.147" Title="Would a dimpled heat shield reduce heat transfer during reentry?" Tags="|reentry|heat-shield|aerodynamics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23491" PostTypeId="1" CreationDate="2017-10-25T08:52:50.550" Score="15" ViewCount="2435" Body="&lt;p&gt;This might be a stupid question, but I can't find and don't know the answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Everyone knows that Neil Armstrong and Edwin &quot;Buzz&quot; Aldrin were the first men to land on/ walk on the moon. But were they the first to physically touch the moon? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I assume dust particles contaminated their suits and the LM and no doubt they inadvertently made contact, but did they actually deliberately skin-to-sample touch (as opposed to through an EVA suit) the moon rock samples? Or were they preserved for analysis on Earth?&lt;/p&gt;&#xA;" OwnerUserId="723" LastEditorUserId="12102" LastEditDate="2021-09-05T23:09:05.590" LastActivityDate="2021-09-06T08:03:19.047" Title="Who was the first person to touch the moon rocks?" Tags="|the-moon|history|apollo-11|sample-return|spaceflight-firsts|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23492" PostTypeId="2" ParentId="23490" CreationDate="2017-10-25T12:10:37.843" Score="17" Body="&lt;p&gt;The dimpled sphere may produce lower drag (though as Uwe points out I’m not sure if that is true in hypersonic flight). However, this does not mean that it will be better at surviving reentry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Higher drag implies a faster reentry and shorter exposure to extreme temperatures. Conducting heat to the core takes time, so even if the outer layer is ablating, the core may be less affected by brief exposure to extreme temperatures than by long exposure to moderate temperatures.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wake is relatively cool and doesn’t need much shielding - if the hypersonic flow does resemble that diagram, the second ball would need approximately double the shielding of the first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Turbulent flow is generally avoided in heat shield design - turbulence implies convection, providing a way for more heat to reach the surface of the sphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dimpled sphere has a larger surface area. More surface exposed to hot air means more heat absorbed.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-10-25T12:10:37.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23495" PostTypeId="2" ParentId="23491" CreationDate="2017-10-25T17:40:57.500" Score="10" Body="&lt;p&gt;First of all, there are many Lunar Meteorites that have been found, but I'm assuming you don't mean those.&lt;/p&gt;&#xA;&lt;p&gt;The first bits of the Apollo moon rocks to be touched were dust inadvertently touched by the 2 astronauts that landed on the Moon. All of them touched at least some dust samples on the return trip that were contained outside of the container. Some of it might have been deliberate, see the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11transscript_cm.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;transcripts&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;05 11 13 55 CMP How big are the rocks that you just scurried around and picked up with the tongs? Good gravy! Beautiful! Just crack those guys open and get a - you know, virgin interior of them in a vacuum, and they'll have a ball. ...&lt;/p&gt;&#xA;&lt;p&gt;05 11 14 19 CMP Hey, the Velcro - the Velcro around them sort of ...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In addition, the workers who helped the astronauts get out of the Apollo capsule are known to have touched at least some dust. One of their jobs was to get dust off their suits by using tape.&lt;/p&gt;&#xA;&lt;p&gt;The next candidate are the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_11_lunar_sample_display&quot; rel=&quot;nofollow noreferrer&quot;&gt;samples&lt;/a&gt; that were sent to world leaders from Apollo 11. While there is no record of the dust that was put in to the display being touched, it seems quite likely that it was at some point in time.&lt;/p&gt;&#xA;&lt;p&gt;I have searched for some kind of a record of a scientist mishandling one of the moon rocks, and I can't find any definitive record of such.&lt;/p&gt;&#xA;&lt;p&gt;There are &lt;a href=&quot;https://www.airspacemag.com/space/in-the-museum-the-rock-28431174/&quot; rel=&quot;nofollow noreferrer&quot;&gt;3 rocks&lt;/a&gt; that the public may touch with their bare hands. The first of these went on display in 1976.&lt;/p&gt;&#xA;&lt;p&gt;In addition, there are a few Apollo rocks that have been stolen, and likely touched.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="10259" LastEditDate="2021-08-12T15:01:46.093" LastActivityDate="2021-08-12T15:01:46.093" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="23496" PostTypeId="1" AcceptedAnswerId="35527" CreationDate="2017-10-25T18:28:51.737" Score="7" ViewCount="351" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/High_Altitude_Venus_Operational_Concept&quot; rel=&quot;noreferrer&quot;&gt;HAVOC&lt;/a&gt; concept for exploring Venus with airships has been made public almost three years ago now, but the scientific publication seems to never have been cited, neither am I aware of any official information about a follow-up study. Is the concept currently completely out of focus of NASA or is it being developed further?&lt;/p&gt;&#xA;" OwnerUserId="6917" LastActivityDate="2019-04-15T14:11:56.043" Title="Are there any updates / follow ups on the HAVOC Venus exploration concept?" Tags="|nasa|venus|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23497" PostTypeId="2" ParentId="12803" CreationDate="2017-10-26T12:01:00.710" Score="0" Body="&lt;p&gt;From the command line in a Linux/Unix (possible in windows with the wget installed)&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;wget URL&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;So get the TLE distributed from amsat.org&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;wget http://www.amsat.org/amsat/ftp/keps/current/nasa.all&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;However there are some annoying headers to this so...&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;tail +16 nasa.all  &amp;gt; nasa.tle&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;All together&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;wget http://www.amsat.org/amsat/ftp/keps/current/nasa.all&#xA;tail +16 nasa.all  &amp;gt; nasa.tle&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;You now just need to put it in a script file and run this as a cron (windows read scheduled task).&lt;/p&gt;&#xA;" OwnerUserId="21453" LastActivityDate="2017-10-26T12:01:00.710" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23500" PostTypeId="1" AcceptedAnswerId="23503" CreationDate="2017-10-26T15:16:46.373" Score="6" ViewCount="277" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/A/2017_U1&quot; rel=&quot;noreferrer&quot;&gt;C/2017 U1 PANSTARRS&lt;/a&gt; is a comet with an eccentricity of close to 1.2, so it is a strong candidate for having interstellar origin. It's perihelion was at 0.25 AU.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What speed did it have relative to the Sun when it swung around it at the closest?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2017-10-26T21:03:30.090" Title="What speed did the (interstellar) comet C/2017 U1 PANSTARRS have at perihelion?" Tags="|comet|escape-velocity|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23501" PostTypeId="1" CreationDate="2017-10-26T17:05:55.363" Score="1" ViewCount="69" Body="&lt;p&gt;I'm not a physicist or rocket man, but I laid awake thinking about this all night and I just wanted expert corrections to help with my basic understanding. So... 483,000 miles per hour is how fast our sun is moving through it's galactic orbit. What my thought is about is this: given my layman's understanding of relativity, it would appear to me that if you're flying, say, 20k mph towards mars, you aren't actually &quot;flying towards&quot; mars at all. Instead you've simply managed to slow down enough for Mars to catch up to you. Even at full thrust you are still technically moving &quot;backwards.&quot; If you reverse this logic and fly towards the sun, you are still traveling at 483k mph plus whatever speed the shuttle can add. Can you slingshot around the sun/venus, without sacrificing your speed, to reach the outer solar system incredibly fast? The basic thought is, if you fly away from the sun you're slowing down, if you fly towards the sun you're speeding up. If you want to reach the outer solar system, fly towards the sun first. If you leave orbit and can succeed in a gravity slingshot around the sun or venus, would you not pass earth traveling a relative speed of almost 1000k mph? I'm sorry if this is a bad question, but I have tried to learn a little about relativity and to me it would seem this makes sense. &lt;/p&gt;&#xA;" OwnerUserId="21456" LastActivityDate="2017-10-26T17:05:55.363" Title="Can the sun/Venus be used to slingshot back out into the outer solar system fast?" Tags="|crewed-spaceflight|interstellar-travel|" AnswerCount="0" CommentCount="0" ClosedDate="2017-10-26T22:37:10.973" ContentLicense="CC BY-SA 3.0" />
  <row Id="23502" PostTypeId="2" ParentId="1028" CreationDate="2017-10-26T19:13:42.680" Score="2" Body="&lt;p&gt;While living in China (2004 to 2007), I overheard that China was hiring every space engineer its universities were producing every year. So, at the time, I heard that their center in Gansu province was the place for over 300000 space related workers. Of course no one told me if they were working under the same entity... Could include many &quot;private&quot; enterprise as well...&lt;/p&gt;&#xA;" OwnerUserId="21457" LastActivityDate="2017-10-26T19:13:42.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23503" PostTypeId="2" ParentId="23500" CreationDate="2017-10-26T20:58:03.777" Score="8" Body="&lt;p&gt;Taking &lt;code&gt;q = 0.2531011&lt;/code&gt; AU (perihelion radius) and &lt;code&gt;e = 1.1937160&lt;/code&gt; from the &lt;a href=&quot;http://www.minorplanetcenter.net/mpec/K17/K17UI5.html&quot; rel=&quot;noreferrer&quot;&gt;latest ephemeris for A/2017 U1&lt;/a&gt;, we get a semimajor axis of $$a = \frac{r_p}{1-e} = -1.3065575~\textrm{AU}.$$ Using the definitions of 1 AU and $\mu_{\textrm{Sun}}$ from &lt;a href=&quot;https://ssd.jpl.nasa.gov/?constants&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;, we get the heliocentric velocity at perihelion of $$v_p = \sqrt{\mu_{\textrm{Sun}}\left(\frac{2}{r_p} - \frac{1}{a}\right)} = \boxed{87.6872~\frac{\textrm{km}}{\textrm{s}}}$$ (assuming I did my math right). This is subject to change as the orbit gets refined with further observations.&lt;/p&gt;&#xA;" OwnerUserId="9060" LastEditorUserId="9060" LastEditDate="2017-10-26T21:03:30.090" LastActivityDate="2017-10-26T21:03:30.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23506" PostTypeId="2" ParentId="23470" CreationDate="2017-10-27T02:20:21.283" Score="2" Body="&lt;p&gt;I have done some internet research on this before out of interest. I'll list the things I found or thought of that could effect a space tether bellow. Hopefully some one else with some real knowledge on the subject can give you an answer, I would also like to know what outside effects would affect a space tether.  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Condensation: this could add a lot of extra weight, and cause a lot of water to run down the tether.&lt;/li&gt;&#xA;&lt;li&gt;Earths magnetic field: if it is conductive and if you run power through it. It would generate electricity or force moving through the earths magnetic field&lt;/li&gt;&#xA;&lt;li&gt;Lightning/electrostatic: &lt;/li&gt;&#xA;&lt;li&gt;temperature: cable could expand and contract in different places at different speeds &lt;/li&gt;&#xA;&lt;li&gt;Moon/tide: gravitation effect on the cable would change for different heights at different times of the day.&lt;/li&gt;&#xA;&lt;li&gt;radio waves: If conductive I assume would act as a big antenna &lt;/li&gt;&#xA;&lt;li&gt;wind, rain, ice: all your normal weather activity as well.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What I think would also be interesting with some of the effects on a space tether would be how you would stop small effects from building up and destroying the tether like happened in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tacoma Bridge&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example if the tether got a wave traveling up the tether created by tidal forces could if have harmonic build up? Would you need to add dampener like they do with tall buildings?  &lt;/p&gt;&#xA;" OwnerUserId="10094" LastActivityDate="2017-10-27T02:20:21.283" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23507" PostTypeId="2" ParentId="22006" CreationDate="2017-10-27T08:14:15.143" Score="2" Body="&lt;p&gt;I think that your guess addressing the decreasing with the orbital radius $(1/r)^n$, $n=3...\infty$ of the non spherical terms for the gravity potential are probably the clue to the problem. This effect increases when you are more close to the major body. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be of interest to analyze the eigenvalues of the rotational dynamics system. When considering vehicle shape one can find plots like this one&#xA;&lt;a href=&quot;https://i.stack.imgur.com/fbT44.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fbT44.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This plot was generated using a spherical gravity model, but considering the assimetries in the orbiter. $k_1, k_3 = f(I_1, I_2, I_3)$, and eigenvalues are proportional to $\sqrt{\mu/a^3}$, so if we are in the unstable case the unstabilities grows faster when going closer to the central body. A similar analysis could be performed including a non spherical gravity field body.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More a comment than an answer. Nice question by the way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: link to explanation video&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://es.coursera.org/learn/spacecraft-dynamics-kinetics/lecture/DAQum/3-gravity-gradient-linear-stability-about-equilibria&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://es.coursera.org/learn/spacecraft-dynamics-kinetics/lecture/DAQum/3-gravity-gradient-linear-stability-about-equilibria&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21262" LastEditorUserId="21262" LastEditDate="2017-10-27T13:45:29.250" LastActivityDate="2017-10-27T13:45:29.250" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23508" PostTypeId="2" ParentId="22006" CreationDate="2017-10-27T15:43:46.090" Score="6" Body="&lt;p&gt;There is one wheel still operable (at least it was operable the last time they operated it), but Dawn no longer uses it. The third wheel failure was in April of this year (2017 for those reading this answer thousands of years from now).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes, gravity tugging on the very long lever arms called solar panels is a significant driver on hydrazine usage, made much worse the lower the orbit is. (Tidal forces go as $1/r^3$ or higher powers of $r$, so the forces are a strong function of altitude.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That hydrazine expenditure is mainly about attitude, not maintaining the orbit parameters. They do still use the ion engines to adjust the orbit. But that operation also consumes hydrazine to point the ion engines in the desired direction, and for roll control during the maneuver.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://dawn.jpl.nasa.gov/mission/journal_05_24_17.html&quot; rel=&quot;noreferrer&quot;&gt;blog post by Marc&lt;/a&gt; explains more about the reaction wheel situation. If you want to know more about the end-game hydrazine budget, you could try to email Marc with your questions. I'd wager that he'll answer since he loves to talk about this stuff. You can find his email on people.nasa.gov.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2017-10-27T15:43:46.090" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23509" PostTypeId="2" ParentId="21146" CreationDate="2017-10-27T18:31:23.720" Score="0" Body="&lt;p&gt;Orion reentry from on board camera:&#xA;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/j1vmVJKqUFE?start=8&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="14191" LastActivityDate="2017-10-27T18:31:23.720" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23510" PostTypeId="1" CreationDate="2017-10-27T23:12:37.493" Score="2" ViewCount="374" Body="&lt;p&gt;Are there any protocols the space station (or any space explorer) follows if there is a massive catastrophe on Earth where the responsive parties on the ground would not be available for support/control? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Something like large scale nuclear war or natural disaster where communication and satellites are inoperable or there’s no one left to communicate with? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps it doesn’t matter at that point, but what would they do? &lt;/p&gt;&#xA;" OwnerUserId="6627" LastActivityDate="2017-11-27T18:24:38.847" Title="Are there any protocols the space station follows if there is an Earth-wide catastrophe?" Tags="|communication|space-station|communication-satellite|earth|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23511" PostTypeId="2" ParentId="23510" CreationDate="2017-10-28T01:00:59.373" Score="0" Body="&lt;p&gt;I don't know, but I'd be surprised if there's anything they could do to help continue civilization or our species. The ISS doesn't have nearly enough people or resources for that.&lt;/p&gt;&#xA;" OwnerUserId="21464" LastActivityDate="2017-10-28T01:00:59.373" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23512" PostTypeId="1" AcceptedAnswerId="23529" CreationDate="2017-10-28T01:47:15.840" Score="3" ViewCount="213" Body="&lt;p&gt;The NYTimes article &lt;a href=&quot;https://www.nytimes.com/2017/10/26/movies/bill-nye-science-guy-review-documentary.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Review: ‘Bill Nye: Science Guy,’ a Portrait of a Fighter for Facts&lt;/a&gt; includes a link to the film's &lt;a href=&quot;https://youtu.be/aQC2xqUXQb4&quot; rel=&quot;nofollow noreferrer&quot;&gt;trailer on YouTube&lt;/a&gt; and a photo of Bill Nye with a model of a 3U cubesat attached to a solar sail.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think this is one of the &lt;a href=&quot;http://www.planetary.org/explore/projects/lightsail-solar-sailing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planetary Society's LightSail spacecraft&lt;/a&gt; and the demonstration of the sail for orbit raising in LEO is illustrated in this &lt;a href=&quot;https://youtu.be/-OmGvycgNCg?t=156&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planetary Society video&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the four solar panels are deployed so that they are almost perpendicular to the plane of the sail, and face four directions, so that one or two of them will receive solar radiation when the sail is oriented perpendicular to the sun for minimal photon pressure, but much less light when the sail is tilted to receive maximum photon pressure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Will the panels be articulated so that all four could receive reflected light simultaneously from the sail when oriented for maximum thrust, or will they remain static in the orientation shown in all of these illustrations? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6M6a6.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6M6a6.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2dpjs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2dpjs.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jKuWQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jKuWQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Bill Nye, from &lt;a href=&quot;https://www.nytimes.com/2017/10/26/movies/bill-nye-science-guy-review-documentary.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NYTimes&lt;/a&gt; &lt;em&gt;Lindsay Mann/Structure Films, PBS,&lt;/em&gt; cropped.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6Wwct.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6Wwct.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-30T05:46:33.700" LastActivityDate="2017-10-30T05:46:33.700" Title="Will the Planetary Society's LightSail spacecraft's solar panels be articulated during each orbit?" Tags="|solar-power|cubesat|solar-sail|planetary-society|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23513" PostTypeId="1" AcceptedAnswerId="23517" CreationDate="2017-10-28T02:48:29.537" Score="3" ViewCount="292" Body="&lt;p&gt;The image is from NASA Media Advisory M17-124 &lt;a href=&quot;https://www.nasa.gov/press-release/georgia-students-to-speak-with-nasa-astronauts-on-space-station&quot; rel=&quot;nofollow noreferrer&quot;&gt;Georgia Students to Speak with NASA Astronauts on Space Station&lt;/a&gt; but there is no mention of the cool shirts. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &lt;em&gt;The six Expedition 53 crew members gather together in the Destiny laboratory module for a group portrait. From left are astronauts Joe Acaba, Paolo Nespoli, and Mark Vande Hei, Commander Randy Bresnik and cosmonauts Sergey Ryazanskiy and Alexander Misurkin. On Monday, Acaba, Vande Hei and Bresnik will speak with students from New Prospect Elementary School in Alpharetta, Georgia&lt;/em&gt;. Credits: NASA&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yH9SC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yH9SC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Cropped detail of same image.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1pPls.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1pPls.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-10-28T14:58:10.570" Title="Expedition 53 group portrait - what's up with the cool shirts?" Tags="|iss|space-art|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23514" PostTypeId="2" ParentId="17609" CreationDate="2017-10-28T03:12:59.100" Score="2" Body="&lt;p&gt;It is the volume problem that must be solved to leave LEO.  LEO means you can be resupplied and abort back to Earth in an emergency.  With Cis lunar or farther you have a series of non-linear scaling problems that go in a step-wise fashion that greatly increase mass and volume required to &quot;bring everything with you&quot;.  Neither of these can be expanded in a way that is meaningful, regardless of the good points on limitations the comments point out like radiation shielding.  In order to solve the volume and mass problem, given the rocket equation and the limitations it places on launches and mass to orbit on each launch, there is basically no way to build a deep space capable craft without making it in section and assembling it in LEO just like the ISS.  Once the parts are all together and proven to function, more supplies and fuel need to be sent up, along with an second propulsion module because one failure that leads to total loss of propulsion that far away leads to total loss of crew, which is unacceptable.  Same goes for other critical functions, so you must bring spare parts and the tools and expertise to replace them.  On the ISS they remove and replace, sending back to Earth for repair, not an option for far beyond LEO.  Estimates for a 900 day journey (to allow the planets to align and use reasonable fuel) requires about eight fully loaded Cygnus extended capacity missions, which carry 3500 KG and are 25 m^3 each all connected together.  This means that even if the Boeing or SpaceX capsules could be connected together in this way, they would need about the same of them 8-10 all connected together.  This does NOT allow for MARS habitats on the ground or ascent/descent vehicles.  Lets say we almost DOUBLE the mass and volume needed for that.  That makes 8-9 missions sent three years in advance to pre-position in Mars orbit THEN 10 launches assembled in LEO to go with a crew of 6.  So, NO, they cannot be expanded in a meaningful way to allow the occupants to live beyond LEO without another 10 missions all connected together to support.&lt;/p&gt;&#xA;" OwnerUserId="21465" LastEditorUserId="25" LastEditDate="2017-12-19T19:21:08.247" LastActivityDate="2017-12-19T19:21:08.247" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23517" PostTypeId="2" ParentId="23513" CreationDate="2017-10-28T14:58:10.570" Score="5" Body="&lt;p&gt;Randy Bresnik tweeted about it! Practicing 'Aloha Friday' wearing space themed Aloha Shirts at work.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Our Earth is but an island in the cosmos. Exp 53 crew is embracing the island lifestyle with our @Space_Station Hawaiians on #Aloha Friday!&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/AstroKomrade/status/911191244469977089&quot; rel=&quot;noreferrer&quot;&gt;https://twitter.com/AstroKomrade/status/911191244469977089&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Aloha_shirt#Aloha_Friday&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Aloha_shirt#Aloha_Friday&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19014" LastActivityDate="2017-10-28T14:58:10.570" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23518" PostTypeId="1" CreationDate="2017-10-28T16:08:54.460" Score="2" ViewCount="676" Body="&lt;p&gt;I've seen cubesat projects talk about the altitude of their satellite.  Starting orbit is, presumably, known based on deployment conditions.  But how does one track the altitude once it's been in orbit for a while?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Do the satellites have some some way to determine altitude and transmit it as part of the telemetry downlink? Presumably from GPS data, as direct measurement sensors seem hard to do - a barometer would need to be incredibly sensitive to be meaningful at that low pressure, and radar altimeters would use too much power.  Some other sensor?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or are there ground tracking systems that offer this info?  Is the info public? How good is the resolution on such?&lt;/p&gt;&#xA;" OwnerUserId="21469" LastActivityDate="2017-10-28T19:46:15.933" Title="Determining altitude of a cubesat/nanosat" Tags="|orbital-mechanics|cubesat|altitude|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23519" PostTypeId="2" ParentId="23518" CreationDate="2017-10-28T17:46:48.117" Score="3" Body="&lt;p&gt;There are ground tracking systems that provide this info, usually in the form of TLEs. It is public, except where it is secret. Amateur tracking provides some information on secret satellites. &lt;/p&gt;&#xA;" OwnerDisplayName="user20636" LastActivityDate="2017-10-28T17:46:48.117" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23520" PostTypeId="2" ParentId="23510" CreationDate="2017-10-28T17:49:46.373" Score="1" Body="&lt;p&gt;What should the ISS astronauts do? Staying longer on ISS is not possible without supply from Earth. Returning to Earth may be done without ground control, but after landing the astronauts would definitly need help to leave the Soyuz capsule and to return from the landing spot in the desert. Just after landing, the fitness of the astronauts is much worse than before their start to the ISS.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-10-28T17:49:46.373" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23521" PostTypeId="1" AcceptedAnswerId="23523" CreationDate="2017-10-28T18:09:53.230" Score="43" ViewCount="4517" Body="&lt;p&gt;Attached to the Voyager probes are the &lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_Golden_Record&quot; rel=&quot;nofollow noreferrer&quot;&gt;golden records&lt;/a&gt;, which are essentially messages-in-a-bottle sent out into the vast sea of interstellar space, containing, of course, the &quot;message of humanity&quot;. Although the primary purposes of these records may have merely been commemorative - and not an actual attempt to come in contact with extraterrestrial life - I think most people have an infinitesimal hope that someday in the far future an extraterrestrial civilization will retrieve these records and &quot;learn of the human species&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, suppose another extraterrestrial species has done the same thing, and one of their &quot;Voyager probes&quot; (carrying it's own &quot;golden record&quot;) comes travelling straight towards us. For simplicity lets assume that its composition is very similar, if not identical, to our Voyager probes. My question is, if it came near Earth, would we be able to retrieve it? An important qualifying question would be, how close would it need to come for us to even detect it in the first place?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know this question is quite speculative and loose, but I think it's an interesting thought.&lt;/p&gt;&#xA;" OwnerUserId="10123" LastEditorUserId="10123" LastEditDate="2019-03-15T16:08:15.237" LastActivityDate="2019-03-15T16:25:39.733" Title="Would we be able to detect an extraterrestrial &quot;Voyager&quot; if it came near us?" Tags="|artificial-satellite|probe|voyager|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="23523" PostTypeId="2" ParentId="23521" CreationDate="2017-10-28T18:58:57.283" Score="18" Body="&lt;p&gt;We should assume that there is no possibility to communicate with the extraterrestrial Voyager. The probe would be out of power, the antenna is not directed to Earth and we don’t know the frequency, modulation and protocol for communication. Therefore the probe may be detected by radar from Earth only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The asteroid Apophis was detected by radar over a distance of 0.192 AU, see this NASA &lt;a href=&quot;https://cneos.jpl.nasa.gov/doc/apophis/image1.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;page&lt;/a&gt;. But the probe is much smaller than Apophis and the radar echo would be very weak. I guess radar detection would be possible for a distance of some hundred kilometers distance from Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But to retrieve the probe we would need to build a special spacecraft to fly to the probe, enclose it into a return capsule with heat shield for reentry and to fly back to earth. For the design of the return capsule we should know the size (width, depth and height) and mass of the probe with good precision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will the necessary spacecraft be ready before the extraterrestrial probe is to far away? I think it would be too difficult for our available technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if we detect the probe by radar, would we recognise it as an extraterrestrial probe? We would need much more information than only a radar echo. An optical image from a very close distance would be necessary, less than kilometers. A picture with much more resolution than a picture of the ISS made from ground.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2019-03-15T16:25:39.733" LastActivityDate="2019-03-15T16:25:39.733" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="23524" PostTypeId="1" AcceptedAnswerId="23586" CreationDate="2017-10-28T19:40:41.643" Score="6" ViewCount="1424" Body="&lt;p&gt;In the &lt;a href=&quot;https://youtu.be/ugl8q8btMAs?t=56&quot; rel=&quot;noreferrer&quot;&gt;YouTube Trailer&lt;/a&gt; for the PBS program &lt;a href=&quot;http://www.pbs.org/a-year-in-space/home/?utm_source=youtube&amp;amp;utm_medium=pbsofficial&amp;amp;utm_campaign=beyondyearinspace_2017&quot; rel=&quot;noreferrer&quot;&gt;A Year in Space&lt;/a&gt; at &lt;code&gt;00:56&lt;/code&gt; a small spinning object can be seen, attached to &lt;em&gt;a spacecraft well above&lt;/em&gt; the ISS. I think the video is significantly speeded up so the object looks blurred.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It reminds me a bit of the &lt;a href=&quot;https://winds.jpl.nasa.gov/missions/RapidScat/&quot; rel=&quot;noreferrer&quot;&gt;RapidScat&lt;/a&gt; (shown below), a microwave scatterometer that measures near-surface wind speed and direction, mentioned &lt;a href=&quot;https://space.stackexchange.com/q/16569/12102&quot;&gt;here&lt;/a&gt; and explained &lt;a href=&quot;https://space.stackexchange.com/a/8742/12102&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this an antenna of some kind or something else? What spacecraft is it attached to?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; RapidScat&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cdkOW.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cdkOW.gif&quot; alt=&quot;RapidScat&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; GIF from the &lt;a href=&quot;https://youtu.be/ugl8q8btMAs?t=56&quot; rel=&quot;noreferrer&quot;&gt;YouTube Trailer&lt;/a&gt; at about &lt;code&gt;00:56&lt;/code&gt;. The motion is clearer in the original video linked below it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KLIMG.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KLIMG.gif&quot; alt=&quot;GIF from video&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/ugl8q8btMAs?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-29T04:12:50.837" LastActivityDate="2018-06-09T23:18:22.633" Title="What is this spinning object seen on a spacecraft above the ISS?" Tags="|iss|antenna|identify-this-object|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23525" PostTypeId="2" ParentId="23518" CreationDate="2017-10-28T19:46:15.933" Score="3" Body="&lt;p&gt;To communicate with the cubesat from a ground station, it is necessary to know before the date and time interval and the direction of the antenna. If the altitude of the satellite is not known before, the orbit and timing of the cubesat is not known and the necessary direction of the ground station antenna is unknown.&lt;br&gt;&#xA;A mathematical model for the orbit of the cubesat is necessary and also the variation of the orbit due to drag of the high atmosphere. There is some uncertainity about the actual drag due to atmospheric variations.&lt;br&gt;&#xA;If there is communication with ground station, distance to cubesat and its speed may be measured.&lt;br&gt;&#xA;It was possible to watch the orbits of satellites in the decades of space science before GPS was available. Before the GPS system was operative, the GPS satellites should be inserted into very precise orbits.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-10-28T19:46:15.933" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23526" PostTypeId="2" ParentId="8698" CreationDate="2017-10-28T22:41:43.950" Score="5" Body="&lt;p&gt;Can't add a comment, so here is an answer instead:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Xenon grade is quite pure - research or the aptly named propulsion grade.&lt;/li&gt;&#xA;&lt;li&gt;Costs ~$15/liter.  Some quotes can be higher.  I would imagine if you were purchasing many 10's of kiloliters, you could get a price break.&lt;/li&gt;&#xA;&lt;li&gt;Xenon capture/recovery is for terrestrial testing.  As Swiatek's paper details, NASA Glenn does this.  During testing, the xenon is captured on the cryo surfaces.  These panels do not work in atmosphere.  The recovery process captures that xenon during the cryo pump regeneration instead of just discharging it to the atmosphere.&lt;/li&gt;&#xA;&lt;li&gt;Yes, xenon for a deep space mission costs a lot.  It is also quite expensive for testing.&lt;/li&gt;&#xA;&lt;li&gt;As pointed out, these missions cost a lot, so the xenon cost is a small percentage.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="21471" LastActivityDate="2017-10-28T22:41:43.950" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23529" PostTypeId="2" ParentId="23512" CreationDate="2017-10-29T09:12:49.600" Score="4" Body="&lt;p&gt;No. The position of the panels seems to be fixed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar panels are &lt;a href=&quot;http://www.planetary.org/blogs/jason-davis/2017/20171020-ls2-closeout.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;spring-loaded&lt;/a&gt;, and have &lt;a href=&quot;http://www.planetary.org/blogs/jason-davis/2015/20150602-panels-wednesday-sails-friday.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;no actuator&lt;/a&gt; for exact positioning. It looks like the spring pushes the solar panel against an end stop.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dXChh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dXChh.jpg&quot; alt=&quot;LightSail 2 solar panels&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-10-29T09:12:49.600" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23530" PostTypeId="2" ParentId="23521" CreationDate="2017-10-29T10:30:53.443" Score="4" Body="&lt;p&gt;I think the other answers have made an unnecessary assumption. Namely, that upon detection we have to retrieve it before it starts moving away from us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think that's necessary (albeit obviously preferable). All we'd have to do is plot its course with sufficient accuracy to be able to predict its future course. Assuming it passes close enough for detection &lt;a href=&quot;https://space.stackexchange.com/a/23523/12941&quot;&gt;which this answer addresses&lt;/a&gt;, then it should be feasible to predict its future motion given that we can do this for asteroids.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having done that, we can then work on the technology to overtake and retrieve it at our leisure.&lt;/p&gt;&#xA;" OwnerUserId="12941" LastActivityDate="2017-10-29T10:30:53.443" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23531" PostTypeId="2" ParentId="23524" CreationDate="2017-10-29T12:58:43.520" Score="7" Body="&lt;p&gt;This is a rotating antenna, part of the Kurs docking system on Soyuz and Progress spacecraft &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://youtu.be/CoLRdl4bzsw&quot; rel=&quot;noreferrer&quot;&gt;See e.g. This video of a docking&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="548" LastActivityDate="2017-10-29T12:58:43.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23532" PostTypeId="2" ParentId="4211" CreationDate="2017-10-29T14:46:29.910" Score="2" Body="&lt;p&gt;I had a similair question, and using pyephem as suggested by zdRan worked great, except for one thing:  Those are instructions for Debian/Ubuntu/etc. distributions and I was on a bare-bones CentOS install.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In case anyone else runs into this, here's the install instructions, to be used in place of the first block in zdRan's post:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;sudo yum install python&#xA;sudo yum install python-devel&#xA;sudo yum upgrade python-setuptools&#xA;sudo yum install epel-release&#xA;sudo yum install python-pip&#xA;sudo yum install gcc&#xA;sudo pip install pyephem&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;The &lt;code&gt;epel-release&lt;/code&gt; is needed since pip isn't part of the core pacakges for CentOS, but is part of the extended packages.  &lt;code&gt;gcc&lt;/code&gt; only needs to be installed if you don't already have it (you can use &lt;code&gt;whereis gcc&lt;/code&gt; to check, but yum won't install if already there, so not strictly necessary).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From there, proceed as above, works great.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(This was on CentOS 7, but a similair procedure should work for most CentOS/Red Hat/Fedora distributions)&lt;/p&gt;&#xA;" OwnerUserId="21469" LastActivityDate="2017-10-29T14:46:29.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23534" PostTypeId="2" ParentId="14002" CreationDate="2017-10-29T20:41:51.817" Score="3" Body="&lt;p&gt;To offer some supplementary information and perspective on viewing satellites, some years ago two Amateur astronomers got a &quot;knock on the door&quot;, after using a 12&quot; guided telescope they 'stacked' over 100 images of a US spy satellite. One can read further at &lt;a href=&quot;https://www.scientificamerican.com/article/amateur-astronomers-spy-satellite/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scientific American&lt;/a&gt; and Phil Plait's &lt;a href=&quot;http://www.slate.com/blogs/bad_astronomy/2010/05/28/amateur_astronomer_spies_on_spy_satellites.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bad Astronomy&lt;/a&gt; and links therein.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The article they published (which I'm still looking for) was something like &quot;Why are US Spy Satellites painted Orange&quot;, with modern day digital cameras and optics there are a lot of people doing this as a hobby it seems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So in some respects there is no need to go to space, although getting above the atmosphere and &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_seeing&quot; rel=&quot;nofollow noreferrer&quot;&gt;seeing&lt;/a&gt;&quot; effects does offer the best resolution with all else being equal. &lt;/p&gt;&#xA;" OwnerUserId="21453" LastEditorUserId="12102" LastEditDate="2017-10-31T04:16:57.070" LastActivityDate="2017-10-31T04:16:57.070" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23535" PostTypeId="1" CreationDate="2017-10-29T22:06:23.403" Score="5" ViewCount="443" Body="&lt;p&gt;I'm facing a problem I can't explain and I would like to know if someone has more information about it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm drawing a solar system and when I plot earth with the Horizon cartesian coordinates, it does not match the orbit plotted via the Horizon orbital elements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Here is the query of the orbital elements&lt;/strong&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/OfVV2.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OfVV2.png&quot; alt=&quot;Query of earth&amp;#39;s orbital elements&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Here is the answer&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&#xA;2458055.500000000 = A.D. 2017-Oct-29 00:00:00.0000 TDB &#xA; EC= 1.693936505240274E-02 QR= 9.828535727121969E-01 IN= 1.619499879517213E-03&#xA; OM= 1.180185004251146E+02 W = 3.460022671282024E+02 Tp=  2458123.158659660257&#xA; N = 9.859206292830410E-01 MA= 2.932939316914255E+02 TA= 2.914962922586066E+02&#xA; A = 9.997893698231427E-01 AD= 1.016725166934089E+00 PR= 3.651409548675242E+02&#xA;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I'm using EC and A to plot the ellipse. Here are my calculations :&#xA;Center of ellipse X : -EC * A&#xA;Center of ellipse Y : 0&#xA;Radius of ellipse X : A&#xA;Radius of ellipse Y : A * sqrt(1 - EC^2)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Here is the query of the cartesian coordinates&lt;/strong&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/T6LQC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/T6LQC.png&quot; alt=&quot;Query of earth&amp;#39;s cartesian coordinates&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;And here is the answer :&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&#xA;2458055.500000000 = A.D. 2017-Oct-29 00:00:00.0000 TDB &#xA; X = 8.085189297377026E-01 Y = 5.770742450594002E-01 Z =-2.783714659030374E-05&#xA; VX=-1.027876320855459E-02 VY= 1.393441165141858E-02 VZ= 7.146303246417249E-08&#xA; LT= 5.737033579585715E-03 RG= 9.933365717771512E-01 RR=-2.711916121500719E-04&#xA;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;I'm plotting earth at the given X, Y, Z coordinates&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And I don't understand the result. Earth is almost on its orbit but not exactly. It's off by 1,275... % and I don't understand why.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fxexQ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fxexQ.png&quot; alt=&quot;Wrong position of the orbit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it because the keplerian elements are approximations or is it because my drawing of the orbit is not very well made atm ? In other words, should the earth be exactly on its orbit when using the Horizon data or is this error expected ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you very much&lt;/p&gt;&#xA;" OwnerUserId="21481" LastEditorUserId="23870" LastEditDate="2018-03-14T08:59:09.873" LastActivityDate="2018-06-12T10:49:28.677" Title="Compatibility of osculating elements and cartesian vectors given by JPL Horizons?" Tags="|orbital-mechanics|jpl-horizons|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23536" PostTypeId="2" ParentId="23535" CreationDate="2017-10-30T01:30:42.860" Score="1" Body="&lt;p&gt;You must use &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements#Required_parameters&quot; rel=&quot;nofollow noreferrer&quot;&gt;all six the orbital parameters&lt;/a&gt;. @RussellBorogove &lt;a href=&quot;https://space.stackexchange.com/questions/23535/#comment67170_23535&quot;&gt;points out one&lt;/a&gt;, but $e, a, i, \Omega, \omega, L$ are all needed to obtain the position of an object at a single point in time in three dimensions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several other answers here in SXSE that explain how to get the position in 3D from these parameters. Check the following; &lt;a href=&quot;https://space.stackexchange.com/a/14110/12102&quot;&gt;1&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/a/1919/12102&quot;&gt;2&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/a/1905/12102&quot;&gt;3&lt;/a&gt;. &lt;a href=&quot;https://space.stackexchange.com/a/19335/12102&quot;&gt;4&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/a/20089/12102&quot;&gt;5&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/16894/12102&quot;&gt;6&lt;/a&gt;, listed in no particular order.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;As an asside, Keplerian &lt;a href=&quot;https://en.wikipedia.org/wiki/Osculating_orbit&quot; rel=&quot;nofollow noreferrer&quot;&gt;osculating orbits&lt;/a&gt; are handy, but the reality of gravity is that in a complex system like this — Earth having such a big moon, the Sun not fixed because of Jupiter and the other giant planets — they are just approximations. It's why the numbers change daily, and it's why if you use these osculating elements to predict a position at any other time, even for example half way between one day and the next in your example, you'll get a small error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &lt;em&gt;Vector diagram of Encke's Method of perturbation analysis, showing the osculating orbit, the perturbed orbit, and a perturbing body&lt;/em&gt;. From &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Enckes_method-vector.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2LSRN.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2LSRN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-10-30T04:41:25.800" LastActivityDate="2017-10-30T04:41:25.800" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23537" PostTypeId="1" AcceptedAnswerId="23542" CreationDate="2017-10-30T02:47:39.407" Score="1" ViewCount="505" Body="&lt;p&gt;Can we use drones for planetary exploration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rovers have the inherent problem of getting stuck in terrain. and have limited range. why have space agencies not considered the use of drones for exploring a planet.&lt;/p&gt;&#xA;" OwnerUserId="21485" LastActivityDate="2017-11-05T11:12:18.487" Title="Can we use drones in planetary exploration?" Tags="|probe|mission-design|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23538" PostTypeId="1" AcceptedAnswerId="27943" CreationDate="2017-10-30T05:44:14.603" Score="7" ViewCount="271" Body="&lt;p&gt;The Planetary Society's new 3U cubesat &lt;a href=&quot;http://www.planetary.org/explore/projects/lightsail-solar-sailing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;LightSail spacecraft&lt;/a&gt; (mentioned in &lt;a href=&quot;https://space.stackexchange.com/a/23529/12102&quot;&gt;this answer&lt;/a&gt;) has corner-cube retroreflectors on on the 10x10 cm +z end, facing directly away from the sail, which will be Earth-facing as the spacecraft starts to enter night time (eclipse) as it orbits the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As discussed in this &lt;a href=&quot;https://physics.stackexchange.com/a/305210/83380&quot;&gt;Physics SE answer&lt;/a&gt;, light reflected by an object moving with significant velocity in a given frame will experience a small deflection because of the finite speed of light. This is often called astronomical &lt;a href=&quot;https://en.wikipedia.org/wiki/Aberration_of_light&quot; rel=&quot;nofollow noreferrer&quot;&gt;aberration&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The deflection in radians is about $2 \ v_{\bot} / c$, so for LEO it's about 10 or 11 arcseconds when at the zenith. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If each retroreflector is 12 mm in diameter and the wavelength is 532 nm (&lt;a href=&quot;https://en.wikipedia.org/wiki/Diode-pumped_solid-state_laser&quot; rel=&quot;nofollow noreferrer&quot;&gt;frequency doubled Nd:YAG DPSS laser&lt;/a&gt;), then the spread in the reflected beam due to diffraction is smaller than the deflection, and so the signal might be weak and difficult to receive. Actually &lt;strong&gt;it's worse than that&lt;/strong&gt;, see the update at the end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is the actual diameter of the reflectors (I've just estimated a half-inch from the images below), and have the internal reflecting faces been ground and polished a few arcseconds away from orthogonal (discussed &lt;a href=&quot;https://physics.stackexchange.com/a/305210/83380&quot;&gt;here&lt;/a&gt;) to spread the beam out, &lt;a href=&quot;https://ilrs.cddis.eosdis.nasa.gov/docs/Arnold_1978.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;as it was done&lt;/a&gt; for &lt;a href=&quot;https://en.wikipedia.org/wiki/LAGEOS&quot; rel=&quot;nofollow noreferrer&quot;&gt;LAGEOS&lt;/a&gt;'s retroreflectors?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;update 1:&lt;/strong&gt; It looks like I may have understated the problem (for LEO at least) when I first posted. I'll do the diffraction more rigorously now: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\theta_{ab} = 2 \ v_{\bot} / c \approx $ 2 x 7670 / 3E+08 $\approx$ 5.07E-05 $\approx$ &lt;strong&gt;10.5 arcsec&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\theta_{Airy} = 1.22 \lambda / d \approx $ 1.22 x 532E-09 / 0.0127 $\approx$ 5.11E-05 $\approx$ &lt;strong&gt;10.5 arcsec also!&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\theta_{Airy}$ is in fact &lt;em&gt;the first zero&lt;/em&gt; of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Airy_disk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Airy&lt;/a&gt; diffraction pattern, and so diffracted light is minimal at this angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$I(\theta) = I_0 \left( \frac{2 J_1(x)}{x} \right)^2  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$x = k \ a \ sin(\theta)  $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/kkNh9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/kkNh9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from scipy.special import j1&#xA;import matplotlib.pyplot as plt&#xA;import numpy as np&#xA;&#xA;halfpi, pi, twopi = [f*np.pi for f in (0.5, 1, 2)]&#xA;degs, rads        = 180/pi, pi/180&#xA;&#xA;a   = 0.5 * 12.7E-03&#xA;lam = 532E-09&#xA;ka  = (twopi/lam) * a&#xA;&#xA;arcsecs = np.arange(0, 30, 0.01)&#xA;theta   = (arcsecs/3600.) * rads&#xA;x       = ka * np.sin(theta)&#xA;&#xA;I       = (2. * j1(x) / x)**2  # https://en.wikipedia.org/wiki/Airy_disk&#xA;Inorm   = I/np.nanmax(I)&#xA;&#xA;plt.figure()&#xA;fs1, fs2, lw = 16, 20, 2&#xA;plt.plot(arcsecs, Inorm, linewidth=lw)&#xA;plt.xlabel('theta (arcsec)', fontsize=fs1)&#xA;plt.ylabel('I/I0 (Airy)', fontsize=fs1)&#xA;plt.title('12.7mm aperture and 532nm', fontsize=fs1)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; cropped and original (but smaller size than original) image of the +z end of the Planetary Society's LightSail spacecraft from &lt;a href=&quot;http://www.planetary.org/blogs/jason-davis/2017/20171020-ls2-closeout.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this blogpost&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IPt1i.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IPt1i.jpg&quot; alt=&quot;Planetary Society&amp;#39;s LightSail spacecraft retroreflector&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tPaYr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tPaYr.jpg&quot; alt=&quot;Planetary Society&amp;#39;s LightSail spacecraft +z end&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screen shot from this &lt;a href=&quot;https://youtu.be/-OmGvycgNCg?t=166&quot; rel=&quot;nofollow noreferrer&quot;&gt;Planetary Society YouTube video&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vSKYs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vSKYsm.jpg&quot; alt=&quot;Planetary Society&amp;#39;s LightSail spacecraft screenshot&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-06-18T06:02:18.087" LastActivityDate="2018-06-18T06:02:18.087" Title="Planetary Society's LightSail Spacecraft's corner cube reflectors; how large, and corrected for aberration?" Tags="|solar-sail|light-sail|retroreflector|planetary-society|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23539" PostTypeId="2" ParentId="23392" CreationDate="2017-10-30T09:15:17.287" Score="4" Body="&lt;p&gt;If you want to keep your formulation and optimize the problem as a TBVP, I would suggest you to use a &lt;a href=&quot;https://en.wikipedia.org/wiki/Shooting_method&quot; rel=&quot;nofollow noreferrer&quot;&gt;shooting method&lt;/a&gt;. Since you have a first order system with six differential equations (and you have the relation between the control variable and adjoint variables), you can integrate your dynamic system with a known initial condition with ode45 or similar. As you will notice the problem is that you do not know the initial values for your adjoint variables and time, so you have to made a guess on them. The idea is to match your terminal conditions, $\mathbf{x}(t_f)$ and the transversality hamiltonian condition due to final time (which you do not have take into account), in your case is $H(t_f)=0$. This could be done with a Newton's method using fsolve MATLAB function. Resuming the process:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Make a guess on your unknown initial conditions, $\pmb{\lambda}(t_0)$ and $t_f$.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;With those parameters simulate your problem with an integrator as for example, ode45.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;Use a Newton method with a sensitivity matrix to change your guess and rerun step 2 with a better guess to match the desired $\mathbf{x}(t_f)$ and $H(t_f)=0$. This could be done easily using fsolve MATLAB function embedding the ode45 integration into it. &lt;/li&gt;&#xA;&lt;li&gt;The output of fsolve function is a candidate to the optimal control problem.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Now, I will tell you some advices, considering your problem I would consider to integrate backwards because you only have to meet the initial state since you can guarantee $H(t_f)=0$ with your initial guess. Also, it is not guaranteed that the problem is extremely sensitive to initial conditions (this has happened to me in some cases) and a more brute force method to solve the TPBVP have to be used like bvp4c or bvp5c MATLAB functions (but, I would recommend that only as the last option).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another advice is that I would consider to solve the problem with direct methods rather than an indirect one because as you will notice obtaining a good initial guess of the adjoint variables is not a trivial issue. &lt;/p&gt;&#xA;" OwnerUserId="21262" LastEditorUserId="12102" LastEditDate="2017-10-30T11:39:01.730" LastActivityDate="2017-10-30T11:39:01.730" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23540" PostTypeId="2" ParentId="23537" CreationDate="2017-10-30T11:08:47.923" Score="2" Body="&lt;p&gt;NASA have considered &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?feature=6686&quot; rel=&quot;nofollow noreferrer&quot;&gt;underwater drones for exploring Europa&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="user20636" LastActivityDate="2017-10-30T11:08:47.923" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23541" PostTypeId="1" AcceptedAnswerId="23546" CreationDate="2017-10-30T11:52:42.477" Score="2" ViewCount="229" Body="&lt;p&gt;I am using the NASA NEO object data:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://api.nasa.gov/neo/rest/v1/feed?start_date=2015-09-07&amp;amp;end_date=2015-09-08&amp;amp;api_key=DEMO_KEY&quot; rel=&quot;nofollow noreferrer&quot;&gt;listing of NEOs&lt;/a&gt;;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://api.nasa.gov/neo/rest/v1/neo/3726710?api_key=DEMO_KEY&quot; rel=&quot;nofollow noreferrer&quot;&gt;specific NEO Data&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Given the information in this JSON data, is it possible to calculate the position of the NEO in relation to Earth? It doesn't seem to offer any positional information that I can see.&lt;/p&gt;&#xA;" OwnerUserId="21488" LastEditorUserId="47149" LastEditDate="2022-05-10T00:18:42.197" LastActivityDate="2022-05-10T00:18:42.197" Title="Is it possible to calculate NEO position with NASA near-Earth object data?" Tags="|celestial-mechanics|calculation|near-earth-object|" AnswerCount="1" CommentCount="3" FavoriteCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23542" PostTypeId="2" ParentId="23537" CreationDate="2017-10-30T15:59:50.607" Score="4" Body="&lt;p&gt;If you are talking about flying drones, then the answer is that we haven't tried to explore a body with a suitable atmosphere yet. Mars' atmosphere is so thin that a winged drone is difficult to engineer. I believe such a thing has been proposed, but there are a lot of easier missions NASA (and other agencies) would want to try first. And Mars is the only body beyond the moon that we have even placed a rover on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But, as for other bodies, Venus is so hot that a drone flying low enough to observe the surface well would be nearly impossible to engineer. Even stationary landers have only ever lasted a few tens of minutes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Titan is quite cold, but gravity is low and the atmosphere is thicker than Earth's. I bet it could be made to work, but Titan is very far away and we won't be sending probes there very often. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gas giants are probably better candidates to be explored by dirigibles, but that would be an expensive mission.&lt;/p&gt;&#xA;" OwnerUserId="10259" LastEditorUserId="10259" LastEditDate="2017-11-02T03:08:46.123" LastActivityDate="2017-11-02T03:08:46.123" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23544" PostTypeId="2" ParentId="23445" CreationDate="2017-10-30T18:21:53.123" Score="4" Body="&lt;p&gt;If we just look at rough numbers, we know the gravity on the Moon is ⅙&lt;sup&gt;th&lt;/sup&gt; that of Earth so a mine shaft should go to a depth six times as deep as on Earth. So if the deepest shaft on Earth is just under 4 km, then the deepest shaft on the moon should be shy of 24 km.&lt;/p&gt;&#xA;" OwnerUserId="21491" LastEditorUserId="7062" LastEditDate="2022-07-05T17:13:26.190" LastActivityDate="2022-07-05T17:13:26.190" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="23545" PostTypeId="2" ParentId="20849" CreationDate="2017-10-30T19:57:21.577" Score="4" Body="&lt;p&gt;I just watched a SpaceX launch from Tampa Florida.  Used Google Earth to determine the heading to look.  Cool clear day and was able to see it just fine.  Binoculars helped, but could see it without. &lt;/p&gt;&#xA;" OwnerUserId="21492" LastEditorUserId="25" LastEditDate="2017-12-19T13:16:15.307" LastActivityDate="2017-12-19T13:16:15.307" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23546" PostTypeId="2" ParentId="23541" CreationDate="2017-10-30T23:56:33.977" Score="0" Body="&lt;p&gt;If you would like &lt;a href=&quot;https://i.stack.imgur.com/TpMcx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Horizons&lt;/a&gt; to calculate it for you, I've found the following setup the most useful. Instead of using those orbital parameters, a pre-calculated trajectory can usually be found that is made by direct numerical integration, which is more accurate than using static parameters. It's nice to see the eccentricity and semi-major axis on the screen, but for calculating the orbit into the future and past, it's better to stick with state vectors.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;You can type several different names or designators and Horizons will make a guess for you. Usually it does a good job, but double check! Here I chose the ID number &lt;code&gt;3727181&lt;/code&gt; from the file in your link, and it seems to have found the right object, also called &lt;code&gt;Asteroid (2015 RC)&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TpMcx.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TpMcx.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I choose the state vector with both x, y, z and vx, vy, vz. Most of the time I don't use the velocities, but sometimes they come in handy, so I save them at the same time. I choose km and km/sec for units since they directly compatible with off-line numerical integration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ecliptic plane (rather than J2000.0 cartesian plane) is the default, and is usually what looks the nicest when plotting, and the CSV format is easy to read in to another program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The actual table of numbers begins after the line with &lt;code&gt;$$SOE&lt;/code&gt; and ends before the line with &lt;code&gt;$$EOE&lt;/code&gt;. I assume those are &lt;em&gt;Start Of Ephemeris&lt;/em&gt; and &lt;em&gt;End Of Ephemeris&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cGxs2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cGxs2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;HTML format will generate text on your screen that you can copy/paste while keeping the menu options. &lt;/li&gt;&#xA;&lt;li&gt;plain text format creates a window in your browser with plain text you can save as a text file or copy/paste.&lt;/li&gt;&#xA;&lt;li&gt;download/save opens a dialog box and allows you to save the text file directly to your hard drive&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QASzr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QASzr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-10-30T23:56:33.977" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23548" PostTypeId="2" ParentId="19091" CreationDate="2017-10-31T06:29:15.377" Score="0" Body="&lt;p&gt;Nukes are a horrible idea to use for asteroids. First nuclear detonations do not work the same way in space as they do in atmosphere. In space a nuclear device creates a large deadly cloud of radiation but not much more. It loses all of its blast component and with no atmosphere to heat, most of its thermal component. So basically if you use a Nuke on an asteroid all that will result is you will have a very large highly irradiated asteroid on the same course. Maybe you might be able to use them if you coul drill a hole and fill it with a reactive gas like oxygen or hydrogen so you could get some of the blast and thermal component back, but this would take weeks possibly months to accomplish and the end result isn't guaranteed and would only work on asteroids that weren't very dense. Against a nickle/iron asteroid it would have very reduced to no effect at all.&lt;/p&gt;&#xA;" OwnerUserId="21495" LastActivityDate="2017-10-31T06:29:15.377" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23549" PostTypeId="1" CreationDate="2017-10-31T09:07:59.287" Score="6" ViewCount="209" Body="&lt;p&gt;Minotaur V might become a candidate for the smallest current launcher to put a payload in GEO, but it has only launched LADEE yet. The first satellite in GEO seems to have been &lt;a href=&quot;https://en.wikipedia.org/wiki/Syncom&quot; rel=&quot;noreferrer&quot;&gt;Syncom 1&lt;/a&gt; which launched on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta_(rocket_family)#Delta_B&quot; rel=&quot;noreferrer&quot;&gt;Delta B&lt;/a&gt; in 1963. There are different ways of being the smallest to consider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;North Korea has announced that they will launch geostationary satellites. How does the capability of their launchers thus far meet that ambition?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastEditorUserId="12102" LastEditDate="2019-01-21T04:36:58.870" LastActivityDate="2019-01-21T04:36:58.870" Title="What is the smallest launch vehicle that has put a satellite in a geostationary orbit?" Tags="|history|launch-vehicle|geosynchronous|north-korea|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23550" PostTypeId="1" CreationDate="2017-10-31T10:59:24.667" Score="4" ViewCount="257" Body="&lt;p&gt;There are a lot of walks like the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sagan_Planet_Walk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sagan Planet Walk&lt;/a&gt; (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_System_model&quot; rel=&quot;nofollow noreferrer&quot;&gt;the list on Wikipedia&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But are there any of these walks that include the position of the Voyager space probes?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some days ago I was preparing for a 10 km run and I calculated the positions of the planets along the trail when Pluto is located at the end. At this scale, the Voyager probes would be at about 35 and 29 km.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-11-02T17:37:37.277" LastActivityDate="2017-11-02T17:37:37.277" Title="Is there a planet walk with the positions of Voyager 1 and 2?" Tags="|voyager|planet|solar-system|space-art|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23552" PostTypeId="2" ParentId="23550" CreationDate="2017-10-31T18:44:31.337" Score="3" Body="&lt;p&gt;I found one including the position of Voyager 1, the Solar System lawn model of &lt;a href=&quot;http://www.griffithobservatory.org/exhibits/solar_system_lawn_model.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Griffith Observatory&lt;/a&gt;&lt;br&gt;&#xA;Very remarkable model including the elliptical orbit of Pluto.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But a solar system model in Houston should in my opinion include the position of Voyager 1: &lt;a href=&quot;http://voyagesolarsystem.org/community-network/houston-texas-voyage-mark-i/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyage in Houston&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-10-31T18:44:31.337" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23554" PostTypeId="2" ParentId="9421" CreationDate="2017-10-31T23:40:18.310" Score="1" Body="&lt;p&gt;It was reportedly used in Planet's SkySats launched by the first Minotaur C; see Space.com's &lt;a href=&quot;https://www.space.com/38629-frankenstein-rocket-launches-on-halloween.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbital ATK's 'Frankenstein' Minotaur C Rocket Blasts Off on Halloween&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="21499" LastEditorUserId="12102" LastEditDate="2017-11-01T00:59:27.403" LastActivityDate="2017-11-01T00:59:27.403" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23555" PostTypeId="1" AcceptedAnswerId="24582" CreationDate="2017-11-01T01:18:18.720" Score="1" ViewCount="252" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/a/23554/12102&quot;&gt;new update&lt;/a&gt; to the story of LMP-103s, the &amp;quot;greener&amp;quot; alternative to hydrazine links to Space.com's &lt;a href=&quot;https://space.stackexchange.com/a/23554/12102&quot;&gt;Orbital ATK's 'Frankenstein' Minotaur C Rocket Blasts Off on Halloween&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;According to the article, &lt;em&gt;Frankenstein's payload&lt;/em&gt; is six &lt;a href=&quot;http://spaceflight101.com/vega-vv07/skysat-satellites/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SkySats&lt;/a&gt; and four &lt;a href=&quot;https://i.stack.imgur.com/CyZxM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Doves&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Orbital ATK's Minotaur C lifted off from Space Launch Complex 576E at Vandenberg Air Force Base in California at 5:37 p.m. EDT (2:37 p.m. local time, 2137 GMT).&lt;/p&gt;&#xA;&lt;p&gt;About 15 minutes later, the rocket deployed six SkySat Earth-imaging satellites and four Dove cubesats for the commercial imaging company Planet. &amp;quot;Liftoff of Minotaur C, carrying the SkySat and Dove satellites for Planet,&amp;quot; Sean Wilson, director of corporate communications for Orbital ATK, said during a live webcast of the launch. [Gallery: Planet Labs Photos of Earth from Space]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The six SkySats appear to be SkySat-2 numbers 6 through 11 according to &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/skysat-3.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;. Most Dove's are deployed in larger groups, why only four on this launch? Just tests for comparison to SKySats? Perhaps this is an unusual orbit for them? Or perhaps they will be &amp;quot;watching&amp;quot; the SkySats deploy? (slightly related: &lt;a href=&quot;https://space.stackexchange.com/q/14002/12102&quot;&gt;Could a spy satellite image another satellite?&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; A suite of SkySat and Dove satellites is encapsulated into the payload fairing of an Orbital ATK Minotaur-C rocket prior to launch. Credit: Orbital ATK, from &lt;a href=&quot;https://space.stackexchange.com/a/23554/12102&quot;&gt;Space.com&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/J1JGc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/J1JGc.jpg&quot; alt=&quot;payload&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; (Earlier launch, for scale) SkySats 4 through 7 take their places atop the Vega VV07 payload adapter – Photo: Arianespace/ESA/CNES/Optique Video du CSG, from &lt;a href=&quot;http://spaceflight101.com/vega-vv07/skysat-satellites/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight101&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Wz5dG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Wz5dG.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-22T21:50:20.627" Title="&quot;Frankenstein's payload&quot;; Why only four Doves along with the six SkySats?" Tags="|doves|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23558" PostTypeId="1" CreationDate="2017-11-01T12:37:48.720" Score="4" ViewCount="228" Body="&lt;p&gt;Most probably, we'll be able to mass manufacture diamond nano-threads and carbon nano-tubes in the near future, but they might not be strong enough. Is there a way to significantly reduce the tensile strength requirements? We also need to take heavy payloads, on board safety and maintenance equipment and climbers into account.&lt;/p&gt;&#xA;" OwnerUserId="21504" LastActivityDate="2017-11-01T21:20:25.097" Title="Can the modern space elevator design be altered so as to reduce the tensile strength requirement of the material?" Tags="|space-elevator|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23560" PostTypeId="2" ParentId="21557" CreationDate="2017-11-01T14:53:17.850" Score="1" Body="&lt;p&gt;The original .gif that the graphic from the question was based on can be found on the &lt;a href=&quot;https://en.wikipedia.org/wiki/J002E3&quot; rel=&quot;nofollow noreferrer&quot;&gt;wikipedia page for J002E3&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/s6ms7.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s6ms7.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As noted by the other two answers, this is the actual path taken by J002E3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For those who play KSP, there are different options for how the patched conics are displayed.  While it cannot accurately model the path, it &lt;em&gt;can&lt;/em&gt; show the behaviour shown in the graphic near L1, which is the cause of the confusion leading to this question.&lt;/p&gt;&#xA;" OwnerUserId="6241" LastActivityDate="2017-11-01T14:53:17.850" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23561" PostTypeId="2" ParentId="23558" CreationDate="2017-11-01T15:50:40.493" Score="0" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/22499/space-elevator-orbit-below-gso-but-slow-enough-to-be-caught-by-an-airplane/22502#22502&quot;&gt;Space elevator orbit below GSO but slow enough to be caught by an airplane.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I asked the question above previously and found that if you have your space elevator not at GSO but at a lower orbit and traveling and it was slow enough to be caught by an airplane you could reduce the total length of the elevator by 30%.  This sounds like a lot but is still 25,744 km but better than 35,800 km (plus extra to counter balance the elevator.)  This reduced length would reduce the tensile strength needed for the elevator.  &lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="20287" LastEditDate="2017-11-01T18:22:06.640" LastActivityDate="2017-11-01T18:22:06.640" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23562" PostTypeId="1" CreationDate="2017-11-01T16:12:17.980" Score="4" ViewCount="502" Body="&lt;p&gt;If you added a dome to a mars base, greenhouses too, and added 1/10th Earth pressure (10 kPa) you would be able to grow lichen which could be a food source and a source of oxygen from the carbon dioxide in the air.  In addition you could reduce the need for radiators for cooling the habitats, since it is likely they will have to expel heat similar to the ISS, the higher pressure would increase convection and increase the temperature within the dome.  Lastly you would reduce the pressure difference of Mars's atmosphere of 0.6 kPa and the dome air pressure of 10 kPa and the habitats pressure of 50 kPa which would reduce structural issues.  (Greenhouses might be set in between 10 and 50 kPa.)&lt;/p&gt;&#xA;&lt;p&gt;Edit: I've removed the part about not needing a pressure suit within the domes, which I see now you'll need at least 33 kPa.&lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="20287" LastEditDate="2020-09-28T20:19:51.307" LastActivityDate="2020-09-29T06:34:29.633" Title="What downsides are there to a low pressure dome over a Mars base?" Tags="|mars|colonization|planetary-science|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="23563" PostTypeId="2" ParentId="23562" CreationDate="2017-11-01T16:26:59.340" Score="3" Body="&lt;p&gt;Assuming a pure oxygen breathing environment, and using the charts found at &lt;a href=&quot;http://www.emsworld.com/article/10915304/the-dangers-of-giving-too-much-oxygen&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site&lt;/a&gt;,  we can determine a few things.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Assuming you can get 100% oxygen through a breathing apparatus, and assuming that you want to be in the middle of the safe zone, that would lead to a min pressure of 5 pounds/ square inch, or about 35 kPa.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;An outer greenhouse is actually a good idea because it provides a second layer of protection against impacts. If the inner dome was broken somehow, but the outer intact, it would provide a layer of safety. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I think it isn't a bad idea. The downside, of course, is that you have to build a much larger dome to contain the two layers of growth. Having a layer that is perhaps a bit bigger and has some way of automatically collecting any growth might be a good idea, as it adds an extra layer of protection.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-11-01T16:26:59.340" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23564" PostTypeId="1" CreationDate="2017-11-01T16:50:37.993" Score="2" ViewCount="305" Body="&lt;p&gt;It is already mentioned that ISS orbit the earth 15 times a day, how we design a dish network to stay communicate with it along its path? Are we build a dish in Atlantic Ocean too?&lt;/p&gt;&#xA;" OwnerUserId="21384" LastActivityDate="2017-11-01T17:52:45.697" Title="Dish For Tracking and Communicating with ISS" Tags="|iss|" AnswerCount="2" CommentCount="2" ClosedDate="2017-11-02T17:48:58.307" ContentLicense="CC BY-SA 3.0" />
  <row Id="23565" PostTypeId="2" ParentId="23564" CreationDate="2017-11-01T17:50:18.567" Score="5" Body="&lt;p&gt;NASA communicates with the ISS through their Space Network, which consists of a constellation of satellites including ones in geosynchronous orbit, low earth satellites, and ground facilities that relay between NASA and the satellites.  It's available 24/7 and allows constant communication with the ISS, including voice, telemetry, etc.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More at: &lt;a href=&quot;https://www.nasa.gov/directorates/heo/scan/services/networks/sn&quot; rel=&quot;noreferrer&quot;&gt;https://www.nasa.gov/directorates/heo/scan/services/networks/sn&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21469" LastActivityDate="2017-11-01T17:50:18.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23566" PostTypeId="2" ParentId="23564" CreationDate="2017-11-01T17:52:45.697" Score="4" Body="&lt;p&gt;While the Apollo-era space program had to rely on ground stations for communication, a network of geostationary satellites called &lt;a href=&quot;https://en.wikipedia.org/wiki/Tracking_and_data_relay_satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;TDRSS&lt;/a&gt; has been in use since the 1980s to provide continuous communication with spacecraft in orbit (as well as to the south pole!).&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-11-01T17:52:45.697" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23567" PostTypeId="1" AcceptedAnswerId="23570" CreationDate="2017-11-01T18:27:16.373" Score="7" ViewCount="2008" Body="&lt;p&gt;I vaguely remember hearing the descent engines in the sky crane of curiosity were monopropellants and ran on hydrazine. Is this true and is there any other information on the engines available?&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="9092" LastEditDate="2021-02-19T02:05:23.467" LastActivityDate="2021-02-19T02:05:23.467" Title="What were the Curiosity sky-crane’s descent engines?" Tags="|mars|propulsion|probe|skycrane|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23568" PostTypeId="2" ParentId="21051" CreationDate="2017-11-01T18:49:20.340" Score="5" Body="&lt;p&gt;NASA Orion / SLS is still using PEG:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150001961.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150001961.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that &quot;linear tangent steering&quot; is a provably optimal guidance law from calculus of variations.  It is going to be used in any finite-burn guidance program.  What you're probably asking is if PEG is being used--which is a predictor-corrector method that uses analytical approximations instead of integration of trajectories (e.g. using RKF45 or whatever), which is fast enough to be used closed-loop.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that guidance programs like PEG are not used for the atmospheric part of ascent.  They're still used only used (AFAIK) exoatmospherically.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ascent profiles for the in-atmosphere (endoatmospheric) are still calculated more laboriously on the ground.  And a lot of mission planning is still done from the ground in plugged into PEG using the &quot;external delta-V&quot; mode.  I don't know what is currently being used on the ground, in the late 60s it was the calculus of variations approach using runge-kutta integration and newton's method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To confuse things slightly PEG's predictor could be replaced by discrete integration with Runge-Kutta which would improve accuracy and remove all the small-angle approximations and gravity integral approximations involved in PEG but at the expense of additional computation.  I don't know what the current state-of-the-art is with the predictor and the gravity integral calculations with PEG.&lt;/p&gt;&#xA;" OwnerUserId="14420" LastActivityDate="2017-11-01T18:49:20.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23570" PostTypeId="2" ParentId="23567" CreationDate="2017-11-01T19:33:24.390" Score="12" Body="&lt;p&gt;That's correct, they're hydrazine monopropellant thrusters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Per &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Science_Laboratory#Powered_descent&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Each rocket thruster, called a Mars Lander Engine (MLE), produces 400 to 3,100 N (90 to 697 lbf) of thrust and were derived from those used on the Viking landers.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The citation links to an &lt;a href=&quot;http://www.rocket.com/Aerojet%20Ships%20Propulsion%20for%20Mars%20Science%20Laboratory&quot; rel=&quot;noreferrer&quot;&gt;Aerojet press release&lt;/a&gt; which contains a little more information:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;After deployment of a parachute, MSL will perform final descent to near the Martian surface using its eight Aerojet 700-pound thrust engines.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Aerojet's model number for these thrusters is MR-80B. The specs for this thruster (as well as many others) are on a &lt;a href=&quot;http://www.rocket.com/files/aerojet/documents/Capabilities/PDFs/Monopropellant%20Data%20Sheets.pdf&quot; rel=&quot;noreferrer&quot;&gt;datasheet&lt;/a&gt; available from Aerojet. Here's &lt;a href=&quot;http://matthewwturner.com/uah/IPT2008_summer/baselines/LOW%20Files/Payload/Downloads/AIAA-2007-5481-979.pdf&quot; rel=&quot;noreferrer&quot;&gt;some deeper technical info on the development of the thrusters&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-11-01T19:33:24.390" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23571" PostTypeId="2" ParentId="23558" CreationDate="2017-11-01T21:20:25.097" Score="1" Body="&lt;p&gt;The simplest way to reduce the tensile strength requirements of a space elevator is to taper the cable instead of using a constant cross section.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First off to start thinking correctly about a traditional space elevator, you have to remember it is literally hanging down from orbit and all the weight of the cable is supported from above.  The problem with a space elevator is that at some point a cable of any material is going to weigh too much to support itself, the concept is known as &lt;A HREF=&quot;https://en.wikipedia.org/wiki/Specific_strength&quot; rel=&quot;nofollow noreferrer&quot;&gt;breaking length&lt;/a&gt;.  In choosing materials for this application you try to optimize these factors: low density (light weight) and high tensile strength, to get the longest breaking length possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the breaking length is not long enough to reach to geosynchronous orbit (or beyond for the counterweight) for a traditional style space elevator you can lengthen the maximum length possible by using a tapering cable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine the following scenario, I have an allow steel cable ~8 km long (this is the breaking length) supported from the top.  If I add any more cable to the bottom, the increased weight will break the cable.  How can we make it longer?  Take the original cable and cut it in half, you now have 2x 4 km cables held together.  Each shorter cable can support the weight of a 4km long section of cable at the bottom, or 8 km cable total together.  So by tapering the cable I can now build a steel cable that is 12 km long that won't break under its own weight.  This tapering can continue to lengthen the maximum cable at the expense of requiring more and more material to support it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tKUMt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tKUMt.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is of course a simplified example, assuming a constant gravitational field over the length of the cable and neglecting the centripetal forces acting on a space elevator (both of which should actually increase the breaking length of a material).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a very detailed discussion of the use of space tethers, including the effects of tapered cables in non-uniform gravity fields, read &lt;A HREF=&quot;https://books.google.com/books?id=MpUjqAQHGG8C&amp;amp;pg=PR5&amp;amp;dq=Dynamics+of+Space+Tether+Systems,+Volume+83&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwiPrvmapp7XAhUFwGMKHTYeBdkQ6AEIKDAA#v=onepage&amp;amp;q=Dynamics%20of%20Space%20Tether%20Systems%2C%20Volume%2083&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;the book&lt;/A&gt;.&lt;/p&gt;&#xA;" OwnerUserId="15824" LastActivityDate="2017-11-01T21:20:25.097" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23573" PostTypeId="1" AcceptedAnswerId="23575" CreationDate="2017-11-02T06:22:00.573" Score="1" ViewCount="434" Body="&lt;p&gt;I am compiling a table of data on price-mass ratios of various modern rockets, and I have been unable to find any information on how much it costs to launch a Zenit rocket. Does anybody know how much it costs to launch a Zenit rocket of any kind?&lt;/p&gt;&#xA;" OwnerUserId="21512" LastActivityDate="2018-10-29T16:39:46.953" Title="Zenit Launch Costs" Tags="|rockets|cost|russia|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23574" PostTypeId="2" ParentId="22691" CreationDate="2017-11-02T07:39:18.857" Score="-3" Body="&lt;p&gt;darkness does not affect geostationary satellites, however they have been affected by lack of solar energy coming from sun since they have a large amount of equipment attached to the satellite for functioning which cannot rely on batteries for such long period of time during eclipse.&lt;/p&gt;&#xA;" OwnerUserId="21514" LastEditorDisplayName="user20636" LastEditDate="2017-11-02T14:48:24.293" LastActivityDate="2017-11-02T14:48:24.293" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23575" PostTypeId="2" ParentId="23573" CreationDate="2017-11-02T08:18:35.357" Score="0" Body="&lt;p&gt;This &lt;a href=&quot;http://www.dglr.de/fileadmin/inhalte/dglr/fb/r1/r1_2/06-Raumtransportsysteme-Kosten.pdf#page=6&quot; rel=&quot;nofollow noreferrer&quot;&gt;slide&lt;/a&gt; from DGLR, which is the non-profit space community of Germany, can help you further. What you want can be found in page 6. For the exact number you can contact the authors. They will definitely give a better answer.&lt;/p&gt;&#xA;" OwnerUserId="19601" LastActivityDate="2017-11-02T08:18:35.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23576" PostTypeId="1" CreationDate="2017-11-02T11:46:08.933" Score="14" ViewCount="5533" Body="&lt;p&gt;With the expected advances in reusable rocket technology within the next decade, will constructing a space elevator become meaningless? Or will it still be way cheaper, safer and convenient to use them?&lt;/p&gt;&#xA;" OwnerUserId="21504" LastEditorUserId="21504" LastEditDate="2017-11-03T09:49:14.103" LastActivityDate="2017-11-05T13:36:17.027" Title="Would reusable rockets render the idea of space elevators useless?" Tags="|reuse|space-elevator|economics|" AnswerCount="7" CommentCount="13" ClosedDate="2017-11-06T02:08:49.827" ContentLicense="CC BY-SA 3.0" />
  <row Id="23577" PostTypeId="2" ParentId="23576" CreationDate="2017-11-02T12:24:06.577" Score="12" Body="&lt;p&gt;Whatever concept of space elevator you want to build, it requires you to transport massive amounts of material into space, so no. Having cheap launch vehicles is actually a requirement to building a space elevator. Whether the elevator could ever break even given its high mass is another matter altogether.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastActivityDate="2017-11-02T12:24:06.577" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23578" PostTypeId="1" CreationDate="2017-11-02T15:45:30.600" Score="24" ViewCount="4666" Body="&lt;p&gt;I know that the Space Shuttle delivered a lot of modules and supplies to the International Space Station, but I also know that other rockets were just as capable of performing the task at that time during construction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it have been more feasible or less feasible to build the station without the Space Shuttle?  Was it really that critical to the construction of the space station to have the Shuttle perform deliveries as opposed to other, perhaps less costly rockets?  Or was it the only American technology available (with contractual funding) at the time to do it?  Was there anything particularly unique to the Space Shuttle that made it crucial to the construction of the ISS?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="11649" LastEditDate="2017-11-02T21:42:14.997" LastActivityDate="2017-11-05T22:35:46.430" Title="Would it have been any more difficult to build the space station without the space shuttle?" Tags="|iss|space-shuttle|mission-design|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23579" PostTypeId="2" ParentId="23578" CreationDate="2017-11-02T16:44:37.827" Score="34" Body="&lt;p&gt;I do not think your assertion that &quot;other rockets were just as capable of performing the task at that time&quot; is correct at all. I'd love to hear your counterexamples, if you have any. Perhaps you are referring simply to their payload-to-LEO capability? If so, that is only the tip of the iceberg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No other vehicle at the time provided all the elements of EVA capability, 50klb payloads, and a manipulator arm to carry out the assembly tasks.  This image from STS-88 (the first assembly flight) shows all of those elements in play.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Ft9v7.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Ft9v7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, the ISS (US side) was designed to be assembled by the shuttle.  If you started with a clean sheet design, it is certainly possible to come up with a configuration that could be built by other vehicles. But that is a purely hypothetical exercise.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-11-02T17:02:32.523" LastActivityDate="2017-11-02T17:02:32.523" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23580" PostTypeId="2" ParentId="23576" CreationDate="2017-11-02T17:04:33.727" Score="29" Body="&lt;h1&gt;A Space Elevator would still be amazingly useful&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The two factors that come to mind are forms of power and scale:&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Power&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;With a space elevator connected to the ground, you could use the energy in your power grid to lift everything up. By doing this, we can use green and renewable energies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With rockets, baring any massive advancements, we are restricted to using chemical energy. So not only to we have to specifically manufacture the fuel, but the entire process maybe less efficient. Not to mention the environmental effects of burning millions of tons of methane if you want something big in orbit.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Scale&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Most likely, we'd be able to bring much larger masses into orbit at a time with a space elevator. Even with SpaceX's BFR system, a large ship in orbit has to refuel multiple time to get anywhere. With a space elevator, we could potentially bring a fully fueled ship (without any need to have in-atmosphere propulsion on-board) into orbit in one go.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this being said, we still need &lt;em&gt;HUGE&lt;/em&gt; advances in material science for any space elevator to be viable.&lt;/p&gt;&#xA;" OwnerUserId="21521" LastActivityDate="2017-11-02T17:04:33.727" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="23581" PostTypeId="2" ParentId="23576" CreationDate="2017-11-02T17:52:11.463" Score="4" Body="&lt;p&gt;A space elevator, no matter what sort of unobtanium it's built from or how it works, has one unbeatable advantage over a rocket: the elevator cars don't need to carry their fuel with them.  The vast majority of the energy involved in a rocket launch goes into carrying rocket fuel to ever-higher speeds and altitudes; being able to leave your energy source on the ground is a huge efficiency gain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The phrase &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;tyranny of the rocket equation&lt;/a&gt; isn't just poetry.&lt;/p&gt;&#xA;" OwnerUserId="2847" LastActivityDate="2017-11-02T17:52:11.463" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23582" PostTypeId="2" ParentId="23576" CreationDate="2017-11-02T18:02:00.777" Score="15" Body="&lt;p&gt;Rockets suffer from the tyrrany of the rocket equation.  While a reusable rocket is great, you need to burn a &lt;em&gt;lot&lt;/em&gt; of propellant for each kg of mass to reach orbital velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A space elevator can steal momentum from the planet itself to provide the speed required to reach orbital velocity (or beyond).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, fast energy expenditure is less efficient than slow.  Elevators allow efficient energy (solar power or whatever) to be used to lift cargo (electric climbing cars).  Rockets require high energy density fuel (rocket fuel) to be burned &lt;em&gt;and lifted to the spot where it is used&lt;/em&gt; by earlier rocket fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 'hard' part of a space evevator is the materials science required; we lack matter that has the tensile strength required on human scales in a controlled lab, let alone industrial scales in the open air.  Once that is solved, we have the problem of building it in place, building a factory in orbit, dodging debris and weather, designing lifters, etc, each of which are problems larger in scale than the Apollo Program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reusable rockets would reduce the cost of lifting stuff to orbit, but the fuel costs would remain high.  Such cheap launch technology would open up the possibility of building (expensive) factories in space, and/or move asteroids into position to provide the other end of the space elevator.  So quite possibly they are a prerequisite for the elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you have the elevator, lifting to orbit energy cost plummets and it becomes basically electricity (at current prices of 1$ per kg to orbital velocity).  Currently it costs on the order of 10000\$ per kg.  Reliable reusable rockets might cut that down, but probably to 1000\$ or 100\$s of dollars per kg, not single-dollars per kg.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Space elevators have limited capacity, so while the energy costs might be 1\$ per kg, prices may be similar to competing technology until demand saturates the pipe.  However, the first space elevator can then be used to build the second, and you'd have something similar to an industrial revolution exponential economic expansion as the cost of reaching space converges on the energy cost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 100\$-1000\$/kg to orbit, space travel of humans requires a small fortune per person.  Space may open up to &lt;em&gt;industry&lt;/em&gt; and robots, but not to migration.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At 1\$/kg to orbit, and 2\$/kg to Lithobraking on Mars (not including travel time), the solar system becomes our back yard.&lt;/p&gt;&#xA;" OwnerUserId="11023" LastActivityDate="2017-11-02T18:02:00.777" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23583" PostTypeId="1" AcceptedAnswerId="24023" CreationDate="2017-11-02T19:05:39.503" Score="5" ViewCount="954" Body="&lt;p&gt;I have &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLE&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/RINEX&quot; rel=&quot;nofollow noreferrer&quot;&gt;RINEX&lt;/a&gt; navigation file for GPS satellite PRN 20, for 01.04.17r. 19:56:56 and 20:00:00 respectively. See also &lt;a href=&quot;ftp://igs.org/pub/data/format/rinex303.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Receiver Independent Exchange Format Version 3.03&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems that the value of longitude of the ascending node (Ω0) differs for both while all the other parameters are relatively similar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;RINEX file:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;M0 = -0.41907076212790001&#xA;e  =  0.0044262305600569997&#xA;ω  =  1.5754052384999999&#xA;i  =  0.92697366533970005&#xA;Ω0 = -0.4446281969834&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;TLE file:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;M0 =  5.8389169820721856&#xA;e  =  0.0042077&#xA;ω  =  1.5770882387483363&#xA;i  =  0.92682044735729674&#xA;Ω0 =  2.7589292150900362&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;What can be the reason for this? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the values for &lt;code&gt;M0&lt;/code&gt; are close modulo 2π, the values for Ω0 seem to differ by only π, or about 180°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Checking also other datasets the result is similar, although the difference between values differs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;TLE:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;GPS BIIR-4  (PRN 20)    &#xA;1 26360U 00025A   17091.83120691 -.00000002  00000-0  00000-0 0  9995&#xA;2 26360  53.1029 158.0750 0042077  90.3605 334.5453  2.00565359123815&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;RINEX:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;20 17  4  1 20  0  0.0 4.665344022214D-04 1.591615728103D-12 0.000000000000D+00&#xA;5.600000000000D+01-1.231250000000D+01 5.538802141913D-09-4.190707621279D-01&#xA;-5.606561899185D-07 4.426230560057D-03 6.150454282761D-06 5.153646507263D+03&#xA; 5.904000000000D+05-6.705522537231D-08-4.446281969834D-01 3.725290298462D-08&#xA; 9.269736653397D-01 2.442500000000D+02 1.575405238500D+00-8.669646839876D-09&#xA; 2.714398779890D-11 0.000000000000D+00 1.942000000000D+03 0.000000000000D+00&#xA; 2.000000000000D+00 0.000000000000D+00-8.381903171539D-09 5.600000000000D+01&#xA; 5.899500000000D+05 4.000000000000D+00&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="21522" LastEditorUserId="21522" LastEditDate="2017-11-13T20:36:52.477" LastActivityDate="2021-12-10T16:50:39.373" Title="TLE and RINEX gps differences" Tags="|artificial-satellite|orbit|gps|two-line-elements|rinex|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23584" PostTypeId="2" ParentId="23578" CreationDate="2017-11-02T20:14:36.850" Score="32" Body="&lt;p&gt;Launching and assembling the ISS &lt;em&gt;in its current design&lt;/em&gt; could not have been accomplished without the STS (Shuttle system), but that is largely because the ISS was designed with the STS in mind as its launch vehicle. The ISS &lt;em&gt;could&lt;/em&gt; be designed to be assembled in orbit using a large number of spacewalks because designers knew they would have the shuttle available to host EVAs. While the STS boasts a payload capacity to LEO of 27,500 kg, the largest ISS components it delivered were all under 16,000 kg. The largest ISS components at 19,000 to 20,000kg were delivered by Russian Proton rockets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has an excellent article on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Assembly_of_the_International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;Assembly of the International Space Station&lt;/a&gt; which describes module sizes, delivery methods, and the number of components and related spacewalks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However a space station of a different design could have been launched and assembled by different rocket families. In fact, two other space stations were. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Skylab was launched in 1973 on a modified Saturn V booster. Skylab weighed in at 77,000 kg, four times the weight of the largest ISS modules, and had an internal volume of 12,417 cubic feet, slightly more than 1/3 of the ISS' current volume. So in the broadest terms, a space station with similar stats to the ISS could have been launched and assembled using three Skylab-class modules. Skylab's crew, launched separately in an Apollo module, transferred to Skylab and then used it to host EVAs, most notably to repair the lab, as it was damaged seriously during launch. &#xA;In 1986 the Soviet Union used a Proton heavy-lift rocket to launch the first component of their space station &quot;Mir.&quot; Mir would eventually be composed of seven pressurized and several unpressurized modules with a total pressurized volume of 350 m^3 (12,360 cu ft). Although eventually decommissioned as Russia joined the US as an international partner on the ISS, Mir was a very capable space station and was assembled in orbit and crewed using Soyuz capsules without the Shuttle-like capabilities of the Canadarm and extensive EVA support. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In summary, the ISS components could have been (and frequently were) launched on non-STS rockets, but assembly in its current design required the support of the Shuttle. Other, earlier space stations were launched by other systems and assembled by different methods. Given the lessons of Skylab and Mir, it stands to reason that a space station with capabilities similar to those of the ISS could have been built, launched, and assembled without the STS. &quot;Easier&quot; is difficult to quantify. If we had stuck with the Saturn V-based launcher and Skylab-class modules, we would have faced far fewer assembly EVAs but with much larger, more massive modules. In lieu of the Shuttle's arm and EVA support we would have had to develop and qualify a docking system that allowed an 85-ton module to rendezvous and dock with another 85-ton module as &quot;easily&quot; as a 32-ton Apollo CSM docked with its LEM.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastEditorUserId="-1" LastEditDate="2017-11-05T22:35:46.430" LastActivityDate="2017-11-05T22:35:46.430" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23585" PostTypeId="2" ParentId="22041" CreationDate="2017-11-03T00:53:38.773" Score="5" Body="&lt;p&gt;I see different versions of Cyrillic transliterations in my sources; I have chosen consistency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are 6 Kurs antennae. For some reason NASA labeled two of them with the single number &quot;3&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Refer to the images for antenna location (yellow numbers on the graphic, red numbers in the photo):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/qjNsq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qjNsq.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/WCAdi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WCAdi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The descriptions come straight out of the linked paper. They appear to have been copied verbatim from the Soyuz Crew Operations Manual, which sadly is not online.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;ACΦ1 - measures range, range rate, target line-of-sight (LOS) heading and pitch angles, relative roll angle, LOS angular rate&lt;/li&gt;&#xA;&lt;li&gt;ACΦ2 - receives parameters transmitted from the station (heading and pitch)&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;AKR2 - omnidirectional range and range rate antenna&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AKR3 - operates with 2AO antenna in the attitude control mode&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;2AO - measures LOS heading and pitch angles&lt;/li&gt;&#xA;&lt;li&gt;AKR1 - omnidirectional range and range rate antenna&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Sources&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/press/DA_Schlottke.pdf&quot; rel=&quot;noreferrer&quot;&gt;Modelling of Soyuz Docking and Radar Systems&#xA;for Implementation in the IRS Simulator&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/expeditions/expedition14/kurs_antenna_system.html&quot; rel=&quot;noreferrer&quot;&gt;Obsolete NASA page for graphic&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Personal notes&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-11-03T01:02:45.223" LastActivityDate="2017-11-03T01:02:45.223" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23586" PostTypeId="2" ParentId="23524" CreationDate="2017-11-03T00:55:44.197" Score="5" Body="&lt;p&gt;That is the &quot;Kurs system 2AO&quot; antenna on a Soyuz. It measures line-of-sight heading and pitch angles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://orbit.medphys.ucl.ac.uk/press/DA_Schlottke.pdf&quot; rel=&quot;noreferrer&quot;&gt;Modelling of Soyuz Docking and Radar Systems&#xA;for Implementation in the IRS Simulator&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/station/expeditions/expedition14/kurs_antenna_system.html&quot; rel=&quot;noreferrer&quot;&gt;Obsolete NASA page on Kurs&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Personal notes&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-11-03T00:55:44.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23587" PostTypeId="2" ParentId="23576" CreationDate="2017-11-03T14:23:46.277" Score="1" Body="&lt;p&gt;Another interesting point - the hard part isn't getting &lt;em&gt;up&lt;/em&gt;, it's going &lt;em&gt;fast&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably, since the elevator is tethered to the ground it's already going fast - fast enough to stay in geostationary uh... orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since by the time whatever you were moving into space got to the top of the elevator, it's going to be going the same speed, it should be a lot easier since you just need to keep moving.&lt;/p&gt;&#xA;" OwnerUserId="6836" LastActivityDate="2017-11-03T14:23:46.277" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23588" PostTypeId="2" ParentId="23576" CreationDate="2017-11-03T16:08:01.810" Score="1" Body="&lt;p&gt;In the book &quot;The Fountains of Paradise&quot;, by Arthur C. Clarke, it is mentioned that a space elevator can regenerate some of the electricity it uses to move material up by electronically braking as it brings material down, like regenerative braking on electric cars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is also the possibility of actual rescue, should something go wrong.  There is a very intense section of the book that involves a rescue maneuver, which also involves &quot;thinking outside the box&quot;.  I'd explain more, but I don't want to drop any spoilers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/The_Fountains_of_Paradise&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/The_Fountains_of_Paradise&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe the book also mentions that with the space elevator, you don't need to reach escape velocity until you reach the end of the elevator.  Since that's in space, escape velocity from that point should be considerably less than it is from the surface.  Less air resistance should also make it easier to reach.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rockets work on a exponential thrust restriction.  The more you want to put into space, the way more fuel you need to lift off.  It's not just that you are lifting 10 lbs into orbit, but you are also lifting the all fuel to get you there.  The rocket crews have to make decisions of how important items are to a mission, because that 10 lbs of cargo may mean something like 10,000 lbs more fuel.  Maybe this is explained by the tyranny of the rocket equation, but I don't have the chops to understand that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Because there's so much fuel going into a rocket launch, it's expensive.  Electricity, on the other hand, is relatively cheap.  Renewable and nuclear generators have pretty much guaranteed that.  Plus, with the extra height of the space elevator to get past the atmosphere, solar panels might be able to be added to the tower itself, allowing it to generate some of it's own power, beyond the regenerative braking mentioned earlier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond launch things into space, a space elevator could be a starting point for a ring around the earth, at orbital height.  This could in turn be a platform for a full planet enclosure.  Both ideas are referenced in several books I've read but can't recall right now.  Both have their advantages in economy and beyond.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Going back to &quot;The Fountains of Paradise&quot;, it tells about families going for sight seeing trips, including elderly people.  Sightseeing is very big business, and if even the baby and the older generation of a family can join in, then you've really got something that'll draw visitors.   Well, you know, besides being able to see a significant portion of the earth all at once, being &quot;in space&quot;, and lots of other &quot;cool factors.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The book also mentions being able to launch capsules in very short order, like multiples a day.  Right now, that's not possible with rockets, even with airplane launched rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The book is basically an essay in how space elevators are pretty much necessary for us to continue to be in space, couched in a science fiction novel.  It's a great book, winning both the Hugo and Nebula awards.  I've enjoyed reading it multiple times.&lt;/p&gt;&#xA;" OwnerUserId="21537" LastActivityDate="2017-11-03T16:08:01.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23589" PostTypeId="1" AcceptedAnswerId="23592" CreationDate="2017-11-03T16:45:43.007" Score="16" ViewCount="4881" Body="&lt;p&gt;The James Webb space telescope will be positioned very close to L2. According to JPL, Webb will have a large solar-array to power itself. I don't understand how this works, since L2 is positioned &quot;behind&quot; the earth relative to the sun.&lt;/p&gt;&#xA;" OwnerUserId="16006" LastActivityDate="2017-11-04T05:50:51.077" Title="What are the sources of light at L2? How will the James Webb telescope be powered?" Tags="|solar-power|lagrangian-points|james-webb-telescope|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23590" PostTypeId="1" AcceptedAnswerId="23594" CreationDate="2017-11-03T17:24:24.503" Score="9" ViewCount="549" Body="&lt;p&gt;I'd like to figure out how to determine where something is in space (solar system) based only solely on measurements that can be preformed on location visually/with a camera. Let's say I'm floating in an orbit around the earth and I can make the following measurements:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Apparent size of planet below&lt;/li&gt;&#xA;&lt;li&gt;Locations of stars relative to me&lt;/li&gt;&#xA;&lt;li&gt;Location of the Sun relative to me (when not obscured by planet)&lt;/li&gt;&#xA;&lt;li&gt;Know the exact time&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;My process/ what I've figured out so far:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;By knowing the diameter of the planet below and knowing camera optics specs I can calculate my altitude above the planet to a high degree of accuracy based on the resolution of the camera and the apparent size of the planet&lt;/li&gt;&#xA;&lt;li&gt;By matching up the image stars I can see with a database of stars, I can determine which point on the planet I am above, like using a sextant&lt;/li&gt;&#xA;&lt;li&gt;If I know which point on the planet I am above and my altitude, I can find out exactly where I am in space&lt;/li&gt;&#xA;&lt;li&gt;By performing this measurement again after a certain elapsed time, I can calculate my relative velocity&lt;/li&gt;&#xA;&lt;li&gt;Using velocity and location I can calculate orbit apogee and perigee&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Questions: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does this approach make sense?&lt;/li&gt;&#xA;&lt;li&gt;Is this how it's done in modern day satellites?&lt;/li&gt;&#xA;&lt;li&gt;How accurate can this type of location-finding get?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="21538" LastEditorUserId="6816" LastEditDate="2017-11-04T01:51:58.330" LastActivityDate="2020-06-27T07:17:11.187" Title="Navigating in space with only cameras" Tags="|orbital-mechanics|artificial-satellite|navigation|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23592" PostTypeId="2" ParentId="23589" CreationDate="2017-11-03T19:02:35.623" Score="23" Body="&lt;p&gt;The planned orbit for the JWST is quite a large halo orbit around Sun-Earth L2. It's very roughly elliptical, with dimensions of about +/- 350,000 km &quot;vertically&quot; (perpendicular to the Earth's orbital plane) and about +/- 750,000 km &quot;horizontally&quot; (in the Ecliptic plane).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see a drawings in (for example) &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140008868.pdf&quot; rel=&quot;noreferrer&quot;&gt;James Webb Space Telescope Initial Mid-Course Correction Monte Carlo Implementation using Task Parallelism&lt;/a&gt; and also in &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20140007519&quot; rel=&quot;noreferrer&quot;&gt;Stationkeeping Monte Carlo Simulation for the James Webb Space Telescope&lt;/a&gt;, shown below. The period of the larger Sun-Earth L1 and L2 halo orbits are roughly six months, or about half of the Sun-Earth orbital period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/a/23249/12102&quot;&gt;In this answer&lt;/a&gt; I've plotted an example of a potential orbit, done in roughly 2012 and found in the &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; database. Included those as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the JWST won't get close enough to the Sun-Earth line to even consider the issue. But if you wanted to do the math, then yes &lt;strong&gt;IF&lt;/strong&gt; the JWST was in an orbit that took it very close to the Earth-Sun line, it could get partially but very significantly eclipsed by the Earth's shadow and the light from the Sun would be dramatically reduced. It wouldn't go to zero, and so if it shut down most systems it might survive this, but it's not going to happen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cwB3o.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cwB3o.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140008868.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. &lt;em&gt;Figure 2. A sample trajectory for the JWST observatory in the RLP frame (the Sun is located along the –x axis). The orbit of the Moon appears for scale in the top two plots. L2 is approximately 235 Earth radii (Re) from the Earth&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xZth3.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xZth3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20140007519&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;. &lt;em&gt;Figure 1. Plots of the JWST Design Reference Mission LPO about SEM L2. The top plot gives a skew 3D view, while the bottom plot gives a view of the RLP xy-plane. The orange line in the top view is the direction of the Sun from Earth&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I've added these here because the axes are labeled in kilometers, not Re (Earth radii).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL Horizons&lt;/a&gt; there is a sample calculation of a Halo orbit for JWST that was done in 2014. It orbits for about eleven years, and presumably includes small station-keeping maneuvers to do so. This real-world orbit is more complex than a pure CR3BP (Circular Restricted Three-body problem) halo orbit because the Earth is in an elliptical orbit around the Sun, and the Moon adds further perturbations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first figure below shows the complete, eleven-year orbit with it's final wander away from L2 when station keeping ends. This is a &quot;top-down&quot; view of the Ecliptic plane, with the Sun to the left.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next three images are just one year's worth, showing the two loops of the six-month period halo orbit. The Blue dot is the Earth, the green torus is the Moon's orbit, and the red line is the orbit of JWST. The coordinates are in the rotating frame of the Earth-Moon barycenter around the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vu82S.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vu82S.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Top-down view, eleven years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/M4erN.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M4erN.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Top-down view, one year. (Sun to the left)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rcx7c.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rcx7c.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; View from the Sun, one year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rSuD8.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rSuD8.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; View from the side, one year. (Sun to the left)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-04T05:50:51.077" LastActivityDate="2017-11-04T05:50:51.077" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23593" PostTypeId="1" AcceptedAnswerId="23595" CreationDate="2017-11-04T00:57:00.343" Score="2" ViewCount="153" Body="&lt;p&gt;Saying &quot;port port&quot; is clumsy. Should I refer to it as the port berthing mechanism? Is there specific terminology for what I'm referring to or are there a few options? Also, I'm specifically referencing modules berthed to each other in this case, not supply ships docking to modules. Is there a difference? (FYI, port:left :: starboard:right)&lt;/p&gt;&#xA;" OwnerUserId="21546" LastEditorUserId="6816" LastEditDate="2017-11-04T01:48:24.753" LastActivityDate="2017-11-04T05:01:37.513" Title="How does one refer to the &quot;port port&quot; on the ISS?" Tags="|iss|terminology|docking|docking-port|berthing|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23594" PostTypeId="2" ParentId="23590" CreationDate="2017-11-04T03:57:28.853" Score="3" Body="&lt;p&gt;&lt;strong&gt;edit 2:&lt;/strong&gt; I just ran across this page; &lt;a href=&quot;https://www.mdpi.com/2072-4292/11/15/1747/htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Improvement of EPIC/DSCOVR Image Registration by Means of Automatic Coastline Detection&lt;/a&gt; and this paper &lt;a href=&quot;https://www.researchgate.net/publication/334682797_Improvement_of_EPICDSCOVR_Image_Registration_by_Means_of_Automatic_Coastline_Detection&quot; rel=&quot;nofollow noreferrer&quot;&gt;Improvement of EPIC/DSCOVR Image Registration by Means of Automatic Coastline Detection&lt;/a&gt;. While it doesn't directly apply here, certainly automatic coastline detection or other surface feature detection could be useful if it can be put into a spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit 1:&lt;/strong&gt; I believe the Mars 2020 Rover landing technique has been discussed in an answer or question here, but I can't find it right now. Here's a quick link, &lt;a href=&quot;http://www.spaceflightinsider.com/space-centers/jet-propulsion-laboratory/nasa-mars-2020-rover-equipped-23-eyes/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight Insider&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GwK5i.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GwK5i.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;A spacecraft in Earth orbit can indeed determine its own position relative to the Earth's center (to several kilometers, not &amp;quot;exactly&amp;quot;) using terminator and star images, and a clock that measures elapsed time. I'll just answer the conceptual aspect of your question, assuming your spacecraft has modern electronics, imaging and computing (say a really nice, &amp;quot;price is no object&amp;quot; 3U cubesat) but you don't have a ground link to send location data to it from Earth, nor have GPS.&lt;/p&gt;&#xA;&lt;p&gt;If you have several fixes spaced around the Earth's terminator, and knowledge of the shape of the Earth, you can indeed get the distance from the Earth and the direction of the Earth's center. Since the atmosphere has say 5 km of &amp;quot;noise&amp;quot; (clouds) it won't be perfect, but a given measurement should be reliable to about 1 part per thousand in distance.&lt;/p&gt;&#xA;&lt;p&gt;Combine the vector (direction plus distance) towards the Earth's center with the spacecraft's attitude with respect to the stars using two star camera fixes (direction-only of course), and you can determine your 3D location in space with respect to the Earth's &lt;em&gt;geometrical center&lt;/em&gt; to that 1 part per thousand; 7km in LEO or 40 km in GEO for example.&lt;/p&gt;&#xA;&lt;p&gt;However keep making these measurements for a period of time of one orbit, and you have measured your distance much more precisely using a Keplerian (or better) model and the known mass of the Earth and tightened up the uncertainty substantially. (The more you sample the terminator, the more the clouds &lt;em&gt;will tend to&lt;/em&gt; average out as well.) This just relies on your clock giving you elapsed time between measurements, not necessarily absolute time and date.&lt;/p&gt;&#xA;&lt;p&gt;As soon as you are measuring for roughly the period of an orbit, you only need the direction to the center of the Earth, not the size. As the Earth's direction with respect to the stars is tracked and timestamped, you can fit it with a model for all of the orbital parameters, eccentricity, inclination, nodes, etc.&lt;/p&gt;&#xA;&lt;p&gt;This all assumes you have well-calibrated cameras and the orientations of each camera's field of view with respect to all other cameras is known, or can be cross-calibrated in real time - depending how the spacecraft rotates and how dedicated vs flexible each camera is. It also assumes the cameras are built well to handle star or terminator imaging, don't get burned out by the Sun (star cameras often/usually have safety shutters), and you can manage your spacecraft's rotation (attitude) sufficiently to make sure the right cameras point at the right things.&lt;/p&gt;&#xA;&lt;p&gt;Having fixes on the Sun and Moon will help you calculate the Earth's position relative position within the solar system barycenter if you need that. Of course if you have an ephemeris and your clock knows the actual time and date, you can have that already.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-06-27T07:17:11.187" LastActivityDate="2020-06-27T07:17:11.187" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23595" PostTypeId="2" ParentId="23593" CreationDate="2017-11-04T04:54:15.233" Score="2" Body="&lt;p&gt;They would most likely use a TLA (Three Letter Acronym) instead of the noun &quot;port&quot;.  In the example you give, they would say the &quot;port CBM&quot; (Common Berthing Mechanism).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an example from the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/420433main_FLT_PLN_130_B.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-130 Flight Plan&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tDP3H.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tDP3H.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-11-04T05:01:37.513" LastActivityDate="2017-11-04T05:01:37.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23596" PostTypeId="1" AcceptedAnswerId="23611" CreationDate="2017-11-04T05:30:20.507" Score="7" ViewCount="3913" Body="&lt;p&gt;I have no idea about this thing, but since Mars is on the table, why not Venus? It has comparatively better gravity and a big enough surface. Also, maybe we can harvest its atmosphere and use it elsewhere. I'm not talking about small colonies but terraforming the whole planet. Secondary question: What would be the benefits of Venus over Mars.&lt;/p&gt;&#xA;" OwnerUserId="21504" LastEditorUserId="690" LastEditDate="2017-11-05T13:56:08.420" LastActivityDate="2019-01-17T06:45:58.767" Title="Will thinning the atmosphere of Venus make its surface habitable?" Tags="|colonization|venus|terraforming|" AnswerCount="4" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23598" PostTypeId="2" ParentId="8623" CreationDate="2017-11-04T09:15:12.317" Score="0" Body="&lt;p&gt;If Venus were not rotating, then the incident sunlight would heat the upper atmosphere, and the Hadley cells would transport the heat away uniformly in all directions.  However, because Venus &lt;em&gt;is&lt;/em&gt; rotating, the flow at the top of the Hadley cells ahead of the subsolar point is traveling faster than the flow behind the subsolar point.  This differential in speed results in an acceleration in the direction of Venus' rotation, with the result of the upper atmosphere rotating faster than the planet.  Thus, the Hadley cells are effectively a lubricating layer between the planet and upper atmosphere which, &lt;em&gt;en masse&lt;/em&gt;, must be considered a separate, enclosing body that is rotating faster than the planet itself.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastActivityDate="2017-11-04T09:15:12.317" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23599" PostTypeId="2" ParentId="23338" CreationDate="2017-11-04T09:15:46.260" Score="2" Body="&lt;p&gt;&lt;strong&gt;No&lt;/strong&gt;, concrete is far too heavy to be a feasible structural material. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;BOE calculation of the weight of a concrete Shuttle hull:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle has 250 m&lt;sup&gt;2&lt;/sup&gt; wing area. Assuming we make a solid wing, 7 cm thick, that's 250*0.07 = 17.5 m&lt;sup&gt;3&lt;/sup&gt; of concrete for the wing structure, for a weight of 38.5 tons (assuming a density of 2.2 tons/m&lt;sup&gt;3&lt;/sup&gt;). &#xA;Simplifying the body to a cylinder with diameter 5 m and length 37 m, that's 620 m&lt;sup&gt;2&lt;/sup&gt; surface area, 43 m&lt;sup&gt;3&lt;/sup&gt; of concrete and 95 tons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So already our structural weight is 132.5 tons and we haven't added any internals. &#xA;The Shuttle had an empty weight of 68 tons including all internal systems. Guesstimating the structural weight to be half that, you've added 100 tons of dead weight to your Shuttle stack, and the stack now has to be twice as large (6 SSME, 4 SRB) to get off the ground.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Buoyancy in water is much higher than in air, so ships aren't a good comparison. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-11-04T10:23:17.520" LastActivityDate="2017-11-04T10:23:17.520" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23600" PostTypeId="1" CreationDate="2017-11-04T09:19:26.147" Score="8" ViewCount="742" Body="&lt;p&gt;If an astronaut gets unconscious or ill I suppose the colleague will help her to the airlock. But are there any other measures that could be taken already outdoors? Do they have duct tape to seal a leak in the suit? Can any kind of first aid be provided?&lt;/p&gt;&#xA;" OwnerUserId="1457" LastActivityDate="2019-08-22T23:05:57.790" Title="What first aid options do astronauts have during an EVA?" Tags="|spacesuits|eva|safety|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23601" PostTypeId="2" ParentId="1334" CreationDate="2017-11-04T09:46:29.257" Score="1" Body="&lt;p&gt;Photosynthesis won't work, as AlanSE has explained.  A better option would be figuring out how to extract the carbon from the CO&lt;sub&gt;2&lt;/sub&gt; to make the carbon nanontubes that we're going to need to build the space elevators.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="7062" LastEditDate="2022-11-19T05:28:08.333" LastActivityDate="2022-11-19T05:28:08.333" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23602" PostTypeId="2" ParentId="23600" CreationDate="2017-11-04T13:02:45.237" Score="10" Body="&lt;p&gt;None, really.  If you examine the EVA Cuff Checklist (the one that is actually taken outside, shown here on Suni Williams' suit - note customized cover) &#xA;&lt;a href=&quot;https://i.stack.imgur.com/lw7Uy.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lw7Uy.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;you will see that all emergencies end in either TERMINATE EVA or ABORT EVA (ABORT EVA being faster) if working the procedure doesn't fix the problem. In other words, get back inside where you can actually do something about the problem.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's the Cuff Checklist table of contents.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/eALJJ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eALJJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a sample of one of the procedures.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/7YVNh.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7YVNh.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you get the Suit Pressure Emergency alarm:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check the hardware gauge to confirm the reading (a redundant sensor to the one that sets off the alarm). If it shows low, check that your valves are closed and the O2 Actuator is in the proper position.  Then abort the EVA. (The check mark means check something and fix it if it's wrong. If you find a valve open and closing it works, you're done.)  If the hardware gauge shows the pressure is OK, ignore the alarm. But keep checking that gauge!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read the whole thing &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/492872main_EVA_G_H_20.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-11-04T13:10:55.390" LastActivityDate="2017-11-04T13:10:55.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23603" PostTypeId="1" AcceptedAnswerId="23604" CreationDate="2017-11-04T14:12:47.637" Score="4" ViewCount="357" Body="&lt;p&gt;A &lt;a href=&quot;https://space.stackexchange.com/questions/22041/#comment67353_23585&quot;&gt;comment&lt;/a&gt; below &lt;a href=&quot;https://space.stackexchange.com/a/23585/12102&quot;&gt;this great answer&lt;/a&gt; about the Kurs navigation and docking system linked to the following page on the Ukrainian company Elimz website:  &lt;a href=&quot;http://www.elmiz.com/en/product/orbitalnyj-servis-eng/product/view/2/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.elmiz.com/en/product/orbitalnyj-servis-eng/product/view/2/1&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is Elimz currently manufacturing Kurs systems and is Russia currently buying systems built in the Ukraine for ongoing Soyuz missions to the ISS? I ask partly because the respective governments have had some &quot;disagreements&quot; in recent years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/xGCtfODf7Yg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-11-04T22:43:06.450" Title="Do the Soyuz spacecraft bringing astronauts and supplies to the ISS use Ukraine-built Kurs systems?" Tags="|soyuz-spacecraft|docking|kurs|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23604" PostTypeId="2" ParentId="23603" CreationDate="2017-11-04T16:08:18.703" Score="3" Body="&lt;p&gt;Progress and Soyuz are now using a &lt;a href=&quot;http://www.russianspaceweb.com/soyuz-ms-kurs-na.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;new digital system called Kurs-NA&lt;/a&gt; which is Russian-designed and -built.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-04T22:43:06.450" LastActivityDate="2017-11-04T22:43:06.450" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23606" PostTypeId="2" ParentId="23537" CreationDate="2017-11-04T20:34:55.763" Score="1" Body="&lt;p&gt;If you're looking for flying drones, NASA is currently developing a flying scout to accompany a driving rover. This scout would have only a couple of minutes of flight time a day and take a while to recharge. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/vpBsFzjyRO8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, let's just say we are designing a similar drone for martian exploration, lets consider some of the advantages and disadvantages we have, specifically on mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Difficulties:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Martian atmosphere is very thin (1% of earth pressure at sea level)&lt;/li&gt;&#xA;&lt;li&gt;Dust kicked up by prop wash certainly isn't good for fast moving parts&lt;/li&gt;&#xA;&lt;li&gt;Active propulsion flying systems (propellers, ducted fans, etc...) require constant power to stay in the air and would have huge power requirements&lt;/li&gt;&#xA;&lt;li&gt;Recharging via the sun takes longer as there is less sunlight and larger solar panels mean more mass&lt;/li&gt;&#xA;&lt;li&gt;A flying drone is much more likely to be completely unusable if it lands badly&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Advantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Martian gravity is lower&lt;/li&gt;&#xA;&lt;li&gt;Less atmospheric weather conditions need to be dealt with (winds weaker, no rain, etc...)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;In short, I think that there are certain valuable applications for a flying drone on mars (or other planets) and they will be used but only when there's people there to pick it up if it lands on its side. Right now, putting a drone on the surface of mars would be extremely expensive and high risk to gather data that could be gathered just as well abet at a slower pace (driving rover or high resolution satellite imagery). &lt;/p&gt;&#xA;" OwnerUserId="21538" LastEditorUserId="21538" LastEditDate="2017-11-05T11:12:18.487" LastActivityDate="2017-11-05T11:12:18.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23607" PostTypeId="1" AcceptedAnswerId="23685" CreationDate="2017-11-04T20:53:55.983" Score="6" ViewCount="3542" Body="&lt;p&gt;SpaceX has been announcing very ambitious goals such as starting to send manned missions to Mars in order to (eventually) create a colony, &lt;em&gt;and recently&lt;/em&gt; there has been a substantial release of more technical details about this plan (see links at the the end). As quoted in Spaceflight Now's article &lt;a href=&quot;https://spaceflightnow.com/2016/09/27/spacexs-elon-musk-announces-vision-for-colonizing-mars/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX’s Elon Musk announces vision for colonizing Mars&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;SpaceX plans to launch its first mission to Mars, a robotic test flight with a modified Dragon capsule, as soon as May 2018&lt;/strong&gt;. After that “Red Dragon” flight, Musk said SpaceX’s goal is to send at least one spacecraft to Mars during every interplanetary launch opportunity, which come every 26 months or so.&lt;/p&gt;&#xA;&lt;p&gt;The concept detailed Tuesday features a huge rocket standing 400 feet (122 meters) tall, and a fleet of passenger-carrying spaceships and refueling tankers.&lt;/p&gt;&#xA;&lt;p&gt;Musk’s long-term vision is to build a self-sustaining civilization on Mars, complete with “iron foundries and pizza joints.” Eventually, it might have a million residents or more.&lt;/p&gt;&#xA;&lt;p&gt;“When will we reach that million person threshold? It’s probably between 20 and 50 total Mars rendezvouses,” Musk said, counting Mars launch windows occurring every other year. “It’s probably anywhere from 40 to 100 years to fully achieve a self-sustaining civilization on Mars.”&lt;/p&gt;&#xA;&lt;p&gt;He counts those numbers from the time of the first crewed flight, which might some as soon as the 2020s.&lt;/p&gt;&#xA;&lt;p&gt;“&lt;strong&gt;We aspire to launch in late 2024 with an arrival in early 2025&lt;/strong&gt;,” Musk told reporters after his presentation at the International Astronautical Congress in Guadalajara, Mexico. “That’s optimistic, so I would describe that as an aspiration and within the realm of possibility, but a lot of things need to go right. That said, I don’t think it would be significantly beyond that if it did go later.” (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This seems really soon. Has technology advanced enough for this type of mission to be fulfilled - transporting astronauts all the way to Mars, and (presumably) returning them safely home again? And what about the radiation problem in the trip?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Based on the more detailed information available by SpaceX recently, what are the most difficult challenges that SpaceX faces accomplishing this goal by 2025, or not &amp;quot;significantly beyond that&amp;quot;?  Are there any obvious technology or logistic &amp;quot;show-stoppers&amp;quot; that could make this goal convincingly untenable?&lt;/p&gt;&#xA;&lt;p&gt;More about the plan can be found on the SpaceX page &lt;a href=&quot;http://www.spacex.com/mars&quot; rel=&quot;noreferrer&quot;&gt;http://www.spacex.com/mars&lt;/a&gt; and a link to a PDF of Elon Musk's position paper &lt;em&gt;Making Humans a Multi-Planetary Species&lt;/em&gt; can be found at &lt;a href=&quot;http://online.liebertpub.com/doi/full/10.1089/space.2017.29009.emu&quot; rel=&quot;noreferrer&quot;&gt;http://online.liebertpub.com/doi/full/10.1089/space.2017.29009.emu&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21551" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2021-04-25T01:49:47.080" Title="What are the most difficult challenges SpaceX will face getting astronauts to Mars by about 2025?" Tags="|spacex|mars|crewed-spaceflight|human-rating|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23609" PostTypeId="1" AcceptedAnswerId="40444" CreationDate="2017-11-05T08:10:00.407" Score="10" ViewCount="602" Body="&lt;p&gt;According to Space.com's article &lt;a href=&quot;https://www.space.com/38668-juno-8th-jupiter-science-flyby-success.html&quot; rel=&quot;noreferrer&quot;&gt;Juno Phones Home: Jupiter Probe Reconnects with Earth After 8th Flyby&lt;/a&gt;, Juno's recent close flyby of Jupiter and data collection happened while Jupiter was too close to the Sun as seen from Earth for the data to be reliably received by the Deep Space Network.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a GIF made from some &lt;a href=&quot;https://sohodata.nascom.nasa.gov/cgi-bin/data_query&quot; rel=&quot;noreferrer&quot;&gt;SOHO LASCO C3&lt;/a&gt; images; you can see Jupiter disappear behind the central occultation disk that protects each SOHO imager from the Sun. (Incidentally, that's Comet 96P on the right; see the NASA Goddard feature &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2017/return-of-the-comet-96p-spotted-by-esa-nasa-satellites&quot; rel=&quot;noreferrer&quot;&gt;Return of the Comet: 96P Spotted by ESA, NASA Satellites&lt;/a&gt;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IrA7a.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IrA7a.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Solar conjunctions of Jupiter — when Earth's and Jupiter's orbits take the planets on opposite sides of the sun — mean that a spacecraft orbiting Jupiter can't transmit to Earth without the &lt;a href=&quot;https://www.space.com/22215-solar-wind.html&quot; rel=&quot;noreferrer&quot;&gt;charged particles the sun emits&lt;/a&gt; corrupting the probe's signal. The last solar conjunction of Jupiter was in August 2015, before Juno had arrived at Jupiter, and the next will be in November 2018, according to &lt;a href=&quot;https://in-the-sky.org/news.php?id=20181126_12_100&quot; rel=&quot;noreferrer&quot;&gt;in-the-sky.org&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Of course the article is wrong, and the previous conjunction would have been in 2016.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've plotted the angles below; it looks like about 1.5 degrees is too close, but 4 degrees is OK. Those correspond to a closest approach of the line-of sight to the surface of the Sun of 3 vs 10 million kilometers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Is this a simple plasma density effect? When the line-of-sight passes too close to the Sun the cut-off frequency drops below that used by the spacectraft for downlink? Is the density of the solar wind at this distance quantitatively high enough to become opaque at this frequency? Or is the problem more complicated, and perhaps also involves too much dispersion? Or perhaps a geometrical problem; it's too hard to point so close to the Sun without equipment damage, or even a radio interference problem; there is too much spillover of radio noise from the Sun and the weak spacecraft signal can not be separated well enough to allow for a high-bandwidth downlink?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; I'm looking for some level of explanation, and not just an answer that says &quot;Because of interference from the Sun.&quot; Thanks!&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Plot of calculated positions and separation using the python package &lt;a href=&quot;http://rhodesmill.org/skyfield/api.html&quot; rel=&quot;noreferrer&quot;&gt;Skyfield&lt;/a&gt;. I'm not sure of the exact time of the beginning of successful downlink, so I've just added seven days to the time of flyby (estimated from &lt;a href=&quot;https://ssd.jpl.nasa.gov/horizons.cgi&quot; rel=&quot;noreferrer&quot;&gt;JPL's Horizons&lt;/a&gt;.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tiny dots are spaced at 1 day intervals, medium-sized red dot on the left is the moment of flyby, large-sized green dot near the right is roughly the time of successful downlink.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Y4OJk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Y4OJk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;from skyfield.api import Loader  # http://rhodesmill.org/skyfield/&#xA;&#xA;degs = 180./np.pi&#xA;&#xA;load = Loader('~/Documents/SkyData')&#xA;data = load('de421.bsp')&#xA;ts   = load.timescale()&#xA;sun, earth, jupiter = data['sun'], data['earth'], data['jupiter barycenter']&#xA;&#xA;ddays = np.arange(0, 10, 0.1)  # ten days by 0.1 day steps&#xA;&#xA;times = ts.utc(2017, 10, 24+ddays, 17, 44) # with respect to 17:44 UTC, October 24th, 2017&#xA;&#xA;observations = [earth.at(times).observe(thing) for thing in sun, jupiter]&#xA;separation   = degs*observations[1].separation_from(observations[0]).radians&#xA;&#xA;if True:&#xA;    fig = plt.figure()&#xA;&#xA;    ax1 = fig.add_subplot(2, 1, 1)&#xA;&#xA;    for obs in observations:&#xA;        RAdegs, Decdegs = [degs*thing.radians for thing in obs.radec()[:2]]&#xA;        ax1.plot(RAdegs, Decdegs)&#xA;        ax1.plot(RAdegs[::10],  Decdegs[::10],  '.k'                 )&#xA;        ax1.plot(RAdegs[:1],    Decdegs[:1],    'or', markersize =  8)&#xA;        ax1.plot(RAdegs[70:71], Decdegs[70:71], 'og', markersize = 12)&#xA;&#xA;    ax1.set_xlim(208, 220)&#xA;    ax1.set_ylim(-16, -10)&#xA;    ax1.set_aspect(1.0) # https://stackoverflow.com/a/18576329/3904031&#xA;    ax1.set_xlabel('RA (degs)')&#xA;    ax1.set_ylabel('Dec (degs)')&#xA;    ax1.set_title('Sun and Jupiter observed from Earth geocenter, start 2017-10-24, 17:44 UTC')&#xA;    ax1.text(212, -11, 'Sun')&#xA;    ax1.text(212, -14, 'Jupiter')&#xA;&#xA;    ax2  = fig.add_subplot(2, 1, 2)&#xA;&#xA;    ax2.plot(ddays, separation)&#xA;    ax2.plot(ddays[:1],    separation[:1],    'or', markersize =  8)&#xA;    ax2.plot(ddays[70:71], separation[70:71], 'og', markersize = 12)&#xA;    ax2.set_xlim(-0.5, None)&#xA;    ax2.set_ylim(0, None)&#xA;    ax2.set_xlabel('days since flyby')&#xA;    ax2.set_ylabel('Jupiter/Sun separation (degs)')&#xA;&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-03-02T05:27:41.623" LastActivityDate="2019-12-14T07:05:18.700" Title="What exactly is the interaction that blocked Juno's data downlink near solar conjunction?" Tags="|jupiter|juno|deep-space-network|data-transmission|skyfield|" AnswerCount="1" CommentCount="19" ContentLicense="CC BY-SA 3.0" />
  <row Id="23610" PostTypeId="2" ParentId="22006" CreationDate="2017-11-05T09:26:41.203" Score="2" Body="&lt;p&gt;To answer the question about &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/marc-rayman/dawn-journal-second-extended-mission.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;staying at Ceres or going somewhere else:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dawn does not have enough Xenon propellant left to enter orbit around another asteroid. It can leave Ceres, but can only do a flyby after that.   &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The promising potential for observing Ceres in elliptical orbits from closer than ever before makes a second extended mission there extremely attractive. NASA and the panel of scientists and engineers convened to provide an independent, objective assessment concluded that further exploration of Ceres would be the most valuable assignment for the spacecraft. It is noteworthy that Dawn is the only spacecraft ever to orbit two extraterrestrial destinations and even now, having significantly exceeded its original objectives, has the capability to leave Ceres and pay a brief visit to a third (although it does not have enough xenon left to orbit a third), but the prospects for new discoveries at Ceres are too great to pass up.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For the second mission extension, NASA is looking at putting Dawn into an elliptical orbit that has a lower periapsis than previous orbits: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For several months, the flight team has been studying the feasibility of flying the spaceship closer to Ceres than had ever been seriously considered. Dawn spent more than eight months in 2015-2016 circling about 240 miles (385 kilometers) above the dwarf planet. It had spectacular views of mysterious landscapes and acquired a wealth of data far beyond what the team had anticipated. Then Dawn flew to a higher altitude during its first extended mission for new observations. Now engineers are making progress on ways to operate the spacecraft in an elliptical orbit that would allow it to swoop down to below 125 miles (200 kilometers) for a few minutes on each revolution. Their results so far are very encouraging. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-11-05T11:01:34.817" LastActivityDate="2017-11-05T11:01:34.817" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23611" PostTypeId="2" ParentId="23596" CreationDate="2017-11-05T10:17:44.333" Score="11" Body="&lt;p&gt;There's several advantages and disadvantages to Venus compared to Mars terraforming and colonization:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Advantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Venus's gravity is ~0.9 G which is very similar to Earth's&lt;/li&gt;&#xA;&lt;li&gt;Venus is closer to the earth with less communication lag and more frequent launch windows (around every 600 days instead of 800 days to mars)&lt;/li&gt;&#xA;&lt;li&gt;At high altitudes, temperatures are in manageable ranges (75C-27C)&lt;/li&gt;&#xA;&lt;li&gt;At same high altitudes, air pressures are OK for humans meaning you'd only need a respirator and no pressure suit (but maybe thermal control)&lt;/li&gt;&#xA;&lt;li&gt;The atmosphere is mostly carbon dioxide which could be used for plants&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Z3UGr.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Z3UGr.png&quot; alt=&quot;Venus Atmosphere&quot;&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Disadvantages:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Venus's atmosphere contains sulfur dioxide which is bad for humans and plants&lt;/li&gt;&#xA;&lt;li&gt;Venus's atmosphere contains almost no oxygen and water will be hard to come by&lt;/li&gt;&#xA;&lt;li&gt;With current technology the corrosive conditions of the surface can't be survived for very long meaning it would be extremley difficult to transport materials from the surface. This means all materials would have to be brought to the planet. The surface is under extreme pressure, temperature, and chemical conditions. On mars, the surface can easily be mined for resources including water.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/23MRI.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/23MRI.jpg&quot; alt=&quot;Surface of venus&quot;&gt;&lt;/a&gt;&#xA;The surface of venus&lt;/p&gt;&#xA;&#xA;&lt;p&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for terraforming efforts, talking about &quot;harvesting the atmosphere&quot; is so far out of our reach currently that making guesses is borderline fantasy however given &quot;magical future technology&quot; we'd have to worry about a couple things:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Removing toxic chemicals from the atmosphere&lt;/li&gt;&#xA;&lt;li&gt;Reduce carbon dioxide content and increase nitrogen and oxygen content&lt;/li&gt;&#xA;&lt;li&gt;Figure out how to cool the planet (shoot hot gas into space?)&lt;/li&gt;&#xA;&lt;li&gt;Figure out how to keep the planet cool (large orbital mirrors between the sun and venus?&lt;/li&gt;&#xA;&lt;li&gt;Figure out how to make the surface non-toxic&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Basically, you'd need to redo and remove a large part of the atmosphere and then redo the entire surface (or a large part of it) and install some sort of planetary thermal management system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think terraforming Mars is much more realistic as you don't have many of the problems that you have with venus:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Raising the temperature of mars will release frozen water which will form an atmosphere. This could be achieved by detonating nukes in the icecaps, pointing large mirrors at mars, or redirecting asteroids with 'good chemistry' to impact mars&lt;/li&gt;&#xA;&lt;li&gt;Reestablish Mars' magnetic field. This could be done by positioning a bunch of magnet-equipped satellites at good positions between the sun and Mars (some scientists say this is feasible with current technology&#xA;&lt;a href=&quot;https://i.stack.imgur.com/ykzl5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ykzl5.jpg&quot; alt=&quot;Mars magnetic shield&quot;&gt;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Terraforming_of_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Terraforming_of_Mars&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Terraforming_of_Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Terraforming_of_Venus&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21538" LastEditorUserId="21538" LastEditDate="2018-10-25T21:56:18.750" LastActivityDate="2018-10-25T21:56:18.750" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23613" PostTypeId="2" ParentId="23576" CreationDate="2017-11-05T13:36:17.027" Score="0" Body="&lt;p&gt;Escape velocity is greater than geosynchronous velocity.  The asteroid or whatever at the end of a tether would have to travel a bit &lt;em&gt;faster&lt;/em&gt; than geosynchronous velocity, because of all the mass &lt;em&gt;below&lt;/em&gt; the orbit of the asteroid moving at &lt;em&gt;slower&lt;/em&gt; than (for the altitude of each bit) orbital velocity.  Perhaps this would be close, or at least close enough, to escape velocity to allow easy exit from Earth's gravity well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But even so, the tether itself would have to be very strong to resist the tension it would have to bear.  A lot of people have suggested that carbon nanotubes would do the trick; I suspect even that would not be enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And even if we could invent the materials, how would we construct such a thing?  If we capture an asteroid, put it into low-Earth orbit, and move the materials to it, we would still have to make a connection somehow.  We can't just lower the cable into the atmosphere -- it would be whipping around until we (somehow) grab onto the end and attach it onto the Earth station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps we will solve all these problems someday.  But it's a little more difficult than I believe most people think.&lt;/p&gt;&#xA;" OwnerUserId="21557" LastActivityDate="2017-11-05T13:36:17.027" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23615" PostTypeId="1" AcceptedAnswerId="23617" CreationDate="2017-11-05T17:04:53.567" Score="2" ViewCount="445" Body="&lt;p&gt;Lets say I want to create a satellite constellation similar to earth's GPS network around the moon or mars with around 24 4U-6U cubesats as shown in this image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eAAzF.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eAAzF.gif&quot; alt=&quot;GPS constellation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering is it possible/practical to launch these small cubesats into their appropriate orbital inclination from an equatorial orbit using some sort of magnetic coil gun or &quot;Gauss cannon&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as a step by step this would look like this:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Larger ship is in a circular, equatorial orbit of a planet (or moon) and is carrying a bunch of small cubesats and two large magnetic accelerators&lt;/li&gt;&#xA;&lt;li&gt;The ship points one cannon towards the normal vector and the other at the anti-normal vector&lt;/li&gt;&#xA;&lt;li&gt;Simultaneously two cubesats are shot out of the cannons (perhaps inside a sabot) and the two shots cancel out the inertia&lt;/li&gt;&#xA;&lt;li&gt;The larger ship waits the appropriate time and preforms this maneuver again and again until all satellites are launched and positioned&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I see several advantages to this system such as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Cubesats can be cheap and don't need a propulsion system&lt;/li&gt;&#xA;&lt;li&gt;A network with many satellites can be set up very quickly&lt;/li&gt;&#xA;&lt;li&gt;The cannon vehicle can be reused&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is such a system feasible and practical compared to current methods? &lt;/li&gt;&#xA;&lt;li&gt;Could the cannons reach enough speed to set the orbits properly?&lt;/li&gt;&#xA;&lt;li&gt;What would be the accuracy on such a system?&lt;/li&gt;&#xA;&lt;li&gt;Would the satellites encounter extreme heat/magnetic buildup during launch or damage to electronics when fired due to the strong magnetic field? Would encasing them in some sort of sabot prevent this?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="21538" LastActivityDate="2019-07-02T01:45:21.557" Title="Rapid deployment of cubesat network with Gauss cannons" Tags="|orbital-mechanics|artificial-satellite|cubesat|communication-satellite|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23616" PostTypeId="1" CreationDate="2017-11-05T19:25:24.430" Score="0" ViewCount="50" Body="&lt;p&gt;Could anyone provide an estimate for the mass of on-orbit servicing vehicles such as SMART-OLEV etc &#xA;The mass of the whole vehicle is estimated to be in the range of 1000-1200 kg but what would be the mass of just the docking system&lt;/p&gt;&#xA;" OwnerUserId="21559" LastActivityDate="2017-11-05T19:25:24.430" Title="Mass of docking system for On-orbit servicing vehicles?" Tags="|docking|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23617" PostTypeId="2" ParentId="23615" CreationDate="2017-11-05T19:30:58.923" Score="5" Body="&lt;p&gt;I've run the math! Although it's possible, the instantaneous energy required to do this is just insane. Basically you'd need the power supply of a Nimitz-class Aircraft Carrier to power the thing or a huge bank of capacitors. An orbital inclination change like this for a 5kg satellite would require around 16 megajoules of power or the energy equivalency of around 400ml of kerosene.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6XM1Y.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6XM1Y.jpg&quot; alt=&quot;Calculations&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note calculation assumes circular orbits&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you can see, the instant power supply capability required is very high and designing a spacecraft which survives an acceleration this extreme would probably become heavier and thus defeat the purpose of the satellites (being light and easy to launch). It's much easier add the extra propulsion systems to position the satellites traditionally.&lt;/p&gt;&#xA;" OwnerUserId="21538" LastEditorUserId="195" LastEditDate="2019-07-02T01:45:21.557" LastActivityDate="2019-07-02T01:45:21.557" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="23621" PostTypeId="1" AcceptedAnswerId="23622" CreationDate="2017-11-06T08:52:20.980" Score="28" ViewCount="5233" Body="&lt;p&gt;Why does the cargo fairing have a much larger diameter than the lower part of the rocket?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course a larger diameter lets thicker payloads fit. But then the first, inter, and second stage could have been designed lower in order to decrease overall surface area (and thus aerodynamic drag) and also to ease the landing (lower center of gravity -&gt; easier balancing).&lt;/p&gt;&#xA;" OwnerUserId="21562" LastEditorUserId="21562" LastEditDate="2017-11-07T17:00:15.953" LastActivityDate="2017-11-07T17:00:15.953" Title="Falcon 9 design: Why is the cargo fairing so much wider than the lower parts?" Tags="|design|falcon-9|falcon-heavy|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23622" PostTypeId="2" ParentId="23621" CreationDate="2017-11-06T09:04:30.907" Score="55" Body="&lt;p&gt;The diameter of the stages &lt;a href=&quot;https://space.stackexchange.com/questions/9273/why-is-falcon-9s-shape-so-tall-and-skinny/9274#9274&quot;&gt;is the largest size that can be transported by road&lt;/a&gt; without extensive &quot;outsize load&quot; issues (permits, having to move traffic lights and signs out of the way etc.). This makes the rocket much cheaper to transport.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fairing size (5.4 m) is dictated by the standard satellite diameter set by the Shuttle and Ariane 5. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="2784" LastEditDate="2017-11-06T18:19:50.507" LastActivityDate="2017-11-06T18:19:50.507" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23623" PostTypeId="1" CreationDate="2017-11-06T10:06:41.230" Score="-10" ViewCount="334" Body="&lt;p&gt;This is for my end project. Thank you.&lt;/p&gt;&#xA;" OwnerUserId="21563" LastActivityDate="2017-11-06T11:27:10.500" Title="What is the required velocity that a rocket should have to escape earth's gravity?" Tags="|orbit|" AnswerCount="1" CommentCount="3" ClosedDate="2021-04-27T05:51:48.080" ContentLicense="CC BY-SA 3.0" />
  <row Id="23624" PostTypeId="2" ParentId="23623" CreationDate="2017-11-06T11:27:10.500" Score="1" Body="&lt;p&gt;Minimum velocity required from Earth is&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;v=sqrt(2gR) &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;which you could've easily looked up on the Web.&lt;/p&gt;&#xA;" OwnerUserId="21564" LastActivityDate="2017-11-06T11:27:10.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23625" PostTypeId="2" ParentId="20933" CreationDate="2017-11-06T11:41:38.440" Score="2" Body="&lt;p&gt;Although an 'E-sail&quot; can probably protect against some of the protons emitted from the sun, there are far more efficient methods available than a fleet of large sails. For example, by providing each ship with it's own 'E-sail' wouldn't be that hard. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some calculations:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/hN8KM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hN8KM.jpg&quot; alt=&quot;Calculations for E-Sheild&quot;&gt;&lt;/a&gt;&#xA;I used numbers for the size of the E-sail from Wikipedia (beware, no citation)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conclusions: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;A solar sail system can be extremely light, in this example, the 'sail' part of the system only has a mass of &lt;strong&gt;35 kg&lt;/strong&gt; for a shield that has an area of over 1000 square kilometers!&lt;/li&gt;&#xA;&lt;li&gt;With such a minimal weight, I don't think it would make sense to have a fleet of ships where the sole purpose is to carry one of these. You'd need a full complement of spacecraft systems (power generation, attitude control, etc...) for every 'Oasis' ship while these systems already all exist on the ships which are traveling&lt;/li&gt;&#xA;&lt;li&gt;Even if the mass of the system is off by an order of magnitude, I still think it's economical to attach one of these units to one of the ships traveling during the transit. With one ship protecting the area of LA, you'd have plenty of space to spare&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These are however provided that an 'E-sail' would work as solar wind shielding. Regardless of whether they do or don't, there's more established and practical methods of radiation shielding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to NASA:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Throughout the entire trip [to mars], astronauts must be protected from two sources of radiation. The first comes from the sun, which regularly releases a steady stream of solar particles, as well as occasional larger bursts in the wake of giant explosions, such as solar flares and coronal mass ejections, on the sun. These energetic particles are almost all protons, and, though the sun releases an unfathomably large number of them, the proton energy is low enough that they can &lt;strong&gt;almost all be physically shielded by the structure of the spacecraft.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;An alternative method for providing radiation shielding from the sun for a ship would be a large, variable power, electromagnet on a gimbal somewhere inside the ship. This magnet would just point perpendicular to the sun and in the event of a larger solar event, it's power would be increased for more deflection. Unfortunately, these are not the hard ones to shield against:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The second source of energetic particles is harder to shield. These particles come from galactic cosmic rays, often known as GCRs. They’re particles accelerated to near the speed of light that shoot into our solar system from other stars in the Milky Way or even other galaxies. Like solar particles, galactic cosmic rays are mostly protons. However, some of them are heavier elements, ranging from helium up to the heaviest elements. These more energetic particles can knock apart atoms in the material they strike, such as in the astronaut, the metal walls of a spacecraft, habitat, or vehicle, causing sub-atomic particles to shower into the structure. This secondary radiation, as it is known, can reach a dangerous level.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This type of radiation can't really be blocked using a single electromagnet due to the fact that all magnets are dipoles and can only redirect particles if the're coming in from a certain direction. Blocking this second type of radiation with current technology requires &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Lots of mass or &quot;thickness&quot; between the radiation and the passengers&lt;/li&gt;&#xA;&lt;li&gt;Advanced materials (often layered) which absorb radiation especially well&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sources/Further reading:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3772/how-much-power-would-a-spacecrafts-magnetic-shield-require&quot;&gt;How much power would a spacecraft&amp;#39;s magnetic shield require?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Electric_sail&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Electric_sail&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21538" LastEditorUserId="21538" LastEditDate="2017-11-06T16:04:55.143" LastActivityDate="2017-11-06T16:04:55.143" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23626" PostTypeId="1" AcceptedAnswerId="27299" CreationDate="2017-11-06T13:17:26.753" Score="16" ViewCount="611" Body="&lt;p&gt;&lt;a href=&quot;https://worldbuilding.stackexchange.com/questions/96984/whats-the-biggest-reasonable-natural-planet-or-moon-with-earth-like-surface-gra/97020#97020&quot;&gt;Here is a question originating on the Worldbuilding stack&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hyperion is porous, with a density of 0.55 g/cc; a little more than half that of water.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KuJZx.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KuJZx.jpg&quot; alt=&quot;Hyperion is porous&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;from &lt;a href=&quot;https://www.space.com/20770-hyperion-moon.html&quot; rel=&quot;noreferrer&quot;&gt;https://www.space.com/20770-hyperion-moon.html&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A body with Earth's gravity and the density of Hyperion would be huge.  But the question for this stack: in celestial bodies, does porosity scale up?  Is there a limit to how large a porous celestial body can be before the pores crunch down?  Are there enough celestial bodies that are porous to even deduce a rule?&lt;/p&gt;&#xA;" OwnerUserId="21565" LastEditorUserId="6816" LastEditDate="2017-11-06T15:27:15.660" LastActivityDate="2018-05-16T13:09:52.153" Title="How large could Hyperion be and stay porous?" Tags="|gravity|planetary-science|moons|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23628" PostTypeId="1" CreationDate="2017-11-06T15:35:31.087" Score="6" ViewCount="1343" Body="&lt;p&gt;I recently calculated using $E = 0.5mv^2$ that one kilogram of mass traveling at one tenth of the speed of light would have a kinetic energy of 449.4 trillion joules.  Relativistic effects are insignificant at this speed.  This is the amount of energy equivalent to an earthquake of &lt;a href=&quot;https://en.wikipedia.org/wiki/Richter_magnitude_scale&quot; rel=&quot;noreferrer&quot;&gt;7 on the Richter scale or a large hydrogen bomb&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is an enormous amount of energy from the perspective of designing a propulsion system to accelerate this 1 kg space craft to this speed from rest even if it is started from space outside of earth's gravity.  Conventional chemical rockets would be out of the question and much slower accelerations using alternate propulsion systems would be needed and even then present a very significant challenge to eventually obtain said velocity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We know that Voyager 1 and 2 have used gravity assist flybys of planets Jupiter, Saturn, Uranus, and Neptune to eventually increase their velocities to 17000m/sec for Voyager 1 and 15000m/sec for Voyager 2.  While this is fine escaping the solar system and enter deep space, it is still very slow for reaching Proxima Centauri, the closest star group after our sun, in any practical time.  Even at one tenth the speed of light, 29,979,246 m/sec, this would take 40 years.  40 years is still a long time but a practical and manageable long term goal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was wondering if a great many chained gravity assists within our solar system could be used to eventually accelerate the 1 kg space craft to said speed.  &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html&quot; rel=&quot;noreferrer&quot;&gt;This article&lt;/a&gt; explains gravity assists well, and how they can be used to speed up or slow down spacecraft relative to the solar frame of reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have in mind oscillating gravity assists on planets in orbits on opposite sides of the sun, back and forth to eventually reach this speed.  I realize that the spacecraft can only gain speed if it the exit trajectory of the gravity assist is in the forward orbital tangent from the planet.  This would present a challenge if we would like to keep oscillating the gravity assists from planet to planet, but it doesn't seem impossible.&lt;/p&gt;&#xA;" OwnerUserId="19730" LastEditorUserId="6816" LastEditDate="2017-11-06T15:44:06.903" LastActivityDate="2017-11-07T03:34:30.523" Title="Can we speed up spacecraft to suitable interstellar travel speed using oscillating gravity assists on planets on opposite sides of the solar system?" Tags="|interstellar-travel|gravity-assist|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23632" PostTypeId="2" ParentId="23628" CreationDate="2017-11-07T03:23:51.587" Score="14" Body="&lt;p&gt;Accumulating 0.1c (30000 km/s) with gravity turns alone within the bounds of Solar system isn't possible. Reason is: system escape velocity (sometimes referred to as third cosmic velocity) is about 42km/s (at Earth orbit, and the farther the lower). Once craft reaches this speed, it still can accumulate a bit more with right escape path, but generally not much more.&lt;/p&gt;&#xA;" OwnerUserId="21572" LastEditorUserId="21572" LastEditDate="2017-11-07T03:34:30.523" LastActivityDate="2017-11-07T03:34:30.523" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23633" PostTypeId="1" AcceptedAnswerId="23634" CreationDate="2017-11-07T11:48:29.193" Score="1" ViewCount="211" Body="&lt;p&gt;Recently Isro's mars orbitor has completed 4 years in mars orbit I guess. I am proud of that as I am Indian. but as curious person I still wonder &#xA;for proofs of that mars orbitor is still in mars orbit . Isro still havenot possesed capabilities like nasa to produce good images , so its hard a &#xA;bit for common people like me to believe in that. I am not questioning credibilities of ISRO. Before few years same questiones were raised for NASA moon mission.&#xA;So big questions is that is there any oraganization which take care of whether achievement of missions are authenticate or not ?&lt;/p&gt;&#xA;" OwnerUserId="16949" LastEditorUserId="16949" LastEditDate="2017-11-07T12:22:56.463" LastActivityDate="2017-11-07T14:19:18.113" Title="Is there any central world organization which authenticate achievement of various space agencies missions?" Tags="|mars|future-missions|isro|mars-orbiter-mission|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="23634" PostTypeId="2" ParentId="23633" CreationDate="2017-11-07T14:19:18.113" Score="6" Body="&lt;p&gt;No. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some spaceflight records are authenticated by the &lt;a href=&quot;https://en.wikipedia.org/wiki/F%C3%A9d%C3%A9ration_A%C3%A9ronautique_Internationale&quot; rel=&quot;noreferrer&quot;&gt;FAI&lt;/a&gt; (Fédération Aéronautique Internationale) under its sub-body FAI Astronautic Records Commission but these are generally highest/longest/fastest type records involving human spaceflight. A &lt;a href=&quot;https://airandspace.si.edu/stories/editorial/why-yuri-gagarin-remains-first-man-space-even-though-he-did-not-land-inside-his&quot; rel=&quot;noreferrer&quot;&gt;famous controversy&lt;/a&gt; arose over Yuri Gagarin's flight because of FAI rules stating that the pilot had to land in his vehicle to be eligible for a record.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the general facts of space accomplishments are typically announced by the agency that performs them, and it is up to everyone who doubts them to use their critical thinking and research skills to confirm or deny them for their own purposes.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-11-07T14:19:18.113" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23635" PostTypeId="1" AcceptedAnswerId="23636" CreationDate="2017-11-07T16:10:11.047" Score="9" ViewCount="2478" Body="&lt;p&gt;Landing of a rocket seems impossible with vertical flight control only. How the rocket achieves horizontal flight control with engines in line with the booster?&lt;/p&gt;&#xA;" OwnerUserId="21400" LastActivityDate="2017-11-08T02:47:33.790" Title="How does the Falcon 9 achieves horizontal flight control" Tags="|spacex|falcon-9|" AnswerCount="1" CommentCount="0" ClosedDate="2017-11-08T09:25:47.220" ContentLicense="CC BY-SA 3.0" />
  <row Id="23636" PostTypeId="2" ParentId="23635" CreationDate="2017-11-07T16:17:33.910" Score="16" Body="&lt;p&gt;The engines, like engines on most large launchers, are &lt;a href=&quot;https://en.wikipedia.org/wiki/Gimbaled_thrust&quot; rel=&quot;noreferrer&quot;&gt;gimbaled&lt;/a&gt; to steer in any direction. By rapidly adjusting the gimbal angle, the rocket can stay balanced vertically while adjusting its path horizontally. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 also has &lt;a href=&quot;https://en.wikipedia.org/wiki/Grid_fin&quot; rel=&quot;noreferrer&quot;&gt;aerodynamic grid fins&lt;/a&gt; at the top of the first stage, which provides steering control throughout the unpowered portion of the reentry and return flight.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, the Falcon also has pressurized nitrogen thrusters at the top of the stage which it can use for steering. These are primarily intended for use at very high altitude, where the air is so thin that the fins aren't effective, but they're still &lt;a href=&quot;https://space.stackexchange.com/questions/8804/why-did-the-rcs-thrusters-on-falcon-9-fire-in-the-wrong-direction&quot;&gt;potentially useful all the way to touchdown&lt;/a&gt;. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-07T16:28:06.550" LastActivityDate="2017-11-07T16:28:06.550" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23637" PostTypeId="2" ParentId="17606" CreationDate="2017-11-07T16:49:09.020" Score="4" Body="&lt;p&gt;The necessity for assigning GEO slots is mostly driven by two factors: 1) the need to prevent signal interference resulting in a physical distance of about 800 - 1600 miles between satellites in GEO and 2) the geographic coverage of each satellite associated with its orbit drives the demand for certain GEO slots. This results in an irregular density of GEO satellite orbital distributions. For example, a highly competitive slot would be one which could see much of the South East Asia or alternatively the United States, Canada, and the Caribbean. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The International Telecommunications Union (ITU) as a part of the United Nations, however, only enforces GEO placement as it involves signal interference between satellites. They do not consider the orbital placement of satellites. Furthermore, since GEO slots overlap across many nations it is not entirely feasible for a nation to assign all the slots which cover its geographic area. For instance, the United States Federal Aviation Administration monitors and regulates launch and reentry activities within the United States as it pertains to the health and safety of people on the ground. However, it does not formally assign slots for a satellite in GEO. It is important to note that international disputes could arise if a nation state or a foreign company placed a “malicious” satellite in GEO above another nation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some level of self-selection must occur since it is currently very expensive to launch and operate a GEO satellite. It is financially disadvantageous for a commercial or national entity to launch to an orbital slot where signal interference would cause it interfere with another satellite hindering its operational use or alternatively to a slot where orbital drift could possibly result in a collision with another satellite.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Thus, a company that wants a GEO satellite currently only needs to petition the ITU and to indicate to them that their satellite does not interfere with the signals of other satellites.&lt;/strong&gt; It must launch the satellite in accordance with national launch regulations. The ambiguity of this answer goes to illustrate the need for more compressive international agreements for the assignment of satellite placements in Earth orbit. This is particularly true as the access to space gets cheaper and more entities wish to launch to GEO orbits.&lt;/p&gt;&#xA;" OwnerDisplayName="user18817" LastActivityDate="2017-11-07T16:49:09.020" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23638" PostTypeId="2" ParentId="20956" CreationDate="2017-11-07T19:08:51.597" Score="4" Body="&lt;p&gt;Assuming that you are talking about a propulsive Landing only, without parachutes, you would need around 3.8 kilometers per second of Delta V (&lt;a href=&quot;https://www.google.com/amp/s/amp.reddit.com/r/space/comments/29cxi6/i_made_a_deltav_subway_map_of_the_solar_system/&quot; rel=&quot;nofollow noreferrer&quot;&gt;from the map below&lt;/a&gt;). It should be noted that the amount of Delta V required to land on a body from orbit roughly equals the amount required to reach orbit. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/2V441.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2V441.png&quot; alt=&quot;The Solar System - A subway map&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;&lt;a href=&quot;https://www.reddit.com/r/space/comments/29cxi6/i_made_a_deltav_subway_map_of_the_solar_system/?ref=share&amp;amp;ref_source=link&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source of image&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To land:&#xA;There is some air at Mars which will slow you down somewhat, so 3.8km/s of Delta V is a ballpark number which is higher than what it is in reality. As the drag due to air resistance will slow the vehicle down and therefore require less Delta V to land.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One major factor in knowing how much fuel you will actually need to perform the landing is the efficiency of the fuels. Taking into account the Apollo LEM which used Aerozine 50 and N2O4 and got an isp of 311 (3047m/s), it had around a 60% fuel mass to total mass ratio. But the moon requires much less delta V to land on than Mars. So a vehicle running the with the same fuels (N2O4 and Aerozine) would need a ratio of .72 fuel mass to total mass. However this ratio really depends on the efficiency of the engine, as a more efficient engine can have a lower ratio, but a less efficient engine will need a higher ratio. &#xA;.72 which was calculated via the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket equation&lt;/a&gt; does not take into account air resistance during landing (which will help in this case). So a lander would need at most a ratio of .72 for a propulsive landing only, neglecting air resistance.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get to orbit from the surface:&#xA;Since the amount of Delta V required to land from orbit is roughly the same to ascend from the surface to orbit, we will assume 3.8km/s of Delta V needed. It is helpful to not that this number is actually a lot closer to reality as air resistance will play much less of a role in ascent than descent. Let’s use the same fuels as the Apollo LEM once again and assume an efficiency of 311s (3047m/s).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3800=3047ln(10/Mf)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass final in this case is around 2.8, giving a percentage of 72% (as mentioned earlier) for the fuel mass to total mass to reach Mars orbit. This is definitely lower than what would actually be used as this is the bare minimum to reach orbit, once in orbit the vehicle may need to rendezvous and use more fuel. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once again, this ratio of .72 is only for the LEM descent engine. Use a more efficient engine and get lower ratios and vice versa.&lt;/p&gt;&#xA;" OwnerUserId="18458" LastEditorUserId="58" LastEditDate="2017-11-08T22:06:58.263" LastActivityDate="2017-11-08T22:06:58.263" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23640" PostTypeId="2" ParentId="21042" CreationDate="2017-11-08T07:42:31.910" Score="2" Body="&lt;p&gt;First, note that Soyuz &lt;em&gt;launcher&lt;/em&gt; and Soyuz &lt;em&gt;spacecraft&lt;/em&gt; are two distinct entities. The launcher is based on soviet strategic R-7 rocket, and all of its modifications are definitely not powerful enough to put its namesake spacecraft anywhere near Moon orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, the Soyuz spacecraft is another matter. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-L1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz 7K-L1&lt;/a&gt; spacecraft, in link with Proton launcher, was modified for testing if piloted circumlunar flight was possible, although the launcher was not man-rated. Zond-7 mission using this spacecraft was succesful in Moon flyby and returning to Earth, so it is possible to reach Moon orbit and return to Earth using modern, more powerful Proton launcher and a modified Soyuz vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ria.ru/science/20060721/51672033.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Another mission&lt;/a&gt; was proposed in 2008 by Director of Russian NPO &quot;Energia&quot; Nikolai Sevastianov, based on original Korolev plan from 1964. It involved docking a standard Soyuz with separately launched booster module. This mission was slated for 2012, and as we know, never reached any results.&lt;/p&gt;&#xA;" OwnerUserId="21168" LastEditorUserId="21168" LastEditDate="2017-11-09T01:24:49.747" LastActivityDate="2017-11-09T01:24:49.747" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23641" PostTypeId="1" CreationDate="2017-11-08T13:53:53.743" Score="3" ViewCount="610" Body="&lt;p&gt;I have heard that SpaceX uses some kind of phased array system. I guess, this is reasonable because the speed of the rocket makes usage of any movable parts impossible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I am not an engineer and want to know, how they position the system around the starting and landing pads to allow 3D positioning of the tracked device. I also wonder which frequency and tx power is necessary and what hardware the rocket has to interact with the system?  &lt;/p&gt;&#xA;" OwnerUserId="21400" LastEditorUserId="1457" LastEditDate="2017-11-09T14:56:11.177" LastActivityDate="2017-11-21T14:49:12.793" Title="How does SpaceX phased array system work to keep track on the rocket" Tags="|spacex|falcon-9|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23642" PostTypeId="1" CreationDate="2017-11-08T15:45:00.800" Score="2" ViewCount="178" Body="&lt;p&gt;I read there are sometimes intra-satellite links.&#xA;Is it already used or just a project in the future ? How is is used and why ? and for what services ?&#xA;thanks&#xA;david&lt;/p&gt;&#xA;&#xA;&lt;p&gt;thanks for interesting answers. can we say it is common use for tv links?&lt;/p&gt;&#xA;" OwnerUserId="19869" LastEditorUserId="19869" LastEditDate="2017-11-11T06:37:22.887" LastActivityDate="2017-11-11T06:37:22.887" Title="Satellite Intra link" Tags="|communication-satellite|radio-communication|" AnswerCount="0" CommentCount="4" ClosedDate="2017-11-09T13:35:38.607" ContentLicense="CC BY-SA 3.0" />
  <row Id="23643" PostTypeId="1" CreationDate="2017-11-08T20:35:57.730" Score="2" ViewCount="128" Body="&lt;p&gt;Where can one find asteroids or comets that contain water ice suitable for a human mission. Perhaps there is comprehensive catalog of &lt;strong&gt;small bodies&lt;/strong&gt; that can be searched for characteristics such as:&#xA;&lt;br&gt;&lt;br&gt;&#xA;mass: &lt;br&gt;&#xA;size: &lt;br&gt;&#xA;rotation period: &lt;br&gt;&#xA;rotation axis: &lt;br&gt;&#xA;composition: &lt;br&gt;&#xA;albedo: &lt;br&gt;&#xA;orbit: &lt;br&gt;&#xA;location: &lt;br&gt;&#xA;distance to Earth: &lt;br&gt;&#xA;surface characteristics: &lt;br&gt;&#xA;&lt;br&gt;&lt;/p&gt;&#xA;" OwnerUserId="21584" LastEditorUserId="21584" LastEditDate="2017-11-10T04:21:33.120" LastActivityDate="2017-11-10T04:21:33.120" Title="Asteroid Catalog" Tags="|asteroid|rotation|asteroid-belt|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23645" PostTypeId="1" CreationDate="2017-11-09T12:47:15.310" Score="19" ViewCount="2878" Body="&lt;p&gt;The interesting question &lt;a href=&quot;https://space.stackexchange.com/q/23607/12102&quot;&gt;What are the most difficult challenges SpaceX will face getting astronauts to Mars by about 2025?&lt;/a&gt; links to Elon Musk's position paper &lt;em&gt;Making Humans a Multi-Planetary Species&lt;/em&gt; which can be found (in both HTML and PDF formats) at:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://online.liebertpub.com/doi/full/10.1089/space.2017.29009.emu&quot; rel=&quot;noreferrer&quot;&gt;http://online.liebertpub.com/doi/full/10.1089/space.2017.29009.emu&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Figure 9 gives you a more direct comparison. The thrust level is enormous. We are talking about a lift-off thrust of 13,000 tons, so it will be quite tectonic when it takes off. &lt;strong&gt;However, it does fit on Pad 39A, which NASA has been kind enough to allow us to use because they oversized the pad in doing Saturn V. As a result, we can use a much larger vehicle on that same launchpad&lt;/strong&gt;. In the future, we expect to add additional launch locations, probably adding one on the south coast of Texas, but this gives you a sense of the relative capability. However, these vehicles have very different purposes. This is really intended to carry huge numbers of people, ultimately millions of tons of cargo to Mars. Therefore, you really need something quite large in order to do that. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;It's an interesting read, and a good way to start to understand the issues involved and how SpaceX amy approach them. However one item caught my eye. Stating that the proposed &lt;a href=&quot;https://en.wikipedia.org/wiki/ITS_launch_vehicle&quot; rel=&quot;noreferrer&quot;&gt;ITS rocket&lt;/a&gt; design (now downsized to the Mars vehicle version of the &lt;a href=&quot;https://en.wikipedia.org/wiki/BFR_(rocket)&quot; rel=&quot;noreferrer&quot;&gt;BFR&lt;/a&gt; (Big Falcon Rocket)) would fit on — and presumably be launch-able from — NASA's Launchpad 39A, it mentions in passing that NASA &quot;oversized the pad in doing Saturn V&quot;, and that as a result SpaceX will be able to &quot;use a much larger vehicle on that same launch pad.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; I am wondering if there is any information, or even &lt;em&gt;educated speculation&lt;/em&gt; about the thinking at the time. Was 39A simply over-designed for Saturn V's weight and thrust out of an abundance of engineering caution, or was it in fact built to be able to accommodate a rocket substantially larger than Saturn V?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the linked article and Figure 9 (shown below), SpaceX believes, and presumably NASA agrees that 39A (of course with suitable modifications to some systems) should be able to handle a rocket with about 3.5 times the weight and 3.6 times the thrust of the Saturn V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HYw0C.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HYw0C.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="195" LastEditDate="2020-05-28T21:31:16.750" LastActivityDate="2022-01-17T09:42:09.277" Title="Was LC39A built with a rocket much larger than the Saturn V in mind?" Tags="|nasa|history|saturn-v|launchpad|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="23646" PostTypeId="1" AcceptedAnswerId="27109" CreationDate="2017-11-09T13:25:11.087" Score="3" ViewCount="652" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/q/23641/12102&quot;&gt;This interesting question&lt;/a&gt; got me started. I have these vignettes in my Apollo-era memory of antenna systems that tracked launches for telemetry. They had maybe four end-fire helical antennas pointing toward the rocket, and were motorized for tracking. I seem to remember a person sat on them to do manual visual tracking with a telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Am I mixing two different systems together (an optical telescope with human tracking and an automatic-tracking antenna) or were there antennas that were human guided?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could someone let me know if they existed, and perhaps post an image and/or link to a system description?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Here are images of each aspect separately; a human-guided optical tracking camera, and an articulated quad array of end-fire helical antennas. Were they ever combined?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7eYWM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7eYWM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; Contraves-Goerz Kineto Tracking Mount (KTM), from &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Contraves-Goerz_Kineto_Tracking_Mount.jpeg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/u6CMg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/u6CMg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; &quot;Satellite tracking-aquisition antenna, Pleumeur-Bodou, France. This type of antenna is called a Helical antenna. It transmits and receives circularly polarized radio waves, which are widely used for satellite communication.&quot; From &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Traqueur_acquisition.JPG&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-09T21:53:43.630" LastActivityDate="2018-05-06T00:38:51.110" Title="What were those motorized human-piloted platforms with helical antennas called (tracking launches)?" Tags="|antenna|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23647" PostTypeId="1" AcceptedAnswerId="23654" CreationDate="2017-11-09T14:28:57.300" Score="6" ViewCount="8474" Body="&lt;p&gt;One of the ways that the Falcon 9's attitude is controlled is through thrust vectoring of the main Merlin engine(s?). In images of older thrusters i've seen hydraulic or electric linear actuators to adjust the angle of the nozzle &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/c/c1/Mercury_Atlas_vernier_motor%2C_Truax_variant.jpg&quot; rel=&quot;noreferrer&quot;&gt;such as in this vernier motor&lt;/a&gt; however I can't find any concrete info on how this is achieved in the Merlin class of engines. The closest I could find was an entry on wikipedia which states that:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Propellants are fed via a single shaft, dual impeller turbopump. The turbo-pump also provides high-pressure fluid for the hydraulic actuators, which then recycles into the low-pressure inlet. This eliminates the need for a separate hydraulic drive system and means that thrust vectoring control failure by running out of hydraulic fluid is not possible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Merlin engine for Falcon 1 had a movable turbopump exhaust assembly which was used to provide roll control by vectoring the exhaust&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I'm not quite sure what to make of these entries. Does this mean that the thrust vectoring is achieved through 3 hydraulic pistons similar to the 3 electric 'pistons' in the Vernier Motor? Also can all Merlin engines thrust vector and what's the thrust vectoring capability of the capable Merlin engines (in deg)?&lt;/p&gt;&#xA;" OwnerUserId="21538" LastActivityDate="2017-11-10T01:16:38.500" Title="Falcon 9 Merlin engine thrust vectoring" Tags="|spacex|falcon-9|thrust-vectoring|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23648" PostTypeId="1" CreationDate="2017-11-09T15:03:31.777" Score="9" ViewCount="200" Body="&lt;p&gt;Elon Musk has recently announced plans to send two unmanned Starships to Mars in 2022. If the timeline is met, what robots will be sent? As I understand, the tasks will be &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;[...] landing two cargo missions on the red planet by 2022, [SpaceX] hopes to be able to confirm sources of water and potential hazards by that time. SpaceX also plans to start building mining, power and life support infrastructure that year [...]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.engadget.com/2017/09/29/spacex-mars-bfr/&quot; rel=&quot;nofollow noreferrer&quot;&gt;[source]&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These ambitions definitely need thorough preparation themselves, maybe just as much as Starship and Super Heavy. Take for example the Mars rovers: they required much more development time. And that was just for cruising around and communicating outputs of cameras and other instruments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any information out there about what kind of robots will be used, and whether SpaceX intends to develop them itself or to buy them?&lt;/p&gt;&#xA;" OwnerUserId="21562" LastEditorUserId="21562" LastEditDate="2019-08-08T13:17:40.527" LastActivityDate="2019-08-08T13:17:40.527" Title="What kind of robots will be sent to Mars with the first Starships(s)? (ex BFR)" Tags="|spacex|mars|spacex-starship|robotic-missions|" AnswerCount="0" CommentCount="2" ClosedDate="2017-11-13T07:37:54.510" ContentLicense="CC BY-SA 4.0" />
  <row Id="23651" PostTypeId="1" CreationDate="2017-11-09T18:12:01.660" Score="3" ViewCount="617" Body="&lt;p&gt;We build as much of the space elevator under compression as possible and the remaining part using carbon based materials. Is this trick going to work or am I missing something?&lt;/p&gt;&#xA;" OwnerUserId="21504" LastEditorDisplayName="user20636" LastEditDate="2017-11-11T11:03:20.133" LastActivityDate="2017-11-12T20:48:10.123" Title="If a space elevator uses both compression and tension design, will we still need new materials?" Tags="|space-elevator|" AnswerCount="4" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23652" PostTypeId="2" ParentId="23651" CreationDate="2017-11-09T19:30:35.667" Score="5" Body="&lt;p&gt;The tallest building we can build is &lt;a href=&quot;https://www.citylab.com/design/2012/08/there-limit-how-tall-buildings-can-get/2963/&quot; rel=&quot;noreferrer&quot;&gt;only a few kilometers&lt;/a&gt; high. Just like a space elevator is limited by tensile strength, a building is limited by the compressive strength of its materials.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few km isn't going to make the space elevator (which needs to be 36,000 km long, plus another longer section for the counterweight) any easier to build. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-11-09T19:30:35.667" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23653" PostTypeId="2" ParentId="23651" CreationDate="2017-11-09T19:42:09.037" Score="4" Body="&lt;p&gt;The original concept of a space elevator, a Tsiolkovsky tower, was a compressive structure that would just reach just to geosynchronous altitude. Unfortunately, there is no material that can come anywhere close to reaching that high. Structures over a few kilometers high undergo compressive yield -- they collapse in on themselves. (This is, for example, why Mount Everest is about as high as a mountain can be on Earth.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The modern concept of a space elevator has a counterweight well beyond geosynchronous altitude, making the elevator under tension throughout its length. The tensile strength required is almost within reach of known materials, and should be attainable using materials that are not in the realm of science fantasy. Something much less exotic than &lt;em&gt;unobtainium&lt;/em&gt; will do the trick.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-11-09T19:42:09.037" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23654" PostTypeId="2" ParentId="23647" CreationDate="2017-11-09T21:05:06.607" Score="5" Body="&lt;p&gt;The Merlins are gimbaled hydraulically, with two pistons per engine. The actuators are the white (diagram) or light blue (photo) cylinders at the top of the long gray linkage rods, labeled &quot;TVC actuator&quot;. This is Merlin 1C rather than the current Merlin 1D, but I believe the actuators are similar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sIVyi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sIVyi.jpg&quot; alt=&quot;schematic and photo of Merlin 1C engine&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure of the gimbal range, but it's probably close to 5-10º in any direction. It may be a little larger on the first-stage center engine (which is offset longitudinally from the others), as it's the most critical to landing maneuvers.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Merlin engine for Falcon 1 had a movable turbopump exhaust assembly which was used to provide roll control by vectoring the exhaust&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;For a single-engine stage, it's not possible to control roll by gimbaling the engine mounted on the centerline -- you need a source of torque offset from the center. For multi-engine stages, you pivot engines on opposite sides in opposite directions. Falcon 1 got around this problem by pivoting the turbopump exhaust, which is off-center from the main engine. The single-engine second stage of the Falcon 9 has a reaction control system using compressed nitrogen to control the attitude and roll of the stage, so doesn't need this scheme, so the turbopump exhaust on current Merlin engines is fixed, saving weight and complexity.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-10T01:16:38.500" LastActivityDate="2017-11-10T01:16:38.500" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23658" PostTypeId="1" AcceptedAnswerId="23676" CreationDate="2017-11-10T16:55:07.780" Score="17" ViewCount="1781" Body="&lt;p&gt;What's the normal leakage rate for a space suit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a modern (as in, the model is still in use today) EVA suit in normal condition, undamaged, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there is a wide variation in leak rates for different suit versions, national designs, etc. that would all be interesting detail for an answer.&lt;/p&gt;&#xA;" OwnerUserId="21600" LastEditorUserId="58" LastEditDate="2017-11-10T20:47:00.333" LastActivityDate="2022-07-22T02:13:44.190" Title="What's the normal leakage rate for a space suit?" Tags="|spacesuits|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23660" PostTypeId="2" ParentId="23651" CreationDate="2017-11-11T02:07:10.680" Score="3" Body="&lt;p&gt;I don't think this is quite what you are asking for, but in the broadest sense, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_fountain&quot; rel=&quot;nofollow noreferrer&quot;&gt;space fountain&lt;/a&gt; is a compression structure. These are towers held aloft by a constant upward matter stream shooting up from the base and being deflected by the structure. The upward force on the structure from the deflection is what holds it up instead of the compression strength of the material. These are far more complicated engineering feats then standard tension elevators, but as far as I am aware, they don't require any new materials, though better superconductors would make things easier. There is no strict limit to how tall these can be, so you could find your compromise between tension and compression. In the next paragraph I will go over why I think you would never build it this way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can build a large enough space fountain to help support an elevator, you could also build a &lt;a href=&quot;https://en.wikipedia.org/wiki/Launch_loop&quot; rel=&quot;nofollow noreferrer&quot;&gt;launch loop&lt;/a&gt;. Which is basically the same type structure but ending with an electromagnetic launcher right above the atmosphere to fling payloads up to orbital velocity instead of dragging them all the way to GEO. Though personally my favorite design is hanging tension space elevators from an &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_ring&quot; rel=&quot;nofollow noreferrer&quot;&gt;orbital ring&lt;/a&gt; in LEO. Then the matter stream is in vacuum the whole time so you don't need to worry about vacuum sheaths. I guess you could also use a series of orbital rings to help take the weight off a geosynchronous tether, but I am not sure that there is much of a point.&lt;/p&gt;&#xA;" OwnerUserId="21390" LastActivityDate="2017-11-11T02:07:10.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23661" PostTypeId="1" AcceptedAnswerId="41025" CreationDate="2017-11-11T11:48:06.853" Score="4" ViewCount="318" Body="&lt;p&gt;Reading this &lt;a href=&quot;https://space.stackexchange.com/questions/23650/saturn-likely-traps-more-antimatter-than-other-planets-in-the-solar-system-can&quot;&gt;question&lt;/a&gt; and then this &lt;a href=&quot;http://www.niac.usra.edu/files/studies/final_report/1071Bickford.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;report&lt;/a&gt; in a comment by @called2voyage &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At some point they talk about possibility to send (using antimatter as propellant) a space mission to solar gravity lens focus, using it as a giant telescope to observe exoplanets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since this solar focus is quite far ( 550 UA ) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is gravitational light bending capability of the earth enough to think of a smaller mission to earth gravity lens focus? How far would this focus be? And how magnifying this virtual lens would be? What about other planets focii in the solar system?&lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="19132" LastEditDate="2017-11-11T12:13:20.743" LastActivityDate="2020-01-24T02:02:19.900" Title="Earth gravity lens focus" Tags="|gravity|mission-design|space-telescope|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23663" PostTypeId="1" CreationDate="2017-11-12T00:17:12.757" Score="2" ViewCount="130" Body="&lt;p&gt;In 1984 JPL engineer Paul Penzo described a ZRVTO (Zero Relative Velocity Transfer Orbit) between an elevator anchored on Deimos and an elevator anchored on Phobos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a Penzo’s illustration:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YAHz9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YAHz9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Release a payload from the top of Phobos’  940 kilometer elevator and the payload will follow an elliptical path and arrive at the bottom of Deimos 2960 kilometer elevator. On arrival the relative velocity between payload and elevator terminus is zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It works the other direction as well. Drop a payload from the bottom of Deimos’ 2960 kilometer tether and it will arrive at the top of Phobos’ 940 km elevator. Again the relative velocity will be zero at the instant of rendezvous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With less than 4000 kilometers of tether, the two moons can exchange payloads using nearly zero reaction mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given two tidelocked moons in circular, coplanar orbits, is there a general method for finding elevator lengths that allow a ZRVTO between the moons' elevators?&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-11-12T18:08:30.177" Title="How do you find ZRVTOs between tethers from coplanar, tidelocked bodies?" Tags="|space-elevator|phobos|space-tether|deimos|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23664" PostTypeId="2" ParentId="23663" CreationDate="2017-11-12T00:19:17.180" Score="4" Body="&lt;p&gt;Yes. Given the radii of the upper and lower moons and knowing their angular velocities, there are simple expressions that give the periapsis and apoapsis radius of the Zero Relative Velocity Transfer Orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The specific angular momentum, h, of an orbit is constant. At any point on the orbit the specific angular momentum $\overrightarrow{h}$ is the cross product between the position and velocity vectors:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\overrightarrow{h} = \overrightarrow{v}$ X $\overrightarrow{r}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And so &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\overrightarrow{h}  = \overrightarrow{v}_{periapsis}$  X $\overrightarrow{v}_{periapsis}=\overrightarrow{v}_{apoapsis}$  X $\overrightarrow{v}_{apoapsis}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Position and velocity vectors are at right angles to one another at periapsis and apoapsis.  So a cross product gives us the same magnitudes with ordinary multiplication.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$h = v_{periapsis} * r_{periapsis} = v_{apoapsis} * r_{apoapsis}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What’s the length of $r_{periapsis}$ and  $r_{apoapsis}$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And what’s $v_{periapsis}$ and $v_{apoapsis}$?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Speed of a point on a vertical elevator  is  $\omega * r$ where r is the distance from center of rotation and $\omega$ is angular velocity of the elevator.  In this case the Phobos elevator has the angular velocity of Phobos and the Deimos elevator has the angular velocity of Deimos.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$v_{periapsis}=\omega_{phobos}*r_{periapsis}$&lt;br&gt;&#xA;$v_{apoapsis}=\omega_{deimos}*r_{apoapsis}$  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recall that&lt;br&gt;&#xA;$v_{periapsis} * r_{periapsis} = v_{apoapsis} * r_{apoapsis}$  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting&lt;br&gt;&#xA;$v_{periapsis}=\omega_{phobos}*r_{periapsis}$ and $v_{periapsis}=\omega_{deimos}*r_{apoapsis}$,&lt;br&gt;&#xA;$\omega_{phobos} * r_{periapsis} * r_{periapsis}= \omega_{deimos}*r_{apoapsis}*r_{apoapsis}$&lt;br&gt;&#xA;$\omega_{phobos} * r_{periapsis}^2= \omega_{deimos}*r_{apoapsis}^2$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting&lt;br&gt;&#xA;$r_{periapsis}=(1-e)a$ and $r_{apoapsis}=(1+e)a$,&lt;br&gt;&#xA;$\omega_{phobos} *((1-e)a)^2= \omega_{deimos}*((1+e)a)^2$&lt;br&gt;&#xA;$\omega_{phobos} *(1-e)^2= \omega_{deimos}*(1+e)^2$&lt;br&gt;&#xA;$((1-e)/(1+e))^2= \omega_{deimos}/\omega_{phobos}$&lt;br&gt;&#xA;$(1-e)/(1+e)= \sqrt{\omega_{deimos}/\omega_{phobos}}$  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;And now we can find eccentricity of the ZRVTO given angular velocities of each moon:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{e=(1-(\omega_{deimos}/\omega_{phobos})^{1/2})/(1+(\omega_{deimos}/\omega_{phobos})^{1/2})}$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Now we need to find the periapsis and apoapsis of the ZRVTO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At peri and apoapsis, magnitude of specific angular momemtnum is $v*r$.&lt;br&gt;&#xA;Since $v = \omega*r$ we can say &#xA;$h=\omega*r^2$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specific angular momentum is also twice the area swept by the orbit over an orbital period or  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$h=2\pi ab/T$&lt;br&gt;&#xA;$h=2\pi ab/(2\pi\sqrt{a^3/\mu})$&lt;br&gt;&#xA;$h=ab/\sqrt{a^3/\mu}$&lt;br&gt;&#xA;$h=a*a\sqrt{1-e^2}/(a^{3/2}\sqrt{\mu}$&lt;br&gt;&#xA;$h=a^2\sqrt{1-e^2}/(a^{3/2}\sqrt{\mu})$&lt;br&gt;&#xA;$h=a^{1/2}\sqrt{1-e^2}\sqrt{\mu}$&lt;br&gt;&#xA;$h=(a(1-e^2)\mu)^{1/2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The angular momentum of the ZRVTO from the top of the Phobos tether is&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$h=\omega_{phobos}*r_{periapsis}^2=(a(1-e^2)\mu)^{1/2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now&lt;br&gt;&#xA;$r_{periapsis}=(1-e)a$&lt;br&gt;&#xA;so $a=r/(1-e)$.&lt;br&gt;&#xA;Substituting for a…&lt;br&gt;&#xA;$\omega_{phobos}*r_{periapsis}^2=((r/(1-e))(1-e^2)\mu)^{1/2}$&lt;br&gt;&#xA;$\omega_{phobos}*r_{periapsis}^2=((r(1+e)\mu)^{1/2}$&lt;br&gt;&#xA;$r_{periapsis}^2=((r(1+e)\mu)^{1/2}/\omega_{phobos}$&lt;br&gt;&#xA;$r_{periapsis}^4=(r(1+e)\mu)/\omega_{phobos}^2$&lt;br&gt;&#xA;$r_{periapsis}^3=(1+e)\mu/\omega_{phobos}^2$  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now&lt;br&gt;&#xA;$\omega_{phobos} = 2\pi/T_{phobos}$&lt;br&gt;&#xA;Where $T_{phobos}$ is Phobos' orbital period.&lt;br&gt;&#xA;$T_{phobos}=2\pi\sqrt{r_{phobos}^3/\mu}$&lt;br&gt;&#xA;where $r_{phobos}$ is the distance from Phobos to the center of Mars.&#xA;So&lt;br&gt;&#xA;$\omega_{phobos} = 2\pi/((2\pi\sqrt{r_{phobos}^3/\mu})$&lt;br&gt;&#xA;The $2\pi$ in the numerator and denominator cancel out&lt;br&gt;&#xA;$\omega_{phobos} = 1/\sqrt{r_{phobos}^3/\mu})$  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Substituting this Value for $\omega_{phobos}$ into&lt;br&gt;&#xA;$r_{periapsis}^3=(1+e)\mu/\omega_{phobos}^2$&lt;br&gt;&#xA;we get&lt;br&gt;&#xA;$r_{periapsis}^3=(1+e)\mu\sqrt{r_{phobos}^3/\mu}^2$&lt;br&gt;&#xA;$r_{periapsis}^3=(1+e)\mu r_{phobos}^3/\mu$&lt;br&gt;&#xA;The $\mu$ s cancel out&lt;br&gt;&#xA;$r_{periapsis}^3=(1+e)r_{phobos}^3$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Which brings to the the periapsis radius in terms of the e we've already found and the orbital radius of Phobos.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{r_{periapsis}=(1+e)^{1/3}r_{phobos}}$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In a similar fashion apoapsis of our Zero Relative Velocity Transfer orbit is (1+e)a and&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{r_{apoapsis}=(1-e)^{1/3}r_{deimos}}$&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;I put these expressions into a spreadsheet &lt;a href=&quot;https://clowder.net/hop/TMI/ZRVTO_finder.xlsx&quot; rel=&quot;nofollow noreferrer&quot;&gt;ZRVTO_finder.xslx&lt;/a&gt;. A screenshot:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YrLZ0.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YrLZ0.png&quot; alt=&quot;ZRVTO spreadsheet&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a pic drawn from the spreadsheet's numbers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JIxst.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JIxst.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is reassuring my numbers are close to Penzo's. I'm just an amateur but Penzo was a very competent aerospace engineer working for JPL.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;There are many tidelocked moons in nearly circular, coplanar orbits. The above expressions can be generalized:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{e=(1-(\omega_{UpperMoon}/\omega_{LowerMoon})^{1/2})/(1+(\omega_{UpperMoon}/\omega_{LowerMoon})^{1/2})}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{r_{periapsis}=(1+e)^{1/3}r_{LowerMoon}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$\mathbf{r_{apoapsis}=(1-e)^{1/3}r_{UpperMoon}}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper and lower moon could also be artificial objects, man made anchors for vertical orbital tethers. If we know the moons' angular velocities and orbital radii, it is easy to figure the tether lengths to accommodate Zero Relative Velocity Transfer Orbits between the two.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastEditorUserId="2848" LastEditDate="2017-11-12T18:08:30.177" LastActivityDate="2017-11-12T18:08:30.177" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23665" PostTypeId="1" AcceptedAnswerId="23667" CreationDate="2017-11-12T03:11:58.877" Score="2" ViewCount="260" Body="&lt;p&gt;I saw the following statement in the BBC's short article and video &lt;a href=&quot;http://www.bbc.com/future/story/20171106-soyuz-the-soviet-marvel-that-remains-on-duty-after-50-years&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz: the Soviet marvel that remains on duty after 50 years&lt;/a&gt;. As a subtitle in the video:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;And it will remain on duty for years: the Russians and the Chinese still work on upgrades to its design.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;While it's intuitive that Russia would work on upgrades to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz spacecraft&lt;/a&gt;, I'm wondering about the mention of China. Are they collaborating on the Russian Soyuz, or is there a distinct vehicle in China which is similar enough to b called a Soyuz?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FSFA6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FSFA6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-12T05:18:22.950" LastActivityDate="2017-11-12T05:18:22.950" Title="Is China collaborating with Russia on the Soyuz, or perhaps developing their own?" Tags="|russia|soyuz-launcher|china|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23666" PostTypeId="1" CreationDate="2017-11-12T03:55:39.963" Score="-3" ViewCount="124" Body="&lt;p&gt;When a spacecraft is moving through outerspace does the movement of the craft create a headwind and if so could that headwind charge or create electricity by an engine that is moved by the headwind i.e. a converted aircraft engine with  with a large alternator or several alternators that help charge the spacecraft &lt;/p&gt;&#xA;" OwnerUserId="21607" LastActivityDate="2017-11-12T04:48:24.933" Title="Space movement or Propulsion" Tags="|propulsion|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23667" PostTypeId="2" ParentId="23665" CreationDate="2017-11-12T04:15:29.223" Score="3" Body="&lt;p&gt;As far as I know, there are no ongoing collaborations on Soyuz design. While it seems true that Russians have &lt;a href=&quot;http://www.spacedaily.com/news/china-99r.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;sold Soyuz designs&lt;/a&gt; to China in the middle of 90s, and those became a basis for &lt;a href=&quot;https://en.wikipedia.org/wiki/Shenzhou_spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;Chinese Shenzhou craft&lt;/a&gt;, nowadays China space program is still very closed enterprise. There was even a &lt;a href=&quot;http://www.nbcnews.com/id/22082431/ns/technology_and_science-space/t/russian-space-exec-convicted-aiding-china/&quot; rel=&quot;nofollow noreferrer&quot;&gt;lawsuit&lt;/a&gt; about 10 years ago against those who have sold the technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for rocket itself, Long March rockets have no common roots with Soyuz rocket family, though very far predecessors were of Soviet origin as well.&lt;/p&gt;&#xA;" OwnerUserId="21572" LastEditorUserId="21572" LastEditDate="2017-11-12T05:03:21.573" LastActivityDate="2017-11-12T05:03:21.573" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23668" PostTypeId="2" ParentId="23666" CreationDate="2017-11-12T04:48:24.933" Score="2" Body="&lt;p&gt;On Earth, a headwind occurs because there is air and a relative velocity between the moving object and the surrounding air. &lt;a href=&quot;https://en.wikipedia.org/wiki/Outer_space&quot; rel=&quot;nofollow noreferrer&quot;&gt;Outer space&lt;/a&gt; is essentially empty, unless you count the odd hydrogen atom per cubic meter. It's hardly going to create a headwind at any conceivable speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even if there was a headwind, and the means to use it to generate power, what is going on is that energy of motion of the vehicle is being taken from that motion and converted to something else (whatever you needed it for). That will cause the vehicle to slow down or require an extra push to maintain speed. You don't get &lt;a href=&quot;https://en.wikipedia.org/wiki/Perpetual_motion&quot; rel=&quot;nofollow noreferrer&quot;&gt;something for nothing&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is usually simpler, easier, and more efficient to generate power by other means than by taking it from the motion of the vehicle, no matter what sort of vehicle we are talking about, or what medium it is traveling through.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-11-12T04:48:24.933" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23669" PostTypeId="2" ParentId="20071" CreationDate="2017-11-12T08:24:21.443" Score="2" Body="&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; This is a partial answer, I'm still hoping for an Apollo-era historian (er... a historian of the Apollo-era) to answer the main question.&lt;/p&gt;&#xA;&lt;p&gt;It turns out that YuTu DID take a photograph of the peculiar backscattering effect from lunar regolith!&lt;/p&gt;&#xA;&lt;p&gt;In the Planetary Society's Senior Editor and Planetary Evangelist &lt;a href=&quot;http://www.planetary.org/about/staff/emily-lakdawalla.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Emily Lakdawalla&lt;/a&gt;'s article &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2016/01281656-fun-with-a-new-data-set-change.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fun with a new data set: Chang'e 3 lander and Yutu rover camera data&lt;/a&gt; there are several photos from Chang'e-3 and the Yutu rover. In one of them, a halo is seen around the camera's own self-photographed shadow. The caption attributes this to &lt;a href=&quot;https://en.wikipedia.org/wiki/Opposition_surge#Shadow_hiding&quot; rel=&quot;nofollow noreferrer&quot;&gt;shadow-hiding&lt;/a&gt; which is a much more plausible and simple phenomenon than &lt;a href=&quot;https://en.wikipedia.org/wiki/Coherent_backscattering&quot; rel=&quot;nofollow noreferrer&quot;&gt;coherent backscattering&lt;/a&gt;. Open the image in a new window to view full size.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Yutu Self-Portrait in Shadow&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Yutu took this photo of its own shadow on January 12, 2014. A halo appears around the right side of the rover's head because in that area, with the Sun precisely behind the right-eye camera, there are no visible shadows.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &lt;em&gt;Chinese Academy of Sciences / China National Space Administration / The Science and Application Center for Moon and Deep Space Exploration&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DeCFH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DeCFH.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; view of Yutu from Chang'e-3 lander&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ElSjH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ElSjH.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2019-12-14T01:59:56.017" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23670" PostTypeId="1" AcceptedAnswerId="23671" CreationDate="2017-11-12T11:27:08.773" Score="4" ViewCount="503" Body="&lt;p&gt;The video in this 27-Oct-2017 &lt;a href=&quot;https://twitter.com/45thSpaceWing&quot; rel=&quot;nofollow noreferrer&quot;&gt;45th Space Wing&lt;/a&gt; tweet uses the phrase &quot;That's how we drive to 48!&quot; What does it mean?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;T-3 days: Our 45 Operations Group is ready to take on this Epic Launch from the Eastern Range! @AFSpace  @AFSpaceCC&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Tweet:&lt;/strong&gt; &lt;a href=&quot;https://twitter.com/45thSpaceWing/status/923901931113902080&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://twitter.com/45thSpaceWing/status/923901931113902080&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v0RUr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v0RUr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-04-12T07:07:29.957" LastActivityDate="2020-04-12T07:07:29.957" Title="What does &quot;That's how we drive to 48!&quot; mean?" Tags="|launch|cape-canaveral|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23671" PostTypeId="2" ParentId="23670" CreationDate="2017-11-12T12:54:36.467" Score="7" Body="&lt;p&gt;&quot;drive to 48&quot; refers to the 45th Space Wing's goal of being able to support &lt;a href=&quot;http://www.patrick.af.mil/Portals/14/documents/8-23%20Priorities%20Commitments%20Booklet.pdf?ver=2017-08-23-174650-240&quot; rel=&quot;nofollow noreferrer&quot;&gt;48 launches per year from Cape Canaveral.&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the &quot;2017 Priorities and Commitments strategic plan&quot; (a document that outlines how the 45th Space Wing will respond to the changes in the launcher market): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Just two years ago, we anticipated Cape Canaveral Air Force Station (CCAFS) and neighboring Kennedy Space Center (which relies heavily upon us for range support) to be running at its full launch capacity in the near future, and began the process to transform the Eastern Range from its original design as a government-led space program to a new design to accommodate and support the needs of new commercial space carriers as well. And today, as a result of our commitment to our 2015 plan, we are positioning the Eastern Range and the services we provide to accommodate up to forty-eight launch operations a year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-11-12T19:30:36.467" LastActivityDate="2017-11-12T19:30:36.467" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23672" PostTypeId="1" CreationDate="2017-11-12T16:18:54.313" Score="1" ViewCount="187" Body="&lt;p&gt;I understand that all satellites must have burn corrections to keep their orbits where they are needed. However my question is specifically about what those changes are in magnitude from the desired orbit for a geostationary satellite, and specifically the altitude changes. If uncorrected, would not the satellite simply return to its &quot; main&quot; altitude in a periodic path? If so, what is the time period  - above its main, and below its main, during one day? (Is this period induced mainly by the moon's gravity?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: No matter how small the altitude changes might be during an average mass satellite's geostationary orbit, are the changes mostly predictable and periodic (mainly Moon's gravity?) - and allow the orbit to continue - and what is the range of those altitude changes (X meters, kms, etc) ?&lt;/p&gt;&#xA;" OwnerUserId="21609" LastEditorUserId="12102" LastEditDate="2017-11-28T00:09:50.610" LastActivityDate="2017-11-28T00:09:51.277" Title="Satellite altitude variances in orbit" Tags="|artificial-satellite|gravity|geostationary|station-keeping|altitude|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23673" PostTypeId="2" ParentId="23012" CreationDate="2017-11-12T18:32:12.730" Score="6" Body="&lt;p&gt;Satellites are routinely monitored by the likes of US, Russia and China. China for example is known to blind American spy satellites with lasers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The goal is to track the approx 23000 (sources vary from 10 to 25000) objects, which could be anything from a satellite, debris from launches and even lost items from EVAs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;US monitoring systems&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;radar&lt;/strong&gt;&#xA;&lt;strong&gt;USSSN - US space surveillance network&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Monitoring is accomplished in the US by the USSSN, the US space surveillance network. The majority of sensors are radars. They allow tracking of smaller objects, that cannot be tracked optically, they are distributed around the globe, they can track under any weather conditions. Facilities of the USSSN are located everywhere around the world,  Alaska, California, Spain, Norway, Greenland, UK, and various islands in the atlantic, indian and pacific ocean. The objective is cpmplete coverage of the geosynchronous belt. There are dedicated sensors as well as civilian sensors that share their data. Optical sensors operate in favorable weather at night or during sun set/rise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The organizational unit responsible is the US Strategic Command's Joint Functional Component Command for Space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;optical&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;GEODSS - Ground based electro-optical deep space surveillance&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;is a dedicated optical system and is part of the USSSN. They can detect 20cm objects in geo orbit, which is not bad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The telescopes can either track the stars, then the stars appear as dots and orbiting objects appear as streaks. This is SOP. The complementary mode is target track, which means, that the telescope tracks a space object, all other objects, such as stars, appear as streaks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Radar is just radar. Depending on the system the output is either a data stream of tracks, video or pictures.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The tracking information of all these sensors is sent in realtime to Cheyenne Mountain Air Station, Colorado, where they.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;facilities&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The list of observing facilities in the screenshot is identical to the USSSN locations. Clear is Clear AFB in Alaska. Xuanhua is a chinese phased array radar on a mountain in northern china. The RUS sensors are from the SKKP.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;US space catalogs&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;military&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All the data is fed into the US space catalog, which tracks everything from small debris left over from space walks to the space station. The Air Force and the Navy maintain separate catalogs.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;civilian&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA publishes all non-classified objects. The offical catalog is available at &lt;a href=&quot;https://www.space-track.org/&quot; rel=&quot;noreferrer&quot;&gt;https://www.space-track.org/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Classified tracks are not available, but it will come as no surprise that this information is readily available on the internet as this seems to be a rather popular hobby in astronomy circles.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;A reasonably proficient hobbyist can track satellites with an optical telescope&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tracking is not as complicated as it may sound at first. Two observations are usually enough to determine the track. Usually everything stays in the same orbit or as a decaying orbit, if it is in a low orbit. Some satellites can maneuver and are therefore harder to track, but far from impossible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Tracking every piece in orbit is very important to ensure safe access to space and safe operation in space. The ISS had to make maneuvers to avoid debris in the past.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Russia&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Radar&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Russia operates a similar system to the USSSN named SKKP. No further information is available&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;China&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Chinese satellite telemetry tracking and control network consists of the Xian satellite control center (XSCC), a number of fixed posts and three mobile units under the command of XSCC and ocean going instrumentation ships.&#xA;source &lt;a href=&quot;http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA306630&quot; rel=&quot;noreferrer&quot;&gt;NAIC&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More information can be found at &lt;a href=&quot;https://en.m.wikipedia.org/wiki/United_States_Space_Surveillance_Network&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia site for USSSN&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="12712" LastEditDate="2017-11-14T14:55:47.220" LastActivityDate="2017-11-14T14:55:47.220" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23674" PostTypeId="2" ParentId="23651" CreationDate="2017-11-12T20:48:10.123" Score="0" Body="&lt;p&gt;For a number of reasons the standard Clarke tower from earth's surface is implausible. I believe even a Kim Stanley Robinson tower from Mars' surface is implausible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Compressive towers would only cut a tiny fraction of the length from a planet's surface to synchronous orbit. They don't help much.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Elevators for Smaller Bodies&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But elevators from smaller bodies are plausible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ceres with its shallow gravity well and high angular velocity is amenable to elevators. A Ceres elevator could be built with Zylon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://hopsblog-hop.blogspot.com/2016/08/pluto-charon-elevator.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;An elevator from Pluto to Charon&lt;/a&gt; could be made from Zylon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Newtons felt by a Pluto-Charon elevator sky rocket as the elevator nears a body surface. See this chart:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rMuKK.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rMuKK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I had an interest in guesstimating how tall compressive towers could be built on these bodies. So I whomped up a &lt;a href=&quot;http://clowder.net/hop/TMI/CompressiveTowers.xslx&quot; rel=&quot;nofollow noreferrer&quot;&gt;Compressive Tower spreadsheet&lt;/a&gt;. Screen capture of the sheet with input cells containing the numbers for earth:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fYjxv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fYjxv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On compressive strength I read from &lt;a href=&quot;https://en.wikipedia.org/wiki/Compressive_strength#Comparison_of_compressive_and_tensile_strengths&quot; rel=&quot;nofollow noreferrer&quot;&gt;a Wikipedia article&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Concrete and ceramics typically have much higher compressive strengths&#xA;  than tensile strengths. Composite materials, such as glass fiber epoxy&#xA;  matrix composite, tend to have higher tensile strengths than&#xA;  compressive strengths. &lt;strong&gt;Metals tend to have tensile and compressive&#xA;  strengths that are very similar&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So I went with metal tensile strengths from &lt;a href=&quot;https://en.wikipedia.org/wiki/Ultimate_tensile_strength#Typical_tensile_strengths&quot; rel=&quot;nofollow noreferrer&quot;&gt;this chart for tensile strengths&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even the above I am skeptical of. A 28 kilometer steel tower would push in the crust of the earth, I believe. And even that implausibly high tower is a tiny fraction of the 36,000 km altitude GSO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But now let's look at a compressive tower for Pluto:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LbWaW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LbWaW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would a 533 kilometer tower push in Pluto's crust? To be honest I don't know how much weight Pluto's crust can take without being deformed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If Pluto's crust could support a 533 kilometer tower then that would take a chunk of weight off of the elevator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9GTk7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9GTk7.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It should be noted this spreadsheet assumes constant gravity. But at altitude of 533 kilometers, a kilogram would weigh less than half of what it would on Pluto's surface. So in this regard the spreadsheet errs on the side of caution.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-11-12T20:48:10.123" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23675" PostTypeId="2" ParentId="23658" CreationDate="2017-11-12T22:50:17.860" Score="7" Body="&lt;p&gt;The only number I can quote you is the requirement for the Apollo suit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which was, &quot;less than 180 standard cubic centimeters/minute at either 0.2 or 3.7 psi&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2Ik04.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Ik04.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is from the &quot;Early Apollo Spacesuit Development, A-7L Suit Requirements, and Design Details&quot; presentation, which you can find on the &lt;a href=&quot;https://nescacademy.nasa.gov/category/5/sub/27&quot; rel=&quot;noreferrer&quot;&gt;Spacesuit Knowledge Capture&lt;/a&gt; page, which is an incredible trove of knowledge for those willing to sit through it. &lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-11-12T23:01:41.090" LastActivityDate="2017-11-12T23:01:41.090" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23676" PostTypeId="2" ParentId="23658" CreationDate="2017-11-12T23:54:05.440" Score="18" Body="&lt;p&gt;Updated with current EVM suit documentation:&lt;/p&gt;&#xA;&lt;p&gt;I was finally able to get the full EMU Systems Workbook (&lt;a href=&quot;https://www.dropbox.com/s/cxbdhnwkiqdlqbd/ESOC-13%20Rev.%20V%2C%20EMU%20Hardware%20Data%20Book.pdf?dl=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;click here to download - 22 MB&lt;/a&gt;, this document is awesome and chock full of info too!)&lt;/p&gt;&#xA;&lt;p&gt;There is all the info you could want on leakage on pages 1-23 and 1-24 (PDF pages 39 and 40) I have circled the relevant number below (in an excerpt from page 1-24&lt;a href=&quot;https://i.stack.imgur.com/jzC4F.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jzC4F.png&quot; alt=&quot;EMU Systems workbook with HAND DRAWN red circle&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Edit:&lt;/p&gt;&#xA;&lt;p&gt;I have just now received an answer to my FOIA request from NASA requesting documents on the current generation space suits. One of the documents of interest was the Extravehicular Mobility Unit Systems Training Workbook (&lt;a href=&quot;https://www.dropbox.com/s/kkly76htyt19x60/EMU%20Systems%20Workbook_JSC19450B.pdf?dl=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;click here to download - 52 MB&lt;/a&gt;), it contains a ton of information on the current generation space suits.&lt;/p&gt;&#xA;&lt;p&gt;If you look on page 131, it states that the automated leak check fails if the suit is losing more than &lt;span class=&quot;math-container&quot;&gt;$0.3\ psi/min$&lt;/span&gt;, with a nominal suit pressure of &lt;span class=&quot;math-container&quot;&gt;$4.3 \pm0.1\ psid$&lt;/span&gt;. Unfortunately, the suit volume is not in this document so the mass flow cannot be determined.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The &amp;quot;Apollo Operations Handbook Extravehicular Mobility Unit&amp;quot; (&lt;a href=&quot;https://www.lpi.usra.edu/lunar/documents/NASA%20TM-X-69516.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;click here to download - 4.4 MB&lt;/a&gt;), Revision 5, Table 2-I quotes the Maximum leak rate at 180 cubic centimeters per minute.&lt;/p&gt;&#xA;&lt;p&gt;The Extravehicular Mobility Unit is the suit used for the Lunar Landings (the linked document contains a wealth of awesome information).&lt;/p&gt;&#xA;&lt;p&gt;Though the current generation of suits is also called the &lt;a href=&quot;https://en.wikipedia.org/wiki/Extravehicular_Mobility_Unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Extravehicular Mobility Unit&lt;/a&gt; and I cannot find out what the relationship between the Apollo era suits and the current generation ones.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/x5Yyj.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/x5Yyj.jpg&quot; alt=&quot;Extravehicular Mobility Unit&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Extravehicular Mobility Unit&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/91qEn.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/91qEn.png&quot; alt=&quot;Table 2-I&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Table 2-I, (scc stands for Standard Cubic Centimeters)&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="18663" LastEditorUserId="18663" LastEditDate="2022-07-22T02:13:44.190" LastActivityDate="2022-07-22T02:13:44.190" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="23678" PostTypeId="2" ParentId="14002" CreationDate="2017-11-13T05:06:36.403" Score="3" Body="&lt;p&gt;The &lt;a href=&quot;https://fas.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Federation of American Scientists&lt;/a&gt; has a page about satellite observation, and &lt;a href=&quot;https://fas.org/spp/military/program/track/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here is one page&lt;/a&gt; with a list of satellite observation programs. On it there is an image and a link:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;On occasion, an imaging satellite will fly close enough to another satellite to obtain a resolved picture of it. This technique, called &amp;quot;satellite-to-satellite&amp;quot; imaging or &amp;quot;sat-squared,&amp;quot; has been rumored to be in use for several years using reconnaissance satellites. An unclassified example is this optical image of the ERS-1 radar satellite imaged by the SPOT-4 remote sensing satellite in May of 1998. For further details, see &lt;a href=&quot;http://spot4.cnes.fr/spot4_gb/im-ers-0.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spot4.cnes.fr/spot4_gb/im-ers-0.htm&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;At that link, you can see the following:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The viewing conditions were calculated at CNES and required complete knowledge of the satellites' respective positions. SPOT 4's orbit was determined using the facilities at the &lt;a href=&quot;http://spot4.cnes.fr/spot4_gb/le%20cmp.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;operations control centre&lt;/strong&gt;&lt;/a&gt; for SPOT satellites at CNES/Toulouse, while the orbit for ERS-1 was determined by the ESA/ESOC/Darmstadt centre which controls it. The SPOT 4 telescopes, with their oblique viewing mirror, took this image above the Ténéré desert in Niger, on 6 May 1998 at 9:56 UT.&lt;/p&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;http://spot4.cnes.fr/spot4_gb/orbite.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;its orbit&lt;/strong&gt;&lt;/a&gt; at an altitude of 820 km, SPOT 4 is able to pick out details of 10 metres on the Earth's surface, which is moving at a speed of 24,000 km per hour. The ERS-1 orbit is 41 km below that of SPOT, which means that the latter can distinguish details of 50 cm in size on ERS-1. However, since ERS-1 flies much lower it flies faster than SPOT 4 and overtakes it with a relative velocity of about 250 km/h. These exceptional viewing conditions explain why the raw image (18 columns by 125 rows) is so distorted. &lt;a href=&quot;http://spot4.cnes.fr/spot4_gb/acquisit.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;Corrective processing&lt;/strong&gt;&lt;/a&gt; was applied at CNES to restore the normal appearance of ERS-1.&lt;/p&gt;&#xA;&lt;p&gt;ERS-1 is a radar observation satellite. On the right-hand side of the image may be seen the platform and the solar array, which are the same as SPOT's. On the left-hand side is the long, rectangular, synthetic aperture radar antenna of 10m x 1m, as well as two of the three scatterometer antennae used to measure wind fields. These two small antennae, inclined at 45°, are shorter and narrower (with dimensions of 3.6 m x 0.25m) than that of the radar, but stand out quite clearly in the image. The very bright areas in the image are due to the sun's rays bounced off the highly reflective outer layer which protects the satellite.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Extract from the image of the Ténéré desert (Niger) with full resolution M+XI (10m by 10m pixels)&amp;quot;:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/viwZk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/viwZk.jpg&quot; alt=&quot;ERS-1 from SPOT-4&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5qWrG.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5qWrG.png&quot; alt=&quot;ERS-1 from SPOT-4&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/2iG03.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2iG03.png&quot; alt=&quot;ERS-1 from SPOT-4&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-13T05:06:36.403" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23680" PostTypeId="2" ParentId="23641" CreationDate="2017-11-13T14:05:16.153" Score="3" Body="&lt;p&gt;KSC has a &lt;a href=&quot;https://www.fastcompany.com/3048337/making-rocket-launches-a-breeze-nasa-unveils-a-new-wind-measurement-system&quot; rel=&quot;nofollow noreferrer&quot;&gt;640 antenna system&lt;/a&gt; to track wind shear at the launch site up to about 60000 feet. It is not used for tracking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The air force at Cape Canaveral air force station houses the 45th space wing, which operates the TEL-4 tracking station. It seems to be vital for launches and there are no launches when it doesn't work. Unfortunately, no more information available on this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a TDRSS tracking and data satellite system, which is a network of satellites to track rockets and satellites and it is the current system for tracking and launch support.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, since you wrote you &lt;em&gt;heard&lt;/em&gt; this, sometimes many antennas are referred to as an &quot;array of antennas&quot;, but this does not mean, they are operating together or are phased.&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="12712" LastEditDate="2017-11-21T14:49:12.793" LastActivityDate="2017-11-21T14:49:12.793" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23681" PostTypeId="2" ParentId="21306" CreationDate="2017-11-13T15:21:35.347" Score="2" Body="&lt;p&gt;It isn't as important as you would think, unless you are doing military type stuff. Here's a list of things that you would need to know to uplink to the satellite:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The frequency (Fairly easy)&lt;/li&gt;&#xA;&lt;li&gt;Polarization (Fairly easy)&lt;/li&gt;&#xA;&lt;li&gt;Data format (Difficult, pretty much requires inside information)&lt;/li&gt;&#xA;&lt;li&gt;Timing information (Moderately difficult)&lt;/li&gt;&#xA;&lt;li&gt;A large enough antenna.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;And even if you did manage to do all of this, you would then have to access the satellite at a time when it wasn't otherwise in communication with Earth, or else the odds that you were successful would be quite low.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next comes the question of receiving a signal and decoding it. That is a fair bit easier to do. You need most of the same information, but you don't have to broadcast any signals, and you can get away with a smaller antenna. In fact, there have been some NASA missions which amateurs have received and decoded some of the signals. A few instances of spacecraft which have had their signals decoded are &lt;a href=&quot;https://www.theregister.co.uk/2011/11/28/radio_hams_curiosity_signals/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity&lt;/a&gt;, &lt;a href=&quot;https://amsat-uk.org/2014/01/23/radio-amateurs-receive-rosetta-signals/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rosetta&lt;/a&gt;, and a host of others seen at &lt;a href=&quot;http://www.amateurradio.com/the-pursuit-of-the-elser-mathes-cup/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it would be rather difficult to make this work overall. No doubt a large power (IE, government) could make it happen if required, but they would only consider doing this if the mission was of particular importance to them. NASA usually doesn't encrypt it's signals, as it is more hassle then it is worth. If one wanted to hack, say, &lt;a href=&quot;https://www.extremetech.com/extreme/134334-could-you-hack-into-mars-rover-curiosity&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity&lt;/a&gt;, it could be done, if one is devoted enough to do so. But it would be quite expensive to do so.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-11-13T15:21:35.347" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23683" PostTypeId="1" CreationDate="2017-11-13T15:49:58.857" Score="4" ViewCount="726" Body="&lt;p&gt;Knowing that contracts come and go over time, where can I find a list of current companies who have contracts with NASA to either do research or build stuff for them?  My goal is to get an overall feel for who is doing what in the space industry, and I think that looking to the contracts that NASA gives out is a good way to find one piece of the puzzle.  I'm not only looking for a list of specific contractors, but also a description of what each contractor is doing for NASA.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm hoping that, if such a list exists, that it is a dynamic list that gets updated over time as contracts are completed, rebidded, and won.  Any help obtaining such information  would be greatly appreciated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If such information is limited to NASA personnel, who can I contact at NASA for such information?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="12102" LastEditDate="2017-11-14T06:16:16.010" LastActivityDate="2019-03-15T16:42:48.543" Title="Is there a comprehensive list of NASA contractors with a list of projects that they are assigned to do?" Tags="|nasa|space-industry|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23684" PostTypeId="2" ParentId="23683" CreationDate="2017-11-13T15:59:33.587" Score="4" Body="&lt;p&gt;You can see all the external spending by Agency for the government at &lt;a href=&quot;https://www.usaspending.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;usaspending.gov&lt;/a&gt;, Just do an advanced search limiting the awarding agency to NASA.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: Search links go stale after a while, &lt;a href=&quot;https://archive.org/details/all_contracts_prime_transactions_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; is an archive of all Awards made by NASA in FY2018.&lt;/p&gt;&#xA;" OwnerUserId="18663" LastEditorUserId="18663" LastEditDate="2019-03-15T16:42:48.543" LastActivityDate="2019-03-15T16:42:48.543" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23685" PostTypeId="2" ParentId="23607" CreationDate="2017-11-13T17:11:03.500" Score="11" Body="&lt;p&gt;Since there have been no answers in over a week, I'll venture a &lt;em&gt;partial&lt;/em&gt; answer.&lt;/p&gt;&#xA;&lt;p&gt;There are of course several challenges. Among them in no particular order:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Crew selection and training&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Putting together, evaluating, and training the crew would be the first challenge.&#xA;They need to train together on &lt;em&gt;accurate replicas of the systems to be used in the future&lt;/em&gt;.&#xA;Just as a comparison: astronauts for the ISS train for almost two years. The apollo astronauts trained for at least five years.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Landing site&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Landing site selection is a difficult process. See for example a summary of the &lt;a href=&quot;https://marsnext.jpl.nasa.gov/documents/3rd%202020%20Workshop%20Outcome%20Letter_for_Michael_Meyer_2-16-17.pdf&quot; rel=&quot;noreferrer&quot;&gt;third Mars 2020 Landing Site Workshop held in Monrovia, CA, from February 8-10, 2017&lt;/a&gt; for an example from NASA. And thats just for rovers, not a site that allows for the relaunch of a large space craft.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Space suits reliable for EVAs and Mars-walks&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;What kind of space suit are they going to use? The one they are&#xA;building now is the &amp;quot;indoor&amp;quot; space suit for use inside the space craft.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Dust, toxic perchlorates&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;On the moon missions, regolith caused a few problems. It stuck to the suits, entered the living space, and it caused a lot of wear and tear. See Air and Space's article &lt;a href=&quot;https://www.airspacemag.com/space/stronger-than-dirt-8944228/&quot; rel=&quot;noreferrer&quot;&gt;Stronger Than Dirt; Lunar explorers will have to battle an insidious enemy—dust.&lt;/a&gt; for more.&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_soil&quot; rel=&quot;noreferrer&quot;&gt;Martian regolith&lt;/a&gt; is also known to contain a substantial amount of &lt;a href=&quot;https://en.wikipedia.org/wiki/Perchlorate&quot; rel=&quot;noreferrer&quot;&gt;perchlorate&lt;/a&gt;, a nasty, toxic combination of chlorine and oxygen. The Space.com article &lt;a href=&quot;https://www.space.com/21554-mars-toxic-perchlorate-chemicals.html&quot; rel=&quot;noreferrer&quot;&gt;Toxic Mars: Astronauts Must Deal with Perchlorate on the Red Planet&lt;/a&gt; covers this nicely, with a good overview of the challenges.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Devilishly dangerous&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The high levels of perchlorate found on Mars would be toxic to humans, Smith said.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Anybody who is saying they want to go live on the surface of Mars better think about the interaction of perchlorate with the human body,&amp;quot; he warned. &amp;quot;At one-half percent, that's a huge amount. Very small amounts are considered toxic. So you'd better have a plan to deal with the poisons on the surface.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The value of one-half percent is mentioned in the abstract of the 2013 paper &lt;a href=&quot;https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/perchlorate-on-mars-a-chemical-hazard-and-a-resource-for-humans/E4906FAD7F45A9AE8212B9198C6FD4AB&quot; rel=&quot;noreferrer&quot;&gt;Perchlorate on Mars: a chemical hazard and a resource for humans&lt;/a&gt;, while a lower but significant value of 0.6 g/kg is found in the abstract of the 2017 paper &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/2016GL072199/full&quot; rel=&quot;noreferrer&quot;&gt;The nitrate/(per)chlorate relationship on Mars&lt;/a&gt; at a different location.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1sA2j.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1sA2j.png&quot; alt=&quot;perchlorate molecule&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/GBz9R.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GBz9R.png&quot; alt=&quot;perchlorate molecule&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Proposed solutions include leaving EVA suits docked on the &lt;em&gt;outside of the spacecraft/habitat&lt;/em&gt;, though it's not clear how practical this could be, another proposed there is a water wash down in the airlock.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;strong&gt;Refueling station&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;While in theory, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Sabatier_reaction&quot; rel=&quot;noreferrer&quot;&gt;Sabatier process&lt;/a&gt; will work, they have to research and develop it at first for a practical and reliable implementation on mars, because nobody currently knows how to really do that.&#xA;An often overlooked aspect in this regard is, that you are going to need mining equipment to bring the reactants into a proper state for processing.&#xA;NASA does have a research project on that. See for example the SBIR summary &lt;a href=&quot;https://sbir.nasa.gov/content/situ-resource-utilization-mars-atmospheregas-chemical-processing&quot; rel=&quot;noreferrer&quot;&gt;In-Situ Resource Utilization - Mars Atmosphere/Gas Chemical Processing&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;State of the art (SOA) technologies for these ISRU processes either do not exist or are too complex, heavy, inefficient, or consume too much power.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;https://mepag.jpl.nasa.gov/index.cfm&quot; rel=&quot;noreferrer&quot;&gt;Mars Exploration Program Analysis Group&lt;/a&gt; seems to be a good source for further reading. There should be a risk analysis for a mars mission there, but I was unable to locate it.&lt;/p&gt;&#xA;&lt;p&gt;As an aside; Buzz Aldrin thinks that the SpaceX plan is &lt;a href=&quot;https://www.wfyi.org/news/articles/purdue-students-to-buzz-aldrin-elon-musks-200k-mars-plan-unfeasible&quot; rel=&quot;noreferrer&quot;&gt;unfeasible&lt;/a&gt;. More on this can be found in Purdue University's course introduction page &lt;a href=&quot;https://engineering.purdue.edu/AAECourses/aae450/2015/spring&quot; rel=&quot;noreferrer&quot;&gt;Project Aldrin-Purdue, Mission to Mars&lt;/a&gt;. The page also links to the nicely produced video &lt;a href=&quot;https://www.youtube.com/watch?v=CX93F0qqAUQ&quot; rel=&quot;noreferrer&quot;&gt;Project Aldrin-Purdue, Mission to Mars&lt;/a&gt; which includes Aldrin's discussion.&lt;/p&gt;&#xA;&lt;p&gt;I hope that I was able to at least provide a starting point and encourage more people to answer. This is a very interesting and developing topic and there should be a place to collect current information.&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-14T06:19:02.897" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23686" PostTypeId="1" CreationDate="2017-11-13T18:41:04.863" Score="12" ViewCount="6260" Body="&lt;p&gt;So I am in fourth year and doing the &lt;a href=&quot;http://btyoungscientist.com/&quot; rel=&quot;noreferrer&quot;&gt;BT Young Scientist&lt;/a&gt; (Ireland’s version for a science fair but bigger and there is a cash prize) and came up with the idea to launch a space shuttle from a rail gun, and was wondering if a 45 degree angle would be the most effective way to orbit around the earth. &lt;/p&gt;&#xA;" OwnerUserId="21618" LastEditorUserId="6816" LastEditDate="2017-11-14T15:42:37.663" LastActivityDate="2017-11-15T21:12:21.287" Title="Would launching a shuttle from a 45 degree angle be the most fuel efficient for a rail gun?" Tags="|launch|rail-gun|" AnswerCount="6" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23687" PostTypeId="1" CreationDate="2017-11-13T18:47:27.577" Score="1" ViewCount="163" Body="&lt;p&gt;There is a fundamental limit to the specific impulse of a chemical rocket because the propellant is also the fuel so you are limited by the fuels energy density. What if you burnt some fuel in a closed cycle, and then through the use of heat exchangers heated up the rocket exhaust? The exhaust temperature would be higher, so the exhaust velocity would be higher and you would get a higher specific impulse. You would then need to crack the spent fuel from the closed cycle back into fuel and oxidizer before the next thrust maneuver. This could be done with solar or RTG. Though this would be less efficient then hooking the power source directly to an ion drive, it seems like the bursts of thrust would allow more efficient orbital maneuvers. Has this idea ever been studied, or are the thermodynamic losses to to much to have any feasibility?&lt;/p&gt;&#xA;" OwnerUserId="21390" LastEditorUserId="21390" LastEditDate="2017-11-13T22:12:09.143" LastActivityDate="2017-11-13T22:12:09.143" Title="Could a high specific impulse chemical rocket be built by using some of the fuel in a closed cycle to heat the exhaust?" Tags="|engine-design|" AnswerCount="0" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="23688" PostTypeId="2" ParentId="23686" CreationDate="2017-11-13T20:18:25.210" Score="28" Body="&lt;p&gt;Two issues:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Any orbit achieved will (discounting perturbations) pass through the point at which it was last altered by some external force.  In the case of your rail gun, if it's on the ground, any possible non-escape trajectory it will take will also intersect the ground.  For a sustaining orbit, you need a second impulse near apogee to raise the perigee above the atmosphere.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Any launch based on rail guns is going to result in acceleration magnitudes so great (thousands of Gs) that anything resembling a recognizable spacecraft will quite simply be crushed.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="512" LastActivityDate="2017-11-13T20:18:25.210" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23689" PostTypeId="2" ParentId="23686" CreationDate="2017-11-13T20:39:31.480" Score="18" Body="&lt;p&gt;To keep the acceleration manageable, you need a very long rail gun: say 8000 m/s of speed needed, 40 m/s&lt;sup&gt;2&lt;/sup&gt; acceleration (4G), that's 8000/40= 200 seconds of 'flight' on the rail. That's 800 km. That's impossible to build at an angle of 45º (highest we can build is about 10 km) so that will dictate a much shallower angle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This ignores the heat load in the atmosphere, as Uwe said, it's almost impossible to accelerate a spacecraft to orbital speed at sea level without it burning up.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also ignores the drag from the atmosphere: as soon as it leaves the rail, the spacecraft will start to slow down. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-11-14T08:58:56.853" LastActivityDate="2017-11-14T08:58:56.853" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23690" PostTypeId="2" ParentId="17176" CreationDate="2017-11-14T02:59:13.710" Score="9" Body="&lt;p&gt;You can calculate the hover power required out of ground effect by using the following formulas:&lt;/p&gt;&#xA;&lt;p&gt;Given &lt;span class=&quot;math-container&quot;&gt;$m$&lt;/span&gt;, the mass of the helicopter, the required lift force is  &lt;span class=&quot;math-container&quot;&gt;$L = g_{Mars}\cdot m$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;The required shaft power is:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$P = \frac{\frac{L^{\frac{3}{2}}}{R} \cdot \sqrt{\frac {2} {\pi \cdot \rho} }}{FM}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;where &lt;span class=&quot;math-container&quot;&gt;$R$&lt;/span&gt; is rotor diameter and FM is the &amp;quot;figure of merit&amp;quot;. For a small helicopter the FM is smaller than 0.66, say 0.55.&lt;/p&gt;&#xA;&lt;p&gt;The density at low altitude on Mars is  0.0152 kg/m&lt;sup&gt;3&lt;/sup&gt;&lt;/p&gt;&#xA;&lt;p&gt;The gravitational acceleration &lt;span class=&quot;math-container&quot;&gt;$g_{Mars}$&lt;/span&gt; is 3.8 m/s&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Example:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;mass = 2 kg&lt;/li&gt;&#xA;&lt;li&gt;lift = 7.6 newton&lt;/li&gt;&#xA;&lt;li&gt;FM = 0.55&lt;/li&gt;&#xA;&lt;li&gt;rotor diameter = 1 meter&lt;/li&gt;&#xA;&lt;li&gt;density = 0.0152 kg/m&lt;sup&gt;3&lt;/sup&gt;&lt;/li&gt;&#xA;&lt;li&gt;pi = 3.1416&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The result is 264 watts.&lt;/p&gt;&#xA;&lt;p&gt;The fact that the counter rotating rotors are coaxial does not significantly reduce the hover power. The effective diameter is nearly the same.&#xA;Note that, when the mass is reduced to 1 kg the required power would be only 87 watts!&lt;/p&gt;&#xA;&lt;p&gt;In summary: power required is proportional to (lift to the &lt;span class=&quot;math-container&quot;&gt;$\frac{3}{2}$&lt;/span&gt; power exponent), inversely proportional to rotor diameter and inversely proportional to the square root of the density.&lt;/p&gt;&#xA;&lt;p&gt;When flying forward instead of hovering the required power goes down significantly for a well shaped streamlined fuselage.&#xA;In  1929 Glauert came up with an approximate formula (the solution of a quartic) which is still used today. A good reference text book is: B. W. McCormick: &lt;em&gt;Aerodynamics of V/STOL Flight&lt;/em&gt;.&lt;/p&gt;&#xA;" OwnerUserId="21624" LastEditorUserId="30164" LastEditDate="2022-05-22T23:46:31.273" LastActivityDate="2022-05-22T23:46:31.273" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23691" PostTypeId="1" AcceptedAnswerId="23701" CreationDate="2017-11-14T03:10:19.980" Score="3" ViewCount="416" Body="&lt;p&gt;I saw this project linked within this &lt;a href=&quot;https://space.stackexchange.com/a/23685/12102&quot;&gt;excellent answer&lt;/a&gt;. Why is this proposed balloon (assisting in deceleration of the proposed Martian atmosphere entry) donut-shaped? Are there aerodynamic, stability and/or weight advantages? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, can it be articulated for steering by adjusting lengths of the lines?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the word ballute is used in the video (balloon parachute) but that seems to be something different. See for example NASA Tech Memo TM X-1773 &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690017080.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flight-Test Results from Supersonic Deployment of an 18-foot-diameter (5.49-meter) Towed Ballute Decelerator&lt;/a&gt; (figure shown at the end).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the Purdue University website page for the project at &lt;a href=&quot;https://engineering.purdue.edu/AAECourses/aae450/2015/spring&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://engineering.purdue.edu/AAECourses/aae450/2015/spring&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/D5Sf7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/D5Sf7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/CX93F0qqAUQ?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OQ4vf.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OQ4vf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-14T04:49:47.863" LastActivityDate="2017-11-14T16:53:33.883" Title="Why a donut-shaped balloon decelerator in the Aldrin-Purdue proposed Mars lander descent?" Tags="|mars|reentry|lander|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23692" PostTypeId="2" ParentId="23686" CreationDate="2017-11-14T03:46:18.320" Score="10" Body="&lt;h2&gt;45 degrees is probably not ideal.&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;...but it depends on your shuttle design. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To get to orbit, you need to go &lt;em&gt;fast&lt;/em&gt;, not high. It's hard to go fast at low altitudes though, because there is a lot of air in the way. If you go higher up, it's easier to go fast because there is less air. But even after you get out of the air, you still have to get going fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Air resistance is a force (i.e. acceleration) which scales opposite velocity. So, the faster you go, the more the wind pushes you backwards. That resistance also scales with how dense the air is, because denser air means more air molecules to get in the way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we assume that your shuttle has only a &quot;go&quot; button on it's propulsion, and then will provide a pre-set thrust, you maximize the speed you get out of that thrust by minimizing the amount of force the air applies to your shuttle. For most rockets, that looks like launching (almost) straight up, then slowly curving further and further toward the orbit they want. For anything basically like a rocket, this is probably the optimal method.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But your shuttle may have other constraints. Maybe you're launching it super fast, but have a way to minimize drag enough that it will fly out of the atmosphere using the energy from the launch, then rocket it's way to orbit. Maybe 45 degrees is your maximum elevation, because after that you run out of mountain. Maybe you can launch the shuttle really, &lt;em&gt;really&lt;/em&gt; fast, and are launching it only into elliptical orbits(like not-quite-Molniya orbits). There may be any number of reasons that 45 degrees is good for your application. In general though, I would not expect 45 degrees to be optimal.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The best primer I know of on everything space is the US Air Force's primer, which is available free: &lt;a href=&quot;http://space.au.af.mil/au-18-2009/index.htm&quot; rel=&quot;noreferrer&quot;&gt;http://space.au.af.mil/au-18-2009/index.htm&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to understand orbits in detail, including getting to and from orbit, Sellers' &lt;em&gt;Understanding Space&lt;/em&gt; is good. &lt;/p&gt;&#xA;" OwnerUserId="19757" LastActivityDate="2017-11-14T03:46:18.320" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23693" PostTypeId="1" AcceptedAnswerId="33407" CreationDate="2017-11-14T04:06:02.870" Score="7" ViewCount="330" Body="&lt;p&gt;THE STF-1 cubesat project, in cooperation with NASA, has developed a whole suite of simulation software for small satellites.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Info here: &lt;a href=&quot;http://stf1.com/simulation.php&quot; rel=&quot;noreferrer&quot;&gt;http://stf1.com/simulation.php&lt;/a&gt;&#xA;And here: &lt;a href=&quot;https://www.nasa.gov/centers/ivv/jstar/nos3.html&quot; rel=&quot;noreferrer&quot;&gt;https://www.nasa.gov/centers/ivv/jstar/nos3.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does anyone have any info on the software stack?  The NOS3 site says it'll eventually be available as open source - does anyone know when?  And what about the rest of the components?  Is any of it available to the general public?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE:  I've found most of the elements (see below), but there are two &quot;missing&quot;:  NOS3 itself, and OOPP.  OIPP sounds like something they've developed for that project, not sure if it's inteded to be public.  I'd love to see NOS3 though.  Any info on that would be welcome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The rest of the &quot;stack&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;cFS(core Flight System) community home page is here: &lt;a href=&quot;http://coreflightsystem.org&quot; rel=&quot;noreferrer&quot;&gt;http://coreflightsystem.org&lt;/a&gt; and the source code is available here: &lt;a href=&quot;https://github.com/nasa/cFE&quot; rel=&quot;noreferrer&quot;&gt;https://github.com/nasa/cFE&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;42 is found here: &lt;a href=&quot;https://sourceforge.net/projects/fortytwospacecraftsimulation/&quot; rel=&quot;noreferrer&quot;&gt;https://sourceforge.net/projects/fortytwospacecraftsimulation/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;COSMOS is here: &lt;a href=&quot;http://cosmosrb.com/&quot; rel=&quot;noreferrer&quot;&gt;http://cosmosrb.com/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Vagrant is here: &lt;a href=&quot;https://www.vagrantup.com/&quot; rel=&quot;noreferrer&quot;&gt;https://www.vagrantup.com/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21469" LastEditorUserId="21469" LastEditDate="2017-11-14T14:26:24.817" LastActivityDate="2019-01-07T20:43:34.173" Title="Info on NASA's NOS3 software and the rest of the suite used by STF-1" Tags="|cubesat|software|simulation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23694" PostTypeId="1" AcceptedAnswerId="30255" CreationDate="2017-11-14T06:30:41.863" Score="9" ViewCount="628" Body="&lt;p&gt;While oxygen recovery and reuse is well developed, the ISS does receive regular shipments of make-up oxygen. See for example &lt;a href=&quot;https://space.stackexchange.com/a/19300/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/17677/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/19303/12102&quot;&gt;this&lt;/a&gt; answer.&lt;/p&gt;&#xA;&lt;p&gt;For an early, manned presence on Mars, a good source of make-up oxygen available early-on would be desirable. Losses from incomplete recovery, leakage, EVA suit venting(?), accidents, catastrophes, and others that I can't think of may not always be covered by oxygen supplies brought from Earth.&lt;/p&gt;&#xA;&lt;p&gt;The three local sources of oxygen on Mars that I can think of are&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;atmospheric CO&lt;SUB&gt;2&lt;/SUB&gt;;&lt;/li&gt;&#xA;&lt;li&gt;sub-surface H&lt;SUB&gt;2&lt;/SUB&gt;O (eventually a source for fuel for return to Earth);&lt;/li&gt;&#xA;&lt;li&gt;ClO&lt;SUB&gt;4&lt;/SUB&gt;&lt;SUP&gt;-&lt;/SUP&gt; (perchlorate) in Martian regolith, order of magnitude of 1 part-per-thousand (see &lt;a href=&quot;https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/perchlorate-on-mars-a-chemical-hazard-and-a-resource-for-humans/E4906FAD7F45A9AE8212B9198C6FD4AB&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/2016GL072199/full&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Of these, which one is most likely to be used first as a source of breathable oxygen for make-up and emergency needs?&lt;/p&gt;&#xA;&lt;p&gt;Answer should evaluate required electrical or thermal power, mass and reliability of equipment hauled from Earth, and challenges collecting the raw materials where these turn out to be limiting parameters.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="47149" LastEditDate="2022-05-07T17:43:15.660" LastActivityDate="2022-05-07T17:43:15.660" Title="Sources for make-up breathable oxygen on Mars; of H₂O, CO₂ and ClO₄⁻, which is likely to be used first?" Tags="|mars|crewed-spaceflight|" AnswerCount="3" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="23695" PostTypeId="1" AcceptedAnswerId="23699" CreationDate="2017-11-14T09:46:51.340" Score="2" ViewCount="473" Body="&lt;p&gt;The BBC News item &lt;a href=&quot;http://www.bbc.com/news/science-environment-41945654&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skylark: The unsung hero of British space&lt;/a&gt; remembers the 441st and final launch of the British rocket &lt;a href=&quot;https://en.wikipedia.org/wiki/Skylark_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skylark&lt;/a&gt; in May 2005, to commemorate the 60th anniversary of its first launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is some interesting historical information and a second video at the bottom that are worth checking out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This one was launched from Sweden, not the UK, and the Wikipedia article linked above suggests it has been launched from sites around the world.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Launches had been carried out from sites in Europe, Australia, and South America, with use far beyond the UK by NASA, the European Space Research Organisation (ESRO), and German and Swedish space organizations&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Did the Skylark &lt;em&gt;ever launch from the UK?&lt;/em&gt; If so, roughly what fraction of the launches?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Image from &lt;a href=&quot;http://www.bbc.com/news/science-environment-41945654&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0IpzY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0IpzY.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorDisplayName="user20636" LastEditDate="2017-11-14T16:52:24.563" LastActivityDate="2017-11-14T16:52:24.563" Title="Did the British Skylark ever launch from the UK?" Tags="|history|sounding-rocket|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23696" PostTypeId="1" AcceptedAnswerId="23698" CreationDate="2017-11-14T10:17:57.103" Score="13" ViewCount="16392" Body="&lt;p&gt;First I was going to ask how many rocket bodies lay at the bottom of the ocean, but I simplified further.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember, this includes sounding rockets; anything passing the Kármán line.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for a good guesstimate. I'm sure it's more than a thousand and less than a million, and guessing that it's probably about half-way between on a log scale (~31,600).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Food for thought; Gunter's Space Page has a map of launch sites worldwide &lt;a href=&quot;http://space.skyrocket.de/directories/launchsites.htm&quot; rel=&quot;noreferrer&quot;&gt;http://space.skyrocket.de/directories/launchsites.htm&lt;/a&gt; Here's a screenshot at maximum zoom-out:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/y0IF8.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/y0IF8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-15T04:17:48.323" LastActivityDate="2021-04-10T10:49:09.413" Title="How many rockets have been launched into space? (roughly, of course)" Tags="|launch|history|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23697" PostTypeId="2" ParentId="23686" CreationDate="2017-11-14T12:20:17.637" Score="7" Body="&lt;p&gt;Ignoring the atmosphere, let's say you're launching from the moon, the best angle to launch at would be 0 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This way you can accelerate to orbital speed at sea-level plus however much you need to get straight into basically a &lt;a href=&quot;https://en.wikipedia.org/wiki/Hohmann_transfer_orbit&quot; rel=&quot;noreferrer&quot;&gt;Hohmann transfer orbit&lt;/a&gt; using only the rail gun. After this you only need a small amount of fuel from the shuttle itself to circularize the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launching at a higher angle will make you reach the height of your desired orbit faster but at a lower velocity. Meaning you get less energy out of the rail gun and would need more from the shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As others have mentioned, once an atmosphere comes into play the whole idea of using a rail gun becomes very difficult.&lt;/p&gt;&#xA;" OwnerUserId="21627" LastActivityDate="2017-11-14T12:20:17.637" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23698" PostTypeId="2" ParentId="23696" CreationDate="2017-11-14T13:13:59.473" Score="15" Body="&lt;p&gt;About 35,000-40,000, including sounding rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ordinarily I'd provide a hard source, but since you specifically asked for a guesstimate I'll just cite myself: My answer &lt;a href=&quot;https://space.stackexchange.com/a/13402/195&quot;&gt;here&lt;/a&gt; claims ~5,000 &lt;em&gt;orbital&lt;/em&gt; launches; unfortunately I failed to cite my source there and can't recall where I got that data -- I don't &lt;em&gt;think&lt;/em&gt; I added up all the entries linked from Wikipedia's &lt;a href=&quot;https://en.wikipedia.org/wiki/Timeline_of_spaceflight&quot; rel=&quot;noreferrer&quot;&gt;Timeline of Spaceflight&lt;/a&gt; article, but it's possible that I did. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Jonathan McDowell's list of &lt;a href=&quot;http://www.planet4589.org/space/lvdb/index.html&quot; rel=&quot;noreferrer&quot;&gt;suborbital space launches&lt;/a&gt;, updated 9/2017, includes this summary:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This database forms the background data for a comprehensive launch list of 70536 launches. The launches include 5704 orbital launch attempts, 28466 suborbital launches, and 36366 endoatmospheric flights&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So this would be between 28466 and 34170 depending on how many of the orbital launch attempts reached space. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-15T15:41:55.480" LastActivityDate="2017-11-15T15:41:55.480" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="23699" PostTypeId="2" ParentId="23695" CreationDate="2017-11-14T16:01:51.523" Score="4" Body="&lt;p&gt;It appears that &lt;a href=&quot;http://www.reocities.com/CapeCanaveral/launchpad/6133/launchsites.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;two Skylarks were launched from the UK&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Aberporth Rocket Range is located near Aberystwyth, on the south coast of Wales. Originally, it was intended to be used for the testing of large rockets, and for Skylark sounding rocket launches. However, the large rocket programme was moved to Australia for safety reasons, as was Skylark, as the remote Woomera base provided extra security. &lt;strong&gt;Two test launches of Skylark did, however, take place from Aberporth in 1971.&lt;/strong&gt; The Range is currently used by the Defence Evaluation and Research Agency (DERA).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.rafaberporth.org.uk/page6.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Another site suggests&lt;/a&gt; that they were originally planned to launch from Aberporth in 1957, before being sent off to Woomera:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;During 1957 attempts were made to fire Skylark Meteorological Research rockets from Aberporth. Ballistic wind measurements had to be taken shortly before the rocket could be fired, which was a very complicated procedure affected by up-draughts from cliffs etc. It took a long time and wind conditions were liable to change throughout the period of measurement up to the point of firing. Winds had to be measured at 100 ft intervals up to 1000 ft and a vast programme took place with the co-operation of the Meteorological Office at Aberporth. Special measuring instruments were made locally, but the measuring procedure could not be perfected at that time so as to take account of all Range Safety requirements. Firing of Skylark from Aberporth had therefore to be finally abandoned although these missiles were successfully fired from other ranges outside the RAE organization, with less stringent safety requirements due to their location.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-11-14T16:01:51.523" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23701" PostTypeId="2" ParentId="23691" CreationDate="2017-11-14T16:41:09.193" Score="3" Body="&lt;p&gt;Yes, that is one of many ballute configurations. That configuration seems rather unsuited to a Mars entry though, due to heating in the supersonic regime where it would potentially be useful, That results in a heavy ballute with not a whole lot of drag in return. The artist's conception looks particularly inefficient, where the torus has a very small minor radius. But that might just be the artist. It would need to be a lot further back to not lose potential drag due to being blocked by the large heat shield. Maybe another artist thing. Also it is not clear how that thing gets inflated, or how the inflation is maintained.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An attached torus works better, as does the lower ballute configuration you show, both of which were successfully tested by LDSD. I've usually seen the trailing torus configuration discussed with &lt;a href=&quot;https://solarsystem.nasa.gov/docs/54_rohrschneider.ppt.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;reference&lt;/a&gt; to &lt;a href=&quot;https://solarsystem.nasa.gov/docs/SummaryofUltralightweightBallute_Masciarelli.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerocapture&lt;/a&gt;, where you just need something big and light for a very high, very low-density drag pass without any significant heating, and that you can cut loose easily.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No, I have not seen any discussion of steering with the lines. You'd be worried enough about the torus buckling that you don't need to help it buckle by pulling on one of the lines. Drag control for such a device being used for aerocapture is effected very simply: by cutting the whole thing loose at the right time.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2017-11-14T16:53:33.883" LastActivityDate="2017-11-14T16:53:33.883" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23703" PostTypeId="1" AcceptedAnswerId="23714" CreationDate="2017-11-14T18:10:13.587" Score="9" ViewCount="1858" Body="&lt;p&gt;Due to the miracle of biology a bamboo plant can transform water, atmosphere, sunlight, and nutrients into a long and stable rod or tube at a incredible growth speeds (some species can grow 90cm/35 inches a day). This could be useful in a variety of practical ways in space such as:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Cheap and disposable scaffolding &lt;/li&gt;&#xA;&lt;li&gt;Temporary structural support&lt;/li&gt;&#xA;&lt;li&gt;Excess can be used as biofuel or burned for energy&lt;/li&gt;&#xA;&lt;li&gt;Requires no metal smelting or ore refinery facilities&lt;/li&gt;&#xA;&lt;li&gt;Can help clean atmosphere because it's a plant&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Specific things you could construct with bamboo (example mars or moon):&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Temporary frames to keep large amounts of solar panels off the surface and angled properly&lt;/li&gt;&#xA;&lt;li&gt;Scaffolding to help human (or robot) workers work on hard to reach places&lt;/li&gt;&#xA;&lt;li&gt;Poles to be used as &quot;telephone poles&quot; keeping cables and pipelines off the ground over long distances&lt;/li&gt;&#xA;&lt;li&gt;Poles to be used as antenna masts for temporary installations&lt;/li&gt;&#xA;&lt;li&gt;Bridges to traverse crevices or natural surface dangers&lt;/li&gt;&#xA;&lt;li&gt;Road building across sandy areas where wheels can get stuck&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Questions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Are alternatives such as aluminum or other metals better because of these versatility?&lt;/li&gt;&#xA;&lt;li&gt;Does bamboo as a temporary building material make sense or is it more economical to just use metal?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Examples of the use of Bamboo in real world construction in Earth gravity can be viewed in &lt;a href=&quot;http://edition.cnn.com/style/gallery/hong-kong-bamboo/index.html&quot; rel=&quot;noreferrer&quot;&gt;this slide show&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4plh6.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4plh6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/S23uT.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/S23uT.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NM4Sl.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NM4Sl.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21538" LastEditorUserId="21538" LastEditDate="2017-11-15T11:34:27.137" LastActivityDate="2024-03-18T07:35:49.653" Title="Using bamboo as space construction material" Tags="|construction|plants|" AnswerCount="2" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="23704" PostTypeId="2" ParentId="23686" CreationDate="2017-11-14T18:49:34.413" Score="1" Body="&lt;p&gt;As others have mentioned, orbits pass through the last point where a force acted upon the body, so a 45 degree angle shot will (attempt) to pass back through the ground at the point of launch at the same angle. Obviously that's not going to work because there's a lot of Earth in between.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now what is the best angle? Again, because orbits pass back through the last point of acceleration (yeah I know that there's always acceleration, you know what I mean), you're going to have to accelerate again once you're in space, or your shuttle will re-enter the atmosphere and not achieve orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So your aim should be to make it to space and then fire engines to accelerate to orbital velocity. The ideal scenario here is that you shoot your rail gun at 0 degrees (tangential to the surface) and aim for your highest point to be exactly on the opposite side of the earth, at the height you want to orbit. The problem with that is that there happens to be a lot of air in the way, so you're going to be streaking through the sky like a meteor for hundreds of miles. Not very efficient, and very likely to destroy your shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's try and minimize the amount of in-atmosphere flight time. The best way to do that is to go straight up. So let's try shooting the rail gun at a 90 degree angle. You're going to fly way up high into space until you reach the highest point, where you'll burn your engines until you reach orbital velocity. The problem with this scenario is that, at the top of your flight, you'll have very little horizontal velocity (you'll have gotten some from the earth's rotation, but because you're much higher than the earth's surface, and if you don't fire your engines, the surface will pass you by and you'll land &lt;em&gt;behind&lt;/em&gt; where you lifted off from). The upshot (get it?) here is that you have to burn more fuel to achieve orbital speed than if you had fired at the 0 degree orbit to make up for the lack of horizontal speed (that's assuming there's no atmosphere; it's likely not possible to launch at 0 degrees with an atmosphere). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you're going to need to do something in between. Finding that in-between is very hard; you'll need to balance how much drag from the atmosphere you are willing to take against how much fuel you need to achieve orbital velocity. The lower the angle, the less fuel you'll need, but the faster you'll have to launch your shuttle from the rail-gun, and the more atmosphere you'll have to pass through to get to space. Keep in mind that the faster you launch the shuttle the more likely it is to burn up in the atmosphere, and that the more fuel you need to get to orbital velocity, the larger your shuttle will need to be, and therefore the more drag will act on it in the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally all of this is assuming you have a rail-gun that can fire your shuttle to begin with. The amount of energy it takes to take a shuttle from a standstill to orbital speed is astronomical (sorry for the puns). Expending all that energy in such a short time as you're considering would take ludicrous amounts of power, and that will likely be your limiting factor when it comes to making this plan work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might want to try posting this question on the physics stack exchange. It might be that someone with much better aerodynamics knowledge than myself can at least give you better guess. That being said I'm going to say that, as an educated guess, you're likely going to need to fire almost straight up, if this is possible at all, or you're going to burn up in the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="20672" LastEditorUserId="20672" LastEditDate="2017-11-15T14:01:33.823" LastActivityDate="2017-11-15T14:01:33.823" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23706" PostTypeId="1" AcceptedAnswerId="23722" CreationDate="2017-11-15T01:57:34.450" Score="6" ViewCount="1058" Body="&lt;p&gt;For this question, block-quoting all of the relevant information would be unwieldy (even for me) so I will just link four articles (plus one quoted section from the last item):&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;NASA Spaceflight 16-Oct-2017 &lt;a href=&quot;https://www.nasaspaceflight.com/2017/10/spacex-zuma-iridium-4-aims-vandenberg-landing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX adds mystery “Zuma” mission, Iridium-4 aims for Vandenberg landing&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;NASA Spaceflight 11-Nov-2017 &lt;a href=&quot;https://www.nasaspaceflight.com/2017/11/spacex-static-fire-zuma-falcon-9-engine-no-issue-manifest/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX static fires Zuma Falcon 9; engine test anomaly no issue for manifest&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Florida Today 14-Nov-2017 &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/2017/11/14/spacex-ready-mystery-zuma-falcon-9-launch-kennedy-space-center-ksc-florida-landing-cape-canaveral/862491001/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX ready for mystery 'Zuma' launch from KSC and landing at Cape Canaveral&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Spaceflight Now 14-Nov-2017 &lt;a href=&quot;https://spaceflightnow.com/2017/11/15/top-secret-zuma-payload-puzzles-satellite-trackers/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Top secret ‘Zuma’ mission puzzles satellite trackers&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A top secret payload developed in secrecy for the U.S. government is set to ride into orbit from Florida’s Space Coast aboard a SpaceX Falcon 9 rocket Thursday.&lt;/p&gt;&#xA;&lt;p&gt;Known only by the codename Zuma, the payload is likely heading for an orbit less than 1,000 miles in altitude, and perhaps much lower. &lt;strong&gt;The existence of the mission was not revealed until it was disclosed in regulatory filings last month&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;A disclosure so close to launch is rare in the space industry, and little information about the mission has been released in the last few weeks&lt;/strong&gt;. Clandestine spacecraft owned by the National Reconnaissance Office — the U.S. government’s spy satellite agency — receive codenames years before they lift off. They’re also listed in public procurement documents and launch manifests. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It looks like NASA was going through normal procedures to approve the use of &amp;quot;used rockets&amp;quot; (&amp;quot;once-flown Falcon 9 booster that lofted a payload to Low Earth Orbit (LEO)&amp;quot; item 1.) to supply the ISS, but I'm not sure this was complete.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt;  Did the sudden appearance of Zuma on the schedule expedite this approval, by &amp;quot;bumping&amp;quot; NASA from business class and encouraging them to accept an empty seat in coach (putting CRS on pre-flown boosters)? Or would NASA have approved the use of once-used-to-LEO F9's in time for the CRS launches anyway?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Bonus:&lt;/strong&gt; Will &lt;a href=&quot;http://paw-patrol.wikia.com/wiki/Zuma&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zuma&lt;/a&gt; appear on the mission patch?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-16T20:20:44.083" Title="Did Zuma &quot;bump&quot; NASA?" Tags="|spacex|nasa|zuma|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23707" PostTypeId="2" ParentId="18475" CreationDate="2017-11-15T03:10:59.653" Score="22" Body="&lt;p&gt;There are 8 active Mars missions at the time of writing (11/14/17)&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Odyssey&quot; rel=&quot;noreferrer&quot;&gt;Mars Odyssey&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Design Downlink Requirement: 3.6 kb/s minium&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;On-Orbit Downlink Performance: up to 110,600 b/s for 34-m DSN passes&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Source: &lt;a href=&quot;https://descanso.jpl.nasa.gov/DPSummary/odyssey_telecom.pdf&quot; rel=&quot;noreferrer&quot;&gt;DESCANSO Design and Performance Summary Series, Article 6, Odyssey Telecommunications&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Express&quot; rel=&quot;noreferrer&quot;&gt;Mars Express&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;Mars Reconnaissance Orbiter&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Design Downlink Requirement: 500 kbps minium&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;On-Orbit Downlink Performance: up to 5.22 Mbps using the X-Band system&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Source: &lt;a href=&quot;https://descanso.jpl.nasa.gov/DPSummary/MRO_092106.pdf&quot; rel=&quot;noreferrer&quot;&gt;DESCANSO Design and Performance Summary Series, Article 12, Mars Reconnaissance Orbiter Telecommunications&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Opportunity_rover&quot; rel=&quot;noreferrer&quot;&gt;Opportunity&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Design Downlink Requirement: 10 b/s minimum&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;On-Orbit Downlink Performance: 11,850 b/s to 3,555 b/s with HGA&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Source: &lt;a href=&quot;https://descanso.jpl.nasa.gov/monograph/series13/DeepCommo_Chapter7--141030.pdf&quot; rel=&quot;noreferrer&quot;&gt;MER Telecommunications Chapter 7: Mars Exploration Rover Telecommunications&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Orbiter_Mission&quot; rel=&quot;noreferrer&quot;&gt;Mars Orbiter Mission&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/MAVEN&quot; rel=&quot;noreferrer&quot;&gt;MAVEN&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Design Downlink Speed: 2048 kbps (max)&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Source: &lt;a href=&quot;https://trs.jpl.nasa.gov/bitstream/handle/2014/44998/11-5643_A1b.pdf&quot; rel=&quot;noreferrer&quot;&gt;MAVEN Relay Operations Concept&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/ExoMars_Trace_Gas_Orbiter&quot; rel=&quot;noreferrer&quot;&gt;ExoMars Trace Gas Orbiter&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Curiosity_rover&quot; rel=&quot;noreferrer&quot;&gt;Curiosity&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Design Downlink Requirement:  at least at 160 bits per second (bps) to a 34-m station or 800 bps to a 70-m station.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Source: &lt;a href=&quot;https://descanso.jpl.nasa.gov/DPSummary/Descanso14_MSL_Telecom.pdf&quot; rel=&quot;noreferrer&quot;&gt;DESCANSO Design and Performance Summary Series, Article 14, Mars Science Laboratory Telecommunications System Design&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;This is all the information that I could find on these missions, based on this the winner is the MRO at 5.22 Mbps, this makes sense as it's main mission is taking high-resolution images.&lt;/p&gt;&#xA;" OwnerUserId="18663" LastEditorUserId="18663" LastEditDate="2022-07-22T02:11:41.070" LastActivityDate="2022-07-22T02:11:41.070" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="23709" PostTypeId="1" AcceptedAnswerId="23710" CreationDate="2017-11-15T12:45:13.297" Score="9" ViewCount="1531" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Evacuated_Airship_for_Mars_Missions&quot; rel=&quot;noreferrer&quot;&gt;There's an article published by NASA in which they discuss the practicality of using an evacuated blimp or airship on Mars. Their main points are that since the exterior pressure is so much lower, this could be constructed with a lattice of existing materials in a large sphere. They also predict that:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Through a more in-depth analysis of the vacuum airship model, it can be shown that the vacuum airship may theoretically carry more than twice as much payload as a modeled dirigible of the same size, a 40-meter radius, in the Martian atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;However it seems to me like the numbers don't add up...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If 1 cubic meter of atmosphere on Earth has a mass of about of about 1.225 kg/m^3 (asl) and the atmosphere on Mars (asl) is 0.6 percent of that then every cubic meter of vacuum can only displace 0.00735kg of Martian atmosphere. Now sure, Mars' gravity is also less but I'm still not quite sure how someone would construct a solid structure for less than 0.018kg which can contain a vacuum, even in mars' low pressure environment. Am I missing something? Where did my assumptions go wrong?&lt;/p&gt;&#xA;" OwnerUserId="21538" LastActivityDate="2021-01-05T19:29:58.417" Title="Mars Vacuum-blimp Feasability" Tags="|mars|colonization|vacuum|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23710" PostTypeId="2" ParentId="23709" CreationDate="2017-11-15T15:58:35.080" Score="9" Body="&lt;p&gt;For the sake of this answer I'm going to use the numbers I get from NASA and Wolfram Alpha. These are:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Density of Earth Atmosphere: 1.204 kg/m^3&#xA;Density of Martian Atmosphere: .020 kg/m^3&#xA;Density of Helium on Mars: .001458 kg/m^3&#xA;Density of Helium on Earth: .1663 kg/m^3&#xA;&#xA;Volume of a 40m radius sphere: 268083 m^3&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;These assume 1 atm pressure and 20 degrees C on earth and 6.36 mbar and 210 degrees K on Mars (sorry for the unit mix up).&lt;/p&gt;&#xA;&lt;p&gt;That means that the Helium craft displaces:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Earth: (1.204 - .1663) * 268083 = 278185 kg&#xA;Mars:  (.020 - .001458) * 268083 = 4971 kg&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;And the Vacuum craft displaces:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;Earth: 1.204 * 268083 = 322772 kg&#xA;Mars:  .020 * 268083 = 5362 kg&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;These numbers will change dramatically depending on the temperature, which can vary widely on Mars, so even though I didn't get the same result that Nasa did, it's entirely possible they're are assuming a colder than average temperature.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Something I didn't note earlier but I think is actually kind of important: the density of a vacuum doesn't change (by definition), so the weight of the craft will not vary with temperature, while the helium craft will have to add or remove helium to maintain a stable pressure. This means that it will perform much better as it gets colder in comparison to the helium craft, which becomes more dense as the temperature decreases.&lt;/p&gt;&#xA;" OwnerUserId="20672" LastEditorUserId="20672" LastEditDate="2021-01-05T19:29:58.417" LastActivityDate="2021-01-05T19:29:58.417" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23711" PostTypeId="2" ParentId="23686" CreationDate="2017-11-15T21:12:21.287" Score="1" Body="&lt;p&gt;As many before me have commented, there is a trade-off between gaining altitude (for purposes of reducing drag and raising the highest possible perigee) and gaining horizontal speed (reducing spacecraft Delta-v reqs to manageable levels, raising actual perigee). Knowing that you have to have a very long railgun to reduce G-loads, you can actually model the ideal path of travel in a very similar way to normal rocket launch trajectories. You would just substitute the gun's acceleration for the increasing acceleration of rocketry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't see the cost of such an enormous structure being less than the cost of the inefficiencies of the rocket equation any time soon especially considering that you would need multiple guns or a rotating gun to service different orbital inclinations without an enormous amount of spacecraft Delta-v.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Essentially, as infeasible as the project is and probably will remain for quite some time, we can say that your gun's optimum shape is not a line at a given angle, but a curve quite like a gravity turn.&lt;/p&gt;&#xA;" OwnerUserId="21643" LastActivityDate="2017-11-15T21:12:21.287" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23712" PostTypeId="1" CreationDate="2017-11-16T08:10:00.467" Score="6" ViewCount="1205" Body="&lt;p&gt;The NASA Science video &lt;a href=&quot;https://www.youtube.com/watch?v=sQH6Oo4hn94&quot; rel=&quot;noreferrer&quot;&gt;ScienceCasts: A Display of Lights Above the Storm&lt;/a&gt; shows several examples of &lt;a href=&quot;http://vlf.stanford.edu/research/transient-luminous-events&quot; rel=&quot;noreferrer&quot;&gt;Transient Luminous Events&lt;/a&gt; (TLEs) (&lt;a href=&quot;https://en.wikipedia.org/wiki/Upper-atmospheric_lightning&quot; rel=&quot;noreferrer&quot;&gt;Upper Atmospheric Lightning&lt;/a&gt;) being filmed by high-sensitivity cameras on the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has an astronaut ever seen any of these with the unaided eye?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Taglt.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Taglt.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8VZ5j.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8VZ5j.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sphcr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sphcr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-16T08:56:20.703" LastActivityDate="2017-12-21T21:13:17.060" Title="Has an astronaut ever seen pixies, ELVEs, sprites or blue jets (without a camera)?" Tags="|iss|atmosphere|observation|earth-observation|weather|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23713" PostTypeId="1" AcceptedAnswerId="23721" CreationDate="2017-11-16T08:55:08.710" Score="49" ViewCount="14198" Body="&lt;p&gt;The &lt;em&gt;music video&lt;/em&gt; on the NASA Johnson YouTube channel &lt;a href=&quot;https://www.youtube.com/watch?v=rgBKFEeXfww&quot; rel=&quot;noreferrer&quot;&gt;The Sound (&amp;amp; Visions) of Silence&lt;/a&gt; shows images and video of Earth taken from the ISS by  astronauts Sergey Ryazanskiy, Paolo Nespoli, and Commander Randy Bresnik.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Between &lt;code&gt;01:02&lt;/code&gt; and &lt;code&gt;01:10&lt;/code&gt; there is shown a photo of a city(?) area with bright sodium street lighting and dimmer white lights, and in the center is a bright lit white area and what looks like light shining straight up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any ideas where in the world this is, and if this is a concert, or a brightly lit public area, or what else has produced all of this light? Are these spotlights, or is this somehow reflected moonlight?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HpERa.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HpERa.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JCrZj.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JCrZj.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Captured by astronauts Sergey Ryazanskiy, Paolo Nespoli, and Commander Randy Bresnik, the video features stunning footage of Earth and outer space from the International Space Station in low-earth orbit during the months of August through October in 2017.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As an aside, this is a re-recording of the Simon and Garfunkel classic &lt;em&gt;Sound of Silence&lt;/em&gt; by the band Disturbed, and their performance of it on &lt;em&gt;Conan&lt;/em&gt; received a Grammy nomination. (if you're particularly fond of the original, a caution that this one is very different)&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-11-24T07:20:18.933" LastActivityDate="2020-06-09T08:28:46.183" Title="What caused this bright light from the ground at night seen from the ISS?" Tags="|iss|low-earth-orbit|identify-this-object|earth-observation|space-art|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23714" PostTypeId="2" ParentId="23703" CreationDate="2017-11-16T09:17:07.037" Score="20" Body="&lt;p&gt;This is a remarkably interesting exchange. I would like to add a few concerns (as an engineer, I go straight for the problems)...&lt;/p&gt;&#xA;&lt;p&gt;Full culm bamboo is a remarkably inefficient product to transport and thus any use in space would require bamboo to be grown in space. On Earth, we consider bamboo inefficient if it is not grown within about 200 km of its end use.&lt;/p&gt;&#xA;&lt;p&gt;This raises the issue that the remarkable strength and unique stiffness properties of bamboo result from its growth. Both gravity and wind (lateral loads) effect the resulting culm geometry. This is demonstrated by bamboo grown on steep hillsides having a distinct bias to their culm geometry, for instance. Bamboo grown in conditions of lower gravity and not subject to lateral load will likley be 'weaker'. That said, if used in a lower gravity conditions, perhaps the bamboo will be 'tailored' appropropriately. Additionally, bamboo grown in conditions of zero gravity is unlikley to be as uniformly straight as we exect the tallest species to be.&lt;/p&gt;&#xA;&lt;p&gt;While many species do indeed grow very rapidly, this does not mean that they are ready for use. Strength of bamboo increases with age (as the plant lignifies), attains an optimum value at 2.5 to 4 years and then decreases following maturity (reported to be greater than about 6 years). Thus there is a growing/harvest cycle on the order of three years. I am not sure anyone knows how this may be affected in an artificial environment with reduced gravity (there is liklely some space-station research out there on lignifying plants).&lt;/p&gt;&#xA;&lt;p&gt;My greatest concern with the question of using bamboo in a vacuum is that this  would imply an equilibrium moisture content of zero. While this will improve some material properties, those associated with transverse-to-longitudinal axis behaviour may be significantly degraded. These are the properties that affect splitting of the culm. This also affects 'toughness'. It is possible that bamboo in a vacuum may behave as though it is decayed.&lt;/p&gt;&#xA;&lt;p&gt;The issue of UV is also a concern. While the bamboo epidermis provides protection while the culm is alive, this protection is degraded after harvest. Similar to many polymers, bamboo degrades under conditions of UV light, showing signs of (to use a polymer term) 'crazing'.&lt;/p&gt;&#xA;&lt;p&gt;The issue of extreme temperature is less certain (I am aware of only limited data that does report a loss of stength at 50 degrees C), although extreme CHANGES in temperature would likely be devastating; causing splitting even with 0% mositure.&lt;/p&gt;&#xA;&lt;p&gt;Finally... As has been noted, bamboo is a high volume plant. To be useful, we likley require structural elements at least 6 m long which imply a culm at least twice that. Unless we are sending Pandas to Mars, I would propose that the volume used to grow bamboo would be much more efficiently put to use growing food crops.&lt;/p&gt;&#xA;" OwnerUserId="21648" LastEditorUserId="46357" LastEditDate="2024-03-18T07:35:49.653" LastActivityDate="2024-03-18T07:35:49.653" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23715" PostTypeId="1" AcceptedAnswerId="23720" CreationDate="2017-11-16T09:38:48.467" Score="6" ViewCount="3798" Body="&lt;p&gt;According to Spaceflight Now's article &lt;a href=&quot;https://spaceflightnow.com/2017/11/15/top-secret-zuma-payload-puzzles-satellite-trackers/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Top secret ‘Zuma’ mission puzzles satellite trackers&lt;/a&gt;, SpaceX's mission patch for the Zuma Falcon 9 mission is that shown below (and in the article).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I noticed the four-leafed clover at the bottom, then doing a search I've found that this [appears frequently but not always in &lt;a href=&quot;http://spacexpatchlist.space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX mission patches&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there any known or announced significance for it, beyond a general hope for good luck? Why would it appear on some patches and not others?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QLVkl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QLVkl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, as far as I can see, no trace of &lt;a href=&quot;http://paw-patrol.wikia.com/wiki/Zuma&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zuma&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="26446" LastEditDate="2020-09-23T04:19:46.817" LastActivityDate="2020-09-23T04:19:46.817" Title="What does the four-leafed clover on mission patches produced by SpaceX signify?" Tags="|spacex|space-art|insignia|zuma|mission-patch|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23716" PostTypeId="1" AcceptedAnswerId="26047" CreationDate="2017-11-16T10:04:56.503" Score="1" ViewCount="261" Body="&lt;p&gt;Researching for the question &lt;a href=&quot;https://space.stackexchange.com/q/23715/12102&quot;&gt;What does the four-leafed clover on Zuma's (and others') mission patch signify?&lt;/a&gt; I came across the site &lt;a href=&quot;http://spacexpatchlist.space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spacexpatchlist.space/&lt;/a&gt; I can't be sure of the validity of the site, but it seems somewhat believable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While looking for four-leaved clovers there, I saw these two patches. Both designs include the Sunflower aspect of the Taiwan flag, but also other images that intrigue me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One has the &quot;all seeing eye&quot; (with a &lt;em&gt;blue colored iris&lt;/em&gt; no less, and the other has something that looks like R2D2 with tentacles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What does this imagery represent? Who generates and approves these graphic elements? Is there any further documentation? Theories?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PFB3b.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PFB3b.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZegPL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZegPL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="26446" LastEditDate="2020-09-23T04:20:40.680" LastActivityDate="2020-09-23T04:20:40.680" Title="What do these Formosat-5 patch images mean - who designed them?" Tags="|space-art|insignia|mission-patch|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23720" PostTypeId="2" ParentId="23715" CreationDate="2017-11-16T16:51:10.213" Score="7" Body="&lt;p&gt;I checked the &lt;a href=&quot;http://spacexpatchlist.space/&quot; rel=&quot;noreferrer&quot;&gt;SpaceX Patch List&lt;/a&gt; (by &lt;a href=&quot;https://www.reddit.com/user/ticklestuff/&quot; rel=&quot;noreferrer&quot;&gt;/u/ticklestuff&lt;/a&gt;) and found only 4 patches issued by SpaceX without the symbol, they are F1 flights 1-3 and &lt;a href=&quot;http://spacexpatchlist.space/patches/sts_133_dragoneye.png&quot; rel=&quot;noreferrer&quot;&gt;STS-133 Dragoneye&lt;/a&gt;&#xA;(&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-133#SpaceX_DragonEye_sensor&quot; rel=&quot;noreferrer&quot;&gt;wiki&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;So definitely a significant symbol for the company launches.&lt;/p&gt;&#xA;&lt;p&gt;SpaceX wrote about it on their FB page in a &lt;a href=&quot;https://www.facebook.com/SpaceX/posts/10157191762440131:0&quot; rel=&quot;noreferrer&quot;&gt;St. Patrick post&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The four leaf clover on SpaceX mission patches began with the fourth flight of Falcon 1, which launched September 28, 2008.&lt;/p&gt;&#xA;&lt;p&gt;This mission marked the first time Falcon 1 successfully achieved orbit and the first successful orbital launch of any privately funded and developed liquid-propelled payload-carrying rocket.&lt;/p&gt;&#xA;&lt;p&gt;When the flight succeeded, the clover stuck and has been with us ever since.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The four-leaf clover shows in other places too. You can see it on one of the walls of the ASDS Of Course I Still Love You (OCISLY) in &lt;a href=&quot;https://i.stack.imgur.com/hgRwV.jpg&quot; rel=&quot;noreferrer&quot;&gt;this gallery&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-16T16:51:10.213" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23721" PostTypeId="2" ParentId="23713" CreationDate="2017-11-16T17:50:11.603" Score="55" Body="&lt;p&gt;Well I confirmed via Google Maps that this is Mecca.  As shown in the map and image below the roads align with those lighted in the image.  The dark areas in the first image are steep hills to the East.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/arkpS.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/arkpS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The brightly lit region is the Kaaba and large Masjid al-Haram Mosque, and the bright up light is indeed the Makkah Royal Clock Tower.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RTHjF.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RTHjF.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15824" LastActivityDate="2017-11-16T17:50:11.603" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23722" PostTypeId="2" ParentId="23706" CreationDate="2017-11-16T20:20:44.083" Score="9" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasaspaceflight.com/2017/11/spacex-falcon-9-launch-clandestine-zuma-satellite/&quot; rel=&quot;noreferrer&quot;&gt;NSF: SpaceX switches Falcon launch of clandestine Zuma satellite to Friday&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;While the contract was signed in 2015, a launch date was not&#xA;  determined until late-April 2017 when the government customer notified&#xA;  Northrop Grumman of the mandatory launch period for Zuma of 1-30&#xA;  November 2017.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Once the launch date was known, it is understood by&#xA;  NASASpaceflight.com that SpaceX assigned Falcon 9 first stage B1043 to&#xA;  Zuma – a core that was originally supposed to launch the CRS-13/Dragon&#xA;  mission for NASA to the International Space Station.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Due to the U.S. government’s need to launch Zuma before 30 November,&#xA;  SpaceX’s manifest was rearranged to meet the customer’s short-notice&#xA;  launch need – representing a rapid launch response capability for&#xA;  SpaceX that has been greatly aided by the company’s immensely&#xA;  successful reuse of the Falcon 9 first stage booster.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;That sounds like NASA/CRS-13 really being bumped, just not on a such short notice, with half a year to plan accordingly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasaspaceflight.com/2017/10/falcon-9-koreasat-5a-nasa-approves-flown-boosters/&quot; rel=&quot;noreferrer&quot;&gt;Flight Proven for CRS status:&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The NASA review into flight proven boosters has been ongoing for months, involving a final review and approval process at NASA HQ to asscertain if flight proven boosters could be used on ISS resupply missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So going preflown may have been the plan since that April bump, supposed all the reviews work out fine.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2017-11-16T20:20:44.083" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23723" PostTypeId="1" AcceptedAnswerId="23724" CreationDate="2017-11-16T21:22:46.253" Score="3" ViewCount="293" Body="&lt;p&gt;I saw this image in Spaceflight Now's 30-Oct-2017 article &lt;a href=&quot;https://www.nasaspaceflight.com/2017/10/falcon-9-koreasat-5a-nasa-approves-flown-boosters/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX Falcon 9 successfully launches Koreasat 5A&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pitch-black background and rocket body with extremely bright grid fins. Is this an infrared image showing how hot they are? I can't understand how they could be lit visibly from below, unless from below by the Sun perhaps?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can someone figure out the orientation of the F9 booster and the source of the lighting for this image?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/jgpS6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jgpS6.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2020-06-14T05:57:20.097" Title="Is this an IR or visible light image? What's illuminating the F9's grid fins?" Tags="|spacex|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23724" PostTypeId="2" ParentId="23723" CreationDate="2017-11-16T22:28:53.613" Score="5" Body="&lt;p&gt;This is not Koreasat-5A, but SES-11/EchoStar 105, at &lt;a href=&quot;https://www.youtube.com/watch?v=iv1zeGSvhIw&quot; rel=&quot;nofollow noreferrer&quot;&gt;about 21:11 in this video&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first stage rocket body is nearly aligned with the sun setting behind the rocket. We're looking &quot;down&quot; the rocket body, toward the East. We're beginning reentry. The upper side of the rocket body and at least the base of the fins are in shadow, and the fins are glowing red hot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lens flare on the right-hand (second-stage) portion of the frame tells us it's late in the day and the sun is setting in the West behind the eastbound first and second stages. Koreasat-5A launched at about 2:30pm Eastern time (and nothing in its video looked like this), but the immediately previous  EchoStar 105/SES-11 launched at 6:53 pm -- close to sunset (which led me to check its video).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The left side is the same camera angle we see in every broadcast; it's a color visible-light camera, and I haven't ever seen any indication of them switching to an IR camera in their webcasts.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-16T22:51:27.823" LastActivityDate="2017-11-16T22:51:27.823" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23725" PostTypeId="1" CreationDate="2017-11-17T06:10:09.000" Score="5" ViewCount="199" Body="&lt;p&gt;Casually reading “orbital mechanics for engineering students” on rigid body attitude dynamics, i see the following passage:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$M_{net}=\dot{H}_{rel} + \Omega\times H$$&#xA;&lt;em&gt;Keep in mind that, whereas $\Omega$ (the angular velocity of the moving xyz coordinate system) and $\omega$ (the angular velocity of the rigid body itself) are both absolute kinematic quantities, ... If the comoving frame is rigidly attached to the body frame, then ... $\Omega=\omega$.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can’t fathom any plausible reason why one would have $\Omega\ne\omega$ at all, much less for a satellite.  Is there a situation that comes up where it would be useful to describe a rigid body satellite with a rotating coordinate axis that differs from the rotation of the body itself?  It seems like the authors are extremely careful to avoid saying that $\Omega=\omega$ universally, which makes me suspect that there may be cases that come up where it is best to keep them separate.  What are those situations in the context of a satellite’s attitude dynamics?&lt;/p&gt;&#xA;" OwnerUserId="6494" LastEditorUserId="6494" LastEditDate="2017-11-17T13:50:30.350" LastActivityDate="2020-12-20T10:37:40.893" Title="In what circumstance would it be useful for a satellite’s angular velocity to differ from its reference frame’s angular velocity?" Tags="|artificial-satellite|attitude|rotation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23726" PostTypeId="1" AcceptedAnswerId="23727" CreationDate="2017-11-17T07:19:12.960" Score="3" ViewCount="226" Body="&lt;p&gt;Sónar Calling GJ273b is in the news, at the time of this question, for sending messages into space in an attempt to contact aliens. Some news sites are calling the messages &amp;quot;secret&amp;quot;. Does anyone know why? Was the sending of the messages supposed to be secret?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://nypost.com/2017/11/16/scientists-take-risk-sending-secret-message-to-aliens/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scientists take risk sending secret message to aliens&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.foxnews.com/science/2017/11/16/scientists-just-sent-message-to-aliens-but-response-could-take-25-years.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scientists send secret message to aliens in new search for intelligent life&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Background:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;From the Newsweek article (caution on annoying auto-play media) &lt;a href=&quot;http://www.newsweek.com/advanced-alien-civilization-send-messages-meti-713186&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scientists have Sent Messages to Advanced Alien Civilizations - and ar Hoping for a Reply in 25 years&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Despite these concerns, they forged ahead and it was announced the messages had been sent at Sónar, a festival of music, creativity and technology held in Barcelona, Spain. Sónar initially came up with the idea for the project, and asked METI and the Institute of Space Studies of Catalonia (IEEC) to collaborate on it.&lt;/p&gt;&#xA;&lt;p&gt;They sent out messages to GJ273—or Luyten’s Star—a red dwarf with a potentially habitable exoplanet called GJ273b. The message was beamed towards the system on October 16, 17, and 18. It took 11 minutes each day, with the total transmission lasting 33 minutes.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Image caption &amp;quot;The messages were sent from the EISCAT antenna in Tromsø, Norway. SÓNAR INTERNATIONAL&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iZoz4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iZoz4.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Linked video:&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/g9GeBKB-0EY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="21659" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-17T10:25:30.270" Title="Why were Sónar Calling GJ273b messages called secret?" Tags="|communication|seti|space-art|meti|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23727" PostTypeId="2" ParentId="23726" CreationDate="2017-11-17T10:25:30.270" Score="8" Body="&lt;p&gt;They were called &quot;secret&quot; because the media doesn't really understand what &quot;encoded&quot; means, and in any case &quot;secret&quot; attracts more attention in a headline, which is what sells.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is nothing secret about the message. The encoding is really a set of logical structures to hopefully make it understandable to another intelligence that may not have anything in common with us culturally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact previous ones, including the message on the Voyager plaques have also included forms of encoding. Voyager has encoded pulsar locations relative to our solar system.&lt;/p&gt;&#xA;" OwnerUserId="38" LastActivityDate="2017-11-17T10:25:30.270" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23729" PostTypeId="2" ParentId="23725" CreationDate="2017-11-17T18:16:37.343" Score="3" Body="&lt;p&gt;Obviously this depends on the purpose of the satellite.&lt;/p&gt;&#xA;&lt;p&gt;If the satellite is a space telescope, it needs to focus on distant stars; &lt;span class=&quot;math-container&quot;&gt;$\omega = 0$&lt;/span&gt;.&lt;/p&gt;&#xA;&lt;p&gt;If it's a spy satellite focusing on a specific point on the surface, it will turn to aim at that point.&lt;/p&gt;&#xA;&lt;p&gt;In case of satellites in highly eccentric orbit, angular velocity is often controlled on ad-hoc basis - since &lt;span class=&quot;math-container&quot;&gt;$\Omega$&lt;/span&gt; is non-constant, the satellite traveling much slower near apoapsis than near periapsis; meanwhile &lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt; &amp;quot;left to its own devices&amp;quot; would be constant; if the satellite is to focus at Earth, it needs to be rotated actively (example: Molniya orbit satellites).&lt;/p&gt;&#xA;&lt;p&gt;There are many other applications where specific spin, or lack of spin is desirable - orienting panels towards the Sun, scanning surface in narrow strips through narrow beam, inertial stabilization, artificial gravity - in all these cases &lt;span class=&quot;math-container&quot;&gt;$\omega$&lt;/span&gt; will be &amp;quot;custom&amp;quot;.&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="33864" LastEditDate="2020-12-20T10:37:40.893" LastActivityDate="2020-12-20T10:37:40.893" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23730" PostTypeId="1" AcceptedAnswerId="23731" CreationDate="2017-11-17T19:47:57.050" Score="6" ViewCount="915" Body="&lt;p&gt;In watching some Saturn rocket launches from the perspective of the camera being on the launch tower, it seems as if there was relatively little distance between the launching rocket and the tower.  I realize this &quot;little distance&quot; between the rocket and tower is relative, but it looks by eyeball measurements to not really be more than the diameter of the 1st stage of the rocket itself.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Were there precautions built into the guidance software that would prevent gimbaling the motors in such a way as to accidentally move the rocket towards the tower at least until some sort of elapsed time or altitude had been reached to guarantee the tower had been cleared?  I know it's highly unlikely that a gust of wind or other external influence could have actually had enough of an effect to move the huge rocket that much. But I am wondering if that contingency had been built into the software to reduce that possibility of an accidental launch collision with the tower.  &lt;/p&gt;&#xA;" OwnerUserId="16735" LastEditorUserId="6944" LastEditDate="2017-11-17T20:36:25.467" LastActivityDate="2017-11-17T20:46:22.973" Title="Saturn launch precautions for clearing tower?" Tags="|launch|saturn-v|guidance|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23731" PostTypeId="2" ParentId="23730" CreationDate="2017-11-17T20:26:24.400" Score="11" Body="&lt;p&gt;Just as with Shuttle, the first stage guidance was pre-programmed to fly a specific flight profile. For Saturn V &quot;guidance &#xA;corrections &#xA;are &#xA;not &#xA;intro­duced &#xA;before &#xA;the &#xA;early &#xA;part &#xA;of &#xA;the &#xA;second &#xA;stage &#xA;flight.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Reference: &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700023342.pdf&quot; rel=&quot;noreferrer&quot;&gt;DESCRIPTION &#xA;AND &#xA;PERFORMANCE &#xA;OF &#xA;THE &#xA;SATURN &#xA;LAUNCH &#xA;VEHICLE'S &#xA;NAVIGATION, &#xA;GUIDANCE &#xA;,&#xA;AND &#xA;CONTROL &#xA;SYSTEM&lt;/a&gt; (first page of text)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was a 1.25 degree tilt programmed in to steer away from the tower until tower clear.  Then the roll program kicked in to maneuver to the flight azimuth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: &lt;a href=&quot;https://books.google.com/books?id=x-taL4N0sjIC&amp;amp;pg=PA86&amp;amp;lpg=PA86&amp;amp;dq=saturn%20ascent%20guidance%20tower%20clear&amp;amp;source=bl&amp;amp;ots=v92rRYoQVN&amp;amp;sig=AD4ZokbqbaH2PDF4d_xroxmXvKw&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjD6MyxuMbXAhUE7WMKHYhZCL4Q6AEIRTAJ#v=onepage&amp;amp;q=saturn%20ascent%20guidance%20tower%20clear&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;How Apollo Flew to the Moon&lt;/a&gt; page 86&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To my eye at least, the tilt is visible at 8:45 in this video of Apollo 8, then you can see it straighten out at 8:52, just before the camera cuts away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Vn00BvWwke0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-11-17T20:46:22.973" LastActivityDate="2017-11-17T20:46:22.973" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23733" PostTypeId="1" AcceptedAnswerId="23735" CreationDate="2017-11-18T01:32:41.687" Score="32" ViewCount="5426" Body="&lt;p&gt;The ISS occasionally fires thrusters to correct orbit and uses reaction control wheels to adjust attitude.  It also experiences various external forces - tidal, aerodynamic, etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From the outside, it seems very flimsy - modules connected to each other just by the docking rings.  Is that really all that holds it together?  The docking rings?  Or is there some sort of additional structure to give the whole thing rigidity?&lt;/p&gt;&#xA;" OwnerUserId="21469" LastEditorUserId="6944" LastEditDate="2020-09-04T13:11:46.240" LastActivityDate="2020-09-04T13:11:46.240" Title="ISS Structural Integrity" Tags="|iss|mechanisms|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23734" PostTypeId="2" ParentId="23733" CreationDate="2017-11-18T02:20:10.553" Score="56" Body="&lt;p&gt;&quot;Is that really all that holds it together?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, no.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are several different attach mechanisms used to hold the various parts of the ISS together.  The mechanism used depends on what kind of component is being used (pressurized, unpressurized) and who built it (US, Russia, Japan).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The US pressurized modules are held to each other by &lt;a href=&quot;https://en.wikipedia.org/wiki/Common_Berthing_Mechanism&quot; rel=&quot;noreferrer&quot;&gt;Common Berthing&#xA;Mechanisms&lt;/a&gt; (CBMs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WOIpU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WOIpU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The interface between the US pressurized modules and the truss is at the&#xA;lab and the S0 truss segment and is called the &lt;a href=&quot;http://pages.erau.edu/~ericksol/projects/issa/destiny.html&quot; rel=&quot;noreferrer&quot;&gt;Lab Cradle Assembly&lt;/a&gt; aka the Module to Truss Segment Attach System (MTSAS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/87BiK.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/87BiK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The truss segments are held to each other by the &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110010964.pdf&quot; rel=&quot;noreferrer&quot;&gt;Segment-to-Segment&#xA;Attach System&lt;/a&gt; (SSAS) or the Rocketdyne Truss Attachment System (RTAS).&#xA;&lt;a href=&quot;https://i.stack.imgur.com/MqyMA.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MqyMA.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/AZj94.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AZj94.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The US side is connected to the Russian side by an &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110010964.pdf&quot; rel=&quot;noreferrer&quot;&gt;Androgynous&#xA;Peripheral Attachment System&lt;/a&gt; (APAS).&#xA;&lt;a href=&quot;https://i.stack.imgur.com/TLYtY.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TLYtY.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Russian side uses Probe &amp;amp; Drogue and Hybrid interface mechanisms.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/F5lQd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/F5lQd.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/4I1Yo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4I1Yo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The &quot;porch&quot; aka the JEM-EF is attached to the JEM by the unique&#xA;Exposed Facility Berthing Mechanism (EFBM).&#xA;&lt;a href=&quot;https://i.stack.imgur.com/iB9oL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iB9oL.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-11-18T02:31:24.583" LastActivityDate="2017-11-18T02:31:24.583" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23735" PostTypeId="2" ParentId="23733" CreationDate="2017-11-18T02:21:00.327" Score="39" Body="&lt;p&gt;The ISS is pressurized to nominally Earth sea-level atmospheric pressure - about 1013hPa (14.7 pounds per square inch). Docking rings look to be about 2.2 meters in diameter, so appear to have a cross-sectional area of about 3.8m² (5880 square inches). That means each docking ring (assuming they are all that size) is constantly under about 385 kN (86,500 lbs) of force as the internal atmosphere tries to push the mated sections apart. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put that in perspective, the total mass of the station is about 420 metric tons (924,740 lb), (equivalent to ~4120 kN on the earth surface).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Structures needed to contain a human-friendly atmosphere are by no means flimsy, especially those built large - the forces involved can be quite substantial.&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="3460" LastEditDate="2017-11-23T16:19:04.757" LastActivityDate="2017-11-23T16:19:04.757" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23736" PostTypeId="2" ParentId="20546" CreationDate="2017-11-18T04:06:34.673" Score="9" Body="&lt;p&gt;In the video of the Keynote Speaker (and former NASA ARC director) &lt;a href=&quot;https://breakthroughinitiatives.org/Leaders/3&quot; rel=&quot;noreferrer&quot;&gt;Pete Worden&lt;/a&gt; (also &lt;a href=&quot;https://en.wikipedia.org/wiki/Pete_Worden&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;)'s talk &lt;a href=&quot;https://www.youtube.com/watch?v=RxlSUlvF6wo&quot; rel=&quot;noreferrer&quot;&gt;Breakthrough Discuss 2016 – Breakthrough Starshot Challenges&lt;/a&gt; the challenges of using the deployed photon sail (for the initial laser-based acceleration) as a reflector &amp;quot;dish&amp;quot; for an optical transmission back to Earth is discussed a bit. The speaker felt that this might be one of the most difficult technological challenges in the short term, possibly with no current or near-future solution.&lt;/p&gt;&#xA;&lt;p&gt;Getting a tiny spacecraft to somehow get the 300 angstrom thin membrane to form an accurate optical surface to collimate an optical transmission does indeed sound pretty difficult.&lt;/p&gt;&#xA;&lt;p&gt;Let's use a burst of 100W and a 10 meter reflector and a distance of 5 light years (5E+16 meters).&lt;/p&gt;&#xA;&lt;p&gt;800nm photons from a simple III-V solid state laser have an energy of about 2.5E-19 Joules each, so that's 4E+20 photons per second. An &lt;a href=&quot;https://en.wikipedia.org/wiki/Airy_disk#Size&quot; rel=&quot;noreferrer&quot;&gt;Airy disk&lt;/a&gt; half-width of 1.22 &lt;span class=&quot;math-container&quot;&gt;$\lambda$&lt;/span&gt;/d of 8E-08 radians means the footprint at Earth has a radius of 8E-08 x 5E+16 = 4E+09 meters. The radius of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Extremely_Large_Telescope&quot; rel=&quot;noreferrer&quot;&gt;Extremely Large Telescope&lt;/a&gt;'s aperture is about 20 meters, so the ratio of the areas is 2.5E-17.&lt;/p&gt;&#xA;&lt;p&gt;Since we're transmitting 4E+20 photons per second, that's about 10,000 photons per second at the receiver!&lt;/p&gt;&#xA;&lt;p&gt;Your milage may vary, but in terms of energy, is quantitatively realistic.&lt;/p&gt;&#xA;&lt;p&gt;However, from an engineering point of view, one would have to back off from this starting point. Forming a 10 meter parabola with &lt;span class=&quot;math-container&quot;&gt;$\lambda$&lt;/span&gt;/4 from a 300 angstrom film automatically using a chip-size satellite is quite a challenge - probably nobody really knows how it might be done at the moment.&lt;/p&gt;&#xA;&lt;p&gt;Pointing is also important; what kind of star cameras could one include within a tiny chip that would provide 0.02 arcsec pointing accuracy is currently unknown.&lt;/p&gt;&#xA;&lt;p&gt;So the real challenge will be building something even remotely close to this. In order to transmit meaningful data, millions of photons would have to be collected. Probably dedicated, lower-tech photon collectors in the microgravity of space would be better than expensive Earth-based telescopes.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The inaugural 2016 Breakthrough Discuss Workshop was held April 15-16, 2016 at Stanford University and sponsored by the Breakthrough Initiatives and the Stanford Physics Department. Breakthrough Discuss is a forum for scientists to present and discuss leading edge ideas in space exploration. The 2016 workshop focused on the following three areas:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Optical SETI and the Detectability of Directed Energy Systems – Chaired by Jill Tarter&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Exo-planet Detection Programs Focused on Alpha Centauri – Chaired by Olivier Guyon&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The Sun as a Gravitational Lens – Chaired by Avi Loeb&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/RxlSUlvF6wo?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-11-21T00:59:37.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23737" PostTypeId="1" AcceptedAnswerId="24569" CreationDate="2017-11-18T13:05:21.203" Score="4" ViewCount="686" Body="&lt;p&gt;AAReST was a really neat project by Caltech and University of Surrey (UK) to experiment with a self-assembling space telescope mirror.  There are some interesting articles and papers about it circa 2015, but nothing since then.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Curious to know if it is going to get launched.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some links: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://weebau.com/satellite/A/aarest.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://weebau.com/satellite/A/aarest.htm&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/a/aarest&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://directory.eoportal.org/web/eoportal/satellite-missions/a/aarest&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21469" LastActivityDate="2019-06-12T07:01:23.893" Title="Anyone know what happened to the AAReST project?" Tags="|cubesat|space-telescope|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23738" PostTypeId="1" AcceptedAnswerId="23740" CreationDate="2017-11-18T14:11:56.687" Score="1" ViewCount="238" Body="&lt;p&gt;Since tardigrades are almost extremophilic, and so are &lt;a href=&quot;https://en.wikipedia.org/wiki/Deinococcus_radiodurans&quot; rel=&quot;nofollow noreferrer&quot;&gt;these&lt;/a&gt;, should we use them as shielding material? A thick enough layer outside the spacecraft will shield it and we may use breeding techniques so that the shield is automatically renewed.&lt;/p&gt;&#xA;" OwnerUserId="21504" LastActivityDate="2017-11-18T21:53:23.447" Title="Using tardigrades as space shielding" Tags="|shielding|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="23739" PostTypeId="1" AcceptedAnswerId="23741" CreationDate="2017-11-18T21:46:11.213" Score="6" ViewCount="1740" Body="&lt;p&gt;Given that the $\Delta V$ required to perform an orbital inclination change is proportional to velocity, I was wondering when the best time to perform such a maneuver would be. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it be best to start the plane change maneuver as soon as possible in the ascent trajectory so that you'll end up with the correct orbital velocity and inclination when your engines finally cut off (i.e. incorporate the plane change as part of the ascent itself), or would it be better to first get into an inclined circular orbit at a higher altitude than your planned orbit, perform a plane change at this higher orbit (due to its lower velocity) and then change your orbit to the lower desired altitude? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, would the worst case be to actually perform the plane change at your desired altitude once you're in an inclined circular orbit? As you can see, I'm slightly confused as to the best approach to take, so any guidance would be great.&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="12102" LastEditDate="2017-11-18T23:22:29.050" LastActivityDate="2017-11-20T00:12:42.633" Title="Best time to perform an orbital inclination change — during ascent, or once in circular LEO?" Tags="|orbital-mechanics|orbit|orbital-maneuver|inclination-change|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23740" PostTypeId="2" ParentId="23738" CreationDate="2017-11-18T21:53:23.447" Score="7" Body="&lt;p&gt;&lt;strong&gt;This would not be very feasible.&lt;/strong&gt; There are several reasons. First, living creatures are unpredictable and uncontrollable.  How could you be sure that all your tardigrades would maintain an even layer across the ship?  What if they chose to all group up at one location?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second, because tardigrades are not as dense as, say, a sheet of iron, you would need to have an extremely thick layer of them to provide adequate shielding.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, in order to survive extremely harsh climates, tardigrades undergo a &lt;a href=&quot;https://en.wikipedia.org/wiki/Tardigrade#Physiology&quot; rel=&quot;noreferrer&quot;&gt;form of stasis&lt;/a&gt; where they suspend all metabolic processes. So they would not be reproducing at all. And even though they have extremely high survivability, studies where tardigrades were exposed to outer space for 10 days yielded a &lt;a href=&quot;http://www.cell.com/current-biology/fulltext/S0960-9822(08)00805-1?_returnURL=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982208008051%3Fshowall%3Dtrue&quot; rel=&quot;noreferrer&quot;&gt;~30% mortality rate&lt;/a&gt;.  So you would still have lots of them die.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, even if they did maintain their metabolic processes, you would still need to provide enough food for them to survive and reproduce. So you'd need to have even more material sent up. Moreover, while tardigrades can survive extremely high doses of radiation, tardigrade eggs exposed to radiation have &lt;a href=&quot;http://www.jlimnol.it/index.php/jlimnol/article/view/750&quot; rel=&quot;noreferrer&quot;&gt;extremely low hatch rates&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately, this plan would be extremely complicated, not very useful, and not have any real advantages.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2017-11-18T21:53:23.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23741" PostTypeId="2" ParentId="23739" CreationDate="2017-11-19T05:32:54.187" Score="8" Body="&lt;p&gt;If you're going to a &lt;em&gt;higher&lt;/em&gt; inclination than your launch latitude (for example, going from Cape Canaveral at 28º to ISS at 51º), you'll want to do it as early as possible, while your horizontal velocity is minimal -- essentially immediately when you start your gravity turn. The velocity you start with from Earth's rotation is less than that of even a very high orbit, so the inclination change is nearly free. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're going to a high altitude, lower inclination orbit, for example to equatorial GSO, there are two options: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Option A: Combine the plane change with the circularization/&quot;apogee kick&quot; burn, which makes the total burn requirement equal to the hypotenuse of the triangle made by the normal (plane change) and prograde (circularization) components of the burn, potentially yielding large savings;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;or &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Option B: Launch into a orbit higher than the destination orbit and do the plane change at apogee when you're going very slow. &lt;a href=&quot;https://github.com/KSP-RO/RealismOverhaul/wiki/Equatorial-Orbits-From-Launchsites-Off-The-Equator&quot; rel=&quot;nofollow noreferrer&quot;&gt;This writeup&lt;/a&gt; claims (without citation or derivation) this is more efficient than the first option for plane changes of more than 45 degrees. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The tricky case is when you're trying to reach a low orbit at lower inclination than your launch latitude, like equatorial LEO. In this case, to minimize the size of the plane change, you want to launch into the minimum inclination reachable from the launch site (i.e. fly a due-East initial ascent, making your inclination equal to your launch latitude). Circularization for LEO has to be done some 10-15 minutes into flight, but the descending node where you need to make the plane change is a quarter-orbit (23-30 minutes) from the launch site, so the burns can't be combined for efficiency. By launching into a more steeply inclined orbit (e.g. flying southeast instead of east from Canaveral) you can arrange for the circularization and plane change to coincide, but the plane change will have to be greater; I'm not sure how it optimizes.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-20T00:12:42.633" LastActivityDate="2017-11-20T00:12:42.633" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23743" PostTypeId="2" ParentId="6572" CreationDate="2017-11-19T09:06:50.093" Score="3" Body="&lt;p&gt;You can now see the Lissajous path of DSCOVR using the Blueturn app:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://app.blueturn.earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://app.blueturn.earth&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just zoom out (a lot) from the default EPIC view...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, this app interpolates EPIC images received from DSCOVR using real-time 3D projection techniques. In such this is the first and only interactive video of the Whole Earth, with 2+ year of data. Very recommended!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WmaRR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WmaRR.jpg&quot; alt=&quot;DSCOVR Lissajous path (behind view)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WGTx2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WGTx2.jpg&quot; alt=&quot;DSCOVR Lissajous path (side view)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21677" LastEditorUserId="21677" LastEditDate="2017-11-20T10:59:54.037" LastActivityDate="2017-11-20T10:59:54.037" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23744" PostTypeId="2" ParentId="22833" CreationDate="2017-11-19T09:12:07.703" Score="2" Body="&lt;p&gt;You can now see a smooth animation of the last 8/21 eclipse using the Blueturn app:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://app.blueturn.earth/?date=2017-08-21_15-35-54&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://app.blueturn.earth/?date=2017-08-21_15-35-54&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally, this app interpolates EPIC images received from DSCOVR using relat-time 3D projection techniques. In such this is the first and only interactive video of the Whole Earth, with 2+ year of data. Very recommended!&lt;/p&gt;&#xA;" OwnerUserId="21677" LastActivityDate="2017-11-19T09:12:07.703" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23746" PostTypeId="1" CreationDate="2017-11-19T14:31:29.890" Score="5" ViewCount="133" Body="&lt;p&gt;The importance of Martian caves for life past and present, volatiles being trapped, and for future habitats would seem to put their exploration high on the list but I don't see any missions to do so.  Same with the moon minus the life part. &lt;/p&gt;&#xA;" OwnerUserId="20287" LastEditorUserId="20287" LastEditDate="2017-11-21T10:34:45.893" LastActivityDate="2017-11-21T10:34:45.893" Title="Does NASA, or any other group, have any plans to directly explore caves on Mars or the moon?" Tags="|mars|the-moon|future-missions|caves|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23747" PostTypeId="1" CreationDate="2017-11-19T16:46:36.843" Score="3" ViewCount="445" Body="&lt;p&gt;I've been reading about High Altitude Balloons (HAB) lately, and it seems that they take much greater pains to fight cold temperatures than cubesats do.  Loads of insulation, especially on batteries, and stories of failures because the batteries got too cold.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While thermal management for cubesats is certainly important, it seems that there's much less insulation and less concern about batteries, while more concern about shedding heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this represent a significant difference in the environments the two operate in?  Is it a factor of cubeage being cheaper on a HAB?  Or am I just missing something?&lt;/p&gt;&#xA;" OwnerUserId="21469" LastActivityDate="2019-11-21T20:23:24.563" Title="Temperatures in &quot;near&quot; space compared to LEO" Tags="|low-earth-orbit|temperature|environment|balloons|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23748" PostTypeId="2" ParentId="23747" CreationDate="2017-11-19T16:53:18.960" Score="6" Body="&lt;p&gt;A vacuum is a pretty good insulator, much better than the air at the altitudes balloons operate at. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-11-19T16:53:18.960" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23749" PostTypeId="2" ParentId="19979" CreationDate="2017-11-19T18:30:25.723" Score="1" Body="&lt;p&gt;If you want to use GMAT, I would recommend writing your own GMAT script to interface with Python. The good thing here is that GMAT is high fidelity, so you can leverage that without having to recode it. Moreover, if your thrust is to be applied based on the orientation of the spacecraft, you can define a spacecraft body reference frame and define your thrust vector on each thrust arc in that frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you want to code it up from scratch, which can be a fun challenge, maybe will &lt;a href=&quot;https://github.com/ChristopherRabotin/smd/blob/master/spacecraft.go&quot; rel=&quot;nofollow noreferrer&quot;&gt;my code&lt;/a&gt; be handy. It has locally optimal control laws for continuous thrust propulsion and finite burns, all in the RNC frame.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-11-19T18:30:25.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23752" PostTypeId="1" CreationDate="2017-11-20T02:27:29.227" Score="-1" ViewCount="166" Body="&lt;p&gt;11/19/2017&#xA;Q:Is sun is in the middle of voyager(1&amp;amp;2)?&#xA; I've recorded this data yesterday.&#xA;According to Nasa.gov(VoyagerMissionStatus) &#xA;&lt;strong&gt;Distance of Voyager From Earth In Miles&lt;/strong&gt;&#xA;           1575359 miles (in 24hrs)&#xA;&lt;strong&gt;Distance of Probe From Sun In Miles&lt;/strong&gt;&#xA;           908566  miles (in 24hrs)&lt;/p&gt;&#xA;" OwnerUserId="21688" LastActivityDate="2017-11-20T08:34:58.020" Title="Voyager location" Tags="|artificial-satellite|nasa|probe|" AnswerCount="1" CommentCount="8" FavoriteCount="0" ClosedDate="2017-11-20T08:40:03.343" ContentLicense="CC BY-SA 3.0" />
  <row Id="23753" PostTypeId="1" AcceptedAnswerId="23855" CreationDate="2017-11-20T07:24:31.250" Score="11" ViewCount="749" Body="&lt;p&gt;The old adage goes &quot;It's easy to get to space, much harder to stay there.&quot; Yet the first manned soviet flight was orbital - Americans took a good several suborbital flights before reaching the orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What factors caused/allowed this? Much faster development of launchers vs spacecraft? Political pressure? Something behind the scenes? Why did they not have suborbital manned launches?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-11-30T17:51:07.343" Title="Why did the Soviets skip the suborbital phase?" Tags="|crewed-spaceflight|soviet-union|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23754" PostTypeId="1" AcceptedAnswerId="23756" CreationDate="2017-11-20T07:33:06.723" Score="13" ViewCount="613" Body="&lt;p&gt;Gagarin went directly to the orbit - the first manned flight was orbital. I don't know about any other Soviet or Russian spaceflights that wouldn't be orbital - except for some odd aborted launch or other unforeseen difficulties.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wonder though, if they performed any manned suborbital launches at a later date - something like what Blue Origin plans to do commercially. Or maybe do they plan something like this? Or is Russia &quot;Orbital only&quot;?&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2019-04-01T03:48:32.267" Title="Did Soviets/Russians perform any crewed (intentionally) suborbital flights?" Tags="|crewed-spaceflight|russia|sub-orbital|soviet-union|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23755" PostTypeId="2" ParentId="23752" CreationDate="2017-11-20T08:34:58.020" Score="3" Body="&lt;p&gt;Position of the two Voyager probes:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Loctp.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Loctp.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(from &lt;a href=&quot;https://space.stackexchange.com/questions/3585/is-there-any-way-to-find-and-track-pioneer-10-11-and-voyager-1-2&quot;&gt;this question&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So they're both in the same general direction seen from Earth, not on opposite sides of the solar system. What you can't see in this image is that both Voyagers are moving out of the plane of the ecliptic (&lt;a href=&quot;https://space.stackexchange.com/questions/14317/how-well-can-voyager-1-separate-earth-signals-from-solar-noise-these-days&quot;&gt;this question&lt;/a&gt; has a nice animation that shows that), with Voyager 1 going 'up' and Voyager 2 going 'down'.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-11-20T08:34:58.020" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23756" PostTypeId="2" ParentId="23754" CreationDate="2017-11-20T10:28:22.123" Score="14" Body="&lt;p&gt;There were only four manned Russian programs:&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Vostok&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;No suborbital flights were made. &lt;a href=&quot;https://en.wikipedia.org/wiki/Vostok_programme#Manned_flights&quot; rel=&quot;nofollow noreferrer&quot;&gt;See&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Voskhod&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;No suborbital flights were made. &lt;a href=&quot;https://en.wikipedia.org/wiki/Voskhod_programme&quot; rel=&quot;nofollow noreferrer&quot;&gt;See&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Soyuz&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;There was a two suborbital mission: a failed Soyuz launch in 1975, &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_7K-T_No.39&quot; rel=&quot;nofollow noreferrer&quot;&gt;Soyuz 7K-T No.39&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Its apogee was 192km high, for a flight time of 21min.&lt;/li&gt;&#xA;&lt;li&gt;Made it to space but clearly short of orbit.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The failure was due to an improper separation from stage 2 and 3.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And &lt;a href=&quot;https://en.wikipedia.org/wiki/Soyuz_MS-10&quot; rel=&quot;nofollow noreferrer&quot;&gt;MS-10&lt;/a&gt;, a failure in 2018, not quite reaching space (93km).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another one comes to mind: the failed launch of Soyouz T-10-1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this case, the rocket caught fire on the launchpad and the escape tower saved the cosmonaut by making a short hop (but not reaching space).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Buran&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;A few manned &lt;a href=&quot;https://en.wikipedia.org/wiki/Buran_programme#Atmospheric_test_flights&quot; rel=&quot;nofollow noreferrer&quot;&gt;atmospheric flight&lt;/a&gt;s were made.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The longest one lasted 36min.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;One orbital robotic flight was made.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;So we have one accidental sub-orbital flight, and few atmospheric flights. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer is &lt;strong&gt;no&lt;/strong&gt; for intentional sub-orbital flights.&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="6642" LastEditDate="2019-04-01T03:48:32.267" LastActivityDate="2019-04-01T03:48:32.267" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="23757" PostTypeId="2" ParentId="8116" CreationDate="2017-11-20T11:15:57.940" Score="1" Body="&lt;p&gt;I suggest you take a look at my answer here:&#xA;&lt;a href=&quot;https://space.stackexchange.com/a/23743/21677&quot;&gt;https://space.stackexchange.com/a/23743/21677&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I made an app that interpolates EPIC images on a 3D globe, and also displays DSCOVR and its path at any time in the past 2 years since DSCOVR started its service. It's a web app, or a mobile app (Android+iOS) if you open this link from your phone or ablet. It's free and ad-free:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://app.blueturn.earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://app.blueturn.earth&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Best,&#xA;Michael&lt;/p&gt;&#xA;" OwnerUserId="21677" LastActivityDate="2017-11-20T11:15:57.940" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23758" PostTypeId="1" AcceptedAnswerId="23853" CreationDate="2017-11-20T14:35:33.823" Score="9" ViewCount="1746" Body="&lt;p&gt;I just looked at this NASA &quot;specials&quot; page; &lt;a href=&quot;https://www.nasa.gov/specials/wheels/&quot; rel=&quot;noreferrer&quot;&gt;https://www.nasa.gov/specials/wheels/&lt;/a&gt; and watched the two videos linked below. These experimental wheels use a special stoichiometric Nickel Titanium alloy which sounds (in the video) like it is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Shape-memory_alloy&quot; rel=&quot;noreferrer&quot;&gt;shape memory alloy&lt;/a&gt;, rather than &lt;a href=&quot;https://en.wikipedia.org/wiki/Spring_steel&quot; rel=&quot;noreferrer&quot;&gt;spring steel&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The alloy metal mesh in these wheels may be useful for a future Mars rover designed to handle either more rugged terrain, or higher speeds, or possibly a heavier vehicle than current rover technology.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But does this require that operation be done in some range of temperatures? Would the wheels need to be heated regularly to recover (&quot;remember&quot;) their original shape?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, is there any possibility this technology would be used on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_2020&quot; rel=&quot;noreferrer&quot;&gt;Mars 2020 rover&lt;/a&gt; or is that design fairly well fixed now?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Read more:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;NASA Glenn Research Center: &lt;a href=&quot;https://www.nasa.gov/specials/wheels/&quot; rel=&quot;noreferrer&quot;&gt;Reinventing the Wheel&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Goodyear corporate: &lt;a href=&quot;https://corporate.goodyear.com/en-US/media/news/Goodyear-and-NASA-Invent-Spring-Tire-for-Moon-Possibly-Earth.html&quot; rel=&quot;noreferrer&quot;&gt;Goodyear and NASA invent 'Sprint tire' for Moon, Possibly Earth&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Engaget: &lt;a href=&quot;https://www.engadget.com/2017/11/24/nasa-superelastic-chainmail-tires/&quot; rel=&quot;noreferrer&quot;&gt;NASA goes back to the middle ages for its rover tire design&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;Huffington Post: &lt;a href=&quot;http://www.huffingtonpost.co.uk/entry/nasa-has-quite-literally-reinvented-the-wheel_uk_5a12ae23e4b0e97dffee4d44&quot; rel=&quot;noreferrer&quot;&gt;NASA Has Quite Literally Reinvented The Wheel&#xA;&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Credit: NASA, from &lt;a href=&quot;https://www.nasa.gov/specials/wheels/&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0aEL0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0aEL0.jpg&quot; alt=&quot;NASA chain wheel&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Credit: NASA, from &lt;a href=&quot;https://www.engadget.com/2017/11/24/nasa-superelastic-chainmail-tires/&quot; rel=&quot;noreferrer&quot;&gt;Engaget&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1DUs4.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1DUs4.jpg&quot; alt=&quot;NASA chain wheel&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The NiTi stoichiometric alloy is mentioned in Part 2:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/m3F42ttA3o8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/sCZthtgql6U?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-25T01:53:18.700" LastActivityDate="2017-11-29T19:20:51.493" Title="Are these experimental stoichiometric NiTi wire mesh wheels made from &quot;memory alloy&quot;? Need to run at certain temperature?" Tags="|mars|nasa|rovers|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23759" PostTypeId="1" AcceptedAnswerId="23760" CreationDate="2017-11-20T22:26:35.177" Score="3" ViewCount="539" Body="&lt;p&gt;In this impressive &lt;a href=&quot;https://www.space.com/38836-mars-2020-parachute-test-flight-video.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt;, between 40s and one minute, one can see white &quot;dots&quot; traveling very fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QmQji.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QmQji.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/wNwi9.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wNwi9.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are these? My guess is that the separation with the payload is done thanks to pyrotechnic means and these dots are residue from the explosion. But I'm curious if this is the correct answer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that particular example, the payload is released at a very low altitude (50 km), but I was wondering it this could be the source of micro meteoroids when done higher.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/mTAbj8aRVvg?start=30&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="15769" LastEditorUserId="12102" LastEditDate="2019-06-07T05:30:10.293" LastActivityDate="2019-06-07T05:30:10.293" Title="What are the white dots traveling fast in this video?" Tags="|debris|identify-this-object|separation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23760" PostTypeId="2" ParentId="23759" CreationDate="2017-11-20T23:09:39.873" Score="3" Body="&lt;p&gt;I believe the sparks are coming from propellant residue from the solid-fueled rocket (a Black Brant IX &lt;a href=&quot;https://www.space.com/38353-nasa-s-predawn-rocket-launch-today-will-test-a-supersonic-parachute.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;according to this article&lt;/a&gt;). You can see some similar sparks flying immediately at launch. Modern solid rocket propellant typically &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid-propellant_rocket#Composite_propellants&quot; rel=&quot;nofollow noreferrer&quot;&gt;consists of powdered aluminum and other components&lt;/a&gt; in a rubber binder; for a short period after burnout you could expect some glowing-hot particles would be shedding from the walls of the rocket motor casing.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-21T15:44:39.900" LastActivityDate="2017-11-21T15:44:39.900" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23763" PostTypeId="1" AcceptedAnswerId="23781" CreationDate="2017-11-21T03:42:33.100" Score="7" ViewCount="1143" Body="&lt;p&gt;&lt;strong&gt;EDIT:&lt;/strong&gt; I am not asking if you think finding water is necessary for a return flight. I'm asking for the scientific evidence that there is mineable water on Mars that could be used for this purpose. Please direct your answers to the question as asked. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one is inclined to write an answer about the necessity of finding mineable water on Mars for this purpose, one can always ask it as a new and separate question, and answer there.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Reviewing:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Nature Geoscience &lt;a href=&quot;https://www.nature.com/articles/s41561-017-0012-5&quot; rel=&quot;nofollow noreferrer&quot;&gt;Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;ibid. &lt;a href=&quot;https://static-content.springer.com/esm/art%3A10.1038%2Fs41561-017-0012-5/MediaObjects/41561_2017_12_MOESM1_ESM.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Supplementary data&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;NASA JPL &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?release=2017-299&quot; rel=&quot;nofollow noreferrer&quot;&gt;Recurring Martian Streaks: Flowing Sand, Not Water?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Space.com &lt;a href=&quot;https://www.space.com/38843-mars-dark-streaks-not-water.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Weird Dark Streaks on Mars May Not Be Flowing Water After All&lt;/a&gt; (a relatively concise summary)&lt;/li&gt;&#xA;&lt;li&gt;Wikipedia &lt;a href=&quot;https://en.wikipedia.org/wiki/Seasonal_flows_on_warm_Martian_slopes&quot; rel=&quot;nofollow noreferrer&quot;&gt;Seasonal flows on warm Martian slopes&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It seems that one significant body of evidence for accessible subsurface water on Mars that could be potentially used along with atmospheric CO2 to produce methalox propellants (CH4 and LOX) is substantially called into question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;THE QUESTION:&lt;/strong&gt; What is the (remaining) body of scientific evidence of potential, mineable sources of water for return trip propellant production on Mars?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;As &lt;em&gt;just one example&lt;/em&gt; of a plan to use water on Mars to produce methane, see Elon Musk's article &lt;a href=&quot;http://online.liebertpub.com/doi/full/10.1089/space.2017.29009.emu&quot; rel=&quot;nofollow noreferrer&quot;&gt;Making Humans a Multi-Planetary Species&lt;/a&gt; or the &lt;a href=&quot;https://youtu.be/tdUX3ypDVwI?t=1996&quot; rel=&quot;nofollow noreferrer&quot;&gt;video of the&lt;/a&gt; 29-Sep-2017 talk at the International Astronautical Congress (IAC) in Adelaide, Australia, starting at &lt;code&gt;33:20&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Spaceflight Now's &lt;a href=&quot;https://spaceflightnow.com/2016/09/27/spacexs-elon-musk-announces-vision-for-colonizing-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX’s Elon Musk announces vision for colonizing Mars&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The moon is not an good option because it lacks an atmosphere, has weaker gravity than Mars, and lacks resources like large reservoirs of frozen ice and carbon dioxide that could be converted into water, air and rocket propellant on the red planet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Warm Season Flows on Slope in Newton Crater (animated), from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Warm_Season_Flows_on_Slope_in_Newton_Crater_(animated).gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9RIQ2.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9RIQ2.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-02-04T00:36:47.517" LastActivityDate="2021-02-04T00:36:47.517" Title="What's the scientific evidence of water for return trip methalox on Mars?" Tags="|mars|planetary-science|water|mining|isru|" AnswerCount="6" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23764" PostTypeId="2" ParentId="23763" CreationDate="2017-11-21T04:42:12.380" Score="2" Body="&lt;p&gt;I don't think liquid water is needed.  My understanding of both NASA's and SpaceX's plan for ISRU is to use the Sabatier process to create the fuel from the Martian atmosphere.  They'd bring H2 feed-stock, and combine it with CO2 from the atmosphere in a catalytic reactor to create methane and water.  The water would then be electrolyzed into O2 (for oxidized) and H2 (to feed back into the reactor).&lt;/p&gt;&#xA;&lt;p&gt;More here: &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Sabatier_reaction#Manufacturing_propellant_on_Mars&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.m.wikipedia.org/wiki/Sabatier_reaction#Manufacturing_propellant_on_Mars&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Edit to add:&lt;/p&gt;&#xA;&lt;p&gt;The &amp;quot;bring H2 with you&amp;quot; approach was an element of the &amp;quot;Mars Direct&amp;quot; proposal from Zubrin, laid out in his 1996 book &amp;quot;The Case for Mars&amp;quot;.  This is summarized in the Wikipedia page here: &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Direct#First_launch&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Mars_Direct#First_launch&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The first flight of the Ares rocket (not to be confused with the&#xA;similarly named rocket of the now defunct Constellation program) would&#xA;take an unmanned Earth Return Vehicle to Mars after a 6-month cruise&#xA;phase, with a supply of hydrogen, a chemical plant and a small nuclear&#xA;reactor. Once there, a series of chemical reactions (the Sabatier&#xA;reaction coupled with electrolysis) would be used to combine a small&#xA;amount of hydrogen (8 tons) carried by the Earth Return Vehicle with&#xA;the carbon dioxide of the Martian atmosphere to create up to 112&#xA;tonnes of methane and oxygen. This relatively simple&#xA;chemical-engineering procedure was used regularly in the 19th and 20th&#xA;centuries,[8] and would ensure that only 7% of the return propellant&#xA;would need to be carried to the surface of Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Here's a paper form 2001 discussing it in the context of a planned-but-cancelled NASA mission: &lt;a href=&quot;http://ufdc.ufl.edu/UF00091523/00106/3j&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Sizing of a Combined Sabatier Reaction and Water Electrolysis Plant for Use in In-Situ Resource Utilization on Mars&quot;&gt;&amp;quot;Sizing of a Combined Sabatier Reaction and Water Electrolysis Plant for Use in In-Situ Resource Utilization on Mars&amp;quot;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The carbon dioxide used in the reaction will be obtained from the&#xA;Martian atmosphere, but the hydrogen used will be imported from&#xA;Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;However, it seems that this approach is now outdated, as it was formulated before we knew much about the presence of water on Mars.&lt;/p&gt;&#xA;&lt;p&gt;More recent approaches seem to be to use Martian water.  For example, there's this paper from 2017 specifically addressing the SpaceX program: &lt;a href=&quot;https://arc.aiaa.org/doi/pdfplus/10.2514/6.2017-5335&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Proposed ITS Pressurized Cargo Modules To Initiate a Chemical Industry on Mars&quot;&gt;&amp;quot;Proposed ITS Pressurized Cargo Modules To Initiate a Chemical Industry on Mars&amp;quot;&lt;/a&gt; p. 13.  This approach assumes a small amount of H2 brought from Earth, followed by more H2 extracted from Martian water,  as well as that reclaimed from the Sabatier process itself.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The H2 input may initially be brought from Earth to start up the&#xA;process, but after start-up the H2 will be derived from water&#xA;electrolysis...&lt;/p&gt;&#xA;&lt;p&gt;The input H2O will derive from a water mining&#xA;operation. Water is known to be abundant on Mars, but unlike Earth, it&#xA;occurs in solid state rather than liquid. Water mining for underground&#xA;ice would proceed by  employing soil-moving machines analogous to&#xA;strip mining equipment. Alternatively, wells may be drilled to reach&#xA;buried glacial ice, heat  and pressurize the water to liquid state,&#xA;and pump the liquid into a storage  module.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Another interesting approach would skip water altogether and generate O2 and H2 from hematite, which is abundant on the surface: &lt;a href=&quot;https://jour.space/html/issue-5-2015_en.html&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;On the in situ production of oxygen and hydrogen from martian hematite deposits via a two-step thermochemical CO2/H2O splitting process.&quot;&gt;&amp;quot;On the in situ production of oxygen and hydrogen from martian hematite deposits via a two-step thermochemical CO2/H2O splitting process.&amp;quot;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Now, opinion time:  I suspect that the very earliest missions will still use the &amp;quot;BYOH2&amp;quot; approach.  It requires only 7% of the material by mass, and is way simpler, mechanically and technologically, than mining ice or collecting hematite.  But once we have a longer-term presence (manned or unmanned), there will be a shift to other approaches.&lt;/p&gt;&#xA;" OwnerUserId="21469" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-19T13:34:40.600" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23765" PostTypeId="2" ParentId="23763" CreationDate="2017-11-21T11:02:02.350" Score="1" Body="&lt;p&gt;The strongest evidence would probably be that the &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Phoenix_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Phoenix lander&lt;/a&gt; actually dug some up, which suggests that there is enough water present to be useful for colonization.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, jgalak’s answer is also correct. Mining local ice is important for long term plans to build an independent colony, but for early missions, a pump and a hydrogen tank is much lighter and less complex than mining equipment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that hydrogen is only a small part of the fuel / oxidizer mix by mass. The reaction CH4 + 2O2 =&gt; 2H2O + CO2 uses 4 parts hydrogen to 14 carbon and 64 oxygen. Even if you bring the hydrogen from Earth, using local carbon and oxygen reduces imported fuel mass by over 95 percent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If storage is a serious problem, you can also bring water from earth for a more modest 56 percent reduction, though you can accept quite a lot of hydrogen leakage before that becomes the better option.&lt;/p&gt;&#xA;" OwnerUserId="18207" LastActivityDate="2017-11-21T11:02:02.350" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23766" PostTypeId="2" ParentId="20546" CreationDate="2017-11-21T12:17:48.577" Score="2" Body="&lt;p&gt;I think that the problem of communication could be done without needing an interstellar power source. What if the super powered telescopes of earth are used to retrieve signal from the ship, and the ships signals are how it reflects incoming beams from a high-powered emitter on earth.&#xA;Or&#xA;Since they are micro-ships, have a specific maneuver or pack pattern as a signal.&lt;/p&gt;&#xA;" OwnerUserId="19048" LastActivityDate="2017-11-21T12:17:48.577" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23767" PostTypeId="2" ParentId="23712" CreationDate="2017-11-21T15:04:32.687" Score="2" Body="&lt;p&gt;They have been observed by astronauts according to &lt;a href=&quot;http://www.sciencedirect.com/science/article/pii/S1364682613001508&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt; and &lt;a href=&quot;http://www.esa.int/Our_Activities/Human_Spaceflight/iriss/Blue_jets_studied_from_Space_Station&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;. Both articles do not make a note on wether the astronauts could see the TLEs without aid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second article also mentions the &lt;a href=&quot;http://www.asim.dk/payload.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;ASIM&lt;/a&gt;, which is a dedicated sensor to permanently look out for this phenomena. It is planned to launch in 2018 on a Falcon 9.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Red sprites can be observed with the naked eye, blue jets are very short-lived and are unlikely to be seen without aid according to &lt;a href=&quot;https://weather.com/news/news/transient-luminous-events-mysteries-sky-20130731&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;. However, like in the other sources, no mention of an unaided sighting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This work does mention sightings by pilots. It can reasonably be assumed, they where unaided:&#xA;Vaughan, O. H., and B. Vonnegut, Recent observations of lightning discharges from the top of a thundercloud into the clear air above, J. Geophys. Res., 94(D11), 13,179-12,182, 1989. &lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="12712" LastEditDate="2017-11-21T20:29:05.627" LastActivityDate="2017-11-21T20:29:05.627" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23768" PostTypeId="2" ParentId="22608" CreationDate="2017-11-21T15:27:37.273" Score="3" Body="&lt;p&gt;According to &lt;a href=&quot;https://books.google.de/books?id=rrdVPtCNL9AC&amp;amp;pg=PA253&amp;amp;lpg=PA253&amp;amp;dq=cosmonauts%20suicide%20pills&amp;amp;source=bl&amp;amp;ots=5RQUED5drc&amp;amp;sig=nN-WmVzCTNCs19zUxhmG9LoGFUk&amp;amp;hl=de&amp;amp;sa=X&amp;amp;ved=0ahUKEwiutJW5-s_XAhWCyKQKHXipC3M4ChDoAQhPMAQ#v=onepage&amp;amp;q=cosmonauts%20suicide%20pills&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;The First Soviet Cosmonaut Team: Their Lives and Legacies&lt;/a&gt;, in the book The Rocket Men, Leonov is said to have revealed, that he indeed had a pill in the helmet. His space walk was a close call according to the account given. The original source is not available, at least not online. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a side note, U2 pilots where given suicide devices, so it is not too far fetched, that astronauts carried them as well.&lt;/p&gt;&#xA;" OwnerUserId="12712" LastActivityDate="2017-11-21T15:27:37.273" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23769" PostTypeId="1" CreationDate="2017-11-21T16:17:30.223" Score="2" ViewCount="507" Body="&lt;p&gt;Usually, when there is a &quot;classified&quot; payload, there are hints as to what it would be. Why is it different on this mission? Why is no government entity taking credit for it?&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="12102" LastEditDate="2020-06-05T00:58:30.863" LastActivityDate="2020-06-05T00:58:30.863" Title="What kind of payload is Mission 1390 (Zuma) going to launch?" Tags="|spacex|launch|classified|zuma|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23770" PostTypeId="2" ParentId="23769" CreationDate="2017-11-21T18:09:07.043" Score="5" Body="&lt;p&gt;There's really no way to know. There are a number of articles out there speculating, but there isn't a lot really known. What we do know is it was manufactured by Northrup Grumman, will enter Low Earth Orbit with an estimated inclination of 40-60 degrees, and was payed for by an unnammed government agency. There are 2 launches in recent memory that have followed similar conventions. Of these, one of them was later determined to be NSA (I believe I read, can't find the source). It does not belong to the NRO (According to them), who usually launches top secret US satellites. See &lt;a href=&quot;http://www.wearethemighty.com/news/spacex-launching-a-third-top-secret-satellite&quot; rel=&quot;noreferrer&quot;&gt;this article&lt;/a&gt; for more information.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I had to guess, it belongs either to the NSA, CIA, or some not very well known government agency. The purpose could really be anything, from proof of concept to spying to communications, or perhaps more. They probably aren't taking credit because the first thing that anyone will have problems with is identifying who is using it, which makes it even more difficult to figure out what the purpose is. With NRO missions, one knows they are some kind of spy satellites, these are less well known. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-11-21T18:20:31.363" LastActivityDate="2017-11-21T18:20:31.363" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23771" PostTypeId="2" ParentId="23763" CreationDate="2017-11-21T18:44:05.513" Score="1" Body="&lt;p&gt;There are two possibilities. The first, proposed by Robert Zubrin in the Case for Mars, is as has been suggested, bring a feedstock of Hydrogen, and use it to make the fuel. Hydrogen is by far the lightest part of a methalox system, and could easily be included. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second is using liquid water to make the hydrogen. There's a &lt;a href=&quot;https://en.wikipedia.org/wiki/Water_on_Mars#Present_water_ice&quot; rel=&quot;nofollow noreferrer&quot;&gt;large number of sources&lt;/a&gt; that support that there is liquid water near the surface of Mars. The following image shows a rough distribution of the water on Mars near the surface, based on neutron flux data. The poles have a much higher portion that is water ice. These have been partially confirmed by the landers on the surface. One of the key objective of any lander is to provide ground truth to help verify orbital observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, we could either bring hydrogen (Likely for the first missions), or look for water ice to use, but either will likely get us what we need to make rocket fuel to get home.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/b/b0/Water_equivalent_hydrogen_abundance_in_the_lower_latitudes_of_Mars_01.jpg/1200px-Water_equivalent_hydrogen_abundance_in_the_lower_latitudes_of_Mars_01.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/thumb/b/b0/Water_equivalent_hydrogen_abundance_in_the_lower_latitudes_of_Mars_01.jpg/1200px-Water_equivalent_hydrogen_abundance_in_the_lower_latitudes_of_Mars_01.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-11-21T18:44:05.513" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23773" PostTypeId="2" ParentId="23769" CreationDate="2017-11-22T03:14:14.657" Score="3" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; apparently there will indeed be a Zuma launch:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://spaceflightnow.com/2018/01/02/falcon-9-zuma-mission-status-center-2/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://spaceflightnow.com/2018/01/02/falcon-9-zuma-mission-status-center-2/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://arstechnica.com/science/2018/01/spacex-will-try-to-launch-the-mysterious-zuma-satellite-again-thursday/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://arstechnica.com/science/2018/01/spacex-will-try-to-launch-the-mysterious-zuma-satellite-again-thursday/&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Some speculate there is no spacecraft for the Zuma mission to begin with.&lt;/p&gt;&#xA;&lt;p&gt;From Florida Today's article &lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/2017/11/20/spacex-launch-secret-zuma-mission-hold-until-after-thanksgiving/880972001/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX launch of secret Zuma mission on hold until after Thanksgiving&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Some have gone as far as to suggest that there really is no Zuma mission — that the &lt;strong&gt;pre-launch preparations were an exercise to prove SpaceX's ability to fly on short notice&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;p&gt;SpaceX put a Falcon 9 on the pad at KSC and test-fired its main engines on Nov. 11. The rocket went vertical again before last week's planned launch attempts, but has since returned to its hangar.&lt;/p&gt;&#xA;&lt;p&gt;Under that scenario, the payload fairing tests SpaceX cited &lt;strong&gt;might merely be an excuse to stand down from a launch that was never really going to happen&lt;/strong&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;p&gt;Time will tell if SpaceX confirms a new launch date for the mission after the holiday.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-03T03:55:54.840" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23774" PostTypeId="2" ParentId="22608" CreationDate="2017-11-22T03:34:28.423" Score="27" Body="&lt;p&gt;I went on a quest to try and get the sources for all the citations for that article; I was partially successful. I have FOIA requests out for the documents I was not able to find on the internet, and thanks to a fantastic librarian at UC San Diego I got a scan of the document that I think spurred this.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;“The First Egress of Man Into Space” (NASA TT F-9727), by Alexei Leonov, translation of “Pervyy vykhod chelovka v kosmicheskoye prostranstvo,” presented at the XIVth International Astronautics Congress, Athens, September 13-18, 1965&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does not contain any information relevant to the technical aspects of the mission or any information relevant to the use of suicide pills&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;“Orbital Castling: Life of Mir Station Will Be Prolonged Three More Years,” Segodnya in Russian, May 11, 1995, p. 9. Translated in JPRS Report, Central Eurasia. Space, August 2, 1995 (FBIS-UST-95-030), p. 20&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;This is a translation of an article in the Moscow SEGODNYA (Moscow Today) by Mikhail Chernyshov&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Despite the fact that the type of work done in open space this year has been carried out for three decades, each new space walk invariably brings its surprises. Aleksey Leonov during his first &quot;five meter&quot; walk almost got stuck in the hatch upon return to the airlock. Several methods were worked out to assist the cosmonaut. If nothing had assisted Leonov, not entering into the airlock, he would have been transformed into a &quot;living satellite&quot;; the last recourse would have been &quot;self liquidation&quot; by means of an ampul or tablet similar to those which have been supplied to professionals of SMERSH and other espionage services. The cosmonaut decided to tell about this quite recently. This delicate detail, judging from everything, belongs among the greatest medical secrets. In any case, until recently not even Flight Control specialists knew about this.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&lt;em&gt;(Relevant passage, the whole document can be found &lt;a href=&quot;https://www.dropbox.com/s/fr10036f3nu8lp9/FBIS-95-030.pdf?dl=1&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;)&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;“Man In Open Space,” Soviet Military Review interview, March, 1980, pp. 14-17. In USSR Report: Space, No. 11 (JPRS 78264), June 10, 1981, p. 19&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Thanks to the help of another amazing Librarian I was able to get a copy of this article (you can find it &lt;a href=&quot;https://www.dropbox.com/s/uaz2iavlh8j4gof/updated_JPRS%20Report%2078264.pdf?dl=1&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;), it does not make any mention of a suicide pill during a spacewalk&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;“The Friendly Solar Wind,” Komsomol’skaya Pravda in Russian, Alexei Leonov, March 8, 1983, p. 4. Translated in USSR Report: Space, No. 24 (JPRS 84161), August 22, 1983, pp. 11-13 (&lt;a href=&quot;http://handle.dtic.mil/100.2/ADA355243&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Does not contain any information relevant to the technical aspects of the mission mostly just about his experience&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;“Voskhod 2 Flight Recalled,” Spaceflight, June 1990, p. 193; Astronautics and Aeronautics, 1965, NASA SP 4006, 1966, pp. 132, 138. (&lt;a href=&quot;https://history.nasa.gov/AAchronologies/1965.pdf&quot; rel=&quot;noreferrer&quot;&gt;source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Some technical information about the orbit and the spacewalk itself&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I think that this idea was most likely spurred initially by the article &quot;Orbital Castling: Life of Mir Station Will Be Prolonged Three More Years&quot; I am continuing to try and find the source for the statement in the article, but based on the amount of copy-pasting references that can be found by searching for that document I think that all the current texts that are out there can be traced back to that article.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also purchased &quot;Two Sides of the Moon: Our Story of the Cold War Space Race&quot; a dual autobiography by Alexei Leonov and David Scott, in the book there was no mention of the suicide pill.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to a lack of primary sources mentioning it, and the only reference being a newspaper that alluded to another source I would say that it is implausible that it really happened.&lt;/p&gt;&#xA;" OwnerUserId="18663" LastEditorUserId="18663" LastEditDate="2018-11-05T18:26:53.783" LastActivityDate="2018-11-05T18:26:53.783" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="23775" PostTypeId="1" CreationDate="2017-11-21T18:19:45.437" Score="0" ViewCount="200" Body="&lt;p&gt;You may imagine that having an upper fuel tank still having heavy liquid inside while landing, as today some reusable rockets do in a vertical attitude, may ease the machine falling on its side. Is this a practical concern, and is this concept applied in actual landings of rockets? Having the weight in the low part of the cylinder will help stability. Will it?&lt;/p&gt;&#xA;" OwnerUserId="20053" OwnerDisplayName="Urquiola" LastActivityDate="2017-11-23T15:11:19.590" Title="Is it necessary for a vertical landing rocket to have the upper fuel tank empty?" Tags="|landing|rockets|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23777" PostTypeId="1" AcceptedAnswerId="23807" CreationDate="2017-11-22T10:30:56.363" Score="18" ViewCount="1256" Body="&lt;p&gt;When performing space rendezvous operations close to the target vehicle (0-5 m relative distance), pursuer thruster escape nozzle gases can hit the target if firings are done in the direction of docking axis. Eventually, this firings could damage the target surfaces (by heating or erosion), or provoke some attitude and orbit changes of the target (slightly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0HqkP.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0HqkP.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: how current and past space vehicles with rendezvous capabilities (Soyuz, ATV, Dragon, Space Shuttle,etc) have deal with this issue since following an R-bar or V-bar rendezvous requires to perform braking firings when the pursuer is very close to the target?. Are target surfaces (ISS,etc) designed to cope with this plumes?. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Figure extracted from: &quot;Fast, Safe, and Propellant-Efficient Spacecraft Planning under Clohessy-Wiltshire-Hill Dynamics&quot;&lt;/p&gt;&#xA;" OwnerUserId="21262" LastActivityDate="2017-11-25T00:08:31.997" Title="How important is plume impingement in rendezvous operations?" Tags="|spacecraft|propulsion|rendezvous|astrodynamics|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23778" PostTypeId="1" CreationDate="2017-11-22T15:27:43.780" Score="1" ViewCount="243" Body="&lt;p&gt;Suppose that during the boost phase a Falcon 9 lost enough engines that it could not deploy its payload into a useful orbit or trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there some scenarios where it could conceivably return to Earth with it's presumably very expensive or valuable payload intact, rather than deploy into a useless orbit or put it in the ocean?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the purposes of this question assume that there is both a downrange landing ship in the ocean and the pad on land is also available and ready. Also assume there is clearance from government agencies, the insurers, customer(s) etc, and that it's really worth it to bring this particular payload back rather than scrap it and build another.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One abstracted, hypothetical example might be that the payload is &quot;the last RTG on Earth&quot; because there's no current replacement supply for any feasible radioisotope source.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also assume that the rocket has sufficient programming to recalculate this dramatically different trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If everyone is &quot;on board&quot; with this, is it possible? Are there any show-stoppers for the current Falcon-9? Below about 110 km the fairing should still be intact, but would that even be necessary?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-11-22T18:27:42.260" Title="Could a Falcon 9 booster return to Earth with its payload safely in an emergency?" Tags="|spacex|falcon-9|landing|" AnswerCount="1" CommentCount="2" ClosedDate="2017-11-23T09:13:06.947" ContentLicense="CC BY-SA 3.0" />
  <row Id="23780" PostTypeId="2" ParentId="23778" CreationDate="2017-11-22T16:22:14.397" Score="1" Body="&lt;p&gt;No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper stage, full of fuel, with a payload on top of it, is heavier than the entire first stage it would be incredibly top heavy, significantly longer and worse all the weight at the far end of the pendulum that it could never control the stack to land. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spaceflightinsider.com/hangar/falcon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceFlight Insider&lt;/a&gt; has the empty masses as:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First Stage Dry Mass = 25,600kg&#xA;Second Stage Dry Mass = 3,900KG&#xA;Second Stage Prop Mass = 92,670KG&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So a second stage, ignoring payload and fairing mass, is close to 4 times the mass of the entire empty first stage. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;While attached to the first stage, you could not vent the fuel, which takes about 8 minutes when using the engines main turbo pumps, to dump the fuel by burning it up.  Hard to imagine a faster way to vent the fuel in the case of a rocket. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-11-22T18:27:42.260" LastActivityDate="2017-11-22T18:27:42.260" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23781" PostTypeId="2" ParentId="23763" CreationDate="2017-11-22T16:37:03.307" Score="8" Body="&lt;p&gt;There is an interesting podcast that I listen to - We Martians.  Last Nov they had an episode that touches heavily on this.  The episode is here: &lt;a href=&quot;http://www.wemartians.com/home/015&quot; rel=&quot;noreferrer&quot;&gt;http://www.wemartians.com/home/015&lt;/a&gt; and it goes into far more detail than I can, but here's a brief summary:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SHARAD (SHAllow RADar) instrument on the Mars Reconnaissance Orbiter used ground penetrating radar on the area of Mars known as Utopia Planitia.  The radar results, while not definitive, strongly suggest &quot;a mixture of ice, air, and dust&quot; in the area.  &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1002/2016GL070138/abstract&quot; rel=&quot;noreferrer&quot; title=&quot;SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars&quot;&gt;&quot;SHARAD detection and characterization of subsurface water ice deposits in Utopia Planitia, Mars&quot;&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This information was presented in context with info from the Phoenix lander. The lander was equipped with a robot arm which dug in the soil.  This article describes it further: &lt;a href=&quot;https://www.space.com/6918-dirt-mars-lander-soil-findings.html&quot; rel=&quot;noreferrer&quot; title=&quot;The Dirt on Mars Lander Soil Findings&quot;&gt;&quot;The Dirt on Mars Lander Soil Findings&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Using its robotic arm, Phoenix dug into the Martian surface to see if&#xA;  it could reach the ice below.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;In one trench, dubbed &quot;Dodo-Goldilocks,&quot; the lander exposed what&#xA;  mission scientists described as &quot;bright material&quot; about 4 to 5&#xA;  centimeters below the surface (a similar patch was seen below the&#xA;  lander itself, likely exposed by the spacecraft's landing thrusters).&#xA;  Over the next two months, the team watched as Phoenix's cameras showed&#xA;  that the material was sublimating away, which would be expected of&#xA;  water ice exposed to the Martian atmosphere.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The tendency of soil samples scooped up by the lander to clump&#xA;  together made it difficult to get the samples into Phoenix's onboard&#xA;  instruments, but after several attempts, a sample was coaxed in and&#xA;  the lander's detectors confirmed that there was indeed water ice&#xA;  hiding under the regolith. The confirmation was originally announced&#xA;  on July 31.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Interestingly, the ice seems to occur at different depths under the&#xA;  surface depending on the terrain. The plains where Phoenix landed&#xA;  feature polygonal mounds surrounded by troughs that result from the&#xA;  seasonal expansion and contraction of the ice underneath the surface,&#xA;  which creates cracks and crevices.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And from this related article: &lt;a href=&quot;https://www.space.com/5686-water-ice-mars-confirmed.html&quot; rel=&quot;noreferrer&quot; title=&quot;Water Ice on Mars Confirmed&quot;&gt;&quot;Water Ice on Mars Confirmed&quot;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NASA's Phoenix Mars Lander has confirmed the existence of water ice on&#xA;  Mars.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Mission scientists celebrated the news after a sample of the ice was&#xA;  finally delivered to one of the lander's instruments.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;later in the article:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;I'm very happy to announce that we've gotten an ice sample,&quot; said the&#xA;  University of Arizona's William Boynton, co-investigator for Phoenix's&#xA;  Thermal and Evolved-Gas Analyzer (TEGA), which heats up samples and&#xA;  analyzes the vapors they give off to determine their composition.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;&quot;We have water,&quot; Boynton added. &quot;We've seen evidence for this water&#xA;  ice before in observations by the Mars Odyssey orbiter and in&#xA;  disappearing chunks observed by Phoenix last month, but this is the&#xA;  first time Martian water has been touched and tasted.&quot;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The news that ice had fallen into TEGA came on Thursday morning,&#xA;  surprising scientists who had run into problems delivering a sample of&#xA;  the icy dirt because of its unexpected stickiness.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Opinion:  I'm not sure any of this is 100% definitive proof of there being sufficient mineable water for producing Methalox, but it does strongly suggest that at least in this part of the planet there is significant water just below the surface. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Future missions will study this issue as well.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Trace Gas Orbiter may provide more information once it's fully online - it has the ability to detect neutrons&lt;a href=&quot;http://exploration.esa.int/mars/48523-trace-gas-orbiter-instruments/?fbodylongid=2217&quot; rel=&quot;noreferrer&quot; title=&quot;FREND - Fine Resolution Epithermal Neutron Detector&quot;&gt;4&lt;/a&gt; from sub-surface hydrogen (which may or may not be water) and trace H2O in the atmosphere&lt;a href=&quot;http://exploration.esa.int/mars/48523-trace-gas-orbiter-instruments/?fbodylongid=2216&quot; rel=&quot;noreferrer&quot; title=&quot;ACS - Atmospheric Chemistry Suite&quot;&gt;5&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ExoMars rover and NASA's Rover 2020 will both carry instruments that can contribute to this.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ExoMars Rover, specifically, will be carrying the WISDOM ground-penetrating radar which should be able to see subsurface ice&lt;a href=&quot;http://exploration.esa.int/mars/45103-rover-instruments/?fbodylongid=2128&quot; rel=&quot;noreferrer&quot; title=&quot;WISDOM - Water Ice and Subsurface Deposit Observation on Mars&quot;&gt;6&lt;/a&gt;, and it's core drill is equipped with Ma-MISS, an infrared spectrometer that should be able to determine the presence of water or ice in the drill site&lt;a href=&quot;http://exploration.esa.int/mars/45103-rover-instruments/?fbodylongid=2133&quot; rel=&quot;noreferrer&quot; title=&quot;Ma_MISS - Mars Multispectral Imager for Subsurface Studies&quot;&gt;7&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA's Rover 2020 will also carry the RIMFAX ground penetrating radar&lt;a href=&quot;https://mars.nasa.gov/mars2020/mission/instruments/rimfax/&quot; rel=&quot;noreferrer&quot; title=&quot;RIMFAX&quot;&gt;8&lt;/a&gt;, as well as other instruments for chemical analysis - I am not sure if any of them are specifically aimed at ice detection.  Of related interest is the MOXIE experiment, which will attempt to extract O2 from the martian atmosphere.&lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Oxygen_ISRU_Experiment&quot; rel=&quot;noreferrer&quot; title=&quot;Mars Oxygen ISRU Experiment&quot;&gt;9&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21469" LastEditorUserId="21469" LastEditDate="2017-11-23T01:43:58.127" LastActivityDate="2017-11-23T01:43:58.127" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23785" PostTypeId="1" AcceptedAnswerId="23786" CreationDate="2017-11-23T03:46:27.007" Score="21" ViewCount="11932" Body="&lt;p&gt;As far as I understand, when we view the ISS, it is because the sunrays get reflected from the solar panels of the ISS and reach us at the appropriate angle. So that would be equivalent to viewing the Sun through a mirror except that solar panels are not 100% reflecting mirrors.&lt;/p&gt;&#xA;&lt;p&gt;So, is it safe to watch the ISS with the naked eye on a regular basis?&lt;/p&gt;&#xA;" OwnerUserId="21725" LastEditorUserId="47149" LastEditDate="2022-05-11T00:49:59.233" LastActivityDate="2022-05-11T00:49:59.233" Title="Is it safe to observe the ISS with the naked eye?" Tags="|iss|safety|observation|" AnswerCount="5" CommentCount="23" ContentLicense="CC BY-SA 4.0" />
  <row Id="23786" PostTypeId="2" ParentId="23785" CreationDate="2017-11-23T04:04:37.867" Score="47" Body="&lt;p&gt;It's safe. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS is about 100 meters across at its widest, and it's 400km away; &lt;a href=&quot;http://www.1728.org/angsize.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this calculator tells me&lt;/a&gt; that makes an angular size of ~0.014 degrees.  The sun's angular size is about 0.53 degrees. If both the ISS and the sun were square, the area ratio would be some 1400:1; the aspect ratio of the ISS is in roughly the ballpark of π/4, so the non-squarenesses pretty much cancel out, and you'd expect a &lt;em&gt;perfectly&lt;/em&gt; reflecting ISS to hit your eye with something like 1/1400 the brightness of the sun. As noted in other answers here, the ISS is nowhere near perfectly reflecting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe the reflected light from the ISS could still be damaging if it was focused on the same part of your retina for a long duration, but there are two things protecting you: first, the ISS travels across the sky at a pretty good clip, and second, even if you try and track ISS with your eyeball, your eye's involuntary &lt;a href=&quot;https://en.wikipedia.org/wiki/Saccade&quot; rel=&quot;nofollow noreferrer&quot;&gt;saccades&lt;/a&gt; keep the image of the ISS moving around your retina rapidly, spreading the received light energy.  &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-26T18:30:12.887" LastActivityDate="2017-11-26T18:30:12.887" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23787" PostTypeId="2" ParentId="23785" CreationDate="2017-11-23T07:08:43.963" Score="16" Body="&lt;p&gt;In addition to Russel's answer:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS' albedo is far lower than 1, so the brightness is going to be less than 1/1400. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Those solar panels convert 30% of the sunlight into electricity, and another part into heat. The &lt;a href=&quot;https://www.solarchoice.net.au/blog/solar-panels-near-airports-glare-issue/&quot; rel=&quot;nofollow noreferrer&quot;&gt;amount of light reflected&lt;/a&gt; is only about 20% (less than the reflection coefficient of e.g. grass). &lt;/li&gt;&#xA;&lt;li&gt;these specific solar panels are not flat. They're flexible, so the surface is going to be pointing in different directions. This means the reflected light will be spread over a larger area, reducing intensity.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;For the ISS to be dangerous to look at, it'd have to be as bright as the sun, meaning it'd be visible during the day instead of just another star at night. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same goes for other objects too. The Moon is far larger than the ISS, but it's safe to look at because it's not a perfect mirror. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a closeup of one of the arrays: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4XhS8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4XhS8.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the lower section you can see there's a zigzag pattern. The upper should be zigzagging as well but was damaged during a deployment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not visible in this image: I also get the impression the end of the array is not always perfectly aligned with the base of the array: the telescoping truss allows a certain amount of rotation between the ends. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="6473" LastEditDate="2017-11-26T14:21:24.970" LastActivityDate="2017-11-26T14:21:24.970" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23788" PostTypeId="2" ParentId="23785" CreationDate="2017-11-23T09:43:48.470" Score="28" Body="&lt;h1&gt;No problem, even with a telescope&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Not only is it perfectly safe with the naked eye, it's also not a problem at all with a large aperture telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first, I tried looking at the ISS with a 80-200mm f/2.8 Nikkor attached to a &lt;a href=&quot;https://nikonrumors.com/2011/10/20/how-to-convert-your-nikon-lens-into-a-telescope-or-a-microscope-nikon-lens-scope-converters.aspx/&quot; rel=&quot;noreferrer&quot;&gt;scope converter&lt;/a&gt;. Handheld, it was pretty hard to find the ISS at all, let alone track it for more than 0.1s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/mKoZd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mKoZd.jpg&quot; alt=&quot;Small dobsonian&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I then bought a small dobsonian telescope (&lt;a href=&quot;https://www.telescope.com/Orion-SkyQuest-XT45-Classic-Dobsonian-Telescope/p/102009.uts&quot; rel=&quot;noreferrer&quot;&gt;Orion XT4.5&lt;/a&gt;). With a bit of patience and practice, I could watch the ISS at 135x magnification for a few seconds. It helps to have an assistant looking through the finderscope. If the ISS goes too far away from the view through the eyepiece, it can be really hard to find it again alone. The assistant can move the telescope back on track and you can then finetune the movement while looking through the eyepiece. The dobsonian mount makes it relatively easy to track the ISS while still maintaining a stable view in the eyepiece. It's tricky at first but very rewarding!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a clear sky and ISS passing close to our location, I could perfectly see its &lt;strong&gt;H&lt;/strong&gt; shape. The ISS in itself was never bright enough to be dangerous for the eyes. It can cause glare with a low magnification though (e.g. 20mm eyepiece for a 45x magnification). It's obviously much easier to track the ISS but it requires either a &lt;a href=&quot;https://www.telescope.com/125-Orion-13-Transmission-Moon-Filter/p/5662.uts&quot; rel=&quot;noreferrer&quot;&gt;moon filter&lt;/a&gt; or sunglasses to better see the details.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It helps a lot to have a good eyepiece with a large apparent field of view. I used an &lt;a href=&quot;https://explorescientificusa.com/products/82-6-7mm&quot; rel=&quot;noreferrer&quot;&gt;Explore Scientific 82° 6.7mm&lt;/a&gt;. This way, the ISS stays much longer in the field of view and it's easier to track.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DqWBr.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DqWBr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last year, I bought a larger telescope : a 10-inch &lt;a href=&quot;https://www.cloudynights.com/articles/cat/user-reviews/zhumell-z10-10-dobsonian-review-r2505&quot; rel=&quot;noreferrer&quot;&gt;Zhumell Z10&lt;/a&gt; dobsonian. Looking at a full moon with this aperture is painfully bright. The telescope is brighter, it supports higher magnifications, it shows more detail and has a smoother rotation than the smaller one. Still, the ISS was bright but not too bright with it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a really good night last July, I could count the 16 photovoltaic arrays and see the whole middle structure with distinct modules. That really was an incredible sight!&lt;/p&gt;&#xA;" OwnerUserId="19229" LastEditorUserId="19229" LastEditDate="2017-11-27T07:41:01.577" LastActivityDate="2017-11-27T07:41:01.577" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23789" PostTypeId="1" CreationDate="2017-11-23T12:21:48.040" Score="3" ViewCount="1238" Body="&lt;p&gt;I know that the Merlin 1D is used in the Falcon first stage, and the Merlin 1D Vacuum in the second stage, but I can't find any information as to why the nozzle has to be bigger in vacuum.&lt;/p&gt;&#xA;&lt;p&gt;I guess that a bigger nozzle increases the area ratio but what does that do? Does it increase thrust? Or does it increase velocity (if that is any different)? Or does it do something else?&lt;/p&gt;&#xA;" OwnerUserId="21731" LastEditorUserId="47149" LastEditDate="2022-06-04T04:59:58.143" LastActivityDate="2022-06-04T04:59:58.143" Title="Why is the Merlin 1D Vacuum nozzle bigger than the Merlin 1D nozzle?" Tags="|spacex|engine-design|nozzle|stages|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="23790" PostTypeId="2" ParentId="23789" CreationDate="2017-11-23T13:01:00.623" Score="9" Body="&lt;p&gt;For maximum thrust, the pressure of the exhaust at the &quot;exit plane&quot; (the end of the nozzle) should be equal to the ambient air pressure it's exhausting into. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The larger the area of the nozzle's cross section, the lower the pressure of the exhaust stream at that point. Intuitively, if the exhaust is at higher pressure than the air when it leaves the nozzle, you would be able extract a little more work from it by containing it in a slightly bigger nozzle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, when exhausting into vacuum -- near-zero pressure -- you should be able to expand the exhaust stream nearly infinitely and still get additional thrust. However, you very rapidly reach the point where the thrust gains are canceled out by the additional mass of the nozzle, and the nozzle has to be limited in diameter to the size of the rocket in any case, so vacuum engines remain finite in size. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ratio between the area of the nozzle throat and the end of the nozzle determines the pressure drop from the combustion chamber pressure to the exit pressure. Typical first-stage engines which have to exhaust into sea-level air pressure commonly have an area ratio of 10:1-20:1. Engines designed for vacuum operation commonly have ratios above 50:1; the version of the &lt;a href=&quot;https://en.wikipedia.org/wiki/RL10&quot; rel=&quot;noreferrer&quot;&gt;RL10&lt;/a&gt; used on the Delta IV second stage has a ratio of 280:1!&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="51" LastEditDate="2017-11-23T13:04:42.887" LastActivityDate="2017-11-23T13:04:42.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23791" PostTypeId="1" AcceptedAnswerId="23977" CreationDate="2017-11-23T13:20:42.283" Score="18" ViewCount="1998" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Flyby_anomaly&quot; rel=&quot;noreferrer&quot;&gt;Flyby Anomaly&lt;/a&gt; is a name given to an unexplained, unexpected difference between the best theoretical calculation of a change in several different spacecraft's velocities due to flyby (&lt;a href=&quot;https://en.wikipedia.org/wiki/Gravity_assist&quot; rel=&quot;noreferrer&quot;&gt;Gravity Assist&lt;/a&gt;) maneuvers, and the best analysis of the measured changes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it's summarized in the linked Wikipedia article, and a long list of possible explanations is given, is there at least some movement towards a consensus on the source of the discrepancy? Or has it been (mostly) explained and the Wikipedia article needs to be updated?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Or is this still, in 2017, a &lt;em&gt;credible, outstanding unexplained phenomenon in spaceflight?&lt;/em&gt; Are there any plans to try to detect or reproduce this anomaly with future space missions that include flybys?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also Phys.org's &lt;a href=&quot;https://phys.org/news/2014-09-anomaly-satellite-flybys-confounds-scientists.html&quot; rel=&quot;noreferrer&quot;&gt;An anomaly in satellite flybys confounds scientists&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Below:&lt;/strong&gt; From &lt;a href=&quot;http://issfd.org/2015/files/downloads/papers/153_Jouannic.pdf&quot; rel=&quot;noreferrer&quot;&gt;The Flyby Anomaly: An Investigation into Potential Causes&lt;/a&gt;. Note the anomalous difference $\Delta$ is much larger than the estimated error $\sigma$ for flyby's &lt;em&gt;before about 2005&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7ASJ6.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7ASJ6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="12102" LastEditDate="2018-02-14T04:12:20.157" LastActivityDate="2022-01-20T09:29:00.110" Title="Is the Flyby Anomaly still a thing?" Tags="|orbital-mechanics|physics|flyby|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23792" PostTypeId="1" CreationDate="2017-11-23T13:56:28.713" Score="6" ViewCount="548" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L61K4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L61K4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This picture caused a lot of laughter and mocking, when it happened. I've since noticed this sign in many pictures of flight hardware.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It has been my understanding, that satellites are built in cleanrooms anyway. As this memento and others seem to suggest, not everything is assembled in a cleanroom. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if they don't need the cleanroom, why are there still signs not to touch anything? Most of it should be aluminum, so there is no concern for corrosion, is there?&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="25" LastEditDate="2018-03-14T19:35:51.777" LastActivityDate="2018-03-14T19:47:51.553" Title="Satellite manufacture and cleanliness" Tags="|artificial-satellite|manufacturing|equipment|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23793" PostTypeId="1" CreationDate="2017-11-23T14:41:12.217" Score="0" ViewCount="145" Body="&lt;p&gt;For class I have an orbit around Mars at an inclination of 63.4 deg and a base site of 55 deg North and was wondering how to work out the communication window, using orbital mechanics. &#xA;Orbital parameters are: a = 20423.45km, e = 0.8, T = 24.62 hrs.&#xA;Any advice would be helpful, thanks.&lt;/p&gt;&#xA;" OwnerUserId="21734" LastEditorUserId="6537" LastEditDate="2018-01-06T00:20:55.733" LastActivityDate="2018-01-06T00:20:55.733" Title="How to determine an available planetary communication window using orbital mechanics?" Tags="|mars|artificial-satellite|communication|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23794" PostTypeId="2" ParentId="23775" CreationDate="2017-11-23T15:11:19.590" Score="1" Body="&lt;p&gt;No, it is not necessary and in fact impossible for a number of reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the mission allows for the first stage to return, the proper amount of fuel is calculated to achieve the landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore the engines are throttled down to 60-40% of the thrust. This requires a certain minimum pressure for the turbo pumps to work, so the tanks can never be totally empty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper tank is the oxygen tank and makes up about 2/3 of the stage, so when there is not much fuel left, it is still located in the lower third of the rocket. If the oxygen tank was empty, the engines would not work anymore.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing computer for the first stage calculates the return path to the launch pad and accounts for the fuel left. You can read more about that in a &lt;a href=&quot;https://qz.com/915702/the-spacex-falcon-9-rocket-you-see-landing-on-earth-is-really-a-sophisticated-flying-robot/&quot; rel=&quot;nofollow noreferrer&quot;&gt;nice article from qz&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nine engines and the octaweb mounting structure have a much larger weight and are located below the mounting point of the legs. This makes the rocket very stable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once the rocket is down, SpaceX has been observed to either weld it to the landing ship or tie it down. There are also efforts under way to use a robot for this.&lt;/p&gt;&#xA;" OwnerUserId="12712" LastActivityDate="2017-11-23T15:11:19.590" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23795" PostTypeId="2" ParentId="23785" CreationDate="2017-11-23T18:33:35.213" Score="5" Body="&lt;p&gt;First, the assumption that we can see ISS because its solar panels reflect the sun, like a mirror does, is &lt;strong&gt;incorrect&lt;/strong&gt;. Some satellites, most notably Iridium, are known for &lt;a href=&quot;https://en.wikipedia.org/wiki/Specular_reflection&quot; rel=&quot;noreferrer&quot;&gt;specular reflections&lt;/a&gt;, called &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_flare&quot; rel=&quot;noreferrer&quot;&gt;flares&lt;/a&gt;, but I haven't seen any reports of ISS doing this. In the best case it is an extremely rare event and I haven't never observed it, despite seeing ISS occasionally.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, now that we know that ISS is more like a &lt;a href=&quot;https://en.wikipedia.org/wiki/Diffuse_reflection&quot; rel=&quot;noreferrer&quot;&gt;regular everyday object illuminated by the sun&lt;/a&gt; rather than a mirror, the answer is obvious:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Can we safely see sunlit objects with the naked eye? Yes, we can.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Yes, it would be safe with a mirrory sun reflecting ISS, too, but that's a different question!)&lt;/p&gt;&#xA;" OwnerUserId="6522" LastActivityDate="2017-11-23T18:33:35.213" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23797" PostTypeId="1" AcceptedAnswerId="23800" CreationDate="2017-11-24T07:22:15.680" Score="3" ViewCount="988" Body="&lt;p&gt;To me it seems surprising, frightening, and sadly typical that before humans set foot on a celestial body, they plan to nuke it (&lt;a href=&quot;https://en.wikipedia.org/wiki/Project_A119&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Project_A119&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;&lt;/a&gt;, &lt;a href=&quot;http://nationalinterest.org/blog/the-buzz/russia-america-had-plans-attack-the-moon-nuclear-weapons-21734&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;a href=&quot;http://nationalinterest.org/blog/the-buzz/russia-america-had-plans-attack-the-moon-nuclear-weapons-21734&quot; rel=&quot;nofollow noreferrer&quot;&gt;2&lt;/a&gt;&lt;/a&gt;, &lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-3229433/Could-BOMBING-Mars-make-habitable-Nuclear-warheads-heat-red-planet-make-Earth-like-claims-Elon-Musk.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;a href=&quot;http://www.dailymail.co.uk/sciencetech/article-3229433/Could-BOMBING-Mars-make-habitable-Nuclear-warheads-heat-red-planet-make-Earth-like-claims-Elon-Musk.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;3&lt;/a&gt;&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would a big explosion on Moon or Mars look like?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suppose that on Moon there would be no mushroom cloud, since it has virtually no atmosphere. Also nothing would burn since there is no oxygen. Would there be any flash? And how about Mars?&lt;/p&gt;&#xA;" OwnerUserId="21562" LastEditorUserId="21562" LastEditDate="2017-11-28T10:32:46.907" LastActivityDate="2017-11-28T10:32:46.907" Title="Bombing Moon and Mars: What would it look like?" Tags="|mars|the-moon|terraforming|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23798" PostTypeId="1" CreationDate="2017-11-24T08:06:55.233" Score="2" ViewCount="259" Body="&lt;p&gt;how &quot;off&quot; (as a percentage) would an attempt at geostationary orbit need to be (in either distance or speed) for a satellite to fall to earth in 100, 1000 or 10,000 years?  (assumes initial setup only, and no subsequent adjustments).&lt;/p&gt;&#xA;" OwnerUserId="21742" LastActivityDate="2017-11-27T19:21:08.783" Title="how &quot;off&quot; (as a percentage) would an attempt at geostationary orbit need to be (in either distance or speed) for a satellite to fall" Tags="|orbit|accuracy|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23800" PostTypeId="2" ParentId="23797" CreationDate="2017-11-24T09:10:12.903" Score="2" Body="&lt;p&gt;&lt;strong&gt;Nuking the Moon:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you detonate a nuke on the surface of the moon, it probably wouldn't be as spectacular as a detonation on earth. Most of the energy released in a nuclear blast is in the form of a pressure wave and heat with only a small portion in radiation. If you detonate the bomb above the surface of the moon the explosion would be extremely bright (instantly blinding/destroying unprotected eyes and cameras in the vicinity along with bombarding it's surroundings with other forms of radiation. The infrared emissions would probably make things pretty hot in the surrounding area and if detonated above frozen water, possibly instantaneously vaporize it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you detonate on or under the surface of the moon, the effects would probably be similar except that the blast wave would propagate through the mun's surface similar to an earthquake and the bomb would blow a large amount of material away at high speeds. This high speed moon debris would also never slow down until it hits something (probably the moon) and it would be traveling so fast that large pieces would probably be on suborbital trajectories and might even land on the other side of the moon (although it's hard to gauge as there's no exact numbers on how fast objects could be launched by a nuclear explosion in a vacuum. For reference, nuclear shock waves on earth reach up to 0.5km/s and lunar escape velocity is around 2.4km/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Nuking Mars:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming we have a nuke that can detonate on Mars, the explosion would probably be similar to one on earth with a couple key differences. Mars' atmosphere is much thinner and is mostly c02 and Mars' gravity is lower. Detonating a nuke on mars would release a large amount of heat into the atmosphere along with shooting debris and creating a shock wave. The &quot;size&quot; of the explosion probably wouldn't be as big as on the earth as the atmosphere is thinner but there would probably still be a mushroom cloud as the atmosphere that is heated up by the explosion would be less dense than the very cold atmosphere of Mars. &lt;/p&gt;&#xA;" OwnerUserId="21538" LastEditorUserId="21538" LastEditDate="2017-11-24T12:55:25.813" LastActivityDate="2017-11-24T12:55:25.813" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23801" PostTypeId="5" CreationDate="2017-11-24T09:12:17.620" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2017-11-24T09:12:17.620" LastActivityDate="2017-11-24T09:12:17.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23802" PostTypeId="4" CreationDate="2017-11-24T09:12:17.620" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2017-11-24T09:12:17.620" LastActivityDate="2017-11-24T09:12:17.620" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23803" PostTypeId="2" ParentId="23797" CreationDate="2017-11-24T11:28:43.333" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;To me it seems surprising, frightening, and sadly typical&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Bombing the moon was mostly a &quot;just because we can&quot; demonstration of military superiority.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using nuclear bombs on Mars is the only way we have of creating a planetwide effect without requiring an impossibly large amount of resources. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Would there be any flash?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From the article on the A119 proposed nuclear detonation on the Moon:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If the explosive device detonated on the surface, not in a lunar crater, the flash of explosive light would have been faintly visible to people on earth with their naked eye,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A surface detonation would not result in a mushroom, but there would be a 'cloud' of high-velocity debris. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On Mars, you would get a mushroom, although I suspect it'd be less visible than on Earth due to the lower atmospheric pressure, so less dust gets picked up. A quick search didn't turn up any studies on the effect of a large detonation on Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There have been &lt;a href=&quot;http://www.ibtimes.co.uk/elon-musk-wants-nuke-mars-scientists-say-its-not-crazy-it-sounds-1519430&quot; rel=&quot;nofollow noreferrer&quot;&gt;some reactions&lt;/a&gt; by scientists to Musk's plan, citing two major problems:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;it wouldn't be as quick as Musks hopes&lt;/li&gt;&#xA;&lt;li&gt;the fallout would be a major problem if you hope to make the planet habitable.  &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-11-24T13:44:51.537" LastActivityDate="2017-11-24T13:44:51.537" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23804" PostTypeId="2" ParentId="23798" CreationDate="2017-11-24T14:29:41.150" Score="8" Body="&lt;p&gt;Being off the nominal geostationary orbital velocity won't cause the satellite to fall to Earth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a given altitude, there's one orbital velocity that yields a circular orbit. If your velocity is higher or lower, you instead get an elliptical orbit. As long as that elliptical orbit doesn't intersect the Earth or a substantial amount of the Earth's atmosphere, it will remain stable for quite a while (although, because of disturbances from solar wind, the unevenness of Earth's gravitational field, and the influence of other planets in the solar system, it may not stay up for tens of thousands of years). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Vanguard_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vanguard 1&lt;/a&gt; has remained in orbit for over fifty years, and at the low end of its orbit its altitude is about ~660 km. Even at apogee it's much, much lower than geosynchronous. Vanguard is expected to stay in orbit for 240 years. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Circular orbit velocity for geosynchronous orbit altitude is 3.07 km/s. If you hit that speed, due East, at an altitude of 35,786km above the equator, you're in geostationary orbit. If you're going slower than that, you're in an elliptical orbit, with that point as your apogee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume that Klassensat-1 reached geosynchronous altitude by doing a transfer from low Earth orbit but its circularization burn failed, leaving it in a 660km perigee, 35,786 km apogee orbit. This should last at least as long as Vanguard. &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_speed#Precise_orbital_speed&quot; rel=&quot;nofollow noreferrer&quot;&gt;The speed at apogee for this orbit works out to about 1644 m/s.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if your orbital velocity is only 54% of geostationary velocity, you'll still stay up for well over 240 years.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it stays above 900km, your satellite's lifetime will be on the order of 1000 years; this requires only a little more speed at apogee, 1668m/s. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-11-25T01:20:48.847" LastActivityDate="2017-11-25T01:20:48.847" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23805" PostTypeId="1" CreationDate="2017-11-24T18:37:04.087" Score="5" ViewCount="674" Body="&lt;p&gt;We know from the answers to the question &lt;a href=&quot;https://space.stackexchange.com/questions/3153&quot;&gt;If a human is being sent to space in a rocket, does the seating angle matter?&lt;/a&gt; that seating position at launch is of less concern than landing. We also learn that some craft (&lt;em&gt;Soyuz&lt;/em&gt;) have seats that change position between launch and landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is the best (&lt;em&gt;safest&lt;/em&gt;) position for a human at launch?   &lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2017-12-18T01:50:53.763" Title="What is the best position for a human to be in during launch?" Tags="|launch|safety|humans|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23806" PostTypeId="1" CreationDate="2017-11-24T22:15:48.197" Score="10" ViewCount="650" Body="&lt;p&gt;Is anyone familiar with the chemical/thermodynamic properties of Otto II fuel (a monopropellant used in torpedoes)?  It is composed of nitrated propylene gycol with a couple additives that make it non-explosive: &lt;a href=&quot;https://en.wikipedia.org/wiki/Otto_fuel_II&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Otto_fuel_II&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is a stable, non-cryogenic, low vapor pressure, &quot;shirt sleeves&quot; chemical blend...not exactly benign, but not nearly as dangerous as hydrazine or even high test peroxide.  It does not require a catalyst to burn.  I have no idea what its ISP would be in a rocket thruster application.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As one of the gaseous products is hydrogen cyanide, it's not hard to understand why it's not commercially available.  Can anyone shed some light on this propellant and why it is not used in spaceflight applications?&lt;/p&gt;&#xA;" OwnerUserId="20219" LastEditorUserId="12102" LastEditDate="2021-08-09T01:36:54.723" LastActivityDate="2021-08-09T01:36:54.723" Title="Why is the Otto II monopropellant fuel not used in spaceflight applications?" Tags="|design-alternative|monopropellant|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23807" PostTypeId="2" ParentId="23777" CreationDate="2017-11-25T00:08:31.997" Score="11" Body="&lt;p&gt;&lt;strong&gt;NOTE: My answer applies specifically to Space Shuttle (STS) operations.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, it is quite safe to say that it is never desireable that the chaser plume the target to any significant degree during rendezvous/proximity operations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A cursory overview of the Space Shuttle Orbiter's Reaction Control System (RCS) is shown below (page taken from a 2002 version of the Shuttle Crew Operations Manual).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/spvy0.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/spvy0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The next diagram is a slide presented during a 1996 briefing given to an Astronaut Candidate class.  It gives the reader an idea about the orientation of the various jets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ORiyf.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ORiyf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;normal&quot; way to thrust toward the Orbiter's +Z axis was to utilize the upward pointing RCS jets, naturally (the next two images below are extracted from the Rendezvous/Proximity Operations Workbook (dated 1985):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FfbKH.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FfbKH.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When suitably close to the target, the crew would select &quot;LOW Z&quot; mode, which operated as explained below:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GyDQJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GyDQJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12422" LastActivityDate="2017-11-25T00:08:31.997" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23809" PostTypeId="1" AcceptedAnswerId="23819" CreationDate="2017-11-25T05:43:23.267" Score="10" ViewCount="1282" Body="&lt;p&gt;I've noticed that in various photos and videos the orientation of the ISS' eight independent solar arrays are often moving, and usually some are different than others.&lt;/p&gt;&#xA;&lt;p&gt;0th-order naive thinking would be that they would all be tilted for maximum power all the time which would put them all at the same orientation, with their most efficient front surface pointed directly at the Sun when the ISS is not in eclipse. Of course the ISS does not always (or possibly ever) need maximum power and many other 1st-order considerations might come into play. Those might include motor wear-and-tear, aging of the photovoltaic cells, drag, mutual shadowing, possibly even angular momentum transfer to the rest of the ISS.&lt;/p&gt;&#xA;&lt;p&gt;Is it possible to describe the thinking behind the algorithm that controls the orientation of each of the independently moving arrays at each point in time, and all of the major considerations behind it?&lt;/p&gt;&#xA;&lt;p&gt;The two answers to the question &lt;a href=&quot;https://space.stackexchange.com/q/1211/12102&quot;&gt;How does the ISS adjust its solar panels?&lt;/a&gt; describe the alpha and beta gimbalings and mention the &amp;quot;Sun Slicer&amp;quot; and &amp;quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Night_Glider_mode&quot; rel=&quot;nofollow noreferrer&quot;&gt;Night Glider&lt;/a&gt;&amp;quot; modes but I'm also wondering &lt;em&gt;if the algorithm also takes into account&lt;/em&gt; many other considerations, including arrays shadowing each other, additional reflected light from the Earth, and solar damage accumulated by panels when not required for power production, especially during major solar events as described in &lt;a href=&quot;https://space.stackexchange.com/a/9907/12102&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Screenshot from the Science at NASA YouTube video &lt;a href=&quot;https://youtu.be/dWuEVSCw8B8&quot; rel=&quot;nofollow noreferrer&quot;&gt;ScienceCasts: Space Coffee&lt;/a&gt; that I just happened to be watching at the moment.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qPzJB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qPzJB.jpg&quot; alt=&quot;Solar Arrays on the ISS&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uppxh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uppxh.jpg&quot; alt=&quot;Solar Arrays on the ISS&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Below:&lt;/strong&gt; Screen shot from the opening scene from CSA Astronaut Chris Hadfield's YouTube music video &lt;a href=&quot;https://www.youtube.com/watch?v=KaOC9danxNo&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Oddity&lt;/a&gt;. Two of the eight arrays are rotated by about 90 degrees relative to the other six, the one on the right is edge-on and nearly invisible.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/w1GET.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/w1GET.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2020-10-06T15:33:11.950" LastActivityDate="2020-10-06T15:33:11.950" Title="How are the orientations of the ISS' eight independent solar arrays optimized?" Tags="|iss|solar-power|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23810" PostTypeId="1" AcceptedAnswerId="46346" CreationDate="2017-11-25T07:32:53.820" Score="5" ViewCount="1880" Body="&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_perturbations_models&quot; rel=&quot;nofollow noreferrer&quot;&gt;Simplified Perturbations model&lt;/a&gt; SGP4 is used to calculate Earth satellite state vectors (position and velocity) using standard ephemeris data encoded as &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;TLEs&lt;/a&gt; (Two Line Elements). According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Simplified_perturbations_models&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Current code libraries have merged SGP4 and SDP4 algorithms into a single codebase handling the range of orbital periods which are usually referred to generically as SGP4.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;where SDP4 is the deep-space partner to the original SGP4, using only the simplest drag model but now also accounting for other perturbative effects, including the gravitational perturbations of the Moon and the Sun (as well as resonant effects near 1 and 2 orbits per day).&lt;/p&gt;&#xA;&lt;p&gt;Published TLEs are calculated specifically to work with the appropriate SGP predictor. According to the original 1980/1988 version of &lt;a href=&quot;https://celestrak.org/NORAD/documentation/spacetrk.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spacetrack Report No. 3, Models for Propagation of NORAD Element Sets&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;All space objects are classified by NORAD as near-Earth (period less than 225 minutes) or deep-space (period greater than or equal 225 minutes). Depending on the period, the NORAD element sets are automatically generated with the near-Earth or deep-space model. The user can then calculate the satellite period and know which prediction model to use.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In SGP4 the initialization uses the TLE's mean motion to set a flag that determines which propagation method is used later in the execution. For example something along the lines of:&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;if ((2*pi / satrec.no) &amp;gt;= 225.0)&#xA;  {&#xA;    satrec.method = 'd';&#xA;    satrec.isimp  = 1;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;or&lt;/p&gt;&#xA;&lt;pre&gt;&lt;code&gt;IF((TWOPI/XNODP/XMNPDA) .GE. .15625) IDEEP=1&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&lt;p&gt;where 0.15625 is exactly 225/(24*60).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;QUESTION:&lt;/strong&gt; Can someone explain how SDP4 mathematically estimates the gravitational perturbations from the Sun and Moon? Does it contain a &amp;quot;mini-ephemeris&amp;quot; for the relative positions of the Sun, Earth, Moon system as a function of epoch, or at least their average periods, and propagate the satellite's motion including these forces, or does it use some average perturbation model?&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; I'm not looking for a general answer like &amp;quot;it uses perturbation theory&amp;quot;, I'd like to know roughly how SGP4 actually does it.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Just for one particular example&lt;/em&gt;, in January the Sun will pull in one direction, but in July it will pull in the opposite direction. If the orbit is highly elliptical, does this matter for the perturbation calculation? Does it matte if the Sun pulls in the direction of periapsis, apoapsis or to the side?&lt;/p&gt;&#xA;&lt;p&gt;SGP4 is also discussed in the 2006 report &lt;a href=&quot;https://celestrak.org/publications/AIAA/2006-6753/AIAA-2006-6753-Rev2.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Revisiting Spacetrack Report #3: Rev 2&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:51.853" LastActivityDate="2023-01-07T20:19:49.850" Title="How do SDP4's &quot;Deep space&quot; corrections to SGP4 account for the Sun's and Moon's gravity?" Tags="|artificial-satellite|mathematics|two-line-elements|sgp4|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="23812" PostTypeId="2" ParentId="23805" CreationDate="2017-11-25T09:40:59.130" Score="4" Body="&lt;p&gt;&lt;em&gt;(Caveat: just googled stuff. Please correct me where I'm wrong.)&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The direction in which the G-forces are applied matters:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/File:Human_linear_acceleration_tolerance.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/9/97/Human_linear_acceleration_tolerance.svg&quot; width=&quot;300&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(See also &lt;a href=&quot;https://what-if.xkcd.com/116/&quot; rel=&quot;nofollow noreferrer&quot;&gt;obligatory related &lt;em&gt;XKCD/What If?&lt;/em&gt; link with a nice image&lt;/a&gt;) The &lt;a href=&quot;https://en.wikipedia.org/wiki/G-force#Human_tolerance&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia article on &lt;em&gt;G-force&lt;/em&gt;&lt;/a&gt; has some explanations on the effects to the human body depending on the direction. The human body is best able to tolerate acceleration &quot;forward&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110016361.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA has a short paper about the considerations for the seating&lt;/a&gt; but I didn't spot anything about the medical aspects of how the astronaut is seated (e.g. &quot;lying&quot; vs. &quot;crouch&quot;). Although current seats all position the astronaut in a slightly crouched position, it would be interesting whether lying straight might actually be preferable if you are only considering the launch and ignore the landing. However, I wasn't able to find anything about that topic.&lt;/p&gt;&#xA;" OwnerUserId="14549" LastActivityDate="2017-11-25T09:40:59.130" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23813" PostTypeId="2" ParentId="23785" CreationDate="2017-11-25T15:25:10.793" Score="8" Body="&lt;p&gt;There's a lot of poor information out there regarding looking directly at the sun (or looking at it through a mirror). As a general rule-of-thumb, it's a bad idea to look directly at the sun, but that is very overstated and the effects that it has on the eye depend on some very clearly-defined factors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not &lt;em&gt;actually&lt;/em&gt; harmful to look at the sun briefly - otherwise every human (as well as all apes and probably many other mammals) would accidentally blind themselves during early childhood. But there are &lt;strong&gt;several different types&lt;/strong&gt; of eye damage that can occur. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;1: over-saturation of the receptors. This is the most common type of damage, and it is the bluish or greenish thing that appears in your vision whenever you look at the sun or any bright light for a few seconds, or after a camera flash. The small light receptor cells in the retina can only output a certain amount of signal when they receive light. If they receive too much or receive it for too long, they get a bit overloaded and need to rest (they're cells, they can only handle so much). You will tend to see a reversed 'afterimage' (like a photographic negative) while these cells refresh. This is generally regarded as harmless, though it's probably not a good idea to overload the cells frequently because that's simply not how they are supposed to be used. Some more information is in the first section at &lt;a href=&quot;http://www.healthguidance.org/entry/9913/1/Spots-Dots-and-Floaters-Seeing-Whats-Inside-Your-Eyes.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.healthguidance.org/entry/9913/1/Spots-Dots-and-Floaters-Seeing-Whats-Inside-Your-Eyes.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2: burning or extreme over-saturation of small portions of the eye. This is the primary concern with lasers. A bright small light can cause direct damage (not mere overuse, but outright damage) to small parts of the retina, either through radiation damage or by concentrating heat on a very small area. This WILL cause long-lasting damage; sometimes it can heal over weeks or months, but sometimes it can be permanent. However, it is only to a portion of the retina, so the eye will still work, but vision won't be as good, and it can create significant blindspots. A powerful laser can do this, and looking at the sun through binoculars or a telescope can also do this on a rather large portion of the retina.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;3: burning of the entire retina - &lt;strong&gt;this&lt;/strong&gt; is the type of damage that people worry about the most from the sun. If you stare at the sun with your naked eye for a long time (several minutes to several hours), you can overheat the entire retina and kill many of the cells just from the heat. As the eye is mostly water, and has blood vessels that circulate liquid, it is good at handling and dissipating heat. But it can only handle so much. Additional cells can also be killed by the UV radiation that the eye concentrates onto the retina - so those cells may die hours or days later in addition to the ones that were killed more quickly by the heat. Most of us have the instinctive reaction of not staring directly at the sun, so this isn't a problem, unless someone forces themselves to do it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;4: long-term UV damage. If you stare at the sun often, or are in a profession where you are often at very high altitudes (such as a pilot) where UV isn't filtered from sunlight as much, or often use a tanning bed without eye protection, then you can accumulate damage from gradual UV exposure. The UV damages the cells on the molecular level. This damage is very gradual and often isn't instantly noticeable, and sometimes isn't even felt until well after the event because the damage can be in forms that prevent cells from repairing or reproducing.  This is a problem common among old career pilots from the era when cockpit windows didn't have anti-UV tinting. It also tends to tie into other vision-reducing problems, because it's radiation damage and basically radiation damage harms everything - hence it will make any other (or latent) problems worse.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the sun, the real worry is from the last two cases. For your question, the panels don't focus light onto a tiny pinpoint, so you won't get any damage from the concentrated light. And the light travels through the atmosphere on its way down to you, so most of that UV radiation is filtered like normal, in addition to its the dispersion in the reflection. And the panels don't reflect nearly enough light to overheat any part of the retina.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you CAN look at the sun, as long as you follow certain precautions: don't stare at it for a very long time (and especially don't hold it in the same part of your vision if you do), don't magnify the amount of light that your eye collects (like with a telescope or binoculars), don't concentrate that light down to a very small point, and don't look at it from very high in the atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Source: I spent many days trying to get real answers about eye damage before the eclipse, because &quot;don't look at the sun. Ever.&quot; is simply an illogical statement when you consider that all land creatures occasionally glance at the sun accidentally. I can dig up specific, more credible sources if needed - but many of those sources will be generalized statements from eye doctors (at least for the first 3 kinds of damage) - medical studies almost always focus on very, very specific things, whereas these types of eye damage are much broader and beyond the scope of any single study. The closest study to &quot;does looking at the sun for a long time cause eye damage?&quot; was a report accumulating eye damage data from the prior eclipse, where patients went to the doctor after they got vision damage because they stared at it for too long (thus, a very self-selected group, and no one knows how many people watched it without eye damage or without going to the doctor)&lt;/p&gt;&#xA;" OwnerUserId="21757" LastEditorUserId="21757" LastEditDate="2017-11-25T15:37:23.133" LastActivityDate="2017-11-25T15:37:23.133" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23814" PostTypeId="1" CreationDate="2017-11-25T15:41:54.263" Score="0" ViewCount="111" Body="&lt;p&gt;A lot of space debris on LEO are small 1 to 2 cm pieces.  Removing these threats is a concern to anyone using LEO.  Can a cube/micro satellite use collected small debris and propel them to both deorbit the debris and gain orbital velocity itself?  A small rail gun perhaps.&lt;/p&gt;&#xA;" OwnerUserId="20287" LastActivityDate="2017-11-26T12:33:37.520" Title="If a cube or micro satellite could collect 1-2 cm space debris in Earth orbit could they be deorbited while raising the sats orbit?" Tags="|artificial-satellite|propulsion|low-earth-orbit|debris|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23817" PostTypeId="2" ParentId="23814" CreationDate="2017-11-26T11:35:40.367" Score="3" Body="&lt;p&gt;In theory, yes. But in reality, no - not yet, and probably not ever.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you could build an ideal collector, something to sweep debris into an inlet where it could then be accelerated to raise the satellite's orbit, then this could work. But in reality, to try and catch debris with a different vector to the satellite's (and possibly dramatically different velocity) would require manoeuvres, and these would consume a large amount of propellant, and probably require a very large collector, whether it be a net, a magnetic funnel of some kind...which also requires more impulse to move etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are numerous concepts to try to de-orbit debris, ranging from &lt;a href=&quot;https://arxiv.org/pdf/1110.3835.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;lasers&lt;/a&gt; to &lt;a href=&quot;http://edition.cnn.com/2017/06/13/tech/space-sweepers-astroscale-japan/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnets&lt;/a&gt; to mirrors and various others.&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2017-11-26T12:33:37.520" LastActivityDate="2017-11-26T12:33:37.520" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23819" PostTypeId="2" ParentId="23809" CreationDate="2017-11-26T19:43:16.600" Score="8" Body="&lt;p&gt;The basic desire would be to keep all eight arrays perpendicular to the sun.  However, it's a bit more complicated due to other, more detailed requirements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the not too distant past NASA held a &lt;a href=&quot;http://www.republicsystems.com/wp-content/uploads/2015/04/Longeron-Challenge-Merged.pdf&quot; rel=&quot;noreferrer&quot;&gt;coding challenge&lt;/a&gt; to address just this problem.  Here are the requirements from that challenge:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The amount of power generated by each SAW depends on its orientation&#xA;  with respect to the sun and on any shadows on the solar collectors.&#xA;  Code to calculate the power generated at a specific orientation will&#xA;  be provided for the use by contestants in the off-line&#xA;  tester/visualizer. In addition to maximizing the total power output&#xA;  there are some constraints on the possible movements: &lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;Each SARJ and BGA is limited to a maximum angular velocity and to a maximum angular&#xA;  acceleration.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Each SAW must produce at least some minimum average&#xA;  power over the orbit (which is different for each SAW).&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;The sequence&#xA;  of positions must be cyclic, so it can be repeated on the next orbit.&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;The maximum amount of BGA rotation is not limited, but exceeding a&#xA;  threshold will result in a score penalty. &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;Some structural members of&#xA;  the SAW mast (called Longerons) have restrictions on how they can be&#xA;  shadowed. &lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;  &#xA;  &lt;p&gt;Longerons are the four long components of the SAW’s mast. If&#xA;  a longeron is shadowed for a period of time it will cool and shrink.&#xA;  If some longerons shrink while others do not, this may stress the&#xA;  longerons and weaken them. This weakening would eventually lead to the&#xA;  failure of the mast so this must be avoided at all cost.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Acronymology:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SARJ = &lt;strong&gt;S&lt;/strong&gt;olar &lt;strong&gt;A&lt;/strong&gt;rray (or &lt;strong&gt;A&lt;/strong&gt;lpha) &lt;strong&gt;R&lt;/strong&gt;otary &lt;strong&gt;J&lt;/strong&gt;oint (the joint between the central ISS truss and the outboard truss segments from which the solar arrays sprout)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BGA = &lt;strong&gt;B&lt;/strong&gt;eta &lt;strong&gt;G&lt;/strong&gt;imbal &lt;strong&gt;A&lt;/strong&gt;ssembly (the joint between the array and the truss - allows the array to pivot on its long axis)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SAW = &lt;strong&gt;S&lt;/strong&gt;olar &lt;strong&gt;A&lt;/strong&gt;rray &lt;strong&gt;W&lt;/strong&gt;ing&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solar arrays may also be moved to specific positions to mitigate loads caused by docking and undocking vehicles - this is the reason for the rather odd positioning seen in the photos taken from visiting vehicles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a more technical discussion of such constraints from &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=2&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwjJvu-igd3XAhVF_4MKHaAQCDcQFgguMAE&amp;amp;url=https%3A%2F%2Fti.arc.nasa.gov%2Fpublications%2F134%2Fdownload%2F&amp;amp;usg=AOvVaw28DOCJl9YNDwpmevr0gAJ8&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Solar Array Constraints &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Solar array constraints fall into the&#xA;  following categories: power, loading (array and SARJ are treated&#xA;  separately), contamination and longeron shadowing. The power&#xA;  availability due to any array will be the maximum if it automatically&#xA;  tracks the sun, but this is not always a safe mode in which to&#xA;  operate. If power availability drops too low, some ISS subsystems must&#xA;  be shut off. As described earlier, the attitude of the station needs&#xA;  to be changed periodically to account for various events by firing a&#xA;  combination of different thrusters, which in turn imposes structural&#xA;  loads on the solar arrays as well as the joints, especially the SARJs.&#xA;  Further, the thruster plumes and water dumps can cause contamination&#xA;  of the arrays, reducing their power generation. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Additionally,&#xA;  differential shading of longerons, which are structural elements that&#xA;  keep the array blankets in tension, put stresses on the arrays, with&#xA;  the magnitude of the stresses depending on a complex set of&#xA;  calculations. An array’s longerons can be shadowed by its own blanket&#xA;  or those of a neighboring array, the amount of shadowing depending on&#xA;  the solar beta (the elevation of the sun relative to the orbit plane&#xA;  of the ISS), and the orientations of the adjacent arrays. The same&#xA;  factors also impact the power generation by the solar arrays; to&#xA;  improve power, the arrays should not be shadowed, but to keep the&#xA;  arrays from shadowing each other or the longerons, they are no longer&#xA;  in an orientation for producing maximum power.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-11-26T19:54:26.863" LastActivityDate="2017-11-26T19:54:26.863" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23820" PostTypeId="1" AcceptedAnswerId="23821" CreationDate="2017-11-26T22:08:01.223" Score="3" ViewCount="172" Body="&lt;p&gt;I am using this Atmospheric Properties &lt;a href=&quot;http://www.aerospaceweb.org/design/scripts/atmosphere/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Calculator&lt;/a&gt; to find the condition a &lt;a href=&quot;https://i.stack.imgur.com/0mCbG.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;grid fin&lt;/a&gt; would encounter when it has a altitude of 86km, and velocity of 7996 km/h. Would this be the total surface area of the part of the grid fin that is encountering the air for the friction coefficient (AKA the underside of the fin)? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And what does it mean by &quot;Temperature Increment&quot;? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many thanks!&lt;/p&gt;&#xA;" OwnerUserId="21769" LastActivityDate="2017-11-26T23:07:12.850" Title="What would the reference length be for a grid fin?" Tags="|spacex|grid-fins|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23821" PostTypeId="2" ParentId="23820" CreationDate="2017-11-26T23:07:12.850" Score="3" Body="&lt;p&gt;A reference &quot;length&quot; can't be a surface area, the units are wrong. I suggest using the major dimension of the fin. Example: For an airfoil, it's the chord length.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The introduction to the web page says &quot; An optional temperature increment can be added or subtracted to model a hot or cold day.  For example, enter &quot;10&quot; to see the effect of a 10° increase above the standard temperature at the current altitude. &quot; So you can leave it zero. &lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-11-26T23:07:12.850" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23822" PostTypeId="1" CreationDate="2017-11-27T02:06:48.950" Score="9" ViewCount="4064" Body="&lt;p&gt;With the correct rocket setup would it be possible to thrust around in a circle, like you would do donuts in a car, to generate centrifugal gravity? &lt;/p&gt;&#xA;" OwnerUserId="1087" LastActivityDate="2017-11-27T22:59:44.180" Title="Is is possible to generate centrifugal gravity by doing donuts?" Tags="|gravity|artificial-gravity|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23823" PostTypeId="2" ParentId="23822" CreationDate="2017-11-27T02:44:44.680" Score="25" Body="&lt;p&gt;Thrust won't generate gravity, but it will produce acceleration which may be indistinguishable from gravity to an occupant. Yes, it's possible to simulate gravity by having a spaceship constantly thrusting to travel in a circle, but it would be an awful waste of fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rotating a large object, or pair of objects connected by a tether, to simulate gravity is more efficient because, once started, it will continue to rotate with no further input of energy.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-11-27T02:44:44.680" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23824" PostTypeId="2" ParentId="10562" CreationDate="2017-11-27T03:27:11.613" Score="0" Body="&lt;p&gt;To simplify the explanation, let's imaging you are standing on the inside surface of a large rotating drum in space. If you jump straight &quot;up&quot; (toward the center of rotation), you will have a velocity vector which will be the sum of your original motion along the circumference plus the &quot;inward&quot; motion you gave yourself by jumping. Excluding the effects of friction with an internal atmosphere, you will continue to travel in a straight line along this new vector (at uniform velocity). This path will be a chord line intersecting the drum at two points, one of which is the point of your &quot;lift-off&quot;, the other will be your inevitable &quot;landing&quot; back on the drum surface.&lt;/p&gt;&#xA;" OwnerUserId="429" LastActivityDate="2017-11-27T03:27:11.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23825" PostTypeId="2" ParentId="23822" CreationDate="2017-11-27T09:58:38.113" Score="5" Body="&lt;p&gt;For a space based object, there is absolutely no problem with the energy need - you can get that from solar panels, RTG (fission heat to electricity via thermoelectric effect). Each comes with a different set of tradeoffs, but they all work now in one spaceship or another (Mars rovers, satellites, deep space probes, ...).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you can't have in space is reaction mass (as in Newton's action and reaction). Even the most mass-efficient thruster (electrically accelerated ion thrusters) use up their fuel - and in deep space you can't get any more (except for the - currently theoretical - &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bussard Ramjet&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, any practical gravity (let's say half Earth gravity) is high in terms of fuel burn rate. I didn't find fuel burn values, but the trans-lunar injection was a 350 seconds burn for 3 km/s delta-v, or about one gravity. The fuel flow was (according to this page: &lt;a href=&quot;https://www.nasa.gov/centers/marshall/pdf/499245main_J2_Engine_fs.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/centers/marshall/pdf/499245main_J2_Engine_fs.pdf&lt;/a&gt;) about 530 lb/sec, or 250 kg per second. In monetary terms, that's more than half a million dolar per second - but the real issue might be one of availability not one of cost (rockets can only lift so much).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, while the theoretical answer to your question is yes, the practical one right now is no.&lt;/p&gt;&#xA;" OwnerUserId="21775" LastEditorUserId="-1" LastEditDate="2017-11-27T22:59:44.180" LastActivityDate="2017-11-27T22:59:44.180" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23827" PostTypeId="2" ParentId="23822" CreationDate="2017-11-27T13:12:49.950" Score="3" Body="&lt;p&gt;Sure, though I can't think of a reason you'd want to do that. Burning the engines in &lt;em&gt;any&lt;/em&gt; direction produces acceleration that feels like gravity. Given the &lt;a href=&quot;https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;tremendous cost of bringing more fuel&lt;/a&gt;, if you're in a spacecraft you are probably trying to get somewhere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spinning in a circle gets you nothing. There isn't a tether or similar to supply the centripetal force, so you'll be supplying it with the engines. And after you've spun around in a circle your net change in velocity is zero.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More likely, you'd just burn prograde or retrograde, in a straight-ish line, turning only to keep the ship aligned with your orbit. Besides using fuel to get you to where you are going rather than burning in circles, the occupants will be more comfortable as they won't need to deal with significant &lt;a href=&quot;https://en.wikipedia.org/wiki/Coriolis_force&quot; rel=&quot;nofollow noreferrer&quot;&gt;Coriolis forces&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="2712" LastActivityDate="2017-11-27T13:12:49.950" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23828" PostTypeId="2" ParentId="16684" CreationDate="2017-11-27T18:23:27.183" Score="1" Body="&lt;p&gt;When ion propulsion is used to travel huge distances while accelerating slowly to very high speed, a flexible power distribution is needed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The amount of available electrical power from solar cells decreases with the square of the distance to sun. The power of RTGs decreases with time due to the decay of nuclear fuel and the degradation of thermocouples. If much power is available, ion thrusters should work with full thrust. But after many years much less power is available and should be reservated for control and communication of the space craft as well as scientific payload. If some power is left, ion thrusters may work at reduced thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if some solar cells or thermocuples are reserved for generation of high voltage only, the may not be used later exclusively for control and communication and no longer for thrust. Solar cell arrays may generate some kilowatt at a distance of 1 AU, but at a distance of 5 AU only 1/25 of initial power is available, only some hundred watts.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using a single voltage of about 28 V allows a flexible power distribution to high and low priority current consumers.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-11-27T18:23:27.183" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23830" PostTypeId="2" ParentId="22551" CreationDate="2017-11-27T19:47:33.163" Score="3" Body="&lt;p&gt;There are block diagrams available for PEG-4 code here which deals with onorbit and deorbit maneuvers:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760024151.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760024151.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is necessary in the TURNR code to have the phi max limiting to &amp;lt; 45 degrees.  And it is also necessary to implement the lambdaDot_xz magnitude limiting to 0.35 of the schuler frequency at the burn.  There are more details in the 1979 paper on the PEG algorithm:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19790048206&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/search.jsp?R=19790048206&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Shuttle code used some subroutines called LTVCON and TRANSTIME to handle burn-coast targeting to integrate through the burn and the coast, including J2 effects of Earth's oblateness, in order to hit a r, v target.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I've found is that it is sufficient (in KSP) to implement in the corrector:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;project rp into the target iy plane normally&lt;/li&gt;&#xA;&lt;li&gt;set rd = rp (no restriction on the burnout position)&lt;/li&gt;&#xA;&lt;li&gt;compute the signed angle between rd and the periapsis of the target orbit to get the true anomaly of the burnout position (periapsis cross rd should be in the -iy direction; dot product of that with -iy should be positive; or else flip the sign on the angle)&lt;/li&gt;&#xA;&lt;li&gt;set vd equal to the velocity of the target orbit at that true anomaly&lt;/li&gt;&#xA;&lt;li&gt;set the gain equal to 1.0 (since there is no integration through any coast phase)&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Most of the LTVCON and TRANSTIME complexity seems to be around using the precise predictor in the shuttle code to account for J2 effects during the coast phase (or most of the coast phase -- as the burnout position changes due to PEG the time of the coast varies by delta-coast and I think that is fed through precise predict with J2=0 -- likely for either numerical efficiency reasons or numerical stability reasons?).  I'm also still somewhat baffled by the C1 and C2 constants in the LTVCON routine (particularly what C1 is used for).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the lead time I've found that simply leading the burn by tgo/2 works fine although it might not be perfectly optimal.  I suspect that letting PEG run continuously and then executing the burn at J/L before the burn could be slightly more precise (in the absence of having a better calculus-of-variations-based trajectory planner).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;UPDATE:  I think I understand a bit better now that the lambdaDot_xz constraints in the TURNR routine are used for Abort Once Around and deorbit burns.  What I wound up doing was to use the standard TURNR routine that computes rgo from &lt;code&gt;rgo = rd - ( r + v * tgo + rgrav ) + rbias&lt;/code&gt; (which uses rd which is where I believe the position constraint feeds back into the routine) and then computes lambdaDot normally and uses the rthrust and vthrust thrust integrals.  Where I ran into difficulty is that even 50-60 seconds out from the end of the burn the lambdaDot computation becomes completely unstable.  I wound up clamping lambdaDot to the 0.35 * the schuler frequency value -- so the clamp is applied to lambdaDot and it goes to zero as tgo goes to zero.  Forcing that clamp throughout the entire burn (as deorbit or AOA does) meant that the burnout location had a large biasing value so it was not accurate.  By allowing the first part of the burn to be driven unclamped then rbias was zero and the position constraints were correct up until they were released in the last 60-ish seconds of the burn as the computed lambdaDot became unstable and exceeded the schuler frequency clamp.&lt;/p&gt;&#xA;" OwnerUserId="14420" LastEditorUserId="14420" LastEditDate="2017-12-11T23:36:19.797" LastActivityDate="2017-12-11T23:36:19.797" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23831" PostTypeId="1" CreationDate="2017-11-27T21:06:28.023" Score="2" ViewCount="187" Body="&lt;p&gt;I am currently working on a college senior design team that is looking at developing a small modular habitat that could sustain plant growth. These small modules would not include humans, so we only need to design around the requirements for the plants. This allows us to explore lower pressure environments where the structural engineering requirements are relaxes. It also allows us to vary the CO2 and O2 partial pressures to optimize for the plants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, I see in the answers to &lt;a href=&quot;https://space.stackexchange.com/q/13331/12102&quot;&gt;this question&lt;/a&gt; that as long as the partial pressure of O2 is very high, human space suit pressures can go as low as 4 and even possibly 3 psi, or about 0.2 bar.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understand that the atmospheric pressure and composition requirements  may vary significantly between varieties, so for the sake of answering this question, I will ask about two different types of plants; legumes and fruit-bearing plants.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My expectation is that legumes would be less sensitive, but let's say we are trying to grow lima beans in one pot, and tomatoes in another. To what degree can I change the fractions of CO2 and O2 in the atmosphere and its pressure, and still have a reasonable expectation that they can still grow?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I understanding that a typical Earth atmosphere is ~78% N2, ~21% O2, and ~.05% CO2 (according to &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;), that on Earth there are some plants capable of living at altitudes &lt;a href=&quot;https://en.wikipedia.org/wiki/Organisms_at_high_altitude#Plants&quot; rel=&quot;nofollow noreferrer&quot;&gt;as high as 6,000 meters&lt;/a&gt; where the pressure is only about &lt;a href=&quot;https://www.mide.com/pages/air-pressure-at-altitude-calculator&quot; rel=&quot;nofollow noreferrer&quot;&gt;0.45 bar&lt;/a&gt;, but I'd like to have a better understanding of how legumes and fruit-bearing plants (more useful to humans) could be grown in an atmosphere with both variable pressure and composition. &lt;/p&gt;&#xA;" OwnerUserId="21783" LastEditorUserId="12102" LastEditDate="2017-11-28T08:24:39.377" LastActivityDate="2017-11-28T08:24:39.377" Title="Range of atmospheric pressure and composition for plant survival?" Tags="|atmosphere|habitat|plants|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23833" PostTypeId="2" ParentId="6548" CreationDate="2017-11-28T02:47:28.803" Score="9" Body="&lt;p&gt;I think it's possible that the airship can go hypersonic which is already a major achievement if it does. If they can reach Mach 12 then it could be combined with momentum exchange tether assist to get to orbit. That's still a big ask though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[&lt;img src=&quot;https://i.stack.imgur.com/iIhkW.jpg&quot; alt=&quot;Orbital momentum exchange tether assist, credit Tethers unlimited - shows tether rendezvous with a hypersonic plane and a payload transfer from plane to tether tip&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;see also &lt;a href=&quot;https://www.fossilhunters.xyz/disruptive-technology/from-the-ground-up.html&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For details of HASTOL system see &lt;a href=&quot;http://www.tethers.com/papers/HASTOLAIAAPaper.pdf&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not so sure about accelerating to orbit because the size of their airship needed is so vast - it's a major challenge.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It doesn't help that his figures, which are given in feet, seem to need to be reinterpreted as meters to get the required buoyancy. E.g. he says his 6000 foot airship would be neutrally buoyant at 200,000 feet and carry 20 tons to orbit, but it works out as only enough buoyancy lift for 2.6 tons. If it is 6000 meters then it works out fine because you multiply all the numbers by 35 making it 91 tons making it much more feasible to take 20 tons to orbit even with skin, engine, crew, and fuel. See my &lt;a href=&quot;http://www.science20.com/robert_walker/can_jp_aerospaces_future_giant_airships_slowly_accelerate_to_orbit_looking_at_the_numbers-225651&quot; rel=&quot;noreferrer&quot;&gt;Can JP Aerospace's Future Giant Airships Slowly Accelerate To Orbit? Looking At The Numbers&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the strength of the airship - they plan to use airbeams and given its vast kilometer scale, and the very thin atmosphere - and that the skin is not intended to give structural support (a bit like the Skylon) I don't see why that would be impossible. The JAXA balloon skin is very strong - with a thickness of 3.4 microns it can withstand a breaking strain of 400 kilograms per square centimeter. It would seem to be way overkill for this application, given equal pressure inside and out. It's overkill for JAXA too - the main reason they have it so thick is because it is hard to make it uniform in thickness if it is very thin. So if that can be cracked, then the skin can be much thinner and still very strong. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for meteorite impacts - then the outer skin is kept in equilibrium with the atmosphere outside and the inner lifting balloons are inflated to only 2 millibars. So if there is a breach in the lifting balloons - a small hole from a micrometeorite - the hydrogen will escape very slowly. They would be designed to be replaced easily - and also he could pump out the hydrogen to equalize pressure inside and out as it ascends for the lifting balloons too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So anyway - I think the strength of the airship is a secondary issue. Other very important but secondary issues are the issue of maintainability - and of inflating it at a height of 140,000 feet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here I want to focus on what seem to be the most essential points - basic points in physics that have to work to make ot possible at all. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My conclusion is that if it can achieve a high lift to drag ratio through hypersonic flight and a high ISP ratio then it could be feasible. So let's look at this in detail&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;NEED FOR HIGH ISP&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it has an ISP of 2000 with his dirty ion thruster then so long as at most 50% of the rocket thrust is used to overcome drag, then it's an easy calculation for the total fuel to orbit. It has the same effect as doubling the delta v with an assumption of no drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I just input double the delta v to orbit ( which we are taking as 7,844 m/s), or 15,688 m/s into the ideal rocket equation, assuming an isp of 2000, mass of, say 100 tons. It turns out that the dry mass you get to orbit is 44.9 tons. So you need 55.1 tons of fuel. Here is &lt;a href=&quot;http://www.quantumg.net/rocketeq.html&quot; rel=&quot;noreferrer&quot;&gt;an ideal rocket equation calculator&lt;/a&gt; to check the calculations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;CONSIDERATIONS OF LIFT TO DRAG RATIO&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also work out the acceleration needed for a given lift to drag ratio assuming at most half of the rocket thrust is used to overcome drag. For instance if the lift to drag ratio is 4, then if half of the thrust is used to overcome drag it needs a rocket capable of accelerating it at 1/2 g in free space, and the airship would accelerate at 1/4 g. Anything less than that and it's very wasteful of fuel and it would surely be worthwhile adding an extra engine to increase the thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If half the thrust is used to overcome drag then that's the rocket equation with double the delta v to orbit we just used. If, say, two thirds of the thrust is used to overcome drag, that's equivalent to tripling the delta v to orbit in the dragless rocket equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, a 4 : 1 lift to drag ratio is reasonable if airships can be as good as a typical hypersonic plane. Skylon can achieve close to that. But 1/2 g of thrust is a big ask for his dirty ion thruster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can also work out the time to orbit from the lift to drag ratio - assuming half the thrust is used to overcome drag - and the lift to drag ratio from the time to orbit. The equations are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Time to orbit from Lift to Drag ratio L:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;7,844 * L / 9.80655 secs&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example, if the average lift to drag ratio is 4, the time to orbit from rest should be  7,844 / (9.80655/4) = 3199 seconds or about 53 minutes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now, suppose that our orbital airship can somehow manage a lift to drag ratio of 70 like the best glider, the ETA glider, and again uses 50% of its thrust to offset drag. Then it would need 1/35 g of thrust, and it would get to orbit in 7,844 *70 / 9.80655= 55991 seconds, or about 15 hours 33 minutes. (You can use &lt;a href=&quot;https://www.tools4noobs.com/online_tools/seconds_to_hh_mm_ss/&quot; rel=&quot;noreferrer&quot;&gt;this online tool&lt;/a&gt; to help convert seconds to hours and minutes quickly).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Lift to drag ratio from time to orbit:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;L = t * 9.80655 / 7,844 where t is the time to get to orbit in seconds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More generally,  L = t * 9.80655 * (R-1) / 7,844 where R is the thrust to drag ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since JP Aerospace says their airship would take three days to get to orbit, this suggests a very high lift to drag ratio of over 300 - either that or they have a very low thrust to drag ratio of not much  more than 1 which doesn't make a lot of sense to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The airship will automatically rise to whatever level in the atmosphere gives it the amount of drag corresponding to its lift to drag ratio. For instance, if it has a lift to drag ratio of 10 and a weight of 1,000,000 newtons, say, then if the drag is more than 100,000 newtons then the balance of forces will cause it to rise until it reaches a level in the atmosphere where the drag is no more than 100,000 newtons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, the higher the lift to drag ratio, the lower the drag, and so the lower the thrust needed. The total fuel is the same, so long as only half of the thrust is used to overcome drag. However their dirty ion thruster is going to be low thrust, so it needs a high lift.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sadly he doesn't give any figures for its performance either. But if his targeted lift to drag ratio is indeed over 300 then it only needs to have enough thrust to accelerate at 1/150 g in free space. Which would indeed put it somewhere between a standard ion thruster and a chemical rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;HIGH LIFT TO DRAG RATIO AIRSHIPS?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So are such high lift to drag ratios possible?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think it is hard to say because it is simply outside of any real world experience. There doesn't seem to be any literature to consult on this. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Airships can be counter intuitive even at subsonic speeds. The Airlander 10 has a huge cross section but the addition of those tiny fins is enough to let it achieve a lift to drag ratio of 3.8.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/rSb71.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rSb71.jpg&quot; alt=&quot;Airlander 10 - achieves lift to drag ratio of 3.8, is slighly heavier than air and depends on lift to stay afloat&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what happens with a hypersonic airship? First of all there's the objection that it has to cross the sound barrier - but he answers this, saying that it will happen so high in the atmosphere at such low densities that it's not an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So let's leave that for now (I discuss it some more in my article about it) and focus on what I see as the most fundamental issue - the lift to drag ratio.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thing is that the orbital airship is designed as a wave rider. When flying at hypersonic speeds then it rides on top of the shock wave caused by its hypersonic flight. The shock wave actually touches its body beneath it. Then - he would have drag reduction technology that lifts the shock wave away from its nose. So there is little drag from the front, and the V shape is designed to be optimized as a wave rider.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;A HYPERSONIC WAVE RIDER GETS LIFT FROM PRESSURE DIFFERENCE, NOT BULK AIR MOMENTUM CHANGE&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I hadn't heard of a hypersonic wave rider until I read about it in JP's book, but it's a well established concept goes back to work that started in 1951 theoretically and has been tested many times in practice. Here is &lt;a href=&quot;https://en.wikipedia.org/wiki/WaveRider&quot; rel=&quot;noreferrer&quot;&gt;the wikipedia page about it with lots of links&lt;/a&gt;. It depends on &lt;a href=&quot;https://en.wikipedia.org/wiki/Compression_lift&quot; rel=&quot;noreferrer&quot;&gt;&quot;compression lift&quot; which is described here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then - it gets lift not so much from change of momentum of bulk air as it hits the skin, like a normal airship. Instead it gets it from the density and pressure difference between the air beneath it and the air above it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now bear in mind that his airships are very low density.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using his figures as meters instead of feet, assuming a 100 ton airship - actually it's 91 tons for 6000 meters length and neutral buoyancy if the calculations are right - but let's assume 100 as a round number.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, taking its weight in force units as 1,000,000 newtons, and the very low density, with the lifting bags inside pressurized to 2 millibars, I calculated  his airship requires only 3.7 newtons per square meter upwards pressure to offset its weight completely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's using an estimate of the surface area of the lower part of the airship -&#xA; the part in contact with the shockwave, as over 270,000 square meters (reading feet as meters again) - there I just multiplied the 6000 meters by 2 and by the width of 300 meters, and subtracted 300 * 300 for the overlap between the two wings, as an estimate for the area for only one side. It under estimates because the cross section is oval, not flat, so would be a bit more than that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since the excess pressure from the shockwave would be acting upwards only, it doesn't need to be much to offset the weight of the airship. It doesn't have to be as much as the neutral buoyancy pressure of 2 millibars at 200,000 foot.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Atmospheric pressure is 10.3 metric tons per square meter, or around 101,000 newtons per square meter. So his 3.7 newtons corresponds to an overpressure of 0.0366 millibars. Just a tiny overpressure in the shockwave would be enough to offset the entire weight of his airship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When you start to think about it like that - an ultralightweight hypersonic waverider presenting a huge surface area to the shockwave and traveling in the upper atmosphere in what would normally be called a hard vacuum - well it is something totally out of any normal aerodynamic experience.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - could he achieve high lift to drag ratios in this way - far better even than our best gliders?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although he never talks about the lift to drag ratio anywhere, it seems from these simple arguments that he must be targeting a very high lift to drag ratio for the concept to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;OPEN QUESTION - IS SUCH A HIGH LIFT TO DRAG RATIO POSSIBLE FOR A HYPERSONIC WAVERIDER&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a very unusual situation of an ultra lightweight hypersonic wave rider operating in air so thin it would count as a hard vacuum. Could it have a high lift to drag ratio. I leave this as an open question - I don't know the answer and haven't found any research papers that tackle the question in this unusual physics setting.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So - that's my main insight into it. The calculation of the buoyancy is due to James Fincannon though the rest of this here is my own work and I wouldn't like to suggest he supports any of the rest of it. The ideas here come out from a very stimulating discussion with him in the SpaceShow comments section&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More background and calculations. &lt;a href=&quot;http://www.science20.com/robert_walker/can_jp_aerospaces_future_giant_airships_slowly_accelerate_to_orbit_looking_at_the_numbers-225651&quot; rel=&quot;noreferrer&quot;&gt;Can JP Aerospace's Future Giant Airships Slowly Accelerate To Orbit? Looking At The Numbers&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="3038" LastEditDate="2017-11-28T04:56:19.143" LastActivityDate="2017-11-28T04:56:19.143" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23836" PostTypeId="2" ParentId="22973" CreationDate="2017-11-28T06:25:54.380" Score="9" Body="&lt;p&gt;Oh man, I wanted to asnwer this but someone got to it first, and look at all those pictures! Here's the short answer:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So Neil and Buzz are on the surface, and the CSM is the &quot;return pod&quot; in orbit above the moon. The lunar lander could never make it back home on its own. So they launch the lander off the surface of the moon, but just the top half. The bottom is empty and useless, it's just the bare essentials. The top half has enough fuel to get into orbit, no more. Once it's in the same orbit as the CSM with Michael, they physically connect tips, the pointy end of the rocket shaped craft fitting into the concave top of the lander, and snap together like a seatbelt. They open the docking hatch, a small door on top. Neil, Buzz and a whole bunch of moon rocks go into the silver CSM, and they disconnect from the lander. The CSM rockets home and the lander is left to sit. &lt;/p&gt;&#xA;" OwnerUserId="21786" LastEditorDisplayName="user20636" LastEditDate="2018-05-25T15:03:52.117" LastActivityDate="2018-05-25T15:03:52.117" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23838" PostTypeId="2" ParentId="21809" CreationDate="2017-11-28T07:11:47.273" Score="1" Body="&lt;p&gt;Yes. While you can change your inclination over a burn, in a classic &quot;plain&quot; launch your starting latitude is the inclination peaks where your orbit is directly  above them relative to the launch site location. If you launch from Cape Canaveral and go &quot;straight&quot; from it with no immediate major change in longitude, the antipode will be the opposite coordinates, so 30°N is 30° south. And if you do have a change in longitude it makes your orbit even more inclined. If you do an inclination changing burn in ascent you can avoid any major inclination from launch sites' latitudes.&lt;/p&gt;&#xA;" OwnerUserId="21786" LastActivityDate="2017-11-28T07:11:47.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23839" PostTypeId="2" ParentId="21223" CreationDate="2017-11-28T07:33:12.980" Score="-1" Body="&lt;p&gt;A great piece of software is called &lt;a href=&quot;http://orbit.medphys.ucl.ac.uk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orbiter&lt;/a&gt;, and can help educate in a very hands-on manner the exact procedures for such a mission.&lt;/p&gt;&#xA;" OwnerUserId="21786" LastActivityDate="2017-11-28T07:33:12.980" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23841" PostTypeId="1" CreationDate="2017-11-28T07:49:29.300" Score="2" ViewCount="483" Body="&lt;p&gt;Is there any specific reason especially for the remote sensing satellites passing at the same local time over a particular place each day?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have some experience measuring the radiance received by vegetation at different local times for example, and the variation can be quite large. In the morning it was low, and at noon it was high. I was wondering if there is any specific reason for satellite measurements &lt;em&gt;at the same time each day&lt;/em&gt; rather than sampling at different times on different days, in order to better understand daily variations. &lt;/p&gt;&#xA;" OwnerUserId="21789" LastEditorUserId="12102" LastEditDate="2017-11-28T08:30:13.667" LastActivityDate="2017-11-28T10:32:24.047" Title="Reason for satellite passes at the same local time each day?" Tags="|artificial-satellite|earth-observation|sun-synchronous|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23842" PostTypeId="1" AcceptedAnswerId="23847" CreationDate="2017-11-28T09:55:54.177" Score="29" ViewCount="12541" Body="&lt;p&gt;The altitude of a satellite is the distance between the Earth's surface and the satellite, but the Earth itself is not spherical. At the equator the Earth's radius is 21 km more than at the poles, and in fact the &lt;a href=&quot;https://en.wikipedia.org/wiki/Figure_of_the_Earth&quot; rel=&quot;noreferrer&quot;&gt;shape of the Earth&lt;/a&gt; is not even a perfect oblate spheroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how is the altitude of a satellite actually defined?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a satellite with a circular orbit which is constant - the distance between the Earth's surface to the satellite or the distance between the Earth's center and the satellite?&lt;/p&gt;&#xA;" OwnerUserId="21399" LastEditorUserId="-1" LastEditDate="2017-11-29T15:29:31.930" LastActivityDate="2019-12-31T02:32:25.923" Title="How is the altitude of a satellite defined, given that the Earth is not spherical?" Tags="|orbital-mechanics|orbit|low-earth-orbit|altitude|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23843" PostTypeId="2" ParentId="23842" CreationDate="2017-11-28T10:21:57.133" Score="6" Body="&lt;p&gt;Earth is approximated as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Point_particle&quot; rel=&quot;noreferrer&quot;&gt;point-mass&lt;/a&gt; when doing this calculation. The oblatness of the planet and different gravity potentials can then be computed using spherical harmonics or &lt;a href=&quot;https://ccar.colorado.edu/grace/&quot; rel=&quot;noreferrer&quot;&gt;&quot;mascon&quot;&lt;/a&gt; (short for mass concentration).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When designing special orbits, such as those around the &lt;a href=&quot;http://www.esa.int/Our_Activities/Operations/What_are_Lagrange_points&quot; rel=&quot;noreferrer&quot;&gt;Lagrange points&lt;/a&gt;, it isn't uncommon to define a new reference frame whose center is at barycenter of the two celestial bodies considered for the problem.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-11-28T10:21:57.133" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23844" PostTypeId="2" ParentId="23841" CreationDate="2017-11-28T10:29:36.663" Score="5" Body="&lt;p&gt;These are called &lt;a href=&quot;https://en.wikipedia.org/wiki/Sun-synchronous_orbit&quot; rel=&quot;noreferrer&quot;&gt;&quot;Sun-synchronous orbits&quot;&lt;/a&gt; and extensively used for remote sensing, where satellites are looking down at the Earth's surface. One of the main reasons for these orbits is that the shadows on the ground are always the same (i.e. the illumination angle is constant between each flyby). This is especially useful for imaging since the changes in terrain can be assessed with ease: the shadows are always the same, so if something looks like it has changed, then it is very likely not an abberation but indeed a change in the terrain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example of missions in these orbits include spy satellites.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="15906" LastEditDate="2017-11-28T10:32:24.047" LastActivityDate="2017-11-28T10:32:24.047" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23845" PostTypeId="2" ParentId="23842" CreationDate="2017-11-28T10:34:00.333" Score="8" Body="&lt;p&gt;In orbital mechanics, the position of a satellite can be defined using &lt;a href=&quot;https://en.wikipedia.org/wiki/Orbital_elements&quot; rel=&quot;noreferrer&quot;&gt;orbital elements&lt;/a&gt; (which have some advantages over other coordinate systems, for typical orbits).  Orbital elements describe a position in relation to the &lt;em&gt;centre of mass&lt;/em&gt; of the Earth or any other body.  For example, one of the orbital elements is the semimajor axis, which for a circular orbit is identical to the distance between the satellite and the Earth centre of mass — not the Earth surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As you say, the distance to the Earth surface changes over even a circular orbit.  One can calculate this based on the geoid, and it's useful for many purposes, but it's rather a diagnostic than a prognostic variable.&lt;/p&gt;&#xA;" OwnerUserId="33" LastEditorUserId="33" LastEditDate="2017-11-28T10:41:32.593" LastActivityDate="2017-11-28T10:41:32.593" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23846" PostTypeId="2" ParentId="21223" CreationDate="2017-11-28T10:51:12.933" Score="3" Body="&lt;p&gt;How much do you know about programming? If you're willing to get your hands somewhat dirty by looking at a few examples and coding up your own script, biased-me would recommend you use the software I developed and validated last year for my thesis: &lt;a href=&quot;https://github.com/ChristopherRabotin/smd&quot; rel=&quot;nofollow noreferrer&quot;&gt;space mission design (or smd)&lt;/a&gt;. I have used it successfully to design literally dozens (if not hundreds) of interplanetary trajectories between the Earth and Mars (and back), but also to design a trajectory to Neptune using resonant orbits and several gravity assist maneuvers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Concerning your specific project though, if I may, I would recommend doing an initial analysis using a tool like &lt;a href=&quot;https://github.com/ChristopherRabotin/smd/tree/master/cmd/pcpplots&quot; rel=&quot;nofollow noreferrer&quot;&gt;mine&lt;/a&gt; to find the cheapest $\Delta v$ trajectory from Earth to Mars, and then plugging in the returned values as $C_3$, right ascension and declination of launch into GMAT, and using GMAT to iterate and optimize these values. Also note that you don't need the delta-V expressed in the VNC frame: the C3, DLA and RLA values are enough if you assume that you spacecraft will be sent on an interplanetary trajectory by the launch vehicle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect this sounds complicated at first sight: there should be a GMAT tutorial on most of this, and the rest should be covered by lab assignments of the University of Colorado at Boulder &quot;Space Mission Design&quot; 6008 class of 2017. I'm pretty sure these assignments are available for free online.&lt;/p&gt;&#xA;" OwnerUserId="1391" LastActivityDate="2017-11-28T10:51:12.933" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23847" PostTypeId="2" ParentId="23842" CreationDate="2017-11-28T11:01:30.987" Score="33" Body="&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; 6378.137 km is what I use now.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By convention the altitude of a spacecraft is the distance to the center of the Earth minus roughly 6378 kilometers, or some reference radius that is representative of the equatorial radius of the Earth. Spacecraft altitude is not really used as a precise description of a satellite's position, since its only a scalar and it requires a definition, but if you have to state some value for an altitude, use the distance to the Earth's center of mass minus some reference radius which is generally an average equatorial radius. For example I see the following equation a lot (e.g. &lt;a href=&quot;https://answers.yahoo.com/question/index?qid=20080622164751AAZzoWQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;, &lt;a href=&quot;http://keisan.casio.com/exec/system/1224665242&quot; rel=&quot;nofollow noreferrer&quot;&gt;2&lt;/a&gt;, &lt;a href=&quot;https://www.orbiter-forum.com/showthread.php?t=26682&quot; rel=&quot;nofollow noreferrer&quot;&gt;3&lt;/a&gt;, &lt;a href=&quot;http://www.atmosp.physics.utoronto.ca/people/strong/phy499/section2_05.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;4&lt;/a&gt;, &lt;a href=&quot;http://www.braeunig.us/space/problem.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;5&lt;/a&gt;, &lt;a href=&quot;https://www.quora.com/What-is-a-geostationary-orbit-and-why-is-it-at-35-786-km&quot; rel=&quot;nofollow noreferrer&quot;&gt;6&lt;/a&gt; and in &lt;a href=&quot;https://physics.stackexchange.com/a/48710&quot;&gt;this answer&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$\text{altitude} = |\mathbf{r}| - 6378 \ \text{ or } \ 6378.1\text{ km}$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;s&gt;Another way you can confirm this is to simply &lt;a href=&quot;https://space.stackexchange.com/search?q=6378%22&quot;&gt;search this SE site for &quot;6378&quot;&lt;/a&gt;&lt;/s&gt;. (it turns out most of the time those are from me, so it doesn't count). A more accurate value for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth_radius#Equatorial_radius&quot; rel=&quot;nofollow noreferrer&quot;&gt;equatorial radius of the Earth&lt;/a&gt; could be used, such as &lt;a href=&quot;https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;6378.137&lt;/a&gt;, but the difference is not meaningful because a spacecraft's altitude &lt;em&gt;per se&lt;/em&gt; is not really a precisely defined quantity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read more about the nadir and the question of precisely what point on Earth's surface is actually &quot;directly beneath&quot; the satellite at a given moment, and the difference between the geodetic subsatellite point rather than the geocentric subsatellite point in &lt;a href=&quot;https://space.stackexchange.com/a/19729/12102&quot;&gt;this excellent answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the last part of the question:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For the satellite with circular orbit which is constant - the distance between earth surface to satellite or the distance between earth center and satellite?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The short answer is &quot;neither&quot;, but the answer that's likely to be closest to correct is that the distance from the satellite to &lt;em&gt;the center of mass of the Earth&lt;/em&gt; is likely to be more constant than the distance to the surface, assuming you figure out whether you mean the geodetic or geocentric subsatellite point. This is because the the higher order gravitational terms beyond the monopole are weak and have only a small effect.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A quick way you can confirm that is to note that the Earth's radius varies by more than 20 kilometers from equatorial to polar, and yet satellites in circular orbits can have periapsis and apoapsis within a kilometer of each other. For example I chose two of the satellites in the &lt;a href=&quot;https://en.wikipedia.org/wiki/A-train_(satellite_constellation)&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;A-Train&quot; constellation&lt;/a&gt;, which has a nearly polar orbit. &lt;a href=&quot;https://en.wikipedia.org/wiki/Aqua_(satellite)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aqua&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Aura_(satellite)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aura&lt;/a&gt; and looked up their info in Wikipedia. Both of them have a difference between periapsis and apoapsis of only 2 kilometers or less.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;A useful convention:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using this soft definition, it is common to see orbits described using the altitudes at periapsis and apoapsis. For example, &lt;a href=&quot;https://www.reddit.com/r/spacex/comments/5klz9g/iridium_corporate_satellites_have_been_fueled/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this discussion&lt;/a&gt; of a &lt;em&gt;625 x 625 km&lt;/em&gt; orbit of an Iridium satellite means that it's circular with a radius of 625 + 6378 km, and a &lt;a href=&quot;https://en.wikipedia.org/wiki/Geostationary_transfer_orbit#Other_considerations&quot; rel=&quot;nofollow noreferrer&quot;&gt;Geostationary Transfer Orbit&lt;/a&gt; of &lt;em&gt;185 km x 35,786 km&lt;/em&gt; would have its apses also at about those values plus 6378 km added to each.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-12-31T02:32:25.923" LastActivityDate="2019-12-31T02:32:25.923" CommentCount="11" ContentLicense="CC BY-SA 4.0" />
  <row Id="23848" PostTypeId="1" AcceptedAnswerId="31443" CreationDate="2017-11-28T13:41:49.543" Score="12" ViewCount="1474" Body="&lt;p&gt;What does the software quality process for NASA's SLS look like? What are the relevant standards that they have to comply to? Do they have to show that they are &lt;a href=&quot;https://en.wikipedia.org/wiki/Capability_Maturity_Model_Integration&quot; rel=&quot;noreferrer&quot;&gt;CMMI&lt;/a&gt; compliant, for example? I'm interested in avionics in particular, but basically everything that is going to space. What kind of tools and software are they employing to reach their goal?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am aware of this &lt;a href=&quot;https://www.fastcompany.com/28121/they-write-right-stuff&quot; rel=&quot;noreferrer&quot;&gt;fantastic piece&lt;/a&gt; over at fastcompany, but it describes the space shuttle era. There does not seem to be current information available.&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="198" LastEditDate="2017-11-29T10:44:31.127" LastActivityDate="2019-07-12T20:53:46.860" Title="What does the software quality process for NASA's SLS look like?" Tags="|software|sls|avionics|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23850" PostTypeId="2" ParentId="21809" CreationDate="2017-11-28T18:12:37.443" Score="1" Body="&lt;p&gt;For a direct injection into orbit (i.e. no dog leg later), then by simple geometry, the orbit inclination will be greater than or equal to the absolute value of the latitude of the location of that injection, which is very close to the latitude of the launch site. A launch azimuth of zero (due East or due West) would result in an inclination equal to the latitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There may be further constraints on the inclination due to the allowed launch azimuth to avoid overflight of populated areas, or to provide telemetry coverage with available downtrack ground stations.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;A nice illustration of dog leg maneuver during launch can be found in &lt;a href=&quot;https://space.stackexchange.com/a/21355/12102&quot;&gt;this answer&lt;/a&gt;, and dog legs are mentioned &lt;a href=&quot;https://space.stackexchange.com/a/2682/12102&quot;&gt;here&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/a/2937/12102&quot;&gt;here&lt;/a&gt;, and &lt;a href=&quot;https://space.stackexchange.com/q/20095/12102&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="12102" LastEditDate="2017-11-30T01:38:12.663" LastActivityDate="2017-11-30T01:38:12.663" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23851" PostTypeId="1" AcceptedAnswerId="23858" CreationDate="2017-11-28T22:34:20.387" Score="8" ViewCount="1129" Body="&lt;p&gt;ISS astronaut Randy Bresnik recently posted a video on social media from his spacewalk, shot with a GoPro.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can watch his clip here (Embedding the clip didn't work):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/AstroKomrade/status/935278434158333952&quot; rel=&quot;noreferrer&quot;&gt;https://twitter.com/AstroKomrade/status/935278434158333952&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there is obviously sounds coming from his movements and his suit and so forth. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But why is there a constant noise in the audio?&lt;/strong&gt; To me it sounds like a really bad recording. Is that just common with GoPros or what is the thing here? Any clever ideas?&lt;/p&gt;&#xA;" OwnerUserId="21811" OwnerDisplayName="lthz" LastEditorUserId="12102" LastEditDate="2017-11-29T00:15:19.250" LastActivityDate="2017-11-29T17:37:15.327" Title="Where does the noise on spacewalks come from?" Tags="|sound|eva|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23853" PostTypeId="2" ParentId="23758" CreationDate="2017-11-29T03:30:43.867" Score="9" Body="&lt;p&gt;The answer broadly is yes, but there are a ton of caveats.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used to work with applying Nickel-Titanium alloys (usually known as Nitinol) as actuators in the biomedical field. Before going any further I'll relay the actual advice I was given by an applications engineer from a leading supplier of specialist metals: &quot;If you can find any other way to achieve your goal without using Nitinol, you should do it. [...] Often we find that the models are unreliable and we just have to try it and see if it works&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I still harbour a burning hatred for this damnable metal. I'm convinced it hates me. I have no idea what I did to offend it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Back to your question:&lt;/strong&gt; Nickel-Titanium (in the right proportions) is a shape-memory-allow (SMA), however in this application the SMA property is not being used directly, rather the Nitinol's superelasticity is what gives the wheels their resiliency to deformation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a video showing Nitinol's SMA properties: &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/VDT7a5twx28?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nickel_titanium&quot; rel=&quot;nofollow noreferrer&quot;&gt;really good definition of Nitinol&lt;/a&gt; and it's SMA and superelastic properties: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Shape memory is the ability of nitinol to undergo deformation at one temperature, then recover its original, undeformed shape upon heating above its &quot;transformation temperature&quot;. Superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10-30 times that of ordinary metal.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Nitinol is an &lt;a href=&quot;https://en.wikipedia.org/wiki/Intermetallic&quot; rel=&quot;nofollow noreferrer&quot;&gt;intermetallic compound&lt;/a&gt;, which gives it a highly regular crystalline structure compared to standard alloys. The crystal structure takes on two separate forms at different temperatures, martinsitic at low temperatures, and austenitic at high temperature. The ability to switch between crystalline states is what gives the alloys it's shape memory properties (I'm oversimplifying, but this is close enough). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image below shows the Nitinol hysteresis loop, again, &lt;a href=&quot;https://en.wikipedia.org/wiki/Nickel_titanium#Mechanism&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia has the best explanation&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;There are four transition temperatures associated to the austenite-to-martensite and martensite-to-austenite transformations. Starting from full austenite, martensite begins to form as the alloy is cooled to the so-called martensite start temperature, or Ms, and the temperature at which the transformation is complete is called the martensite finish temperature, or Mf. When the alloy is fully martensite and is subjected to heating, austenite starts to form at the austenite start temperature, As, and finishes at the austenite finish temperature, Af.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fqQGW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fqQGW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The key thing is that the temperature at which the shape-memory and superelasticity occur can be controlled by precisely controlling the ratio of Nickel to Titanium in the alloy, and thus shifting the point of martinsitic transformation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/20NMk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/20NMk.jpg&quot; alt=&quot;https://en.wikipedia.org/wiki/File:Nitinol_Ms_vs_Ni_content.jpg&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This means that it is possible to create a version of Nitnol that exhibits superelasticity at room temperature, or even at 100 degrees below zero. Thus, no heating is needed to reset the wheels as it could be said that they are in a constant state of &quot;resetting&quot; if the Mf temperature is set correctly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All that said, the temperature swings on Mars are huge, and Nitinol's superelastic window is only ~40 degrees above the Af temperature. How the designers plan to maintain the superelastic properties over the ~150 degree temperature swings on Mars is unclear. Maybe a thermal management system would be used, or alternatively the rover could be operated only when the temperature would allow optimum conditions for the wheels. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the other part of your question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars 2020 is intended to be a sister robot to Curiosity with the only main difference being the instruments. This far through the mission campaign it's probably too late to change major components like wheels, especially when the TRL (&lt;a href=&quot;https://en.wikipedia.org/wiki/Technology_readiness_level&quot; rel=&quot;nofollow noreferrer&quot;&gt;Technology Readiness Level&lt;/a&gt;) for these new wheels is probably only around 5-6 (technology demonstrator) at the moment.&lt;/p&gt;&#xA;" OwnerUserId="6312" LastEditorUserId="18085" LastEditDate="2017-11-29T19:20:51.493" LastActivityDate="2017-11-29T19:20:51.493" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23854" PostTypeId="1" AcceptedAnswerId="23856" CreationDate="2017-11-29T14:16:49.997" Score="-6" ViewCount="228" Body="&lt;p&gt;I have a answer from the last question that I asked (yes the rail-gun one) and was wondering if it could possible work (My spelling is shit by the way&#xA;) sorry about not saying this there is mettle ball going before a satellite. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;This is not a space gun people!&lt;/strong&gt; just a consept for when it is possible &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;I wrote that one as a guest not on this account&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="21639" LastEditorUserId="21639" LastEditDate="2017-11-29T14:51:45.800" LastActivityDate="2017-11-29T15:54:25.993" Title="would launching from a vacuum tunnel in a mountain from a 45 degree angle be the best way to launch a space shuttle" Tags="|rail-gun|" AnswerCount="1" CommentCount="8" ClosedDate="2017-11-30T01:01:07.230" ContentLicense="CC BY-SA 3.0" />
  <row Id="23855" PostTypeId="2" ParentId="23753" CreationDate="2017-11-29T14:19:55.607" Score="6" Body="&lt;p&gt;The Soviets had the very first intercontinential ballistic missile, the R-7 Semyorka. This rocket was designed for very heavy nuclear warheads and could be used for manned orbital missions too. Cold war and space race led to the decision to start with orbital manned launches and to skip suborbital flights.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_missiles#R&quot; rel=&quot;nofollow noreferrer&quot;&gt;list&lt;/a&gt; the R-7 was the first russian two stage rocket.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The R-7 and their sucessors were very successful and reliable rockets. With some modifications, these rockets were also used for robotic moon missions. The transport of astronauts to the ISS is still done with this rocket family.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the R-7 was a liquid fuel rocket using liquid oxygen and rocket petrol. Preparation for launch took many hours, later ICBM rockets used solid fuel or storable hypergolic liquid fuel to be ready for launch in a very short time.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-11-30T17:51:07.343" LastActivityDate="2017-11-30T17:51:07.343" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23856" PostTypeId="2" ParentId="23854" CreationDate="2017-11-29T15:54:25.993" Score="2" Body="&lt;p&gt;&lt;strong&gt;Yes&lt;/strong&gt;, &lt;strong&gt;No&lt;/strong&gt;, &lt;strong&gt;Maybe&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, you have to define what is the &quot;best way&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Considering the technology for such a railgun is science fiction it's impossible to answer you without detailed parameters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firsts points to consider:&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;If the barrel end within the atmosphere, the shuttle will be subject depending on the altitude to:&lt;/h3&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;High drag&lt;/li&gt;&#xA;&lt;li&gt;Forces suffisant to kill any occupant &lt;/li&gt;&#xA;&lt;li&gt;Forces suffisant to destroy it&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h3&gt;If the final speed is high enough, you are still suborbital (unless you reach escape velocity.&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;To reach space you need a horizontal component. Your perigee (lowest point of your orbit) will ALWAYS be at the lowest acceleration altitude. One other way to put it is that you always go back where you where originally, even if you accelerate a lot. You need a &quot;circularization burn&quot;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So now to try to answer you I'd say:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;If you want to escape earth, a &lt;strong&gt;vertical&lt;/strong&gt; railgun is best&lt;/li&gt;&#xA;&lt;li&gt;If you want to orbit a &lt;strong&gt;horizontal&lt;/strong&gt; railgun is best: you will gain the most speed from it (assuming you are vertical but do NOT follow the earth curvature, you'll still reach space).&lt;/li&gt;&#xA;&lt;li&gt;If you don't have illimited materials, the best angle will be &lt;strong&gt;somewhere in between&lt;/strong&gt;, depending on your engineering requirements.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Now about 45 degrees, one would need detailed parameters to answer you, but yes this is a &quot;good&quot; angle. If I were to write a SF book with railguns on earth this would be my pick. Or the horizontal one, because that would look really awesome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course it would be made of &lt;a href=&quot;https://en.wikipedia.org/wiki/Unobtainium&quot; rel=&quot;nofollow noreferrer&quot;&gt;Unobtainium&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6642" LastActivityDate="2017-11-29T15:54:25.993" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23857" PostTypeId="2" ParentId="23842" CreationDate="2017-11-29T17:07:55.900" Score="7" Body="&lt;p&gt;To expand on the other answers, it really depends on what you want to use the altitude for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Shuttle Mission Simulator, we calculated the orbital altitude several different ways.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Altitude above the Fischer Ellipsoid&lt;/li&gt;&#xA;&lt;li&gt;Altitude above Mean Sea Level&lt;/li&gt;&#xA;&lt;li&gt;Altitude above the selected landing site&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-11-29T17:07:55.900" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23858" PostTypeId="2" ParentId="23851" CreationDate="2017-11-29T17:14:27.650" Score="6" Body="&lt;p&gt;&lt;strong&gt;Short answer:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These &quot;sounds&quot; consist of vibrations produced by the astronaut's movements and his suit systems and transferred through his space suit, camera enclosure and PCB the microphone is soldered to plus electrical and digital noise added by the sound processing pipeline.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Long answer:&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Microphone is an electromechanical device, that converts vibrations into electricity. &lt;a href=&quot;http://jayrose.com/tutorial/archive/0007/fig01a.jpg&quot; rel=&quot;noreferrer&quot;&gt;Different&lt;/a&gt; &lt;a href=&quot;http://hyperphysics.phy-astr.gsu.edu/hbase/Audio/imgaud/conm.gif&quot; rel=&quot;noreferrer&quot;&gt;types&lt;/a&gt; of &lt;a href=&quot;https://www.njr.com/products/MEMS/images/MEMS_structure_e.jpg&quot; rel=&quot;noreferrer&quot;&gt;microphones&lt;/a&gt; contain different mechanical parts, that can easily perceive air density oscillations. Usually it is a sort of thin foil membrane fixed around edges and vibrating in the middle. Since its so lightweight, it picks all sorts of vibrations that reach it - taps of your fingers touching the enclosure and buttons, your steps, ultrasounds that you can't even hear - everything (that's why on Earth studio microphones are frequently vibroisolated using so called &lt;a href=&quot;https://images-na.ssl-images-amazon.com/images/I/41XL-xUY5PL.jpg&quot; rel=&quot;noreferrer&quot;&gt;shock mounts&lt;/a&gt;). You don't need air to conduct them, because dense materials usually conduct vibrations even better than air (think about echolocation on the submarines). Also membrane sensitivity should be somewhat stronger in space, where the amplitude of membrane's vibrations isn't limited by the pull of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This membrane is one half of variable capacitor or a coil, which produces small oscillations of current in the input circuit of the microphone pre-amplifier, thus converting them into electricity. You need pre-amp, because those oscillations are really small, in the millivolts range. Pre-amp produces amplified signal suitable for analog-to-digital conversion and recording, but all the pipeline parts - amplification, ADC and sound compression - add their own noise to the microphone signal. That's why you hear something even when there are no obvious vibrations produced by the astronaut himself.&lt;/p&gt;&#xA;" OwnerUserId="21572" LastEditorUserId="21572" LastEditDate="2017-11-29T17:37:15.327" LastActivityDate="2017-11-29T17:37:15.327" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="23859" PostTypeId="1" CreationDate="2017-11-29T17:47:18.897" Score="0" ViewCount="118" Body="&lt;p&gt;Specifically the number of engines simultaneously ignited. So not summing up over different stages. I am interested in the max for both number of engines and number of combustion chambers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I have found so far:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Soyuz: 20 chambers (+12 vernier chambers) / 5 engines&lt;/li&gt;&#xA;&lt;li&gt;Energia: 20 chambers / 8 engines&lt;/li&gt;&#xA;&lt;li&gt;Falcon 9: 9 chambers / 9 engines&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Disqualified rockets:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;N1: 30 chambers / 30 engines / No successful launches&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.space.com/33810-nasa-world-record-most-rocket-engines.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;This&lt;/a&gt;: 44 engines / Sounding rocket; majority of engines were part of scientific payload; this is an upper bound though&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Will the Falcon Heavy have more chambers / engines then any successful rocket to date? Does any other successful rocket have 9 or more engines? or has SpaceX already claimed that record?&lt;/p&gt;&#xA;" OwnerUserId="21390" LastEditorUserId="21390" LastEditDate="2017-11-30T06:55:26.187" LastActivityDate="2017-11-30T06:55:26.187" Title="What is the largest number of engines ever used on an orbital class rocket?" Tags="|rockets|history|engine-design|" AnswerCount="0" CommentCount="6" ClosedDate="2017-11-30T01:04:47.527" ContentLicense="CC BY-SA 3.0" />
  <row Id="23860" PostTypeId="1" AcceptedAnswerId="23862" CreationDate="2017-11-30T10:27:33.270" Score="-1" ViewCount="790" Body="&lt;p&gt;The question &lt;a href=&quot;https://astronomy.stackexchange.com/q/23563/7982&quot;&gt;Astronomical ABCD…!&lt;/a&gt; suggests a corollary for SXSE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for one example of a rocket with $n$ first-stage engines, for each $n$ for which you can think of an example for. It should be at least built, or very seriously proposed and investigated at a minimum.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll start off with some suggestions, items of recent popularity to get the ball rolling, but you don't have to use them.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;1&lt;/strong&gt; - &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon-1&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;9&lt;/strong&gt; - &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9&quot; rel=&quot;nofollow noreferrer&quot;&gt;Falcon-9&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Let's first try without jumping immediately to vehicles with variable strap-on number options, and see how that goes?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-30T10:56:16.157" LastActivityDate="2020-03-03T00:29:19.447" Title="Rockets with n-engines, for n = 1, 2, 3..." Tags="|rockets|booster|" AnswerCount="2" CommentCount="8" ClosedDate="2017-11-30T15:39:50.507" ContentLicense="CC BY-SA 3.0" />
  <row Id="23861" PostTypeId="1" CreationDate="2017-11-30T10:44:00.777" Score="1" ViewCount="932" Body="&lt;p&gt;Don't be distracted by the 1960 Mercury shown below. Reading about &lt;a href=&quot;https://www.nap.edu/read/12884/chapter/16&quot; rel=&quot;nofollow noreferrer&quot;&gt;Irene Fischer&lt;/a&gt; (also &lt;a href=&quot;https://en.wikipedia.org/wiki/Irene_Fischer&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;, and mentioned in &lt;a href=&quot;https://space.stackexchange.com/a/23857/12102&quot;&gt;this answer&lt;/a&gt;) and seeing the Fischer 1960 Mercury Ellipsoid turn up in tables of ellipsoid piqued my interest.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is the Fischer 1960 Mercury Ellipsoid, and why is it called that?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;The other kind of 1960 Mercury, &lt;a href=&quot;http://hooniverse.com/2013/01/02/the-last-full-sized-mercury-with-a-unique-body-and-it-was-for-sale-on-ebay/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this particular specimen&lt;/a&gt; believe it or not came with factory AC and power windows! (click for larger)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lfEFc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lfEFcm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;a href=&quot;https://i.stack.imgur.com/jqmQB.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/jqmQBm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-11-30T11:01:09.880" LastActivityDate="2017-12-02T05:39:41.647" Title="What is the Fischer 1960 Mercury Ellipsoid, and why is it called that?" Tags="|low-earth-orbit|earth|altitude|" AnswerCount="2" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="23862" PostTypeId="2" ParentId="23860" CreationDate="2017-11-30T11:12:39.140" Score="3" Body="&lt;p&gt;British, very seriously proposed and investigated (and the Black Arrow orbited a satellite after it was cancelled): &lt;span class=&quot;math-container&quot;&gt;$$\begin{array} {r l}&#xA;\text{Black Arrow}^1 &amp;amp; \text{1 engine with 8 combustion chambers} \\&#xA;\text{Europa}^2 &amp;amp; \text{2 rocket engines} \\&#xA;\text{MUSTARD}^3 &amp;amp; \text{3 engines (one in each of the triamese craft)} \\&#xA;\text{Interim Hotol}^4 &amp;amp; \text{4 engines} \\&#xA;\text{Skylon D1}^5 &amp;amp; \text{6 engines with 12 combustion chambers/nozzles.}^* \\&#xA;\hline&#xA;\text{The BIS Space Ship (1939)}^6 &amp;amp; \text{168 solid motors}^{**}&#xA;\end{array}$$&lt;/span&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;* two engines per nacelle, two SOMA engines in the tail. All running during the latter parts of the ascent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;** OK, perhaps not &lt;em&gt;so&lt;/em&gt; seriously&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Black_Arrow&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Black_Arrow&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Europa_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Europa_(rocket)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;umm...&lt;/li&gt;&#xA;&lt;li&gt;err...&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://web.archive.org/web/20160328220308/http://www.reactionengines.co.uk/tech_docs/SKYLON_Users_Manual_Rev_2.1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://web.archive.org/web/20160328220308/http://www.reactionengines.co.uk/tech_docs/SKYLON_Users_Manual_Rev_2.1.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.bis-space.com/what-we-do/projects/bis-lunar-spaceship&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.bis-space.com/what-we-do/projects/bis-lunar-spaceship&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerDisplayName="user20636" LastEditorDisplayName="user20636" LastEditDate="2020-03-03T00:29:19.447" LastActivityDate="2020-03-03T00:29:19.447" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="23864" PostTypeId="2" ParentId="23860" CreationDate="2017-11-30T13:53:39.253" Score="2" Body="&lt;ul&gt;&#xA;&lt;li&gt;5: &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_V&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn V&lt;/a&gt;, first and second stages.&lt;/li&gt;&#xA;&lt;li&gt;8: &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_I&quot; rel=&quot;nofollow noreferrer&quot;&gt;Saturn I&lt;/a&gt;, first stage.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="13208" LastActivityDate="2017-11-30T13:53:39.253" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23865" PostTypeId="2" ParentId="23861" CreationDate="2017-11-30T15:56:44.050" Score="5" Body="&lt;p&gt;The U.S. Department of Defense (DOD) 1960 World Geodetic System was produced by Irene Fischer and published in 1960. Today they are commonly known as 1960 Fisher Ellipsoids, as they are called today, is a mapping of the shape of the Earth, published by Irene Fischer in 1960. This was the best unclassified ellipsoid available at the time. As a result, NASA used them for the Mercury project, which had it's first manned launch in 1961. Thus they gained the nickname &quot;Mercury Datum&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://books.google.com/books?id=B97bWOGPHIUC&amp;amp;pg=PT225&amp;amp;lpg=PT225&amp;amp;dq=Mercury+Datum+Ellipsoid&amp;amp;source=bl&amp;amp;ots=SzXFVvB8Zs&amp;amp;sig=2h30pt-mkIEWZr11UopREFvx8ro&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjZ-4Wi0-bXAhXFKCYKHR8cAWoQ6AEIWzAJ#v=onepage&amp;amp;q=Mercury%20Datum%20Ellipsoid&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://books.google.com/books?id=B97bWOGPHIUC&amp;amp;pg=PT225&amp;amp;lpg=PT225&amp;amp;dq=Mercury+Datum+Ellipsoid&amp;amp;source=bl&amp;amp;ots=SzXFVvB8Zs&amp;amp;sig=2h30pt-mkIEWZr11UopREFvx8ro&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjZ-4Wi0-bXAhXFKCYKHR8cAWoQ6AEIWzAJ#v=onepage&amp;amp;q=Mercury%20Datum%20Ellipsoid&amp;amp;f=false&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.nap.edu/read/12884/chapter/16#79&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nap.edu/read/12884/chapter/16#79&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://books.google.com/books?id=QeGL4Rw9nxgC&amp;amp;pg=PA153&amp;amp;lpg=PA153&amp;amp;dq=Mercury+Datum+Ellipsoid+Fischer&amp;amp;source=bl&amp;amp;ots=cGX_w5QZeo&amp;amp;sig=1NQHBYXcJf4OSxHdFik-dLmIRUA&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjqyafR1ObXAhWMSSYKHb1sCIEQ6AEIQzAD#v=onepage&amp;amp;q=Mercury%20Datum%20Ellipsoid%20Fischer&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://books.google.com/books?id=QeGL4Rw9nxgC&amp;amp;pg=PA153&amp;amp;lpg=PA153&amp;amp;dq=Mercury+Datum+Ellipsoid+Fischer&amp;amp;source=bl&amp;amp;ots=cGX_w5QZeo&amp;amp;sig=1NQHBYXcJf4OSxHdFik-dLmIRUA&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwjqyafR1ObXAhWMSSYKHb1sCIEQ6AEIQzAD#v=onepage&amp;amp;q=Mercury%20Datum%20Ellipsoid%20Fischer&amp;amp;f=false&lt;/a&gt;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6944" LastEditDate="2017-12-01T01:00:58.153" LastActivityDate="2017-12-01T01:00:58.153" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23867" PostTypeId="1" AcceptedAnswerId="36510" CreationDate="2017-11-30T19:22:11.587" Score="12" ViewCount="841" Body="&lt;p&gt;The first US two stage rocket was &lt;a href=&quot;https://en.wikipedia.org/wiki/RTV-G-4_Bumper&quot; rel=&quot;nofollow noreferrer&quot;&gt;bumper&lt;/a&gt;, a combination of a german V2 liquid fuel rocket with an US second &lt;a href=&quot;https://en.wikipedia.org/wiki/WAC_Corporal&quot; rel=&quot;nofollow noreferrer&quot;&gt;stage&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Soviet Union had the &lt;a href=&quot;https://en.wikipedia.org/wiki/R-1_(missile)&quot; rel=&quot;nofollow noreferrer&quot;&gt;R-1&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/R-2_(missile)&quot; rel=&quot;nofollow noreferrer&quot;&gt;R-2&lt;/a&gt; based on the V-2.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In this &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_missiles#R&quot; rel=&quot;nofollow noreferrer&quot;&gt;list&lt;/a&gt;, the &lt;a href=&quot;https://en.wikipedia.org/wiki/R-7_Semyorka&quot; rel=&quot;nofollow noreferrer&quot;&gt;R-7&lt;/a&gt; is the first two stage rocket (or one and a half stage) of the Soviet Union. Did they never try an experimental two stage rocket using the R-1 or R-2 as first stage? &lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="27091" LastEditDate="2019-06-03T15:40:22.007" LastActivityDate="2019-06-03T15:40:22.007" Title="Was the R-7 the first two stage rocket of the Soviet Union?" Tags="|rockets|history|soviet-union|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="23868" PostTypeId="1" CreationDate="2017-11-30T20:06:36.783" Score="5" ViewCount="1294" Body="&lt;p&gt;One idea that's often floated when discussing ways of constructing martian habitats is that of &quot;doming&quot; over small craters or valleys and pressurizing the whole area. This would have several advantages such as having a large open space for scientists/colonists to spend time out of their spacesuit and protecting the people from the harsh conditions on the surface. One problem that this presents though is that underneath a transparent dome the people would be exposed to more radiation from the sun and cosmic background radiation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Question: What would be the most effective building material that is transparent, easy to obtain on mars, and protects the people inside from radiation and other martian dangers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My idea: Glass can be produced on Mars with the abundant silica deposits and reinforced with layers of plastic brought from earth. Then, these glass panels are cut into triangular shapes and two geodesic dome structures are built inside each other. The space in between these two domes is filled with water providing an excellent radiation shield and automatically &quot;repairing&quot; any punctures or cracks due to micrometeorite impacts by freezing upon contact with the martian atmosphere. Water is either insulated so it doesn't freeze inside the dome or is actively heated (maybe as coolant for a nuclear reactor).&lt;/p&gt;&#xA;" OwnerUserId="21538" LastActivityDate="2019-02-14T15:15:09.727" Title="Transparent radiation shielding for martian surface bases" Tags="|mars|colonization|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="23869" PostTypeId="2" ParentId="22899" CreationDate="2017-11-30T20:06:38.237" Score="4" Body="&lt;p&gt;This answer clarifies a detail over Russell Borogove's answer.&lt;/p&gt;&#xA;&lt;p&gt;At typical surface conditions N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; is colourless but dis-associates readily into NO&lt;sub&gt;2&lt;/sub&gt;.  The latter is the gas that has the reddish brown colour; it is also a hazardous (toxic) gas.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="47149" LastEditDate="2022-05-29T10:25:50.127" LastActivityDate="2022-05-29T10:25:50.127" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23870" PostTypeId="2" ParentId="22018" CreationDate="2017-11-30T20:36:05.960" Score="1" Body="&lt;p&gt;If you are looking perpendicular to the orbital plane, instead of radially outward, you can have your secondary sensor at a lower orbit by your chosen distance &lt;em&gt;d&lt;/em&gt;. It will drift ahead of the craft by about &lt;em&gt;3 pi d&lt;/em&gt; for every orbit. For LEO it only moves ahead by about a tenth of a second per orbit. So the images from the secondary sensor slightly earlier so that the positions in orbit match up.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For 7000km orbit you have ~100 orbits per week. With 60m separation that is 7.5 seconds of separation per week. You could always compensate with correction burns between measurements or launching a new sensor once the first is to far away. probably the ideal configuration though would be a tidally locked tether, but that does not satisfy the question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The obvious con to this configuration is that you get a more limited view of the sky.&lt;/p&gt;&#xA;" OwnerUserId="21390" LastActivityDate="2017-11-30T20:36:05.960" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="23872" PostTypeId="2" ParentId="20601" CreationDate="2017-12-01T10:47:55.680" Score="6" Body="&lt;p&gt;The different brightness is caused by the fuel. The liquid fuel rockets burn hydrogen with oxygen, both are gases and the reaction product is hot water vapour, also a gas. But pure gas flames emit very little light, that is why &lt;a href=&quot;https://en.wikipedia.org/wiki/Gas_mantle&quot; rel=&quot;nofollow noreferrer&quot;&gt;gas mantles&lt;/a&gt; are used for camping gas lanterns. The hot solid mesh emits far more light than the hot gas heating the mesh.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The solid fuel boosters contain aluminium powder and an oxidizer. Aluminium and magnesium are known to burn very bright.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason is &lt;a href=&quot;https://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kirchhoff's law of thermal radiation&lt;/a&gt;; the emissivity for visible light is much bigger for solids than for gases.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="6816" LastEditDate="2017-12-02T03:52:53.120" LastActivityDate="2017-12-02T03:52:53.120" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="23873" PostTypeId="1" CreationDate="2017-12-01T13:38:10.943" Score="5" ViewCount="2093" Body="&lt;p&gt;A previous question I asked here got clarification on the Earth-passing phases of the Lunar Cycler orbits that use the &quot;backflip&quot; maneuver. It came up in the book &lt;a href=&quot;https://rads.stackoverflow.com/amzn/click/0553448129&quot; rel=&quot;noreferrer&quot;&gt;Artemis&lt;/a&gt;, and it used the name Uphoff-Crouch cycler (Uphoff &amp;amp; Crouch were the authors of the linked paper), so I will use that here. From the previous question, the key information was:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Earth-return trajectory that has a period of 1/2 month (or 1/3 month is some cases) in order to ensure return to the Moon after 2 &lt;strong&gt;(or 3)&lt;/strong&gt; revolutions of the Cycler in its Earth-return orbit&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(bold emphasis mine)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3889/what-does-the-backflip-lunar-cycler-do-in-its-pass-by-earth&quot;&gt;What does the BackFlip lunar cycler do in its pass by Earth?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I started trying to run through the math, and the computations are put in this worksheet:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://github.com/AlanCoding/Lunar-Cycler/blob/master/Lunar%20Cycler%20math.ipynb&quot; rel=&quot;noreferrer&quot;&gt;https://github.com/AlanCoding/Lunar-Cycler/blob/master/Lunar%20Cycler%20math.ipynb&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I came to an impasse on the 3-orbit variety. I'll try to express the problem in light math here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Say the moon's orbit is roughly circular, Ra=Rp. The elliptical transfer orbit is high elliptical, Rp=0 (we can relax both of these with numbers, later). For the 3-pass case, the transfer orbit's period must divide the moon's orbit's period by 3... but at first glance, it looks like &lt;em&gt;it can't do that&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starting with the equation for orbital period...&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;a - semi-major axis&lt;/li&gt;&#xA;&lt;li&gt;T - orbital period&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/w2Zr9.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/w2Zr9.png&quot; alt=&quot;math&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Problem: the factor above is below unity for f=3. Simply, 2/3^(2/3)=0.96&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is saying that the apogee of a 3-pass transfer orbit would not reach the moon. Was it just carelessness that the Uphoff-Crouch paper assumed a 3-pass orbit would work? I did play with numbers breaking the assumption of circularity of the moon's orbit, and making perigee of the transfer orbit equal to Earth's radius, and that changed 0.96 to 0.94, so that's not helping.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Surely, a 3-pass orbit would be superior to a 2-pass orbit (which I have still to prove to myself works), because it would be lower-energy.&lt;/p&gt;&#xA;" OwnerUserId="26" LastActivityDate="2017-12-02T03:40:13.843" Title="Is the Uphoff-Crouch cycler orbit with 3 passes impossible?" Tags="|orbital-mechanics|the-moon|low-energy-transfer|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23875" PostTypeId="1" CreationDate="2017-12-01T17:57:28.760" Score="3" ViewCount="794" Body="&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/mission_pages/sunearth/news/flare-impacts.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;When a solar flare hits the earth it can damage certain electrical equipment&lt;/a&gt;. If one manages to strike the SpaceX Mars rocket it could possibly damage some significant equipment on board (like navigation and life support). So what can these rockets do to protect themselves from these harmful solar flares and other radiation.&lt;/p&gt;&#xA;" OwnerUserId="21864" LastEditorUserId="15906" LastEditDate="2017-12-14T10:42:58.637" LastActivityDate="2018-09-24T09:39:16.510" Title="How can SpaceX's Mars spacecraft be protected against solar flares?" Tags="|spacex|radiation|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23877" PostTypeId="2" ParentId="23873" CreationDate="2017-12-02T03:40:13.843" Score="3" Body="&lt;p&gt;Set your time and length units to LD and lunar period and you can lose the 2 pi and mu.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$T=a^{3/2}$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if you wanted an ellipse whose period was 2/3 that of the moon you'd have&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$2/3=a^{3/2}$&lt;br&gt;&#xA;$2/3^{2/3}=a$&lt;br&gt;&#xA;$a=.763$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;.763 of an LD is .763 * 384400 = 293352 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If perigee were 6678 km then apogee would be (2*293352)-6678. Which is 580026 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you wanted a lower apogee you could raise the perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you wanted an elliptical orbit whose period is 1/3 that of the moon's you'd have:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$1/3=a^{3/2}$&lt;br&gt;&#xA;$1/3^{2/3}=a$&lt;br&gt;&#xA;$a=.481$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;.481 of 384400 is 184800 km&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again apogee would be 2a - perigee. If perigee were 6678 km, apogee would be 362922.5 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In my opinion this apogee is too close to the moon. If the cycler goes deep inside the lunar Hill sphere, lunar perturbation will wreck the cycler orbit (I believe). But again apogee could be lowered by raising perigee.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-12-02T03:40:13.843" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23880" PostTypeId="2" ParentId="23861" CreationDate="2017-12-02T05:23:53.440" Score="-1" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Irene_Fischer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; seems to suggest that the relationship was more than &quot;&lt;a href=&quot;https://space.stackexchange.com/a/23865/12102&quot;&gt;This was the best unclassified ellipsoid available at the time. As a result, NASA used them...&lt;/a&gt;&quot;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Fischer became one of two internationally known women scientists in the field of geodesy during the golden age of the Mercury and Apollo moon missions. Her Mercury Datum, or Fischer Ellipsoid 1960 and 1968, as well as her work on the lunar parallax, were instrumental in conducting these missions. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Since the Mercury and Apollo missions required accurate orbital calculations both around the Earth and to the moon, the geometrical relationships and accurate 3 dimensional vectors between locations on Earth — especially the tracking stations — was extremely important.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The use of satellite data in Geodesy was in its very earliest stages at this point obviously, but using satellite tracking to define positions on Earth was critical to subsequent use of those positions on Earth to track Mercury and Apollo spacecraft!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll expand on this answer later today; in the mean time &lt;a href=&quot;https://en.wikipedia.org/wiki/Irene_Fischer#Bibliography_of_Publications&quot; rel=&quot;nofollow noreferrer&quot;&gt;this bibliography&lt;/a&gt; offers some clues.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-12-02T05:39:41.647" LastActivityDate="2017-12-02T05:39:41.647" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23883" PostTypeId="1" AcceptedAnswerId="23903" CreationDate="2017-12-02T15:03:08.467" Score="7" ViewCount="2702" Body="&lt;p&gt;Elon Musk made another characteristic splash recently with his plan to &lt;a href=&quot;https://arstechnica.com/science/2017/12/with-bowie-playing-on-the-radio-elon-musk-plans-to-launch-his-tesla-to-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;launch a Falcon Heavy into orbit around Mars&lt;/a&gt;, with the bonus payload of his personal Tesla Roadster inside.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, it's an experimental launch, and quite possibly might blow up on launch. If it doesn't, it will head into orbit around Mars, apparently indefinitely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Musk was quoted in the above article as saying it&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&quot;Will be in deep space for a billion years or so if it doesn’t blow up on ascent&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Assuming he means Mars orbit when he says &quot;deep space&quot;, and assuming the rocket makes it there, how long would it &lt;em&gt;actually&lt;/em&gt; stay in orbit, without refueling or any kind of human interference? I would assume the rocket might have some fuel left for correcting its trajectory, and it would need far less than, say, the ISS in LEO, where there's more drag due to Earth's dense atmosphere. But would it really last a billion years? Or will its orbit degrade and its body disintegrate before then?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I admit I am imagining a stereotypical circular orbit around Mars... could Elon be talking about a larger Elliptical orbit akin to a comet? In that case, I imagine there are even more variables that would make a billion-year lifespan unpredictable.&lt;/p&gt;&#xA;" OwnerUserId="4592" LastEditorUserId="4592" LastEditDate="2017-12-02T22:58:45.270" LastActivityDate="2017-12-06T08:43:45.013" Title="How long would a Falcon Heavy last in orbit around Mars before degrading?" Tags="|orbital-mechanics|spacex|mars|orbit|falcon-heavy|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23886" PostTypeId="1" CreationDate="2017-12-02T21:28:10.797" Score="3" ViewCount="1453" Body="&lt;p&gt;Say we have a guy orbiting Earth in LEO with no spacesuit (I know, I know), what would be the millisieverts he would be receiving?&lt;/p&gt;&#xA;" OwnerUserId="21885" LastActivityDate="2017-12-16T18:12:23.160" Title="What's the unshielded radiation dose in mSv in Low Earth Orbit?" Tags="|orbit|low-earth-orbit|radiation|earth|cosmic-radiation|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23887" PostTypeId="1" AcceptedAnswerId="23889" CreationDate="2017-12-03T02:42:49.770" Score="27" ViewCount="11537" Body="&lt;p&gt;Specifically, I'm interested in how closely the models used to calculate the various burns and course corrections represented reality. Was standard Newtonian mechanics sufficient or were relativistic effects included? Were the Earth, Moon, and spacecraft modelled as point masses or more complicated bodies? Were forces such as solar wind included in the calculations? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Approaching the question a different way: to what extent was the control of the spacecraft pre-calculated, vs. calculated in real time by spacecraft computers, vs. done manually by the astronauts? &lt;/p&gt;&#xA;" OwnerUserId="21891" LastActivityDate="2017-12-13T17:48:58.317" Title="How complex was the math and physics necessary to place Apollo 11 on the moon?" Tags="|orbital-mechanics|apollo-program|physics|mathematics|flight-computer|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23889" PostTypeId="2" ParentId="23887" CreationDate="2017-12-03T05:24:36.190" Score="29" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Was standard Newtonian mechanics sufficient or were relativistic effects included? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Relativistic effects didn't have to be modeled; other sources of error&#xA;would have swamped the effects of relativity, and midcourse corrections were made. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Were the Earth, Moon, and spacecraft modelled as point masses or more complicated bodies? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The moon's gravity was modeled with the &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19700042008&quot; rel=&quot;noreferrer&quot;&gt;L-1 potential model&lt;/a&gt; consisting of &lt;a href=&quot;http://ccar.colorado.edu/asen5050/projects/projects_2014/Hardy_Ryan/&quot; rel=&quot;noreferrer&quot;&gt;&quot;5 coefficients out to a maximum degree of 3&quot;&lt;/a&gt;. I don't know a lot about &lt;a href=&quot;https://en.wikipedia.org/wiki/Geopotential_model&quot; rel=&quot;noreferrer&quot;&gt;geopotential modeling&lt;/a&gt;; it's discussed a little bit in &lt;a href=&quot;https://space.stackexchange.com/questions/2840/what-was-the-mascon-fix-used-by-apollo-12&quot;&gt;this Q/A&lt;/a&gt; and the &quot;Tindallgrams&quot; linked therefrom. Apparently &lt;a href=&quot;https://en.wikipedia.org/wiki/Mass_concentration_%28astronomy%29&quot; rel=&quot;noreferrer&quot;&gt;the model was updated between Apollo 11 and 12&lt;/a&gt; with better data obtained from &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Orbiter_program&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbiter&lt;/a&gt; data.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A gravity field is modeled (and can be visualized as) deviations from the gravity of a perfect sphere. The visualization of the L-1 Lunar potential looks like this (with red indicating increased gravity and blue decreased):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/uifAi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uifAi.jpg&quot; alt=&quot;Lunar L-1 Gravity Anomalies (degree=3 order=3)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the Apollo 12 gravity model looks like this:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/MTWOJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MTWOJ.jpg&quot; alt=&quot;Lunar ML 1.2 Gravity Anomalies (degree=4 order=3)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, for reference our current lunar gravity model, based on GRAIL data, looks like this:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/o8sVq.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/o8sVq.jpg&quot; alt=&quot;Lunar GRAIL Gravity Anomalies (degree=400 order=400)&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft's attitude control system required good estimates of its mass, center of mass location, and &lt;a href=&quot;https://en.wikipedia.org/wiki/Moment_of_inertia&quot; rel=&quot;noreferrer&quot;&gt;moment of inertia&lt;/a&gt; in order to maneuver efficiently. Without that level of detail in the modeling, the attitude control system would likely have been less responsive or more wasteful of propellant or both. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure about their Earth gravitational model; they may not have needed much detail since they would only be in parking orbit for a few hours on the way out and re-entering aerodynamically on the way back. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't believe solar wind was factored into navigation, again because any effect it would have was easily corrected for. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Approaching the question a different way: to what extent was the control of the spacecraft pre-calculated, vs. calculated in real time by spacecraft computers, vs. done manually by the astronauts?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Mostly navigation was done by computers on Earth. The position of the spacecraft was accurately tracked throughout the mission, and correction maneuvers would be calculated on ground-side computers and called up to the crew to execute as needed; the guidance computer would execute the maneuver with a crewman ready to hit the shutdown button if necessary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A few parts of the flight were flown manually. The terminal phase of the moon landing was one of them; the capability existed to land semi-automatically using a radar altimeter, with the commander able to adjust the targeted landing spot, but in every landing the computer was switched into a more manual mode at around 500 feet altitude and landed with the commander's hands on the controls while the LM pilot managed the computer and called out altitude and speed figures to the commander. Lovell intended to try to use the automatic mode on Apollo 13, but didn't get the chance to attempt the landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The transposition-docking-extraction maneuver to pull the LM away from the booster was flown manually by the command module pilot (CMP). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the LM separated in lunar orbit, the commander would manually turn the LM to allow the CMP to visually inspect it; on the LM's return from the moon's surface, the commander would manually fly the very last part of the approach and docking, with the CMP ready to take over the active role if a problem developed on the LM. &lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="12483" LastEditDate="2017-12-13T17:48:58.317" LastActivityDate="2017-12-13T17:48:58.317" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="23891" PostTypeId="2" ParentId="3004" CreationDate="2017-12-03T20:03:26.300" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Firefly_Space_Systems&quot; rel=&quot;nofollow noreferrer&quot;&gt;Firefly Aerospace&lt;/a&gt; will use an (annular) aerospike engine with their Alpha rocket.  They have a planned launch in 2018.&lt;/p&gt;&#xA;" OwnerUserId="21899" LastEditorUserId="6816" LastEditDate="2017-12-03T21:37:09.917" LastActivityDate="2017-12-03T21:37:09.917" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23892" PostTypeId="1" CreationDate="2017-12-03T22:44:30.713" Score="0" ViewCount="293" Body="&lt;p&gt;How can astronauts conduct EVAs without the ISS falling away and leaving them behind? Similarly; a spaceship returning to the Earth from the Moon, should it not undershoot the earth given Earth's relative speed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The answer probably has to do with Earth's gravity, but I think it's an interesting issue: given everything's relative speed (earth, sun, galaxy) should it not be easy to be left behind? Or how difficult would it be to catch up with anything?&lt;/p&gt;&#xA;" OwnerUserId="21901" LastEditorUserId="6816" LastEditDate="2017-12-03T23:07:50.193" LastActivityDate="2017-12-04T00:10:11.127" Title="Relative speeds" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23893" PostTypeId="2" ParentId="23892" CreationDate="2017-12-03T23:04:00.403" Score="0" Body="&lt;p&gt;It does and doesn't have anything to do with gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Far out in intergalactic space are vast regions with almost no matter, not merely millimeters between atoms, or even meters, but kilometers or more. In the middle of these, anything larger than a diffuse cloud of cold gas may be many parsecs away. Even out there, though, a hypothetical astronaut leaving their hypothetical spacecraft for an EVA would simply hang around very near the craft unless they accelerated relative to it. There is nothing that would make the craft &quot;fall away and leave them behind&quot;. Simple inertia is enough to keep them close together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in Earth orbit, of course, there are quite a few quite large masses quite nearby. However, in each case, the distance between the center of gravity of the large body (Earth, the Sun, the Moon, etc etc etc) and the CG of the spacecraft is almost exactly the same as the distance between that same CG and the CG of the astronaut. And likewise, the mass of the spacecraft or astronaut is effectively irrelevant when considering the acceleration. So in Earth orbit, the ISS &lt;strong&gt;does&lt;/strong&gt; &quot;fall away&quot;, but so does the astronaut, at almost exactly the same speed. That is in fact what an orbit is: continuously falling away from the straight-line course that would otherwise be taken, at such a high speed that the ballistic curve of the fall matches the curve of the planet. So here, inertia plus gravity keeps them together.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gravitational environment in orbit is sometimes referred to as microgravity. This is more accurate than the other term, zero-g, but both are an approximation of the &lt;em&gt;relative&lt;/em&gt; accelerations; because everything around you is in constant freefall at the same acceleration and velocity, there's no large force to make you move &lt;em&gt;relative to your immediate surroundings&lt;/em&gt; if you let go of the wall or ceiling or floor. But because there are very small differences in distance and therefore gravity, &quot;microgravity&quot; is more accurate: an astronaut in the closest-to-Earth (or farthest-from-Earth) sections of the ISS could very slowly start drifting relative to the whole structure. But this is so slow that rather than taking a fraction of a second to fall a meter or a yard, it could very easily take many hours.&lt;/p&gt;&#xA;" OwnerUserId="6816" LastActivityDate="2017-12-03T23:04:00.403" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23894" PostTypeId="2" ParentId="23892" CreationDate="2017-12-04T00:10:11.127" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion&quot; rel=&quot;noreferrer&quot;&gt;Newton's first law of motion&lt;/a&gt; says that an object in motion remains in motion unless acted on by some force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An astronaut leaving the hatchway of the ISS starts off moving at the same speed, in the same direction, as the ISS; both she and the space station continue to move at the same speed as she steps out of the hatch. Gravity acts on both of them to change their direction of motion, but it does so equally, so again they move together. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the astronaut were to push away from the station slightly, she would drift away from it slowly, but astronauts are tethered to the station for safety.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This situation is different from what your intuition says about, say, a skydiver leaving the doorway of an airplane in flight. In that case, both the aircraft and the skydiver are moving rapidly relative to the air around them; the resistance of the air produces a very large force on both the skydiver and the aircraft which tends to slow them both down. The engines of the aircraft provide a force which counteracts the air drag force, so the aircraft continues on at the same speed; the skydiver has no engine, so is slowed down, and rapidly falls -- backward relative to the plane, but still moving forward for a while relative to the air. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-12-04T00:10:11.127" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23895" PostTypeId="1" AcceptedAnswerId="23896" CreationDate="2017-12-04T04:39:28.360" Score="15" ViewCount="3320" Body="&lt;p&gt;There are several popular news items about the recent test of Voyager 1's Trajectory Control Thrusters to see if they can be used for attitude control to facilitate communication with Earth. The original attitude control thrusters are slowly degrading. Read more in the NASA News item &lt;a href=&quot;https://www.nasa.gov/feature/jpl/voyager-1-fires-up-thrusters-after-37&quot; rel=&quot;noreferrer&quot;&gt;Voyager 1 Fires Up Thrusters After 37 Years&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In what ways do the two sets of thrusters differ? An answer should include at least their locations and (design) minimum possible impulse per firing for each type. I say design because this may be more available, the actual minimum of the attitude thrusters is changing with time (which is the problem).&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="9060" LastEditDate="2017-12-05T01:56:58.487" LastActivityDate="2021-06-09T21:37:14.073" Title="How do Voyager 1's Trajectory Control Thrusters differ from its Attitude Control Thrusters?" Tags="|attitude|engines|voyager|attitude-determination-and-control|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23896" PostTypeId="2" ParentId="23895" CreationDate="2017-12-04T05:44:32.700" Score="25" Body="&lt;p&gt;The attitude thrusters and TCMs are mechanically identical, all Aerojet &lt;a href=&quot;http://www.rocket.com/propulsion-systems/monopropellant-rockets&quot; rel=&quot;nofollow noreferrer&quot;&gt;MR-103s&lt;/a&gt;. From the Voyager Press Kit:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The 16 thrusters on the mission module each deliver 0.89 N (0.2-lb.) thrust. Four are used to execute trajectory correction maneuvers; the others in two redundant six-thruster branches, to stabilize the spacecraft on its three axes. Only one branch of attitude control thrusters is needed at any time.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The MR-103 is almost a commodity part, used on many spacecraft over 4 decades; the modern version is quite versatile, rated for 0.027 N-sec minimum impulse with a 15ms pulse, and also for 5000 seconds of continuous fire; Astronautix mentions an &lt;em&gt;8ms&lt;/em&gt; minimum duty cycle, presumably for an earlier version of the thruster.&lt;/p&gt;&#xA;&lt;p&gt;Here's a diagram showing most of the thrusters:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IkDI4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IkDI4.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image source: &lt;a href=&quot;https://planetary.s3.amazonaws.com/voyager_msb/VoyagerMSB13.PDF&quot; rel=&quot;nofollow noreferrer&quot;&gt;Voyager Bulletin #13&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;I've highlighted thruster groups in color. There are short lines  drawn out from the thruster blocks indicating their orientation (although the pitch thrusters are obscured by the struts). Based on their position and orientation, their functions are:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The four in blue would be the TCM thrusters (all facing the same direction, not paired).&lt;/li&gt;&#xA;&lt;li&gt;The purple set would provide roll control, with two branches each of +roll and -roll&lt;/li&gt;&#xA;&lt;li&gt;The green set would be yaw control; again two branches for +yaw and -yaw&lt;/li&gt;&#xA;&lt;li&gt;The orange set would be 2 branches of +pitch and -pitch; there are two pairs back-to-back mounted here, as shown in the following diagram:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/q1LMg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/q1LMg.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Image source: &lt;a href=&quot;https://space.stackexchange.com/questions/19331/how-are-the-voyagers-thrusters-configured-in-a-novel-way-to-minimize-accelerati/49839#49839&quot;&gt;How are the Voyagers&amp;#39; thrusters configured in a novel way to minimize accelerations along Earth-spacecraft axis?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;You can see the positive yaw thrusters marked +Y1 and +Y2, and the pitch thrusters marked +P1, -P1, +P2, -P2.&lt;/p&gt;&#xA;&lt;p&gt;It's easy to see that, by firing one of the TCM thrusters at a time, you produce a small amount of torque for pitch or yaw control.&lt;/p&gt;&#xA;&lt;p&gt;Pitch and yaw control alone (without roll) are probably sufficient to maintain communication with Earth. Since Voyager's antenna bore sight is aligned with the roll axis, roll would not be particularly necessary to maintain communication with Earth. Also, since (according to Table 2-2 on page 10 of the DESCANSO Design and Performance Summary Series &lt;a href=&quot;https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Article 4: Voyager Telecommunications&lt;/a&gt;) Voyager uses circular polarization rather than linear, arbitrary rotation would not have a significant impact on the polarization of the received signal.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ElypC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ElypC.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0CQBX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0CQBX.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-06-09T21:37:14.073" LastActivityDate="2021-06-09T21:37:14.073" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="23897" PostTypeId="2" ParentId="3004" CreationDate="2017-12-04T10:10:02.203" Score="16" Body="&lt;p&gt;Aerospikes are notoriously difficult to cool efficiently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With a bell nozzle, you have a minor part of rapidly expanding(+cooling)  exhaust touching the broad, actively cooled nozzle - that means little conductive heat transfer, lower temperature gradient, lots of area for coolant plumbing on the outside (or within) the bell, and outer area radiating a lot of heat out (or passing it to air while in atmosphere) besides coolant drawing it away. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In aerospike the pressure (and temperature) of the gas remains very high all along the spike surface, and the sharp tip leaves very little room for cooling systems. You have a lot of extra-hot, very dense gas in contact with the narrow &lt;em&gt;spike&lt;/em&gt; that must pass all the coolant and dissipate the heat somehow, not to melt.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That means short test runs of aerospike engines, proving all the benefits, are perfectly viable - but the experimental rigs are shut down before they could overheat and suffer critical damage. They just couldn't run continuously for as long as a typical rocket needs to lift payload to orbit. Work on efficient, fault-proof cooling of the engines is ongoing, but it's not nearly as easy as in case of bell nozzles - a major challenge that seriously throws a wrench into broad adoption of this engine type.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-12-04T10:10:02.203" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23898" PostTypeId="1" AcceptedAnswerId="23899" CreationDate="2017-12-04T16:54:43.883" Score="4" ViewCount="835" Body="&lt;p&gt;I'm a MSC student in computer science. For my thesis I'm using SGP4 tool to propagate a LEO satellite.&lt;/p&gt;&#xA;&lt;p&gt;If I have understood rightly, the SPG4 returns the two state vectors r and v in TEME format. (&lt;a href=&quot;https://celestrak.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;celestrak&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;Here my problem: I need to obtain the Keplerian orbital parameter from those two vectors but I have some conversion trouble. I can't figure the difference between TEME and ECI. Are they the same? Can I convert from TEME with &lt;a href=&quot;https://space.stackexchange.com/questions/1904/how-to-programmatically-calculate-orbital-elements-using-position-velocity-vecto&quot;&gt;this method&lt;/a&gt;?&lt;/p&gt;&#xA;&lt;p&gt;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="21916" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:51.853" LastActivityDate="2017-12-04T18:45:55.450" Title="Can I convert from TEME to Keplerian orbital parameters?" Tags="|orbital-mechanics|orbital-elements|orbit-determination|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23899" PostTypeId="2" ParentId="23898" CreationDate="2017-12-04T18:45:55.450" Score="2" Body="&lt;p&gt;ECI doesn't have an actual reference time, it's just an arbitrary one that is useful. It works well for rockets and things with limited lifespans, but for satellites it is a bit trickier. As &lt;a href=&quot;https://celestrak.org/columns/v04n05/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt; states:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;What is the reference frame of the resulting coordinates? This question is a bit more technical than most we have covered. To be precise, the reference frame of the Earth-centered inertial (ECI) coordinates produced by the SGP4/SDP4 orbital model is true equator, mean equinox (TEME) of epoch.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;There is a &lt;a href=&quot;https://celestrak.org/columns/v02n01/&quot; rel=&quot;nofollow noreferrer&quot;&gt;whole article&lt;/a&gt; on how to properly use the coordinates. Using that article, and the question you linked, you should be able to make the conversion you require.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2017-12-04T18:45:55.450" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23900" PostTypeId="2" ParentId="23883" CreationDate="2017-12-05T01:52:43.053" Score="5" Body="&lt;p&gt;The short answer is, it really does depend heavily on the orbit. It is not exactly known at this time what kind of orbit it will be in, but there are some requirements if it is to last as long as Musk claims.&#xA;If the periapsis of the orbit is fairly high, like several thousand kilometers, and the apoapsis is reasonably low, it could stay for an extremely long time, quite possibly over a billion years as Musk tweeted. This would take a good bit of $\Delta$v, however, compared to a simple low-periapse, eccentric orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Phobos, which orbits at about 6000km, is predicted to either break up or impact Mars in about 50 million years. So we'll need a higher orbit than that. If this car is going to enter Martian orbit, they would most likely bring their periapsis very low, likely below 1000km, because the lower your periapsis, the faster you go at that point, and the faster you're moving, the more power you can get for a given amount of thrust (by the Oberth effect). This, of course, means you can use less fuel to get captured. So the best way to get into a super-stable orbit is to come in on a very close approach, burn retrograde to get captured, coast to apoapsis, and use the remaining fuel to raise your periapsis to a respectable altitude. I have not crunched the numbers on how much $\Delta$v it will actually have or how much it will need, however.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, either there is going to be enough leftover fuel in that second stage (or I suppose maybe it's a third stage now?) to be able to get it into an orbit with a high periapsis, which could indeed last over a billion years, or he's only doing a flyby, or perhaps it won't actually last over a billion years. I don't know which of these is the case, but it will be easy enough to tell which of these is the case if it sends videos or pictures back, which I'm sure it will.&lt;/p&gt;&#xA;" OwnerUserId="20555" LastEditorUserId="4592" LastEditDate="2017-12-05T20:45:12.523" LastActivityDate="2017-12-05T20:45:12.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23903" PostTypeId="2" ParentId="23883" CreationDate="2017-12-05T12:07:53.303" Score="12" Body="&lt;p&gt;The Falcon Heavy has no capability to put a passive object (i.e. without its own propulsion) into Mars orbit. The lifetime of the second stage is on the order of 8 hours. After the cruise to Mars, the batteries on the stage will be dead.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So there's no option to change the trajectory once you get to Mars. The best they can do is aim the rocket in the general direction of Mars, for an elliptical heliocentric orbit that intersects Mars' orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;They would have to modify the stage to allow it to function in Mars orbit: Larger batteries, thermal management, insulation for the main propellant tanks to reduce boiloff, custom software for the Mars orbit injection burn. All that sounds like too much effort for a one-off demonstration mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More importantly: they're doing this mission as part of the certification effort for Falcon Heavy. That means they need to demonstrate the standard stage. Modifying the stage for this mission would make the mission useless. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-12-06T08:43:45.013" LastActivityDate="2017-12-06T08:43:45.013" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23904" PostTypeId="2" ParentId="22018" CreationDate="2017-12-05T16:24:46.060" Score="2" Body="&lt;p&gt;You could use a main satellite with 2 subsidiaries. The subsidiaries use orbits with the same shape as the main orbit, but in a plane that has a slight angle relative to the main (plus a minimal difference in altitude to prevent collisions). The point where sub 1 crosses the main's orbit should be 90 degrees away from where sub 2 crosses the main's orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With 2 subs, you always have one that is more than half the maximum distance away from the main. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-12-05T16:24:46.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23905" PostTypeId="1" CreationDate="2017-12-05T17:55:05.950" Score="3" ViewCount="208" Body="&lt;p&gt;Digital technology is for sure an important component. DevOps is the practice allowing reducing time to market of digital products and increasing their quality.&lt;/p&gt;&#xA;&lt;p&gt;Now, is there a special, specific branch of DevOps in the space exploration industry (e.g. with much more influence from system engineering), or not?&lt;/p&gt;&#xA;&lt;p&gt;An update: according to &lt;a href=&quot;https://www.coderskitchen.com/spacex-software-development-and-testing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this recent report&lt;/a&gt;, as 2019 SpaceX has had 50 software engineers following DevOps principles doing work of roughly 2500 to serve all vehicles AND custom enterprise systems.&lt;/p&gt;&#xA;" OwnerUserId="19946" LastEditorUserId="19946" LastEditDate="2020-10-25T06:08:21.160" LastActivityDate="2020-10-25T06:08:21.160" Title="Role of DevOps in space exploration related digital systems" Tags="|reference-request|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23906" PostTypeId="1" AcceptedAnswerId="23907" CreationDate="2017-12-05T19:55:57.407" Score="7" ViewCount="2344" Body="&lt;p&gt;With the Voyager 1 attitude correction all over the news (related: &lt;a href=&quot;https://space.stackexchange.com/questions/23895/how-do-voyager-1s-trajectory-control-thrusters-differ-from-its-attitude-control&quot;&gt;How do Voyager 1&amp;#39;s Trajectory Control Thrusters differ from its Attitude Control Thrusters?&lt;/a&gt;), I wonder with what instrument such a spacecraft can measure its own attitude towards earth in the first place.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How do we know exactly what its attitude is so that the thrusters can accurately be fired?&lt;/p&gt;&#xA;" OwnerUserId="746" LastActivityDate="2017-12-05T23:00:28.080" Title="How does Voyager 1 keep track of its orientation?" Tags="|attitude|voyager|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23907" PostTypeId="2" ParentId="23906" CreationDate="2017-12-05T20:22:07.537" Score="8" Body="&lt;p&gt;Voyager 1 and Voyager 2 use the same Attitude and Articulation Control Subsystem (AACS) to maintain the crafts orientation; facing the antenna towards the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager's AACS system uses a sun sensor for yaw and pitch reference, and a star tracker trained continuously on a bright star at right angles to the sun point for a roll reference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Links:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1#Spacecraft_components&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Voyager_1#Spacecraft_components&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_program#Computers_and_data_processing&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Voyager_program#Computers_and_data_processing&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.au.af.mil/au/awc/awcgate/jplbasic/bsf11-2.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.au.af.mil/au/awc/awcgate/jplbasic/bsf11-2.htm&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="15824" LastEditorUserId="5946" LastEditDate="2017-12-05T23:00:28.080" LastActivityDate="2017-12-05T23:00:28.080" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23908" PostTypeId="1" AcceptedAnswerId="23910" CreationDate="2017-12-05T21:35:47.763" Score="7" ViewCount="945" Body="&lt;p&gt;Based on today's technology (requiring no new engineering hurdles to be solved), could a new interstellar space probe overtake Voyager 1's distance? It doesn't need to go in the same direction, but just achieve a further distance from Earth than Voyager 1? I assume the answer is yes, and if so, with today's technology how quickly could this be accomplished?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ultimately, I'm curious if Voyager 1 is destined to be the furthest man-made object from Earth in my lifetime.&lt;/p&gt;&#xA;" OwnerUserId="21932" LastEditorUserId="21932" LastEditDate="2017-12-05T22:16:52.180" LastActivityDate="2018-12-11T09:31:16.557" Title="Shortest time to place a probe further than Voyager 1?" Tags="|probe|interstellar-travel|voyager|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="23910" PostTypeId="2" ParentId="23908" CreationDate="2017-12-05T23:07:15.410" Score="10" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Voyager_1&quot; rel=&quot;noreferrer&quot;&gt;Voyager 1&lt;/a&gt; received gravitational assists increasing its velocity from both Jupiter and Saturn. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Opportunities for such a Jupiter-Saturn trajectory occur about every 20 years, so we don't have to wait too long to launch, at least; in order to outperform Voyager we merely need to launch a probe of about the same total mass as Voyager, at the same initial velocity, with the same gravity-assist parameters, but including a propulsion system in place of the 105kg of scientific instruments carried by the earlier probe. An ion engine would give the most ∆v for the mass; something like an &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness&quot; rel=&quot;noreferrer&quot;&gt;NSTAR&lt;/a&gt; thruster of 25kg plus 80kg of Xenon propellant, on a total spacecraft mass of 773 kg, firing for about a year, would yield an additional 3km/sec above Voyager's speed. (Power would be a bit of a problem; Voyager produced only ~430W from its radiothermal generators, while NSTAR wants 2100W for maximum efficiency, but we can handwave this away by assuming a smaller thruster operating for a longer period of time.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Improvements in computer and other technologies over the last 40 years should make it possible to shave off additional mass from the probe, of course. Those mass savings could pay for additional propellant on the probe. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_IIIE&quot; rel=&quot;noreferrer&quot;&gt;Titan IIIE&lt;/a&gt; booster that launched Voyager is no longer in service; the &lt;a href=&quot;https://en.wikipedia.org/wiki/Atlas_V&quot; rel=&quot;noreferrer&quot;&gt;Atlas V&lt;/a&gt; 551 configuration that launched &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons&quot; rel=&quot;noreferrer&quot;&gt;New Horizons&lt;/a&gt; slightly outperforms the Titan in mass-to-LEO, so I assume it could serve this role.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Voyager 1 does have a substantial head start, however, so it could take something like 200 years for this speedy little probe to outdistance it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A larger booster -- Delta IV Heavy, Falcon Heavy, SLS, or SpaceX's future rockets -- could of course lift a very powerful upper stage that could give the probe another substantial boost, perhaps cutting the outdistancing time in half or further.    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, with no major technological advances, I estimate it should be no more than 20 years from budget approval to launch, and 100-200 years from launch to outdistancing depending on the budget. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-12-05T23:07:15.410" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23911" PostTypeId="2" ParentId="23908" CreationDate="2017-12-05T23:25:00.767" Score="0" Body="&lt;p&gt;We know how to build mass drivers (basically, a very high power maglev train that doesn't stop accelerating until it runs out of track.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Long ago I saw a 100g version on TV.  There's basically zero issues with making it longer until the surface curvature of the body you mounted it on matters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus we could build such a system on the moon and launch something that can overtake Voyager.  There's nothing we don't know how to do but it's a pretty massive engineering project.  (However, unless it becomes obsolete before being built I think it's something that will be done.  While using it to overtake Voyager would be a waste it can boost payloads elsewhere in the solar system.)&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-12-05T23:25:00.767" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23912" PostTypeId="1" CreationDate="2017-12-06T00:23:43.270" Score="0" ViewCount="309" Body="&lt;p&gt;To travel once in space, not only using gravity as a method of propelling, could pressurised gases work? The ubiquity of gases such as helium or hydrogen being brought into the craft, once compressed could they in turn be used to release pressure in a manner to generate thrust? The limited friction could enable gradual inclines of release to build momentum each time for speed. Reserving solid fuel purely for launch and recovery. &lt;/p&gt;&#xA;" OwnerUserId="21935" LastActivityDate="2017-12-07T11:26:54.507" Title="Use of compressed gases for space travel" Tags="|interstellar-travel|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="23914" PostTypeId="1" AcceptedAnswerId="23915" CreationDate="2017-12-06T01:14:32.477" Score="3" ViewCount="111" Body="&lt;p&gt;What is regarded as the current single biggest challenge/hurdle limiting space exploration?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once that tipping point has past it would pave the way for future exploration to dramatically increase. &lt;/p&gt;&#xA;" OwnerUserId="21935" LastActivityDate="2017-12-06T04:11:12.453" Title="Current biggest challenge" Tags="|interstellar-travel|travel|experiment|" AnswerCount="1" CommentCount="2" ClosedDate="2017-12-06T13:10:24.583" ContentLicense="CC BY-SA 3.0" />
  <row Id="23915" PostTypeId="2" ParentId="23914" CreationDate="2017-12-06T01:56:01.973" Score="1" Body="&lt;p&gt;It may likely  be a simple one: Money. It's what killed the last 3 Apollo missions, postponed the JWST, and plagues NASA to this day. Financial shortages to the space agencies is like poor internet; it only slows things down. While public interest goes hand in hand with funding for space exploration, not having the cash to put up often cuts missions short or out entirely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Historically, the hurdles were a little more interesting. Serious attempts were considered to use artillery technology to send men into space. Never forget Project Orion, the proposed method to use a small nuclear bomb to kick a spaceship forward. Working models were made, but it was just that: a nuclear bomb going off every few seconds. In 1958 a treaty was signed with the Soviet Union to agree to not have any nuclear wars in space, which kinda made Orion internationally banned, even if it was capable of speeds half that of light. &lt;/p&gt;&#xA;" OwnerUserId="21786" LastEditorUserId="21786" LastEditDate="2017-12-06T04:11:12.453" LastActivityDate="2017-12-06T04:11:12.453" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23917" PostTypeId="2" ParentId="2339" CreationDate="2017-12-06T09:39:05.380" Score="0" Body="&lt;p&gt;There is a simple answer to this problem. No seals!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The issues with most spacecraft designs is they are done by aircraft engineers. Ask a ship builder how to make a pressure hull for space, their design will be quite different. All too often do I watch a sci-fi movie and see a space ship with all its key components exposed to the hard environment of space. This looks nice, but both wrong and impractical. If any component failed it would mean sending a person out into open space to fix it.  Space walks look like fun but they are extremely risky to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So using this line of thinking and the issues of keeping a seal between moving parts in a vacuum. If we designed a spaceship like a super tanker there is no need to put in complex joints that can fail or leak. You simply put the rotating section inside the pressure hull. In fact, you put all the key components inside the hull.  Some parts would be unpressurised like the engine space, but all other parts of the cabin and engineering spaces would / could be pressurised (with air or an inert gases) to enable a service engineer to access the area in an environment suit (a more flexible suit that supplies heat, O2).&#xA;The rotating area within the pressure hull would be in two sections each rotating in the opposing direction, that way the torque effects could be managed by gyroscopes and not by thruster activity.&lt;/p&gt;&#xA;" OwnerUserId="21948" LastEditorUserId="6816" LastEditDate="2017-12-06T17:23:06.330" LastActivityDate="2017-12-06T17:23:06.330" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23921" PostTypeId="2" ParentId="247" CreationDate="2017-12-06T23:03:47.073" Score="2" Body="&lt;p&gt;Interestingly, this does not apply to all spacecraft. The Flock satellites by Planet Labs are actually pretty cutting-edge as stated by one of the developers on &lt;a href=&quot;https://theamphour.com/220-an-interview-with-shaun-meehan-doctiloquent-dove-deployer/&quot; rel=&quot;nofollow noreferrer&quot;&gt;The AmpHour podcast&lt;/a&gt;. In fact, the testing of new satellite designs was slowed down by the time it took to actually launch the satellites once they were manufactured.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suggest listening to the podcast, this episode was quite interesting.&lt;/p&gt;&#xA;" OwnerUserId="16099" LastActivityDate="2017-12-06T23:03:47.073" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23922" PostTypeId="2" ParentId="23912" CreationDate="2017-12-07T11:26:54.507" Score="3" Body="&lt;p&gt;Collecting interstellar gas, compressing it and then releasing it (via decompression) takes more power than it yields. You have drag losses in the ram scoop, and compression losses. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Burning the interstellar gas would extract more energy, but oxygen seems to be far less abundant than hydrogen. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One variant is at least theoretically possible, and scientists have examined this method. This method is known as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bussard_ramjet&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bussard ramjet&lt;/a&gt;, after the physicist who proposed it in the 1960s. This takes the hydrogen and runs it through a fusion reactor. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Since the time of Bussard's original proposal, it has been discovered that the region surrounding the Solar System has a much lower density of hydrogen than was believed at that time (see Local Interstellar Cloud). John Ford Fishback made an important contribution to the details for the Bussard ramjet in 1969,[2] T. A. Heppenheimer analyzed Bussard's original suggestion of fusing protons, but found the bremsstrahlung losses from compressing protons to fusion densities was greater than the power that could be produced by a factor of about 1 billion, thus indicating that the proposed version of the Bussard ramjet was infeasible.[3] However Daniel P. Whitmire's 1975 analysis[4] indicates that a ramjet may achieve net power via the CNO cycle, which produces fusion at a much higher rate (~10&lt;sup&gt;16&lt;/sup&gt; times higher) than the proton-proton chain.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-12-07T11:26:54.507" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23923" PostTypeId="1" AcceptedAnswerId="23928" CreationDate="2017-12-07T12:56:14.933" Score="36" ViewCount="5786" Body="&lt;p&gt;A &lt;a href=&quot;https://en.wikipedia.org/wiki/Tea_cosy&quot; rel=&quot;noreferrer&quot;&gt;tea cozy&lt;/a&gt; keeps the tea in a teapot warmer longer by insulating it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why does the nose of the rocket shown in Apollo-era file footage used in &lt;a href=&quot;https://youtu.be/rfVfRWv7igg&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; (&lt;em&gt;lower your volume before watching&lt;/em&gt;) have a thick cover that's pulled off about eight seconds before launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IVavA.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IVavA.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; as in the video in the as-yet unanswered question &lt;a href=&quot;https://space.stackexchange.com/q/18880/12102&quot;&gt;Is this one coherent set of images or a mixture of several missions?&lt;/a&gt; it appears that the footage in the video is from mixed missions.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-12-07T15:10:24.117" LastActivityDate="2019-04-30T16:54:31.510" Title="Why does this rocket have a nose cone &quot;cozy&quot;?" Tags="|rockets|apollo-program|space-art|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23928" PostTypeId="2" ParentId="23923" CreationDate="2017-12-07T16:01:44.527" Score="42" Body="&lt;p&gt;That video frame shows the top of the Apollo LES. The object under the cover is the &lt;a href=&quot;http://www.collectspace.com/ubb/Forum29/HTML/000928.html&quot; rel=&quot;noreferrer&quot;&gt;Q-ball&lt;/a&gt;. The cover is removed at T - 9 s.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Q-ball is an angle of attack sphere perched above the launch escape system,&lt;/p&gt;&#xA;&lt;p&gt;The Q-ball is not unlike a pitot tube on an aircraft that measures airspeed. It consists of eight openings at the top of the Launch Escape Tower. These openings lead to instruments which gauge air pressure.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.hq.nasa.gov/alsj/CSM15_Launch_Escape_Subsystem_pp137-146.pdf&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; has more information:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The Q-ball has eight static ports (openings) through which pressure changes are measured. The instru­ments use this information to determine aerody­namic incidence angle and dynamic pressure data. The instruments send information on the angle of attack to an indicator of the CM's main display console and to the launch vehicle guidance.&lt;/p&gt;&#xA;&lt;p&gt;The pitch and yaw pressure-change signals are electronically summed in the Q-ball and dis­played on the indicator. The Q-ball information provides a basis for crew abort decision in the event of slow launch vehicle divergence.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://heroicrelics.org/cosmosphere/q-ball/index.html&quot; rel=&quot;noreferrer&quot;&gt;Herorelics&lt;/a&gt; has photos and can tell us why there's a cover:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;While on the pad, the Q-ball is protected by a cover to keep the ports free of dust and debris.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CU0k1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CU0k1.jpg&quot; alt=&quot;Q-ball&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This is the LES &lt;a href=&quot;https://history.nasa.gov/afj/ap08fj/photos/ground/19681217-s68-55415.jpg&quot; rel=&quot;noreferrer&quot;&gt;with the cover on top:&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/O2mLg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/O2mLg.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=SDWEpNx05uA&quot; rel=&quot;noreferrer&quot;&gt;Vintage Space&lt;/a&gt; has a video that explains the LES, the Q-ball and how the cover retraction mechanism worked. And the origin of the name: Q is the symbol used to indicate air pressure (think max-Q).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-08T15:25:45.503" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="23929" PostTypeId="1" CreationDate="2017-12-07T18:05:16.940" Score="8" ViewCount="948" Body="&lt;p&gt;One of the benefits of the SpaceX Raptor full-flow staged combustion engine is the extra power generated by having all of the fuel and all of the oxidizer flow through the turbines to produce power to run the fuel and oxidizer pumps.&lt;/p&gt;&#xA;&lt;p&gt;At first, I thought having two pre-combustion chambers and two turbo pumps would double the power compared to an RD-180 style oxygen-rich-only staged combustion engine. But then I did this calculation (see below) of fuel mass and concluded that the fuel has only 25% of the mass of the oxygen, and so would only add 25% to the total pump power available.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;${CH_4 +2O_2 = CO_2 + 2H_2O}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;${CH_4}$&lt;/span&gt;molecular weight = 16&lt;br/&gt;&#xA;&lt;span class=&quot;math-container&quot;&gt;$2O_2$&lt;/span&gt; molecular weight = 64&lt;/p&gt;&#xA;&lt;p&gt;Fuel to Oxidizer ratio is 1:4   (Assuming Stoichiometric for simplicity)&lt;/p&gt;&#xA;&lt;p&gt;Conclusion: Using the fuel as well as the oxygen to drive the turbines adds 25% to the available pump power.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Is that the correct conclusion? If so, is it worth the extra effort to get this 25% extra power?&lt;/p&gt;&#xA;" OwnerUserId="21963" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-07-02T05:46:27.987" Title="How does turbine power output of a full-flow staged combustion engine compare to Oxygen-rich staged combustion?" Tags="|spacex|engine-design|raptor|" AnswerCount="1" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="23931" PostTypeId="1" AcceptedAnswerId="23932" CreationDate="2017-12-07T21:34:32.193" Score="6" ViewCount="811" Body="&lt;p&gt;I found different versions in the &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_6&quot; rel=&quot;nofollow noreferrer&quot;&gt;English&lt;/a&gt; and &lt;a href=&quot;https://de.wikipedia.org/wiki/Apollo_6&quot; rel=&quot;nofollow noreferrer&quot;&gt;German&lt;/a&gt; Wikipedia articles about the Apollo 6 flight. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Das zweite Triebwerk wurde vom Steuerungssystem fälschlicherweise deaktiviert, weil die Steuerungsleitungen zwischen den beiden Triebwerken vertauscht waren.&quot; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Translation: &lt;em&gt;The second engine was shut down erroneously due to swapped control cables between the two engines.&lt;/em&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The English version is just about two independent engine shut downs.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found no other sources confirming the German version. Such a cabling error would be difficult to prove using telemetry data only. But what else could prove the error after the flight? Is there any other information about the Apollo 6 engine shut down?&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="195" LastEditDate="2019-05-09T17:37:44.410" LastActivityDate="2019-05-09T18:21:45.460" Title="Apollo 6, reason for premature engine shutdown of two engines of second stage of Saturn V?" Tags="|apollo-program|engines|saturn-v|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23932" PostTypeId="2" ParentId="23931" CreationDate="2017-12-07T21:49:15.217" Score="10" Body="&lt;p&gt;The official NASA history of the Saturn vehicles &quot;&lt;a href=&quot;https://history.nasa.gov/SP-4206/sp4206.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Stages to Saturn&lt;/a&gt;&quot;, pp. 361-362 agrees that there was a miswired control cable.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;..the low-thrust sensing equipment triggered a sequence to shut down&#xA;  the engine by closing the fuel and oxidizer valves. The electrical&#xA;  sequence to close number two LOX valve went erroneously to number&#xA;  three.  Closing the fuel valve for engine number two and the oxygen&#xA;  valve for engine number three shut down both engines.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(The second occurrence of &quot;sequence&quot; appears to be an error; it should probably read &quot;signal&quot;.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have the print version of the book, but in the online one it's on &lt;a href=&quot;https://history.nasa.gov/SP-4206/ch12.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this page&lt;/a&gt; (look for the [362]).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This chapter also discusses the ground investigation via the telemetry data in some detail.  The root cause was a rupture of the igniter propellant lines due to resonant vibration.  Ground testing had not identified the failure mode because when the engines were fired in an atmosphere, air that liquefied on the lines dampened the oscillation enough to prevent failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is an extremely detailed analysis of the failure in this &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680026390.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;document&lt;/a&gt;,  which I just noticed in the excellent &lt;a href=&quot;https://space.stackexchange.com/a/30895/6944&quot;&gt;answer&lt;/a&gt; to this question: &lt;a href=&quot;https://space.stackexchange.com/questions/30894/how-common-is-the-ability-to-compensate-for-a-lost-engine-through-gimbaling&quot;&gt;How common is the ability to compensate for a lost engine through gimbaling?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2019-05-09T18:21:45.460" LastActivityDate="2019-05-09T18:21:45.460" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="23934" PostTypeId="1" CreationDate="2017-12-07T22:36:44.573" Score="5" ViewCount="348" Body="&lt;p&gt;In the new SMAD (Chapter 18 Space Mission Engineering and Analysis) they have the equation for thrust of a solar sail and a reduced version. Variables are described in &lt;a href=&quot;https://space.stackexchange.com/q/4587/12102&quot;&gt;this question&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to reproduce this simplification. Can someone explain how they got from the first one to the simplified version? What numerical values (the &lt;em&gt;actual values too&lt;/em&gt;, not only their descriptions) were likely to have been used to get the numerical value of $9.113 \times 10^{-6}$? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/h1M8v.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h1M8v.png&quot; alt=&quot;Chapter 18 Space Mission Engineering and Analysis&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21968" LastEditorUserId="12102" LastEditDate="2017-12-08T03:42:26.283" LastActivityDate="2017-12-08T18:43:35.703" Title="Thrust for solar sail equation derivation" Tags="|orbital-mechanics|mathematics|solar-sail|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23935" PostTypeId="1" AcceptedAnswerId="24481" CreationDate="2017-12-07T22:55:22.443" Score="3" ViewCount="395" Body="&lt;p&gt;Bringing spacecraft to higher orbits than LEO is quite expensive and as rocket reusability gains traction, it will probably get even more expensive when compared to a standard LEO deployment since the first stage does not have enough surplus fuel to go back to earth (at least the current designs).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrodynamic_tether&quot; rel=&quot;nofollow noreferrer&quot;&gt;electrodynamic tether&lt;/a&gt; is a device which can exchange electrical energy for kinetic energy and vice versa by exerting a Lorentz force against the Earth's magnetic field. Since a spacecraft using this technology would not need any fuel once it's in orbit, it would be significantly cheaper to deploy. Combined with a magnetorquer, it would prolong the lifespan of a satellite significantly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is this technology not being used all the time? Are there any issues with practicality of such a system?&lt;/p&gt;&#xA;" OwnerUserId="16099" LastEditorUserId="12102" LastEditDate="2020-01-24T07:48:04.263" LastActivityDate="2020-01-24T07:48:04.263" Title="Why are electrodynamic tethers not used to propel payloads beyond LEO?" Tags="|orbital-maneuver|reuse|electric-propulsion|space-tether|electrodynamic-tether|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23938" PostTypeId="1" CreationDate="2017-12-08T08:22:15.363" Score="5" ViewCount="735" Body="&lt;p&gt;Propagating a state vector seems easier, you just account for the gravity, change the velocity and update the position.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also allows to easily apply perturbations, etc.&#xA;Additionally, the computation may be faster that way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So why are Keplerian elements preferred for propagation of orbits?&lt;/p&gt;&#xA;" OwnerUserId="19223" LastActivityDate="2017-12-08T14:39:49.760" Title="Why are Keplerian elements used in TLEs instead of Cartesian state vectors?" Tags="|orbit|two-line-elements|orbit-determination|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23939" PostTypeId="1" CreationDate="2017-12-08T11:53:18.223" Score="4" ViewCount="800" Body="&lt;p&gt;The launch azimuth for Apollo missions to the general area of the lunar equator was approximately 70 degrees. Could someone explain in simple terms how this figure is arrived at? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I presume that if the moon and earth equators  were both aligned with the ecliptic plane and your trip was from equator to equator you would use a 90 degree azimuth. If so, is the 70 degree Apollo figure just this 90 degrees adjusted for the actual alignment of the two bodies?&lt;/p&gt;&#xA;" OwnerUserId="21971" LastEditorDisplayName="user10509" LastEditDate="2017-12-08T16:15:13.503" LastActivityDate="2020-02-17T00:52:20.233" Title="Calculating Apollo launch azimuth" Tags="|launch|apollo-program|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23940" PostTypeId="2" ParentId="23938" CreationDate="2017-12-08T14:07:07.350" Score="4" Body="&lt;p&gt;For a body in orbit in 2-body scenario, for Keplerian elements only a single variable (true anomaly) changes over time. For Carthesian the whole state vector is in constant flux.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When working with orbital mechanics in Keplerian coordinates, e.g preparing maneuvers, you don't need to work on a body in motion - 2-degree differential equations of motion all the time, as typical to most of Newtonian mechanics and similar. You just stash time (and true anomaly) temporarily forgetting about them, and work out the completely time-independent, static geometry, finding the needed maneuvers and trajectories. When you have the entire plan ready, just retrieve your time/true anomaly data, substitute values and get the moments of maneuvers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also means in Keplerian mechanics, the vector state holds quite explicit history and future of the position, besides position &lt;em&gt;at the moment&lt;/em&gt;. Things like eccentricity or inclination tell a lot about the orbit at a glance. Meanwhile, given a velocity and position vector of a satellite, and mass of the central body, telling whether it's nearly on escape trajectory, or in equatorial circular orbit takes a good bit of calculations. &lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-12-08T14:07:07.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23941" PostTypeId="2" ParentId="23938" CreationDate="2017-12-08T14:39:49.760" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Propagating a state vector seems easier, you just account for the gravity, change the velocity and update the position.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You described the symplectic Euler method (aka semi-implicit Euler method, Euler–Cromer, Newton–Størmer–Verlet, and other names). This is a rather lousy numerical integrator. It loses accuracy rather quickly.  Extremely small time steps are needed to have any semblance of an accurate propagation. The only saving graces of this technique are that it's extremely easy to program and that it conserves angular momentum.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Why are Keplerian elements used in TLEs instead of Cartesian state vectors?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;TLEs do &lt;strong&gt;not&lt;/strong&gt; use Keplerian elements. They instead use a mean orbital element set that accounts for the non-spherical shape of the Earth, atmospheric drag, and perturbations from the Moon and the Sun. Four key advantages of using TLEs over a numerical integrator are&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;There are multiple free implementations of SGP4, in multiple languages.&lt;/li&gt;&#xA;&lt;li&gt;The SGP4 model accounts for the perturbations mentioned above. You'll need to explicitly model these perturbations in a numerical integrator.&lt;/li&gt;&#xA;&lt;li&gt;With SGP4, calculating where the satellite will be one week from now has the same computational cost as does calculating where it will be one second&#xA;from now (the same is not true for a numerical integrator), and&lt;/li&gt;&#xA;&lt;li&gt;Epoch states of satellites orbiting the Earth orbit in TLE form are widely available.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It certainly is possible to do better than SGP4. You'll need to use a fairly good numerical integrator (i.e., not symplectic Euler) and models of those perturbations. Finally, you'll need to have a good initial state.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-12-08T14:39:49.760" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23942" PostTypeId="2" ParentId="23887" CreationDate="2017-12-08T16:16:19.833" Score="5" Body="&lt;p&gt;As a complement to the good answers provided above, I remember learning in a control system's graduate course that the Saturn V launch was modelled via a 23rd order equation. This gives an indication of the complexity of the math involved in the control of the full up Saturn V.&lt;/p&gt;&#xA;" OwnerUserId="21974" LastActivityDate="2017-12-08T16:16:19.833" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23943" PostTypeId="2" ParentId="23939" CreationDate="2017-12-08T16:49:26.923" Score="1" Body="&lt;p&gt;&lt;a href=&quot;https://history.nasa.gov/afj/ap10fj/pdf/as-505-postflight-trajectory.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This document&lt;/a&gt; says (in rather general terms, as you requested)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;This flight azimuth, dependent on the launch time, launch day and&#xA;  month, is calculated using polynomial coefficients taken from the&#xA;  guidance presettings in order to achieve the desired translunar targeting parameters. The translunar targeting parameters are functions&#xA;  of the moon position, earth parking orbit inclination, earth-moon&#xA;  distance, and moon travel rate.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-12-08T16:49:26.923" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23944" PostTypeId="2" ParentId="23934" CreationDate="2017-12-08T18:34:36.393" Score="4" Body="&lt;h1&gt;Unit&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;When dealing with a magic number, it's important to know what its unit should be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$F=\frac{2RSA}{c}\sin^2\theta=9.113\times10^{-6}\frac{RA}{D^2}\sin^2\theta$$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$F$ is a thrust (=Force) in Newton.&lt;/li&gt;&#xA;&lt;li&gt;$R$, $2$ and $\sin^2$ are unitless.&lt;/li&gt;&#xA;&lt;li&gt;$A$ is an area in $m^2$.&lt;/li&gt;&#xA;&lt;li&gt;It means that $\frac{S}{c}$ is a pressure in Pascal.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;By simplifying $R$, $A$ and $\sin$, we get:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\frac{2S}{c}=9.113\times10^{-6}\frac{1}{D^2}$$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$D$ is a distance in $\mathrm{AU}$&lt;/li&gt;&#xA;&lt;li&gt;It means that $9.113\times10^{-6}$ is a force in a non-conventional unit : $\mathrm{Pascal} * \mathrm{AU}^2$&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;Value&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;This equation should be valid anywhere in the solar system. In particular, it should work on Earth ($D = 1 \mathrm{AU}$), where $S$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_constant&quot; rel=&quot;nofollow noreferrer&quot;&gt;solar constant&lt;/a&gt; : $1361 \mathrm{W/m²}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using &lt;a href=&quot;https://qalculate.github.io/&quot; rel=&quot;nofollow noreferrer&quot;&gt;qalculate&lt;/a&gt; with $2*S/c$: &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;&amp;gt; 2 * 1361W/m² / c to Pa&#xA;&#xA;  ((2 * (1361 * watt)) / (meter^2)) / speed_of_light = approx. 9.0796147 uPa&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Close enough! It seems that $1366 \mathrm{W/m²}$ has been used to get $9.113\times10^{-6} \mathrm{Pa}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using &lt;a href=&quot;https://en.wikipedia.org/wiki/Inverse-square_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;inverse-square law&lt;/a&gt;, you get the desired equation.&lt;/p&gt;&#xA;" OwnerUserId="19229" LastEditorUserId="19229" LastEditDate="2017-12-08T18:43:35.703" LastActivityDate="2017-12-08T18:43:35.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23945" PostTypeId="1" CreationDate="2017-12-08T20:27:29.857" Score="5" ViewCount="124" Body="&lt;p&gt;I'm currently writing fictional story which is happening at ISS and I have so many questions about this topic, but this one is the most important to me. How many possibilities of communication they have? Is there's a special &quot;communication place&quot; for every space agenture at ISS or there's a just one big communication centre for all? &lt;/p&gt;&#xA;" OwnerUserId="21976" LastActivityDate="2017-12-08T20:27:29.857" Title="How many ways of communication with ground can ISS crew use?" Tags="|iss|communication|" AnswerCount="0" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23946" PostTypeId="1" AcceptedAnswerId="23948" CreationDate="2017-12-09T00:21:00.827" Score="18" ViewCount="1684" Body="&lt;p&gt;The solar panel modules of ISS use an extending central truss structure to stretch two &quot;ribbons&quot; of solar panels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can that structure be re-folded in space, if need arose (for whatever reason), or is it &quot;extend only&quot;, irreversible operation that would result in damage or just a mess if reversing it was attempted?&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="6944" LastEditDate="2020-09-04T13:22:31.537" LastActivityDate="2020-09-04T13:22:31.537" Title="Can ISS fold its solar panels?" Tags="|iss|solar-power|mechanisms|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23948" PostTypeId="2" ParentId="23946" CreationDate="2017-12-09T02:08:18.257" Score="24" Body="&lt;p&gt;The long boom holding the panels can be retracted (the term used instead of &quot;folding&quot;).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is not done lightly, for fear of not being able to reverse it.  It has only been done a few times, once when a truss (the P6 truss) was relocated, and once to work out an issue.    During the extension of the boom during the relocation task, the panels tore, requiring an EVA to repair - another reason retraction is not done unless absolutely necessary.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: the P6 arrays were retracted long before the relocation, as the outboard arrays were installed. See the last video in this answer. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's an animation of the relocation plans. The arrays are already retracted at the beginning, but it does show the extension at the end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4iOKEv6MeAk?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a real video of that later event, to work out the issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/-bTcodL0awg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Supplementary information: the P6 truss had to be relocated because it was launched early in the ISS assembly sequence to provide power for the proto-ISS. It was attached to the Z1 truss near the center of the station.  Much later in the assembly, after the other solar arrays were launched and attached, the P6 truss was relocated to its final resting place at the end of the port truss.  Here's a picture of the proto-ISS showing the original placement of the P6 truss.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2p9Mg.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2p9Mg.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This video shows the whole assembly.  The P6 array retractions are at 0:47 and 0:53.  The relocation is at 1:03.  The last Russian module added in this video is the long-delayed Nauka module, it has not actually been launched yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/USrYp1ziYvw?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-12-09T02:47:14.470" LastActivityDate="2017-12-09T02:47:14.470" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23950" PostTypeId="2" ParentId="23939" CreationDate="2017-12-09T08:06:34.510" Score="1" Body="&lt;p&gt;One important contributing parameter is the inclination of the Earth, around 23 degrees (this causes seasons).  Without that inclination,  the launch azimuth would be around 90. Other contributing factors are of course the plane in which the Moon moves, and more.&lt;/p&gt;&#xA;" OwnerUserId="5980" LastEditorUserId="5980" LastEditDate="2017-12-11T20:59:43.947" LastActivityDate="2017-12-11T20:59:43.947" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23951" PostTypeId="2" ParentId="20925" CreationDate="2017-12-09T11:32:15.867" Score="9" Body="&lt;p&gt;Before the question is closed I'll post my original comment as an answer.&lt;/p&gt;&#xA;&lt;p&gt;The problem of simply &lt;em&gt;calculating&lt;/em&gt; the gravitational field of a cube is already interesting in part because more complex objects such as asteroids can be built-up from many smaller cubes, for example using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Marching_cubes&quot; rel=&quot;nofollow noreferrer&quot;&gt;Marching Cubes&lt;/a&gt; technique.&lt;/p&gt;&#xA;&lt;p&gt;See also:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://scicomp.stackexchange.com/q/26537/17869&quot;&gt;Another way to evaluate the gravitational force from a uniform cube?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The idea of using the &amp;quot;Polyhedra Method&amp;quot; to approximate an arbitrarily shaped object as a series of polyhedra seems to go back to the book &lt;em&gt;The Theory of the Potential&lt;/em&gt;, MacMillan W. D. 1930, New York: McGraw-Hill. Republished by Dover, New York, 1958. This method was used in the very interesting paper by JPL’s Robert A. Werner and Daniel J. Scheeres  &lt;em&gt;Exterior gravitation of a polyhedron derived and compared with harmonic and mascon gravitation representations of asteroid 4769 Castalia&lt;/em&gt; &lt;a href=&quot;https://link.springer.com/article/10.1007/BF00053511&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celest.&#xA;Mech. Dyn. Astron. 65(3), 313–344 (1997)&lt;/a&gt; where they compare to using a spherical harmonic technique (e.g. &lt;span class=&quot;math-container&quot;&gt;$J_2$&lt;/span&gt;...) as well as trying to define the potential in terms of a central or spherical mass plus a series of &amp;quot;mascons&amp;quot; or mass concentrations.&lt;/p&gt;&#xA;&lt;p&gt;The first discussion to the problem that &lt;em&gt;also considers the orbits about cubes&lt;/em&gt; that I've found was by James M. Chappell, Mark J. Chappell, Azhar Iqbal and Derek Abbott in their 2012 paper &lt;a href=&quot;https://arxiv.org/abs/1206.3857&quot; rel=&quot;nofollow noreferrer&quot;&gt;The gravitational field of a cube&lt;/a&gt; (also &lt;a href=&quot;https://research-repository.griffith.edu.au/bitstream/handle/10072/52169/86009_1.pdf?sequence=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;). Here they also explore what a &amp;quot;lake&amp;quot; would look like as well as some of the possible orbits. However &lt;em&gt;they were not able to identify stable, closed orbits.&lt;/em&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hvHCD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hvHCD.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The problem was studied further in the 2011 paper by Xiaodong Liu, Hexi Baoyin, Xingrui Ma &lt;a href=&quot;https://arxiv.org/abs/1108.4635&quot; rel=&quot;nofollow noreferrer&quot;&gt;Periodic orbits in the gravity field of a fixed homogeneous cube&lt;/a&gt;. They identified orbits which they state are closed and stable around a fixed, non rotating cube, both in the symmetry plane (one the cube's &amp;quot;equators&amp;quot;) as well as in the plane of the inscribed hexagon with the cube.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/et6cZ.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/et6cZ.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gRpxd.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gRpxd.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/K64Fb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/K64Fb.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;In the companion paper by the same authors around the same time &lt;a href=&quot;https://arxiv.org/abs/1108.4636&quot; rel=&quot;nofollow noreferrer&quot;&gt;Equilibria, periodic orbits around equilibria, and heteroclinic connections in the gravity field of a rotating homogeneous cube&lt;/a&gt; they analyze orbits about a &lt;em&gt;rotating cube.&lt;/em&gt; They find stable orbits, some of which &amp;quot;hover&amp;quot; or orbit around a certain region above the rotating cube, like a geosynchronous orbit would.&lt;/p&gt;&#xA;&lt;p&gt;It is always difficult to draw orbits in rotating frames in 2D. I'll include one &amp;quot;sample&amp;quot; figure which gives a flavor of their results, but one has to read the paper in depth to appreciate what the orbits will actually be like.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vyFpz.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vyFpz.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-08-19T01:58:05.350" LastActivityDate="2021-08-19T01:58:05.350" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="23952" PostTypeId="1" AcceptedAnswerId="23953" CreationDate="2017-12-09T12:04:06.220" Score="6" ViewCount="2015" Body="&lt;p&gt;In an answer to my previous question, OrganicMarble posted a video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/-bTcodL0awg?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The booms like through the center of ISS solar panels are quite common in space use - supporting magnetometers, keeping modules away from the core craft, etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I always thought they extend by &quot;scisoring&quot;, pantograph mechanism action, whole boom getting slimmer but longer simultaneously. Apparently this is not the case - in the video you can see the still extended part of the boom remains &quot;immutable&quot;, same unchanging structure. So what happens to the part of the boom that is being pulled into the assembly? Vanishing into a wormhole? Getting disassembled into atoms by nanomachines, to be reassembled later? How does that extending/retracting mechanism work?  &lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="6944" LastEditDate="2020-09-04T13:27:21.710" LastActivityDate="2020-09-04T13:27:21.710" Title="How do the booms on ISS (and other spacecraft) extend and retract?" Tags="|iss|engineering|mechanisms|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23953" PostTypeId="2" ParentId="23952" CreationDate="2017-12-09T13:50:37.693" Score="12" Body="&lt;p&gt;tl;dr - each bay of the mast collapses as it is pulled into the cylinder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS solar array mast is a truly ingenious mechanical structure which can be retracted into a remarkably small canister. It was developed by ATK and is called a Folding Articulated Square Truss (FAST) mast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This picture shows a FAST mast without arrays.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/beFwz.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/beFwz.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mast is composed of aluminum&#xA;longerons, stainless steel wire rope diagonals, aluminum rigid battens, and flexible fiberglass battens.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every second batten ring is &#xA;flexible.  Therefore,  the  battens  are  able  to  buckle  and  two  cells  of &#xA;the  boom  can  be  collapsed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ha00a.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ha00a.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The  deployment  is  driven  by  the  strain  energy  of  the  buckled  battens  and  is  controlled  by  a &#xA;retaining &#xA;mechanism in the container. This mechanism is a rotating nut&#xA;with four helical grooves. These helical grooves engage roller bearings located on&#xA;the corners of each mast section in a manner similar to that of a screw being pulled&#xA;through a rotating nut. It requires two revolutions of the nut to fully extend one mast&#xA;bay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ElAj1.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ElAj1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This image shows a cross section of the deployment mechanism. It's a bit complicated, but you can see the nut at the top (gray area), the collapsed truss sections at the bottom, and the drum/lanyard/cradle mechanism that feeds the sections into the bottom of the nut.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OTcX3.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OTcX3.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;References&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170003919.pdf&quot; rel=&quot;noreferrer&quot;&gt;ADVANCED DEPLOYABLE STRUCTURAL SYSTEMS FOR SMALL SATELLITES&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960000328.pdf&quot; rel=&quot;noreferrer&quot;&gt;Static Stability of the Space Station Array Fastmast structure&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950021309.pdf&quot; rel=&quot;noreferrer&quot;&gt;Static Stability of a Three-dimensional Space Truss&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Personal Notes&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-12-09T17:27:17.227" LastActivityDate="2017-12-09T17:27:17.227" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23954" PostTypeId="1" CreationDate="2017-12-09T14:52:09.853" Score="3" ViewCount="246" Body="&lt;p&gt;Elon Musk has said that he's planning to launch two uncrewed BFR rockets to Mars by 2022. And of course there would be some test launches even before that. For such an enormous project, I don't see how this is even possible. Also, the Falcon Heavy got delayed for a long time due to unexpected engineering difficulties so I don't see why this shouldn't be the same. So what do you guys think is a reasonable expectation for the first launch of the BFR? I'm not saying first Mars mission, just first launch.&lt;/p&gt;&#xA;" OwnerUserId="20555" LastActivityDate="2017-12-09T14:52:09.853" Title="How long can it be reasonably expected to wait for the first flight of SpaceX's BFR?" Tags="|spacex|rockets|engineering|spacex-starship|" AnswerCount="0" CommentCount="4" ClosedDate="2017-12-10T01:34:46.497" ContentLicense="CC BY-SA 3.0" />
  <row Id="23955" PostTypeId="1" AcceptedAnswerId="23959" CreationDate="2017-12-09T15:07:38.047" Score="0" ViewCount="385" Body="&lt;p&gt;The Tech Times article &lt;a href=&quot;http://www.techtimes.com/articles/216842/20171209/nasa-fails-spot-whale-sized-asteroid-skimmed-past-earth.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Fails To Spot Whale-Sized Asteroid That Skimmed Past Earth&lt;/a&gt; says that Asteroid 2017 VL2 passed the Earth within about 120,000 km, or about 1/3 the distance of the Moon on November 9 (2017) but wasn't spotted until November 10 by Mauna Loa observatory in Hawaii. It is estimated to be about 16 to 32 meters in diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this below the threshold where there is good data on NEOs that may impact Earth, or in fact is there not yet good data on that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How safe are we? More specifically, is there in fact little data on potential NEOs in this size range (~30 meters)?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Slight irony in the timing - see the BBC's &lt;a href=&quot;http://www.bbc.com/news/science-environment-41583704&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asteroid close approach to test warning systems&lt;/a&gt; where the &lt;em&gt;similarly sized&lt;/em&gt; and well documented 2012 TC4 which passed the Earth two months ago.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, the much larger 3200 Phaethon will pass much farther from the Earth next week, and is expected to produce &lt;a href=&quot;https://echo.jpl.nasa.gov/asteroids/Phaethon/Phaethon_planning.2017.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;excellent radar imaging&lt;/a&gt;. See also (&lt;a href=&quot;http://www.skyandtelescope.com/observing/3200-phaethon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sky and Telescope&lt;/a&gt; and &lt;a href=&quot;https://astronomynow.com/2017/12/04/see-asteroid-phaethon-source-of-the-geminids-in-a-close-brush-with-earth/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astronomy Now&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AsteroidDay.org shorts:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://asteroidday.org/video/brian-may/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Brian May&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://asteroidday.org/video/update-august-2016/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scott Manley&lt;/a&gt; &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=G0NhCzeebjI&quot; rel=&quot;nofollow noreferrer&quot;&gt;Asteroid Day short video 1st of 7 linked&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=D_DjaH4ciXA&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jimmy Kimmel&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; from &lt;a href=&quot;http://metro.co.uk/2017/12/08/asteroid-2017-vl2-zoomed-terrifyingly-close-earth-nasa-didnt-see-coming-7144051/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qGdgW.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qGdgW.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-12-09T17:25:22.843" LastActivityDate="2017-12-10T19:52:28.457" Title="Yikes! How did we miss 2017 VL2?" Tags="|asteroid|near-earth-object|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23956" PostTypeId="1" AcceptedAnswerId="23972" CreationDate="2017-12-09T15:56:38.323" Score="4" ViewCount="6153" Body="&lt;p&gt;Has there been any current estimates for the cost of putting a man on the moon, today? I'm curious how much the price has dropped with the improvement of technology, compared to the original moon landing's cost.&lt;/p&gt;&#xA;" OwnerUserId="21984" LastEditorUserId="1054" LastEditDate="2017-12-10T03:42:06.167" LastActivityDate="2017-12-11T00:10:20.793" Title="With today's technology, how much would it cost to put a man on the Moon again?" Tags="|the-moon|cost|lunar-landing|" AnswerCount="2" CommentCount="4" ClosedDate="2017-12-12T10:38:00.517" ContentLicense="CC BY-SA 3.0" />
  <row Id="23958" PostTypeId="1" AcceptedAnswerId="23965" CreationDate="2017-12-09T20:07:07.297" Score="33" ViewCount="11341" Body="&lt;p&gt;I've recently become interested in amateur satellite packet radio, and I've seen that the bandwidth and thus the bitrate is extremely low on most amateur satellites, not even factoring in noise. So I wanted to create my own (extremely small) amateur satellite which has an uplink and downlink in the gigahertz frequency range, which should provide the necessary bandwidth and bitrate I and many other amateur satellite packet radio enthusiasts are craving.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So how do we get a satellite in orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would it cost to piggyback on a space shuttle mission?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is amateur rocketry a viable option to get a satellite into orbit and what would be the cost of such a project?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any regulatory bodies that I should be aware of that enforce activities in the air and in space?&lt;/p&gt;&#xA;" OwnerUserId="21986" LastActivityDate="2021-02-08T21:31:20.980" Title="What would one need to do to get an amateur satellite into orbit?" Tags="|artificial-satellite|amateur-rocketry|amateur-radio|" AnswerCount="4" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="23959" PostTypeId="2" ParentId="23955" CreationDate="2017-12-09T20:23:13.760" Score="3" Body="&lt;p&gt;There are estimated to be around 1 million Near Earth Objects (NEOs) of around in the same size as 2017 VL2, of which around 1% are known (see &lt;a href=&quot;https://resources.asteroidday.org/ed/rustys-planetary-defense-6-part-series/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rusty's Planetary Defense 6-part series&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's only much larger asteroids which are better tracked: we're estimated to know well over 90% of the NEOs larger than 1 km, and are aiming to get up to 90% of the 140 m+ ones over the next few years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to how we missed it, remember that in the weeks leading up to 9th November it was roughly in the direction of the Sun (as can be seen from the animated gif in the question), so impossible to see against the daytime sky.&lt;/p&gt;&#xA;" OwnerUserId="12831" LastEditorUserId="12831" LastEditDate="2017-12-10T19:52:28.457" LastActivityDate="2017-12-10T19:52:28.457" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23960" PostTypeId="2" ParentId="23958" CreationDate="2017-12-09T21:21:40.223" Score="13" Body="&lt;p&gt;The Space Shuttle retired in 2011, and no amateur launch has ever reached orbit, but there are plenty of opportunities to piggyback on commercial and government launch services. Mostly this is arranged through a broker, for example &lt;a href=&quot;http://spaceflight.com/&quot; rel=&quot;noreferrer&quot; title=&quot;Spaceflight Industries&quot;&gt;Spaceflight Industries&lt;/a&gt;. A rough order of magnitude estimate for price would be US$50,000 / kg.&lt;/p&gt;&#xA;" OwnerUserId="12831" LastActivityDate="2017-12-09T21:21:40.223" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23962" PostTypeId="2" ParentId="23958" CreationDate="2017-12-09T23:20:38.987" Score="12" Body="&lt;p&gt;A TV show addressed this some years ago.  What appears to be the full video on You-tube:&#xA;(Link gone--tv show &amp;quot;How Hard Can It Be&amp;quot;, Episode 3 -- &amp;quot;Homemade Rocket&amp;quot;.)&#xA;Owner's website with a few minutes of video: &lt;a href=&quot;https://www.natgeotv.com/ca/how-hard-can-it-be/videos&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.natgeotv.com/ca/how-hard-can-it-be/videos&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;This was a TV production, they obviously have a budget well above what most amateurs could bring to it.  They quickly concluded that reaching orbit was out of the question within the scope of what they could do, so they set out to do what they could.&lt;/p&gt;&#xA;&lt;p&gt;Their rocket--which cost thousands of dollars to build--made it something like 10 miles up.  That's only a drop in the bucket compared to what it would take to reach orbit.  (Simple illustration:  To push a rocket to the edge of space costs you about 1,400 m/s minimum.  To put a rocket in low orbit costs you about 9,800 m/s minimum.)&lt;/p&gt;&#xA;&lt;p&gt;Also, by the time you're talking 9,800 m/s the rocket equation has become brutal.  Good solid motors have an ISP of about 250, vs 450 for LH2/LO2.  This means that pushing something to orbit with solids means a rocket several times bigger than with the good stuff--and note how big even the smallest orbital rockets are.&lt;/p&gt;&#xA;&lt;p&gt;There are also major permission issues involved in such a launch.  The normal safety laws of amateur rocketry can't handle an orbital launch, thus you have to convince the FAA and the government that your systems are safe even when they don't function as intended.  You'll also need FCC licenses to talk to your rocket and a blaster's license for the range safety system.  (Yes, there are high explosives on every rocket that goes up.  The charges are fairly small as they are only meant to burst it, drag &amp;amp; the fuel finish the job.  Watch the video of the failed SpaceX CRS-7 mission--the control computer detected a catastrophic failure and destroyed it.  Likewise, the first Ariane 5 launch, the rocket tips, when it starts to break up the computer destroys it.  Manned missions are not exempt--look hard enough and you can find video of the commanded destruct of the boosters in the Challenger disaster.  This is hard to find as most of the video focuses on the disaster itself.  The solid boosters were still burning after the big boom, though, but no longer had a guidance system.  Note how their flight terminates in a big puff--that was a destruct order.)&lt;/p&gt;&#xA;&lt;p&gt;Since the comment mentioned abbreviations:&#xA;ISP:  Specific impulse. (It's usually written as &lt;strong&gt;I&lt;/strong&gt;mpulse [sub]&lt;strong&gt;SP&lt;/strong&gt;ecific[/sub], hence the order of the letters.)  This is a measure of a rocket's performance, the bigger the number the better.  While the ratio is linear the ratio between performance and size is exponential.&lt;/p&gt;&#xA;&lt;p&gt;LH2:  Liquid hydrogen.  (Hydrogen exists as a molecule of two hydrogen atoms, hence the 2.)  Energetic but very nasty to deal with, enough so that it's not unusual to see a lesser fuel used in the first stage where performance isn't so critical.&lt;/p&gt;&#xA;&lt;p&gt;LO2:  Liquid oxygen.  Again, a molecule of two atoms.  Note that this is also sometimes labeled LOX.  Not nearly so nasty as LH2 but still it needs some pretty careful handling because it can make little fires very big very quickly.&lt;/p&gt;&#xA;" OwnerUserId="276" LastEditorUserId="276" LastEditDate="2021-02-08T21:31:20.980" LastActivityDate="2021-02-08T21:31:20.980" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="23963" PostTypeId="2" ParentId="18368" CreationDate="2017-12-10T00:00:48.273" Score="2" Body="&lt;p&gt;The ISS does not have a fax machine.  There are two Epson 800 inkjet printers onboard connected to the ISS local area network.  Any document sent up from the ground that the crew needs to print, can be printed on this printer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next year, the aging Epson 800s are planned to be replaced with a new HP Envy, specially modified for free-fall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://mashable.com/2017/11/02/nasa-updates-international-space-station-printer/#uZf0lrmJbPqC&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-12-10T00:00:48.273" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23964" PostTypeId="2" ParentId="23958" CreationDate="2017-12-10T01:48:43.580" Score="2" Body="&lt;p&gt;Obligatory &lt;a href=&quot;https://what-if.xkcd.com/58/&quot; rel=&quot;nofollow noreferrer&quot;&gt;XKCD What If&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting to orbit is not a a problem of just vertical distance.  You also need to go &lt;strong&gt;really&lt;/strong&gt; fast to stay in orbit (I assume you want your satellite to stay up there).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, you have to go up about 100 KM, then you need to reach a speed of 8 &lt;strong&gt;km/s&lt;/strong&gt; to stay in orbit.  That's going to require a massive rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Randal compared the orbital speed of ISS to a rifle bullet:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The ISS moves so quickly that if you fired a rifle bullet from one end&#xA;  of a football field, the International Space Station could cross the&#xA;  length of the field before the bullet traveled 10 yards.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8474" LastActivityDate="2017-12-10T01:48:43.580" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="23965" PostTypeId="2" ParentId="23958" CreationDate="2017-12-10T03:09:58.193" Score="45" Body="&lt;p&gt;Astronautical engineer here, welcome to the small satellite community!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Firstly, it's important to recognize that the FCC tightly regulates the radio spectra available to satellites and amateur broadcasters, so before even considering &lt;strong&gt;how&lt;/strong&gt; to launch your spacecraft, you should make sure your payload is legal. The FCC tables on frequency allocations can be found &lt;a href=&quot;https://transition.fcc.gov/oet/spectrum/table/fcctable.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;; you'll be interested in all the bands with allocations to &quot;Amateur-Satellite&quot;. Note that some of these frequency allocations are not bi-directional, so double check in the detailed notes on each band to make sure you understand the limits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to launch costs, there are a couple of launch service providers who offer payload space for small satellites to ride alongside a much larger payload, which they term &quot;secondary payloads&quot;. A service provider can typically squeeze half a dozen or more secondary payloads into a launch, which means these smaller payloads only pay a fraction of the overall cost of the launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Launch providers generally don't make their costs available to the public, for competitive reasons; you'd usually have to contact one to request a quote, and the cost may vary depending on the launch window, vehicle, and how many other secondary payloads will be sharing the launch. For a ballpark estimate, you can use the table provided &lt;a href=&quot;http://spaceflight.com/schedule-pricing/#pricing&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It sounds like your spacecraft will be relatively small (what we would term a &quot;nano satellite&quot;), so you'd probably be most interested in everything to the left of the 50kg bracket in their table, which lists launch prices from 295 thousand dollars up to about 2 million. If your satellite is &lt;em&gt;really&lt;/em&gt; small (e.g. a 1U CubeSat), the price may be even lower. As I said, your actual price will vary, so you really need to request a quote from a launch provider to get accurate numbers for your specific case.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most launch providers also place a number of requirements on their customers, to ensure that they won't pose a safety risk to the launch vehicle or other payloads. Your satellite will have to meet certain structural criteria for how it mates with the launch vehicle (e.g. via &lt;a href=&quot;http://www.planetarysystemscorp.com/?post_type=product&amp;amp;p=449&quot; rel=&quot;noreferrer&quot;&gt;lightband&lt;/a&gt; adapter, &lt;a href=&quot;http://mstl.atl.calpoly.edu/~workshop/archive/2004/Spring/04c-Toorian(CubeSat)-P-POD.pdf&quot; rel=&quot;noreferrer&quot;&gt;CubeSat Pod&lt;/a&gt;, etc.), the location of its center of mass, and vibration frequency response, which are usually spelled out in the launch vehicle technical data manual, such as &lt;a href=&quot;https://www.orbitalatk.com/flight-systems/space-launch-vehicles/minotaur/docs/MinotaurIV_V_UG.pdf&quot; rel=&quot;noreferrer&quot;&gt;this one&lt;/a&gt; for the Minotaur IV. Fortunately some of these requirements are less severe for secondary payloads, due to their small size, but you'll still need to do some level of analysis and/or testing to satisfy the launch provider that your design meets the requirements.&lt;/p&gt;&#xA;" OwnerUserId="21989" LastEditorUserId="2784" LastEditDate="2017-12-11T22:28:16.297" LastActivityDate="2017-12-11T22:28:16.297" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="23967" PostTypeId="2" ParentId="23259" CreationDate="2017-12-10T14:43:46.913" Score="4" Body="&lt;p&gt;Yes, it would.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The base, for sustenance, would require a lot of electric power, which would dissipate as heat upon use. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Greenhouses, to produce useful, edible plants, room-efficient plants would require extra lighting, and quite a bit of it. Natural light levels on Mars are sufficient to grow some ferns or other plants accustomed to deep shade. You might get some berries. You wouldn't get nearly enough to feed the crew. You need actual, efficient cultivars - legumes, potatoes, maybe grains - plants that provide abundant, highly nutritious crop. And their light requirements are so high you'll need a lot of growth lights - which will produce a lot of excess heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The greenhouses are hardly mass- and volume-efficient where it comes to production of oxygen. Scrubbing excess CO2 and production of oxygen through &lt;a href=&quot;https://mars.nasa.gov/mars2020/mission/instruments/moxie/&quot; rel=&quot;nofollow noreferrer&quot;&gt;MOXIE&lt;/a&gt; would produce another couple kilowatts per human.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Add heat from all the electronic equipment, heat from losses on charging/discharging batteries (to last the night), human body heat, machinery for purifying water and so on - you'll quickly find the heat budget of the base goes way above expenditure - especially if the base is at least partially covered by soil to protect it from cosmic radiation, which severely limits radiative dissipation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How much heat dissipation capacity the base would need is still subject to study - we don't have complete plans, we don't know all the variables, but we know already radiators will be a must.&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2017-12-10T14:43:46.913" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="23968" PostTypeId="2" ParentId="23956" CreationDate="2017-12-10T16:49:13.970" Score="1" Body="&lt;p&gt;There are many ways to guess a cost, as there are to interpret your question.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To recreate exactly you'd need somewhere to make outdated equipment, so that's not cost efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To update everything and use current technology has a cost that can be estimated but it's more than a day's worth of work to give you a guess; what would the guess mean to you, would you then ask to go for that price, a guaranteed estimate takes longer (and wouldn't be free).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX plans on sending two people for &lt;a href=&quot;http://www.spacex.com/news/2017/02/27/spacex-send-privately-crewed-dragon-spacecraft-beyond-moon-next-year&quot; rel=&quot;nofollow noreferrer&quot;&gt;a spin around the Moon and back&lt;/a&gt; (no landing) next year. You need to be approved and pass the health and fitness test results; you don't have to fly yourself since you get a crew.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They haven't decided upon the price per person but they've &lt;a href=&quot;https://www.inverse.com/article/28424-spacex-moon-mission-ticket-cost-elon-musk&quot; rel=&quot;nofollow noreferrer&quot;&gt;hinted&lt;/a&gt; that it's over $35m. So if you don't plan on putting your foot on the Moon it's not extremely expensive (cheaper than before).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we assume they're making a small profit, then $50M with no landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're going to land you might as well do something useful while you're there. So you could double or triple the $50M estimate. You're asking for a guess unless you're more exact on everything.&lt;/p&gt;&#xA;" OwnerUserId="20766" LastActivityDate="2017-12-10T16:49:13.970" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23969" PostTypeId="2" ParentId="20012" CreationDate="2017-12-10T17:27:35.300" Score="24" Body="&lt;p&gt;Launch vehicle operators (or at least the major ones) all seem to drop their fairings such that the heat produced by the remaining atmosphere remains below 1135 W/m$^2$. Not all the operators provide details on how heat is calculated, but those that do say that the heat flux is calculated using a free molecular flow through a plane perpendicular to the direction of travel, so I suspect that this is more or less a standard definition.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Sources:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.arianespace.com/wp-content/uploads/2015/09/Vega-Users-Manual_Issue-04_April-2014.pdf&quot; rel=&quot;noreferrer&quot;&gt;Vega User's Manual, page 3-14&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nominal time for jettisoning the fairing is determined in order not to exceed a maximum instantaneous flux of 1135 W/m$^2$. This flux is calculated as a free molecular flow acting on a plane surface perpendicular to the velocity direction ($\frac{1}{2}\rho V^3$).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf&quot; rel=&quot;noreferrer&quot;&gt;Falcon 9 Payload User's Guide, page 32&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The payload fairing will nominally be deployed when free molecular aero-thermal heating is less than 1,135 W/m$^2$.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.arianespace.com/wp-content/uploads/2011/07/Ariane5_Users-Manual_October2016.pdf&quot; rel=&quot;noreferrer&quot;&gt;Ariane 5 User's Manual, page 3-11&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nominal time for jettisoning the fairing is determined in order to not exceed the aerothermal flux of 1135 W/m$^2$. This flux is calculated as a free molecular flow acting on a plane surface perpendicular to the velocity direction, and based on the atmospheric model US66, latitude 15° North.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Launch_Vehicles/Delta_IV_Users_Guide_June_2013.pdf&quot; rel=&quot;noreferrer&quot;&gt;Delta IV Launch Services User's Guide, page 3-19&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Unless otherwise requested, fairing jettison for Delta IV missions will occur shortly after the 3-sigma high theoretical free molecular heating for a flat plate normal to the free stream drops below 1135 W/m$^2$ (360 Btu/hr ft$^2$) based on the 1962 United States Standard Atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/DeltaIIPayloadPlannersGuide2007.pdf&quot; rel=&quot;noreferrer&quot;&gt;Delta II Payload Planners Guide, page 4-13&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Unless otherwise requested, fairing jettison will occur shortly after the theoretical free molecular heating for a flat plate normal to the free stream drops below 0.1 Btu/ft$^2$-sec (1135 W/m$^2$) based on the 1962 U.S. standard atmosphere.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/AtlasVUsersGuide2010.pdf&quot; rel=&quot;noreferrer&quot;&gt;Atlas V Launch Services User's Guide, page 3-27&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;PLF [payload fairing] jettison typically occurs when the 3-sigma maximum free molecular heat flux decreases to 1,135 W/m$^2$ (360 Btu/hr-ft$^2$).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.arianespace.com/wp-content/uploads/2015/09/Soyuz-Users-Manual-March-2012.pdf&quot; rel=&quot;noreferrer&quot;&gt;Arianespace Soyuz User's Manual, page 3-15&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The nominal time for jettisoning the Fairing is determined in order to not exceed the aerothermal flux of 1135 W/m$^2$. This flux is calculated as a free molecular flow acting on a plane surface perpendicular to the velocity direction and based on the atmospheric model US 66, latitude 15° North. After Fairing jettisoning, the aerothermal flux varies from 1135 W/m$^2$ to less than 200 W/m$^2$ within 20 seconds.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.ilslaunch.com/sites/default/files/pdf/Proton%20Mission%20Planner%27s%20Guide%20Revision%207%20%28LKEB-9812-1990%29.pdf&quot; rel=&quot;noreferrer&quot;&gt;Proton Launch System Mission Planner's Guide, page 3-6&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The PLF [payload fairing] is jettisoned at approximately 340 to 350 seconds into flight (at an altitude of 121 to 125km or more), and the maximum FMHF [free molecular heat flux] does not exceed 1135 W/m$^2$ at any time following PLF jettison.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="21703" LastActivityDate="2017-12-10T17:27:35.300" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23970" PostTypeId="1" CreationDate="2017-12-10T18:05:01.463" Score="4" ViewCount="361" Body="&lt;p&gt;What do you call the kind of transfer that takes an orbiting body from orbiting the planet of the system to orbiting a moon of the system.&#xA;I am merely looking for the name of this kind of transfer orbit (if such a name for it exists). For example, a Hohmann transfer takes an orbiting body from one orbital radius to another orbital radius.&lt;/p&gt;&#xA;" OwnerUserId="21997" LastActivityDate="2017-12-11T23:24:42.523" Title="Specific kind of orbital transfer?" Tags="|orbital-mechanics|orbit|orbital-maneuver|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23971" PostTypeId="2" ParentId="7897" CreationDate="2017-12-10T19:23:29.500" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_9&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Falcon_9&lt;/a&gt;&#xA;Falcon 9 payload&#xA;  to LEO: 22.8tons&#xA;  to GTO: 8.3tons&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_Heavy&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Falcon_Heavy&lt;/a&gt;&#xA;Falcon Heavy payload&#xA;   to LEO: 63.8tons&#xA;   to GTO: 26.7tons&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FH performance is just 180% more for LEO, but 220% more for GTO&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, no, its NOT 4x more to LEO, that was using cross feed, which has been abandoned because its too risky and complex and is not necessary for existing missions SpaceX sees enough market opportunities for&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With cross feed, FH might throw 70+ tons to LEO, perhaps over 4x as much&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is that ? The two additional side boosters don't add performance linearly. The 3 boosters function as FH first stage and then when the side boosters are jettisoned, the center booster alone functions as a second stage. This improves performance non linearly.&#xA;Without cross feed, the side boosters must stay attached longer, reducing the burn time of the 2nd stage configuration. With cross feed, the side boosters feed the center booster fuel until separation, providing a fully fuelled center booster upon separation, substantially increasing the 2nd stage burn time&#xA;In this thought process the upper stage is now a 3rd stage&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The further reason cross feed was abandoned is with it the center booster finishes burning going way too fast and way too high, that in order to recover it, an ASDS platform might have be be positioned 2x to 3x as far from the launch pad as with F9. Since the revised Block V non cross feed FH can now handle all missions SpaceX has been hired to perform, its logical to make it easier to recover all 3 boosters which is far more important than having performance no customer is willing to pay for.&lt;/p&gt;&#xA;" OwnerUserId="18764" LastActivityDate="2017-12-10T19:23:29.500" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23972" PostTypeId="2" ParentId="23956" CreationDate="2017-12-10T19:42:25.290" Score="3" Body="&lt;p&gt;The cost of putting a man on the moon involves more than just the cost of launch -you also have to consider the cost to develop the rocket to get there, the capsule to carry the crew, and the landing module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA attempted to do this with the &lt;a href=&quot;https://www.nasa.gov/pdf/163092main_constellation_program_overview.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Constellation&lt;/a&gt; program, the direct successor to the Apollo lunar program. The Constellation program essentially tried to develop a lunar (and eventually interplanetary) launch system from scratch, in the same vein as the original Saturn V, LEM, and Apollo capsule system. However, significant technical problems and immature, changing requirements led the projected cost to soar above &lt;a href=&quot;http://www.gao.gov/new.items/d09844.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;$97 billion dollars&lt;/a&gt;. Constellation was later cancelled by the Obama administration, citing lack of progress, continued uncertainty, and the enormous price tag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Constellation was replaced by the &lt;a href=&quot;https://www.cbsnews.com/news/nasa-commits-to-heavy-lift-mega-rocket/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Space Launch System (SLS)&lt;/a&gt;, which is currently projected to cost $18-35 billion to develop. SLS is more geared toward interplanetary missions, but it could conceivably carry a crew and landed to lunar orbit, though a modern lunar lander has not yet been developed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The SpaceX Falcon Heavy is the next-closest contender for a modern equivalent to the Apollo program. Development costs for the Falcon Heavy are unknown since SpaceX is a private company, but what we do know is that the rocket is expected to be able to carry &lt;a href=&quot;http://www.businessinsider.com/spacex-falcon-heavy-vs-apollo-saturn-v-rocket-2016-7&quot; rel=&quot;nofollow noreferrer&quot;&gt;roughly 16 metric tons to Trans-Lunar Injection (TLI)&lt;/a&gt;. This is roughly one-third of the payload capacity of a Saturn V rocket, and one-quarter the expected capacity of the SLS. The crew dragon is expected to have a dry mass of around 5 metric tons, but the original lunar module had a dry mass of over 15 metric tons, so it is likely beyond the capability of the Falcon Heavy to carry a crew to the lunar surface -at least in the near term.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, assuming the SLS program isn't cancelled and the current price estimates are accurate, it would be reasonable to assume that it would cost around $35 billion plus the cost to develop a new lunar lander.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Apollo program spent around $2.2 billion developing the original lunar lander, or about &lt;a href=&quot;https://history.nasa.gov/SP-4029/Apollo_18-16_Apollo_Program_Budget_Appropriations.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;11% of the total budget for the program&lt;/a&gt;. If we assume that ratio is comparable to what it would take for a modern lander to be developed, then you're adding around 3-5 billion dollars to the overall price tag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cost for a single SLS launch is currently projected at around &lt;a href=&quot;http://www.nbcnews.com/id/49019843/ns/technology_and_science-space/#.UFIyOxii5i4&quot; rel=&quot;nofollow noreferrer&quot;&gt;$500 million&lt;/a&gt;. By contrast, a Saturn V launch cost &lt;a href=&quot;https://history.nasa.gov/SP-4221/ch6.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;somewhere around a billion (2017) dollars&lt;/a&gt; to launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, in the neighborhood of $40.5 billion.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I'm doing my math right, the Apollo program cost roughly &lt;a href=&quot;http://www.in2013dollars.com/1968-dollars-in-2017?amount=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;$85 billion in 2017 dollars&lt;/a&gt; to get to the first landing on the moon. So the cost to land men on the moon using modern technology is, theoretically, &lt;strong&gt;about half&lt;/strong&gt; what it was back at the dawn of space travel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are of course additional considerations for the fact that Apollo was doing something that had never been done before, but we also have a greater degree of bureaucracy and regulation in present times (e.g. the safety requirements on manned vehicles are substantially higher now than in the 1960s), so by my reckoning any advantage we might have from our past experience with Apollo would be negated by the increased red tape.&lt;/p&gt;&#xA;" OwnerUserId="21989" LastEditorUserId="21989" LastEditDate="2017-12-11T00:10:20.793" LastActivityDate="2017-12-11T00:10:20.793" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23973" PostTypeId="2" ParentId="23970" CreationDate="2017-12-10T20:19:57.300" Score="2" Body="&lt;p&gt;The closest I know of is &lt;a href=&quot;https://en.wikipedia.org/wiki/Trans-lunar_injection&quot; rel=&quot;nofollow noreferrer&quot;&gt;Trans-Lunar Injection&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;A trans-lunar injection (TLI) is a propulsive maneuver used to set a spacecraft on a trajectory that will cause it to arrive at the Moon.&lt;/p&gt;&#xA;&lt;p&gt;A spacecraft performs TLI to begin a lunar transfer from a low circular parking orbit around Earth. The large TLI burn, usually performed by a chemical rocket engine, increases the spacecraft's velocity, changing its orbit from a circular low Earth orbit to a highly eccentric orbit. As the spacecraft begins coasting on the lunar transfer arc, its trajectory approximates an elliptical orbit about the Earth with an apogee near to the radius of the Moon's orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;I'm not aware of any specific term to involve the entire combination of TLI and the capture burn (which is the maneuver performed near the moon, in order to change the flyby trajectory into a lunar orbit).&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-10T20:19:57.300" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23974" PostTypeId="2" ParentId="23970" CreationDate="2017-12-11T02:20:49.533" Score="6" Body="&lt;p&gt;You're looking for injection transfers, which is a type of orbital transfer whereby a spacecraft (or body) is moved from one sphere of influence to another. Specifically, in the Earth-Moon system, this is called a &lt;a href=&quot;https://en.wikipedia.org/wiki/Trans-lunar_injection&quot; rel=&quot;nofollow noreferrer&quot;&gt;Trans-Lunar Injection&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Injection transfers are not unique to Planet-Moon systems though — for example a Trans-Mars Injection is essentially a Hohmann Transfer between two planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that this is not the only type of Earth-Moon transfer however: trajectory designs can and have exploited weak boundary effects between the Earth and the Moon called a &lt;a href=&quot;http://ccar.colorado.edu/geryon/papers/Misc/Parker_Thesis.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Low Energy Ballistic Lunar Transfer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It has been found that low-energy BLTs may be produced by intersecting the stable manifold of an unstable Earth-Moon three-body orbit with the Earth. A spacecraft following such a trajectory is only required to perform a single maneuver, namely, the Trans-Lunar Injection maneuver, in order to complete the transfer. After the Trans-Lunar Injection maneuver, the spacecraft follows an entirely ballistic trajectory that asymptotically approaches and arrives at the target lunar three-body orbit. Because these orbit transfers require no orbit insertion maneuver at the three-body orbit, the transfers may be used to send spacecraft 25 – 40% more massive than spacecraft sent to the same orbits via conventional, direct transfers. From the targeted three-body orbits, the spacecraft may transfer to nearly any region within the Earth-Moon system, including any location on the surface of the Moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This effect was first exploited by the Japanese Hiten probe in 1991:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The trajectory Belbruno and Miller developed for Hiten used Weak Stability Boundary Theory and required only a small perturbation to the elliptical swing-by orbit, sufficiently small to be achievable by the spacecraft's thrusters. This course would result in the probe being captured into temporary lunar orbit using zero delta-v (called a ballistic transfer), but required five months instead of the usual three days for a Hohmann transfer. This was the first time a satellite had used low energy transfer to transfer to a moon orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;- &lt;a href=&quot;https://en.wikipedia.org/wiki/Hiten#cite_note-Capture-7&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Hiten&lt;/em&gt;, Wikipedia&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer your question narrowly: &lt;strong&gt;no, there is no unique name for these specific types of transfers, although a number of transfer types form a subset of what you're describing.&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6816" LastEditDate="2017-12-11T23:24:42.523" LastActivityDate="2017-12-11T23:24:42.523" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23976" PostTypeId="1" AcceptedAnswerId="24099" CreationDate="2017-12-11T12:47:11.337" Score="1" ViewCount="71" Body="&lt;p&gt;In an application I have to analyse I found this trajectory description for the space probe cassini, unfortunately header and comments have been removed:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;2004 6 01 00.0         16669287.46        -13098482.32        -10225933.05          1313468.24&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The file format is .trj / .traj. Supposedly, there was a tool called SKYMAP which created these?&#xA;I would like to know which tool created this data set. From the source code I gather that the table is in the format &lt;/p&gt;&#xA;&#xA;&lt;p&gt;year month day hour ignore x y z&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The column ignore is ..ignored. I tried to recreate the output with JPL's horizons system but it does not quite fit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit:&#xA;When we develop the application further I want it to be ready for future space missions. I might replace the trajectory description completely. I know asking for opinions is frowned upon, but what would you choose: spice toolkit or horizons toolkit? The spice toolkit seems more extensive and well-kept.&lt;/p&gt;&#xA;" OwnerUserId="22000" LastEditorUserId="22000" LastEditDate="2017-12-12T07:42:33.083" LastActivityDate="2017-12-19T12:29:58.567" Title="Need help recreating Cassini trajectory description from SKYMAP trj file" Tags="|probe|trajectory|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23977" PostTypeId="2" ParentId="23791" CreationDate="2017-12-11T13:16:30.697" Score="16" Body="&lt;p&gt;Yes, the Flyby Anomaly is still an unsolved problem. Studies have found several candidates for the cause of the anomaly. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;https://arxiv.org/pdf/1711.08893.pdf&quot; rel=&quot;noreferrer&quot;&gt;recent study&lt;/a&gt; examined Juno's behavior on its Jupiter orbits, and found the anomaly there as well: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;“Our conclusion is that an anomalous acceleration is also acting upon the Juno spacecraft in the vicinity of the perijove (in this case, the asymptotic velocity is not a useful concept because the trajectory is closed). This acceleration is almost one hundred times larger than the typical anomalous accelerations responsible for the anomaly in the case of the Earth flybys. This was already expected in connection with Anderson et al.’s initial intuition that the effect increases with the angular rotational velocity of the planet (a period of 9.8 hours for Jupiter vs the 24 hours of the Earth), the radius of the planet and probably its mass.” &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Some possible origins of the anomaly have been discarded: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It is important to emphasize that this anomaly is also observed in the ranging data and cannot be attributed to a conventional or unconventional issue related entirely to the Doppler tracking. A primary evaluation of the possible conventional physical effects with could be contributing to the anomaly was carried out by Lämmerzahl et al. [31]. Ocean tides and a coupling of the spacecraft to the tesseral harmonic terms in the geopotential model have also recently been studied &lt;a href=&quot;https://trs.jpl.nasa.gov/handle/2014/45519&quot; rel=&quot;noreferrer&quot;&gt;2&lt;/a&gt;. Atmospheric friction can also be dismissed except for flybys at altitudes of 300 km or lower [4]. The same can be said of the corrections corresponding to General Relativity [28, 26], thermal effects [41] or other [14].&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;But the study found no conclusive cause for the anomaly:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Summarizing, we can say that in this paper: (i) We have found evidence&#xA;  that an anomaly could be operating also during the Juno flybys of Jupiter (ii) We have developed a theoretical model to compare with the orbital model&#xA;  fitted to telemetry data in order to disclose the form of the possible anomalous acceleration field acting upon the spacecraft. A significant radial component was found and this decays with the distance to the center of Jupiter as expected from an unknown physical interaction. (iii) The anomaly shows an asymmetry among the incoming and outgoing branches of the trajectory and this could be suggestive of a non-conservative interaction. The confirmation of these conclusions would require further independent analysis and we hope that our work will stimulate future research in this and other planetary flybys.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Another &lt;a href=&quot;https://trs.jpl.nasa.gov/handle/2014/45519&quot; rel=&quot;noreferrer&quot;&gt;paper studied Juno's Earth flyby&lt;/a&gt;, and found that inaccuracies of the geopotential model (Earth's gravity field) used to model the flyby could account for the flyby anomaly:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The only perturbation that we examined that was found to be capable of producing something detectable in real-time and comparable to the predicted flyby velocity anomaly was truncation in the Earth’s geopotential model. Previously, we had found a 10x10 degree/order field sufficient for supporting launch operations. The uncertainties in the trajectory during that phase were large enough as to not require a higher fidelity model. However, for the EGA flyby we found that the perturbation caused by including the additional effect of the higher degree and order terms was as large as 4.5 mm/s when using a 50x50 field. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://issfd.org/2015/files/downloads/papers/153_Jouannic.pdf&quot; rel=&quot;noreferrer&quot;&gt;Another paper&lt;/a&gt; concluded that solar radiation pressure is another candidate:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Solar radiation pressure presented more interesting results: small changes in the solar radiation pressure coefficient C&lt;sub&gt;R&lt;/sub&gt; could explain the anomaly. In more detail, uncertainties in the values of the reflectivity coefficients of the materials that cover the spacecraft could be held responsible for the anomalous velocity change observed. Actually, we proved that modifications of the specular and diffuse reflectivity coefficients by an amount ranging from 10&lt;sup&gt;−4&lt;/sup&gt; to 4*10&lt;sup&gt;−2&lt;/sup&gt; are sufficient to successfully eliminate the unexpected velocity shift at perigee.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-12-11T13:42:55.480" LastActivityDate="2017-12-11T13:42:55.480" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="23978" PostTypeId="1" AcceptedAnswerId="23997" CreationDate="2017-12-11T14:00:06.357" Score="6" ViewCount="992" Body="&lt;p&gt;Why does the Dream Chaser use a skid as the nose landing gear?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qjcvE.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qjcvE.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wouldn't this increase the risk of accidents or damage to the runway when landing on a paved runway?&lt;/p&gt;&#xA;" OwnerUserId="12685" LastActivityDate="2017-12-12T08:31:53.557" Title="Why use skids for landing gear on an orbiter?" Tags="|spacecraft|landing|landing-gear|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23979" PostTypeId="1" CreationDate="2017-12-11T14:34:33.930" Score="4" ViewCount="671" Body="&lt;p&gt;Some rockets use strap-on boosters to match the payload requirements, even sometimes launching with only one strap on. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this a plausible scenario for Falcon to be strapped together with only one side booster?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorDisplayName="user8269" LastEditDate="2017-12-27T22:57:09.163" LastActivityDate="2018-03-08T13:09:51.597" Title="Is a Falcon Semi-Heavy plausible?" Tags="|spacex|falcon-heavy|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23980" PostTypeId="1" AcceptedAnswerId="23981" CreationDate="2017-12-11T14:37:45.450" Score="2" ViewCount="317" Body="&lt;p&gt;The two side boosters will need some kind of aerodynamic protection, which could very much be recovered with the rocket in one piece. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will they be recovered and reused?&lt;/p&gt;&#xA;" OwnerUserId="6642" LastEditorUserId="12102" LastEditDate="2021-09-13T00:15:37.257" LastActivityDate="2021-09-13T00:15:37.257" Title="Will the Falcon Heavy make the first full fairing reuse ever?" Tags="|spacex|reuse|falcon-heavy|spaceflight-firsts|fairings|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23981" PostTypeId="2" ParentId="23980" CreationDate="2017-12-11T15:23:23.970" Score="5" Body="&lt;p&gt;The caps on the side boosters of the Falcon Heavy are attached to the cores and will not detach. Not sure that counts as a fairing in the traditional sense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The term fairing, is traditionally used to refer to the protections around the second stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the Go Searcher recovery ships has been seen with a steel structure on the deck that is suggested to be a storage rack for either the recovered fairings, or a underpinning for the 'bouncy castle' designed to catch them.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/mD1p9.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mD1p9.jpg&quot; alt=&quot;Go Searcher with possible recovery structure&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bouncy castle approach seems to have been more fully implemented on Mr Steven, a fast recovery ship SpaceX is trying to use to catch fairings.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FH2K0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FH2K0.jpg&quot; alt=&quot;Mr Steven underway - nets ho&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another view:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CRyvo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CRyvo.jpg&quot; alt=&quot;Showing some net&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They tried to catch the fairings from the Falcon Heavy demo flight, and missed.  They will get it next time I am sure. Here is the fairing floating along, minding its own business before being hijacked by Mr Steven.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j3WZW.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j3WZW.jpg&quot; alt=&quot;Peaceful fairing floating&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then they dragged it aboard, kicking and screaming I am sure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ITV43.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ITV43.jpg&quot; alt=&quot;Is it there by choice?  Hard to tell&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the other half clearly fought back and did not make it so cleanly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/G86ru.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/G86ru.jpg&quot; alt=&quot;That is what happens when you struggle&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-02-23T18:36:34.853" LastActivityDate="2018-02-23T18:36:34.853" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23982" PostTypeId="2" ParentId="23979" CreationDate="2017-12-11T16:30:05.977" Score="3" Body="&lt;p&gt;As @jgalak notes, the problem would be asymmetric thrust, but I believe the vectored thrust on the Falcon would be sufficient to keep things balanced. Since the rocket wouldn't be going straight up, there would be a net sideways aerodynamic force on the stack, but Atlas V seems able to deal with that in its asymmetric single-booster configuration. At first glance it seems plausible to me. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-12-11T16:30:05.977" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="23983" PostTypeId="2" ParentId="23979" CreationDate="2017-12-11T17:58:21.493" Score="6" Body="&lt;p&gt;With first stage reuse, there is a simpler configuration: Keep both strap-on boosters, but only fill them part way with fuel. Since each of the two boosters fires for about half as long as if there was one, they only accrue about half the wear. You could also fire only some of the engines on each booster as another way of limiting wear. This seems easier to manage than asymmetric thrust. Also of note is how small the boosters are on the Atlas V compared to a second complete first stage.&lt;/p&gt;&#xA;" OwnerUserId="21390" LastEditorUserId="6816" LastEditDate="2017-12-11T23:09:55.347" LastActivityDate="2017-12-11T23:09:55.347" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23984" PostTypeId="2" ParentId="23979" CreationDate="2017-12-11T20:47:58.573" Score="5" Body="&lt;p&gt;This is very unlikely to happen for a number of reasons. The biggest reason is that the Falcon Heavy in particular is designed to be reused. If you would only need 2 cores instead of 3, then just land the 2 cores back at your main base, which would save a lot of fuel. It is thought that it will be cheaper to fly a Falcon Heavy with 3 core reuse then it would be to fly a Falcon 9 with the same payload that requires the Falcon 9 to use up it's landing fuel reserve. I can't find performance on the expendable Falcon 9 configuration, but here is the performance differences between the various Falcon configurations for C3, per &lt;a href=&quot;https://elvperf.ksc.nasa.gov/Pages/Results.aspx&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Launch Vehicle Performance website&lt;/a&gt;. Note that there isn't a huge difference between a Falcon 9 expendable and a Falcon Heavy Recoverable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yx4Gb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yx4Gb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-03-08T13:09:51.597" LastActivityDate="2018-03-08T13:09:51.597" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="23985" PostTypeId="1" AcceptedAnswerId="23986" CreationDate="2017-12-11T21:35:53.330" Score="7" ViewCount="1440" Body="&lt;p&gt;I just watched a documentary about the Mercury program and was wondering what those &quot;side rockets&quot; (don't know how to call them properly) are for? Further stability or generating some extra thrust?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Talking about those four &quot;little&quot; engines above the main ones. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Mercury-Atlas_5#/media/File:Atlas_D_with_Mercury-Atlas_5_(Nov._29_1961).jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://upload.wikimedia.org/wikipedia/commons/b/bb/Atlas_D_with_Mercury-Atlas_5_%28Nov._29_1961%29.jpg&quot; alt=&quot;&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;By NASA &amp;mdash; &lt;a href=&quot;http://spaceflight.nasa.gov/gallery/images/mercury/mercury_ov/html/s62-08310.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://spaceflight.nasa.gov/gallery/images/mercury/mercury_ov/html/s62-08310.html&lt;/a&gt;, Public Domain, &lt;a href=&quot;https://commons.wikimedia.org/w/index.php?curid=1475653&quot; rel=&quot;nofollow noreferrer&quot;&gt;Link&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="22007" LastEditorUserId="34345" LastEditDate="2020-02-26T18:37:17.947" LastActivityDate="2020-02-26T18:37:17.947" Title="What are the thrusters present on the side of the Mercury-Atlas launch vehicle?" Tags="|rockets|nasa|history|project-mercury|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="23986" PostTypeId="2" ParentId="23985" CreationDate="2017-12-11T22:43:29.517" Score="10" Body="&lt;p&gt;These are &lt;strong&gt;vernier thrusters&lt;/strong&gt;! They're small (relative to the main engine) rocket engines used to finely control the heading and alignment and form part of the Mercury-Atlas' attitude &amp;amp; roll control system. These days, systems tend to use gimbaling of the main motor to maintain attitude control; but if you couldn't gimbal, introducing changes in the spacecraft's or rocket's attitude by pushing the vehicle with smaller rockets is preferable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Specifically, the vernier engines are &lt;a href=&quot;http://heroicrelics.org/info/lr-101/lr-101.html&quot; rel=&quot;noreferrer&quot;&gt;LR-101&lt;/a&gt;'s developed by Aerojet Rocketdyne and certified for use in 1958:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The LR-101 was a single-start, fixed-thrust engine with an expansion ration of 6:1. In its Block 2 version, ignition occurred by means of pyrophoric (hypergolic) fluid (a mixture of tri-ethyl aluminum and tri-ethyl boron) which ignites spontaneously in the presence of oxygen.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;img src=&quot;https://i.stack.imgur.com/TK4cz.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://heroicrelics.org&quot; rel=&quot;noreferrer&quot;&gt;http://heroicrelics.org&lt;/a&gt; has some additional specific information about the engine, but its variants usually produced between 528 and 1000 lbs of thrust each. Mercury-Atlas had 2 of these at 180 degree intervals around the booster.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2017-12-11T23:11:44.063" LastActivityDate="2017-12-11T23:11:44.063" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23987" PostTypeId="1" AcceptedAnswerId="23989" CreationDate="2017-12-11T23:43:32.587" Score="4" ViewCount="1062" Body="&lt;p&gt;Today, as per the tabloids, President Donald Trump signed the &quot;Space policy directive 1&quot;.   Where can I find the original/raw document?   (I get the gist from the tabloids, but wanted to read the raw document)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/press-release/new-space-policy-directive-calls-for-human-expansion-across-solar-system&quot; rel=&quot;nofollow noreferrer&quot;&gt;The president Monday signed at the White House Space Policy Directive 1&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="8653" LastActivityDate="2017-12-21T11:39:49.600" Title="Where can I find &quot;Space policy directive 1&quot;?" Tags="|the-moon|nasa|policy|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23988" PostTypeId="1" AcceptedAnswerId="24489" CreationDate="2017-12-12T00:30:27.300" Score="6" ViewCount="11978" Body="&lt;p&gt;An exercise that was left unsolved from last year's class gives me this equation :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;t-t_{p} = \sqrt{\frac{a^3}{\mu}}*(\arcsin(X) - e*X)&#xA;$$&#xA;where :&#xA;$$&#xA;X = \frac{\sqrt{1-e^2}*\sin(v)}{1+e*\cos(v)}.&#xA;$$&#xA;It uses the following notations :&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$t$ is current time on orbit&lt;/li&gt;&#xA;&lt;li&gt;$t_{p}$ is time at perigee&lt;/li&gt;&#xA;&lt;li&gt;$a$ is the semimajor axis&lt;/li&gt;&#xA;&lt;li&gt;$\mu$ is the standard gravitationnal parameter, with $\mu = GM$&lt;/li&gt;&#xA;&lt;li&gt;$e$ is the eccentricity of the orbit&lt;/li&gt;&#xA;&lt;li&gt;$v$ is the true anomaly &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;To finish the software I started writing, I would need to extract the true anomaly, $v$, knowing that all other parameters are given, but after a long time trying I still have no clue.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, I think it can be reduced to an equation in the form of $\alpha*\arcsin(X) + \beta*X + \gamma = 0$ but I couldn't find any reference to solve such an equation anywhere online, and WolframAlpha doesn't give anything useful.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I thought I could use a power series or a Taylor series to approximate a result, but it looks like it brings more trouble than solves anything...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If anyone has skills to help, I'd be grateful !&#xA;Thank you&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;EDIT : I fixed my program, if anyone is interested, &lt;a href=&quot;https://gist.github.com/MagixInTheAir/7c2a55f204b6ab9d4de4e386abbe82e1&quot; rel=&quot;noreferrer&quot;&gt;here is the implementation&lt;/a&gt;.&#xA;Thanks to everyone who helped ! &lt;/p&gt;&#xA;" OwnerUserId="10836" LastEditorUserId="10836" LastEditDate="2017-12-18T18:41:39.073" LastActivityDate="2018-01-16T13:19:48.337" Title="How to get true anomaly from time?" Tags="|orbital-mechanics|orbit|trajectory|orbital-elements|calculation|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23989" PostTypeId="2" ParentId="23987" CreationDate="2017-12-12T00:52:51.677" Score="5" Body="&lt;p&gt;The full text of &lt;code&gt;Space Policy Directive-1&lt;/code&gt; can be found on &lt;a href=&quot;https://www.whitehouse.gov/the-press-office/2017/12/11/presidential-memorandum-reinvigorating-americas-human-space-exploration&quot; rel=&quot;noreferrer&quot;&gt;whitehouse.gov&lt;/a&gt; (and soon, in the &lt;a href=&quot;https://www.federalregister.gov&quot; rel=&quot;noreferrer&quot;&gt;Federal Register&lt;/a&gt;).&lt;/p&gt;&#xA;&lt;p&gt;It reads:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SUBJECT:        Reinvigorating America's Human Space Exploration Program&lt;/p&gt;&#xA;&lt;p&gt;Section 1.  Amendment to Presidential Policy Directive-4.&lt;/p&gt;&#xA;&lt;p&gt;Presidential Policy Directive-4 of June 28, 2010 (National Space Policy), is amended as follows:&lt;/p&gt;&#xA;&lt;p&gt;The paragraph beginning &amp;quot;Set far-reaching exploration milestones&amp;quot; is deleted and replaced with the following:&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Lead an innovative and sustainable program of exploration with commercial and international partners to enable human expansion across the solar system and to bring back to Earth new knowledge and opportunities.  Beginning with missions beyond low-Earth orbit, the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations;&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;Sec. 2.  General Provisions.  (a)  Nothing in this memorandum shall be construed to impair or otherwise affect:&lt;/p&gt;&#xA;&lt;p&gt;(i)   the authority granted by law to an executive department or agency, or the head thereof; or&lt;/p&gt;&#xA;&lt;p&gt;(ii)  the functions of the Director of the Office of Management and Budget relating to budgetary, administrative, or legislative proposals.&lt;/p&gt;&#xA;&lt;p&gt;(b)  This memorandum shall be implemented consistent with applicable law and subject to the availability of appropriations.&lt;/p&gt;&#xA;&lt;p&gt;(c)  This memorandum is not intended to, and does not, create any right or benefit, substantive or procedural, enforceable at law or in equity by any party against the United States, its departments, agencies, or entities, its officers, employees, or agents, or any other person.&lt;/p&gt;&#xA;&lt;p&gt;(d)  This memorandum shall be published in the Federal Register.&lt;/p&gt;&#xA;&lt;p&gt;Donald J. Trump&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-12T00:52:51.677" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23990" PostTypeId="1" AcceptedAnswerId="23994" CreationDate="2017-12-12T02:29:44.540" Score="1" ViewCount="144" Body="&lt;p&gt;How far did the Mars Polar Lander travel in its journey to the red planet?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I couldn't find the answer on &lt;a href=&quot;https://en.wikipedia.org/wiki/Mars_Polar_Lander&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; or on the &lt;a href=&quot;https://mars.nasa.gov/msp98/lander/&quot; rel=&quot;nofollow noreferrer&quot;&gt;official NASA site&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd prefer some answer based on a good source, or rough arc-length calculation, rather than just multiplying its velocity times 11 months or something like that.&lt;/p&gt;&#xA;" OwnerUserId="22008" LastEditorUserId="6816" LastEditDate="2017-12-12T03:09:15.847" LastActivityDate="2017-12-12T05:29:36.963" Title="How far did the Mars Polar Lander travel?" Tags="|mars|spacecraft|travel|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23991" PostTypeId="2" ParentId="23988" CreationDate="2017-12-12T03:00:15.657" Score="5" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; @DavidHammen has just posted a much more thorough and insightful &lt;a href=&quot;https://space.stackexchange.com/a/24489/12102&quot;&gt;answer&lt;/a&gt;, which also points out some problems applying Newton's method to the current form.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm pretty sure there are has never been an analytical expression discovered to solve $\nu(t-t_p)$, but solving using &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_method&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's method&lt;/a&gt; applied to &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ \sqrt{\frac{a^3}{\mu}}*(\arcsin(X) - e*X) - (t-t_p) = 0 $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;should converge nicely to values of $X$ (or of course $\nu$ if you do the replacement) in a half-dozen iterations, at least for an elliptical orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't actually verified your equations though, just taking your word for it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You could also calculate an array of points solving for time, flip it, and interpolate with a spline, but the accuracy is not predictable/reliable.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-16T13:19:48.337" LastActivityDate="2018-01-16T13:19:48.337" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="23992" PostTypeId="1" AcceptedAnswerId="24018" CreationDate="2017-12-12T03:38:18.077" Score="13" ViewCount="6752" Body="&lt;p&gt;Why is a near rectilinear halo orbit a good place for a &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Gateway&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbital Platform-Gateway&lt;/a&gt; (formerly known as Deep Space Gateway)? Are there &lt;em&gt;specific logistical and orbital-mechanical advantages&lt;/em&gt; over other possible orbits in cis-lunar space; other Lagrange point associated orbits or even high or elliptical lunar orbits for example? &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Deep Space Gateway would be initially placed in a near rectilinear halo orbit (NRHO) around the Moon.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/X5O77OV9_ek?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-04-07T10:51:06.800" LastActivityDate="2023-03-07T14:53:40.830" Title="Why is a near rectilinear halo orbit proposed for LOP-G (formerly known as Deep Space Gateway?)" Tags="|orbital-mechanics|halo-orbit|cislunar|lunar-orbit|lunar-gateway|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="23993" PostTypeId="1" CreationDate="2017-12-12T03:59:33.560" Score="2" ViewCount="79" Body="&lt;p&gt;could communication with the voyagers eventually be lost due to the solar systems &quot;Bow Wave&quot;  Seems this Bow Wave looks like the one that astronaut's capsule's create upon reentry into earths atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="22009" LastActivityDate="2017-12-12T07:51:39.500" Title="solar systems &quot;Bow Wave&quot;" Tags="|solar-wind|shockwave|" AnswerCount="1" CommentCount="2" ClosedDate="2017-12-13T04:20:40.190" ContentLicense="CC BY-SA 3.0" />
  <row Id="23994" PostTypeId="2" ParentId="23990" CreationDate="2017-12-12T05:29:36.963" Score="4" Body="&lt;p&gt;For those not aware, Mars Polar Lander took what is known as a &quot;Type II&quot; transfer orbit to Mars, a lower-energy transfer that took over 11 months, whereby the spacecraft traveled more than 180° around the sun. This is illustrated in this image on the &lt;a href=&quot;https://mars.nasa.gov/msp98/lander/cruise.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Polar Lander — Cruise Phase&lt;/a&gt; webpage:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/izNlO.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/izNlO.gif&quot; alt=&quot;Mars Polar Lander Trajectoy Design&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Mars Polar Lander will spend 11 months in cruise, entering Mars' atmosphere December 3, 1999. The spacecraft's flight path is called a Type 2 trajectory because it will take the lander more than 180 degrees around the Sun, enabling it to target a landing zone near Mars' south pole. By comparison, Mars Pathfinder followed a Type 1 trajectory which took it less than 180 degrees around the Sun, reaching Mars in only seven months. During the first leg of its trip, Mars Polar Lander will fly slightly inward toward the Sun before spiralling out beyond Earth's orbit to Mars. Toward the end of cruise, it will fly slightly out past the orbit of Mars before returning inward to intersect the planet's orbit.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;As far as I can tell, this is the longest Earth-Mars transfer trajectory ever taken for a Mars-bound spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your answer, however, lies in the official &lt;a href=&quot;https://www.jpl.nasa.gov/news/press_kits/mplds2hq.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars Polar Lander/Deep Space 2 press kit&lt;/a&gt; on page six: &lt;strong&gt;470 million miles (757 million km)&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sadly, the last 40 metres of that journey were a bit problematic.   &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2017-12-12T05:29:36.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23995" PostTypeId="1" CreationDate="2017-12-12T07:20:40.910" Score="9" ViewCount="638" Body="&lt;p&gt;I am making an exoplanet for a game and was wondering whether a planet without an atmosphere (or at least with a very weak atmosphere) could support caves that do have an atmosphere, or whether the air would escape through all the cracks immediately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The caves are connected to the surface with some small holes. They also have liquid water and black smokers.&lt;/p&gt;&#xA;" OwnerUserId="22010" LastEditorUserId="58" LastEditDate="2020-09-14T15:58:39.567" LastActivityDate="2020-09-14T15:58:39.567" Title="Can there be an atmosphere in a cave on an atmosphere-less planet?" Tags="|atmosphere|planetary-science|exoplanet|caves|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="23996" PostTypeId="2" ParentId="23993" CreationDate="2017-12-12T07:51:39.500" Score="3" Body="&lt;p&gt;Interesting idea. It seems unlikely to me though. So without doing any research:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;if the bow wave was a barrier to radio, we wouldn't be able to to radio astronomy.&lt;/li&gt;&#xA;&lt;li&gt;the bow wave has a much lower density than our atmosphere, so plasma effects are going to be much weaker. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;More later.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-12-12T07:51:39.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="23997" PostTypeId="2" ParentId="23978" CreationDate="2017-12-12T08:31:53.557" Score="6" Body="&lt;p&gt;Sierra Nevada Corporation's answer to NASASpaceFlight on this topic in 2012 was the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Speaking of the rationale for opting to use a skid strip, SNC noted it is a simple, light, safe option. They also added that there had been some issues with the analysis of the performance of tires in the space environment and this eliminates one of the tires – with the other two tires easier to control than the nose wheel if there’s a problem with one of them.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;- &lt;a href=&quot;https://www.nasaspaceflight.com/2012/06/snc-dream-chasers-enterprise-test-approach/&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;SNC outline Dream Chaser’s Enterprise-style landing test approach&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Antzi mentioned in the comments, the overall cost-benefit analysis seems to favour a skid over an additional wheel. I'd love to hear more from the SNC engineers on this, but they seem to be tightlipped on the design choice. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2017-12-12T08:31:53.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24000" PostTypeId="1" CreationDate="2017-12-12T10:44:47.963" Score="7" ViewCount="609" Body="&lt;p&gt;Many orbital launch vehicles use a combination of a chilled oxidiser (usually LOX when dealing with liquid-fuelled rockets) and a fuel (commonly RP-1, but with many designs now opting for Methane). Both LOX &amp;amp; Methane require super-cooled temperatures to maintain liquidity: the boiling point for LOX is −182.96°C, while Methane's is -161.5°C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX has attempted to take this lower (with varying success...) by super-chilling their LOX and RP-1 (to the point where the latter starts acting most similarly to a watery gel) &lt;a href=&quot;https://twitter.com/elonmusk/status/677666779494248449&quot; rel=&quot;noreferrer&quot;&gt;down to -206.67°C &amp;amp; -6.67°C respectively&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Falcon 9 however, is painted in a proprietary blend of white on top of its Aluminum-Lithium alloy. The understanding, at least within casual spaceflight fan circles, is that this reflective coating is enough to prevent excessive heat buildup and eliminate or reduce propellant boiloff.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CRS-13 mission looks set to change this slightly — with the Falcon 9 first stage continuing to sport its soot from its previous flight:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4u12b.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4u12b.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;The CRS-13 vehicle at SLC-40 in Florida, with its soot still present from the last landing.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does this added layer of darker material influence the thermal properties of the vehicle with respect to its propellant?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, RocketLab's Electron launch vehicle completely bucks the trend of white rocket designs; instead opting to stick with the sleek &amp;amp; shiny black of its Carbon-composite fibre body:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L3yYm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L3yYm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;sup&gt;The second Electron launch vehicle on the launchpad at Mahia Peninsula, New Zealand; sporting its black carbon-composite design.&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Propellant thermal management should surely be in full play with this vehicle, especially considering it has a much higher surface to volume ratio, when compared to Falcon 9.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;So, just how much does the exterior color of the vehicle affect thermal management protocols, the propellant, and rates of boiloff? Is this a genuine concern or something that has endlessly made the rounds of spaceflight-geek circles with no substantial evidence?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ideally, I'm looking for an answer which has citations to prior work, or studies conducted recently on modern launch vehicles.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2019-07-17T00:52:25.270" Title="To what extent, if any, does the exterior paint color of a launch vehicle affect its propellant &amp; vehicle thermal management?" Tags="|spacex|materials|launch-vehicle|composite-materials|rocketlab|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24001" PostTypeId="5" CreationDate="2017-12-12T10:56:07.907" Score="0" Body="" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2017-12-12T10:56:07.907" LastActivityDate="2017-12-12T10:56:07.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24002" PostTypeId="4" CreationDate="2017-12-12T10:56:07.907" Score="0" Body="RocketLab, is a private American &amp; New Zealand space exploration company that design &amp; build the Electron launch vehicle. " OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2017-12-12T10:56:07.907" LastActivityDate="2017-12-12T10:56:07.907" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24003" PostTypeId="2" ParentId="23995" CreationDate="2017-12-12T14:43:08.480" Score="6" Body="&lt;p&gt;Yes, in fact it can happen. It happens on Earth to some extent. The term is a &quot;Gas Pocket&quot;, and is most commonly associated with oil and natural gas deposits. There are other times of gasses that are sometimes stored as well under the Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there is a connection with the outside world in the form of a small hole, then the likelihood of there still being any gas in it is much lower. The only way this would work is if the gas was very heavy. Again, this happens to some extend on Earth, most notably in Radon being concentrated in some basements. This likely wouldn't lead to a thick atmosphere by any means, but could produce something.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only way that it could sustain long term an atmosphere is if it was completely sealed up, or if there was a source generating the atmosphere on a regular basis. There are a few things that could cause this to happen. It could come from outgassing due to heat, from some kind of biological process, or possibly from some subterranean vents, which is really just an extension of the outgassing problem. It could also be caused by some kind of chemical reaction. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-12-12T19:29:02.733" LastActivityDate="2017-12-12T19:29:02.733" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24005" PostTypeId="1" AcceptedAnswerId="24006" CreationDate="2017-12-12T19:56:55.237" Score="2" ViewCount="178" Body="&lt;p&gt;If I was in a circular 10km orbit (theoretical) around Earth, what will occur earlier? Will I burn up in atmosphere or be stopped by atmospheric drag?&lt;/p&gt;&#xA;" OwnerUserId="22025" LastEditorUserId="6537" LastEditDate="2017-12-12T22:28:04.810" LastActivityDate="2017-12-13T17:33:17.640" Title="Atmospheric drag vs. atmospheric heating. Which occurs first?" Tags="|orbit|reentry|atmospheric-drag|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24006" PostTypeId="2" ParentId="24005" CreationDate="2017-12-12T20:03:29.803" Score="3" Body="&lt;p&gt;If you're really asking about moving at orbital speed while at 10km altitude above the Earth's surface, you'll do both at the same time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You will plow into the air in front of you, compressing it and slowing you down; the compression of the air will create extreme heat and you'll burst into flames. Since you have a great deal of speed to shed, you'll be pretty well incinerated before you come to a stop. &lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-12-12T20:03:29.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24007" PostTypeId="1" CreationDate="2017-12-12T20:05:05.277" Score="2" ViewCount="304" Body="&lt;p&gt;I've heard an argument that moon landing was at best improbable as it would have required too much more fuel than simply achieving LEO. It seems to me that it would be relatively little more. I'm just looking for a percentage of fuel burned after clearing the atmosphere by a craft headed for the Lagrange point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To further simplify the problem, we can assume that the launch is straight up, the Moon is stationary with respect to the launch point and the final velocity upon reaching the Lagrange point is 0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For practical purposes, I think fuel weight would have to be included in the ratio, but that stage separation can be neglected as it tremendously complicates things and the entire launch mass, minus all fuel, reaches the Lagrange point.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus: To what degree does it matter whether the moon is new or full?&lt;/p&gt;&#xA;" OwnerUserId="22024" LastEditorUserId="22024" LastEditDate="2017-12-26T06:51:20.407" LastActivityDate="2017-12-26T06:51:20.407" Title="What ratio of fuel is needed for launching a mass up to low earth orbit compared to the Lagrange point?" Tags="|rockets|lunar-landing|" AnswerCount="0" CommentCount="17" ClosedDate="2017-12-14T06:49:00.527" ContentLicense="CC BY-SA 3.0" />
  <row Id="24010" PostTypeId="1" CreationDate="2017-12-12T23:01:50.633" Score="5" ViewCount="1407" Body="&lt;p&gt;For reference, Hubble's mirror is 2.4 meters wide, the upcoming James Webb's 6.5 meters, and the proposed ATLAST 8 or 16 meters.  Let's assume a mirror nearly ten times Hubble's size, 21 meters, is sent up and benefits from a coronagraph. Could it detect the planets of stars like Proxima Centauri, at 4.2 light years, and likewise Epsilon Eridani, at ~10.5 light years?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By detect, not so much a pretty picture but enough light and data to confirm orbits and overall characteristics. Only other assumption would be that this telescope observes UV-O-NIR spectra, say between 200 to 1000 nanometer wavelengths.&lt;/p&gt;&#xA;" OwnerUserId="22028" LastActivityDate="2018-01-02T22:13:06.173" Title="Could a 21 meter space telescope detect the nearest exoplanets?" Tags="|mission-design|technology|space-telescope|exoplanet|telescope|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24011" PostTypeId="2" ParentId="24010" CreationDate="2017-12-13T01:06:31.817" Score="2" Body="&lt;p&gt;The &lt;a href=&quot;http://www.astro.cornell.edu/academics/courses/astro201/diff_limit.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;resolution of a telescope&lt;/a&gt; in radians is given (to a very good first-order approximation) by $\theta_{res}=1.22\frac{\lambda}{D}$ where $\lambda$ is the wavelength and $D$ is the diameter of the telescope aperture. Using the numbers you give, a 21 meter telescope at 200 nanometers would have a resolution of 0.002 arcseconds. Now, there are many other factors which will limit our resolution, but you can consider this, the diffraction-limited resolution, to be the highest possible resolution you can achieve. Anything with a smaller angular size than this will be indistinguishable. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So our next step is to figure out what angular size an exoplanet will subtend on the sky. Let's go with a Jupiter-size planet, to increase our chances. This means a planetary diameter of $~1.4*10^8$ meters. This object's angular size (in radians) will be given by $\theta_{size}=\frac{diameter}{distance}$. We can actually set this expression equal to our equation for resolution and solve for distance. This will give us an approximation for how far away a planet can be before we won't be able to resolve it. Solving for $distance$ yields:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;distance = \frac{diameter\cdot D}{1.22 \lambda}&#xA;$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, using our numbers, this gives us a distance of $1.2*10^{16}$ meters, or 1.3 light years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, ultimately the answer is no. A 21 meter primary aperture will not be enough to resolve an exoplanet. So let's see how big our mirror would have to be, by rearranging our equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$&#xA;D = \frac{1.22\cdot \lambda \cdot distance}{diameter}&#xA;$$&#xA;If we want to resolve a Jupiter size exoplanet at the distance of Epsilon Eridani, it would need to be 173 meters in diameter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, this is all assuming that we don't have to worry about other things, like glare from the star, which presents its own set of problems. But we can get around this by doing things like &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_optical_interferometry&quot; rel=&quot;nofollow noreferrer&quot;&gt;optical interferometry&lt;/a&gt;, which allows us to increase the effective size of our telescope without having to build bigger mirrors.&lt;/p&gt;&#xA;" OwnerUserId="13710" LastActivityDate="2017-12-13T01:06:31.817" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24012" PostTypeId="2" ParentId="24000" CreationDate="2017-12-13T04:54:48.313" Score="2" Body="&lt;p&gt;Wernher von Braun always painted their rockets in large black &amp;amp; white blocks or checkers, to enable them to see if the rocket rolled during launch. This continued on to the Saturn series. There were some instances where the tanks under the black gained too much pressure. I don't know if there were any painting scheme changes as a result.&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2017-12-13T04:54:48.313" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24014" PostTypeId="2" ParentId="24010" CreationDate="2017-12-13T07:22:50.207" Score="2" Body="&lt;p&gt;Yes. The ESO's &lt;a href=&quot;https://en.wikipedia.org/wiki/European_Southern_Observatory#Search_for_extrasolar_planets&quot; rel=&quot;nofollow noreferrer&quot;&gt;VLT&lt;/a&gt; used the &lt;a href=&quot;https://en.wikipedia.org/wiki/Doppler_spectroscopy&quot; rel=&quot;nofollow noreferrer&quot;&gt;wobble method&lt;/a&gt; to detect &lt;a href=&quot;https://en.wikipedia.org/wiki/Proxima_Centauri_b&quot; rel=&quot;nofollow noreferrer&quot;&gt;Proxima Centauri b&lt;/a&gt;, a planet with a radius estimated at 0.8–1.5 R⊕ and a semi-major axis estimated at 0.0485 (+0.0041,−0.0051) AU, at a distance of 4.224 ly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Hubble Telescope &lt;a href=&quot;https://en.wikipedia.org/wiki/Proxima_Centauri_b#Gallery&quot; rel=&quot;nofollow noreferrer&quot;&gt;can see&lt;/a&gt; the Proxima Centauri system's Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ground based VLT consists of four individual telescopes, each with a primary mirror 8.2 m across, they can achieve an angular resolution of about 0.001 arc-second. In single telescope mode of operation the angular resolution is about 0.05 arc-second.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Best case ground based conditions give a &lt;a href=&quot;https://en.wikipedia.org/wiki/Astronomical_seeing&quot; rel=&quot;nofollow noreferrer&quot;&gt;seeing disk diameter of ~0.4 arcseconds&lt;/a&gt; and are found at high-altitude observatories on small islands such as Mauna Kea or La Palma.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the best high-altitude mountaintop observatories, the wind brings in stable air which has not previously been in contact with the ground, sometimes providing seeing as good as 0.4&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Under bad conditions a ground based telescope over 10 meters with poor seeing can limit the resolution to be about the same as given by a space-based 10–20 cm telescope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ground based telescopes must look through the atmosphere, which is opaque in many infrared bands (see figure of atmospheric transmission). Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, also exist in the Earth's atmosphere, vastly complicating analysis. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Existing space telescopes such as &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Overview&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hubble cannot study these bands&lt;/a&gt; since their mirrors are not cool enough (the Hubble mirror is maintained at about 15 degrees C) and hence the telescope itself radiates strongly in the IR bands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Optics&quot; rel=&quot;nofollow noreferrer&quot;&gt;JWST telescope&lt;/a&gt; has an expected mass about half of Hubble Space Telescope's, but its primary mirror (a 6.5 meter diameter gold-coated beryllium reflector) will have a collecting area about five times as large (25 m^2 or 270 sq ft vs. 4.5 m^2 or 48 sq ft). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The JWST is oriented toward near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;JWST's primary mirror is a 6.5-meter-diameter gold-coated beryllium reflector with a collecting area of 25 m^2.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;From the &lt;a href=&quot;https://jwst.nasa.gov/faq.html#wavelengths&quot; rel=&quot;nofollow noreferrer&quot;&gt;JWST FAQ&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;At which wavelengths will Webb observe?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Webb will work from 0.6 to 28 micrometers, ranging from visible gold-colored light through the invisible mid-infrared. The short wavelength end is set by the gold coating on the primary mirror. The long wavelength cut-off is set by the sensitivity of the detectors in the Mid-Infrared Instrument.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How faint can Webb see?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Webb is designed to discover and study the first stars and galaxies that formed in the early Universe. To see these faint objects, it must be able to detect things that are ten billion times as faint as the faintest stars visible without a telescope. This is 10 to 100 times fainter than Hubble can see.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;What are the main science goals of Webb?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Webb has four mission science goals:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Search for the first galaxies or luminous objects that formed after the Big Bang.&lt;/li&gt;&#xA;&lt;li&gt;Determine how galaxies evolved from their formation until the present.&lt;/li&gt;&#xA;&lt;li&gt;Observe the formation of stars from the first stages to the formation of planetary systems.&lt;/li&gt;&#xA;&lt;li&gt;Measure the physical and chemical properties of planetary systems and investigate the potential for life in those systems. &lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How far will Webb look?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One of the main goals of Webb is to detect some of the very first star formation in the Universe. This is thought to happen somewhere between redshift 15 and 30 (redshift explained in question 45). At those redshifts, the Universe was only one or two percent of its current age. The Universe is now 13.7 billion years old, and these redshifts correspond to 100 to 250 million years after the Big Bang. The light from the first galaxies has traveled for about 13.5 billion years, over a distance of 13.5 billion light-years.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Will Webb see planets around other stars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Webb will be able to detect the presence of planetary systems around nearby stars from their infrared light (heat). It will be able to see directly the reflected light of large planets - the size of Jupiter - orbiting around nearby stars. It will also be possible to see very young planets in formation, while they are still hot. Webb will have coronagraphic capability, which blocks out the light of the parent star of the planets. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is needed, as the parent star will be millions of times brighter than the planets orbiting it. Webb will not have the resolution to see any details on the planets; it will only be able to detect a faint light speckle next to the bright parent star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Webb will also study planets that transit across their parent star. When the planet goes between the star and Webb, the total brightness will drop slightly. The amount that the brightness drops tells us the size of the planet. Webb can even see starlight that passes through the planet's atmosphere, measure its constituent gasses and determine whether the planet has liquid water on its surface. When the planet passes behind the star, the total brightness drops, and we can again determine more of the planet's characteristics.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Super short version: They're launching a slightly larger telescope than you have asked for that will reach (&quot;detect&quot;, not provide close-up photos) virtually to the known edge of the universe.&lt;/p&gt;&#xA;" OwnerUserId="20766" LastActivityDate="2017-12-13T07:22:50.207" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24015" PostTypeId="2" ParentId="24005" CreationDate="2017-12-13T07:57:11.597" Score="2" Body="&lt;p&gt;Much depends on your heatshield - if it is strong enough, you might survive until you have slowed down enough to that further heating is no longer a question. If it is not - you will burn... Besides - you will never be stopped completely by atmospheric drag as it is a function of your speed - the slower you get, the less it decelarates you - to the point where gravity and drag are equal - called terminal velocity.&lt;/p&gt;&#xA;" OwnerUserId="17398" LastActivityDate="2017-12-13T07:57:11.597" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24016" PostTypeId="1" AcceptedAnswerId="24022" CreationDate="2017-12-13T08:07:57.523" Score="5" ViewCount="214" Body="&lt;p&gt;How is rocket energy (or fuel consumption) is distributed between&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;gaining orbital velocity&lt;/li&gt;&#xA;&lt;li&gt;gaining orbital height&lt;/li&gt;&#xA;&lt;li&gt;fight aerodynamic drag &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="11046" LastEditorUserId="6537" LastEditDate="2018-01-05T06:54:46.203" LastActivityDate="2018-01-05T06:54:46.203" Title="What is the distribution of ∆V amongst different stages of rocket flight?" Tags="|power|energy|rocket-equation|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24018" PostTypeId="2" ParentId="23992" CreationDate="2017-12-13T09:57:24.977" Score="11" Body="&lt;p&gt;In fact, they are the preferred option among other staging orbits by analyzing multiple factors:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uFqqi.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uFqqi.png&quot; alt=&quot;Candidate orbits&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;EARTH ACCESS:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;A study considering NASA SLS and Orion performances was carried out. Since SLS places Orion in a trans-lunar trajectory, Orion vehicle has to carried out orbital maneuvers to reach a orbit near the Moon, so the limiting factor will Orion's propellant load limitations. These makes difficult to access low-lunar orbits since an estimated budget of &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt;=1250 m/s has to be enough for entering and leaving the orbit.&lt;/p&gt;&#xA;&lt;p&gt;Analyzing the previous constraint, although for some epochs moon Keplerian orbits were affordable, they were discarded because they were problematic for continuous access. This reduces the candidates to orbits within the Restricted 3-Body Model: nominal L1/L2 halos (637-811 m/s), near-rectilinear halo orbit (NRO) (751-840 m/s) and distant retrograde orbit (DRO) (841-957 m/s).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9yrqU.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9yrqU.png&quot; alt=&quot;NRO transfer cost&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;LUNAR SURFACE ACCESS&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;This metric only measured transfer of the lander located at the staging orbit to a low lunar orbit (LLO) (the cost of descending and ascending from Moon surface to the LLO is assumed constant). Also, transfer time should be reasonable for the crew.&lt;/p&gt;&#xA;&lt;p&gt;Obviously with this consideration, the preferred staging orbits would be Keplerian orbits near the Moon, but they were discarded by Earth-access. It is not difficult to see that between the 3-Body Model orbits candidates the NRO provides the best metric by terms of &lt;span class=&quot;math-container&quot;&gt;$\Delta V$&lt;/span&gt; and time, if a polar LLO is chosen (note that polar orbits guarantees global surface access waiting the required amount of time).&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;ADDITIONAL CONSIDERATIONS&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Considering station-keeping, for all the analyzed orbits, \Delta V budget per year was affordable (&amp;lt;10 m/s), so this was not a determinant factor.&lt;/p&gt;&#xA;&lt;p&gt;When taking into account communications to Earth, 3-body problem orbits got another point for that since a direct line-of-sight with Earth is all time available. Communications with the Moon where most favourable (at percentage) with NRO and L2 orbits, although they favored one pole or one side.&lt;/p&gt;&#xA;&lt;p&gt;Another preliminary analysis was done with thermal environment which also favored 3-body problem orbits, but it was not conclusive at all.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;CONCLUSION&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Staging orbit access from Earth with the Orion module discarded all the Moon Keplerian orbits. Among the considered 3-body problem orbits, the option that got most points was the NRO orbit.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/hsruM.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hsruM.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;My answer is entirely based on this nice paper about the subject: &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019648.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019648.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="21262" LastEditorUserId="6944" LastEditDate="2023-03-07T14:53:40.830" LastActivityDate="2023-03-07T14:53:40.830" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="24019" PostTypeId="1" AcceptedAnswerId="24025" CreationDate="2017-12-13T10:41:14.787" Score="15" ViewCount="18365" Body="&lt;p&gt;A proposed option for the lunar orbit of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Gateway&quot; rel=&quot;noreferrer&quot;&gt;Lunar Orbital Platform-Gateway&lt;/a&gt;  (formerly known as Deep Space Gateway) is a near rectilinear halo orbit, or NRHO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/24018/12102&quot;&gt;excellent answer&lt;/a&gt; discusses the various tradeoffs between different lunar orbit options and explains why the NRHO appears to be such a good candidate.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But &lt;em&gt;what is&lt;/em&gt; a near rectilinear halo orbit to begin with? Is it different than a &quot;normal&quot; halo orbit or just a subclass? Is there such a thing as a &quot;true&quot; rectilinear halo orbit as well? What makes it so special?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some visual clues in the video, especially &lt;a href=&quot;https://i.stack.imgur.com/mZGHE.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://i.stack.imgur.com/pHQpO.jpg&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; but the captions are minimal. These might be good places to start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For convenience I've managed to find some links to some of the papers listed at the end of the video.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;R. Whitley and R. Martinez, 2015, &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019648.pdf&quot; rel=&quot;noreferrer&quot;&gt;Options for Staging Orbits in Cis-Lunar Space&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;D. Davis, S. Bhatt, K. Howell, J-W. Jang, R. Whitley, F. Clark, D. Guzzetti, E. Zimovan, G. Barton, 2017, &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170001347.pdf&quot; rel=&quot;noreferrer&quot;&gt;Orbit Maintenance and Navigation of Human Spacecraft at Cislunar Near Rectilinear Halo Orbits&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;J. Williams, D. E. Lee, R. J. Whitey, K. A. Bokelmann, D. C. Davis, D. F. Berry, 2017, &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170001352.pdf&quot; rel=&quot;noreferrer&quot;&gt;Targeting Cislunar Near Rectilinear Halo Orbits for Human Space Exploration&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/X5O77OV9_ek?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt; &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-04-07T10:52:20.317" LastActivityDate="2018-04-07T10:52:20.317" Title="What is a near rectilinear halo orbit?" Tags="|orbital-mechanics|halo-orbit|cislunar|lunar-orbit|lunar-gateway|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24020" PostTypeId="2" ParentId="24010" CreationDate="2017-12-13T11:35:56.307" Score="8" Body="&lt;p&gt;As of 2017, &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets&quot; rel=&quot;nofollow noreferrer&quot;&gt;22 exoplanets have been imaged directly&lt;/a&gt;. The most distant of those is 1200 ly away. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This shows 4 of them orbiting HR 8799, which is 128 ly away: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NpK4s.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NpK4s.gif&quot; alt=&quot;HR 8799&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These observations are good enough to determine planet orbits, and to do &lt;a href=&quot;https://en.wikipedia.org/wiki/HR_8799_e&quot; rel=&quot;nofollow noreferrer&quot;&gt;spectroscopy&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So we can already observe large exoplanets at much longer distances, if they orbit far enough away from their star. Smaller planets closer to their star are more difficult to see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the image above there's a black disk at the center. This is used to block out the star's light. It covers the star, plus a radius of about 5 AU, so any Earth-like planets in the habitable zone are masked in this image. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is necessary because a telescope's optics and imaging system aren't perfect: the star's light is not confined to the star's diameter, it gets spread out a bit. This spread-out starlight is brighter than planets in close orbits, so those planets are lost in the noise. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;One way to combat this is an &lt;a href=&quot;https://en.wikipedia.org/wiki/Occulting_disk&quot; rel=&quot;nofollow noreferrer&quot;&gt;occulter&lt;/a&gt;, i.e. a physical object in front of the telescope that blocks out the star's light. Most of these are small disks at the front end of the telescope.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA is working on a &lt;a href=&quot;https://science.nasa.gov/technology/technology-stories/starshade-enable-first-images-earth-sized-exoplanets&quot; rel=&quot;nofollow noreferrer&quot;&gt;starshade&lt;/a&gt;: a much larger disk placed 50,000 km in front of a space telescope. This would be able to block out the star's light more accurately, allowing the telescope to see exoplanets in the habitable zone (so at distances of ~1 AU for smallish stars).&#xA;The plan was to use the starshade with WFIRST (a 2.4 m telescope), but &lt;a href=&quot;https://spaceflightnow.com/2017/11/03/wfirst-astronomy-mission-faces-cuts-to-contain-rising-costs/&quot; rel=&quot;nofollow noreferrer&quot;&gt;recent concerns about costs&lt;/a&gt; could see the starshade removed from this mission. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the solution for direct imaging of exoplanets may not have to be a larger mirror. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-02T10:19:01.103" LastActivityDate="2018-01-02T10:19:01.103" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24021" PostTypeId="2" ParentId="23848" CreationDate="2017-12-13T12:25:00.083" Score="6" Body="&lt;p&gt;A good starting point is NASA Software Engineering Requirements &lt;a href=&quot;https://standards.nasa.gov/standard/nasadir/npr-71502&quot; rel=&quot;noreferrer&quot;&gt;NPR 7150.2B&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;3.6 Software Assurance and Software IV&amp;amp;V&#xA;  3.6.1 The project manager shall plan and implement software assurance per NASA-STD-8739.8. [SWE-022]&#xA;  Note: Software assurance activities occur throughout the life of the project. Some of the actual analyses and activities may be performed by engineering or the project.&#xA;  3.6.2 For projects reaching Key Decision Point (KDP) A after the effective date of this directive's revision, the program manager shall ensure that software IV&amp;amp;V is performed on the following categories of projects: [SWE-141]&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;a. Category 1 projects as defined in NPR 7120.5.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;b. Category 2 projects as defined in NPR 7120.5 that have Class A or Class B payload risk classification per NPR NPR 7150.2B -- Chapter3 &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;c. Projects specifically selected by the NASA CSMA to have software IV&amp;amp;V.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;3.6.3 If software IV&amp;amp;V is performed on a project, the project manager shall ensure that an IV&amp;amp;V Project Execution Plan (IPEP) is developed. [SWE-131]&#xA;  Note: The scope of IV&amp;amp;V services is determined by the project and the IV&amp;amp;V provider, and is documented in the IPEP. The IPEP is developed by the IV&amp;amp;V provider and serves as the operational document that will be shared with the project receiving IV&amp;amp;V support. In accordance with the responsibilities defined in NPD 7120.4, section 5.J.(5), projects ensure that software providers allow access to software and associated artifacts to enable implementation of IV&amp;amp;V. A template and instructions for an IPEP may be found in the NASA IV&amp;amp;V Management System, accessible at &lt;a href=&quot;http://www.nasa.gov/centers/ivv/ims/home/index.html&quot; rel=&quot;noreferrer&quot;&gt;http://www.nasa.gov/centers/ivv/ims/home/index.html&lt;/a&gt;&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;3.7.1 When a project is determined to have safety-critical software, the project manager shall implement the requirements of NASA-STD-8719.13. [SWE-023] &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;SLS would fall in class A (human-rated). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More on the &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140008731.pdf&quot; rel=&quot;noreferrer&quot;&gt;flight control system&lt;/a&gt; for SLS: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The fundamental design paradigms that shaped the development of the architecture are as follows:&lt;br&gt;&#xA;  1. Rely on simple, proven, flight-tested algorithms and processes. Based on a heritage of more than fifty years of successful NASA flight controls development for large boosters, the use of classical PID control, multi-station rate gyro blending, linear optimal bending filters, and gain scheduling is retained.&lt;br&gt;&#xA;  2. Enhance algorithm capability when warranted with compact and verifiable methods. The use of optimal reconfigurable linear control allocation has been employed to maximize control authority and enhance fault tolerance, and a novel mechanization of classical load relief has been applied. In addition, robustness enhancement is obtained through the use of a simple model reference adaptive control scheme.&lt;br&gt;&#xA;  3. Maximize robustness to failures. As a program requirement, tolerance of at least one engine failure at any point in the flight regime with negligible impact to flight control performance is supported by the architecture. Robustness to sensor failures and severe off-nominal conditions has been demonstrated through rigorous simulation analysis.&lt;br&gt;&#xA;  4. Seamlessly integrate with the SLS program to facilitate flight certification. In support of concise communication of the design margin from the flight controls element, new metrics, such as the flight control Technical Performance Metric (TPM), are used to facilitate straightforward assessment of the design’s merit and the vehicle capability at the system engineering level.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20150022333&amp;amp;hterms=SLS%20avionics%20software&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3DSLS%2520avionics%2520software%26Ntx%3Dmode%2520matchallpartial&quot; rel=&quot;noreferrer&quot;&gt;This presentation&lt;/a&gt; gives a high-level overview of the development process. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-12-13T16:51:13.913" LastActivityDate="2017-12-13T16:51:13.913" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24022" PostTypeId="2" ParentId="24016" CreationDate="2017-12-13T16:27:17.733" Score="12" Body="&lt;p&gt;Considering fuel consumption or energy expenditure may be misleading, because of the huge change in mass over the flight as fuel is expended. 2/3 of the fuel is expended by the first stage, which only produces 1/3 of the total velocity, for example.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another way to look at the question is through delta-v expenditure; according to &lt;a href=&quot;https://web.archive.org/web/20170313142729/http://www.braeunig.us/apollo/saturnV.htm&quot; rel=&quot;noreferrer&quot;&gt;Bob Braeunig's simulation of the Apollo 11 launch&lt;/a&gt; (now offline but available on archive.org), the Saturn V produced 9,194 m/s of ∆v; Earth's rotation contributed 390 m/s of ∆v, for a total budget of 9584 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Gravity losses account for 1743 m/s; drag losses 48 m/s, and the velocity on orbital insertion is 7793 m/s. If you treat gravity loss as the cost of reaching orbital height, the breakdown is thus 18.2% gravity loss, 0.5% drag loss, 81.3% orbital velocity.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-12-13T16:27:17.733" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24023" PostTypeId="2" ParentId="23583" CreationDate="2017-12-13T18:46:32.860" Score="6" Body="&lt;p&gt;They are in different reference frames.  The TLE element set is defined relative to the True Equator, Mean Equinox (&lt;a href=&quot;https://en.wikipedia.org/wiki/Earth-centered_inertial#TEME&quot; rel=&quot;noreferrer&quot;&gt;TEME&lt;/a&gt;) reference frame, and the elements in the RINEX Nav message are defined relative to Earth Centered, Earth Fixed (ECEF).  Since both frames use the true equator, other elements (such as inclination) don't vary as much as the location of the node.&lt;/p&gt;&#xA;" OwnerUserId="12483" LastActivityDate="2017-12-13T18:46:32.860" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24024" PostTypeId="1" AcceptedAnswerId="24030" CreationDate="2017-12-13T19:13:43.060" Score="4" ViewCount="1798" Body="&lt;p&gt;The question says it all.  How much does NASA pay for a new Dragon spacecraft compared to a used one?  Similarly, how much would a regular corporation pay for such cargo missions?&lt;/p&gt;&#xA;" OwnerUserId="21424" LastEditorUserId="21424" LastEditDate="2017-12-13T19:52:03.750" LastActivityDate="2017-12-14T05:38:19.183" Title="How much does it cost to fly a Dragon that is new compared to one that is used?" Tags="|spacex|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24025" PostTypeId="2" ParentId="24019" CreationDate="2017-12-13T20:35:25.743" Score="17" Body="&lt;p&gt;Halo orbit families exist near the L1, L2, and L3 librations points.  This video focuses on the L1 and L2 halo families.  There are northern and southern families at each of the libration points.  The northern family is identical to the southern family but mirrored across the x-y plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At each point, the family bifurcates from the planar Lyapunov family of orbits.  That is, the first halo in the family is planar, and it is also a member of the kidney-bean-shaped Lyapunov family.  You can step up in the z direction and find another member of the halo family.  And step up again to find the next.  The family thus evolves out of plane, as you see in the video, and it keeps growing and approaches the smaller primary (the Moon if we're working in the Earth-Moon system).  The Near Rectilinear Halo Orbits are the tall, nearly-polar ones that get really close to the Moon.  How are they defined?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We've defined the NRHOs as those members of the halo families whose stability indices are bounded.  That is, they're marginally stable, or nearly so, in a linear analysis.  You can see a plot of the stability indices of the L1 and L2 halo families, a zoom of the NRHO portions of the two families, and the butterflies as a bonus, in Figure 2 of this paper:&#xA;&lt;a href=&quot;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Conferences/2017_AAS_DavPhiHow.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Conferences/2017_AAS_DavPhiHow.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check out the top plot in Figure 2b.  See how the stability indices are bounded (with a value of 3 or less) until you hit a perilune radius (rp) of 16,000 or 18,000 km, and then the stability indices begin to grow quickly?  We've defined &quot;NRHO&quot; to lie to the left of that bifurcation- those halos with bounded stability properties.  There's another bifurcation on the far left of those plots, where the stability index is equal to 1.  You can see it for the red L2 line; for the blue L1 line that point is below the lunar surface and isn't included in the plot.  That bifurcation marks the lower limit of the NRHO portion of the halo families.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why are they special?  As discussed in Ryan Whitley's paper you linked to, they're favorable for transferring into and out of, which is nice when your spacecraft is meant to be a staging ground for exploration, with other ships coming and going.  They also provide really great lunar south pole coverage, since they whip quickly around the north pole and spend almost all of their time in the southern hemisphere.  See: &lt;a href=&quot;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Journals/2008_JSR_GreOziHowFol.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Journals/2008_JSR_GreOziHowFol.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, they're close to stable.  In a flatter, less stable halo, if you miss a stationkeeping maneuver or have an unexpected perturbation, you might escape from the halo in just a few weeks or less.  In an NRHO, the lower stability index means that a missed maneuver or a perturbation affects your orbit less, and there's more time to recover before the spacecraft escapes the orbit.  So- orbit maintenance (or stationkeeping) is required, but there's more recovery time than in a flatter Earth-Moon halo like Artemis flew.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Illustration for clarity:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;&lt;a href=&quot;https://i.stack.imgur.com/T5Jwe.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/T5Jwe.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Figure 2b from &lt;a href=&quot;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Conferences/2017_AAS_DavPhiHow.pdf&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/eJIjm.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/eJIjm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22036" LastEditorUserId="22036" LastEditDate="2017-12-14T19:52:59.737" LastActivityDate="2017-12-14T19:52:59.737" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="24026" PostTypeId="2" ParentId="22548" CreationDate="2017-12-13T20:43:46.810" Score="6" Body="&lt;p&gt;As the halo orbits grow out of plane, their orbital periods generally decrease. You can see a plot of the period (in days) of the L1 and L2 halo families at the Moon as a function of the closest approach distance from the Moon (rp) in Figure 2 of this paper:&#xA;&lt;a href=&quot;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Conferences/2017_AAS_DavPhiHow.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Conferences/2017_AAS_DavPhiHow.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the Earth-Moon system, the range from around 2 weeks for nearly-planar halos to less than a week for the smallest NRHOs at L2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The orbits actually get &lt;em&gt;more&lt;/em&gt; stable as the families grow out of plane.  The flatter halos are unstable.&lt;/p&gt;&#xA;" OwnerUserId="22036" LastActivityDate="2017-12-13T20:43:46.810" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24027" PostTypeId="2" ParentId="20583" CreationDate="2017-12-13T23:21:27.933" Score="12" Body="&lt;p&gt;LRO was inserted into a polar frozen orbit for commissioning, which required no stationkeeping.  This orbit was a stable 31.5 km x 199 km polar orbit with periapsis over the South Pole.  By &quot;frozen&quot; they mean that the line of apsides and eccentricity remain fixed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It then moved into a 50km circular polar orbit (+/- 20 km) for its science mission.  This orbit required a stationkeeping maneuver once a month, with a budget of about 150 m/s for the year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then after the mapping phase, LRO moved again into a polar frozen orbit, where no stationkeeping is required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;references:&#xA;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070021535.pdf&quot; rel=&quot;noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070021535.pdf&lt;/a&gt;&#xA;&lt;a href=&quot;https://arc.aiaa.org/doi/pdfplus/10.2514/6.2010-1985&quot; rel=&quot;noreferrer&quot;&gt;https://arc.aiaa.org/doi/pdfplus/10.2514/6.2010-1985&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Circular polar orbits are &lt;em&gt;not&lt;/em&gt; stable.  The orbit maintenance budget to stay in one varies significantly based on your selected insertion state- a 110 km orbit at 84 deg inclination might stay around for a year without needing a maneuver, but a 100 km orbit at 90 deg inclination can require 100 m/s annually to stay within +/- 30 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But there are elliptical polar frozen orbits, like the one that LRO is in, that don't need orbit maintenance.  As here: &lt;a href=&quot;https://arc.aiaa.org/doi/abs/10.2514/6.2006-6749&quot; rel=&quot;noreferrer&quot;&gt;https://arc.aiaa.org/doi/abs/10.2514/6.2006-6749&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22036" LastActivityDate="2017-12-13T23:21:27.933" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24028" PostTypeId="1" CreationDate="2017-12-14T00:26:20.870" Score="5" ViewCount="580" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Lambert%27s_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lambert's problem&lt;/a&gt; (also &lt;a href=&quot;http://ccar.colorado.edu/asen5050/ASEN5050/Lectures_files/lecture16.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;http://ccar.colorado.edu/imd/2015/documents/LambertHandout.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{r_1}, \mathbf{r_2}, t \rightarrow \mathbf{v_1}, \mathbf{v_2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;was solved by &lt;a href=&quot;https://en.wikipedia.org/wiki/Johann_Heinrich_Lambert&quot; rel=&quot;nofollow noreferrer&quot;&gt;Johann Heinrich Lambert&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/ivzXC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ivzXC.png&quot; alt=&quot;Lambert&quot;&gt;&lt;/a&gt; via Lambert's Theorem (see references below).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm wondering if there exists in the literature a (solved) problem where the given parameters are&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{v_1}, \mathbf{v_2}, t \rightarrow \mathbf{r_1}, \mathbf{r_2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\text{or}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\mathbf{v_1}, \mathbf{r_2}, t \rightarrow \mathbf{r_1}, \mathbf{v_2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $\mathbf{r}$ and $\mathbf{v}$ are position and velocity vectors.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;For the second case, &lt;a href=&quot;https://www.shadertoy.com/view/ltlfWB&quot; rel=&quot;nofollow noreferrer&quot;&gt;this shader&lt;/a&gt; gives (if I didn't mess up the math) the locus of $\mathbf{r_1}$ for all the $t$ with a given $\mathbf{v_1} = (1,0,0)$ and a varying $\mathbf{r_2}$ (the red cross).&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Helpful references for Lambert's Theorem:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;NASA technical note &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690027552.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;A Unified Form of Lambert's Theorem&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Erich Weissstein's World of Physics &lt;a href=&quot;http://scienceworld.wolfram.com/physics/LambertsTheorem.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lambert's Theorem&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;James F. Jordan, 1964, JPL Technical Report 32-521 &lt;a href=&quot;http://www.gravityassist.com/IAF1/Ref.%201-77.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Application of Lambert’s Theorem to the Solution of Interplanetary Transfer Problems&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;hr&gt;&#xA;" OwnerUserId="22038" LastEditorUserId="22038" LastEditDate="2017-12-19T23:33:48.843" LastActivityDate="2017-12-20T02:31:01.377" Title="Solutions to a &quot;Lambert's problem in velocity&quot;" Tags="|orbital-mechanics|" AnswerCount="0" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="24029" PostTypeId="2" ParentId="23253" CreationDate="2017-12-14T03:37:49.313" Score="7" Body="&lt;p&gt;The points are numbered according to their Jacobi constants.  The zero velocity curves open first at L1, the libration point with with the lowest energy (highest value of Jacobi.)  They open next at L2, and then at L3.  The ZVCs leave the plane at last at the L4 and L5 points simultaneously.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;JC_L1 &gt; JC_L2 &gt; JC_L3 &gt; JC_L4 = JC_L5&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uQrP1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uQrP1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://engineering.purdue.edu/people/kathleen.howell.1/Publications/Dissertations/2011_CraigDavis.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="22036" LastEditorUserId="12102" LastEditDate="2019-03-20T13:18:53.627" LastActivityDate="2019-03-20T13:18:53.627" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="24030" PostTypeId="2" ParentId="24024" CreationDate="2017-12-14T03:43:28.193" Score="7" Body="&lt;p&gt;NASA hires transportation service from SpaceX.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is not the customer that saves until the savings is passed on. SpaceX &lt;a href=&quot;http://spacenews.com/spacexs-reusable-falcon-9-what-are-the-real-cost-savings-for-customers/&quot; rel=&quot;noreferrer&quot;&gt;expects to save&lt;/a&gt; ~30%.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Much like hiring a new or used taxi, it's the same rate per kg for each km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Prices for the Falcon 9 and Falcon Heavy with a cargo hold (a Dragon) from &lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;noreferrer&quot;&gt;SpaceX's price page&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Falcon 9 costs $62M and can carry up to 22,800 kg (LEO), 8,300 kg (GTO), or 4,020 kg to Mars.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Falcon Heavy costs $90M and can carry up to 63,800 kg (LEO), 26,700 kg (GTO), or 16,800 kg to Mars.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Previously NASA used the Space Shuttle, at a cost of $10K / lb.&#xA;Now NASA pays more per pound but less overall. Much like taking a ride on an old or new bus costs the same bus fare but avoids purchase, storage, maintenance, staffing and regulatory costs of owning a bus.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;In an &lt;a href=&quot;http://www.businessinsider.com/spacex-rocket-cargo-price-by-weight-2016-6/#bottle-of-water-9100-to-43180-1&quot; rel=&quot;noreferrer&quot;&gt;interview with payload specialist&lt;/a&gt; and space station engineer Ravi Margasahayam he crunched some numbers and said:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2008, NASA signed contracts with SpaceX and its rival aerospace company Orbital Sciences, to the tune of 1.6 billion for 12 launches and 1.9 billion for eight rocket launches, respectively.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While these new missions cost hundreds of millions of taxpayer dollars less than a space shuttle launch, the price of sending cargo into space didn't go down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Margasahayam told Tech Insider &quot;My cost per pound went up with these rockets, on the shuttle it would be much less.&quot; (Margasahayam spoke to Tech Insider as a private citizen and engineer, rather than as a representative of NASA.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Margasahayam points out that, while the space shuttles were more expensive — a whopping 500 million per launch (or possibly 1.5 billion, according to one analysis we've seen) — each mission carried about 50,000 lbs. (plus seven astronauts!). That means each pound of cargo used to cost about $10,000 to ship on a shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Orbital Science's Cygnus spacecraft costs over 43 thousand per pound to send things up, dividing the $1.9 billion contract by the maximum 20 metric tons of cargo the company is supposed to supply.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For SpaceX — the cheapest of NASA's new carriers — dividing the cost of each launch (133 million dollars) by the cargo weight of its most recent resupply mission (5,000 lbs.) gives you about $27,000 per pound.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But that's a high estimate. SpaceX told Tech Insider that its Dragon cargo spacecraft launched on a Falcon 9 rocket can carry up to 7,300 lbs. — and that you could bring just as much cargo back to Earth, too (something Cygnus can't do). So if a Dragon is full of supplies at launch and on landing, the cost dips to $9,100 per pound.&lt;/p&gt;&#xA;" OwnerUserId="20766" LastEditorUserId="20766" LastEditDate="2017-12-14T05:38:19.183" LastActivityDate="2017-12-14T05:38:19.183" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24031" PostTypeId="1" AcceptedAnswerId="54271" CreationDate="2017-12-14T14:31:44.993" Score="7" ViewCount="181" Body="&lt;p&gt;14 December 1972 05:40:56 UTC&lt;/p&gt;&#xA;&#xA;&lt;p&gt;US astronaut Gene Cernan returned to the Apollo 17 Lunar Excursion Module, becoming &quot;the last man on the Moon&quot; for forty-five years, so far.  (as reported in my &lt;a href=&quot;http://www.l5development.com/i_resource/history/DayInHistory.php?d=1214&quot; rel=&quot;noreferrer&quot;&gt;Space History&lt;/a&gt; newsletter)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems like there should be something done to honor the man who had that dubious title foisted upon him:  I'm sure he didn't expect that to be an accurate description when he passed away on 16 January this past year.&lt;/p&gt;&#xA;" OwnerUserId="19916" LastActivityDate="2021-07-23T05:44:17.227" Title="Is anything being done to honor Gene Cernan today? (14 Dec. 2017)" Tags="|apollo-program|lunar-landing|history|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24032" PostTypeId="2" ParentId="7846" CreationDate="2017-12-14T15:38:02.163" Score="2" Body="&lt;p&gt;You also had reference books like &lt;em&gt;Formulas for Stress and Strain&lt;/em&gt; by Roark (and Young) and &lt;em&gt;Stress Concentration Factors&lt;/em&gt; by Peterson.  Prediction might not have been as strong as it is today but often enough comparison was possible.&lt;/p&gt;&#xA;" OwnerUserId="22041" LastActivityDate="2017-12-14T15:38:02.163" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24033" PostTypeId="1" CreationDate="2017-12-14T16:38:53.193" Score="6" ViewCount="496" Body="&lt;p&gt;Satellites like &lt;a href=&quot;https://en.wikipedia.org/wiki/Deep_Space_Climate_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSCOVR&lt;/a&gt; are used to predict solar storms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How does that work? If the satellite is orbiting L1 and &quot;watches&quot; the sun, how can the signal be back on earth before a storm arrives?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How are these storms predicted exactly? Are the models like for weather on earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Can these storms be predicted several days before, like with weather on earth?&lt;/p&gt;&#xA;" OwnerUserId="12712" LastEditorUserId="18879" LastEditDate="2017-12-16T08:08:28.407" LastActivityDate="2021-09-02T14:04:16.280" Title="How can a solar storm be predicted?" Tags="|artificial-satellite|the-sun|solar-wind|space-weather|prediction|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24034" PostTypeId="1" CreationDate="2017-12-14T16:55:58.370" Score="3" ViewCount="218" Body="&lt;p&gt;I am interested in long-term data preservation on the Moon in style of Lunar Mission One. To attract the attention of the next civilization on Earth to the message, I think that a beacon similar to Nasca Lines should be drawn on the surface. What will be the most cost-effective way to do so? I already have the following ideas:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;• Using craters as dots&#xA;•   Electrically powered tractor could slowly “print” a message by boring dots or channels on surface, &#xA;•   A robot could collect stones and put them into lines or pyramids, &#xA;•   Drawing by creating tracks in lunar dust. &#xA;•   The beacon could be also created by a gradient of a rare and clearly artificial element (but not radioactive)&#xA;•   Aluminum foil could be rolled into image, &#xA;•   Aluminum powder could be spayed by a lander around its location in some geometric form&lt;br&gt;&#xA;•   Very stable and visibly dye. Which one?&lt;/p&gt;&#xA;" OwnerUserId="22042" LastActivityDate="2017-12-14T19:08:54.797" Title="How to draw a picture on the surface of the Moon so it could be visible for 100 million years?" Tags="|the-moon|" AnswerCount="1" CommentCount="2" ClosedDate="2017-12-15T00:27:18.340" ContentLicense="CC BY-SA 3.0" />
  <row Id="24035" PostTypeId="2" ParentId="24033" CreationDate="2017-12-14T17:14:26.500" Score="3" Body="&lt;p&gt;You may be asking about a couple of different things here.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;1  Events at the surface of the sun&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Events at the Sun itself may or may not head in the direction of the Earth, but this is step one of a prediction system. I believe there are two approaches: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;a) look at what happened about a month ago and predict that it will be still there when that part of the Sun rotates back into view. I think this has been tried for a while as it is conceptually straightforward. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;b) try and forecast an event (e.g. flare or CME) from the appearance of other measurable features on the Sun. I believe this is pretty involved in modelling terms and has not yet come of age (anyone know better?)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;2  Events close to the Earth&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A geomagnetic storm is the term given to phenomena measurable at the Earth but caused by previous events on the Sun.   Whilst a flare (e.g. X rays) might travel quickly to Earth the accompanying release of particulate matter can take of the order of three days to transit.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;a)  Having a clear view of the sun in several wavebands does therefore allow quite an advanced warning but does not prove that the event seen at the sun is heading towards the Earth (cue more modelling work) or will connect magnitically with the Earth's field when it does (local measurements needed). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;b) Having instruments at L1 for sampling of the local environment there may well say much more about the disturbances in the local region of space but unfortunately at not much advanced a warning (a few hours).&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-12-14T17:14:26.500" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24036" PostTypeId="1" CreationDate="2017-12-14T18:05:05.207" Score="3" ViewCount="668" Body="&lt;p&gt;Ive always wanted the answer to an specific question as you saw in the title &quot;How deep does the moon craters go&quot;. Please leave your answers down below possibly link a good source.&lt;/p&gt;&#xA;" OwnerUserId="22045" LastActivityDate="2017-12-18T10:49:34.573" Title="Moon craters. How deep do they get?" Tags="|the-moon|crater|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24038" PostTypeId="2" ParentId="24034" CreationDate="2017-12-14T19:08:54.797" Score="2" Body="&lt;p&gt;According to &lt;a href=&quot;https://astronomy.stackexchange.com/q/2398&quot;&gt;Is the moon only 60 pixels?&lt;/a&gt; on Astronomy.SE, it would appear that to the unaided eye you can't see much more than would fit in 40 to 60 pixels (height &amp;amp; width).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia, &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon&quot; rel=&quot;nofollow noreferrer&quot;&gt;the Moon&lt;/a&gt; has a diameter of 3,476 km.  Therefore, each &quot;pixel&quot; would have to be dot nearly 58 km in diameter for you to see it from Earth without a telescope.  (3,476 / 60 = 57.8 km) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even a small image will have to be hundreds of kilometers on a side.  For example, a simple image consisting of 2 lines, each 10 dots long would require almost 72,000 km&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're going to mechanically alter the face of the Moon you'll need a lot of really big robots and a lot of paint.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would be a lot cheaper to bury a time capsule and plant either a radioactive marker above it&lt;sup&gt;1&lt;/sup&gt; or a nuclear powered radio transmitting &quot;I am here&quot; in Morse code.  Neither would be visible from Earth but any civilization with the technology to actually visit the Moon would certainly notice the marker.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;1 Yes, you explicitly excluded radioactive markers, but they're probably the best option available&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="12978" LastActivityDate="2017-12-14T19:08:54.797" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24039" PostTypeId="2" ParentId="24033" CreationDate="2017-12-14T19:33:49.957" Score="3" Body="&lt;p&gt;Solar flares travel at about 2000 km/s. Radio travels at 300,000 km/s, so a spacecraft at L1 (1.5 million km out) like DISCOVR gives about 12 minutes of warning when the solar flare reaches it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spacecraft like &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOHO&lt;/a&gt; observe the sun, and these observations are used by e.g. &lt;a href=&quot;http://www.swpc.noaa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NOAA&lt;/a&gt; to try and &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_flare#Prediction&quot; rel=&quot;nofollow noreferrer&quot;&gt;predict&lt;/a&gt; flares: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Current methods of flare prediction are problematic, and there is no certain indication that an active region on the Sun will produce a flare. However, many properties of sunspots and active regions correlate with flaring. For example, magnetically complex regions (based on line-of-sight magnetic field) called delta spots produce the largest flares. A simple scheme of sunspot classification due to McIntosh, or related to fractal complexity.[57] is commonly used as a starting point for flare prediction.[58] Predictions are usually stated in terms of probabilities for occurrence of flares above M or X GOES class within 24 or 48 hours. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-12-14T19:33:49.957" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24040" PostTypeId="2" ParentId="13876" CreationDate="2017-12-14T23:02:31.817" Score="5" Body="&lt;p&gt;The 800 km orbit is much more important than the LTAN. At 800 km orbit, the satellite is going to have a  orbital period of roughly 100 minutes.  For a few  of those successive 100 minute orbits, it's going to spend close to 1/2 of the orbit in eclipse.  Without designing for any safety margin, the power system needs to be able to fully charge the batteries in roughly 50 minutes, and the batteries need to be able to hold enough charge to power the spacecraft for roughly 50 minutes.&lt;/p&gt;&#xA;" OwnerUserId="22050" LastEditorUserId="6816" LastEditDate="2017-12-15T00:22:08.553" LastActivityDate="2017-12-15T00:22:08.553" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24041" PostTypeId="1" AcceptedAnswerId="24045" CreationDate="2017-12-14T23:32:49.300" Score="21" ViewCount="2737" Body="&lt;p&gt;I watched a documentary on Voyager 1 and 2 last night and wondered &quot;how is the digital equipment on them still operating&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I work in IT, and decided to look up what the longest running servers were - recently there have been servers that ran 17 years and 24 years respectively before requiring reboot. These are records AFAIK. Now, being in IT and having dealt with a variety of legacy applications, I can say that I'm often astonished by how long things can last, and am keen to know how Voyager is still alive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the Voyager missions, I imagine the following would have potentially fatal effects on the equipment:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fatal or irrecoverable errors in operating code.&lt;/li&gt;&#xA;&lt;li&gt;Radiation from the Sun, Jupiter, and Saturn - despite shielding, surely some would still get through?&lt;/li&gt;&#xA;&lt;li&gt;Graviational forces entering slingshot trajectories with major planetary bodies could cause fracture or stress on physical components.&lt;/li&gt;&#xA;&lt;li&gt;Magnetic forces when approaching planetary magnetic fields could play havoc with electronic equipment.&lt;/li&gt;&#xA;&lt;li&gt;Temperature changes, while not fast, would still cause components to cool to very low temperates. This may be enough to cause the &quot;fixed on&quot; metal components on circuit boards to contract enough to fracture or crack.&lt;/li&gt;&#xA;&lt;li&gt;Firing up thrusters for short periods could cause immediate local spikes on temperature in various components, which may have stayed at a reasonably stable temperature for long periods. The heat stress could again cause components to fracture or crack.&lt;/li&gt;&#xA;&lt;li&gt;Electronic components running continuously for long periods run the risk of shorting or burning out.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Can anyone explain how these stresses have been catered for and/or mitigated for so long?&lt;/p&gt;&#xA;" OwnerUserId="22051" LastEditorUserId="12102" LastEditDate="2017-12-15T04:14:01.333" LastActivityDate="2017-12-15T16:25:28.150" Title="How is Voyager 1 still operating?" Tags="|voyager|deep-space|flight-computer|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24042" PostTypeId="2" ParentId="16670" CreationDate="2017-12-14T23:45:51.197" Score="4" Body="&lt;p&gt;If the original spin is clockwise: the weights will accelerate a counterclockwise spin as they begin to unwind. This will slow the clockwise spin to a stop (if the cable is long enough; and the weights have enough mass).   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the rocket has stopped spinning and the cables are still attached, and the cable has not reached 90° to tangent, then the counterclockwise acceleration will continue. Until 90° to tangent the force in the cable is always counterclockwise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At first the momentum is given from the rocket to the weights; but there comes a point where the weights give the motion back to the rocket. If the weights are not released the counterclockwise acceleration continues; up to 90° to tangent.   &lt;/p&gt;&#xA;" OwnerUserId="22049" LastActivityDate="2017-12-14T23:45:51.197" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24043" PostTypeId="1" AcceptedAnswerId="24066" CreationDate="2017-12-15T04:13:22.640" Score="10" ViewCount="475" Body="&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/24041/12102&quot;&gt;How is Voyager 1 still operating?&lt;/a&gt; and the interesting comparison to the record times that Earthbound servers have run without reboot got me thinking about deep space spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm tentatively excluding Earth orbit because these may include legacy spacecraft with simple systems (and there are so many!), but lunar orbit is fine, and landers and rovers count as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't know how to handle periods of sleep (see answers below &lt;a href=&quot;https://space.stackexchange.com/q/15371/12102&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/15374/12102&quot;&gt;here&lt;/a&gt; for Rosetta's 2.5 year snooze for example) but my bias would be to either count them as part of the functioning period, or at most subtract the sleep time but not use them to break one period into two periods.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-06-02T23:49:07.800" LastActivityDate="2019-06-02T23:49:07.800" Title="What's the record for the longest time a deep space craft has functioned without a reboot?" Tags="|rovers|deep-space|lander|flight-computer|record|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24044" PostTypeId="1" AcceptedAnswerId="25527" CreationDate="2017-12-15T09:16:33.540" Score="12" ViewCount="2082" Body="&lt;p&gt;Until food can be grown or synthesized reliably and effectively in space, a manned deep space mission needs to bring all of the food with it, or cache some of it ahead of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Roughly what would be the mass of &quot;space food&quot; per year per astronaut? Possibly an answer for current supplies used on the ISS would be larger than a supply optimized for absolute minimum weight - I don't know.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, if I take 5 Calories per gram (carb/protein/fat mix) and 2500 Calories/day as ballpark figures I get about 200 kg/year, or about 3x the mass of the astronaut per year, but food is not necessarily pure carb/protein/fat. The water content could be ignored here and considered as part of the &quot;make up&quot; water necessary for system losses.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How would that ballpark number compare to reality?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; the answer needs to include some estimate of the &quot;mass overhead&quot; — additional items necessary to contain, maintain, and eat the food; anything that scales with time. Reusable items that don't scale with time (like a heating device or utensil) don't count.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The motivation of the question stems from reading Kate Greene's 2014 Slate.com article &lt;a href=&quot;http://www.slate.com/articles/health_and_science/space_20/2014/10/manned_mission_to_mars_female_astronauts_are_cheaper_to_launch_into_outer.html&quot; rel=&quot;noreferrer&quot;&gt;An All-Female Mission to Mars; As a NASA guinea pig, I verified that women would be cheaper to launch than men.&lt;/a&gt; where the conclusion comes in part from an overall lower caloric requirement.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2017-12-15T11:14:17.257" LastActivityDate="2018-03-23T12:01:58.723" Title="Mass of food per astronaut per year for an extended deep space excursion?" Tags="|crewed-spaceflight|deep-space|food|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24045" PostTypeId="2" ParentId="24041" CreationDate="2017-12-15T09:53:44.343" Score="28" Body="&lt;p&gt;The Voyagers have been so reliable due to careful design, plus &lt;a href=&quot;https://history.nasa.gov/computers/Ch6-2.html&quot; rel=&quot;noreferrer&quot;&gt;lots of redundancy&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Voyager employs three dual-redundant computer systems per spacecraft. The first, the CCS, is nearly identical to that flown on Viking, performing sequencing and spacecraft health functions along with new ones necessitated by the addition of the other computers. Telemetry data formatting and transmission handled by the Flight Data System are done on Voyager with the help of a custom-built computer. Attitude control and articulation of the scan platform are accomplished with the third computer system. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;One concept from the STAR computer proposed for the TOPS, applicable to Voyager, is dormancy. JPL's project staff believed that equipment would last longer if unpowered4. Although both CCSs are always powered, rarely are both Flight Data Systems running, and both attitude control computers are never turned on at the same time. Full bit-for-bit redundancy is not maintained in the dual memories. For example, &quot;expended&quot; algorithms, such as the deployment sequence executed shortly after separation from the booster, need not be maintained5. Both memories are accessed by the single active processor in each system. The Flight Data System keeps a copy of its instructions in both memories, but intermediate data and variables can be stored in either memory. This seemingly casual attitude toward memory duplication tightens up considerably near encounter periods, which is one time that both CCS processors are in tandem mode.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The Voyagers are regularly updated in-flight with new software:  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In-flight programming allowed for new routines and programs to be uploaded regularly in non-volatile memory and eliminated the need for large amounts of memory to be required onboard.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You're assuming these computers can't be rebooted. Voyager 2's FDS was rebooted in &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?release=2010-151&quot; rel=&quot;noreferrer&quot;&gt;2010&lt;/a&gt;, for example. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Engineers successfully reset a computer onboard Voyager 2 that caused an unexpected data pattern shift, and the spacecraft resumed sending properly formatted science data back to Earth on Sunday, May 23. Mission managers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., had been operating the spacecraft in engineering mode since May 6. They took this action as they traced the source of the pattern shift to the flip of a single bit in the flight data system computer that packages data to transmit back to Earth. In the next week, engineers will be checking the science data with Voyager team scientists to make sure instruments onboard the spacecraft are processing data correctly. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Spacecraft in general are built to assure operations can continue even if errors occur. They usually have watchdog systems: a simple circuit outside the main computer that can command a reboot if the main computer is unresponsive for too long. They also have emergency modes: simple programs that make sure the probe can still communicate if the main programs have failed.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As pioneered on Mariner X, a disaster backup sequence was stored in the Voyager 2 CCS memory for the Uranus encounter, and later for the Neptune encounter. Required because of the loss of redundancy after the primary radio receiver developed an internal short, the backup sequence will execute minimum experiment sequences and transmit data to earth; it occupies 20% of the 4K memory.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Reliability starts in the &lt;a href=&quot;http://archive.eetasia.com/www.eetasia.com/ART_8800682256_499495_NT_76ec95b3.HTM&quot; rel=&quot;noreferrer&quot;&gt;design phase&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;they used high-quality, space-rated parts&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;they designed the spacecraft so that parts were always well within their tolerances (i.e. parts were not used at full power or near their temperature limits)  &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If a silicon-based component was designed to operate at a junction temperature of 110°C, JPL engineers would use a combination of junction temperature derating and high proto-flight temperatures so that junction temperatures never rose above 65°C. A similar temperature reduction was made for GaAs parts, which were typically designed to operate at 130°C.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;they minimized thermal cycling: the spacecraft was kept at a constant temperature, and electronics were left on instead of being switched off&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;the design was subjected to worst case analysis: &#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A circuit is analysed to determine whether the circuit will properly function if all of the environmental and parts-related parameters are &quot;stacked&quot; at their worst-case levels. Jones explained how this works: &quot;If the gain of a part is at its worst case at high temperature, then you select that gain, and if the part next door performs poorest at a very low temperature, then you select that parameter.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The Voyager engineering team &lt;a href=&quot;https://www.wired.com/2013/09/vintage-voyager-probes/&quot; rel=&quot;noreferrer&quot;&gt;ignored their superiors&lt;/a&gt; who just wanted a design that would last for the 4 planetary encounters: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Looking at engineers' testimonials from when it was built, Dodd said the original designers were told not to worry about reaching interstellar space and focus on making sure the Voyagers could observe Jupiter and Saturn.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;“Basically they kind of ignored those directions, nodded their heads and did what they wanted to make it capable of getting to interstellar space,” she said.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-12-15T12:40:38.717" LastActivityDate="2017-12-15T12:40:38.717" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24046" PostTypeId="1" AcceptedAnswerId="24049" CreationDate="2017-12-15T11:12:01.747" Score="7" ViewCount="3373" Body="&lt;p&gt;What cause the difference in communication delay, about 11 seconds in this video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/zEPCeXWQtyM?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and apparently almost no delay on this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/AvAnfi8WpVE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thank you!&lt;/p&gt;&#xA;" OwnerUserId="22055" LastEditorUserId="12102" LastEditDate="2017-12-15T11:46:29.260" LastActivityDate="2020-02-18T13:13:24.667" Title="Difference in communication delay on ISS" Tags="|iss|communication|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24047" PostTypeId="1" AcceptedAnswerId="24069" CreationDate="2017-12-15T11:43:02.723" Score="4" ViewCount="349" Body="&lt;p&gt;The TLEs accuracy for PSLV-C39/IRNSS-1H seems low when compared to that of the other LEO satellites (ISS, HST, Tiangong, Sfera 2, ...). Look, for example, at the following graph that shows the radius vector at perigee:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3tMgR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3tMgR.png&quot; alt=&quot;http://cristianopi.altervista.org/IRNSS-1H_TIA1_peri.png&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;the perigee for the Tiangong-1 is much smoother (the scale is 24 km for both the objects).&#xA;Is there some particular reasons for the low accuracy of the PSLV-C39/IRNSS-1H TLEs?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&#xA;The following graph show the same thing, but without linked points:&#xA;&lt;a href=&quot;https://i.stack.imgur.com/mZO6z.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mZO6z.png&quot; alt=&quot;http://cristianopi.altervista.org/IRNSS-1H_TIA1_peri_dot.png&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;1 point is 1 TLE. Each TLE is propagated (via SGP4) for at most 1 orbit (until I find the smallest radius vector for that TLE).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I do the same kind of graph for the semi-major axis or for the apogee, the shape is much smoother.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT #2: the next graph shows the apogee (the shape for the sma is the same):&#xA;&lt;a href=&quot;https://i.stack.imgur.com/JPheb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JPheb.png&quot; alt=&quot;http://cristianopi.altervista.org/IRNSS_Apo.png&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graph also shows the interpolating line for the decay rate (9.3 km/day).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The radius vector is calculated with a time step = 10 s.&lt;/p&gt;&#xA;" OwnerUserId="20397" LastEditorUserId="20397" LastEditDate="2017-12-16T11:21:41.423" LastActivityDate="2017-12-17T07:47:30.897" Title="PSLV-C39/IRNSS-1H TLE accuracy" Tags="|low-earth-orbit|two-line-elements|tiangong-1|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24048" PostTypeId="1" AcceptedAnswerId="24060" CreationDate="2017-12-15T15:41:10.830" Score="4" ViewCount="209" Body="&lt;p&gt;After the AMOS-6 explosion on the pad at SLC-40 in September 2015, SpaceX spent a lot of time rebuilding SLC-40, including a very big update on the TEL (Transporter/Erector/Launcher). Whenever they speak of it (Gwynne Shotwell or on the CRS-13 launch webcast) they mention that the original TEL was built before they ever launched a Falcon 9, and they have since built Vandenberg's SLC-4E, and LC-39A. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus they have a lot of experience now on how a TEL should be built.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When they revealed the LC-39A TEL it was very different from the previous looks and was mostly enclosed. But on the CRS-13 launch it because clear that the newest TEL was still a lattice design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Initially I was wondering why they did not enclose it, but perhaps a better question might be, why was the LC-39A TEL enclosed? &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-12-16T11:06:16.097" Title="Why is LC39A's TEL enclosed, but the newer SLC40 TEL is not?" Tags="|launch|spacex|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24049" PostTypeId="2" ParentId="24046" CreationDate="2017-12-15T15:45:38.460" Score="10" Body="&lt;p&gt;I think there's a bit of an X/Y problem here -- the question you're asking isn't really the question you want an answer to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;First, a direct answer to the question you're asking:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The long delay is chiefly a result of several sources.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Starting from the ISS, you have a delay from the video encoder on board, you have a delay from the satellite downlink that goes from ISS to a TDRS satellite, down to the TDRS terminal at White Sands, New Mexico, you have a delay from the TDRS terminal to the video processing facility at JSC building 8, then you have the delay associated with pushing this video stream from JSC to Las Vegas, and finally the decoder delay associated with actually displaying the video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the way back, you have the VoIP audio feed from Las Vegas that travels back to JSC, then to White Sands, then back up to ISS via TDRS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The delay you see is the sum of all of those.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;4K video is a lot of data to push through a satellite link that was put into place long before 4K video was ever a thing.  The second video is &quot;only&quot; HD, which I would suspect suffers less of a codec delay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;That said, here's a better explanation of why the delay is less apparent in the second video:&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second video doesn't require round-trip communication from space to ground to be real-time to keep the music in sync. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Timing issues like that are actually very similar to what marching bands and drum corps have to deal with all the time.  The solution is pretty simple: the most delayed member of the ensemble (Chris Hadfield in this instance, or the farthest backfield members of the marching group) are the timing sources, and everyone else just plays with what they hear.  I suspect there was also a click track on BNL's in-ear monitors to help.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were to see a recording of this from Chris Hadfield's perspective, it would have sounded horribly out of sync (again, just like in marching band and drum corps).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also suspect this was produced from multiple takes, which would cover any other ensemble tears there may have been.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2017-12-15T16:00:53.793" LastActivityDate="2017-12-15T16:00:53.793" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24050" PostTypeId="2" ParentId="24041" CreationDate="2017-12-15T16:18:44.010" Score="7" Body="&lt;p&gt;Concerning mechanical Stress:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As Uwe has pointed out in the comments, there is no large mechanical stress on the probe at any point in the mission after launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The thrusters cause stress, but not more than a theoretical combined maximum of 13.3N distributed among 16 thrusters. The load-bearing structures are designed to handle this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During the entirety of a gravity assist, the spacecraft is in free fall, so it will not experience any acceleration. There will only be a tiny tidal force due to one end of the spacecraft being slightly closer to the central body than the opposite end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for thermal stress, heat soak is an issue, but the manufacturer of the thruster makes sure that the thrusters are adequately cooled by radiation. Other components can see some heat radiation from the thrusters' plume, but it is important to note that these things are tiny, and do not produce a lot of heat to begin with.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2017-12-15T16:25:28.150" LastActivityDate="2017-12-15T16:25:28.150" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24051" PostTypeId="1" CreationDate="2017-12-15T17:33:12.793" Score="1" ViewCount="698" Body="&lt;p&gt;What types of propellant are used in the second stage of multi-stage rockets? For example, the Falcon 9. Is it the same combination as the first stage? I've found plenty of information about the first stage but not the second stage.&lt;/p&gt;&#xA;" OwnerUserId="22059" LastEditorUserId="6537" LastEditDate="2017-12-16T10:02:27.047" LastActivityDate="2017-12-19T18:01:48.880" Title="What fuel is used in the second stage of orbital launch vehicles?" Tags="|rockets|falcon-9|fuel|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24052" PostTypeId="2" ParentId="24051" CreationDate="2017-12-15T17:55:56.263" Score="3" Body="&lt;p&gt;Falcon 9: second stage uses the same propellant as the first stage (RP-1). This is somewhat unusual, many launchers have a second stage optimized for performance, using liquid hydrogen. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-12-15T17:55:56.263" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24053" PostTypeId="1" CreationDate="2017-12-15T23:34:09.390" Score="4" ViewCount="1081" Body="&lt;p&gt;I watched the start of the CRS-13 and the view of the engine and other elements of the rocket are shown from CCD cameras in sd. The resolution ok, does not have to be large but why is it still used for this color-washed and susceptible to vertical smear cameras of the old generation and not CMOS? From what you can now observe, CMOS provides a better picture and no CCD flaws.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At old NASA launches video feed was analog (CCD output = analog), spaceX uses digital transmission so what is the matter with these old technology cameras?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In picture characteristic for ccd overexposed pixels vertical smear lines&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lm8CK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lm8CK.jpg&quot; alt=&quot;characteristic for ccd overexposed pixel vertical smear line&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22062" LastEditorUserId="22062" LastEditDate="2017-12-15T23:44:22.363" LastActivityDate="2018-05-11T20:49:56.487" Title="Why are CCD cameras still used, not CMOS for live view from the rocket?" Tags="|spacex|falcon-9|camera|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24054" PostTypeId="2" ParentId="24053" CreationDate="2017-12-16T01:26:28.190" Score="7" Body="&lt;p&gt;CCDs aren't actually analog, they are a type of digital camera. There are a number of them out there, but the two most popular methods of manufacturing a digital camera are CCD and CMOS. CCD cameras have a higher ratio of sensor to space ratio, and thus tend to be a bit better at low light, but at the cost of being more expensive. In many other areas, CMOS will perform better, however.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the end, I suspect they want the better low light performance, and thus went with a CCD. But both can be good sensors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.teledynedalsa.com/imaging/knowledge-center/appnotes/ccd-vs-cmos/&quot; rel=&quot;noreferrer&quot;&gt;http://www.teledynedalsa.com/imaging/knowledge-center/appnotes/ccd-vs-cmos/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-12-16T01:26:28.190" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24055" PostTypeId="1" AcceptedAnswerId="24059" CreationDate="2017-12-16T05:02:29.047" Score="10" ViewCount="343" Body="&lt;p&gt;I was surprised to find that the Voyager spacecraft use tape to store data pending transmission to Earth. Amazing that they have enjoyed such a long operating life, given that they are mechanical devices and presumably consume a significant amount of power from an ever-diminishing supply. Have more modern space probe designs e.g. New Horizons continued to use tape, or have they switched to solid-state? This is in regard to storage needed for the largest amounts of data, presumably science data - images and such. Are there benefits to tape-based data storage aboard robotic spacecraft still favoring their use despite the drawbacks?&lt;/p&gt;&#xA;" OwnerUserId="429" LastEditorUserId="18879" LastEditDate="2017-12-16T13:19:39.040" LastActivityDate="2017-12-16T15:11:54.517" Title="What is the preferred technology for bulk data storage on a modern space probe?" Tags="|probe|technology|raw-data|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24059" PostTypeId="2" ParentId="24055" CreationDate="2017-12-16T08:37:12.130" Score="8" Body="&lt;p&gt;The Voyagers used tape because that was the state of the art in the 1970s. Rotating harddrives were available, but are less robust than tape drives (and would act like a gyroscope, complicating attitude changes). &#xA;Non-volatile &lt;a href=&quot;https://en.wikipedia.org/wiki/Magnetic-core_memory&quot; rel=&quot;noreferrer&quot;&gt;solid state storage&lt;/a&gt; was available at that time, but primitive: you had e.g. magnetic core memory. The Voyagers used this as &lt;a href=&quot;https://codingrelic.geekhold.com/2010/07/voyager-2-soft-error.html&quot; rel=&quot;noreferrer&quot;&gt;RAM&lt;/a&gt;. &#xA;A magnetic core memory with enough capacity to replace the tape unit may have been heavier than the tape unit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rotating harddisks were rarely used: &lt;a href=&quot;https://space.stackexchange.com/questions/2272/on-which-device-data-gets-saved-on-spacecrafts&quot;&gt;this related question&lt;/a&gt; mentions one mission that used them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More recent spacecraft like Cassini and New Horizons use Flash storage. The switch from tape to solid stage was done somewhere between the Galileo and Cassini missions. &lt;a href=&quot;https://en.wikipedia.org/wiki/Galileo_(spacecraft)#Tape_recorder_anomalies_and_remote_repair&quot; rel=&quot;noreferrer&quot;&gt;Galileo&lt;/a&gt; used a tape drive, which developed problems right around the Jupiter orbit insertion. For &lt;a href=&quot;https://www.spiedigitallibrary.org/conference-proceedings-of-spie/2803/1/Cassini-solid-state-recorder--a-high-capacity-radiation-tolerant/10.1117/12.253426.short?SSO=1&quot; rel=&quot;noreferrer&quot;&gt;Cassini&lt;/a&gt;, a tape drive was considered but rejected: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A high-capacity, high-performance solid-state recorder (SSR) design was chosen for the main software and data-storage system of the Cassini spacecraft. The SSR design was selected rather than candidate moving-tape data-transport designs, which have a demonstrated lifetime. In contrast, the SSR design offers unlimited read and write opportunities. Additionally, the design of the SSR permits definition of up to 16 individual partitions, each controllable and accessible independently of the others. The Cassini SSR is designed to operate during intense radiation events, including passage through Saturn's trapped-proton belt, with indiscernible impact on data. Custom application- specific integrated circuits are used throughout the SSR. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-12-16T15:11:54.517" LastActivityDate="2017-12-16T15:11:54.517" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24060" PostTypeId="2" ParentId="24048" CreationDate="2017-12-16T11:06:16.097" Score="3" Body="&lt;p&gt;The LC-39A TEL is also internally a lattice, just has panels attached to protect it from rocket exhaust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately I can't find the links now, but I recall that there were pictures of LC-39A TEL in the few months/weeks before its first launch, and it lacked the protective panels. They were only attached at the last moment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We also know that SpaceX continues to iterate on their TEL&amp;amp;GSE. They attached some of the Falcon Heavy hold-down clamps and tail service masts &lt;em&gt;in between Falcon 9 launches&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Therefore, my best guess is that they simply haven't done it &lt;strong&gt;yet&lt;/strong&gt;. They already slated CRS-13 to launch from LC-40, and the panels aren't &lt;em&gt;necessary&lt;/em&gt;, so they skipped that part to launch sooner.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alternative guess: LC-39A was designed for Falcon Heavy which has higher thrust, so that TEL needs additional protection.&lt;/p&gt;&#xA;" OwnerUserId="10274" LastActivityDate="2017-12-16T11:06:16.097" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24061" PostTypeId="2" ParentId="24047" CreationDate="2017-12-16T17:25:03.977" Score="2" Body="&lt;p&gt;The perigee altitude for PSLV-C39/IRNSS-1H is very low (&amp;lt; 200 km) in the timespan you selected. This makes it much more sensitive to atmospheric drag than an object at higher altitude. Atmospheric drag is notoriously difficult to predict due to atmospheric density fluctuations and spacecraft attitude variation. Also, the lower altitude means fewer opportunities for radar and visual observations.  These factors combine to give inherently noisier ephemerides.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2017-12-16T17:25:03.977" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24062" PostTypeId="2" ParentId="23886" CreationDate="2017-12-16T18:12:23.160" Score="4" Body="&lt;p&gt;After a lot of reasearch, I've found this NASA page &lt;a href=&quot;https://srag.jsc.nasa.gov/Publications/TM104782/techmemo.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://srag.jsc.nasa.gov/Publications/TM104782/techmemo.htm&lt;/a&gt; where it states that &quot;The integral GCR dose rate in free space is approximately a factor of 2.5 higher at solar minimum than at solar maximum. During solar minimum, the unshielded dose to the blood forming organs (BFO) is approximately 60 rem/year.&quot; and &quot;Compared to low inclination orbits, higher inclination orbits are exposed to increased GCR levels as the spacecraft transits the higher latitudes. In the proposed Space Station orbit, the magnetic field provides a factor of 10 reduction in total GCR exposure relative to the free space environment.&quot; So according to that site the short answer to my question is: the guy would be roughly receiving an average dose of 0.00685 mSv/h in LEO in low inclination orbits at solar minimum.&lt;/p&gt;&#xA;" OwnerUserId="21885" LastActivityDate="2017-12-16T18:12:23.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24064" PostTypeId="1" CreationDate="2017-12-17T00:53:54.300" Score="12" ViewCount="1113" Body="&lt;p&gt;I had noticed this before, but when landing, the legs do not open at exactly the same time or speed. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the CRS-13 mission, it was really obvious in this video around 50 seconds into the video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=UmD0vltNbvQ?t=50s&quot; rel=&quot;noreferrer&quot;&gt;CRS-13 first stage landing video&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Clearly it does not matter, since this is the 20th successfully landed first stage, 16th in a row, so I am quibbling.  But I wonder why the difference in speed?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Presumably the mechanism for all 4 legs is identical and operates the same.  Failure to lock like in one of the ASDS landings would be terrible, but so long as the slowest leg opens in time it may not really matter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xHo8S.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xHo8S.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="12102" LastEditDate="2017-12-17T08:58:16.443" LastActivityDate="2018-02-09T15:31:18.620" Title="Why does one leg open slower than others on Falcon 9 landings?" Tags="|spacex|falcon-9|landing|recovery|" AnswerCount="3" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24065" PostTypeId="2" ParentId="13036" CreationDate="2017-12-17T00:54:13.313" Score="2" Body="&lt;p&gt;Is it possible? Yes, but not with &lt;em&gt;current&lt;/em&gt; technology (sorry not sorry for that pun). Right now railguns can't shoot projectiles fast enough to launch picosats from altitude. At least that's what my back of the envelope calculations say, maybe someone will check my math.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's use the General Atomics 32 megajoule (MJ) Multimission Medium Range Railgun Weapon System (MMRRWS). It's the bigger one pictured below and one of the more powerful railguns out there.&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/Bb4pi.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Bb4pi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Escape velocity on the surface of the earth is 11km/s. The 32 MJ can launch a projectile at about 2.5km/s (5500mph).  That's not going to do the trick so let's load the whole rig into the back of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lockheed_Martin_C-130J_Super_Hercules&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lockheed Martin c130-30J.&lt;/a&gt; The pulsed power system that runs it can fit in a 20 foot shipping container.  So the gun itself and the power system can fit in the 55 foot compartment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to wikipedia the c130j can get up to 12km in the air.  The wikipedia article on &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;escape velocity&lt;/a&gt; says that at 9km is you need 7.1km/sec to get escape velocity so this won't quite work out.  You'll either need to get higher or build a more powerful railgun. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You might be able to do it in a hybrid airship like the &lt;a href=&quot;https://www.lockheedmartin.com/us/products/HybridAirship.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;one lockheed martin is making&lt;/a&gt;. By my calculations you would need to get up to about 22km to be able to use the railgun.      &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of interest, NASA researched a program to do this but in reverse.  The idea was to use a 2 mile long railgun at Kennedy Space Center that would launch a scramjet that would then deploy a rocket stage with the payload.  You can read more about it here &lt;a href=&quot;https://www.nasa.gov/topics/technology/features/horizontallaunch.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/topics/technology/features/horizontallaunch.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a pretty good infographic with how railguns work. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/YP5IC.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YP5IC.jpg&quot; alt=&quot;Railgun info&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="16606" LastActivityDate="2017-12-17T00:54:13.313" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24066" PostTypeId="2" ParentId="24043" CreationDate="2017-12-17T04:06:10.263" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; &quot;What's the record for the longest time a deep space craft has functioned without a reboot?&quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Source:&lt;/strong&gt; This &lt;a href=&quot;https://www.nasaspaceflight.com/2017/07/pioneer-10-first-probe-inner-precursor-juno/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pioneer 10 webpage from NASA&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Pioneer 10: first probe to leave the inner solar system &amp;amp; precursor to Juno&lt;/p&gt;&#xA;&#xA;&lt;p&gt;July 15, 2017 by Chris Gebhardt&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first spacecraft to leave the inner solar system sailed into the asteroid belt 45 years ago today, 15 July 1972, on a mission that would mark many “firsts” for NASA’s exploration of the solar system.  Pioneer 10, the first outer solar system mission, became the first probe not only to leave the inner solar system, but also the first probe to be launched on an escape trajectory from the solar system and the first craft to visit the planet Jupiter.  Today&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;and&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;Unlike the previous Pioneer probes, Pioneers 10 and 11 were specifically designed for exploration of the outer solar system, with enhanced communications systems and hardened radiation shielding to protect their instruments and systems from the damaging radiation fields they would encounter at Jupiter.&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;and&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;On 6 November 1973, while still 25 million km (15.5 million miles) from Jupiter, direct observations of the Jovian system began.&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;and&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;By 29 November [1973], Pioneer 10 was &lt;strong&gt;still operating flawlessly&lt;/strong&gt; as its instruments collected data point after data point and image after image of Jupiter.&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;and&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;On 3 December, the radiation began to take its toll on Pioneer 10, with the spacecraft generating several false commands.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thankfully, Pioneer 10’s controllers had prepared for just such a contingency, and most of the false commands were able to be countermanded by contingency commands to the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the radiation-induced false commands did result in the loss of one image of Io and several close-ups of Jupiter.&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;and&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;The final signal received from Pioneer 10 arrived on Earth through the Deep Space Network on 23 January 2003 from a distance of ~82.2 AU.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All further attempts to contact the spacecraft were unsuccessful, with the final attempt made on 4 March 2006 – 34 years and 1 day after the craft left Earth on its historic mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Today, Pioneer 10 – assuming it hasn’t collided with anything – is ~118.5 AU from the sun and is travelling outward at 2.54 AU per year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is currently the second farthest human-made object from the sun – a position it will hold until April 2019 when Voyager 2 overtakes it.&quot;.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Answer&lt;/strong&gt;: Pioneer 10 worked flawlessly until it had &quot;false commands [that] were able to be countermanded by contingency commands&quot;, it operated (with power) for almost 31 years (until 23 January 2003), and continues travel to deep space 45 1/2 years later. Nothing in NASA's article mentions shutdown, sleep, or a reboot - since it no longer has power a reboot is impossible. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, 31 or &lt;em&gt;technically&lt;/em&gt; (the answer, as the question is written), 45 1/2 years. The &quot;&lt;strong&gt;function&lt;/strong&gt;&quot; of Pioneer 10 after it looses power is to transport the Systems Diagram, so it's still functioning (until it crashes).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3zDRv.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3zDRv.png&quot; alt=&quot;Pioneer 10&amp;#39;s System Diagram&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My understanding is that the decades old Voyager probes have multiple computers, some of which have not been rebooted; but I don't have an authoritative source.&lt;/p&gt;&#xA;" OwnerUserId="20766" LastEditorUserId="20766" LastEditDate="2017-12-17T05:51:46.060" LastActivityDate="2017-12-17T05:51:46.060" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24068" PostTypeId="1" CreationDate="2017-12-17T07:39:14.297" Score="7" ViewCount="5919" Body="&lt;p&gt;I'm relatively new to orbital mechanics and I know there are a ton of resources and similar questions on the web, but I can't seem to get a straight answer/guidance from any of them.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have a set of data regarding any particular satellite which consists of the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Mean motion&lt;/li&gt;&#xA;&lt;li&gt;Mean anomaly&lt;/li&gt;&#xA;&lt;li&gt;Argument of perigee&lt;/li&gt;&#xA;&lt;li&gt;Eccentricity &lt;/li&gt;&#xA;&lt;li&gt;Inclination&lt;/li&gt;&#xA;&lt;li&gt;Right Ascension of Ascending Node (RAAN) AKA Longitude of Ascending Node&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This data is extracted using PyEphem library.&#xA;What I would like to calculate is the longitude, latitude and altitude of the satellite at any given time. I know what each parameter means but I don't know how they relate to my unknowns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I tried reading papers, tried several simulations and checked out lots of articles about Keplerian elements conversion but I still can't figure out how to connect the dots. The main resources I have been studying are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://fgg-web.fgg.uni-lj.si/~/mkuhar/Pouk/SG/Seminar/Vrste_tirnic_um_Zemljinih_sat/Keplerian%20Elements%20Tutorial.html#epoch&quot; rel=&quot;noreferrer&quot;&gt;http://fgg-web.fgg.uni-lj.si/~/mkuhar/Pouk/SG/Seminar/Vrste_tirnic_um_Zemljinih_sat/Keplerian%20Elements%20Tutorial.html#epoch&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://au.mathworks.com/matlabcentral/fileexchange/54875-geostationary-satellites-tracking&quot; rel=&quot;noreferrer&quot;&gt;https://au.mathworks.com/matlabcentral/fileexchange/54875-geostationary-satellites-tracking&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://au.mathworks.com/matlabcentral/fileexchange/982-gui-based-satellite-tracking-system&quot; rel=&quot;noreferrer&quot;&gt;https://au.mathworks.com/matlabcentral/fileexchange/982-gui-based-satellite-tracking-system&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;and I also read most of the related questions on this website. Would it be easier if I used real-time tracking data instead of using TLE to predict my unknown?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks in advance!&lt;/p&gt;&#xA;" OwnerUserId="22072" LastEditorUserId="22072" LastEditDate="2017-12-17T22:31:52.810" LastActivityDate="2018-01-27T02:55:39.440" Title="Calculating longitude, latitude and altitude using RAAN, argument of perigee etc" Tags="|orbital-mechanics|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24069" PostTypeId="2" ParentId="24047" CreationDate="2017-12-17T07:47:30.897" Score="2" Body="&lt;p&gt;I'll elaborate on &lt;a href=&quot;https://space.stackexchange.com/a/24061/12102&quot;&gt;@pericynthion's answer&lt;/a&gt; a bit, with regard to what the &quot;noise&quot; may be, and speculate on the cause.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite the visual appearance in the question, a plot of apogee is not any smoother than one of perigee. Currently you show one plot with a range of 24 kilometers and the other with a range of 1,000 kilometers. Below, I've plotted the incremental changes from one TLE to the next of the perigee, apogee and average of the two as a proxy for semi-major axis (e.g. &lt;code&gt;apo[1:] - apo[:-1]&lt;/code&gt; etc. in Python). It's a little bit like the derivative except I haven't normalized to the uneven time increment from one TLE to the next.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that the semi-major axis more stable than either apo or peri, indicating that much of the &quot;noise&quot; is in the determination of the eccentricity. The third plot is the change in eccentricity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's difficult to imagine a physical process that can &quot;tickle&quot; the eccentricity in this particular way, making it sometimes less circular and then more circular immediately afterward. If it were space-plane shaped, perhaps there's a way, but these objects are not space-planes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, what I believe is happening is that the reentering spacecraft is being observed mostly from a single location on Earth. If you look at the argument of periapsis, it's extremely stable as it should be. So this one location on Earth, wherever it may be, &lt;em&gt;might&lt;/em&gt; always be seeing a similar section of the elliptical orbit. Mean motion can be extracted with high precision and independence from the other parameters from timing, but without several samples around the ellipse, the other parameters can have a &lt;a href=&quot;https://en.wikipedia.org/wiki/Correlation_and_dependence&quot; rel=&quot;nofollow noreferrer&quot;&gt;high degree of correlation&lt;/a&gt;. Correlated parameters in fitting limited data can create all kinds of errors and noise.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Incidentally you can see that the change eccentricity of PSLV-C39 has a negative offset, as well as the plot of its &lt;em&gt;perigee&lt;/em&gt; but not &lt;em&gt;apogee&lt;/em&gt;. This is characteristic of what happens to an eccentric re-entry. At first the drag impulse lowers apogee until the orbit almost circularizes. Then the constant drag lowers the orbit much more quickly, as described in &lt;a href=&quot;https://space.stackexchange.com/a/21226/12102&quot;&gt;this answer&lt;/a&gt; and illustrated in the final figure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NPeCb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NPeCb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Simple simulation of a spacecraft in elliptical orbit with low perigee. The orbit first circularizes, then decays. See &lt;a href=&quot;https://space.stackexchange.com/a/21226/12102&quot;&gt;this answer&lt;/a&gt; for a more thorough discussion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TqIXT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TqIXT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-12-17T07:47:30.897" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24070" PostTypeId="2" ParentId="24051" CreationDate="2017-12-17T16:54:48.890" Score="2" Body="&lt;p&gt;Propellants of second stage varies from UDMH to RP1. Usually propellant is selected on the basis of required characteristics of launch vehicle, its efficiency, and its ease of handling. For example ARCA is using RP1/HP due to its higher ISP. Falcon uses RP1/LOX. Whereas Indian PSLV uses N2O4/UDMH in its second stage. In comparison to this Japan's SS520 uses solid propellant.&lt;/p&gt;&#xA;" OwnerUserId="22074" LastEditorUserId="6816" LastEditDate="2017-12-17T20:44:36.613" LastActivityDate="2017-12-17T20:44:36.613" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24071" PostTypeId="1" AcceptedAnswerId="24078" CreationDate="2017-12-17T22:14:16.177" Score="1" ViewCount="1226" Body="&lt;p&gt;Quick disclaimer; I'm not that well informed on many of the mechanics on space in general so sorry if this question is rather easy/basic.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is the center of mass in a object a factor in space when there is no gravity? By gravity I mean planets or any other large body of mass that could interfere. For example if I had a rocket in space where the bottom was significantly heavier than the top and exerted force at the top would it rotate around the center of mass or the center of the object shape/volume wise?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is all assuming its an enclosed system and the rocket had no velocity prior to the addition of the force exerted.&lt;/p&gt;&#xA;" OwnerUserId="22076" LastActivityDate="2017-12-17T23:29:05.230" Title="Is centre of mass a factor in space without gravity?" Tags="|mass|rotation|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24072" PostTypeId="1" AcceptedAnswerId="24083" CreationDate="2017-12-17T22:29:15.297" Score="7" ViewCount="2334" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/List_of_crewed_spacecraft&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, ESA has never had a crewed spacecraft. While this is reasonable, it's striking that no ESA crewed spacecraft is in development right now. What's the rationale behind this decision? &lt;/p&gt;&#xA;" OwnerUserId="16049" LastEditorUserId="-1" LastEditDate="2017-12-20T21:22:38.880" LastActivityDate="2021-10-07T23:12:10.323" Title="Why has ESA never flown a crewed spacecraft?" Tags="|crewed-spaceflight|spacecraft|history|esa|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24073" PostTypeId="2" ParentId="24071" CreationDate="2017-12-17T22:33:28.023" Score="0" Body="&lt;p&gt;You have a number of misunderstandings. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First, there is no place anywhere, in space or not, where there is no gravity. Secondly, when you say &quot;the rocket has no velocity&quot; what does that mean? Velocity is a relative thing, so you can only define it relative to some other object, e.g. a planet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, Newtons laws of motion are completely independent of the location where the force happens. In other words, the presence of gravity is irrelevant. As a result, yes the centre of mass is equally important in space and on earth. Any force applied to an object will cause it to rotate, unless that force is directed through the centre of mass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In fact, this is exactly how attitude control jets work: their force does not pass through the centre of mass of the spaceship, hence they apply a force that causes it to rotate around its centre of mass. &lt;/p&gt;&#xA;" OwnerUserId="17390" LastActivityDate="2017-12-17T22:33:28.023" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24074" PostTypeId="2" ParentId="13036" CreationDate="2017-12-17T22:43:03.227" Score="2" Body="&lt;p&gt;Another problem that would rear it's ugly head if you were to try this:  Wake turbulence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your satellite will be leaving the barrel at a stupendous velocity.  It's going to slam into the atmosphere in front of the plane as it goes.  Your launching aircraft is going to slam into that turbulence.  I would be very surprised if that wasn't followed by pieces of the aircraft slamming into the ground.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And that's assuming you've already managed to overcome:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Puffin's point that you need a circularization motor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ViennaCodex's point that the railguns of today are nowhere near powerful enough.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The fact that drag is going to be brutal.  Drag goes at at least the square of velocity and the smaller the craft the higher the drag/mass ratio and thus the faster it's brought to a stop.  Realistically, you need something pretty big and heavy to hope to push it through the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's also a little problem of recoil.  Using the same plane as ViennaCodex I find a max weight of 67,000kg.  Lets figure your satellite launcher weighs 100kg and you're kicking it out at 7,000m/s.  (I'm ignoring drag here, reality is &lt;strong&gt;much&lt;/strong&gt; worse) You just jolted your plane 10 m/s.  So what, you say?  The cargo compartment is 12.5m long.  To boost it you need 200,000g (note that this is beyond even gun-rated stuff!), applied for 3.5ms.  While the gun is boosting your plane is being pushed backwards at 290g.  The railgun rips the plane apart.  You need a humongous recoil buffer to survive this--and where are you going to put it since the railgun is already as long as the plane?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, to power this.  You need 2.45GJ assuming 100% efficiency.  The best capacitors available are somewhat under 10J/cc.  I'll assume 10 as I'm having to pull a number off a graph.  You need 245 m^3 of capacitor to power this.  That's more than 3x the total cargo capacity of our C130j.  Those beefy capacitors didn't come with weight information but looking elsewhere I'm getting around 2g/cc.  Your capacitors are something like 490,000kg--about 15x the total payload of the plane.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2017-12-17T22:43:03.227" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24075" PostTypeId="1" CreationDate="2017-12-17T23:11:14.717" Score="5" ViewCount="1241" Body="&lt;p&gt;With the Block 4 version of the Falcon 9, one of the new features is the Titanium Grid fins, vs the previously used Aluminium fins. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, not all Block 4 flights have used the Ti vs Al fins. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a reason?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Check &lt;a href=&quot;https://space.stackexchange.com/questions/22096/how-are-the-spacex-falcon-9-mod-3-and-mod-4-grid-fins-different&quot;&gt;this question and answer&lt;/a&gt; for a great set of images of the two types of fins.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-12-17T23:13:04.707" Title="Why are the new Titanium Grid fins not used on all Falcon 9 landings?" Tags="|spacex|falcon-9|landing|recovery|grid-fins|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24076" PostTypeId="2" ParentId="24075" CreationDate="2017-12-17T23:13:04.707" Score="8" Body="&lt;p&gt;Musk tweeted an answer to a question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/942526513731260417&quot; rel=&quot;noreferrer&quot;&gt;Musk Tweet&lt;/a&gt; in which he wrote:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For now, we only use those on super hot reentry missions. Will go to all Ti with Falcon 9 V5, which is a few months away.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So the Block 5 (v5/mod5/Falcon 9 1.3) that is due soon enough, designed for rapid reuse will be the full time target. Until then they are using the cheaper ones, which undoubtedly they have a supply of, to use up in flights on easier landing missions. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-12-17T23:13:04.707" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24077" PostTypeId="1" CreationDate="2017-12-17T23:20:32.213" Score="2" ViewCount="59" Body="&lt;p&gt;I have just been wondering if is there any company in Europe which launch scientific experiments into an altitude of about 100 km?&lt;/p&gt;&#xA;" OwnerUserId="22077" LastEditorUserId="1054" LastEditDate="2020-12-23T22:57:39.537" LastActivityDate="2020-12-23T22:57:39.537" Title="Where can I book a flight for scientific experiment to 100 km high?" Tags="|launch|rockets|experiment|" AnswerCount="0" CommentCount="4" ClosedDate="2017-12-18T03:08:48.010" ContentLicense="CC BY-SA 4.0" />
  <row Id="24078" PostTypeId="2" ParentId="24071" CreationDate="2017-12-17T23:29:05.230" Score="1" Body="&lt;p&gt;The c.g./center of mass is the center of rotation even for a free-falling spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many spacecraft are rather symmetrical and so the difference between the c.g. and the volumetric center isn't too pronounced.  A notable exception was the shuttle orbiter, especially when its payload bay was empty.  You can see in this diagram from an old training manual how the maneuvering engines are gimbaled to point through the center of gravity, to keep the orbiter from rotating during orbital adjustment burns.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3gYLl.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3gYLl.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-12-17T23:29:05.230" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24079" PostTypeId="2" ParentId="23805" CreationDate="2017-12-18T01:50:53.763" Score="7" Body="&lt;p&gt;&lt;strong&gt;Transverse forces supine&lt;/strong&gt; +Gx &quot;Lying on your back, Eye Balls In&quot; &lt;em&gt;Recommended high acceleration position&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Transverse forces prone&lt;/strong&gt; -Gx &quot;Lying face down, Eye Balls Out&quot; &lt;em&gt;Second-best high acceleration position&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Positive longitudinal&lt;/strong&gt; +Gz &quot;Sitting with head above heart, Eye Balls Up&quot; &#xA; &lt;em&gt;Third-best high acceleration position&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Negative longitudinal&lt;/strong&gt; -Gz &quot;Standing on your head, Eye Balls Down&quot; &lt;em&gt;Worst high acceleration position&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4eePt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4eePt.jpg&quot; alt=&quot;Acceptable Acceleration Limits&quot;&gt;&lt;/a&gt;&#xA;Yellow is seconds until unconsciousness, Orange is seconds of &quot;tolerance&quot; (whatever that means), Red is seconds until injury. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, an astronaut in the recommended +Gx lying on back position would have 5 seconds of consciousness at 14g, 5 seconds of tolerance at 27g and 10 seconds until injury at 48g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chart from a 1963 Douglas report&lt;/p&gt;&#xA;" OwnerUserId="22079" LastActivityDate="2017-12-18T01:50:53.763" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24083" PostTypeId="2" ParentId="24072" CreationDate="2017-12-18T09:41:58.750" Score="12" Body="&lt;p&gt;From 1985 to 1992, ESA worked on a human space program, using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Hermes_%28spacecraft%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hermes&lt;/a&gt; spaceplane (to be launched by the Ariane 5 then in development). This program was cancelled due to rising cost and the loss of a sense of urgency: having an independent crewed spaceflight program was seen as non-critical.&#xA;Basically they had 3 expensive programs: Hermes, Ariane 5 and the Spacelab laboratory (for use on the Space Shuttle) and could afford only 2 of those.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The project was subject to numerous delays and funding issues. In 1992, Hermes was cancelled, in part due to unachievable cost and performance goals, as well as the formation of a partnership with the Russian Aviation and Space Agency (RKA), which reduced the demand for an independent crewed spaceplane.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;After that, the ISS program was built on almost world-wide cooperation in human spaceflight. ESA's crewed program supports the ISS with astronauts, training facilities, ISS modules and (a few years ago) a series of cargo flights using the ATV.&lt;/p&gt;&#xA;&lt;p&gt;ESA's long-term plans are visible in e.g. the &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Transportation/New_Technologies/FLPP_preparing_for_Europe_s_next-generation_launcher&quot; rel=&quot;nofollow noreferrer&quot;&gt;Future Launchers Preparatory Programme&lt;/a&gt;, and that does not include a crewed capability.&lt;/p&gt;&#xA;&lt;p&gt;More in general, the 2016 &amp;quot;&lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Transportation/Space_Transportation_strategy&quot; rel=&quot;nofollow noreferrer&quot;&gt;Shared Vision and Goals for the Future of Europe in Space&lt;/a&gt;&amp;quot; contained (among others) these goals, which point to uncrewed launch capability only:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;ensure availability and foster the competitiveness and reliability of Ariane, Vega and Soyuz from Europe’s Spaceport;&lt;/li&gt;&#xA;&lt;li&gt;ensure that Europe can respond to evolving market demands by developing Ariane 6 and Vega C and their ground infrastructures;&lt;/li&gt;&#xA;&lt;li&gt;prepare a future for Europe to better serve institutional and commercial markets by focusing on innovative technologies, investigating future launcher evolutions, demonstrating technical capabilities and preparing routine access to and return from space.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;As to why ESA isn't working on a human spaceflight program, I think cost is the main reason. It took years of negotiation among member states to reach agreement on the development of Ariane 6 - a development that's both relatively cheap and aimed at cutting the cost of a launch by 50%. An independent crewed program would be a bigger investment than member countries are willing to make. Participation in the ISS gets ESA most of what they want/need (access to a research lab in space).&lt;/p&gt;&#xA;&lt;p&gt;Finally, ESA is involved in NASA's crewed spaceflight program: they're building the service module for Orion, based on the ATV.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="1391" LastEditDate="2021-10-07T23:12:10.323" LastActivityDate="2021-10-07T23:12:10.323" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="24084" PostTypeId="2" ParentId="24036" CreationDate="2017-12-18T10:49:34.573" Score="4" Body="&lt;p&gt;According to &lt;a href=&quot;https://www.sciencedaily.com/releases/2010/03/100304165918.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;, the deepest crater (and the widest) is the South Pole-Aitken basin, almost 1,500 miles (2500 km) across and more than five miles (8 km) deep.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia mentions &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton_%28lunar_crater%29&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton&lt;/a&gt; at 6.1 km, but &lt;a href=&quot;http://www.spacegrant.hawaii.edu/futureflight/ffh2001/MoonFAQs/deepcrat.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;another source&lt;/a&gt; has the depth of this crater at 8.8 km. Might be different definitions of 'depth', though.&#xA;The links provided by Uwe disagree as well, providing a depth of 9864 m for an unnamed crater. These measurements are done via photo analysis, by measuring the length of shadows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The best source data seems to be the &lt;a href=&quot;https://astrogeology.usgs.gov/search/details/Moon/LRO/LOLA/Lunar_LRO_LOLA_Global_LDEM_118m_Mar2014/cub&quot; rel=&quot;nofollow noreferrer&quot;&gt;altitude measurements&lt;/a&gt; done via laser ranging by the &lt;a href=&quot;https://lola.gsfc.nasa.gov/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;LOLA&lt;/a&gt; instrument on NASA's LRO. I haven't found a summary of findings from these data though.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2017-12-18T10:49:34.573" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24086" PostTypeId="2" ParentId="24068" CreationDate="2017-12-18T15:07:58.540" Score="4" Body="&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Calculate eccentric anomaly. It can be done by solving &lt;a href=&quot;https://en.wikipedia.org/wiki/Kepler%27s_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;Kepler's equation&lt;/a&gt;. Example Python code for Newton's method:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;M   = ...           # your mean anomaly&#xA;ecc = ...           # your eccentricity&#xA;maxIter  = 15       # maximum number of iterations&#xA;maxError = 1e-11    # maximum error&#xA;&#xA;i = 0&#xA;E = M if (ecc &amp;lt; 0.8) else math.pi&#xA;F = E - ecc * math.sin(M) - M&#xA;while ((abs(F) &amp;gt; maxError) and (i &amp;lt; maxIter)):&#xA;    E = E - F / (1 - ecc * math.cos(E))&#xA;    F = E - ecc * math.sin(E) - M&#xA;    i += 1&#xA;&#xA;return E&#xA;&lt;/code&gt;&lt;/pre&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Use &lt;a href=&quot;http://ysc.sm.bmstu.ru/microsat/e-library/Ballistics/kepler.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this paper&lt;/a&gt; to calculate the cartesian position from eccentric anomaly and other elements. You'll need to rotate some vectors, you can do that with rotation matrices described &lt;a href=&quot;https://en.wikipedia.org/wiki/Rotation_matrix#Basic_rotations&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Now you need to convert your cartesian position to spherical coordinates. The Pyhton expressions are as follows:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;r   = math.sqrt(x**2 + y**2 + z**2)&#xA;lat = math.asin(z / r)&#xA;lon = math.atan2(y, x)&#xA;if lon &amp;lt; 0:&#xA;    lon += 2 * math.pi&#xA;&lt;/code&gt;&lt;/pre&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Note that &lt;code&gt;r&lt;/code&gt; here is the distance from the Earth barycenter. You should subtract Earth radius from it to calculate altitude. You can just take 6371 km as a mean value or use a more precise one from some ellipsoid models described &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth_ellipsoid&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Reference_ellipsoid&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also note that &lt;code&gt;lon&lt;/code&gt; is relative to the equinox vector, not Earth's prime meridian, so you'll need to account for Earth rotation (get current Earth rotation angle about its axis relative to the equinox vector and subtract it from your derived longitude).&lt;/p&gt;&#xA;" OwnerUserId="17295" LastEditorUserId="17295" LastEditDate="2017-12-18T15:20:21.497" LastActivityDate="2017-12-18T15:20:21.497" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24087" PostTypeId="1" AcceptedAnswerId="24158" CreationDate="2017-12-18T03:07:29.087" Score="5" ViewCount="312" Body="&lt;p&gt;When a spacecraft is built and prepared for a future launch, its parts and the whole spacecraft usually goes through a lot of testing - for example, the James Webb telescope has already been through the &lt;a href=&quot;https://phys.org/news/2017-11-james-webb-space-telescope-cryogenic.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cryogenic Testing&lt;/a&gt; under extreme cold temperatures, the &lt;a href=&quot;https://www.nasa.gov/feature/goddard/2017/nasas-james-webb-space-telescope-completes-acoustic-and-vibration-tests/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Acoustic Vibration tests&lt;/a&gt; in a special acoustic test chamber etc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All of these tests put a lot of &quot;pressure&quot;, load and stress on a spacecraft and its hardware.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does all of this come at some cost? Is not by applying stress and checking how would it perform in extreme conditions we are reducing the durability and fault tolerance of a spacecraft?&lt;/p&gt;&#xA;" OwnerUserId="12778" OwnerDisplayName="alecxe" LastEditorUserId="12778" LastEditDate="2017-12-18T16:15:27.043" LastActivityDate="2017-12-25T23:59:11.673" Title="Does stress-testing a spacecraft affect its durability?" Tags="|spacecraft|spacecraft-development|testing|development|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24088" PostTypeId="2" ParentId="24087" CreationDate="2017-12-18T05:48:02.420" Score="4" Body="&lt;p&gt;Each part of a space craft has a life span of x number of hours. Parts are changed out on space craft long before the x number of hour life span, long before failure, due to prevent any malfunction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermal Stress testing on proto-flight hardware over the temperature range of +75°C/-20°C is done for 24 hours at the cold end and 144 to 288 hours at the hot end but the hardware has many hours of life left after stress testing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thermal stress tests, coupled with rigorous design practices, provides ahigh confidence that the hardware design is not marginal during its intended long life, high reliability mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some of the tests performed by NASA can be viewed at &lt;a href=&quot;http://www.klabs.org/DEI/References/design_guidelines/nasa_reliability_preferred_practices.htm#1404&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.klabs.org/DEI/References/design_guidelines/nasa_reliability_preferred_practices.htm#1404&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerDisplayName="Brian McCarthy" LastEditorUserId="6537" LastEditDate="2017-12-18T19:48:50.627" LastActivityDate="2017-12-18T19:48:50.627" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24090" PostTypeId="1" AcceptedAnswerId="25219" CreationDate="2017-12-18T20:59:52.883" Score="13" ViewCount="675" Body="&lt;p&gt;I'd recently read Roving Mars by Steven Squyres and in it he talks about how one of the heaters in the arm of the Opportunity Rover was drawing a half-amp over night and it was severely costly to it's power usage. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Opportunity’s stuck-on heater is still sucking 170 watt-hours out of the rover every sol, and we’re starting to feel it badly.&lt;br&gt;&#xA;  &lt;sub&gt;Roving Mars: Spirit, Opportunity, and the Exploration of the Red Planet.&lt;/sub&gt; &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;They decided to implement Deep Sleep to save power overnight when the Heater is usually stuck on. The downfalls of Deep Sleep, however, is that the Rover wouldn't be able to carry out long Mössbauer and APXS measurements because the battery was off and was likely to kill Mini-TES.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Although for the rest of the book Mini-TES seemed to survive, and as far as I can tell it continues to survive (even though winter temperatures were dropping dangerously close to it's operational limits).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did they simply continue to use Deep Sleep and put Mini-TES at risk or have they found a new solution to saving power while keeping Mini-TES safe?&lt;/p&gt;&#xA;" OwnerUserId="18175" LastEditorUserId="18175" LastEditDate="2018-06-14T17:01:26.127" LastActivityDate="2018-06-14T17:01:26.127" Title="What was the solution to the half-amp the heater on Opportunity was drawing?" Tags="|mars|rovers|opportunity|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="24092" PostTypeId="2" ParentId="22980" CreationDate="2017-12-18T23:13:10.593" Score="-1" Body="&lt;p&gt;Everything about a mission its scheduled, but the re-entries of any space vehicle relies on pre-re-entire, meaning they have to do checks and re-checks, and the angle of descent its a lot more important then position. And of course there are a lot more that comes in to consideration. But in the case of the article, they might wanted for the Soyuz to go trough there.&#xA;Its almost impossible to present a danger, especially being that rare. In the situation, which the article presents, the Soyuz (with 2.4 km/s or 28,000 mph at 135 km altitude) would appear! above a commercial plane, but would land a long way away.&lt;/p&gt;&#xA;" OwnerUserId="22084" LastEditorUserId="22084" LastEditDate="2017-12-18T23:18:11.000" LastActivityDate="2017-12-18T23:18:11.000" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24093" PostTypeId="2" ParentId="18655" CreationDate="2017-12-19T00:01:43.493" Score="2" Body="&lt;p&gt;Short explanation: Treat Venus as though it rotates 224.701 times per one orbit of the Sun (instead of one rotation per 243.025 Earth days, retrograde). Alternatively make Venusian minutes longer? than &amp;quot;Earth minutes&amp;quot; so there are 24*60 Venusian minutes per day with 224.701 Earth days per Venusian year.&lt;/p&gt;&#xA;&lt;p&gt;That's an attempt to assign a reasonable period of time to each (modified) &amp;quot;Venus day&amp;quot; for the purposes of dividing each day into a reasonable number of chunks.&lt;/p&gt;&#xA;&lt;p&gt;A standard 12 or 24 &lt;em&gt;hour&lt;/em&gt; clock could be used by simply adjusting the speed that it runs at. Once 24 (modified) hours pass it would be called a &amp;quot;&lt;em&gt;day&lt;/em&gt;&amp;quot; and 224.701 of those periods would be one orbit around the Sun.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;On &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Earth&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;One year = 365.256363004 (Earth) days&lt;/li&gt;&#xA;&lt;li&gt;One day = 23 hours, 56 minutes, 4.0916 seconds&lt;/li&gt;&#xA;&lt;li&gt;One second is the duration of &lt;a href=&quot;https://www.scientificamerican.com/article/how-does-one-arrive-at-th/&quot; rel=&quot;nofollow noreferrer&quot;&gt;9,192,631,770 periods&lt;/a&gt; of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;On &lt;a href=&quot;https://en.wikipedia.org/wiki/Venus&quot; rel=&quot;nofollow noreferrer&quot;&gt;Venus&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;One year = 224.701 Earth days&lt;/li&gt;&#xA;&lt;li&gt;One day = −243.025 Earth days (retrograde)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Earth: 365.256363004 / 365 = 1.00070236439 days, or 365.256363004 /  365.256363004 = 1 day or 365 / 365.256363004 = 0.99929812857 days.&lt;/p&gt;&#xA;&lt;p&gt;Venus: 224.701 /  224.701 = 1, but 365.256363004 / 224.701 = 1.62552175114 Earth days, or 1.62552175114 * 23 hours, 56 minutes, 4.0916 seconds = 1.62108339 Earth days, alternatively 224.701 / 365.256363004 = 0.61518709257 Earth days, or 0.61518709257 * 23 hours, 56 minutes, 4.0916 second = 14.7241769 Earth hours. [Shhh, don't tell anyone at math.stackexchange.com]. No doubt there's something wrong with that math. :)&lt;/p&gt;&#xA;&lt;p&gt;So I propose that either 1.62108339 Earth days (1 day, 14 hours, 54 Earth minutes), or for the other different method 14.7241769 Earth hours, equals one Venusion day.&lt;/p&gt;&#xA;&lt;p&gt;Don't rely on wind speed, rotation, or even solely Venus' orbital period; do a mashup of the math (a mathematical abomination) to derive a reasonable number of &amp;quot;Earth hours&amp;quot; per &amp;quot;Venus day&amp;quot;, or change the duration of an hour (relative to &amp;quot;Earth hours&amp;quot;) and use &amp;quot;Venus hours&amp;quot; per &amp;quot;Venus day&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;Work it out so 224.701 Earth days (one &amp;quot;Venus year&amp;quot;) equals 224.701 * one &amp;quot;Venus day&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Rationale:&lt;/strong&gt; It is probably most useful for one orbit of the Sun to equal one year. It's not particularly useful for either 243.025 Earth days to equal a single day on Venus. Using 1.62552175114 Earth days or instead using 14.7241769 Earth hours to represent a day provides a day of reasonable length.&lt;/p&gt;&#xA;&lt;p&gt;[Spot an error in the pseudo-math it's OK to edit, as long as the means to derive a reasonable daylength isn't altered.]&lt;/p&gt;&#xA;&lt;p&gt;I'll scratch my head and take another look at this suggestion in an hour or two.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Sidereal_time&quot; rel=&quot;nofollow noreferrer&quot;&gt;Interesting background info&lt;/a&gt;:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Sidereal time is the angle, measured from the observer's meridian, along the celestial equator, to the great circle that passes through the March equinox and both poles, and is usually expressed in hours, minutes, and seconds.&lt;/p&gt;&#xA;&lt;p&gt;Common time on a typical clock measures a slightly longer cycle, accounting not only for Earth's axial rotation but also for Earth's annual revolution around the Sun of slightly less than 1 degree per day (in fact to the nearest arcsecond, it takes 365.2422 days to revolve, therefore 360 degrees/365.2422 days = 0.9856° or 59′ 8″ per day, i.e., slightly less than 1 degree per day).&lt;/p&gt;&#xA;&lt;p&gt;A mean sidereal day is 23 hours, 56 minutes, 4.0916 seconds (23.9344699 hours or 0.99726958 mean solar days), the time it takes Earth to make one rotation relative to the vernal equinox. (Due to nutation, an actual sidereal day is not quite so constant.)&lt;/p&gt;&#xA;&lt;p&gt;The vernal equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 26,000 years, so the misnamed sidereal day (&amp;quot;sidereal&amp;quot; is derived from the Latin sidus meaning &amp;quot;star&amp;quot;) is 0.0084 seconds shorter than Earth's period of rotation relative to the fixed stars.&lt;/p&gt;&#xA;&lt;p&gt;The slightly longer &amp;quot;true&amp;quot; sidereal period is called a stellar day by the International Earth Rotation and Reference Systems Service (IERS). It is also referred to as the sidereal period of rotation, or simply as the period of rotation or the rotational period.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="20766" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-19T00:22:04.380" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24094" PostTypeId="2" ParentId="22927" CreationDate="2017-12-19T00:58:48.993" Score="2" Body="&lt;p&gt;The Trump Administration's policy on Deep Space Exploration is currently defined in terms of the executive action &lt;code&gt;Space Policy Directive-1&lt;/code&gt;, signed by President Trump on 11 December, 2017. It reads:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;SUBJECT:        Reinvigorating America's Human Space Exploration Program&lt;/p&gt;&#xA;&lt;p&gt;Section 1.  Amendment to Presidential Policy Directive-4.&lt;/p&gt;&#xA;&lt;p&gt;Presidential Policy Directive-4 of June 28, 2010 (National Space Policy), is amended as follows:&lt;/p&gt;&#xA;&lt;p&gt;The paragraph beginning &amp;quot;Set far-reaching exploration milestones&amp;quot; is deleted and replaced with the following:&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;Lead an innovative and sustainable program of exploration with commercial and international partners to enable human expansion across the solar system and to bring back to Earth new knowledge and opportunities.  Beginning with missions beyond low-Earth orbit, the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations;&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;Sec. 2.  General Provisions.  (a)  Nothing in this memorandum shall be construed to impair or otherwise affect:&lt;/p&gt;&#xA;&lt;p&gt;(i)   the authority granted by law to an executive department or agency, or the head thereof; or&lt;/p&gt;&#xA;&lt;p&gt;(ii)  the functions of the Director of the Office of Management and Budget relating to budgetary, administrative, or legislative proposals.&lt;/p&gt;&#xA;&lt;p&gt;(b)  This memorandum shall be implemented consistent with applicable law and subject to the availability of appropriations.&lt;/p&gt;&#xA;&lt;p&gt;(c)  This memorandum is not intended to, and does not, create any right or benefit, substantive or procedural, enforceable at law or in equity by any party against the United States, its departments, agencies, or entities, its officers, employees, or agents, or any other person.&lt;/p&gt;&#xA;&lt;p&gt;(d)  This memorandum shall be published in the Federal Register.&lt;/p&gt;&#xA;&lt;p&gt;Donald J. Trump&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;You can find this document in the &lt;a href=&quot;https://www.federalregister.gov/documents/2017/12/14/2017-27160/reinvigorating-americas-human-space-exploration-program&quot; rel=&quot;nofollow noreferrer&quot;&gt;Federal Register&lt;/a&gt;. No other official policy by the Trump Administration relating to deep space or NASA has been enacted as of December 2017, nor has any funding been allotted to fulfil what is discussed is Directive 1.&lt;/p&gt;&#xA;&lt;p&gt;Tangential to deep space however, Trump did reestablish the existence of the National Space Council in June 2017 as per &lt;a href=&quot;https://www.gpo.gov/fdsys/pkg/FR-2017-07-07/pdf/2017-14378.pdf#page=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Executive Order 13803&lt;/a&gt;, and escaping from policy for a minute, Trump &lt;a href=&quot;https://www.theguardian.com/science/2017/apr/30/trump-mission-mars-nasa-space-flight&quot; rel=&quot;nofollow noreferrer&quot;&gt;has expressed passing interest&lt;/a&gt; in deep-space exploration:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;“Well, we want to try and do it [send humans to Mars] during my first term or, at worst, during my second term, so we’ll have to speed that up a little bit, OK?”&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-19T02:31:10.277" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24095" PostTypeId="2" ParentId="22980" CreationDate="2017-12-19T04:15:48.350" Score="2" Body="&lt;p&gt;For at least some vehicles, yes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This &lt;a href=&quot;https://sattrackcam.blogspot.com/2017/05/analysis-re-entry-of-cz-4b-rb-2014-049c.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;blog post&lt;/a&gt; discusses a re-entry event.  Although it was initially misidentified, it confirms that the second stage reentry of some Soyuz craft do get placed on a NOTAM (although perhaps with such a large window of time as to be less useful).  The author states that NOTAMs are common for launch vehicle re-entry.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;NOTAMS or Area Warnings are however generally only issued for controlled de-orbits, and first and second stage splashdowns during launches.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;A Nasaspaceflight &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=29976.msg959184#msg959184&quot; rel=&quot;nofollow noreferrer&quot;&gt;forum post&lt;/a&gt; discussed the specific NOTAM that was sent for the ATV-3 vehicle re-entry in 2012.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;B4742/12 -  DANGER AREA NZD021 (AUCKLAND OCEANIC FIR) ACT. SFC - FL999, 03 OCT 00:30 2012 UNTIL 03 OCT 03:35 2012. CREATED: 27 SEP 20:21 2012&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This one seems to be much more useful with a 3-hour window.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2017-12-19T04:15:48.350" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24096" PostTypeId="1" CreationDate="2017-12-19T04:32:21.710" Score="10" ViewCount="1067" Body="&lt;p&gt;The depictions of the BFR for earth to earth travel include the 1st stage booster. I would think if you are just going ballistic, the upper stage would have enough delta V to go it alone, for closer destinations at least. That would simplify the operation a lot. Can anyone calculate the range it would have as a single stage to not-quite-orbit passenger vehicle?&lt;/p&gt;&#xA;" OwnerUserId="16669" LastEditorUserId="51" LastEditDate="2017-12-19T11:27:23.660" LastActivityDate="2018-03-14T16:37:41.913" Title="Range of BFR as single stage" Tags="|spacex|rockets|trajectory|ballistics|spacex-starship|" AnswerCount="2" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24097" PostTypeId="2" ParentId="13409" CreationDate="2017-12-19T09:31:40.947" Score="2" Body="&lt;p&gt;We currently do not know the first date of the first commercial launch, other than that they have fast tracked their testing program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;#StillTesting&lt;/em&gt;, their 2nd test launch, &lt;a href=&quot;https://www.nbr.co.nz/article/rocket-labs-second-test-launch-carry-weather-mapping-satellites-ck-p-208114&quot; rel=&quot;nofollow noreferrer&quot;&gt;however does include a number of commercial payloads onboard, including 2 Dove satellites for Planet, and 2 Lemur-2 satellites from Spire&lt;/a&gt;, &lt;strong&gt;and launched successfully on 21 January 2018&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether this qualifies as a commercial launch is up to you — future launches may be &quot;all commercial&quot; but will absolutely include testing elements, as RocketLab models its agile methodology after bigger newspace companies such as SpaceX. So they will always be testing. They'll just be making money doing it :) &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-21T02:43:13.080" LastActivityDate="2018-01-21T02:43:13.080" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24098" PostTypeId="2" ParentId="20754" CreationDate="2017-12-19T10:16:56.207" Score="2" Body="&lt;p&gt;It is all about angular momentum.  A spinning object has angular momentum -- which can be visualized as a vector around which the object spins.  It takes a force to change that angular momentum vector and therefore the axis around which the object spins.&lt;/p&gt;&#xA;" OwnerUserId="63" LastActivityDate="2017-12-19T10:16:56.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24099" PostTypeId="2" ParentId="23976" CreationDate="2017-12-19T12:29:58.567" Score="0" Body="&lt;p&gt;It seems the traj format was a custom format created from a spice kernel just for the application. In the end, we managed to load the original spice kernels using a meta-kernel file and the spice tools making the traj file format obsolete.&lt;/p&gt;&#xA;" OwnerUserId="22000" LastActivityDate="2017-12-19T12:29:58.567" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24100" PostTypeId="1" CreationDate="2017-12-19T16:15:40.503" Score="1" ViewCount="108" Body="&lt;p&gt;I kind of feel that light is older than stars. But is there any scientific theory or hypothesis on it? &lt;/p&gt;&#xA;" OwnerUserId="22092" LastEditorUserId="58" LastEditDate="2019-05-15T13:13:40.440" LastActivityDate="2019-05-15T13:13:40.440" Title="Which one is older light or stars?" Tags="|time|stars|" AnswerCount="0" CommentCount="6" ClosedDate="2017-12-19T18:48:03.347" ContentLicense="CC BY-SA 3.0" />
  <row Id="24102" PostTypeId="2" ParentId="24051" CreationDate="2017-12-19T18:01:48.880" Score="2" Body="&lt;p&gt;Second stage fuel usage is often a historical artifact. That is, while some boosters are developed in toto (Falcon 9) others are built out of parts of other systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some are liquid fueled with anything from LH2 to solid fuels. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even a new booster, like Antares, starts with a Kerosene first stage (Started with NK-33 engines, moved onto RD-191) but then pairs it with a solid upper stage. In that case, the developer Orbital ATK was known as a solid booster house, and they effectively outsourced the first stage components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the first stage you want more efficient thrust to get off the ground, and for upper stages you care much more about efficiency to get you to orbit.  &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-12-19T18:01:48.880" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24103" PostTypeId="2" ParentId="24096" CreationDate="2017-12-19T19:31:32.163" Score="3" Body="&lt;p&gt;Let's see if this is even possible. There are &lt;a href=&quot;https://en.wikipedia.org/wiki/BFR_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;3 engines&lt;/a&gt; on the upper stage of the BFR that are rated for atmospheric. Each engine, at ground level, has about 3 MN of thrust. The fully loaded weight is about 1335 tons. Gravity is pulling on that mass at a force of about 12 MN.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, the second stage can't lift off of the ground fully fueled. It could lift off if it doesn't have a full fuel load, but that would severely limit the range.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-12-19T19:31:32.163" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24104" PostTypeId="2" ParentId="21303" CreationDate="2017-12-19T20:56:37.133" Score="1" Body="&lt;p&gt;I could not find a good live imagery of planetary positions. Also try this question in Astronomy SE. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.astrosoftware.com/cpnew/software/sirius/features/skymap.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.astrosoftware.com/cpnew/software/sirius/features/skymap.html&lt;/a&gt; works well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The examples in the question and the comments are valid ways to track the planets. I do see plausibility in converting the coordinates into a visual grid. &lt;/p&gt;&#xA;" OwnerUserId="18879" LastEditorUserId="18879" LastEditDate="2017-12-19T21:01:57.463" LastActivityDate="2017-12-19T21:01:57.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24105" PostTypeId="2" ParentId="20754" CreationDate="2017-12-19T22:29:06.423" Score="13" Body="&lt;p&gt;This answer answers the title &quot;Why does spinning help stabilisation&quot; rather than specifically for launches. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Context&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Before jumping into the answer it is worth looking at the background. With rigid bodies then adding a spin will help average out asymmetries. However neither rockets or satellites are rigid, they contain liquids and flexible appendages and these are both means of energy damping, and means by which energy can transfer from one axis of rotation to another. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The basic ideas of stability in this context (stability is a big subject, even in maths before one tries to apply it to any real object) are that objects that spin can be stable if they spin around their axes of maximum or minimum inertia. Good examples would be the central axis of a coin (like a windmill) and pencil (like a barbeque) respectively. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This doesn't hold for bodies with energy transfer modes from propellant sloshing and appendages.  A pencil like rocket spinning longitudinally can, through flexible modes, transfer all of its rotational energy into a rotation about its axis of maximum inertia. This means it will adopt an &quot;end over end&quot; motion. Please note I'm not claiming that its angular momentum has changed here.   There is a case of this really happening with an early small launch vehicle (late 50's I think).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The answer&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there are flexible modes then the only rotation that is stable will be about the axis of maximum inertial, i.e. a windmill rotation rather than barbeque mode. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Spin stabilisation is used on many satellite designs though in every case the designers have to pay close attention to the shape of the satellite so that it is behaving firmly in windmill mode, i.e. spinning around its axis of maximum inertia.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Extra detail (EDIT)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The early small launch vehicle I mentioned was the Explorer 1 /  Juno 1 mission the first US satellite. It was intended to spin at 750 rpm along the long axis but was found to be spinning at 7.5 - 8 rpm which corresponded to the energy having transferred into one of the transverse axes. Its well known enough to be a class room dynamics example. This image below shows the whip antennas which are, I believe, the physical mechanism for the energy transfer between axes. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/MjtKR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MjtKR.jpg&quot; alt=&quot;Explorer 1 from the wiki page citing this NASA page http://www.nasa.gov/mission_pages/explorer/explorer-hold_prt.htm&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-12-26T23:15:13.423" LastActivityDate="2017-12-26T23:15:13.423" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24106" PostTypeId="2" ParentId="13440" CreationDate="2017-12-19T23:04:37.723" Score="0" Body="&lt;p&gt;&lt;strong&gt;Short Answer:&lt;/strong&gt; Yes, there is an existing rocket that has separate fuel and reaction mass. As already mentioned, the &lt;a href=&quot;https://dawn.jpl.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA DAWN spacecraft&lt;/a&gt; uses solar photons as fuel and xenon gas as reaction mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Long Answer:&lt;/strong&gt; Almost &lt;em&gt;all&lt;/em&gt; rocket engines have separate fuel and reaction mass. Chemical rockets like those used by NASA, SpaceX, and Roscosmos are the rare exception.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rocket propulsion uses &lt;a href=&quot;https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Newton&amp;#39;s_3rd_Law&quot; rel=&quot;nofollow noreferrer&quot;&gt;Newton's third law&lt;/a&gt;, the law of Action and Reaction. The reaction mass is thrown at high velocity out the rocket nozzle. This is the action. The reaction of this propels the spacecraft forward.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It takes energy to throw the reaction mass out the nozzle at high velocity. This is obtained from the rocket's fuel.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;strong&gt;CHEMICAL ENGINE:&lt;/strong&gt; &lt;em&gt;fuel:&lt;/em&gt; chemical fuel and oxidizer, &lt;em&gt;rmass:&lt;/em&gt; chemical reaction by-products&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;SOLID CORE NUCLEAR THERMAL ROCKET (NERVA):&lt;/strong&gt; &lt;em&gt;fuel:&lt;/em&gt; uranium 235 in the reactor, &lt;em&gt;rmass:&lt;/em&gt; liquid hydrogen&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;SOLAR ELECTRIC ION DRIVE:&lt;/strong&gt; &lt;em&gt;fuel:&lt;/em&gt; solar energy, &lt;em&gt;rmass:&lt;/em&gt; typically xenon gas&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;NUCLEAR ELECTRIC ION DRIVE:&lt;/strong&gt; &lt;em&gt;fuel:&lt;/em&gt; uranium 235 in the reactor, &lt;em&gt;rmass:&lt;/em&gt; typically xenon gas&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;MAGNETICALLY CONFINED FUSION:&lt;/strong&gt; &lt;em&gt;fuel:&lt;/em&gt; typically deuterium and tritium, &lt;em&gt;rmass:&lt;/em&gt; sometimes the reaction by-products, sometimes injected liquid hydrogen&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;ORION NUCLEAR PULSE:&lt;/strong&gt; &lt;em&gt;fuel:&lt;/em&gt; weapons-grade plutonium in each pulse unit, &lt;em&gt;rmass:&lt;/em&gt; layer of tungsten vaporized and aimed at the pusher-plate by the nuclear shaped charge&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;GAS CORE ANTIMATTER:&lt;/strong&gt; &lt;em&gt;fuel:&lt;/em&gt; antiprotons, &lt;em&gt;remass:&lt;/em&gt; typically water&lt;/li&gt;&#xA;&lt;li&gt;&lt;strong&gt;LASER LAUNCH:&lt;/strong&gt; &lt;em&gt;fuel:&lt;/em&gt; whatever is on the other end of the electrical power grid (coal-fired plant, reactor, hydroelectric, wind farm, etc), &lt;em&gt;rmass:&lt;/em&gt; atmospheric gas or slab of solid propellant strapped to rear of rocket &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="22079" LastActivityDate="2017-12-19T23:04:37.723" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24107" PostTypeId="2" ParentId="23763" CreationDate="2017-12-20T03:40:18.980" Score="3" Body="&lt;p&gt;Some of the &lt;a href=&quot;https://en.wikipedia.org/wiki/SHARAD&quot; rel=&quot;nofollow noreferrer&quot;&gt;SHARAD&lt;/a&gt; evidence discussed in &lt;a href=&quot;https://space.stackexchange.com/a/23781/12102&quot;&gt;this answer&lt;/a&gt; is also described in Phys.org's &lt;a href=&quot;https://phys.org/news/2016-11-mars-ice-deposit-lake-superior.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mars ice deposit holds as much water as Lake Superior&lt;/a&gt;. This image shows evidence of subsurface ice at Utopia Planitia at about 47° N latitude.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/OgSOO.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/OgSOO.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;For comparison, here is some SHARAD data for Mars' northern polar ice cap, &lt;a href=&quot;https://photojournal.jpl.nasa.gov/catalog/PIA13164&quot; rel=&quot;nofollow noreferrer&quot;&gt;from here&lt;/a&gt;. Note that CO2 ice and H2O ice have very different dielectric properties and can be distinguished using SHARAD's chirped radar.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ajHuk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ajHuk.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;This image shows a cross-section of a portion of the north polar ice cap of Mars, derived from data acquired by the Mars Reconnaissance Orbiter's Shallow Radar (SHARAD), one of six instruments on the spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;The data depict the region's internal ice structure, with annotations describing different layers. The ice depicted in this graphic is approximately 2 kilometers (1.2 miles) thick and 250 kilometers (155 miles) across. White lines show reflection of the radar signal back to the spacecraft. Each line represents a place where a layer sits on top of another. Scientists study how thick the pancake-like layers are, where they bulge and how they tilt up or down to understand what the surface of the ice sheet was like in the past as each new layer was deposited.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;As far as the direct evidence of water (as ice) from Phoenix is concerned, I've found non-paywalled copies of the following papers:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Arvidson, R. E. et al. (2009): &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.581.4018&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Results from the Mars Phoenix Lander Robotic Arm experiment&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Smith, P. T. et al. (2009): &lt;a href=&quot;http://aolab.phys.dal.ca/publications/science2009a.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;H2O at the Phoenix Landing Site&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://science.sciencemag.org/content/sci/suppl/2009/07/02/325.5936.58.DC1/Smith.SOM.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Supporting Online Material&lt;/a&gt; for Smith et al.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;The TEGA mass spectroscopy data for mass = 18 AMU shown in Figure S6 of the Supporting Online Material of Smith at al. is a little complicated to interpret/explain, but the melting of ice shown in TEGA's &lt;a href=&quot;https://en.wikipedia.org/wiki/Differential_scanning_calorimetry&quot; rel=&quot;nofollow noreferrer&quot;&gt;differential scanning calorimetry&lt;/a&gt; data in Figure S9 of the data is pretty easy to grasp. Once the oven's ramping temperature reached -2°C the sample became &amp;quot;endothermic&amp;quot; and absorbed more heating power for a given change in temperature.&lt;/p&gt;&#xA;&lt;p&gt;The area under the curve is 0.35 Joules, (or 0.35/4.2 = 0.08 calories). If it were pure ice that value would correspond to only about &lt;em&gt;one milligram of water&lt;/em&gt; based on a &lt;a href=&quot;https://en.wikipedia.org/wiki/Enthalpy_of_fusion&quot; rel=&quot;nofollow noreferrer&quot;&gt;heat of fusion&lt;/a&gt; of 334 J/g.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Fig. S9. TEGA finds ice and aqueous minerals. (A) Thermal analysis for “Wicked Witch” subsurface sample, sublimation lag in the Snow White trench. The endothermic peak with an onset of -2ºC indicates the melting of water ice.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/MOlvt.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/MOlvt.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2017-12-20T06:24:24.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24108" PostTypeId="1" AcceptedAnswerId="34511" CreationDate="2017-12-20T14:03:36.317" Score="9" ViewCount="1830" Body="&lt;p&gt;After reading the really interesting question and discussion &lt;a href=&quot;https://astronomy.stackexchange.com/q/24048/7982&quot;&gt;Terrestrial Exoplanet Skies – I've Built a Visual Sky Chart. Is it Accurate?&lt;/a&gt; I was reminded that in &lt;a href=&quot;https://youtu.be/S5V7R_se1Xc?t=40m20s&quot; rel=&quot;noreferrer&quot;&gt;this video&lt;/a&gt; at &lt;code&gt;40:20&lt;/code&gt; Elon Musk mentions that at dawn and dusk Mars' sky (would appear) &lt;em&gt;blue&lt;/em&gt;, and &lt;em&gt;red&lt;/em&gt; during the middle of the day, the reverse of Earth's sky.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this true? If so, why?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-25T22:30:40.320" LastActivityDate="2021-05-22T22:51:31.863" Title="Why would Mars' sky appear blue at dawn and dusk, but red during the middle of the day (reverse of Earth)?" Tags="|mars|visualization|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24109" PostTypeId="1" CreationDate="2017-12-20T17:01:02.650" Score="18" ViewCount="3496" Body="&lt;p&gt;Elon Musk &lt;a href=&quot;https://twitter.com/elonmusk/status/943420026593337344&quot; rel=&quot;noreferrer&quot;&gt;showed a picture&lt;/a&gt; of the almost ready to fly Falcon Heavy in the HIF (Horizontal Integration Facility, aka rocket barn). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NmER2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NmER2.jpg&quot; alt=&quot;Falcon Heavy in the barn&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the darker, larger grid fins on the side cores, and the lighter smaller Aluminium fins on the center core.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan is for the two side boosters to fly back (RTLS) to LZ-1 and land on the two landing pads there.  (You could see the completed second pad in &lt;a href=&quot;https://twitter.com/DigitalGlobe/status/941814212686311424&quot; rel=&quot;noreferrer&quot;&gt;Digiglobe's tweet showing it after landing&lt;/a&gt;). What is kind of cool is to zoom in, and you can see them hosing down the booster. Commercial spy satellites are fun!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VMAkL.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VMAkL.jpg&quot; alt=&quot;Spy photo of landed CRS-13 booster and second pad&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The core booster will land downrange on the ASDS OCISLY. In theory one would expect that to be the more difficult landing than RTLS. The side boosters stage, lower and slower so should be easier?&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="9060" LastEditDate="2017-12-20T17:52:38.063" LastActivityDate="2018-02-13T21:39:09.237" Title="On the Falcon Heavy, why are the side boosters using Ti Grid fins, but not the center core?" Tags="|spacex|landing|falcon-heavy|grid-fins|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24110" PostTypeId="2" ParentId="24109" CreationDate="2017-12-20T17:03:16.617" Score="10" Body="&lt;p&gt;I know, I know, I am asking, so I can answer. But these are such great photos, and they are interesting questions...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Facebook a possible answer (Not authoritative, I get that) the following:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;It's the nose cones. They make it more unstable, so they need the increased control authority from the Ti gridfins. The center core's interstage acts like the feathers on a shuttlecock, stabilizing it, so you only need the Ti fins on it for high speed reentries. This won't be, since the payload is very light.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;You can see some good photos of the differences in size and shape between the two types in this &lt;a href=&quot;https://space.stackexchange.com/questions/22096/how-are-the-spacex-falcon-9-mod-3-and-mod-4-grid-fins-different&quot;&gt;question/answer.&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-12-20T17:48:41.437" LastActivityDate="2017-12-20T17:48:41.437" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="24111" PostTypeId="2" ParentId="24096" CreationDate="2017-12-20T18:51:46.210" Score="5" Body="&lt;p&gt;From &lt;a href=&quot;https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk AMA on r/space&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Will be starting with a full-scale Ship doing short hops of a few&#xA;  hundred kilometers altitude and lateral distance. Those are fairly&#xA;  easy on the vehicle, as no heat shield is needed, we can have a large&#xA;  amount of reserve propellant and don't need the high area ratio, deep&#xA;  space Raptor engines.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Next step will be doing orbital velocity Ship flights, which will need&#xA;  all of the above. Worth noting that BFS is capable of reaching orbit&#xA;  by itself with low payload, but having the BF Booster increases&#xA;  payload by more than an order of magnitude. Earth is the wrong planet&#xA;  for single stage to orbit. No problemo on Mars.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/&quot; rel=&quot;noreferrer&quot;&gt;https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &quot;vacuum&quot; or high area ratio Raptors can operate at full thrust at&#xA;  sea level. Not recommended.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodhsw6/&quot; rel=&quot;noreferrer&quot;&gt;https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodhsw6/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for testing purposes even longer hops should be possible if they plan orbital launches for it. Maybe they will produce &quot;for testing&quot; vac nozzles which will be safer to use at sea level?&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2017-12-20T18:51:46.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24112" PostTypeId="1" CreationDate="2017-12-20T19:22:18.877" Score="2" ViewCount="54" Body="&lt;p&gt;I was wondering why mars missions do not choose to land on the poles, as it should be safer weather-wise.&lt;/p&gt;&#xA;" OwnerUserId="22100" LastActivityDate="2017-12-20T19:22:18.877" Title="How do large dust storms impact the Martian poles?" Tags="|mars|" AnswerCount="0" CommentCount="3" ClosedDate="2017-12-23T04:42:42.770" ContentLicense="CC BY-SA 3.0" />
  <row Id="24113" PostTypeId="2" ParentId="5463" CreationDate="2017-12-20T21:49:04.867" Score="2" Body="&lt;p&gt;We do not use catapults because we don't want to destroy the payload before reaching the orbit. The immense acceleration necessary would destroy every payload build in the same way as for a rocket launch. If a very robust payload would survive the catapult, it would be destroyed by the immense heat load of orbital speed within the dense lower parts of the atmosphere.&lt;br&gt;&#xA;A reentry is possible because the orbital speed is only within the very thin upper parts of the atmosphere.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2017-12-20T21:49:04.867" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24116" PostTypeId="2" ParentId="8806" CreationDate="2017-12-21T04:47:06.293" Score="7" Body="&lt;p&gt;There are several reasons for having deep throttling in the vacuum engine but not the first stage engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(1) The first stage cannot use a large expansion bell, and couldn't even if the mechanicals allowed it, because the exhaust flow would separate from the bell. This would cause flow instability/backflow/... that would likely destroy or damage the bell, or worse. Any rocket bell's outflow pressure should match the ambient atmospheric pressure for maximum efficiency. (This does mean that any traditional rocket nozzle with best efficiency at sea level is less efficient at all higher altitudes.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(2) The first stage only needs to throttle down slightly near the end of its burn to limit payload acceleration to five or six g (I forget... 6 I think). Even without throttling, this could be accomplished by shutting down engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(3) The second stage needs to throttle down much more and earlier (percentage-wise; the burn is considerably longer) to limit payload g forces. For lighter payloads, it has to throttle down even more and earlier. Because the engine is firing in empty space, there's no limit to the size of the nozzle or concern about flow separation.&lt;/p&gt;&#xA;" OwnerUserId="22103" LastActivityDate="2017-12-21T04:47:06.293" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24117" PostTypeId="1" AcceptedAnswerId="24131" CreationDate="2017-12-21T11:08:28.470" Score="3" ViewCount="3040" Body="&lt;p&gt;There is a lot of discussion about refueling rockets in deep space. This is to be managed by electrolysis of water powered by solar cells. This requires a lot of energy so one needs a lot of solar cells. This might increase the costs of such missions dramatically. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what is actually the price for solar cells for space, i.e. the solar panels for a cubesat or a large com sat?&lt;/p&gt;&#xA;" OwnerUserId="20251" LastActivityDate="2017-12-28T23:29:21.813" Title="How much do solar cells for space cost?" Tags="|solar-power|cubesat|cost|communication-satellite|" AnswerCount="1" CommentCount="8" ClosedDate="2017-12-23T14:47:55.210" ContentLicense="CC BY-SA 3.0" />
  <row Id="24119" PostTypeId="2" ParentId="12935" CreationDate="2017-12-21T12:31:53.713" Score="2" Body="&lt;p&gt;A reason for increasing the fuel density may also be to maximize the acceleration in the early part of the flight. The time a rocket spends at low speed is very wasteful of fuel and worsens the payload to rocket weight ratio. For this reason, heavy rockets have two or more additional boosters which improve the acceleration in the first phase of the flight. Improving the power by using densified fuel helps and is mostly useful at the beginning, while the rocket is most heavy.&lt;/p&gt;&#xA;" OwnerUserId="22106" LastActivityDate="2017-12-21T12:31:53.713" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24120" PostTypeId="1" CreationDate="2017-12-21T17:11:23.870" Score="33" ViewCount="6319" Body="&lt;p&gt;NORAD has concluded from their observations of Santa that he must have some faster-than-light travel mechanism and space-time warping capabilities. However, the &lt;a href=&quot;https://sacmuseum.org/event/santa-goes-to-space/&quot; rel=&quot;noreferrer&quot;&gt;Santa Goes to Space&lt;/a&gt; event at the Strategic Air Command and Aerospace Museum seems to indicate that Santa has visited a galaxy far, far away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Even with the imaginable means of FTL travel and space-time warping, it is hard to conceive of a way Santa could have visited another galaxy and returned in any reasonable amount of time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did Santa accomplish this feat?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bonus: Why would Santa want to visit other galaxies?&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2017-12-26T07:36:53.393" Title="How does Santa travel to other galaxies?" Tags="|ftl|santa-claus|galactic|" AnswerCount="5" CommentCount="9" ClosedDate="2017-12-26T16:16:09.163" ContentLicense="CC BY-SA 3.0" />
  <row Id="24121" PostTypeId="2" ParentId="24120" CreationDate="2017-12-21T19:22:43.463" Score="43" Body="&lt;p&gt;This picture seems to imply that interstellar travel planning is enough of a headache without throwing in other galaxies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Me7il.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Me7il.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However this picture may provide a clue: perhaps the galaxy(ies) are serviced by local Santa equivalents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nKsqC.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nKsqC.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2017-12-21T19:22:43.463" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24123" PostTypeId="2" ParentId="24120" CreationDate="2017-12-22T01:06:01.890" Score="9" Body="&lt;h2&gt;Time travel&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Even with &quot;warp&quot; or any other FTL technology (including the alcubierre drive) backwards time travel is not just possible, but &lt;em&gt;necessary&lt;/em&gt;. From Santa's reference frame, millions of year surely pass each Christmas (which explains why he is so old), whereas from our reference frame, it will only be a day. It will also appear from our reference frame that Santa is bouncing forward and backwards in time delivering presents.&lt;/p&gt;&#xA;" OwnerUserId="22114" LastActivityDate="2017-12-22T01:06:01.890" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24126" PostTypeId="2" ParentId="24120" CreationDate="2017-12-22T13:27:19.727" Score="14" Body="&lt;p&gt;Looks like he had the Space Gem. This funny webcomic already spilled the secret:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3BU1y.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3BU1y.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(found at &lt;a href=&quot;http://pvponline.com/comic/2017-12-20&quot; rel=&quot;noreferrer&quot;&gt;http://pvponline.com/comic/2017-12-20&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="22117" LastEditorUserId="22126" LastEditDate="2017-12-23T16:10:05.417" LastActivityDate="2017-12-23T16:10:05.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24128" PostTypeId="2" ParentId="20754" CreationDate="2017-12-22T17:14:26.477" Score="3" Body="&lt;p&gt;It's counter-intuitive, but true as per the laws of physics, that rotating objects will be harder to change direction in the axis perpendicular to their rotation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's the same principle of the bicycle. Almost no one can balance on a stationary bicycle, but, when the wheels are spinning, it tumbles slowly enough to allow a trained human to sense it and shift their weight in time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A coin will roll on a smooth surface, but will not stay upright on its side if not rolling.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The easiest way to experience it directly is with one of those fidget spinner toys that are in fashion. It takes more muscular effort to roll one around while it's spinning.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same principle is used in rockets. A given force asymmetry will change the direction of the rocket much more slowly if the rocket is spinning. This gives the control system more time to react.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(the fact that the force came from either thrust or aerodynamics is not relevant)&lt;/p&gt;&#xA;" OwnerUserId="2765" LastEditorUserId="2765" LastEditDate="2018-01-07T18:36:24.737" LastActivityDate="2018-01-07T18:36:24.737" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24131" PostTypeId="2" ParentId="24117" CreationDate="2017-12-23T06:59:02.253" Score="5" Body="&lt;p&gt;I couldn't find it on the wayback machine, but Andrews Space (now Spaceflight Industries) used to have listed on their website a 6U form-factor solar panel for $$17,500 that produced about 20 W max power. (Link to old &lt;a href=&quot;http://www.parabolicarc.com/2014/03/27/andrews-space-completes-qualification-6u-cubesat-solar-panel&quot; rel=&quot;nofollow noreferrer&quot;&gt;press release&lt;/a&gt;) Given the number of cells per panel that's about $1,100 per cell on average, integrated into a space-qualified and acceptance-tested panel that was roughly 20 × 30 cm.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Raw cells themselves are much cheaper around 1/3 to 1/4 the cost; unfortunately I don't have any quotes that I can post.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Disclosure: I worked at Andrews Space when the above mentioned product was developed and for sale. I’m no longer there and it no longer is listed for sale on their website.&lt;/p&gt;&#xA;" OwnerUserId="127" LastEditorUserId="127" LastEditDate="2017-12-28T23:29:21.813" LastActivityDate="2017-12-28T23:29:21.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24133" PostTypeId="1" AcceptedAnswerId="24137" CreationDate="2017-12-23T09:58:23.907" Score="5" ViewCount="257" Body="&lt;p&gt;i.e., from what parts of the Earth would it have been possible to directly listen to the broadcast with &lt;strike&gt;a shortwave&lt;/strike&gt; an amateur radio receiver?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On a related note, which of NASA's tracking stations were in view?&lt;/p&gt;&#xA;" OwnerUserId="19916" LastEditorUserId="19916" LastEditDate="2017-12-24T05:27:48.097" LastActivityDate="2017-12-24T12:05:06.637" Title="What part of the Earth was visible to Apollo 8 when the Christmas message was broadcast in 1968?" Tags="|historical-data|history|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24134" PostTypeId="1" AcceptedAnswerId="24136" CreationDate="2017-12-23T14:57:07.423" Score="14" ViewCount="2612" Body="&lt;p&gt;There are several news items about the recent passing of astronaut Bruce McCandless II, and they generally refer to his work developing, and then using the &lt;a href=&quot;https://en.wikipedia.org/wiki/Manned_Maneuvering_Unit&quot; rel=&quot;nofollow noreferrer&quot;&gt;Manned Maneuvering Unit&lt;/a&gt; (MMU).&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;BBC: &lt;a href=&quot;http://www.bbc.com/news/world-us-canada-42465059&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bruce McCandless, who made first untethered space flight, dies at 80&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;NBC: &lt;a href=&quot;https://www.nbcnews.com/news/us-news/nasa-astronaut-bruce-mccandless-first-fly-untethered-space-dies-80-n832236&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA astronaut Bruce McCandless, first to fly untethered in space, dies at 80&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;NPR: &lt;a href=&quot;https://www.npr.org/2017/12/22/517135809/bruce-mccandless-first-astronaut-to-fly-untethered-in-space-has-died&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bruce McCandless, First Astronaut To Fly Untethered In Space, Has Died&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Access to the original New York Times article describing McCandless' first use of the MMU is provided in a free sample from their archive &lt;a href=&quot;https://timesmachine.nytimes.com/timesmachine/1984/02/08/231504.html?pageNumber=1&quot; rel=&quot;nofollow noreferrer&quot;&gt;TimesMachine&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is the farthest distance that an astronaut has been &lt;em&gt;free-floating in space&lt;/em&gt; from any spacecraft other than a space suit, presumably with an attached propulsion unit? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure if a &quot;how many times&quot; question should be asked separately, or if it's better to include here. I'm guessing this hasn't happened very many times.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;NASA video of McCandless: &lt;a href=&quot;https://www.youtube.com/watch?v=RvnC--JjDBw&quot; rel=&quot;nofollow noreferrer&quot;&gt;Astronaut Bruce McCandless II Floats Free in Space&lt;/a&gt;, video and much more: &lt;a href=&quot;https://www.nasa.gov/astronautprofiles/mccandless&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Remembers Astronaut Bruce McCandless II&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;This Feb. 7, 1984 photo made available by NASA shows astronaut Bruce McCandless II participating in a spacewalk a few meters away from the cabin of the Earth-orbiting space shuttle Challenger, using a nitrogen-propelled Manned Maneuvering Unit.&quot; Photo: AP. From &lt;a href=&quot;https://www.nbcnews.com/news/us-news/nasa-astronaut-bruce-mccandless-first-fly-untethered-space-dies-80-n832236&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AR5s2.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AR5s2.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;On Feb. 12, 1984, Bruce McCandless ventured away unrestrained from the safety of his spaceship, which no previous astronaut had done. He could do it because of a brand-new, jet-powered backpack.&quot; Photo: NASA. &lt;em&gt;Cropped&lt;/em&gt; from &lt;a href=&quot;https://www.npr.org/2017/12/22/517135809/bruce-mccandless-first-astronaut-to-fly-untethered-in-space-has-died&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dEqDX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dEqDX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="26446" LastEditDate="2021-07-05T03:50:44.913" LastActivityDate="2021-07-05T03:50:44.913" Title="What is the farthest that a &quot;human satellite&quot; has been from their spacecraft?" Tags="|eva|record|astronaut-propulsion|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="24135" PostTypeId="1" AcceptedAnswerId="24141" CreationDate="2017-12-23T16:00:14.197" Score="5" ViewCount="2471" Body="&lt;p&gt;For example on the Iridium-4 launch from SpaceX on 22-12-2017, 10 satellites are deployed almost at the same time. How do the satellites go into their own separate orbits after they are deployed? When they are deployed like this, don't they just stay together in the same orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the question &lt;a href=&quot;https://space.stackexchange.com/q/19807&quot;&gt;How were the 10 Iridium NEXT satellites deployed by SpaceX?&lt;/a&gt; and its answer address the mechanisms of deployment, I'm asking here about the orbits of the satellites. The deployment leaves all ten satellites in nearly identical orbits, but surely this would not be correct for a constellation providing global coverage.&lt;/p&gt;&#xA;" OwnerUserId="22128" LastEditorUserId="6816" LastEditDate="2017-12-24T05:07:44.520" LastActivityDate="2018-07-23T03:51:56.760" Title="How do multiple satellites deployed in the same launch reach their different final orbits?" Tags="|spacex|artificial-satellite|orbit|orbital-maneuver|" AnswerCount="3" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24136" PostTypeId="2" ParentId="24134" CreationDate="2017-12-23T16:03:40.463" Score="11" Body="&lt;p&gt;None of them were particularly far. The &lt;a href=&quot;http://spacefacts.de/mission/english/sts-41b.htm&quot; rel=&quot;noreferrer&quot;&gt;first flight&lt;/a&gt; shows a distance of 325 feet (99 m). The best record I can find for &lt;a href=&quot;http://spacefacts.de/mission/english/sts-41c.htm&quot; rel=&quot;noreferrer&quot;&gt;STS-41-C&lt;/a&gt; says the shuttle came within 200 feet (63 m) of the satellite, and the last was done via the MMU. and the best guess I have from STS-51-A is &lt;a href=&quot;http://spacefacts.de/mission/english/sts-51a.htm&quot; rel=&quot;noreferrer&quot;&gt;35 feet&lt;/a&gt;. The tests of SAFER on STS-64 was just around the robotic arm, so I'm assuming less then that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Best guess: 325 feet (99 m). Small chance of a further distance.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2017-12-24T02:00:06.133" LastActivityDate="2017-12-24T02:00:06.133" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24137" PostTypeId="2" ParentId="24133" CreationDate="2017-12-23T16:25:49.123" Score="8" Body="&lt;p&gt;The reading began at 03 14 06 56 of the mission, which is to say, 3 days, 14 hours, and almost 7 minutes after launch. They launched at 12:51 UTC on Dec 21, 1968 (just before 8 a.m. at Cape Canaveral). So, doing the conversion, that was 2 minutes before 10 p.m. on the east coast of the US, and just before 7 p.m. on the west coast. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://spacelog.org/&quot; rel=&quot;noreferrer&quot;&gt;Spacelog.org&lt;/a&gt; has the full transcript of the mission in searchable and linkable format. The reading occurred &lt;a href=&quot;http://apollo8.spacelog.org/03:14:04:53/#log-line-309893&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;https://www.timeanddate.com/worldclock/sunearth.html?month=12&amp;amp;day=25&amp;amp;year=1968&amp;amp;hour=2&amp;amp;min=57&amp;amp;sec=0&amp;amp;n=&amp;amp;ntxt=&amp;amp;earth=0&quot; rel=&quot;noreferrer&quot;&gt;Day and Night World Map of that moment&lt;/a&gt;, the Moon was directly over Hawaii. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EY0qc.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EY0qc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the proper equipment, someone in the Americas or on the Pacific Rim could have listened in. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2017-12-23T16:25:49.123" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24138" PostTypeId="1" AcceptedAnswerId="24142" CreationDate="2017-12-23T17:24:47.243" Score="16" ViewCount="8065" Body="&lt;p&gt;Following the links in &lt;a href=&quot;https://space.stackexchange.com/questions/24133/#comment69169_24133&quot;&gt;this comment&lt;/a&gt; by @Uwe I've found some interesting sources of information on Ham (amateur) radio operators trying to listen in on Apollo transmissions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the bottom of Sven Grahn's &lt;a href=&quot;http://www.svengrahn.pp.se/trackind/Apollo17/APOLLO17.htm&quot; rel=&quot;noreferrer&quot;&gt;Tracking Apollo-17 from Florida&lt;/a&gt; there is a table of time points and events. Near the bottom is an item for December 12, 1972 at 22:12 UT. &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;2212    Perfect voice: &quot;The barber pole - it is gray&quot;.:&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;It links to &lt;a href=&quot;http://www.svengrahn.pp.se/sounds/AP17SB.mp3&quot; rel=&quot;noreferrer&quot;&gt;this audio recording&lt;/a&gt; of the Apollo-17 transmission received by the amateur station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is a barber pole, and what did &quot;it is gray&quot; mean?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="195" LastEditDate="2017-12-24T03:37:07.527" LastActivityDate="2017-12-24T21:58:01.017" Title="Apollo 17; what is a barber pole, and what did &quot;it is gray&quot; mean?" Tags="|spacecraft|apollo-program|communication|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24139" PostTypeId="2" ParentId="24138" CreationDate="2017-12-23T17:33:03.443" Score="12" Body="&lt;p&gt;I don't know specifically which one, but I'd lay large odds the quote refers to a &quot;talkback&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Shuttle and Apollo used electromechanical &quot;talkbacks&quot; to provide status indications on the control panels.  The talkbacks showed various statuses like ON, OFF, OP, CL, sometimes just gray, etc.  If the signals driving them were lost, or in a conflicting state, the talkback would show &quot;barberpole&quot; - a pattern of diagonal black and white stripes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A gray talkback indicated nominal operating status.  Here's a fuel cell procedure from the &lt;a href=&quot;https://www.nasa.gov/centers/johnson/pdf/359853main_OPCL_G_M_10.pdf&quot; rel=&quot;noreferrer&quot;&gt;Orbit Pocket Checklist&lt;/a&gt; asking the crew to check that some talkbacks (tb) are gray or barberpole (bp).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/L5MTf.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/L5MTf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's are some pictures I took in an orbiter in the OPF a while back. You can see various talkback statuses including a barberpole talkback on the Emergency Oxygen isolation valve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WEsGU.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WEsGU.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/E88M8.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/E88M8.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-12-23T17:45:06.867" LastActivityDate="2017-12-23T17:45:06.867" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24140" PostTypeId="1" AcceptedAnswerId="24144" CreationDate="2017-12-23T17:56:02.370" Score="6" ViewCount="1096" Body="&lt;p&gt;Following the links in &lt;a href=&quot;https://space.stackexchange.com/questions/24133/#comment69169_24133&quot;&gt;this comment&lt;/a&gt; by @Uwe I've found some interesting sources of information on Ham (amateur) radio operators trying to listen in on Apollo transmissions. One of them is Sven Grahn's &lt;a href=&quot;http://www.svengrahn.pp.se/trackind/Apollo17/APOLLO17.htm&quot; rel=&quot;noreferrer&quot;&gt;Tracking Apollo-17 from Florida&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Ham's setting up a 9 meter dish to receive signals from the Moon, and a doppler shift measurement (offset) of the received signal at around 2287.5 MHz as the spacecraft orbited the near side of the Moon. From &lt;a href=&quot;http://www.svengrahn.pp.se/trackind/Apollo17/APOLLO17.htm&quot; rel=&quot;noreferrer&quot;&gt;Tracking Apollo-17 from Florida&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like there will be at least 50 kHz of doppler shift for one orbit of the near hemisphere of the moon, eyeballing the graph.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;50 kHz divided by 2287.5 MHz is about 22.9 ppm. Multiply that by the speed of light and I get a velocity change of 6557 m/s. Using a GM_moon of 4.905E+12 m^3/s^2 and an altitude of 60km, I esitmate the orbital speed of only 1650 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That makes the estimate of the change in line-of-sight velocity &lt;em&gt;four times larger&lt;/em&gt; than the orbital velocity. I can understand twice, since it changes from coming towards to going away. But not four times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is the explanation for such a large doppler shift?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Images from Sven Grahn's &lt;a href=&quot;http://www.svengrahn.pp.se/trackind/Apollo17/APOLLO17.htm&quot; rel=&quot;noreferrer&quot;&gt;Tracking Apollo-17 from Florida&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PeJWO.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PeJWO.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1yjjP.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1yjjP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/j9oH1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/j9oH1.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2017-12-23T19:04:28.257" Title="Why such a large observed doppler shift from Apollo 17 orbiting the moon?" Tags="|apollo-program|radio-communication|doppler-ranging|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24141" PostTypeId="2" ParentId="24135" CreationDate="2017-12-23T18:01:26.537" Score="5" Body="&lt;p&gt;According to a reply to the the same question on &lt;a href=&quot;https://youtu.be/wtdjCwo6d3Q&quot; rel=&quot;noreferrer&quot;&gt;youtube&lt;/a&gt; by &quot;How does it really work&quot;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&quot;They will all remain in the same circular orbit, but they will spread out all around it to very accurately controlled positions. To do that, they have 8 tiny engines and enough fuel for 15 years. (141 kg of fuel each.)&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I found some additional information &lt;a href=&quot;https://www.eumetsat.int/website/home/Satellites/LaunchesandOrbits/SatelliteOrbits/Satellitemanoeuvres/index.html&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22128" LastEditorUserId="22128" LastEditDate="2017-12-23T18:11:47.330" LastActivityDate="2017-12-23T18:11:47.330" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24142" PostTypeId="2" ParentId="24138" CreationDate="2017-12-23T18:21:29.287" Score="23" Body="&lt;p&gt;Here's a closeup of some Apollo CM switches and talkbacks from the &lt;a href=&quot;https://history.nasa.gov/afj/ap15fj/02earth_orbit_tli.html&quot; rel=&quot;noreferrer&quot;&gt;Apollo 15 flight journal&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NlFie.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NlFie.jpg&quot; alt=&quot;Apollo CM panel switches and talkbacks, highlighted&quot;&gt;&lt;/a&gt;&#xA;From that page:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The &quot;gray talkback&quot; referred to by Dave Scott is an example of the indicators which were mounted around some of the instrument panels. Each talkback consisted of a small window with a black and white stripe pattern behind it - the &quot;barber pole&quot; often referred to during the mission. A gray flag could move in front of the barber pole stripes to indicate the status of a particular system. The gray flag would be driven by a control signal from the system in question and it told the crew what was going on in that system, therefore the indicator was called a talkback. Gray was essentially a normal or &quot;doing nothing&quot; indication. A barber pole in the talkback usually meant an abnormal or transient status.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This photograph, kindly supplied by Bruce Yarbro, is of the RCS talkbacks in the Apollo 13 Command Module, Odyssey. The two talkbacks to the left are for the Command Module's RCS system, which will not be activated until re-entry. The four talkbacks to the right show half-barber pole in this photo and are the same as those in question.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;[A switch is highlighted in purple; that switch's corresponding talkback, currently gray, is highlighted in orange. Another talkback showing &quot;half barberpole&quot; is highlighted in blue.]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_17&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia gives the mission start time as December 7, 1972, 05:33:00 UTC&lt;/a&gt;, so December 12 22:12 UTC should be 5 days, 16 hours, 39 minutes into the mission (or 136 hours, 39 minutes -- some records break out the days, some don't). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I went looking for that particular call in the Apollo 17 transcripts -- no joy on the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a17/AS17_CM.PDF&quot; rel=&quot;noreferrer&quot;&gt;Command Module onboard recorder transcript&lt;/a&gt;, but the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a17/AS17_TEC.PDF&quot; rel=&quot;noreferrer&quot;&gt;air-to-ground&lt;/a&gt; recording has it at 5:16:37, I think:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;(CMP is Command Module Pilot Evans; CC is Capcom on Earth)&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;05:16:35:02&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CMP: MARK it. OFF.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CC: Roger. We got it.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CMP: ...wherever that is.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CMP: Okay. RECORDER is going OFF, not the HEATERS. RADAR is OFF. DATA &#xA;  SYSTEM is ON. Uh-oh. Should have the SM/AC POWER switch up - up there (laughter) at 230, as much as we use it.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CC: Roger. I concur with that one. I thought it would only be in the simulation that it would get to you on that one.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CMP†: (Laughter) Yes. Okay. SM/AC POWER is ON. The old MAPPER's going to STANDBY. IR is going ON. SELF TEST is going to HEATERS. UV is going ON. Now, we're going to open IR and we're going to wait on UV.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;05:16:37:19 &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CC: We concur with that, Ron.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CMP: Okay.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CC: We'll give you a cue on that UV COVER, OPEN, here.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;05:16:37:33&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;CMP: Okay, there's the IR. &lt;strong&gt;Barber pole. And a gray.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;† Incorrectly attributed to CDR; this exchange occurred while the LM was on the surface so it seems unlikely the commander would have jumped into the checklist. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So our ham operator's watch may have been off by a minute or two. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These operations seem to be in reference to &lt;a href=&quot;https://www.lpi.usra.edu/lunar/missions/apollo/apollo_17/experiments/&quot; rel=&quot;noreferrer&quot;&gt;scientific instruments carried in the service module SIM bay&lt;/a&gt;, including an ultraviolet (UV) spectrometer to study the moon's (extremely tenuous) atmosphere and an infrared (IR) radiometer to study the cooling of the moon's surface soil at night. &quot;Mapper&quot; is probably the laser altimeter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a number of calls of the form &quot;[some control], barberpole... gray&quot; where a crew member is throwing a switch, observing the &quot;in-progress&quot; barberpole indicator, then reporting the operation completed, e.g. this one where Evans is operating the docking probe circuit breaker at 01:10:20 MET:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;CMP: DOCKING PROBE, EXTEND/RELEASE; EXTEND/RELEASE until probe extension. Okay, I'm going to hit it real lightly .... barber pole and gray. I hear a - I thought it went bump, didn't it?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's also a lot of &quot;barberpole plus three&quot; or &quot;plus four&quot; in relation to camera settings; in those cases each throw of the switch would advance one step through a cycle of possible settings, one of which would show barberpole so that the operator would know where in the cycle they were. &quot;Barberpole plus three&quot; would mean toggling the switch as many times as necessary until barberpole appeared, then toggling three more times. (h/t @hobbs for that clarification.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2017-12-24T21:58:01.017" LastActivityDate="2017-12-24T21:58:01.017" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24143" PostTypeId="2" ParentId="24044" CreationDate="2017-12-23T18:48:04.733" Score="2" Body="&lt;p&gt;By searching &quot;ISS food intake&quot; on Google, I found this, which mostly answers you question: &lt;a href=&quot;https://www.nasa.gov/vision/earth/everydaylife/jamestown-needs-fs.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/vision/earth/everydaylife/jamestown-needs-fs.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22130" LastActivityDate="2017-12-23T18:48:04.733" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24144" PostTypeId="2" ParentId="24140" CreationDate="2017-12-23T19:04:28.257" Score="5" Body="&lt;p&gt;From your link: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;If the orbit had been perfectly circular at the 128.2 minute period the doppler shift for a simple transmitter would have been = 2287.5 x 1000 x 1.58/300000= ± 12 kHz. &lt;strong&gt;For a coherent transponder the doppler shift would be almost double this number (doppler shift on both uplink and downlink), i.e. 46 kHz.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-12-23T19:04:28.257" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24145" PostTypeId="2" ParentId="24120" CreationDate="2017-12-23T23:27:18.673" Score="5" Body="&lt;p&gt;In the Start Trek universe (happened in Star Trek Voyager), Santa could break the transwarp barrier to visit other galaxies:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Achieving warp 10 would allow a person to be in every point in the&#xA;  universe, simultaneously.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://memory-alpha.wikia.com/wiki/Threshold_(episode)&quot; rel=&quot;noreferrer&quot;&gt;Threshold (episode)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are some side effects though like accelerated form of natural evolution. This side effect could be an answer to your bonus question, he might look to becoming the ultimate human being. This could also explain the depictions of a Santa with four arms in Organic Marble's post. Not sure about the time travel implications though.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Food for thought.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: Apparently Darren H beat me to it 7 hours ago with this theory, sorry I didn't notice it. I hope my explanation to the reasons is some kind of contribution.&lt;/p&gt;&#xA;" OwnerUserId="22133" LastEditorUserId="22133" LastEditDate="2017-12-23T23:34:37.447" LastActivityDate="2017-12-23T23:34:37.447" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24146" PostTypeId="1" CreationDate="2017-12-23T23:59:54.530" Score="7" ViewCount="1049" Body="&lt;p&gt;The Falcon 9 stages, by themselves, are usually stored on rotisserie style mounts, that allow the stage to be rotated and worked on easily.  That is, if the engineer needs access to the top, instead of bringing a ladder or scaffolding over, and possibly dropping something on the stage, they roll the stage till the components are at the bottom. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the mount, on a the Orbcomm stage, in the factory, with the engine install rig (The double ring of metal) being pulled back.  Look just north of the engines, there is an aluminum ring around the stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RPbUx.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RPbUx.jpg&quot; alt=&quot;Orbcomm stage in the factory&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here you can see three recovered stages with the ring in site and in various states of engine removal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IeHyd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IeHyd.jpg&quot; alt=&quot;Three recovered cores, some engines removed&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is another recovered stage, where you can see the rings at both ends.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nw3kd.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nw3kd.jpg&quot; alt=&quot;F9 flight 21 in LC-39A&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a shot where you can see the ring from above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3GR0i.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3GR0i.jpg&quot; alt=&quot;F9 in hangar&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see the huge amount of scaffolding they had to deploy for the Shuttle to work in the payload bay, leading to much slower turn around times.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fm0k2.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fm0k2.jpg&quot; alt=&quot;Shuttle Atlantis in processing facility&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1p9JH.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1p9JH.jpg&quot; alt=&quot;Endevour in the processing facility&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Saturn V had custom scaffolding in the VAB as well. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iXEbD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iXEbD.jpg&quot; alt=&quot;Saturn V in VAB&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the Falcon Heavy, at some point they have to attach the three cores.  Do they attach the cores on the rotisserie stands, or do they wait, bring in the TEL and assemble them together on the TEL itself? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2017-12-25T04:04:57.097" LastActivityDate="2021-09-18T05:12:38.000" Title="Do they assemble the Falcon Heavy on the TEL?" Tags="|spacex|falcon-heavy|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24147" PostTypeId="2" ParentId="24146" CreationDate="2017-12-24T00:04:16.290" Score="6" Body="&lt;p&gt;Elon Musk &lt;a href=&quot;https://twitter.com/elonmusk/status/943420026593337344&quot; rel=&quot;noreferrer&quot;&gt;tweeted photos of the connected cores&lt;/a&gt;, in the HIF, and at the same time, the TEL was still reportadly on the launch pad. Since there is but a single TEL it is obviously NOT assembled initially on the TEL.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2017-12-24T00:04:16.290" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24149" PostTypeId="1" AcceptedAnswerId="37276" CreationDate="2017-12-24T06:37:08.223" Score="5" ViewCount="236" Body="&lt;p&gt;Phys.org's &lt;a href=&quot;https://phys.org/news/2017-12-esa-satellite-propelled-butane.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA's next satellite propelled by butane&lt;/a&gt; describes GomX-4B's butane powered cold gas thrusters, and how it will test them and use them to navigate to different distances from its partner GomX-4A. See &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/gomx-4.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt;. Also &lt;a href=&quot;http://www.esa.int//Our_Activities/Space_Engineering_Technology/ESA_s_next_satellite_propelled_by_butane&quot; rel=&quot;nofollow noreferrer&quot;&gt;see ESA&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image below is shown in the article. If I understand correctly this represents one sixth of the 6U spacecraft - or 2x0.5 U at one end. It's said that there are four thrusters, and one pair will be used primarily with the other pair as backup.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Where will the thrusters be located? Can they vector, or are they fixed wrt the spacecraft's center of mass? How will the spacecraft ensure controlled, directed thrust without picking up spin around any axis, using only two fixed thrusters? Are there reaction wheels for this, and other types of thrusters for unloading of accumulated angular momentum?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;Thruster chip for the GomX-4B CubeSat’s propulsion system, designed by Nanospace in Sweden. Elements such as flow channels and sensors, chamber and nozzle are fitted into a 1x2 cm chip, just 1 mm thick by using microelectromechanical systems technology, otherwise known as MEMS. In terrestrial terms, MEMS is already a very mature technology platform: there are such devices all around us, in our cellphones, watches and cars.&quot; From &lt;a href=&quot;http://www.esa.int/spaceinimages/Images/2017/12/Thruster_chip&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Open in new window for full size:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/XGD4d.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/XGD4dm.jpg&quot; alt=&quot;Butane Thruster Chip&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Cropped and reduced from &lt;a href=&quot;https://phys.org/news/2017-12-esa-satellite-propelled-butane.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. Open in new window for larger size.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;GomX-4B’s cold-gas thruster system takes up two half-CubeSat units at one side of the nanosatellite, with two spherical titanium tanks filled with liquid butane. It has four 1 mN thrusters, typically to be fired in pairs while keeping one set in reserve. Credit: Nanospace&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/v1C5s.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/v1C5s.jpg&quot; alt=&quot;GomX-4B Butane Thruster System&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-01T10:37:46.123" LastActivityDate="2019-07-12T05:12:52.747" Title="How will GomX-4B manage angular momentum when using butane thrusters to move?" Tags="|engines|attitude-determination-and-control|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24150" PostTypeId="2" ParentId="24133" CreationDate="2017-12-24T09:33:38.970" Score="4" Body="&lt;p&gt;The other answers show in more visible terms but there is a more direct way of seeing who could have heard it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remember that radio and light are both electromagnetic waves and we have a direct picture of where light would be visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bill Anders's photograph&#xA;&lt;a href=&quot;https://i.stack.imgur.com/LWtXP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LWtXP.jpg&quot; alt=&quot;Earthrise&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;from &lt;a href=&quot;https://www.nasa.gov/centers/johnson/home/earthrise.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/centers/johnson/home/earthrise.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OK not exactly when the verse was given. I wrote this based on my memory and thought this was taken at the exact time but it was worth getting this photo in :)&lt;/p&gt;&#xA;" OwnerUserId="1282" LastEditorUserId="1282" LastEditDate="2017-12-24T12:05:06.637" LastActivityDate="2017-12-24T12:05:06.637" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24154" PostTypeId="1" AcceptedAnswerId="24161" CreationDate="2017-12-25T11:36:44.470" Score="5" ViewCount="3080" Body="&lt;p&gt;Almost all space settlement designs featuring artificial gravity by centrifugal force use the inner side of the outer part of the torus:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/03zWX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/03zWX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is it that I see hardly any cylindrical-shaped artificial gravity habitation areas like this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/lhC7m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/lhC7m.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Isn't the surface area under 1g of centrifugal acceleration for cylinders more than that of tori? Won't tori have variable fluctuations of gravity?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If my point is valid, then why do people suggest tori for space settlement designs?&lt;/p&gt;&#xA;" OwnerDisplayName="user20905" LastActivityDate="2018-12-10T12:21:19.927" Title="Why do space settlement designers use tori instead of cylinders for space settlements?" Tags="|artificial-gravity|habitat|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24155" PostTypeId="2" ParentId="24120" CreationDate="2017-12-25T11:44:12.513" Score="4" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Bonus: Why would Santa want to visit other galaxies?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I can't imagine all other galaxies in the universe are devoid of children, nor can I imagine that all children of other galaxies are naughty. There must be nice children in other galaxies who deserve Christmas presents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now pure speculation leads me to suggest that it's possible, even likely, that there are galaxies where the cookie ingredients available are much more flavorful and offer amazing flavors not available in the Milky Way. For all we know, the Milky Way could be Santa's &lt;em&gt;least&lt;/em&gt; favorite galaxy to visit, and right now there is a question on the Andromedan Stack Exchange asking why Santa would ever visit the Milky Way.&lt;/p&gt;&#xA;" OwnerUserId="11228" LastActivityDate="2017-12-25T11:44:12.513" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24156" PostTypeId="1" CreationDate="2017-12-25T14:43:57.407" Score="6" ViewCount="574" Body="&lt;p&gt;The upper stage of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Juno_I&quot; rel=&quot;nofollow noreferrer&quot;&gt;Juno 1 rocket&lt;/a&gt; was spun up to a high speed prior to launch so it would be spin stabilized and not require a guidance system. After the upper stage separated from the 1st stage it coasted to apoapsis, oriented itself to point at the horizon with air jets, and then ignited to put the Explorer satellite in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How did the upper stage orient itself to point at the horizon while it was spinning? Wouldn't the spinning upper stage be very resistant to changes in its orientation? Were the air jets fired in such a way as to take advantage of the gyroscopic nature of the spinning upper stage to make orienting it easier?&lt;/p&gt;&#xA;" OwnerUserId="11820" LastEditorUserId="12102" LastEditDate="2017-12-25T17:00:40.740" LastActivityDate="2017-12-27T11:22:24.477" Title="How did the upper stage of the Juno 1 rocket change its orientation while it was spinning?" Tags="|juno|guidance|spin-stabilisation|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24158" PostTypeId="2" ParentId="24087" CreationDate="2017-12-25T23:59:11.673" Score="5" Body="&lt;p&gt;The simple answer to &quot;Does stress-testing a spacecraft affect its durability?&quot; is Yes, it does, in the classical western view of test programmes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can't answer for the James Webb Telescope specifics but a common approach is to divide a test programme conceptually into design verification and workmanship verification.   As an aside, the design verification tests are called the &quot;qualification&quot; tests where as the workmanship verification tests are also known as &quot;acceptance&quot; tests. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Qualification tests are aimed at showing that the system works at some given range of environment and can survive at some greater range. These will all be the expected mission environment (temperature, radiation, vibration plus some margin, all for some given length of time). Acceptance tests are similar but with lower margins.   The usual philosophy is that a flight model can not be subjected to qualification levels and so a special qualification model has to be built for the qual tests.   This fits the usual serial production well, the qual model is tested hard but never flies, the flight models only see acceptance testing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the James Webb case the production is just one satellite and there may not be a qualification model at satellite level. In this case sometimes the mission management agree with their customer that the flight model will be tested to qualification levels of temperature and vibration stress but only for durations consistent with acceptance testing. &lt;strong&gt;This is called &quot;proto-flight&quot; testing and it explicitly recognises that testing affects durability.&lt;/strong&gt;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside I often think that protoflight testing is a bit of a fudge that both undertests for qualification purposes and over tests for acceptance (workmanship purposes), but that's just my tuppence.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2017-12-25T23:59:11.673" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24160" PostTypeId="1" CreationDate="2017-12-26T13:45:00.743" Score="1" ViewCount="242" Body="&lt;p&gt;Is it a feasible method of obtaining different gases - such as nitrogen and argon - in space to form the composition of a normal atmosphere in the space settlement?&lt;/p&gt;&#xA;" OwnerUserId="22158" LastEditorUserId="1054" LastEditDate="2017-12-27T14:49:21.917" LastActivityDate="2017-12-27T14:49:21.917" Title="Can fractional distillation of pure air take place in a space settlement?" Tags="|space-station|" AnswerCount="1" CommentCount="5" ClosedDate="2017-12-31T20:52:48.340" ContentLicense="CC BY-SA 3.0" />
  <row Id="24161" PostTypeId="2" ParentId="24154" CreationDate="2017-12-26T14:26:29.313" Score="6" Body="&lt;p&gt;Despite the obvious reason - a torus looks much more elegant than a barrel - I can come up with several points that favor a toroid:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Rounded shapes can withstand the pressure difference between inside and outside much more easily. Large areas with sharp corners like at the end caps of the cylinder are difficult to make really tight. The parts of the ISS are actually cylindrical, but their diameter is tiny compared to that of a rotating station.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Usually a fixed, easy to access, non-rotating part of the station is needed for thrusters, docks, experiments in micro gravity. This comes naturally with a torus, but is harder to build with a cylindrical shape.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;To get the same inhabitable space, the radius of a torus is larger than the one of a cylinder. This makes the toroid rotate more slowly which helps with joints and traversing between fixed and rotating parts. Also, it reduces the curvature of the floor.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You mention the floor area at precisely 1g is larger in a cylinder. That's true, but there is no actual reason that the same acceleration is needed everywhere. Even if it varies by 10% throughout the station, I doubt that this can be felt by any of the astronauts. In your example double-toroid the inner ring has a reduced gravity, but this might even be a good thing, e.g. to have storage places where heavy objects can be moved more easily.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12024" LastActivityDate="2017-12-26T14:26:29.313" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24162" PostTypeId="1" CreationDate="2017-12-26T15:13:36.037" Score="6" ViewCount="470" Body="&lt;p&gt;The Cypress PSoC line of microcontrollers, especially the PSOC 4 family are very versatile and capable, and seem a natural for use in cubesats.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, at least one experiment: &lt;a href=&quot;http://www.dtic.mil/get-tr-doc/pdf?AD=ADA616531&quot; rel=&quot;noreferrer&quot;&gt;http://www.dtic.mil/get-tr-doc/pdf?AD=ADA616531&lt;/a&gt; showed a PSOC failing immediately in a radiation environment.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, that article has several problems:  1) it doesn't identify the model of the PSOC (and from the image it appears to be an older board - I can't find any info on it, and the manufacturer of the board no longer has it on their website), 2) they only ran one test on one board before rejecting it, so it's not clear if it was a fluke, and 3) they don't specify whether they shielded the other components on the board, as they did for the MSP430.  Finally, the report is now 2 years old, and several new PSOC chips have come out since then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Several cubesat proposals seem to plan on using PSOC chips, but it is not clear to me if these proposals were ever built or launched.  For example: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://2017.qcrypt.net/wp-content/uploads/2017/09/We418.pdf&quot; rel=&quot;noreferrer&quot;&gt;http://2017.qcrypt.net/wp-content/uploads/2017/09/We418.pdf&lt;/a&gt;&#xA;&lt;a href=&quot;https://digitalcommons.usu.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&amp;amp;httpsredir=1&amp;amp;article=3229&amp;amp;context=smallsat&quot; rel=&quot;noreferrer&quot;&gt;https://digitalcommons.usu.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&amp;amp;httpsredir=1&amp;amp;article=3229&amp;amp;context=smallsat&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's obviously not a rad-hard chip, but neither are the Atmel AVRs, or several other off-the-shelf microcontrollers that have been used successfully in short-duration cubesat missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am wondering if anyone knows of more complete and/or more recent data on how the PSOC, especially the PSOC 4 FAMILY, is likely to behave in a space environment.  I would be especially interested in any cases where the chips were actually flown as part of a space mission.&lt;/p&gt;&#xA;" OwnerUserId="21469" LastActivityDate="2018-08-10T20:10:38.047" Title="Cypress PSOC chips in a Cubesat" Tags="|radiation|cubesat|flight-computer|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24164" PostTypeId="2" ParentId="24160" CreationDate="2017-12-26T16:58:13.313" Score="3" Body="&lt;p&gt;Fractional distillation requires a way to separate liquids from gases. The easiest way to do this is with gravity (with hot gases rising, and liquids falling to collect). In freefall, you'd get bubbles of liquid mixed in with the gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the ISS, a distillation unit is part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/ISS_ECLSS#Water_recovery_systems&quot; rel=&quot;nofollow noreferrer&quot;&gt;Environmental Control and Life Support System&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Urine Processor Assembly uses a low pressure vacuum distillation process that uses a centrifuge to compensate for the lack of gravity and thus aid in separating liquids and gasses.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2017-12-26T19:23:32.317" LastActivityDate="2017-12-26T19:23:32.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24165" PostTypeId="2" ParentId="24108" CreationDate="2017-12-26T20:43:15.233" Score="1" Body="&lt;p&gt;The Martian atmosphere is too thin. When the sun is high in the Martian sky the thin atmosphere and suspended dust particles make the sky red. As the sun 'sets'the light must penetrate more atmosphere and this is when all colors but the color blue are absorbed. Hence a blue sunset / sunrise.&lt;/p&gt;&#xA;" OwnerUserId="19727" LastActivityDate="2017-12-26T20:43:15.233" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24166" PostTypeId="2" ParentId="24156" CreationDate="2017-12-26T23:16:57.687" Score="1" Body="&lt;p&gt;This answer addresses the part of the question: &quot;&lt;em&gt;How did the upper stage orient itself to point at the horizon while it was spinning? Wouldn't the spinning upper stage be very resistant to changes in its orientation?&lt;/em&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This manoeuvre is called precession of the spin axis. You are right that the spin stabilisation does exactly that, it makes the spin axis more stable to small perturbations, however that doesn't mean that its not viable to do such a manoeuvre.   Off the top of my head I couldn't say how much more it would cost in propellant though obviously it needs careful phasing from the thruster driving the precession so that it only fires at the right time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;[&lt;strong&gt;EDIT 1&lt;/strong&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a typical precession manoeuvre (not necessarily that of Explorer 1, works like this, picture this concept:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;i) a radially mounted thruster that does not point through the Centre of Mass and is firing continually - there will be no precession, &lt;/p&gt;&#xA;&#xA;&lt;p&gt;then imagine &lt;/p&gt;&#xA;&#xA;&lt;p&gt;ii) that it is synchronised only to fire when pointing in one direction - now there will be a turning force (though the reaction has to be consistent with gyroscopic behaviour, my memory is rusty on this). As it will only fire for a small part of the spin period the manoeuvre will take longer than for a non spinning body. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT 1/&lt;/strong&gt;]&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that the Juno 1 / Explorer 1 mission was also famous because of a misunderstanding about the nature of spin stabilisation at the time. The answer to this question &lt;a href=&quot;https://space.stackexchange.com/a/24105/10791&quot;&gt;Explorer 1&lt;/a&gt; describes that point in more detail (I just found and added some relevant material) as the satellite tumbled because of energy transfer due to flexible modes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;EDIT 2&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I accept the foregoing stops short of saying what actually was the design for Explorer 1. An obvious query would be what combination of sensors and control loops was used for the Explorer 1 mission. What I have been trying to describe is the physics of the problem from a blank sheet of paper perspective.  &lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2017-12-27T11:22:24.477" LastActivityDate="2017-12-27T11:22:24.477" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24167" PostTypeId="1" CreationDate="2017-12-27T01:37:59.873" Score="11" ViewCount="597" Body="&lt;p&gt;I was recently looking at the specs of the Antares launch vehicle and I was surprised to find out that they were using a solid fueled motor as their upper stage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've always learned that in the upper stage of an LV the ISP is the most important parameter to design for, but solid fueled motors aren't know for their high ISP numbers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my question is, why did they do it in the case of the Antares vehicle?&lt;/p&gt;&#xA;" OwnerUserId="19789" LastActivityDate="2019-06-08T16:10:04.470" Title="Advantages of a Solid Fueled Upper Stage" Tags="|design|upper-stage|antares|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24168" PostTypeId="1" CreationDate="2017-12-27T02:27:54.703" Score="1" ViewCount="179" Body="&lt;p&gt;If a Space Station designed around centrifugal forces being essential for gravity was to be constructed similar to the fans of a helicopter blade like the image below, how would it differ from a regular torus design of space settlement?&lt;a href=&quot;https://i.stack.imgur.com/ViSz3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ViSz3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would building off to the sides cause the centrifuge to dis-align and knock it off course if it was orbiting a planetary body at a stable orbit?&#xA;Please provide serious answers. &lt;/p&gt;&#xA;" OwnerUserId="22156" LastActivityDate="2017-12-27T02:27:54.703" Title="Centrifugal Space Station design" Tags="|space-station|station-keeping|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24170" PostTypeId="2" ParentId="4959" CreationDate="2017-12-27T05:02:22.250" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;[W]hy does Delta IV Heavy have rocket engines on its side cores? Engines which are dropped when there's fuel left in the central core. Why not fix all rocket engines to the central core and just use drop tanks on the side?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Most rockets have a mass ratio between 10 to 20. A mass ratio of 10 means a 100 ton rocket will weigh only 10 tons when all the rocket propellant is burned up. If the thrust at liftoff is 200 tons, the acceleration at liftoff is 2G. At burnout, the acceleration will be 20G. 20G is excessive.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So as you can see, you don't need to keep all the engines for the entire burn. The first Atlas rocket did exactly this, it dropped 2 of the 3 engines it launched with and was able to proceed to orbit with the one remaining engine.&lt;/p&gt;&#xA;" OwnerDisplayName="user8269" LastEditorUserId="6816" LastEditDate="2017-12-27T09:25:07.650" LastActivityDate="2017-12-27T09:25:07.650" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24175" PostTypeId="1" AcceptedAnswerId="24194" CreationDate="2017-12-27T14:37:58.413" Score="9" ViewCount="1355" Body="&lt;p&gt;The Falcon Heavy has 27 engines (Suck on it N-1!) in three sets of 9 (per core booster). SpaceX initially suggested they might need a couple of static fires to nail down the ignition sequence, since it turns out to be hard to get that many engines all running in very short time windows.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had asked in a question about &lt;a href=&quot;https://space.stackexchange.com/questions/3002/how-does-spacex-ignite-their-engines&quot;&gt;how the engines start&lt;/a&gt; whether the burning engines next to each other ignite the following engines or if they use TEA-TEB on all of them.  The answer was they use ignitors on all the engines.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now Falcon Heavy is of course, really running as three seperate boosters, connected. Thus no doubt the same is true.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if an engine failed to ignite via TEA-TEB would the fact the other engines are rapidly burning ignite the Kerosene and LOX that would be pumping out of the unstarted engine? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am sure they would shut the stack down when the sensors detect a failed start, but would the missed engine actually start from external ignition sources?&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2018-01-06T17:41:18.023" Title="What if an engine fails to ignite on Falcon Heavy?" Tags="|launch|spacex|falcon-heavy|ignition|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24176" PostTypeId="1" CreationDate="2017-12-27T14:49:47.063" Score="-4" ViewCount="148" Body="&lt;p&gt;I always wondered:&#xA; would a 10,000 lb, 40 feet small rocket with retractable delta wings winched from a cargo plane,propelled by a jet similar to a f22 to mach 3 before igniting its own  rocket &#xA;My question:  is how much fuel would i need  to get to space?&lt;/p&gt;&#xA;" OwnerUserId="22175" LastEditorUserId="22175" LastEditDate="2017-12-31T19:00:38.297" LastActivityDate="2017-12-31T19:00:38.297" Title="Retractable delta wing rocket" Tags="|launch|" AnswerCount="1" CommentCount="7" ClosedDate="2017-12-28T17:29:08.600" ContentLicense="CC BY-SA 3.0" />
  <row Id="24177" PostTypeId="2" ParentId="24176" CreationDate="2017-12-27T15:00:19.943" Score="1" Body="&lt;p&gt;Your proposed design isn't too much different from the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pegasus_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pegasus&lt;/a&gt; rocket, although Pegasus is quite a bit heavier (18,500 kg=40,785 lb), the wing isn't retractable, and Pegasus doesn't use an initial jet engine.&lt;/p&gt;&#xA;" OwnerUserId="17059" LastActivityDate="2017-12-27T15:00:19.943" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24178" PostTypeId="2" ParentId="20918" CreationDate="2017-12-27T15:04:23.633" Score="7" Body="&lt;p&gt;Have a look at this video.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see one of the fairings stabilizing itself with a thruster around &lt;code&gt;04:00&lt;/code&gt;. Start watching after &lt;code&gt;02:44&lt;/code&gt; when the 2nd stage engine ignites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It looks like the bright spot at the top is the 1st stage, using cold gas thrusters, and the bright one at the bottom is the 2nd stage continuing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The two smaller spots are the fairing halves, and the upper one shows a cold gas thruster plume in the center panel:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7IgZ1.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7IgZ1.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are cropped screenshots from the video:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/UU5KpfOT-_Y?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="22176" LastEditorUserId="12102" LastEditDate="2017-12-27T21:23:24.840" LastActivityDate="2017-12-27T21:23:24.840" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24179" PostTypeId="2" ParentId="445" CreationDate="2017-12-27T16:30:17.183" Score="2" Body="&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: An alternative for building a huge ship to sustain a whole colony for thousands of years of travel would be to send cryogenized people, and unfreeze them once arrived; a middle term solution in case we do not manage to handle adult unfreezing shortly is to send frozen embryos and have robots hatch them once arrived on / near habitable land. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A baby girl recently &lt;a href=&quot;http://www.manilachannel.com/blog/baby-girl-born-from-donor-embryo-frozen-for-25-years-breaks-world-record/&quot; rel=&quot;nofollow noreferrer&quot;&gt;was born from a 25 years frozen embryo&lt;/a&gt;. Plus babies as young as 24 gestational weeks are now able to live a normal life after months of careful mothering... nothing seems impossible when you've seen this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know this is horrible for the children who will come into life on this other stellar system.&lt;/p&gt;&#xA;" OwnerUserId="18615" LastEditorUserId="18615" LastEditDate="2017-12-28T07:06:46.163" LastActivityDate="2017-12-28T07:06:46.163" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24180" PostTypeId="1" AcceptedAnswerId="24229" CreationDate="2017-12-27T18:18:17.353" Score="5" ViewCount="803" Body="&lt;p&gt;I searched for this question but couldn't find it, so I'm asking. Lets just ignore turning into a liquid at high accelerations and the fact that an object with mass cannot travel at the speed of light (I say that last one because as your velocity approaches the speed of light, we could just say it is the speed of light, and then just say its less than the answer you get from the math). So since the distances in the universe change over time, and the velocity it takes to travel those distances also changes, there exists places that are impossible to reach, as it would require faster than light travel. My question then becomes, if I intended to return to my starting point, how far could I travel until returning would become impossible?&lt;/p&gt;&#xA;" OwnerUserId="19098" LastActivityDate="2018-01-03T17:06:13.703" Title="How far can you travel until you can't get back to where you started?" Tags="|time|" AnswerCount="3" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="24182" PostTypeId="2" ParentId="445" CreationDate="2017-12-27T20:53:47.093" Score="1" Body="&lt;p&gt;I'd say that we need the construction capability to build an O'Neill cylinder that is strong enough to survive thrust along it's axis, that contains a balanced biosphere, and an energy source that will last the journey.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This will give you a generation ship or, another way to look at it, an independent space station that can move from star to star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We could do some of that today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The cylinder itself is just an engineering and resource problem.  Once we have the infrastructure in space to make it, we can make it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The closed cycle biosphere is a bit tricky but there is no real new science involved, just experience balancing things and finding stable balances so things self correct instead of spinning out of control like we have seen in several experiments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The kicker will be the energy source.  Solar power only really helps out to the asteroid belt.  After that, we need a different power source.  &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;You could use fissionables with a long half live but you will need a&#xA;lot of it (the longer the half life the less energy it puts out at&#xA;any given time).&lt;/li&gt;&#xA;&lt;li&gt;Another possibility would be fusion once we figure out how to make&#xA;that self sustaining and can store enough hydrogen/deuterium/tritium&#xA;for the trip.&lt;/li&gt;&#xA;&lt;li&gt;A new branch of science for a new power source.  If there are other options that I missed, please post them in the comments. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Point 3, while possible, is not something we should try to plan for.   To use it in a story you have to invent handwavium (though less handwavium, possibly than FTL).&lt;/p&gt;&#xA;" OwnerUserId="18842" LastEditorUserId="18842" LastEditDate="2017-12-28T18:38:45.043" LastActivityDate="2017-12-28T18:38:45.043" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24183" PostTypeId="2" ParentId="24180" CreationDate="2017-12-27T20:53:51.340" Score="0" Body="&lt;p&gt;Nobody knows for sure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Recent WMAP experiments suggest a planar Universe in this, the answer for your question is infinite. If the Universe has a spherical space-like geometry, its radius seems to be at least &lt;a href=&quot;https://physics.stackexchange.com/q/88786/32426&quot;&gt;300billion light years&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note, this is only a theoretical thing, because even if you are going with $c$, the Universe expands faster. It is not known, what will happen to it on the long term. Currently an accelerating expansion is visible, but also &lt;a href=&quot;https://en.wikipedia.org/wiki/Physical_cosmology&quot; rel=&quot;nofollow noreferrer&quot;&gt;decelerating expansion and exponential acceleration&lt;/a&gt; were already happened.&lt;/p&gt;&#xA;" OwnerUserId="1054" LastActivityDate="2017-12-27T20:53:51.340" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24184" PostTypeId="1" CreationDate="2017-12-27T21:25:14.787" Score="-3" ViewCount="36672" Body="&lt;p&gt;If I make a rocket and want to reach a point in space above sea level where I don't need to burn fuel to rise, how far is that point? space or zero gravity area?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am just trying to figure out distance where I can shut my rockets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Put another way, how much fuel do I need, or how much distance do I need to cover so that I can orbit earth like ISS?&lt;/p&gt;&#xA;" OwnerUserId="22178" LastEditorUserId="6816" LastEditDate="2017-12-29T07:10:35.100" LastActivityDate="2017-12-29T07:10:35.100" Title="How far is zero-gravity from ground?" Tags="|rockets|space-shuttle|" AnswerCount="1" CommentCount="13" ClosedDate="2017-12-28T22:19:03.580" ContentLicense="CC BY-SA 3.0" />
  <row Id="24186" PostTypeId="2" ParentId="24184" CreationDate="2017-12-27T22:39:02.437" Score="10" Body="&lt;p&gt;There is gravity everywhere. It doesn't work so that enough far away from the Earth, there won't be gravity any more.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In Low Earth Orbit (i.e. things orbiting Earth), they are in nearly the same gravitational field as we are. For example, the ISS orbits roughly 400 km above the Earth, which is only 400/6378 $\sim$ 6.3 % farther from the center of the Earth than the surface. Since gravity scales as $1/r^2$ that means at that altitude, gravity is still about 88% as strong as on the surface.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The people of the ISS are in weightlessness because they are also flying 7.8km/s speed (around 28000 km/h). This results in a centripetal force which compensates the gravity of the Earth. This is why the ISS doesn't fall down.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Farther away from the Earth, the gravity of the Moon, or other planets, or the Sun would affect more. These are typically far lesser effects as we have on the Earth, but they still exist. Planning the trajectories of space probes, all of them should be calculated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, if you simply fly upward and shut down the engines, you will fall down. If you fly faster than 11.2km/s, then your spaceship will leave Earth and never come back. If you want to orbit Earth, you have to leave the atmosphere (around 100km height) and accelerate to the speed of at least 7.8km/s sideways around the Earth (because there is a little air even there, 300km is more practical).&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="6816" LastEditDate="2017-12-28T20:08:32.437" LastActivityDate="2017-12-28T20:08:32.437" CommentCount="15" ContentLicense="CC BY-SA 3.0" />
  <row Id="24187" PostTypeId="2" ParentId="22233" CreationDate="2017-12-28T01:40:31.983" Score="3" Body="&lt;p&gt;It is nitrogen snow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nitrogen gas enters the rocket nozzle around room temperature.  The expansion converts the gas' internal energy to kinetic energy of bulk motion.  The large pressure ratio means almost all the internal energy is converted and the gas becomes extremely cold.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It cannot be droplets of liquid nitrogen, since that would not be stable at the low pressure existing at that altitude.  It would spontaneously boil, become colder while doing so, making a combination of gaseous and solid nitrogen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Liquid hydrogen vented from some of the Saturn V third stages, after they were jettisoned by the Apollo missions on the way to the moon, produced clouds of frozen hydrogen by this mechanism.&lt;/p&gt;&#xA;" OwnerUserId="22180" LastActivityDate="2017-12-28T01:40:31.983" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24188" PostTypeId="2" ParentId="13597" CreationDate="2017-12-28T10:38:06.647" Score="4" Body="&lt;p&gt;Combustibles (oil, fat or grease, but also asphalt) in contact with liquid oxygen may explode on ignition or impact. See these links &lt;a href=&quot;http://www.cryoservice.co.uk/uploads/documents/Oxygen%20Refrigerated%20Liquid.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;1&lt;/a&gt;, &lt;a href=&quot;http://www.airgas.com/msds/001190.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;2&lt;/a&gt;, &lt;a href=&quot;http://www.puritygas.com/SDS/LiquidOxygen.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;3&lt;/a&gt;, &lt;a href=&quot;http://www.airproducts.com/~/media/Files/PDF/company/safetygram-33.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;4&lt;/a&gt;, &lt;a href=&quot;http://www.serto.com/fileadmin/serto/PDF_Dateien/5_Tech_Infos/TEC_DOK_Sauerstoff_en.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;5&lt;/a&gt;, &lt;a href=&quot;http://blog.wika.com/products/oxygen-oil-and-grease-free-in-calibration-technology/&quot; rel=&quot;nofollow noreferrer&quot;&gt;6&lt;/a&gt;, &lt;a href=&quot;http://www.baesg.org/loxhaz.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;7&lt;/a&gt;. This NASA &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740002978.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;document&lt;/a&gt; is about tests on compatibility of runways to LOX.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A very impressive cite from the NASA document: &quot;A detonation was reported to have occured when a man walked across a gravelled surface on which liquid oxygen was spilled.&#xA;Ignition was attributed to friction between gravel and the asphalt beneath.&quot;&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2017-12-28T21:28:45.517" LastActivityDate="2017-12-28T21:28:45.517" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24189" PostTypeId="2" ParentId="445" CreationDate="2017-12-28T17:12:04.563" Score="2" Body="&lt;p&gt;The question assumes interstellar travel, so we're already indulging in far fetched science fiction. So I'll put aside my pessimism and indulge my inner space cadet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have a greater incentive to open a new frontier when our population growth becomes limited by resources available. Then the apparently exponential growth levels out and an S curve asymptotically approaches our ceiling. This is called &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2016/02/limits-to-growth-logistic-vs-exponential.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;logistic growth&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we manage to break free of Cradle Earth, there is enough real estate and resources in the Main Belt to postpone bumping into the logistic growth ceiling for centuries or even millennia. This would be a great setting for gaining experience in maintaining biomes. Sunlight is still fairly abundant. The asteroid city states need not be entirely self sufficient as they could trade with other asteroid city states as well Earth, Moon and Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Beyond the Main Belt are two healthy asteroid populations: the Trojans residing at the sun earth L4 and L5 points. The &lt;a href=&quot;http://hopsblog-hop.blogspot.com/2016/07/hildas-as-cyclers.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Hilda asteroids are natural cyclers&lt;/a&gt; between the Main Belt and the Trojans. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Building asteroid biomes would give some experience in burrowing. The moons of the Jupiter, Saturn, Uranus and Neptune could host subsurface biomes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But as we get further from the sun, solar energy is harder to come by. At Jupiter and Trojan distance, insolation is about 1/25 what we enjoy on earth. At Saturn 1/100. At Neptune nearly a 1/1000.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I believe developing fusion energy sources would be prerequisite for developing real estate in the Kuiper Belt and the Oort Cloud. Perhaps non stellar fusion power is possible, who knows?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Oort city states would be more isolated than Main Belt or Trojan city states. There is more incentive for self sufficiency.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By the time human kinds expands to the outer Oort we would already have plenty of experience in maintaining biomes that are self sufficient for centuries or even millennia. These city states could easily drift off into interstellar space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given fusion power and lots of water ice (as many Oort bodies are thought to have), the city sates could send off hydrogen ions at a good fraction of light speed. The thrust would be pathetic but given centuries to do a burn, the interstellar city states could manage substantial delta V.&lt;/p&gt;&#xA;" OwnerUserId="2848" LastActivityDate="2017-12-28T17:12:04.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24191" PostTypeId="1" CreationDate="2017-12-28T20:32:01.627" Score="3" ViewCount="62" Body="&lt;p&gt;I am looking at entry dynamics for a convex blunt aeroshell. We use Heading angle, bank angle and flight path angle for description of attitude from local horizon axes system. While it is sufficiently clear from vectorial definition what these angles are, it is difficult to describe them while explaining it in person. More so, in describing angle of attack and bank angle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a good way to visualize these angles when thinking of an axisymmetric aeroshell? &#xA;It has proven difficult for me to explain these, especially to fellow aerospace engineering professionals from aeronautics side (almost all  around me), who are corrupted by clear visualisation w.r.t forward direction and wing orientation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How to paint a complete picture of an aero-assist scenario that describes wind frame angles, angle of attack and lift and drag vectors on an a convention aeroshell in a fairly intuitive manner?&lt;/p&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2017-12-28T20:32:01.627" Title="Atmospheric Entry Vehicle: Wind frame representation and Visualization" Tags="|reentry|aerodynamics|entry-descent-landing|" AnswerCount="0" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24192" PostTypeId="2" ParentId="24154" CreationDate="2017-12-29T04:58:34.683" Score="3" Body="&lt;p&gt;There has been a space colonization discussion started by Gerard O'Neill. The idea was to not colonize a planet or moon, but rather orbital space, particularly L5 or the trojan asteroids. They envisioned massive spinning colonies, miles across. They came in all designs, including torus and sphere, but most of them were cylinders. (with dome end caps) &#xA;The advantage of the cylinder or sphere was efficiency of materials use. This worked well for huge structures because, with the radius of 1 to 3 miles, the rotation period was low enough to reduce the coriolis effect to negligible levels. The coriolis effect can cause a kind of sea sickness.&#xA;In smaller structures, a torus, or better yet dumbbells can increase the radius for the same purpose. So that may be why you see a lot of toruses. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A spinning object is not stable if it is spinning around it's long axis. So if you imagine a colony the proportions of a long can, spinning around it's middle, after a while it would wobble and wind up spinning end over end. So that favors a torus, dumbbells, a sphere, or squat cylinder. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However O'Neill favored pairs of long cylinders attached together with beams at the poles. That way they would balance out and be able to keep spinning on the long axis. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2017-12-29T04:58:34.683" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24193" PostTypeId="1" CreationDate="2017-12-29T20:27:37.583" Score="7" ViewCount="466" Body="&lt;p&gt;I've been trying to implement a PI (&lt;a href=&quot;https://en.wikipedia.org/wiki/PID_controller#PI_controller&quot; rel=&quot;nofollow noreferrer&quot;&gt;proportional-integral&lt;/a&gt;) controller to allow a launch vehicle to track 2 reference states simultaneous, namely position and velocity, so that the launch vehicle will end its ascent burn in a stable circular orbit at a 600km orbital altitude. I tried doing this by first solving an optimal guidance problem which provides the required reference position, reference velocity and reference control input (which is used as feedforward control) for the rocket to follow along its ascent. The PI controller is then used primarily to reject disturbances, with the feedforward control providing the bulk of the control input.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What I've noticed is that the PI controller does a good job at tracking 1 reference at a time (i.e. tracking a position reference or a velocity reference), but if I try and track both position &lt;em&gt;and&lt;/em&gt; velocity at the same time, it gives disappointing results, with only 1 of the reference states being tracked accurately.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Attached is an example of implementing the controller to track only a position reference. Radial position is given in the left plot, and total velocity is given in the right plot, where the blue plots are the references to follow, and the orange plots are the actual trajectories taken by the rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zhEE3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zhEE3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As can be seen, the controller tracks the position reference very well, but isn't able to track the velocity reference with good accuracy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we then try to track just the velocity reference, we have the following plots, where the velocity is tracked well, but the position deviates from our desired trajectory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2WNYf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2WNYf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As such, I was wondering if there is a way to accurately track both position and velocity for the launch vehicle's ascent to orbit. How would tracking multiple reference targets be achieved during a real launch event?&lt;/p&gt;&#xA;" OwnerUserId="1064" LastEditorUserId="12102" LastEditDate="2017-12-30T06:27:00.533" LastActivityDate="2019-10-14T20:36:24.490" Title="Implementing control system to track position and velocity references simultaneously" Tags="|launch|trajectory|mathematics|launch-vehicle|flight-control|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24194" PostTypeId="2" ParentId="24175" CreationDate="2017-12-29T21:39:21.010" Score="10" Body="&lt;p&gt;In order to start the engine, you first have to start the gas generator to run the turbo pump. This means getting a flame up past the turbine and igniting the mixture in the gas generator combustion chamber. This seems unlikely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the gas generator is somehow running but the main combustion chamber hasn't ignited, the propellant leaving the nozzle will burn outside the nozzle but will not generate much thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Wikipedia the launch mass of a falcon 9 FT is 549,054&amp;nbsp;kg. That means a first stage propellant mass of around 400 metric tons, divided between 9 engines so around 44 metric tons each, with a burn time of 162 seconds, so 274&amp;nbsp;kg/s through a throat a few inches in diameter. It seems unlikely that the flame front will move back into the engine against such a flow, especially when you consider that the pressure gradient in the nozzle is 97&amp;nbsp;bar when the engine is operating nominally. That's a strong flow! (Which is kind of the point of a rocket engine.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whatever happens, it will not be under proper control, which is why the control system will shut an engine down if it is not operating nominally. If the engine did ignite (probably at reduced flow) it could go off with a big bang (&quot;hard start&quot;) which could destroy it, as indicated in the other answer you linked. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I work with rather more mundane combustion equipment (boilers) and although this is not directly relevant, the control system on boilers always purges the combustion chamber of fuel before attempting ignition to avoid such an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Note that the mixture from the unignited engine will probably vaporise before it begins to combust. Under normal ambient conditions (which is not the case in a rocket engine) a flame will propagate backwards along a pipe filled with an air/hydrocarbon explosive mixture at about 1&amp;nbsp;ft/s.&lt;/em&gt;&lt;/p&gt;&#xA;" OwnerUserId="6117" LastEditorUserId="716" LastEditDate="2018-01-06T17:41:18.023" LastActivityDate="2018-01-06T17:41:18.023" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24195" PostTypeId="1" AcceptedAnswerId="24197" CreationDate="2017-12-29T23:22:02.263" Score="3" ViewCount="223" Body="&lt;p&gt;There is a lot of footage of SpaceX's Iridium 4 mission of 22 Dec 2017. The rocket's vapor trails look very different from those of airplanes, since it gains a lot of height. Anyhow, the result looks somewhat unexpected to me.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;After stage separation, why does the booster eject so many short thrusts? See, e.g.,  &lt;a href=&quot;https://www.youtube.com/watch?v=JRzZl_nq6fk&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. I guess these are maneuver impulses by the small engines on top. But why are they so &quot;pulsative&quot; and not in a more continuous manner?&lt;/li&gt;&#xA;&lt;li&gt;Concerning the same short thrusts: Can anyone explain the sequence of their directions?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Apart from these specific questions, explanations of other noteworthy details is highly appreciated.&lt;/p&gt;&#xA;" OwnerUserId="21562" LastEditorUserId="25" LastEditDate="2017-12-30T01:05:04.453" LastActivityDate="2017-12-30T04:05:40.467" Title="SpaceX's Iridium 4: Explanation of Trail Footage" Tags="|falcon-9|iridium|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24196" PostTypeId="2" ParentId="4140" CreationDate="2017-12-29T23:47:00.360" Score="1" Body="&lt;p&gt;Look ar following animation of a Hohmann transfer, it qhows the vehicle's velocity at anytime so its a nice visual answer on the topic starter's question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/gmR6W-c-kOE?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="22195" LastActivityDate="2017-12-29T23:47:00.360" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24197" PostTypeId="2" ParentId="24195" CreationDate="2017-12-30T01:06:31.997" Score="2" Body="&lt;p&gt;The short thrusts are shots of Nitrogen, used for attitude adjustment. They are used when required, which is determined in real time, and thus has no pattern.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of some note, you might watch this video. &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/CJ6nn8fZOmc?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2017-12-30T01:06:31.997" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24198" PostTypeId="2" ParentId="24195" CreationDate="2017-12-30T04:05:40.467" Score="2" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;After stage separation, why does the booster eject so many short thrusts? &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;One of the many, many ways that launches can result in failure occurs shortly after stage separation when residual fuel in the separated stage causes that lower stage to collide with the upper stage. SpaceX itself suffered this failure in 2008. To avoid this failure, the lower stage needs to quickly point itself away from the upper stage. This direction change makes the lower stage fly past the upper stage rather than collide with it if residual fuel remains and happens to ignite.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;But why are they so &quot;pulsative&quot; and not in a more continuous manner?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Attitude control thrusters typically are either on or off, with no in-between. (This is quite typical. The heater and air conditioner in your house or appartment most likely function on the same basis: On or off, with no in-between.) This on or off behavior is directed by a &lt;a href=&quot;https://en.wikipedia.org/wiki/Bang%E2%80%93bang_control&quot; rel=&quot;nofollow noreferrer&quot;&gt;bang-bang controller&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Concerning the same short thrusts: Can anyone explain the sequence of their directions?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The thrusters are fired in whatever the attitude controller deems necessary. Bang-bang control is fairly simple: There's a direction in which the vehicle is supposed to point (plus or minus a deadband), or a change in direction in which the vehicle is supposed move (plus or minus a deadband). Nothing happens when the attitude / attitude rates remain within these headbands. When the attitude error or attitude rate error goes outside the deadbands, the controller activates thrusters based on a jet select algorithm.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2017-12-30T04:05:40.467" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24200" PostTypeId="2" ParentId="20054" CreationDate="2017-12-30T12:56:53.290" Score="1" Body="&lt;p&gt;It should be possible to extract the fuel by using this formula for the $\Delta V$ of a rocket's lift off:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$\Delta V = g I_{sp} \ln(m_i/m_f) - \int_{t_i}^{t_f}dt (g \sin(\gamma) +D/m)$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where the contributions are respectively  Tsiolkovsky rocket equation, gravity loss and drag loss. &lt;a href=&quot;https://prod-edxapp.edx-cdn.org/assets/courseware/v1/9bce1811288b5b1915eb7754c3d8dd1f/asset-v1:EPFLx+EE585x+1T2017+type@asset+block/4.5_Ascent_to_space_and_re-entry.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;See Reference&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The idea is to look at the position $(x(t),y(t),z(t))$ of  a known rocket launch. From there it should be possible to extract the angle $\gamma$ as a function of time and the $\Delta V$ at any given time interval. Moreover we can estimate the drag force $D = \tfrac12\, \rho\, V^2\, C_D\, A$ at any time (knowing the drag coefficent C_D and the shape of the rocket. As pointed out, $\rho = \rho(y)$ ). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we can numerically solve for the remaining wet mass $m_f$ mass at some given $y(t)$. &lt;/p&gt;&#xA;" OwnerUserId="16049" LastEditorUserId="16049" LastEditDate="2017-12-31T12:16:53.270" LastActivityDate="2017-12-31T12:16:53.270" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24201" PostTypeId="1" CreationDate="2017-12-30T16:14:41.143" Score="15" ViewCount="2920" Body="&lt;p&gt;We know there is no sound in space in a vacuum, but when an astronaut is outside on a spacewalk doing repairs, can they hear the tapping when they use a hammer or the sound of a power screwdriver etc.?&lt;/p&gt;&#xA;" OwnerUserId="22197" LastEditorUserId="12102" LastEditDate="2017-12-31T06:29:54.393" LastActivityDate="2020-09-10T12:00:36.833" Title="Can astronauts hear sounds during space walks for repairs?" Tags="|iss|sound|" AnswerCount="2" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="24202" PostTypeId="1" AcceptedAnswerId="24203" CreationDate="2017-12-30T18:05:53.753" Score="2" ViewCount="519" Body="&lt;p&gt;If someone were to go to the moon and set up their own camp. Would NASA and governments be okay with it? Or would they not care?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a rule that says I can't do anything to the moon or else people will put me in prison or something?&lt;/p&gt;&#xA;" OwnerUserId="22199" LastEditorUserId="6816" LastEditDate="2017-12-31T03:36:27.240" LastActivityDate="2017-12-31T03:36:27.240" Title="Are there rules and ethics to follow in space?" Tags="|nasa|law|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24203" PostTypeId="2" ParentId="24202" CreationDate="2017-12-30T18:16:13.853" Score="1" Body="&lt;p&gt;This is highly dependent on who the parties in question answer to. Also rules, ethics, and laws are three quite different (though sometimes related) things.  To address the &lt;em&gt;ethics&lt;/em&gt; part of the question, here are a few examples.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a &lt;a href=&quot;http://www.esa.int/esapub/bulletin/bullet105/bul105_6.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Code of Conduct for ISS Crewmembers&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...the  Code has  been  implemented  in  Europe  through  a directive&#xA;  of the ESA Director General addressed individually   to   members   of&#xA;  the   European Astronaut  Corps  (EAC),  in  which  they  are invited &#xA;  to  agree  in  writing  to  the  terms  and conditions  in  the  Code,&#xA;  a  process  that  is consistent  with  the  ESA  Staff  Regulations &#xA;  and the  decision  taken  in  March  1998  by  the  ESA Council  on &#xA;  the  modalities  for  building  up  the EAC. In  the  United  States,  the  Code  has  become&#xA;  part of the US astronauts’ terms and conditions&#xA;  of   employment   through   the   adoption   on  1  October  2000  of  corresponding  regulations&#xA;  under NASA’s existing legislation (14 CFR Part 214). In Japan, the Code will be incorporated&#xA;  into regulations consistent with the terms of the&#xA;  legislation  that  established  the  National  Space&#xA;  Development  Agency  (NASDA)  and  Japanese&#xA;  astronauts,  as  employees  of  NASDA,  will  be&#xA;  invited  to  sign  up  to  the  terms  of  the  Code.&#xA;  Similarly  in  Russia,  the  cosmonauts  will  be&#xA;  invited to sign up individually to the terms of the&#xA;  Code  once  it  becomes  part  of  the  regulations&#xA;  and policies applicable to the Russian Aviation&#xA;  and  Space  Agency  (RSA).  In  Canada,  where&#xA;  astronauts  are  appointed  by  decision  of  the&#xA;  Cabinet, the Code will become part of the terms&#xA;  and  conditions  of  astronauts’  employment  in the  same  manner,  i.e.  through  an  Order in Council issued by the Cabinet.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;And a &lt;a href=&quot;https://www.jsc.nasa.gov/Bios/AstronautCodeEarthFINAL.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Astronaut Code of Professional Responsibility.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Vz7pQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Vz7pQ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2017-12-30T18:22:52.370" LastActivityDate="2017-12-30T18:22:52.370" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24204" PostTypeId="2" ParentId="24201" CreationDate="2017-12-31T06:17:43.793" Score="14" Body="&lt;p&gt;According to WIRED Magazine's article and video &lt;a href=&quot;https://www.wired.com/story/watch-astronauts-answer-your-burning-questions-about-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Watch Astronauts Answer Your Burning Questions about Space&lt;/a&gt; (also viewable on &lt;a href=&quot;https://youtu.be/dikG4vMJG1s&quot; rel=&quot;nofollow noreferrer&quot;&gt;YouTube&lt;/a&gt;):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Sounds exist in space, but humans can not hear them.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BU538.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BU538.jpg&quot; alt=&quot;Watch Astronauts Answer Your Burning Questions about Space&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2Kser.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2Kser.jpg&quot; alt=&quot;Watch Astronauts Answer Your Burning Questions about Space&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;As pointed out in the question, there is not going to be any significant, perceptible sound transmitted through space at the altitude of the ISS (now roughly 400 km). The density is so low and the mean free path so long that any tiny, residual amounts of sound that could potentially be transmitted (by an anomalously high local pressure (see below, &lt;a href=&quot;https://youtu.be/gpwvJzcfL1w?t=63&quot; rel=&quot;nofollow noreferrer&quot;&gt;or &lt;strong&gt;especially here&lt;/strong&gt;&lt;/a&gt; but lower your volume a bit first)) would be lost at the first interface between the extremely small atmosphere and the solid materials of the astronaut's helmet due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Acoustic_impedance&quot; rel=&quot;nofollow noreferrer&quot;&gt;acoustic impedance&lt;/a&gt; mismatch.&lt;/p&gt;&#xA;&lt;p&gt;In the &lt;a href=&quot;https://youtu.be/9yRWhu0UGYw?t=25&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt; referenced by &lt;a href=&quot;https://space.stackexchange.com/q/19436/12102&quot;&gt;this question&lt;/a&gt; you can hear both acoustic sound transmitted through the low pressure atmosphere recorded by the camera's microphone at the lower altitudes, as well as some mechanical or &amp;quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Microphonics&quot; rel=&quot;nofollow noreferrer&quot;&gt;microphonics&lt;/a&gt;&amp;quot; as discussed in the &lt;a href=&quot;https://space.stackexchange.com/a/19440/12102&quot;&gt;excellent answer&lt;/a&gt;. Microphonics can also be heard in &lt;a href=&quot;https://youtu.be/26QWMMT1c8Y&quot; rel=&quot;nofollow noreferrer&quot;&gt;the video&lt;/a&gt; discussed in the question &lt;a href=&quot;https://space.stackexchange.com/q/20786/12102&quot;&gt;What are the sounds in this GoPro-in-orbit ISS video?&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;If the astronaut were working in the vicinity of a serious, large gas leak, or in any other highly unusual situation where there was a much larger atmosphere, one should not be absolutely certain no sound perception could be possible. It should not be explicitly ruled out, though very unlikely.&lt;/p&gt;&#xA;&lt;p&gt;However, for &lt;em&gt;hand tools&lt;/em&gt; held by an astronaut, there is a possibility of microphonics via &lt;a href=&quot;https://www.nytimes.com/2017/12/08/science/why-we-hear-some-silent-gifs.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;bone conduction&lt;/a&gt;, though this would require conduction through the suit's gloves, so the chances are very low.&lt;/p&gt;&#xA;&lt;p&gt;Finally, as an aside, there is always a possibility of perceived, or mis-assigned sound cues. See for example the New York Times article &lt;a href=&quot;https://www.nytimes.com/2017/12/08/science/why-we-hear-some-silent-gifs.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Why We ‘Hear’ Some Silent GIFs&lt;/a&gt; and the scientific references within, as well as &lt;a href=&quot;https://twitter.com/lisadebruine/status/937302328184594432&quot; rel=&quot;nofollow noreferrer&quot;&gt;this tweet&lt;/a&gt; by Lisa Debruine, a researcher at the Institute of Neuroscience and Psychology at the University of Glasgow.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cQPyV.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cQPyV.gif&quot; alt=&quot;GIF from https://twitter.com/lisadebruine/status/937302328184594432&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="58" LastEditDate="2020-09-10T12:00:36.833" LastActivityDate="2020-09-10T12:00:36.833" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24205" PostTypeId="1" AcceptedAnswerId="24238" CreationDate="2017-12-31T11:59:56.300" Score="3" ViewCount="2693" Body="&lt;p&gt;I've seen the company &lt;a href=&quot;http://www.positrondynamics.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;Positron Dynamics&lt;/a&gt; claim they have &lt;em&gt;a potential&lt;/em&gt; solution to interstellar travel through their proposed antimatter propulsion system.  More often then not these advanced propulsion proposals are met with widespread criticism, but I can't find anyone «debunking» their plans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In short: are they on to something?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From Next Big Future:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://www.positrondynamics.com&quot; rel=&quot;nofollow noreferrer&quot;&gt;Billionaire Peter Thiel funds Positron Dynamics who are developing a 10 microgram per week antimatter factory&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nextbigfuture.com/2016/10/positron-dynamics-vision-of-antimatter.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Positron Dynamics Vision of Antimatter Catalyzed Fusion&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="21901" LastEditorUserId="12102" LastEditDate="2017-12-31T17:58:53.943" LastActivityDate="2020-02-01T17:51:52.517" Title="Is antimatter feasable?" Tags="|interstellar-travel|antimatter|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24206" PostTypeId="1" AcceptedAnswerId="24208" CreationDate="2017-12-31T13:53:59.527" Score="145" ViewCount="41729" Body="&lt;p&gt;Currently, &lt;a href=&quot;https://space.stackexchange.com/questions/653/washing-clothes-in-space-are-there-any-technical-challenges&quot;&gt;doing laundry in microgravity is an unsolved engineering problem&lt;/a&gt;. The result is that clean clothes on the International Space Station &lt;a href=&quot;https://space.stackexchange.com/questions/13893/laundry-on-the-iss&quot;&gt;have become a consumable resource&lt;/a&gt;. Clean clothes get launched to the space station regularly. When they are too dirty to wear, they just get stuffed into a used cargo capsule as trash.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a more long-term mission where supply flights are not an option, like a manned Mars mission for example, this would be a problem. Shipping enough clothes for 9 months (even more when you don't want to wash clothes while staying on the Mars surface) would require quite a lot of valuable payload capacity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about the simplest solution: &lt;strong&gt;Just don't wear any clothes&lt;/strong&gt;. Human body hygiene in space is a solved problem. When astronauts stay nude during most of their intra-vehicular activities, the need for fresh clothes could be drastically reduced.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The astronauts are in a controlled environment, so protection from the weather is not required. &lt;/li&gt;&#xA;&lt;li&gt;The everyday clothes ISS astronauts are wearing aren't going to protect them against any of the dangers of space travel, so safety is not an argument either.&lt;/li&gt;&#xA;&lt;li&gt;The cultural stigma against nudity between the astronauts can be overcome. Nudist movements all around the world show that it is perfectly possible to have normal social interactions without wearing clothes.&lt;/li&gt;&#xA;&lt;li&gt;The modesty of the astronauts towards the general public can be preserved by not livestreaming the whole mission. The long transfer phases between Earth and Mars are boring anyway. Public interest in actually seeing the astronauts can be limited to the critical mission phases. But these would just be a tiny fraction of the overall mission time. The astronauts could get dressed just for these occasions.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Am I overlooking some problems which would arise if astronauts would not wear clothes at all times and which would outweigh the problems caused by managing laundry?&lt;/p&gt;&#xA;" OwnerUserId="335" LastActivityDate="2022-02-08T17:22:02.657" Title="Nudism in space: Why wear clothes anyway?" Tags="|water|humans|long-duration|waste|hygiene|" AnswerCount="5" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="24207" PostTypeId="1" CreationDate="2017-12-31T15:34:27.027" Score="8" ViewCount="751" Body="&lt;p&gt;Using the SGP4 propagator, I calculated the minimum distance between ISS and Tiangong-2 for this year.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I obtained 12.32 km for &lt;code&gt;2017-09-27 17:29:36.376 UTC&lt;/code&gt; (which is the day &lt;code&gt;270.7289&lt;/code&gt;). I used the TLE &lt;code&gt;17270.72310185&lt;/code&gt; for the ISS and the TLE &lt;code&gt;17270.58030751&lt;/code&gt; for the Tiangong-2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That distance seems very small. Is there any minimum safe distance to keep?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: I uploaded the graphs that I use to analyze the close encounter geometry: &lt;a href=&quot;http://cristianopi.altervista.org/_Incontri.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://cristianopi.altervista.org/_Incontri.html&lt;/a&gt; (Sat1 = ISS, Sat2 = TG2).&lt;/p&gt;&#xA;" OwnerUserId="20397" LastEditorUserId="20397" LastEditDate="2019-09-21T16:16:09.910" LastActivityDate="2019-09-21T16:16:09.910" Title="ISS - Tiangong 2 minimum safe distance" Tags="|iss|safety|collision-avoidance|tiangong-2|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="24208" PostTypeId="2" ParentId="24206" CreationDate="2017-12-31T18:06:25.633" Score="199" Body="&lt;p&gt;Clothes require laundry because they have accumulated dirt and other materials from the environment and their wearer. If the astronaut was not wearing those clothes then that material they captured would be released to the environment. In the space station it is a closed environment where such dirt and material would contribute to the accumulated hazards.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The purpose of clothes it to capture that material, such as dead skin, bodily fluids (such as sweat and so forth) and enable it to be disposed of. On earth we dispose of that material by recycling the clothes in a process we call laundry. In space it is more efficient to dispose of the hazardous material (we call dirt) within the capture container (we call clothes).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other purpose of the clothes is to protect the astronauts from other day-to-day hazards to their body, such as abrasions and impacts. Socks, in particular, are quite important on the ISS as they protect the feet from the abrasions when hooked around the holding-bars when they are using their hand for work. Without this they would either be unable to work or would have sore upper feet.&lt;/p&gt;&#xA;" OwnerUserId="8162" LastEditorUserId="-1" LastEditDate="2019-10-16T10:38:48.503" LastActivityDate="2019-10-16T10:38:48.503" CommentCount="17" ContentLicense="CC BY-SA 4.0" />
  <row Id="24209" PostTypeId="2" ParentId="24205" CreationDate="2017-12-31T18:18:34.527" Score="4" Body="&lt;p&gt;As written, this question is far too broad. &quot;Are they on to something?&quot; is a nearly meaningless question. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is &lt;a href=&quot;https://en.wikipedia.org/wiki/Antimatter_rocket&quot; rel=&quot;nofollow noreferrer&quot;&gt;antimatter propulsion theoretically feasible&lt;/a&gt;? Yes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it make interstellar travel possible? Not by itself. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does Positron Dynamics know how to make an antimatter-powered thruster useful for interstellar travel? No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Will Positron Dynamics demonstrate an antimatter-powered thruster in the near future? I have no idea.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2017-12-31T18:18:34.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24210" PostTypeId="2" ParentId="24206" CreationDate="2017-12-31T18:35:49.837" Score="87" Body="&lt;p&gt;Clothing performs essential duties on the station in addition to modesty. They are an easy way to organize stuff. In addition to pockets, clothing is festooned with velcro strips for attaching tools, pens etc. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/daYqk.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/daYqk.jpg&quot; alt=&quot;astronaut clothing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.space.com/13798-space-laundry-machine-microwaves.html&quot; rel=&quot;noreferrer&quot;&gt;NASA is working on methods&lt;/a&gt; for washing clothes in space. Not surprisingly, &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150003039.pdf&quot; rel=&quot;noreferrer&quot;&gt;they've studied&lt;/a&gt; the tradeoffs between including a washing machine in the mission and just supplying enough clean clothes to last the mission. The breakeven point for a washing machine is missions of about 1 year. For the ISS, missions are shorter than that and there are frequent resupplies, so it wasn't necessary to develop a washing machine earlier. For a Mars mission, it may become necessary. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The same argument for not wearing clothes in space is true for life on Earth: it'd be cheaper to run around naked. Less frequent washing and reduced wear would save money. Yet, nobody's doing that here on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nudists are a small subset of humans, suggesting modesty is not that easy to dispense with. I think we can file this idea under N for 'non-starter'. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional possible rationales for not having a washing machine on the ISS: Adding a washing machine would place additional loads on the station's systems (water recycling, electricity), plus it'd be another non-science item to develop, operate and maintain.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-02T09:13:29.460" LastActivityDate="2018-01-02T09:13:29.460" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="24211" PostTypeId="2" ParentId="88" CreationDate="2017-12-31T22:01:17.500" Score="9" Body="&lt;p&gt;My son modified the program to dump out data yearly, and also report the number of launches with unknown payload mass. I corrected the data using:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$$m_{estimate} = m_{known}*{n\over n-n_{unknown}}$$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$m_{estimate}$&lt;/span&gt; is the estimate of mass launched in a given year&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; is the number of launches in the year&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$n_{unknown}$&lt;/span&gt; is the number of launches where we don't know the payload mass in the year&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$m_{known}$&lt;/span&gt; is the total payload mass for those launches that list the payload mass in a year.&lt;/p&gt;&#xA;&lt;p&gt;With this correction factor, the total tally for all years becomes 17,976,052 kg.&lt;/p&gt;&#xA;&lt;p&gt;(Update)&lt;/p&gt;&#xA;&lt;p&gt;I also plotted this data, fit a line to it, and asked Excel to display the line fit formula.&lt;/p&gt;&#xA;&lt;p&gt;Finally, I found some data on objects tracked by the US Space Surveillance Network and plotted that as well, just to get some indication as to whether the estimate was in the right ballpark.&lt;/p&gt;&#xA;&lt;p&gt;The two curves together suggest that roughly 60% of the payloads we have sent up have either de-orbited or are not being tracked for some reason.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qrwJg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qrwJg.png&quot; alt=&quot;Mass In Orbit Versus Year&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Some words of caution: I do recall that a year or two after we did this work, I tried to update the estimate I didn't like the new results. When I went looking, I found several minor examples of the data in the database not being organized and sorted perfectly. This could have been due to restructuring of the database that occurred after the original attempt was made, or it could have been errors that were there in the original attempt - I just don't know. Therefore, I would advise taking these results with a grain of salt. I think this represents an 80/20 rule estimate. I would not be surprised if someone in the future generates a more accurate estimate by putting in a lot of effort, or by training an AI to sift through the available data sources.&lt;/p&gt;&#xA;" OwnerUserId="22203" LastEditorUserId="22203" LastEditDate="2023-12-03T06:54:23.927" LastActivityDate="2023-12-03T06:54:23.927" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="24212" PostTypeId="2" ParentId="24206" CreationDate="2018-01-01T13:58:40.410" Score="26" Body="&lt;p&gt;In addition to capture of contaminants, such as dead skin, hair, sweat, etc, and abrasion/cut protection, clothing forms a basic thermal layer that allows the human body to better regulate its internal temperature and perceived comfort level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every human has variations in temperature, and even in a perfectly controlled environment once you place two humans inside it you'll find it's only perfect for one or the other, and that this can change over time based on activity.  They can adapt in a variety of ways, but the simplest is to wear clothing which conveniently serves several purposes in addition to easier thermal regulation.&lt;/p&gt;&#xA;" OwnerUserId="248" LastActivityDate="2018-01-01T13:58:40.410" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24213" PostTypeId="1" AcceptedAnswerId="24215" CreationDate="2018-01-01T15:53:23.587" Score="22" ViewCount="3664" Body="&lt;p&gt;As far as I understand, NASA's Space Shuttle was initially conceived as a cheap way of launching people and cargo into orbit, with one-week vehicle turnaround time and dozens of missions per year. Then the politicians/USAF intervened, demanding unreasonably large payload and oversized wings for glide to Vandenberg, among other things — and that's why the Space Shuttle programme ended as an economic disaster.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But my question is: was the initial concept really feasible in the first place? Would the Space Shuttle have been as good as advertised, if only the design had not been changed to accommodate unnecessary features?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If not, would it be possible to build such vehicle with modern tech?&lt;/p&gt;&#xA;" OwnerUserId="22215" LastEditorUserId="6537" LastEditDate="2018-01-05T20:26:27.567" LastActivityDate="2018-01-05T20:26:27.567" Title="Cost-effective Space Shuttle: was it feasible?" Tags="|spacecraft|space-shuttle|cost|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24214" PostTypeId="2" ParentId="24213" CreationDate="2018-01-01T16:55:07.843" Score="10" Body="&lt;p&gt;The Shuttle had many design issues. Some of them could have been fixed with modern technology, for example better heat shielding that did not require tens of thousands of tiles. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Others were endemic to the way the requirements fell out. The Air Force, to keep their funding in, required large cross range abilities, and the possibility of launch, deploy payload, and land, in a single orbit. These required a large delta wing, which added weight and complexity. (Which was related to one of the losses when the wing leading edge was punctured on the Columbia loss).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also the Air Force wanted to launch very large radar/optical recon satellite, which meant a very large payload bay.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lack of funding meant they settled on SRB's instead of liquid fueled boosters (Possibly using F-1 engines) which was related to the other shuttle loss of Challenger. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then political issues meant work needed to be split up and Thiokol/Morton/ATK (whatever the name was back then) got work in Utah, so the SRB's had to be segmented and shipped by rail, which meant joints and so on which was part of the issue for Challenger.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dream Chaser is an attempt at a lifting body reusable vehicle. Time will tell if scaling down the absolute size, the wing size, using a simpler booster, etc will be feasible. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2018-01-01T16:55:07.843" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24215" PostTypeId="2" ParentId="24213" CreationDate="2018-01-01T16:55:18.940" Score="24" Body="&lt;p&gt;Necessarily speculative, but here goes...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's look at what &quot;the initial concept&quot; really was.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The post-Apollo vision was to build space infrastructure that would enable the manned exploration and exploitation of the solar system.  The surface-to-low-Earth-orbit reusable shuttle was but one part of this grand plan, as shown in this 1970 slide from Marshall Space Flight Center.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/noKNR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/noKNR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Notice the many orbital stations around Earth, the Moon, and Mars.  Reusable nuclear thermal vehicles. Reusable space tugs. Also the smaller, fully reusable shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When budget realties and realpolitik got into the process, the fully reusable shuttle was changed to partly reusable, trading development costs for operational costs later down the road. All other elements of the plan were deferred, vastly reducing the number of shuttle launches and really calling the entire rationale for a shuttle into question. Desperate to keep a manned vehicle, NASA resorted to joining with DOD and handwaving the economics of the shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it have been as good as advertised?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A smaller, totally reusable vehicle would have cost less per flight. A much larger fleet flying many more missions would have vastly increased the experience base and perhaps reduced the political impact of accidents.  If an F-16 crashes, it makes the news, but it's not a national tragedy, and the Air Force doesn't grind to a halt for years while it's investigated.  The shuttles we got were treated as national treasures and their low flight rate and extreme cost made the experience base small and the opportunities to improve them scanty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You're probably really asking &quot;was 1970s technology up to the task of building a smaller, fully reusable, shuttle that could crank out a high flight rate&quot;.  Speculative, but my guess is...probably. The shuttle we got was so close to the margins every flight that an enormous amount of inspection, test, and overhaul was required after every mission to make sure it was OK to fly again. It was probably possible to design in extra margin at the cost of performance...but that's largely an economic decision, and a tough one to make if you don't have the grand vision.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some aspects of your question are covered in answers to these questions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/12156/technology-how-would-a-state-of-the-art-space-shuttle-be-built/12157#12157&quot;&gt;Technology: How would a state-of-the-art space shuttle be built?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/22640/why-did-nasa-close-the-space-shuttle-program/22641#22641&quot;&gt;Why did NASA close the space shuttle program?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/10350/why-didnt-nasa-use-the-shuttle-to-make-a-profit/10352#10352&quot;&gt;Why didn&amp;#39;t NASA use the shuttle to make a profit?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/14499/why-were-four-space-shuttle-orbiters-initially-built/14500#14500&quot;&gt;Why were four Space Shuttle orbiters initially built?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you have a serious interest in this topic, I highly suggest you read &quot;The Space Shuttle Decision&quot; by Heppenheimer. It's available online &lt;a href=&quot;https://history.nasa.gov/SP-4221/contents.htm&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-01-01T17:00:37.650" LastActivityDate="2018-01-01T17:00:37.650" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24217" PostTypeId="1" AcceptedAnswerId="24220" CreationDate="2018-01-01T20:15:58.170" Score="18" ViewCount="3137" Body="&lt;p&gt;Seemingly as a joke, Elon Musk suggested &lt;a href=&quot;https://twitter.com/elonmusk/status/936782477502246912&quot; rel=&quot;noreferrer&quot;&gt;in a tweet&lt;/a&gt; that the first payload of Falcon Heavy demo flight would be his Midnight Cherry Tesla Roadster. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first payload of Falcon 9 with a Dragon test article was a wheel of cheese (playing off &lt;a href=&quot;https://www.youtube.com/watch?v=cWDdd5KKhts&quot; rel=&quot;noreferrer&quot;&gt;Monty Python's Cheese Shop skit&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Many assumed he was joking, so of course he did it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DdC2j.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DdC2j.png&quot; alt=&quot;Roadster in a fairing&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i1uD1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i1uD1.jpg&quot; alt=&quot;Roadster on a payload adapter&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the annals of space history this has to pretty high up there in ridiculous   payloads. To be clear, this is totally awesome.  And playing Space Oddity (Must be Hadfields ISS version) till the battery runs out is even better!  In space no one can hear your stereo playing music, but who cares!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there other examples that are as silly? &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="25" LastEditDate="2018-01-09T15:01:54.980" LastActivityDate="2019-09-04T04:17:38.183" Title="Has any payload been as silly, as the Falcon Heavy's Tesla Roadster?" Tags="|spacex|falcon-heavy|payload|tesla-roadster|nonstandard-payload|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="24220" PostTypeId="2" ParentId="24217" CreationDate="2018-01-02T03:08:35.803" Score="18" Body="&lt;p&gt;Update: February 24, 2018&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The '&lt;strong&gt;Disco Ball&lt;/strong&gt;': Created by Rocket Lab founder and CEO Peter Beck, the &lt;a href=&quot;https://web.archive.org/web/20181031134615/http://thehumanitystar.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Humanity Star&lt;/a&gt; is a geodesic sphere made from carbon fibre with 76 highly reflective panels. It spins rapidly, reflecting the sun’s rays back to Earth, creating a flashing light that can be seen against a backdrop of stars. Visible for a few seconds, every 90 minutes, each night, for the next several months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Criticisms: &lt;a href=&quot;https://en.wikipedia.org/wiki/Humanity_Star#Criticism&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, Caleb Scharf in &lt;a href=&quot;https://blogs.scientificamerican.com/life-unbounded/twinkle-twinkle-satellite-vermin/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scientific America&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vIg4W.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vIg4W.jpg&quot; alt=&quot;Oh The Humanity!&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: February 21, 2018&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Throughout history a number of odd or useless things have been sent into space. Commercial ventures seem to top the list for 'bad ideas' placing profit and sanity over professionalism and safety.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Examples are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 2005 a gold-plated scandium six-iron golf club from &lt;a href=&quot;https://www.e21golfshafts.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Element 21&lt;/a&gt;, several golf balls, an equipment bag, tee and specially-built platform were hauled to the station last September on a Russian space freighter. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;See anything wrong with this picture?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6suY6.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6suY6.jpg&quot; alt=&quot;Four, or five million dollars damage&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Indeed there were &lt;a href=&quot;https://www.space.com/2091-iss-commander-spacewalk-golf-shot-raises-concerns.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;delays approving the stunt&lt;/a&gt; and it was &lt;a href=&quot;https://www.space.com/2428-astronauts-golf-shot-pulled-iss-spacewalk.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;cancelled&lt;/a&gt; eventually due to safety concerns. The idea was that the balls were equipped with transmitters that would enable golfers to follow its flight around the world at the company's Web site, because that's something golfers would like to do.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Suggested by user &quot;&lt;a href=&quot;https://space.stackexchange.com/users/6944/organic-marble&quot;&gt;Organic Marble&lt;/a&gt;&quot; are the Coke dispensing machines and the Pepsi can shaped inflatable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The yellow tinted Coke machine, an unfortunate color choice considering where the water comes from.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BEfoH.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BEfoH.jpg&quot; alt=&quot;Coke or Peepsee?&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Later flights included a more traditional color scheme with a &lt;a href=&quot;http://www.collectspace.com/ubb/Forum14/HTML/000692.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;new set of problems&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JIU3z.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JIU3z.jpg&quot; alt=&quot;It says: &amp;quot;Please insert 16 Quarters, or 40 Dimes, or 80 Nickels, or ... Did any of you bring any change?&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The machine also included a number of complicated accessories necessary for its operation. Personally autographed photo of Vladimir G. Titov examining the refills for the pop machine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xDdiy.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xDdiy.jpg&quot; alt=&quot;Vladimir Titov&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Rare (and low resolution) &lt;a href=&quot;https://youtu.be/KgBeD8GPnZM?t=2m30s&quot; rel=&quot;nofollow noreferrer&quot;&gt;footage from YouTube showing BTS of the inflatable Pepsi can commercial&lt;/a&gt;, with interviews and extra info (15 mins.). Video footage of the can and banner was shot both inside and outside Mir space station but &lt;a href=&quot;http://www.whitelabelspace.com/2008/08/greatest-space-ads-part-iii-pepsi-on.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;the commercial never aired&lt;/a&gt; because Pepsi later changed the design of the can. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3a7oK.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3a7oK.gif&quot; alt=&quot;Inflatable Pepsi can&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would really have to be noodles to think Nissin Noodles would be a good idea for space consumption, boiling water, chop sticks, or just mash them and eat it like toothpaste.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nI2qK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nI2qK.jpg&quot; alt=&quot;Nissin Noodles, at least they didn&amp;#39;t think they meant Nissan.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Earlier space flights were kinder to their human occupants, it was non-human lifeforms that suffered indignities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Russian_tortoise#1968_Moon_flight&quot; rel=&quot;nofollow noreferrer&quot;&gt;Russian Tortoise&lt;/a&gt; is an endangered species so it seems as though a better choice could have been made. The launch of &lt;a href=&quot;https://en.wikipedia.org/wiki/Animals_in_space#1940s&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fruit Flies&lt;/a&gt; didn't seem particularly adventurous.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a photo of &lt;a href=&quot;https://en.wikipedia.org/wiki/Miss_Baker#The_flight&quot; rel=&quot;nofollow noreferrer&quot;&gt;Miss Baker&lt;/a&gt;, she was accompanied by Neurospora, samples of human blood; E. coli; onions; mustard and corn seeds; Drosophila pupae; yeast, and sea urchin eggs and sperm. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which is sillier, traveling with E. Coli or urchin sperm?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PT5wY.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PT5wY.jpg&quot; alt=&quot;Miss Baker&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.businessinsider.com/spacex-dragon-cheese-rocket-2010-2017-3&quot; rel=&quot;nofollow noreferrer&quot;&gt;reason cheese was sent into orbit&lt;/a&gt; is because it was a maiden flight with many untested calculations, so they didn't want to risk anything too valuable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other silly things included:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;The Astronaut's Checklist (&lt;a href=&quot;http://www.hq.nasa.gov/alsj/a12/a12_cdrcuff.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Link to a .PDF&lt;/a&gt; of one of the EVA Checklists) for Apollo 12 had centerfolds of Playboy Playmates added to it. The photos and captions are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Angela Dorian (Miss September 1967) with the line &quot;Seen any interesting hills &amp;amp; valleys?&quot; &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Reagan Wilson (Miss October 1967) was displayed as a &quot;Preferred tether partner.&quot; &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Cynthia Myers (Miss December 1968) and &quot;Don't forget—describe the protuberances.&quot;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Leslie Bianchini (Miss January 1969) and &quot;Survey her activity.&quot;&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;DeDe Lind (Miss August 1967) titled &quot;Map of a Heavenly Body&quot;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Coca Cola spent \$250K followed by Pepsi who spent $14M to develop cans for dispensing their beverages in space. The Challenger shuttle was sent into orbit in 1985 with four cans of each cola. The five-person crew tested the drinks out and &lt;strong&gt;hated both&lt;/strong&gt; brands.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I'm not so sure it was &lt;em&gt;silly&lt;/em&gt; to take Luke Skywalker's Light Saber into space and the &lt;a href=&quot;http://listverse.com/2014/12/14/10-oddities-sent-into-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Russians carry TP-28&lt;/a&gt; triple barreled guns in case they run into bears and wolves upon landing.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The most useless thing was a &lt;a href=&quot;http://newsfeed.time.com/2012/11/13/watch-five-guys-from-harvard-send-a-hamburger-into-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;shellaced hamburger&lt;/a&gt;.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/STS-124#Mission_payloads&quot; rel=&quot;nofollow noreferrer&quot;&gt;Buzz Lightyear&lt;/a&gt; joined the crew of the &lt;a href=&quot;https://www.nasa.gov/audience/foreducators/buzz-lightyears-dream-come-true.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;STS-124 space shuttle&lt;/a&gt; mission, launched May 31, 2008. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Also silly was a Obama Bobblehead but it &lt;a href=&quot;http://newsfeed.time.com/2012/09/21/watch-hey-lets-launch-an-obama-bobblehead-into-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;didn't pass the stratosphere&lt;/a&gt; and it's reason was to raise funds for reelection.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Update: February 21, 2018 - Starman Payload. New upload to Musk's Instagram account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gn15U.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gn15U.jpg&quot; alt=&quot;Starman seated in a red Tesla Roadster mounted for launch in the  payload capsule.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Update: February 6, 2018: Here's the video, car closeup at the end.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/wbSwFU6tY1c?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="20766" LastEditorUserId="20766" LastEditDate="2019-09-04T04:17:38.183" LastActivityDate="2019-09-04T04:17:38.183" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="24221" PostTypeId="1" CreationDate="2018-01-02T08:31:28.840" Score="4" ViewCount="279" Body="&lt;p&gt;Specific impulse of exhaust velocity depends on the temperature in the combustion chamber. Won't regenerative cooling lower the chamber temperature, causing the exhaust velocity to decrease?&lt;/p&gt;&#xA;" OwnerUserId="22224" LastEditorUserId="6816" LastEditDate="2018-01-02T08:36:26.523" LastActivityDate="2018-01-02T14:12:27.433" Title="Does regenerative cooling reduce specific impulse?" Tags="|rockets|engine-design|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24223" PostTypeId="1" AcceptedAnswerId="24226" CreationDate="2018-01-02T10:43:03.010" Score="9" ViewCount="1092" Body="&lt;p&gt;In &lt;a href=&quot;https://space.stackexchange.com/a/24215/90&quot;&gt;an answer to a recent question&lt;/a&gt; we see a plan for many space stations as part of a space infrastructure, with a surface-to-low-Earth-orbit reusable shuttle as another part.  This shuttle eventually turned into what became the space shuttle.  As far as I understand, the initial design specified a cargo bay 12 feet wide and 40 feet long.  Due to &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Shuttle_design_process#Air_Force_involvement&quot; rel=&quot;noreferrer&quot;&gt;involvement of the Air Force&lt;/a&gt; this was changed to a design with 15 feet width and 60 feet length.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looking at the &lt;a href=&quot;https://en.wikipedia.org/wiki/Assembly_of_the_International_Space_Station#Assembly_sequence&quot; rel=&quot;noreferrer&quot;&gt;modules of the ISS&lt;/a&gt; we see that many modules have a diameter of 4 meter or more (12 feet = ~3.6 meter).  Also many modules where launched by the space shuttle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming the smaller shuttle with a smaller cargo bay would have been built and used, I guess space station modules with a maximal diameter of 12 feet would have been kind of an issue (think of &lt;a href=&quot;https://i.stack.imgur.com/daYqk.jpg&quot; rel=&quot;noreferrer&quot;&gt;this image&lt;/a&gt; but with 1 meter less space).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Would have these planned stations be built in the same way the ISS was built, by sending up -smaller- modules with a shuttle to be linked in space, or by sending smaller parts (walls) with a shuttle?&lt;/strong&gt; Or were there other means of getting these stations up planned?&lt;/p&gt;&#xA;" OwnerUserId="90" LastActivityDate="2018-01-15T09:06:21.153" Title="Building space stations with the original, smaller shuttle" Tags="|space-shuttle|space-station|orbital-assembly|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24224" PostTypeId="2" ParentId="24223" CreationDate="2018-01-02T13:17:38.760" Score="3" Body="&lt;p&gt;Prior to the Shuttle approach, there were really two modes of station building.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Skylab first, which used a Saturn-IVB third stage outfitted before launch, and was basically all done in a single launch.  (Dry workshop. Wet workshop idea was launch it full of fuel with a big payload and have the stuff submerged in the fuel/oxidizer). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Salyut/Mir second, used a module approach, but the key technology was a maneuvering system, small enough to attach to a module, that did not detract from the total payload. For Pirs/Poisk at ISS this was a modified Progress. Proton launched modules had their own maneuvering systems. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Had the shuttle payload bay not been big enough, no doubt big modules would have been launched on a Titan or Atlas and maneuvered to dock. Or else Shuttle-C style approaches where a singular large payload would be delivered.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2018-01-02T13:17:38.760" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24225" PostTypeId="2" ParentId="24213" CreationDate="2018-01-02T13:41:55.663" Score="1" Body="&lt;p&gt;It's certainly possible to build a reusable path to orbit with modern technology. That's precisely what &lt;a href=&quot;http://www.spacex.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX&lt;/a&gt; have been doing for several years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would it look like a Space Shuttle?  Nope, which is why SpaceX has taken the well-proven &lt;a href=&quot;https://en.wikipedia.org/wiki/Big_dumb_booster&quot; rel=&quot;nofollow noreferrer&quot;&gt;big dumb booster&lt;/a&gt; design and added only such complexity (for landing and reuse) as is absolutely necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A simple rocket is simple (relatively so!), and the mechanics and control theory are reasonably well understood.  Politically though, it had to look like an aeroplane because the Air Force contributed such a large element of funding and manpower.  The very concept of wings was a failure from the start, because every element of a wing for is pretty much the opposite of what you want for space flight and for a reentry vehicle.  The utter failure of &lt;a href=&quot;http://www.scaled.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Scaled Composites&lt;/a&gt; to deliver anything beyond their initial simple shot to low altitude is a further demonstration of how faulty the thinking behind an aeroplane-based design is.  Looks cool, ticks all the boxes from your 50s sci-fi pulp novel covers - but fails in the face of physics.&lt;/p&gt;&#xA;" OwnerUserId="15976" LastActivityDate="2018-01-02T13:41:55.663" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24226" PostTypeId="2" ParentId="24223" CreationDate="2018-01-02T14:07:24.900" Score="10" Body="&lt;p&gt;A mix of manned shuttle missions and unmanned boosters carrying larger modules.  They planned to use Saturn Vs to launch pre-assembled stations (like Skylab) and have the shuttles bring up experiment and logistics modules.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rbBNK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rbBNK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/bgmRi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bgmRi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/LfxLh.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LfxLh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This whole report is available &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700026519.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt; but the illustrations are poorly reproduced in black-and-white.&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-01-02T14:16:20.563" LastActivityDate="2018-01-02T14:16:20.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24227" PostTypeId="2" ParentId="24221" CreationDate="2018-01-02T14:12:27.433" Score="3" Body="&lt;p&gt;In a regeneratively cooled engine the heat extracted from the chamber and/or nozzle walls is absorbed by the fuel, which is later burned.  So compared to e.g. an ablatively cooled engine, the reactants start combustion at a higher temperature and therefore finish combustion at a higher temperature, with more total energy available for conversion to velocity. Therefore regenerative cooling actually increases Isp compared to other cooling methods, by recapturing some of the energy that would otherwise be lost (radiated or ablated away) and returning it to where it is wanted.&lt;/p&gt;&#xA;" OwnerUserId="3065" LastActivityDate="2018-01-02T14:12:27.433" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="24228" PostTypeId="2" ParentId="1570" CreationDate="2018-01-02T14:36:53.727" Score="2" Body="&lt;p&gt;Modern solid fueled missile systems use so called &quot;blow off ports&quot; near the top of the motor in order to decrease the pressure of the combustion chamber and effectively &quot;shut down&quot; the thrust generation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The burning rate of a solid propellant is dictated by: rate = a*P**n&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if the P (chamber pressure) could be brought to a near vacuum, the burning rate will decrease to near 0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So yes.&lt;/p&gt;&#xA;" OwnerUserId="19789" LastEditorUserId="19789" LastEditDate="2019-04-03T12:54:16.493" LastActivityDate="2019-04-03T12:54:16.493" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="24229" PostTypeId="2" ParentId="24180" CreationDate="2018-01-02T14:58:58.697" Score="2" Body="&lt;p&gt;In the expanding universe the interval is,&#xA;\begin{equation}&#xA;ds^2=c^2 dt^2-a^2(t) d\vec{x}^2&#xA;\end{equation}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The interval along lightlike trajectory is zero, so from that we get,&#xA;\begin{equation}&#xA;\frac{dx}{dt}=\frac{c}{a(t)},\quad \Delta x(t_0,t_1)=c\int_{t_0}^{t_1}\frac{dt}{a(t)}&#xA;\end{equation}&#xA;If you as usually done choose that $a$ at present time ($t=0$) is equal to $1$. the coordinate $x$ will tell you how far this is at the present. To learn how far it is at $t=t_1$ you should multiply it on $a(t_1)$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the equation above require you to know how exactly scale factor $a(t)$ will evolve in the future which is determined by the matter. Luckily the problem is much simplified as we entered the dark energy dominated era with dark energy most likely being just a cosmological constant. We then assume that the Hubble parameter is approximately constant (actually it will gradually drop by about 20% as matter will disperse and its contribution will decrease),&#xA;\begin{equation}&#xA;H\equiv\frac{1}{t_H}\equiv\frac{\dot{a}}{a}\simeq \mathrm{const},\Rightarrow a(t)=e^{t/t_H}&#xA;\end{equation}&#xA;With $t_H\approx 14.4 Gy$ (where Gy stands for gigayear i.e. billion years) Then,&#xA;\begin{equation}&#xA;\Delta x(t_0,t_1)=ct_H\Big(e^{-t_0/t_H}-e^{-t_1/t_H}\Big)&#xA;\end{equation}&#xA;Note that if we take $t_0=0$ (i.e. shoot the light pulse now) the fartherst point it may get is $c t_H$. This is the cosmological event horizon - anything further can't be reached because of the universe expansion. The point of no return for light is defined in such a way that when light reaches it the point of departure is exactly on its cosmologial event horizon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general case the cosmological horizon depends on $t_0$. However when $H=\mathrm{const}$ we get highly symmetric de Sitter spacetime for which $a(t+\Delta t)=a(t)a(\Delta t)$ and $a(t_0)\Delta x(t_0,t_1)=\Delta x(0,t_1-t_0)$. So to find the point of no return we simply take,&#xA;\begin{equation}&#xA;a(t_{NR})\Delta x(0,t_{NR})=ct_H&#xA;\end{equation}&#xA;This gives us $t_{NR}=t_H\ln{2}\approx 10 Gy$. At the present this point is at $\Delta x=\frac{ct_H}{2}\approx 7.2 Gly$ ($7.2$ billion light-years) from us.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To put those numbers into perspective the age of the universe is 13.8 Gy, the radius of the observable universe is 45.7 Gly.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;UPD: By the way there's a very simple way to understand that for any FRLW cosmology with event horizon the sphere of no return will always have the radius equal to half of the horizon radius. To do so you need to use conformal time,&#xA;\begin{equation}&#xA;\tau=\int \frac{dt}{a(t)},\quad ds^2=a^2(\tau)\Big(c^2 d\tau^2-d\vec{x}^2\Big)&#xA;\end{equation}&#xA;The nice thing about the conformal time is that lightcones look very simple, just like in the flat spacetime $\Delta x=c \Delta \tau$. But how cosmological horizons then appear, what prevents lightcones from intersecting with some wordline $x=\mathrm{const}$? This happens when $a$ grows sufficiently fast, then $\tau$ happens to be restricted by some finite $\tau_{max}$ where $a\rightarrow+\infty$. So for lightcones this looks just like a Minkowski spacetime where you cut off everything after $\tau_{max}$. If the intersection lies exactly at $\tau_{max}$ this is a horizon, if it happens after that then no signal can pass.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From that it's obvious that the cosmological horizon for $x=0$ is the past lightcone originating at $\tau_{max}$ i.e. $|\vec{x}_H|=c(\tau_{max}-\tau)$ To find the point of no return we send lightsignal $x=c(\tau-\tau_0)$ and look where it intersects with the horizon lightcone. It's pretty obvious that the intersection will happen at $x_{NR}=x_H/2=\frac{c}{2}(\tau_{max}-\tau_0)$. For better understanding I add the illustration. Again I remind you that this is true for any $a(t)$ growing sufficiently fast in time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KqYYT.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KqYYT.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="18138" LastEditorUserId="18138" LastEditDate="2018-01-02T20:31:00.743" LastActivityDate="2018-01-02T20:31:00.743" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="24230" PostTypeId="2" ParentId="24206" CreationDate="2018-01-02T15:40:59.793" Score="18" Body="&lt;p&gt;A picture might be worth a thousand words. Just imagine this situation without clothes:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/c5y7r.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/c5y7r.jpg&quot; alt=&quot;one woman upside down between two men, board the ISS&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19229" LastEditorUserId="19229" LastEditDate="2022-02-08T17:22:02.657" LastActivityDate="2022-02-08T17:22:02.657" CommentCount="16" ContentLicense="CC BY-SA 4.0" />
  <row Id="24231" PostTypeId="2" ParentId="2823" CreationDate="2018-01-02T17:11:54.693" Score="13" Body="&lt;p&gt;from &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19960014824.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;the document&lt;/a&gt; referred to in &lt;a href=&quot;https://space.stackexchange.com/a/2824/22233&quot;&gt;TidalWave's answer&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dr. Robert C. Seamans, Jr.&lt;br /&gt;&#xA;Associate Administrator&lt;br /&gt;&#xA;National Aeronautics and Space Administration&lt;br /&gt;&#xA;1520 K Street, N.W.&lt;br /&gt;&#xA;Washington 25, D. C.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dear Dr. Seamans :&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Somewhat as a voice in the wilderness, I would like to pass on a few thoughts on matters that have been of deep concern to me over recent months. This concern may be phrased in terms of two questions: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;If you were told that we can put men on the moon with safe return with a&#xA;single C-3, its equivalent or something less, would you judge this statement&#xA;with the critical skepticism that others have? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is the establishment of a sound booster program really so difficult?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I would like to comment on both these questions, and more, would&#xA;like to forward as attachments condensed versions of plans which embody&#xA;ideas and suggestions which I believe are so fundamentally sound and&#xA;important that we cannot afford to overlook them. You will recall I&#xA;wrote to you on a previous occasion. I fully realize that contacting&#xA;you in this manner is somewhat unorthodox; but the issues at stake are&#xA;crucial enough to us all that an unusual course is warranted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since we have had only occasional and limited contact, and because&#xA;you therefore probably do not know me very well, it is conceivable that&#xA;after reading this you may feel that you are dealing with a crank. Do&#xA;not be afraid of this. The thoughts expressed here may not be stated&#xA;in as diplomatic a fashion as they might be, or as I would normally try&#xA;to do, but this is by choice and at the moment is not important. The&#xA;important point is that you hear the ideas directly, not after they have&#xA;filtered through a score or more of other people, with the attendant risk&#xA;that they may not even reach you.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Manned Lunar Landing Through Use of Lunar Orbit Rendezvous&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The plan.&lt;/strong&gt;- The first attachment outlines in brief the plan by which&#xA;we may accomplish a manned lunar landing through use of a lunar rendezvous,&#xA;and shows a number of schemes for doing this by means of a single&#xA;C-3, its equivalent, or even something less. The basic ideas of the&#xA;plan were presented before various NASA people well over a year ago, and&#xA;were since repeated at numerous interlaboratory meetings. A lunar landing&#xA;program utilizing rendezvous concepts was even suggested back In April.&#xA;Essentially, it had three basic points: &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;the establishment of an&#xA;early rendezvous program involving Mercury, &lt;/li&gt;&#xA;&lt;li&gt;the specific inclusion of&#xA;rendezvous in Apollo developments, and &lt;/li&gt;&#xA;&lt;li&gt;the accomplishment of lunar&#xA;landing through use of C-2's. &lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;It was indicated then that two C-2's could&#xA;do the job, C-2 being referred to simply because NASA booster plans did&#xA;not go beyond the C-2 at that time; it was mentioned, however, that with&#xA;a C-3 the number of boosters required would be cut in half, specifically&#xA;only one.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Regrettably, there was little interest shown in the idea - indeed, if&#xA;any, it was negative.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also (for the record), the scheme was presented before the Lundin&#xA;Committee. It received only bare mention in the final report and was&#xA;not discussed further (see comments below in section entitled &quot;Grandiose&#xA;Plans &quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It was presented before the Heaton Committee, accepted as a good idea,&#xA;then dropped, mainly on the irrelevant basis that it did not conform to&#xA;the ground rules. I even argued against presenting the main plan considered&#xA;by the Heaton Committee, largely because it would only bring harm to&#xA;the rendezvous cause, and further argued that if the committee did not&#xA;want to consider lunar rendezvous, at least they should make a strong&#xA;recommendation that it looks promising enough that it deserves a separate&#xA;treatment by itself - but to no avail. In fact, it was mentioned that if&#xA;I felt sufficiently strong about the matter, I should make a minority report.&#xA;This is essentially what I am doing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have given the plan to the presently meeting Golovin Committee&#xA;on several occasions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a rehearsal of a talk on rendezvous for the recent Apollo Conference,&#xA;I gave a brief reference to the plan, indicating the benefit&#xA;derivable therefrom, knowing full well that the reviewing committee&#xA;would ask me to withdraw any reference to this idea. As expected, this&#xA;was the only item I was asked to delete.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan has been presented to the Space Task Group personnel several&#xA;times, dating back to more than a year ago. The interest expressed has&#xA;been completely negative.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Ground rules.&lt;/strong&gt;- The greatest objection that has been raised about our&#xA;lunar rendezvous plan is that it does not conform to the &quot;ground rules&quot;.&#xA;This to me is nonsense; the important question is, &lt;strong&gt;&quot;Do we want to get to&#xA;the moon or not?&quot;&lt;/strong&gt;, and, if so, why do we have to restrict our thinking&#xA;along a certain narrow channel. I feel very fortunate that I do not have&#xA;to confine my thinking to arbitrarily set up ground rules which only serve&#xA;to constrain and preclude possible equally good or perhaps better approaches.&#xA;Too often thinking goes along the following vein: ground rules are set up,&#xA;and then the question is tacitly asked, &quot;Now, with these ground rules what&#xA;does it take or what is necessary to do the job?&quot;. A design begins and&#xA;shortly it is realized that a booster system way beyond present plans is&#xA;necessary. Then a scare factor is thrown in; the proponents of the plan&#xA;suddenly become afraid of the growth problem or that perhaps they haven't&#xA;computed so well, and so they make the system even larger as an &quot;insurance&quot;&#xA;that no matter what happens the booster will be large enough to meet the&#xA;contingency. Somehow, the fact is completely ignored that they are now&#xA;dealing with a ponderous development that goes far beyond the state-of-the-art.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why is there not more thinking along the following lines: Thus,&#xA;with this given booster, or this one, is there anything we can do to&#xA;do the job? In other words, why can't we also think along the lines of&#xA;deriving a plan to fit a booster rather than derive a booster to fit&#xA;a plan?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Three ground rules in particular are worthy of mention: three men,&#xA;direct landing, and storable return. These are &lt;strong&gt;very&lt;/strong&gt; restrictive requirements.  If two men can do the job, and if the use of only two men allows&#xA;the job to be done, then why not do it this way? If relaxing the direct&#xA;requirements allows the job to be done with a C-3, then why not relax it?&#xA;Further, when a hard objective look is taken at the use of storables,&#xA;then it is soon realized that perhaps they aren't so desirable or advantageous&#xA;after all in comparison with some other fuels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Grandiose plans, one-sided objections, and bias.&lt;/strong&gt;- For some inexplicable&#xA;reason, everyone seems to want to avoid simple schemes. The majority&#xA;always seems to be thinking in terms of grandiose plans, giving all&#xA;sort of argents for long-range plans, etc. Why is there not more&#xA;thinking in the direction of developing the simplest scheme possible?&#xA;Figuratively, why not go buy a Chevrolet instead of a Cadillac? Surely&#xA;a Chevrolet gets one from one place to another just as well as a Cadillac,&#xA;and in many respects with marked advantages. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have been appalled at the thinking of individuals and committees&#xA;on these matters. For example, comments of the following type have been&#xA;made: &quot;Houbolt has a scheme that has a 50 percent chance of getting a&#xA;man to the moon, and a 1 percent chance of getting him back.&quot; This comment was made by a Headquarters individual at 'high level who never&#xA;really has taken the time to hear about the scheme, never has had the&#xA;scheme explained to him fully, or possible even correctly, and yet he&#xA;feels free to pass judgement on the work. I am bothered by stupidity of&#xA;this type being displayed by individuals who are in a position to make&#xA;decisions which affect not only the NASA, but the fate of the nation as&#xA;well. I have even grown to be concerned about the merits of all the committees that have been considering the problem. Because of bias, the&#xA;intent of the committee is destroyed even before it starts and, further,&#xA;the outcome is usually obvious from the beginning. We knew what the&#xA;Fleming Committee results would be before it started. After one day&#xA;it was clear what decisions the Lundin Committee would reach. After a&#xA;couple days it was obvious what the main decision of the Heaton Committee&#xA;would be. In connection with the Lundin Committee, I would like to cite&#xA;a specific example. Considered by this committee was one of the most&#xA;hair-brained ideas I have ever heard, and yet it received one first place&#xA;vote. in contrast, our lunar rendezvous scheme, which I am positive is a&#xA;much more workable idea, received only bare mention in a negative vein,&#xA;as was mentioned earlier. Thus, committees are no better than the bias&#xA;of the men composing them. We might then ask, why are men who are not&#xA;competent to judge ideas, allowed to judge them?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps the substance of this section might be summarized this way.&#xA;Why is NOVA, with its ponderous ideas, whether in size, manufacturing,&#xA;erection, site location, etc., simply just accepted, and why is a much&#xA;less grandiose scheme involving rendezvous ostracized or put on the&#xA;defensive?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;PERT chart folly.&lt;/strong&gt;- When one examines the various program schedules&#xA;that have been advanced, he cannot help from being impressed by the&#xA;optimism shown. The remarkable aspect is that the more remote the year,&#xA;the bolder the schedule becomes. This is, in large measure, due to the&#xA;PERT chart craze. It has become the vogue to subject practically everything&#xA;to a PERT chart analysis, whether it means anything or not. Those&#xA;who apply or make use of it seen to be overcome by a form of self-hypnosis,&#xA;more or less accepting the point of view, &quot;Because the PERT chart says so,&#xA;it is so.&quot; Somehow, perhaps unfortunately, the year 1967 was mentioned&#xA;as the target year for putting a man on the moon. The Fleming report&#xA;through extensive PERT chart analysis then &quot;proved&quot; this could be done.&#xA;One cannot help but get the feeling that if the year 1966 had been&#xA;mentioned, then this would have been the date proven; likewise, if 1968&#xA;had been the year mentioned.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My quarrel is not with the basic theory of PERT chart analysis; I&#xA;am fully aware of its usefulness, when properly applied. I have been&#xA;nominally in charge of a facility development and know the merits,&#xA;utility, and succinctness by which it is helpful in keeping a going job&#xA;moving, uncovering bottlenecks, and so forth. But when it is used in&#xA;the nature of a crystal ball, then I begin to object. Thus, when we&#xA;scrutinize various schedules and programs, we have to be very careful&#xA;to ask how realistic the plan really is. often simple common sense&#xA;tells us much more than all the machines in the world.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I make the above points because, as you will see, we have a very&#xA;strong point to make about the possibility of coming up with a realistic&#xA;schedule; the plan we offer is exceptionally clean and simple in vehicle&#xA;and booster requirements relative to other plans.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Booster is pacing item.&lt;/strong&gt;- In working out a paper schedule we have&#xA;adopted the C-3 development schedule used by Fleming and Eeaton, not&#xA;necessarily because we feel the schedule is realistic, but simply to&#xA;make a comparison on a parallel basis. But whether the date is right,&#xA;or not, doesn't matter. Here, I just want to point out that for the&#xA;lunar rendezvous scheme the C-3 booster is the pacing item. Thus, we&#xA;can phrase our lunar landing date this way. We can put a man on the&#xA;moon as soon as the C-3 is developed and the number of C-3's required&#xA;is very small. (In fact, as I mentioned earlier, I would not be surprised&#xA;to have the plan criticized on the basis that it is not grandiose&#xA;enough.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Abort.&lt;/strong&gt;- An item which perhaps deserves special attention is abort.&#xA;People have leveled criticism, again erroneously and with no knowledge&#xA;of the situation, that the lunar rendezvous scheme offers no abort&#xA;possibilities. Along with our many technical studies we have also&#xA;studied the abort problem quite thoroughly. We find that there is no&#xA;problem in executing an abort maneuver at any point in the mission.&#xA;In fact, a very striking result comes out, just the reverse of the&#xA;impression many people try to create. When one compares, for example,&#xA;the lunar rendezvous scheme with a direct approach, he finds that on&#xA;every count the lunar rendezvous method offers a degree of safety and&#xA;reliability far greater then that possible by the direct approach.&#xA;These items are touched upon to a limted extent in the attached plan.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Booster Program&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;My comments on a booster program will be relatively short, since&#xA;the second attachment more or less speaks for itself. There are, however,&#xA;a few points worthy of embellishment. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Booster design.&lt;/strong&gt;- In the course of participating in meetings dealing&#xA;with vehicle design, I have sometimes had to sit back completely awed&#xA;mud astonished at what I was seeing take place. I have seen the course&#xA;of an entire meeting change because of an individual not connected with&#xA;the meeting walking in, looking over shoulders, shaking his head in a&#xA;negative sense, and then walking out without uttering a word. I have&#xA;seen people agree on velocity increments, engine performance, and&#xA;structural data, and after a booster design was made to these figures,&#xA;have seen some of the people then derate the vehicle simply because they&#xA;couldn't believe the numbers. I just cannot cater to proceedings of this&#xA;type. The situation is very much akin to a civil engineer who knows full&#xA;well that the material he is using will withstand 60,000 psi. He then&#xA;applies a factor of safety of 2.5, makes a design, then after looking at&#xA;the results, arbitrarily doubles the size of every member because he&#xA;isn't quite sure that the design is strong enough. A case in point is&#xA;the C-3. In my initial contacts with this vehicle, we were assured that&#xA;it had a payload capability in the neighborhood of ll0,000-120,000 lbs.&#xA;Then it was derated. The value used by the Heaton Committee was&#xA;105,000 lbs. By the time the vehicle had reached the Golovin Committee&#xA;I was amazed to find that it had a capability of only 82,570 lbs. Perhaps the only comment that can be made to this is that if we can't do&#xA;any better on making elementary computations of this type, then we&#xA;deserve to be in the pathetic situation we are. I also wonder where&#xA;we will stand after NOVA is derated similarly.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;&quot;Quantizing&quot; bad.&lt;/strong&gt;- One of the reasons our booster situation is in&#xA;such a sad state is the lack of appropriate engines, more specifically&#xA;the lack of an orderly stepping in engine sizes. Booster progress is&#xA;virtually at a standstill because there are no engines available, Just&#xA;as engines were the major pacing item in the development of aircraft.&#xA;Aside from the engines on our smaller boosters and the H-1 being used&#xA;on the C-1, the only engines we have in development are:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{array} { r|r }&#xA;\text{Capability} &amp;amp;\text{Ratio} \\&#xA;\hline&#xA;15,000 \\&#xA;&amp;amp;13.3 \\&#xA;200,000 \\&#xA;&amp;amp;7.5 \\&#xA;1,500,000 \\&#xA;\end{array}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The attempt to make boosters out of this stock of engines, having very&#xA;large ratios in capability, can only result in boosters of grotesque and&#xA;unwieldy configurations, and which require many, many in-flight engine&#xA;starts. What is needed are engines which step up in size at a lower&#xA;ratio. Consideration of the staging of an &quot;ideal&quot; rocket system indicates&#xA;that whether accelerating to orbit speed or to escape speed, the ratio of&#xA;engine sizes needed is in the order of 3. Logically then we ought to&#xA;have engines that step in capability by a factor of around 2, 3, or 4.&#xA;An every-day analog that can be mentioned is outboard motors. There&#xA;is a motor to serve nearly every need, and in the extreme cases the&#xA;process of doubling up is even used.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Booster program.&lt;/strong&gt;- In light of the preceding paragraph, and taking&#xA;into account the engines under development, we should add the following two:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{array} { r r }&#xA;80,000 - 100,000 &amp;amp;\textrm{H}_2 - \textrm{O}_2 \\&#xA;400,000 - 500,000 &amp;amp;\textrm{H}_2 - \textrm{O}_2 \\&#xA;\end{array}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would then give a line-up as follows:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{array} { r r r }&#xA;15,000 &amp;amp; &amp;amp;\textrm{H}/\textrm{O} \\&#xA;80,000 &amp;amp;- 100,000 &amp;amp;\textrm{H}/\textrm{O} \\&#xA;200,000 &amp;amp; &amp;amp;\textrm{H}/\textrm{O} \\&#xA;400,000 &amp;amp;- 500,000 &amp;amp;\textrm{H}/\textrm{O} \\&#xA;1500,000 &amp;amp; &amp;amp;\textrm{RP}/\textrm{O} \\&#xA;\end{array}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;with the 15,000-lb. engine really not needed. This array (plus those&#xA;mentioned immediately below) would allow the construction of almost all&#xA;types of boosters conceivable. For example, a single 80,000-100,000 engine would take the place of the six L-115 engines being used on S-IV;&#xA;not only is the arrangement of six engines on this vehicle bad, but&#xA;these engines have very poor starting characteristics.  The 400_000-500,000 would be used to replace the four J-2's on the S-II.  Thus,&#xA;C-3 would change from a messy 12-engined vehicle requiring 10 in-flight&#xA;engine starts to a fairly simple 5-engine vehicle with only 3 in-flight&#xA;engine starts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition, the following engines should be included in a program:&#xA;\begin{array} { l r r r }&#xA;1,000,000 &amp;amp;- 1,500,000 &amp;amp;\textrm{lb.} &amp;amp;\textrm{Solid} \\&#xA;&amp;amp;5,000,000 &amp;amp;&amp;amp;\textrm{Solid} \\&#xA;\textrm{and/or} &amp;amp;5,000,000 &amp;amp;&amp;amp;\textrm{Storable}\\&#xA;\end{array}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The 1,000,000 lb. solid would in itself be a good building&#xA;block and would probably work in nicely to extend the capabilities of&#xA;vehicles, such as Titan. The 5,000,000 solid and/or storable would also&#xA;be good building blocks and specifically would serve as alternate first-stage&#xA;boosters for C-3, aiming at simplicity and reliability.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may be said that there is nothing new here and that all of the&#xA;above is obvious. Indeed, it seems so obvious that one wonders why such&#xA;a program has not started 5 years ago. But the fact that it may be&#xA;obvious doesn't help us; what is necessary is putting the obvious into&#xA;effect. In this connection, there may be some who ask, &quot;But are the&#xA;plans optimum and the best?&quot;. This question is really not pertinent.&#xA;There will never be an optimized booster or program. We might have an&#xA;optimum booster for a given situation, but there is none that is optimum&#xA;for all situations. To seek one, would just cause deliberation to string&#xA;out indefinitely with little, if any, progress being made. The Dyna-Soar&#xA;case is a good example of this.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A criticism that undoubtedly will be leveled at the above suggestions&#xA;is that I'm not being realistic in that there is just not enough money&#xA;around to do all these things. If this is the situation, then the answer&#xA;is simply that's why we have Webb and his staff. That's why he was&#xA;chosen to head the organization, this is one of his major functions,&#xA;to ask the question, do we want to do a job or not?, and, if so, then&#xA;to find out where the gaps or holes are, and then to go about doing what&#xA;is necessary to fill the gaps to make sure the job gets done.  Further,&#xA;the load doesn't have to be carried by the NASA alone. The Air Force and&#xA;NASA can work together and share the load, and I'm sure that if this is&#xA;done, the necessary money can be found. Even if some project, say, for&#xA;example, the 5,000,000-lb. storable engine has to be dropped for some&#xA;reason after it gets started; no harm will be done. This happens every&#xA;day. On the contrary, some good, some new knowledge, will have been&#xA;uncovered, even if it turns out to be the discovery of the next obstacle which prevents such a booster from being built.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;Nuclear booster and booster size.&lt;/em&gt;- Although not 'mentioned in the&#xA;previous section, work on nuclear engines should, of course, continue.&#xA;Any progress made here will integrate very nicely into the booster plans&#xA;indicated in the attachment.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As regards booster size, the following comment is offered. Excluding&#xA;for the moment NOVA type vehicles, we should strive for boosters which&#xA;make use of the engines mentioned in the preceding section and which&#xA;are the biggest that can be made and yet still be commensurate with&#xA;existing test-stand sites and with the use of launch sites that are&#xA;composed of an array of assembly buildings and multiple launch pads.&#xA;The idea behind launch sites of this type is an excellent one. It keeps&#xA;real estate demands to a minimum, allows for ease in vehicle assembly&#xA;and check-out, end greatly eases the launch rate problem. Thus, C-3 or&#xA;C-4, should be designed accordingly. We would then have a nice work-horse&#xA;type vehicle having relative ease of handling, and which would permit a&#xA;lunar landing mission, as indicated earlier in the lunar rendezvous&#xA;write-up section. From my point of view, I would much rather confine&#xA;my spending to a single versatile launch site of the type mentioned, save&#xA;money in real estate acquisition and launch site development necessary for&#xA;the huge vehicles, and put the money saved into an engine development program.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Concluding Remarks&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;It is one thing to gripe, another to offer constructive criticism.&#xA;Thus, in making a few final remarks I would like to offer what I feel&#xA;would be a sonud integrated overall program. I think we should:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Get a rearmed rendezvous experiment going with the&#xA;Mark II Mercury.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Firm up the engine program suggested in this letter&#xA;and attachment; converting the booster to these engines as&#xA;soon as possible.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Establish the concept of using a C-3 and lunar rendezvous&#xA;to accomplish the manned lunar landing as a firm program.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Naturally, in discussing matters of the type touched upon herein,&#xA;one cannot make comments without having them smack somewhat against&#xA;NOVA. I want to assure you, however, I'm not trying to say NOVA should&#xA;not be built, i'm simply trying to establish that our scheme deserves&#xA;a parallel front-line position. As a matter of fact, because the lunar&#xA;rendezvous approach is easier, quicker, less costly, requires less&#xA;development, less new sites and facilities, it would appear more&#xA;appropriate to say that this is the way to go, and that we will use&#xA;NOVA as a follow on. Give us the go-ahead, and a C-3, and we will put&#xA;men on the moon in very short order - and we don't need any Houston&#xA;empire to do it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In closing, Dr. Seamens, let me say that should you desire to discuss&#xA;the points covered in this letter in more detail, I would welcome&#xA;the opportunity to come up to Headquarters to discuss them with you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Respectfully yours,&lt;/p&gt;&#xA;&#xA;&lt;p&gt;John C. Houbolt&lt;/p&gt;&#xA;" OwnerUserId="22233" LastEditorUserId="22233" LastEditDate="2018-08-15T01:14:30.127" LastActivityDate="2018-08-15T01:14:30.127" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24232" PostTypeId="1" CreationDate="2018-01-02T17:17:29.253" Score="20" ViewCount="6177" Body="&lt;p&gt;I wonder if triangle stack would be a better design to handle thrust and stability of the flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/G0ziD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/G0ziD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.flickr.com/photos/spacex/38583830575/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Photo source (public domain)&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="22237" LastEditorUserId="19891" LastEditDate="2018-01-04T03:35:22.527" LastActivityDate="2018-01-04T09:37:03.727" Title="Why are the Falcon Heavy boosters stacked in a row instead of a triangle?" Tags="|launch|spacex|rockets|falcon-heavy|" AnswerCount="3" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24233" PostTypeId="2" ParentId="10837" CreationDate="2018-01-02T17:20:08.470" Score="8" Body="&lt;p&gt;The short answers is to ensure ground track repeatability. &#xA;And the period is not 12 hours but half a sidereal day (that is about 4 minutes shorter), so that when the earth have done one rotation, the satellites have done two and the geometry of the whole constellation relative to earth is the same than one sidereal day before.&#xA;Repeatability is important for multiple reasons, one of them been that some errors related to the atmosphere or ground reflections (i.e. multipath) are dependent on geometry. If the geometry is the same each sidereal day the errors will be similar, therefore the displacements computed in a sidereal-day-to-sidereal-day basis are very accurate, because been the errors so similar they cancel out when computing displacements (or speeds).&#xA;Also corrections of atmospheric effects or multipath effects are much easier to calculate and reuse if the ground tracks repeat (which is the same than saying that the satellites return to the same positions in the sky every one sidereal day).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now another question is why to choose half sidereal day instead of a third or a quarter. I'm not 100% sure about this but I'm pretty confident it is due to the fact that in contrast with other satellites, for GPS satellite to be useful their position has to be know with really high accuracy and in real time, so for this to be achieved, the bigger the orbit the easier, because of slower speed and smaller perturbations due to the non-central gravity field of the Earth, and atmospheric drag.&#xA;So why not orbits with one full sidereal day period? Probably due to cost (to get them to the orbit and to transmit with more power), so half a sidereal day was the cheaper that still allowed to meet the satellite position accuracy specifications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This paper have a good treatment and explain of how multipath repeatability is important for solution quality and how such repeatability can be used to improve GPS solutions. Also explains that the period is close to one Sidereal day: &lt;a href=&quot;http://xenon.colorado.edu/larsonetal_2007.pdf&quot; rel=&quot;noreferrer&quot;&gt;Improving the precision of high-rate GPS&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22234" LastEditorUserId="22234" LastEditDate="2018-01-05T16:38:34.583" LastActivityDate="2018-01-05T16:38:34.583" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24234" PostTypeId="2" ParentId="24232" CreationDate="2018-01-02T17:22:47.587" Score="10" Body="&lt;p&gt;The row works better for the intended purpose. The center booster will not be consuming fuel as quickly as the side boosters. This will allow it to continue on for a time after the side boosters have been dropped off.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one were to do a triangle, it would be more stable, but would not allow for releasing two of the boosters separate from the third.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-02T17:22:47.587" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24235" PostTypeId="2" ParentId="10860" CreationDate="2018-01-02T17:34:32.733" Score="3" Body="&lt;p&gt;\begin{align}&#xA;T&amp;amp;=24\times60^2&amp;amp;&amp;amp;=86400\,s\\&#xA;\omega&amp;amp;=2\pi f&amp;amp;&amp;amp;={2\pi\over T}\\&#xA;F&amp;amp;={m v^2\over r}&amp;amp;&amp;amp;=m\omega^2r\\&#xA;\therefore F&amp;amp;=m\left({2\pi\over T}\right)^2r&amp;amp;&amp;amp;= {4\pi^2mr\over T^2}\\&#xA;\text{And }F&amp;amp;={GMm\over r^2}\\&#xA;&amp;amp; \text{For height to be maintained: }\sum f=0\\&#xA;{4\pi^2mr\over T^2}&amp;amp;={Gm\over r^2}\\&#xA;\therefore r^3&amp;amp;={T^2GM\over4\pi^2}\\&#xA;\therefore r&amp;amp;=\root 3\of{T^2GM\over4\pi^2}\\&#xA;T&amp;amp;=86400, G=6.67\times10^{-11},M=5.97\times10^{24}\\&#xA;\therefore r&amp;amp;=\root 3\of{86400^2\times6.67\times10^{-11}\times5.97\times10^{24}\over4\pi^2}\\&#xA;r&amp;amp;=42,226km\;\text{from centre of Earth}\\&#xA;h&amp;amp;=r-R\\&#xA;\therefore h&amp;amp;=42,226km-6370km=35856km&#xA;\end{align}&#xA;$M$ is the mass of Earth. $R$ is the radius of Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is my attempt at getting the value. It is off by a little bit but this may be due to accuracy of numbers used and considering the orbit perfectly circular.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, in order for it to orbit correctly it must have the same angular velocity as earth (rotate at the same speed), which means having the same frequency or time period of rotation as the earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The weight of the object orbiting must then be equal to the centripetal force it has acting on it due to the circular motion. As others have said if these two forces are not equal then it will either crash into earth or fly off. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this point onward it is just maths to calculate the actual value, remembering that this value of r gives the radius of orbit which is distance from the centre of the earth, so you must subtract R to get the height above earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;From this you could calculate a velocity that the satellite is traveling at but in this area generally angular velocity is used more. Most people would not know what to do with this velocity either as it doesn't mean much and isn't useful.&lt;/p&gt;&#xA;" OwnerUserId="22235" LastEditorUserId="6816" LastEditDate="2018-01-08T00:30:27.660" LastActivityDate="2018-01-08T00:30:27.660" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24236" PostTypeId="2" ParentId="24232" CreationDate="2018-01-02T18:44:41.220" Score="30" Body="&lt;p&gt;The boosters are in a row because it's easy to assemble them horizontally when they're in a row. If they were stacked, the lower 2 stages would have to carry the weight of the third, so they'd need to be made stronger.   &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan is also to have the center core to run at a lower thrust setting. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a triangle setup with the 3 cores as they are now, i.e. 1 two-stage rocket with the payload on top, plus 2 boosters), total thrust (as well as weight) would be asymmetric, which is undesirable: to compensate, you'd have to gimbal the engines and thrust at an angle, which is less efficient than thrusting along the velocity vector. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Putting the 3 cores in a triangle has another drawback: the attachment points now overlap each other, making it more difficult to have 2 identical &quot;side&quot; cores. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Underfilling one of the side cores to stagger separation is possible, but reduces performance because the core will shut down earlier. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Back in the 1960s, British company BAC was working on &lt;a href=&quot;https://www.secretprojects.co.uk/forum/index.php?topic=2260.0&quot; rel=&quot;noreferrer&quot;&gt;MUSTARD&lt;/a&gt;, a two-stage to orbit fully reusable vehicle. This consisted of 3 lifting bodies of identical shape. 2 were the boosters, the third was the spacecraft. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were 2 ways to stack the 3 vehicles: in line, or in a triangle. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4uwwu.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4uwwu.jpg&quot; alt=&quot;Mustard takeoff&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;BAC studied both configurations (plus lots of others) and found the in-line configuration was the better one, in part because of the balance problems of the triangle (from 'British secret projects volume 5: Britain's space shuttle', by Dan Sharp, page 149). Given that Mustard was a low-budget R&amp;amp;D program, the fact that different booster configurations have been studied tells me the Falcon Heavy configuration was not arrived at by accident, IOW, SpaceX will have looked at the alternatives and discarded them because they did not offer a benefit over the one they chose to implement. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, consider history: every rocket with 2 boosters has them in line instead of in a triangle. Ariane 4, 5, 6. Titan IIC, 34D, Delta IV Heavy, Space Shuttle, to name a few. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Edit&lt;/strong&gt;: &#xA;A configuration with 3 first stages in a triangle, plus a second stage and payload in the center of that triangle would be worse: now you have to run all 3 stages at the same power, and have to ditch them at the same time. This means the first stage has a shorter total run time compared to the real-world configuration.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In RW, the center core is run at a lower thrust setting. So when the 2 boosters separate, the center core still has fuel left and can keep running for a while. This increases performance over running the 3 cores at the same thrust. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a triangle with the second stage in the middle, you'd need a new, larger second stage to compensate for the lower performance of the first stage.&lt;br&gt;&#xA;This configuration reduces commonality with the F9: second stage is unique, you'd need a new interstage to connect the 3 cores with the second stage, this interstage needs separation mechanisms to separate it both from the cores and from the second stage (while the F9 interstage stays attached to the core). You can't recover the interstage any more. Interstage gets much heavier, reducing performance. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-04T09:37:03.727" LastActivityDate="2018-01-04T09:37:03.727" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24237" PostTypeId="2" ParentId="1570" CreationDate="2018-01-02T18:51:50.207" Score="3" Body="&lt;p&gt;Liquid rocket motors (esp. ones that use turbopumps) are usually shut down before fuel/oxidizer depletion. When a turbopump runs dry, it'll overspeed and disintegrate, and that's best avoided. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-01-02T18:51:50.207" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24238" PostTypeId="2" ParentId="24205" CreationDate="2018-01-02T18:52:55.893" Score="6" Body="&lt;p&gt;They are on to something... else than they try to claim on their site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://www.nextbigfuture.com/2016/10/positron-dynamics-vision-of-antimatter.html&quot; rel=&quot;noreferrer&quot;&gt;paper&lt;/a&gt; on the actual science explains the concepts.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is not intended to be the &quot;holy grail&quot;, genuine antimatter drive (a photon drive utilizing gamma rays from annihilation of positron and electon; specific impulse of the maximal ${c}\over{g_0}$ and 100% mass to energy conversion), but a ion thruster, with positrons used to produce muons for &lt;a href=&quot;https://en.wikipedia.org/wiki/Muon-catalyzed_fusion&quot; rel=&quot;noreferrer&quot;&gt;muon-catalyzed fusion&lt;/a&gt; as source of ion propulsion (instead of electric field).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Muon-catalyzed fusion is a &quot;cold fusion&quot; process which, while working, was considered (similarly to many other similar) useless practically, the reason being a relatively low number of deuterium or tritium atoms catalyzed into fusion to helium, per positron, and obviously prohibitive price of antimatter-generating isotopes. In other words, the wattage of the process is minuscule, and energy needed to produce the anti-matter - way higher than the energy output of this fusion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Still, products of the muon-catalyzed fusion are extremely hot (=fast) ions (= charged particles which can be directed using magnetic fields). In other words, it can be used for a ion engine of quite extreme specific impulse - heat of nuclear fusion replacing accelerating ions through electric field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unfortunately, the &lt;em&gt;poor wattage&lt;/em&gt; problem remains. The site the asker linked provides beautiful visions, &quot;months to Pluto, Proxima Centauri in 5 years, a craft accelerating at sustained 1g&quot;. The paper makes much more modest claims:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;the 6U cubesat that they will use to test the propulsion in space will be generating 100s of watts&lt;/li&gt;&#xA;&lt;li&gt;the propulsion will have delta V of 1 to 10 km/second&lt;/li&gt;&#xA;&lt;li&gt;Later systems will have more delta V and enable cubesats and small satellites to stay in orbit for years instead of days&lt;/li&gt;&#xA;&lt;li&gt;the cubesats with propulsion will enable very low orbit internet satellites&lt;/li&gt;&#xA;&lt;li&gt;in the 2020s if things go well they will be able to scale to 10 km/second to 100 km/second with 10-100 kilogram payloads for small probe exploration of the solar system&lt;/li&gt;&#xA;&lt;li&gt;Later beyond 2030, they will have regenerative isotopes for a lot more power and achieve ten million ISP and several kilonewtons of propulsive force&lt;/li&gt;&#xA;&lt;li&gt;could enable 1G acceleration and deceleration propulsion which would 3.5 weeks to Pluto&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;One significant actual breakthrough they claim to have made is production of &quot;cold positrons&quot; (usable for catalyzing the fusion) at a rate an order of magnitude greater than currently available, and with promise for improvement. This could indeed reduce the amount (=cost) of antimatter-producing isotope needed for this to work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's still a lot of very tall claims, especially for the schedule, without even a decent test article device for the thruster. It's still a viable (if not realistically near) concept for a fusion-powered ion drive, which would indeed surpass everything we have in terms of specific impulse and delta-V total. Regardless, there's nothing that would suggest thrust in newton range, never mind the claimed kilonewtons. While the energy density of the fuel (deuterium+tritium) is fantastic, the catalyst-producing isotopes (e.g $^{22}Na$) are still prohibitively expensive and must be used in macroscopic amounts to trigger release of any macroscopic amounts of energy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(and this is just a tip of the iceberg of problems, like half-life of the fuel, powering magnetic fields to handle the high-energy ions, all the accidental hard gamma radiation from positron-electron annihilation (which WILL happen, wanted or not), and lots and lots the authors of the proposal conveniently remained silent about.)&lt;/p&gt;&#xA;" OwnerUserId="103" LastActivityDate="2018-01-02T18:52:55.893" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24239" PostTypeId="2" ParentId="24232" CreationDate="2018-01-02T19:30:29.587" Score="17" Body="&lt;p&gt;The other answers have excellent reasons for why the 3 boosters on the Falcon Heavy are in a line, however they haven't mentioned the asymmetrical weight distribution that making a triangle would cause.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Consider: 2 regular, &quot;unloaded&quot; boosters (no fairing/payload) and one &quot;loaded&quot; booster in a triangle. The booster / corner with the payload would be heavier.&lt;/p&gt;&#xA;" OwnerUserId="22240" LastEditorUserId="6816" LastEditDate="2018-01-02T19:41:59.093" LastActivityDate="2018-01-02T19:41:59.093" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24240" PostTypeId="1" AcceptedAnswerId="24242" CreationDate="2018-01-02T20:11:42.057" Score="5" ViewCount="461" Body="&lt;p&gt;Why would the falcon heavy sit on the launch pad if it is not yet ready to be launched?  Wouldn't this just make it susceptible to damage to inclement weather and possible (although not probable due to lightening rods) lightening strikes?&lt;/p&gt;&#xA;" OwnerUserId="13032" LastActivityDate="2018-01-03T01:48:55.057" Title="Why was the falcon heavy already on the pad?" Tags="|spacex|falcon-heavy|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24241" PostTypeId="2" ParentId="24240" CreationDate="2018-01-02T20:40:37.313" Score="5" Body="&lt;p&gt;Because this will be the first Falcon Heavy launch, they did a &lt;a href=&quot;https://www.nasaspaceflight.com/2017/12/falcon-heavy-maiden-static-fire-test/&quot; rel=&quot;noreferrer&quot;&gt;fit check&lt;/a&gt;: basically they ran through the process of moving the rocket to the launch pad and erecting it, to see if everything works as designed, a.k.a. validation.&lt;br&gt;&#xA;The rocket will be rolled out again for a static fire (i.e. running the first stage engines while holding the rocket down): another test for rocket, pad and prelaunch procedures.&lt;br&gt;&#xA;When that's done, there will be a final inspection (vehicle will be rolled back into the hangar for this) and then it's time to launch. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Sometimes a separate, non-flight rocket is used for fit checks. There were 5 &lt;a href=&quot;https://en.wikipedia.org/wiki/SA-500F&quot; rel=&quot;noreferrer&quot;&gt;non-flight Saturn V rockets&lt;/a&gt;, for instance. While this reduces the risk to flight hardware, it's an expensive way to work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Risk to the hardware from weather can be mitigated by using the same procedures as for a launch: the weather forecast is checked, and if weather is outside of preset parameters, the rollout doesn't take place. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-02T21:22:46.847" LastActivityDate="2018-01-02T21:22:46.847" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24242" PostTypeId="2" ParentId="24240" CreationDate="2018-01-02T20:44:32.943" Score="9" Body="&lt;p&gt;This is a new booster (three cores vs one is a &lt;strong&gt;&lt;em&gt;big&lt;/em&gt;&lt;/strong&gt; change).  The TEL was updated to hold it.  Clearly it 'fits' on the TEL, since they mounted it on the TEL in the HIF.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But then they raised it at the pad to make sure all the connections are where they need to be placed, and that they all fit.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next up is the wet dress rehearsal (WDR) in which they then actually fill the boosters up with fuel and oxidizer to ensure that they can handle the load (enough prechilling for all the LOX and RP1). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then they drain it (which is also a test, since if they have to abort they need to be able to drain the tanks safely) and roll it back into the HIF to ensure all is still well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If, however, the WDR goes well they are suggesting that they would go straight into a static fire, since they are all dressed up with nowhere to go. If not, they will fix whatever the issue requires.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then, finally, they will get ready for a real launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is possible, likely even, that the payload will be removed for the WDR/Static fire as a holdover from the AMOS-6 RUD. Or not. Time will tell.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="-1" LastEditDate="2018-01-03T01:48:55.057" LastActivityDate="2018-01-03T01:48:55.057" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24243" PostTypeId="2" ParentId="24010" CreationDate="2018-01-02T22:13:06.173" Score="1" Body="&lt;p&gt;At the present time, there &lt;em&gt;is&lt;/em&gt; a plan to incorporate a planetary coronagraph on WFIRST based on the PISCES integral field spectrograph (IFS).  The instrument will use a local occulter, rather than a starshade.  The coronagraph will not resolve the planetary disk, but will be able to see a point image (PSF) separate from the host star.  This will enable the IFS to extract a spectrum.  The &quot;smoking gun&quot; for a habitable planet is a combination of water vapor, oxygen, and methane.  [Methane can not endure over geologic time in the presence of free oxygen, so there must be a constant source (life?)].&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lessons learned from the coronagraph on WFIRST will be used to optimize the design of a revised planetary coronagraph on LUVOIR, or whatever the eventual name of the follow on to Hubble.  A starshade is not only expensive (as Hobbs pointed out), but it also takes a long time to re-target.  In an ideal world, and with sufficient funds, the local occulter would be used to get a first order spectrum of promising planets, then the most promising would be re-visited with a starshade to extract more detailed spectra.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additionally, JPL is planning to propose the HabEx Mission which would incorporate a planetary coronagraph, also including an IFS.  I believe it's important to point out that their site says: &lt;em&gt;... directly image planetary systems around Sun-like stars&lt;/em&gt;.  This means they will be able to image luminous points (PSFs) that are distinguishable from the host star and adjacent planets--not planetary disks.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, priorities change and funds come and go, so none of this is written in stone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;WFIRST/PISCES Integral Spectrograph: &lt;a href=&quot;https://arxiv.org/abs/1707.07779&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://arxiv.org/abs/1707.07779&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Proposed HabEx Mission: &lt;a href=&quot;https://www.jpl.nasa.gov/habex/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.jpl.nasa.gov/habex/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17341" LastActivityDate="2018-01-02T22:13:06.173" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24244" PostTypeId="1" CreationDate="2018-01-02T22:48:03.070" Score="4" ViewCount="507" Body="&lt;p&gt;I'm curious about this picture taken from Osiris-Rex. There is a faint vertical line nearly the width of Earth going from the top to the bottom of the image. What is it? Does it have something to do with the camera, or is it some sort of leftovers from cleaning up the image, or something else?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the image with &quot;Shadows Enhanced&quot;: &lt;a href=&quot;https://i.stack.imgur.com/W0scW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://i.stack.imgur.com/W0scW.png&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Probable source: &lt;a href=&quot;https://www.nasa.gov/image-feature/from-the-earth-moon-and-beyond&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/image-feature/from-the-earth-moon-and-beyond&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;More info: &lt;a href=&quot;https://earthobservatory.nasa.gov/IOTD/view.php?id=91494&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://earthobservatory.nasa.gov/IOTD/view.php?id=91494&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Explanation: &lt;a href=&quot;https://www.asteroidmission.org/?attachment_id=3229#main&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.asteroidmission.org/?attachment_id=3229#main&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FWC1z.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FWC1z.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="10234" LastEditorUserId="12102" LastEditDate="2018-01-03T03:14:54.510" LastActivityDate="2018-01-03T22:30:34.443" Title="What is the line in this picture?" Tags="|imaging|osiris-rex|image-processing|" AnswerCount="4" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24245" PostTypeId="2" ParentId="24244" CreationDate="2018-01-02T23:03:43.627" Score="7" Body="&lt;p&gt;The line you are referencing is because the Earth is very bright on a &quot;black&quot; background. It happens as the light overpowers the image chip, similar to the Starburst pattern bright headlights make on dark nights.&lt;/p&gt;&#xA;" OwnerUserId="22238" LastActivityDate="2018-01-02T23:03:43.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24246" PostTypeId="2" ParentId="20986" CreationDate="2018-01-02T23:19:15.583" Score="4" Body="&lt;p&gt;Nothing can be perfectly inflexible without causing undue stress on the hold downs. Similar to using a longer crowbar to gain leverage making any wind load transfer directly to the hold-down clamps, etc. The motion displayed in the image is likely a light breeze as the clamping mechanism lets go and wouldn't affect the launch vehicle itself. The Falcon 9 fairing is listed as 5.2m in diameter, the motion I estimate about a .1m in 10kt wind with a 30kt wind limit would put it at about 25% of the Acceptable loading giving an estimated flex of about .4m for the Stack.  The maximum flexion would vary due to the construction materials and design. &lt;/p&gt;&#xA;" OwnerUserId="22238" LastEditorUserId="22238" LastEditDate="2018-01-03T00:47:21.237" LastActivityDate="2018-01-03T00:47:21.237" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24247" PostTypeId="2" ParentId="24244" CreationDate="2018-01-02T23:57:19.963" Score="7" Body="&lt;p&gt;The image was taked by the OCAMS instrument.  OCAMS uses CCD (Charge Coupled Device) focal planes.  The charges generated by the incident light in each frame are transferred down the columns one pixel at a time.  If there is an especially bright object, it may affect the remainder of the column.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note: OCAMS is a suite of cameras: PolyCam, MapCam, and SamCam. which have narrow, medium, and wide fields of view respectively. All use the same CCD focal plane design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Charge Coupled Device (CCD):&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Charge-coupled_device&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Charge-coupled_device&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;OSIRIS-Rex OCAMS Instrument:&#xA;&lt;a href=&quot;https://arxiv.org/ftp/arxiv/papers/1704/1704.04531.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://arxiv.org/ftp/arxiv/papers/1704/1704.04531.pdf&lt;/a&gt;&lt;br&gt;&#xA;   (Note that the figures are at the end of the .PDF).&lt;/p&gt;&#xA;" OwnerUserId="17341" LastEditorUserId="17341" LastEditDate="2018-01-03T22:30:34.443" LastActivityDate="2018-01-03T22:30:34.443" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24248" PostTypeId="1" AcceptedAnswerId="24256" CreationDate="2018-01-03T00:40:07.223" Score="7" ViewCount="5081" Body="&lt;p&gt;This &lt;a href=&quot;https://physics.stackexchange.com/a/367808/83380&quot;&gt;excellent answer&lt;/a&gt; nicely describes both &lt;a href=&quot;https://en.wikipedia.org/wiki/Tundra_orbit&quot; rel=&quot;noreferrer&quot;&gt;Tundra orbits&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/Molniya_orbit&quot; rel=&quot;noreferrer&quot;&gt;Molniya orbits&lt;/a&gt;. They are both &lt;a href=&quot;https://space.stackexchange.com/a/18079/12102&quot;&gt;repeat groundtrack orbits&lt;/a&gt; with periods of rational fractions of a sidereal day, and both have inclinations of $\arcsin(\sqrt{4/5}) \approx 63.4°$ to zero the precession of the argument of perigee.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The linked articles say that Tundra orbits have eccentricity of about 0.2 to 0.3 (though QZSS are only about 0.075) and Molniya orbits are about 0.7. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What are the &lt;em&gt;orbital mechanical&lt;/em&gt; and &lt;em&gt;functional&lt;/em&gt; differences between these two classes of orbits? Is a Molniya the same as a Tundra, except that the eccentricity of the latter is high enough to be useful to Moscow while the Tundra is good enough for Tokyo (e.g. &lt;a href=&quot;https://en.wikipedia.org/wiki/Quasi-Zenith_Satellite_System&quot; rel=&quot;noreferrer&quot;&gt;QZSS&lt;/a&gt;, see also &lt;a href=&quot;http://qzss.go.jp/en/technical/technology/orbit.html&quot; rel=&quot;noreferrer&quot;&gt;the orbit&lt;/a&gt;)? Or are there other meaningful distinctions?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-03T03:09:38.253" LastActivityDate="2020-12-18T14:55:31.540" Title="Difference between Tundra and Molniya orbits?" Tags="|orbital-mechanics|communication-satellite|geosynchronous|molniya|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24249" PostTypeId="1" AcceptedAnswerId="24260" CreationDate="2018-01-03T02:10:36.357" Score="1" ViewCount="157" Body="&lt;p&gt;BNNTs are a upcoming and promising nano-tube for space exploration. Featuring a young's modulus similar to diamond [1] and offering extensive radiation protection [2],&#xA;Could BNNTs be utilized in a Whipple Shield-like design for interplanetary spacecraft? Current designs of the Whipple Shield use Kevlar, but I believe that BNNTs could potentially be stronger than Kevlar [3].&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This question is mostly for fact-checking the possibility of the replacement of Kevlar in standard Whipple Shields with Boron-Nitrate Nano-tubes for increased ballistic and radiation protection. I would prefer hard-science answers, but any response would be appreciated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[1] &#xA;(&lt;a href=&quot;http://www.nims.go.jp/eng/news/press/2010/09/p201009140.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.nims.go.jp/eng/news/press/2010/09/p201009140.html&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[2] (&lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/niac_2011_phasei_thibeault_radiationshieldingmaterials_tagged.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/sites/default/files/atoms/files/niac_2011_phasei_thibeault_radiationshieldingmaterials_tagged.pdf&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;[3]&#xA;(&lt;a href=&quot;http://www.dupont.com/content/dam/dupont/products-and-services/fabrics-fibers-and-nonwovens/fibers/documents/Kevlar_Technical_Guide.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.dupont.com/content/dam/dupont/products-and-services/fabrics-fibers-and-nonwovens/fibers/documents/Kevlar_Technical_Guide.pdf&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="22156" LastActivityDate="2021-03-26T15:49:46.813" Title="Could BNNTs be used for Whipple Shield designs?" Tags="|spacecraft|interplanetary|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24250" PostTypeId="2" ParentId="24244" CreationDate="2018-01-03T02:36:50.160" Score="5" Body="&lt;p&gt;The streak is perfectly aligned to the vertical axis. Assuming that it hasn't been rotated, it is likely to be an artifact from imager chip, which may have saturated or overloaded - more charge/pixel than they can linearly process. However I noticed that the color of the streak is pure white, which is hard to understand unless it is caused specifically by the white clouds. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The image of the Earth is slightly saturated, and it can be seen that the bumps in the vertically collapsed image of the earth correlate well with the bumps in the vertically collapsed streak, so while somehow this is likely the cause, the particulars may lie in the processing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It is important to note that while &lt;a href=&quot;https://en.wikipedia.org/wiki/OSIRIS-REx&quot; rel=&quot;noreferrer&quot;&gt;Orisis&lt;/a&gt;' target &lt;a href=&quot;https://en.wikipedia.org/wiki/101955_Bennu&quot; rel=&quot;noreferrer&quot;&gt;Bennu&lt;/a&gt; has a similar distance from the Sun as the Earth, it's albedo is only about 0.05. The camera is optimized for darker images, so those white clouds may simply be brighter than the design range of the whole system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your &lt;a href=&quot;https://earthobservatory.nasa.gov/IOTD/view.php?id=91494&quot; rel=&quot;noreferrer&quot;&gt;last link&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;On October 2, 2017, the MapCam instrument on OSIRIS-REx captured the data for a composite image (above) of the Earth and Moon. The spacecraft was approximately 5 million kilometers (3 million miles) from Earth at the time, about 13 times the distance between the Earth and Moon. (&lt;a href=&quot;https://www.asteroidmission.org/?attachment_id=3229#main&quot; rel=&quot;noreferrer&quot;&gt;Click here&lt;/a&gt; to see the geometry of the shot.) Three images (different color wavelengths) were combined and color-corrected to make the composite, and the Moon was “stretched” (brightened) to make it more easily visible.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;and according to &lt;a href=&quot;http://spaceflight101.com/osiris-rex/osiris-rex-instrument-posters/ocams-schematic-mapcam/&quot; rel=&quot;noreferrer&quot;&gt;Spaceflight 101&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;MapCam is a medium-range camera. It searches for satellites and outgassing plumes around Bennu, maps the asteroid in color, and provides images to construct topographic maps. Its filter wheel and five-element lens system allows both panchromatic (clear) and wide-band spectral imaging in the blue, green, red, and infrared.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Read more technical information in &lt;a href=&quot;https://arxiv.org/abs/1704.04531&quot; rel=&quot;noreferrer&quot;&gt;OCAMS: The OSIRIS-REx Camera Suite&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oAvtS.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oAvtS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KVqZE.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KVqZE.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;img = plt.imread('FWC1z.png')[...,:3]  # get rid of alpha&#xA;&#xA;print img.shape&#xA;&#xA;earth  =   img[244:284, 150:250]&#xA;streak = 2*img[400:800, 150:250].reshape(40, 10, 100, -1).sum(axis=1) # vert bin by 10&#xA;&#xA;plt.figure()&#xA;plt.subplot(2, 2, 1)&#xA;plt.imshow(earth) &#xA;&#xA;plt.subplot(2, 2, 3)&#xA;for thing in earth.sum(axis=0).T:  # plot r, g, b separately&#xA;    plt.plot(thing)&#xA;plt.ylim(0, None)&#xA;&#xA;plt.subplot(2, 2, 2)&#xA;plt.imshow(streak) &#xA;&#xA;plt.subplot(2, 2, 4)&#xA;for thing in streak.sum(axis=0).T:  # plot r, g, b separately&#xA;    plt.plot(thing)&#xA;plt.ylim(0, None)&#xA;&#xA;plt.show()&#xA;&#xA;earth_2 = earth[:, 20:80]&#xA;&#xA;plt.figure()&#xA;plt.subplot(2, 1, 1)&#xA;plt.imshow(earth_2)&#xA;&#xA;titles = &quot;red&quot;, &quot;green&quot;, &quot;blue&quot;&#xA;for i, (color, title) in enumerate(zip(np.rollaxis(earth_2, 2), titles)):&#xA;    plt.subplot(6, 1, i+1+3)&#xA;    print &quot;i = &quot;, i, &quot;title = &quot;, title&#xA;    for line in color:&#xA;        plt.plot(line)&#xA;    plt.ylim(0, 1.05)&#xA;    plt.text(3, 0.6, title, fontsize=16)&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-03T03:11:32.813" LastActivityDate="2018-01-03T03:11:32.813" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24251" PostTypeId="1" CreationDate="2018-01-03T02:38:49.063" Score="1" ViewCount="92" Body="&lt;p&gt;What is the most efficient scenario for a total of 700 Newtons of thrust on three lined up boosters? If the air pressure for the payload booster isn't the same, can they make an efficient air turbulence pattern which generates more lift?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;233, 233, 233&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;200, 300, 200 &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;266, 166, 266 &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="17876" LastEditorUserId="17876" LastEditDate="2018-01-03T10:06:35.433" LastActivityDate="2018-01-03T10:06:35.433" Title="Efficiency of 3 in line boosters of a shuttle?" Tags="|space-shuttle|fuel|physics|booster|" AnswerCount="0" CommentCount="3" ClosedDate="2018-01-03T14:18:03.810" ContentLicense="CC BY-SA 3.0" />
  <row Id="24252" PostTypeId="1" AcceptedAnswerId="24255" CreationDate="2018-01-03T08:36:33.200" Score="26" ViewCount="5893" Body="&lt;p&gt;On the &lt;a href=&quot;https://www.youtube.com/watch?v=wtdjCwo6d3Q&amp;amp;t=16m11s&quot; rel=&quot;nofollow noreferrer&quot;&gt;official webcast&lt;/a&gt; of SpaceX's Iridium-4 mission a &amp;quot;throttle down and throttle back up of the Falcon 9 first stage engines&amp;quot; is mentioned. Why would one throttle down at a crucial time during steep ascent, way before MECO (Main Engine Cut-Off)? Is it to weaken the effect of &lt;a href=&quot;https://en.wikipedia.org/wiki/Max_q&quot; rel=&quot;nofollow noreferrer&quot;&gt;max-Q&lt;/a&gt;?&lt;/p&gt;&#xA;" OwnerUserId="21562" LastEditorUserId="6449" LastEditDate="2023-10-01T21:55:53.483" LastActivityDate="2023-10-01T21:55:53.483" Title="Why are Falcon 9 main engines throttled down and up shortly after launch?" Tags="|launch|spacex|falcon-9|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="24253" PostTypeId="1" AcceptedAnswerId="24254" CreationDate="2018-01-03T08:50:31.993" Score="4" ViewCount="840" Body="&lt;p&gt;Overshadowed by a silly, yet awesome primary payload — 'a cool colored cool electric sports car playing a cool track' on to a heliocentric orbit, are there other satellites/secondary payload Falcon Heavy would lift on it's maiden flight?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know of Planetary Society's LightSail 2, for one.&lt;sup&gt;1&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Has there been a mention of any other secondary payloads? &#xA;Also, information on orbits of secondary payloads and speculation of deployment sequence, if possible.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;EDIT&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;LightSail 2 is scheduled for and April launch.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="9092" LastEditDate="2021-03-26T14:28:30.817" LastActivityDate="2021-03-26T14:28:30.817" Title="What secondary payloads, if any, are launching on the inaugural flight of Falcon Heavy?" Tags="|spacex|launch|falcon-heavy|payload|secondary-payload|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="24254" PostTypeId="2" ParentId="24253" CreationDate="2018-01-03T08:56:39.693" Score="8" Body="&lt;p&gt;There are no known secondary payloads flying on the Falcon Heavy maiden flight. &lt;sup&gt;Excluding what's in the Roadster's glove box... ;)&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Planetary Society's LightSail 2 &lt;a href=&quot;http://www.planetary.org/blogs/jason-davis/2017/20170721-lightsail-2-updates-prox-1-launch-dates.html&quot; rel=&quot;noreferrer&quot;&gt;is flying&lt;/a&gt; on the Space Test Program-2 (STP-2) mission of Falcon Heavy, the third flight, notionally penciled in for April 15, 2018. Other smallsats flying on this launch include the &lt;a href=&quot;https://en.wikipedia.org/wiki/Green_Propellant_Infusion_Mission&quot; rel=&quot;noreferrer&quot;&gt;Green Propellant Infusion Mission&lt;/a&gt;, which itself is quite interesting.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2018-01-03T08:56:39.693" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24255" PostTypeId="2" ParentId="24252" CreationDate="2018-01-03T09:00:45.317" Score="40" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;Why would one throttle down at a crucial time during steep ascent, way before MECO? Is it to weaken the effect of max Q?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yes, exactly that. The engines are throttled up to the highest extent possible during all phases of flight to reduce gravity losses; but must be throttled down during at least two points during flight:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;When Falcon passes through Maximum Aerodynamic Pressure (Max-Q) which is a combination of atmospheric density and velocity that puts the greatest strain on the vehicle. This effect is more prominent during Dragon launches due to the frontal shape of Dragon.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;As Falcon nears MECO to limit acceleration on the payload, if necessary.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;These two flight aspects can be seen very clearly in this exceptionally detailed graph produced by &lt;a href=&quot;https://www.reddit.com/user/veebay&quot; rel=&quot;noreferrer&quot;&gt;/u/veebay&lt;/a&gt; on &lt;a href=&quot;https://www.reddit.com/r/spacex/&quot; rel=&quot;noreferrer&quot;&gt;r/spacex&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/IC61v.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/IC61v.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graph on the right hand side shows a reduction in net acceleration for Dragon missions around Max-Q, along with a tailoff in net acceleration just before booster cutoff.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="9060" LastEditDate="2018-01-03T16:19:54.630" LastActivityDate="2018-01-03T16:19:54.630" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24256" PostTypeId="2" ParentId="24248" CreationDate="2018-01-03T09:48:35.050" Score="16" Body="&lt;p&gt;They are definitely not identical.&lt;/p&gt;&#xA;&lt;p&gt;Tundra is geosynchronous; period = 1 day. The eccentricity allows it to spend most of the time over a region of Earth off the equator, something not possible for geostationary (sit always over one point on equator) or circular geosynchronous orbits (sinusoidal function with slow-down at both extremes). Tundra is intended for a single satellite to observe one specific point of Earth.&lt;/p&gt;&#xA;&lt;p&gt;Molniya is an orbit intended for 3-satellite constellations and it has a period of half a day. It's also used to observe two off-equator regions of Earth, 180 degrees of longitude apart. It utilizes the similar &amp;quot;dwelling&amp;quot; effect of a satellite staying for a long time near apogee, above &lt;em&gt;roughly&lt;/em&gt; the same region of Earth (not nearly as narrow as in case of Tundra though), and by the time the satellite leaves the apogee, another from the constellation approaches it, and enters region of interest for the observation.&lt;/p&gt;&#xA;&lt;p&gt;The most important part is about the trajectory over ground:&lt;/p&gt;&#xA;&lt;p&gt;Tundra:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/p7nMq.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/p7nMq.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Molniya:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yc33C.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yc33C.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In Tundra, the satellite spends most time over the &amp;quot;narrow&amp;quot; top part of the figure 8, then dashes through the lower part, to return to the top.&lt;/p&gt;&#xA;&lt;p&gt;In Molniya, there are three satellites on exactly the same track over ground, 1/3 period apart. They dash through the long southern stretches, then dwell on the northern &amp;quot;peaks&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;Due to Earth rotation, the ground track rotates, so their RAAN are actually 120 degrees apart; also, their true anomaly is such, that their epoch would differ +8/0/-8 hours if the true anomaly was identical.&lt;/p&gt;&#xA;&lt;p&gt;Summing up:&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Orbital&lt;/em&gt;:&lt;/p&gt;&#xA;&lt;p&gt;Tundra: 1 satellite, 1 day period, 0.2 to 0.3 eccentricity.&lt;/p&gt;&#xA;&lt;p&gt;Molniya: 3 satellites, 12h period, 0.7 eccentricity, RAAN and True Anomaly spread 1/3 period apart.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;Functional:&lt;/em&gt;&lt;/p&gt;&#xA;&lt;p&gt;Tundra: observing &lt;em&gt;one&lt;/em&gt; point of Earth, for &lt;em&gt;most of the time.&lt;/em&gt; (also possible to use two or three satellites for 100% time coverage).&lt;/p&gt;&#xA;&lt;p&gt;Molniya: observing &lt;em&gt;two&lt;/em&gt; areas on Earth 180 degrees apart, &lt;em&gt;at all times&lt;/em&gt;.&lt;/p&gt;&#xA;&lt;p&gt;As for main &lt;em&gt;technical&lt;/em&gt; differences, Molniya requires three separate launches, but Tundra requires a stronger booster to reach the higher orbit. Also, Tundra's somewhat higher apogee may require stronger radio connection or result in worse results due to higher distance; the difference is not big though (about 15%).&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="33864" LastEditDate="2020-12-18T14:55:31.540" LastActivityDate="2020-12-18T14:55:31.540" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="24258" PostTypeId="1" CreationDate="2018-01-03T15:28:24.370" Score="6" ViewCount="164" Body="&lt;p&gt;I am struggling with the implementation of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_laser_ranging&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satellite Laser Ranging&lt;/a&gt; (SLR) validation tool.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The situation is as follows: I am provided with a framework which gives me the position of a SLR observation station and the position of a satellite at a specified GPS time. Those positions can be assumed to be sufficiently correct for the given task. From those positions I calculate a reference distance which I will use for comparison with given SLR observations. The observation files I use as input are in CRD format and provided by data centers like &lt;a href=&quot;http://edc.dgfi.tum.de/en/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://edc.dgfi.tum.de/en/&lt;/a&gt;. From those files I extract, for each observation record, the GPS time stamp and time of flight in seconds. According to the simplest observation equation for SLR given by&#xA; $$d = c * \frac{\Delta t}{2}$$&#xA;I calculate the distance between station and satellite and compare it to the distance provided by my framework. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The problem is the distances deviate seemingly randomly with deviations ranging from 1 to 300 meters in both directions. I am aware that the simple observation equation is missing all correction terms but even without those an accuracy of up to 10 meters should be feasible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So far I thought I was dealing with a station clock or observation clock bias but adding offsets in positive as well as negative direction to the observation times always resulted in greater deviations of the two distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing I am constantly experiencing in all observations is (surprisingly) a greater deviation at steeper elevation angles, which is in contrast to my expectations, since this results in far shorter absolute distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One example of my results is the following &lt;a href=&quot;http://edc.dgfi.tum.de/en/data/npt/1443408/&quot; rel=&quot;nofollow noreferrer&quot;&gt;observation from Graz Lustbühel of the 22 June 2017 monitoring the GRACE A satellite&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt; | standard Deviation 156.32185&#xA; |  Time               | Ref Distance| Obs Distance| Deviation   | Elevation Angle&#xA; | ------------------------------------------------------------------------------&#xA; | 2017-06-22_13-57-47 | 573630.53   | 573510.16   | -124.37518  | 34.1093&#xA; | 2017-06-22_13-57-51 | 550922.4    | 550796.16   | -130.05672  | 35.989845&#xA; | 2017-06-22_13-57-56 | 528348.25   | 528215.69   | -136.19948  | 38.060675&#xA; | 2017-06-22_13-58-00 | 507464.43   | 507325.52   | -142.39058  | 40.19135&#xA; | 2017-06-22_13-58-06 | 484607.77   | 484461.27   | -149.80929  | 42.81356&#xA; | 2017-06-22_13-58-11 | 465610.45   | 465457.04   | -156.57817  | 45.279134&#xA; | 2017-06-22_13-58-16 | 449075.77   | 448915.81   | -162.99763  | 47.689646&#xA; | 2017-06-22_13-58-20 | 435233      | 435067.11   | -168.8336   | 49.94278&#xA; | 2017-06-22_13-58-25 | 424230.88   | 424059.89   | -173.84746  | 51.921769&#xA; | 2017-06-22_13-58-30 | 415972.62   | 415797.47   | -177.94548  | 53.540971&#xA; | 2017-06-22_13-58-35 | 411432.82   | 411255.13   | -180.45426  | 54.491375&#xA; | 2017-06-22_13-58-40 | 409363.89   | 409184.61   | -182.0285   | 54.951053&#xA; | 2017-06-22_13-58-46 | 411421.31   | 411242.31   | -181.76171  | 54.532429&#xA; | 2017-06-22_13-58-50 | 416021.81   | 415844.36   | -180.23949  | 53.598194&#xA; | 2017-06-22_13-58-56 | 427050      | 426876.64   | -176.23183  | 51.499173&#xA; | 2017-06-22_13-58-59 | 433701.19   | 433530.31   | -173.80953  | 50.323281&#xA; | 2017-06-22_13-59-06 | 452168.09   | 452003.85   | -167.28999  | 47.357755&#xA; | 2017-06-22_13-59-11 | 468417.38   | 468258.68   | -161.8747   | 45.049964&#xA; | 2017-06-22_13-59-15 | 487200.93   | 487048.25   | -155.9925   | 42.663639&#xA; | 2017-06-22_13-59-21 | 509791.27   | 509645.32   | -149.43676  | 40.116813&#xA; | 2017-06-22_13-59-26 | 532186.88   | 532047.1    | -143.43855  | 37.873845&#xA; | 2017-06-22_13-59-30 | 555171.16   | 555037.25   | -137.7555   | 35.809979&#xA; | 2017-06-22_13-59-35 | 580787.28   | 580659.4    | -131.91771  | 33.744171&#xA; | 2017-06-22_13-59-40 | 607376.04   | 607253.94   | -126.36003  | 31.816812&#xA; | 2017-06-22_14-00-11 | 792767.05   | 792675.37   | -97.569278  | 22.258777&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;Is there any phenomenon explaining this varying offsets?&lt;/p&gt;&#xA;" OwnerUserId="22266" LastEditorUserId="25" LastEditDate="2018-01-03T19:30:13.613" LastActivityDate="2018-01-03T19:30:13.613" Title="Inexplicable Offset in Satellite Laser Ranging (SLR) observations" Tags="|artificial-satellite|attitude|navigation|gps|earth-observation|" AnswerCount="1" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="24259" PostTypeId="2" ParentId="24244" CreationDate="2018-01-03T15:56:25.903" Score="4" Body="&lt;p&gt;Largest original: &lt;a href=&quot;https://www.asteroidmission.org/wp-content/uploads/2017/10/EGA_Plus_10_mapcam_Earth_Moon.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.asteroidmission.org/wp-content/uploads/2017/10/EGA_Plus_10_mapcam_Earth_Moon.png&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Blooming is vertical while streaking is horizontal:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6daBb.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6daBb.gif&quot; alt=&quot;Blooming vs. Streaking&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Some people use blooming and streaking &lt;a href=&quot;https://en.wikipedia.org/wiki/Charge-coupled_device#Blooming&quot; rel=&quot;nofollow noreferrer&quot;&gt;interchangeably&lt;/a&gt;, while others use them as synonymous with each other. Some CCDs have &lt;a href=&quot;http://www.astrosurf.com/re/ccdrev.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;anti-blooming gates&lt;/a&gt; which mostly inhibit the problem but can result in as little as 70% fill factor with resulting loss of sensitivity and well depth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The charge capacity of an image sensor can be limited by either the individual photodiode characteristics (pixels) or the CCD itself, and is defined by the maximum amount of charge that the image sensor can collect and transfer while still maintaining all of its design performance specifications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This capacity limit is termed the saturation charge level, and when this limit is reached, the pixel or CCD is described as being saturated. Exceeding the saturation level results in the generation of blooming artifacts in captured images. Blooming refers to the overflow of excess confined photo-generated charge from a photodiode well into adjacent structures when the maximum well charge capacity is exceeded.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Smear is generated directly or indirectly in the vertical shift registers (VCCD) in the interline transfer CCD (IL-CCD).  The VCCD is a light-shielded area of the image sensor used to transfer the charge off of the sensor. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Crop of enhanced image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wzfw8.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wzfw8.png&quot; alt=&quot;Image enhanced and tone mapped on blue channel&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hamamatsu article on &quot;&lt;a href=&quot;http://hamamatsu.magnet.fsu.edu/articles/ccdsatandblooming.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;CCD Saturation and Blooming&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Video: &quot;&lt;a href=&quot;https://www.youtube.com/watch?v=-DoTgQbALU0&quot; rel=&quot;nofollow noreferrer&quot;&gt;What is the Blooming effect in CCD chips?&lt;/a&gt;&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that there is a band on the left side and numerous speckles which tends to indicate that a minimal amount of processing was done on the image; knocking down the blooming was likely the extent of what they wanted to accomplish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Calculations of the effect in CCDs: &lt;a href=&quot;https://www.pco.de/fileadmin/user_upload/pco-publications/pco_pub_20000107_smear_e.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.pco.de/fileadmin/user_upload/pco-publications/pco_pub_20000107_smear_e.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is an example and explanation of why the bloom is vertical:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DUzpL.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DUzpL.jpg&quot; alt=&quot;CCD vs. CMOS Imagers&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The CCD (although best in class -120 dB) shows the problem while it's not visible in CMOS (no vertical charge transport).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See also Apogee Instruments article on &lt;a href=&quot;http://apogee-systems.net/apogee/ccd102.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Blooming vs. Anti-blooming CCDs&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;FSU's &quot;&lt;a href=&quot;http://zeiss-campus.magnet.fsu.edu/articles/basics/digitalimaging.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Understanding Digital Imaging&lt;/a&gt;&quot; webpage shows this image:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/qOUlk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qOUlk.jpg&quot; alt=&quot;CCD internal structure&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="20766" LastEditorUserId="20766" LastEditDate="2018-01-03T17:35:22.943" LastActivityDate="2018-01-03T17:35:22.943" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24260" PostTypeId="2" ParentId="24249" CreationDate="2018-01-03T16:20:35.860" Score="3" Body="&lt;p&gt;I wouldn't be able to judge without seeing how it actually performs under test, but it's important to point out that young's modulus actually has very little effect on the ballistic performance of a material under hypervelocity impact conditions.&lt;/p&gt;&#xA;&lt;p&gt;The suitability of a material for MMOD shielding depends on different things, depending on where in the shield layup you want to use it.  For the outermost layer, not a whole lot matters beyond environmental degradation performance, as it's going to get penetrated.  You just want to make sure it is there.  For intermediate layers, thermal resistance and toughness are important, as you need to be able to absorb the incoming energy.  Extremely high stiffness materials are often quite brittle, which means they don't actually absorb much energy.&lt;/p&gt;&#xA;&lt;p&gt;TL;DR: Maybe.  We won't know for sure until we actually build some shields out of the material and test them.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2021-03-26T15:49:46.813" LastActivityDate="2021-03-26T15:49:46.813" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24261" PostTypeId="2" ParentId="24180" CreationDate="2018-01-03T17:00:22.287" Score="0" Body="&lt;p&gt;I'm very much not an expert on the subject, but my understanding of the problem is this:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The universe, to our knowledge expands at a steady rate of 0.007% per million years.&#xA;All of it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Doesn't sound like much. but ask a financier about the value of compound interest.. essentially, the more money you have, the better 0.007% sounds.&#xA;If I have 7000 dollarpounds in my bank account, that'll be an extra dollarpound per million years.&#xA;But if I have $£7,000,000, then I'll have an extra 1000 dollarpounds.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, if it's 2.5 million lightyears from here to Andromeda. then in a million years that distance will have grown by 17,500 lightyears. (ignoring that andromeda is in reality moving &lt;em&gt;towards&lt;/em&gt; us under the effects of mutual gravity)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Measure a wide enough region and the overall growth of that region per million years will exceed a million lightyears at a time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's your limit. you can't go faster than light, but the universe's expansion is not limited by that. so eventually every body in the universe will pass that limit relative to one another and be isolated. forever.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;By my math, Assuming you can travel at the speed of light, the furthest away an object can be and still be reachable at all is 142,857,142.857 Lightyears away.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anything further away than that is accelerating away faster than you can ever catch up to.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Provided that your destination will still be within 142.86 million lightyears of your starting point when you get there, you should still be able to get home after the 284 million year round-trip.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is however thrown off kilter by things like your own acceleration, I'm assuming you can accelerate instantly and decelerate just as fast.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What worries me is that I've come to a substantially smaller conclusion to all these much more knowledgable people who are also answering the question.&#xA;I think I'm missing something important.&lt;/p&gt;&#xA;" OwnerUserId="22270" LastEditorUserId="22270" LastEditDate="2018-01-03T17:06:13.703" LastActivityDate="2018-01-03T17:06:13.703" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24262" PostTypeId="2" ParentId="24253" CreationDate="2018-01-03T17:09:14.097" Score="4" Body="&lt;p&gt;Let's look at the picture as seen from the side (Not official photograph, but...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FtV2v.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FtV2v.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there was going to be a secondary payload, it would have to be between the car and the payload adapter. There clearly isn't room there to do so.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've heard rumors that there could be a second point where secondary payloads might reside. There is an open space near the engine that could host a small payload. This was a possibility some years ago, but I haven't found anything to be the case more recently. As Kuldeenp answered, there doesn't seem to be any other payloads indicated on this particular flight.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/F514l.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/F514l.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-03T17:09:14.097" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24263" PostTypeId="2" ParentId="2025" CreationDate="2018-01-03T17:11:32.863" Score="7" Body="&lt;p&gt;Supplemental answer to this un-addressed part of the question &quot;Were the shuttles capable of back-to-back missions?&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It happened throughout the program. More frequently early on as orbiters were introduced into the fleet and others went in for maintenance.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The first five flights were back-to-back missions of Columbia&#xA;(OV-102).&lt;/li&gt;&#xA;&lt;li&gt;The next three flights were back-to-back missions of Challenger&#xA;(OV-099).&lt;/li&gt;&#xA;&lt;li&gt;The 10th and 11th flights were back-to-back missions of Challenger.&lt;/li&gt;&#xA;&lt;li&gt;The 14th, 15th, and 16th flights were back-to-back missions of&#xA;Discovery (OV-103).&lt;/li&gt;&#xA;&lt;li&gt;The 35th and 36th flights were back-to-back missions of Discovery.&lt;/li&gt;&#xA;&lt;li&gt;The 91st and 92nd flights were back-to-back missions of Discovery.&lt;/li&gt;&#xA;&lt;li&gt;The 98th and 99th flights were back-to-back missions of Atlantis&#xA;(OV-104).&lt;/li&gt;&#xA;&lt;li&gt;The 114th and 115th flights were back-to-back missions of Discovery.  &lt;/li&gt;&#xA;&lt;li&gt;Endeavour (OV-105) never flew back-to-back missions.&lt;/li&gt;&#xA;&lt;li&gt;The shortest interval on a back-to-back mission was 63 days, between&#xA;mission 10 (STS-41B) and mission 11 (STS-41C), using Challenger.&lt;/li&gt;&#xA;&lt;li&gt;The longest interval on a back-to-back mission was almost a year, during the recovery from the STS-107 accident. These (STS-114 and STS-121, both flown by Discovery) were the last back-to-back missions in the program.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="-1" LastEditDate="2018-03-17T19:37:44.933" LastActivityDate="2018-03-17T19:37:44.933" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24265" PostTypeId="1" CreationDate="2018-01-03T18:58:20.767" Score="5" ViewCount="233" Body="&lt;p&gt;Being aeronautical engineering graduate, I have gone through huge literature regarding design of Liquid propellant engines(LPRs). But as I am planning to develop small LPR based on gasoline and oxygen. I want to listen to people here who have already done this. Specifically challenges, their suggestions and any kind of tips. &#xA;Moreover I have come across this book &quot;How to Design, Build and Test Small Liquid-Fuel Rocket Engines Leroy J. Krzycki – 1967&quot;, I'm curious to know about trustworthiness of this book, if anyone has developed a working engine based on this book. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(apology for mistakes, this is my first question). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thanks&lt;/p&gt;&#xA;" OwnerUserId="18070" LastEditorUserId="6944" LastEditDate="2020-02-13T13:02:47.677" LastActivityDate="2020-02-13T13:02:47.677" Title="Challenges in Development of small scale liquid propellant engines" Tags="|rockets|propulsion|liquid-fuel|amateur-rocketry|krzycki-text|" AnswerCount="0" CommentCount="0" ClosedDate="2018-01-04T03:36:24.213" ContentLicense="CC BY-SA 3.0" />
  <row Id="24266" PostTypeId="2" ParentId="24258" CreationDate="2018-01-03T19:29:09.267" Score="4" Body="&lt;p&gt;A few things to consider:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Refraction- The atmosphere will bend the light somewhat, which might cause it to take longer.&lt;/li&gt;&#xA;&lt;li&gt;Speed of light changes- Light moves a bit slower when going through more of the atmosphere&lt;/li&gt;&#xA;&lt;li&gt;Timing accuracy- The position of the satellite might be off in time slightly, which would result in a deviation.&lt;/li&gt;&#xA;&lt;li&gt;Positional accuracy- This results if you don't know the location of your source point exactly.&lt;/li&gt;&#xA;&lt;li&gt;Floating point errors. Floating points are accurate to about 7 decimal digits. That means it is accurate to about a meter, give or take. Not likely to be the source of your error, but it could crop up in the time estimates.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;To narrow this down a bit, try the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;See if there is a correlation between the error and the line of sight angle. If the distance is most accurate when straight overhead, it could be a refraction or speed of light error.&lt;/li&gt;&#xA;&lt;li&gt;Make sure your GPS location and time are accurate.&lt;/li&gt;&#xA;&lt;li&gt;Try plotting various of your values vs others.&lt;/li&gt;&#xA;&lt;li&gt;Determine the distance to a known source. I recommend using the Lunar laser reflectors on the moon. With these, you can get great results. If you are able to do this with satellites, consider using one of the GPS satellites, as their exact positions can easily be calculated.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Looking at your data, I found a few things of some interest.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The higher the elevation, the higher the error. This would indicate an overcorrection.&lt;/li&gt;&#xA;&lt;li&gt;The relative error is highest at the highest angles.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Thinking about this, that indicates that when the overhead motion of the satellite is the most, the error is the most. Also when the Doppler change is the most, the error is the most. I rather strongly suspect the issue is some kind of a timing issue, where the exact position of an object isn't as well determined as you think it is. Try some of the tests I indicated (Moon and GPS satellites) if you can to further validate this.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-03T19:29:09.267" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24267" PostTypeId="2" ParentId="2473" CreationDate="2018-01-03T20:16:52.150" Score="11" Body="&lt;p&gt;Revisiting this question 4 years later, it seems clear that while they had this estimate of the engine build rate, they did not hit it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There was an &lt;a href=&quot;https://www.instagram.com/p/Bc8lfJ0l2X3/&quot; rel=&quot;noreferrer&quot;&gt;Instagram post of the 400th completed engine&lt;/a&gt; Dec 2017. So they clearly did not hit this run rate, nor sustain it if they meant Merlin engines. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in that interim they have launched 45 Falcon 9 flights, which consumed 450 engines. Except of course 4 of those missions (as of this writing) were reflown first stages, freeing up 36 engines. Some of the early landed vehicles, are thought to have had engines removed and used on other launches for testing reuse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The change to successful reuse is likely a major change for SpaceX. No doubt they planned for it, expected it, but had to plan as well for a failure to achieve it, and thus build as if there is no tomorrow.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Conversely with 18 launches in 2017, and a clear push to increase that yet again in 2018 as well as multiple Falcon Heavy launches planned, the demand for engines may spike initially as enough Block 5 builds are completed to fill out the fleet. Falcon 9 - Block 5 is claimed to support very rapid reuse with minimal refurbishment, as the penultimate design based on experience from landing 20 stages, and reflying at least 4 (more every month!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With the announcement that part of the business plan for Mars will be to fund the &lt;a href=&quot;https://en.wikipedia.org/wiki/BFR_(rocket)&quot; rel=&quot;noreferrer&quot;&gt;BFR/BFS&lt;/a&gt; development by moving all resources to BFR and away from Falcon 9, then moving all the business (launch of satellites, ISS missions, etc) to BFR once ready, we can assume that engine build rate will slow down (for Merlins, of course Raptors will scale up, especially since each BFR needs 31 and BFS needs 7).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It may be this was an aspirational goal to reach a build rate, but not necessarily run at that rate long term.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2018-01-03T20:16:52.150" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24270" PostTypeId="2" ParentId="7849" CreationDate="2018-01-04T00:35:41.457" Score="4" Body="&lt;p&gt;It should be like about 5-10 minutes after sunset on Earth. At that time the global horizontal irradiance (with clear skies) is typically measured (e.g. with the NOAA SurfRAD network) to be about 1 W/m**2, about .001 times the mid-day value. Similarly, about 10% of the light incident on Titan reaches the surface via scattering, based on a green light optical depth of 8 and reasonable assumptions of backscatter fraction and single scattering albedo. This combines with a baseline value of 1% solar irradiance at the top of Titan's atmosphere compared with Earth's (due to being 10AU from the Sun).&lt;/p&gt;&#xA;" OwnerUserId="22283" LastEditorUserId="22283" LastEditDate="2018-01-04T23:20:15.233" LastActivityDate="2018-01-04T23:20:15.233" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24271" PostTypeId="1" AcceptedAnswerId="36224" CreationDate="2018-01-04T01:18:35.720" Score="4" ViewCount="919" Body="&lt;p&gt;The title refers to the promotional slogan &quot;What happens in Vegas, &lt;em&gt;stays&lt;/em&gt; in Vegas.&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question &lt;a href=&quot;https://space.stackexchange.com/q/23403/12102&quot;&gt;How much CO2 would city-to-city rocket flight produce compared to airliners?&lt;/a&gt; seems to be focused on CO2 contribution to the Earth's atmosphere. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an exercise to better understand the effects of rocket exhaust on the atmosphere, is there some point &lt;em&gt;above which&lt;/em&gt; most rocket exhaust would not become part of Earth's atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, at sea-level, all exhaust remains, and probably by the time one is passing the moon, nearly none of it has a chance to enter the atmosphere due mostly to geometry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But roughly where would be the cross-over point for a typical LOX/RP-1 launch vehicle's exhaust, where roughy half escapes and half remains?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2019-05-17T23:34:46.897" Title="Does &quot;What happens beyond Kármán, stay beyond Kármán&quot;?" Tags="|rockets|atmosphere|exhaust|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24272" PostTypeId="1" AcceptedAnswerId="24281" CreationDate="2018-01-04T01:38:23.970" Score="1" ViewCount="306" Body="&lt;p&gt;According to &lt;a href=&quot;https://space.stackexchange.com/a/7858/12102&quot;&gt;this cool answer&lt;/a&gt; the &lt;a href=&quot;https://en.wikipedia.org/wiki/Huygens_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Huygens lander&lt;/a&gt; on Saturn's moon &lt;a href=&quot;https://en.wikipedia.org/wiki/Titan_(moon)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Titan&lt;/a&gt; had a flood light to aid in photography of the surface because of the very low ambient light levels due to the 9.5x farther distance than Earth from the Sun, and also absorption and scattering of the already low sunlight level from Titan's thick atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Since images exist that show foreground together with background, I'm assuming the light was not always necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Did it work? Was it ever used? Are there photos that are taken using this light? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; unsourced image from &lt;a href=&quot;https://space.stackexchange.com/a/7858/12102&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1BV5f.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1BV5f.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Huygens image of the surface of Titan, from &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Huygens_surface_color_sr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/H4CgD.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/H4CgD.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2018-01-04T10:28:58.440" Title="Did the Huygens lander have a flood-light for photography? Was it used?" Tags="|imaging|photography|titan|huygens-lander|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24273" PostTypeId="1" AcceptedAnswerId="24274" CreationDate="2018-01-04T01:45:29.983" Score="2" ViewCount="128" Body="&lt;p&gt;I've asked about the flood-light that apparently was included on the &lt;a href=&quot;https://en.wikipedia.org/wiki/Huygens_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Huygens lander&lt;/a&gt; in the question &lt;a href=&quot;https://space.stackexchange.com/q/24272/12102&quot;&gt;Did the Huygens lander have a flood-light for photography? Was it used?&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Have there been any other landers or rovers that have been at least launched into space which were equipped with flood-lights for photography/imaging? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;note:&lt;/strong&gt; I'm not asking about laser scanners. This is about lights which are somewhat uncollimated and illuminate an extended area so that cameras can record images in low light levels.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2018-01-04T03:15:52.803" Title="Has any lander or rover (besides Huygens) used a flood-light for photography?" Tags="|imaging|photography|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24274" PostTypeId="2" ParentId="24273" CreationDate="2018-01-04T03:15:52.803" Score="4" Body="&lt;p&gt;Yes. The &lt;strong&gt;MAHLI&lt;/strong&gt; (Mars Hand Lens Imager) instrument on Mars Science Laboratory (also known as Curiosity) has both white visible light LEDs and ultraviolet LEDs for nighttime imaging and promotion of UV fluorescence, respectively. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VjZm0.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VjZm0.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The MAHLI includes two sets of two white light LEDs to permit nighttime imaging. Each pair can be independently commanded on/off. MAHLI also has two ultraviolet (365 nm) LEDs to look for materials that fluoresce under longwave UV illumination. The UV LEDs are included on an exploratory, &quot;best efforts&quot; basis and are not a calibrated investigative tool. The MSL Project is required to accommodate night operations of MAHLI, but thermal and power constraints might preclude more than just occasional night-time operation of the instrument and the robotic arm.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;- MAHLI Instruments Page, &lt;a href=&quot;https://msl-scicorner.jpl.nasa.gov/Instruments/MAHLI/&quot; rel=&quot;nofollow noreferrer&quot;&gt;courtesy JPL&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The four white LEDs produced by Avago Technologies from their HSMW-10x series (&lt;a href=&quot;http://www.farnell.com/datasheets/1855424.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Product Data Sheet&lt;/a&gt;)&lt;/li&gt;&#xA;&lt;li&gt;The two UV LEDs are produced by Nichia (NSHU550B's). &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These were first used on 22 January 2013:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7Dvwn.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7Dvwn.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;https://www.nasa.gov/mission_pages/msl/news/msl20130124.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA Press Release&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2018-01-04T03:15:52.803" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24276" PostTypeId="1" AcceptedAnswerId="24278" CreationDate="2018-01-04T03:39:00.667" Score="4" ViewCount="238" Body="&lt;p&gt;I did try to think this through before posting, but I am going to need some explanation. In my (perhaps naïve) view, an ellipse will have the same eccentricity no matter what frame it is in. In Horizons, if you ask for elements for Venus on 2018-1-3 00:00 TDB, if you set the origin to 500@0 (solar system barycenter) you get: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EC= 1.701169124295256E-02 (0.01701) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;and if you set to 500@Sun you get: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;EC= 6.802128501627439E-03 (0.0068) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is confusing to me. Changing the reference frame doesn't change the focus or center of the ellipse - I should be able to set the coordinate origin to anything in the universe and still see an ellipse with a particular singular eccentricity so long as the frame remains Euclidean. &#xA;Thanks! &lt;/p&gt;&#xA;" OwnerUserId="22285" LastEditorUserId="12102" LastEditDate="2018-02-11T02:42:25.327" LastActivityDate="2018-02-11T02:42:25.327" Title="Why does the eccentricity of Venus's (and other) orbits as reported by Horizons vary based on the frame origin and basis?" Tags="|orbital-elements|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24278" PostTypeId="2" ParentId="24276" CreationDate="2018-01-04T05:49:57.603" Score="6" Body="&lt;p&gt;State vectors ($x, y, z, v_x, v_y, v_z$) are the real output of the ephemeris for the planets and moons, and the orbit propagators for the minor bodies and spacecraft that Horizons provides. What you are looking at are &lt;a href=&quot;https://en.wikipedia.org/wiki/Osculating_orbit&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;Osculating Orbital Elements&lt;/em&gt;&lt;/a&gt;, which are the instantaneous values of a &lt;em&gt;hypothetical, elliptical&lt;/em&gt; orbit that would be tangent to the &lt;em&gt;real orbit&lt;/em&gt; at that moment in time. Every new line in the osculating elements table is a totally different hypothetical orbit that is perfectly tangent to the real orbit at that moment only.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An excellent clue to this would be the fact that every point in time has new values for all of the orbital elements. If these were fixed, elliptical orbits instead of instantaneous osculating elements, the numbers would not have to be recalculated for each point in time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read more about osculating vs Keplerian orbits in &lt;a href=&quot;https://space.stackexchange.com/a/14752/12102&quot;&gt;this excellent answer&lt;/a&gt;. You can also see &quot;live&quot; osculating orbits changing moment-by-moment in the excellent YouTube video &lt;a href=&quot;https://youtu.be/rr0JpgKPKgg&quot; rel=&quot;noreferrer&quot;&gt;Pythagorean 3-Body Problem With Osculating Orbits&lt;/a&gt;.  Here is a screen shot: (read more about this mathematical problem in &lt;a href=&quot;https://space.stackexchange.com/q/15364/12102&quot;&gt;this question&lt;/a&gt;.) While the orbits (thick lines) are chaotic, the &lt;em&gt;osculating orbits&lt;/em&gt; are always elliptical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/yGFZV.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/yGFZV.png&quot; alt=&quot;Pythagorean 3-Body Problem With Osculating Orbits&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;No solar system orbit is perfectly elliptical because of the mutual gravitational attraction between all bodies, including central and tidal forces. However, for the period of one year, Venus' orbit &lt;em&gt;around the Sun itself&lt;/em&gt; is much closer to elliptical than is it's path around the solar system barycenter. Over years, the Sun slowly moves around by a few hundred thousand kilometers in response to the motion of the large outer planets Jupiter, Saturn, Uranus and Neptune. However, the inner planets keep fairly close to Kelperian orbits around the Sun &lt;em&gt;wherever it happens to be&lt;/em&gt; at the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is the setup I used to extract daily osculating elements for Venus around first the Sun (body centered) and then the solar system barycenter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LPFUf.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LPFUf.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5xium.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5xium.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is a plot of some of those elements, including eccentricity. The straight (blue) lines are the osculating parameters for the Sun-centered data, and the wavy (green) lines are the osculating parameters calculated about the Solar System Barycenter. You can see that for the barycentric osculating elements they vary quite a bit, while the osculating elements for orbits about the Sun are almost constant, meaning that the real orbit of Venus is extremely close to a hypothetical elliptical orbit around the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see your values of ~0.017 and ~0.007 as the starting points on the left side of the top (eccentricity) plot!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I've also included the Python script that made the plot, so you can check how it's done.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/0WQEb.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/0WQEb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;import numpy as np&#xA;import matplotlib.pyplot as plt&#xA;&#xA;fnames = (&quot;Venus Sun 2018 daily horizons_results.txt&quot;,&#xA;          &quot;Venus Bary 2018 daily horizons_results.txt&quot;)&#xA;&#xA;&quot;&quot;&quot;&#xA;JDTDB  Julian Day Number, Barycentric Dynamical Time&#xA;EC     Eccentricity, e&#xA;QR     Periapsis distance, q (au)&#xA;IN     Inclination w.r.t XY-plane, i (degrees)&#xA;OM     Longitude of Ascending Node, OMEGA, (degrees)&#xA;W      Argument of Perifocus, w (degrees)&#xA;Tp     Time of periapsis (Julian Day Number)&#xA;N      Mean motion, n (degrees/day)&#xA;MA     Mean anomaly, M (degrees)&#xA;TA     True anomaly, nu (degrees)&#xA;A      Semi-major axis, a (au)&#xA;AD     Apoapsis distance (au)&#xA;PR     Sidereal orbit period (day)&#xA;&quot;&quot;&quot;&#xA;&#xA;blobs = []&#xA;element_sets = []&#xA;for fname in fnames:&#xA;    with open(fname, 'r') as infile:&#xA;        blob = infile.read().splitlines()&#xA;    print fname&#xA;    istart = [i for i, line in enumerate(blob) if &quot;$$SOE&quot; in line][0]&#xA;    iend   = [i for i, line in enumerate(blob) if &quot;$$EOE&quot; in line][0]&#xA;    print len(blob), istart, iend&#xA;&#xA;    blob = [line.split(',') for line in blob[istart+1: iend]]&#xA;    blobs.append(blob)&#xA;    JD = np.array([float(line[0]) for line in blob])&#xA;    elements = np.array([[float(x) for x in line[2:14]] for line in blob])&#xA;    data = np.vstack((JD, elements.T))&#xA;    element_sets.append(data)&#xA;&#xA;Data = np.stack(element_sets, axis=1)[..., :225] # just 225 days&#xA;print Data.shape&#xA;&#xA;if 1 == 1:&#xA;    plt.figure()&#xA;    indices = (1, 2, 10, 11, 12)&#xA;    names = ('eccentricity', 'periapsis (AU)', 'semi-major axis (AU)',&#xA;             'apoapsis AU', 'period (days)')&#xA;    for i, (j, name) in enumerate(zip(indices, names)):&#xA;        plt.subplot(len(indices), 1, i+1)&#xA;        for k, thing in enumerate(Data[j]):&#xA;            plt.plot(Data[0, k]-Data[0, k, 0], thing)&#xA;        plt.title(name)&#xA;    plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-04T06:03:38.680" LastActivityDate="2018-01-04T06:03:38.680" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24279" PostTypeId="1" CreationDate="2018-01-04T07:26:14.453" Score="10" ViewCount="2631" Body="&lt;p&gt;So for instance, for the space shuttle,liquid hydrogen will react with liquid oxygen to produce water vapour. Is only water vapour ejected from the rocket? Or will the ejected mass be a mix of water and hydrogen?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the ejected mass is a mix, is it done on purpose? Or it is not possible to eject purely water even with the right stoichiometric ratio? What is the desired mix then?&lt;/p&gt;&#xA;" OwnerUserId="22224" LastEditorUserId="25" LastEditDate="2018-01-04T11:20:38.220" LastActivityDate="2018-01-04T13:22:22.940" Title="What do rockets eject?" Tags="|rockets|fuel|engines|" AnswerCount="2" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24281" PostTypeId="2" ParentId="24272" CreationDate="2018-01-04T07:56:31.750" Score="2" Body="&lt;p&gt;The plan was to &lt;a href=&quot;http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Huygens_descent_timeline&quot; rel=&quot;nofollow noreferrer&quot;&gt;switch the lamp on 4 minutes before landing&lt;/a&gt; and leave it on. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Descent Imager/Spectral Radiometer lamp turned on&lt;br&gt;&#xA;  Close to the surface, Huygens's camera instrument will turn on a light. The light is particularly important for the 'Spectral Radiometer' part of the instrument to determine the composition of Titan's surface accurately.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There are &lt;a href=&quot;http://sci.esa.int/cassini-huygens/31193-instruments/?fbodylongid=734&quot; rel=&quot;nofollow noreferrer&quot;&gt;2 lights&lt;/a&gt;? &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Surface Science Lamp&lt;/li&gt;&#xA;&lt;li&gt;Lamp for measurement of reflection spectrum of surface below 100 m altitude&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;But from 'The &lt;a href=&quot;http://lasp.colorado.edu/~horanyi/graduate_seminar/Descent_Imager.pdf?fobjectid=31193&quot; rel=&quot;nofollow noreferrer&quot;&gt;Descent Imager/Spectral Radiometer (DISR) experiment on the Huygens entry probe of Titan&lt;/a&gt;', these might be one and the same. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A 20 watt lamp and collimator are used to provide spectrally continuous illumination of the surface during the last 100 m of the descent for measurements of the reflection spectrum of the surface.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Surface Science Lamp&#xA;  The purpose of the surface science lamp (SSL) is to illuminate the surface of Titan in spectral regions where strong atmospheric absorption bands prevent sunlight from penetrating to Titan’s surface. The SSL permits continuous measurements of the spectral reflectivity of the surface to be made throughout the entire spectral range. The SSL is a 20 Watt lamp with a parabolic reflector which illuminates the surface and fills the narrow 3 × 9 degrees FOV of the IR spectrometer with enough light to give a S/N of 50 at 60 m altitude within the strong methane bands even if the surface reflectivity is as low as 0.05.&#xA;  The lamp system is activated when an altitude of 700 meters is reached (given&#xA;  by the radar altimeter) and is operated during the last several minutes of the descent where a continuous sampling of the reflection spectrum of the surface is obtained using both the DLVS and DLIS.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Approximately two minutes after impact, the surface science lamp is switched off. After that it is alternately switched on and off every two minutes. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So about half the photos taken by Huygens should be with the lamp on. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The study '&lt;a href=&quot;https://arxiv.org/abs/1702.00653&quot; rel=&quot;nofollow noreferrer&quot;&gt;The reflectance spectrum of Titan’s surface at the Huygens landing site determined by the Descent Imager/Spectral Radiometer&lt;/a&gt;' indicates that the light was used as planned. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We reconstruct the reflectance spectrum of the landing site in the 500-1500 nm range from Downward Looking Visual and Infrared Spectrometer data that show evidence of lamp light.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-04T10:28:58.440" LastActivityDate="2018-01-04T10:28:58.440" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24282" PostTypeId="2" ParentId="24279" CreationDate="2018-01-04T11:20:01.703" Score="5" Body="&lt;p&gt;Basically they do testing on the ground to determine the optimal mixture, and use that mixture. &lt;a href=&quot;http://www.braeunig.us/space/comb.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;This website&lt;/a&gt; shows a few of them, of which I will post one chart here. Note that these are mass ratios.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/P0P2b.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/P0P2b.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So you figure out what your chamber pressure should be, and then you mix the gasses as is optimal. That will often not be a pure mixture of the two elements to do a complete burn. For LOX/ LH, the optimal point will have some unused fuels. For other fuels, the mixture may favor a different mixture as the pressure increases in general.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-04T13:22:22.940" LastActivityDate="2018-01-04T13:22:22.940" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24283" PostTypeId="2" ParentId="23403" CreationDate="2018-01-04T12:04:37.283" Score="4" Body="&lt;p&gt;Loose estimate based on preliminary data for BFR: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;BFR first stage gross weight is &lt;a href=&quot;https://en.wikipedia.org/wiki/BFR_%28rocket%29&quot; rel=&quot;noreferrer&quot;&gt;4400 tons&lt;/a&gt; (IAC presentation 2017). If I set the mass fraction to 90% that's 4000 tons of methane + LOX. The second stage has another 1100 tons. &#xA;Assuming all of that is burned on a flight (worst case).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reaction is CH&lt;sub&gt;4&lt;/sub&gt; + 2 O&lt;sub&gt;2&lt;/sub&gt; = CO&lt;sub&gt;2&lt;/sub&gt; + 2 H&lt;sub&gt;2&lt;/sub&gt;O, ratio 42:36, is 2746 tons of CO&lt;sub&gt;2&lt;/sub&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Payload to LEO is 150 tons, that's (my estimate) enough to carry 800 passengers. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For comparison, a Boeing 747-400 uses &lt;a href=&quot;http://www.carbonindependent.org/sources_aviation.html&quot; rel=&quot;noreferrer&quot;&gt;60 tons of fuel&lt;/a&gt; for a 5500 km long flight carrying 400 passengers, producing 189 tons of CO&lt;sub&gt;2&lt;/sub&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So for a transatlantic flight, a BFR produces 7x as much CO&lt;sub&gt;2&lt;/sub&gt; as an airliner. For the longest flights (London-Australia, 20,000 km), fuel consumption of the Boeing increases by 4x. &#xA;I already assumed the BFR consumes its entire tank capacity above, so for this flight the BFR produces 1.4x as much CO&lt;sub&gt;2&lt;/sub&gt; as an airliner.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-04T20:46:51.087" LastActivityDate="2018-01-04T20:46:51.087" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24284" PostTypeId="2" ParentId="24279" CreationDate="2018-01-04T12:08:58.213" Score="11" Body="&lt;p&gt;In rocket engines, the objective of the combustion is to produce the maximum ISP, which is to say that the gasses exiting the nozzle have the highest possible speed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a typical scenario, changing the chamber pressure does not influence the ISP much. However, the exit velocity is influenced by the molecular weight of the gases, since the same exit Mach number means a higher speed for a light gas than it means for a heavy gas.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is part of the optimization, which should ultimately lead to optimal performance parameters of the rocket, not the engine, while also taking other factors into account.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For hydrogen, stoichiometric is at a ratio of 8:1, and optimal ISP is attained at a mixture ratio as low as 3.5:1, meaning that more than half the hydrogen is ejected unburnt. This would lead to extremely large and heavy hydrogen tanks, however, so an optimal choice will be something in between.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For hydrocarbon fuels, the same is true to a lesser extent. CO2 is a quite heavy molecule, so rockets also eject a lot of CO, in order to obtain higher ISP.&lt;/p&gt;&#xA;" OwnerUserId="3185" LastEditorUserId="3185" LastEditDate="2018-01-04T12:16:22.993" LastActivityDate="2018-01-04T12:16:22.993" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24285" PostTypeId="1" CreationDate="2018-01-04T13:38:42.900" Score="4" ViewCount="192" Body="&lt;p&gt;I have never come across any papers which talks about flutter analysis of gridded fins in general, does it not under go flutter behavior or is it technically incorrect to talk about flutter for gridded fins?&lt;/p&gt;&#xA;" OwnerUserId="22296" LastEditorUserId="6537" LastEditDate="2018-01-05T21:48:01.167" LastActivityDate="2018-01-05T21:48:01.167" Title="Is there any flutter analysis available regarding Falcon's grid fins?" Tags="|spacex|falcon-9|grid-fins|analysis|" AnswerCount="0" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24286" PostTypeId="1" CreationDate="2018-01-04T18:36:06.027" Score="1" ViewCount="218" Body="&lt;p&gt;Could architectures seeking human missions to Mars not put caches of water in orbit around Earth prior to launching of a manned mission? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This would allow a later-launched manned mission to dock with and collect from the water caches placed in orbit, thus needing less thrust to get underway towards the destination. &lt;/p&gt;&#xA;" OwnerUserId="22307" LastEditorUserId="6537" LastEditDate="2018-01-05T21:10:14.833" LastActivityDate="2018-01-05T21:10:14.833" Title="Water supplies needed during travel to mars" Tags="|mars|water|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24287" PostTypeId="2" ParentId="24286" CreationDate="2018-01-04T20:53:09.210" Score="1" Body="&lt;p&gt;This question is a bit vague since it seems to simply be proposing a solution, but I will do my best to give an explanation. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the plan to travel to Mars, much of the plan revolves around what is called the space railroad, I think. Now while it is not a literal railroad, the plan calls for an orbiting lunar station, at which, assembly of a much larger rocket would be possible. This concept of assembling in space is a well know, but not commonly executed way of creating spacecraft. Until recently, the need for such large spacecraft has only really been limited to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To bring in a semi-realisitic situation, in the video game, Kerbal Space Program, from what I understand, this is common practice for missions to other planets. But, since we have not sent a mission to another terrestrial body other than our moon that required the ability to get back, the need for a heavy spacecraft could be overcome by simply launching a bigger ship from the ground.&lt;/p&gt;&#xA;" OwnerUserId="21783" LastActivityDate="2018-01-04T20:53:09.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24288" PostTypeId="2" ParentId="24286" CreationDate="2018-01-04T20:54:02.573" Score="1" Body="&lt;p&gt;Your idea is possible, and it's being looked at for various supplies, not just water. The SpaceX &lt;a href=&quot;https://en.wikipedia.org/wiki/BFR_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;BFR&lt;/a&gt;, for example, is designed to launch into Earth orbit, where the spaceship is refueled from tankers before it sets off to Mars. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But when you assemble a spacecraft in Earth orbit, you still have to accelerate the entire stack into a transfer orbit to Mars. So while it becomes easier to launch the stack in pieces, you still need to spend a lot of energy. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So NASA is looking into closed-loop environmental systems that can recycle most of the water. A &lt;a href=&quot;https://en.wikipedia.org/wiki/ISS_ECLSS&quot; rel=&quot;nofollow noreferrer&quot;&gt;prototype&lt;/a&gt; is installed on the ISS. This reduces the amount of water you need to bring along by a huge amount, making it a cheaper solution than multiple launches to supply water to the spacecraft.  &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-01-04T20:54:02.573" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24289" PostTypeId="1" AcceptedAnswerId="24290" CreationDate="2018-01-04T22:59:12.463" Score="3" ViewCount="415" Body="&lt;p&gt;Many satellites (such as the recent Iridium launch) need to be in either slight retrograde or polar orbit. Since Kennedy Space Center is located on the east coast of Florida, rockets that need to go west or north/south have to be launched from Vandenberg in California.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, during the early days of the Shuttle, it had a monopoly on all US launches (until the Challenger accident, when the EELV program started).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As no Shuttles were ever launched from the West coast, were there simply no polar orbiting satellites launched between 1981-1986?&lt;/p&gt;&#xA;" OwnerUserId="22313" LastActivityDate="2018-01-04T23:40:01.280" Title="How were polar or retrograde satellites launched in the early 80s?" Tags="|space-shuttle|history|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24290" PostTypeId="2" ParentId="24289" CreationDate="2018-01-04T23:33:27.357" Score="5" Body="&lt;p&gt;You can look up the list of space launches in those years in wikipedia&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/1981_in_spaceflight&quot; rel=&quot;noreferrer&quot;&gt;1981 in spaceflight&lt;/a&gt;,&lt;a href=&quot;https://en.wikipedia.org/wiki/1982_in_spaceflight&quot; rel=&quot;noreferrer&quot;&gt;1982 in spaceflight&lt;/a&gt;,&lt;a href=&quot;https://en.wikipedia.org/wiki/1983_in_spaceflight&quot; rel=&quot;noreferrer&quot;&gt;1983 in spaceflight&lt;/a&gt;,&lt;a href=&quot;https://en.wikipedia.org/wiki/1984_in_spaceflight&quot; rel=&quot;noreferrer&quot;&gt;1984 in spaceflight&lt;/a&gt;,&lt;a href=&quot;https://en.wikipedia.org/wiki/1985_in_spaceflight&quot; rel=&quot;noreferrer&quot;&gt;1985 in spaceflight&lt;/a&gt; and &lt;a href=&quot;https://en.wikipedia.org/wiki/1986_in_spaceflight&quot; rel=&quot;noreferrer&quot;&gt;1986 in spaceflight&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you will discover is that while Space Shuttles indeed covered the majority of the american space launches in those years, they never had absolute monopoly. Some satellites (obviously high inclination ones but some geosynchronous too) were still launched using Atlas and Titan rockets. For example Block I GPS used Atlas.&lt;/p&gt;&#xA;" OwnerUserId="18138" LastActivityDate="2018-01-04T23:33:27.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24291" PostTypeId="2" ParentId="24289" CreationDate="2018-01-04T23:40:01.280" Score="2" Body="&lt;p&gt;They were launched from Vandenberg on expendable boosters.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Example: KH9-17 launched on May 11, 1982 into a 96.4 degree orbit.  The booster was a Titan IIID.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/KH-9_Hexagon&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2018-01-04T23:40:01.280" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24292" PostTypeId="1" CreationDate="2018-01-05T02:20:15.077" Score="3" ViewCount="613" Body="&lt;p&gt;Historically, most launches from U.S. soil that have headed into a polar or near-polar orbit have been launched from Vandenberg Air Force Base in California. However, in a recent article written by James Dean of Florida Today, titled &quot;&lt;a href=&quot;http://www.floridatoday.com/story/tech/science/space/2017/12/31/southbound-cape-rockets-may-fly-new-path-toward-poles/975027001/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Southbound? Cape rockets may fly new path toward poles&lt;/a&gt;&quot;, in which the possibility of flying into a polar orbit from Florida is now considered feasible, that in the early days of the space race, launches were indeed conducted on polar trajectories from Florida:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Cape launches most often head east to send satellites on their way around the equator. Polar trajectories have been avoided since a 1960 Navy launch inadvertently dropped a Thor rocket stage on Cuba, reportedly killing a cow.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Do we have a list of these satellites, their launching vehicle, and the date of their launch? Ideally, please only cite first hand sources, not forums. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-05T21:26:32.917" LastActivityDate="2018-01-05T21:26:32.917" Title="What satellites were launched into a polar orbit from Cape Canaveral in the 1950s and 1960s?" Tags="|history|polar-orbit|cape-canaveral|launch-trajectories|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24293" PostTypeId="5" CreationDate="2018-01-05T02:21:39.210" Score="0" Body="" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-05T02:21:39.210" LastActivityDate="2018-01-05T02:21:39.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24294" PostTypeId="4" CreationDate="2018-01-05T02:21:39.210" Score="0" Body="For questions discussing trajectories taken by rockets during launch." OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-05T02:21:39.210" LastActivityDate="2018-01-05T02:21:39.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24295" PostTypeId="2" ParentId="2920" CreationDate="2018-01-05T02:37:47.420" Score="8" Body="&lt;p&gt;To revisit this question, it does appear that the during the 1950s &amp;amp; 1960s, a number of extreme inclination and polar launches did indeed take place from Cape Canaveral in Florida.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The range safety limitations placed on launches from Florida was only enacted &lt;em&gt;after&lt;/em&gt; a number of polar launches had already taken a place (possibly for good reason at the time too, as it is reported a 1960 Thor launch killed a cow in Cuba). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/PV6pN.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/PV6pN.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;The launch of ESSA9 onboard a Delta 1E rocket from LC-17B at Cape Canaveral&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most extreme of these being ESSA9 (1969-016A), a 145kg meteorological satellite which launched on 26 February, 1969 on a Delta E1 launch vehicle from Cape Canaveral LC-17B, with the orbital parameters of:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Periapsis: 1427km&lt;/li&gt;&#xA;&lt;li&gt;Apoapsis: 1508km&lt;/li&gt;&#xA;&lt;li&gt;Inclination: &lt;strong&gt;101.79°&lt;/strong&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;https://nssdc.gsfc.nasa.gov/nmc/spacecraftOrbit.do?id=1969-016A&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This inclination actually &lt;em&gt;exceeds&lt;/em&gt; a typical polar orbit, and is technically a very weak retrograde orbit. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2018-01-05T02:37:47.420" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24296" PostTypeId="2" ParentId="24271" CreationDate="2018-01-05T12:20:13.520" Score="-2" Body="&lt;p&gt;&lt;strong&gt;Orbit (EDIT: more like escape velocity)&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Well, kind of. Gas is similar to anything in space: if it's in orbit, it won't come down until the atmosphere or solar radiation decays it's trajectory enough. But no matter what, it will settle onto the top of the atmosphere if it stays in orbit. If it escapes the Earth's gravitational pull then it will never come down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's not necessarily a matter of altitude, but more of the orbital trajectory of the particles.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As for the crossover value, it depends on the engine, namely its exhaust velocity and plume dispersion, as well as the ascent profile of the rocket. So the answer is that there isn't really any set threshold that will define, for all rockets on LP-1/LOX or any other fuel combination, when 50% of the exhaust will fall back to Earth and 50% will not. It is entirely dependent on the particular rocket launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let me rethink my answer...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume a Merlin 1D from SpaceX as an example, running on kerosene and oxygen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In a vacuum, Merlin 1D has a specific impulse of 311 seconds, so our exhaust velocity $v_e=g_0\times I_{sp}$, so about 3,050 m/s. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the rocket is at the end of its orbital insertion burn into LEO, then it will be traveling at orbital velocity, 7,800 m/s. If the exhaust leaves the engine the other way at 3,050 m/s, then the final velocity of the exhaust would be 4750 m/s, well under orbital velocity. It will be recaptured by the atmosphere.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the exhaust is to leave the Earth forever, then it must be traveling away from Earth at escape velocity, 11,200 m/s, and then some to account for other forces. That means that the rocket itself must be traveling at escape velocity plus the exhaust velocity: 14,250 m/s. The rocket must be well on its way out of the Earth's gravitational influence if it is to stay in space. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, this leads to the same answer that there is no practical 50%. It is more of a binary returns-to-earth or doesn't-return-to-earth outcome. Altitude is not important, it is velocity and direction of the exhaust that matters.&lt;/p&gt;&#xA;" OwnerUserId="17272" LastEditorUserId="17272" LastEditDate="2018-01-05T22:01:11.503" LastActivityDate="2018-01-05T22:01:11.503" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24297" PostTypeId="2" ParentId="24292" CreationDate="2018-01-05T13:43:31.493" Score="3" Body="&lt;p&gt;Looking at &lt;a href=&quot;http://www.zarya.info/Diaries/Launches/Launches.php?year=1960&quot; rel=&quot;nofollow noreferrer&quot;&gt;Zarya&lt;/a&gt;, which lists all attempted launches and the orbits of all successful launches, there was never a successful launch from Canaveral to polar orbit. The highest inclination I saw was the Transit 2A, which had a 67 degree inclination.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It seems likely the cow killing incident was the Transit 3A, presumably which would have been launched in to a similar orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I suspect the only remotely polar launches from Canaveral thus far have been the Transit 2A and 3A. The Transit 1A/1B were at an inclination of about 50 degrees.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I should add that I looked at each of the failed orbits to see if I could determine if they would have been a higher inclination by looking at the replacement launches, and couldn't find anything.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect the article should have stated southward launches, as opposed to the normal North or Northeast launches that typically happen from Canaveral.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-05T13:43:31.493" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24298" PostTypeId="2" ParentId="24271" CreationDate="2018-01-05T15:02:52.553" Score="3" Body="&lt;p&gt;Virtually all exhaust will stay inside the atmosphere, most of it in the very high atmosphere. In order for it to leave, it would have to reach escape velocity. It's actually going the wrong direction, and thus won't have the energy to escape. Some of it that is light enough, like leftover hydrogen, might escape, but it won't be that much. The only exhaust that might escape Earth is for interplanetary missions, which depending on the trajectory the final bit of exhaust might escape the atmosphere, or retrograde exhaust from an orbital mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that almost all launches from Earth will leave the rocket exhaust in Earth orbit, soon to return to Earth, particularly because most rocket firings happen nearly in the prograde direction, which means the exhaust will have less energy then the rest of what's around.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of all of the environmental affects that are on the watch list for rocket launches, Ozone depletion is the largest concern, followed by soot/ particles, and then on to CO2 emissions. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-05T15:25:30.463" LastActivityDate="2018-01-05T15:25:30.463" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24299" PostTypeId="1" AcceptedAnswerId="24300" CreationDate="2018-01-05T16:20:19.967" Score="15" ViewCount="2328" Body="&lt;p&gt;I was looking at the pictures of the newly rolled out Falcon Heavy and I noticed these odd yellow extensions on the payload faring. What are they?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/i6tWU.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/i6tWU.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22326" LastEditorUserId="51" LastEditDate="2018-01-08T01:44:28.823" LastActivityDate="2018-01-08T01:44:28.823" Title="What are these yellow devices attached to the Falcon Heavy?" Tags="|launch|spacex|rockets|falcon-heavy|tel|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24300" PostTypeId="2" ParentId="24299" CreationDate="2018-01-05T17:50:37.210" Score="20" Body="&lt;p&gt;Those are part of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Transporter_erector&quot; rel=&quot;noreferrer&quot;&gt;TEL (Transporter, Erector, Launch tower)&lt;/a&gt;. Here's a &lt;a href=&quot;https://www.instagram.com/p/BdeEU2glMJT/?taken-by=spacex&quot; rel=&quot;noreferrer&quot;&gt;video&lt;/a&gt; that shows the rocket and TEL from various angles. A shot from that video: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8SjvQ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8SjvQ.png&quot; alt=&quot;Falcon Heavy seen from the side&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They are clamps that support the fairing &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=44376.40&quot; rel=&quot;noreferrer&quot;&gt;when the TEL is horizontal&lt;/a&gt;. They retract once the TEL is vertical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VwoLD.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VwoLD.jpg&quot; alt=&quot;Tel horizontal, clamps in place&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Originally, the acronym &lt;a href=&quot;https://en.wikipedia.org/wiki/Transporter_erector_launcher&quot; rel=&quot;noreferrer&quot;&gt;TEL&lt;/a&gt; was used for vehicles that carried and launched ballistic missiles. Some people have started using the acronym to refer to SpaceX's launch towers (which are somewhat unusual for Western rockets - most rockets are assembled and transported vertically, this one is assembled and transported horizontally, on the TEL).&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-06T08:02:36.563" LastActivityDate="2018-01-06T08:02:36.563" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24301" PostTypeId="1" CreationDate="2018-01-05T18:00:40.310" Score="4" ViewCount="303" Body="&lt;p&gt;I know the rocket temperature can go up to 3500 degree Celsius especially at the combustion chamber where it is the hottest. However I know there are cooling measures to cool the surface of the chamber and nozzle and thus they will not reach stoichiometric temperature. However how effective are these measures in reducing the heat transfer to the chamber and nozzle surface? Must the materials have a minimum melting point of above 2000 degree Celsius?&lt;/p&gt;&#xA;" OwnerUserId="22224" LastActivityDate="2018-01-05T18:00:40.310" Title="What is the minimum melting point a material need to have in order for it to be used for the rocket chamber and nozzle?" Tags="|rockets|" AnswerCount="0" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24302" PostTypeId="1" AcceptedAnswerId="24310" CreationDate="2018-01-05T19:45:43.190" Score="27" ViewCount="5630" Body="&lt;p&gt;So the payload of the maiden Falcon Heavy flight will be... Elon Musks's Tesla&#xA;&lt;a href=&quot;https://i.stack.imgur.com/VsTIo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VsTIo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is one part publicity stunt (it's going to be playing David Bowie's &lt;a href=&quot;https://www.youtube.com/watch?v=iYYRH4apXDo&quot; rel=&quot;noreferrer&quot;&gt;Space Oddity&lt;/a&gt;, if for nothing more than the irony) as well as one part test flight. So, normally I'd expect this to just go up, get a few viral photos, and make a flashy re-entry. But Musk is aiming higher. &lt;a href=&quot;https://twitter.com/elonmusk/status/936782477502246912&quot; rel=&quot;noreferrer&quot;&gt;Much higher&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Destination is Mars orbit. Will be in deep space for a billion years or so if it doesn’t blow up on ascent.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Now, you can send anything you want into orbit or into space (no air resistance, corrosion, etc). However, I'm curious how he intends to do this. Normally you send a smaller rocket into orbit and it breaks orbit with said rocket and goes on its merry way thanks to inertia and gravity. Is the FH booster going to burn longer? Is there some second booster hidden somewhere? (seems unlikely for a mission with 50/50 odds of it blowing up) Or is it just going to aim as far out as possible to send it on it's merry way?&lt;/p&gt;&#xA;" OwnerUserId="22326" LastEditorUserId="25" LastEditDate="2018-01-09T15:05:50.257" LastActivityDate="2018-01-09T15:05:50.257" Title="How can Elon Musk's Tesla actually reach Mars orbit?" Tags="|spacex|mars|falcon-heavy|tesla-roadster|nonstandard-payload|" AnswerCount="3" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24303" PostTypeId="2" ParentId="24302" CreationDate="2018-01-05T19:51:43.650" Score="12" Body="&lt;p&gt;It can't reach Mars Orbit. That would require some kind of a long stable rocket fuel on board, and the ability to separate from the second stage, none of which appear to be the case in the rocket. Not to mention that it would require approval from the US government to do so under the planetary protection act, it doesn't have any indicator of power to keep it going for that long, and that would add a huge expense overall to the mission. Lastly, it would require that the launch occurs in a Mars transfer window, which won't happen until March.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think the desire not to orbit Mars, but reach the orbit of Mars around the Sun. Perhaps an elliptical orbit with one end at Earth's distance, and a second at Mars. A close flyby of Mars might be possible, but orbiting Mars isn't.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit- Just to add a bit more. The cost to send the Telsa in to space without any &quot;smarts&quot; is probably under a million. They would have to do a vacuum test, vibration test, manufacture the stand, and probably do a few modifications, but all in all not a huge amount of engineering would be required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to orbit Mars, it would need the following:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A long life time rocket fuel, of a variable amount. It would need to make some kind of course corrections mid-way. This would also need to be centered on the payload's mass, to allow for it to work. Note that would require some small mid-course adjustments, as well as a longer burn to orbit.&lt;/li&gt;&#xA;&lt;li&gt;Long term power- Most likely the batteries, while large, would not be sufficient.&lt;/li&gt;&#xA;&lt;li&gt;Attitude control- Would have to involve some nozzles for adjustment.&lt;/li&gt;&#xA;&lt;li&gt;Approval from NASA- In order to comply with Planetary Protection laws.&lt;/li&gt;&#xA;&lt;li&gt;Long range communication- Some kind of a dish.&lt;/li&gt;&#xA;&lt;li&gt;Some kind of a processor to connect everything together.&lt;/li&gt;&#xA;&lt;li&gt;Thermal control.&lt;/li&gt;&#xA;&lt;li&gt;Launching at the proper launch window.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Basically, they would need to convert the Roadster into a proper satellite. There is zero evidence that ANY of this has been done. The cost to do so would be in the $10+ million range, and would take considerable engineering work to accomplish.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it would be awesome, but it's far too much work and cost to do on a publicity stunt that has such a high chance of failure.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-09T14:52:49.457" LastActivityDate="2018-01-09T14:52:49.457" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="24304" PostTypeId="2" ParentId="24302" CreationDate="2018-01-05T20:10:58.520" Score="4" Body="&lt;p&gt;It will be in an elliptical Heliocentric orbit with a perihelion at the Earth's distance from the sun, and an aphelion at Mars' distance from the sun. But it shouldn't get close enough to either planet to be captured by their gravity and impact, or be flung outwards by a gravity assist. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Falcon 9 second stage will put it onto this elliptical orbit. &lt;/p&gt;&#xA;" OwnerUserId="22274" LastActivityDate="2018-01-05T20:10:58.520" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="24305" PostTypeId="2" ParentId="22539" CreationDate="2018-01-05T20:27:45.070" Score="6" Body="&lt;p&gt;In &lt;a href=&quot;https://www.businessinsider.nl/is-elon-musk-tesla-car-falcon-heavy-payload-real-2017-12/?international=true&amp;amp;r=US&quot; rel=&quot;noreferrer&quot;&gt;Dec 2017&lt;/a&gt;, Musk revealed they'd be sending his old Tesla Roadster as the payload:  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VsTIo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VsTIo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-01-05T20:27:45.070" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24306" PostTypeId="2" ParentId="4176" CreationDate="2018-01-05T20:52:04.773" Score="2" Body="&lt;p&gt;Probably the best tool to use for this sort of application would be &lt;a href=&quot;https://www.flightclub.io&quot; rel=&quot;nofollow noreferrer&quot;&gt;FlightClub&lt;/a&gt;, developed by Declan Murphy. Launches have estimated trajectory data which the developer ingests into the system prior to launch to produce a viable simulation of the rocket's flight path. Post-launch completion, the developer then adjusts the trajectory to match the actual values (as shown on webcasts). You can opt to track these launches in &quot;realtime&quot; by following along with the launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/e6JG3.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/e6JG3.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Simulation of the CRS-9 launch in FlightClub&lt;/sup&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This data is overlaid onto &lt;a href=&quot;https://cesiumjs.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cesium&lt;/a&gt;, a WebGL-based 3D earth viewing application, alongside mission &quot;vitals&quot; such as velocity, altitude, downrange distance, and more. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.youtube.com/watch?v=NT50R2dLht8&quot; rel=&quot;nofollow noreferrer&quot;&gt;Here is an example&lt;/a&gt; of Flight Club simulating the SpaceX CRS-9 resupply mission to the International Space Station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sup&gt;Full disclosure: I am friends with the developer of this application.&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2018-01-05T20:52:04.773" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24307" PostTypeId="2" ParentId="17898" CreationDate="2018-01-05T21:40:32.280" Score="5" Body="&lt;p&gt;There is a good manual &amp;quot;Soyuz Crew Operations Manual (SoyCOM) (ROP-19) (Final)&#xA;, Yu. A. Gagarin Cosmonaut's Training Center, NAS15-10110, 0004AE7a&#xA;(ROP-19), April 1998&amp;quot; which is in English, although translated, it's an OK job.&lt;/p&gt;&#xA;&lt;p&gt;You can find a copy at this &lt;a href=&quot;https://www.infosondas.com/2017/04/pdf-soyuz-crew-operations-manual-soycomrop-19final/&quot; rel=&quot;nofollow noreferrer&quot;&gt;page&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6CG6h.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6CG6h.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2020-08-05T14:15:30.090" LastActivityDate="2020-08-05T14:15:30.090" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24308" PostTypeId="1" CreationDate="2018-01-05T22:10:09.900" Score="6" ViewCount="1221" Body="&lt;p&gt;With the Space Shuttle system being as complex as it was I was wondering the following; did the initial engineering teams in the 70's use any form of CAD/CAE or did they simply utilize the same drafting tables as the Saturn V engineers?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's difficult to imagine the world's most complicated mechanical machine at the time being designed with just pencils and rulers.&lt;/p&gt;&#xA;" OwnerUserId="19789" LastActivityDate="2019-04-03T09:33:24.807" Title="Was computer aided design used on the Space Shuttle Program?" Tags="|apollo-program|space-shuttle|design|engineering|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24309" PostTypeId="2" ParentId="24308" CreationDate="2018-01-06T01:22:03.253" Score="9" Body="&lt;p&gt;Rockwell International, lead contractor for the Shuttle, at least had a database which stored vehicle configuration data.  &quot;&lt;a href=&quot;http://hdl.handle.net/2060/19850008628&quot; rel=&quot;noreferrer&quot;&gt;Space Shuttle Technical Conference&lt;/a&gt;&quot; (warning, giant pdf) refers to this when discussing manufacture of the tiles.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Rockwell master dimension engineering data base and vehicle&#xA;  configuration coordinates were converted into computer tapes that&#xA;  drove the numerically controlled mills that machined the tiles to&#xA;  precise dimensions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(page 1070 of the linked document)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://books.google.com/books?id=X_3_D4RqzvIC&amp;amp;pg=PA14&amp;amp;lpg=PA14&amp;amp;dq=space%20shuttle%20manufacturing%20process%20rockwell&amp;amp;source=bl&amp;amp;ots=-Z07sow_QF&amp;amp;sig=xpi0a1t9XAkQCz0knvPK_XEowc4&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwiD6YaNjMLYAhVCLSYKHZU4CmY4ChDoAQhEMAc#v=onepage&amp;amp;q&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;Contemporary advertising in Computerworld magazine&lt;/a&gt; mentions the database and computer simulation applications used by Rockwell, but does not refer to any CAD-CAM systems.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b0UY4.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b0UY4.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.quora.com/Are-the-space-shuttle-design-documents-available-online&quot; rel=&quot;noreferrer&quot;&gt;An unsourced statement&lt;/a&gt; in response to a quora.com question says the company was transitioning to CAM at the time, but drawings were done in the traditional manner.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;When I worked for Rockwell, they were preparing for computer&#xA;  controlled/aided manufacturing (this was pre-AUTOCAD so they were&#xA;  developing their own formats, this was before I seriously got into&#xA;  programming, I was shown the machine but can’t recall the hardware). I&#xA;  drew on company coated Mylar, so I’d expect to see NAR Rockwell logos.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;So, it's difficult to find a definitive answer,but it appears that CAD-CAM wasn't used, at least in the early design days, but that the design information was entered into a large database (&quot;Completed designs are stored online&quot;).&lt;/p&gt;&#xA;" OwnerUserId="6944" LastActivityDate="2018-01-06T01:22:03.253" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24310" PostTypeId="2" ParentId="24302" CreationDate="2018-01-06T01:33:52.557" Score="29" Body="&lt;p&gt;I haven't been able to find any statements on the exact target orbit, but the general consensus is that the payload won't be in orbit around Mars, it will be in an eliptical orbit around the sun and 'touch' the area of space that Mars orbits within. Reaching an orbit around Mars requires much more fuel, the real goal of this launch is to test the rockets capabilities so it makes sense to be able to use some fuel to land the boosters rather than use that fuel to insert a rather useless payload into Mars orbit.&lt;/p&gt;&#xA;&lt;p&gt;As suggested in the comments below, Musk may have been purposefully vague about the target orbit in an attempt to make us assume it will be in orbit around Mars, and therefore a grander achievement than it really is. &lt;a href=&quot;http://www.spacex.com/about/capabilities&quot; rel=&quot;noreferrer&quot;&gt;The SpaceX website makes the same statement&lt;/a&gt;, claiming that Falcon Heavy can take a 16,800kg payload 'to Mars'. &lt;a href=&quot;https://en.wikipedia.org/wiki/Tesla_Roadster_(2008)#Performance&quot; rel=&quot;noreferrer&quot;&gt;The weight of a Tesla is around 1,300kg.&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;From Musk's &lt;a href=&quot;https://www.instagram.com/p/BdA94kVgQhU/&quot; rel=&quot;noreferrer&quot;&gt;Instagram post&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Test flights of new rockets usually contain mass simulators in the form of concrete or steel blocks. That seemed extremely boring.&lt;/p&gt;&#xA;&lt;p&gt;Of course, anything boring is terrible, especially companies, so we decided to send something unusual, something that made us feel.&lt;/p&gt;&#xA;&lt;p&gt;The payload will be an original Tesla Roadster, playing Space Oddity, &lt;strong&gt;on a billion year elliptic Mars orbit&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The phrase &amp;quot;billion year elliptic Mars orbit&amp;quot; sounds like it could be an elliptical Mars orbit, (orbiting &lt;em&gt;around Mars&lt;/em&gt;) in a technical context, but in a social media post, the interpretation could be a bit more loose.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5LGGt.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5LGGt.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;To explain the expected orbit, the image below is a diagram of a Hohmann Transfer, the most fuel efficient method of transfering between two objects that orbit the same body.&#xA;The grey circle is the Sun, the green ring is Earths orbit around the sun, and the red ring is Mars's orbit around the sun. The transfer is done with two burns, first you move from the green orbit to the red orbit, then once you reach the target orbit, you circularize your orbit.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JaKrj.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JaKrj.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;SpaceX plans to make only the first burn, and orbit in the yellow ring, allowing them to reach the orbit of Mars around the Sun, but not stay there forever. The 'Half-Hohmann Transfer' allows them to do this as cheaply possible and still claim to reach Mars.&lt;/p&gt;&#xA;&lt;p&gt;The tweet is a little misleading, rocket science is hard enough without the 140 character limit per tweet.&lt;/p&gt;&#xA;" OwnerUserId="22333" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-06T02:25:56.527" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24311" PostTypeId="1" AcceptedAnswerId="24312" CreationDate="2018-01-06T01:37:23.160" Score="2" ViewCount="691" Body="&lt;p&gt;Many/most early artificial satellites were spheres with mirror-like polished outer surfaces.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am guessing that for lower orbits the spherical shape minimized drag as well as simplified the analysis and interpretation of rate of decay, since this was critical to understand in the early days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the metal could have been roughened, anodized, or even painted, rather than having a mirror-like finish. Was this done for reasons of visibility and optical tracking? Or was it done for reasons of thermal management, where the emissivity in the visible determined heat absorption and in the IR determined radiation? Or another reason, or for no particular reason at all beyond aesthetics?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm looking for &lt;em&gt;an answer supported with a source that addresses the design intention at the time&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Sputnik_asm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sputnik-1&lt;/a&gt;, &lt;a href=&quot;https://en.wikipedia.org/wiki/File:RIAN_archive_510848_Interplanetary_station_Luna_1_-_blacked.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Luna-1&lt;/a&gt;, and &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Vanguard_1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vanguard-1&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/efHmm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/efHmm.jpg&quot; alt=&quot;Sputnik-1&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/JjgJ5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JjgJ5.jpg&quot; alt=&quot;Luna-1&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/hTegp.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/hTegp.jpg&quot; alt=&quot;Vanguard-1&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Exploring the &quot;aesthetics&quot; angle, &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:Vanguard_1_composite.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;rendering of Vanguard-1&lt;/a&gt; versus &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Disco_ball4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;disco mirror ball&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/WkIk7m.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/WkIk7m.jpg&quot; alt=&quot;rendering of Vanguard-1&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/Yw7n1.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Yw7n1.jpg&quot; alt=&quot;disco mirror ball&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2018-01-06T13:29:58.390" Title="Why were early satellites produced with polished &quot;mirror-like&quot; surfaces?" Tags="|thermal|tracking|thermal-control|visibility-of-spacecraft|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24312" PostTypeId="2" ParentId="24311" CreationDate="2018-01-06T01:43:48.587" Score="6" Body="&lt;p&gt;Per &lt;a href=&quot;https://history.nasa.gov/SP-4202.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;the official NASA history of Vanguard&lt;/a&gt; it was for visibility and thermal control.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Bolding mine)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Tousey  had  made  some  of  the  first  calculations  in  the  fall &#xA;  of  1955,  contributing  his  knowledge  of  optics  &lt;strong&gt;to  ensure &#xA;  that  protective  coatings  on  the  exterior  of  the  booster  and &#xA;  on  the  satellite  shell  would  have  sufficient  reflec­tivity  to&#xA;  permit  telescopic  observation  of  the  course  of  the  rocket  as &#xA;  it  rose  and  then  optical  acquisition  and  tracking  of  the &#xA;  satellite  in  space.&lt;/strong&gt;   Schach  undertook  the  &quot;thermal  design,&quot; &#xA;  that  is,  the  calculations  of  what  tempera­tures  to  expect  at&#xA;  various  points  in  the  satellite's  orbit,  in  darkness  and  in &#xA;  daylight,  &lt;strong&gt;the  selection  of  the  optimum  thickness  of  coating &#xA;  materials  to  emphasize  their  emissiveness  of  solar  heat &#xA;  radiation,&lt;/strong&gt;  and  methods  of  keeping  the  satellite's  surface  free&#xA;  of  contaminating  substances  such  as  soot  which  would &#xA;  ultimately  raise  the  satellite's  temperature.  Hass  worked  out &#xA;  the  techniques  of  applying  the  successive  surface  coatings-the &#xA;  gold  plating,  the  chromium  evaporated  to  vapor  and  deposited &#xA;  to  serve  as  a  primer,  the  silicon  oxide  to  serve  as  a&#xA;  barrier,  the  thin  layer  of  evaporated  aluminum  to  give  a &#xA;  mirror-like  finish,  and  finally  a  film  of  silicon  oxide  to &#xA;  control  emitted  radiation.  Drummeter  and  Schach  were  chiefly &#xA;  responsible  for  developing  the  sunlight  simulator  with  carbon &#xA;  arcs  as  the  source  of  high­ intensity  light.  Through  windows &#xA;  in  the  large  cylindrical  vacuum  tank  in  which  the  coated &#xA;  sphere  sat  for  two  or  three  days  of  testing,  the  simulated &#xA;  sunlight  beat  upon  the  satellite's  surface  and  indirectly &#xA;  heated  the  inner  pot  of  instruments.  Measurements  of  the &#xA;  effects  furnished  means  of  determining  the  most  desirable &#xA;  material  and  thickness  of  the  layering  required.  LaGow  acted &#xA;  as  advisor  and  monitor  on  all  these  operations.  Every  man &#xA;  concerned  with  temperature  control  worked  closely  with  every &#xA;  other.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-01-06T13:29:58.390" LastActivityDate="2018-01-06T13:29:58.390" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24314" PostTypeId="1" AcceptedAnswerId="35533" CreationDate="2018-01-06T04:49:50.613" Score="4" ViewCount="1112" Body="&lt;p&gt;This &lt;a href=&quot;https://space.stackexchange.com/a/8205/12102&quot;&gt;excellent answer&lt;/a&gt; to the question &lt;a href=&quot;https://space.stackexchange.com/q/8201/12102&quot;&gt;Why has no TLE been published for the DSCOVR satellite and the Falcon 9 R/B?&lt;/a&gt; suggests that to the best of the writer's knowledge TLEs are only issued for spacecraft in Earth orbit, presumably meaning a gravitationally bound orbit (rather than a hyperbolic one).&lt;/p&gt;&#xA;&lt;p&gt;However, I'm thinking that it is certainly conceivable that one might be issued for a spacecraft executing a flyby maneuver of Earth if it is low enough and could potentially interact with other satellites as a precaution.&lt;/p&gt;&#xA;&lt;p&gt;Then again, I'm thinking that this might be useless/meaningless since the format of a TLE requires a value for mean motion (revolutions/day) and there is no way the other parameters have enough information to describe an orbit without some way to communicate the semi-major axis, which would be negative for a hyperbolic fly-by.&lt;/p&gt;&#xA;&lt;p&gt;Since Two-Line Element sets actually contain a blank space at column 52 (see &lt;a href=&quot;https://en.wikipedia.org/wiki/Two-line_element_set&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt; and &lt;a href=&quot;https://celestrak.org/NORAD/documentation/tle-fmt.asp&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak&lt;/a&gt;) where a minus sign could potentially be inserted, in some universe it might be possible to actually do this. Also, Three-Line Element sets are at least defined (see &lt;a href=&quot;https://celestrak.org/NORAD/documentation/ADCOM%20DO%20Form%2012.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Celestrak PDF&lt;/a&gt;) though I am not sure how often they are used, and columns 11 and 12 of card 3 are explicitly labeled &amp;quot;orbit type&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; So I'd like to know if a Two-Line or Three-Line Element set has ever been issued for a spacecraft trajectory not bound to Earth orbit, or if in fact none have, &lt;em&gt;then in that case&lt;/em&gt; if one &lt;em&gt;could be issued&lt;/em&gt; if necessary. Looking for a well-supported, factual answer, not just a &amp;quot;not to my knowledge&amp;quot; response.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:27:30.220" LastActivityDate="2019-04-18T18:17:26.813" Title="Has a TLE ever been issued for a spacecraft trajectory not bound to Earth orbit?" Tags="|orbital-mechanics|artificial-satellite|two-line-elements|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24317" PostTypeId="2" ParentId="24314" CreationDate="2018-01-06T15:21:04.653" Score="2" Body="&lt;p&gt;No — TLEs are meant for a particular family of algorithms that are designed for trajectory propagation of Earth-orbiting spacecraft (the Simplified Perturbation Models). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What you can find instead, are state fixes (t, r, v) in a given frame, relative to a given center, issued as some kind of ephemerides. For example, JPL produces SPK files for all of its spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can propagate any point-mass trajectory as long as you know the initial state (six variables) and time, and an “appropriate” algorithm. “Appropriate” can be as easy as a two-body problem, or as complex as a proprietary algorithm that considers all known forces and attitude dynamics. &lt;/p&gt;&#xA;" OwnerUserId="18918" LastActivityDate="2018-01-06T15:21:04.653" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24318" PostTypeId="2" ParentId="17916" CreationDate="2018-01-06T16:26:45.483" Score="3" Body="&lt;p&gt;From the point you are, you need to:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Calculate the XYZ coordinates of your ground station&lt;/li&gt;&#xA;&lt;li&gt;Find the line in space between the satellite and the ground station&lt;/li&gt;&#xA;&lt;li&gt;Find the XYZ coordinates of the point at the intersection of that line with the ellipsoid defined by all the points with elevations of 300 km&lt;/li&gt;&#xA;&lt;li&gt;Find the latitude and longitude of that point.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;For (1) you can use this matlab function:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;function [X,Y,Z] = ll2xyz(lat,lon,alt)&#xA;    b=6356752.3141;&#xA;    a=6378137.0;&#xA;    lat=lat*pi/180;&#xA;    %transformation between geografic and geocentric latitude&#xA;    gclat=atan(b^2*tan(lat)/a^2);&#xA;    lon=lon*pi/180;&#xA;    R=sqrt(1./(tan(gclat).^2/b^2 + 1/a^2));&#xA;    Z=R.*tan(gclat);&#xA;    Z=Z+alt.*sin(lat);&#xA;    R=R+alt.*cos(lat);&#xA;    X=R.*cos(lon);&#xA;    Y=R.*sin(lon);&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;For (2) and (3) this function will do the trick&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;function [X Y Z] = rectaxelip(x1,x2,y1,y2,z1,z2,alt)&#xA;%rectaxelip calculates the point XYZ were a straight line that pass by points P1&#xA;%and P2, intersects a ellipsoid alt meters bigger than WGS84 &#xA;%ellispsoid (on each axis)&#xA;&#xA;%The director cosines of the line are &#xA;    d=sqrt((x1-x2).^2.*(y1-y2).^2.*(z1-z2).^2);&#xA;    L=(x1-x2)./d;&#xA;    M=(y1-y2)./d;&#xA;    N=(z1-z2)./d;&#xA;    %its parametric  for is&#xA;    %x = x1 + L * t&#xA;    %y = y1 + M * t&#xA;    %z = z1 + N * t&#xA;    %Now we look for the intersection&#xA;    %the ellipsoid would have axes bp and ap&#xA;&#xA;    b=6356752.3141; &#xA;    a=6378137.0;&#xA;    bp=b+alt; &#xA;    ap=a+alt;&#xA;&#xA;    %The equation of such ellipsod would be&#xA;    %  x^2/ap^2+y^2/ap^2+z^2/bp^2=1&#xA;    %subtituting and reorganizng you get a quadratic equation like&#xA;    %   A x^2 + B x + C = 0&#xA;    %with&#xA;    A=(L./ap).^2+(M./ap).^2+(N./bp).^2;&#xA;    B=2*((x1.*L./(ap.^2))+(y1.*M./(ap.^2))+(z1.*N./(bp.^2)));&#xA;    C=((x1./ap).^2+(y1./ap).^2+(z1./bp).^2)-1;&#xA;    ta=(-B+sqrt(B.^2 - (4*A.*C)))./(2*A);&#xA;    tb=(-B-sqrt(B.^2 - (4*A.*C)))./(2*A);&#xA;&#xA;    %then&#xA;    x1a = x1 + L .* ta;&#xA;    y1a = y1 + M .* ta;&#xA;    z1a = z1 + N .* ta;&#xA;    x1b = x1 + L .* tb;&#xA;    y1b = y1 + M .* tb;&#xA;    z1b = z1 + N .* tb;&#xA;    %Now the distance from each point to the satellite is&#xA;    da=sqrt((x1a-x2).^2+(y1a-y2).^2+(z1a-z2).^2);&#xA;    db=sqrt((x1b-x2).^2+(y1b-y2).^2+(z1b-z2).^2);&#xA;    ESta=da&amp;lt;db;&#xA;    t=ta.*ESta+tb.*(~ESta);&#xA;&#xA;    %then&#xA;    X = x1 + L .* t;&#xA;    Y = y1 + M .* t;&#xA;    Z = z1 + N .* t;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;And for (4) you just need a function that calculate lat lon coordinates from XYZ positions, and this one would do it:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;function [lat,lon,alt] = xyz2ll(x,y,z1)&#xA;%For some reason the error spikes at +-45&#xA;a=6356752.3141; &#xA;b=6378137.0;&#xA;&#xA;signos=sign(z1);&#xA;z1=-abs(z1);&#xA;&#xA;lon=atan2(y,x)*180/pi; %Longitude&#xA;d1=sqrt(x.^2+y.^2);&#xA;z2=-sqrt(1./((1/(a^2))+(1./(b^2*(z1./d1).^2))));&#xA;d2=z2./(z1./d1);&#xA;p2=-a*d2./(b^2*sqrt(1-(d2.^2/(b^2)))); %slope of the tangent to the ellipse in the point&#xA;for i=1:5&#xA;    %points in the palne&#xA;    dp=(z1-z2-(d1./p2)+(p2.*d2))./(p2-(1./p2));&#xA;    zp=(p2.*dp)+z2-(p2.*d2);&#xA;    %points in the ellipse&#xA;    z2=-sqrt(1./((1/(a^2))+(1./(b^2*(zp./dp).^2))));&#xA;    d2=z2./(zp./dp);&#xA;    p2=-a*d2./(b^2*sqrt(1-(d2.^2/(b^2))));&#xA;end&#xA;lat=-90-(atan(p2)*180/pi);   %geographic latitude&#xA;lat=abs(lat).*signos;&#xA;ecuador=find(abs(z1)&amp;lt;0.001);&#xA;lat(ecuador)=0;&#xA;alt=sqrt(d1.^2+z1.^2)-sqrt(d2.^2+z2.^2);&#xA;alt(ecuador)=d1(ecuador)-a;&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;And then you are ready and you have the ground position of the point were the GPS signal reaching the base station crosses the 300 km altitude.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I wrote these functions myself and I've tested them extensively so I know they work, but be aware that the last function, despite it has very good accuracy, for a reason I haven't investigated much, seem to produce large errors at latitudes of +-45°.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Last thing to note. If you want more accuracy and avoid calculating the XYZ coordinates of the GPS satellites, you can use the sp3 ephemeris files. They are easy to download from &lt;a href=&quot;http://igscb.jpl.nasa.gov/components/prods_cb.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;. There is one file per day, and each file have the XYZ positions of each satellite tabulated  every 15 minutes. To get positions any time spline interpolation gives a very good solution, pretty much the same as the recommended Neville's polynomial interpolation.&lt;/p&gt;&#xA;" OwnerUserId="22234" LastActivityDate="2018-01-06T16:26:45.483" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24319" PostTypeId="1" CreationDate="2018-01-06T22:48:01.807" Score="1" ViewCount="2103" Body="&lt;p&gt;I have come across a problem that has had me stumped for a while now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A spacecraft is in a Sun-synchronous Earth orbit with a =1.40 R⊕ and&#xA;e = 0.2. The argument of periapsis is ω = 0◦&#xA;, and the Right Ascension&#xA;of the Ascending Node, Ω, is equal to the Sun’s Right Ascension plus 12&#xA;hours. Calculate the length of time per orbit for which the spacecraft is&#xA;in Earth’s shadow, assuming that the Earth is at an equinox.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not fully sure how to interpret this, as everywhere I read, Sun-synchronous orbits are always sunlit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Any help or a point in the right direction would be much appreciated.&lt;/p&gt;&#xA;" OwnerUserId="22348" LastActivityDate="2019-12-27T19:02:45.870" Title="Length of time in a satellite spends in Earths shadow" Tags="|orbital-mechanics|sun-synchronous|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24321" PostTypeId="2" ParentId="17562" CreationDate="2018-01-07T03:16:18.670" Score="10" Body="&lt;p&gt;The heat shield (TPS) is not meant to protect the spacecraft from dust impacts. The statement is not quite accurate and (from a recent search) seems to have been removed from site.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an engineer on the Parker Solar Probe team, I have been involved in the Preliminary and Critical Design Reviews for the mission which include presentations on Dust Protection and the TPS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The dust protection studies evaluate potential damage to all spacecraft surfaces, including the TPS, but do not assign any unique role to any one surface as a protective device.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The TPS design requirements are focused on the thermal management of the spacecraft; there are no requirements for protection of the other spacecraft elements from dust damage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was something of an overstatement and seems to have been removed.&lt;/p&gt;&#xA;" OwnerUserId="22352" LastActivityDate="2018-01-07T03:16:18.670" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24322" PostTypeId="2" ParentId="21721" CreationDate="2018-01-07T03:38:43.743" Score="2" Body="&lt;p&gt;The largest torque affecting the Parker Solar Probe during its closest approach is caused by the solar wind blasting against the heat shield (Thermal Protection System, a.k.a TPS).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The TPS is like an umbrella held facing into a strong wind. The wind whips against it, and tries to force it around away from the wind. The spacecraft has to &quot;fight&quot; against this force to maintain it's proper attitude with the TPS pointed right at the sun and right into the solar wind.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To &quot;fight&quot; against the solar wind, the primary mechanism is the spacecraft's reaction wheels. They are mounted at angles to each other (there are four) and they are spun up or slowed down to create the counter force to the solar wind acting on the TPS. The spacecraft subsystem that controls the wheels is the Guidance and Control (G&amp;amp;C) system. Fifty times a second (50 Hz) the inertial measurement unit provides change-in-attitude readings and the G&amp;amp;C system calculates the reaction wheel response required to counter act the change and bring the spacecraft back to the correct attitude with the TPS pointing at the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In addition to controlling attitude with the reaction wheels, the spacecraft thrusters can also be used, but this is not normally done because it degrades the science measurements. However, as the reaction wheels counter the torques from the solar wind, they wind up and spin faster and faster. In order to bring the speed back down, the spacecraft periodically performs a &quot;monmentum dump&quot; by firing the thrusters to counter-act the wheel's momentum and allow them to slow down to a manageable rate.&lt;/p&gt;&#xA;" OwnerUserId="22352" LastActivityDate="2018-01-07T03:38:43.743" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24323" PostTypeId="2" ParentId="10060" CreationDate="2018-01-07T04:08:23.287" Score="6" Body="&lt;p&gt;As one scientist said to me &quot;It would be a &lt;strong&gt;&lt;em&gt;sin&lt;/em&gt;&lt;/strong&gt; to fly by Venus and not do science&quot;. So, yes, scientifically valuable data will be collected during the Venus flybys.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(As it turns out this data collection will be very limited - see Dr. Fox's answer below - to the extent that the &quot;short answer&quot; is &quot;none&quot;) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;HOWEVER, designing and building a spacecraft that will fly through the sun's corona 24 times over 7 years is a very complex, very challenging engineering feat. Everything, and I mean every single, minute detail, of the spacecraft design has to be focused specifically and only on the mission's primary science goals. Nothing, and I mean absolutely nothing, other than that can be accommodated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, useful and interesting data will be collected at Venus, but only what can be incidentally obtained with a spacecraft that is designed specifically and only for the challenge of unlocking the secrets of the Sun's corona.&lt;/p&gt;&#xA;" OwnerUserId="22352" LastEditorUserId="22352" LastEditDate="2018-10-06T22:59:24.167" LastActivityDate="2018-10-06T22:59:24.167" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="24324" PostTypeId="5" CreationDate="2018-01-07T04:23:01.163" Score="0" Body="&lt;p&gt;The Parker Solar Probe is a NASA mission to explore the sun and answer key questions about its corona and the solar wind. It was formerly known as the Solar Probe Plus, and is often just called the Solar Probe. It is recommended to use &quot;Parker Solar Probe&quot;, now it is the official NASA designation.&lt;/p&gt;&#xA;" OwnerUserId="22352" LastEditorUserId="58" LastEditDate="2019-02-08T15:07:42.440" LastActivityDate="2019-02-08T15:07:42.440" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="24325" PostTypeId="4" CreationDate="2018-01-07T04:23:01.163" Score="0" Body="The Parker Solar Probe is a NASA mission to explore the sun and answer key questions about its corona and the solar wind. This tag should be used to ask questions about the Parker Solar Probe." OwnerUserId="22352" LastEditorUserId="22352" LastEditDate="2018-01-07T13:44:29.417" LastActivityDate="2018-01-07T13:44:29.417" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24326" PostTypeId="1" AcceptedAnswerId="24342" CreationDate="2018-01-07T13:08:40.843" Score="5" ViewCount="724" Body="&lt;p&gt;I read in a lot of places that for some reason the moon lacks carbonates, and thus carbon. Part of the reason seems to be the lack of biological origin and cycles. But are there no other sources for getting carbon on the moon if the need arises?&lt;/p&gt;&#xA;" OwnerUserId="16627" LastActivityDate="2019-03-04T07:00:04.177" Title="Why aren't there carbonate rocks on the moon?" Tags="|the-moon|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24327" PostTypeId="1" CreationDate="2018-01-07T13:53:28.370" Score="4" ViewCount="2443" Body="&lt;p&gt;Let's suppose there is a wifi router or a cell phone or similar transmitter in an orbit of 3000 km. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;How could one make a dish antenna to connect and receive the signal from that wifi router or cell phone or signal transmitting device?&lt;/p&gt;&#xA;" OwnerUserId="22358" LastEditorUserId="6537" LastEditDate="2018-01-11T09:27:24.277" LastActivityDate="2018-02-15T12:01:41.377" Title="How to design a ground antenna for sending &amp; receiving from a satellite?" Tags="|communication|communication-satellite|antenna|ground-station|" AnswerCount="2" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="24328" PostTypeId="1" CreationDate="2018-01-07T15:15:30.297" Score="5" ViewCount="154" Body="&lt;p&gt;If satellites were in place with space elevator type cable holding a &quot;friction generator&quot; close to the Martian atmosphere, how far from the surface would the said device have to be?  I don't need an exact answer.  I am wondering if such a device might be seen with the naked eye while standing on the planets surface.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The improbability of such a device is not in question at the moment.  Thank you.&lt;/p&gt;&#xA;" OwnerUserId="22340" LastActivityDate="2018-05-05T04:15:06.230" Title="How close to the planet Mars would an object have to be to generate friction in its atmosphere?" Tags="|mars|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24329" PostTypeId="1" AcceptedAnswerId="24331" CreationDate="2018-01-07T15:20:42.600" Score="25" ViewCount="5287" Body="&lt;p&gt;Why did SpaceX retire Falcon 1? Wouldn't it have been more cost effective then Falcon 9 for lighter missions?&lt;/p&gt;&#xA;" OwnerUserId="22359" LastEditorUserId="25" LastEditDate="2018-01-08T15:32:44.240" LastActivityDate="2018-01-08T15:32:44.240" Title="Why did SpaceX retire Falcon 1?" Tags="|spacex|rockets|falcon-9|design|falcon-1|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24330" PostTypeId="2" ParentId="24329" CreationDate="2018-01-07T15:52:49.440" Score="14" Body="&lt;p&gt;Viewing SpaceX's history, I get the distinct impression that Falcon 1 was a development vehicle: it allowed SpaceX to test the Merlin engine, and it allowed them to demonstrate they were a viable business. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;From early on in their history, SpaceX has been trying to lower launch cost through &lt;a href=&quot;http://www.spacex.com/falcon9&quot; rel=&quot;noreferrer&quot;&gt;reuse&lt;/a&gt;. There were plans to recover the first stage of the Falcon 1 (via &lt;a href=&quot;https://en.wikipedia.org/wiki/Falcon_1&quot; rel=&quot;noreferrer&quot;&gt;parachute landings&lt;/a&gt;) but those plans were shelved. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My guess is that the extra hardware needed would reduce F1's payload by too much, or they realized refurbishing a booster after a parachute landing in water wasn't going to be cheap. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX said the market for Falcon 1-sized payloads was too small to pursue. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, development effort was concentrated on Falcon 9, which has much larger margins for recovery. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX believes rocket reusability is the key breakthrough needed to reduce the cost of access to space and enable people to live on other planets.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Running 2 programs in parallel (F1/F9) would have spread engineering resources, delaying the F9 program. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-01-07T15:52:49.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24331" PostTypeId="2" ParentId="24329" CreationDate="2018-01-07T16:19:34.720" Score="38" Body="&lt;p&gt;The Falcon 1 was less profitable to maintain, and it didn't have the customer base to support using it. A Falcon 1 launch cost around $10 million, of which about 10% was profit. They also considered a Falcon 1e design, which would carry slightly more to orbit, but only 1100 kg tops.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There were a few people who are known to have booked a Falcon 1 launch, namely DoD (TacSAT-1), MDA Corp, Swedish Space Corp, the US Air Force, and ORBCOMM. Most of these could be done with a Falcon 9, and many were given a huge discount to use the larger rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Keep in mind that the Falcon 1 had a development cost of around \$100 million. At best it would make \$1 million profit per launch. Without considerable demand, it wasn't worth continuing to develop that rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the same time, NASA came knocking on SpaceX's door with a $400 million contract to build a ISS resupply mission. SpaceX was broke, and it would have been hard to justify continuing the development of Falcon 1 which would never see a huge return on it's money. In the end, they opted to pursue NASA's money, which pretty much required them to abandon the Falcon 1 design entirely.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, Falcon 1 wasn't profitable enough to continue with, and had to be abandoned, in order to make room for the more capable and profitable Falcon 9.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-07T16:19:34.720" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24332" PostTypeId="1" CreationDate="2018-01-07T17:00:28.150" Score="3" ViewCount="90" Body="&lt;p&gt;I am working on an implementation of an SLR tool. At some points of the &lt;a href=&quot;https://ilrs.cddis.eosdis.nasa.gov/docs/2009/crd_v1.01.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;documentation of the CRD format&lt;/a&gt; the term &lt;strong&gt;telescope invariant point&lt;/strong&gt; shows up. Apparently it's a point on an ILRS antenna system. From what I have got so far it is some (arbitrary?) point of the antenna system which is not known beforehand but has to be determined through observations.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The documentation states&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Note  that  station  transmit  and  receive  times  are  nominally  with  respect  to  the  system  reference  point &#xA;  (SRP) which will in many cases be the telescope invariant point. This requires a knowledge of both the  transmit delay and receive delay, which is critical for transponder ranging. It is less critical for normal &#xA;  satellite  (two-way)  ranging  since  errors  in  distributing  the  system  delay  to  these  components  will &#xA;  cancel. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If the telescope invariant point is coincident with the station reference point (SRP) I do not have to take care of any corrections at this point?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also do not get the sentence &quot;This requires a knowledge of both the transmit delay and receive delay&quot;. In how far could the transmit and receive delay of an antenna system influence a reference point?&lt;/p&gt;&#xA;" OwnerUserId="22266" LastEditorUserId="33" LastEditDate="2018-08-15T10:44:57.890" LastActivityDate="2018-08-15T10:44:57.890" Title="What is the Telescope Invariant Point?" Tags="|earth-observation|antenna|ground-station|" AnswerCount="0" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="24333" PostTypeId="5" CreationDate="2018-01-07T19:50:47.587" Score="0" Body="" OwnerUserId="-1" LastEditorUserId="-1" LastEditDate="2018-01-07T19:50:47.587" LastActivityDate="2018-01-07T19:50:47.587" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24334" PostTypeId="4" CreationDate="2018-01-07T19:50:47.587" Score="0" Body="&quot;Solar Probe&quot; refers to the &quot;Parker Solar Probe&quot;, a NASA mission to explore the sun and answer key questions about its corona and the solar wind. The Parker Solar Probe is also referred to as the &quot;Solar Probe Plus&quot;. Use the &quot;Parker Solar Probe&quot; tag instead of &quot;Solar Probe&quot; since that is now the official NASA designation." OwnerUserId="22352" LastEditorUserId="22352" LastEditDate="2018-01-07T21:44:46.510" LastActivityDate="2018-01-07T21:44:46.510" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24335" PostTypeId="1" AcceptedAnswerId="24349" CreationDate="2018-01-08T00:30:41.420" Score="4" ViewCount="553" Body="&lt;p&gt;Permanent magnet as passive stabilization and magnetorquer with solid core as active stabilization has been used over the year in satellites.But the would have magnetic field lines, wouldn't they affect the reading of magnetometer. The magnetometer is used for taking the reading of earth magnetic field.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is possibility of switching off the magnetorquer with air core while taking reading of magnetometer, but what about the other two case of actuator one with permanent magnets and second with the solid core (Ferrite) magnetorquer ?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is placement of magnetometer important? If yes, where should one put the magnetometer ideally?&#xA;Can one take reading of Permanent magnet and solid core, and subtract it from overall reading of magnetometer? but how reliable would be that, as the earth magnetic field magnitude would be quite less than that of permanent magnet.&lt;/p&gt;&#xA;" OwnerUserId="21399" LastActivityDate="2018-01-14T18:45:23.230" Title="Effect of permanent magnet and solid core magnetorquer on magnetometer reading" Tags="|magnetic-field|attitude-determination-and-control|magnetorquer|sensors|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24336" PostTypeId="1" CreationDate="2018-01-08T01:20:08.200" Score="3" ViewCount="285" Body="&lt;p&gt;I believe that the SGP4 model for orbit estimation for LEO satellites uses TLE data as initial condition. Some CubeSats chose to use up-link TLE data than installing GPS. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is for the case when just after launch and detumbling, there is no communication has been made between the CubeSat and Ground station and the satellite has no TLE data, then how the ADCS would work without position of satellite. As there is no GPS and SGP4 would not work until someone feed it TLE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If somehow the communication is possible then by up-link the TLE the SGP4 would run, I understand but for having that communication the satellite has to be in proper orientation and for that ADCS (sun model and IGRF model) need position in orbit. Sort of chicken-egg problem.&lt;/p&gt;&#xA;" OwnerUserId="21399" LastActivityDate="2018-01-09T20:25:07.513" Title="Initial condition for SGP4 in satellites without GPS, before the uplink of TLE data" Tags="|orbital-mechanics|cubesat|orbital-elements|gps|attitude-determination-and-control|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24337" PostTypeId="2" ParentId="24327" CreationDate="2018-01-08T01:36:57.230" Score="6" Body="&lt;p&gt;This is an interesting question! There are (at least) two communications issues to consider. One is signal strength as the question asks about, the other is latency or delay. But the &lt;strong&gt;tl;dr&lt;/strong&gt; is that this won't work using standard WiFi or cellular data equipment because the antenna would have to be kilometer sized, and the delays are incompatible with the protocols.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A &lt;a href=&quot;https://en.wikipedia.org/wiki/Link_budget&quot; rel=&quot;nofollow noreferrer&quot;&gt;link budget&lt;/a&gt; calculation (which you can see &lt;a href=&quot;https://space.stackexchange.com/a/24343/12102&quot;&gt;in this answer&lt;/a&gt;) would show that a smaller ground-based antenna would work, but you would still have to modify the protocol to deal with low data rates and long latency, and that means specialized hardware in your satellite and ground station rather than COTS WiFi or cellular.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Signal Strength&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;A very quick way to &lt;em&gt;roughly estimate&lt;/em&gt; the size of the collecting area needed would be to take simple ratios. Let's choose 2 GHz as an approximate working frequency. The corresponding wavelength from $\lambda = c/f$ is 3E+08 m s^-1 / 2E+09 s^-1 or 15 centimeters. A WiFi dipole antenna can be assumed to have an effective receiving area of roughly 1 wavelength squared, or 0.02 m^2, and a cell phone tower antenna about 1 x 10 wavelengths or 0.2 m^2.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's say for rough, round numbers that normal WiFi range in free space (no obstructions) is 100 meters, and useful, high speed cellular &lt;em&gt;data&lt;/em&gt; range is 3 km. Cell phones may be able to receive data out to 30 km, but for &lt;em&gt;transmitting&lt;/em&gt; high speed data the range will be less. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your distance is 3,000 km. Since signal drops as $1/r^2$ those require antenna areas that are increased by factors of 10^9 and 10^6 respectively. That means ground antennas with an area of 2E+07 and 2E+05 m^2, or sizes of 4 km and 400 meters, respectively. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These are huge! In other words, if you keep everything about the equipment the same, and try to tackle the large distance by simply scaling the Earth-based antenna size, it's not going to work. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Square_Kilometre_Array&quot; rel=&quot;nofollow noreferrer&quot;&gt;Square Kilometer Array&lt;/a&gt; is an example of a project where the total area of all the antennas together is roughly 1km x 1km, and that's a world-class project.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The largest dish used in the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Network&lt;/a&gt; system is only 70 meters in diameter, and yet it can communicate to distances of billions of kilometers. Why? The data rates at these distances are far, far slower than the minimum data rates for standard, commercial WiFi or cellular data. They use special protocols to beat the weak signals and large noise at these distances. In fact, you can trace the development of some commercial wireless data protocols &lt;em&gt;directly from deep space communications technology&lt;/em&gt; by people who worked at &lt;a href=&quot;https://en.wikipedia.org/wiki/CSIRO&quot; rel=&quot;nofollow noreferrer&quot;&gt;CSIRO&lt;/a&gt;. See [WiFi History].&lt;a href=&quot;https://en.wikipedia.org/wiki/Wi-Fi#History&quot; rel=&quot;nofollow noreferrer&quot;&gt;6&lt;/a&gt; They also use shorter wavelengths (improving gain) and cryogenic (cooled) receiver front ends to go deeper into the noise.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Latency&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;The way commercial cell phone voice and data channels and WiFi router communications work normally, they need to establish two way communications first before they start sending data. At large distances, the time delay due to the finite speed of light will confuse commercial WiFi and cellular data communications. For example, &lt;a href=&quot;https://en.wikipedia.org/wiki/Cell_site#Range&quot; rel=&quot;nofollow noreferrer&quot;&gt;GSM has a hard cut off at 35 km&lt;/a&gt; due to the maximum time-shifting allowed in the protocol. At 3,000 km the delay is &lt;em&gt;TEN milliseconds&lt;/em&gt;, and for high-speed data, that's forever!&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-08T05:27:27.200" LastActivityDate="2018-01-08T05:27:27.200" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24338" PostTypeId="1" AcceptedAnswerId="24343" CreationDate="2018-01-08T01:43:30.093" Score="17" ViewCount="10169" Body="&lt;p&gt;I've read the following passage in the answer to the Quora question &lt;a href=&quot;https://www.quora.com/How-can-Voyager-send-a-signal-strong-enough-for-us-to-receive-in-spite-of-its-enormous-distance-from-us-And-how-can-it-have-the-power-to-do-so-more-than-20-years-after-its-launch&quot; rel=&quot;noreferrer&quot;&gt;How can Voyager send a signal strong enough for us to receive, in spite of its enormous distance from us? And how can it have the power to do so more than 20 years after its launch?&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;For example: &lt;/p&gt;&#xA;  &#xA;  &lt;ul&gt;&#xA;  &lt;li&gt;&lt;p&gt;TODAY, when Voyager (I) sends a signal at 22W (13.42dBW) power:&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;We can receive the signal power as 7.22e-19Watt (-181.4dBW). &lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Very weak signal because path losses are too big (316.77 dB).&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Voyager I’s data bit rate was 21.6 kbps at the beginning, now it is decreased to 160 bit per second (so slow)&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;li&gt;&lt;p&gt;Received energy per bit is 4.5e-21 Joule (-203.4 dBJ)&lt;/p&gt;&lt;/li&gt;&#xA;  &lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Is there a simple way to understand mathematically the relationship between these numbers? I understand that these were reported values by the Deep Space Network system, but is there a way ton at least estimate the resulting low received power and data rates from known information like Voyager's power and the receiving station's capabilities?&lt;/p&gt;&#xA;" OwnerUserId="22365" LastEditorUserId="12102" LastEditDate="2018-01-08T02:13:07.593" LastActivityDate="2018-12-12T18:13:01.380" Title="How to calculate data rate of Voyager 1?" Tags="|voyager|deep-space|radio-communication|data-transmission|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24339" PostTypeId="2" ParentId="24299" CreationDate="2018-01-08T01:44:06.463" Score="3" Body="&lt;p&gt;They are part of the TEL that is used to hold the fairing for extra stability while in transit.  You can see the almost identical fittings on the LC-40 TEL from the Zuma mission, Jan 7, 2017.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/mMafo.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/mMafo.jpg&quot; alt=&quot;Fairing supports on Zuma mission F9&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see below it there is a set of arms or maybe a collar that is holding the top of the stage as well.  These open before launch, but also help hold the stage to the TEL as it rolls from the HIF to the launch pad. The ones higher up are just for additional support of the fairing.&lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2018-01-08T01:44:06.463" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24340" PostTypeId="2" ParentId="3868" CreationDate="2018-01-08T02:51:06.557" Score="1" Body="&lt;p&gt;The limitation of 10 cm or so here only applies for a single mirror. I don't know if it is possible but if you can fly two spy satellites in formation and combine the images using optical interferometry and given that the images from Earth are very bright- maybe, just maybe, they could improve on that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, have to say, I don't know, rather than that it's totally impossible. This is not my own idea though obvious once you think of it, I got it from the conversation on Snopes &lt;a href=&quot;http://msgboard.snopes.com/cgi-bin/ultimatebb.cgi?ubb=print_topic;f=39;t=000991&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="3038" LastEditorUserId="3038" LastEditDate="2018-01-08T03:05:00.487" LastActivityDate="2018-01-08T03:05:00.487" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24342" PostTypeId="2" ParentId="24326" CreationDate="2018-01-08T04:08:40.627" Score="3" Body="&lt;p&gt;There is a lot of carbon in the solar system without biological origin. Carbonaceous chondrites make up about 5% of meteorite falls. Astronomers have matched the spectral signature of asteroids to c chondrites. There are several categories but they look like coal. &#xA;These contain plant nutrients and plants have been grown in them. &#xA;&lt;a href=&quot;https://www.newscientist.com/article/mg22430004-900-asteroid-soil-could-fertilise-farms-in-space/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.newscientist.com/article/mg22430004-900-asteroid-soil-could-fertilise-farms-in-space/&lt;/a&gt;&#xA;So if this kind of thing exists on the moon, it would be great for colonists. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2018-01-08T04:08:40.627" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24343" PostTypeId="2" ParentId="24338" CreationDate="2018-01-08T05:16:27.757" Score="19" Body="&lt;p&gt;OK let's first understand units. The decibel (dB) is a base-10 log scale without units and dBm is a similar decibel scale &lt;em&gt;for power&lt;/em&gt; referenced to 1 milliwatt. They also include a factor of 10, so for example 10 dB is a ratio of 10^1, 20 dB is a ratio of 10^2, etc, while 10 and 20 dBm would be 10 mW and 100 mW.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But in the block quote, they use dBW instead of mW, so $log_{10}(22)$ = 1.342 and it's shown as 13.42 dbW. While dBm is more common, let's stick with Watts here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The standard way to calculate the received power on Earth is to use a &lt;a href=&quot;https://en.wikipedia.org/wiki/Link_budget&quot; rel=&quot;noreferrer&quot;&gt;link budget&lt;/a&gt; calculation. This is one way to calculate the received power in a standardized format so Engineers can understand each part of the link separately, and to share the information with each other. Since the calculation is a series of multiplications and division, when you use dB, these become addition and subtraction of logarithms. I'm going to leave out the smaller corrections from the big equation &lt;a href=&quot;https://en.wikipedia.org/wiki/Link_budget#Equation&quot; rel=&quot;noreferrer&quot;&gt;shown here&lt;/a&gt; since this is an approximate calculation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P_{RX} = P_{TX} + G_{TX} - L_{FS} + G_{RX} $$&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;$P_{RX}$: received power on Earth&lt;/li&gt;&#xA;&lt;li&gt;$P_{TX}$: transmitted power by Voyager&lt;/li&gt;&#xA;&lt;li&gt;$G_{TX}$: Gain of Voyagers transmitting antenna (compared to isotropic)&lt;/li&gt;&#xA;&lt;li&gt;$L_{FS}$: Free space Loss, what we usually call $1/r^2$&lt;/li&gt;&#xA;&lt;li&gt;$G_{RX}$: Gain of Earth's receiving antenna (compared to isotropic)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;We know that $P_{TX}$ is 13.4 dBW already, and on page 17 of the DESCANSO Design and Performance Summary Series &lt;a href=&quot;https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf&quot; rel=&quot;noreferrer&quot;&gt;Article 4: Voyager Telecommunications&lt;/a&gt; we can see that Voyager's high gain (X-band, around 8.4 GHz) antenna has a $G_{TX}$ gain of 48 dBi, where the &quot;i&quot; means relative to a theoretical isotropic radiator.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The gain of the receiving dish antenna $G_{RX}$ can be caluladed (from here) as&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$G_{Dish} \sim \left( \frac{\pi d}{\lambda} \right)^2 e_A$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;where $d$ is the diameter of the dish, $\lambda$ is the wavelength, which is the speed of light of 3E+08 m/s divided by the frequency of 8.4E+09 Hz or about 0.036 meter (3.6 centimeters), and $e_A$ is some aperture efficiency term between 0 and 1 for a realistic dish, which we'll set to 1 to make things simple. For the &lt;a href=&quot;https://en.wikipedia.org/wiki/NASA_Deep_Space_Network&quot; rel=&quot;noreferrer&quot;&gt;Deep Space Network&lt;/a&gt;'s largest diameter dish antenna of 70 meters, this becomes about 1.9E+07 which after applying $10 \times \log_{10}$ becomes about 73 dB.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Free Space path loss is calculated by calculating the fraction of an expanding spherical wave (from an isotropic radiator) that would be received by an area &lt;em&gt;similar to&lt;/em&gt; one square wavelength. The exact equation in dB is:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$L_{FS} = 20 \times \log_{10}\left( 4 \pi \frac{R}{\lambda} \right).$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason the fraction flipped, but a minus sign did not appear outside is because by convention, loss is expressed in positive dB, and then &lt;em&gt;subtracted&lt;/em&gt; by the minus sign in the &quot;master equation&quot;. Currently Voyager 1 is about 2.1E+13 meters (yes, 21 &lt;em&gt;billion&lt;/em&gt; kilometers!) away, so $L_{FS}$ is about 7.3E+16 or 317 dB.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$ P_{RX} \ dBW = 13.4 \ dBW  + 48 \ dB - 317 \ dB + 73 \ dB =  -182.6 \ dBW$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;which is darn close to the -181.4 dBW shown in the question!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;When receiving the signal, the limit to the data rate is the ratio of received signal power to the total noise power (received plus system). We calculate both for a fixed range in frequency, which should be roughly the bandwidth that Voyager is using.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For an effective receiver temperature of say 20 Kelvin, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Noise-equivalent_power&quot; rel=&quot;noreferrer&quot;&gt;noise equivalent power&lt;/a&gt; will be about $k_B T \times \Delta f$ where $k_B$ is the &lt;a href=&quot;https://en.wikipedia.org/wiki/Boltzmann_constant&quot; rel=&quot;noreferrer&quot;&gt;Boltzmann constant&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll do some handwaving${}^†$ here and just estimate the bandwidth used by Voyager's &lt;s&gt;spread-spectrum&lt;/s&gt; transmission to be about 1 kHz, a few times larger than the quoted bit rate of 160 bits/second would require. &lt;s&gt;That makes the noise effective power about 1.3E-20 Watts or -199 dBW, and that gives a signal to noise ratio (S/N) of -182.6 dBW minus -199 dBW of 16.4 dB, which is more than ample for good reception!&lt;/s&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;update:&lt;/strong&gt; Thanks to @TomSpilker's &lt;a href=&quot;https://space.stackexchange.com/questions/24338/how-to-calculate-data-rate-of-voyager-1/24343?noredirect=1#comment80413_24343&quot;&gt;careful review&lt;/a&gt;: That makes the noise effective power about 2.7E-19 or -182.6 dBW minus -185.6 dBW = 3 dB, which is sufficient when used with some combination of redundancy and error correction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;†&lt;strong&gt;edit:&lt;/strong&gt; @Hobbes' &lt;a href=&quot;https://space.stackexchange.com/questions/24338/how-to-calculate-data-rate-of-voyager-1/24343#comment75613_24343&quot;&gt;comment&lt;/a&gt; points out that I don't really know if Voyager uses spread-spectrum for data communications or not, since I've recently asked &lt;a href=&quot;https://space.stackexchange.com/q/25780/12102&quot;&gt;Have deep-space spacecraft always used some form of spread-spectrum for data downlink?&lt;/a&gt;. I had assumed that it would have been used to improve the S/N ratio, but that was a groundless assumption. &lt;em&gt;Stay tuned&lt;/em&gt; for more updates!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CDDGJ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CDDGJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-05-06T06:33:33.623" LastActivityDate="2018-05-06T06:33:33.623" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="24345" PostTypeId="2" ParentId="1140" CreationDate="2018-01-08T13:34:54.513" Score="0" Body="&lt;p&gt;To answer the second part of your question, the highest resolution camera proposed in Mars orbit would be one built off of one of the &lt;a href=&quot;https://en.wikipedia.org/wiki/2012_National_Reconnaissance_Office_space_telescope_donation_to_NASA&quot; rel=&quot;nofollow noreferrer&quot;&gt;two NRO donated spy satellite&lt;/a&gt; frames. As of yet, it has not been determined what the second of these will be used for. The Mars proposal, called &lt;a href=&quot;https://www.space.com/21064-nasa-donated-spy-telescope-mars.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;MOST&lt;/a&gt;, would allow for a camera with higher spatial resolution then the HiRISE camera, and the proposal was put forward by the same Principal Investigator as HiRISE, Alfred McEwen.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The truth is, however, that we very rarely need higher resolution photos then HiRISE can provide, and in fact it is quite common to bin the HiRISE images to effectively lower it's resolution, which allows it to take more photographs in slightly lower resolution. The highest resolution images are saved primarily for mission support tasks. Data is really the limiting factor, not the resolution. The original 2 year primary mission of MRO called for imaging 1% of Mars's surface with HiRISE, I'm sure the number now is closer to 10%, but I don't have that number handy.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-08T13:34:54.513" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24347" PostTypeId="1" AcceptedAnswerId="24348" CreationDate="2018-01-08T14:08:23.730" Score="17" ViewCount="2607" Body="&lt;p&gt;Assuming both launches are successful, if JWST tries to solve Tesla's cherry red body paint light spectrum, what would it tell about it's composition? Could it be confusing if it was an unknown object?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: If it was one 10km radius object on same heliocentric orbit, how could JWST tell it's made out of composite material or concrete, or wood, or ice, since it has some thin misleading coating or paint?&lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="25" LastEditDate="2018-01-09T15:02:11.100" LastActivityDate="2018-01-09T15:02:11.100" Title="JWST observing cherry red Tesla roadster" Tags="|spacex|materials|space-telescope|james-webb-telescope|tesla-roadster|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24348" PostTypeId="2" ParentId="24347" CreationDate="2018-01-08T14:24:31.213" Score="19" Body="&lt;p&gt;First of all, can the color even be seen? James Webb has a spectrum of 600 nm at the lowest end, which means it can just barely see the color red. In addition, it could potentially be seen in other wavelengths that aren't visible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spatial resolution is around 70 milli-arc seconds. That means that the Roadster, being about 4m in size (roughly) in it's longest dimension could be resolved at about 11,800 km. That would require an exceedingly close pass! Of course, it can be seen even if not resolved, but it will still be quite dark.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically James Webb would treat a Tesla Roadster similar to a 4m asteroid. From &lt;a href=&quot;https://arxiv.org/abs/1510.08414&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;, it seems like even the much larger 275m sized asteroids would be difficult to see.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, it might show up as an interesting speck, but is unlikely to actually be observed unless intentionally attempted at a particularly close approach.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-09T10:46:30.077" LastActivityDate="2018-01-09T10:46:30.077" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24349" PostTypeId="2" ParentId="24335" CreationDate="2018-01-08T14:40:55.423" Score="2" Body="&lt;p&gt;This is an excellent question, and in fact one that plagues many small satellite designs! There are a number of means that are typically done to adjust for the effect of a magnetorquer/ magnetometer reading issues.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Don't use the magnetometer while the magnetorquer is being used. If you don't require a 100% duty cycle of the magnetorquers, this can be the simplest solution. &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://github.com/kriswiner/MPU6050/wiki/Simple-and-Effective-Magnetometer-Calibration&quot; rel=&quot;nofollow noreferrer&quot;&gt;Bias out the magnetorquer&lt;/a&gt; by measuring it's effect on the ground, and using math to remove the effect. This isn't as perfect, but is useful in particular with permanent magnets.&lt;/li&gt;&#xA;&lt;li&gt;Put the &lt;a href=&quot;https://en.wikipedia.org/wiki/Spacecraft_magnetometer&quot; rel=&quot;nofollow noreferrer&quot;&gt;magnetometer&lt;/a&gt; as far away from the main body of the spacecraft as possible. This is often done by sticking it on the end of a boom, sometimes shared by the antenna, other times as a stand alone boon. This is usually used in conjunction with the other items.&lt;/li&gt;&#xA;&lt;li&gt;Use more than one, and figure out the correct reading.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://books.google.com/books?id=t8gcDQAAQBAJ&amp;amp;pg=PA2&amp;amp;lpg=PA2&amp;amp;dq=magnetometer%20faraday%20cage&amp;amp;source=bl&amp;amp;ots=bJ025q7eJy&amp;amp;sig=6zktLntodAyV1AClgL5h0x0uIuw&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=2ahUKEwitjtOnodLYAhWFnOAKHUhPBG0Q6AEwGXoECAwQAQ#v=onepage&amp;amp;q=magnetometer%20faraday%20cage&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Place the magnetometer outside of the Faraday cage of the satellite&lt;/a&gt;. This will prevent the inner electronics from affecting things. This won't protect against permanent magnets, as blocking magnetism is really hard, but it can protect against some spurious readings created by electric fields, to a degree.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;In practice, a combination of these is typically done. The best bet is to place it outside of the spacecraft as far away from any magnetic materials as you can do, and test it in a number of orientations on the ground to try and determine the truth from whatever it is reading.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-14T18:45:23.230" LastActivityDate="2018-01-14T18:45:23.230" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24350" PostTypeId="2" ParentId="21207" CreationDate="2018-01-08T15:55:18.210" Score="12" Body="&lt;p&gt;Yes, it could be. The ISP for &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910014990.pdf&quot; rel=&quot;noreferrer&quot;&gt;CO/ O2 is about 200&lt;/a&gt;. Compare that to Methane, with a specific impulse of 299, and you can see it's really not that great.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of some related interest is a hot Carbon Dioxide rocket, with a theoretical ISP of about 260. This would work for short surface to surface hops, but not beyond orbit. &lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-08T15:55:18.210" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24351" PostTypeId="2" ParentId="19681" CreationDate="2018-01-08T16:04:45.053" Score="3" Body="&lt;p&gt;According to &quot;A Case for Mars&quot;, adequate protection can be achieved by simply filling sand bags and placing them over the shelter. Thus only a few inches (7-10 cm) of Mars soil would be required to provide adequate protection. A more accurate guess can be made with &lt;a href=&quot;https://eps.utk.edu/faculty/taylor/Miller-radiation%20measurement.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;lunar soil&lt;/a&gt;, which is similar enough to Mars to give at least a first order guess. The lunar soil required to provide adequate radiation protection is 46 cm (18 inches). Mars would likely require less due to the atmosphere, which will protect it somewhat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A bit more detail about the proposal from &quot;A Case for Mars&quot;. The assumed Cosmic Ray radiation received is about 6 rem/year. The unsheltered radiation is assumed to be 9 rem/year. It seems like thicker shielding would be required for a truly long duration settlement, but these values are considerably below what the radiation exposure is in deep space, which Mars soil won't protect against.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And as Mark pointed out, to achieve the full protection of the Earth's atmosphere, one would need 14.7 psi/ Mars Soil Density, or about 12 feet (3.3 m) (roughly) of packed soil. Of course, one could get away with much less, say, 30% (Same as an airplane). That would give a thickness of about 4 feet( 1.2m)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I would go for 46 cm for &quot;good&quot; protection, and about 10 cm for &quot;adequate&quot; protection.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-08T18:45:00.843" LastActivityDate="2018-01-08T18:45:00.843" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="24352" PostTypeId="2" ParentId="1536" CreationDate="2018-01-08T16:05:21.317" Score="3" Body="&lt;p&gt;No, gradual burning is not delta-v efficient. Basically if you want to change your apopsis it's most efficient to do it by burning at periapsis and vice-versa.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If there was no air and we had infinite thrust then the most efficient launch would have two burns, both horizontal. One on the ground to set the apopsis and then one at apopsis to circularise. Basically a hohmman transfer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However we do air and we do we have limited thrust. So a real launch profile looks more like.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Vertical ascent, to get out of the worst of the atmosphere quickly while reducing losses to atmospheric drag.&lt;/li&gt;&#xA;&lt;li&gt;A gradual transition to horizontal burning to efficiently raise the apopsis. If we had infinite thrust this could be purely horizontal but since we have finite thurst we need a vertical component to avoid falling back to earth while we build up horizontal velocity.&lt;/li&gt;&#xA;&lt;li&gt;A coast phase until we reach the desired orbit height.&lt;/li&gt;&#xA;&lt;li&gt;A burn to insert into orbit.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There may also be additional burns to fine-tune the orbit or to control where along the orbit the sattelite ends up. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In many cases (including nearly all GEO launches) the final burn is performed by an engine on the sattelite itself rather than the launch vehicle. It's a much smaller engine and may come hours after the launch, so you are unlikely to notice it.&lt;/p&gt;&#xA;" OwnerUserId="12039" LastActivityDate="2018-01-08T16:05:21.317" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24353" PostTypeId="1" CreationDate="2018-01-08T16:46:35.097" Score="3" ViewCount="212" Body="&lt;p&gt;Elon Musk has a plan for satellite internet that involves LEO and VLEO satellites. The question I have is how long would a satellite at these altitudes actually be within range before having to hand off to another? And how many satellites would it take at these altitudes to provide 24x7 coverage of a specific area?&lt;/p&gt;&#xA;" OwnerDisplayName="kevin freels" LastEditorUserId="7062" LastEditDate="2023-12-25T07:53:09.877" LastActivityDate="2024-01-25T22:02:35.323" Title="For how long is a satellite orbiting at 300 miles going to be visible from horizon to horizon?" Tags="|communication|internet|artificial-satellite|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="24354" PostTypeId="1" AcceptedAnswerId="24357" CreationDate="2018-01-08T20:53:32.980" Score="19" ViewCount="8011" Body="&lt;p&gt;The BBC article &lt;a href=&quot;http://www.bbc.com/future/story/20180105-the-astronaut-fighting-to-save-our-home-in-space&quot; rel=&quot;noreferrer&quot;&gt;The Astronaut Fighting to Save our Home in Space&lt;/a&gt; discusses “experienced astronaut, and astrophysicist Dr Michael Foale”'s exciting career aboard the Space Shuttle, Soyuz, Mir, and ISS, and now current efforts to find future backers for the ISS after its current mission (and funding) might come to a close.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;But the station’s days are numbered. Funding by the various space agencies involved is only agreed until 2024. This means in just six years’ time, the most expensive structure ever built will be pushed out of orbit by a Progress spacecraft to disintegrate over the Pacific.&lt;/p&gt;&#xA;&lt;p&gt;And the countdown clock is ticking.&lt;/p&gt;&#xA;&lt;p&gt;“&lt;strong&gt;Year by year, Russia is launching the fuel to fill up the tanks of the ISS service module to enable the space station to be deorbited&lt;/strong&gt;,” says Foale. “That’s the current plan – I think it’s a bad plan, a massive waste of a fantastic resource.” (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Since the ISS' large cross-sectional area including the solar panels already leads to continuous loss of altitude requiring regular &lt;em&gt;orbit-raising burns&lt;/em&gt;, why would there be such a requirement for so much fuel for de-orbit? Has there a specific plan been developed to de-orbit (if decided) in such a way that it comes down over the Pacific? How much fuel is thought to be needed to do this?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-11T00:32:04.760" Title="Why would extra fuel be needed to de-orbit the ISS (if it comes to that)?" Tags="|iss|deorbit|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24355" PostTypeId="1" CreationDate="2018-01-08T20:55:09.223" Score="3" ViewCount="185" Body="&lt;p&gt;In orbit propellant depots could be valuable in the near future. Assume they would be used for storing fuel harvested in situ from the moon and NEOs (So primarily H2, O2 and maybe CH4). Assume in orbit assembly techniques on par with what was available for the ISS. Is there any way to design fuel tanks such that they are modular/collapsible to allow the tanks to be transported to orbit and assembled there? The goal is to have tanks with much higher total capacity then could be held within assembled tanks on the same launcher.&lt;/p&gt;&#xA;" OwnerUserId="21390" LastEditorUserId="6816" LastEditDate="2018-01-08T23:38:33.187" LastActivityDate="2023-12-24T02:01:15.160" Title="Is it possible to build a collapsible fuel tank for in-orbit assembly?" Tags="|orbital-assembly|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24356" PostTypeId="1" AcceptedAnswerId="24358" CreationDate="2018-01-08T21:02:07.747" Score="4" ViewCount="741" Body="&lt;p&gt;Things in space are not always what they seem. The question &lt;a href=&quot;https://space.stackexchange.com/q/18529/12102&quot;&gt;Why does there appear to be a 180-degree stereo microphone array outside the ISS?&lt;/a&gt; makes that clear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is this object shown to the right of astronaut Michael Foale in the BBC article &lt;a href=&quot;http://www.bbc.com/future/story/20180105-the-astronaut-fighting-to-save-our-home-in-space&quot; rel=&quot;noreferrer&quot;&gt;The Astronaut Fighting to Save our Home in Space&lt;/a&gt;?&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Michael Foale has spent more than a year in space on various missions (Credit: Nasa)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/tMj4m.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/tMj4m.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastActivityDate="2018-01-08T23:56:08.787" Title="What is this object outside the ISS that looks like a loudspeaker?" Tags="|iss|identify-this-object|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24357" PostTypeId="2" ParentId="24354" CreationDate="2018-01-08T21:20:36.857" Score="45" Body="&lt;p&gt;The extent of Earth's atmosphere and thus the amount of drag on the station varies greatly with solar weather; the relationship between drag and time-to-deorbit is very nonlinear. Combined, this means that reentry time is not very predictable on the scale of hours, which means the ISS could come down anywhere on its ground track and we wouldn't know where it was headed until a short time before it hit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By doing a substantial deorbit burn at a precise time, the reentry can be controlled much more closely, and the ISS can land in the middle of an ocean instead of in my back yard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to the &lt;a href=&quot;https://www.nasa.gov/pdf/578543main_asap_eol_plan_2010_101020.pdf&quot; rel=&quot;noreferrer&quot;&gt;2010 NASA deorbit plan&lt;/a&gt;, a Progress spacecraft would be modified to be able to thrust while taking propellant directly from the station's tanks (the ATV version of the plan is obviously a non-starter now). Apparently the &lt;a href=&quot;https://en.wikipedia.org/wiki/Zvezda_(ISS_module)&quot; rel=&quot;noreferrer&quot;&gt;Zvezda&lt;/a&gt; module's thrusters, while more powerful, can't burn continuously for the durations envisioned here. A Progress would be modified with sustained-fire engines and used for deorbit burns. &lt;a href=&quot;https://www.nasa.gov/pdf/167129main_Systems.pdf&quot; rel=&quot;noreferrer&quot;&gt;Zvezda can store up to 860kg of propellant and the FGB/Zarya module 5760kg&lt;/a&gt;. Using the rocket equation, we can calculate that this comes to something like 50 m/s of ∆v applied to the station. This is enough to drop from a fairly stable circular 270km orbit to something like a 270km apogee x 100km perigee, which will produce a prompt and predictable reentry. This version of the plan has a map suggesting an Indian Ocean disposal rather than Pacific; this gives the longest available safe ground track (about half of Earth's circumference!), passing South of Australia and NZ before coming back north to the West Coast of North America.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The plan seems to have been &lt;a href=&quot;https://www.nasaspaceflight.com/2012/08/asap-discuss-deorbit-iss-two-progress-ships/&quot; rel=&quot;noreferrer&quot;&gt;updated in 2012 to make use of 2 Progress spacecraft&lt;/a&gt;, but I can't find more detail on that version of the plan.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-01-11T00:32:04.760" LastActivityDate="2018-01-11T00:32:04.760" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24358" PostTypeId="2" ParentId="24356" CreationDate="2018-01-08T23:17:44.883" Score="8" Body="&lt;p&gt;It's a light, or a luminaire if you're from Quebec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The lights are part of the ETVCG (&lt;strong&gt;E&lt;/strong&gt;xternal &lt;strong&gt;T&lt;/strong&gt;ele&lt;strong&gt;v&lt;/strong&gt;ision &lt;strong&gt;C&lt;/strong&gt;amera &lt;strong&gt;G&lt;/strong&gt;roup). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Sb9Dv.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Sb9Dv.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The picture is from a presentation on the new high-def cameras, which can be found &lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/V_Studer-External_High_Definition_Camera.pdf&quot; rel=&quot;noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The light pans and tilts with the cameras, as seen in this picture from the same presentation.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/qxaho.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qxaho.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see an ETVCG at the right of this picture from the STS-127 mission. It's watching the on-going robotic ops, probably the clearance between the shuttle arm and the Japanese module.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/AKsPR.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/AKsPR.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And here's a cropped version of that. You can see that the light appears to be on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1FHYK.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1FHYK.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-01-08T23:56:08.787" LastActivityDate="2018-01-08T23:56:08.787" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24359" PostTypeId="1" AcceptedAnswerId="24361" CreationDate="2018-01-09T01:18:03.837" Score="8" ViewCount="1371" Body="&lt;p&gt;History provides enough examples of failed missions due to software failure.&#xA;If the mission system is still physically intact (ISS best example but very close to Earth making the examole maybe less challengung), do they provide &quot;update and reboot&quot; nowadays? How is this done, are there regular updates as well?&#xA;To narrow the scope of the question: what are examples more on that (specs), if they exist? Doubt there is an &quot;apt get&quot; our there?&lt;/p&gt;&#xA;" OwnerUserId="19946" LastEditorUserId="19946" LastEditDate="2018-01-11T12:44:30.410" LastActivityDate="2018-01-11T12:44:30.410" Title="Are software updates to satellites or in general space craft commonplace during scientific missions?" Tags="|software|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24361" PostTypeId="2" ParentId="24359" CreationDate="2018-01-09T03:32:11.370" Score="7" Body="&lt;p&gt;Nearly all, if not all, modern spacecraft are designed to accept software updates. Updates are done often to enhance capability and to work around failures of hardware.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to how its done, after &lt;em&gt;extensive&lt;/em&gt; testing on the ground either a patch or a complete new copy of the flight software is uploaded to the spacecraft. The upload is carefully verified. Then the patch is applied or the vehicle reboots to the new image. There is always a safeguard in place, where for example either the previous image or a golden image is kept on board, which is automatically rebooted to in the event of a fatal software error in the new or patched load.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2018-01-09T03:32:11.370" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24363" PostTypeId="2" ParentId="1308" CreationDate="2018-01-09T07:22:07.247" Score="3" Body="&lt;p&gt;Actually, the rotating space station in &quot;2001&quot; is a pretty accurate rendition of the size and rotation speed needed.  No center hub around which to rotate - hence no seals or bearings to go bad - and a large central docking bay.  Of course, it should be noted that the fictional space station is part of a larger transportation system, to and from the moon; so there's that.  As far as trying to use centripetal force to simulate gravity in a long-term space mission, that's a little trickier - for all the reasons already listed - but it seems to me that there's actually no reason the whole spacecraft couldn't rotate, making it, in essence, a real &quot;flying saucer&quot;.&lt;/p&gt;&#xA;" OwnerUserId="22382" LastActivityDate="2018-01-09T07:22:07.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24364" PostTypeId="1" AcceptedAnswerId="24371" CreationDate="2018-01-09T07:27:06.313" Score="5" ViewCount="537" Body="&lt;p&gt;Reading &lt;a href=&quot;https://space.stackexchange.com/questions/24354/why-would-extra-fuel-be-needed-to-de-orbit-the-iss-if-it-comes-to-that&quot;&gt;this&lt;/a&gt; question, what is the best way to make sure most of ISS (or any other large object planned to be destroyed in the atmosphere) will burn and break into relatively small pieces in the atmosphere? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, will it be a retrograde burn? Or will there be some radial composant in this burn, in order to raise apogee, and make a more brutal/steeper entry? For a given amount of de-orbit fuel, what's the best de-orbit burn profile?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: It may be completely irrelevant in comparison with reality, but:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I tested something using stock KSP. Initial conditions (using orbit cheat) is perfectly circular orbit at 86750m &#xA;Spacecraft setup is one large command pod (white apollo like) one heatshield, two sepratrons (simulating identical de-orbit delta V) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;First de-orbit burn was retrograde. resulting perigee was 52966m, ablative material used during re-entry : 153&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second de-orbit burn was 45 deg. between retrograde and radial. Resulting perigee was 60355m, ablative material used : 161&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Which would mean, for identical de-orbit delta V, burning with radial composant raises apogee immediately after burn, but leads to higher perigee too (less steep entry) More heat is produced, and landing site is less predictable. &lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="19132" LastEditDate="2018-01-12T14:55:06.903" LastActivityDate="2018-01-12T14:55:06.903" Title="For a given amount of fuel, what is the best de-orbit profile for the ISS?" Tags="|iss|reentry|deorbit|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24365" PostTypeId="1" AcceptedAnswerId="24366" CreationDate="2018-01-09T11:23:13.843" Score="66" ViewCount="25957" Body="&lt;p&gt;We know that the &lt;a href=&quot;https://space.stackexchange.com/questions/24302&quot;&gt;the payload of the maiden Falcon Heavy flight will be... Elon Musks's Tesla&lt;/a&gt; Which will be placed in &quot;Mars Orbit&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VsTIo.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VsTIo.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Assuming it is serviced and road ready when launched with the keys in the ignition...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;100 Years later, someone plucks it out of orbit and lands it safely on a planet (Earth?), would it still be road ready?  What if any service or repairs would likely be required to drive it?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Would 10,000 years or a billion years make a significant difference it is road readyness?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="21993" LastEditDate="2018-01-11T13:05:28.283" LastActivityDate="2018-02-07T21:55:30.357" Title="Can I drive Elon Musk's Tesla after it's been in space for 100 Years?" Tags="|spacex|spacecraft|falcon-heavy|nonstandard-payload|tesla-roadster|" AnswerCount="6" CommentCount="19" ContentLicense="CC BY-SA 3.0" />
  <row Id="24366" PostTypeId="2" ParentId="24365" CreationDate="2018-01-09T11:44:07.283" Score="93" Body="&lt;p&gt;Making a car run when it's been stored on Earth for 10 years can be a challenge. Storing it in space makes things worse. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;All lubricants will have evaporated. Cold welding is a possibility. The thermal environment is a variable. If the car + payload adapter tumble, the car will spend time in the shadow of the adapter, and you get thermal cycling which will eventually break up the electronics. The Roadster has a glued chassis, the glue bonds may fail at those low temperatures. &#xA;The battery will be dead. Rubber (tires, seals) will degrade (it'll do that on Earth in 100 years, let alone in space).&#xA;If the cooling system hasn't been drained prior to launch, the coolant will freeze. Depending on coolant type, the coolant may expand and rupture piping, and distort everything around the piping (batteries, power electronics).&#xA;Radiation will degrade the electronics. The car could take micrometeoroid damage too, but at least the orbit it's in won't be as dirty as LEO. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Getting the car to run again will require a full restoration: it has to be stripped to bare metal, and every component tested. Expect to replace lots of moving parts, plus all the electronics and the battery.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-09T18:14:58.890" LastActivityDate="2018-01-09T18:14:58.890" CommentCount="14" ContentLicense="CC BY-SA 3.0" />
  <row Id="24367" PostTypeId="2" ParentId="24365" CreationDate="2018-01-09T11:45:10.580" Score="13" Body="&lt;p&gt;Would you be able to just jump in the car and drive it? &lt;strong&gt;No.&lt;/strong&gt; Would you be able to get it roadworthy again with a little work? &lt;em&gt;Maybe.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The state of the car itself will come down to how well protected it is in the capsule, it's going to experience some pretty extreme fluctuations in temperature which will not be kind to electronics that aren't designed for those sorts of conditions and the radiation levels it will pass through could do nasty things to the same components unless the capsule is providing some level of protection but even assuming that it's adequately shielded from any environmental concerns that would cause it to deteriorate such as radiation, space debris etc. then you'd still face most of the same challenges as any other attempt to park a car up for ten years:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;Flat tires - while you won't really have a problem of flat spots forming on the contact patch (because the car will be in microgravity) you'll still have the problem of the air used in the inflation escaping naturally over time (no tire/wheel is 100% airtight) so you'll have to reinflate them before you go anywhere. The rubber of the tires is also likely to degrade significantly over that length of time, maybe not to the point where you couldn't move it around but I certainly wouldn't want to be doing long distances or high speeds on them.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Batteries - All batteries lose charge over time, and in 100 years the car's main drive batteries, any ancillary batteries and probably even the battery in the key will likely be either flat or in a really low state of charge. While it might have enough juice to start and run I highly doubt it and it certainly won't be going very far. Depending on the battery chemistry spending a large amount of time in the cold can actually do good things to preserve the condition of the batteries but cycling back up to hot and back down to cold won't do them any favors.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Perished rubber - while it won't have as many engine-critical seals as a car with a conventional ICE there will still be things like shock absorber seals etc that will perish over time (the large temperature variations will &lt;strong&gt;not&lt;/strong&gt; be kind to them!)&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="19967" LastActivityDate="2018-01-09T11:45:10.580" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24368" PostTypeId="2" ParentId="24365" CreationDate="2018-01-09T14:00:43.717" Score="-6" Body="&lt;p&gt;Elon Musk is a man of his word. Mea Culpa:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.faa.gov/about/office_org/headquarters_offices/ast/licenses_permits/media/LLS%2018-107%20Falcon%20Heavy%20Demo%20License%20and%20Orders%20FINAL%202018_02_02.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;FAA Launch License Including Payload&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It would not survive launch. Upon orbit insertion, it would be a jumble of loosely assembled broken parts. Things would go downhill from there. If they are saying that they will do this, it is just a publicity stunt. Or maybe not a stunt, so much as &quot;earned publicity&quot; since they haven't actually done anything yet.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In order to substantiate this assessment, I spoke with two experts in the field of spacecraft design and launch. One is a Spacecraft System Engineer with multi-mission experience (PHD, MIT), the other is an Mechanical Engineer with experience designing spacecraft instruments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both had seen the press releases and pictures and both were incredulous. They expressed concern that during the launch phase the car would not be able to withstand the mechanical forces, particularly the vibration, and would come apart, endangering the launch vehicle and causing catastrophic failure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The wheels in particular were called out as one egregiously dangerous example. In the pictures of the mounting, the wheels are neither supported nor constrained. They seem to be hanging freely by their suspension. During launch the wheels will be severely vibrated and would bounce up and down rapidly, stressing the suspension. The chances are good that the suspension will fail and the wheels would be set free to bounce around in the fairing like loose cannons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Articles on this subject note that the FAA requires that a &quot;non-traditional&quot; payload undergo a review to make sure launching the payload “does not jeopardize public health and safety, safety of property, U.S. national security or foreign policy interests, or international obligations of the United States...” including the 1967 Outer Space Treaty.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The results of such a review are made public by the FAA (e.g., &lt;a href=&quot;https://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=20595&quot; rel=&quot;nofollow noreferrer&quot;&gt;Moon Express Payload Determination&lt;/a&gt;). There is no news yet of such a review determination having been made for the Tesla payload.&lt;/p&gt;&#xA;" OwnerUserId="22352" LastEditorUserId="22352" LastEditDate="2018-02-05T17:48:31.903" LastActivityDate="2018-02-05T17:48:31.903" CommentCount="16" ContentLicense="CC BY-SA 3.0" />
  <row Id="24369" PostTypeId="2" ParentId="24365" CreationDate="2018-01-09T14:44:42.917" Score="29" Body="&lt;p&gt;Let's look at some of the biggest stressors in the Tesla-Probe's lifetime-&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Launch&lt;/strong&gt;- This will be a very stressful time. The car will be subject to around 3g for a few minutes, in a direction that it isn't accustomed to having any kind of force. Luckily the unofficial side-view of the Roadster shows that it is almost certainly mounted by the frame of the car, not the tires. It should be able to better hold up in that configuration. It does seem quite likely, however, that there was some &quot;damage&quot; to get it mounted there, and taking it off, even today on Earth, would likely significantly affect it's impact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, it seems highly likely that they did testing to ensure that the Roaster won't come apart on launch. The last thing that SpaceX would want is to have their publicity stunt turn sour as it caused the failure of the Falcon Heavy. They must have done at least basic testing, vibration, thermal, and even shock testing, to ensure it won't break apart. It is entirely possibly that there were some parts that were welded together to keep it together, such as the wheels. These would have to be undone to drive it again.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CQp6V.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CQp6V.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Thermal&lt;/strong&gt;- The thermal environment will actually be fairly benign. It will be warmer then on Earth, but not subject to the sudden changes in sunlight that, say, a LEO satellite will face. There will be some tumbling, so there will be some variation in temperature. I don't think this will be a major concern. It is quite likely, however, that the whole &quot;probe&quot; will at some point in time be subject to high temperatures, in the vicinity of 40-50 C. This could cause battery lifetime issues.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Vacuum&lt;/strong&gt;- As has been noted by others, any exposed liquid will be evaporated, and cold welding is likely. I personally suspect they will have removed any exposed liquids prior to launch, to reduce the likelihood of incident. The seats are particularly likely to have issues being in a vacuum. They will outgas, and probably not be recognizable as the seats that they are.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Time&lt;/strong&gt;- The batteries will almost certainly be completely depleted and would need to be completely replaced.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Micrometeorites/ etc&lt;/strong&gt;- Not likely to be much of an issue, but there will likely be a few dents.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Sun Exposure&lt;/strong&gt;- The color will probably be off, and again, more damage to the wheels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Finding it&lt;/strong&gt;- It will likely be very difficult to find it in 100 years. We have only found 90% of objects 1 km in size. Granted we should know a rough trajectory, but just to give you a comparable task, we don't know that well where all of the Apollo hardware is. For instance, there is the &quot;&lt;a href=&quot;https://www.space.com/13010-snoopy-nasa-lost-apollo-10-lunar-module-search.html&quot; rel=&quot;noreferrer&quot;&gt;Search for Snoopy&lt;/a&gt;&quot;, trying to find where Apollo 10's LEM is now. We will only be able to track the Roadster Probe when it passes close to Earth, which won't be that often.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I rather suspect that this is a fairly heavily modified Tesla Roadster to get it to work. Glass and liquids might have been removed, items have probably been welded together for improved stability, all of which would make it complex to drive even if it was taken off the rocket today, let alone in 100 years. But sure, if you spend the time, you could probably drive it, but it would involve a lot of work and new parts. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-09T20:44:02.757" LastActivityDate="2018-01-09T20:44:02.757" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24371" PostTypeId="2" ParentId="24364" CreationDate="2018-01-09T15:11:34.607" Score="6" Body="&lt;p&gt;There is no need to have a steeper angle, and in fact, that's probably counter-productive. As explained in &lt;a href=&quot;https://space.stackexchange.com/a/24357/12102&quot;&gt;this thorough answer&lt;/a&gt;; the quickest deorbit would be to have a roughly circular orbit that is stable for a few months (270 km was proposed), and then lower the perigee to below 100 km, the lower the better. This lowering should happen such that the station won't reach below, say, 180 km or so until it reaches the start of the safe path. This will have the lowest energy, and the most time, to allow for deorbiting the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If one makes the path steeper, it increases the likelihood of some part of it making it through the entire process. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, wait until the station will only be stable for a month or so, and then burn as hard as you can at the apogee, until you are out of fuel. That will give you the best deorbiting capability for something like the space station.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="12102" LastEditDate="2018-01-12T03:55:51.990" LastActivityDate="2018-01-12T03:55:51.990" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24373" PostTypeId="2" ParentId="20342" CreationDate="2018-01-09T17:57:19.720" Score="5" Body="&lt;p&gt;At an elemental level, Mars will contain at least to some degree all of the elements found on Earth, albeit at different proportions. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars is primarily igneous. Thus any kind of sedimentary and metamorphose minerals will not be found in any great abundance. Thus marble, slate, quartz, flint, clay, limestone, and others will be rare, if found at all. These could theoretically be manufactured, but aren't really required for most applications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give you an idea, the most important elements can be seen below, from &lt;a href=&quot;https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Earth&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;. Mars has all of these, to some extent.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/M9WoR.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M9WoR.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one thing that I can't find is Helium. Nitrogen also appears much rarer then on Earth, as does most noble gases.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-10T16:06:45.390" LastActivityDate="2018-01-10T16:06:45.390" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24374" PostTypeId="2" ParentId="1308" CreationDate="2018-01-09T17:58:20.913" Score="1" Body="&lt;p&gt;have a look at these:&lt;br&gt;&#xA;1966, first radial acceleration / centripetal force experiment by Gemini XI (11) using a tether (&lt;a href=&quot;https://www.youtube.com/watch?v=j6EWMjgRTdg&quot; rel=&quot;nofollow noreferrer&quot;&gt;youtube reference&lt;/a&gt;)&lt;br&gt;&#xA;1996, Tethered Satellite System experiment during STS-75 (&lt;a href=&quot;https://www.youtube.com/watch?v=34oXawqtVeA&quot; rel=&quot;nofollow noreferrer&quot;&gt;youtube reference&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have read the issues but are still not sure why they they did not try to improve this. Cost may be higher due to primary &amp;amp; secondary backup modules, the need to duplicate life support, docking issues, ...  However, alternative features to improve may be: (dynamic/variable) counterbalanced module, strutted modules with center section for docking, ...&lt;/p&gt;&#xA;" OwnerUserId="22393" LastActivityDate="2018-01-09T17:58:20.913" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24375" PostTypeId="2" ParentId="24336" CreationDate="2018-01-09T18:15:23.527" Score="2" Body="&lt;p&gt;This response addresses separate parts of the question in turn. &lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;TLEs are produced by ground measurements which are processed using the SGP4 model. Whilst it may well be helpful to use a previous TLE there is no absolute need. Every satellite has to have a first measurement to start the process.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;There is no special reason for a cubesat to know where it is in order for the ADCS to work. Consider that ADCS means Attitude Determination and Control Subsystem, it mentions nothing about position. For attitude control one needs attitude sensors, e.g. sun/earth/star sensors. Solar cells, in concert, can provide very helpful attitude indications. &lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2018-01-09T18:15:23.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24379" PostTypeId="1" CreationDate="2018-01-10T03:56:41.157" Score="7" ViewCount="1139" Body="&lt;p&gt;I am a &lt;a href=&quot;https://en.wikipedia.org/wiki/Civil_engineering&quot; rel=&quot;nofollow noreferrer&quot;&gt;civil engineer&lt;/a&gt; and would like to do a study on the design and construction of launching platforms for large lift vehicles, one example of which is the &lt;a href=&quot;https://en.wikipedia.org/wiki/ELA-3&quot; rel=&quot;nofollow noreferrer&quot;&gt;ELA 3&lt;/a&gt; (Ariane Launch Area 3) in &lt;a href=&quot;https://en.wikipedia.org/wiki/French_Guiana&quot; rel=&quot;nofollow noreferrer&quot;&gt;French Guyana&lt;/a&gt;, a near equatorial location particularly useful for launching heavy payloads to geostationary orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously a lot of &lt;a href=&quot;https://en.wikipedia.org/wiki/Geotechnical_engineering&quot; rel=&quot;nofollow noreferrer&quot;&gt;geotechnical engineering&lt;/a&gt; goes into the design of a launch platform to ensure that it is robust against both the rocket exhaust and vibrations during normal launches and any potential anomaly (explosions or &quot;fast fires&quot;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would like to know if there is a structural launchpad design available on the internet, drawings, dimensions, and possibly even some calculations. Based on this, and using information already widely available on rockets, I would do some modeling and study both the thermal and dynamic behaviors that the reinforced concrete is subjected to. So I need the structural design of the platform or information with the structure dimensions.&lt;/p&gt;&#xA;" OwnerUserId="22410" LastEditorUserId="12102" LastEditDate="2018-01-10T13:31:26.343" LastActivityDate="2020-05-29T19:02:10.270" Title="Launchpad platform structural design" Tags="|launch|launchpad|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24380" PostTypeId="2" ParentId="23241" CreationDate="2018-01-10T13:05:17.690" Score="2" Body="&lt;p&gt;The early ICBM Reentry Vehicles, which while not orbital at least approach orbital velocity, used a heat sink design, where the heat was stored in to the system to keep it from being used. The problem with these is that if you exceed the load slightly, they will not work, and it adds considerable mass&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of some interest is &lt;a href=&quot;http://www.dtic.mil/dtic/tr/fulltext/u2/439449.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this DoD paper&lt;/a&gt; that discusses reentry techniques for hypersonic and orbital planes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, there are 3 techniques that one can use to mitigate the heat of reentry:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Ablative Shielding- The most commonly used technique.&lt;/li&gt;&#xA;&lt;li&gt;Absorb the heat- Might add considerable mass to the spacecraft, works well for low speeds. Also might cause stress, as metals could get quite hot.&lt;/li&gt;&#xA;&lt;li&gt;Spread out the reentry heat more gradually.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;If there was going to be such a system, it would most likely be a plane design, as that could glide more and spread out it's heat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, I would challenge your assumption that it would be easier to maintain. There is a high amount of heat that happens in reentry, and that heat has to go somewhere. It could warm metals up considerably, which can cause a number of issues, which might affect the lifetime of not just an external component, but the internal frames, which are likely to be more difficult to replace. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the one orbital system well designed, the &lt;a href=&quot;https://books.google.com/books?id=dUXKgSYZPV8C&amp;amp;pg=PA143&amp;amp;lpg=PA143&amp;amp;dq=dynasoar%20hot%20structure&amp;amp;source=bl&amp;amp;ots=PjwiDL9XVX&amp;amp;sig=MlfU1yxZwWPYJ7KFEK0uz5UYjuM&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwieovyvwdXWAhVl64MKHfimB1EQ6AEIOzAG#v=onepage&amp;amp;q=dynasoar%20hot%20structure&amp;amp;f=false&quot; rel=&quot;nofollow noreferrer&quot;&gt;Dyna-soar&lt;/a&gt;, it was expected that the structure could withstand about 4 worst-case reentries. I'm not sure what the distribution of worst-case reentries is likely to be, but this is still below the SpaceX estimation of 8-10 reentries for it's heat shields.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of some note is the Space Shuttle. It is a plane, and thus can maneuver, but at the same time it has a shielding that puts it somewhere between a &quot;hot&quot; and &quot;cold&quot; reentry system. The tiles can be reused many times, but need to be examined after every flight, and some of them needed to be replaced. So it achieved partial reusability, and were easily the most reused vehicles to orbit in space as a result, although much work was required to refit each shuttle between launches.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The bottom line, if you can slow reentry, and absorb a certain amount of heat into the structure, you can reduce the maintenance cost. You might be better off still using at least some ablative shielding to further improve the lifetime rather then absorbing all of the heat.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-10T17:30:56.670" LastActivityDate="2018-01-10T17:30:56.670" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24381" PostTypeId="2" ParentId="23645" CreationDate="2018-01-10T13:26:44.153" Score="11" Body="&lt;p&gt;Launch Pads 39A and 39B were initially built for either the &lt;a href=&quot;https://en.wikipedia.org/wiki/Saturn_C-8&quot; rel=&quot;noreferrer&quot;&gt;Saturn C-8&lt;/a&gt; or the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nova_(rocket)&quot; rel=&quot;noreferrer&quot;&gt;Nova&lt;/a&gt; rockets, each of which has a lift off thrust of about 7000 tons, about twice that of the Saturn V. These were alternative designs to allow for a return directly from the Moon to Earth, but ultimately scrapped in favor of the Saturn V Lunar Rendezvous design.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note also that the Space Shuttle actually has a higher thrust on lift off, namely about 6000 tons, then the Saturn V.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect that when the pad was designed, it was designed to support 7000 tons of thrust at liftoff, with a fair bit of margin. I'm not sure that the design can fully support the BFR, but it should be close. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-10T13:51:37.323" LastActivityDate="2018-01-10T13:51:37.323" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24382" PostTypeId="1" AcceptedAnswerId="24383" CreationDate="2018-01-10T14:04:42.920" Score="70" ViewCount="34009" Body="&lt;p&gt;ISS might be destroyed &lt;em&gt;soon&lt;/em&gt;, &lt;a href=&quot;https://space.stackexchange.com/q/24354&quot;&gt;de-orbiting for a controlled reentry&lt;/a&gt; into the Pacific Ocean.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why isn't simply pushed &lt;em&gt;away&lt;/em&gt; from Earth to an outer stable orbit (not sure if it's possible) or even into an interstellar space voyage? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Maybe it has not any scientific value (specific probes are better and specifically designed for this task) but at least they will not destroy that amazing technological achievement and it may still contribute to research during its sunset (specific instruments may even be carried on board).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think it will use more propellant than a controlled re-entry.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Edit: yes, I'm thinking about all those experiments involving bacterial, fungi and degradation of materials. Also &lt;strong&gt;viruses&lt;/strong&gt;, no safety concerns because there won't be any crew.  A slowly moving ISS can run (some of) those experiments for the next 10 years sending results back to Earth. Also new experiments for a possible long-time trip to Mars...&lt;/p&gt;&#xA;" OwnerUserId="22415" LastEditorUserId="22415" LastEditDate="2018-01-10T15:59:00.623" LastActivityDate="2022-04-28T15:15:55.003" Title="Why does the ISS have to be destroyed?" Tags="|iss|deorbit|" AnswerCount="6" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24383" PostTypeId="2" ParentId="24382" CreationDate="2018-01-10T14:16:09.130" Score="75" Body="&lt;p&gt;The ISS is not designed to be run unmanned, entirely. The staff on board, when there are 6 astronauts, between exercise, sleeping, and maintenance get a single person-day of science work completed. (That is an 8 hour days' worth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ISS is at a fairly low orbit, so that Soyuz, Dragon, CST-100, Cygnus, and the Shuttle can reach it. De-orbiting it will take significantly less fuel than boosting its orbit to something that will not reenter in the long term. Even more so compared to boosting it to escape velocity from Earth. Nor is it designed to sustain the kind of thrust that would be needed to get to escape velocity using existing hardware on station, or existing technologies. (I am ignore VASIMIR and relying on existing technologies.) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why not keep it manned longer?  We have discussed this in many other questions and answers on the site before:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/8514/why-is-nasa-planning-to-deorbit-the-iss-instead-of-reusing-its-newer-modules-li&quot;&gt;Reusing the US Modules like the Russians plan&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/20197/how-do-orbital-boosts-affect-the-structural-integrity-of-the-iss&quot;&gt;Boosting the ISS and structural Integrity&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/24354/why-would-extra-fuel-be-needed-to-de-orbit-the-iss-if-it-comes-to-that&quot;&gt;How much fuel to deorbit the ISS&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/3606/how-feasible-would-it-be-to-move-the-iss-to-the-surface-of-the-moon-to-recycle-i&quot;&gt;Moving ISS to the Moon&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/22849/what-would-happen-to-the-iss-if-we-stopped-maintaining-it&quot;&gt;What would happen if we stopped maintaining the ISS?&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-01-11T21:37:16.073" LastActivityDate="2018-01-11T21:37:16.073" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="24384" PostTypeId="2" ParentId="24382" CreationDate="2018-01-10T14:30:21.167" Score="39" Body="&lt;p&gt;Let's explore the options:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Try to keep it flying: due to structural and other stress, you would need to replace parts of the ISS. You might end up with a situation like Mir where more time is spent patching the station than actually making use of it and waste a lot of money and time.&lt;/li&gt;&#xA;&lt;li&gt;Just abandon the ISS: Not a good idea, its orbit decays and the ISS would thus fall back to earth uncontrolled. Parts may hit inhabited areas. Maybe even &lt;em&gt;large&lt;/em&gt; areas.&lt;/li&gt;&#xA;&lt;li&gt;Increase its orbit a bit so it won't hit earth in the next few years: just a temporary solution that only wastes money and creates headaches.&lt;/li&gt;&#xA;&lt;li&gt;Push the ISS far out (leave earth orbit): a very expensive option, it would require a lot of fuel to push that much mass into interstellar space. AFAIK no currently available solution would be available. &lt;a href=&quot;https://space.stackexchange.com/a/24357/14549&quot;&gt;The on-board tanks can only hold enough fuel for a measly ∆v of 50m/s&lt;/a&gt;, but a &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;noreferrer&quot;&gt;∆v of about 3220m/s is required to reach earth escape velocity from LEO&lt;/a&gt;. You would need to develop and build a solution for that and fly a lot of missions.&lt;/li&gt;&#xA;&lt;li&gt;Make the ISS take a controlled dive into the atmosphere where it burns up: Since it's very costly to keep it up, it must come down. With this option, you get to control &lt;em&gt;where&lt;/em&gt; and &lt;em&gt;when&lt;/em&gt; it comes down and &quot;only&quot; requires to develop and build a modified Progress.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="14549" LastEditorUserId="14549" LastEditDate="2018-01-11T07:46:15.880" LastActivityDate="2018-01-11T07:46:15.880" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24385" PostTypeId="2" ParentId="21833" CreationDate="2018-01-10T15:06:22.167" Score="7" Body="&lt;p&gt;According to &quot;Ask an Astronaut&quot; by Tim Peake, the Soyuz is flown entirely by computer during a typical launch to the ISS, but is designed to be flown manually in close control &amp;amp; docking as a backup, which occurred on his flight because a pressure sensor in a thruster failed so the automatic approach aborted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd presume there's no manual control for the launcher though other than an abort switch.&lt;/p&gt;&#xA;" OwnerUserId="22417" LastEditorUserId="22417" LastEditDate="2018-01-10T20:47:00.160" LastActivityDate="2018-01-10T20:47:00.160" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24387" PostTypeId="2" ParentId="23252" CreationDate="2018-01-10T15:49:53.570" Score="4" Body="&lt;p&gt;There were a few things that were done, namely:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;A number of news articles.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://uwm.edu/planetarium/event/60-years-of-sputnik/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Events at planetariums&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://airandspace.si.edu/events/60th-anniversary-sputnik&quot; rel=&quot;nofollow noreferrer&quot;&gt;Events at the Smithsonian/ other museums&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I'm sure there were other events too, but I can't find references. I can't find any in Russia itself, interestingly enough.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-10T15:49:53.570" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24388" PostTypeId="2" ParentId="14750" CreationDate="2018-01-10T17:58:26.267" Score="4" Body="&lt;p&gt;With respect to an equatorial ring of propulsion engines, realistic spacecraft cannot afford redundant mass. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Every extra gram of engine mass means many more grams of propellant must be carried due to the &lt;a href=&quot;https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;tyranny of the rocket equation&lt;/a&gt;. Since NASA does not have access to Arkonide Impuls engines, NASA spacecraft have one engine, or one engine cluster aimed in a single direction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft uses its reaction control system to change its orientation and thus the direction its engine is pointed at &lt;em&gt;rather&lt;/em&gt; than the STARDUST class solution of having multiple engines pointed in all directions.&lt;/p&gt;&#xA;" OwnerUserId="22079" LastActivityDate="2018-01-10T17:58:26.267" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24389" PostTypeId="2" ParentId="24382" CreationDate="2018-01-10T18:42:16.913" Score="18" Body="&lt;p&gt;The problem with the ISS is that it cannot simply be left in its current orbit, because there is a (miniscule) amount of atmospheric drag acting on the structure to continually slow it down. Since the first module was placed in orbit, we've had to periodically boost the ISS back up to its nominal orbit (e.g. using the space shuttle's RCS engines while it was docked, or the station's main engines on the Zvezda module) to compensate for the effects of drag. There are additional forces which perturb the orbit (irregular density of the Earth, solar wind, variations in atmospheric density and composition, etc.) but drag is by far the biggest factor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If left alone, this drag force will eventually slow the ISS so much that it falls from orbit on its own, and we won't be able to control when and where that occurs. Something that massive falling from orbit poses a safety hazard, so by intentionally de-orbiting the station at a time and place of our choosing, we can control where it falls so that nobody is put at risk.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now the considerations for saving the station:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;We would have to configure the station such that it can remain intact and safe if left unmanned and unmaintained for a long time&lt;/li&gt;&#xA;&lt;li&gt;We would have to move the station into a stable orbit that is out of the way of any other operational vehicles&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The first point is actually more difficult than it first appears. The ISS is very heavily dependent upon its crew for repairs, and wouldn't be able to function for more than a few months without maintenance, so most, if not all systems would have to be shut down. You might be able to run some low-power, self-contained experiments on board, but that runs into the next problem:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The station is &lt;em&gt;huge,&lt;/em&gt; but actually very delicate. Things like radio antennas and solar arrays would have to be stowed or removed, otherwise they could exacerbate the effects of getting hit by small debris. The solar arrays in particular can also contribute to undesirable rotation of the station (due to gravitational gradient and solar wind effects) which could ultimately damage the station. Without solar arrays, there is no power, and without power, there is no attitude control, monitoring or communications, and most importantly, no &lt;em&gt;thermal control&lt;/em&gt;. The station would have to rely entirely on passive heating and cooling, which aren't the most stable things in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, the station contains a lot of volatile materials such as coolant liquids, water, air, thruster gases, etc. Most of these would have to be vented from the station beforehand, because any leakage later on could act like a small thruster, causing the station to rotate or change orbit in an undesired way. Batteries and fuel cells would have to be removed or fully discharged to avoid undesirable (and potentially explosive) chemical reactions in the uncontrolled environment aboard the station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now to the second point: getting the thing out of the way. You actually don't have to go to interstellar space to preserve the station, as some other answerers have suggested; the Moon has been in stable orbit for millions of years and will happily remain so for millions to come. The question is: how far do you have to go to avoid bothering other satellites and rockets? The answer is a &lt;a href=&quot;https://en.wikipedia.org/wiki/Graveyard_orbit&quot; rel=&quot;noreferrer&quot;&gt;graveyard orbit&lt;/a&gt;, an orbit a few hundred to a few thousand kilometers above geostationary where old satellites go to die. Objects placed in such an orbit can theoretically stay there forever.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is the bugaboo: because the ISS weighs a staggering 420 metric tons, it would require an &lt;em&gt;enormous&lt;/em&gt; amount of propellant to move that bulk from 400km to 37,000km! (Think Saturn-V levels of propellant.) Nobody wants to pay for the rocket(s) and fuel to do that, or all of the preparatory work for Bullet 1 above.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So while I and many others would agree that we should preserve the ISS as a matter of World Heritage, unfortunately the cost to do so would be, quite literally, astronomical.&lt;/p&gt;&#xA;" OwnerUserId="21989" LastEditorUserId="21989" LastEditDate="2018-01-10T23:12:11.307" LastActivityDate="2018-01-10T23:12:11.307" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24390" PostTypeId="1" AcceptedAnswerId="24400" CreationDate="2018-01-10T19:24:37.373" Score="4" ViewCount="1044" Body="&lt;p&gt;A number of comments and some answers at the question &lt;a href=&quot;https://space.stackexchange.com/questions/24365/can-i-drive-elon-musks-tesla-in-100-years&quot;&gt;Can I drive Elon Musk&amp;#39;s Tesla in 100 Years?&lt;/a&gt; suggets the Tesla Roadster &lt;a href=&quot;https://space.stackexchange.com/questions/24365/can-i-drive-elon-musks-tesla-in-100-years/24368#24368&quot;&gt;&quot;would not survive launch... it would be a jumble of loosely assembled broken parts.&quot;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But if we look at pictures of the lunar rover it appears much more fragile than the Tesla.  How or Why did the Lunar Rover survive launch while the Tesla would not? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/04rGm.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/04rGm.jpg&quot; alt=&quot;Lunar Rover&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Above: The U.S. Apollo Lunar Roving Vehicle from &lt;a href=&quot;https://en.wikipedia.org/wiki/Apollo_15&quot; rel=&quot;noreferrer&quot;&gt;Apollo 15&lt;/a&gt; on the moon in 1971. Photograph source: &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle&quot; rel=&quot;noreferrer&quot;&gt;Wikipedia on Lunar Roving Vehicle&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Below: Tesla Roadster in preparation for launch to deep space&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CQp6V.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CQp6V.jpg&quot; alt=&quot;Tesla Roadster to Space&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="6944" LastEditDate="2020-10-09T00:45:49.907" LastActivityDate="2020-10-09T00:45:49.907" Title="Reasons why the Lunar Rover DID NOT become &quot;a jumble of loosely assembled broken parts on Launch&quot;?" Tags="|launch|apollo-program|lunar-rover|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24391" PostTypeId="2" ParentId="24390" CreationDate="2018-01-10T19:26:19.223" Score="5" Body="&lt;p&gt;Because the lunar rover was well packed up. See the simulated unpacking below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/NBNhUNROV5U?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case the rover was designed from the ground up to survive launch, unlike other strange payloads.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-10T19:36:48.347" LastActivityDate="2018-01-10T19:36:48.347" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24392" PostTypeId="5" CreationDate="2018-01-10T19:38:15.570" Score="0" Body="&lt;p&gt;The &lt;a href=&quot;/questions/tagged/falcon-heavy&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;falcon-heavy&amp;#39;&quot; rel=&quot;tag&quot;&gt;falcon-heavy&lt;/a&gt; rocket (constructed by &lt;a href=&quot;/questions/tagged/spacex&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;spacex&amp;#39;&quot; rel=&quot;tag&quot;&gt;spacex&lt;/a&gt;) made her inaugural flight on Feb 6, 2018. Since this was for test purposes, the payload was Elon Musk's Midnight Cherry Tesla Roadster, with a SpaceX suit named &quot;Starman&quot; in the drivers seat.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/GZvDM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GZvDM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The successful launch placed the &lt;a href=&quot;/questions/tagged/nonstandard-payload&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;nonstandard-payload&amp;#39;&quot; rel=&quot;tag&quot;&gt;nonstandard-payload&lt;/a&gt; into orbit for a short time (the payload needed to complete 9 orbits to meet Air Force requirements for future launches) and was then boosted into a heliocentric orbit. &lt;a href=&quot;https://www.youtube.com/watch?v=3sMiNr9MEYI&quot; rel=&quot;nofollow noreferrer&quot;&gt;Video of the payload in orbit around the Earth on launch day&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="22326" LastEditDate="2018-02-07T21:27:45.927" LastActivityDate="2018-02-07T21:27:45.927" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24393" PostTypeId="4" CreationDate="2018-01-10T19:38:15.570" Score="0" Body="For questions related to Elon Musk's Midnight Cherry Tesla Roadster (and its &quot;pilot&quot;, an empty SpaceX suit named Starman), launched as the payload on the inaugural Falcon Heavy flight on February 6, 2018. May also be used in conjunction with the [nonstandard-payload] tag. " OwnerUserId="6537" LastEditorUserId="22326" LastEditDate="2018-02-07T21:27:37.680" LastActivityDate="2018-02-07T21:27:37.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24394" PostTypeId="2" ParentId="23171" CreationDate="2018-01-10T21:25:59.590" Score="4" Body="&lt;p&gt;Previous commenters have mentioned the biggest issue being the direct thermal radiation from the sun, which I agree with.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The sun, modelled as a 5800K black body, releases heat at 64MW per square meter (Stefan-Boltzmann const * T^4). Wikipedia describes the transition region as &quot;tens to hundreds of kilometers thick&quot;, so this hypothetical spacecraft is essentially so close to the sun that one side is directly exposed to 64MW/m^2$, as you can pretty much ignore circular geometry.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If we coat the sunlit side with the shiniest material feasible (optical solar reflector with a visible light absorbtance of 0.07) and paint the shaded side black for an infrared emmittance of 0.9, we get via some slightly fudged maths&#xA;   $ T^4 = \frac{solar\_intensity * absorbtance}{SB const * emmittance} $&#xA;an equilibrium temperature of 5443K, due to those tricksy 4th power terms.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You of course asked about a cryogenically cooled, ablatively shielded craft, but I'm gonna put the maths aside and say we don't have cooling technologies that could preserve something that wants to settle at well over 5000K.&lt;/p&gt;&#xA;" OwnerUserId="22417" LastActivityDate="2018-01-10T21:25:59.590" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24395" PostTypeId="1" AcceptedAnswerId="24406" CreationDate="2018-01-11T03:45:34.217" Score="19" ViewCount="858" Body="&lt;p&gt;Now, I realise this question is inherently unanswerable due to the classified nature of the mission, and speculation is also not desirable, but that's okay — because we can at least assemble official sources into something that resembles the most up to date information we have. So...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What was the outcome of the launch of ZUMA by SpaceX, to the best of the public's knowledge?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For those unaware, SpaceX conducted its 3rd secretive national security launch on January 4^th 2018, launching a payload known by the name of &lt;em&gt;ZUMA&lt;/em&gt;, which was developed (including its payload adaptor) by Northrop Grumman.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Soon after launch, however, rumors started swirling that some kind of mishap had occurred. These were fanned hardest by Are Technica's Senior Space Editor, &lt;a href=&quot;https://twitter.com/SciGuySpace&quot; rel=&quot;noreferrer&quot;&gt;Eric Berger&lt;/a&gt;, who published an article entitled &quot;&lt;em&gt;It’s not official, but sources say the secretive Zuma satellite was lost&lt;/em&gt;&quot;, who suggested a number of possibilities, ranging from the payload failing to deploy, or some mishap with the Falcon 9 upper stage occurring.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/SeLIM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/SeLIM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Despite this, Elon Musk was more than happy to share mission success on social media, taking to Twitter &amp;amp; Instagram afterwards. Sources state that prior to the mission, he had also made comments about the importance of this specific payload to the company.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, what do we know?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Again, please, no speculation. Hard knowledge and sources only — the more the better. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastActivityDate="2020-09-13T08:56:39.440" Title="What was the outcome of the launch of ZUMA by SpaceX, to the best of the public's knowledge?" Tags="|spacex|falcon-9|payload|payload-deployment|classified|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24396" PostTypeId="2" ParentId="24365" CreationDate="2018-01-11T06:26:23.223" Score="7" Body="&lt;p&gt;In addition to all the structural and electrical perspectives, there is also the possiblity of the firmware in the ECUs being corrupted due to cosmic rays. &#xA;The automobile is definitely certified for errr.. to be used on earth and not-space-hardened memory will not survive space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The car is going nowhere when the the mechanics and electrics are ready and there is a ECU checksum error :)&lt;/p&gt;&#xA;" OwnerUserId="22437" LastActivityDate="2018-01-11T06:26:23.223" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24397" PostTypeId="2" ParentId="24395" CreationDate="2018-01-11T07:08:57.493" Score="6" Body="&lt;p&gt;SpaceX published a message from Gwynne Shotwell explaining that the first and second stage performed nominally and they cannot comment upon the status of a classified mission.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/ChrisG_NSF/status/950731607693983744&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gwynne's statement, retweeted by ChrisB from NSF&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Northrup Grumman also supplied the payload adapter, the connection between the payload and the second stage. If the failure was there, then the second stage could have properly separated but the payload adapter might have been the issue, or the satellite itself. But SpaceX is standing firm that the booster performed properly, without commenting on the payload itself.&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-01-11T13:55:09.447" LastActivityDate="2018-01-11T13:55:09.447" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24400" PostTypeId="2" ParentId="24390" CreationDate="2018-01-11T08:37:47.090" Score="9" Body="&lt;p&gt;Let's see what was needed to make sure the LRV didn't disintegrate during launch. These are the same requirements that apply to the rest of the Saturn V stack, by the way. &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;You have to know what the environment will be. This is initially done by &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=20100042567&amp;amp;hterms=Saturn%20vibration&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3D%2522Saturn%2520V%2522%2520vibration%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA&quot; rel=&quot;noreferrer&quot;&gt;testing&lt;/a&gt; individual stages and the entire rocket: early launches are instrumented to measure noise, vibration and acceleration. &lt;/li&gt;&#xA;&lt;li&gt;once you've done this for one rocket, you can develop models that help &lt;a href=&quot;https://ntrs.nasa.gov/search.jsp?R=19730012187&amp;amp;hterms=TN%20D-7159&amp;amp;qs=N%3D0%26Ntk%3DAll%26Ntt%3DTN%2520D-7159%26Ntx%3Dmode%2520matchallpartial%26Nm%3D123%7CCollection%7CNASA%2520STI%7C%7C17%7CCollection%7CNACA&quot; rel=&quot;noreferrer&quot;&gt;predict&lt;/a&gt; what the noise and vibration will be for your new design. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;These days, noise and vibration data is published in a rocket's &lt;a href=&quot;http://www.arianespace.com/wp-content/uploads/2011/07/Ariane5_Users-Manual_October2016.pdf&quot; rel=&quot;noreferrer&quot;&gt;user manual&lt;/a&gt;. In the linked Ariane 5 user manual, chapter 3 lists the environmental data:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;steady-state acceleration&lt;/li&gt;&#xA;&lt;li&gt;regular and random vibration&lt;/li&gt;&#xA;&lt;li&gt;noise&lt;/li&gt;&#xA;&lt;li&gt;shocks&lt;/li&gt;&#xA;&lt;li&gt;temperature&lt;/li&gt;&#xA;&lt;li&gt;electromagnetic interference&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Space agencies have &lt;a href=&quot;https://en.wikipedia.org/wiki/Space_Power_Facility&quot; rel=&quot;noreferrer&quot;&gt;test chambers&lt;/a&gt; that can simulate this environment. &lt;a href=&quot;https://www.nas.nasa.gov/SC11/demos/demo44.html&quot; rel=&quot;noreferrer&quot;&gt;Computer simulation&lt;/a&gt; is also possible these days.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you know the noise and vibration environment, you can design for it. Noise and vibration exert a known load on a structure, you just have to make the structure strong enough to withstand this load. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The assumption that a car would not survive launch is incorrect. Cars are subject to noise and vibration analysis during development (to ensure passenger comfort and durability of the car), and SpaceX would have easy access to that data for a Tesla car. Even without that data, they will have put the car through noise and vibration testing (just like any other payload) because you really don't want bits dropping off and damaging the launcher. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-11T08:48:28.410" LastActivityDate="2018-01-11T08:48:28.410" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24401" PostTypeId="1" CreationDate="2018-01-11T08:42:16.410" Score="5" ViewCount="191" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/W2wZU.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/W2wZU.jpg&quot; alt=&quot;Astronaut Norishige Kanai (Left) in space suit&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I recently saw this picture, and it puzzles me as to what they are wearing. At first thought, I imagined that they would be space suits. But, they went in a Soyuz craft, and the suits for a Soyuz don't look like that, so either they are new spacesuits, or they are wearing something different. What are they?&lt;/p&gt;&#xA;" OwnerUserId="21480" LastEditorDisplayName="user20636" LastEditDate="2018-04-06T03:22:36.547" LastActivityDate="2018-04-06T03:22:36.547" Title="What space suit is this?" Tags="|soyuz-spacecraft|spacesuits|identify-this-object|jaxa|roscosmos|" AnswerCount="0" CommentCount="2" ClosedDate="2018-01-11T14:34:07.120" ContentLicense="CC BY-SA 3.0" />
  <row Id="24404" PostTypeId="5" CreationDate="2018-01-11T13:52:10.940" Score="0" Body="&lt;p&gt;This tag is used to discuss non-standard payloads which don't usually fit into the set of normal orbital payloads, such as satellites, spacecraft, and scientific instruments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Non-standard payloads are unusual as rockets (&lt;a href=&quot;/questions/tagged/rockets&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;rockets&amp;#39;&quot; rel=&quot;tag&quot;&gt;rockets&lt;/a&gt;) are expensive to launch, and are thus reserved for similarly expensive or important payloads. This makes it pretty rare to see things like Elon Musk's Midnight-Cherry Tesla Roadster(&lt;a href=&quot;/questions/tagged/roadster&quot; class=&quot;post-tag&quot; title=&quot;show questions tagged &amp;#39;roadster&amp;#39;&quot; rel=&quot;tag&quot;&gt;roadster&lt;/a&gt;) as a payload.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other non-standard payloads include paper planes sent to the ISS for flights back to Earth, and a wheel of cheese on the first SpaceX cargo flight. &lt;/p&gt;&#xA;" OwnerUserId="22326" LastEditorUserId="6537" LastEditDate="2018-01-11T20:22:55.837" LastActivityDate="2018-01-11T20:22:55.837" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24405" PostTypeId="4" CreationDate="2018-01-11T13:52:10.940" Score="0" Body="Payloads (cargo) that are launched for unusual reasons. Use this tag to discuss payloads that are not normal for space-bound vehicles." OwnerUserId="22326" LastEditorUserId="4660" LastEditDate="2018-01-12T15:49:45.527" LastActivityDate="2018-01-12T15:49:45.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24406" PostTypeId="2" ParentId="24395" CreationDate="2018-01-11T14:04:58.560" Score="21" Body="&lt;p&gt;What is known:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SpaceX issued a statement saying that the Falcon 9 performed as intended&lt;/li&gt;&#xA;&lt;li&gt;An &lt;a href=&quot;https://twitter.com/Marco_Langbroek/status/950509102970621957/photo/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;object was seen with spinning plumes&lt;/a&gt; that is assumed to be a Falcon 9 upper stage post-thrust fuel dump. This occurred at about 2 hours, 15 minutes after launch, over Africa.&lt;/li&gt;&#xA;&lt;li&gt;The &lt;a href=&quot;http://www.spacex.com/sites/spacex/files/zumapresskit.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;official timeline/ press brief&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;The customer of Zuma was an unnamed US Government agency.&lt;/li&gt;&#xA;&lt;li&gt;Space Track has an official ID for this object, USA 280, 43098. They have not listed it as decayed, as of now.&lt;/li&gt;&#xA;&lt;li&gt;There has been (or soon will be) one or more &lt;a href=&quot;https://www.bloomberg.com/news/articles/2018-01-10/lawmakers-look-into-spacex-launch-that-ended-with-lost-satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;classified congressional briefings&lt;/a&gt; on the presumed failure.&lt;/li&gt;&#xA;&lt;li&gt;Thus far, it has not been seen by &lt;a href=&quot;http://www.satobs.org/seesat/Jan-2018/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;amateurs&lt;/a&gt;. That isn't really a surprise, it will take time to rule out anything being there.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;What is strongly rumored. These occur in multiple news outlets, but there isn't anything that one can confirm.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;There was a failure of the Zuma vehicle, either in separating from the upper stage, or else it appeared dead on orbit. Some anonymous reports have come out that the satellite failed.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.wired.com/story/spacexs-top-secret-zuma-mission-launches-today/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Northrup Grumman supplied the payload adapter&lt;/a&gt;. This isn't explicitly stated, but Northup Grumman has seemed to agree with that statement.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.reddit.com/r/spacex/comments/7dgvlz/spacex_launch_of_secretive_zuma_mission_from_ksc/dpy98qs/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elon Musk claimed that Zuma&lt;/a&gt; was the &amp;quot;Most important and expensive payload&amp;quot; that SpaceX has ever launched.&lt;/li&gt;&#xA;&lt;li&gt;At at least one of the congressional briefings, finger pointing between SpaceX and Northrup Grumman has occurred as to who is responsible for the failure of the Zuma satellite.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Other source material that seems confusing&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Fairing separation was announced much after it was expected to happen. It is believed that it was observed to occur on time &lt;a href=&quot;https://s13.postimg.org/641l5img7/zuma_2ndfairing.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;via visual observation&lt;/a&gt;, however.&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://youtu.be/uK4dELV4b9Q?t=7m28s&quot; rel=&quot;nofollow noreferrer&quot;&gt;USLaunchReport&lt;/a&gt; reported SECO at 7:15, which is much earlier then it should have been (Expected closer to 9:00)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The best article on just the facts I've seen is &lt;a href=&quot;https://www.nytimes.com/2018/01/09/science/spacex-zuma-satellite.html?rref=collection%2Fsectioncollection%2Fscience&quot; rel=&quot;nofollow noreferrer&quot;&gt;this one&lt;/a&gt;. &lt;a href=&quot;http://spaceflight101.com/falcon-9-zuma/zuma-potential-mission-failure/&quot; rel=&quot;nofollow noreferrer&quot;&gt;This one&lt;/a&gt; also does a good job at pointing of the validity of rumors.&lt;/p&gt;&#xA;&lt;p&gt;In April 2018, it was &lt;a href=&quot;https://www.theverge.com/2018/4/9/17215016/spacex-falcon-9-zuma-spacecraft-northrop-grumman-payload-adapter&quot; rel=&quot;nofollow noreferrer&quot;&gt;reported&lt;/a&gt; the Wall Street Journal &lt;a href=&quot;https://www.wsj.com/articles/probes-point-to-northrop-grumman-errors-in-january-spy-satellite-failure-1523220500&quot; rel=&quot;nofollow noreferrer&quot;&gt;saw a report&lt;/a&gt; which reached the tentative conclusion Zuma failed to separate from The F9 upper stage, and so re-entered after one and a half orbits.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorDisplayName="user20636" LastEditDate="2020-09-13T08:56:39.440" LastActivityDate="2020-09-13T08:56:39.440" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="24407" PostTypeId="2" ParentId="24365" CreationDate="2018-01-11T16:40:57.007" Score="6" Body="&lt;p&gt;Nope.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Silicon has a mortal enemy: radiation.&lt;br&gt;&#xA;Space has a lot of radiation, including charged nuclear particles coming off the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The computers, motor controllers, heads-up display, autopilot, and 1000 other important sub-systems in the car are driven by silicon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While it might be a very good museum piece in 100 years, it is profoundly unlikely to ever drive again.   .... assuming it doesn't blow up on launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After 100 years in space, the silicon based technology, not wrapped in very high atomic number materials, are going to be dead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Polymers (think plastic cladding on wires) has &quot;plasticizers&quot;, which make it elastic instead of brittle.  Those are going to evaporate.  Nothing made of plastic is going to be able to take tiny shocks after 100 years.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update:&lt;/strong&gt;    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Chemical bonds are on the scale of a few electron volts (Chemistry).  The solar wind's particles are on the order of keV, or about 1000x more powerful than the bonds &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_wind&quot; rel=&quot;nofollow noreferrer&quot;&gt;(wikipedia- solar wind)&lt;/a&gt;.   Think about this like putting the device at the end of a kilovolt particle accelerator and ask about EMI/RFI consequences.  In terms of particle interactions, not all materials are created equal, which drove the idea of &quot;barns&quot;, particle capture cross-section, and thermal-neutron producing moderators (like carbon).  &lt;a href=&quot;https://en.wikipedia.org/wiki/Neutron_capture&quot; rel=&quot;nofollow noreferrer&quot;&gt;(wikipedia - neutron capture)&lt;/a&gt; &lt;a href=&quot;https://en.wikipedia.org/wiki/Neutron_moderator&quot; rel=&quot;nofollow noreferrer&quot;&gt;(wikipedia - thermal moderator)&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I had a buddy who made dna-scale precious metal bar-codes by putting silicon at the working end of a cyclotron, blowing holes through it, then depositing alternating layers of metals.  High velocity protons can blow holes in silicon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another buddy of mine got his work on the space station when he vapor-deposited tungsten on carbon fiber for super-light radiation protection hardware.  What he described looks like &lt;a href=&quot;https://technology.nasa.gov/patent/LAR-TOPS-201&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;. Tungsten and other high-Z materials are used to shield electronics from total radiation dose.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here are NASA links on the topic:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/sites/default/files/files/SMIII_Problem25.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/sites/default/files/files/SMIII_Problem25.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990116210.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990116210.pdf&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://www.nasa.gov/sites/default/files/atoms/files/small_spacecraft_technology_state_of_the_art_2015_tagged.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.nasa.gov/sites/default/files/atoms/files/small_spacecraft_technology_state_of_the_art_2015_tagged.pdf&lt;/a&gt;  &amp;lt;-- check out section 6.4.    &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Here is an article saying how bipolar FETs die super early at super low doses of radiation.&#xA;&lt;a href=&quot;http://www.spacedaily.com/news/radiation-98a.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spacedaily.com/news/radiation-98a.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is another, about power-down of satellites during solar storms because Van Allen belts accelerate charged particles.&#xA;&lt;a href=&quot;http://www.spacedaily.com/news/radiation-98d.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.spacedaily.com/news/radiation-98d.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Update2:&lt;/strong&gt;&lt;br&gt;&#xA;Thanks James for the related answer on plasticizers evaporating. (&lt;a href=&quot;https://space.stackexchange.com/questions/24410/do-plasticizers-evaporate-in-space&quot;&gt;link&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="18449" LastEditorUserId="18449" LastEditDate="2018-02-07T21:04:10.017" LastActivityDate="2018-02-07T21:04:10.017" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24408" PostTypeId="1" CreationDate="2018-01-11T17:02:28.413" Score="5" ViewCount="415" Body="&lt;p&gt;The Space.com article &lt;a href=&quot;https://www.space.com/39338-spacex-zuma-rocket-sky-spiral-photos.html&quot; rel=&quot;noreferrer&quot;&gt;Strange Sky Spiral May Come from &lt;strong&gt;Secretive SpaceX Zuma Launch&lt;/strong&gt;&lt;/a&gt; shows images from &lt;a href=&quot;https://twitter.com/Samcornwell/status/950499540666331136&quot; rel=&quot;noreferrer&quot;&gt;this Tweet&lt;/a&gt; and &lt;a href=&quot;https://twitter.com/Marco_Langbroek/status/950509102970621957&quot; rel=&quot;noreferrer&quot;&gt;this tweet&lt;/a&gt;, the second of which was posted by Dr Marco Langbroek (@Marco_Langbroek): &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Palaeolithic archaeologist (PhD), spy satellite tracker, asteroid discoverer. Tweets about space, archaeology, astronomy &amp;amp; other interests. Asteroid (183294).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There is a lot more Zuma-related orbital mechanics Dr. Langbroek's twitter (&lt;a href=&quot;https://sattrackcam.blogspot.com/&quot; rel=&quot;noreferrer&quot;&gt;and extensive blogposting&lt;/a&gt;) as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is this unburned propellant, if so, why would this need to be done, and in any event, what gives it this particularly distinctive shape?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/xkQZJ.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/xkQZJ.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/8nalb.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8nalb.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-11T17:18:08.770" LastActivityDate="2018-01-11T18:37:09.113" Title="Why did the &quot;Secretive SpaceX Zuma Launch&quot; make a big &quot;S&quot; or yin-yang shape in the sky?" Tags="|spacex|falcon-9|fuel|zuma|visual-signature|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24409" PostTypeId="2" ParentId="24408" CreationDate="2018-01-11T18:37:09.113" Score="4" Body="&lt;p&gt;All of the reports are that this was expending the unspent fuel from the second stage. There have been other sightings of unburnt fuel dumping, such as this one:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/7WTx5k_63ZY?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the other instances, there appeared to be more of a cloud, in this instance, it appeared to spin.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, why the difference? The most likely culprits:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Previous missions, the fuel dumping was more balanced. One of the valves might have been stuck.&lt;/li&gt;&#xA;&lt;li&gt;The weight distribution wasn't as expected. This could cause the satellite to spin while dumping fuel. This could be caused by a number of things, from the Northrup Grumman provided PAF being a different mass distribution then the SpaceX PAF, something still being stuck on to the stage.&lt;/li&gt;&#xA;&lt;li&gt;It could have been rotating for some reason. This could be caused by a separation that didn't occur quite as expected.&lt;/li&gt;&#xA;&lt;li&gt;SpaceX was trying something different for some reason&lt;/li&gt;&#xA;&lt;li&gt;Something could have bumped the second stage between the burn and the defuel.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I suspect it was one of these, but it's really hard to know for sure. I doubt SpaceX will release the reasoning behind this, so for now, one will have to live in the realm of guesses.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-11T18:37:09.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24410" PostTypeId="1" CreationDate="2018-01-11T19:11:29.303" Score="4" ViewCount="265" Body="&lt;p&gt;A recent statement in an answer got me wondering.  I looked and could not find an answer on space.se&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Polymers (think plastic cladding on wires) has &quot;plasticizers&quot;, which make it elastic instead of brittle. Those are going to evaporate. Nothing made of plastic is going to be able to take tiny shocks after 100 years.  &lt;a href=&quot;https://space.stackexchange.com/questions/24365/can-i-drive-elon-musks-tesla-after-its-been-in-space-for-100-years/24407#24407&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Do plasticizers evaporate in space?&lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="58" LastEditDate="2018-01-11T19:30:53.803" LastActivityDate="2018-01-12T03:24:33.887" Title="Do plasticizers evaporate in space?" Tags="|materials|vacuum|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24411" PostTypeId="2" ParentId="23452" CreationDate="2018-01-11T19:18:36.530" Score="4" Body="&lt;p&gt;It could be, assuming it is in fact sealed. But the likelihood of &lt;a href=&quot;https://www.google.com/url?sa=t&amp;amp;rct=j&amp;amp;q=&amp;amp;esrc=s&amp;amp;source=web&amp;amp;cd=6&amp;amp;cad=rja&amp;amp;uact=8&amp;amp;ved=0ahUKEwiA0ZLyzdDYAhVBZN8KHTKeDKgQFgg3MAU&amp;amp;url=https%3A%2F%2Fsservi.nasa.gov%2Farticles%2Flava-tube-lunar-base%2F&amp;amp;usg=AOvVaw3cEH8ypCSdDWEiEj2XedkK&quot; rel=&quot;nofollow noreferrer&quot;&gt;it being completely sealed&lt;/a&gt; are unlikely. There are a couple of workarounds to such habitats that I have seen:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Use some kind of an inflatable habitat inside of the lava tube - this would offer protection against radiation, thermal regularity, etc.&lt;/li&gt;&#xA;&lt;li&gt;Seal the walls somehow. There are a few ways that this could be done. One could imagine spraying some kind of a solution on the entire exposed walls, or melting the surface, or something like that to give it some kind of seal.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Personally, I can't imagine humans being in such a surface without at least 2 layers of protection against atmospheric loss. The structure of the cave will probably be shored up via some kind of coating, followed by at least one interior bubble, and possibly a second one. The cave would need to be made sturdy enough where it wouldn't fall, and to have any sharp pieces dulled down some.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="7062" LastEditDate="2021-10-06T16:01:28.430" LastActivityDate="2021-10-06T16:01:28.430" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="24413" PostTypeId="1" AcceptedAnswerId="24415" CreationDate="2018-01-11T22:07:02.540" Score="7" ViewCount="1511" Body="&lt;p&gt;Many websites consistently claim that the first stage of the enormous &lt;a href=&quot;https://en.wikipedia.org/wiki/Sea_Dragon_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sea Dragon&lt;/a&gt; rocket was intended to be reusable. While recovering a giant booster from sea after a splashdown may seem tricky, it does not break my suspension of disbelief - I am willing to believe that the terminal velocity of a huge, but (mostly) empty tank is low enough to achieve this feat.&lt;/p&gt;&#xA;&lt;p&gt;But yesterday I stumbled upon the Vintage Space blogpost &lt;a href=&quot;http://blogs.discovermagazine.com/vintagespace/2017/08/11/sea-dragon-biggest-rocket-ever-dreamed/#.WlaM5jciHIV&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sea Dragon is the Biggest Rocket We Ever Dreamed Of&lt;/a&gt;. Out of a lot of interesting design details, one seems ridiculous for me - second stage reuse!&#xA;Behold:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Second Stage recovery would be similar, though its return to Earth would begin with small retrorockets in the nose since it would be coming home from orbit. After atmospheric reentry, a skirt similar to the one on the first stage would slow it to the desired impact speed [...]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Is it even remotely possible? I mean, the second (and last) stage should have (if I understand correctly) a velocity pretty close to the orbital velocity. How is it supposed to survive reentry, then? Especially when retrorockets are described as &amp;quot;small&amp;quot;? (at least &amp;quot;small&amp;quot; by Sea Dragon standards)&lt;/p&gt;&#xA;" OwnerUserId="22215" LastEditorUserId="5946" LastEditDate="2021-02-14T01:10:43.787" LastActivityDate="2021-02-14T01:10:43.787" Title="Was Sea Dragon second stage reusability ever plausible?" Tags="|launch|reuse|launch-vehicle|sea-dragon|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24414" PostTypeId="1" CreationDate="2018-01-12T01:39:43.603" Score="13" ViewCount="761" Body="&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/24408/why-did-the-secretive-spacex-zuma-launch-make-a-big-s-or-yin-yang-shape-in-t?noredirect=1#comment70190_24408&quot;&gt;This comment&lt;/a&gt; mentions the amazing double-spiral event seen in the night sky from Norway that Wikipedia calls the &lt;a href=&quot;https://en.wikipedia.org/wiki/2009_Norwegian_spiral_anomaly&quot; rel=&quot;nofollow noreferrer&quot;&gt;2009 Norwegian spiral anomaly&lt;/a&gt;. The Gizmodo article &lt;a href=&quot;https://gizmodo.com/5422574/giant-mysterious-spiral-takes-over-the-skies-of-norway&quot; rel=&quot;nofollow noreferrer&quot;&gt;Giant Mysterious Spiral Takes Over the Skies of Norway&lt;/a&gt; includes the image shown below, and the Gizmodo article &lt;a href=&quot;https://gizmodo.com/5422792/this-is-how-the-mysterious-giant-spiral-happened&quot; rel=&quot;nofollow noreferrer&quot;&gt;This Is How the Mysterious Giant Spiral Happened&lt;/a&gt; includes a link to a YouTube video explaining some basic physics behind the mechanism (shown below). Loosely paraphrasing, one is mostly axial, the other more tangential.&lt;/p&gt;&#xA;&lt;p&gt;So far I have not figured out or find an explanation for the reason for the strong, uniform blue coloration of the (presumably) reflected sunlight from one of the two spirals, while the other appears to be white.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Why is one of these two concurrent fuel-dump spirals blue?&lt;/p&gt;&#xA;&lt;p&gt;Looking for authoritative, sourced answer if possible. I know that bulk liquid LOX has a slight blue color, but I don't think that LOX dumps are generally blue.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/M9Did.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M9Did.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; From &lt;a href=&quot;https://gizmodo.com/5422574/giant-mysterious-spiral-takes-over-the-skies-of-norway&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gizmodo&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/Zx8i5EfmYU4?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-11-14T10:39:14.683" LastActivityDate="2021-11-14T10:39:14.683" Title="Why is one of these two concurrent fuel-dump spirals blue?" Tags="|fuel|visual-signature|" AnswerCount="1" CommentCount="12" ContentLicense="CC BY-SA 4.0" />
  <row Id="24415" PostTypeId="2" ParentId="24413" CreationDate="2018-01-12T02:37:40.163" Score="6" Body="&lt;p&gt;The linked blogpost in the question is from a series of Vintage Space posts in Discover Magazine. Amy Shira Teitel also has an extensive series of informative and enjoyable videos on her &lt;a href=&quot;https://www.youtube.com/channel/UCw95T_TgbGHhTml4xZ9yIqg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Vintage Space&lt;/a&gt; YouTube channel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The video &lt;a href=&quot;https://www.youtube.com/watch?v=yiTjbsiKu6g&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sea Dragon is the Massive Rocket of Spaceflight Dreams&lt;/a&gt; is worth watching for a helpful breakdown of the Sea Dragon proposal. I've transcribed her discussion of the recovery of both the first and second stages between &lt;code&gt;06:07&lt;/code&gt; to &lt;code&gt;07:19&lt;/code&gt;. The last paragraph indicates that Aerojet asserts that the proposed Sea Dragon implementation would be a series of solvable engineering challenges.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The first and second stages were both recovered at first just using just simple aerodynamic braking as they fell through the upper atmosphere, but the active stage of recovery would be using an aerodynamic deceleration device. &lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;This decelerator was a reliable, and very easy system. It was a large conical flare 300 feet in diameter, that could be pressurized with the same pressurizing gas as was used in the pressure-fed engines for each stage. This also ensured that both stages would hit the water at the correct nose-down orientation, a way that would minimize the forces and stresses on the body such that it could be refurbished easily and reused, as opposed to building a new one. After the first and second stages both splashed down, they would be recovered and towed back to the assembly lagoon. There they would be refurbished, and any parts that need replacing could be replaced. But at the end of the day, most of the rocket could be just refurbished, mated together again in that lagoon, and used again on another Sea Dragon launch.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;As Aerojet noted in its 1963 report, the sea dragon definitely represented a massive leap in technology, but it wasn’t one that was completely impossible. Every challenge with Sea dragon was actually just an engineering challenge, and engineering challenges could be solved.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The report &lt;a href=&quot;http://neverworld.net/truax/Sea_Dragon_Concept_Volume_1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA-CR-52817 Sea Dragon Concept. Volume 1: Summary (Aerojet-General Corp.) N88-71080m 28-Jan-1963&lt;/a&gt; discusses recovery of both stages in section III-C, and the second stage in particular on pages III-c-19 to 21.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Additional discussion of recovery development can be found throughout &lt;a href=&quot;http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.694.7611&amp;amp;rep=rep1&amp;amp;type=pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this report&lt;/a&gt; dated a few weeks later.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The second-stage auxiliary engines provide orbital injection thrust for the payload, and the second stage will go into&#xA;  orbit with the payload. During ascent into orbit and after the second stage main engine has ceased firing, the expandable nozzle skirt will be separated and ejected from the stage. After attaining orbital condition, the payload and expended second stage will be separated. When the second stage has reached the desired position, small retro rockets in the nose will be fired to eject the vehicle out of orbit. A velocity impulse of 480 ft/sec will be required to give an initial re-entry angle of 2.5 ° from the local horizontal.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Aerodynamic stabilization and deceleration of the stage will be attained with an inflatable flare similar to the one used for first stage recovery. The size of the flare will be smaller than the flare used on first stage. The available tank pressure is lower, 50 psi, however, the recovered weight is also lower, 1.2 x 10E6 lb. This 50 psi tank pressure, available for axial load reactions,corresponds&#xA;  to a terminal velocity of 210 ft/sec for which a flare diameter of 240 ft is required.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Aerodynamic heating for second stage recovery will be more severe than for first stage. Use of flare material with higher thermal resistance_ such as Rene 41 mesh and ablative coatings will be necessary.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LEcg7.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LEcg7.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bl9Ra.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bl9Ra.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/yiTjbsiKu6g?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/INutKAWisX0?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2019-07-13T02:35:03.970" LastActivityDate="2019-07-13T02:35:03.970" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="24416" PostTypeId="2" ParentId="24410" CreationDate="2018-01-12T03:24:33.887" Score="2" Body="&lt;p&gt;Some (and probably most) of them do. Modern plastics use different plasticizers, but all of them have the same characteristic - they are not bound in the polymer matrix, and thus evaporate  even in atmosphere: if you ever owned an object made out of plastic for a long period of time (think about ten years) you might've noticed that its plastic parts slightly changed color and became a bit brittle - exactly because of this reason. It is more obvious in older plastics, but modern materials are susceptible to this process, too. And in the vacuum of space the process of plasticizers migrating from polymers will be much faster due to the pressure differential.&lt;/p&gt;&#xA;" OwnerUserId="21168" LastActivityDate="2018-01-12T03:24:33.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24420" PostTypeId="2" ParentId="22554" CreationDate="2018-01-12T12:46:06.393" Score="3" Body="&lt;p&gt;Pure Space Exploration is a bit tricky to have an open source project, as many of the items fall under the realm of ITAR restrictions, which probits the sharing of technical data. That being said, let me point you in a few directions:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AMSAT- The amateur radio satellite group. This group has been around for a long while, building amateur radio satellites. They have a list of &lt;a href=&quot;http://www.amsat.org/amsat-new/tools/software.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;tracking software&lt;/a&gt;, which includes a number of open sourced tasks. AMSAT-NA has a &lt;a href=&quot;http://www.amsat.org/amsat-new/tools/softwareArchive.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;larger list of software projects&lt;/a&gt; as well, which has a few open sourced items.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://code.nasa.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA&lt;/a&gt;- Has a list of open source software endeavors.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://essr.esa.int/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ESA&lt;/a&gt;- Same thing, open source projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure there are university projects out there as well, but these should give you a good start.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure that all of the projects will accept pull requests, but take a look at the ones with git hubs and see if they have accepted pull requests.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-12T23:29:55.913" LastActivityDate="2018-01-12T23:29:55.913" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24421" PostTypeId="1" AcceptedAnswerId="24422" CreationDate="2018-01-12T13:45:05.120" Score="5" ViewCount="1183" Body="&lt;p&gt;The BBC News item &lt;a href=&quot;http://www.bbc.com/news/science-environment-42654281&quot; rel=&quot;nofollow noreferrer&quot;&gt;UK satellite to make movies from space&lt;/a&gt; describes this newly deployed satellite:&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;caption:&lt;/strong&gt; Artwork: Manufacturer SSTL calls it Carbonite-2, but Earth-i refers to the satellite as VividX2&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dLMtS.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dLMtS.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/cbnt-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gunter's Space Page&lt;/a&gt; it has a 25 centimeter aperture and at about 500 km altitude should have a ground resolution of about 1.5 meters. However, according to the BBC article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;We can collect up to 50 frames per second which is a lot of information,&amp;quot; explained Earth-i CEO Richard Blain.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;That allows us to stack the individual images and increase our effective resolution, achieving somewhere around 65cm to 75cm,&amp;quot; he told BBC News.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;How would a high video frame rate improve ground resolution to 65 centimeters for a ten-inch telescope at 500 km?&lt;/p&gt;&#xA;&lt;p&gt;I have heard of techniques such as &amp;quot;lucky imaging&amp;quot; (it's a real thing) but this is usually to combat atmospheric seeing effects, not the diffraction limit. Also, this &lt;a href=&quot;https://space.stackexchange.com/a/20111/12102&quot;&gt;excellent answer&lt;/a&gt; points out that atmospheric effects are much less important looking down at the surface than looking up from the surface for apertures well below 2 meters, confirmed by &lt;a href=&quot;https://space.stackexchange.com/a/20116/12102&quot;&gt;this answer&lt;/a&gt; as well.&lt;/p&gt;&#xA;&lt;p&gt;So here, I don't understand how the high frame rate can have such a profound effect so as to push well beyond the &lt;a href=&quot;https://en.wikipedia.org/wiki/Airy_disk&quot; rel=&quot;nofollow noreferrer&quot;&gt;Airy Disk&lt;/a&gt; diffraction limit of say &lt;span class=&quot;math-container&quot;&gt;$1.22 \lambda / D$&lt;/span&gt; which would be about 2.7E-06 radians, or 1.36 meters at a 505 km closest approach (assuming a 550nm central wavelength).&lt;/p&gt;&#xA;&lt;p&gt;There are other techniques to push beyond this limit, such as &lt;a href=&quot;https://en.wikipedia.org/wiki/Aperture_masking_interferometry&quot; rel=&quot;nofollow noreferrer&quot;&gt;Aperture Mask Interferometry&lt;/a&gt; as described in &lt;a href=&quot;https://astronomy.stackexchange.com/a/24182/7982&quot;&gt;this answer&lt;/a&gt;, but that wouldn't be related to high frame rate.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-12T14:44:40.033" Title="Can image-stacking allow this 0.25 m satellite telescope achieve 0.65m resolution?" Tags="|artificial-satellite|imaging|space-telescope|reconnaissance-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24422" PostTypeId="2" ParentId="24421" CreationDate="2018-01-12T14:44:40.033" Score="7" Body="&lt;p&gt;The technique is known as &quot;Super Resolution&quot;. Basically this involves a few steps, which the general outline is as follows:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Knowing the point source performance to a sub-pixel level of your system. You have to move the test target a fraction of a pixel at a time, and calculate the Point Source Function (PSF) for each point. The smaller you can do, the better.&lt;/li&gt;&#xA;&lt;li&gt;Convert this PSF into a smooth function.&lt;/li&gt;&#xA;&lt;li&gt;Take your source image, and deconvolve it, removing the diffraction errors.&lt;/li&gt;&#xA;&lt;li&gt;Combine these with other photos slightly offset.&lt;/li&gt;&#xA;&lt;li&gt;Solve for a solution such that the root image gives the desired value.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;This is somewhat similar to images from the &lt;a href=&quot;http://www.stsci.edu/hst/HST_overview/documents/RestorationofHSTImagesandSpectra.pdf&quot; rel=&quot;noreferrer&quot;&gt;deblurred Hubble Space Telescope&lt;/a&gt;. The best image I can find of a deblurred Hubble image is &lt;a href=&quot;https://books.google.com/books?id=MFHLut0E2y8C&amp;amp;pg=PA23&amp;amp;lpg=PA23&amp;amp;dq=Hubble%20spherical%20aberration%20image%20processing&amp;amp;source=bl&amp;amp;ots=AXHIz12ZI8&amp;amp;sig=T57Qt8dN71UYlx5FxpYsUDrdMoE&amp;amp;hl=en&amp;amp;sa=X&amp;amp;ved=0ahUKEwilvLbx09LYAhWk5YMKHczyBZs4FBDoAQgxMAE#v=onepage&amp;amp;q=Hubble%20spherical%20aberration%20image%20processing&amp;amp;f=false&quot; rel=&quot;noreferrer&quot;&gt;this one&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5CiFx.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5CiFx.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To give a bit better example of how super resolution is used, take a look at &lt;a href=&quot;https://amundtveit.com/2017/06/04/deep-learning-for-image-super-resolution-scale-up/&quot; rel=&quot;noreferrer&quot;&gt;this site&lt;/a&gt;. Image not included because I'm not sure of the copyright, but it shows that you can add multiple images together to get a reasonable image out. You need to have at least the same number of pixels of data to get a reasonable output, so to double the resolution, you would need at least 4-5 images. With 50 images per second, it seems reasonable that they could achieve the indicated resolution, albeit not as good as a single image would be with a larger telescope.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-12T14:44:40.033" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24423" PostTypeId="1" CreationDate="2018-01-12T17:45:21.127" Score="11" ViewCount="588" Body="&lt;p&gt;Data transmission using an optical laser between ground station and a GEO satellite may offer very high data rates, up to 1.8 GBit/s for &lt;a href=&quot;http://www.esa.int/Our_Activities/Telecommunications_Integrated_Applications/EDRS/Overview&quot; rel=&quot;noreferrer&quot;&gt;instance&lt;/a&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;But what about a transmission from Earth to Mars without any repeater between the planets? Will the huge distance decrease the maximum data rate to MBit/s or even kBit/s? Using a much more powerful laser and larger aperture sizes for transmitter and receiver may increase the possible data rate, but to what extent? At least some photons per data bit are needed for an acceptable error rate.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastEditorUserId="15906" LastEditDate="2018-01-13T10:29:16.850" LastActivityDate="2023-08-10T06:52:09.720" Title="Is it possible to extend high speed data transmission with lasers to the distance Earth to Mars?" Tags="|mars|data-transmission|laser|" AnswerCount="3" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24424" PostTypeId="1" AcceptedAnswerId="24431" CreationDate="2018-01-12T18:38:38.093" Score="4" ViewCount="250" Body="&lt;p&gt;They always talk about how there might be life on Enceladus, but how old is this lake really? I would imagine water being ejected from the moon is not a process that would last forever (because the water would reach an &quot;equilibrium&quot; in which it cannot be ejected). Since moon orbits can move, and planets change, it would not be unreasonable to imagine a time in which this process would not occur. But that leaves the question, how old is this lake? Its pretty important to know since if its young, there probably is no life, or just simple life. If it is old, then what has kept this process going, how long will it last, and is there life? (rhetorically; thats not the question).&lt;/p&gt;&#xA;" OwnerUserId="19098" LastEditorUserId="19098" LastEditDate="2018-01-16T18:22:39.760" LastActivityDate="2018-05-15T23:42:05.560" Title="How old is the underground lake on Enceladus?" Tags="|planetary-science|water|saturn|enceladus|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24425" PostTypeId="1" CreationDate="2018-01-12T20:15:50.040" Score="-1" ViewCount="155" Body="&lt;p&gt;Is it possible to launch a spacecraft into space with a magnetic accelerator? If so, what materials would it require?&lt;/p&gt;&#xA;" OwnerUserId="22479" LastEditorUserId="6944" LastEditDate="2018-01-12T22:40:40.550" LastActivityDate="2018-01-12T22:40:40.550" Title="Magnetic Launcher for a Spacecraft" Tags="|rail-gun|" AnswerCount="1" CommentCount="0" ClosedDate="2018-01-12T21:01:35.093" ContentLicense="CC BY-SA 3.0" />
  <row Id="24426" PostTypeId="2" ParentId="24425" CreationDate="2018-01-12T20:18:08.047" Score="-2" Body="&lt;p&gt;It is completely possible, with the right materials. Carbon nonofibers can be used to make a space elevator, and the magnetic ramp could accelerate the spacecraft to 11km per second, which will make it escape earth gravity&lt;/p&gt;&#xA;" OwnerUserId="22479" LastActivityDate="2018-01-12T20:18:08.047" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24429" PostTypeId="2" ParentId="24382" CreationDate="2018-01-13T00:46:58.620" Score="8" Body="&lt;p&gt;It doesn't, not really, but there will come a time when it will simply not be worth keeping it up. Eventually the solar panels will not produce enough power, some airtight joint will break, or something similar. Right now the station is planned to be used until 2024. &lt;a href=&quot;https://www.popsci.com/how-many-years-does-space-station-have-left&quot; rel=&quot;noreferrer&quot;&gt;There is a study going&lt;/a&gt; on to see if it can last until 2028. The things that they are the most worried about are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Thermal cycling abuse- It seems likely this won't be an issue from studies so far.&lt;/li&gt;&#xA;&lt;li&gt;Solar panels- By the year 2028, the solar panels will have degraded about 30%. This might be an issue, and is the most likely item to cause problems.&lt;/li&gt;&#xA;&lt;li&gt;Electronics are out of date- Might need some fairly major updates. But this could be avoided if required.&lt;/li&gt;&#xA;&lt;li&gt;Cost- The ISS costs NASA bout $3.5 billion to maintain per year. That money could go elsewhere that might be more productive.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The bottom line is, I expect that the ISS will continue to be in use for as long as it makes sense to keep it operating. It cost a large amount of money to get it up there, and I expect until the station could be replaced, it will remain up there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, this could all be moot. If SpaceX managed to get the BFR working, it will essentially be the size of the space station. That would reduce the operating cost of the ISS somewhat, but it would also make it redundant. There are other companies talking about building their own space stations as well. Mars will likely be a major target then.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6Gj3U.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6Gj3U.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6944" LastEditDate="2018-01-13T01:40:27.780" LastActivityDate="2018-01-13T01:40:27.780" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24430" PostTypeId="1" CreationDate="2018-01-13T02:12:20.917" Score="1" ViewCount="134" Body="&lt;p&gt;There once was a website called &lt;em&gt;Space Films&lt;/em&gt;, I think that was it. The web site featured restored official video footage from NASA and others of various space launches over the years (i.e. Gemini, Mercury, Apollo, etc.).&lt;/p&gt;&#xA;&lt;p&gt;I think the proprietor of the website even sold &amp;quot;space stuff&amp;quot;..&lt;/p&gt;&#xA;&lt;p&gt;I can't find the site or remember what the name was. Yes, I've done a web search w/ no luck.&lt;/p&gt;&#xA;&lt;p&gt;Any one of the space fans here remember what this website was called and if it still exists?&lt;/p&gt;&#xA;" OwnerUserId="22487" LastEditorUserId="58" LastEditDate="2022-06-03T15:54:39.797" LastActivityDate="2022-06-03T15:54:39.797" Title="Looking for information about a space-related footage and film resource" Tags="|apollo-program|history|project-gemini|" AnswerCount="1" CommentCount="5" ClosedDate="2018-01-13T16:55:59.137" ContentLicense="CC BY-SA 4.0" />
  <row Id="24431" PostTypeId="2" ParentId="24424" CreationDate="2018-01-13T02:25:26.193" Score="2" Body="&lt;p&gt;I'm assuming you mean the underground ocean, as only Titan and Earth have actual lakes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's really hard to know the age of a geological feature, especially with limited opportunity to study it directly. Taking a photograph of the Moon doesn't really tell you how old it is, but getting samples of the Moon gives a pretty good idea. The best estimate to the age of Enceladus was published in &lt;a href=&quot;https://www.nature.com/articles/s41550-017-0289-8&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nature magazine&lt;/a&gt;. A few quotes:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The final exciting observation that comes from the model is that the amount of heat produced by the internal tide is sufficient to maintain Enceladus' subsurface ocean for billions of years&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;We know that it could be around for billions of years, and we know that it is now. There is some question as the age of Enceladus itself, which might only be 100 million years old. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Odds are that it's at least in the tens of millions of years, more likely billions. Geologically, it's been around for a long time. And in any event, it likely will remain around for some time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm not sure that short of a sample mission that we could refine this any further, but it is an active area of research, so perhaps we will know more soon!&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-13T02:25:26.193" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24432" PostTypeId="2" ParentId="24430" CreationDate="2018-01-13T05:06:41.653" Score="4" Body="&lt;p&gt;Found it! Someone provided me with a contact.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The website is &lt;a href=&quot;http://www.spacecraftfilms.com&quot; rel=&quot;nofollow noreferrer&quot; title=&quot;Spacecraft Films&quot;&gt;Spacecraft Films&lt;/a&gt;.. They provide an extensive, historically accurate collection of space related films &amp;amp; DVDs. They do a lot of research on their films. Hence, the reason that I think it meets the standards of space stackexchange.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Trying to find a small space-related site in a web search is terribly painful.&lt;/p&gt;&#xA;" OwnerUserId="22487" LastActivityDate="2018-01-13T05:06:41.653" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24433" PostTypeId="2" ParentId="9837" CreationDate="2018-01-13T06:55:55.053" Score="4" Body="&lt;p&gt;Although I and many other authors of the 2007 Enceladus study would love to see such a mission happen, that seems very unlikely in the near future. The first issue is that Enceladus orbits only four Saturn radii out, so is in a very deep gravitational well. Second, assuming the probe orbits with its pariapsis at Enceladus and its apoapsis at Titan, the relative speed when passing by Enceladus would be about 4 km/sec.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The distance to the Saturn system compared to Jupiter's and the deep gravity well means a lot more fuel and gravity assists, which translates into more $$. The distance and number of gravity assists also makes arrival at Saturn something like ten years after launch, which is longer that the nominal life of many of the components. The principle advantage, compared to Europa, is Enceladus squirts samples of its (presumed) ocean out into space. The large relative speed of the orbiter relative to Enceladus' surface limits the mass that can be slowed down enough to survive landing (using an ATK STAR series solid rocket motor).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was interested to read about the IceMole referenced in 1227joe's answer. However, it seems to me it would be very challenging to include one on an Enceladus mission. The power requirement for the proposed EnEx-IceMole is on the order of 10 KW. Solar flux at Saturn is only 1.1% of that at Earth and RTGs (Radioisotope Thermoelectric Generators), like those used on Cassini and New Horizons, only put out a little over 200 watts each.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The primary ways proposed to search for biomarkers (or life itself) was to sample the effluent from the tiger strips from orbit or buried in the snow just under the surface with a drill on a lander.  &lt;strong&gt;Since there would be no deep drilling, the only communication would be in data bursts from the lander when the probe passed by that would eventually be relayed to Earth.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, there was another proposed mission, TANDEM, that, initially, was planned to study both Titan and Enceladus (hence the name). However, it morphed into primarily a Titan mission with maybe some flybys of Enceladus. The coolest thing about the mission was its proposed nuclear powered Montgolfiere that would float over Titan's surface! The proposal was dropped in 2009 when a (much less expensive) Jupiter/Europa mission was chosen as the primary effort.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Link to Enceladus Study Final Report (thanks TildalWave):&lt;br&gt;&#xA;www.lpi.usra.edu/opag/Enceladus_Public_Report.pdf&lt;/p&gt;&#xA;&#xA;&lt;p&gt;ESA/NASA proposed TANDEM mission:&lt;br&gt;&#xA;&lt;a href=&quot;http://sci.esa.int/tandem-tssm/44033-tssm-nasa-esa-joint-summary-report/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://sci.esa.int/tandem-tssm/44033-tssm-nasa-esa-joint-summary-report/&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="17341" LastEditorUserId="17341" LastEditDate="2018-01-13T08:02:30.760" LastActivityDate="2018-01-13T08:02:30.760" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24435" PostTypeId="1" AcceptedAnswerId="24437" CreationDate="2018-01-13T08:35:15.943" Score="3" ViewCount="448" Body="&lt;p&gt;These are screen shots from &lt;a href=&quot;http://www.dailymotion.com/video/x5v2ny7&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;s&gt;a video of&lt;/s&gt;&lt;/a&gt; (link broken, &lt;a href=&quot;https://www.youtube.com/watch?v=wD3T1myZ6rU&amp;amp;t=190s&quot; rel=&quot;nofollow noreferrer&quot;&gt;this will have to do for now&lt;/a&gt;) the &lt;a href=&quot;http://memory-alpha.wikia.com/wiki/Assignment:_Earth_(episode)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Assignment Earth&lt;/a&gt; episode of Star Trek (TOS). It was filmed in January 1968. I would assume that the image shown on the screen is NASA file footage, and that this would be somewhere in Florida.&lt;/p&gt;&#xA;&lt;p&gt;Are all of these of LC-39? Is it clear which pad(s)? Is there any chance one can tell &lt;em&gt;which mission&lt;/em&gt; these might be of?&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zeHUz.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zeHUz.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/VG2K9.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VG2K9.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/gDE0j.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gDE0j.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fVLIX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fVLIX.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;  &lt;a href=&quot;https://i.stack.imgur.com/CmA0R.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CmA0R.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2022-01-17T09:17:53.610" LastActivityDate="2022-01-17T09:17:53.610" Title="Identify this 1960's launch site and mission?" Tags="|launch-site|saturn-v|identify-this-object|kennedy-space-center|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24436" PostTypeId="1" AcceptedAnswerId="24440" CreationDate="2018-01-13T10:13:10.970" Score="5" ViewCount="2072" Body="&lt;p&gt;It sound like standard electrical wire as you would use in earth bound transportation would have a significantly shorter life in the vacuum of space. &lt;em&gt;(See related &lt;a href=&quot;https://space.stackexchange.com/questions/24410/do-plasticizers-evaporate-in-space&quot;&gt;Do plasticizers evaporate in space?&lt;/a&gt;)&lt;/em&gt; Mostly due to the insulation becoming fragile with age. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Google is telling me that standard household wiring has a life expectancy of 40 to 80 years. With technological improvements adding to the expected, life each generation. I did not find a life expectancy for automotive wire, it seems like the cars useful life is generally less then the electrical wire. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;If I want to plan for long term (&lt;em&gt;decades or centuries&lt;/em&gt;) in space, what type of electrical wiring provides the longest life in space?&lt;/p&gt;&#xA;" OwnerUserId="109" LastActivityDate="2018-01-13T21:30:27.420" Title="What type of electrical wiring provides the longest life in space?" Tags="|materials|energy|electronics|vacuum|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24437" PostTypeId="2" ParentId="24435" CreationDate="2018-01-13T10:22:39.690" Score="12" Body="&lt;p&gt;This is a Saturn V! Although from which mission I'm a bit uncertain of. Note however in the first two images, the photos have been stretched vertically by an incredible proportion, making the vehicle look preposterously tall.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given this episode was filmed in January 1968, this places some upper bounds on what specific vehicle we're seeing here. It's either:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;SA-500F, first vertical: May 25, 1966 (at LC-39A). Fit checks.&lt;/li&gt;&#xA;&lt;li&gt;SA-500D, first vertical: ~1966. Vibration checks.&lt;/li&gt;&#xA;&lt;li&gt;SA-501 (Apollo 4), first vertical: Unknown. Launched: November 9, 1967.&lt;/li&gt;&#xA;&lt;li&gt;SA-502 (Apollo 6), first vertical: March 17, 1967. Launched: April 4, 1968.&lt;/li&gt;&#xA;&lt;li&gt;SA-503 (Apollo 8), first vertical: December 20, 1967. Launched: December 21, 1968.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;All five of these vehicles were erected on LC-39A specifically. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The unofficial Star Trek wiki has this to say about the episode:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The rocket stock footage in this episode is actually of three Saturn Vs: footage of the rocket on the ground is a combination of the SA-500F Test Vehicle (the only Saturn V to feature &quot;USA&quot; markings on the third stage) and Apollo 6 (the only Saturn V launched with a white service module). Footage of the rocket launching is of Apollo 4.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-14T02:34:32.260" LastActivityDate="2018-01-14T02:34:32.260" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24439" PostTypeId="1" CreationDate="2018-01-13T11:20:42.190" Score="4" ViewCount="197" Body="&lt;p&gt;I am trying to make an atmospheric entry/aero-assist simulation. For the propagator, I have used conventional equations of motions in planet fixed reference frame:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/2ojn7.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/2ojn7.png&quot; alt=&quot;Equations of Motion&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where the scalar states are- longitude( $\theta\ $), latitude ( $\phi\ $), radial distance(&lt;em&gt;r&lt;/em&gt;), Velocity (&lt;em&gt;V&lt;/em&gt;) Horizontal Flight Path Angle( $\gamma\ $) and vehicle heading angle ( $\psi\ $).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://hdl.handle.net/2060/19900004125&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reference: Optimal trajectories for the Aeroassisted Flight Experiment. Part 1: Equations of motion in an Earth-fixed system&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In that, if I were to neglect aerodynamic terms (L,D,Q) and obtain propagated states, they should comply with extra-atmospheric flight/ two body orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The result is decent to look at in the inertial frame but I see an Angular momentum and orbital specific energy variation of cyclic nature with variation in magnitude shown below. (for r = 4396.19, v=2.80963)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Jlxur.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Jlxur.png&quot; alt=&quot;Angular Momentum and Energy&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That's 54 $ \ km^2/s \ $ (min to max) in about 4000 seconds. If I were to make sense of this for a constant radial distance, it comes up to be a velocity addition of 0.012 km/s. Matter is worse for high elliptical orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I double checked the equations used in the solver and the transformation between frames. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So my questions are:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Can this be due to numerical approximation in MATLAB? Why is the variation so ordered?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;I understand that the equations contain terms that are 'large' and then some that are comparatively smaller in orders of magnitude. Can this be a plausible reason behind such a variation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If that is the case, what solution do I have, other than deriving canonical state variables and re-writing the differential equations?&lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;Apart from validation, I would use this propagator for an aero-assist scenario, where I would just need it for atmospheric flight regime. Is the error acceptable?&#xA;If so, The error would cause states to diverge (which I've observed for longer time intervals). If I were to use this, what maximum time interval should I consider executing this solver?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="5975" LastEditorUserId="5975" LastEditDate="2018-01-15T16:48:50.513" LastActivityDate="2018-01-15T16:48:50.513" Title="Atmospheric Entry Simulation: Numerical Inaccuracy" Tags="|orbital-mechanics|aerobraking|simulation|entry-descent-landing|analysis|" AnswerCount="0" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24440" PostTypeId="2" ParentId="24436" CreationDate="2018-01-13T12:36:53.103" Score="5" Body="&lt;p&gt;This is a rather major issue. As you pointed out, plastics etc don't cope well in space. That said, there are numerous insulator materials which will do just fine. But atomic oxygen and solar radiation degrades many substances exposed to space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But this is really only an issue for external wires that require insulation... So let's look at the challenges as detailed by NASA:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Arc tracking&lt;/li&gt;&#xA;&lt;li&gt;Offgassing&lt;/li&gt;&#xA;&lt;li&gt;Flammability&lt;/li&gt;&#xA;&lt;li&gt;Thermal decomposition&lt;/li&gt;&#xA;&lt;li&gt;Microbial growth&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Of these, arc tracking is the most relevant to cables in vacuum, and NASA have looked at the effect of ageing on this issue.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;communication wiring: in general you'd want this inside, partly for ease of maintenance, and partly for the slight additional shielding the capsule walls give against stray particles and interference. Electrical comms is expected to be replaced with optical comms in future anyway.&lt;/li&gt;&#xA;&lt;li&gt;power wiring: most of this will be internal, except for connections to solar panels etc. The insulation requirements here are relatively straightforward, and can be delivered with a range of solutions from plastics and ceramics to none at all. (Although ceramics are brittle, so have their own challenges during launch...)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The specification for cables such as the FCC connector to solar panels etc is for a 15 year exposure to atomic oxygen and 15 years elapsed exposure to the sun (10 effective years)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See &#xA;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940024194.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940024194.pdf&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="38" LastEditorUserId="38" LastEditDate="2018-01-13T21:30:27.420" LastActivityDate="2018-01-13T21:30:27.420" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24441" PostTypeId="2" ParentId="24423" CreationDate="2018-01-13T17:24:04.810" Score="9" Body="&lt;p&gt;No, you don't need &quot;at least some photons per data bit&quot;. &lt;a href=&quot;https://trs.jpl.nasa.gov/bitstream/handle/2014/44268/13-0735_A1b.pdf&quot; rel=&quot;noreferrer&quot;&gt;13 bits&lt;/a&gt; &lt;a href=&quot;http://spie.org/Publications/Proceedings/Paper/10.1117/12.2007000&quot; rel=&quot;noreferrer&quot;&gt;&lt;em&gt;per photon&lt;/em&gt;&lt;/a&gt; has been demonstrated with laser communications. You calculate the data rate capability the same way you do with any other wavelength, which is using the power, range, transmit and receive apertures, noise, modulation scheme, and coding gain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070017310.pdf&quot; rel=&quot;noreferrer&quot;&gt;This paper&lt;/a&gt; summarizes detailed analyses of the power and mass comparisons of spacecraft radios for the same data rate at Ka-band and laser comm at $1.55\,\mathrm{\mu m}$. The ground stations were equivalenced based on the cost of construction, which ended up being an array of radio dishes with an aperture equivalent to an $80.5\,\mathrm m$ antenna, and an optical telescope with a $10\,\mathrm m$ aperture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also for apples-to-apples comparison, both systems assumed the same pointing accuracy requirement for the spacecraft, with the laser terminal responsible for fine-tuning its telescope pointing with the additional accuracy needed for the smaller beam width.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The benefit is much more dramatic farther than Mars, but at maximum Mars distance, the mass of a 1 Gbps RF system would be more than twice that of a laser system, and the power required for that RF system would be 13 times the laser system. There is no question that at 1 Gbps, even at Mars, you would use a laser system.&lt;/p&gt;&#xA;" OwnerUserId="265" LastEditorUserId="265" LastEditDate="2018-01-14T04:06:40.700" LastActivityDate="2018-01-14T04:06:40.700" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24443" PostTypeId="1" AcceptedAnswerId="24447" CreationDate="2018-01-13T22:36:28.687" Score="3" ViewCount="503" Body="&lt;p&gt;I am writing a &lt;a href=&quot;https://github.com/crmathieu/Orbital&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;&lt;em&gt;3D solar system simulator&lt;/em&gt;&lt;/strong&gt;&lt;/a&gt; that features planets, asteroids, comets and Trans-Neptunian objects. It also includes daily &quot;Close Approach&quot; bodies from the Jet Propulsion Laboratory database. I am calculating planets positions using the “&lt;a href=&quot;https://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;&lt;em&gt;Keplerian Elements for Approximate Positions of the Major Planets&lt;/em&gt;&lt;/strong&gt;&lt;/a&gt;” document from JPL/Caltech with formulas valid for the 3000 BC to 3000 AD range. It looks like the calculations give me a position that is not accurate enough when dealing with close approach objects: In the simulation, the time of closest distance between earth and an object doesn’t match the theoretical time. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I also tried formulas for the 1800 AD to 2050 AD range (which are supposed to be more accurate) with no significant difference.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is: Is there a more accurate method to calculate planets positions than the method I am using.&lt;/p&gt;&#xA;" OwnerUserId="22490" LastEditorUserId="22490" LastEditDate="2018-01-14T16:50:39.000" LastActivityDate="2018-01-16T05:00:24.470" Title="Planets positions determination" Tags="|orbital-mechanics|celestial-mechanics|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24444" PostTypeId="1" AcceptedAnswerId="24445" CreationDate="2018-01-13T23:16:01.697" Score="4" ViewCount="2850" Body="&lt;p&gt;Is the website &lt;a href=&quot;http://space-track.org&quot; rel=&quot;nofollow noreferrer&quot;&gt;space-track.org&lt;/a&gt; a free service ? &lt;br&gt;&#xA;If not can someone give other free website to track space debris.&lt;/p&gt;&#xA;" OwnerUserId="22498" LastActivityDate="2018-01-13T23:21:42.670" Title="is space-track.org a free service?" Tags="|debris|tracking|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24445" PostTypeId="2" ParentId="24444" CreationDate="2018-01-13T23:21:42.670" Score="6" Body="&lt;p&gt;It is free, however, you have create an account, and the request must be approved. You have to include a reason on the account to request it. There are quite a few acceptable reasons, however, so feel free to create an account if you wish, many of which aren't really professional at all.&lt;/p&gt;&#xA;&lt;p&gt;Another service that doesn't require an account, but is quite useful, is &lt;a href=&quot;https://celestrak.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://celestrak.org/&lt;/a&gt; . They may get their information direction from the operators, so it might even be more accurate then space-track.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="-1" LastEditDate="2022-09-27T22:28:57.327" LastActivityDate="2018-01-13T23:21:42.670" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24447" PostTypeId="2" ParentId="24443" CreationDate="2018-01-14T03:09:47.373" Score="2" Body="&lt;p&gt;The &quot;more accurate method&quot; will be to either numerically integrate orbital motion to make your own approximate ephemeris (see &lt;a href=&quot;https://space.stackexchange.com/a/23409/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/a/22960/12102&quot;&gt;this&lt;/a&gt; answer) or as @Vince49 &lt;a href=&quot;https://space.stackexchange.com/questions/24443/planets-positions-determination#comment70289_24443&quot;&gt;points out&lt;/a&gt;, download and interpolate existing ephemerides, in this case for a very large number of minor bodies! Neither is a pretty or elegant solution but because gravity is long ranged and &quot;everything pulls on everything&quot;, Keplerian orbits won't do here.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Realistic solar system orbits are only approximately Keplerian.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll mention that integrating yourself is fun and educational if you really enjoy that kind of challenge, but doing it right is much more complicated than shown in those linked answers. I was able to match the JPL ephemerides to &lt;em&gt;1 km&lt;/em&gt; over &lt;em&gt;1 year&lt;/em&gt; for a dozen large bodies, but without correctly addressing the tidal effects between the Earth and Moon, the Earth's position will continue to degrade each year. You would need to do a more sophisticated calculation than the approximation shown there. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For smaller bodies, non-gravitational forces such as radiation from the sun and by the body itself, as well as outgassing when near the sun are also important. Those can be approximated as I've described in &lt;a href=&quot;https://space.stackexchange.com/q/18474/12102&quot;&gt;this question&lt;/a&gt; and in &lt;a href=&quot;https://space.stackexchange.com/a/18438/12102&quot;&gt;this answer&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So I think it is likely that sooner or later you will probably decide to use existing ephemerides. If you like to program in C, there should be documentation out there to use things like the &lt;a href=&quot;https://naif.jpl.nasa.gov/naif/toolkit.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spice toolkit&lt;/a&gt; to interpolate &lt;a href=&quot;ftp://ssd.jpl.nasa.gov/pub/eph/planets/bsp&quot; rel=&quot;nofollow noreferrer&quot;&gt;JPL kernels&lt;/a&gt;. Once you get good at it you may decide to construct less accurate much smaller tables to interpolate, considering you may need to do thousands or tens of thousands of minor bodies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;@RoryAlsop's &lt;a href=&quot;https://astronomy.stackexchange.com/a/20379/7982&quot;&gt;excellent answer&lt;/a&gt; includes several helpful links, including a Python library &lt;a href=&quot;https://pypi.python.org/pypi/jplephem&quot; rel=&quot;nofollow noreferrer&quot;&gt;jplephem&lt;/a&gt; that interpolates Spice kernels. This might be particularly helpful to you.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Python package &lt;a href=&quot;http://rhodesmill.org/skyfield/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Skyfield&lt;/a&gt; is already set up to automatically download and interpolate the JPL &lt;a href=&quot;https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris&quot; rel=&quot;nofollow noreferrer&quot;&gt;Developmental Ephemerides&lt;/a&gt;, but these are only for the major solar system bodies. Since you are already using Python, I'd guess you would really enjoy reading through that package. It's a joy to use.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the Python package &lt;a href=&quot;http://rhodesmill.org/pyephem/&quot; rel=&quot;nofollow noreferrer&quot;&gt;PyEphem&lt;/a&gt; performs a wider range of orbit functionality for you. I'm less familliar with it so you'll have to investigate yourself. It's been around quite a long time, and I have found out is based on &lt;a href=&quot;https://stackoverflow.com/a/34487339/3904031&quot;&gt;XEphem and VSOP87&lt;/a&gt; (and &lt;a href=&quot;https://stackoverflow.com/q/34530288/3904031&quot;&gt;see also&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Python packages SkyField and PyEphem are both supported by the same person. &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="265" LastEditDate="2018-01-16T05:00:24.470" LastActivityDate="2018-01-16T05:00:24.470" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24448" PostTypeId="1" AcceptedAnswerId="24450" CreationDate="2018-01-14T05:46:50.243" Score="6" ViewCount="444" Body="&lt;p&gt;Computers on board spacecraft can determine their positions using a combination of uplinked data from the ground and for those closer to Earth, GPS/GNSS. (See &lt;a href=&quot;https://space.stackexchange.com/a/5553/12102&quot;&gt;this answer&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/14737/12102&quot;&gt;this question&lt;/a&gt; for resources).&lt;/p&gt;&#xA;&lt;p&gt;The Science Daily article &lt;a href=&quot;https://www.sciencedaily.com/releases/2018/01/180111223914.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;X-ray navigation could open up new frontiers for robotic spacecraft&lt;/a&gt; describes NICER's use of timing data collected from millisecond pulsars using its X-ray telescope to generate a solution for it's orbit around Earth, without help. Of course it may use a built-in ephemeris (for the Solar System as well as pulsar rates and epochs) and Earth orbit propagation model (something like SPG4) but I believe it was &amp;quot;blindfolded&amp;quot; and asked to generate it's orbit and real-time position within it using only X-rays.&lt;/p&gt;&#xA;&lt;p&gt;NICER is currently a payload on the ISS, not technically a spacecraft itself yet, and so it really solves for the orbit and position of the ISS.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; I'm wondering if this is the first time that a civilian system in space &lt;em&gt;accurately solved&lt;/em&gt; for its own &lt;em&gt;orbit and position&lt;/em&gt; using only ephemerides and its own observations in space, without using data transmissions from Earth stations or GPS or other artificially generated navigation signals?&lt;/p&gt;&#xA;&lt;p&gt;I've mentioned &amp;quot;civilian&amp;quot; a few times because I'd like to exclude discussions of ICBMs etc. as well as inertial navigation. I've added the deep space tag because the utility of X-ray navigation is especially important far from Earth, though it could certainly be used in emergency or unusual situations in cis-lunar space.&lt;/p&gt;&#xA;&lt;p&gt;You can see several images of NICER (as well as a video) in &lt;a href=&quot;https://space.stackexchange.com/q/21737/12102&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://space.stackexchange.com/q/19882/12102&quot;&gt;this&lt;/a&gt; question.&lt;/p&gt;&#xA;&lt;p&gt;Excerpts from the article, the full article is certainly worth reading for additional information:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;In a technology first, a team of NASA engineers has demonstrated fully autonomous X-ray navigation in space -- a capability that could revolutionize NASA's ability in the future to pilot robotic spacecraft to the far reaches of the solar system and beyond.&lt;/p&gt;&#xA;&lt;p&gt;The demonstration, which the team carried out with an experiment called Station Explorer for X-ray Timing and Navigation Technology, or SEXTANT, showed that millisecond pulsars could be used to accurately determine the location of an object moving at thousands of miles per hour in space -- similar to how the Global Positioning System, widely known as GPS, provides positioning, navigation, and timing services to users on Earth with its constellation of 24 operating satellites.&lt;/p&gt;&#xA;&lt;p&gt;&amp;quot;This demonstration is a breakthrough for future deep space exploration,&amp;quot; said SEXTANT Project Manager Jason Mitchell, an aerospace technologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. &amp;quot;As the first to demonstrate X-ray navigation fully autonomously and in real-time in space, we are now leading the way.&amp;quot;&lt;/p&gt;&#xA;&lt;p&gt;This technology provides a new option for deep space navigation that could work in concert with existing spacecraft-based radio and optical systems.&lt;/p&gt;&#xA;&lt;p&gt;[...]&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Veteran's Day Demonstration&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the SEXTANT demonstration that occurred over the Veteran's Day holiday in 2017, the SEXTANT team selected four millisecond pulsar targets -- J0218+4232, B1821-24, J0030+0451, and J0437-4715 -- and directed NICER to orient itself so it could detect X-rays within their sweeping beams of light. The millisecond pulsars used by SEXTANT are so stable that their pulse arrival times can be predicted to accuracies of microseconds for years into the future.&lt;/p&gt;&#xA;&lt;p&gt;During the two-day experiment, the payload generated 78 measurements to get timing data, which the SEXTANT experiment fed into its specially developed onboard algorithms to autonomously stitch together a navigational solution that revealed the location of NICER in its orbit around Earth as a space station payload. The team compared that solution against location data gathered by NICER's onboard GPS receiver.&lt;/p&gt;&#xA;&lt;p&gt;The goal was to demonstrate that the system could locate NICER within a 10-mile radius as the space station sped around Earth at slightly more than 17,500 mph. Within eight hours of starting the experiment on November 9, the system converged on a location within the targeted range of 10 miles and remained well below that threshold for the rest of the experiment, Mitchell said. In fact, &amp;quot;a good portion&amp;quot; of the data showed positions that were accurate to within three miles.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-09-13T00:14:31.387" LastActivityDate="2021-09-13T00:14:31.387" Title="Is NICER/SEXTANT the first civilian &quot;spacecraft&quot; to determine it's own position in space without GPS or uplinked data?" Tags="|deep-space|navigation|spaceflight-firsts|nicer|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24450" PostTypeId="2" ParentId="24448" CreationDate="2018-01-14T12:02:52.113" Score="1" Body="&lt;p&gt;So far as I can tell, it is the first time it has actually been demonstrated on orbit. There are a number of other ways that have been done in at least a simulation or on paper. These include:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;a href=&quot;http://ieeexplore.ieee.org/document/371675/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Using the Earth's magnetic field&lt;/a&gt;. If one has an accurate enough map of the field, it can be used to refine the orbit. Requires the orbit to be close to Earth.&lt;/li&gt;&#xA;&lt;li&gt;Using the &lt;a href=&quot;http://dnc.tamu.edu/drjunkins/yearwise/2000/conference/AIAA_GNC_2000_Orbit_Navigation.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun direction and relative motion with the Sun&lt;/a&gt;. If one can determine where the Earth is as well, the accuracy goes up considerably.&lt;/li&gt;&#xA;&lt;li&gt;Most rocket providers (At least the ones that I am familiar with) know where they are in orbit without using GPS, using the on board IMUs. These of course only work for the initial orbit, and will fade away eventually, but they can be used as a starting point.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-14T12:02:52.113" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24451" PostTypeId="1" CreationDate="2018-01-14T12:52:07.637" Score="7" ViewCount="286" Body="&lt;p&gt;In &lt;a href=&quot;https://arstechnica.com/science/2017/12/apollo-guts/&quot; rel=&quot;noreferrer&quot;&gt;Ars' series on the Apollo program&lt;/a&gt; Eric tells a story about the Lunar Module from NASA lore...&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;...recounted in Apollo: The Race to the Moon, a technician working with a test version of the vehicle absentmindedly clicked the cap of his ballpoint pen against a filled fuel tank. This minute amount of pressure opened a small leak in the tank, and the thin stream of propellant sliced off his finger with surgical precision.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Did this really happen? And if so how was the tank designed to be strong enough to withstand take-off, but so weak another way?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.amazon.co.uk/dp/B006Q2D5DK/ref=dp-kindle-redirect?_encoding=UTF8&amp;amp;btkr=1&quot; rel=&quot;noreferrer&quot;&gt;Tom Kelly's book&lt;/a&gt; talks a lot about the weight saving efforts and the strict QA around the tanks due to the danger they could pose (going so far as to reject a tank that passed all the tests because it was inexplicably off-colour!) but it seemed this was precisely so they would be anything but fragile..&lt;/p&gt;&#xA;" OwnerUserId="6173" LastActivityDate="2018-01-14T14:18:57.140" Title="Did a fuel tank ruptured by a ballpoint pen really sever an engineers finger?" Tags="|apollo-program|history|design|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24452" PostTypeId="1" CreationDate="2018-01-14T13:31:49.680" Score="1" ViewCount="102" Body="&lt;p&gt;Does anyone know of any company that is using CubeSats for telecommunication services or is this a new idea not yet implemented into the commercial arena?&lt;/p&gt;&#xA;" OwnerUserId="22509" LastEditorUserId="6816" LastEditDate="2018-01-14T13:58:03.750" LastActivityDate="2018-01-14T15:02:59.957" Title="Telecommunication CubeSats" Tags="|low-earth-orbit|cubesat|communication-satellite|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24453" PostTypeId="1" CreationDate="2018-01-14T13:59:34.460" Score="0" ViewCount="39" Body="&lt;p&gt;Specifically, a nuclear fusion rocket. And no pulse propulsion, because I know doing that will be very hard.&lt;/p&gt;&#xA;" OwnerUserId="21504" LastEditorUserId="21504" LastEditDate="2018-01-14T15:01:58.037" LastActivityDate="2018-01-14T15:01:58.037" Title="How can a nuclear spacecraft be in compliance with the Test Ban Treaty?" Tags="|nuclear|advanced-propulsion|" AnswerCount="0" CommentCount="11" ClosedDate="2018-01-14T14:53:45.903" ContentLicense="CC BY-SA 3.0" />
  <row Id="24454" PostTypeId="2" ParentId="24451" CreationDate="2018-01-14T14:18:57.140" Score="2" Body="&lt;p&gt;The tanks of the LM descent module seem to have been designed for a pressure of up to &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730011150.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;300 psi&lt;/a&gt; (20 bar). Water jet cutters operate at pressures &lt;a href=&quot;https://en.wikipedia.org/wiki/Water_jet_cutter&quot; rel=&quot;nofollow noreferrer&quot;&gt;100 times higher&lt;/a&gt;. 20 bar is lower than today's pressure washers. Those can reach 200 bar, and that's enough to cut off a finger, &lt;a href=&quot;http://www.tractorbynet.com/forums/safety/127906-pressure-washer-dangers-2.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;it seems.&lt;/a&gt; Whether 20 bar would be enough? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.scribd.com/document/46941424/Apollo-Experience-Report-Ascent-Propulsion-System&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wall thickness&lt;/a&gt; of the ascent engine propellant lines was 1.2 mm (0.049&quot;), these ran at similar pressures. (couldn't find wall thickness of the tanks). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's certainly possible to design very thin structures for use at high pressure, close to their breaking strength. A slight increase in pressure can overstress the tank and create a leak. Compare to a balloon: a tiny puncture can rip the balloon apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I haven't been able to find other sources for the story though. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-01-14T14:18:57.140" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24455" PostTypeId="1" CreationDate="2018-01-14T14:58:37.753" Score="4" ViewCount="1303" Body="&lt;p&gt;Best means maximum thrust, safest, easy to maintain. And it's not opinion based because I'm looking for a way that's theoretically feasible and may be practical in a few years. However, if you have an idea of your own (howsoever wacky it might be, because you never know if that's the best way in the future), please tell me about it.&lt;/p&gt;&#xA;" OwnerUserId="21504" LastActivityDate="2018-10-18T22:24:15.923" Title="What will be the best way to convert nuclear fusion energy into thrust for a rocket?" Tags="|nuclear|advanced-propulsion|" AnswerCount="5" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24456" PostTypeId="2" ParentId="24452" CreationDate="2018-01-14T15:02:59.957" Score="3" Body="&lt;p&gt;It depends on what you mean by Telecom.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Amsat has a two-way radio transponder cubesat, called &lt;a href=&quot;https://www.amsat.org/meet-the-fox-project/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fox.&lt;/a&gt; , for amateur radio operators.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.spacequest.com/ais-payloads/sts-300-ais-receiver&quot; rel=&quot;nofollow noreferrer&quot;&gt;AIS signals&lt;/a&gt; from ships can be received and retransmitted.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are other applications that are on going as well, although these are the two projects with which I am familiar.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-14T15:02:59.957" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24458" PostTypeId="1" AcceptedAnswerId="40776" CreationDate="2018-01-14T15:45:15.897" Score="5" ViewCount="224" Body="&lt;p&gt;Suppose that I am launching a cubesat into space. Let the cubesat be launched from the P-POD deployer. Since the satellite will have a huge velocity, hence the detumbling mode will be operating. As soon as the cubesat has completely detumbled, I wish to determine the initial quarternion of the cubesat with respect to an inertial frame of reference. How should I go about it? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I cannot determine it using TRIAD algorithm as the satellite will just immediately be launched, hence the sun sensors and magnetometers might not be active by the time the satellite has detumbled. Also how will I find the relation between the satellite frame of reference and the inertial frame of reference?&lt;/p&gt;&#xA;" OwnerUserId="21017" LastEditorUserId="9060" LastEditDate="2018-01-14T16:26:29.197" LastActivityDate="2020-01-07T22:42:30.873" Title="Initial attitude determination after deployment" Tags="|attitude|cubesat|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24459" PostTypeId="2" ParentId="24455" CreationDate="2018-01-14T16:18:11.300" Score="10" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)&quot; rel=&quot;noreferrer&quot;&gt;Exploding thermonuclear bombs&lt;/a&gt; behind a big, thick plate, with the payload on giant shock absorbers behind that, referred to as Project Orion, is practical today, and has been for decades.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is no other practical way to generate net energy from fusion, and there won't be for decades.&lt;/p&gt;&#xA;" OwnerUserId="265" LastActivityDate="2018-01-14T16:18:11.300" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24461" PostTypeId="1" AcceptedAnswerId="24640" CreationDate="2018-01-14T17:17:26.107" Score="5" ViewCount="340" Body="&lt;p&gt;How would you be able to extract aluminium and iron in the moon for example? Is there some electrolytic technique that does not use carbon?&lt;/p&gt;&#xA;" OwnerUserId="16627" LastActivityDate="2018-01-26T18:01:01.270" Title="What would be the way to extract iron and aluminium in space without carbon?" Tags="|materials|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24463" PostTypeId="1" CreationDate="2018-01-14T21:48:58.723" Score="1" ViewCount="1710" Body="&lt;p&gt;I've read from a page that you can find the distance from the Sun to the Earth via the following equation:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$r = a\frac{1-e^2}{1+e\cdot \cos(θ)}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;where r is the distance from the Sun, a is the semi-major axis, e is the eccentricity and θ is the angle around from perihelion. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I am trying to create a model of the orbit of Earth around the Sun and I have to find the distance between the Earth and the Sun at multiple points and also its speed.&#xA;Can anyone explain the maths?&lt;/p&gt;&#xA;" OwnerUserId="22514" LastEditorUserId="6816" LastEditDate="2018-07-08T20:28:41.730" LastActivityDate="2018-07-08T20:28:41.730" Title="Finding the true anomaly" Tags="|orbital-mechanics|earth|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="24464" PostTypeId="1" CreationDate="2018-01-14T22:20:22.970" Score="0" ViewCount="252" Body="&lt;p&gt;I need some help on my science experiment. I have tried researching it on the internet but nothing seemed to have answered my question. I am currently doing a science fair project on &quot;How does Rocket Fuel affect Rockets&quot;. By that I mean any variables such air resistance, weight, drag, thrust,and lift. I'm wondering if I use different rocket fuels, if it will have an impact on the previous variables. I  am more than happy to receive any information regarding rockets.&lt;/p&gt;&#xA;" OwnerUserId="22515" LastEditorDisplayName="user20636" LastEditDate="2018-01-14T22:50:03.553" LastActivityDate="2018-01-14T23:40:11.873" Title="What affects the flight of rockets?" Tags="|rockets|experiment|science|" AnswerCount="2" CommentCount="3" ClosedDate="2018-01-15T12:45:35.930" ContentLicense="CC BY-SA 3.0" />
  <row Id="24466" PostTypeId="2" ParentId="24455" CreationDate="2018-01-14T22:38:20.300" Score="0" Body="&lt;p&gt;Theoretically whatever might be done to make controlled fusion work in a reactor could be adapted to a rocket. &#xA;One fusion reactor approach is to detonate micro pellets of deuterium/ tritium in a cross fire of lasers. If the reactor is open to space on one end, the resulting plasma would exit out and create thrust. It could be diluted with additional reaction mass to increase thrust and cool the chamber walls. &#xA;A fusion reactor in a magnetic bottle could become a rocket by bleeding some plasma out a magnetic nozzle. Again adding reaction mass to the plasma would increase thrust.  &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2018-01-14T22:38:20.300" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24467" PostTypeId="2" ParentId="24464" CreationDate="2018-01-14T22:59:22.997" Score="2" Body="&lt;p&gt;The choice of rocket fuels directly affect the weight of a rocket, due to the difference in efficiency, measured in &lt;a href=&quot;https://en.wikipedia.org/wiki/Specific_impulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;ISP&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on the choice of fuel, the size of engine will change to produce the required thrust - this will affect the weight, and potentially the aerodynamic cross-section of the vehicle, which will affect the lift and drag.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The choice of fuel will affect the size of the storage tanks, and what they are made of, and whether then need insulation - this will affect the weight and aerodynamic properties of the vehicle, in turn affecting the lift and drag.&lt;/p&gt;&#xA;" OwnerDisplayName="user20636" LastEditorUserId="429" LastEditDate="2018-01-14T23:00:04.527" LastActivityDate="2018-01-14T23:00:04.527" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24468" PostTypeId="2" ParentId="24464" CreationDate="2018-01-14T23:40:11.873" Score="1" Body="&lt;p&gt;This question has massive scope, but it is clear that the OP is at a very novice level, so i will attempt answer at this level.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The typical way rocket fuels are compared is by considering their &lt;em&gt;specific impulse&lt;/em&gt;.  Different propellant/oxidizer combos only offer a certain amount of specific impulse, which ultimately affects the maximum possible velocity achievable. In general, solid propellants have lower specific impulse than liquid propellants.  However, liquid propellants are more complicated to create (pumps, cooling, slosh control, etc.) which adds weight and complexity to the overall system compared to solid propellant systems.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You may want to look at Tsiolkovsky’s rocket equation for a simple estimate of how propellant mass and specific impulse can be used to estimate the maximum velocity acheivable for a given rocket (ignoring drag &amp;amp; gravity losses).&lt;/p&gt;&#xA;" OwnerUserId="6494" LastActivityDate="2018-01-14T23:40:11.873" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24469" PostTypeId="2" ParentId="22554" CreationDate="2018-01-15T01:05:16.367" Score="8" Body="&lt;p&gt;You might be interested in the work of &lt;a href=&quot;https://libre.space&quot; rel=&quot;noreferrer&quot;&gt;Libre Space Foundation&lt;/a&gt; it's a non-profit organization that develops open-source technologies (they are based in Greece).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;LSF builds several open-source projects for space applications. Including &lt;a href=&quot;https://satnogs.org&quot; rel=&quot;noreferrer&quot;&gt;SatNOGS&lt;/a&gt; a network of satellite ground-stations and has also build &lt;a href=&quot;https://upsat.gr&quot; rel=&quot;noreferrer&quot;&gt;UPSat&lt;/a&gt; the first open-hardware and software satellite currently in orbit. They run an open-source technologies event together with ESA and the Librecube project &lt;a href=&quot;https://oscw.space&quot; rel=&quot;noreferrer&quot;&gt;OSCW&lt;/a&gt;. They maintain a forum for open-source space technologies at their &lt;a href=&quot;https://community.libre.space&quot; rel=&quot;noreferrer&quot;&gt;community website&lt;/a&gt;. And you might want to check out their &lt;a href=&quot;https://gitlab.com/librespacefoundation/&quot; rel=&quot;noreferrer&quot;&gt;gitlab pages&lt;/a&gt; for more projects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;PS: I'm actively involved in their organization&lt;/p&gt;&#xA;" OwnerUserId="6674" LastActivityDate="2018-01-15T01:05:16.367" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24471" PostTypeId="1" AcceptedAnswerId="24486" CreationDate="2018-01-15T01:36:30.693" Score="5" ViewCount="524" Body="&lt;p&gt;&lt;a href=&quot;https://satnogs.org/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SatNOGS&lt;/a&gt;, the Satellite Networked Open Ground Station (mentioned &lt;a href=&quot;https://space.stackexchange.com/a/24469/12102&quot;&gt;in this answer&lt;/a&gt;) appears to be a distribution of independently maintained amateur DIY satellite receiving ground stations, linked by a single network.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How is it run? Do members log in via the internet and then reserve a time slot on whichever station they would like to use? I noticed that there are amateur radio callsigns associated with the stations. Does this suggest that &lt;em&gt;uplink&lt;/em&gt; capabilities are available in addition to downlink?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1mDFM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1mDFM.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-16T04:17:48.367" LastActivityDate="2018-01-16T04:17:48.367" Title="How does the Satellite Networked Open Ground Station operate? How is it used?" Tags="|cubesat|radio-communication|ground-station|public-involvement|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24472" PostTypeId="2" ParentId="24223" CreationDate="2018-01-15T09:06:21.153" Score="0" Body="&lt;p&gt;Other way to think: The modules were designed to fit into the shuttle - had it been smaller, the modules itself would probably been designed smaller (so still to fit the shuttle) - perhaps limiting its use, but had the shuttle been larger, I'd bet my money the modules (at least some of them) might even been larger...&lt;/p&gt;&#xA;" OwnerUserId="17398" LastActivityDate="2018-01-15T09:06:21.153" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24474" PostTypeId="2" ParentId="24455" CreationDate="2018-01-15T09:25:10.523" Score="6" Body="&lt;h3&gt;The current situation about the fusion&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;Today, there is no working fusion reactor today which could produce energy. The current seemingly best prototype is the &lt;a href=&quot;https://en.wikipedia.org/wiki/ITER&quot; rel=&quot;noreferrer&quot;&gt;ITER&lt;/a&gt; reactor, which still won't produce energy for the European energy network, but it will produce at least more energy (from fusion) as it takes (for maintaining plasma). It is currently under production, the first plasma reaction is planned for 2025 (which is already delay to the originally planned 2018).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;After the ITER demonstrated the capability to produce fusion energy, the next planned reactor (&lt;a href=&quot;https://en.wikipedia.org/wiki/DEMOnstration_Power_Station&quot; rel=&quot;noreferrer&quot;&gt;DEMO&lt;/a&gt;, existing currently as preliminary plans) will already produce electricity for the continental power system. This is estimated for 2040.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note, the ITER tokamak weighs 23000 tons, and there is no reason to think that the DEMO will be lighter. Compare this to the &lt;a href=&quot;https://en.wikipedia.org/wiki/International_Space_Station&quot; rel=&quot;noreferrer&quot;&gt;ISS&lt;/a&gt;, which is the most massive thing currently in space, with its 450 tons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Thus, after the DEMO is ready and working, probably it will require further decades to miniaturize the reactor and making it space-capable.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note, the only currently working industrial fusion doesn't fuse Hydrogen. It uses Lithium and Deuterium, produces partially Tritium from them, and then uses this Tritium to create fusion.&lt;/p&gt;&#xA;&#xA;&lt;h3&gt;Current situation about the nuclear-capable space propulsion&lt;/h3&gt;&#xA;&#xA;&lt;p&gt;The mass of the decay products of the nuclear energy is very small, compared to the chemical rockets. This rules out the current chemical rocket-like solutions (although there are some &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket&quot; rel=&quot;noreferrer&quot;&gt;ideas&lt;/a&gt; even for that).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Having the very high energy level and the very small fuel mass, ion drives are the most obvious choice. Between them, &lt;a href=&quot;https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket&quot; rel=&quot;noreferrer&quot;&gt;VASIMR&lt;/a&gt; has the most fruitful prototype.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;VASIMR is essentially a very effective ion engine. If it gets (electrical) energy and it has fuel, it can work with an effectivity around 70%. It can get its electric input from batteries, from solar power or from a fusion reactor.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is currently a working VASIMR prototype which could reduce the orbital boost costs of the ISS to the 5% of the today, and there was a plan to install it on the ISS in 2015. The NASA stepped back from it in the last moment, on unclear  reasons. Their this decision makes probable, that the &lt;a href=&quot;https://space.stackexchange.com/q/24382/1054&quot;&gt;ISS will be simply crashed into the Pacific&lt;/a&gt; sometimes in the 2020s, for example on cost reasons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.adastrarocket.com/aarc/&quot; rel=&quot;noreferrer&quot;&gt;company behind the VASIMR&lt;/a&gt; still works for the NASA, although it is dubious how can they do any useful, considering that currently there is no plan to use VASIMR anywhere.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;A fusion reactor, converting Lithium and Deuterium to Helium4, Hydrogen and energy, driving VASIMR drives seem the most realistic outcome.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It couldn't run before 2060 even in the best scenario.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the latest decades, the developments in the space exploration, the actually  realized scenarios were more near to the worst.&lt;/p&gt;&#xA;" OwnerUserId="1054" LastEditorUserId="1054" LastEditDate="2018-01-15T09:31:21.180" LastActivityDate="2018-01-15T09:31:21.180" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24475" PostTypeId="1" AcceptedAnswerId="24476" CreationDate="2018-01-15T11:34:39.713" Score="5" ViewCount="1479" Body="&lt;p&gt;From the book &lt;a href=&quot;http://www.nytimes.com/2010/08/09/books/excerpt-packing-for-mars.html&quot; rel=&quot;noreferrer&quot;&gt;Packing for Mars&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An entire odd universe of mock outer space has grown up here on&#xA;  earth. Capsules that never blast off; &lt;strong&gt;hospital wards where healthy&#xA;  people spend months on their backs, masquerading zero gravity&lt;/strong&gt;;&#xA;  crash labs where cadavers drop to earth in simulated splash-downs.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What does &quot;masquerading zero gravity&quot; do? Does it have any relation with masquerade ball?&lt;/p&gt;&#xA;" OwnerUserId="12270" LastEditorUserId="12270" LastEditDate="2018-01-16T03:32:04.147" LastActivityDate="2018-01-16T03:32:04.147" Title="What does &quot;masquerading zero gravity&quot; mean?" Tags="|mars|microgravity|health|simulation|language|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24476" PostTypeId="2" ParentId="24475" CreationDate="2018-01-15T12:14:03.000" Score="9" Body="&lt;p&gt;'masquerading' is a bit of a misnomer. Lying in bed simulates some of the effects that zero gravity has on the body. &lt;a href=&quot;https://www.popsci.com/military-aviation-amp-space/article/2009-06/pillownaut-stays-bed-sake-science&quot; rel=&quot;nofollow noreferrer&quot;&gt;This article&lt;/a&gt; has a decent introduction:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;NASA needs more volunteers for its bed-rest study, which helps doctors understand what a lack of gravity does to the body.&lt;/p&gt;&#xA;&lt;p&gt;In the lunar study, you lie down for 21 days, with your body tilted so your feet support one-sixth of your weight, which mimics gravity on the moon. The longer-term study simulates the lack of gravity you would experience in a space station sojourn or while traveling to Mars. In that study, Archuletta lay in bed for 50 days, with her head declined, six degrees below the rest of her body.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;A hospital ward is a good place to run these tests, because they're already rigged with monitoring equipment. If you run experiments on multiple subjects, it makes sense to stick them in the same ward.&lt;/p&gt;&#xA;&lt;p&gt;Effects of lying down:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Lying on your back for long periods simulates what happens to your muscles and bones in space. When you have no gravity pulling you down, your muscles don't have to work hard to move you across the room, and your bones don't have much weight to support, so they weaken.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The results of these studies can be found on &lt;a href=&quot;https://ntrs.nasa.gov/?N=0&amp;amp;Ntk=All&amp;amp;Ntt=bedrest&amp;amp;Ntx=mode%20matchallpartial&amp;amp;Nm=123%7CCollection%7CNASA%20STI%7C%7C17%7CCollection%7CNACA&quot; rel=&quot;nofollow noreferrer&quot;&gt;NTRS&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;crash labs where cadavers drop to earth in simulated splash-downs&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;That sounds like a crash test for a landing capsule, in this case &lt;a href=&quot;https://www.universetoday.com/15691/nasas-use-of-cadavers-to-test-the-orion-capsule/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Orion&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-15T15:27:13.710" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24477" PostTypeId="2" ParentId="24455" CreationDate="2018-01-15T15:11:27.577" Score="3" Body="&lt;p&gt;Hard to say if it's &lt;em&gt;best&lt;/em&gt;, but it's certainly both very promising, very efficient - both fantastic specific impulse, and very little fuel wasted (unlike most fusion based propulsions where only small percentage of the fuel undergoes fusion, here nearly all deuterium and tritium is used up; resulting alpha particles being the propellant). Also, unfortunately fantastically expensive: &lt;a href=&quot;https://space.stackexchange.com/a/24238/103&quot;&gt;Muon-catalyzed fusion&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike proposals like Project Orion, or ITER, it scales very well, working equally well for nanosatelites as small as 6U cubesats and craft as big as generation ships (...although the cost scales linearly; for bigger craft other proposals may be more economically viable). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The one bad problem is it depends on anti-matter as catalyst of the fusion. Unlike in annihilation, where one positron + one electron produce two photons, the end, here one positron can catalyze fusion of several hundred and more of deuterium or tritium atoms, releasing much more energy than its own annihilation would produce. That makes requirements for the amount of anti-matter (or more specifically, an isotope of a common element, that produces positrons on radioactive decay) needed to run quite small. Quite small still doesn't mean insignificant though, and production of these isotopes is awfully expensive. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Let me add what way of converting the fusion energy into propulsion would be objectively best, disregarding the source of the fusion and the current technology readiness level:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Direct.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As two atoms/particles undergo fusion (e.g. deuterium+tritium turning into helium), their products are extremely hot. If you are able to focus these products into an exhaust nozzle and just eject them, you're getting the optimal propulsion - least losses, the highest exhaust momentum possible, shedding inert mass most rapidly. Anything else will be more lossy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, &lt;em&gt;how&lt;/em&gt; to direct the fusion products into the nozzle (and not vent all the substrates simultaneously) is one challenge on par with achieving sustainable fusion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;sub&gt;BTW, the fusion doesn't need to be energy-positive! You can deliver a surplus energy, say, beaming it, lose it on causing fusion, produce less through fusion than you lost, and still if you can eject the helium in a coherent 'exhaust flame' you're good - Fusion is a fantastic way of accelerating particles to huge speeds, which is an essential last step of any reaction drive, completely regardless of the source of power!&lt;/sub&gt;&lt;/p&gt;&#xA;" OwnerUserId="103" LastEditorUserId="103" LastEditDate="2018-10-18T22:24:15.923" LastActivityDate="2018-10-18T22:24:15.923" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24478" PostTypeId="2" ParentId="21294" CreationDate="2018-01-15T17:09:21.687" Score="6" Body="&lt;p&gt;There are basically 3 means of tracking satellites:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Radar- Can hide from this, see anything about stealth aircraft out there (Same principals)&lt;/li&gt;&#xA;&lt;li&gt;Visible- To hide from visible, it would basically be necessary to absorb any light for anything that might be pointed to the ground. This would heat the satellite up, and might make it less useful. In theory one could dissipate the heat upward, but this would make it easy to spot by IR searching satellites.&lt;/li&gt;&#xA;&lt;li&gt;Infrared- The ideal way of achieving this is to make it as cool as possible. This is challenging, to say the least, but tends to work best by reflecting most light.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;As a result, it is very difficult to hide from both Visible and IR. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The truth is, it's very difficult to hide a satellite. But to use any of these detection methods, it helps to have at least a rough idea of where it is. The best thing you can do to hide a satellite is to not let it be known where it is, and make it as stealthy as you can.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This was done previously with the &lt;a href=&quot;https://www.space.com/637-anatomy-spy-satellite.html&quot; rel=&quot;noreferrer&quot;&gt;Misty satellites&lt;/a&gt;. Of course, these were identified by amateurs eventually, but it did take some time.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-15T17:09:21.687" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24479" PostTypeId="2" ParentId="23626" CreationDate="2018-01-15T18:24:33.067" Score="0" Body="&lt;p&gt;There are limits of how large a body can be without being &quot;round&quot;. The limits depend on &lt;a href=&quot;https://www.spaceanswers.com/space-exploration/does-the-us-own-the-moon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;what type of materials&lt;/a&gt; make up the object. For an icy object, the size is about 400 km, for a rocky object, it is closer to 600 km. Note that Hyperion is mostly ice, and about 135 km at it's widest dimension.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-15T18:24:33.067" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24480" PostTypeId="1" AcceptedAnswerId="24482" CreationDate="2018-01-15T18:40:23.693" Score="23" ViewCount="4178" Body="&lt;p&gt;Being a secret government launch, there has been speculation maybe the Zuma satellite wasn't lost, and claims to the contrary are a ruse to throw off parties who'd like to track it. There is possible precedent for this, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Misty_(satellite)&quot; rel=&quot;noreferrer&quot;&gt;Misty satellites&lt;/a&gt;, one of which was &lt;a href=&quot;https://www.space.com/637-anatomy-spy-satellite.html&quot; rel=&quot;noreferrer&quot;&gt;reported lost&lt;/a&gt; a few weeks after launch. However, amateur astronomers &lt;a href=&quot;http://www.armaghplanet.com/blog/the-real-mystery-satellites.html&quot; rel=&quot;noreferrer&quot;&gt;tracked an object&lt;/a&gt; that could have been that satellite. From the link:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A week after launch, reports were released from the Soviets that six&#xA;  bits of debris had been detected suggesting an explosion had occurred.&#xA;  The Pentagon announced that any debris would decay after six weeks.&#xA;  The amateur astronomers and observers that were tracking this object&#xA;  only catalogued five out of the six pieces. Six months later an&#xA;  unidentified satellite was discovered in orbit on a similar trajectory&#xA;  to that of the classified payload was released, leading the satellite&#xA;  spotters to suspect it was the missing piece, nick-named Misty.&#xA;  However a couple of noticeable manoeuvres later, Misty disappeared&#xA;  again. Perhaps the ‘explosion’ was a decoy to put Misty into place&#xA;  unbeknownst to the Russians.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Taking the general idea of hiding a satellite by reporting it lost after changing its orbit, what are the limits on how successful such an approach can be? How much could its orbit plausibly have been changed? Would searchers need to scan the whole sky or could they still narrow the search to certain latitudes and orbits? To what degree is it plausible that the search could be hampered by reducing the satellite's visibility?&lt;/p&gt;&#xA;" OwnerUserId="4660" LastEditorUserId="12102" LastEditDate="2020-06-05T00:58:17.013" LastActivityDate="2020-06-05T00:58:17.013" Title="Suppose Zuma wasn't &quot;lost&quot; — what would it take to detect it?" Tags="|tracking|security|classified|detection|zuma|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24481" PostTypeId="2" ParentId="23935" CreationDate="2018-01-15T18:47:23.307" Score="2" Body="&lt;p&gt;Parsing information from &lt;a href=&quot;https://en.wikipedia.org/wiki/Electrodynamic_tether&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;. Space tethers are extremely slow, and are somewhat unweildly. The main thought has been for use to deorbit them. In order to make them work, one has to have a very long tether.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give a few numbers, as seen from &lt;a href=&quot;http://www.daviddarling.info/encyclopedia/P/ProSEDS.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this site&lt;/a&gt;, a tether to raise the ISS would be about 25 km long. It would produce a continual 0.5-0.8 N of thrust. But the chance of a collision is much higher for such a long tether, and it makes operations more difficult.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the effect stops working at about 1500 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All in all, it might be able to save a bit of energy, but it hasn't been well tested. It has a lot of potential, but also a fair bit of risk, with such long thin structures dramatically increasing the likelihood of space collisions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The break even point is actually probably fairly low. It could be used to save fuel for most of the low altitude satellites, like Spy Satellites and the ISS, that have a significant deorbit thrust that shows up over time. I don't have an exact number, but think large LEO satellites in general.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-15T18:47:23.307" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24482" PostTypeId="2" ParentId="24480" CreationDate="2018-01-15T19:05:29.490" Score="20" Body="&lt;p&gt;First of all, what do we know about it's orbit? From the direction that the rocket took off, we know it's inclination was about 50 degrees. That could be adjusted higher in flight at a time that is difficult, but that's probably about right. That information and the launch time effectively confines the orbit into a plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Less known is the altitude. The altitude makes two primary differences. The first is the tendency to precess depends on the altitude. The effect is that the plane of the satellite will tend to rotate with time. The second, and far more noticeable, is that the location within the plane will change considerably given a small variation in speed. As altitude is speed, that can vary quite a bit. From the observed fuel dumping maneuver, it is assumed that the satellite is at the higher range, around 900-1000 km, but that is a bit difficult to verify. If that was incorrect, the altitude could be as low as 400 km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Next, how can amateurs find it? The best way is to look at the visible light. The easiest time to see that is when the satellite is in the light, but it is local night time. For most LEO satellites, this window is about an hour and a half from sunset/ sunrise. It's really hard to say how bright it will be, because we don't know much about it, but likely it will be at the edge of visibility, maybe magnitude 4-7. The time where it will pass overhead in the light, but with Earth in dark, makes it very difficult to observe for the first week after launch, at least for most of the amateur classified satellite observing groups.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A nodal precession of a satellite's orbit can take a considerable time. The orbital altitude has a large range, 400-1000 km orbit. Most likely it will tend towards the 1000 km range, but 400 km is at least possible. Using the &lt;a href=&quot;http://www.calctool.org/CALC/phys/astronomy/earth_orbit&quot; rel=&quot;noreferrer&quot;&gt;following&lt;/a&gt; &lt;a href=&quot;https://www.fxsolver.com/solve/&quot; rel=&quot;noreferrer&quot;&gt;calculators&lt;/a&gt;, one can get a fair idea of the nodal precession. If I've done this right, the precession rate varies between 5.58 deg/day and  3.92 deg/day for the provided orbital range. That means the drift rate between the two is 1.66 deg/day.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so how can one find Zuma today? Essentially, we know the plane relatively certainly, with the plane possibly having drifted 10 degrees between worst case assumptions. More worrisome is that we don't know at all where in the plane the satellite might be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Okay, so what does all of that mean? The best bet to finding Zuma is to search along the entire plane of the orbit, and assume a few related planes as well. If it isn't found within a few weeks, it will become increasingly more difficult to spot at all, and if it isn't found in a few months, then it is effectively lost.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If an initial observation can be made of the satellite, then one can narrow the altitude considerably, which will improve the likelihood of observation for all future dates, even if it does have significant propulsion capabilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If it isn't observed within a few weeks, or even a few months, one can keep looking for unknown satellites with the 50 degree inclination. That is going to be somewhat difficult to do, but it might turn up at some point in time. There aren't a huge number of classified satellites known to be at that inclination, so it would be more likely to stick out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Best bet, if you want to find it, know if it was found, etc, take a look at &lt;a href=&quot;http://www.satobs.org/seesat/Jan-2018/index.html&quot; rel=&quot;noreferrer&quot;&gt;SeeSat&lt;/a&gt;, which tracks classified satellites. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-16T14:20:05.197" LastActivityDate="2018-01-16T14:20:05.197" CommentCount="13" ContentLicense="CC BY-SA 3.0" />
  <row Id="24483" PostTypeId="2" ParentId="23374" CreationDate="2018-01-15T20:11:57.630" Score="2" Body="&lt;p&gt;I just saw a documentary about it. According to it the Apollo astronauts did know Luna 15 was near them and I think it said some of the astronauts finishing their moon walk and getting ready to leave saw it flying around as it did something like 42 orbits Of the moon while our astronauts were doing the walk. I’m not sure if they saw the crash but they def saw the spacecraft orbiting around them which just had to be crazy i imagine. And as someone kinda mentioned the US and USSR even talked to each other about their missions. They both knew it was happening and made plans so their radios and channels wouldn’t interfere with each other’s and to make sure the trajectories were not going to interfere with each other’s either. It was pretty cool for them at that time to put the Cold War on the back burner so to speak. One famous cosmonaut said the whole USSR space program for one moment forgot we were different countries in a war and they all cheered and applauded our astronauts landing on the moon as one human race Rather than Americans vs. Russians.. which if just awesome  &lt;/p&gt;&#xA;" OwnerUserId="22520" LastActivityDate="2018-01-15T20:11:57.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24484" PostTypeId="2" ParentId="23259" CreationDate="2018-01-16T00:10:01.067" Score="-1" Body="&lt;p&gt;Would it not be possible to use semiconductor sandwiches using the Seebeck-Peltier (thermoelectric) effect to derive heat from the excess heat?  Pump the warmed air into a holding container (or use an air conditioner to cool the air, which unit then expels much hotter air out its exhaust) to collect and concentrate the heat, which could then be used for various commercial and industrial processes--clothes washing, cooking, waste treatment, kilning, preheating of blast furnaces, etc.  All that is needed for a thermoelectric effect is a temperature difference, and if the gradient is not strong, then the efficiency will be low, but it will still produce a nonzero amount of electricity, which can be used to supplement or replace the output of the main generator or reactor, allowing it to be run less often or for reduced duration, thus lowering the total heat output.  Eventually, the system (including human consumers of ambient heat in water and air) would reach an energy-heat economic equilibrium, even if it never reaches a classical &quot;thermal equilibrium.&quot;  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most likely use to which the colony will put the &quot;excess&quot; heat is in expansion of the colony by pre-conditioning as-yet-empty lavatubes, heating the perimeter and the pressurized CO2 they have pumped into it.  Add in some drip irrigation, sunlamps (once these reach sufficient output density, you can redirect the heat), and soil/hydroponic vats, and add seedlings.  Come back in a month to bleed off the excess oxygen, rinse and repeat.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I daresay that a colony determined to enact both self-sufficiency and expansion will always find a use for excess heat, to the extent that Mars may even have to import heat from Earth (/humor/).  &lt;/p&gt;&#xA;" OwnerUserId="22523" LastActivityDate="2018-01-16T00:10:01.067" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24485" PostTypeId="2" ParentId="23374" CreationDate="2018-01-16T01:13:57.093" Score="2" Body="&lt;p&gt;Almost certainly not.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;First of all, &lt;a href=&quot;https://astrobob.areavoices.com/2016/05/18/can-astronauts-see-stars-from-the-space-station-the-moon/&quot; rel=&quot;nofollow noreferrer&quot;&gt;only very bright stars were visible&lt;/a&gt;, when standing in the shadow of the LEM. The size of it was quite small, so most likely it would be too dim to view.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The deorbit burn took place at 15:47 UT on 21 July 1969. It crashed about 17° north latitude and 60° east. The difference in longitude is about 37 degrees, meaning it wasn't overhead during the burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It could have happened, but it would have required a dedicated look, or extremely good luck. As there is no record of such having happened...&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-16T01:13:57.093" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24486" PostTypeId="2" ParentId="24471" CreationDate="2018-01-16T02:01:04.663" Score="3" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;members log in via the internet and then reserve a time slot on whichever&#xA;  station they would like to use&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Yeah,that's the idea.SatNOGS is primarily designed to receive data. In most  jurisdictions receiving data is allowed even by non-licensed individuals.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But.. having said that, if a station owner needs to send data to a satellite and has the needed hardware to do so (p.e. a radio or a Tx enabled SDR) the provided SatNOGS client software allows to the user open a console and transmit data to their satellite via their own ground-station.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Hope that helps.&lt;/p&gt;&#xA;" OwnerUserId="6674" LastActivityDate="2018-01-16T02:01:04.663" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24487" PostTypeId="2" ParentId="8599" CreationDate="2018-01-16T08:58:45.990" Score="0" Body="&lt;p&gt;I think using 24 Kjoules of energy to produce the equivalent of 1 Joule of momentum is the killer here..&#xA;If the ion engine is ever going to be truly usefull, it'll need to go up..&#xA;And in that case I think even 2 Joules of momentum or 10, wouldn't be a breakthrough, but a fraction of efficiency that is measured in the percentage range would be, instead of the current of 41.67 per million..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(even if the ISP would drop..)&#xA;(but still, if the ISP is but a fraction, while the efficiency would go up&#xA;several decimals, it would be worth it..)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More fuel in this case yes, but much less engine weight,&#xA;much less reactor weight..(or solar panels for that matter..)&#xA;On the latter I'd suggest a solar sail rather than panels..&#xA;Also, a solar sail would be much less prone to efficiency loss&#xA;due to damage..&#xA;A solar sail would still be useful even at 10+ au..&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(the solar sil would function very similar to a solar panel, but would be flexible, with much bigger cells..unlike the standard definitions of a solar sail, which would yield momentum through the physical impact of 'solar particles' onto it's surface, rather than electrical current..)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(thinking of that now, it may also be possible to use both, at least to certain extent, being moving away from the sun..)&lt;/p&gt;&#xA;" OwnerUserId="22527" LastEditorUserId="22527" LastEditDate="2018-01-16T09:24:02.790" LastActivityDate="2018-01-16T09:24:02.790" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24488" PostTypeId="1" AcceptedAnswerId="34216" CreationDate="2018-01-16T09:50:50.877" Score="10" ViewCount="1138" Body="&lt;p&gt;CNN's &lt;em&gt;Tomorrow's Hero&lt;/em&gt; profile &lt;a href=&quot;http://edition.cnn.com/2018/01/15/health/amber-yang-tomorrows-hero/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Meet Amber Yang. She's trying to prevent a space debris catastrophe&lt;/a&gt; describes an investigation into the use of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Convolutional_neural_network&quot; rel=&quot;nofollow noreferrer&quot;&gt;convolutional neural network&lt;/a&gt; (CNN) to predict possible collisions in low Earth orbit.&lt;/p&gt;&#xA;&lt;p&gt;If I understand correctly, searches for and predictions of potential collisions (such as &lt;a href=&quot;https://celestrak.org/SOCRATES/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SOCRATES&lt;/a&gt; for example) are done by raw number crunching, propagation of all known objects with something like SGP4 combined with some basic algorithms that only scrutinize pairs of objects with some possibility of intersecting orbits. One might not compute distance of closest approach between a pair of circular orbits if one were GEO and the other MEO for example.&lt;/p&gt;&#xA;&lt;p&gt;How might one apply a CNN to this problem? Plausible speculation is welcome, as this is a somewhat new approach.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; I've found a short but interesting &lt;a href=&quot;https://indico.cern.ch/event/539439/contributions/2190841/attachments/1293121/1926942/CERN_presentation.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;introduction&lt;/a&gt; to the problem (see page 6 for a flow chart), but I don't think it's meant to be a complete description of the technique.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; plot from the &lt;a href=&quot;http://edition.cnn.com/2018/01/15/health/amber-yang-tomorrows-hero/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;CNN article&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/b4RN9.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/b4RN9.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Good video, if you are short of time, watch the last two minutes at least!&lt;/p&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/lQQk9XqTMAY?start=0&quot;&gt;&lt;/iframe&gt;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2022-09-27T22:30:51.853" LastActivityDate="2022-03-06T05:22:19.143" Title="How might one approach using AI (convolutional neural network) to predict collisions in orbit?" Tags="|orbital-mechanics|simulation|impact|collision-avoidance|artificial-intelligence|" AnswerCount="1" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="24489" PostTypeId="2" ParentId="23988" CreationDate="2018-01-16T11:15:56.210" Score="17" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;An exercise that was left unsolved from last year's class gives me this equation :&#xA;  $$&#xA;t-t_{p} = \sqrt{\frac{a^3}{\mu}}*(\arcsin(X) - e*X)&#xA;$$&#xA;  where :&#xA;  $$&#xA;X = \frac{\sqrt{1-e^2}*\sin(v)}{1+e*\cos(v)}.&#xA;$$&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;This is just Kepler's equation $M = E-e\sin E$, but written in terms of $X = \sin E$, where $E$ is the eccentric anomaly. We don't have a derivation of Kepler's equation on this site, so here goes. I'll start with a picture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/GJWpA.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/GJWpA.png&quot; alt=&quot;Image showing portions of an elliptical orbit by a body P about a central body F and its circumscribing circle, with the center denoted as C.&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The above image portrays a body $P$ in an elliptical orbit about a body $F$ that occupies one of the foci of the ellipse. The ellipse has a semi-major axis $a$ along the horizontal axis and an eccentricity $e$. The center of the ellipse and its circumscribing circle are at $C$. The vertical projection of the current location onto the circumscribing circle is denoted $P'$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kepler's second law says that the area of the elliptical sector $ZFP$ is a linear function of time: $A(ZFP) = k(t-t_p)$, where $A(ZFP)$ is the area in question, $k$ is some constant, $t$ is the time at which the orbiting object reaches the position $P$, and $t_p$ is the time of periapsis passage. In a full orbit, the area swept by that elliptical sector is the area of the ellipse: $A(2\pi) = \pi a b$. Thus $\pi a b = kT$, where $T$ is the orbital period,or $k = \frac{\pi a b}T$. Kepler's third law combined with Newtonian gravity in turn tells us that $\frac{2\pi}T = \sqrt{\frac \mu {a^3}}$, where $\mu$ is the system's gravitational coefficient $G(M+m)$. Defining $n = \sqrt{\frac \mu {a^3}}$, we have $$A(ZFP) = \frac12ab\,n(t-t_p)\tag{1}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We need an expression for $A(f)$. To get there, it's best to introduce the concept of &lt;a href=&quot;https://en.wikipedia.org/wiki/Eccentric_anomaly&quot; rel=&quot;noreferrer&quot;&gt;eccentric anomaly&lt;/a&gt;. This is portrayed in the image as the angle $E$. This is formed by projecting the point $P$ vertically to the intersection with the circumscribing circle, denoted $P'$. Given a point $x,y'$ on the circumscribing circle expressed relative to the center $C$, the corresponding point on the ellipse $x,y$ results by scaling the $y$ coordinate by $\frac b a$: $y=\frac b a y'$. This scaling means that the area of the elliptical sector $ZCP$ is the area of the circular sector $ZCP'$ scaled by the same scale factor. Since the area of the circular sector $ZCP$ is $\frac 1 2 a^2 E$ with $E$ expressed in radians, the area of the elliptical sector $ZCP$ is $\frac 1 2 ab E$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The area in question, that of the elliptical sector $ZFP$, is the area of the elliptical sector $ZCP$ less the area of the triangle $FCP$. The latter is $\frac12 \, ae \, b\sin E$ (1/2 * base * height), or $\frac12 ab\,e\sin E$. Thus $A(ZFP) = \frac12 ab (E - e\sin E)$. Combining this with equation (1) yields&#xA;$$E-e\sin E = n(t-t_p) \equiv M \tag{2}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is Kepler's equation. It provides a simple mechanism for computing time as a function of position. Computing position as a function of time requires inverting this transcendental function of the two variables $E$ and $e$. This inverse function cannot be expressed in terms of the elementary functions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A very simple, guaranteed to work, method to find $E$ given $M$ and $e$ is to use the fixed point iteration scheme $E_{n+1} = M + e\sin E_n$. Any starting guess $E_0$ will do, but typically $M$ is used as the initial guess. This converges for all $M$ and all eccentricities between 0 (inclusive) and 1 (exclusive). The convergence is very slow, particularly for large eccentricities. A better approach is to use Newton's method, which exhibits quadratic convergence &lt;em&gt;when it converges&lt;/em&gt;. A good initial guess is needed for large eccentricities to ensure convergence. Even better approaches, and even better initial guesses than $E_0=M$ have been found over the centuries. Kepler's equation is the subject of hundreds of scientific papers.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Inverting Kepler's equation gives us the eccentric anomaly as a function of time. But what about the true anomaly $f$? The relationship between $E$ and $f$ is easily found using the tangent half angle formula, $\tan^2 \frac x2 = \frac{1-\cos x}{1+\cos x}$. The coordinates of the point $P$ with respect to the focus $F$ are $x=a(\cos E-e)=r\cos f$, $y=a\sqrt{1-e^2}\sin E = r \sin f$. Thus&#xA;$$\tan^2 \frac f 2 = \frac{1+e}{1-e} \,\tan^2 \frac E 2$$&#xA;(Deriving this requires deriving $r=a(\cos E - e)$, not shown.) Since the true anomaly and eccentric anomaly are always on the same side of the $x$ axis, the tangents of their half angles will always have the same sign, yielding&#xA;$$\tan \frac f 2 = \sqrt{\frac{1+e}{1-e}} \,\tan \frac E 2 \tag{3}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;br&gt;&#xA;What about using $X=\sin E$ instead of $E$, as is done in the question? That's a &quot;don't do that, then&quot; kind of situation (from the joke, &quot;Doctor, it hurts when I hit myself like this:《 bonk 》.&quot;) This only gets less than half of the ellipse, and the convergence of $\arcsin X - e X = M$ is terrible (if it converges at all). Use Kepler's equation (equation (2)) to solve for $E$, then solve for the true anomaly $f$ (alternatively written as $\nu$ or $\theta$) via equation (3).&lt;/p&gt;&#xA;" OwnerUserId="2752" LastEditorUserId="2752" LastEditDate="2018-01-16T12:25:24.347" LastActivityDate="2018-01-16T12:25:24.347" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24491" PostTypeId="2" ParentId="20603" CreationDate="2018-01-16T20:41:03.890" Score="4" Body="&lt;p&gt;The field strength of an &lt;a href=&quot;https://en.wikipedia.org/wiki/Coulomb%27s_law&quot; rel=&quot;nofollow noreferrer&quot;&gt;electromagnetic field&lt;/a&gt; decreases with r&lt;sup&gt;2&lt;/sup&gt;. This has two effects:  &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;small variations in distance create huge changes in the attractive force. This means it's difficult to accurately control e.g. two ships flying towards each other. &lt;/li&gt;&#xA;&lt;li&gt;if you want this field to have a measurable effect over the distances used in space, you need a very strong source. &lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;A strong source means a large static charge. The achievable charge is limited by the &lt;a href=&quot;https://en.wikipedia.org/wiki/Relative_permittivity&quot; rel=&quot;nofollow noreferrer&quot;&gt;relative permittivity&lt;/a&gt; of the materials involved, if I remember correctly. Too much charge and you get arcing across your insulator to the nearest object with a different potential. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A particle beam won't be useful as a means to project an electromagnetic force over a distance. If the beam hits the target, it's a weapon. If you aim the beam just to the side of the target, beam and target could conceivably attract each other. The result then depends on the relative mass of target and beam. Since the target will be tens of orders of magnitude heavier than the beam, you'll end up using lots of energy to deflect the target a tiny bit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-17T08:57:59.603" LastActivityDate="2018-01-17T08:57:59.603" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24492" PostTypeId="2" ParentId="21768" CreationDate="2018-01-17T00:29:34.170" Score="5" Body="&lt;p&gt;Uhoh, I'll jump in the deep end and take a shot at this since you've been waiting 2 months for an answer, there certainly are people better qualified than I to answer this as I've limited my efforts to what I could come up with over several hours. Improving edits welcome.&lt;/p&gt;&#xA;&lt;p&gt;The fabric is simply a means to hold something like &lt;a href=&quot;http://www.enbiomaterials.com/products/&quot; rel=&quot;nofollow noreferrer&quot;&gt;CoBlasted Solarblack&lt;/a&gt; (they might use a different brand). It's a heatshielding compound that is transparent to radio waves (unlike carbon fiber); combined with the fabric it provides lightweight stiffening and thermal protection to the metal honeycomb dish reflector underneath.&lt;/p&gt;&#xA;&lt;p&gt;Based on your comments your question is not so much &amp;quot;Why is TESS' high gain antenna made of undulating &lt;strong&gt;BLACK&lt;/strong&gt; fabric rather than metal?&amp;quot; and more about &lt;strong&gt;why the surface is corrugated&lt;/strong&gt;. They also make &lt;a href=&quot;http://www.enbiomaterials.com/solar-orbiter/&quot; rel=&quot;nofollow noreferrer&quot;&gt;SolarWhite&lt;/a&gt; which &lt;strong&gt;is&lt;/strong&gt; electrically conductive.&lt;/p&gt;&#xA;&lt;p&gt;Solar&lt;strong&gt;black&lt;/strong&gt; (which you wrote in all caps) was chosen over Solarwhite because the corrugated surface must hold precise tolerances and the white coating is conductive, thus it would be the reflector rather than the precisely shaped metal underneath.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Q: Why isn't it smooth?&lt;/p&gt;&#xA;&lt;p&gt;It's a Floquet corrugated surface: &lt;a href=&quot;http://www.academia.edu/13976543/Periodic_Structures_and_Floquets_Theorem&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.academia.edu/13976543/Periodic_Structures_and_Floquets_Theorem&lt;/a&gt; or &lt;a href=&quot;http://www.chebfun.org/examples/ode-linear/Floquet.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.chebfun.org/examples/ode-linear/Floquet.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;A really easy way to demonstrate this without complicated mathematics is to show a photo from a simple experiment.&lt;/p&gt;&#xA;&lt;p&gt;You'll need a flashlight, a &lt;a href=&quot;https://en.wikipedia.org/wiki/Fresnel_lens&quot; rel=&quot;nofollow noreferrer&quot;&gt;Fresnel lens&lt;/a&gt; (like a &amp;quot;Magnifying Sheet&amp;quot; used to enlarge small type), a mirror, and a wall.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Simply shine the flashlight at the wall. Notice that there is a brighter central spot but the flashlight also illuminates an area around the central spot, that's the sidelobe.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;Place the magnifying sheet on the mirror, hold the reflective surface facing the wall. Now point the flashlight towards yourself, directed at the center of the Fresnel lens.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Compare 1 and 2, notice how the sidelobes are greatly reduced and the central spot is much brighter. The central black spot in the right side photo is caused by the beam being blocked by the flashlight. The same camera settings were used for both photos.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/HkDZQ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/HkDZQ.jpg&quot; alt=&quot;Foquet Demo&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Foquet surface on a parabolic dish is &lt;strong&gt;like&lt;/strong&gt; (not the same as) a first surface mirror on a Fresnel lens.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Interesting facts and other info: &lt;a href=&quot;https://tess.gsfc.nasa.gov/overview.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://tess.gsfc.nasa.gov/overview.html&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;TESS Twitter: &lt;a href=&quot;https://mobile.twitter.com/tessatmit&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://mobile.twitter.com/tessatmit&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The mission is scheduled to last 2 years but the orbit was designed to remain stable for decades with minor course corrections: &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/t/tess&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://directory.eoportal.org/web/eoportal/satellite-missions/t/tess&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;TESS still needs to use it's hydrazine thrusters for unloading angular momentum build-up induced by solar radiation pressure but requires no propulsion for station-keeping.&lt;/p&gt;&#xA;&lt;p&gt;The payload consists of four cameras of  20.4 cm^2, 16.8 Mpixel, low-noise, low-power, MIT/LL CCID-80 detectors.&lt;/p&gt;&#xA;&lt;p&gt;The four CCDs (Charge Coupled Devices) arrays are attached to the spacecraft such that their FOVs are lined up to form a rectangle measuring 24º x 96º on the sky.&lt;/p&gt;&#xA;&lt;p&gt;They capture the spectrum from 600 nm to 1000 nm through 4 lenses with seven 10.5 cm f/1.4 optical elements each.&lt;/p&gt;&#xA;&lt;p&gt;With a payload like that you'll want a first rate means to transmit the data: &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/t/tess#spacecraft&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://directory.eoportal.org/web/eoportal/satellite-missions/t/tess#spacecraft&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Answer:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Making &lt;a href=&quot;http://www.composites.com/Gallery.html#Reflector&quot; rel=&quot;nofollow noreferrer&quot;&gt;a &lt;strong&gt;smooth&lt;/strong&gt; carbon fiber Ka band satellite dish&lt;/a&gt; is not too difficult. Electromagneticly transparent composites are are available from companies like &lt;a href=&quot;https://www.tencatecomposites.com/markets/aerospace/radome-antenna-systems&quot; rel=&quot;nofollow noreferrer&quot;&gt;TenCate&lt;/a&gt; and are used in radomes. They wanted to squeeze better performance out of the relatively small dish.&lt;/p&gt;&#xA;&lt;p&gt;The high gain antenna (HGA) is a Cassegrain (2 reflector, convex secondary) antenna &lt;a href=&quot;https://en.m.wikipedia.org/wiki/Cassegrain_antenna&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.m.wikipedia.org/wiki/Cassegrain_antenna&lt;/a&gt; with a &lt;a href=&quot;https://spiral.imperial.ac.uk/handle/10044/1/21679&quot; rel=&quot;nofollow noreferrer&quot;&gt;Floquet&lt;/a&gt; on the primary surface.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zxg8r.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zxg8r.png&quot; alt=&quot;Floquet Surface Sidelobe Reduction&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The presence of a second reflecting surface in the signal path allows additional opportunities for tailoring the radiation pattern for maximum performance and permits placing a larger feed outside the path of the signal. They don't appear to do that, opting for a more conventional convex conical secondary.&lt;/p&gt;&#xA;&lt;p&gt;The feed's sidelobes that miss the secondary can be placed out of phase with correctly reflected signals (cutting interference) and the Floquet corrugations reduce the sidelobes on the main dish (concentrating the beam).&lt;/p&gt;&#xA;&lt;p&gt;It is also possible to use a modified electromagnetic-bandgap (M-EBG) structure in the feed to both allow ultrawide bandwidth with a center notch and orient the lobes of the feed to avoid the secondary's supports - since there are 3 supports rather than 4 one would presume that is not being done either.&lt;/p&gt;&#xA;&lt;p&gt;In addition it's also possible to use a Curved-aperture Corrugated Conical Horn: &lt;a href=&quot;http://www.antennamagus.com/newsletter.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.antennamagus.com/newsletter.php&lt;/a&gt; to further narrow the beamwidth.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/9YipM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/9YipM.jpg&quot; alt=&quot;TESS HGA&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The dish appears that it &lt;em&gt;could&lt;/em&gt; contain 14 corrugations in half it's diameter, it has fewer since the feedhorn occupies the central portion.&lt;/p&gt;&#xA;&lt;p&gt;The diameter (0.7 meters) / (14*2) = 2.5 centimeters, peak to peak. A wavelength of 1.25 centimetres = 23.98339664 gigahertz, (1.00 cm = 29.98 GHz). So the peak to peak distance is a bit under 2.5 cm.&lt;/p&gt;&#xA;&lt;p&gt;Floquet Theorem: &lt;a href=&quot;https://en.wikipedia.org/wiki/Floquet_theory&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Floquet_theory&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Uhoh's question: &amp;quot;Are there Optical Magnetic Mirrors (OMMs) which actually reflect via interaction with the magnetic field?&amp;quot; &lt;a href=&quot;https://physics.stackexchange.com/questions/273422/are-there-optical-magnetic-mirrors-omms-which-actually-reflect-via-interaction&quot;&gt;https://physics.stackexchange.com/questions/273422/are-there-optical-magnetic-mirrors-omms-which-actually-reflect-via-interaction&lt;/a&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Update January 19, 2018:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://aip.scitation.org/doi/abs/10.1063/1.372241&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://aip.scitation.org/doi/abs/10.1063/1.372241&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the paper: &amp;quot;The coupling of microwave radiation to surface plasmon polaritons and guided modes via dielectric gratings&amp;quot; by Hibbinsa, Sambles, and Lawrence they write:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;It is shown that an absorbing dielectric layer, sinusoidally modulated in height, on top of a planar metal substrate, may be used to provide coupling between both s-and p-polarized incident microwave photons and surface plasmon polaritons, which propagate along the metal-dielectric interface.&amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://ieeexplore.ieee.org/document/1548109/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://ieeexplore.ieee.org/document/1548109/&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the paper &amp;quot;Mirror that does not change the phase of reflected waves&amp;quot; by Fedotov, Rogacheva, and Zheludeva 2006 they write:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Wavelength sensitive transmission and reflection of a structured thin metal layers in the optical and microwave parts of the spectrum currently attract considerable attention.&lt;/p&gt;&#xA;&lt;p&gt;Such selectivity results from patterning the interface on a subwavelength scale in a way that makes electromagnetic excitation couple with the structure in a resonant fashion.&lt;/p&gt;&#xA;&lt;p&gt;In this letter, we report on the first experimental observation of the “magnetic wall” property of novel structured metallic surfaces. The magnetic wall is a mirror that imposes extremely unusual electromagnetic boundary conditions see Fig. 1b:&lt;/p&gt;&#xA;&lt;p&gt;It does not change the phase of the electric field upon reflection, but reverses the phase of the magnetic field. This property is in sharp contrast to the reflection from a dielectric interface or metal mirror, which reverses the phase of the electric field of the reflected electromagnetic wave and &lt;strong&gt;preserves the phase of the magnetic field&lt;/strong&gt; instead Fig. 1a.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;What differs from the parabolic under discussion is the shape of the pattern. It provides proof that a sinusoidal pattern can affect the EM wave, though it's orientation is rotated in respect to what is used on the TESS' reflector.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://iopscience.iop.org/article/10.1088/1674-1056/21/1/017301/meta&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://iopscience.iop.org/article/10.1088/1674-1056/21/1/017301/meta&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the paper &amp;quot;Anomalous microwave reflection from a metal surface induced by spoof surface plasmon&amp;quot; by Liang, Jin-Xiang, Yin-Chang and Jian they write:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;... metallic subwavelength structures have become one of the main research branches, also in the studies of the microwave regime and THz frequencies.&lt;/p&gt;&#xA;&lt;p&gt;In the wavelength range of microwave frequencies, the electromagnetic surface modes are referred to as spoof surface plasmons using language borrowed from optical technology.&lt;/p&gt;&#xA;&lt;p&gt;As is known, metal surfaces are outstanding ref lectors for incident electromagnetic waves. However, if SPs are launched in a certain wavelength region at the metal–dielectric interface, the energy of the incident electromagnetic waves will convert into that of SPs, giving rise to the reflection minimum. In this study, the influence of spoof SPs on the microwave reflection from a metallic aluminum (Al) surface was examined.&lt;/p&gt;&#xA;&lt;p&gt;The spoof SPs at the Al surface were excited by patterning the metal surface with periodic corrugated grooves, i.e., to form a surface grating coupler. Extraordinary attenuation of microwave reflection from the periodic corrugated Al surface was observed when spoof SPs are excited successfully. This grating coupling method of exciting SPs to decrease microwave reflection has also recently been employed in experiments performed with overdense plasma.&lt;/p&gt;&#xA;&lt;p&gt;The &lt;strong&gt;groove gratings are clearly acting as resonators&lt;/strong&gt;, and the experimental phenomenon is similar to the famous anomalies in diffraction from metal gratings (Wood’s anomaly) in the optical regime.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://arxiv.org/abs/1610.04780&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://arxiv.org/abs/1610.04780&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;In the paper &amp;quot;Flat Engineered Multi-Channel Reflectors&amp;quot; by Asadchy, D´ıaz-Rubio, Tcvetkova, Kwon, Elsakka, Albooyeh, and Tretyakov they write:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Here we introduce a concept of multi-channel functional metasurfaces, which are able to control incoming and outgoing waves in a number of propagation directions simultaneously. In particular, we reveal a possibility to engineer multi-channel reflectors.&lt;/p&gt;&#xA;&lt;p&gt;Under the assumption of reciprocity and energy conservation, we find that there exist three basic functionalities of such reflectors: Specular, anomalous, and retro reflections. Multi-channel response of a general flat reflector can be described by a combination of these functionalities.&lt;/p&gt;&#xA;&lt;p&gt;To demonstrate the potential of the introduced concept, we design and experimentally test three different multi-channel reflectors: Three-and five-channel retro-reflectors and a three-channel power splitter. Furthermore, by extending the concept to reflectors supporting higher-order &lt;strong&gt;Floquet harmonics&lt;/strong&gt;, we forecast the emergence of other multiple-channel flat devices, such as isolating mirrors, complex splitters, and multi-functional gratings.&amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;This is demonstrated on a flat surface and would need to be rotated around the central point to create a round structure.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.inatel.br/biblioteca/artigos-cientificos/2014/7536-design-of-a-high-gain-frequency-selective-surface-antenna-system/file&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.inatel.br/biblioteca/artigos-cientificos/2014/7536-design-of-a-high-gain-frequency-selective-surface-antenna-system/file&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The paper: &amp;quot;Design of a High-Gain Frequency Selective Surface Antenna System&amp;quot; by Alves, Barros, Mologni, Juliano F.; da Silva, Siqueira, Cesareo , Cerqueira and Ribas has this abstract:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;An idealized planar infinite array of circle elements is evaluated using &lt;strong&gt;Floquet&lt;/strong&gt; theory based on only a single unit cell to determine the reflection and transmission coefficients in the frequency band from 800MHz to 4GHz. The circle elements are then projected on the parabolic surface made of polyethylene and a complete hybrid full-wave simulation is carried out using a horn antenna as excitation. The proposed methodology results in a parabolic reflector made of circular frequency selective surface elements that provide high gain and is able to filter electromagnetic waves from a given frequency band. Numerical results demonstrate a 4.5 dB gain improvement compared to a parabolic reflector made of perfect electric conductor for the operational frequency of 3.1GHz.&amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Update January 21st, 2018:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The &lt;a href=&quot;http://www.satellitetoday.com/technology/2015/12/28/vanguard-space-technologies-delivers-components-for-tess-satellite/&quot; rel=&quot;nofollow noreferrer&quot;&gt;TESS HGA was made&lt;/a&gt; by Vanguard Space Technologies Inc. which was &lt;a href=&quot;https://www.compositesworld.com/news/solaero-technologies-acquires-vanguard-space&quot; rel=&quot;nofollow noreferrer&quot;&gt;acquired by SolAero Technologies Corp.&lt;/a&gt; and operates under the name of &lt;a href=&quot;https://alliancespacesystems.com/products/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alliance Space Systems&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;Vanguard was &lt;a href=&quot;https://www.prnewswire.com/news-releases/vanguard-space-technologies-delivers-primary-components-for-tess-satellite-300196956.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;responsible for structural engineering, fabrication, assembly and test of the High Gain Antenna (HGA)&lt;/a&gt;, and fabrication and assembly of the TESS Spacecraft Primary Bus Structure as well as the Camera Accommodation Structure (CAS).&amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Unfortunately none of those websites explain the manufacturing of the antenna, the search engine on the &lt;a href=&quot;https://alliancespacesystems.com/?s=TESS&quot; rel=&quot;nofollow noreferrer&quot;&gt;Alliance site returns no hits&lt;/a&gt; for &amp;quot;TESS&amp;quot; (but it's clear that their search engine doesn't return as many hits as Google).&lt;/p&gt;&#xA;&lt;p&gt;Their webpage says that &lt;a href=&quot;https://alliancespacesystems.com/capabilities/manufacturing/&quot; rel=&quot;nofollow noreferrer&quot;&gt;they use Astroquartz® fabric and Kevlar&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;See page 37: &lt;a href=&quot;http://www.jpscm.com/jps/wp-content/uploads/2017/10/2017-Data-Book-Small-1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.jpscm.com/jps/wp-content/uploads/2017/10/2017-Data-Book-Small-1.pdf&lt;/a&gt; for info on &lt;a href=&quot;http://www.jpscm.com/astroquartz/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.jpscm.com/astroquartz/&lt;/a&gt; .&lt;/p&gt;&#xA;&lt;p&gt;They use Bismaleimide Polyamide &lt;a href=&quot;http://www.huntsman.com/advanced_materials/a/Our%20Technologies/High%20Performance%20Components/Imides%20and%20Benzoxazines/Bismaleimides%20%20%20Polyimides&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.huntsman.com/advanced_materials/a/Our%20Technologies/High%20Performance%20Components/Imides%20and%20Benzoxazines/Bismaleimides%20%20%20Polyimides&lt;/a&gt; as the resin for Astroquartz. I see no reason that SolarBlack couldn't be used on the fabric since it's 99% silicon.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;An very &lt;a href=&quot;https://en.wikipedia.org/wiki/Parabolic_antenna#Parabolic_reflector&quot; rel=&quot;nofollow noreferrer&quot;&gt;simple explanation of parabolic antenna design&lt;/a&gt; is provided at Wikipedia. It mentions 2 points relevant to the question(s):&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&amp;quot;Since a shiny metal parabolic reflector can also focus the sun's rays, and most dishes could concentrate enough solar energy on the feed structure to severely overheat it if they happened to be pointed at the sun, solid reflectors are always given a coat of flat paint.&amp;quot;.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Black is a less reflective color than white.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&amp;quot;The reflector can be of sheet metal, metal screen, or wire grill construction, and it can be either a circular &amp;quot;dish&amp;quot; or various other shapes to create different beam shapes. A metal screen reflects radio waves as well as a solid metal surface as long as the holes are &lt;strong&gt;smaller than one-tenth of a wavelength&lt;/strong&gt;, so screen reflectors are often used to reduce weight and wind loads on the dish. To achieve the maximum gain, &lt;strong&gt;it is necessary that the shape of the dish be accurate within a small fraction of a wavelength&lt;/strong&gt;, to ensure the waves from different parts of the antenna arrive at the focus in phase. Large dishes often require a supporting truss structure behind them to provide the required stiffness.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;A reflector made of a grill of parallel wires or bars oriented in one direction acts as a polarizing filter as well as a reflector. It only reflects linearly polarized radio waves, with the electric field parallel to the grill elements.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The reflector dish could have been made of wire spaced less than one-tenth of a wavelength&lt;/p&gt;&#xA;&lt;p&gt;A solid dish was likely chosen to ensure that every portion of the dish was fully supported, wind loading is not a consideration in outer space and precise accuracy improves performance; a sinusoidal surface serves to increase the performance at &lt;strong&gt;one&lt;/strong&gt; specific frequency.&lt;/p&gt;&#xA;&lt;p&gt;A reflector made of wire or solid metal consisting of rings (rather than parallel wires) reflects circularly polarized radio waves (rather than linearly polarized waves).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.m.wikipedia.org/wiki/Ka_band&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia describes the Ka band&lt;/a&gt; as: &amp;quot;frequencies in the range 26.5–40 gigahertz (GHz), i.e. wavelengths from slightly over one centimeter down to 7.5 millimeters&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;26.5 gigahertz = a wavelength of 11.3129229434 millimetres.&lt;/p&gt;&#xA;&lt;p&gt;40 gigahertz = a wavelength of 7.49481145 millimetres.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;NASA's frequencies are given below.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasa.gov/content/goddard/nasa-team-develops-new-ka-band-communications-system-to-break-through-the-noise&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA's Ka band webpage&lt;/a&gt; purports that 3rd generation technology is capable of delivering 2400 Mbps (2.4 Gbps), with a higher transmitted power option of 10 watts.&lt;/p&gt;&#xA;&lt;p&gt;The solar arrays are rated at 500W (EoL). &lt;a href=&quot;https://tess.gsfc.nasa.gov/documents/tess-litho.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Material available from NASA (.PDF)&lt;/a&gt; explains that TESS will use a Ka-band with a 100 Mbps science downlink (some 24 times slower than is possible at such a high frequency).&lt;/p&gt;&#xA;&lt;p&gt;In addition to the &amp;quot;science data&amp;quot; &lt;a href=&quot;https://tess.gsfc.nasa.gov/groundops.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;there is also commanding and telemetry data&lt;/a&gt; (which I presume would be less than the science data).&lt;/p&gt;&#xA;&lt;p&gt;It is claimed that TESS has a 192 GB solid-state buffer that needs to be transferred every 13.7 days, (192 gigabytes) / (100 Mbps) = 4.26666667 hours,  so &lt;a href=&quot;https://tess.mit.edu/science/&quot; rel=&quot;nofollow noreferrer&quot;&gt;science data&lt;/a&gt; alone will require 4 hours and 16 minutes to transmit (if the buffer is full, but the following video link, next paragraph, says that they expect most transmissions to last only 3 hours).&lt;/p&gt;&#xA;&lt;p&gt;TESS's orbit and a bit about the mission is explained in the Video you linked to above (so I removed my duplicate link here) - the closeup animations show a smooth faced antenna.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Information on the Relay Satellite:&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://digitalcommons.usu.edu/smallsat/2017/PosterS1/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;NASA’s Tracking and Data Relay Satellite&lt;/a&gt; (TDRS) Ka-band Single Access (KaSA)&lt;/p&gt;&#xA;&lt;p&gt;KaSA Service via large steerable antennas in auto-track mode:&lt;/p&gt;&#xA;&lt;p&gt;–Return (from spacecraft) of mission data and spacecraft telemetry; G/T: 26.5 dB/K; 25.25-27.5 GHz&lt;/p&gt;&#xA;&lt;p&gt;–Forward (toward) command and control EIRP: 63.0 dBW; 22.55-23.55 GHz – Field of View +76.8oE-W; +30.5oN-S&lt;/p&gt;&#xA;&lt;p&gt;Transmitter: &lt;a href=&quot;http://www.spacemicro.com/assets/datasheets/rf-and-microwave/%C2%B5Tx-300_Ka-Band_Transmitter.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;µKaTx-300 Ka-band Transmitter (.PDF)&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Frequency: 25.25-27.5GHz (Frequency Agile) Occupied bandwidth up to 850 MHz at a maximum of 5 watts output (requires 68W @ 18-40 Vdc) with a data rate of 100kbps – 4.0Gbps.&lt;/p&gt;&#xA;&lt;p&gt;29 gigahertz = a wavelength of 10.337671 millimetres.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.microwavejournal.com/articles/29572-antenna-technologies-for-the-future&quot; rel=&quot;nofollow noreferrer&quot;&gt;It's &lt;strong&gt;far&lt;/strong&gt; from the most advanced antenna technology&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;While that's a simplified version, &lt;strong&gt;a fair bit more knowledge and effort is needed to design an aperture antenna suitable for reliable communication.&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Acoustic noise dissipation:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Two examples explaining that tri-woven fabric is often used since it can be stretched over a parabolic mold (while 'traditionally woven' fabric usually only stretches in a single direction).&lt;/p&gt;&#xA;&lt;p&gt;Patent US 20040113863 A1 &amp;quot;Microwave frequency antenna reflector&amp;quot; (Jun 17, 2004) &lt;a href=&quot;http://www.google.com/patents/US20040113863&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.google.com/patents/US20040113863&lt;/a&gt; explains:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;The high levels of acoustic noise produced during launch of the spacecraft can result in high structural loading of lightweight antenna reflector structures, and has been a major problem in the past.&lt;/p&gt;&#xA;&lt;p&gt;Holes in both the microwave reflective layer along with the holes in the tri-axial woven fabric second layer are desirable, because the acoustic noise that is produced during launch of the reflector on a spacecraft into space and during maneuvers in space are transmitted and dissipated through the holes in the two layers without causing structural damage or failure that would affect the ability of the reflector to reflect microwaves.&lt;/p&gt;&#xA;&lt;p&gt;It will continue to be a problem with thin and lightweight space structures having large unstiffened surface areas, with this acoustic noise environment sometimes resulting in structural failures of the reflector and its materials. &amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Patent US 9685710 B1 &amp;quot;Reflective and permeable metalized laminate&amp;quot; (Jun 20, 2017) &lt;a href=&quot;http://www.google.com/patents/US9685710&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.google.com/patents/US9685710&lt;/a&gt; explains:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&amp;quot;TECHNICAL FIELD&lt;/p&gt;&#xA;&lt;p&gt;This invention relates generally to a laminated material, and more particularly to an antenna reflector configured as a laminated structure where a first layer includes an acoustically permeable, nonwoven metallized fiber matte and a second layer includes an acoustically permeable, open weave fabric.&lt;/p&gt;&#xA;&lt;p&gt;BACKGROUND OF THE INVENTION&lt;/p&gt;&#xA;&lt;p&gt;The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services from geostationary orbit. During launch, such spacecraft experience environmental dynamic loads, particularly acoustic launch loads.&lt;/p&gt;&#xA;&lt;p&gt;Spacecraft components, including particularly radio frequency (RF) antenna reflectors, are required to be compatible with such launch loads, but must also comply, subsequent to launch, with challenging performance specifications in the face of substantial temperature variations and solar radiation exposure, typical of a space environment. Furthermore, such structures must be designed in view of stringent mass and cost objectives.&lt;/p&gt;&#xA;&lt;p&gt;...&lt;/p&gt;&#xA;&lt;p&gt;In another implementation, the antenna reflector may include a honeycomb core. The honeycomb core may be sandwiched between the laminated structure and a rear skin, the laminated structure being disposed proximate to a first surface of the honeycomb core and the rear skin being disposed proximate to a second surface of the honeycomb core, the second surface being opposite to the first surface.&lt;/p&gt;&#xA;&lt;p&gt;In a yet further implementation, the fiber matte may include one or more of carbon fibers, carbon composite fibers, polyamide fibers and glass fibers.&lt;/p&gt;&#xA;&lt;p&gt;In another implementation, the second layer may include a triaxially woven open weave fiber. The triaxially woven open weave fiber may include one or more of carbon fibers, carbon composite fibers, polyamide fibers and glass fibers.&amp;quot;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Necessary Accuracy:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Ruze%27s_equation&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Ruze%27s_equation&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=2002ASPC..278...45G&amp;amp;db_key=AST&amp;amp;page_ind=20&amp;amp;plate_select=NO&amp;amp;data_type=GIF&amp;amp;type=SCREEN_GIF&amp;amp;classic=YES&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=2002ASPC..278...45G&amp;amp;db_key=AST&amp;amp;page_ind=20&amp;amp;plate_select=NO&amp;amp;data_type=GIF&amp;amp;type=SCREEN_GIF&amp;amp;classic=YES&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The Ruze equation gives the often-cited result that the aperture efficiency is reduced by 3 dB (a factor of 2) for an rms surface error 'e' equal to 1/16.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.cv.nrao.edu/course/astr534/2DApertures.html#SurfaceEfficiency&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.cv.nrao.edu/course/astr534/2DApertures.html#SurfaceEfficiency&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;The surface efficiency \eta_{\rm s} declines rapidly as the rms error in wavelengths \sigma / \lambda exceeds 1/16 \approx 0.06.&lt;/p&gt;&#xA;&lt;p&gt;27.5  GHz * 16 = 440 GHz&lt;/p&gt;&#xA;&lt;p&gt;440 gigahertz = a tolerance for error of 0.6813464954545 millimetres (0.0268 inches).&lt;/p&gt;&#xA;&lt;p&gt;So if the surface is off by ~ 0.7 mm (rms) the signal strength will be reduced 50%.&lt;/p&gt;&#xA;&lt;p&gt;Offsetting the surface in a controlled manner would act like a Single-Frequency Mirror (&lt;a href=&quot;https://physics.aps.org/articles/v8/20&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://physics.aps.org/articles/v8/20&lt;/a&gt;) except it would amplify one particular frequency while reflecting the others to a lesser extent.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Beam Width:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;The angular half-power beam width (HPBW) of a parabolic reflector can be estimated from the diameter of the dish and the frequency of operation:&lt;/p&gt;&#xA;&lt;p&gt;Angular beam width (degrees)=70 degrees/(D/lambda)&lt;/p&gt;&#xA;&lt;p&gt;29 gigahertz = a wavelength of 10.337671 millimetres.&lt;/p&gt;&#xA;&lt;p&gt;70 / (70 / 10.337671) = 10.337671 degrees HPBW.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Link Budget:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;[I'll wisely leave this calculation to someone else.]&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;" OwnerUserId="20766" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-21T23:29:29.393" CommentCount="11" ContentLicense="CC BY-SA 3.0" />
  <row Id="24493" PostTypeId="1" CreationDate="2018-01-17T11:21:40.547" Score="1" ViewCount="398" Body="&lt;p&gt;I'm trying to find out the meaning of azimuth and elevation form the perspective of a geosynchronous satellite. So if I'm trying to direct a radio beam toward earth from a communication satellite and I'm given azimuth and elevation values &lt;em&gt;from the perspective of the satellite&lt;/em&gt; what do they mean, exactly?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: I'm not sure I can specify the details of the system I'm working on, as it's proprietary. But what I am trying to do (well model, it's not real world) is define a beam footprint on earth from a communication satellite. The software allows the user to define a beam using a number of parameters, but the ones that relate to the position of the centre of the (circular) beam are Azimuth and Elevation. I believe that these refer to the frame of reference of the satellite (or possibly the antenna for the particular beam on the satellite), but I'm not sure how. Since different satellites will be oriented differently, I can't understand how this will work. Maybe these terms have a standard meaning for satellite operators? &lt;/p&gt;&#xA;" OwnerUserId="22537" LastEditorUserId="12102" LastEditDate="2018-01-18T03:59:32.867" LastActivityDate="2018-01-18T03:59:32.867" Title="Azimuth and elevation meaning relative to satellite (not earth)" Tags="|orbital-mechanics|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24494" PostTypeId="1" AcceptedAnswerId="24510" CreationDate="2018-01-17T12:08:48.620" Score="0" ViewCount="203" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; more photos of a &lt;em&gt;model&lt;/em&gt; of Mariner 2 here at Historic Spacecraft's &lt;a href=&quot;https://historicspacecraft.com/Probes_Mariner.html#Mariner_2&quot; rel=&quot;nofollow noreferrer&quot;&gt;Mariner_2&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The images and quote are from the book &lt;em&gt;Space Probes and Planetary Exploration&lt;/em&gt;  William R. Corliss (written under the sponsorship of the National Aeronautics and Space Administration), Princeton, NJ: Van Nostrand, 1965.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How exactly does this ridged surface &quot;diffuse infrared radiation without defocusing the microwaves&quot;? These are millimeter scale, with increasing pitch at larger radius, so I'm guessing that they contribute to the focusing &lt;em&gt;of the microwaves&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd really prefer a supporting link that explains how these ridges work to maintain focus simultaneously on &lt;em&gt;two different wavelengths&lt;/em&gt;  I can guess how (19 and 13.5 mm are close to the ratio 3/2), but an official explanation would be much better!&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;A solid, aluminum, parabolic dish 48.5 cm in diameter was used on Mariner 2 (Fig. 14-1). Its field of view was 2.2° and 2.5° in half angle for the 13.5- and 19-mm channels, respectively. The ridged surface seen in the photograph diffuses infrared radiation without defocusing the microwaves. Infrared heating of the radiometer is minimized in this way. Ten hours prior to the Venus encounter, the whole dish was driven by a scanning motor in a search mode at 1°/sec. When the limb of the planet was acquired, the scan rate was reduced to 0.1°/sec.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The first figure is a check that the radii do increase exactly as the square root of the ring number, which is what one might expect for a diffractive surface. The line is just 55 * sqrt(r) where r is in pixels.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fqzRc.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fqzRc.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/CRqTf.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/CRqTf.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DGund.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DGund.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-08-29T11:44:32.943" LastActivityDate="2018-08-29T11:44:32.943" Title="How did Mariner 2's ridged radiometer dish focus two microwave bands but diffuse IR?" Tags="|planetary-science|instrument|mariner|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24496" PostTypeId="1" AcceptedAnswerId="24526" CreationDate="2018-01-17T14:55:17.107" Score="4" ViewCount="605" Body="&lt;p&gt;Since Full-Flow Staged Engine uses preburned fuel/OX-rich gases in combustion chamber, don't those exhaust fumes produced by the preburner somehow unbalance or 'pollute' the pure combustion of fuel and oxygen in the chamber? I know there is still coming a lot of leftover fuel and OX through the preburner but what about the other gases produced by the combustion?&lt;/p&gt;&#xA;&lt;p&gt;For instance in &lt;a href=&quot;https://en.wikipedia.org/wiki/Raptor_(rocket_engine_family)&quot; rel=&quot;nofollow noreferrer&quot;&gt;SpaceX's Raptor Engine&lt;/a&gt; which uses liquid methane and LOX, preburner produces some &lt;em&gt;carbon dioxide and water steam&lt;/em&gt;. Don't these gases spoil the pure combustion? Or is the amount of these fumes so insignificantly small?&lt;/p&gt;&#xA;" OwnerUserId="22540" LastEditorUserId="11262" LastEditDate="2022-08-10T00:32:59.570" LastActivityDate="2022-08-10T00:32:59.570" Title="Don't Preburner exhaust fumes disturb the combustion in a Full-flow Staged Engine?" Tags="|engine-design|engines|liquid-fuel|combustion|raptor|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24497" PostTypeId="1" AcceptedAnswerId="24498" CreationDate="2018-01-17T16:16:19.997" Score="7" ViewCount="566" Body="&lt;p&gt;Yesterday, NASA Stennis successfully completed a hot fire test of Aerojet Rocketdyne RS-25 a.k.a SSME. It was noted in the webcast before the test started that they were testing the 'engine controller'. I also read about testing the new 3d printed 'pogo' for the engine.&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;What are these components and what function do they serve? Why have a test dedicated to these?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Looking at the live test video one could gather they tested for about 7 mins in which the back pressure area of blue radiation or the first shock was noticably varying, which would imply variable thrusting. &lt;/p&gt;&#xA;&#xA;&lt;ol start=&quot;2&quot;&gt;&#xA;&lt;li&gt;&lt;p&gt;So, while doing that how did they exactly test the two components? What data were they looking for? &lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;How does passing this test pave way for further tests that are planned, ultimately to the  EM-1 mission?&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="5975" LastActivityDate="2018-01-19T05:52:57.590" Title="RS-25 Hot Fire Test" Tags="|rockets|nasa|testing|sls|ssme|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24498" PostTypeId="2" ParentId="24497" CreationDate="2018-01-17T16:41:51.043" Score="11" Body="&lt;p&gt;The engine controller is a computer mounted on the SSME which monitors and controls the engine.  It, for example, takes throttle commands as a % of power level from the vehicle and translates them into control commands for the engine valve control loops.  The SLS engine controller is a new hardware device and presumably new software as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The pogo accumulator is a spherical tank charged initially with helium and later with GO2 which acts as a standpipe and provides the capability to damp out oscillations in the LO2 feed system.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can read about the shuttle era pogo system &lt;a href=&quot;https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-mps.html#sts-pogo&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; and the engine controller &lt;a href=&quot;https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-mps.html#sts-mps-mec&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.nasaspaceflight.com/wp-content/uploads/2017/12/NSF_20171213_181107-350x297.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Pogo accumulator drawing&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/7xNyP.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7xNyP.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It makes sense to hotfire an engine after incorporating changes. I have no detailed information on the test objectives, but they would have been testing the engine controller hardware and software in an operational environment.  Same for the pogo accumulator hardware. &lt;/p&gt;&#xA;" OwnerUserId="6944" LastEditorUserId="6944" LastEditDate="2018-01-19T05:52:57.590" LastActivityDate="2018-01-19T05:52:57.590" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24499" PostTypeId="1" CreationDate="2018-01-17T16:52:58.897" Score="4" ViewCount="659" Body="&lt;p&gt;I'm sure it's possible, but would it be wise to place a hex-cluster of SpaceX boosters around a central booster to get a very large payload, or one going to a very high orbit, into space? Maybe even to the extent of the central booster staying in orbit with a payload, like a personnel/supply capsule, that will later deorbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Anybody heard of any concepts, designs, plans or rumors regarding this idea?&lt;/p&gt;&#xA;" OwnerUserId="22541" LastEditorUserId="6816" LastEditDate="2018-01-18T03:50:22.180" LastActivityDate="2018-01-18T03:50:22.180" Title="Would a hex-cluster of SpaceX boosters around a central booster lift a large payload into space?" Tags="|launch|spacex|design|booster|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24500" PostTypeId="2" ParentId="24499" CreationDate="2018-01-17T17:18:10.447" Score="11" Body="&lt;p&gt;First issue is that even though Falcon Heavy is claimed to have a max payload of 50,000+ lbs to LEO, it turns out the payload adapter is much more limited, with two models, the bigger limited to 24,000 lbs range. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/YbC9d.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/YbC9d.jpg&quot; alt=&quot;Users manual on Payload adapters&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Second issue is that the fairing is volume limited, today, and increasing the payload would not help, since the density is usually not high enough to max out the mass allowed, before hitting volume limits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Of course, if you are developing an &lt;a href=&quot;https://wiki.kerbalspaceprogram.com/wiki/Asparagus_staging&quot; rel=&quot;noreferrer&quot;&gt;asparagus booster&lt;/a&gt; as suggested, a new payload adapter or new fairing seem like likely developments.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the fairing is tricky as it is already a hammerhead fairing, with a diameter of about 5m, and the core stage being 3m or so. Thus it already overhangs 1m on either side. It would be tricky to make it very much larger. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Length wise the booster is almost entirely maxed out in terms of bending and aerodynamic stability. Thus a bigger fairing is tricky with the current core stages, which your suggestion would be to keep in service, just more per launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Third, the current method of assembling and managing and launching the booster is designed to work in a flat layout. This would require much ground support equipment changes. Again, if needed, changes can be made.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However the real question seems to be, is SpaceX looking into any of these options, and there the answer is a clear &quot;No&quot;.  As Elon discussed at the IAC in 2017, they plan on replacing everything with BFR/BFS as a way of funding their Mars program.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I.e. They will launch satellites, ISS cargo and crew missions on BFR/BFS and use funding from those missions to help fund the development of the booster and ship. &lt;/p&gt;&#xA;" OwnerUserId="51" LastActivityDate="2018-01-17T17:18:10.447" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24501" PostTypeId="1" CreationDate="2018-01-17T17:45:54.113" Score="3" ViewCount="911" Body="&lt;p&gt;According to &lt;a href=&quot;https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt;, the Martian atmosphere can be saved from being dispersed into space through an artificial magnetic shield.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm interested in knowing the costs and mechanics that can make it happen today. &#xA;My question is not limited to solar winds, it includes cosmic radiation as well. Will superconducting magnets be more efficient? Is it better to put magnets at the north and south poles?&lt;/p&gt;&#xA;" OwnerUserId="21504" LastEditorUserId="21504" LastEditDate="2018-06-26T09:12:32.947" LastActivityDate="2018-06-26T09:12:32.947" Title="Artificial magnetic shielding for Mars feasible?" Tags="|mars|terraforming|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="24502" PostTypeId="1" CreationDate="2018-01-17T18:25:07.020" Score="30" ViewCount="14480" Body="&lt;p&gt;My science teacher asked us this, and nobody in my class knew. We guessed it would be three, but with no evidence.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Edit: The asker has not returned to clarify if they meant leaving the Earth-Moon system, or re-entry from orbit. As it has been a few days, and this question has received lots of attention, I have edited the title to reflect the common interpretation of 'leave orbit', and the content of most answers. Note that answers below also address the possibility it refers to re-entry. It would be a shame to close this, and so I think this minor modicum of interpretation is justified.)&lt;/p&gt;&#xA;" OwnerUserId="22542" LastEditorUserId="4660" LastEditDate="2018-01-22T01:42:43.487" LastActivityDate="2018-01-22T01:42:43.487" Title="How many times do you have to circle the Earth to break orbit?" Tags="|orbit|escape-velocity|" AnswerCount="10" CommentCount="22" ContentLicense="CC BY-SA 3.0" />
  <row Id="24503" PostTypeId="2" ParentId="24502" CreationDate="2018-01-17T19:06:13.057" Score="87" Body="&lt;p&gt;This depends on how much thrust you have available. With enough thrust, you don't need to be in Earth orbit at all: you can launch straight into an escape trajectory. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;New Horizons &lt;a href=&quot;http://www.boulder.swri.edu/pkb/ssr/ssr-mission-design.pdf&quot; rel=&quot;noreferrer&quot;&gt;did this, more or less&lt;/a&gt;: after launch it did &lt;a href=&quot;https://en.wikipedia.org/wiki/New_Horizons#Launch&quot; rel=&quot;noreferrer&quot;&gt;about 1/4&lt;/a&gt; orbit before the second stage was ignited again and insertion into its trajectory towards Jupiter began. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;With very little thrust, it can take months: &lt;a href=&quot;https://en.wikipedia.org/wiki/SMART-1&quot; rel=&quot;noreferrer&quot;&gt;SMART-1, one of the first missions to use an ion engine took 14 months&lt;/a&gt; to get into Moon orbit. The same principle can be used to leave Earth orbit. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-18T09:53:56.857" LastActivityDate="2018-01-18T09:53:56.857" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24504" PostTypeId="1" CreationDate="2018-01-17T20:42:00.930" Score="8" ViewCount="1742" Body="&lt;p&gt;I believe that currently, SpaceX only has two landing barges - one in the Atlantic (Of Course I Still Love You (OCISLY)), and one barge in the Pacific (Just Read The Instructions (JRTI)).  How long would it realistically take to cycle a barge from having a rocket landing on it, returning to port, offloading the rocket, and deploying it back to the landing zone ready for another ocean landing?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Obviously part of the cycle time is how far downrange the landing zones are, but although a return to land scenario is going to provide a faster cycle time, I am guessing that there will still be launches that require an ocean landing for various reasons.  Just wondering if there are announced plans for multiple landing barges to provide redundancy or additional landing ocean landing capacity?  &lt;/p&gt;&#xA;" OwnerUserId="16735" LastEditorUserId="6537" LastEditDate="2018-01-17T22:05:08.313" LastActivityDate="2018-01-18T12:44:18.307" Title="What is the turnaround time for SpaceX's barges?" Tags="|spacex|falcon-9|landing|reuse|droneship|" AnswerCount="2" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24505" PostTypeId="2" ParentId="24504" CreationDate="2018-01-17T20:47:16.437" Score="6" Body="&lt;p&gt;Towing speed is usually around 5-8 knots. The ASDS Barges do not have engines for travelling, rather the Thrustmasters at each corner are used almost exclusively for station keeping as the booster is coming in for a landing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The towing ships are different on each coast, and have different capabilities.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The folk on NasaSpaceflight.com forums usually track (Via the AIS system, land based is free, satellite data you have to pay for) and predict when the returning barge should come in sight of the various web cams.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Usually it is a day or two to return. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The real interesting question is how much work is required to get the barge ready to head back out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;They need to remove the stage from the barge, which has happened within the first day most of the time now that it has become more regular. Once the stage is off the barge, they need to refuel the Thrustmasters, unweld and clean up the brackets they used to hold the stage down. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;No clear answer has emerged, since there have been no really close landings to see how fast they can do the work. &lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-01-17T22:43:28.787" LastActivityDate="2018-01-17T22:43:28.787" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24506" PostTypeId="2" ParentId="24504" CreationDate="2018-01-17T20:52:24.403" Score="8" Body="&lt;p&gt;Let's look at some of the past missions to get an idea. In particular, let's look at the Echostar 105 mission. The mission launched on &lt;a href=&quot;http://www.spacex.com/missions&quot; rel=&quot;nofollow noreferrer&quot;&gt;October 11&lt;/a&gt;. It arrived in port on &lt;a href=&quot;https://www.floridatoday.com/story/tech/science/space/2017/10/15/spacex-falcon-9-booster-arrives-port-canaveral-hours-after-atlas-v-launch-cape-canaveral-florida/766014001/&quot; rel=&quot;nofollow noreferrer&quot;&gt;October 15th&lt;/a&gt;. Unloading the booster takes &lt;a href=&quot;https://www.popsci.com/this-is-how-spacex-unloads-15-story-rocket-booster-off-drone-ship#page-6&quot; rel=&quot;nofollow noreferrer&quot;&gt;less than a day&lt;/a&gt;. There is some amount of time to refurbish, and then the ship goes back out for the next mission. It then could take about the same amount of time to leave. So the lower limit is a week.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The upper limit can be done by seeing how long between missions. The &lt;a href=&quot;https://en.wikipedia.org/wiki/Autonomous_spaceport_drone_ship&quot; rel=&quot;nofollow noreferrer&quot;&gt;closest pair&lt;/a&gt; I believe was the Oct 11/ Oct 30 2017 landings, with only 19 days apart. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I suspect it could be done faster if required, but 1 to 3 weeks seems to be the fastest turnaround, if there aren't any issues with the landing.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-18T12:44:18.307" LastActivityDate="2018-01-18T12:44:18.307" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24507" PostTypeId="2" ParentId="24502" CreationDate="2018-01-17T21:29:29.003" Score="46" Body="&lt;h2&gt;Zero&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;See at: &lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;noreferrer&quot;&gt;https://en.wikipedia.org/wiki/Escape_velocity&lt;/a&gt; for theory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once you build enough velocity to surpass gravitational attraction, you will leave planetary orbit.  A spacecraft simply circling the earth in orbit is not inherently doing anything to contribute to escaping that orbit.&lt;/p&gt;&#xA;" OwnerUserId="22545" LastEditorUserId="18903" LastEditDate="2018-01-17T23:22:47.353" LastActivityDate="2018-01-17T23:22:47.353" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24508" PostTypeId="1" CreationDate="2018-01-18T00:40:50.373" Score="6" ViewCount="205" Body="&lt;h2&gt;Background&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As detailed in &lt;a href=&quot;https://space.stackexchange.com/questions/16604/thrust-and-rotation-strategy-to-circularize-a-standard-gto-orbit-using-ion-propu/21513#21513&quot;&gt;this answer&lt;/a&gt; low thrust optimization is subject of ongoing research. There are a few closed-loop control laws which allow to achieve a locally optimal (but close to globally optimal) transfer solutions between two orbits. Petropoulos, Naasz and Ruggiero have defined such control laws (and possibly others). The Ruggiero laws (1) are relatively simple in their implementation and have been shown to lead to almost identical results for fuel optimal and time optimal transfers the Petropoulos laws (2).&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Problem&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Applying the locally optimal control laws of Ruggiero or Naasz for LEO transfers works like a charm in two body dynamics or with non periodic perturbations (such as Sun gravity). This has been shown in their respective papers and many have demonstrated this in software libraries (including in my own).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, as soon as periodic dynamics are added to the propagated dynamics (specifically J2 in addition to two-body dynamics in as point-masses), the osculating orbital elements start to oscillate, causing these control laws to oscillate as well, and preventing them from converging in a reasonable time span.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;How to use these control laws with higher fidelity dynamics such that they converge in a reasonable transfer time?&lt;/strong&gt; I've tried computing the osculating orbital elements only at given spots in the orbit (true anomaly of zero, 45, 90, 270 degrees) but that doesn't help. Moreover, I'm concerned about using the Brouwer Mean orbital elements as these only smooth away the effects of J2: I assume that the control laws will start failing again once I enable spherical harmonics.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Follow-up question: &lt;strong&gt;Has anyone used the Eckstein Ustinov orbital element set developed in Spiridonova et al. 2014 for low thrust transfers?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;References&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;(1) Ruggiero, A., P. Pergola, S. Marcuccio, and M. Andrenucci. &quot;Low-thrust maneuvers for the efficient correction of orbital elements.&quot; In 32nd International Electric Propulsion Conference, pp. 11-15. 2011.&#xA;(2) Rabotin C. &quot;Feasibility of Reusable Continuous Thrust Spacecraft for Cargo Resupply Missions to Mars&quot; (2017). Aerospace Engineering Sciences Graduate Theses &amp;amp; Dissertations. &lt;a href=&quot;https://scholar.colorado.edu/asen_gradetds/184/&quot; rel=&quot;nofollow noreferrer&quot;&gt;link&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="1391" LastEditorUserId="1391" LastEditDate="2018-03-27T05:23:00.990" LastActivityDate="2018-03-27T05:23:00.990" Title="Locally optimal control for low thrust transfers with J2 and other Earth perturbations" Tags="|orbital-mechanics|orbital-elements|low-energy-transfer|" AnswerCount="0" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24509" PostTypeId="1" CreationDate="2018-01-18T03:02:36.410" Score="8" ViewCount="2527" Body="&lt;p&gt;This engine was making news few years ago, but I have not heard much about it lately. Were there any updates regarding its tests, further development or anything?&lt;/p&gt;&#xA;" OwnerUserId="12158" LastActivityDate="2018-05-21T17:32:35.663" Title="What is the current status of EmDrive engine?" Tags="|engine-design|emdrive|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24510" PostTypeId="2" ParentId="24494" CreationDate="2018-01-18T05:16:58.897" Score="3" Body="&lt;p&gt;Sorry, I have no documentation.  I can only offer you a physics explanation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The larger wavelength of the microwaves is too big to &quot;see&quot; the ridges.  The wavefront is reflected by the general curve of the dish to arrive at the focus.  The ridges are obviously much smaller than 13mm (assuming that's a 485mm diameter dish).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's the same reason your microwave oven door shield can have holes in it.  Since they're smaller than the wavelength, the energy is reflected efficiently.  The ridges don't contribute to the focusing of the microwaves, they are ignored by the microwaves.  As such, there's nothing special about how two different wavelengths are focused.  Anything around that size or larger would be similarly focused.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The shorter-wavelength IR is affected by the ridges.  The surfaces of the ridges do not focus to the same spot as the general curve.  This reduces the IR power that reaches the parabolic focus.&lt;/p&gt;&#xA;" OwnerUserId="5979" LastActivityDate="2018-01-18T05:16:58.897" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24511" PostTypeId="1" AcceptedAnswerId="24549" CreationDate="2018-01-18T06:57:20.273" Score="7" ViewCount="2061" Body="&lt;p&gt;I've been reading about some concerns about the densified (sub-cooled) propellants for Falcon 9 crewed missions in Spaceflight Now's article &lt;a href=&quot;https://spaceflightnow.com/2018/01/17/lawmakers-question-safety-schedule-for-boeing-spacex-commercial-crew-ships/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lawmakers question safety, schedule for Boeing/SpaceX commercial crew ships&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could this concern be sidestepped by launching with ambient temperature RP-1 and boiling point LOX? Taking a payload hit of course. If so, would doing so prevent reuse?&lt;/p&gt;&#xA;" OwnerUserId="16505" LastEditorUserId="12102" LastEditDate="2018-01-20T14:56:05.967" LastActivityDate="2018-05-08T13:25:30.587" Title="Could the SpaceX Falcon 9 Block 5 work without sub-cooling the propellants?" Tags="|spacex|falcon-9|crewed-spaceflight|fuel|" AnswerCount="1" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24513" PostTypeId="2" ParentId="24502" CreationDate="2018-01-18T10:44:06.707" Score="10" Body="&lt;p&gt;In addition to @Hobbes answer, even if you have high thrust you can use the fuel more efficiently by accelerating when the rocket is moving the fastest due to the &lt;a href=&quot;https://en.wikipedia.org/wiki/Oberth_effect&quot; rel=&quot;noreferrer&quot;&gt;Oberth effect&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Explanation in terms of work&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Rocket engines produce the same force regardless of their velocity. A rocket acting on a fixed object, as in a static firing, does no useful work at all; the rocket's stored energy is entirely expended on accelerating its propellant in the form of exhaust. But when the rocket moves, its thrust acts through the distance it moves. Force multiplied by distance is the definition of mechanical energy or work. So the farther the rocket and payload move during the burn (i.e. the faster they move), the greater the kinetic energy imparted to the rocket and its payload and the less to its exhaust.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The closer you are to orbital periapsis (the closest point to Earth), the faster your rocket moves, and the more efficient the burns. However the burn isn't instant so your rocket will quickly move out of the most efficient zone. To maximize efficiency you'd then circle the Earth many times, each time firing the rocket near the periapsis.&lt;/p&gt;&#xA;" OwnerUserId="19692" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-18T10:44:06.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24514" PostTypeId="2" ParentId="19568" CreationDate="2018-01-18T12:51:25.770" Score="8" Body="&lt;p&gt;You could, but it wouldn't make sense. The most fuel efficient way to enter an areosynchronous (same as geosynchronous, but Mars) would be to do a close pass of Mars to orbit, burning until the apoaerion is at the areosynchronous orbital height, and then burn again to have it be at a true areosynchronous orbital plane.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="6816" LastEditDate="2018-01-19T02:39:01.040" LastActivityDate="2018-01-19T02:39:01.040" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24515" PostTypeId="2" ParentId="24502" CreationDate="2018-01-18T13:17:12.963" Score="25" Body="&lt;p&gt;Think of this as a slightly different question, and the answer becomes more clear.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;How many times do you have to circle the Sun to leave orbit?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth has been orbiting our Sun for about &lt;a href=&quot;https://en.wikipedia.org/wiki/Age_of_the_Earth&quot; rel=&quot;noreferrer&quot;&gt;4.5 Billion years&lt;/a&gt; with each &lt;a href=&quot;https://en.wikipedia.org/wiki/Earth%27s_orbit&quot; rel=&quot;noreferrer&quot;&gt;year being one orbit.&lt;/a&gt;  The Earth is expected to stay in orbit around the Sun for the &lt;a href=&quot;https://en.wikipedia.org/wiki/Stability_of_the_Solar_System&quot; rel=&quot;noreferrer&quot;&gt;next few billion years.&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also consider the moon has been orbiting the Earth for about the &lt;a href=&quot;https://en.wikipedia.org/wiki/Moon&quot; rel=&quot;noreferrer&quot;&gt;same 4.5 billion years.&lt;/a&gt;  ~12 orbits a year for 4.5 billion is ~54 billion orbits and it is still there.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The number of orbits is not relevant to leaving orbit.  &lt;/p&gt;&#xA;" OwnerUserId="109" LastEditorUserId="109" LastEditDate="2018-01-18T13:49:58.550" LastActivityDate="2018-01-18T13:49:58.550" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24516" PostTypeId="2" ParentId="24502" CreationDate="2018-01-18T13:57:49.127" Score="6" Body="&lt;p&gt;Here is an example of going to deep space without orbiting the Earth once.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As can be heard in &lt;a href=&quot;https://www.youtube.com/watch?v=OvHJSIKP0Hg&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; SpaceX's launch of the DSCOVR spacecraft to &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun-Earth L1&lt;/a&gt; went there almost directly. The first secondary engine cut-off (SECO-1) was at about &lt;code&gt;T+ 00:08:40&lt;/code&gt;, and after what the announcer says (at about &lt;code&gt;T+ 00:09:50&lt;/code&gt;) would be &quot;in about 21 minutes&quot;, the secondary stage's &lt;em&gt;second burn&lt;/em&gt; would put it in a Heliocentric orbit on its way to the SE L1. Considering that a LEO orbit is about 90 minutes, DSCOVR was in Earth orbit for &lt;em&gt;roughly half of one Earth orbit&lt;/em&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is more to read in the answers to the question &lt;a href=&quot;https://space.stackexchange.com/q/8081/12102&quot;&gt;Why would a mission to Sun-Earth L1 have an instantaneous launch window?&lt;/a&gt; The fractional Earth orbit was used only to minimize the required Δv based on launching from Florida and getting to L1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; From Spaceflight 101's &lt;a href=&quot;http://spaceflight101.com/dscovr/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Space Climate Observatory&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ILwuX.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ILwuX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Xc32J.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xc32Jm.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-21T03:52:37.243" LastActivityDate="2018-01-21T03:52:37.243" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24517" PostTypeId="1" CreationDate="2018-01-18T14:29:08.870" Score="2" ViewCount="411" Body="&lt;p&gt;I just wrote (and have since removed) the following paragraph in &lt;a href=&quot;https://space.stackexchange.com/a/24516/12102&quot;&gt;this answer to &lt;em&gt;How many times do you have to circle the Earth to break orbit?&lt;/em&gt;&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;As can be heard in &lt;a href=&quot;https://www.youtube.com/watch?v=OvHJSIKP0Hg&quot; rel=&quot;nofollow noreferrer&quot;&gt;this video&lt;/a&gt; SpaceX's launch of the DSCOVR spacecraft to &lt;a href=&quot;https://en.wikipedia.org/wiki/Lagrangian_point&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sun-Earth L1&lt;/a&gt; went there almost directly. The first secondary engine cut-off (SECO-1) was at about &lt;code&gt;T+ 00:08:40&lt;/code&gt;, and after what the announcer says (at about &lt;code&gt;T+ 00:09:50&lt;/code&gt;) would be &amp;quot;in about 21 minutes&amp;quot;, the secondary stage's &lt;em&gt;second burn&lt;/em&gt; would put it in to a Heliocentric orbit (&lt;strong&gt;along the stable manifold of its future SE L1 Halo orbit&lt;/strong&gt;) on its way to the SE L1. Considering that a LEO orbit is about 90 minutes, DSCOVR was in Earth orbit for &lt;em&gt;roughly half of one Earth orbit&lt;/em&gt;. (emphasis added)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;DSCOVR would need about four months to arrive in its final orbit, and explained in the answers to &lt;a href=&quot;https://space.stackexchange.com/q/8081/12102&quot;&gt;Why would a mission to Sun-Earth L1 have an instantaneous launch window?&lt;/a&gt; was probably traveling along the most efficient (lowest Δv) path to arrive there.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Based on comments, I should explain that my use of &amp;quot;probably traveling along the most efficient&amp;quot; comes from a reading of the paper &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150022915.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Resurrected DSCOVR Propulsion System – Challenges and Lessons Learned&lt;/a&gt; that nicely describes DSCOVR's propulsion system and lists the two mid-course corrections used:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The first planned thruster maneuver was the MCC burn, which was designed to correct any launch dispersion in the spacecraft trajectory. Since the Falcon 9 trajectory resulted in a low launch dispersion, the MCC #1 burn only required a burn duration of 37 seconds (0.4895 m/s). The maneuver was performed successfully 32 hours after launch.&lt;/p&gt;&#xA;&lt;p&gt;MCC #2 was performed on April 28, 2015, as a +Z maneuver with a burn duration of 3.1 minutes (about 2.449 m/s).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Launch was 11-Feb-2015, so MCC #2 happened at around day ~77.&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Is the idea that DSCOVR travelled on a trajectory along a &lt;a href=&quot;https://en.wikipedia.org/wiki/Three-body_problem&quot; rel=&quot;nofollow noreferrer&quot;&gt;CR3BP&lt;/a&gt; manifold at least generally correct?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Is the wording at least fairly accurate, or is there a better way to say this? If I understand correctly, DSCOVR is actually in a Lissajous orbit, not technically a halo orbit, although sometimes they get lumped together. In that case is there still a manifold?&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;Possibly helpful items:&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150018272.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lissajous Orbit Control for the Deep Space Climate Observatory Sun-Earth L1 Libration Point Mission&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Fig. 1 of &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019786.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Early Mission Maneuver Operations for the Deep Space Climate Observatory Sun-Earth L1 Libration Point Mission&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;page 3 of &lt;a href=&quot;http://www.swpc.noaa.gov/sites/default/files/images/u33/DSCOVR-Halfway-to-L1.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;DSCOVR Halfway to L1&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Xc32J.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Xc32J.png&quot; alt=&quot;illustration of DSCOVR near Sun-Earth L1&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-12-26T19:49:25.260" LastActivityDate="2021-12-26T19:49:25.260" Title="Did DSCOVR travel &quot;along the stable manifold of its future SE L1 Halo orbit&quot; to get there?" Tags="|orbital-mechanics|halo-orbit|three-body|lissajous-orbit|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 4.0" />
  <row Id="24519" PostTypeId="1" AcceptedAnswerId="24533" CreationDate="2018-01-18T18:16:21.150" Score="7" ViewCount="522" Body="&lt;p&gt;Pictures of SpaceX's &quot;&lt;a href=&quot;https://en.m.wikipedia.org/wiki/SpaceX_Mars_transportation_infrastructure&quot; rel=&quot;nofollow noreferrer&quot;&gt;Big Falcon Spaceship&lt;/a&gt;&quot; (BFS) &lt;a href=&quot;http://res.cloudinary.com/spontany-com/image/upload/v1508082275/spontany/elon-musks-spacex-ama-mars-bfr-bfs-spaceship/pic_21.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;delta wing&lt;/a&gt; don't seem to show presence of any aerodynamic control surfaces. How will it's attitude be controlled when encountering the atmosphere?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To go a bit further, &lt;a href=&quot;https://2.bp.blogspot.com/-SguUW20piOk/WbxRyY_I-jI/AAAAAAAACWM/9ZCyHZ7x-aYYxfi0Ve9Yhc2G7lBIFHDQQCLcBGAs/s2048/Fleet%2Bof%2BSpaceX%2BITS%2Bspaceships%2Bon%2BMars%2Bby%2BSam%2BTaylor.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;BFS V1&lt;/a&gt; looked like center of lift was very close to center of gravity, where BFS V2 looks like center of lift is very aft from center of gravity.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;One thing to notice on both versions is the use of blunt wedge at trailing edge, like seen on &lt;a href=&quot;https://i.ytimg.com/vi/cfnXs03FA5s/hqdefault.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;X-15&lt;/a&gt; vertical stabilizer.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: @PearsonArtPhoto As far as I try to understand, movable ballast sled won't act on the same axis on both BFS and Dragon. I think either vernier nor reaction wheels can match aerodynamic forces on delta wings, and grid fins should be as fat as delta wing to turn it 180° Anyway you're right we'll see the solution soon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/BaFp4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/BaFp4.jpg&quot; alt=&quot;movable ballast sled&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="19132" LastEditDate="2018-01-19T15:53:48.387" LastActivityDate="2018-01-19T16:01:45.930" Title="How will BFS maneuver during aerobraking?" Tags="|spacex|reentry|aerobraking|spacex-starship|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24520" PostTypeId="2" ParentId="24502" CreationDate="2018-01-18T20:13:47.220" Score="6" Body="&lt;p&gt;Note that &quot;leave orbit&quot; can mean achieving escape velocity (to go to the Moon, Mars, or beyond), or it can mean slowing down and re-entering the atmosphere and eventually landing (or splashing down, or in extreme cases, lithobraking).  Or, to be ridiculously pedantic, you can leave one orbit to enter another (i.e., GTO to GEO).  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;What matters is what your teacher meant by &quot;circle the Earth&quot;.  As currently phrased, it's not clear.  Orbital maneuvers are expressed in terms of change in velocity (&amp;Delta;V), not the number of times it takes to go around the planet.  To go up (to higher orbit or escape velocity), most chemical rockets do it in less than a single orbit.  Rockets or spacecraft using electrical propulsion (ion drives) may take several dozen (or several dozens of) orbits to do it.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;To hit a specific target, you may have to go into a parking orbit briefly and wait for things to line up (such as on the Apollo missions).  But if all you care about is &quot;leaving orbit&quot;, regardless of direction, then it only depends on how much oomph you have in your rocket.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To come back down, most rockets and spacecraft thrust just enough to bring their perigee into the uppermost atmosphere, then let drag and friction take over.  That usually takes less than a single orbit as well for chemical rockets.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;or&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your teacher is being funny and asking a trick question:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&quot;In orbit&quot; around &lt;em&gt;what&lt;/em&gt;?  The Earth?  The Sun?  The center of the Milky Way?&lt;/li&gt;&#xA;&lt;li&gt;Is he/she counting on you &quot;orbiting&quot; the center of the Earth by virtue of standing on its surface, and the Earth spinning on its axis (which isn't really an &lt;em&gt;orbit&lt;/em&gt; for a number of reasons, but maybe he/she is using a &quot;horseshoes and hand grenades&quot; definition of orbit)?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="12922" LastActivityDate="2018-01-18T20:13:47.220" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24521" PostTypeId="2" ParentId="287" CreationDate="2018-01-18T20:30:44.627" Score="1" Body="&lt;p&gt;Clearly, the answer has to be a probability distribution.  The actual distribution would necessarily &quot;encode&quot; the past history of the galaxy, neighboring galaxies, etc.  Since a fast-moving object has a longer trajectory, it has a greater probability of hitting something.  Consequently, the faster-moving, longer-path objects will collide sooner and will be culled from the system sooner, leaving more slower objects.  Thus, the probability distribution of number of objects vs speed will shift over time toward slower objects.  But, again, it is a probability distribution,, so there is some chance of a very fast-moving object left over from earlier days, or one that was sling-shoted in some less-probable way.&lt;/p&gt;&#xA;" OwnerUserId="22563" LastActivityDate="2018-01-18T20:30:44.627" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24523" PostTypeId="1" AcceptedAnswerId="24524" CreationDate="2018-01-18T21:34:41.973" Score="4" ViewCount="1916" Body="&lt;p&gt;I mean something like Kerbal Space Program style - payload between 2 parallel rockets e.g. &lt;a href=&quot;https://i.stack.imgur.com/fAZpX.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fAZpX.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, space shuttle configuration comes to mind where we have the large fuel tank between 2 solid boosters: &lt;a href=&quot;http://www.britmodeller.com/walkarounds/misc/extank/t%2001.JPG&quot; rel=&quot;nofollow noreferrer&quot;&gt;space shuttle rocket&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I imagine such a payload between 2 Falcon 9 or Proton M rockets e.g.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I know this will have issues judging on Falcon heavy where they combined 3 boosters that are separately tested and needed years of tests to make it work, but is it feasible and has anyone considered something like that?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;edit: I know the Kerbal example won't work in real life, my point is instead of developing new super heavy lifting vehicles can't we just use few of our reliable rockets to go to the Moon or Mars.&lt;/p&gt;&#xA;" OwnerUserId="19321" LastEditorUserId="19321" LastEditDate="2018-01-18T22:16:25.117" LastActivityDate="2018-10-27T07:59:52.130" Title="Has anyone considered using multiple rockets to lift a single payload?" Tags="|launch|rockets|propulsion|" AnswerCount="4" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24524" PostTypeId="2" ParentId="24523" CreationDate="2018-01-18T22:00:38.177" Score="14" Body="&lt;p&gt;Apart from edge cases like the Space Shuttle or the Titan III series (where the SRM boosters fire at launch and the core stage ignites later; you could consider the core stage to be &quot;payload&quot; slung between two rockets), I don't think it's been done. There's no particular reason it can't work. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your Kerbal example won't work because it's not aerodynamic, of course. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There would be a weight penalty relative to a single-stick launcher of equivalent capability; you need inert nose cones on each stick, the payload module needs a tail cone to reduce drag, and a more complex structure to properly distribute load to the sticks. The launch pad needs to be designed to accommodate the launcher.&lt;/p&gt;&#xA;" OwnerUserId="195" LastActivityDate="2018-01-18T22:00:38.177" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24525" PostTypeId="2" ParentId="24502" CreationDate="2018-01-18T22:32:52.007" Score="5" Body="&lt;p&gt;Orbiting in circles cannot get you away from earth. To leave earth's orbit requires accelerating away from the orbit. The acceleration is usually by firing rocket motors that burn fuel expanding hot gases at high speed from a nozzle.&lt;/p&gt;&#xA;" OwnerUserId="22567" LastEditorUserId="6816" LastEditDate="2018-01-19T02:09:30.673" LastActivityDate="2018-01-19T02:09:30.673" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24526" PostTypeId="2" ParentId="24496" CreationDate="2018-01-18T22:35:09.160" Score="1" Body="&lt;p&gt;It's not a problem. The thermodynamic and chemical properties of the fuel and oxidizer suffer a very small penalty from the H&lt;sub&gt;2&lt;/sub&gt;O and CO&lt;sub&gt;2&lt;/sub&gt; gases they contain after pre-burning. Meanwhile, you get to feed your combustion chamber with hot gasses, ensuring efficient mixing and combustion. To steal a line from Real Genius, &lt;em&gt;&amp;quot;It's like lasing a stick of dynamite.&amp;quot;&lt;/em&gt; Note that the SSME (RS-25) feeds its combustion chamber with preburned fuel and oxidizer (with low concentration of exhaust products) and it's the gold standard for ISP.&lt;/p&gt;&#xA;" OwnerUserId="8247" LastEditorUserId="47149" LastEditDate="2022-05-29T16:03:03.350" LastActivityDate="2022-05-29T16:03:03.350" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="24530" PostTypeId="1" CreationDate="2018-01-19T14:24:30.377" Score="1" ViewCount="439" Body="&lt;p&gt;we are building a cubesat and we have considered using an arduino as the computer. However I'm not sure if that is smart. I mean will the arduino get damaged in some sort? &#xA;Is arduino suitable to do cubesats?&#xA;Is it effective or will we face problems?&#xA;And is there better alternative?&lt;/p&gt;&#xA;" OwnerUserId="22578" LastActivityDate="2018-01-19T14:24:30.377" Title="Can I use Arduino for a cubesat?" Tags="|cubesat|" AnswerCount="0" CommentCount="1" ClosedDate="2018-01-19T14:54:56.433" ContentLicense="CC BY-SA 3.0" />
  <row Id="24531" PostTypeId="2" ParentId="24519" CreationDate="2018-01-19T15:09:07.210" Score="1" Body="&lt;p&gt;It isn't known exactly, there are a couple of possibilities. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dragon uses &quot;Movable ballast sled&quot; in order to control it's attitude when reentering. Basically it moves a weight around to control the angle in which it is reentering. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second possibility would be using control jets. This seems unlikely. It might also use the main rocket engine thrust gimbaling to control the attitude, or even a grid fin design similar to Falcon 9.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I suspect it will use the movable ballast sled design, or else add control surfaces to the wings. We will probably know more in July or August, when another update will be made in regard to the BFR.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-19T15:09:07.210" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24532" PostTypeId="2" ParentId="21737" CreationDate="2018-01-19T15:29:22.563" Score="1" Body="&lt;blockquote&gt;&#xA;  &lt;p&gt;There is a mission patch on the side. I had always thought mission patches were for missions identified with a particular launch, but the NICER-SEXTANT mission is really a research program if I understand correctly. If so, how common is it for research projects to have mission patches?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://wind.nasa.gov&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wind spacecraft&lt;/a&gt; has its own mission patch/logo (logo found at &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Windlogo.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://en.wikipedia.org/wiki/File:Windlogo.gif&lt;/a&gt;) as do the &lt;a href=&quot;http://parkersolarprobe.jhuapl.edu&quot; rel=&quot;nofollow noreferrer&quot;&gt;Parker Solar Probe&lt;/a&gt; (logo at &lt;a href=&quot;http://parkersolarprobe.jhuapl.edu/Multimedia/Images.php&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://parkersolarprobe.jhuapl.edu/Multimedia/Images.php&lt;/a&gt;) and &lt;a href=&quot;https://www.nasa.gov/mission_pages/mms/index.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;MMS&lt;/a&gt; missions.  I think all NASA missions at least have their own logo.  Whether they have an actual fabric patch made that can be sewn onto something like a shirt or jacket is another thing, but they definitely have their own logo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance, the &lt;a href=&quot;http://fields.ssl.berkeley.edu&quot; rel=&quot;nofollow noreferrer&quot;&gt;FIELDS&lt;/a&gt; instrument on Parker Solar Probe has its own logo (I think the other three instrument suites do as well).  I know the ISIS instrument (from &lt;a href=&quot;https://en.wikipedia.org/wiki/Isis&quot; rel=&quot;nofollow noreferrer&quot;&gt;Egyptian mythology&lt;/a&gt;, not the misguided terrorist organization), which is now called IS$\odot$IS, has its own logo (found here &lt;a href=&quot;https://helios.gsfc.nasa.gov/ace/erchome.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://helios.gsfc.nasa.gov/ace/erchome.html&lt;/a&gt;).  It was named ISIS first and then the terrorist organization came along and ruined the name, so they renamed it to IS$\odot$IS to avoid confusion and bad press (the instrument paper can be found at &lt;a href=&quot;https://link.springer.com/article/10.1007/s11214-014-0059-1&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://link.springer.com/article/10.1007/s11214-014-0059-1&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In general, most missions and instruments have requirements for some fraction of the total budget going to education and public outreach, which often involves generating a logo/design used for visual association with said mission/instrument.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As an aside, when a university gets $5+ million to build something, you can bet the art department will join in ;)&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Is there a write-up somewhere or some information about the design of the patch?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;I am fairly certain there is but it might not be searchable unless you are part of the organization that designed it.  I was recently involved with a mission proposal and we have lots of documents about the logo and the acronym we chose, but these are not searchable things.  It would be interesting to see all the brainstorming sessions on mission/instrument names and logo designs.&lt;/p&gt;&#xA;" OwnerUserId="12508" LastActivityDate="2018-01-19T15:29:22.563" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24533" PostTypeId="2" ParentId="24519" CreationDate="2018-01-19T16:01:45.930" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/SpaceXLounge/comments/73h70h/transcript_of_elon_musks_iac_2017_talk_4_parts/&quot; rel=&quot;noreferrer&quot;&gt;According to Elon&lt;/a&gt;, mainly with aerodynamic control surfaces.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Compared to the design I showed last time, you can see that there’s a small delta wing at the back of the rocket. The reason for that is in order to expand the mission envelope of the BFR Spaceship. Depending on whether you’re [landing on] a planet or a moon that has no atmosphere, a thin atmosphere, or a dense atmosphere, and depending on whether you’re reentering with no payload in the front, a small payload, or a heavy payload, you have to balance the rocket out as it’s coming in. &lt;strong&gt;So the delta wing at the back, which also includes a split flap for pitch and roll control allows us to control the pitch angle&lt;/strong&gt; despite having a wide range of payloads in the nose and a wide range of atmospheric densities. We were trying to avoid having to have the delta wing, but it was necessary in order to generalize the capability of the Spaceship such that it could land anywhere in the Solar System.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(Bold mine) &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are also meant to be attitude control thrusters operating on gaseous methane and oxygen that would make them much more powerful than the cold gas nitrogen thrusters used on current vehicles, but he didn't emphasize them much in terms of entry control.&lt;/p&gt;&#xA;" OwnerUserId="3044" LastActivityDate="2018-01-19T16:01:45.930" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24534" PostTypeId="2" ParentId="24502" CreationDate="2018-01-19T17:15:35.790" Score="1" Body="&lt;p&gt;Doesn't have anything to do with it. The critial parameter is your (parabolic) escape velocity... for earth it's appr. 11,2 km/s. As soon as you exceed this velocity you will leave earth's gravitational field... &lt;/p&gt;&#xA;" OwnerUserId="22584" LastActivityDate="2018-01-19T17:15:35.790" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24535" PostTypeId="1" AcceptedAnswerId="24536" CreationDate="2018-01-19T19:49:15.757" Score="8" ViewCount="475" Body="&lt;p&gt;Please help me understand: As far as I understand it, when an object gets thrown off the surface of a planet by an impact and doesn't reach escape velocity it is on a suborbital trajectory and therefore will impact on the planet eventually. If that is true, how can debris from an impact reach a stable orbit? How can the periapsis be higher than the point of the impact?&lt;/p&gt;&#xA;" OwnerUserId="22587" LastEditorUserId="22587" LastEditDate="2018-01-19T20:16:21.183" LastActivityDate="2018-01-20T02:44:10.840" Title="How can debris from an impact escape to a stable orbit?" Tags="|orbital-mechanics|the-moon|orbit|planetary-science|impact|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24536" PostTypeId="2" ParentId="24535" CreationDate="2018-01-19T20:06:38.850" Score="6" Body="&lt;p&gt;You are correct that if a single object is thrown off of a planet that it will either escape, or else return to that object, that orbiting isn't a solution. The way that an object can orbit is by interacting with something after it has launched. For instance, it might pass close to another large clump that will use gravity to change it's path. Another solution is that it could impact another object, causing it to enter an orbit. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The third solution is that the impact could change the orbit of the host body itself. I believe this is what was actually done in the case of forming the Moon. Also of some note is that the velocity transfer isn't an instantaneous value, but rather happens over some period of time. The periapsis can be as late as the latest change. So for a side impacting asteroid, much of the speed will remain even after it has left the Earth's surface, and will continue to accelerate pieces that went ahead of the event.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To see how this could actually happen, take a look at &lt;a href=&quot;https://courses.seas.harvard.edu/climate/eli/Courses/EPS281r/Sources/Origin-of-the-Moon/more/Canup-Ashphaug-2001.pdf&quot; rel=&quot;noreferrer&quot;&gt;this paper&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-19T20:06:38.850" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24537" PostTypeId="1" AcceptedAnswerId="24539" CreationDate="2018-01-19T22:02:38.820" Score="9" ViewCount="340" Body="&lt;p&gt;When an asteroid enters the sphere of influence of a planet and the periapsis is above the atmosphere (if it wasn't and the object doesn't leave the atmosphere at escape velocity, the object would eventually fall back to the planet), how can it go into an orbit? If it wasn't slowed down, wouldn't it have to leave the sphere of influence with the same velocity as it entered it? And if it was slowed down, how did that happen?&lt;/p&gt;&#xA;" OwnerUserId="22587" LastActivityDate="2018-01-29T17:21:46.493" Title="How can a planet “catch” an asteroid?" Tags="|orbital-mechanics|orbit|asteroid|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24538" PostTypeId="1" CreationDate="2018-01-19T22:14:19.583" Score="13" ViewCount="946" Body="&lt;p&gt;The date of the &quot;Pale Blue Dot&quot; photo is 14 February 1990, but is its exact time of capture (or at least hour) known?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7454G.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7454G.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I was a school child back then, and I am trying to figure out what I was most likely doing at the time the pale blue dot image was taken.&lt;/p&gt;&#xA;" OwnerUserId="22590" LastEditorUserId="6537" LastEditDate="2018-01-23T21:25:19.600" LastActivityDate="2020-02-13T11:44:26.810" Title="At what exact time was the Pale Blue Dot image taken?" Tags="|voyager|pale-blue-dot|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24539" PostTypeId="2" ParentId="24537" CreationDate="2018-01-19T22:51:15.673" Score="7" Body="&lt;p&gt;The short of it is, because gravity is a complex interaction between thousands of objects rather than the simple two body problems commonly presented. The influences of some distant world could apply the proper mild braking force at the right moment, and there you are. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Wikipedia has an article on captures without insertion burns, or &lt;a href=&quot;https://en.wikipedia.org/wiki/Ballistic_capture&quot; rel=&quot;noreferrer&quot;&gt;ballistic capture&lt;/a&gt;. One scenario described is entering an orbit just below the orbit of the body you wish to transfer to, and letting it's gravity pull you in as it approaches from behind. This leaves you in a very high orbit of the body that caught up to you, but evidently isn't stable. As a navigational tool it doesn't seem very practical, but it could explain the temporary capture of asteroids by a planet.&lt;/p&gt;&#xA;" OwnerUserId="3044" LastActivityDate="2018-01-19T22:51:15.673" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24540" PostTypeId="1" CreationDate="2018-01-19T23:16:05.497" Score="4" ViewCount="230" Body="&lt;p&gt;I am trying to see how frequently I can acquire a link for 1,2,3,4,5 or more satellites.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;When I began, I just decided to separate satellites by True anomoly, because that made the most sense. But now I'm wondering if I can try out different RAAN of each satellite, because that might be better.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So if I have 4 satellites, can I put them at the same inclination but with different RAAN (90,180,270,360)? This would be done in one launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is that possible, or does that make for a much more costly launch, or multiple launches? This is a LEO I'm looking at right now.&lt;/p&gt;&#xA;" OwnerUserId="22594" LastEditorUserId="25" LastEditDate="2018-01-20T00:55:20.987" LastActivityDate="2018-01-21T02:46:32.173" Title="Can a launch vehicle put several different satellites into different RAAN in one launch?" Tags="|orbital-mechanics|launch|satellite-constellation|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24541" PostTypeId="2" ParentId="24540" CreationDate="2018-01-20T00:54:51.990" Score="3" Body="&lt;p&gt;You can, but it will take time and fuel.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Right Ascension changes with time. The higher you are, the longer an orbit takes, and the slower your drift will be. Thus, you can start drifting your satellites by setting them at different altitude. Once you have reached your desired separation, you then have the satellites burn to be in the same plane.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Depending on how much fuel you are willing to spend, this could take a year or more to happen. It would take about a year to drift 90 degrees if separated by 100 km or so, depending on the inclination.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-20T00:54:51.990" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24542" PostTypeId="2" ParentId="24535" CreationDate="2018-01-20T02:44:10.840" Score="2" Body="&lt;p&gt;You can actually have this happen in freespace. (I.e. no other bodies.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Imagine the following in 2d, to make things easier.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine you kick off a rotating piece of debris &quot;North&quot; at somewhat under escape velocity. Then somewhere near apoapsis it breaks into two pieces - one flung &quot;West&quot; and the other &quot;East&quot;. The two pieces each end up with enough tangential velocity to form an orbit around the parent body.&lt;/p&gt;&#xA;" OwnerUserId="12210" LastActivityDate="2018-01-20T02:44:10.840" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24544" PostTypeId="2" ParentId="19568" CreationDate="2018-01-20T05:19:35.617" Score="2" Body="&lt;p&gt;Space Flight 101:  Do your burns as deep in a gravity well as you can to maximize the Oberth effect.  That means your capture burn should be done as close to the planet as you dare no matter what your intended final orbit.  Note that this very well might be in the outer fringes of the planet's atmosphere.  While in theory there might be some scenario where you were approaching slowly and your objective was a high orbit where this isn't the right I do not believe any such cases exist within our solar system.&lt;/p&gt;&#xA;" OwnerUserId="276" LastActivityDate="2018-01-20T05:19:35.617" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24545" PostTypeId="1" AcceptedAnswerId="24554" CreationDate="2018-01-20T14:09:45.940" Score="7" ViewCount="203" Body="&lt;p&gt;In &lt;a href=&quot;https://www.youtube.com/watch?list=PLPp8jXxLy0rPNUprqw9jN97n3MSCVuGNO&amp;amp;params=EAEYATgBSAFYBWILOWs4Y0RuM0p5OE1oAw%253D%253D&amp;amp;v=FudFBMLC0C4&quot; rel=&quot;noreferrer&quot;&gt;this talk&lt;/a&gt;, J. L. Galache mentions that asteroid miners are looking for NEAs with:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;low delta-v (so the asteroid is easy to get to/from from Earth)&lt;/li&gt;&#xA;&lt;li&gt;low synodic period (for frequent launch/return-windows)&lt;/li&gt;&#xA;&lt;li&gt;low albedo, i.e. dark&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;I don't understand the last point. The low albedo is apparently thought to be correlated with a carbonaceous and water-rich composition. Why? Are there some good resources about this stuff online?&lt;/p&gt;&#xA;" OwnerUserId="4558" LastEditorUserId="4558" LastEditDate="2018-01-20T14:38:18.587" LastActivityDate="2018-01-21T09:16:51.357" Title="Why are low-albedo asteroids thought to be more water-rich?" Tags="|asteroid|water|mining|near-earth-asteroid|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24546" PostTypeId="1" AcceptedAnswerId="24560" CreationDate="2018-01-20T15:28:47.163" Score="3" ViewCount="905" Body="&lt;p&gt;The PRNewsWire release &lt;a href=&quot;https://www.prnewswire.com/news-releases/first-iceye-x1-radar-image-from-space-published-300583982.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;First ICEYE-X1 Radar Image from Space Published&lt;/a&gt; says:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The full image transmitted to the ground from ICEYE-X1 exceeded 1.2GB of raw data and spans an area of roughly 80 x 40 km on the ground. ICEYE-X1 obtained the image in the span of ten seconds, traveling at a speed of more than 7.5 km/s and at an altitude exceeding 500 km. Matching what ICEYE simulated prior to the launch, the final data resolution from the first satellite reaches 10 x 10 meters.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Synthetic-aperture_radar&quot; rel=&quot;nofollow noreferrer&quot;&gt;Synthetic Aperture Radar&lt;/a&gt; works by mathematically combining radar data recored from a number of different physical locations, effectively synthesizing a large effective aperture defined by the cluster of different locations. For imaging the Earth from space, this is done using the movement of the spacecraft itself along-track, and if I understand correctly, for full 2D imaging of the surface the cross-track sampling comes either from a large physical span of antennas (see for example &lt;a href=&quot;https://space.stackexchange.com/q/22018/12102&quot;&gt;this question&lt;/a&gt;) or from closely-spaced ground tracks of completely different orbits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The quoted passage says that the image was taken in only tens of seconds, which I believe means that there are not multiple passes used here. Since the full width of the spacecraft is only about 3.2 meters, and the wavelength is probably not much less than a centimeter, the cross-track resolution at an altitude of 500 km should be about 1km.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But the released 1600 x 1600 pixel image seems to be getting fairly close to 1 pixel resolution. I don't think this is a 1600 x 1600 km patch of Alaska. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; How can ICEYE-X1 capture 2D high resolution SAR images in &quot;tens of seconds&quot;?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Looking a the images, edges facing towards the top are much brighter than those facing the bottom, suggesting reflections were recorded obliquely rather than top-down (nadir). I'm wondering if the rate of change of path length being different for different cross-track positions might be the distinguishing feature producing resolution in that direction.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;See also:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;BBC News &lt;a href=&quot;http://www.bbc.com/news/science-environment-42725349&quot; rel=&quot;nofollow noreferrer&quot;&gt;ICEYE's 'suitcase space radar' returns first image&lt;/a&gt; &lt;/li&gt;&#xA;&lt;li&gt;Ars Technica &lt;a href=&quot;https://arstechnica.com/science/2017/08/satellites-that-can-image-earth-at-night-and-through-clouds-near-launch/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Satellites that can image Earth at night, and through clouds, near launch&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Spaceflight 101 &lt;a href=&quot;http://spaceflight101.com/pslv-c40/iceye-x1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;ICEYE X1 (ICEYE POC-1)&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; &quot;First Light&quot; image from the &lt;a href=&quot;https://www.prnewswire.com/news-releases/first-iceye-x1-radar-image-from-space-published-300583982.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;press release&lt;/a&gt;. Open in new window to see full size. The SatCat hasn't even sorted out the payload objects of 2018-004, and one of them is already &quot;&lt;a href=&quot;https://twitter.com/iceyefi/status/953674312820711425&quot; rel=&quot;nofollow noreferrer&quot;&gt;Tweeting&lt;/a&gt;&quot; images back to Earth! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;caption:&lt;/strong&gt; First ICEYE-X1 Radar Image from Space of Noatak National Preserve, Alaska&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/sZvuk.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/sZvuk.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-21T01:26:51.523" LastActivityDate="2020-01-30T09:28:02.810" Title="How can ICEYE-X1 capture 2D high resolution SAR images in &quot;tens of seconds&quot;?" Tags="|imaging|radar|synthetic-aperture-radar|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24548" PostTypeId="1" CreationDate="2018-01-20T21:28:07.353" Score="9" ViewCount="417" Body="&lt;p&gt;The Spaceflight Now article &lt;a href=&quot;https://spaceflightnow.com/2018/01/20/rocket-lab-test-launch-halted-by-ship-traffic-deteriorating-weather/&quot; rel=&quot;noreferrer&quot;&gt;Rocket Lab test launch halted by ship traffic, deteriorating weather&lt;/a&gt; mentions that the recent planned launch of the Electron rocket was cancelled due to a combination of factors, including weather.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Are there any significant differences between launch vehicles in terms of &quot;launchability&quot; in less than ideal weather? Would the Electron, or even smaller sounding rockets require better weather (for example lower wind speed or gradients) than larger rockets? Or vice versa?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-20T23:14:03.053" LastActivityDate="2018-03-08T00:57:16.853" Title="Do smaller rockets require better weather at launch than larger ones?" Tags="|launch|weather|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24549" PostTypeId="2" ParentId="24511" CreationDate="2018-01-20T21:34:25.570" Score="5" Body="&lt;p&gt;&lt;a href=&quot;https://www.reddit.com/r/spacex/comments/3xkk6g/propellant_densification_and_f9_v11_to_v12/cy5hgv2/&quot; rel=&quot;nofollow noreferrer&quot;&gt;enter link description here&lt;/a&gt;The LOX is densified by 9.9% and the RP-1 by 2.6%. This means that if you bring both back to regular temperatures, for the same flow rates, the engines will see a different fuel ratio (producing a different amount of thrust and possibly being less stable, or a much different temperature). You will also consume different ratios of propellant than your tankage - this means you carry extra dead weight, either in unused, empty tankage or unused, surplus propellant. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Warming the propellants and retaining the same fuel ratio could require changes to propellant feed valves in the engines, the turbopump itself, and the common bulkhead location between the LOX &amp;amp; RP-1 tanks. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;These aren't small changes, the rocket would require redesign to a hybrid of the current model and the v1.1 model. You could bring the LOX densification up to match the RP-1 densification, but you wouldn't avoid having a small window after fueling to launch - you would just change the size of that window.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;These changes would cost them payload mass on regular flights and reduce the number of missions that could recover, and flying these changes on only Dragon 2 flights would mean it wasn't of a common design and SpaceX would have to expend more flights proving the design's safety and reliability, rather than simply using paid for commercial flights to prove the design.&lt;/p&gt;&#xA;" OwnerUserId="3044" LastEditorUserId="3044" LastEditDate="2018-05-08T13:25:30.587" LastActivityDate="2018-05-08T13:25:30.587" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="24552" PostTypeId="1" CreationDate="2018-01-21T05:01:33.933" Score="2" ViewCount="648" Body="&lt;p&gt;What components are needed for a rocket launch pad besides the following?&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Tanks for Fuel and Water&lt;/li&gt;&#xA;&lt;li&gt;Lightning protection rods&lt;/li&gt;&#xA;&lt;li&gt;Hangar&lt;/li&gt;&#xA;&lt;li&gt;Launch Pad&lt;/li&gt;&#xA;&lt;li&gt;Duct&lt;/li&gt;&#xA;&lt;li&gt;Tracking Facility&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Is there a good resource to estimate the costs of each?&lt;/p&gt;&#xA;" OwnerUserId="21504" LastEditorUserId="4660" LastEditDate="2018-01-21T20:37:27.147" LastActivityDate="2018-01-21T20:37:27.147" Title="What are the components of a rocket launch complex?" Tags="|launchpad|" AnswerCount="0" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24553" PostTypeId="2" ParentId="24502" CreationDate="2018-01-21T05:35:25.680" Score="0" Body="&lt;p&gt;I'm no professional but I think I have a clear answer.  I would say the simple answer is a fraction of one orbit.  Exiting orbit would require enough thrust to escape earth's gravitational pull.  You could try going out without some form of orbit but I think it would be a waste of fuel.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The question is at what time do you engage the thrust to send the object on the right path to its destination. If you missed your opportunity to exit orbit, you may let it go around for a second orbit and try again once you're absolutely sure it's headed in the right direction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's like football.  If you're trying to send something from Earth to Mars, Earth is like the quarterback and Mars is like the wide receiver.  You need to know the route the receiver is running so you can place the ball where he's going to be, not where he's at upon release.  Once You figure that out, you'll know the trajectory in which the object should leave orbit.  You use the momentum from its short orbit around the earth with the extra thrust to help escape orbit at just the right time. Without the thrust, the object will just continue orbiting or fall back to the earth if it loses speed.&lt;/p&gt;&#xA;" OwnerUserId="22616" LastActivityDate="2018-01-21T05:35:25.680" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24554" PostTypeId="2" ParentId="24545" CreationDate="2018-01-21T09:16:51.357" Score="5" Body="&lt;p&gt;It's a Bayesian thing. If one spots a low albedo asteroid, the odds are good that it's a carbonaceous chondrite asteroid that formed beyond the frost line and hence contains minerals that have become hydrated.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most of the observed low albedo asteroids are carbonaceous chondrites, and most of carbonaceous chondrite meteorites that have fallen to Earth have hydrated minerals. However, some of the observed low albedo asteroids are not carbonaceous chondrites, some of which are pretty much anhydrous, and some of the carbonaceous chondrite meteorites that have fallen to Earth are pretty much anhydrous.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2018-01-21T09:16:51.357" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24556" PostTypeId="2" ParentId="18578" CreationDate="2018-01-21T12:44:15.227" Score="5" Body="&lt;p&gt;The Delta rocket started out with separate versions of the aft end for different numbers solid rocket boosters attached. Eventually McDonnell Douglas standardized, introducing the Universal Boattail Thor (UBT) with with a full complement of nine attach points regardless of how many solid boosters were to be flown on a mission. The positions for the boosters were not evenly spaced on the UBT, though unlike for some Atlas V configurations the aggregate thrust was always symmetrical. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As I'm sure was the case with Atlas V, MDAC's decision to use the same aft end regardless of the number of solid boosters flown on a mission was founded in engineering analysis which traded performance increments against program cost increments, and took consideration of reliability intangibles such as that the more versions of something you fly, the more chance for problems and the less flights each version gets compared to if there is just a single one... diminishing the demonstrated reliability for any of the various configurations relative to only a single type.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, it appears from inspection (i.e. I don't specifically know, but I do have a structural engineering background) that these &quot;attach fittings&quot; are structural elements which carry loads for the frustum.  That would allow the frustum and associated internal structure to be made lighter, so the attach fittings would thus not be deadweight even if no booster is present at a given position.  More like that they are external &quot;ribs&quot;, which then do double-duty when there is a booster attached.&lt;/p&gt;&#xA;" OwnerUserId="22623" LastEditorUserId="22623" LastEditDate="2018-01-23T03:07:18.547" LastActivityDate="2018-01-23T03:07:18.547" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24557" PostTypeId="1" CreationDate="2018-01-21T13:01:59.957" Score="1" ViewCount="677" Body="&lt;p&gt;So, I've been doing some digging, and I found that the chamber pressure for the raptor engine is more than 4 times that of Merlin (~4,000 psi to ~980 psi). How would SpaceX manage to pressurize the fuel/oxidizer enough to be efficient? And if they're planning for their liquid methane to be self pressurizing wouldn't that take too much of the fuel supply?&lt;/p&gt;&#xA;" OwnerUserId="22624" LastEditorUserId="6816" LastEditDate="2018-01-21T13:36:43.160" LastActivityDate="2018-01-21T13:36:43.160" Title="Chamber pressure for Merlin vs Raptor" Tags="|rockets|spacecraft|physics|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24558" PostTypeId="2" ParentId="24557" CreationDate="2018-01-21T13:27:40.227" Score="5" Body="&lt;p&gt;From the wording of the question I suspect you think the entire fuel/ox tanks are maintained at higher than the chamber pressure. They're not: tank pressure is a few bar. The fuel and oxidizer are pressurized in the turbopump.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Self-pressurizing means they have to boil some of the fuel (which is easy, when you have a rocket engine providing all the heat you could possibly want) and run the gases back into the tank. &#xA;This means some of the fuel is not available for combustion, but it's more efficient than the alternative: a heavy tank (strong enough to withstand ~200 bar) filled with enough helium to fill the entire fuel tank at a few bar. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-01-21T13:27:40.227" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24559" PostTypeId="2" ParentId="24546" CreationDate="2018-01-21T14:16:25.817" Score="3" Body="&lt;p&gt;A few issues with what you included:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The resolution is stated as 10m in the bit you included.&lt;/li&gt;&#xA;&lt;li&gt;The size of the image was 80 km/ 40 km&lt;/li&gt;&#xA;&lt;li&gt;The frequency is &lt;a href=&quot;http://spaceflight101.com/pslv-c40/iceye-x1/&quot; rel=&quot;nofollow noreferrer&quot;&gt;10 GHz&lt;/a&gt;, meaning a wavelength of about 3 cm.&lt;/li&gt;&#xA;&lt;li&gt;The SAR antenna is deployable, meaning the effective beam width can be greater then the satellite. Of course, the total width is about 3.2 m, as you indicated.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;The actual theoretical resolution of an image can be found from &lt;a href=&quot;https://igppweb.ucsd.edu/~fialko/insar/sar_summary.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this source&lt;/a&gt;, and is actually the size of the antenna, meaning a smaller antenna will give higher resolution. Thus, the maximum resolution is actually 1.6 m, or half of the antenna size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It also seems that the actual beamwidth might be 200 km. If that is the case, it seems likely that they concentrate it on the 80x40km section, but have increased resolution otherwise. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that repetition time is 6 kHz, meaning that 6k images are used per second, or at least 60k during the time period over which the satellite passes overhead.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-21T18:05:27.197" LastActivityDate="2018-01-21T18:05:27.197" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24560" PostTypeId="2" ParentId="24546" CreationDate="2018-01-21T17:26:41.037" Score="3" Body="&lt;p&gt;ICEYE-X1 does not have a vertically large antenna, so it can only provide 2D data. To interpret the image correctly, you can imagine the satellite crossing over the top of the image from left to right, looking sideways. The axis are:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;x-axis: position along track&lt;/li&gt;&#xA;&lt;li&gt;y-axis: slant range&lt;/li&gt;&#xA;&lt;li&gt;color axis: reflectivity&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;None of these measurements requires a large antenna. Resolution along track is provided by the synthetic aperture and resolution in distance is naturally good with any radar (it only depends on bandwidth).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A 3D-capable radar would be able to give ground distance and height instead of just slant range.&lt;/p&gt;&#xA;" OwnerUserId="16943" LastEditorUserId="16943" LastEditDate="2018-01-21T19:31:07.050" LastActivityDate="2018-01-21T19:31:07.050" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24561" PostTypeId="1" CreationDate="2018-01-21T19:37:13.297" Score="8" ViewCount="554" Body="&lt;p&gt;If we could capture an asteroid or meteor, can we use it as a vehicle for space travel? From what I understand, propulsion is a result of mass flow, so we could use a rail gun configuration to propel the rock mass from the asteroid or meteor. The asteroid could also provide some protection from flying into micro meteors. Is this a possibility or am I a dreamer? (I am an absolute novice.)&lt;/p&gt;&#xA;" OwnerUserId="22630" LastEditorUserId="47149" LastEditDate="2022-06-01T17:17:20.013" LastActivityDate="2022-06-01T17:17:20.013" Title="Using asteroids as propellant source" Tags="|propulsion|fuel|physics|asteroid|interstellar-travel|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 4.0" />
  <row Id="24562" PostTypeId="2" ParentId="24502" CreationDate="2018-01-21T19:43:09.207" Score="2" Body="&lt;h1&gt;It doesn't matter how many times you circle. If you go faster than the velocity calculated, you will escape the object's gravitational field.&lt;/h1&gt;&#xA;&lt;h2&gt;Part 1: formula derivation&lt;/h2&gt;&#xA;&lt;p&gt;We can calculate escape velocity using the formula:&lt;/p&gt;&#xA;&lt;p&gt;G = &lt;span class=&quot;math-container&quot;&gt;$6.67*10^-$$^1$$^1$&lt;/span&gt; &lt;span class=&quot;math-container&quot;&gt;$N m^2/kg^2$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;M = mass of planet&lt;/p&gt;&#xA;&lt;p&gt;m = mass of sattelite&lt;/p&gt;&#xA;&lt;p&gt;r = the distance between both masses&lt;/p&gt;&#xA;&lt;p&gt;K = Kinetic energy, U = Potential Energy&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$ K + U = 0 $&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$-GMm/r + 1/2 mv^2 = 0$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$2GM/r = v^2$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$v = \sqrt {2GM/r}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;h2&gt;Part 2: solving for escape velocity&lt;/h2&gt;&#xA;&lt;p&gt;So let's plug in our numbers:&lt;/p&gt;&#xA;&lt;p&gt;M = mass of earth, &lt;span class=&quot;math-container&quot;&gt;$5.98*10^2$$^4$&lt;/span&gt; &lt;span class=&quot;math-container&quot;&gt;$kg$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;r= radius of earth, &lt;span class=&quot;math-container&quot;&gt;$6.38 * 10^6$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$v = \sqrt {2GM/r}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$v = \sqrt {2* 6.67*10^-11 N m^2/kg^2 * (5.98*10^24 kg)/6.38 * 10^6}$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;span class=&quot;math-container&quot;&gt;$v = 1.12 * 10^4 m/s$&lt;/span&gt;&lt;/p&gt;&#xA;&lt;p&gt;Therefore, it doesnt matter how many times you circle, you need to be going faster than &lt;span class=&quot;math-container&quot;&gt;$1.12*10 ^4$&lt;/span&gt; m/s or about 11.2 km/s to escape the earth's gravitational field.&lt;/p&gt;&#xA;" OwnerUserId="22629" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-21T23:51:50.383" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24563" PostTypeId="1" CreationDate="2018-01-21T20:33:31.780" Score="13" ViewCount="2744" Body="&lt;p&gt;The Falcon Heavy is &lt;a href=&quot;https://spaceflightnow.com/2018/01/09/falcon-heavy-demo-flight-preparations/&quot; rel=&quot;noreferrer&quot;&gt;currently undergoing testing&lt;/a&gt; at Kennedy Space Center's Pad 39A, which SpaceX is &lt;a href=&quot;https://www.nasa.gov/content/nasa-spacex-sign-property-agreement-for-historic-launch-pad&quot; rel=&quot;noreferrer&quot;&gt;leasing from NASA&lt;/a&gt;.  The hold-down test firing, according to the previously-linked article, may occur as early as Monday (tomorrow, as of posting).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At the same time, the &lt;a href=&quot;https://www.washingtonpost.com/powerpost/trump-launches-new-round-of-partisan-attacks-as-government-shutdown-hits-day-2/2018/01/21/0560aece-fe35-11e7-a46b-a3614530bd87_story.html&quot; rel=&quot;noreferrer&quot;&gt;current shutdown of the U.S. government&lt;/a&gt; may well continue through Monday.  The last time this happened, &lt;a href=&quot;https://space.stackexchange.com/questions/2238/how-did-the-us-government-shutdown-of-2013-affect-nasa-operations&quot;&gt;97% of NASA employees were furloughed&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It appears that SpaceX conducted fueling tests on the Falcon Heavy on Saturday (yesterday), which would indicate that they are able to access KSC facilities despite the shutdown.  However, if the hold-down firing requires any coordination with NASA (and the launch certainly does), could the shutdown delay further testing and possibly delay the launch?&lt;/p&gt;&#xA;" OwnerUserId="8996" LastActivityDate="2018-01-22T15:21:44.703" Title="Will the January 2018 government shutdown affect the Falcon Heavy launch?" Tags="|spacex|nasa|testing|procedure|kennedy-space-center|" AnswerCount="3" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24564" PostTypeId="2" ParentId="24563" CreationDate="2018-01-21T20:53:09.567" Score="10" Body="&lt;p&gt;Yes, some testing and all launches except for those critical to national security &lt;a href=&quot;http://www.patrick.af.mil/2018-Government-Shutdown/&quot; rel=&quot;noreferrer&quot;&gt;have to be postponed&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;By now, I’m sure you are well aware that a budget agreement was not reached last night resulting in a government-wide shutdown effective today.  Many in our workforce recall the impacts of the 2013 shutdown, and now 5 years later, we find ourselves in a similar situation.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The unfortunate reality of the current situation is that lives and operations are disrupted. That said, I want you to know I am committed to each and every one of you and believe that communication will be the key to working through this together. I encourage you to read this page as it contains information on the current shutdown, how it will impact base resources, and what financial resource options you have, should you need them.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;If the government does not reopen by the end of the weekend, everyone will report at their regularly scheduled times on Monday, and we will begin carrying out an orderly shutdown and doing official furlough notifications. This day will be difficult on all of us, but know that I value all of you for the effort you make day in and day out. We simply cannot accomplish our primary mission of assured access to space without our incredible civilian workforce.  This challenging time should not divide but unite us. My hope is that the furlough’s duration will be brief and become a mere speed bump in our Drive to 48 launches a year.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to discussions over at NasaSpaceflight, &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=44698.0;topicseen&quot; rel=&quot;noreferrer&quot;&gt;test firings and launches are affected&lt;/a&gt;. For the Falcon Heavy mission, there was a &lt;a href=&quot;https://forum.nasaspaceflight.com/index.php?topic=44376.100&quot; rel=&quot;noreferrer&quot;&gt;wet dress rehearsal over the weekend&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;45th cannot support SpaceX commerical static fire tests&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-22T07:17:54.107" LastActivityDate="2018-01-22T07:17:54.107" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24565" PostTypeId="2" ParentId="24563" CreationDate="2018-01-21T22:46:43.963" Score="14" Body="&lt;h2&gt;Currently unclear&lt;/h2&gt;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://www.theverge.com/2018/1/19/16910378/government-shutdown-2018-nasa-spacex-iss-falcon-heavy&quot; rel=&quot;noreferrer&quot;&gt;the Verge&lt;/a&gt;:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;It's possible that the [static fire] test could come early next week. But the Falcon Heavy’s launchpad is located at NASA’s Kennedy Space Center, and limited staffing at the site could pose a problem. SpaceX told The Verge on Friday that the company is not expecting the shutdown to affect its operations.&lt;/p&gt;&#xA;&lt;p&gt;However, the 45th Space Wing says it will not be able to support commercial static fires at KSC during the shutdown. It’s unclear if that means the static fire can’t happen or if SpaceX can still do the test without the 45th’s support. We’ve reached out to both SpaceX and the 45th for further clarification and will update the story when we find out more.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="8996" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-21T22:46:43.963" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24566" PostTypeId="2" ParentId="24561" CreationDate="2018-01-22T00:20:59.500" Score="3" Body="&lt;p&gt;There have been many discussions about using asteroids for reaction mass. One obvious way would be to process material from icy asteroids into rocket fuel. Much of them are largely water, and that gives hydrogen and oxygen. If a carbon source were also accessed (like a carbonations contrite) then methane could be made. Or some may contain frozen methane. An obvious way to do this would be to transport small asteroids to L5 in the earth / moon system and process them there. Another way would be to use any solids as reaction mass, fired from a mass driver (magnetic rail gun). Even if large reusable rockets become common, the cost of lifting fuel will continue to encourage people to find sources that are already in orbit.&lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2018-01-22T00:20:59.500" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24567" PostTypeId="2" ParentId="24561" CreationDate="2018-01-22T00:57:17.033" Score="0" Body="&lt;p&gt;The main problem with using the asteroid as reaction mass is that you need a large power source. Science fiction stories usually assume that a nuclear fusion (or even anti-matter) reactor is available to heat up the asteroid material to a high enough temperature so that its momentum can propel the asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In reality it will be quite a long time before such power sources become available - if ever. Until then, the only possible power source is solar power. Near earth, approximately $1 kW$ is available per square metre. As the asteroid belt is more than twice as far away from the sun as the earth is, the available power there is less than a quarter, or less than $250W/m^2$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now let's do some calculations. Assume an ice-based asteroid of 50 m diameter. It would have a mass of about 500,000 tons. To accelerate this at $1m/s^2 $ we need a thrust of 500,000 tons. The Saturn first stage engines generated a total of about 750 tons thrust or 40 MW each. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Allowing for some inefficiency, let's assume we need 50 MW to generate 500 tons of thrust, or 40 GW for to achieve our $1m/s^2$ acceleration. We would then need a solar panel with a total area of $160,000,000 m^2$, a square of more than 12 km on a side. Reducing the acceleration to $1mm/s^2$ still leaves us with a square of 400 m. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Then we need to ship all this to the asteroid. At $100kg/m^2$ for the solar panel, we are talking about sending 1,600 tons to the asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;All this may become feasible in the distant future, but don't hold your breath.&lt;/p&gt;&#xA;" OwnerUserId="17390" LastActivityDate="2018-01-22T00:57:17.033" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24568" PostTypeId="2" ParentId="15167" CreationDate="2018-01-22T03:51:45.783" Score="3" Body="&lt;p&gt;Additional useful property of large orbit is easier avoidance of eclipses:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Gaia_(spacecraft)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Gaia&lt;/a&gt; paper &lt;a href=&quot;http://issfd.org/ISSFD_2014/ISSFD24_Paper_S2-5_Renk.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;GAIA: TRAJECTORY DESIGN WITH TIGHTENING CONSTRAINTS&quot; 2014&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Due to the size of the Lissajous orbit to be limited to less than 15◦ unfavourable conditions with&#xA;  respect to the moon can occur, where the moon is causing a partial eclipse for a prolonged period of&#xA;  time (up to 42 hours). While not problematic with respect to solar power generation it is undesirable&#xA;  with respect to the thermal balance  ... this situation could not&#xA;  occur anymore due to changed insertion conditions, which resulted in more favourable conditions&#xA;  with respect to the trajectory of the moon due to &lt;em&gt;larger out-of-plane amplitudes&lt;/em&gt;.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;Earth Eclipse Avoidance: &lt;em&gt;Any small amplitude Lissajous orbit about SEL2 will have an eclipse following the natural motion&lt;/em&gt;. By choosing appropriate initial conditions the time to an eclipse can usually be extended to 6.5 years for orbits with SSCE ∼ 15◦ , if appropriate initial phase angles are selected&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope&quot; rel=&quot;nofollow noreferrer&quot;&gt;JWST&lt;/a&gt; paper &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160001318.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;&quot;LAUNCH WINDOW TRADE ANALYSIS FOR&#xA;THE JAMES WEBB SPACE TELESCOPE&quot; 2016&lt;/a&gt; lists some constraints on Sun-Earth L2 orbit:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;&lt;pre&gt;&lt;code&gt;Table 1. JWST Launch Window Constraints.&#xA;Constraint. Value. Requirements/Constraint Driver(s)&#xA;LPO Box: Maximum RLP Y - 832,000 km - Communications&#xA;LPO Box: Maximum RLP Z - 520,000 km - Science (Stray light)&#xA;Maximum SCAT finite-burn duration - 11,425 sec - Propulsion &#xA;Minimum precision of SCAT finite-burn duration - 0.2 sec - Propulsion &#xA;MCC Available ΔV - 58.5 m/s - Mass &amp;amp; Propulsion&#xA;Mission Lifetime Goal - 10.5 years - Science&#xA;Lunar / Earth Eclipse - None allowed - Power and Thermal&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;  &#xA;  &lt;p&gt;The LPO restrictions define a box centered at the L2 point with three dimensions in the RLP X, Y and Z directions ...  The Y and Z&#xA;  constraints are based on communications and stray light requirements. The stray light&#xA;  requirement protects the JWST instruments from solar, lunar and Earth stray light. As seen in&#xA;  Fig. 3, the LPO requirements can be fulfilled by a general LPO category, which includes tori,&#xA;  quasi-halo, and Lissajous. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="2843" LastActivityDate="2018-01-22T03:51:45.783" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24569" PostTypeId="2" ParentId="23737" CreationDate="2018-01-22T07:00:28.580" Score="4" Body="&lt;p&gt;Based on the research papers published related to the AAReST Mission and &lt;a href=&quot;http://epubs.surrey.ac.uk/844913/1/Future%20Rendezvous%20and%20Docking%20Missions%20enabled%20by%20low-cost%20but%20safety%20compliant%20Guidance%20Navigation%20and%20Control%20%28GNC%29%20architectures.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;this recent paper&lt;/a&gt; by Surrey Space Center, it can be said that various subsystems for the mission are being developed and the mission is set to launch in 2018 or later.&#xA;&lt;br&gt;&lt;br&gt;&#xA;Demonstrating in-space docking of small satellites is the key to in-space assembly and this mission is a stepping stone to achieve the same. There is another such mission which focuses on a Universal Docking Port(UDP) mechanism by NASA and Tyvak, called CPOD, which is also set to launch in 2018. Check out &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/cpod.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; and &lt;a href=&quot;https://www.nasa.gov/directorates/spacetech/small_spacecraft/cpod_project.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22635" LastActivityDate="2018-01-22T07:00:28.580" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24570" PostTypeId="1" AcceptedAnswerId="24571" CreationDate="2018-01-22T07:06:41.883" Score="38" ViewCount="7703" Body="&lt;p&gt;For those that watched the second launch of RocketLab's Electron launch vehicle, out of Mahia Peninsula, New Zealand on 21 January 2018; in which the vehicle successfully made it to orbit for the first time, this curious sequence of events occurred approximately four minutes into stage two burn.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Operators first called out a number of updates, before two items physically detached from the aft-end of the second stage of the vehicle.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;T+6:26: &quot;Coming up on hot swap&quot;.&lt;br&gt;&#xA;  T+6:32: &quot;Hotswapping...&quot;&lt;br&gt;&#xA;  T+6:36: &quot;Eject&quot;.&lt;br&gt;&#xA;  T+6:38: &lt;em&gt;Two items detach and fall away from the vehicle&lt;/em&gt;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;This can be seen in this frame captured from the live broadcast of the launch on YouTube. In it, one of the items that fell away can be seen to the left, just below the Rutherford engine bell. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/3pCLi.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/3pCLi.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;What are these two items falling away from the vehicle, and what is &quot;hot swapping&quot;?&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-22T07:14:52.100" LastActivityDate="2018-01-29T19:54:50.187" Title="What was ejected from RocketLab's Electron launch vehicle at T+6:25 during stage two burn?" Tags="|battery|rocketlab|electron|rutherford-engine|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24571" PostTypeId="2" ParentId="24570" CreationDate="2018-01-22T07:07:25.200" Score="50" Body="&lt;p&gt;I’ve continued to do a bit of research on this, and what we’re seeing falling away are &lt;strong&gt;two of the lithium-ion battery packs that power the Rutherford engine turbopumps&lt;/strong&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Rutherford engine is unique in that its small size allows it to utilise an electric pump-fed cycle. On the second stage in particular, there are three lithium-ion batteries, two of which—the ones that are ejected—can be seen in this diagram from RocketLab’s payload user guide as the rectangular containers on either side of the second stage engine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/EucQJ.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/EucQJ.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;As these two batteries become depleted of charge during flight, the third battery begins supplying power and driving the engine cycle. This process is referred to as &lt;strong&gt;hotswapping&lt;/strong&gt;. Once the hot swap is completed, the two batteries are ejected to increase payload to orbit. Spaceflight101.com &lt;a href=&quot;http://spaceflight101.com/spacerockets/electron/&quot; rel=&quot;noreferrer&quot;&gt;further states&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Electron’s second stage hosts a total of three Lithium-Polymer battery packs to power the engine during its burn. Two battery packs are jettisoned as they become exhausted during propulsive flight to shed dead weight from the vehicle and increase Electron’s payload capacity. One battery pack is carried into orbit and remains with the second stage until re-entry. With a low auto-ignition temperature of only 150°C, the second stage batteries will burn up in their entirety during re-entry while the first stage batteries are also likely to be incinerated to some extent during their sub-orbital re-entry.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;The remaining batteries—13 of them— are installed and similarly provide power to the first stage’s nine engines. These are not ejected, and are designed to provide enough power for all of first stage flight. &lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-22T07:15:31.167" LastActivityDate="2018-01-22T07:15:31.167" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24572" PostTypeId="5" CreationDate="2018-01-22T07:09:35.797" Score="0" Body="" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-22T07:09:35.797" LastActivityDate="2018-01-22T07:09:35.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24573" PostTypeId="4" CreationDate="2018-01-22T07:09:35.797" Score="0" Body="Electron is a small, two-stage, orbital launch vehicle developed by the New Zealand &amp; United States company RocketLab. Developed almost entirely out of carbon-fibre composites, it has nine Rutherford engines on its first stage, and a single Rutherford engine on its second stage, and can take 150kg to a 500km sun-synchronous orbit. " OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-22T07:09:35.797" LastActivityDate="2018-01-22T07:09:35.797" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24574" PostTypeId="1" CreationDate="2018-01-22T07:46:05.710" Score="7" ViewCount="434" Body="&lt;p&gt;Now that Rocket Labs has proven that orbit is well within the range of small nations and medium-sized companies, I have to wonder about the strategic implications.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK the Electron rocket has an LEO capacity of 150kg. From SpaceX' history, one could even conclude that this number is not the final state of affairs for that rocket.&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Is it possible to construct a &quot;rod from god&quot; within that weight class? &lt;/li&gt;&#xA;&lt;li&gt;What is actually necessary for a targeted and controlled reentry? Does shape suffice or is some form of active control needed?&lt;/li&gt;&#xA;&lt;li&gt;What would be the reentry speed of a, say, 100kg cone decelerated enough to hit a target from LEO?&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;edit: To clarify a point that was raised in the comments: I am implicitly assuming that lowering the launch price by an order of magnitude also lowers the upfront investment and scale of the whole operation. Hence what we know as a strategic weapon moves towards a more tactical sphere. My question is if it is actually fit for such a purpose or if the scale is simply too small.&lt;/p&gt;&#xA;" OwnerUserId="11256" LastEditorUserId="6537" LastEditDate="2018-01-22T10:02:57.210" LastActivityDate="2018-01-23T09:52:41.467" Title="Is it possible to develop a small orbital rocket, like RocketLab's Electron, for a weapons delivery system?" Tags="|reentry|weapon|electron|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24575" PostTypeId="2" ParentId="24574" CreationDate="2018-01-22T09:53:41.613" Score="11" Body="&lt;p&gt;You're quite right that 150kg is unlikely to be the final maximum payload for Electron: SpaceX's Falcon 1 was almost immediately uprated with the addition of a Merlin-1C engine, and was nominally planned to be offered in an extended variant, Falcon 1e, that could take 1010kg to LEO.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The most likely upgrade path probably consists uprating the Rutherford engine and extending the tankage structure in length to accomodate for propellant.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Now: how feasible is it to develop, or otherwise convert such a small payload orbital-rocket design to that of an offensive weapon?&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll try to roughly address your points case-by-case.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;1. Development costs &amp;amp; timeline&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;The question presumes that this technology would be falling into the hands of a company or country. It's worth noting even RocketLab have required significant injections of money for operation: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;In late 2013, RocketLab secured venture capital Series A funding from Silicon Valley company Khosla Ventures.&lt;/li&gt;&#xA;&lt;li&gt;They were heavily backed in 2014 by U.S. defence conglomerate Lockheed Martin. &lt;/li&gt;&#xA;&lt;li&gt;The New Zealand government &lt;a href=&quot;https://www.callaghaninnovation.govt.nz/news-and-events/140m-rd-grants-business-growth&quot; rel=&quot;noreferrer&quot;&gt;has subsidised their development&lt;/a&gt; through innovation awards, totalling some eight digits of funding.&lt;/li&gt;&#xA;&lt;li&gt;In May 2017, &lt;a href=&quot;https://www.rocketlabusa.com/news/updates/series-d-financing/&quot; rel=&quot;noreferrer&quot;&gt;they raised&lt;/a&gt; $75 million as part of a private Series D funding round.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Make no mistake: rocketry is an expensive business. Your actor will need either deep pockets or very good friends to make it through development hell. Even ignoring finances, it takes a lot of people and a lot of time to get off the ground:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;RocketLab &lt;a href=&quot;https://www.nbr.co.nz/article/10-things-about-rocket-lab-ck-203485&quot; rel=&quot;noreferrer&quot;&gt;currently employ&lt;/a&gt; 200 engineers, developers, and other staff.&lt;/li&gt;&#xA;&lt;li&gt;Development started on Electron in 2014. You'll need a very stable political and physical environment to make sure development is seen through.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;h1&gt;2. Operations needed&lt;/h1&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;If you're a company, &lt;a href=&quot;http://www.mbie.govt.nz/about/whats-happening/news/2016/govt-signs-contract-authorising-rocket-lab-launches&quot; rel=&quot;noreferrer&quot;&gt;you're going to need approval&lt;/a&gt; of your country first to authorise and sign off your launches:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The Government has today signed a contract authorising Rocket Lab’s space activities from New Zealand. [...] It is an interim measure to allow Rocket Lab to commence launching rockets before the Bill establishing a regulatory regime comes into force.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;I hope you've got considerable sources of liquid oxygen and your preferred choice of fuel, as well as power available!&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You'll need steady access to range assets both on ground at your launch site, and around the globe to ensure tracking and communication with your vehicle. A ground tracking error resulted in the flight termination of the first Electron flight in 2017.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;You'll probably need access to an unlocked GPS unit, which requires special authorisation and exports licensing from the U.S. government.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Which leads me to my next point: &lt;strong&gt;RocketLab were incredibly lucky to make it to orbit on their second flight&lt;/strong&gt;. It took SpaceX four attempts to do so. The failure rate on maiden rocket launches is very high, so you will need the money and resources to continue manufacturing, because a single shot success cannot be guaranteed. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;3. Weaponization &amp;amp; miniaturization&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;What weapon fits into 150kg? The obvious answer to maximise a destruction to weight ratio would be minituarized nuclear warhead. The United States have some! (Good luck &quot;acquiring&quot; them though)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you were able to scale down one ~164kg &lt;a href=&quot;https://en.wikipedia.org/wiki/W76&quot; rel=&quot;noreferrer&quot;&gt;W76 warhead&lt;/a&gt;, designed for the Trident I &amp;amp; II ballistic missiles, you'd have a very nice sub-100 kiloton nuclear weapon available at your disposal.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This might not be enough however... &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;4. Reentry&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;But, somehow, you managed to make it into orbit on your first try with a prohibited device onboard — well done! &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's safe to say that most, if not all orbital vehicle designs currently, are not well suited for reentry. If you've ever watched a SpaceX launch and landing, you'll be aware that during reentry:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;The steerable grid fins on the vehicle heavily abate and catch fire as the soupy atmosphere pushes through them. &lt;/li&gt;&#xA;&lt;li&gt;The vehicle is covered in its own soot as it reenters the atmosphere, and must endure high-g decelerations as it hits the stratosphere.&lt;/li&gt;&#xA;&lt;li&gt;Unless specifically designed for reuse, the vehicle is &lt;a href=&quot;https://www.youtube.com/watch?v=wtdjCwo6d3Q&quot; rel=&quot;noreferrer&quot;&gt;practically throwaway&lt;/a&gt; after 1-2 flights — reuse isn't easy on a rocket.&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;What really drives home the point here is that Falcon 9's first stage is only reentering from a relatively mediocre speed of ~1-2km/s. And as kinetic energy increases at the square of velocity, going from a suborbital reentry to orbital reentry will strain even the best designed architecture: &lt;strong&gt;you're looking at at a 31x increase in kinetic energy&lt;/strong&gt;.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, your going to need to have your weapon encapsulated in a blunt body with sufficient heat shield. Small blunt bodies have been designed before: the Stardust capsule was targeted to land back on Earth in a &lt;a href=&quot;https://www.jpl.nasa.gov/news/press_kits/stardust-return.pdf&quot; rel=&quot;noreferrer&quot;&gt;76 by 44 kilometre ellipse&lt;/a&gt; in the Utah Test &amp;amp; Training Range. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You'll need to be a bit more accurate than that though, a 100 kiloton nuclear weapon &lt;a href=&quot;http://nuclearsecrecy.com/nukemap/&quot; rel=&quot;noreferrer&quot;&gt;will only create a thermal radiation blast with a ~3.9km radius&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;5. Speeds involved&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Say your bad actor gets this far. As we're in orbit, the minimum reentry speed you're going to get without significant propulsive reductions is that of orbital velocity: &lt;strong&gt;~7.6km/s&lt;/strong&gt;. You might be able to lower this slightly by ensuring you hit your target on first pass and aim for a suborbital trajectory, but there isn't much difference here unless your target is very close by, and you wouldn't use an orbital rocket for that anyway — far more conventional forms of attack are more suited to this subdomain.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You would need to make that 150kg of payload capability almost all fuel to get any meaningful reductions out of it. You'll also be slowing yourself down, making it easier for the enemy to target you with anti-ballistic missile technology. Any rational actor with the capability to develop this technology would likely prefer to reenter at high velocity, and shed the speeds using an ablative heatshield. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;At such a speed, you'll be able to target anywhere on Earth in just under 90 minutes. A very useful capability for sure — but my personal expectation is that they'll be either sabotaged, forcibly shutdown, or reasoned with beforehand :).&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;In Summary&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Yep, this is technically possible — but it has been for a while. RocketLab are not the first to pioneer any of this small orbital technology, beyond some carbon composites manufacturing or a novel engine design. There simply just hasn't been a market for it before now. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The reason you don't see bad actors popping up everywhere with their own small orbital offensive weapons is because &lt;strong&gt;it's still damn hard to do&lt;/strong&gt;.&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="6537" LastEditDate="2018-01-23T09:52:41.467" LastActivityDate="2018-01-23T09:52:41.467" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24576" PostTypeId="2" ParentId="24563" CreationDate="2018-01-22T15:21:44.703" Score="5" Body="&lt;p&gt;Yes, &lt;a href=&quot;https://www.usatoday.com/story/tech/nation-now/2018/01/22/shutdown-no-spacex-cant-test-fire-falcon-heavy-rocket/1052975001/&quot; rel=&quot;noreferrer&quot;&gt;it will&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;SpaceX will be unable to test fire its three-core Falcon Heavy rocket at Kennedy Space Center because of the government shutdown, further delaying checkout operations ahead of the rocket's demonstration flight, the 45th Space Wing said Sunday.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-22T15:21:44.703" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24577" PostTypeId="1" AcceptedAnswerId="24579" CreationDate="2018-01-22T15:51:02.737" Score="7" ViewCount="237" Body="&lt;p&gt;Last year, ISRO launched 104 satellites successfully into orbit. People praised it as an 'astounding achievement'.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides a more complicated-than-usual satellite deployment, what truly was astounding here? This may be out of &lt;strong&gt;my ignorance of the complexities that might have be involved&lt;/strong&gt;, but to me, as long as you ensure that all the satellites are separated (deployed) from the final stage safely without colliding into each other, there isn't really much to do. This is likely far more difficult than it may sound to me.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In other words, I'd like to know why this feat was &lt;em&gt;exceptionally&lt;/em&gt; difficult.&lt;/p&gt;&#xA;" OwnerDisplayName="user20905" LastEditorUserId="12102" LastEditDate="2019-12-19T00:27:33.003" LastActivityDate="2019-12-19T00:27:33.003" Title="How was it difficult for the PSLV to launch a record of 104 satellites?" Tags="|launch|artificial-satellite|isro|payload-deployment|pslv|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 4.0" />
  <row Id="24579" PostTypeId="2" ParentId="24577" CreationDate="2018-01-22T16:20:28.143" Score="9" Body="&lt;p&gt;The key phrase is &quot;satellites separated from the final stage safely without colliding into each other&quot;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's break that into two pieces:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Satellites Separated From Each other- In order to do this, it takes time, power, and attitude adjustment.Also, they have to be arranged such that all of the satellites can be separated from each other. Most of these were nano satellites, so they can be put together into a single dispenser that could contain at least 3 satellites, but there were still a considerable number of them. Still, that requires some work.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;More interesting is the second part, without colliding into each other. There is something about separating satellites that isn't well considered. If you put a satellite ahead of you, then that increases it's speed, and it will actually orbit slightly slower, ultimately having it be at the end of the pack. Satellites released into a retrograde direction will have the opposite problem. Bottom line, they will have some period in the first orbit where they will be very close to each other, and with 100+ satellites, one has to ensure that no pair of them collides with the other, which isn't a small task.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way that they accomplished this was to release most of the satellites in the same direction. The primary satellites were released in a different direction from the nanosats. This allowed them to separate successfully.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As a point of comparison, SpaceX, the last I heard, will only allow up to 12 satellites on the same launch, although that number might have increased since I last heard.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more information, &lt;a href=&quot;https://www.space.com/35709-india-rocket-launches-record-104-satellites.html&quot; rel=&quot;noreferrer&quot;&gt;check this out&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-22T16:20:28.143" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24580" PostTypeId="2" ParentId="24064" CreationDate="2018-01-22T18:00:20.747" Score="5" Body="&lt;p&gt;The Falcon 9 landing legs are actuated by pneumatic cylinders powered by tank(s) of high-pressure helium. My guess is that very slight pressure deviations in the cylinders cause the legs to fully deploy at different times.&lt;/p&gt;&#xA;" OwnerDisplayName="user22645" LastActivityDate="2018-01-22T18:00:20.747" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24581" PostTypeId="2" ParentId="3585" CreationDate="2018-01-22T18:13:51.757" Score="1" Body="&lt;p&gt;Pretend we are using the best we have (Hubble?) at 0.05 arcsecond resolution (meaning that's the smallest object that can be detected above the background noise)... at P10 of roughly 120AU, the current radius being imaged by Hubble at 0.05 arcseconds, is roughly 4500 km. Obviously Pioneer 10 is much smaller.  Another perspective, Hubble can't see the lunar landers on the moon's surface, here is an article covering that...&#xA;&lt;a href=&quot;http://hubblesite.org/explore_astronomy/hubbles_universe_unfiltered/blogs/angular-resolution-and-what-hubble-cant-see&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://hubblesite.org/explore_astronomy/hubbles_universe_unfiltered/blogs/angular-resolution-and-what-hubble-cant-see&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22646" LastEditorUserId="21553" LastEditDate="2019-08-25T08:41:09.243" LastActivityDate="2019-08-25T08:41:09.243" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="24582" PostTypeId="2" ParentId="23555" CreationDate="2018-01-22T21:50:20.627" Score="5" Body="&lt;p&gt;Rocket performance taking 6 SkySats to 500 km SSO limited the number of Doves that could be integrated. Also, they were not originally manifested on the mission, but were 'swapped in' after Planet's acquisition of Terra Bella from Google &lt;a href=&quot;https://www.planet.com/pulse/terra-bella-officially-joins-planet/&quot; rel=&quot;noreferrer&quot; title=&quot;here&quot;&gt;here&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So the number was the result of performance limits &amp;amp; logistics given the timeframe from acquisition to launch.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(by SkySat propulsion engineer)&lt;/p&gt;&#xA;" OwnerUserId="22648" LastActivityDate="2018-01-22T21:50:20.627" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24583" PostTypeId="1" CreationDate="2018-01-23T01:45:39.707" Score="1" ViewCount="218" Body="&lt;p&gt;What is the best propulsion for an interstellar ship? with the current technology&#xA;If we sent a spacecraft in this decade or the next, what would be the best propulsion for that ship? I speak of a ship with twice as much as the space station, with technology from now on in 20 years&lt;/p&gt;&#xA;" OwnerUserId="22650" LastEditorUserId="20937" LastEditDate="2018-01-23T03:29:26.497" LastActivityDate="2018-01-23T03:29:26.497" Title="What is the best propulsion for an interstellar ship?" Tags="|spacecraft|propulsion|interstellar-travel|" AnswerCount="0" CommentCount="7" ClosedDate="2018-01-23T03:29:31.863" ContentLicense="CC BY-SA 3.0" />
  <row Id="24584" PostTypeId="2" ParentId="9421" CreationDate="2018-01-23T03:25:41.897" Score="4" Body="&lt;p&gt;I'm the SkySat propulsion lead and have been flying LMP-103S since June 2016 when SkySat-3 launched from India.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To answer the question:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Yes it is viable and has a number of advantages, mostly the (Isp * density) is much better than hydrazine. But, right now the engines are more expensive to build than those running hydrazine (due to higher combustion temperature, cost of propellant not really an issue). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Really it comes down to what your mission values (cost, complexity of CONOPS, lots of flight heritage, or minimizing size), but it is a serious contender for missions which previously would be hydrazine.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I put a more technical comparison &lt;a href=&quot;https://www.totalimpulse.net/blog/hydrazine-vs-lmp-103s/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22648" LastEditorUserId="22648" LastEditDate="2018-01-23T19:05:14.707" LastActivityDate="2018-01-23T19:05:14.707" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24585" PostTypeId="1" AcceptedAnswerId="24587" CreationDate="2018-01-23T03:59:22.740" Score="11" ViewCount="2631" Body="&lt;p&gt;If a rocket launches from Florida, how far around the world does it travel before it reaches orbit? Is it in space before it completes crossing the Atlantic?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Does it vary for different powered rockets, or is it typically the same trajectory?&lt;/p&gt;&#xA;" OwnerUserId="22581" LastActivityDate="2021-04-20T23:34:46.800" Title="How far around the world does a rocket travel before it enters orbit?" Tags="|launch|trajectory|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24586" PostTypeId="1" CreationDate="2018-01-23T04:00:26.967" Score="20" ViewCount="3499" Body="&lt;p&gt;A rocket engine is basically a device that generates a high pressure inside the chamber and the pressure difference creates a force. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;A nozzle helps in effectively accelerating the flow and increases the thrust.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;But where exactly would the thrust force act?&lt;/strong&gt; &lt;em&gt;Is it on the nozzle, or top of the combustion chamber, or all over the body, or by any other place?&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, the answer might vary based on fuel type, as liquid engine and solid motors have a different construction. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, &lt;strong&gt;how is the force transmitted to the entire rocket structure?&lt;/strong&gt; A structure capable of withstanding such concentrated load has to be really strong and well designed. What materials are generally used for this and what is the generally adopted design?&lt;/p&gt;&#xA;" OwnerUserId="511" LastActivityDate="2018-01-24T15:27:22.363" Title="Where does the thrust act in a rocket engine and how is it transferred to rocket structure?" Tags="|rockets|propulsion|design|thrust|nozzle|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24587" PostTypeId="2" ParentId="24585" CreationDate="2018-01-23T04:53:16.260" Score="14" Body="&lt;p&gt;It varies with different rockets, but typically insertion to low Earth orbit is achieved around 1200-1500 miles (~2000-2500 km) downrange distance — something less than halfway across the Atlantic for an Eastward launch from Cape Canaveral.&lt;/p&gt;&#xA;&lt;p&gt;The longer the ascent, the further downrange it will be before orbital insertion.&lt;/p&gt;&#xA;&lt;p&gt;Here's one example for a 10-minute ascent (note that the horizontal axis is linear in downrange distance, not in time):&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/DEhCX.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/DEhCX.gif&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Note that the rocket will be &lt;em&gt;in space&lt;/em&gt; long before this point; the 100km Kármán line is reached at less than 100km downrange distance in the trajectory shown.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2021-04-20T23:34:46.800" LastActivityDate="2021-04-20T23:34:46.800" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="24588" PostTypeId="2" ParentId="20234" CreationDate="2018-01-23T14:55:29.390" Score="0" Body="&lt;p&gt;Mayby possibly answer is: to catch a docking spaceship using long robotic arm, and, by manipulating of them, changing motion vectors of space ship gradually (by a lot of motors of the arm), and simply to pull down in lock in a Torus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is my crazy idea for docking to rotating space station without docking port in axis of Torus. &lt;/p&gt;&#xA;" OwnerUserId="22653" LastActivityDate="2018-01-23T14:55:29.390" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24589" PostTypeId="2" ParentId="24586" CreationDate="2018-01-23T15:35:00.763" Score="16" Body="&lt;p&gt;The thrust acts on the nozzle and combustion chamber walls by virtue of the pressure differential they contain.  Using the RS-25 (SSME) as an example, with illustrations from &lt;a href=&quot;http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&quot; rel=&quot;noreferrer&quot;&gt;this pdf&lt;/a&gt;, I can highlight a few of the components.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Page 11 (pdf page 17) shows the gimbal bearing assembly, a ball-and-socket joint assembly made from a titanium forging.  That is where more or less all of the thrust load is transferred to the vehicles thrust structure.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Page 23 (pdf page 29) shows the Block IIA powerhead, which is the combustion chamber and turbopump assembly.  At the top center of the combustion chamber, you can see a flower-shaped flange that is bolted to the gimbal bearing assembly and is where load is transferred from the combustion chamber/nozzle assembly.  You can also see this on Page 26 (pdf page 32) and in closer detail on Page 42 (pdf page 48) -- note the term &quot;Thrust Cone&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Pages 45 and 46 (pdf pages 51 and 52) show detail of the combustion chamber.  Load from the nozzle is sent up mostly through the throat ring, except not really, because it's also being reacted by the actual exhaust gas present in the combustion chamber.  In all likelihood, the throat ring is probably under tension when the engine is firing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The nozzle detail is seen on Pages 49 and 51 (pdf pages 55 and 57), you can see the flange through which it attaches to the combustion chamber at the top of the illustration.  The Hatbands react the hoop stresses generated by the pressure inside the nozzle and prevent it from just blowing apart, much like the iron bands on a whiskey barrel.  The conical(ish) shape of the nozzle means that the net result of the pressure is directed upward.&lt;/p&gt;&#xA;" OwnerUserId="512" LastEditorUserId="512" LastEditDate="2018-01-23T16:24:00.470" LastActivityDate="2018-01-23T16:24:00.470" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24590" PostTypeId="1" AcceptedAnswerId="24594" CreationDate="2018-01-23T18:41:35.393" Score="18" ViewCount="3473" Body="&lt;p&gt;I will give an example with the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If some spacecraft is put either in orbit around both earth and moon or in highly eccentric orbit that intersects with the moon orbit, some other spacecraft can dock to it and &quot;ride&quot; it to the moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A problem will be docking with so much faster craft on different orbit and also slowing down of this craft over time, but it can gain speed from gravitational slingshots from the moon, or it can be reboosted with cheap fuel from the moon for example?&lt;/p&gt;&#xA;" OwnerUserId="19321" LastEditorUserId="4660" LastEditDate="2018-01-23T19:11:29.393" LastActivityDate="2018-01-24T05:13:20.820" Title="Can we set up an orbiting transporter between Earth and the Moon?" Tags="|the-moon|spacecraft|trajectory|gravity-assist|" AnswerCount="4" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24591" PostTypeId="1" AcceptedAnswerId="24592" CreationDate="2018-01-23T18:53:10.017" Score="14" ViewCount="4180" Body="&lt;p&gt;What are some of the measures engineers would take to ensure that the LOX and other similar fuels are kept below their boiling point during flight?&lt;/p&gt;&#xA;" OwnerDisplayName="user22645" LastActivityDate="2018-01-24T01:30:32.673" Title="How do rockets keep their fuel in a liquid state?" Tags="|rockets|fuel|liquid-fuel|lox|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24592" PostTypeId="2" ParentId="24591" CreationDate="2018-01-23T19:14:58.120" Score="21" Body="&lt;p&gt;As small amounts of LOX boil off heat is removed from the remaining volume. Boiloff actively cools the fluid and helps keep the remainder liquid. Rockets typically have vents to aid in managing the boiloff rate and fluid feed lines to replace lost fluid while waiting on the pad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During flight the consumption rate outpaces any boiloff losses and it is not an issue.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Cryogenic propellants aren't used for long term functions in flight, so it is not an issue after launch either. Storable propellants like solid fuel or hydrazine are used when long term storage is necessary.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Superchilled rockets like the Falcon 9 cannot manage propellant temperatures as easily. As the propellant warms it expands before boiling off, so they cannot hold indefinitely while topping off fluids like a near-boiling-point rocket. Instead they must time propellant loading to end shortly before launch, and a delayed launch would typically result in a scrub while the warmed propellant is drained and replaced with cold propellant.&lt;/p&gt;&#xA;" OwnerUserId="3044" LastEditorUserId="3044" LastEditDate="2018-01-23T23:32:14.823" LastActivityDate="2018-01-23T23:32:14.823" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24593" PostTypeId="2" ParentId="24590" CreationDate="2018-01-23T19:18:07.150" Score="6" Body="&lt;p&gt;Yes. This concept is known as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_cycler&quot; rel=&quot;noreferrer&quot;&gt;Lunar Cycler&lt;/a&gt;. Especially interesting is the &lt;a href=&quot;https://space.stackexchange.com/q/3889/4660&quot;&gt;Backflip Lunar Cycler&lt;/a&gt;, which meets the Moon once a month, and the Earth twice a month. A search for 'lunar cycler' will bring up several papers. &lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2018-01-23T19:18:07.150" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24594" PostTypeId="2" ParentId="24590" CreationDate="2018-01-23T19:22:41.503" Score="29" Body="&lt;p&gt;This sort of spacecraft is known as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunar_cycler&quot; rel=&quot;noreferrer&quot;&gt;&quot;cycler&quot;&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You hit on the problem with it: you have to match its trajectory/velocity exactly in order to dock with it, so if you can reach the cycler, you could already reach the cycler's destination. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There's no slowing down of the cycler for the same reason. In principle, if you connected to the cycler with a very elastic tether, you could borrow velocity from it, but that's completely impractical.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The potential advantage of the cycler is that it can provide space and non-consumable amenities for the journey, thus saving mass on the shuttle that rides the cycler. That's not a huge win for translunar flight, where it's only a 3-5 day trip each way, but for e.g. routine flights between Earth and Mars, it's more attractive. The mothership &lt;em&gt;Hermes&lt;/em&gt; in &quot;The Martian&quot; is acting as an Earth-Mars cycler, though that wasn't its original mission design.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-01-23T20:57:05.940" LastActivityDate="2018-01-23T20:57:05.940" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24595" PostTypeId="2" ParentId="24590" CreationDate="2018-01-23T19:25:23.080" Score="7" Body="&lt;p&gt;The problem is that when you rendezvous with such cycler (and possibly dock) you are already on the same orbit and you had to spend the fuel to get you to the Moon. So you mostly do not need the second craft for anything.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the Moon trip that does not make much sense. On the other hand cyclers are often suggested for Mars trips: when you put one big cycler in Earth-Mars loop, you can then possibly use much smaller crafts at each planet to move people and supplies (needing less propellant hopefully) but you can enjoy the big ship on the long trip.&lt;/p&gt;&#xA;" OwnerUserId="10829" LastActivityDate="2018-01-23T19:25:23.080" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24596" PostTypeId="2" ParentId="24064" CreationDate="2018-01-23T20:42:07.650" Score="9" Body="&lt;p&gt;The most likely explanation seems to be that there is no need for the legs to open simultaneously. The only requirement is that they open and lock in place reliably prior to touchdown. Therefor no effort was expended ensuring precise synchronization of the leg movements, and they are allowed some fuzziness in timing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drag, minute differences in pipe length and resistance or imperfections in cylinder sizing will all potentially produce different deployment rates.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A system feeding four leg cylinders from a common gas supply would tend to extend the leg with the least resistance most rapidly, and as each leg reaches maximum extension and locks in place the pressure available for the remaining legs increases, ensuring that any leg that &quot;sticks&quot; at partial deployment will receive additional force to ensure it catches up to the others by landing.&lt;/p&gt;&#xA;" OwnerUserId="3044" LastActivityDate="2018-01-23T20:42:07.650" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24597" PostTypeId="2" ParentId="17176" CreationDate="2018-01-23T22:53:08.393" Score="3" Body="&lt;p&gt;I do not object to Mark's answer, but I am concerned with the rotor blades rotating so fast that they will be entirely within a supersonic flow. Maintaining helicopter blades within a subsonic flow is a limiting factor for helicopter speeds here on earth. How would the rotor's lift be affected if it were entirely behind a shockwave? On earth most subsonic wings placed in such conditions simply stall. Also because Mars' atmosphere is so cold, the local sound velocity should be quite low, hence a rotor for Mars better be designed to function within transonic and supersonic regimes. A good example of this problem: the U2 spy plane (granted not a helicopter) at its highest altitude it was at the combined limits of not flying fast enough to generate enough lift (in atmosphere), or flying too fast and going supersonic (low local sound velocity) which was not good for the type of plane it was.&lt;/p&gt;&#xA;" OwnerUserId="22658" LastEditorUserId="6816" LastEditDate="2018-01-23T23:09:52.577" LastActivityDate="2018-01-23T23:09:52.577" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24598" PostTypeId="2" ParentId="24591" CreationDate="2018-01-24T01:30:32.673" Score="4" Body="&lt;p&gt;Adding to Saibogu's answer, ULA is currently developing technology called &lt;a href=&quot;http://www.ulalaunch.com/uploads/docs/Published_Papers/Upper_Stages/ACES-Stage_Concept-AIAASpace_2015.pdf&quot; rel=&quot;noreferrer&quot;&gt;Integrated Vehicle Fluids&lt;/a&gt; (IVF) that will use boiloff gas normally vented during second stage coast phases. The waste gas will be used generate electricity using an internal combustion engine and also to power reaction control thrusters and pressurize tanks.&lt;/p&gt;&#xA;" OwnerUserId="6804" LastActivityDate="2018-01-24T01:30:32.673" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24599" PostTypeId="2" ParentId="24590" CreationDate="2018-01-24T05:13:20.820" Score="3" Body="&lt;p&gt;Besides cyclers, a reason to have dedicated vehicles to go from orbit to orbit is to be able to use the best vehicle design for each leg of the trip. One can imagine a winged shuttle bringing people up from earth to an orbiter. Then the orbiter goes from low earth orbit to lunar orbit. There people transfer to a lunar lander to reach the lunar colony on the surface. The orbiter never lands, and the lunar lander never goes further than lunar orbit. The orbiter could have low thrust electric propulsion and heavy shielding. The lunar lander would have legs and higher thrust engines. Also fuel and equipment is not carried unnecessarily through a leg of the trip it is not needed for. The orbiter need not carry the lander or fuel for it. &lt;/p&gt;&#xA;" OwnerUserId="16669" LastActivityDate="2018-01-24T05:13:20.820" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24600" PostTypeId="1" AcceptedAnswerId="24611" CreationDate="2018-01-24T06:17:18.090" Score="17" ViewCount="2257" Body="&lt;p&gt;A recent (Sept. 2017) news item on the company website of Gas Sensing Solutions &lt;a href=&quot;https://www.gassensing.co.uk/news/innovative-sensor-technology-assists-nasa-with-vital-wearable-co2-monitoring-equipment/&quot; rel=&quot;noreferrer&quot;&gt;Innovative sensor technology assists NASA with vital wearable CO2 monitoring equipment&lt;/a&gt; caught my eye. A section of the article is quoted below, with the accompanying photo of Expedition 49 crewmember Kate Rubins of NASA working &amp;quot;...with the airlock inside of Kibo, the Japanese Experiment Module. Rubins was installing the Robotics External Leak Locator (RELL), a technology demonstration designed to locate external ISS ammonia (NH3) leaks.&amp;quot; (quoted caption from the copy of this image found in &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:ISS-49_Kate_Rubins_works_with_the_airlock_inside_of_Kibo.jpg&quot; rel=&quot;noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt; and in NASA Johnson's &lt;a href=&quot;https://www.flickr.com/photos/nasa2explore/30138140905/&quot; rel=&quot;noreferrer&quot;&gt;Flikr&lt;/a&gt; where it is dated 27-Sep-2016).&lt;/p&gt;&#xA;&lt;p&gt;See also SST Sensing's &lt;a href=&quot;http://www.sstsensing.com/innovative-sensor-technology-assists-nasa-with-vital-wearable-co2-monitoring-equipment/&quot; rel=&quot;noreferrer&quot;&gt;Innovative Sensor Technology Assists NASA with Vital Wearable CO2 Monitoring Equipment&lt;/a&gt; and CO2Meter.com's &lt;a href=&quot;https://www.co2meter.com/blogs/news/co2-sensor-nasa-wearable-co2-monitor&quot; rel=&quot;noreferrer&quot;&gt;NASA Wearable CO2 Monitor uses CozIR Sensor&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;p&gt;I know that CO2 buildup is an issue in confined spaces in microgravity, but the article suggests that even the baseline levels experienced by ISS astronauts might be a problem.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Do ISS astronauts notice, or experience symptoms of elevated CO2 levels? Do they wear monitors regularly? I wonder if some confided sections, like end modules or the Bigelow Expandable Activity Module might be more at risk, and astronauts are more likely to use personal monitors there for safety, or if they are used strictly for data collection and archiving.&lt;/p&gt;&#xA;&lt;p&gt;These are commercial websites selling or advocating products. I'm wondering if there are NASA or other non-commercial, authoritative sources that address this question.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Though extensive terrestrial studies have shown that this increased level will have no effect on the continued good health of those exposed to it, there must be some acknowledgement of the fact that crew members onboard the ISS still regularly experience symptoms of acute CO2 toxicity – such as headaches or lethargy.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Per this &lt;a href=&quot;https://space.stackexchange.com/a/26209/12102&quot;&gt;answer&lt;/a&gt;, the image is that of astronaut and molecular biologist &lt;a href=&quot;https://www.nasa.gov/astronauts/biographies/kathleen-rubins/biography&quot; rel=&quot;noreferrer&quot;&gt;Dr. Kathleen &amp;quot;Kate&amp;quot; Rubins (Ph.D.)&lt;/a&gt;, the first person to sequence DNA in space.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Akma4.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Akma4.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;It is generally accepted that human adaptations to microgravity conditions will cause astronauts in space to become more sensitive to elevated CO2 levels. This can result not only in physical discomfort, but also impinge on their cognitive skills and reaction times – thereby potentially leading to safety risks. On average crew members stay on the ISS for a period of around 6 months, so having a good grasp of the ongoing implications is clearly of great value.&lt;/p&gt;&#xA;&lt;p&gt;With operational practicalities meaning that it is not possible (either technically or financially speaking) to remove enough CO2 to replicate normal conditions here on Earth, the ISS has to function with relatively high ambient CO2 concentrations present in the air. Yet its occupants have to live and work in an acceptable environment.&lt;/p&gt;&#xA;&lt;p&gt;NASA scientists have, for many years, wanted to embark on a thorough investigation of how elevated CO2 levels impact on ISS crew members’ ability to carry out their allotted tasks, but it has taken a long time for them to be in a position to employ a method for acquiring data to a high enough degree of precision. Given that, as already mentioned, air does not mix well without the influence of gravity, wall-mounted sensors had proved themselves to be an unsuitable means by which to get an accurate reflection of a crew member’s exposure to CO2. What was needed instead were wearable CO2 sensors.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2022-02-13T17:29:38.857" Title="Do ISS astronauts notice, or experience symptoms of elevated CO2 levels? Do they wear monitors regularly?" Tags="|iss|astronauts|" AnswerCount="3" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24601" PostTypeId="1" AcceptedAnswerId="24615" CreationDate="2018-01-24T08:09:35.080" Score="1" ViewCount="334" Body="&lt;p&gt;Looking through NASA Johnson's &lt;a href=&quot;https://www.flickr.com/photos/nasa2explore/29659250095/in/photostream/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flickr&lt;/a&gt; I saw this photo. The complete caption is shown below.&lt;/p&gt;&#xA;&lt;p&gt;What are these? How do they work? Are those tractor wheels at the bottom? Why are they orange and so futuristic yet retro-looking? Am I just imagining it, or is there a distinctly stylized aspect to those consoles?&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Expedition 49 Preflight&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;jsc2008e040362 Photographic documentation of the Lunar Rover tour for Center Director in the Mars Yard. View is of JSC Center Director Mike Coats and Ellen Ochoa take a ride on the NASA ground prototype vehicle for lunar transportation.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; NASA image from &lt;a href=&quot;https://www.flickr.com/photos/nasa2explore/29659250095/in/photostream/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; with &lt;a href=&quot;https://creativecommons.org/licenses/by-nc/2.0/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Attribution-NonCommercial 2.0 Generic (CC BY-NC 2.0)&lt;/a&gt; license.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/Eb0Fs.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/Eb0Fs.jpg&quot; alt=&quot;NASA personal Lunar rover&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-25T02:26:28.973" Title="Segways on the Moon?" Tags="|the-moon|nasa|rovers|space-art|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24602" PostTypeId="1" AcceptedAnswerId="24608" CreationDate="2018-01-24T08:16:17.427" Score="4" ViewCount="241" Body="&lt;p&gt;Looking through NASA Johnson's &lt;a href=&quot;https://www.flickr.com/photos/nasa2explore/29714622641/in/photostream/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flickr&lt;/a&gt; I saw this photo (and also &lt;a href=&quot;https://www.flickr.com/photos/nasa2explore/29684334372/in/photostream/&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; one). The complete caption is shown below.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;Expedition 49 Preflight&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;nhq201609160011 (09/16/2016) --- Expedition 49 flight engineer Shane Kimbrough of NASA left, and Soyuz commander Sergey Ryzhikov take part in the traditional tree planting ceremony at the Cosmonaut Hotel on Friday, Sept. 16, 2016 in Baikonur, Kazakhstan. Kimbrough, Ryzhikov and flight engineer Andrey Borisenko of Roscosmos are scheduled to launch to the International Space Station aboard the Soyuz MS-02 spacecraft from the Baikonur Cosmodrome on September 24 Kazakh time. Photo Credit: (NASA/Victor Zelentsov)&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; NASA image from &lt;a href=&quot;https://www.flickr.com/photos/nasa2explore/29714622641/in/photostream/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; with &lt;a href=&quot;https://creativecommons.org/licenses/by-nc/2.0/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Attribution-NonCommercial 2.0 Generic (CC BY-NC 2.0)&lt;/a&gt; license.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5yahh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5yahh.jpg&quot; alt=&quot;Cosmonaut Tree Planting Ceremony&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-24T11:25:14.543" Title="Origin and history of the traditional tree planting ceremony at the Cosmonaut Hotel?" Tags="|history|astronauts|baikonur-cosmodrome|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24603" PostTypeId="1" AcceptedAnswerId="26277" CreationDate="2018-01-24T08:50:36.500" Score="7" ViewCount="1418" Body="&lt;p&gt;The image Photo ID: ISS049-E-003166 can be found in &lt;a href=&quot;https://www.flickr.com/photos/nasa2explore/29323401864/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Flikr&lt;/a&gt; and &lt;a href=&quot;https://commons.wikimedia.org/wiki/File:ISS-49_Soyuz_MS-01_over_the_Earth.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikimedia Commons&lt;/a&gt; as well as in &lt;a href=&quot;https://spaceflightnow.com/2016/10/13/nasa-has-no-plans-to-buy-more-soyuz-seats-and-it-may-be-too-late-anyway/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Spaceflight Now&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a long (perhaps 1 meter?) green structure that is sticking out from the side of a Soyuz spacecraft. It has windows on the end that look like they might be for the mirrors of a periscope used for docking.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is it for? What kind of optics are in there? Presumably during launch it is not deployed like this; is it mechanically extended like a submarine periscope, or does it fold out after a fairing deploys after launch?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fB5oh.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fB5oh.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/ZJz5x.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/ZJz5x.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-09-07T09:34:56.477" LastActivityDate="2020-08-05T14:43:06.527" Title="Soyuz has a Space Periscope?" Tags="|launch|reentry|soyuz-spacecraft|navigation|docking|" AnswerCount="3" CommentCount="7" ContentLicense="CC BY-SA 4.0" />
  <row Id="24604" PostTypeId="1" AcceptedAnswerId="24605" CreationDate="2018-01-24T09:59:32.287" Score="13" ViewCount="2777" Body="&lt;p&gt;I'm a bit confused about how the fuel manages to get from wherever its being stored in a spacecraft into the engines, seeing as it is supposedly solid and so seems like it wouldn't be easy to move. could anyone explain how the solid propellant works for this?&lt;/p&gt;&#xA;" OwnerUserId="22662" LastActivityDate="2018-01-24T15:06:11.520" Title="How does a solid propellant in a rocket work?" Tags="|propulsion|solid-fuel|" AnswerCount="1" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24605" PostTypeId="2" ParentId="24604" CreationDate="2018-01-24T10:07:44.813" Score="18" Body="&lt;p&gt;The solid propellant is &lt;a href=&quot;https://en.wikipedia.org/wiki/Solid-propellant_rocket&quot; rel=&quot;noreferrer&quot;&gt;stored in the engine&lt;/a&gt;, so it doesn't have to be moved. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The engine consists of a large cylinder that contains the propellant (indicated as 'grain' in the image, but it's a solid block with a grainy structure), with the nozzle at one end of the cylinder.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/pYOrc.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/pYOrc.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's &lt;a href=&quot;https://www.nasa.gov/mission_pages/constellation/multimedia/ares/10_042.html&quot; rel=&quot;noreferrer&quot;&gt;one segment&lt;/a&gt; of a Space Shuttle solid booster, with the void down the middle clearly visible: &#xA;&lt;a href=&quot;https://i.stack.imgur.com/7GmN1.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7GmN1.jpg&quot; alt=&quot;Shuttle SRB segment&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The propellant is shaped as a hollow cylinder: the void down the middle is exposed to the elements. When the engine starts, this surface is ignited. The hot gases escape down the central void and out the nozzle. &lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="897" LastEditDate="2018-01-24T15:06:11.520" LastActivityDate="2018-01-24T15:06:11.520" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24606" PostTypeId="1" AcceptedAnswerId="24607" CreationDate="2018-01-24T10:09:24.760" Score="18" ViewCount="1870" Body="&lt;p&gt;The solar panels on the Phoenix Lander and InSight rover seem to take on an iconic new look of circular solar panels. As far as I could see solar panels on landers and rover took up the maximum surface area that they could be packaged into. For Pathfinder, that meant &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Mars_Pathfinder_Lander_preparations.jpg&quot; rel=&quot;noreferrer&quot;&gt;three triangular panels&lt;/a&gt;, for its accompanied rover, Sojourner, that was a simple &lt;a href=&quot;https://en.wikipedia.org/wiki/File:Sojourner_on_Mars_PIA01122.jpg&quot; rel=&quot;noreferrer&quot;&gt;rectangle on its top&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Mars Exploration Rovers has a slightly more difficult design of Solar Panels, but ended up with a &lt;a href=&quot;http://pancam.sese.asu.edu/images/Sol322B_DeckPan.jpg&quot; rel=&quot;noreferrer&quot;&gt;similar triangular wing shaped design&lt;/a&gt; to that of Pathfinder (as it used a similar tetrahedral lander to Pathfinder, I believe).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, with Phoenix (2007) and InSight (2018) a circular solar panel arrangement has been used for whichever reasons. A &lt;a href=&quot;https://blogs.nasa.gov/spacestation/wp-content/uploads/sites/240/2016/04/blog_iss046e044220.jpg&quot; rel=&quot;noreferrer&quot;&gt;similar design&lt;/a&gt; can also be seen Enhanced Cygnus spacecraft, as well as some mock-ups of the &lt;a href=&quot;https://upload.wikimedia.org/wikipedia/commons/f/f3/Orion_lunar_orbit_%28Sept_2006%29.jpg&quot; rel=&quot;noreferrer&quot;&gt;Orion spacecraft&lt;/a&gt;, where it is almost expected (by amateurs/ the casual public) to have rectangularsolar arrays.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm curious as to what led to the decision of the use of circular arrays in the Phoenix Lander and InSight Rover as opposed to trying to pack in as much solar array area as possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NB: I'm marginally aware that the Enhanced Cyngus uses circular panels to reduce its weight and increase its max payload by ~700kg &lt;sup&gt;&lt;a href=&quot;https://www.nasaspaceflight.com/2012/02/orbital-upbeat-ahead-of-antares-debut/&quot; rel=&quot;noreferrer&quot;&gt;1&lt;/a&gt;&lt;/sup&gt;&lt;/p&gt;&#xA;" OwnerUserId="18175" LastEditorUserId="79" LastEditDate="2018-01-25T03:37:16.043" LastActivityDate="2018-01-26T08:44:30.457" Title="What inspired the circular panels on the Phoenix Lander and InSight rover?" Tags="|rovers|solar-power|insight|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24607" PostTypeId="2" ParentId="24606" CreationDate="2018-01-24T11:17:21.227" Score="19" Body="&lt;p&gt;Good catch noting that Cygnus has the same solar panel design! Orbital ATK, developer of Cygnus, builds these panels under the &amp;quot;Ultraflex&amp;quot; and &amp;quot;Megaflex&amp;quot; brands, and did indeed supply them to JPL for two Mars missions:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Mars Phoenix Lander&lt;/li&gt;&#xA;&lt;li&gt;Mars InSight&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;p&gt;The Mars Polar Lander program successfully qualified the panels for flight; but were not used on that particular mission.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vi4Fv.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vi4Fv.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;You can find more about the panels in &lt;a href=&quot;https://www.orbitalatk.com/space-systems/space-components/solar-arrays/docs/UltraFlex_Factsheet.pdf&quot; rel=&quot;noreferrer&quot;&gt;this PDF&lt;/a&gt; by Orbital-ATK. With panels on SpaceX's Dragon, where you have a number of rectangular panels that expand individually, this necessitates the need to have at least &lt;span class=&quot;math-container&quot;&gt;$n-1$&lt;/span&gt; hinges or joints for &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; panels. Additionally, on a platform such as a Mars lander, these hinges will be operating in the direction of gravity, which is much harder than operating perpendicular to the force of gravity. A way to solve this is to have a hinge able to pivot the entire array at the body of the lander, but this just means you now have &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; hinges for &lt;span class=&quot;math-container&quot;&gt;$n$&lt;/span&gt; panels.&lt;/p&gt;&#xA;&lt;p&gt;With a Ultraflex panel, the entire boom can be mounted statically, and a single motor can unfurl the entire panel, as seen in this deployment annotation:&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/68Zkp.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/68Zkp.jpg&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;These panels had enough determined about their reliability that NASA felt confident in using them, again, reading from the Orbital ATK PDF:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;Lightweight: ⅓ total mass of a rigid solar array of the same power&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;High strength: &amp;gt; 10x max on-orbit acceleration&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;High deployed stiffness: 3-8x higher 1st mode&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Compact: ¼ stowage volume and footprint compared to a rigid solar array of the same power&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Reliable: flight proven; deployment powered by a single redundant motor.&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-26T08:44:30.457" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24608" PostTypeId="2" ParentId="24602" CreationDate="2018-01-24T11:25:14.543" Score="3" Body="&lt;p&gt;As most of cosmonauts traditions, this started from Yuri Gagarin (at least the first tree there was planted by him).&#xA;Source: &lt;a href=&quot;http://www.bbc.com/russian/science/2014/06/140613_vert_fut_strange_space_rituals&quot; rel=&quot;nofollow noreferrer&quot;&gt;Traditions and rituals of Russian cosmonauts&lt;/a&gt; (BBC Russian, in russian, use google translate)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit: here is &lt;a href=&quot;http://www.bbc.com/future/story/20140610-the-strange-rituals-of-cosmonauts&quot; rel=&quot;nofollow noreferrer&quot;&gt;English version&lt;/a&gt; of the text, but it does not contain words about the tree planted by Gagarin&lt;/p&gt;&#xA;" OwnerUserId="11046" LastActivityDate="2018-01-24T11:25:14.543" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24609" PostTypeId="2" ParentId="24586" CreationDate="2018-01-24T15:27:22.363" Score="5" Body="&lt;p&gt;The force from the rocket engine is transmitted to the rocket structure by the Thrust Frame Adapter. This is a framework rated to handle the newtons of force created by the engine. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The spacecraft's load-bearing skeleton is built on top of the thrust frame adapter, and the spacecraft components are attached to the skeleton. &#xA;&lt;a href=&quot;https://i.stack.imgur.com/6qO1k.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6qO1k.jpg&quot; alt=&quot;Thrust Frame Adapter&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(&lt;a href=&quot;https://www.nasa.gov/exploration/systems/sls/multimedia/gallery/rs25_test_1.html&quot; rel=&quot;noreferrer&quot;&gt;image source&lt;/a&gt;)&lt;/p&gt;&#xA;" OwnerUserId="22079" LastActivityDate="2018-01-24T15:27:22.363" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24610" PostTypeId="2" ParentId="24603" CreationDate="2018-01-24T15:58:26.070" Score="7" Body="&lt;p&gt;It is used for star alignments, which is a backup system in case of serious navigation errors. It can also be used for manual docking operations, if required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As is mentioned in &lt;a href=&quot;https://space.stackexchange.com/questions/3232/how-is-the-periscope-port-on-soyuz-spacecraft-secured-for-the-atmospheric-reentr&quot;&gt;this question&lt;/a&gt;, it is discarded prior to reentry. During launch, it is under the shroud, as seen below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/bfRMe.gif&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/bfRMe.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-24T16:32:27.140" LastActivityDate="2018-01-24T16:32:27.140" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24611" PostTypeId="2" ParentId="24600" CreationDate="2018-01-24T18:44:32.313" Score="17" Body="&lt;p&gt;I don't have anything public I can point to, but yes, they do notice.  The CO2 target is a frequent topic of conversation between the crew and ground personnel, and the crew haven't been shy about pointing out when CO2 concentrations are too high for their liking.&lt;/p&gt;&#xA;" OwnerUserId="512" LastActivityDate="2018-01-24T18:44:32.313" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24612" PostTypeId="1" CreationDate="2018-01-24T21:08:27.187" Score="10" ViewCount="315" Body="&lt;p&gt;Given a new, unprepared &quot;in the field&quot; landing site for a rocket, what is the SpaceX strategy? Are the landing legs intended to be rated to handle that kind of instability? How can they possibly be?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a plan to prepare a landing site ahead of time, or what? Has this been addressed in the released information?&lt;/p&gt;&#xA;" OwnerUserId="5965" LastEditorUserId="51" LastEditDate="2018-01-24T22:40:10.577" LastActivityDate="2018-10-08T01:15:09.493" Title="How, in the SpaceX Mars stack, do they plan to address de novo landing sites?" Tags="|spacex|mars|landing|" AnswerCount="2" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24613" PostTypeId="1" CreationDate="2018-01-24T21:09:05.390" Score="5" ViewCount="2182" Body="&lt;p&gt;Upon news of RocketLab's successful flight of the Electron launch vehicle, it was announced that not only had they deployed the three onboard payloads into the desired orbit, they had done so using a newly developed kick stage powered by an engine called &lt;em&gt;Curie&lt;/em&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The successful launch of Rocket Lab’s Electron rocket Jan. 20 also tested a kick stage designed to circularize the orbits of its satellite payloads. [...] The heart of the kick stage is Curie, an engine developed by Rocket Lab.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From SpaceNews' article: &lt;em&gt;&lt;a href=&quot;http://spacenews.com/rocket-lab-launch-also-tested-new-kick-stage/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Lab launch also tested new kick stage&lt;/a&gt;&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An image of the &lt;a href=&quot;https://en.wikipedia.org/wiki/Curie_(rocket_engine)&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curie&lt;/a&gt; kick stage can be seen in this photo supplied by RocketLab:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/M0rcm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M0rcm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, the &lt;em&gt;kicker&lt;/em&gt; is that this doesn't appear to be using the typical monopropellant of Hydrazine:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The heart of the kick stage is Curie, an engine developed by Rocket Lab. Beck described the engine, capable of performing multiple burns, as 3D-printed &lt;strong&gt;and using an unspecified “green” monopropellant&lt;/strong&gt;. [Emphasis mine]&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;What is this green monopropellant? Is it similar to &lt;a href=&quot;https://space.stackexchange.com/questions/8396/why-did-spacex-choose-to-use-hydrazine-over-newer-green-propellants-for-dragon&quot;&gt;AF-M315E&lt;/a&gt;, &lt;a href=&quot;https://space.stackexchange.com/questions/10032/is-there-a-derivative-of-lmp-103s-called-lmp-103-that-has-even-higher-performa/20463#20463&quot;&gt;LMP-103S&lt;/a&gt;, or something else entirely?&lt;/p&gt;&#xA;" OwnerUserId="6537" LastEditorUserId="46507" LastEditDate="2023-03-21T00:45:05.583" LastActivityDate="2023-12-19T00:08:31.017" Title="What &quot;green&quot; propellant powers Curie, the engine on RocketLab's payload kickstage?" Tags="|fuel|rocketlab|monopropellant|green-propellant|curie|" AnswerCount="1" CommentCount="9" ContentLicense="CC BY-SA 4.0" />
  <row Id="24614" PostTypeId="2" ParentId="24523" CreationDate="2018-01-24T21:21:08.303" Score="2" Body="&lt;p&gt;Technically, it's possible. But it's very hard and not worth it. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's very important for rockets to have their center of mass (COM) exactly in the center. If it isn't, you rocket is going to get off course. This means the payload has to be in the middle a weird adapter (like in your image) so you do not have to add weight to the other rockets to counter-balance the COM. But that structure holding the payload in the middle weighs a lot too.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, it's very complex. For example, three rockets strapped together are much more complex to control. Because they are so close to each other, there's a lot of vibrations. And there's a weird gap in the middle of a couple of rockets which could lead to problems. For example when the boosters are expanding or shrinking or when air gets too dense. And don't forget, each rocket has a chance of failing. If one rocket of three rockets strapped together fails, the entire launch failed. The chance of the launch failing from failed engines gets much bigger. So reliability is pretty bad.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Seperation of stages is also difficult. The engine shutdown and stage separation of all the rocket boosters has to occur at the exact same time, or the rocket is flying of course. If one of the engines is stil on, you've got a problem. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;That doesn't mean we haven't tried, though: look at this &lt;a href=&quot;https://en.wikipedia.org/wiki/Conestoga_(rocket)#Conestoga_1620&quot; rel=&quot;nofollow noreferrer&quot;&gt;rocket&lt;/a&gt;. Sadly, it exploded and never flew again.&lt;/p&gt;&#xA;" OwnerUserId="22670" LastActivityDate="2018-01-24T21:21:08.303" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24615" PostTypeId="2" ParentId="24601" CreationDate="2018-01-24T21:27:22.843" Score="2" Body="&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/nOyVm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/nOyVm.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;&lt;a href=&quot;https://commons.wikimedia.org/wiki/File:LER_deploying_K10_robot.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Source&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I think that this is the same vehicle from another angle, it is the &lt;a href=&quot;https://www.nasa.gov/multimedia/imagegallery/image_feature_1106.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;Crew Mobility Chassis&lt;/a&gt;. So Segway is not an accurate description. As for the 'distinctly stylized aspect', that is subjective, but at least in part, seeing the whole vehicle gives more of a utilitarian feeling. I think at least in part, it seems stylized because the photographer intentionally cropped the picture in such a way to give that effect. Also the exaggerated controls which are necessary when wearing a space suit seem to add to the effect. &lt;/p&gt;&#xA;" OwnerUserId="21390" LastEditorUserId="21390" LastEditDate="2018-01-25T02:26:28.973" LastActivityDate="2018-01-25T02:26:28.973" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24616" PostTypeId="1" AcceptedAnswerId="25418" CreationDate="2018-01-25T04:33:55.697" Score="6" ViewCount="745" Body="&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Rocket_Lab&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Lab&lt;/a&gt; has successfully launched a rocket from New Zealand and placed several objects in orbit, see &lt;a href=&quot;https://qz.com/1187991/rocket-lab-electron-private-rocketeers-put-their-own-humanity-star-into-orbit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt; for example. One payload is New Zealand's first satellite the Humanity Star, a reflective geodesic that can be &lt;a href=&quot;http://www.thehumanitystar.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;seen at night&lt;/a&gt; by reflected light. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Unlike &lt;a href=&quot;https://space.stackexchange.com/q/22476/12102&quot;&gt;Mayak&lt;/a&gt; which was supposed to be deployed from a 3U cubesat, then inflated (which it seems &lt;a href=&quot;https://space.stackexchange.com/a/22661/12102&quot;&gt;didn't happen&lt;/a&gt;), the &quot;secret payload&quot; Humanity Star is rigid and filled most of the payload space within the Electron's 2nd stage payload fairing, described in &lt;a href=&quot;https://www.tvnz.co.nz/one-news/new-zealand/star-humanity-revealed-secret-object-launched-into-space-rocket-labs&quot; rel=&quot;nofollow noreferrer&quot;&gt;TVNZ's interview&lt;/a&gt; of Peter Beck. This suggests it may already be visible! &lt;a href=&quot;http://www.n2yo.com/satellite/?s=43166&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.n2yo.com/satellite/?s=43166&lt;/a&gt; seems to be Humanity Star; it correlates with the tracking option shown at &lt;a href=&quot;http://www.thehumanitystar.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://www.thehumanitystar.com/&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Ars Technica's &lt;a href=&quot;https://arstechnica.com/science/2018/01/rocket-lab-launched-a-secret-payload-into-space-last-weekend/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Lab launched a secret payload into space last weekend&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Humanity Star is something of an experiment. Beck isn't sure it will actually work. Made of carbon fiber materials, the sphere has 65 highly reflective panels, each about the size of a laptop. It truly is intended to act like a giant disco ball, to reflect the Sun's light back to Earth as it flies around the planet in a 300×500km orbit. It will last about nine months before its orbit begins to decay, and Humanity Star begins to fall back toward its cradle.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to this &lt;a href=&quot;https://math.stackexchange.com/a/2620121/284619&quot;&gt;Math SE answer&lt;/a&gt;, the shape can be described as a pentakis &lt;a href=&quot;https://en.wikipedia.org/wiki/Pentagonal_orthobirotunda&quot; rel=&quot;nofollow noreferrer&quot;&gt;pentagonal orthobirotunda&lt;/a&gt;, and so should have 80 total sides as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Pentakis_icosidodecahedron&quot; rel=&quot;nofollow noreferrer&quot;&gt;pentakis icosidodecahedron&lt;/a&gt; would. In addition to the 65 highly reflective panels and the mounting face (which would account for what looks like five triangles at the bottom as seen in the 2nd image) there may be something like &lt;em&gt;ten triangular panels with other functions&lt;/em&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Does Humanity Star have non-reflective triangular panels, and if so, what are their characteristics and/or purpose?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Open images in new window for full size (&lt;a href=&quot;https://qz.com/1187991/rocket-lab-electron-private-rocketeers-put-their-own-humanity-star-into-orbit/&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt; and &lt;a href=&quot;https://www.theverge.com/2018/1/24/16926426/rocket-lab-humanity-star-secret-satellite-electron-test-launch&quot; rel=&quot;nofollow noreferrer&quot;&gt;source&lt;/a&gt;):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/JZ9bg.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/JZ9bgm.jpg&quot; alt=&quot;Humanity Star 1&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/VngsM.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/VngsMm.jpg&quot; alt=&quot;Humanity Star 2&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-02-01T04:15:51.017" LastActivityDate="2018-02-25T05:54:42.630" Title="Does Humanity Star have non-reflective triangular panels? If so, what are their characteristics?" Tags="|artificial-satellite|space-art|rocketlab|humanitystar|" AnswerCount="2" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24617" PostTypeId="2" ParentId="24523" CreationDate="2018-01-25T09:22:53.557" Score="2" Body="&lt;p&gt;Something similar, the parallel staging is possible if you do it right. There is the &lt;a href=&quot;https://en.wikipedia.org/wiki/R-7_(rocket_family)&quot; rel=&quot;nofollow noreferrer&quot;&gt;R-7&lt;/a&gt; rocket family, a very sucessful rocket used over many decades. It used a central core stage and four boosters, all equipped with the same type of engine. It was used for the first satellite in orbit and the first man in space. It is still used today for the transport of astronauts to the ISS.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The boosters should be very close and symmetrically mounted to the core stage like in &lt;a href=&quot;https://en.wikipedia.org/wiki/File:R-7_(7A)_misil.svg&quot; rel=&quot;nofollow noreferrer&quot;&gt;this&lt;/a&gt; picture. Each booster and the central core stage has its own nose cone, a very simple design with a good aerodynamic shape.&lt;/p&gt;&#xA;" OwnerUserId="15906" LastActivityDate="2018-01-25T09:22:53.557" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24618" PostTypeId="1" AcceptedAnswerId="24623" CreationDate="2018-01-25T13:36:31.853" Score="31" ViewCount="4889" Body="&lt;p&gt;Recent plans on human missions to Mars have sparked many discussions, one of which is about if we should colonize Venusian upper atmosphere with Zeppelin like floating ships and cities first.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;NASA even has a mission developed - the &lt;a href=&quot;https://sacd.larc.nasa.gov/smab/havoc/&quot; rel=&quot;noreferrer&quot;&gt;High Altitude Venus Operational Concept (HAVOC)&lt;/a&gt;&#xA;&lt;a href=&quot;https://i.stack.imgur.com/qIFBX.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/qIFBX.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why are people so fascinated about Mars and not about Venus at all?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is there a new space race to Mars, and why don't some of the major players like Nasa, Cnsa, Roscosmos or Boeing consider Venus as a target for robotic exploration?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We have explored only 2 celestial bodies with robotic probes - The Moon and Mars. I don't talk about orbiters and impactors, but about real, driveable drones that are in Venus' atmosphere for months or years to come, and while we used rovers because of the lack of atmosphere on the Moon and to big extent on Mars, we can use all kinds of flying machines on Venus, so why wouldn't we send probes to Venus and explore it, like we have done with Mars?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;P.S. These are somehow similar to this question : &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/13607/is-there-any-plan-to-send-balloon-like-probes-to-gas-giants-to-explore-interior&quot;&gt;Is there any plan to send balloon like probes to gas giants to explore interior of gas giants?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://space.stackexchange.com/questions/104/have-there-been-any-official-feasability-studies-into-descending-balloons-into-o&quot;&gt;Have there been any official feasibility studies into descending balloons into outer planetary atmospheres?&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="19321" LastEditorUserId="109" LastEditDate="2019-02-26T19:10:26.503" LastActivityDate="2021-11-13T06:09:46.243" Title="When will we send floating probes to Venus?" Tags="|spacecraft|probe|colonization|venus|balloons|" AnswerCount="4" CommentCount="6" ContentLicense="CC BY-SA 4.0" />
  <row Id="24619" PostTypeId="1" AcceptedAnswerId="24638" CreationDate="2018-01-25T13:37:13.483" Score="5" ViewCount="1998" Body="&lt;p&gt;The video &lt;a href=&quot;https://www.youtube.com/watch?v=RsrnCeRmW3Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;Apollo Program Guidance and Navigation System 79974&lt;/a&gt; explains that the spacecraft's sextant could be used together with the navigation computer to determine a &lt;em&gt;position fix&lt;/em&gt; while in Earth orbit, and an &lt;em&gt;attitude fix&lt;/em&gt; when en-route to the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.ph.unimelb.edu.au/~wbelcher/lunacypapers/mit_il-01.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;This paper&lt;/a&gt; states on page 14:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The sextant articulating star line of sight makes the precision measurement of star direction for the IMU alignment. For the entire Apollo 8 mission the IMU was scheduled to remain operating continuously. Periodic realignment was performed 30 times with the sextant each time using the automatic star pointing acquisition of program P52.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; This describes &lt;em&gt;attitude&lt;/em&gt; determination using two stars. But was the sextant ever used or even demonstrated to help make &lt;em&gt;position&lt;/em&gt; determinations using a point on the Earth in combination with a star?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Screen shots from &lt;a href=&quot;https://www.youtube.com/watch?v=RsrnCeRmW3Q&quot; rel=&quot;nofollow noreferrer&quot;&gt;video&lt;/a&gt;, open in new window for full size.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/rGLaJ.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/rGLaJm.jpg&quot; alt=&quot;Apollo Sextant 4&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/zi07r.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zi07rm.jpg&quot; alt=&quot;Apollo Sextant 1&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/vxyeF.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/vxyeFm.jpg&quot; alt=&quot;Apollo Sextant 2&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/d4NmR.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/d4NmRm.jpg&quot; alt=&quot;Apollo Sextant 3&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-26T12:44:26.893" LastActivityDate="2018-01-28T04:08:51.910" Title="Was the Apollo sextant used (or tested) to determine position while in Earth orbit?" Tags="|apollo-program|navigation|" AnswerCount="1" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24620" PostTypeId="1" CreationDate="2018-01-25T14:39:44.960" Score="4" ViewCount="513" Body="&lt;p&gt;Is any viewing of the falcon heavy launch visible from the $75 rocket garden ticket? Just trying to decide if it would be worth the money.&lt;/p&gt;&#xA;" OwnerUserId="22678" LastEditorUserId="25" LastEditDate="2018-01-25T16:52:24.947" LastActivityDate="2018-04-25T07:45:00.123" Title="Can the launch of Falcon Heavy be seen well from the Rocket Garden?" Tags="|launch|falcon-heavy|kennedy-space-center|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24621" PostTypeId="2" ParentId="24620" CreationDate="2018-01-25T15:28:25.463" Score="6" Body="&lt;p&gt;From the &lt;a href=&quot;https://www.kennedyspacecenter.com/info/tickets#&quot; rel=&quot;noreferrer&quot;&gt;KSC Rocket Garden&lt;/a&gt; website, a $75 ticket gets you access to the KSC Visitor Complex. More expensive tickets get you closer (down to about 4 km).&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Where:&lt;/p&gt;&#xA;&lt;p&gt;Kennedy Space Center Visitor Complex, approximately 7.5 miles/ 12 kilometers with rocket visible shortly after lift off&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;12 km is very far away, the rocket will be a speck on the horizon, comparable with an airliner flying over at cruising altitude.&lt;/p&gt;&#xA;" OwnerUserId="897" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-25T16:37:35.633" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24622" PostTypeId="1" AcceptedAnswerId="24625" CreationDate="2018-01-25T15:33:30.063" Score="3" ViewCount="1289" Body="&lt;p&gt;A quote near the end of the BBC's article &lt;a href=&quot;http://www.bbc.com/news/science-environment-42808180&quot; rel=&quot;nofollow noreferrer&quot;&gt;'Disco ball' put into space from NZ&lt;/a&gt; says: &lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Jonathan McDowell, a satellite-tracker and astronomer at the Harvard-Smithsonian Center for Astrophysics, commented: &quot;The irony is that it's poorly placed for observation right now - low on the horizon for evening passes in New Zealand and not visible from the USA until March - if it stays up that long.&quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;http://www.n2yo.com/satellite/?s=43166&quot; rel=&quot;nofollow noreferrer&quot;&gt;N2YO says&lt;/a&gt; that it is 43166, 2018-010D, in a 299 x 532 km orbit at 82.9°. This LEO orbit has a mean motion of close to 15.5 per day, is there some freaky resonance that puts it over the US only during the daytime somehow?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt;  How could it remain invisible from North America (or USA at least) for two months?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A secondary question would be if this is intentional - is this a side-effect of making its visibility somehow more from New Zealand?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-29T04:11:06.853" LastActivityDate="2018-01-29T04:11:06.853" Title="Why would Humanity Star not be visible from North America (USA?) until March?" Tags="|orbital-mechanics|artificial-satellite|rocketlab|humanitystar|" AnswerCount="1" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24623" PostTypeId="2" ParentId="24618" CreationDate="2018-01-25T16:50:40.097" Score="47" Body="&lt;p&gt;When will we send? &lt;strong&gt;We already did.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/LeGBT.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/LeGBT.jpg&quot; alt=&quot;Vega mission description&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In 1985 and 1986 the Soviet Union sent two Vega probes to Venus. Both included a robotic balloon (usually called &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobot&quot; rel=&quot;noreferrer&quot;&gt;aerobots&lt;/a&gt;).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/RNC8v.jpg&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/RNC8v.jpg&quot; alt=&quot;Vega balloon probe on display at the Udvar-Hazy Center of the Smithsonian Institution. Photo by Geoffrey A. Landis. Wikipedia&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;The two balloon aerobots were designed to float at 54 km from the&#xA;  surface, in the most active layer of the Venusian cloud system. The&#xA;  instrument pack had enough battery power for sixty hours of operation&#xA;  and measured temperature, pressure, wind speed and aerosol density.&#xA;  The balloon envelopes were surfaced with &lt;a href=&quot;https://en.wikipedia.org/wiki/Polytetrafluoroethylene&quot; rel=&quot;noreferrer&quot;&gt;polytetrafluoroethylene&lt;/a&gt;&#xA;  to resist attack by the corrosive atmosphere. Both Vega-1 and Vega-2&#xA;  balloons operated for more than 46 hrs from injection to the final&#xA;  transmission.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The balloons were spherical superpressure types with a diameter of&#xA;  3.54 metres (11.6 ft) and filled with helium. A gondola assembly weighing 6.9 kilograms (15.2 pounds) and 1.3 meters (4.26 ft) long was&#xA;  connected to the balloon envelope by a tether 13 metres (42.6 ft)&#xA;  long. Total mass of the entire assembly was 21 kilograms (46 pounds).&#xA;  [...]&#xA;  The balloons were dropped onto the planet's darkside and deployed at&#xA;  an altitude of about 50 kilometres (31 mi). They then floated upward a&#xA;  few kilometres to their equilibrium altitude. At this altitude,&#xA;  pressure and temperature conditions of Venus are similar to those of&#xA;  Earth, though the planet's winds moved at hurricane velocity and the&#xA;  carbon dioxide atmosphere is laced with sulfuric acid, along with&#xA;  smaller concentrations of hydrochloric and hydrofluoric acid.&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;The balloons moved swiftly across the night side of the planet into&#xA;  the light side, where their batteries finally ran down and contact was&#xA;  lost. Tracking indicated that the motion of the balloons included a&#xA;  surprising vertical component, revealing vertical motions of air&#xA;  masses that had not been detected by earlier probe missions.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Vega_program#Balloon&quot; rel=&quot;noreferrer&quot;&gt;(Wikipedia)&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="13643" LastEditorUserId="-1" LastEditDate="2018-01-26T02:24:07.297" LastActivityDate="2018-01-26T02:24:07.297" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24624" PostTypeId="2" ParentId="24618" CreationDate="2018-01-25T17:16:05.270" Score="35" Body="&lt;blockquote&gt;&#xA;&lt;p&gt;Why are people so fascinated about Mars and not about Venus at all?&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The trivial answer - Mars is moderate place.&lt;/p&gt;&#xA;&lt;p&gt;A human could easily get around with a spacesuit, and it is trivial for robots and equipment to exist on Mars.&lt;/p&gt;&#xA;&lt;p&gt;Venus is a swirling hell.&lt;/p&gt;&#xA;&lt;p&gt;The &amp;quot;atmosphere&amp;quot; is a &lt;strong&gt;&lt;em&gt;sea&lt;/em&gt; of incredibly hot, ultra-boiling, 800+ °F sulfuric acid&lt;/strong&gt; (!!!)&lt;/p&gt;&#xA;&lt;p&gt;(Indeed, Venus is by far the &lt;em&gt;&lt;strong&gt;most incredibly hot&lt;/strong&gt;&lt;/em&gt; planet in our system!)&lt;/p&gt;&#xA;&lt;p&gt;The very few things we have sent to Venus which landed there, &lt;strong&gt;were totally destroyed in an hour or so&lt;/strong&gt; - the first ones in a few minutes.&lt;/p&gt;&#xA;&lt;p&gt;The excellent Russian balloon-robots which stayed up high, with excellent '80s technology, (mentioned above) only lasted 2 days.&lt;/p&gt;&#xA;&lt;p&gt;Venus is the solar system's hell.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;http://coolcosmos.ipac.caltech.edu/ask/44-Has-a-spacecraft-ever-landed-on-Venus-&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://coolcosmos.ipac.caltech.edu/ask/44-Has-a-spacecraft-ever-landed-on-Venus-&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;[a few...] landers from the former Soviet Union have landed on Venus. They were only able to send us information for a short time because the extremely high temperature and pressure on the surface of Venus melted and crushed the landers...&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="3289" LastEditorUserId="7062" LastEditDate="2021-11-13T06:09:46.243" LastActivityDate="2021-11-13T06:09:46.243" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="24625" PostTypeId="2" ParentId="24622" CreationDate="2018-01-25T17:48:23.507" Score="2" Body="&lt;p&gt;The mean motion isn't really what causes it to not be visible for a long time, but the nodal precession. High inclination orbits tend to have a slower rate of nodal precession, which is why Sun Synchronous orbits are nearly completely polar. Using this &lt;a href=&quot;https://www.fxsolver.com/solve/&quot; rel=&quot;nofollow noreferrer&quot;&gt;calculator&lt;/a&gt;, and calculating for a 400 km orbit, I calculate a precession of about a degree per day. That being the case, if they launched it poorly, it could indeed take months to switch to a more favorable viewing location.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I don't think this was intentional. It largely depends on the time of day they launched the rocket. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For such a nearly polar orbit, the satellite will basically pass over twice per day, separated by 12 hours. The launch happened at 2:43 PM in New Zealand. The orbit happens a few minutes later, so let's say 2:00.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Actual pass times are closer to 12-2, AM and PM. The exact time can vary. For a really good New Zealand pass, it will just clip the sun at the earlier portion of the window. Due to the fact that New Zealand is far south, the &quot;twilight&quot; period where the satellite is in sun, but the Earth in shadow, is expanded more then normal, and allows for it to be seen, barely in middle of the night passes, as can be seen at &lt;a href=&quot;http://heavens-above.com/PassSummary.aspx?satid=43168&amp;amp;lat=-40.9006&amp;amp;lng=174.886&amp;amp;loc=New%20Zealand&amp;amp;alt=0&amp;amp;tz=NZST&quot; rel=&quot;nofollow noreferrer&quot;&gt;Heavens-Above&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the US, it is the winter. This time of the night has no visibility. Thus, it is pretty much a guarantee that the satellite is eclipsed when it will pass overhead.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only thing that could have been done differently would be to launch at a different time. I assume at this was a test flight, they choose to launch at a time of day that they could clearly see what was happening.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-25T19:23:52.490" LastActivityDate="2018-01-25T19:23:52.490" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24626" PostTypeId="1" CreationDate="2018-01-25T18:42:13.763" Score="3" ViewCount="674" Body="&lt;p&gt;Has SpaceX provided any details on their specific plans for the trajectory of the Falcon Heavy payload propulsion on its trip out as far as Mars orbit?  I'm wondering if they have made public whether they are launching directly towards Mars orbit or if they will orbit the earth for a few rotations first.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, will the 2nd stage Merlin Vacuum engine provide all of the thrust necessary to get to Earth orbit and then use the same engine to get out of Earth's gravity towards Mars orbit?  I know the Merlin Vacuum engine can be re-fired multiple times, but I do not know if getting to earth orbit, then getting out of Earth's gravity field will run the motor significantly longer than it's ever run before on a flight.  &lt;/p&gt;&#xA;" OwnerUserId="16735" LastEditorUserId="25" LastEditDate="2018-01-25T19:23:13.980" LastActivityDate="2018-02-11T11:34:28.357" Title="Falcon Heavy path and propulsion to Mars" Tags="|launch|spacex|mars|trajectory|falcon-heavy|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24627" PostTypeId="2" ParentId="24626" CreationDate="2018-01-25T19:23:01.483" Score="6" Body="&lt;p&gt;It is pretty much decided that it won't actually orbit Mars, but rather touch the orbit of Mars going around the Sun. See for example the answers to &lt;a href=&quot;https://space.stackexchange.com/q/23883/12102&quot;&gt;this question&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As far as it rotating the Earth for a few rotations, that also won't happen. The only reason someone would do that is if one was testing out a system prior to committing, like a manned mission. There isn't a payload to check out, thus the sooner they can get it going to its final destination, the better. There is a chance that it will have some kind of a &quot;Drift&quot; phase, where it needs to wait for a few minutes between firings of the second stage, but that hasn't been made public yet. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line, I'm quite sure they will get to the final trajectory as quickly as they can. The timeline isn't public yet, but should be a day or two before launch, if not longer. There might be a small coast phase.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Edit- So it turns out there was a large coast, but it was to demonstrate a new capability. There is a desire to put a satellite directly to  GEO. This requires a coast of about 6 hours. The mission tested that ability. It was not to check out the payload.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-02-11T11:34:28.357" LastActivityDate="2018-02-11T11:34:28.357" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24628" PostTypeId="1" CreationDate="2018-01-25T19:51:40.653" Score="4" ViewCount="1188" Body="&lt;p&gt;The cost of an SLS launch is significantly more expensive then a Falcon Heavy. Could an Orion spacecraft be mated to the Falcon Heavy to save launch costs?&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-25T20:13:38.980" Title="Could an Orion Spacecraft launch from a Falcon Heavy?" Tags="|launch|falcon-heavy|orion-spacecraft|" AnswerCount="2" CommentCount="0" ClosedDate="2018-01-25T20:35:43.623" ContentLicense="CC BY-SA 3.0" />
  <row Id="24629" PostTypeId="2" ParentId="24628" CreationDate="2018-01-25T20:04:36.733" Score="4" Body="&lt;p&gt;Based on weight to LEO, the falcon heavy could lift the Orion.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The current fairing OD is 5.2 meters and the Orion capsule is 5 meters wide. Aerodynamically it'd be pretty similar to  the standard fairing.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another issue would be the launch abort system. NASA typically wants these so SpaceX would need to include it and make an adapter to step up the diameter of the falcon 9 core.&lt;/p&gt;&#xA;" OwnerUserId="21538" LastActivityDate="2018-01-25T20:04:36.733" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24630" PostTypeId="2" ParentId="24628" CreationDate="2018-01-25T20:13:38.980" Score="2" Body="&lt;p&gt;One of the challenges Boeing has faced with CST-100 was the &lt;a href=&quot;http://www.spaceflightinsider.com/organizations/ula/aeroskirt-added-to-atlas-v-configuration-for-cst-100-starliner/&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerodynamics of a larger diameter capsule on a skinnier stack&lt;/a&gt;. This would be similar to the problems encountered placing Orion on Falcon Heavy, and would require aerodynamic studies to determine if it is stable, and what modifications might be required.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Besides the windtunnel and simulation tests, the government may require a test of the inflight abort system since A) the current system (tested standalone) was designed for SLS and B) They are aware SpaceX could realistically perform this test much cheaper than other providers, due to cheaper launch costs and possibilities of reuse (a cheaper used booster, and maybe saving the booster). &lt;/p&gt;&#xA;" OwnerUserId="3044" LastActivityDate="2018-01-25T20:13:38.980" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24631" PostTypeId="2" ParentId="24538" CreationDate="2018-01-25T20:29:22.117" Score="9" Body="&lt;p&gt;As far as I know, the raw data used to make the Pale Blue Dot image have not been preserved in NASA's Planetary Data System, which is why other commenters can't find it online. You might try contacting someone at the PDS Rings Node to see if anybody there has the data and metadata.&lt;/p&gt;&#xA;" OwnerUserId="22684" LastActivityDate="2018-01-25T20:29:22.117" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24632" PostTypeId="1" AcceptedAnswerId="24635" CreationDate="2018-01-25T21:21:44.253" Score="0" ViewCount="262" Body="&lt;p&gt;IF we launched a series of nuclear strikes on the polar ice caps on Mars, would it affect anyone in orbit around mars (assuming we could have them in orbit around Mars in such circumstances)?&lt;/p&gt;&#xA;" OwnerUserId="22340" LastActivityDate="2018-01-28T11:58:32.753" Title="Can people be in orbit around Mars were they to blast the poles with Nuclear bombs?" Tags="|mars|nuclear|" AnswerCount="3" CommentCount="3" ClosedDate="2018-01-26T19:05:13.223" ContentLicense="CC BY-SA 3.0" />
  <row Id="24633" PostTypeId="2" ParentId="21403" CreationDate="2018-01-25T22:10:48.283" Score="10" Body="&lt;p&gt;I just saw this and recognized my research, ha ha. I realize this is an older question but I wanted to give my two cents. The published article is a bit deceitful in describing the technology (which frustrates me) so I wanted to straighten things out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You are exactly right in that the finite surface area will offset almost any gains from increases in radiative surface properties. The first plot below shows how the apparent absorptivity and apparent emissivity of the surface openings will change as the V collapses from fully open (phi of 180 degrees) to fully closed (0 degrees) for a diffusely-reflecting surface. Each line is for a different intrinsic absorptivity that is inherent to the material. As you can see, the apparent absorptivity approaches one for all cases. However, as you have pointed out, the surface area of the radiator is decreasing. The net effect is that the total net radiative heat does indeed decrease as the surface collapses (see Figure 2 which is for the net radiative heat transfer of a diffusely-reflecting surface and several different intrinsic absorptivities). This still gives us significant control, though, over the heat transfer from the radiator just in the opposite direction that we originally intended. We are also exploring a variety of surfaces that either don't collapse to zero surface area or that maintain a given surface area while changing the cavity angle. See the presentation linked below for more of that information. I have also included Figure 3, which shows how the heat transfer of a specularly-reflecting (or mirror-like reflection) radiator behaves as the radiator collapses if the incident irradiation is collimated, or all coming from one direction (as is the case generally for solar irradiation). Here the multiple reflections combined with the singular direction of irradiation gives very erratic behavior and significant changes in heat transfer are possible over small actuation distances.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See slides 23 - 29 in &lt;a href=&quot;https://tfaws.nasa.gov/files/TFAWS2017-PT-06.pdf&quot; rel=&quot;noreferrer&quot;&gt;this&lt;/a&gt; presentation for more of the math, experimental procedures, etc. Should have a paper soon talking about the behavior of net radiative heat transfer with actuation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/dYHCM.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/dYHCM.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;Figure 1&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/h9wZ6.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/h9wZ6.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;Figure 2&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/FXyWS.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/FXyWS.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&#xA;Figure 3&lt;/p&gt;&#xA;" OwnerUserId="22689" LastActivityDate="2018-01-25T22:10:48.283" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24634" PostTypeId="2" ParentId="24632" CreationDate="2018-01-25T23:29:38.333" Score="0" Body="&lt;p&gt;Can people be in orbit around Mars and launch nukes at the poles? Theoretically it's possible.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Should this be done? No.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll assume the idea here is to use the nukes to release the water and carbon dioxide frozen the poles to start greenhouse-effect warming and thicken the atmosphere a bit. There are two problems with this approach: nukes aren't nearly as powerful as most people think they are, and you've just dispersed fallout over a large portion of Mars, concentrated in the areas where your most valuable resource (water) is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using a 1-Megaton bomb as a reference nuclear warhead, and the information on  Wikipedia, the thermal effects would have a area of around 700-1200 sq km or so. By comparison, the rough area of the northern polar ice cap is 900000 sq km. You'll need about 1000 warheads to cover just the northern ice cap.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now, you've irradiated the planet and melted some, but not all of the ice cap (it's kilometers-thick in places). And you've also caused a nuclear winter -- a lot of dust will be kicked into the atmosphere from detonating 1000 nukes. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The water will quickly refreeze and precipitate out of the air (not even accounting for the nuclear winter), but the fallout does not. So no surface operations are possible, the water supply is contaminated, and its now colder than when you started.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bad idea all around.&lt;/p&gt;&#xA;" OwnerUserId="19192" LastActivityDate="2018-01-25T23:29:38.333" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24635" PostTypeId="2" ParentId="24632" CreationDate="2018-01-26T03:05:49.713" Score="6" Body="&lt;p&gt;No, it wouldn't affect anything in orbit significantly. First of all, there won't be that many people in an orbit that is that high of inclination where it even passes over the poles. Secondly, it is too far of a distance to have any significant effects. Assuming a 1 Megaton bomb, there is $4.184\times10^{15}$ J of energy. The energy in an actual blast will be directed more along the horizon, but for simplicity let's use the worst case of equal direction in all direction. Let's even assume all of the energy that hits the ground will be reflected. The lowest orbit around Mars is about 300 km, anything lower then that and you will significantly change your orbit. The energy is then divided by the surface area of the half-sphere with that as a radius, or about 4000 J/m^2. That is the same amount of energy that a spacecraft receives from the sun over about 3 seconds in Earth orbit, which should be quite survivable. And that would be if the spacecraft was directly overhead. Of course, while a good chunk of that will be in the visible spectrum, higher energy portions could be more damaging.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &lt;a href=&quot;https://en.wikipedia.org/wiki/Effects_of_nuclear_explosions#Ionizing_radiation&quot; rel=&quot;nofollow noreferrer&quot;&gt;amount of the nuclear weapon's energy&lt;/a&gt; that is radiation is about 5%. According to Wikipedia, the survivable distance for a nuclear explosion due to the radiation effects is about 3 km from a 1 MTon bomb. Of course, much of that would be because the atmosphere of Earth will absorb the energy. The values there given are slant range, which means it is an absolute distance. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The other effect that could damage any satellite, not just including manned ones, is &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;Electromagnetic Pulse&lt;/a&gt;. EMP is an interaction with a number of things, including the magnetic field of the Earth. The use of a thermonuclear weapon will further decrease it's effectiveness. It could affect satellites directly overhead, but probably would not break a satellite overhead. Here the thinner atmosphere of the planet actually helps, there will be fewer electrons being sent then a comparable burst on Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, it isn't that likely that a manned spacecraft would be in an orbit that will pass over the poles, and even if it was, it wouldn't likely cause any real damage. The timing of the explosives should be such that no satellite is directly overhead, out of caution, but that shouldn't really affect anything.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-28T11:58:32.753" LastActivityDate="2018-01-28T11:58:32.753" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24637" PostTypeId="2" ParentId="24618" CreationDate="2018-01-26T12:36:10.403" Score="16" Body="&lt;p&gt;No, there is no space race to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original space race was to orbit, for entirely military reasons.  Sputnik was simply a demonstration of &quot;we can drop a nuke anywhere we want, any time&quot;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The second space race to the Moon was a publicity exercise.  It served no military purpose, and it didn't serve any scientific purpose either.  (Science builds off previous work.  NASA deliberately destroyed all plans for the Atlas rockets, so that they could not be reproduced.)  Primarily the reason was national pride (basically a willy-waving exercise with the USSR); and a strong secondary reason was a bread-and-circuses exercise in distraction from the war in Vietnam.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For Mars, we do not have either justification.  A mission to Mars clearly isn't militarily useful.  More importantly, the cost of the mission and the parlous state of the US and Russian economies mean that there isn't going to be any significant competition.  China are actually in the strongest position nationally, since they have no political obstacles to redirecting funding at will.  If there was an economic justification for going to Mars, then we could still get a space race from private investors trying to get a slice of the pie, but there is no economic reason.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Instead, what we have is a small collection of incredibly wealthy individuals funding private space research, with a fairly significant amount of government funding too.  Inevitably someone &lt;em&gt;will&lt;/em&gt; be first to get to Mars, but it isn't particularly a race, because all sides recognise that the important factor isn't getting there but &lt;em&gt;surviving&lt;/em&gt; there.  As private individuals without the artificial obstacles imposed by politics, this means we see a lot more collaboration.  SpaceX may have the best rockets, but it's likely someone else will have the best hab, for example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Venus certainly hasn't been missed out on exploration.  Previous posts have pointed out how harsh the environment is - you'd need to &lt;em&gt;really&lt;/em&gt; want to have a rover there, and currently there's no good reason to do that.  On Mars we have a good reason for wanting a close-up view of the surface, because we hope someday to send people there.  Venus is interesting from a pure curiosity point of view, but the cost-benefit appears low when orbiters have already done high-quality mapping and observation work, and there is no prospect of a human ever walking on the surface.  The airship concept is interesting, but it fails to explain why it is necessary to put humans on the airship, when the purpose is only to do more observation work which can easily be done remotely.&lt;/p&gt;&#xA;" OwnerUserId="15976" LastEditorUserId="6537" LastEditDate="2019-06-30T21:25:16.917" LastActivityDate="2019-06-30T21:25:16.917" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="24638" PostTypeId="2" ParentId="24619" CreationDate="2018-01-26T14:30:24.787" Score="8" Body="&lt;p&gt;IMU realignments using the sextant were done regularly, by all Apollo missions. Using the sextant for a position update was also done by most missions, or at least it was practiced for the case of loss-of-communication with the ground. Then the sextant and onboard computer were needed to for navigation to get the astronauts home.&lt;/p&gt;&#xA;&lt;p&gt;There were two separate programs in the Command Module Computer for this type of position update. Program 22 (Orbital Navigation) and Program 23 (Cislunar Navigation). P22 in low lunar and Earth orbit did not involve any of the navigation stars. Instead you would point the sextant at a predetermined landmark with known latitude and longitude and take five marks on it at short time intervals. This was practiced successfully during the Earth orbital missions Apollo 7 (see the &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a410/A07_MissionReport.pdf&quot; rel=&quot;noreferrer&quot;&gt;Mission Report&lt;/a&gt; page 5-95) and Apollo 9. The same method could be used in lunar orbit, either for updating the position of the spacecraft in the computer or to calculate the coordinates of a landmark on the lunar surface. This second method was used for the lunar landing missions to get more accurate coordinates of the landing site, before and after the actual landing.&lt;/p&gt;&#xA;&lt;p&gt;The other Program, P23, is the one that uses the position update method you were describing, pointing the sextant at a star and a position on Earth. This program actually had a few different modes: Earth horizon, Earth landmark, Lunar horizon, Lunar landmark. So you could give the computer coordinates of a known landmark on Earth or Moon or you could also use the horizon closest to the navigation star. This last method is the one that was used the most, usually a few hours after TLI, while still close to the Earth. For example, &lt;a href=&quot;https://www.hq.nasa.gov/alsj/a11/a11fltpln_final_reformat.pdf&quot; rel=&quot;noreferrer&quot;&gt;Apollo 11 Flight Plan&lt;/a&gt;, page 3-7.&lt;/p&gt;&#xA;&lt;p&gt;Program 23 best works during the mission phase a few hours after TLI or TEI and a few hours before LOI or reentry. And that's when Apollo 8 practiced these procedures. Later missions like Apollo 11 only tested P23s in the hours after TLI.&lt;/p&gt;&#xA;&lt;p&gt;Now, Program 23 was also attempted in Earth orbit by Apollo 7. But it didn't work very well. Here the quote from the Mission Report, page 5-98:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Midcourse navigation/star horizon/landmark.- A number of star/earth horizon measurements were scheduled, but all attempts to perform these sightings were unsuccessful. This failure resulted partially from the difficulty of the control task at the relatively high earth-orbital rates, but primarily from the crew's inability to define a horizon locator, which was the primary purpose of these tests. The dichroic filter in the sextant landmark line-of-sight did not aid in land/sea definition and actually smoothed out the horizon such that it was impossible at earth orbital ranges to define a locator for repeatable sightings. The crew stated that at longer ranges, the sightings should be accomplished with ease. The capability for performing star/lunar landmark sighting was demonstrated using the star Alphard and lunar landmark 5 (crater Diophantus).&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;" OwnerUserId="13716" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-26T20:21:56.057" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24639" PostTypeId="2" ParentId="24632" CreationDate="2018-01-26T15:45:48.380" Score="1" Body="&lt;p&gt;Yes, a nuclear detonation &lt;em&gt;certainly might affect things in orbit!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;Nuclear EMP&lt;/a&gt; could potentially be a serious issue for anything electronic which had &lt;em&gt;line of sight to a nuclear detonation&lt;/em&gt;. That would include any GPS-like satellites, or those that provided comms links to Earth, as well as the electronics of the spacecraft that contained the people.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK on Earth, a detonation on, &lt;em&gt;and especially under&lt;/em&gt; the surface would be much less of an issue to satellites than an atmospheric burst. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However this is Mars, and so the atmosphere at the surface is like the &quot;upper atmosphere&quot; of Earth, so I think anyone who's not particularly versed in the weaponization of nukes may not be able to definitively state how bad of an EMP would be caused by one, or even a series of ice-melting nukes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;21st century space-worthy electronics is likely to be more robust than 20th century stuff, but an EMP is not at all the same as a &lt;a href=&quot;https://en.wikipedia.org/wiki/Coronal_mass_ejection&quot; rel=&quot;nofollow noreferrer&quot;&gt;CME&lt;/a&gt;, so EMP protection would need to be implemented specifically, considering how sensitive all those deep-space receivers need to be.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Since Mars' atmosphere is only roughly 1% that of Earth, the gamma-ray attenuation length is much larger, and so the characteristics of the resulting EMP from an atmospheric detonation will be very different than on Earth. So I've asked the follow-up question &lt;a href=&quot;https://space.stackexchange.com/q/24652/12102&quot;&gt;Would the EMP from atmospheric polar nuclear detonations on Mars felt by orbiting spacecraft be larger or smaller than (if it were) on Earth?&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-28T08:58:24.630" LastActivityDate="2018-01-28T08:58:24.630" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24640" PostTypeId="2" ParentId="24461" CreationDate="2018-01-26T18:01:01.270" Score="5" Body="&lt;p&gt;Don Sadoway of MIT has led a team developing &lt;a href=&quot;https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090018064.pdf&quot; rel=&quot;noreferrer&quot;&gt;molten oxide electrolysis methods for use on the Moon&lt;/a&gt;. In 2008 he &lt;a href=&quot;https://www.google.com/patents/US20080023321&quot; rel=&quot;noreferrer&quot;&gt;received a patent&lt;/a&gt; for a process using an anode made of iridium and a cathode of tungsten or molybdenum. This apparatus will produce a puddle of molten metals at the cathode, placed at the bottom of a crucible, and bubbles of oxygen around the anode at the top. The metals will be a mixed alloy of all the metals present in the feedstock. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Selection of favorable feedstock from the lunar highlands, comprised mostly of anorthite, CaAl&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;8&lt;/sub&gt;, allows the alloy produced to be mostly aluminum and silicon. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More recently, Sadoway's team have developed an &lt;a href=&quot;https://www.technologyreview.com/s/515411/eco-friendly-steelmaking/&quot; rel=&quot;noreferrer&quot;&gt;anode made of chromium and iron&lt;/a&gt; for use in producing iron on the Moon. This process also has potential application on Earth, as a method of steel production that doesn't release carbon gases as a by-product. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For iron production, other methods have also been investigated. &lt;a href=&quot;http://adsabs.harvard.edu/full/1980LPSC...11.1697M&quot; rel=&quot;noreferrer&quot;&gt;Elemental iron is present in lunar regolith&lt;/a&gt; at a level of up to 1% in the maria. Simply passing magnets through a stream of the smallest regolith particles could produce a feedstock very high in iron, but with substantial contaminants. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Processing of ilmenite, FeTiO&lt;sub&gt;3&lt;/sub&gt;, with hydrogen produces water, iron, and titanium dioxide. Concentrations of ilmenite in the maria can be around 10%. Reactors to process ilmenite with hydrogen have been investigated principally as a &lt;a href=&quot;https://www.lpi.usra.edu/publications/books/lunar_bases/LSBchapter08.pdf#pagemode=bookmarks&amp;amp;page=14&quot; rel=&quot;noreferrer&quot;&gt;means of producing oxygen&lt;/a&gt;, but iron is another product.&lt;/p&gt;&#xA;" OwnerUserId="4660" LastActivityDate="2018-01-26T18:01:01.270" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24641" PostTypeId="1" CreationDate="2018-01-26T22:32:17.537" Score="6" ViewCount="116" Body="&lt;p&gt;I'm looking for a map/model of the movements of our solar system and associated moons.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What are the 'best' algorythmic models to represent our solar system. Looking to write a game/edutainment app for users to find solar system objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm sure there is something out there, keywords googlefu fail ATM.&lt;/p&gt;&#xA;" OwnerUserId="19896" LastEditorUserId="2752" LastEditDate="2018-01-27T00:21:52.943" LastActivityDate="2018-01-27T00:21:52.943" Title="Any algorithmic models of our solar system to predict positions relative to earth?" Tags="|solar-system|ephemeris|prediction|" AnswerCount="0" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24642" PostTypeId="2" ParentId="24068" CreationDate="2018-01-27T00:19:27.533" Score="2" Body="&lt;p&gt;if you're using pyephem, you can get the latitude and longitude directly from the body object.  &lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;body = ephem.EarthSatellite() &#xA;body._raan = ....  &#xA;body._ap = ....  &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;&#xA;&lt;p&gt;then get the lat, lon at datetime dt&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;body.compute(dt)  &#xA;body.sublat  &#xA;body.sublon &#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="22704" LastEditorUserId="12102" LastEditDate="2018-01-27T02:55:39.440" LastActivityDate="2018-01-27T02:55:39.440" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24643" PostTypeId="1" CreationDate="2018-01-27T05:17:06.847" Score="11" ViewCount="1532" Body="&lt;p&gt;I've written a MATLAB script to use the bvp4c two point boundary equation solver to implement a Calculus of Variations trajectory optimization of the problem of a single finite burn with a free coast to pick the optimum ignition point.  I'd like any input on mistakes that I've made or ways to improve the code.  What it does is solve for the optimum finite burn from a 185x185 km orbit to a 185x1000 km orbit, with an optional inclination change.  It sets up a multipoint boundary value problem to solve a burn-coast problem using the primer vector equation.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The code is based off of MATLAB code in the appendices of &lt;em&gt;Optimal Control with Aerospace Applications&lt;/em&gt; by Longuski, Guzman and Prussing.  The coast time and burn times have been implemented as free parameters and the boundary value problem uses normalized time (tau) so that [0,1] is the coast period and [1,2] is the burn period.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The ODE implements the equations of motion and the primer vector equation for central force motion in 3D cartesian coordinates.  I think I've gotten those correct, but could use someone double checking my work. The multiplication by the coast or burn times in the return value of the ODE is necessary to convert the derivative from $\frac{d}{dt}$ to $\frac{d}{d \tau}$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Boundary Conditions are something that I've got a question about if I've done correctly.  The initial position, velocity and mass are trivial conditions.  There are also 14 conditions which impose continuity across all the variables on the lefthand side and righthand side in between the coast and the burn (tau = 1).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The remaining conditions are the 5 constraints on the terminal orbital conditions, and then the 3 transversality conditions to account for the true anomaly of the attachment point being free, and the times of the coast and the burn being free.  For the terminal orbital conditions I'm using the angular momentum vector and the first two components of the eccentricity vector being equal to the target orbit.  What I'm using for transverality conditions are that if the hamiltonian is broken up into $H = H_0 + T S$ where T is the thrust and S the switching function, then I set $H_0(t_2) = 0$, $H_0(t_0) = 0$ and then with $S = ( \lvert p_v \rvert - 1)$ set $\lvert p_v(t_1) \rvert = 1$.  I lack some confidence that I've gotten that entirely correct.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Results seem to to be pretty decent, but I'm a little surprised that I don't see $\lvert p_v(t_2) \rvert = 1$.  I also see convergence issues for any problems involving an inclination above about 25 degrees (but that gets into 300 sec / 3000 dV burns).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I have not yet normalized the units for position and velocity so they're still in $m$ and $m/s$.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For more moderate burns it seems to perform pretty well and gives burntimes within about a second of the impulsive burn model and looks like it leads the burn correctly with a negative coast of roughly half the burntime.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'd appreciate any help though in finding mistakes that I've made or ways to improve this code to make it more robust.&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;close all; clear all; clc;&#xA;&#xA;global r0 v0 m0 rT vT g0&#xA;global MU Thrust Mdot&#xA;&#xA;MU = 3.9860044189e+14;  % earth&#xA;Thrust = 232.7 * 1000;  % N; LR-91 232.7 kN&#xA;m0 = 32.74 * 1000;      % kg; 32.74t - 1g start accel&#xA;isp = 316;              % sec; LR-91&#xA;g0 = 9.80665;           % m/s; standard gravity&#xA;ve = isp * g0;          % m/s&#xA;a0 = Thrust / m0        % m/s^2; initial accel&#xA;tau = ve / a0           % s; time to burn rocket to zero&#xA;Mdot = Thrust / ve;     % kg/sec&#xA;&#xA;rearth = 6.371e+6;      % m; earth radius&#xA;r185 = rearth + 0.185e+6;&#xA;r1000 = rearth + 1.000e+6;&#xA;&#xA;% initial position (185x185)&#xA;r0 = [ r185, 0, 0 ];&#xA;v185 = sqrt(MU/r185);&#xA;v0 = [ 0, v185, 0 ];&#xA;&#xA;% target orbital parameters (185x1000)&#xA;rT = [ r185, 0, 0 ];&#xA;smaT = ( r185 + r1000 ) / 2;&#xA;inc = 10;  % degrees&#xA;vTm = sqrt(MU * (2/r185 - 1/smaT) );&#xA;vT = [ 0, vTm * cosd(inc), vTm * sind(inc)];&#xA;&#xA;vBurn = vT - v0;&#xA;dV = norm(vBurn)&#xA;&#xA;% list initial conds&#xA;yinit = [r0 v0 vBurn/norm(vBurn) 0 0 0 m0 ];  % initial state and costate&#xA;tb_guess = tau * (1 - exp(-dV/ve) )&#xA;tc_guess = - tb_guess / 2&#xA;&#xA;% sanity checks on the impulsive burn model&#xA;mf_impulsive = m0 - tb_guess * Mdot&#xA;af_impulsive = Thrust / mf_impulsive / g0&#xA;&#xA;% parameters&#xA;parameters = [ tc_guess, tb_guess ];&#xA;&#xA;% number of timeslices&#xA;Nt = 41;&#xA;tau = [&#xA;  linspace(0,1,Nt)'&#xA;  linspace(1,2,Nt)'&#xA;];&#xA;&#xA;% initial guess&#xA;solinit = bvpinit(tau, yinit, parameters);&#xA;&#xA;% bump up the mesh by a bit&#xA;option=bvpset('Nmax',50000);&#xA;&#xA;% solve&#xA;sol = bvp4c(@Burn_odes, @Burn_bcs, solinit, option);&#xA;&#xA;% extract times&#xA;tc = sol.parameters(1)&#xA;tb = sol.parameters(2)&#xA;&#xA;% pull out values for times&#xA;Z = deval(sol, tau);&#xA;&#xA;% convert taus to times&#xA;for i=1:length(tau)&#xA;  if tau(i) &amp;lt;= 1&#xA;    time(i) = tau(i) * tc;&#xA;  else&#xA;    time(i) = tc + ( tau(i) - 1 ) * tb;&#xA;  end&#xA;end&#xA;&#xA;% extract solution vars&#xA;x_sol = Z(1,:);&#xA;y_sol = Z(2,:);&#xA;z_sol = Z(3,:);&#xA;r_sol = sqrt( x_sol.^2 + y_sol.^2 + z_sol.^2 );&#xA;vx_sol = Z(4,:);&#xA;vy_sol = Z(5,:);&#xA;vz_sol = Z(6,:);&#xA;v_sol = sqrt( vx_sol.^2 + vy_sol.^2 + vz_sol.^2 );&#xA;pvx_sol = Z(7,:);&#xA;pvy_sol = Z(8,:);&#xA;pvz_sol = Z(9,:);&#xA;pv_sol = sqrt( pvx_sol.^2 + pvy_sol.^2 + pvz_sol.^2 );&#xA;m_sol = Z(13,:);&#xA;&#xA;for i = 1:length(tau)&#xA;  r = Z(1:3, i);&#xA;  v = Z(4:6, i);&#xA;  h(i) = norm(cross(r,v));&#xA;end&#xA;&#xA;% plots&#xA;&#xA;figure;&#xA;subplot(3,2,1);&#xA;plot(time,m_sol/1000);&#xA;ylabel('mass, tons', 'fontsize', 14);&#xA;xlabel('Time, sec', 'fontsize', 14);&#xA;hold on;&#xA;grid on;&#xA;&#xA;subplot(3,2,2);&#xA;plot(time,r_sol/1000);&#xA;ylabel('radius, km', 'fontsize', 14);&#xA;xlabel('Time, sec', 'fontsize', 14);&#xA;hold on;&#xA;grid on;&#xA;&#xA;subplot(3,2,3);&#xA;plot(time,v_sol/1000);&#xA;ylabel('velocity, km/s', 'fontsize', 14);&#xA;xlabel('Time, sec', 'fontsize', 14);&#xA;hold on;&#xA;grid on;&#xA;&#xA;subplot(3,2,4);&#xA;plot(time,h);&#xA;ylabel('h', 'fontsize', 14);&#xA;xlabel('Time, sec', 'fontsize', 14);&#xA;hold on;&#xA;grid on;&#xA;&#xA;subplot(3,2,5);&#xA;plot(time,pv_sol);&#xA;ylabel('pv magnitude', 'fontsize', 14);&#xA;xlabel('Time, sec', 'fontsize', 14);&#xA;hold on;&#xA;grid on;&#xA;&#xA;figure;&#xA;plot(x_sol/1000, y_sol/1000);&#xA;grid on;&#xA;axis equal&#xA;xlabel('x, km', 'fontsize', 14);&#xA;ylabel('y, km', 'fontsize', 14);&#xA;&#xA;&#xA;%&#xA;% Boundary conditions&#xA;%&#xA;&#xA;function PSI = Burn_bcs(yleft, yright, parameters)&#xA;&#xA;Y0 = yleft(:,1);&#xA;Y1 = yleft(:,2);  % == yright(:,1)&#xA;Y2 = yright(:,2);&#xA;&#xA;global r0 v0 m0 rT vT MU g0 Thrust&#xA;&#xA;ri = Y0(1:3);&#xA;vi = Y0(4:6);&#xA;pvi = Y0(7:9);&#xA;pri = Y0(10:12);&#xA;mi = Y0(13);&#xA;ri3 = norm(ri)^3;&#xA;&#xA;rf = Y2(1:3);&#xA;vf = Y2(4:6);&#xA;pvf = Y2(7:9);&#xA;prf = Y2(10:12);&#xA;mf = Y2(13);&#xA;rf3 = norm(rf)^3;&#xA;&#xA;r1 = Y1(1:3);&#xA;v1 = Y1(4:6);&#xA;pv1 = Y1(7:9);&#xA;pr1 = Y1(10:12);&#xA;r13 = norm(r1)^3;&#xA;&#xA;hT = cross(rT, vT);&#xA;hf = cross(rf, vf);&#xA;&#xA;eT = - (rT/norm(rT) + cross(hT,vT)/MU);&#xA;ef = - (rf/norm(rf) + cross(hf,vf)/MU);&#xA;&#xA;emiss = ef - eT';&#xA;&#xA;uf = pvf / norm(pvf);&#xA;&#xA;H0ti = dot(pri, vi) - MU * dot(pvi, ri) / ri3;&#xA;H0t1 = dot(pr1, v1) - MU * dot(pv1, r1) / r13;&#xA;H0tf = dot(prf, vf) - MU * dot(pvf, rf) / rf3;&#xA;Htf = H0tf - Thrust * ( norm(pvf) - 1 );&#xA;&#xA;PSI = [&#xA;    Y0(1:3) - r0'                       % 3 - initial position&#xA;    Y0(4:6) - v0'                       % 3 - initial velocity&#xA;    Y0(13) - m0                         % 1 - initial mass&#xA;    norm(Y1(7:9)) - 1                   % 1 - norm(pv1) = 1 (so H(t1) = 0 on both sides of t1)&#xA;    hf - hT'                            % 3 - terminal constraint on angular momentum&#xA;    emiss(1:2)                          % 2 - terminal constraint on ecc vector&#xA;    H0ti                                % 1 - H0(t0) = 0&#xA;    H0tf                                % 1 - H0(tf) = 0&#xA;    yleft(:,2) - yright(:,1)            % 14 - values at t1 are continuous&#xA;    ];&#xA;&#xA;end&#xA;&#xA;%&#xA;% Equations of motion&#xA;%&#xA;&#xA;function dX_dtau = Burn_odes(tau, X, k, parameters)&#xA;&#xA;global MU Thrust Mdot&#xA;&#xA;if k == 1&#xA;  thr = 0;&#xA;  md = 0;&#xA;  tinterval = parameters(1);  % tc&#xA;else&#xA;  thr = Thrust;&#xA;  md = Mdot;&#xA;  tinterval = parameters(2);  % tb&#xA;end&#xA;&#xA;r = X(1:3);&#xA;v = X(4:6);&#xA;pv = X(7:9);&#xA;pr = X(10:12);&#xA;m = X(13);&#xA;&#xA;u = pv / norm(pv);&#xA;&#xA;Fm = thr / m;&#xA;r3 = norm(r)^3;&#xA;r2 = dot(r,r);&#xA;r5 = norm(r)^5;&#xA;&#xA;rdot  = v;&#xA;vdot  = - MU * r / r3 + Fm * u;&#xA;pvdot = pr;&#xA;prdot = - MU / r5 * ( 3 * r' * r - r2 * eye(3,3) ) * pv;&#xA;&#xA;dX_dtau = tinterval*[rdot', vdot', pvdot', prdot', -md ];&#xA;&#xA;end&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="14420" LastEditorUserId="14420" LastEditDate="2018-01-27T22:27:44.417" LastActivityDate="2018-06-08T02:13:13.553" Title="Improvements to script for Finite Burn Trajectory Optimization with MATLAB" Tags="|orbital-mechanics|orbital-maneuver|trajectory|" AnswerCount="1" CommentCount="18" ContentLicense="CC BY-SA 3.0" />
  <row Id="24645" PostTypeId="2" ParentId="8281" CreationDate="2018-01-27T15:30:43.840" Score="3" Body="&lt;p&gt;Not necessarily. Here's a few reasons why:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;The Falcon Heavy can't actually lift 60 tons. It has the fuel and thrust required, however, structural limitations means the heaviest payload it'll fly with in the foreseeable future will most likely only be around 10 tons or less. Meanwhile, the SLS, at its most basic configuration can loft 70 tons. The FH could lift 60 tons if SpaceX decides to incorporate fuel cross-feed into their first stage boosters. This is an extremely difficult stunt to pull off but so is landing 13 story tall rockets on barges so we can't be too quick to assume this will never happen.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Evolved versions of the SLS have  diameters of 8.5-10 meters as opposed to the Falcon Heavy's 3 meter diameter. This is a major limitation for interplanetary missions on the FH. 3m is simply not a wide enough area for even a moderately sized crew to live for 3-9 months. However, SpaceX's 3m diameter boosters are crucial to making their rockets more affordable. Any wider and they'd have to transport the boosters by sea rather than on the road.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;We're too far ahead to cancel the SLS, especially with Trump in office. He'll do anything to get that JFK moment so I doubt the SLS' development will be hindered under the current government's rule.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerDisplayName="user22645" LastEditorDisplayName="user22645" LastEditDate="2018-01-27T17:52:43.683" LastActivityDate="2018-01-27T17:52:43.683" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24646" PostTypeId="1" CreationDate="2018-01-27T21:01:43.920" Score="4" ViewCount="947" Body="&lt;p&gt;I've written 2 functions to convert elliptical orbital elements to state vectors and vice-versa. Here is a web page containing them:&#xA;&lt;a href=&quot;http://orbitsimulator.com/formulas/OrbitalElements.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;http://orbitsimulator.com/formulas/OrbitalElements.html&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;But my conversion does not work if I input hyperbolic elements (e &gt; 1, a &amp;lt; 1).&#xA;I need to know how to take orbital elements for a hyperbolic object passing the Sun and convert them into x,y,z, vx,vy,vz where the Sun sits motionless at 0,0,0.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For example, here are the orbital elements for  the interstellar asteroid 'Oumuamua for November 2, 2017 as given by JPL Horizons:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;2458059.500000000 = A.D. 2017-Nov-02 00:00:00.0000 TDB &#xA;   EC= 1.199512420116503E+00 QR= 2.553431944164113E-01 IN= 1.226872051262464E+02&#xA;   OM= 2.459921097817110E+01 W = 2.417029828623325E+02 Tp=  2458005.990518697072&#xA;   N = 6.807263284370278E-01 MA= 3.642531274396101E+01 TA= 1.221047887790030E+02&#xA;   A =-1.279836083725045E+00 AD= 6.684586453809735E+91 PR= 1.157407291666667E+95&#xA;  $$EOE&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;JDTDB    Julian Day Number, Barycentric Dynamical Time&#xA;        EC     Eccentricity, e&lt;br&gt;&#xA;        QR     Periapsis distance, q (au)&lt;br&gt;&#xA;        IN     Inclination w.r.t XY-plane, i (degrees)&lt;br&gt;&#xA;        OM     Longitude of Ascending Node, OMEGA, (degrees)&lt;br&gt;&#xA;        W      Argument of Perifocus, w (degrees)&lt;br&gt;&#xA;        Tp     Time of periapsis (Julian Day Number)&lt;br&gt;&#xA;        N      Mean motion, n (degrees/day)&lt;br&gt;&#xA;        MA     Mean anomaly, M (degrees)&lt;br&gt;&#xA;        TA     True anomaly, nu (degrees)&lt;br&gt;&#xA;        A      Semi-major axis, a (au)&lt;br&gt;&#xA;        AD     Apoapsis distance (au)&lt;br&gt;&#xA;        PR     Sidereal orbit period (day) &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;If I ask JPL Horizons for vectors instead, they give me:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;X = 213737716048.4732, &#xA;  Y = 88710852026.16885,&#xA;  Z = 12954584936.530249&lt;/p&gt;&#xA;  &#xA;  &lt;p&gt;VX = 39601.19506830577,&#xA;  VY = 7999.088132812113,&#xA;  VZ = 14355.61617225209,&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;(They provide distances in AU and speed in AU/day. The numbers I've provided here are translated into meters and m/s.)&#xA;I'd like to know how to do this conversion myself.&#xA;Thanks!&lt;/p&gt;&#xA;" OwnerUserId="22642" LastEditorUserId="265" LastEditDate="2018-01-28T04:22:56.203" LastActivityDate="2018-01-28T04:22:56.203" Title="Finding x,y,z, vx,vy,vz from Hyperbolic orbital elements" Tags="|orbital-mechanics|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24647" PostTypeId="2" ParentId="24537" CreationDate="2018-01-27T21:20:50.807" Score="5" Body="&lt;p&gt;2006 RH120 got captured by Earth in 2006 through the Earth-Sun L2 point. It completed a few orbits before escaping in the general direction of L1.&#xA;This is an example of a ballistic capture.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here is a simulation of this event that will run in your browser. Earth is held stationary in a rotating frame to make it easier to visualize the orientation of the Lagrange points. You can tell the direction to the Sun by the phases of the objects:&#xA;&lt;a href=&quot;http://orbitsimulator.com/gravitySimulatorCloud/simulations/1517087032751_2006RH120.html&quot; rel=&quot;noreferrer&quot;&gt;http://orbitsimulator.com/gravitySimulatorCloud/simulations/1517087032751_2006RH120.html&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22642" LastActivityDate="2018-01-27T21:20:50.807" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24649" PostTypeId="2" ParentId="24646" CreationDate="2018-01-28T00:49:34.520" Score="4" Body="&lt;p&gt;The equations for an elliptical orbit work just fine for hyperbolic orbits -- so long as you use the complex valued trigonometric functions. Hyperbolic orbits are characterized with a semimajor axis that is negative. This means that the standard formula for mean motion, $n = \sqrt{\mu/a^3}$ becomes imaginary. This in turn means that the various anomalies (mean anomaly, eccentric anomaly, and true anomaly) also take on imaginary values. There is an easier way.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The easy way to solve this problem is to use relationships between the trigonometric and hyperbolic functions, $\sin(ix) = i\sinh x, \cos(ix) = \cosh x, \tan(ix) = i\tanh x$. This, along with defining mean motion as $n = \sqrt{\mu/|a|^3}$, means that all of the values are once again real. The expression for mean anomaly remains unchanged ($M = n(t-t_p)$ (but using the modified expression for mean motion). The hyperbolic version of Kepler's equation is $M = e\sinh H - H$ (as opposed to $E - e\sin E$), where $H$ is the hyperbolic anomaly. Finally, true anomaly is related to hyperbolic anomaly via $\tan\frac\theta 2 = \sqrt{\frac{e+1}{e-1}} \tanh \frac H 2$.&lt;/p&gt;&#xA;" OwnerUserId="2752" LastActivityDate="2018-01-28T00:49:34.520" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24650" PostTypeId="1" AcceptedAnswerId="24660" CreationDate="2018-01-28T06:45:33.847" Score="6" ViewCount="860" Body="&lt;p&gt;&amp;quot;Egg Rock&amp;quot; is a smooth, shiny nickel-iron meteorite found sitting on the surface of Mars on Curiosity's Sol 1505, or 30-Oct-2016. It is described the next day in Arizona State University's Red Planet Report website: &lt;a href=&quot;http://redplanet.asu.edu/?p=20977&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity: Egg Rock, a small meteorite&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;Did Curiosity's software flag it as potentially interesting (e.g. &amp;quot;hey, what's that?), or was it noticed/spotted first by researchers, who then directed Curiosity to take a closer look?&lt;/p&gt;&#xA;&lt;p&gt;See also:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Debora Bird in Earthsky.org: &lt;a href=&quot;http://earthsky.org/space/curiosity-meteorite-mars-october-30-2016&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;Curiosity Finds&lt;/strong&gt; Small Meteorite on Mars&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Gizmodo: &lt;a href=&quot;https://www.gizmodo.com.au/2016/11/curiosity-just-found-a-freaky-metal-meteorite-on-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;strong&gt;Curiosity Just Found&lt;/strong&gt; A Freaky Metal Meteorite On Mars&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Images from &lt;a href=&quot;http://earthsky.org/space/curiosity-meteorite-mars-october-30-2016&quot; rel=&quot;nofollow noreferrer&quot;&gt;EarthSky&lt;/a&gt;, credit NASA/JPL/&lt;a href=&quot;http://redplanet.asu.edu/?p=20977&quot; rel=&quot;nofollow noreferrer&quot;&gt;ASU&lt;/a&gt;, captions &amp;quot;October 30, 2016 image via Curiosity rover on Mars&amp;quot; (click for larger size):&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7Nuup.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7Nuupm.jpg&quot; alt=&quot;&amp;quot;Egg Rock&amp;quot; meteorite on Mars&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/QshE4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QshE4m.jpg&quot; alt=&quot;&amp;quot;Egg Rock&amp;quot; meteorite on Mars&quot; /&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/gZS9p.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gZS9pm.jpg&quot; alt=&quot;&amp;quot;Egg Rock&amp;quot; meteorite on Mars&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="39182" LastEditDate="2021-04-30T06:42:22.133" LastActivityDate="2021-04-30T06:42:22.133" Title="Who discovered &quot;Egg Rock&quot;? The Curiosity rover or people?" Tags="|mars|rovers|curiosity|meteorites|" AnswerCount="1" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="24651" PostTypeId="1" AcceptedAnswerId="24687" CreationDate="2018-01-28T06:57:52.077" Score="3" ViewCount="438" Body="&lt;p&gt;&quot;Egg Rock&quot; is a smooth, shiny nickel-iron meteorite found sitting on the surface of Mars on 30-Oct-2016. It is described the next day in Arizona State University's Red Planet Report website: &lt;a href=&quot;http://redplanet.asu.edu/?p=20977&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity: Egg Rock, a small meteorite&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;According to Gizmodo's &lt;a href=&quot;https://www.gizmodo.com.au/2016/11/curiosity-just-found-a-freaky-metal-meteorite-on-mars/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Curiosity Just Found A Freaky Metal Meteorite On Mars&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;As Deborah Byrd &lt;a href=&quot;http://earthsky.org/space/curiosity-meteorite-mars-october-30-2016&quot; rel=&quot;nofollow noreferrer&quot;&gt;notes&lt;/a&gt; in EarthSky, the surface of Mars is peppered with meteorites, so discoveries like this aren't all that uncommon. Meteorites can last for millions of years on the Red Planet, free from the oxidising and weathering effects of moisture and oxygen.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Martian_soil#Observations&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;, Martian soil (and presumably blowing dust) contains plenty of &lt;a href=&quot;https://en.wikipedia.org/wiki/Perchlorate&quot; rel=&quot;nofollow noreferrer&quot;&gt;perchlorate&lt;/a&gt;, a strong oxidizer. Perchlorate on Mars's surface is discussed in this &lt;a href=&quot;https://space.stackexchange.com/a/4708/12102&quot;&gt;answer&lt;/a&gt;, and both perchlorate and &lt;a href=&quot;https://en.wikipedia.org/wiki/Hydrogen_peroxide&quot; rel=&quot;nofollow noreferrer&quot;&gt;peroxide&lt;/a&gt; are discussed in this &lt;a href=&quot;https://space.stackexchange.com/q/5335/12102&quot;&gt;question&lt;/a&gt;, with reference to deleterious chemical effects on space suits.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Why then would metallic meteorites remain unoxidized on Mars?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Images from &lt;a href=&quot;http://redplanet.asu.edu/?p=20977&quot; rel=&quot;nofollow noreferrer&quot;&gt;EarthSky&lt;/a&gt;, credit NASA/JPL/&lt;a href=&quot;http://redplanet.asu.edu/?p=20977&quot; rel=&quot;nofollow noreferrer&quot;&gt;ASU&lt;/a&gt;, captions &quot;October 30, 2016 image via Curiosity rover on Mars&quot; (click for larger size):&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7Nuup.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7Nuupm.jpg&quot; alt=&quot;&amp;quot;Egg Rock&amp;quot; meteorite on Mars&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/QshE4.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QshE4m.jpg&quot; alt=&quot;&amp;quot;Egg Rock&amp;quot; meteorite on Mars&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/gZS9p.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/gZS9pm.png&quot; alt=&quot;&amp;quot;Egg Rock&amp;quot; meteorite on Mars&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-28T07:21:09.820" LastActivityDate="2018-01-31T14:03:36.030" Title="Why doesn't the perchlorate on Mars' surface oxidize metallic meteorites?" Tags="|mars|rovers|curiosity|meteorites|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24652" PostTypeId="1" CreationDate="2018-01-28T08:23:02.100" Score="1" ViewCount="193" Body="&lt;p&gt;Gizmodo's &lt;a href=&quot;https://gizmodo.com/heres-how-elon-musk-described-terraforming-mars-to-step-1729792899&quot; rel=&quot;nofollow noreferrer&quot;&gt;Watch Elon Musk Describe Terraforming Mars to Stephen Colbert&lt;/a&gt; links to the video (shown below). Their second article &lt;a href=&quot;https://gizmodo.com/elon-musk-clarifies-his-plan-to-nuke-mars-1734457751&quot; rel=&quot;nofollow noreferrer&quot;&gt;Elon Musk Clarifies His Plan to &amp;quot;Nuke Mars&amp;quot;&lt;/a&gt; quotes Musk from a SolarCity launch event in Times Square New York a few weeks later in late 2015:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;“What I was talking about,” said Musk, “was having a series of very large, by our standards, but very small by calamity standards, essentially having two tiny pulsing suns over the poles. They’re really above the planet. Not on the planet.&lt;/p&gt;&#xA;&lt;p&gt;“Every several seconds,” Musk continued “send large fusion bombs over the poles. A lot of people don’t appreciate that our sun is a giant fusion explosion,” he added, to a reportedly silent audience.&lt;/p&gt;&#xA;&lt;p&gt;What Musk keeps trying to explain here is a scheme for heating up the surface of Mars quickly, in order to make the planet hospitable to plant life. There’s a lot of carbon dioxide locked up in Mars’ poles as dry ice, and CO2, as we know, is a powerful global warming agent. If we release enough energy over the poles, we might just be able to send all that CO2 skyward, warming the atmosphere enough to kickstart a positive feedback loop and a runaway greenhouse effect.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;So&lt;/p&gt;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;The detonation would be atmospheric, not on the surface.&lt;/li&gt;&#xA;&lt;li&gt;Mars' atmosphere is roughly speaking 1% that of Earth, so the gamma-ray attenuation length is roughly 100x longer.&lt;/li&gt;&#xA;&lt;li&gt;The &amp;quot;Compton current&amp;quot; (&lt;a href=&quot;https://en.wikipedia.org/wiki/Compton_scattering&quot; rel=&quot;nofollow noreferrer&quot;&gt;Compton&lt;/a&gt; electrons produced by the gamma-ray pulse) would be produced over a far larger shell, creating a &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse&quot; rel=&quot;nofollow noreferrer&quot;&gt;nuclear electromagnetic pulse&lt;/a&gt; with far different characteristics than the atmospheric detonation of a &lt;a href=&quot;https://en.wikipedia.org/wiki/Neutron_bomb&quot; rel=&quot;nofollow noreferrer&quot;&gt;neutron bomb&lt;/a&gt; on Earth would.&lt;/li&gt;&#xA;&lt;li&gt;While characteristics of terrestrial EMPs are affected by Earth's magnetic field, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse#E1&quot; rel=&quot;nofollow noreferrer&quot;&gt;E1 pulse&lt;/a&gt; certainly happens with or without one.&lt;/li&gt;&#xA;&lt;li&gt;For a start on how to think about the Compton current and the generation of the prompt electromagnetic pulse, see for example &lt;a href=&quot;http://ece-research.unm.edu/summa/notes/TheoreticalPDFs/TN368.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;Justification and verification of High Altitude EMP Theory, Part 1&lt;/a&gt; as well as the Rand Corporation report &lt;a href=&quot;https://www.rand.org/content/dam/rand/pubs/research_memoranda/2008/RM4151.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;The Compton Current and the Energy Deposition Rate from Gamma Quanta — A Monte Carlo Calculation&lt;/a&gt;.&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&lt;p&gt;Using nuclear devices for terraforming might be considered for other purposes besides liberating gas from the polar ice caps, and very low yield devices make handy (but radioactive) tools for large scale excavation. I'm not advocating any of this or asking about utility here. The question is about the EMP felt by orbiting spacecraft.&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; Would the EMP from atmospheric polar nuclear detonations on Mars felt by orbiting spacecraft be larger or smaller than (if it were) on Earth?&lt;/p&gt;&#xA;&lt;p&gt;For this question (originally inspired by discussion below this &lt;a href=&quot;https://space.stackexchange.com/a/24635/12102&quot;&gt;answer&lt;/a&gt;) I'm looking particularly for sourced answers that draw upon science.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/gV6hP9wpMW8?start=0&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-28T08:48:37.280" Title="Would the EMP from atmospheric polar nuclear detonations on Mars felt by orbiting spacecraft be larger or smaller than (if it were) on Earth?" Tags="|mars|nuclear|terraforming|" AnswerCount="0" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24653" PostTypeId="1" AcceptedAnswerId="46933" CreationDate="2018-01-28T10:32:23.660" Score="4" ViewCount="353" Body="&lt;p&gt;According to &lt;a href=&quot;https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Clouds&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;Venusian clouds are thick and are composed mainly (75-96%) of sulfuric acid droplets. These clouds obscure the surface of Venus from optical imaging, and reflect about 75% of the sunlight that falls on them. The geometric albedo, a common measure of reflectivity, is the highest of any planet in the Solar System. This high reflectivity potentially enables any probe exploring the cloud tops sufficient solar energy such that solar cells can be fitted anywhere on the craft. The density of the clouds is highly variable with the densest layer at about 48.5 km, reaching 0.1 g/m^3 similar to the lower range of cumulonimbus storm clouds on Earth.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;Can the typical size of the sulfuric acid droplets in the atmosphere of Venus be estimated from observation, or from calculation based on models of the atmosphere?  Would the average size be expected to vary with altitude?&lt;/p&gt;&#xA;" OwnerUserId="19335" LastEditorUserId="12102" LastEditDate="2018-01-28T11:05:56.943" LastActivityDate="2020-10-10T04:49:58.877" Title="What is the typical size of the sulfuric acid droplets in the atmosphere of Venus?" Tags="|atmosphere|venus|" AnswerCount="1" CommentCount="4" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24655" PostTypeId="2" ParentId="24616" CreationDate="2018-01-29T04:02:21.587" Score="2" Body="&lt;p&gt;I can't actually find any definitive source on if there are 10 panels that aren't highly reflective or not. You are correct that there should be 75 panels, with the bottom 5 being removed due to the mounting location. None of the photos or articles talking about it indicate anything differently.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As to why they could be missing, there is one thing that comes to mind in particular. The articles mention that it is spinning quickly. There are a few basic ways in which one could make it spin. The first would be to have the rocket spin when dropping it off. I find that unlikely. The second would be to have some kind of a magnet in it to have it line up with the magnetic field of the Earth. This doesn't tend to actually create the rotation, but rather align what rotation there is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The more likely explanation, in my opinion, is that the non-reflective panels are there to induce rotation. Essentially if one side of a satellite absorbs sunlight, while the other side reflects it, it will create a very small rotation speed. This will be enhanced with the small size of the spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Another explanation could be that there is some panels removed to induce the rotation through thrusters or perhaps spin-induced motion similar to what New Horizons did to start spinning. Thrusters were used by &lt;a href=&quot;http://space.skyrocket.de/doc_sdat/starshine-1.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;Starshine&lt;/a&gt; to induce spin, which was a similar project at the turn of the century.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Other explanations could be that some panels were removed to allow it to better fit into the spacecraft, or others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In any case, in the very likely event of 10 missing panels, I'm pretty sure they have something to do with the spinning of the ball, although I'm not sure how that work work.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-29T04:02:21.587" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24656" PostTypeId="1" AcceptedAnswerId="24662" CreationDate="2018-01-29T04:10:28.407" Score="2" ViewCount="500" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; This must be some kind of record!  (cf. &lt;a href=&quot;https://space.stackexchange.com/q/20191/12102&quot;&gt;ISRO's PSLV-C37 with 104 satellites - after 56 hours only six TLEs, which factor is greatest challenge?&lt;/a&gt; This time its only six &lt;em&gt;total!&lt;/em&gt;)&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;At the end of every episode of old US TV game show called &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/To_Tell_the_Truth&quot; rel=&quot;nofollow noreferrer&quot;&gt;To Tell the Truth&lt;/a&gt;&quot; the moderator would ask for the real contestant (one of three) to &quot;please, stand up&quot;. (&lt;a href=&quot;https://youtu.be/GVRSJc2JF08?t=1316&quot; rel=&quot;nofollow noreferrer&quot;&gt;example ending&lt;/a&gt;)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As of 29-Jan-2018 03:25 UTC:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Both &lt;a href=&quot;http://www.n2yo.com/satellite/?s=43166&quot; rel=&quot;nofollow noreferrer&quot;&gt;N2YO&lt;/a&gt; and &lt;a href=&quot;https://de.wikipedia.org/wiki/Humanity_Star&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia (German)&lt;/a&gt; say that Humanity Star is &lt;code&gt;43166&lt;/code&gt;, &lt;code&gt;2018-010D&lt;/code&gt;, but &lt;a href=&quot;https://en.wikipedia.org/wiki/Humanity_Star&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia (English) says&lt;/a&gt; it is &lt;code&gt;43168&lt;/code&gt;, &lt;code&gt;2018-010F&lt;/code&gt;, and NY2O's current ground track positions for &lt;code&gt;D&lt;/code&gt; and &lt;code&gt;F&lt;/code&gt; both disagree with the position on &lt;a href=&quot;http://thehumanitystar.com/&quot; rel=&quot;nofollow noreferrer&quot;&gt;TheHumanityStar.com&lt;/a&gt;'s tracking page, and Celestrak's SatCat listing does not show names for any of the 2018-010 objects yet,  although Celestrak's TLE for the &lt;code&gt;A&lt;/code&gt; object shows the name &lt;code&gt;DOVE PIONEER&lt;/code&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;Question:&lt;/strong&gt; What is the Humanity Star's actual identity? Will the real Humanity Star please stand up?&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Simultaneous screen shots for &lt;code&gt;D&lt;/code&gt; (labeled Humanity Star) and &lt;code&gt;F&lt;/code&gt; via N2YO and Humanity Star via the official site:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/1iOG5.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/1iOG5.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/s7X0W.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/s7X0W.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;a href=&quot;https://i.stack.imgur.com/22wzW.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/22wzW.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-02-05T01:23:13.603" LastActivityDate="2018-02-08T17:34:45.893" Title="It's been over two weeks now, will the real Humanity Star please stand up?" Tags="|artificial-satellite|tracking|two-line-elements|humanitystar|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24657" PostTypeId="1" AcceptedAnswerId="24661" CreationDate="2018-01-29T04:32:45.687" Score="2" ViewCount="1203" Body="&lt;p&gt;This &lt;a href=&quot;https://www.reddit.com/r/RocketLab/comments/7s3xxi/6_objects_from_the_still_testing_launch_can_be/&quot; rel=&quot;nofollow noreferrer&quot;&gt;reddit&lt;/a&gt; points to Rocket Lab's news page &lt;a href=&quot;https://www.rocketlabusa.com/news/updates/rocket-lab-successfully-circularizes-orbit-with-new-electron-kick-stage/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Rocket Lab successfully circularizes orbit with new Electron kick stage&lt;/a&gt;. It is named Curie, presumably after &lt;a href=&quot;https://en.wikipedia.org/wiki/Marie_Curie&quot; rel=&quot;nofollow noreferrer&quot;&gt;Marie&lt;/a&gt;?&lt;/p&gt;&#xA;&lt;p&gt;According to the Rocket Lab's article:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;The kick stage is designed for use on the Electron launch vehicle with a payload capacity of up to 150 kg and will be used to disperse CubeSat constellations faster and more accurately, enabling satellite data to be received and utilized sooner after launch.&lt;/p&gt;&#xA;&lt;p&gt;Equipped with a precision pointing cold gas reaction control system, the kick stage also has its own avionics, power and communications systems.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;It sounds a little bit like a rocket-propelled cubesat dispenser. Is that what it is? What sort of propulsion does it use?&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-29T11:07:30.623" Title="What is Rocket Lab's new Curie &quot;Kick Stage&quot;?" Tags="|upper-stage|attitude-determination-and-control|rocketlab|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24658" PostTypeId="1" AcceptedAnswerId="24663" CreationDate="2018-01-29T05:50:19.947" Score="3" ViewCount="96" Body="&lt;p&gt;I'm personally interested in the &lt;a href=&quot;http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0169296&quot; rel=&quot;nofollow noreferrer&quot;&gt;biological implications&lt;/a&gt; of different isotope variations on biology. This can be tested on Earth by separating them and just observing, but determining the isotope variations on other planets is what I'm not sure about.&lt;/p&gt;&#xA;&lt;p&gt;I know the Curiosity rover can do a range of experiments and chemical analyses, but I'm not super familiar with them.&lt;/p&gt;&#xA;&lt;p&gt;Can Curiosity test for isotope variations in Martian soil?&lt;/p&gt;&#xA;&lt;p&gt;If not, is there any other known way of testing for isotope variations, maybe through observation through a telescope (I really don't know), or possibly knowledge about the historical formation of the universe and solar system (I'm not an astronomer)?&lt;/p&gt;&#xA;" OwnerUserId="22722" LastEditorUserId="7062" LastEditDate="2022-06-09T01:34:28.630" LastActivityDate="2022-06-09T01:34:28.630" Title="How (if possible) can we assess the isotope variations on Martian soils?" Tags="|mars|planetary-science|life|curiosity|chemistry|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 4.0" />
  <row Id="24660" PostTypeId="2" ParentId="24650" CreationDate="2018-01-29T09:19:08.320" Score="7" Body="&lt;p&gt;&lt;strong&gt;People did!&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;Re-reading the linked article in ASU's Red Planet Report, I noticed at the end it links to an additional article; &amp;quot;&lt;em&gt;More on Egg Rock at &lt;a href=&quot;http://redplanet.asu.edu/?p=21047&quot; rel=&quot;nofollow noreferrer&quot;&gt;redplanet.asu.edu/?p=21047&lt;/a&gt;&lt;/em&gt;&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;That link says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Laser-zapping of a globular, golf-ball-size object on Mars by NASA’s Curiosity rover confirms that it is an &lt;a href=&quot;https://www.jpl.nasa.gov/news/news.php?feature=6667&quot; rel=&quot;nofollow noreferrer&quot;&gt;iron-nickel meteorite&lt;/a&gt; fallen from the Red Planet’s sky.&lt;/p&gt;&#xA;&lt;p&gt;Iron-nickel meteorites are a common class of space rocks found on Earth, and previous examples have been seen on Mars, but this one, called “Egg Rock,” is the first on Mars examined with a laser-firing spectrometer. To do so, the rover team used Curiosity’s Chemistry and Camera (ChemCam) instrument.&lt;/p&gt;&#xA;&lt;p&gt;Scientists of the Mars Science Laboratory (MSL) project, which operates the rover, first noticed the odd-looking rock in images taken by Curiosity’s Mast Camera (Mastcam) at at a site the rover reached by an Oct. 27 drive.&lt;/p&gt;&#xA;&lt;p&gt;“The dark, smooth and lustrous aspect of this target, and its sort of spherical shape attracted the attention of some MSL scientists when we received the Mastcam images at the new location,” said ChemCam team member Pierre-Yves Meslin, at the Research Institute in Astrophysics and Planetology (IRAP), of France’s National Center for Scientific Research (CNRS) and the University of Toulouse, France.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;The iron-nickel meteorite link above points to the same information as well as a further discussion of the meteorite.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; cropped section of image of &amp;quot;Egg Rock&amp;quot; from &lt;a href=&quot;http://redplanet.asu.edu/?p=21047&quot; rel=&quot;nofollow noreferrer&quot;&gt;redplanet.asu.edu/?p=21047&lt;/a&gt; showing the spots where Curiosity's ChemCam laser has ablated material.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/KJ2nK.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/KJ2nK.jpg&quot; alt=&quot;Egg rock on Mars showing the spots where Curiosity's ChemCam laser has ablated material.&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2021-04-30T03:22:47.237" LastActivityDate="2021-04-30T03:22:47.237" CommentCount="4" ContentLicense="CC BY-SA 4.0" />
  <row Id="24661" PostTypeId="2" ParentId="24657" CreationDate="2018-01-29T11:07:30.623" Score="2" Body="&lt;p&gt;From the &lt;a href=&quot;https://www.rocketlabusa.com/electron/&quot; rel=&quot;nofollow noreferrer&quot;&gt;RocketLab site&lt;/a&gt;: &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(this is a view of the bottom of the kick stage, with a tiny rocket nozzle in the center)&#xA;&lt;a href=&quot;https://i.stack.imgur.com/M0rcm.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/M0rcm.jpg&quot; alt=&quot;Curie kick stage &quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;An apogee kick stage that can execute multiple burns to place numerous payloads into different, circularized orbits. It opens up significantly more orbital options, particularly for rideshare customers that have traditionally been limited to the primary payload’s designated orbit. Powered by Rocket Lab's 3D printed liquid propellant Curie engine capable of 120N of thrust and multiple burns. &lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;From the images, it looks like the stage fits underneath the payload adapter. According to &lt;a href=&quot;https://www.rocketlabusa.com/news/updates/rocket-lab-successfully-circularizes-orbit-with-new-electron-kick-stage/&quot; rel=&quot;nofollow noreferrer&quot;&gt;the news article you linked to&lt;/a&gt;, the primary payload was deployed, then the stage+payload adapter+secondary payloads flew to the deployment location for the secondary payloads. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Propellant is &quot;a green mono-propellant stored in four spherical tanks&quot;, but &lt;a href=&quot;https://space.stackexchange.com/questions/24613/what-green-propellant-powers-curie-the-engine-on-rocketlabs-payload-kickstag&quot;&gt;composition is unknown at this point&lt;/a&gt;.&lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-01-29T11:07:30.623" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24662" PostTypeId="2" ParentId="24656" CreationDate="2018-01-29T13:55:05.400" Score="4" Body="&lt;p&gt;First of all, it isn't really known yet which object it is, at least to the general public. Space Track (JSpOC) no doubt knows which object it is, but it would require them confirming it to the public to get the positive identification. In order for them to release this information to the public, it requires a high level expert to confirm the information, who normally doesn't work on weekends or nights. I expect the confirmation will be done today.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, there are a few bits of information we can use. Of the 6 objects that are being tracked, 3 of them are in a lower orbit then the others. We know from publicly released information that Humanity Star is supposed to be in the lowest orbit. One of these objects is positively identified, meaning that it must be one of the two remaining objects, D or F. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Given that Humanity Star was a secret payload, not announced to the general public until after launch, I suspect that it was given the last designated payload slot. In addition, &quot;F&quot; is the lowest object at the moment, and is given a &quot;Large&quot; Radar Cross Section (RCS) size by JSpOC, as is object &quot;D&quot;. It seems like Small, Medium, and Large are the most commonly given values for the RCS size. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lastly, the common convention of JSpOC is to give all payloads the first levels, followed by any booster stages, and finally any debris objects. It is worth noting that there are 4 &quot;Payloads&quot; that were launched, in addition to 2 &quot;Booster&quot; objects.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That being said, I'm confident that JSpOC things that &quot;D&quot; is Humanity Star&quot;, and &quot;F&quot; is the spent second stage booster. &quot;E&quot; would be the spent third stage booster, and &quot;B&quot; and &quot;C&quot; should be the two Lemur satellites. I wouldn't make anything official until JSpOC releases the designations, but I believe 43166 is the proper designation for Humanity's Star.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So, I was wrong, the real one is the &quot;F&quot;, 2018-010F 43168. I suspect that the two looked alike, and they initially guessed wrong between Humanity Star and the booster stage. &lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-02-08T17:34:45.893" LastActivityDate="2018-02-08T17:34:45.893" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24663" PostTypeId="2" ParentId="24658" CreationDate="2018-01-29T14:47:13.893" Score="4" Body="&lt;p&gt;Curiosity's &lt;a href=&quot;https://mars.nasa.gov/msl/mission/instruments/spectrometers/sam/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Sample Analysis at Mars (SAM) Instrument Suite&lt;/a&gt; has both a mass spectrometer, which can identify different isotopes, and a tunable laser spectrometer, which can also be used for isotope abundance measurements, as explained in Emily Lakdawalla's detailed blogpost &lt;a href=&quot;http://www.planetary.org/blogs/emily-lakdawalla/2012/curiosity-instrument-sam.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;More than you probably wanted to know about Curiosity's SAM instrument&lt;/a&gt;. Of particular note are the Carbon, Hydrogen, and Oxygen isotopes, but it should be able to detect others.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Remote sensing of isotopes is difficult. It hasn't been demonstrated in terrestrial, yet alone extraterrestrial applications, but it &lt;a href=&quot;https://link.springer.com/chapter/10.1007/978-90-481-3354-3_3&quot; rel=&quot;nofollow noreferrer&quot;&gt;is being studied&lt;/a&gt; currently.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="12102" LastEditDate="2018-01-31T01:21:18.100" LastActivityDate="2018-01-31T01:21:18.100" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24664" PostTypeId="1" AcceptedAnswerId="24666" CreationDate="2018-01-29T15:55:00.533" Score="16" ViewCount="3010" Body="&lt;p&gt;In a YouTube video &lt;a href=&quot;https://www.youtube.com/watch?v=0W7IRBeDkj4&quot; rel=&quot;noreferrer&quot;&gt;Colonizing the Solar System, part 2: the Outer Solar System&lt;/a&gt;, Isaac Arthur who is a YouTuber and also a physicist says something like &quot;&lt;em&gt;you don't spin an asteroid for artificial gravity because it will cause it to fly away&lt;/em&gt;&quot;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In more than few instances, he also explains how easy it is to dig an asteroid as it is more of a loosely held together dust and debris than something solid. So,&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Why doesn't an asteroid turn to dust as soon as it enters the earth's atmosphere?&lt;/li&gt;&#xA;&lt;li&gt;Can't we spin an asteroid in order to pulverize it?&lt;/li&gt;&#xA;&lt;li&gt;What about mechanically grinding it, if spinning is not an option? When it is so soft, would that mean we surely can send moderately small spacecraft that can grind few cubic kilometers in few years?&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;" OwnerUserId="19321" LastEditorUserId="8859" LastEditDate="2018-01-30T04:51:43.563" LastActivityDate="2020-12-07T22:54:10.697" Title="Can we destroy an asteroid by spinning it?" Tags="|spacecraft|gravity|asteroid|near-earth-asteroid|" AnswerCount="3" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24665" PostTypeId="1" AcceptedAnswerId="24667" CreationDate="2018-01-29T16:01:11.710" Score="3" ViewCount="517" Body="&lt;p&gt;Lifetime of satellites is affected by orbital decay. By using electric propulsion which is more efficient than chemical propulsion, this means that we got more fuel correct the satellite from orbital decay. Does this mean that satellites with electric propulsion last longer? And how much longer?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For instance lifetime of GEO satellite is around 8-15 years. What about GEO satellite using ion thruster?&lt;/p&gt;&#xA;" OwnerUserId="22224" LastActivityDate="2018-02-01T01:33:10.470" Title="Does using electric propulsion increase the lifetime of satellites?" Tags="|artificial-satellite|propulsion|electric-propulsion|" AnswerCount="2" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24666" PostTypeId="2" ParentId="24664" CreationDate="2018-01-29T16:05:43.553" Score="10" Body="&lt;p&gt;Not really. If an asteroid that was a loose collection of rubble was spinning, it would fall apart. The question is, how do you get it spinning? And would it do any good anyways?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To make it spin one would have to apply an asymmetric thrust. The problem is, most likely if you can do this, then it will actually end up only affecting the area you are thrusting, still leaving the main body of the asteroid intact. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An easier solution almost certainly would be the nuclear option. With a loose collection of rocks, digging into it should be easy enough, just place the nuclear weapon in the middle of the pile and it would scatter quite effectively. This would be quite a bit easier then trying to spin the rock, and impart more energy. The amount of energy required to blow something up is known as the &lt;a href=&quot;https://en.wikipedia.org/wiki/Gravitational_binding_energy&quot; rel=&quot;nofollow noreferrer&quot;&gt;gravitational binding energy&lt;/a&gt;. Suffieceth to say, a 100 MTon bomb could blow up a 10 km asteroid, barely. That is the largest nuclear weapon ever built. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;And the more interesting question, would it do any good? That is difficult to tell really, but the answer is probably not. A single dense asteroid will hit the Earth mostly intact, which would cause local severe damage, but probably not global damage, unless the rock was huge. A loose collection of objects hitting the Earth all over, or even worse dust, could cause a different problem. If all of that dust hit the atmosphere, it would probably stay up there. It is very difficult to get things from the upper atmosphere down, thus any dust reaching that high could stay up there for years. This could cause a massive cooling event, similar to a &lt;a href=&quot;https://en.wikipedia.org/wiki/Volcanic_winter&quot; rel=&quot;nofollow noreferrer&quot;&gt;volcanic winter&lt;/a&gt;. That could have global repercussions. Note that it would take a large amount of dust to cause this kind of problem, &lt;a href=&quot;https://www.popsci.com/60-tons-cosmic-dust-fall-earth-every-day&quot; rel=&quot;nofollow noreferrer&quot;&gt;60 tons&lt;/a&gt; of dust enters Earth's atmosphere every day. If, say, over a million tons of dust entered the atmosphere in one day, it would probably cause issues. Also, if instead you convert it into a number of smaller rocks, the area of affect can be greatly increased. Instead of one large event, you might have a dozen smaller events, but overall increase the area of destruction.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Bottom line is, the best thing is to get the entire rock out. If you can't do that, then unless you have enough time to allow for a wide separation of  the rocks, on the order of a million or more miles, you are probably better off just letting the rock hit the Earth intact. If you have the opportunity to do something tens or even a hundred years in advance, then trying to get it to disperse might be feasible, but otherwise, moving the thing completely at once is your best bet.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-30T12:44:12.980" LastActivityDate="2018-01-30T12:44:12.980" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24667" PostTypeId="2" ParentId="24665" CreationDate="2018-01-29T16:15:41.650" Score="4" Body="&lt;p&gt;It can, but more commonly it is used to reduce the cost of the satellite. For instance, &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/a/all-electric&quot; rel=&quot;nofollow noreferrer&quot;&gt;this article&lt;/a&gt; shows two satellites stacked on each other heading to GTO, which is only possible due to a significant weight savings that is achieved by using ion propulsion. In this configuration, one essentially gets 2 satellites for the price of 1.&lt;/p&gt;&#xA;" OwnerUserId="25" LastActivityDate="2018-01-29T16:15:41.650" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24669" PostTypeId="2" ParentId="24664" CreationDate="2018-01-29T21:40:31.207" Score="1" Body="&lt;p&gt;Actually yes an asteroid could (to an extent) be destroyed by spinning. I like the idea. It would at worst likely split in half and the two halves would go safely past either side of the target to be avoided (eg: Earth), or at best, pieces would keep coming off the periphery (and they should fly off well clear with centrifugal force).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The method of getting it to spin would be gyroscopic (a very fast spinning small weight forced to gradually change it's axis would cause an opposite spin on the asteroid if that's what the spinning weight's cradle was attached to. Theoretically, with not much power, the asteroid could be made to eventually reach a high RPM. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;So what would be more difficult? Transporting an explosive to the surface of the asteroid, along with drilling equipment, or sending a gyroscope? The thing about the spinning destruction is it would be likely more reliable as explosives can be hit and miss. On the other hand, if the asteroid were iron, it would resist destruction either way. Another problem is if the asteroid is already spinning on an unfavourable axis.... &lt;/p&gt;&#xA;" OwnerUserId="22728" LastEditorUserId="22728" LastEditDate="2018-02-10T15:34:06.503" LastActivityDate="2018-02-10T15:34:06.503" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24670" PostTypeId="1" AcceptedAnswerId="24673" CreationDate="2018-01-29T23:32:04.247" Score="3" ViewCount="275" Body="&lt;p&gt;If one fictitious probe mission was to impact an asteroid fast enough to destroy it using its kinetic energy, could the asteroid's trajectory be predicted accurately enough (plumes or emanations or external forces may change its course) for the probe to hit it exactly where it should (aligned with center of mass)? If not, what kind of trajectory correction system would be required? &lt;/p&gt;&#xA;" OwnerUserId="19132" LastEditorUserId="13710" LastEditDate="2018-01-29T23:34:10.327" LastActivityDate="2018-01-30T12:48:28.383" Title="How predictable is one asteroid's trajectory?" Tags="|orbital-mechanics|asteroid|impact|collision-avoidance|" AnswerCount="1" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24671" PostTypeId="2" ParentId="21297" CreationDate="2018-01-29T23:47:31.370" Score="2" Body="&lt;p&gt;I believe the first mission that intended to use a stereo pair of cameras was the &lt;a href=&quot;https://en.wikipedia.org/wiki/Luna_13&quot; rel=&quot;nofollow noreferrer&quot;&gt;Luna 13&lt;/a&gt; mission, although one of the cameras failed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Likely the first mission that intended to capture stereo photos was the Apollo 12 mission, which contained a &lt;a href=&quot;https://www.history.nasa.gov/FINAL%20Catalogue%20of%20Manmade%20Material%20on%20the%20Moon.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;close up Stereo camera&lt;/a&gt;. There was also a deliberate pair of images captured with a single camera on Apollo 11, AS11-40-5943 and AS11-40-5944.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The first unmanned mission that was successful was probably the &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunokhod_1&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunokhod 1&lt;/a&gt;. Images show that it has a pair of cameras that appears to be a stereo pair, and there is no mention of the cameras failing. As the Soviets previously had attempted a stereo pair, I have to assume this one was successful. I've included a replica of &lt;a href=&quot;https://en.wikipedia.org/wiki/Lunokhod_programme#/media/File:Lunokhod-2_model.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;Lunokhod 2&lt;/a&gt; below, which is supposed to be a replica. Like &lt;a href=&quot;http://mentallandscape.com/C_CatalogMoon.htm&quot; rel=&quot;nofollow noreferrer&quot;&gt;most Soviet images of the era&lt;/a&gt;, records weren't carefully kept, and thus it is difficult to determine more then early in the mission when the photo might have been taken.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/oWCgr.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/oWCgr.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2019-04-20T12:10:00.633" LastActivityDate="2019-04-20T12:10:00.633" CommentCount="8" ContentLicense="CC BY-SA 4.0" />
  <row Id="24673" PostTypeId="2" ParentId="24670" CreationDate="2018-01-30T03:42:13.027" Score="5" Body="&lt;p&gt;It might be, but more likely one would do some kind of a correction burn on route. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;When we are going to a far away body, such as Pluto, a KBO, or asteroid, guidance from Earth will only get it so far. Eventually the spacecraft will start to image the target body, and use those images to better map the trajectory it will take. They all have a slight amount of ability to divert, which would allow it to do a course correction and hit the correct spot. Most of the time, these go through Earth, but not all of the time. &lt;a href=&quot;https://cosmosmagazine.com/space/why-nasa-plans-crash-spacecraft-asteroid&quot; rel=&quot;nofollow noreferrer&quot;&gt;AIM&lt;/a&gt; is an upcoming mission that is very similar to what you want, and &lt;a href=&quot;https://link.springer.com/article/10.1007/s11214-005-3394-4&quot; rel=&quot;nofollow noreferrer&quot;&gt;Deep Impact&lt;/a&gt; is a previous one that worked before.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A couple of missions that have done similar things are Deep Space One and New Horizons. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a nice chart from &lt;a href=&quot;https://space.stackexchange.com/a/19087/25&quot;&gt;this answer&lt;/a&gt; that shows the accuracy vs time of an asteroid. With radar we can get close, but the uncertainty is still quite a bit further then the size of the asteroid.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/QdGQw.gif&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/QdGQw.gif&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-01-30T12:48:28.383" LastActivityDate="2018-01-30T12:48:28.383" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24676" PostTypeId="1" CreationDate="2018-01-30T22:58:04.293" Score="2" ViewCount="348" Body="&lt;p&gt;How and where can I determine the direction of a launch?  I live on the South Carolina coast and sometimes see a launch and sometimes it goes Southeast.&lt;/p&gt;&#xA;" OwnerUserId="22737" LastEditorUserId="6537" LastEditDate="2018-02-02T07:00:27.057" LastActivityDate="2020-09-18T19:04:27.220" Title="How and where can I determine the direction of a launch prior to liftoff?" Tags="|launch-trajectories|" AnswerCount="1" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24677" PostTypeId="2" ParentId="24480" CreationDate="2018-01-30T23:21:16.920" Score="6" Body="&lt;p&gt;There are other wavelengths than those discussed in this &lt;a href=&quot;https://space.stackexchange.com/a/24482/12102&quot;&gt;answer&lt;/a&gt;. On the off-chance that Zuma might broadcast data or telemetry, Canadian amateur astronomer Scott Tilley used a nice antenna and &lt;a href=&quot;https://en.wikipedia.org/wiki/Software-defined_radio&quot; rel=&quot;nofollow noreferrer&quot;&gt;SDR&lt;/a&gt; to probe the sky for signals.&lt;/p&gt;&#xA;&lt;p&gt;The first post in his extensive blogging on the signal in &lt;a href=&quot;https://skyriddles.wordpress.com/2018/01/21/nasas-long-dead-image-satellite-is-alive/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Riddles in the Sky, A blog dedicated to observing, mostly classified, satellites.&lt;/a&gt; Tilley says:&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Over the past week the station has been dedicated to an S-band scan looking for new targets and refreshing the frequency list, triggered by the recent launch of the mysterious ZUMA mission.  This tends to be a semi-annual activity as it can eat up a lot of observing resources even with much of the data gathering automated the data reviewing is tedious.&lt;/p&gt;&#xA;&lt;p&gt;Upon reviewing the data from January 20, 2018, I noticed a curve consistent with an satellite in High Earth Orbit (HEO) on 2275.905MHz, darn not ZUMA… This is not uncommon during these searches.  So I set to work to identify the source.&lt;/p&gt;&#xA;&lt;p&gt;A quick identity scan using ‘strf’ (sat tools rf) revealed the signal to come from 2000-017A, 26113, called &lt;a href=&quot;https://directory.eoportal.org/web/eoportal/satellite-missions/i/image-mission&quot; rel=&quot;nofollow noreferrer&quot;&gt;IMAGE&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; Doppler analysis from subsequent &lt;a href=&quot;https://skyriddles.wordpress.com/2018/01/21/nasas-long-dead-image-satellite-is-alive/&quot; rel=&quot;nofollow noreferrer&quot;&gt;observation&lt;/a&gt; and TLE search result (click for full size).&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TUYWq.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TUYWqm.png&quot; alt=&quot;SDR analysis of signals from IMAGE satellite&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;That this method discovered a satellite that was still transmitting that was otherwise (at least generally*) unknown indicates the veracity of the method.&lt;/p&gt;&#xA;&lt;p&gt;Of course, a stealthy spacecraft that did rely on RF would likely use stealthy techniques - careful timing, short, irregular bursts at odd frequencies, tightest possible RF beam consistent with diffraction, etc. and since optical communication is now an established (though not yet commonplace) technology, there's no guarantee a stealthy spacecraft would generate &lt;em&gt;any&lt;/em&gt; intentionally-transmitted RF, but like the slogan for the &lt;a href=&quot;https://en.wikipedia.org/wiki/New_York_Lottery&quot; rel=&quot;nofollow noreferrer&quot;&gt;New York State Lottery&lt;/a&gt; so aptly summarizes, &amp;quot;&lt;a href=&quot;http://nylottery.ny.gov/&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;em&gt;Hey, you never know!&lt;/em&gt;&lt;/a&gt;&amp;quot;&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Science: &lt;a href=&quot;http://www.sciencemag.org/news/2018/01/amateur-astronomer-discovers-revived-nasa-satellite&quot; rel=&quot;nofollow noreferrer&quot;&gt;Amateur astronomer discovers a revived NASA satellite&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;CBC: &lt;a href=&quot;http://www.cbc.ca/news/canada/british-columbia/nasa-satellite-discovery-1.4506955&quot; rel=&quot;nofollow noreferrer&quot;&gt;Amateur astronomer in B.C. may have discovered long-lost NASA satellite&lt;/a&gt;&lt;/li&gt;&#xA;&lt;li&gt;Gizmodo: &lt;a href=&quot;https://gizmodo.com/over-a-decade-later-nasas-long-dead-image-satellite-ma-1822479068&quot; rel=&quot;nofollow noreferrer&quot;&gt;Over a Decade Later, NASA's Long-Dead IMAGE Satellite May Have Come Back to Life&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;From &lt;a href=&quot;https://wattsupwiththat.com/2018/01/26/long-dead-zombie-satellite-wakes-up-13-years-later/&quot; rel=&quot;nofollow noreferrer&quot;&gt;What's Up with That?&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;That is, until Scott Tilley started looking for Zuma. “When I saw the radio signature, I ran a program called STRF to identify it,” he says. Developed by &lt;a href=&quot;https://twitter.com/cgbassa?lang=en&quot; rel=&quot;nofollow noreferrer&quot;&gt;Cees Bassa&lt;/a&gt;, a professional astronomer at the Netherlands Institute for Radio Astronomy, STRF treats Earth-orbiting satellites much like binary pulsars–deducing their orbital elements from the Doppler shifts of their radio signals. “The program immediately matched the orbit of the satellite I saw to IMAGE. It was that easy,” says Tilley.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Tilley communicated the find to NASA. According to &lt;a href=&quot;http://www.wral.com/amateur-astronomer-finds-long-lost-satellite/17292165/&quot; rel=&quot;nofollow noreferrer&quot;&gt;WRAL&lt;/a&gt;&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;Rather than wait for NASA's Deep Space Network, already busy with communications for active missions, a team of engineers assembled the gear necessary and set-up a receiving station of their own behind their GSFC office building. Richard Burley, mission manager for the joint NASA/NOAA polar satellite program, confirmed that from multiple acquisitions of RF signal that match the orbit, spin and signal characteristics, &amp;quot;all indications so far suggest that, in fact this is IMAGE.&amp;quot;&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;Click the image below for better view of the receiving station(?)&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/fjyw8.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/fjyw8.jpg&quot; alt=&quot;Goddard Space Flight Center engineers&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;p&gt;&lt;strong&gt;above:&lt;/strong&gt; a team of Goddard Space Flight Center engineers, confirming transmissions from [IMAGE]. From &lt;a href=&quot;https://skyriddles.wordpress.com/2018/01/21/nasas-long-dead-image-satellite-is-alive/&quot; rel=&quot;nofollow noreferrer&quot;&gt;here&lt;/a&gt;.&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;Moral of the story? As &amp;quot;Star Hustler&amp;quot; &lt;a href=&quot;https://en.wikipedia.org/wiki/Jack_Horkheimer&quot; rel=&quot;nofollow noreferrer&quot;&gt;Jack Horkheimer&lt;/a&gt; always said; &amp;quot;&lt;a href=&quot;https://www.youtube.com/watch?v=hwuN4MJxZIQ&quot; rel=&quot;nofollow noreferrer&quot;&gt;Keep, Looking, Up!&lt;/a&gt;&amp;quot;&lt;/p&gt;&#xA;&lt;hr /&gt;&#xA;&lt;p&gt;&lt;strong&gt;Footnotes:&lt;/strong&gt;&lt;/p&gt;&#xA;&lt;p&gt;*at least generally: As Tilley &lt;a href=&quot;https://skyriddles.wordpress.com/2018/01/21/nasas-long-dead-image-satellite-is-alive/&quot; rel=&quot;nofollow noreferrer&quot;&gt;mentions&lt;/a&gt; Cees Bassa confirms that IMAGE had been transmitting for years.&lt;/p&gt;&#xA;&lt;blockquote&gt;&#xA;&lt;p&gt;20:06 UTC – Dr. Cees Bassa a fellow satellite tracking enthusiast completes review of his data archive which shows IMAGE signal as early as October 2016 and no detection in January/February 2014:&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/039OC.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/039OC.png&quot; alt=&quot;SDR analysis for IMAGE spacecraft 2016&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-01-31T11:46:34.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24678" PostTypeId="2" ParentId="24676" CreationDate="2018-01-30T23:45:26.040" Score="3" Body="&lt;p&gt;welcome to Space Stack Exchange.&lt;/p&gt;&#xA;&lt;p&gt;I'm going to take a guess that you mean &amp;quot;how can I determine the direction prior to the event?&amp;quot;.&lt;/p&gt;&#xA;&lt;p&gt;Go to the launch providers website and see what advance information they give as a press kit. It may well not give anything obvious, but have a look at the inclination of the target orbit.&lt;/p&gt;&#xA;&lt;p&gt;The rest of this answer provides some ideas of how to understand the inclination:&lt;/p&gt;&#xA;&lt;p&gt;Very roughly, if a launch goes due East, without any later dog-legs, then the orbit inclination achieved will be close, numerically, to the latitude of the launch site. For example, launching due East from Cape Canaveral would result in an inclination of about 28 degrees. Hence you can work back from the inclination to work out its direction from the pad.&lt;/p&gt;&#xA;&lt;p&gt;Its more complicated if the launch goes North East or South East. In either case, and still assuming no dog-leg, then the achieved inclination will be higher than the latitude. This takes a diagram to explain better.&lt;/p&gt;&#xA;&lt;p&gt;If there are headlands in the way of the trajectory it gets complicated again as the launch vehicle will need to do an early dog-leg.  The later the dog-leg in the mission the more expensive it is.&lt;/p&gt;&#xA;&lt;p&gt;For those curious, here's an example of a launch with a &lt;a href=&quot;https://space.stackexchange.com/a/21355/12102&quot;&gt;dog-leg&lt;/a&gt; .&lt;/p&gt;&#xA;" OwnerUserId="10791" LastEditorUserId="10791" LastEditDate="2020-09-18T19:04:27.220" LastActivityDate="2020-09-18T19:04:27.220" CommentCount="3" ContentLicense="CC BY-SA 4.0" />
  <row Id="24679" PostTypeId="2" ParentId="18938" CreationDate="2018-01-31T07:47:54.940" Score="2" Body="&lt;p&gt;It appears that the actuators are there for initial alignment.  Should the need arise, and I bet it will, the servomotors should be and will be used for adaptive optics during observations as well.  Why waste all the hardware/software that went into alignment?  NASA's always been good at killing several birds with one stone.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Kenny, P. Eng.&lt;/p&gt;&#xA;" OwnerUserId="22742" LastActivityDate="2018-01-31T07:47:54.940" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24680" PostTypeId="1" CreationDate="2018-01-31T10:01:13.680" Score="0" ViewCount="378" Body="&lt;p&gt;What if some future Falcon Heavy's launch put a regular boulder in orbit around Earth (as a secondary payload)? Well, not arbitrary one that lies by the edge of the road, rather something resembling potentially useful asteroids, with water ice, carbon and metal inclusions and so on. Tesla roadster is no doubts cool advertisement, but a boulder in the orbit can be a test yard for developing microgravity anchoring, drilling techniques, tugging, maybe even resource processing, and all that for a small price, so even students with cubesats technically could participate. If there was a boulder, then probably Philae lander components could have been tested before departing to the rendezvous with a comet, and we'd also knew better how to make use of NEO resources. So, the question is: do you think it's a worthy thing to do?&lt;/p&gt;&#xA;" OwnerUserId="21572" LastActivityDate="2018-01-31T23:35:19.460" Title="Most useless artificial Earth satellite" Tags="|spacecraft-development|mining|near-earth-asteroid|" AnswerCount="2" CommentCount="11" ClosedDate="2018-02-02T09:00:29.913" ContentLicense="CC BY-SA 3.0" />
  <row Id="24681" PostTypeId="2" ParentId="8281" CreationDate="2018-01-31T10:33:40.397" Score="4" Body="&lt;p&gt;No. Whether the SLS will be continued or not is a purely &lt;strong&gt;political decision&lt;/strong&gt; (as pointed out in the comment by @LocalFluff). One of the main &lt;strong&gt;political reasons&lt;/strong&gt; to keep SLS alive, as long as costs are not dramatically exploding, will be to &lt;strong&gt;ensure military&lt;/strong&gt; (and other governmental) &lt;strong&gt;access to space independently of international relationships and of the condition of private companies&lt;/strong&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The US are very unhappy to currently be dependent on the Russians for manned missions (e.g. &lt;div class=&quot;youtube-embed&quot;&gt;&lt;div&gt;&#xD;&#xA;                &lt;iframe width=&quot;640px&quot; height=&quot;395px&quot; src=&quot;https://www.youtube.com/embed/4o3H0hcneFc?start=300&quot;&gt;&lt;/iframe&gt;&#xD;&#xA;            &lt;/div&gt;&lt;/div&gt;). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Dependency on a single firm (even if it's from the US) is also not what is desired.&lt;/p&gt;&#xA;" OwnerUserId="21562" LastActivityDate="2018-01-31T10:33:40.397" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24682" PostTypeId="2" ParentId="24680" CreationDate="2018-01-31T12:05:49.173" Score="0" Body="&lt;p&gt;There's probably no real reason for this:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;to test landers, robots, that work on hard surfaces - you just test them on Earth!&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;as @Hobbes explains, actual comets/asteroids are in fact &quot;loose bunches&quot; of stuff. So your &quot;Boulder Experiment #1 !&quot; really wouldn't help.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;Note that the one and only reason the Falcon guys had for adding a Tesla car, was &lt;strong&gt;publicity&lt;/strong&gt;. It's exactly like running TV ads or simply painting a sign on the side - it was simply a &quot;publicity stunt&quot;.  (And a good one!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's worth noting too that boulders are incredibly heavier than Tesla cars, or any other robots / spacecraft.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A boulder the same size as a Tesla would weigh 10? 20? X more than a Tesla.  A 1-tonne piece of stone is actually quite small. (Ask any stonemason!)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would say that for some &lt;em&gt;very specific use&lt;/em&gt;, your surprising idea mighty actually be implemented! (If we had to test some very specific robot, which, for some reason simply couldn't be tested in gravity.) I'd say it's unlikely to be useful as a sort of &quot;general facility&quot;.  And again, bear in mind that stone is extremely dense.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Mind you, some folks argue that the ISS achieved - nothing - other than just showing it could be done, so - who knows!)&lt;/p&gt;&#xA;" OwnerUserId="3289" LastActivityDate="2018-01-31T12:05:49.173" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24683" PostTypeId="1" CreationDate="2018-01-31T12:10:37.827" Score="4" ViewCount="1733" Body="&lt;p&gt;For e.g. if we want a telescope in space, can't we assemble it in space? Send the constituent assembly parts up there (unmanned). Have the individual units, assemble (unmanned) to constitute the actual thing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The assemblies themselves constitutes a small payload so it won't be very cost prohibitive to get it into space. The building of assembly parts themselves can be distributed across nations in some way. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;There could be small shuttle space ships that carry small payloads, put, those payloads, into orbit, and re-enter back into earth once their missions are over. They can be subsequently re-used for more such missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And there could be one global body to coordinate all of those activities. Why are there not any projects in this direction?&lt;/p&gt;&#xA;" OwnerUserId="8810" LastEditorUserId="6537" LastEditDate="2018-02-02T10:02:41.933" LastActivityDate="2018-02-02T10:02:41.933" Title="Would space exploration benefit by sending small drones instead of huge rockets?" Tags="|cubesat|orbital-assembly|" AnswerCount="3" CommentCount="5" FavoriteCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24684" PostTypeId="2" ParentId="24680" CreationDate="2018-01-31T12:34:14.393" Score="1" Body="&lt;p&gt;&lt;strong&gt;edit:&lt;/strong&gt; Actually, now that I re-read your question and think about this happening in Earth orbit, there's another problem. The dynamics in Earth orbit are very different than in deep space, far from any local sources of gravity. The rock and the spacecraft are both in orbit around the Earth, and so their relationship will constantly be changing as they move in their own Earth orbits. If one is above, below, or to the side of the other, then they are not going to stay like that for more than a few minutes. Even if one is &lt;em&gt;behind&lt;/em&gt; the other, once the spacecraft starts accelerating toward the rock, it will start drifting up, away from Earth. &lt;em&gt;There are no real parallel Earth orbits&lt;/em&gt;, despite the discussion in the question &lt;a href=&quot;https://space.stackexchange.com/q/22018/12102&quot;&gt;Parallel orbits around the Earth - effectively?&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you get all the way out to a heliocentric orbit, then you can do your maneuvers over a period of hours without having to think much about the Sun.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;It's always good to have a reminder just how weak gravity really is... so:&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Gravity is really, &lt;em&gt;really,&lt;/em&gt; weak!&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;$$G \approx 6.6741 \times 10^{-11} \ \ \text{m}^{3} \text{kg}^{-1} \text{s}^{-2}$$ &lt;/p&gt;&#xA;&#xA;&lt;p&gt;For a body with spherical symmetry you can calculate the gravitational force as if all the mass were at the center, instead of integrating. So for a body of radius $R$ and density $\rho$, the gravitational acceleration (the thing that's 9.8 m/s^2 on Earth) would be:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$M = \frac{4}{3} \pi R^3 \rho $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$a = GM \frac{1}{R^2} = \frac{4}{3}G \pi R \rho   $$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Plug in an example &quot;rock&quot; density of 5 g/cm^3 (5,000 kg/m^3) and the 4.5 meter radius of a &lt;a href=&quot;https://en.wikipedia.org/wiki/BFR_(rocket)&quot; rel=&quot;nofollow noreferrer&quot;&gt;BFR&lt;/a&gt; (BFRocklifter) and you get 6.29E-06 m/s^2. That is a bit less than one millionth of Earth gravity, but more importantly &lt;em&gt;it is only about 1% as strong&lt;/em&gt; as a characteristic gravitational attraction to the crazy-shaped object &lt;a href=&quot;https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko&quot; rel=&quot;nofollow noreferrer&quot;&gt;67P&lt;/a&gt; that Philae landed on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That is, if the BFRocklifter could even lift it.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;em&gt;And,&lt;/em&gt; you'd have to bring up &lt;em&gt;two of them&lt;/em&gt; in order to simulate the crazily-shaped gravity field associated with 67P. See the image below.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Also, as @Uwe points out in &lt;a href=&quot;https://space.stackexchange.com/questions/24680/most-useless-artificial-earth-satellite/24684#comment71113_24684&quot;&gt;comments&lt;/a&gt;, the escape velocity is going to be much lower as well. In fact it also scales linearly with radius (for a sphere of fixed density) so it will be 100 times lower than that near 67P as well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;$$v_E = \sqrt{\frac{2GM}{R}} = 2 R \sqrt{\frac{2}{3} G \pi \rho}$$&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At this point, if it's a low mass cubesat with large solar panels, the photon pressure from the Sun is starting to become non-negligible. &lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/f417n.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/f417n.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-01-31T23:35:19.460" LastActivityDate="2018-01-31T23:35:19.460" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24685" PostTypeId="2" ParentId="24683" CreationDate="2018-01-31T12:44:13.797" Score="3" Body="&lt;p&gt;The building of things is already split across nations - but it's much easer to assemble them here on Earth than remotely in space.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Due to various factors, but mostly due to surfaces increasing as the square of size, and volumes increasing as the cube, it is more expensive to have two launches of one tonne than one launch of two tonnes.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There are a lot of things where having a single global body to coordinate activities would be beneficial, but human nature wont allow it. Space is one of the areas that has better coordination than most.&lt;/p&gt;&#xA;" OwnerDisplayName="user20636" LastEditorDisplayName="user20636" LastEditDate="2018-01-31T14:42:30.210" LastActivityDate="2018-01-31T14:42:30.210" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24686" PostTypeId="2" ParentId="24683" CreationDate="2018-01-31T13:04:02.967" Score="11" Body="&lt;p&gt;Let's say you have a &lt;a href=&quot;https://en.wikipedia.org/wiki/James_Webb_Space_Telescope&quot; rel=&quot;noreferrer&quot;&gt;6500kg piece of equipment&lt;/a&gt; you want to launch into space, but your rocket design can only lift 2500kg to the desired orbit. You can:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Build five of these&lt;/li&gt;&#xA;&lt;li&gt;Strap them together&lt;/li&gt;&#xA;&lt;li&gt;Put said piece of equipment on the nose of the middle one.&lt;/li&gt;&#xA;&lt;li&gt;Launch&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;That's already worse than designing a new rocket, but at least it cuts down on R&amp;amp;D time. The vital part is to realize that five rockets glued together are almost as powerful as five rockets launched separately (you need some extra mass to hold the rockets together and the aerodynamics are affected as well).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you can figure out a way to transfer fuel from one rocket to another, the combined rocket has even more delta-V than the original.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Your suggestion is to:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Build five of these&lt;/li&gt;&#xA;&lt;li&gt;Chop up the payload to five equally sized parts in a way that it can be assembled back together in space.&lt;/li&gt;&#xA;&lt;li&gt;Add an assembly machine with one of the launches, or add a sixth one&lt;/li&gt;&#xA;&lt;li&gt;Arrange the launches so that they meet at the same orbit. This will likely require maneuvers after reaching orbit (costs fuel).&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;You saved: the cost and R&amp;amp;D time for strapping the rockets together; the cost for a launchpad that can carry the bigger rocket (both insignificant); mass of the struts holding five rockets together (possibly less mass than the assembly drone); you lose only 1/5 of the payload, not all of it, if a rocket fails. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;You introduced: the assembler cost; orbital maneuvering fuel and logistical challenge; R&amp;amp;D how to chop up the payload and assemble it. I'm not sure if automated docking is already a thing, but at the very least someone has to program the drone - or send a human (bad for PR if the rocket fails, and they can't handle as much acceleration).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Not a win.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Perhaps you could decide to build one rocket and reuse it five times. Unfortunately, recycling a used rocket takes time, and it still does cost money (though less than a brand new rocket). Also, it's kind of a new thing and not very thoroughly tested, meaning you might not want to rely on this. You also don't save the cost of the fuel. Overall, you might save money (or not), but I'm afraid the extra time it takes makes the idea not worth it.&lt;/p&gt;&#xA;" OwnerUserId="604" LastEditorUserId="604" LastEditDate="2018-01-31T16:44:41.920" LastActivityDate="2018-01-31T16:44:41.920" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="24687" PostTypeId="2" ParentId="24651" CreationDate="2018-01-31T14:03:36.030" Score="6" Body="&lt;p&gt;As it turns out, perchlorate is actually rather unreactive under Martian surface conditions, and this actually explains the unusual abundance of perchlorate there:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;In any case, most of the chlorine has been oxidized as far as possible because the chlorine within perchlorate has a maximal +7 oxidation number. From a thermodynamic perspective, this makes perchlorate a strong oxidizing agent but kinetically, perchlorate is very inert at typical planetary surface temperatures. The energetically stable tetrahedral structure of oxygen atoms in perchlorate around the central, electrophilic chlorine atom renders perchlorate unreactive [Brown and Gu, 2006]. Hence, perchlorate, once formed, is prone to persist in soil provided that liquid water does not flush the perchlorate away, given that perchlorate salts are generally highly soluble. In contrast, if an environment is very wet, perchlorate is liable to be leached out of soil and end up diluted to trace concentrations in groundwater or rivers. In anaerobic conditions on Earth, the perchlorate is then microbially degraded back to chloride [Coates and Achenbach, 2004]. In short, perchlorate will accumulate in soil if its rate of deposition exceeds the rate of dissolution, and it will then persist if conditions remain free of perchlorate reduction. Thus, the oxidizing aridity of Mars, combined with perchlorate being an unreactive cul-de-sac of oxidation chemistry, probably explains the unusual abundance of perchlorate in Martian soil.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;-- &lt;a href=&quot;http://onlinelibrary.wiley.com/doi/10.1029/2009JE003425/full&quot; rel=&quot;noreferrer&quot;&gt;Atmospheric origins of perchlorate on Mars and in the Atacama&lt;/a&gt;; Catling, et al.&lt;/p&gt;&#xA;" OwnerUserId="58" LastActivityDate="2018-01-31T14:03:36.030" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24688" PostTypeId="1" AcceptedAnswerId="36641" CreationDate="2018-01-31T16:48:19.207" Score="9" ViewCount="408" Body="&lt;p&gt;Many people would like to know if any form of life could survive anywhere in the atmosphere of Venus.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Researchers have simulated conditions on Mars and found that after a month some cyanobacteria were still alive and active ! &lt;a href=&quot;http://www.planetary.org/blogs/guest-blogs/20120515-earth-life-survive-mars.html&quot; rel=&quot;noreferrer&quot;&gt;(Reference)&lt;/a&gt;    &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Is it not important that researchers could also study if CO2 consuming bacteria could thrive in some regions of the atmosphere of Venus by simulating the conditions in those regions ?&lt;br&gt;&#xA;Or has there been such research already ?  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Nostoc commune,a species of Cyanobacterium, could be a good candidate for the harsh conditions that have to be simulated. &lt;/p&gt;&#xA;" OwnerUserId="19335" LastEditorUserId="19335" LastEditDate="2018-02-02T16:02:59.203" LastActivityDate="2020-10-17T16:44:55.573" Title="Why not simulate certain regions of the atmosphere of Venus?" Tags="|atmosphere|venus|astrobiology|simulation|" AnswerCount="2" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24689" PostTypeId="1" AcceptedAnswerId="24699" CreationDate="2018-01-31T17:01:10.393" Score="3" ViewCount="612" Body="&lt;p&gt;I am trying to calculate the delta-v between Kerbin and Duna from a 70 km circular orbit around Kerbin just to do a flyby of Duna. However, I am running into a couple of difficulties with my calculations. Everything that I read tells me to deal with the heliocentric transfer orbit first, and that is where my problem lies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The periapsis of the transfer orbit between Kerbin and Duna is the orbit of Kerbin which is 13,599,840,256 m.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;the transfer apoapsis is Duna's semimajor axis: 20,726,155,264 m.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives a semimajor axis of the transfer orbit of 17,162,997,760 m.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To find the delta-v of the transfer, I need to find the velocity at periapsis of the transfer orbit and subtract Kerbin's orbital velocity from that.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Using $v^2=GM(2/r-1/a)$, I get a transfer velocity at periapsis of 10203 m/s.&#xA;Kerbin's orbital velocity is 9284.5 m/s.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This gives me a delta-v of 919 m/s. However, the delta-v maps that I am reading say it is 130 m/s from Kerbin escape to Duna intercept.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What am I doing wrong? Why do my numbers not match?&lt;/p&gt;&#xA;" OwnerUserId="22745" LastEditorUserId="195" LastEditDate="2018-01-31T18:30:05.507" LastActivityDate="2018-01-31T21:45:22.140" Title="How do I calculate the delta-v with patched conics?" Tags="|orbital-mechanics|orbital-maneuver|interplanetary|kerbal-space-program|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24691" PostTypeId="1" AcceptedAnswerId="24692" CreationDate="2018-01-31T18:31:16.107" Score="5" ViewCount="2338" Body="&lt;p&gt;Does it require the same delta-v to reach the same heliocentric orbit that Mars has as it does to reach Mars? Ignoring entering Mars' atmosphere, is the difference only timing of the launch?&lt;/p&gt;&#xA;" OwnerUserId="22747" LastActivityDate="2018-02-01T21:12:23.387" Title="What is the difference between a Trans-Mars Injection orbit and a Mars Orbit Insertion?" Tags="|mars|" AnswerCount="1" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24692" PostTypeId="2" ParentId="24691" CreationDate="2018-01-31T18:45:43.030" Score="6" Body="&lt;p&gt;Your question hinges on what you mean by &quot;reach Mars&quot;. Trans-Mars &lt;em&gt;injection&lt;/em&gt; takes you to Mars' orbit, and if your timing is right, takes you close to Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Mars orbit insertion is a &lt;em&gt;separate&lt;/em&gt; maneuver, executed close to Mars, which leaves you in orbit around Mars. Without the orbit insertion, you will fly by Mars at about 4800 m/s without stopping. To get into orbit around Mars, you have to do both. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;(Besides &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerocapture&quot; rel=&quot;nofollow noreferrer&quot;&gt;aerocapture&lt;/a&gt;, there's also a trick called &lt;a href=&quot;https://en.wikipedia.org/wiki/Ballistic_capture&quot; rel=&quot;nofollow noreferrer&quot;&gt;ballistic capture&lt;/a&gt; which can achieve a very high and barely-stable orbit around your destination without an insertion burn, but it's not useful in itself for planetary exploration.)&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-02-01T21:12:23.387" LastActivityDate="2018-02-01T21:12:23.387" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24693" PostTypeId="2" ParentId="19232" CreationDate="2018-01-31T19:45:24.887" Score="3" Body="&lt;p&gt;On the Saturn V, if all engines are operative at Max Thrust at T=0, the Lock Down Arms detach from the Rocket Body in the First Stage of the &quot;Soft Release&quot;. When this is confirmed on the ground, an engineer in the Control Room yells &quot;Launch Commit!&quot;. A second or so later, When positive movement off the pad is established, the Commentator calls &quot;Liftoff&quot;. &lt;/p&gt;&#xA;" OwnerUserId="22748" LastActivityDate="2018-01-31T19:45:24.887" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24694" PostTypeId="1" CreationDate="2018-01-31T19:58:39.413" Score="44" ViewCount="30883" Body="&lt;p&gt;Which is closer to Mars, Earth or the Moon? &lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm writing a novel and I'm so grateful for the wealth of information. The only answers I find refer to Phobos and Demos Mars moons.&lt;/p&gt;&#xA;" OwnerUserId="22686" LastEditorUserId="25" LastEditDate="2018-02-03T13:20:44.610" LastActivityDate="2018-02-05T17:58:57.547" Title="Which is closer to Mars, Earth or the Moon?" Tags="|orbital-mechanics|mars|the-moon|" AnswerCount="10" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24695" PostTypeId="2" ParentId="24683" CreationDate="2018-01-31T20:10:47.397" Score="0" Body="&lt;p&gt;I will not discuss the use of smaller or bigger space crafts, that is already done here by others. I will discuss the use of drones.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Drones will never make it into space. At least not a drone with propeller blades or wings. The blades need air to generate some lift. And the higher you get, the thinner the air is. So there will always be a 'ceiling' where a drone can get, ignoring the amount of energy it needs to get that high (bigger batteries = bigger drones = bigger batteries, etc.). The lack of air is why airplanes cannot make it to space either.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;AFAIK there are only two ways of escaping from earth: with a deflating balloon (propulsion) or a canon ball (no propulsion). A canon ball needs to get the &quot;&lt;a href=&quot;https://en.wikipedia.org/wiki/Escape_velocity&quot; rel=&quot;nofollow noreferrer&quot;&gt;escape velocity&lt;/a&gt;&quot; which is for earth a little more than 11 km/s. This is not practical, since you will have to release all the energy at once to the space craft. So it has to withstand a huge acceleration (and forces). Although you do not need to reach the escape velocity when you go into orbit (but you will get close), the principles are the same.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;That leaves only a rocket as being usable for escaping the earth. A rocket can release the energy it needs to escape the earth in a controlled manner. It doesn't need air to get lift, but it exhausts gases to propel itself.&lt;/p&gt;&#xA;" OwnerUserId="22749" LastEditorUserId="22749" LastEditDate="2018-02-02T08:35:09.997" LastActivityDate="2018-02-02T08:35:09.997" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24696" PostTypeId="2" ParentId="24694" CreationDate="2018-01-31T20:12:14.190" Score="127" Body="&lt;p&gt;As you said, it varies.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Imagine I'm in Chicago and you're in London.  My little dog is running circles around me.  Which is closer to you, me or my dog?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;While the correct answer is &quot;it depends on where the dog is in its orbit around me&quot;, I'd argue that a better answer is &quot;it doesn't matter&quot; - the distance between you and me is so great that any little extra distance one way or the other is essentially nothing more than a rounding error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For purposes of your novel, &lt;strong&gt;the Earth and the Moon are the same distance from Mars&lt;/strong&gt;&lt;/p&gt;&#xA;" OwnerUserId="12978" LastActivityDate="2018-01-31T20:12:14.190" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24697" PostTypeId="2" ParentId="24694" CreationDate="2018-01-31T20:38:10.117" Score="11" Body="&lt;p&gt;To get a sense of distances in the solar system: &lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;find a &lt;a href=&quot;https://en.wikipedia.org/wiki/Solar_System_model&quot; rel=&quot;noreferrer&quot;&gt;planet walk&lt;/a&gt; in your area&lt;/li&gt;&#xA;&lt;li&gt;or go to this page and prepare to &lt;a href=&quot;http://www.joshworth.com/dev/pixelspace/pixelspace_solarsystem.html&quot; rel=&quot;noreferrer&quot;&gt;scroll your way across the solar system&lt;/a&gt;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;The distance Earth-Moon is 380 thousand km. The distance Earth-Mars &lt;a href=&quot;https://www.universetoday.com/14824/distance-from-earth-to-mars/&quot; rel=&quot;noreferrer&quot;&gt;varies between 50 and 400 &lt;em&gt;million&lt;/em&gt; km&lt;/a&gt;, i.e. 3 orders of magnitude more. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-01-31T20:38:10.117" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24698" PostTypeId="2" ParentId="24694" CreationDate="2018-01-31T21:14:26.603" Score="107" Body="&lt;p&gt;&lt;strong&gt;Traveling from Mars's surface to Earth's surface requires less energy than traveling from Mars's surface to Luna's surface, but traveling from Luna's surface to Mars's surface requires &lt;em&gt;much&lt;/em&gt; less energy than traveling from Earth's surface to Mars's surface.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the purposes of space-travel, the actual physical distance is much less important than the relative energy needed for the trip. This energy is called &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v&quot; rel=&quot;noreferrer&quot;&gt;Δ&lt;em&gt;v&lt;/em&gt;&lt;/a&gt; (&quot;delta-vee&quot;) and is measured in km/s:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;\begin{array}{c c|c c}&#xA;&amp;amp; \Delta\textbf{V} &amp;amp; \textbf{D} \hskip{1em} \textbf{E} \hskip{1em} \textbf{S} &amp;amp; \textbf{T} \hskip{1em} \textbf{I} \hskip{1em} \textbf{N} \hskip{1em} \textbf{A} \hskip{1em} \textbf{T} &amp;amp; \textbf{I} \hskip{1em} \textbf{O} \hskip{1em} \textbf{N} \\&#xA;&amp;amp; \textbf{(km/s)} &amp;amp; \textbf{Earth} &amp;amp; \textbf{Luna} &amp;amp; \textbf{Mars} \\ \hline&#xA;\textbf{S} &amp;amp; \textbf{Earth} &amp;amp; - &amp;amp; 16.1 &amp;amp; 13.5^* \\&#xA;\textbf{T} \\&#xA;\textbf{A} &amp;amp; \textbf{Luna} &amp;amp; 2.3^* &amp;amp; - &amp;amp; 2.9^* \\&#xA;\textbf{R} \\&#xA;\textbf{T} &amp;amp; \textbf{Mars} &amp;amp; 6.4^* &amp;amp; 9.3 &amp;amp; - \\&#xA;\end{array}&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Values are approximate and assume optimal routes.&lt;br&gt;&#xA;$^*$ indicates maximum utilisation of &lt;a href=&quot;https://en.wikipedia.org/wiki/Aerobraking&quot; rel=&quot;noreferrer&quot;&gt;aerobraking&lt;/a&gt;.&lt;br&gt;&#xA;Actual energy expenditure will be more.&lt;br&gt;&#xA;&lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget#Delta-vs_between_Earth,_Moon_and_Mars&quot; rel=&quot;noreferrer&quot;&gt;Source&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If the characters and/or goods in your novel need to round-trip (or at least there is an approximately equal transfer in each direction), then since Mars-Luna-Mars costs 12.2 km/s and Mars-Earth-Mars costs 19.9 km/s, &lt;strong&gt;Luna's surface is significantly closer to Mars's surface than to Earth's surface.&lt;/strong&gt; Note that in practice, landing on Mars requires retro-propulsion, but that overhead will be approximately equal for travelers arriving from Earth and Luna, so the round-trip comparison stays the same.&lt;/p&gt;&#xA;" OwnerUserId="14585" LastEditorUserId="14585" LastEditDate="2018-02-05T17:58:57.547" LastActivityDate="2018-02-05T17:58:57.547" CommentCount="23" ContentLicense="CC BY-SA 3.0" />
  <row Id="24699" PostTypeId="2" ParentId="24689" CreationDate="2018-01-31T21:45:22.140" Score="5" Body="&lt;p&gt;The numbers on the map are not actually independent, you should treat it as saying &quot;950+130&quot; from LKO to Duna flyby.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The number you got, 919 m/s, is the amount of dV you'd need if you were going for a Duna intercept from an orbit similar to Kerbin's, but not actually from Kerbin. The number on the map, 1080 m/s, is a single burn at the right point in the right direction from LKO to Duna intercept. This is taking into account your orbital velocity around Kerbin, Kerbin's gravity and Oberth gains from doing the burn in a gravity well.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;We should actually be able to calculate an optimistic estimate of that number:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;&lt;p&gt;You already calculated that at the point of leaving Kerbin's sphere of influence (SOI), we should be going 919 m/s relative to Kerbin.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Since this is patched conics, we have a discrete point of changing SOI - 84 159 271 m, at which we should be going at 919 m/s.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Our starting point is LKO, which, from the top of my head, I'm going to say is 80 km high (for an orbital radius of 680 000 m) at a speed of 2 300 m/s.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;At this point we use the orbital energy conservation equation on step 2, so the orbit energy is $\epsilon = v^2/2 - \mu/r = 380317$.&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;Plugging this into step 3, we know that in LKO $\epsilon_p = 5193527$, so $v = \sqrt{2(\epsilon+\epsilon_p)} = 3339 m/s$&lt;/p&gt;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;The estimated required delta-V is then $3339 - 2300 = 1039 m/s$&lt;/p&gt;&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;Our number does not match the map exactly, but it is an optimistic estimate. Then again, the KSP wiki suggests that &lt;a href=&quot;https://wiki.kerbalspaceprogram.com/wiki/Kerbin#Orbits&quot; rel=&quot;noreferrer&quot;&gt;we need 1060 m/s to get to Duna&lt;/a&gt;, not 1080 m/s. And there will probably be inefficiencies in implementation, e.g. the burn will not be instant, the starting orbit will vary a bit, we probably won't be going in exactly the right direction upon leaving Kerbin's SOI, and we'll probably be at a bit lower periapsis than Kerbin's. These will likely eat up 20-40 m/s.&lt;/p&gt;&#xA;" OwnerUserId="17667" LastActivityDate="2018-01-31T21:45:22.140" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24700" PostTypeId="2" ParentId="8801" CreationDate="2018-02-01T00:23:57.807" Score="6" Body="&lt;p&gt;Well, they seem to have managed it, albeit by accident, today (31 January 2018).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX were testing a 3-engine suicide burn with a booster they didn't plan to reuse, so they were landing on the sea rather than the droneship.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And, well, it wasn't destroyed on contact with the ocean:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://twitter.com/elonmusk/status/958847818583584768&quot; rel=&quot;noreferrer&quot;&gt;Elon Musk's twitter with a remarkable photo&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="22754" LastActivityDate="2018-02-01T00:23:57.807" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24701" PostTypeId="2" ParentId="24665" CreationDate="2018-02-01T01:33:10.470" Score="1" Body="&lt;p&gt;This question seems to be aimed at two different things a) what is possible and b) what has commonly happened.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Its quite correct that electric propulsion has been used for all of the following purposes:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Life extension&lt;/li&gt;&#xA;&lt;li&gt;Launch mass reduction&lt;/li&gt;&#xA;&lt;li&gt;Flying more payload mass for the same launch mass and lifetime&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;There are no general rules to this and it is certainly not true that mass saving has been more common than payload maximisation or life extension. There are only a couple of examples of clear mass reduction and the large bulk of previous missions are rather more mixed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The following examples address each of these points and in some cases its hard to tell which of the purposes was adopted as the driving reason to use electric propulsion (EP). By the way, for that term we can include gridded ion thrusters, hall effect thrusters and also hydrazine arcjets and resistojets.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;For the last fifteen years or more a 15 year life has been the standard adopted for most commercial geostationary communications satellite. A key life-limiting feature is inclination drift and so satellites with EP for North South station-keeping often have a larger propellant life margin than those with chemical propulsion only, often of the order of 5 - 15 years over the initial 15. i.e. there &lt;strong&gt;really are&lt;/strong&gt; satellites with &lt;strong&gt;25-30&lt;/strong&gt; year propellant lives! Recently longer design lifetimes have begun to appear as the baseline mission length, though typically this has only crept up a year or so from 15. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The mass reduction argument has changed recently.  For NSSK over the last decades it depends upon the EP type. The satellites that have gridded ion engines and hall effect EP for NSSK only have tended to be the larger ones, in the range 4 - 6 tonnes. The point is that the overhead of using the higher Isp systems for NSSK is such that its more worthwhile for larger satellites. Satellites using Arcjets or Resistojets have been smaller.     &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the last couple of years, with the introduction of EP for GTO-GEO transfer there have been different flight examples. &lt;strong&gt;ABS 3A&lt;/strong&gt; and &lt;strong&gt;Eutelsat 115 West B&lt;/strong&gt; achieved a given payload at a small mass to get on a dual launch on a Falcon 9. &lt;strong&gt;Viasat 2&lt;/strong&gt; appears to have gone for a maximum payload.&lt;/p&gt;&#xA;" OwnerUserId="10791" LastActivityDate="2018-02-01T01:33:10.470" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24702" PostTypeId="1" AcceptedAnswerId="24720" CreationDate="2018-02-01T01:36:27.557" Score="6" ViewCount="196" Body="&lt;p&gt;I'm piecing together a chronological timeline of Mir station including manned and unmanned visits to the ship, time of docking and undocking, and location of docking.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;My question is around the docking location of the Soyuz spaceflights and the Progress resupply missions to the station.  It would appear from the information I've gathered so far that (nearly) every flight to station had the Soyuz docking with the end of the Kvant-1 or the Core aft, with four exceptions:&lt;/p&gt;&#xA;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;Soyuz TM-16 docking with Kristall to test the APAS docking system for upcoming Shuttle visit&lt;/li&gt;&#xA;&lt;li&gt;Soyuz T-15, Soyuz TM-1, and Soyuz TM-2 docking with the Core forward port (Kvant-1 hadn't been docked yet)&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;&#xA;&lt;p&gt;It seems most Progress flights docked with Kvant-1 after it was installed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Was there a common location to each flight?&#xA;Is there a definitive and reliable source for docking locations of each flight to the station I can use for reference?&lt;/p&gt;&#xA;" OwnerUserId="6773" LastActivityDate="2018-02-01T20:00:20.247" Title="Where did Soyuz and Progress dock with Mir?" Tags="|crewed-spaceflight|soyuz-spacecraft|docking|mir|progress|" AnswerCount="1" CommentCount="6" ContentLicense="CC BY-SA 3.0" />
  <row Id="24703" PostTypeId="1" AcceptedAnswerId="24704" CreationDate="2018-02-01T03:36:25.357" Score="14" ViewCount="2903" Body="&lt;p&gt;Supposedly SpaceX did not intend to recover the first stage of the GovSat-1 launch. If so, what was the point of doing a re-entry burn and a landing burn, if they were just planning on dumping the rocket in the ocean anyway?&lt;/p&gt;&#xA;" OwnerUserId="22757" LastEditorUserId="6537" LastEditDate="2018-02-02T10:03:47.680" LastActivityDate="2018-02-02T10:03:47.680" Title="What was the point of the re-entry and landing burn of the first stage of the GovSat-1 launch?" Tags="|spacex|reuse|first-stage|recovery|" AnswerCount="2" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24704" PostTypeId="2" ParentId="24703" CreationDate="2018-02-01T03:46:50.273" Score="20" Body="&lt;p&gt;SpaceX was testing a more efficient, faster landing technique. The test produced useful telemetry data. See Musk's &lt;a href=&quot;https://twitter.com/elonmusk/status/958847818583584768/photo/1&quot; rel=&quot;nofollow noreferrer&quot;&gt;tweet&lt;/a&gt; and photo.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Normally, the terminal phase of Falcon 9 first-stage landing finishes with a single engine firing. This attempt used three engines for the landing. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;By firing three engines instead of one, the final deceleration of the stage can be performed over a shorter period of time, which provides a net fuel savings, because gravity has less time to act on the stage. Less fuel needed = more mass budget available for the payload. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, in this mode, everything is happening faster, and the stage has less time to correct for any trajectory errors. SpaceX didn't want to put a barge under it for this test, because of the possibility of a hard landing that would damage the barge; they have only &lt;em&gt;Of Course I Still Love You&lt;/em&gt; in the Atlantic, and are planning to land the center core of the Falcon Heavy on it in the test flight scheduled for February 6th -- which, come to think of it, is likely too short a time frame to bring the landed stage back, clean up, and go back out to catch the Falcon Heavy even if there wasn't any damage.&lt;/p&gt;&#xA;" OwnerUserId="195" LastEditorUserId="195" LastEditDate="2018-02-01T21:07:10.640" LastActivityDate="2018-02-01T21:07:10.640" CommentCount="8" ContentLicense="CC BY-SA 3.0" />
  <row Id="24705" PostTypeId="2" ParentId="24694" CreationDate="2018-02-01T04:13:40.563" Score="45" Body="&lt;p&gt;In terms of distance, the two swap considerably. But perhaps a more interesting question is, which of the two is closer in terms of the energy required to land. For that, let's look at our friend, the &lt;a href=&quot;https://en.wikipedia.org/wiki/Delta-v_budget&quot; rel=&quot;noreferrer&quot;&gt;delta-v table&lt;/a&gt;. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/iYZNk.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/iYZNk.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Once one is approaching Earth from Mars, things only become different at the point labeled&lt;code&gt;Earth C3=0&lt;/code&gt; (See &lt;a href=&quot;https://space.stackexchange.com/a/4057/12102&quot;&gt;$C_3$&lt;/a&gt;). From there it is about 2.3 km/s to land on the Moon. To land on Earth requires no rocket propellant, as it can all be lost via the atmosphere, thus it is easier to get from Mars to Earth's Surface than to the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the reverse direction, it is far easier to get from the Moon to Mars then it is from Earth to Mars. The energy required to leave the Earth is considerable, while it isn't that much from the Moon, relatively speaking.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="25" LastEditDate="2018-02-02T10:34:04.997" LastActivityDate="2018-02-02T10:34:04.997" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24706" PostTypeId="1" CreationDate="2018-02-01T04:15:31.613" Score="0" ViewCount="116" Body="&lt;p&gt;Using the Humanity Star (see this &lt;a href=&quot;https://space.stackexchange.com/a/24655/12102&quot;&gt;answer&lt;/a&gt; for example for background/context), consider two possibilities for spinning a small spherical shell at a low rate, say perhaps in the neighborhood of 2 to 10 RMP.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To spin a spacectraft in LEO, one could use tangential thrusters as discussed in &lt;a href=&quot;https://space.stackexchange.com/q/19436/12102&quot;&gt;How did the tangential thrusters for the 2014 LDSD test spin-up then spin-down so nicely?&lt;/a&gt; and also &lt;a href=&quot;https://space.stackexchange.com/q/16944/12102&quot;&gt;What ever happened to SpinSat - did it work?&lt;/a&gt; for example, or one could use a battery, motor, and flywheel. In either case a timer or signal would activate the spin-up once it is safely in orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What would be the tradeoffs for each? Perhaps mass, reliability, safety? Are there others?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Let's assume it's a 1.3 meters in diameter shell (most mass at the surface) perhaps a mass of 10 kg.&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="6816" LastEditDate="2018-02-01T06:43:59.720" LastActivityDate="2018-02-01T21:00:26.913" Title="Tradeoffs using a battery/motor/gyro versus tangential thrusters to spin a spherical shell in space?" Tags="|artificial-satellite|space-art|attitude-determination-and-control|humanitystar|" AnswerCount="2" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24708" PostTypeId="1" CreationDate="2018-02-01T08:51:00.303" Score="8" ViewCount="540" Body="&lt;p&gt;Beyond theory, are any organizations doing experimental work on Mars terraforming? In laboratory environments.&lt;/p&gt;&#xA;" OwnerUserId="22686" LastEditorUserId="21562" LastEditDate="2018-03-02T17:02:45.203" LastActivityDate="2018-03-04T20:45:07.797" Title="Is there any experimental work being done on terraforming?" Tags="|mars|terraforming|" AnswerCount="3" CommentCount="4" ContentLicense="CC BY-SA 3.0" />
  <row Id="24709" PostTypeId="2" ParentId="24694" CreationDate="2018-02-01T09:17:15.747" Score="0" Body="&lt;p&gt;Earth and Mars both orbit the Sun - because Mars is further out, its orbital path/circle is slightly larger.  So think of two circles on a piece of paper - the slightly larger one enveloping the smaller is like Mars' orbit around the sun compared to Earth's orbit around the Sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now for the moon - while Earth is orbiting the Sun in its &quot;circle&quot;, the moon is making a bunch of laps around Earth in a small circle.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Now on the paper diagram, you can pick a point on Earth's orbital circle and draw a small circle around that point.  This represents the moon's orbit around Earth.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;With all this, it's clear now that the moon and Earth take turns being closer to Mars.  When the moon is in between the Sun and Earth on its mini-circle, Earth is obviously closer to Mars.  Conversly, when the moon is on the other side of its circle (putting it in between Earth's and Mars' big circles) it is obviously closer to Mars than Earth is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;So it really depends on how Earth, Mars, and the moon happen to be positioned in their orbits at any given time.  &lt;em&gt;On average&lt;/em&gt; though, the Moon and Earth are about an equal distance from Mars.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Best of luck on your novel!&lt;/p&gt;&#xA;" OwnerUserId="22763" LastActivityDate="2018-02-01T09:17:15.747" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24710" PostTypeId="1" CreationDate="2018-02-01T09:45:10.083" Score="5" ViewCount="567" Body="&lt;p&gt;The following graph shows the Tiangong-1 radius vector at the perigee, semi-major axis and apogee (scaled to 6371 km just to show an approximate altitude) and its orbit eccentricity:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/wk4tK.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/wk4tK.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I used the SGP4 propagator to do all the calculations and 1 dot is 1 TLE.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;There is a big discontinuity on 2018-01-27 (706 m on the apogee).&#xA;It can also be clearly seen a big change in the apogee decay rate.&#xA;Finally, there are no TLEs for the day 30 (as if the tracking data were uncorrelated).&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I can only say that all the three “strangeness” are generated by the Tiangong-1 RCS engines or vernier engines (the acceleration is very small).&#xA;Is that possible? Can anyone give me another explanation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;EDIT: here is another suspect change:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/uiYia.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/uiYia.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The graph shows the orbit inclination (the min and max is because of the earth's oblateness). It’s clearly showed a sudden change exactly were the eccentricity changes and the new inclination is confirmed by (at least) 4 TLEs. I’m not able to give any explanation for that other than engines firing.&lt;/p&gt;&#xA;" OwnerUserId="20397" LastEditorUserId="20397" LastEditDate="2018-02-02T17:37:55.130" LastActivityDate="2018-04-01T17:58:58.417" Title="Did Tiangong-1 turn the engines on?" Tags="|orbital-mechanics|orbit|attitude|space-station|tiangong-1|" AnswerCount="2" CommentCount="7" ContentLicense="CC BY-SA 3.0" />
  <row Id="24711" PostTypeId="2" ParentId="24708" CreationDate="2018-02-01T11:08:16.037" Score="4" Body="&lt;p&gt;What do you mean &quot;beyond research&quot; ? Such studies are done in laboratories, or they are just games. You have a few &quot;societies&quot;, such as the Mars Society, where people try to advance in such studies. Farming is a good subject, quite an easy one, lost cost, still important, and can eventualy not require huge instrumentation. Therefore, it has focussed some attention and still keeps going on.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At a less serious level, last year, I proposed such an experiment to young students (14-15) that were interested to do some experiments. They proceeded at one, about resistance to an important daily variation of temperature. On Mars the difference between night and day is about 70 to 80°C. We did a few &quot;studies&quot; between sub-0° at night and about 25 to 30°C at day, starting with different type of seeds, and chosing a comparative approach with witnesses. No major discovery ;-) Apart a cute intro for these young student to science reasoning :-).&lt;/p&gt;&#xA;" OwnerUserId="19714" LastEditorUserId="19714" LastEditDate="2018-03-01T23:02:48.770" LastActivityDate="2018-03-01T23:02:48.770" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24712" PostTypeId="2" ParentId="24694" CreationDate="2018-02-01T11:19:47.930" Score="80" Body="&lt;h1&gt;Distances to Mars&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;You can answer the question with &lt;a href=&quot;http://www.astropy.org/&quot; rel=&quot;noreferrer&quot;&gt;Astropy&lt;/a&gt;, a Python library for astronomy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a diagram of distances from Earth to Mars and Moon to Mars, between 2000 and 2030.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/cBBDC.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/cBBDC.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You can see that the two curves are so close from each other that they look like one single curve.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Here's a zoom around the last &lt;a href=&quot;https://mars.nasa.gov/allaboutmars/nightsky/opposition/&quot; rel=&quot;noreferrer&quot;&gt;opposition&lt;/a&gt;, in &lt;a href=&quot;http://earthsky.org/space/mars-opposition-may-22-2016-how-to-see&quot; rel=&quot;noreferrer&quot;&gt;May 2016&lt;/a&gt;:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/4KHLB.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/4KHLB.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Relative difference&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;Following @gerrit's advice, here's a plot showing the relative difference between the distances Moon-Mars and Earth-Mars. The envelope of the curve oscillates between ±0.089% (during conjunction, when Mars is the furthest) and ±0.69% (during opposition). This difference is positive when Earth is closer to Mars than the Moon.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/zwm8N.png&quot; rel=&quot;noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/zwm8N.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;During this period (2010-2020), Earth is closest 49.6% of the time. Over a longer period of time, this percentage gets very close to 50%.&lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Code&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;For reference, here's the code that's been used for the first diagram:&lt;/p&gt;&#xA;&#xA;&lt;pre&gt;&lt;code&gt;from astropy.time import Time&#xA;from astropy.coordinates import solar_system_ephemeris, get_body&#xA;&#xA;import numpy as np&#xA;from datetime import datetime, timedelta&#xA;&#xA;import matplotlib.pyplot as plt&#xA;&#xA;start_time = datetime(2000,1,1)&#xA;end_time = datetime(2030,1,1)&#xA;time_step = timedelta(days=1)&#xA;&#xA;times = np.arange(start_time, end_time, time_step).astype(datetime)&#xA;astro_times = Time(times)&#xA;with solar_system_ephemeris.set('builtin'):&#xA;    mars = get_body('mars', astro_times)&#xA;    earth = get_body('earth', astro_times)&#xA;    moon = get_body('moon', astro_times)&#xA;earth_to_mars = earth.separation_3d(mars).AU&#xA;moon_to_mars = moon.separation_3d(mars).AU&#xA;&#xA;plt.plot(times, earth_to_mars, '--', label='From Earth')&#xA;plt.plot(times, moon_to_mars, '--', label='From Moon')&#xA;plt.legend()&#xA;&#xA;plt.xlabel('Time')&#xA;plt.ylabel('Distance (AU)')&#xA;plt.title('Distance to Mars')&#xA;&#xA;plt.savefig('earth_moon_mars.png')&#xA;plt.show()&#xA;&lt;/code&gt;&lt;/pre&gt;&#xA;" OwnerUserId="19229" LastEditorUserId="19229" LastEditDate="2018-02-03T11:03:41.137" LastActivityDate="2018-02-03T11:03:41.137" CommentCount="10" ContentLicense="CC BY-SA 3.0" />
  <row Id="24715" PostTypeId="2" ParentId="24710" CreationDate="2018-02-01T13:54:43.543" Score="3" Body="&lt;p&gt;&lt;strong&gt;Edit:&lt;/strong&gt; The question has been updated and the plot showing a &quot;blip&quot; in inclination which has the same shape as the &quot;blib&quot; in eccentricity. I've incorporated my comments here:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The only phenomenon I'm really proposing is that when limited measure data is fitted and parameters are extracted, there are sources of noise in the extracted parameters, and the noise can be enhanced when there are statistical correlations between parameters. Put simply, a simultaneous change in two (or more) parameters in a correlated way can result in a nearly identical &lt;a href=&quot;https://en.wikipedia.org/wiki/Goodness_of_fit&quot; rel=&quot;nofollow noreferrer&quot;&gt;goodness of fit&lt;/a&gt;, and this can amplify the ever-present noise in the initial measurements.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;You've now flagged &lt;em&gt;two parameters that changed at the same time&lt;/em&gt; (eccentricity and inclination on 28-01-2018) and &lt;em&gt;in a beautifully correlated way&lt;/em&gt;. The &quot;blip&quot; in inclination matches the shape of the blip in eccentricity. This is a &lt;em&gt;textbook example of correlation in fitted parameters!&lt;/em&gt; Applying &lt;a href=&quot;https://en.wikipedia.org/wiki/Occam%27s_razor&quot; rel=&quot;nofollow noreferrer&quot;&gt;Occam's razor&lt;/a&gt; to the choice between a well-understood mathematical behavior when fitting imperfect, under-sampled, noisy data, and conspiracy theories of &quot;secret spacecraft reactivations&quot;, and what's going to be the most likely explanation?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The way to proceed in a systematic and scientific way would be for you to look at all parameters (rather than &quot;revealing them&quot; one at a time), plotted over much longer periods of time, for many spacecraft in similar decaying orbits. I am guessing you will find blips and wiggles are the rule, rather than the exception. Public TLEs are approximate. They are provided as &lt;em&gt;guidelines&lt;/em&gt; for judging roughly where spacecraft are likely to be with many km uncertainty at epoch, and more when propagating to other times. They have never been more than that, and this has been stated time and again.&lt;/p&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;hr&gt;&#xA;&#xA;&lt;p&gt;Probably not. This kind of noise, at the level of 700 meters in SPG4 propagated orbits, probably happens all the time.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;See the discussion in &lt;a href=&quot;https://space.stackexchange.com/a/24069/12102&quot;&gt;this answer&lt;/a&gt;; it seems to be the same exact phenomenon. There is a &quot;blipping&quot; of the eccentricity, but not a change in the semimajor axis, so the periapsis goes one way, and the apoapsis goes the opposite way, then everything goes back to the way it was before the blip (except of course for the steady decay in this case). The explanation is probably the same as well; a small error in position can always happen, especially if at a point in the orbit where there are statistical correlations between fitted parameters. As you point out, the positional error is only several hundred meters. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;An error that large would be &lt;em&gt;much less likely&lt;/em&gt; in the semimajor axis, because this is very strongly linked to mean motion. As your data the data for the object shown below both show, that doesn't happen. A mis-measurement of a few hundred meters than change the fitted eccentricity easily, but since satellites are observed over and over, timing and phasing but a much stronger lock on the semimajor axis.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll bet if one were to start looking, noise and blips in eccentricity is probably fairly common. It's likely a non-event. &lt;strong&gt;EDIT:&lt;/strong&gt; I started looking, and sure enough, it's quite noisy!&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'll add some more about ellipticity noise in a few minutes...&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; This plot from &lt;a href=&quot;https://space.stackexchange.com/a/24069/12102&quot;&gt;here&lt;/a&gt; shows a &lt;strong&gt;much larger&lt;/strong&gt; blip in eccentricity for another spacecraft In this case it's maybe 20x larger than that shown in the question. By coincidence, in this plot Tiangong-1 was used as a reference, and does not show significant noise at this scale.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is data from about 01-oct-2017 to 31-dec-2017:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/NPeCb.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/NPeCb.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;below:&lt;/strong&gt; This is only the Tiangong-1 data, showing the change in peri, apo, semi (top, scale = km) and eccentricity (bottom) from one TLE to the next, over the same three months.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This shows that noise at the 1km scale for Tiangong-1 is commonplace.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/6IyJW.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/6IyJW.png&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="12102" LastEditorUserId="12102" LastEditDate="2018-04-01T00:47:39.700" LastActivityDate="2018-04-01T00:47:39.700" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24716" PostTypeId="2" ParentId="24706" CreationDate="2018-02-01T15:21:34.890" Score="2" Body="&lt;p&gt;In general, mechanisms such as reaction wheels are the most &quot;undesired&quot; parts of a spacecraft as they are statistically the ones most prone to failure. This is why they are typically used for station-keeping requirements like instrument pointing and attitude corrections. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the other hand, small solid boosters are relatively inexpensive (compared to mechanisms), much more reliable and flight proven. They also do not require batteries, a bus and perhaps solar arrays. Thus, if the objective is to only spin-up and/or spin down a spherical spacecraft, this would be the obvious choice. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Finally, as a side note, for spin-stabilised or other satellites that require spinning to fulfil their mission, the spin is usually provided by the upper stage of the launch vehicle before it releases the payload. &lt;/p&gt;&#xA;" OwnerUserId="22773" LastActivityDate="2018-02-01T15:21:34.890" CommentCount="5" ContentLicense="CC BY-SA 3.0" />
  <row Id="24717" PostTypeId="1" CreationDate="2018-02-01T16:30:51.423" Score="27" ViewCount="6181" Body="&lt;p&gt;Most commercial satellites are insured in case something &lt;strong&gt;bad&lt;/strong&gt; happens on launch. Although the Ariane 5 is a fairly reliable rocket, a launch failure is not out of the question.&lt;/p&gt;&#xA;&lt;p&gt;While &lt;a href=&quot;https://space.stackexchange.com/q/4902/22775&quot;&gt;other questions&lt;/a&gt; have said that if the JWST fails to deploy it's basically SOL (&amp;quot;Sadly Out of Luck&amp;quot;) and I would assume uninsurable, rocket launches are regularly insured. Is the JWST insured against the failure of the Ariane 5?&lt;/p&gt;&#xA;&lt;p&gt;For more info see:&lt;/p&gt;&#xA;&lt;ul&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.wired.com/2017/03/yup-rockets-need-insurance-way-feds-think/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wired, 31 March 2017: &lt;em&gt;Yup, Rockets Need Insurance, Too. But Way More Than the Feds Think&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://www.reuters.com/article/us-space-insurance/space-insurance-costs-to-rocket-after-satellite-crash-idUSKCN1UQ1SK&quot; rel=&quot;nofollow noreferrer&quot;&gt;Reuters, 31 July 2019: &lt;em&gt;Space insurance costs to rocket after satellite crash&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://en.wikipedia.org/wiki/Satellite_insurance&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wikipedia: &lt;em&gt;Satellite insurance&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;li&gt;&lt;p&gt;&lt;a href=&quot;https://observer.com/2019/09/space-insurance-rocket-satellite-industry-analysis/&quot; rel=&quot;nofollow noreferrer&quot;&gt;Observer.com, 20 September 2019: &lt;em&gt;A Closer Look at Rocket Insurance, a Peculiar Branch of the Space Industry&lt;/em&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;/li&gt;&#xA;&lt;/ul&gt;&#xA;" OwnerUserId="22775" LastEditorUserId="45864" LastEditDate="2021-12-25T17:09:23.913" LastActivityDate="2021-12-25T17:09:23.913" Title="Will the James Webb Space Telescope be insured against launch failure?" Tags="|launch|failure|james-webb-telescope|ariane|insurance|" AnswerCount="2" CommentCount="10" ContentLicense="CC BY-SA 4.0" />
  <row Id="24718" PostTypeId="2" ParentId="24717" CreationDate="2018-02-01T16:43:25.680" Score="41" Body="&lt;p&gt;NASA tends not to insure its missions, nor do any government missions. These missions are one-of-a-kind, and so expensive that the satellite insurance market would have a hard time making it work. They simply triple-check everything they can, and expect to lose a few missions, so called &quot;Self-insurance&quot;. They have considered insuring things like the &lt;a href=&quot;https://motherboard.vice.com/en_us/article/wnjyjb/nasa-once-considered-insuring-the-international-space-station&quot; rel=&quot;noreferrer&quot;&gt;ISS&lt;/a&gt; before, but it's never worked out.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Just to give you an idea, the cost of insurance is about 10-15% of the insurance amount for the space market. The success rate of Space Shuttle launches was 99%. Since 2000, there has been 19 successful NASA interplanetary mission launches, and 1 failure (CONTOUR). &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Basically, it would cost NASA more to insure the missions then the benefit it would gain from insurance, and thus they have chosen not to insure their missions.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, the requirements tend to change for missions over time. It might take another 4 years to re-build the spacecraft, and another year or two waiting for the right opportunity to launch. In that length of time, things will have changed considerably.&lt;/p&gt;&#xA;" OwnerUserId="25" LastEditorUserId="22796" LastEditDate="2018-02-02T13:38:50.313" LastActivityDate="2018-02-02T13:38:50.313" CommentCount="12" ContentLicense="CC BY-SA 3.0" />
  <row Id="24719" PostTypeId="2" ParentId="24717" CreationDate="2018-02-01T17:13:23.023" Score="16" Body="&lt;p&gt;Insurance is done when losing a mission would mean an unacceptable financial loss, e.g. when a launch failure would bankrupt your company. The government is large enough to absorb such losses, so no insurance is necessary. &lt;/p&gt;&#xA;" OwnerUserId="897" LastActivityDate="2018-02-01T17:13:23.023" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24720" PostTypeId="2" ParentId="24702" CreationDate="2018-02-01T19:45:29.137" Score="5" Body="&lt;p&gt;There has been a book on the topic I can not seem to find from my memory.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However in searching I found a good resource, in the PDF at NASA called Mir Heritage.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.hq.nasa.gov/pao/History/SP-4225/documentation/mhh/mirheritage.pdf&quot; rel=&quot;nofollow noreferrer&quot;&gt;https://www.hq.nasa.gov/pao/History/SP-4225/documentation/mhh/mirheritage.pdf&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Looks like someone Wiki-ized the PDF into web pages:&#xA;&lt;a href=&quot;https://en.wikisource.org/wiki/Mir_Hardware_Heritage/Figures&quot; rel=&quot;nofollow noreferrer&quot;&gt;Wiki'ed version&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;What is interesting is how they depict the state of Mir at different points, which I think is what the question wants, but also I enjoyed since it makes visualization so much easier.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Alas, it ends in 1995, and would be nice to have a more up to date version.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;An example page, showing some of the 1993 configurations is a nice example.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/7wigD.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/7wigD.png&quot; alt=&quot;Sample page&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;And just for fun, since I love this style of photo, here is a Mir line art drawing from the doc.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5qdhE.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5qdhE.png&quot; alt=&quot;Mir Complex&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;" OwnerUserId="51" LastEditorUserId="51" LastEditDate="2018-02-01T20:00:20.247" LastActivityDate="2018-02-01T20:00:20.247" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24721" PostTypeId="2" ParentId="1316" CreationDate="2018-02-01T19:48:18.897" Score="1" Body="&lt;p&gt;Desirable qualities of a launch site:&lt;/p&gt;&#xA;&#xA;&lt;ol&gt;&#xA;&lt;li&gt;Near the equator, to take advantage of the Earth's rotation speed, allowing a very slightly greater payload for the same cost in fuel weight&lt;/li&gt;&#xA;&lt;li&gt;Good access to rail and/or port, because big rockets require lots of big pieces to be brought in for assembly&lt;/li&gt;&#xA;&lt;li&gt;Acceptable safety zone, because are slow burning bombs that do not always politely burn slowly&lt;/li&gt;&#xA;&lt;/ol&gt;&#xA;&#xA;&lt;p&gt;NASA (Florida) and ESA (French Guiana) fit the pattern.  Sea Launch was designed with these factors in mind.&lt;/p&gt;&#xA;" OwnerUserId="22780" LastEditorUserId="22780" LastEditDate="2018-02-03T17:45:20.577" LastActivityDate="2018-02-03T17:45:20.577" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24722" PostTypeId="2" ParentId="24706" CreationDate="2018-02-01T21:00:26.913" Score="2" Body="&lt;p&gt;Any acceleration of the spacecraft requires a reaction mass. With a rocket, that reaction mass is immediately expelled. With a flywheel, that reaction mass is retained. If one wishes to repeatedly increase and decrease the rotational velocity, then retaining the reaction mass means that it can be reused. If we're simply accelerating the spacecraft once, then there's no advantage to retaining the reaction mass. Since the change in momentum in the reaction mass must match the change in momentum of the spacecraft, increasing the velocity of the reaction mass decreases the amount of reaction mass required. Rocket exhaust can be accelerated to greater velocity than a flywheel , and so is more efficient.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Furthermore, if the purpose of the spinning is to create gyroscopic stability, then retaining the flywheel would mean that the spacecraft as a whole has zero angular momentum, and thus would defeat the purpose.&lt;/p&gt;&#xA;" OwnerUserId="21050" LastActivityDate="2018-02-01T21:00:26.913" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24723" PostTypeId="2" ParentId="24694" CreationDate="2018-02-01T21:21:41.963" Score="5" Body="&lt;p&gt;I think Adam and PearsonArtPhoto have given you the best answers in terms of the effort of getting from place to place.  However, since you are asking from the perspective of writing fiction, I want to give a slightly different angle for consideration.  Namely economics, and how they might impact the process.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It sounds like you are trying to decide where your characters departed from on their trip to Mars.  My response is that it would depend on how advanced/common space travel is in your setting.  So the questions I would ask are: How often are such trips made?  What percentage of the lift capacity from Earth is intended for Luna, Mars, or elsewhere?  How do those numbers compare with trips from those places back to Earth?  What about transshipping between such points without landing on Earth?&lt;/p&gt;&#xA;&#xA;&lt;p&gt;As explained in other answers, getting from Earth's surface to orbit is the most energy intensive step of any of the potential trips under discussion.  Between atmospheric density and acceleration forces, it is also the phase that has the greatest challenges in terms of engineering. Once we have reached a stable orbit, many considerations that govern the design of a launch vehicle don't apply to one that is intended to remain in space, or land on bodies with lighter gravity and/or a thinner atmosphere.  (Compare the Apollo rockets to the lunar lander &amp;amp; ascent module designs for a real example of this.)&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The point to all of this is, if your society has reached a point where space travel is frequent enough to make it economical, they will stop trying to build craft that go from the ground on one body to the ground on the other and back.  They will instead establish some form of transfer station.  A launch vessel would get passengers/cargo from Earth to orbit, where they would transfer to another vessel designed to make the trip to Luna, Mars, or wherever, while the launch vessel returned to Earth for another load.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;Whether such a transfer station exists at the other end of any given trip would depend on how much traffic goes there.  When the volume reaches a point where it is more cost effective to have dedicated surface to orbit craft, and the personnel to operate them, you can expect one to exist.  &lt;/p&gt;&#xA;&#xA;&lt;p&gt;In the case of Earth and Luna, they are close enough together that the same station would serve both.  For someplace like Mars, there would almost have to be a permanent human presence there, and frequent trips there and back, to make it worth while.  Until such a time, the transfer station at Earth would be used.&lt;/p&gt;&#xA;" OwnerUserId="16717" LastEditorUserId="6816" LastEditDate="2018-02-01T22:14:01.910" LastActivityDate="2018-02-01T22:14:01.910" CommentCount="0" ContentLicense="CC BY-SA 3.0" />
  <row Id="24724" PostTypeId="1" AcceptedAnswerId="24783" CreationDate="2018-02-01T22:02:20.750" Score="5" ViewCount="1820" Body="&lt;p&gt;I'm trying to determine an algorithm for orbit propagation for a satellite on &lt;code&gt;LEO&lt;/code&gt;, with high accuracy in a &lt;code&gt;six month&lt;/code&gt; time interval. The propagation should consider the Sun and Moon, and any other effects worth accounting for.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I'm going to integrate the orbit numerically. Now I'm using the equation, that describes a Keplerian motion perturbed by main effects of an oblate Earth.&#xA;&lt;a href=&quot;https://i.stack.imgur.com/8I7hg.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/8I7hg.png&quot; alt=&quot;Equation&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;However, I have to consider also other effects to achieve realistic results.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Could anyone suggest literature, or provide some links &amp;amp; references?&#xA;I'm trying to implement the methods and algorithms myself, please, don't suggest SW for this purpose. &lt;/p&gt;&#xA;" OwnerUserId="19219" LastEditorUserId="19219" LastEditDate="2018-02-03T19:24:42.613" LastActivityDate="2018-05-26T23:29:21.543" Title="How to determine an algorithm for Low Earth Orbit propagation, considering perturbation from the moon, sun, etc?" Tags="|orbital-mechanics|low-earth-orbit|n-body-problem|" AnswerCount="5" CommentCount="3" ContentLicense="CC BY-SA 3.0" />
  <row Id="24725" PostTypeId="2" ParentId="24546" CreationDate="2018-02-01T23:26:16.123" Score="3" Body="&lt;p&gt;&lt;em&gt;Full disclosure: ICEYE employee, I'll try my best to answer. Thank you for asking and being interested in ICEYE-X1!&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The &quot;amount of seconds&quot; refers to the amount of time the satellite is actively imaging data for an individual frame.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;I would not recommend referring to the current Spaceflight101.com article for technical specifications on ICEYE-X1.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The original Alaska image in the CISION/PR Newswire press release has unfortunately been upscaled by the service itself to 1600x1600 pixel size, which may have caused unnecessary confusion. The &quot;original&quot; released image on the ICEYE website press release section is the correct one (&lt;a href=&quot;https://www.iceye.com/hubfs/PNG-ICEYE-X1-Alaska-Noatak-National-Preserve-compressed.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;925x925 pixel png file&lt;/a&gt;) to look into, but that file is compressed down from the true original. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;More &lt;a href=&quot;https://www.iceye.com/lp/iceye-x1-sar-imagery-example-set-jan-2018&quot; rel=&quot;nofollow noreferrer&quot;&gt;ICEYE-X1 SAR imagery at a far higher resolution has recently been made available&lt;/a&gt; on the company website (for free but requires filling in a form), which you may find to be informative and valuable to answer these questions.&lt;/p&gt;&#xA;" OwnerUserId="22784" LastActivityDate="2018-02-01T23:26:16.123" CommentCount="9" ContentLicense="CC BY-SA 3.0" />
  <row Id="24727" PostTypeId="2" ParentId="24694" CreationDate="2018-02-02T01:11:24.960" Score="1" Body="&lt;p&gt;A couple of other answers have pointed out that the orbit of Mars is outside that of the Earth&amp;mdash;that is, it's farther away from the sun.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;To be more specific about &lt;em&gt;when&lt;/em&gt; the moon will be closer to Mars: a full Moon is a full Moon because we're seeing the side of it that's illuminated by the sun. If we drew it out on a plane as viewed from above, when there was a straight line directly from the center of the sun, going through the center of the Earth, then out to the center of the Moon, that would be a full Moon, and the point at which the Moon is closest to the path of Mars' orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;At new Moon, there would be a straight line from the sun, through the Moon, to the Earth. That's the time at which the Moon is farthest away from the path of Mars' orbit.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Note that wording though: &quot;the path of Mars' orbit&quot;, not just &quot;closest to Mars&quot; or &quot;farthest from Mars&quot;. Depending on exactly &lt;em&gt;when&lt;/em&gt; you look, Mars could be on the other side of the sun from the Earth. In this case, a new moon (closer to the sun, farther away from Mars' orbit) is &lt;em&gt;also&lt;/em&gt; closer to Mars itself than the Earth is.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Earth's orbit is about 149.6 million kilometers in diameter and Mars' orbit is about 228 million kilometers in diameter (average, in both cases). At their closest, the Earth and Mars are about 78 million kilometers apart. At their farthest, they're about 189 million kilometers apart.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;The Moon's orbit around the earth is about 385 thousand kilometers. So even when the Earth and Mars are at their closest, the distance saved by being on the Moon instead of Earth is so small that in normal calculations, it's pretty much lost to rounding error.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;If you're interested in traveling from the Earth to Mars, the actual distance is often nearly irrelevant. When sending rockets over long distances, we often use &quot;slingshot&quot; maneuvers in which the rocket travels (what appears to be) a long ways out of its way. For example, consider &lt;a href=&quot;http://www.theplanetstoday.com/juno_flight_path.html&quot; rel=&quot;nofollow noreferrer&quot;&gt;this animation of the path Juno followed to get from the earth to Jupiter&lt;/a&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's launched from Earth, travels outward for a ways, then comes back inward toward the sun, and goes quite a bit closer to the sun than the Earth (or Moon) ever does, before finally spiraling back outward to meet up with Jupiter.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;A spacecraft won't normally even try to just follow a straight line from point A to point B, so the closest approach between the Earth and Mars won't necessarily be the time that it's fastest or easiest to travel from one to the other.&lt;/p&gt;&#xA;" OwnerUserId="3508" LastEditorUserId="6816" LastEditDate="2018-02-02T01:22:20.840" LastActivityDate="2018-02-02T01:22:20.840" CommentCount="1" ContentLicense="CC BY-SA 3.0" />
  <row Id="24728" PostTypeId="2" ParentId="24694" CreationDate="2018-02-02T01:54:32.807" Score="0" Body="&lt;p&gt;Here's probably the best thing for your &lt;strong&gt;story&lt;/strong&gt;.&lt;/p&gt;&#xA;&lt;p&gt;&lt;em&gt;During the time period &lt;strong&gt;of your story&lt;/strong&gt;&lt;/em&gt;, the Earth is always &lt;strong&gt;about the same&lt;/strong&gt; distance from Mars, whereas the moon spins around Earth.&lt;/p&gt;&#xA;&lt;p&gt;Every 14 days, it gets 1/2 million miles closer to Mars, then every 14 days, 1/2 million miles further away.&lt;/p&gt;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TNHny.png&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TNHny.png&quot; alt=&quot;enter image description here&quot; /&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&lt;h1&gt;Now just to be clear........&lt;/h1&gt;&#xA;&lt;p&gt;over the course of long periods of time - &lt;strong&gt;years&lt;/strong&gt; - &amp;quot;all of the Earth and Moon&amp;quot; and &amp;quot;all of Mars&amp;quot; of course change distances dramatically.&lt;/p&gt;&#xA;&lt;p&gt;But the fact is &lt;em&gt;&lt;strong&gt;during the time period of your story&lt;/strong&gt;&lt;/em&gt; it will be &amp;quot;about the same&amp;quot; distance from Mars to Earth.&lt;/p&gt;&#xA;&lt;h1&gt;- but -&lt;/h1&gt;&#xA;&lt;p&gt;every 14 days the Moon moves &amp;quot;back and fore&amp;quot; 1/2 million miles.&lt;/p&gt;&#xA;&lt;p&gt;Whereas &lt;em&gt;&lt;strong&gt;during the time period of your story&lt;/strong&gt;&lt;/em&gt; the Earth is always about the same distance.&lt;/p&gt;&#xA;&lt;p&gt;No matter now far the distance between Mars/Earth - of course, as the years pass that changes dramatically - every 14 days the Moon moves &amp;quot;closer and further away&amp;quot; by 1/2 million miles.&lt;/p&gt;&#xA;&lt;p&gt;Got it? Good! :)&lt;/p&gt;&#xA;" OwnerUserId="3289" LastEditorUserId="-1" LastEditDate="2020-06-17T08:54:11.483" LastActivityDate="2018-02-02T13:33:29.823" CommentCount="2" ContentLicense="CC BY-SA 3.0" />
  <row Id="24729" PostTypeId="2" ParentId="24703" CreationDate="2018-02-02T02:05:57.850" Score="7" Body="&lt;p&gt;&lt;em&gt;Preface: By popular request, converting my comment into an answer. As a professional SpaceX nerd, this is going to have to be a multi-phase answer as I don't have the resources to fill it out all in one sitting.&lt;/em&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Most are unaware that SpaceX has actually performed post-separation experimental tests to determine the feasibility, and later, the acceptable envelope of recovery &lt;strong&gt;on every Falcon 9 vehicle&lt;/strong&gt;, except for Falcon 9 Flight 5&lt;sup&gt;&lt;strong&gt;†&lt;/strong&gt;&lt;/sup&gt;.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;This is true even when SpaceX wasn't pursuing retropropulsion as a means of recovering Falcon, or when it was even thought impossible; and SpaceX still do it to this day. Even missions publicly marked as &quot;expendable&quot; have experimental post-mission testing performed, with the remaining fuel available, and current recovery program direction, informing what tests can be done. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;strong&gt;The overarching goal is to increasingly expand the envelope for recovery, trialling new techniques that improve margin, recoverability, or a myriad of other factors; and GovSat is just a single example of this.&lt;/strong&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;Lars Blackmore, Principal Rocket Landing Engineer at SpaceX, obviously would be one of the most authoritative sources on this topic. Luckily for us, &lt;a href=&quot;https://ee.stanford.edu/event/department/ee-distinguished-lecture-lars-blackmore-spacex&quot; rel=&quot;nofollow noreferrer&quot;&gt;he recently gave a talk at Stanford&lt;/a&gt; on Falcon recovery on 12 January, 2018. Here a paraphrased statement he made during that talk:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;We needed to add additional componentry to the rocket to make this [recovery] possible, including grid fins, landing legs, upgrades to shielding; as well as developing new untested techniques, flipping around, retropropuvlsive burns, aerodynamic guidance, and more.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;There's no way you can do this all in one mission. It's iteratively developed over dozens of missions, figuring out what worked, what didn't, and revising from there. &lt;/p&gt;&#xA;&#xA;&lt;h1&gt;Examples&lt;/h1&gt;&#xA;&#xA;&lt;p&gt;As SpaceX has learnt more about their vehicle, and how it performs when coming back to Earth, there's some fairly distinct phases throughout the program which become apparent. I'll cover GovSat first, then work chronologically through Falcon 9's recovery program to present. &lt;/p&gt;&#xA;&#xA;&lt;h2&gt;GovSat (2018)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;As mentioned by Russell, the specific post-mission testing for this flight was to trial a triple-engine landing burn to reduce gravity losses on approach — gravity losses referring to the Δv that must be spent to fight gravity. Usually this is taught in the context of a rocket going uphill, into orbit, as an example of why your trajectory should be as horizontal as possible, but it applies to Falcon 9 coming in to land too. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;The landing burn is the longest of the three recovery burns Falcon 9 performs, and every second it continues, there are effectively close to 9.81ms⁻¹ of Δv eliminated from your performance budget. An example landing burn is that of the Orbcomm OG2-2 landing profile, &lt;a href=&quot;https://space.stackexchange.com/a/13708/6537&quot;&gt;where 32 seconds were spent&lt;/a&gt; slowing the stage down from subsonic to zero velocity at zero metres. If you can halve that 32 seconds to 16 seconds, you've effectively saved 160ms⁻¹ of Δv from gravity losses alone. &lt;/p&gt;&#xA;&#xA;&lt;p&gt;This extra Δv—effectively extra propellant—can be spent either improving payload to orbit, delivering a fixed payload further, spent elsewhere in the recovery profile as desired, or justify the addition of heavier hardware if needed.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/TdSKv.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/TdSKv.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt;&lt;/p&gt;&#xA;&#xA;&lt;p&gt;SpaceX have already performed triple engine landing burns before, notably on GTO missions where it is effectively a requirement. Based off what &lt;a href=&quot;https://twitter.com/elonmusk/status/958847818583584768&quot; rel=&quot;nofollow noreferrer&quot;&gt;@elonmusk has noted on Twitter&lt;/a&gt;, my expectation is this was a triple engine landing all the way down to the barge — previous triple engine landings resulted in outboard engine shutoff in the last few seconds.&lt;/p&gt;&#xA;&#xA;&lt;h2&gt;Falcon 9 Early Days (2008–2011)&lt;/h2&gt;&#xA;&#xA;&lt;p&gt;Initially, the plan was, like Falcon 1 (which also wasn't successful in this regard), to use a series of large chutes to slow the vehicle down, ensuring a &quot;gentle&quot; splashdown in the ocean under canopy.&lt;/p&gt;&#xA;&#xA;&lt;p&gt;On the first few flights, this manifested as four parachutes present in the interstage of the first stage:&lt;/p&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://i.stack.imgur.com/5QZZ9.jpg&quot; rel=&quot;nofollow noreferrer&quot;&gt;&lt;img src=&quot;https://i.stack.imgur.com/5QZZ9.jpg&quot; alt=&quot;enter image description here&quot;&gt;&lt;/a&gt; &lt;/p&gt;&#xA;&#xA;&lt;p&gt;It's worth noting this never worked; the stage always broke up before the parachutes had a chance to deploy. Courtesy Gwynne Shotwell:&lt;/p&gt;&#xA;&#xA;&lt;blockquote&gt;&#xA;  &lt;p&gt;At the Space Access conference in April, Gwynne Shotwell, SpaceX's president, admitted: &quot;We have recovered pieces of the first stages.&quot; The first stages weren't even getting as far as deploying their parachutes – they were breaking up during atmospheric re-entry.&lt;/p&gt;&#xA;&lt;/blockquote&gt;&#xA;&#xA;&lt;p&gt;&lt;a href=&quot;https://www.newscientist.com/blogs/shortsharpscience/2011/09/falcon-rockets-to-land-on-thei.html&quot; rel=&quot;nofollow noreferrer&quot;